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ABSTRACT. Abstract
(This cover sheet to be replaced by ISO.)
This document specifies the form and establishes the interpretation of programs expressed in the
programming language C. Its purpose is to promote portability, reliability, maintainability, and
efficient execution of C language programs on a variety of computing systems.
Clauses are included that detail the C language itself and the contents of the C language execution
library. Annexes summarize aspects of both of them, and enumerate factors that influence the
portability of C programs.
Although this document is intended to guide knowledgeable C language programmers as well as
implementors of C language translation systems, the document itself is not designed to serve as a
tutorial.
Recipients of this draft are invited to submit, with their comments, notification of any relevant
patent rights of which they are aware and to provide supporting documentation.
The following documents, for all intents and purposes, have been applied to this draft from before
and during the October 2019 Meeting:
DR 476 volatile semantics for lvalues
DR 488 c16rtomb() on wide characters encoded as multiple char16_t
DR 494 Part 1: Alignment specifier expression evaluation
DR 496 offsetof and subobjects (with editorial modification)
DR 497 "white-space character" defined in two places
DR 499 Anonymous structure in union behavior
DR 500 Ambiguous specification for FLT_EVAL_METHOD
DR 501 make DECIMAL_DIG obsolescent
FP DR 13 totalorder parameters
FP DR 20 changes for obsolescing DECIMAL_DIG
FP DR 21 printf of one-digit character string
FP DR 22 changes for obsolescing DECIMAL_DIG, Part 2
FP DR 23 llquantexp invalid case
FP DR 24 remainder NaN case
FP DR 25 totalorder parameters
N2124 and N2319 rounding direction macro FE_TONEARESTFROMZERO
N2186 Alternative to N2166
N2212 type generic cbrt (with editorial changes)
N2260 Clarifying the restrict Keyword v2
N2265 Harmonizing static_assert with C++
N2267 nodiscard attribute
N2270 maybe_unused attribute
N2271 CR for pow divide-by-zero case
N2293 Alignment requirements for memory management functions
N2314 TS 18661-1 plus CR/DRs for C2X
N2322 preprocessor line numbers unspecified
N2325 DBL_NORM_MAX etc
N2326 floating-point zero and other normalization
N2334 deprecated attribute
N2335 attributes
N2337 strftime, with ’b’ and ’B’ swapped
N2338 error indicator for encoding errors in fgetwc
N2341 TS 18661-2 plus CR/DRs for C2X
N2345 editors, resolve ambiguity of a semicolon
N2349 the memccpy function
N2350 defining new types in offsetof
N2353 the strdup and strndup functions
N2356 update for payload functions
N2358 no internal state for mblen
N2359 part 2 (remove WANT macros from numbered clauses) and part 3 (version macros for
changed library clauses)
N2401 TS 18661-4a for C2X
N2408 The fallthrough attribute
N2412 Two’s complement sign representation for C2x
N2417 Section 6: Add time conversion functions that are relatively thread-safe
N2418 Adding the u8 character prefix
N2432 Remove support for function definitions with identifier lists
N2508 Free Positioning of Labels Inside Compound Statements
N2554 Minor attribute wording cleanups
The following documents have been applied to this draft from the October 2019 Meeting:
N2379 *_IS_IEC_60559 Feature Test Macros.
N2416 Floating Point Negation and Conversion.
N2384 Annex F.8 Update for Implementation Extensions and Rounding.
N2424 Why logp1 as a Function Name.
N2406 Signaling NaN Initializers.
N2393 _Bool Definitions For true and false.
The following documents have been applied to this draft from the March/April 2020 Virtual
Meeting:
N2444 More optionally per-thread state for the library.
N2446 printf of NAN().
N2448 [[Nodiscard("should have a reason")]].
N2459 Add an interface to query resolution of time bases, v3.
N2464 Zero-size Reallocations are Undefined Behavior.
N2476 Names and Locations of Floating Point Entities.
N2480 Allowing unnamed parameters in function definitions.
N2490 Why no wide string strfrom functions.
The following documents have been applied to this draft from the August 2020 Virtual Meeting:
N2491 powr justification
N2492 Note About Math Function Properties.
N2506 Range Errors in Math Functions.
N2508 Free Positioning of Labels.
N2517 Clarification Request for C17 Example of Undefined Behavior.
N2532 Min-max Functions.
N2553 Querying Attribute Support.
N2554 Minor Attribute Wording Cleanup.
The following documents have been applied to this draft from the October and November 2020
Virtual Meetings:
N2546 Missing DEC_EVAL_METHOD
N2547 Missing const in decimal getpayload functions
N2548 intmax_t removal from FP functions
N2549 Binary Literals
N2552 Editorial cleanup for rounding macros
N2557 Allow Duplicate Attributes
N2560 FP hex formatting precision
N2562 Unclear type relationship between a format specifier and its argument
N2563 Character encoding of diagnostic text
N2564 Range errors and math functions (updated previous version, N2506)
N2570 Feature and WANT macros for Annex F functions
N2571 snprintf nonnegative clarification
N2572 What We Think We Reserve
N2580 Decimal Floating Point Triples
N2586 Sufficient Formatting Precision
N2594 Remove Mixed Wide String Literal Concatenation
N2559 Update to IEC 60559:2020
N2600 Update to IEC 60559:2020 (updates previous version, N2559)
N2602 Infinity/NAN Macros, Editorial Fixes
N2607 Compatibility of Pointers to Arrays with Qualifiers
The following documents have been applied to this draft from the March/April 2021 Virtual
Meeting:
N2524 String Functions for Freestanding Implementations
N2626 Digit Separators
N2630 Formatting Input/Output of Binary Integer Numbers
N2640 Missing DEC_EVAL_METHOD, Take 2
N2641 Missing +(x) in Table
N2643 Negative vs. Less Than Zero
N2645 Add Support for Preprocessing Directives #elifdef and #elifndef
N2680 Specific Width Length Modifier for Formatting
The following documents have been applied to this draft from the June 2021 Virtual Meeting:
N2651 fabs and copysign Cleanup
N2662 [[maybe_unused]] for Labels
N2665 Zero-size Reallocations Are No Longer an Obsolescent Feature
N2670 Zeros Compare Equal
N2671 Negative Values
N2672 §5.2.4.2.2 Cleanup
N2683 Towards Integer Safety
N2751 signbit Cleanup
N2763 Adding a Fundamental Type for N-bit Integers
The following documents have been applied to this draft from the August/September 2021 Virtual
Meeting:
N2686 #warning Directive
N2688 Sterile Characters
N2710 SNAN Fixes
N2711 fmin, fmax
N2713 Integer Constant Expressions
N2714 hypot Changes
N2715 cr_ Prefix Potentially Reserved for Identifiers
N2716 Fix "numerically"/"numerically equal" Usage
N2726 _Imaginary_I and _Complex_I Qualifiers
N2728 char16_t & char32_t String Literals Shall be UTF-16 & UTF-32
N2745 Range Error Definition
N2748 Effects of fenv Exception Functions
N2749 IEC 60559 Bindings
N2755 Static Initialization of Decimal Floating Point
N2776 ckd_* Identifiers Should be Potentially Reserved Identifiers
N2799 __has_include for C
The following documents have been applied to this draft from the November/December 2021
Virtual Meeting:
N2747 Annex F Overflow and Underflow
N2770 Remove UB from Incomplete Types in Function Parameters
N2778 Require Variably-Modified Types
N2781 Types do not have Types (with meeting-agreed changes plus some editorial changes)
N2790 "remquo" Changes
N2805 Overflow and Underflow Definitions
N2806 §5.2.4.2.2 Cleanup, Again
N2808 Allow 16-bit ptrdiff_t
N2823 Freestanding CFP Functions
N2838 Types and Sizes
N2837 Clarifying Integer Terms (also, delete Annex H and replace with the Floating Point TS
/ Annex merge)
N2842 Normal and Subnormal Classification
N2843 Clarification of Max Exponent Macros
N2845 feraiseexcept Update
N2846 Clarification about Expression Transformations
N2848 INFINITY Macro Contradictions (Wording 1 only!)
N2872 Require Exact-Width Integer Type Interfaces, Part I (Change from proposal’s §3.1 only)
The following documents have been applied to this draft from the January/February 2022 Virtual
Meeting, Parts 1 and 2:
N2653 char8_t: A type for UTF-8 characters and strings
N2701 @, $, and ‘ in the source/execution character set
N2754 Decimal Floating Point: Quantum Exponent of NaN
N2762 Fixes for Potentially Reserved Identifiers
N2764 The _Noreturn Attribute
N2775 Literal Suffixes for Bit-Precise Integers
N2797 *_HAS_SUBNORM == 0 Implies What?
N2810 calloc Overflow Handling
N2819 Disambiguate the Storage Class of Some Compound Literals
N2826 unreachable()
N2828 Unicode Sequences More Than 21 Bits are a Constraint Violation
N2829 Make assert() user friendly in C
N2836 Unicode Syntax Identifiers for C
N2840 Make call_once() Mandatory
N2841 No Function Declarators without Prototypes
N2844 Remove default promotions for _FloatN Types
N2847 Revised Suggestions of Change for Numerically Equal / Equivalent
N2879 5.2.4.2.2 Cleanup, Again Again
N2880 Overflow and Underflow Definitions Update
N2881 Normal and Subnormal Classification Update
N2882 Clarification for the Max Exponent Macros
N2900 Consistent, Warningless, and Intuitive Initialization with {}
N2927 Not-So-Magic: typeof(...)
N2931 Macros and Macro Spellings from C Floating Point Integration
N2934 Revised Spelling of Keywords
N2935 Make false and true Language Features
N2937 Properly Define Blocks in the Grammar
The following documents have been applied to this draft from the May 2022 Virtual Meeting:
N2601 Annex X (replacing Annex H) for IEC 60599 Interchange (ratified early 2021 but
integrated over a long period of time).
N2861 Indeterminate Values and Trap Representations
N2867 Checked N-Bit Integers? (Not Now)
N2886 Remove ATOMIC_VAR_INIT
N2888 Require Exact-width Integer Type Interfaces, Part II
N2897 memset_explicit
N2992 Wording Clarification for Variably-Modified Types
The following documents have been applied to this draft from the July 2022 Virtual Meeting:
N2930 Change remove_quals to typeof_unqual
N2939 Identifier Syntax Fixes
N2940 Remove Trigraphs??!
N2969 (nice) Bit-Precise Bit Fields
N2974 Queryable Pointer Alignment
N3029 Improved Normal Enumerations
N2975 Relax requirements for va_start
N2993 Make *_HAS_SUBNORM Obsolete
N3011 Oops, Empty Initializers in Compound Literals
N3030 Enhanced Enumerations
N2951 Freestanding C and IEC 60559 Conformance Scope Reduction
N2956 Unsequenced Functions
N3033 Comma Ommission and Deletion (__VA_OPT__ and Preprocessor Wording Improve-
ments)
N3035 _BitInt(...) Fixes
N3006 Underspecified Object Declarations
N3007 Type Inference for Object Declarations
N3018 constexpr for Object Definitions
N3038 Introduce Storage Class Specifiers for Compound Literals
N3034 Identifier Primary Expressions
N3042 Introduce the nullptr_t constant, nullptr
N2929 Memory Layout of union s
N3037 Improved Tag Compatibility
N3020 Qualifier-preserving Standard Functions
N3022 Modern Bit Utilities - without Rotate Left/Right, Memory Reversal ("byteswap"), or
Endian-Aware Load/Store
N3017 #embed
N2957 New Optional Time Bases
In addition to these, the document has undergone some editorial changes, including the following.
— The synopsis lists in Annex B are now generated automatically and classified according to
the feature test or WANT macros that are required to make them available.
— A new non-normative clause J.6 added to Annex J categorizes identifiers used by this
document.
— Renaming of the syntax term "struct declaration", "struct declaration list" "struct declarator",
and "struct declarator list" to the more appropriate "member declaration", "member declaration
list", "member declarator" and "member declarator list", respectively.
— Misspelling of "invokation" fixed to "invocation".
— A positional reference to a table was changed to be a more direct reference due to unfortunate
page breaks.
— Missing macros were added to from <float.h> and <limits.h>.
— A footnote added for simple atomic assignment (6.5.16).
— The _Bool expansion macros were properly defined and fixed for true and false.
— An issue with "modifying object" being removed from an earlier draft was fixed. This was a
mistake: side effects do include modifying an object.
— The Decimal Floating Point Initialization text was not well-worded. It was fixed after the
paper adding the wording was integrated.
— Examples using poor phrasing for objects and their types were fixed to say "object(s) of type
int" and similar.
— The terms "floating-point type" and "floating-point constant" were changed to just be
"floating type" and "floating constant", as are defined in the standard, respectively.
— The wording "thread-local storage" was normalized to be "thread storage" everywhere, as
intended (this is the word defined by the standard, the other just fell naturally out of casual
usage and thought).
— A footnote clarifying the role for valid pointers with zero size was added to the library
frontmatter, specifically concerning functions like memcpy and memset.
— Various duplicate spellings (e.g. "function functions" and similar) were removed and typos
were fixed (e.g., "stirng" and similar).
— The pp-number production was incorrect for digit separators. Adjusted and fixed.
— The wording for freestanding heads for <string.h> were very poorly done. It was changed
to have better wording.
— The introductory sentence for the implementation limits was very wordy and deeply confus-
ing to normal users. The sentence was adjusted to read much better and more clearly.
— In a sentence using "respectively" for fmin and fmax descriptions, the order of the respective
items was swapped. This gave the wrong definitions to each item. They were put in the
proper order.
— A missing closing parenthesis in Annex J was fixed.
— The term "floating-point multiply add" was changed to "fused multiply add", matching
naming conventions in reality.
CONTENTS. Contents
Foreword
Introduction
~~1. Scope
~~
~~2. Normative references
~~
~~3. Terms, definitions, and symbols
~~
~~4. Conformance
~~
~~5. Environment
~~ 5.1 Conceptual models
~~ 5.1.1 Translation environment
~~ 5.1.2 Execution environments
~~ 5.2 Environmental considerations
~~ 5.2.1 Character sets
~~ 5.2.2 Character display semantics
~~ 5.2.3 Signals and interrupts
~~ 5.2.4 Environmental limits
~~
~~
~~ 6.1 Notation
~~ 6.2 Concepts
~~ 6.2.1 Scopes of identifiers
~~ 6.2.2 Linkages of identifiers
~~ 6.2.3 Name spaces of identifiers
~~ 6.2.4 Storage durations of objects
~~ 6.2.5 Types
~~ 6.2.6 Representations of types
~~ 6.2.7 Compatible type and composite type
~~ 6.2.8 Alignment of objects
~~ 6.2.9 Encodings
~~ 6.3 Conversions
~~ 6.3.1 Arithmetic operands
~~ 6.3.2 Other operands
~~ 6.4 Lexical elements
~~ 6.4.1 Keywords
~~ 6.4.2 Identifiers
~~ 6.4.3 Universal character names
~~ 6.4.4 Constants
~~ 6.4.5 String literals
~~ 6.4.6 Punctuators
~~ 6.4.7 Header names
~~ 6.4.8 Preprocessing numbers
~~ 6.4.9 Comments
~~ 6.5 Expressions
~~ 6.5.1 Primary expressions
~~ 6.5.2 Postfix operators
~~ 6.5.3 Unary operators
~~ 6.5.4 Cast operators
~~ 6.5.5 Multiplicative operators
~~ 6.5.6 Additive operators
~~ 6.5.7 Bitwise shift operators
~~ 6.5.8 Relational operators
~~ 6.5.9 Equality operators
~~ 6.5.10 Bitwise AND operator
~~ 6.5.11 Bitwise exclusive OR operator
~~ 6.5.12 Bitwise inclusive OR operator
~~ 6.5.13 Logical AND operator
~~ 6.5.14 Logical OR operator
~~ 6.5.15 Conditional operator
~~ 6.5.16 Assignment operators
~~ 6.5.17 Comma operator
~~ 6.6 Constant expressions
~~ 6.7 Declarations
~~ 6.7.1 Storage-class specifiers
~~ 6.7.2 Type specifiers
~~ 6.7.3 Type qualifiers
~~ 6.7.4 Function specifiers
~~ 6.7.5 Alignment specifier
~~ 6.7.6 Declarators
~~ 6.7.7 Type names
~~ 6.7.8 Type definitions
~~ 6.7.9 Type inference
~~ 6.7.10 Initialization
~~ 6.7.11 Static assertions
~~ 6.7.12 Attributes
~~ 6.8 Statements and blocks
~~ 6.8.1 Labeled statements
~~ 6.8.2 Compound statement
~~ 6.8.3 Expression and null statements
~~ 6.8.4 Selection statements
~~ 6.8.5 Iteration statements
~~ 6.8.6 Jump statements
~~ 6.9 External definitions
~~ 6.9.1 Function definitions
~~ 6.9.2 External object definitions
~~ 6.10 Preprocessing directives
~~ 6.10.1 Conditional inclusion
~~ 6.10.2 Source file inclusion
~~ 6.10.3 Binary resource inclusion
~~ 6.10.4 Macro replacement
~~ 6.10.5 Line control
~~ 6.10.6 Diagnostic directives
~~ 6.10.7 Pragma directive
~~ 6.10.8 Null directive
~~ 6.10.9 Predefined macro names
~~ 6.10.10 Pragma operator
~~ 6.11 Future language directions
~~ 6.11.1 Floating types
~~ 6.11.2 Linkages of identifiers
~~ 6.11.3 External names
~~ 6.11.4 Character escape sequences
~~ 6.11.5 Storage-class specifiers
~~ 6.11.6 Pragma directives
~~ 6.11.7 Predefined macro names
~~
~~
~~ 7.1 Introduction
~~ 7.1.1 Definitions of terms
~~ 7.1.2 Standard headers
~~ 7.1.3 Reserved identifiers
~~ 7.1.4 Use of library functions
~~ 7.2 Diagnostics <assert.h>
~~ 7.2.1 Program diagnostics
~~ 7.3 Complex arithmetic <complex.h>
~~ 7.3.1 Introduction
~~ 7.3.2 Conventions
~~ 7.3.3 Branch cuts
~~ 7.3.4 The CX_LIMITED_RANGE pragma
~~ 7.3.5 Trigonometric functions
~~ 7.3.6 Hyperbolic functions
~~ 7.3.7 Exponential and logarithmic functions
~~ 7.3.8 Power and absolute-value functions
~~ 7.3.9 Manipulation functions
~~ 7.4 Character handling <ctype.h>
~~ 7.4.1 Character classification functions
~~ 7.4.2 Character case mapping functions
~~ 7.5 Errors <errno.h>
~~ 7.6 Floating-point environment <fenv.h>
~~ 7.6.1 The FENV_ACCESS pragma
~~ 7.6.2 The FENV_ROUND pragma
~~ 7.6.3 The FENV_DEC_ROUND pragma
~~ 7.6.4 Floating-point exceptions
~~ 7.6.5 Rounding and other control modes
~~ 7.6.6 Environment
~~ 7.7 Characteristics of floating types <float.h>
~~ 7.8 Format conversion of integer types <inttypes.h>
~~ 7.8.1 Macros for format specifiers
~~ 7.8.2 Functions for greatest-width integer types
~~ 7.9 Alternative spellings <iso646.h>
~~ 7.10 Characteristics of integer types <limits.h>
~~ 7.11 Localization <locale.h>
~~ 7.11.1 Locale control
~~ 7.11.2 Numeric formatting convention inquiry
~~ 7.12 Mathematics <math.h>
~~ 7.12.1 Treatment of error conditions
~~ 7.12.2 The FP_CONTRACT pragma
~~ 7.12.3 Classification macros
~~ 7.12.4 Trigonometric functions
~~ 7.12.5 Hyperbolic functions
~~ 7.12.6 Exponential and logarithmic functions
~~ 7.12.7 Power and absolute-value functions
~~ 7.12.8 Error and gamma functions
~~ 7.12.9 Nearest integer functions
~~ 7.12.10 Remainder functions
~~ 7.12.11 Manipulation functions
~~ 7.12.12 Maximum, minimum, and positive difference functions
~~ 7.12.13 Fused multiply-add
~~ 7.12.14 Functions that round result to narrower type
~~ 7.12.15 Quantum and quantum exponent functions
~~ 7.12.16 Decimal re-encoding functions
~~ 7.12.17 Comparison macros
~~ 7.13 Non-local jumps <setjmp.h>
~~ 7.13.1 Save calling environment
~~ 7.13.2 Restore calling environment
~~ 7.14 Signal handling <signal.h>
~~ 7.14.1 Specify signal handling
~~ 7.14.2 Send signal
~~ 7.15 Alignment <stdalign.h>
~~ 7.16 Variable arguments <stdarg.h>
~~ 7.16.1 Variable argument list access macros
~~ 7.17 Atomics <stdatomic.h>
~~ 7.17.1 Introduction
~~ 7.17.2 Initialization
~~ 7.17.3 Order and consistency
~~ 7.17.4 Fences
~~ 7.17.5 Lock-free property
~~ 7.17.6 Atomic integer types
~~ 7.17.7 Operations on atomic types
~~ 7.17.8 Atomic flag type and operations
~~ 7.18 Bit and byte utilities <stdbit.h>
~~ 7.18.1 General
~~ 7.18.2 Endian
~~ 7.18.3 Count Leading Zeros
~~ 7.18.4 Count Leading Ones
~~ 7.18.5 Count Trailing Zeros
~~ 7.18.6 Count Trailing Ones
~~ 7.18.7 First Leading Zero
~~ 7.18.8 First Leading One
~~ 7.18.9 First Trailing Zero
~~ 7.18.10 First Trailing One
~~ 7.18.11 Count Ones
~~ 7.18.12 Count Zeros
~~ 7.18.13 Single-bit Check
~~ 7.18.14 Bit Width
~~ 7.18.15 Bit Floor
~~ 7.18.16 Bit Ceiling
~~ 7.19 Boolean type and values <stdbool.h>
~~ 7.20 Checked Integer Arithmetic <stdckdint.h>
~~ 7.20.1 The ckd_ Checked Integer Operation Macros
~~ 7.21 Common definitions <stddef.h>
~~ 7.21.1 The unreachable macro
~~ 7.21.2 The nullptr_t type
~~ 7.22 Integer types <stdint.h>
~~ 7.22.1 Integer types
~~ 7.22.2 Widths of specified-width integer types
~~ 7.22.3 Width of other integer types
~~ 7.22.4 Macros for integer constants
~~ 7.22.5 Maximal and minimal values of integer types
~~ 7.23 Input/output <stdio.h>
~~ 7.23.1 Introduction
~~ 7.23.2 Streams
~~ 7.23.3 Files
~~ 7.23.4 Operations on files
~~ 7.23.5 File access functions
~~ 7.23.6 Formatted input/output functions
~~ 7.23.7 Character input/output functions
~~ 7.23.8 Direct input/output functions
~~ 7.23.9 File positioning functions
~~ 7.23.10 Error-handling functions
~~ 7.24 General utilities <stdlib.h>
~~ 7.24.1 Numeric conversion functions
~~ 7.24.2 Pseudo-random sequence generation functions
~~ 7.24.3 Memory management functions
~~ 7.24.4 Communication with the environment
~~ 7.24.5 Searching and sorting utilities
~~ 7.24.6 Integer arithmetic functions
~~ 7.24.7 Multibyte/wide character conversion functions
~~ 7.24.8 Multibyte/wide string conversion functions
~~ 7.24.9 Alignment of memory
~~ 7.25 _Noreturn <stdnoreturn.h>
~~ 7.26 String handling <string.h>
~~ 7.26.1 String function conventions
~~ 7.26.2 Copying functions
~~ 7.26.3 Concatenation functions
~~ 7.26.4 Comparison functions
~~ 7.26.5 Search functions
~~ 7.26.6 Miscellaneous functions
~~ 7.27 Type-generic math <tgmath.h>
~~ 7.28 Threads <threads.h>
~~ 7.28.1 Introduction
~~ 7.28.2 Initialization functions
~~ 7.28.3 Condition variable functions
~~ 7.28.4 Mutex functions
~~ 7.28.5 Thread functions
~~ 7.28.6 Thread-specific storage functions
~~ 7.29 Date and time <time.h>
~~ 7.29.1 Components of time
~~ 7.29.2 Time manipulation functions
~~ 7.29.3 Time conversion functions
~~ 7.30 Unicode utilities <uchar.h>
~~ 7.30.1 Restartable multibyte/wide character conversion functions
~~ 7.31 Extended multibyte and wide character utilities <wchar.h>
~~ 7.31.1 Introduction
~~ 7.31.2 Formatted wide character input/output functions
~~ 7.31.3 Wide character input/output functions
~~ 7.31.4 General wide string utilities
~~ 7.31.4.1 Wide string numeric conversion functions
~~ 7.31.4.2 Wide string copying functions
~~ 7.31.4.3 Wide string concatenation functions
~~ 7.31.4.4 Wide string comparison functions
~~ 7.31.4.5 Wide string search functions
~~ 7.31.4.6 Introduction
~~ 7.31.4.7 Miscellaneous functions
~~ 7.31.5 Wide character time conversion functions
~~ 7.31.6 Extended multibyte/wide character conversion utilities
~~ 7.31.6.1 Single-byte/wide character conversion functions
~~ 7.31.6.2 Conversion state functions
~~ 7.31.6.3 Restartable multibyte/wide character conversion functions
~~ 7.31.6.4 Restartable multibyte/wide string conversion functions
~~ 7.32 Wide character classification and mapping utilities <wctype.h>
~~ 7.32.1 Introduction
~~ 7.32.2 Wide character classification utilities
~~ 7.32.2.1 Wide character classification functions
~~ 7.32.2.2 Extensible wide character classification functions
~~ 7.32.3 Wide character case mapping utilities
~~ 7.32.3.1 Wide character case mapping functions
~~ 7.32.3.2 Extensible wide character case mapping functions
~~ 7.33 Future library directions
~~ 7.33.1 Complex arithmetic <complex.h>
~~ 7.33.2 Character handling <ctype.h>
~~ 7.33.3 Errors <errno.h>
~~ 7.33.4 Floating-point environment <fenv.h>
~~ 7.33.5 Characteristics of floating types <float.h>
~~ 7.33.6 Format conversion of integer types <inttypes.h>
~~ 7.33.7 Localization <locale.h>
~~ 7.33.8 Mathematics <math.h>
~~ 7.33.9 Signal handling <signal.h>
~~ 7.33.10 Atomics <stdatomic.h>
~~ 7.33.11 Boolean type and values <stdbool.h>
~~ 7.33.12 Bit and byte utilities <stdbit.h>
~~ 7.33.13 Checked Arithmetic Functions <stdckdint.h>
~~ 7.33.14 Integer types <stdint.h>
~~ 7.33.15 Input/output <stdio.h>
~~ 7.33.16 General utilities <stdlib.h>
~~ 7.33.17 String handling <string.h>
~~ 7.33.18 Date and time <time.h>
~~ 7.33.19 Threads <threads.h>
~~ 7.33.20 Extended multibyte and wide character utilities <wchar.h>
~~ 7.33.21 Wide character classification and mapping utilities <wctype.h>
~~
~~Annex A (informative) Language syntax summary
~~
~~Annex B (informative) Library summary
~~
~~Annex C (informative) Sequence points
~~
~~Annex D (informative) Universal character names for identifiers
~~
~~Annex E (informative) Implementation limits
~~
~~Annex F (normative) IEC 60559 floating-point arithmetic
~~
~~Annex G (normative) IEC 60559-compatible complex arithmetic
~~
~~Annex H (normative) IEC 60559 interchange and extended types
~~
~~Annex I (informative) Common warnings
~~
~~Annex J (informative) Portability issues
~~ Annex K (normative) Bounds-checking interfaces
~~
~~Annex L (normative) Analyzability
~~
~~Annex M (informative) Change History
FOREWORD. Foreword
1 ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that
are member of ISO or IEC participate in the development of International Standards through
technical committees established by the respective organization to deal with particular fields of
technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with ISO and IEC, also
take part in the work. In the field of information technology, ISO and IEC have established a joint
technical committee, ISO/IEC JTC 1.
2 The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for
the different types of document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
3 Attention is drawn to the possibility that some of the elements of this document may be the subject
of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent
rights. Details of any patent rights identified during the development of the document will be in the
Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).
4 Any trade name used in this document is information given for the convenience of users and does
not constitute an endorsement.
5 For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see the
following URL: www.iso.org/iso/foreword.html.
6 This document was prepared by Technical Committee ISO/IEC JTC 1, Information technology, Sub-
committee SC 22, Programming languages, their environments and system software interfaces.
7 This fifth edition cancels and replaces the fourth edition, ISO/IEC 9899:2018. A complete change
history can be found in Annex M.
INTRO. Introduction
1 With the introduction of new devices and extended character sets, new features could be added to
this document. Subclauses in the language and library clauses warn implementors and programmers
of usages which, though valid in themselves, could conflict with future additions.
2 Certain features are obsolescent, which means that they could be considered for withdrawal in future
revisions of this document. They are retained because of their widespread use, but their use in
new implementations (for implementation features) or new programs (for language [6.11] or library
features [7.33]) is discouraged.
3 This document is divided into four major subdivisions:
— preliminary elements (Clauses 1–4);
— the characteristics of environments that translate and execute C programs (Clause 5);
— the language syntax, constraints, and semantics (Clause 6);
— the library facilities (Clause 7).
4 Examples are provided to illustrate possible forms of the constructions described. Footnotes are
provided to emphasize consequences of the rules described in that subclause or elsewhere in this
document. References are used to refer to other related subclauses. Recommendations are provided
to give advice or guidance to implementors. Annexes define optional features, provide additional
information and summarize the information contained in this document. A bibliography lists
documents that were referred to during the preparation of this document.
5 The language clause (Clause 6) is derived from "The C Reference Manual".
6 The library clause (Clause 7) is based on the 1984 /usr/group Standard.
7 The Working Group responsible for this document (WG 14) maintains a site on the World Wide Web
at http://www.open-std.org/JTC1/SC22/WG14/ containing ancillary information that may be of
interest to some readers such as a Rationale for many of the decisions made during its preparation
and a log of Defect Reports and Responses.
1. Scope
1 This document specifies the form and establishes the interpretation of programs written in the C
programming language.1) It specifies
— the representation of C programs;
— the syntax and constraints of the C language;
— the semantic rules for interpreting C programs;
— the representation of input data to be processed by C programs;
— the representation of output data produced by C programs;
— the restrictions and limits imposed by a conforming implementation of C.
2 This document does not specify
— the mechanism by which C programs are transformed for use by a data-processing system;
— the mechanism by which C programs are invoked for use by a data-processing system;
— the mechanism by which input data are transformed for use by a C program;
— the mechanism by which output data are transformed after being produced by a C program;
— the size or complexity of a program and its data that will exceed the capacity of any specific
data-processing system or the capacity of a particular processor;
— all minimal requirements of a data-processing system that is capable of supporting a conform-
ing implementation.
1) This document is designed to promote the portability of C programs among a variety of data-processing systems. It is
intended for use by implementors and programmers. Annex J gives an overview of portability issues that a C program might
encounter.
2. Normative references
1 The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any amendments)
applies.
2 ISO/IEC 2382:2015, Information technology — Vocabulary. Available from the ISO online browsing
platform at http://www.iso.org/obp.
3 ISO 4217, Codes for the representation of currencies and funds.
4 ISO 8601, Data elements and interchange formats — Information interchange — Representation of dates and
times.
5 ISO/IEC 10646, Information technology —Universal Coded Character Set (UCS). Available from the
ISO/IEC Information Technology Task Force (ITTF) web site at http://isotc.iso.org/livelink/
livelink/fetch/2000/2489/Ittf_Home/PubliclyAvailableStandards.htm.
6 ISO/IEC 60559:2020, Floating-point arithmetic.
7 ISO 80000–2, Quantities and units — Part 2: Mathematical signs and symbols to be used in the natural
sciences and technology.
8 The Unicode Consortium. Unicode Standard Annex, UAX #44, Unicode Character Database [online].
Edited by Ken Whistler and Laurentiu Iancu. Available at http://www.unicode.org/reports/
tr44.
9 The Unicode Consortium. The Unicode Standard, Derived Core Properties. Available at https:
//www.unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt.
3. Terms, definitions, and symbols
1 For the purposes of this document, the terms and definitions given in ISO/IEC 2382, ISO 80000–2,
and the following apply.
2 ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3 Additional terms are defined where they appear in italic type or on the left side of a syntax rule.
Terms explicitly defined in this document are not to be presumed to refer implicitly to similar terms
defined elsewhere.
3.1
1 access (verb)
⟨execution-time action⟩ to read or modify the value of an object
2 Note 1 to entry: Where only one of these two actions is meant, "read" or "modify" is used.
3 Note 2 to entry: "Modify" includes the case where the new value being stored is the same as the previous value.
4 Note 3 to entry: Expressions that are not evaluated do not access objects.
3.2
1 alignment
requirement that objects of a particular type be located on storage boundaries with addresses that
are particular multiples of a byte address
3.3
1 argument
actual argument (DEPRECATED: actual parameter)
expression in the comma-separated list bounded by the parentheses in a function call expression, or
a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a
function-like macro invocation
3.4
1 behavior
external appearance or action
3.4.1
1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made
2 Note 1 to entry: J.3 gives an overview over properties of C programs that lead to implementation-defined behavior.
3 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer
is shifted right.
3.4.2
1 locale-specific behavior
behavior that depends on local conventions of nationality, culture, and language that each implemen-
tation documents
2 Note 1 to entry: J.4 gives an overview over properties of C programs that lead to locale-specific behavior.
3 EXAMPLE An example of locale-specific behavior is whether the islower function returns true for characters other than
the 26 lowercase Latin letters.
3.4.3
1 undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which
this document imposes no requirements
2 Note 1 to entry: Possible undefined behavior ranges from ignoring the situation completely with unpredictable results,
to behaving during translation or program execution in a documented manner characteristic of the environment (with or
without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic
message).
3 Note 2 to entry: J.2 gives an overview over properties of C programs that lead to undefined behavior.
4 EXAMPLE An example of undefined behavior is the behavior on dereferencing a null pointer.
3.4.4
1 unspecified behavior
behavior, that results from the use of an unspecified value, or other behavior upon which this
document provides two or more possibilities and imposes no further requirements on which is
chosen in any instance
2 Note 1 to entry: J.1 gives an overview over properties of C programs that lead to unspecified behavior.
3 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.
3.5
1 bit
unit of data storage in the execution environment large enough to hold an object that can have one
of two values
2 Note 1 to entry: It need not be possible to express the address of each individual bit of an object.
3.6
1 byte
addressable unit of data storage large enough to hold any member of the basic character set of the
execution environment
2 Note 1 to entry: It is possible to express the address of each individual byte of an object uniquely.
3 Note 2 to entry: A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The
least significant bit is called the low-order bit; the most significant bit is called the high-order bit.
3.7
1 character
⟨abstract⟩ member of a set of elements used for the organization, control, or representation of data
3.7.1
1 character
single-byte character
⟨C⟩ bit representation that fits in a byte
3.7.2
1 multibyte character
sequence of one or more bytes representing a member of the extended character set of either the
source or the execution environment
2 Note 1 to entry: The extended character set is a superset of the basic character set.
3.7.3
1 wide character
value representable by an object of type wchar_t, capable of representing any character in the
current locale
3.8
1 constraint
restriction, either syntactic or semantic, by which the exposition of language elements is to be
interpreted
3.9
1 correctly rounded result
representation in the result format that is nearest in value, subject to the current rounding mode, to
what the result would be given unlimited range and precision
2 Note 1 to entry: In this document, when the words "correctly rounded" are not immediately followed by "result", this is the
intended usage.
3 Note 2 to entry: IEC 60559 or implementation-defined rules apply for extreme magnitude results if the result format contains
infinity.
3.10
1 diagnostic message
message belonging to an implementation-defined subset of the implementation’s message output
3.11
1 forward reference
reference to a later subclause of this document that contains additional information relevant to this
subclause
3.12
1 implementation
particular set of software, running in a particular translation environment under particular con-
trol options, that performs translation of programs for, and supports execution of functions in, a
particular execution environment
3.13
1 implementation limit
restriction imposed upon programs by the implementation
3.14
1 memory location
either an object of scalar type, or a maximal sequence of adjacent bit-fields all having nonzero width
2 Note 1 to entry: Two threads of execution can update and access separate memory locations without interfering with each
other.
3 Note 2 to entry: A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to
two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a
zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration. It is not safe to concurrently
update two non-atomic bit-fields in the same structure if all members declared between them are also (nonzero-length)
bit-fields, no matter what the sizes of those intervening bit-fields happen to be.
4 EXAMPLE A structure declared as
struct {
char a;
int b:5, c:11,:0, d:8;
struct { int ee:8; } e;
}
contains four separate memory locations: The member a, and bit-fields d and e.ee are each separate memory locations,
and can be modified concurrently without interfering with each other. The bit-fields b and c together constitute the fourth
memory location. The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be.
3.15
1 object
region of data storage in the execution environment, the contents of which can represent values
2 Note 1 to entry: When referenced, an object can be interpreted as having a particular type; see 6.3.2.1.
3.16
1 parameter
formal parameter
DEPRECATED: formal argument
object declared as part of a function declaration or definition that acquires a value on entry to the
function, or an identifier from the comma-separated list bounded by the parentheses immediately
following the macro name in a function-like macro definition
3.17
1 recommended practice
specification that is strongly recommended as being in keeping with the intent of the standard, but
that might be impractical for some implementations
3.18
1 runtime-constraint
requirement on a program when calling a library function
2 Note 1 to entry: Despite the similar terms, a runtime-constraint is not a kind of constraint as defined by 3.8, and need not be
diagnosed at translation time.
3 Note 2 to entry: Implementations that support the extensions in Annex K are required to verify that the runtime-constraints
for a library function are not violated by the program; see K.3.1.4.
4 Note 3 to entry: Implementations that support Annex L are permitted to invoke a runtime-constraint handler when they
perform a trap.
3.19
1 value
precise meaning of the contents of an object when interpreted as having a specific type
3.19.1
1 implementation-defined value
unspecified value where each implementation documents how the choice is made
3.19.2
1 indeterminate representation
object representation that either represents an unspecified value or is a non-value representation
3.19.3
1 unspecified value
valid value of the relevant type where this document imposes no requirements on which value is
chosen in any instance
3.19.4
1 non-value representation
an object representation that does not represent a value of the object type
3.19.5
1 perform a trap
interrupt execution of the program such that no further operations are performed2)
2 Note 1 to entry: Implementations that support Annex L are permitted to invoke a runtime-constraint handler when they
perform a trap.
3.20
1 ⌈x⌉
ceiling of x
the least integer greater than or equal to x
2 EXAMPLE ⌈2.4⌉ is 3, ⌈−2.4⌉ is −2.
3.21
1 ⌊x⌋
floor of x
the greatest integer less than or equal to x
2 EXAMPLE ⌊2.4⌋ is 2, ⌊−2.4⌋ is −3.
3.22
1 wraparound
the process by which a value is reduced modulo 2N , where N is the width of the resulting type
2) Note that fetching a non-value representation might perform a trap but is not required to (see 6.2.6.1).
4. Conformance
1 In this document, "shall" is to be interpreted as a requirement on an implementation or on a program;
conversely, "shall not" is to be interpreted as a prohibition.
2 If a "shall" or "shall not" requirement that appears outside of a constraint or runtime-constraint is
violated, the behavior is undefined. Undefined behavior is otherwise indicated in this document by
the words "undefined behavior" or by the omission of any explicit definition of behavior. There is
no difference in emphasis among these three; they all describe "behavior that is undefined".
3 A program that is correct in all other aspects, operating on correct data, containing unspecified
behavior shall be a correct program and act in accordance with 5.1.2.3.
4 The implementation shall not successfully translate a preprocessing translation unit containing a
#error preprocessing directive unless it is part of a group skipped by conditional inclusion.
5 A strictly conforming program shall use only those features of the language and library specified
in this document.3) It shall not produce output dependent on any unspecified, undefined, or
implementation-defined behavior, and shall not exceed any minimum implementation limit.
6 The two forms of conforming implementation are hosted and freestanding. A conforming hosted
implementation shall accept any strictly conforming program. A conforming freestanding implementation
shall accept any strictly conforming program in which the use of the features specified in the library
clause (Clause 7) is confined to the contents of the standard headers <float.h>, <iso646.h>,
<limits.h>, <stdalign.h>, <stdarg.h>, <stdbit.h>, <stdbool.h>, <stddef.h>, <stdint.h>,
and <stdnoreturn.h>. Additionally, a conforming freestanding implementation shall accept any
strictly conforming program where:
— the features specified in the header <string.h> are used, except the following functions:
strdup, strndup, strcoll, strxfrm, strerror; and/or,
the selected function memalignment from <stdlib.h> is used.
A conforming implementation may have extensions (including additional library functions), pro-
vided they do not alter the behavior of any strictly conforming program4) .
7 The strictly conforming programs that shall be accepted by a conforming freestanding implementa-
tion that defines __STDC_IEC_60559_BFP__ or __STDC_IEC_60559_DFP__ may also use features in
the contents of the standard headers <fenv.h>, <math.h>, and the strto * floating-point numeric
conversion functions (7.24.1) of the standard header <stdlib.h>, provided the program does not
set the state of the FENV_ACCESS pragma to "ON".
All identifiers that are reserved when <stdlib.h> is included in a hosted implementation are
reserved when it is included in a freestanding implementation.
8 A conforming program is one that is acceptable to a conforming implementation. 5)
3) A strictly conforming program can use conditional features (see 6.10.9.3) provided the use is guarded by an appropriate
conditional inclusion preprocessing directive using the related macro. For example:
#ifdef __STDC_IEC_60559_BFP__ /* FE_UPWARD defined */
/* ... */
fesetround(FE_UPWARD);
/* ... */
#endif
4) This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this
document.
5) Strictly conforming programs are intended to be maximally portable among conforming implementations. Conforming
programs can depend upon nonportable features of a conforming implementation.
9 An implementation shall be accompanied by a document that defines all implementation-defined
and locale-specific characteristics and all extensions.
Forward references: conditional inclusion (6.10.1), error directive (6.10.6), characteristics of floating
types <float.h> (7.7), alternative spellings <iso646.h> (7.9), sizes of integer types <limits.h>
(7.10), alignment <stdalign.h> (7.15), variable arguments <stdarg.h> (7.16), boolean type and
values <stdbool.h> (7.19), common definitions <stddef.h> (7.21), integer types <stdint.h> (7.22),
<stdnoreturn.h> (7.25).
5. Environment
1 An implementation translates C source files and executes C programs in two data-processing-system
environments, which will be called the translation environment and the execution environment in this
document. Their characteristics define and constrain the results of executing conforming C programs
constructed according to the syntactic and semantic rules for conforming implementations.
Forward references: In this clause, only a few of many possible forward references have been
noted.
5.1 Conceptual models
5.1.1 Translation environment
5.1.1.1 Program structure
1 A C program need not all be translated at the same time. The text of the program is kept in units
called source files, (or preprocessing files) in this document. A source file together with all the headers
and source files included via the preprocessing directive #include is known as a preprocessing
translation unit. After preprocessing, a preprocessing translation unit is called a translation unit.
Previously translated translation units may be preserved individually or in libraries. The separate
translation units of a program communicate by (for example) calls to functions whose identifiers have
external linkage, manipulation of objects whose identifiers have external linkage, or manipulation
of data files. Translation units may be separately translated and then later linked to produce an
executable program.
Forward references: linkages of identifiers (6.2.2), external definitions (6.9), preprocessing direc-
tives (6.10).
5.1.1.2 Translation phases
1 The precedence among the syntax rules of translation is specified by the following phases.6)
ZZZ1. Physical source file multibyte characters are mapped, in an implementation-defined manner, to
the source character set (introducing new-line characters for end-of-line indicators) if necessary.
ZZZ2. Each instance of a backslash character (\ ) immediately followed by a new-line character is
deleted, splicing physical source lines to form logical source lines. Only the last backslash on
any physical source line shall be eligible for being part of such a splice. A source file that is
not empty shall end in a new-line character, which shall not be immediately preceded by a
backslash character before any such splicing takes place.
ZZZ3. The source file is decomposed into preprocessing tokens7) and sequences of white-space
characters (including comments). A source file shall not end in a partial preprocessing token or
in a partial comment. Each comment is replaced by one space character. New-line characters
are retained. Whether each nonempty sequence of white-space characters other than new-line
is retained or replaced by one space character is implementation-defined.
ZZZ4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary
operator expressions are executed. If a character sequence that matches the syntax of a univer-
sal character name is produced by token concatenation (6.10.4.3), the behavior is undefined. A
#include preprocessing directive causes the named header or source file to be processed from
phase 1 through phase 4, recursively. All preprocessing directives are then deleted.
6) This requires implementations to behave as if these separate phases occur, even though many are typically folded
together in practice. Source files, translation units, and translated translation units need not necessarily be stored as files,
nor need there be any one-to-one correspondence between these entities and any external representation. The description is
conceptual only, and does not specify any particular implementation.
7) As described in 6.4, the process of dividing a source file’s characters into preprocessing tokens is context-dependent. For
example, see the handling of < within a #include preprocessing directive.
ZZZ5. Each source character set member and escape sequence in character constants and string
literals is converted to the corresponding member of the execution character set. Each instance
of a source character or escape sequence for which there is no corresponding member is
converted in an implementation-defined manner to some member of the execution character
set other than the null (wide) character.8)
ZZZ6. Adjacent string literal tokens are concatenated.
ZZZ7. White-space characters separating tokens are no longer significant. Each preprocessing token
is converted into a token. The resulting tokens are syntactically and semantically analyzed
and translated as a translation unit.
ZZZ8. All external object and function references are resolved. Library components are linked to
satisfy external references to functions and objects not defined in the current translation. All
such translator output is collected into a program image which contains information needed
for execution in its execution environment.
Forward references: universal character names (6.4.3), lexical elements (6.4), preprocessing direc-
tives (6.10), external definitions (6.9).
5.1.1.3 Diagnostics
1 A conforming implementation shall produce at least one diagnostic message (identified in an
implementation-defined manner) if a preprocessing translation unit or translation unit contains a
violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined
or implementation-defined. Diagnostic messages need not be produced in other circumstances.9)
2 EXAMPLE An implementation is required to issue a diagnostic for the translation unit:
char i;
int i;
because in those cases where wording in this document describes the behavior for a construct as being both a constraint error
and resulting in undefined behavior, the constraint error is still required to be diagnosed.
5.1.2 Execution environments
1 Two execution environments are defined: freestanding and hosted. In both cases, program startup
occurs when a designated C function is called by the execution environment. All objects with static
storage duration shall be initialized (set to their initial values) before program startup. The manner
and timing of such initialization are otherwise unspecified. Program termination returns control to
the execution environment.
Forward references: storage durations of objects (6.2.4), initialization (6.7.10).
5.1.2.1 Freestanding environment
1 In a freestanding environment (in which C program execution may take place without any ben-
efit of an operating system), the name and type of the function called at program startup are
implementation-defined. Any library facilities available to a freestanding program, other than the
minimal set required by Clause 4, are implementation-defined.
2 The effect of program termination in a freestanding environment is implementation-defined.
5.1.2.2 Hosted environment
1 A hosted environment need not be provided, but shall conform to the following specifications if
present.
8) An implementation may convert each instance of the same non-corresponding source character to a different member of
the execution character set.
9) An implementation is encouraged to identify the nature of, and where possible localize, each violation. Of course, an
implementation is free to produce any number of diagnostic messages, often referred to as warnings, as long as a valid
program is still correctly translated. It can also successfully translate an invalid program. Annex I lists a few of the more
common warnings.
5.1.2.2.1 Program startup
1 The function called at program startup is named main. The implementation declares no prototype
for this function. It shall be defined with a return type of int and with no parameters:
int main(void) { /* ... */ }
or with two parameters (referred to here as argc and argv, though any names may be used, as they
are local to the function in which they are declared):
int main(int argc, char *argv[]) { /* ... */ }
or equivalent10) ; or in some other implementation-defined manner.
2 If they are declared, the parameters to the main function shall obey the following constraints:
— The value of argc shall be nonnegative.
— argv[argc] shall be a null pointer.
— If the value of argc is greater than zero, the array members argv[0] through argv[argc-1]
inclusive shall contain pointers to strings, which are given implementation-defined values
by the host environment prior to program startup. The intent is to supply to the program
information determined prior to program startup from elsewhere in the hosted environment.
If the host environment is not capable of supplying strings with letters in both uppercase and
lowercase, the implementation shall ensure that the strings are received in lowercase.
— If the value of argc is greater than zero, the string pointed to by argv[0] represents the
program name; argv[0][0] shall be the null character if the program name is not available
from the host environment. If the value of argc is greater than one, the strings pointed to by
argv[1] through argv[argc-1] represent the program parameters.
— The parameters argc and argv and the strings pointed to by the argv array shall be modifiable
by the program, and retain their last-stored values between program startup and program
termination.
5.1.2.2.2 Program execution
1 In a hosted environment, a program may use all the functions, macros, type definitions, and objects
described in the library clause (Clause 7).
5.1.2.2.3 Program termination
1 If the return type of the main function is a type compatible with int, a return from the initial call
to the main function is equivalent to calling the exit function with the value returned by the main
function as its argument;11) reaching the } that terminates the main function returns a value of 0. If
the return type is not compatible with int, the termination status returned to the host environment
is unspecified.
Forward references: definition of terms (7.1.1), the exit function (7.24.4.4).
5.1.2.3 Program execution
1 The semantic descriptions in this document describe the behavior of an abstract machine in which
issues of optimization are irrelevant.
2 An access to an object through the use of an lvalue of volatile-qualified type is a volatile access. A
volatile access to an object, modifying an object, modifying a file, or calling a function that does any
10) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char
** argv, and so
on.
11) In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main will have ended in the
former case, even where they would not have in the latter.
of those operations are all side effects12) , which are changes in the state of the execution environment.
Evaluation of an expression in general includes both value computations and initiation of side effects.
Value computation for an lvalue expression includes determining the identity of the designated
object.
3 Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a
single thread, which induces a partial order among those evaluations. Given any two evaluations
A and B, if A is sequenced before B, then the execution of A shall precede the execution of B.
(Conversely, if A is sequenced before B, then B is sequenced after A.) If A is not sequenced before or
after B, then A and B are unsequenced. Evaluations A and B are indeterminately sequenced when A is
sequenced either before or after B, but it is unspecified which.13) The presence of a sequence point
between the evaluation of expressions A and B implies that every value computation and side effect
associated with A is sequenced before every value computation and side effect associated with B. (A
summary of the sequence points is given in Annex C.)
4 In the abstract machine, all expressions are evaluated as specified by the semantics. An actual
implementation need not evaluate part of an expression if it can deduce that its value is not used
and that no needed side effects are produced (including any caused by calling a function or through
volatile access to an object).
5 When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects
that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified, as is
the state of the dynamic floating-point environment. The representation of any object modified by
the handler that is neither a lock-free atomic object nor of type volatile sig_atomic_t becomes
indeterminate when the handler exits, as does the state of the dynamic floating-point environment if
it is modified by the handler and not restored to its original state.
6 The least requirements on a conforming implementation are:
— Volatile accesses to objects are evaluated strictly according to the rules of the abstract machine.
— At program termination, all data written into files shall be identical to the result that execution
of the program according to the abstract semantics would have produced.
— The input and output dynamics of interactive devices shall take place as specified in 7.23.3.
The intent of these requirements is that unbuffered or line-buffered output appear as soon as
possible, to ensure that prompting messages actually appear prior to a program waiting for
input.
This is the observable behavior of the program.
7 What constitutes an interactive device is implementation-defined.
8 More stringent correspondences between abstract and actual semantics may be defined by each
implementation.
9 EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual semantics: at every
sequence point, the values of the actual objects would agree with those specified by the abstract semantics. The keyword
volatile would then be redundant.
10 Alternatively, an implementation might perform various optimizations within each translation unit, such that the actual
semantics would agree with the abstract semantics only when making function calls across translation unit boundaries. In
such an implementation, at the time of each function entry and function return where the calling function and the called
function are in different translation units, the values of all externally linked objects and of all objects accessible via pointers
therein would agree with the abstract semantics. Furthermore, at the time of each such function entry the values of the
parameters of the called function and of all objects accessible via pointers therein would agree with the abstract semantics. In
this type of implementation, objects referred to by interrupt service routines activated by the signal function would require
explicit specification of volatile storage, as well as other implementation-defined restrictions.
12) The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status flags and control
modes. Floating-point operations implicitly set the status flags; modes affect result values of floating-point operations.
Implementations that support such floating-point state are required to regard changes to it as side effects — see Annex F for
details. The floating-point environment library <fenv.h> provides a programming facility for indicating when these side
effects matter, freeing the implementations in other cases.
13) The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations cannot interleave, but
can be executed in any order.
11 EXAMPLE 2 In executing the fragment
char c1, c2;
/* ... */
c1 = c1 + c2;
the "integer promotions" require that the abstract machine promote the value of each variable to int size and then add the
two ints and truncate the sum. Provided the addition of two chars can be done without integer overflow, or with integer
overflow wrapping silently to produce the correct result, the actual execution need only produce the same result, possibly
omitting the promotions.
12 EXAMPLE 3 Similarly, in the fragment
float f1, f2;
double d;
/* ... */
f1 = f2 * d;
the multiplication can be executed using single-precision arithmetic if the implementation can ascertain that the result would
be the same as if it were executed using double-precision arithmetic (for example, if d were replaced by the constant 2.0,
which has type double).
13 EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate semantics. Values are
independent of whether they are represented in a register or in memory. For example, an implicit spilling of a register is
not permitted to alter the value. Also, an explicit store and load is required to round to the precision of the storage type. In
particular, casts and assignments are required to perform their specified conversion. For the fragment
double d1, d2;
float f;
d1 = f = expression;
d2 = (float) expression;
the values assigned to d1 and d2 are required to have been converted to float.
14 EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in precision as well as
range. The implementation cannot generally apply the mathematical associative rules for addition or multiplication, nor
the distributive rule, because of roundoff error, even in the absence of overflow and underflow. Likewise, implementations
cannot generally replace decimal constants in order to rearrange expressions. In the following fragment, rearrangements
suggested by mathematical rules for real numbers are often not valid (see F.9).
double x, y, z;
/* ... */
x = (x * y) * z; // not equivalent to x *= y * z;
z = (x - y) + y; // not equivalent to z = x;
z = x + x * y; // not equivalent to z = x * (1.0 + y);
y = x / 5.0; // not equivalent to y = x * 0.2;
15 EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment
int a, b;
/* ... */
a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is next added to b, and
that result is then added to 5 which results in the value assigned to a. On a machine in which integer overflows produce
an explicit trap and in which the range of values representable by an int is [−32768, +32767], the implementation cannot
rewrite this expression as
a = ((a + b) + 32765);
since if the values for a and b were, respectively, −32754 and −15, the sum a + b would produce a trap while the original
expression would not; nor can the expression be rewritten either as
a = ((a + 32765) + b);
or
a = (a + (b + 32765));
since the values for a and b might have been, respectively, 4 and −8 or −17 and 12. However, on a machine in which integer
overflow silently generates some value and where positive and negative integer overflows cancel, the above expression
statement can be rewritten by the implementation in any of the above ways because the same result will occur.
16 EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the following fragment
#include <stdio.h>
int sum;
char *p;
/* ... */
sum = sum * 10 - ’0’ + (*p++ = getchar());
the expression statement is grouped as if it were written as
sum = (((sum * 10) - ’0’) + ((*(p++)) = (getchar())));
but the actual increment of p can occur at any time between the previous sequence point and the next sequence point (the ;),
and the call to getchar can occur at any point prior to the need of its returned value.
Forward references: expressions (6.5), type qualifiers (6.7.3), statements (6.8), floating-point envi-
ronment <fenv.h> (7.6), the signal function (7.14), files (7.23.3).
5.1.2.4 Multi-threaded executions and data races
1 Under a hosted implementation, a program can have more than one thread of execution (or thread)
running concurrently. The execution of each thread proceeds as defined by the remainder of this
document. The execution of the entire program consists of an execution of all of its threads.14)
Under a freestanding implementation, it is implementation-defined whether a program can have
more than one thread of execution.
2 The value of an object visible to a thread T at a particular point is the initial value of the object, a
value stored in the object by T , or a value stored in the object by another thread, according to the
rules below.
3 NOTE 1 In some cases, there could instead be undefined behavior. Much of this section is motivated by the desire to support
atomic operations with explicit and detailed visibility constraints. However, it also implicitly supports a simpler view for
more restricted programs.
4 Two expression evaluations conflict if one of them modifies a memory location and the other one
reads or modifies the same memory location.
5 The library defines a number of atomic operations (7.17) and operations on mutexes (7.28.4) that are
specially identified as synchronization operations. These operations play a special role in making
assignments in one thread visible to another. A synchronization operation on one or more memory
locations is one of an acquire operation, a release operation, both an acquire and release operation, or a
consume operation. A synchronization operation without an associated memory location is a fence and
can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there
are relaxed atomic operations, which are not synchronization operations, and atomic read-modify-write
operations, which have special characteristics.
6 NOTE 2 For example, a call that acquires a mutex will perform an acquire operation on the locations composing the mutex.
Correspondingly, a call that releases the same mutex will perform a release operation on those same locations. Informally,
performing a release operation on A forces prior side effects on other memory locations to become visible to other threads
that later perform an acquire or consume operation on A. Relaxed atomic operations are not included as synchronization
operations although, like synchronization operations, they cannot contribute to data races.
7 All modifications to a particular atomic object M occur in some particular total order, called the
14) The execution can usually be viewed as an interleaving of all of the threads. However, some kinds of atomic operations,
for example, allow executions inconsistent with a simple interleaving as described below.
modification order of M . If A and B are modifications of an atomic object M , and A happens before B,
then A shall precede B in the modification order of M , which is defined below.
8 NOTE 3 This states that the modification orders are expected to respect the "happens before" relation.
9 NOTE 4 There is a separate order for each atomic object. There is no requirement that these can be combined into a single
total order for all objects. In general this will be impossible since different threads can observe modifications to different
variables in inconsistent orders.
10 A release sequence headed by a release operation A on an atomic object M is a maximal contiguous
sub-sequence of side effects in the modification order of M , where the first operation is A and every
subsequent operation either is performed by the same thread that performed the release or is an
atomic read-modify-write operation.
11 Certain library calls synchronize with other library calls performed by another thread. In particular,
an atomic operation A that performs a release operation on an object M synchronizes with an atomic
operation B that performs an acquire operation on M and reads a value written by any side effect in
the release sequence headed by A.
12 NOTE 5 Except in the specified cases, reading a later value does not necessarily ensure visibility as described below. Such a
requirement would sometimes interfere with efficient implementation.
13 NOTE 6 The specifications of the synchronization operations define when one reads the value written by another. For atomic
variables, the definition is clear. All operations on a given mutex occur in a single total order. Each mutex acquisition "reads
the value written" by the last mutex release.
14 An evaluation A carries a dependency15) to an evaluation B if:
— the value of A is used as an operand of B, unless:
• B is an invocation of the kill_dependency macro,
• A is the left operand of a && or || operator,
• A is the left operand of a ?: operator, or
• A is the left operand of a , operator;
or
— A writes a scalar object or bit-field M , B reads from M the value written by A, and A is
sequenced before B, or
— for some evaluation X, A carries a dependency to X and X carries a dependency to B.
15 An evaluation A is dependency-ordered before16) an evaluation B if:
— A performs a release operation on an atomic object M , and, in another thread, B performs a
consume operation on M and reads a value written by any side effect in the release sequence
headed by A, or
— for some evaluation X, A is dependency-ordered before X and X carries a dependency to B.
16 An evaluation A inter-thread happens before an evaluation B if A synchronizes with B, A is
dependency-ordered before B, or, for some evaluation X:
— A synchronizes with X and X is sequenced before B,
— A is sequenced before X and X inter-thread happens before B, or
— A inter-thread happens before X and X inter-thread happens before B.
15) The "carries a dependency" relation is a subset of the "sequenced before" relation, and is similarly strictly intra-thread.
16) The "dependency-ordered before" relation is analogous to the "synchronizes with" relation, but uses release/consume in
place of release/acquire.
17 NOTE 7 The "inter-thread happens before" relation describes arbitrary concatenations of "sequenced before", "synchronizes
with", and "dependency-ordered before" relationships, with two exceptions. The first exception is that a concatenation is
not permitted to end with "dependency-ordered before" followed by "sequenced before". The reason for this limitation is
that a consume operation participating in a "dependency-ordered before" relationship provides ordering only with respect
to operations to which this consume operation actually carries a dependency. The reason that this limitation applies only
to the end of such a concatenation is that any subsequent release operation will provide the required ordering for a prior
consume operation. The second exception is that a concatenation is not permitted to consist entirely of "sequenced before".
The reasons for this limitation are (1) to permit "inter-thread happens before" to be transitively closed and (2) the "happens
before" relation, defined below, provides for relationships consisting entirely of "sequenced before".
18 An evaluation A happens before an evaluation B if A is sequenced before B or A inter-thread happens
before B. The implementation shall ensure that no program execution demonstrates a cycle in the
"happens before" relation.
19 NOTE 8 This cycle would otherwise be possible only through the use of consume operations.
20 A visible side effect A on an object M with respect to a value computation B of M satisfies the
conditions:
— A happens before B, and
— there is no other side effect X to M such that A happens before X and X happens before B.
The value of a non-atomic scalar object M , as determined by evaluation B, shall be the value stored
by the visible side effect A.
21 NOTE 9 If there is ambiguity about which side effect to a non-atomic object is visible, then there is a data race and the
behavior is undefined.
22 NOTE 10 This states that operations on ordinary variables are not visibly reordered. This is not actually detectable without
data races, but it is necessary to ensure that data races, as defined here, and with suitable restrictions on the use of atomics,
correspond to data races in a simple interleaved (sequentially consistent) execution.
23 The value of an atomic object M , as determined by evaluation B, shall be the value stored by some
side effect A that modifies M , where B does not happen before A.
24 NOTE 11 The set of side effects from which a given evaluation might take its value is also restricted by the rest of the rules
described here, and in particular, by the coherence requirements below.
25 If an operation A that modifies an atomic object M happens before an operation B that modifies M ,
then A shall be earlier than B in the modification order of M .
26 NOTE 12 The requirement above is known as "write-write coherence".
27 If a value computation A of an atomic object M happens before a value computation B of M , and A
takes its value from a side effect X on M , then the value computed by B shall either be the value
stored by X or the value stored by a side effect Y on M , where Y follows X in the modification
order of M .
28 NOTE 13 The requirement above is known as "read-read coherence".
29 If a value computation A of an atomic object M happens before an operation B on M , then A shall
take its value from a side effect X on M , where X precedes B in the modification order of M .
30 NOTE 14 The requirement above is known as "read-write coherence".
31 If a side effect X on an atomic object M happens before a value computation B of M , then the
evaluation B shall take its value from X or from a side effect Y that follows X in the modification
order of M .
32 NOTE 15 The requirement above is known as "write-read coherence".
33 NOTE 16 This effectively disallows compiler reordering of atomic operations to a single object, even if both operations are
"relaxed" loads. By doing so, it effectively makes the "cache coherence" guarantee provided by most hardware available to C
atomic operations.
34 NOTE 17 The value observed by a load of an atomic object depends on the "happens before" relation, which in turn depends
on the values observed by loads of atomic objects. The intended reading is that there exists an association of atomic loads
with modifications they observe that, together with suitably chosen modification orders and the "happens before" relation
derived as described above, satisfy the resulting constraints as imposed here.
35 The execution of a program contains a data race if it contains two conflicting actions in different
threads, at least one of which is not atomic, and neither happens before the other. Any such data
race results in undefined behavior.
36 NOTE 18 It can be shown that programs that correctly use simple mutexes and memory_order_seq_cst operations to
prevent all data races, and use no other synchronization operations, behave as though the operations executed by their
constituent threads were simply interleaved, with each value computation of an object being the last value stored in that
interleaving. This is normally referred to as "sequential consistency". However, this applies only to data-race-free programs,
and data-race-free programs cannot observe most program transformations that do not change single-threaded program
semantics. In fact, most single-threaded program transformations continue to be allowed, since any program that behaves
differently as a result necessarily has undefined behavior even before such a transformation is applied.
37 NOTE 19 Compiler transformations that introduce assignments to a potentially shared memory location that would not
be modified by the abstract machine are generally precluded by this document, since such an assignment might overwrite
another assignment by a different thread in cases in which an abstract machine execution would not have encountered a
data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory
locations. Reordering of atomic loads in cases in which the atomics in question might alias is also generally precluded, since
this could violate the coherence requirements.
38 NOTE 20 Transformations that introduce a speculative read of a potentially shared memory location might not preserve
the semantics of the program as defined in this document, since they potentially introduce a data race. However, they are
typically valid in the context of an optimizing compiler that targets a specific machine with well-defined semantics for data
races. They would be invalid for a hypothetical machine that is not tolerant of races or provides hardware race detection.
5.2 Environmental considerations
5.2.1 Character sets
1 Two sets of characters and their associated collating sequences shall be defined: the set in which source
files are written (the source character set), and the set interpreted in the execution environment (the
execution character set). Each set is further divided into a basic character set, whose contents are given
by this subclause, and a set of zero or more locale-specific members (which are not members of the
basic character set) called extended characters. The combined set is also called the extended character
set. The values of the members of the execution character set are implementation-defined.
2 In a character constant or string literal, members of the execution character set shall be represented by
corresponding members of the source character set or by escape sequences consisting of the backslash
\ followed by one or more characters. A byte with all bits set to 0, called the null character, shall exist
in the basic execution character set; it is used to terminate a character string.
Both the basic source and basic execution character sets shall have the following members: the 26
uppercase letters of the Latin alphabet
3
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
the 26 lowercase letters of the Latin alphabet
a b c d e f g h i j k l m
n o p q r s t u v w x y z
the 10 decimal digits
0 1 2 3 4 5 6 7 8 9
the following 29 graphic characters
! " # % & ’ ( ) * + , - . / :
; < = > ? [ \ ] ^ _ { | } ~
the space character, and control characters representing horizontal tab, vertical tab, and form feed.
The representation of each member of the source and execution basic character sets shall fit in a
byte. In both the source and execution basic character sets, the value of each character after 0 in
the above list of decimal digits shall be one greater than the value of the previous. In source files,
there shall be some way of indicating the end of each line of text; this document treats such an
end-of-line indicator as if it were a single new-line character. In the basic execution character set,
there shall be control characters representing alert, backspace, carriage return, and new line. If any
other characters are encountered in a source file (except in an identifier, a character constant, a string
literal, a header name, a comment, or a preprocessing token that is never converted to a token), the
behavior is undefined.
4 A letter is an uppercase letter or a lowercase letter as defined above; in this document the term does
not include other characters that are letters in other alphabets.
5 The universal character name construct provides a way to name other characters.
Forward references: universal character names (6.4.3), character constants (6.4.4.4), preprocessing
directives (6.10), string literals (6.4.5), comments (6.4.9), string (7.1.1).
5.2.1.1 Multibyte characters
1 The source character set may contain multibyte characters, used to represent members of the
extended character set. The execution character set may also contain multibyte characters, which
need not have the same encoding as for the source character set. For both character sets, the following
shall hold:
— The basic character set, @, $, and ` shall be present and each character shall be encoded as a
single byte.
— The presence, meaning, and representation of any additional members is locale-specific.
— A multibyte character set may have a state-dependent encoding, wherein each sequence of
multibyte characters begins in an initial shift state and enters other locale-specific shift states
when specific multibyte characters are encountered in the sequence. While in the initial shift
state, all single-byte characters retain their usual interpretation and do not alter the shift state.
The interpretation for subsequent bytes in the sequence is a function of the current shift state.
— A byte with all bits zero shall be interpreted as a null character independent of shift state. Such
a byte shall not occur as part of any other multibyte character.
2 For source files, the following shall hold:
— An identifier, comment, string literal, character constant, or header name shall begin and end
in the initial shift state.
— An identifier, comment, string literal, character constant, or header name shall consist of a
sequence of valid multibyte characters.
5.2.2 Character display semantics
1 The active position is that location on a display device where the next character output by the
fputc function would appear. The intent of writing a printing character (as defined by the isprint
function) to a display device is to display a graphic representation of that character at the active
position and then advance the active position to the next position on the current line. The direction
of writing is locale-specific. If the active position is at the final position of a line (if there is one), the
behavior of the display device is unspecified.
2 Alphabetic escape sequences representing non-graphic characters in the execution character set are
intended to produce actions on display devices as follows:
\a (alert) Produces an audible or visible alert without changing the active position.
\b (backspace) Moves the active position to the previous position on the current line. If the active
position is at the initial position of a line, the behavior of the display device is unspecified.
\f (form feed) Moves the active position to the initial position at the start of the next logical page.
\n (new line) Moves the active position to the initial position of the next line.
\r (carriage return) Moves the active position to the initial position of the current line.
\t (horizontal tab) Moves the active position to the next horizontal tabulation position on the current
line. If the active position is at or past the last defined horizontal tabulation position, the behavior
of the display device is unspecified.
\v (vertical tab) Moves the active position to the initial position of the next vertical tabulation
position. If the active position is at or past the last defined vertical tabulation position, the
behavior of the display device is unspecified.
3 Each of these escape sequences shall produce a unique implementation-defined value which can be
stored in a single char object. The external representations in a text file need not be identical to the
internal representations, and are outside the scope of this document.
Forward references: the isprint function (7.4.1.8), the fputc function (7.23.7.3).
5.2.3 Signals and interrupts
1 Functions shall be implemented such that they may be interrupted at any time by a signal, or may be
called by a signal handler, or both, with no alteration to earlier, but still active, invocations’ control
flow (after the interruption), function return values, or objects with automatic storage duration.
All such objects shall be maintained outside the function image (the instructions that compose the
executable representation of a function) on a per-invocation basis.
5.2.4 Environmental limits
1 Both the translation and execution environments constrain the implementation of language trans-
lators and libraries. The following summarizes the language-related environmental limits on a
conforming implementation; the library-related limits are discussed in Clause 7.
5.2.4.1 Translation limits
1 The implementation shall be able to translate and execute a program that uses but does not exceed
the following limitations for these constructs and entities17) :
— 127 nesting levels of blocks
— 63 nesting levels of conditional inclusion
— 12 pointer, array, and function declarators (in any combinations) modifying an arithmetic,
structure, union, or void type in a declaration
— 63 nesting levels of parenthesized declarators within a full declarator
— 63 nesting levels of parenthesized expressions within a full expression
— 63 significant initial characters in an internal identifier or a macro name(each universal charac-
ter name or extended source character is considered a single character)
— 31 significant initial characters in an external identifier (each universal character name specify-
ing a short identifier of 0000FFFF or less is considered 6 characters, each universal character
name specifying a short identifier of 00010000 or more is considered 10 characters, and each
extended source character is considered the same number of characters as the corresponding
universal character name, if any)18)
— 4095 external identifiers in one translation unit
— 511 identifiers with block scope declared in one block
— 4095 macro identifiers simultaneously defined in one preprocessing translation unit
— 127 parameters in one function definition
— 127 arguments in one function call
— 127 parameters in one macro definition
— 127 arguments in one macro invocation
17) Implementations are encouraged to avoid imposing fixed translation limits whenever possible.
18) See "future language directions" (6.11.3).
— 4095 characters in a logical source line
— 4095 characters in a string literal (after concatenation)
— 32767 bytes in an object (in a hosted environment only)
— 15 nesting levels for #included files
— 1023 case labels for a switch statement (excluding those for any nested switch statements)
— 1023 members in a single structure or union
— 1023 enumeration constants in a single enumeration
— 63 levels of nested structure or union definitions in a single member declaration list
5.2.4.2 Numerical limits
1 An implementation is required to document all the limits specified in this subclause, which are
specified in the headers <limits.h> and <float.h>. Additional limits are specified in <stdint.h>.
Forward references: integer types <stdint.h> (7.22).
5.2.4.2.1 Characteristics of integer types <limits.h>
1 The values given below shall be replaced by constant expressions suitable for use in #if preprocess-
ing directives. Their implementation-defined values shall be equal or greater to those shown.
— width for an object of type bool19)
BOOL_WIDTH 1
— number of bits for smallest object that is not a bit-field (byte)
CHAR_BIT 8
The macros CHAR_WIDTH, SCHAR_WIDTH, and UCHAR_WIDTH that represent the width of the
types char, signed char and unsigned char shall expand to the same value as CHAR_BIT.
— width for an object of type unsigned short int
USHRT_WIDTH 16
The macro SHRT_WIDTH represents the width of the type short int and shall expand to the
same value as USHRT_WIDTH.
— width for an object of type unsigned int
UINT_WIDTH 16
The macro INT_WIDTH represents the width of the type int and shall expand to the same value
as UINT_WIDTH.
— width for an object of type unsigned long int
ULONG_WIDTH 32
The macro LONG_WIDTH represents the width of the type long int and shall expand to the
same value as ULONG_WIDTH.
— width for an object of type unsigned long long int
19) This value is exact.
ULLONG_WIDTH 64
The macro LLONG_WIDTH represents the width of the type long long int and shall expand to
the same value as ULLONG_WIDTH.
— maximum width for an object of type _BitInt or unsigned _BitInt
BITINT_MAXWIDTH /* see below */
The macro BITINT_MAXWIDTH represents the maximum width N supported by the declaration
of a bit-precise integer (6.2.5) in the type specifier _BitInt( N). The value BITINT_MAXWIDTH
shall expand to a value that is greater than or equal to the value of ULLONG_WIDTH.
— maximum number of bytes in a multibyte character, for any supported locale
MB_LEN_MAX 1
2 For all unsigned integer types for which <limits.h> or <stdint.h> define a macro with suffix
_WIDTH holding its width N , there is a macro with suffix _MAX holding the maximal value 2N − 1
that is representable by the type and that has the same type as would an expression that is an object
of the corresponding type converted according to the integer promotions. If the value is in the range
of the type uintmax_t (7.22.1.5) the macro is suitable for use in #if preprocessing directives.
3 For all signed integer types for which <limits.h> or <stdint.h> define a macro with suffix _WIDTH
holding its width N , there are macros with suffix _MIN and _MAX holding the minimal and maximal
values −2N −1 and 2N −1 − 1 that are representable by the type and that have the same type as
would an expression that is an object of the corresponding type converted according to the integer
promotions. If the values are in the range of the type intmax_t (7.22.1.5) the macros are suitable for
use in #if preprocessing directives.
4 If an object of type char can hold negative values, the value of CHAR_MIN shall be the same as that of
SCHAR_MIN and the value of CHAR_MAX shall be the same as that of SCHAR_MAX. Otherwise, the value
of CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX.20)
Forward references: representations of types (6.2.6), conditional inclusion (6.10.1), integer types
<stdint.h> (7.22).
5.2.4.2.2 Characteristics of floating types <float.h>
1 The characteristics of floating types are defined in terms of a model that describes a repre-
sentation of floating-point numbers and allows other values. The characteristics provide in-
formation about an implementation’s floating-point arithmetic21) . An implementation that de-
fines __STDC_IEC_60559_BFP__ or __STDC_IEC_559__ shall implement floating-point types and
arithmetic conforming to IEC 60559 as specified in Annex F. An implementation that defines
__STDC_IEC_60559_COMPLEX__ or __STDC_IEC_559_COMPLEX__ shall implement complex types
and arithmetic conforming to IEC 60559 as specified in Annex G.
2 The following parameters are used to define the model for each floating type:
s sign (±1)
b base or radix of exponent representation (an integer > 1)
e exponent (an integer between a minimum emin and a maximum emax )
p precision (the number of base-b digits in the significand)
fk nonnegative integers less than b (the significand digits)
For each floating type, the parameters b, p, emin , and emax are fixed constants.
3 For each floating type, a floating-point number (x) is defined by the following model:
20) See 6.2.5.
21) The floating-point model is intended to clarify the description of each floating-point characteristic and does not require
the floating-point arithmetic of the implementation to be identical.
p
x = sbe fk b−k ,
SIGMA
emin ≤ e ≤ emax
k=1
4 Model floating-point numbers x with f1 > 0 are called normalized floating-point numbers.
5 Model floating-point numbers x ̸= 0 with f1 = 0 and e = emin are called subnormal floating-point
numbers.
6 Model floating-point numbers x ̸= 0 with f1 = 0 and e > emin are called unnormalized floating-point
numbers.
7 Model floating-point numbers x with all fk = 0 are zeros.
8 Floating types shall be able to represent signed zeros or an unsigned zero and all normalized floating-
point numbers. In addition, floating types may be able to contain other kinds of floating-point
numbers22) , such as subnormal floating-point numbers and unnormalized floating-point numbers,
and values that are not floating-point numbers, such as NaNs and (signed and unsigned) infinities.
A NaN is a value signifying Not-a-Number. A quiet NaN propagates through almost every arithmetic
operation without raising a floating-point exception; a signaling NaN generally raises a floating-point
exception when occurring as an arithmetic operand23) .
9 Wherever values are unsigned, any requirement in this document to get the sign shall produce an
unspecified sign, and any requirement to set the sign shall be ignored, unless otherwise specified24) .
10 Whether and in what cases subnormal numbers are treated as zeros is implementation-defined.
Subnormal numbers that in some cases are treated by arithmetic operations as zeros are properly
classified as subnormal. However, object representations that could represent subnormal numbers
but that are always treated by arithmetic operations as zeros are non-canonical zeros, and the
values are properly classified as zero, not subnormal. IEC 60559 arithmetic (with default exception
handling) always treats subnormal numbers as nonzero.
11 A value is negative if and only if it compares less than 0. Thus, negative zeros and NaNs are not
negative values.
12 An implementation may prefer particular representations of values that have multiple representa-
tions in a floating type, 6.2.6.1 not withstanding.25) The preferred representations of a floating type,
including unique representations of values in the type, are called canonical. A floating type may also
contain non-canonical representations, for example, redundant representations of some or all of its
values, or representations that are extraneous to the floating-point model.26) Typically, floating-point
operations deliver results with canonical representations. IEC 60559 operations deliver results with
canonical representations, unless specified otherwise.
13 The minimum range of representable values for a floating type is the most negative finite floating-
point number representable in that type through the most positive finite floating-point number
representable in that type. In addition, if negative infinity is representable in a type, the range of
that type is extended to all negative real numbers; likewise, if positive infinity is representable in a
type, the range of that type is extended to all positive real numbers.
14 The accuracy of the floating-point operations (+ ,- , * , / ) and of the library functions in <math.h>
and <complex.h> that return floating-point results is implementation-defined, as is the accuracy of
the conversion between floating-point internal representations and string representations performed
by the library functions in <stdio.h>, <stdlib.h>, and <wchar.h>. The implementation may state
that the accuracy is unknown. Decimal floating-point operations have stricter requirements.
15 All integer values in the <float.h> header, except FLT_ROUNDS, shall be constant expressions
22) Some implementations have types that include finite numbers with range and/or precision that are not covered by the
model.
23) IEC 60559 specifies quiet and signaling NaNs. For implementations that do not support IEC 60559, the terms quiet NaN
and signaling NaN are intended to apply to values with similar behavior.
24) Bit representations of floating-point values might include a sign bit, even if the values can be regarded as unsigned.
IEC 60559 NaNs are such values.
25) The library operations iscanonical and canonicalize distinguish canonical (preferred) representations, but this
distinction alone does not imply that canonical and non-canonical representations are of different values.
26) Some of the values in the IEC 60559 decimal formats have non-canonical representations (as well as a canonical
representation).
suitable for use in #if preprocessing directives; all floating values shall be constant expressions.
All except CR_DECIMAL_DIG (F.5), DECIMAL_DIG, DEC_EVAL_METHOD, FLT_EVAL_METHOD, FLT_RADIX,
and FLT_ROUNDS have separate names for all floating types. The floating-point model representation
is provided for all values except DEC_EVAL_METHOD, FLT_EVAL_METHOD and FLT_ROUNDS.
16 The remainder of this subclause specifies characteristics of standard floating types.
17 The rounding mode for floating-point addition for standard floating types is characterized by the
implementation-defined value of FLT_ROUNDS. Evaluation of FLT_ROUNDS correctly reflects any
execution-time change of rounding mode through the function fesetround in <fenv.h>.
−1 indeterminable
0 toward zero
1 to nearest, ties to even
2 toward positive infinity
3 toward negative infinity
4 to nearest, ties away from zero
All other values for FLT_ROUNDS characterize implementation-defined rounding behavior.
18 Whether a type matches an IEC 60559 format (and perhaps, operations) is characterized
by the implementation-defined values of FLT_IS_IEC_60559, DBL_IS_IEC_60559, and
LDBL_IS_IEC_60559 (this does not imply conformance to Annex F):
0 type does not match an IEC 60559 format
1 type matches an IEC 60559 format
2 type matches an IEC 60559 format and operations
19 The values of floating type yielded by operators subject to the usual arithmetic conversions, including
the values yielded by the implicit conversion of operands, and the values of floating constants are
evaluated to a format whose range and precision may be greater than required by the type. Such a
format is called an evaluation format. In all cases, assignment and cast operators yield values in the
format of the type. The extent to which evaluation formats are used is characterized by the value of
FLT_EVAL_METHOD:27)
−1 indeterminable;
0 evaluate all operations and constants just to the range and precision of the type;
1 evaluate operations and constants of type float and double to the range and precision of
the double type, evaluate long double operations and constants to the range and precision
of the long double type;
2 evaluate all operations and constants to the range and precision of the long double type.
All other negative values for FLT_EVAL_METHOD characterize implementation-defined behavior. The
value of FLT_EVAL_METHOD does not characterize values returned by function calls (see 6.8.6.4, F.6).
20 The presence or absence of subnormal numbers is characterized by the implementation-defined
values of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and LDBL_HAS_SUBNORM:
−1 indeterminable
27) The evaluation method determines evaluation formats of expressions involving all floating types, not just real
types. For example, if FLT_EVAL_METHOD is 1, then the product of two float _Complex operands is represented in the
double _Complex format, and its parts are evaluated to double.
0 absent (type does not support subnormal numbers)
1 present (type does support subnormal numbers)
The use of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and LDBL_HAS_SUBNORM macros is an obsolescent
feature.
21 The signaling NaN macros
FLT_SNAN
DBL_SNAN
LDBL_SNAN
each is defined if and only if the respective type contains signaling NaNs. They expand to a constant
expression of the respective type representing a signaling NaN. If an optional unary + or - operator
followed by a signaling NaN macro is used as the initializer for initializing an object of the same
type that has static or thread storage duration, the object is initialized with a signaling NaN value.
22 The macro
INFINITY
is defined if and only if the implementation supports an infinity for the type float. It expands to a
constant expression of type float representing positive or unsigned infinity.
23 The macro
NAN
is defined if and only if the implementation supports quiet NaNs for the float type. It expands to a
constant expression of type float representing a quiet NaN.
24 The values given in the following list shall be replaced by constant expressions with implementation-
defined values that are greater or equal in magnitude (absolute value) to those shown, with the
same sign:
— radix of exponent representation, b
FLT_RADIX 2
— number of base-FLT_RADIX digits in the floating-point significand, p
FLT_MANT_DIG
DBL_MANT_DIG
LDBL_MANT_DIG
— number of decimal digits, n, such that any floating-point number with p radix b digits can be
rounded to a floating-point number with n decimal digits and back again without change to
the value,
(
p log10 b if b is a power of 10
⌈1 + p log10 b⌉ otherwise
FLT_DECIMAL_DIG 6
DBL_DECIMAL_DIG 10
LDBL_DECIMAL_DIG 10
— number of decimal digits, n, such that any floating-point number in the widest of the supported
floating types and the supported IEC 60559 encodings with pmax radix b digits can be rounded
to a floating-point number with n decimal digits and back again without change to the value,
(
pmax log10 b if b is a power of 10
⌈1 + pmax log10 b⌉ otherwise
DECIMAL_DIG 10
This is an obsolescent feature, see 7.33.8.
— number of decimal digits, q, such that any floating-point number with q decimal digits can be
rounded into a floating-point number with p radix b digits and back again without change to
the q decimal digits,
(
p log10 b if b is a power of 10
⌊(p − 1) log10 b⌋ otherwise
FLT_DIG 6
DBL_DIG 10
LDBL_DIG 10
— minimum negative integer such that FLT_RADIX raised to one less than that power is a normal-
ized floating-point number, emin
FLT_MIN_EXP
DBL_MIN_EXP
LDBL_MIN_EXP
— minimum negative integer
 such that10 raised to that power is in the range of normalized
floating-point numbers, log10 bemin −1
FLT_MIN_10_EXP -37
DBL_MIN_10_EXP -37
LDBL_MIN_10_EXP -37
— maximum integer such that FLT_RADIX raised to one less than that power is a representable
finite floating-point number; if that representable finite floating-point number is normalized,
the value of the macro is emax
FLT_MAX_EXP
DBL_MAX_EXP
LDBL_MAX_EXP
— maximum integer such that 10 raised to that power is in the range of representable finite
floating-point numbers, ⌊log10 ((1 − b−p )bemax )⌋
FLT_MAX_10_EXP +37
DBL_MAX_10_EXP +37
LDBL_MAX_10_EXP +37
25 The values given in the following list shall be replaced by constant expressions with implementation-
defined values that are greater than or equal to those shown:
— maximum representable finite floating-point number; if that number is normalized, its value is
(1 − b−p )bemax
FLT_MAX 1E+37
DBL_MAX 1E+37
LDBL_MAX 1E+37
— maximum normalized floating-point number, (1 − b−p )bemax
FLT_NORM_MAX 1E+37
DBL_NORM_MAX 1E+37
LDBL_NORM_MAX 1E+37
26 The values given in the following list shall be replaced by constant expressions with implementation-
defined (positive) values that are less than or equal to those shown:
— the difference between 1 and the least normalized value greater than 1 that is representable in
the given floating type, b1−p
FLT_EPSILON 1E-5
DBL_EPSILON 1E-9
LDBL_EPSILON 1E-9
— minimum normalized positive floating-point number, bemin −1
FLT_MIN 1E-37
DBL_MIN 1E-37
LDBL_MIN 1E-37
— minimum positive floating-point number28)
FLT_TRUE_MIN 1E-37
DBL_TRUE_MIN 1E-37
LDBL_TRUE_MIN 1E-37
Recommended practice
27 Conversion between real floating type and decimal character sequence with at most T_DECIMAL_DIG
digits should be correctly rounded, where T is the macro prefix for the type. This assures conversion
from real floating type to decimal character sequence with T_DECIMAL_DIG digits and back, using
to-nearest rounding, is the identity function.
28 EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum requirements of this
document, and the appropriate values in a <float.h> header for type float:
6
x = s16e fk 16−k ,
P
−31 ≤ e ≤ +32
k=1
FLT_RADIX 16
FLT_MANT_DIG 6
FLT_EPSILON 9.53674316E-07F
FLT_DECIMAL_DIG 9
FLT_DIG 6
FLT_MIN_EXP -31
FLT_MIN 2.93873588E-39F
FLT_MIN_10_EXP -38
FLT_MAX_EXP +32
FLT_MAX 3.40282347E+38F
FLT_MAX_10_EXP +38
28) If the presence or absence of subnormal numbers is indeterminable, then the value is intended to be a positive number
no greater than the minimum normalized positive number for the type.
29 EXAMPLE 2 The following describes floating-point representations that also meet the requirements for single-precision and
double-precision numbers in IEC 60559,29) and the appropriate values in a <float.h> header for types float and double:
24
xf = s2e fk 2−k ,
P
−125 ≤ e ≤ +128
k=1
53
xd = s2e fk 2−k ,
P
−1021 ≤ e ≤ +1024
k=1
FLT_IS_IEC_60559 2
FLT_RADIX 2
FLT_MANT_DIG 24
FLT_EPSILON 1.19209290E-07F // decimal constant
FLT_EPSILON 0X1P-23F // hex constant
FLT_DECIMAL_DIG 9
FLT_DIG 6
FLT_MIN_EXP -125
FLT_MIN 1.17549435E-38F // decimal constant
FLT_MIN 0X1P-126F // hex constant
FLT_TRUE_MIN 1.40129846E-45F // decimal constant
FLT_TRUE_MIN 0X1P-149F // hex constant
FLT_HAS_SUBNORM 1
FLT_MIN_10_EXP -37
FLT_MAX_EXP +128
FLT_MAX 3.40282347E+38F // decimal constant
FLT_MAX 0X1.fffffeP127F // hex constant
FLT_MAX_10_EXP +38
DBL_MANT_DIG 53
DBL_IS_IEC_60559 2
DBL_EPSILON 2.2204460492503131E-16 // decimal constant
DBL_EPSILON 0X1P-52 // hex constant
DBL_DECIMAL_DIG 17
DBL_DIG 15
DBL_MIN_EXP -1021
DBL_MIN 2.2250738585072014E-308 // decimal constant
DBL_MIN 0X1P-1022 // hex constant
DBL_TRUE_MIN 4.9406564584124654E-324 // decimal constant
DBL_TRUE_MIN 0X1P-1074 // hex constant
DBL_HAS_SUBNORM 1
DBL_MIN_10_EXP -307
DBL_MAX_EXP +1024
DBL_MAX 1.7976931348623157E+308 // decimal constant
DBL_MAX 0X1.fffffffffffffP1023 // hex constant
DBL_MAX_10_EXP +308
Forward references: conditional inclusion (6.10.1), predefined macro names (6.10.9), complex arith-
metic <complex.h> (7.3), extended multibyte and wide character utilities <wchar.h> (7.31), floating-
point environment <fenv.h> (7.6), general utilities <stdlib.h> (7.24), input/output <stdio.h>
(7.23), mathematics <math.h> (7.12), IEC 60559 floating-point arithmetic (Annex F), IEC 60559-
compatible complex arithmetic (Annex G).
5.2.4.2.3 Characteristics of decimal floating types in <float.h>
1 This subclause specifies macros in <float.h> that provide characteristics of decimal floating types
(an optional feature) in terms of the model presented in 5.2.4.2.2. An implementation that does not
support decimal floating types shall not provide these macros. The prefixes DEC32_, DEC64_, and
DEC128_ denote the types _Decimal32 , _Decimal64 , and _Decimal128 respectively.
2 DEC_EVAL_METHOD is the decimal floating-point analog of FLT_EVAL_METHOD (5.2.4.2.2). Its
implementation-defined value characterizes the use of evaluation formats for decimal floating
29) The floating-point model in that standard sums powers of b from zero, so the values of the exponent limits are one less
than shown here.
types:
−1 indeterminable;
0 evaluate all operations and constants just to the range and precision of the type;
1 evaluate operations and constants of type _Decimal32 and _Decimal64 to the range and
precision of the _Decimal64 type, evaluate _Decimal128 operations and constants to the
range and precision of the _Decimal128 type;
2 evaluate all operations and constants to the range and precision of the _Decimal128 type.
3 Each of the decimal signaling NaN macros
DEC32_SNAN
DEC64_SNAN
DEC128_SNAN
expands to a constant expression of the respective decimal floating type representing a signaling
NaN. If an optional unary + or - operator followed by a signaling NaN macro is used for initializing
an object of the same type that has static or thread storage duration, the object is initialized with a
signaling NaN value.
4 The macro
DEC_INFINITY
expands to a constant expression of type _Decimal32 representing positive infinity.
5 The macro
DEC_NAN
expands to a constant expression of type _Decimal32 representing a quiet NaN.
6 The integer values given in the following lists shall be replaced by constant expressions suitable for
use in #if preprocessing directives:
— radix of exponent representation, b(=10)
For the standard floating types, this value is implementation-defined and is specified by the
macro FLT_RADIX. For the decimal floating types there is no corresponding macro, since the
value 10 is an inherent property of the types. Wherever FLT_RADIX appears in a description
of a function that has versions that operate on decimal floating types, it is noted that for the
decimal floating-point versions the value used is implicitly 10, rather than FLT_RADIX.
— number of digits in the coefficient
DEC32_MANT_DIG 7
DEC64_MANT_DIG 16
DEC128_MANT_DIG 34
— minimum exponent
DEC32_MIN_EXP -94
DEC64_MIN_EXP -382
DEC128_MIN_EXP -6142
— maximum exponent
DEC32_MAX_EXP 97
DEC64_MAX_EXP 385
DEC128_MAX_EXP 6145
— maximum representable finite decimal floating-point number (there are 6, 15 and 33 9’s after
the decimal points respectively)
DEC32_MAX 9.999999E96DF
DEC64_MAX 9.999999999999999E384DD
DEC128_MAX 9.999999999999999999999999999999999E6144DL
— the difference between 1 and the least value greater than 1 that is representable in the given
floating type
DEC32_EPSILON 1E-6DF
DEC64_EPSILON 1E-15DD
DEC128_EPSILON 1E-33DL
— minimum normalized positive decimal floating-point number
DEC32_MIN 1E-95DF
DEC64_MIN 1E-383DD
DEC128_MIN 1E-6143DL
— minimum positive subnormal decimal floating-point number
DEC32_TRUE_MIN 0.000001E-95DF
DEC64_TRUE_MIN 0.000000000000001E-383DD
DEC128_TRUE_MIN 0.000000000000000000000000000000001E-6143DL
7 For decimal floating-point arithmetic, it is often convenient to consider an alternate equivalent
model where the significand is represented with integer rather than fraction digits. With s, b, e, p,
and fk as defined in 5.2.4.2.2, a floating-point number x is defined by the model:
p
X
(e−p)
x=s·b fk · b(p−k)
k=1
8 With b fixed to 10, a decimal floating-point number x is thus:
p
X
(e−p)
x = s · 10 fk · 10(p−k)
k=1
The quantum exponent is q = e − p and the coefficient is c = f1 f2 · · · fp , which is an integer between
0 and 10(p−1) , inclusive. Thus, x = s · c · 10q is represented by the triple of integers (s, c, q). The
quantum of x is 10q , which is the value of a unit in the last place of the coefficient.
Quantum exponent ranges
Type _Decimal32 _Decimal64 _Decimal128
Maximum Quantum Exponent (qmax ) 90 369 6111
Minimum Quantum Exponent (qmin ) −101 −398 −6176
9 For binary floating-point arithmetic following IEC 60559, representations in the model described
in 5.2.4.2.2 that have the same numerical value are indistinguishable in the arithmetic. However, for
decimal floating-point arithmetic, representations that have the same numerical value but different
quantum exponents, e.g., (+1, 10, −1) representing 1.0 and (+1, 100, −2) representing 1.00, are
distinguishable. To facilitate exact fixed-point calculation, operation results that are of decimal
floating type have a preferred quantum exponent, as specified in IEC 60559, which is determined
by the quantum exponents of the operands if they have decimal floating types (or by specific
rules for conversions from other types). The table below gives rules for determining preferred
quantum exponents for results of IEC 60559 operations, and for other operations specified in
this document. When exact, these operations produce a result with their preferred quantum
exponent, or as close to it as possible within the limitations of the type. When inexact, these
operations produce a result with the least possible quantum exponent. For example, the preferred
quantum exponent for addition is the minimum of the quantum exponents of the operands. Hence
(+1, 123, −2) + (+1, 4000, −3) = (+1, 5230, −3) or 1.23 + 4.000 = 5.230.
10 The following table shows, for each operation delivering a result in decimal floating-point format,
how the preferred quantum exponents of the operands, Q(x), Q(y), etc., determine the preferred
quantum exponent of the operation result, provided the table formula is defined for the arguments.
For the cases where the formula is undefined and the function result is ±∞, the preferred quantum
exponent is immaterial because the quantum exponent of ±∞ is defined to be infinity. For the
other cases where the formula is undefined and the function result is finite, the preferred quantum
exponent is unspecified30) .
Preferred quantum exponents
Operation Preferred quantum exponent of result
roundeven, round, trunc, ceil, floor, max(Q(x), 0)
rint, nearbyint
nextup, nextdown, nextafter, nexttoward least possible
remainder min(Q(x), Q(y))
fmin, fmax, fminimum, fmaximum, Q(x) if x gives the result, Q(y) if y gives the result
fminimum_mag, fmaximum_mag,
fminimum_num, fmaximum_num,
fminimum_mag_num, fmaximum_mag_num
scalbn, scalbln Q(x) + n
ldexp Q(x) + p
logb 0
+ , d32add, d64add min(Q(x), Q(y))
- , d32sub, d64sub min(Q(x), Q(y))
* , d32mul, d64mul Q(x) + Q(y)
/ , d32div, d64div Q(x) − Q(y)
sqrt, d32sqrt, d64sqrt ⌊Q(x)/2⌋
fma, d32fma, d64fma min(Q(x) + Q(y), Q(z))
conversion from integer type 0
exact conversion from non-decimal floating 0
type
inexact conversion from non-decimal floating least possible
type
conversion between decimal floating types Q(x)
*cx returned by canonicalize Q(*x )
strto, wcsto, scanf, floating constants of see 7.24.1.6
decimal floating type
-(x) , +(x) Q(x)
fabs Q(x)
copysign Q(x)
quantize Q(y)
30) Although unspecified in IEC 60559, a preferred quantum exponent of 0 for these cases would be a reasonable implemen-
tation choice.
quantum Q(x)
*encptr returned by encodedec, encodebin Q(*xptr )
*xptr returned by decodedec, decodebin Q(*encptr )
fmod min(Q(x), Q(y))
fdim min((Q(x), Q(y)) if x > y, 0 if x ≤ y
cbrt ⌊Q(x)/3⌋
hypot min(Q(x), Q(y))
pow ⌊y × Q(x)⌋
modf Q(value)
*iptr returned by modf max(Q(value), 0)
frexp Q(value) if value = 0, –(length of coefficient of
value) otherwise
*res returned by setpayload, 0 if pl does not represent a valid payload, not
setpayloadsig applicable otherwise (NaN returned)
getpayload 0 if *x is a NaN, unspecified otherwise
compoundn ⌊n × min(0, Q(x))⌋
pown ⌊n × Q(x)⌋
powr ⌊y × Q(x)⌋
rootn ⌊Q(x)/n⌋
rsqrt −⌊Q(x)/2⌋
transcendental functions 0
A function family listed in the table above indicates the functions for all decimal floating types,
where the function family is represented by the name of the functions without a suffix. For example,
ceil indicates the functions ceild32, ceild64, and ceild128.
Forward references: extended multibyte and wide character utilities <wchar.h> (7.31), floating-
point environment <fenv.h> (7.6), general utilities <stdlib.h> (7.24), input/output <stdio.h>
(7.23), mathematics <math.h> (7.12), type-generic mathematics <tgmath.h> (7.27), IEC 60559
floating-point arithmetic (Annex F).
6. Language
6.1 Notation
1 In the syntax notation used in this clause, syntactic categories (nonterminals) are indicated by italic
type, and literal words and character set members (terminals) by bold type. A colon (:) following
a nonterminal introduces its definition. Alternative definitions are listed on separate lines, except
when prefaced by the words "one of". An optional symbol is indicated by the subscript "opt", so
that
{ expressionopt }
indicates an optional expression enclosed in braces.
2 When syntactic categories are referred to in the main text, they are not italicized and words are
separated by spaces instead of hyphens.
3 A summary of the language syntax is given in Annex A.
6.2 Concepts
6.2.1 Scopes of identifiers
1 An identifier can denote:
— an object; a function;
— a tag or a member of a structure, union, or enumeration;
— a typedef name;
— a label name;
— a macro name;
— or, a macro parameter.
The same identifier can denote different entities at different points in the program. A member
of an enumeration is called an enumeration constant. Macro names and macro parameters are not
considered further here, because prior to the semantic phase of program translation any occurrences
of macro names in the source file are replaced by the preprocessing token sequences that constitute
their macro definitions.
2 For each different entity that an identifier designates, the identifier is visible (i.e., can be used) only
within a region of program text called its scope. Different entities designated by the same identifier
either have different scopes, or are in different name spaces. There are four kinds of scopes: function,
file, block, and function prototype. (A function prototype is a declaration of a function.)
3 A label name is the only kind of identifier that has function scope. It can be used (in a goto statement)
anywhere in the function in which it appears, and is declared implicitly by its syntactic appearance
(followed by a : and a statement).
4 Every other identifier has scope determined by the placement of its declaration (in a declarator or
type specifier). If the declarator or type specifier that declares the identifier appears outside of any
block or list of parameters, the identifier has file scope, which terminates at the end of the translation
unit. If the declarator or type specifier that declares the identifier appears inside a block or within the
list of parameter declarations in a function definition, the identifier has block scope, which terminates
at the end of the associated block. If the declarator or type specifier that declares the identifier
appears within the list of parameter declarations in a function prototype (not part of a function
definition), the identifier has function prototype scope, which terminates at the end of the function
declarator. If an identifier designates two different entities in the same name space, the scopes might
overlap. If so, the scope of one entity (the inner scope) will end strictly before the scope of the other
entity (the outer scope). Within the inner scope, the identifier designates the entity declared in the
inner scope; the entity declared in the outer scope is hidden (and not visible) within the inner scope.
5 Unless explicitly stated otherwise, where this document uses the term "identifier" to refer to some
entity (as opposed to the syntactic construct), it refers to the entity in the relevant name space whose
declaration is visible at the point the identifier occurs.
6 Two identifiers have the same scope if and only if their scopes terminate at the same point.
7 Structure, union, and enumeration tags have scope that begins just after the appearance of the tag
in a type specifier that declares the tag. Each enumeration constant has scope that begins just after
the appearance of its defining enumerator in an enumerator list. An ordinary identifier that has an
underspecified definition has scope that starts when the definition is completed; if the same ordinary
identifier declares another entity with a scope that encloses the current block, that declaration is
hidden as soon as the inner declarator is completed31) . Any other identifier has scope that begins
just after the completion of its declarator.
8 As a special case, a type name (which is not a declaration of an identifier) is considered to have
a scope that begins just after the place within the type name where the omitted identifier would
appear were it not omitted.
Forward references: declarations (6.7), function calls (6.5.2.2), function definitions (6.9.1), identifiers
(6.4.2), macro replacement (6.10.4), name spaces of identifiers (6.2.3), source file inclusion (6.10.2),
statements and blocks (6.8).
6.2.2 Linkages of identifiers
1 An identifier declared in different scopes or in the same scope more than once can be made to refer
to the same object or function by a process called linkage32) . There are three kinds of linkage: external,
internal, and none.
2 In the set of translation units and libraries that constitutes an entire program, each declaration of a
particular identifier with external linkage denotes the same object or function. Within one translation
unit, each declaration of an identifier with internal linkage denotes the same object or function. Each
declaration of an identifier with no linkage denotes a unique entity.
3 If the declaration of a file scope identifier for:
— an object contains any of the storage-class specifiers static or constexpr;
— or, a function contains the storage-class specifier static,
then the identifier has internal linkage33) .
4 For an identifier declared with the storage-class specifier extern in a scope in which a prior dec-
laration of that identifier is visible34) , if the prior declaration specifies internal or external linkage,
the linkage of the identifier at the later declaration is the same as the linkage specified at the prior
declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the
identifier has external linkage.
5 If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined
exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier
for an object has file scope and no storage-class specifier or only the specifier auto, its linkage is
external.
6 The following identifiers have no linkage: an identifier declared to be anything other than an object
or a function; an identifier declared to be a function parameter; a block scope identifier for an object
declared without the storage-class specifier extern.
7 If, within a translation unit, the same identifier appears with both internal and external linkage, the
behavior is undefined.
31) That means, that the outer declaration is not visible for the initializer
32) There is no linkage between different identifiers.
33) A function declaration can contain the storage-class specifier static only if it is at file scope; see 6.7.1.
34) As specified in 6.2.1, the later declaration might hide the prior declaration.
Forward references: declarations (6.7), expressions (6.5), external definitions (6.9), statements (6.8).
6.2.3 Name spaces of identifiers
1 If more than one declaration of a particular identifier is visible at any point in a translation unit, the
syntactic context disambiguates uses that refer to different entities. Thus, there are separate name
spaces for various categories of identifiers, as follows:
— label names (disambiguated by the syntax of the label declaration and use);
— the tags of structures, unions, and enumerations (disambiguated by following any35) of the
keywords struct, union, or enum);
— the members of structures or unions; each structure or union has a separate name space for its
members (disambiguated by the type of the expression used to access the member via the . or
-> operator);
— standard attributes and attribute prefixes (disambiguated by the syntax of the attribute specifier
and name of the attribute token) (6.7.12);
— the trailing identifier in an attribute prefixed token; each attribute prefix has a separate name
space for the implementation-defined attributes that it introduces (disambiguated by the
attribute prefix and the trailing identifier token);
— all other identifiers, called ordinary identifiers (declared in ordinary declarators or as enumera-
tion constants).
Forward references: enumeration specifiers (6.7.2.2), labeled statements (6.8.1), structure and union
specifiers (6.7.2.1), structure and union members (6.5.2.3), tags (6.7.2.3), the goto statement (6.8.6.1).
6.2.4 Storage durations of objects
1 An object has a storage duration that determines its lifetime. There are four storage durations: static,
thread, automatic, and allocated. Allocated storage is described in 7.24.3.
2 The lifetime of an object is the portion of program execution during which storage is guaranteed
to be reserved for it. An object exists, has a constant address36) , and retains its last-stored value
throughout its lifetime37) . If an object is referred to outside of its lifetime, the behavior is undefined.
If a pointer value is used in an evaluation after the object the pointer points to (or just past) reaches
the end of its lifetime, the behavior is undefined. The representation of a pointer object becomes
indeterminate when the object the pointer points to (or just past) reaches the end of its lifetime.
3 An object whose identifier is declared without the storage-class specifier thread_local, and either
with external or internal linkage or with the storage-class specifier static, has static storage duration.
Its lifetime is the entire execution of the program and its stored value is initialized only once, prior
to program startup.
4 An object whose identifier is declared with the storage-class specifier thread_local has thread
storage duration. Its lifetime is the entire execution of the thread for which it is created, and its
stored value is initialized when the thread is started. There is a distinct object per thread, and use of
the declared name in an expression refers to the object associated with the thread evaluating the
expression. The result of attempting to indirectly access an object with thread storage duration from
a thread other than the one with which the object is associated is implementation-defined.
5 An object whose identifier is declared with no linkage and without the storage-class specifier static
has automatic storage duration, as do some compound literals. The result of attempting to indirectly
access an object with automatic storage duration from a thread other than the one with which the
object is associated is implementation-defined.
35) There is only one name space for tags even though three are possible.
36) The term "constant address" means that two pointers to the object constructed at possibly different times will compare
equal. The address can be different during two different executions of the same program.
37) In the case of a volatile object, the last store need not be explicit in the program.
6 For such an object that does not have a variable length array type, its lifetime extends from entry
into the block with which it is associated until execution of that block ends in any way. (Entering
an enclosed block or calling a function suspends, but does not end, execution of the current block.)
If the block is entered recursively, a new instance of the object is created each time. The initial
representation of the object is indeterminate. If an initialization is specified for the object, it is
performed each time the declaration or compound literal is reached in the execution of the block;
otherwise, the representation of the object becomes indeterminate each time the declaration is
reached.
7 For such an object that does have a variable length array type, its lifetime extends from the declaration
of the object until execution of the program leaves the scope of the declaration38) . If the scope is
entered recursively, a new instance of the object is created each time. The initial representation of
the object is indeterminate.
8 A non-lvalue expression with structure or union type, where the structure or union contains a
member with array type (including, recursively, members of all contained structures and unions)
refers to an object with automatic storage duration and temporary lifetime.39) Its lifetime begins
when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends
when the evaluation of the containing full expression ends. Any attempt to modify an object with
temporary lifetime results in undefined behavior. An object with temporary lifetime behaves as if it
were declared with the type of its value for the purposes of effective type. Such an object need not
have a unique address.
Forward references: array declarators (6.7.6.2), compound literals (6.5.2.5), declarators (6.7.6),
function calls (6.5.2.2), initialization (6.7.10), statements (6.8), effective type (6.5).
6.2.5 Types
1 The meaning of a value stored in an object or returned by a function is determined by the type of the
expression used to access it. (An identifier declared to be an object is the simplest such expression;
the type is specified in the declaration of the identifier.) Types are partitioned into object types (types
that describe objects) and function types (types that describe functions). At various points within a
translation unit an object type may be incomplete40) (lacking sufficient information to determine the
size of objects of that type) or complete (having sufficient information)41) .
2 An object declared as type bool is large enough to store the values false and true.
3 An object declared as type char is large enough to store any member of the basic execution char-
acter set. If a member of the basic execution character set is stored in a char object, its value is
guaranteed to be nonnegative. If any other character is stored in a char object, the resulting value is
implementation-defined but shall be within the range of values that can be represented in that type.
4 There are five standard signed integer types, designated as signed char, short int, int, long int,
and long long int. (These and other types may be designated in several additional ways, as
described in 6.7.2.)
5 A bit-precise signed integer type is designated as _BitInt( N) where N is an integer constant expression
that specifies the number of bits that are used to represent the type, including the sign bit. Each
value of N designates a distinct type42) . There may also be implementation-defined extended signed
integer types 43) . The standard signed integer types, bit-precise signed integer types, and extended
signed integer types are collectively called signed integer types. 44)
38) Leaving the innermost block containing the declaration, or jumping to a point in that block or an embedded block prior
to the declaration, leaves the scope of the declaration.
39) The address of such an object is taken implicitly when an array member is accessed.
40) An incomplete type can only be used when the size of an object of that type is not needed. It is not needed, for example,
when a typedef name is declared to be a specifier for a structure or union, or when a pointer to or a function returning a
structure or union is being declared. The specification has to be complete before such a function is called or defined.
41) A type can be incomplete or complete throughout an entire translation unit, or it can change states at different points
within a translation unit.
42) Thus, _BitInt(3) is not the same type as _BitInt(4) .
43) Implementation-defined keywords have the form of an identifier reserved for any use as described in 7.1.3.
44) Any statement in this document about signed integer types also applies to the bit-precise signed integer types and the
extended signed integer types, unless otherwise noted.
6 An object declared as type signed char occupies the same amount of storage as a "plain" char
object. A "plain" int object has the natural size suggested by the architecture of the execution
environment (large enough to contain any value in the range INT_MIN to INT_MAX as defined in the
header <limits.h>).
7 For each of the signed integer types, there is a corresponding (but different) unsigned integer type
(designated with the keyword unsigned) that uses the same amount of storage (including sign
information) and has the same alignment requirements. The type bool and the unsigned integer
types that correspond to the standard signed integer types are the standard unsigned integer types. The
unsigned integer types that correspond to the extended signed integer types are the extended unsigned
integer types. The unsigned integer types that correspond to the bit-precise signed integer types
are the bit-precise unsigned integer types. The standard unsigned integer types, bit-precise unsigned
integer types, and extended unsigned integer types are collectively called unsigned integer types.45)
8 The standard signed integer types and standard unsigned integer types are collectively called the
standard integer types; the bit-precise signed integer types and bit-precise unsigned integer types
are collectively called the bit-precise integer types the extended signed integer types and extended
unsigned integer types are collectively called the extended integer types.
9 For any two integer types with the same signedness and different integer conversion rank (see
6.3.1.1), the range of values of the type with smaller integer conversion rank is a subrange of the
values of the other type.
10 The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned
integer type, and the representation of the same value in each type is the same.46) The range of
representable values for the unsigned type is 0 to 2N − 1 (inclusive). A computation involving
unsigned operands can never produce an overflow, because arithmetic for the unsigned type is
performed modulo 2N .
11 There are three standard floating types, designated as float, double, and long double. 47) The set
of values of the type float is a subset of the set of values of the type double; the set of values of the
type double is a subset of the set of values of the type long double.
12 There are three decimal floating types, designated as _Decimal32 , _Decimal64 , and _Decimal128 .
Respectively, they have the IEC 60559 formats: decimal32,48) decimal64, and decimal128. Decimal
floating types are real floating types.
13 The standard floating types and the decimal floating types are collectively called the real floating
types.
14 There are three complex types, designated as float _Complex, double _Complex, and long double
_Complex .49) (Complex types are a conditional feature that implementations need not support; see
6.10.9.3.) The real floating and complex types are collectively called the floating types.
15 For each floating type there is a corresponding real type, which is always a real floating type. For real
floating types, it is the same type. For complex types, it is the type given by deleting the keyword
_Complex from the type name.
16 Each complex type has the same representation and alignment requirements as an array type
containing exactly two elements of the corresponding real type; the first element is equal to the real
part, and the second element to the imaginary part, of the complex number.
17 The type char, the signed and unsigned integer types, and the floating types are collectively called
the basic types. The basic types are complete object types. Even if the implementation defines two or
more basic types to have the same representation, they are nevertheless different types.50)
45) Any statement in this document about unsigned integer types also applies to the bit-precise unsigned integer types and
the extended unsigned integer types, unless otherwise specified.
46) The same representation and alignment requirements are meant to imply interchangeability as arguments to functions,
return values from functions, and members of unions.
47) See "future language directions" (6.11.1).
48) IEC 60559 specifies decimal32 as a data-interchange format that does not require arithmetic support; however,
_Decimal32 is a fully supported arithmetic type.
49) A specification for imaginary types is in Annex G.
50) An implementation can define new keywords that provide alternative ways to designate a basic (or any other) type; this
18 The three types char, signed char, and unsigned char are collectively called the character types.
The implementation shall define char to have the same range, representation, and behavior as either
signed char or unsigned char.51)
19 An enumeration comprises a set of named integer constant values. Each distinct enumeration
constitutes a different enumerated type.
20 The type char, the signed and unsigned integer types, and the enumerated types are collectively
called integer types. The integer and real floating types are collectively called real types.
21 Integer and floating types are collectively called arithmetic types. Each arithmetic type belongs to
one type domain: the real type domain comprises the real types, the complex type domain comprises the
complex types.
22 The void type comprises an empty set of values; it is an incomplete object type that cannot be
completed.
23 Any number of derived types can be constructed from the object and function types, as follows:
— An array type describes a contiguously allocated nonempty set of objects with a particular
member object type, called the element type. The element type shall be complete whenever the
array type is specified. Array types are characterized by their element type and by the number
of elements in the array. An array type is said to be derived from its element type, and if its
element type is T, the array type is sometimes called "array of T". The construction of an array
type from an element type is called "array type derivation".
— A structure type describes a sequentially allocated nonempty set of member objects (and, in
certain circumstances, an incomplete array), each of which has an optionally specified name
and possibly distinct type.
— A union type describes an overlapping nonempty set of member objects, each of which has an
optionally specified name and possibly distinct type.
— A function type describes a function with specified return type. A function type is characterized
by its return type and the number and types of its parameters. A function type is said to
be derived from its return type, and if its return type is T, the function type is sometimes
called "function returning T". The construction of a function type from a return type is called
"function type derivation".
— A pointer type may be derived from a function type or an object type, called the referenced type. A
pointer type describes an object whose value provides a reference to an entity of the referenced
type. A pointer type derived from the referenced type T is sometimes called "pointer to T".
The construction of a pointer type from a referenced type is called "pointer type derivation".
A pointer type is a complete object type.
— An atomic type describes the type designated by the construct _Atomic (type-name). (Atomic
types are a conditional feature that implementations need not support; see 6.10.9.3.)
These methods of constructing derived types can be applied recursively.
24 Arithmetic types, pointer types, and the nullptr_t type are collectively called scalar types. Array
and structure types are collectively called aggregate types52) .
25 An array type of unknown size is an incomplete type. It is completed, for an identifier of that type,
by specifying the size in a later declaration (with internal or external linkage). A structure or union
type of unknown content (as described in 6.7.2.3) is an incomplete type. It is completed, for all
does not violate the requirement that all basic types be different. Implementation-defined keywords have the form of an
identifier reserved for any use as described in 7.1.3.
51) CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the
two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.
52) Note that aggregate type does not include union type because an object with union type can only contain one member at
a time.
declarations of that type, by declaring the same structure or union tag with its defining content later
in the same scope.
26 A complete type shall have a size that is less than or equal to SIZE_MAX. A type has known constant
size if it is complete and is not a variable length array type.
27 Array, function, and pointer types are collectively called derived declarator types. A declarator type
derivation from a type T is the construction of a derived declarator type from T by the application of
an array-type, a function-type, or a pointer-type derivation to T.
28 A type is characterized by its type category, which is either the outermost derivation of a derived
type (as noted above in the construction of derived types), or the type itself if the type consists of no
derived types.
29 Any type so far mentioned is an unqualified type. Each unqualified type has several qualified versions
of its type53) , corresponding to the combinations of one, two, or all three of the const, volatile, and
restrict qualifiers. The qualified or unqualified versions of a type are distinct types that belong to
the same type category and have the same representation and alignment requirements.54) An array
and its element type are always considered to be identically qualified55) . Any other derived type is
not qualified by the qualifiers (if any) of the type from which it is derived.
30 Further, there is the _Atomic qualifier. The presence of the _Atomic qualifier designates an atomic
type. The size, representation, and alignment of an atomic type need not be the same as those of
the corresponding unqualified type. Therefore, this document explicitly uses the phrase "atomic,
qualified, or unqualified type" whenever the atomic version of a type is permitted along with the
other qualified versions of a type. The phrase "qualified or unqualified type", without specific
mention of atomic, does not include the atomic types.
31 A pointer to void shall have the same representation and alignment requirements as a pointer to a
character type.54) Similarly, pointers to qualified or unqualified versions of compatible types shall
have the same representation and alignment requirements. All pointers to structure types shall have
the same representation and alignment requirements as each other. All pointers to union types shall
have the same representation and alignment requirements as each other. Pointers to other types
need not have the same representation or alignment requirements.
32 EXAMPLE 1 The type designated as "float *" has type "pointer to float". Its type category is pointer, not a floating type.
The const-qualified version of this type is designated as "float * const" whereas the type designated as "const float *"
is not a qualified type — its type is "pointer to const-qualified float" and is a pointer to a qualified type.
33 EXAMPLE 2 The type designated as "struct tag (*[5])(float)" has type "array of pointer to function returning
struct tag". The array has length five and the function has a single parameter of type float. Its type category is array.
Forward references: compatible type and composite type (6.2.7), declarations (6.7).
6.2.6 Representations of types
6.2.6.1 General
1 The representations of all types are unspecified except as stated in this subclause.
2 Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number,
order, and encoding of which are either explicitly specified or implementation-defined.
3 Values stored in unsigned bit-fields and objects of type unsigned char shall be represented using a
pure binary notation.56)
4 Values stored in non-bit-field objects of any other object type are represented using n× CHAR_BIT bits,
where n is the size of an object of that type, in bytes. An object that has the value may be copied into
53) See 6.7.3 regarding qualified array and function types.
54) The same representation and alignment requirements are meant to imply interchangeability as arguments to functions,
return values from functions, and members of unions.
55) This does not apply to the _Atomic qualifier. Note that qualifiers do not have any direct effect on the array type itself,
but affect conversion rules for pointer types that reference an array type.
56) A positional representation for integers that uses the binary digits 0 and 1, in which the values represented by successive
bits are additive, begin with 1, and are multiplied by successive integral powers of 2, except perhaps the bit with the highest
position. (Adapted from the American National Dictionary for Information Processing Systems.) A byte contains CHAR_BIT bits,
_
and the values of type unsigned char range from 0 to 2CHAR BIT − 1.
an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object
representation of the value. Values stored in bit-fields consist of m bits, where m is the size specified
for the bit-field. The object representation is the set of m bits the bit-field comprises in the addressable
storage unit holding it. Two values (other than NaNs) with the same object representation compare
equal, but values that compare equal may have different object representations.
5 Certain object representations need not represent a value of the object type. If such a representation
is read by an lvalue expression that does not have character type, the behavior is undefined. If such
a representation is produced by a side effect that modifies all or any part of the object by an lvalue
expression that does not have character type, the behavior is undefined57) . Such a representation is
called a non-value representation.
6 When a value is stored in an object of structure or union type, including in a member object, the bytes
of the object representation that correspond to any padding bytes take unspecified values58) . The
object representation of a structure or union object is never a non-value representation, even though
the byte range corresponding to a member of the structure or union object may be a non-value
representation for that member.
7 When a value is stored in a member of an object of union type, the bytes of the object representation
that do not correspond to that member but do correspond to other members take unspecified values.
8 Where an operator is applied to a value that has more than one object representation, which object
representation is used shall not affect the value of the result.59) Where a value is stored in an object
using a type that has more than one object representation for that value, it is unspecified which
representation is used, but a non-value representation shall not be generated.
9 Loads and stores of objects with atomic types are done with memory_order_seq_cst semantics.
Forward references: declarations (6.7), expressions (6.5), lvalues, arrays, and function designators
(6.3.2.1), order and consistency (7.17.3).
6.2.6.2 Integer types
1 For unsigned integer types the bits of the object representation shall be divided into two groups:
value bits and padding bits. If there are N value bits, each bit shall represent a different power of
2 between 1 and 2N −1 , so that objects of that type shall be capable of representing values from 0
to 2N − 1 using a pure binary representation; this shall be known as the value representation. The
values of any padding bits are unspecified. The number of value bits N is called the width of the
unsigned integer type. The type bool shall have one value bit and (sizeof(bool)*CHAR_BIT)- 1
padding bits. Otherwise, there need not be any padding bits; unsigned char shall not have any
padding bits.
2 For signed integer types, the bits of the object representation shall be divided into three groups:
value bits, padding bits, and the sign bit. If the corresponding unsigned type has width N , the
signed type uses the same number of N bits, its width, as value bits and sign bit. N − 1 are value
bits and the remaining bit is the sign bit. Each bit that is a value bit shall have the same value as the
same bit in the object representation of the corresponding unsigned type. If the sign bit is zero, it
shall not affect the resulting value. If the sign bit is one, it has value −(2N −1 ). There need not be any
padding bits; signed char shall not have any padding bits.
3 The values of any padding bits are unspecified. A valid object representation of a signed integer
type where the sign bit is zero is a valid object representation of the corresponding unsigned type,
and shall represent the same value. For any integer type, the object representation where all the bits
are zero shall be a representation of the value zero in that type.
4 The precision of an integer type is the number of value bits.
57) Thus, an automatic variable can be initialized to a non-value representation without causing undefined behavior, but the
value of the variable cannot be used until a proper value is stored in it.
58) Thus, for example, structure assignment need not copy any padding bits.
59) It is possible for objects x and y with the same effective type T to have the same value when they are accessed as objects
of type T, but to have different values in other contexts. In particular, if == is defined for type T, then x == y does not imply
that memcmp(&x, &y, sizeof (T))== 0. Furthermore, x == y does not necessarily imply that x and y have the same value;
other operations on values of type T might distinguish between them.
5 NOTE 1 Some combinations of padding bits might generate non-value representations, for example, if one padding bit is a
parity bit. Regardless, no arithmetic operation on valid values can generate a non-value representation other than as part of
an exceptional condition such as an integer overflow, and this cannot occur with unsigned types. All other combinations of
padding bits are alternative object representations of the value specified by the value bits.
6 NOTE 2 The sign representation defined in this document is called two’s complement. Previous revisions of this document
additionally allowed other sign representations.
7 NOTE 3 For unsigned integer types the width and precision are the same, while for signed integer types the width is one
greater than the precision.
6.2.7 Compatible type and composite type
1 Two types are compatible types if they are the same. Additional rules for determining whether two
types are compatible are described in 6.7.2 for type specifiers, in 6.7.3 for type qualifiers, and in 6.7.6
for declarators60) . Moreover, two complete structure, union, or enumerated types declared with the
same tag are compatible if members satisfy the following requirements:
— there shall be a one-to-one correspondence between their members such that each pair of
corresponding members are declared with compatible types;
— if one member of the pair is declared with an alignment specifier, the other is declared with an
equivalent alignment specifier;
— and, if one member of the pair is declared with a name, the other is declared with the same
name.
For two structures, corresponding members shall be declared in the same order. For two structures or
unions, corresponding bit-fields shall have the same widths. For two enumerations, corresponding
members shall have the same values; if one has a fixed underlying type, then the other shall have a
compatible fixed underlying type. For determining type compatibility, anonymous structures and
unions are considered a regular member of the containing structure or union type, and the type
of an anonymous structure or union is considered compatible to the type of another anonymous
structure or union, respectively, if their members fulfill the above requirements.
Furthermore, two structure, union, or enumerated types declared in separate translation units are
compatible in the following cases:
— both are declared without tags and they fulfill the requirements above;
— both have the same tag and are completed somewhere in their respective translation units and
they fulfill the requirements above;
— both have the same tag and at least one of the two types is not completed in its translation unit.
Otherwise, the structure, union, or enumerated types are incompatible61) .
2 All declarations that refer to the same object or function shall have compatible type; otherwise, the
behavior is undefined.
3 A composite type can be constructed from two types that are compatible; it is a type that is compatible
with both of the two types and satisfies the following conditions:
— If both types are array types, the following rules are applied:
• If one type is an array of known constant size, the composite type is an array of that size.
• Otherwise, if one type is a variable length array whose size is specified by an expression
that is not evaluated, the behavior is undefined.
• Otherwise, if one type is a variable length array whose size is specified, the composite
type is a variable length array of that size.
60) Two types need not be identical to be compatible.
61) A structure, union, or enumerated type without a tag or an incomplete structure, union or enumerated type is not
compatible with any other structure, union or enum type declared in the same translation unit.
• Otherwise, if one type is a variable length array of unspecified size, the composite type is
a variable length array of unspecified size.
• Otherwise, both types are arrays of unknown size and the composite type is an array of
unknown size.
The element type of the composite type is the composite type of the two element types.
— If both types are function types, the type of each parameter in the composite parameter type
list is the composite type of the corresponding parameters.
— If one of the types has a standard attribute, the composite type also has that attribute.
These rules apply recursively to the types from which the two types are derived.
4 For an identifier with internal or external linkage declared in a scope in which a prior declaration of
that identifier is visible62) , if the prior declaration specifies internal or external linkage, the type of
the identifier at the later declaration becomes the composite type.
5 EXAMPLE Given the following two file scope declarations:
int f(int (*)(char *), double (*)[3]);
int f(int (*)(char *), double (*)[]);
The resulting composite type for the function is:
int f(int (*)(char *), double (*)[3]);
Forward references: array declarators (6.7.6.2).
6.2.8 Alignment of objects
1 Complete object types have alignment requirements which place restrictions on the addresses at
which objects of that type may be allocated. An alignment is an implementation-defined integer
value representing the number of bytes between successive addresses at which a given object can be
allocated. An object type imposes an alignment requirement on every object of that type: stricter
alignment can be requested using the alignas keyword.
2 A fundamental alignment is a valid alignment less than or equal to alignof (max_align_t). Funda-
mental alignments shall be supported by the implementation for objects of all storage durations.
The alignment requirements of the following types shall be fundamental alignments:
— all atomic, qualified, or unqualified basic types;
— all atomic, qualified, or unqualified enumerated types;
— all atomic, qualified, or unqualified pointer types;
— all array types whose element type has a fundamental alignment requirement;
— all types specified in Clause 7 as complete object types;
— all structure or union types all of whose elements have types with fundamental alignment
requirements and none of whose elements have an alignment specifier specifying an alignment
that is not a fundamental alignment.
3 An extended alignment is represented by an alignment greater than alignof (max_align_t). It is
implementation-defined whether any extended alignments are supported and the storage durations
for which they are supported. A type having an extended alignment requirement is an over-aligned
type.63)
62) As specified in 6.2.1, the later declaration might hide the prior declaration.
63) Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has
been applied.
4 Alignments are represented as values of the type size_t. Valid alignments include only fundamental
alignments, plus an additional implementation-defined set of values, which may be empty. Every
valid alignment value shall be a nonnegative integral power of two.
5 Alignments have an order from weaker to stronger or stricter alignments. Stricter alignments have
larger alignment values. An address that satisfies an alignment requirement also satisfies any weaker
valid alignment requirement.
6 The alignment requirement of a complete type can be queried using an alignof expression. The
types char, signed char, and unsigned char shall have the weakest alignment requirement.
7 Comparing alignments is meaningful and provides the obvious results:
— Two alignments are equal when their numeric values are equal.
— Two alignments are different when their numeric values are not equal.
— When an alignment is larger than another it represents a stricter alignment.
6.2.9 Encodings
1 The literal encoding is an implementation-defined mapping of the characters of the execution character
set to the values in a character constant (6.4.4.4) or string literal (6.4.5). It shall support a mapping
from all the basic execution character set values into the implementation-defined encoding. It may
contain multibyte character sequences (5.2.1.1).
2 The wide literal encoding is an implementation-defined mapping of the characters of the execution
character set to the values in a wchar_t character constant (6.4.4.4) or a wchar_t string literal (6.4.5).
It shall support a mapping from all the basic execution character set values into the implementation-
defined encoding. The mapping shall produce values identical to the literal encoding for all the basic
execution character set values if an implementation does not define __STDC_MB_MIGHT_NEQ_WC__ .
One or more values may map to one or more values of the extended execution character set.
6.3 Conversions
1 Several operators convert operand values from one type to another automatically. This subclause
specifies the result required from such an implicit conversion, as well as those that result from a cast
operation (an explicit conversion). The list in 6.3.1.8 summarizes the conversions performed by most
ordinary operators; it is supplemented as required by the discussion of each operator in 6.5.
2 Unless explicitly stated otherwise, conversion of an operand value to a compatible type causes no
change to the value or the representation.
Forward references: cast operators (6.5.4).
6.3.1 Arithmetic operands
6.3.1.1 Boolean, characters, and integers
1 Every integer type has an integer conversion rank defined as follows:
— No two signed integer types shall have the same rank, even if they have the same representa-
tion.
— The rank of a signed integer type shall be greater than the rank of any signed integer type with
less precision.
— The rank of long long int shall be greater than the rank of long int, which shall be greater
than the rank of int, which shall be greater than the rank of short int, which shall be greater
than the rank of signed char.
— The rank of a bit-precise signed integer type shall be greater than the rank of any standard
integer type with less width or any bit-precise integer type with less width.
— The rank of any unsigned integer type shall equal the rank of the corresponding signed integer
type, if any.
— The rank of any standard integer type shall be greater than the rank of any extended integer
type with the same width or bit-precise integer type with the same width.
— The rank of any bit-precise integer type relative to an extended integer type of the same width
is implementation-defined.
— The rank of char shall equal the rank of signed char and unsigned char.
— The rank of bool shall be less than the rank of all other standard integer types.
— The rank of any enumerated type shall equal the rank of the compatible integer type (see
6.7.2.2).
— The rank of any extended signed integer type relative to another extended signed integer
type with the same precision is implementation-defined, but still subject to the other rules for
determining the integer conversion rank.
— For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has greater rank than
T3 , then T1 has greater rank than T3.
2 The following may be used in an expression wherever an int or unsigned int may be used:
— An object or expression with an integer type (other than int or unsigned int) whose integer
conversion rank is less than or equal to the rank of int and unsigned int.
— A bit-field of type bool, int, signed int, or unsigned int.
The value from a bit-field of a bit-precise integer type is converted to the corresponding bit-precise
type. If the original type is not a bit-precise integer type (6.2.5) and if an int can represent all values
of the original type (as restricted by the width, for a bit-field), the value is converted to an int64) ;
otherwise, it is converted to an unsigned int. These are called the integer promotions65) . All other
types are unchanged by the integer promotions.
3 The integer promotions preserve value including sign. As discussed earlier, whether a "plain" char
can hold negative values is implementation-defined.
Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1).
6.3.1.2 Boolean type
1 When any scalar value is converted to bool, the result is false if the value is a zero (for arithmetic
types), null (for pointer types), or the scalar has type nullptr_t; otherwise, the result is true.
6.3.1.3 Signed and unsigned integers
1 When a value with integer type is converted to another integer type other than bool, if the value
can be represented by the new type, it is unchanged.
2 Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting
one more than the maximum value that can be represented in the new type until the value is in the
range of the new type.66)
3 Otherwise, the new type is signed and the value cannot be represented in it; either the result is
implementation-defined or an implementation-defined signal is raised.
64) E.g.,
unsigned _BitInt(7): 2 is a bit-field that can hold the values −2, −1, 0, 1, and converts to
unsigned _BitInt(7).
65) The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to
the operands of the unary + ,- , and ~ operators, and to both operands of the shift operators, as specified by their respective
subclauses.
66) The rules describe arithmetic on the mathematical value, not the value of a given type of expression.
6.3.1.4 Real floating and integer
1 When a finite value of standard floating type is converted to an integer type other than bool, the
fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part
cannot be represented by the integer type, the behavior is undefined.67)
2 When a finite value of decimal floating type is converted to an integer type other than bool, the
fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part
cannot be represented by the integer type, the "invalid" floating-point exception shall be raised and
the result of the conversion is unspecified.
3 When a value of integer type is converted to a standard floating type, if the value being converted
can be represented exactly in the new type, it is unchanged. If the value being converted is in the
range of values that can be represented but cannot be represented exactly, the result is either the
nearest higher or nearest lower representable value, chosen in an implementation-defined manner.
If the value being converted is outside the range of values that can be represented, the behavior is
undefined. Results of some implicit conversions may be represented in greater range and precision
than that required by the new type (see 6.3.1.8 and 6.8.6.4).
4 When a value of integer type is converted to a decimal floating type, if the value being converted
can be represented exactly in the new type, it is unchanged. If the value being converted cannot
be represented exactly, the result shall be correctly rounded with exceptions raised as specified in
IEC 60559.
6.3.1.5 Real floating types
1 When a value of real floating type is converted to a real floating type, if the value being converted
can be represented exactly in the new type, it is unchanged.
2 When a value of real floating type is converted to a standard floating type, if the value being
converted is in the range of values that can be represented but cannot be represented exactly, the
result is either the nearest higher or nearest lower representable value, chosen in an implementation-
defined manner. If the value being converted is outside the range of values that can be represented,
the behavior is undefined.
3 When a value of real floating type is converted to a decimal floating type, if the value being converted
cannot be represented exactly, the result is correctly rounded with exceptions raised as specified in
IEC 60559.
4 Results of some implicit conversions may be represented in greater range and precision than that
required by the new type (see 6.3.1.8 and 6.8.6.4).
6.3.1.6 Complex types
1 When a value of complex type is converted to another complex type, both the real and imaginary
parts follow the conversion rules for the corresponding real types.
6.3.1.7 Real and complex
1 When a value of real type is converted to a complex type, the real part of the complex result value is
determined by the rules of conversion to the corresponding real type and the imaginary part of the
complex result value is a positive zero or an unsigned zero.
2 When a value of complex type is converted to a real type other than bool,68) the imaginary part of
the complex value is discarded and the value of the real part is converted according to the conversion
rules for the corresponding real type.
6.3.1.8 Usual arithmetic conversions
1 Many operators that expect operands of arithmetic type cause conversions and yield result types in
a similar way. The purpose is to determine a common real type for the operands and result. For the
specified operands, each operand is converted, without change of type domain, to a type whose
67) The remaindering operation performed when a value of integer type is converted to unsigned type need not be
performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is
(−1, Utype_MAX + 1).
68) See 6.3.1.2.
corresponding real type is the common real type. Unless explicitly stated otherwise, the common
real type is also the corresponding real type of the result, whose type domain is the type domain of
the operands if they are the same, and complex otherwise. This pattern is called the usual arithmetic
conversions:
If one operand has decimal floating type, the other operand shall not have standard floating,
complex, or imaginary type.
First, if the type of either operand is _Decimal128 , the other operand is converted to
_Decimal128 .
Otherwise, if the type of either operand is _Decimal64 , the other operand is converted to
_Decimal64 .
Otherwise, if the type of either operand is _Decimal32 , the other operand is converted to
_Decimal32 .
Otherwise, if the corresponding real type of either operand is long double, the other operand
is converted, without change of type domain, to a type whose corresponding real type is
long double.
Otherwise, if the corresponding real type of either operand is double, the other operand is
converted, without change of type domain, to a type whose corresponding real type is double.
Otherwise, if the corresponding real type of either operand is float, the other operand is
converted, without change of type domain, to a type whose corresponding real type is float.69)
Otherwise, the integer promotions are performed on both operands. Then the following rules
are applied to the promoted operands:
If both operands have the same type, then no further conversion is needed.
Otherwise, if both operands have signed integer types or both have unsigned integer
types, the operand with the type of lesser integer conversion rank is converted to the type
of the operand with greater rank.
Otherwise, if the operand that has unsigned integer type has rank greater or equal to
the rank of the type of the other operand, then the operand with signed integer type is
converted to the type of the operand with unsigned integer type.
Otherwise, if the type of the operand with signed integer type can represent all of the
values of the type of the operand with unsigned integer type, then the operand with
unsigned integer type is converted to the type of the operand with signed integer type.
Otherwise, both operands are converted to the unsigned integer type corresponding to
the type of the operand with signed integer type.
2 The values of floating operands and of the results of floating expressions may be represented in
greater range and precision than that required by the type; the types are not changed thereby.
See 5.2.4.2.2 regarding evaluation formats.
3 EXAMPLE 1 One consequence of _BitInt being exempt from the integer promotion rules (6.3.1) is that a _BitInt operand
of a binary operator is not always promoted to an int or unsigned int as part of the usual arithmetic conversions. Instead,
a lower-ranked operand is converted to the higher-rank operand type and the result of the operation is the higher-ranked
type.
_BitInt(2) a2 = 1;
_BitInt(3) a3 = 2;
_BitInt(33) a33 = 1;
char c = 3;
a2 * a3 /* As part of the multiplication, a2 is converted to
69) For example, addition of a double _Complex and a float entails just the conversion of the float operand to double
(and yields a double _Complex result).
_BitInt(3) and the result type is _BitInt(3). */
a2 * c /* As part of the multiplication, c is promoted to int,
a2 is converted to int and the result type is int. */
a33 * c /* As part of the multiplication, c is promoted to int,
then converted to _BitInt(33) and the result type
is _BitInt(33). */
void func(_BitInt(8) a1, _BitInt(24) a2) {
/* Cast one of the operands to 32-bits to guarantee the
result of the multiplication can contain all possible
values. */
_BitInt(32) a3 = a1 * (_BitInt(32))a2;
}
6.3.2 Other operands
6.3.2.1 Lvalues, arrays, and function designators
1 An lvalue is an expression (with an object type other than void) that potentially designates an
object;70) if an lvalue does not designate an object when it is evaluated, the behavior is undefined.
When an object is said to have a particular type, the type is specified by the lvalue used to designate
the object. A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete
type, does not have a const-qualified type, and if it is a structure or union, does not have any
member (including, recursively, any member or element of all contained aggregates or unions) with
a const-qualified type.
2 Except when it is the operand of the sizeof operator, or the typeof operators, the unary & operator,
the ++ operator, the-- operator, or the left operand of the . operator or an assignment operator, an
lvalue that does not have array type is converted to the value stored in the designated object (and is
no longer an lvalue); this is called lvalue conversion. If the lvalue has qualified type, the value has the
unqualified version of the type of the lvalue; additionally, if the lvalue has atomic type, the value has
the non-atomic version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the
lvalue has an incomplete type and does not have array type, the behavior is undefined. If the lvalue
designates an object of automatic storage duration that could have been declared with the register
storage class (never had its address taken), and that object is uninitialized (not declared with an
initializer and no assignment to it has been performed prior to use), the behavior is undefined.
3 Except when it is the operand of the sizeof operator, or typeof operators, or the unary & operator,
or is a string literal used to initialize an array, an expression that has type "array of type" is converted
to an expression with type "pointer to type" that points to the initial element of the array object and
is not an lvalue. If the array object has register storage class, the behavior is undefined.
4 A function designator is an expression that has function type. Except when it is the operand of the
sizeof operator71) , a typeof operator, or the unary & operator, a function designator with type
"function returning type" is converted to an expression that has type "pointer to function returning
type".
Forward references: address and indirection operators (6.5.3.2), assignment operators (6.5.16),
common definitions <stddef.h> (7.21), initialization (6.7.10), postfix increment and decrement
operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), the sizeof and alignof
operators (6.5.3.4), structure and union members (6.5.2.3).
70) The name "lvalue" comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to
be a (modifiable) lvalue. It is perhaps better considered as representing an object "locator value". What is sometimes called
"rvalue" is in this document described as the "value of an expression".
An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary expression that is a
pointer to an object, *E is an lvalue that designates the object to which E points.
71) Because this conversion does not occur, the operand of the sizeof operator remains a function designator and violates
the constraints in 6.5.3.4.
6.3.2.2 void
1 The (nonexistent) value of a void expression (an expression that has type void) shall not be used in any
way, and implicit or explicit conversions (except to void) shall not be applied to such an expression.
If an expression of any other type is evaluated as a void expression, its value or designator is
discarded. (A void expression is evaluated for its side effects.)
6.3.2.3 Pointers
1 A pointer to void may be converted to or from a pointer to any object type. A pointer to any object
type may be converted to a pointer to void and back again; the result shall compare equal to the
original pointer.
2 For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to the q-qualified
version of the type; the values stored in the original and converted pointers shall compare equal.
3 An integer constant expression with the value 0, such an expression cast to type void *, or the
predefined constant nullptr is called a null pointer constant72) . If a null pointer constant or a value
of the type nullptr_t (which is necessarily the value nullptr) is converted to a pointer type, the
resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or
function.
4 Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null
pointers shall compare equal.
5 An integer may be converted to any pointer type. Except as previously specified, the result is
implementation-defined, might not be correctly aligned, might not point to an entity of the referenced
type, and might produce an indeterminate representation when stored into an object73) .
6 Any pointer type may be converted to an integer type. Except as previously specified, the result
is implementation-defined. If the result cannot be represented in the integer type, the behavior is
undefined. The result need not be in the range of values of any integer type.
7 A pointer to an object type may be converted to a pointer to a different object type. If the resulting
pointer is not correctly aligned74) for the referenced type, the behavior is undefined. Otherwise,
when converted back again, the result shall compare equal to the original pointer. When a pointer to
an object is converted to a pointer to a character type, the result points to the lowest addressed byte
of the object. Successive increments of the result, up to the size of the object, yield pointers to the
remaining bytes of the object.
8 A pointer to a function of one type may be converted to a pointer to a function of another type and
back again; the result shall compare equal to the original pointer. If a converted pointer is used to
call a function whose type is not compatible with the referenced type, the behavior is undefined.
6.3.2.4 nullptr_t
1 The type nullptr_t may be converted to bool or to a pointer type. The result is false or the null
pointer value, respectively.
2 The type nullptr_t may be converted to itself.
Forward references: cast operators (6.5.4), equality operators (6.5.9), integer types capable of
holding object pointers (7.22.1.4), simple assignment (6.5.16.1).
72) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.21.
73) The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to be consistent with
the addressing structure of the execution environment.
74) In general, the concept "correctly aligned" is transitive: if a pointer to type A is correctly aligned for a pointer to type B,
which in turn is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C.
6.4 Lexical elements
Syntax
1 token:
keyword
identifier
constant
string-literal
punctuator
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each universal-character-name that cannot be one of the above
each non-white-space character that cannot be one of the above
Constraints
2 Each preprocessing token that is converted to a token shall have the lexical form of a keyword, an
identifier, a constant, a string literal, or a punctuator. A single universal character name shall match
one of the other preprocessing token categories.
Semantics
3 A token is the minimal lexical element of the language in translation phases 7 and 8. The categories of
tokens are: keywords, identifiers, constants, string literals, and punctuators. A preprocessing token
is the minimal lexical element of the language in translation phases 3 through 6. The categories of
preprocessing tokens are: header names, identifiers, preprocessing numbers, character constants,
string literals, punctuators, and both single universal character names as well as single non-white-
space characters that do not lexically match the other preprocessing token categories.75) If a ’ or a "
character matches the last category, the behavior is undefined. Preprocessing tokens can be separated
by white space; this consists of comments (described later), or white-space characters (space, horizontal
tab, new-line, vertical tab, and form-feed), or both. As described in 6.10, in certain circumstances
during translation phase 4, white space (or the absence thereof) serves as more than preprocessing
token separation. White space may appear within a preprocessing token only as part of a header
name or between the quotation characters in a character constant or string literal.
4 If the input stream has been parsed into preprocessing tokens up to a given character, the next
preprocessing token is the longest sequence of characters that could constitute a preprocessing token.
There is one exception to this rule: header name preprocessing tokens are recognized only within
#include and #embed preprocessing directives, in __has_include and __has_embed expressions,
as well as in implementation-defined locations within #pragma directives. In such contexts, a
sequence of characters that could be either a header name or a string literal is recognized as the
former.
5 EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer
constant token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for
example, if Ex were a macro defined as +1 ). Similarly, the program fragment 1E1 is parsed as a preprocessing number (one
that is a valid floating constant token), whether or not E is a macro name.
6 EXAMPLE 2 The program fragment x+++++y is parsed as x ++ ++ + y, which violates a constraint on increment operators,
even though the parse x ++ + ++ y might yield a correct expression.
Forward references: character constants (6.4.4.4), comments (6.4.9), expressions (6.5), floating
75) An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.4.3); it cannot occur in source
files.
constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.4), postfix increment and decrement
operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), preprocessing directives (6.10),
preprocessing numbers (6.4.8), string literals (6.4.5).
6.4.1 Keywords
Syntax
1 keyword: one of
alignas enum short void
alignof extern signed volatile
auto false sizeof while
bool float static _Atomic
break for static_assert _BitInt
case goto struct _Complex
char if switch _Decimal128
const inline thread_local _Decimal32
constexpr int true _Decimal64
continue long typedef _Generic
default nullptr typeof _Imaginary
do register typeof_unqual _Noreturn
double restrict union
else return unsigned
Semantics
2 The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as keywords
except in an attribute token, and shall not be used otherwise. The keyword _Imaginary is reserved
for specifying imaginary types.76)
3 The following table provides alternate spellings for certain keywords. These can be used wherever
the keyword can77) .
Keyword Alternative Spelling
alignas _Alignas
alignof _Alignof
bool _Bool
static_assert _Static_assert
thread_local _Thread_local
The spelling of these keywords, their alternate forms, and of false and true inside expressions that
are subject to the # and ## preprocessing operators is unspecified78) .
6.4.2 Identifiers
6.4.2.1 General
Syntax
1 identifier:
identifier-start
identifier identifier-continue
identifier-start:
nondigit
XID_Start character
universal-character-name of class XID_Start
76) One possible specification for imaginary types appears in Annex G.
77) These alternative keywords are obsolescent features and should not be used for new code and development.
78) The intent of this specification is to allow but not force the implementation of the corresponding feature by means of a
predefined macro.
identifier-continue:
digit
nondigit
XID_Continue character
universal-character-name of class XID_Continue
nondigit: one of
_ a b c d e f g h i j k l m
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
digit: one of
0 1 2 3 4 5 6 7 8 9
Semantics
2 An XID_Start character is an implementation-defined character whose corresponding code point
in ISO/IEC 10646 has the XID_Start property. An XID_Continue character is an implementation-
defined character whose corresponding code point in ISO/IEC 10646 has the XID_Continue property.
An identifier is a sequence of one identifier start character followed by 0 or more identifier continue
characters, which designates one or more entities as described in 6.2.1. Lowercase and uppercase
letters are distinct. There is no specific limit on the maximum length of an identifier.
3 The character classes XID_Start and XID_Continue are Derived Core Properties as described by
UAX #4479) . Each character and universal character name in an identifier shall designate a character
whose encoding in ISO/IEC 10646 has the XID_Continue property. The initial character (which
may be a universal character name) shall designate a character whose encoding in ISO/IEC 10646
has the XID_Start property. An identifier shall conform to Normalization Form C as specified in
ISO/IEC 10646. Annex D provides an overview of the conforming identifiers.
4 NOTE 1 Uppercase and lowercase letters are considered different for all identifiers.
5 NOTE 2 In translation phase 4 (4), the term identifier also includes those preprocessing tokens (6.4.8) differentiated as
keywords (6.4.1) in the later translation phase 7 (7).
6 When preprocessing tokens are converted to tokens during translation phase 7, if a preprocessing
token could be converted to either a keyword or an identifier, it is converted to a keyword except in
an attribute token.
7 Some identifiers are reserved.
— All identifiers that begin with a double underscore (__ ) or begin with an underscore (_ )
followed by an uppercase letter are reserved for any use, except those identifiers which are
lexically identical to keywords80) .
— All identifiers that begin with an underscore are reserved for use as identifiers with file scope
in both the ordinary and tag name spaces.
Other identifiers may be reserved, see 7.1.3.
8 If the program declares or defines an identifier in a context in which it is reserved (other than as
allowed by 7.1.4), the behavior is undefined.
79) On systems that cannot accept extended characters in external identifiers, an encoding of the universal-character-name
may be used in forming such identifiers. For example, some otherwise unused character or sequence of characters may be
used to encode the u in a universal character name.
80) This allows a reserved identifier that matches the spelling of a keyword to be used as a macro name by the program.
9 If the program defines a reserved identifier or attribute token described in 6.7.12.1 as a macro name,
or removes (with #undef) any macro definition of an identifier in the first group listed above or
attribute token described in 6.7.12.1, the behavior is undefined.
10 Some identifiers may be potentially reserved. A potentially reserved identifier is an identifier which is
not reserved unless made so by an implementation providing the identifier (7.1.3) but is anticipated
to become reserved by an implementation or a future version of this document.
Recommended Practice
11 Implementations are encouraged to issue a diagnostic message when a potentially reserved identifier
is declared or defined for any use that is not implementation-compatible (see below) in a context
where the potentially reserved identifier may be reserved under a conforming implementation. This
brings attention to a potential conflict when porting a program to a future revision of this document.
12 An implementation-compatible use of a potentially reserved identifier is a declaration of an external
name where the name is provided by the implementation as an external name and where the
declaration declares an object or function with a type that is compatible with the type of the object
or function provided by the implementation under that name.
Implementation limits
13 As discussed in 5.2.4.1, an implementation may limit the number of significant initial characters
in an identifier; the limit for an external name (an identifier that has external linkage) may be more
restrictive than that for an internal name (a macro name or an identifier that does not have external
linkage). The number of significant characters in an identifier is implementation-defined.
14 Any identifiers that differ in a significant character are different identifiers. If two identifiers differ
only in nonsignificant characters, the behavior is undefined.
Forward references: universal character names (6.4.3), macro replacement (6.10.4), reserved library
identifiers (7.1.3), use of library functions (7.1.4), attributes (6.7.12.1).
6.4.2.2 Predefined identifiers
Semantics
1 The identifier __func__ shall be implicitly declared by the translator as if, immediately following
the opening brace of each function definition, the declaration
static const char __func__[] = "function-name";
appeared, where function-name is the name of the lexically-enclosing function.81)
2 This name is encoded as if the implicit declaration had been written in the source character set and
then translated into the execution character set as indicated in translation phase 5.
3 EXAMPLE Consider the code fragment:
#include <stdio.h>
void myfunc(void)
{
printf("%s\n", __func__);
/* ... */
}
Each time the function is called, it will print to the standard output stream:
myfunc
Forward references: function definitions (6.9.1).
81) Since the name __func__ is reserved for any use by the implementation (7.1.3), if any other identifier is explicitly declared
using the name __func__ , the behavior is undefined.
6.4.3 Universal character names
Syntax
1 universal-character-name:
\u hex-quad
\U hex-quad hex-quad
hex-quad:
hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit
Constraints
2 A universal character name shall not designate a code point where the hexadecimal value is:
— less than 00A0 other than 0024 ($), 0040 (@), or 0060 (` );
— in the range D800 through DFFF inclusive; or
— greater than 10FFFF82) .
Description
3 Universal character names may be used in identifiers, character constants, and string literals to
designate characters that are not in the basic character set.
Semantics
4 The universal character name \U nnnnnnnn designates the character whose eight-digit short identifier
(as specified by ISO/IEC 10646) is nnnnnnnn.83) Similarly, the universal character name \u nnnn
designates the character whose four-digit short identifier is nnnn (and whose eight-digit short
identifier is 0000nnnn).
82) The disallowed characters are the characters in the basic character set and the code positions reserved by ISO/IEC 10646
for control characters, the character DELETE, the S-zone (reserved for use by UTF-16), and characters too large to be encoded
by ISO/IEC 10646. Disallowed universal character escape sequences can still be specified with hexadecimal and octal escape
sequences (6.4.4.4).
83) Short identifiers for characters were first specified in ISO/IEC 10646–1:1993/Amd 9:1997.
6.4.4 Constants
Syntax
1 constant:
integer-constant
floating-constant
enumeration-constant
character-constant
predefined-constant
Constraints
2 Each constant shall have a type and the value of a constant shall be in the range of representable
values for its type.
Semantics
3 Each constant has a type, determined by its form and value, as detailed later.
6.4.4.1 Integer constants
Syntax
1 integer-constant:
decimal-constant integer-suffixopt
octal-constant integer-suffixopt
hexadecimal-constant integer-suffixopt
binary-constant integer-suffixopt
decimal-constant:
nonzero-digit
decimal-constant ’opt digit
octal-constant:
0
octal-constant ’opt octal-digit
hexadecimal-constant:
hexadecimal-prefix hexadecimal-digit-sequence
binary-constant:
binary-prefix binary-digit
binary-constant ’opt binary-digit
hexadecimal-prefix: one of
0x 0X
binary-prefix: one of
0b 0B
nonzero-digit: one of
1 2 3 4 5 6 7 8 9
octal-digit: one of
0 1 2 3 4 5 6 7
hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence ’opt hexadecimal-digit
hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
binary-digit: one of
0 1
integer-suffix:
unsigned-suffix long-suffixopt
unsigned-suffix long-long-suffix
unsigned-suffix bit-precise-int-suffix
long-suffix unsigned-suffixopt
long-long-suffix unsigned-suffixopt
bit-precise-int-suffix unsigned-suffixopt
bit-precise-int-suffix: one of
wb WB
unsigned-suffix: one of
u U
long-suffix: one of
l L
long-long-suffix: one of
ll LL
Description
2 An integer constant begins with a digit, but has no period or exponent part. It may have a prefix that
specifies its base and a suffix that specifies its type. An optional separating single quote character (
’ ) in an integer or floating constant is called a digit separator. Digit separators are ignored when
determining the value of the constant.
3 EXAMPLE
0b11’10’11’01 /* 0b11101101 */
’1’2 /* character constant ’1’ followed by integer constant 2,
not the integer constant 12 */
11’22 /* 1122 */
0x’FFFF’FFFF /* invalid hexadecimal constant (’ cannot appear after 0x) */
0x1’2’3’4AB’C’D /* 0x1234ABCD */
4 A decimal constant begins with a nonzero digit and consists of a sequence of decimal digits. An
octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7
only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal
digits and the letters a (or A) through f (or F) with values 10 through 15 respectively. A binary
constant consists of the prefix 0b or 0B followed by a sequence of the digits 0 or 1.
Semantics
5 The value of a decimal constant is computed base 10; that of an octal constant, base 8; that of a
hexadecimal constant, base 16; that of a binary constant, base 2. The lexically first digit is the most
significant.
6 The type of an integer constant is the first of the corresponding list in which its value can be
represented.
Octal, Hexadecimal or Binary
Suffix Decimal Constant Constant
none int int
long int unsigned int
long long int long int
unsigned long int
long long int
unsigned long long int
u or U unsigned int unsigned int
unsigned long int unsigned long int
unsigned long long int unsigned long long int
l or L long int long int
long long int unsigned long int
long long int
unsigned long long int
Both u or U unsigned long int unsigned long int
and l or L unsigned long long int unsigned long long int
ll or LL long long int long long int
unsigned long long int
Both u or U unsigned long long int unsigned long long int
and ll or LL
wb or WB _BitInt(N) where the width N _BitInt(N) where the width N
is the smallest N greater than is the smallest N greater than
1 which can accommodate 1 which can accommodate
the value and the sign bit. the value and the sign bit.
Both u or U unsigned _BitInt(N) where the unsigned _BitInt(N) where the
and wb or WB width N is the smallest N width N is the smallest N
greater than 0 which can greater than 0 which can
accommodate the value. accommodate the value.
7 If an integer constant cannot be represented by any type in its list, it may have an extended integer
type, if the extended integer type can represent its value. If all of the types in the list for the constant
are signed, the extended integer type shall be signed. If all of the types in the list for the constant
are unsigned, the extended integer type shall be unsigned. If the list contains both signed and
unsigned types, the extended integer type may be signed or unsigned. If an integer constant cannot
be represented by any type in its list and has no extended integer type, then the integer constant has
no type.
8 EXAMPLE 1 The wb suffix results in an _BitInt that includes space for the sign bit even if the value of the constant is
positive or was specified in hexadecimal or octal notation.
-3wb /* Yields an _BitInt(3) that is then negated; two value
bits, one sign bit */
-0x3wb /* Yields an _BitInt(3) that is then negated; two value
bits, one sign bit */
3wb /* Yields an _BitInt(3); two value bits, one sign bit */
3uwb /* Yields an unsigned _BitInt(2) */
-3uwb /* Yields an unsigned _BitInt(2) that is then negated,
resulting in wrap-around */
Forward references: preprocessing numbers (6.4.8), numeric conversion functions (7.24.1).
6.4.4.2 Floating constants
Syntax
1 floating-constant:
decimal-floating-constant
hexadecimal-floating-constant
decimal-floating-constant:
fractional-constant exponent-partopt floating-suffixopt
digit-sequence exponent-part floating-suffixopt
hexadecimal-floating-constant:
hexadecimal-prefix hexadecimal-fractional-constant
binary-exponent-part floating-suffixopt
hexadecimal-prefix hexadecimal-digit-sequence
binary-exponent-part floating-suffixopt
fractional-constant:
digit-sequenceopt . digit-sequence
digit-sequence .
exponent-part:
e signopt digit-sequence
E signopt digit-sequence
sign: one of
+ -
digit-sequence:
digit
digit-sequence ’opt digit
hexadecimal-fractional-constant:
hexadecimal-digit-sequenceopt . hexadecimal-digit-sequence
hexadecimal-digit-sequence .
binary-exponent-part:
p signopt digit-sequence
P signopt digit-sequence
floating-suffix: one of
f l F L df dd dl DF DD DL
Constraints
2 A floating suffix df, dd, dl, DF, DD, or DL shall not be used in a hexadecimal floating constant.
Description
3 A floating constant has a significand part that may be followed by an exponent part and a suffix that
specifies its type. The components of the significand part may include a digit sequence representing
the whole-number part, followed by a period ( .), followed by a digit sequence representing the
fraction part. Digit separators (6.4.4.1) are ignored when determining the value of the constant. The
components of the exponent part are an e, E, p, or P followed by an exponent consisting of an
optionally signed digit sequence. Either the whole-number part or the fraction part has to be present;
for decimal floating constants, either the period or the exponent part has to be present.
Semantics
4 The significand part is interpreted as a (decimal or hexadecimal) rational number; the digit sequence
in the exponent part is interpreted as a decimal integer. For decimal floating constants, the exponent
indicates the power of 10 by which the significand part is to be scaled. For hexadecimal floating
constants, the exponent indicates the power of 2 by which the significand part is to be scaled. For
decimal floating constants, and also for hexadecimal floating constants when FLT_RADIX is not a
power of 2, the result is either the nearest representable value, or the larger or smaller representable
value immediately adjacent to the nearest representable value, chosen in an implementation-defined
manner. For hexadecimal floating constants when FLT_RADIX is a power of 2, the result is correctly
rounded.
5 An unsuffixed floating constant has type double. If suffixed by a floating suffix it has a type
according to the following table:
Suffixes for floating constants
Suffix Type
f, F float
l, L long double
df, DF _Decimal32
dd, DD _Decimal64
dl, DL _Decimal128
6 The values of floating constants may be represented in greater range and precision than that required
by the type (determined by the suffix); the types are not changed thereby. See 5.2.4.2.2 regarding
evaluation formats. 84)
84) Hexadecimal floating constants can be used to obtain exact values in the semantic type that are independent of the
evaluation format. Casts produce values in the semantic type, though depend on the rounding mode and may raise the
inexact floating-point exception.
7 Floating constants of decimal floating type that have the same numerical value but different quantum
exponents have distinguishable internal representations. The value shall be correctly rounded as
specified in IEC 60559. The coefficient c and the quantum exponent q of a finite converted decimal
floating-point number (see 5.2.4.2.3) are determined as follows:
— q is set to the value of signopt digit-sequence in the exponent part, if any, or to 0, otherwise.
— If there is a fractional constant, q is decreased by the number of digits to the right of the period
and the period is removed to form a digit sequence.
— c is set to the value of the digit sequence (after any period has been removed).
— Rounding required because of insufficient precision or range in the type of the result will
round c to the full precision available in the type, and will adjust q accordingly within the
limits of the type, provided the rounding does not yield an infinity (in which case the result
is an appropriately signed internal representation of infinity). If the full precision of the type
would require q to be smaller than the minimum for the type, then q is pinned at the minimum
and c is adjusted through the subnormal range accordingly, perhaps to zero.
8 Floating constants are converted to internal format as if at translation-time. The conversion of a
floating constant shall not raise an exceptional condition or a floating-point exception at execution
time. All floating constants of the same source form 85) shall convert to the same internal format
with the same value.
9 EXAMPLE Following are floating constants of type _Decimal64 and their values as triples (s, c, q). Note that for
_Decimal64 , the precision (maximum coefficient length) is 16 and the quantum exponent range is −398 ≤ q ≤ 369.
0.dd (+1, 0, 0)
0.00dd (+1, 0, −2)
123.dd (+1, 123, 0)
1.23E3dd (+1, 123, 1)
1.23E+3dd (+1, 123, 1)
12.3E+7dd (+1, 123, 6)
12.0dd (+1, 120, −1)
12.3dd (+1, 123, −1)
0.00123dd (+1, 123, −5)
1.23E-12dd (+1, 123, −14)
1234.5E-4dd (+1, 12345, −5)
0E+7dd (+1, 0, 7)
12345678901234567890.dd (+1, 1234567890123457, 4) assuming default rounding and DEC_EVAL_METHOD is 0
or 186)
1234E-400dd (+1, 12, −398) assuming default rounding and DEC_EVAL_METHOD is 0 or 1
1234E-402dd (+1, 0, −398) assuming default rounding and DEC_EVAL_METHOD is 0 or 1
1000.dd (+1, 1000, 0)
.0001dd (+1, 1, −4)
1000.e0dd (+1, 1000, 0)
.0001e0dd (+1, 1, −4)
1000.0dd (+1, 10000, −1)
0.0001dd (+1, 1, −4)
1000.00dd (+1, 100000, −2)
00.0001dd (+1, 1, −4)
001000.dd (+1, 1000, 0)
001000.0dd (+1, 10000, −1)
001000.00dd (+1, 100000, −2)
00.00dd (+1, 0, −2)
00.dd (+1, 0, 0)
.00dd (+1, 0, −2)
00.00e-5dd (+1, 0, −7)
00.e-5dd (+1, 0, −5)
.00e-5dd (+1, 0, −7)
85) 1.23 , 1.230 , 123e-2 , 123e-02 , and 1.23L are all different source forms and thus need not convert to the same internal
format and value.
86) That is, assuming the default translation rounding-direction mode is not changed by an FENV_DEC_ROUND pragma (7.6.3).
Recommended practice
10 The implementation should produce a diagnostic message if a hexadecimal constant cannot be
represented exactly in its evaluation format; the implementation should then proceed with the
translation of the program.
11 The translation-time conversion of floating constants should match the execution-time conversion
of character strings by library functions, such as strtod, given matching inputs suitable for both
conversions, the same result format, and default execution-time rounding. 87)
12 NOTE Floating constants do not include a sign and are negated by the unary - operator (6.5.3.3) which negates the rounded
value of the constant. In contrast, the numeric conversion functions in the strto family (7.24.1.5, 7.24.1.6) include the sign as
part of the input value and convert and round the negated input. Negating before rounding and negating after rounding
might yield different results, depending on the rounding direction and whether the results are correctly rounded. For
example, the results are the same when both are correctly rounded using rounding to nearest or rounding toward zero, but
the results are different when they are inexact and correctly rounded using rounding toward positive infinity or rounding
toward negative infinity.
Conversions yielding exact results require no rounding, so are not affected by the order of negating and rounding. For
types with radix 10, decimal floating constants expressed within the precision and range of the evaluation format convert
exactly. For types whose radix is a power of 2, hexadecimal floating constants expressed within the precision and range of the
evaluation format convert exactly.
Forward references: preprocessing numbers (6.4.8), numeric conversion functions (7.24.1), the
strto function family (7.24.1.5, 7.24.1.6).
6.4.4.3 Enumeration constants
Syntax
1 enumeration-constant:
identifier
Semantics
2 An identifier declared as an enumeration constant for an enumeration without a fixed underlying
type has either type int or the enumerated type, as defined in 6.7.2.2. An identifier declared
as an enumeration constant for an enumeration with a fixed underlying type has the associated
enumeration type.
3 An enumeration constant may be used in an expression (or constant expression) wherever a value
of an integer type may be used.
Forward references: enumeration specifiers (6.7.2.2).
6.4.4.4 Character constants
Syntax
1 character-constant:
encoding-prefixopt ’ c-char-sequence ’
encoding-prefix:
u8
u
U
L
c-char-sequence:
c-char
c-char-sequence c-char
87) The specification for the library functions recommends more accurate conversion than required for floating constants
(see 7.24.1.5).
c-char:
any member of the source character set except
the single-quote ’, backslash \ , or new-line character
escape-sequence
escape-sequence:
simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence
universal-character-name
simple-escape-sequence: one of
\’ \" \? \\
\a \b \f \n \r \t \v
octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit
Description
2 An integer character constant is a sequence of one or more multibyte characters enclosed in single-
quotes, as in ’x’ . A UTF-8 character constant is the same, except prefixed by u8. A wchar_t character
constant is prefixed by the letter L. A UTF-16 character constant is prefixed by the letter u. A UTF-32
character constant is prefixed by the letter U. Collectively, wchar_t, UTF-16, and UTF-32 character
constants are called wide character constants. With a few exceptions detailed later, the elements of
the sequence are any members of the source character set; they are mapped in an implementation-
defined manner to members of the execution character set.
3 The single-quote ’, the double-quote ", the question-mark ?, the backslash \, and arbitrary integer
values are representable according to the following table of escape sequences:
single quote ’ \’
double quote " \"
question mark ? \?
backslash \ \\
octal character \ octal digits
hexadecimal character \x hexadecimal digits
4 The double-quote " and question-mark ? are representable either by themselves or by the escape
sequences \" and \?, respectively, but the single-quote ’ and the backslash \ shall be represented,
respectively, by the escape sequences \’ and \\ .
5 The octal digits that follow the backslash in an octal escape sequence are taken to be part of the
construction of a single character for an integer character constant or of a single wide character for a
wide character constant. The numerical value of the octal integer so formed specifies the value of
the desired character or wide character.
6 The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape sequence
are taken to be part of the construction of a single character for an integer character constant or of a
single wide character for a wide character constant. The numerical value of the hexadecimal integer
so formed specifies the value of the desired character or wide character.
7 Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute
the escape sequence.
8 In addition, characters not in the basic character set are representable by universal character names
and certain non-graphic characters are representable by escape sequences consisting of the back-
slash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t, and \v.88)
Constraints
9 The value of an octal or hexadecimal escape sequence shall be in the range of representable values
for the corresponding type:
Prefix Corresponding Type
none unsigned char
u8 char8_t
L the unsigned type corresponding to wchar_t
u char16_t
U char32_t
10 A UTF-8, UTF-16, or UTF-32 character constant shall not contain more than one character.89) The
value shall be representable with a single UTF-8, UTF-16, or UTF-32 code unit.
Semantics
11 An integer character constant has type int. The value of an integer character constant containing
a single character that maps to a single value in the literal encoding (6.2.9) is the numerical value
of the representation of the mapped character in the literal encoding interpreted as an integer.
The value of an integer character constant containing more than one character (e.g., ’ab’ ), or
containing a character or escape sequence that does not map to a single value in the literal encoding,
is implementation-defined. If an integer character constant contains a single character or escape
sequence, its value is the one that results when an object with type char whose value is that of the
single character or escape sequence is converted to type int.
12 A UTF-8 character constant has type char8_t. If the UTF8 character constant is not produced
through a hexadecimal or octal escape sequence, the value of a UTF-8 character constant is equal to
its ISO/IEC 10646 code point value, provided that the code point value can be encoded as a single
UTF-8 code unit. Otherwise, the value of the UTF8 character constant is the numeric value specified
in the hexadecimal or octal escape sequence.
13 A UTF-16 character constant has type char16_t which is an unsigned integer types defined in the
<uchar.h> header. If the UTF-16 character constant is not produced through a hexadecimal or octal
escape sequence, the value of a UTF-16 character constant is equal to its ISO/IEC 10646 code point
value, provided that the code point value can be encoded as a single UTF-16 code unit. Otherwise,
the value of the UTF-16 character constant is the numeric value specified in the hexadecimal or octal
escape sequence.
14 A UTF-32 character constant has type char32_t which is an unsigned integer types defined in the
<uchar.h> header. If the UTF-32 character constant is not produced through a hexadecimal or octal
escape sequence, the value of a UTF-32 character constant is equal to its ISO/IEC 10646 code point
value, provided that the code point value can be encoded as a single UTF-32 code unit. Otherwise,
the value of the UTF-32 character constant is the numeric value specified in the hexadecimal or octal
escape sequence.
15 A wchar_t character constant prefixed by the letter L has type wchar_t, an integer type defined in
the <stddef.h> header. The value of a wchar_t character constant containing a single multibyte
88) The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash, the result is not a
token and a diagnostic is required. See "future language directions" (6.11.4).
89) For example u8’ab’ violates this constraint.
character that maps to a single member of the extended execution character set is the wide character
corresponding to that multibyte character in the implementation-defined wide literal encoding
(6.2.9). The value of a wchar_t character constant containing more than one multibyte character or a
single multibyte character that maps to multiple members of the extended execution character set,
or containing a multibyte character or escape sequence not represented in the extended execution
character set, is implementation-defined.
16 EXAMPLE 1 The construction ’\0’ is commonly used to represent the null character.
17 EXAMPLE 2 Consider implementations that use eight bits for objects that have type char. In an implementation in which
type char has the same range of values as signed char, the integer character constant ’\xFF’ has the value −1; if type
char has the same range of values as unsigned char, the character constant ’\xFF’ has the value +255.
18 EXAMPLE 3 Even if eight bits are used for objects that have type char, the construction ’\x123’ specifies an integer character
constant containing only one character, since a hexadecimal escape sequence is terminated only by a non-hexadecimal
character. To specify an integer character constant containing the two characters whose values are ’\x12’ and ’3’ , the
construction ’\0223’ can be used, since an octal escape sequence is terminated after three octal digits. (The value of this
two-character integer character constant is implementation-defined.)
19 EXAMPLE 4 Even if 12 or more bits are used for objects that have type wchar_t, the construction L’\1234’ specifies the
implementation-defined value that results from the combination of the values 0123 and ’4’ .
Forward references: common definitions <stddef.h> (7.21), the mbtowc function (7.24.7.2), Uni-
code utilities <uchar.h> (7.30).
6.4.4.5 Predefined constants
Syntax
1 predefined-constant:
false
true
nullptr
Description
2 Some keywords represent constants of a specific value and type.
3 The keywords false and true are constants of type bool with a value of 0 for false and 1 for
true90) .
4 The keyword nullptr represents a null pointer constant. Details of its type are described in 7.21.2.
6.4.5 String literals
Syntax
1 string-literal:
encoding-prefixopt " s-char-sequenceopt "
s-char-sequence:
s-char
s-char-sequence s-char
s-char:
any member of the source character set except
the double-quote ", backslash \, or new-line character
escape-sequence
90) The constants false and true promote to type int, see 6.3.1.1. When used for arithmetic, in translation phase 4, they are
signed values and the result of such arithmetic is consistent with the results of later translation phases.
Constraints
2 If a sequence of adjacent string literal tokens includes prefixed string literal tokens, the prefixed
tokens shall all have the same prefix.
Description
3 A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes,
as in "xyz". A UTF-8 string literal is the same, except prefixed by u8. A wchar_t string literal is the
same, except prefixed by L. A UTF-16 string literal is the same, except prefixed by u. A UTF-32 string
literal is the same, except prefixed by U. Collectively, wchar_t, UTF-16, and UTF-32 string literals are
called wide string literals.
4 The same considerations apply to each element of the sequence in a string literal as if it were in an
integer character constant (for a character or UTF-8 string literal) or a wide character constant (for a
wide string literal), except that the single-quote ’ is representable either by itself or by the escape
sequence \’, but the double-quote " shall be represented by the escape sequence \".
Semantics
5 In translation phase 6, the multibyte character sequences specified by any sequence of adjacent
character and identically-prefixed string literal tokens are concatenated into a single multibyte
character sequence. If any of the tokens has an encoding prefix, the resulting multibyte character
sequence is treated as having the same prefix; otherwise, it is treated as a character string literal.
6 In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence
that results from a string literal or literals. 91) The multibyte character sequence is then used to
initialize an array of static storage duration and length just sufficient to contain the sequence. For
character string literals, the array elements have type char, and are initialized with the individual
bytes of the multibyte character sequence corresponding to the literal encoding (6.2.9). For UTF-8
string literals, the array elements have type char8_t, and are initialized with the characters of the
multibyte character sequence, as encoded in UTF-8. For wide string literals prefixed by the letter
L, the array elements have type wchar_t and are initialized with the sequence of wide characters
corresponding to the wide literal encoding. For wide string literals prefixed by the letter u or U,
the array elements have type char16_t or char32_t, respectively, and are initialized sequence of
wide characters corresponding to UTF-16 and UTF-32 encoded text, respectively. The value of a
string literal containing a multibyte character or escape sequence not represented in the execution
character set is implementation-defined. Any hexadecimal escape sequence or octal escape sequence
specified in a u8, u, or U string specifies a single char8_t, char16_t, or char32_t value and may
result in the full character sequence not being valid UTF-8, UTF-16, or UTF-32.
7 It is unspecified whether these arrays are distinct provided their elements have the appropriate
values. If the program attempts to modify such an array, the behavior is undefined.
8 EXAMPLE 1 This pair of adjacent character string literals
"\x12" "3"
produces a single character string literal containing the two characters whose values are ’\x12’ and ’3’ , because escape
sequences are converted into single members of the execution character set just prior to adjacent string literal concatenation.
9 EXAMPLE 2 Each of the sequences of adjacent string literal tokens
"a" "b" L"c"
"a" L"b" "c"
L"a" "b" L"c"
L"a" L"b" L"c"
is equivalent to the string literal
L"abc"
Likewise, each of the sequences
91) A string literal might not be a string (see 7.1.1), because a null character can be embedded in it by a \0 escape sequence.
"a" "b" u"c"
"a" u"b" "c"
u"a" "b" u"c"
u"a" u"b" u"c"
is equivalent to
u"abc"
Forward references: common definitions <stddef.h> (7.21), the mbstowcs function (7.24.8.1),
Unicode utilities <uchar.h> (7.30).
6.4.6 Punctuators
Syntax
1 punctuator: one of
[ ] ( ) { } . ->
++ -- & * + - ~ !
/ % << >> < > <= >= == != ^ | && ||
? : :: ; ...
= *= /= %= += -= <<= >>= &= ^= |=
, # ##
<: :> <% %> %: %:%:
Semantics
2 A punctuator is a symbol that has independent syntactic and semantic significance. Depending on
context, it may specify an operation to be performed (which in turn may yield a value or a function
designator, produce a side effect, or some combination thereof) in which case it is known as an
operator (other forms of operator also exist in some contexts). An operand is an entity on which an
operator acts.
3 In all aspects of the language, the six tokens92)
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens
[ ] { } # ##
except for their spelling.93)
Forward references: expressions (6.5), declarations (6.7), preprocessing directives (6.10), statements
(6.8).
6.4.7 Header names
Syntax
1 header-name:
< h-char-sequence >
" q-char-sequence "
h-char-sequence:
h-char
h-char-sequence h-char
92) These tokens are sometimes called "digraphs".
93) Thus [ and <: behave differently when "stringized" (see 6.10.4.2), but can otherwise be freely interchanged.
h-char:
any member of the source character set except
the new-line character and >
q-char-sequence:
q-char
q-char-sequence q-char
q-char:
any member of the source character set except
the new-line character and "
Semantics
2 The sequences in both forms of header names are mapped in an implementation-defined manner to
headers or external source file names as specified in 6.10.2.
3 If the characters ’ , \ , ", // , or /* occur in the sequence between the < and > delimiters, the behavior
is undefined. Similarly, if the characters ’ , \ , // , or /* occur in the sequence between the "
delimiters, the behavior is undefined.94) Header name preprocessing tokens are recognized only
within #include preprocessing directives and in implementation-defined locations within #pragma
directives.95)
4 EXAMPLE The following sequence of characters:
0x3<1/a.h>1e2
#include <1/a.h>
#define const.member@$
forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited by a { on the left
and a } on the right).
{0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
{#}{include} {<1/a.h>}
{#}{define} {const}{.}{member}{@}{$}
Forward references: source file inclusion (6.10.2).
6.4.8 Preprocessing numbers
Syntax
1 pp-number:
digit
. digit
pp-number identifier-continue
pp-number ’ digit
pp-number ’ nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
94) Thus, sequences of characters that resemble escape sequences cause undefined behavior.
95) For an example of a header name preprocessing token used in a #pragma directive, see 6.10.10.
Description
2 A preprocessing number begins with a digit optionally preceded by a period (.) and may be followed
by valid identifier characters and the character sequences e+, e-, E+, E-, p+, p-, P+, or P-.
3 Preprocessing number tokens lexically include all floating and integer constant tokens.
Semantics
4 A preprocessing number does not have type or a value; it acquires both after a successful conversion
(as part of translation phase 7) to a floating constant token or an integer constant token.
6.4.9 Comments
1 Except within a character constant, a string literal, or a comment, the characters /* introduce a
comment. The contents of such a comment are examined only to identify multibyte characters and
to find the characters */ that terminate it.96)
2 Except within a character constant, a string literal, or a comment, the characters // introduce a
comment that includes all multibyte characters up to, but not including, the next new-line character.
The contents of such a comment are examined only to identify multibyte characters and to find the
terminating new-line character.
3 EXAMPLE
"a//b" // four-character string literal
#include "//e" // undefined behavior
// */ // comment, not syntax error
f = g/**//h; // equivalent to f = g / h;
//\
i(); // part of a two-line comment
/\
/ j(); // part of a two-line comment
#define glue(x,y) x##y
glue(/,/) k(); // syntax error, not comment
/*//*/ l(); // equivalent to l();
m = n//**/o
+ p; // equivalent to m = n + p;
96) Thus, /
* . . . */ comments do not nest.
6.5 Expressions
1 An expression is a sequence of operators and operands that specifies computation of a value, or that
designates an object or a function, or that generates side effects, or that performs a combination
thereof. The value computations of the operands of an operator are sequenced before the value
computation of the result of the operator.
2 If a side effect on a scalar object is unsequenced relative to either a different side effect on the
same scalar object or a value computation using the value of the same scalar object, the behavior
is undefined. If there are multiple allowable orderings of the subexpressions of an expression, the
behavior is undefined if such an unsequenced side effect occurs in any of the orderings.97)
3 The grouping of operators and operands is indicated by the syntax.98) Except as specified later, side
effects and value computations of subexpressions are unsequenced.99)
4 Some operators (the unary operator ~ , and the binary operators << , >> , &, ^, and |, collectively
described as bitwise operators) are required to have operands that have integer type. These operators
yield values that depend on the internal representations of integers, and have implementation-
defined and undefined aspects for signed types.
5 If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not
mathematically defined or not in the range of representable values for its type), the behavior is
undefined.
6 The effective type of an object for an access to its stored value is the declared type of the object, if
any.100) If a value is stored into an object having no declared type through an lvalue having a type
that is not a non-atomic character type, then the type of the lvalue becomes the effective type of the
object for that access and for subsequent accesses that do not modify the stored value. If a value
is copied into an object having no declared type using memcpy or memmove, or is copied as an array
of character type, then the effective type of the modified object for that access and for subsequent
accesses that do not modify the value is the effective type of the object from which the value is
copied, if it has one. For all other accesses to an object having no declared type, the effective type of
the object is simply the type of the lvalue used for the access.
7 An object shall have its stored value accessed only by an lvalue expression that has one of the
following types:101)
— a type compatible with the effective type of the object,
— a qualified version of a type compatible with the effective type of the object,
— a type that is the signed or unsigned type corresponding to the effective type of the object,
97) This paragraph renders undefined statement expressions such as
i = ++i + 1;
a[i++] = i;
while allowing
i = i + 1;
a[i] = i;
98) The syntax specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the
major subclauses of this subclause, highest precedence first. Thus, for example, the expressions allowed as the operands
of the binary + operator (6.5.6) are those expressions defined in 6.5.1 through 6.5.6. The exceptions are cast expressions
(6.5.4) as operands of unary operators (6.5.3), and an operand contained between any of the following pairs of operators:
grouping parentheses () (6.5.1), subscripting brackets [] (6.5.2.1), function-call parentheses () (6.5.2.2), and the conditional
operator ?: (6.5.15).
Within each major subclause, the operators have the same precedence. Left- or right-associativity is indicated in each
subclause by the syntax for the expressions discussed therein.
99) In an expression that is evaluated more than once during the execution of a program, unsequenced and indeterminately
sequenced evaluations of its subexpressions need not be performed consistently in different evaluations.
100) Allocated objects have no declared type.
101) The intent of this list is to specify those circumstances in which an object can or cannot be aliased.
— a type that is the signed or unsigned type corresponding to a qualified version of the effective
type of the object,
— an aggregate or union type that includes one of the aforementioned types among its members
(including, recursively, a member of a subaggregate or contained union), or
— a character type.
8 A floating expression may be contracted, that is, evaluated as though it were a single opera-
tion, thereby omitting rounding errors implied by the source code and the expression evalua-
tion method.102) The FP_CONTRACT pragma in <math.h> provides a way to disallow contracted
expressions. Otherwise, whether and how expressions are contracted is implementation-defined.103)
9 Operators involving decimal floating types are evaluated according to the semantics of IEC 60559,
including production of results with the preferred quantum exponent as specified in IEC 60559.
Forward references: the FP_CONTRACT pragma (7.12.2), copying functions (7.26.2).
6.5.1 Primary expressions
Syntax
1 primary-expression:
identifier
constant
string-literal
( expression )
generic-selection
Constraints
The identifier in an identifier primary expression shall have a visible declaration as an ordinary
identifier that declares an object or a function104) .
Semantics
2 An identifier primary expression designating an object is an lvalue. An identifier primary expression
designating a function is a function designator.
3 A constant is a primary expression. Its type depends on its form and value, as detailed in 6.4.4.
4 A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5.
5 A parenthesized expression is a primary expression. Its type, value, and semantics are identical to
those of the unparenthesized expression.
6 A generic selection is a primary expression. Its type, value, and semantics depend on the selected
generic association, as detailed in the following subclause.
Forward references: declarations (6.7).
6.5.1.1 Generic selection
Syntax
1 generic-selection:
_Generic ( assignment-expression , generic-assoc-list )
generic-assoc-list:
generic-association
102) The intermediate operations in the contracted expression are evaluated as if to infinite range and precision, while the
final operation is rounded to the format determined by the expression evaluation method. A contracted expression might
also omit the raising of floating-point exceptions.
103) This license is specifically intended to allow implementations to exploit fast machine instructions that combine multiple
C operators. As contractions potentially undermine predictability, and can even decrease accuracy for containing expressions,
their use needs to be well-defined and clearly documented.
104) An identifier designating an enumeration constant is a primary expression through the constant production, not the
identifier production.
generic-assoc-list , generic-association
generic-association:
type-name : assignment-expression
default : assignment-expression
Constraints
2 A generic selection shall have no more than one default generic association. The type name in a
generic association shall specify a complete object type other than a variably modified type. No two
generic associations in the same generic selection shall specify compatible types. The type of the
controlling expression is the type of the expression as if it had undergone an lvalue conversion,105)
array to pointer conversion, or function to pointer conversion. That type shall be compatible with at
most one of the types named in the generic association list. If a generic selection has no default
generic association, its controlling expression shall have type compatible with exactly one of the
types named in its generic association list.
Semantics
3 The controlling expression of a generic selection is not evaluated. If a generic selection has a generic
association with a type name that is compatible with the type of the controlling expression, then the
result expression of the generic selection is the expression in that generic association. Otherwise, the
result expression of the generic selection is the expression in the default generic association. None
of the expressions from any other generic association of the generic selection is evaluated.
4 The type and value of a generic selection are identical to those of its result expression. It is an
lvalue, a function designator, or a void expression if its result expression is, respectively, an lvalue, a
function designator, or a void expression.
5 EXAMPLE The cbrt type-generic macro could be implemented as follows:
#define cbrt(X) _Generic((X), \
long double: cbrtl, \
default: cbrt, \
float: cbrtf \
)(X)
See 7.27 how such a macro could be implemented with the required rounding properties.
6.5.2 Postfix operators
Syntax
1 postfix-expression:
primary-expression
postfix-expression [ expression ]
postfix-expression ( argument-expression-listopt )
postfix-expression . identifier
postfix-expression -> identifier
postfix-expression ++
postfix-expression --
compound-literal
argument-expression-list:
assignment-expression
argument-expression-list , assignment-expression
105) An lvalue conversion drops type qualifiers.
6.5.2.1 Array subscripting
Constraints
1 One of the expressions shall have type "pointer to complete object type", the other expression shall
have integer type, and the result has type "type".
Semantics
2 A postfix expression followed by an expression in square brackets [] is a subscripted designation of
an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical
to (*((E1)+(E2))) . Because of the conversion rules that apply to the binary + operator, if E1 is an
array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer,
E1[E2] designates the E2 -th element of E1 (counting from zero).
3 Successive subscript operators designate an element of a multidimensional array object. If E is an
n-dimensional array (n ≥ 2) with dimensions i × j × · · · × k, then E (used as other than an lvalue) is
converted to a pointer to an (n − 1)-dimensional array with dimensions j × · · · × k. If the unary *
operator is applied to this pointer explicitly, or implicitly as a result of subscripting, the result is the
referenced (n − 1)-dimensional array, which itself is converted into a pointer if used as other than an
lvalue. It follows from this that arrays are stored in row-major order (last subscript varies fastest).
4 EXAMPLE Consider the array object defined by the declaration
int x[3][5];
Here x is a 3 × 5 array of objects of type int; more precisely, x is an array of three element objects, each of which is an array of
five objects of type int. In the expression x[i], which is equivalent to (*((x)+(i))) , x is first converted to a pointer to the
initial array of five objects of type int. Then i is adjusted according to the type of x, which conceptually entails multiplying i
by the size of the object to which the pointer points, namely an array of five int objects. The results are added and indirection
is applied to yield an array of five objects of type int. When used in the expression x[i][j], that array is in turn converted
to a pointer to the first of the objects of type int, so x[i][j] yields an int.
Forward references: additive operators (6.5.6), address and indirection operators (6.5.3.2), array
declarators (6.7.6.2).
6.5.2.2 Function calls
Constraints
1 The expression that denotes the called function106) shall have type pointer to function returning
void or returning a complete object type other than an array type.
2 The number of arguments shall agree with the number of parameters. Each argument shall have a
type such that its value may be assigned to an object with the unqualified version of the type of its
corresponding parameter
Semantics
3 A postfix expression followed by parentheses () containing a possibly empty, comma-separated
list of expressions is a function call. The postfix expression denotes the called function. The list of
expressions specifies the arguments to the function.
4 An argument may be an expression of any complete object type. In preparing for the call to a
function, the arguments are evaluated, and each parameter is assigned the value of the corresponding
argument.107)
5 If the expression that denotes the called function has type pointer to function returning an object
type, the function call expression has the same type as that object type, and has the value determined
as specified in 6.8.6.4. Otherwise, the function call has type void.
6 The arguments are implicitly converted, as if by assignment, to the types of the corresponding
parameters, taking the type of each parameter to be the unqualified version of its declared type. The
ellipsis notation in a function prototype declarator causes argument type conversion to stop after the
106) Most often, this is the result of converting an identifier that is a function designator.
107) A function can change the values of its parameters, but these changes cannot affect the values of the arguments. On the
other hand, it is possible to pass a pointer to an object, and the function can then change the value of the object pointed to. A
parameter declared to have array or function type is adjusted to have a pointer type as described in 6.7.6.3.
last declared parameter, if present. The integer promotions are performed on each trailing argument,
and trailing arguments that have type float are promoted to double. These are called the default
argument promotions. No other conversions are performed implicitly.
7 If the function is defined with a type that is not compatible with the type (of the expression) pointed
to by the expression that denotes the called function, the behavior is undefined.
8 There is a sequence point after the evaluations of the function designator and the actual arguments
but before the actual call. Every evaluation in the calling function (including other function calls)
that is not otherwise specifically sequenced before or after the execution of the body of the called
function is indeterminately sequenced with respect to the execution of the called function.108)
9 Recursive function calls shall be permitted, both directly and indirectly through any chain of other
functions.
10 EXAMPLE In the function call
(*pf[f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 can be called in any order. All side effects have to be completed before the function pointed
to by pf[f1()] is called.
Forward references: function declarators (6.7.6.3), function definitions (6.9.1), the return statement
(6.8.6.4), simple assignment (6.5.16.1).
6.5.2.3 Structure and union members
Constraints
1 The first operand of the . operator shall have an atomic, qualified, or unqualified structure or union
type, and the second operand shall name a member of that type.
2 The first operand of the-> operator shall have type "pointer to atomic, qualified, or unqualified
structure" or "pointer to atomic, qualified, or unqualified union", and the second operand shall
name a member of the type pointed to.
Semantics
3 A postfix expression followed by the . operator and an identifier designates a member of a structure
or union object. The value is that of the named member,109) and is an lvalue if the first expression is
an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type
of the designated member.
4 A postfix expression followed by the-> operator and an identifier designates a member of a structure
or union object. The value is that of the named member of the object to which the first expression
points, and is an lvalue.110) If the first expression is a pointer to a qualified type, the result has the
so-qualified version of the type of the designated member.
5 Accessing a member of an atomic structure or union object results in undefined behavior.111)
6 One special guarantee is made in order to simplify the use of unions: if a union contains several
structures that share a common initial sequence (see below), and if the union object currently contains
one of these structures, it is permitted to inspect the common initial part of any of them anywhere
that a declaration of the completed type of the union is visible. Two structures share a common initial
sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a
sequence of one or more initial members.
7 EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or union, f().x is a valid
postfix expression but is not an lvalue.
108) In other words, function executions do not "interleave" with each other.
109) If the member used to read the contents of a union object is not the same as the member last used to store a value in the
object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new
type as described in 6.2.6 (a process sometimes called "type punning"). This might be a non-value representation.
110) If &E is a valid pointer expression (where & is the "address-of" operator, which generates a pointer to its operand), the
expression (&E)->MOS is the same as E.MOS.
111) For example, a data race would occur if access to the entire structure or union in one thread conflicts with access to a
member from another thread, where at least one access is a modification. Members can be safely accessed using a non-atomic
object which is assigned to or from the atomic object.
8 EXAMPLE 2 In:
struct s { int i; const int ci; };
struct s s;
const struct s cs;
volatile struct s vs;
the various members have the types:
s.i int
s.ci const int
cs.i const int
cs.ci const int
vs.i volatile int
vs.ci volatile const int
9 EXAMPLE 3 The following is a valid fragment:
union {
struct {
int alltypes;
} n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
} u;
u.nf.type = 1;
u.nf.doublenode = 3.14;
/* ... */
if (u.n.alltypes == 1)
if (sin(u.nf.doublenode) == 0.0)
/* ... */
The following is not a valid fragment (because the union type is not visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f(struct t1 *p1, struct t2 *p2)
{
if (p1->m < 0)
p2->m = -p2->m;
return p1->m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
} u;
/* ... */
return f(&u.s1, &u.s2);
}
Forward references: address and indirection operators (6.5.3.2), structure and union specifiers
(6.7.2.1).
6.5.2.4 Postfix increment and decrement operators
Constraints
1 The operand of the postfix increment or decrement operator shall have atomic, qualified, or unquali-
fied real or pointer type, and shall be a modifiable lvalue.
Semantics
2 The result of the postfix ++ operator is the value of the operand. As a side effect, the value of the
operand object is incremented (that is, the value 1 of the appropriate type is added to it). See the
discussions of additive operators and compound assignment for information on constraints, types,
and conversions and the effects of operations on pointers. The value computation of the result is
sequenced before the side effect of updating the stored value of the operand. With respect to an
indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation. Postfix
++ on an object with atomic type is a read-modify-write operation with memory_order_seq_cst
memory order semantics.112)
3 The postfix-- operator is analogous to the postfix ++ operator, except that the value of the operand
is decremented (that is, the value 1 of the appropriate type is subtracted from it).
Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).
6.5.2.5 Compound literals
Syntax
1 compound-literal:
( storage-class-specifiersopt type-name ) braced-initializer
storage-class-specifiers:
storage-class-specifier
storage-class-specifiers storage-class-specifier
Constraints
2 The type name shall specify a complete object type or an array of unknown size, but not a variable
length array type.
3 All the constraints for initializer lists in 6.7.10 also apply to compound literals.
4 If the compound literal is evaluated outside the body of a function and outside of any parameter list,
it is associated with file scope; otherwise, it is associated with the enclosing block. Depending on
this association, the storage-class specifiers SC (possibly empty)113) , type name T, and initializer list,
if any, shall be such that they are valid specifiers for an object definition in file scope or block scope,
respectively, of the following form,
SC typeof(T) ID = { IL };
where ID is an identifier that is unique for the whole program and where IL is a (possibly empty)
112) Where a pointer to an atomic object can be formed and E has integer type, E++ is equivalent to the following code
sequence where T is the type of E:
T *addr = &E;
T old = *addr;
T new;
do {
new = old + 1;
} while (!atomic_compare_exchange_strong(addr, &old, new));
with old being the result of the operation.
Special care is necessary if E has floating type; see 6.5.16.2.
113) If the storage-class specifiers contain the same storage-class specifier more than once, the following constraint is violated.
initializer list with nested structure, designators, values and types as the initializer list of the
compound literal. All the constraints for storage class specifiers in 6.7.1 also apply correspondingly
to compound literals.
Semantics
5 A compound literal provides an unnamed object whose value, type, storage duration and other
properties are as if given by the definition syntax in the constraints; if the storage duration is
automatic, the lifetime of the instance of the unnamed object is the current execution of the enclosing
block114) . If the storage-class specifiers contain other specifiers than constexpr, static, register,
or thread_local the behavior is undefined.
6 The value of the compound literal is that of an lvalue corresponding to the unnamed object.
7 All the semantic rules for initializer lists in 6.7.10 also apply to compound literals115) .
8 EXAMPLE 1 Consider the following 2 functions:
int f(int*);
int g(char * para[f((int[27]){ 0, })]) {
/* ... */
return 0;
}
Here, each call to g creates an unnamed object of type int[27] to determine the variably-modified type of para for the
duration of the call. During that determination, a pointer to the object is passed into a call to the function f. If a pointer to the
object is kept by f, access to that object is possible during the whole execution of the call to g. The lifetime of the object ends
with the end of the call to g; for any access after that, the behavior is undefined.
9 String literals, and compound literals with const-qualified types, need not designate distinct ob-
jects.116)
10 EXAMPLE 2 The file scope definition
int *p = (int []){2, 4};
initializes p to point to the first element of an array of two ints, the first having the value two and the second, four. The
expressions in this compound literal are required to be constant. The unnamed object has static storage duration.
11 EXAMPLE 3 In contrast, in
void f(void)
{
int *p;
/*...*/
p = (int [2]){*p};
/*...*/
}
p is assigned the address of the first element of an array of two ints, the first having the value previously pointed to by p and
the second, zero. The expressions in this compound literal need not be constant. The unnamed object has automatic storage
duration.
12 EXAMPLE 4 Initializers with designations can be combined with compound literals. Structure objects created using
compound literals can be passed to functions without depending on member order:
drawline((struct point){.x=1, .y=1},
(struct point){.x=3, .y=4});
Or, if drawline instead expected pointers to struct point:
drawline(&(struct point){.x=1, .y=1},
114) Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types or void only, and
the result of a cast expression is not an lvalue.
115) For example, subobjects without explicit initializers are initialized to zero.
116) This allows implementations to share storage for string literals and constant compound literals with the same or
overlapping representations.
&(struct point){.x=3, .y=4});
13 EXAMPLE 5 A read-only compound literal can be specified through constructions like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
14 EXAMPLE 6 The following three expressions have different meanings:
"/tmp/fileXXXXXX"
(char []){"/tmp/fileXXXXXX"}
(const char []){"/tmp/fileXXXXXX"}
The first always has static storage duration and has type array of char, but need not be modifiable; the last two have
automatic storage duration when they occur within the body of a function, and the first of these two is modifiable.
15 EXAMPLE 7 Like string literals, const-qualified compound literals can be placed into read-only memory and can even be
shared. For example,
(const char []){"abc"} == "abc"
might yield 1 if the literals’ storage is shared.
16 EXAMPLE 8 Since compound literals are unnamed, a single compound literal cannot specify a circularly linked object. For
example, there is no way to write a self-referential compound literal that could be used as the function argument in place of
the named object endless_zeros below:
struct int_list { int car; struct int_list *cdr; };
struct int_list endless_zeros = {0, &endless_zeros};
eval(endless_zeros);
17 EXAMPLE 9 Each compound literal creates only a single object in a given scope:
struct s { int i; };
int f (void)
{
struct s *p = 0, *q;
int j = 0;
again:
q = p, p = &((struct s){ j++ });
if (j < 2) goto again;
return p == q && q->i == 1;
}
The function f() always returns the value 1.
18 Note that if an iteration statement were used instead of an explicit goto and a label, the lifetime of the unnamed object would
be the body of the loop only, and on entry next time around p would have indeterminate representation, which would result
in undefined behavior.
Forward references: type names (6.7.7), initialization (6.7.10).
6.5.3 Unary operators
Syntax
1 unary-expression:
postfix-expression
++ unary-expression
-- unary-expression
unary-operator cast-expression
sizeof unary-expression
sizeof ( type-name )
alignof ( type-name )
unary-operator: one of
& * + - ~ !
6.5.3.1 Prefix increment and decrement operators
Constraints
1 The operand of the prefix increment or decrement operator shall have atomic, qualified, or unquali-
fied real or pointer type, and shall be a modifiable lvalue.
Semantics
2 The value of the operand of the prefix ++ operator is incremented. The result is the new value of the
operand after incrementation. The expression ++E is equivalent to (E+=1) . See the discussions of
additive operators and compound assignment for information on constraints, types, side effects,
and conversions and the effects of operations on pointers.
3 The prefix-- operator is analogous to the prefix ++ operator, except that the value of the operand is
decremented.
Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).
6.5.3.2 Address and indirection operators
Constraints
1 The operand of the unary & operator shall be either a function designator, the result of a [] or unary
* operator, or an lvalue that designates an object that is not a bit-field and is not declared with the
register storage-class specifier.
2 The operand of the unary * operator shall have pointer type.
Semantics
3 The unary & operator yields the address of its operand. If the operand has type "type", the result has
type "pointer to type". If the operand is the result of a unary * operator, neither that operator nor
the & operator is evaluated and the result is as if both were omitted, except that the constraints on
the operators still apply and the result is not an lvalue. Similarly, if the operand is the result of a []
operator, neither the & operator nor the unary * that is implied by the [] is evaluated and the result
is as if the & operator were removed and the [] operator were changed to a + operator. Otherwise,
the result is a pointer to the object or function designated by its operand.
4 The unary * operator denotes indirection. If the operand points to a function, the result is a function
designator; if it points to an object, the result is an lvalue designating the object. If the operand has
type "pointer to type", the result has type "type". If an invalid value has been assigned to the pointer,
the behavior of the unary * operator is undefined.117)
Forward references: storage-class specifiers (6.7.1), structure and union specifiers (6.7.2.1).
6.5.3.3 Unary arithmetic operators
Constraints
1 The operand of the unary + or- operator shall have arithmetic type; of the ~ operator, integer type;
of the ! operator, scalar type.
Semantics
2 The result of the unary + operator is the value of its (promoted) operand. The integer promotions
are performed on the operand, and the result has the promoted type.
117) Thus, & E is equivalent to E (even if E is a null pointer), and &(E1[E2]) to ((E1)+(E2)) . It is always true that if E is a
*
function designator or an lvalue that is a valid operand of the unary & operator, *&E is a function designator or an lvalue
equal to E. If *P is an lvalue and T is the name of an object pointer type, *(T)P is an lvalue that has a type compatible with
that to which T points.
Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately
aligned for the type of object pointed to, and the address of an object after the end of its lifetime.
3 The result of the unary- operator is the negative of its (promoted) operand. The integer promotions
are performed on the operand, and the result has the promoted type.
4 The result of the ~ operator is the bitwise complement of its (promoted) operand (that is, each bit in
the result is set if and only if the corresponding bit in the converted operand is not set). The integer
promotions are performed on the operand, and the result has the promoted type. If the promoted
type is an unsigned type, the expression ~E is equivalent to the maximum value representable in
that type minus E.
5 The result of the logical negation operator ! is 0 if the value of its operand compares unequal to
0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is
equivalent to (0==E) .
6.5.3.4 The sizeof and alignof operators
Constraints
1 The sizeof operator shall not be applied to an expression that has function type or an incomplete
type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.
The alignof operator shall not be applied to a function type or an incomplete type.
Semantics
2 The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the
parenthesized name of a type. The size is determined from the type of the operand. The result
is an integer. If the type of the operand is a variable length array type, the operand is evaluated;
otherwise, the operand is not evaluated and the result is an integer constant.
3 The alignof operator yields the alignment requirement of its operand type. The operand is not
evaluated and the result is an integer constant expression. When applied to an array type, the result
is the alignment requirement of the element type.
4 When sizeof is applied to an operand that has type char, unsigned char, or signed char, (or
a qualified version thereof) the result is 1. When applied to an operand that has array type, the
result is the total number of bytes in the array.118) When applied to an operand that has structure or
union type, the result is the total number of bytes in such an object, including internal and trailing
padding.
5 The value of the result of both operators is implementation-defined, and its type (an unsigned
integer type) is size_t, defined in <stddef.h> (and other headers).
6 EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage allocators and I/O
systems. A storage-allocation function might accept a size (in bytes) of an object to allocate and return a pointer to void. For
example:
extern void *alloc(size_t);
double *dp = alloc(sizeof *dp);
The implementation of the alloc function presumably ensures that its return value is aligned suitably for conversion to a
pointer to double.
7 EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:
sizeof array / sizeof array[0]
8 EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a function:
#include <stddef.h>
size_t fsize3(int n)
{
char b[n+3]; // variable length array
return sizeof b; // execution time sizeof
}
118) When applied to a parameter declared to have array or function type, the sizeof operator yields the size of the adjusted
(pointer) type (see 6.9.1).
int main()
{
size_t size;
size = fsize3(10); // fsize3 returns 13
return 0;
}
Forward references: common definitions <stddef.h> (7.21), declarations (6.7), structure and union
specifiers (6.7.2.1), type names (6.7.7), array declarators (6.7.6.2).
6.5.4 Cast operators
Syntax
1 cast-expression:
unary-expression
( type-name ) cast-expression
Constraints
2 Unless the type name specifies a void type, the type name shall specify atomic, qualified, or
unqualified scalar type, and the operand shall have scalar type.
3 Conversions that involve pointers, other than where permitted by the constraints of 6.5.16.1, shall be
specified by means of an explicit cast.
4 A pointer type shall not be converted to any floating type. A floating type shall not be converted to
any pointer type. The type nullptr_t shall not be converted to any type other than void, bool or a
pointer type. No type other than nullptr_t shall be converted to nullptr_t.
Semantics
5 Preceding an expression by a parenthesized type name converts the value of the expression to the
unqualified version of the named type. This construction is called a cast119) . A cast that specifies no
conversion has no effect on the type or value of an expression.
6 If the value of the expression is represented with greater range or precision than required by the type
named by the cast (6.3.1.8), then the cast specifies a conversion even if the type of the expression is
the same as the named type and removes any extra range and precision.
Forward references: equality operators (6.5.9), function declarators (6.7.6.3), simple assignment
(6.5.16.1), type names (6.7.7).
6.5.5 Multiplicative operators
Syntax
1 multiplicative-expression:
cast-expression
multiplicative-expression * cast-expression
multiplicative-expression / cast-expression
multiplicative-expression % cast-expression
Constraints
2 Each of the operands shall have arithmetic type. The operands of the % operator shall have integer
type.
3 If either operand has decimal floating type, the other operand shall not have standard floating type,
complex type, or imaginary type.
119) A cast does not yield an lvalue.
Semantics
4 The usual arithmetic conversions are performed on the operands.
5 The result of the binary * operator is the product of the operands.
6 The result of the / operator is the quotient from the division of the first operand by the second; the
result of the % operator is the remainder. In both operations, if the value of the second operand is
zero, the behavior is undefined.
7 When integers are divided, the result of the / operator is the algebraic quotient with any fractional
part discarded.120) If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a;
otherwise, the behavior of both a/b and a%b is undefined.
6.5.6 Additive operators
Syntax
1 additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression - multiplicative-expression
Constraints
2 For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a
complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)
3 For subtraction, one of the following shall hold:
— both operands have arithmetic type;
— both operands are pointers to qualified or unqualified versions of compatible complete object
types; or
— the left operand is a pointer to a complete object type and the right operand has integer type.
(Decrementing is equivalent to subtracting 1.)
4 If either operand has decimal floating type, the other operand shall not have standard floating type,
complex type, or imaginary type.
Semantics
5 If both operands have arithmetic type, the usual arithmetic conversions are performed on them.
6 The result of the binary + operator is the sum of the operands.
7 The result of the binary- operator is the difference resulting from the subtraction of the second
operand from the first.
8 For the purposes of these operators, a pointer to an object that is not an element of an array behaves
the same as a pointer to the first element of an array of length one with the type of the object as its
element type.
9 When an expression that has integer type is added to or subtracted from a pointer, the result has the
type of the pointer operand. If the pointer operand points to an element of an array object, and the
array is large enough, the result points to an element offset from the original element such that the
difference of the subscripts of the resulting and original array elements equals the integer expression.
In other words, if the expression P points to the i-th element of an array object, the expressions
(P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i + n-th
and i − n-th elements of the array object, provided they exist. Moreover, if the expression P points to
the last element of an array object, the expression (P)+1 points one past the last element of the array
120) This is often called "truncation toward zero".
object, and if the expression Q points one past the last element of an array object, the expression
(Q)-1 points to the last element of the array object. If the pointer operand and the result do not point
to elements of the same array object or one past the last element of the array object, the behavior is
undefined. If the addition or subtraction produces an overflow, the behavior is undefined. If the
result points one past the last element of the array object, it shall not be used as the operand of a
unary * operator that is evaluated.
10 When two pointers are subtracted, both shall point to elements of the same array object, or one past
the last element of the array object; the result is the difference of the subscripts of the two array
elements. The size of the result is implementation-defined, and its type (a signed integer type) is
ptrdiff_t defined in the <stddef.h> header. If the result is not representable in an object of that
type, the behavior is undefined. In other words, if the expressions P and Q point to, respectively, the
i-th and j-th elements of an array object, the expression (P)-(Q) has the value i − j provided the
value fits in an object of type ptrdiff_t. Moreover, if the expression P points either to an element of
an array object or one past the last element of an array object, and the expression Q points to the last
element of the same array object, the expression ((Q)+1)-(P) has the same value as ((Q)-(P))+1
and as-((P)-((Q)+1)) , and has the value zero if the expression P points one past the last element
of the array object, even though the expression (Q)+1 does not point to an element of the array
object.121)
11 EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.
{
int n = 4, m = 3;
int a[n][m];
int (*p)[m] = a; // p == &a[0]
p += 1; // p == &a[1]
(*p)[2] = 99; // a[1][2] == 99
n = p - a; // n == 1
}
12 If array a in the above example were declared to be an array of known constant size, and pointer p were declared to be a
pointer to an array of the same known constant size (pointing to a), the results would be the same.
Forward references: array declarators (6.7.6.2), common definitions <stddef.h> (7.21).
6.5.7 Bitwise shift operators
Syntax
1 shift-expression:
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The integer promotions are performed on each of the operands. The type of the result is that of the
promoted left operand. If the value of the right operand is negative or is greater than or equal to the
width of the promoted left operand, the behavior is undefined.
4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has
an unsigned type, the value of the result is E1 × 2E2 , wrapped around. If E1 has a signed type and
121) Another way to approach pointer arithmetic is first to convert the pointer(s) to character pointer(s): In this scheme the
integer expression added to or subtracted from the converted pointer is first multiplied by the size of the object originally
pointed to, and the resulting pointer is converted back to the original type. For pointer subtraction, the result of the difference
between the character pointers is similarly divided by the size of the object originally pointed to.
When viewed in this way, an implementation need only provide one extra byte (which can overlap another object in the
program) just after the end of the object in order to satisfy the "one past the last element" requirements.
nonnegative value, and E1 × 2E2 is representable in the result type, then that is the resulting value;
otherwise, the behavior is undefined.
5 The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a
signed type and a nonnegative value, the value of the result is the integral part of the quotient of
E1/2E2 . If E1 has a signed type and a negative value, the resulting value is implementation-defined.
6.5.8 Relational operators
Syntax
1 relational-expression:
shift-expression
relational-expression < shift-expression
relational-expression > shift-expression
relational-expression <= shift-expression
relational-expression >= shift-expression
Constraints
2 One of the following shall hold:
— both operands have real type; or
— both operands are pointers to qualified or unqualified versions of compatible object types.
3 If either operand has decimal floating type, the other operand shall not have standard floating type.
Semantics
4 If both of the operands have arithmetic type, the usual arithmetic conversions are performed.
Positive zeros compare equal to negative zeros.
5 For the purposes of these operators, a pointer to an object that is not an element of an array behaves
the same as a pointer to the first element of an array of length one with the type of the object as its
element type.
6 When two pointers are compared, the result depends on the relative locations in the address space
of the objects pointed to. If two pointers to object types both point to the same object, or both point
one past the last element of the same array object, they compare equal. If the objects pointed to
are members of the same aggregate object, pointers to structure members declared later compare
greater than pointers to members declared earlier in the structure, and pointers to array elements
with larger subscript values compare greater than pointers to elements of the same array with lower
subscript values. All pointers to members of the same union object compare equal. If the expression
P points to an element of an array object and the expression Q points to the last element of the same
array object, the pointer expression Q+1 compares greater than P. In all other cases, the behavior is
undefined.
7 Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or
equal to) shall yield 1 if the specified relation is true and 0 if it is false.122) . The result has type int.
6.5.9 Equality operators
Syntax
1 equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression
122) The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c ; in other
words, "if a is less than b, compare 1 to c; otherwise, compare 0 to c".
Constraints
2 One of the following shall hold:
— both operands have arithmetic type;
— both operands are pointers to qualified or unqualified versions of compatible types;
— one operand is a pointer to an object type and the other is a pointer to a qualified or unqualified
version of void;
— both operands have type nullptr_t;
— one operand has type nullptr_t and the other is a null pointer constant; or,
— one operand is a pointer and the other is a null pointer constant.
3 If either operand has decimal floating type, the other operand shall not have standard floating type,
complex type, or imaginary type.
Semantics
4 The == (equal to) and != (not equal to) operators are analogous to the relational operators except for
their lower precedence123) Each of the operators yields 1 if the specified relation is true and 0 if it is
false. The result has type int. For any pair of operands, exactly one of the relations is true.
5 If both of the operands have arithmetic type, the usual arithmetic conversions are performed.
Positive zeros compare equal to negative zeros. Values of complex types are equal if and only if both
their real parts are equal and also their imaginary parts are equal. Any two values of arithmetic
types from different type domains are equal if and only if the results of their conversions to the
(complex) result type determined by the usual arithmetic conversions are equal. If both operands
have type nullptr_t or one operand has type nullptr_t and the other is a null pointer constant,
they compare equal.
6 Otherwise, at least one operand is a pointer. If one operand is a pointer and the other is a null
pointer constant, the null pointer constant is converted to the type of the pointer. If one operand is a
pointer to an object type and the other is a pointer to a qualified or unqualified version of void, the
former is converted to the type of the latter.
7 Two pointers compare equal if and only if both are null pointers, both are pointers to the same object
(including a pointer to an object and a subobject at its beginning) or function, both are pointers to
one past the last element of the same array object, or one is a pointer to one past the end of one array
object and the other is a pointer to the start of a different array object that happens to immediately
follow the first array object in the address space124) .
8 For the purposes of these operators, a pointer to an object that is not an element of an array behaves
the same as a pointer to the first element of an array of length one with the type of the object as its
element type.
6.5.10 Bitwise AND operator
Syntax
1 AND-expression:
equality-expression
AND-expression & equality-expression
Constraints
2 Each of the operands shall have integer type.
123) Because of the precedences, a<b == c<d is 1 whenever a<b and c<d have the same truth-value.
124) Two objects can be adjacent in memory because they are adjacent elements of a larger array or adjacent members
of a structure with no padding between them, or because the implementation chose to place them so, even though they
are unrelated. If prior invalid pointer operations (such as accesses outside array bounds) produced undefined behavior,
subsequent comparisons also produce undefined behavior.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the binary & operator is the bitwise AND of the operands (that is, each bit in the result
is set if and only if each of the corresponding bits in the converted operands is set).
6.5.11 Bitwise exclusive OR operator
Syntax
1 exclusive-OR-expression:
AND-expression
exclusive-OR-expression ^ AND-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the ^ operator is the bitwise exclusive OR of the operands (that is, each bit in the result
is set if and only if exactly one of the corresponding bits in the converted operands is set).
6.5.12 Bitwise inclusive OR operator
Syntax
1 inclusive-OR-expression:
exclusive-OR-expression
inclusive-OR-expression | exclusive-OR-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the | operator is the bitwise inclusive OR of the operands (that is, each bit in the result
is set if and only if at least one of the corresponding bits in the converted operands is set).
6.5.13 Logical AND operator
Syntax
1 logical-AND-expression:
inclusive-OR-expression
logical-AND-expression && inclusive-OR-expression
Constraints
2 Each of the operands shall have scalar type.
Semantics
3 The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0. The
result has type int.
4 Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; if the
second operand is evaluated, there is a sequence point between the evaluations of the first and
second operands. If the first operand compares equal to 0, the second operand is not evaluated.
6.5.14 Logical OR operator
Syntax
1 logical-OR-expression:
logical-AND-expression
logical-OR-expression || logical-AND-expression
Constraints
2 Each of the operands shall have scalar type.
Semantics
3 The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0.
The result has type int.
4 Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the second
operand is evaluated, there is a sequence point between the evaluations of the first and second
operands. If the first operand compares unequal to 0, the second operand is not evaluated.
6.5.15 Conditional operator
Syntax
1 conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
Constraints
2 The first operand shall have scalar type.
3 One of the following shall hold for the second and third operands125) :
— both operands have arithmetic type;
— both operands have the same structure or union type;
— both operands have void type;
— both operands are pointers to qualified or unqualified versions of compatible types;
— both operands have nullptr_t type;
— one operand is a pointer and the other is a null pointer constant or has type nullptr_t; or
— one operand is a pointer to an object type and the other is a pointer to a qualified or unqualified
version of void.
4 If either of the second or third operands has decimal floating type, the other operand shall not have
standard floating type, complex type, or imaginary type.
Semantics
5 The first operand is evaluated; there is a sequence point between its evaluation and the evaluation
of the second or third operand (whichever is evaluated). The second operand is evaluated only if
the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0;
the result is the value of the second or third operand (whichever is evaluated), converted to the type
described below126) .
125) If a second or third operand of type nullptr_t is used that is not a null pointer constant and the other operand is not a
pointer or does not have type nullptr_t itself, a constraint is violated even if that other operand is a null pointer constant
such as 0.
126) A conditional expression does not yield an lvalue.
6 If both the second and third operands have arithmetic type, the result type that would be determined
by the usual arithmetic conversions, were they applied to those two operands, is the type of the
result. If both the operands have structure or union type, the result has that type. If both operands
have void type, the result has void type.
7 If both the second and third operands are pointers, the result type is a pointer to a type qualified
with all the type qualifiers of the types referenced by both operands; if one is a null pointer constant
(other than a pointer) or has type nullptr_t type, the result also has that type. Furthermore, if
both operands are pointers to compatible types or to differently qualified versions of compatible
types, the result type is a pointer to an appropriately qualified version of the composite type; if
one operand is a null pointer constant, the result has the type of the other operand; otherwise, one
operand is a pointer to void or a qualified version of void, in which case the result type is a pointer
to an appropriately qualified version of void.
8 EXAMPLE The common type that results when the second and third operands are pointers is determined in two independent
stages. The appropriate qualifiers, for example, do not depend on whether the two pointers have compatible types.
9 Given the declarations
const void *c_vp;
void *vp;
const int *c_ip;
volatile int *v_ip;
int *ip;
const char *c_cp;
the third column in the following table is the common type that is the result of a conditional expression in which the first two
columns are the second and third operands (in either order):
c_vp c_ip const void *
v_ip 0 volatile int *
c_ip v_ip const volatile int *
vp c_cp const void *
ip c_ip const int *
vp ip void *
6.5.16 Assignment operators
Syntax
1 assignment-expression:
conditional-expression
unary-expression assignment-operator assignment-expression
assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
Constraints
2 An assignment operator shall have a modifiable lvalue as its left operand.
Semantics
3 An assignment operator stores a value in the object designated by the left operand. An assignment
expression has the value of the left operand after the assignment,127) but is not an lvalue. The type of
an assignment expression is the type the left operand would have after lvalue conversion. The side
effect of updating the stored value of the left operand is sequenced after the value computations of
the left and right operands. The evaluations of the operands are unsequenced.
127) The implementation is permitted to read the object to determine the value but is not required to, even when the object
has volatile-qualified type.
6.5.16.1 Simple assignment
Constraints
1 One of the following shall hold128) :
— the left operand has atomic, qualified, or unqualified arithmetic type, and the right has
arithmetic type;
— the left operand has an atomic, qualified, or unqualified version of a structure or union type
compatible with the type of the right;
— the left operand has atomic, qualified, or unqualified pointer type, and (considering the type
the left operand would have after lvalue conversion) both operands are pointers to qualified
or unqualified versions of compatible types, and the type pointed to by the left has all the
qualifiers of the type pointed to by the right;
— the left operand has atomic, qualified, or unqualified pointer type, and (considering the type
the left operand would have after lvalue conversion) one operand is a pointer to an object type,
and the other is a pointer to a qualified or unqualified version of void, and the type pointed to
by the left has all the qualifiers of the type pointed to by the right;
— the left operand has an atomic, qualified, or unqualified version of the nullptr_t type and
the type of the right is nullptr_t129) ;
— the left operand is an atomic, qualified, or unqualified pointer, and the type of the right is
nullptr_t
— the left operand is an atomic, qualified, or unqualified bool, and the type of the right is
nullptr_t;
— the left operand is an atomic, qualified, or unqualified pointer, and the right is a null pointer
constant; or
— the left operand has type atomic, qualified, or unqualified bool, and the right is a pointer.
Semantics
2 In simple assignment (=), the value of the right operand is converted to the type of the assignment
expression and replaces the value stored in the object designated by the left operand. 130)
3 If the value being stored in an object is read from another object that overlaps in any way the
storage of the first object, then the overlap shall be exact and the two objects shall have qualified or
unqualified versions of a compatible type; otherwise, the behavior is undefined.
4 EXAMPLE 1 In the program fragment
int f(void);
char c;
/* ... */
if ((c = f()) == -1)
/* ... */
the int value returned by the function could be truncated when stored in the char, and then converted back to int width
prior to the comparison. In an implementation in which "plain" char has the same range of values as unsigned char (and
char is narrower than int), the result of the conversion cannot be negative, so the operands of the comparison can never
compare equal. Therefore, for full portability, the variable c would be declared as int.
5 EXAMPLE 2 In the fragment:
128) The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion (specified
in 6.3.2.1) that changes lvalues to "the value of the expression" and thus removes any type qualifiers that were applied to the
type category of the expression (for example, it removes const but not volatile from the type int volatile * const).
129) The assignment of an object of type nullptr_t with a value of another type, even if the value is a null pointer constant,
is a constraint violation.
130) As described in 6.2.6.1, a store to an object with atomic type is done with memory_order_seq_cst semantics.
char c;
int i;
long l;
l = (c = i);
the value of i is converted to the type of the assignment expression c = i, that is, char type. The value of the expression
enclosed in parentheses is then converted to the type of the outer assignment expression, that is, long int type.
6 EXAMPLE 3 Consider the fragment:
const char **cpp;
char *p;
const char c = ’A’;
cpp = &p; // constraint violation
*cpp = &c; // valid
*p = 0; // valid
The first assignment is unsafe because it would allow the following valid code to attempt to change the value of the const
object c.
6.5.16.2 Compound assignment
Constraints
1 For the operators += and-= only, either the left operand shall be an atomic, qualified, or unqualified
pointer to a complete object type, and the right shall have integer type; or the left operand shall have
atomic, qualified, or unqualified arithmetic type, and the right shall have arithmetic type.
2 For the other operators, the left operand shall have atomic, qualified, or unqualified arithmetic type,
and (considering the type the left operand would have after lvalue conversion) each operand shall
have arithmetic type consistent with those allowed by the corresponding binary operator.
3 If either operand has decimal floating type, the other operand shall not have standard floating type,
complex type, or imaginary type.
Semantics
4 A compound assignment of the form E1 op= E2 is equivalent to the simple assignment expression
E1 = E1 op (E2) , except that the lvalue E1 is evaluated only once, and with respect to an inde-
terminately-sequenced function call, the operation of a compound assignment is a single evalu-
ation. If E1 has an atomic type, compound assignment is a read-modify-write operation with
memory_order_seq_cst memory order semantics.
5 NOTE Where a pointer to an atomic object can be formed and E1 and E2 have integer type, this is equivalent to the following
code sequence where T1 is the type of E1 and T2 is the type of E2:
T1 *addr = &E1;
T2 val = (E2);
T1 old = *addr;
T1 new;
do {
new = old op val;
} while (!atomic_compare_exchange_strong(addr, &old, new));
with new being the result of the operation.
If E1 or E2 has floating type, then exceptional conditions or floating-point exceptions encountered during discarded
evaluations of new would also be discarded in order to satisfy the equivalence of E1 op= E2 and E1 = E1 op (E2) . For
example, if Annex F is in effect, the floating types involved have IEC 60559 binary formats, and FLT_EVAL_METHOD is 0, the
equivalent code would be:
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
/* ... */
fenv_t fenv;
T1 *addr = &E1;
T2 val = E2;
T1 old = *addr;
T1 new;
feholdexcept(&fenv);
for (;;) {
new = old op val;
if (atomic_compare_exchange_strong(addr, &old, new))
break;
feclearexcept(FE_ALL_EXCEPT);
}
feupdateenv(&fenv);
If FLT_EVAL_METHOD is not 0, then T2 is expected to be a type with the range and precision to which E2 is evaluated in order
to satisfy the equivalence.
6.5.17 Comma operator
Syntax
1 expression:
assignment-expression
expression , assignment-expression
Semantics
2 The left operand of a comma operator is evaluated as a void expression; there is a sequence point
between its evaluation and that of the right operand. Then the right operand is evaluated; the result
has its type and value.131)
3 EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where
a comma is used to separate items in a list (such as arguments to functions or lists of initializers). On the other hand, it can be
used within a parenthesized expression or within the second expression of a conditional operator in such contexts. In the
function call
f(a, (t=3, t+2), c)
the function has three arguments, the second of which has the value 5.
Forward references: initialization (6.7.10).
131) A comma operator does not yield an lvalue.
6.6 Constant expressions
Syntax
1 constant-expression:
conditional-expression
Description
2 A constant expression can be evaluated during translation rather than runtime, and accordingly
may be used in any place that a constant may be.
Constraints
3 Constant expressions shall not contain assignment, increment, decrement, function-call, or comma
operators, except when they are contained within a subexpression that is not evaluated132) .
4 Each constant expression shall evaluate to a constant that is in the range of representable values for
its type.
Semantics
5 An expression that evaluates to a constant is required in several contexts. If a floating expression
is evaluated in the translation environment, the arithmetic range and precision shall be at least as
great as if the expression were being evaluated in the execution environment. 133)
6 A compound literal with storage-class specifier constexpr is a compound literal constant. A com-
pound literal constant is a constant expression with the type and value of the unnamed object.
7 An identifier that is:
— an enumeration constant;
— a predefined constant;
— or, declared with storage-class specifier constexpr and has an object type,
is a named constant, as is a postfix expression that applies the . member access operator to a named
constant of structure or union type, even recursively. For enumeration and predefined constants,
their value and type are defined in the respective clauses; for constexpr objects, such a named
constant is a constant expression with the type and value of the declared object.
8 An integer constant expression134) shall have integer type and shall only have operands that are
integer constants, named and compound literal constants of integer type, character constants,
sizeof expressions whose results are integer constants, alignof expressions, and floating, named,
or compound literal constants of arithmetic type that are the immediate operands of casts. Cast
operators in an integer constant expression shall only convert arithmetic types to integer types,
except as part of an operand to the typeof operators, sizeof operator, or alignof operator.
9 More latitude is permitted for constant expressions in initializers. Such a constant expression shall
be, or evaluate to, one of the following:
— a named constant,
— a compound literal constant,
— an arithmetic constant expression,
132) The operand of a the typeof 6.7.2.5, sizeof, or alignof operator is usually not evaluated (6.5.3.4).
133) The use of evaluation formats as characterized by FLT_EVAL_METHOD and DEC_EVAL_METHOD also applies to evaluation in
the translation environment.
134) An integer constant expression is required in a number of contexts such as the size of a bit-field member of a structure,
the value of an enumeration constant, and the size of a non-variable length array. Further constraints that apply to the integer
constant expressions used in conditional-inclusion preprocessing directives are discussed in 6.10.1.
— a null pointer constant,
— an address constant, or
— an address constant for a complete object type plus or minus an integer constant expression.
10 An arithmetic constant expression shall have arithmetic type and shall only have operands that are
integer constants, floating constants, named or compound literal constants of arithmetic type, char-
acter constants, sizeof expressions whose results are integer constants, and alignof expressions.
Cast operators in an arithmetic constant expression shall only convert arithmetic types to arithmetic
types, except as part of an operand to the typeof operators, sizeof operator, or alignof operator.
11 An address constant is a null pointer135) , a pointer to an lvalue designating an object of static storage
duration, or a pointer to a function designator; it shall be created explicitly using the unary &
operator or an integer constant cast to pointer type, or implicitly by the use of an expression of array
or function type.
12 The array-subscript [] and member-access -> operator, the address & and indirection * unary
operators, and pointer casts may be used in the creation of an address constant, but the value of an
object shall not be accessed by use of these operators136) .
13 A structure or union constant is a named constant or compound literal constant with structure or
union type, respectively.
14 An implementation may accept other forms of constant expressions, however, they are not an integer
constant expression.137)
15 Starting from a structure or union constant, the member-access . operator may be used to form a
named constant or compound literal constant as described above.
16 If the member-access operator . accesses a member of a union constant, the access member shall be
the same as the member that is initialized by the union constant’s initializer.
17 The semantic rules for the evaluation of a constant expression are the same as for nonconstant
expressions138) .
Forward references: array declarators (6.7.6.2), initialization (6.7.10).
135) A named constant or compound literal constant of integer type and value zero is a null pointer constant. A named
constant or compound literal constant with a pointer type and a value null is a null pointer but not a null pointer constant; it
may only be used to initialize a pointer object if its type implicitly converts to the target type.
136) Named constant or compound literal constants with arithmetic type, including names of constexpr objects, are valid in
offset computations such as array subscripts or int pointer casts, as long as the expression in which they occur form integer
constant expressions. In contrast, names of other objects, even if const-qualified and with static storage duration, are not
valid.
137) For example, in the statement int arr_or_vla[(int)+1.0];, while possible to be computed by some implementations
as an array with a size of one, still results in a variable length array declaration of automatic storage duration.
138) Thus, in the following initialization,
static int i = 2 || 1 / 0;
the expression is a valid integer constant expression with value one.
6.7 Declarations
Syntax
1 declaration:
declaration-specifiers init-declarator-listopt ;
attribute-specifier-sequence declaration-specifiers init-declarator-list ;
static_assert-declaration
attribute-declaration
declaration-specifiers:
declaration-specifier attribute-specifier-sequenceopt
declaration-specifier declaration-specifiers
declaration-specifier:
storage-class-specifier
type-specifier-qualifier
function-specifier
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
attribute-declaration:
attribute-specifier-sequence ;
Constraints
2 A declaration other than a static_assert or attribute declaration shall declare at least a declarator
(other than the parameters of a function or the members of a structure or union), a tag, or the
members of an enumeration.
3 If an identifier has no linkage, there shall be no more than one declaration of the identifier (in a
declarator or type specifier) with the same scope and in the same name space, except that:
— a typedef name may be redefined to denote the same type as it currently does, provided that
type is not a variably modified type;
— enumeration constants and tags may be redeclared as specified in 6.7.2.2 6.7.2.3.
4 All declarations in the same scope that refer to the same object or function shall specify compatible
types.
5 In an underspecified declaration all declared identifiers that do not have a prior declaration shall be
ordinary identifiers.
Semantics
6 A declaration specifies the interpretation and properties of a set of identifiers. A definition of an
identifier is a declaration for that identifier that:
— for an object, causes storage to be reserved for that object;
— for a function, includes the function body139) ;
— for an enumeration constant, is the (only) declaration of the identifier;
— for a typedef name, is the first (or only) declaration of the identifier.
7 The declaration specifiers consist of a sequence of specifiers, followed by an optional attribute
specifier sequence, that indicate the linkage, storage duration, and part of the type of the entities that
139) Function definitions have a different syntax, described in 6.9.1.
the declarators denote. The init declarator list is a comma-separated sequence of declarators, each
of which may have additional type information, or an initializer, or both. The declarators contain
the identifiers (if any) being declared. The optional attribute specifier sequence in a declaration
appertains to each of the entities declared by the declarators of the init declarator list.
8 If an identifier for an object is declared with no linkage, the type for the object shall be complete
by the end of its declarator, or by the end of its init-declarator if it has an initializer; in the case of
function parameters, it is the adjusted type (see 6.7.6.3) that is required to be complete.
9 The optional attribute specifier sequence terminating a sequence of declaration specifiers appertains
to the type determined by the preceding sequence of declaration specifiers. The attribute specifier
sequence affects the type only for the declaration it appears in, not other declarations involving the
same type.
10 Except where specified otherwise, the meaning of an attribute declaration is implementation-defined.
11 EXAMPLE In the declaration for an entity, attributes appertaining to that entity may appear at the start of the declaration
and after the identifier for that declaration.
[[deprecated]] void f [[deprecated]] (void); // valid
12 A declaration such that the declaration specifiers contain no type specifier or that is declared with
constexpr is said to be underspecified. If such a declaration is not a definition, if it declares no or more
than one ordinary identifier, if the declared identifier already has a declaration in the same scope, or
if the declared entity is not an object, the behavior is undefined.
Forward references: declarators (6.7.6), enumeration specifiers (6.7.2.2), initialization (6.7.10), type
names (6.7.7), type qualifiers (6.7.3).
6.7.1 Storage-class specifiers
Syntax
1 storage-class-specifier:
auto
constexpr
extern
register
static
thread_local
typedef
Constraints
2 At most, one storage-class specifier may be given in the declaration specifiers in a declaration, except
that:
— thread_local may appear with static or extern;
— auto may appear with all the others except typedef140) ;
— and, constexpr may appear with auto, register, or static.
3 In the declaration of an object with block scope, if the declaration specifiers include thread_local,
they shall also include either static or extern. If thread_local appears in any declaration of an
object, it shall be present in every declaration of that object.
4 thread_local shall not appear in the declaration specifiers of a function declaration. auto shall
only appear in the declaration specifiers of an identifier with file scope or along with other storage
class specifiers if the type is to be inferred from an initializer.
140) See "future language directions" (6.11.5).
5 An object declared with storage-class specifier constexpr or any of its members, even recursively,
shall not have an atomic type, or a variably modified type, or a type that is volatile or restrict
qualified. The declaration shall be a definition and shall have an initializer141) . The value of
any constant expressions or of any character in a string literal of the initializer shall be exactly
representable in the corresponding target type; no change of value shall be applied142) . If an object
or subobject declared with storage-class specifier constexpr has pointer, integer, or arithmetic type,
the implicit or explicit initializer value for it shall be a null pointer constant143) , an integer constant
expression, or an arithmetic constant expression, respectively.
Semantics
6 Storage-class specifiers specify various properties of identifiers and declared features; storage
duration (static in block scope, thread_local, auto, register), linkage (extern, static and
constexpr in file scope, typedef), value (constexpr), and type (typedef). The meanings of the
various linkages and storage durations were discussed in 6.2.2 and 6.2.4, typedef is discussed in
6.7.8, and type inference is discussed in 6.7.9.
7 A declaration of an identifier for an object with storage-class specifier register suggests that
access to the object be as fast as possible. The extent to which such suggestions are effective is
implementation-defined144) .
8 The declaration of an identifier for a function that has block scope shall have no explicit storage-class
specifier other than extern.
9 If an aggregate or union object is declared with a storage-class specifier other than typedef, the
properties resulting from the storage-class specifier, except with respect to linkage, also apply to the
members of the object, and so on recursively for any aggregate or union member objects.
10 If auto appears with another storage-class specifier, or if it appears in a declaration at file scope, it is
ignored for the purposes of determining a storage duration of linkage. It then only indicates that the
declared type may be inferred.
11 An object declared with a storage-class specifier constexpr has its value permanently fixed at
translation-time; if not yet present, a const-qualification is implicitly added to the object’s type. The
declared identifier is considered a constant expression of the respective kind, see ??.
12 NOTE An object declared in block scope with a storage-class specifier constexpr and without static has automatic storage
duration, the identifier has no linkage, and each instance of the object has a unique address obtainable with & (if it is not
declared with the register specifier), if any. Such an object in file scope has static storage duration, the corresponding identifier
has internal linkage, and each translation unit that sees the same textual definition implements a separate object with a
distinct address.
13 NOTE The constraints for constexpr objects are intended to enforce checks for portability at translation time.
constexpr unsigned int minusOne = -1; // constraint violation
constexpr unsigned int uint_max = -1U; // ok
constexpr char string[] = { "\xFF", }; // possible constraint
violation
constexpr unsigned char unstring[] = { "\xFF", }; // ok
constexpr char8_t u8string[] = { u8"\xFF", }; // ok
constexpr double onethird = 1.0/3.0; // possible constraint
violation
constexpr double onethirdtrunc = (double)(1.0/3.0); // ok
141) The right operand of all assignment expressions of such an initializer, if any, are constant expressions or string literals,
see 6.7.10
142) In the context of arithmetic conversions, 6.3.1 describes the details of changes of value that occur if values of arithmetic
expressions are stored in the objects that for example have a different signedness, excess precision or quantum exponent.
Whenever such a change of value is necessary, the constraint is violated.
143) The named constant or compound literal constant corresponding to an object declared with storage-class specifier
constexpr and pointer type is a constant expression with a value null, and thus a null pointer and an address constant.
However, even if it has type void* it is not a null pointer constant.
144) The implementation can treat any register declaration simply as an auto declaration. However, whether or not
addressable storage is actually used, the address of any part of an object declared with storage-class specifier register
cannot be computed, either explicitly (by use of the unary & operator as discussed in 6.5.3.2) or implicitly (by converting
an array name to a pointer as discussed in 6.3.2.1). Thus, the only operator that can be applied to an array declared with
storage-class specifier register is sizeof and the typeof operators.
constexpr _Decimal32 small = DEC64_TRUE_MIN * 0; // constraint violation
Using an octal or hexadecimal escape character sequence with a value greater than the largest representable value of the target
character type (such as for unstring) possibly violates a constraint. Equally, an implementation that uses excess precision for
floating-point constants violates the constraint for onethird; a diagnostic is required if a truncation of the mantissa occurs.
In contrast to that, the explicit conversion in the initializer for onethirdtrunc ensures that the definition is valid. Similarly,
the initializer of small has a quantum exponent that is larger than the largest possible quantum exponent for _Decimal32 .
14 EXAMPLE 1 An identifier declared with the constexpr specifier may have its value used in constant expressions:
constexpr int K = 47;
enum {
A = K, // valid, constant initialization
};
constexpr int L = K; // valid, constexpr initialization
static int b = K + 1; // valid, static initialization
int array[K]; // not a VLA
15 EXAMPLE 2 An object declared with the constexpr specifier stores the exact value of its initializer, no implicit value change
is applied:
#include <float.h>
constexpr int A = 42LL; // valid, 42 always fits in an int
constexpr signed short B = ULLONG_MAX; // constraint violation, value never
// fits
constexpr float C = 47u; // valid, exactly representable
// in single precision
#if FLT_MANT_DIG > 24
constexpr float D = 432000000; // constraint violation if float is
// 32-bit single-precision IEC 60559
#endif
#if (FLT_MANT_DIG == DBL_MANT_DIG) && (0 <= FLT_EVAL_METHOD) && (FLT_EVAL_METHOD
<= 1)
constexpr float E = 1.0 / 3.0; // only valid if double expressions
// and float objects have the same
precision
#endif
#if FLT_EVAL_METHOD == 0
constexpr float F = 1.0f / 3.0f; // valid, same type and precision
#else
constexpr float F = (float)(1.0f / 3.0f); // needs cast to truncate the
// excess precision
#endif
16 EXAMPLE 3 EXAMPLE 3 This recursively applies to initializers for all elements of an aggregate object declared with the
constexpr specifier:
constexpr static unsigned short array[] = {
3000, // valid, fits in unsigned short range
300000, // constraint violation if short is 16-bit
-1 // constraint violation, target type is unsigned
};
struct S {
int x, y;
};
constexpr struct S s = {
.x = INT_MAX, // valid
.y = UINT_MAX, // constraint violation
};
Forward references: type definitions (6.7.8), type definitions (6.7.9).
6.7.2 Type specifiers
Syntax
1 type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_BitInt ( constant-expression )
bool
_Complex
_Decimal32
_Decimal64
_Decimal128
atomic-type-specifier
struct-or-union-specifier
enum-specifier
typedef-name
typeof-specifier
Constraints
2 Except where the type is inferred (6.7.9), at least one type specifier shall be given in the declaration
specifiers in each declaration, and in the specifier-qualifier list in each member declaration and type
name. Each list of type specifiers shall be one of the following multisets (delimited by commas,
when there is more than one multiset per item); the type specifiers may occur in any order, possibly
intermixed with the other declaration specifiers.
— void
— char
— signed char
— unsigned char
— short, signed short, short int, or signed short int
— unsigned short, or unsigned short int
— int, signed, or signed int
— unsigned, or unsigned int
— long, signed long, long int, or signed long int
— unsigned long, or unsigned long int
— long long, signed long long, long long int, or signed long long int
— unsigned long long, or unsigned long long int
— _BitInt( constant-expression), or signed _BitInt( constant-expression)
— unsigned _BitInt( constant-expression)
— float
— double
— long double
— _Decimal32
— _Decimal64
— _Decimal128
— bool
— float _Complex
— double _Complex
— long double _Complex
— atomic type specifier
— struct or union specifier
— enum specifier
— typedef name
— typeof specifier
3 The type specifier _Complex shall not be used if the implementation does not support complex
types, and the type specifiers _Decimal32 , _Decimal64 , and _Decimal128 shall not be used if the
implementation does not support decimal floating types (see 6.10.9.3).
4 The parenthesized constant expression that follows the _BitInt keyword shall be an integer constant
expression N that specifies the width (6.2.6.2) of the type. The value of N for unsigned _BitInt
shall be greater than or equal to 1. The value of N for _BitInt shall be greater than or equal to 2.
The value of N shall be less than or equal to the value of BITINT_MAXWIDTH (see 5.2.4.2.1).
Semantics
5 Specifiers for structures, unions, enumerations, atomic types, and typeof specifiers are discussed
in 6.7.2.1 through 6.7.2.5. Declarations of typedef names are discussed in 6.7.8. The characteristics of
the other types are discussed in 6.2.5.
6 For a declaration such that the declaration specifiers contain no type specifier a mechanism to infer
the type from an initializer is discussed in 6.7.9. In such a declaration, optional elements, if any,
of a sequence of declaration specifiers appertain to the inferred type (for qualifiers and attribute
specifiers) or to the declared objects (for alignment specifiers).
7 Each of the comma-separated multisets designates the same type, except that for bit-fields, it is
implementation-defined whether the specifier int designates the same type as signed int or the
same type as unsigned int.
Forward references: atomic type specifiers (6.7.2.4), enumeration specifiers (6.7.2.2), structure and
union specifiers (6.7.2.1), tags (6.7.2.3), type definitions (6.7.8).
6.7.2.1 Structure and union specifiers
Syntax
1 struct-or-union-specifier:
struct-or-union attribute-specifier-sequenceopt identifieropt { member-declaration-list }
struct-or-union attribute-specifier-sequenceopt identifier
struct-or-union:
struct
union
member-declaration-list:
member-declaration
member-declaration-list member-declaration
member-declaration:
attribute-specifier-sequenceopt specifier-qualifier-list member-declarator-listopt ;
static_assert-declaration
specifier-qualifier-list:
type-specifier-qualifier attribute-specifier-sequenceopt
type-specifier-qualifier specifier-qualifier-list
type-specifier-qualifier:
type-specifier
type-qualifier
alignment-specifier
member-declarator-list:
member-declarator
member-declarator-list , member-declarator
member-declarator:
declarator
declaratoropt : constant-expression
Constraints
2 A member declaration that does not declare an anonymous structure or anonymous union shall
contain a member declarator list.
3 A structure or union shall not contain a member with incomplete or function type (hence, a structure
shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that
the last member of a structure with more than one named member may have incomplete array type;
such a structure (and any union containing, possibly recursively, a member that is such a structure)
shall not be a member of a structure or an element of an array.
4 The expression that specifies the width of a bit-field shall be an integer constant expression with a
nonnegative value that does not exceed the width of an object of the type that would be specified
were the colon and expression omitted145) . If the value is zero, the declaration shall have no
declarator.
5 A bit-field shall have a type that is a qualified or unqualified version of bool, signed int, unsigned
int, a bit-precise integer type, or some other implementation-defined type. It is implementation-
defined whether atomic types are permitted.
6 An attribute specifier sequence shall not appear in a struct-or-union specifier without a member
declaration list, except in a declaration of the form:
struct-or-union attribute-specifier-sequence identifier ;
The attributes in the attribute specifier sequence, if any, are thereafter considered attributes of the
struct or union whenever it is named.
Semantics
7 As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose storage is
allocated in an ordered sequence, and a union is a type consisting of a sequence of members whose
storage overlap.
145) While the number of bits in a bool object is at least CHAR_BIT, the width of a bool can be just 1 bit.
8 Structure and union specifiers have the same form. The keywords struct and union indicate that
the type being specified is, respectively, a structure type or a union type.
9 The optional attribute specifier sequence in a struct-or-union specifier appertains to the structure
or union type being declared. The optional attribute specifier sequence in a member declaration
appertains to each of the members declared by the member declarator list; it shall not appear if the
optional member declarator list is omitted. The optional attribute specifier sequence in a specifier
qualifier list appertains to the type denoted by the preceding type specifier qualifiers. The attribute
specifier sequence affects the type only for the member declaration or type name it appears in, not
other types or declarations involving the same type.
10 The member declaration list is a sequence of declarations for the members of the structure or union.
If the member declaration list does not contain any named members, either directly or via an
anonymous structure or anonymous union, the behavior is undefined146) .
11 A member of a structure or union may have any complete object type other than a variably modified
type147) . In addition, a member may be declared to consist of a specified number of bits (including a
sign bit, if any). Such a member is called a bit-field 148) ; its width is preceded by a colon.
12 A bit-field is interpreted as having a signed or unsigned integer type consisting of the specified
number of bits149) . If the value 0 or 1 is stored into a nonzero-width bit-field of type bool, the value
of the bit-field shall compare equal to the value stored; a bool bit-field has the semantics of a bool.
13 An implementation may allocate any addressable storage unit large enough to hold a bit-field. If
enough space remains, a bit-field that immediately follows another bit-field in a structure shall be
packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that
does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The
order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is
implementation-defined. The alignment of the addressable storage unit is unspecified.
14 A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed
bit-field.150) As a special case, a bit-field structure member with a width of 0 indicates that no
further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed.
15 An unnamed member whose type specifier is a structure specifier with no tag is called an anonymous
structure; an unnamed member whose type specifier is a union specifier with no tag is called an
anonymous union. The members of an anonymous structure or union are considered to be members
of the containing structure or union, keeping their structure or union layout. This applies recursively
if the containing structure or union is also anonymous.
16 Each non-bit-field member of a structure or union object is aligned in an implementation-defined
manner appropriate to its type.
17 Within a structure object, the non-bit-field members and the units in which bit-fields reside have
addresses that increase in the order in which they are declared. A pointer to a structure object,
suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in
which it resides), and vice versa. There may be unnamed padding within a structure object, but not
at its beginning.
18 The size of a union is sufficient to contain the largest of its members. The value of at most one of the
members can be stored in a union object at any time. A pointer to a union object, suitably converted,
points to each of its members (or if a member is a bit-field, then to the unit in which it resides),
and vice versa. The members of a union object overlap in such a way that pointers to them when
converted to pointers to character types point to the same byte. There may be unnamed padding at
146) For further rules affecting compatibility and completeness of structure or union types, see 6.2.7 and 6.7.2.3.
147) A structure or union cannot contain a member with a variably modified type because member names are not ordinary
identifiers as defined in 6.2.3.
148) The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to or arrays of bit-field
objects.
149) As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int, then it is
implementation-defined whether the bit-field is signed or unsigned. This includes an int type specifier produced by
the use of the typeof specifier (6.7.2.5).
150) An unnamed bit-field structure member is useful for padding to conform to externally imposed layouts.
the end of a union object, but not at its beginning.
19 There may be unnamed padding at the end of a structure or union.
20 As a special case, the last member of a structure with more than one named member may have an
incomplete array type; this is called a flexible array member. In most situations, the flexible array
member is ignored. In particular, the size of the structure is as if the flexible array member were
omitted except that it may have more trailing padding than the omission would imply. However,
when a . (or-> ) operator has a left operand that is (a pointer to) a structure with a flexible array
member and the right operand names that member, it behaves as if that member were replaced with
the longest array (with the same element type) that would not make the structure larger than the
object being accessed; the offset of the array shall remain that of the flexible array member, even if
this would differ from that of the replacement array. If this array would have no elements, it behaves
as if it had one element but the behavior is undefined if any attempt is made to access that element
or to generate a pointer one past it.
21 EXAMPLE 1 The following declarations illustrate the behavior when an attribute is written on a tag declaration:
struct [[deprecated]] S; // valid, [[deprecated]] appertains to struct S
void f(struct S *s); // valid, the struct S type has the [[deprecated]]
// attribute
struct S { // valid, struct S inherits the [[deprecated]] attribute
int a; // from the previous declaration
};
void g(struct [[deprecated]] S s); // invalid
22 EXAMPLE 2 The following illustrates anonymous structures and unions:
struct v {
union { // anonymous union
struct { int i, j; }; // anonymous structure
struct { long k, l; } w;
};
int m;
} v1;
v1.i = 2; // valid
v1.k = 3; // invalid: inner structure is not anonymous
v1.w.k = 5; // valid
23 EXAMPLE 3 After the declaration:
struct s { int n; double d[]; };
the structure struct s has a flexible array member d. A typical way to use this is:
int m = /* some value */;
struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));
and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been
declared as:
struct { int n; double d[m]; } *p;
(there are circumstances in which this equivalence is broken; in particular, the offsets of member d might not be the same).
24 Following the above declaration:
struct s t1 = { 0 }; // valid
struct s t2 = { 1, { 4.2 }}; // invalid
t1.n = 4; // valid
t1.d[0] = 4.2; // might be undefined behavior
The initialization of t2 is invalid (and violates a constraint) because struct s is treated as if it did not contain member d.
The assignment to t1.d[0] is probably undefined behavior, but it is possible that
sizeof (struct s) >= offsetof(struct s, d) + sizeof (double)
in which case the assignment would be legitimate. Nevertheless, it cannot appear in strictly conforming code.
25 After the further declaration:
struct ss { int n; };
the expressions:
sizeof (struct s) >= sizeof (struct ss)
sizeof (struct s) >= offsetof(struct s, d)
are always equal to 1.
26 If sizeof (double) is 8, then after the following code is executed:
struct s *s1;
struct s *s2;
s1 = malloc(sizeof (struct s) + 64);
s2 = malloc(sizeof (struct s) + 46);
and assuming that the calls to malloc succeed, the objects pointed to by s1 and s2 behave, for most purposes, as if the
identifiers had been declared as:
struct { int n; double d[8]; } *s1;
struct { int n; double d[5]; } *s2;
27 Following the further successful assignments:
s1 = malloc(sizeof (struct s) + 10);
s2 = malloc(sizeof (struct s) + 6);
they then behave as if the declarations were:
struct { int n; double d[1]; } *s1, *s2;
and:
double *dp;
dp = &(s1->d[0]); // valid
*dp = 42; // valid
dp = &(s2->d[0]); // valid
*dp = 42; // undefined behavior
28 The assignment:
*s1 = *s2;
only copies the member n; if any of the array elements are within the first sizeof (struct s) bytes of the structure, they
are set to an indeterminate representation, that may or may not coincide with a copy of the representation of the elements of
the source array.
29 EXAMPLE 4 Because members of anonymous structures and unions are considered to be members of the containing
structure or union, struct s in the following example has more than one named member and thus the use of a flexible array
member is valid:
struct s {
struct { int i; };
int a[];
};
Forward references: declarators (6.7.6), tags (6.7.2.3).
6.7.2.2 Enumeration specifiers
Syntax
1 enum-specifier:
enum attribute-specifier-sequenceopt identifieropt enum-type-specifieropt
{ enumerator-list }
enum attribute-specifier-sequenceopt identifieropt enum-type-specifieropt
{ enumerator-list , }
enum identifier enum-type-specifieropt
enumerator-list:
enumerator
enumerator-list , enumerator
enumerator:
enumeration-constant attribute-specifier-sequenceopt
enumeration-constant attribute-specifier-sequenceopt = constant-expression
enum-type-specifier:
: specifier-qualifier-list
2 All enumerations have an underlying type. The underlying type can be explicitly specified using an
enum type specifier and is its fixed underlying type. If it is not explicitly specified, the underlying type
is the enumeration’s compatible type, which is either a signed or unsigned integer type (excluding
the bit-precise integer types), or char.
Constraints
3 For an enumeration with a fixed underlying type, the integer constant expression defining the value
of the enumeration constant shall be representable in that fixed underlying type. The definition of an
enumeration constant without a defining constant expression shall neither overflow nor wraparound
the fixed underlying type by adding 1 to the previous enumeration constant.
4 For an enumeration without a fixed underlying type, the expression that defines the value of an
enumeration constant shall be an integer constant expression. For all the integer constant expressions
which make up the values of the enumeration constants, there shall be a signed or unsigned integer
type (excluding the bit-precise integer types) capable of representing all of the values.
5 If an enum type specifier is present, then the longest possible sequence of tokens that can be
interpreted as a specifier qualifier list is interpreted as part of the enum type specifier. It shall name
an integer type that is neither an enumeration nor a bit-precise integer type.
6 An enum specifier of the form
enum identifier enum-type-specifier
may not appear except in a declaration of the form
enum identifier enum-type-specifier ;
unless it is immediately followed by an opening brace, an enumerator list (with an optional ending
comma), and a closing brace.
7 If two enum specifiers that include an enum type specifier declare the same type, the underlying
types shall be compatible.
Semantics
8 The optional attribute specifier sequence in the enum specifier appertains to the enumeration; the
attributes in that attribute specifier sequence are thereafter considered attributes of the enumeration
whenever it is named. The optional attribute specifier sequence in the enumerator appertains to that
enumerator.
9 The identifiers in an enumerator list are declared as constants of the types specified below and may
appear wherever such are permitted151) . An enumerator with = defines its enumeration constant as
the value of the constant expression. If the first enumerator has no =, the value of its enumeration
constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value
of the constant expression obtained by adding 1 to the value of the previous enumeration constant.
(The use of enumerators with = may produce enumeration constants with values that duplicate
other values in the same enumeration.) The enumerators of an enumeration are also known as its
members.
10 The type for the members of an enumeration is called the enumeration member type.
11 During the processing of each enumeration constant in the enumerator list, the type of the enumera-
tion constant shall be:
— the previously declared type, if it is a redeclaration of the same enumeration constant; or,
— the enumerated type, for an enumeration with fixed underlying type; or,
— int, if there are no previous enumeration constants in the enumerator list and no explicit =
with a defining integer constant expression; or,
— int, if given explicitly with = and the value of the integer constant expression is representable
by an int; or,
— the type of the integer constant expression, if given explicitly with = and if the value of the
integer constant expression is not representable by int; or,
— the type of the value from the previous enumeration constant with 1 added to it. If such
an integer constant expression would overflow or wraparound the value of the previous
enumeration constant from the addition of 1, the type takes on either:
— a suitably sized signed integer type (excluding the bit-precise signed integer types)
capable of representing the value of the previous enumeration constant plus 1; or,
— a suitably sized unsigned integer type (excluding the bit-precise unsigned integer types)
capable of representing the value of the previous enumeration constant plus 1.
A signed integer type is chosen if the previous enumeration constant being added is of signed
integer type. An unsigned integer type is chosen if the previous enumeration constant is of
unsigned integer type. If there is no suitably sized integer type described previously which can
represent the new value, then the enumeration has no type which is capable of representing
all of its values152) .
12 For all enumerations without a fixed underlying type, each enumerated type shall be compatible
with char, a signed integer type, or an unsigned integer type (excluding the bit-precise integer
types). The choice of type is implementation-defined153) , but shall be capable of representing the
values of all the members of the enumeration154) .
13 Enumeration constants can be redefined in the same scope with the same value as part of a redecla-
ration of the same enumerated type.
14 The enumeration member type for an enumerated type without fixed underlying type upon comple-
tion is:
— int if all the values of the enumeration are representable as an int; or,
— the enumerated type155) .
151) Thus, the identifiers of enumeration constants declared in the same scope are all required to be distinct from each other
and from other identifiers declared in ordinary declarators.
152) Therefore, a constraint has been violated.
153) An implementation can delay the choice of which integer type until all enumeration constants have been seen.
154) For further rules affecting compatibility and completeness of enumerated types see 6.2.7 and 6.7.2.3.
155) The integer type selected during processing of the enumerator list (before completion) of the enumeration may not be the
same as the compatible implementation-defined integer type selected for the completed enumeration.
15 The enumeration member type for an enumerated type with fixed underlying type is the enumerated
type. The enumerated type is compatible with the underlying type of the enumeration. After possible
lvalue conversion a value of the enumerated type behaves the same as the value with the underlying
type, in particularly with all aspects of promotion, conversion, and arithmetic156) .
16 EXAMPLE The following fragment:
enum hue { chartreuse, burgundy, claret=20, winedark };
enum hue col, *cp;
col = claret;
cp = &col;
if (*cp != burgundy)
/* ... */
makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a pointer to an object
that has that type. The enumerated values are in the set {0, 1, 20, 21}.
17 EXAMPLE Even if the value of an enumeration constant is generated by the implicit addition of 1, an enumeration with a
fixed underlying type does not exhibit typical overflow behavior:
#include <limits.h>
enum us : unsigned short {
us_max = USHRT_MAX,
us_violation, /* Constraint violation:
USHRT_MAX + 1 would wraparound. */
us_violation_2 = us_max + 1, /* Maybe constraint violation:
USHRT_MAX + 1 may be promoted to "int", and
result is too wide for the
underlying type. */
us_wrap_around_to_zero = (unsigned short)(USHRT_MAX + 1) /* Okay:
conversion done in constant expression
before conversion to underlying type:
unsigned semantics okay. */
};
enum ui : unsigned int {
ui_max = UINT_MAX,
ui_violation, /* Constraint violation:
UINT_MAX + 1 would wraparound. */
ui no violation = ui_max + 1, /* Okay: Arithmetic performed as typical
_ _
unsigned integer arithmetic: conversion
from a value that is already 0 to 0. */
ui_wrap_around_to_zero = (unsigned int)(UINT_MAX + 1) /* Okay: conversion
done in constant expression before conversion to
underlying type: unsigned semantics okay. */
};
int main () {
// Same as return 0;
return ui_wrap_around_to_zero
+ us_wrap_around_to_zero;
}
18 EXAMPLE The following fragment:
#include <limits.h>
enum E1: short;
enum E2: short;
156) This means in particular that if the compatible type is bool, values of the enumerated type behave in all aspects the
same as bool and the members only have values 0 and 1. If it is a signed integer type and the constant expression of an
enumeration constant overflows, a constraint for constant expressions (6.6) is violated.
enum E3; /* Constraint violation: E3 forward declaration. */
enum E4 : unsigned long long;
enum E1 : short { m11, m12 };
enum E1 x = m11;
enum E2 : long { m21, m22 }; /* Constraint violation: different underlying types
*/
enum E3 {
m31,
m32,
m33 = sizeof(enum E3) /* Constraint violation: E3 is not complete here. */
};
enum E3 : int; /* Constraint violation: E3 previously had no underlying type */
enum E4 : unsigned long long {
m40 = sizeof(enum E4),
m41 = ULLONG_MAX,
m42 /* Constraint violation: unrepresentable value (wraparound) */
};
enum E5 y; /* Constraint violation: incomplete type */
enum E6 : long int z; /* Constraint violation: enum-type-specifier
with identifier in declarator */
enum E7 : long int = 0; /* Syntax violation:
enum-type-specifier with initializer */
demonstrates many of the properties of multiple declarations of enumerations with underlying types. Particularly, enum E3
is declared and defined without an underlying type first, therefore a redeclaration with an underlying type second is a
violation. Because it not complete at that time within its enumerator list, sizeof(enum E3) is a constraint violation within
the enum E3 definition. enum E4 is complete as it is being defined, therefore sizeof(enum E4) is not a constraint violation.
19 EXAMPLE The following fragment:
enum no_underlying {
a0
};
int main () {
int a = _Generic(a0,
int: 2,
unsigned char: 1,
default: 0
);
int b = _Generic((enum no_underlying)a0,
int: 2,
unsigned char: 1,
default: 0
);
return a + b;
}
demonstrates the implementation-defined nature of the underlying type of enumerations using generic selection (6.5.1.1).
The value of a after its initialization is 2. The value of b after its initialization is implementation-defined: the enumeration
must be compatible with a type large enough to fit the values of its enumeration constants. Since the only value is 0 for a0, b
may hold any of 2, 1, or 0.
Now, consider a similar fragment, but using a fixed underlying type:
enum underlying : unsigned char {
b0
};
int main () {
int a = _Generic(b0,
int: 2,
unsigned char: 1,
default: 0
);
int b = _Generic((enum underlying)b0,
int: 2,
unsigned char: 1,
default: 0
);
return 0;
}
Here, we are guaranteed that a and b are both initialized to 1. This makes enumerations with a fixed underlying type more
portable.
20 EXAMPLE Enumerations with a fixed underlying type must have their braces and the enumerator list specified as part of
their declaration if they are not a standalone declaration:
void f1 (enum a : long b); /* Constraint violation */
void f2 (enum c : long { x } d);
enum e : int f3(); /* Constraint violation */
typedef enum t u; /* Constraint violation: forward declaration of t. */
typedef enum v : short W; /* Constraint violation */
typedef enum q : short { s } R;
struct s1 {
int x;
enum e : int : 1; /* Constraint violation */
int y;
};
enum forward; /* Constraint violation */
extern enum forward fwd_val0; /* Constraint violation: incomplete type */
extern enum forward* fwd_ptr0; /* Constraint violation: enums cannot be
used like other incomplete types */
extern int* fwd_ptr0; /* Constraint violation: incompatible
with incomplete type. */
enum forward1 : int;
extern enum forward1 fwd_val1;
extern int fwd_val1;
extern enum forward1* fwd_ptr1;
extern int* fwd_ptr1;
int main () {
enum e : short;
enum e : short f = 0; /* Constraint violation */
enum g : short { y } h = y;
return 0;
}
21 EXAMPLE Enumerations with a fixed underlying type are complete when the enum type specifier for that specific
enumeration is complete. The enumeration e in this snippet:
enum e : typeof ((enum e : short { A })0, (short)0);
enum e is considered complete by the first opening brace within the typeof in this snippet.
Forward references: generic selection (6.5.1.1), tags (6.7.2.3), declarations (6.7), declarators (6.7.6),
function declarators (6.7.6.3), type names (6.7.7).
6.7.2.3 Tags
Constraints
1 Where two declarations that use the same tag declare the same type, they shall both use the same
choice of struct, union, or enum. If two declarations of the same type have a member-declaration
or enumerator-list, one shall not be nested within the other and both declarations shall fulfill
all requirements of compatible types (6.2.7) with the additional requirement that corresponding
members of structure or union types shall have the same (and not merely compatible) types.
2 A type specifier of the form
enum identifier
without an enumerator list shall only appear after the type it specifies is complete.
3 A type specifier of the form
struct-or-union attribute-specifier-sequenceopt identifier
shall not contain an attribute specifier sequence157) .
Semantics
4 All declarations of structure, union, or enumerated types that have the same scope and use the same
tag declare the same type.
5 Irrespective of whether there is a tag or what other declarations of the type are in the same translation
unit, a type (except enumerated types with a fixed underlying type) is incomplete from the beginning
of the specifier until immediately after the closing brace of the list defining the content for the first
time, and complete thereafter until the beginning of the next specifier that redeclares the same type
later in the same translation unit (if any) or otherwise until the end of the translation unit.
6 Enumerated types with fixed underlying type (6.7.2.2) are complete immediately after their first
associated enum type specifier ends.
7 EXAMPLE 1 The following example shows allowed redeclarations of the same structure, union, or enumerated type in the
same scope:
struct foo { struct { int x; }; };
struct foo { struct { int x; }; };
union bar { int x; float y; };
union bar { float y; int x; };
typedef struct q { int x; } q_t;
typedef struct q { int x; } q_t;
void foo(void)
{
struct S { int x; };
struct T { struct S s; };
struct S { int x; };
struct T { struct S s; };
}
enum X { A = 1, B = 1 + 1 };
enum X { B = 2, A = 1 };
8 EXAMPLE 2 The following example shows invalid redeclarations of the same structure, union, or enumerated type in the
same scope:
struct foo { int (*p)[3]; };
struct foo { int (*p)[]; }; // member has different type
union bar { int x; float y; };
union bar { int z; float y; }; // member has different name
typedef struct { int x; } q_t;
typedef struct { int x; } q_t; // not the same type
157) As specified in 6.7.2.1 above, the type specifier may be followed by a ; or a member declaration list.
struct S { int x; };
void foo(void)
{
struct T { struct S s; };
struct S { int x; };
struct T { struct S s; }; // struct S not the same type
}
enum X { A = 1, B = 2 };
enum X { A = 1, B = 3 }; // different enumeration constant
enum R { C = 1 };
enum Q { C = 1 }; // conflicting enumeration constant
enum Q { C = C }; // ok!
9 Two declarations of structure, union, or enumerated types which are in different scopes or use
different tags declare distinct types. Each declaration of a structure, union, or enumerated type
which does not include a tag declares a distinct type.
10 A type specifier of the form
struct-or-union attribute-specifier-sequenceopt identifieropt { member-declaration-list }
or
enum attribute-specifier-sequenceopt identifieropt { enumerator-list }
or
enum attribute-specifier-sequenceopt identifieropt { enumerator-list , }
declares a structure, union, or enumerated type. The list defines the structure content, union content,
or enumeration content. If an identifier is provided158) , the type specifier also declares the identifier to
be the tag of that type. The optional attribute specifier sequence appertains to the structure, union,
or enumeration type being declared; the attributes in that attribute specifier sequence are thereafter
considered attributes of the structure, union, or enumeration type whenever it is named.
11 A declaration of the form
struct-or-union attribute-specifier-sequenceopt identifier ;
specifies a structure or union type and declares the identifier as a tag of that type159) . The optional
attribute specifier sequence appertains to the structure or union type being declared; the attributes
in that attribute specifier sequence are thereafter considered attributes of the structure or union type
whenever it is named.
12 If a type specifier of the form
struct-or-union attribute-specifier-sequenceopt identifier
occurs other than as part of one of the above forms, and no other declaration of the identifier as a
tag is visible, then it declares an incomplete structure or union type, and declares the identifier as
the tag of that type.159)
13 If a type specifier of the form
struct-or-union attribute-specifier-sequenceopt identifier
or
enum identifier
158) If there is no identifier, the type can, within the translation unit, only be referred to by the declaration of which it is a part.
Of course, when the declaration is of a typedef name, subsequent declarations can make use of that typedef name to declare
objects having the specified structure, union, or enumerated type.
159) A similar construction with enum does not exist.
occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is
visible, then it specifies the same type as that other declaration, and does not redeclare the tag.
14 EXAMPLE 3 This mechanism allows declaration of a self-referential structure.
struct tnode {
int count;
struct tnode *left, *right;
};
specifies a structure that contains an integer and two pointers to objects of the same type. Once this declaration has been
given, the declaration
struct tnode s, *sp;
declares s to be an object of the given type and sp to be a pointer to an object of the given type. With these declarations, the
expression sp->left refers to the left struct tnode pointer of the object to which sp points; the expression s.right->count
designates the count member of the right struct tnode pointed to from s.
15 The following alternative formulation uses the typedef mechanism:
typedef struct tnode TNODE;
struct tnode {
int count;
TNODE *left, *right;
};
TNODE s, *sp;
16 EXAMPLE 4 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the
declarations
struct s1 { struct s2 *s2p; /* ... */ }; // D1
struct s2 { struct s1 *s1p; /* ... */ }; // D2
specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already declared as a tag in an
enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in D2. To eliminate this context sensitivity, the
declaration
struct s2;
can be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then completes the specification
of the new type.
Forward references: declarators (6.7.6), type definitions (6.7.8).
6.7.2.4 Atomic type specifiers
Syntax
1 atomic-type-specifier:
_Atomic ( type-name )
Constraints
2 Atomic type specifiers shall not be used if the implementation does not support atomic types (see
6.10.9.3).
3 The type name in an atomic type specifier shall not refer to an array type, a function type, an atomic
type, or a qualified type.
Semantics
4 The properties associated with atomic types are meaningful only for expressions that are lvalues.
If the _Atomic keyword is immediately followed by a left parenthesis, it is interpreted as a type
specifier (with a type name), not as a type qualifier.
6.7.2.5 Typeof specifiers
Syntax
1 typeof-specifier:
typeof ( typeof-specifier-argument )
typeof_unqual ( typeof-specifier-argument )
typeof-specifier-argument:
expression
type-name
2 The typeof and typeof_unqual tokens are collectively called the typeof operators.
Constraints
3 The typeof operators shall not be applied to an expression that designates a bit-field member.
Semantics
4 The typeof specifier applies the typeof operators to an expression (6.5) or a type name. If the typeof
operators are applied to an expression, they yield the type-name representing the type of their
operand160) . Otherwise, they produce the type name with any nested typeof specifier evaluated161) .
If the type of the operand is a variably modified type, the operand is evaluated; otherwise, the
operand is not evaluated.
5 All qualifiers (6.7.3) on the type from the result of a typeof_unqual operation are removed, including
the _Atomic qualifier162) . Otherwise, for typeof operations, all qualifiers are preserved.
6 EXAMPLE 1 Type of an expression.
typeof(1+1) main () {
return 0;
}
is equivalent to this program:
int main () {
return 0;
}
7 EXAMPLE 2 The following program:
const _Atomic int purr = 0;
const int meow = 1;
const char* const mew[] = {
"aardvark",
"bluejay",
"catte",
};
typeof_unqual(meow) main (int argc, char* argv[]) {
typeof_unqual(purr) plain_purr;
typeof( Atomic typeof(meow)) atomic_meow;
_
typeof(mew) mew_array;
typeof_unqual(mew) mew2_array;
return 0;
}
160) When applied to a parameter declared to have array or function type, the typeof operators yield the adjusted (pointer)
type (see 6.9.1).
161) If the typeof specifier argument is itself a typeof specifier, the operand will be evaluated before evaluating the current
typeof operation. This happens recursively until a typeof specifier is no longer the operand.
162)_Atomic ( type-name ) , with parentheses, is considered an _Atomic -qualified type.
is equivalent to this program:
const _Atomic int purr = 0;
const int meow = 1;
const char* const mew[] = {
"aardvark",
"bluejay",
"catte",
};
int main (int argc, char* argv[]) {
int plain_purr;
const _Atomic int atomic_meow;
const char* const mew_array[3];
const char* mew2_array[3];
return 0;
}
8 EXAMPLE 3 The equivalence between sizeof and typeof’s deduction of the type means this program has no constraint
violations:
int main (int argc, char* argv[]) {
static_assert(sizeof(typeof(’p’)) == sizeof(int));
static_assert(sizeof(typeof(’p’)) == sizeof(’p’));
static_assert(sizeof(typeof((char)’p’)) == sizeof(char));
static_assert(sizeof(typeof((char)’p’)) == sizeof((char)’p’));
static_assert(sizeof(typeof("meow")) == sizeof(char[5]));
static_assert(sizeof(typeof("meow")) == sizeof("meow"));
static_assert(sizeof(typeof(argc)) == sizeof(int));
static_assert(sizeof(typeof(argc)) == sizeof(argc));
static_assert(sizeof(typeof(argv)) == sizeof(char**));
static_assert(sizeof(typeof(argv)) == sizeof(argv));
static_assert(sizeof(typeof_unqual(’p’)) == sizeof(int));
static_assert(sizeof(typeof_unqual(’p’)) == sizeof(’p’));
static_assert(sizeof(typeof_unqual((char)’p’)) == sizeof(char));
static_assert(sizeof(typeof_unqual((char)’p’)) == sizeof((char)’p’));
static_assert(sizeof(typeof_unqual("meow")) == sizeof(char[5]));
static_assert(sizeof(typeof_unqual("meow")) == sizeof("meow"));
static_assert(sizeof(typeof_unqual(argc)) == sizeof(int));
static_assert(sizeof(typeof_unqual(argc)) == sizeof(argc));
static_assert(sizeof(typeof_unqual(argv)) == sizeof(char**));
static_assert(sizeof(typeof_unqual(argv)) == sizeof(argv));
return 0;
}
9 EXAMPLE 4 The following program with nested typeof(...):
int main (int argc, char*[]) {
float val = 6.0f;
return (typeof(typeof_unqual(typeof(argc))))val;
}
is equivalent to this program:
int main (int argc, char*[]) {
float val = 6.0f;
return (int)val;
}
10 EXAMPLE 5 Variable length arrays with typeof operators performs the operation at execution time rather than translation
time.
#include <stddef.h>
size_t vla_size (int n) {
typedef char vla_type[n + 3];
vla_type b; // variable length array
return sizeof(
typeof_unqual(b)
); // execution-time sizeof, translation-time typeof operation
}
int main () {
return (int)vla_size(10); // vla_size returns 13
}
11 EXAMPLE 6 Nested typeof operators, arrays, and pointers do not perform array to pointer decay.
int main () {
typeof(typeof(const char*)[4]) y = {
"a",
"b",
"c",
"d"
}; // 4-element array of "pointer to const char"
return 0;
}
12 EXAMPLE 7 Function, pointer, and array types may be substituted with typeof operations.
void f(int);
typeof(f(5)) g(double x) { // g has type "void(double)"
printf("value %g\n", x);
}
typeof(g)* h; // h has type "void(*)(double)"
typeof(true ? g : NULL) k; // k has type "void(*)(double)"
void j(double A[5], typeof(A)* B); // j has type "void(double*, double**)"
extern typeof(double[]) D; // D has an incomplete type
typeof(D) C = { 0.7, 99 }; // C has type "double[2]"
typeof(D) D = { 5, 8.9, 0.1, 99 }; // D is now completed to "double[4]"
typeof(D) E; // E has type "double[4]" from D’s completed type
6.7.3 Type qualifiers
Syntax
1 type-qualifier:
const
restrict
volatile
_Atomic
Constraints
2 Types other than pointer types whose referenced type is an object type and (possibly multi-
dimensional) array types with such pointer types as element type shall not be restrict-qualified.
3 The _Atomic qualifier shall not be used if the implementation does not support atomic types
(see 6.10.9.3).
4 The type modified by the _Atomic qualifier shall not be an array type or a function type.
Semantics
5 The properties associated with qualified types are meaningful only for expressions that are lval-
ues.163)
6 If the same qualifier appears more than once in the same specifier-qualifier list or as declaration
specifiers, either directly, via one or more typeof specifiers, or via one or more typedefs, the behavior
is the same as if it appeared only once. If other qualifiers appear along with the _Atomic qualifier
the resulting type is the so-qualified atomic type.
7 If an attempt is made to modify an object defined with a const-qualified type through use of an
lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an
object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified
type, the behavior is undefined164) .
8 An object that has volatile-qualified type may be modified in ways unknown to the implementation
or have other unknown side effects. Therefore any expression referring to such an object shall be
evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3. Furthermore,
at every sequence point the value last stored in the object shall agree with that prescribed by the
abstract machine, except as modified by the unknown factors mentioned previously.165) What
constitutes an access to an object that has volatile-qualified type is implementation-defined.
9 An object that is accessed through a restrict-qualified pointer has a special association with that
pointer. This association, defined in 6.7.3.1 below, requires that all accesses to that object use, directly
or indirectly, the value of that particular pointer.166) The intended use of the restrict qualifier (like
the register storage class) is to promote optimization, and deleting all instances of the qualifier
from all preprocessing translation units composing a conforming program does not change its
meaning (i.e., observable behavior).
10 If the specification of an array type includes any type qualifiers, both the array and the element type
is so-qualified. If the specification of a function type includes any type qualifiers, the behavior is
undefined.167)
11 For two qualified types to be compatible, both shall have the identically qualified version of a
compatible type; the order of type qualifiers within a list of specifiers or qualifiers does not affect the
specified type.
12 EXAMPLE 1 An object declared
extern const volatile int real_time_clock;
might be modifiable by hardware, but cannot be assigned to, incremented, or decremented.
13 EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate
type:
const struct s { int mem; } cs = { 1 };
struct s ncs; // the object ncs is modifiable
typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
163) The implementation can place a const object that is not volatile in a read-only region of storage. Moreover, the
implementation need not allocate storage for such an object if its address is never used.
164) This applies to those objects that behave as if they were defined with qualified types, even if they are never actually
defined as objects in the program (such as an object at a memory-mapped input/output address).
165) A volatile declaration can be used to describe an object corresponding to a memory-mapped input/output port or an
object accessed by an asynchronously interrupting function. Actions on objects so declared are not allowed to be "optimized
out" by an implementation or reordered except as permitted by the rules for evaluating expressions.
166) For example, a statement that assigns a value returned by malloc to a single pointer establishes this association between
the allocated object and the pointer.
167) This can occur through the use of typedef s. Note that this rule does not apply to the _Atomic qualifier, and that
qualifiers do not have any direct effect on the array type itself, but affect conversion rules for pointer types that reference an
array type.
int *pi;
const int *pci;
ncs = cs; // valid
cs = ncs; // violates modifiable lvalue constraint for =
pi = &ncs.mem; // valid
pi = &cs.mem; // violates type constraints for =
pci = &cs.mem; // valid
pi = a[0]; // invalid: a[0] has type "const int *"
14 EXAMPLE 3 The declaration
_Atomic volatile int *p;
specifies that p has the type "pointer to volatile atomic int", a pointer to a volatile-qualified atomic type.
6.7.3.1 Formal definition of restrict
1 Let D be a declaration of an ordinary identifier that provides a means of designating an object P as a
restrict-qualified pointer to type T.
2 If D appears inside a block and does not have storage class extern, let B denote the block. If D
appears in the list of parameter declarations of a function definition, let B denote the associated block.
Otherwise, let B denote the block of main (or the block of whatever function is called at program
startup in a freestanding environment).
3 In what follows, a pointer expression E is said to be based on object P if (at some sequence point in
the execution of B prior to the evaluation of E) modifying P to point to a copy of the array object into
which it formerly pointed would change the value of E.168) Note that "based" is defined only for
expressions with pointer types.
4 During each execution of B, let L be any lvalue that has &L based on P. If L is used to access the
value of the object X that it designates, and X is also modified (by any means), then the following
requirements apply: T shall not be const-qualified. Every other lvalue used to access the value of
X shall also have its address based on P. Every access that modifies X shall be considered also to
modify P, for the purposes of this subclause. If P is assigned the value of a pointer expression E that
is based on another restricted pointer object P2, associated with block B2, then either the execution
of B2 shall begin before the execution of B, or the execution of B2 shall end prior to the assignment.
If these requirements are not met, then the behavior is undefined.
5 Here an execution of B means that portion of the execution of the program that would correspond to
the lifetime of an object with scalar type and automatic storage duration associated with B.
6 A translator is free to ignore any or all aliasing implications of uses of restrict.
7 EXAMPLE 1 The file scope declarations
int * restrict a;
int * restrict b;
extern int c[];
assert that if an object is accessed using one of a, b, or c, and that object is modified anywhere in the program, then it is never
accessed using either of the other two.
8 EXAMPLE 2 The function parameter declarations in the following example
void f(int n, int * restrict p, int * restrict q)
{
while (n-- > 0)
*p++ = *q++;
}
assert that, during each execution of the function, if an object is accessed through one of the pointer parameters, then it is not
also accessed through the other. The translator can make this no-aliasing inference based on the parameter declarations alone,
without analyzing the function body.
9 The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence analysis of function f
without examining any of the calls of f in the program. The cost is that the programmer has to examine all of those calls to
ensure that none give undefined behavior. For example, the second call of f in g has undefined behavior because each of
d[1] through d[49] is accessed through both p and q.
void g(void)
{
extern int d[100];
f(50, d + 50, d); // valid
f(50, d + 1, d); // undefined behavior
}
168) In other words, E depends on the value of P itself rather than on the value of an object referenced indirectly through P.
For example, if identifier p has type (int **restrict) , then the pointer expressions p and p+1 are based on the restricted
pointer object designated by p, but the pointer expressions *p and p[1] are not.
10 EXAMPLE 3 The function parameter declarations
void h(int n, int * restrict p, int * restrict q, int * restrict r)
{
int i;
for (i = 0; i < n; i++)
p[i] = q[i] + r[i];
}
illustrate how an unmodified object can be aliased through two restricted pointers. In particular, if a and b are disjoint arrays,
a call of the form h(100, a, b, b) has defined behavior, because array b is not modified within function h.
11 EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a function call and
an equivalent nested block. With one exception, only "outer-to-inner" assignments between restricted pointers declared in
nested blocks have defined behavior.
{
int * restrict p1;
int * restrict q1;
p1 = q1; // undefined behavior
{
int * restrict p2 = p1; // valid
int * restrict q2 = q1; // valid
p1 = q2; // undefined behavior
p2 = q2; // undefined behavior
}
}
12 The one exception allows the value of a restricted pointer to be carried out of the block in which it (or, more precisely, the
ordinary identifier used to designate it) is declared when that block finishes execution. For example, this permits new_vector
to return a vector.
typedef struct { int n; float * restrict v; } vector;
vector new_vector(int n)
{
vector t;
t.n = n;
t.v = malloc(n * sizeof (float));
return t;
}
13 EXAMPLE 5 Suppose that a programmer knows that references of the form p[i] and q[j] are never aliases in the body of a
function:
void f(int n, int *p, int *q) { /* ... */ }
There are several ways that this information could be conveyed to a translator using the restrict qualifier. Example 2 shows
the most effective way, qualifying all pointer parameters, and can be used provided that neither p nor q becomes based on
the other in the function body. A potentially effective alternative is:
void f(int n, int * restrict p, int * const q) { /* ... */ }
Again it is possible for a translator to make the no-aliasing inference based on the parameter declarations alone, though now
it must use subtler reasoning: that the const-qualification of q precludes it becoming based on p. There is also a requirement
that q is not modified, so this alternative cannot be used for the function in Example 2, as written.
14 EXAMPLE 6 Another potentially effective alternative is:
void f(int n, int *p, int const * restrict q) { /* ... */ }
Again it is possible for a translator to make the no-aliasing inference based on the parameter declarations alone, though
now it must use even subtler reasoning: that this combination of restrict and const means that objects referenced using q
cannot be modified, and so no modified object can be referenced using both p and q.
15 EXAMPLE 7 The least effective alternative is:
void f(int n, int * restrict p, int *q) { /* ... */ }
Here the translator can make the no-aliasing inference only by analyzing the body of the function and proving that q cannot
become based on p. Some translator designs may choose to exclude this analysis, given availability of the more effective
alternatives above. Such a translator is required to assume that aliases are present because assuming that aliases are not
present may result in an incorrect translation. Also, a translator that attempts the analysis may not succeed in all cases and
thus need to conservatively assume that aliases are present.
6.7.4 Function specifiers
Syntax
1 function-specifier:
inline
_Noreturn
Constraints
2 Function specifiers shall be used only in the declaration of an identifier for a function.
3 An inline definition of a function with external linkage shall not contain a definition of a modifiable
object with static or thread storage duration, and shall not contain a reference to an identifier with
internal linkage.
4 In a hosted environment, no function specifier(s) shall appear in a declaration of main.
Semantics
5 A function specifier may appear more than once; the behavior is the same as if it appeared only
once.
6 A function declared with an inline function specifier is an inline function. Making a function an
inline function suggests that calls to the function be as fast as possible.169) The extent to which such
suggestions are effective is implementation-defined.170)
7 Any function with internal linkage can be an inline function. For a function with external linkage,
the following restrictions apply: If a function is declared with an inline function specifier, then it
shall also be defined in the same translation unit. If all of the file scope declarations for a function in
a translation unit include the inline function specifier without extern, then the definition in that
translation unit is an inline definition. An inline definition does not provide an external definition
for the function, and does not forbid an external definition in another translation unit. An inline
definition provides an alternative to an external definition, which a translator may use to implement
any call to the function in the same translation unit. It is unspecified whether a call to the function
uses the inline definition or the external definition.171)
8 A function declared with a _Noreturn function specifier shall not return to its caller. The _Noreturn
function specifier is an obsolescent feature (6.7.12.6).
Recommended practice
9 The implementation should produce a diagnostic message for a function declared with a _Noreturn
function specifier that appears to be capable of returning to its caller.
10 EXAMPLE 1 The declaration of an inline function with external linkage can result in either an external definition, or a
definition available for use only within the translation unit. A file scope declaration with extern creates an external definition.
The following example shows an entire translation unit.
169) By using, for example, an alternative to the usual function call mechanism, such as "inline substitution". Inline
substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro
used within the body of the function uses the definition it had at the point the function body appears, and not where the
function is called; and identifiers refer to the declarations in scope where the body occurs. Likewise, the function has a single
address, regardless of the number of inline definitions that occur in addition to the external definition.
170) For example, an implementation might never perform inline substitution, or might only perform inline substitutions to
calls in the scope of an inline declaration.
171) Since an inline definition is distinct from the corresponding external definition and from any other corresponding inline
definitions in other translation units, all corresponding objects with static storage duration are also distinct in each of the
definitions.
inline double fahr(double t)
{
return (9.0 * t) / 5.0 + 32.0;
}
inline double cels(double t)
{
return (5.0 * (t - 32.0)) / 9.0;
}
extern double fahr(double); // creates an external definition
double convert(int is_fahr, double temp)
{
/* A translator may perform inline substitutions */
return is_fahr ? cels(temp): fahr(temp);
}
11 Note that the definition of fahr is an external definition because fahr is also declared with extern, but the definition of cels
is an inline definition. Because cels has external linkage and is referenced, an external definition has to appear in another
translation unit (see 6.9); the inline definition and the external definition are distinct and either can be used for the call.
Forward references: function definitions (6.9.1).
6.7.5 Alignment specifier
Syntax
1 alignment-specifier:
alignas ( type-name )
alignas ( constant-expression )
Constraints
2 An alignment specifier shall appear only in the declaration specifiers of a declaration, or in the
specifier-qualifier list of a member declaration, or in the type name of a compound literal. An
alignment specifier shall not be used in conjunction with either of the storage-class specifiers
typedef or register, nor in a declaration of a function or bit-field.
3 The constant expression shall be an integer constant expression. It shall evaluate to a valid funda-
mental alignment, or to a valid extended alignment supported by the implementation for an object
of the storage duration (if any) being declared, or to zero.
4 An object shall not be declared with an over-aligned type with an extended alignment requirement
not supported by the implementation for an object of that storage duration.
5 The combined effect of all alignment specifiers in a declaration shall not specify an alignment that is
less strict than the alignment that would otherwise be required for the type of the object or member
being declared.
Semantics
6 The first form is equivalent to alignas(alignof( type-name)).
7 The alignment requirement of the declared object or member is taken to be the specified alignment.
An alignment specification of zero has no effect.172) When multiple alignment specifiers occur in a
declaration, the effective alignment requirement is the strictest specified alignment.
8 If the definition of an object has an alignment specifier, any other declaration of that object shall
either specify equivalent alignment or have no alignment specifier. If the definition of an object does
not have an alignment specifier, any other declaration of that object shall also have no alignment
specifier. If declarations of an object in different translation units have different alignment specifiers,
172) An alignment specification of zero also does not affect other alignment specifications in the same declaration.
the behavior is undefined.
6.7.6 Declarators
Syntax
1 declarator:
pointeropt direct-declarator
direct-declarator:
identifier attribute-specifier-sequenceopt
( declarator )
array-declarator attribute-specifier-sequenceopt
function-declarator attribute-specifier-sequenceopt
array-declarator:
direct-declarator [ type-qualifier-listopt assignment-expressionopt ]
direct-declarator [ static type-qualifier-listopt assignment-expression ]
direct-declarator [ type-qualifier-list static assignment-expression ]
direct-declarator [ type-qualifier-listopt * ]
function-declarator:
direct-declarator ( parameter-type-listopt )
pointer:
* attribute-specifier-sequenceopt type-qualifier-listopt
* attribute-specifier-sequenceopt type-qualifier-listopt pointer
type-qualifier-list:
type-qualifier
type-qualifier-list type-qualifier
parameter-type-list:
parameter-list
parameter-list , ...
...
parameter-list:
parameter-declaration
parameter-list , parameter-declaration
parameter-declaration:
attribute-specifier-sequenceopt declaration-specifiers declarator
attribute-specifier-sequenceopt declaration-specifiers abstract-declaratoropt
Semantics
2 Each declarator declares one identifier, and asserts that when an operand of the same form as
the declarator appears in an expression, it designates a function or object with the scope, storage
duration, and type indicated by the declaration specifiers.
3 A full declarator is a declarator that is not part of another declarator. If, in the nested sequence of
declarators in a full declarator, there is a declarator specifying a variable length array type, the type
specified by the full declarator is said to be variably modified. Furthermore, any type derived by
declarator type derivation from a variably modified type is itself variably modified.
4 In the following subclauses, consider a declaration
T D1
where T contains the declaration specifiers that specify a type T (such as int) and D1 is a declarator
that contains an identifier ident. The type specified for the identifier ident in the various forms of
declarator is described inductively using this notation.
5 If, in the declaration "T D1", D1 has the form
identifier attribute-specifier-sequenceopt
then the type specified for ident is T and the optional attribute specifier sequence appertains to the
entity that is declared.
6 If, in the declaration "T D1", D1 has the form
(D )
then ident has the type specified by the declaration "T D". Thus, a declarator in parentheses is
identical to the unparenthesized declarator, but the binding of complicated declarators may be
altered by parentheses.
Implementation limits
7 As discussed in 5.2.4.1, an implementation may limit the number of pointer, array, and function
declarators that modify an arithmetic, structure, union, or void type, either directly or via one or
more typedef s.
Forward references: array declarators (6.7.6.2), type definitions (6.7.8).
6.7.6.1 Pointer declarators
Semantics
1 If, in the declaration "T D1", D1 has the form
* attribute-specifier-sequenceopt type-qualifier-listopt D
and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the
type specified for ident is "derived-declarator-type-list type-qualifier-list pointer to T". For each type
qualifier in the list, ident is a so-qualified pointer. The optional attribute specifier sequence appertains
to the pointer and not the object pointed to.
2 For two pointer types to be compatible, both shall be identically qualified and both shall be pointers
to compatible types.
3 EXAMPLE The following pair of declarations demonstrates the difference between a "variable pointer to a constant value"
and a "constant pointer to a variable value".
const int *ptr_to_constant;
int *const constant_ptr;
The contents of any object pointed to by ptr_to_constant cannot be modified through that pointer, but ptr_to_constant
itself can be changed to point to another object. Similarly, the contents of the int pointed to by constant_ptr can be
modified, but constant_ptr itself always points to the same location.
4 The declaration of the constant pointer constant_ptr can be clarified by including a definition for the type "pointer to int".
typedef int *int_ptr;
const int_ptr constant_ptr;
declares constant_ptr as an object that has type "const-qualified pointer to int".
6.7.6.2 Array declarators
Constraints
1 In addition to optional type qualifiers and the keyword static, the [ and ] may delimit an expres-
sion or * . If they delimit an expression (which specifies the size of an array), the expression shall
have an integer type. If the expression is a constant expression, it shall have a value greater than
zero. The element type shall not be an incomplete or function type. The optional type qualifiers and
the keyword static shall appear only in a declaration of a function parameter with an array type,
and then only in the outermost array type derivation.
2 If an identifier is declared as having a variably modified type, it shall be an ordinary identifier (as
defined in 6.2.3), have no linkage, and have either block scope or function prototype scope. If an
identifier is declared to be an object with static or thread storage duration, it shall not have a variable
length array type.
Semantics
3 If, in the declaration "T D1", D1 has one of the forms:
D [ type-qualifier-listopt assignment-expressionopt ] attribute-specifier-sequenceopt
D [ static type-qualifier-listopt assignment-expression ] attribute-specifier-sequenceopt
D [ type-qualifier-list static assignment-expression ] attribute-specifier-sequenceopt
D [ type-qualifier-listopt * ] attribute-specifier-sequenceopt
and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the
type specified for ident is "derived-declarator-type-list array of T".173)174) The optional attribute specifier
sequence appertains to the array. (See 6.7.6.3 for the meaning of the optional type qualifiers and the
keyword static.)
4 If the size is not present, the array type is an incomplete type. If the size is * instead of being an
expression, the array type is a variable length array type of unspecified size, which can only be used in
declarations or type names with function prototype scope175) ; such arrays are nonetheless complete
types. If the size is an integer constant expression and the element type has a known constant
size, the array type is not a variable length array type; otherwise, the array type is a variable length
array type. (Variable length arrays with automatic storage duration are a conditional feature that
implementations need not support; see 6.10.9.3.)
5 If the size is an expression that is not an integer constant expression: if it occurs in a declaration at
function prototype scope, it is treated as if it were replaced by * ; otherwise, each time it is evaluated
it shall have a value greater than zero. The size of each instance of a variable length array type does
not change during its lifetime. Where a size expression is part of the operand of a typeof or sizeof
operator and changing the value of the size expression would not affect the result of the operator, it
is unspecified whether or not the size expression is evaluated. Where a size expression is part of the
operand of an alignof operator, that expression is not evaluated.
6 For two array types to be compatible, both shall have compatible element types, and if both size
specifiers are present, and are integer constant expressions, then both size specifiers shall have
the same constant value. If the two array types are used in a context which requires them to be
compatible, it is undefined behavior if the two size specifiers evaluate to unequal values.
7 EXAMPLE 1
float fa[11], *afp[17];
declares an array of float numbers and an array of pointers to float numbers.
8 EXAMPLE 2 Note the distinction between the declarations
extern int *x;
extern int y[];
The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size (an incomplete type),
the storage for which is defined elsewhere.
9 EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.
extern int n;
extern int m;
void fcompat(void)
{
int a[n][6][m];
int (*p)[4][n+1];
int c[n][n][6][m];
int (*r)[n][n][n+1];
p = a; // invalid: not compatible because 4 != 6
r = c; // compatible, but defined behavior only if
173) When several "array of" specifications are adjacent, a multidimensional array is declared.
174) The array is considered identically qualified to T according to 6.2.5.
175) Thus,
* can be used only in function declarations that are not definitions (see 6.7.6.3).
// n == 6 and m == n+1
}
10 EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or function prototype scope.
Array objects declared with the thread_local, static, or extern storage-class specifier cannot have a variable length array
(VLA) type. However, an object declared with the static storage-class specifier can have a VM type (that is, a pointer to a
VLA type). Finally, all identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be members
of structures or unions.
extern int n;
int A[n]; // invalid: file scope VLA
extern int (*p2)[n]; // invalid: file scope VM
int B[100]; // valid: file scope but not VM
void fvla(int m, int C[m][m]); // valid: VLA with prototype scope
void fvla(int m, int C[m][m]) // valid: adjusted to auto pointer to VLA
{
typedef int VLA[m][m]; // valid: block scope typedef VLA
struct tag {
int (*y)[n]; // invalid: y not ordinary identifier
int z[n]; // invalid: z not ordinary identifier
};
int D[m]; // valid: auto VLA
static int E[m]; // invalid: static block scope VLA
extern int F[m]; // invalid: F has linkage and is VLA
int (*s)[m]; // valid: auto pointer to VLA
extern int (*r)[m]; // invalid: r has linkage and points to VLA
static int (*q)[m] = &B; // valid: q is a static block pointer to VLA
}
Forward references: function declarators (6.7.6.3), function definitions (6.9.1), initialization (6.7.10).
6.7.6.3 Function declarators
Constraints
1 A function declarator shall not specify a return type that is a function type or an array type.
2 The only storage-class specifier that shall occur in a parameter declaration is register.
3 After adjustment, the parameters in a parameter type list in a function declarator that is part of a
definition of that function shall not have incomplete type.
Semantics
4 If, in the declaration "T D1", D1 has the form
D ( parameter-type-listopt ) attribute-specifier-sequenceopt
and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the
type specified for ident is "derived-declarator-type-list function returning the unqualified version of T".
The optional attribute specifier sequence appertains to the function type.
5 A parameter type list specifies the types of, and may declare identifiers for, the parameters of the
function.
6 A declaration of a parameter as "array of type" shall be adjusted to "qualified pointer to type", where
the type qualifiers (if any) are those specified within the [ and ] of the array type derivation. If the
keyword static also appears within the [ and ] of the array type derivation, then for each call to
the function, the value of the corresponding actual argument shall provide access to the first element
of an array with at least as many elements as specified by the size expression.
7 A declaration of a parameter as "function returning type" shall be adjusted to "pointer to function
returning type", as in 6.3.2.1.
8 If the list terminates with an ellipsis (...), no information about the number or types of the
parameters after the comma is supplied. 176)
9 The special case of an unnamed parameter of type void as the only item in the list specifies that the
function has no parameters.
10 If, in a parameter declaration, an identifier can be treated either as a typedef name or as a parameter
name, it shall be taken as a typedef name.
11 If the function declarator is not part of a definition of that function, parameters may have incomplete
type and may use the [*] notation in their sequences of declarator specifiers to specify variable
length array types.
12 The storage class specifier in the declaration specifiers for a parameter declaration, if present, is
ignored unless the declared parameter is one of the members of the parameter type list for a function
definition. The optional attribute specifier sequence in a parameter declaration appertains to the
parameter.
13 For a function declarator without a parameter type list: the effect is as if it were declared with a
parameter type list consisting of the keyword void. A function declarator provides a prototype for
the function177) .
14 For two function types to be compatible, both shall specify compatible return types. Moreover,
the parameter type lists shall agree in the number of parameters and in use of the final ellipsis;
corresponding parameters shall have compatible types. In the determination of type compatibility
and of a composite type, each parameter declared with function or array type is taken as having the
adjusted type and each parameter declared with qualified type is taken as having the unqualified
version of its declared type.
15 EXAMPLE 1 The declaration
int f(void), *fip(), (*pfi)();
declares a function f with no parameters returning an int, a function fip with no parameters returning a pointer to an int,
and a pointer pfi to a function with no parameters returning an int. It is especially useful to compare the last two. The
binding of *fip() is *(fip()) , so that the declaration suggests, and the same construction in an expression requires, the
calling of a function fip, and then using indirection through the pointer result to yield an int. In the declarator (*pfi)() ,
the extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function designator,
which is then used to call the function; it returns an int.
16 If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the declaration
occurs inside a function, the identifiers of the functions f and fip have block scope and either internal or external linkage
(depending on what file scope declarations for these identifiers are visible), and the identifier of the pointer pfi has block
scope and no linkage.
17 EXAMPLE 2 The declaration
int (*apfi[3])(int *x, int *y);
declares an array apfi of three pointers to functions returning int. Each of these functions has two parameters that are
pointers to int. The identifiers x and y are declared for descriptive purposes only and go out of scope at the end of the
declaration of apfi.
18 EXAMPLE 3 The declaration
int (*fpfi(int (*)(long), int))(int, ...);
declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two parameters: a
pointer to a function returning an int (with one parameter of type long int), and an int. The pointer returned by fpfi
points to a function that has one int parameter and accepts zero or more additional arguments of any type.
19 EXAMPLE 4 The following prototype has a variably modified parameter.
void addscalar(int n, int m,
double a[n][n*m+300], double x);
176) The macros defined in the <stdarg.h> header (7.16) can be used to access arguments that correspond to the ellipsis.
177) This implies that a function definition without a parameter list provides a prototype, and that subsequent calls to that
function in the same translation unit are constrained not to provide any argument to the function call. Thus a definition of a
function without parameter list and one that has such a list consisting of the keyword void are fully equivalent.
int main()
{
double b[4][308];
addscalar(4, 2, b, 2.17);
return 0;
}
void addscalar(int n, int m,
double a[n][n*m+300], double x)
{
for (int i = 0; i < n; i++)
for (int j = 0, k = n*m+300; j < k; j++)
// a is a pointer to a VLA with n*m+300 elements
a[i][j] += x;
}
20 EXAMPLE 5 The following are all compatible function prototype declarators.
double maximum(int n, int m, double a[n][m]);
double maximum(int n, int m, double a[*][*]);
double maximum(int n, int m, double a[ ][*]);
double maximum(int n, int m, double a[ ][m]);
as are:
void f(double (* restrict a)[5]);
void f(double a[restrict][5]);
void f(double a[restrict 3][5]);
void f(double a[restrict static 3][5]);
(Note that the last declaration also specifies that the argument corresponding to a in any call to f can be expected to be a
non-null pointer to the first of at least three arrays of 5 doubles, which the others do not.)
Forward references: function definitions (6.9.1), type names (6.7.7).
6.7.7 Type names
Syntax
1 type-name:
specifier-qualifier-list abstract-declaratoropt
abstract-declarator:
pointer
pointeropt direct-abstract-declarator
direct-abstract-declarator:
( abstract-declarator )
array-abstract-declarator attribute-specifier-sequenceopt
function-abstract-declarator attribute-specifier-sequenceopt
array-abstract-declarator:
direct-abstract-declaratoropt [ type-qualifier-listopt assignment-expressionopt ]
direct-abstract-declaratoropt [ static type-qualifier-listopt assignment-expression ]
direct-abstract-declaratoropt [ type-qualifier-list static assignment-expression ]
direct-abstract-declaratoropt [ * ]
function-abstract-declarator:
direct-abstract-declaratoropt ( parameter-type-listopt )
Semantics
2 In several contexts, it is necessary to specify a type. This is accomplished using a type name, which is
syntactically a declaration for a function or an object of that type that omits the identifier.178) The
optional attribute specifier sequence in a direct abstract declarator appertains to the preceding array
or function type. The attribute specifier sequence affects the type only for the declaration it appears
in, not other declarations involving the same type.
3 EXAMPLE The constructions
(a) int
(b) int *
(c) int *[3]
(d) int (*)[3]
(e) int (*)[*]
(f) int *()
(g) int (*)(void)
(h) int (*const [])(unsigned int, ...)
name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an array of three
int s, (e) pointer to a variable length array of an unspecified number of int s, (f) function with no parameters returning
a pointer to int, (g) pointer to function with no parameters returning an int, and (h) array of an unspecified number of
constant pointers to functions, each with one parameter that has type unsigned int and an unspecified number of other
parameters, returning an int.
6.7.8 Type definitions
Syntax
1 typedef-name:
identifier
Constraints
2 If a typedef name specifies a variably modified type then it shall have block scope.
Semantics
3 In a declaration whose storage-class specifier is typedef, each declarator defines an identifier to
be a typedef name that denotes the type specified for the identifier in the way described in 6.7.6.
Any array size expressions associated with variable length array declarators are evaluated each time
the declaration of the typedef name is reached in the order of execution. A typedef declaration
does not introduce a new type, only a synonym for the type so specified. That is, in the following
declarations:
typedef T type_ident;
type_ident D;
type_ident is defined as a typedef name with the type specified by the declaration specifiers in T
(known as T), and the identifier in D has the type "derived-declarator-type-list T" where the derived-
declarator-type-list is specified by the declarators of D. A typedef name shares the same name space
as other identifiers declared in ordinary declarators. If the identifier is redeclared in an enclosed
block the inner declaration shall not be such that the type is inferred (6.7.9).
4 EXAMPLE 1 After
typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;
the constructions
178) As indicated by the syntax, empty parentheses in a type name are interpreted as "function with no parameter specifica-
tion", rather than redundant parentheses around the omitted identifier.
MILES distance;
extern KLICKSP *metricp;
range x;
range z, *zp;
are all valid declarations. The type of distance is int, that of metricp is "pointer to function with no parameters returning
int", and that of x and z is the specified structure; zp is a pointer to such a structure. The object distance has a type
compatible with any other int object.
5 EXAMPLE 2 After the declarations
typedef struct s1 { int x; } t1, *tp1;
typedef struct s2 { int x; } t2, *tp2;
type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct s1, but not compatible
with the types struct s2, t2, the type pointed to by tp2, or int.
6 EXAMPLE 3 The following obscure constructions
typedef signed int t;
typedef int plain;
struct tag {
unsigned t:4;
const t:5;
plain r:5;
};
declare a typedef name t with type signed int, a typedef name plain with type int, and a structure with three bit-field
members, one named t that contains values in the range [0, 15], an unnamed const-qualified bit-field which (if it could
be accessed) would contain values in either the range [−15, +15] or [−16, +15], and one named r that contains values in
one of the ranges [0, 31], [−15, +15], or [−16, +15]. (The choice of range is implementation-defined.) The first two bit-field
declarations differ in that unsigned is a type specifier (which forces t to be the name of a structure member), while const is
a type qualifier (which modifies t which is still visible as a typedef name). If these declarations are followed in an inner scope
by
t f(t (t));
long t;
then a function f is declared with type "function returning signed int with one unnamed parameter with type pointer
to function returning signed int with one unnamed parameter with type signed int", and an identifier t with type
long int.
7 EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the following
declarations of the signal function specify exactly the same type, the first without making use of any typedef names.
typedef void fv(int), (*pfv)(int);
void (*signal(int, void (*)(int)))(int);
fv *signal(int, fv *);
pfv signal(int, pfv);
8 EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef
name is defined, not each time it is used:
void copyt(int n)
{
typedef int B[n]; // B is n ints, n evaluated now
n += 1;
B a; // a is n ints, n without += 1
int b[n]; // a and b are different sizes
for (int i = 1; i < n; i++)
a[i-1] = b[i];
}
6.7.9 Type inference
Constraints
1 A declaration for which the type is inferred shall contain the storage-class specifier auto.
Description
2 For such a declaration that is the definition of an object the init-declarator shall have one of the forms
direct-declarator = assignment-expression
direct-declarator = { assignment-expression }
direct-declarator = { assignment-expression , }
The declared type is the type of the assignment expression after lvalue, array to pointer or function
to pointer conversion, additionally qualified by qualifiers and amended by attributes as they appear
in the declaration specifiers, if any179) . If the direct declarator is not of the form
identifier attribute-specifier-sequenceopt
, possibly enclosed in balanced pairs of parentheses, the behavior is undefined.
3 NOTE Such a declaration that also defines a structure or union type violates a constraint. Here, the identifier a which is not
ordinary but in the name space of the structure type is declared.
auto p = (struct { int a; } *)0;
Even a forward declaration of a structure tag
struct s;
auto p = (struct s { int a; } *)0;
would not change that situation. A direct use of the structure definition as the type specifier ensures the validity of the
declaration.
struct s { int a; } * p = 0;
4 EXAMPLE 1 Consider the following file scope definitions:
static auto a = 3.5;
auto p = &a;
They are interpreted as if they had been written as:
static double a = 3.5;
double * p = &a;
So effectively a is a double and p is a double*. Note that the restrictions on the syntax of such declarations does not allow the
declarator to be *p , but that the final type here nevertheless is a pointer type.
5 EXAMPLE 2 The scope of the identifier for which the type is inferred only starts after the end of the initializer (6.2.1), so
the assignment expression cannot use the identifier to refer to the object or function that is declared, for example to take its
address. Any use of the identifier in the initializer is invalid, even if an entity with the same name exists in an outer scope.
{
double a = 7;
double b = 9;
{
double b = b * b; // undefined, uses uninitialized
// variable without address
printf("%g\n", a); // valid, uses "a" from outer scope, prints 7
auto a = a * a; // invalid, "a" from outer scope is already
shadowed
}
179) The scope rules as described in 6.2.1 also prohibit the use of the identifier of the declarator within the assignment
expression.
{
auto b = a * a; // valid, uses "a" from outer scope
auto a = b; // valid, shadows "a" from outer scope
// ...
printf("%g\n", a); // valid, uses "a" from inner scope, prints 49
}
// ...
}
6 EXAMPLE 3 In the following, declarations of pA and qA are valid. The type of A after array-to-pointer conversion is a pointer
type, and qA is a pointer to array.
double A[3] = { 0 };
auto pA = A;
auto qA = &A;
7 EXAMPLE 4 Type inference can be used to capture the type of a call to a type-generic function. It ensures that the same type
as the argument x is used.
#include <tgmath.h>
auto y = cos(x);
If instead the type of y is explicitly specified to a different type than x, a diagnosis of the mismatch is not enforced.
8 EXAMPLE 5 A type-generic macro that generalizes the div functions (7.24.6.2) is defined and used as follows.
#define div(X, Y) _Generic((X)+(Y), int: div, long: ldiv, long long: lldiv)((X),
(Y))
auto z = div(x, y);
auto q = z.quot;
auto r = z.rem;
9 EXAMPLE 6 Definitions of objects with inferred type are valid in all contexts that allow the initializer syntax as described.
In particular they can be used to ensure type safety of for-loop controlling expressions.
for (auto i = j; i < 2*j; ++i) {
// ...
}
Here, regardless of the integer rank or signedness of the type of j, i will have the non-atomic unqualified type of j. So, after
lvalue conversion and possible promotion, the two operands of the < operator in the controlling expression are guaranteed to
have the same type, and, in particular, the same signedness.
6.7.10 Initialization
Syntax
1 braced-initializer:
{ }
{ initializer-list }
{ initializer-list , }
initializer:
assignment-expression
braced-initializer
initializer-list:
designationopt initializer
initializer-list , designationopt initializer
designation:
designator-list =
designator-list:
designator
designator-list designator
designator:
[ constant-expression ]
. identifier
2 An empty brace pair ({}) is called an empty initializer and is referred to as empty initialization.
Constraints
3 No initializer shall attempt to provide a value for an object not contained within the entity being
initialized.
4 The type of the entity to be initialized shall be an array of unknown size or a complete object type.
An entity of variable length array type shall not be initialized except by an empty initializer. An
array of unknown size shall not be initialized by an empty initializer.
5 All the expressions in an initializer for an object that has static or thread storage duration or is
declared with the constexpr storage-class specifier shall be constant expressions or string literals.
6 If the declaration of an identifier has block scope, and the identifier has external or internal linkage,
the declaration shall have no initializer for the identifier.
7 If a designator has the form
[ constant-expression ]
then the current object (defined below) shall have array type and the expression shall be an integer
constant expression. If the array is of unknown size, any nonnegative value is valid.
8 If a designator has the form
. identifier
then the current object (defined below) shall have structure or union type and the identifier shall be
the name of a member of that type.
Semantics
9 An initializer specifies the initial value stored in an object. For objects with atomic type additional
restrictions apply, see 7.17.2 and 7.17.8.
10 Except where explicitly stated otherwise, for the purposes of this subclause unnamed members
of objects of structure and union type do not participate in initialization. Unnamed members of
structure objects have indeterminate representation even after initialization.
11 If an object that has automatic storage duration is initialized with an empty initializer, its value
is the same as the initialization of a static storage duration object. Otherwise, if an object that has
automatic storage duration is not initialized explicitly, its representation is indeterminate. If an
object that has static or thread storage duration is not initialized explicitly, or is initialized with an
empty initializer, then default initialization:
— if it has pointer type, it is initialized to a null pointer;
— if it has decimal floating type, it is initialized to (positive or unsigned) zero, and the quantum
exponent is implementation-defined180) ;
— if it has arithmetic type, and it does not have decimal floating type, it is initialized to (positive
or unsigned) zero;
— if it is an aggregate, every member is initialized (recursively) according to these rules, and any
padding is initialized to zero bits;
180) A representation with all bits zero results in a decimal floating-point zero with the most negative exponent.
— if it is a union, the first named member is initialized (recursively) according to these rules, and
any padding is initialized to zero bits;
12 The initializer for a scalar shall be a single expression, optionally enclosed in braces, or it shall be
an empty initializer. If the initializer is the empty initializer, the initial value is the same as the
initialization of a static storage duration object. Otherwise, the initial value of the object is that of the
expression (after conversion); the same type constraints and conversions as for simple assignment
apply, taking the type of the scalar to be the unqualified version of its declared type.
13 The rest of this subclause deals with initializers for objects that have aggregate or union type.
14 The initializer for a structure or union object that has automatic storage duration shall be either
an initializer list as described below, or a single expression that has compatible structure or union
type. In the latter case, the initial value of the object, including unnamed members, is that of the
expression.
15 An array of character type may be initialized by a character string literal or UTF-8 string literal,
optionally enclosed in braces. Successive bytes of the string literal (including the terminating null
character if there is room or if the array is of unknown size) initialize the elements of the array.
16 An array with element type compatible with a qualified or unqualified version of wchar_t, char16_t,
or char32_t may be initialized by a wide string literal with the corresponding encoding prefix (L,
u, or U, respectively), optionally enclosed in braces. Successive wide characters of the wide string
literal (including the terminating null wide character if there is room or if the array is of unknown
size) initialize the elements of the array.
17 Otherwise, the initializer for an object that has aggregate or union type shall be a brace-enclosed list
of initializers for the elements or named members.
18 Each brace-enclosed initializer list has an associated current object. When no designations are present,
subobjects of the current object are initialized in order according to the type of the current object:
array elements in increasing subscript order, structure members in declaration order, and the first
named member of a union.181) In contrast, a designation causes the following initializer to begin
initialization of the subobject described by the designator. Initialization then continues forward in
order, beginning with the next subobject after that described by the designator.182)
19 Each designator list begins its description with the current object associated with the closest sur-
rounding brace pair. Each item in the designator list (in order) specifies a particular member of its
current object and changes the current object for the next designator (if any) to be that member.183)
The current object that results at the end of the designator list is the subobject to be initialized by the
following initializer.
20 The initialization shall occur in initializer list order, each initializer provided for a particular subobject
overriding any previously listed initializer for the same subobject;184) all subobjects that are not
initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.
21 If the aggregate or union contains elements or members that are aggregates or unions, these rules
apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or
contained union begins with a left brace, the initializers enclosed by that brace and its matching right
brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only
enough initializers from the list are taken to account for the elements or members of the subaggregate
or the first member of the contained union; any remaining initializers are left to initialize the next
element or member of the aggregate of which the current subaggregate or contained union is a part.
22 If there are fewer initializers in a brace-enclosed list than there are elements or members of an
181) If the initializer list for a subaggregate or contained union does not begin with a left brace, its subobjects are initialized as
usual, but the subaggregate or contained union does not become the current object: current objects are associated only with
brace-enclosed initializer lists.
182) After a union member is initialized, the next object is not the next member of the union; instead, it is the next subobject of
an object containing the union.
183) Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with the surrounding
brace pair. Note, too, that each separate designator list is independent.
184) Any initializer for the subobject which is overridden and so not used to initialize that subobject might not be evaluated at
all.
aggregate, or fewer characters in a string literal used to initialize an array of known size than there
are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as
objects that have static storage duration.
23 If an array of unknown size is initialized, its size is determined by the largest indexed element with
an explicit initializer. The array type is completed at the end of its initializer list.
24 The evaluations of the initialization list expressions are indeterminately sequenced with respect to
one another and thus the order in which any side effects occur is unspecified.185)
25 EXAMPLE 1 Provided that <complex.h> has been #included, the declarations
int i = 3.5;
double complex c = 5 + 3 * I;
define and initialize i with the value 3 and c with the value 5.0 + i3.0.
26 EXAMPLE 2 The declaration
int x[] = { 1, 3, 5 };
defines and initializes x as a one-dimensional array object that has three elements, as no size was specified and there are three
initializers.
27 EXAMPLE 3 The declaration
int y[4][3] = {
{ 1, 3, 5 },
{ 2, 4, 6 },
{ 3, 5, 7 },
};
is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object y[0]), namely
y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early, so y[3] is
initialized with zeros. Precisely the same effect could have been achieved by
int y[4][3] = {
1, 3, 5, 2, 4, 6, 3, 5, 7
};
The initializer for y[0] does not begin with a left brace, so three items from the list are used. Likewise the next three are
taken successively for y[1] and y[2].
28 EXAMPLE 4 The declaration
int z[4][3] = {
{ 1 }, { 2 }, { 3 }, { 4 }
};
initializes the first column of z as specified and initializes the rest with zeros.
29 EXAMPLE 5 The declaration
struct { int a[3], b; } w[] = { { 1 }, 2 };
is a definition with an inconsistently bracketed initialization. It defines an array with two element structures: w[0].a[0] is 1
and w[1].a[0] is 2; all the other elements are zero.
30 EXAMPLE 6 The declaration
short q[4][3][2] = {
{ 1 },
{ 2, 3 },
{ 4, 5, 6 }
};
185) In particular, the evaluation order need not be the same as the order of subobject initialization.
contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array object: q[0][0][0]
is 1, q[1][0][0] is 2, q[1][0][1] is 3, and 4, 5, and 6 initialize q[2][0][0], q[2][0][1], and q[2][1][0], respectively;
all the rest are zero. The initializer for q[0][0] does not begin with a left brace, so up to six items from the current list
could be used. There is only one, so the values for the remaining five elements are initialized with zero. Likewise, the
initializers for q[1][0] and q[2][0] do not begin with a left brace, so each uses up to six items, initializing their respective
two-dimensional subaggregates. If there had been more than six items in any of the lists, a diagnostic message would have
been issued. The same initialization result could have been achieved by:
short q[4][3][2] = {
1, 0, 0, 0, 0, 0,
2, 3, 0, 0, 0, 0,
4, 5, 6
};
or by:
short q[4][3][2] = {
{
{ 1 },
},
{
{ 2, 3 },
},
{
{ 4, 5 },
{ 6 },
}
};
in a fully bracketed form.
31 Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion.
32 EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the declaration
typedef int A[]; // OK - declared with block scope
the declaration
A a = { 1, 2 }, b = { 3, 4, 5 };
is identical to
int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
due to the rules for incomplete types.
33 EXAMPLE 8 The declaration
char s[] = "abc", t[3] = "abc";
defines "plain" char array objects s and t whose elements are initialized with character string literals. This declaration is
identical to
char s[] = { ’a’, ’b’, ’c’, ’\0’ },
t[] = { ’a’, ’b’, ’c’ };
The contents of the arrays are modifiable. On the other hand, the declaration
char *p = "abc";
defines p with type "pointer to char" and initializes it to point to an object with type "array of char" with length 4 whose
elements are initialized with a character string literal. If an attempt is made to use p to modify the contents of the array, the
behavior is undefined.
34 EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:
enum { member_one, member_two };
const char *nm[] = {
[member_two] = "member two",
[member_one] = "member one",
};
35 EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:
div_t answer = {.quot = 2, .rem = -1 };
36 EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists might be misunder-
stood:
struct { int a[3], b; } w[] =
{ [0].a = {1}, [1].a[0] = 2 };
37 EXAMPLE 12
struct T {
int k;
int l;
};
struct S {
int i;
struct T t;
};
struct T x = {.l = 43, .k = 42, };
void f(void)
{
struct S l = { 1, .t = x, .t.l = 41, };
}
The value of l.t.k is 42, because implicit initialization does not override explicit initialization.
38 EXAMPLE 13 Space can be "allocated" from both ends of an array by using a single designator:
int a[MAX] = {
1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
};
39 In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less than ten, some of
the values provided by the first five initializers will be overridden by the second five.
40 EXAMPLE 14 Any member of a union can be initialized:
union { /* ... */ } u = {.any_member = 42 };
Forward references: common definitions <stddef.h> (7.21).
6.7.11 Static assertions
Syntax
1 static_assert-declaration:
static_assert ( constant-expression , string-literal ) ;
static_assert ( constant-expression ) ;
Constraints
2 The constant expression shall compare unequal to 0.
Semantics
3 The constant expression shall be an integer constant expression. If the value of the constant expres-
sion compares unequal to 0, the declaration has no effect. Otherwise, the constraint is violated and
the implementation shall produce a diagnostic message which should include the text of the string
literal, if present.
Forward references: diagnostics (7.2).
6.7.12 Attributes
1 Attributes specify additional information for various source constructs such as types, variables,
identifiers, or blocks. They are identified by an attribute token, which can either be a attribute prefixed
token (for implementation-specific attributes) or a standard attribute specified by an identifier (for
attributes specified in this document).
2 Support for any of the standard attributes specified in this document is implementation-defined
and optional. For an attribute token (including an attribute prefixed token) not specified in this
document, the behavior is implementation-defined. Any attribute token that is not supported by the
implementation is ignored.
3 Attributes are said to appertain to some source construct, identified by the syntactic context where
they appear, and for each individual attribute, the corresponding clause constrains the syntactic
context in which this appertainance is valid. The attribute specifier sequence appertaining to some
source construct shall contain only attributes that are allowed to apply to that source construct.
4 In all aspects of the language, a standard attribute specified by this document as an identifier attr
and an identifier of the form __attr__ shall behave the same when used as an attribute token,
except for the spelling.186)
Recommended practice
5 It is recommended that implementations support all standard attributes as defined in this document.
6.7.12.1 General
Syntax
1 attribute-specifier-sequence:
attribute-specifier-sequenceopt attribute-specifier
attribute-specifier:
[ [ attribute-list ] ]
attribute-list:
attributeopt
attribute-list , attributeopt
attribute:
attribute-token attribute-argument-clauseopt
attribute-token:
standard-attribute
attribute-prefixed-token
standard-attribute:
identifier
attribute-prefixed-token:
attribute-prefix :: identifier
attribute-prefix:
identifier
attribute-argument-clause:
( balanced-token-sequenceopt )
balanced-token-sequence:
balanced-token
186) Thus, the attributes [[nodiscard]] and [[__nodiscard__]] can be freely interchanged. Implementations are encour-
aged to behave similarly for attribute tokens (including attribute prefixed tokens) they provide.
balanced-token-sequence balanced-token
balanced-token:
( balanced-token-sequenceopt )
[ balanced-token-sequenceopt ]
{ balanced-token-sequenceopt }
any token other than a parenthesis, a bracket, or a brace
Constraints
2 The identifier in a standard attribute shall be one of:
deprecated maybe_unused noreturn unsequenced
fallthrough nodiscard _Noreturn reproducible
Semantics
3 An attribute specifier that contains no attributes has no effect. The order in which attribute tokens
appear in an attribute list is not significant. If a keyword (6.4.1) that satisfies the syntactic require-
ments of an identifier (6.4.2) is contained in an attribute token, it is considered an identifier. A strictly
conforming program using a standard attribute remains strictly conforming in the absence of that
attribute. 187)
4 NOTE For each standard attribute, the form of the balanced token sequence, if any, will be specified.
Recommended Practice
5 Each implementation should choose a distinctive name for the attribute prefix in an attribute
prefixed token. Implementations should not define attributes without an attribute prefix unless it is
a standard attribute as specified in this document.
6 EXAMPLE 1 Suppose that an implementation chooses the attribute prefix hal and provides specific attributes named daisy
and rosie.
[[deprecated, hal::daisy]] double nine1000(double);
[[deprecated]] [[hal::daisy]] double nine1000(double);
[[deprecated]] double nine1000 [[hal::daisy]] (double);
Then all the following declarations should be equivalent aside from the spelling:
[[__deprecated__, __hal__::__daisy__]] double nine1000(double);
[[__deprecated__]] [[__hal__::__daisy__]] double nine1000(double);
[[__deprecated__]] double nine1000 [[__hal__::__daisy__]] (double);
These use the alternate spelling that is required for all standard attributes and recommended for prefixed attributes. These
may be better-suited for use in header files, where the use of the alternate spelling avoids naming conflicts with user-provided
macros.
7 EXAMPLE 2 For the same implementation, the following two declarations are equivalent, because the ordering inside
attribute lists is not important.
[[hal::daisy, hal::rosie]] double nine999(double);
[[hal::rosie, hal::daisy]] double nine999(double);
On the other hand the following two declarations are not equivalent, because the ordering of different attribute specifiers
may affect the semantics.
[[hal::daisy]] [[hal::rosie]] double nine999(double);
[[hal::rosie]] [[hal::daisy]] double nine999(double); // may have different semantics
187) Standard attributes specified by this document can be parsed but ignored by an implementation without changing the
semantics of a correct program; the same is not true for attributes not specified by this document.
6.7.12.2 The nodiscard attribute
Constraints
1 The nodiscard attribute shall be applied to the identifier in a function declaration or to the definition
of a structure, union, or enumeration type. If an attribute argument clause is present, it shall have
the form:
( string-literal )
Semantics
2 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 202003L
when given nodiscard as the pp-tokens operand.
3 A name or entity declared without the nodiscard attribute can later be redeclared with the attribute
and vice versa. An entity is considered marked after the first declaration that marks it.
Recommended Practice
4 A nodiscard call is a function call expression that calls a function previously declared with attribute
nodiscard, or whose return type is a structure, union, or enumeration type marked with attribute
nodiscard. Evaluation of a nodiscard call as a void expression (6.8.3) is discouraged unless explicitly
cast to void. Implementations are encouraged to issue a diagnostic in such cases. This is typically
because immediately discarding the return value of a nodiscard call has surprising consequences.
5 The diagnostic message should include text provided by the string literal within the attribute
argument clause of any nodiscard attribute applied to the name or entity.
6 EXAMPLE 1
struct [[nodiscard]] error_info { /*...*/ };
struct error_info enable_missile_safety_mode(void);
void launch_missiles(void);
void test_missiles(void) {
enable_missile_safety_mode();
launch_missiles();
}
A diagnostic for the call to enable_missile_safety_mode is encouraged.
7 EXAMPLE 2
[[nodiscard]] int important_func(void);
void call(void) {
int i = important_func();
}
No diagnostic for the call to important_func is encouraged despite the value of i not being used.
8 EXAMPLE 3
[[nodiscard("must check armed state")]]
bool arm_detonator(int);
void call(void) {
arm_detonator(3);
detonate();
}
A diagnostic for the call toarm_detonator using the string literal "must check armed state" from the attribute argument
clause is encouraged.
6.7.12.3 The maybe_unused attribute
Constraints
1 The maybe_unused attribute shall be applied to the declaration of a structure, a union, a typedef
name, a variable, a structure or union member, a function, an enumeration, an enumerator, or a label.
No attribute argument clause shall be present.
Semantics
2 The maybe_unused attribute indicates that a name or entity is possibly intentionally unused.
3 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 202106L
when given maybe_unused as the pp-tokens operand.
A name or entity declared without the maybe_unused attribute can later be redeclared with the
attribute and vice versa. An entity is considered marked with the attribute after the first declaration
that marks it.
Recommended Practice
4 For an entity marked maybe_unused, implementations are encouraged not to emit a diagnostic that
the entity is unused, or that the entity is used despite the presence of the attribute.
5 EXAMPLE
[[maybe_unused]] void f([[maybe_unused]] int i) {
[[maybe_unused]] int j = i + 100;
assert(j);
}
Implementations are encouraged not to diagnose that j is unused, whether or not NDEBUG is defined.
6.7.12.4 The deprecated attribute
Constraints
1 The deprecated attribute shall be applied to the declaration of a structure, a union, a typedef name,
a variable, a structure or union member, a function, an enumeration, or an enumerator.
2 If an attribute argument clause is present, it shall have the form:
( string-literal )
Semantics
3 The deprecated attribute can be used to mark names and entities whose use is still allowed, but is
discouraged for some reason. 188)
4 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 201904L
when given deprecated as the pp-tokens operand.
5 A name or entity declared without the deprecated attribute can later be redeclared with the attribute
and vice versa. An entity is considered marked with the attribute after the first declaration that
marks it.
Recommended Practice
6 Implementations should use the deprecated attribute to produce a diagnostic message in case the
program refers to a name or entity other than to declare it, after a declaration that specifies the
attribute, when the reference to the name or entity is not within the context of a related deprecated
entity. The diagnostic message should include text provided by the string literal within the attribute
argument clause of any deprecated attribute applied to the name or entity.
7 EXAMPLE
struct [[deprecated]] S {
int a;
};
enum [[deprecated]] E1 {
one
};
188) In particular, deprecated is appropriate for names and entities that are obsolescent, insecure, unsafe, or otherwise unfit
for purpose.
enum E2 {
two [[deprecated("use ’three’ instead")]],
three
};
[[deprecated]] typedef int Foo;
void f1(struct S s) { // Diagnose use of S
int i = one; // Diagnose use of E1
int j = two; // Diagnose use of two: "use ’three’ instead"
int k = three;
Foo f; // Diagnose use of Foo
}
[[deprecated]] void f2(struct S s) {
int i = one;
int j = two;
int k = three;
Foo f;
}
struct [[deprecated]] T {
Foo f;
struct S s;
};
Implementations are encouraged to diagnose the use of deprecated entities within a context which is not itself deprecated, as
indicated for function f1, but not to diagnose within function f2 and struct T, as they are themselves deprecated.
6.7.12.5 The fallthrough attribute
Constraints
1 The attribute token fallthrough shall only appear in an attribute declaration (6.7); such a declara-
tion is a fallthrough declaration. No attribute argument clause shall be present. A fallthrough decla-
ration may only appear within an enclosing switch statement (6.8.4.2). The next block item(6.8.2)
that would be encountered after a fallthrough declaration shall be a case label or default label
associated with the smallest enclosing switch statement.
Semantics
2 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 201904L
when given fallthrough as the pp-tokens operand.
Recommended Practice
3 The use of a fallthrough declaration is intended to suppress a diagnostic that an implementation
might otherwise issue for a case or default label that is reachable from another case or default
label along some path of execution. Implementations are encouraged to issue a diagnostic if a
fallthrough declaration is not dynamically reachable.
4 EXAMPLE
void f(int n) {
void g(void), h(void), i(void);
switch (n) {
case 1: /* diagnostic on fallthrough discouraged */
case 2:
g();
[[fallthrough]];
case 3: /* diagnostic on fallthrough discouraged */
h();
case 4: /* fallthrough diagnostic encouraged */
i();
[[fallthrough]]; /* constraint violation */
}
}
6.7.12.6 The noreturn and _Noreturn attributes
Description
1 When _Noreturn is used as an attribute token (instead of a function specifier), the constraints and
semantics are identical to that of the noreturn attribute token. Use of _Noreturn as an attribute
token is an obsolescent feature189) .
Constraints
2 The noreturn attribute shall be applied to the identifier in a function declaration. No attribute
argument clause shall be present.
Semantics
3 The first declaration of a function shall specify the noreturn attribute if any declaration of that
function specifies the noreturn attribute. If a function is declared with the noreturn attribute in
one translation unit and the same function is declared without the noreturn attribute in another
translation unit, the behavior is undefined.
4 If a function f is called where f was previously declared with the noreturn attribute and f eventually
returns, the behavior is undefined.
5 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 202202L
when given noreturn as the pp-tokens operand.
Recommended Practice
6 The implementation should produce a diagnostic message for a function declared with a noreturn
attribute that appears to be capable of returning to its caller.
7 EXAMPLE
[[noreturn]] void f(void) {
abort(); // ok
}
[[noreturn]] void g(int i) { // causes undefined behavior if i <= 0
if (i > 0) abort();
}
[[noreturn]] int h(void);
Implementations are encouraged to diagnose the definition of g() because it is capable of returning to its caller. Implementa-
tions are similarly encouraged to diagnose the declaration of h() because it appears capable of returning to its caller due to
the non-void return type.
6.7.12.7 Standard attributes for function types
Constraints
1 The identifier in a standard function type attribute shall be one of:
unsequenced reproducible
2 An attribute for a function type shall be applied to a function declarator190) or to a type specifier that
has a function type. The corresponding attribute is a property of the referred function type191) . No
attribute argument clause shall be present.
189) [[_Noreturn]] and [[noreturn]] are equivalent attributes to support code that includes <stdnoreturn.h>, because
that header defines noreturn as a macro that expands to _Noreturn .
190) That is, they appear in the attributes right after the closing parenthesis of the parameter list, independently if the function
type is, for example, used directly to declare a function or if it is used in a pointer to function type.
191) If several declarations of the same function or function pointer are visible, regardless whether an attribute is present
Description
3 The main purpose of the function type properties and attributes defined in this clause is to provide
the translator with information about the access of objects by a function such that certain properties
of function calls can be deduced; the properties distinguish read operations (stateless and inde-
pendent) and write operations (effectless, idempotent and reproducible) or a combination of both
(unsequenced). Although semantically attached to a function type, the attributes described are not
part of the prototype of a such annotated function, and redeclarations and conversions that drop
such an attribute are valid and constitute compatible types. Conversely, if a definition that does not
have the asserted property is accessed by a function declaration or a function pointer with a type
that has the attribute, the behavior is undefined192) .
4 To allow reordering of calls to functions as they are described here, possible access to objects with a
lifetime that starts before or ends after a call has to be restricted; effects on all objects that are accessed
during a function call are restricted to the same thread as the call and the based-on relation between
pointer parameters and lvalues (6.7.3.1) models the fact that objects do not change inadvertently
during the call. In the following, an operation is said to be sequenced during a function call if it is
sequenced after the start of the function call193) and before the call terminates. An object definition
of an object X in a function f escapes if an access to X happens while no call to f is active. An
object is local to a call to a function f if its lifetime starts and ends during the call or if it is defined
by f but does not escape. A function call and an object X synchronize if all accesses to X that are
not sequenced during the call happen before or after the call. Execution state that is described in
the library clause, such as the floating-point environment, conversion state, locale, input/output
streams, external files or errno account as objects; operations that allow to query this state, even
indirectly, account as lvalue conversions, and operations that allow to change this state account as
store operations.
5 A function definition f is stateless if any definition of an object of static or thread storage duration in
f or in a function that is called by f is const but not volatile qualified.
6 An object X is observed by a function call if both synchronize, if X is not local to the call, if X has a
lifetime that starts before the function call and if an access of X is sequenced during the call; the last
value of X, if any, that is stored before the call is said to be the value of X that is observed by the
call. A function pointer value f is independent if for any object X that is observed by some call to f
through an lvalue that is not based on a parameter of the call, then all accesses to X in all calls to
f during the same program execution observe the same value; otherwise if the access is based on
a pointer parameter, there shall be a unique such pointer parameter P such that any access to X
shall be to an lvalue that is based on P . A function definition is independent if the derived function
pointer value is independent.
7 A store operation to an object X that is sequenced during a function call such that both synchronize
is said to be observable if X is not local to the call, if the lifetime of X ends after the call, if the stored
value is different from the value observed by the call, if any, and if it is the last value written before
the termination of the call. An evaluation of a function call194) is effectless if any store operation
that is sequenced during the call is the modification of an object that synchronizes with the call; if
additionally the operation is observable, there shall be a unique pointer parameter P of the function
such that any access to X shall be to an lvalue that is based on P . A function pointer value f is
effectless if any evaluation of a function call that calls f is effectless. A function definition is effectless
if the derived function pointer value is effectless.
8 An evaluation E is idempotent if a second evaluation of E can be sequenced immediately after the
original one without changing the resulting value, if any, or the observable state of the execution.
at several or just one of the declarators, it is attached to the type of the corresponding function definition, function pointer
object, or function pointer value.
192) That is, the fact that a function has one of these properties is in general not determined by the specification of the
translation unit in which it is found; other translation units and specific run time conditions also condition the possible
assertion of the properties.
193) The initializations of the parameters is sequenced during the function call.
194) This considers the evaluation of the function call itself, not the evaluation of a full function call expression. Such an
evaluation is sequenced after all evaluations that determine f and the call arguments, if any, have been performed.
A function pointer value f is idempotent if any evaluation of a function call195) that calls f is
idempotent. A function definition is idempotent if the derived function pointer value is idempotent.
9 A function is reproducible if it is effectless and idempotent; it is unsequenced if it is stateless, effectless,
idempotent and independent196) .
10 NOTE The synchronization requirements with respect to any accessed object X for the independence of functions provide
boundaries up to which a function call may safely be reordered without changing the semantics of the program. If X is
const but not volatile qualified the reordering is unconstrained. If it is an object that is conditioned in an initialization
phase, for a single threaded program a synchronization is provided by the sequenced before relation and the reordering
may, in principle, move the call just after the initialization. For a multi-threaded program, synchronization guarantees can be
given by calls to synchronizing functions of the <threads.h> header or by an appropriate call to atomic_thread_fence at
the end of the initialization phase. If a function is known to be independent or effectless, adding restrict qualifications to
the declarations of all pointer parameters does not change the semantics of any call. Similarly, changing the memory order to
memory_order_relaxed for all atomic operations during a call to such a function preserves semantics.
11 NOTE In general the functions provided by the <math.h> header do not have the properties that are defined above; many
of them change the floating-point state or errno when they encounter an error (so they have observable side effects) and the
results of most of them depend on execution wide state such as the rounding direction mode (so they are not independent).
Whether a particular C library function is reproducible or unsequenced additionally often depends on properties of the
implementation, such as implementation-defined behavior for certain error conditions.
Recommended Practice
12 If possible, it is recommended that implementations diagnose if an attribute of this clause is applied
to a function definition that does not have the corresponding property. It is recommended that appli-
cations that assert the independent or effectless properties for functions qualify pointer parameters
with restrict.
Forward references: errors <errno.h> (7.5), floating-point environment <fenv.h> (7.6), localiza-
tion <locale.h> (7.11), mathematics <math.h> (7.12), fences (7.17.4), input/output <stdio.h>
(7.23), threads <threads.h> (7.28), extended multibyte and wide character utilities <wchar.h>
(7.31).
6.7.12.7.1 The reproducible type attribute
Description
1 The reproducible type attribute asserts that a function or pointed-to function with that type is
reproducible.
2 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 202207L
when given reproducible as the pp-tokens operand.
3 EXAMPLE 1 The attribute in the following function declaration asserts that two consecutive calls to the function will result
in the same return value. Changes to the abstract state during the call are possible as long as they are not observable, but
no other side effects will occur. Thus the function definition may for example use local objects of static or thread storage
duration to keep track of the arguments for which the function has been called and cache their computed return values.
size_t hash(char const[static 32]) [[reproducible]];
6.7.12.7.2 The unsequenced type attribute
Description
1 The unsequenced type attribute asserts that a function or pointed-to function with that type is
unsequenced.
2 The __has_c_attribute conditional inclusion expression (6.10.1) shall return the value 202207L
when given unsequenced as the pp-tokens operand.
3 NOTE The unsequenced type attribute asserts strong properties for the such typed function, in particular that certain
sequencing requirements for function calls can be relaxed without affecting the state of the abstract machine. Thereby, calls
to such functions are natural candidates for optimization techniques such as common subexpression elimination, local
memoization or lazy evaluation.
195) This considers the evaluation of the function call itself, not the evaluation of a full function call expression. Such an
evaluated is sequenced after all evaluations that determine f and the call arguments, if any, have been performed.
196) A function call of an unsequenced function can be executed as early as the function pointer value, the values of the
arguments and all objects that are accessible through them, and all values of globally accessible state have been determined,
and it can be executed as late as the arguments and the objects they possibly target are unchanged and as any of its return
value or modified pointed-to arguments are accessed.
4 NOTE A proof of validity of the annotation of a function type with the unsequenced attribute may depend on the property
if a derived function pointer escapes the translation unit or not. For a function with internal linkage where no function
pointer escapes the translation unit, all calling contexts are known and it is possible, in principle, to prove that no control flow
exists such that a library function is called with arguments that trigger an exceptional condition. For a function with external
linkage such a proof may not be possible and the use of such a function then has to ensure that no exceptional condition
results from the provided arguments.
5 NOTE The unsequenced property does not necessarily imply that the function is reentrant or that calls can be executed
concurrently. This is because an unsequenced function can read from and write to objects of static storage duration, as long
as no change is observable after a call terminates.
6 EXAMPLE 1 The attribute in the following function declaration asserts that it doesn’t depend on any modifiable state of the
abstract machine. Calls to the function can be executed out of sequence before the return value is needed and two calls to the
function with the same argument value will result in the same return value.
bool tendency(signed char) [[unsequenced]];
Therefore such a call for a given argument value needs only to be executed once and the returned value can be reused when
appropriate. For example, calls for all possible argument values can be executed during program startup and tabulated.
7 EXAMPLE 2 The attribute in the following function declaration asserts that it doesn’t depend on any modifiable state of
the abstract machine. Within the same thread, calls to the function can be executed out of sequence before the return value
is needed and two calls to the function will result in the same pointer return value. Therefore such a call needs only to be
executed once in a given thread and the returned pointer value can be reused when appropriate. For example, a single
call can be executed during thread startup and the return value p and the value of the object *p of type toto const can be
cached.
typedef struct toto toto;
toto const* toto_zero(void) [[unsequenced]];
8 EXAMPLE 3 The unsequenced property of a function f can be locally asserted within a function g that uses it. For example
the library function sqrt is in generally not unsequenced because a negative argument will raise a domain error and because
the result may depend on the rounding mode. Nevertheless in contexts similar to the following function a user can prove
that it will not be called with invalid arguments, and, that the floating-point environment has the same value for all calls.
#include <math.h>
#include <fenv.h>
inline double distance (double const x[static 2]) [[reproducible]] {
#pragma FP_CONTRACT OFF
#pragma FENV_ROUND FE_TONEAREST
// We assert that sqrt will not be called with invalid arguments
// and the result only depends on the argument value.
extern typeof(sqrt) [[unsequenced]] sqrt;
return sqrt(x[0]*x[0] + x[1]*x[1]);
}
The function distance potentially has the side effect of changing the floating-point environment. Nevertheless the floating
environment is thread local, thus a change to that state outside the function is sequenced with the change within and
additionally the observed value is restored when the function returns. Thus this side effect is not observable for a caller.
Overall the function distance is stateless, effectless and idempotent and in particular it is reproducible as the attribute
indicates. Because the function can be called in a context where the floating-point environment has different state, distance
is not independent and thus it is also not unsequenced. Nevertheless, adding an unsequenced attribute where this is justified
may introduce optimization opportunities.
double g (double y[static 1], double const x[static 2]) {
// We assert that distance will not see different states of the floating
// point environment.
extern double distance (double const x[static 2]) [[unsequenced]];
y[0] = distance(x);
...
return distance(x); // replacement by y[0] is valid
}
6.8 Statements and blocks
Syntax
1 statement:
labeled-statement
unlabeled-statement
unlabeled-statement:
expression-statement
attribute-specifier-sequenceopt primary-block
attribute-specifier-sequenceopt jump-statement
primary-block:
compound-statement
selection-statement
iteration-statement
secondary-block:
statement
Semantics
2 A statement specifies an action to be performed. Except as indicated, statements are executed in
sequence. The optional attribute specifier sequence appertains to the respective statement.
3 A block is either a primary block, a secondary block, or the block associated with a function definition;
it allows a set of declarations and statements to be grouped into one syntactic unit. Whenever a
block B appears in the syntax production as part of the definition of an enclosing block A, scopes of
identifiers and lifetimes of objects that are associated with B do not extend to the parts of A that are
outside of B. The initializers of objects that have automatic storage duration, and the variable length
array declarators of ordinary identifiers with block scope, are evaluated and the values are stored in
the objects (the representation of objects without an initializer becomes indeterminate) each time the
declaration is reached in the order of execution, as if it were a statement, and within each declaration
in the order that declarators appear.
4 A full expression is an expression that is not part of another expression, nor part of a declarator
or abstract declarator. There is also an implicit full expression in which the non-constant size
expressions for a variably modified type are evaluated; within that full expression, the evaluation of
different size expressions are unsequenced with respect to one another. There is a sequence point
between the evaluation of a full expression and the evaluation of the next full expression to be
evaluated.
5 NOTE Each of the following is a full expression:
— a full declarator for a variably modified type,
— an initializer that is not part of a compound literal,
— the expression in an expression statement,
— the controlling expression of a selection statement (if or switch),
— the controlling expression of a while or do statement,
— each of the (optional) expressions of a for statement,
— the (optional) expression in a return statement.
While a constant expression satisfies the definition of a full expression, evaluating it does not depend on nor produce any
side effects, so the sequencing implications of being a full expression are not relevant to a constant expression.
Forward references: expression and null statements (6.8.3), selection statements (6.8.4), iteration
statements (6.8.5), the return statement (6.8.6.4).
6.8.1 Labeled statements
Syntax
1 label:
attribute-specifier-sequenceopt identifier :
attribute-specifier-sequenceopt case constant-expression :
attribute-specifier-sequenceopt default :
labeled-statement:
label statement
Constraints
2 A case or default label shall appear only in a switch statement. Further constraints on such labels
are discussed under the switch statement.
3 Label names shall be unique within a function.
Semantics
4 Any statement may be preceded by a prefix that declares an identifier as a label name. The optional
attribute specifier sequence appertains to the label. Labels in themselves do not alter the flow of
control, which continues unimpeded across them.
Forward references: the goto statement (6.8.6.1), the switch statement (6.8.4.2) .
6.8.2 Compound statement
Syntax
1 compound-statement:
{ block-item-listopt }
block-item-list:
block-item
block-item-list block-item
block-item:
declaration
unlabeled-statement
label
Semantics
2 A compound statement that is a function body together with the parameter type list and the optional
attribute specifier sequence between them forms the block associated with the function definition
in which it appears. Otherwise, it is a block that is different from any other block. A label shall be
translated as if it were followed by a null statement.
6.8.3 Expression and null statements
Syntax
1 expression-statement:
expressionopt ;
attribute-specifier-sequence expression ;
Semantics
2 The attribute specifier sequence appertains to the expression. The expression in an expression
statement is evaluated as a void expression for its side effects.197)
3 A null statement (consisting of just a semicolon) performs no operations.
4 EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the discarding of its value can
be made explicit by converting the expression to a void expression by means of a cast:
197) Such as assignments, and function calls which have side effects.
int p(int);
/* ... */
(void)p(0);
5 EXAMPLE 2 In the program fragment
char *s;
/* ... */
while (*s++ != ’\0’)
;
a null statement is used to supply an empty loop body to the iteration statement.
Forward references: iteration statements (6.8.5).
6.8.4 Selection statements
Syntax
1 selection-statement:
if ( expression ) secondary-block
if ( expression ) secondary-block else secondary-block
switch ( expression ) secondary-block
Semantics
2 A selection statement selects among a set of secondary blocks depending on the value of a controlling
expression.
6.8.4.1 The if statement
Constraints
1 The controlling expression of an if statement shall have scalar type.
Semantics
2 In both forms, the first substatement is executed if the expression compares unequal to 0. In the
else form, the second substatement is executed if the expression compares equal to 0. If the first
substatement is reached via a label, the second substatement is not executed.
3 An else is associated with the lexically nearest preceding if that is allowed by the syntax.
6.8.4.2 The switch statement
Constraints
1 The controlling expression of a switch statement shall have integer type.
2 If a switch statement has an associated case or default label within the scope of an identifier with
a variably modified type, the entire switch statement shall be within the scope of that identifier.198)
3 The expression of each case label shall be an integer constant expression and no two of the case
constant expressions in the same switch statement shall have the same value after conversion.
There may be at most one default label in a switch statement. (Any enclosed switch statement
may have a default label or case constant expressions with values that duplicate case constant
expressions in the enclosing switch statement.)
Semantics
4 A switch statement causes control to jump to, into, or past the statement that is the switch body,
depending on the value of a controlling expression, and on the presence of a default label and the
values of any case labels on or in the switch body. A case or default label is accessible only within
the closest enclosing switch statement.
5 The integer promotions are performed on the controlling expression. The constant expression in
198) That is, the declaration either precedes the switch statement, or it follows the last case or default label associated with
the switch that is in the block containing the declaration.
each case label is converted to the promoted type of the controlling expression. If a converted value
matches that of the promoted controlling expression, control jumps to the statement following the
matched case label. Otherwise, if there is a default label, control jumps to the statement following
the default label. If no converted case constant expression matches and there is no default label,
no part of the switch body is executed.
Implementation limits
6 As discussed in 5.2.4.1, the implementation may limit the number of case values in a switch
statement.
7 EXAMPLE In the artificial program fragment
switch (expr)
{
int i = 4;
f(i);
case 0:
i = 17;
/* falls through into default code */
default:
printf("%d\n", i);
}
the object whose identifier is i exists with automatic storage duration (within the block) but is never initialized, and thus if
the controlling expression has a nonzero value, the call to the printf function will access an object with an indeterminate
representation. Similarly, the call to the function f cannot be reached.
6.8.5 Iteration statements
Syntax
1 iteration-statement:
while ( expression ) secondary-block
do secondary-block while ( expression ) ;
for ( expressionopt ; expressionopt ; expressionopt ) secondary-block
for ( declaration expressionopt ; expressionopt ) secondary-block
Constraints
2 The controlling expression of an iteration statement shall have scalar type.
3 The declaration part of a for statement shall only declare identifiers for objects having storage class
auto or register.
Semantics
4 An iteration statement causes a secondary block called the loop body to be executed repeatedly until
the controlling expression compares equal to 0. The repetition occurs regardless of whether the loop
body is entered from the iteration statement or by a jump199) .
5 An iteration statement may be assumed by the implementation to terminate if its controlling
expression is not a constant expression200) , and none of the following operations are performed in its
body, controlling expression or (in the case of a for statement) its expression-3201) :
— input/output operations
— accessing a volatile object
— synchronization or atomic operations.
6.8.5.1 The while statement
1 The evaluation of the controlling expression takes place before each execution of the loop body.
6.8.5.2 The do statement
1 The evaluation of the controlling expression takes place after each execution of the loop body.
6.8.5.3 The for statement
1 The statement
199) Code jumped over is not executed. In particular, the controlling expression of a for or while statement is not evaluated
before entering the loop body, nor is clause-1 of a for statement.
200) An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.
201) This is intended to allow compiler transformations such as removal of empty loops even when termination cannot be
proven.
for (clause-1; expression-2; expression-3) statement
behaves as follows: The expression expression-2 is the controlling expression that is evaluated before
each execution of the loop body. The expression expression-3 is evaluated as a void expression after
each execution of the loop body. If clause-1 is a declaration, the scope of any identifiers it declares
is the remainder of the declaration and the entire loop, including the other two expressions; it is
reached in the order of execution before the first evaluation of the controlling expression. If clause-1
is an expression, it is evaluated as a void expression before the first evaluation of the controlling
expression.202)
2 Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero
constant.
6.8.6 Jump statements
Syntax
1 jump-statement:
goto identifier ;
continue ;
break ;
return expressionopt ;
Semantics
2 A jump statement causes an unconditional jump to another place.
6.8.6.1 The goto statement
Constraints
1 The identifier in a goto statement shall name a label located somewhere in the enclosing function. A
goto statement shall not jump from outside the scope of an identifier having a variably modified
type to inside the scope of that identifier.
Semantics
2 A goto statement causes an unconditional jump to the statement prefixed by the named label in the
enclosing function.
3 EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The following outline
presents one possible approach to a problem based on these three assumptions:
1. The general initialization code accesses objects only visible to the current function.
2. The general initialization code is too large to warrant duplication.
3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by continue statements,
for example.)
/* ... */
goto first_time;
for (;;) {
// determine next operation
/* ... */
if (need to reinitialize) {
// reinitialize-only code
/* ... */
first_time:
// general initialization code
/* ... */
continue;
}
202) Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in the loop; the
controlling expression, expression-2, specifies an evaluation made before each iteration, such that execution of the loop
continues until the expression compares equal to 0; and expression-3 specifies an operation (such as incrementing) that is
performed after each iteration.
// handle other operations
/* ... */
}
4 EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably modified types. A jump
within the scope, however, is permitted.
goto lab3; // invalid: going INTO scope of VLA.
{
double a[n];
a[j] = 4.4;
lab3:
a[j] = 3.3;
goto lab4; // valid: going WITHIN scope of VLA.
a[j] = 5.5;
lab4:
a[j] = 6.6;
}
goto lab4; // invalid: going INTO scope of VLA.
6.8.6.2 The continue statement
Constraints
1 A continue statement shall appear only in or as a loop body.
Semantics
2 A continue statement causes a jump to the loop-continuation portion of the smallest enclosing
iteration statement; that is, to the end of the loop body. More precisely, in each of the statements
while (/* ... */) { do { for (/* ... */) {
/* ... */ /* ... */ /* ... */
continue; continue; continue;
/* ... */ /* ... */ /* ... */
contin: contin: contin:
} } while (/* ... */); }
unless the continue statement shown is in an enclosed iteration statement (in which case it is
interpreted within that statement), it is equivalent to goto contin;.203)
6.8.6.3 The break statement
Constraints
1 A break statement shall appear only in or as a switch body or loop body.
Semantics
2 A break statement terminates execution of the smallest enclosing switch or iteration statement.
6.8.6.4 The return statement
Constraints
1 A return statement with an expression shall not appear in a function whose return type is void. A
return statement without an expression shall only appear in a function whose return type is void .
Semantics
2 A return statement terminates execution of the current function and returns control to its caller. A
function may have any number of return statements.
203) Following the contin: label in the 2nd example is a null statement. The null statement in the first and third example is
implied by the label (6.8.2).
3 If a return statement with an expression is executed, the value of the expression is returned to the
caller as the value of the function call expression. If the expression has a type different from the
return type of the function in which it appears, the value is converted as if by assignment to an
object having the return type of the function.204)
4 EXAMPLE In:
struct s { double i; } f(void);
union {
struct {
int f1;
struct s f2;
} u1;
struct {
struct s f3;
int f4;
} u2;
} g;
struct s f(void)
{
return g.u1.f2;
}
/* ... */
g.u2.f3 = f();
there is no undefined behavior, although there would be if the assignment were done directly (without using a function call
to fetch the value).
204) The return statement is not an assignment. The overlap restriction of 6.5.16.1 does not apply to the case of function
return. The representation of floating-point values can have wider range or precision than implied by the type; a cast can be
used to remove this extra range and precision.
6.9 External definitions
Syntax
1 translation-unit:
external-declaration
translation-unit external-declaration
external-declaration:
function-definition
declaration
Constraints
2 The storage-class specifier register shall not appear in the declaration specifiers in an external
declaration.
3 There shall be no more than one external definition for each identifier declared with internal linkage
in a translation unit. Moreover, if an identifier declared with internal linkage is used in an expression
there shall be exactly one external definition for the identifier in the translation unit, unless it is:
— part of the operand of a sizeof operator whose result is an integer constant;
— part of the operand of an alignof operator whose result is an integer constant;
— or, part of the operand of any typeof operator whose result is not a variably modified type.
Semantics
4 As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which
consists of a sequence of external declarations. These are described as "external" because they
appear outside any function (and hence have file scope). As discussed in 6.7, a declaration that also
causes storage to be reserved for an object or a function named by the identifier is a definition.
5 An external definition is an external declaration that is also a definition of a function (other than an
inline definition) or an object. If an identifier declared with external linkage is used in an expression
(other than as part of the operand of a typeof operator whose result is not a variably modified type,
or a sizeof or alignof operator whose result is an integer constant expression), somewhere in the
entire program there shall be exactly one external definition for the identifier; otherwise, there shall
be no more than one205) .
6.9.1 Function definitions
Syntax
1 function-definition:
attribute-specifier-sequenceopt declaration-specifiers declarator function-body
function-body:
compound-statement
Constraints
2 The identifier declared in a function definition (which is the name of the function) shall have a
function type, as specified by the declarator portion of the function definition.
3 The return type of a function shall be void or a complete object type other than array type.
4 The storage-class specifier, if any, in the declaration specifiers shall be either extern or static.
205) Thus, if an identifier declared with external linkage is not used in an expression, there need be no external definition for
it.
5 If the parameter list consists of a single parameter of type void, the parameter declarator shall not
include an identifier.
Semantics
6 The optional attribute specifier sequence in a function definition appertains to the function.
7 The declarator in a function definition specifies the name of the function being defined and the
types (and optionally the names) of all the parameters; the declarator also serves as a function
prototype for later calls to the same function in the same translation unit. The type of each parameter
is adjusted as described in 6.7.6.3.
8 If a function that accepts a variable number of arguments is defined without a parameter type list
that ends with the ellipsis notation, the behavior is undefined.
9 The parameter type list, the attribute specifier sequence of the declarator that follows the parameter
type list, and the compound statement of the function body form a single block206) . Each parameter
has automatic storage duration; its identifier, if any207) , is an lvalue208) . The layout of the storage for
parameters is unspecified.
10 On entry to the function, the size expressions of each variably modified parameter are evaluated
and the value of each argument expression is converted to the type of the corresponding parameter
as if by assignment. (Array expressions and function designators as arguments were converted to
pointers before the call.)
11 After all parameters have been assigned, the compound statement of the function body is executed.
12 Unless otherwise specified, if the } that terminates the function body is reached, and the value of the
function call is used by the caller, the behavior is undefined.
13 NOTE In a function definition, the type of the function and its prototype cannot be inherited from a typedef:
typedef int F(void); // type F is "function with no parameters
// returning int"
F f, g; // f and g both have type compatible with F
F f { /* ... */ } // WRONG: syntax/constraint error
F g() { /* ... */ } // WRONG: declares that g returns a function
int f(void) { /* ... */ } // RIGHT: f has type compatible with F
int g() { /* ... */ } // RIGHT: g has type compatible with F
F *e(void) { /* ... */ } // e returns a pointer to a function
F *((e))(void) { /* ... */ } // same: parentheses irrelevant
int (*fp)(void); // fp points to a function that has type F
F *Fp; // Fp points to a function that has type F
14 EXAMPLE 1 In the following:
extern int max(int a, int b)
{
return a > b ? a: b;
}
extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the function declarator; and
{ return a > b ? a: b; }
is the function body.
15 EXAMPLE 2 To pass one function to another, one might say
int f(void);
206) The visibility scope of a parameter in a function definition starts when its declaration is completed, extends to following
parameter declarations, to possible attributes that follow the parameter type list, and then to the entire function body. The
lifetime of each instance of a parameter starts when the declaration is evaluated starting a call and ends when that call
terminates.
207) A parameter that has no declared name is inaccessible within the function body.
208) A parameter identifier cannot be redeclared in the function body except in an enclosed block.
/* ... */
g(f);
Then the definition of g might read
void g(int (*funcp)(void))
{
/* ... */
(*funcp)(); /* or funcp(); ...*/
}
or, equivalently,
void g(int func(void))
{
/* ... */
func(); /* or (*func)(); ...*/
}
6.9.2 External object definitions
Semantics
1 If the declaration of an identifier for an object has file scope and an initializer, the declaration is an
external definition for the identifier.
2 A declaration of an identifier for an object that has file scope without an initializer, and without a
storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a
translation unit contains one or more tentative definitions for an identifier, and the translation unit
contains no external definition for that identifier, then the behavior is exactly as if the translation
unit contains a file scope declaration of that identifier, with the composite type as of the end of the
translation unit, with an initializer equal to { 0 } .
3 If the declaration of an identifier for an object is a tentative definition and has internal linkage, the
declared type shall not be an incomplete type.
4 EXAMPLE 1
int i1 = 1; // definition, external linkage
static int i2 = 2; // definition, internal linkage
extern int i3 = 3; // definition, external linkage
int i4; // tentative definition, external linkage
static int i5; // tentative definition, internal linkage
int i1; // valid tentative definition, refers to previous
int i2; // 6.2.2 renders undefined, linkage disagreement
int i3; // valid tentative definition, refers to previous
int i4; // valid tentative definition, refers to previous
int i5; // 6.2.2 renders undefined, linkage disagreement
extern int i1; // refers to previous, whose linkage is external
extern int i2; // refers to previous, whose linkage is internal
extern int i3; // refers to previous, whose linkage is external
extern int i4; // refers to previous, whose linkage is external
extern int i5; // refers to previous, whose linkage is internal
5 EXAMPLE 2 If at the end of the translation unit containing
int i[];
the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to zero on program
startup.
6.10 Preprocessing directives
Syntax
1 preprocessing-file:
groupopt
group:
group-part
group group-part
group-part:
if-section
control-line
text-line
# non-directive
if-section:
if-group elif-groupsopt else-groupopt endif-line
if-group:
# if constant-expression new-line groupopt
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt
elif-groups:
elif-group
elif-groups elif-group
elif-group:
# elif constant-expression new-line groupopt
# elifdef identifier new-line groupopt
# elifndef identifier new-line groupopt
else-group:
# else new-line groupopt
endif-line:
# endif new-line
control-line:
# include pp-tokens new-line
# embed pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-listopt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
# undef identifier new-line
# line pp-tokens new-line
# error pp-tokensopt new-line
# warning pp-tokensopt new-line
# pragma pp-tokensopt new-line
# new-line
text-line:
pp-tokensopt new-line
non-directive:
pp-tokens new-line
lparen:
a ( character not immediately preceded by white space
replacement-list:
pp-tokensopt
pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
new-line:
the new-line character
identifier-list:
identifier
identifier-list , identifier
pp-parameter:
pp-parameter-name pp-parameter-clauseopt
pp-parameter-name:
pp-standard-parameter
pp-prefixed-parameter
pp-standard-parameter:
identifier
pp-prefixed-parameter:
identifier :: identifier
pp-parameter-clause:
( pp-balanced-token-sequenceopt )
pp-balanced-token-sequence:
pp-balanced-token
pp-balanced-token-sequence pp-balanced-token
pp-balanced-token:
( pp-balanced-token-sequenceopt )
[ pp-balanced-token-sequenceopt ]
{ pp-balanced-token-sequenceopt }
any pp-token other than a parenthesis, a bracket, or a brace
embed-parameter-sequence:
pp-parameter
embed-parameter-sequence pp-parameter
Description
2 A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following
constraints: The first token in the sequence is a # preprocessing token that (at the start of translation
phase 4) is either the first character in the source file (optionally after white space containing no
new-line characters) or that follows white space containing at least one new-line character. The last
token in the sequence is the first new-line character that follows the first token in the sequence. 209)
209) Thus, preprocessing directives are commonly called "lines". These "lines" have no other syntactic significance, as all
white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator
A new-line character ends the preprocessing directive even if it occurs within what would otherwise
be an invocation of a function-like macro.
3 A text line shall not begin with a # preprocessing token. A non-directive shall not begin with any of
the directive names appearing in the syntax.
4 Some preprocessing directives take additional information by the use of preprocessor parameters.
A preprocessing parameter (pp-parameter) shall be either a preprocessor prefixed parameter (identified
by a pp-prefixed-parameter, for implementation-defined preprocessor parameters) or a preprocessor
standard parameter (identified with a pp-standard-parameter, for pp-parameters specified by this
document).
5 In all aspects, a preprocessor standard parameter specified by this document as an identifier pp_param
and an identifier of the form __pp_param__ shall behave the same when used as a preprocessor
parameter, except for the spelling.
6 EXAMPLE 1 Thus, the preprocessor parameters on the two binary resource inclusion directives (6.10.3.1):
#embed "boop.h" limit(5)
#embed "boop.h" __limit__(5)
behave the same, and can be freely interchanged. Implementations are encouraged to behave similarly for preprocessor
parameters (including preprocessor prefixed parameters) they provide.
7 When in a group that is skipped (6.10.1), the directive syntax is relaxed to allow any sequence of
preprocessing tokens to occur between the directive name and the following new-line character.
Constraints
8 The only white-space characters that shall appear between preprocessing tokens within a prepro-
cessing directive (from just after the introducing # preprocessing token through just before the
terminating new-line character) are space and horizontal-tab (including spaces that have replaced
comments or possibly other white-space characters in translation phase 3).
9 A preprocessor parameter shall be either a preprocessor standard parameter, or an implementation-
defined preprocessor prefixed parameter210) .
Semantics
10 The implementation can process and skip sections of source files conditionally, include other source
files, and replace macros. These capabilities are called preprocessing, because conceptually they occur
before translation of the resulting translation unit.
11 The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless
otherwise stated.
12 EXAMPLE In:
#define EMPTY
EMPTY # include <file.h>
the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not begin with a # at
the start of translation phase 4, even though it will do so after the macro EMPTY has been replaced.
13 The execution of a non-directive preprocessing directive results in undefined behavior.
6.10.1 Conditional inclusion
Syntax
1 defined-macro-expression:
defined identifier
defined ( identifier )
h-preprocessing-token:
any preprocessing-token other than >
in 6.10.4.2, for example).
210) An unrecognized preprocessor prefixed parameter is a constraint violation, except within has_embed expressions (6.10.1).
h-pp-tokens:
h-preprocessing-token
h-pp-tokens h-preprocessing-token
header-name-tokens:
string-literal
< h-pp-tokens >
has-include-expression:
__has_include ( header-name )
__has_include ( header-name-tokens )
has-embed-expression:
__has_embed ( header-name embed-parameter-sequenceopt )
__has_embed ( header-name-tokens pp-balanced-token-sequenceopt )
has-c-attribute-express:
__has_c_attribute ( pp-tokens )
Constraints
2 The expression that controls conditional inclusion shall be an integer constant expression except that:
identifiers (including those lexically identical to keywords) are interpreted as described below211)
and it may contain zero or more defined macro expressions, has_include expressions, has_embed
expressions, and/or has_c_attribute expressions as unary operator expressions.
3 A defined macro expression evaluates to 1 if the identifier is currently defined as a macro name (that
is, if it is predefined or if it has been the subject of a #define preprocessing directive without an
intervening #undef directive with the same subject identifier), 0 if it is not.
4 The second form of the has_include expression and has_embed expression is considered only if the
first form does not match, in which case the preprocessing tokens are processed just as in normal
text.
5 The header or source file identified by the parenthesized preprocessing token sequence in each
contained has_include expression is searched for as if that preprocessing token were the pp-tokens
in a #include directive, except that no further macro expansion is performed. Such a directive shall
satisfy the syntactic requirements of a #include directive. The has_include expression evaluates to
1 if the search for the source file succeeds, and to 0 if the search fails.
6 The resource (6.10.3.1) identified by the header-name preprocessing token sequence in each contained
has_embed expression is searched for as if those preprocessing token were the pp-tokens in a #embed
directive, except that no further macro expansion is performed. Such a directive shall satisfy the
syntactic requirements of a #embed directive. The has_embed expression evaluates to:
— 0 if the search fails or if any of the embed parameters in the embed parameter sequence
specified are not supported by the implementation for the #embed directive; or,
— 1 if the search for the resource succeeds and all embed parameters in the embed parameter
sequence specified are supported by the implementation for the #embed directive and the
resource is not empty; or,
— 2 if the search for the resource succeeds and all embed parameters in the embed parameter
sequence specified are supported by the implementation for the #embed directive and the
resource is empty.
7 NOTE Unrecognized preprocessor prefixed parameters in has_embed expressions is not a constraint violation and instead
causes the expression to be evaluate to 0, as specified above.
8 Each has_c_attribute expression is replaced by a nonzero pp-number matching the form of an integer
constant if the implementation supports an attribute with the name specified by interpreting the
211) Because the controlling constant expression is evaluated during translation phase 4, all identifiers either are or are not
macro names — there simply are no keywords, enumeration constants, etc.
pp-tokens as an attribute token, and by 0 otherwise. The pp-tokens shall match the form of an
attribute token.
9 Each preprocessing token that remains (in the list of preprocessing tokens that will become the
controlling expression) after all macro replacements have occurred shall be in the lexical form of a
token (6.4).
Semantics
10 The #ifdef, #ifndef, #elifdef, and #elifndef, and the defined conditional inclusion operator,
shall treat __has_include and __has_c_attribute as if they were the name of defined macros.
The identifiers __has_include , __has_embed , and __has_c_attribute shall not appear in any
context not mentioned in this subclause.
11 Preprocessing directives of the forms
# if constant-expression new-line groupopt
# elif constant-expression new-line groupopt
check whether the controlling constant expression evaluates to nonzero.
12 Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the control-
ling constant expression are replaced (except for those macro names modified by the defined unary
operator), just as in normal text. If the token defined is generated as a result of this replacement
process or use of the defined unary operator does not match one of the two specified forms prior to
macro replacement, the behavior is undefined. After all replacements due to macro expansion and
evaluations of defined macro expressions, has_include expressions, and has_c_attribute expressions
have been performed, all remaining identifiers other than true (including those lexically identical
to keywords such as false) are replaced with the pp-number 0, true is replaced with pp-number
1 , and then each preprocessing token is converted into a token. The resulting tokens compose the
controlling constant expression which is evaluated according to the rules of 6.6. For the purposes of
this token conversion and evaluation, all signed integer types and all unsigned integer types act as
if they have the same representation as, respectively, the types intmax_t and uintmax_t defined
in the header <stdint.h>. 212) This includes interpreting character constants, which may involve
converting escape sequences into execution character set members. Whether the numeric value for
these character constants matches the value obtained when an identical character constant occurs in
an expression (other than within a #if or #elif directive) is implementation-defined213) .
Also, whether a single-character character constant may have a negative value is implementation-
defined.
13 Preprocessing directives of the forms
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt
# elifdef identifier new-line groupopt
# elifndef identifier new-line groupopt
check whether the identifier is or is not currently defined as a macro name. Their conditions
are equivalent to #if defined identifier, #if !defined identifier, #elif defined identifier, and
#elif !defined identifier respectively.
14 Each directive’s condition is checked in order. If it evaluates to false (zero), the group that it
controls is skipped: directives are processed only through the name that determines the directive
in order to keep track of the level of nested conditionals; the rest of the directives’ preprocessing
212) Thus, on an implementation where INT_MAX is 0x7FFF and UINT_MAX is 0xFFFF, the constant 0x8000 is signed and
positive within a #if expression even though it would be unsigned in translation phase 7.
213) Thus, the constant expression in the following #if directive and if statement is not guaranteed to evaluate to the same
value in these two contexts.
#if ’z’ - ’a’ == 25
if (’z’ - ’a’ == 25)
tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose
control condition evaluates to true (nonzero) is processed; any following groups are skipped and
their controlling directives are processed as if they were in a group that is skipped. If none of the
conditions evaluates to true, and there is a #else directive, the group controlled by the #else is
processed; lacking a #else directive, all the groups until the #endif are skipped. 214)
15 EXAMPLE This demonstrates a way to include a header file only if it is available.
#if __has_include(<optional.h>)
# include <optional.h>
# define have_optional 1
#elif __has_include(<experimental/optional.h>)
# include <experimental/optional.h>
# define have_optional 1
# define have_experimental_optional 1
#endif
#ifndef have_optional
# define have_optional 0
#endif
16 EXAMPLE
/* Fallback for compilers not yet implementing this feature. */
#ifndef __has_c_attribute
#define __has_c_attribute(x) 0
#endif /* __has_c_attribute */
#if __has_c_attribute(fallthrough)
/* Standard attribute is available, use it. */
#define FALLTHROUGH [[fallthrough]]
#elif __has_c_attribute(vendor::fallthrough)
/* Vendor attribute is available, use it. */
#define FALLTHROUGH [[vendor::fallthrough]]
#else
/* Fallback implementation. */
#define FALLTHROUGH
#endif
17 EXAMPLE
#ifdef __STDC__
#define TITLE "ISO C Compilation"
#elifndef __cplusplus
#define TITLE "Non-ISO C Compilation"
#else
/* C++ */
#define TITLE "C++ Compilation"
#endif
18 EXAMPLE 1 A combination of __FILE__ (6.10.9.1) and __has_embed could be used to check for support of specific implemen-
tation extensions for the #embed (6.10.3.1) directive’s parameters.
#if __has_embed(__FILE__ ext::token(0xB055))
#define DESCRIPTION "Supports extended token embed"
#else
#define DESCRIPTION "Does not support extended token embed"
#endif
19 EXAMPLE 2 The below snippet uses __has_embed to check for support of a specific implementation-defined embed
parameter, and otherwise uses standard behavior to produce the same effect.
214) As indicated by the syntax, no preprocessing tokens are allowed to follow a #else or #endif directive before the
terminating new-line character. However, comments can appear anywhere in a source file, including within a preprocessing
directive.
void parse_into_s(short* ptr, unsigned char* ptr_bytes, unsigned long long size);
int main () {
#if __has_embed ("bits.bin" ds9000::element_type(short))
/* Implementation extension: create short integers from the */
/* translation environment resource into */
/* a sequence of integer constants */
short meow[] = {
#embed "bits.bin" ds9000::element_type(short)
};
#elif __has_embed ("bits.bin")
/* no support for implementation-specific */
/* ds9000::element_type(short) parameter */
const unsigned char meow_bytes[] = {
#embed "bits.bin"
};
short meow[sizeof(meow_bytes) / sizeof(short)] = {};
/* parse meow_bytes into short values by-hand! */
parse_into_s(meow, meow_bytes, sizeof(meow_bytes));
#else
#error "cannot find bits.bin resource"
#endif
return (int)(meow[0] + meow[(sizeof(meow) / sizeof(*meow)) - 1]);
}
20 EXAMPLE 3 This resource is considered empty due to the limit(0) embed parameter, always, including in __has_embed
expressions.
int main () {
#if __has_embed(</owo/uwurandom> limit(0)) == 2
// if </owo/uwurandom> exits, this
// token sequence is always taken.
return 0;
#else
// the resource does not exist
#error "The resource does not exist"
#endif
}
Forward references: macro replacement (6.10.4), source file inclusion (6.10.2), mandatory macros
(6.10.9.1), largest integer types (7.22.1.5).
6.10.2 Source file inclusion
Constraints
1 A #include directive shall identify a header or source file that can be processed by the implementa-
tion.
Semantics
2 A preprocessing directive of the form
# include < h-char-sequence > new-line
searches a sequence of implementation-defined places for a header identified uniquely by the
specified sequence between the < and > delimiters, and causes the replacement of that directive
by the entire contents of the header. How the places are specified or the header identified is
implementation-defined.
3 A preprocessing directive of the form
# include " q-char-sequence " new-line
causes the replacement of that directive by the entire contents of the source file identified by
the specified sequence between the " delimiters. The named source file is searched for in an
implementation-defined manner. If this search is not supported, or if the search fails, the directive is
reprocessed as if it read
# include < h-char-sequence > new-line
with the identical contained sequence (including > characters, if any) from the original directive.
4 A preprocessing directive of the form
# include pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing tokens after
include in the directive are processed just as in normal text. (Each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after
all replacements shall match one of the two previous forms.215) The method by which a sequence
of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is
combined into a single header name preprocessing token is implementation-defined.
5 The implementation shall provide unique mappings for sequences consisting of one or more nondig-
its or digits (6.4.2.1) followed by a period (.) and a single nondigit. The first character shall not be a
digit. The implementation may ignore distinctions of alphabetical case and restrict the mapping to
eight significant characters before the period.
6 A #include preprocessing directive may appear in a source file that has been read because of a
#include directive in another file, up to an implementation-defined nesting limit (see 5.2.4.1).
7 EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following:
#include <stdio.h>
#include "myprog.h"
8 EXAMPLE 2 This illustrates macro-replaced #include directives:
#if VERSION == 1
#define INCFILE "vers1.h"
#elif VERSION == 2
#define INCFILE "vers2.h" // and so on
#else
#define INCFILE "versN.h"
#endif
#include INCFILE
Forward references: macro replacement (6.10.4).
6.10.3 Binary resource inclusion
6.10.3.1 #embed preprocessing directive
Description
1 A resource is a source of data accessible from the translation environment. An embed parameter is a
single preprocessor parameter in the embed parameter sequence. It has an implementation resource
width, which is the implementation-defined size in bits of the located resource. It also has a resource
width, which is either:
— the number of bits as computed from the optionally-provided limit embed parameter (??), if
present; or,
— the implementation resource width.
215) Note that adjacent string literals are not concatenated into a single string literal (see the translation phases in 5.1.1.2);
thus, an expansion that results in two string literals is an invalid directive.
2 An embed parameter sequence is a whitespace-delimited list of preprocessor parameters which may
modify the result of the replacement for the #embed preprocessing directive.
Constraints
3 An #embed directive shall identify a resource that can be processed by the implementation as a
binary data sequence given the provided embed parameters.
4 Embed parameters not specified in this document shall be implementation-defined. Implementation-
defined embed parameters may change the below-defined semantics of the directive; otherwise,
#embed directives which do not contain implementation-defined embed parameters shall behave as
described in this document.
5 A resource is considered empty when its resource width is zero.
6 Let embed element width be either:
— an integer constant expression greater than zero determined by an implementation-defined
embed parameter; or,
— CHAR_BIT (5.2.4.2.1).
The result of (resource width) % (embed element width) shall be zero216) .
Semantics
7 The expansion of a #embed directive is a token sequence formed from the list of integer constant
expressions described below. The group of tokens for each integer constant expression in the list
is separated in the token sequence from the group of tokens for the previous integer constant
expression in the list by a comma. The sequence neither begins nor ends in a comma. If the list of
integer constant expressions is empty, the token sequence is empty. The directive is replaced by its
expansion and, with the presence of certain embed parameters, additional or replacement token
sequences.
8 A preprocessing directive of the form
# embed < h-char-sequence > embed-parameter-sequenceopt new-line
searches a sequence of implementation-defined places for a resource identified uniquely by the spec-
ified sequence between the < and > . The search for the named resource is done in an implementation-
defined manner.
9 A preprocessing directive of the form
# embed " q-char-sequence " embed-parameter-sequenceopt new-line
searches a sequence of implementation-defined places for a resource identified uniquely by the
specified sequence between the " delimiters. The search for the named resource is done in an
implementation-defined manner. If this search is not supported, or if the search fails, the directive is
reprocessed as if it read
# embed < h-char-sequence > embed-parameter-sequenceopt new-line
with the identical contained q-char-sequence (including > characters, if any) from the original
directive.
10 Either form of the #embed directive specified previously behave as specified below. The values of the
integer constant expressions in the expanded sequence is determined by an implementation-defined
mapping of the resource’s data. Each integer constant expression’s value is in the range from 0 to
(2embed element width ) − 1, inclusive217) . If the list of integer constant expressions:
— is used to initialize an array of a type compatible with unsigned char, or compatible with
char if char cannot hold negative values; or,
216) This constraint helps ensure data is neither filled with padding values nor truncated in a given environment, and helps
ensure the data is portable with respect to usages of memcpy (7.26.2.1) with character type arrays initialized from the data.
217) For example, an embed element width of 8 will yield a range of values from 0 to 255, inclusive.
— the embed element width is equal to CHAR_BIT (??env-consider-characteristics-of-integer-
types-limits-h)),
then the contents of the initialized elements of the array are as-if the resource’s binary data is fread
(7.23.8.1) into the array at translation time.
11 A preprocessing directive of the form
# embed pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing tokens after
embed in the directive are processed just as in normal text. (Each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after
all replacements shall match one of the two previous forms218) . The method by which a sequence
of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is
combined into a single resource name preprocessing token is implementation-defined.
12 An embed parameter with a preprocessor parameter token that is one of the following is a standard
embed parameter:
limit prefix suffix if_empty
The significance of these standard embed parameters is specified below.
Recommended practice
13 The #embed directive is meant to translate binary data in resources to sequence of integer constant
expressions in a way that preserves the value of the resource’s bit stream where possible.
14 A mechanism similar to, but distinct from, the implementation-defined search paths used for source
file inclusion (6.10.2) is encouraged.
15 Implementations should take into account translation-time bit and byte orders as well as execution
time bit and byte orders to more appropriately represent the resource’s binary data from the directive.
This maximizes the chance that, if the resource referenced at translation time through the #embed
directive is the same one accessed through execution-time means, the data that is e.g. fread or
similar into contiguous storage will compare bit-for-bit equal to an array of character type initialized
from an #embed directive’s expanded contents.
16 EXAMPLE 1 Placing a small image resource.
#include <stddef.h>
void have_you_any_wool(const unsigned char*, size_t);
int main (int, char*[]) {
static const unsigned char baa_baa[] = {
#embed "black_sheep.ico"
};
have_you_any_wool(baa_baa, sizeof(baa_baa));
return 0;
}
17 EXAMPLE 2 This snippet:
int main (int, char*[]) {
static const unsigned char coefficients[] = {
#embed "only_8_bits.bin" // potential constraint violation
};
218) Note that adjacent string literals are not concatenated into a single string literal (see the translation phases in 5.1.1.2);
thus, an expansion that results in two string literals is an invalid directive.
return 0;
}
may violate the constraint that (resource width) % (embed element width) must be 0. The 8 bits might not be evenly
divisible by the embed element width (e.g., on a system where CHAR_BIT is 16). Issuing a diagnostic in this case may aid in
portability by calling attention to potentially incompatible expectations between implementations and their resources.
18 EXAMPLE 3 Initialization of non-arrays.
int main () {
/* Braces may be kept or elided as per normal initialization rules */
int i = {
#embed "i.dat"
}; /* i value is [0, 2^(embed element width)) first entry */
int i2 =
#embed "i.dat"
; /* valid if i.dat produces 1 value,
i2 value is [0, 2^(embed element width)) */
struct s {
double a, b, c;
struct { double e, f, g; };
double h, i, j;
};
struct s x = {
/* initializes each element in
order according to initialization rules with
comma-separated list of integer constant expressions
inside of braces */
#embed "s.dat"
};
return 0;
}
Non-array types can still be initialized since the directive produces a comma-delimited lists of integer constant expressions, a
single integer constant expression, or nothing.
19 EXAMPLE 4 Equivalency of bit sequence and bit order between a translation-time read and an execution-time read of the
same resource/file.
#include <string.h>
#include <stddef.h>
#include <stdio.h>
int main() {
static const unsigned char embed_data[] = {
#embed <data.dat>
};
const size_t f_size = sizeof(embed_data);
unsigned char f_data[f_size];
FILE* f_source = fopen("data.dat", "rb");
if (f_source == NULL);
return 1;
char* f_ptr = (char*)&f_data[0];
if (fread(f_ptr, 1, f_size, f_source) != f_size) {
fclose(f_source);
return 1;
}
fclose(f_source);
int is_same = memcmp(&embed_data[0], f_ptr, f_size);
// if both operations refers to the same resource/file at
// execution time and translation time, "is_same" should be 0
return is_same == 0 ? 0 : 1;
}
6.10.3.2 limit parameter
Constraints
1 The limit standard embed parameter may appear zero times or one time in the embed parameter
sequence. Its preprocessor argument clause shall be present and have the form:
( constant-expression )
and shall be an integer constant expression. The integer constant expression shall not evaluate to a
value less than 0.
2 The token defined shall not appear within the constant expression.
Semantics
3 The embed parameter with a preprocessor parameter token limit denotes a balanced preprocessing
token sequence that will be used to compute the resource width. Independently of any macro
replacement done previously (e.g. when matching the form of #embed), the constant expression is
evaluated after the balanced preprocessing token sequence is processed as in normal text, using
the rules specified for conditional inclusion (6.10.1), with the exception that any defined macro
expressions are not permitted.
4 The resource width is:
— 0, if the integer constant expression evaluates to 0; or,
— the implementation resource width if it is less than the embed element width multiplied by
the integer constant expression; or,
— the embed element width multiplied by the integer constant expression, if it is less than or
equal to the implementation resource width.
5 EXAMPLE 1 Checking the first 4 elements of a sound resource.
#include <assert.h>
int main (int, char*[]) {
static const char sound_signature[] = {
#embed <sdk/jump.wav> limit(2+2)
};
static_assert((sizeof(sound_signature) / sizeof(*sound_signature)) == 4,
"There should only be 4 elements in this array.");
// verify PCM WAV resource
assert(sound_signature[0] == ’R’);
assert(sound_signature[1] == ’I’);
assert(sound_signature[2] == ’F’);
assert(sound_signature[3] == ’F’);
assert(sizeof(sound_signature) == 4);
return 0;
}
6 EXAMPLE 2 Similar to a previous example, except it illustrates macro expansion specifically done for the limit(...)
parameter.
#include <assert.h>
#define TWO_PLUS_TWO 2+2
int main (int, char*[]) {
const char sound_signature[] = {
/* the token sequence within the parentheses
for the "limit" parameter undergoes macro
expansion, at least once, resulting in
#embed <sdk/jump.wav> limit(2+2)
*/
#embed <sdk/jump.wav> limit(TWO_PLUS_TWO)
};
static_assert((sizeof(sound_signature) / sizeof(*sound_signature)) == 4,
"There should only be 4 elements in this array.");
// verify PCM WAV resource
assert(sound_signature[0] == ’R’);
assert(sound_signature[1] == ’I’);
assert(sound_signature[2] == ’F’);
assert(sound_signature[3] == ’F’);
assert(sizeof(sound_signature) == 4);
return 0;
}
7 EXAMPLE 3 A potential constraint violation from a resource that may not have enough information in an environment that
has a CHAR_BIT greater than 24.
int main (int, char*[]) {
const unsigned char arr[] = {
#embed "24_bits.bin" limit(1) // may be a constraint violation
};
return 0;
}
6.10.3.3 suffix parameter
Constraints
The suffix standard embed parameter may appear zero times or one time in the embed parameter
sequence. Its preprocessor argument clause shall be present and have the form:
( pp-balanced-token-sequenceopt )
Semantics
1 The embed parameter with a preprocessing parameter token suffix denotes a balanced preprocess-
ing token sequence within its preprocessor argument clause that will be placed immediately after
the result of the associated #embed directive’s expansion.
2 If the resource is empty, then suffix has no effect and is ignored.
3 EXAMPLE 1 Extra elements added to array initializer.
#include <string.h>
#ifndef SHADER_TARGET
#define SHADER_TARGET "edith-impl.glsl"
#endif
extern char* null_term_shader_data;
void fill_in_data () {
const char internal_data[] = {
#embed SHADER_TARGET \
suffix(,)
0
};
strcpy(null_term_shader_data, internal_data);
}
6.10.3.4 prefix parameter
Constraints
1 The prefix standard embed parameter may appear zero times or one time in the embed parameter
sequence. Its preprocessor parameter clause shall be present and have the form:
( pp-balanced-token-sequenceopt )
Semantics
2 The embed parameter with a preprocessor parameter token prefix denotes a balanced preprocessing
token sequence within its preprocessor argument clause that will be placed immediately before the
result of the associated #embed directive’s expansion, if any.
3 If the resource is empty, then prefix has no effect and is ignored.
4 EXAMPLE 1 A null-terminated character array with prefixed and suffixed tokens of additional tokens when the resource is
not empty, providing null termination and a byte order mark.
#include <string.h>
#include <assert.h>
#ifndef SHADER_TARGET
#define SHADER_TARGET "ches.glsl"
#endif
extern char* merp;
void init_data () {
const char whl[] = {
#embed SHADER_TARGET \
prefix(0xEF, 0xBB, 0xBF, ) /* UTF-8 BOM */ \
suffix(,)
0
};
// always null terminated,
// contains BOM if not-empty
int is_good = (sizeof(whl) == 1 && whl[0] == ’\0’)
|| (whl[0] == ’\xEF’ && whl[1] == ’\xBB’
&& whl[2] == ’\xBF’ && whl[sizeof(whl) - 1] == ’\0’);
assert(is_good);
strcpy(merp, whl);
}
6.10.3.5 if_empty parameter
Constraints
The if_empty standard embed parameter may appear zero times or one time in the embed parameter
sequence. Its preprocessor argument clause shall be present and have the form:
( pp-balanced-token-sequenceopt )
Semantics
1 The embed parameter with a preprocessing parameter token if_empty denotes a balanced pre-
processing token sequence within its preprocessor argument clause that will replace the #embed
directive entirely.
If the resource is not empty, then if_empty has no effect and is ignored.
2 EXAMPLE 1 This resource is considered empty due to the limit(0) embed parameter, always. This program always returns 0,
even if the resource is searched for and found successfully by the implementation.
int main () {
return
#embed </owo/uwurandom> limit(0) prefix(1) if_empty(0)
;
// becomes:
// return 0;
}
3 EXAMPLE 2 An example similar to using the suffix embed parameter, but changed slightly.
#include <string.h>
#ifndef SHADER_TARGET
#define SHADER_TARGET "edith-impl.glsl"
#endif
extern char* null_term_shader_data;
void fill_in_data () {
const char internal_data[] = {
#embed SHADER_TARGET \
suffix(, 0) \
if_empty(0)
};
strcpy(null_term_shader_data, internal_data);
}
4 EXAMPLE 3 This resource is considered empty due to the limit(0) embed parameter, always, which means any if_empty
expressions replace the directive as specified above.
int main () {
return
#include </owo/uwurandom> limit(0) if_empty(45540)
;
}
becomes:
int main () {
return 45540;
}
6.10.4 Macro replacement
Constraints
1 Two replacement lists are identical if and only if the preprocessing tokens in both have the same
number, ordering, spelling, and white-space separation, where all white-space separations are
considered identical.
2 An identifier currently defined as an object-like macro shall not be redefined by another #define
preprocessing directive unless the second definition is an object-like macro definition and the two
replacement lists are identical. Likewise, an identifier currently defined as a function-like macro
shall not be redefined by another #define preprocessing directive unless the second definition is a
function-like macro definition that has the same number and spelling of parameters, and the two
replacement lists are identical.
3 There shall be white space between the identifier and the replacement list in the definition of an
object-like macro.
4 If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments
(including those arguments consisting of no preprocessing tokens) in an invocation of a function-like
macro shall equal the number of parameters in the macro definition. Otherwise, there shall be at
least as many arguments in the invocation as there are parameters in the macro definition (excluding
the ...). There shall exist a ) preprocessing token that terminates the invocation.
5 The identifiers __VA_ARGS__ and __VA_OPT__ shall occur only in the replacement-list of a function-
like macro that uses the ellipsis notation in the parameters.
6 A parameter identifier in a function-like macro shall be uniquely declared within its scope.
Semantics
7 The identifier immediately following the define is called the macro name. There is one name
space for macro names. Any white-space characters preceding or following the replacement list of
preprocessing tokens are not considered part of the replacement list for either form of macro.
8 If a # preprocessing token, followed by an identifier, occurs lexically at the point at which a prepro-
cessing directive could begin, the identifier is not subject to macro replacement.
9 A preprocessing directive of the form
# define identifier replacement-list new-line
defines an object-like macro that causes each subsequent instance of the macro name219) to be replaced
by the replacement list of preprocessing tokens that constitute the remainder of the directive. The
replacement list is then rescanned for more macro names as specified below.
10 A preprocessing directive of the form
# define identifier lparen identifier-listopt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
defines a function-like macro with parameters, whose use is similar syntactically to a function call. The
parameters are specified by the optional list of identifiers, whose scope extends from their declaration
in the identifier list until the new-line character that terminates the #define preprocessing directive.
Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing
token introduces the sequence of preprocessing tokens that is replaced by the replacement list
in the definition (an invocation of the macro). The replaced sequence of preprocessing tokens is
terminated by the matching ) preprocessing token, skipping intervening matched pairs of left and
right parenthesis preprocessing tokens. Within the sequence of preprocessing tokens making up an
invocation of a function-like macro, new-line is considered a normal white-space character.
11 The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms
the list of arguments for the function-like macro. The individual arguments within the list are
separated by comma preprocessing tokens, but comma preprocessing tokens between matching
inner parentheses do not separate arguments. If there are sequences of preprocessing tokens within
the list of arguments that would otherwise act as preprocessing directives,220) the behavior is
undefined.
12 If there is a ... in the identifier-list in the macro definition, then the trailing arguments (if any),
including any separating comma preprocessing tokens, are merged to form a single item: the variable
arguments. The number of arguments so combined is such that, following merger, the number of
arguments is one more than the number of parameters in the macro definition (excluding the ...),
except that if there are as many arguments as named parameters, the macro invocation behaves as if
a comma token has been appended to the argument list such that variable arguments are formed
that contain no pp-tokens.
219) Since, by macro-replacement time, all character constants and string literals are preprocessing tokens, not sequences
possibly containing identifier-like subsequences (see 5.1.1.2, translation phases), they are never scanned for macro names or
parameters.
220) Despite the name, a non-directive is a preprocessing directive.
6.10.4.1 Argument substitution
Syntax
1 va-opt-replacement:
__VA_OPT__ ( pp-tokensopt )
Description
2 Argument substitution is a process during macro expansion in which identifiers corresponding to
the parameters of the macro definition and the special constructs __VA_ARGS__ and __VA_OPT__
are replaced with token sequences from the arguments of the macro invocation and possibly of the
argument of the feature __VA_OPT__ . The latter process allows to control a substitute token sequence
that is only expanded if the argument list that corresponds to a trailing ... of the parameter list is
present and has a non-empty substitution.
Constraints
3 The identifier __VA_OPT__ shall always occur as part of the preprocessing token sequence va-opt-
replacement; its closing ) is determined by skipping intervening pairs of matching left and right
parentheses in its pp-tokens. The pp-tokens of a va-opt-replacement shall not contain __VA_OPT__ .
The pp-tokens shall form a valid replacement list for the current function-like macro.
Semantics
4 After the arguments for the invocation of a function-like macro have been identified, argument
substitution takes place. A va-opt-replacement is treated as if it were a parameter. For each parameter
in the replacement list that is neither preceded by a # or ## preprocessing token nor followed by a
## preprocessing token, the preprocessing tokens naming the parameter are replaced by a token
sequence determined as follows:
— If the parameter is of the form va-opt-replacement, the replacement preprocessing tokens are
the preprocessing token sequence for the corresponding argument, as specified below.
— Otherwise, the replacement preprocessing tokens are the preprocessing tokens of the corre-
sponding argument after all macros contained therein have been expanded. The argument’s
preprocessing tokens are completely macro replaced before being substituted as if they formed
the rest of the preprocessing file with no other preprocessing tokens being available.
5 EXAMPLE 1
#define LPAREN() (
#define G(Q) 42
#define F(R, X, ...) __VA_OPT__(G R X) )
int x = F(LPAREN(), 0, <:-); // replaced by int x = 42;
6 An identifier __VA_ARGS__ that occurs in the replacement list is treated as if it were a parameter,
and the variable arguments form the preprocessing tokens used to replace it.
7 The preprocessing token sequence for the corresponding argument of a va-opt-replacement is
defined as follows. If a (hypothetical) substitution of __VA_ARGS__ as neither an operand of # nor
## consists of no preprocessing tokens, the argument consists of a single placemarker preprocessing
token (6.10.4.3, 6.10.4.4). Otherwise, the argument consists of the results of the expansion of the
contained pp-tokens as the replacement list of the current function-like macro before removal of
placemarker tokens, rescanning, and further replacement.
8 NOTE The placemarker tokens are removed before stringization (6.10.4.2), and can be removed by rescanning and further
replacement (6.10.4.4).
9 EXAMPLE 2
#define F(...) f(0 __VA_OPT__(,) __VA_ARGS__)
#define G(X, ...) f(0, X __VA_OPT__(,) __VA_ARGS__)
#define SDEF(sname, ...) S sname __VA_OPT__(= { __VA_ARGS__ })
#define EMP
F(a, b, c) // replaced by f(0, a, b, c)
F() // replaced by f(0)
F(EMP) // replaced by f(0)
G(a, b, c) // replaced by f(0, a, b, c)
G(a, ) // replaced by f(0, a)
G(a) // replaced by f(0, a)
SDEF(foo); // replaced by S foo;
SDEF(bar, 1, 2); // replaced by S bar = { 1, 2 };
#define H1(X, ...) X __VA_OPT__(##) __VA_ARGS__
// error: ## on line above
// may not appear at the beginning of a replacement
// list (6.10.4.3)
#define H2(X, Y, ...) __VA_OPT__(X ## Y,) __VA_ARGS__
H2(a, b, c, d) // replaced by ab, c, d
#define H3(X, ...) #__VA_OPT__(X##X X##X)
H3(, 0) // replaced by ""
#define H4(X, ...) __VA_OPT__(a X ## X) ## b
H4(, 1) // replaced by a b
#define H5A(...) __VA_OPT__()/**/__VA_OPT__()
#define H5B(X) a ## X ## b
#define H5C(X) H5B(X)
H5C(H5A()) // replaced by ab
6.10.4.2 The # operator
Constraints
1 Each # preprocessing token in the replacement list for a function-like macro shall be followed by a
parameter as the next preprocessing token in the replacement list.
Semantics
2 If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both
are replaced by a single character string literal preprocessing token that contains the spelling of the
preprocessing token sequence for the corresponding argument (excluding placemarker tokens). Let
the stringizing argument be the preprocessing token sequence for the corresponding argument with
placemarker tokens removed. Each occurrence of white space between the stringizing argument’s
preprocessing tokens becomes a single space character in the character string literal. White space
before the first preprocessing token and after the last preprocessing token composing the stringizing
argument is deleted. Otherwise, the original spelling of each preprocessing token in the stringizing
argument is retained in the character string literal, except for special handling for producing the
spelling of string literals and character constants: a \ character is inserted before each " and \
character of a character constant or string literal (including the delimiting " characters), except that
it is implementation-defined whether a \ character is inserted before the \ character beginning a
universal character name. If the replacement that results is not a valid character string literal, the
behavior is undefined. The character string literal corresponding to an empty stringizing argument
is "". The order of evaluation of # and ## operators is unspecified.
6.10.4.3 The ## operator
Constraints
1 A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for
either form of macro definition.
Semantics
2 If, in the replacement list of a function-like macro, a parameter is immediately preceded or followed
by a ## preprocessing token, the parameter is replaced by the corresponding argument’s preprocess-
ing token sequence; however, if an argument consists of no preprocessing tokens, the parameter is
replaced by a placemarker preprocessing token instead.221)
3 For both object-like and function-like macro invocations, before the replacement list is reexamined
for more macro names to replace, each instance of a ## preprocessing token in the replacement list
(not from an argument) is deleted and the preceding preprocessing token is concatenated with the
following preprocessing token. Placemarker preprocessing tokens are handled specially: concatena-
tion of two placemarkers results in a single placemarker preprocessing token, and concatenation
of a placemarker with a non-placemarker preprocessing token results in the non-placemarker pre-
processing token. If the result is not a valid preprocessing token, the behavior is undefined. The
resulting token is available for further macro replacement. The order of evaluation of ## operators is
unspecified.
4 EXAMPLE In the following fragment:
#define hash_hash # ## #
#define mkstr(a) # a
#define in_between(a) mkstr(a)
#define join(c, d) in_between(c hash_hash d)
char p[] = join(x, y); // equivalent to
// char p[] = "x ## y";
The expansion produces, at various stages:
join(x, y)
in_between(x hash_hash y)
in_between(x ## y)
mkstr(x ## y)
"x ## y"
In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is
not the ## operator.
6.10.4.4 Rescanning and further replacement
1 After all parameters in the replacement list have been substituted and # and ## processing has
taken place, all placemarker preprocessing tokens are removed. The resulting preprocessing token
sequence is then rescanned, along with all subsequent preprocessing tokens of the source file, for
more macro names to replace.
2 If the name of the macro being replaced is found during this scan of the replacement list (not
including the rest of the source file’s preprocessing tokens), it is not replaced. Furthermore, if any
nested replacements encounter the name of the macro being replaced, it is not replaced. These
nonreplaced macro name preprocessing tokens are no longer available for further replacement even
if they are later (re)examined in contexts in which that macro name preprocessing token would
otherwise have been replaced.
221) Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that exist only within
translation phase 4.
3 The resulting completely macro-replaced preprocessing token sequence is not processed as a prepro-
cessing directive even if it resembles one, but all pragma unary operator expressions within it are
then processed as specified in 6.10.10 below.
4 EXAMPLE There are cases where it is not clear whether a replacement is nested or not. For example, given the following
macro definitions:
#define f(a) a*g
#define g(a) f(a)
the invocation
f(2)(9)
could expand to either
2*f(9)
or
2*9*g
Strictly conforming programs are not permitted to depend on such unspecified behavior.
6.10.4.5 Scope of macro definitions
1 A macro definition lasts (independent of block structure) until a corresponding #undef directive is
encountered or (if none is encountered) until the end of the preprocessing translation unit. Macro
definitions have no significance after translation phase 4.
2 A preprocessing directive of the form
# undef identifier new-line
causes the specified identifier no longer to be defined as a macro name. It is ignored if the specified
identifier is not currently defined as a macro name.
3 EXAMPLE 1 The simplest use of this facility is to define a "manifest constant", as in
#define TABSIZE 100
int table[TABSIZE];
4 EXAMPLE 2 The following defines a function-like macro whose value is the maximum of its arguments. It has the advantages
of working for any compatible types of the arguments and of generating in-line code without the overhead of function calling.
It has the disadvantages of evaluating one or the other of its arguments a second time (including side effects) and generating
more code than a function if invoked several times. It also cannot have its address taken, as it has none.
#define max(a, b) ((a) > (b) ? (a): (b))
The parentheses ensure that the arguments and the resulting expression are bound properly.
5 EXAMPLE 3 To illustrate the rules for redefinition and reexamination, the sequence
#define x 3
#define f(a) f(x * (a))
#undef x
#define x 2
#define g f
#define z z[0]
#define h g(\~{ }
#define m(a) a(w)
#define w 0,1
#define t(a) a
#define p() int
#define q(x) x
#define r(x,y) x ## y
#define str(x) # x
f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
g(x+(3,4)-w) | h 5) & m
(f)^m(m);
p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
char c[2][6] = { str(hello), str() };
results in
f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
f(2 * (2+(3,4)-0,1)) | f(2 * (\~{ } 5)) & f(2 * (0,1))^m(0,1);
int i[] = { 1, 23, 4, 5, };
char c[2][6] = { "hello", "" };
6 EXAMPLE 4 To illustrate the rules for creating character string literals and concatenating tokens, the sequence
#define str(s) # s
#define xstr(s) str(s)
#define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
x ## s, x ## t)
#define INCFILE(n) vers ## n
#define glue(a, b) a ## b
#define xglue(a, b) glue(a, b)
#define HIGHLOW "hello"
#define LOW LOW ", world"
debug(1, 2);
fputs(str(strncmp("abc\0d", "abc", ’\4’) // this goes away
== 0) str(: @\n), s);
#include xstr(INCFILE(2).h)
glue(HIGH, LOW);
xglue(HIGH, LOW)
results in
printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
fputs(
"strncmp(\"abc\\0d\", \"abc\", ’\\4’) == 0" ": @\n",
s);
#include "vers2.h" (after macro replacement, before file access)
"hello";
"hello" ", world"
or, after concatenation of the character string literals,
printf("x1= %d, x2= %s", x1, x2);
fputs(
"strncmp(\"abc\\0d\", \"abc\", ’\\4’) == 0: @\n",
s);
#include "vers2.h" (after macro replacement, before file access)
"hello";
"hello, world"
Space around the # and ## tokens in the macro definition is optional.
7 EXAMPLE 5 To illustrate the rules for placemarker preprocessing tokens, the sequence
#define t(x,y,z) x ## y ## z
int j[] = { t(+1,2,3), t(,4,5), t(6,,7), t(8,9,),
t(10,,), t(,11,), t(,,12), t(,,) };
results in
int j[] = { 123, 45, 67, 89,
10, 11, 12, };
8 EXAMPLE 6 To demonstrate the redefinition rules, the following sequence is valid.
#define OBJ_LIKE (1-1)
#define OBJ_LIKE /* white space */ (1-1) /* other */
#define FUNC_LIKE(a) (a)
#define FUNC_LIKE(a)( /* note the white space */ \
a /* other stuff on this line
*/)
But the following redefinitions are invalid:
#define OBJ_LIKE (0) // different token sequence
#define OBJ_LIKE (1 - 1) // different white space
#define FUNC_LIKE(b) (a) // different parameter usage
#define FUNC_LIKE(b) (b) // different parameter spelling
9 EXAMPLE 7 Finally, to show the variable argument list macro facilities:
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define showlist(...) puts(#__VA_ARGS__)
#define report(test, ...) ((test)?puts(#test):\
printf(__VA_ARGS__))
debug("Flag");
debug("X = %d\n", x);
showlist(The first, second, and third items.);
report(x>y, "x is %d but y is %d", x, y);
results in
fprintf(stderr, "Flag");
fprintf(stderr, "X = %d\n", x);
puts("The first, second, and third items.");
((x>y)?puts("x>y"):
printf("x is %d but y is %d", x, y));
6.10.5 Line control
Constraints
1 The string literal of a #line directive, if present, shall be a character string literal.
Semantics
2 The line number of the current source line is one greater than the number of new-line characters read
or introduced in translation phase 1 (5.1.1.2) while processing the source file to the current token.
3 If a preprocessing token (in particular __LINE__ ) spans two or more physical lines, it is unspecified
which of those line numbers is associated with that token. If a preprocessing directive spans two or
more physical lines, it is unspecified which of those line numbers is associated with the preprocessing
directive. If a macro invocation spans multiple physical or logical lines, it is unspecified which of
those line numbers is associated with that invocation. The line number of a preprocessing token is
independent of the context (in particular, as a macro argument or in a preprocessing directive). The
line number of a __LINE__ in a macro body is the line number of the macro invocation.
4 A preprocessing directive of the form
# line digit-sequence new-line
causes the implementation to behave as if the following sequence of source lines begins with a
source line that has a line number as specified by the digit sequence (interpreted as a decimal integer,
ignoring any optional digit separators (6.4.4.1) in the digit sequence). The digit sequence shall not
specify zero, nor a number greater than 2147483647.
5 A preprocessing directive of the form
# line digit-sequence " s-char-sequenceopt " new-line
sets the presumed line number similarly and changes the presumed name of the source file to be the
contents of the character string literal.
6 A preprocessing directive of the form
# line pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing tokens after
line on the directive are processed just as in normal text (each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens). The directive resulting after
all replacements shall match one of the two previous forms and is then processed as appropriate.222)
Recommended practice
7 The line number associated with a pp-token should be the line number of the first character of the
pp-token. The line number associated with a preprocessing directive should be the line number of
the line with the first # token. The line number associated with a macro invocation should be the
line number of the first character of the macro name in the invocation.
6.10.6 Diagnostic directives
Semantics
1 A preprocessing directive of either form
# error pp-tokensopt new-line # warning pp-tokensopt new-line
causes the implementation to produce a diagnostic message that includes the specified sequence of
preprocessing tokens.
6.10.7 Pragma directive
Semantics
1 A preprocessing directive of the form
# pragma pp-tokensopt new-line
where the preprocessing token STDC does not immediately follow pragma in the directive (prior to
any macro replacement)223) causes the implementation to behave in an implementation-defined man-
ner. The behavior might cause translation to fail or cause the translator or the resulting program to
behave in a non-conforming manner. Any such pragma that is not recognized by the implementation
is ignored.
2 If the preprocessing token STDC does immediately follow pragma in the directive (prior to any macro
replacement), then no macro replacement is performed on the directive, and the directive shall have
one of the following forms224) whose meanings are described elsewhere:
standard-pragma:
# pragma STDC FP_CONTRACT on-off-switch
# pragma STDC FENV_ACCESS on-off-switch
# pragma STDC FENV_DEC_ROUND dec-direction
# pragma STDC FENV_ROUND direction
# pragma STDC CX_LIMITED_RANGE on-off-switch
on-off-switch: one of
ON OFF DEFAULT
direction: one of
FE_DOWNWARD FE_TONEAREST FE_TONEARESTFROMZERO
FE_TOWARDZERO FE_UPWARD FE_DYNAMIC
dec-direction: one of
FE_DEC_DOWNWARD FE_DEC_TONEAREST FE_DEC_TONEARESTFROMZERO
FE_DEC_TOWARDZERO FE_DEC_UPWARD FE_DEC_DYNAMIC
222) Because a new-line is explicitly included as part of the #line directive, the number of new-line characters read while
processing to the first pp-token can be different depending on whether or not the implementation uses a one-pass preprocessor.
Therefore, there are two possible values for the line number following a directive of the form #line __LINE__ new-line.
223) An implementation is not required to perform macro replacement in pragmas, but it is permitted except for in standard
pragmas (where STDC immediately follows pragma). If the result of macro replacement in a non-standard pragma has the
same form as a standard pragma, the behavior is still implementation-defined; an implementation is permitted to behave as
if it were the standard pragma, but is not required to.
224) See "future language directions" (6.11.6).
Forward references: the FP_CONTRACT pragma (7.12.2), the FENV_ACCESS pragma
(7.6.1), the FENV_DEC_ROUND pragma (7.6.3), the FENV_ROUND pragma (7.6.2), the
CX_LIMITED_RANGE pragma (7.3.4).
6.10.8 Null directive
Semantics
1 A preprocessing directive of the form
# new-line
has no effect.
6.10.9 Predefined macro names
1 The values of the predefined macros listed in the following subclauses225) (except for __FILE__ and
__LINE__ ) remain constant throughout the translation unit.
2 None of these macro names, nor the identifiers defined or __has_c_attribute , shall be the subject
of a #define or a #undef preprocessing directive. Any other predefined macro names: shall begin
with a leading underscore followed by an uppercase letter; or, a second underscore; or, shall be any
of the identifiers alignas, alignof, bool, false, static_assert, thread_local, or true.
3 The implementation shall not predefine the macro __cplusplus , nor shall it define it in any standard
header.
Forward references: standard headers (7.1.2).
6.10.9.1 Mandatory macros
1 The following macro names shall be defined by the implementation:
__DATE__ The date of translation of the preprocessing translation unit: a character string literal of
the form "Mmm dd yyyy", where the names of the months are the same as those generated
by the asctime function, and the first character of dd is a space character if the value is
less than 10. If the date of translation is not available, an implementation-defined valid
date shall be supplied.
__FILE__ The presumed name of the current source file (a character string literal).226)
__LINE__ The presumed line number (within the current source file) of the current source line (an
integer constant).226)
__STDC__ The integer constant 1 , intended to indicate a conforming implementation.
__STDC_HOSTED__ The integer constant 1 if the implementation is a hosted implementation or the
integer constant 0 if it is not.
__STDC_UTF_16__ The integer constant 1 , intended to indicate that values of type char16_t are
UTF–16 encoded.
__STDC_UTF_32__ The integer constant 1 , intended to indicate that values of type char32_t are
UTF–32 encoded.
__STDC_VERSION__ The integer constant 202311L.227)
__TIME__ The time of translation of the preprocessing translation unit: a character string literal of
the form "hh:mm:ss" as in the time generated by the asctime functions. If the time of
translation is not available, an implementation-defined valid time shall be supplied.
Forward references: the asctime functions (7.29.3.1).
225) See "future language directions" (6.11.7).
226) The presumed source file name and line number can be changed by the #line directive.
227) See Annex M for the values in previous revisions. The intention is that this will remain an integer constant of type
long int that is increased with each revision of this document.
6.10.9.2 Environment macros
1 The following macro names are conditionally defined by the implementation:
__STDC_ISO_10646__ An integer constant of the form yyyymmL (for example, 199712L ). If this
symbol is defined, then every character in the Unicode required set, when stored in an
object of type wchar_t, has the same value as the short identifier of that character. The
Unicode required set consists of all the characters that are defined by ISO/IEC 10646, along
with all amendments and technical corrigenda, as of the specified year and month. If
some other encoding is used, the macro shall not be defined and the actual encoding
used is implementation-defined.
__STDC_MB_MIGHT_NEQ_WC__ The integer constant 1 , intended to indicate that, in the encoding for
wchar_t , a member of the basic character set need not have a code value equal to its
value when used as the lone character in an integer character constant.
Forward references: common definitions (7.21), Unicode utilities (7.30).
6.10.9.3 Conditional feature macros
1 The following macro names are conditionally defined by the implementation:
__STDC_ANALYZABLE__ The integer constant 1 , intended to indicate conformance to the specifica-
tions in Annex L (Analyzability).
__STDC_IEC_60559_BFP__ The integer constant 202311L, intended to indicate conformance to
Annex F (IEC 60559 floating-point arithmetic) for binary floating-point arithmetic.
__STDC_IEC_559__ The integer constant 1 , intended to indicate conformance to the specifications
in Annex F (IEC 60559 floating-point arithmetic) for binary floating-point arithmetic. Use
of this macro is an obsolescent feature.
__STDC_IEC_60559_DFP__ The integer constant 202311L, intended to indicate support of decimal
floating types and conformance to Annex F (IEC 60559 floating-point arithmetic) for
decimal floating-point arithmetic.
__STDC_IEC_60559_COMPLEX__ The integer constant 202311L, intended to indicate conformance
to the specifications in Annex G (IEC 60559 compatible complex arithmetic).
__STDC_IEC_60559_TYPES__ The integer constant 202311L, intended to indicate conformance to
the specification in Annex H (IEC 60559 interchange and extended types).
__STDC_IEC_559_COMPLEX__ The integer constant 1 , intended to indicate adherence to the specifi-
cations in Annex G (IEC 60559 compatible complex arithmetic). Use of this macro is an
obsolescent feature.
__STDC_LIB_EXT1__ The integer constant 202311L, intended to indicate support for the extensions
defined in Annex K (Bounds-checking interfaces)228) .
__STDC_NO_ATOMICS__ The integer constant 1, intended to indicate that the implementation does
not support atomic types (including the _Atomic type qualifier) and the <stdatomic.h>
header.
__STDC_NO_COMPLEX__ The integer constant 1, intended to indicate that the implementation does
not support complex types or the <complex.h> header.
__STDC_NO_THREADS__ The integer constant 1, intended to indicate that the implementation does
not support the <threads.h> header.
228) The intention is that this will remain an integer constant of type long int that is increased with each revision of this
document.
__STDC_NO_VLA__ The integer constant 1 , intended to indicate that the implementation does not
support variable length arrays with automatic storage duration. Parameters declared
with variable length array types are adjusted and then define objects of automatic storage
duration with pointer types. Thus, support for such declarations is mandatory.
2 An implementation that defines __STDC_NO_COMPLEX__ shall not define __STDC_IEC_60559_COMPLEX__
or __STDC_IEC_559_COMPLEX__ .
6.10.10 Pragma operator
Semantics
1 A unary operator expression of the form:
_Pragma ( string-literal )
is processed as follows: The string literal is destringized by deleting any encoding prefix, deleting
the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and
replacing each escape sequence \\ by a single backslash. The resulting sequence of characters
is processed through translation phase 3 to produce preprocessing tokens that are executed as if
they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary
operator expression are removed.
2 EXAMPLE A directive of the form:
#pragma listing on "..\listing.dir"
can also be expressed as:
_Pragma ("listing on \"..\\listing.dir\"")
The latter form is processed in the same way whether it appears literally as shown, or results from macro replacement, as in:
#define LISTING(x) PRAGMA(listing on #x)
#define PRAGMA(x) _Pragma(#x)
LISTING (..\listing.dir)
6.11 Future language directions
6.11.1 Floating types
1 Future standardization may include additional floating types, including those with greater range,
precision, or both than long double.
6.11.2 Linkages of identifiers
1 Declaring an identifier with internal linkage at file scope without the static storage-class specifier
is an obsolescent feature.
6.11.3 External names
1 Restriction of the significance of an external name to fewer than 255 characters (considering each
universal character name or extended source character as a single character) is an obsolescent feature
that is a concession to existing implementations.
6.11.4 Character escape sequences
1 Lowercase letters as escape sequences are reserved for future standardization. Other characters may
be used in extensions.
6.11.5 Storage-class specifiers
1 The placement of a storage-class specifier other than at the beginning of the declaration specifiers in
a declaration is an obsolescent feature.
6.11.6 Pragma directives
1 Pragmas whose first preprocessing token is STDC are reserved for future standardization.
6.11.7 Predefined macro names
1 Macro names beginning with __STDC_ are reserved for future standardization.
2 Uses of the __STDC_IEC_559__ and __STDC_IEC_559_COMPLEX__ macros are obsolescent features.
7. Library
7.1 Introduction
7.1.1 Definitions of terms
1 A string is a contiguous sequence of characters terminated by and including the first null character.
The term multibyte string is sometimes used instead to emphasize special processing given to
multibyte characters contained in the string or to avoid confusion with a wide string. A pointer to
a string is a pointer to its initial (lowest addressed) character. The length of a string is the number
of bytes preceding the null character and the value of a string is the sequence of the values of the
contained characters, in order.
2 The decimal-point character is the character used by functions that convert floating-point numbers
to or from character sequences to denote the beginning of the fractional part of such character
sequences.229) It is represented in the text and examples by a period, but may be changed by the
setlocale function.
3 A null wide character is a wide character with code value zero.
4 A wide string is a contiguous sequence of wide characters terminated by and including the first null
wide character. A pointer to a wide string is a pointer to its initial (lowest addressed) wide character.
The length of a wide string is the number of wide characters preceding the null wide character and the
value of a wide string is the sequence of code values of the contained wide characters, in order.
5 A shift sequence is a contiguous sequence of bytes within a multibyte string that (potentially) causes
a change in shift state (see 5.2.1.1). A shift sequence shall not have a corresponding wide character;
it is instead taken to be an adjunct to an adjacent multibyte character.230) In this clause, references to
"white-space character" refer to (execution) white-space character as defined by isspace. References to
"white-space wide character" refer to (execution) white-space wide character as defined by iswspace.
Forward references: character handling (7.4), the setlocale function (7.11.1.1).
7.1.2 Standard headers
1 Each library function is declared in a header,231) whose contents are made available by the #include
preprocessing directive. The header declares a set of related functions, plus any necessary types
and additional macros needed to facilitate their use. In addition to the provisions given in this
clause, an implementation that defines __STDC_LIB_EXT1__ shall conform to the specifications in
Annex K and Subclause K.3 should be read as if it were merged into the parallel structure of named
subclauses of this clause. Declarations of types described here or in Annex K shall not include type
qualifiers, unless explicitly stated otherwise.
2 An implementation that does not support decimal floating types (6.10.9.3) need not support inter-
faces or aspects of interfaces that are specific to these types.
3 The standard headers are232)
<assert.h> <fenv.h> <limits.h>
<complex.h> <float.h> <locale.h>
<ctype.h> <inttypes.h> <math.h>
<errno.h> <iso646.h> <setjmp.h>
229) The functions that make use of the decimal-point character are the numeric conversion functions (7.24.1, 7.31.4.1) and the
formatted input/output functions (7.23.6, 7.31.2).
230) For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX are thus required to be large enough to
count all the bytes in any complete multibyte character plus at least one adjacent shift sequence of maximum length. Whether
these counts provide for more than one shift sequence is the implementation’s choice.
231) A header is not necessarily a source file, nor are the < and > delimited sequences in header names necessarily valid source
file names.
232) The headers <complex.h>, <stdatomic.h>, and <threads.h> are conditional features that implementations need not
support; see 6.10.9.3.
<signal.h> <stddef.h> <tgmath.h>
<stdalign.h> <stdint.h> <threads.h>
<stdarg.h> <stdio.h> <time.h>
<stdatomic.h> <stdlib.h> <uchar.h>
<stdbool.h> <stdnoreturn.h> <wchar.h>
<stdckdint.h> <string.h> <wctype.h>
4 If a file with the same name as one of the above < and > delimited sequences, not provided as part of
the implementation, is placed in any of the standard places that are searched for included source
files, the behavior is undefined.
5 Standard headers may be included in any order; each may be included more than once in a given
scope, with no effect different from being included only once, except that the effect of including
<assert.h> depends on the definition of NDEBUG (see 7.2). If used, a header shall be included outside
of any external declaration or definition, and it shall first be included before the first reference to
any of the functions or objects it declares, or to any of the types or macros it defines. However, if
an identifier is declared or defined in more than one header, the second and subsequent associated
headers may be included after the initial reference to the identifier. The program shall not have any
macros with names lexically identical to keywords currently defined prior to the inclusion of the
header or when any macro defined in the header is expanded.
6 Some standard headers define or declare identifiers that had not been present in previous versions
of this document. To allow implementations and users to adapt to that situation, they also define a
version macro for feature test of the form __STDC_VERSION_XXXX_H__ which expands to 202311L,
where XXXX is the all-caps spelling of the corresponding header <xxxx.h>.
7 Any definition of an object-like macro described in this clause or Annex K shall expand to code that
is fully protected by parentheses where necessary, so that it groups in an arbitrary expression as if it
were a single identifier.
8 Any declaration of a library function shall have external linkage.
9 A summary of the contents of the standard headers is given in Annex B.
Forward references: diagnostics (7.2).
7.1.3 Reserved identifiers
1 Each header declares or defines all identifiers listed in its associated subclause, and optionally
declares or defines identifiers listed in its associated future library directions subclause and identifiers
which are always reserved either for any use or for use as file scope identifiers.
— All potentially reserved identifiers (including ones listed in the future library directions) that
are provided by an implementation with an external definition are reserved for any use. An
implementation shall not provide an external definition of a potentially reserved identifier
unless that identifier is reserved for a use where it would have external linkage. All other
potentially reserved identifiers that are provided by an implementation (including in the
form of a macro) are reserved for any use when the associated header is included. No other
potentially reserved identifiers are reserved.233)
— Each macro name in any of the following subclauses (including the future library directions)
is reserved for use as specified if any of its associated headers is included; unless explicitly
stated otherwise (see 7.1.4).
— All identifiers with external linkage in any of the following subclauses (including the future
library directions) and errno are always reserved for use as identifiers with external linkage234) .
— Each identifier with file scope listed in any of the following subclauses (including the future
233) A potentially reserved identifier becomes a reserved identifier when an implementation begins using it or a future
standard reserves it, but is otherwise available for use by the programmer.
234) The list of reserved identifiers with external linkage includes math_errhandling, setjmp, va_copy , and va_end .
library directions) is reserved for use as a macro name and as an identifier with file scope in
the same name space if any of its associated headers is included.
7.1.4 Use of library functions
1 Each of the following statements applies unless explicitly stated otherwise in the detailed descrip-
tions that follow:
— If an argument to a function has an invalid value (such as a value outside the domain of the
function, or a pointer outside the address space of the program, or a null pointer, or a pointer
to non-modifiable storage when the corresponding parameter is not const-qualified) or a type
(after default argument promotion) not expected by a function with a variable number of
arguments, the behavior is undefined.
— If a function argument is described as being an array, the pointer actually passed to the function
shall have a value such that all address computations and accesses to objects (that would be
valid if the pointer did point to the first element of such an array) are in fact valid.235)
— Any function declared in a header may be additionally implemented as a function-like macro
defined in the header, so if a library function is declared explicitly when its header is included,
one of the techniques shown below can be used to ensure the declaration is not affected by
such a macro. Any macro definition of a function can be suppressed locally by enclosing
the name of the function in parentheses, because the name is then not followed by the left
parenthesis that indicates expansion of a macro function name. For the same syntactic reason,
it is permitted to take the address of a library function even if it is also defined as a macro.236)
The use of #undef to remove any macro definition will also ensure that an actual function is
referred to.
— Any invocation of a library function that is implemented as a macro shall expand to code that
evaluates each of its arguments exactly once, fully protected by parentheses where necessary,
so it is generally safe to use arbitrary expressions as arguments.237)
— Likewise, those function-like macros described in the following subclauses may be invoked in
an expression anywhere a function with a compatible return type could be called. 238)
— All object-like macros listed as expanding to integer constant expressions shall additionally be
suitable for use in #if preprocessing directives.
2 Provided that a library function can be declared without reference to any type defined in a header, it
is also permissible to declare the function and use it without including its associated header.
3 There is a sequence point immediately before a library function returns.
4 The functions in the standard library are not guaranteed to be reentrant and may modify objects
with static or thread storage duration. 239)
235) This includes, for example, passing a valid pointer that points one-past-the-end of an array along with a size of 0, or
using any valid pointer with a size of 0.
236) This means that an implementation is required to provide an actual function for each library function, even if it also
provides a macro for that function.
237) Such macros might not contain the sequence points that the corresponding function calls do.
238) Because external identifiers and some macro names beginning with an underscore are reserved, implementations can
provide special semantics for such names. For example, the identifier _BUILTIN_abs could be used to indicate generation of
in-line code for the abs function. Thus, the appropriate header could specify
#define abs(x) _BUILTIN_abs(x)
for a compiler whose code generator will accept it.
In this manner, a user desiring to guarantee that a given library function such as abs will be a genuine function can write
#undef abs
whether the implementation’s header provides a macro implementation of abs or a built-in implementation. The prototype
for the function, which precedes and is hidden by any macro definition, is thereby revealed also.
239) Thus, a signal handler cannot, in general, call standard library functions.
5 Unless explicitly stated otherwise in the detailed descriptions that follow, library functions shall
prevent data races as follows: A library function shall not directly or indirectly access objects
accessible by threads other than the current thread unless the objects are accessed directly or
indirectly via the function’s arguments. A library function shall not directly or indirectly modify
objects accessible by threads other than the current thread unless the objects are accessed directly
or indirectly via the function’s non-const arguments. 240) Implementations may share their own
internal objects between threads if the objects are not visible to users and are protected against data
races.
6 Unless otherwise specified, library functions shall perform all operations solely within the current
thread if those operations have effects that are visible to users.241)
7 EXAMPLE The function atoi can be used in any of several ways:
— by use of its associated header (possibly generating a macro expansion)
#include <stdlib.h>
const char *str;
/* ... */
i = atoi(str);
— by use of its associated header (assuredly generating a true function reference)
#include <stdlib.h>
#undef atoi
const char *str;
/* ... */
i = atoi(str);
or
#include <stdlib.h>
const char *str;
/* ... */
i = (atoi)(str);
— by explicit declaration
extern int atoi(const char *);
const char *str;
/* ... */
i = atoi(str);
240) This means, for example, that an implementation is not permitted to use a static object for internal purposes without
synchronization because it could cause a data race even in programs that do not explicitly share objects between threads.
Similarly, an implementation of memcpy is not permitted to copy bytes beyond the specified length of the destination object
and then restore the original values because it could cause a data race if the program shared those bytes between threads.
241) This allows implementations to parallelize operations if there are no visible side effects.
7.2 Diagnostics <assert.h>
1 The header <assert.h> defines the assert and static_assert macros and refers to another
macro,
NDEBUG
which is not defined by <assert.h>. If NDEBUG is defined as a macro name at the point in the source
file where <assert.h> is included, the assert macro is defined simply as
#define assert(...) ((void)0)
The assert macro is redefined according to the current state of NDEBUG each time that <assert.h>
is included.
2 The assert macro shall be implemented as a macro with an ellipsis parameter, not as an actual
function. If the macro definition is suppressed in order to access an actual function, the behavior is
undefined.
7.2.1 Program diagnostics
7.2.1.1 The assert macro
Synopsis
1 #include <assert.h>
void assert(scalar expression);
Description
2 The assert macro puts diagnostic tests into programs; it expands to a void expression. When it
is executed, if expression (which shall have a scalar type) is false (that is, compares equal to 0),
the assert macro writes information about the particular call that failed (including the text of the
argument, the name of the source file, the source line number, and the name of the enclosing function
— the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__ and of
the identifier __func__ ) on the standard error stream in an implementation-defined format.242)
It then calls the abort function.
Returns
3 The assert macro returns no value.
Forward references: the abort function (7.24.4.1).
242) The message written might be of the form:
Assertion failed: expression, function abc, file xyz, line nnn.
7.3 Complex arithmetic <complex.h>
7.3.1 Introduction
1 The header <complex.h> defines macros and declares functions that support complex arithmetic.243)
2 Implementations that define the macro __STDC_NO_COMPLEX__ need not provide this header nor
support any of its facilities.
3 Each synopsis, other than for the CMPLX macros, specifies a family of functions consisting of a princi-
pal function with one or more double complex parameters and a double complex or double return
value; and other functions with the same name but with f and l suffixes which are corresponding
functions with float and long double parameters and return values.
4 The macro
complex
expands to _Complex ; the macro
_Complex_I
expands to a constant expression of type float _Complex, with the value of the imaginary unit.244)
5 The macros
imaginary
and
_Imaginary_I
are defined if and only if the implementation supports imaginary types;245) if defined, they expand
to _Imaginary and a constant expression of type float _Imaginary with the value of the imaginary
unit.
6 The macro
I
expands to either _Imaginary_I or _Complex_I . If _Imaginary_I is not defined, I shall expand to
_Complex_I .
7 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the
macros complex, imaginary, and I.
Forward references: the CMPLX macros (7.3.9.3), IEC 60559-compatible complex arithmetic (An-
nex G).
7.3.2 Conventions
1 Values are interpreted as radians, not degrees. An implementation may set errno but is not required
to.
7.3.3 Branch cuts
1 Some of the functions below have branch cuts, across which the function is discontinuous. For
implementations with a signed zero (including all IEC 60559 implementations) that follow the
specifications of Annex G, the sign of zero distinguishes one side of a cut from another so the
function is continuous (except for format limitations) as the cut is approached from either side. For
example, for the square root function, which has a branch cut along the negative real axis, the top of
243) See "future library directions" (7.33.1).
244) The imaginary unit is a number i such that i2 = −1.
245) A specification for imaginary types is in Annex G.
the cut, with imaginary part +0 , maps to the positive imaginary axis, and the bottom of the cut, with
imaginary part-0 , maps to the negative imaginary axis.
2 Implementations that do not support a signed zero (see Annex F) cannot distinguish the sides of
branch cuts. These implementations shall map a cut so the function is continuous as the cut is
approached coming around the finite endpoint of the cut in a counter clockwise direction. (Branch
cuts for the functions specified here have just one finite endpoint.) For example, for the square root
function, coming counter clockwise around the finite endpoint of the cut along the negative real axis
approaches the cut from above, so the cut maps to the positive imaginary axis.
7.3.4 The CX_LIMITED_RANGE pragma
Synopsis
1 #include <complex.h>
#pragma STDC CX_LIMITED_RANGE on-off-switch
Description
2 The usual mathematical formulas for complex multiply, divide, and absolute value are problem-
atic because of their treatment of infinities and because of undue overflow and underflow. The
CX_LIMITED_RANGE pragma can be used to inform the implementation that (where the state is "on")
the usual mathematical formulas are acceptable.246) The pragma can occur either outside external
declarations or preceding all explicit declarations and statements inside a compound statement.
When outside external declarations, the pragma takes effect from its occurrence until another
CX_LIMITED_RANGE pragma is encountered, or until the end of the translation unit. When inside a
compound statement, the pragma takes effect from its occurrence until another CX_LIMITED_RANGE
pragma is encountered (including within a nested compound statement), or until the end of the
compound statement; at the end of a compound statement the state for the pragma is restored to
its condition just before the compound statement. If this pragma is used in any other context, the
behavior is undefined. The default state for the pragma is "off".
7.3.5 Trigonometric functions
7.3.5.1 The cacos functions
Synopsis
1 #include <complex.h>
double complex cacos(double complex z);
float complex cacosf(float complex z);
long double complex cacosl(long double complex z);
Description
2 The cacos functions compute the complex arc cosine of z, with branch cuts outside the interval
[−1, +1] along the real axis.
Returns
3 The cacos functions return the complex arc cosine value, in the range of a strip mathematically
unbounded along the imaginary axis and in the interval [0, π] along the real axis.
246) The purpose of the pragma is to allow the implementation to use the formulas:
(x + iy) × (u + iv) = (xu − yv) + i(yu + xv)
(x + iy) / (u + iv) = [(xu + yv) + i(yu − xv)]/(u2 + v 2 )
p
|x + iy| = x2 + y 2
where the programmer can determine they are safe.
7.3.5.2 The casin functions
Synopsis
1 #include <complex.h>
double complex casin(double complex z);
float complex casinf(float complex z);
long double complex casinl(long double complex z);
Description
2 The casin functions compute the complex arc sine of z, with branch cuts outside the interval
[−1, +1] along the real axis.
Returns
3 The casin functions return the complex arc sine value, in the range of a strip mathematically
unbounded along the imaginary axis and in the interval [− π2 , + π2 ] along the real axis.
7.3.5.3 The catan functions
Synopsis
1 #include <complex.h>
double complex catan(double complex z);
float complex catanf(float complex z);
long double complex catanl(long double complex z);
Description
2 The catan functions compute the complex arc tangent of z, with branch cuts outside the interval
[−i, +i] along the imaginary axis.
Returns
3 The catan functions return the complex arc tangent value, in the range of a strip mathematically
unbounded along the imaginary axis and in the interval [− π2 , + π2 ] along the real axis.
7.3.5.4 The ccos functions
Synopsis
1 #include <complex.h>
double complex ccos(double complex z);
float complex ccosf(float complex z);
long double complex ccosl(long double complex z);
Description
2 The ccos functions compute the complex cosine of z.
Returns
3 The ccos functions return the complex cosine value.
7.3.5.5 The csin functions
Synopsis
1 #include <complex.h>
double complex csin(double complex z);
float complex csinf(float complex z);
long double complex csinl(long double complex z);
Description
2 The csin functions compute the complex sine of z.
Returns
3 The csin functions return the complex sine value.
7.3.5.6 The ctan functions
Synopsis
1 #include <complex.h>
double complex ctan(double complex z);
float complex ctanf(float complex z);
long double complex ctanl(long double complex z);
Description
2 The ctan functions compute the complex tangent of z.
Returns
3 The ctan functions return the complex tangent value.
7.3.6 Hyperbolic functions
7.3.6.1 The cacosh functions
Synopsis
1 #include <complex.h>
double complex cacosh(double complex z);
float complex cacoshf(float complex z);
long double complex cacoshl(long double complex z);
Description
2 The cacosh functions compute the complex arc hyperbolic cosine of z, with a branch cut at values
less than 1 along the real axis.
Returns
3 The cacosh functions return the complex arc hyperbolic cosine value, in the range of a half-strip of
nonnegative values along the real axis and in the interval [−iπ, +iπ] along the imaginary axis.
7.3.6.2 The casinh functions
Synopsis
1 #include <complex.h>
double complex casinh(double complex z);
float complex casinhf(float complex z);
long double complex casinhl(long double complex z);
Description
2 The casinh functions compute the complex arc hyperbolic sine of z, with branch cuts outside the
interval [−i, +i] along the imaginary axis.
Returns
3 The casinh functions return the complex arc hyperbolic sine value, in the range of a strip mathe-
matically unbounded along the real axis and in the interval [− iπ iπ
2 , + 2 ] along the imaginary axis.
7.3.6.3 The catanh functions
Synopsis
1 #include <complex.h>
double complex catanh(double complex z);
float complex catanhf(float complex z);
long double complex catanhl(long double complex z);
Description
2 The catanh functions compute the complex arc hyperbolic tangent of z, with branch cuts outside
the interval [−1, +1] along the real axis.
Returns
3 The catanh functions return the complex arc hyperbolic tangent value, in the range of a strip
mathematically unbounded along the real axis and in the interval [− iπ iπ
2 , + 2 ] along the imaginary
axis.
7.3.6.4 The ccosh functions
Synopsis
1 #include <complex.h>
double complex ccosh(double complex z);
float complex ccoshf(float complex z);
long double complex ccoshl(long double complex z);
Description
2 The ccosh functions compute the complex hyperbolic cosine of z.
Returns
3 The ccosh functions return the complex hyperbolic cosine value.
7.3.6.5 The csinh functions
Synopsis
1 #include <complex.h>
double complex csinh(double complex z);
float complex csinhf(float complex z);
long double complex csinhl(long double complex z);
Description
2 The csinh functions compute the complex hyperbolic sine of z.
Returns
3 The csinh functions return the complex hyperbolic sine value.
7.3.6.6 The ctanh functions
Synopsis
1 #include <complex.h>
double complex ctanh(double complex z);
float complex ctanhf(float complex z);
long double complex ctanhl(long double complex z);
Description
2 The ctanh functions compute the complex hyperbolic tangent of z.
Returns
3 The ctanh functions return the complex hyperbolic tangent value.
7.3.7 Exponential and logarithmic functions
7.3.7.1 The cexp functions
Synopsis
1 #include <complex.h>
double complex cexp(double complex z);
float complex cexpf(float complex z);
long double complex cexpl(long double complex z);
Description
2 The cexp functions compute the complex base-e exponential of z.
Returns
3 The cexp functions return the complex base-e exponential value.
7.3.7.2 The clog functions
Synopsis
1 #include <complex.h>
double complex clog(double complex z);
float complex clogf(float complex z);
long double complex clogl(long double complex z);
Description
2 The clog functions compute the complex natural (base-e) logarithm of z, with a branch cut along
the negative real axis.
Returns
3 The clog functions return the complex natural logarithm value, in the range of a strip mathematically
unbounded along the real axis and in the interval [−iπ, +iπ] along the imaginary axis.
7.3.8 Power and absolute-value functions
7.3.8.1 The cabs functions
Synopsis
1 #include <complex.h>
double cabs(double complex z);
float cabsf(float complex z);
long double cabsl(long double complex z);
Description
2 The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude)
of z.
Returns
3 The cabs functions return the complex absolute value.
7.3.8.2 The cpow functions
Synopsis
1 #include <complex.h>
double complex cpow(double complex x, double complex y);
float complex cpowf(float complex x, float complex y);
long double complex cpowl(long double complex x, long double complex y);
Description
2 The cpow functions compute the complex power function xy , with a branch cut for the first parameter
along the negative real axis.
Returns
3 The cpow functions return the complex power function value.
7.3.8.3 The csqrt functions
Synopsis
1 #include <complex.h>
double complex csqrt(double complex z);
float complex csqrtf(float complex z);
long double complex csqrtl(long double complex z);
Description
2 The csqrt functions compute the complex square root of z, with a branch cut along the negative
real axis.
Returns
3 The csqrt functions return the complex square root value, in the range of the right half-plane
(including the imaginary axis).
7.3.9 Manipulation functions
7.3.9.1 The carg functions
Synopsis
1 #include <complex.h>
double carg(double complex z);
float cargf(float complex z);
long double cargl(long double complex z);
Description
2 The carg functions compute the argument (also called phase angle) of z, with a branch cut along
the negative real axis.
Returns
3 The carg functions return the value of the argument in the interval [−π, +π].
7.3.9.2 The cimag functions
Synopsis
1 #include <complex.h>
double cimag(double complex z);
float cimagf(float complex z);
long double cimagl(long double complex z);
Description
2 The cimag functions compute the imaginary part of z.247)
Returns
3 The cimag functions return the imaginary part value (as a real).
7.3.9.3 The CMPLX macros
Synopsis
1 #include <complex.h>
double complex CMPLX(double x, double y);
float complex CMPLXF(float x, float y);
long double complex CMPLXL(long double x, long double y);
247) For a variable z of complex type, z == creal(z)+cimag(z) I.
*
Description
2 The CMPLX macros expand to an expression of the specified complex type, with the real part having
the (converted) value of x and the imaginary part having the (converted) value of y. The resulting
expression shall be suitable for use as an initializer for an object with static or thread storage duration,
provided both arguments are likewise suitable.
Returns
3 The CMPLX macros return the complex value x + iy.
4 NOTE These macros act as if the implementation supported imaginary types and the definitions were:
#define CMPLX(x, y) ((double complex)((double)(x) + \
_Imaginary_I * (double)(y)))
#define CMPLXF(x, y) ((float complex)((float)(x) + \
_Imaginary_I * (float)(y)))
#define CMPLXL(x, y) ((long double complex)((long double)(x) + \
_Imaginary_I * (long double)(y)))
7.3.9.4 The conj functions
Synopsis
1 #include <complex.h>
double complex conj(double complex z);
float complex conjf(float complex z);
long double complex conjl(long double complex z);
Description
2 The conj functions compute the complex conjugate of z, by reversing the sign of its imaginary part.
Returns
3 The conj functions return the complex conjugate value.
7.3.9.5 The cproj functions
Synopsis
1 #include <complex.h>
double complex cproj(double complex z);
float complex cprojf(float complex z);
long double complex cprojl(long double complex z);
Description
2 The cproj functions compute a projection of z onto the Riemann sphere: z projects to z except that
all complex infinities (even those with one infinite part and one NaN part) project to positive infinity
on the real axis. If z has an infinite part, then cproj(z) is equivalent to
INFINITY + I * copysign(0.0, cimag(z))
Returns
3 The cproj functions return the value of the projection onto the Riemann sphere.
7.3.9.6 The creal functions
Synopsis
1 #include <complex.h>
double creal(double complex z);
float crealf(float complex z);
long double creall(long double complex z);
Description
2 The creal functions compute the real part of z.248)
Returns
3 The creal functions return the real part value.
248) For a variable z of complex type, z == creal(z)+cimag(z) I.
*
7.4 Character handling <ctype.h>
1 The header <ctype.h> declares several functions useful for classifying and mapping characters.249)
In all cases the argument is an int, the value of which shall be representable as an unsigned char
or shall equal the value of the macro EOF. If the argument has any other value, the behavior is
undefined.
2 The behavior of these functions is affected by the current locale. Those functions that have locale-
specific aspects only when not in the "C" locale are noted below.
3 The term printing character refers to a member of a locale-specific set of characters, each of which
occupies one printing position on a display device; the term control character refers to a member of a
locale-specific set of characters that are not printing characters.250) All letters and digits are printing
characters.
Forward references: EOF (7.23.1), localization (7.11).
7.4.1 Character classification functions
1 The functions in this subclause return nonzero (true) if and only if the value of the argument c
conforms to that in the description of the function.
7.4.1.1 The isalnum function
Synopsis
1 #include <ctype.h>
int isalnum(int c);
Description
2 The isalnum function tests for any character for which isalpha or isdigit is true.
7.4.1.2 The isalpha function
Synopsis
1 #include <ctype.h>
int isalpha(int c);
Description
2 The isalpha function tests for any character for which isupper or islower is true, or any character
that is one of a locale-specific set of alphabetic characters for which none of iscntrl, isdigit,
ispunct, or isspace is true.251) In the "C" locale, isalpha returns true only for the characters for
which isupper or islower is true.
7.4.1.3 The isblank function
Synopsis
1 #include <ctype.h>
int isblank(int c);
Description
2 The isblank function tests for any character that is a standard blank character or is one of a locale-
specific set of characters for which isspace is true and that is used to separate words within a line
of text. The standard blank characters are the following: space (’ ’ ), and horizontal tab (’\t’ ). In
the "C" locale, isblank returns true only for the standard blank characters.
249) See "future library directions" (7.33.2).
250) In an implementation that uses the seven-bit US ASCII character set, the printing characters are those whose values lie
from 0x20 (space) through 0x7E (tilde); the control characters are those whose values lie from 0 (NUL) through 0x1F (US),
and the character 0x7F (DEL).
251) The functions islower and isupper test true or false separately for each of these additional characters; all four combina-
tions are possible.
7.4.1.4 The iscntrl function
Synopsis
1 #include <ctype.h>
int iscntrl(int c);
Description
2 The iscntrl function tests for any control character.
7.4.1.5 The isdigit function
Synopsis
1 #include <ctype.h>
int isdigit(int c);
Description
2 The isdigit function tests for any decimal-digit character (as defined in 5.2.1).
7.4.1.6 The isgraph function
Synopsis
1 #include <ctype.h>
int isgraph(int c);
Description
2 The isgraph function tests for any printing character except space (’ ’ ).
7.4.1.7 The islower function
Synopsis
1 #include <ctype.h>
int islower(int c);
Description
2 The islower function tests for any character that is a lowercase letter or is one of a locale-specific set
of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale,
islower returns true only for the lowercase letters (as defined in 5.2.1).
7.4.1.8 The isprint function
Synopsis
1 #include <ctype.h>
int isprint(int c);
Description
2 The isprint function tests for any printing character including space (’ ’ ).
7.4.1.9 The ispunct function
Synopsis
1 #include <ctype.h>
int ispunct(int c);
Description
2 The ispunct function tests for any printing character that is one of a locale-specific set of punctuation
characters for which neither isspace nor isalnum is true. In the "C" locale, ispunct returns true
for every printing character for which neither isspace nor isalnum is true.
7.4.1.10 The isspace function
Synopsis
1 #include <ctype.h>
int isspace(int c);
Description
2 The isspace function tests for any character that is a standard white-space character or is one of
a locale-specific set of characters for which isalnum is false. The standard white-space characters
are the following: space (’ ’ ), form feed (’\f’ ), new-line (’\n’ ), carriage return (’\r’ ), horizontal
tab (’\t’ ), and vertical tab (’\v’ ). In the "C" locale, isspace returns true only for the standard
white-space characters.
7.4.1.11 The isupper function
Synopsis
1 #include <ctype.h>
int isupper(int c);
Description
2 The isupper function tests for any character that is an uppercase letter or is one of a locale-specific
set of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale,
isupper returns true only for the uppercase letters (as defined in 5.2.1).
7.4.1.12 The isxdigit function
Synopsis
1 #include <ctype.h>
int isxdigit(int c);
Description
2 The isxdigit function tests for any hexadecimal-digit character (as defined in 6.4.4.1).
7.4.2 Character case mapping functions
7.4.2.1 The tolower function
Synopsis
1 #include <ctype.h>
int tolower(int c);
Description
2 The tolower function converts an uppercase letter to a corresponding lowercase letter.
Returns
3 If the argument is a character for which isupper is true and there are one or more corresponding
characters, as specified by the current locale, for which islower is true, the tolower function returns
one of the corresponding characters (always the same one for any given locale); otherwise, the
argument is returned unchanged.
7.4.2.2 The toupper function
Synopsis
1 #include <ctype.h>
int toupper(int c);
Description
2 The toupper function converts a lowercase letter to a corresponding uppercase letter.
Returns
3 If the argument is a character for which islower is true and there are one or more corresponding
characters, as specified by the current locale, for which isupper is true, the toupper function returns
one of the corresponding characters (always the same one for any given locale); otherwise, the
argument is returned unchanged.
7.5 Errors <errno.h>
1 The header <errno.h> defines several macros, all relating to the reporting of error conditions.
2 The macros are
EDOM
EILSEQ
ERANGE
which expand to integer constant expressions with type int, distinct positive values, and which are
suitable for use in #if preprocessing directives; and
errno
which expands to a modifiable lvalue252) that has type int and thread storage duration, the value
of which is set to a positive error number by several library functions. If a macro definition is
suppressed in order to access an actual object, or a program defines an identifier with the name
errno, the behavior is undefined.
3 The value of errno in the initial thread is zero at program startup (the initial representation of the
object designated by errno in other threads is indeterminate), but is never set to zero by any library
function253) . The value of errno may be set to nonzero by a library function call whether or not there
is an error, provided the use of errno is not documented in the description of the function in this
document.
4 Additional macro definitions, beginning with E and a digit or E and an uppercase letter,254) may also
be specified by the implementation.
252) The macro errno need not be the identifier of an object. It might expand to a modifiable lvalue resulting from a function
call (for example, *errno() ).
253) Thus, a program that uses errno for error checking would set it to zero before a library function call, then inspect it
before a subsequent library function call. Of course, a library function can save the value of errno on entry and then set it to
zero, as long as the original value is restored if errno’s value is still zero just before the return.
254) See "future library directions" (7.33.3).
7.6 Floating-point environment <fenv.h>
1 The header <fenv.h> defines several macros, and declares types and functions that provide access to
the floating-point environment. The floating-point environment refers collectively to any floating-point
status flags and control modes supported by the implementation.255)
A floating-point status flag is a system variable whose value is set (but never cleared) when a floating-
point exception is raised, which occurs as a side effect of exceptional floating-point arithmetic to
provide auxiliary information.256) A floating-point control mode is a system variable whose value may
be set by the user to affect the subsequent behavior of floating-point arithmetic.
2 A floating-point control mode may be constant (7.6.2) or dynamic. The dynamic floating-point en-
vironment includes the dynamic floating-point control modes and the floating-point status flags.
3 The dynamic floating-point environment has thread storage duration. The initial state for a thread’s
dynamic floating-point environment is the current state of the dynamic floating-point environment
of the thread that creates it at the time of creation.
4 Certain programming conventions support the intended model of use for the dynamic floating-point
environment:257)
— a function call does not alter its caller’s floating-point control modes, clear its caller’s floating-
point status flags, nor depend on the state of its caller’s floating-point status flags unless the
function is so documented;
— a function call is assumed to require default floating-point control modes, unless its documen-
tation promises otherwise;
— a function call is assumed to have the potential for raising floating-point exceptions, unless its
documentation promises otherwise.
5 The feature test macro __STDC_VERSION_FENV_H__ expands to the token 202311L.
6 The type
fenv_t
represents the entire dynamic floating-point environment.
7 The type
femode_t
represents the collection of dynamic floating-point control modes supported by the implementation,
including the dynamic rounding direction mode.
8 The type
fexcept_t
represents the floating-point status flags collectively, including any status the implementation
associates with the flags.
9 Each of the macros
255) This header is designed to support the floating-point exception status flags and rounding-direction control modes
required by IEC 60559, and other similar floating-point state information. It is also designed to facilitate code portability
among all systems.
256) A floating-point status flag is not an object and can be set more than once within an expression.
257) With these conventions, a programmer can safely assume default floating-point control modes (or be unaware of them).
The responsibilities associated with accessing the floating-point environment fall on the programmer or program that does so
explicitly.
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
is defined if and only if the implementation supports the floating-point exception by means of
the functions in 7.6.4.258) Additional implementation-defined floating-point exceptions, with
macro definitions beginning with FE_ and an uppercase letter,259) may also be specified by the
implementation. The defined macros expand to integer constant expressions with values such that
bitwise ORs of all combinations of the macros result in distinct values, and furthermore, bitwise
ANDs of all combinations of the macros result in zero.260)
10 Decimal floating-point operations and IEC 60559 binary floating-point operations (Annex F) access
the same floating-point exception status flags.
11 The macro
FE_DFL_MODE
represents the default state for the collection of dynamic floating-point control modes sup-
ported by the implementation – and has type "pointer to const-qualified femode_t". Additional
implementation-defined states for the dynamic mode collection, with macro definitions beginning
with FE_ and an uppercase letter, and having type "pointer to const-qualified femode_t", may also
be specified by the implementation.
12 The macro
FE_ALL_EXCEPT
is simply the bitwise OR of all floating-point exception macros defined by the implementation. If no
such macros are defined, FE_ALL_EXCEPT shall be defined as 0.
13 Each of the macros
FE_DOWNWARD
FE_TONEAREST
FE_TONEARESTFROMZERO
FE_TOWARDZERO
FE_UPWARD
is defined if and only if the implementation supports getting and setting the represented rounding
direction by means of the fegetround and fesetround functions. Additional implementation-
defined rounding directions, with macro definitions beginning with FE_ and an uppercase letter,261)
may also be specified by the implementation.262)
14 If the implementation supports decimal floating types, each of the macros
FE_DEC_DOWNWARD
FE_DEC_TONEAREST
FE_DEC_TONEARESTFROMZERO
FE_DEC_TOWARDZERO
FE_DEC_UPWARD
258) The implementation supports a floating-point exception if there are circumstances where a call to at least one of the
functions in 7.6.4, using the macro as the appropriate argument, will succeed. It is not necessary for all the functions to
succeed all the time.
259) See "future library directions" (7.33.4).
260) The macros are typically distinct powers of two.
261) See "future library directions" (7.33.4).
262) Even though the rounding direction macros might expand to constants corresponding to the values of FLT_ROUNDS, they
are not required to do so.
is defined for use with the fe_dec_getround and fe_dec_setround functions for getting and
setting the dynamic rounding direction mode, and with the FENV_DEC_ROUND rounding control
pragma (7.6.3) for specifying a constant rounding direction, for decimal floating-point operations.
The decimal rounding direction affects all (inexact) operations that produce a result of decimal
floating type and all operations that produce an integer or character sequence result and have an
operand of decimal floating type, unless stated otherwise. The macros expand to integer constant
expressions whose values are distinct nonnegative values.
15 During translation, constant rounding direction modes for decimal floating-point arithmetic are
in effect where specified. Elsewhere, during translation the decimal rounding direction mode is
FE_DEC_TONEAREST.
16 At program startup the dynamic rounding direction mode for decimal floating-point arithmetic is
initialized to FE_DEC_TONEAREST.
17 The macro
FE_DFL_ENV
represents the default dynamic floating-point environment — the one installed at program startup
— and has type "pointer to const-qualified fenv_t". It can be used as an argument to <fenv.h>
functions that manage the dynamic floating-point environment.
18 Additional implementation-defined environments, with macro definitions beginning with FE_ and
an uppercase letter,263) and having type "pointer to const-qualified fenv_t", may also be specified
by the implementation.
7.6.1 The FENV_ACCESS pragma
Synopsis
1 #include <fenv.h>
#pragma STDC FENV_ACCESS on-off-switch
Description
2 The FENV_ACCESS pragma provides a means to inform the implementation when a program might
access the floating-point environment to test floating-point status flags or run under non-default
floating-point control modes.264) The pragma shall occur either outside external declarations or
preceding all explicit declarations and statements inside a compound statement. When outside
external declarations, the pragma takes effect from its occurrence until another FENV_ACCESS pragma
is encountered, or until the end of the translation unit. When inside a compound statement, the
pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered (including
within a nested compound statement), or until the end of the compound statement; at the end of a
compound statement the state for the pragma is restored to its condition just before the compound
statement. If this pragma is used in any other context, the behavior is undefined. If part of a
program tests floating-point status flags or establishes non-default floating-point mode settings
using any means other than the FENV_ROUND pragmas, but was translated with the state for the
FENV_ACCESS pragma "off", the behavior is undefined. The default state ("on" or "off") for the
pragma is implementation-defined. (When execution passes from a part of the program translated
with FENV_ACCESS "off" to a part translated with FENV_ACCESS "on", the state of the floating-point
status flags is unspecified and the floating-point control modes have their default settings.)
3 EXAMPLE
#include <fenv.h>
void f(double x)
263) See "future library directions" (7.33.4).
264) The purpose of the FENV_ACCESS pragma is to allow certain optimizations that could subvert flag tests and mode changes
(e.g., global common subexpression elimination, code motion, and constant folding). In general, if the state of FENV_ACCESS
is "off", the translator can assume that the flags are not tested, and that default modes are in effect, except where specified
otherwise by an FENV_ROUND pragma.
{
#pragma STDC FENV_ACCESS ON
void g(double);
void h(double);
/* ... */
g(x + 1);
h(x + 1);
/* ... */
}
4 If the function g might depend on status flags set as a side effect of the first x + 1, or if the second x + 1 might depend on
control modes set as a side effect of the call to function g, then the program has to contain an appropriately placed invocation
of #pragma STDC FENV_ACCESS ON as shown.265)
7.6.2 The FENV_ROUND pragma
Synopsis
1 #include <fenv.h>
#pragma STDC FENV_ROUND direction
#pragma STDC FENV_ROUND FE_DYNAMIC
Description
2 The FENV_ROUND pragma provides a means to specify a constant rounding direction for floating-
point operations for standard floating types within a translation unit or compound statement. The
pragma shall occur either outside external declarations or preceding all explicit declarations and
statements inside a compound statement. When outside external declarations, the pragma takes
effect from its occurrence until another FENV_ROUND pragma is encountered, or until the end of the
translation unit. When inside a compound statement, the pragma takes effect from its occurrence
until another FENV_ROUND pragma is encountered (including within a nested compound statement),
or until the end of the compound statement; at the end of a compound statement the static rounding
mode is restored to its condition just before the compound statement. If this pragma is used in any
other context, its behavior is undefined.
3 direction shall be one of the names of the supported rounding direction macros for operations for
standard floating types (7.6), or FE_DYNAMIC. If any other value is specified, the behavior is unde-
fined. If no FENV_ROUND pragma is in effect, or the specified constant rounding mode is FE_DYNAMIC,
rounding is according to the mode specified by the dynamic floating-point environment, which is the
dynamic rounding mode that was established either at thread creation or by a call to fesetround,
fesetmode, fesetenv, or feupdateenv. If the FE_DYNAMIC mode is specified and FENV_ACCESS is
"off", the translator may assume that the default rounding mode is in effect.
4 The FENV_ROUND pragma affects operations for standard floating types. Within the scope of an
FENV_ROUND pragma establishing a mode other than FE_DYNAMIC, floating-point operators, implicit
conversions (including the conversion of a value represented in a format wider than its semantic
types to its semantic type, as done by classification macros), and invocations of functions indicated
in the table below, for which macro replacement has not been suppressed (7.1.4), shall be evaluated
according to the specified constant rounding mode (as though no constant mode was specified
and the corresponding dynamic rounding mode had been established by a call to fesetround).
Invocations of functions for which macro replacement has been suppressed and invocations of
functions other than those indicated in the table below shall not be affected by constant rounding
modes – they are affected by (and affect) only the dynamic mode. Floating constants (6.4.4.2) of
a standard floating type that occur in the scope of a constant rounding mode shall be interpreted
according to that mode.
265) The side effects impose a temporal ordering that requires two evaluations of x + 1 . On the other hand, without the
#pragma STDC FENV_ACCESS ON pragma, and assuming the default state is "off", just one evaluation of x + 1 would suffice.
Functions affected by constant rounding modes – for standard
floating types
Header Function families
<math.h> acos, acospi, asin, asinpi, atan, atan2, atan2pi, atanpi
<math.h> cos, cospi, sin, sinpi, tan, tanpi
<math.h> acosh, asinh, atanh
<math.h> cosh, sinh, tanh
<math.h> exp, exp10, exp10m1, exp2, exp2m1, expm1
<math.h> log, log10, log10p1, log1p, log2, log2p1, logp1
<math.h> scalbn, scalbln, ldexp
<math.h> cbrt, compoundn, hypot, pow, pown, powr, rootn, rsqrt, sqrt
<math.h> erf, erfc
<math.h> lgamma, tgamma
<math.h> rint, nearbyint, lrint, llrint
<math.h> fdim
<math.h> fma
<math.h> fadd, dadd, fsub, dsub, fmul, dmul, fdiv, ddiv, ffma, dfma, fsqrt, dsqrt
<stdlib.h> atof, strfrom, strto
<wchar.h> wcsto
<stdio.h> printf and scanf families
<wchar.h> wprintf and wscanf families
A function family listed in the table above indicates the functions for all standard floating types,
where the function family is represented by the name of the functions without a suffix. For example,
acos indicates the functions acos, acosf, and acosl.
5 NOTE Constant rounding modes (other than FE_DYNAMIC) could be implemented using dynamic rounding modes as
illustrated in the following example:
{
#pragma STDC FENV_ROUND direction
// compiler inserts:
// #pragma STDC FENV_ACCESS ON
// int __savedrnd;
// __savedrnd = __swapround(direction);
... operations affected by constant rounding mode ...
// compiler inserts:
// __savedrnd = __swapround(__savedrnd);
... operations not affected by constant rounding mode ...
// compiler inserts:
// __savedrnd = __swapround(__savedrnd);
... operations affected by constant rounding mode ...
// compiler inserts:
// __swapround(__savedrnd);
}
where __swapround is defined by:
static inline int __swapround(const int new) {
const int old = fegetround();
fesetround(new);
return old;
}
7.6.3 The FENV_DEC_ROUND pragma
Synopsis
1 #include <fenv.h>
#ifdef __STDC_IEC_60559_DFP__
#pragma STDC FENV_DEC_ROUND dec-direction
#endif
Description
2 The FENV_DEC_ROUND pragma is a decimal floating-point analog of the FENV_ROUND pragma. If
FLT_RADIX is not 10, the FENV_DEC_ROUND pragma affects operators, functions, and floating con-
stants only for decimal floating types. The affected functions are listed in the table below. If
FLT_RADIX is 10, whether the FENV_ROUND and FENV_DEC_ROUND pragmas alter the rounding direc-
tion of both standard and decimal floating-point operations is implementation-defined. dec-direction
shall be one of the decimal rounding direction macro names (FE_DEC_DOWNWARD, FE_DEC_TONEAREST,
FE_DEC_TONEARESTFROMZERO, FE_DEC_TOWARDZERO, and FE_DEC_UPWARD) defined in 7.6, to specify
a constant rounding mode, or FE_DEC_DYNAMIC, to specify dynamic rounding. The corresponding
dynamic rounding mode can be established by a call to fe_dec_setround.
Functions affected by constant rounding modes – for decimal float-
ing types
Header Function families
<math.h> acos, acospi, asin, asinpi, atan, atan2, atan2pi, atanpi
<math.h> cos, cospi, sin, sinpi, tan, tanpi
<math.h> acosh, asinh, atanh
<math.h> cosh, sinh, tanh
<math.h> exp, exp10, exp10m1, exp2, exp2m1, expm1
<math.h> log, log10, log10p1, log1p, log2, log2p1, logp1
<math.h> scalbn, scalbln, ldexp
<math.h> cbrt, compoundn, hypot, pow, pown, powr, rootn, rsqrt, sqrt
<math.h> erf, erfc
<math.h> lgamma, tgamma
<math.h> rint, nearbyint, lrint, llrint
<math.h> quantize
<math.h> fdim
<math.h> fma
<math.h> d32add, d64add, d32sub, d64sub, d32mul, d64mul, d32div, d64div,
d32fma, d64fma, d32sqrt, d64sqrt
<stdlib.h> strfrom, strto
<wchar.h> wcsto
<stdio.h> printf and scanf families
<wchar.h> wprintf and wscanf families
A function family listed in the table above indicates the functions for all decimal floating types,
where the function family is represented by the name of the functions without a suffix. For example,
acos indicates the functions acosd32, acosd64, and acosd128.
7.6.4 Floating-point exceptions
1 The following functions provide access to the floating-point status flags.266) The int input argument
for the functions represents a subset of floating-point exceptions, and can be zero or the bitwise
OR of one or more floating-point exception macros, for example FE_OVERFLOW | FE_INEXACT. For
other argument values, the behavior of these functions is undefined.
7.6.4.1 The feclearexcept function
266) The functions fetestexcept, feraiseexcept, and feclearexcept support the basic abstraction of flags that are either
set or clear. An implementation can endow floating-point status flags with more information — for example, the address of
the code which first raised the floating-point exception; the functions fegetexceptflag and fesetexceptflag deal with
the full content of flags.
Synopsis
1 #include <fenv.h>
int feclearexcept(int excepts);
Description
2 The feclearexcept function attempts to clear the supported floating-point exceptions represented
by its argument.
Returns
3 The feclearexcept function returns zero if the excepts argument is zero or if all the specified
exceptions were successfully cleared. Otherwise, it returns a nonzero value.
7.6.4.2 The fegetexceptflag function
Synopsis
1 #include <fenv.h>
int fegetexceptflag(fexcept_t *flagp, int excepts);
Description
2 The fegetexceptflag function attempts to store an implementation-defined representation of the
states of the floating-point status flags indicated by the argument excepts in the object pointed to
by the argument flagp.
Returns
3 The fegetexceptflag function returns zero if the representation was successfully stored. Otherwise,
it returns a nonzero value.
7.6.4.3 The feraiseexcept function
Synopsis
1 #include <fenv.h>
int feraiseexcept(int excepts);
Description
2 The feraiseexcept function attempts to raise the supported floating-point exceptions represented
by its argument. 267) The order in which these floating-point exceptions are raised is unspecified,
except as stated in F.8.6. Whether the feraiseexcept function additionally raises the "inexact"
floating-point exception whenever it raises the "overflow" or "underflow" floating-point exception
is implementation-defined.
Returns
3 The feraiseexcept function returns zero if the excepts argument is zero or if all the specified
exceptions were successfully raised. Otherwise, it returns a nonzero value.
Recommended Practice
Implementation extensions associated with raising a floating-point exception (for example, enabled
traps or IEC 60559 alternate exception handling) should be honored by this function.
7.6.4.4 The fesetexcept function
Synopsis
1 #include <fenv.h>
int fesetexcept(int excepts);
267) The effect is intended to be similar to that of floating-point exceptions raised by arithmetic operations. Hence, implemen-
tation extensions associated with raising a floating-point exception (for example, enabled traps or IEC 60559 alternate
exception handling) should be honored. The specification in F.8.6 is in the same spirit.
Description
2 The fesetexcept function attempts to set the supported floating-point exception flags represented
by its argument. This function does not clear any floating-point exception flags. This function
changes the state of the floating-point exception flags, but does not cause any other side effects that
might be associated with raising floating-point exceptions. 268)
Returns
3 The fesetexcept function returns zero if all the specified exceptions were successfully set or if the
excepts argument is zero. Otherwise, it returns a nonzero value.
7.6.4.5 The fesetexceptflag function
Synopsis
1 #include <fenv.h>
int fesetexceptflag(const fexcept_t *flagp, int excepts);
Description
2 The fesetexceptflag function attempts to set the floating-point status flags indicated by the
argument excepts to the states stored in the object pointed to by flagp. The value of *flagp
shall have been set by a previous call to fegetexceptflag whose second argument represented at
least those floating-point exceptions represented by the argument excepts. Like fesetexcept, this
function does not raise floating-point exceptions, but only sets the state of the flags.
Returns
3 The fesetexceptflag function returns zero if the excepts argument is zero or if all the specified
flags were successfully set to the appropriate state. Otherwise, it returns a nonzero value.
7.6.4.6 The fetestexceptflag function
Synopsis
1 #include <fenv.h>
int fetestexceptflag(const fexcept_t * flagp, int excepts);
Description
2 The fetestexceptflag function determines which of a specified subset of the floating-point excep-
tion flags are set in the object pointed to by flagp. The value of *flagp shall have been set by a
previous call to fegetexceptflag whose second argument represented at least those floating-point
exceptions represented by the argument excepts. The excepts argument specifies the floating-point
status flags to be queried.
Returns
3 The fetestexceptflag function returns the value of the bitwise OR of the floating-point exception
macros included in excepts corresponding to the floating-point exceptions set in *flagp .
7.6.4.7 The fetestexcept function
Synopsis
1 #include <fenv.h>
int fetestexcept(int excepts);
Description
2 The fetestexcept function determines which of a specified subset of the floating-point excep-
tion flags are currently set. The excepts argument specifies the floating-point status flags to be
queried.269)
268) Implementation extensions like traps for floating-point exceptions and IEC 60559 exception handling do not occur.
269) This mechanism allows testing several floating-point exceptions with just one function call.
Returns
3 The fetestexcept function returns the value of the bitwise OR of the floating-point exception
macros corresponding to the currently set floating-point exceptions included in excepts.
4 EXAMPLE Call f if "invalid" is set, then g if "overflow" is set:
#include <fenv.h>
/* ... */
{
#pragma STDC FENV_ACCESS ON
int set_excepts;
feclearexcept(FE_INVALID | FE_OVERFLOW);
// maybe raise exceptions
set_excepts = fetestexcept(FE_INVALID | FE_OVERFLOW);
if (set_excepts & FE_INVALID) f();
if (set_excepts & FE_OVERFLOW) g();
/* ... */
}
7.6.5 Rounding and other control modes
1 The fegetround and fesetround functions provide control of rounding direction modes. The
fegetmode and fesetmode functions manage all the implementation’s dynamic floating-point
control modes collectively.
7.6.5.1 The fegetmode function
Synopsis
1 #include <fenv.h>
int fegetmode(femode_t *modep);
Description
2 The fegetmode function attempts to store all the dynamic floating-point control modes in the object
pointed to by modep.
Returns
3 The fegetmode function returns zero if the modes were successfully stored. Otherwise, it returns a
nonzero value.
7.6.5.2 The fegetround function
Synopsis
1 #include <fenv.h>
int fegetround(void);
Description
2 The fegetround function gets the current value of the dynamic rounding direction mode.
Returns
3 The fegetround function returns the value of the rounding direction macro representing the current
dynamic rounding direction or a negative value if there is no such rounding direction macro or the
current dynamic rounding direction is not determinable.
7.6.5.3 The fe_dec_getround function
Synopsis
1 #include <fenv.h>
#ifdef __STDC_IEC_60559_DFP__
int fe_dec_getround(void);
#endif
Description
2 The fe_dec_getround function gets the current value of the dynamic rounding direction mode for
decimal floating-point operations.
Returns
3 The fe_dec_getround function returns the value of the rounding direction macro representing the
current dynamic rounding direction for decimal floating-point operations, or a negative value if
there is no such rounding macro or the current rounding direction is not determinable.
7.6.5.4 The fesetmode function
Synopsis
1 #include <fenv.h>
int fesetmode(const femode_t *modep);
Description
2 The fesetmode function attempts to establish the dynamic floating-point modes represented by the
object pointed to by modep. The argument modep shall point to an object set by a call to fegetmode,
or equal FE_DFL_MODE or a dynamic floating-point mode state macro defined by the implementation.
Returns
The fesetmode fesetmode function returns zero if the modes were successfully established. Other-
wise, it returns a nonzero value.
7.6.5.5 The fesetround function
Synopsis
1 #include <fenv.h>
int fesetround(int rnd);
Description
2 The fesetround function establishes the rounding direction represented by its argument rnd. If
the argument is not equal to the value of a rounding direction macro, the rounding direction is not
changed.
Returns
3 The fesetround function returns zero if and only if the dynamic rounding direction mode was set
to the requested rounding direction.
4 EXAMPLE Save, set, and restore the rounding direction. Report an error and abort if setting the rounding direction fails.
#include <fenv.h>
#include <assert.h>
void f(int rnd_dir)
{
#pragma STDC FENV_ACCESS ON
int save_round;
int setround_ok;
save_round = fegetround();
setround_ok = fesetround(rnd_dir);
assert(setround_ok == 0);
/* ... */
fesetround(save_round);
/* ... */
}
7.6.5.6 The fe_dec_setround function
Synopsis
1 #include <fenv.h>
#ifdef __STDC_IEC_60559_DFP__
int fe_dec_setround(int rnd);
#endif
Description
2 The fe_dec_setround function sets the dynamic rounding direction mode for decimal floating-
point operations to be the rounding direction represented by its argument rnd. If the argument is
not equal to the value of a decimal rounding direction macro, the rounding direction is not changed.
3 If FLT_RADIX is not 10, the rounding direction altered by the fesetround function is independent
of the rounding direction altered by the fe_dec_setround function; otherwise if FLT_RADIX is
10, whether the fesetround and fe_dec_setround functions alter the rounding direction of both
standard and decimal floating-point operations is implementation-defined.
Returns
4 The fe_dec_setround function returns a zero value if and only if the argument is equal to a decimal
rounding direction macro (that is, if and only if the dynamic rounding direction mode for decimal
floating-point operations was set to the requested rounding direction).
7.6.6 Environment
1 The functions in this section manage the floating-point environment — status flags and control
modes — as one entity.
7.6.6.1 The fegetenv function
Synopsis
1 #include <fenv.h>
int fegetenv(fenv_t *envp);
Description
2 The fegetenv function attempts to store the current dynamic floating-point environment in the
object pointed to by envp.
Returns
3 The fegetenv function returns zero if the environment was successfully stored. Otherwise, it returns
a nonzero value.
7.6.6.2 The feholdexcept function
Synopsis
1 #include <fenv.h>
int feholdexcept(fenv_t *envp);
Description
2 The feholdexcept function saves the current dynamic floating-point environment in the object
pointed to by envp, clears the floating-point status flags, and then installs a non-stop (continue on
floating-point exceptions) mode, if available, for all floating-point exceptions.270)
Returns
3 The feholdexcept function returns zero if and only if non-stop floating-point exception handling
was successfully installed.
270) IEC 60559 systems have a default non-stop mode, and typically at least one other mode for trap handling or aborting; if
the system provides only the non-stop mode then installing it is trivial. For such systems, the feholdexcept function can be
used in conjunction with the feupdateenv function to write routines that hide spurious floating-point exceptions from their
callers.
7.6.6.3 The fesetenv function
Synopsis
1 #include <fenv.h>
int fesetenv(const fenv_t *envp);
Description
2 The fesetenv function attempts to establish the dynamic floating-point environment represented by
the object pointed to by envp. The argument envp shall point to an object set by a call to fegetenv or
feholdexcept, or equal a dynamic floating-point environment macro. Note that fesetenv merely
installs the state of the floating-point status flags represented through its argument, and does not
raise these floating-point exceptions.
Returns
3 The fesetenv function returns zero if the environment was successfully established. Otherwise, it
returns a nonzero value.
7.6.6.4 The feupdateenv function
Synopsis
1 #include <fenv.h>
int feupdateenv(const fenv_t *envp);
Description
2 The feupdateenv function attempts to save the currently raised floating-point exceptions in its
automatic storage, install the dynamic floating-point environment represented by the object pointed
to by envp, and then raise the saved floating-point exceptions. The argument envp shall point to an
object set by a call to feholdexcept or fegetenv, or equal a dynamic floating-point environment
macro.
Returns
3 The feupdateenv function returns zero if all the actions were successfully carried out. Otherwise, it
returns a nonzero value.
4 EXAMPLE Hide spurious underflow floating-point exceptions:
#include <fenv.h>
double f(double x)
{
#pragma STDC FENV_ACCESS ON
double result;
fenv_t save_env;
if (feholdexcept(&save_env))
return /* indication of an environmental problem */;
// compute result
if (/* test spurious underflow */)
if (feclearexcept(FE_UNDERFLOW))
return /* indication of an environmental problem */;
if (feupdateenv(&save_env))
return /* indication of an environmental problem */;
return result;
}
7.7 Characteristics of floating types <float.h>
1 The header <float.h> defines several macros that expand to various limits and parameters of the
real floating types.
2 The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.2
and 5.2.4.2.3. A summary is given in Annex E.
7.8 Format conversion of integer types <inttypes.h>
1 The header <inttypes.h> includes the header <stdint.h> and extends it with additional facilities
provided by hosted implementations.
2 It declares functions for manipulating greatest-width integers and converting numeric character
strings to greatest-width integers, and it declares the type
imaxdiv_t
which is a structure type that is the type of the value returned by the imaxdiv function. For each
type declared in <stdint.h>, it defines corresponding macros for conversion specifiers for use with
the formatted input/output functions.271)
Forward references: integer types <stdint.h> (7.22), formatted input/output functions (7.23.6),
formatted wide character input/output functions (7.31.2).
7.8.1 Macros for format specifiers
1 Each of the following object-like macros expands to a character string literal containing a conversion
specifier, possibly modified by a length modifier, suitable for use within the format argument of a
formatted input/output function when converting the corresponding integer type. These macro
names have the general form of PRI (character string literals for the fprintf and fwprintf family)
or SCN (character string literals for the fscanf and fwscanf family),272) followed by the conversion
specifier, followed by a name corresponding to a similar type name in 7.22.1. In these names, N
represents the width of the type as described in 7.22.1. For example, PRIdFAST32 can be used in a
format string to print the value of an integer of type int_fast32_t.
2 The fprintf macros for signed integers are:
PRIdN PRIdLEASTN PRIdFASTN PRIdMAX PRIdPTR
PRIiN PRIiLEASTN PRIiFASTN PRIiMAX PRIiPTR
3 The fprintf macros for unsigned integers are:
PRIoN PRIoLEASTN PRIoFASTN PRIoMAX PRIoPTR
PRIuN PRIuLEASTN PRIuFASTN PRIuMAX PRIuPTR
PRIxN PRIxLEASTN PRIxFASTN PRIxMAX PRIxPTR
PRIXN PRIXLEASTN PRIXFASTN PRIXMAX PRIXPTR
4 The fscanf macros for signed integers are:
SCNdN SCNdLEASTN SCNdFASTN SCNdMAX SCNdPTR
SCNiN SCNiLEASTN SCNiFASTN SCNiMAX SCNiPTR
5 The fscanf macros for unsigned integers are:
SCNoN SCNoLEASTN SCNoFASTN SCNoMAX SCNoPTR
SCNuN SCNuLEASTN SCNuFASTN SCNuMAX SCNuPTR
SCNxN SCNxLEASTN SCNxFASTN SCNxMAX SCNxPTR
6 For each type that the implementation provides in <stdint.h>, the corresponding fprintf macros
shall be defined and the corresponding fscanf macros shall be defined unless the implementation
does not have a suitable fscanf length modifier for the type.
7 EXAMPLE
#include <inttypes.h>
#include <wchar.h>
int main(void)
{
uintmax_t i = UINTMAX_MAX; // this type always exists
wprintf(L"The largest integer value is %020"
271) See "future library directions" (7.33.6).
272) Separate macros are given for use with fprintf and fscanf functions because, in the general case, different format
specifiers might be required for fprintf and fscanf, even when the type is the same.
PRIxMAX "\n", i);
return 0;
}
7.8.2 Functions for greatest-width integer types
7.8.2.1 The imaxabs function
Synopsis
1 #include <inttypes.h>
intmax_t imaxabs(intmax_t j);
Description
2 The imaxabs function computes the absolute value of an integer j. If the result cannot be represented,
the behavior is undefined.273)
Returns
3 The imaxabs function returns the absolute value.
7.8.2.2 The imaxdiv function
Synopsis
1 #include <inttypes.h>
imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);
Description
2 The imaxdiv function computes numer / denom and numer % denom in a single operation.
Returns
3 The imaxdiv function returns a structure of type imaxdiv_t comprising both the quotient and the
remainder. The structure shall contain (in either order) the members quot (the quotient) and rem
(the remainder), each of which has type intmax_t. If either part of the result cannot be represented,
the behavior is undefined.
7.8.2.3 The strtoimax and strtoumax functions
Synopsis
1 #include <inttypes.h>
intmax_t strtoimax(const char * restrict nptr, char ** restrict endptr, int base);
uintmax_t strtoumax(const char * restrict nptr, char ** restrict endptr, int base);
Description
2 The strtoimax and strtoumax functions are equivalent to the strtol, strtoll, strtoul, and
strtoull functions, except that the initial portion of the string is converted to intmax_t and
uintmax_t representation, respectively.
Returns
3 The strtoimax and strtoumax functions return the converted value, if any. If no conversion could
be performed, zero is returned. If the correct value is outside the range of representable values,
INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the
value, if any), and the value of the macro ERANGE is stored in errno.
Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.24.1.7).
7.8.2.4 The wcstoimax and wcstoumax functions
273) The absolute value of the most negative number may not be representable.
Synopsis
1 #include <stddef.h> // for wchar_t
#include <inttypes.h>
intmax_t wcstoimax(const wchar_t *restrict nptr, wchar_t **restrict endptr, int base);
uintmax_t wcstoumax(const wchar_t *restrict nptr, wchar_t **restrict endptr, int base);
Description
2 The wcstoimax and wcstoumax functions are equivalent to the wcstol, wcstoll, wcstoul, and
wcstoull functions except that the initial portion of the wide string is converted to intmax_t and
uintmax_t representation, respectively.
Returns
3 The wcstoimax function returns the converted value, if any. If no conversion could be performed,
zero is returned. If the correct value is outside the range of representable values, INTMAX_MAX,
INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the value, if any),
and the value of the macro ERANGE is stored in errno.
Forward references: the wcstol, wcstoll, wcstoul, and wcstoull functions (7.31.4.1.4).
7.9 Alternative spellings <iso646.h>
1 The header <iso646.h> defines the following eleven macros (on the left) that expand to the corre-
sponding tokens (on the right):
and &&
and_eq &=
bitand &
bitor |
compl ~
not !
not_eq !=
or ||
or_eq |=
xor ^
xor_eq ^=
7.10 Characteristics of integer types <limits.h>
1 The header <limits.h> defines several macros that expand to various limits and parameters of the
standard integer types.
2 The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.1.
A summary is given in Annex E.
7.11 Localization <locale.h>
1 The header <locale.h> declares two functions, one type, and defines several macros.
2 The type is
struct lconv
which contains members related to the formatting of numeric values. The structure shall contain
at least the following members, in any order. The semantics of the members and their normal
ranges are explained in 7.11.2.1. In the "C" locale, the members shall have the values specified in the
comments.
char *decimal_point; // "."
char *thousands_sep; // ""
char *grouping; // ""
char *mon_decimal_point; // ""
char *mon_thousands_sep; // ""
char *mon_grouping; // ""
char *positive_sign; // ""
char *negative_sign; // ""
char *currency_symbol; // ""
char frac_digits; // CHAR_MAX
char p_cs_precedes; // CHAR_MAX
char n_cs_precedes; // CHAR_MAX
char p_sep_by_space; // CHAR_MAX
char n_sep_by_space; // CHAR_MAX
char p_sign_posn; // CHAR_MAX
char n_sign_posn; // CHAR_MAX
char *int_curr_symbol; // ""
char int_frac_digits; // CHAR_MAX
char int_p_cs_precedes; // CHAR_MAX
char int_n_cs_precedes; // CHAR_MAX
char int_p_sep_by_space; // CHAR_MAX
char int_n_sep_by_space; // CHAR_MAX
char int_p_sign_posn; // CHAR_MAX
char int_n_sign_posn; // CHAR_MAX
3 The macros defined are NULL (described in 7.21); and
LC_ALL
LC_COLLATE
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_TIME
which expand to integer constant expressions with distinct values, suitable for use as the first argu-
ment to the setlocale function.274) Additional macro definitions, beginning with the characters
LC_ and an uppercase letter,275) may also be specified by the implementation.
7.11.1 Locale control
7.11.1.1 The setlocale function
Synopsis
1 #include <locale.h>
char *setlocale(int category, const char *locale);
274) ISO/IEC 9945–2 specifies locale and charmap formats that can be used to specify locales for C.
275) See "future library directions" (7.33.7).
Description
2 The setlocale function selects the appropriate portion of the program’s locale as specified by
the category and locale arguments. The setlocale function may be used to change or query
the program’s entire current locale or portions thereof. The value LC_ALL for category names
the program’s entire locale; the other values for category name only a portion of the program’s
locale. LC_COLLATE affects the behavior of the strcoll and strxfrm functions. LC_CTYPE affects
the behavior of the character handling functions276) and the multibyte and wide character functions.
LC_MONETARY affects the monetary formatting information returned by the localeconv function.
LC_NUMERIC affects the decimal-point character for the formatted input/output functions and the
string conversion functions, as well as the nonmonetary formatting information returned by the
localeconv function. LC_TIME affects the behavior of the strftime and wcsftime functions.
3 A value of "C" for locale specifies the minimal environment for C translation; a value of "" for
locale specifies the locale-specific native environment. Other implementation-defined strings may
be passed as the second argument to setlocale.
4 At program startup, the equivalent of
setlocale(LC_ALL, "C");
is executed.
5 A call to the setlocale function may introduce a data race with other calls to the setlocale
function or with calls to functions that are affected by the current locale. The implementation shall
behave as if no library function calls the setlocale function.
Returns
6 If a pointer to a string is given for locale and the selection can be honored, the setlocale function
returns a pointer to the string associated with the specified category for the new locale. If the
selection cannot be honored, the setlocale function returns a null pointer and the program’s locale
is not changed.
7 A null pointer for locale causes the setlocale function to return a pointer to the string associated
with the category for the program’s current locale; the program’s locale is not changed.277)
8 The pointer to string returned by the setlocale function is such that a subsequent call with that
string value and its associated category will restore that part of the program’s locale. The string
pointed to shall not be modified by the program. The behavior is undefined if the returned value
is used after a subsequent call to the setlocale function, or after the thread which called the
setlocale function to obtain the returned value has exited.
Forward references: formatted input/output functions (7.23.6), multibyte/wide character conver-
sion functions (7.24.7), multibyte/wide string conversion functions (7.24.8), numeric conversion
functions (7.24.1), the strcoll function (7.26.4.3), the strftime function (7.29.3.5), the strxfrm
function (7.26.4.5).
7.11.2 Numeric formatting convention inquiry
7.11.2.1 The localeconv function
Synopsis
1 #include <locale.h>
struct lconv *localeconv(void);
Description
2 The localeconv function sets the components of an object with type struct lconv with values
appropriate for the formatting of numeric quantities (monetary and otherwise) according to the
rules of the current locale.
276) The only functions in 7.4 whose behavior is not affected by the current locale are isdigit and isxdigit.
277) The implementation is thus required to arrange to encode in a string the various categories due to a heterogeneous locale
when category has the value LC_ALL.
3 The members of the structure with type char * are pointers to strings, any of which (except
decimal_point) can point to "", to indicate that the value is not available in the current locale or is
of zero length. Apart from grouping and mon_grouping, the strings shall start and end in the initial
shift state. The members with type char are nonnegative numbers, any of which can be CHAR_MAX
to indicate that the value is not available in the current locale. The members include the following:
char *decimal_point
The decimal-point character used to format nonmonetary quantities.
char *thousands_sep
The character used to separate groups of digits before the decimal-point character in
formatted nonmonetary quantities.
char *grouping
A string whose elements indicate the size of each group of digits in formatted nonmon-
etary quantities.
char *mon_decimal_point
The decimal-point used to format monetary quantities.
char *mon_thousands_sep
The separator for groups of digits before the decimal-point in formatted monetary
quantities.
char *mon_grouping
A string whose elements indicate the size of each group of digits in formatted monetary
quantities.
char *positive_sign
The string used to indicate a nonnegative-valued formatted monetary quantity.
char *negative_sign
The string used to indicate a negative-valued formatted monetary quantity.
char *currency_symbol
The local currency symbol applicable to the current locale.
char frac_digits
The number of fractional digits (those after the decimal-point) to be displayed in a
locally formatted monetary quantity.
char p_cs_precedes
Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a
nonnegative locally formatted monetary quantity.
char n_cs_precedes
Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a
negative locally formatted monetary quantity.
char p_sep_by_space
Set to a value indicating the separation of the currency_symbol, the sign string, and
the value for a nonnegative locally formatted monetary quantity.
char n_sep_by_space
Set to a value indicating the separation of the currency_symbol, the sign string, and
the value for a negative locally formatted monetary quantity.
char p_sign_posn
Set to a value indicating the positioning of the positive_sign for a nonnegative locally
formatted monetary quantity.
char n_sign_posn
Set to a value indicating the positioning of the negative_sign for a negative locally
formatted monetary quantity.
char *int_curr_symbol
The international currency symbol applicable to the current locale. The first three
characters contain the alphabetic international currency symbol in accordance with
those specified in ISO 4217. The fourth character (immediately preceding the null
character) is the character used to separate the international currency symbol from the
monetary quantity.
char int_frac_digits
The number of fractional digits (those after the decimal-point) to be displayed in an
internationally formatted monetary quantity.
char int_p_cs_precedes
Set to 1 or 0 if the int_curr_symbol respectively precedes or succeeds the value for a
nonnegative internationally formatted monetary quantity.
char int_n_cs_precedes
Set to 1 or 0 if the int_curr_symbol respectively precedes or succeeds the value for a
negative internationally formatted monetary quantity.
char int_p_sep_by_space
Set to a value indicating the separation of the int_curr_symbol, the sign string, and
the value for a nonnegative internationally formatted monetary quantity.
char int_n_sep_by_space
Set to a value indicating the separation of the int_curr_symbol, the sign string, and
the value for a negative internationally formatted monetary quantity.
char int_p_sign_posn
Set to a value indicating the positioning of the positive_sign for a nonnegative
internationally formatted monetary quantity.
char int_n_sign_posn
Set to a value indicating the positioning of the negative_sign for a negative interna-
tionally formatted monetary quantity.
4 The elements of grouping and mon_grouping are interpreted according to the following:
CHAR_MAX No further grouping is to be performed.
0 The previous element is to be repeatedly used for the remainder of the digits.
other The integer value is the number of digits that compose the current group. The next
element is examined to determine the size of the next group of digits before the current
group.
5 The values of p_sep_by_space, n_sep_by_space, int_p_sep_by_space, and
int_n_sep_by_space are interpreted according to the following:
0 No space separates the currency symbol and value.
1 If the currency symbol and sign string are adjacent, a space separates them from the value;
otherwise, a space separates the currency symbol from the value.
2 If the currency symbol and sign string are adjacent, a space separates them; otherwise, a space
separates the sign string from the value.
For int_p_sep_by_space and int_n_sep_by_space, the fourth character of int_curr_symbol is
used instead of a space.
6 The values of p_sign_posn, n_sign_posn, int_p_sign_posn, and int_n_sign_posn are inter-
preted according to the following:
0 Parentheses surround the quantity and currency symbol.
1 The sign string precedes the quantity and currency symbol.
2 The sign string succeeds the quantity and currency symbol.
3 The sign string immediately precedes the currency symbol.
4 The sign string immediately succeeds the currency symbol.
7 The implementation shall behave as if no library function calls the localeconv function.
Returns
8 The localeconv function returns a pointer to the filled-in object. The structure pointed to by the
return value shall not be modified by the program, but may be overwritten by a subsequent call
to the localeconv function. In addition, calls to the setlocale function with categories LC_ALL,
LC_MONETARY, or LC_NUMERIC may overwrite the contents of the structure.
9 EXAMPLE 1 The following table illustrates rules which might well be used by four countries to format monetary quantities.
Local format International format
Country Positive Negative Positive Negative
Country1 1.234,56 mk -1.234,56 mk FIM 1.234,56 FIM -1.234,56
Country2 L.1.234 -L.1.234 ITL 1.234 -ITL 1.234
Country3 ƒ 1.234,56 ƒ -1.234,56 NLG 1.234,56 NLG -1.234,56
Country4 SFrs.1,234.56 SFrs.1,234.56C CHF 1,234.56 CHF 1,234.56C
10 For these four countries, the respective values for the monetary members of the structure returned by localeconv could be:
Country1 Country2 Country3 Country4
mon_decimal_point "," "" "," "."
mon_thousands_sep "." "." "." ","
mon_grouping "\3" "\3" "\3" "\3"
positive_sign "" "" "" ""
negative_sign "-" "-" "-" "C"
currency_symbol "mk" "L." "\u0192" "SFrs."
frac_digits 2 0 2 2
p_cs_precedes 0 1 1 1
n_cs_precedes 0 1 1 1
p_sep_by_space 1 0 1 0
n_sep_by_space 1 0 2 0
p_sign_posn 1 1 1 1
n_sign_posn 1 1 4 2
int_curr_symbol "FIM " "ITL " "NLG " "CHF "
int_frac_digits 2 0 2 2
int_p_cs_precedes 1 1 1 1
int_n_cs_precedes 1 1 1 1
int_p_sep_by_space 1 1 1 1
int_n_sep_by_space 2 1 2 1
int_p_sign_posn 1 1 1 1
int_n_sign_posn 4 1 4 2
11 EXAMPLE 2 The following table illustrates how the cs_precedes, sep_by_space, and sign_posn members affect the
formatted value.
p_sep_by_space
p_cs_precedes p_sign_posn 0 1 2
0 0 (1.25$) (1.25 $) (1.25$)
1 +1.25$ +1.25 $ + 1.25$
2 1.25$+ 1.25 $+ 1.25$ +
3 1.25+$ 1.25 +$ 1.25+ $
4 1.25$+ 1.25 $+ 1.25$ +
1 0 ($1.25) ($ 1.25) ($1.25)
1 +$1.25 +$ 1.25 + $1.25
2 $1.25+ $ 1.25+ $1.25 +
3 +$1.25 +$ 1.25 + $1.25
4 $+1.25 $+ 1.25 $ +1.25
7.12 Mathematics <math.h>
1 The header <math.h> declares two types and many mathematical functions and defines several
macros. Most synopses specify a family of functions consisting of a principal function with one
or more double parameters, a double return value, or both; and other functions with the same
name but with f and l suffixes, which are corresponding functions with float and long double
parameters, return values, or both.278) Integer arithmetic functions and conversion functions are
discussed later.
2 The feature test macro __STDC_VERSION_MATH_H__ expands to the token 202311L.
3 The types
float_t
double_t
are floating types at least as wide as float and double, respectively, and such that double_t is
at least as wide as float_t. If FLT_EVAL_METHOD equals 0, float_t and double_t are float and
double, respectively; if FLT_EVAL_METHOD equals 1, they are both double; if FLT_EVAL_METHOD
equals 2, they are both long double; and for other values of FLT_EVAL_METHOD, they are otherwise
implementation-defined.279)
4 The types
_Decimal32_t
_Decimal64_t
are decimal floating types at least as wide as _Decimal32 and _Decimal64 , respectively,
and such that _Decimal64_t is at least as wide as _Decimal32_t . If DEC_EVAL_METHOD
equals 0, _Decimal32_t and _Decimal64_t are _Decimal32 and _Decimal64 , respectively; if
DEC_EVAL_METHOD equals 1, they are both _Decimal64 ; if DEC_EVAL_METHOD equals 2, they are
both _Decimal128 ; and for other values of DEC_EVAL_METHOD, they are otherwise implementation-
defined.
5 The macro
HUGE_VAL
expands to a double constant expression, not necessarily representable as a float, whose value is
the maximum value returned by library functions when a floating result of type double overflows
under the default rounding mode, either maximum finite number in the type or positive or unsigned
infinity. The macros
HUGE_VALF
HUGE_VALL
are respectively float and long double analogs of HUGE_VAL280) .
6 The macro
HUGE_VAL_D32
expands to a constant expression of type _Decimal32 representing positive infinity. The macros
HUGE_VAL_D64
278) Particularly on systems with wide expression evaluation, a <math.h> function might pass arguments and return values
in wider format than the synopsis prototype indicates.
279) The types float_t and double_t are intended to be the implementation’s most efficient types at least as wide as
float and double, respectively. For FLT_EVAL_METHOD equal 0, 1, or 2, the type float_t is the narrowest type used by the
implementation to evaluate floating expressions.
280) HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that supports infinities.
HUGE_VAL_D128
are respectively _Decimal64 and _Decimal128 analogs of HUGE_VAL_D32.
7 The macro
INFINITY
is defined if and only if the implementation supports an infinity for the type float. It expands to a
constant expression of type float representing positive or unsigned infinity.
8 The macro
DEC_INFINITY
expands to a constant expression of type _Decimal32 representing positive infinity.
9 The macro
NAN
is defined if and only if the implementation supports quiet NaNs for the float type. It expands to a
constant expression of type float representing a quiet NaN.
10 The macro
DEC_NAN
expands to a constant expression of type _Decimal32 representing a quiet NaN.
11 Use of the macros INFINITY, DEC_INFINITY, NAN, and DEC_NAN in <math.h> is an obsolescent
feature. Instead, use the same macros in <float.h>.
12 The number classification macros
FP_INFINITE
FP_NAN
FP_NORMAL
FP_SUBNORMAL
FP_ZERO
represent mutually exclusive kinds of floating-point values. They expand to integer constant
expressions with distinct values. Additional implementation-defined floating-point classifications,
with macro definitions beginning with FP_ and an uppercase letter, may also be specified by the
implementation.
13 The math rounding direction macros
FP_INT_UPWARD
FP_INT_DOWNWARD
FP_INT_TOWARDZERO
FP_INT_TONEARESTFROMZERO
FP_INT_TONEAREST
represent the rounding directions of the functions ceil, floor, trunc, round, and roundeven,
respectively, that convert to integral values in floating-point formats. They expand to integer
constant expressions with distinct values suitable for use as the second argument to the fromfp,
ufromfp, fromfpx, and ufromfpx functions.
14 The macro
FP_FAST_FMA
is optionally defined. If defined, it indicates that the fma function generally executes about as fast as,
or faster than, a multiply and an add of double operands.281) The macros
FP_FAST_FMAF
FP_FAST_FMAL
are, respectively, float and long double analogs of FP_FAST_FMA. If defined, these macros expand
to the integer constant 1.
15 The macros
FP_FAST_FMAD32
FP_FAST_FMAD64
FP_FAST_FMAD128
are, respectively, _Decimal32 , _Decimal64 , and _Decimal128 analogs of FP_FAST_FMA.
16 Each of the macros
FP_FAST_FADD FP_FAST_DSUBL FP_FAST_FDIVL FP_FAST_FFMA
FP_FAST_FADDL FP_FAST_FMUL FP_FAST_DDIVL FP_FAST_FFMAL
FP_FAST_DADDL FP_FAST_FMULL FP_FAST_FSQRT FP_FAST_DFMAL
FP_FAST_FSUB FP_FAST_DMULL FP_FAST_FSQRTL
FP_FAST_FSUBL FP_FAST_FDIV FP_FAST_DSQRTL
is optionally defined. If defined, it indicates that the corresponding function generally executes
about as fast, or faster, than the corresponding operation or function of the argument type with
result type the same as the argument type followed by conversion to the narrower type. For
FP_FAST_FFMA, FP_FAST_FFMAL, and FP_FAST_DFMAL, the comparison is to a call to fma or fmal
followed by a conversion, not to separate multiply, add, and conversion. If defined, these macros
expand to the integer constant 1.
17 The macros
FP_FAST_D32ADDD64 FP_FAST_D32MULD64 FP_FAST_D32FMAD64
FP_FAST_D32ADDD128 FP_FAST_D32MULD128 FP_FAST_D32FMAD128
FP_FAST_D64ADDD128 FP_FAST_D64MULD128 FP_FAST_D64FMAD128
FP_FAST_D32SUBD64 FP_FAST_D32DIVD64 FP_FAST_D32SQRTD64
FP_FAST_D32SUBD128 FP_FAST_D32DIVD128 FP_FAST_D32SQRTD128
FP_FAST_D64SUBD128 FP_FAST_D64DIVD128 FP_FAST_D64SQRTD128
are analogs of FP_FAST_FADD, FP_FAST_FADDL, FP_FAST_DADDL, etc., for decimal floating types.
18 The macros
FP_ILOGB0
FP_ILOGBNAN
expand to integer constant expressions whose values are returned by ilogb(x) if x is zero or
NaN, respectively. The value of FP_ILOGB0 shall be either INT_MIN or -INT_MAX . The value of
FP_ILOGBNAN shall be either INT_MAX or INT_MIN.
19 The macros
FP_LLOGB0
FP_LLOGBNAN
281) Typically, the FP_FAST_FMA macro is defined if and only if the fma function is implemented directly with a hardware
multiply-add instruction. Software implementations are expected to be substantially slower.
expand to integer constant expressions whose values are returned by llogb(x) if x is zero or NaN, re-
spectively. The value of FP_LLOGB0 shall be LONG_MIN if the value of FP_ILOGB0 is INT_MIN, and shall
be-LONG_MAX if the value of FP_ILOGB0 is-INT_MAX . The value of FP_LLOGBNAN shall be LONG_MAX
if the value of FP_ILOGBNAN is INT_MAX, and shall be LONG_MIN if the value of FP_ILOGBNAN is
INT_MIN.
20 The macros
MATH_ERRNO
MATH_ERREXCEPT
expand to the integer constants 1 and 2, respectively; the macro
math_errhandling
expands to an expression that has type int and the value MATH_ERRNO, MATH_ERREXCEPT, the
bitwise OR of both, or 0; the value shall not be 0 in a hosted implementation. The value
of math_errhandling is constant for the duration of the program. It is unspecified whether
math_errhandling is a macro or an identifier with external linkage. If a macro definition is sup-
pressed or a program defines an identifier with the name math_errhandling, the behavior is
undefined. If the expression math_errhandling & MATH_ERREXCEPT can be nonzero, the implemen-
tation shall define the macros FE_DIVBYZERO, FE_INVALID, and FE_OVERFLOW in <fenv.h>.
7.12.1 Treatment of error conditions
1 The behavior of each of the functions in <math.h> is specified for all representable values of its
input arguments, except where explicitly stated otherwise. Each function shall execute as if it were a
single operation without raising SIGFPE and without generating any of the floating-point exceptions
"invalid", "divide-by-zero", or "overflow" except to reflect the result of the function.
2 For all functions, a domain error occurs if and only if an input argument is outside the domain over
which the mathematical function is defined. The description of each function lists any required
domain errors; an implementation may define additional domain errors, provided that such errors
are consistent with the mathematical definition of the function.282) Whether a signaling NaN
input causes a domain error is implementation-defined. On a domain error, the function returns
an implementation-defined value; if the integer expression math_errhandling & MATH_ERRNO
is nonzero, the integer expression errno acquires the value EDOM; if the integer expression
math_errhandling & MATH_ERREXCEPT is nonzero, the "invalid" floating-point exception is raised.
3 Similarly, a pole error (also known as a singularity or infinitary) occurs if and only if the mathematical
function has an exact infinite result as the finite input argument(s) are approached in the limit (for ex-
ample, log(0.0)). The description of each function lists any required pole errors; an implementation
may define additional pole errors, provided that such errors are consistent with the mathematical
definition of the function. On a pole error, the function returns an implementation-defined value;
if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression
errno acquires the value ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT
is nonzero, the "divide-by-zero" floating-point exception is raised.
4 Likewise, a range error occurs if and only if the result overflows or underflows, as defined below.
The description of each function lists any required range errors; an implementation may define
additional range errors, provided that such errors are consistent with the mathematical definition of
the function and are the result of either overflow or underflow283) .
5 A floating result overflows if a finite result value with ordinary accuracy284) would have magnitude
(absolute value) too large for the representation with full precision in the specified type. A result
that is exactly an infinity does not overflow. If a floating result overflows and default rounding
282) In an implementation that supports infinities, this allows an infinity as an argument to be a domain error if the
mathematical domain of the function does not include the infinity.
283) Range errors that are required or implementation-defined shall or may be reported, as specified in this subclause.
284) Ordinary accuracy is determined by the implementation. It refers to the accuracy of the function where results are not
compromised by extreme magnitude.
is in effect, then the function returns the value of the macro HUGE_VAL, HUGE_VALF, or HUGE_VALL
according to the return type, with the same sign as the correct value of the function; however, for
the types with reduced-precision representations of numbers beyond the overflow threshold, the
function may return a representation of the result with less than full precision for the type. If a
floating result overflows and the integer expression math_errhandling & MATH_ERRNO is nonzero,
the integer expression errno acquires the value ERANGE. If a floating result overflows and the
integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the "overflow" floating-
point exception is raised.
6 The result underflows if a nonzero result value with ordinary accuracy would have magnitude (abso-
lute value) less than the minimum normalized number in the type; however a zero result that is spec-
ified to be an exact zero does not underflow. Also, a result with ordinary accuracy and the magnitude
of the minimum normalized number may underflow285) . If the result underflows, the function re-
turns an implementation-defined value whose magnitude is no greater than the smallest normalized
positive number in the specified type; if the integer expression math_errhandling & MATH_ERRNO is
nonzero, whether errno acquires the value ERANGE is implementation-defined; if the integer expres-
sion math_errhandling & MATH_ERREXCEPT is nonzero, whether the "underflow" floating-point
exception is raised is implementation-defined.
7 If a domain, pole, or range error occurs and the integer expression math_errhandling & MATH_ERRNO
is zero,286) then errno shall either be set to the value corresponding to the error or left unmodified. If
no such error occurs, errno shall be left unmodified regardless of the setting of math_errhandling.
7.12.2 The FP_CONTRACT pragma
Synopsis
1 #include <math.h>
#pragma STDC FP_CONTRACT on-off-switch
Description
2 The FP_CONTRACT pragma can be used to allow (if the state is "on") or disallow (if the state is
"off") the implementation to contract expressions (6.5). Each pragma can occur either outside
external declarations or preceding all explicit declarations and statements inside a compound
statement. When outside external declarations, the pragma takes effect from its occurrence until
another FP_CONTRACT pragma is encountered, or until the end of the translation unit. When inside
a compound statement, the pragma takes effect from its occurrence until another FP_CONTRACT
pragma is encountered (including within a nested compound statement), or until the end of the
compound statement; at the end of a compound statement the state for the pragma is restored to
its condition just before the compound statement. If this pragma is used in any other context, the
behavior is undefined. The default state ("on" or "off") for the pragma is implementation-defined.
7.12.3 Classification macros
1 Floating-point values can be classified as NaN, infinite, normal, subnormal, or zero, or into other
implementation-defined categories. Numbers whose magnitude is at least bemin −1 (the minimum
magnitude of normalized floating-point numbers in the type) and at most (1 − b−p )bemax (the
maximum magnitude of normalized floating-point numbers in the type), where b, p, emin , and emax
are as in 5.2.4.2.2, are classified as normal. Larger magnitude finite numbers represented with full
precision in the type may also be classified as normal. Nonzero numbers whose magnitude is less
than bemin −1 are classified as subnormal.
2 In the synopses in this subclause, real-floating indicates that the argument shall be an expression of
real floating type.
7.12.3.1 The fpclassify macro
Synopsis
285) The term underflow here is intended to encompass both "gradual underflow" as in IEC 60559 and also "flush-to-zero"
1
underflow. IEC 60559 underflow can occur in cases where the magnitude of the rounded result (accurate to the full precision
of the type) equals the minimum normalized number in the format.
286) Math errors are being indicated by the floating-point exception flags rather than by errno.
#include <math.h>
int fpclassify(real-floating x);
Description
2 The fpclassify macro classifies its argument value as NaN, infinite, normal, subnormal, zero, or
into another implementation-defined category. First, an argument represented in a format wider
than its semantic type is converted to its semantic type. Then classification is based on the type of
the argument.287)
Returns
3 The fpclassify macro returns the value of the number classification macro appropriate to the value
of its argument.
7.12.3.2 The iscanonical macro
Synopsis
1 #include <math.h>
int iscanonical(real-floating x);
Description
2 The iscanonical macro determines whether its argument value is canonical (5.2.4.2.2). First, an
argument represented in a format wider than its semantic type is converted to its semantic type.
Then, determination is based on the type of the argument.
Returns
3 The iscanonical macro returns a nonzero value if and only if its argument is canonical.
7.12.3.3 The isfinite macro
Synopsis
1 #include <math.h>
int isfinite(real-floating x);
Description
2 The isfinite macro determines whether its argument has a finite value (zero, subnormal, or
normal, and not infinite or NaN). First, an argument represented in a format wider than its semantic
type is converted to its semantic type. Then determination is based on the type of the argument.
Returns
3 The isfinite macro returns a nonzero value if and only if its argument has a finite value.
7.12.3.4 The isinf macro
Synopsis
1 #include <math.h>
int isinf(real-floating x);
Description
2 The isinf macro determines whether its argument value is (positive or negative) infinity. First, an
argument represented in a format wider than its semantic type is converted to its semantic type.
Then determination is based on the type of the argument.
287) Since an expression can be evaluated with more range and precision than its type has, it is important to know the type
that classification is based on. For example, a normal long double value might become subnormal when converted to
double, and zero when converted to float.
Returns
3 The isinf macro returns a nonzero value if and only if its argument has an infinite value.
7.12.3.5 The isnan macro
Synopsis
1 #include <math.h>
int isnan(real-floating x);
Description
2 The isnan macro determines whether its argument value is a NaN. First, an argument represented
in a format wider than its semantic type is converted to its semantic type. Then determination is
based on the type of the argument.288)
Returns
3 The isnan macro returns a nonzero value if and only if its argument has a NaN value.
7.12.3.6 The isnormal macro
Synopsis
1 #include <math.h>
int isnormal(real-floating x);
Description
2 The isnormal macro determines whether its argument value is normal (neither zero, subnormal,
infinite, nor NaN). First, an argument represented in a format wider than its semantic type is
converted to its semantic type. Then determination is based on the type of the argument.
Returns
3 The isnormal macro returns a nonzero value if and only if its argument has a normal value.
7.12.3.7 The signbit macro
Synopsis
1 #include <math.h>
int signbit(real-floating x);
Description
2 The signbit macro determines whether the sign of its argument value is negative289) . If the
argument value is an unsigned zero, its sign is regarded as positive. Otherwise, if the argument
value is unsigned, the result value (zero or nonzero) is implementation-defined.
Returns
3 The signbit macro returns a nonzero value if and only if the sign of its argument value is determined
to be negative.
7.12.3.8 The issignaling macro
Synopsis
1 #include <math.h>
int issignaling(real-floating x);
Description
2 The issignaling macro determines whether its argument value is a signaling NaN.
288) For the isnan macro, the type for determination does not matter unless the implementation supports NaNs in the
evaluation type but not in the semantic type.
289) The signbit macro determines the sign of all values, including infinities, zeros, and NaNs.
Returns
3 The issignaling macro returns a nonzero value if and only if its argument is a signaling NaN.290)
7.12.3.9 The issubnormal macro
Synopsis
1 #include <math.h>
int issubnormal(real-floating x);
Description
2 The issubnormal macro determines whether its argument value is subnormal. First, an argument
represented in a format wider than its semantic type is converted to its semantic type. Then
determination is based on the type of the argument.
Returns
3 The issubnormal macro returns a nonzero value if and only if its argument is subnormal.
7.12.3.10 The iszero macro
Synopsis
1 #include <math.h>
int iszero(real-floating x);
Description
2 The iszero macro determines whether its argument value is (positive, negative, or unsigned) zero.
First, an argument represented in a format wider than its semantic type is converted to its semantic
type. Then, determination is based on the type of the argument.
Returns
3 The iszero macro returns a nonzero value if and only if its argument is zero.
7.12.4 Trigonometric functions
7.12.4.1 The acos functions
Synopsis
1 #include <math.h>
double acos(double x);
float acosf(float x);
long double acosl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 acosd32(_Decimal32 x);
_Decimal64 acosd64(_Decimal64 x);
_Decimal128 acosd128(_Decimal128 x);
#endif
Description
2 The acos functions compute the principal value of the arc cosine of x. A domain error occurs for
arguments not in the interval [−1, +1].
Returns
3 The acos functions return arccos x in the interval [0, π] radians.
290) F.3 specifies that issignaling (and all the other classification macros), raise no floating-point exception if the argument
is a variable, or any other expression whose value is represented in the format of its semantic type, even if the value is a
signaling NaN.
7.12.4.2 The asin functions
Synopsis
1 #include <math.h>
double asin(double x);
float asinf(float x);
long double asinl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 asind32(_Decimal32 x);
_Decimal64 asind64(_Decimal64 x);
_Decimal128 asind128(_Decimal128 x);
#endif
Description
2 The asin functions compute the principal value of the arc sine of x. A domain error occurs for
arguments not in the interval [−1, +1]. A range error occurs if nonzero x is too close to zero.
Returns
3 The asin functions return arcsin x in the interval [− π2 , + π2 ] radians.
7.12.4.3 The atan functions
Synopsis
1 #include <math.h>
double atan(double x);
float atanf(float x);
long double atanl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 atand32(_Decimal32 x);
_Decimal64 atand64(_Decimal64 x);
_Decimal128 atand128(_Decimal128 x);
#endif
Description
2 The atan functions compute the principal value of the arc tangent of x. A range error occurs if
nonzero x is too close to zero.
Returns
3 The atan functions return arctan x in the interval [− π2 , + π2 ] radians.
7.12.4.4 The atan2 functions
Synopsis
1 #include <math.h>
double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 atan2d32(_Decimal32 y, _Decimal32 x);
_Decimal64 atan2d64(_Decimal64 y, _Decimal64 x);
_Decimal128 atan2d128(_Decimal128 y, _Decimal128 x);
#endif
Description
2 The atan2 functions compute the value of the arc tangent of y/x, using the signs of both arguments
to determine the quadrant of the return value. A domain error may occur if both arguments are zero.
A range error occurs if x is positive and nonzero xy is too close to zero.
Returns
3 The atan2 functions return arctan(y/x) in the interval [−π, +π] radians.
7.12.4.5 The cos functions
Synopsis
1 #include <math.h>
double cos(double x);
float cosf(float x);
long double cosl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 cosd32(_Decimal32 x);
_Decimal64 cosd64(_Decimal64 x);
_Decimal128 cosd128(_Decimal128 x);
#endif
Description
2 The cos functions compute the cosine of x (measured in radians).
Returns
3 The cos functions return cos x.
7.12.4.6 The sin functions
Synopsis
1 #include <math.h>
double sin(double x);
float sinf(float x);
long double sinl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 sind32(_Decimal32 x);
_Decimal64 sind64(_Decimal64 x);
_Decimal128 sind128(_Decimal128 x);
#endif
Description
2 The sin functions compute the sine of x (measured in radians). A range error occurs if nonzero x is
too close to zero.
Returns
3 The sin functions return sin x.
7.12.4.7 The tan functions
Synopsis
1 #include <math.h>
double tan(double x);
float tanf(float x);
long double tanl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 tand32(_Decimal32 x);
_Decimal64 tand64(_Decimal64 x);
_Decimal128 tand128(_Decimal128 x);
#endif
Description
2 The tan functions return the tangent of x (measured in radians). A range error occurs if nonzero x is
too close to zero.
Returns
3 The tan functions return tan x.
7.12.4.8 The acospi functions
Synopsis
1 #include <math.h>
double acospi(double x);
float acospif(float x);
long double acospil(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 acospid32(_Decimal32 x);
_Decimal64 acospid64(_Decimal64 x);
_Decimal128 acospid128(_Decimal128 x);
#endif
Description
2 The acospi functions compute the principal value of the arc cosine of x, divided by π, thus measur-
ing the angle in half-revolutions. A domain error occurs for arguments not in the interval [−1, +1].
Returns
3 The acospi functions return arccos(x)/π in the interval [0, 1].
7.12.4.9 The asinpi functions
Synopsis
1 #include <math.h>
double asinpi(double x);
float asinpif(float x);
long double asinpil(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 asinpid32(_Decimal32 x);
_Decimal64 asinpid64(_Decimal64 x);
_Decimal128 asinpid128(_Decimal128 x);
#endif
Description
2 The asinpi functions compute the principal value of the arc sine of x, divided by π, thus measuring
the angle in half-revolutions. A domain error occurs for arguments not in the interval [−1, +1]. A
range error occurs if nonzero x is too close to zero.
Returns
3 The asinpi functions return arcsin(x)/π in the interval [− 12 , + 12 ].
7.12.4.10 The atanpi functions
Synopsis
1 #include <math.h>
double atanpi(double x);
float atanpif(float x);
long double atanpil(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 atanpid32(_Decimal32 x);
_Decimal64 atanpid64(_Decimal64 x);
_Decimal128 atanpid128(_Decimal128 x);
#endif
Description
2 The atanpi functions compute the principal value of the arc tangent of x, divided by π, thus
measuring the angle in half-revolutions. A range error occurs if nonzero x is too close to zero.
Returns
3 The atanpi functions return arctan(x)/π. in the interval [− 12 , + 12 ].
7.12.4.11 The atan2pi functions
Synopsis
1 #include <math.h>
double atan2pi(double y, double x);
float atan2pif(float y, float x);
long double atan2pil(long double y, long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 atan2pid32(_Decimal32 y, _Decimal32 x);
_Decimal64 atan2pid64(_Decimal64 y, _Decimal64 x);
_Decimal128 atan2pid128(_Decimal128 y, _Decimal128 x);
#endif
Description
2 The atan2pi functions compute the angle, measured in half-revolutions, subtended at the origin by
the point (x, y) and the positive x-axis. Thus, the atan2pi functions compute arctan( xy )/π, in the
range [−1, +1]. A domain error may occur if both arguments are zero. A range error occurs if x is
positive and nonzero xy is too close to zero.
Returns
3 The atan2pi functions return the computed angle, in the interval [−1, +1].
7.12.4.12 The cospi functions
Synopsis
1 #include <math.h>
double cospi(double x);
float cospif(float x);
long double cospil(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 cospid32(_Decimal32 x);
_Decimal64 cospid64(_Decimal64 x);
_Decimal128 cospid128(_Decimal128 x);
#endif
Description
2 The cospi functions compute the cosine of π × x, thus regarding x as a measurement in half-
revolutions.
Returns
3 The cospi functions return cos(π × x).
7.12.4.13 The sinpi functions
Synopsis
1 #include <math.h>
double sinpi(double x);
float sinpif(float x);
long double sinpil(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 sinpid32(_Decimal32 x);
_Decimal64 sinpid64(_Decimal64 x);
_Decimal128 sinpid128(_Decimal128 x);
#endif
Description
2 The sinpi functions compute the sine of π× x, thus regarding x as a measurement in half-revolutions.
A range error occurs if nonzero x is too close to zero.
Returns
3 The sinpi functions return sin(π × x).
7.12.4.14 The tanpi functions
Synopsis
1 #include <math.h>
double tanpi(double x);
float tanpif(float x);
long double tanpil(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 tanpid32(_Decimal32 x);
_Decimal64 tanpid64(_Decimal64 x);
_Decimal128 tanpid128(_Decimal128 x);
#endif
Description
2 The tanpi functions compute the tagent of π × x, thus regarding x as a measurement in half-
revolutions. A range error occurs if nonzero x is too close to zero.
Returns
3 The tanpi functions return tan(π × x).
7.12.5 Hyperbolic functions
7.12.5.1 The acosh functions
Synopsis
1 #include <math.h>
double acosh(double x);
float acoshf(float x);
long double acoshl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 acoshd32(_Decimal32 x);
_Decimal64 acoshd64(_Decimal64 x);
_Decimal128 acoshd128(_Decimal128 x);
#endif
Description
2 The acosh functions compute the (nonnegative) arc hyperbolic cosine of x. A domain error occurs
for arguments less than 1.
Returns
3 The acosh functions return arcosh x in the interval [0, +∞].
7.12.5.2 The asinh functions
Synopsis
1 #include <math.h>
double asinh(double x);
float asinhf(float x);
long double asinhl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 asinhd32(_Decimal32 x);
_Decimal64 asinhd64(_Decimal64 x);
_Decimal128 asinhd128(_Decimal128 x);
#endif
Description
2 The asinh functions compute the arc hyperbolic sine of x. A range error occurs if nonzero x is too
close to zero.
Returns
3 The asinh functions return arsinh x.
7.12.5.3 The atanh functions
Synopsis
1 #include <math.h>
double atanh(double x);
float atanhf(float x);
long double atanhl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 atanhd32(_Decimal32 x);
_Decimal64 atanhd64(_Decimal64 x);
_Decimal128 atanhd128(_Decimal128 x);
#endif
Description
2 The atanh functions compute the arc hyperbolic tangent of x. A domain error occurs for arguments
not in the interval [−1, +1]. A pole error may occur if the argument equals-1 or +1 . A range error
occurs if nonzero x is too close to zero.
Returns
3 The atanh functions return artanh x.
7.12.5.4 The cosh functions
Synopsis
1 #include <math.h>
double cosh(double x);
float coshf(float x);
long double coshl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 coshd32(_Decimal32 x);
_Decimal64 coshd64(_Decimal64 x);
_Decimal128 coshd128(_Decimal128 x);
#endif
Description
2 The cosh functions compute the hyperbolic cosine of x. A range error occurs if the magnitude of
finite x is too large.
Returns
3 The cosh functions return cosh x.
7.12.5.5 The sinh functions
Synopsis
1 #include <math.h>
double sinh(double x);
float sinhf(float x);
long double sinhl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 sinhd32(_Decimal32 x);
_Decimal64 sinhd64(_Decimal64 x);
_Decimal128 sinhd128(_Decimal128 x);
#endif
Description
2 The sinh functions compute the hyperbolic sine of x. A range error occurs if the magnitude of finite
x is too large or if nonzero x is too close to zero.
Returns
3 The sinh functions return sinh x.
7.12.5.6 The tanh functions
Synopsis
1 #include <math.h>
double tanh(double x);
float tanhf(float x);
long double tanhl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 tanhd32(_Decimal32 x);
_Decimal64 tanhd64(_Decimal64 x);
_Decimal128 tanhd128(_Decimal128 x);
#endif
Description
2 The tanh functions compute the hyperbolic tangent of x. A range error occurs if nonzero x is too
close to zero.
Returns
3 The tanh functions return tanh x.
7.12.6 Exponential and logarithmic functions
7.12.6.1 The exp functions
Synopsis
1 #include <math.h>
double exp(double x);
float expf(float x);
long double expl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 expd32(_Decimal32 x);
_Decimal64 expd64(_Decimal64 x);
_Decimal128 expd128(_Decimal128 x);
#endif
Description
2 The exp functions compute the base-e exponential of x. A range error occurs if the magnitude of
finite x is too large.
Returns
3 The exp functions return ex .
7.12.6.2 The exp10 functions
Synopsis
1 #include <math.h>
double exp10(double x);
float exp10f(float x);
long double exp10l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 exp10d32(_Decimal32 x);
_Decimal64 exp10d64(_Decimal64 x);
_Decimal128 exp10d128(_Decimal128 x);
#endif
Description
2 The exp10 functions compute the base-10 exponential of x. A range error occurs if the magnitude of
finite x is too large.
Returns
3 The exp10 functions return 10x .
7.12.6.3 The exp10m1 functions
Synopsis
1 #include <math.h>
double exp10m1(double x);
float exp10m1f(float x);
long double exp10m1l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 exp10m1d32(_Decimal32 x);
_Decimal64 exp10m1d64(_Decimal64 x);
_Decimal128 exp10m1d128(_Decimal128 x);
#endif
Description
2 The exp10m1 functions compute the base-10 exponential of the argument, minus 1. A range error
occurs if positive finite x is too large or if nonzero x is too close to zero.
Returns
3 The exp10m1 functions return 10x − 1.
7.12.6.4 The exp2 functions
Synopsis
1 #include <math.h>
double exp2(double x);
float exp2f(float x);
long double exp2l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 exp2d32(_Decimal32 x);
_Decimal64 exp2d64(_Decimal64 x);
_Decimal128 exp2d128(_Decimal128 x);
#endif
Description
2 The exp2 functions compute the base-2 exponential of x. A range error occurs if the magnitude of
finite x is too large.
Returns
3 The exp2 functions return 2x .
7.12.6.5 The exp2m1 functions
Synopsis
1 #include <math.h>
double exp2m1(double x);
float exp2m1f(float x);
long double exp2m1l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 exp2m1d32(_Decimal32 x);
_Decimal64 exp2m1d64(_Decimal64 x);
_Decimal128 exp2m1d128(_Decimal128 x);
#endif
Description
2 The exp2m1 functions compute the base-2 exponential of the argument, minus 1. A range error
occurs if positive finite x is too large or if nonzero x is too close to zero.
Returns
3 The exp2m1 functions return 2x − 1.
7.12.6.6 The expm1 functions
Synopsis
1 #include <math.h>
double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 expm1d32(_Decimal32 x);
_Decimal64 expm1d64(_Decimal64 x);
_Decimal128 expm1d128(_Decimal128 x);
#endif
Description
2 The expm1 functions compute the base-e exponential of the argument, minus 1. A range error occurs
if positive finite x is too large or if nonzero x is too close to zero. 291)
Returns
3 The expm1 functions return ex − 1.
7.12.6.7 The frexp functions
Synopsis
1 #include <math.h>
double frexp(double value, int *p);
float frexpf(float value, int *p);
long double frexpl(long double value, int *p);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 frexpd32(_Decimal32 value, int *p);
_Decimal64 frexpd64(_Decimal64 value, int *p);
_Decimal128 frexpd128(_Decimal128 value, int *p);
#endif
Description
2 The frexp functions break a floating-point number into a normalized fraction and an integer
exponent. They store the integer in the int object pointed to by p. If the type of the function is a
291) For small magnitude x , expm1(x) is expected to be more accurate than exp(x)-1.
standard floating type, the exponent is an integral power of 2. If the type of the function is a decimal
floating type, the exponent is an integral power of 10.
Returns
3 If value is not a floating-point number or if the integral power is outside the range of int, the results
are unspecified. Otherwise, the frexp functions return the value x, such that x has a magnitude
in the interval [ 12 , 1) or zero, and value equals x × 2*p , when the type of the function is a standard
floating type; or x has a magnitude in the interval [1/10, 1) or zero, and value equals x × 10*p , when
the type of the function is a decimal floating type. If value is zero, both parts of the result are zero.
7.12.6.8 The ilogb functions
Synopsis
1 #include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
#ifdef __STDC_IEC_60559_DFP__
int ilogbd32(_Decimal32 x);
int ilogbd64(_Decimal64 x);
int ilogbd128(_Decimal128 x);
#endif
Description
2 The ilogb functions extract the exponent of x as a signed int value. If x is zero they compute the
value FP_ILOGB0; if x is infinite they compute the value INT_MAX; if x is a NaN they compute the
value FP_ILOGBNAN; otherwise, they are equivalent to calling the corresponding logb function and
converting the returned value to type int. A domain error or range error may occur if x is zero,
infinite, or NaN. If the correct value is outside the range of the return type, the numeric result is
unspecified and a domain error or range error may occur.
Returns
3 The ilogb functions return the exponent of x as a signed int value.
Forward references: the logb functions (7.12.6.17).
7.12.6.9 The ldexp functions
Synopsis
1 #include <math.h>
double ldexp(double x, int p);
float ldexpf(float x, int p);
long double ldexpl(long double x, int p);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 ldexpd32(_Decimal32 x, int p);
_Decimal64 ldexpd64(_Decimal64 x, int p);
_Decimal128 ldexpd128(_Decimal128 x, int p);
#endif
Description
2 The ldexp functions multiply a floating-point number by an integral power of 2 when the type of
the function is a standard floating type, or by an integral power of 10 when the type of the function
is a decimal floating type. A range error occurs for some finite x, depending on p.
Returns
3 The ldexp functions return x × 2p when the type of the function is a standard floating type, or return
x × 10p when the type of the function is a decimal floating type.
7.12.6.10 The llogb functions
Synopsis
1 #include <math.h>
long int llogb(double x);
long int llogbf(float x);
long int llogbl(long double x);
#ifdef __STDC_IEC_60559_DFP__
long int llogbd32(_Decimal32 x);
long int llogbd64(_Decimal64 x);
long int llogbd128(_Decimal128 x);
#endif
Description
2 The llogb functions extract the exponent of x as a signed long int value. If x is zero they compute
the value FP_LLOGB0; if x is infinite they compute the value LONG_MAX; if x is a NaN they compute
the value FP_LLOGBNAN; otherwise, they are equivalent to calling the corresponding logb function
and converting the returned value to type long int. A domain error or range error may occur if x is
zero, infinite, or NaN. If the correct value is outside the range of the return type, the numeric result
is unspecified.
Returns
3 The llogb functions return the exponent of x as a signed long int value.
Forward references: the logb functions (7.12.6.17).
7.12.6.11 The log functions
Synopsis
1 #include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 logd32(_Decimal32 x);
_Decimal64 logd64(_Decimal64 x);
_Decimal128 logd128(_Decimal128 x);
#endif
Description
2 The log functions compute the base-e (natural) logarithm of x. A domain error occurs if the
argument is less than zero. A pole error may occur if the argument is zero.
Returns
3 The log functions return loge x.
7.12.6.12 The log10 functions
Synopsis
1 #include <math.h>
double log10(double x);
float log10f(float x);
long double log10l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 log10d32(_Decimal32 x);
_Decimal64 log10d64(_Decimal64 x);
_Decimal128 log10d128(_Decimal128 x);
#endif
Description
2 The log10 functions compute the base-10 (common) logarithm of x. A domain error occurs if the
argument is less than zero. A pole error may occur if the argument is zero.
Returns
3 The log10 functions return log10 x.
7.12.6.13 The log10p1 functions
Synopsis
1 #include <math.h>
double log10p1(double x);
float log10p1f(float x);
long double log10p1l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 log10p1d32(_Decimal32 x);
_Decimal64 log10p1d64(_Decimal64 x);
_Decimal128 log10p1d128(_Decimal128 x);
#endif
Description
2 The log10p1 functions compute the base-10 logarithm of 1 plus the argument. A domain error
occurs if the argument is less than −1. A pole error may occur if the argument equals −1. A range
error occurs if nonzero x is too close to zero.
Returns
3 The log10p1 functions return log10 (1 + x).
7.12.6.14 The log1p and logp1 functions
Synopsis
1 #include <math.h>
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
double logp1(double x);
float logp1f(float x);
long double logp1l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 log1pd32(_Decimal32 x);
_Decimal64 log1pd64(_Decimal64 x);
_Decimal128 log1pd128(_Decimal128 x);
_Decimal32 logp1d32(_Decimal32 x);
_Decimal64 logp1d64(_Decimal64 x);
_Decimal128 logp1d128(_Decimal128 x);
#endif
Description
2 The log1p functions are equivalent to the logp1 functions.292) These functions compute the base-e
(natural) logarithm of 1 plus the argument.293) A domain error occurs if the argument is less than
−1. A pole error may occur if the argument equals −1. A range error occurs if nonzero x is too close
to zero.
Returns
3 The log1p and logp1 functions return loge (1 + x).
292) The logp1 functions are preferred for name consistency with the log10p1 and log2p1 functions.
293) For small magnitude x , logp1(x) is expected to be more accurate than log(1 + x).
7.12.6.15 The log2 functions
Synopsis
1 #include <math.h>
double log2(double x);
float log2f(float x);
long double log2l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 log2d32(_Decimal32 x);
_Decimal64 log2d64(_Decimal64 x);
_Decimal128 log2d128(_Decimal128 x);
#endif
Description
2 The log2 functions compute the base-2 logarithm of x. A domain error occurs if the argument is less
than zero. A pole error may occur if the argument is zero.
Returns
3 The log2 functions return log2 x.
7.12.6.16 The log2p1 functions
Synopsis
1 #include <math.h>
double log2p1(double x);
float log2p1f(float x);
long double log2p1l(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 log2p1d32(_Decimal32 x);
_Decimal64 log2p1d64(_Decimal64 x);
_Decimal128 log2p1d128(_Decimal128 x);
#endif
Description
2 The log2p1 functions compute the base-2 logarithm of 1 plus the argument. A domain error occurs
if the argument is less than −1. A pole error may occur if the argument equals −1. A range error
occurs if nonzero x is too close to zero.
Returns
3 The log2p1 functions return log2 (1+x).
7.12.6.17 The logb functions
Synopsis
1 #include <math.h>
double logb(double x);
float logbf(float x);
long double logbl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 logbd32(_Decimal32 x);
_Decimal64 logbd64(_Decimal64 x);
_Decimal128 logbd128(_Decimal128 x);
#endif
Description
2 The logb functions extract the exponent of x, as a signed integer value in floating-point format. If x
is subnormal it is treated as though it were normalized; thus, for positive finite x,
1 ≤ x × b−logb(x) < b
where b = FLT_RADIX if the type of the function is a standard floating type, or b = 10 if the type of
the function is a decimal floating type. A domain error or pole error may occur if the argument is
zero.
Returns
3 The logb functions return the signed exponent of x.
7.12.6.18 The modf functions
Synopsis
1 #include <math.h>
double modf(double value, double *iptr);
float modff(float value, float *iptr);
long double modfl(long double value, long double *iptr);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 modfd32(_Decimal32 x, _Decimal32 *iptr);
_Decimal64 modfd64(_Decimal64 x, _Decimal64 *iptr);
_Decimal128 modfd128(_Decimal128 x, _Decimal128 *iptr);
#endif
Description
2 The modf functions break the argument value into integral and fractional parts, each of which has
the same type and sign as the argument. They store the integral part (in floating-point format) in the
object pointed to by iptr.
Returns
3 The modf functions return the signed fractional part of value.
7.12.6.19 The scalbn and scalbln functions
Synopsis
1 #include <math.h>
double scalbn(double x, int n);
float scalbnf(float x, int n);
long double scalbnl(long double x, int n);
double scalbln(double x, long int n);
float scalblnf(float x, long int n);
long double scalblnl(long double x, long int n);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 scalbnd32(_Decimal32 x, int n);
_Decimal64 scalbnd64(_Decimal64 x, int n);
_Decimal128 scalbnd128(_Decimal128 x, int n);
_Decimal32 scalblnd32(_Decimal32 x, long int n);
_Decimal64 scalblnd64(_Decimal64 x, long int n);
_Decimal128 scalblnd128(_Decimal128 x, long int n);
#endif
Description
2 The scalbn and scalbln functions compute x × bn , where b = FLT_RADIX if the type of the function
is a standard floating type, or b = 10 if the type of the function is a decimal floating type. A range
error occurs for some finite x, depending on n.
Returns
3 The scalbn and scalbln functions return x × bn .
7.12.7 Power and absolute-value functions
7.12.7.1 The cbrt functions
Synopsis
1 #include <math.h>
double cbrt(double x);
float cbrtf(float x);
long double cbrtl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 cbrtd32(_Decimal32 x);
_Decimal64 cbrtd64(_Decimal64 x);
_Decimal128 cbrtd128(_Decimal128 x);
#endif
Description
2 The cbrt functions compute the real cube root of x.
Returns
1
3 The cbrt functions return x 3 .
7.12.7.2 The compoundn functions
Synopsis
1 #include <stdint.h>
#include <math.h>
double compoundn(double x, long long int n);
float compoundnf(float x, long long int n);
long double compoundnl(long double x, long long int n);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 compoundnd32(_Decimal32 x, long long int n);
_Decimal64 compoundnd64(_Decimal64 x, long long int n);
_Decimal128 compoundnd128(_Decimal128 x, long long int n);
#endif
Description
2 The compoundn functions compute 1 plus x, raised to the power n. A domain error occurs if x < −1.
Depending on n, a range error occurs if either positive finite x is too large or if x is too near but not
equal to-1 . A pole error may occur if x equals −1 and n < 0.
Returns
3 The compoundn functions return (1 + x)n .
7.12.7.3 The fabs functions
Synopsis
1 #include <math.h>
double fabs(double x);
float fabsf(float x);
long double fabsl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fabsd32(_Decimal32 x);
_Decimal64 fabsd64(_Decimal64 x);
_Decimal128 fabsd128(_Decimal128 x);
#endif
Description
2 The fabs functions compute the absolute value of x.
Returns
3 The fabs functions return |x|.
7.12.7.4 The hypot functions
Synopsis
1 #include <math.h>
double hypot(double x, double y);
float hypotf(float x, float y);
long double hypotl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 hypotd32(_Decimal32 x, _Decimal32 y);
_Decimal64 hypotd64(_Decimal64 x, _Decimal64 y);
_Decimal128 hypotd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The hypot functions compute the square root of the sum of the squares of x and y, without undue
overflow or underflow. A range error occurs for some finite arguments.
3
Returns
p
4 The hypot functions return x 2 + y2 .
7.12.7.5 The pow functions
Synopsis
1 #include <math.h>
double pow(double x, double y);
float powf(float x, float y);
long double powl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 powd32(_Decimal32 x, _Decimal32 y);
_Decimal64 powd64(_Decimal64 x, _Decimal64 y);
_Decimal128 powd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The pow functions compute x raised to the power y. A domain error occurs if x is finite and less than
zero and y is finite and not an integer value. A domain error may occur if x is zero and y is zero.
Depending on y, a range error occurs if either the magnitude of nonzero finite x is too large or too
near zero. A domain error or pole error may occur if x is zero and y is less than zero.
Returns
3 The pow functions return xy .
7.12.7.6 The pown functions
Synopsis
1 #include <stdint.h>
#include <math.h>
double pown(double x, long long int n);
float pownf(float x, long long int n);
long double pownl(long double x, long long int n);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 pownd32(_Decimal32 x, long long int n);
_Decimal64 pownd64(_Decimal64 x, long long int n);
_Decimal128 pownd128(_Decimal128 x, long long int n);
#endif
Description
2 The pown functions compute x raised to the nth power. A pole error may occur if x equals 0 and
n < 0. Depending on n, a range error occurs if either the magnitude of nonzero finite x is too large
or too near zero.
Returns
3 The pown functions return xn .
7.12.7.7 The powr functions
Synopsis
1 #include <math.h>
double powr(double y, double x);
float powrf(float y, float x);
long double powrl(long double y, long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 powrd32(_Decimal32 y, _Decimal32 x);
_Decimal64 powrd64(_Decimal64 y, _Decimal64 x);
_Decimal128 powrd128(_Decimal128 y, _Decimal128 x);
#endif
Description
2 The powr functions compute x raised to the power y as ey loge x .294) A domain error occurs if x < 0
or if x and y are both zero. Depending on y, a range error occurs if either positive nonzero finite x is
too large or too near zero. A pole error may occur if x equals zero and finite y < 0.
Returns
3 The powr functions return ey loge x .
7.12.7.8 The rootn functions
Synopsis
1 #include <stdint.h>
#include <math.h>
double rootn(double x, long long int n);
float rootnf(float x, long long int n);
long double rootnl(long double x, long long int n);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 rootnd32(_Decimal32 x, long long int n);
_Decimal64 rootnd64(_Decimal64 x, long long int n);
_Decimal128 rootnd128(_Decimal128 x, long long int n);
#endif
Description
2 The rootn functions compute the principal nth root of x. A domain error occurs if n is 0 or if x < 0
and n is even. If n is −1, a range error occurs if either the magnitude of nonzero finite x is too large
or too near zero. A pole error may occur if x equals zero and n < 0.
Returns
1
3 The rootn functions return x n .
7.12.7.9 The rsqrt functions
Synopsis
1 #include <math.h>
double rsqrt(double x);
294) Restricting the domain to that of the formula ey loge x is intended to better meet expectations for a continuous power
function and to allow implementations with fewer tests for special cases.
float rsqrtf(float x);
long double rsqrtl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 rsqrtd32(_Decimal32 x);
_Decimal64 rsqrtd64(_Decimal64 x);
_Decimal128 rsqrtd128(_Decimal128 x);
#endif
Description
2 The rsqrt functions compute the reciprocal of the nonnegative square root of the argument. A
domain error occurs if the argument is less than zero. A pole error may occur if the argument equals
zero.
Returns
3 The rsqrt functions return √1x .
7.12.7.10 The sqrt functions
Synopsis
1 #include <math.h>
double sqrt(double x);
float sqrtf(float x);
long double sqrtl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 sqrtd32(_Decimal32 x);
_Decimal64 sqrtd64(_Decimal64 x);
_Decimal128 sqrtd128(_Decimal128 x);
#endif
Description
2 The sqrt functions compute the nonnegative square root of x. A domain error occurs if the argument
is less than zero.
Returns
√
3 The sqrt functions return x.
7.12.8 Error and gamma functions
7.12.8.1 The erf functions
Synopsis
1 #include <math.h>
double erf(double x);
float erff(float x);
long double erfl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 erfd32(_Decimal32 x);
_Decimal64 erfd64(_Decimal64 x);
_Decimal128 erfd128(_Decimal128 x);
#endif
Description
2 The erf functions compute the error function of x. A range error occurs if nonzero x is too close to
zero.
Returns
Rx −t2
3 The erf functions return erf x = √2π e dt.
0
7.12.8.2 The erfc functions
Synopsis
1 #include <math.h>
double erfc(double x);
float erfcf(float x);
long double erfcl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 erfcd32(_Decimal32 x);
_Decimal64 erfcd64(_Decimal64 x);
_Decimal128 erfcd128(_Decimal128 x);
#endif
Description
2 The erfc functions compute the complementary error function of x. A range error occurs if positive
finite x is too large.
Returns
R∞ −t2
3 The erfc functions return erfc x = 1 − erf x = √2π e dt.
x
7.12.8.3 The lgamma functions
Synopsis
1 #include <math.h>
double lgamma(double x);
float lgammaf(float x);
long double lgammal(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 lgammad32(_Decimal32 x);
_Decimal64 lgammad64(_Decimal64 x);
_Decimal128 lgammad128(_Decimal128 x);
#endif
Description
2 The lgamma functions compute the natural logarithm of the absolute value of gamma of x. A range
error occurs if positive finite x is too large. A pole error may occur if x is a negative integer or zero.
Returns
3 The lgamma functions return loge |Γ(x)|.
7.12.8.4 The tgamma functions
Synopsis
1 #include <math.h>
double tgamma(double x);
float tgammaf(float x);
long double tgammal(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 tgammad32(_Decimal32 x);
_Decimal64 tgammad64(_Decimal64 x);
_Decimal128 tgammad128(_Decimal128 x);
#endif
Description
2 The tgamma functions compute the gamma function of x. A domain error or pole error may occur
if x is a negative integer or zero. A range error occurs for some negative finite x less than zero, if
positive finite x is too large, or nonzero x is too close to zero.
Returns
3 The tgamma functions return Γ(x).
7.12.9 Nearest integer functions
7.12.9.1 The ceil functions
Synopsis
1 #include <math.h>
double ceil(double x);
float ceilf(float x);
long double ceill(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 ceild32(_Decimal32 x);
_Decimal64 ceild64(_Decimal64 x);
_Decimal128 ceild128(_Decimal128 x);
#endif
Description
2 The ceil functions compute the smallest integer value not less than x.
Returns
3 The ceil functions return ⌈x⌉, expressed as a floating-point number.
7.12.9.2 The floor functions
Synopsis
1 #include <math.h>
double floor(double x);
float floorf(float x);
long double floorl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 floord32(_Decimal32 x);
_Decimal64 floord64(_Decimal64 x);
_Decimal128 floord128(_Decimal128 x);
#endif
Description
2 The floor functions compute the largest integer value not greater than x.
Returns
3 The floor functions return ⌊x⌋, expressed as a floating-point number.
7.12.9.3 The nearbyint functions
Synopsis
1 #include <math.h>
double nearbyint(double x);
float nearbyintf(float x);
long double nearbyintl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 nearbyintd32(_Decimal32 x);
_Decimal64 nearbyintd64(_Decimal64 x);
_Decimal128 nearbyintd128(_Decimal128 x);
#endif
Description
2 The nearbyint functions round their argument to an integer value in floating-point format, using
the current rounding direction and without raising the "inexact" floating-point exception.
Returns
3 The nearbyint functions return the rounded integer value.
7.12.9.4 The rint functions
Synopsis
1 #include <math.h>
double rint(double x);
float rintf(float x);
long double rintl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 rintd32(_Decimal32 x);
_Decimal64 rintd64(_Decimal64 x);
_Decimal128 rintd128(_Decimal128 x);
#endif
Description
2 The rint functions differ from the nearbyint functions (7.12.9.3) only in that the rint functions
may raise the "inexact" floating-point exception if the result differs in value from the argument.
Returns
3 The rint functions return the rounded integer value.
7.12.9.5 The lrint and llrint functions
Synopsis
1 #include <math.h>
long int lrint(double x);
long int lrintf(float x);
long int lrintl(long double x);
long long int llrint(double x);
long long int llrintf(float x);
long long int llrintl(long double x);
#ifdef __STDC_IEC_60559_DFP__
long int lrintd32(_Decimal32 x);
long int lrintd64(_Decimal64 x);
long int lrintd128(_Decimal128 x);
long long int llrintd32(_Decimal32 x);
long long int llrintd64(_Decimal64 x);
long long int llrintd128(_Decimal128 x);
#endif
Description
2 The lrint and llrint functions round their argument to the nearest integer value, rounding
according to the current rounding direction. If the rounded value is outside the range of the return
type, the numeric result is unspecified and a domain error or range error may occur.
Returns
3 The lrint and llrint functions return the rounded integer value.
7.12.9.6 The round functions
Synopsis
1 #include <math.h>
double round(double x);
float roundf(float x);
long double roundl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 roundd32(_Decimal32 x);
_Decimal64 roundd64(_Decimal64 x);
_Decimal128 roundd128(_Decimal128 x);
#endif
Description
2 The round functions round their argument to the nearest integer value in floating-point format,
rounding halfway cases away from zero, regardless of the current rounding direction.
Returns
3 The round functions return the rounded integer value.
7.12.9.7 The lround and llround functions
Synopsis
1 #include <math.h>
long int lround(double x);
long int lroundf(float x);
long int lroundl(long double x);
long long int llround(double x);
long long int llroundf(float x);
long long int llroundl(long double x);
#ifdef __STDC_IEC_60559_DFP__
long int lroundd32(_Decimal32 x);
long int lroundd64(_Decimal64 x);
long int lroundd128(_Decimal128 x);
long long int llroundd32(_Decimal32 x);
long long int llroundd64(_Decimal64 x);
long long int llroundd128(_Decimal128 x);
#endif
Description
2 The lround and llround functions round their argument to the nearest integer value, rounding
halfway cases away from zero, regardless of the current rounding direction. If the rounded value is
outside the range of the return type, the numeric result is unspecified and a domain error or range
error may occur.
Returns
3 The lround and llround functions return the rounded integer value.
7.12.9.8 The roundeven functions
Synopsis
1 #include <math.h>
double roundeven(double x);
float roundevenf(float x);
long double roundevenl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 roundevend32(_Decimal32 x);
_Decimal64 roundevend64(_Decimal64 x);
_Decimal128 roundevend128(_Decimal128 x);
#endif
Description
2 The roundeven functions round their argument to the nearest integer value in floating-point format,
rounding halfway cases to even (that is, to the nearest value that is an even integer), regardless of
the current rounding direction.
Returns
3 The roundeven functions return the rounded integer value.
7.12.9.9 The trunc functions
Synopsis
1 #include <math.h>
double trunc(double x);
float truncf(float x);
long double truncl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 truncd32(_Decimal32 x);
_Decimal64 truncd64(_Decimal64 x);
_Decimal128 truncd128(_Decimal128 x);
#endif
Description
2 The trunc functions round their argument to the integer value, in floating format, nearest to but no
larger in magnitude than the argument.
Returns
3 The trunc functions return the truncated integer value.
7.12.9.10 The fromfp and ufromfp functions
Synopsis
1 #include <stdint.h>
#include <math.h>
double fromfp(double x, int rnd, unsigned int width);
float fromfpf(float x, int rnd, unsigned int width);
long double fromfpl(long double x, int rnd, unsigned int width);
double ufromfp(double x, int rnd, unsigned int width);
float ufromfpf(float x, int rnd, unsigned int width);
long double ufromfpl(long double x, int rnd, unsigned int width);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fromfpd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 fromfpd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 fromfpd128(_Decimal128 x, int rnd, unsigned int width);
_Decimal32 ufromfpd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 ufromfpd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 ufromfpd128(_Decimal128 x, int rnd, unsigned int width);
#endif
Description
2 The fromfp and ufromfp functions round x, using the math rounding direction indicated by rnd, to
a signed or unsigned integer, respectively. If width is nonzero and the resulting integer is within the
range
— [−2(width−1) , 2(width−1) − 1], for signed
— [0, 2width − 1], for unsigned
then the functions return the integer value (represented in floating type). Otherwise, if width is
zero or x does not round to an integer within the range, the functions return a NaN (of the type of
the x argument, if available), else the value of x, and a domain error occurs. If the value of the
rnd argument is not equal to the value of a math rounding direction macro (7.12), the direction of
rounding is unspecified. The fromfp and ufromfp functions do not raise the "inexact" floating-point
exception.
Returns
3 The fromfp and ufromfp functions return the rounded integer value.
4 EXAMPLE Upward rounding of double x to type int, without raising the "inexact" floating-point exception, is achieved by
(int)fromfp(x, FP_INT_UPWARD, INT_WIDTH)
5 EXAMPLE Unsigned integer wrapping is not performed in
ufromfp(-3.0, FP_INT_UPWARD, UINT_WIDTH) /* domain error */
7.12.9.11 The fromfpx and ufromfpx functions
Synopsis
1 #include <stdint.h>
#include <math.h>
double fromfpx(double x, int rnd, unsigned int width);
float fromfpxf(float x, int rnd, unsigned int width);
long double fromfpxl(long double x, int rnd, unsigned int width);
double ufromfpx(double x, int rnd, unsigned int width);
float ufromfpxf(float x, int rnd, unsigned int width);
long double ufromfpxl(long double x, int rnd, unsigned int width);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fromfpxd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 fromfpxd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 fromfpxd128(_Decimal128 x, int rnd, unsigned int width);
_Decimal32 ufromfpxd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 ufromfpxd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 ufromfpxd128(_Decimal128 x, int rnd, unsigned int width);
#endif
Description
2 The fromfpx and ufromfpx functions differ from the fromfp and ufromfp functions, respectively,
only in that the fromfpx and ufromfpx functions raise the "inexact" floating-point exception if a
rounded result not exceeding the specified width differs in value from the argument x.
Returns
3 The fromfpx and ufromfpx functions return the rounded integer value.
4 NOTE Conversions to integer types that are not required to raise the inexact exception can be done simply by rounding to
integral value in floating type and then converting to the target integer type. For example, the conversion of long double x
to uint64_t, using upward rounding, is done by
(uint64_t)ceill(x)
7.12.10 Remainder functions
7.12.10.1 The fmod functions
Synopsis
1 #include <math.h>
double fmod(double x, double y);
float fmodf(float x, float y);
long double fmodl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmodd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmodd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmodd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmod functions compute the floating-point remainder of x/y.
Returns
3 The fmod functions return the value x − ny, for some integer n such that, if y is nonzero, the result
has the same sign as x and magnitude less than the magnitude of y. If y is zero, whether a domain
error occurs or the fmod functions return zero is implementation-defined.
7.12.10.2 The remainder functions
Synopsis
1 #include <math.h>
double remainder(double x, double y);
float remainderf(float x, float y);
long double remainderl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 remainderd32(_Decimal32 x, _Decimal32 y);
_Decimal64 remainderd64(_Decimal64 x, _Decimal64 y);
_Decimal128 remainderd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The remainder functions compute the remainder x REM y required by IEC 60559. 295)
Returns
3 The remainder functions return x REM y. If y is zero, whether a domain error occurs or the functions
return zero is implementation-defined.
7.12.10.3 The remquo functions
Synopsis
1 #include <math.h>
double remquo(double x, double y, int *quo);
float remquof(float x, float y, int *quo);
long double remquol(long double x, long double y, int *quo);
Description
2 The remquo functions compute the same remainder as the remainder functions. In the object pointed
to by quo they store a value whose sign is the sign of x/y and whose magnitude is congruent modulo
2n to the magnitude of the integral quotient of x/y, where n is an implementation-defined integer
greater than or equal to 3.
Returns
3 The remquo functions return x REM y. If y is zero, the value stored in the object pointed to by quo
is unspecified and whether a domain error occurs or the functions return zero is implementation-
defined.
4 NOTE There are no decimal floating-point versions of the remquo functions.
7.12.11 Manipulation functions
7.12.11.1 The copysign functions
Synopsis
1 #include <math.h>
double copysign(double x, double y);
295) "When y ̸= 0, the remainder r = x REM y is defined regardless of the rounding mode by the mathematical relation
r = x − ny, where n is the integer nearest the exact value of xy
; whenever |n − x
y
| = 12 , then n is even. If r = 0, its sign shall
be that of x." This definition is applicable for all implementations.
float copysignf(float x, float y);
long double copysignl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 copysignd32(_Decimal32 x, _Decimal32 y);
_Decimal64 copysignd64(_Decimal64 x, _Decimal64 y);
_Decimal128 copysignd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The copysign functions produce a value with the magnitude of x and the sign of y. If x or y is an
unsigned value, the sign (if any) of the result is implementation-defined. On implementations that
represent a signed zero but do not treat negative zero consistently in arithmetic operations, the
copysign functions should regard the sign of zero as positive.
Returns
3 The copysign functions return a value with the magnitude of x and the sign of y.
7.12.11.2 The nan functions
Synopsis
1 #include <math.h>
double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 nand32(const char *tagp);
_Decimal64 nand64(const char *tagp);
_Decimal128 nand128(const char *tagp);
#endif
Description
2 The nan, nanf, and nanl functions convert the string pointed to by tagp according to the following
rules. The call nan("n-char-sequence") is equivalent to strtod("NAN(n-char-sequence)", nullptr);
the call nan("") is equivalent to strtod("NAN()", nullptr). If tagp does not point to an empty
string or an n-char sequence, the call is equivalent to strtod("NAN", nullptr). Calls to nanf and
nanl are equivalent to the corresponding calls to strtof and strtold.
Returns
3 The nan functions return a quiet NaN, if available, with content indicated through tagp. If the
implementation does not support quiet NaNs, the functions return zero.
Forward references: the strtod, strtof, and strtold functions (7.24.1.5).
7.12.11.3 The nextafter functions
Synopsis
1 #include <math.h>
double nextafter(double x, double y);
float nextafterf(float x, float y);
long double nextafterl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 nextafterd32(_Decimal32 x, _Decimal32 y);
_Decimal64 nextafterd64(_Decimal64 x, _Decimal64 y);
_Decimal128 nextafterd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The nextafter functions determine the next representable value, in the type of the function, after x
in the direction of y, where x and y are first converted to the type of the function296) . The nextafter
functions return y if x equals y.
A range error occurs if the magnitude of x is the largest finite value representable in the type and the
result is infinite or not representable in the type.
Returns
3 The nextafter functions return the next representable value in the specified format after x in the
direction of y.
7.12.11.4 The nexttoward functions
Synopsis
1 #include <math.h>
double nexttoward(double x, long double y);
float nexttowardf(float x, long double y);
long double nexttowardl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 nexttowardd32(_Decimal32 x, _Decimal128 y);
_Decimal64 nexttowardd64(_Decimal64 x, _Decimal128 y);
_Decimal128 nexttowardd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The nexttoward functions are equivalent to the nextafter functions except that the second param-
eter has type long double or _Decimal128 and the functions return y converted to the type of the
function if x equals y.297)
7.12.11.5 The nextup functions
Synopsis
1 #include <math.h>
double nextup(double x);
float nextupf(float x);
long double nextupl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 nextupd32(_Decimal32 x);
_Decimal64 nextupd64(_Decimal64 x);
_Decimal128 nextupd128(_Decimal128 x);
#endif
Description
2 The nextup functions determine the next representable value, in the type of the function, greater
than x. If x is the negative number of least magnitude in the type of x, nextup(x) is −0 if the
type has signed zeros and is 0 otherwise. If x is zero, nextup(x) is the positive number of least
magnitude in the type of x. If x is the positive number (finite or infinite) or maximum magnitude in
the type, nextup(x) is x.
Returns
3 The nextup functions return the next representable value in the specified type greater than x.
7.12.11.6 The nextdown functions
296) The argument values are converted to the type of the function, even by a macro implementation of the function.
297) The result of the nexttoward functions is determined in the type of the function, without loss of range or precision in a
floating second argument.
Synopsis
1 #include <math.h>
double nextdown(double x);
float nextdownf(float x);
long double nextdownl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 nextdownd32(_Decimal32 x);
_Decimal64 nextdownd64(_Decimal64 x);
_Decimal128 nextdownd128(_Decimal128 x);
#endif
Description
2 The nextdown functions determine the next representable value, in the type of the function, less than
x . If x is the positive number of least magnitude in the type of x , nextdown(x) is +0 if the type has
signed zeros and is 0 otherwise. If x is zero, nextdown(x) is the negative number of least magnitude
in the type of x. If x is the negative number (finite or infinite) of maximum magnitude in the type,
nextdown(x) is x .
Returns
3 The nextdown functions return the next representable value in the specified type less than x.
7.12.11.7 The canonicalize functions
Synopsis
1 #include <math.h>
int canonicalize(double * cx, const double * x);
int canonicalizef(float * cx, const float * x);
int canonicalizel(long double * cx, const long double * x);
#ifdef __STDC_IEC_60559_DFP__
int canonicalized32(_Decimal32 cx, const _Decimal32 * x);
int canonicalized64(_Decimal64 cx, const _Decimal64 * x);
int canonicalized128(_Decimal128 cx, const _Decimal128 * x);
#endif
Description
2 The canonicalize functions attempt to produce a canonical version of the floating-point repre-
sentation in the object pointed to by the argument x, as if to a temporary object of the specified
type, and store the canonical result in the object pointed to by the argument cx.298) If the input *x
is a signaling NaN, the canonicalize functions are intended to store a canonical quiet NaN. If a
canonical result is not produced the object pointed to by cx is unchanged.
Returns
3 The canonicalize functions return zero if a canonical result is stored in the object pointed to by cx.
Otherwise they return a nonzero value.
7.12.12 Maximum, minimum, and positive difference functions
7.12.12.1 The fdim functions
Synopsis
1 #include <math.h>
double fdim(double x, double y);
float fdimf(float x, float y);
long double fdiml(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fdimd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fdimd64(_Decimal64 x, _Decimal64 y);
298) Arguments x and cx may point to the same object.
_Decimal128 fdimd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fdim functions determine the positive difference between their arguments:
(
x − y if x > y
+0 if x ≤ y
A range error may occur.
Returns
3 The fdim functions return the positive difference value.
7.12.12.2 The fmax functions
Synopsis
1 #include <math.h>
double fmax(double x, double y);
float fmaxf(float x, float y);
long double fmaxl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmaxd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaxd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaxd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmax functions determine the maximum numeric value of their arguments.299)
Returns
3 The fmax functions return the maximum numeric value of their arguments.
7.12.12.3 The fmin functions
Synopsis
1 #include <math.h>
double fmin(double x, double y);
float fminf(float x, float y);
long double fminl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmind32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmind64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmind128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmin functions determine the minimum numeric value of their arguments. 300)
Returns
3 The fmin functions return the minimum numeric value of their arguments.
4 NOTE 1 The fmax and fmin functions are similar to the fmaximum_num and fminimum_num functions, though may differ in
which signed zero is returned when the arguments are differently signed zeros and in their treatment of signaling NaNs
(see F.10.9.5).
299) Quiet NaN arguments are treated as missing data: if one argument is a quiet NaN and the other numeric, then the fmax
functions choose the numeric value. See F.10.9.2.
300) The fmin functions are analogous to the fmax functions in their treatment of quiet NaNs.
7.12.12.4 The fmaximum functions
Synopsis
1 #include <math.h>
double fmaximum(double x, double y);
float fmaximumf(float x, float y);
long double fmaximuml(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmaximumd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximumd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximumd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmaximum functions determine the maximum value of their arguments. For these functions, +0
is considered greater than −0. These functions differ from the fmaximum_num functions only in their
treatment of NaN arguments (see F.10.9.4, F.10.9.5).
Returns
3 The fmaximum functions return the maximum value of their arguments.
7.12.12.5 The fminimum functions
Synopsis
1 #include <math.h>
double fminimum(double x, double y);
float fminimumf(float x, float y);
long double fminimuml(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fminimumd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimumd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimumd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fminimum functions determine the minimum value of their arguments. For these functions, −0
is considered less than +0. These functions differ from the fminimum_num functions only in their
treatment of NaN arguments (see F.10.9.4, F.10.9.5).
Returns
3 The fminimum functions return the minimum value of their arguments.
7.12.12.6 The fmaximum_mag functions
Synopsis
1 #include <math.h>
double fmaximum_mag(double x, double y);
float fmaximum_magf(float x, float y);
long double fmaximum_magl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmaximum_magd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximum_magd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximum_magd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmaximum_mag functions determine the value of the argument of maximum magnitude:
x if |x | > |y|, y if |y| > |x |, and fmaximum(x, y) otherwise. These functions differ from the
fmaximum_mag_num functions only in their treatment of NaN arguments (see F.10.9.4, F.10.9.5).
Returns
3 The fmaximum_mag functions return the value of the argument of maximum magnitude.
7.12.12.7 The fminimum_mag functions
Synopsis
1 #include <math.h>
double fminimum_mag(double x, double y);
float fminimum_magf(float x, float y);
long double fminimum_magl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fminimum_magd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimum_magd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimum_magd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fminimum_mag functions determine the value of the argument of minimum magnitude:
x if |x | < |y|, y if |y| < |x |, and fminimum(x, y) otherwise. These functions differ from the
fminimum_mag_num functions only in their treatment of NaN arguments (see F.10.9.4, F.10.9.5).
Returns
3 The fminimum_mag functions return the value of the argument of minimum magnitude.
7.12.12.8 The fmaximum_num functions
Synopsis
1 #include <math.h>
double fmaximum_num(double x, double y);
float fmaximum_numf(float x, float y);
long double fmaximum_numl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmaximum_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximum_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximum_numd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmaximum_num functions determine the maximum value of their numeric arguments. They
determine the number if one argument is a number and the other is a NaN. These functions differ
from the fmaximum functions only in their treatment of NaN arguments (see F.10.9.4, F.10.9.5).
Returns
3 The fmaximum_num functions return the maximum value of their numeric arguments.
7.12.12.9 The fminimum_num functions
Synopsis
1 #include <math.h>
double fminimum_num(double x, double y);
float fminimum_numf(float x, float y);
long double fminimum_numl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fminimum_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimum_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimum_numd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fminimum_num functions determine the minimum value of their numeric arguments. They
determine the number if one argument is a number and the other is a NaN. These functions differ
from the fminimum functions only in their treatment of NaN arguments (see F.10.9.4, F.10.9.5).
Returns
3 The fminimum_num functions return the minimum value of their numeric arguments.
7.12.12.10 The fmaximum_mag_num functions
Synopsis
1 #include <math.h>
double fmaximum_mag_num(double x, double y);
float fmaximum_mag_numf(float x, float y);
long double fmaximum_mag_numl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmaximum_mag_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximum_mag_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximum_mag_numd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fmaximum_mag_num functions determine the value of a numeric argument of maximum mag-
nitude. They determine the number if one argument is a number and the other is a NaN. These
functions differ from the fmaximum_mag functions only in their treatment of NaN arguments
(see F.10.9.4, F.10.9.5).
Returns
3 The fmaximum_mag_num functions return the value of a numeric argument of maximum magnitude.
7.12.12.11 The fminimum_mag_num functions
Synopsis
1 #include <math.h>
double fminimum_mag_num(double x, double y);
float fminimum_mag_numf(float x, float y);
long double fminimum_mag_numl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fminimum_mag_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimum_mag_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimum_mag_numd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The fminimum_mag_num functions determine the value of a numeric argument of minimum mag-
nitude. They determine the number if one argument is a number and the other is a NaN. These
functions differ from the fminimum_mag functions only in their treatment of NaN arguments
(see F.10.9.4, F.10.9.5).
Returns
3 The fminimum_mag_num functions return the value of a numeric argument of mimum minagnitude.
7.12.13 Fused multiply-add
7.12.13.1 The fma functions
Synopsis
1 #include <math.h>
double fma(double x, double y, double z);
float fmaf(float x, float y, float z);
long double fmal(long double x, long double y, long double z);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 fmad32(_Decimal32 x, _Decimal32 y, _Decimal32 z);
_Decimal64 fmad64(_Decimal64 x, _Decimal64 y, _Decimal64 z);
_Decimal128 fmad128(_Decimal128 x, _Decimal128 y, _Decimal128 z);
#endif
Description
2 The fma functions compute (x × y) + z, rounded as one ternary operation: they compute the value
(as if) to infinite precision and round once to the result format, according to the current rounding
mode. A range error occurs for some finite arguments.
Returns
3 The fma functions return (x × y) + z, rounded as one ternary operation.
7.12.14 Functions that round result to narrower type
1 The functions in this subclause round their results to a type typically narrower301) than the parameter
types.
7.12.14.1 Add and round to narrower type
Synopsis
1 #include <math.h>
float fadd(double x, double y);
float faddl(long double x, long double y);
double daddl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32addd64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32addd128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64addd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 These functions compute the sum of x + y, rounded to the type of the function. They compute
the sum (as if) to infinite precision and round once to the result format, according to the current
rounding mode. A range error occurs for some finite arguments. A domain error may occur for
infinite arguments.
Returns
3 These functions return the sum of x + y, rounded to the type of the function.
7.12.14.2 Subtract and round to narrower type
Synopsis
1 #include <math.h>
float fsub(double x, double y);
float fsubl(long double x, long double y);
double dsubl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32subd64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32subd128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64subd128(_Decimal128 x, _Decimal128 y);
#endif
301) In some cases the destination type might not be narrower than the parameter types. For example, double might not be
narrower than long double.
Description
2 These functions compute the difference of x − y, rounded to the type of the function. They compute
the difference (as if) to infinite precision and round once to the result format, according to the current
rounding mode. A range error occurs for some finite arguments. A domain error may occur for
infinite arguments.
Returns
3 These functions return the difference of x − y, rounded to the type of the function.
7.12.14.3 Multiply and round to narrower type
Synopsis
1 #include <math.h>
float fmul(double x, double y);
float fmull(long double x, long double y);
double dmull(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32muld64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32muld128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64muld128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 These functions compute the product x × y, rounded to the type of the function. They compute the
product (as if) to infinite precision and round once to the result format, according to the current
rounding mode. A range error occurs for some finite arguments. A domain error occurs for one
infinite argument and one zero argument.
Returns
3 These functions return the product of x × y, rounded to the type of the function.
7.12.14.4 Divide and round to narrower type
Synopsis
1 #include <math.h>
float fdiv(double x, double y);
float fdivl(long double x, long double y);
double ddivl(long double x, long double y);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32divd64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32divd128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64divd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 These functions compute the quotient x ÷ y, rounded to the type of the function. They compute the
quotient (as if) to infinite precision and round once to the result format, according to the current
rounding mode. A range error occurs for some finite arguments. A domain error occurs for either
both arguments infinite or both arguments zero. A pole error occurs for a finite x and a zero y.
Returns
3 These functions return the quotient x ÷ y, rounded to the type of the function.
7.12.14.5 Fused multiply-add and round to narrower type
Synopsis
1 #include <math.h>
float ffma(double x, double y, double z);
float ffmal(long double x, long double y, long double z);
double dfmal(long double x, long double y, long double z);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32fmad64(_Decimal64 x, _Decimal64 y, _Decimal64 z);
_Decimal32 d32fmad128(_Decimal128 x, _Decimal128 y, _Decimal128 z);
_Decimal64 d64fmad128(_Decimal128 x, _Decimal128 y, _Decimal128 z);
#endif
Description
2 These functions compute (x × y) + z, rounded to the type of the function. They compute (x × y) + z
(as if) to infinite precision and round once to the result format, according to the current rounding
mode. A range error occurs for some finite arguments. A domain error may occur for an infinite
argument.
Returns
3 These functions return (x × y) + z, rounded to the type of the function.
7.12.14.6 Square root rounded to narrower type
Synopsis
1 #include <math.h>
float fsqrt(double x);
float fsqrtl(long double x);
double dsqrtl(long double x);
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32sqrtd64(_Decimal64 x);
_Decimal32 d32sqrtd128(_Decimal128 x);
_Decimal64 d64sqrtd128(_Decimal128 x);
#endif
Description
2 These functions compute the square root of x, rounded to the type of the function. They compute the
square root (as if) to infinite precision and round once to the result format, according to the current
rounding mode. A range error occurs for some finite positive arguments. A domain error occurs if
the argument is less than zero.
Returns
3 These functions return the nonnegative square root of x, rounded to the type of the function.
7.12.15 Quantum and quantum exponent functions
7.12.15.1 The quantizedN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 quantized32(_Decimal32 x, _Decimal32 y);
_Decimal64 quantized64(_Decimal64 x, _Decimal64 y);
_Decimal128 quantized128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The quantizedN functions compute, if possible, a value with the numerical value of x and the
quantum exponent of y. If the quantum exponent is being increased, the value shall be correctly
rounded; if the result does not have the same value as x, the "inexact" floating-point exception shall
be raised. If the quantum exponent is being decreased and the significand of the result has more
digits than the type would allow, the result is NaN, the "invalid" floating-point exception is raised,
and a domain error occurs. If one or both operands are NaN the result is NaN. Otherwise if only one
operand is infinite, the result is NaN, the "invalid" floating-point exception is raised, and a domain
error occurs. If both operands are infinite, the result is DEC_INFINITY with the sign of x, converted
to the type of the function. The quantizedN functions do not raise the "overflow" and "underflow"
floating-point exceptions.
Returns
3 The quantizedN functions return a value with the numerical value of x (except for any rounding)
and the quantum exponent of y.
7.12.15.2 The samequantumdN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
bool samequantumd32(_Decimal32 x, _Decimal32 y);
bool samequantumd64(_Decimal64 x, _Decimal64 y);
bool samequantumd128(_Decimal128 x, _Decimal128 y);
#endif
Description
2 The samequantumdN functions determine if the quantum exponents of x and y are the same. If both
x and y are NaN, or both infinite, they have the same quantum exponents; if exactly one operand
is infinite or exactly one operand is NaN, they do not have the same quantum exponents. The
samequantumdN functions raise no floating-point exception.
Returns
3 The samequantumdN functions return nonzero (true) when x and y have the same quantum expo-
nents, zero (false) otherwise.
7.12.15.3 The quantumdN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 quantumd32(_Decimal32 x);
_Decimal64 quantumd64(_Decimal64 x);
_Decimal128 quantumd128(_Decimal128 x);
#endif
Description
2 The quantumdN functions compute the quantum (5.2.4.2.3) of a finite argument. If x is infinite, the
result is +∞.
Returns
3 The quantumdN functions return the quantum of x.
7.12.15.4 The llquantexpdN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
long long int llquantexpd32(_Decimal32 x);
long long int llquantexpd64(_Decimal64 x);
long long int llquantexpd128(_Decimal128 x);
#endif
Description
2 The llquantexpdN functions compute the quantum exponent (5.2.4.2.3) of a finite argument. If x is
infinite or NaN, they compute LLONG_MIN, the "invalid" floating-point exception is raised, and a
domain error occurs.
Returns
3 The llquantexpdN functions return the quantum exponent of x.
7.12.16 Decimal re-encoding functions
1 IEC 60559 specifies two different schemes to encode significands in the object representation of a
decimal floating-point object: one based on decimal encoding (which packs three decimal digits
into 10 bits), the other based on binary encoding (as a binary integer). An implementation may use
either of these encoding schemes for its decimal floating types. The re-encoding functions in this
subclause provide conversions between external decimal data with a given encoding scheme and
the implementation’s corresponding decimal floating type.
7.12.16.1 The encodedecdN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
void encodedecd32(unsigned char encptr[restrict static 4],
const _Decimal32*restrict xptr);
void encodedecd64(unsigned char encptr[restrict static 8],
const _Decimal64*restrict xptr);
void encodedecd128(unsigned char encptr[restrict static 16],
const _Decimal128*restrict xptr);
#endif
Description
2 The encodedecdN functions convert *xptr into an IEC 60559 decimalN encoding in the encoding
scheme based on decimal encoding of the significand and store the resulting encoding as an N/8
element array, with 8 bits per array element, in the object pointed to by encptr. The order of bytes
in the array is implementation-defined. These functions preserve the value of *xptr and raise no
floating-point exceptions. If *xptr is non-canonical, these functions may or may not produce a
canonical encoding.
Returns
3 The encodedecdN functions return no value.
7.12.16.2 The decodedecdN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
void decodedecd32(_Decimal32 * restrict xptr,
const unsigned char encptr[restrict static 4]);
void decodedecd64(_Decimal64 * restrict xptr,
const unsigned char encptr[restrict static 8]);
void decodedecd128(_Decimal128 * restrict xptr,
const unsigned char encptr[restrict static 16]);
#endif
Description
15
2 The decodedecdN functions interpret the N/8 element array pointed to by encptr as an IEC 60559
decimalN encoding, with 8 bits per array element, in the encoding scheme based on decimal
encoding of the significand. The order of bytes in the array is implementation-defined. These
functions convert the given encoding into a value of the decimal floating type, and store the result in
the object pointed to by xptr. These functions preserve the encoded value and raise no floating-point
exceptions. If the encoding is non-canonical, these functions may or may not produce a canonical
representation.
Returns
3 The decodedecdN functions return no value.
7.12.16.3 The encodebindN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
void encodebind32(unsigned char encptr[restrict static 4],
const _Decimal32 * restrict xptr);
void encodebind64(unsigned char encptr[restrict static 8],
const _Decimal64 * restrict xptr);
void encodebind128(unsigned char encptr[restrict static 16],
const _Decimal128 * restrict xptr);
#endif
Description
2 The encodebindN functions convert *xptr into an IEC 60559 decimalN encoding in the encoding
scheme based on binary encoding of the significand and store the resulting encoding as an N/8
element array, with 8 bits per array element, in the object pointed to by encptr. The order of bytes
in the array is implementation-defined. These functions preserve the value of *xptr and raise no
floating-point exceptions. If *xptr is non-canonical, these functions may or may not produce a
canonical encoding.
Returns
3 The encodebindN functions return no value.
7.12.16.4 The decodebindN functions
Synopsis
1 #include <math.h>
#ifdef __STDC_IEC_60559_DFP__
void decodebind32(_Decimal32 * restrict xptr,
const unsigned char encptr[restrict static 4]);
void decodebind64(_Decimal64 * restrict xptr,
const unsigned char encptr[restrict static 8]);
void decodebind128(_Decimal128 * restrict xptr,
const unsigned char encptr[restrict static 16]);
#endif
Description
2 The decodebindN functions interpret the N/8 element array pointed to by encptr as an IEC 60559
decimalN encoding, with 8 bits per array element, in the encoding scheme based on binary encoding
of the significand. The order of bytes in the array is implementation-defined. These functions convert
the given encoding into a value of decimal floating type, and store the result in the object pointed to
by xptr. These functions preserve the encoded value and raise no floating-point exceptions. If the
encoding is non-canonical, these functions may or may not produce a canonical representation.
Returns
3 The decodebindN functions return no value.
7.12.17 Comparison macros
1 The relational and equality operators support the usual mathematical relationships between numeric
values. For any ordered pair of numeric values exactly one of the relationships — less, greater, and
equal — is true. Relational operators may raise the "invalid" floating-point exception when argument
values are NaNs. For a NaN and a numeric value, or for two NaNs, just the unordered relationship
is true.302) Subclauses 7.12.17.1 through 7.12.17.6 provide macros that are quiet versions of the
relational operators: the macros do not raise the "invalid" floating-point exception as an effect
of quiet NaN arguments. The comparison macros facilitate writing efficient code that accounts
for quiet NaNs without suffering the "invalid" floating-point exception. In the synopses in this
subclause, real-floating indicates that the argument shall be an expression of real floating type303)
(both arguments need not have the same type).304) If either argument has decimal floating type, the
other argument shall have decimal floating type as well.
7.12.17.1 The isgreater macro
Synopsis
1 #include <math.h>
int isgreater(real-floating x, real-floating y);
Description
2 The isgreater macro determines whether its first argument is greater than its second argu-
ment. The value of isgreater(x,y) is always equal to (x)> (y) ; however, unlike (x)> (y) ,
isgreater(x,y) does not raise the "invalid" floating-point exception when x and y are unordered
and neither is a signaling NaN.
Returns
3 The isgreater macro returns the value of (x)> (y) .
7.12.17.2 The isgreaterequal macro
Synopsis
1 #include <math.h>
int isgreaterequal(real-floating x, real-floating y);
Description
2 The isgreaterequal macro determines whether its first argument is greater than or equal to its
second argument. The value of isgreaterequal(x,y) is always equal to (x)>= (y) ; however,
unlike (x)>= (y) , isgreaterequal(x,y) does not raise the "invalid" floating-point exception
when x and y are unordered and neither is a signaling NaN.
Returns
3 The isgreaterequal macro returns the value of (x)>= (y) .
7.12.17.3 The isless macro
Synopsis
1 #include <math.h>
int isless(real-floating x, real-floating y);
Description
2 The isless macro determines whether its first argument is less than its second argument. The value
of isless(x,y) is always equal to (x)< (y) ; however, unlike (x)< (y) , isless(x,y) does not
raise the "invalid" floating-point exception when x and y are unordered and neither is a signaling
NaN.
Returns
3 The isless macro returns the value of (x) < (y).
302) IEC 60559 requires that the built-in relational operators raise the "invalid" floating-point exception if the operands
compare unordered, as an error indicator for programs written without consideration of NaNs; the result in these cases is
false.
303) If any argument is of integer type, or any other type that is not a real floating type, the behavior is undefined.
304) Whether an argument represented in a format wider than its semantic type is converted to the semantic type is unspecified.
7.12.17.4 The islessequal macro
Synopsis
1 #include <math.h>
int islessequal(real-floating x, real-floating y);
Description
2 The islessequal macro determines whether its first argument is less than or equal to its sec-
ond argument. The value of islessequal(x,y) is always equal to (x)<= (y) ; however, unlike
(x)<= (y) , islessequal(x,y) does not raise the "invalid" floating-point exception when x and y
are unordered and neither is a signaling NaN.
Returns
3 The islessequal macro returns the value of (x)<= (y) .
7.12.17.5 The islessgreater macro
Synopsis
1 #include <math.h>
int islessgreater(real-floating x, real-floating y);
Description
2 The islessgreater macro determines whether its first argument is less than or greater than its
second argument. The islessgreater(x,y) macro is similar to (x)< (y)|| (x)> (y) ; however,
islessgreater(x,y) does not raise the "invalid" floating-point exception when x and y are un-
ordered and neither is a signaling NaN (nor does it evaluate x and y twice).
Returns
3 The islessgreater macro returns the value of (x)< (y)|| (x)> (y) .
7.12.17.6 The isunordered macro
Synopsis
1 #include <math.h>
int isunordered(real-floating x, real-floating y);
Description
2 The isunordered macro determines whether its arguments are unordered. It raises no floating-point
exceptions if neither argument is a signaling NaN.
Returns
3 The isunordered macro returns 1 if its arguments are unordered and 0 otherwise.
7.12.17.7 The iseqsig macro
Synopsis
1 #include <math.h>
int iseqsig(real-floating x, real-floating y);
Description
2 The iseqsig macro determines whether its arguments are equal. If an argument is a NaN, a domain
error occurs for the macro, as if a domain error occurred for a function (7.12.1).
Returns
3 The iseqsig macro returns 1 if its arguments are equal and 0 otherwise.
7.13 Non-local jumps <setjmp.h>
1 The header <setjmp.h> defines the macro setjmp, and declares one function and one type, for
bypassing the normal function call and return discipline.305)
2 The type declared is
jmp_buf
which is an array type suitable for holding the information needed to restore a calling environment.
The environment of a call to the setjmp macro consists of information sufficient for a call to the
longjmp function to return execution to the correct block and invocation of that block, were it called
recursively. It does not include the state of the floating-point status flags, of open files, or of any
other component of the abstract machine.
3 It is unspecified whether setjmp is a macro or an identifier declared with external linkage. If a
macro definition is suppressed in order to access an actual function, or a program defines an external
identifier with the name setjmp, the behavior is undefined.
7.13.1 Save calling environment
7.13.1.1 The setjmp macro
Synopsis
1 #include <setjmp.h>
int setjmp(jmp_buf env);
Description
2 The setjmp macro saves its calling environment in its jmp_buf argument for later use by the
longjmp function.
Returns
3 If the return is from a direct invocation, the setjmp macro returns the value zero. If the return is
from a call to the longjmp function, the setjmp macro returns a nonzero value.
Environmental limits
4 An invocation of the setjmp macro shall appear only in one of the following contexts:
— the entire controlling expression of a selection or iteration statement;
— one operand of a relational or equality operator with the other operand an integer constant
expression, with the resulting expression being the entire controlling expression of a selection
or iteration statement;
— the operand of a unary ! operator with the resulting expression being the entire controlling
expression of a selection or iteration statement; or
— the entire expression of an expression statement (possibly cast to void).
5 If the invocation appears in any other context, the behavior is undefined.
7.13.2 Restore calling environment
7.13.2.1 The longjmp function
Synopsis
1 #include <setjmp.h>
[[noreturn]] void longjmp(jmp_buf env, int val);
305) These functions are useful for dealing with unusual conditions encountered in a low-level function of a program.
Description
2 The longjmp function restores the environment saved by the most recent invocation of the setjmp
macro in the same invocation of the program with the corresponding jmp_buf argument. If there
has been no such invocation, or if the invocation was from another thread of execution, or if the
function containing the invocation of the setjmp macro has terminated execution306) in the interim,
or if the invocation of the setjmp macro was within the scope of an identifier with variably modified
type and execution has left that scope in the interim, the behavior is undefined.
3 All accessible objects have values, and all other components of the abstract machine307) have state,
as of the time the longjmp function was called, except that the representation of objects of automatic
storage duration that are local to the function containing the invocation of the corresponding
setjmp macro that do not have volatile-qualified type and have been changed between the setjmp
invocation and longjmp call is indeterminate.
Returns
4 After longjmp is completed, thread execution continues as if the corresponding invocation of the
setjmp macro had just returned the value specified by val. The longjmp function cannot cause the
setjmp macro to return the value 0; if val is 0, the setjmp macro returns the value 1.
5 EXAMPLE The longjmp function that returns control back to the point of the setjmp invocation might cause memory
associated with a variable length array object to be squandered.
#include <setjmp.h>
jmp_buf buf;
void g(int n);
void h(int n);
int n = 6;
void f(void)
{
int x[n]; // valid: f is not terminated
setjmp(buf);
g(n);
}
void g(int n)
{
int a[n]; // a may remain allocated
h(n);
}
void h(int n)
{
int b[n]; // b may remain allocated
longjmp(buf, 2); // might cause memory loss
}
306) For example, by executing a return statement or because another longjmp call has caused a transfer to a setjmp
invocation in a function earlier in the set of nested calls.
307) This includes, but is not limited to, the floating-point status flags and the state of open files.
7.14 Signal handling <signal.h>
1 The header <signal.h> declares a type and two functions and defines several macros, for handling
various signals (conditions that may be reported during program execution).
2 The type defined is
sig_atomic_t
which is the (possibly volatile-qualified) integer type of an object that can be accessed as an atomic
entity, even in the presence of asynchronous interrupts.
3 The macros defined are
SIG_DFL
SIG_ERR
SIG_IGN
which expand to constant expressions with distinct values that have type compatible with the second
argument to, and the return value of, the signal function, and whose values compare unequal to
the address of any declarable function; and the following, which expand to positive integer constant
expressions with type int and distinct values that are the signal numbers, each corresponding to
the specified condition:
SIGABRT abnormal termination, such as is initiated by the abort function
SIGFPE an erroneous arithmetic operation, such as zero divide or an operation resulting in
overflow
SIGILL detection of an invalid function image, such as an invalid instruction
SIGINT receipt of an interactive attention signal
SIGSEGV an invalid access to storage
SIGTERM a termination request sent to the program
4 An implementation need not generate any of these signals, except as a result of explicit calls to the
raise function. Additional signals and pointers to undeclarable functions, with macro definitions
beginning, respectively, with the letters SIG and an uppercase letter or with SIG_ and an uppercase
letter,308) may also be specified by the implementation. The complete set of signals, their semantics,
and their default handling is implementation-defined; all signal numbers shall be positive.
7.14.1 Specify signal handling
7.14.1.1 The signal function
Synopsis
1 #include <signal.h>
void (*signal(int sig, void (*func)(int)))(int);
Description
2 The signal function chooses one of three ways in which receipt of the signal number sig is to
be subsequently handled. If the value of func is SIG_DFL, default handling for that signal will
occur. If the value of func is SIG_IGN, the signal will be ignored. Otherwise, func shall point to a
function to be called when that signal occurs. An invocation of such a function because of a signal, or
(recursively) of any further functions called by that invocation (other than functions in the standard
library),309) is called a signal handler.
308) See "future library directions" (7.33.9). The names of the signal numbers reflect the following terms (respectively): abort,
floating-point exception, illegal instruction, interrupt, segmentation violation, and termination.
309) This includes functions called indirectly via standard library functions (e.g., a SIGABRT handler called via the abort
function).
3 When a signal occurs and func points to a function, it is implementation-defined whether the equiva-
lent of signal(sig, SIG_DFL); is executed or the implementation prevents some implementation-
defined set of signals (at least including sig) from occurring until the current signal handling has
completed; in the case of SIGILL, the implementation may alternatively define that no action is taken.
Then the equivalent of (*func)(sig); is executed. If and when the function returns, if the value
of sig is SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to a
computational exception, the behavior is undefined; otherwise the program will resume execution
at the point it was interrupted.
4 If the signal occurs as the result of calling the abort or raise function, the signal handler shall not
call the raise function.
5 If the signal occurs other than as the result of calling the abort or raise function, the behavior is
undefined if the signal handler refers to any object with static or thread storage duration that is
not a lock-free atomic object other than by assigning a value to an object declared as volatile
sig_atomic_t, or the signal handler calls any function in the standard library other than
— the abort function,
— the _Exit function,
— the quick_exit function,
— the functions in <stdatomic.h> (except where explicitly stated otherwise) when the atomic
arguments are lock-free,
— the atomic_is_lock_free function with any atomic argument, or
— the signal function with the first argument equal to the signal number corresponding to the
signal that caused the invocation of the handler. Furthermore, if such a call to the signal
function results in a SIG_ERR return, the object designated by errno has an indeterminate
representation310) .
6 At program startup, the equivalent of
signal(sig, SIG_IGN);
may be executed for some signals selected in an implementation-defined manner; the equivalent of
signal(sig, SIG_DFL);
is executed for all other signals defined by the implementation.
7 Use of this function in a multi-threaded program results in undefined behavior. The implementation
shall behave as if no library function calls the signal function.
Returns
8 If the request can be honored, the signal function returns the value of func for the most recent
successful call to signal for the specified signal sig. Otherwise, a value of SIG_ERR is returned and
a positive value is stored in errno.
Forward references: the abort function (7.24.4.1), the exit function (7.24.4.4), the _Exit function
(7.24.4.5), the quick_exit function (7.24.4.7).
7.14.2 Send signal
7.14.2.1 The raise function
Synopsis
1 #include <signal.h>
int raise(int sig);
310) If any signal is generated by an asynchronous signal handler, the behavior is undefined.
Description
2 The raise function carries out the actions described in 7.14.1.1 for the signal sig. If a signal handler
is called, the raise function shall not return until after the signal handler does.
Returns
3 The raise function returns zero if successful, nonzero if unsuccessful.
7.15 Alignment <stdalign.h>
1 The header <stdalign.h> provides no content.
7.16 Variable arguments <stdarg.h>
1 The header <stdarg.h> declares a type and defines four macros, for advancing through a list of
arguments whose number and types are not known to the called function when it is translated.
2 A function may be called with a variable number of arguments of varying types if its parameter
type list ends with an ellipsis.
3 The type declared is
va_list
which is a complete object type suitable for holding information needed by the macros va_start,
va_arg, va_end, and va_copy . If access to the varying arguments is desired, the called function
shall declare an object (generally referred to as ap in this subclause) having type va_list. The object
ap may be passed as an argument to another function; if that function invokes the va_arg macro
with parameter ap, the representation of ap in the calling function is indeterminate and shall be
passed to the va_end macro prior to any further reference to ap311) .
7.16.1 Variable argument list access macros
1 The va_start and va_arg macros described in this subclause shall be implemented as macros,
not functions. It is unspecified whether va_copy and va_end are macros or identifiers declared
with external linkage. If a macro definition is suppressed in order to access an actual function,
or a program defines an external identifier with the same name, the behavior is undefined. Each
invocation of the va_start and va_copy macros shall be matched by a corresponding invocation of
the va_end macro in the same function.
7.16.1.1 The va_arg macro
Synopsis
1 #include <stdarg.h>
type va_arg(va_list ap, type);
Description
2 The va_arg macro expands to an expression that has the specified type and the value of the next
argument in the call. The parameter ap shall have been initialized by the va_start or va_copy
macro (without an intervening invocation of the va_end macro for the same ap). Each invocation
of the va_arg macro modifies ap so that the values of successive arguments are returned in turn.
The behavior is undefined if there is no actual next argument. The parameter type shall be a type
name specified such that the type of a pointer to an object that has the specified type can be obtained
simply by postfixing a * to type. If type is not compatible with the type of the actual next argument
(as promoted according to the default argument promotions), the behavior is undefined, except for
the following cases:
— both types are pointers to qualified or unqualified versions of compatible types;
— one type is a signed integer type, the other type is the corresponding unsigned integer type,
and the value is representable in both types;
— one type is pointer to qualified or unqualified void and the other is a pointer to a qualified or
unqualified character type;
— or, the type of the next argument is nullptr_t and type is a pointer type that has the same
representation and alignment requirements as a pointer to a character type312) .
311) It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original
function can make further use of the original list after the other function returns.
312) Such types are in particular pointers to qualified or unqualified versions of void .
Returns
3 The first invocation of the va_arg macro after that of the va_start macro returns the value of
the first argument without an explicitly parameter, which matches the position of the ... in the
parameter list. Successive invocations return the values of the remaining arguments in succession.
7.16.1.2 The va_copy macro
Synopsis
1 #include <stdarg.h>
void va_copy(va_list dest, va_list src);
Description
2 The va_copy macro initializes dest as a copy of src, as if the va_start macro had been applied
to dest followed by the same sequence of uses of the va_arg macro as had previously been used
to reach the present state of src. Neither the va_copy nor va_start macro shall be invoked to
reinitialize dest without an intervening invocation of the va_end macro for the same dest.
Returns
3 The va_copy macro returns no value.
7.16.1.3 The va_end macro
Synopsis
1 #include <stdarg.h>
void va_end(va_list ap);
Description
2 The va_end macro facilitates a normal return from the function whose variable argument list was
referred to by the expansion of the va_start macro, or the function containing the expansion of
the va_copy macro, that initialized the va_list ap. The va_end macro may modify ap so that it
is no longer usable (without being reinitialized by the va_start or va_copy macro). If there is no
corresponding invocation of the va_start or va_copy macro, or if the va_end macro is not invoked
before the return, the behavior is undefined.
Returns
3 The va_end macro returns no value.
7.16.1.4 The va_start macro
Synopsis
1 #include <stdarg.h>
void va_start(va_list ap, ...);
Description
2 The va_start macro shall be invoked before any access to the unnamed arguments.
3 The va_start macro initializes ap for subsequent use by the va_arg and va_end macros. Neither the
va_start nor va_copy macro shall be invoked to reinitialize ap without an intervening invocation
of the va_end macro for the same ap.
4 Only the first argument passed to va_start is evaluated. Any additional arguments are not used by
the macro and will not be expanded or evaluated for any reason.
5 NOTE The macro allows additional arguments to be passed for va_start for compatibility with older versions of the library
only.
Returns
6 The va_start macro returns no value.
7 EXAMPLE 1 The function f1 gathers into an array a list of arguments that are pointers to strings (but not more than MAXARGS
arguments), then passes the array as a single argument to function f2. The number of pointers is specified by the first
argument to f1.
#include <stdarg.h>
#define MAXARGS 31
void f1(int n_ptrs, ...)
{
va_list ap;
char *array[MAXARGS];
int ptr_no = 0;
if (n_ptrs > MAXARGS)
n_ptrs = MAXARGS;
va_start(ap);
while (ptr_no < n_ptrs)
array[ptr_no++] = va_arg(ap, char *);
va_end(ap);
f2(n_ptrs, array);
}
Each call to f1 is required to have visible the definition of the function or a declaration such as
void f1(int, ...);
8 EXAMPLE 2 The function f3 is similar, but saves the status of the variable argument list after the indicated number of
arguments; after f2 has been called once with the whole list, the trailing part of the list is gathered again and passed to
function f4.
#include <stdarg.h>
#define MAXARGS 31
void f3(int n_ptrs, int f4_after, ...)
{
va_list ap, ap_save;
char *array[MAXARGS];
int ptr_no = 0;
if (n_ptrs > MAXARGS)
n_ptrs = MAXARGS;
_
va start(ap);
while (ptr_no < n_ptrs) {
array[ptr_no++] = va_arg(ap, char *);
if (ptr_no == f4_after)
va_copy(ap_save, ap);
}
va_end(ap);
f2(n_ptrs, array);
// Now process the saved copy.
n_ptrs -= f4_after;
ptr_no = 0;
while (ptr_no < n_ptrs)
array[ptr_no++] = va_arg(ap_save, char *);
va end(ap_save);
_
f4(n_ptrs, array);
}
9 EXAMPLE 3 The function f5 is similar to f1, but instead of passing an explicit number of strings as the first argument, the
argument list is terminated with a null pointer.
#include <stdarg.h>
#define MAXARGS 31
void f5(...)
{
va_list ap;
char *array[MAXARGS];
int ptr_no = 0;
va_start(ap);
while (ptr_no < MAXARGS)
{
char *ptr = va_arg(ap, char *);
if (!ptr)
break;
array[ptr_no++] = ptr;
}
va_end(ap);
f6(ptr_no, array);
}
Each call to f5 is required to have visible the definition of the function or a declaration such as
void f5(...);
and implicitly requires the last argument to be a null pointer.
7.17 Atomics <stdatomic.h>
7.17.1 Introduction
1 The header <stdatomic.h> defines several macros and declares several types and functions for
performing atomic operations on data shared between threads.313)
2 Implementations that define the macro __STDC_NO_ATOMICS__ need not provide this header nor
support any of its facilities.
3 The macros defined are the atomic lock-free macros
ATOMIC_BOOL_LOCK_FREE
ATOMIC_CHAR_LOCK_FREE
ATOMIC_CHAR8_T_LOCK_FREE
ATOMIC_CHAR16_T_LOCK_FREE
ATOMIC_CHAR32_T_LOCK_FREE
ATOMIC_WCHAR_T_LOCK_FREE
ATOMIC_SHORT_LOCK_FREE
ATOMIC_INT_LOCK_FREE
ATOMIC_LONG_LOCK_FREE
ATOMIC_LLONG_LOCK_FREE
ATOMIC_POINTER_LOCK_FREE
which expand to constant expressions suitable for use in #if preprocessing directives and which
indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and
ATOMIC_FLAG_INIT
which expands to an initializer for an object of type atomic_flag.
4 The types include
memory_order
which is an enumerated type whose enumerators identify memory ordering constraints;
atomic_flag
which is a structure type representing a lock-free, primitive atomic flag; and several atomic analogs
of integer types.
5 In the following synopses:
— An A refers to an atomic type.
— A C refers to its corresponding non-atomic type.
— An M refers to the type of the other argument for arithmetic operations. For atomic integer
types, M is C. For atomic pointer types, M is ptrdiff_t.
— The functions not ending in _explicit have the same semantics as the corresponding
_explicit function with memory_order_seq_cst for the memory_order argument.
6 It is unspecified whether any generic function declared in <stdatomic.h> is a macro or an identifier
declared with external linkage. If a macro definition is suppressed in order to access an actual
function, or a program defines an external identifier with the name of a generic function, the
behavior is undefined.
7 NOTE Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard.
This qualification means that volatility is preserved when applying these operations to volatile objects.
313) See "future library directions" (7.33.10).
7.17.2 Initialization
1 An atomic object with automatic storage duration that is not initialized or such an object with
allocated storage duration initially has an indeterminate representation; equally, a non-atomic store
to any byte of the representation (either directly or, for example, by calls to memcpy or memset) makes
any atomic object have an indeterminate representation. Explicit or default initialization for atomic
objects with static or thread storage duration that do not have the type atomic_flag is guaranteed
to produce a valid state.314) .
2 Concurrent access to an atomic object before it is set to a valid state, even via an atomic operation,
constitutes a data race. If a signal occurs other than as the result of calling the abort or raise
functions, the behavior is undefined if the signal handler reads or modifies an atomic object that has
an indeterminate representation.
3 EXAMPLE The following definition ensure valid states for guide and head regardless if these are found in file scope or
block scope. Thus any atomic operation that is performed on them after their initialization has been met is well defined.
_Atomic int guide = 42;
static _Atomic(void*) head;
7.17.2.1 The atomic_init generic function
Synopsis
1 #include <stdatomic.h>
void atomic_init(volatile A *obj, C value);
Description
2 The atomic_init generic function initializes the atomic object pointed to by obj to the value value,
while also initializing any additional state that the implementation might need to carry for the
atomic object. If the object has no declared type, after the call the effective type is the atomic type A.
3 Although this function initializes an atomic object, it does not avoid data races; concurrent access to
the variable being initialized, even via an atomic operation, constitutes a data race.
4 If a signal occurs other than as the result of calling the abort or raise functions, the behavior is
undefined if the signal handler calls the atomic_init generic function.
Returns
5 The atomic_init generic function returns no value.
6 EXAMPLE
atomic_int guide;
atomic_init(&guide, 42);
7.17.3 Order and consistency
1 The enumerated type memory_order specifies the detailed regular (non-atomic) memory synchro-
nization operations as defined in 5.1.2.4 and may provide for operation ordering. Its enumeration
constants are as follows:315)
memory_order_relaxed
memory_order_consume
memory_order_acquire
memory_order_release
memory_order_acq_rel
memory_order_seq_cst
2 For memory_order_relaxed, no operation orders memory.
314) See "future library directions" (7.33.10).
315) See "future library directions" (7.33.10).
3 For memory_order_release, memory_order_acq_rel, and memory_order_seq_cst, a store opera-
tion performs a release operation on the affected memory location.
4 For memory_order_acquire, memory_order_acq_rel, and memory_order_seq_cst, a load opera-
tion performs an acquire operation on the affected memory location.
5 For memory_order_consume, a load operation performs a consume operation on the affected mem-
ory location.
6 There shall be a single total order S on all memory_order_seq_cst operations, consistent with the
"happens before" order and modification orders for all affected locations, such that each
memory_order_seq_cst operation B that loads a value from an atomic object M observes one of
the following values:
— the result of the last modification A of M that precedes B in S, if it exists, or
— if A exists, the result of some modification of M that is not memory_order_seq_cst and that
does not happen before A, or
— if A does not exist, the result of some modification of M that is not memory_order_seq_cst.
7 NOTE 1 Although it is not explicitly required that S include lock operations, it can always be extended to an order that does
include lock and unlock operations, since the ordering between those is already included in the "happens before" ordering.
8 NOTE 2 Atomic operations specifying memory_order_relaxed are relaxed only with respect to memory ordering. Imple-
mentations still guarantee that any given atomic access to a particular atomic object is indivisible with respect to all other
atomic accesses to that object.
9 For an atomic operation B that reads the value of an atomic object M , if there is a
memory_order_seq_cst fence X sequenced before B, then B observes either the last
memory_order_seq_cst modification of M preceding X in the total order S or a later mod-
ification of M in its modification order.
10 For atomic operations A and B on an atomic object M , where A modifies M and B takes its value, if
there is a memory_order_seq_cst fence X such that A is sequenced before X and B follows X in S,
then B observes either the effects of A or a later modification of M in its modification order.
11 For atomic modifications A and B of an atomic object M , B occurs later than A in the modification
order of M if:
— there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes
B in S, or
— there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes
Y in S, or
— there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is
sequenced before B, and X precedes Y in S.
12 Atomic read-modify-write operations shall always read the last value (in the modification order)
stored before the write associated with the read-modify-write operation.
13 An atomic store shall only store a value that has been computed from constants and program input
values by a finite sequence of program evaluations, such that each evaluation observes the values of
variables as computed by the last prior assignment in the sequence. The ordering of evaluations in
this sequence shall be such that
— If an evaluation B observes a value computed by A in a different thread, then B does not
happen before A.
— If an evaluation A is included in the sequence, then all evaluations that assign to the same
variable and happen before A are also included.
14 NOTE 3 The second requirement disallows "out-of-thin-air", or "speculative" stores of atomics when relaxed atomics are
used. Since unordered operations are involved, evaluations can appear in this sequence out of thread order. For example,
with x and y initially zero,
// Thread 1:
r1 = atomic_load_explicit(&y, memory_order_relaxed);
atomic_store_explicit(&x, r1, memory_order_relaxed);
// Thread 2:
r2 = atomic_load_explicit(&x, memory_order_relaxed);
atomic_store_explicit(&y, 42, memory_order_relaxed);
is allowed to produce r1 == 42 && r2 == 42. The sequence of evaluations justifying this consists of:
atomic_store_explicit(&y, 42, memory_order_relaxed);
r1 = atomic_load_explicit(&y, memory_order_relaxed);
atomic_store_explicit(&x, r1, memory_order_relaxed);
r2 = atomic_load_explicit(&x, memory_order_relaxed);
On the other hand,
// Thread 1:
r1 = atomic_load_explicit(&y, memory_order_relaxed);
atomic_store_explicit(&x, r1, memory_order_relaxed);
// Thread 2:
r2 = atomic_load_explicit(&x, memory_order_relaxed);
atomic_store_explicit(&y, r2, memory_order_relaxed);
is not allowed to produce r1 == 42 && r2 == 42, since there is no sequence of evaluations that results in the computation
of 42. In the absence of "relaxed" operations and read-modify-write operations with weaker than memory_order_acq_rel
ordering, the second requirement has no impact.
Recommended practice
15 The requirements do not forbid r1 == 42 && r2 == 42 in the following example, with x and y
initially zero:
// Thread 1:
r1 = atomic_load_explicit(&x, memory_order_relaxed);
if (r1 == 42)
atomic_store_explicit(&y, r1, memory_order_relaxed);
// Thread 2:
r2 = atomic_load_explicit(&y, memory_order_relaxed);
if (r2 == 42)
atomic_store_explicit(&x, 42, memory_order_relaxed);
However, this is not useful behavior, and implementations should not allow it.
16 Implementations should make atomic stores visible to atomic loads within a reasonable amount of
time.
7.17.3.1 The kill_dependency macro
Synopsis
1 #include <stdatomic.h>
type kill_dependency(type y);
Description
2 The kill_dependency macro terminates a dependency chain; the argument does not carry a depen-
dency to the return value.
Returns
3 The kill_dependency macro returns the value of y.
7.17.4 Fences
1 This subclause introduces synchronization primitives called fences. Fences can have acquire seman-
tics, release semantics, or both. A fence with acquire semantics is called an acquire fence; a fence with
release semantics is called a release fence.
2 A release fence A synchronizes with an acquire fence B if there exist atomic operations X and Y ,
both operating on some atomic object M , such that A is sequenced before X, X modifies M , Y is
sequenced before B, and Y reads the value written by X or a value written by any side effect in the
hypothetical release sequence X would head if it were a release operation.
3 A release fence A synchronizes with an atomic operation B that performs an acquire operation on an
atomic object M if there exists an atomic operation X such that A is sequenced before X, X modifies
M , and B reads the value written by X or a value written by any side effect in the hypothetical
release sequence X would head if it were a release operation.
4 An atomic operation A that is a release operation on an atomic object M synchronizes with an
acquire fence B if there exists some atomic operation X on M such that X is sequenced before B
and reads the value written by A or a value written by any side effect in the release sequence headed
by A.
7.17.4.1 The atomic_thread_fence function
Synopsis
1 #include <stdatomic.h>
void atomic_thread_fence(memory_order order);
Description
2 Depending on the value of order, this operation:
— has no effects, if order == memory_order_relaxed;
— is an acquire fence, if order == memory_order_acquire or order == memory_order_consume;
— is a release fence, if order == memory_order_release;
— is both an acquire fence and a release fence, if order == memory_order_acq_rel;
— is a sequentially consistent acquire and release fence, if order == memory_order_seq_cst.
Returns
3 The atomic_thread_fence function returns no value.
7.17.4.2 The atomic_signal_fence function
Synopsis
1 #include <stdatomic.h>
void atomic_signal_fence(memory_order order);
Description
2 Equivalent to atomic_thread_fence(order), except that the resulting ordering constraints are
established only between a thread and a signal handler executed in the same thread.
3 NOTE 1 The atomic_signal_fence function can be used to specify the order in which actions performed by the thread
become visible to the signal handler.
4 NOTE 2 Compiler optimizations and reorderings of loads and stores are inhibited in the same way as with
atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would have inserted are not
emitted.
Returns
5 The atomic_signal_fence function returns no value.
7.17.5 Lock-free property
1 The atomic lock-free macros indicate the lock-free property of integer and address atomic types. A
value of 0 indicates that the type is never lock-free; a value of 1 indicates that the type is sometimes
lock-free; a value of 2 indicates that the type is always lock-free.
Recommended practice
2 Operations that are lock-free should also be address-free. That is, atomic operations on the same
memory location via two different addresses will communicate atomically. The implementation
should not depend on any per-process state. This restriction enables communication via memory
mapped into a process more than once and memory shared between two processes.
7.17.5.1 The atomic_is_lock_free generic function
Synopsis
1 #include <stdatomic.h>
bool atomic_is_lock_free(const volatile A *obj);
Description
2 The atomic_is_lock_free generic function indicates whether or not atomic operations on objects
of the type pointed to by obj are lock-free.
Returns
3 The atomic_is_lock_free generic function returns nonzero (true) if and only if atomic operations
on objects of the type pointed to by the argument are lock-free. In any given program execution, the
result of the lock-free query shall be consistent for all pointers of the same type.316)
7.17.6 Atomic integer types
1 For each line in the following table,317) the atomic type name is declared as a type that has the same
representation and alignment requirements as the corresponding direct type.318)
Atomic type name Direct type
atomic_bool _Atomic bool
atomic_char _Atomic char
atomic_schar _Atomic signed char
atomic_uchar _Atomic unsigned char
atomic_short _Atomic short
atomic_ushort _Atomic unsigned short
atomic_int _Atomic int
atomic_uint _Atomic unsigned int
atomic_long _Atomic long
atomic_ulong _Atomic unsigned long
atomic_llong _Atomic long long
atomic_ullong _Atomic unsigned long long
atomic_char8_t _Atomic char8_t
atomic_char16_t _Atomic char16_t
atomic_char32_t _Atomic char32_t
atomic_wchar_t _Atomic wchar_t
atomic_int_least8_t _Atomic int_least8_t
atomic_uint_least8_t _Atomic uint_least8_t
atomic_int_least16_t _Atomic int_least16_t
atomic_uint_least16_t _Atomic uint_least16_t
atomic_int_least32_t _Atomic int_least32_t
316) obj can be a null pointer.
317) See "future library directions" (7.33.10).
318) The same representation and alignment requirements are meant to imply interchangeability as arguments to functions,
return values from functions, and members of unions.
Atomic type name Direct type
atomic_uint_least32_t _Atomic uint_least32_t
atomic_int_least64_t _Atomic int_least64_t
atomic_uint_least64_t _Atomic uint_least64_t
atomic_int_fast8_t _Atomic int_fast8_t
atomic_uint_fast8_t _Atomic uint_fast8_t
atomic_int_fast16_t _Atomic int_fast16_t
atomic_uint_fast16_t _Atomic uint_fast16_t
atomic_int_fast32_t _Atomic int_fast32_t
atomic_uint_fast32_t _Atomic uint_fast32_t
atomic_int_fast64_t _Atomic int_fast64_t
atomic_uint_fast64_t _Atomic uint_fast64_t
atomic_intptr_t _Atomic intptr_t
atomic_uintptr_t _Atomic uintptr_t
atomic_size_t _Atomic size_t
atomic_ptrdiff_t _Atomic ptrdiff_t
atomic_intmax_t _Atomic intmax_t
atomic_uintmax_t _Atomic uintmax_t
Recommended practice
2 The representation of an atomic integer type is not required to have the same size as the correspond-
ing regular type but it should have the same size whenever possible, as it eases effort required to
port existing code.
7.17.7 Operations on atomic types
1 There are only a few kinds of operations on atomic types, though there are many instances of those
kinds. This subclause specifies each general kind.
7.17.7.1 The atomic_store generic functions
Synopsis
1 #include <stdatomic.h>
void atomic_store(volatile A *object, C desired);
void atomic_store_explicit(volatile A *object, C desired, memory_order order);
Description
2 The order argument shall not be memory_order_acquire, memory_order_consume, nor
memory_order_acq_rel. Atomically replace the value pointed to by object with the value of
desired. Memory is affected according to the value of order.
Returns
3 The atomic_store generic functions return no value.
7.17.7.2 The atomic_load generic functions
Synopsis
1 #include <stdatomic.h>
C atomic_load(const volatile A *object);
C atomic_load_explicit(const volatile A *object, memory_order order);
Description
2 The order argument shall not be memory_order_release nor memory_order_acq_rel. Memory is
affected according to the value of order.
Returns
3 Atomically returns the value pointed to by object.
7.17.7.3 The atomic_exchange generic functions
Synopsis
1 #include <stdatomic.h>
C atomic_exchange(volatile A *object, C desired);
C atomic_exchange_explicit(volatile A *object, C desired, memory_order order);
Description
2 Atomically replace the value pointed to by object with desired. Memory is affected according to
the value of order. These operations are read-modify-write operations (5.1.2.4).
Returns
3 Atomically returns the value pointed to by object immediately before the effects.
7.17.7.4 The atomic_compare_exchange generic functions
Synopsis
1 #include <stdatomic.h>
bool atomic_compare_exchange_strong(volatile A *object, C *expected, C desired);
bool atomic_compare_exchange_strong_explicit(volatile A *object, C *expected,
C desired, memory_order success, memory_order failure);
bool atomic_compare_exchange_weak(volatile A *object, C *expected, C desired);
bool atomic_compare_exchange_weak_explicit(volatile A *object, C *expected,
C desired, memory_order success, memory_order failure);
Description
2 The failure argument shall not be memory_order_release nor memory_order_acq_rel. The
failure argument shall be no stronger than the success argument.
3 Atomically, compares the contents of the memory pointed to by object for equality with that
pointed to by expected, and if true, replaces the contents of the memory pointed to by object
with desired, and if false, updates the contents of the memory pointed to by expected with that
pointed to by object. Further, if the comparison is true, memory is affected according to the value
of success, and if the comparison is false, memory is affected according to the value of failure.
These operations are atomic read-modify-write operations (5.1.2.4).
4 NOTE 1 For example, the effect of atomic_compare_exchange_strong is
if (memcmp(object, expected, sizeof (*object)) == 0)
memcpy(object, &desired, sizeof (*object));
else
memcpy(expected, object, sizeof (*object));
5 A weak compare-and-exchange operation may fail spuriously. That is, even when the contents
of memory referred to by expected and object are equal, it may return zero and store back to
expected the same memory contents that were originally there.
6 NOTE 2 This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g.
load-locked store-conditional machines.
7 EXAMPLE A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.
exp = atomic_load(&cur);
do {
des = function(exp);
} while (!atomic_compare_exchange_weak(&cur, &exp, des));
When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak
compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.
Returns
8 The result of the comparison.
7.17.7.5 The atomic_fetch and modify generic functions
1 The following operations perform arithmetic and bitwise computations. All of these operations
are applicable to an object of any atomic integer type. None of these operations is applicable to
atomic_bool. The key, operator, and computation correspondence is:
key op computation
add + addition
sub - subtraction
or | bitwise inclusive or
xor ^ bitwise exclusive or
and & bitwise and
Synopsis
2 #include <stdatomic.h>
C atomic_fetch_key(volatile A *object, M operand);
C atomic_fetch_key_explicit(volatile A *object, M operand, memory_order order);
Description
3 Atomically replaces the value pointed to by object with the result of the computation applied to
the value pointed to by object and the given operand. Memory is affected according to the value
of order. These operations are atomic read-modify-write operations (5.1.2.4). For signed integer
types, arithmetic performs silent wraparound on integer overflow; there are no undefined results.
For address types, the result may be an undefined address, but the operations otherwise have no
undefined behavior.
Returns
4 Atomically, the value pointed to by object immediately before the effects.
5 NOTE The operation of the atomic_fetch and modify generic functions are nearly equivalent to the operation of the
corresponding op= compound assignment operators. The only differences are that the compound assignment operators are
not guaranteed to operate atomically, and the value yielded by a compound assignment operator is the updated value of the
object, whereas the value returned by the atomic_fetch and modify generic functions is the previous value of the atomic
object.
7.17.8 Atomic flag type and operations
1 The atomic_flag type provides the classic test-and-set functionality. It has two states, set and clear.
2 Operations on an object of type atomic_flag shall be lock free.
3 NOTE Hence, as per 7.17.5, the operations should also be address-free. No other type requires lock-free operations, so the
atomic_flag type is the minimum hardware-implemented type needed to conform to this document. The remaining types
can be emulated with atomic_flag, though with less than ideal properties.
4 The macro ATOMIC_FLAG_INIT may be used to initialize an atomic_flag to the clear state. An
atomic_flag that is not explicitly initialized with ATOMIC_FLAG_INIT has initially an indeterminate
representation.
5 EXAMPLE
atomic_flag guard = ATOMIC_FLAG_INIT;
7.17.8.1 The atomic_flag_test_and_set functions
Synopsis
1 #include <stdatomic.h>
bool atomic_flag_test_and_set(volatile atomic_flag *object);
bool atomic_flag_test_and_set_explicit(volatile atomic_flag *object,
memory_order order);
Description
2 Atomically places the atomic flag pointed to by object in the set state and returns the value
corresponding to the immediately preceding state. Memory is affected according to the value of
order. These operations are atomic read-modify-write operations (5.1.2.4).
Returns
3 The atomic_flag_test_and_set functions return the value that corresponds to the state of the
atomic flag immediately before the effects. The return value true corresponds to the set state and the
return value false corresponds to the clear state.
7.17.8.2 The atomic_flag_clear functions
Synopsis
1 #include <stdatomic.h>
void atomic_flag_clear(volatile atomic_flag *object);
void atomic_flag_clear_explicit(volatile atomic_flag *object,
memory_order order);
Description
2 The order argument shall not be memory_order_acquire nor memory_order_acq_rel. Atomically
places the atomic flag pointed to by object into the clear state. Memory is affected according to the
value of order.
Returns
3 The atomic_flag_clear functions return no value.
7.18 Bit and byte utilities <stdbit.h>
7.18.1 General
1 The header <stdbit.h> defines the following macros, types, and functions, to work with the byte
and bit representation of many types, typically integer types. This header makes available the
size_t type name (7.21) and any uintN_t, intN_t, uint_leastN_t, or int_leastN_t type names
defined by the implementation (7.22).
2 The most significant index is the 0-based index counting from the most significant bit, 0, to the
least significant bit, w − 1, where w is the width of the type that is having its most significant index
computed.
3 The least significant index is the 0-based index counting from the least significant bit, 0, to the
most significant bit, w − 1, where w is the width of the type that is having its least significant index
computed.
4 It is unspecified whether any generic function declared in <stdbit.h> is a macro or an identifier
declared with external linkage. If a macro definition is suppressed in order to access an actual
function, or a program defines an external identifier with the name of a generic function, the
behavior is unspecified.
7.18.2 Endian
1 Two common methods of byte ordering in multi-byte scalar types are little-endian and big-endian.
Little-endian is a format for storage of binary data in which the least significant byte is placed
first, with the rest in ascending order. Or, that the least significant byte is stored at the smallest
memory address. Big-endian is a format for storage or transmission of binary data in which the
most significant byte is placed first, with the rest in descending order. Or, that the most significant
byte is stored at the smallest memory address. Other byte orderings are also possible.
2 The macros are:
__STDC_ENDIAN_LITTLE__
which represents a method of byte order storage least significant byte is placed first and the rest are
in ascending order, and is an integer constant expression;
__STDC_ENDIAN_BIG__
which represents a method of byte order storage most significant byte is placed first and the rest are
in descending order, and is an integer constant expression;
__STDC_ENDIAN_NATIVE__ /* see below */
which represents the method of byte order storage for the execution environment and is an integer
constant expression.
3 __STDC_ENDIAN_NATIVE__ shall expand to an integer constant expression whose value is equiv-
alent to the value of __STDC_ENDIAN_LITTLE__ if the execution environment is little-endian.
Otherwise, __STDC_ENDIAN_NATIVE__ shall expand to an integer constant expression whose
value is equivalent to the value of __STDC_ENDIAN_BIG__ if the execution environment is big-
endian. If __STDC_ENDIAN_NATIVE__ is not equivalent to either, then the byte order for the exe-
cution environment is implementation-defined. The value of the integer constant expression for
__STDC_ENDIAN_LITTLE__ and __STDC_ENDIAN_BIG__ are not equal.
7.18.3 Count Leading Zeros
Synopsis
1 #include <stdbit.h>
int stdc_leading_zerosuc(unsigned char value);
int stdc_leading_zerosus(unsigned short value);
int stdc_leading_zerosui(unsigned int value);
int stdc_leading_zerosul(unsigned long value);
int stdc_leading_zerosull(unsigned long long value);
generic_return_type stdc_leading_zeros(generic_value_type value);
Returns
Returns the number of consecutive 0 bits in value, starting from the most significant bit.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.4 Count Leading Ones
Synopsis
1 #include <stdbit.h>
int stdc_leading_onesuc(unsigned char value);
int stdc_leading_onesus(unsigned short value);
int stdc_leading_onesui(unsigned int value);
int stdc_leading_onesul(unsigned long value);
int stdc_leading_onesull(unsigned long long value);
generic_return_type stdc_leading_ones(generic_value_type value);
Returns
Returns the number of consecutive 1 bits in value, starting from the most significant bit.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.5 Count Trailing Zeros
Synopsis
1 #include <stdbit.h>
int stdc_trailing_zerosuc(unsigned char value);
int stdc_trailing_zerosus(unsigned short value);
int stdc_trailing_zerosui(unsigned int value);
int stdc_trailing_zerosul(unsigned long value);
int stdc_trailing_zerosull(unsigned long long value);
generic_return_type stdc_trailing_zeros(generic_value_type value);
Returns
Returns the number of consecutive 0 bits in value, starting from the least significant bit.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.6 Count Trailing Ones
Synopsis
1 #include <stdbit.h>
int stdc_trailing_onesuc(unsigned char value);
int stdc_trailing_onesus(unsigned short value);
int stdc_trailing_onesui(unsigned int value);
int stdc_trailing_onesul(unsigned long value);
int stdc_trailing_onesull(unsigned long long value);
generic_return_type stdc_trailing_ones(generic_value_type value);
Returns
Returns the number of consecutive 1 bits in value, starting from the least significant bit.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.7 First Leading Zero
Synopsis
1 #include <stdbit.h>
int stdc_first_leading_zerouc(unsigned char value);
int stdc_first_leading_zerous(unsigned short value);
int stdc_first_leading_zeroui(unsigned int value);
int stdc_first_leading_zeroul(unsigned long value);
int stdc_first_leading_zeroull(unsigned long long value);
generic_return_type stdc_first_leading_zero(generic_value_type value);
Returns
Returns the most significant index of the first 0 bit in value, plus 1. If it is not found, this function
returns 0.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.8 First Leading One
Synopsis
1 #include <stdbit.h>
int stdc_first_leading_oneuc(unsigned char value);
int stdc_first_leading_oneus(unsigned short value);
int stdc_first_leading_oneui(unsigned int value);
int stdc_first_leading_oneul(unsigned long value);
int stdc_first_leading_oneull(unsigned long long value);
generic_return_type stdc_first_leading_one(generic_value_type value);
Returns
Returns the most significant index of the first 1 bit in value, plus 1. If it is not found, this function
returns 0.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.9 First Trailing Zero
Synopsis
1 #include <stdbit.h>
int stdc_first_trailing_zerouc(unsigned char value);
int stdc_first_trailing_zerous(unsigned short value);
int stdc_first_trailing_zeroui(unsigned int value);
int stdc_first_trailing_zeroul(unsigned long value);
int stdc_first_trailing_zeroull(unsigned long long value);
generic_return_type stdc_first_trailing_zero(generic_value_type value);
Returns
Returns the least significant index of the first 0 bit in value, plus 1. If it is not found, this function
returns 0.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.10 First Trailing One
Synopsis
1 #include <stdbit.h>
int stdc_first_trailing_oneuc(unsigned char value);
int stdc_first_trailing_oneus(unsigned short value);
int stdc_first_trailing_oneui(unsigned int value);
int stdc_first_trailing_oneul(unsigned long value);
int stdc_first_trailing_oneull(unsigned long long value);
generic_return_type stdc_first_trailing_one(generic_value_type value);
Returns
Returns the least significant index of the first 1 bit in value, plus 1. If it is not found, this function
returns 0.
The type-generic function (marked by its generic_value_type argument) returns the appropriate
value based on the type of the input value, so long as it is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.11 Count Ones
Synopsis
1 #include <stdbit.h>
int stdc_count_onesuc(unsigned char value);
int stdc_count_onesus(unsigned short value);
int stdc_count_onesui(unsigned int value);
int stdc_count_onesul(unsigned long value);
int stdc_count_onesull(unsigned long long value);
generic_return_type stdc_count_ones(generic_value_type value);
Returns
Returns the total number of 1 bits within the given value.
The type-generic function (marked by its generic_value_type argument) returns the previously
described result for a given input value so long as the generic_value_type is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type shall be a suitably large signed integer type capable of representing
the computed result.
7.18.12 Count Zeros
Synopsis
1 #include <stdbit.h>
int stdc_count_zerosuc(unsigned char value);
int stdc_count_zerosus(unsigned short value);
int stdc_count_zerosui(unsigned int value);
int stdc_count_zerosul(unsigned long value);
int stdc_count_zerosull(unsigned long long value);
generic_return_type stdc_count_zeros(generic_value_type value);
Returns
Returns the total number of 0 bits within the given value.
The type-generic function (marked by its generic_value_type argument) returns the previously
described result for a given input value so long as the generic_value_type is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type for the type-generic function need not be the same as the type of
value. It shall be suitably large unsigned integer type capable of representing the computed result.
7.18.13 Single-bit Check
Synopsis
1 #include <stdbit.h>
bool stdc_has_single_bituc(unsigned char value);
bool stdc_has_single_bitus(unsigned short value);
bool stdc_has_single_bitui(unsigned int value);
bool stdc_has_single_bitul(unsigned long value);
bool stdc_has_single_bitull(unsigned long long value);
bool stdc_has_single_bit(generic_value_type value);
Returns
The stdc_has_single_bit functions returns true if and only if there is a single 1 bit in value.
The type-generic function (marked by its generic_value_type argument) returns the previously
described result for a given input value so long as the generic_value_type is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
7.18.14 Bit Width
Synopsis
1 #include <stdbit.h>
int stdc_bit_widthuc(unsigned char value);
int stdc_bit_widthus(unsigned short value);
int stdc_bit_widthui(unsigned int value);
int stdc_bit_widthul(unsigned long value);
int stdc_bit_widthull(unsigned long long value);
generic_return_type stdc_bit_width(generic_value_type value);
Description
The stdc_bit_width functions compute the smallest number of bits needed to store value.
Returns
The stdc_bit_width functions return 0 if value is 0. Otherwise, they return 1 + ⌊log2(value)⌋.
The type-generic function (marked by its generic_value_type argument) returns the previously
described result for a given input value so long as the generic_value_type is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
The generic_return_type type for the type-generic function need not be the same as the type of
value. It shall be suitably large signed integer type capable of representing the computed result.
7.18.15 Bit Floor
Synopsis
1 #include <stdbit.h>
unsigned char stdc_bit_flooruc(unsigned char value);
unsigned short stdc_bit_floorus(unsigned short value);
unsigned int stdc_bit_floorui(unsigned int value);
unsigned long stdc_bit_floorul(unsigned long value);
unsigned long long stdc_bit_floorull(unsigned long long value);
generic_value_type stdc_bit_floor(generic_value_type value);
Description
The stdc_bit_floor functions compute the largest integral power of 2 that is not greater than
value.
Returns
The stdc_bit_floor functions return 0 if value is 0. Otherwise, they return the largest integral
power of 2 that is not greater than value.
The type-generic function (marked by its generic_value_type argument) returns the previously
described result for a given input value so long as the generic_value_type is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
7.18.16 Bit Ceiling
Synopsis
1 #include <stdbit.h>
unsigned char stdc_bit_ceiluc(unsigned char value);
unsigned short stdc_bit_ceilus(unsigned short value);
unsigned int stdc_bit_ceilui(unsigned int value);
unsigned long stdc_bit_ceilul(unsigned long value);
unsigned long long stdc_bit_ceilull(unsigned long long value);
generic_value_type stdc_bit_ceil(generic_value_type value);
Description
The stdc_bit_ceil functions compute the smallest integral power of 2 that is not less than value.
If the computation does not fit in the given return type, the behavior is undefined.
Returns
The stdc_bit_ceil functions return the smallest integral power of 2 that is not less than value.
The type-generic function (marked by its generic_value_type argument) returns the previously
described result for a given input value so long as the generic_value_type is an:
— standard unsigned integer type, excluding bool;
— extended unsigned integer type;
— or, bit-precise unsigned integer type whose width matches a standard or extended integer
type, excluding bool.
7.19 Boolean type and values <stdbool.h>
1 The header <stdbool.h> provides the obsolescent macro __bool_true_false_are_defined which
expands to the integer constant 1.
7.20 Checked Integer Arithmetic <stdckdint.h>
1 The header <stdckdint.h> defines several macros for performing checked integer arithmetic.
7.20.1 The ckd_ Checked Integer Operation Macros
Synopsis
1 #include <stdckdint.h>
bool ckd_add(type1 *result, type2 a, type3 b);
bool ckd_sub(type1 *result, type2 a, type3 b);
bool ckd_mul(type1 *result, type2 a, type3 b);
Description
2 These macros perform addition, subtraction, or multiplication of the mathematical values of a and b,
storing the result of the operation in *result , (that is, *result is assigned the result of computing
a + b, a - b, or a * b). Each operation is performed as if both operands were represented in a
signed integer type with infinite range, and the result was then converted from this integer type to
type1.
3 Both type2 and type3 shall be any integer type other than plain char, bool, a bit-precise integer
type, or an enumeration type, and they need not be the same. *result shall be a modifiable lvalue
of any integer type other than plain char, bool, a bit-precise integer type, or an enumeration type.
Recommended practice
4 It is recommended to produce a diagnostic message if type2 or type3 are not suitable integer types,
or if *result is not a modifiable lvalue of a suitable integer type.
Returns
5 If these macros return false, the value assigned to *result correctly represents the mathematical
result of the operation. Otherwise, these macros return true. In this case, the value assigned to
*result is the mathematical result of the operation wrapped around to the width of *result .
6 EXAMPLE 1 If a and b are values of type signed int, and result is a signed long, then
ckd_sub(&result, a, b);
will indicate if a - b can be expressed as a signed long. If signed long has a greater width than signed int, this will
always be possible and this macro will return false.
7.21 Common definitions <stddef.h>
1 The header <stddef.h> defines the following macros and declares the following types. Some are
also defined in other headers, as noted in their respective subclauses.
2 The types are
ptrdiff_t
which is the signed integer type of the result of subtracting two pointers;
size_t
which is the unsigned integer type of the result of the sizeof operator;
max_align_t
which is an object type whose alignment is the greatest fundamental alignment;
wchar_t
which is an integer type whose range of values can represent distinct codes for all members of the
largest extended character set specified among the supported locales; the null character shall have
the code value zero. Each member of the basic character set shall have a code value equal to its
value when used as the lone character in an integer character constant if an implementation does
not define __STDC_MB_MIGHT_NEQ_WC__ ; and,
nullptr_t
which is the type of the nullptr predefined constant, see below.
3 The macros are
NULL
which expands to an implementation-defined null pointer constant;
unreachable()
which expands to a void expression that invokes undefined behavior if it is reached during execution;
and
offsetof(type, member-designator)
which expands to an integer constant expression that has type size_t, the value of which is the
offset in bytes, to the subobject (designated by member-designator), from the beginning of any object
of type type. The type and member designator shall be such that given
static type t;
then the expression &(t. member-designator) evaluates to an address constant. If the specified type
defines a new type or if the specified member is a bit-field, the behavior is undefined.
Recommended practice
4 The types used for size_t and ptrdiff_t should not have an integer conversion rank greater than
that of signed long int unless the implementation supports objects large enough to make this
necessary.
7.21.1 The unreachable macro
Synopsis
1 #include <stddef.h>
void unreachable(void);
Description
2 A call to the function-like macro unreachable indicates that the particular flow control that leads to
the call will never be taken; it receives no arguments and expands to a void expression. The program
execution shall not reach such a call.
Returns
3 If a macro call unreachable() is reached during execution, the behavior is undefined.
4 EXAMPLE 1 The following program assumes that each execution is provided with at least one command line argument.
The behavior of an execution with no arguments is undefined.
#include <stddef.h>
#include <stdio.h>
int main (int argc, char* argv[static argc + 1]) {
if (argc <= 2)
unreachable();
else
return printf("%s: we see %s", argv[0], argv[1]);
return puts("this should never be reached");
}
Here, the static array size expression and the annotation of the control flow with unreachable indicates that the pointed-to
parameter array argv will hold at least three elements, regardless of the circumstances. A possible optimization is that the
resulting executable never performs the comparison and unconditionally executes a tail call to printf that never returns to
the main function. In particular, the entire call and reference to puts can be omitted from the executable. No diagnostic is
expected.
7.21.2 The nullptr_t type
Synopsis
1 #include <stddef.h>
typedef typeof_unqual(nullptr) nullptr_t;
Description
2 The nullptr_t type is the type of the nullptr predefined constant. It has only a very limited use
in contexts where this type is needed to distinguish nullptr from other expression types. It is an
unqualified complete scalar type that is different from all pointer or arithmetic types and is neither
an atomic or array type and has exactly one value, nullptr. Default initialization of an object of this
type is equivalent to an initialization by nullptr.
3 The size and alignment of nullptr_t is the same as for a pointer to character type. An object
representation of the value nullptr is the same as the object representation of a null pointer value of
type void*. An lvalue conversion of an object of type nullptr_t with such an object representation
has the value nullptr; if the object representation is different, the behavior is undefined319) .
4 NOTE Because it is considered to be a scalar type, nullptr_t may appear in many context where (void*)0 would be valid,
for example,
— as the operand of alignas, sizeof or typeof operators,
— as the operand of an implicit or explicit conversion to a pointer type,
— as the assignment expression in an assignment or initialization of an object of type nullptr_t,
— as an argument to a parameter of type nullptr_t or in a variable argument list,
— as a void expression,
319) Thus, during the whole program execution an object of type nullptr_t evaluates to the assumed value nullptr.
— as the operand of an implicit or explicit conversion to bool,
— as an operand of a _Generic primary expression,
— as an operand of the !, &&, || or conditional operators, or
— as the controlling expression of an if or iteration statement.
7.22 Integer types <stdint.h>
1 The header <stdint.h> declares sets of integer types having specified widths, and defines corre-
sponding sets of macros.320) It also defines macros that specify limits of integer types corresponding
to types defined in other standard headers.
2 Types are defined in the following categories:
— integer types having certain exact widths;
— integer types having at least certain specified widths;
— fastest integer types having at least certain specified widths;
— integer types wide enough to hold pointers to objects;
— integer types having greatest width.
(Some of these types may denote the same type.)
3 Corresponding macros specify limits of the declared types and construct suitable constants.
4 For each type described herein that the implementation provides,321) <stdint.h> shall declare that
typedef name and define the associated macros. Conversely, for each type described herein that
the implementation does not provide, <stdint.h> shall not declare that typedef name nor shall it
define the associated macros. An implementation shall provide those types described as "required",
but need not provide any of the others (described as "optional"). None of the types shall be defined
as a synonym for a bit-precise integer type.
5 The feature test macro __STDC_VERSION_STDINT_H__ expands to the token 202311L.
7.22.1 Integer types
1 When typedef names differing only in the absence or presence of the initial u are defined, they shall
denote corresponding signed and unsigned types as described in 6.2.5; an implementation providing
one of these corresponding types shall also provide the other.
2 In the following descriptions, the symbol N represents an unsigned decimal integer with no leading
zeros (e.g., 8 or 24, but not 04 or 048).
7.22.1.1 Exact-width integer types
1 The typedef name intN_t designates a signed integer type with width N and no padding bits. Thus,
int8_t denotes such a signed integer type with a width of exactly 8 bits.
2 The typedef name uintN_t designates an unsigned integer type with width N and no padding bits.
Thus, uint24_t denotes such an unsigned integer type with a width of exactly 24 bits.
3 If an implementation provides standard or extended integer types with a particular width and no
padding bits, it shall define the corresponding typedef names.
7.22.1.2 Minimum-width integer types
1 The typedef name int_leastN_t designates a signed integer type with a width of at least N, such
that no signed integer type with lesser size has at least the specified width. Thus, int_least32_t
denotes a signed integer type with a width of at least 32 bits.
2 The typedef name uint_leastN_t designates an unsigned integer type with a width of at least
N, such that no unsigned integer type with lesser size has at least the specified width. Thus,
uint_least16_t denotes an unsigned integer type with a width of at least 16 bits.
3 If the typedef name intN_t is defined, int_leastN_t designates the same type. If the typedef
name uintN_t is defined, uint_leastN_t designates the same type.
4 The following types are required:
320) See "future library directions" (7.33.14).
321) Some of these types might denote implementation-defined extended integer types.
int_least8_t uint_least8_t
int_least16_t uint_least16_t
int_least32_t uint_least32_t
int_least64_t uint_least64_t
All other types of this form are optional.
7.22.1.3 Fastest minimum-width integer types
1 Each of the following types designates an integer type that is usually fastest322) to operate with
among all integer types that have at least the specified width.
2 The typedef name int_fastN_t designates the fastest signed integer type with a width of at least N.
The typedef name uint_fastN_t designates the fastest unsigned integer type with a width of at
least N.
3 The following types are required:
int_fast8_t uint_fast8_t
int_fast16_t uint_fast16_t
int_fast32_t uint_fast32_t
int_fast64_t uint_fast64_t
All other types of this form are optional.
7.22.1.4 Integer types capable of holding object pointers
1 The following type designates a signed integer type, other than a bit-precise integer type, with the
property that any valid pointer to void can be converted to this type, then converted back to pointer
to void, and the result will compare equal to the original pointer:
intptr_t
The following type designates an unsigned integer type, other than a bit-precise integer type, with
the property that any valid pointer to void can be converted to this type, then converted back to
pointer to void, and the result will compare equal to the original pointer:
uintptr_t
These types are optional.
7.22.1.5 Greatest-width integer types
1 The following type designates a signed integer type, other than a bit-precise integer type, capable of
representing any value of any signed integer type with the possible exceptions of signed bit-precise
integer types and of signed extended integer types that are wider than long long and that are
referred by the type definition for an exact width integer type:
intmax_t
The following type designates the unsigned integer type that corresponds to intmax_t323) :
uintmax_t
These types are required.
322) The designated type is not guaranteed to be fastest for all purposes; if the implementation has no clear grounds for
choosing one type over another, it will simply pick some integer type satisfying the signedness and width requirements.
323) Thus this type is capable of representing any value of any unsigned integer type with the possible exception of particular
extended integer types that are wider than unsigned long long.
7.22.2 Widths of specified-width integer types
1 The following object-like macros specify the width of the types declared in <stdint.h>. Each macro
name corresponds to a similar type name in 7.22.1.
2 Each instance of any defined macro shall be replaced by a constant expression suitable for use in
#if preprocessing directives. Its implementation-defined value shall be equal to or greater than
the value given below, except where stated to be exactly the given value. An implementation shall
define only the macros corresponding to those typedef names it actually provides.324)
7.22.2.1 Width of exact-width integer types
1 INTN_WIDTH exactly N
UINTN_WIDTH exactly N
7.22.2.2 Width of minimum-width integer types
1 INT_LEASTN_WIDTH exactly UINT_LEASTN_WIDTH
UINT_LEASTN_WIDTH N
7.22.2.3 Width of fastest minimum-width integer types
1 INT_FASTN_WIDTH exactly UINT_FASTN_WIDTH
UINT_FASTN_WIDTH N
7.22.2.4 Width of integer types capable of holding object pointers
1 INTPTR_WIDTH exactly UINTPTR_WIDTH
UINTPTR_WIDTH 16
7.22.2.5 Width of greatest-width integer types
1 INTMAX_WIDTH exactly UINTMAX_WIDTH
UINTMAX_WIDTH 64
7.22.3 Width of other integer types
1 The following object-like macros specify the width of integer types corresponding to types defined
in other standard headers.
2 Each instance of these macros shall be replaced by a constant expression suitable for use in #if
preprocessing directives. Its implementation-defined value shall be equal to or greater than the
corresponding value given below. An implementation shall define only the macros corresponding
to those typedef names it actually provides.325)
7.22.3.1 Width of ptrdiff_t
1 PTRDIFF_WIDTH 16
7.22.3.2 Width of sig_atomic_t
1 SIG_ATOMIC_WIDTH 8
324) The exact-width and pointer-holding integer types are optional.
325) A freestanding implementation need not provide all of these types.
7.22.3.3 Width of size_t
1 SIZE_WIDTH 16
7.22.3.4 Width of wchar_t
1 WCHAR_WIDTH 8
7.22.3.5 Width of wint_t
1 WINT_WIDTH 16
7.22.4 Macros for integer constants
1 The following function-like macros expand to integer constants suitable for initializing objects that
have integer types corresponding to types defined in <stdint.h>. Each macro name corresponds to
a similar type name in 7.22.1.2 or 7.22.1.5.
2 The argument in any instance of these macros shall be an unsuffixed integer constant (as defined
in 6.4.4.1) with a value that does not exceed the limits for the corresponding type.
3 Each invocation of one of these macros shall expand to an integer constant expression. The type of
the expression shall have the same type as would an expression of the corresponding type converted
according to the integer promotions. The value of the expression shall be that of the argument. If
the value is in the range of the type intmax_t (for a signed type) or the type uintmax_t (for an
unsigned type), see 7.22.1.5, the expression is suitable for use in #if preprocessing directives.
7.22.4.1 Macros for minimum-width integer constants
1 The macro INTN_C( value) expands to an integer constant expression corresponding to the type
int_leastN_t . The macro UINTN_C( value) expands to an integer constant expression corre-
sponding to the type uint_leastN_t . For example, if uint_least64_t is a name for the type
unsigned long long int, then UINT64_C(0x123) might expand to the integer constant 0x123ULL.
7.22.4.2 Macros for greatest-width integer constants
1 The following macro expands to an integer constant expression having the value specified by its
argument and the type intmax_t:
INTMAX_C(value)
The following macro expands to an integer constant expression having the value specified by its
argument and the type uintmax_t:
UINTMAX_C(value)
7.22.5 Maximal and minimal values of integer types
1 For all integer types for which there is a macro with suffix _WIDTH holding the width, maximum
macros with suffix _MAX and, for all signed types, minimum macros with suffix _MIN are defined as
by 5.2.4.2. If it is unspecified if a type is signed or unsigned and the implementation has it as an
unsigned type, a minimum macro with extension _MIN , and value 0 of the corresponding type is
defined.
7.23 Input/output <stdio.h>
7.23.1 Introduction
1 The header <stdio.h> defines several macros, and declares three types and many functions for
performing input and output.
2 The types declared are size_t (described in 7.21);
FILE
which is an object type capable of recording all the information needed to control a stream, including
its file position indicator, a pointer to its associated buffer (if any), an error indicator that records
whether a read/write error has occurred, and an end-of-file indicator that records whether the end of
the file has been reached; and
fpos_t
which is a complete object type other than an array type capable of recording all the information
needed to specify uniquely every position within a file.
3 The macros are NULL (described in 7.21);
_IOFBF
_IOLBF
_IONBF
which expand to integer constant expressions with distinct values, suitable for use as the third
argument to the setvbuf function;
BUFSIZ
which expands to an integer constant expression that is the size of the buffer used by the setbuf
function;
EOF
which expands to an integer constant expression, with type int and a negative value, that is returned
by several functions to indicate end-of-file, that is, no more input from a stream;
FOPEN_MAX
which expands to an integer constant expression that is the minimum number of files that the
implementation guarantees can be open simultaneously;
FILENAME_MAX
which expands to an integer constant expression that is the size needed for an array of char large
enough to hold the longest file name string that the implementation guarantees can be opened or, if
the implementation imposes no practical limit on the length of file name strings, the recommended
size of an array intended to hold a file name string326) ;
326) Of course, file name string contents are subject to other system-specific constraints; therefore all possible strings of length
FILENAME_MAX cannot be expected to be opened successfully.
_PRINTF_NAN_LEN_MAX
which expands to an integer constant expression (suitable for use in #if preprocessing directives)
that is the maximum number of characters output for any
[-]NAN(n-char-sequence)
sequence.327) If an implementation has no support for NaNs, it shall be 0. _PRINTF_NAN_LEN_MAX
shall be less than 64;
L_tmpnam
which expands to an integer constant expression that is the size needed for an array of char large
enough to hold a temporary file name string generated by the tmpnam function;
SEEK_CUR
SEEK_END
SEEK_SET
which expand to integer constant expressions with distinct values, suitable for use as the third
argument to the fseek function;
TMP_MAX
which expands to an integer constant expression that is the minimum number of unique file names
that can be generated by the tmpnam function;
stderr
stdin
stdout
which are expressions of type "pointer to FILE" that point to the FILE objects associated, respectively,
with the standard error, input, and output streams.
4 The header <wchar.h> declares a number of functions useful for wide character input and output.
The wide character input/output functions described in that subclause provide operations analogous
to most of those described here, except that the fundamental units internal to the program are
wide characters. The external representation (in the file) is a sequence of "generalized" multibyte
characters, as described further in 7.23.3.
5 The input/output functions are given the following collective terms:
— The wide character input functions — those functions described in 7.31 that perform input
into wide characters and wide strings: fgetwc, fgetws, getwc, getwchar, fwscanf, wscanf,
vfwscanf , and vwscanf .
— The wide character output functions — those functions described in 7.31 that perform output from
wide characters and wide strings: fputwc, fputws, putwc, putwchar, fwprintf, wprintf,
vfwprintf , and vwprintf.
— The wide character input/output functions — the union of the ungetwc function, the wide charac-
ter input functions, and the wide character output functions.
— The byte input/output functions — those functions described in this subclause that perform
input/output: fgetc, fgets, fprintf, fputc, fputs, fread, fscanf, fwrite, getc, getchar,
printf, putc, putchar, puts, scanf, ungetc, vfprintf , vfscanf , vprintf , and vscanf.
Forward references: files (7.23.3), the fseek function (7.23.9.2), streams (7.23.2), the tmpnam func-
tion (7.23.4.4), <wchar.h> (7.31).
327) If the implementation only uses the [-]NAN style, then _PRINTF_NAN_LEN_MAX would have the value 4.
7.23.2 Streams
1 Input and output, whether to or from physical devices such as terminals and tape drives, or whether
to or from files supported on structured storage devices, are mapped into logical data streams, whose
properties are more uniform than their various inputs and outputs. Two forms of mapping are
supported, for text streams and for binary streams.328)
2 A text stream is an ordered sequence of characters composed into lines, each line consisting of
zero or more characters plus a terminating new-line character. Whether the last line requires a
terminating new-line character is implementation-defined. Characters may have to be added, altered,
or deleted on input and output to conform to differing conventions for representing text in the host
environment. Thus, there need not be a one-to-one correspondence between the characters in a
stream and those in the external representation. Data read in from a text stream will necessarily
compare equal to the data that were earlier written out to that stream only if: the data consist only
of printing characters and the control characters horizontal tab and new-line; no new-line character
is immediately preceded by space characters; and the last character is a new-line character. Whether
space characters that are written out immediately before a new-line character appear when read in
is implementation-defined.
3 A binary stream is an ordered sequence of characters that can transparently record internal data.
Data read in from a binary stream shall compare equal to the data that were earlier written out to
that stream, under the same implementation. Such a stream may, however, have an implementation-
defined number of null characters appended to the end of the stream.
4 Each stream has an orientation. After a stream is associated with an external file, but before any
operations are performed on it, the stream is without orientation. Once a wide character input/out-
put function has been applied to a stream without orientation, the stream becomes a wide-oriented
stream. Similarly, once a byte input/output function has been applied to a stream without orien-
tation, the stream becomes a byte-oriented stream. Only a call to the freopen function or the fwide
function can otherwise alter the orientation of a stream. (A successful call to freopen removes any
orientation.)329)
5 Byte input/output functions shall not be applied to a wide-oriented stream and wide character
input/output functions shall not be applied to a byte-oriented stream. The remaining stream
operations do not affect, and are not affected by, a stream’s orientation, except for the following
additional restrictions:
— Binary wide-oriented streams have the file-positioning restrictions ascribed to both text and
binary streams.
— For wide-oriented streams, after a successful call to a file-positioning function that leaves the
file position indicator prior to the end-of-file, a wide character output function can overwrite a
partial multibyte character; any file contents beyond the byte(s) written may henceforth not
consist of valid multibyte characters.
6 Each wide-oriented stream has an associated mbstate_t object that stores the current parse state
of the stream. A successful call to fgetpos stores a representation of the value of this mbstate_t
object as part of the value of the fpos_t object. A later successful call to fsetpos using the same
stored fpos_t value restores the value of the associated mbstate_t object as well as the position
within the controlled stream.
7 Each stream has an associated lock that is used to prevent data races when multiple threads of
execution access a stream, and to restrict the interleaving of stream operations performed by multiple
threads. Only one thread may hold this lock at a time. The lock is reentrant: a single thread may
hold the lock multiple times at a given time.
8 All functions that read, write, position, or query the position of a stream lock the stream before
accessing it. They release the lock associated with the stream when the access is complete.
328) An implementation need not distinguish between text streams and binary streams. In such an implementation, there
need be no new-line characters in a text stream nor any limit to the length of a line.
329) The three predefined streams stdin, stdout, and stderr are unoriented at program startup.
Environmental limits
9 An implementation shall support text files with lines containing at least 254 characters, including
the terminating new-line character. The value of the macro BUFSIZ shall be at least 256.
Forward references: the freopen function (7.23.5.4), the fwide function (7.31.3.5), mbstate_t
(7.31.1), the fgetpos function (7.23.9.1), the fsetpos function (7.23.9.3).
7.23.3 Files
1 A stream is associated with an external file (which may be a physical device) by opening a file, which
may involve creating a new file. Creating an existing file causes its former contents to be discarded,
if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal),
then a file position indicator associated with the stream is positioned at the start (character number
zero) of the file, unless the file is opened with append mode in which case it is implementation-
defined whether the file position indicator is initially positioned at the beginning or the end of the
file. The file position indicator is maintained by subsequent reads, writes, and positioning requests,
to facilitate an orderly progression through the file.
2 Binary files are not truncated, except as defined in 7.23.5.3. Whether a write on a text stream causes
the associated file to be truncated beyond that point is implementation-defined.
3 When a stream is unbuffered, characters are intended to appear from the source or at the destination
as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host
environment as a block. When a stream is fully buffered, characters are intended to be transmitted
to or from the host environment as a block when a buffer is filled. When a stream is line buffered,
characters are intended to be transmitted to or from the host environment as a block when a new-line
character is encountered. Furthermore, characters are intended to be transmitted as a block to the
host environment when a buffer is filled, when input is requested on an unbuffered stream, or when
input is requested on a line buffered stream that requires the transmission of characters from the
host environment. Support for these characteristics is implementation-defined, and may be affected
via the setbuf and setvbuf functions.
4 A file may be disassociated from a controlling stream by closing the file. Output streams are
flushed (any unwritten buffer contents are transmitted to the host environment) before the stream
is disassociated from the file. The lifetime of a FILE object ends when the associated file is closed
(including the standard text streams). Whether a file of zero length (on which no characters have
been written by an output stream) actually exists is implementation-defined.
5 The file may be subsequently reopened, by the same or another program execution, and its contents
reclaimed or modified (if it can be repositioned at its start). If the main function returns to its original
caller, or if the exit function is called, all open files are closed (hence all output streams are flushed)
before program termination. Other paths to program termination, such as calling the abort function,
need not close all files properly.
6 The address of the FILE object used to control a stream may be significant; a copy of a FILE object
need not serve in place of the original.
7 At program startup, three text streams are predefined and need not be opened explicitly — standard
input (for reading conventional input), standard output (for writing conventional output), and standard
error (for writing diagnostic output). As initially opened, the standard error stream is not fully
buffered; the standard input and standard output streams are fully buffered if and only if the stream
can be determined not to refer to an interactive device.
8 Functions that open additional (nontemporary) files require a file name, which is a string. The
rules for composing valid file names are implementation-defined. Whether the same file can be
simultaneously open multiple times is also implementation-defined.
9 Although both text and binary wide-oriented streams are conceptually sequences of wide characters,
the external file associated with a wide-oriented stream is a sequence of multibyte characters,
generalized as follows:
— Multibyte encodings within files may contain embedded null bytes (unlike multibyte encod-
ings valid for use internal to the program).
— A file need not begin nor end in the initial shift state.330)
10 Moreover, the encodings used for multibyte characters may differ among files. Both the nature and
choice of such encodings are implementation-defined.
11 The wide character input functions read multibyte characters from the stream and convert them
to wide characters as if they were read by successive calls to the fgetwc function. Each conversion
occurs as if by a call to the mbrtowc function, with the conversion state described by the stream’s
own mbstate_t object. The byte input functions read characters from the stream as if by successive
calls to the fgetc function.
12 The wide character output functions convert wide characters to multibyte characters and write them
to the stream as if they were written by successive calls to the fputwc function. Each conversion
occurs as if by a call to the wcrtomb function, with the conversion state described by the stream’s
own mbstate_t object. The byte output functions write characters to the stream as if by successive
calls to the fputc function.
13 In some cases, some of the byte input/output functions also perform conversions between multibyte
characters and wide characters. These conversions also occur as if by calls to the mbrtowc and
wcrtomb functions.
14 An encoding error occurs if the character sequence presented to the underlying mbrtowc function
does not form a valid (generalized) multibyte character, or if the code value passed to the underlying
wcrtomb does not correspond to a valid (generalized) multibyte character. The wide character
input/output functions and the byte input/output functions store the value of the macro EILSEQ in
errno if and only if an encoding error occurs.
Environmental limits
15 The value of FOPEN_MAX shall be at least eight, including the three standard text streams.
Forward references: the exit function (7.24.4.4), the fgetc function (7.23.7.1), the fopen function
(7.23.5.3), the fputc function (7.23.7.3), the setbuf function (7.23.5.5), the setvbuf function (7.23.5.6),
the fgetwc function (7.31.3.1), the fputwc function (7.31.3.3), conversion state (7.31.6), the mbrtowc
function (7.31.6.3.2), the wcrtomb function (7.31.6.3.3).
7.23.4 Operations on files
7.23.4.1 The remove function
Synopsis
1 #include <stdio.h>
int remove(const char *filename);
Description
2 The remove function causes the file whose name is the string pointed to by filename to be no longer
accessible by that name. A subsequent attempt to open that file using that name will fail, unless it is
created anew. If the file is open, the behavior of the remove function is implementation-defined.
Returns
3 The remove function returns zero if the operation succeeds, nonzero if it fails.
7.23.4.2 The rename function
Synopsis
1 #include <stdio.h>
int rename(const char *old, const char *new);
330) Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a
binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not
assuredly end in the initial shift state.
Description
2 The rename function causes the file whose name is the string pointed to by old to be henceforth
known by the name given by the string pointed to by new. The file named old is no longer accessible
by that name. If a file named by the string pointed to by new exists prior to the call to the rename
function, the behavior is implementation-defined.
Returns
3 The rename function returns zero if the operation succeeds, nonzero if it fails,331) in which case if the
file existed previously it is still known by its original name.
7.23.4.3 The tmpfile function
Synopsis
1 #include <stdio.h>
FILE *tmpfile(void);
Description
2 The tmpfile function creates a temporary binary file that is different from any other existing file
and that will automatically be removed when it is closed or at program termination. If the program
terminates abnormally, whether an open temporary file is removed is implementation-defined. The
file is opened for update with "wb+" mode.
Recommended practice
3 It should be possible to open at least TMP_MAX temporary files during the lifetime of the program
(this limit may be shared with tmpnam) and there should be no limit on the number simultaneously
open other than this limit and any limit on the number of open files (FOPEN_MAX).
Returns
4 The tmpfile function returns a pointer to the stream of the file that it created. If the file cannot be
created, the tmpfile function returns a null pointer.
Forward references: the fopen function (7.23.5.3).
7.23.4.4 The tmpnam function
Synopsis
1 #include <stdio.h>
char *tmpnam(char *s);
Description
2 The tmpnam function generates a string that is a valid file name and that is not the same as the name
of an existing file.332) The function is potentially capable of generating at least TMP_MAX different
strings, but any or all of them may already be in use by existing files and thus not be suitable return
values.
3 The tmpnam function generates a different string each time it is called.
4 Calls to the tmpnam function with a null pointer argument may introduce data races with each other.
The implementation shall behave as if no library function calls the tmpnam function.
Returns
5 If no suitable string can be generated, the tmpnam function returns a null pointer. Otherwise, if
the argument is a null pointer, the tmpnam function leaves its result in an internal static object and
returns a pointer to that object (subsequent calls to the tmpnam function may modify the same object).
331) Among the reasons the implementation could cause the rename function to fail are that the file is open or that it is
necessary to copy its contents to effectuate its renaming.
332) Files created using strings generated by the tmpnam function are temporary only in the sense that their names are not
expected to collide with those generated by conventional naming rules for the implementation. It is still necessary to use the
remove function to remove such files when their use is ended, and before program termination.
If the argument is not a null pointer, it is assumed to point to an array of at least L_tmpnam chars;
the tmpnam function writes its result in that array and returns the argument as its value.
Environmental limits
6 The value of the macro TMP_MAX shall be at least 25.
7.23.5 File access functions
7.23.5.1 The fclose function
Synopsis
1 #include <stdio.h>
int fclose(FILE *stream);
Description
2 A successful call to the fclose function causes the stream pointed to by stream to be flushed and
the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host
environment to be written to the file; any unread buffered data are discarded. Whether or not the
call succeeds, the stream is disassociated from the file and any buffer set by the setbuf or setvbuf
function is disassociated from the stream (and deallocated if it was automatically allocated).
Returns
3 The fclose function returns zero if the stream was successfully closed, or EOF if any errors were
detected.
7.23.5.2 The fflush function
Synopsis
1 #include <stdio.h>
int fflush(FILE *stream);
Description
2 If stream points to an output stream or an update stream in which the most recent operation was
not input, the fflush function causes any unwritten data for that stream to be delivered to the host
environment to be written to the file; otherwise, the behavior is undefined.
3 If stream is a null pointer, the fflush function performs this flushing action on all streams for which
the behavior is defined above.
Returns
4 The fflush function sets the error indicator for the stream and returns EOF if a write error occurs,
otherwise it returns zero.
Forward references: the fopen function (7.23.5.3).
7.23.5.3 The fopen function
Synopsis
1 #include <stdio.h>
FILE *fopen(const char * restrict filename, const char * restrict mode);
Description
2 The fopen function opens the file whose name is the string pointed to by filename, and associates
a stream with it.
3 The argument mode points to a string. If the string is one of the following, the file is open in the
indicated mode. Otherwise, the behavior is undefined.333)
333) If the string begins with one of the listed mode sequences, the implementation might choose to ignore the remaining
characters, or it might use them to select different kinds of a file (some of which might not conform to the properties in 7.23.2).
r open text file for reading
w truncate to zero length or create text file for writing
wx create text file for writing
a append; open or create text file for writing at end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for writing
wbx create binary file for writing
ab append; open or create binary file for writing at end-of-file
r+ open text file for update (reading and writing)
w+ truncate to zero length or create text file for update
w+x create text file for update
a+ append; open or create text file for update, writing at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for update
w+bx or wb+x create binary file for update
a+b or ab+ append; open or create binary file for update, writing at end-of-file
4 Opening a file with read mode (’r’ as the first character in the mode argument) fails if the file does
not exist or cannot be read.
5 Opening a file with exclusive mode (’x’ as the last character in the mode argument) fails if the file
already exists or cannot be created. Otherwise, the file is created with exclusive (also known as
non-shared) access to the extent that the underlying system supports exclusive access.
6 Opening a file with append mode (’a’ as the first character in the mode argument) causes all
subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening
calls to the fseek function. In some implementations, opening a binary file with append mode (’b’
as the second or third character in the above list of mode argument values) may initially position the
file position indicator for the stream beyond the last data written, because of null character padding.
7 When a file is opened with update mode (’+’ as the second or third character in the above list
of mode argument values), both input and output may be performed on the associated stream.
However, output shall not be directly followed by input without an intervening call to the fflush
function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly
followed by output without an intervening call to a file positioning function, unless the input
operation encounters end-of-file. Opening (or creating) a text file with update mode may instead
open (or create) a binary stream in some implementations.
8 When opened, a stream is fully buffered if and only if it can be determined not to refer to an
interactive device. The error and end-of-file indicators for the stream are cleared.
Returns
9 The fopen function returns a pointer to the object controlling the stream. If the open operation fails,
fopen returns a null pointer.
Forward references: file positioning functions (7.23.9).
7.23.5.4 The freopen function
Synopsis
1 #include <stdio.h>
FILE *freopen(const char * restrict filename, const char * restrict mode,
FILE * restrict stream);
Description
2 The freopen function opens the file whose name is the string pointed to by filename and associates
the stream pointed to by stream with it. The mode argument is used just as in the fopen function.334)
3 If filename is a null pointer, the freopen function attempts to change the mode of the stream to
that specified by mode, as if the name of the file currently associated with the stream had been
used. It is implementation-defined which changes of mode are permitted (if any), and under what
circumstances.
4 The freopen function first attempts to close any file that is associated with the specified stream.
Failure to close the file is ignored. The error and end-of-file indicators for the stream are cleared.
Returns
5 The freopen function returns a null pointer if the open operation fails. Otherwise, freopen returns
the value of stream.
7.23.5.5 The setbuf function
Synopsis
1 #include <stdio.h>
void setbuf(FILE * restrict stream, char * restrict buf);
Description
2 Except that it returns no value, the setbuf function is equivalent to the setvbuf function invoked
with the values _IOFBF for mode and BUFSIZ for size, or (if buf is a null pointer), with the value
_IONBF for mode.
Returns
3 The setbuf function returns no value.
Forward references: the setvbuf function (7.23.5.6).
7.23.5.6 The setvbuf function
Synopsis
1 #include <stdio.h>
int setvbuf(FILE * restrict stream, char * restrict buf, int mode, size_t size);
Description
2 The setvbuf function may be used only after the stream pointed to by stream has been associated
with an open file and before any other operation (other than an unsuccessful call to setvbuf) is
performed on the stream. The argument mode determines how stream will be buffered, as follows:
_IOFBF causes input/output to be fully buffered;
_IOLBF causes input/output to be line buffered;
_IONBF causes input/output to be unbuffered.
If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by the
setvbuf function335) and the argument size specifies the size of the array; otherwise, size may
determine the size of a buffer allocated by the setvbuf function. The members of the array at any
time have unspecified values.
334) The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin,
or stdout), as those identifiers need not be modifiable lvalues to which the value returned by the fopen function could be
assigned.
335) The buffer has to have a lifetime at least as great as the open stream, so not closing the stream before a buffer that has
automatic storage duration is deallocated upon block exit results in undefined behavior.
Returns
3 The setvbuf function returns zero on success, or nonzero if an invalid value is given for mode or if
the request cannot be honored.
7.23.6 Formatted input/output functions
1 The formatted input/output functions shall behave as if there is a sequence point after the actions
associated with each specifier.336)
7.23.6.1 The fprintf function
Synopsis
1 #include <stdio.h>
int fprintf(FILE * restrict stream, const char * restrict format, ...);
Description
2 The fprintf function writes output to the stream pointed to by stream, under control of the string
pointed to by format that specifies how subsequent arguments are converted for output. If there are
insufficient arguments for the format, the behavior is undefined. If the format is exhausted while
arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The
fprintf function returns when the end of the format string is encountered.
3 The format shall be a multibyte character sequence, beginning and ending in its initial shift state.
The format is composed of zero or more directives: ordinary multibyte characters (not %), which
are copied unchanged to the output stream; and conversion specifications, each of which results
in fetching zero or more subsequent arguments, converting them, if applicable, according to the
corresponding conversion specifier, and then writing the result to the output stream.
4 Each conversion specification is introduced by the character %. After the %, the following appear in
sequence:
— Zero or more flags (in any order) that modify the meaning of the conversion specification.
— An optional minimum field width. If the converted value has fewer characters than the field
width, it is padded with spaces (by default) on the left (or right, if the left adjustment flag,
described later, has been given) to the field width. The field width takes the form of an asterisk
* (described later) or a nonnegative decimal integer.337)
— An optional precision that gives the minimum number of digits to appear for the b, d, i, o, u, x,
and X conversions, the number of digits to appear after the decimal-point character for a, A, e,
E, f, and F conversions, the maximum number of significant digits for the g and G conversions,
or the maximum number of bytes to be written for s conversions. The precision takes the form
of a period (.) followed either by an asterisk * (described later) or by an optional nonnegative
decimal integer; if only the period is specified, the precision is taken as zero. If a precision
appears with any other conversion specifier, the behavior is undefined.
— An optional length modifier that specifies the size of the argument.
— A conversion specifier character that specifies the type of conversion to be applied.
5 As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case,
an int argument supplies the field width or precision. The arguments specifying field width, or
precision, or both, shall appear (in that order) before the argument (if any) to be converted. A
negative field width argument is taken as a - flag followed by a positive field width. A negative
precision argument is taken as if the precision were omitted.
6 The flag characters and their meanings are:
336) The fprintf functions perform writes to memory for the %n specifier.
337) Note that 0 is taken as a flag, not as the beginning of a field width.
- The result of the conversion is left-justified within the field. (It is right-justified if this flag is
not specified.)
+ The result of a signed conversion always begins with a plus or minus sign. (It begins with a
sign only when a value with a negative sign is converted if this flag is not specified.) 338)
space If the first character of a signed conversion is not a sign, or if a signed conversion results in
no characters, a space is prefixed to the result. If the space and + flags both appear, the space
flag is ignored.
# The result is converted to an "alternative form". For o conversion, it increases the precision, if
and only if necessary, to force the first digit of the result to be a zero (if the value and precision
are both 0, a single 0 is printed). For b conversion, a nonzero result has 0b prefixed to it. For
x (or X) conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G
conversions, the result of converting a floating-point number always contains a decimal-point
character, even if no digits follow it. (Normally, a decimal-point character appears in the
result of these conversions only if a digit follows it.) For g and G conversions, trailing zeros
are not removed from the result. For other conversions, the behavior is undefined.
0 For b, d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, leading zeros (following any
indication of sign or base) are used to pad to the field width rather than performing space
padding, except when converting an infinity or NaN. If the 0 and - flags both appear, the
0 flag is ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag is
ignored. For other conversions, the behavior is undefined.
7 The length modifiers and their meanings are:
hh Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a
signed char or unsigned char argument (the argument will have been promoted
according to the integer promotions, but its value shall be converted to signed char or
unsigned char before printing); or that a following n conversion specifier applies to a
pointer to a signed char argument.
h Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a short int
or unsigned short int argument (the argument will have been promoted accord-
ing to the integer promotions, but its value shall be converted to short int or
unsigned short int before printing); or that a following n conversion specifier applies
to a pointer to a short int argument.
l (ell) Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a long int
or unsigned long int argument; that a following n conversion specifier applies to
a pointer to a long int argument; that a following c conversion specifier applies to
a wint_t argument; that a following s conversion specifier applies to a pointer to a
wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion
specifier.
ll (ell-ell) Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a
long long int or unsigned long long int argument; or that a following n con-
version specifier applies to a pointer to a long long int argument.
j Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to an intmax_t
or uintmax_t argument; or that a following n conversion specifier applies to a pointer
to an intmax_t argument.
z Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a size_t
or the corresponding signed integer type argument; or that a following n conversion
specifier applies to a pointer to a signed integer type corresponding to size_t argument.
338) The results of all floating conversions of a negative zero, and of negative values that round to zero, include a minus sign.
t Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t
or the corresponding unsigned integer type argument; or that a following n conversion
specifier applies to a pointer to a ptrdiff_t argument.
wN Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to an integer
argument with a specific width where N is a positive decimal integer with no leading
zeros (the argument will have been promoted according to the integer promotions, but
its value shall be converted to the unpromoted type); or that a following n conversion
specifier applies to a pointer to an integer type argument with a width of N bits. All
minimum-width integer types (7.22.1.2) and exact-width integer types (7.22.1.1) de-
fined in the header <stdint.h> shall be supported. Other supported values of N are
implementation-defined.
wfN Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a fastest
minimum-width integer argument with a specific width where N is a positive decimal
integer with no leading zeros (the argument will have been promoted according to
the integer promotions, but its value shall be converted to the unpromoted type); or
that a following n conversion specifier applies to a pointer to a fastest minimum-width
integer type argument with a width of N bits. All fastest minimum-width integer types
(7.22.1.3) defined in the header <stdint.h> shall be supported. Other supported values
of N are implementation-defined.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
long double argument.
H Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
_Decimal32 argument.
D Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
_Decimal64 argument.
DD Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
_Decimal128 argument.
If a length modifier appears with any conversion specifier other than as specified above, the behavior
is undefined.
8 The conversion specifiers and their meanings are:
d,i The int argument is converted to signed decimal in the style [-]dddd. The precision
specifies the minimum number of digits to appear; if the value being converted can be
represented in fewer digits, it is expanded with leading zeros. The default precision is 1.
The result of converting a zero value with a precision of zero is no characters.
b, o,u,x,X The unsigned int argument is converted to unsigned binary (b), unsigned octal (o),
unsigned decimal (u), or unsigned hexadecimal notation (x or X) in the style dddd; the
letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The
precision specifies the minimum number of digits to appear; if the value being converted
can be represented in fewer digits, it is expanded with leading zeros. The default precision
is 1. The result of converting a zero value with a precision of zero is no characters.
f,F A double argument representing a floating-point number is converted to decimal notation
in the style [-]ddd.ddd, where the number of digits after the decimal-point character is
equal to the precision specification. If the precision is missing, it is taken as 6; if the
precision is zero and the # flag is not specified, no decimal-point character appears. If a
decimal-point character appears, at least one digit appears before it. The value is rounded
to the appropriate number of digits.
A double argument representing an infinity is converted in one of the styles [-]inf or
[-]infinity — which style is implementation-defined. A double argument representing a
NaN is converted in one of the styles [-]nan or [-]nan(n-char-sequence) — which style, and
the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier
produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.339)
e,E A double argument representing a floating-point number is converted in the style
[-]d.ddde±dd, where there is one digit (which is nonzero if the argument is nonzero) before
the decimal-point character and the number of digits after it is equal to the precision; if the
precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified,
no decimal-point character appears. The value is rounded to the appropriate number of
digits. The E conversion specifier produces a number with E instead of e introducing the
exponent. The exponent always contains at least two digits, and only as many more digits
as necessary to represent the exponent. If the value is zero, the exponent is zero.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
g,G A double argument representing a floating-point number is converted in style f or e (or
in style F or E in the case of a G conversion specifier), depending on the value converted
and the precision. Let P equal the precision if nonzero, 6 if the precision is omitted, or 1 if
the precision is zero. Then, if a conversion with style E would have an exponent of X:
if P > X ≥ −4, the conversion is with style f (or F) and precision P − (X + 1).
otherwise, the conversion is with style e (or E) and precision P − 1.
Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion
of the result and the decimal-point character is removed if there is no fractional portion
remaining.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
a,A A double argument representing a floating-point number is converted in the style
[-]0xh.hhhhp±d, where there is one hexadecimal digit (which is nonzero if the argument is a
normalized floating-point number and is otherwise unspecified) before the decimal-point
character340) and the number of hexadecimal digits after it is equal to the precision; if the
precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an
exact representation of the value; if the precision is missing and FLT_RADIX is not a power
of 2, then the precision is sufficient to distinguish341) values of type double, except that
trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no
decimal-point character appears. The letters abcdef are used for a conversion and the
letters ABCDEF for A conversion. The A conversion specifier produces a number with X and
P instead of x and p. The exponent always contains at least one digit, and only as many
more digits as necessary to represent the decimal exponent of 2. If the value is zero, the
exponent is zero.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
339) When applied to infinite and NaN values, the -, +, and space flag characters have their usual meaning; the # and 0 flag
characters have no effect.
340) Binary implementations can choose the hexadecimal digit to the left of the decimal-point character so that subsequent
digits align to nibble (4-bit) boundaries. This implementation choice affects numerical values printed with a precision P
that is insufficient to represent all values exactly. Implementations with different conventions about the most significant
hexadecimal digit will round at different places, affecting the numerical value of the hexadecimal result. For example,
possible printed output for the code
#include <stdio.h>
/* ... */
double x = 123.0;
printf("%.1a", x);
include "0x1.fp+6 " and "0xf.6p+3 " whose numerical values are 124 and 123, respectively. Portable code seeking identical
numerical results on different platforms should avoid precisions P that require rounding.
341) The formatting precision P is sufficient to distinguish values of the source type if 16P > bp where b (not a power of 2)
and p are the base and precision of the source type (5.2.4.2.2). A smaller P might suffice depending on the implementation’s
scheme for determining the digit to the left of the decimal-point character.
If an H, D, or DD modifier is present and the precision is missing, then for a decimal
floating type argument represented by a triple of integers (s, c, q), where n is the number
of significant digits in the coefficient c,
— if −(n + 5) ≤ q ≤ 0, use style f (or style F in the case of an A conversion specifier)
with formatting precision equal to −q,
— otherwise, use style e (or style E in the case of an A conversion specifier) with format-
ting precision equal to n − 1, with the exceptions that if c = 0 then the digit-sequence
in the exponent-part shall have the value q (rather than 0), and that the exponent is
always expressed with the minimum number of digits required to represent its value
(the exponent never contains a leading zero).
If the precision P is present (in the conversion specification) and is zero or at least as
large as the precision p (5.2.4.2.2) of the decimal floating type, the conversion is as if the
precision were missing. If the precision P is present (and nonzero) and less than the
precision p of the decimal floating type, the conversion first obtains an intermediate result
as follows, where n is the number of significant digits in the coefficient:
— If n ≤ P , set the intermediate result to the input.
— If n > P , round the input value, according to the current rounding direction for
decimal floating-point operations, to P decimal digits, with unbounded exponent
range, representing the result with a P -digit integer coefficient when in the form
(s, c, q).
Convert the intermediate result in the manner described above for the case where the
precision is missing.
c If no l length modifier is present, the int argument is converted to an unsigned char,
and the resulting character is written.
If an l length modifier is present, the wint_t argument is converted as if by an ls
conversion specification with no precision and an argument that points to storage suitably
sized for at least two wchar_t elements, the first element containing the wint_t argument
to the lc conversion specification and the second a null wide character.
s If no l length modifier is present, the argument shall be a pointer to storage of character
type.342) Characters from the storage are written up to (but not including) the terminating
null character. If the precision is specified, no more than that many bytes are written. If
the precision is not specified or is greater than the size of the storage, the storage shall
contain a null character.
If an l length modifier is present, the argument shall be a pointer to storage of wchar_t
type. Wide characters from the storage are converted to multibyte characters (each as if
by a call to the wcrtomb function, with the conversion state described by an mbstate_t
object initialized to zero before the first wide character is converted) up to and including
a terminating null wide character. The resulting multibyte characters are written up to
(but not including) the terminating null character (byte). If no precision is specified, the
storage shall contain a null wide character. If a precision is specified, no more than that
many bytes are written (including shift sequences, if any), and the storage shall contain
a null wide character if, to equal the multibyte character sequence length given by the
precision, the function would need to access a wide character one past the end of the array.
In no case is a partial multibyte character written.343)
p The argument shall be a pointer to void or a pointer to a character type. The value of the
pointer is converted to a sequence of printing characters, in an implementation-defined
manner.
342) No special provisions are made for multibyte characters.
343) Redundant shift sequences can result if multibyte characters have a state-dependent encoding.
n The argument shall be a pointer to signed integer whose type is specified by the length
modifiers, if any, for the conversion specification, or shall be int if no length modifiers are
specified for the conversion specification. The number of characters written to the output
stream so far by this call to fprintf is stored into the integer object pointed to by the
argument. No argument is converted, but one is consumed. If the conversion specification
includes any flags, a field width, or a precision, the behavior is undefined.
% A % character is written. No argument is converted. The complete conversion specification
shall be %%.
9 If a conversion specification is invalid, the behavior is undefined.344) fprintf shall behave as if it
uses va_arg with a type argument naming the type resulting from applying the default argument
promotions to the type corresponding to the conversion specification and then converting the result
of the va_arg expansion to the type corresponding to the conversion specification.345)
10 In no case does a nonexistent or small field width cause truncation of a field; if the result of a
conversion is wider than the field width, the field is expanded to contain the conversion result.
11 For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal
floating number with the given precision.
Recommended practice
12 For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable
in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating
style with the given precision, with the extra stipulation that the error should have a correct sign for
the current rounding direction.
13 For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most the maximum
value M of the T_DECIMAL_DIG macros (defined in <float.h>), then the result should be correctly
rounded.346) If the number of significant decimal digits is more than M but the source value is
exactly representable with M digits, then the result should be an exact representation with trailing
zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having
M significant digits; the value of the resultant decimal string D should satisfy L ≤ D ≤ U, with the
extra stipulation that the error should have a correct sign for the current rounding direction.
14 An uppercase B format specifier is not covered by the description above, because it used to be
available for extensions in previous versions of this standard.
Implementations that did not use an uppercase B as their own extension before are encouraged to
implement it similar to conversion specifier b as standardized above, with the alternative form (#B)
generating 0B as prefix for nonzero values.
Returns
15 The fprintf function returns the number of characters transmitted, or a negative value if an output
or encoding error occurred or if the implementation does not support a specified width length
modifier.
Environmental limits
16 The number of characters that can be produced by any single conversion shall be at least 4095.
17 EXAMPLE 1 To print a date and time in the form "Sunday, July 3, 10:02" followed by π to five decimal places:
#include <math.h>
#include <stdio.h>
/* ... */
char *weekday, *month; // pointers to strings
int day, hour, min;
344) See "future library directions" (7.33.15).
345) The behavior is undefined when the types differ as specified for va_arg 7.16.1.1.
346) For binary-to-decimal conversion, the result format’s values are the numbers representable with the given format specifier.
The number of significant digits is determined by the format specifier, and in the case of fixed-point conversion by the source
value as well.
fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
weekday, month, day, hour, min);
fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
18 EXAMPLE 2 In this example, multibyte characters do not have a state-dependent encoding, and the members of the extended
character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a □
and the second by an uppercase letter.
19 Given the following wide string with length seven,
static wchar_t wstr[] = L"□X□Yabc□Z□W";
the seven calls
fprintf(stdout, "|1234567890123|\n");
fprintf(stdout, "|%13ls|\n", wstr);
fprintf(stdout, "|%-13.9ls|\n", wstr);
fprintf(stdout, "|%13.10ls|\n", wstr);
fprintf(stdout, "|%13.11ls|\n", wstr);
fprintf(stdout, "|%13.15ls|\n", &wstr[2]);
fprintf(stdout, "|%13lc|\n", (wint_t) wstr[5]);
will print the following seven lines:
|1234567890123|
| □X□Yabc□Z□W|
|□X□Yabc□Z |
| □X□Yabc□Z|
| □X□Yabc□Z□W|
| abc□Z□W|
| □Z|
20 EXAMPLE 3 Following are representations of _Decimal64 arguments as triples (s, c, q) and the corresponding character
sequences fprintf produces with "%Da":
(+1, 123, 0) 123
(−1, 123, 0) -123
(+1, 123, −2) 1.23
(+1, 123, 1) 1.23e+3
(−1, 123, 1) -1.23e+3
(+1, 123, −8) 0.00000123
(+1, 123, −9) 1.23e-7
(+1, 120, −8) 0.00000120
(+1, 120, −9) 1.20e-7
(+1, 1234567890123456, 0) 1234567890123456
(+1, 1234567890123456, 1) 1.234567890123456e+16
(+1, 1234567890123456, −1) 123456789012345.6
(+1, 1234567890123456, −21) 0.000001234567890123456
(+1, 1234567890123456, −22) 1.234567890123456e-7
(+1, 0, 0) 0
(−1, 0, 0) -0
(+1, 0, −6) 0.000000
(+1, 0, −7) 0e-7
(+1, 0, 2) 0e+2
(+1, 5, −6) 0.000005
(+1, 50, −7) 0.0000050
(+1, 5, −7) 5e-7
To illustrate the effects of a precision specification, the sequence:
_Decimal32 x = 6543.00DF; // (+1, 654300, -2)
fprintf(stdout, "%Ha\n", x);
fprintf(stdout, "%.6Ha\n", x);
fprintf(stdout, "%.5Ha\n", x);
fprintf(stdout, "%.4Ha\n", x);
fprintf(stdout, "%.3Ha\n", x);
fprintf(stdout, "%.2Ha\n", x);
fprintf(stdout, "%.1Ha\n", x);
fprintf(stdout, "%.0Ha\n", x);
assuming default rounding, results in:
6543.00
6543.00
6543.0
6543
6.54e+3
6.5e+3
7e+3
6543.00
To illustrate the effects of the exponent range, the sequence:
_Decimal32 x = 9543210e87DF; // (+1, 9543210, 87)
_Decimal32 y = 9500000e90DF; // (+1, 9500000, 90)
fprintf(stdout, "%.6Ha\n", x);
fprintf(stdout, "%.5Ha\n", x);
fprintf(stdout, "%.4Ha\n", x);
fprintf(stdout, "%.3Ha\n", x);
fprintf(stdout, "%.2Ha\n", x);
fprintf(stdout, "%.1Ha\n", x);
fprintf(stdout, "%.1Ha\n", y);
assuming default rounding, results in:
9.54321e+93
9.5432e+93
9.543e+93
9.54e+93
9.5e+93
1e+94
1e+97
To further illustrate the effects of the exponent range, the sequence:
_Decimal32 x = 9512345e90DF; // (+1, 9512345, 90)
_Decimal32 y = 9512345e86DF; // (+1, 9512345, 86)
fprintf(stdout, "%.3Ha\n", x);
fprintf(stdout, "%.2Ha\n", x);
fprintf(stdout, "%.1Ha\n", x);
fprintf(stdout, "%.2Ha\n", y);
assuming default rounding, results in:
9.51e+96
9.5e+96
1e+97
9.5e+92
Forward references: conversion state (7.31.6), the wcrtomb function (7.31.6.3.3).
7.23.6.2 The fscanf function
Synopsis
1 #include <stdio.h>
int fscanf(FILE * restrict stream, const char * restrict format, ...);
Description
2 The fscanf function reads input from the stream pointed to by stream, under control of the string
pointed to by format that specifies the admissible input sequences and how they are to be converted
for assignment, using subsequent arguments as pointers to the objects to receive the converted
input. If there are insufficient arguments for the format, the behavior is undefined. If the format
is exhausted while arguments remain, the excess arguments are evaluated (as always) but are
otherwise ignored.
3 The format shall be a multibyte character sequence, beginning and ending in its initial shift state.The
format is composed of zero or more directives: one or more white-space characters, an ordinary
multibyte character (neither % nor a white-space character), or a conversion specification. Each
conversion specification is introduced by the character %. After the %, the following appear in
sequence:
— An optional assignment-suppressing character *.
— An optional decimal integer greater than zero that specifies the maximum field width (in
characters).
— An optional length modifier that specifies the size of the receiving object.
— A conversion specifier character that specifies the type of conversion to be applied.
4 The fscanf function executes each directive of the format in turn. When all directives have been
executed, or if a directive fails (as detailed below), the function returns. Failures are described as
input failures (due to the occurrence of an encoding error or the unavailability of input characters),
or matching failures (due to inappropriate input).
5 A directive composed of white-space character(s) is executed by reading input up to the first non-
white-space character (which remains unread), or until no more characters can be read. The directive
never fails.
6 A directive that is an ordinary multibyte character is executed by reading the next characters of the
stream. If any of those characters differ from the ones composing the directive,the directive fails and
the differing and subsequent characters remain unread. Similarly, if end-of-file, an encoding error,
or a read error prevents a character from being read, the directive fails.
7 A directive that is a conversion specification defines a set of matching input sequences, as described
below for each specifier. A conversion specification is executed in the following steps:
8 Input white-space characters are skipped, unless the specification includes a [, c, or n specifier.347)
9 An input item is read from the stream, unless the specification includes an n specifier. An input
item is defined as the longest sequence of input characters which does not exceed any specified
field width and which is, or is a prefix of, a matching input sequence.348) The first character, if any,
after the input item remains unread. If the length of the input item is zero, the execution of the
directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read
error prevented input from the stream, in which case it is an input failure.
10 Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input
characters) is converted to a type appropriate to the conversion specifier. If the input item is not a
matching sequence, the execution of the directive fails: this condition is a matching failure. Unless
assignment suppression was indicated by a *, the result of the conversion is placed in the object
pointed to by the first argument following the format argument that has not already received a
conversion result. If this object does not have an appropriate type, or if the result of the conversion
cannot be represented in the object, the behavior is undefined.
11 The length modifiers and their meanings are:
hh Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to signed char or unsigned char.
h Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to short int or unsigned short int.
l (ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F,
347) These white-space characters are not counted against a specified field width.
348) fscanf pushes back at most one input character onto the input stream. Therefore, some sequences that are acceptable to
strtod, strtol, etc., are unacceptable to fscanf.
g, or G conversion specifier applies to an argument with type pointer to double; or that
a following c, s, or [ conversion specifier applies to an argument with type pointer to
wchar_t .
ll (ell-ell) Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to long long int or unsigned long long int.
j Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to intmax_t or uintmax_t.
z Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to size_t or the corresponding signed integer type.
t Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to ptrdiff_t or the corresponding unsigned integer type.
wN Specifies that a following b, d, i, o, u, x, or X, or n conversion specifier applies to an
argument which is a pointer to an integer with a specific width where N is a positive
decimal integer with no leading zeros. All minimum-width integer types (7.22.1.2) and
exact-width integer types (7.22.1.1) defined in the header <stdint.h> shall be supported.
Other supported values of N are implementation-defined.
wfN Specifies that a following b, d, i, o, u, x, or X, or n conversion specifier applies to an
argument which is a pointer to a fastest minimum-width integer with a specific width
where N is a positive decimal integer with no leading zeros. All fastest minimum-width
integer types (7.22.1.3) defined in the header <stdint.h> shall be supported. Other
supported values of N are implementation-defined.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to long double.
H Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to _Decimal32 .
D Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to _Decimal64 .
DD Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to _Decimal128 .
If a length modifier appears with any conversion specifier other than as specified above, the behavior
is undefined.
12 In the following, the type of the corresponding argument for a conversion specifier shall be a pointer
to a type determined by the length modifiers, if any, or specified by the conversion specifier. The
conversion specifiers and their meanings are:
d Matches an optionally signed decimal integer, whose format is the same as expected for
the subject sequence of the strtol function with the value 10 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
int.
b Matches an optionally signed binary integer, whose format is the same as expected for the
subject sequence of the strtol function with the value 2 for the base argument. Unless a
length modifier is specified, the corresponding argument shall be a pointer to unsigned
int.
i Matches an optionally signed integer, whose format is the same as expected for the subject
sequence of the strtol function with the value 0 for the base argument. Unless a length
modifier is specified, the corresponding argument shall be a pointer to int.
o Matches an optionally signed octal integer, whose format is the same as expected for
the subject sequence of the strtoul function with the value 8 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
unsigned int.
u Matches an optionally signed decimal integer, whose format is the same as expected for
the subject sequence of the strtoul function with the value 10 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
unsigned int.
x Matches an optionally signed hexadecimal integer, whose format is the same as expected
for the subject sequence of the strtoul function with the value 16 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
unsigned int.
a,e,f,g Matches an optionally signed floating-point number, infinity, or NaN, whose format is
the same as expected for the subject sequence of the strtod function. Unless a length
modifier is specified, the corresponding argument shall be a pointer to float.
c Matches a sequence of characters of exactly the number specified by the field width (1 if
no field width is present in the directive).349)
If no l length modifier is present, the corresponding argument shall be a pointer to char,
signed char, unsigned char, or void that points to storage large enough to accept the
sequence. No null character is added.
If an l length modifier is present, the input shall be a sequence of multibyte characters that
begins in the initial shift state. Each multibyte character in the sequence is converted to a
wide character as if by a call to the mbrtowc function, with the conversion state described
by an mbstate_t object initialized to zero before the first multibyte character is converted.
The corresponding argument shall be a pointer to storage of wchar_t large enough to
accept the resulting sequence of wide characters.No null wide character is added.
s Matches a sequence of non-white-space characters.349)
If no l length modifier is present, the corresponding argument shall be a pointer to char,
signed char, unsigned char, or void that points to storage large enough to accept the
sequence and a terminating null character, which will be added automatically.
If an l length modifier is present, the input shall be a sequence of multibyte characters
that begins in the initial shift state. Each multibyte character is converted to a wide
character as if by a call to the mbrtowc function, with the conversion state described by an
mbstate_t object initialized to zero before the first multibyte character is converted. The
corresponding argument shall be a pointer to storage of wchar_t large enough to accept
the sequence and the terminating null wide character, which will be added automatically.
[ Matches a nonempty sequence of characters from a set of expected characters (the
scanset).349)
If no l length modifier is present, the corresponding argument shall be a pointer to char,
signed char, unsigned char, or void that points to storage large enough to accept the
sequence and a terminating null character, which will be added automatically.
If an l length modifier is present, the input shall be a sequence of multibyte characters
that begins in the initial shift state. Each multibyte character is converted to a wide
character as if by a call to the mbrtowc function, with the conversion state described by
an mbstate_t object initialized to zero before the first multibyte character is converted.
The corresponding argument shall be a pointer that points to storage of wchar_t large
349) No special provisions are made for multibyte characters in the matching rules used by the c, s, and [ conversion specifiers
— the extent of the input field is determined on a byte-by-byte basis. The resulting field is nevertheless a sequence of multibyte
characters that begins in the initial shift state.
enough to accept the sequence and the terminating null wide character, which will be
added automatically.
The conversion specifier includes all subsequent characters in the format string, up to
and including the matching right bracket (]). The characters between the brackets (the
scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^),
in which case the scanset contains all characters that do not appear in the scanlist between
the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the
right bracket character is in the scanlist and the next following right bracket character is
the matching right bracket that ends the specification; otherwise the first following right
bracket character is the one that ends the specification. If a - character is in the scanlist
and is not the first, nor the second where the first character is a ^, nor the last character,
the behavior is implementation-defined.
p Matches an implementation-defined set of sequences, which should be the same as the
set of sequences that may be produced by the %p conversion of the fprintf function.
The corresponding argument shall be a pointer to a pointer of void. The input item is
converted to a pointer value in an implementation-defined manner. If the input item is a
value converted earlier during the same program execution, the pointer that results shall
compare equal to that value; otherwise the behavior of the %p conversion is undefined.
n No input is consumed. The corresponding argument shall be a pointer of a signed integer
type. The number of characters read from the input stream so far by this call to the fscanf
function is stored into the integer object pointed to by the argument. Execution of a %n
directive does not increment the assignment count returned at the completion of execution
of the fscanf function. No argument is converted, but one is consumed. If the conversion
specification includes an assignment-suppressing character or a field width, the behavior
is undefined.
% Matches a single % character; no conversion or assignment occurs. The complete conversion
specification shall be %%.
13 If a conversion specification is invalid, the behavior is undefined.350)
14 The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f,
g, and x.
15 Trailing white-space characters(including new-line characters) are left unread unless matched by a
directive. The success of literal matches and suppressed assignments is not directly determinable
other than via the %n directive.
Returns
16 The fscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the function returns the number of input items
assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure or if the implementation does not support a specific width length modifier.
17 EXAMPLE 1 The call:
#include <stdio.h>
/* ... */
int n, i; float x; char name[50];
n = fscanf(stdin, "%d%f%s", &i, &x, name);
with the input line:
25 54.32E-1 thompson
will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.
18 EXAMPLE 2 The call:
350) See "future library directions" (7.33.15).
#include <stdio.h>
/* ... */
int i; float x; char name[50];
fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name);
with input:
56789 0123 56a72
will assign to i the value 56 and to x the value 789.0, will skip 0123, and will assign to name the sequence 56\0. The next
character read from the input stream will be a.
19 EXAMPLE 3 To accept repeatedly from stdin a quantity, a unit of measure, and an item name:
#include <stdio.h>
/* ... */
int count; float quant; char units[21], item[21];
do {
count = fscanf(stdin, "%f%20s of %20s", &quant, units, item);
fscanf(stdin,"%*[^\n]");
} while (!feof(stdin) && !ferror(stdin));
20 If the stdin stream contains the following lines:
2 quarts of oil
-12.8degrees Celsius
lots of luck
10.0LBS of
dirt
100ergs of energy
the execution of the above example will be analogous to the following assignments:
quant = 2; strcpy(units, "quarts"); strcpy(item, "oil");
count = 3;
quant = -12.8; strcpy(units, "degrees");
count = 2; // "C" fails to match "o"
count = 0; // "l" fails to match "%f"
quant = 10.0; strcpy(units, "LBS"); strcpy(item, "dirt");
count = 3;
count = 0; // "100e" fails to match "%f"
count = EOF;
21 EXAMPLE 4 In:
#include <stdio.h>
/* ... */
int d1, d2, n1, n2, i;
i = sscanf("123", "%d%n%n%d", &d1, &n1, &n2, &d2);
the value 123 is assigned to d1 and the value 3 to n1. Because %n can never get an input failure, the value of 3 is also assigned
to n2. The value of d2 is not affected. The value 1 is assigned to i.
22 EXAMPLE 5 The call:
#include <stdio.h>
/* ... */
int n, i;
n = sscanf("foo %bar 42", "foo%%bar%d", &i);
will assign to n the value 1 and to i the value 42 because input white-space characters are skipped for both the % and d
conversion specifiers.
23 EXAMPLE 6 In these examples, multibyte characters do have a state-dependent encoding, and the members of the extended
character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a □
and the second by an uppercase letter, but are only recognized as such when in the alternate shift state. The shift sequences
are denoted by ↑ and ↓, in which the first causes entry into the alternate shift state.
24 After the call:
#include <stdio.h>
/* ... */
char str[50];
fscanf(stdin, "a%s", str);
with the input line:
a↑□X□Y↓ bc
str will contain ↑□X□Y↓\\0 assuming that none of the bytes of the shift sequences (or of the multibyte characters, in the
more general case) appears to be a single-byte white-space character.
25 In contrast, after the call:
#include <stdio.h>
#include <stddef.h>
/* ... */
wchar_t wstr[50];
fscanf(stdin, "a%ls", wstr);
with the same input line, wstr will contain the two wide characters that correspond to □X and □Y and a terminating null
wide character.
26 However, the call:
#include <stdio.h>
#include <stddef.h>
/* ... */
wchar_t wstr[50];
fscanf(stdin, "a↑□X↓%ls", wstr);
with the same input line will return zero due to a matching failure against the ↓ sequence in the format string.
27 Assuming that the first byte of the multibyte character □X is the same as the first byte of the multibyte character □Y, after the
call:
#include <stdio.h>
#include <stddef.h>
/* ... */
wchar_t wstr[50];
fscanf(stdin, "a↑□Y↓%ls", wstr);
with the same input line, zero will again be returned, but stdin will be left with a partially consumed multibyte character.
Forward references: the strtod, strtof, and strtold functions (7.24.1.5), the strtol, strtoll,
strtoul, and strtoull functions (7.24.1.7), conversion state (7.31.6), the wcrtomb function
(7.31.6.3.3).
7.23.6.3 The printf function
Synopsis
1 #include <stdio.h>
int printf(const char * restrict format, ...);
Description
2 The printf function is equivalent to fprintf with the argument stdout interposed before the
arguments to printf.
Returns
3 The printf function returns the number of characters transmitted, or a negative value if an output
or encoding error occurred.
7.23.6.4 The scanf function
Synopsis
1 #include <stdio.h>
int scanf(const char * restrict format, ...);
Description
2 The scanf function is equivalent to fscanf with the argument stdin interposed before the argu-
ments to scanf.
Returns
3 The scanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the scanf function returns the number of input items
assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.23.6.5 The snprintf function
Synopsis
1 #include <stdio.h>
int snprintf(char * restrict s, size_t n, const char * restrict format, ...);
Description
2 The snprintf function is equivalent to fprintf, except that the output is written into an array
(specified by argument s) rather than to a stream. If n is zero, nothing is written, and s may be a
null pointer. Otherwise, output characters beyond the n-1st are discarded rather than being written
to the array, and a null character is written at the end of the characters actually written into the array.
If copying takes place between objects that overlap, the behavior is undefined.
Returns
3 The snprintf function returns the number of characters that would have been written had n been
sufficiently large, not counting the terminating null character, or a negative value if an encoding
error occurred. Thus, the null-terminated output has been completely written if and only if the
returned value is both nonnegative and less than n.
7.23.6.6 The sprintf function
Synopsis
1 #include <stdio.h>
int sprintf(char * restrict s, const char * restrict format, ...);
Description
2 The sprintf function is equivalent to fprintf, except that the output is written into an array
(specified by the argument s) rather than to a stream. A null character is written at the end of the
characters written; it is not counted as part of the returned value. If copying takes place between
objects that overlap, the behavior is undefined.
Returns
3 The sprintf function returns the number of characters written in the array, not counting the
terminating null character, or a negative value if an encoding error occurred.
7.23.6.7 The sscanf function
Synopsis
1 #include <stdio.h>
int sscanf(const char * restrict s, const char * restrict format, ...);
Description
2 The sscanf function is equivalent to fscanf, except that input is obtained from a string (specified
by the argument s) rather than from a stream. Reaching the end of the string is equivalent to
encountering end-of-file for the fscanf function. If copying takes place between objects that overlap,
the behavior is undefined.
Returns
3 The sscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the sscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.23.6.8 The vfprintf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vfprintf(FILE * restrict stream, const char * restrict format, va_list arg);
Description
2 The vfprintf function is equivalent to fprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfprintf function does not invoke the va_end macro351) .
Returns
3 The vfprintf function returns the number of characters transmitted, or a negative value if an
output or encoding error occurred.
4 EXAMPLE The following shows the use of the vfprintf function in a general error-reporting routine.
#include <stdarg.h>
#include <stdio.h>
void error(char *function_name, char *format, ...)
{
va_list args;
va_start(args, format);
// print out name of function causing error
fprintf(stderr, "ERROR in %s: ", function_name);
// print out remainder of message
vfprintf(stderr, format, args);
va_end(args);
}
7.23.6.9 The vfscanf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vfscanf(FILE * restrict stream, const char * restrict format, va_list arg);
351) As the functions vfprintf , vfscanf , vprintf , vscanf , vsnprintf , vsprintf , and vsscanf invoke the va_arg macro,
arg after the return has an indeterminate representation.
Description
2 The vfscanf function is equivalent to fscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfscanf function does not invoke the va_end macro.351)
Returns
3 The vfscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vfscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.23.6.10 The vprintf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vprintf(const char * restrict format, va_list arg);
Description
2 The vprintf function is equivalent to printf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vprintf function does not invoke the va_end macro.351)
Returns
3 The vprintf function returns the number of characters transmitted, or a negative value if an output
or encoding error occurred.
7.23.6.11 The vscanf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vscanf(const char * restrict format, va_list arg);
Description
2 The vscanf function is equivalent to scanf, with the variable argument list replaced by arg, which
shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The
vscanf function does not invoke the va_end macro.351)
Returns
3 The vscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.23.6.12 The vsnprintf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vsnprintf(char * restrict s, size_t n, const char * restrict format, va_list
arg);
Description
2 The vsnprintf function is equivalent to snprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsnprintf function does not invoke the va_end macro.351) If copying takes place between
objects that overlap, the behavior is undefined.
Returns
3 The vsnprintf function returns the number of characters that would have been written had n been
sufficiently large, not counting the terminating null character, or a negative value if an encoding
error occurred. Thus, the null-terminated output has been completely written if and only if the
returned value is both nonnegative and less than n.
7.23.6.13 The vsprintf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vsprintf(char * restrict s, const char * restrict format, va_list arg);
Description
2 The vsprintf function is equivalent to sprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsprintf function does not invoke the va_end macro.351) If copying takes place between objects
that overlap, the behavior is undefined.
Returns
3 The vsprintf function returns the number of characters written in the array, not counting the
terminating null character, or a negative value if an encoding error occurred.
7.23.6.14 The vsscanf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vsscanf(const char * restrict s, const char * restrict format, va_list arg);
Description
2 The vsscanf function is equivalent to sscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsscanf function does not invoke the va_end macro.351)
Returns
3 The vsscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vsscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.23.7 Character input/output functions
7.23.7.1 The fgetc function
Synopsis
1 #include <stdio.h>
int fgetc(FILE *stream);
Description
2 If the end-of-file indicator for the input stream pointed to by stream is not set and a next character
is present, the fgetc function obtains that character as an unsigned char converted to an int and
advances the associated file position indicator for the stream (if defined).
Returns
3 If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file
indicator for the stream is set and the fgetc function returns EOF. Otherwise, the fgetc function
returns the next character from the input stream pointed to by stream. If a read error occurs, the
error indicator for the stream is set and the fgetc function returns EOF.352)
7.23.7.2 The fgets function
Synopsis
1 #include <stdio.h>
char *fgets(char * restrict s, int n, FILE * restrict stream);
Description
2 The fgets function reads at most one less than the number of characters specified by n from the
stream pointed to by stream into the array pointed to by s. No additional characters are read after a
new-line character (which is retained) or after end-of-file. A null character is written immediately
after the last character read into the array.
Returns
3 The fgets function returns s if successful. If end-of-file is encountered and no characters have been
read into the array, the contents of the array remain unchanged and a null pointer is returned. If a
read error occurs during the operation, the members of the array have unspecified values and a null
pointer is returned.
7.23.7.3 The fputc function
Synopsis
1 #include <stdio.h>
int fputc(int c, FILE *stream);
Description
2 The fputc function writes the character specified by c (converted to an unsigned char) to the
output stream pointed to by stream, at the position indicated by the associated file position indicator
for the stream (if defined), and advances the indicator appropriately. If the file cannot support
positioning requests, or if the stream was opened with append mode, the character is appended to
the output stream.
Returns
3 The fputc function returns the character written. If a write error occurs, the error indicator for the
stream is set and fputc returns EOF.
7.23.7.4 The fputs function
Synopsis
1 #include <stdio.h>
int fputs(const char * restrict s, FILE * restrict stream);
Description
2 The fputs function writes the string pointed to by s to the stream pointed to by stream. The
terminating null character is not written.
Returns
3 The fputs function returns EOF if a write error occurs; otherwise it returns a nonnegative value.
7.23.7.5 The getc function
352) An end-of-file and a read error can be distinguished by use of the feof and ferror functions.
Synopsis
1 #include <stdio.h>
int getc(FILE *stream);
Description
2 The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate
stream more than once, so the argument should never be an expression with side effects.
Returns
3 The getc function returns the next character from the input stream pointed to by stream. If the
stream is at end-of-file, the end-of-file indicator for the stream is set and getc returns EOF. If a read
error occurs, the error indicator for the stream is set and getc returns EOF.
7.23.7.6 The getchar function
Synopsis
1 #include <stdio.h>
int getchar(void);
Description
2 The getchar function is equivalent to getc with the argument stdin.
Returns
3 The getchar function returns the next character from the input stream pointed to by stdin. If the
stream is at end-of-file, the end-of-file indicator for the stream is set and getchar returns EOF. If a
read error occurs, the error indicator for the stream is set and getchar returns EOF.
7.23.7.7 The putc function
Synopsis
1 #include <stdio.h>
int putc(int c, FILE *stream);
Description
2 The putc function is equivalent to fputc, except that if it is implemented as a macro, it may evaluate
stream more than once, so that argument should never be an expression with side effects.
Returns
3 The putc function returns the character written. If a write error occurs, the error indicator for the
stream is set and putc returns EOF.
7.23.7.8 The putchar function
Synopsis
1 #include <stdio.h>
int putchar(int c);
Description
2 The putchar function is equivalent to putc with the second argument stdout.
Returns
3 The putchar function returns the character written. If a write error occurs, the error indicator for
the stream is set and putchar returns EOF.
7.23.7.9 The puts function
Synopsis
1 #include <stdio.h>
int puts(const char *s);
Description
2 The puts function writes the string pointed to by s to the stream pointed to by stdout, and appends
a new-line character to the output. The terminating null character is not written.
Returns
3 The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative value.
7.23.7.10 The ungetc function
Synopsis
1 #include <stdio.h>
int ungetc(int c, FILE *stream);
Description
2 The ungetc function pushes the character specified by c (converted to an unsigned char) back
onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent
reads on that stream in the reverse order of their pushing. A successful intervening call (with the
stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards
any pushed-back characters for the stream. The external storage corresponding to the stream is
unchanged.
3 One character of pushback is guaranteed. If the ungetc function is called too many times on the
same stream without an intervening read or file positioning operation on that stream, the operation
may fail.
4 If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged.
5 A successful call to the ungetc function clears the end-of-file indicator for the stream. The value
of the file position indicator for the stream after reading or discarding all pushed-back characters
shall be the same as it was before the characters were pushed back.353) For a text stream, the value
of its file position indicator after a successful call to the ungetc function is unspecified until all
pushed-back characters are read or discarded. For a binary stream, its file position indicator is
decremented by each successful call to the ungetc function; if its value was zero before a call, it has
an indeterminate representation after the call354) .
Returns
6 The ungetc function returns the character pushed back after conversion, or EOF if the operation
fails.
Forward references: file positioning functions (7.23.9).
7.23.8 Direct input/output functions
7.23.8.1 The fread function
Synopsis
1 #include <stdio.h>
size_t fread(void * restrict ptr, size_t size, size_t nmemb,
FILE * restrict stream);
353) Note that a file positioning function could further modify the file position indicator after discarding any pushed-back
characters.
354) See "future library directions" (7.33.15).
Description
2 The fread function reads, into the array pointed to by ptr, up to nmemb elements whose size is
specified by size, from the stream pointed to by stream. For each object, size calls are made to
the fgetc function and the results stored, in the order read, in an array of unsigned char exactly
overlaying the object. The file position indicator for the stream (if defined) is advanced by the
number of characters successfully read. If an error occurs, the resulting representation of the file
position indicator for the stream is indeterminate. If a partial element is read, its representation is
indeterminate.
Returns
3 The fread function returns the number of elements successfully read, which may be less than nmemb
if a read error or end-of-file is encountered. If size or nmemb is zero, fread returns zero and the
contents of the array and the state of the stream remain unchanged.
7.23.8.2 The fwrite function
Synopsis
1 #include <stdio.h>
size_t fwrite(const void * restrict ptr, size_t size, size_t nmemb,
FILE * restrict stream);
Description
2 The fwrite function writes, from the array pointed to by ptr, up to nmemb elements whose size is
specified by size, to the stream pointed to by stream. For each object, size calls are made to the
fputc function, taking the values (in order) from an array of unsigned char exactly overlaying the
object. The file position indicator for the stream (if defined) is advanced by the number of characters
successfully written. If an error occurs, the resulting representation of the file position indicator for
the stream is indeterminate.
Returns
3 The fwrite function returns the number of elements successfully written, which will be less than
nmemb only if a write error is encountered. If size or nmemb is zero, fwrite returns zero and the
state of the stream remains unchanged.
7.23.9 File positioning functions
7.23.9.1 The fgetpos function
Synopsis
1 #include <stdio.h>
int fgetpos(FILE * restrict stream, fpos_t * restrict pos);
Description
2 The fgetpos function stores the current values of the parse state (if any) and file position indicator
for the stream pointed to by stream in the object pointed to by pos. The values stored contain
unspecified information usable by the fsetpos function for repositioning the stream to its position
at the time of the call to the fgetpos function.
Returns
3 If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero
and stores an implementation-defined positive value in errno.
Forward references: the fsetpos function (7.23.9.3).
7.23.9.2 The fseek function
Synopsis
1 #include <stdio.h>
int fseek(FILE *stream, long int offset, int whence);
Description
2 The fseek function sets the file position indicator for the stream pointed to by stream. If a read or
write error occurs, the error indicator for the stream is set and fseek fails.
3 For a binary stream, the new position, measured in characters from the beginning of the file, is
obtained by adding offset to the position specified by whence. The specified position is the
beginning of the file if whence is SEEK_SET, the current value of the file position indicator if
SEEK_CUR, or end-of-file if SEEK_END. A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END.
4 For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier
successful call to the ftell function on a stream associated with the same file and whence shall be
SEEK_SET.
5 After determining the new position, a successful call to the fseek function undoes any effects of the
ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes
the new position. After a successful fseek call, the next operation on an update stream may be
either input or output.
Returns
6 The fseek function returns nonzero only for a request that cannot be satisfied.
Forward references: the ftell function (7.23.9.4).
7.23.9.3 The fsetpos function
Synopsis
1 #include <stdio.h>
int fsetpos(FILE *stream, const fpos_t *pos);
Description
2 The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream
pointed to by stream according to the value of the object pointed to by pos, which shall be a value
obtained from an earlier successful call to the fgetpos function on a stream associated with the
same file. If a read or write error occurs, the error indicator for the stream is set and fsetpos fails.
3 A successful call to the fsetpos function undoes any effects of the ungetc function on the stream,
clears the end-of-file indicator for the stream, and then establishes the new parse state and position.
After a successful fsetpos call, the next operation on an update stream may be either input or
output.
Returns
4 If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero
and stores an implementation-defined positive value in errno.
7.23.9.4 The ftell function
Synopsis
1 #include <stdio.h>
long int ftell(FILE *stream);
Description
2 The ftell function obtains the current value of the file position indicator for the stream pointed to
by stream. For a binary stream, the value is the number of characters from the beginning of the file.
For a text stream, its file position indicator contains unspecified information, usable by the fseek
function for returning the file position indicator for the stream to its position at the time of the ftell
call; the difference between two such return values is not necessarily a meaningful measure of the
number of characters written or read.
Returns
3 If successful, the ftell function returns the current value of the file position indicator for the stream.
On failure, the ftell function returns −1L and stores an implementation-defined positive value in
errno.
7.23.9.5 The rewind function
Synopsis
1 #include <stdio.h>
void rewind(FILE *stream);
Description
2 The rewind function sets the file position indicator for the stream pointed to by stream to the
beginning of the file. It is equivalent to
(void)fseek(stream, 0L, SEEK_SET)
except that the error indicator for the stream is also cleared.
Returns
3 The rewind function returns no value.
7.23.10 Error-handling functions
7.23.10.1 The clearerr function
Synopsis
1 #include <stdio.h>
void clearerr(FILE *stream);
Description
2 The clearerr function clears the end-of-file and error indicators for the stream pointed to by
stream.
Returns
3 The clearerr function returns no value.
7.23.10.2 The feof function
Synopsis
1 #include <stdio.h>
int feof(FILE *stream);
Description
2 The feof function tests the end-of-file indicator for the stream pointed to by stream.
Returns
3 The feof function returns nonzero if and only if the end-of-file indicator is set for stream.
7.23.10.3 The ferror function
Synopsis
1 #include <stdio.h>
int ferror(FILE *stream);
Description
2 The ferror function tests the error indicator for the stream pointed to by stream.
Returns
3 The ferror function returns nonzero if and only if the error indicator is set for stream.
7.23.10.4 The perror function
Synopsis
1 #include <stdio.h>
void perror(const char *s);
Description
2 The perror function maps the error number in the integer expression errno to an error message.
It writes a sequence of characters to the standard error stream thus: first (if s is not a null pointer
and the character pointed to by s is not the null character), the string pointed to by s followed by a
colon (:) and a space; then an appropriate error message string followed by a new-line character.
The contents of the error message strings are the same as those returned by the strerror function
with argument errno.
Returns
3 The perror function returns no value.
Forward references: the strerror function (7.26.6.3).
7.24 General utilities <stdlib.h>
1 The header <stdlib.h> declares five types and several functions of general utility, and defines
several macros.355)
2 The feature test macro __STDC_VERSION_STDLIB_H__ expands to the token 202311L.
3 The types declared are size_t and wchar_t (both described in 7.21), once_flag (described in 7.28),
div_t
which is a structure type that is the type of the value returned by the div function,
ldiv_t
which is a structure type that is the type of the value returned by the ldiv function, and
lldiv_t
which is a structure type that is the type of the value returned by the lldiv function.
4 The macros defined are NULL (described in 7.21); ONCE_FLAG_INIT (described in 7.28);
EXIT_FAILURE
and
EXIT_SUCCESS
which expand to integer constant expressions that can be used as the argument to the exit function
to return unsuccessful or successful termination status, respectively, to the host environment;
RAND_MAX
which expands to an integer constant expression that is the maximum value returned by the rand
function; and
MB_CUR_MAX
which expands to a positive integer expression with type size_t that is the maximum number of
bytes in a multibyte character for the extended character set specified by the current locale (category
LC_CTYPE), which is never greater than MB_LEN_MAX.
5 The function
#include <stdlib.h>
void call_once(once_flag *flag, void (*func)(void));
is described in 7.28.2.
7.24.1 Numeric conversion functions
1 The functions atof, atoi, atol, and atoll need not affect the value of the integer expression errno
on an error. If the value of the result cannot be represented, the behavior is undefined.
7.24.1.1 The atof function
Synopsis
1 #include <stdlib.h>
double atof(const char *nptr);
355) See "future library directions" (7.33.16).
Description
2 The atof function converts the initial portion of the string pointed to by nptr to double representa-
tion. Except for the behavior on error, it is equivalent to
strtod(nptr, nullptr)
Returns
3 The atof function returns the converted value.
Forward references: the strtod, strtof, and strtold functions (7.24.1.5).
7.24.1.2 The atoi, atol, and atoll functions
Synopsis
1 #include <stdlib.h>
int atoi(const char *nptr);
long int atol(const char *nptr);
long long int atoll(const char *nptr);
Description
2 The atoi, atol, and atoll functions convert the initial portion of the string pointed to by nptr to
int, long int, and long long int representation, respectively. Except for the behavior on error,
they are equivalent to
atoi: (int)strtol(nptr, nullptr, 10)
atol: strtol(nptr, nullptr, 10)
atoll: strtoll(nptr, nullptr, 10)
Returns
3 The atoi, atol, and atoll functions return the converted value.
Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.24.1.7).
7.24.1.3 The strfromd, strfromf, and strfroml functions
Synopsis
1 #include <stdlib.h>
int strfromd(char *restrict s, size_t n, const char *restrict format, double fp);
int strfromf(char *restrict s, size_t n, const char *restrict format, float fp);
int strfroml(char *restrict s, size_t n, const char *restrict format, long double fp);
Description
2 The strfromd, strfromf, and strfroml functions are equivalent to snprintf(s, n, format, fp)
(7.23.6.5), except that the format string shall only contain the character %, an optional precision that
does not contain an asterisk *, and one of the conversion specifiers a, A, e, E, f, F, g, or G, which
applies to the type (double, float, or long double) indicated by the function suffix (rather than by
a length modifier).
Returns
3 The strfromd, strfromf, and strfroml functions return the number of characters that would
have been written had n been sufficiently large, not counting the terminating null character. Thus,
the null-terminated output has been completely written if and only if the returned value is both
nonnegative and less than n.
7.24.1.4 The strfromdN functions
Synopsis
1 #include <stdlib.h>
#ifdef __STDC_IEC_60559_DFP__
int strfromd32(char*restrict s, size_t n, const char*restrict format, _Decimal32 fp);
int strfromd64(char*restrict s, size_t n, const char*restrict format, _Decimal64 fp);
int strfromd128(char*restrict s, size_t n, const char*restrict format, _Decimal128 fp);
#endif
Description
2 The strfromdN functions are equivalent to snprintf(s, n, format, fp) (7.23.6.5), except the
format string contains only the character %, an optional precision that does not contain an asterisk *,
and one of the conversion specifiers a, A, e, E, f, F, g, or G, which applies to the type (_Decimal32 ,
_Decimal64 , or _Decimal128 ) indicated by the function suffix (rather than by a length modifier).
Use of these functions with any other format string results in undefined behavior.
Returns
3 The strfromdN functions return the number of characters that would have been written had n been
sufficiently large, not counting the terminating null character. Thus, the null-terminated output has
been completely written if and only if the returned value is less than n.
7.24.1.5 The strtod, strtof, and strtold functions
Synopsis
1 #include <stdlib.h>
double strtod(const char *restrict nptr, char **restrict endptr);
float strtof(const char *restrict nptr, char **restrict endptr);
long double strtold(const char *restrict nptr, char **restrict endptr);
Description
2 The strtod, strtof, and strtold functions convert the initial portion of the string pointed to by
nptr to double, float, and long double representation, respectively. First, they decompose the
input string into three parts: an initial, possibly empty, sequence of white-space characters, a subject
sequence resembling a floating constant or representing an infinity or NaN; and a final string of one
or more unrecognized characters, including the terminating null character of the input string. Then,
they attempt to convert the subject sequence to a floating-point number, and return the result.
3 The expected form of the subject sequence is an optional plus or minus sign, then one of the
following:
— a nonempty sequence of decimal digits optionally containing a decimal-point character, then
an optional exponent part as defined in 6.4.4.2, excluding any digit separators (6.4.4.1);
— a 0x or 0X, then a nonempty sequence of hexadecimal digits optionally containing a decimal-
point character, then an optional binary exponent part as defined in 6.4.4.2, excluding any digit
separators;
— INF or INFINITY, ignoring case
— NAN or NAN(n-char-sequenceopt ), ignoring case in the NAN part, where: n-char-sequence:
digit
nondigit
n-char-sequence digit
n-char-sequence nondigit
The subject sequence is defined as the longest initial subsequence of the input string, starting with
the first non-white-space character, that is of the expected form. The subject sequence contains no
characters if the input string is not of the expected form.
4 If the subject sequence has the expected form for a floating-point number, the sequence of characters
starting with the first digit or the decimal-point character (whichever occurs first) is interpreted as a
floating constant according to the rules of 6.4.4.2, except that the decimal-point character is used
in place of a period, and that if neither an exponent part nor a decimal-point character appears in
a decimal floating-point number, or if a binary exponent part does not appear in a hexadecimal
floating-point number, an exponent part of the appropriate type with value zero is assumed to
follow the last digit in the string. If the subject sequence begins with a minus sign, the sequence is
interpreted as negated.356)
A character sequence INF or INFINITY is interpreted as an infinity, if representable in the return type,
else like a floating constant that is too large for the range of the return type. A character sequence
NAN or NAN(n-char-sequenceopt ) is interpreted as a quiet NaN, if supported in the return type, else like
a subject sequence part that does not have the expected form; the meaning of the n-char sequence
is implementation-defined.357) A pointer to the final string is stored in the object pointed to by
endptr, provided that endptr is not a null pointer.
5 If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, the value resulting
from the conversion is correctly rounded.
6 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.
7 If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
Recommended practice
8 If the subject sequence has the hexadecimal form, FLT_RADIX is not a power of 2, and the result is
not exactly representable, the result should be one of the two numbers in the appropriate internal
format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the
error should have a correct sign for the current rounding direction.
9 If the subject sequence has the decimal form and at most M significant digits, where M is the
maximum value of the T_DECIMAL_DIG macros (defined in <float.h>), the result should be correctly
rounded. If the subject sequence D has the decimal form and more than M significant digits, consider
the two bounding, adjacent decimal strings L and U, both having M significant digits, such that the
values of L, D, and U satisfy L ≤ D ≤ U. The result should be one of the (equal or adjacent) values
that would be obtained by correctly rounding L and U according to the current rounding direction,
with the extra stipulation that the error with respect to D should have a correct sign for the current
rounding direction.358)
Returns
10 The functions return the converted value, if any. If no conversion could be performed, zero is
returned.
If the correct value overflows and default rounding is in effect (7.12.1), plus or minus HUGE_VAL,
HUGE_VALF, or HUGE_VALL is returned (according to the return type and sign of the value); if the
integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno
acquires the value of ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is
nonzero, the "overflow" floating-point exception is raised.
If the result underflows (7.12.1), the functions return a value whose magnitude is no greater
than the smallest normalized positive number in the return type; if the integer expression
math_errhandling & MATH_ERRNO is nonzero, whether errno acquires the value ERANGE is
implementation-defined; if the integer expression math_errhandling & MATH_ERREXCEPT is
nonzero, whether the "underflow" floating-point exception is raised is implementation-defined.
356) It is unspecified whether a minus-signed sequence is converted to a negative number directly or by negating the value
resulting from converting the corresponding unsigned sequence (see F.5); the two methods could yield different results if
rounding is toward positive or negative infinity. In either case, the functions honor the sign of zero if floating-point arithmetic
supports signed zeros.
357) An implementation can use the n-char sequence to determine extra information to be represented in the NaN’s significand.
358) M is sufficiently large that L and U will usually correctly round to the same internal floating value, but if not will correctly
round to adjacent values.
7.24.1.6 The strtodN functions
Synopsis
1 #include <stdlib.h>
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 strtod32(const char * restrict nptr, char ** restrict endptr);
_Decimal64 strtod64(const char * restrict nptr,char ** restrict endptr);
_Decimal128 strtod128(const char * restrict nptr,char ** restrict endptr);
#endif
Description
2 The strtodN functions convert the initial portion of the string pointed to by nptr to decimal floating
type representation. First, they decompose the input string into three parts: an initial, possibly
empty, sequence of white-space characters; a subject sequence resembling a floating constant or
representing an infinity or NaN; and a final string of one or more unrecognized characters, including
the terminating null character of the input string. Then, they attempt to convert the subject sequence
to a floating-point number, and return the result.
3 The expected form of the subject sequence is an optional plus or minus sign, then one of the
following:
— a nonempty sequence of decimal digits optionally containing a decimal-point character, then
an optional exponent part as defined in 6.4.4.2, excluding any digit separators (6.4.4.1)
— INF or INFINITY, ignoring case
— NAN or NAN(d-char-sequenceopt ), ignoring case in the NAN part, where: d-char-sequence:
digit
nondigit
d-char-sequence digit
d-char-sequence nondigit
The subject sequence is defined as the longest initial subsequence of the input string, starting with
the first non-white-space character, that is of the expected form. The subject sequence contains no
characters if the input string is not of the expected form.
4 If the subject sequence has the expected form for a floating-point number, the sequence of characters
starting with the first digit or the decimal-point character (whichever occurs first) is interpreted as a
floating constant according to the rules of 6.4.4.2, including correct rounding and determination of
the coefficient c and the quantum exponent q, with the following exceptions:
— It is not a hexadecimal floating number.
— The decimal-point character is used in place of a period.
— If neither an exponent part nor a decimal-point character appears in a decimal floating-point
number, an exponent part of the appropriate type with value zero is assumed to follow the
last digit in the string.
If the subject sequence begins with a minus sign, the sequence is interpreted as negated (before
rounding) and the sign s is set to −1, else s is set to 1. A character sequence INF or INFINITY is
interpreted as an infinity. A character sequence NAN or NAN(d-char-sequenceopt ), is interpreted as a
quiet NaN; the meaning of the d-char sequence is implementation-defined.359) A pointer to the final
string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
359) An implementation may use the d-char sequence to determine extra information to be represented in the NaN’s
significand.
5 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.
6 If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
Returns
7 The strtodN functions return the correctly rounded converted value, if any. If no conversion could
be performed, the value of the triple (+1, 0, 0) is returned. If the correct value overflows:
— the value of the macro ERANGE is stored in errno if the integer expression
math_errhandling & MATH_ERRNO is nonzero;
— the "overflow" floating-point exception is raised if the integer expression
math_errhandling & MATH_ERREXCEPT is nonzero.
If the result underflows (7.12.1), whether errno acquires the value ERANGE if the integer expression
math_errhandling & MATH_ERRNO is nonzero is implementation-defined; if the integer expres-
sion math_errhandling & MATH_ERREXCEPT is nonzero, whether the "underflow" floating-point
exception is raised is implementation-defined.
8 EXAMPLE Following are subject sequences of the decimal form and the resulting triples (s, c, q) produced by strtod64.
Note that for _Decimal64 , the precision (maximum coefficient length) is 16 and the quantum exponent range is −398 ≤ q ≤ 369.
"0" (+1, 0, 0)
"0.00" (+1, 0, −2)
"123" (+1, 123, 0)
"-123" (−1, 123, 0)
"1.23E3" (+1, 123, 1)
"1.23E+3" (+1, 123, 1)
"12.3E+7" (+1, 123, 6)
"12.0" (+1, 120, −1)
"12.3" (+1, 123, −1)
"0.00123" (+1, 123, −5)
"-1.23E-12" (−1, 123, −14)
"1234.5E-4" (+1, 12345, −5)
"-0" (−1, 0, 0)
"-0.00" (−1, 0, −2)
"0E+7" (+1, 0, 7)
"-0E-7" (−1, 0, −7)
"12345678901234567890" (+1, 1234567890123457, 4) or (+1, 1234567890123456, 4) depending on rounding
mode
"1234E-400" (+1, 12, −398) or (+1, 13, −398) depending on rounding mode
"1234E-402" (+1, 0, −398) or (+1, 1, −398) depending on rounding mode
"1000." (+1, 1000, 0)
".0001" (+1, 1, −4)
"1000.e0" (+1, 1000, 0)
".0001e0" (+1, 1, −4)
"1000.0" (+1, 10000, −1)
"0.0001" (+1, 1, −4)
"1000.00" (+1, 100000, −2)
"00.0001" (+1, 1, −4)
"001000." (+1, 1000, 0)
"001000.0" (+1, 10000, −1)
"001000.00" (+1, 100000, −2)
"00.00" (+1, 0, −2)
"00." (+1, 0, 0)
".00" (+1, 0, −2)
"00.00e-5" (+1, 0, −7)
"00.e-5" (+1, 0, −5)
".00e-5" (+1, 0, −7)
"0x1.8p+4" (+1, 0, 0), and a pointer to "x1.8p+4" is stored in the object pointed to by endptr,
provided endptr is not a null pointer
"infinite" infinity, and a pointer to "inite" is stored in the object pointed to by endptr, provided
endptr is not a null pointer
7.24.1.7 The strtol, strtoll, strtoul, and strtoull functions
Synopsis
1 #include <stdlib.h>
long int strtol(const char *restrict nptr, char **restrict endptr, int base);
long long int strtoll(const char *restrict nptr, char **restrict endptr, int base);
unsigned long int strtoul(const char *restrict nptr, char **restrict endptr, int base);
unsigned long long int strtoull(const char *restrict nptr, char **restrict endptr, int
base);
Description
2 The strtol, strtoll, strtoul, and strtoull functions convert the initial portion of
the string pointed to by nptr to long int, long long int, unsigned long int, and
unsigned long long int representation, respectively. First, they decompose the input
string into three parts: an initial, possibly empty, sequence of white-space characters, a subject
sequence resembling an integer represented in some radix determined by the value of base, and a
final string of one or more unrecognized characters, including the terminating null character of the
input string. Then, they attempt to convert the subject sequence to an integer, and return the result.
3 If the value of base is zero, the expected form of the subject sequence is that of an integer constant as
described in 6.4.4.1, optionally preceded by a plus or minus sign, but not including an integer suffix
or any optional digit separators. If the value of base is between 2 and 36 (inclusive), the expected
form of the subject sequence is a sequence of letters and digits representing an integer with the radix
specified by base, optionally preceded by a plus or minus sign, but not including an integer suffix
or any optional digit separators. The letters from a (or A) through z (or Z) are ascribed the values 10
through 35; only letters and digits whose ascribed values are less than that of base are permitted. If
the value of base is 2, the characters 0b or 0B may optionally precede the sequence of letters and
digits, following the sign if present. If the value of base is 16, the characters 0x or 0X may optionally
precede the sequence of letters and digits, following the sign if present.
4 The subject sequence is defined as the longest initial subsequence of the input string, starting with
the first non-white-space character, that is of the expected form. The subject sequence contains no
characters if the input string is empty or consists entirely of white-space characters, or if the first
non-white-space character is other than a sign or a permissible letter or digit.
5 If the subject sequence has the expected form and the value of base is zero, the sequence of characters
starting with the first digit is interpreted as an integer constant according to the rules of 6.4.4.1. If
the subject sequence has the expected form and the value of base is between 2 and 36, it is used as
the base for conversion, ascribing to each letter its value as given above. If the subject sequence
begins with a minus sign, the value resulting from the conversion is negated (in the return type). A
pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a
null pointer.
6 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.
7 If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
Returns
8 The strtol, strtoll, strtoul, and strtoull functions return the converted value, if any. If
no conversion could be performed, zero is returned. If the correct value is outside the range of
representable values, LONG_MIN, LONG_MAX, LLONG_MIN, LLONG_MAX, ULONG_MAX, or ULLONG_MAX is
returned (according to the return type and sign of the value, if any), and the value of the macro
ERANGE is stored in errno.
7.24.2 Pseudo-random sequence generation functions
7.24.2.1 The rand function
Synopsis
1 #include <stdlib.h>
int rand(void);
Description
2 The rand function computes a sequence of pseudo-random integers in the range 0 to RAND_MAX
inclusive.
3 The rand function is not required to avoid data races with other calls to pseudo-random sequence
generation functions. The implementation shall behave as if no library function calls the rand
function.
Recommended practice
4 There are no guarantees as to the quality of the random sequence produced and some implementa-
tions are known to produce sequences with distressingly non-random low-order bits. Applications
with particular requirements should use a generator that is known to be sufficient for their needs.
Returns
5 The rand function returns a pseudo-random integer.
Environmental limits
6 The value of the RAND_MAX macro shall be at least 32767.
7.24.2.2 The srand function
Synopsis
1 #include <stdlib.h>
void srand(unsigned int seed);
Description
2 The srand function uses the argument as a seed for a new sequence of pseudo-random numbers
to be returned by subsequent calls to rand. If srand is then called with the same seed value, the
sequence of pseudo-random numbers shall be repeated. If rand is called before any calls to srand
have been made, the same sequence shall be generated as when srand is first called with a seed
value of 1.
3 The srand function is not required to avoid data races with other calls to pseudo-random sequence
generation functions. The implementation shall behave as if no library function calls the srand
function.
Returns
4 The srand function returns no value.
5 EXAMPLE The following functions define a portable implementation of rand and srand.
static unsigned long int next = 1;
int rand(void) // RAND_MAX assumed to be 32767
{
next = next * 1103515245 + 12345;
return (unsigned int)(next/65536) % 32768;
}
void srand(unsigned int seed)
{
next = seed;
}
7.24.3 Memory management functions
1 The order and contiguity of storage allocated by successive calls to the aligned_alloc, calloc,
malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is
suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental
alignment requirement and size less than or equal to the size requested. It may then be used to
access such an object or an array of such objects in the space allocated (until the space is explicitly
deallocated). The lifetime of an allocated object extends from the allocation until the deallocation.
Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer
returned points to the start (lowest byte address) of the allocated space. If the space cannot be
allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is
implementation-defined: either a null pointer is returned to indicate an error, or the behavior is as if
the size were some nonzero value, except that the returned pointer shall not be used to access an
object.
2 For purposes of determining the existence of a data race, memory allocation functions behave as
though they accessed only memory locations accessible through their arguments and not other
static duration storage. These functions may, however, visibly modify the storage that they allocate
or deallocate. Calls to these functions that allocate or deallocate a particular region of memory
shall occur in a single total order, and each such deallocation call shall synchronize with the next
allocation (if any) in this order.
7.24.3.1 The aligned_alloc function
Synopsis
1 #include <stdlib.h>
void *aligned_alloc(size_t alignment, size_t size);
Description
2 The aligned_alloc function allocates space for an object whose alignment is specified by
alignment, whose size is specified by size, and whose representation is indeterminate. If the
value of alignment is not a valid alignment supported by the implementation the function shall fail
by returning a null pointer.
Returns
3 The aligned_alloc function returns either a null pointer or a pointer to the allocated space.
7.24.3.2 The calloc function
Synopsis
1 #include <stdlib.h>
void *calloc(size_t nmemb, size_t size);
Description
2 The calloc function allocates space for an array of nmemb objects, each of whose size is size. The
space is initialized to all bits zero360) .
Returns
3 The calloc function returns either a pointer to the allocated space or a null pointer if the space
cannot be allocated or if the mathematical product nmemb * size is not representable as a value of
type size_t.
7.24.3.3 The free function
Synopsis
1 #include <stdlib.h>
void free(void *ptr);
Description
2 The free function causes the space pointed to by ptr to be deallocated, that is, made available
for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does
360) Note that this need not be the same as the representation of floating-point zero or a null pointer constant.
not match a pointer earlier returned by a memory management function, or if the space has been
deallocated by a call to free or realloc, the behavior is undefined.
Returns
3 The free function returns no value.
7.24.3.4 The free_sized function
Synopsis
1 #include <stdlib.h>
void free_sized(void *ptr, size_t size);
Description
2 If ptr is a null pointer or the result obtained from a call to malloc, realloc, or calloc, where size
size is equal to the requested allocation size, this function is equivalent to free(ptr). Otherwise,
the behavior is undefined.
3 NOTE 1 A conforming implementation may ignore size and call free.
4 NOTE 2 The result of an aligned_alloc call may not be passed to free_sized.
Recommended practice
5 Implementations may provide extensions to query the usable size of an allocation, or to determine
the usable size of the allocation that would result if a request for some other size were to succeed.
Such implementations should allow passing the resulting usable size as the size parameter, and
provide functionality equivalent to free in such cases.
Returns
6 The free_sized function returns no value.
7.24.3.5 The free_aligned_sized function
Synopsis
1 #include <stdlib.h>
void free_aligned_sized(void *ptr, size_t alignment, size_t size);
Description
2 If ptr is a null pointer or the result obtained from a call to aligned_alloc, where alignment is
equal to the requested allocation alignment and size is equal to the requested allocation size, this
function is equivalent to free(ptr). Otherwise, the behavior is undefined.
3 NOTE 1 A conforming implementation may ignore alignment and size and call free.
4 NOTE 2 The result of an malloc, calloc, or realloc call may not be passed to free_aligned_sized.
Recommended practice
5 Implementations may provide extensions to query the usable size of an allocation, or to determine
the usable size of the allocation that would result if a request for some other size were to succeed.
Such implementations should allow passing the resulting usable size as the size parameter, and
provide functionality equivalent to free in such cases.
Returns
6 The free_aligned_sized function returns no value.
7.24.3.6 The malloc function
Synopsis
1 #include <stdlib.h>
void *malloc(size_t size);
Description
2 The malloc function allocates space for an object whose size is specified by size and whose
representation is indeterminate.
Returns
3 The malloc function returns either a null pointer or a pointer to the allocated space.
7.24.3.7 The realloc function
Synopsis
1 #include <stdlib.h>
void *realloc(void *ptr, size_t size);
Description
2 The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new
object that has the size specified by size. The contents of the new object shall be the same as that of
the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new
object beyond the size of the old object have unspecified values.
3 If ptr is a null pointer, the realloc function behaves like the malloc function for the specified size.
Otherwise, if ptr does not match a pointer earlier returned by a memory management function, or
if the space has been deallocated by a call to the free or realloc function, or if the size is zero, the
behavior is undefined. If memory for the new object is not allocated, the old object is not deallocated
and its value is unchanged.
Returns
4 The realloc function returns a pointer to the new object (which may have the same value as a
pointer to the old object), or a null pointer if the new object has not been allocated.
7.24.4 Communication with the environment
7.24.4.1 The abort function
Synopsis
1 #include <stdlib.h>
[[noreturn]] void abort(void);
Description
2 The abort function causes abnormal program termination to occur, unless the signal SIGABRT is
being caught and the signal handler does not return. Whether open streams with unwritten buffered
data are flushed, open streams are closed, or temporary files are removed is implementation-
defined. An implementation-defined form of the status unsuccessful termination is returned to the
host environment by means of the function call raise(SIGABRT).
Returns
3 The abort function does not return to its caller.
7.24.4.2 The atexit function
Synopsis
1 #include <stdlib.h>
int atexit(void (*func)(void));
Description
2 The atexit function registers the function pointed to by func, to be called without arguments at
normal program termination.361) It is unspecified whether a call to the atexit function that does
361) The atexit function registrations are distinct from the at_quick_exit registrations, so applications might need to call
both registration functions with the same argument.
not happen before the exit function is called will succeed.
Environmental limits
3 The implementation shall support the registration of at least 32 functions.
Returns
4 The atexit function returns zero if the registration succeeds, nonzero if it fails.
Forward references: the at_quick_exit function (7.24.4.3), the exit function (7.24.4.4).
7.24.4.3 The at_quick_exit function
Synopsis
1 #include <stdlib.h>
int at_quick_exit(void (*func)(void));
Description
2 The at_quick_exit function registers the function pointed to by func, to be called without argu-
ments should quick_exit be called.362) It is unspecified whether a call to the at_quick_exit
function that does not happen before the quick_exit function is called will succeed.
Environmental limits
3 The implementation shall support the registration of at least 32 functions.
Returns
4 The at_quick_exit function returns zero if the registration succeeds, nonzero if it fails.
Forward references: the quick_exit function (7.24.4.7).
7.24.4.4 The exit function
Synopsis
1 #include <stdlib.h>
[[noreturn]] void exit(int status);
Description
2 The exit function causes normal program termination to occur. No functions registered by the
at_quick_exit function are called. If a program calls the exit function more than once, or calls the
quick_exit function in addition to the exit function, the behavior is undefined.
3 First, all functions registered by the atexit function are called, in the reverse order of their registra-
tion,363) except that a function is called after any previously registered functions that had already
been called at the time it was registered. If, during the call to any such function, a call to the longjmp
function is made that would terminate the call to the registered function, the behavior is undefined.
4 Next, all open streams with unwritten buffered data are flushed, all open streams are closed, and all
files created by the tmpfile function are removed.
5 Finally, control is returned to the host environment. If the value of status is zero or EXIT_SUCCESS,
an implementation-defined form of the status successful termination is returned. If the value of
status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is
returned. Otherwise the status returned is implementation-defined.
Returns
6 The exit function cannot return to its caller.
362) The at_quick_exit function registrations are distinct from the atexit registrations, so applications might need to call
both registration functions with the same argument.
363) Each function is called as many times as it was registered, and in the correct order with respect to other registered
functions.
7.24.4.5 The _Exit function
Synopsis
1 #include <stdlib.h>
[[noreturn]] void _Exit(int status);
Description
2 The _Exit function causes normal program termination to occur and control to be returned to the
host environment. No functions registered by the atexit function, the at_quick_exit function,
or signal handlers registered by the signal function are called. The status returned to the host
environment is determined in the same way as for the exit function (7.24.4.4). Whether open
streams with unwritten buffered data are flushed, open streams are closed, or temporary files are
removed is implementation-defined.
Returns
3 The _Exit function cannot return to its caller.
7.24.4.6 The getenv function
Synopsis
1 #include <stdlib.h>
char *getenv(const char *name);
Description
2 The getenv function searches an environment list, provided by the host environment, for a string that
matches the string pointed to by name. The set of environment names and the method for altering
the environment list are implementation-defined. The getenv function need not avoid data races
with other threads of execution that modify the environment list.364)
3 The implementation shall behave as if no library function calls the getenv function.
Returns
4 The getenv function returns a pointer to a string associated with the matched list member. The
string pointed to shall not be modified by the program, but may be overwritten by a subsequent call
to the getenv function. If the specified name cannot be found, a null pointer is returned.
7.24.4.7 The quick_exit function
Synopsis
1 #include <stdlib.h>
[[noreturn]] void quick_exit(int status);
Description
2 The quick_exit function causes normal program termination to occur. No functions registered by
the atexit function or signal handlers registered by the signal function are called. If a program calls
the quick_exit function more than once, or calls the exit function in addition to the quick_exit
function, the behavior is undefined. If a signal is raised while the quick_exit function is executing,
the behavior is undefined.
3 The quick_exit function first calls all functions registered by the at_quick_exit function, in the
reverse order of their registration,365) except that a function is called after any previously registered
functions that had already been called at the time it was registered. If, during the call to any such
function, a call to the longjmp function is made that would terminate the call to the registered
function, the behavior is undefined.
4 Then control is returned to the host environment by means of the function call _Exit(status) .
364) Many implementations provide non-standard functions that modify the environment list.
365) Each function is called as many times as it was registered, and in the correct order with respect to other registered
functions.
Returns
5 The quick_exit function cannot return to its caller.
7.24.4.8 The system function
Synopsis
1 #include <stdlib.h>
int system(const char *string);
Description
2 If string is a null pointer, the system function determines whether the host environment has a
command processor. If string is not a null pointer, the system function passes the string pointed to
by string to that command processor to be executed in a manner which the implementation shall
document; this might then cause the program calling system to behave in a non-conforming manner
or to terminate.
Returns
3 If the argument is a null pointer, the system function returns nonzero only if a command processor
is available. If the argument is not a null pointer, and the system function does return, it returns an
implementation-defined value.
7.24.5 Searching and sorting utilities
1 These utilities make use of a comparison function to search or sort arrays of unspecified type. Where
an argument declared as size_t nmemb specifies the length of the array for a function, nmemb can
have the value zero on a call to that function; the comparison function is not called, a search finds no
matching element, and sorting performs no rearrangement. Pointer arguments on such a call shall
still have valid values, as described in 7.1.4.
2 The implementation shall ensure that the second argument of the comparison function (when called
from bsearch), or both arguments (when called from qsort), are pointers to elements of the array.366)
The first argument when called from bsearch shall equal key.
3 The comparison function shall not alter the contents of the array. The implementation may reorder
elements of the array between calls to the comparison function, but shall not alter the contents of
any individual element.
4 When the same objects (consisting of size bytes, irrespective of their current positions in the array)
are passed more than once to the comparison function, the results shall be consistent with one
another. That is, for qsort they shall define a total ordering on the array, and for bsearch the same
object shall always compare the same way with the key.
5 A sequence point occurs immediately before and immediately after each call to the comparison
function, and also between any call to the comparison function and any movement of the objects
passed as arguments to that call.
7.24.5.1 The bsearch generic function
Synopsis
1 #include <stdlib.h>
void *bsearch(const void *key, const void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
366) That is, if the value passed is p, then the following expressions are always nonzero:
((char *)p - (char *)base) % size == 0
(char *)p >= (char *)base
(char *)p < (char *)base + nmemb * size
Description
2 The bsearch generic function searches an array of nmemb objects, the initial element of which is
pointed to by base, for an element that matches the object pointed to by key. The size of each
element of the array is specified by size.
3 The comparison function pointed to by compar is called with two arguments that point to the key
object and to an array element, in that order. The function shall return an integer less than, equal to,
or greater than zero if the key object is considered, respectively, to be less than, to match, or to be
greater than the array element. The array shall consist of: all the elements that compare less than, all
the elements that compare equal to, and all the elements that compare greater than the key object, in
that order.367)
Returns
4 The bsearch generic function returns a pointer to a matching element of the array, or a null pointer
if no match is found. If two elements compare as equal, which element is matched is unspecified.
5 The bsearch function is generic in the qualification of the type pointed to by the argument to base.
If this argument is a pointer to a const-qualified object type, the returned pointer will be a pointer
to const-qualified void. Otherwise, the argument shall be a pointer to an unqualified object type or
a null pointer constant368) , and the returned pointer will be a pointer to unqualified void.
The external declaration of bsearch has the concrete type:
void * (const void *, const void *, size_t, size_t, int (*) (const void *, const
void *))
, which supports all correct uses. If a macro definition of this generic function is suppressed in order
to access an actual function, the external declaration with this concrete type is visible369) .
367) In practice, the entire array is sorted according to the comparison function.
368) If the argument is a null pointer and the call is executed, the behavior is undefined.
369) This is an obsolescent feature.
7.24.5.2 The qsort function
Synopsis
1 #include <stdlib.h>
void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
Description
2 The qsort function sorts an array of nmemb objects, the initial element of which is pointed to by
base. The size of each object is specified by size.
3 The contents of the array are sorted into ascending order according to a comparison function pointed
to by compar, which is called with two arguments that point to the objects being compared. The
function shall return an integer less than, equal to, or greater than zero if the first argument is
considered to be respectively less than, equal to, or greater than the second.
4 If two elements compare as equal, their order in the resulting sorted array is unspecified.
Returns
5 The qsort function returns no value.
7.24.6 Integer arithmetic functions
7.24.6.1 The abs, labs, and llabs functions
Synopsis
1 #include <stdlib.h>
int abs(int j);
long int labs(long int j);
long long int llabs(long long int j);
Description
2 The abs, labs, and llabs functions compute the absolute value of an integer j. If the result cannot
be represented, the behavior is undefined370) .
Returns
3 The abs, labs, and llabs, functions return the absolute value.
7.24.6.2 The div, ldiv, and lldiv functions
Synopsis
1 #include <stdlib.h>
div_t div(int numer, int denom);
ldiv_t ldiv(long int numer, long int denom);
lldiv_t lldiv(long long int numer, long long int denom);
Description
2 The div, ldiv, and lldiv, functions compute numer/denom and numer%denom in a single operation.
Returns
3 The div, ldiv, and lldiv functions return a structure of type div_t, ldiv_t, and lldiv_t, respec-
tively, comprising both the quotient and the remainder. The structures shall contain (in either order)
the members quot (the quotient) and rem (the remainder), each of which has the same type as
the arguments numer and denom. If either part of the result cannot be represented, the behavior is
undefined.
370) The absolute value of the most negative number may not be representable.
7.24.7 Multibyte/wide character conversion functions
1 The behavior of the multibyte character functions is affected by the LC_CTYPE category of the current
locale. For a state-dependent encoding, each of the mbtowc and wctomb functions is placed into its
initial conversion state prior to the first call to the function and can be returned to that state by a
call for which its character pointer argument, s, is a null pointer. Subsequent calls with s as other
than a null pointer cause the internal conversion state of the function to be altered as necessary. It is
implementation-defined whether internal conversion state has thread storage duration, and whether
a newly created thread has the same state as the current thread at the time of creation, or the initial
conversion state. A call with s as a null pointer causes these functions to return a nonzero value if
encodings have state dependency, and zero otherwise.
Changing the LC_CTYPE category causes the internal object describing the conversion state of the
mbtowc and wctomb functions to have an indeterminate representation.
7.24.7.1 The mblen function
Synopsis
1 #include <stdlib.h>
int mblen(const char *s, size_t n);
Description
2 If s is not a null pointer, the mblen function determines the number of bytes contained in the
multibyte character pointed to by s. Except that the conversion state of the mbtowc function is not
affected, it is equivalent to
mbtowc((wchar_t *)0, (const char *)0, 0);
mbtowc((wchar_t *)0, s, n);
Returns
3 If s is a null pointer, the mblen function returns a nonzero or zero value, if multibyte character
encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the
mblen function either returns 0 (if s points to the null character), or returns the number of bytes
that are contained in the multibyte character (if the next n or fewer bytes form a valid multibyte
character), or returns-1 (if they do not form a valid multibyte character).
Forward references: the mbtowc function (7.24.7.2).
7.24.7.2 The mbtowc function
Synopsis
1 #include <stdlib.h>
int mbtowc(wchar_t * restrict pwc, const char * restrict s, size_t n);
Description
2 If s is not a null pointer, the mbtowc function inspects at most n bytes beginning with the byte
pointed to by s to determine the number of bytes needed to complete the next multibyte character
(including any shift sequences). If the function determines that the next multibyte character is
complete and valid, it determines the value of the corresponding wide character and then, if pwc
is not a null pointer, stores that value in the object pointed to by pwc. If the corresponding wide
character is the null wide character, the function is left in the initial conversion state.
3 The implementation shall behave as if no library function calls the mbtowc function.
Returns
4 If s is a null pointer, the mbtowc function returns a nonzero or zero value, if multibyte character
encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the
mbtowc function either returns 0 (if s points to the null character), or returns the number of bytes
that are contained in the converted multibyte character (if the next n or fewer bytes form a valid
multibyte character), or returns-1 (if they do not form a valid multibyte character).
5 In no case will the value returned be greater than n or the value of the MB_CUR_MAX macro.
7.24.7.3 The wctomb function
Synopsis
1 #include <stdlib.h>
int wctomb(char *s, wchar_t wc);
Description
2 The wctomb function determines the number of bytes needed to represent the multibyte character
corresponding to the wide character given by wc (including any shift sequences), and stores the
multibyte character representation in the array whose first element is pointed to by s (if s is not a
null pointer). At most MB_CUR_MAX characters are stored. If wc is a null wide character, a null byte is
stored, preceded by any shift sequence needed to restore the initial shift state, and the function is
left in the initial conversion state.
3 The implementation shall behave as if no library function calls the wctomb function.
Returns
4 If s is a null pointer, the wctomb function returns a nonzero or zero value, if multibyte character
encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the
wctomb function returns-1 if the value of wc does not correspond to a valid multibyte character, or
returns the number of bytes that are contained in the multibyte character corresponding to the value
of wc.
5 In no case will the value returned be greater than the value of the MB_CUR_MAX macro.
7.24.8 Multibyte/wide string conversion functions
1 The behavior of the multibyte string functions is affected by the LC_CTYPE category of the current
locale.
7.24.8.1 The mbstowcs function
Synopsis
1 #include <stdlib.h>
size_t mbstowcs(wchar_t * restrict pwcs, const char * restrict s, size_t n);
Description
2 The mbstowcs function converts a sequence of multibyte characters that begins in the initial shift
state from the array pointed to by s into a sequence of corresponding wide characters and stores not
more than n wide characters into the array pointed to by pwcs. No multibyte characters that follow
a null character (which is converted into a null wide character) will be examined or converted. Each
multibyte character is converted as if by a call to the mbtowc function, except that the conversion
state of the mbtowc function is not affected.
3 No more than n elements will be modified in the array pointed to by pwcs. If copying takes place
between objects that overlap, the behavior is undefined.
Returns
4 If an invalid multibyte character is encountered, the mbstowcs function returns (size_t)(-1) .
Otherwise, the mbstowcs function returns the number of array elements modified, not including a
terminating null wide character, if any.371)
371) The array will not be null-terminated if the value returned is n.
7.24.8.2 The wcstombs function
Synopsis
1 #include <stdlib.h>
size_t wcstombs(char * restrict s, const wchar_t * restrict pwcs, size_t n);
Description
2 The wcstombs function converts a sequence of wide characters from the array pointed to by pwcs
into a sequence of corresponding multibyte characters that begins in the initial shift state, and stores
these multibyte characters into the array pointed to by s, stopping if a multibyte character would
exceed the limit of n total bytes or if a null character is stored. Each wide character is converted
as if by a call to the wctomb function, except that the conversion state of the wctomb function is not
affected.
3 No more than n bytes will be modified in the array pointed to by s. If copying takes place between
objects that overlap, the behavior is undefined.
Returns
4 If a wide character is encountered that does not correspond to a valid multibyte character, the
wcstombs function returns (size_t)(-1) . Otherwise, the wcstombs function returns the number
of bytes modified, not including a terminating null character, if any.371)
7.24.9 Alignment of memory
7.24.9.1 The memalignment function
Synopsis
1 #include <stdlib.h>
size_t memalignment(const void * p);
Description
2 The memalignment function accepts a pointer to any object and returns the maximum alignment
satisfied by its address value. The alignment may be an extended alignment and may also be beyond
the range supported by the implementation for explicit use by alignas372) . If so, it will satisfy all
alignments usable by the implementation. The value returned can be compared to the result of
alignof, and if it is greater or equal, the alignment requirement for the type operand is satisfied.
Returns
3 The alignment of the pointer p, which is a power of two. If p is a null pointer, an alignment of zero is
returned.
4 NOTE An alignment of zero indicates that the tested pointer cannot be used to access an object of any type.
372) The actual alignment of an object may be stricter than the alignment requested for an object by alignas or (implicitly) by
an allocation function, but will always satisfy it.
7.25 _Noreturn <stdnoreturn.h>
1 The header <stdnoreturn.h> defines the macro
noreturn
which expands to _Noreturn .
2 The noreturn macro and the <stdnoreturn.h> header are obsolescent features.
7.26 String handling <string.h>
7.26.1 String function conventions
1 The header <string.h> declares one type and several functions, and defines one macro useful
for manipulating arrays of character type and other objects treated as arrays of character type.373)
The type is size_t and the macro is NULL (both described in 7.21). Various methods are used for
determining the lengths of the arrays, but in all cases a char * or void * argument points to the
initial (lowest addressed) character of the array. If an array is accessed beyond the end of an object,
the behavior is undefined.
2 Where an argument declared as size_t n specifies the length of the array for a function, n can have
the value zero on a call to that function. Unless explicitly stated otherwise in the description of a
particular function in this subclause, pointer arguments on such a call shall still have valid values, as
described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function
that compares two character sequences returns zero, and a function that copies characters copies
zero characters.
3 For all functions in this subclause, each character shall be interpreted as if it had the type
unsigned char (and therefore every possible object representation is valid and has a different
value).
7.26.2 Copying functions
7.26.2.1 The memcpy function
Synopsis
1 #include <string.h>
void *memcpy(void * restrict s1, const void * restrict s2, size_t n);
Description
2 The memcpy function copies n characters from the object pointed to by s2 into the object pointed to
by s1. If copying takes place between objects that overlap, the behavior is undefined.
Returns
3 The memcpy function returns the value of s1.
7.26.2.2 The memccpy function
Synopsis
1 #include <string.h>
void *memccpy(void * restrict s1, const void * restrict s2, int c, size_t n);
Description
2 The memccpy function copies characters from the object pointed to by s2 into the object pointed to
by s1, stopping after the first occurrence of character c (converted to an unsigned char) is copied,
or after n characters are copied, whichever comes first. If copying takes place between objects that
overlap, the behavior is undefined.
Returns
3 The memccpy function returns a pointer to the character after the copy of c in s1, or a null pointer if
c was not found in the first n characters of s2.
7.26.2.3 The memmove function
Synopsis
1 #include <string.h>
void *memmove(void *s1, const void *s2, size_t n);
373) See "future library directions" (7.33.17).
Description
2 The memmove function copies n characters from the object pointed to by s2 into the object pointed to
by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied
into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and
then the n characters from the temporary array are copied into the object pointed to by s1.
Returns
3 The memmove function returns the value of s1.
7.26.2.4 The strcpy function
Synopsis
1 #include <string.h>
char *strcpy(char * restrict s1, const char * restrict s2);
Description
2 The strcpy function copies the string pointed to by s2 (including the terminating null character)
into the array pointed to by s1. If copying takes place between objects that overlap, the behavior is
undefined.
Returns
3 The strcpy function returns the value of s1.
7.26.2.5 The strncpy function
Synopsis
1 #include <string.h>
char *strncpy(char * restrict s1, const char * restrict s2, size_t n);
Description
2 The strncpy function copies not more than n characters (characters that follow a null character are
not copied) from the array pointed to by s2 to the array pointed to by s1.374) If copying takes place
between objects that overlap, the behavior is undefined.
3 If the array pointed to by s2 is a string that is shorter than n characters, null characters are appended
to the copy in the array pointed to by s1, until n characters in all have been written.
Returns
4 The strncpy function returns the value of s1.
7.26.2.6 The strdup function
Synopsis
1 #include <string.h>
char *strdup(const char *s);
Description
2 The strdup function creates a copy of the string pointed to by s in a space allocated as if by a call to
malloc.
Returns
3 The strdup function returns a pointer to the first character of the duplicate string. The returned
pointer can be passed to free. If no space can be allocated the strdup function returns a null pointer.
7.26.2.7 The strndup function
374) Thus, if there is no null character in the first n characters of the array pointed to by s2, the result will not be null-
terminated.
Synopsis
1 #include <string.h>
char *strndup(const char *s, size_t size);
Description
2 The strndup function creates a string initialized with no more than size initial characters of the
array pointed to by s and up to the first null character, whichever comes first, in a space allocated
as if by a call to malloc. If the array pointed to by s does not contain a null within the first size
characters, a null is appended to the copy of the array.
Returns
3 The strndup function returns a pointer to the first character of the created string. The returned
pointer can be passed to free. If space cannot be allocated the strndup function returns a null
pointer.
7.26.3 Concatenation functions
7.26.3.1 The strcat function
Synopsis
1 #include <string.h>
char *strcat(char * restrict s1, const char * restrict s2);
Description
2 The strcat function appends a copy of the string pointed to by s2 (including the terminating null
character) to the end of the string pointed to by s1. The initial character of s2 overwrites the null
character at the end of s1. If copying takes place between objects that overlap, the behavior is
undefined.
Returns
3 The strcat function returns the value of s1.
7.26.3.2 The strncat function
Synopsis
1 #include <string.h>
char *strncat(char * restrict s1, const char * restrict s2, size_t n);
Description
2 The strncat function appends not more than n characters (a null character and characters that
follow it are not appended) from the array pointed to by s2 to the end of the string pointed to by
s1. The initial character of s2 overwrites the null character at the end of s1. A terminating null
character is always appended to the result.375) If copying takes place between objects that overlap,
the behavior is undefined.
Returns
3 The strncat function returns the value of s1.
Forward references: the strlen function (7.26.6.4).
7.26.4 Comparison functions
1 The sign of a nonzero value returned by the comparison functions memcmp, strcmp, and strncmp
is determined by the sign of the difference between the values of the first pair of characters (both
interpreted as unsigned char) that differ in the objects being compared.
7.26.4.1 The memcmp function
375) Thus, the maximum number of characters that can end up in the array pointed to by s1 is strlen(s1)+n+1.
Synopsis
1 #include <string.h>
int memcmp(const void *s1, const void *s2, size_t n);
Description
2 The memcmp function compares the first n characters of the object pointed to by s1 to the first n
characters of the object pointed to by s2376) .
Returns
3 The memcmp function returns an integer greater than, equal to, or less than zero, accordingly as the
object pointed to by s1 is greater than, equal to, or less than the object pointed to by s2.
7.26.4.2 The strcmp function
Synopsis
1 #include <string.h>
int strcmp(const char *s1, const char *s2);
Description
2 The strcmp function compares the string pointed to by s1 to the string pointed to by s2.
Returns
3 The strcmp function returns an integer greater than, equal to, or less than zero, accordingly as the
string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2.
7.26.4.3 The strcoll function
Synopsis
1 #include <string.h>
int strcoll(const char *s1, const char *s2);
Description
2 The strcoll function compares the string pointed to by s1 to the string pointed to by s2, both
interpreted as appropriate to the LC_COLLATE category of the current locale.
Returns
3 The strcoll function returns an integer greater than, equal to, or less than zero, accordingly as the
string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2 when both
are interpreted as appropriate to the current locale.
7.26.4.4 The strncmp function
Synopsis
1 #include <string.h>
int strncmp(const char *s1, const char *s2, size_t n);
Description
2 The strncmp function compares not more than n characters (characters that follow a null character
are not compared) from the array pointed to by s1 to the array pointed to by s2.
Returns
3 The strncmp function returns an integer greater than, equal to, or less than zero, accordingly as the
possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly
376) The unused bytes used as padding for purposes of alignment within structure objects take on unspecified values when a
value is stored in the object (see 6.2.6.1). Strings shorter than their allocated space and unions can also cause problems in
comparison.
null-terminated array pointed to by s2.
7.26.4.5 The strxfrm function
Synopsis
1 #include <string.h>
size_t strxfrm(char * restrict s1, const char * restrict s2, size_t n);
Description
2 The strxfrm function transforms the string pointed to by s2 and places the resulting string into
the array pointed to by s1. The transformation is such that if the strcmp function is applied to two
transformed strings, it returns a value greater than, equal to, or less than zero, corresponding to the
result of the strcoll function applied to the same two original strings. No more than n characters
are placed into the resulting array pointed to by s1, including the terminating null character. If n is
zero, s1 is permitted to be a null pointer. If copying takes place between objects that overlap, the
behavior is undefined.
Returns
3 The strxfrm function returns the length of the transformed string (not including the terminating
null character). If the value returned is n or more, the members of the array pointed to by s1 have
an indeterminate representation.
4 EXAMPLE The value of the following expression is the size of the array needed to hold the transformation of the string
pointed to by s.
1 + strxfrm(NULL, s, 0)
7.26.5 Search functions
7.26.5.1 Introduction
1 The stateless search functions in this section (memchr, strchr, strpbrk, strrchr, strstr) are
generic functions. These functions are generic in the qualification of the array to be searched and
will return a result pointer to an element with the same qualification as the passed array. If the array
to be searched is const- qualified, the result pointer will be to a const-qualified element. If the array
to be searched is not const-qualified377) , the result pointer will be to an unqualified element.
2 The external declarations of these generic functions have a concrete function type that returns a
pointer to an unqualified element (of type char when specified as QChar, and void when specified
as QVoid), and accepts a pointer to a const-qualified array of the same type to search. This signature
supports all correct uses. If a macro definition of any of these generic functions is suppressed in
order to access an actual function, the external declaration with the corresponding concrete type is
visible378) .
3 The volatile and restrict qualifiers are not accepted on the elements of the array to search.
7.26.5.2 The memchr generic function
Synopsis
1 #include <string.h>
QVoid *memchr(QVoid *s, int c, size_t n);
Description
2 The memchr generic function locates the first occurrence of c (converted to an unsigned char)
in the initial n characters (each interpreted as unsigned char) of the object pointed to by s. The
implementation shall behave as if it reads the characters sequentially and stops as soon as a matching
character is found.
377) The null pointer constant is not a pointer to a const-qualified type, and therefore the result expression has the type of a
pointer to an unqualified element; however, evaluating such a call is undefined.
378) This is an obsolescent feature.
Returns
3 The memchr generic function returns a pointer to the located character, or a null pointer if the
character does not occur in the object.
7.26.5.3 The strchr generic function
Synopsis
1 #include <string.h>
QChar *strchr(QChar *s, int c);
Description
2 The strchr generic function locates the first occurrence of c (converted to a char) in the string
pointed to by s. The terminating null character is considered to be part of the string.
Returns
3 The strchr generic function returns a pointer to the located character, or a null pointer if the
character does not occur in the string.
7.26.5.4 The strcspn function
Synopsis
1 #include <string.h>
size_t strcspn(const char *s1, const char *s2);
Description
2 The strcspn function computes the length of the maximum initial segment of the string pointed to
by s1 which consists entirely of characters not from the string pointed to by s2.
Returns
3 The strcspn function returns the length of the segment.
7.26.5.5 The strpbrk generic function
Synopsis
1 #include <string.h>
QChar *strpbrk(QChar *s1, const char *s2);
Description
2 The strpbrk generic function locates the first occurrence in the string pointed to by s1 of any
character from the string pointed to by s2.
Returns
3 The
tcodestrpbrk generic function returns a pointer to the character, or a null pointer if no character from
s2 occurs in s1.
7.26.5.6 The strrchr generic function
Synopsis
1 #include <string.h>
QChar *strrchr(QChar *s, int c);
Description
2 The strrchr generic function locates the last occurrence of c (converted to a char) in the string
pointed to by s. The terminating null character is considered to be part of the string.
Returns
3 The strrchr generic function returns a pointer to the character, or a null pointer if c does not occur
in the string.
7.26.5.7 The strspn function
Synopsis
1 #include <string.h>
size_t strspn(const char *s1, const char *s2);
Description
2 The strspn function computes the length of the maximum initial segment of the string pointed to
by s1 which consists entirely of characters from the string pointed to by s2.
Returns
3 The strspn function returns the length of the segment.
7.26.5.8 The strstr generic function
Synopsis
1 #include <string.h>
QChar *strstr(QChar *s1, const char *s2);
Description
2 The strstr generic function locates the first occurrence in the string pointed to by s1 of the sequence
of characters (excluding the terminating null character) in the string pointed to by s2.
Returns
3 The strstr generic function returns a pointer to the located string, or a null pointer if the string is
not found. If s2 points to a string with zero length, the function returns s1.
7.26.5.9 The strtok function
Synopsis
1 #include <string.h>
char *strtok(char * restrict s1, const char * restrict s2);
Description
2 A sequence of calls to the strtok function breaks the string pointed to by s1 into a sequence of
tokens, each of which is delimited by a character from the string pointed to by s2. The first call
in the sequence has a non-null first argument; subsequent calls in the sequence have a null first
argument. If any of the subsequent calls in the sequence is made by a different thread than the first,
the behavior is undefined. The separator string pointed to by s2 may be different from call to call.
3 The first call in the sequence searches the string pointed to by s1 for the first character that is not
contained in the current separator string pointed to by s2. If no such character is found, then there
are no tokens in the string pointed to by s1 and the strtok function returns a null pointer. If such a
character is found, it is the start of the first token.
4 The strtok function then searches from there for a character that is contained in the current separator
string. If no such character is found, the current token extends to the end of the string pointed to by
s1, and subsequent searches for a token will return a null pointer. If such a character is found, it is
overwritten by a null character, which terminates the current token. The strtok function saves a
pointer to the following character, from which the next search for a token will start.
5 Each subsequent call, with a null pointer as the value of the first argument, starts searching from the
saved pointer and behaves as described above.
6 The strtok function is not required to avoid data races with other calls to the strtok function.379)
The implementation shall behave as if no library function calls the strtok function.
Returns
7 The strtok function returns a pointer to the first character of a token, or a null pointer if there is no
token.
8 EXAMPLE
#include <string.h>
static char str[] = "?a???b,,,#c";
char *t;
t = strtok(str, "?"); // t points to the token "a"
t = strtok(NULL, ","); // t points to the token "??b"
t = strtok(NULL, "#,"); // t points to the token "c"
t = strtok(NULL, "?"); // t is a null pointer
Forward references: The strtok_s function (K.3.7.3.1).
7.26.6 Miscellaneous functions
7.26.6.1 The memset function
Synopsis
1 #include <string.h>
void *memset(void *s, int c, size_t n);
Description
2 The memset function copies the value of c (converted to an unsigned char) into each of the first n
characters of the object pointed to by s.
Returns
3 The memset function returns the value of s.
7.26.6.2 The memset_explicit function
Synopsis
1 #include <string.h>
void *memset_explicit(void *s, int c, size_t n);
Description
2 The memset_explicit function copies the value of c (converted to an unsigned char) into each of
the first n characters of the object pointed to by s. The purpose of this function is to make sensitive
information stored in the object inaccessible380) .
Returns
3 The memset_explicit function returns the value of s.
7.26.6.3 The strerror function
Synopsis
1 #include <string.h>
char *strerror(int errnum);
379) The strtok_s function can be used instead to avoid data races.
380) The intention is that the memory store is always performed (i.e., never elided), regardless of optimizations. This is in
contrast to calls to the memset function (7.26.6.1)
Description
2 The strerror function maps the number in errnum to a message string. Typically, the values for
errnum come from errno, but strerror shall map any value of type int to a message.
3 The strerror function is not required to avoid data races with other calls to the strerror func-
tion.381) The implementation shall behave as if no library function calls the strerror function.
Returns
4 The strerror function returns a pointer to the string, the contents of which are locale-specific. The
array pointed to shall not be modified by the program. The behavior is undefined if the returned
value is used after a subsequent call to the strerror function, or after the thread which called the
function to obtain the returned value has exited.
Forward references: The strerror_s function (K.3.7.4.2).
381) The strerror_s function can be used instead to avoid data races.
7.26.6.4 The strlen function
Synopsis
1 #include <string.h>
size_t strlen(const char *s);
Description
2 The strlen function computes the length of the string pointed to by s.
Returns
3 The strlen function returns the number of characters that precede the terminating null character.
7.27 Type-generic math <tgmath.h>
1 The header <tgmath.h> includes the headers <math.h> and <complex.h> and defines several
type-generic macros.
2 The feature test macro __STDC_VERSION_TGMATH_H__ expands to the token 202311L.
3 This clause specifies a many-to-one correspondence of functions in <math.h> and <complex.h> with
type-generic macros.382) Use of a type-generic macro invokes a corresponding function whose type is
determined by the types of the arguments for particular parameters called the generic parameters.383)
4 Of the <math.h> and <complex.h> functions without an f (float) or l (long double) suffix, several
have one or more parameters whose corresponding real type is double. For each such function,
except the functions that round result to narrower type (7.12.14) (which are covered below) and
modf,
there is a corresponding type-generic macro. The parameters whose corresponding real type is
double in the function synopsis are generic parameters.
5 Some of the <math.h> functions for decimal floating types have no unsuffixed counterpart. Of these
functions with a d64 suffix, some have one or more parameters whose type is _Decimal64 . For each
such function, except decodedecd64, encodedecd64, decodebind64, and encodebind64, there is a
corresponding type-generic macro. The parameters whose real type is _Decimal64 in the function
synopsis are generic parameters.
6 If arguments for generic parameters of a type-generic macro are such that some argument has a
corresponding real type that is of standard floating type and another argument is of decimal floating
type, the behavior is undefined.
7 Except for the macros for functions that round result to a narrower type (7.12.14), use of a type-
generic macro invokes a function whose generic parameters have the corresponding real type
determined by the types of the arguments for the generic parameters as follows:
— Arguments of integer type are regarded as having type _Decimal64 if any argument has
decimal floating type, and as having type double otherwise.
— If the function has exactly one generic parameter, the type determined is the corresponding
real type of the argument for the generic parameter.
— If the function has exactly two generic parameters, the type determined is the corresponding
real type determined by the usual arithmetic conversions (6.3.1.8) applied to the arguments for
the generic parameters.
— If the function has more than two generic parameters, the type determined is the corresponding
real type determined by repeatedly applying the usual arithmetic conversions, first to the first
two arguments for generic parameters, then to that result type and the next argument for a
generic parameter, and so forth until the usual arithmetic conversions have been applied to
the last argument for a generic parameter.
If neither <math.h> and <complex.h> define a function whose generic parameters have the deter-
mined corresponding real type, the behavior is undefined.
8 For each unsuffixed function in <math.h> for which there is a function in <complex.h> with the
same name except for a c prefix, the corresponding type-generic macro (for both functions) has the
same name as the function in <math.h>. The corresponding type-generic macro for fabs and cabs
is fabs.
382) Like other function-like macros in standard libraries, each type-generic macro can be suppressed to make available the
corresponding ordinary function.
383) If the type of the argument is not compatible with the type of the parameter for the selected function, the behavior is
undefined.
<math.h> <complex.h> type-generic
function function macro
acos cacos acos
asin casin asin
atan catan atan
acosh cacosh acosh
asinh casinh asinh
atanh catanh atanh
cos ccos cos
sin csin sin
tan ctan tan
cosh ccosh cosh
sinh csinh sinh
tanh ctanh tanh
exp cexp exp
log clog log
pow cpow pow
sqrt csqrt sqrt
fabs cabs fabs
If at least one argument for a generic parameter is complex, then use of the macro invokes a complex
function; otherwise, use of the macro invokes a real function.
9 For each unsuffixed function in <math.h> without a c-prefixed counterpart in <complex.h> (except
functions that round result to narrower type, modf, and canonicalize), the corresponding type-
generic macro has the same name as the function. These type-generic macros are:
acospi exp2 fmod log2 rootn
asinpi expm1 frexp logb roundeven
atan2pi fdim fromfpx logp1 round
atan2 floor fromfp lrint rsqrt
atanpi fmax hypot lround scalbln
cbrt fmaximum ilogb nearbyint scalbn
ceil fmaximum_mag ldexp nextafter sinpi
compoundn fmaximum_num lgamma nextdown tanpi
copysign fmaximum_mag_num llogb nexttoward tgamma
cospi fma llrint nextup trunc
erfc fmin llround pown ufromfpx
erf fminimum log10p1 powr ufromfp
exp10m1 fminimum_mag log10 remainder
exp10 fminimum_num log1p remquo
exp2m1 fminimum_mag_num log2p1 rint
If all arguments for generic parameters are real, then use of the macro invokes a real function
(provided <math.h> defines a function of the determined type); otherwise, use of the macro is
undefined.
10 For each unsuffixed function in <complex.h> that is not a c-prefixed counterpart to a function
in <math.h>, the corresponding type-generic macro has the same name as the function. These
type-generic macros are:
carg cimag conj cproj creal
Use of the macro with any argument of standard floating or complex type invokes a complex
function. Use of the macro with an argument of decimal floating type is undefined.
11 The functions that round result to a narrower type have type-generic macros whose names are
obtained by omitting any suffix from the function names. Thus, the macros with f or d prefix are:
fadd fsub fmul fdiv ffma fsqrt
dadd dsub dmul ddiv dfma dsqrt
and the macros with d32 or d64 prefix are:
d32add d32sub d32mul d32div d32fma d32sqrt
d64add d64sub d64mul d64div d64fma d64sqrt
All arguments shall be real. If the macro prefix is f or d, use of an argument of decimal floating
type is undefined. If the macro prefix is d32 or d64, use of an argument of standard floating type is
undefined. The function invoked is determined as follows:
— If any argument has type _Decimal128 , or if the macro prefix is d64, the function invoked has
the name of the macro, with a d128 suffix.
— Otherwise, if the macro prefix is d32, the function invoked has the name of the macro, with a
d64 suffix.
— Otherwise, if any argument has type long double, or if the macro prefix is d, the function
invoked has the name of the macro, with an l suffix.
— Otherwise, the function invoked has the name of the macro (with no suffix).
12 For each d64-suffixed function in <math.h>, except decodedecd64, encodedecd64, decodebind64,
and encodebind64, that does not have an unsuffixed counterpart, the corresponding type-generic
macro has the name of the function, but without the suffix. These type-generic macros are:
<math.h> type-generic
function macro
quantizedN quantize
samequantumdN samequantum
quantumdN quantum
llquantexpdN llquantexp
Use of the macro with an argument of standard floating or complex type or with only integer type
arguments is undefined.
13 A type-generic macro corresponding to a function indicated in the table in 7.6.2 is affected by
constant rounding modes (7.6.4).
14 NOTE The type-generic macro definition in the example in 6.5.1.1 does not conform to this specification. A conforming
macro could be implemented as follows:
#define cbrt(X) \
_Generic((X), \
long double: _Roundwise_cbrtl, \
default: _Roundwise_cbrt, \
float: _Roundwise_cbrtf \
)(X)
where where _Roundwise_cbrtl , _Roundwise_cbrt , and _Roundwise_cbrtf are pointers to functions that are equivalent
to cbrtl, cbrt, and cbrtf, respectively, but that are guaranteed to be affected by constant rounding modes (7.6.2).
15 EXAMPLE With the declarations
#include <tgmath.h>
int n;
float f;
double d;
long double ld;
float complex fc;
double complex dc;
long double complex ldc;
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 d32;
_Decimal64 d64;
_Decimal128 d128;
#endif
functions invoked by use of type-generic macros are shown in the following table:
macro use invocation
exp(n) exp(n) , the function
acosh(f) acoshf(f)
sin(d) sin(d) , the function
atan(ld) atanl(ld)
log(fc) clogf(fc)
sqrt(dc) csqrt(dc)
pow(ldc, f) cpowl(ldc, f)
remainder(n, n) remainder(n, n) , the function
nextafter(d, f) nextafter(d, f) , the function
nexttoward(f, ld) nexttowardf(f, ld)
copysign(n, ld) copysignl(n, ld)
ceil(fc) undefined
rint(dc) undefined
fmaximum(ldc, ld) undefined
carg(n) carg(n) , the function
cproj(f) cprojf(f)
creal(d) creal(d) , the function
cimag(ld) cimagl(ld)
fabs(fc) cabsf(fc)
carg(dc) carg(dc) , the function
cproj(ldc) cprojl(ldc)
fsub(f, ld) fsubl(f, ld)
fdiv(d, n) fdiv(d, n) , the function
dfma(f, d, ld) dfmal(f, d, ld)
dadd(f, f) daddl(f, f)
dsqrt(dc) undefined
exp(d64) expd64(d64)
sqrt(d32) sqrtd32(d32)
fmaximum(d64, d128) fmaximumd128(d64, d128)
pow(d32, n) powd64(d32, n)
remainder(d64, d) undefined
creal(d64) undefined
remquo(d32, d32, &n) undefined
llquantexp(d) undefined
quantize(dc) undefined
samequantum(n, n) undefined
d32sub(d32, d128) d32subd128(d32, d128)
d32div(d64, n) d32divd64(d64, n)
d64fma(d32, d64, d128) d64fmad128(d32, d64, d128)
d64add(d32, d32) d64addd128(d32, d32)
d64sqrt(d) undefined
dadd(n, d64) undefined
acospi
asinpi
atan2pi
atan2
atanpi
cbrt
ceil
compoundn
copysign
cospi
erfc
erf
exp10m1
exp10
exp2m1
exp2
expm1
fdim
floor
fmax
fmaximum
fmaximum_mag
fmaximum_num
fmaximum_mag_num
fma
fmin
fminimum
fminimum_mag
fminimum_num
fminimum_mag_num
fmod
frexp
fromfpx
fromfp
hypot
ilogb
ldexp
lgamma
llogb
llrint
llround
log10p1
log10
log1p
log2p1
log2
logb
logp1
lrint
lround
nearbyint
nextafter
nextdown
nexttoward
nextup
pown
powr
remainder
remquo
rint
rootn
roundeven
round
rsqrt
scalbln
scalbn
sinpi
tanpi
tgamma
trunc
ufromfpx
ufromfp carg
cimag
conj
cproj
creal d32add
d64add
d32sub
d64sub
d32mul
d64mul
d32div
d64div
d32fma
d64fma
d32sqrt
d64sqrt fadd
dadd
fsub
dsub
fmul
dmul
fdiv
ddiv
ffma
dfma
fsqrt
dsqrt
7.28 Threads <threads.h>
7.28.1 Introduction
1 The header <threads.h> includes the header <time.h>, defines macros, and declares types, enu-
meration constants, and functions that support multiple threads of execution384) .
2 Implementations that define the macro __STDC_NO_THREADS__ need not provide this header nor
support any of its facilities.
3 The macros are
ONCE_FLAG_INIT
which expands to a value that can be used to initialize an object of type once_flag; and
TSS_DTOR_ITERATIONS
which expands to an integer constant expression representing the maximum number of times that
destructors will be called when a thread terminates.
4 The types are
cnd_t
which is a complete object type that holds an identifier for a condition variable;
thrd_t
which is a complete object type that holds an identifier for a thread;
tss_t
which is a complete object type that holds an identifier for a thread-specific storage pointer;
mtx_t
which is a complete object type that holds an identifier for a mutex;
tss_dtor_t
which is the function pointer type void (*)(void*), used for a destructor for a thread-specific
storage pointer;
thrd_start_t
which is the function pointer type int (*)(void*) that is passed to thrd_create to create a new
thread; and
once_flag
which is a complete object type that holds a flag for use by call_once.
5 The enumeration constants are
mtx_plain
which is passed to mtx_init to create a mutex object that does not support timeout;
mtx_recursive
384) See "future library directions" (7.33.19).
which is passed to mtx_init to create a mutex object that supports recursive locking;
mtx_timed
which is passed to mtx_init to create a mutex object that supports timeout;
thrd_timedout
which is returned by a timed wait function to indicate that the time specified in the call was reached
without acquiring the requested resource;
thrd_success
which is returned by a function to indicate that the requested operation succeeded;
thrd_busy
which is returned by a function to indicate that the requested operation failed because a resource
requested by a test and return function is already in use;
thrd_error
which is returned by a function to indicate that the requested operation failed; and
thrd_nomem
which is returned by a function to indicate that the requested operation failed because it was unable
to allocate memory.
Forward references: date and time (7.29).
7.28.2 Initialization functions
7.28.2.1 The call_once function
Synopsis
1 #include <threads.h>
void call_once(once_flag *flag, void (*func)(void));
Description
2 The call_once function uses the once_flag pointed to by flag to ensure that func is called exactly
once, the first time the call_once function is called with that value of flag. Completion of an
effective call to the call_once function synchronizes with all subsequent calls to the call_once
function with the same value of flag.
Returns
3 The call_once function returns no value.
7.28.3 Condition variable functions
7.28.3.1 The cnd_broadcast function
Synopsis
1 #include <threads.h>
int cnd_broadcast(cnd_t *cond);
Description
2 The cnd_broadcast function unblocks all of the threads that are blocked on the condition variable
pointed to by cond at the time of the call. If no threads are blocked on the condition variable pointed
to by cond at the time of the call, the function does nothing.
Returns
3 The cnd_broadcast function returns thrd_success on success, or thrd_error if the request could
not be honored.
7.28.3.2 The cnd_destroy function
Synopsis
1 #include <threads.h>
void cnd_destroy(cnd_t *cond);
Description
2 The cnd_destroy function releases all resources used by the condition variable pointed to by cond.
The cnd_destroy function requires that no threads be blocked waiting for the condition variable
pointed to by cond.
Returns
3 The cnd_destroy function returns no value.
7.28.3.3 The cnd_init function
Synopsis
1 #include <threads.h>
int cnd_init(cnd_t *cond);
Description
2 The cnd_init function creates a condition variable. If it succeeds it sets the variable pointed to by
cond to a value that uniquely identifies the newly created condition variable. A thread that calls
cnd_wait on a newly created condition variable will block.
Returns
3 The cnd_init function returns thrd_success on success, or thrd_nomem if no memory could be
allocated for the newly created condition, or thrd_error if the request could not be honored.
7.28.3.4 The cnd_signal function
Synopsis
1 #include <threads.h>
int cnd_signal(cnd_t *cond);
Description
2 The cnd_signal function unblocks one of the threads that are blocked on the condition variable
pointed to by cond at the time of the call. If no threads are blocked on the condition variable at the
time of the call, the function does nothing and returns success.
Returns
3 The cnd_signal function returns thrd_success on success or thrd_error if the request could not
be honored.
7.28.3.5 The cnd_timedwait function
Synopsis
1 #include <threads.h>
int cnd_timedwait(cnd_t *restrict cond, mtx_t *restrict mtx,
const struct timespec *restrict ts);
Description
2 The cnd_timedwait function atomically unlocks the mutex pointed to by mtx and blocks until the
condition variable pointed to by cond is signaled by a call to cnd_signal or to cnd_broadcast, or
until after the TIME_UTC-based calendar time pointed to by ts, or until it is unblocked due to an
unspecified reason. When the calling thread becomes unblocked it locks the variable pointed to by
mtx before it returns. The cnd_timedwait function requires that the mutex pointed to by mtx be
locked by the calling thread.
Returns
3 The cnd_timedwait function returns thrd_success upon success, or thrd_timedout if the time
specified in the call was reached without acquiring the requested resource, or thrd_error if the
request could not be honored.
7.28.3.6 The cnd_wait function
Synopsis
1 #include <threads.h>
int cnd_wait(cnd_t *cond, mtx_t *mtx);
Description
2 The cnd_wait function atomically unlocks the mutex pointed to by mtx and blocks until the condi-
tion variable pointed to by cond is signaled by a call to cnd_signal or to cnd_broadcast, or until it
is unblocked due to an unspecified reason. When the calling thread becomes unblocked it locks the
mutex pointed to by mtx before it returns. The cnd_wait function requires that the mutex pointed
to by mtx be locked by the calling thread.
Returns
3 The cnd_wait function returns thrd_success on success or thrd_error if the request could not be
honored.
7.28.4 Mutex functions
1 For purposes of determining the existence of a data race, lock and unlock operations behave as
atomic operations. All lock and unlock operations on a particular mutex occur in some particular
total order.
2 NOTE This total order can be viewed as the modification order of the mutex.
7.28.4.1 The mtx_destroy function
Synopsis
1 #include <threads.h>
void mtx_destroy(mtx_t *mtx);
Description
2 The mtx_destroy function releases any resources used by the mutex pointed to by mtx. No threads
can be blocked waiting for the mutex pointed to by mtx.
Returns
3 The mtx_destroy function returns no value.
7.28.4.2 The mtx_init function
Synopsis
1 #include <threads.h>
int mtx_init(mtx_t *mtx, int type);
Description
2 The mtx_init function creates a mutex object with properties indicated by type, which shall have
one of these values:
mtx_plain for a simple non-recursive mutex,
mtx_timed for a non-recursive mutex that supports timeout,
mtx_plain | mtx_recursive for a simple recursive mutex, or
mtx_timed | mtx_recursive for a recursive mutex that supports timeout.
3 If the mtx_init function succeeds, it sets the mutex pointed to by mtx to a value that uniquely
identifies the newly created mutex.
Returns
4 The mtx_init function returns thrd_success on success, or thrd_error if the request could not
be honored.
7.28.4.3 The mtx_lock function
Synopsis
1 #include <threads.h>
int mtx_lock(mtx_t *mtx);
Description
2 The mtx_lock function blocks until it locks the mutex pointed to by mtx. If the mutex is non-
recursive, it shall not be locked by the calling thread. Prior calls to mtx_unlock on the same mutex
synchronize with this operation.
Returns
3 The mtx_lock function returns thrd_success on success, or thrd_error if the request could not
be honored.
7.28.4.4 The mtx_timedlock function
Synopsis
1 #include <threads.h>
int mtx_timedlock(mtx_t *restrict mtx, const struct timespec *restrict ts);
Description
2 The mtx_timedlock function endeavors to block until it locks the mutex pointed to by mtx or
until after the TIME_UTC-based calendar time pointed to by ts. The specified mutex shall support
timeout. If the operation succeeds, prior calls to mtx_unlock on the same mutex synchronize with
this operation.
Returns
3 The mtx_timedlock function returns thrd_success on success, or thrd_timedout if the time
specified was reached without acquiring the requested resource, or thrd_error if the request could
not be honored.
7.28.4.5 The mtx_trylock function
Synopsis
1 #include <threads.h>
int mtx_trylock(mtx_t *mtx);
Description
2 The mtx_trylock function endeavors to lock the mutex pointed to by mtx. If the mutex is already
locked, the function returns without blocking. If the operation succeeds, prior calls to mtx_unlock
on the same mutex synchronize with this operation.
Returns
3 The mtx_trylock function returns thrd_success on success, or thrd_busy if the resource requested
is already in use, or thrd_error if the request could not be honored. mtx_trylock may spuriously
fail to lock an unused resource, in which case it returns thrd_busy.
7.28.4.6 The mtx_unlock function
Synopsis
1 #include <threads.h>
int mtx_unlock(mtx_t *mtx);
Description
2 The mtx_unlock function unlocks the mutex pointed to by mtx. The mutex pointed to by mtx shall
be locked by the calling thread.
Returns
3 The mtx_unlock function returns thrd_success on success or thrd_error if the request could not
be honored.
7.28.5 Thread functions
7.28.5.1 The thrd_create function
Synopsis
1 #include <threads.h>
int thrd_create(thrd_t *thr, thrd_start_t func, void *arg);
Description
2 The thrd_create function creates a new thread executing func(arg). If the thrd_create function
succeeds, it sets the object pointed to by thr to the identifier of the newly created thread. (A thread’s
identifier may be reused for a different thread once the original thread has exited and either been
detached or joined to another thread.) The completion of the thrd_create function synchronizes
with the beginning of the execution of the new thread.
3 Returning from func has the same behavior as invoking thrd_exit with the value returned from
func.
Returns
4 The thrd_create function returns thrd_success on success, or thrd_nomem if no memory could
be allocated for the thread requested, or thrd_error if the request could not be honored.
7.28.5.2 The thrd_current function
Synopsis
1 #include <threads.h>
thrd_t thrd_current(void);
Description
2 The thrd_current function identifies the thread that called it.
Returns
3 The thrd_current function returns the identifier of the thread that called it.
7.28.5.3 The thrd_detach function
Synopsis
1 #include <threads.h>
int thrd_detach(thrd_t thr);
Description
2 The thrd_detach function tells the operating system to dispose of any resources allocated to the
thread identified by thr when that thread terminates. The thread identified by thr shall not have
been previously detached or joined with another thread.
Returns
3 The thrd_detach function returns thrd_success on success or thrd_error if the request could
not be honored.
7.28.5.4 The thrd_equal function
Synopsis
1 #include <threads.h>
int thrd_equal(thrd_t thr0, thrd_t thr1);
Description
2 The thrd_equal function will determine whether the thread identified by thr0 refers to the thread
identified by thr1.
Returns
3 The thrd_equal function returns zero if the thread thr0 and the thread thr1 refer to different
threads. Otherwise the thrd_equal function returns a nonzero value.
7.28.5.5 The thrd_exit function
Synopsis
1 #include <threads.h>
[[noreturn]] void thrd_exit(int res);
Description
2 For every thread-specific storage key which was created with a non-null destructor and for which
the value is non-null, thrd_exit sets the value associated with the key to a null pointer value and
then invokes the destructor with its previous value. The order in which destructors are invoked is
unspecified.
3 If after this process there remain keys with both non-null destructors and values, the implementation
repeats this process up to TSS_DTOR_ITERATIONS times.
4 Following this, the thrd_exit function terminates execution of the calling thread and sets its result
code to res.
5 The program terminates normally after the last thread has been terminated. The behavior is as if the
program called the exit function with the status EXIT_SUCCESS at thread termination time.
Returns
6 The thrd_exit function returns no value.
7.28.5.6 The thrd_join function
Synopsis
1 #include <threads.h>
int thrd_join(thrd_t thr, int *res);
Description
2 The thrd_join function joins the thread identified by thr with the current thread by blocking until
the other thread has terminated. If the parameter res is not a null pointer, it stores the thread’s result
code in the integer pointed to by res. The termination of the other thread synchronizes with the
completion of the thrd_join function. The thread identified by thr shall not have been previously
detached or joined with another thread.
Returns
3 The thrd_join function returns thrd_success on success or thrd_error if the request could not
be honored.
7.28.5.7 The thrd_sleep function
Synopsis
1 #include <threads.h>
int thrd_sleep(const struct timespec *duration, struct timespec *remaining);
Description
2 The thrd_sleep function suspends execution of the calling thread until either the interval specified
by duration has elapsed or a signal which is not being ignored is received. If interrupted by a signal
and the remaining argument is not null, the amount of time remaining (the requested interval
minus the time actually slept) is stored in the interval it points to. The duration and remaining
arguments may point to the same object.
3 The suspension time may be longer than requested because the interval is rounded up to an integer
multiple of the sleep resolution or because of the scheduling of other activity by the system. But,
except for the case of being interrupted by a signal, the suspension time will not be less than that
specified, as measured by the system clock TIME_UTC.
Returns
4 The thrd_sleep function returns zero if the requested time has elapsed, −1 if it has been interrupted
by a signal, or a negative value (which may also be −1) if it fails.
7.28.5.8 The thrd_yield function
Synopsis
1 #include <threads.h>
void thrd_yield(void);
Description
2 The thrd_yield function endeavors to permit other threads to run, even if the current thread would
ordinarily continue to run.
Returns
3 The thrd_yield function returns no value.
7.28.6 Thread-specific storage functions
7.28.6.1 The tss_create function
Synopsis
1 #include <threads.h>
int tss_create(tss_t *key, tss_dtor_t dtor);
Description
2 The tss_create function creates a thread-specific storage pointer with destructor dtor, which may
be null.
3 A null pointer value is associated with the newly created key in all existing threads. Upon subsequent
thread creation, the value associated with all keys is initialized to a null pointer value in the new
thread.
4 Destructors associated with thread-specific storage are not invoked at program termination.
5 The tss_create function shall not be called from within a destructor.
Returns
6 If the tss_create function is successful, it sets the thread-specific storage pointed to by key to a
value that uniquely identifies the newly created pointer and returns thrd_success; otherwise,
thrd_error is returned and the thread-specific storage pointed to by key is set to an indeterminate
representation.
7.28.6.2 The tss_delete function
Synopsis
1 #include <threads.h>
void tss_delete(tss_t key);
Description
2 The tss_delete function releases any resources used by the thread-specific storage identified by
key. The tss_delete function shall only be called with a value for key that was returned by a call
to tss_create before the thread commenced executing destructors.
3 If tss_delete is called while another thread is executing destructors, whether this will affect the
number of invocations of the destructor associated with key on that thread is unspecified.
4 Calling tss_delete will not result in the invocation of any destructors.
Returns
5 The tss_delete function returns no value.
7.28.6.3 The tss_get function
Synopsis
1 #include <threads.h>
void *tss_get(tss_t key);
Description
2 The tss_get function returns the value for the current thread held in the thread-specific storage
identified by key. The tss_get function shall only be called with a value for key that was returned
by a call to tss_create before the thread commenced executing destructors.
Returns
3 The tss_get function returns the value for the current thread if successful, or zero if unsuccessful.
7.28.6.4 The tss_set function
Synopsis
1 #include <threads.h>
int tss_set(tss_t key, void *val);
Description
2 The tss_set function sets the value for the current thread held in the thread-specific storage
identified by key to val. The tss_set function shall only be called with a value for key that was
returned by a call to tss_create before the thread commenced executing destructors.
3 This action will not invoke the destructor associated with the key on the value being replaced.
Returns
4 The tss_set function returns thrd_success on success or thrd_error if the request could not be
honored.
7.29 Date and time <time.h>
7.29.1 Components of time
1 The header <time.h> defines several macros, and declares types and functions for manipulating
time. Many functions deal with a calendar time that represents the current date (according to the
Gregorian calendar) and time. Some functions deal with local time, which is the calendar time
expressed for some specific time zone, and with Daylight Saving Time, which is a temporary change
in the algorithm for determining local time. The local time zone and Daylight Saving Time are
implementation-defined.
2 The feature test macro __STDC_VERSION_TIME_H__ expands to the token 202311L. The other macros
defined are NULL (described in 7.21);
CLOCKS_PER_SEC
which expands to an expression with type clock_t (described below) that is the number per second
of the value returned by the clock function;
TIME_UTC
TIME_MONOTONIC
which expand to integer constants greater than 0 that designates the calendar time and monotonic
time bases, respectively. Additional time base macro definitions, beginning with TIME_ and an
uppercase letter, may also be specified by the implementation385) ; and,
TIME_ACTIVE
TIME_THREAD_ACTIVE
which, if defined, expand to integer values, designating overall execution and thread-specific active
processing time bases, respectively.
3 The definition of macros for time bases other than TIME_UTC are optional. If defined, the correspond-
ing time bases are supported by timespec_get and timespec_getres, and their values are positive.
If defined, the value of the optional macro TIME_ACTIVE shall be different from the constants
TIME_UTC and TIME_MONOTONIC and shall not change during the same program invocation. The
optional macro TIME_THREAD_ACTIVE shall not be defined if the implementation does not support
threads; its value shall be different from TIME_UTC, TIME_MONOTONIC, and TIME_ACTIVE, it shall be
the same for all expansions of the macro for the same thread, and the value provided for one thread
shall not be used by a different thread as the base argument of timespec_get or timespec_getres.
4 The types declared are size_t (described in 7.21);
clock_t
and
time_t
which are real types capable of representing times;
struct timespec
which holds an interval specified in seconds and nanoseconds (which may represent a calendar time
based on a particular epoch); and
struct tm
385) See future library directions (7.33). Implementations can define additional time bases, but are only required to support a
real time clock based on UTC.
which holds the components of a calendar time, called the broken-down time.
5 The range and precision of times representable in clock_t and time_t are implementation-defined.
The timespec structure shall contain at least the following members, in any order. The semantics of
the members and their normal ranges are expressed in the comments.386)
time_t tv_sec; // whole seconds -- ≥ 0
long tv_nsec; // nanoseconds -- [0, 999999999]
The tm structure shall contain at least the following members, in any order. The semantics of the
members and their normal ranges are expressed in the comments.387)
int tm_sec; // seconds after the minute -- [0, 60]
int tm_min; // minutes after the hour -- [0, 59]
int tm_hour; // hours since midnight -- [0, 23]
int tm_mday; // day of the month -- [1, 31]
int tm_mon; // months since January -- [0, 11]
int tm_year; // years since 1900
int tm_wday; // days since Sunday -- [0, 6]
int tm_yday; // days since January 1 -- [0, 365]
int tm_isdst; // Daylight Saving Time flag
The value of tm_isdst is positive if Daylight Saving Time is in effect, zero if Daylight Saving Time
is not in effect, and negative if the information is not available.
7.29.2 Time manipulation functions
7.29.2.1 The clock function
Synopsis
1 #include <time.h>
clock_t clock(void);
Description
2 The clock function determines the processor time used.
Returns
3 The clock function returns the implementation’s best approximation of the active processing time
associated with the program execution since the beginning of an implementation-defined era related
only to the program invocation. To determine the time in seconds, the value returned by the clock
function should be divided by the value of the macro CLOCKS_PER_SEC. If the processor time used
is not available, the function returns the value (clock_t) (−1). If the value cannot be represented,
the function returns an unspecified value388) .
7.29.2.2 The difftime function
Synopsis
1 #include <time.h>
double difftime(time_t time1, time_t time0);
Description
2 The difftime function computes the difference between two calendar times: time1 - time0.
Returns
3 The difftime function returns the difference expressed in seconds as a double.
386) The tv_sec member is a linear count of seconds and might not have the normal semantics of a time_t.
387) The range [0, 60] for tm_sec allows for a positive leap second.
388) This could be due to overflow of the clock_t type.
7.29.2.3 The mktime function
Synopsis
1 #include <time.h>
time_t mktime(struct tm *timeptr);
Description
2 The mktime function converts the broken-down time, expressed as local time, in the structure
pointed to by timeptr into a calendar time value with the same encoding as that of the values
returned by the time function. The original values of the tm_wday and tm_yday components of the
structure are ignored, and the original values of the other components are not restricted to the ranges
indicated above. 389) On successful completion, the values of the tm_wday and tm_yday components
of the structure are set appropriately, and the other components are set to represent the specified
calendar time, but with their values forced to the ranges indicated above; the final value of tm_mday
is not set until tm_mon and tm_year are determined.
Returns
3 The mktime function returns the specified calendar time encoded as a value of type time_t. If the
calendar time cannot be represented, the function returns the value (time_t) (−1).
4 EXAMPLE What day of the week is July 4, 2001?
#include <stdio.h>
#include <time.h>
static const char *const wday[] = {
"Sunday", "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "-unknown-"
};
struct tm time_str;
/* ... */
time_str.tm_year = 2001 - 1900;
time_str.tm_mon = 7 - 1;
time_str.tm_mday = 4;
time_str.tm_hour = 0;
time_str.tm_min = 0;
time_str.tm_sec = 1;
time_str.tm_isdst = -1;
if (mktime(&time_str) == (time_t)(-1))
time_str.tm_wday = 7;
printf("%s\n", wday[time_str.tm_wday]);
7.29.2.4 The timegm function
Synopsis
1 #include <time.h>
time_t timegm(struct tm *timeptr);
Description
2 The timegm function converts the broken-down time, expressed as UTC time, in the structure
pointed to by timeptr into a calendar time value with the same encoding as that of the values
returned by the time function. The original values of the tm_wday and tm_yday components of the
structure are ignored, and the original values of the other components are not restricted to the ranges
indicated above. On successful completion, the values of the tm_wday and tm_yday components
of the structure are set appropriately, and the other components are set to represent the specified
389) Thus, a positive or zero value for tm_isdst causes the mktime function to presume initially that Daylight Saving Time,
respectively, is or is not in effect for the specified time. A negative value causes it to attempt to determine whether Daylight
Saving Time is in effect for the specified time.
calendar time, but with their values forced to the ranges indicated above; the final value of tm_mday
is not set until tm_mon and tm_year are determined.
Returns
3 The timegm function returns the specified calendar time encoded as a value of type time_t. If the
calendar time cannot be represented, the function returns the value (time_t)(-1) .
7.29.2.5 The time function
Synopsis
1 #include <time.h>
time_t time(time_t *timer);
Description
2 The time function determines the current calendar time. The encoding of the value is unspecified.
Returns
3 The time function returns the implementation’s best approximation to the current calendar time.
The value (time_t) (−1) is returned if the calendar time is not available. If timer is not a null
pointer, the return value is also assigned to the object it points to.
7.29.2.6 The timespec_get function
Synopsis
1 #include <time.h>
int timespec_get(struct timespec *ts, int base);
Description
2 The timespec_get function sets the interval pointed to by ts to hold the current calendar time
based on the specified time base.
3 If base is TIME_UTC, the tv_sec member is set to the number of seconds since an implementation-
defined epoch, truncated to a whole value and the tv_nsec member is set to the integral num-
ber of nanoseconds, rounded to the resolution of the system clock390) . The optional time base
TIME_MONOTONIC is the same, but the reference point is an implementation-defined time point;
different program invocations need not refer to the same reference points391) . For the same program
invocation, the results of two calls to timespec_get with TIME_MONOTONIC such that the first hap-
pens before the second shall not be decreasing. It is implementation-defined if TIME_MONOTONIC
accounts for time during which the execution environment is suspended392) . For the optional time
bases TIME_ACTIVE and TIME_THREAD_ACTIVE the result is similar, but the call measures the amount
of active processing time associated with the whole program invocation or with the calling thread,
respectively.
Returns
4 If the timespec_get function is successful it returns the nonzero value base; otherwise, it returns
zero.
Recommended practice
5 It is recommended practice that timing results of calls to timespec_get with TIME_ACTIVE, if
defined, and of calls to clock are as close to each other as their types, value ranges, and resolutions
(obtained with timespec_getres and CLOCKS_PER_SEC, respectively) allow. Because of its wider
value range and improved indications on error, timespec_get with time base TIME_ACTIVE should
be used instead of clock by new code whenever possible.
390) Although a struct timespec object describes times with nanosecond resolution, the available resolution is system
dependent and could even be greater than 1 second.
391) Commonly, this reference point is the boot time of the execution environment or the start of the execution.
392) The execution environment may, for example, not be able to track physical time that elapsed during suspension in a low
power consumption mode.
7.29.2.7 The timespec_getres function
Synopsis
1 #include <time.h>
int timespec_getres(struct timespec *ts, int base);
Description
2 If ts is non-null and base is supported by the timespec_get function, the timespec_getres
function returns the resolution of the time provided by the timespec_get function for base
in the timespec structure pointed to by ts. For each supported base, multiple calls to the
timespec_getres function during the same program execution shall have identical results.
Returns
3 If the value base is supported by the timespec_get function, the timespec_getres function returns
the nonzero value base; otherwise, it returns zero.
7.29.3 Time conversion functions
1 Functions with a _r suffix place the result of the conversion into the buffer referred by buf and
return that pointer. These functions and the function strftime shall not be subject to data races,
unless the time or calendar state is changed in a multi-thread execution.393)
2 Functions asctime, ctime, gmtime, and localtime are the same as their counterparts suffixed with
_r. In place of the parameter buf, these functions use a pointer to an object and return it: one or two
broken-down time structures (for gmtime and localtime) or an array of char (commonly used by
asctime and ctime). Execution of any of the functions that return a pointer to one of these static
objects may overwrite the information returned from any previous call to one of these functions that
uses the same object. These functions are not reentrant and are not required to avoid data races with
each other. Accessing the returned pointer after the thread that called the function that returned
it has exited results in undefined behavior. The implementation shall behave as if no other library
functions call these functions.
7.29.3.1 The asctime function
Synopsis
1 #include <time.h>
[[deprecated]] char *asctime(const struct tm *timeptr);
Description
2 This function is obsolescent and should be avoided in new code.
3 The asctime function converts the broken-down time in the structure pointed to by timeptr into a
string in the form
Sun Sep 16 01:03:52 1973\n\0
using the equivalent of the following algorithm.
[[deprecated]] char *asctime(const struct tm *timeptr)
{
static const char wday_name[7][3] = {
"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
};
static const char mon_name[12][3] = {
"Jan", "Feb", "Mar", "Apr", "May", "Jun",
"Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
393) This does not mean that these functions may not read global state that describes the time and calendar settings of the
execution, such as the LC_TIME locale or the implementation-defined specification of the local time zone. Only the setting of
that state by setlocale or by means of implementation-defined functions may constitute races.
};
static char result[26];
snprintf(result, 26, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
wday_name[timeptr->tm_wday],
mon_name[timeptr->tm_mon],
timeptr->tm_mday, timeptr->tm_hour,
timeptr->tm_min, timeptr->tm_sec,
1900 + timeptr->tm_year);
return result;
}
4 If any of the members of the broken-down time contain values that are outside their normal ranges394) ,
the behavior of the asctime function is undefined. Likewise, if the calculated year exceeds four
digits or is less than the year 1000, the behavior is undefined.
Returns
5 The asctime function returns a pointer to the string.
7.29.3.2 The ctime function
Synopsis
1 #include <time.h>
[[deprecated]] char *ctime(const time_t *timer);
Description
2 This function is obsolescent and should be avoided in new code.
3 The ctime function converts the calendar time pointed to by timer to local time in the form of a
string. They are equivalent to:
asctime(localtime(timer))
Returns
4 The ctime function returns the pointer returned by the asctime functions with that broken-down
time as argument.
Forward references: the localtime functions (7.29.3.4).
7.29.3.3 The gmtime functions
Synopsis
1 #include <time.h>
struct tm *gmtime(const time_t *timer);
struct tm *gmtime_r(const time_t *timer, struct tm *buf);
Description
2 The gmtime functions convert the calendar time pointed to by timer into a broken-down time,
expressed as UTC.
Returns
3 The gmtime functions return a pointer to the broken-down time, or a null pointer if the specified
time cannot be converted to UTC.
7.29.3.4 The localtime functions
394) See 7.29.1.
Synopsis
1 #include <time.h>
struct tm *localtime(const time_t *timer);
struct tm *localtime_r(const time_t *timer, struct tm *buf);
Description
2 The localtime functions converts the calendar time pointed to by timer into a broken-down time,
expressed as local time.
Returns
3 The localtime functions return a pointer to the broken-down time, or a null pointer if the specified
time cannot be converted to local time.
7.29.3.5 The strftime function
Synopsis
1 #include <time.h>
size_t strftime(char * restrict s, size_t maxsize, const char * restrict format,
const struct tm * restrict timeptr);
Description
2 The strftime function places characters into the array pointed to by s as controlled by the string
pointed to by format. The format shall be a multibyte character sequence, beginning and ending in
its initial shift state. The format string consists of zero or more conversion specifiers and ordinary
multibyte characters. A conversion specifier consists of a % character, possibly followed by an E or O
modifier character (described below), followed by a character that determines the behavior of the
conversion specifier. All ordinary multibyte characters (including the terminating null character) are
copied unchanged into the array. If copying takes place between objects that overlap, the behavior is
undefined. No more than maxsize characters are placed into the array.
3 Each conversion specifier shall be replaced by appropriate characters as described in the following
list. The appropriate characters shall be determined using the LC_TIME category of the current
locale and by the values of zero or more members of the broken-down time structure pointed to
by timeptr, as specified in brackets in the description. If any of the specified values is outside the
normal range, the characters stored are unspecified.
%a is replaced by the locale’s abbreviated weekday name. [tm_wday]
%A is replaced by the locale’s full weekday name. [tm_wday]
%b is replaced by the locale’s abbreviated month name. [tm_mon]
%B is replaced by the locale’s full month name. [tm_mon]
%c is replaced by the locale’s appropriate date and time representation. [all specified in 7.29.1]
%C is replaced by the year divided by 100 and truncated to an integer, as a decimal number (00–99).
[tm_year]
%d is replaced by the day of the month as a decimal number (01–31). [tm_mday]
%D is equivalent to "%m/%d/%y". [tm_mon, tm_mday, tm_year]
%e is replaced by the day of the month as a decimal number (1–31); a single digit is preceded by a
space. [tm_mday]
%F is equivalent to "%Y-%m-%d" (the ISO 8601 date format). [tm_year, tm_mon, tm_mday]
%g is replaced by the last 2 digits of the week-based year (see below) as a decimal number (00–99).
[tm_year, tm_wday, tm_yday]
%G is replaced by the week-based year (see below) as a decimal number (e.g., 1997). [tm_year,
tm_wday, tm_yday]
%h is equivalent to "%b". [tm_mon]
%H is replaced by the hour (24-hour clock) as a decimal number (00–23). [tm_hour]
%I is replaced by the hour (12-hour clock) as a decimal number (01–12). [tm_hour]
%j is replaced by the day of the year as a decimal number (001–366). [tm_yday]
%m is replaced by the month as a decimal number (01–12). [tm_mon]
%M is replaced by the minute as a decimal number (00–59). [tm_min]
%n is replaced by a new-line character.
%p is replaced by the locale’s equivalent of the AM/PM designations associated with a 12-hour
clock. [tm_hour]
%r is replaced by the locale’s 12-hour clock time. [tm_hour, tm_min, tm_sec]
%R is equivalent to "%H:%M". [tm_hour, tm_min ]
%S is replaced by the second as a decimal number (00–60). [tm_sec]
%t is replaced by a horizontal-tab character.
%T is equivalent to "%H:%M:%S" (the ISO 8601 time format). [tm_hour, tm_min, tm_sec]
%u is replaced by the ISO 8601 weekday as a decimal number (1–7), where Monday is 1. [tm_wday]
%U is replaced by the week number of the year (the first Sunday as the first day of week 1) as a
decimal number (00–53). [tm_year, tm_wday, tm_yday]
%V is replaced by the ISO 8601 week number (see below) as a decimal number (01–53). [tm_year,
tm_wday, tm_yday]
%w is replaced by the weekday as a decimal number (0–6), where Sunday is 0. [tm_wday]
%W is replaced by the week number of the year (the first Monday as the first day of week 1) as a
decimal number (00–53). [tm_year, tm_wday, tm_yday]
%x is replaced by the locale’s appropriate date representation. [all specified in 7.29.1]
%X is replaced by the locale’s appropriate time representation. [all specified in 7.29.1]
%y is replaced by the last 2 digits of the year as a decimal number (00–99). [tm_year]
%Y is replaced by the year as a decimal number (e.g., 1997). [tm_year]
%z is replaced by the offset from UTC in the ISO 8601 format "-0430" (meaning 4 hours 30
minutes behind UTC, west of Greenwich), or by no characters if no time zone is determinable.
[tm_isdst]
%Z is replaced by the locale’s time zone name or abbreviation, or by no characters if no time zone is
determinable. [tm_isdst]
%% is replaced by %.
4 Some conversion specifiers can be modified by the inclusion of an E or O modifier character to
indicate an alternative format or specification. If the alternative format or specification does not
exist for the current locale, the modifier is ignored.
%Ec is replaced by the locale’s alternative date and time representation.
%EC is replaced by the name of the base year (period) in the locale’s alternative representation.
%Ex is replaced by the locale’s alternative date representation.
%EX is replaced by the locale’s alternative time representation.
%Ey is replaced by the offset from %EC (year only) in the locale’s alternative representation.
%EY is replaced by the locale’s full alternative year representation.
%Ob is replaced by the locale’s abbreviated alternative month name.
%OB is replaced by the locale’s alternative appropriate full month name.
%Od is replaced by the day of the month, using the locale’s alternative numeric symbols (filled as
needed with leading zeros, or with leading spaces if there is no alternative symbol for zero).
%Oe is replaced by the day of the month, using the locale’s alternative numeric symbols (filled as
needed with leading spaces).
%OH is replaced by the hour (24-hour clock), using the locale’s alternative numeric symbols.
%OI is replaced by the hour (12-hour clock), using the locale’s alternative numeric symbols.
%Om is replaced by the month, using the locale’s alternative numeric symbols.
%OM is replaced by the minutes, using the locale’s alternative numeric symbols.
%OS is replaced by the seconds, using the locale’s alternative numeric symbols.
%Ou is replaced by the ISO 8601 weekday as a number in the locale’s alternative representation,
where Monday is 1.
%OU is replaced by the week number, using the locale’s alternative numeric symbols.
%OV is replaced by the ISO 8601 week number, using the locale’s alternative numeric symbols.
%Ow is replaced by the weekday as a number, using the locale’s alternative numeric symbols.
%OW is replaced by the week number of the year, using the locale’s alternative numeric symbols.
%Oy is replaced by the last 2 digits of the year, using the locale’s alternative numeric symbols.
5 %g, %G, and %V give values according to the ISO 8601 week-based year. In this system, weeks begin
on a Monday and week 1 of the year is the week that includes January 4th, which is also the week
that includes the first Thursday of the year, and is also the first week that contains at least four days
in the year. If the first Monday of January is the 2nd, 3rd, or 4th, the preceding days are part of the
last week of the preceding year; thus, for Saturday 2nd January 1999, %G is replaced by 1998 and %V
is replaced by 53. If December 29th, 30th, or 31st is a Monday, it and any following days are part of
week 1 of the following year. Thus, for Tuesday 30th December 1997, %G is replaced by 1998 and %V
is replaced by 01.
6 If a conversion specifier is not one of the above, the behavior is undefined.
7 In the "C" locale, the E and O modifiers are ignored and the replacement strings for the following
specifiers are:
%a the first three characters of %A.
%A one of "Sunday", "Monday", . . . , "Saturday".
%b the first three characters of %B.
%B one of "January", "February", . . . , "December".
%c equivalent to "%a %b %e %T %Y".
%p one of "AM" or "PM".
%r equivalent to "%I:%M:%S %p".
%x equivalent to "%m/%d/%y".
%X equivalent to %T.
%Z implementation-defined.
Returns
8 If the total number of resulting characters including the terminating null character is not more than
maxsize, the strftime function returns the number of characters placed into the array pointed to
by s not including the terminating null character. Otherwise, zero is returned and the members of
the array have an indeterminate representation.
7.30 Unicode utilities <uchar.h>
1 The header <uchar.h> declares types and functions for manipulating Unicode characters.
2 The types declared are mbstate_t (described in 7.31.1) and size_t (described in 7.21);
char8_t
which is an unsigned integer type used for 8-bit characters and is the same type as unsigned char;
char16_t
which is an unsigned integer type used for 16-bit characters and is the same type as uint_least16_t
(described in 7.22.1.2); and
char32_t
which is an unsigned integer type used for 32-bit characters and is the same type as uint_least32_t
(also described in 7.22.1.2).
7.30.1 Restartable multibyte/wide character conversion functions
1 These functions have a parameter, ps, of type pointer to mbstate_t that points to an object that can
completely describe the current conversion state of the associated multibyte character sequence,
which the functions alter as necessary. If ps is a null pointer, each function uses its own internal
mbstate_t object instead, which is initialized prior to the first call to the function to the initial
conversion state; the functions are not required to avoid data races with other calls to the same
function in this case. It is implementation-defined whether the internal mbstate_t object has thread
storage duration; if it has thread storage duration, it is initialized to the initial conversion state
prior to the first call to the function on the new thread. The implementation behaves as if no library
function calls these functions with a null pointer for ps.
7.30.1.1 The mbrtoc8 function
Synopsis
1 #include <uchar.h>
size_t mbrtoc8(char8_t * restrict pc8, const char * restrict s, size_t n,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the mbrtoc8 function is equivalent to the call:
mbrtoc8(NULL, "", 1, ps)
In this case, the values of the parameters pc8 and n are ignored.
3 If s is not a null pointer, the mbrtoc8 function function inspects at most n bytes beginning with
the byte pointed to by s to determine the number of bytes needed to complete the next multibyte
character (including any shift sequences). If the function determines that the next multibyte character
is complete and valid, it determines the values of the corresponding characters and then, if pc8 is
not a null pointer, stores the value of the first (or only) such character in the object pointed to by pc8.
Subsequent calls will store successive characters without consuming any additional input until all
the characters have been stored. If the corresponding character is the null character, the resulting
state described is the initial conversion state.
Returns
4 The mbrtoc8 function returns the first of the following that applies (given the current conversion
state):
0 if the next n or fewer bytes complete the multibyte character that corresponds to
the null character (which is the value stored).
between 1 and n inclusive if the next n or fewer bytes complete a valid multibyte character (which
is the value stored); the value returned is the number of bytes that complete the
multibyte character.
(size_t) (−3) if the next character resulting from a previous call has been stored (no bytes from
the input have been consumed by this call).
(size_t) (−2) if the next n bytes contribute to an incomplete (but potentially valid) multibyte
character, and all n bytes have been processed (no value is stored).395)
(size_t) (−1) if an encoding error occurs, in which case the next n or fewer bytes do not contribute
to a complete and valid multibyte character (no value is stored); the value of the
macro EILSEQ is stored in errno, and the conversion state is unspecified.
7.30.1.2 The c8rtomb function
Synopsis
1 #include <uchar.h>
size_t c8rtomb(char * restrict s, char8_t c8, mbstate_t * restrict ps);
Description
2 If s is a null pointer, the c8rtomb function is equivalent to the call
c8rtomb(buf, u8’\0’, ps)
where buf is an internal buffer.
3 If s is not a null pointer, the c8rtomb function determines the number of bytes needed to represent
the multibyte character that corresponds to the character given or completed by c8 (including any
shift sequences), and stores the multibyte character representation in the array whose first element is
pointed to by s, or stores nothing if c8 does not represent a complete character. At most MB_CUR_MAX
bytes are stored. If c8 is a null character, a null byte is stored, preceded by any shift sequence needed
to restore the initial shift state; the resulting state described is the initial conversion state.
Returns
4 The c8rtomb function returns the number of bytes stored in the array object (including any shift
sequences). When c8 is not a valid character, an encoding error occurs: the function stores the value
of the macro EILSEQ in errno and returns (size_t) (−1); the conversion state is unspecified.
7.30.1.3 The mbrtoc16 function
Synopsis
1 #include <uchar.h>
size_t mbrtoc16(char16_t * restrict pc16, const char * restrict s, size_t n,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the mbrtoc16 function is equivalent to the call:
mbrtoc16(NULL, "", 1, ps)
In this case, the values of the parameters pc16 and n are ignored.
3 If s is not a null pointer, the mbrtoc16 function inspects at most n bytes beginning with the byte
pointed to by s to determine the number of bytes needed to complete the next multibyte character
(including any shift sequences). If the function determines that the next multibyte character is
395) When n has at least the value of the MB_CUR_MAX macro, this case can only occur if s points at a sequence of redundant
shift sequences (for implementations with state-dependent encodings).
complete and valid, it determines the values of the corresponding wide characters and then, if pc16
is not a null pointer, stores the value of the first (or only) such character in the object pointed to by
pc16. Subsequent calls will store successive wide characters without consuming any additional
input until all the characters have been stored. If the corresponding wide character is the null wide
character, the resulting state described is the initial conversion state.
Returns
4 The mbrtoc16 function returns the first of the following that applies (given the current conversion
state):
0 if the next n or fewer bytes complete the multibyte character that corresponds to
the null wide character (which is the value stored).
between 1 and n inclusive if the next n or fewer bytes complete a valid multibyte character (which
is the value stored); the value returned is the number of bytes that complete the
multibyte character.
(size_t) (−3) if the next character resulting from a previous call has been stored (no bytes from
the input have been consumed by this call).
(size_t) (−2) if the next n bytes contribute to an incomplete (but potentially valid) multibyte
character, and all n bytes have been processed (no value is stored).396)
(size_t) (−1) if an encoding error occurs, in which case the next n or fewer bytes do not contribute
to a complete and valid multibyte character (no value is stored); the value of the
macro EILSEQ is stored in errno, and the conversion state is unspecified.
7.30.1.4 The c16rtomb function
Synopsis
1 #include <uchar.h>
size_t c16rtomb(char * restrict s, char16_t c16, mbstate_t * restrict ps);
Description
2 If s is a null pointer, the c16rtomb function is equivalent to the call
c16rtomb(buf, u’\0’, ps)
where buf is an internal buffer.
3 If s is not a null pointer, the c16rtomb function determines the number of bytes needed to represent
the multibyte character that corresponds to the wide character given or completed by c16 (including
any shift sequences), and stores the multibyte character representation in the array whose first
element is pointed to by s, or stores nothing if c16 does not represent a complete character. At
most MB_CUR_MAX bytes are stored. If c16 is a null wide character, a null byte is stored, preceded by
any shift sequence needed to restore the initial shift state; the resulting state described is the initial
conversion state.
Returns
4 The c16rtomb function returns the number of bytes stored in the array object (including any shift
sequences). When c16 is not a valid wide character, an encoding error occurs: the function stores the
value of the macro EILSEQ in errno and returns (size_t) (−1); the conversion state is unspecified.
7.30.1.5 The mbrtoc32 function
396) When n has at least the value of the MB_CUR_MAX macro, this case can only occur if s points at a sequence of redundant
shift sequences (for implementations with state-dependent encodings).
Synopsis
1 #include <uchar.h>
size_t mbrtoc32(char32_t * restrict pc32, const char * restrict s, size_t n,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the mbrtoc32 function is equivalent to the call:
mbrtoc32(NULL, "", 1, ps)
In this case, the values of the parameters pc32 and n are ignored.
3 If s is not a null pointer, the mbrtoc32 function inspects at most n bytes beginning with the byte
pointed to by s to determine the number of bytes needed to complete the next multibyte character
(including any shift sequences). If the function determines that the next multibyte character is
complete and valid, it determines the values of the corresponding wide characters and then, if pc32
is not a null pointer, stores the value of the first (or only) such character in the object pointed to by
pc32. Subsequent calls will store successive wide characters without consuming any additional
input until all the characters have been stored. If the corresponding wide character is the null wide
character, the resulting state described is the initial conversion state.
Returns
4 The mbrtoc32 function returns the first of the following that applies (given the current conversion
state):
0 if the next n or fewer bytes complete the multibyte character that corresponds to
the null wide character (which is the value stored).
between 1 and n inclusive if the next n or fewer bytes complete a valid multibyte character (which
is the value stored); the value returned is the number of bytes that complete the
multibyte character.
(size_t) (−3) if the next character resulting from a previous call has been stored (no bytes from
the input have been consumed by this call).
(size_t) (−2) if the next n bytes contribute to an incomplete (but potentially valid) multibyte
character, and all n bytes have been processed (no value is stored).397)
(size_t) (−1) if an encoding error occurs, in which case the next n or fewer bytes do not contribute
to a complete and valid multibyte character (no value is stored); the value of the
macro EILSEQ is stored in errno, and the conversion state is unspecified.
7.30.1.6 The c32rtomb function
Synopsis
1 #include <uchar.h>
size_t c32rtomb(char * restrict s, char32_t c32, mbstate_t * restrict ps);
Description
2 If s is a null pointer, the c32rtomb function is equivalent to the call
c32rtomb(buf, U’\0’, ps)
where buf is an internal buffer.
397) When n has at least the value of the MB_CUR_MAX macro, this case can only occur if s points at a sequence of redundant
shift sequences (for implementations with state-dependent encodings).
3 If s is not a null pointer, the c32rtomb function determines the number of bytes needed to represent
the multibyte character that corresponds to the wide character given by c32 (including any shift
sequences), and stores the multibyte character representation in the array whose first element is
pointed to by s. At most MB_CUR_MAX bytes are stored. If c32 is a null wide character, a null byte is
stored, preceded by any shift sequence needed to restore the initial shift state; the resulting state
described is the initial conversion state.
Returns
4 The c32rtomb function returns the number of bytes stored in the array object (including any shift
sequences). When c32 is not a valid wide character, an encoding error occurs: the function stores the
value of the macro EILSEQ in errno and returns (size_t) (−1);the conversion state is unspecified.
7.31 Extended multibyte and wide character utilities <wchar.h>
7.31.1 Introduction
1 The header <wchar.h> defines four macros, and declares four data types, one tag, and many
functions.398)
2 The types declared are wchar_t and size_t (both described in 7.21);
mbstate_t
which is a complete object type other than an array type that can hold the conversion state informa-
tion necessary to convert between sequences of multibyte characters and wide characters;
wint_t
which is an integer type unchanged by default argument promotions that can hold any value
corresponding to members of the extended character set, as well as at least one value that does not
correspond to any member of the extended character set (see WEOF below);399) and
struct tm
which is declared as an incomplete structure type (the contents are described in 7.29.1).
3 The macros defined are NULL (described in 7.21); WCHAR_MIN, WCHAR_MAX, and WCHAR_WIDTH (de-
scribed in 7.22); and
WEOF
which expands to a constant expression of type wint_t whose value does not correspond to any
member of the extended character set.400) It is accepted (and returned) by several functions in
this subclause to indicate end-of-file, that is, no more input from a stream. It is also used as a wide
character value that does not correspond to any member of the extended character set.
4 The functions declared are grouped as follows:
— Functions that perform input and output of wide characters, or multibyte characters, or both;
— Functions that provide wide string numeric conversion;
— Functions that perform general wide string manipulation;
— Functions for wide string date and time conversion; and
— Functions that provide extended capabilities for conversion between multibyte and wide
character sequences.
5 Arguments to the functions in this subclause may point to arrays containing wchar_t values that do
not correspond to members of the extended character set. Such values shall be processed according
to the specified semantics, except that it is unspecified whether an encoding error occurs if such a
value appears in the format string for a function in 7.31.2 or 7.31.5 and the specified semantics do
not require that value to be processed by wcrtomb.
6 Unless explicitly stated otherwise, if the execution of a function described in this subclause causes
copying to take place between objects that overlap, the behavior is undefined.
7.31.2 Formatted wide character input/output functions
1 The formatted wide character input/output functions shall behave as if there is a sequence point
after the actions associated with each specifier.401)
398) See "future library directions" (7.33.20).
399) wchar_t and wint_t can be the same integer type.
400) The value of the macro WEOF can differ from that of EOF and need not be negative.
401) The fwprintf functions perform writes to memory for the %n specifier.
7.31.2.1 The fwprintf function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
int fwprintf(FILE * restrict stream, const wchar_t * restrict format, ...);
Description
2 The fwprintf function writes output to the stream pointed to by stream, under control of the wide
string pointed to by format that specifies how subsequent arguments are converted for output. If
there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted
while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored.
The fwprintf function returns when the end of the format string is encountered.
3 The format is composed of zero or more directives: ordinary wide characters (not %), which are
copied unchanged to the output stream; and conversion specifications, each of which results in
fetching zero or more subsequent arguments, converting them, if applicable, according to the
corresponding conversion specifier, and then writing the result to the output stream.
4 Each conversion specification is introduced by the wide character %. After the %, the following
appear in sequence:
— Zero or more flags (in any order) that modify the meaning of the conversion specification.
— An optional minimum field width. If the converted value has fewer wide characters than the
field width, it is padded with spaces (by default) on the left (or right, if the left adjustment flag,
described later, has been given) to the field width. The field width takes the form of an asterisk
* (described later) or a nonnegative decimal integer.402)
— An optional precision that gives the minimum number of digits to appear for the b, d, i, o, u,
x, and X conversions, the number of digits to appear after the decimal-point wide character
for a, A, e, E, f, and F conversions, the maximum number of significant digits for the g and G
conversions, or the maximum number of wide characters to be written for s conversions. The
precision takes the form of a period (.) followed either by an asterisk * (described later) or by
an optional nonnegative decimal integer; if only the period is specified, the precision is taken
as zero. If a precision appears with any other conversion specifier, the behavior is undefined.
— An optional length modifier that specifies the size of the argument.
— A conversion specifier wide character that specifies the type of conversion to be applied.
5 As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case,
an int argument supplies the field width or precision. The arguments specifying field width, or
precision, or both, shall appear (in that order) before the argument (if any) to be converted. A
negative field width argument is taken as a - flag followed by a positive field width. A negative
precision argument is taken as if the precision were omitted.
6 The flag wide characters and their meanings are:
- The result of the conversion is left-justified within the field. (It is right-justified if this flag is
not specified.)
+ The result of a signed conversion always begins with a plus or minus sign. (It begins with a
sign only when a value with a negative sign is converted if this flag is not specified.) 403)
space If the first wide character of a signed conversion is not a sign, or if a signed conversion results
in no wide characters, a space is prefixed to the result. If the space and + flags both appear,
the space flag is ignored.
402) Note that 0 is taken as a flag, not as the beginning of a field width.
403) The results of all floating conversions of a negative zero, and of negative values that round to zero, include a minus sign.
# The result is converted to an "alternative form". For o conversion, it increases the precision, if
and only if necessary, to force the first digit of the result to be a zero (if the value and precision
are both 0, a single 0 is printed). For b conversion, a nonzero result has 0b prefixed to it. For
x (or X) conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G
conversions, the result of converting a floating-point number always contains a decimal-point
wide character, even if no digits follow it. (Normally, a decimal-point wide character appears
in the result of these conversions only if a digit follows it.) For g and G conversions, trailing
zeros are not removed from the result. For other conversions, the behavior is undefined.
0 For b, d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, leading zeros (following any
indication of sign or base) are used to pad to the field width rather than performing space
padding, except when converting an infinity or NaN. If the 0 and - flags both appear, the
0 flag is ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag is
ignored. For other conversions, the behavior is undefined.
7 The length modifiers and their meanings are:
hh Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a
signed char or unsigned char argument (the argument will have been promoted
according to the integer promotions, but its value shall be converted to signed char or
unsigned char before printing); or that a following n conversion specifier applies to a
pointer to a signed char argument.
h Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a short int
or unsigned short int argument (the argument will have been promoted accord-
ing to the integer promotions, but its value shall be converted to short int or
unsigned short int before printing); or that a following n conversion specifier applies
to a pointer to a short int argument.
l (ell) Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a long int
or unsigned long int argument; that a following n conversion specifier applies to
a pointer to a long int argument; that a following c conversion specifier applies to
a wint_t argument; that a following s conversion specifier applies to a pointer to a
wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion
specifier.
ll (ell-ell) Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a
long long int or unsigned long long int argument; or that a following n con-
version specifier applies to a pointer to a long long int argument.
j Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to an intmax_t
or uintmax_t argument; or that a following n conversion specifier applies to a pointer
to an intmax_t argument.
z Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a size_t
or the corresponding signed integer type argument; or that a following n conversion
specifier applies to a pointer to a signed integer type corresponding to size_t argument.
t Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t
or the corresponding unsigned integer type argument; or that a following n conversion
specifier applies to a pointer to a ptrdiff_t argument.
wN Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to an integer
argument with a specific width where N is a positive decimal integer with no leading
zeros (the argument will have been promoted according to the integer promotions, but
its value shall be converted to the unpromoted type); or that a following n conversion
specifier applies to a pointer to an integer type argument with a width of N bits. All
minimum-width integer types (7.22.1.2) and exact-width integer types (7.22.1.1) de-
fined in the header <stdint.h> shall be supported. Other supported values of N are
implementation-defined.
wfN Specifies that a following b, d, i, o, u, x, or X conversion specifier applies to a fastest
minimum-width integer argument with a specific width where N is a positive decimal
integer with no leading zeros (the argument will have been promoted according to
the integer promotions, but its value shall be converted to the unpromoted type); or
that a following n conversion specifier applies to a pointer to a fastest minimum-width
integer type argument with a width of N bits. All fastest minimum-width integer types
(7.22.1.3) defined in the header <stdint.h> shall be supported. Other supported values
of N are implementation-defined.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
long double argument.
H Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
_Decimal32 argument.
D Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
_Decimal64 argument.
DD Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
_Decimal128 argument.
If a length modifier appears with any conversion specifier other than as specified above, the behavior
is undefined.
8 The conversion specifiers and their meanings are:
d,i The int argument is converted to signed decimal in the style [-]dddd. The precision
specifies the minimum number of digits to appear; if the value being converted can be
represented in fewer digits, it is expanded with leading zeros. The default precision is 1.
The result of converting a zero value with a precision of zero is no wide characters.
b, o,u,x,X The unsigned int argument is converted to unsigned binary (b), unsigned octal (o),
unsigned decimal (u), or unsigned hexadecimal notation (x or X) in the style dddd; the
letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The
precision specifies the minimum number of digits to appear; if the value being converted
can be represented in fewer digits, it is expanded with leading zeros. The default precision
is 1. The result of converting a zero value with a precision of zero is no wide characters.
f,F A double argument representing a floating-point number is converted to decimal notation
in the style [-]ddd.ddd, where the number of digits after the decimal-point wide character
is equal to the precision specification. If the precision is missing, it is taken as 6; if the
precision is zero and the # flag is not specified, no decimal-point wide character appears.
If a decimal-point wide character appears, at least one digit appears before it. The value is
rounded to the appropriate number of digits.
A double argument representing an infinity is converted in one of the styles [-]inf or
[-]infinity — which style is implementation-defined. A double argument representing
a NaN is converted in one of the styles [-]nan or [-]nan(n-wchar-sequence) — which style,
and the meaning of any n-wchar-sequence, is implementation-defined. The F conversion
specifier produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.404)
e,E A double argument representing a floating-point number is converted in the style
[-]d.ddde±dd, where there is one digit (which is nonzero if the argument is nonzero)
before the decimal-point wide character and the number of digits after it is equal to the
precision; if the precision is missing, it is taken as 6; if the precision is zero and the #
flag is not specified, no decimal-point wide character appears. The value is rounded to
the appropriate number of digits. The E conversion specifier produces a number with E
instead of e introducing the exponent. The exponent always contains at least two digits,
404) When applied to infinite and NaN values, the -, +, and space flag wide characters have their usual meaning; the # and 0
flag wide characters have no effect.
and only as many more digits as necessary to represent the exponent. If the value is zero,
the exponent is zero.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
g,G A double argument representing a floating-point number is converted in style f or e (or
in style F or E in the case of a G conversion specifier), depending on the value converted
and the precision. Let P equal the precision if nonzero, 6 if the precision is omitted, or 1 if
the precision is zero. Then, if a conversion with style E would have an exponent of X:
if P > X ≥ −4, the conversion is with style f (or F) and precision P − (X + 1).
otherwise, the conversion is with style e (or E) and precision P − 1.
Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion
of the result and the decimal-point wide character is removed if there is no fractional
portion remaining.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
a,A A double argument representing a floating-point number is converted in the style
[-]0xh.hhhhp±d, where there is one hexadecimal digit (which is nonzero if the argument is a
normalized floating-point number and is otherwise unspecified) before the decimal-point
wide character405) and the number of hexadecimal digits after it is equal to the precision;
if the precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient
for an exact representation of the value; if the precision is missing and FLT_RADIX is not a
power of 2, then the precision is sufficient to distinguish406) values of type double, except
that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no
decimal-point wide character appears. The letters abcdef are used for a conversion and
the letters ABCDEF for A conversion. The A conversion specifier produces a number with
X and P instead of x and p. The exponent always contains at least one digit, and only as
many more digits as necessary to represent the decimal exponent of 2. If the value is zero,
the exponent is zero.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
If an H, D, or DD modifier is present and the precision is missing, then for a decimal
floating type argument represented by a triple of integers (s, c, q), where n is the number
of significant digits in the coefficient c,
— if −(n + 5) ≤ q ≤ 0, use style f (or style F in the case of an A conversion specifier)
with formatting precision equal to −q,
— otherwise, use style e (or style E in the case of an A conversion specifier) with format-
ting precision equal to n − 1, with the exceptions that if c = 0 then the digit-sequence
in the exponent-part shall have the value q (rather than 0), and that the exponent is
405) Binary implementations can choose the hexadecimal digit to the left of the decimal-point wide character so that subsequent
digits align to nibble (4-bit) boundaries. This implementation choice affects numerical values printed with a precision P
that is insufficient to represent all values exactly. Implementations with different conventions about the most significant
hexadecimal digit will round at different places, affecting the numerical value of the hexadecimal result. For example,
possible printed output for the code
#include <stdio.h>
/* ... */
double x = 123.0;
printf("%.1a", x);
include "0x1.fp+6 " and "0xf.6p+3 " whose numerical values are 124 and 123, respectively. Portable code seeking identical
numerical results on different platforms should avoid precisions P that require rounding.
406) The formatting precision P is sufficient to distinguish values of the source type if 16P > bp where b (not a power of 2)
and p are the base and precision of the source type (5.2.4.2.2). A smaller P might suffice depending on the implementation’s
scheme for determining the digit to the left of the decimal-point wide character.
always expressed with the minimum number of digits required to represent its value
(the exponent never contains a leading zero).
If the precision P is present (in the conversion specification) and is zero or at least as
large as the precision p (5.2.4.2.2) of the decimal floating type, the conversion is as if the
precision were missing. If the precision P is present (and nonzero) and less than the
precision p of the decimal floating type, the conversion first obtains an intermediate result
as follows, where n is the number of significant digits in the coefficient:
— If n ≤ P , set the intermediate result to the input.
— If n > P , round the input value, according to the current rounding direction for
decimal floating-point operations, to P decimal digits, with unbounded exponent
range, representing the result with a P -digit integer coefficient when in the form
(s, c, q).
Convert the intermediate result in the manner described above for the case where the
precision is missing.
c If no l length modifier is present, the int argument is converted to a wide character as if
by calling btowc and the resulting wide character is written.
If an l length modifier is present, the wint_t argument is converted to wchar_t and
written.
s If no l length modifier is present, the argument shall be a pointer to storage of character
type containing a multibyte character sequence beginning in the initial shift state. Charac-
ters from the storage are converted as if by repeated calls to the mbrtowc function, with
the conversion state described by an mbstate_t object initialized to zero before the first
multibyte character is converted, and written up to (but not including) the terminating
null wide character. If the precision is specified, no more than that many wide characters
are written. If the precision is not specified or is greater than the size of the converted
storage, the converted storage shall contain a null wide character.
If an l length modifier is present, the argument shall be a pointer to storage of wchar_t
type. Wide characters from the storage are written up to (but not including) a terminating
null wide character. If the precision is specified, no more than that many wide characters
are written. If the precision is not specified or is greater than the size of the array, the
storage shall contain a null wide character.
p The argument shall be a pointer to void or a pointer to a character type. The value of
the pointer is converted to a sequence of printing wide characters, in an implementation-
defined manner.
n The argument shall be a pointer to signed integer whose type is specified by the length
modifiers, if any, for the conversion specification, or shall be int if no length modifiers
are specified for the conversion specification. The number of wide characters written to
the output stream so far by this call to fwprintf is stored into the integer object pointed
to by the argument. No argument is converted, but one is consumed. If the conversion
specification includes any flags, a field width, or a precision, the behavior is undefined.
% A % wide character is written. No argument is converted. The complete conversion
specification shall be %%.
9 If a conversion specification is invalid, the behavior is undefined.407) fwprintf shall behave as if it
uses va_arg with a type argument naming the type resulting from applying the default argument
promotions to the type corresponding to the conversion specification and then converting the result
of the va_arg expansion to the type corresponding to the conversion specification.408)
10 In no case does a nonexistent or small field width cause truncation of a field; if the result of a
conversion is wider than the field width, the field is expanded to contain the conversion result.
407) See "future library directions" (7.33.20).
408) The behavior is undefined when the types differ as specified for va_arg 7.16.1.1.
11 For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal
floating number with the given precision.
Recommended practice
12 For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable
in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating
style with the given precision, with the extra stipulation that the error should have a correct sign for
the current rounding direction.
13 For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most the maximum
value M of the T_DECIMAL_DIG macros (defined in <float.h>), then the result should be correctly
rounded.409) If the number of significant decimal digits is more than M but the source value is
exactly representable with M digits, then the result should be an exact representation with trailing
zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having
M significant digits; the value of the resultant decimal string D should satisfy L ≤ D ≤ U, with the
extra stipulation that the error should have a correct sign for the current rounding direction.
14 An uppercase B format specifier is not covered by the description above, because it used to be
available for extensions in previous versions of this standard.
Implementations that did not use an uppercase B as their own extension before are encouraged to
implement it similar to conversion specifier b as standardized above, with the alternative form (#B)
generating 0B as prefix for nonzero values.
Returns
15 The fwprintf function returns the number of wide characters transmitted, or a negative value if
an output or encoding error occurred or if the implementation does not support a specified width
length modifier.
Environmental limits
16 The number of wide characters that can be produced by any single conversion shall be at least 4095.
17 EXAMPLE To print a date and time in the form "Sunday, July 3, 10:02" followed by π to five decimal places:
#include <math.h>
#include <stdio.h>
#include <wchar.h>
/* ... */
wchar_t *weekday, *month; // pointers to wide strings
int day, hour, min;
fwprintf(stdout, L"%ls, %ls %d, %.2d:%.2d\n",
weekday, month, day, hour, min);
fwprintf(stdout, L"pi = %.5f\n", 4 * atan(1.0));
18 EXAMPLE 1 In this example, multibyte characters do not have a state-dependent encoding, and the members of the extended
character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a □
and the second by an uppercase letter.
19 Given the following wide string with length seven,
static wchar_t wstr[] = L"□X□Yabc□Z□W";
the seven calls
fprintf(stdout, "|1234567890123|\n");
fprintf(stdout, "|%13ls|\n", wstr);
fprintf(stdout, "|%-13.9ls|\n", wstr);
fprintf(stdout, "|%13.10ls|\n", wstr);
fprintf(stdout, "|%13.11ls|\n", wstr);
fprintf(stdout, "|%13.15ls|\n", &wstr[2]);
409) For binary-to-decimal conversion, the result format’s values are the numbers representable with the given format specifier.
The number of significant digits is determined by the format specifier, and in the case of fixed-point conversion by the source
value as well.
fprintf(stdout, "|%13lc|\n", (wint_t) wstr[5]);
will print the following seven lines:
|1234567890123|
| □X□Yabc□Z□W|
|□X□Yabc□Z |
| □X□Yabc□Z|
| □X□Yabc□Z□W|
| abc□Z□W|
| □Z|
20 EXAMPLE 2 Following are representations of _Decimal64 arguments as triples (s, c, q) and the corresponding character
sequences fprintf produces with "%Da":
(+1, 123, 0) 123
(−1, 123, 0) -123
(+1, 123, −2) 1.23
(+1, 123, 1) 1.23e+3
(−1, 123, 1) -1.23e+3
(+1, 123, −8) 0.00000123
(+1, 123, −9) 1.23e-7
(+1, 120, −8) 0.00000120
(+1, 120, −9) 1.20e-7
(+1, 1234567890123456, 0) 1234567890123456
(+1, 1234567890123456, 1) 1.234567890123456e+16
(+1, 1234567890123456, −1) 123456789012345.6
(+1, 1234567890123456, −21) 0.000001234567890123456
(+1, 1234567890123456, −22) 1.234567890123456e-7
(+1, 0, 0) 0
(−1, 0, 0) -0
(+1, 0, −6) 0.000000
(+1, 0, −7) 0e-7
(+1, 0, 2) 0e+2
(+1, 5, −6) 0.000005
(+1, 50, −7) 0.0000050
(+1, 5, −7) 5e-7
To illustrate the effects of a precision specification, the sequence:
_Decimal32 x = 6543.00DF; // (+1, 654300, -2)
fprintf(stdout, "%Ha\n", x);
fprintf(stdout, "%.6Ha\n", x);
fprintf(stdout, "%.5Ha\n", x);
fprintf(stdout, "%.4Ha\n", x);
fprintf(stdout, "%.3Ha\n", x);
fprintf(stdout, "%.2Ha\n", x);
fprintf(stdout, "%.1Ha\n", x);
fprintf(stdout, "%.0Ha\n", x);
assuming default rounding, results in:
6543.00
6543.00
6543.0
6543
6.54e+3
6.5e+3
7e+3
6543.00
To illustrate the effects of the exponent range, the sequence:
_Decimal32 x = 9543210e87DF; // (+1, 9543210, 87)
_Decimal32 y = 9500000e90DF; // (+1, 9500000, 90)
fprintf(stdout, "%.6Ha\n", x);
fprintf(stdout, "%.5Ha\n", x);
fprintf(stdout, "%.4Ha\n", x);
fprintf(stdout, "%.3Ha\n", x);
fprintf(stdout, "%.2Ha\n", x);
fprintf(stdout, "%.1Ha\n", x);
fprintf(stdout, "%.1Ha\n", y);
assuming default rounding, results in:
9.54321e+93
9.5432e+93
9.543e+93
9.54e+93
9.5e+93
1e+94
1e+97
To further illustrate the effects of the exponent range, the sequence:
_Decimal32 x = 9512345e90DF; // (+1, 9512345, 90)
_Decimal32 y = 9512345e86DF; // (+1, 9512345, 86)
fprintf(stdout, "%.3Ha\n", x);
fprintf(stdout, "%.2Ha\n", x);
fprintf(stdout, "%.1Ha\n", x);
fprintf(stdout, "%.2Ha\n", y);
assuming default rounding, results in:
9.51e+96
9.5e+96
1e+97
9.5e+92
Forward references: the btowc function (7.31.6.1.1), the mbrtowc function (7.31.6.3.2).
7.31.2.2 The fwscanf function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
int fwscanf(FILE * restrict stream, const wchar_t * restrict format, ...);
Description
2 The fwscanf function reads input from the stream pointed to by stream, under control of the wide
string pointed to by format that specifies the admissible input sequences and how they are to be
converted for assignment, using subsequent arguments as pointers to the objects to receive the
converted input. If there are insufficient arguments for the format, the behavior is undefined. If the
format is exhausted while arguments remain, the excess arguments are evaluated (as always) but
are otherwise ignored.
3 The format is composed of zero or more directives: one or more white-space wide characters, an
ordinary wide character (neither % nor a white-space wide character), or a conversion specification.
Each conversion specification is introduced by the wide character %. After the %, the following
appear in sequence:
— An optional assignment-suppressing wide character *.
— An optional decimal integer greater than zero that specifies the maximum field width (in wide
characters).
— An optional length modifier that specifies the size of the receiving object.
— A conversion specifier wide character that specifies the type of conversion to be applied.
4 The fwscanf function executes each directive of the format in turn. When all directives have been
executed, or if a directive fails (as detailed below), the function returns. Failures are described as
input failures (due to the occurrence of an encoding error or the unavailability of input characters),
or matching failures (due to inappropriate input).
5 A directive composed of white-space wide character(s) is executed by reading input up to the first
non-white-space wide character (which remains unread), or until no more wide characters can be
read. The directive never fails.
6 A directive that is an ordinary wide character is executed by reading the next wide character of
the stream. If that wide character differs from the directive,the directive fails and the differing and
subsequent wide characters remain unread. Similarly, if end-of-file, an encoding error, or a read
error prevents a wide character from being read, the directive fails.
7 A directive that is a conversion specification defines a set of matching input sequences, as described
below for each specifier. A conversion specification is executed in the following steps:
8 Input white-space wide characters are skipped, unless the specification includes a [, c, or n speci-
fier.410)
9 An input item is read from the stream, unless the specification includes an n specifier. An input item
is defined as the longest sequence of input wide characters which does not exceed any specified
field width and which is, or is a prefix of, a matching input sequence.411) The first wide character, if
any, after the input item remains unread. If the length of the input item is zero, the execution of the
directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read
error prevented input from the stream, in which case it is an input failure.
10 Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input
wide characters) is converted to a type appropriate to the conversion specifier. If the input item is
not a matching sequence, the execution of the directive fails: this condition is a matching failure.
Unless assignment suppression was indicated by a *, the result of the conversion is placed in the
object pointed to by the first argument following the format argument that has not already received
a conversion result. If this object does not have an appropriate type, or if the result of the conversion
cannot be represented in the object, the behavior is undefined.
11 The length modifiers and their meanings are:
hh Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to signed char or unsigned char.
h Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to short int or unsigned short int.
l (ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F,
g, or G conversion specifier applies to an argument with type pointer to double; or that
a following c, s, or [ conversion specifier applies to an argument with type pointer to
wchar_t .
ll (ell-ell) Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to long long int or unsigned long long int.
j Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to intmax_t or uintmax_t.
z Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to size_t or the corresponding signed integer type.
t Specifies that a following b, d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to ptrdiff_t or the corresponding unsigned integer type.
410) These white-space wide characters are not counted against a specified field width.
411) fwscanf pushes back at most one input wide character onto the input stream. Therefore, some sequences that are
acceptable to wcstod, wcstol, etc., are unacceptable to fwscanf.
wN Specifies that a following b, d, i, o, u, x, or X, or n conversion specifier applies to an
argument which is a pointer to an integer with a specific width where N is a positive
decimal integer with no leading zeros. All minimum-width integer types (7.22.1.2) and
exact-width integer types (7.22.1.1) defined in the header <stdint.h> shall be supported.
Other supported values of N are implementation-defined.
wfN Specifies that a following b, d, i, o, u, x, or X, or n conversion specifier applies to an
argument which is a pointer to a fastest minimum-width integer with a specific width
where N is a positive decimal integer with no leading zeros. All fastest minimum-width
integer types (7.22.1.3) defined in the header <stdint.h> shall be supported. Other
supported values of N are implementation-defined.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to long double.
H Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to _Decimal32 .
D Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to _Decimal64 .
DD Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to _Decimal128 .
If a length modifier appears with any conversion specifier other than as specified above, the behavior
is undefined.
12 In the following, the type of the corresponding argument for a conversion specifier shall be a pointer
to a type determined by the length modifiers, if any, or specified by the conversion specifier. The
conversion specifiers and their meanings are:
d Matches an optionally signed decimal integer, whose format is the same as expected for
the subject sequence of the wcstol function with the value 10 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
int.
b Matches an optionally signed binary integer, whose format is the same as expected for the
subject sequence of the wcstol function with the value 2 for the base argument. Unless a
length modifier is specified, the corresponding argument shall be a pointer to unsigned
int.
i Matches an optionally signed integer, whose format is the same as expected for the subject
sequence of the wcstol function with the value 0 for the base argument. Unless a length
modifier is specified, the corresponding argument shall be a pointer to int.
o Matches an optionally signed octal integer, whose format is the same as expected for
the subject sequence of the wcstoul function with the value 8 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
unsigned int.
u Matches an optionally signed decimal integer, whose format is the same as expected for
the subject sequence of the wcstoul function with the value 10 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
unsigned int.
x Matches an optionally signed hexadecimal integer, whose format is the same as expected
for the subject sequence of the wcstoul function with the value 16 for the base argument.
Unless a length modifier is specified, the corresponding argument shall be a pointer to
unsigned int.
a,e,f,g Matches an optionally signed floating-point number, infinity, or NaN, whose format is
the same as expected for the subject sequence of the wcstod function. Unless a length
modifier is specified, the corresponding argument shall be a pointer to float.
c Matches a sequence of wide characters of exactly the number specified by the field width
(1 if no field width is present in the directive).
If no l length modifier is present, characters from the input field are converted as if
by repeated calls to the wcrtomb function, with the conversion state described by an
mbstate_t object initialized to zero before the first wide character is converted. The
corresponding argument shall be a pointer to char, signed char, unsigned char, or
void that points to storage large enough to accept the sequence. No null character is
added.
If an l length modifier is present, the corresponding argument shall be a pointer to storage
of wchar_t large enough to accept the sequence.No null wide character is added.
s Matches a sequence of non-white-space wide characters.
If no l length modifier is present, characters from the input field are converted as if
by repeated calls to the wcrtomb function, with the conversion state described by an
mbstate_t object initialized to zero before the first wide character is converted. The
corresponding argument shall be a pointer to char, signed char, unsigned char, or
void that points to storage large enough to accept the sequence and a terminating null
character, which will be added automatically.
If an l length modifier is present, the corresponding argument shall be a pointer to storage
of wchar_t large enough to accept the sequence and the terminating null wide character,
which will be added automatically.
[ Matches a nonempty sequence of wide characters from a set of expected characters (the
scanset).
If no l length modifier is present, characters from the input field are converted as if
by repeated calls to the wcrtomb function, with the conversion state described by an
mbstate_t object initialized to zero before the first wide character is converted. The
corresponding argument shall be a pointer to char, signed char, unsigned char, or
void that points to storage large enough to accept the sequence and a terminating null
character, which will be added automatically.
If an l length modifier is present, the corresponding argument shall be a pointer that
points to storage of wchar_t large enough to accept the sequence and the terminating null
wide character, which will be added automatically.
The conversion specifier includes all subsequent wide characters in the format string,
up to and including the matching right bracket (]). The wide characters between the
brackets (the scanlist) compose the scanset, unless the wide character after the left bracket
is a circumflex (^), in which case the scanset contains all wide characters that do not
appear in the scanlist between the circumflex and the right bracket. If the conversion
specifier begins with [] or [^], the right bracket wide character is in the scanlist and
the next following right bracket wide character is the matching right bracket that ends
the specification; otherwise the first following right bracket wide character is the one
that ends the specification. If a - wide character is in the scanlist and is not the first, nor
the second where the first wide character is a ^, nor the last character, the behavior is
implementation-defined.
p Matches an implementation-defined set of sequences, which should be the same as the
set of sequences that may be produced by the %p conversion of the fwprintf function.
The corresponding argument shall be a pointer to a pointer of void. The input item is
converted to a pointer value in an implementation-defined manner. If the input item is a
value converted earlier during the same program execution, the pointer that results shall
compare equal to that value; otherwise the behavior of the %p conversion is undefined.
n No input is consumed. The corresponding argument shall be a pointer of a signed integer
type. The number of wide characters read from the input stream so far by this call to the
fwscanf function is stored into the integer object pointed to by the argument. Execution
of a %n directive does not increment the assignment count returned at the completion of
execution of the fwscanf function. No argument is converted, but one is consumed. If
the conversion specification includes an assignment-suppressing wide character or a field
width, the behavior is undefined.
% Matches a single % wide character; no conversion or assignment occurs. The complete
conversion specification shall be %%.
13 If a conversion specification is invalid, the behavior is undefined.412)
14 The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f,
g, and x.
15 Trailing white-space wide characters(including new-line wide characters) are left unread unless
matched by a directive. The success of literal matches and suppressed assignments is not directly
determinable other than via the %n directive.
Returns
16 The fwscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the function returns the number of input items
assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure or if the implementation does not support a specific width length modifier.
412) See "future library directions" (7.33.20).
17 EXAMPLE 1 The call:
#include <stdio.h>
#include <wchar.h>
/* ... */
int n, i; float x; wchar_t name[50];
n = fwscanf(stdin, L"%d%f%ls", &i, &x, name);
with the input line:
25 54.32E-1 thompson
will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.
18 EXAMPLE 2 The call:
#include <stdio.h>
#include <wchar.h>
/* ... */
int i; float x; double y;
fwscanf(stdin, L"%2d%f%*d %lf", &i, &x, &y);
with input:
56789 0123 56a72
will assign to i the value 56 and to x the value 789.0, will skip past 0123, and will assign to y the value 56.0. The next wide
character read from the input stream will be a.
Forward references: the wcstod, wcstof, and wcstold functions (7.31.4.1.2), the wcstol, wcstoll,
wcstoul , and wcstoull functions (7.31.4.1.4), the wcrtomb function (7.31.6.3.3).
7.31.2.3 The swprintf function
Synopsis
1 #include <wchar.h>
int swprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format,
...);
Description
2 The swprintf function is equivalent to fwprintf, except that the argument s specifies an array of
wide characters into which the generated output is to be written, rather than written to a stream.
No more than n wide characters are written, including a terminating null wide character, which is
always added (unless n is zero).
Returns
3 The swprintf function returns the number of wide characters written in the array, not counting the
terminating null wide character, or a negative value if an encoding error occurred or if n or more
wide characters were requested to be written.
7.31.2.4 The swscanf function
Synopsis
1 #include <wchar.h>
int swscanf(const wchar_t * restrict s, const wchar_t * restrict format, ...);
Description
2 The swscanf function is equivalent to fwscanf, except that the argument s specifies a wide string
from which the input is to be obtained, rather than from a stream. Reaching the end of the wide
string is equivalent to encountering end-of-file for the fwscanf function.
Returns
3 The swscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the swscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.31.2.5 The vfwprintf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
#include <wchar.h>
int vfwprintf(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
Description
2 The vfwprintf function is equivalent to fwprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfwprintf function does not invoke the va_end macro413) .
Returns
3 The vfwprintf function returns the number of wide characters transmitted, or a negative value if
an output or encoding error occurred.
4 EXAMPLE The following shows the use of the vfwprintf function in a general error-reporting routine.
#include <stdarg.h>
#include <stdio.h>
#include <wchar.h>
void error(char *function_name, wchar_t *format, ...)
{
va_list args;
va_start(args, format);
// print out name of function causing error
fwprintf(stderr, L"ERROR in %s: ", function_name);
// print out remainder of message
vfwprintf(stderr, format, args);
va_end(args);
}
7.31.2.6 The vfwscanf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
#include <wchar.h>
int vfwscanf(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
Description
2 The vfwscanf function is equivalent to fwscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfwscanf function does not invoke the va_end macro.413)
413) As the functions vfwprintf , vswprintf , vfwscanf , vwprintf , vwscanf , and vswscanf invoke the va_arg macro, the
representation of arg after the return is indeterminate.
Returns
3 The vfwscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vfwscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.31.2.7 The vswprintf function
Synopsis
1 #include <stdarg.h>
#include <wchar.h>
int vswprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format,
va_list arg);
Description
2 The vswprintf function is equivalent to swprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vswprintf function does not invoke the va_end macro.413)
Returns
3 The vswprintf function returns the number of wide characters written in the array, not counting
the terminating null wide character, or a negative value if an encoding error occurred or if n or more
wide characters were requested to be generated.
7.31.2.8 The vswscanf function
Synopsis
1 #include <stdarg.h>
#include <wchar.h>
int vswscanf(const wchar_t * restrict s, const wchar_t * restrict format,
va_list arg);
Description
2 The vswscanf function is equivalent to swscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vswscanf function does not invoke the va_end macro.413)
Returns
3 The vswscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vswscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.31.2.9 The vwprintf function
Synopsis
1 #include <stdarg.h>
#include <wchar.h>
int vwprintf(const wchar_t * restrict format, va_list arg);
Description
2 The vwprintf function is equivalent to wprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vwprintf function does not invoke the va_end macro.413)
Returns
3 The vwprintf function returns the number of wide characters transmitted, or a negative value if an
output or encoding error occurred.
7.31.2.10 The vwscanf function
Synopsis
1 #include <stdarg.h>
#include <wchar.h>
int vwscanf(const wchar_t * restrict format, va_list arg);
Description
2 The vwscanf function is equivalent to wscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vwscanf function does not invoke the va_end macro.413)
Returns
3 The vwscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vwscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.31.2.11 The wprintf function
Synopsis
1 #include <wchar.h>
int wprintf(const wchar_t * restrict format, ...);
Description
2 The wprintf function is equivalent to fwprintf with the argument stdout interposed before the
arguments to wprintf.
Returns
3 The wprintf function returns the number of wide characters transmitted, or a negative value if an
output or encoding error occurred.
7.31.2.12 The wscanf function
Synopsis
1 #include <wchar.h>
int wscanf(const wchar_t * restrict format, ...);
Description
2 The wscanf function is equivalent to fwscanf with the argument stdin interposed before the
arguments to wscanf.
Returns
3 The wscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the wscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.
7.31.3 Wide character input/output functions
7.31.3.1 The fgetwc function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wint_t fgetwc(FILE *stream);
Description
2 If the end-of-file indicator for the input stream pointed to by stream is not set and a next wide
character is present, the fgetwc function obtains that wide character as a wchar_t converted to a
wint_t and advances the associated file position indicator for the stream (if defined).
Returns
3 If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file
indicator for the stream is set and the fgetwc function returns WEOF. Otherwise, the fgetwc function
returns the next wide character from the input stream pointed to by stream. If a read error occurs,
the error indicator for the stream is set and the fgetwc function returns WEOF. If an encoding error
occurs (including too few bytes), the error indicator for the stream is set and the value of the macro
EILSEQ is stored in errno and the fgetwc function returns WEOF.414)
7.31.3.2 The fgetws function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wchar_t *fgetws(wchar_t * restrict s, int n, FILE * restrict stream);
Description
2 The fgetws function reads at most one less than the number of wide characters specified by n from
the stream pointed to by stream into the array pointed to by s. No additional wide characters are
read after a new-line wide character (which is retained) or after end-of-file. A null wide character is
written immediately after the last wide character read into the array.
Returns
3 The fgetws function returns s if successful. If end-of-file is encountered and no characters have
been read into the array, the contents of the array remain unchanged and a null pointer is returned.
If a read or encoding error occurs during the operation, the array members have an indeterminate
representation and a null pointer is returned.
7.31.3.3 The fputwc function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wint_t fputwc(wchar_t c, FILE *stream);
Description
2 The fputwc function writes the wide character specified by c to the output stream pointed to by
stream, at the position indicated by the associated file position indicator for the stream (if defined),
and advances the indicator appropriately. If the file cannot support positioning requests, or if the
stream was opened with append mode, the character is appended to the output stream.
Returns
3 The fputwc function returns the wide character written. If a write error occurs, the error indicator
for the stream is set and fputwc returns WEOF. If an encoding error occurs, the value of the macro
EILSEQ is stored in errno and fputwc returns WEOF .
7.31.3.4 The fputws function
414) An end-of-file and a read error can be distinguished by use of the feof and ferror functions. Also, errno will be set to
EILSEQ by input/output functions only if an encoding error occurs.
Synopsis
1 #include <stdio.h>
#include <wchar.h>
int fputws(const wchar_t * restrict s, FILE * restrict stream);
Description
2 The fputws function writes the wide string pointed to by s to the stream pointed to by stream. The
terminating null wide character is not written.
Returns
3 The fputws function returns EOF if a write or encoding error occurs; otherwise, it returns a nonnega-
tive value.
7.31.3.5 The fwide function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
int fwide(FILE *stream, int mode);
Description
2 The fwide function determines the orientation of the stream pointed to by stream. If mode is greater
than zero, the function first attempts to make the stream wide oriented. If mode is less than zero,
the function first attempts to make the stream byte oriented.415) Otherwise, mode is zero and the
function does not alter the orientation of the stream.
Returns
3 The fwide function returns a value greater than zero if, after the call, the stream has wide orientation,
a value less than zero if the stream has byte orientation, or zero if the stream has no orientation.
7.31.3.6 The getwc function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wint_t getwc(FILE *stream);
Description
2 The getwc function is equivalent to fgetwc, except that if it is implemented as a macro, it may
evaluate stream more than once, so the argument should never be an expression with side effects.
Returns
3 The getwc function returns the next wide character from the input stream pointed to by stream, or
WEOF .
7.31.3.7 The getwchar function
Synopsis
1 #include <wchar.h>
wint_t getwchar(void);
Description
2 The getwchar function is equivalent to getwc with the argument stdin.
415) If the orientation of the stream has already been determined, fwide does not change it.
Returns
3 The getwchar function returns the next wide character from the input stream pointed to by stdin,
or WEOF.
7.31.3.8 The putwc function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wint_t putwc(wchar_t c, FILE *stream);
Description
2 The putwc function is equivalent to fputwc, except that if it is implemented as a macro, it may
evaluate stream more than once, so that argument should never be an expression with side effects.
Returns
3 The putwc function returns the wide character written, or WEOF.
7.31.3.9 The putwchar function
Synopsis
1 #include <wchar.h>
wint_t putwchar(wchar_t c);
Description
2 The putwchar function is equivalent to putwc with the second argument stdout.
Returns
3 The putwchar function returns the character written, or WEOF.
7.31.3.10 The ungetwc function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wint_t ungetwc(wint_t c, FILE *stream);
Description
2 The ungetwc function pushes the wide character specified by c back onto the input stream pointed
to by stream. Pushed-back wide characters will be returned by subsequent reads on that stream
in the reverse order of their pushing. A successful intervening call (with the stream pointed to by
stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed-back wide
characters for the stream. The external storage corresponding to the stream is unchanged.
3 One wide character of pushback is guaranteed, even if the call to the ungetwc function follows just
after a call to a formatted wide character input function fwscanf, vfwscanf, vwscanf, or wscanf. If
the ungetwc function is called too many times on the same stream without an intervening read or
file positioning operation on that stream, the operation may fail.
4 If the value of c equals that of the macro WEOF, the operation fails and the input stream is unchanged.
5 A successful call to the ungetwc function clears the end-of-file indicator for the stream. The value of
the file position indicator for the stream after reading or discarding all pushed-back wide characters
is the same as it was before the wide characters were pushed back.416) For a text or binary stream,
the value of its file position indicator after a successful call to the ungetwc function is unspecified
until all pushed-back wide characters are read or discarded.
416) Note that a file positioning function could further modify the file position indicator after discarding any pushed-back
wide characters.
Returns
6 The ungetwc function returns the wide character pushed back, or WEOF if the operation fails.
7.31.4 General wide string utilities
1 The header <wchar.h> declares a number of functions useful for wide string manipulation. Various
methods are used for determining the lengths of the arrays, but in all cases a wchar_t* argument
points to the initial (lowest addressed) element of the array. If an array is accessed beyond the end
of an object, the behavior is undefined.
2 Where an argument declared as size_t n determines the length of the array for a function, n can
have the value zero on a call to that function. Unless explicitly stated otherwise in the description of
a particular function in this subclause, pointer arguments on such a call shall still have valid values,
as described in 7.1.4. On such a call, a function that locates a wide character finds no occurrence, a
function that compares two wide character sequences returns zero, and a function that copies wide
characters copies zero wide characters.
7.31.4.1 Wide string numeric conversion functions
7.31.4.1.1 General
This subclause describes wide string analogs of the strtod family of functions (7.24.1.5, 7.24.1.6)417) .
7.31.4.1.2 The wcstod, wcstof, and wcstold functions
#include <wchar.h>
double wcstod(const wchar_t * restrict nptr, wchar_t ** restrict endptr);
float wcstof(const wchar_t * restrict nptr, wchar_t ** restrict endptr);
long double wcstold(const wchar_t * restrict nptr, wchar_t ** restrict endptr);
Description
1 The wcstod, wcstof, and wcstold functions convert the initial portion of the wide string pointed to
by nptr to double, float, and long double representation, respectively. First, they decompose the
input string into three parts: an initial, possibly empty, sequence of white-space wide characters, a
subject sequence resembling a floating constant or representing an infinity or NaN; and a final wide
string of one or more unrecognized wide characters, including the terminating null wide character
of the input wide string. Then, they attempt to convert the subject sequence to a floating-point
number, and return the result.
2 The expected form of the subject sequence is an optional plus or minus sign, then one of the
following:
— a nonempty sequence of decimal digits optionally containing a decimal-point wide character,
417) Wide string analogs of the strfromd family of functions (7.24.1.5, 7.24.1.6) are not provided because those conversions can
be done by using mbstowcs (7.24.8.1) to convert the result of strfromd, strfromf, and similar to wide string. For example,
the following converts double d to wide string ws with at most n-1 non-null wide characters, using style g formatting, and
computes the number nc of wide characters that would have been written had n been sufficiently large, not counting the
terminating null wide character.
#include <stdlib.h>
const size_t n = 20;
double d;
//...
// convert d to single-byte character string s
char s[n];
int nc = strfromd(s, n, "%g", d);
// convert s (regarded as a multi-byte character
// string) to wide string ws
wchar_t ws[n];
(void)mbstowcs(ws, s, n);
then an optional exponent part as defined for the corresponding single-byte characters in
6.4.4.2, excluding any digit separators (6.4.4.1);
— a 0x or 0X, then a nonempty sequence of hexadecimal digits optionally containing a decimal-
point wide character, then an optional binary exponent part as defined in 6.4.4.2, excluding
any digit separators (6.4.4.1);
— INF or INFINITY, or any other wide string equivalent except for case
— NAN or NAN(n-wchar-sequenceopt ), or any other wide string equivalent except for case in the NAN
part, where:
n-wchar-sequence:
digit
nondigit
n-wchar-sequence digit
n-wchar-sequence nondigit
The subject sequence is defined as the longest initial subsequence of the input wide string, starting
with the first non-white-space wide character, that is of the expected form. The subject sequence
contains no wide characters if the input wide string is not of the expected form.
3 If the subject sequence has the expected form for a floating-point number, the sequence of wide
characters starting with the first digit or the decimal-point wide character (whichever occurs first) is
interpreted as a floating constant according to the rules of 6.4.4.2, except that the decimal-point wide
character is used in place of a period, and that if neither an exponent part nor a decimal-point wide
character appears in a decimal floating-point number, or if a binary exponent part does not appear
in a hexadecimal floating-point number, an exponent part of the appropriate type with value zero is
assumed to follow the last digit in the string.
If the subject sequence begins with a minus sign, the sequence is interpreted as negated.418)
A wide character sequence INF or INFINITY is interpreted as an infinity, if representable in the
return type, else like a floating constant that is too large for the range of the return type. A wide
character sequence NAN or NAN(n-wchar-sequenceopt ) is interpreted as a quiet NaN, if supported in
the return type, else like a subject sequence part that does not have the expected form; the meaning
of the n-wchar sequence is implementation-defined.419)
A pointer to the final wide string is stored in the object pointed to by endptr, provided that endptr
is not a null pointer.
4 If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, the value resulting
from the conversion is correctly rounded.
5 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.
6 If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
Recommended practice
7 If the subject sequence has the hexadecimal form, FLT_RADIX is not a power of 2, and the result is
not exactly representable, the result should be one of the two numbers in the appropriate internal
format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the
error should have a correct sign for the current rounding direction.
8 If the subject sequence has the decimal form and at most M significant digits, where M is the
maximum value of the T_DECIMAL_DIG macros (defined in <float.h>), the result should be correctly
418) It is unspecified whether a minus-signed sequence is converted to a negative number directly or by negating the value
resulting from converting the corresponding unsigned sequence (see F.5); the two methods could yield different results if
rounding is toward positive or negative infinity. In either case, the functions honor the sign of zero if floating-point arithmetic
supports signed zeros.
419) An implementation can use the n-wchar sequence to determine extra information to be represented in the NaN’s
significand.
rounded. If the subject sequence D has the decimal form and more than M significant digits, consider
the two bounding, adjacent decimal strings L and U, both having M significant digits, such that the
values of L, D, and U satisfy L ≤ D ≤ U. The result should be one of the (equal or adjacent) values
that would be obtained by correctly rounding L and U according to the current rounding direction,
with the extra stipulation that the error with respect to D should have a correct sign for the current
rounding direction.420)
Returns
9 The functions return the converted value, if any. If no conversion could be performed, zero is
returned.
If the correct value overflows and default rounding is in effect (7.12.1), plus or minus HUGE_VAL,
HUGE_VALF, or HUGE_VALL is returned (according to the return type and sign of the value); if the
integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno
acquires the value of ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is
nonzero, the "overflow" floating-point exception is raised.
If the result underflows (7.12.1), the functions return a value whose magnitude is no greater
than the smallest normalized positive number in the return type; if the integer expression
math_errhandling & MATH_ERRNO is nonzero, whether errno acquires the value ERANGE is
implementation-defined; if the integer expression math_errhandling & MATH_ERREXCEPT is
nonzero, whether the "underflow" floating-point exception is raised is implementation-defined.
7.31.4.1.3 The wcstodN functions
Synopsis
1 #include <wchar.h>
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 wcstod32(const wchar_t * restrict nptr, char ** restrict endptr);
_Decimal64 wcstod64(const wchar_t * restrict nptr,char ** restrict endptr);
_Decimal128 wcstod128(const wchar_t * restrict nptr,char ** restrict endptr);
#endif
Description
2 The wcstodN functions convert the initial portion of the wide string pointed to by nptr to decimal
floating type representation. First, they decompose the input wide string into three parts: an initial,
possibly empty, sequence of white-space wide characters; a subject sequence resembling a floating
constant or representing an infinity or NaN; and a final wide string of one or more unrecognized
wide characters, including the terminating null wide character of the input wide string. Then, they
attempt to convert the subject sequence to a floating-point number, and return the result.
3 The expected form of the subject sequence is an optional plus or minus sign, then one of the
following:
— a nonempty sequence of decimal digits optionally containing a decimal-point wide character,
then an optional exponent part as defined in 6.4.4.2, excluding any digit separators (6.4.4.1)
— INF or INFINITY, ignoring case
— NAN or NAN(d-wchar-sequenceopt ), ignoring case in the NAN part, where: d-wchar-
sequence:
digit
nondigit
d-wchar-sequence digit
d-wchar-sequence nondigit
420) M is sufficiently large that L and U will usually correctly round to the same internal floating value, but if not will correctly
round to adjacent values.
The subject sequence is defined as the longest initial subsequence of the input wide string, starting
with the first non-white-space wide character, that is of the expected form. The subject sequence
contains no wide characters if the input wide string is not of the expected form.
4 If the subject sequence has the expected form for a floating-point number, the sequence of wide
characters starting with the first digit or the decimal-point wide character (whichever occurs first) is
interpreted as a floating constant according to the rules of 6.4.4.2, including correct rounding and
determination of the coefficient c and the quantum exponent q, with the following exceptions:
— It is not a hexadecimal floating number.
— The decimal-point wide character is used in place of a period.
— If neither an exponent part nor a decimal-point wide character appears in a decimal floating-
point number, an exponent part of the appropriate type with value zero is assumed to follow
the last digit in the wide string.
If the subject sequence begins with a minus sign, the sequence is interpreted as negated (before
rounding) and the sign s is set to −1, else s is set to 1. A wide character sequence INF or INFINITY is
interpreted as an infinity. A wide character sequence NAN or NAN(d-wchar-sequenceopt ), is interpreted
as a quiet NaN; the meaning of the d-wchar sequence is implementation-defined.421) A pointer to
the final wide string is stored in the object pointed to by endptr, provided that endptr is not a null
pointer.
5 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.
6 If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
Returns
7 The wcstodN functions return the correctly rounded converted value, if any. If no conversion could
be performed, the value of the triple (+1, 0, 0) is returned. If the correct value overflows:
— the value of the macro ERANGE is stored in errno if the integer expression
math_errhandling & MATH_ERRNO is nonzero;
— the "overflow" floating-point exception is raised if the integer expression
math_errhandling & MATH_ERREXCEPT is nonzero.
If the result underflows (7.12.1), whether errno acquires the value ERANGE if the integer expression
math_errhandling & MATH_ERRNO is nonzero is implementation-defined; if the integer expres-
sion math_errhandling & MATH_ERREXCEPT is nonzero, whether the "underflow" floating-point
exception is raised is implementation-defined.
7.31.4.1.4 The wcstol, wcstoll, wcstoul, and wcstoull functions
Synopsis
1 #include <wchar.h>
long int wcstol(const wchar_t * restrict nptr, wchar_t ** restrict endptr,
int base);
long long int wcstoll(const wchar_t * restrict nptr, wchar_t ** restrict endptr,
int base);
unsigned long int wcstoul(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
unsigned long long int wcstoull(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
421) An implementation may use the d-wchar sequence to determine extra information to be represented in the NaN’s
significand.
Description
2 The wcstol, wcstoll, wcstoul, and wcstoull functions convert the initial portion of the
wide string pointed to by nptr to long int, long long int, unsigned long int, and
unsigned long long int representation, respectively. First, they decompose the input string into
three parts: an initial, possibly empty, sequence of white-space wide characters, a subject sequence
resembling an integer represented in some radix determined by the value of base, and a final wide
string of one or more unrecognized wide characters, including the terminating null wide character
of the input wide string. Then, they attempt to convert the subject sequence to an integer, and return
the result.
3 If the value of base is zero, the expected form of the subject sequence is that of an integer constant
as described for the corresponding single-byte characters in 6.4.4.1, optionally preceded by a plus or
minus sign, but not including an integer suffix or any optional digit separators (6.4.4.1). If the value
of base is between 2 and 36 (inclusive), the expected form of the subject sequence is a sequence of
letters and digits representing an integer with the radix specified by base, optionally preceded by a
plus or minus sign, but not including an integer suffix or any optional digit separators. The letters
from a (or A) through z (or Z) are ascribed the values 10 through 35; only letters and digits whose
ascribed values are less than that of base are permitted. If the value of base is 2, the characters 0b or
0B may optionally precede the sequence of letters and digits, following the sign if present. If the
value of base is 16, the wide characters 0x or 0X may optionally precede the sequence of letters and
digits, following the sign if present.
4 The subject sequence is defined as the longest initial subsequence of the input wide string, starting
with the first non-white-space wide character, that is of the expected form. The subject sequence
contains no wide characters if the input wide string is empty or consists entirely of white-space
wide characters, or if the first non-white-space wide character is other than a sign or a permissible
letter or digit.
5 If the subject sequence has the expected form and the value of base is zero, the sequence of wide
characters starting with the first digit is interpreted as an integer constant according to the rules
of 6.4.4.1. If the subject sequence has the expected form and the value of base is between 2 and 36, it
is used as the base for conversion, ascribing to each letter its value as given above. If the subject
sequence begins with a minus sign, the value resulting from the conversion is negated (in the return
type). A pointer to the final wide string is stored in the object pointed to by endptr, provided that
endptr is not a null pointer.
6 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.
7 If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
Returns
8 The wcstol, wcstoll, wcstoul, and wcstoull functions return the converted value, if any. If
no conversion could be performed, zero is returned. If the correct value is outside the range of
representable values, LONG_MIN, LONG_MAX, LLONG_MIN, LLONG_MAX, ULONG_MAX, or ULLONG_MAX is
returned (according to the return type sign of the value, if any), and the value of the macro ERANGE
is stored in errno.
7.31.4.2 Wide string copying functions
7.31.4.2.1 The wcscpy function
Synopsis
1 #include <wchar.h>
wchar_t *wcscpy(wchar_t * restrict s1, const wchar_t * restrict s2);
Description
2 The wcscpy function copies the wide string pointed to by s2 (including the terminating null wide
character) into the array pointed to by s1.
Returns
3 The wcscpy function returns the value of s1.
7.31.4.2.2 The wcsncpy function
Synopsis
1 #include <wchar.h>
wchar_t *wcsncpy(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
Description
2 The wcsncpy function copies not more than n wide characters (those that follow a null wide character
are not copied) from the array pointed to by s2 to the array pointed to by s1.422)
3 If the array pointed to by s2 is a wide string that is shorter than n wide characters, null wide
characters are appended to the copy in the array pointed to by s1, until n wide characters in all have
been written.
Returns
4 The wcsncpy function returns the value of s1.
7.31.4.2.3 The wmemcpy function
Synopsis
1 #include <wchar.h>
wchar_t *wmemcpy(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
Description
2 The wmemcpy function copies n wide characters from the object pointed to by s2 to the object pointed
to by s1.
Returns
3 The wmemcpy function returns the value of s1.
7.31.4.2.4 The wmemmove function
Synopsis
1 #include <wchar.h>
wchar_t *wmemmove(wchar_t *s1, const wchar_t *s2, size_t n);
Description
2 The wmemmove function copies n wide characters from the object pointed to by s2 to the object
pointed to by s1. Copying takes place as if the n wide characters from the object pointed to by s2
are first copied into a temporary array of n wide characters that does not overlap the objects pointed
to by s1 or s2, and then the n wide characters from the temporary array are copied into the object
pointed to by s1.
Returns
3 The wmemmove function returns the value of s1.
7.31.4.3 Wide string concatenation functions
7.31.4.3.1 The wcscat function
Synopsis
1 #include <wchar.h>
wchar_t *wcscat(wchar_t * restrict s1, const wchar_t * restrict s2);
422) Thus, if there is no null wide character in the first n wide characters of the array pointed to by s2, the result will not be
null-terminated.
Description
2 The wcscat function appends a copy of the wide string pointed to by s2 (including the terminating
null wide character) to the end of the wide string pointed to by s1. The initial wide character of s2
overwrites the null wide character at the end of s1.
Returns
3 The wcscat function returns the value of s1.
7.31.4.3.2 The wcsncat function
Synopsis
1 #include <wchar.h>
wchar_t *wcsncat(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
Description
2 The wcsncat function appends not more than n wide characters (a null wide character and those
that follow it are not appended) from the array pointed to by s2 to the end of the wide string pointed
to by s1. The initial wide character of s2 overwrites the null wide character at the end of s1. A
terminating null wide character is always appended to the result.423)
Returns
3 The wcsncat function returns the value of s1.
7.31.4.4 Wide string comparison functions
1 Unless explicitly stated otherwise, the functions described in this subclause order two wide charac-
ters the same way as two integers of the underlying integer type designated by wchar_t.
7.31.4.4.1 The wcscmp function
Synopsis
1 #include <wchar.h>
int wcscmp(const wchar_t *s1, const wchar_t *s2);
Description
2 The wcscmp function compares the wide string pointed to by s1 to the wide string pointed to by s2.
Returns
3 The wcscmp function returns an integer greater than, equal to, or less than zero, accordingly as the
wide string pointed to by s1 is greater than, equal to, or less than the wide string pointed to by s2.
7.31.4.4.2 The wcscoll function
Synopsis
1 #include <wchar.h>
int wcscoll(const wchar_t *s1, const wchar_t *s2);
Description
2 The wcscoll function compares the wide string pointed to by s1 to the wide string pointed to by
s2, both interpreted as appropriate to the LC_COLLATE category of the current locale.
Returns
3 The wcscoll function returns an integer greater than, equal to, or less than zero, accordingly as the
wide string pointed to by s1 is greater than, equal to, or less than the wide string pointed to by s2
when both are interpreted as appropriate to the current locale.
7.31.4.4.3 The wcsncmp function
423) Thus, the maximum number of wide characters that can end up in the array pointed to by s1 is wcslen(s1)+n+1 .
Synopsis
1 #include <wchar.h>
int wcsncmp(const wchar_t *s1, const wchar_t *s2, size_t n);
Description
2 The wcsncmp function compares not more than n wide characters (those that follow a null wide
character are not compared) from the array pointed to by s1 to the array pointed to by s2.
Returns
3 The wcsncmp function returns an integer greater than, equal to, or less than zero, accordingly as the
possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly
null-terminated array pointed to by s2.
7.31.4.4.4 The wcsxfrm function
Synopsis
1 #include <wchar.h>
size_t wcsxfrm(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
Description
2 The wcsxfrm function transforms the wide string pointed to by s2 and places the resulting wide
string into the array pointed to by s1. The transformation is such that if the wcscmp function is
applied to two transformed wide strings, it returns a value greater than, equal to, or less than zero,
corresponding to the result of the wcscoll function applied to the same two original wide strings.
No more than n wide characters are placed into the resulting array pointed to by s1, including the
terminating null wide character. If n is zero, s1 is permitted to be a null pointer.
Returns
3 The wcsxfrm function returns the length of the transformed wide string (not including the terminat-
ing null wide character). If the value returned is n or greater, the members of the array pointed to by
s1 have an indeterminate representation.
4 EXAMPLE The value of the following expression is the length of the array needed to hold the transformation of the wide
string pointed to by s:
1 + wcsxfrm(NULL, s, 0)
7.31.4.4.5 The wmemcmp function
Synopsis
1 #include <wchar.h>
int wmemcmp(const wchar_t *s1, const wchar_t *s2, size_t n);
Description
2 The wmemcmp function compares the first n wide characters of the object pointed to by s1 to the first
n wide characters of the object pointed to by s2.
Returns
3 The wmemcmp function returns an integer greater than, equal to, or less than zero, accordingly as the
object pointed to by s1 is greater than, equal to, or less than the object pointed to by s2.
7.31.4.5 Wide string search functions
7.31.4.6 Introduction
1 The stateless search functions in this section (wcschr, wcspbrk, wcsrchr, wmemchr, wcsstr) are
generic functions. These functions are generic in the qualification of the array to be searched and
will return a result pointer to an element with the same qualification as the passed array. If the array
to be searched is const-qualified, the result pointer will be to a const-qualified element. If the array
to be searched is not const-qualified424) , the result pointer will be to an unqualified element.
2 The external declarations of these generic functions have a concrete function type that returns a
pointer to an unqualified element of type wchar_t (named QWchar_t), and accepts a pointer to a
const-qualified array of the same type to search. This signature supports all correct uses. If a macro
definition of any of these generic functions is suppressed in order to access an actual function, the
external declaration with this concrete type is visible425) .
3 The volatile and restrict qualifiers are not accepted on the elements of the array to search.
7.31.4.6.1 The wcschr generic function
Synopsis
1 #include <wchar.h>
QWchar_t *wcschr(QWchar_t *s, wchar_t c);
Description
2 The wcschr generic function locates the first occurrence of c in the wide string pointed to by s. The
terminating null wide character is considered to be part of the wide string.
Returns
3 The wcschr generic function returns a pointer to the located wide character, or a null pointer if the
wide character does not occur in the wide string.
7.31.4.6.2 The wcscspn function
Synopsis
1 #include <wchar.h>
size_t wcscspn(const wchar_t *s1, const wchar_t *s2);
Description
2 The wcscspn function computes the length of the maximum initial segment of the wide string
pointed to by s1 which consists entirely of wide characters not from the wide string pointed to by
s2.
Returns
3 The wcscspn function returns the length of the segment.
424) The null pointer constant is not a pointer to a const-qualified type, and therefore the result expression has the type of a
pointer to an unqualified element; however, evaluating such a call is undefined.
425) This is an obsolescent feature.
7.31.4.6.3 The wcspbrk generic function
Synopsis
1 #include <wchar.h>
QWchar_t *wcspbrk(QWchar_t *s1, const wchar_t *s2);
Description
2 The wcspbrk generic function locates the first occurrence in the wide string pointed to by s1 of any
wide character from the wide string pointed to by s2.
Returns
3 The wcspbrk generic function returns a pointer to the wide character in s1, or a null pointer if no
wide character from s2 occurs in s1.
7.31.4.6.4 The wcsrchr generic function
Synopsis
1 #include <wchar.h>
QWchar_t *wcsrchr(const wchar_t *s, wchar_t c);
Description
2 The wcsrchr generic function locates the last occurrence of c in the wide string pointed to by s. The
terminating null wide character is considered to be part of the wide string.
Returns
3 The wcsrchr generic function returns a pointer to the wide character, or a null pointer if c does not
occur in the wide string.
7.31.4.6.5 The wcsspn function
Synopsis
1 #include <wchar.h>
size_t wcsspn(const wchar_t *s1, const wchar_t *s2);
Description
2 The wcsspn function computes the length of the maximum initial segment of the wide string pointed
to by s1 which consists entirely of wide characters from the wide string pointed to by s2.
Returns
3 The wcsspn function returns the length of the segment.
7.31.4.6.6 The wcsstr generic function
Synopsis
1 #include <wchar.h>
QWchar_t *wcsstr(QWchar_t *s1, const wchar_t *s2);
Description
2 The wcsstr generic function locates the first occurrence in the wide string pointed to by s1 of the
sequence of wide characters (excluding the terminating null wide character) in the wide string
pointed to by s2.
Returns
3 The wcsstr generic function returns a pointer to the located wide string, or a null pointer if the
wide string is not found. If s2 points to a wide string with zero length, the function returns s1.
7.31.4.6.7 The wcstok function
Synopsis
1 #include <wchar.h>
wchar_t *wcstok(wchar_t * restrict s1, const wchar_t * restrict s2,
wchar_t ** restrict ptr);
Description
2 A sequence of calls to the wcstok function breaks the wide string pointed to by s1 into a sequence
of tokens, each of which is delimited by a wide character from the wide string pointed to by s2. The
third argument points to a caller-provided wchar_t pointer into which the wcstok function stores
information necessary for it to continue scanning the same wide string.
3 The first call in a sequence has a non-null first argument and stores an initial value in the object
pointed to by ptr. Subsequent calls in the sequence have a null first argument and the object pointed
to by ptr is required to have the value stored by the previous call in the sequence, which is then
updated. The separator wide string pointed to by s2 may be different from call to call.
4 The first call in the sequence searches the wide string pointed to by s1 for the first wide character
that is not contained in the current separator wide string pointed to by s2. If no such wide character
is found, then there are no tokens in the wide string pointed to by s1 and the wcstok function
returns a null pointer. If such a wide character is found, it is the start of the first token.
5 The wcstok function then searches from there for a wide character that is contained in the current
separator wide string. If no such wide character is found, the current token extends to the end of the
wide string pointed to by s1, and subsequent searches in the same wide string for a token return
a null pointer. If such a wide character is found, it is overwritten by a null wide character, which
terminates the current token.
6 In all cases, the wcstok function stores sufficient information in the pointer pointed to by ptr so
that subsequent calls, with a null pointer for s1 and the unmodified pointer value for ptr, shall start
searching just past the element overwritten by a null wide character (if any).
Returns
7 The wcstok function returns a pointer to the first wide character of a token, or a null pointer if there
is no token.
8 EXAMPLE
#include <wchar.h>
static wchar_t str1[] = L"?a???b,,,#c";
static wchar_t str2[] = L"\t \t";
wchar_t *t, *ptr1, *ptr2;
t = wcstok(str1, L"?", &ptr1); // t points to the token L"a"
t = wcstok(NULL, L",", &ptr1); // t points to the token L"??b"
t = wcstok(str2, L" \t", &ptr2); // t is a null pointer
t = wcstok(NULL, L"#,", &ptr1); // t points to the token L"c"
t = wcstok(NULL, L"?", &ptr1); // t is a null pointer
7.31.4.6.8 The wmemchr generic function
Synopsis
1 #include <wchar.h>
QWchar_t *wmemchr(QWchar_t *s, wchar_t c, size_t n);
Description
2 The wmemchr generic function locates the first occurrence of c in the initial n wide characters of the
object pointed to by s.
Returns
3 The wmemchr generic function returns a pointer to the located wide character, or a null pointer if the
wide character does not occur in the object.
7.31.4.7 Miscellaneous functions
7.31.4.7.1 The wcslen function
Synopsis
1 #include <wchar.h>
size_t wcslen(const wchar_t *s);
Description
2 The wcslen function computes the length of the wide string pointed to by s.
Returns
3 The wcslen function returns the number of wide characters that precede the terminating null wide
character.
7.31.4.7.2 The wmemset function
Synopsis
1 #include <wchar.h>
wchar_t *wmemset(wchar_t *s, wchar_t c, size_t n);
Description
2 The wmemset function copies the value of c into each of the first n wide characters of the object
pointed to by s.
Returns
3 The wmemset function returns the value of s.
7.31.5 Wide character time conversion functions
7.31.5.1 The wcsftime function
Synopsis
1 #include <time.h>
#include <wchar.h>
size_t wcsftime(wchar_t * restrict s, size_t maxsize,
const wchar_t * restrict format, const struct tm * restrict timeptr);
Description
2 The wcsftime function is equivalent to the strftime function, except that:
— The argument s points to the initial element of an array of wide characters into which the
generated output is to be placed.
— The argument maxsize indicates the limiting number of wide characters.
— The argument format is a wide string and the conversion specifiers are replaced by corre-
sponding sequences of wide characters.
— The return value indicates the number of wide characters.
Returns
3 If the total number of resulting wide characters including the terminating null wide character is not
more than maxsize, the wcsftime function returns the number of wide characters placed into the
array pointed to by s not including the terminating null wide character. Otherwise, zero is returned
and the members of the array have an indeterminate representation.
7.31.6 Extended multibyte/wide character conversion utilities
1 The header <wchar.h> declares an extended set of functions useful for conversion between multibyte
characters and wide characters.
2 Most of the following functions — those that are listed as "restartable", 7.31.6.3 and 7.31.6.4 — take
as a last argument a pointer to an object of type mbstate_t that is used to describe the current
conversion state from a particular multibyte character sequence to a wide character sequence (or the
reverse) under the rules of a particular setting for the LC_CTYPE category of the current locale.
3 The initial conversion state corresponds, for a conversion in either direction, to the beginning of a
new multibyte character in the initial shift state. A zero-valued mbstate_t object is (at least) one
way to describe an initial conversion state. A zero-valued mbstate_t object can be used to initiate
conversion involving any multibyte character sequence, in any LC_CTYPE category setting. If an
mbstate_t object has been altered by any of the functions described in this subclause, and is then
used with a different multibyte character sequence, or in the other conversion direction, or with a
different LC_CTYPE category setting than on earlier function calls, the behavior is undefined.426)
4 On entry, each function takes the described conversion state (either internal or pointed to by an
argument) as current. The conversion state described by the referenced object is altered as needed
to track the shift state, and the position within a multibyte character, for the associated multibyte
character sequence.
7.31.6.1 Single-byte/wide character conversion functions
7.31.6.1.1 The btowc function
Synopsis
1 #include <wchar.h>
wint_t btowc(int c);
Description
2 The btowc function determines whether c constitutes a valid single-byte character in the initial shift
state.
Returns
3 The btowc function returns WEOF if c has the value EOF or if (unsigned char)c does not constitute
a valid single-byte character in the initial shift state. Otherwise, it returns the wide character
representation of that character.
7.31.6.1.2 The wctob function
Synopsis
1 #include <wchar.h>
int wctob(wint_t c);
Description
2 The wctob function determines whether c corresponds to a member of the extended character set
whose multibyte character representation is a single byte when in the initial shift state.
Returns
3 The wctob function returns EOF if c does not correspond to a multibyte character with length one
in the initial shift state. Otherwise, it returns the single-byte representation of that character as an
unsigned char converted to an int.
7.31.6.2 Conversion state functions
7.31.6.2.1 The mbsinit function
Synopsis
1 #include <wchar.h>
426) Thus, a particular mbstate_t object can be used, for example, with both the mbrtowc and mbsrtowcs functions as long
as they are used to step sequentially through the same multibyte character string.
int mbsinit(const mbstate_t *ps);
Description
2 If ps is not a null pointer, the mbsinit function determines whether the referenced mbstate_t object
describes an initial conversion state.
Returns
3 The mbsinit function returns nonzero if ps is a null pointer or if the referenced object describes an
initial conversion state; otherwise, it returns zero.
7.31.6.3 Restartable multibyte/wide character conversion functions
1 These functions differ from the corresponding multibyte character functions of 7.24.7 (mblen, mbtowc,
and wctomb) in that they have an extra parameter, ps, of type pointer to mbstate_t that points
to an object that can completely describe the current conversion state of the associated multibyte
character sequence. If ps is a null pointer, each function uses its own internal mbstate_t object
instead, which is initialized prior to the first call to the function to the initial conversion state; the
functions are not required to avoid data races with other calls to the same function in this case. It
is implementation-defined whether the internal mbstate_t object has thread storage duration; if
it has thread storage duration, it is initialized to the initial conversion state prior to the first call to
the function on the new thread. The implementation behaves as if no library function calls these
functions with a null pointer for ps.
2 Also unlike their corresponding functions, the return value does not represent whether the encoding
is state-dependent.
7.31.6.3.1 The mbrlen function
Synopsis
1 #include <wchar.h>
size_t mbrlen(const char * restrict s, size_t n, mbstate_t * restrict ps);
Description
2 The mbrlen function is equivalent to the call:
mbrtowc(NULL, s, n, ps != NULL ? ps: &internal)
where internal is the mbstate_t object for the mbrlen function, except that the expression desig-
nated by ps is evaluated only once.
Returns
3 The mbrlen function returns a value between zero and n, inclusive, (size_t) (−2), or (size_t) (−1).
Forward references: the mbrtowc function (7.31.6.3.2).
7.31.6.3.2 The mbrtowc function
Synopsis
1 #include <wchar.h>
size_t mbrtowc(wchar_t * restrict pwc, const char * restrict s, size_t n,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the mbrtowc function is equivalent to the call:
mbrtowc(NULL, "", 1, ps)
In this case, the values of the parameters pwc and n are ignored.
3 If s is not a null pointer, the mbrtowc function inspects at most n bytes beginning with the byte
pointed to by s to determine the number of bytes needed to complete the next multibyte character
(including any shift sequences). If the function determines that the next multibyte character is
complete and valid, it determines the value of the corresponding wide character and then, if pwc
is not a null pointer, stores that value in the object pointed to by pwc. If the corresponding wide
character is the null wide character, the resulting state described is the initial conversion state.
Returns
4 The mbrtowc function returns the first of the following that applies (given the current conversion
state):
0 if the next n or fewer bytes complete the multibyte character that corresponds to
the null wide character (which is the value stored).
between 1 and n inclusive if the next n or fewer bytes complete a valid multibyte character (which
is the value stored); the value returned is the number of bytes that complete the
multibyte character.
(size_t) (−2) if the next n bytes contribute to an incomplete (but potentially valid) multibyte
character, and all n bytes have been processed (no value is stored).427)
(size_t)(-1) if an encoding error occurs, in which case the next n or fewer bytes do not contribute
to a complete and valid multibyte character (no value is stored); the value of the
macro EILSEQ is stored in errno, and the conversion state is unspecified.
7.31.6.3.3 The wcrtomb function
Synopsis
1 #include <wchar.h>
size_t wcrtomb(char * restrict s, wchar_t wc, mbstate_t * restrict ps);
Description
2 If s is a null pointer, the wcrtomb function is equivalent to the call
wcrtomb(buf, L’\0’, ps)
where buf is an internal buffer.
3 If s is not a null pointer, the wcrtomb function determines the number of bytes needed to represent
the multibyte character that corresponds to the wide character given by wc (including any shift
sequences), and stores the multibyte character representation in the array whose first element is
pointed to by s. At most MB_CUR_MAX bytes are stored. If wc is a null wide character, a null byte is
stored, preceded by any shift sequence needed to restore the initial shift state; the resulting state
described is the initial conversion state.
Returns
4 The wcrtomb function returns the number of bytes stored in the array object (including any shift
sequences). When wc is not a valid wide character, an encoding error occurs: the function stores the
value of the macro EILSEQ in errno and returns (size_t) (−1); the conversion state is unspecified.
7.31.6.4 Restartable multibyte/wide string conversion functions
1 These functions differ from the corresponding multibyte string functions of 7.24.8 (mbstowcs and
wcstombs) in that they have an extra parameter, ps, of type pointer to mbstate_t that points to
an object that can completely describe the current conversion state of the associated multibyte
character sequence. If ps is a null pointer, each function uses its own internal mbstate_t object
instead, which is initialized prior to the first call to the function to the initial conversion state; the
427) When n has at least the value of the MB_CUR_MAX macro, this case can only occur if s points at a sequence of redundant
shift sequences (for implementations with state-dependent encodings).
functions are not required to avoid data races with other calls to the same function in this case. It
is implementation-defined whether the internal mbstate_t object has thread storage duration; if
it has thread storage duration, it is initialized to the initial conversion state prior to the first call to
the function on the new thread. The implementation behaves as if no library function calls these
functions with a null pointer for ps.
2 Also unlike their corresponding functions, the conversion source parameter, src, has a pointer-to-
pointer type. When the function is storing the results of conversions (that is, when dst is not a null
pointer), the pointer object pointed to by this parameter is updated to reflect the amount of the
source processed by that invocation.
7.31.6.4.1 The mbsrtowcs function
Synopsis
1 #include <wchar.h>
size_t mbsrtowcs(wchar_t * restrict dst, const char ** restrict src, size_t len,
mbstate_t * restrict ps);
Description
2 The mbsrtowcs function converts a sequence of multibyte characters that begins in the conversion
state described by the object pointed to by ps, from the array indirectly pointed to by src into a
sequence of corresponding wide characters. If dst is not a null pointer, the converted characters are
stored into the array pointed to by dst. Conversion continues up to and including a terminating
null character, which is also stored. Conversion stops earlier in two cases: when a sequence of bytes
is encountered that does not form a valid multibyte character, or (if dst is not a null pointer) when
len wide characters have been stored into the array pointed to by dst.428) Each conversion takes
place as if by a call to the mbrtowc function.
3 If dst is not a null pointer, the pointer object pointed to by src is assigned either a null pointer (if
conversion stopped due to reaching a terminating null character) or the address just past the last
multibyte character converted (if any). If conversion stopped due to reaching a terminating null
character and if dst is not a null pointer, the resulting state described is the initial conversion state.
Returns
4 If the input conversion encounters a sequence of bytes that do not form a valid multibyte character,
an encoding error occurs: the mbsrtowcs function stores the value of the macro EILSEQ in errno
and returns (size_t)(-1) ; the conversion state is unspecified. Otherwise, it returns the number of
multibyte characters successfully converted, not including the terminating null character (if any).
7.31.6.4.2 The wcsrtombs function
Synopsis
1 #include <wchar.h>
size_t wcsrtombs(char * restrict dst, const wchar_t ** restrict src, size_t len,
mbstate_t * restrict ps);
Description
2 The wcsrtombs function converts a sequence of wide characters from the array indirectly pointed to
by src into a sequence of corresponding multibyte characters that begins in the conversion state
described by the object pointed to by ps. If dst is not a null pointer, the converted characters are then
stored into the array pointed to by dst. Conversion continues up to and including a terminating null
wide character, which is also stored. Conversion stops earlier in two cases: when a wide character
is reached that does not correspond to a valid multibyte character, or (if dst is not a null pointer)
when the next multibyte character would exceed the limit of len total bytes to be stored into the
array pointed to by dst. Each conversion takes place as if by a call to the wcrtomb function.429)
428) Thus, the value of len is ignored if dst is a null pointer.
429) If conversion stops because a terminating null wide character has been reached, the bytes stored include those necessary
to reach the initial shift state immediately before the null byte.
3 If dst is not a null pointer, the pointer object pointed to by src is assigned either a null pointer (if
conversion stopped due to reaching a terminating null wide character) or the address just past the
last wide character converted (if any). If conversion stopped due to reaching a terminating null wide
character, the resulting state described is the initial conversion state.
Returns
4 If conversion stops because a wide character is reached that does not correspond to a valid multibyte
character, an encoding error occurs: the wcsrtombs function stores the value of the macro EILSEQ
in errno and returns (size_t) (−1); the conversion state is unspecified. Otherwise, it returns the
number of bytes in the resulting multibyte character sequence, not including the terminating null
character (if any).
7.32 Wide character classification and mapping utilities <wctype.h>
7.32.1 Introduction
1 The header <wctype.h> defines one macro, and declares three data types and many functions.430)
2 The types declared are wint_t described in 7.31.1;
wctrans_t
which is a scalar type that can hold values which represent locale-specific character mappings; and
wctype_t
which is a scalar type that can hold values which represent locale-specific character classifications.
3 The macro defined is WEOF (described in 7.31.1).
4 The functions declared are grouped as follows:
— Functions that provide wide character classification;
— Extensible functions that provide wide character classification;
— Functions that provide wide character case mapping;
— Extensible functions that provide wide character mapping.
5 For all functions described in this subclause that accept an argument of type wint_t, the value shall
be representable as a wchar_t or shall equal the value of the macro WEOF. If this argument has any
other value, the behavior is undefined.
6 The behavior of these functions is affected by the LC_CTYPE category of the current locale.
7.32.2 Wide character classification utilities
1 The header <wctype.h> declares several functions useful for classifying wide characters.
2 The term printing wide character refers to a member of a locale-specific set of wide characters, each of
which occupies at least one printing position on a display device. The term control wide character
refers to a member of a locale-specific set of wide characters that are not printing wide characters.
7.32.2.1 Wide character classification functions
1 The functions in this subclause return nonzero (true) if and only if the value of the argument wc
conforms to that in the description of the function.
2 Each of the following functions returns true for each wide character that corresponds (as if by a call
to the wctob function) to a single-byte character for which the corresponding character classification
function from 7.4.1 returns true, except that the iswgraph and iswpunct functions may differ with
respect to wide characters other than L’ ’ that are both printing and white-space wide characters.431)
Forward references: the wctob function (7.31.6.1.2).
430) See "future library directions" (7.33.21).
431) For example, if the expression isalpha(wctob(wc)) evaluates to true, then the call iswalpha(wc) also returns true.
But, if the expression isgraph(wctob(wc)) evaluates to true (which cannot occur for wc == L’’ of course), then either
iswgraph(wc) or iswprint(wc)&& iswspace(wc) is true, but not both.
7.32.2.1.1 The iswalnum function
Synopsis
1 #include <wctype.h>
int iswalnum(wint_t wc);
Description
2 The iswalnum function tests for any wide character for which iswalpha or iswdigit is true.
7.32.2.1.2 The iswalpha function
Synopsis
1 #include <wctype.h>
int iswalpha(wint_t wc);
Description
2 The iswalpha function tests for any wide character for which iswupper or iswlower is true, or any
wide character that is one of a locale-specific set of alphabetic wide characters for which none of
iswcntrl, iswdigit, iswpunct, or iswspace is true.432)
7.32.2.1.3 The iswblank function
Synopsis
1 #include <wctype.h>
int iswblank(wint_t wc);
Description
2 The iswblank function tests for any wide character that is a standard blank wide character or is one
of a locale-specific set of wide characters for which iswspace is true and that is used to separate
words within a line of text. The standard blank wide characters are the following: space (L’ ’),
and horizontal tab (L’\t’). In the "C" locale, iswblank returns true only for the standard blank
characters.
7.32.2.1.4 The iswcntrl function
Synopsis
1 #include <wctype.h>
int iswcntrl(wint_t wc);
Description
2 The iswcntrl function tests for any control wide character.
7.32.2.1.5 The iswdigit function
Synopsis
1 #include <wctype.h>
int iswdigit(wint_t wc);
Description
2 The iswdigit function tests for any wide character that corresponds to a decimal-digit character (as
defined in 5.2.1).
432) The functions iswlower and iswupper test true or false separately for each of these additional wide characters; all four
combinations are possible.
7.32.2.1.6 The iswgraph function
Synopsis
1 #include <wctype.h>
int iswgraph(wint_t wc);
Description
2 The iswgraph function tests for any wide character for which iswprint is true and iswspace is
false.433)
7.32.2.1.7 The iswlower function
Synopsis
1 #include <wctype.h>
int iswlower(wint_t wc);
Description
2 The iswlower function tests for any wide character that corresponds to a lowercase letter or is one
of a locale-specific set of wide characters for which none of iswcntrl, iswdigit, iswpunct, or
iswspace is true.
7.32.2.1.8 The iswprint function
Synopsis
1 #include <wctype.h>
int iswprint(wint_t wc);
Description
2 The iswprint function tests for any printing wide character.
7.32.2.1.9 The iswpunct function
Synopsis
1 #include <wctype.h>
int iswpunct(wint_t wc);
Description
2 The iswpunct function tests for any printing wide character that is one of a locale-specific set of
punctuation wide characters for which neither iswspace nor iswalnum is true.433)
7.32.2.1.10 The iswspace function
Synopsis
1 #include <wctype.h>
int iswspace(wint_t wc);
Description
2 The iswspace function tests for any wide character that corresponds to a locale-specific set of
white-space wide characters for which none of iswalnum, iswgraph, or iswpunct is true.
7.32.2.1.11 The iswupper function
Synopsis
1 #include <wctype.h>
int iswupper(wint_t wc);
433) Note that the behavior of the iswgraph and iswpunct functions can differ from their corresponding functions in 7.4.1
with respect to printing, white-space, single-byte execution characters other than ’ ’ .
Description
2 The iswupper function tests for any wide character that corresponds to an uppercase letter or is
one of a locale-specific set of wide characters for which none of iswcntrl, iswdigit, iswpunct, or
iswspace is true.
7.32.2.1.12 The iswxdigit function
Synopsis
1 #include <wctype.h>
int iswxdigit(wint_t wc);
Description
2 The iswxdigit function tests for any wide character that corresponds to a hexadecimal-digit
character (as defined in 6.4.4.1).
7.32.2.2 Extensible wide character classification functions
1 The functions wctype and iswctype provide extensible wide character classification as well as
testing equivalent to that performed by the functions described in the previous subclause (7.32.2.1).
7.32.2.2.1 The iswctype function
Synopsis
1 #include <wctype.h>
int iswctype(wint_t wc, wctype_t desc);
Description
2 The iswctype function determines whether the wide character wc has the property described by
desc. The current setting of the LC_CTYPE category shall be the same as during the call to wctype
that returned the value desc.
3 Each of the following expressions has a truth-value equivalent to the call to the wide character
classification function (7.32.2.1) in the comment that follows the expression:
iswctype(wc, wctype("alnum")) // iswalnum(wc)
iswctype(wc, wctype("alpha")) // iswalpha(wc)
iswctype(wc, wctype("blank")) // iswblank(wc)
iswctype(wc, wctype("cntrl")) // iswcntrl(wc)
iswctype(wc, wctype("digit")) // iswdigit(wc)
iswctype(wc, wctype("graph")) // iswgraph(wc)
iswctype(wc, wctype("lower")) // iswlower(wc)
iswctype(wc, wctype("print")) // iswprint(wc)
iswctype(wc, wctype("punct")) // iswpunct(wc)
iswctype(wc, wctype("space")) // iswspace(wc)
iswctype(wc, wctype("upper")) // iswupper(wc)
iswctype(wc, wctype("xdigit")) // iswxdigit(wc)
Returns
4 The iswctype function returns nonzero (true) if and only if the value of the wide character wc has
the property described by desc. If desc is zero, the iswctype function returns zero (false).
Forward references: the wctype function (7.32.2.2.2).
7.32.2.2.2 The wctype function
Synopsis
1 #include <wctype.h>
wctype_t wctype(const char *property);
Description
2 The wctype function constructs a value with type wctype_t that describes a class of wide characters
identified by the string argument property.
3 The strings listed in the description of the iswctype function shall be valid in all locales as property
arguments to the wctype function.
Returns
4 If property identifies a valid class of wide characters according to the LC_CTYPE category of the
current locale, the wctype function returns a nonzero value that is valid as the second argument to
the iswctype function; otherwise, it returns zero.
7.32.3 Wide character case mapping utilities
1 The header <wctype.h> declares several functions useful for mapping wide characters.
7.32.3.1 Wide character case mapping functions
7.32.3.1.1 The towlower function
Synopsis
1 #include <wctype.h>
wint_t towlower(wint_t wc);
Description
2 The towlower function converts an uppercase letter to a corresponding lowercase letter.
Returns
3 If the argument is a wide character for which iswupper is true and there are one or more correspond-
ing wide characters, as specified by the current locale, for which iswlower is true, the towlower
function returns one of the corresponding wide characters (always the same one for any given
locale); otherwise, the argument is returned unchanged.
7.32.3.1.2 The towupper function
Synopsis
1 #include <wctype.h>
wint_t towupper(wint_t wc);
Description
2 The towupper function converts a lowercase letter to a corresponding uppercase letter.
Returns
3 If the argument is a wide character for which iswlower is true and there are one or more correspond-
ing wide characters, as specified by the current locale, for which iswupper is true, the towupper
function returns one of the corresponding wide characters (always the same one for any given
locale); otherwise, the argument is returned unchanged.
7.32.3.2 Extensible wide character case mapping functions
1 The functions wctrans and towctrans provide extensible wide character mapping as well as case
mapping equivalent to that performed by the functions described in the previous subclause (7.32.3.1).
7.32.3.2.1 The towctrans function
Synopsis
1 #include <wctype.h>
wint_t towctrans(wint_t wc, wctrans_t desc);
Description
2 The towctrans function maps the wide character wc using the mapping described by desc. The
current setting of the LC_CTYPE category shall be the same as during the call to wctrans that returned
the value desc.
3 Each of the following expressions behaves the same as the call to the wide character case mapping
function (7.32.3.1) in the comment that follows the expression:
towctrans(wc, wctrans("tolower")) // towlower(wc)
towctrans(wc, wctrans("toupper")) // towupper(wc)
Returns
4 The towctrans function returns the mapped value of wc using the mapping described by desc. If
desc is zero, the towctrans function returns the value of wc .
7.32.3.2.2 The wctrans function
Synopsis
1 #include <wctype.h>
wctrans_t wctrans(const char *property);
Description
2 The wctrans function constructs a value with type wctrans_t that describes a mapping between
wide characters identified by the string argument property.
3 The strings listed in the description of the towctrans function shall be valid in all locales as
property arguments to the wctrans function.
Returns
4 If property identifies a valid mapping of wide characters according to the LC_CTYPE category of the
current locale, the wctrans function returns a nonzero value that is valid as the second argument to
the towctrans function; otherwise, it returns zero.
7.33 Future library directions
1 Although grouped under individual headers, all of the external names identified as reserved
identifiers or potentially reserved identifiers in this subclause remain so regardless of which headers
are included in the program.
7.33.1 Complex arithmetic <complex.h>
1 The function names
cacospi cexp10m1 clog10 crootn
casinpi cexp10 clog1p crsqrt
catanpi cexp2m1 clog2p1 csinpi
ccompoundn cexp2 clog2 ctanpi
ccospi cexpm1 clogp1 ctgamma
cerfc clgamma cpown
cerf clog10p1 cpowr
and the same names suffixed with f or l are potentially reserved identifiers and may be added to
the declarations in the <complex.h> header.
7.33.2 Character handling <ctype.h>
1 Function names that begin with either is or to, and a lowercase letter are potentially reserved
identifiers and may be added to the declarations in the <ctype.h> header.
7.33.3 Errors <errno.h>
1 Macros that begin with E and a digit or E and an uppercase letter may be added to the macros
defined in the <errno.h> header by a future revision of this document or by an implementation.
7.33.4 Floating-point environment <fenv.h>
1 Macros that begin with FE_ and an uppercase letter may be added to the macros defined in the
<fenv.h> header by a future revision of this document.
7.33.5 Characteristics of floating types <float.h>
1 Macros that begin with DBL_, DEC32_, DEC64_, DEC128_, DEC_, FLT_, or LDBL_ and an uppercase
letter are potentially reserved identifiers and may be added to the macros defined in the <float.h>
header.
2 Use of the DECIMAL_DIG macro is an obsolescent feature. A similar type-specific macro, such as
LDBL_DECIMAL_DIG, can be used instead.
3 The use of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and LDBL_HAS_SUBNORM macros is an obsolescent
feature.
7.33.6 Format conversion of integer types <inttypes.h>
1 Macros that begin with either PRI or SCN, and either a lowercase letter or X are potentially reserved
identifiers and may be added to the macros defined in the <inttypes.h> header.
2 Function names that begin with str, or wcs and a lowercase letter are potentially reserved identifiers
may be added to the declarations in the <inttypes.h> header.
7.33.7 Localization <locale.h>
1 Macros that begin with LC_ and an uppercase letter may be added to the macros defined in the
<locale.h> header by a future revision of this document or by an implementation.
7.33.8 Mathematics <math.h>
1 Macros that begin with FP_ or MATH_ and an uppercase letter may be added to the macros defined
in the <math.h> header by a future revision of this document or by an implementation.
2 Macros that begin with MATH_ and an uppercase letter are potentially reserved identifiers and may
be added to the macros in the <math.h> header.
3 Function names that begin with is and a lowercase letter are potentially reserved identifiers and
may be added to the declarations in the <math.h> header.
4 Function names that begin with cr_ are potentially reserved identifiers and may be added to the
<math.h> header. The cr_ prefix is intended to indicate a correctly rounded version of the function.
5 Use of the macros INFINITY, DEC_INFINITY, NAN, and DEC_NAN in <math.h> is an obsolescent
feature. Instead, use the same macros in <float.h>.
7.33.9 Signal handling <signal.h>
1 Macros that begin with either SIG and an uppercase letter or SIG_ and an uppercase letter may be
added to the macros defined in the <signal.h> header by a fture revision of this document or by an
implementation.
7.33.10 Atomics <stdatomic.h>
1 Macros that begin with ATOMIC_ and an uppercase letter are potentially reserved identifiers and
may be added to the macros defined in the <stdatomic.h> header. Typedef names that begin with
either atomic_ or memory_, and a lowercase letter are potentially reserved identifiers and may be
added to the declarations in the <stdatomic.h> header. Enumeration constants that begin with
memory_order_ and a lowercase letter are potentially reserved identifiers and may be added to
the definition of the memory_order type in the <stdatomic.h> header. Function names that begin
with atomic_ and a lowercase letter are potentially reserved identifiers and may be added to the
declarations in the <stdatomic.h> header.
7.33.11 Boolean type and values <stdbool.h>
1 The macro __bool_true_false_are_defined is an obsolescent feature.
7.33.12 Bit and byte utilities <stdbit.h>
1 Type and function names that begin with stdc_ are potentially reserved identifiers and may be
added to the declarations in the <stdbit.h> header.
7.33.13 Checked Arithmetic Functions <stdckdint.h>
1 Type and function names that begin with ckd_ are potentially reserved identifiers and may be added
to the declarations in the <stdckdint.h> header.
7.33.14 Integer types <stdint.h>
1 Typedef names beginning with int or uint and ending with _t are potentially reserved identifiers
and may be added to the types defined in the <stdint.h> header. Macro names beginning with
INT or UINT and ending with _MAX , _MIN , _WIDTH , or _C are potentially reserved identifiers and may
be added to the macros defined in the <stdint.h> header.
7.33.15 Input/output <stdio.h>
1 Lowercase letters may be added to the conversion specifiers and length modifiers in fprintf and
fscanf. Other characters may be used in extensions.
2 The use of ungetc on a binary stream where the file position indicator is zero prior to the call is an
obsolescent feature.
7.33.16 General utilities <stdlib.h>
1 Function names that begin with str or wcs and a lowercase letter are potentially reserved identifiers
and may be added to the declarations in the <stdlib.h> header.
2 Suppressing the macro definition of bsearch in order to access the actual function is an obsolescent
feature.
7.33.17 String handling <string.h>
1 Function names that begin with str, mem, or wcs and a lowercase letter are potentially reserved
identifiers and may be added to the declarations in the <string.h> header.
2 Suppressing the macro definitions of memchr, strchr, strpbrk, strrchr, or strstr in order to
access the corresponding actual function is an obsolescent feature.
7.33.18 Date and time <time.h>
Macros beginning with TIME_ and an uppercase letter may be added to the macros in the <time.h>
header by a future revision of this document or by an implementation.
7.33.19 Threads <threads.h>
1 Function names, type names, and enumeration constants that begin with either cnd_, mtx_, thrd_, or
tss_, and a lowercase letter are potentially reserved identifiers and may be added to the declarations
in the <threads.h> header.
7.33.20 Extended multibyte and wide character utilities <wchar.h>
1 Function names that begin with wcs and a lowercase letter are potentially reserved identifiers and
may be added to the declarations in the <wchar.h> header.
2 Lowercase letters may be added to the conversion specifiers and length modifiers in fwprintf and
fwscanf. Other characters may be used in extensions.
3 Suppressing the macro definitions of wcschr, wcspbrk, wcsrchr, wmemchr, or wcsstr in order to
access the corresponding actual function is an obsolescent feature.
7.33.21 Wide character classification and mapping utilities <wctype.h>
1 Function names that begin with is or to and a lowercase letter are potentially reserved identifiers
and may be added to the declarations in the <wctype.h> header.
A. Annex A (informative) Language syntax summary
1 NOTE The notation is described in 6.1.
A.1 Lexical grammar
A.1.1 Lexical elements
(6.4) token:
keyword
identifier
constant
string-literal
punctuator
(6.4) preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each universal-character-name that cannot be one of the above
each non-white-space character that cannot be one of the above
A.1.2 Keywords
(6.4.1) keyword: one of
alignas enum short void
alignof extern signed volatile
auto false sizeof while
bool float static _Atomic
break for static_assert _BitInt
case goto struct _Complex
char if switch _Decimal128
const inline thread_local _Decimal32
constexpr int true _Decimal64
continue long typedef _Generic
default nullptr typeof _Imaginary
do register typeof_unqual _Noreturn
double restrict union
else return unsigned
A.1.3 Identifiers
(6.4.2.1) identifier:
identifier-start
identifier identifier-continue
(6.4.2.1) identifier-start:
nondigit
XID_Start character
universal-character-name of class XID_Start
(6.4.2.1) identifier-continue:
digit
nondigit
XID_Continue character
universal-character-name of class XID_Continue
(6.4.2.1) nondigit: one of
_ a b c d e f g h i j k l m
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
(6.4.2.1) digit: one of
0 1 2 3 4 5 6 7 8 9
A.1.4 Universal character names
(6.4.3) universal-character-name:
\u hex-quad
\U hex-quad hex-quad
(6.4.3) hex-quad:
hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit
A.1.5 Constants
(6.4.4) constant:
integer-constant
floating-constant
enumeration-constant
character-constant
predefined-constant
(6.4.4.1) integer-constant:
decimal-constant integer-suffixopt
octal-constant integer-suffixopt
hexadecimal-constant integer-suffixopt
binary-constant integer-suffixopt
(6.4.4.1) decimal-constant:
nonzero-digit
decimal-constant ’opt digit
(6.4.4.1) octal-constant:
0
octal-constant ’opt octal-digit
(6.4.4.1) hexadecimal-constant:
hexadecimal-prefix hexadecimal-digit-sequence
(6.4.4.1) binary-constant:
binary-prefix binary-digit
binary-constant ’opt binary-digit
(6.4.4.1) hexadecimal-prefix: one of
0x 0X
(6.4.4.1) binary-prefix: one of
0b 0B
(6.4.4.1) nonzero-digit: one of
1 2 3 4 5 6 7 8 9
(6.4.4.1) octal-digit: one of
0 1 2 3 4 5 6 7
hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence ’opt hexadecimal-digit
(6.4.4.1) hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
(6.4.4.1) binary-digit: one of
0 1
(6.4.4.1) integer-suffix:
unsigned-suffix long-suffixopt
unsigned-suffix long-long-suffix
unsigned-suffix bit-precise-int-suffix
long-suffix unsigned-suffixopt
long-long-suffix unsigned-suffixopt
bit-precise-int-suffix unsigned-suffixopt
(6.4.4.1) bit-precise-int-suffix: one of
wb WB
(6.4.4.1) unsigned-suffix: one of
u U
(6.4.4.1) long-suffix: one of
l L
(6.4.4.1) long-long-suffix: one of
ll LL
(6.4.4.2) floating-constant:
decimal-floating-constant
hexadecimal-floating-constant
(6.4.4.2) decimal-floating-constant:
fractional-constant exponent-partopt floating-suffixopt
digit-sequence exponent-part floating-suffixopt
(6.4.4.2) hexadecimal-floating-constant:
hexadecimal-prefix hexadecimal-fractional-constant
binary-exponent-part floating-suffixopt
hexadecimal-prefix hexadecimal-digit-sequence
binary-exponent-part floating-suffixopt
(6.4.4.2) fractional-constant:
digit-sequenceopt . digit-sequence
digit-sequence .
(6.4.4.2) exponent-part:
e signopt digit-sequence
E signopt digit-sequence
(6.4.4.2) sign: one of
+ -
(6.4.4.2) digit-sequence:
digit
digit-sequence ’opt digit
(6.4.4.2) hexadecimal-fractional-constant:
hexadecimal-digit-sequenceopt . hexadecimal-digit-sequence
hexadecimal-digit-sequence .
(6.4.4.2) binary-exponent-part:
p signopt digit-sequence
P signopt digit-sequence
(6.4.4.2) floating-suffix: one of
f l F L df dd dl DF DD DL
(6.4.4.3) enumeration-constant:
identifier
(6.4.4.4) character-constant:
encoding-prefixopt ’ c-char-sequence ’
(6.4.4.4) encoding-prefix:
u8
u
U
L
(6.4.4.4) c-char-sequence:
c-char
c-char-sequence c-char
(6.4.4.4) c-char:
any member of the source character set except
the single-quote ’, backslash \ , or new-line character
escape-sequence
(6.4.4.4) escape-sequence:
simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence
universal-character-name
(6.4.4.4) simple-escape-sequence: one of
\’ \" \? \\
\a \b \f \n \r \t \v
(6.4.4.4) octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
(6.4.4.4) hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit
(6.4.4.5) predefined-constant:
false
true
nullptr
A.1.6 String literals
(6.4.5) string-literal:
encoding-prefixopt " s-char-sequenceopt "
(6.4.5) s-char-sequence:
s-char
s-char-sequence s-char
(6.4.5) s-char:
any member of the source character set except
the double-quote ", backslash \, or new-line character
escape-sequence
A.1.7 Punctuators
(6.4.6) punctuator: one of
[ ] ( ) { } . ->
++ -- & * + - ~ !
/ % << >> < > <= >= == != ^ | && ||
? : :: ; ...
= *= /= %= += -= <<= >>= &= ^= |=
, # ##
<: :> <% %> %: %:%:
A.1.8 Header names
(6.4.7) header-name:
< h-char-sequence >
" q-char-sequence "
(6.4.7) h-char-sequence:
h-char
h-char-sequence h-char
(6.4.7) h-char:
any member of the source character set except
the new-line character and >
(6.4.7) q-char-sequence:
q-char
q-char-sequence q-char
(6.4.7) q-char:
any member of the source character set except
the new-line character and "
A.1.9 Preprocessing numbers
(6.4.8) pp-number:
digit
. digit
pp-number identifier-continue
pp-number ’ digit
pp-number ’ nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
(6.5.1) primary-expression:
A.2 Phrase structure grammar
A.2.1 Expressions
identifier
constant
string-literal
( expression )
generic-selection
(6.5.1.1) generic-selection:
_Generic ( assignment-expression , generic-assoc-list )
(6.5.1.1) generic-assoc-list:
generic-association
generic-assoc-list , generic-association
(6.5.1.1) generic-association:
type-name : assignment-expression
default : assignment-expression
(6.5.2) postfix-expression:
primary-expression
postfix-expression [ expression ]
postfix-expression ( argument-expression-listopt )
postfix-expression . identifier
postfix-expression -> identifier
postfix-expression ++
postfix-expression --
compound-literal
(6.5.2) argument-expression-list:
assignment-expression
argument-expression-list , assignment-expression
(6.5.2.5) compound-literal:
( storage-class-specifiersopt type-name ) braced-initializer
(6.5.2.5) storage-class-specifiers:
storage-class-specifier
storage-class-specifiers storage-class-specifier
(6.5.3) unary-expression:
postfix-expression
++ unary-expression
-- unary-expression
unary-operator cast-expression
sizeof unary-expression
sizeof ( type-name )
alignof ( type-name )
(6.5.3) unary-operator: one of
& * + - ~ !
(6.5.4) cast-expression:
unary-expression
( type-name ) cast-expression
(6.5.5) multiplicative-expression:
cast-expression
multiplicative-expression * cast-expression
multiplicative-expression / cast-expression
multiplicative-expression % cast-expression
(6.5.6) additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression - multiplicative-expression
(6.5.7) shift-expression:
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression
(6.5.8) relational-expression:
shift-expression
relational-expression < shift-expression
relational-expression > shift-expression
relational-expression <= shift-expression
relational-expression >= shift-expression
(6.5.9) equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression
(6.5.10) AND-expression:
equality-expression
AND-expression & equality-expression
(6.5.11) exclusive-OR-expression:
AND-expression
exclusive-OR-expression ^ AND-expression
(6.5.12) inclusive-OR-expression:
exclusive-OR-expression
inclusive-OR-expression | exclusive-OR-expression
(6.5.13) logical-AND-expression:
inclusive-OR-expression
logical-AND-expression && inclusive-OR-expression
(6.5.14) logical-OR-expression:
logical-AND-expression
logical-OR-expression || logical-AND-expression
(6.5.15) conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
(6.5.16) assignment-expression:
conditional-expression
unary-expression assignment-operator assignment-expression
(6.5.16) assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
(6.5.17) expression:
assignment-expression
expression , assignment-expression
(6.6) constant-expression:
conditional-expression
(6.7) declaration:
A.2.2 declaration-specifiers init-declarator-listopt ;
Declarations
attribute-specifier-sequence declaration-specifiers init-declarator-list ;
static_assert-declaration
attribute-declaration
(6.7) declaration-specifiers:
declaration-specifier attribute-specifier-sequenceopt
declaration-specifier declaration-specifiers
(6.7) declaration-specifier:
storage-class-specifier
type-specifier-qualifier
function-specifier
(6.7) init-declarator-list:
init-declarator
init-declarator-list , init-declarator
(6.7) init-declarator:
declarator
declarator = initializer
(6.7) attribute-declaration:
attribute-specifier-sequence ;
(6.7.1) storage-class-specifier:
auto
constexpr
extern
register
static
thread_local
typedef
(6.7.2) type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_BitInt ( constant-expression )
bool
_Complex
_Decimal32
_Decimal64
_Decimal128
atomic-type-specifier
struct-or-union-specifier
enum-specifier
typedef-name
typeof-specifier
(6.7.2.1) struct-or-union-specifier:
struct-or-union attribute-specifier-sequenceopt identifieropt { member-declaration-list }
struct-or-union attribute-specifier-sequenceopt identifier
(6.7.2.1) struct-or-union:
struct
union
(6.7.2.1) member-declaration-list:
member-declaration
member-declaration-list member-declaration
(6.7.2.1) member-declaration:
attribute-specifier-sequenceopt specifier-qualifier-list member-declarator-listopt ;
static_assert-declaration
(6.7.2.1) specifier-qualifier-list:
type-specifier-qualifier attribute-specifier-sequenceopt
type-specifier-qualifier specifier-qualifier-list
(6.7.2.1) type-specifier-qualifier:
type-specifier
type-qualifier
alignment-specifier
(6.7.2.1) member-declarator-list:
member-declarator
member-declarator-list , member-declarator
(6.7.2.1) member-declarator:
declarator
declaratoropt : constant-expression
(6.7.2.2) enum-specifier:
enum attribute-specifier-sequenceopt identifieropt enum-type-specifieropt
{ enumerator-list }
enum attribute-specifier-sequenceopt identifieropt enum-type-specifieropt
{ enumerator-list , }
enum identifier enum-type-specifieropt
(6.7.2.2) enumerator-list:
enumerator
enumerator-list , enumerator
(6.7.2.2) enumerator:
enumeration-constant attribute-specifier-sequenceopt
enumeration-constant attribute-specifier-sequenceopt = constant-expression
(6.7.2.2) enum-type-specifier:
: specifier-qualifier-list
(6.7.2.4) atomic-type-specifier:
_Atomic ( type-name )
(6.7.2.5) typeof-specifier:
typeof ( typeof-specifier-argument )
typeof_unqual ( typeof-specifier-argument )
(6.7.2.5) typeof-specifier-argument:
expression
type-name
(6.7.3) type-qualifier:
const
restrict
volatile
_Atomic
(6.7.4) function-specifier:
inline
_Noreturn
(6.7.5) alignment-specifier:
alignas ( type-name )
alignas ( constant-expression )
(6.7.6) declarator:
pointeropt direct-declarator
(6.7.6) direct-declarator:
identifier attribute-specifier-sequenceopt
( declarator )
array-declarator attribute-specifier-sequenceopt
function-declarator attribute-specifier-sequenceopt
(6.7.6) array-declarator:
direct-declarator [ type-qualifier-listopt assignment-expressionopt ]
direct-declarator [ static type-qualifier-listopt assignment-expression ]
direct-declarator [ type-qualifier-list static assignment-expression ]
direct-declarator [ type-qualifier-listopt * ]
(6.7.6) function-declarator:
direct-declarator ( parameter-type-listopt )
(6.7.6) pointer:
* attribute-specifier-sequenceopt type-qualifier-listopt
* attribute-specifier-sequenceopt type-qualifier-listopt pointer
(6.7.6) type-qualifier-list:
type-qualifier
type-qualifier-list type-qualifier
(6.7.6) parameter-type-list:
parameter-list
parameter-list , ...
...
(6.7.6) parameter-list:
parameter-declaration
parameter-list , parameter-declaration
(6.7.6) parameter-declaration:
attribute-specifier-sequenceopt declaration-specifiers declarator
attribute-specifier-sequenceopt declaration-specifiers abstract-declaratoropt
(6.7.7) type-name:
specifier-qualifier-list abstract-declaratoropt
(6.7.7) abstract-declarator:
pointer
pointeropt direct-abstract-declarator
(6.7.7) direct-abstract-declarator:
( abstract-declarator )
array-abstract-declarator attribute-specifier-sequenceopt
function-abstract-declarator attribute-specifier-sequenceopt
(6.7.7) array-abstract-declarator:
direct-abstract-declaratoropt [ type-qualifier-listopt assignment-expressionopt ]
direct-abstract-declaratoropt [ static type-qualifier-listopt assignment-expression ]
direct-abstract-declaratoropt [ type-qualifier-list static assignment-expression ]
direct-abstract-declaratoropt [ * ]
(6.7.7) function-abstract-declarator:
direct-abstract-declaratoropt ( parameter-type-listopt )
(6.7.8) typedef-name:
identifier
(6.7.10) braced-initializer:
{ }
{ initializer-list }
{ initializer-list , }
(6.7.10) initializer:
assignment-expression
braced-initializer
(6.7.10) initializer-list:
designationopt initializer
initializer-list , designationopt initializer
(6.7.10) designation:
designator-list =
(6.7.10) designator-list:
designator
designator-list designator
(6.7.10) designator:
[ constant-expression ]
. identifier
(6.7.11) static_assert-declaration:
static_assert ( constant-expression , string-literal ) ;
static_assert ( constant-expression ) ;
(6.7.12.1) attribute-specifier-sequence:
attribute-specifier-sequenceopt attribute-specifier
(6.7.12.1) attribute-specifier:
[ [ attribute-list ] ]
(6.7.12.1) attribute-list:
attributeopt
attribute-list , attributeopt
(6.7.12.1) attribute:
attribute-token attribute-argument-clauseopt
(6.7.12.1) attribute-token:
standard-attribute
attribute-prefixed-token
(6.7.12.1) standard-attribute:
identifier
(6.7.12.1) attribute-prefixed-token:
attribute-prefix :: identifier
(6.7.12.1) attribute-prefix:
identifier
(6.7.12.1) attribute-argument-clause:
( balanced-token-sequenceopt )
(6.7.12.1) balanced-token-sequence:
balanced-token
balanced-token-sequence balanced-token
(6.7.12.1) balanced-token:
( balanced-token-sequenceopt )
[ balanced-token-sequenceopt ]
{ balanced-token-sequenceopt }
any token other than a parenthesis, a bracket, or a brace
A.2.3 Statements
(6.8) statement:
labeled-statement
unlabeled-statement
(6.8) unlabeled-statement:
expression-statement
attribute-specifier-sequenceopt primary-block
attribute-specifier-sequenceopt jump-statement
(6.8) primary-block:
compound-statement
selection-statement
iteration-statement
(6.8) secondary-block:
statement
(6.8.1) label:
attribute-specifier-sequenceopt identifier :
attribute-specifier-sequenceopt case constant-expression :
attribute-specifier-sequenceopt default :
(6.8.1) labeled-statement:
label statement
(6.8.2) compound-statement:
{ block-item-listopt }
(6.8.2) block-item-list:
block-item
block-item-list block-item
(6.8.2) block-item:
declaration
unlabeled-statement
label
(6.8.3) expression-statement:
expressionopt ;
attribute-specifier-sequence expression ;
(6.8.4) selection-statement:
if ( expression ) secondary-block
if ( expression ) secondary-block else secondary-block
switch ( expression ) secondary-block
(6.8.5) iteration-statement:
while ( expression ) secondary-block
do secondary-block while ( expression ) ;
for ( expressionopt ; expressionopt ; expressionopt ) secondary-block
for ( declaration expressionopt ; expressionopt ) secondary-block
(6.8.6) jump-statement:
goto identifier ;
continue ;
break ;
return expressionopt ;
A.2.4 External definitions
(6.9) translation-unit:
external-declaration
translation-unit external-declaration
(6.9) external-declaration:
function-definition
declaration
(6.9.1) function-definition:
attribute-specifier-sequenceopt declaration-specifiers declarator function-body
(6.9.1) function-body:
compound-statement
A.3 Preprocessing directives
(6.10) preprocessing-file:
groupopt
(6.10) group:
group-part
group group-part
(6.10) group-part:
if-section
control-line
text-line
# non-directive
(6.10) if-section:
if-group elif-groupsopt else-groupopt endif-line
(6.10) if-group:
# if constant-expression new-line groupopt
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt
(6.10) elif-groups:
elif-group
elif-groups elif-group
(6.10) elif-group:
# elif constant-expression new-line groupopt
# elifdef identifier new-line groupopt
# elifndef identifier new-line groupopt
(6.10) else-group:
# else new-line groupopt
(6.10) endif-line:
# endif new-line
(6.10) control-line:
# include pp-tokens new-line
# embed pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-listopt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
# undef identifier new-line
# line pp-tokens new-line
# error pp-tokensopt new-line
# warning pp-tokensopt new-line
# pragma pp-tokensopt new-line
# new-line
(6.10) text-line:
pp-tokensopt new-line
(6.10) non-directive:
pp-tokens new-line
(6.10) lparen:
a ( character not immediately preceded by white space
(6.10) replacement-list:
pp-tokensopt
(6.10) pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
(6.10) new-line:
the new-line character
(6.10) identifier-list:
identifier
identifier-list , identifier
(6.10) pp-parameter:
pp-parameter-name pp-parameter-clauseopt
(6.10) pp-parameter-name:
pp-standard-parameter
pp-prefixed-parameter
(6.10) pp-standard-parameter:
identifier
(6.10) pp-prefixed-parameter:
identifier :: identifier
(6.10) pp-parameter-clause:
( pp-balanced-token-sequenceopt )
(6.10) pp-balanced-token-sequence:
pp-balanced-token
pp-balanced-token-sequence pp-balanced-token
(6.10) pp-balanced-token:
( pp-balanced-token-sequenceopt )
[ pp-balanced-token-sequenceopt ]
{ pp-balanced-token-sequenceopt }
any pp-token other than a parenthesis, a bracket, or a brace
(6.10) embed-parameter-sequence:
pp-parameter
embed-parameter-sequence pp-parameter
defined-macro-expression:
defined identifier
defined ( identifier )
h-preprocessing-token:
any preprocessing-token other than >
h-pp-tokens:
h-preprocessing-token
h-pp-tokens h-preprocessing-token
header-name-tokens:
string-literal
< h-pp-tokens >
has-include-expression:
__has_include ( header-name )
__has_include ( header-name-tokens )
has-embed-expression:
__has_embed ( header-name embed-parameter-sequenceopt )
__has_embed ( header-name-tokens pp-balanced-token-sequenceopt )
has-c-attribute-express:
__has_c_attribute ( pp-tokens )
va-opt-replacement:
__VA_OPT__ ( pp-tokensopt )
(6.10.7) standard-pragma:
# pragma STDC FP_CONTRACT on-off-switch
# pragma STDC FENV_ACCESS on-off-switch
# pragma STDC FENV_DEC_ROUND dec-direction
# pragma STDC FENV_ROUND direction
# pragma STDC CX_LIMITED_RANGE on-off-switch
(6.10.7) on-off-switch: one of
ON OFF DEFAULT
(6.10.7) direction: one of
FE_DOWNWARD FE_TONEAREST FE_TONEARESTFROMZERO
FE_TOWARDZERO FE_UPWARD FE_DYNAMIC
(6.10.7) dec-direction: one of
FE_DEC_DOWNWARD FE_DEC_TONEAREST FE_DEC_TONEARESTFROMZERO
FE_DEC_TOWARDZERO FE_DEC_UPWARD FE_DEC_DYNAMIC
A.4 Floating-point subject sequence
A.4.1 NaN char sequence
(7.24.1.5) n-char-sequence:
digit
nondigit
n-char-sequence digit
n-char-sequence nondigit
A.4.2 NaN wchar_t sequence
(7.31.4.1.2) n-wchar-sequence:
digit
nondigit
n-wchar-sequence digit
n-wchar-sequence nondigit
A.5 Decimal floating-point subject sequence
A.5.1 NaN decimal char sequence
(7.24.1.6) d-char-sequence:
digit
nondigit
d-char-sequence digit
d-char-sequence nondigit
A.5.2 NaN decimal wchar_t sequence
(7.31.4.1.3) d-wchar-sequence:
digit
nondigit
d-wchar-sequence digit
d-wchar-sequence nondigit
B. Annex B (informative) Library summary
B.1 Diagnostics <assert.h>
NDEBUG
void assert(scalar expression);
B.2 Complex <complex.h>
__STDC_NO_COMPLEX__ imaginary
complex _Imaginary_I
_Complex_I I
#pragma STDC CX_LIMITED_RANGE on-off-switch
double complex cacos(double complex z);
float complex cacosf(float complex z);
long double complex cacosl(long double complex z);
double complex casin(double complex z);
float complex casinf(float complex z);
long double complex casinl(long double complex z);
double complex catan(double complex z);
float complex catanf(float complex z);
long double complex catanl(long double complex z);
double complex ccos(double complex z);
float complex ccosf(float complex z);
long double complex ccosl(long double complex z);
double complex csin(double complex z);
float complex csinf(float complex z);
long double complex csinl(long double complex z);
double complex ctan(double complex z);
float complex ctanf(float complex z);
long double complex ctanl(long double complex z);
double complex cacosh(double complex z);
float complex cacoshf(float complex z);
long double complex cacoshl(long double complex z);
double complex casinh(double complex z);
float complex casinhf(float complex z);
long double complex casinhl(long double complex z);
double complex catanh(double complex z);
float complex catanhf(float complex z);
long double complex catanhl(long double complex z);
double complex ccosh(double complex z);
float complex ccoshf(float complex z);
long double complex ccoshl(long double complex z);
double complex csinh(double complex z);
float complex csinhf(float complex z);
long double complex csinhl(long double complex z);
double complex ctanh(double complex z);
float complex ctanhf(float complex z);
long double complex ctanhl(long double complex z);
double complex cexp(double complex z);
float complex cexpf(float complex z);
long double complex cexpl(long double complex z);
double complex clog(double complex z);
float complex clogf(float complex z);
long double complex clogl(long double complex z);
double cabs(double complex z);
float cabsf(float complex z);
long double cabsl(long double complex z);
double complex cpow(double complex x, double complex y);
float complex cpowf(float complex x, float complex y);
long double complex cpowl(long double complex x, long double complex y);
double complex csqrt(double complex z);
float complex csqrtf(float complex z);
long double complex csqrtl(long double complex z);
double carg(double complex z);
float cargf(float complex z);
long double cargl(long double complex z);
double cimag(double complex z);
float cimagf(float complex z);
long double cimagl(long double complex z);
double complex CMPLX(double x, double y);
float complex CMPLXF(float x, float y);
long double complex CMPLXL(long double x, long double y);
double complex conj(double complex z);
float complex conjf(float complex z);
long double complex conjl(long double complex z);
double complex cproj(double complex z);
float complex cprojf(float complex z);
long double complex cprojl(long double complex z);
double creal(double complex z);
float crealf(float complex z);
long double creall(long double complex z);
B.3 Character handling <ctype.h>
int isalnum(int c);
int isalpha(int c);
int isblank(int c);
int iscntrl(int c);
int isdigit(int c);
int isgraph(int c);
int islower(int c);
int isprint(int c);
int ispunct(int c);
int isspace(int c);
int isupper(int c);
int isxdigit(int c);
int tolower(int c);
int toupper(int c);
B.4 Errors <errno.h>
EDOM EILSEQ ERANGE errno
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <errno.h>:
errno_t
B.5 Floating-point environment <fenv.h>
fenv_t FE_OVERFLOW FE_TOWARDZERO
fexcept_t FE_UNDERFLOW FE_UPWARD
FE_DIVBYZERO FE_ALL_EXCEPT FE_DFL_ENV
FE_INEXACT FE_DOWNWARD
FE_INVALID FE_TONEAREST
#pragma STDC FENV_ACCESS on-off-switch
#pragma STDC FENV_ROUND direction
#pragma STDC FENV_ROUND FE_DYNAMIC
int feclearexcept(int excepts);
int fegetexceptflag(fexcept_t *flagp, int excepts);
int feraiseexcept(int excepts);
int fesetexcept(int excepts);
int fesetexceptflag(const fexcept_t *flagp, int excepts);
int fetestexceptflag(const fexcept_t * flagp, int excepts);
int fetestexcept(int excepts);
int fegetmode(femode_t *modep);
int fegetround(void);
int fesetmode(const femode_t *modep);
int fesetround(int rnd);
int fegetenv(fenv_t *envp);
int feholdexcept(fenv_t *envp);
int fesetenv(const fenv_t *envp);
int feupdateenv(const fenv_t *envp);
Only if the implementation defines __STDC_IEC_60559_DFP__ :
FE_DEC_DOWNWARD FE_DEC_TONEARESTFROMZERO FE_DEC_UPWARD
FE_DEC_TONEAREST FE_DEC_TOWARDZERO
#pragma STDC FENV_DEC_ROUND dec-direction
int fe_dec_getround(void);
int fe_dec_setround(int rnd);
B.6 Characteristics of floating types <float.h>
FLT_ROUNDS LDBL_DIG DBL_NORM_MAX
FLT_EVAL_METHOD FLT_MIN_EXP LDBL_NORM_MAX
FLT_HAS_SUBNORM DBL_MIN_EXP FLT_EPSILON
DBL_HAS_SUBNORM LDBL_MIN_EXP DBL_EPSILON
LDBL_HAS_SUBNORM FLT_MIN_10_EXP LDBL_EPSILON
FLT_RADIX DBL_MIN_10_EXP FLT_MIN
FLT_MANT_DIG LDBL_MIN_10_EXP DBL_MIN
DBL_MANT_DIG FLT_MAX_EXP LDBL_MIN
LDBL_MANT_DIG DBL_MAX_EXP FLT_SNAN
FLT_DECIMAL_DIG LDBL_MAX_EXP DBL_SNAN
DBL_DECIMAL_DIG FLT_MAX_10_EXP LDBL_SNAN
LDBL_DECIMAL_DIG DBL_MAX_10_EXP FLT_TRUE_MIN
DECIMAL_DIG LDBL_MAX_10_EXP DBL_TRUE_MIN
FLT_IS_IEC_60559 FLT_MAX LDBL_TRUE_MIN
DBL_IS_IEC_60559 DBL_MAX INFINITY
FLT_DIG LDBL_MAX NAN
DBL_DIG FLT_NORM_MAX
B.6.1 Characteristics of decimal floating types
1 The following macros are provided only if the implementation defines __STDC_IEC_60559_DFP__ .
N is 32, 64 and 128.
DEC_INFINITY DECN_MANT_DIG DECN_MIN_EXP DECN_SNAN
DEC_NAN DECN_MAX_EXP DECN_MIN
DECN_EPSILON DECN_MAX DECN_TRUE_MIN
B.7 Format conversion of integer types <inttypes.h>
imaxdiv_t
PRIdN PRIdLEASTN PRIdFASTN PRIdMAX PRIdPTR
PRIiN PRIiLEASTN PRIiFASTN PRIiMAX PRIiPTR
PRIoN PRIoLEASTN PRIoFASTN PRIoMAX PRIoPTR
PRIuN PRIuLEASTN PRIuFASTN PRIuMAX PRIuPTR
PRIxN PRIxLEASTN PRIxFASTN PRIxMAX PRIxPTR
PRIXN PRIXLEASTN PRIXFASTN PRIXMAX PRIXPTR
SCNdN SCNdLEASTN SCNdFASTN SCNdMAX SCNdPTR
SCNiN SCNiLEASTN SCNiFASTN SCNiMAX SCNiPTR
SCNoN SCNoLEASTN SCNoFASTN SCNoMAX SCNoPTR
SCNuN SCNuLEASTN SCNuFASTN SCNuMAX SCNuPTR
SCNxN SCNxLEASTN SCNxFASTN SCNxMAX SCNxPTR
intmax_t imaxabs(intmax_t j);
imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);
intmax_t strtoimax(const char * restrict nptr, char ** restrict endptr, int base);
uintmax_t strtoumax(const char * restrict nptr, char ** restrict endptr, int base);
intmax_t wcstoimax(const wchar_t *restrict nptr, wchar_t **restrict endptr, int base);
uintmax_t wcstoumax(const wchar_t *restrict nptr, wchar_t **restrict endptr, int base);
B.8 Alternative spellings <iso646.h>
and bitor not_eq xor
and_eq compl or xor_eq
bitand not or_eq
B.9 Sizes of integer types <limits.h>
BOOL_WIDTH UINT_WIDTH UCHAR_MAX INT_MAX
CHAR_BIT LONG_WIDTH CHAR_MIN UINT_MAX
CHAR_WIDTH ULONG_WIDTH CHAR_MAX LONG_MIN
SCHAR_WIDTH LLONG_WIDTH MB_LEN_MAX LONG_MAX
UCHAR_WIDTH ULLONG_WIDTH SHRT_MIN ULONG_MAX
SHRT_WIDTH BOOL_MAX SHRT_MAX LLONG_MIN
USHRT_WIDTH SCHAR_MIN USHRT_MAX LLONG_MAX
INT_WIDTH SCHAR_MAX INT_MIN ULLONG_MAX
B.10 Localization <locale.h>
struct lconv LC_ALL LC_CTYPE LC_NUMERIC
NULL LC_COLLATE LC_MONETARY LC_TIME
char *setlocale(int category, const char *locale);
struct lconv *localeconv(void);
B.11 Mathematics <math.h>
float_t FP_INFINITE FP_FAST_FMAL
double_t FP_NAN FP_ILOGB0
HUGE_VAL FP_NORMAL FP_ILOGBNAN
HUGE_VALF FP_SUBNORMAL MATH_ERRNO
HUGE_VALL FP_ZERO MATH_ERREXCEPT
INFINITY FP_FAST_FMA math_errhandling
NAN FP_FAST_FMAF
#pragma STDC FP_CONTRACT on-off-switch
int fpclassify(real-floating x);
int iscanonical(real-floating x);
int isfinite(real-floating x);
int isinf(real-floating x);
int isnan(real-floating x);
int isnormal(real-floating x);
int signbit(real-floating x);
int issignaling(real-floating x);
int issubnormal(real-floating x);
int iszero(real-floating x);
double acos(double x);
float acosf(float x);
long double acosl(long double x);
double asin(double x);
float asinf(float x);
long double asinl(long double x);
double atan(double x);
float atanf(float x);
long double atanl(long double x);
double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);
double cos(double x);
float cosf(float x);
long double cosl(long double x);
double sin(double x);
float sinf(float x);
long double sinl(long double x);
double tan(double x);
float tanf(float x);
long double tanl(long double x);
double acospi(double x);
float acospif(float x);
long double acospil(long double x);
double asinpi(double x);
float asinpif(float x);
long double asinpil(long double x);
double atanpi(double x);
float atanpif(float x);
long double atanpil(long double x);
double atan2pi(double y, double x);
float atan2pif(float y, float x);
long double atan2pil(long double y, long double x);
double cospi(double x);
float cospif(float x);
long double cospil(long double x);
double sinpi(double x);
float sinpif(float x);
long double sinpil(long double x);
double tanpi(double x);
float tanpif(float x);
long double tanpil(long double x);
double acosh(double x);
float acoshf(float x);
long double acoshl(long double x);
double asinh(double x);
float asinhf(float x);
long double asinhl(long double x);
double atanh(double x);
float atanhf(float x);
long double atanhl(long double x);
double cosh(double x);
float coshf(float x);
long double coshl(long double x);
double sinh(double x);
float sinhf(float x);
long double sinhl(long double x);
double tanh(double x);
float tanhf(float x);
long double tanhl(long double x);
double exp(double x);
float expf(float x);
long double expl(long double x);
double exp10(double x);
float exp10f(float x);
long double exp10l(long double x);
double exp10m1(double x);
float exp10m1f(float x);
long double exp10m1l(long double x);
double exp2(double x);
float exp2f(float x);
long double exp2l(long double x);
double exp2m1(double x);
float exp2m1f(float x);
long double exp2m1l(long double x);
double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);
double frexp(double value, int *p);
float frexpf(float value, int *p);
long double frexpl(long double value, int *p);
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
double ldexp(double x, int p);
float ldexpf(float x, int p);
long double ldexpl(long double x, int p);
long int llogb(double x);
long int llogbf(float x);
long int llogbl(long double x);
double log(double x);
float logf(float x);
long double logl(long double x);
double log10(double x);
float log10f(float x);
long double log10l(long double x);
double log10p1(double x);
float log10p1f(float x);
long double log10p1l(long double x);
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
double logp1(double x);
float logp1f(float x);
long double logp1l(long double x);
double log2(double x);
float log2f(float x);
long double log2l(long double x);
double log2p1(double x);
float log2p1f(float x);
long double log2p1l(long double x);
double logb(double x);
float logbf(float x);
long double logbl(long double x);
double modf(double value, double *iptr);
float modff(float value, float *iptr);
long double modfl(long double value, long double *iptr);
double scalbn(double x, int n);
float scalbnf(float x, int n);
long double scalbnl(long double x, int n);
double scalbln(double x, long int n);
float scalblnf(float x, long int n);
long double scalblnl(long double x, long int n);
double cbrt(double x);
float cbrtf(float x);
long double cbrtl(long double x);
double compoundn(double x, long long int n);
float compoundnf(float x, long long int n);
long double compoundnl(long double x, long long int n);
double fabs(double x);
float fabsf(float x);
long double fabsl(long double x);
double hypot(double x, double y);
float hypotf(float x, float y);
long double hypotl(long double x, long double y);
double pow(double x, double y);
float powf(float x, float y);
long double powl(long double x, long double y);
double pown(double x, long long int n);
float pownf(float x, long long int n);
long double pownl(long double x, long long int n);
double powr(double y, double x);
float powrf(float y, float x);
long double powrl(long double y, long double x);
double rootn(double x, long long int n);
float rootnf(float x, long long int n);
long double rootnl(long double x, long long int n);
double rsqrt(double x);
float rsqrtf(float x);
long double rsqrtl(long double x);
double sqrt(double x);
float sqrtf(float x);
long double sqrtl(long double x);
double erf(double x);
float erff(float x);
long double erfl(long double x);
double erfc(double x);
float erfcf(float x);
long double erfcl(long double x);
double lgamma(double x);
float lgammaf(float x);
long double lgammal(long double x);
double tgamma(double x);
float tgammaf(float x);
long double tgammal(long double x);
double ceil(double x);
float ceilf(float x);
long double ceill(long double x);
double floor(double x);
float floorf(float x);
long double floorl(long double x);
double nearbyint(double x);
float nearbyintf(float x);
long double nearbyintl(long double x);
double rint(double x);
float rintf(float x);
long double rintl(long double x);
long int lrint(double x);
long int lrintf(float x);
long int lrintl(long double x);
long long int llrint(double x);
long long int llrintf(float x);
long long int llrintl(long double x);
double round(double x);
float roundf(float x);
long double roundl(long double x);
long int lround(double x);
long int lroundf(float x);
long int lroundl(long double x);
long long int llround(double x);
long long int llroundf(float x);
long long int llroundl(long double x);
double roundeven(double x);
float roundevenf(float x);
long double roundevenl(long double x);
double trunc(double x);
float truncf(float x);
long double truncl(long double x);
double fromfp(double x, int rnd, unsigned int width);
float fromfpf(float x, int rnd, unsigned int width);
long double fromfpl(long double x, int rnd, unsigned int width);
double ufromfp(double x, int rnd, unsigned int width);
float ufromfpf(float x, int rnd, unsigned int width);
long double ufromfpl(long double x, int rnd, unsigned int width);
double fromfpx(double x, int rnd, unsigned int width);
float fromfpxf(float x, int rnd, unsigned int width);
long double fromfpxl(long double x, int rnd, unsigned int width);
double ufromfpx(double x, int rnd, unsigned int width);
float ufromfpxf(float x, int rnd, unsigned int width);
long double ufromfpxl(long double x, int rnd, unsigned int width);
double fmod(double x, double y);
float fmodf(float x, float y);
long double fmodl(long double x, long double y);
double remainder(double x, double y);
float remainderf(float x, float y);
long double remainderl(long double x, long double y);
double remquo(double x, double y, int *quo);
float remquof(float x, float y, int *quo);
long double remquol(long double x, long double y, int *quo);
double copysign(double x, double y);
float copysignf(float x, float y);
long double copysignl(long double x, long double y);
double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);
double nextafter(double x, double y);
float nextafterf(float x, float y);
long double nextafterl(long double x, long double y);
double nexttoward(double x, long double y);
float nexttowardf(float x, long double y);
long double nexttowardl(long double x, long double y);
double nextup(double x);
float nextupf(float x);
long double nextupl(long double x);
double nextdown(double x);
float nextdownf(float x);
long double nextdownl(long double x);
int canonicalize(double * cx, const double * x);
int canonicalizef(float * cx, const float * x);
int canonicalizel(long double * cx, const long double * x);
double fdim(double x, double y);
float fdimf(float x, float y);
long double fdiml(long double x, long double y);
double fmax(double x, double y);
float fmaxf(float x, float y);
long double fmaxl(long double x, long double y);
double fmin(double x, double y);
float fminf(float x, float y);
long double fminl(long double x, long double y);
double fmaximum(double x, double y);
float fmaximumf(float x, float y);
long double fmaximuml(long double x, long double y);
double fminimum(double x, double y);
float fminimumf(float x, float y);
long double fminimuml(long double x, long double y);
double fmaximum_mag(double x, double y);
float fmaximum_magf(float x, float y);
long double fmaximum_magl(long double x, long double y);
double fminimum_mag(double x, double y);
float fminimum_magf(float x, float y);
long double fminimum_magl(long double x, long double y);
double fmaximum_num(double x, double y);
float fmaximum_numf(float x, float y);
long double fmaximum_numl(long double x, long double y);
double fminimum_num(double x, double y);
float fminimum_numf(float x, float y);
long double fminimum_numl(long double x, long double y);
double fmaximum_mag_num(double x, double y);
float fmaximum_mag_numf(float x, float y);
long double fmaximum_mag_numl(long double x, long double y);
double fminimum_mag_num(double x, double y);
float fminimum_mag_numf(float x, float y);
long double fminimum_mag_numl(long double x, long double y);
double fma(double x, double y, double z);
float fmaf(float x, float y, float z);
long double fmal(long double x, long double y, long double z);
float fadd(double x, double y);
float faddl(long double x, long double y);
double daddl(long double x, long double y);
float fsub(double x, double y);
float fsubl(long double x, long double y);
double dsubl(long double x, long double y);
float fmul(double x, double y);
float fmull(long double x, long double y);
double dmull(long double x, long double y);
float fdiv(double x, double y);
float fdivl(long double x, long double y);
double ddivl(long double x, long double y);
float ffma(double x, double y, double z);
float ffmal(long double x, long double y, long double z);
double dfmal(long double x, long double y, long double z);
float fsqrt(double x);
float fsqrtl(long double x);
double dsqrtl(long double x);
int isgreater(real-floating x, real-floating y);
int isgreaterequal(real-floating x, real-floating y);
int isless(real-floating x, real-floating y);
int islessequal(real-floating x, real-floating y);
int islessgreater(real-floating x, real-floating y);
int isunordered(real-floating x, real-floating y);
int iseqsig(real-floating x, real-floating y);
Only if the implementation defines __STDC_IEC_60559_DFP__ :
_Decimal32 acosd32(_Decimal32 x);
_Decimal64 acosd64(_Decimal64 x);
_Decimal128 acosd128(_Decimal128 x);
_Decimal32 asind32(_Decimal32 x);
_Decimal64 asind64(_Decimal64 x);
_Decimal128 asind128(_Decimal128 x);
_Decimal32 atand32(_Decimal32 x);
_Decimal64 atand64(_Decimal64 x);
_Decimal128 atand128(_Decimal128 x);
_Decimal32 atan2d32(_Decimal32 y, _Decimal32 x);
_Decimal64 atan2d64(_Decimal64 y, _Decimal64 x);
_Decimal128 atan2d128(_Decimal128 y, _Decimal128 x);
_Decimal32 cosd32(_Decimal32 x);
_Decimal64 cosd64(_Decimal64 x);
_Decimal128 cosd128(_Decimal128 x);
_Decimal32 sind32(_Decimal32 x);
_Decimal64 sind64(_Decimal64 x);
_Decimal128 sind128(_Decimal128 x);
_Decimal32 tand32(_Decimal32 x);
_Decimal64 tand64(_Decimal64 x);
_Decimal128 tand128(_Decimal128 x);
_Decimal32 acospid32(_Decimal32 x);
_Decimal64 acospid64(_Decimal64 x);
_Decimal128 acospid128(_Decimal128 x);
_Decimal32 asinpid32(_Decimal32 x);
_Decimal64 asinpid64(_Decimal64 x);
_Decimal128 asinpid128(_Decimal128 x);
_Decimal32 atanpid32(_Decimal32 x);
_Decimal64 atanpid64(_Decimal64 x);
_Decimal128 atanpid128(_Decimal128 x);
_Decimal32 atan2pid32(_Decimal32 y, _Decimal32 x);
_Decimal64 atan2pid64(_Decimal64 y, _Decimal64 x);
_Decimal128 atan2pid128(_Decimal128 y, _Decimal128 x);
_Decimal32 cospid32(_Decimal32 x);
_Decimal64 cospid64(_Decimal64 x);
_Decimal128 cospid128(_Decimal128 x);
_Decimal32 sinpid32(_Decimal32 x);
_Decimal64 sinpid64(_Decimal64 x);
_Decimal128 sinpid128(_Decimal128 x);
_Decimal32 tanpid32(_Decimal32 x);
_Decimal64 tanpid64(_Decimal64 x);
_Decimal128 tanpid128(_Decimal128 x);
_Decimal32 acoshd32(_Decimal32 x);
_Decimal64 acoshd64(_Decimal64 x);
_Decimal128 acoshd128(_Decimal128 x);
_Decimal32 asinhd32(_Decimal32 x);
_Decimal64 asinhd64(_Decimal64 x);
_Decimal128 asinhd128(_Decimal128 x);
_Decimal32 atanhd32(_Decimal32 x);
_Decimal64 atanhd64(_Decimal64 x);
_Decimal128 atanhd128(_Decimal128 x);
_Decimal32 coshd32(_Decimal32 x);
_Decimal64 coshd64(_Decimal64 x);
_Decimal128 coshd128(_Decimal128 x);
_Decimal32 sinhd32(_Decimal32 x);
_Decimal64 sinhd64(_Decimal64 x);
_Decimal128 sinhd128(_Decimal128 x);
_Decimal32 tanhd32(_Decimal32 x);
_Decimal64 tanhd64(_Decimal64 x);
_Decimal128 tanhd128(_Decimal128 x);
_Decimal32 expd32(_Decimal32 x);
_Decimal64 expd64(_Decimal64 x);
_Decimal128 expd128(_Decimal128 x);
_Decimal32 exp10d32(_Decimal32 x);
_Decimal64 exp10d64(_Decimal64 x);
_Decimal128 exp10d128(_Decimal128 x);
_Decimal32 exp10m1d32(_Decimal32 x);
_Decimal64 exp10m1d64(_Decimal64 x);
_Decimal128 exp10m1d128(_Decimal128 x);
_Decimal32 exp2d32(_Decimal32 x);
_Decimal64 exp2d64(_Decimal64 x);
_Decimal128 exp2d128(_Decimal128 x);
_Decimal32 exp2m1d32(_Decimal32 x);
_Decimal64 exp2m1d64(_Decimal64 x);
_Decimal128 exp2m1d128(_Decimal128 x);
_Decimal32 expm1d32(_Decimal32 x);
_Decimal64 expm1d64(_Decimal64 x);
_Decimal128 expm1d128(_Decimal128 x);
_Decimal32 frexpd32(_Decimal32 value, int *p);
_Decimal64 frexpd64(_Decimal64 value, int *p);
_Decimal128 frexpd128(_Decimal128 value, int *p);
int ilogbd32(_Decimal32 x);
int ilogbd64(_Decimal64 x);
int ilogbd128(_Decimal128 x);
_Decimal32 ldexpd32(_Decimal32 x, int p);
_Decimal64 ldexpd64(_Decimal64 x, int p);
_Decimal128 ldexpd128(_Decimal128 x, int p);
long int llogbd32(_Decimal32 x);
long int llogbd64(_Decimal64 x);
long int llogbd128(_Decimal128 x);
_Decimal32 logd32(_Decimal32 x);
_Decimal64 logd64(_Decimal64 x);
_Decimal128 logd128(_Decimal128 x);
_Decimal32 log10d32(_Decimal32 x);
_Decimal64 log10d64(_Decimal64 x);
_Decimal128 log10d128(_Decimal128 x);
_Decimal32 log10p1d32(_Decimal32 x);
_Decimal64 log10p1d64(_Decimal64 x);
_Decimal128 log10p1d128(_Decimal128 x);
_Decimal32 log1pd32(_Decimal32 x);
_Decimal64 log1pd64(_Decimal64 x);
_Decimal128 log1pd128(_Decimal128 x);
_Decimal32 logp1d32(_Decimal32 x);
_Decimal64 logp1d64(_Decimal64 x);
_Decimal128 logp1d128(_Decimal128 x);
_Decimal32 log2d32(_Decimal32 x);
_Decimal64 log2d64(_Decimal64 x);
_Decimal128 log2d128(_Decimal128 x);
_Decimal32 log2p1d32(_Decimal32 x);
_Decimal64 log2p1d64(_Decimal64 x);
_Decimal128 log2p1d128(_Decimal128 x);
_Decimal32 logbd32(_Decimal32 x);
_Decimal64 logbd64(_Decimal64 x);
_Decimal128 logbd128(_Decimal128 x);
_Decimal32 modfd32(_Decimal32 x, _Decimal32 *iptr);
_Decimal64 modfd64(_Decimal64 x, _Decimal64 *iptr);
_Decimal128 modfd128(_Decimal128 x, _Decimal128 *iptr);
_Decimal32 scalbnd32(_Decimal32 x, int n);
_Decimal64 scalbnd64(_Decimal64 x, int n);
_Decimal128 scalbnd128(_Decimal128 x, int n);
_Decimal32 scalblnd32(_Decimal32 x, long int n);
_Decimal64 scalblnd64(_Decimal64 x, long int n);
_Decimal128 scalblnd128(_Decimal128 x, long int n);
_Decimal32 cbrtd32(_Decimal32 x);
_Decimal64 cbrtd64(_Decimal64 x);
_Decimal128 cbrtd128(_Decimal128 x);
_Decimal32 compoundnd32(_Decimal32 x, long long int n);
_Decimal64 compoundnd64(_Decimal64 x, long long int n);
_Decimal128 compoundnd128(_Decimal128 x, long long int n);
_Decimal32 fabsd32(_Decimal32 x);
_Decimal64 fabsd64(_Decimal64 x);
_Decimal128 fabsd128(_Decimal128 x);
_Decimal32 hypotd32(_Decimal32 x, _Decimal32 y);
_Decimal64 hypotd64(_Decimal64 x, _Decimal64 y);
_Decimal128 hypotd128(_Decimal128 x, _Decimal128 y);
_Decimal32 powd32(_Decimal32 x, _Decimal32 y);
_Decimal64 powd64(_Decimal64 x, _Decimal64 y);
_Decimal128 powd128(_Decimal128 x, _Decimal128 y);
_Decimal32 pownd32(_Decimal32 x, long long int n);
_Decimal64 pownd64(_Decimal64 x, long long int n);
_Decimal128 pownd128(_Decimal128 x, long long int n);
_Decimal32 powrd32(_Decimal32 y, _Decimal32 x);
_Decimal64 powrd64(_Decimal64 y, _Decimal64 x);
_Decimal128 powrd128(_Decimal128 y, _Decimal128 x);
_Decimal32 rootnd32(_Decimal32 x, long long int n);
_Decimal64 rootnd64(_Decimal64 x, long long int n);
_Decimal128 rootnd128(_Decimal128 x, long long int n);
_Decimal32 rsqrtd32(_Decimal32 x);
_Decimal64 rsqrtd64(_Decimal64 x);
_Decimal128 rsqrtd128(_Decimal128 x);
_Decimal32 sqrtd32(_Decimal32 x);
_Decimal64 sqrtd64(_Decimal64 x);
_Decimal128 sqrtd128(_Decimal128 x);
_Decimal32 erfd32(_Decimal32 x);
_Decimal64 erfd64(_Decimal64 x);
_Decimal128 erfd128(_Decimal128 x);
_Decimal32 erfcd32(_Decimal32 x);
_Decimal64 erfcd64(_Decimal64 x);
_Decimal128 erfcd128(_Decimal128 x);
_Decimal32 lgammad32(_Decimal32 x);
_Decimal64 lgammad64(_Decimal64 x);
_Decimal128 lgammad128(_Decimal128 x);
_Decimal32 tgammad32(_Decimal32 x);
_Decimal64 tgammad64(_Decimal64 x);
_Decimal128 tgammad128(_Decimal128 x);
_Decimal32 ceild32(_Decimal32 x);
_Decimal64 ceild64(_Decimal64 x);
_Decimal128 ceild128(_Decimal128 x);
_Decimal32 floord32(_Decimal32 x);
_Decimal64 floord64(_Decimal64 x);
_Decimal128 floord128(_Decimal128 x);
_Decimal32 nearbyintd32(_Decimal32 x);
_Decimal64 nearbyintd64(_Decimal64 x);
_Decimal128 nearbyintd128(_Decimal128 x);
_Decimal32 rintd32(_Decimal32 x);
_Decimal64 rintd64(_Decimal64 x);
_Decimal128 rintd128(_Decimal128 x);
long int lrintd32(_Decimal32 x);
long int lrintd64(_Decimal64 x);
long int lrintd128(_Decimal128 x);
long long int llrintd32(_Decimal32 x);
long long int llrintd64(_Decimal64 x);
long long int llrintd128(_Decimal128 x);
_Decimal32 roundd32(_Decimal32 x);
_Decimal64 roundd64(_Decimal64 x);
_Decimal128 roundd128(_Decimal128 x);
long int lroundd32(_Decimal32 x);
long int lroundd64(_Decimal64 x);
long int lroundd128(_Decimal128 x);
long long int llroundd32(_Decimal32 x);
long long int llroundd64(_Decimal64 x);
long long int llroundd128(_Decimal128 x);
_Decimal32 roundevend32(_Decimal32 x);
_Decimal64 roundevend64(_Decimal64 x);
_Decimal128 roundevend128(_Decimal128 x);
_Decimal32 truncd32(_Decimal32 x);
_Decimal64 truncd64(_Decimal64 x);
_Decimal128 truncd128(_Decimal128 x);
_Decimal32 fromfpd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 fromfpd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 fromfpd128(_Decimal128 x, int rnd, unsigned int width);
_Decimal32 ufromfpd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 ufromfpd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 ufromfpd128(_Decimal128 x, int rnd, unsigned int width);
_Decimal32 fromfpxd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 fromfpxd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 fromfpxd128(_Decimal128 x, int rnd, unsigned int width);
_Decimal32 ufromfpxd32(_Decimal32 x, int rnd, unsigned int width);
_Decimal64 ufromfpxd64(_Decimal64 x, int rnd, unsigned int width);
_Decimal128 ufromfpxd128(_Decimal128 x, int rnd, unsigned int width);
_Decimal32 fmodd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmodd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmodd128(_Decimal128 x, _Decimal128 y);
_Decimal32 remainderd32(_Decimal32 x, _Decimal32 y);
_Decimal64 remainderd64(_Decimal64 x, _Decimal64 y);
_Decimal128 remainderd128(_Decimal128 x, _Decimal128 y);
_Decimal32 copysignd32(_Decimal32 x, _Decimal32 y);
_Decimal64 copysignd64(_Decimal64 x, _Decimal64 y);
_Decimal128 copysignd128(_Decimal128 x, _Decimal128 y);
_Decimal32 nand32(const char *tagp);
_Decimal64 nand64(const char *tagp);
_Decimal128 nand128(const char *tagp);
_Decimal32 nextafterd32(_Decimal32 x, _Decimal32 y);
_Decimal64 nextafterd64(_Decimal64 x, _Decimal64 y);
_Decimal128 nextafterd128(_Decimal128 x, _Decimal128 y);
_Decimal32 nexttowardd32(_Decimal32 x, _Decimal128 y);
_Decimal64 nexttowardd64(_Decimal64 x, _Decimal128 y);
_Decimal128 nexttowardd128(_Decimal128 x, _Decimal128 y);
_Decimal32 nextupd32(_Decimal32 x);
_Decimal64 nextupd64(_Decimal64 x);
_Decimal128 nextupd128(_Decimal128 x);
_Decimal32 nextdownd32(_Decimal32 x);
_Decimal64 nextdownd64(_Decimal64 x);
_Decimal128 nextdownd128(_Decimal128 x);
int canonicalized32(_Decimal32 cx, const _Decimal32 * x);
int canonicalized64(_Decimal64 cx, const _Decimal64 * x);
int canonicalized128(_Decimal128 cx, const _Decimal128 * x);
_Decimal32 fdimd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fdimd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fdimd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmaxd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaxd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaxd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmind32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmind64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmind128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmaximumd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximumd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximumd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fminimumd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimumd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimumd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmaximum_magd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximum_magd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximum_magd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fminimum_magd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimum_magd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimum_magd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmaximum_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximum_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximum_numd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fminimum_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimum_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimum_numd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmaximum_mag_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fmaximum_mag_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fmaximum_mag_numd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fminimum_mag_numd32(_Decimal32 x, _Decimal32 y);
_Decimal64 fminimum_mag_numd64(_Decimal64 x, _Decimal64 y);
_Decimal128 fminimum_mag_numd128(_Decimal128 x, _Decimal128 y);
_Decimal32 fmad32(_Decimal32 x, _Decimal32 y, _Decimal32 z);
_Decimal64 fmad64(_Decimal64 x, _Decimal64 y, _Decimal64 z);
_Decimal128 fmad128(_Decimal128 x, _Decimal128 y, _Decimal128 z);
_Decimal32 d32addd64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32addd128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64addd128(_Decimal128 x, _Decimal128 y);
_Decimal32 d32subd64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32subd128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64subd128(_Decimal128 x, _Decimal128 y);
_Decimal32 d32muld64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32muld128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64muld128(_Decimal128 x, _Decimal128 y);
_Decimal32 d32divd64(_Decimal64 x, _Decimal64 y);
_Decimal32 d32divd128(_Decimal128 x, _Decimal128 y);
_Decimal64 d64divd128(_Decimal128 x, _Decimal128 y);
_Decimal32 d32fmad64(_Decimal64 x, _Decimal64 y, _Decimal64 z);
_Decimal32 d32fmad128(_Decimal128 x, _Decimal128 y, _Decimal128 z);
_Decimal64 d64fmad128(_Decimal128 x, _Decimal128 y, _Decimal128 z);
_Decimal32 d32sqrtd64(_Decimal64 x);
_Decimal32 d32sqrtd128(_Decimal128 x);
_Decimal64 d64sqrtd128(_Decimal128 x);
_Decimal32 quantized32(_Decimal32 x, _Decimal32 y);
_Decimal64 quantized64(_Decimal64 x, _Decimal64 y);
_Decimal128 quantized128(_Decimal128 x, _Decimal128 y);
bool samequantumd32(_Decimal32 x, _Decimal32 y);
bool samequantumd64(_Decimal64 x, _Decimal64 y);
bool samequantumd128(_Decimal128 x, _Decimal128 y);
_Decimal32 quantumd32(_Decimal32 x);
_Decimal64 quantumd64(_Decimal64 x);
_Decimal128 quantumd128(_Decimal128 x);
long long int llquantexpd32(_Decimal32 x);
long long int llquantexpd64(_Decimal64 x);
long long int llquantexpd128(_Decimal128 x);
void encodedecd32(unsigned char encptr[restrict static 4],
const _Decimal32*restrict xptr);
void encodedecd64(unsigned char encptr[restrict static 8],
const _Decimal64*restrict xptr);
void encodedecd128(unsigned char encptr[restrict static 16],
const _Decimal128*restrict xptr);
void decodedecd32(_Decimal32 * restrict xptr,
const unsigned char encptr[restrict static 4]);
void decodedecd64(_Decimal64 * restrict xptr,
const unsigned char encptr[restrict static 8]);
void decodedecd128(_Decimal128 * restrict xptr,
const unsigned char encptr[restrict static 16]);
void encodebind32(unsigned char encptr[restrict static 4],
const _Decimal32 * restrict xptr);
void encodebind64(unsigned char encptr[restrict static 8],
const _Decimal64 * restrict xptr);
void encodebind128(unsigned char encptr[restrict static 16],
const _Decimal128 * restrict xptr);
void decodebind32(_Decimal32 * restrict xptr,
const unsigned char encptr[restrict static 4]);
void decodebind64(_Decimal64 * restrict xptr,
const unsigned char encptr[restrict static 8]);
void decodebind128(_Decimal128 * restrict xptr,
const unsigned char encptr[restrict static 16]);
Only if the implementation defines __STDC_IEC_60559_BFP__ or __STDC_IEC_559__ and addition-
ally the user code defines __STDC_WANT_IEC_60559_EXT__ before any inclusion of <math.h>:
int totalorder(const double *x, const double *y);
int totalorderf(const float *x, const float *y);
int totalorderl(const long double *x, const long double *y);
int totalordermag(const double *x, const double *y);
int totalordermagf(const float *x, const float *y);
int totalordermagl(const long double *x, const long double *y);
double getpayload(const double *x);
float getpayloadf(const float *x);
long double getpayloadl(const long double *x);
int setpayload(double *res, double pl);
int setpayloadf(float *res, float pl);
int setpayloadl(long double *res, long double pl);
int setpayloadsig(double *res, double pl);
int setpayloadsigf(float *res, float pl);
int setpayloadsigl(long double *res, long double pl);
Only if the implementation defines __STDC_IEC_60559_DFP__ and additionally the user code
defines __STDC_WANT_IEC_60559_EXT__ before any inclusion of <math.h>:
_Decimal32_t _Decimal64_t HUGE_VAL_D32 HUGE_VAL_D64 HUGE_VAL_D128
int totalorderd32(const _Decimal32 *x, const _Decimal32 *y);
int totalorderd64(const _Decimal64 *x, const _Decimal64 *y);
int totalorderd128(const _Decimal128 *x, const _Decimal128 *y);
int totalordermagd32(const _Decimal32 *x, const _Decimal32 *y);
int totalordermagd64(const _Decimal64 *x, const _Decimal64 *y);
int totalordermagd128(const _Decimal128 *x, const _Decimal128 *y);
_Decimal32 getpayloadd32(const _Decimal32 *x);
_Decimal64 getpayloadd64(const _Decimal64 *x);
_Decimal128 getpayloadd128(const _Decimal128 *x);
int setpayloadd32(_Decimal32 *res, _Decimal32 pl);
int setpayloadd64(_Decimal64 *res, _Decimal64 pl);
int setpayloadd128(_Decimal128 *res, _Decimal128 pl);
int setpayloadsigd32(_Decimal32 *res, _Decimal32 pl);
int setpayloadsigd64(_Decimal64 *res, _Decimal64 pl);
int setpayloadsigd128(_Decimal128 *res, _Decimal128 pl);
B.12 Non-local jumps <setjmp.h>
jmp_buf
int setjmp(jmp_buf env);
[[noreturn]] void longjmp(jmp_buf env, int val);
B.13 Signal handling <signal.h>
sig_atomic_t SIG_IGN SIGILL SIGTERM
SIG_DFL SIGABRT SIGINT
SIG_ERR SIGFPE SIGSEGV
void (*signal(int sig, void (*func)(int)))(int);
int raise(int sig);
B.14 Alignment <stdalign.h>
The header <stdalign.h> provides no content.
B.15 Variable arguments <stdarg.h>
va_list
type va_arg(va_list ap, type);
void va_copy(va_list dest, va_list src);
void va_end(va_list ap);
void va_start(va_list ap, ...);
B.16 Atomics <stdatomic.h>
__STDC_NO_ATOMICS__ ATOMIC_CHAR16_T_LOCK_FREE ATOMIC_SHORT_LOCK_FREE
ATOMIC_BOOL_LOCK_FREE ATOMIC_CHAR32_T_LOCK_FREE ATOMIC_INT_LOCK_FREE
ATOMIC_CHAR_LOCK_FREE ATOMIC_WCHAR_T_LOCK_FREE ATOMIC_LONG_LOCK_FREE
ATOMIC_LLONG_LOCK_FREE atomic_ushort atomic_int_least64_t
ATOMIC_POINTER_LOCK_FREE atomic_int atomic_uint_least64_t
ATOMIC_FLAG_INIT atomic_uint atomic_int_fast8_t
memory_order atomic_long atomic_uint_fast8_t
atomic_flag atomic_ulong atomic_int_fast16_t
memory_order_relaxed atomic_llong atomic_uint_fast16_t
memory_order_consume atomic_ullong atomic_int_fast32_t
memory_order_acquire atomic_char16_t atomic_uint_fast32_t
memory_order_release atomic_char32_t atomic_int_fast64_t
memory_order_acq_rel atomic_wchar_t atomic_uint_fast64_t
memory_order_seq_cst atomic_int_least8_t atomic_intptr_t
atomic_bool atomic_uint_least8_t atomic_uintptr_t
atomic_char atomic_int_least16_t atomic_size_t
atomic_schar atomic_uint_least16_t atomic_ptrdiff_t
atomic_uchar atomic_int_least32_t atomic_intmax_t
atomic_short atomic_uint_least32_t atomic_uintmax_t
void atomic_init(volatile A *obj, C value);
type kill_dependency(type y);
void atomic_thread_fence(memory_order order);
void atomic_signal_fence(memory_order order);
bool atomic_is_lock_free(const volatile A *obj);
void atomic_store(volatile A *object, C desired);
void atomic_store_explicit(volatile A *object, C desired, memory_order order);
C atomic_load(const volatile A *object);
C atomic_load_explicit(const volatile A *object, memory_order order);
C atomic_exchange(volatile A *object, C desired);
C atomic_exchange_explicit(volatile A *object, C desired, memory_order order);
bool atomic_compare_exchange_strong(volatile A *object, C *expected, C desired);
bool atomic_compare_exchange_strong_explicit(volatile A *object, C *expected,
C desired, memory_order success, memory_order failure);
bool atomic_compare_exchange_weak(volatile A *object, C *expected, C desired);
bool atomic_compare_exchange_weak_explicit(volatile A *object, C *expected,
C desired, memory_order success, memory_order failure);
C atomic_fetch_key(volatile A *object, M operand);
C atomic_fetch_key_explicit(volatile A *object, M operand, memory_order order);
bool atomic_flag_test_and_set(volatile atomic_flag *object);
bool atomic_flag_test_and_set_explicit(volatile atomic_flag *object,
memory_order order);
void atomic_flag_clear(volatile atomic_flag *object);
void atomic_flag_clear_explicit(volatile atomic_flag *object,
memory_order order);
B.17 Bit and byte utilities <stdbit.h>
__STDC_ENDIAN_BIG__ __STDC_ENDIAN_LITTLE__ __STDC_ENDIAN_NATIVE__
int stdc_leading_zerosuc(unsigned char value);
int stdc_leading_zerosus(unsigned short value);
int stdc_leading_zerosui(unsigned int value);
int stdc_leading_zerosul(unsigned long value);
int stdc_leading_zerosull(unsigned long long value);
generic_return_type stdc_leading_zeros(generic_value_type value);
int stdc_leading_onesuc(unsigned char value);
int stdc_leading_onesus(unsigned short value);
int stdc_leading_onesui(unsigned int value);
int stdc_leading_onesul(unsigned long value);
int stdc_leading_onesull(unsigned long long value);
generic_return_type stdc_leading_ones(generic_value_type value);
int stdc_trailing_zerosuc(unsigned char value);
int stdc_trailing_zerosus(unsigned short value);
int stdc_trailing_zerosui(unsigned int value);
int stdc_trailing_zerosul(unsigned long value);
int stdc_trailing_zerosull(unsigned long long value);
generic_return_type stdc_trailing_zeros(generic_value_type value);
int stdc_trailing_onesuc(unsigned char value);
int stdc_trailing_onesus(unsigned short value);
int stdc_trailing_onesui(unsigned int value);
int stdc_trailing_onesul(unsigned long value);
int stdc_trailing_onesull(unsigned long long value);
generic_return_type stdc_trailing_ones(generic_value_type value);
int stdc_first_leading_zerouc(unsigned char value);
int stdc_first_leading_zerous(unsigned short value);
int stdc_first_leading_zeroui(unsigned int value);
int stdc_first_leading_zeroul(unsigned long value);
int stdc_first_leading_zeroull(unsigned long long value);
generic_return_type stdc_first_leading_zero(generic_value_type value);
int stdc_first_leading_oneuc(unsigned char value);
int stdc_first_leading_oneus(unsigned short value);
int stdc_first_leading_oneui(unsigned int value);
int stdc_first_leading_oneul(unsigned long value);
int stdc_first_leading_oneull(unsigned long long value);
generic_return_type stdc_first_leading_one(generic_value_type value);
int stdc_first_trailing_zerouc(unsigned char value);
int stdc_first_trailing_zerous(unsigned short value);
int stdc_first_trailing_zeroui(unsigned int value);
int stdc_first_trailing_zeroul(unsigned long value);
int stdc_first_trailing_zeroull(unsigned long long value);
generic_return_type stdc_first_trailing_zero(generic_value_type value);
int stdc_first_trailing_oneuc(unsigned char value);
int stdc_first_trailing_oneus(unsigned short value);
int stdc_first_trailing_oneui(unsigned int value);
int stdc_first_trailing_oneul(unsigned long value);
int stdc_first_trailing_oneull(unsigned long long value);
generic_return_type stdc_first_trailing_one(generic_value_type value);
int stdc_count_onesuc(unsigned char value);
int stdc_count_onesus(unsigned short value);
int stdc_count_onesui(unsigned int value);
int stdc_count_onesul(unsigned long value);
int stdc_count_onesull(unsigned long long value);
generic_return_type stdc_count_ones(generic_value_type value);
int stdc_count_zerosuc(unsigned char value);
int stdc_count_zerosus(unsigned short value);
int stdc_count_zerosui(unsigned int value);
int stdc_count_zerosul(unsigned long value);
int stdc_count_zerosull(unsigned long long value);
generic_return_type stdc_count_zeros(generic_value_type value);
bool stdc_has_single_bituc(unsigned char value);
bool stdc_has_single_bitus(unsigned short value);
bool stdc_has_single_bitui(unsigned int value);
bool stdc_has_single_bitul(unsigned long value);
bool stdc_has_single_bitull(unsigned long long value);
bool stdc_has_single_bit(generic_value_type value);
int stdc_bit_widthuc(unsigned char value);
int stdc_bit_widthus(unsigned short value);
int stdc_bit_widthui(unsigned int value);
int stdc_bit_widthul(unsigned long value);
int stdc_bit_widthull(unsigned long long value);
generic_return_type stdc_bit_width(generic_value_type value);
unsigned char stdc_bit_flooruc(unsigned char value);
unsigned short stdc_bit_floorus(unsigned short value);
unsigned int stdc_bit_floorui(unsigned int value);
unsigned long stdc_bit_floorul(unsigned long value);
unsigned long long stdc_bit_floorull(unsigned long long value);
generic_value_type stdc_bit_floor(generic_value_type value);
unsigned char stdc_bit_ceiluc(unsigned char value);
unsigned short stdc_bit_ceilus(unsigned short value);
unsigned int stdc_bit_ceilui(unsigned int value);
unsigned long stdc_bit_ceilul(unsigned long value);
unsigned long long stdc_bit_ceilull(unsigned long long value);
generic_value_type stdc_bit_ceil(generic_value_type value);
B.18 Boolean type and values <stdbool.h>
__bool_true_false_are_defined
B.19 Common definitions <stddef.h>
ptrdiff_t size_t wchar_t
nullptr_t max_align_t NULL
offsetof(type, member-designator)
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <stddef.h>:
rsize_t
B.20 Integer types <stdint.h>
intN_t UINT_LEASTN_MAX PTRDIFF_MAX
uintN_t UINT_LEASTN_WIDTH SIG_ATOMIC_MIN
int_leastN_t INT_FASTN_MIN SIG_ATOMIC_MAX
uint_leastN_t INT_FASTN_MAX SIG_ATOMIC_WIDTH
int_fastN_t INT_FASTN_WIDTH SIZE_MAX
uint_fastN_t UINT_FASTN_MAX SIZE_WIDTH
intptr_t UINT_FASTN_WIDTH WCHAR_MIN
uintptr_t INTPTR_MIN WCHAR_MAX
intmax_t INTPTR_MAX WCHAR_WIDTH
uintmax_t INTPTR_WIDTH WINT_MIN
INTN_MIN UINTPTR_MAX WINT_MAX
INTN_MAX UINTPTR_WIDTH WINT_WIDTH
INTN_WIDTH INTMAX_MIN INTN_C( value )
UINTN_MAX INTMAX_MAX UINTN_C( value )
UINTN_WIDTH INTMAX_WIDTH INTMAX_C( value )
INT_LEASTN_MIN UINTMAX_MAX UINTMAX_C( value )
INT_LEASTN_MAX UINTMAX_WIDTH
INT_LEASTN_WIDTH PTRDIFF_MIN
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <stdint.h>:
RSIZE_MAX
B.21 Input/output <stdio.h>
size_t _IONBF SEEK_CUR stdout
_PRINTF_NAN_LEN_MAX
FILE BUFSIZ SEEK_END
fpos_t EOF SEEK_SET
NULL FOPEN_MAX TMP_MAX
_IOFBF FILENAME_MAX stderr
_IOLBF L_tmpnam stdin
int remove(const char *filename);
int rename(const char *old, const char *new);
FILE *tmpfile(void);
char *tmpnam(char *s);
int fclose(FILE *stream);
int fflush(FILE *stream);
FILE *fopen(const char * restrict filename, const char * restrict mode);
FILE *freopen(const char * restrict filename, const char * restrict mode,
FILE * restrict stream);
void setbuf(FILE * restrict stream, char * restrict buf);
int setvbuf(FILE * restrict stream, char * restrict buf, int mode, size_t size);
int printf(const char * restrict format, ...);
int scanf(const char * restrict format, ...);
int snprintf(char * restrict s, size_t n, const char * restrict format, ...);
int sprintf(char * restrict s, const char * restrict format, ...);
int sscanf(const char * restrict s, const char * restrict format, ...);
int vfprintf(FILE * restrict stream, const char * restrict format, va_list arg);
int vfscanf(FILE * restrict stream, const char * restrict format, va_list arg);
int vprintf(const char * restrict format, va_list arg);
int vscanf(const char * restrict format, va_list arg);
int vsnprintf(char * restrict s, size_t n, const char * restrict format, va_list arg);
int vsprintf(char * restrict s, const char * restrict format, va_list arg);
int vsscanf(const char * restrict s, const char * restrict format, va_list arg);
int fgetc(FILE *stream);
char *fgets(char * restrict s, int n, FILE * restrict stream);
int fputc(int c, FILE *stream);
int fputs(const char * restrict s, FILE * restrict stream);
int getc(FILE *stream);
int getchar(void);
int putc(int c, FILE *stream);
int putchar(int c);
int puts(const char *s);
int ungetc(int c, FILE *stream);
size_t fread(void * restrict ptr, size_t size, size_t nmemb,
FILE * restrict stream);
size_t fwrite(const void * restrict ptr, size_t size, size_t nmemb,
FILE * restrict stream);
int fgetpos(FILE * restrict stream, fpos_t * restrict pos);
int fseek(FILE *stream, long int offset, int whence);
int fsetpos(FILE *stream, const fpos_t *pos);
long int ftell(FILE *stream);
void rewind(FILE *stream);
void clearerr(FILE *stream);
int feof(FILE *stream);
int ferror(FILE *stream);
void perror(const char *s);
int fprintf(FILE * restrict stream, const char * restrict format, ...);
int fscanf(FILE * restrict stream, const char * restrict format, ...);
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <stdio.h>:
L_tmpnam_s TMP_MAX_S errno_t rsize_t
errno_t tmpfile_s(FILE * restrict * restrict streamptr);
errno_t tmpnam_s(char *s, rsize_t maxsize);
errno_t fopen_s(FILE * restrict * restrict streamptr,
const char * restrict filename, const char * restrict mode);
errno_t freopen_s(FILE * restrict * restrict newstreamptr,
const char * restrict filename, const char * restrict mode,
FILE * restrict stream);
int fprintf_s(FILE * restrict stream, const char * restrict format, ...);
int fscanf_s(FILE * restrict stream, const char * restrict format, ...);
int printf_s(const char * restrict format, ...);
int scanf_s(const char * restrict format, ...);
int snprintf_s(char * restrict s, rsize_t n, const char * restrict format, ...);
int sprintf_s(char * restrict s, rsize_t n, const char * restrict format, ...);
int sscanf_s(const char * restrict s, const char * restrict format, ...);
int vfprintf_s(FILE *restrict stream, const char *restrict format, va_list arg);
int vfscanf_s(FILE *restrict stream, const char *restrict format, va_list arg);
int vprintf_s(const char * restrict format, va_list arg);
int vscanf_s(const char * restrict format, va_list arg);
int vsnprintf_s(char *restrict s, rsize_t n, const char *restrict format,
va_list arg);
int vsprintf_s(char * restrict s, rsize_t n, const char * restrict format,
va_list arg);
int vsscanf_s(const char *restrict s, const char *restrict format, va_list arg);
char *gets_s(char *s, rsize_t n);
B.22 General utilities <stdlib.h>
size_t div_t lldiv_t EXIT_FAILURE RAND_MAX
wchar_t ldiv_t NULL EXIT_SUCCESS MB_CUR_MAX
double atof(const char *nptr);
int atoi(const char *nptr);
long int atol(const char *nptr);
long long int atoll(const char *nptr);
int strfromd(char *restrict s, size_t n, const char *restrict format, double fp);
int strfromf(char *restrict s, size_t n, const char *restrict format, float fp);
int strfroml(char *restrict s, size_t n, const char *restrict format, long double fp);
double strtod(const char *restrict nptr, char **restrict endptr);
float strtof(const char *restrict nptr, char **restrict endptr);
long double strtold(const char *restrict nptr, char **restrict endptr);
long int strtol(const char *restrict nptr, char **restrict endptr, int base);
long long int strtoll(const char *restrict nptr, char **restrict endptr, int base);
unsigned long int strtoul(const char *restrict nptr, char **restrict endptr, int base);
unsigned long long int strtoull(const char *restrict nptr, char **restrict endptr, int
base);
int rand(void);
void srand(unsigned int seed);
void *aligned_alloc(size_t alignment, size_t size);
void *calloc(size_t nmemb, size_t size);
void free(void *ptr);
void free_sized(void *ptr, size_t size);
void free_aligned_sized(void *ptr, size_t alignment, size_t size);
void *malloc(size_t size);
void *realloc(void *ptr, size_t size);
[[noreturn]] void abort(void);
int atexit(void (*func)(void));
int at_quick_exit(void (*func)(void));
[[noreturn]] void exit(int status);
[[noreturn]] void _Exit(int status);
char *getenv(const char *name);
[[noreturn]] void quick_exit(int status);
int system(const char *string);
void *bsearch(const void *key, const void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
int abs(int j);
long int labs(long int j);
long long int llabs(long long int j);
div_t div(int numer, int denom);
ldiv_t ldiv(long int numer, long int denom);
lldiv_t lldiv(long long int numer, long long int denom);
int mblen(const char *s, size_t n);
int mbtowc(wchar_t * restrict pwc, const char * restrict s, size_t n);
int wctomb(char *s, wchar_t wc);
size_t mbstowcs(wchar_t * restrict pwcs, const char * restrict s, size_t n);
size_t wcstombs(char * restrict s, const wchar_t * restrict pwcs, size_t n);
size_t memalignment(const void * p);
Only if the implementation defines __STDC_IEC_60559_DFP__ :
int strfromd32(char*restrict s, size_t n, const char*restrict format, _Decimal32 fp);
int strfromd64(char*restrict s, size_t n, const char*restrict format, _Decimal64 fp);
int strfromd128(char*restrict s, size_t n, const char*restrict format, _Decimal128 fp);
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <stdlib.h>:
errno_t rsize_t constraint_handler_t
constraint_handler_t set_constraint_handler_s(constraint_handler_t handler);
void abort_handler_s(const char * restrict msg, void * restrict ptr,
errno_t error);
void ignore_handler_s(const char * restrict msg, void * restrict ptr,
errno_t error);
errno_t getenv_s(size_t * restrict len, char * restrict value, rsize_t maxsize,
const char * restrict name);
void *bsearch_s(const void *key, QVoid *base, rsize_t nmemb, rsize_t size,
int (*compar)(const void *k, const void *y, void *context),
void *context);
errno_t qsort_s(void *base, rsize_t nmemb, rsize_t size,
int (*compar)(const void *x, const void *y, void *context),
void *context);
errno_t wctomb_s(int *restrict status, char *restrict s, rsize_t smax,
wchar_t wc);
errno_t mbstowcs_s(size_t *restrict retval, wchar_t *restrict dst,
rsize_t dstmax, const char * restrict src, rsize_t len);
errno_t wcstombs_s(size_t * restrict retval, char * restrict dst, rsize_t dstmax,
const wchar_t * restrict src, rsize_t len);
B.23 _Noreturn <stdnoreturn.h>
noreturn
B.24 ckd_ Checked Integer Operations <stdckdint.h>
bool ckd_add(type1 *result, type2 a, type3 b);
bool ckd_sub(type1 *result, type2 a, type3 b);
bool ckd_mul(type1 *result, type2 a, type3 b);
B.25 String handling <string.h>
size_t NULL
void *memcpy(void * restrict s1, const void * restrict s2, size_t n);
void *memccpy(void * restrict s1, const void * restrict s2, int c, size_t n);
void *memmove(void *s1, const void *s2, size_t n);
char *strcpy(char * restrict s1, const char * restrict s2);
char *strncpy(char * restrict s1, const char * restrict s2, size_t n);
char *strdup(const char *s);
char *strndup(const char *s, size_t size);
char *strcat(char * restrict s1, const char * restrict s2);
char *strncat(char * restrict s1, const char * restrict s2, size_t n);
int memcmp(const void *s1, const void *s2, size_t n);
int strcmp(const char *s1, const char *s2);
int strcoll(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
size_t strxfrm(char * restrict s1, const char * restrict s2, size_t n);
QVoid *memchr(QVoid *s, int c, size_t n);
QChar *strchr(QChar *s, int c);
size_t strcspn(const char *s1, const char *s2);
QChar *strpbrk(QChar *s1, const char *s2);
QChar *strrchr(QChar *s, int c);
size_t strspn(const char *s1, const char *s2);
QChar *strstr(QChar *s1, const char *s2);
char *strtok(char * restrict s1, const char * restrict s2);
void *memset(void *s, int c, size_t n);
void *memset_explicit(void *s, int c, size_t n);
char *strerror(int errnum);
size_t strlen(const char *s);
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <string.h>:
errno_t rsize_t
errno_t memcpy_s(void * restrict s1, rsize_t s1max, const void * restrict s2,
rsize_t n);
errno_t memmove_s(void *s1, rsize_t s1max, const void *s2, rsize_t n);
errno_t strcpy_s(char * restrict s1, rsize_t s1max, const char * restrict s2);
errno_t strncpy_s(char * restrict s1, rsize_t s1max, const char * restrict s2,
rsize_t n);
errno_t strcat_s(char * restrict s1, rsize_t s1max, const char * restrict s2);
errno_t strncat_s(char * restrict s1, rsize_t s1max, const char * restrict s2,
rsize_t n);
char *strtok_s(char * restrict s1, rsize_t * restrict s1max,
const char * restrict s2, char ** restrict ptr);
errno_t memset_s(void *s, rsize_t smax, int c, rsize_t n)
errno_t strerror_s(char *s, rsize_t maxsize, errno_t errnum);
size_t strerrorlen_s(errno_t errnum);
size_t strnlen_s(const char *s, size_t maxsize);
B.26 Type-generic math <tgmath.h>
acos atanpi fmin logb tanpi
asin cbrt fminimum logp1 tgamma
atan ceil fminimum_mag lrint trunc
acosh compoundn fminimum_num lround ufromfpx
asinh copysign fminimum_mag_num nearbyint ufromfp
atanh cospi fmod nextafter fadd
cos erfc frexp nextdown dadd
sin erf fromfpx nexttoward fsub
tan exp10m1 fromfp nextup dsub
cosh exp10 hypot pown fmul
sinh exp2m1 ilogb powr dmul
tanh exp2 ldexp remainder fdiv
exp expm1 lgamma remquo ddiv
log fdim llogb rint ffma
pow floor llrint rootn dfma
sqrt fmax llround roundeven fsqrt
fabs fmaximum log10p1 round dsqrt
acospi fmaximum_mag log10 rsqrt
asinpi fmaximum_num log1p scalbln
atan2pi fmaximum_mag_num log2p1 scalbn
atan2 fma log2 sinpi
Only if the implementation does not define __STDC_NO_COMPLEX__ :
carg cimag conj cproj creal
Only if the implementation defines __STDC_IEC_60559_DFP__ :
d32add d64sub d32div d64fma quantize llquantexp
d64add d32mul d64div d32sqrt samequantum
d32sub d64mul d32fma d64sqrt quantum
B.27 Threads <threads.h>
__STDC_NO_THREADS__ mtx_t thrd_timedout
thread_local tss_dtor_t thrd_success
ONCE_FLAG_INIT thrd_start_t thrd_busy
TSS_DTOR_ITERATIONS once_flag thrd_error
cnd_t mtx_plain thrd_nomem
thrd_t mtx_recursive
tss_t mtx_timed
void call_once(once_flag *flag, void (*func)(void));
int cnd_broadcast(cnd_t *cond);
void cnd_destroy(cnd_t *cond);
int cnd_init(cnd_t *cond);
int cnd_signal(cnd_t *cond);
int cnd_timedwait(cnd_t *restrict cond, mtx_t *restrict mtx,
const struct timespec *restrict ts);
int cnd_wait(cnd_t *cond, mtx_t *mtx);
void mtx_destroy(mtx_t *mtx);
int mtx_init(mtx_t *mtx, int type);
int mtx_lock(mtx_t *mtx);
int mtx_timedlock(mtx_t *restrict mtx, const struct timespec *restrict ts);
int mtx_trylock(mtx_t *mtx);
int mtx_unlock(mtx_t *mtx);
int thrd_create(thrd_t *thr, thrd_start_t func, void *arg);
thrd_t thrd_current(void);
int thrd_detach(thrd_t thr);
int thrd_equal(thrd_t thr0, thrd_t thr1);
[[noreturn]] void thrd_exit(int res);
int thrd_join(thrd_t thr, int *res);
int thrd_sleep(const struct timespec *duration, struct timespec *remaining);
void thrd_yield(void);
int tss_create(tss_t *key, tss_dtor_t dtor);
void tss_delete(tss_t key);
void *tss_get(tss_t key);
int tss_set(tss_t key, void *val);
B.28 Date and time <time.h>
NULL size_t struct timespec
CLOCKS_PER_SEC clock_t struct tm
TIME_UTC time_t
clock_t clock(void);
double difftime(time_t time1, time_t time0);
time_t mktime(struct tm *timeptr);
time_t timegm(struct tm *timeptr);
time_t time(time_t *timer);
int timespec_get(struct timespec *ts, int base);
int timespec_getres(struct timespec *ts, int base);
[[deprecated]] char *asctime(const struct tm *timeptr);
[[deprecated]] char *ctime(const time_t *timer);
struct tm *gmtime(const time_t *timer);
struct tm *gmtime_r(const time_t *timer, struct tm *buf);
struct tm *localtime(const time_t *timer);
struct tm *localtime_r(const time_t *timer, struct tm *buf);
size_t strftime(char * restrict s, size_t maxsize, const char * restrict format,
const struct tm * restrict timeptr);
Only if supported by the implementation:
TIME_MONOTONIC
TIME_ACTIVE
Only if threads are supported and it is supported by the implementation:
TIME_THREAD_ACTIVE
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <time.h>:
errno_t rsize_t
errno_t asctime_s(char *s, rsize_t maxsize, const struct tm *timeptr);
errno_t ctime_s(char *s, rsize_t maxsize, const time_t *timer);
struct tm *gmtime_s(const time_t * restrict timer, struct tm * restrict result);
struct tm *localtime_s(const time_t *restrict timer, struct tm *restrict result);
B.29 Unicode utilities <uchar.h>
mbstate_t size_t char16_t char32_t
size_t mbrtoc8(char8_t * restrict pc8, const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t c8rtomb(char * restrict s, char8_t c8, mbstate_t * restrict ps);
size_t mbrtoc16(char16_t * restrict pc16, const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t c16rtomb(char * restrict s, char16_t c16, mbstate_t * restrict ps);
size_t mbrtoc32(char32_t * restrict pc32, const char * restrict s, size_t n,
mbstate_t * restrict ps);
size t c32rtomb(char * restrict s, char32_t c32, mbstate_t * restrict ps);
_
B.30 Extended multibyte/wide character utilities <wchar.h>
wchar_t wint_t WCHAR_MAX
size_t struct tm WCHAR_MIN
mbstate_t NULL WEOF
int fwprintf(FILE * restrict stream, const wchar_t * restrict format, ...);
int fwscanf(FILE * restrict stream, const wchar_t * restrict format, ...);
int swprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format,
...);
int swscanf(const wchar_t * restrict s, const wchar_t * restrict format, ...);
int vfwprintf(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
int vfwscanf(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
int vswprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format,
va_list arg);
int vswscanf(const wchar_t * restrict s, const wchar_t * restrict format,
va_list arg);
int vwprintf(const wchar_t * restrict format, va_list arg);
int vwscanf(const wchar_t * restrict format, va_list arg);
int wprintf(const wchar_t * restrict format, ...);
int wscanf(const wchar_t * restrict format, ...);
wint_t fgetwc(FILE *stream);
wchar_t *fgetws(wchar_t * restrict s, int n, FILE * restrict stream);
wint_t fputwc(wchar_t c, FILE *stream);
int fputws(const wchar_t * restrict s, FILE * restrict stream);
int fwide(FILE *stream, int mode);
wint_t getwc(FILE *stream);
wint_t getwchar(void);
wint_t putwc(wchar_t c, FILE *stream);
wint_t putwchar(wchar_t c);
wint_t ungetwc(wint_t c, FILE *stream);
long int wcstol(const wchar_t * restrict nptr, wchar_t ** restrict endptr,
int base);
long long int wcstoll(const wchar_t * restrict nptr, wchar_t ** restrict endptr,
int base);
unsigned long int wcstoul(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
unsigned long long int wcstoull(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
wchar t *wcscpy(wchar_t * restrict s1, const wchar_t * restrict s2);
_
wchar_t *wcsncpy(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
wchar_t *wmemcpy(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
wchar_t *wmemmove(wchar_t *s1, const wchar_t *s2, size_t n);
wchar_t *wcscat(wchar_t * restrict s1, const wchar_t * restrict s2);
wchar_t *wcsncat(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
int wcscmp(const wchar_t *s1, const wchar_t *s2);
int wcscoll(const wchar_t *s1, const wchar_t *s2);
int wcsncmp(const wchar_t *s1, const wchar_t *s2, size_t n);
size_t wcsxfrm(wchar_t * restrict s1, const wchar_t * restrict s2, size_t n);
int wmemcmp(const wchar_t *s1, const wchar_t *s2, size_t n);
QWchar_t *wcschr(QWchar_t *s, wchar_t c);
size_t wcscspn(const wchar_t *s1, const wchar_t *s2);
QWchar_t *wcspbrk(QWchar_t *s1, const wchar_t *s2);
QWchar_t *wcsrchr(const wchar_t *s, wchar_t c);
size_t wcsspn(const wchar_t *s1, const wchar_t *s2);
QWchar_t *wcsstr(QWchar_t *s1, const wchar_t *s2);
wchar_t *wcstok(wchar_t * restrict s1, const wchar_t * restrict s2,
wchar_t ** restrict ptr);
QWchar_t *wmemchr(QWchar_t *s, wchar_t c, size_t n);
size_t wcslen(const wchar_t *s);
wchar_t *wmemset(wchar_t *s, wchar_t c, size_t n);
size_t wcsftime(wchar_t * restrict s, size_t maxsize,
const wchar_t * restrict format, const struct tm * restrict timeptr);
wint_t btowc(int c);
int wctob(wint_t c);
int mbsinit(const mbstate_t *ps);
size_t mbrlen(const char * restrict s, size_t n, mbstate_t * restrict ps);
size_t mbrtowc(wchar_t * restrict pwc, const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t wcrtomb(char * restrict s, wchar_t wc, mbstate_t * restrict ps);
size_t mbsrtowcs(wchar_t * restrict dst, const char ** restrict src, size_t len,
mbstate_t * restrict ps);
size_t wcsrtombs(char * restrict dst, const wchar_t ** restrict src, size_t len,
mbstate_t * restrict ps);
Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines
__STDC_WANT_LIB_EXT1__ before any inclusion of <wchar.h>:
errno_t rsize_t
int fwprintf_s(FILE * restrict stream, const wchar_t * restrict format, ...);
int fwscanf_s(FILE * restrict stream, const wchar_t * restrict format, ...);
int snwprintf_s(wchar_t * restrict s, rsize_t n, const wchar_t * restrict format,
...);
int swprintf_s(wchar_t * restrict s, rsize_t n, const wchar_t * restrict format,
...);
int swscanf_s(const wchar_t * restrict s, const wchar_t * restrict format, ...);
int vfwprintf_s(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
int vfwscanf_s(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
int vsnwprintf_s(wchar_t *restrict s, rsize_t n, const wchar_t *restrict format,
va_list arg);
int vswprintf_s(wchar_t *restrict s, rsize_t n, const wchar_t *restrict format,
va_list arg);
int vswscanf_s(const wchar_t * restrict s, const wchar_t * restrict format,
va_list arg);
int vwprintf_s(const wchar_t * restrict format, va_list arg);
int vwscanf_s(const wchar_t * restrict format, va_list arg);
int wprintf_s(const wchar_t * restrict format, ...);
int wscanf_s(const wchar_t * restrict format, ...);
errno_t wcscpy_s(wchar_t *restrict s1, rsize_t s1max,
const wchar_t *restrict s2);
errno t wcsncpy_s(wchar_t * restrict s1, rsize_t s1max,
_
const wchar_t * restrict s2, rsize_t n);
errno_t wmemcpy_s(wchar_t *restrict s1, rsize_t s1max,
const wchar_t *restrict s2, rsize_t n);
errno_t wmemmove_s(wchar_t *s1, rsize_t s1max, const wchar_t *s2, rsize_t n);
errno_t wcscat_s(wchar_t * restrict s1, rsize_t s1max,
const wchar_t * restrict s2);
errno_t wcsncat_s(wchar_t * restrict s1, rsize_t s1max,
const wchar_t * restrict s2, rsize_t n);
wchar t *wcstok_s(wchar_t * restrict s1, rsize_t * restrict s1max,
_
const wchar_t * restrict s2, wchar_t ** restrict ptr);
size_t wcsnlen_s(const wchar_t *s, size_t maxsize);
errno_t wcrtomb_s(size_t * restrict retval, char * restrict s, rsize_t smax,
wchar_t wc, mbstate_t * restrict ps);
errno_t mbsrtowcs_s(size_t * restrict retval, wchar_t * restrict dst,
rsize_t dstmax, const char ** restrict src, rsize_t len,
mbstate_t * restrict ps);
errno_t wcsrtombs_s(size_t * restrict retval, char * restrict dst,
rsize_t dstmax, const wchar_t ** restrict src, rsize_t len,
mbstate_t * restrict ps);
B.31 Wide character classification and mapping utilities <wctype.h>
wint_t wctrans_t wctype_t WEOF
int iswalnum(wint_t wc);
int iswalpha(wint_t wc);
int iswblank(wint_t wc);
int iswcntrl(wint_t wc);
int iswdigit(wint_t wc);
int iswgraph(wint_t wc);
int iswlower(wint_t wc);
int iswprint(wint_t wc);
int iswpunct(wint_t wc);
int iswspace(wint_t wc);
int iswupper(wint_t wc);
int iswxdigit(wint_t wc);
int iswctype(wint_t wc, wctype_t desc);
wctype_t wctype(const char *property);
wint_t towlower(wint_t wc);
wint_t towupper(wint_t wc);
wint_t towctrans(wint_t wc, wctrans_t desc);
wctrans_t wctrans(const char *property);
C. Annex C (informative) Sequence points
1 The following are the sequence points described in 5.1.2.3:
— Between the evaluations of the function designator and actual arguments in a function call
and the actual call. (6.5.2.2).
— Between the evaluations of the first and second operands of the following operators: logical
AND && (6.5.13); logical OR || (6.5.14); comma , (6.5.17).
— Between the evaluations of the first operand of the conditional ?: operator and whichever of
the second and third operands is evaluated (6.5.15).
— Between the evaluation of a full expression and the next full expression to be evaluated. The
following are full expressions: a full declarator for a variably modified type; an initializer that
is not part of a compound literal (6.7.10); the expression in an expression statement (6.8.3); the
controlling expression of a selection statement (if or switch) (6.8.4); the controlling expression
of a while or do statement (6.8.5); each of the (optional) expressions of a for statement (6.8.5.3);
the (optional) expression in a return statement (6.8.6.4).
— Immediately before a library function returns (7.1.4).
— After the actions associated with each formatted input/output function conversion specifier
(7.23.6, 7.31.2).
— Immediately before and immediately after each call to a comparison function, and also between
any call to a comparison function and any movement of the objects passed as arguments to
that call (7.24.5).
D. Annex D (informative) Universal character names for identifiers
1 This subclause describes the choices made in application of UAX #31 ("Unicode Identifier and
Pattern Syntax") to C of the requirements from UAX #31 and how they do or do not apply to C.
For UAX #31, C conforms by meeting the requirements "Default Identifiers" (D.1) and "Equivalent
Normalized Identifiers" (D.1). The other requirements, also listed below, are either alternatives not
taken or do not apply to C.
D.1 Default Identifiers
1 UAX #31 specifies a default syntax for identifiers based on properties from the Unicode Character
Database, UAX #44. The general syntax is
<Identifier> := <Start> <Continue>* (<Medial> <Continue>+)*
where <Start> has the XID_Start property, <Continue> has the XID_Continue property, and
<Medial> is a list of characters permitted between continue characters. For C we add the character
U+005F, LOW LINE, or _, to the set of permitted Start characters, the Medial set is empty, and the
Continue characters are unmodified. In the grammar used in UAX #31, this is
<Identifier> := <Start> <Continue>*
<Start> := XID_Start + U+005F
<Continue> := <Start> + XID_Continue
This is described in the C grammar (6.4.2.1), where identifier is formed from identifier-start or identifier
followed by identifier-continue.
D.1.1 Restricted Format Characters
1 If an implementation of UAX #31 wishes to allow format characters such as ZERO WIDTH JOINER
or ZERO WIDTH NON-JOINER it must define a profile allowing them, or describe precisely which
combinations are permitted.
2 C does not allow format characters in identifiers, so this does not apply.
D.1.2 Stable Identifiers
1 An implementation of UAX #31 may choose to guarantee that identifiers are stable across versions
of the Unicode Standard. Once a string qualifies as an identifier it does so in all future versions.
C does not make this guarantee, except to the extent that UAX #31 guarantees the stability of the
XID_Start and XID_Continue properties.
D.2 Immutable Identifiers
1 An implementation may choose to guarantee that the set of identifiers will never change by fixing
the set of code points allowed in identifiers forever.
2 C does not choose to make this guarantee. As scripts are added to Unicode, additional characters in
those scripts may become available for use in identifiers.
D.3 Pattern_White_Space and Pattern_Syntax Characters
1 UAX #31 describes how languages that use or interpret patterns of characters, such as regular
expressions or number formats, may describe that syntax with Unicode properties.
2 C does not do this as part of the language, deferring to library components for such usage of patterns.
This requirement does not apply to C.
D.4 Equivalent Normalized Identifiers
1 UAX #31 requires that implementations describe how identifiers are compared and considered
equivalent.
2 C requires that identifiers be in Normalization Form C and therefore identifiers that compare the
same under NFC are equivalent. This is described in subclause 6.4.2.
D.5 Equivalent Case-Insensitive Identifiers
1 C considers case to be significant in identifier comparison, and does not do any case folding. This
requirement does not apply to C
D.6 Filtered Normalized Identifiers
1 If any characters are excluded from normalization, UAX #31 requires a precise specification of those
exclusions.
2 C does not make any such exclusions.
D.7 Filtered Case-Insensitive Identifiers
1 C identifiers are case sensitive, and therefore this requirement does not apply.
D.8 Hashtag Identifiers
1 There are no hashtags in C, so this requirement does not apply.
E. Annex E (informative) Implementation limits
1 The contents of the header <limits.h> are given below. The values shall all be constant expressions
suitable for use in #if preprocessing directives. The components are described further in 5.2.4.2.1.
2 For the following macros, the minimum values shown shall be replaced by implementation-defined
values.
#define BOOL_WIDTH 1
#define CHAR_BIT 8
#define USHRT_WIDTH 16
#define UINT_WIDTH 16
#define ULONG_WIDTH 32
#define ULLONG_WIDTH 64
#define BITINT_MAXWIDTH 64 // ULLONG_WIDTH
#define MB_LEN_MAX 1
3 For the following macros, the minimum magnitudes shown shall be replaced by implementation-
defined magnitudes with the same sign that are deduced from the macros above as indicated.434)
_
#define BOOL_MAX 1 // 2BOOL WIDTH − 1
#define CHAR_MAX UCHAR_MAX or SCHAR_MAX
#define CHAR_MIN 0 or SCHAR_MIN
_
#define CHAR WIDTH 8 // CHAR_BIT
_
#define INT_MAX +32767 // 2INT WIDTH−1 − 1
INT_WIDTH−1
#define INT_MIN -32768 // −2
#define INT_WIDTH 16 // UINT_WIDTH
_
#define LONG_MAX +2147483647 // 2LONG WIDTH−1 − 1
_
#define LONG_MIN -2147483648 // −2LONG WIDTH−1
#define LONG_WIDTH 32 // ULONG_WIDTH
_
#define LLONG_MAX +9223372036854775807 // 2LLONG WIDTH−1 − 1
LLONG_WIDTH−1
#define LLONG_MIN -9223372036854775808 // −2
#define LLONG_WIDTH 64 // ULLONG_WIDTH
_
#define SCHAR_MAX +127 // 2SCHAR WIDTH−1 − 1
SCHAR_WIDTH−1
#define SCHAR_MIN -128 // −2
#define SCHAR_WIDTH 8 // CHAR_BIT
_
#define SHRT_MAX +32767 // 2SHRT WIDTH−1 − 1
SHRT_WIDTH−1
#define SHRT_MIN -32768 // −2
_
#define UCHAR_MAX 255 // 2UCHAR WIDTH − 1
#define UCHAR_WIDTH 8 _
// CHAR BIT
_
#define USHRT_MAX 65535 // 2USHRT WIDTH − 1
_
#define UINT_MAX 65535 // 2UINT WIDTH − 1
_
#define ULONG_MAX 4294967295 // 2ULONG WIDTH − 1
ULLONG_WIDTH
#define ULLONG_MAX 18446744073709551615 // 2 −1
4 The contents of the header <float.h> are given below. All integer values, except FLT_ROUNDS, shall
be constant expressions suitable for use in #if preprocessing directives; all floating values shall be
constant expressions. The components are described further in 5.2.4.2.2 and 5.2.4.2.3.
5 The values given in the following list shall be replaced by implementation-defined expressions:
#define FLT_EVAL_METHOD
#define FLT_ROUNDS
#ifdef __STDC_IEC_60559_DFP__
#define DEC_EVAL_METHOD
434) For the minimum value of a signed integer type there is no expression consisting of a minus sign and a decimal literal of
that same type. The numbers in the table are only given as indications for the values and do not represent suitable expressions
to be used for these macros.
#endif
6 The values given in the following list shall be replaced by implementation-defined constant ex-
pressions that are greater or equal in magnitude (absolute value) to those shown, with the same
sign:
#define DBL_DECIMAL_DIG 10
#define DBL_DIG 10
#define DBL_MANT_DIG
#define DBL_MAX_10_EXP +37
#define DBL_MAX_EXP
#define DBL_MIN_10_EXP -37
#define DBL_MIN_EXP
#define DECIMAL_DIG 10
#define FLT_DECIMAL_DIG 6
#define FLT_DIG 6
#define FLT_MANT_DIG
#define FLT_MAX_10_EXP +37
#define FLT_MAX_EXP
#define FLT_MIN_10_EXP -37
#define FLT_MIN_EXP
#define FLT_RADIX 2
#define LDBL_DECIMAL_DIG 10
#define LDBL_DIG 10
#define LDBL_MANT_DIG
#define LDBL_MAX_10_EXP +37
#define LDBL_MAX_EXP
#define LDBL_MIN_10_EXP -37
#define LDBL_MIN_EXP
7 The values given in the following list shall be replaced by implementation-defined constant expres-
sions with values that are greater than or equal to those shown:
#define DBL_MAX 1E+37
#define DBL_NORM_MAX 1E+37
#define FLT_MAX 1E+37
#define FLT_NORM_MAX 1E+37
#define LDBL_MAX 1E+37
#define LDBL_NORM_MAX 1E+37
8 The values given in the following list shall be replaced by implementation-defined constant expres-
sions with (positive) values that are less than or equal to those shown:
#define DBL_EPSILON 1E-9
#define DBL_MIN 1E-37
#define FLT_EPSILON 1E-5
#define FLT_MIN 1E-37
#define LDBL_EPSILON 1E-9
#define LDBL_MIN 1E-37
9 If the implementation supports decimal floating types, the following macros provide the parameters
of these types as exact values.
#ifdef __STDC_IEC_60559_DFP__
#define DEC32_EPSILON 1E-6DF
#define DEC32_MANT_DIG 7
#define DEC32_MAX 9.999999E96DF
#define DEC32_MAX_EXP 97
#define DEC32_MIN 1E-95DF
#define DEC32_MIN_EXP -94
#define DEC32_TRUE_MIN 0.000001E-95DF
#define DEC64_EPSILON 1E-15DD
#define DEC64_MANT_DIG 16
#define DEC64_MAX 9.999999999999999E384DD
#define DEC64_MAX_EXP 385
#define DEC64_MIN 1E-383DD
#define DEC64_MIN_EXP -382
#define DEC64_TRUE_MIN 0.000000000000001E-383DD
#define DEC128_EPSILON 1E-33DL
#define DEC128_MANT_DIG 34
#define DEC128_MAX 9.999999999999999999999999999999999E6144DL
#define DEC128_MAX_EXP 6145
#define DEC128_MIN 1E-6143DL
#define DEC128_MIN_EXP -6142
#define DEC128_TRUE_MIN 0.000000000000000000000000000000001E-6143DL
#endif
F. Annex F (normative) IEC 60559 floating-point arithmetic
F.1 Introduction
1 This annex specifies C language support for the IEC 60559 floating-point standard. The IEC 60559
floating-point standard is specifically Floating-point arithmetic (ISO/IEC 60559:2020), also designated
as IEEE Standard for Floating-Point Arithmetic (IEEE 754–2019). IEC 60559 generally refers to the
floating-point standard, as in IEC 60559 operation, IEC 60559 format, etc.
2 The IEC 60559 floating-point standard specifies decimal, as well as binary, floating-point arithmetic.
It supersedes IEEE Standard for Radix-Independent Floating-Point Arithmetic (ANSI/IEEE 854–1987)
which generalized the binary arithmetic standard (IEEE 754-1985) to remove dependencies on radix
and word length.
3 An implementation that defines __STDC_IEC_60559_BFP__ to 202311L shall conform to the specifi-
cations in this annex for binary floating-point arithmetic and shall also define __STDC_IEC_559__
to 1.435)
4 An implementation that defines __STDC_IEC_60559_DFP__ to 202311L shall conform to the
specifications for decimal floating-point arithmetic in the following subclauses of this annex:
— F.2.1 Infinities and NaNs
— F.3 Operations
— F.4 Floating to integer conversions
— F.6 The return statement
— F.7 Contracted expressions
— F.8 Floating-point environment
— F.9 Optimization
— F.10 Mathematics <math.h> and <tgmath.h>
For the purpose of specifying these conformance requirements, the macros, functions, and values
mentioned in the subclauses listed above are understood to refer to the corresponding macros,
functions, and values for decimal floating types. Likewise, the "rounding direction mode" is
understood to refer to the rounding direction mode for decimal floating-point arithmetic.
5 Where a binding between the C language and IEC 60559 is indicated, the IEC 60559-specified
behavior is adopted by reference, unless stated otherwise.
F.2 Types
1 The C floating types match the IEC 60559 formats as follows:
— The float type matches the IEC 60559 binary32 format.
— The double type matches the IEC 60559 binary64 format.
— The long double type matches the IEC 60559 binary128 format, else an IEC 60559 binary64-
extended format, 436) else a non-IEC 60559 extended format, else the IEC 60559 binary64
format.
Any non-IEC 60559 extended format used for the long double type shall have more precision than
IEC 60559 binary64 and at least the range of IEC 60559 binary64.437) The value of FLT_ROUNDS
435) Implementations that do not define either of __STDC_IEC_60559_BFP__ and __STDC_IEC_559__ are not required to
conform to these specifications. New code should not use the obsolescent macro __STDC_IEC_559__ to test for conformance
to this annex.
436) IEC 60559 binary64-extended formats include the common 80-bit IEC 60559 format.
437) A non-IEC 60559 long double type is required to provide infinity and NaNs, as its values include all double values.
applies to all IEC 60559 types supported by the implementation, but need not apply to non-IEC 60559
types.
Recommended practice
2 The long double type should match the IEC 60559 binary128 format, else an IEC 60559 binary64-
extended format.
F.2.1 Infinities and NaNs
1 Since negative and positive infinity are representable in IEC 60559 formats, all real numbers lie
within the range of representable values (5.2.4.2.2).
2 The NAN and INFINITY macros in <float.h> and the nan functions in <math.h> provide designa-
tions for IEC 60559 quiet NaNs and infinities. The FLT_SNAN, DBL_SNAN, and LDBL_SNAN macros in
<float.h> provide designations for IEC 60559 signaling NaNs.
3 This annex does not require the full support for signaling NaNs specified in IEC 60559. This
annex uses the term NaN, unless explicitly qualified, to denote quiet NaNs. Where specification of
signaling NaNs is not provided, the behavior of signaling NaNs is implementation-defined (either
treated as an IEC 60559 quiet NaN or treated as an IEC 60559 signaling NaN). 438)
4 Any operator or <math.h> function that raises an "invalid" floating-point exception, if delivering a
floating type result, shall return a quiet NaN, unless explicitly specified otherwise.
5 In order to support signaling NaNs as specified in IEC 60559, an implementation should adhere to
the following recommended practice.
Recommended practice
6 Any floating-point operator or <math.h> function or macro with a signaling NaN input, unless
explicitly specified otherwise, raises an "invalid" floating-point exception.
7 NOTE Some functions do not propagate quiet NaN arguments. For example, hypot(x, y) returns infinity if x or y is
infinite and the other is a quiet NaN. The recommended practice in this subclause specifies that such functions (and others)
raise the "invalid" floating-point exception if an argument is a signaling NaN, which also implies they return a quiet NaN in
these cases.
8 The <fenv.h> header defines the macro FE_SNANS_ALWAYS_SIGNAL if and only if the implemen-
tation follows the recommended practice in this subclause. If defined, FE_SNANS_ALWAYS_SIGNAL
expands to the integer constant 1.
F.3 Operations
1 C operators, functions, and function-like macros provide operations specified by IEC 60559 as shown
in the following table. In the table, C functions are represented by the function name without a type
suffix. Specifications for the C facilities are provided in the listed clauses. The C specifications are
intended to match IEC 60559, unless stated otherwise.
Operation binding
IEC 60559 operation C operation Clause
roundToIntegralTiesToEven roundeven 7.12.9.8, F.10.6.8
roundToIntegralTiesAway round 7.12.9.6, F.10.6.6
roundToIntegralTowardZero trunc 7.12.9.9, F.10.6.9
roundToIntegralTowardPositive ceil 7.12.9.1, F.10.6.1
roundToIntegralTowardNegative floor 7.12.9.2, F.10.6.2
roundToIntegralExact rint 7.12.9.4, F.10.6.4
nextUp nextup 7.12.11.5, F.10.8.5
nextDown nextdown 7.12.11.6, F.10.8.6
getPayload getpayload F.10.13.1
setPayload setpayload F.10.13.2
438) Since NaNs created by IEC 60559 arithmetic operations are always quiet, quiet NaNs (along with infinities) are sufficient
for closure of the arithmetic.
setPayloadSignaling setpayloadsig F.10.13.3
quantize quantize 7.12.15.1
sameQuantum samequantum 7.12.15.2
quantum quantum 7.12.15.3
encodeDecimal encodedec 7.12.16.1
decodeDecimal decodedec 7.12.16.2
encodeBinary encodebin 7.12.16.3
decodeBinary decodebin 7.12.16.4
remainder remainder, remquo 7.12.10.2, F.10.7.2,
7.12.10.3, F.10.7.3
maximum fmaximum 7.12.12.4, F.10.9.4
minimum fminimum 7.12.12.5, F.10.9.4
maximumMagnitude fmaximum_mag 7.12.12.6, F.10.9.4
minimumMagnitude fminimum_mag 7.12.12.7, F.10.9.4
maximumNumber fmaximum_num 7.12.12.8, F.10.9.5
minimumNumber fminimum_num 7.12.12.9, F.10.9.5
maximumMagnitudeNumber fmaximum_mag_num 7.12.12.10, F.10.9.5
minimumMagnitudeNumber fminimum_mag_num 7.12.12.11, F.10.9.5
scaleB scalbn, scalbln 7.12.6.19, F.10.3.19
logB logb, ilogb, llogb 7.12.6.17, F.10.3.17,
7.12.6.8, F.10.3.8,
7.12.6.10, F.10.3.10
addition + , fadd, faddl, daddl 6.5.6, 7.12.14.1,
F.10.11
subtraction - , fsub, fsubl, dsubl 6.5.6, 7.12.14.2,
F.10.11
multiplication * , fmul, fmull, dmull 6.5.5, 7.12.14.3,
F.10.11
division / , fdiv, fdivl, ddivl 6.5.5, 7.12.14.4,
F.10.11
squareRoot sqrt, fsqrt, fsqrtl, dsqrtl 7.12.7.10, F.10.4.10,
7.12.14.6, F.10.11
fusedMultiplyAdd fma, ffma, ffmal, dfmal 7.12.13.1, F.10.10.1,
7.12.14.5, F.10.11
convertFromInt cast and implicit conversion 6.3.1.4, 6.5.4
convertToIntegerTiesToEven fromfp, ufromfp 7.12.9.10, F.10.6.10
convertToIntegerTowardZero
convertToIntegerTowardPositive
convertToIntegerTowardNegative
convertToIntegerTiesToAway fromfp, ufromfp, lround, 7.12.9.10, F.10.6.10,
llround 7.12.9.7, F.10.6.7
convertToIntegerExactTiesToEven fromfpx, ufromfpx 7.12.9.11, F.10.6.11
convertToIntegerExactTowardZero
convertToIntegerExactTowardPositive
convertToIntegerExactTowardNegative
convertToIntegerExactTiesToAway
convertFormat - different formats cast and implicit conversions 6.3.1.5, 6.5.4
convertFormat - same format canonicalize 7.12.11.7, F.10.8.7
convertFromDecimalCharacter strtod, wcstod, scanf, wscanf , 7.24.1.5, 7.31.4.1.2,
decimal floating constants 7.23.6.4, 7.31.2.12,
F.5
convertToDecimalCharacter printf, wprintf , strfromd 7.23.6.3, 7.31.2.11,
7.24.1.3, F.5
convertFromHexCharacter strtod, wcstod, scanf, wscanf , 7.24.1.5, 7.31.4.1.2,
hexadecimal floating constants 7.23.6.4, 7.31.2.12,
F.5
convertToHexCharacter printf, wprintf , strfromd 7.23.6.3, 7.31.2.11,
7.24.1.3, F.5
copy memcpy, memmove, +(x) 7.26.2.1, 7.26.2.3
negate -(x) 6.5.3.3
abs fabs 7.12.7.3, F.10.4.3
copySign copysign 7.12.11.1, F.10.8.1
compareQuietEqual == 6.5.9, F.9.3
compareQuietNotEqual != 6.5.9, F.9.3
compareSignalingEqual iseqsig 7.12.17.7, F.10.14.1
compareSignalingGreater > 6.5.8, F.9.3
compareSignalingGreaterEqual >= 6.5.8, F.9.3
compareSignalingLess < 6.5.8, F.9.3
compareSignalingLessEqual <= 6.5.8, F.9.3
compareSignalingNotEqual ! iseqsig(x) 7.12.17.7, F.10.14.1
compareSignalingNotGreater ! (x > y) 6.5.8, F.9.3
compareSignalingLessUnordered ! (x >= y) 6.5.8, F.9.3
compareSignalingNotLess ! (x < y) 6.5.8, F.9.3
compareSignalingGreaterUnordered ! (x <= y) 6.5.8, F.9.3
compareQuietGreater isgreater 7.12.17.1
compareQuietGreaterEqual isgreaterequal 7.12.17.2
compareQuietLess isless 7.12.17.3
compareQuietLessEqual islessequal 7.12.17.4
compareQuietUnordered isunordered 7.12.17.6
compareQuietNotGreater ! isgreater(x, y) 7.12.17.1
compareQuietLessUnordered ! isgreaterequal(x, y) 7.12.17.2
compareQuietNotLess ! isless(x, y) 7.12.17.3
compareQuietGreaterUnordered ! islessequal(x, y) 7.12.17.4
compareQuietOrdered ! isunordered(x, y) 7.12.17.6
class fpclassify, signbit, 7.12.3.1, 7.12.3.7,
issignaling 7.12.3.8
isSignMinus signbit 7.12.3.7
isNormal isnormal 7.12.3.6
isFinite isfinite 7.12.3.3
isZero iszero 7.12.3.10
isSubnormal issubnormal 7.12.3.9
isInfinite isinf 7.12.3.4
isNaN isnan 7.12.3.5
isSignaling issignaling 7.12.3.8
isCanonical iscanonical 7.12.3.2
radix FLT_RADIX 5.2.4.2.2
totalOrder totalorder F.10.12.1
totalOrderMag totalordermag F.10.12.2
lowerFlags feclearexcept 7.6.4.1
raiseFlags fesetexcept 7.6.4.4
testFlags fetestexcept 7.6.4.7
testSavedFlags fetestexceptflag 7.6.4.6
restoreFlags fesetexceptflag 7.6.4.5
saveAllFlags fegetexceptflag 7.6.4.2
getBinaryRoundingDirection fegetround 7.6.5.2
setBinaryRoundingDirection fesetround 7.6.5.5
saveModes fegetmode 7.6.5.1
restoreModes fesetmode 7.6.5.4
defaultModes fesetmode(FE_DFL_MODE) 7.6.5.4, 7.6
2 The IEC 60559 requirement that certain of its operations be provided for operands of different
formats (of the same radix) is satisfied by C’s usual arithmetic conversions (6.3.1.8) and function-call
argument conversions (6.5.2.2). For example, the following operations take float f and double d
inputs and produce a long double result:
(long double)f * d
powl(f, d)
3 The functions fmin and fmax have been superseded by fminimum_num and fmaximum_num. The fmin
and fmax functions provide the minNum and maxNum operations specified in (the superseded)
IEC 60559:2011.
4 Whether C assignment (6.5.16) (and conversion as if by assignment) to the same format is an
IEC 60559 convertFormat or copy operation439) is implementation-defined, even if <fenv.h> defines
the macro FE_SNANS_ALWAYS_SIGNAL (F.2.1). If the return expression of a return statement is
evaluated to the floating-point format of the return type, it is implementation-defined whether a
convertFormat operation is applied to the result of the return expression.
5 The unary + and - operators raises no floating-point exceptions, even if the operand is a signaling
NaN.
6 The C classification macros fpclassify, iscanonical, isfinite, isinf, isnan, isnormal,
issignaling, issubnormal, iszero, and signbit provide the IEC 60559 operations indicated
in the table above provided their arguments are in the format of their semantic type. Then these
macros raise no floating-point exceptions, even if an argument is a signaling NaN.
7 The signbit macro, providing the IEC 60559 isSignMinus operation, determines the sign of its
argument value as the sign bit of the value’s representation. This applies to all values, including
NaNs whose sign bit is not generally interpreted by IEC 60559.
8 The C nearbyint functions (7.12.9.3, F.10.6.3) provide the nearbyinteger function recommended in
the Appendix to (superseded) ANSI/IEEE 854.
9 The C nextafter (7.12.11.3, F.10.8.3) and nexttoward (7.12.11.4, F.10.8.4) functions provide the
nextafter function recommended in the Appendix to (superseded) IEC 60559:1989 (but with a
minor change to better handle signed zeros).
10 The macros (7.6) FE_DOWNWARD, FE_TONEAREST, FE_TONEARESTFROMZERO, FE_TOWARDZERO, and
FE_UPWARD, which are used in conjunction with the fegetround and fesetround functions and the
FENV_ROUND pragma, represent the IEC 60559 rounding-direction attributes roundTowardNegative,
roundTiesToEven, roundTiesToAway, roundTowardZero, and roundTowardPositive, respectively,
for binary floating-point arithmetic. Support for the roundTiesToAway attribute for binary floating-
point arithmetic, and hence for the FE_TONEARESTFROMZERO macro, is optional.
11 The C fegetenv (7.6.6.1), feholdexcept (7.6.6.2), fesetenv (7.6.6.3) and feupdateenv (7.6.6.4)
functions provide a facility to manage the dynamic floating-point environment, comprising the
IEC 60559 status flags and dynamic control modes.
12 IEC 60559 requires operations with specified operand and result formats. Therefore, math functions
that are bound to IEC 60559 operations (see table above) must remove any extra range and precision
from arguments or results.
13 IEC 60559 requires operations that round their result to formats the same as and wider than the
operands, in addition to the operations that round their result to narrower formats (see 7.12.14).
Operators (+ , - , * , and / ) whose evaluation formats are wider than the semantic type (5.2.4.2.2)
439) Where the source and destination formats are the same, convertFormat operations differ from copy operations in
that convertFormat operations raise the "invalid" floating-point exception on signaling NaN inputs and do not propagate
non-canonical encodings.
might not support some of the IEEE 60559 operations, because getting a result in a given format
might require a cast that could introduce an extra rounding error. The functions that round result to
narrower type (7.12.14) provide the IEC 60559 operations that round result to same and wider (as
well as narrower) formats, in those cases where built-in operators and casts do not. For example,
ddivl(x, y) computes a correctly rounded double divide of float x by float y, regardless of
the evaluation method.
14 Decimal versions of the remquo library function are not provided. (The decimal remainder functions
provide the remainder operation defined by IEC 60559.)
15 The binding for the convertFormat operation applies to all conversions among IEC 60559 formats.
Therefore, for implementations that conform to Annex F, conversions between decimal floating types
and standard floating types with IEC 60559 formats are correctly rounded and raise floating-point
exceptions as specified in IEC 60559.
16 IEC 60559 specifies the convertFromHexCharacter and convertToHexCharacter operations only for
binary floating-point arithmetic.
17 The integer constant 10 provides the radix operation defined in IEC 60559 for decimal floating-point
arithmetic.
18 The fe_dec_getround (7.6.5.3) and fe_dec_setround (7.6.5.6) functions provide the getDeci-
malRoundingDirection and setDecimalRoundingDirection operations defined in IEC 60559 for
decimal floating-point arithmetic. The macros (7.6) FE_DEC_DOWNWARD, FE_DEC_TONEAREST,
FE_DEC_TONEARESTFROMZERO, FE_DEC_TOWARDZERO, and FE_DEC_UPWARD, which are used in con-
junction with the fe_dec_getround and fe_dec_setround functions and the FENV_DEC_ROUND
pragma, represent the IEC 60559 rounding-direction attributes roundTowardNegative, roundTiesTo-
Even, roundTiesToAway, roundTowardZero, and roundTowardPositive, respectively, for decimal
floating-point arithmetic.
19 The llquantexpdN (7.12.15.4) functions compute the (quantum) exponent q defined in IEC 60559
for decimal numbers viewed as having integer significands.
20 The C functions in the following table correspond to mathematical operations recommended by
IEC 60559. However, correct rounding, which IEC 60559 specifies for its operations, is not required
for the C functions in the table. 7.33.8 (potentially) reserves cr_ prefixed names for functions fully
matching the IEC 60559 mathematical operations. In the table, the C functions are represented by
the function name without a type suffix.
IEC 60559 operation C function Clause
exp exp 7.12.6.1, F.10.3.1
expm1 expm1 7.12.6.6, F.10.3.6
exp2 exp2 7.12.6.4, F.10.3.4
exp2m1 exp2m1 7.12.6.5, F.10.3.5
exp10 exp10 7.12.6.2, F.10.3.2
exp10m1 exp10m1 7.12.6.3, F.10.3.3
log log 7.12.6.11, F.10.3.11
log2 log2 7.12.6.15, F.10.3.15
log10 log10 7.12.6.12, F.10.3.12
logp1 log1p, logp1 7.12.6.14, F.10.3.14
log2p1 log2p1 7.12.6.16, F.10.3.16
log10p1 log10p1 7.12.6.13, F.10.3.13
hypot hypot 7.12.7.4, F.10.4.4
rSqrt rsqrt 7.12.7.9, F.10.4.9
compound compoundn 7.12.7.2, F.10.4.2
rootn rootn 7.12.7.8, F.10.4.8
pown pown 7.12.7.6, F.10.4.6
pow pow 7.12.7.5, F.10.4.5
powr powr 7.12.7.7, F.10.4.7
sin sin 7.12.4.6, F.10.1.6
... continued ...
... continued ...
IEC 60559 operation C function Clause
cos cos 7.12.4.5, F.10.1.5
tan tan 7.12.4.7, F.10.1.7
sinPi sinpi 7.12.4.13, F.10.1.13
cosPi cospi 7.12.4.12, F.10.1.12
tanPi tanpi 7.12.4.14, F.10.1.14
asinPi asinpi 7.12.4.9, F.10.1.9
acosPi acospi 7.12.4.8, F.10.1.8
atanPi atanpi 7.12.4.10, F.10.1.10
atan2Pi atan2pi 7.12.4.11, F.10.1.11
asin asin 7.12.4.2, F.10.1.2
acos acos 7.12.4.1, F.10.1.1
atan atan 7.12.4.3, F.10.1.3
atan2 atan2 7.12.4.4, F.10.1.4
sinh sinh 7.12.5.5, F.10.2.5
cosh cosh 7.12.5.4, F.10.2.4
tanh tanh 7.12.5.6, F.10.2.6
asinh asinh 7.12.5.2, F.10.2.2
acosh acosh 7.12.5.1, F.10.2.1
atanh atanh 7.12.5.3, F.10.2.3
F.4 Floating to integer conversion
1 If the integer type is bool, 6.3.1.2 applies and the conversion raises no floating-point exceptions if
the floating-point value is not a signaling NaN. Otherwise, if the floating value is infinite or NaN
or if the integral part of the floating value exceeds the range of the integer type, then the "invalid"
floating-point exception is raised and the resulting value is unspecified. Otherwise, the resulting
value is determined by 6.3.1.4. Conversion of an integral floating value that does not exceed the
range of the integer type raises no floating-point exceptions; whether conversion of a non-integral
floating value raises the "inexact" floating-point exception is unspecified.440)
F.5 Conversions between binary floating types and decimal character se-
quences
1 The <float.h> header defines the macro
CR_DECIMAL_DIG
which expands to an integer constant expression suitable for use in #if preprocessing directives
whose value is a number such that conversions between all supported IEC 60559 binary formats and
character sequences with at most CR_DECIMAL_DIG significant decimal digits are correctly rounded.
The value of CR_DECIMAL_DIG shall be at least M + 3, where M is the maximum value of the
T_DECIMAL_DIG macros for IEC 60559 binary formats. If the implementation correctly rounds for
all numbers of significant decimal digits, then CR_DECIMAL_DIG shall have the value of the macro
UINTMAX_MAX.
2 Conversions of types with IEC 60559 binary formats to character sequences with more than
CR_DECIMAL_DIG significant decimal digits shall correctly round to CR_DECIMAL_DIG significant
digits and pad zeros on the right.
3 Conversions from character sequences with more than CR_DECIMAL_DIG significant decimal digits
to types with IEC 60559 binary formats shall correctly round to an intermediate character sequence
with CR_DECIMAL_DIG significant decimal digits, according to the applicable rounding direction,
and correctly round the intermediate result (having CR_DECIMAL_DIG significant decimal digits) to
440) IEC 60559 recommends that implicit floating-to-integer conversions raise the "inexact" floating-point exception for
non-integer in-range values. In those cases where it matters, library functions can be used to effect such conversions with or
without raising the "inexact" floating- point exception. See fromfp, ufromfp, fromfpx, ufromfpx, rint, lrint, llrint, and
nearbyint in <math.h>.
the destination type. The "inexact" floating-point exception is raised (once) if either conversion
is inexact.441) (The second conversion may raise the "overflow" or "underflow" floating-point
exception.)
4 The specification in this subclause assures conversion between IEC 60559 binary format and decimal
character sequence follows all pertinent recommended practice. It also assures conversion from
IEC 60559 format to decimal character sequence with at least T_DECIMAL_DIG digits and back, using
to-nearest rounding, is the identity function, where T is the macro prefix for the format.
5 Functions such as strtod that convert character sequences to floating types honor the rounding
direction. Hence, if the rounding direction might be upward or downward, the implementation
cannot convert a minus-signed sequence by negating the converted unsigned sequence.
6 NOTE IEC 60559 specifies that conversion to one-digit character strings using roundTiesToEven when both choices have
an odd least significant digit, shall produce the value with the larger magnitude. For example, this can happen with 9.5e2
whose nearest neighbors are 9.e2 and 1.e3, both of which have a single odd digit in the significand part.
F.6 The return statement
If the return expression is evaluated in a floating-point format different from the return type, the
expression is converted as if by assignment442) to the return type of the function and the resulting
value is returned to the caller.
F.7 Contracted expressions
1 A contracted expression is correctly rounded (once) and treats infinities, NaNs, signed zeros, sub-
normals, and the rounding directions in a manner consistent with the basic arithmetic operations
covered by IEC 60559.
Recommended practice
2 A contracted expression should raise floating-point exceptions in a manner generally consistent
with the basic arithmetic operations.
F.8 Floating-point environment
1 The floating-point environment defined in <fenv.h> includes the IEC 60559 floating-point exception
status flags and rounding-direction control modes. It may also include other floating-point status or
modes that the implementation provides as extensions.443)
2 This annex does not include support for IEC 60559’s optional alternate exception handling. The
specification in this annex assumes IEC 60559 default exception handling: the flag is set, a default
result is delivered, and execution continues. Implementations might provide alternate exception
handling as an extension.
F.8.1 Environment management
1 IEC 60559 requires that floating-point operations implicitly raise floating-point exception status
flags, and that rounding control modes can be set explicitly to affect result values of floating-point
operations. These changes to the floating-point state are treated as side effects which respect
sequence points.444)
F.8.2 Translation
1 During translation, constant rounding direction modes (7.6.2) are in effect where specified. Else-
where, during translation the IEC 60559 default modes are in effect:
— The rounding direction mode is rounding to nearest.
— The rounding precision mode (if supported) is set so that results are not shortened.
441) The intermediate conversion is exact only if all input digits after the first CR_DECIMAL_DIG digits are 0.
442) Assignment removes any extra range and precision.
443) Dynamic rounding precision and trap enablement modes are examples of such extensions.
444) If the state for the FENV_ACCESS pragma is "off", the implementation is free to assume the dynamic floating-point control
modes will be the default ones and the floating-point status flags will not be tested, which allows certain optimizations
(see F.9).
— Trapping or stopping (if supported) is disabled on all floating-point exceptions.
Recommended practice
2 The implementation should produce a diagnostic message for each translation-time floating-point
exception, other than "inexact";445) the implementation should then proceed with the translation of
the program.
F.8.3 Execution
1 At program startup the dynamic floating-point environment is initialized as prescribed by IEC 60559:
— All floating-point exception status flags are cleared.
— The dynamic rounding direction mode is rounding to nearest.
— The dynamic rounding precision mode (if supported) is set so that results are not shortened.
— Trapping or stopping (if supported) is disabled on all floating-point exceptions.
F.8.4 Constant expressions
1 An arithmetic constant expression of floating type, other than one in an initializer for an object that
has static or thread storage duration, is evaluated (as if) during execution; thus, it is affected by any
operative floating-point control modes and raises floating-point exceptions as required by IEC 60559
(provided the state for the FENV_ACCESS pragma is "on").446)
2 EXAMPLE
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(void)
{
float w[] = { 0.0/0.0 }; // raises an exception
static float x = 0.0/0.0; // does not raise an exception
float y = 0.0/0.0; // raises an exception
double z = 0.0/0.0; // raises an exception
/* ... */
}
3 For the static initialization, the division is done at translation time, raising no (execution-time) floating-point exceptions. On
the other hand, for the three automatic initializations the invalid division occurs at execution time.
F.8.5 Initialization
1 All computation for automatic initialization is done (as if) at execution time; thus, it is affected by
any operative modes and raises floating-point exceptions as required by IEC 60559 (provided the
state for the FENV_ACCESS pragma is "on"). All computation for initialization of objects that have
static or thread storage duration is done (as if) at translation time.
2 EXAMPLE
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(void)
445) As floating constants are converted to appropriate internal representations at translation time, their conversion is subject
to constant or default rounding modes and raises no execution-time floating-point exceptions (even where the state of the
FENV_ACCESS pragma is "on"). Library functions, for example strtod, provide execution-time conversion of numeric strings.
446) Where the state for the FENV_ACCESS pragma is "on", results of inexact expressions like 1.0/3.0 are affected by rounding
modes set at execution time, and expressions such as 0.0/0.0 and 1.0/0.0 generate execution-time floating-point exceptions.
The programmer can achieve the efficiency of translation-time evaluation through static initialization, such as
const static double one_third = 1.0/3.0;
{
float u[] = { 1.1e75 }; // raises exceptions
static float v = 1.1e75; // does not raise exceptions
float w = 1.1e75; // raises exceptions
double x = 1.1e75; // may raise exceptions
float y = 1.1e75f; // may raise exceptions
long double z = 1.1e75; // does not raise exceptions
/* ... */
}
3 The static initialization of v raises no (execution-time) floating-point exceptions because its computation is done at translation
time. The automatic initialization of u and w require an execution-time conversion to float of the wider value 1.1e75,
which raises floating-point exceptions. The automatic initializations of x and y entail execution-time conversion; however, in
some expression evaluation methods, the conversions is not to a narrower format, in which case no floating-point exception
is raised.447) The automatic initialization of z entails execution-time conversion, but not to a narrower format, so no
floating-point exception is raised. Note that the conversions of the floating constants 1.1e75 and 1.1e75f to their internal
representations occur at translation time in all cases.
F.8.6 Changing the environment
1 Operations defined in 6.5 and functions and macros defined for the standard libraries change
floating-point status flags and control modes just as indicated by their specifications (including
conformance to IEC 60559). They do not change flags or modes (so as to be detectable by the user) in
any other cases.
2 If the floating-point exceptions represented by the argument to the feraiseexcept function in
<fenv.h> include both "overflow" and "inexect", then "overflow" is raised before "inexact". Simi-
larly, if the represented exceptions include both "underflow" and "inexact", then "underflow" is
raised before "inexact".
F.9 Optimization
1 This section identifies code transformations that might subvert IEC 60559-specified behavior, and
others that do not.
F.9.1 Global transformations
1 Floating-point arithmetic operations and external function calls may entail side effects which
optimization shall honor, at least where the state of the FENV_ACCESS pragma is "on". The flags
and modes in the floating-point environment may be regarded as global variables; floating-point
operations (+ , * , etc.) implicitly read the modes and write the flags.
2 Concern about side effects may inhibit code motion and removal of seemingly useless code. For
example, in
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(double x)
{
/* ... */
for (i = 0; i < n; i++) x + 1;
/* ... */
}
x+1 might raise floating-point exceptions, so cannot be removed. And since the loop body might not
execute (maybe 0 ≥ n), x+1 cannot be moved out of the loop. (Of course these optimizations are
valid if the implementation can rule out the nettlesome cases.)
447) Use of float_t and double_t variables increases the likelihood of translation-time computation. For example, the
automatic initialization
double_t x = 1.1e75;
could be done at translation time, regardless of the expression evaluation method.
3 This specification does not require support for trap handlers that maintain information about
the order or count of floating-point exceptions. Therefore, between function calls, floating-point
exceptions need not be precise: the actual order and number of occurrences of floating-point
exceptions (> 1) may vary from what the source code expresses. Thus, the preceding loop could be
treated as
if (0 < n) x + 1;
F.9.2 Expression transformations
1 Valid expression transformations must preserve numerical values.
2 The equivalences noted below apply to expressions of standard floating types.
x/2 ↔ x × 0.5 Although similar transformations involving inexact constants generally do not
yield equivalent expressions, if the constants are exact then such transforma-
tions can be made on IEC 60559 machines and others that round perfectly.
1 × x and x/1 → x The expressions 1 × x, x/1, and x may be regarded as equivalent (on IEC 60559
machines, among others).448)
x/x → 1.0 The expressions x/x and 1.0 are not equivalent if x can be zero, infinite, or NaN.
x − y ↔ x + (−y) The expressions x − y, x + (−y), and (−y) + x are equivalent (on IEC 60559
machines, among others).
x − y ↔ −(y − x) The expressions x − y and −(y − x) are not equivalent because 1 − 1 is +0 but
−(1 − 1) is −0 (in the default rounding direction).449)
x − x → 0.0 The expressions x − x and 0.0 are not equivalent if x is a NaN or infinite.
0 × x → 0.0 The expressions 0 × x and 0.0 are not equivalent if x is a NaN, infinite, or −0.
x+0→x The expressions x + 0 and x are not equivalent if x is −0, because (−0) + (+0)
yields +0 (in the default rounding direction), not −0.
x−0→x (+0) − (+0) yields −0 when rounding is downward (toward −∞), but +0
otherwise, and (−0)−(+0) always yields −0; so, if the state of the FENV_ACCESS
pragma is "off", promising default rounding, then the implementation can
replace x − 0 by x, even if x might be zero.
−x ↔ 0 − x The expressions −x and 0−x are not equivalent if x is +0, because −(+0) yields
−0, but 0 − (+0) yields +0 (unless rounding is downward).
3 For expressions of decimal floating types, transformations must preserve quantum exponents, as
well as numerical values (5.2.4.2.3).
4 EXAMPLE 1. × x → x is valid for decimal floating-point expressions x, but 1.0 × x → x is not:
1. × 12.34 = (+1, 1, 0) × (+1, 1234, −2) yields (+1, 1234, −2) = 12.34
1.0 × 12.34 = (+1, 10, −1) × (+1, 1234, −2) yields (+1, 12340, −3) = 12.340
In the second case, the factor 12.34 and the result 12.340 have different quantum exponents, demonstrating that 1.0 × x and
x are not equivalent expressions.
448) Implementations might have non-required features that invalidate these and other transformations that remove arithmetic
operators. Examples include strict support for signaling NaNs (an optional feature) and alternate exception handling (not
included in this specification).
449) IEC 60559 prescribes a signed zero to preserve mathematical identities across certain discontinuities. Examples include:
1/(1/±∞) is ±∞
and
conj(csqrt(z)) is csqrt(conj(z)),
for complex z.
F.9.3 Relational operators
1 x ̸= x → false The expression x ̸= x is true if x is a NaN.
x = x → true The expression x = x is false if x is a NaN.
x < y → isless(x, y) (and similarly for ≤, >, ≥) Though equal, these expressions are not equiv-
alent because of side effects when x or y is a NaN and the state of the
FENV_ACCESS pragma is "on". This transformation, which would be de-
sirable if extra code were required to cause the "invalid" floating-point
exception for unordered cases, could be performed provided the state of the
FENV_ACCESS pragma is "off".
The sense of relational operators shall be maintained. This includes handling unordered cases as
expressed by the source code.
2 EXAMPLE
// calls g and raises "invalid" if a and b are unordered
if (a < b)
f();
else
g();
is not equivalent to
// calls f and raises "invalid" if a and b are unordered
if (a >= b)
g();
else
f();
nor to
// calls f without raising "invalid" if a and b are unordered
if (isgreaterequal(a,b))
g();
else
f();
nor, unless the state of the FENV_ACCESS pragma is "off", to
// calls g without raising "invalid" if a and b are unordered
if (isless(a,b))
f();
else
g();
but is equivalent to
if (!(a < b))
g();
else
f();
F.9.4 Constant arithmetic
1 The implementation shall honor floating-point exceptions raised by execution-time constant arith-
metic wherever the state of the FENV_ACCESS pragma is "on". (See F.8.4 and F.8.5.) An operation
on constants that raises no floating-point exception can be folded during translation, except, if the
state of the FENV_ACCESS pragma is "on", a further check is required to assure that changing the
rounding direction to downward does not alter the sign of the result,450) and implementations that
450) 0-0 yields-0 instead of +0 just when the rounding direction is downward.
support dynamic rounding precision modes shall assure further that the result of the operation
raises no floating-point exception when converted to the semantic type of the operation.
F.10 Mathematics <math.h> and <tgmath.h>
1 This subclause contains specifications of <math.h> and <tgmath.h> facilities that are particularly
suited for IEC 60559 implementations.
2 The Standard C macro HUGE_VAL and its float and long double analogs, HUGE_VALF and
HUGE_VALL, expand to expressions whose values are positive infinities.
3 For each single-argument function f in <math.h> whose mathematical counterpart is symmetric
(even), f(-x) is f(x) for all rounding modes and for all x in the (valid) domain of the function. For
each single-argument function f in <math.h> whose mathematical counterpart is antisymmetric
(odd), f(-x) is-f(x) for the IEC 60559 rounding modes roundTiesToEven, roundTiesToAway, and
roundTowardZero, and for all x in the (valid) domain of the function. The atan2 and atan2pi
functions are odd in their first argument.
4 Special cases for functions in <math.h> are covered directly or indirectly by IEC 60559. The functions
that IEC 60559 specifies directly are identified in F.3. The other functions in <math.h> treat infinities,
NaNs, signed zeros, subnormals, and (provided the state of the FENV_ACCESS pragma is "on") the
floating-point status flags in a manner consistent with IEC 60559 operations.
5 The expression math_errhandling & MATH_ERREXCEPT shall evaluate to a nonzero value.
6 The functions bound to operations in IEC 60559 (F.3) are fully specified by IEC 60559, including
rounding behaviors and floating-point exceptions.
7 The "invalid" and "divide-by-zero" floating-point exceptions are raised as specified in subsequent
subclauses of this annex.
8 The "overflow" floating-point exception is raised whenever an infinity — or, because of rounding di-
rection, a maximal-magnitude finite number — is returned in lieu of a finite value whose magnitude
is too large.
9 The "underflow" floating-point exception is raised whenever a computed result is tiny451) and the
returned result is inexact.
10 Whether or when library functions not listed in the "Operation binding" table in F.3 raise the
"inexact" floating-point exception is unspecified, unless stated otherwise.
11 Whether or when library functions not listed in the "Operation binding" table in F.3 raise a spurious
"underflow" floating-point exception is not specified by this annex.452)
12 As implied by F.8.6, library functions do not raise spurious "invalid", "overflow", or "divide-by-zero"
floating-point exceptions (detectable by the user).
13 Whether the functions not listed in the "Operation binding" table in F.3 honor the rounding direction
mode is implementation-defined, unless explicitly specified otherwise.
14 Functions with a NaN argument return a NaN result and raise no floating-point exception, except
where explicitly stated otherwise.
15 The specifications in the following subclauses append to the definitions in <math.h>. For families of
functions, the specifications apply to all of the functions even though only the principal function
is shown. Unless otherwise specified, where the symbol "±" occurs in both an argument and the
result, the result has the same sign as the argument.
Recommended practice
16 IEC 60559 specifies correct rounding for the operations in the F.3 table of operations recommended
by IEC 60559, and thereby preserves useful mathematical properties such as symmetry, monotonicity,
and periodicity. The corresponding functions with (potentially) reserved cr_-prefixed names (7.33.8)
451) Tiny generally indicates having a magnitude in the subnormal range. See IEC 60559 for details about detecting tininess.
452) It is intended that spurious "underflow" and "inexact" floating-point exceptions are raised only if avoiding them would
be too costly. 7.12.1 specifies that if math_errhandling & MATH_ERREXCEPT is nonzero, then an "underflow" floating-point
exception shall not be raised unless an underflow range error occurs.
do the same. The C functions in the table, however, are not required to be correctly rounded, but
implementations should still preserve as many of these useful mathematical properties as possible.
17 If a function with one or more NaN arguments returns a NaN result, the result should be the same
as one of the NaN arguments (after possible type conversion), except perhaps for the sign.
F.10.1 Trigonometric functions
F.10.1.1 The acos functions
1 — acos(1) returns +0.
— acos(x) returns a NaN and raises the "invalid" floating-point exception for |x| > 1.
F.10.1.2 The asin functions
1 — asin(±0) returns ±0.
— asin(x) returns a NaN and raises the "invalid" floating-point exception for |x| > 1.
F.10.1.3 The atan functions
1 — atan(±0) returns ±0.
— atan(±∞) returns ± π2 .
F.10.1.4 The atan2 functions
1 — atan2(±0, −0) returns ±π.453)
— atan2(±0, +0) returns ±0.
— atan2(±0, x) returns ±π for x < 0.
— atan2(±0, x) returns ±0 for x > 0.
— atan2(y, ±0) returns − π2 for y < 0.
— atan2(y, ±0) returns π2 for y > 0.
— atan2(±y, −∞) returns ±π for finite y > 0.
— atan2(±y, +∞) returns ±0 for finite y > 0.
— atan2(±∞, x) returns ± π2 for finite x.
— atan2(±∞, −∞) returns ± 3π
4 .
— atan2(±∞, +∞) returns ± π4 .
F.10.1.5 The cos functions
1 — cos(±0) returns 1.
— cos(±∞) returns a NaN and raises the "invalid" floating-point exception.
F.10.1.6 The sin functions
1 — sin(±0) returns ±0.
— sin(±∞) returns a NaN and raises the "invalid" floating-point exception.
F.10.1.7 The tan functions
1 — tan(±0) returns ±0.
— tan(±∞) returns a NaN and raises the "invalid" floating-point exception.
453) atan2(0, 0) does not raise the "invalid" floating-point exception, nor does atan2(y, 0) raise the "divide-by-zero" floating-
point exception.
F.10.1.8 The acospi functions
1 — acospi(+1) returns +0.
— acospi(x) returns a NaN and raises the "invalid" floating-point exception for |x| > 1.
F.10.1.9 The asinpi functions
1 — asinpi(±0) returns ±0.
— asinpi(x) returns a NaN and raises the "invalid" floating-point exception for |x| > 1.
F.10.1.10 The atanpi functions
1 — atanpi(±0) returns ±0.
— atanpi(±∞) returns ± 21 .
F.10.1.11 The atan2pi functions
1 — atan2pi(±0, −0) returns ±1.454)
— atan2pi(±0, +0) returns ±0.
— atan2pi(±0, x) returns ±1 for x < 0.
— atan2pi(±0, x) returns ±0 for x > 0.
— atan2pi(y, ±0) returns − 12 for y < 0.
— atan2pi(y, ±0) returns + 12 for y > 0.
— atan2pi(±y, −∞) returns ±1 for finite y > 0.
— atan2pi(±y, +∞) returns ±0 for finite y > 0.
— atan2pi(±∞, x) returns ± 12 for finite x.
— atan2pi(±∞, −∞) returns ± 43 .
— atan2pi(±∞, +∞) returns ± 14 .
F.10.1.12 The cospi functions
1 — cospi(±0) returns 1.
— cospi(n + 12 ) returns +0, for integers n.
— cospi(±∞) returns a NaN and raises the "invalid" floating-point exception.
F.10.1.13 The sinpi functions
1 — sinpi(±0) returns ±0.
— sinpi(±n) returns ±0, for positive integers n.
— sinpi(±∞) returns a NaN and raises the "invalid" floating-point exception.
454) atan2pi(0, 0) does not raise the "invalid" floating-point exception, nor does atan2pi(y, 0) raise the "divide-by-zero"
floating-point exception.
F.10.1.14 The tanpi functions
1 — tanpi(±0) returns ±0.
— tanpi(n) returns +0, for positive even and negative odd integers n.
— tanpi(n) returns −0, for positive odd and negative even integers n.
— tanpi(n + 12 ) returns +∞ and raises the "divide-by-zero" floating-point exception, for even
integers n.
— tanpi(n + 12 ) returns −∞ and raises the "divide-by-zero" floating-point exception, for odd
integers n.
— tanpi(±∞) returns a NaN and raises the "invalid" floating-point exception.
F.10.2 Hyperbolic functions
F.10.2.1 The acosh functions
1 — acosh(1) returns +0.
— acosh(x) returns a NaN and raises the "invalid" floating-point exception for x < 1.
— acosh(+∞) returns +∞.
F.10.2.2 The asinh functions
1 — asinh(±0) returns ±0.
— asinh(±∞) returns ±∞.
F.10.2.3 The atanh functions
1 — atanh(±0) returns ±0.
— atanh(±1) returns ±∞ and raises the "divide-by-zero" floating-point exception.
— atanh(x) returns a NaN and raises the "invalid" floating-point exception for |x| > 1.
F.10.2.4 The cosh functions
1 — cosh(±0) returns 1.
— cosh(±∞) returns +∞.
F.10.2.5 The sinh functions
1 — sinh(±0) returns ±0.
— sinh(±∞) returns ±∞.
F.10.2.6 The tanh functions
1 — tanh(±0) returns ±0.
— tanh(±∞) returns ±1.
F.10.3 Exponential and logarithmic functions
F.10.3.1 The exp functions
1 — exp(±0) returns 1.
— exp(−∞) returns +0.
— exp(+∞) returns +∞.
F.10.3.2 The exp10 functions
1 — exp10(±0) returns 1.
— exp10(−∞) returns +0.
— exp10(+∞) returns +∞.
F.10.3.3 The exp10m1 functions
1 — exp10m1(±0) returns ±0.
— exp10m1(−∞) returns −1.
— exp10m1(+∞) returns +∞.
F.10.3.4 The exp2 functions
1 — exp2(±0) returns 1.
— exp2(−∞) returns +0.
— exp2(+∞) returns +∞.
F.10.3.5 The exp2m1 functions
1 — exp2m1(±0) returns ±0.
— exp2m1(−∞) returns −1.
— exp2m1(+∞) returns +∞.
F.10.3.6 The expm1 functions
1 — expm1(±0) returns ±0.
— expm1(−∞) returns −1.
— expm1(+∞) returns +∞.
F.10.3.7 The frexp functions
1 — frexp(±0, exp) returns ±0, and stores 0 in the object pointed to by exp.
— frexp(±∞, exp) returns ±∞, and stores an unspecified value in the object pointed to by exp.
— frexp(NaN, exp) stores an unspecified value in the object pointed to by exp (and returns a
NaN).
2 frexp raises no floating-point exceptions if value is not a signaling NaN.
3 The returned value is independent of the current rounding direction mode.
4 On a binary system, the body of the frexp function might be
{
*exp = (value == 0) ? 0: (int)(1 + logb(value));
return scalbn(value, -(*exp));
}
F.10.3.8 The ilogb functions
1 When the correct result is representable in the range of the return type, the returned value is exact
and is independent of the current rounding direction mode.
2 If the correct result is outside the range of the return type, the numeric result is unspecified and the
"invalid" floating-point exception is raised.
3 ilogb(x), for x zero, infinite, or NaN, raises the "invalid" floating-point exception and returns the
value specified in 7.12.6.8.
F.10.3.9 The ldexp functions
1 On a binary system, ldexp(x, exp) is equivalent to scalbn(x, exp).
F.10.3.10 The llogb functions
1 The llogb functions are equivalent to the ilogb functions, except that the llogb functions determine
a result in the long int type.
F.10.3.11 The log functions
1 — log(±0) returns −∞ and raises the "divide-by-zero" floating-point exception.
— log(1) returns +0.
— log(x) returns a NaN and raises the "invalid" floating-point exception for x < 0.
— log(+∞) returns +∞.
F.10.3.12 The log10 functions
1 — log10(±0) returns −∞ and raises the "divide-by-zero" floating-point exception.
— log10(1) returns +0.
— log10(x) returns a NaN and raises the "invalid" floating-point exception for x < 0.
— log10(+∞) returns +∞.
F.10.3.13 The log10p1 functions
1 — log10p1(±0) returns ±0.
— log10p1(−1) returns −∞ and raises the "divide-by-zero" floating-point exception.
— log10p1(x) returns a NaN and raises the "invalid" floating-point exception for x < −1.
— log10p1(+∞) returns +∞.
F.10.3.14 The log1p and logp1 functions
1 — logp1(±0) returns ±0.
— logp1(−1) returns −∞ and raises the "divide-by-zero" floating-point exception.
— logp1(x) returns a NaN and raises the "invalid" floating-point exception for x < −1.
— logp1(+∞) returns +∞.
The log1p functions are equivalent to the logp1 functions.
F.10.3.15 The log2 functions
1 — log2(±0) returns −∞ and raises the "divide-by-zero" floating-point exception.
— log2(1) returns +0.
— log2(x) returns a NaN and raises the "invalid" floating-point exception for x < 0.
— log2(+∞) returns +∞.
F.10.3.16 The log2p1 functions
1 — log2p1(±0) returns ±0.
— log2p1(−1) returns −∞ and raises the "divide-by-zero" floating-point exception.
— log2p1(x) returns a NaN and raises the "invalid" floating-point exception for x < −1.
— log2p1(+∞) returns +∞.
F.10.3.17 The logb functions
1 — logb(±0) returns −∞ and raises the "divide-by-zero" floating-point exception.
— logb(±∞) returns +∞.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.3.18 The modf functions
1 — modf(±x, iptr) returns a result with the same sign as x.
— modf(±∞, iptr) returns ±0 and stores ±∞ in the object pointed to by iptr.
— modf(NaN, iptr) stores a NaN in the object pointed to by iptr (and returns a NaN).
2 The returned values are exact and are independent of the current rounding direction mode.
3 modf behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double modf(double value, double *iptr)
{
int save_round = fegetround();
fesetround(FE_TOWARDZERO);
*iptr = nearbyint(value);
fesetround(save_round);
return copysign(
isinf(value) ? 0.0:
value - (*iptr), value);
}
F.10.3.19 The scalbn and scalbln functions
1 — scalbn(±0, n) returns ±0.
— scalbn(x, 0) returns x.
— scalbn(±∞, n) returns ±∞.
2 If the calculation does not overflow or underflow, the returned value is exact and independent of
the current rounding direction mode.
F.10.4 Power and absolute value functions
F.10.4.1 The cbrt functions
1 — cbrt(±0) returns ±0.
— cbrt(±∞) returns ±∞.
F.10.4.2 The compoundn functions
1 — compoundn(x, 0) returns 1 for x ≥ −1 or x a NaN.
— compoundn(x, n) returns a NaN and raises the "invalid" floating-point exception for x < −1.
— compoundn(−1, n) returns +∞ and raises the divide-by-zero floating-point exception for n < 0.
— compoundn(−1, n) returns +0 for n > 0.
F.10.4.3 The fabs functions
1 fabs(x) returns a value with the same bit representation as x, except with the sign bit set to 0
(positive), for all values of x (even quiet and signaling NaNs).
2 — fabs(±0) returns +0.
— fabs(±∞) returns +∞.
3 fabs(x) raises no floating-point exceptions, even if x is a signaling NaN. The returned value is
independent of the current rounding direction mode.
F.10.4.4 The hypot functions
1 — hypot(x, y), hypot(y, x), and hypot(x, −y) are equivalent.
— hypot(x, ±0) returns the absolute value of x, if x is not a NaN.
— hypot(±∞, y) returns +∞, even if y is a NaN.
— hypot(x, NaN) returns a NaN, if x is not ±∞.
F.10.4.5 The pow functions
1 — pow(±0, y) returns ±∞ and raises the "divide-by-zero" floating-point exception for y an odd
integer < 0.
— pow(±0, y) returns +∞ and raises the "divide-by-zero" floating-point exception for y < 0,
finite, and not an odd integer.
— pow(±0, −∞) returns +∞.
— pow(±0, y) returns ±0 for y an odd integer > 0.
— pow(±0, y) returns +0 for y > 0 and not an odd integer.
— pow(−1, ±∞) returns 1.
— pow(+1, y) returns 1 for any y, even a NaN.
— pow(x, ±0) returns 1 for any x, even a NaN.
— pow(x, y) returns a NaN and raises the "invalid" floating-point exception for finite x < 0 and
finite non-integer y.
— pow(x, −∞) returns +∞ for |x| < 1.
— pow(x, −∞) returns +0 for |x| > 1.
— pow(x, +∞) returns +0 for |x| < 1.
— pow(x, +∞) returns +∞ for |x| > 1.
— pow(−∞, y) returns −0 for y an odd integer < 0.
— pow(−∞, y) returns +0 for y < 0 and not an odd integer.
— pow(−∞, y) returns −∞ for y an odd integer > 0.
— pow(−∞, y) returns +∞ for y > 0 and not an odd integer.
— pow(+∞, y) returns +0 for y < 0.
— pow(+∞, y) returns +∞ for y > 0.
F.10.4.6 The pown functions
1 — pown(x, 0) returns 1 for all x not a signalling NaN.
— pown(±0, n) returns ±∞ and raises the "divide-by-zero" floating-point exception for odd
n < 0.
— pown(±0, n) returns +∞ and raises the "divide-by-zero" floating-point exception for even
n < 0.
— pown(±0, n) returns +0 for even n > 0.
— pown(±0, n) returns ±0 for odd n > 0.
— pown(±∞, n) is equivalent to pown(±0, −n) for n not 0, except that the "divide-by-zero"
floating-point exception is not raised.
F.10.4.7 The powr functions
1 — powr(x, ±0) returns 1 for finite x > 0.
— powr(±0, y) returns +∞ and raises the "divide-by-zero" floating-point exception for finite
y < 0.
— powr(±0, −∞) returns +∞.
— powr(±0, y) returns +0 for y > 0.
— powr(+1, y) returns 1 for finite y.
— powr(+1, y)
— powr(x, y) returns a NaN and raises the "invalid" floating-point exception for x < 0.
— powr(±0, ±0) returns a NaN and raises the "invalid" floating-point exception.
— powr(+∞, ±0) returns a NaN and raises the "invalid" floating-point exception.
F.10.4.8 The rootn functions
1 — rootn(±0, n) returns ±∞ and raises the "divide-by-zero" floating-point exception for odd
n < 0.
— rootn(±0, n) returns +∞ and raises the "divide-by-zero" floating-point exception for even
n < 0.
— rootn(±0, n) returns +0 for even n > 0.
— rootn(±0, n) returns ±0 for odd n > 0.
— rootn(+∞, n) returns +∞ for n > 0.
— rootn(−∞, n) returns −∞ for odd n > 0.
— rootn(−∞, n) returns a NaN and raises the "invalid" floating-point exception for even n > 0.
— rootn(+∞, n) returns +0 for n < 0.
— rootn(−∞, n) returns −0 for odd n < 0.
— rootn(−∞, n) returns a NaN and raises the "invalid" floating-point exception for even n < 0.
— rootn(x, 0) returns a NaN and raises the "invalid" floating-point exception for all x (including
NaN).
— rootn(x, n) returns a NaN and raises the "invalid" floating-point exception for x < 0 and n
even.
F.10.4.9 The rsqrt functions
1 — rsqrt(±0) returns ±∞ and raises the "divide-by-zero" floating-point exception.
— rsqrt(x) returns a NaN and raises the "invalid" floating-point exception for x < 0.
— rsqrt(+∞) returns +0.
F.10.4.10 The sqrt functions
1 — sqrt(±0) returns ±0.
— sqrt(+∞) returns +∞.
— sqrt(x) returns a NaN and raises the "invalid" floating-point exception for x < 0.
2 The returned value is dependent on the current rounding direction mode.
F.10.5 Error and gamma functions
F.10.5.1 The erf functions
1 — erf(±0) returns ±0.
— erf(±∞) returns ±1.
F.10.5.2 The erfc functions
1 — erfc(−∞) returns 2.
— erfc(+∞) returns +0.
F.10.5.3 The lgamma functions
1 — lgamma(1) returns +0.
— lgamma(2) returns +0.
— lgamma(x) returns +∞ and raises the "divide-by-zero" floating-point exception for x a negative
integer or zero.
— lgamma(−∞) returns +∞.
— lgamma(+∞) returns +∞.
F.10.5.4 The tgamma functions
1 — tgamma(±0) returns ±∞ and raises the "divide-by-zero" floating-point exception.
— tgamma(x) returns a NaN and raises the "invalid" floating-point exception for x a negative
integer.
— tgamma(−∞) returns a NaN and raises the "invalid" floating-point exception.
— tgamma(+∞) returns +∞.
F.10.6 Nearest integer functions
F.10.6.1 The ceil functions
1 — ceil(±0) returns ±0.
— ceil(±∞) returns ±∞.
2 The returned value is exact and is independent of the current rounding direction mode.
3 The double version of ceil behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double ceil(double x)
{
double result;
int save_round = fegetround();
fesetround(FE_UPWARD);
result = nearbyint(x);
fesetround(save_round);
return result;
}
F.10.6.2 The floor functions
1 — floor(±0) returns ±0.
— floor(±∞) returns ±∞.
2 The returned value is exact and is independent of the current rounding direction mode.
3 See the sample implementation for ceil in F.10.6.1.
F.10.6.3 The nearbyint functions
1 The nearbyint functions use IEC 60559 rounding according to the current rounding direction. They
do not raise the "inexact" floating-point exception if the result differs in value from the argument.
— nearbyint(±0) returns ±0 (for all rounding directions).
— nearbyint(±∞) returns ±∞ (for all rounding directions).
F.10.6.4 The rint functions
1 The rint functions differ from the nearbyint functions only in that they do raise the "inexact"
floating-point exception if the result differs in value from the argument.
F.10.6.5 The lrint and llrint functions
1 The lrint and llrint functions provide floating-to-integer conversion as prescribed by IEC 60559.
They round according to the current rounding direction. If the rounded value is outside the range of
the return type, the numeric result is unspecified and the "invalid" floating-point exception is raised.
When they raise no other floating-point exception and the result differs from the argument, they
raise the "inexact" floating-point exception.
F.10.6.6 The round functions
1 — round(±0) returns ±0.
— round(±∞) returns ±∞.
2 The returned value is independent of the current rounding direction mode.
3 The double version of round behaves as though implemented by455)
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double round(double x)
{
double result;
fenv_t save_env;
455) This code does not handle signaling NaNs as required of implementations that define FE_SNANS_ALWAYS_SIGNAL.
feholdexcept(&save_env);
result = rint(x);
if (fetestexcept(FE_INEXACT)) {
fesetround(FE_TOWARDZERO);
result = rint(copysign(0.5 + fabs(x), x));
feclearexcept(FE_INEXACT);
}
feupdateenv(&save_env);
return result;
}
F.10.6.7 The lround and llround functions
1 The lround and llround functions differ from the lrint and llrint functions with the default
rounding direction just in that the lround and llround functions round halfway cases away from
zero and need not raise the "inexact" floating-point exception for non-integer arguments that round
to within the range of the return type.
F.10.6.8 The roundeven functions
1
— roundeven(±0) returns ±0.
— roundeven(±∞) returns ±∞.
2 The returned value is exact and is independent of the current rounding direction mode.
3 See the sample implementation for ceil in F.10.6.1.
F.10.6.9 The trunc functions
1 The trunc functions use IEC 60559 rounding toward zero (regardless of the current rounding
direction).
— trunc(±0) returns ±0.
— trunc(±∞) returns ±∞.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.6.10 The fromfp and ufromfp functions
1 The fromfp and ufromfp functions raise the "invalid" floating-point exception and return a NaN
if the argument width is zero or if the floating-point argument x is infinite or NaN or rounds to an
integral value that is outside the range determined by the argument width (see 7.12.9.10).
2 These functions do not raise the "inexact" floating-point exception.
F.10.6.11 The fromfpx and ufromfpx functions
1 The fromfpx and ufromfpx functions raise the "invalid" floating-point exception and return a NaN
if the argument width is zero or if the floating-point argument x is infinite or NaN or rounds to an
integral value that is outside the range determined by the argument width (see 7.12.9.11).
2 These functions raise the "inexact" floating-point exception if a valid result differs in value from the
floating-point argument x.
F.10.7 Remainder functions
F.10.7.1 The fmod functions
1 — fmod(±0, y) returns ±0 for y not zero.
— fmod(x, y) returns a NaN and raises the "invalid" floating-point exception for x infinite or y
zero (and neither is a NaN).
— fmod(x, ±∞) returns x for x finite x.
2 When subnormal results are supported, the returned value is exact and is independent of the current
rounding direction mode.
3 The double version of fmod behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double fmod(double x, double y)
{
double result;
result = remainder(fabs(x), (y = fabs(y)));
if (signbit(result)) result += y;
return copysign(result, x);
}
F.10.7.2 The remainder functions
1 — remainder(±0, y) returns ±0 for y not zero.
— remainder(x, y) returns a NaN and raises the "invalid" floating-point exception for x infinite
or y zero (and neither is a NaN).
— remainder(x, ±∞) returns x for finite x.
2 When subnormal results are supported, the returned value is exact and is independent of the current
rounding direction mode.
F.10.7.3 The remquo functions
1 The remquo functions follow the specifications for the remainder functions.
2 If a NaN is returned, the value stored in the object pointed to by quo is unspecified.
3 When subnormal results are supported, the returned value is exact and is independent of the current
rounding direction mode.
F.10.8 Manipulation functions
F.10.8.1 The copysign functions
1 copysign(x, y) returns a value with the bit representation of x , except with the sign bit of y, for all
values x and y (even quiet and signaling NaNs).
2 copysign(x, y) raises no floating-point exceptions, even if x or y is a signaling NaN. The returned
value is independent of the current rounding direction mode.
F.10.8.2 The nan functions
1 All IEC 60559 implementations support quiet NaNs, in all floating formats.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.8.3 The nextafter functions
1 — nextafter(x, y) raises the "overflow" and "inexact" floating-point exceptions for x finite and
the function value infinite.
— nextafter(x, y) raises the "underflow" and "inexact" floating-point exceptions for the func-
tion value subnormal or zero and x ̸= y.
2 Even though underflow or overflow can occur, the returned value is independent of the current
rounding direction mode.
F.10.8.4 The nexttoward functions
1 No additional requirements beyond those on nextafter.
2 Even though underflow or overflow can occur, the returned value is independent of the current
rounding direction mode.
F.10.8.5 The nextup functions
1
— nextup(+∞) returns +∞.
— nextup(−∞) returns the largest-magnitude negative finite number in the type of the function.
2 nextup(x) raises no floating-point exceptions if x is not a signaling NaN. The returned value is
independent of the current rounding direction mode.
F.10.8.6 The nextdown functions
1
— nextdown(−∞) returns −∞.
— nextdown(+∞) returns the largest-magnitude positive finite number in the type of the func-
tion.
2 nextdown(x) raises no floating-point exceptions if x is not a signaling NaN. The returned value is
independent of the current rounding direction mode.
F.10.8.7 The canonicalize functions
1 The canonicalize functions produce456) the canonical version of the representation in the object
pointed to by the argument x. If the input *x is a signaling NaN, the "invalid" floating-point
exception is raised and a (canonical) quiet NaN (which should be the canonical version of that
signaling NaN made quiet) is produced. For quiet NaN, infinity, and finite inputs, the functions
raise no floating-point exceptions.
F.10.9 Maximum, minimum, and positive difference functions
F.10.9.1 The fdim functions
1 No additional requirements.
F.10.9.2 The fmax functions
1 If just one argument is a NaN, the fmax functions return the other argument (if both arguments are
NaNs, the functions return a NaN).
2 The returned value is exact and is independent of the current rounding direction mode.
3 The body of the fmax function might be457)
{
double r = (isgreaterequal(x, y) || isnan(y)) ? x : y;
(void) canonicalize(&r, &r);
return r;
}
F.10.9.3 The fmin functions
1 The fmin functions are analogous to the fmax functions (see F.10.9.2).
2 The returned value is exact and is independent of the current rounding direction mode.
456) As if *x * 1e0 were computed. Note also that this implementation does not handle signaling NaNs as required of
implementations that define FE_SNANS_ALWAYS_SIGNAL.
457) Ideally, fmax would be sensitive to the sign of zero, for example fmax(−0.0, +0.0) would return +0; however, implemen-
tation in software might be impractical.
F.10.9.4 The fmaximum, fminimum, fmaximum_mag, and fminimum_mag functions
1 These functions treat NaNs like other functions in <math.h> (see F.10). They differ from the cor-
responding fmaximum_num, fminimum_num, fmaximum_mag_num, and fminimum_mag_num functions
only in their treatment of NaNs.
F.10.9.5 The fmaximum_num, fminimum_num, fmaximum_mag_num, and fminimum_mag_num func-
tions
1 These functions return the number if one argument is a number and the other is a quiet or signaling
NaN. If both arguments are NaNs, a quiet NaN is returned. If an argument is a signaling NaN, the
"invalid" floating-point exception is raised (even though the function returns the number when the
other argument is a number).
F.10.10 Fused multiply-add
F.10.10.1 The fma functions
1 — fma(x, y, z) computes xy + z, correctly rounded once.
— fma(x, y, z) returns a NaN and optionally raises the "invalid" floating-point exception if one
of x and y is infinite, the other is zero, and z is a NaN.
— fma(x, y, z) returns a NaN and raises the "invalid" floating-point exception if one of x and y is
infinite, the other is zero, and z is not a NaN.
— fma(x, y, z) returns a NaN and raises the "invalid" floating-point exception if x times y is an
exact infinity and z is also an infinity but with the opposite sign.
F.10.11 Functions that round result to narrower type
1 The functions that round their result to narrower type (7.12.14) are fully specified in IEC 60559. The
returned value is dependent on the current rounding direction mode.
2 These functions treat zero and infinite arguments like the corresponding operation or function: + ,- ,
* , / , fma, or sqrt.
F.10.12 Total order functions
1 This subclause specifies the total order functions required by IEC 60559.
2 NOTE These functions are specified only in Annex F because they depend on details of IEC 60559 formats that might not be
supported if __STDC_IEC_60559_BFP__ is not defined.
F.10.12.1 The totalorder functions
Synopsis
1 #define __STDC_WANT_IEC_60559_EXT__
#include <math.h>
#ifdef __STDC_IEC_60559_BFP__
int totalorder(const double *x, const double *y);
int totalorderf(const float *x, const float *y);
int totalorderl(const long double *x, const long double *y);
#endif
#ifdef __STDC_IEC_60559_DFP__
int totalorderd32(const _Decimal32 *x, const _Decimal32 *y);
int totalorderd64(const _Decimal64 *x, const _Decimal64 *y);
int totalorderd128(const _Decimal128 *x, const _Decimal128 *y);
#endif
Description
2 The totalorder functions determine whether the total order relationship, defined by IEC 60559, is
true for the ordered pair of *x , *y . These functions are fully specified in IEC 60559. These functions
are independent of the current rounding direction mode and raise no floating-point exceptions, even
if *x or *y is a signaling NaN.
Returns
3 The totalorder functions return nonzero if and only if the total order relation is true for the ordered
pair of *x , *y .
F.10.12.2 The totalordermag functions
Synopsis
1 #define __STDC_WANT_IEC_60559_EXT__
#include <math.h>
#ifdef __STDC_IEC_60559_BFP__
int totalordermag(const double *x, const double *y);
int totalordermagf(const float *x, const float *y);
int totalordermagl(const long double *x, const long double *y);
#endif
#ifdef __STDC_IEC_60559_DFP__
int totalordermagd32(const _Decimal32 *x, const _Decimal32 *y);
int totalordermagd64(const _Decimal64 *x, const _Decimal64 *y);
int totalordermagd128(const _Decimal128 *x, const _Decimal128 *y);
#endif
Description
2 The totalordermag functions determine whether the total order relationship, defined by IEC 60559,
is true for the ordered pair of the magnitudes of *x , *y . These functions are fully specified in
IEC 60559. These functions are independent of the current rounding direction mode and raise no
floating-point exceptions, even if *x or *y is a signaling NaN.
Returns
3 The totalordermag functions return nonzero if and only if the total order relation is true for the
ordered pair of the magnitudes of *x , *y .
F.10.13 Payload functions
1 IEC 60559 defines the payload to be information contained in a quiet or signaling NaN. The payload
is intended for implementation-defined diagnostic information about the NaN, such as where or
how the NaN was created. The implementation interprets the payload as a nonnegative integer
suitable for use with the functions in this subclause, which get and set payloads. The implementation
may restrict which payloads are admissible for the user to set.
2 NOTE These functions are specified only in Annex F because they depend on details of IEC 60559 formats that might not be
supported if __STDC_IEC_60559_BFP__ is not defined.
F.10.13.1 The getpayload functions
Synopsis
1 #define __STDC_WANT_IEC_60559_EXT__
#include <math.h>
#ifdef __STDC_IEC_60559_BFP__
double getpayload(const double *x);
float getpayloadf(const float *x);
long double getpayloadl(const long double *x);
#endif
#ifdef __STDC_IEC_60559_DFP__
_Decimal32 getpayloadd32(const _Decimal32 *x);
_Decimal64 getpayloadd64(const _Decimal64 *x);
_Decimal128 getpayloadd128(const _Decimal128 *x);
#endif
Description
2 The getpayload functions extract the payload of a quiet or signaling NaN input and return it as a
positive-signed floating-point integer. If *x is not a NaN, the return result is −1. These functions
raise no floating-point exceptions, even if *x is a signaling NaN.
Returns
3 The getpayload functions return the payload of the NaN input as a positive-signed floating-point
integer.
F.10.13.2 The setpayload functions
Synopsis
1 #define __STDC_WANT_IEC_60559_EXT__
#include <math.h>
#ifdef __STDC_IEC_60559_BFP__
int setpayload(double *res, double pl);
int setpayloadf(float *res, float pl);
int setpayloadl(long double *res, long double pl);
#endif
#ifdef __STDC_IEC_60559_DFP__
int setpayloadd32(_Decimal32 *res, _Decimal32 pl);
int setpayloadd64(_Decimal64 *res, _Decimal64 pl);
int setpayloadd128(_Decimal128 *res, _Decimal128 pl);
#endif
Description
2 The setpayload functions create a quiet NaN with the payload specified by pl and a zero sign bit
and store that NaN in the object pointed to by *res . If pl is not a floating-point integer representing
an admissible payload, *res is set to +0.
Returns
3 If the setpayload functions stored the specified NaN, they return a zero value, otherwise a nonzero
value (and *res is set to +0).
F.10.13.3 The setpayloadsig functions
Synopsis
1 #define __STDC_WANT_IEC_60559_EXT__
#include <math.h>
#ifdef __STDC_IEC_60559_BFP__
int setpayloadsig(double *res, double pl);
int setpayloadsigf(float *res, float pl);
int setpayloadsigl(long double *res, long double pl);
#endif
#ifdef __STDC_IEC_60559_DFP__
int setpayloadsigd32(_Decimal32 *res, _Decimal32 pl);
int setpayloadsigd64(_Decimal64 *res, _Decimal64 pl);
int setpayloadsigd128(_Decimal128 *res, _Decimal128 pl);
#endif
Description
2 The setpayloadsig functions create a signaling NaN with the payload specified by pl and a zero
sign bit and store that NaN in the object pointed to by *res . If pl is not a floating-point integer
representing an admissible payload, *res is set to +0.
Returns
3 If the setpayloadsig functions stored the specified NaN, they return a zero value, otherwise a
nonzero value (and *res is set to +0).
F.10.14 Comparison macros
1 Relational operators and their corresponding comparison macros (7.12.17) produce equivalent result
values, even if argument values are represented in wider formats. Thus, comparison macro argu-
ments represented in formats wider than their semantic types are not converted to the semantic types,
unless the wide evaluation method converts operands of relational operators to their semantic types.
The standard wide evaluation methods characterized by FLT_EVAL_METHOD and DEC_EVAL_METHOD
equal to 1 or 2 (5.2.4.2.2, 5.2.4.2.3), do not convert operands of relational operators to their semantic
types.
F.10.14.1 The iseqsig macro
1 The equality operator == and the iseqsig macro produce equivalent results, except that the iseqsig
macro raises the "invalid" floating-point exception if an argument is a NaN.
G. Annex G (normative) IEC 60559-compatible complex arithmetic
G.1 Introduction
1 This annex supplements Annex F to specify complex arithmetic for compatibility with IEC 60559
real floating-point arithmetic. An implementation that defines __STDC_IEC_60559_COMPLEX__ or
__STDC_IEC_559_COMPLEX__ shall conform to the specifications in this annex.458)
G.2 Types
1 There is a new keyword _Imaginary , which is used to specify imaginary types. It is used as a type
specifier within declaration specifiers in the same way as _Complex is (thus, _Imaginary float is a
valid type name).
2 There are three imaginary types, designated as float _Imaginary, double _Imaginary, and
long double _Imaginary . The imaginary types (along with the real floating and complex types)
are floating types.
3 For imaginary types, the corresponding real type is given by deleting the keyword _Imaginary
from the type name.
4 Each imaginary type has the same representation and alignment requirements as the corresponding
real type. The value of an object of imaginary type is the value of the real representation times the
imaginary unit.
5 The imaginary type domain comprises the imaginary types.
G.3 Conventions
1 A complex or imaginary value with at least one infinite part is regarded as an infinity (even if its
other part is a quiet NaN). A complex or imaginary value is a finite number if each of its parts is a
finite number (neither infinite nor NaN). A complex or imaginary value is a zero if each of its parts is
a zero.
G.4 Conversions
G.4.1 Imaginary types
1 Conversions among imaginary types follow rules analogous to those for real floating types.
G.4.2 Real and imaginary
1 When a value of imaginary type is converted to a real type other than bool,459) the result is a positive
zero.
2 When a value of real type is converted to an imaginary type, the result is a positive imaginary zero.
G.4.3 Imaginary and complex
1 When a value of imaginary type is converted to a complex type, the real part of the complex result
value is a positive zero and the imaginary part of the complex result value is determined by the
conversion rules for the corresponding real types.
2 When a value of complex type is converted to an imaginary type, the real part of the complex value
is discarded and the value of the imaginary part is converted according to the conversion rules for
the corresponding real types.
458) Implementations that do not define __STDC_IEC_60559_COMPLEX__ or __STDC_IEC_559_COMPLEX__ are not required
to conform to these specifications. The use of __STDC_IEC_559_COMPLEX__ for this purpose is obsolescent and should be
avoided in new code.
459) See 6.3.1.2.
G.5 Binary operators
1 The following subclauses supplement 6.5 in order to specify the type of the result for an operation
with an imaginary operand.
2 For most operand types, the value of the result of a binary operator with an imaginary or complex
operand is completely determined, with reference to real arithmetic, by the usual mathematical
formula. For some operand types, the usual mathematical formula is problematic because of its
treatment of infinities and because of undue overflow or underflow; in these cases the result satisfies
certain properties (specified in G.5.1), but is not completely determined.
G.5.1 Multiplicative operators
Semantics
1 If one operand has real type and the other operand has imaginary type, then the result has imaginary
type. If both operands have imaginary type, then the result has real type. (If either operand has
complex type, then the result has complex type.)
2 If the operands are not both complex, then the result and floating-point exception behavior of the *
operator is defined by the usual mathematical formula:
* u iv u + iv
x xu i(xv) (xu) + i(xv)
iy i(yu) (−y)v ((−y)v) + i(yu)
x + iy (xu) + i(yu) ((−y)v) + i(xv)
3 If the second operand is not complex, then the result and floating-point exception behavior of the /
operator is defined by the usual mathematical formula:
/ u iv
x x/u i((−x)/v)
iy i(y/u) y/v
x + iy (x/u) + i(y/u) (y/v) + i((−x)/v)
4 The * and / operators satisfy the following infinity properties for all real, imaginary, and complex
operands:460)
— if one operand is an infinity and the other operand is a nonzero finite number or an infinity,
then the result of the * operator is an infinity;
— if the first operand is an infinity and the second operand is a finite number, then the result of
the / operator is an infinity;
— if the first operand is a finite number and the second operand is an infinity, then the result of
the / operator is a zero;
— if the first operand is a nonzero finite number or an infinity and the second operand is a zero,
then the result of the / operator is an infinity.
5 If both operands of the * operator are complex or if the second operand of the / operator is complex,
the operator raises floating-point exceptions if appropriate for the calculation of the parts of the
result, and may raise spurious floating-point exceptions.
6 EXAMPLE 1 Multiplication of double _Complex operands could be implemented as follows. Note that the imaginary unit
I has imaginary type (see G.6).
#include <math.h>
#include <complex.h>
/* Multiply z * w ...*/
double complex _Cmultd(double complex z, double complex w)
460) These properties are already implied for those cases covered in the tables, but are required for all cases (at least where the
state for CX_LIMITED_RANGE is "off").
{
#pragma STDC FP_CONTRACT OFF
double a, b, c, d, ac, bd, ad, bc, x, y;
a = creal(z); b = cimag(z);
c = creal(w); d = cimag(w);
ac = a * c; bd = b * d;
ad = a * d; bc = b * c;
x = ac - bd; y = ad + bc;
if (isnan(x) && isnan(y)) {
/* Recover infinities that computed as NaN+iNaN ... */
int recalc = 0;
if (isinf(a) || isinf(b)) { // z is infinite
/* "Box" the infinity and change NaNs in the other factor to 0 */
a = copysign(isinf(a) ? 1.0: 0.0, a);
b = copysign(isinf(b) ? 1.0: 0.0, b);
if (isnan(c)) c = copysign(0.0, c);
if (isnan(d)) d = copysign(0.0, d);
recalc = 1;
}
if (isinf(c) || isinf(d)) { // w is infinite
/* "Box" the infinity and change NaNs in the other factor to 0 */
c = copysign(isinf(c) ? 1.0: 0.0, c);
d = copysign(isinf(d) ? 1.0: 0.0, d);
if (isnan(a)) a = copysign(0.0, a);
if (isnan(b)) b = copysign(0.0, b);
recalc = 1;
}
if (!recalc && (isinf(ac) || isinf(bd) ||
isinf(ad) || isinf(bc))) {
/* Recover infinities from overflow by changing NaNs to 0 ... */
if (isnan(a)) a = copysign(0.0, a);
if (isnan(b)) b = copysign(0.0, b);
if (isnan(c)) c = copysign(0.0, c);
if (isnan(d)) d = copysign(0.0, d);
recalc = 1;
}
if (recalc) {
x = INFINITY * (a * c - b * d);
y = INFINITY * (a * d + b * c);
}
}
return x + I * y;
}
7 This implementation achieves the required treatment of infinities at the cost of only one isnan test in ordinary (finite) cases.
It is less than ideal in that undue overflow and underflow could occur.
8 EXAMPLE 2 Division of two double _Complex operands could be implemented as follows.
#include <math.h>
#include <complex.h>
/* Divide z / w ... */
double complex _Cdivd(double complex z, double complex w)
{
#pragma STDC FP_CONTRACT OFF
double a, b, c, d, logbw, denom, x, y;
int ilogbw = 0;
a = creal(z); b = cimag(z);
c = creal(w); d = cimag(w);
logbw = logb(fmaximum_num(fabs(c), fabs(d)));
if (isfinite(logbw)) {
ilogbw = (int)logbw;
c = scalbn(c, -ilogbw); d = scalbn(d, -ilogbw);
}
denom = c * c + d * d;
x = scalbn((a * c + b * d) / denom, -ilogbw);
y = scalbn((b * c - a * d) / denom, -ilogbw);
/* Recover infinities and zeros that computed as NaN+iNaN; */
/* the only cases are nonzero/zero, infinite/finite, and finite/infinite, ... */
if (isnan(x) && isnan(y)) {
if ((denom == 0.0) &&
(!isnan(a) || !isnan(b))) {
x = copysign(INFINITY, c) * a;
y = copysign(INFINITY, c) * b;
}
else if ((isinf(a) || isinf(b)) &&
isfinite(c) && isfinite(d)) {
a = copysign(isinf(a) ? 1.0: 0.0, a);
b = copysign(isinf(b) ? 1.0: 0.0, b);
x = INFINITY * (a * c + b * d);
y = INFINITY * (b * c - a * d);
}
else if ((logbw == INFINITY) &&
isfinite(a) && isfinite(b)) {
c = copysign(isinf(c) ? 1.0: 0.0, c);
d = copysign(isinf(d) ? 1.0: 0.0, d);
x = 0.0 * (a * c + b * d);
y = 0.0 * (b * c - a * d);
}
}
return x + I * y;
}
9 Scaling the denominator alleviates the main overflow and underflow problem, which is more serious than for multiplication.
In the spirit of the multiplication example above, this code does not defend against overflow and underflow in the calculation
of the numerator. Scaling with the scalbn function, instead of with division, provides better roundoff characteristics.
G.5.2 Additive operators
Semantics
1 If both operands have imaginary type, then the result has imaginary type. (If one operand has real
type and the other operand has imaginary type, or if either operand has complex type, then the
result has complex type.)
2 In all cases the result and floating-point exception behavior of a + or- operator is defined by the
usual mathematical formula:
+ or- u iv u + iv
x x±u x±iv (x±u)±iv
iy ±u + iy i(y±v) ±u + i(y±v)
x + iy (x±u) + iy x + i(y±v) (x±u) + i(y±v)
G.6 Complex arithmetic <complex.h>
1 The macros
imaginary
and
_Imaginary_I
are defined, respectively, as _Imaginary and a constant expression of type float _Imaginary with
the value of the imaginary unit. The macro
I
is defined to be _Imaginary_I (not _Complex_I as stated in 7.3). Notwithstanding the provisions of
7.1.3, a program may undefine and then perhaps redefine the macro imaginary.
2 This subclause contains specifications for the <complex.h> functions that are particularly suited to
IEC 60559 implementations. For families of functions, the specifications apply to all of the functions
even though only the principal function is shown. Unless otherwise specified, where the symbol "±"
occurs in both an argument and the result, the result has the same sign as the argument.
3 The functions are continuous onto both√sides of their branch cuts, taking into account the sign of
zero. For example, csqrt(−2±i0) = ±i 2.
4 Since complex and imaginary values are composed of real values, each function may be regarded as
computing real values from real values. Except as noted, the functions treat real infinities, NaNs,
signed zeros, subnormals, and the floating-point exception flags in a manner consistent with the
specifications for real functions in F.10.461)
5 In subsequent subclauses in G.6 "NaN" refers to a quiet NaN. The behavior of signaling NaNs
in Annex G is implementation-defined.
6 The functions cimag, conj, cproj, and creal are fully specified for all implementations, including
IEC 60559 ones, in 7.3.9. These functions raise no floating-point exceptions.
7 Each of the functions cabs and carg is specified by a formula in terms of a real function (whose
special cases are covered in Annex F):
cabs(x + iy ) = hypot(x, y )
carg(x + iy ) = atan2(y , x)
8 Each of the functions casin, catan, ccos, csin, and ctan is specified implicitly by a formula in
terms of other complex functions (whose special cases are specified below):
casin(z ) = −i casinh(iz )
catan(z ) = −i catanh(iz )
ccos(z ) = ccosh(iz )
csin(z ) = −i csinh(iz )
ctan(z ) = −i ctanh(iz )
9 For the other functions, the following subclauses specify behavior for special cases, including
treatment of the "invalid" and "divide-by-zero" floating-point exceptions. For families of functions,
the specifications apply to all of the functions even though only the principal function is shown. For
a function f satisfying f (conj(z)) = conj(f (z)), the specifications for the upper half-plane imply the
specifications for the lower half-plane; if the function f is also either even, f (−z) = f (z), or odd,
f (−z) = −f (z), then the specifications for the first quadrant imply the specifications for the other
three quadrants.
10 In the following subclauses, cis(y) is defined as cos(y) + i sin(y).
G.6.1 Trigonometric functions
G.6.1.1 The cacos functions
1 — cacos(conj(z)) = conj(cacos(z)).
— cacos(±0 + i0) returns π2 − i0.
— cacos(±0 + iNaN) returns π2 + iNaN.
461) As noted in G.3, a complex value with at least one infinite part is regarded as an infinity even if its other part is a quiet
NaN.
— cacos(x + i∞) returns π2 − i∞, for finite x.
— cacos(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for nonzero finite x.
— cacos(−∞ + iy) returns pi − i∞, for positive-signed finite y.
— cacos(+∞ + iy) returns +0 − i∞, for positive-signed finite y.
— cacos(−∞ + i∞) returns 3 π4 − i∞.
— cacos(+∞ + i∞) returns π4 − i∞.
— cacos(±∞ + iNaN) returns NaN±i∞ (where the sign of the imaginary part of the result is
unspecified).
— cacos(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite y.
— cacos(NaN + i∞) returns NaN − i∞.
— cacos(NaN + iNaN) returns NaN + iNaN.
G.6.2 Hyperbolic functions
G.6.2.1 The cacosh functions
1
— cacosh(conj(z)) = conj(cacosh(z)).
— cacosh(±0 + i0) returns +0 + iπ
2 .
— cacosh(x + i∞) returns +∞ + iπ
2 , for finite x.
— cacosh(0 + iNaN) returns NaN± iπ
2 (where the sign of the imaginary part of the result is
unspecified).
— cacosh(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite nonzero x.
— cacosh(−∞ + iy) returns +∞ + iπ, for positive-signed finite y.
— cacosh(+∞ + iy) returns +∞ + i0, for positive-signed finite y.
— cacosh(−∞ + i∞) returns +∞ + i 3π
4 .
— cacosh(+∞ + i∞) returns +∞ + iπ
4 .
— cacosh(±∞ + iNaN) returns +∞ + iNaN.
— cacosh(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite y.
— cacosh(NaN + i∞) returns +∞ + iNaN.
— cacosh(NaN + iNaN) returns NaN + iNaN.
G.6.2.2 The casinh functions
1 — casinh(conj(z)) = conj(casinh(z)). and casinh is odd.
— casinh(+0 + i0) returns 0 + i0.
— casinh(x + i∞) returns +∞ + iπ
2 for positive-signed finite x.
— casinh(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite x.
— casinh(+∞ + iy) returns +∞ + i0 for positive-signed finite y.
— casinh(+∞ + i∞) returns +∞ + iπ
4 .
— casinh(+∞ + iNaN) returns +∞ + iNaN.
— casinh(NaN + i0) returns NaN + i0.
— casinh(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite nonzero y.
— casinh(NaN + i∞) returns ±∞ + iNaN (where the sign of the real part of the result is
unspecified).
— casinh(NaN + iNaN) returns NaN + iNaN.
G.6.2.3 The catanh functions
1 — catanh(conj(z)) = conj(catanh(z)). and catanh is odd.
— catanh(+0 + i0) returns +0 + i0.
— catanh(+0 + iNaN) returns +0 + iNaN.
— catanh(+1 + i0) returns +∞ + i0 and raises the "divide-by-zero" floating-point exception.
— catanh(x + i∞) returns +0 + iπ
2 , for finite positive-signed x.
— catanh(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for nonzero finite x.
— catanh(+∞ + iy) returns +0 + iπ
2 , for finite positive-signed y.
— catanh(+∞ + i∞) returns +0 + iπ
2 .
— catanh(+∞ + iNaN) returns +0 + iNaN.
— catanh(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite y.
— catanh(NaN + i∞) returns ±0 + iπ
2 (where the sign of the real part of the result is unspecified).
— catanh(NaN + iNaN) returns NaN + iNaN.
G.6.2.4 The ccosh functions
1 — ccosh(conj(z)) = conj(ccosh(z)) and ccosh is even.
— ccosh(+0 + i0) returns 1 + i0.
— ccosh(+0 + i∞) returns NaN±i0 (where the sign of the imaginary part of the result is unspec-
ified) and raises the "invalid" floating-point exception.
— ccosh(+0 + iNaN) returns NaN±i0 (where the sign of the imaginary part of the result is
unspecified).
— ccosh(x + i∞) returns NaN + iNaN and raises the "invalid" floating-point exception, for
finite nonzero x.
— ccosh(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite nonzero x.
— ccosh(+∞ + i0) returns +∞ + i0.
— ccosh(+∞ + iy) returns +∞ cis(y), for finite nonzero y.
— ccosh(+∞+i∞) returns ±∞+iNaN (where the sign of the real part of the result is unspecified)
and raises the "invalid" floating-point exception.
— ccosh(+∞ + iNaN) returns +∞ + iNaN.
— ccosh(NaN + i0) returns NaN±i0 (where the sign of the imaginary part of the result is
unspecified).
— ccosh(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for all nonzero numbers y.
— ccosh(NaN + iNaN) returns NaN + iNaN.
G.6.2.5 The csinh functions
1 — csinh(conj(z)) = conj(csinh(z)). and csinh is odd.
— csinh(+0 + i0) returns +0 + i0.
— csinh(+0 + i∞) returns ±0 + iNaN (where the sign of the real part of the result is unspecified)
and raises the "invalid" floating-point exception.
— csinh(+0 + iNaN) returns ±0 + iNaN (where the sign of the real part of the result is unspeci-
fied).
— csinh(x + i∞) returns NaN + iNaN and raises the "invalid" floating-point exception, for
positive finite x.
— csinh(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite nonzero x.
— csinh(+∞ + i0) returns +∞ + i0.
— csinh(+∞ + iy) returns +∞ cis(y), for positive finite y.
— csinh(+∞+i∞) returns ±∞+iNaN (where the sign of the real part of the result is unspecified)
and raises the "invalid" floating-point exception.
— csinh(+∞ + iNaN) returns ±∞ + iNaN (where the sign of the real part of the result is
unspecified).
— csinh(NaN + i0) returns NaN + i0.
— csinh(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for all nonzero numbers y.
— csinh(NaN + iNaN) returns NaN + iNaN.
G.6.2.6 The ctanh functions
1 — ctanh(conj(z)) = conj(ctanh(z)) and ctanh is odd.
— ctanh(+0 + i0) returns +0 + i0.
— ctanh(0 + i∞) returns 0 + iNaN and raises the "invalid" floating-point exception.
— ctanh(x + i∞) returns NaN + iNaN and raises the "invalid" floating-point exception, for
finite nonzero x.
— ctanh(0 + iNaN) returns 0 + iNaN.
— ctanh(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite nonzero x.
— ctanh(+∞ + iy) returns 1 + i0sin(2y), for positive-signed finite y.
— ctanh(+∞ + i∞) returns 1±i0 (where the sign of the imaginary part of the result is unspeci-
fied).
— ctanh(+∞ + iNaN) returns 1±i0 (where the sign of the imaginary part of the result is unspec-
ified).
— ctanh(NaN + i0) returns NaN + i0.
— ctanh(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for all nonzero numbers y.
— ctanh(NaN + iNaN) returns NaN + iNaN.
G.6.3 Exponential and logarithmic functions
G.6.3.1 The cexp functions
1 — cexp(conj(z)) = conj(cexp(z)).
— cexp(±0 + i0) returns 1 + i0.
— cexp(x + i∞) returns NaN + iNaN and raises the "invalid" floating-point exception, for finite
x.
— cexp(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point excep-
tion, for finite x.
— cexp(+∞ + i0) returns +∞ + i0.
— cexp(−∞ + iy) returns +0 cis(y), for finite y.
— cexp(+∞ + iy) returns +∞ cis(y), for finite nonzero y.
— cexp(−∞ + i∞) returns ±0±i0 (where the signs of the real and imaginary parts of the result
are unspecified).
— cexp(+∞ + i∞) returns ±∞ + iNaN and raises the "invalid" floating-point exception (where
the sign of the real part of the result is unspecified).
— cexp(−∞ + iNaN) returns ±0±i0 (where the signs of the real and imaginary parts of the result
are unspecified).
— cexp(+∞ + iNaN) returns ±∞ + iNaN (where the sign of the real part of the result is unspec-
ified).
— cexp(NaN + i0) returns NaN + i0.
— cexp(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point excep-
tion, for all nonzero numbers y.
— cexp(NaN + iNaN) returns NaN + iNaN.
G.6.3.2 The clog functions
1 — clog(conj(z)) = conj(clog(z)).
— clog(−0 + i0) returns −∞ + iπ and raises the "divide-by-zero" floating-point exception.
— clog(+0 + i0) returns −∞ + i0 and raises the "divide-by-zero" floating-point exception.
— clog(x + i∞) returns +∞ + iπ
2 , for finite x.
— clog(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point excep-
tion, for finite x.
— clog(−∞ + iy) returns +∞ + iπ, for finite positive-signed y.
— clog(+∞ + iy) returns +∞ + i0, for finite positive-signed y.
— clog(−∞ + i∞) returns +∞ + i 3π
4 .
— clog(+∞ + i∞) returns +∞ + iπ
4 .
— clog(±∞ + iNaN) returns +∞ + iNaN.
— clog(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point excep-
tion, for finite y.
— clog(NaN + i∞) returns +∞ + iNaN.
— clog(NaN + iNaN) returns NaN + iNaN.
G.6.4 Power and absolute-value functions
G.6.4.1 The cpow functions
1 The cpow functions raise floating-point exceptions if appropriate for the calculation of the parts of
the result, and may also raise spurious floating-point exceptions.462)
G.6.4.2 The csqrt functions
1 — csqrt(conj(z)) = conj(csqrt(z)).
— csqrt(±0 + i0) returns +0 + i0.
— csqrt(x + i∞) returns +∞ + i∞, for all x (including NaN).
— csqrt(x + iNaN) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite x.
— csqrt(−∞ + iy) returns +0 + i∞, for finite positive-signed y.
— csqrt(+∞ + iy) returns +∞ + i0, for finite positive-signed y.
— csqrt(−∞ + iNaN) returns NaN±i∞ (where the sign of the imaginary part of the result is
unspecified).
— csqrt(+∞ + iNaN) returns +∞ + iNaN.
— csqrt(NaN + iy) returns NaN + iNaN and optionally raises the "invalid" floating-point
exception, for finite y.
— csqrt(NaN + iNaN) returns NaN + iNaN.
462) This allows cpow(z, c) to be implemented as cexp(cclog(z)) without precluding implementations that treat special cases
more carefully.
G.7 Type-generic math <tgmath.h>
1 Type-generic macros that accept complex arguments also accept imaginary arguments. If an argu-
ment is imaginary, the macro expands to an expression whose type is real, imaginary, or complex, as
appropriate for the particular function: if the argument is imaginary, then the types of cos, cosh,
fabs, carg, cimag, and creal are real; the types of sin, tan, sinh, tanh, asin, atan, asinh, and
atanh are imaginary; and the types of the others are complex.
2 Given an imaginary argument, each of the type-generic macros cos, sin, tan, cosh, sinh, tanh,
asin, atan, asinh, atanh is specified by a formula in terms of real functions:
cos(iy ) = cosh(y )
sin(iy ) = i sinh(y )
tan(iy ) = i tanh(y )
cosh(iy ) = cos(y )
sinh(iy ) = i sin(y )
tanh(iy ) = i tan(y )
asin(iy ) = i asinh(y )
atan(iy ) = i atanh(y )
asinh(iy ) = i asin(y )
atanh(iy ) = i atan(y )
H. Annex H (normative) IEC 60559 interchange and extended types
H.1 Introduction
1 This annex specifies extension types for programming language C that have the arithmetic inter-
change and extended floating-point formats specified in ISO/IEC/IEEE 60559. This annex also
includes functions that support the non-arithmetic interchange formats in that standard. This annex
was adapted from ISO/IEC TS 18661-3:2015, Floating-point extensions for C —Interchange and
extended types.
2 An implementation that defines __STDC_IEC_60559_TYPES__ to 202311L shall conform to the
specifications in this annex. An implementation may define __STDC_IEC_60559_TYPES__ only
if it defines __STDC_IEC_60559_BFP__ , indicating support for IEC 60559 binary floating-point
arithmetic, or defines __STDC_IEC_60559_DFP__ , indicating support for IEC 60559 decimal floating-
point arithmetic (or defines both). Where a binding between the C language and IEC 60559 is
indicated, the IEC 60559-specified behavior is adopted by reference, unless stated otherwise.
H.2 Types
This clause specifies types that support IEC 60559 arithmetic interchange and extended formats. The
encoding conversion functions (H.11.3) and numeric conversion functions for encodings (H.12.3,
and H.12.4) support the non-arithmetic interchange formats specified in IEC 60559.
H.2.1 Interchange floating types
IEC 60559 specifies interchange formats, and their encodings, which can be used for the exchange of
floating-point data between implementations. These formats are identified by their radix (binary
or decimal) and their storage width N. The two tables below give the C floating-point model
parameters463) (5.2.4.2.2) for the IEC 60559 interchange formats, where the function round() rounds
to the nearest integer.
Binary interchange format parameters
Parameter binary16 binary32 binary64 binary128 binaryN (N ≥ 128)
N , storage width in bits 16 32 64 128 N, a multiple of 32
p, precision in bits 11 24 53 113 N − round(4 × log2 (N )) + 13
emax , maximum exponent e 16 128 1024 16384 2(N −p−1)
emin , minimum exponent e −13 −125 −1021 −16381 3 − emax
Decimal interchange format parameters
Parameter decimal32 decimal64 decimal128 decimalN (N ≥ 32)
N , storage width in bits 32 64 128 N, a multiple of 32
p, precision in bits 7 16 34 9 × (N ÷ 32) − 2
emax , maximum exponent e 97 385 6145 3 × 2((N ÷16)+3) + 1
emin , minimum exponent e −94 −382 −6142 3 − emax
1 EXAMPLE For the binary160 format, p = 144, emax = 32678 and emin = −32765. For the decimal160 format, p = 43,
emax = 24577 and emin = −24574.
2 Types designated:
_FloatN
where N is 16, 32, 64, or ≥ 128 and a multiple of 32; and, types designated
463) In IEC 60559, normal floating-point numbers are expressed with the first significant digit to the left of the radix point.
Hence the exponent in the C model (shown in the tables) is 1 more than the exponent of the same number in the IEC 60559
model.
_DecimalN
where N ≥ 32 and a multiple of 32, are collectively called the interchange floating types. Each
interchange floating type has the IEC 60559 interchange format corresponding to its width (N ) and
radix (2 for _FloatN, 10 for _DecimalN). Each interchange floating type is not compatible with any
other type.
3 An implementation that defines __STDC_IEC_60559_BFP__ and __STDC_IEC_60559_TYPES__ shall
provide _Float32 and _Float64 as interchange floating types with the same representation and
alignment requirements as float and double, respectively. If the implementation’s long double
type supports an IEC 60559 interchange format of width N > 64, then the implementation shall also
provide the type _FloatN as an interchange floating type with the same representation and alignment
requirements as long double. The implementation may provide other radix-2 interchange floating
types _FloatN; the set of such types supported is implementation-defined.
4 An implementation that defines __STDC_IEC_60559_DFP__ provides the decimal floating
types _Decimal32 , _Decimal64 , and _Decimal128 (6.2.5). If the implementation also defines
__STDC_IEC_60559_TYPES__ , it may provide other radix-10 interchange floating types _DecimalN;
the set of such types supported is implementation-defined.
H.2.2 Non-arithmetic interchange formats
1 An implementation supports IEC 60559 non-arithmetic interchange formats by providing the as-
sociated encoding-to-encoding conversion functions (H.11.3.2) in <math.h> and the string-from-
encoding functions (H.12.3) and string-to-encodng functions (H.12.4) in <stdlib.h>.
2 An implementation that defines __STDC_IEC_60559_BFP__ and __STDC_IEC_60559_TYPES__ sup-
ports some IEC 60559 radix-2 interchange formats as arithmetic formats by providing types _Float
N (as well as float and double) with those formats. The implementation may support other
IEC 60559 radix-2 interchange formats as non-arithmetic formats; the set of such formats supported
is implementation-defined.
3 An implementation that defines __STDC_IEC_60559_DFP__ and __STDC_IEC_60559_TYPES__ sup-
ports some IEC 60559 radix-10 interchange formats as arithmetic formats by providing types
_DecimalN with those formats. The implementations may support other IEC 60559 radix-10 inter-
change formats as non-arithmetic formats; the set of such formats supported is implementation-
defined.
H.2.3 Extended floating types
1 For each of its basic formats, IEC 60559 specifies an extended format whose maximum exponent and
precision exceed those of the basic format it is associated with. Extended formats are intended for
arithmetic with more precision and exponent range than is available in the basic formats used for
the input data. The extra precision and range often mitigate round-off error and eliminate overflow
and underflow in intermediate computations. The table below gives the minimum values of these
parameters, as defined for the C floating-point model (5.2.4.2.2). For all IEC 60559 extended (and
interchange) formats, emin = 3 − emax .
Extended format parameters for floating-point numbers
Extended formats associated with:
Parameter binary32 binary64 binary128 decimal64 decimal128
p digits ≥ 32 64 128 22 40
emax ≥ 1024 16384 65536 6145 24577
2 Types designated _Float32x , _Float64x , _Float128x , _Decimal64x , and _Decimal128x support
the corresponding IEC 60559 extended formats and are collectively called the extended floating
types. The set of values of _Float32x is a subset of the set of values of _Float64x ; the set
of values of _Float64x is a subset of the set of values of _Float128x . The set of values of
_Decimal64x is a subset of the set of values of _Decimal128x . Each extended floating type is
not compatible with any other type. An implementation that defines __STDC_IEC_60559_BFP__
and __STDC_IEC_60559_TYPES__ shall provide _Float32x , and may provide one or both of the
types _Float64x and _Float128x . An implementation that defines __STDC_IEC_60559_DFP__ and
__STDC_IEC_60559_TYPES__ shall provide _Decimal64x , and may provide _Decimal128x . Which
(if any) of the optional extended floating types are provided is implementation-defined.
3 NOTE IEC 60559 does not specify an extended format associated with the decimal32 format, nor does this annex specify an
extended type associated with the _Decimal32 type.
4 NOTE The _Float32x type may have the same format as double. The _Decimal64x type may have the same format as
_Decimal128 .
H.2.4 Classification of real floating types
1 6.2.5 defines standard floating types as a collective name for the types float, double and
long double and it defines decimal floating types as a collective name for the types _Decimal32 ,
_Decimal64 , and _Decimal128 .
2 H.2.1 defines interchange floating types and H.2.3 defines extended floating types.
3 The types _FloatN and _FloatNx are collectively called binary floating types.
4 This subclause broadens decimal floating types to include the types _DecimalN and _DecimalNx,
introduced in this annex, as well as _Decimal32 , _Decimal64 , and _Decimal128 .
5 This sublcause broadens real floating types to include all interchange floating types and extended
floating types, as well as standard floating types.
6 Thus, in this annex, real floating types are classified as follows:
— standard floating types, composed of float, double, long double;
— decimal floating types, composed of _DecimalN, _DecimalNx;
— binary floating types, composed of _FloatN, _FloatNx;
— interchange floating types, composed of _FloatN, _DecimalN; and,
— extended floating types, composed of _FloatNx, _DecimalNx.
7 NOTE Standard floating types (which have an implementation-defined radix) are not included in either binary floating
types (which always have radix 2) or decimal floating types (which always have radix 10).
H.2.5 Complex types
1 This subclause broadens the C complex types (6.2.5) to also include similar types whose correspond-
ing real parts have binary floating types. For the types _FloatN and _FloatNx, there are complex
types designated respectively as _FloatN _Complex and _FloatNx _Complex . (Complex types are a
conditional feature that implementations need not support; see 6.10.9.3.)
H.2.6 Imaginary types
1 This subclause broadens the C imaginary types (G.2) to also include similar types whose correspond-
ing real parts have binary floating types. For the types _FloatN and _FloatNx, there are imaginary
types designated respectively as _FloatN _Imaginary and _FloatNx _Imaginary . The imaginary
types (along with the real floating and complex types) are floating types. (Annex G, including
imaginary types, is a conditional feature that implementations need not support; see 6.10.9.3.)
H.3 Characteristics in <float.h>
1 This subclause enhances the FLT_EVAL_METHOD and DEC_EVAL_METHOD macros to apply to the types
introduced in this annex.
2 If FLT_RADIX is 2, the value of FLT_EVAL_METHOD (5.2.4.2.2) characterizes the use of evaluation
formats for standard floating types and for binary floating types:
-1 indeterminable;
0 evaluate all operations and constants, whose semantic type comprises a set of values
that is a strict subset of the values of float, to the range and precision of float; evaluate
all other operations and constants to the range and precision of the semantic type;
1 evaluate operations and constants, whose semantic type comprises a set of values that
is a strict subset of the values of double, to the range and precision of double; evaluate
all other operations and constants to the range and precision of the semantic type;
2 evaluate operations and constants, whose semantic type comprises a set of values that is
a strict subset of the values of long double, to the range and precision of long double;
evaluate all other operations and constants to the range and precision of the semantic
type;
N where _FloatN is a supported interchange floating type, evaluate operations and con-
stants, whose semantic type comprises a set of values that is a strict subset of the values
of _FloatN, to the range and precision of _FloatN; evaluate all other operations and
constants to the range and precision of the semantic type;
N +1 where _FloatNx is a supported extended floating type, evaluate operations and con-
stants, whose semantic type comprises a set of values that is a strict subset of the values
of _FloatNx, to the range and precision of _FloatNx; evaluate all other operations and
constants to the range and precision of the semantic type.
If FLT_RADIX is not 2, the use of evaluation formats for operations and constants of binary floating
types is implementation-defined.
3 The implementation-defined value of DEC_EVAL_METHOD (5.2.4.2.3) characterizes the use of evalua-
tion formats for decimal floating types:
-1 indeterminable;
0 evaluate all operations and constants just to the range and precision of the type;
1 evaluate operations and constants, whose semantic type comprises a set of values that
is a strict subset of the values of _Decimal64 , to the range and precision of _Decimal64 ;
evaluate all other operations and constants to the range and precision of the semantic
type;
2 evaluate operations and constants, whose semantic type comprises a set of values that is
a strict subset of the values of _Decimal128 , to the range and precision of _Decimal128 ;
evaluate all other operations and constants to the range and precision of the semantic
type;
N where _DecimalN is a supported interchange floating type evaluate operations and
constants, whose semantic type comprises a set of values that is a strict subset of
the values of _DecimalN, to the range and precision of _DecimalN; evaluate all other
operations and constants to the range and precision of the semantic type;
N +1 where _DecimalNx is a supported extended floating type evaluate operations and
constants, whose semantic type comprises a set of values that is a strict subset of the
values of _DecimalNx, to the range and precision of _DecimalNx; evaluate all other
operations and constants to the range and precision of the semantic type.
4 This subclause also specifies <float.h> macros, analogous to the macros for standard floating
types, that characterize binary floating types in terms of the model presented in 5.2.4.2.2. This
subclause generalizes the specification of characteristics in 5.2.4.2.3 to include the decimal floating
types introduced in this annex. The prefix FLTN_ indicates the type _FloatN or the non-arithmetic
binary interchange format of width N . The prefix FLTNX_ indicates the type _FloatNx. The prefix
DECN_ indicates the type _DecimalN or the non-arithmetic decimal interchange format of width
N . The prefix DECNX_ indicates the type _DecimalNx. The type parameters p, emax , and emin for
extended floating types are for the extended floating type itself, not for the basic format that it
extends.
5 If __STDC_WANT_IEC_60559_TYPES_EXT__ is defined (by the user) at the point in the code where
<float.h> is first included, the following applies (H.8). For each interchange or extended floating
type that the implementation provides, <float.h> shall define the associated macros in the fol-
lowing lists. Conversely, for each such type that the implementation does not provide, <float.h>
shall not define the associated macros in the following list, except, the implementation shall define
the macros FLTN_DECIMAL_DIG and FLTN_DIG if it supports the IEC 60559 non-arithmetic binary
interchange format of width N (H.2.2).
6 The signaling NaN macros
The macro
FLTN_SNAN
DECN_SNAN
FLTNX_SNAN
DECNX_SNAN
expand to constant expressions of types _FloatN, _DecimalN, _FloatNx, and _DecimalNx respec-
tively, representing a signaling NaN. If an optional unary + or- operator followed by a signaling
NaN macro is used for initializing an object of the same type that has static or thread storage
duration, the object is initialized with a signaling NaN value.
7 The integer values given in the following lists shall be replaced by integer constant expressions:
— radix of exponent representation, b (2 for binary, 10 for decimal)
For the standard floating types, this value is implementation-defined and is specified
by the macro FLT_RADIX. For the interchange and extended floating types there is no
corresponding macro; the radix is an inherent property of the types.
— The number of bits in the floating-point significand, p
FLTN_MANT_DIG
FLTNX_MANT_DIG
— The number of digits in the coefficient, p
DECN_MANT_DIG
DECNX_MANT_DIG
— number of decimal digits, n, such that any floating-point number with p bits can be rounded
to a floating-point number with n decimal digits and back again without change to the value,
⌈1 + p log10 (2)⌉
FLTN_DECIMAL_DIG
FLTNX_DECIMAL_DIG
— number of decimal digits, q, such that any floating-point number with q decimal digits can
be rounded to a floating-point number with p bits and back again without a change to the q
decimal digits, ⌊(p − 1) log10 (2)⌋
FLTN_DIG
FLTNX_DIG
— minimum negative integer such that the radix raised to one less than that power is a normalized
floating-point number, emin
FLTN_MIN_EXP
FLTNX_MIN_EXP
DECN_MIN_EXP
DECNX_MIN_EXP
— minimum negative integer such that 10 raised to that power is in the range of normalized
floating-point numbers, ⌈ log10 (2)emin −1 ⌉
FLTN_MIN_10_EXP
FLTNX_MIN_10_EXP
— maximum negative integer such that the radix raised to one less than that power is a repre-
sentable finite floating-point number, emax
FLTN_MAX_EXP
FLTNX_MAX_EXP
DECN_MAX_EXP
DECNX_MAX_EXP
— maximum integer such that 10 raised to that power is in the range of representable finite
floating-point numbers, ⌊ log10 ((1 − 2−p )2emax )⌋
FLTN_MAX_10_EXP
FLTNX_MAX_10_EXP
— maximum representable finite floating-pointer number, (1 − b−p )bemax
FLTN_MAX
FLTNX_MAX
DECN_MAX
DECNX_MAX
— the difference between 1 and the least value greater than 1 that is representable in the given
floating-point type, b1−p
FLTN_EPSILON
FLTNX_EPSILON
DECN_EPSILON
DECNX_EPSILON
— minimum normalized positive floating-point number, bemin −1
FLTN_MIN
FLTNX_MIN
DECN_MIN
DECNX_MIN
— minimum positive floating-point number, bemin −p
FLTN_TRUE_MIN
FLTNX_TRUE_MIN
DECN_TRUE_MIN
DECNX_TRUE_MIN
H.4 Conversions
1 This subclause enhances the usual arithmetic conversions (6.3.1.8) to handle interchange and ex-
tended floating types. It supports the IEC 60559 recommendation against allowing implicit conver-
sions of operands to obtain a common type where the conversion is between types where neither is
a subset of (or equivalent to) the other.
2 This subclause also broadens the operation binding in F.3 for the IEC 60559 convertFormat operation
to apply to IEC 60559 arithmetic and non-arithmetic formats.
H.4.1 Real floating and integer
1 When a finite value of interchange or extended floating type is converted to an integer type other
than bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of
the integral part cannot be represented by the integer type, the "invalid" floating-point exception
shall be raised and the result of the conversion is unspecified.
2 When a value of integer type is converted to an interchange or extended floating type, if the value
being converted can be represented exactly in the new type, it is unchanged. If the value being
converted cannot be represented exactly, the result shall be correctly rounded with exceptions raised
as specified in IEC 60559.
H.4.2 Usual arithmetic conversions
1 If either operand is of floating type, the common real type is determined as follows:
— If one operand has decimal floating type, the other operand shall not have standard floating
type, binary floating type, complex type, or imaginary type.
— If only one operand has a floating type, the other operand is converted to the corresponding
real type of the operand of floating type.
— If both operands have the same corresponding real type, no further conversion is needed.
— If both operands have floating types and neither of the sets of values of their corresponding
real types is a subset of (or equivalent to) the other, the behavior is undefined.
— Otherwise, if both operands are floating types and the sets of values of their corresponding
real types are not equivalent, the operand whose set of values of its corresponding real type
is a strict subset of the set of values of the corresponding real type of the other operand is
converted, without change of type domain, to a type with the corresponding real type of that
other operand.
— Otherwise, if both operands are floating types and the sets of values of their corresponding
real types are equivalent, then the following rules are applied:
– If the corresponding real type of either operand is an interchange floating type, the other
operand is converted, without change of type domain, to a type whose corresponding
real type is that same interchange floating type.
– Otherwise, if the corresponding real type of either operand is long double, the other
operand is converted, without change of type domain, to a type whose corresponding
real type is long double.
– Otherwise, if the corresponding real type of either operand is double, the other operand
is converted, without change of type domain, to a type whose corresponding real type is
double464) .
– Otherwise, if the corresponding real type of either operand is _Float128x or
_Decimal128x , the other operand is converted, without change of type domain, to a type
whose corresponding real type is _Float128x or _Decimal128x , respectively.
– Otherwise, if the corresponding real type of either operand is _Float64x or _Decimal64x ,
the other operand is converted, without change of type domain, to a type whose corre-
sponding real type is _Float64x or _Decimal64x , respectively.
464) All cases where float might have the same format as another type are covered above.
H.4.3 Arithmetic and non-arithmetic formats
1 The operation binding in F.3 for the IEC 60559 convertFormat operation applies to IEC 60559
arithmetic and non-arithmetic formats as follows:
— For conversions between arithmetic formats supported by floating types (same or different
radix) – casts and implicit conversions.
— For same-radix conversions between non-arithmetic interchange formats – encoding-to-
encoding conversion functions (H.11.3.2).
— For conversions between non-arithmetic interchange formats (same or different radix) – compo-
sitions of string-from-encoding functions (H.12.3) (converting exactly) and string-to-encoding
functions (H.12.4).
— For same-radix conversions from interchange formats supported by interchange floating types
to non-arithmetic interchange formats – compositions of encode functions (H.11.3.1.1, 7.12.16.1,
7.12.16.3) and encoding-to-encoding functions (H.11.3.2).
— For same radix conversions from non-arithmetic interchange formats to interchange formats
supported by interchange floating types – compositions of encoding-to-encoding conversion
functions (H.11.3.2) and decode functions (H.11.3.1.2, 7.12.16.2, 7.12.16.4). See the example in
H.11.3.2.1.
— For conversions from non-arithmetic interchange formats to arithmetic formats supported
by floating types (same or different radix) – compositions of string-from-encoding functions
(H.12.3) (converting exactly) and numeric conversion functions strtod, etc. (7.24.1.5, 7.24.1.6).
See the example in H.12.2.
— For conversions from arithmetic formats supported by floating types to non-arithmetic in-
terchange formats (same or different radix) – compositions of numeric conversion func-
tions strfromd, etc. (7.24.1.3, 7.24.1.4) (converting exactly) and string-to-encoding functions
(H.12.4).
H.5 Lexical Elements
H.5.1 Keywords
1 This subclause expands the list of keywords (6.4.1) to also include:
— _FloatN, where N is 16, 32, 64, or ≥ 128 and a multiple of 32
— _Float32x
— _Float64x
— _Float128x
— _DecimalN, where N is 96 or > 128 and a multiple of 32
— _Decimal64x
— _Decimal128x
H.5.2 Constants
1 This subclause specifies constants of interchange and extended floating types.
2 This subclause expands floating-suffix (6.4.4.2) to also include: fN, FN, fNx, FNx, dN, DN, dNx, or DNx.
3 A floating suffix dN, DN, dNx, or DNx shall not be used in a hexadecimal-floating-constant.
4 A floating suffix shall not designate a type that the implementation does not provide.
5 If a floating constant is suffixed by fN or FN, it has type _FloatN. If suffixed by fNx or FNx, it has
type _FloatNx. If suffixed by dN or DN, it has type _DecimalN. If suffixed by dNx or DNx, it has type
_DecimalNx .
6 The quantum exponent of a floating constant of decimal floating type is the same as for the result
value of the corresponding strtodN or strtodNx function (H.12.2) for the same numeric string.
7 NOTE For N = 32, 64, and 128, the suffixes dN and DN in this subclause for constants of type _DecimalN are equivalent
alternatives to the suffixes df, dd, dl, DF, DD, and DL in 6.4.4.2 for the same types.
H.6 Expressions
1 This subclause expands the specification of expressions to also cover interchange and extended
floating types.
2 Operators involving operands of interchange or extended floating type are evaluated according to
the semantics of IEC 60559, including production of decimal floating-point results with the preferred
quantum exponent as specified in IEC 60559 (see 5.2.4.2.3).
3 For multiplicative operators (6.5.5), additive operators (6.5.6), relational operators (6.5.8), equality
operators (6.5.9), and compound assignment operators (6.5.16.2), if either operand has decimal
floating type, the other operand shall not have standard floating type, binary floating type, complex
type, or imaginary type.
4 For conditional operators (6.5.15), if the second or third operand has decimal floating type, the
other of those operands shall not have standard floating type, binary floating type, complex type, or
imaginary type.
5 The equivalence of expressions noted in F.9.2 apply to expressions of binary floating types, as well
as standard floating types.
H.7 Declarations
1 This subclause expands the list of type specifiers (6.7.2) to also include:
— _FloatN, where N is 16, 32, 64, or ≥ 128 and a multiple of 32
— _Float32x
— _Float64x
— _Float128x
— _DecimalN, where N is 96 or > 128 and a multiple of 32
— _Decimal64x
— _Decimal128x
2 The type specifiers _FloatN (where N is 16, 32, 64, or ≥ 128 and a multiple of 32), _Float32x ,
_Float64x , _Float128x , _DecimalN (where N is 96 or > 128 and a multiple of 32), _Decimal64x ,
and _Decimal128x shall not be used if the implementation does not support the corresponding
types (see 6.10.9.3 and H.2).
3 This subclause also expands the list under Constraints in 6.7.2 to also include:
— _FloatN, where N is 16, 32, 64, or ≥ 128 and a multiple of 32
— _Float32x
— _Float64x
— _Float128x
— _DecimalN, where N is 96 or > 128 and a multiple of 32
— _Decimal64x
— _Decimal128x
— _FloatN _Complex , where N is 16, 32, 64, or ≥ 128 and a multiple of 32
— _Float32x _Complex
— _Float64x _Complex
— _Float128x _Complex
H.8 Identifiers in standard headers
1 The identifiers added to library headers by this annex are defined or declared by their respective
headers only if the macro __STDC_WANT_IEC_60559_TYPES_EXT__ is defined (by the user) at the
point in the code where the appropriate header is first included.
H.9 Complex arithmetic <complex.h>
1 This subclause specifies complex functions for corresponding real types that are binary floating
types.
2 Each function synopsis in 7.3 specifies a family of functions including a principal function with
one or more double complex parameters and a double complex or double return value. This
subclause expands the synopsis to also include other functions, with the same name as the principal
function but with fN and fNx suffixes, which are corresponding functions whose parameters and
return values have corresponding real types _FloatN and _FloatNx.
3 The following function prototypes are added to the synopses of the respective subclauses in 7.3.
For each binary floating type that the implementation provides, <complex.h> shall declare the
associated functions (see H.8). Conversely, for each such type that the implementation does not
provide, <complex.h> shall not declare the associated functions.
(7.3.5) Trigonometric functions
_FloatN complex cacosfN(_FloatN complex z);
_FloatNx complex cacosfNx(_FloatNx complex z);
_FloatN complex casinfN(_FloatN complex z);
_FloatNx complex casinfNx(_FloatNx complex z);
_FloatN complex catanfN(_FloatN complex z);
_FloatNx complex catanfNx(_FloatNx complex z);
_FloatN complex ccosfN(_FloatN complex z);
_FloatNx complex ccosfNx(_FloatNx complex z);
_FloatN complex csinfN(_FloatN complex z);
_FloatNx complex csinfNx(_FloatNx complex z);
_FloatN complex ctanfN(_FloatN complex z);
_FloatNx complex ctanfNx(_FloatNx complex z);
(7.3.6) Hyperbolic functions
_FloatN complex cacoshfN(_FloatN complex z);
_FloatNx complex cacoshfNx(_FloatNx complex z);
_FloatN complex casinhfN(_FloatN complex z);
_FloatNx complex casinhfNx(_FloatNx complex z);
_FloatN complex catanhfN(_FloatN complex z);
_FloatNx complex catanhfNx(_FloatNx complex z);
_FloatN complex ccoshfN(_FloatN complex z);
_FloatNx complex ccoshfNx(_FloatNx complex z);
_FloatN complex csinhfN(_FloatN complex z);
_FloatNx complex csinhfNx(_FloatNx complex z);
_FloatN complex ctanhfN(_FloatN complex z);
_FloatNx complex ctanhfNx(_FloatNx complex z);
(7.3.7) Exponential and logarithmic functions
_FloatN complex cexpfN(_FloatN complex z);
_FloatNx complex cexpfNx(_FloatNx complex z);
_FloatN complex clogfN(_FloatN complex z);
_FloatNx complex clogfNx(_FloatNx complex z);
(7.3.8) Power and absolute value functions
_FloatN cabsfN(_FloatN complex z);
_FloatNx cabsfNx(_FloatNx complex z);
_FloatN complex cpowfN(_FloatN complex x, _FloatN complex y);
_FloatNx complex cpowfNx(_FloatNx complex x, _FloatNx complex y);
_FloatN complex csqrtfN(_FloatN complex z);
_FloatNx complex csqrtfNx(_FloatNx complex z);
(7.3.9) Manipulation functions
_FloatN cargfN(_FloatN complex z);
_FloatNx cargfNx(_FloatNx complex z);
_FloatN cimagfN(_FloatN complex z);
_FloatNx cimagfNx(_FloatNx complex z);
_FloatN complex CMPLXFN(_FloatN x, _FloatN y);
_FloatNx complex CMPLXFNX(_FloatNx x, _FloatNx y);
_FloatN complex conjfN(_FloatN complex z);
_FloatNx complex conjfNx(_FloatNx complex z);
_FloatN complex cprojfN(_FloatN complex z);
_FloatNx complex cprojfNx(_FloatNx complex z);
_FloatN crealfN(_FloatN complex z);
_FloatNx crealfNx(_FloatNx complex z);
4 For the functions listed in "future library directions" for <complex.h> (7.33.1), the possible suffixes
are expanded to also include fN and fNx.
H.10 Floating-point environment
1 This subclause broadens the effects of the floating-point environment (7.6) to apply to types and
formats specified in this annex.
2 The same floating-point status flags are used by floating-point operations for all floating types,
including those types introduced in this annex, and by conversions for IEC 60559 non-arithmetic
interchange formats.
3 Both the dynamic rounding direction mode accessed by fegetround and fesetround and the
FENV_ROUND rounding control pragma apply to operations for binary floating types, as well as
for standard floating types, and also to conversions for radix-2 non-arithmetic interchange for-
mats. Likewise, both the dynamic rounding direction mode accessed by fe_dec_getround and
fe_dec_setround and the FENV_DEC_ROUND rounding control pragmas apply to operations for all
the decimal floating types, including those decimal floating types introduced in this annex, and to
conversions for radix-10 non-arithmetic interchange formats.
4 In 7.6.2, the table of functions affected by constant rounding modes for standard floating types
applies also for binary floating types. Each <math.h> function family listed in the table indicates
the family of functions of all standard and binary floating types (for example, the acos family
includes acosf, acosl, acosfN, and acosfNx as well as acos). The fMencfN, strfromencfN, and
strtoencfN functions are also affected by these constant rounding modes.
5 In 7.6.3, in the table of functions affected by constant rounding modes for decimal floating types, each
<math.h> function family indicates the family of functions of all decimal floating types (for example,
the acos family includes acosdN and acosdNx). The dMencbindN, dMencdecdN, strfromencbindN,
strfromencdecdN, strtoencbindN, and strtoencdecdN functions are also affected by these con-
stant rounding modes.
H.11 Mathematics <math.h>
1 This subclause specifies types, functions, and macros for interchange and extended floating types,
generally corresponding to those specified in 7.12 and F.10.
2 All classification macros (7.12.3) and comparison macros (7.12.17) naturally extend to handle inter-
change and extended floating types. For comparison macros, if neither of the sets of values of the
argument formats is a subset of (or equivalent to) the other, the behavior is undefined.
3 This subclause also specifies encoding conversion functions that are part of support for the non-
arithmetic interchange formats in IEC 60559 (see H.2.2).
4 Most function synopses in 7.12 specify a family of functions including a principal function with
one or more double parameters, a double return value, or both. The synopses are expanded to
also include functions with the same name as the principal function but with fN, fNx, dN, and dNx
suffixes, which are corresponding functions whose parameters, return values, or both are of types
_FloatN, _FloatNx , _DecimalN, and _DecimalNx, respectively.
5 For each interchange or extended floating type that the implementation provides, <math.h> shall
define the associated types and macros and declare the associated functions (see H.8). Conversely, for
each such type that the implementation does not provide, <math.h> shall not define the associated
types and macros or declare the associated functions unless explicitly specified otherwise.
6 With the types
float_t
double_t
in 7.12 are included the type
long_double_t
and for each supported type _FloatN, the type
_FloatN_t
and for each supported type _DecimalN , the type
_DecimalN_t
These are floating types, such that:
— each of the types has at least the range and precision of the corresponding real floating type;
— long_double_t has at least the range and precision of double_t;
— _FloatN_t
has at least the range and precision of _FloatM_t if N > M ;
— _DecimalN_t
has at least the range and precision of _DecimalM_t if N > M .
If FLT_RADIX is 2 and FLT_EVAL_METHOD (H.3) is nonnegative, then each of the types corresponding
to a standard or binary floating type is the type whose range and precision are specified by
FLT_EVAL_METHOD to be used for evaluating operations and constants of that standard or binary
floating type. If DEC_EVAL_METHOD (H.3) is nonnegative, then each of the types corresponding to a
decimal floating type is the type whose range and precision are specified by DEC_EVAL_METHOD to
be used for evaluating operations and constants of that decimal floating type.
7 EXAMPLE If the supported standard and binary floating types are
Type IEC 60559 format
_Float16 binary16
float, _Float32 binary32
double, _Float64 , _Float32x binary64
long double, _Float64x 80-bit binary64-extended
_Float128 binary128
then the following tables gives the types with _t suffixes for various values for a FLT_EVAL_METHOD of a given value m:
_t type/m 0 1 2 32
_Float16_t float double long double _Float32
float_t float double long double float
_Float32_t _Float32 double long double _Float32
double_t double double long double double
_Float64_t _Float64 _Float64 long double _Float64
long_double_t long double long double long double long double
_Float128_t _Float128 _Float128 _Float128 _Float128
_t type/m 64 128 33 65
_Float16_t _Float64 _Float128 _Float32x _Float64x
float_t _Float64 _Float128 _Float32x _Float64x
_Float32_t _Float64 _Float128 _Float32x _Float64x
double_t double _Float128 double _Float64x
_Float64_t _Float64 _Float128 _Float64 _Float64x
long_double_t long double _Float128 long double long double
_Float128_t _Float128 _Float128 _Float128 _Float128
H.11.1 Macros
1 This subclause adds macros in 7.12 as follows.
2 The macros
HUGE_VAL_FN
HUGE_VAL_DN
HUGE_VAL_FNX
HUGE_VAL_DNX
expand to constant expressions of types _FloatN, _DecimalN, _FloatNx, and _DecimalNx, respec-
tively, representing positive infinity.
3 The macros
FP_FAST_FMAFN
FP_FAST_FMADN
FP_FAST_FMAFNX
FP_FAST_FMADNX
are, respectively, _FloatN, _DecimalN, _FloatNx, and _DecimalNx analogues of FP_FAST_FMA.
4 The macros in the following lists are interchange and extended floating type analogues of
FP_FAST_FADD, FP_FAST_FADDL, FP_FAST_DADDL, etc.
5 For M < N , the macros
FP_FAST_FMADDFN
FP_FAST_FMSUBFN
FP_FAST_FMMULFN
FP_FAST_FMDIVFN
FP_FAST_FMFMAFN
FP_FAST_FMSQRTFN
FP_FAST_DMADDDN
FP_FAST_DMSUBDN
FP_FAST_DMMULDN
FP_FAST_DMDIVDN
FP_FAST_DMFMADN
FP_FAST_DMSQRTDN
characterize the corresponding functions whose arguments are of an interchange floating type of
width N and whose return type is an interchange floating type of width M .
6 For M ≤ N , the macros
FP_FAST_FMADDFNX
FP_FAST_FMSUBFNX
FP_FAST_FMMULFNX
FP_FAST_FMDIVFNX
FP_FAST_FMFMAFNX
FP_FAST_FMSQRTFNX
FP_FAST_DMADDDNX
FP_FAST_DMSUBDNX
FP_FAST_DMMULDNX
FP_FAST_DMDIVDNX
FP_FAST_DMFMADNX
FP_FAST_DMSQRTDNX
characterize the corresponding functions whose arguments are of an extended floating type that
extends a format of width N and whose return type is an interchange floating type of width M .
7 For M < N , the macros
FP_FAST_FMXADDFN
FP_FAST_FMXSUBFN
FP_FAST_FMXMULFN
FP_FAST_FMXDIVFN
FP_FAST_FMXFMAFN
FP_FAST_FMXSQRTFN
FP_FAST_DMXADDDN
FP_FAST_DMXSUBDN
FP_FAST_DMXMULDN
FP_FAST_DMXDIVDN
FP_FAST_DMXFMADN
FP_FAST_DMXSQRTDN
characterize the corresponding functions whose arguments are of an interchange floating type of
width N and whose return type is an extended floating type that extends a format of width M .
8 For M < N , the macros
FP_FAST_FMXADDFNX
FP_FAST_FMXSUBFNX
FP_FAST_FMXMULFNX
FP_FAST_FMXDIVFNX
FP_FAST_FMXFMAFNX
FP_FAST_FMXSQRTFNX
FP_FAST_DMXADDDNX
FP_FAST_DMXSUBDNX
FP_FAST_DMXMULDNX
FP_FAST_DMXDIVDNX
FP_FAST_DMXFMADNX
FP_FAST_DMXSQRTDNX
characterize the corresponding functions whose arguments are of an extended floating type that
extends a format of width N and whose return type is an extended floating type that extends a
format of width M .
H.11.2 Functions
1 This sublause adds the following functions to the synopses of the respective subclauses in 7.12.
(7.12.4) Trigonometric functions
_FloatN acosfN(_FloatN x);
_FloatNx acosfNx(_FloatNx x);
_DecimalN acosdN(_DecimalN x);
_DecimalNx acosdNx(_DecimalNx x);
_FloatN asinfN(_FloatN x);
_FloatNx asinfNx(_FloatNx x);
_DecimalN asindN(_DecimalN x);
_DecimalNx asindNx(_DecimalNx x);
_FloatN atanfN(_FloatN x);
_FloatNx atanfNx(_FloatNx x);
_DecimalN atandN(_DecimalN x);
_DecimalNx atandNx(_DecimalNx x);
_FloatN atan2fN(_FloatN y, _FloatN x);
_FloatNx atan2fNx(_FloatNx y, _FloatNx x);
_DecimalN atan2dN(_DecimalN y, _DecimalN x);
_DecimalNx atan2dNx(_DecimalNx y, _DecimalNx x);
_FloatN cosfN(_FloatN x);
_FloatNx cosfNx(_FloatNx x);
_DecimalN cosdN(_DecimalN x);
_DecimalNx cosdNx(_DecimalNx x);
_FloatN sinfN(_FloatN x);
_FloatNx sinfNx(_FloatNx x);
_DecimalN sindN(_DecimalN x);
_DecimalNx sindNx(_DecimalNx x);
_FloatN tanfN(_FloatN x);
_FloatNx tanfNx(_FloatNx x);
_DecimalN tandN(_DecimalN x);
_DecimalNx tandNx(_DecimalNx x);
_FloatN acospifN(_FloatN x);
_FloatNx acospifNx(_FloatNx x);
_DecimalN acospidN(_DecimalN x);
_DecimalNx acospidNx(_DecimalNx x);
_FloatN asinpifN(_FloatN x);
_FloatNx asinpifNx(_FloatNx x);
_DecimalN asinpidN(_DecimalN x);
_DecimalNx asinpidNx(_DecimalNx x);
_FloatN atanpifN(_FloatN x);
_FloatNx atanpifNx(_FloatNx x);
_DecimalN atanpidN(_DecimalN x);
_DecimalNx atanpidNx(_DecimalNx x);
_FloatN atan2pifN(_FloatN y, _FloatN x);
_FloatNx atan2pifNx(_FloatNx y, _FloatNx x);
_DecimalN atan2pidN(_DecimalN y, _DecimalN x);
_DecimalNx atan2pidNx(_DecimalNx y, _DecimalNx x);
_FloatN cospifN(_FloatN x);
_FloatNx cospifNx(_FloatNx x);
_DecimalN cospidN(_DecimalN x);
_DecimalNx cospidNx(_DecimalNx x);
_FloatN sinpifN(_FloatN x);
_FloatNx sinpifNx(_FloatNx x);
_DecimalN sinpidN(_DecimalN x);
_DecimalNx sinpidNx(_DecimalNx x);
_FloatN tanpifN(_FloatN x);
_FloatNx tanpifNx(_FloatNx x);
_DecimalN tanpidN(_DecimalN x);
_DecimalNx tanpidNx(_DecimalNx x);
(7.12.5) Hyperbolic functions
_FloatN acoshfN(_FloatN x);
_FloatNx acoshfNx(_FloatNx x);
_DecimalN acoshdN(_DecimalN x);
_DecimalNx acoshdNx(_DecimalNx x);
_FloatN asinhfN(_FloatN x);
_FloatNx asinhfNx(_FloatNx x);
_DecimalN asinhdN(_DecimalN x);
_DecimalNx asinhdNx(_DecimalNx x);
_FloatN atanhfN(_FloatN x);
_FloatNx atanhfNx(_FloatNx x);
_DecimalN atanhdN(_DecimalN x);
_DecimalNx atanhdNx(_DecimalNx x);
_FloatN coshfN(_FloatN x);
_FloatNx coshfNx(_FloatNx x);
_DecimalN coshdN(_DecimalN x);
_DecimalNx coshdNx(_DecimalNx x);
_FloatN sinhfN(_FloatN x);
_FloatNx sinhfNx(_FloatNx x);
_DecimalN sinhdN(_DecimalN x);
_DecimalNx sinhdNx(_DecimalNx x);
_FloatN tanhfN(_FloatN x);
_FloatNx tanhfNx(_FloatNx x);
_DecimalN tanhdN(_DecimalN x);
_DecimalNx tanhdNx(_DecimalNx x);
(7.12.6) Exponential and logarithmic functions
_FloatN expfN(_FloatN x);
_FloatNx expfNx(_FloatNx x);
_DecimalN expdN(_DecimalN x);
_DecimalNx expdNx(_DecimalNx x);
_FloatN exp10fN(_FloatN x);
_FloatNx exp10fNx(_FloatNx x);
_DecimalN exp10dN(_DecimalN x);
_DecimalNx exp10dNx(_DecimalNx x);
_FloatN exp10m1fN(_FloatN x);
_FloatNx exp10m1fNx(_FloatNx x);
_DecimalN exp10m1dN(_DecimalN x);
_DecimalNx exp10m1dNx(_DecimalNx x);
_FloatN exp2fN(_FloatN x);
_FloatNx exp2fNx(_FloatNx x);
_DecimalN exp2dN(_DecimalN x);
_DecimalNx exp2dNx(_DecimalNx x);
_FloatN exp2m1fN(_FloatN x);
_FloatNx exp2m1fNx(_FloatNx x);
_DecimalN exp2m1dN(_DecimalN x);
_DecimalNx exp2m1dNx(_DecimalNx x);
_FloatN expm1fN(_FloatN x);
_FloatNx expm1fNx(_FloatNx x);
_DecimalN expm1dN(_DecimalN x);
_DecimalNx expm1dNx(_DecimalNx x);
_FloatN frexpfN(_FloatN value, int *exp);
_FloatNx frexpfNx(_FloatNx value, int *exp);
_DecimalN frexpdN(_DecimalN value, int *exp);
_DecimalNx frexpdNx(_DecimalNx value, int *exp);
int ilogbfN(_FloatN x);
int ilogbfNx(_FloatNx x);
int ilogbdN(_DecimalNx x);
int ilogbdNx(_DecimalNx x);
_FloatN ldexpfN(_FloatN value, int exp);
_FloatNx ldexpfNx(_FloatNx value, int exp);
_DecimalN ldexpdN(_DecimalN value, int exp);
_DecimalNx ldexpdNx(_DecimalNx value, int exp);
long int llogbfN(_FloatN x);
long int llogbfNx(_FloatNx x);
long int llogbdN(_DecimalN x);
long int llogbdNx(_DecimalNx x);
_FloatN logfN(_FloatN x);
_FloatNx logfNx(_FloatNx x);
_DecimalN logdN(_DecimalN x);
_DecimalNx logdNx(_DecimalNx x);
_FloatN log10fN(_FloatN x);
_FloatNx log10fNx(_FloatNx x);
_DecimalN log10dN(_DecimalN x);
_DecimalNx log10dNx(_DecimalNx x);
_FloatN log10p1fN(_FloatN x);
_FloatNx log10p1fNx(_FloatNx x);
_DecimalN log10p1dN(_DecimalN x);
_DecimalNx log10p1dNx(_DecimalNx x);
_FloatN log1pfN(_FloatN x);
_FloatNx log1pfNx(_FloatNx x);
_FloatN logp1fN(_FloatN x);
_FloatNx logp1fNx(_FloatNx x);
_DecimalN log1pdN(_DecimalN x);
_DecimalNx log1pdNx(_DecimalNx x);
_DecimalN logp1dN(_DecimalN x);
_DecimalNx logp1dNx(_DecimalNx x);
_FloatN log2fN(_FloatN x);
_FloatNx log2fNx(_FloatNx x);
_DecimalN log2dN(_DecimalN x);
_DecimalNx log2dNx(_DecimalNx x);
_FloatN log2p1fN(_FloatN x);
_FloatNx log2p1fNx(_FloatNx x);
_DecimalN log2p1dN(_DecimalN x);
_DecimalNx log2p1dNx(_DecimalNx x);
_FloatN logbfN(_FloatN x);
_FloatNx logbfNx(_FloatNx x);
_DecimalN logbdN(_DecimalN x);
_DecimalNx logbdNx(_DecimalNx x);
_FloatN modffN(_FloatN x, _FloatN *iptr);
_FloatNx modffNx(_FloatNx x, _FloatNx *iptr);
_DecimalN modfdN(_DecimalN x, _DecimalN *iptr);
_DecimalNx modfdNx(_DecimalNx x, _DecimalNx *iptr);
_FloatN scalbnfN(_FloatN value, int exp);
_FloatNx scalbnfNx(_FloatNx value, int exp);
_DecimalN scalbndN(_DecimalN value, int exp);
_DecimalNx scalbndNx(_DecimalNx value, int exp);
_FloatN scalblnfN(_FloatN value, long int exp);
_FloatNx scalblnfNx(_FloatNx value, long int exp);
_DecimalN scalblndN(_DecimalN value, long int exp);
_DecimalNx scalblndNx(_DecimalNx value, long int exp);
(7.12.7) Power and absolute-value functions
_FloatN cbrtfN(_FloatN x);
_FloatNx cbrtfNx(_FloatNx x);
_DecimalN cbrtdN(_DecimalN x);
_DecimalNx cbrtdNx(_DecimalNx x);
_FloatN compoundnfN(_FloatN x, long long int n);
_FloatNx compoundnfNx(_FloatNx x, long long int n);
_DecimalN compoundndN(_DecimalN x, long long int n);
_DecimalNx compoundndNx(_DecimalNx x, long long int n);
_FloatN fabsfN(_FloatN x);
_FloatNx fabsfNx(_FloatNx x);
_DecimalN fabsdN(_DecimalN x);
_DecimalNx fabsdNx(_DecimalNx x);
_FloatN hypotfN(_FloatN x, _FloatN y);
_FloatNx hypotfNx(_FloatNx x, _FloatNx y);
_DecimalN hypotdN(_DecimalN x, _DecimalN y);
_DecimalNx hypotdNx(_DecimalNx x, _DecimalNx y);
_FloatN powfN(_FloatN x, _FloatN y);
_FloatNx powfNx(_FloatNx x, _FloatNx y);
_DecimalN powdN(_DecimalN x, _DecimalN y);
_DecimalNx powdNx(_DecimalNx x, _DecimalNx y);
_FloatN pownfN(_FloatN x, long long int n);
_FloatNx pownfNx(_FloatNx x, long long int n);
_DecimalN powndN(_DecimalN x, long long int n);
_DecimalNx powndNx(_DecimalNx x, long long int n);
_FloatN powrfN(_FloatN x, _FloatN y);
_FloatNx powrfNx(_FloatNx x, _FloatNx y);
_DecimalN powrdN(_DecimalN x, _DecimalN y);
_DecimalNx powrdNx(_DecimalNx x, _DecimalNx y);
_FloatN rootnfN(_FloatN x, long long int n);
_FloatNx rootnfNx(_FloatNx x, long long int n);
_DecimalN rootndN(_DecimalN x, long long int n);
_DecimalNx rootndNx(_DecimalNx x, long long int n);
_FloatN rsqrtfN(_FloatN x);
_FloatNx rsqrtfNx(_FloatNx x);
_DecimalN rsqrtdN(_DecimalN x);
_DecimalNx rsqrtdNx(_DecimalNx x);
_FloatN sqrtfN(_FloatN x);
_FloatNx sqrtfNx(_FloatNx x);
_DecimalN sqrtdN(_DecimalN x);
_DecimalNx sqrtdNx(_DecimalNx x);
(7.12.8) Error and gamma functions
_FloatN erffN(_FloatN x);
_FloatNx erffNx(_FloatNx x);
_DecimalN erfdN(_DecimalN x);
_DecimalNx erfdNx(_DecimalNx x);
_FloatN erfcfN(_FloatN x);
_FloatNx erfcfNx(_FloatNx x);
_DecimalN erfcdN(_DecimalN x);
_DecimalNx erfcdNx(_DecimalNx x);
_FloatN lgammafN(_FloatN x);
_FloatNx lgammafNx(_FloatNx x);
_DecimalN lgammadN(_DecimalN x);
_DecimalNx lgammadNx(_DecimalNx x);
_FloatN tgammafN(_FloatN x);
_FloatNx tgammafNx(_FloatNx x);
_DecimalN tgammadN(_DecimalN x);
_DecimalNx tgammadNx(_DecimalNx x);
(7.12.9) Nearest integer functions
_FloatN ceilfN(_FloatN x);
_FloatNx ceilfNx(_FloatNx x);
_DecimalN ceildN(_DecimalN x);
_DecimalNx ceildNx(_DecimalNx x);
_FloatN floorfN(_FloatN x);
_FloatNx floorfNx(_FloatNx x);
_DecimalN floordN(_DecimalN x);
_DecimalNx floordNx(_DecimalNx x);
_FloatN nearbyintfN(_FloatN x);
_FloatNx nearbyintfNx(_FloatNx x);
_DecimalN nearbyintdN(_DecimalN x);
_DecimalNx nearbyintdNx(_DecimalNx x);
_FloatN rintfN(_FloatN x);
_FloatNx rintfNx(_FloatNx x);
_DecimalN rintdN(_DecimalN x);
_DecimalNx rintdNx(_DecimalNx x);
long int lrintfN(_FloatN x);
long int lrintfNx(_FloatNx x);
long int lrintdN(_DecimalN x);
long int lrintdNx(_DecimalNx x);
long long int llrintfN(_FloatN x);
long long int llrintfNx(_FloatNx x);
long long int llrintdN(_DecimalN x);
long long int llrintdNx(_DecimalNx x);
_FloatN roundfN(_FloatN x);
_FloatNx roundfNx(_FloatNx x);
_DecimalN rounddN(_DecimalN x);
_DecimalNx rounddNx(_DecimalNx x);
long int lroundfN(_FloatN x);
long int lroundfNx(_FloatNx x);
long int lrounddN(_DecimalN x);
long int lrounddNx(_DecimalNx x);
long long int llroundfN(_FloatN x);
long long int llroundfNx(_FloatNx x);
long long int llrounddN(_DecimalN x);
long long int llrounddNx(_DecimalNx x);
_FloatN roundevenfN(_FloatN x);
_FloatNx roundevenfNx(_FloatNx x);
_DecimalN roundevendN(_DecimalN x);
_DecimalNx roundevendNx(_DecimalNx x);
_FloatN truncfN(_FloatN x);
_FloatNx truncfNx(_FloatNx x);
_DecimalN truncdN(_DecimalN x);
_DecimalNx truncdNx(_DecimalNx x);
_FloatN fromfpfN(_FloatN x, int rnd, unsigned int width);
_FloatNx fromfpfNx(_FloatNx x, int rnd, unsigned int width);
_DecimalN fromfpdN(_DecimalN x, int rnd, unsigned int width);
_DecimalNx fromfpdNx(_DecimalNx x, int rnd, unsigned int width);
_FloatN ufromfpfN(_FloatN x, int rnd, unsigned int width);
_FloatNx ufromfpfNx(_FloatNx x, int rnd, unsigned int width);
_DecimalN ufromfpdN(_DecimalN x, int rnd, unsigned int width);
_DecimalNx ufromfpdNx(_DecimalNx x, int rnd, unsigned int width);
_FloatN fromfpxfN(_FloatN x, int rnd, unsigned int width);
_FloatNx fromfpxfNx(_FloatNx x, int rnd, unsigned int width);
_DecimalN fromfpxdN(_DecimalN x, int rnd, unsigned int width);
_DecimalNx fromfpxdNx(_DecimalNx x, int rnd, unsigned int width);
_FloatN ufromfpxfN(_FloatN x, int rnd, unsigned int width);
_FloatNx ufromfpxfNx(_FloatNx x, int rnd, unsigned int width);
_DecimalN ufromfpxdN(_DecimalN x, int rnd, unsigned int width);
_DecimalNx ufromfpxdNx(_DecimalNx x, int rnd, unsigned int width);
(7.12.10.2) Remainder functions
_FloatN fmodfN(_FloatN x, _FloatN y);
_FloatNx fmodfNx(_FloatNx x, _FloatNx y);
_DecimalN fmoddN(_DecimalN x, _DecimalN y);
_DecimalNx fmoddNx(_DecimalNx x, _DecimalNx y);
_FloatN remainderfN(_FloatN x, _FloatN y);
_FloatNx remainderfNx(_FloatNx x, _FloatNx y);
_DecimalN remainderdN(_DecimalN x, _DecimalN y);
_DecimalNx remainderdNx(_DecimalNx x, _DecimalNx y);
_FloatN remquofN(_FloatN x, _FloatN y, int *quo);
_FloatNx remquofNx(_FloatNx x, _FloatNx y, int *quo);
(7.12.11) Manipulation functions
_FloatN copysignfN(_FloatN x, _FloatN y);
_FloatNx copysignfNx(_FloatNx x, _FloatNx y);
_DecimalN copysigndN(_DecimalN x, _DecimalN y);
_DecimalNx copysigndNx(_DecimalNx x, _DecimalNx y);
_FloatN nanfN(const char *tagp);
_FloatNx nanfNx(const char *tagp);
_DecimalN nandN(const char *tagp);
_DecimalNx nandNx(const char *tagp);
_FloatN nextafterfN(_FloatN x, _FloatN y);
_FloatNx nextafterfNx(_FloatNx x, _FloatNx y);
_DecimalN nextafterdN(_DecimalN x, _DecimalN y);
_DecimalNx nextafterdNx(_DecimalNx x, _DecimalNx y);
_FloatN nextupfN(_FloatN x);
_FloatNx nextupfNx(_FloatNx x);
_DecimalN nextupdN(_DecimalN x);
_DecimalNx nextupdNx(_DecimalNx x);
_FloatN nextdownfN(_FloatN x);
_FloatNx nextdownfNx(_FloatNx x);
_DecimalN nextdowndN(_DecimalN x);
_DecimalNx nextdowndNx(_DecimalNx x);
int canonicalizefN(_FloatN * cx, const _FloatN * x);
int canonicalizefNx(_FloatNx * cx, const _FloatNx * x);
int canonicalizedN(_DecimalN * cx, const _DecimalN * x);
int canonicalizedNx(_DecimalNx * cx, const _DecimalNx * x);
(7.12.12) Maximum, minimum, and positive difference functions
_FloatN fdimfN(_FloatN x, _FloatN y);
_FloatNx fdimfNx(_FloatNx x, _FloatNx y);
_DecimalN fdimdN(_DecimalN x, _DecimalN y);
_DecimalNx fdimdNx(_DecimalNx x, _DecimalNx y);
_FloatN fmaximumfN(_FloatN x, _FloatN y);
_FloatNx fmaximumfNx(_FloatNx x, _FloatNx y);
_DecimalN fmaximumdN(_DecimalN x, _DecimalN y);
_DecimalNx fmaximumdNx(_DecimalNx x, _DecimalNx y);
_FloatN fminimumfN(_FloatN x, _FloatN y);
_FloatNx fminimumfNx(_FloatNx x, _FloatNx y);
_DecimalN fminimumdN(_DecimalN x, _DecimalN y);
_DecimalNx fminimumdNx(_DecimalNx x, _DecimalNx y);
_FloatN fmaximum_magfN(_FloatN x, _FloatN y);
_FloatNx fmaximum_magfNx(_FloatNx x, _FloatNx y);
_DecimalN fmaximum_magdN(_DecimalN x, _DecimalN y);
_DecimalNx fmaximum_magdNx(_DecimalNx x, _DecimalNx y);
_FloatN fminimum_magfN(_FloatN x, _FloatN y);
_FloatNx fminimum_magfNx(_FloatNx x, _FloatNx y);
_DecimalN fminimum_magdN(_DecimalN x, _DecimalN y);
_DecimalNx fminimum_magdNx(_DecimalNx x, _DecimalNx y);
_FloatN fmaximum_numfN(_FloatN x, _FloatN y);
_FloatNx fmaximum_numfNx(_FloatNx x, _FloatNx y);
_DecimalN fmaximum_numdN(_DecimalN x, _DecimalN y);
_DecimalNx fmaximum_numdNx(_DecimalNx x, _DecimalNx y);
_FloatN fminimum_numfN(_FloatN x, _FloatN y);
_FloatNx fminimum_numfNx(_FloatNx x, _FloatNx y);
_DecimalN fminimum_numdN(_DecimalN x, _DecimalN y);
_DecimalNx fminimum_numdNx(_DecimalNx x, _DecimalNx y);
_FloatN fmaximum_mag_numfN(_FloatN x, _FloatN y);
_FloatNx fmaximum_mag_numfNx(_FloatNx x, _FloatNx y);
_DecimalN fmaximum_mag_numdN(_DecimalN x, _DecimalN y);
_DecimalNx fmaximum_mag_numdNx(_DecimalNx x, _DecimalNx y);
_FloatN fminimum_mag_numfN(_FloatN x, _FloatN y);
_FloatNx fminimum_mag_numfNx(_FloatNx x, _FloatNx y);
_DecimalN fminimum_mag_numdN(_DecimalN x, _DecimalN y);
_DecimalNx fminimum_mag_numdNx(_DecimalNx x, _DecimalNx y);
(7.12.13.1) Fused multiply-add
_FloatN fmafN(_FloatN x, _FloatN y, _FloatN z);
_FloatNx fmafNx(_FloatNx x, _FloatNx y, _FloatNx z);
_DecimalN fmadN(_DecimalN x, _DecimalN y, _DecimalN z);
_DecimalNx fmadNx(_DecimalNx x, _DecimalNx y, _DecimalNx z);
(7.12.14) Functions that round result to narrower type
_FloatM fMaddfN(_FloatN x, _FloatN y); // M < N
_FloatM fMaddfNx(_FloatNx x, _FloatNx y); // M ≤ N
_FloatMx fMxaddfN(_FloatN x, _FloatN y); // M < N
_FloatMx fMxaddfNx(_FloatNx x, _FloatNx y); // M < N
_DecimalM dMadddN(_DecimalN x, _DecimalN y); // M < N
_DecimalM dMadddNx(_DecimalNx x, _DecimalNx y); // M ≤ N
_DecimalMx dMxadddN(_DecimalN x, _DecimalN y); // M < N
_DecimalMx dMxadddNx(_DecimalNx x, _DecimalNx y); // M < N
_FloatM fMsubfN(_FloatN x, _FloatN y); // M < N
_FloatM fMsubfNx(_FloatNx x, _FloatNx y); // M ≤ N
_FloatMx fMxsubfN(_FloatN x, _FloatN y); // M < N
_FloatMx fMxsubfNx(_FloatNx x, _FloatNx y); // M < N
_DecimalM dMsubdN(_DecimalN x, _DecimalN y); // M < N
_DecimalM dMsubdNx(_DecimalNx x, _DecimalNx y); // M ≤ N
_DecimalMx dMxsubdN(_DecimalN x, _DecimalN y); // M < N
_DecimalMx dMxsubdNx(_DecimalNx x, _DecimalNx y); // M < N
_FloatM fMmulfN(_FloatN x, _FloatN y); // M < N
_FloatM fMmulfNx(_FloatNx x, _FloatNx y); // M ≤ N
_FloatMx fMxmulfN(_FloatN x, _FloatN y); // M < N
_FloatMx fMxmulfNx(_FloatNx x, _FloatNx y); // M < N
_DecimalM dMmuldN(_DecimalN x, _DecimalN y); // M < N
_DecimalM dMmuldNx(_DecimalNx x, _DecimalNx y); // M ≤ N
_DecimalMx dMxmuldN(_DecimalN x, _DecimalN y); // M < N
_DecimalMx dMxmuldNx(_DecimalNx x, _DecimalNx y); // M < N
_FloatM fMdivfN(_FloatN x, _FloatN y); // M < N
_FloatM fMdivfNx(_FloatNx x, _FloatNx y); // M ≤ N
_FloatMx fMxdivfN(_FloatN x, _FloatN y); // M < N
_FloatMx fMxdivfNx(_FloatNx x, _FloatNx y); // M < N
_DecimalM dMdivdN(_DecimalN x, _DecimalN y); // M < N
_DecimalM dMdivdNx(_DecimalNx x, _DecimalNx y); // M ≤ N
_DecimalMx dMxdivdN(_DecimalN x, _DecimalN y); // M < N
_DecimalMx dMxdivdNx(_DecimalNx x, _DecimalNx y); // M < N
_FloatM fMfmafN(_FloatN x, _FloatN y, _FloatN z); // M < N
_FloatM fMfmafNx(_FloatNx x, _FloatNx y, _FloatNx z); // M ≤ N
_FloatMx fMxfmafN(_FloatN x, _FloatN y, _FloatN z); // M < N
_FloatMx fMxfmafNx(_FloatNx x, _FloatNx y, _FloatNx z); // M < N
_DecimalM dMfmadN(_DecimalN x, _DecimalN y, _DecimalN z); // M < N
_DecimalM dMfmadNx(_DecimalNx x, _DecimalNx y, _DecimalNx z); // M ≤ N
_DecimalMx dMxfmadN(_DecimalN x, _DecimalN y, _DecimalN z); // M < N
_DecimalMx dMxfmadNx(_DecimalNx x, _DecimalNx y, _DecimalNx z); // M < N
_FloatM fMsqrtfN(_FloatN x); // M < N
_FloatM fMsqrtfNx(_FloatNx x); // M ≤ N
_FloatMx fMxsqrtfN(_FloatN x); // M < N
_FloatMx fMxsqrtfNx(_FloatNx x); // M < N
_DecimalM dMsqrtdN(_DecimalN x); // M < N
_DecimalM dMsqrtdNx(_DecimalNx x); // M ≤ N
_DecimalMx dMxsqrtdN(_DecimalN x); // M < N
_DecimalMx dMxsqrtdNx(_DecimalNx x); // M < N
(7.12.15) Quantum and quantum exponent functions
_DecimalN quantizedN(_DecimalN x, _DecimalN y);
_DecimalNx quantizedNx(_DecimalNx x, _DecimalNx y);
bool samequantumdN(_DecimalN x, _DecimalN y);
bool samequantumdNx(_DecimalNx x, _DecimalNx y);
_DecimalN quantumdN(_DecimalN x);
_DecimalNx quantumdNx(_DecimalNx x);
long long int llquantexpdN(_DecimalN x);
long long int llquantexpdNx(_DecimalNx x);
(7.12.16) Decimal re-encoding functions
void encodedecdN(unsigned char * restrict encptr,
const _DecimalN * restrict xptr);
void decodedecdN(_DecimalN * restrict xptr,
const unsigned char * restrict encptr);
void encodebindN(unsigned char * restrict encptr,
const _DecimalN * restrict xptr);
void decodebindN(_DecimalN * restrict xptr,
const unsigned char * restrict encptr);
(F.10.12) Total order functions
int totalorderfN(const _FloatN *x, const _FloatN *y);
int totalorderfNx(const _FloatNx *x, const _FloatNx *y);
int totalorderdN(const _DecimalN *x, const _DecimalN *y);
int totalorderdNx(const _DecimalNx *x, const _DecimalNx *y);
int totalordermagfN(const _FloatN *x, const _FloatN *y);
int totalordermagfNx(const _FloatNx *x, const _FloatNx *y);
int totalordermagdN(const _DecimalN *x, const _DecimalN *y);
int totalordermagdNx(const _DecimalNx *x, const _DecimalNx *y);
(F.10.13) Payload functions
_FloatN getpayloadfN(const _FloatN *x);
_FloatNx getpayloadfNx(const _FloatNx *x);
_DecimalN getpayloaddN(const _DecimalN *x);
_DecimalNx getpayloaddNx(const _DecimalNx *x);
int setpayloadfN(_FloatN *res, _FloatN pl);
int setpayloadfNx(_FloatNx *res, _FloatNx pl);
int setpayloaddN(_DecimalN *res, _DecimalN pl);
int setpayloaddNx(_DecimalNx *res, _DecimalNx pl);
int setpayloadsigfN(_FloatN *res, _FloatN pl);
int setpayloadsigfNx(_FloatNx *res, _FloatNx pl);
int setpayloadsigdN(_DecimalN *res, _DecimalN pl);
int setpayloadsigdNx(_DecimalNx *res, _DecimalNx pl);
2 The specification of the frexp functions (7.12.6.7) applies to the functions for binary floating types
like those for standard floating types: the exponent is an integral power of 2 and, when applicable,
value equals x × 2*exp .
3 The specification of the ldexp functions (7.12.6.9) applies to the functions for binary floating types
like those for standard floating types: they return x × 2exp .
4 The specification of the logb functions (7.12.6.17) applies to binary floating types, with b = 2.
5 The specification of the scalbn and scalbln functions (7.12.6.19) applies to binary floating types,
with b = 2.
H.11.3 Encoding conversion functions
1 This subclause introduces <math.h> functions that, together with the numerical conversion functions
for encodings in H.12, support the non-arithmetic interchange formats specified by IEC 60559.
Support for these formats is an optional feature of this annex. Implementations that do not support
non-arithmetic interchange formats need not declare the functions in this subclause.
2 Non-arithmetic interchange formats are not associated with floating types. Arrays of element
type unsigned char are used as parameters for conversion functions, to represent encodings in
interchange formats that might be non-arithmetic formats.
H.11.3.1 Encode and decode functions
1 This subclause specifies functions to map representations in binary floating types to and from
encodings in unsigned char arrays.
H.11.3.1.1 The encodefN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <math.h>
void encodefN(unsigned char encptr[restrict static N/8],
const _FloatN * restrict xptr);
Description
2 The encodefN functions convert *xptr into an IEC 60559 binaryN encoding and store the resulting
encoding as an N /8 element array, with 8 bits per array element, in the object pointed to by encptr.
The order of bytes in the array is implementation-defined. These functions preserve the value of
*xptr and raise no floating-point exceptions. If *xptr is non-canonical, these functions may or may
not produce a canonical encoding.
Returns
3 The encodefN functions return no value.
H.11.3.1.2 The decodefN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <math.h>
void decodefN(_FloatN * restrict xptr,
const unsigned char encptr[restrict static N/8]);
Description
2 The decodefN functions interpret the N /8 element array pointed to by encptr as an IEC 60559
binaryN encoding, with 8 bits per array element. The order of bytes in the array is implementation-
defined. These functions convert the given encoding into a representation in the type _FloatN, and
store the result in the object pointed to by xptr. These functions preserve the encoded value and
raise no floating-point exceptions. If the encoding is non-canonical, these functions may or may not
produce a canonical representation.
Returns
3 The decodefN functions return no value.
4 See EXAMPLE in H.11.3.2.1.
H.11.3.2 Encoding-to-encoding conversion functions
1 An implementation shall declare an fMencfN function for each M and N equal to the width of
a supported IEC 60559 arithmetic or non-arithmetic binary interchange format, M ̸= N . An
implementation shall provide both dMencdecdN and dMencbindNfunctions for each M and N equal
to the width of a supported IEC 60559 arithmetic or non-arithmetic decimal interchange format,
M ̸= N .
H.11.3.2.1 The fMencfN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <math.h>
void fMencfN(unsigned char encMptr[restrict static M/8],
const unsigned char encNptr[restrict static N/8]);
Description
2 The fMencfN functions convert between IEC 60559 binary interchange formats. These functions
interpret the N /8 element array pointed to by encNptr as an encoding of width N bits. They
convert the encoding to an encoding of width M bits and store the resulting encoding as an M /8
element array in the object pointed to by encMptr. The conversion rounds and raises floating-point
exceptions as specified in IEC 60559. The order of bytes in the arrays is implementation-defined.
Returns
3 These functions return no value.
4 EXAMPLE If the IEC 60559 binary16 format is supported as a non-arithmetic format, data in binary16 format can be
converted to type float as follows:
#define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <math.h>
unsigned char b16[2]; // for input binary16 datum
float f; // for result
unsigned char b32[4];
_Float32 f32;
// store input binary16 datum in array b16
...
f32encf16(b32, b16);
decodef32(&f32, b32);
f = f32;
...
H.11.3.2.2 The dMencdecdN and dMencbindN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <math.h>
void dMencdecdN(unsigned char encMptr[restrict static M/8],
const unsigned char encNptr[restrict static N/8]);
void dMencbindN(unsigned char encMptr[restrict static M/8],
const unsigned char encNptr[restrict static N/8]);
Description
2 The dMencdecdN and dMencbindN functions convert between IEC 60559 decimal interchange formats
that use the same encoding scheme. The dMencdecdN functions convert between formats using the
encoding scheme based on decimal encoding of the significand. The dMencbindN functions convert
between formats using the encoding scheme based on binary encoding of the significand. These
functions interpret the N /8 element array pointed to by encNptr as an encoding of width N bits.
They convert the encoding to an encoding of width M bits and store the resulting encoding as an M /8
element array in the object pointed to by encMptr. The conversion rounds and raises floating-point
exceptions as specified in IEC 60559. The order of bytes in the arrays is implementation-defined.
Returns
3 These functions return no value.
H.12 Numeric conversion functions <stdlib.h>
1 This clause expands the specification of numeric conversion functions in <stdlib.h> (7.24.1) to also
include conversions of strings from and to interchange and extended floating types. The conversions
from floating are provided by functions analogous to the strfromd function. The conversions to
floating are provided by functions analogous to the strtod function.
2 This clause also specifies functions to convert strings from and to IEC 60559 interchange format
encodings.
3 For each interchange or extended floating type that the implementation provides, <stdlib.h> shall
declare the associated functions specified below in H.12.1 and H.12.2 (see H.8). Conversely, for each
such type that the implementation does not provide, <stdlib.h> shall not declare the associated
functions.
4 For each IEC 60559 arithmetic or non-arithmetic format that the implementation supports,
<stdlib.h> shall declare the associated functions specified below in H.12.3 and H.12.4 (see H.8).
Conversely, for each such format that the implementation does not provide, <stdlib.h> shall not
declare the associated functions.
H.12.1 String from floating
1 This subclause expands 7.24.1.3 and 7.24.1.4 to also include functions for the interchange and
extended floating types. It adds to the synopsis in 7.24.1.3 the prototypes
int strfromfN(char * restrict s, size_t n,
const char * restrict format, _FloatN fp);
int strfromfNx(char * restrict s, size_t n,
const char * restrict format, _FloatNx fp);
It encompasses the prototypes in 7.24.1.4 by replacing them with
int strfromdN(char * restrict s, size_t n,
const char * restrict format, _DecimalN fp);
int strfromdNx(char * restrict s, size_t n,
const char * restrict format, _DecimalNx fp);
2 The descriptions and returns for the added functions are analogous to the ones in 7.24.1.3
and 7.24.1.4.
H.12.2 String to floating
1 This subclause expands 7.24.1.5 and 7.24.1.6 to also include functions for the interchange and
extended floating types. It adds to the synopsis in 7.24.1.5 the prototypes
_FloatN strtofN(const char * restrict nptr,
char ** restrict endptr);
_FloatNx strtofNx(const char * restrict nptr,
char ** restrict endptr);
It encompasses the prototypes in 7.24.1.6 by replacing them with
_DecimalN strtodN(const char * restrict nptr,
char ** restrict endptr);
_DecimalNx strtodNx(const char * restrict nptr,
char ** restrict endptr);
2 The descriptions and returns for the added functions are analogous to the ones in 7.24.1.5 and 7.24.1.6.
3 For implementations that support both binary and decimal floating types and a (binary or dec-
imal) non-arithmetic interchange format, the strtodN and strtodNx functions (and hence the
strtoencdecdN and strtoencbindN functions in H.12.4.2) shall accept subject sequences that have
the form of hexadecimal floating numbers and otherwise meet the requirements of subject sequences
(7.24.1.6). Then the decimal results shall be correctly rounded if the subject sequence has at most
M significant hexadecimal digits, where M ≥ ⌈(P − 1)/4⌉ + 1 is implementation-defined, and P is
the maximum precision of the supported binary floating types and binary non-arithmetic formats.
If all subject sequences of hexadecimal form are correctly rounded, M may be regarded as infinite.
If the subject sequence has more than M significant hexadecimal digits, the implementation may
first round to M significant hexadecimal digits according to the applicable rounding direction mode,
signaling exceptions as though converting from a wider format, then correctly round the result of
the shortened hexadecimal input to the result type.
4 EXAMPLE If the IEC 60559 binary128 format is supported as a non-arithmetic format, data in binary128 format can be
converted to type _Decimal128 as follows:
#define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <stdlib.h>
#define MAXSIZE 41 // > intermediate hex string length
unsigned char b128[16]; // for input binary128 datum
_Decimal128 d128; // for result
char s[MAXSIZE];
// store input binary128 datum in array b128
...
strfromencf128(s, MAXSIZE, "%a", b128);
d128 = strtod128(s, NULL);
...
Use of "%a" for formatting assures an exact conversion of the value in binary format to character sequence. The value of that
character sequence will be correctly rounded to _Decimal128 , as specified above in this subclause. The array s for the output
of strfromencf128 need have no greater size than 41, which is the maximum length of strings of the form
[−]0xh.h . . . hp ± d
where there are up to 29 hexadecimal digits h and d has 5 digits plus 1 for the null character.
H.12.3 String from encoding
1 An implementation shall declare the strfromencfN function for each N equal to the width of a
supported IEC 60559 arithmetic or non-arithmetic binary interchange format. An implementation
shall declare both the strfromencdecdN and strfromencbindN functions for each N equal to the
width of a supported IEC 60559 arithmetic or non-arithmetic decimal interchange format.
H.12.3.1 The strfromencf N functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <stdlib.h>
int strfromencfN(char * restrict s, size_t n, const char * restrict format,
const unsigned char encptr[restrict static N/8]);
Description
2 The strfromencfN functions are similar to the strfromfN functions, except the input is the value of
the N /8 element array pointed to by encptr, interpreted as an IEC 60559 binaryN encoding. The
order of bytes in the arrays is implementation-defined.
Returns
3 The strfromencfN functions return the same values as corresponding strfromfN functions.
H.12.3.2 The strfromencdecdN and strfromencbindN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <stdlib.h>
int strfromencdecdN(char * restrict s, size_t n, const char * restrict format,
const unsigned char encptr[restrict static N/8]);
int strfromencbindN(char * restrict s, size_t n, const char * restrict format,
const unsigned char encptr[restrict static N/8]);
Description
2 The strfromencdecdN functions are similar to the strfromdN functions except the input is the value
of the N /8 element array pointed to by encptr, interpreted as an IEC 60559 decimalN encoding in
the coding scheme based on decimal encoding of the significand. The strfromencbindN functions
are similar to the strfromdN functions except the input is the value of the N /8 element array pointed
to by encptr, interpreted as an IEC 60559 decimalN encoding in the coding scheme based on binary
encoding of the significand. The order of bytes in the arrays is implementation-defined.
Returns
3 The strfromencdecdN and strfromencbindN functions return the same values as corresponding
strfromdN functions.
H.12.4 String to encoding
1 An implementation shall declare the strtoencfN function for each N equal to the width of a
supported IEC 60559 arithmetic or non-arithmetic binary interchange format. An implementation
shall declare both the strtoencdecdN and strtoencbindN functions for each N equal to the width
of a supported IEC 60559 arithmetic or non-arithmetic decimal interchange format.
H.12.4.1 The strtoencfN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <stdlib.h>
void strtoencfN(unsigned char encptr[restrict static N/8],
const char * restrict nptr, char ** restrict endptr);
Description
2 The strtoencfN functions are similar to the strtofN functions, except they store an IEC 60559
encoding of the result as an N /8 element array in the object pointed to by encptr. The order of
bytes in the arrays is implementation-defined.
Returns
3 These functions return no value.
H.12.4.2 The strtoencdecdN and strtoencbindN functions
Synopsis
1 #define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <stdlib.h>
void strtoencdecdN(unsigned char encptr[restrict static N/8],
const char * restrict nptr, char ** restrict endptr);
void strtoencbindN(unsigned char encptr[restrict static N/8],
const char * restrict nptr, char ** restrict endptr);
Description
2 The strtoencdecdN and strtoencbindNfunctions are similar to the strtodN functions, except
they store an IEC 60559 encoding of the result as an N /8 element array in the object pointed to
by encptr. The strtoencdecdN functions produce an encoding in the encoding scheme based on
decimal encoding of the significand. The strtoencbindN functions produce an encoding in the
encoding scheme based on binary encoding of the significand. The order of bytes in the arrays is
implementation-defined.
Returns
3 These functions return no value.
H.13 Type-generic macros <tgmath.h>
1 This clause enhances the specification of type-generic macros in <tgmath.h> (7.27) to apply to
interchange and extended floating types, as well as standard floating types.
2 If arguments for generic parameters of a type-generic macro are such that some argument has
a corresponding real type that is a standard floating type or a binary floating type and another
argument is of decimal floating type, the behavior is undefined.
3 The treatment of arguments of integer type in 7.27 is expanded to cases where another argument
has extended type. Arguments of integer type are regarded as having type:
— _Decimal64x , if any argument has a decimal extended type; otherwise
— _Float32x , if any argument has a binary extended type; otherwise
— _Decimal64 , if any argument has decimal type; otherwise
— double
4 Use of the macros carg, cimag, conj, cproj, or creal with any argument of standard floating type,
binary floating type, complex type, or imaginary type invokes a complex function. Use of the macro
with an argument of a decimal floating type results in undefined behavior.
5 The functions that round results to a narrower type have type-generic macros whose names are
obtained by omitting any suffix from the function names. Thus, the macros with f or d prefix are (as
in 7.27):
fadd fmul ffma
dadd dmul dfma
fsub fdiv fsqrt
dsub ddiv dsqrt
and the macros with fM, fMx, dM, or dMx prefix are:
fMadd fMxmul dMfma
fMsub fMxdiv dMsqrt
fMmul fMxfma dMxadd
fMdiv fMxsqrt dMxsub
fMfma dMadd dMxmul
fMsqrt dMsub dMxdiv
fMxadd dMmul dMxfma
fMxsub dMdiv dMxsqrt
All arguments are generic. If any argument is not real, use of the macro results in undefined behavior.
The following specification uses the notation type1 ⊆ type2 to mean the values of type1 are a subset
of (or the same as) the values of type2. The generic parameter type T for the function invoked by the
macro is determined as follows:
— First, obtain a preliminary type P for the generic parameters: if all arguments are of integer
type, then P is double if the macro prefix is f, d, fN, or fNx and P is _Decimal64 if the macro
prefix is dN or dNx; otherwise (if some argument is not of integer type), apply the rules (for
determining the corresponding real type of the generic parameters) in 7.27 for macros that
do not round result to narrower type, using the usual arithmetic conversion rules in H.4.2, to
obtain P .
— If there exists a corresponding function whose generic parameters have type P , then T is P .
— Otherwise, T is determined from P and the macro prefix as follows:
• For prefix f: if P is a standard or binary floating type, then T is the first standard floating
type of either double or long double, such that P ⊆ T , if such a type T exists. Otherwise
(if no such type T exists or P is a decimal floating type), the behavior is undefined.
• For prefix d: if P is a standard or binary floating type, then T is long double if P ⊆
long double. Otherwise (if P ⊆ long double is false or P is a decimal floating type),
the behavior is undefined.
• For prefix fM: if P is a standard or binary floating type, then T is _FloatN for minimum
N > M such that P ⊆ T , if such a type T is supported; otherwise T is _FloatNx for
minimum N ≥ M such that P ⊆ T , if such a type T is supported. Otherwise (if no
such _FloatN or _FloatNx is supported or P is a decimal floating type), the behavior is
undefined.
• For prefix fMx: if P is a standard or binary floating type, then T is _FloatNx for minimum
N > M such that P ⊆ T , if such a type T is supported; otherwise T is _FloatN for
minimum N > M such that P ⊆ T , if such a type T is supported. Otherwise (if no
such _FloatNx or _FloatN is supported or P is a decimal floating type), the behavior is
undefined.
• For prefix dM: if P is a decimal floating type, then T is _DecimalN for minimum N > M
such that P ⊆ T , if such a type T is supported; otherwise T is _DecimalNx for minimum
N ≥ M such that P ⊆ T . Otherwise (P is a standard or binary floating type), the behavior
is undefined.
• For prefix dMx: if P is a decimal floating type, then T is _DecimalNx for minimum N > M
such that P ⊆ T , if such a type T is supported; otherwise T is _DecimalN for minimum
N > M such that P ⊆ T , if such a type T is supported. Otherwise (P is a standard or
binary floating type), the behavior is undefined.
6 EXAMPLE With the declarations
#define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <tgmath.h>
int n;
double d;
long double ld;
double complex dc;
_Float32x f32x;
_Float64 f64;
_Float64x f64x;
_Float128 f128;
_Float64x complex f64xc;
functions invoked by use of type-generic macros are shown in the following table, where type1 ⊆ type2 means the values of
type1 are a subset of (or the same as) the values of type2, and type1 ⊂ type2 means the values of type1 are a strict subset of
the values of type2:
macro use invokes
cos(f64xc) ccosf64x
pow(dc, f128) cpowf128
pow(f64, d) powf64
pow(d, f32x) pow, the function, if _Float32x ⊆ double, else powf32x if double ⊂
_Float32x , else undefined
pow(f32, n) pow, the function
pow(f32x, n) pow32x
Macros that round the result to a narrower type. . .
macro use invokes
fsub(d, ld) fsubl
dsub(d, f32) dsubl
undefined
fmul(dc, d)
ddiv(ld, f128) ddivl if _Float128 ⊆ long double, else undefined
f32add(f64x, f64) f32addf64x
f32xsqrt(n) f32xsqrtf64
f32mul(f128, f32x) f32mulf128 if _Float32x ⊆ _Float128 , else f32mulf32x if _Float128
⊂ _Float32x , else undefined
f32fma(f32x, n, f32x) f32fmaf32x
f32add(f32, f32) f32addf64
f32xsqrt(f32) f32xsqrtf64x, as declaration above shows _Float64x is supported
f64div(f32x, f32x) f64divf128 if _Float32x ⊆ _Float128 , else f64divf64x
I. Annex I (informative) Common warnings
1 An implementation may generate warnings in many situations, none of which are specified as part
of this document. The following are a few of the more common situations.
2 — A new struct or union type appears in a function prototype (6.2.1, 6.7.2.3).
— A block with initialization of an object that has automatic storage duration is jumped into
(6.2.4).
— An implicit narrowing conversion is encountered, such as the assignment of a long int or a
double to an int, or a pointer to void to a pointer to any type other than a character type (6.3).
— A hexadecimal floating constant cannot be represented exactly in its evaluation format (6.4.4.2).
— An integer character constant includes more than one character or a wide character constant
includes more than one multibyte character (6.4.4.4).
— The characters /* are found in a comment (6.4.7).
— An "unordered" binary operator (not comma, &&, or ||) contains a side effect to an lvalue in
one operand, and a side effect to, or an access to the value of, the identical lvalue in the other
operand (6.5).
— An object is defined but not used (6.7).
— A value is given to an object of an enumerated type other than by assignment of an enumeration
constant that is a member of that type, or an enumeration object that has the same type, or the
value of a function that returns the same enumerated type (6.7.2.2).
— An aggregate has a partly bracketed initialization (6.7.8).
— A statement cannot be reached (6.8).
— A statement with no apparent effect is encountered (6.8).
— A constant expression is used as the controlling expression of a selection statement (6.8.4).
— An incorrectly formed preprocessing group is encountered while skipping a preprocessing
group (6.10.1).
— An unrecognized #pragma directive is encountered (6.10.7).
J. Annex J (informative) Portability issues
1 This annex collects some information about portability that appears in this document.
J.1 Unspecified behavior
1 The following are unspecified:
— The manner and timing of static initialization (5.1.2).
— The termination status returned to the hosted environment if the return type of main is not
compatible with int (5.1.2.2.3).
— The values of objects that are neither lock-free atomic objects nor of type
volatile sig_atomic_t and the state of the floating-point environment, when the
processing of the abstract machine is interrupted by receipt of a signal (5.1.2.3).
— The behavior of the display device if a printing character is written when the active position is
at the final position of a line (5.2.2).
— The behavior of the display device if a backspace character is written when the active position
is at the initial position of a line (5.2.2).
— The behavior of the display device if a horizontal tab character is written when the active
position is at or past the last defined horizontal tabulation position (5.2.2).
— The behavior of the display device if a vertical tab character is written when the active position
is at or past the last defined vertical tabulation position (5.2.2).
— How an extended source character that does not correspond to a universal character name
counts toward the significant initial characters in an external identifier (5.2.4.1).
— Many aspects of the representations of types (6.2.6).
— The value of padding bytes when storing values in structures or unions (6.2.6.1).
— The values of bytes that correspond to union members other than the one last stored into
(6.2.6.1).
— The representation used when storing a value in an object that has more than one object
representation for that value (6.2.6.1).
— The values of any padding bits in integer representations (6.2.6.2).
— Whether two string literals result in distinct arrays (6.4.5).
— The order in which subexpressions are evaluated and the order in which side effects take place,
except as specified for the function-call () , &&, ||, ?:, and comma operators (6.5).
— The order in which the function designator, arguments, and subexpressions within the argu-
ments are evaluated in a function call (6.5.2.2).
— The order of side effects among compound literal initialization list expressions (6.5.2.5).
— The order in which the operands of an assignment operator are evaluated (6.5.16).
— The alignment of the addressable storage unit allocated to hold a bit-field (6.7.2.1).
— Whether a call to an inline function uses the inline definition or the external definition of the
function (6.7.4).
— Whether or not a size expression is evaluated when it is part of the operand of a sizeof
operator and changing the value of the size expression would not affect the result of the
operator (6.7.6.2).
— The order in which any side effects occur among the initialization list expressions in an
initializer (6.7.10).
— The layout of storage for function parameters (6.9.1).
— When a fully expanded macro replacement list contains a function-like macro name as its
last preprocessing token and the next preprocessing token from the source file is a ( , and
the fully expanded replacement of that macro ends with the name of the first macro and the
next preprocessing token from the source file is again a ( , whether that is considered a nested
replacement (6.10.4).
— The order in which # and ## operations are evaluated during macro substitution (6.10.4.2,
and 6.10.4.3).
— The line number of a preprocessing token, in particular __LINE__ , that spans multiple physical
lines (6.10.5).
— The line number of a preprocessing directive that spans multiple physical lines (6.10.5).
— The line number of a macro invocation that spans multiple physical or logical lines (6.10.5).
— The line number following a directive of the form #line __LINE__ new-line (6.10.5).
— The state of the floating-point status flags when execution passes from a part of the program
translated with FENV_ACCESS "off" to a part translated with FENV_ACCESS "on" (7.6.1).
— The order in which feraiseexcept raises floating-point exceptions, except as stated in F.8.6
(7.6.4.3).
— Whether math_errhandling is a macro or an identifier with external linkage (7.12).
— The results of the frexp functions when the specified value is not a floating-point number
(7.12.6.7).
— The numeric result of the ilogb functions when the correct value is outside the range of the
return type (7.12.6.8, F.10.3.8).
— The result of rounding when the value is out of range (7.12.9.5, 7.12.9.7, F.10.6.5).
— The value stored by the remquo functions in the object pointed to by quo when y is zero
(7.12.10.3).
— Whether a comparison macro argument that is represented in a format wider than its semantic
type is converted to the semantic type (7.12.17).
— Whether setjmp is a macro or an identifier with external linkage (7.13).
— Whether va_copy and va_end are macros or identifiers with external linkage (7.16.1).
— The hexadecimal digit before the decimal point when a non-normalized floating-point number
is printed with an a or A conversion specifier (7.23.6.1, 7.31.2.1).
— The value of the file position indicator after a successful call to the ungetc function for a text
stream, or the ungetwc function for any stream, until all pushed-back characters are read or
discarded (7.23.7.10, 7.31.3.10).
— The details of the value stored by the fgetpos function (7.23.9.1).
— The details of the value returned by the ftell function for a text stream (7.23.9.4).
— Whether the strtod, strtof, strtold, wcstod, wcstof, and wcstold functions convert a
minus-signed sequence to a negative number directly or by negating the value resulting from
converting the corresponding unsigned sequence (7.24.1.5, 7.31.4.1.2).
— The order and contiguity of storage allocated by successive calls to the calloc, malloc,
realloc, and aligned_alloc functions (7.24.3).
— The amount of storage allocated by a successful call to the calloc, malloc, realloc, or
aligned_alloc function when 0 bytes was requested (7.24.3).
— Whether a call to the atexit function that does not happen before the exit function is called
will succeed (7.24.4.2).
— Whether a call to the at_quick_exit function that does not happen before the quick_exit
function is called will succeed (7.24.4.3).
— Which of two elements that compare as equal is matched by the bsearch function (7.24.5.1).
— The order of two elements that compare as equal in an array sorted by the qsort function
(7.24.5.2).
— The order in which destructors are invoked by thrd_exit (7.28.5.5).
— Whether calling tss_delete on a key while another thread is executing destructors affects the
number of invocations of the destructors associated with the key on that thread (7.28.6.2).
— The encoding of the calendar time returned by the time function (7.29.2.5).
— The characters stored by the strftime or wcsftime function if any of the time values being
converted is outside the normal range (7.29.3.5, 7.31.5.1).
— Whether an encoding error occurs if a wchar_t value that does not correspond to a member of
the extended character set appears in the format string for a function in 7.31.2 or 7.31.5 and the
specified semantics do not require that value to be processed by wcrtomb (7.31.1).
— The conversion state after an encoding error occurs (7.31.6.3.2, 7.31.6.3.3, 7.31.6.4.1, 7.31.6.4.2,
and 7.30.1.1, 7.30.1.2, 7.30.1.3, 7.30.1.4, 7.30.1.5, 7.30.1.6).
— The resulting value when the "invalid" floating-point exception is raised during IEC 60559
floating to integer conversion (F.4).
— Whether conversion of non-integer IEC 60559 floating values to integer raises the "inexact"
floating-point exception (F.4).
— Whether or when library functions in <math.h> raise the "inexact" floating-point exception in
an IEC 60559 conformant implementation (F.10).
— Whether or when library functions in <math.h> raise an undeserved "underflow" floating-
point exception in an IEC 60559 conformant implementation (F.10).
— The exponent value stored by frexp for a NaN or infinity (F.10.3.7).
— The numeric result returned by the lrint, llrint, lround, and llround functions if the
rounded value is outside the range of the return type (F.10.6.5, F.10.6.7).
— The sign of one part of the complex result of several math functions for certain special cases
in IEC 60559 compatible implementations (G.6.1.1, G.6.2.2, G.6.2.3, G.6.2.4, G.6.2.5, G.6.2.6,
and G.6.3.1, G.6.4.2).
J.2 Undefined behavior
1 The behavior is undefined in the following circumstances:
— A "shall" or "shall not" requirement that appears outside of a constraint is violated (Clause 4).
— A nonempty source file does not end in a new-line character which is not immediately preceded
by a backslash character or ends in a partial preprocessing token or comment (5.1.1.2).
— Token concatenation produces a character sequence matching the syntax of a universal charac-
ter name (5.1.1.2).
— A program in a hosted environment does not define a function named main using one of the
specified forms (5.1.2.2.1).
— The execution of a program contains a data race (5.1.2.4).
— A character not in the basic source character set is encountered in a source file, except in an
identifier, a character constant, a string literal, a header name, a comment, or a preprocessing
token that is never converted to a token (5.2.1).
— An identifier, comment, string literal, character constant, or header name contains an invalid
multibyte character or does not begin and end in the initial shift state (5.2.1.1).
— The same identifier has both internal and external linkage in the same translation unit (6.2.2).
— An object is referred to outside of its lifetime (6.2.4).
— The value of a pointer to an object whose lifetime has ended is used (6.2.4).
— The value of an object with automatic storage duration is used while the object has an indeter-
minate representation (6.2.4, 6.7.10, 6.8).
— A non-value representation is read by an lvalue expression that does not have character type
(6.2.6.1).
— A non-value representation is produced by a side effect that modifies any part of the object
using an lvalue expression that does not have character type (6.2.6.1).
— Two declarations of the same object or function specify types that are not compatible (6.2.7).
— A program requires the formation of a composite type from a variable length array type whose
size is specified by an expression that is not evaluated (6.2.7).
— Conversion to or from an integer type produces a value outside the range that can be repre-
sented (6.3.1.4).
— Demotion of one real floating type to another produces a value outside the range that can be
represented (6.3.1.5).
— An lvalue does not designate an object when evaluated (6.3.2.1).
— A non-array lvalue with an incomplete type is used in a context that requires the value of the
designated object (6.3.2.1).
— An lvalue designating an object of automatic storage duration that could have been declared
with the register storage class is used in a context that requires the value of the designated
object, but the object is uninitialized. (6.3.2.1).
— An lvalue having array type is converted to a pointer to the initial element of the array, and
the array object has register storage class (6.3.2.1).
— An attempt is made to use the value of a void expression, or an implicit or explicit conversion
(except to void) is applied to a void expression (6.3.2.2).
— Conversion of a pointer to an integer type produces a value outside the range that can be
represented (6.3.2.3).
— Conversion between two pointer types produces a result that is incorrectly aligned (6.3.2.3).
— A pointer is used to call a function whose type is not compatible with the referenced type
(6.3.2.3).
— An unmatched ’ or " character is encountered on a logical source line during tokenization
(6.4).
— A reserved keyword token is used in translation phase 7 or 8 for some purpose other than as a
keyword (6.4.1).
— A universal character name in an identifier does not designate a character whose encoding
falls into one of the specified ranges (6.4.2.1).
— The initial character of an identifier is a universal character name designating a digit (6.4.2.1).
— Two identifiers differ only in nonsignificant characters (6.4.2.1).
— The identifier __func__ is explicitly declared (6.4.2.2).
— The program attempts to modify a string literal (6.4.5).
— The characters ’ , \ , ", // , or /* occur in the sequence between the < and > delimiters, or the
characters ’ , \ , // , or /* occur in the sequence between the " delimiters, in a header name
preprocessing token (6.4.7).
— A side effect on a scalar object is unsequenced relative to either a different side effect on the
same scalar object or a value computation using the value of the same scalar object (6.5).
— An exceptional condition occurs during the evaluation of an expression (6.5).
— An object has its stored value accessed other than by an lvalue of an allowable type (6.5).
— A function is defined with a type that is not compatible with the type (of the expression)
pointed to by the expression that denotes the called function (6.5.2.2).
— A member of an atomic structure or union is accessed (6.5.2.3).
— The operand of the unary * operator has an invalid value (6.5.3.2).
— A pointer is converted to other than an integer or pointer type (6.5.4).
— The value of the second operand of the / or % operator is zero (6.5.5).
— If the quotient a/b is not representable, the behavior of both a/b and a%b (6.5.5).
— Addition or subtraction of a pointer into, or just beyond, an array object and an integer type
produces a result that does not point into, or just beyond, the same array object (6.5.6).
— Addition or subtraction of a pointer into, or just beyond, an array object and an integer type
produces a result that points just beyond the array object and is used as the operand of a unary
* operator that is evaluated (6.5.6).
— Pointers that do not point into, or just beyond, the same array object are subtracted (6.5.6).
— An array subscript is out of range, even if an object is apparently accessible with the given
subscript (as in the lvalue expression a[1][7] given the declaration int a[4][5]) (6.5.6).
— The result of subtracting two pointers is not representable in an object of type ptrdiff_t
(6.5.6).
— An expression is shifted by a negative number or by an amount greater than or equal to the
width of the promoted expression (6.5.7).
— An expression having signed promoted type is left-shifted and either the value of the expres-
sion is negative or the result of shifting would not be representable in the promoted type
(6.5.7).
— Pointers that do not point to the same aggregate or union (nor just beyond the same array
object) are compared using relational operators (6.5.8).
— An object is assigned to an inexactly overlapping object or to an exactly overlapping object
with incompatible type (6.5.16.1).
— An expression that is required to be an integer constant expression does not have an integer
type; has operands that are not integer constants, enumeration constants, character constants,
predefined constants, sizeof expressions whose results are integer constants, alignof expres-
sions, or immediately-cast floating constants; or contains casts (outside operands to sizeof
and alignof operators) other than conversions of arithmetic types to integer types (6.6).
— A constant expression in an initializer is not, or does not evaluate to, one of the following: an
arithmetic constant expression, a null pointer constant, an address constant, or an address
constant for a complete object type plus or minus an integer constant expression (6.6).
— An arithmetic constant expression does not have arithmetic type; has operands that are not
integer constants, floating constants, enumeration constants, character constants, predefined
constants, sizeof expressions whose results are integer constants, or alignof expressions; or
contains casts (outside operands to sizeof or alignof operators) other than conversions of
arithmetic types to arithmetic types (6.6).
— The value of an object is accessed by an array-subscript [], member-access . or-> , address &,
or indirection * operator or a pointer cast in creating an address constant (6.6).
— An identifier for an object is declared with no linkage and the type of the object is incomplete
after its declarator, or after its init-declarator if it has an initializer (6.7).
— A function is declared at block scope with an explicit storage-class specifier other than extern
(6.7.1).
— A structure or union is defined without any named members (including those specified
indirectly via anonymous structures and unions) (6.7.2.1).
— An attempt is made to access, or generate a pointer to just past, a flexible array member of a
structure when the referenced object provides no elements for that array (6.7.2.1).
— When the complete type is needed, an incomplete structure or union type is not completed in
the same scope by another declaration of the tag that defines the content (6.7.2.3).
— An attempt is made to modify an object defined with a const-qualified type through use of an
lvalue with non-const-qualified type (6.7.3).
— An attempt is made to refer to an object defined with a volatile-qualified type through use of
an lvalue with non-volatile-qualified type (6.7.3).
— The specification of a function type includes any type qualifiers (6.7.3).
— Two qualified types that are required to be compatible do not have the identically qualified
version of a compatible type (6.7.3).
— An object which has been modified is accessed through a restrict-qualified pointer to a const-
qualified type, or through a restrict-qualified pointer and another pointer that are not both
based on the same object (6.7.3.1).
— A restrict-qualified pointer is assigned a value based on another restricted pointer whose
associated block neither began execution before the block associated with this pointer, nor
ended before the assignment (6.7.3.1).
— A function with external linkage is declared with an inline function specifier, but is not also
defined in the same translation unit (6.7.4).
— A function declared with a _Noreturn function specifier returns to its caller (6.7.4).
— The definition of an object has an alignment specifier and another declaration of that object
has a different alignment specifier (6.7.5).
— Declarations of an object in different translation units have different alignment specifiers
(6.7.5).
— Two pointer types that are required to be compatible are not identically qualified, or are not
pointers to compatible types (6.7.6.1).
— The size expression in an array declaration is not a constant expression and evaluates at
program execution time to a nonpositive value (6.7.6.2).
— In a context requiring two array types to be compatible, they do not have compatible element
types, or their size specifiers evaluate to unequal values (6.7.6.2).
— A declaration of an array parameter includes the keyword static within the [ and ] and the
corresponding argument does not provide access to the first element of an array with at least
the specified number of elements (6.7.6.3).
— A storage-class specifier or type qualifier modifies the keyword void as a function parameter
type list (6.7.6.3).
— In a context requiring two function types to be compatible, they do not have compatible return
types, or their parameters disagree in use of the ellipsis terminator or the number and type of
parameters (after default argument promotion, when there is no parameter type list) (6.7.6.3).
— A declaration for which a type is inferred contains a pointer, array, or function declarators
(6.7.9).
— A declaration for which a type is inferred contains no or more than one declarators (6.7.9).
— The value of an unnamed member of a structure or union is used (6.7.10).
— The initializer for a scalar is neither a single expression nor a single expression enclosed in
braces (6.7.10).
— The initializer for a structure or union object that has automatic storage duration is neither an
initializer list nor a single expression that has compatible structure or union type (6.7.10).
— The initializer for an aggregate or union, other than an array initialized by a string literal, is
not a brace-enclosed list of initializers for its elements or members (6.7.10).
— A function definition that does not have the asserted property is called by a function decla-
ration or a function pointer with a type that has the unsequenced or reproducible attribute
(6.7.12.7).
— An identifier with external linkage is used, but in the program there does not exist exactly
one external definition for the identifier, or the identifier is not used and there exist multiple
external definitions for the identifier (6.9).
— A function that accepts a variable number of arguments is defined without a parameter type
list that ends with the ellipsis notation (6.9.1).
— The } that terminates a function is reached, and the value of the function call is used by the
caller (6.9.1).
— An identifier for an object with internal linkage and an incomplete type is declared with a
tentative definition (6.9.2).
— A non-directive preprocessing directive is executed (6.10).
— The token defined is generated during the expansion of a #if or #elif preprocessing direc-
tive, or the use of the defined unary operator does not match one of the two specified forms
prior to macro replacement (6.10.1).
— The #include preprocessing directive that results after expansion does not match one of the
two header name forms (6.10.2).
— The character sequence in an #include preprocessing directive does not start with a letter
(6.10.2).
— There are sequences of preprocessing tokens within the list of macro arguments that would
otherwise act as preprocessing directives (6.10.4).
— The result of the preprocessing operator # is not a valid character string literal (6.10.4.2).
— The result of the preprocessing operator ## is not a valid preprocessing token (6.10.4.3).
— The #line preprocessing directive that results after expansion does not match one of the two
well-defined forms, or its digit sequence specifies zero or a number greater than 2147483647
(6.10.5).
— A non-STDC #pragma preprocessing directive that is documented as causing translation failure
or some other form of undefined behavior is encountered (6.10.7).
— A #pragma STDC preprocessing directive does not match one of the well-defined forms (6.10.7).
— The name of a predefined macro, or the identifier defined, is the subject of a #define or
#undef preprocessing directive (6.10.9).
— An attempt is made to copy an object to an overlapping object by use of a library function,
other than as explicitly allowed (e.g., memmove) (Clause 7).
— A file with the same name as one of the standard headers, not provided as part of the implemen-
tation, is placed in any of the standard places that are searched for included source files (7.1.2).
— A header is included within an external declaration or definition (7.1.2).
— A function, object, type, or macro that is specified as being declared or defined by some
standard header is used before any header that declares or defines it is included (7.1.2).
— A standard header is included while a macro is defined with the same name as a keyword
(7.1.2).
— The program attempts to declare a library function itself, rather than via a standard header,
but the declaration does not have external linkage (7.1.2).
— The program declares or defines a reserved identifier, other than as allowed by 7.1.4 (7.1.3).
— The program removes the definition of a macro whose name begins with an underscore and
either an uppercase letter or another underscore (7.1.3).
— An argument to a library function has an invalid value or a type not expected by a function
with a variable number of arguments (7.1.4).
— The pointer passed to a library function array parameter does not have a value such that all
address computations and object accesses are valid (7.1.4).
— The macro definition of assert is suppressed in order to access an actual function (7.2).
— The argument to the assert macro does not have a scalar type (7.2).
— The CX_LIMITED_RANGE, FENV_ACCESS, or FP_CONTRACT pragma is used in any context other
than outside all external declarations or preceding all explicit declarations and statements
inside a compound statement (7.3.4, 7.6.1, 7.12.2).
— The value of an argument to a character handling function is neither equal to the value of EOF
nor representable as an unsigned char (7.4).
— A macro definition of errno is suppressed in order to access an actual object, or the program
defines an identifier with the name errno (7.5).
— Part of the program tests floating-point status flags, sets floating-point control modes, or
runs under non-default mode settings, but was translated with the state for the FENV_ACCESS
pragma "off" (7.6.1).
— The exception-mask argument for one of the functions that provide access to the floating-point
status flags has a nonzero value not obtained by bitwise OR of the floating-point exception
macros (7.6.4).
— The fesetexceptflag function is used to set floating-point status flags that were not specified
in the call to the fegetexceptflag function that provided the value of the corresponding
fexcept_t object (7.6.4.5).
— The argument to fesetenv or feupdateenv is neither an object set by a call to fegetenv or
feholdexcept, nor is it an environment macro (7.6.6.3, 7.6.6.4).
— The value of the result of an integer arithmetic or conversion function cannot be represented
(7.8.2.1, 7.8.2.2, 7.8.2.3, 7.8.2.4, 7.24.6.1, 7.24.6.2, 7.24.1).
— The program modifies the string pointed to by the value returned by the setlocale function
(7.11.1.1).
— A pointer returned by the setlocale function is used after a subsequent call to the function,
or after the calling thread has exited (7.11.1.1).
— The program modifies the structure pointed to by the value returned by the localeconv
function (7.11.2.1).
— A macro definition of math_errhandling is suppressed or the program defines an identifier
with the name math_errhandling (7.12).
— An argument to a floating-point classification or comparison macro is not of real floating type
(7.12.3, 7.12.17).
— A macro definition of setjmp is suppressed in order to access an actual function, or the
program defines an external identifier with the name setjmp (7.13).
— An invocation of the setjmp macro occurs other than in an allowed context (7.13.2.1).
— The longjmp function is invoked to restore a nonexistent environment (7.13.2.1).
— After a longjmp, there is an attempt to access the value of an object of automatic storage dura-
tion that does not have volatile-qualified type, local to the function containing the invocation
of the corresponding setjmp macro, that was changed between the setjmp invocation and
longjmp call (7.13.2.1).
— The program specifies an invalid pointer to a signal handler function (7.14.1.1).
— A signal handler returns when the signal corresponded to a computational exception (7.14.1.1).
— A signal handler called in response to SIGFPE, SIGILL, SIGSEGV, or any other implementation-
defined value corresponding to a computational exception returns (7.14.1.1).
— A signal occurs as the result of calling the abort or raise function, and the signal handler
calls the raise function (7.14.1.1).
— A signal occurs other than as the result of calling the abort or raise function, and the signal
handler refers to an object with static or thread storage duration that is not a lock-free atomic
object other than by assigning a value to an object declared as volatile sig_atomic_t, or
calls any function in the standard library other than the abort function, the _Exit function,
the quick_exit function, the functions in <stdatomic.h> (except where explicitly stated
otherwise) when the atomic arguments are lock-free, the atomic_is_lock_free function with
any atomic argument, or the signal function (for the same signal number) (7.14.1.1).
— The value of errno is referred to after a signal occurred other than as the result of calling the
abort or raise function and the corresponding signal handler obtained a SIG_ERR return
from a call to the signal function (7.14.1.1).
— A signal is generated by an asynchronous signal handler (7.14.1.1).
— The signal function is used in a multi-threaded program (7.14.1.1).
— A function with a variable number of arguments attempts to access its varying arguments
other than through a properly declared and initialized va_list object, or before the va_start
macro is invoked (7.16, 7.16.1.1, 7.16.1.4).
— The macro va_arg is invoked using the parameter ap that was passed to a function that
invoked the macro va_arg with the same parameter (7.16).
— A macro definition of va_start, va_arg, va_copy, or va_end is suppressed in order to access
an actual function, or the program defines an external identifier with the name va_copy or
va_end (7.16.1).
— The va_start or va_copy macro is invoked without a corresponding invocation of the va_end
macro in the same function, or vice versa (7.16.1, 7.16.1.2, 7.16.1.3, 7.16.1.4).
— The type parameter to the va_arg macro is not such that a pointer to an object of that type can
be obtained simply by postfixing a * (7.16.1.1).
— The va_arg macro is invoked when there is no actual next argument, or with a specified
type that is not compatible with the promoted type of the actual next argument, with certain
exceptions (7.16.1.1).
— Using a null pointer constant in form of an integer expression as an argument to a ... function
and then interpreting it as a void* or char* (7.16.1.1).
— The va_copy or va_start macro is called to initialize a va_list that was previously initialized
by either macro without an intervening invocation of the va_end macro for the same va_list
(7.16.1.2, 7.16.1.4).
— The macro definition of a generic function is suppressed in order to access an actual function
(7.17.1, 7.18).
— The type parameter of an offsetof macro defines a new type (7.21).
— When program execution reaches an unreachable() macro call (7.21.1).
— Arbitrarily copying or changing the bytes of or copying from a non-null pointer into a
nullptr_t object and then reading that object (7.21.2).
— The member-designator parameter of an offsetof macro is an invalid right operand of the .
operator for the type parameter, or designates a bit-field (7.21).
— The argument in an instance of one of the integer-constant macros is not a decimal, octal, or
hexadecimal constant, or it has a value that exceeds the limits for the corresponding type
(7.22.4).
— A byte input/output function is applied to a wide-oriented stream, or a wide character
input/output function is applied to a byte-oriented stream (7.23.2).
— Use is made of any portion of a file beyond the most recent wide character written to a
wide-oriented stream (7.23.2).
— The value of a pointer to a FILE object is used after the associated file is closed (7.23.3).
— The stream for the fflush function points to an input stream or to an update stream in which
the most recent operation was input (7.23.5.2).
— The string pointed to by the mode argument in a call to the fopen function does not exactly
match one of the specified character sequences (7.23.5.3).
— An output operation on an update stream is followed by an input operation without an
intervening call to the fflush function or a file positioning function, or an input operation
on an update stream is followed by an output operation with an intervening call to a file
positioning function (7.23.5.3).
— An attempt is made to use the contents of the array that was supplied in a call to the setvbuf
function (7.23.5.6).
— There are insufficient arguments for the format in a call to one of the formatted input/output
functions, or an argument does not have an appropriate type (7.23.6.1, 7.23.6.2, 7.31.2.1,
and 7.31.2.2).
— The format in a call to one of the formatted input/output functions or to the strftime or
wcsftime function is not a valid multibyte character sequence that begins and ends in its
initial shift state (7.23.6.1, 7.23.6.2, 7.29.3.5, 7.31.2.1, 7.31.2.2, 7.31.5.1).
— In a call to one of the formatted output functions, a precision appears with a conversion
specifier other than those described (7.23.6.1, 7.31.2.1).
— A conversion specification for a formatted output function uses an asterisk to denote an
argument-supplied field width or precision, but the corresponding argument is not provided
(7.23.6.1, 7.31.2.1).
— A conversion specification for a formatted output function uses a # or 0 flag with a conversion
specifier other than those described (7.23.6.1, 7.31.2.1).
— A conversion specification for one of the formatted input/output functions uses a length
modifier with a conversion specifier other than those described (7.23.6.1, 7.23.6.2, 7.31.2.1,
and 7.31.2.2).
— An s conversion specifier is encountered by one of the formatted output functions, and the
argument is missing the null terminator (unless a precision is specified that does not require
null termination) (7.23.6.1, 7.31.2.1).
— An n conversion specification for one of the formatted input/output functions includes any
flags, an assignment-suppressing character, a field width, or a precision (7.23.6.1, 7.23.6.2,
and 7.31.2.1, 7.31.2.2).
— A % conversion specifier is encountered by one of the formatted input/output functions, but
the complete conversion specification is not exactly %% (7.23.6.1, 7.23.6.2, 7.31.2.1, 7.31.2.2).
— An invalid conversion specification is found in the format for one of the formatted input/out-
put functions, or the strftime or wcsftime function (7.23.6.1, 7.23.6.2, 7.29.3.5, 7.31.2.1,
and 7.31.2.2, 7.31.5.1).
— The number of characters or wide characters transmitted by a formatted output function (or
written to an array, or that would have been written to an array) is greater than INT_MAX
(7.23.6.1, 7.31.2.1).
— The number of input items assigned by a formatted input function is greater than INT_MAX
(7.23.6.2, 7.31.2.2).
— The result of a conversion by one of the formatted input functions cannot be represented in
the corresponding object, or the receiving object does not have an appropriate type (7.23.6.2,
and 7.31.2.2).
— A c, s, or [ conversion specifier is encountered by one of the formatted input functions, and
the array pointed to by the corresponding argument is not large enough to accept the input
sequence (and a null terminator if the conversion specifier is s or [) (7.23.6.2, 7.31.2.2).
— A c, s, or [ conversion specifier with an l qualifier is encountered by one of the formatted
input functions, but the input is not a valid multibyte character sequence that begins in the
initial shift state (7.23.6.2, 7.31.2.2).
— The input item for a %p conversion by one of the formatted input functions is not a value
converted earlier during the same program execution (7.23.6.2, 7.31.2.2).
— The vfprintf, vfscanf, vprintf, vscanf, vsnprintf, vsprintf, vsscanf, vfwprintf,
vfwscanf , vswprintf , vswscanf , vwprintf , or vwscanf function is called with an improperly
initialized va_list argument, or the argument is used (other than in an invocation of va_end)
after the function returns (7.23.6.8, 7.23.6.9, 7.23.6.10, 7.23.6.11, 7.23.6.12, 7.23.6.13, 7.23.6.14,
and 7.31.2.5, 7.31.2.6, 7.31.2.7, 7.31.2.8, 7.31.2.9, 7.31.2.10).
— The contents of the array supplied in a call to the fgets or fgetws function are used after a
read error occurred (7.23.7.2, 7.31.3.2).
— The file position indicator for a binary stream is used after a call to the ungetc function where
its value was zero before the call (7.23.7.10).
— The file position indicator for a stream is used after an error occurred during a call to the
fread or fwrite function (7.23.8.1, 7.23.8.2).
— A partial element read by a call to the fread function is used (7.23.8.1).
— The fseek function is called for a text stream with a nonzero offset and either the offset was
not returned by a previous successful call to the ftell function on a stream associated with
the same file or whence is not SEEK_SET (7.23.9.2).
— The fsetpos function is called to set a position that was not returned by a previous successful
call to the fgetpos function on a stream associated with the same file (7.23.9.3).
— A non-null pointer returned by a call to the calloc, malloc, realloc, or aligned_alloc
function with a zero requested size is used to access an object (7.24.3).
— The value of a pointer that refers to space deallocated by a call to the free or realloc function
is used (7.24.3).
— The pointer argument to the free or realloc function does not match a pointer earlier
returned by a memory management function, or the space has been deallocated by a call to
free or realloc (7.24.3.3, 7.24.3.7).
— The value of the object allocated by the malloc function is used (7.24.3.6).
— The values of any bytes in a new object allocated by the realloc function beyond the size of
the old object are used (7.24.3.7).
— The program calls the exit or quick_exit function more than once, or calls both functions
(7.24.4.4, 7.24.4.7).
— During the call to a function registered with the atexit or at_quick_exit function, a call is
made to the longjmp function that would terminate the call to the registered function (7.24.4.4,
and 7.24.4.7).
— The string set up by the getenv or strerror function is modified by the program (7.24.4.6,
and 7.26.6.3).
— A signal is raised while the quick_exit function is executing (7.24.4.7).
— A command is executed through the system function in a way that is documented as causing
termination or some other form of undefined behavior (7.24.4.8).
— A searching or sorting utility function is called with an invalid pointer argument, even if the
number of elements is zero (7.24.5).
— The comparison function called by a searching or sorting utility function alters the contents of
the array being searched or sorted, or returns ordering values inconsistently (7.24.5).
— The array being searched by the bsearch function does not have its elements in proper order
(7.24.5.1).
— The current conversion state is used by a multibyte/wide character conversion function after
changing the LC_CTYPE category (7.24.7).
— A string or wide string utility function is instructed to access an array beyond the end of an
object (7.26.1, 7.31.4).
— A string or wide string utility function is called with an invalid pointer argument, even if the
length is zero (7.26.1, 7.31.4).
— The contents of the destination array are used after a call to the strxfrm, strftime, wcsxfrm,
or wcsftime function in which the specified length was too small to hold the entire null-
terminated result (7.26.4.5, 7.29.3.5, 7.31.4.4.4, 7.31.5.1).
— A sequence of calls of the strtok function is made from different threads (7.26.5.9).
— The first argument in the very first call to the strtok or wcstok is a null pointer (7.26.5.9,
and 7.31.4.6.7).
— A pointer returned by the strerror function is used after a subsequent call to the function, or
after the calling thread has exited (7.26.6.3).
— The type of an argument to a type-generic macro is not compatible with the type of the
corresponding parameter of the selected function (7.27).
— Arguments for generic parameters of a type-generic macro are such that some argument has a
corresponding real type that is of standard floating type and another argument is of decimal
floating type (7.27).
— Arguments for generic parameters of a type-generic macro are such that neither <math.h> and
<complex.h> define a function whose generic parameters have the determined corresponding
real type (7.27).
— A complex argument is supplied for a generic parameter of a type-generic macro that has no
corresponding complex function (7.27).
— A decimal floating argument is supplied for a generic parameter of a type-generic macro that
expects a complex argument (7.27).
— A standard floating or complex argument is supplied for a generic parameter of a type-generic
macro that expects a decimal floating type argument (7.27).
— A non-recursive mutex passed to mtx_lock is locked by the calling thread (7.28.4.3).
— The mutex passed to mtx_timedlock does not support timeout (7.28.4.4).
— The mutex passed to mtx_unlock is not locked by the calling thread (7.28.4.6).
— The thread passed to thrd_detach or thrd_join was previously detached or joined with
another thread (7.28.5.3, 7.28.5.6).
— The tss_create function is called from within a destructor (7.28.6.1).
— The key passed to tss_delete, tss_get, or tss_set was not returned by a call to tss_create
before the thread commenced executing destructors (7.28.6.2, 7.28.6.3, 7.28.6.4).
— An attempt is made to access the pointer returned by the time conversion functions after the
thread that originally called the function to obtain it has exited (7.29.3).
— At least one member of the broken-down time passed to asctime contains a value outside its
normal range, or the calculated year exceeds four digits or is less than the year 1000 (7.29.3.1).
— The argument corresponding to an s specifier without an l qualifier in a call to the fwprintf
function does not point to a valid multibyte character sequence that begins in the initial shift
state (7.31.2.11).
— In a call to the wcstok function, the object pointed to by ptr does not have the value stored by
the previous call for the same wide string (7.31.4.6.7).
— An mbstate_t object is used inappropriately (7.31.6).
— The value of an argument of type wint_t to a wide character classification or case mapping
function is neither equal to the value of WEOF nor representable as a wchar_t (7.32.1).
— The iswctype function is called using a different LC_CTYPE category from the one in effect for
the call to the wctype function that returned the description (7.32.2.2.1).
— The towctrans function is called using a different LC_CTYPE category from the one in effect
for the call to the wctrans function that returned the description (7.32.3.2.1).
J.3 Implementation-defined behavior
1 A conforming implementation is required to document its choice of behavior in each of the areas
listed in this subclause. The following are implementation-defined:
J.3.1 Translation
1 — How a diagnostic is identified (3.10, 5.1.1.3).
— Whether each nonempty sequence of white-space characters other than new-line is retained or
replaced by one space character in translation phase 3 (5.1.1.2).
J.3.2 Environment
1 — The mapping between physical source file multibyte characters and the source character set in
translation phase 1 (5.1.1.2).
— The name and type of the function called at program startup in a freestanding environment
(5.1.2.1).
— The effect of program termination in a freestanding environment (5.1.2.1).
— An alternative manner in which the main function may be defined (5.1.2.2.1).
— The values given to the strings pointed to by the argv argument to main (5.1.2.2.1).
— What constitutes an interactive device (5.1.2.3).
— Whether a program can have more than one thread of execution in a freestanding environment
(5.1.2.4).
— The set of signals, their semantics, and their default handling (7.14).
— Signal values other than SIGFPE, SIGILL, and SIGSEGV that correspond to a computational
exception (7.14.1.1).
— Signals for which the equivalent of signal(sig, SIG_IGN); is executed at program startup
(7.14.1.1).
— The set of environment names and the method for altering the environment list used by the
getenv function (7.24.4.6).
— The manner of execution of the string by the system function (7.24.4.8).
J.3.3 Identifiers
1 — Which additional multibyte characters may appear in identifiers and their correspondence to
universal character names (6.4.2).
— The number of significant initial characters in an identifier (5.2.4.1, 6.4.2).
J.3.4 Characters
1 — The number of bits in a byte (3.6).
— The values of the members of the execution character set (5.2.1).
— The unique value of the member of the execution character set produced for each of the
standard alphabetic escape sequences (5.2.2).
— The value of a char object into which has been stored any character other than a member of
the basic execution character set (6.2.5).
— Which of signed char or unsigned char has the same range, representation, and behavior
as "plain" char (6.2.5, 6.3.1.1).
— The literal encoding, which maps of the characters of the execution character set to the values
in a character constant or string literal (6.2.9, 6.4.4.4).
— The wide literal encoding, of the characters of the execution character set to the values in a
wchar_t character constant or wchar_t string literal (6.2.9, 6.4.4.4).
— The mapping of members of the source character set (in character constants and string literals)
to members of the execution character set (6.4.4.4, 5.1.1.2).
— The value of an integer character constant containing more than one character or containing a
character or escape sequence that does not map to a single-byte execution character (6.4.4.4).
— The value of a wide character constant containing more than one multibyte character or a
single multibyte character that maps to multiple members of the extended execution character
set, or containing a multibyte character or escape sequence not represented in the extended
execution character set (6.4.4.4).
— The current locale used to convert a wide character constant consisting of a single multibyte
character that maps to a member of the extended execution character set into a corresponding
wide character code (6.4.4.4).
— The current locale used to convert a wide string literal into corresponding wide character
codes (6.4.5).
— The value of a string literal containing a multibyte character or escape sequence not represented
in the execution character set (6.4.5).
— The encoding of any of wchar_t, char16_t, and char32_t where the corresponding stan-
dard encoding macro (__STDC_ISO_10646__ , __STDC_UTF_16__ , or __STDC_UTF_32__ ) is not
defined (6.10.9.2).
J.3.5 Integers
1 — Any extended integer types that exist in the implementation (6.2.5).
— The rank of any extended integer type relative to another extended integer type with the same
precision (6.3.1.1).
— The result of, or the signal raised by, converting an integer to a signed integer type when the
value cannot be represented in an object of that type (6.3.1.3).
— The results of some bitwise operations on signed integers (6.5).
J.3.6 Floating-point
1 — The accuracy of the floating-point operations and of the library functions in <math.h> and
<complex.h> that return floating-point results (5.2.4.2.2).
— The accuracy of the conversions between floating-point internal representations and string
representations performed by the library functions in <stdio.h>, <stdlib.h>, and <wchar.h>
(5.2.4.2.2).
— The rounding behaviors characterized by non-standard values of FLT_ROUNDS (5.2.4.2.2).
— The evaluation methods characterized by non-standard negative values of FLT_EVAL_METHOD
(5.2.4.2.2).
— The evaluation methods characterized by non-standard negative values of DEC_EVAL_METHOD
(5.2.4.2.3).
— If decimal floating types are supported (6.2.5).
— The direction of rounding when an integer is converted to a floating-point number that cannot
exactly represent the original value (6.3.1.4).
— The direction of rounding when a floating-point number is converted to a narrower floating-
point number (6.3.1.5).
— How the nearest representable value or the larger or smaller representable value immediately
adjacent to the nearest representable value is chosen for certain floating constants (6.4.4.2).
— Whether and how floating expressions are contracted when not disallowed by the
FP_CONTRACT pragma (6.5).
— The default state for the FENV_ACCESS pragma (7.6.1).
— Additional floating-point exceptions, rounding modes, environments, and classifications, and
their macro names (7.6, 7.12).
— The default state for the FP_CONTRACT pragma (7.12.2).
J.3.7 Arrays and pointers
1 — The result of converting a pointer to an integer or vice versa (6.3.2.3).
— The size of the result of subtracting two pointers to elements of the same array (6.5.6).
J.3.8 Hints
1 — The extent to which suggestions made by using the register storage-class specifier are
effective (6.7.1).
— The extent to which suggestions made by using the inline function specifier are effective
(6.7.4).
J.3.9 Structures, unions, enumerations, and bit-fields
1 — Whether a "plain" int bit-field is treated as a signed int bit-field or as an unsigned int
bit-field (6.7.2, 6.7.2.1).
— Allowable bit-field types other than bool, signed int, unsigned int, and bit-precise integer
types (6.7.2.1).
— Whether atomic types are permitted for bit-fields (6.7.2.1).
— Whether a bit-field can straddle a storage-unit boundary (6.7.2.1).
— The order of allocation of bit-fields within a unit (6.7.2.1).
— The alignment of non-bit-field members of structures (6.7.2.1). This should present no problem
unless binary data written by one implementation is read by another.
— The integer type compatible with each enumerated type (6.7.2.2).
J.3.10 Qualifiers
1 — What constitutes an access to an object that has volatile-qualified type (6.7.3).
J.3.11 Preprocessing directives
1 — The locations within #pragma directives where header name preprocessing tokens are recog-
nized (6.4, 6.4.7).
— How sequences in both forms of header names are mapped to headers or external source file
names (6.4.7).
— Whether the value of a character constant in a constant expression that controls conditional
inclusion matches the value of the same character constant in the execution character set
(6.10.1).
— Whether the value of a single-character character constant in a constant expression that controls
conditional inclusion may have a negative value (6.10.1).
— The places that are searched for an included < > delimited header, and how the places are
specified or the header is identified (6.10.2).
— How the named source file is searched for in an included " " delimited header (6.10.2).
— The method by which preprocessing tokens (possibly resulting from macro expansion) in a
#include directive are combined into a header name (6.10.2).
— The nesting limit for #include processing (6.10.2).
— Whether the # operator inserts a \ character before the \ character that begins a universal
character name in a character constant or string literal (6.10.4.2).
— The behavior on each recognized non-STDC #pragma directive (6.10.7).
— The definitions for __DATE__ and __TIME__ when respectively, the date and time of translation
are not available (6.10.9.1).
J.3.12 Library functions
1 — Any library facilities available to a freestanding program, other than the minimal set required
by Clause 4 (5.1.2.1).
— The format of the diagnostic printed by the assert macro (7.2.1.1).
— The representation of the floating-point status flags stored by the fegetexceptflag function
(7.6.4.2).
— Whether the feraiseexcept function raises the "inexact" floating-point exception in addition
to the "overflow" or "underflow" floating-point exception (7.6.4.3).
— Strings other than "C" and "" that may be passed as the second argument to the setlocale
function (7.11.1.1).
— The types defined for float_t and double_t when the value of the FLT_EVAL_METHOD macro
is less than 0 (7.12).
— The types defined for _Decimal32_t and _Decimal64_t when the value of the
DEC_EVAL_METHOD macro is less than 0 (7.12).
— Domain errors for the mathematics functions, other than those required by this document
(7.12.1).
— The values returned by the mathematics functions on domain errors or pole errors (7.12.1).
— The values returned by the mathematics functions on underflow range errors, whether errno
is set to the value of the macro ERANGE when the integer expression math_errhandling &
MATH_ERRNO is nonzero, and whether the "underflow" floating-point exception is raised when
the integer expression math_errhandling & MATH_ERREXCEPT is nonzero. (7.12.1).
— Whether a domain error occurs or zero is returned when an fmod function has a second
argument of zero (7.12.10.1).
— Whether a domain error occurs or zero is returned when a remainder function has a second
argument of zero (7.12.10.2).
— The base-2 logarithm of the modulus used by the remquo functions in reducing the quotient
(7.12.10.3).
— The byte order of decimal floating type encodings (7.12.16).
— Whether a domain error occurs or zero is returned when a remquo function has a second
argument of zero (7.12.10.3).
— Whether the equivalent of signal(sig, SIG_DFL); is executed prior to the call of a signal
handler, and, if not, the blocking of signals that is performed (7.14.1.1).
— The value of __STDC_ENDIAN_NATIVE__ if the execution environment is not big-endian or
little-endian (7.18.2)
— The null pointer constant to which the macro NULL expands (7.21).
— Whether the last line of a text stream requires a terminating new-line character (7.23.2).
— Whether space characters that are written out to a text stream immediately before a new-line
character appear when read in (7.23.2).
— The number of null characters that may be appended to data written to a binary stream (7.23.2).
— Whether the file position indicator of an append-mode stream is initially positioned at the
beginning or end of the file (7.23.3).
— Whether a write on a text stream causes the associated file to be truncated beyond that point
(7.23.3).
— The characteristics of file buffering (7.23.3).
— Whether a zero-length file actually exists (7.23.3).
— The rules for composing valid file names (7.23.3).
— Whether the same file can be simultaneously open multiple times (7.23.3).
— The nature and choice of encodings used for multibyte characters in files (7.23.3).
— The effect of the remove function on an open file (7.23.4.1).
— The effect if a file with the new name exists prior to a call to the rename function (7.23.4.2).
— Whether an open temporary file is removed upon abnormal program termination (7.23.4.3).
— Which changes of mode are permitted (if any), and under what circumstances (7.23.5.4).
— The style used to print an infinity or NaN, and the meaning of any n-char or n-wchar sequence
printed for a NaN (7.23.6.1, 7.31.2.1).
— The output for %p conversion in the fprintf or fwprintf function (7.23.6.1, 7.31.2.1).
— The interpretation of a- character that is neither the first nor the last character, nor the second
where a ^ character is the first, in the scanlist for %[ conversion in the fscanf or fwscanf
function (7.23.6.2, 7.31.2.1).
— The set of sequences matched by a %p conversion and the interpretation of the corresponding
input item in the fscanf or fwscanf function (7.23.6.2, 7.31.2.2).
— The value to which the macro errno is set by the fgetpos, fsetpos, or ftell functions on
failure (7.23.9.1, 7.23.9.3, 7.23.9.4).
— The meaning of any n-char or n-wchar sequence in a string representing a NaN that is
converted by the strtod, strtof, strtold, wcstod, wcstof, or wcstold function (7.24.1.5,
and 7.31.4.1.2).
— Whether or not the strtod, strtof, strtold, wcstod, wcstof, or wcstold function sets
errno to ERANGE when underflow occurs (7.24.1.5, 7.31.4.1.2).
— The meaning of any d-char or d-wchar sequence in a string representing a NaN that is con-
verted by the strtod32, strtod64, strtod128, wcstod32, wcstod64, or wcstod128 function
(7.24.1.6, 7.31.4.1.3).
— Whether or not the strtod32, strtod64, strtod128, wcstod32, wcstod64, or wcstod128
function sets errno to ERANGE when underflow occurs (7.24.1.6, 7.31.4.1.3).
— Whether the calloc, malloc, realloc, and aligned_alloc functions return a null pointer or
a pointer to an allocated object when the size requested is zero (7.24.3).
— Whether open streams with unwritten buffered data are flushed, )open streams are closed, or
temporary files are removed when the abort or _Exit function is called (7.24.4.1, 7.24.4.5).
— The termination status returned to the host environment by the abort, exit, _Exit , or
quick_exit function (7.24.4.1, 7.24.4.4, 7.24.4.5, 7.24.4.7).
— The value returned by the system function when its argument is not a null pointer (7.24.4.8).
— Whether the internal state of multibyte/wide character conversion functions has thread-storage
duration, and its initial value in newly created threads (7.24.7).
— The range and precision of times representable in clock_t and time_t (7.29).
— The local time zone and Daylight Saving Time (7.29.1).
— Whether TIME_MONOTONIC or TIME_ACTIVE are supported time bases (7.29.1).
— Whether TIME_THREAD_ACTIVE is a supported time bases (7.29.1, 7.28.1).
— The local time zone and Daylight Saving Time (7.29.1).
— The era for the clock function (7.29.2.1).
— The TIME_UTC epoch (7.29.2.6).
— The replacement string for the %Z specifier to the strftime, and wcsftime functions in the
"C" locale (7.29.3.5, 7.31.5.1).
— Whether internal mbstate_t objects have thread storage duration (7.30.1, 7.31.6.3, 7.31.6.4).
— Whether the functions in <math.h> honor the rounding direction mode in an IEC 60559
conformant implementation, unless explicitly specified otherwise (F.10).
J.3.13 Architecture
1 — The values or expressions assigned to the macros specified in the headers <float.h>,
<limits.h>, and <stdint.h> (5.2.4.2, 7.22).
— The result of attempting to indirectly access an object with automatic or thread storage duration
from a thread other than the one with which it is associated (6.2.4).
— The number, order, and encoding of bytes in any object (when not explicitly specified in this
document) (6.2.6.1).
— Whether any extended alignments are supported and the contexts in which they are supported
(6.2.8).
— Valid alignment values other than those returned by an alignof expression for fundamental
types, if any (6.2.8).
— The value of the result of the sizeof and alignof operators (6.5.3.4).
J.4 Locale-specific behavior
1 The following characteristics of a hosted environment are locale-specific and are required to be
documented by the implementation:
— Additional members of the source and execution character sets beyond the basic character set
(5.2.1).
— The presence, meaning, and representation of additional multibyte characters in the execution
character set beyond the basic character set (5.2.1.1).
— The shift states used for the encoding of multibyte characters (5.2.1.1).
— The direction of writing of successive printing characters (5.2.2).
— The decimal-point character (7.1.1).
— The set of printing characters (7.4, 7.32.2).
— The set of control characters (7.4, 7.32.2).
— The sets of characters tested for by the isalpha, isblank, islower, ispunct, isspace,
isupper, iswalpha, iswblank, iswlower, iswpunct, iswspace, or iswupper functions
(7.4.1.2, 7.4.1.3, 7.4.1.7, 7.4.1.9, 7.4.1.10, 7.4.1.11, 7.32.2.1.2, 7.32.2.1.3, 7.32.2.1.7, 7.32.2.1.9,
7.32.2.1.10, 7.32.2.1.11).
— The native environment (7.11.1.1).
— Additional subject sequences accepted by the numeric conversion functions (7.24.1, 7.31.4.1).
— The collation sequence of the execution character set (7.26.4.3, 7.31.4.4.2).
— The contents of the error message strings set up by the strerror function (7.26.6.3).
— The formats for time and date (7.29.3.5, 7.31.5.1).
— Character mappings that are supported by the towctrans function (7.32.1).
— Character classifications that are supported by the iswctype function (7.32.1).
J.5 Common extensions
1 The following extensions are widely used in many systems, but are not portable to all implemen-
tations. The inclusion of any extension that may cause a strictly conforming program to become
invalid renders an implementation nonconforming. Examples of such extensions are new keywords,
extra library functions declared in standard headers, or predefined macros with names that do not
begin with an underscore.
J.5.1 Environment arguments
1 In a hosted environment, the main function receives a third argument, char *envp[], that points to
a null-terminated array of pointers to char, each of which points to a string that provides information
about the environment for this execution of the program (5.1.2.2.1).
J.5.2 Specialized identifiers
1 Characters other than the underscore _ , letters, and digits, that are not part of the basic source
character set (such as the dollar sign $, or characters in national character sets) may appear in an
identifier (6.4.2).
J.5.3 Lengths and cases of identifiers
1 All characters in identifiers (with or without external linkage) are significant (6.4.2).
J.5.4 Scopes of identifiers
1 A function identifier, or the identifier of an object the declaration of which contains the keyword
extern, has file scope (6.2.1).
J.5.5 Writable string literals
1 String literals are modifiable (in which case, identical string literals should denote distinct objects)
(6.4.5).
J.5.6 Other arithmetic types
1 Additional arithmetic types, such as __int128 or double double, and their appropriate conver-
sions are defined (6.2.5, 6.3.1). Additional floating types may have more range or precision than
long double, may be used for evaluating expressions of other floating types, and may be used to
define float_t or double_t. Additional floating types may also have less range or precision than
float.
J.5.7 Function pointer casts
1 A pointer to an object or to void may be cast to a pointer to a function, allowing data to be invoked
as a function (6.5.4).
2 A pointer to a function may be cast to a pointer to an object or to void, allowing a function to be
inspected or modified (for example, by a debugger) (6.5.4).
J.5.8 Extended bit-field types
1 A bit-field may be declared with a type other than bool, unsigned int, signed int, or a bit-precise
integer type, with an appropriate maximum width (6.7.2.1).
J.5.9 The fortran keyword
1 The fortran function specifier may be used in a function declaration to indicate that calls suitable
for FORTRAN should be generated, or that a different representation for the external name is to be
generated (6.7.4).
J.5.10 The asm keyword
1 The asm keyword may be used to insert assembly language directly into the translator output (6.8).
The most common implementation is via a statement of the form:
asm (character-string-literal);
J.5.11 Multiple external definitions
1 There may be more than one external definition for the identifier of an object, with or without the
explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the
behavior is undefined (6.9.2).
J.5.12 Predefined macro names
1 Macro names that do not begin with an underscore, describing the translation and execution
environments, are defined by the implementation before translation begins (6.10.9).
J.5.13 Floating-point status flags
1 If any floating-point status flags are set on normal termination after all calls to functions registered
by the atexit function have been made (see 7.24.4.4), the implementation writes some diagnostics
indicating the fact to the stderr stream, if it is still open,
J.5.14 Extra arguments for signal handlers
1 Handlers for specific signals are called with extra arguments in addition to the signal number
(7.14.1.1).
J.5.15 Additional stream types and file-opening modes
1 Additional mappings from files to streams are supported (7.23.2).
2 Additional file-opening modes may be specified by characters appended to the mode argument of
the fopen function (7.23.5.3).
J.5.16 Defined file position indicator
1 The file position indicator is decremented by each successful call to the ungetc or ungetwc function
for a text stream, except if its value was zero before a call (7.23.7.10, 7.31.3.10).
J.5.17 Math error reporting
1 Functions declared in <complex.h> and <math.h> raise SIGFPE to report errors instead of, or in
addition to, setting errno or raising floating-point exceptions (7.3, 7.12).
J.6 Reserved identifiers and keywords
1 A lot of identifier preprocessing tokens are used for specific purposes in regular clauses or appendices
from translation phase 3 onwards. Using any of these for a purpose different from their description
in this document, even if the use is in a context where they are normatively permitted, may have an
impact on the portability of code and should thus be avoided.
J.6.1 Rule based identifiers
1 The following 40 regular expressions characterize identifiers that are systematically reserved by
some clause this document.
ATOMIC_[A-Z][a-zA-Z0-9_]* LC_[A-Z][a-zA-Z0-9_]*
DBL_[A-Z][a-zA-Z0-9_]* LDBL_[A-Z][a-zA-Z0-9_]*
DEC128_[A-Z][a-zA-Z0-9_]* MATH_[A-Z][a-zA-Z0-9_]*
DEC32_[A-Z][a-zA-Z0-9_]* PRI[a-zX][a-zA-Z0-9_]*
DEC64_[A-Z][a-zA-Z0-9_]* SCN[a-zX][a-zA-Z0-9_]*
DEC_[A-Z][a-zA-Z0-9_]* SIG[A-Z][a-zA-Z0-9_]*
E[0-9A-Z][a-zA-Z0-9_]* SIG_[A-Z][a-zA-Z0-9_]*
FE_[A-Z][a-zA-Z0-9_]* TIME_[A-Z][a-zA-Z0-9_]*
FLT_[A-Z][a-zA-Z0-9_]* UINT[a-zA-Z0-9_]*_C
FP_[A-Z][a-zA-Z0-9_]* UINT[a-zA-Z0-9_]*_MAX
INT[a-zA-Z0-9_]*_C UINT[a-zA-Z0-9_]*_WIDTH
INT[a-zA-Z0-9_]*_MAX _[a-zA-Z_][a-zA-Z0-9_]*
INT[a-zA-Z0-9_]*_MIN atomic_[a-z][a-zA-Z0-9_]*
INT[a-zA-Z0-9_]*_WIDTH cnd_[a-z][a-zA-Z0-9_]*
cr_[a-z][a-zA-Z0-9_]* str[a-z][a-zA-Z0-9_]*
int[a-zA-Z0-9_]*_t thrd_[a-z][a-zA-Z0-9_]*
is[a-z][a-zA-Z0-9_]* to[a-z][a-zA-Z0-9_]*
mem[a-z][a-zA-Z0-9_]* tss_[a-z][a-zA-Z0-9_]*
mtx_[a-z][a-zA-Z0-9_]* uint[a-zA-Z0-9_]*_t
stdc_[a-zA-Z0-9_]* wcs[a-z][a-zA-Z0-9_]*
2 The following 794 identifiers or keywords match these patterns and have particular semantics
provided by this document.
atomic_bool atomic_is_lock_free
ATOMIC_BOOL_LOCK_FREE atomic_llong
atomic_char ATOMIC_LLONG_LOCK_FREE
atomic_char16_t atomic_load
ATOMIC_CHAR16_T_LOCK_FREE atomic_load_explicit
atomic_char32_t atomic_long
ATOMIC_CHAR32_T_LOCK_FREE ATOMIC_LONG_LOCK_FREE
atomic_char8_t ATOMIC_POINTER_LOCK_FREE
ATOMIC_CHAR8_T_LOCK_FREE atomic_ptrdiff_t
ATOMIC_CHAR_LOCK_FREE atomic_schar
atomic_compare_exchange_strong atomic_short
atomic_compare_exchange_strong_explicit ATOMIC_SHORT_LOCK_FREE
atomic_compare_exchange_weak atomic_signal_fence
atomic_compare_exchange_weak_explicit atomic_size_t
atomic_exchange atomic_store
atomic_exchange_explicit atomic_store_explicit
atomic_fetch_ atomic_thread_fence
atomic_fetch_add atomic_uchar
atomic_fetch_add_explicit atomic_uint
atomic_fetch_and atomic_uintmax_t
atomic_fetch_and_explicit atomic_uintptr_t
atomic_fetch_or atomic_uint_fast16_t
atomic_fetch_or_explicit atomic_uint_fast32_t
atomic_fetch_sub atomic_uint_fast64_t
atomic_fetch_sub_explicit atomic_uint_fast8_t
atomic_fetch_xor atomic_uint_least16_t
atomic_fetch_xor_explicit atomic_uint_least32_t
atomic_flag atomic_uint_least64_t
atomic_flag_clear atomic_uint_least8_t
atomic_flag_clear_explicit atomic_ullong
ATOMIC_FLAG_INIT atomic_ulong
atomic_flag_test_and_set atomic_ushort
atomic_flag_test_and_set_explicit ATOMIC_VAR_INIT
atomic_init atomic_wchar_t
atomic_int ATOMIC_WCHAR_T_LOCK_FREE
atomic_intmax_t cnd_broadcast
atomic_intptr_t cnd_destroy
atomic_int_fast16_t cnd_init
atomic_int_fast32_t cnd_signal
atomic_int_fast64_t cnd_t
atomic_int_fast8_t cnd_timedwait
atomic_int_least16_t cnd_wait
atomic_int_least32_t DBL_DECIMAL_DIG
atomic_int_least64_t DBL_DIG
atomic_int_least8_t DBL_EPSILON
ATOMIC_INT_LOCK_FREE DBL_HAS_SUBNORM
DBL_IS_IEC_60559 FE_DYNAMIC
DBL_MANT_DIG FE_INEXACT
DBL_MAX FE_INVALID
DBL_MAX_10_EXP FE_OVERFLOW
DBL_MAX_EXP FE_SNANS_ALWAYS_SIGNAL
DBL_MIN FE_TONEAREST
DBL_MIN_10_EXP FE_TONEARESTFROMZERO
DBL_MIN_EXP FE_TOWARDZERO
DBL_NORM_MAX FE_UNDERFLOW
DBL_SNAN FE_UPWARD
DBL_TRUE_MIN FLT_DECIMAL_DIG
DEC128_EPSILON FLT_DIG
DEC128_MANT_DIG FLT_EPSILON
DEC128_MAX FLT_EVAL_METHOD
DEC128_MAX_EXP FLT_HAS_SUBNORM
DEC128_MIN FLT_IS_IEC_60559
DEC128_MIN_EXP FLT_MANT_DIG
DEC128_SNAN FLT_MAX
DEC128_TRUE_MIN FLT_MAX_10_EXP
DEC32_EPSILON FLT_MAX_EXP
DEC32_MANT_DIG FLT_MIN
DEC32_MAX FLT_MIN_10_EXP
DEC32_MAX_EXP FLT_MIN_EXP
DEC32_MIN FLT_NORM_MAX
DEC32_MIN_EXP FLT_RADIX
DEC32_SNAN FLT_ROUNDS
DEC32_TRUE_MIN FLT_SNAN
DEC64_EPSILON FLT_TRUE_MIN
DEC64_MANT_DIG FP_CONTRACT
DEC64_MAX FP_FAST_D
DEC64_MAX_EXP FP_FAST_D32ADDD128
DEC64_MIN FP_FAST_D32ADDD64
DEC64_MIN_EXP FP_FAST_D32DIVD128
DEC64_SNAN FP_FAST_D32DIVD64
DEC64_TRUE_MIN FP_FAST_D32FMAD128
DEC_EVAL_METHOD FP_FAST_D32FMAD64
DEC_INFINITY FP_FAST_D32MULD128
DEC_NAN FP_FAST_D32MULD64
EDOM FP_FAST_D32SQRTD128
EILSEQ FP_FAST_D32SQRTD64
EOF FP_FAST_D32SUBD128
EOL FP_FAST_D32SUBD64
ERANGE FP_FAST_D64ADDD128
EXIT_FAILURE FP_FAST_D64DIVD128
EXIT_SUCCESS FP_FAST_D64FMAD128
FE_ALL_EXCEPT FP_FAST_D64MULD128
FE_DEC_DOWNWARD FP_FAST_D64SQRTD128
FE_DEC_DYNAMIC FP_FAST_D64SUBD128
FE_DEC_TONEAREST FP_FAST_DADDL
FE_DEC_TONEARESTFROMZERO FP_FAST_DDIVL
FE_DEC_TOWARDZERO FP_FAST_DFMAL
FE_DEC_UPWARD FP_FAST_DMULL
FE_DFL_ENV FP_FAST_DSQRTL
FE_DFL_MODE FP_FAST_DSUBL
FE_DIVBYZERO FP_FAST_F
FE_DOWNWARD FP_FAST_FADD
FP_FAST_FADDL INTMAX_WIDTH
FP_FAST_FDIV INTPTR_MAX
FP_FAST_FDIVL INTPTR_MIN
FP_FAST_FFMA intptr_t
FP_FAST_FFMAL INTPTR_WIDTH
FP_FAST_FMA int_fast16_t
FP_FAST_FMAD int_fast32_t
FP_FAST_FMAD128 int_fast64_t
FP_FAST_FMAD32 int_fast8_t
FP_FAST_FMAD64 int_least16_t
FP_FAST_FMAF int_least32_t
FP_FAST_FMAL int_least64_t
FP_FAST_FMUL int_least8_t
FP_FAST_FMULL INT_MAX
FP_FAST_FSQRT INT_MIN
FP_FAST_FSQRTL INT_WIDTH
FP_FAST_FSUB isalnum
FP_FAST_FSUBL isalpha
FP_ILOGB0 isblank
FP_ILOGBNAN iscanonical
FP_INFINITE iscntrl
FP_INT_DOWNWARD isdigit
FP_INT_TONEAREST iseqsig
FP_INT_TONEARESTFROMZERO isfinite
FP_INT_TOWARDZERO isgraph
FP_INT_UPWARD isgreater
FP_LLOGB0 isgreaterequal
FP_LLOGBNAN isinf
FP_NAN isless
FP_NORMAL islessequal
FP_SUBNORMAL islessgreater
FP_ZERO islower
INT16_C isnan
INT16_MAX isnormal
INT16_MIN isprint
int16_t ispunct
INT16_WIDTH issignaling
INT32_C isspace
INT32_MAX issubnormal
INT32_MIN isunordered
int32_t isupper
INT32_WIDTH iswalnum
INT64_C iswalpha
INT64_MAX iswblank
INT64_MIN iswcntrl
int64_t iswctype
INT64_WIDTH iswdigit
INT8_C iswgraph
INT8_MAX iswlower
INT8_MIN iswprint
int8_t iswpunct
INT8_WIDTH iswspace
INTMAX_C iswupper
INTMAX_MAX iswxdigit
INTMAX_MIN isxdigit
intmax_t iszero
LC_ALL PRIdLEAST64
LC_COLLATE PRIdMAX
LC_CTYPE PRIdPTR
LC_MONETARY PRIi32
LC_NUMERIC PRIi64
LC_TIME PRIiFAST32
LDBL_DECIMAL_DIG PRIiFAST64
LDBL_DIG PRIiLEAST32
LDBL_EPSILON PRIiLEAST64
LDBL_HAS_SUBNORM PRIiMAX
LDBL_IS_IEC_60559 PRIiPTR
LDBL_MANT_DIG PRIo32
LDBL_MAX PRIo64
LDBL_MAX_10_EXP PRIoFAST32
LDBL_MAX_EXP PRIoFAST64
LDBL_MIN PRIoLEAST32
LDBL_MIN_10_EXP PRIoLEAST64
LDBL_MIN_EXP PRIoMAX
LDBL_NORM_MAX PRIoPTR
LDBL_SNAN PRIu32
LDBL_TRUE_MIN PRIu64
MATH_ERREXCEPT PRIuFAST32
MATH_ERRNO PRIuFAST64
memalignment PRIuLEAST32
memccpy PRIuLEAST64
memchr PRIuMAX
memcmp PRIuPTR
memcpy PRIX32
memcpy_s PRIX64
memmove PRIXFAST32
memmove_s PRIXFAST64
memory_order PRIXLEAST32
memory_order_acquire PRIXLEAST64
memory_order_acq_rel PRIXMAX
memory_order_consume PRIXPTR
memory_order_relaxed SCNdMAX
memory_order_release SCNdPTR
memory_order_seq_cst SCNiMAX
memset SCNiPTR
memset_explicit SCNoMAX
memset_s SCNoPTR
mtx_destroy SCNuMAX
mtx_init SCNuPTR
mtx_lock SCNxMAX
mtx_plain SCNxPTR
mtx_recursive SIGABRT
mtx_t SIGFPE
mtx_timed SIGILL
mtx_timedlock SIGINT
mtx_trylock SIGSEGV
mtx_unlock SIGTERM
PRId32 SIG_ATOMIC_MAX
PRId64 SIG_ATOMIC_MIN
PRIdFAST32 SIG_ATOMIC_WIDTH
PRIdFAST64 SIG_DFL
PRIdLEAST32 SIG_ERR
SIG_IGN stdc_has_single_bituc
stdc_bit_ceil stdc_has_single_bitui
stdc_bit_ceiluc stdc_has_single_bitul
stdc_bit_ceilui stdc_has_single_bitull
stdc_bit_ceilul stdc_has_single_bitus
stdc_bit_ceilull stdc_leading_ones
stdc_bit_ceilus stdc_leading_onesuc
stdc_bit_floor stdc_leading_onesui
stdc_bit_flooruc stdc_leading_onesul
stdc_bit_floorui stdc_leading_onesull
stdc_bit_floorul stdc_leading_onesus
stdc_bit_floorull stdc_leading_zeros
stdc_bit_floorus stdc_leading_zerosuc
stdc_bit_width stdc_leading_zerosui
stdc_bit_widthuc stdc_leading_zerosul
stdc_bit_widthui stdc_leading_zerosull
stdc_bit_widthul stdc_leading_zerosus
stdc_bit_widthull stdc_trailing_ones
stdc_bit_widthus stdc_trailing_onesuc
stdc_count_ones stdc_trailing_onesui
stdc_count_onesuc stdc_trailing_onesul
stdc_count_onesui stdc_trailing_onesull
stdc_count_onesul stdc_trailing_onesus
stdc_count_onesull stdc_trailing_zeros
stdc_count_onesus stdc_trailing_zerosuc
stdc_count_zeros stdc_trailing_zerosui
stdc_count_zerosuc stdc_trailing_zerosul
stdc_count_zerosui stdc_trailing_zerosull
stdc_count_zerosul stdc_trailing_zerosus
stdc_count_zerosull strcat
stdc_count_zerosus strcat_s
stdc_first_leading_one strchr
stdc_first_leading_oneuc strcmp
stdc_first_leading_oneui strcoll
stdc_first_leading_oneul strcpy
stdc_first_leading_oneull strcpy_s
stdc_first_leading_oneus strcspn
stdc_first_leading_zero strdup
stdc_first_leading_zerouc strerror
stdc_first_leading_zeroui strerrorlen_s
stdc_first_leading_zeroul strerror_s
stdc_first_leading_zeroull strfromd
stdc_first_leading_zerous strfromd128
stdc_first_trailing_one strfromd32
stdc_first_trailing_oneuc strfromd64
stdc_first_trailing_oneui strfromencbind
stdc_first_trailing_oneul strfromencdecd
stdc_first_trailing_oneull strfromencf
stdc_first_trailing_oneus strfromencf128
stdc_first_trailing_zero strfromf
stdc_first_trailing_zerouc strfroml
stdc_first_trailing_zeroui strftime
stdc_first_trailing_zeroul strlen
stdc_first_trailing_zeroull strncat
stdc_first_trailing_zerous strncat_s
stdc_has_single_bit strncmp
strncpy totalordermagd
strncpy_s totalordermagd128
strndup totalordermagd32
strnlen_s totalordermagd64
strpbrk totalordermagf
strrchr totalordermagl
strspn toupper
strstr towctrans
strto towlower
strtod towupper
strtod128 tss_create
strtod32 tss_delete
strtod64 tss_dtor_t
strtoencbind tss_get
strtoencdecd tss_set
strtoencf tss_t
strtof UINT16_C
strtoimax UINT16_MAX
strtok uint16_t
strtok_s UINT16_WIDTH
strtol UINT32_C
strtold UINT32_MAX
strtoll uint32_t
strtoul UINT32_WIDTH
strtoull UINT64_C
strtoumax UINT64_MAX
struct uint64_t
strxfrm UINT64_WIDTH
thrd_busy UINT8_C
thrd_create UINT8_MAX
thrd_current uint8_t
thrd_detach UINT8_WIDTH
thrd_equal UINTMAX_C
thrd_error UINTMAX_MAX
thrd_exit uintmax_t
thrd_join UINTMAX_WIDTH
thrd_nomem UINTPTR_MAX
thrd_sleep uintptr_t
thrd_start_t UINTPTR_WIDTH
thrd_success uint_fast16_t
thrd_t uint_fast32_t
thrd_timedout uint_fast64_t
thrd_yield uint_fast8_t
TIME_ACTIVE uint_least16_t
TIME_MONOTONIC uint_least32_t
TIME_THREAD_ACTIVE uint_least64_t
TIME_UTC uint_least8_t
tolower UINT_MAX
totalorder UINT_WIDTH
totalorderd wcscat
totalorderd128 wcscat_s
totalorderd32 wcschr
totalorderd64 wcscmp
totalorderf wcscoll
totalorderl wcscpy
totalordermag wcscpy_s
wcscspn _Float128_t
wcsftime _Float16
wcslen _Float16_t
wcsncat _Float32
wcsncat_s _Float32x
wcsncmp _Float32_t
wcsncpy _Float64
wcsncpy_s _Float64x
wcsnlen_s _Float64_t
wcspbrk _Generic
wcsrchr _Imaginary
wcsrtombs _Imaginary_I
wcsrtombs_s _IOFBF
wcsspn _IOLBF
wcsstr _IONBF
wcsto _MANT_DIG
wcstod _MAX_10_EXP
wcstod128 _MAX_EXP
wcstod32 _MIN_10_EXP
wcstod64 _MIN_EXP
wcstof _Noreturn
wcstoimax _Pragma
wcstok _PRINTF_NAN_LEN_MAX
wcstok_s _SNAN
wcstol _Static_assert
wcstold _Thread_local
wcstoll _TRUE_MIN
wcstombs __alignas_is_defined
wcstombs_s __alignof_is_defined
wcstoul __bool_true_false_are_defined
wcstoull __cplusplus
wcstoumax __DATE__
wcsxfrm __deprecated__
_Alignas __fallthrough__
_Alignof __FILE__
_Atomic __func__
_BitInt __has_c_attribute
_Bool __has_embed
_Complex __has_include
_Complex_I __if_empty__
_Decimal __limit__
_Decimal128 __LINE__
_Decimal128x __maybe_unused__
_Decimal32 __nodiscard__
_Decimal32_t __noreturn__
_Decimal64 __pp_param__
_Decimal64x __prefix__
_Decimal64_t __reproducible__
_DECIMAL_DIG __STDC_ANALYZABLE__
_DIG __STDC_ENDIAN_BIG__
_EPSILON __STDC_ENDIAN_LITTLE__
_Exit __STDC_ENDIAN_NATIVE__
_EXT__ __STDC_HOSTED__
_Float __STDC_IEC_559_COMPLEX__
_Float128 __STDC_IEC_559__
_Float128x __STDC_IEC_60559_BFP__
__STDC_IEC_60559_COMPLEX__ __STDC_VERSION_STDLIB_H__
__STDC_IEC_60559_DFP__ __STDC_VERSION_TGMATH_H__
__STDC_IEC_60559_TYPES__ __STDC_VERSION_TIME_H__
__STDC_ISO_10646__ __STDC_VERSION__
__STDC_LIB_EXT1__ __STDC_WANT_IEC_60559_
__STDC_MB_MIGHT_NEQ_WC__ __STDC_WANT_IEC_60559_EXT__
__STDC_NO_ATOMICS__ __STDC_WANT_IEC_60559_TYPES_EXT__
__STDC_NO_COMPLEX__ __STDC_WANT_LIB_EXT1__
__STDC_NO_THREADS__ __STDC__
__STDC_NO_VLA__ __suffix__
__STDC_UTF_16__ __TIME__
__STDC_UTF_32__ __unsequenced__
__STDC_VERSION_FENV_H__ __VA_ARGS__
__STDC_VERSION_MATH_H__ __VA_OPT__
__STDC_VERSION_STDINT_H__ ___Noreturn__
J.6.2 Particular identifiers or keywords
1 The following 1358 identifiers or keywords are not covered by the above and have particular
semantics provided by this document.
abort_handler_s asind atand
abort asinf atanf
abs asinhd128 atanhd128
acosd128 asinhd32 atanhd32
acosd32 asinhd64 atanhd64
acosd64 asinhd atanhd
acosd asinhf atanhf
acosf asinhl atanhl
acoshd128 asinh atanh
acoshd32 asinl atanl
acoshd64 asinpid128 atanpid128
acoshd asinpid32 atanpid32
acoshf asinpid64 atanpid64
acoshl asinpid atanpid
acosh asinpif atanpif
acosl asinpil atanpil
acospid128 asinpi atanpi
acospid32 asin atan
acospid64 assert atexit
acospid atan2d128 atof
acospif atan2d32 atoi
acospil atan2d64 atoll
acospi atan2d atol
acos atan2f at_quick_exit
addd atan2l auto
addf atan2pid128 bitand
alignas atan2pid32 BITINT_MAXWIDTH
aligned_alloc atan2pid64 bitor
alignof atan2pid BOOL_MAX
and_eq atan2pif BOOL_WIDTH
and atan2pil bool
asctime_s atan2pi break
asctime atan2 bsearch_s
asind128 atand128 bsearch
asind32 atand32 btowc
asind64 atand64 BUFSIZ
c16rtomb ceild32 compoundnl
c32rtomb ceild64 compoundn
c8rtomb ceild conjf
cabsf ceilf conjl
cabsl ceill conj
cabs ceil constexpr
cacosf cerfc constraint_handler_t
cacoshf cerf const
cacoshl cexp10m1 continue
cacosh cexp10 copysignd128
cacosl cexp2m1 copysignd32
cacospi cexp2 copysignd64
cacos cexpf copysignd
calloc cexpl copysignf
call_once cexpm1 copysignl
canonicalized128 cexp copysign
canonicalized32 char16_t cosd128
canonicalized64 char32_t cosd32
canonicalized char8_t cosd64
canonicalizef CHAR_BIT cosd
canonicalizel CHAR_MAX cosf
canonicalize CHAR_MIN coshd128
cargf CHAR_WIDTH coshd32
cargl char coshd64
carg cimagf coshd
case cimagl coshf
casinf cimag coshl
casinhf ckd_add cosh
casinhl ckd_div cosl
casinh ckd_mul cospid128
casinl ckd_sub cospid32
casinpi ckd_ cospid64
casin clearerr cospid
catanf clgamma cospif
catanhf CLOCKS_PER_SEC cospil
catanhl clock_t cospi
catanh clock cos
catanl clog10p1 cpowf
catanpi clog10 cpowl
catan clog1p cpown
cbrtd128 clog2p1 cpowr
cbrtd32 clog2 cpow
cbrtd64 clogf cprojf
cbrtd clogl cprojl
cbrtf clogp1 cproj
cbrtl clog crealf
cbrt CMPLXF creall
ccompoundn CMPLXL creal
ccosf CMPLX CR_DECIMAL_DIG
ccoshf complex csinf
ccoshl compl csinhf
ccosh compoundnd128 csinhl
ccosl compoundnd32 csinh
ccospi compoundnd64 csinl
ccos compoundnd csinpi
ceild128 compoundnf csin
csqrtf decodebind64 erfcl
csqrtl decodebind erfc
csqrt decodebin erfd128
ctanf decodedecd128 erfd32
ctanhf decodedecd32 erfd64
ctanhl decodedecd64 erfd
ctanh decodedecd erff
ctanl decodedec erfl
ctanpi decodef erf
ctan DEC errno_t
ctgamma DEFAULT errno
ctime_s defined error
ctime define exit
currency_symbol deprecated exp10d128
CX_LIMITED_RANGE dfmal exp10d32
d32addd128 dfma exp10d64
d32addd64 difftime exp10d
d32add divd exp10f
d32divd128 divf exp10l
d32divd64 div_t exp10m1d128
d32div div exp10m1d32
d32fmad128 dmull exp10m1d64
d32fmad64 dmul exp10m1d
d32fma double_t exp10m1f
d32muld128 double exp10m1l
d32muld64 do exp10m1
d32mul dsqrtl exp10
d32sqrtd128 dsqrt exp2d128
d32sqrtd64 dsubl exp2d32
d32sqrt dsub exp2d64
d32subd128 elifdef exp2d
d32subd64 elifndef exp2f
d32sub elif exp2l
d64addd128 else exp2m1d128
d64add embed exp2m1d32
d64divd128 encbind exp2m1d64
d64div encdecd exp2m1d
d64fmad128 encf exp2m1f
d64fma encodebind128 exp2m1l
d64muld128 encodebind32 exp2m1
d64mul encodebind64 exp2
d64sqrtd128 encodebind expd128
d64sqrt encodebin expd32
d64subd128 encodedecd128 expd64
d64sub encodedecd32 expd
daddl encodedecd64 expf
dadd encodedecd expl
ddivl encodedec expm1d128
ddiv encodef expm1d32
DECIMAL_DIG endif expm1d64
decimal_point enum expm1d
Decimal erfcd128 expm1f
DECN_ erfcd32 expm1l
DECN erfcd64 expm1
decodebind128 erfcd exp
decodebind32 erfcf extern
fabsd128 float_t fmind64
fabsd32 Float fmind
fabsd64 floord128 fminf
fabsd floord32 fminimumd128
fabsf floord64 fminimumd32
fabsl floord fminimumd64
fabs floorf fminimumd
faddl floorl fminimumf
fadd floor fminimuml
fallthrough FLTN_ fminimum_magd128
false FLTN fminimum_magd32
fclose FLT fminimum_magd64
fdimd128 fmad128 fminimum_magd
fdimd32 fmad32 fminimum_magf
fdimd64 fmad64 fminimum_magl
fdimd fmad fminimum_mag_numd128
fdimf fmaf fminimum_mag_numd32
fdiml fmal fminimum_mag_numd64
fdim fmaxd128 fminimum_mag_numd
fdivl fmaxd32 fminimum_mag_numf
fdiv fmaxd64 fminimum_mag_numl
feclearexcept fmaxd fminimum_mag_num
fegetenv fmaxf fminimum_mag
fegetexceptflag fmaximumd128 fminimum_numd128
fegetmode fmaximumd32 fminimum_numd32
fegetround fmaximumd64 fminimum_numd64
feholdexcept fmaximumd fminimum_numd
femode_t fmaximumf fminimum_numf
FENV_ACCESS fmaximuml fminimum_numl
FENV_DEC_ROUND fmaximum_magd128 fminimum_num
FENV_ROUND fmaximum_magd32 fminimum
fenv_t fmaximum_magd64 fminl
feof fmaximum_magd fmin
feraiseexcept fmaximum_magf fmodd128
ferror fmaximum_magl fmodd32
fesetenv fmaximum_mag_numd128 fmodd64
fesetexceptflag fmaximum_mag_numd32 fmodd
fesetexcept fmaximum_mag_numd64 fmodf
fesetmode fmaximum_mag_numd fmodl
fesetround fmaximum_mag_numf fmod
fetestexceptflag fmaximum_mag_numl fmull
fetestexcept fmaximum_mag_num fmul
feupdateenv fmaximum_mag FOPEN_MAX
fexcept_t fmaximum_numd128 fopen_s
fe_dec_getround fmaximum_numd32 fopen
fe_dec_setround fmaximum_numd64 for
fflush fmaximum_numd fpclassify
ffmal fmaximum_numf fpos_t
ffma fmaximum_numl fprintf_s
fgetc fmaximum_num fprintf
fgetpos fmaximum fputc
fgets fmaxl fputs
fgetwc fmax fputwc
fgetws fma fputws
FILENAME_MAX fmind128 frac_digits
FILE fmind32 fread
free_aligned_sized gets ldexpd32
free_sized getwchar ldexpd64
free getwc ldexpd
freopen_s gmtime_r ldexpf
freopen gmtime_s ldexpl
frexpd128 gmtime ldexp
frexpd32 goto ldiv_t
frexpd64 grouping ldiv
frexpd HUGE_VALF lgammad128
frexpf HUGE_VALL lgammad32
frexpl HUGE_VAL_D128 lgammad64
frexp HUGE_VAL_D32 lgammad
fromfpd128 HUGE_VAL_D64 lgammaf
fromfpd32 HUGE_VAL_D lgammal
fromfpd64 HUGE_VAL_F lgamma
fromfpd HUGE_VAL limit
fromfpf hypotd128 line
fromfpl hypotd32 llabs
fromfpxd128 hypotd64 lldiv_t
fromfpxd32 hypotd lldiv
fromfpxd64 hypotf llogbd128
fromfpxd hypotl llogbd32
fromfpxf hypot llogbd64
fromfpxl ifdef llogbd
fromfpx ifndef llogbf
fromfp if_empty llogbl
fscanf_s if llogb
fscanf ignore_handler_s LLONG_MAX
fseek ilogbd128 LLONG_MIN
fsetpos ilogbd32 LLONG_WIDTH
fsqrtl ilogbd64 llquantexpd128
fsqrt ilogbd llquantexpd32
fsubl ilogbf llquantexpd64
fsub ilogbl llquantexpd
ftell ilogb llquantexp
fwide imaginary llrintd128
fwprintf_s imaxabs llrintd32
fwprintf imaxdiv_t llrintd64
fwrite imaxdiv llrintd
fwscanf_s include llrintf
fwscanf INFINITY llrintl
generic_count_type inline llrint
generic_return_type int_curr_symbol llroundd128
generic_value_type int_frac_digits llroundd32
getchar int_n_cs_precedes llroundd64
getc int_n_sep_by_space llroundd
getenv_s int_n_sign_posn llroundf
getenv int_p_cs_precedes llroundl
getpayloadd128 int_p_sep_by_space llround
getpayloadd32 int_p_sign_posn localeconv
getpayloadd64 I localtime_r
getpayloadd jmp_buf localtime_s
getpayloadf kill_dependency localtime
getpayloadl labs log10d128
getpayload lconv log10d32
gets_s ldexpd128 log10d64
log10d LONG_MIN nanf
log10f LONG_WIDTH nanl
log10l long nan
log10p1d128 lrintd128 NDEBUG
log10p1d32 lrintd32 nearbyintd128
log10p1d64 lrintd64 nearbyintd32
log10p1d lrintd nearbyintd64
log10p1f lrintf nearbyintd
log10p1l lrintl nearbyintf
log10p1 lrint nearbyintl
log10 lroundd128 nearbyint
log1pd128 lroundd32 negative_sign
log1pd32 lroundd64 nextafterd128
log1pd64 lroundd nextafterd32
log1pd lroundf nextafterd64
log1pf lroundl nextafterd
log1pl lround nextafterf
log1p L_tmpnam_s nextafterl
log2d128 L_tmpnam nextafter
log2d32 main nextdownd128
log2d64 malloc nextdownd32
log2d math_errhandling nextdownd64
log2f max_align_t nextdownd
log2l maybe_unused nextdownf
log2p1d128 mblen nextdownl
log2p1d32 mbrlen nextdown
log2p1d64 mbrtoc16 nexttowardd128
log2p1d mbrtoc32 nexttowardd32
log2p1f mbrtoc8 nexttowardd64
log2p1l mbrtowc nexttowardf
log2p1 mbsinit nexttowardl
log2 mbsrtowcs_s nexttoward
logbd128 mbsrtowcs nextupd128
logbd32 mbstate_t nextupd32
logbd64 mbstowcs_s nextupd64
logbd mbstowcs nextupd
logbf mbtowc nextupf
logbl MB_CUR_MAX nextupl
logb MB_LEN_MAX nextup
logd128 mktime nodiscard
logd32 modfd128 noreturn
logd64 modfd32 not_eq
logd modfd64 not
logf modfd nullptr_t
logl modff nullptr
logp1d128 modfl NULL
logp1d32 modf n_cs_precedes
logp1d64 mon_decimal_point n_sep_by_space
logp1d mon_grouping n_sign_posn
logp1f mon_thousands_sep N
logp1l muld offsetof
logp1 mulf OFF
log nand128 ONCE_FLAG_INIT
longjmp nand32 once_flag
long_double_t nand64 ON
LONG_MAX nand or_eq
or QWchar_t rsqrtf
perror raise rsqrtl
positive_sign RAND_MAX rsqrt
powd128 rand samequantumd128
powd32 realloc samequantumd32
powd64 register samequantumd64
powd remainderd128 samequantumd
powf remainderd32 samequantum
powl remainderd64 scalblnd128
pownd128 remainderd scalblnd32
pownd32 remainderf scalblnd64
pownd64 remainderl scalblnd
pownd remainder scalblnf
pownf remove scalblnl
pownl remquof scalbln
pown remquol scalbnd128
powrd128 remquo scalbnd32
powrd32 rename scalbnd64
powrd64 reproducible scalbnd
powrd restrict scalbnf
powrf return scalbnl
powrl rewind scalbn
powr rintd128 scanf_s
pow rintd32 scanf
pp_param rintd64 SCHAR_MAX
pragma rintd SCHAR_MIN
prefix rintf SCHAR_WIDTH
printf_s rintl SEEK_CUR
printf rint SEEK_END
PTRDIFF_MAX rootnd128 SEEK_SET
PTRDIFF_MIN rootnd32 setbuf
ptrdiff_t rootnd64 setjmp
PTRDIFF_WIDTH rootnd setlocale
putchar rootnf setpayloadd128
putc rootnl setpayloadd32
puts rootn setpayloadd64
putwchar roundd128 setpayloadd
putwc roundd32 setpayloadf
p_cs_precedes roundd64 setpayloadl
p_sep_by_space roundd setpayloadsigd128
p_sign_posn roundevend128 setpayloadsigd32
QChar roundevend32 setpayloadsigd64
qsort_s roundevend64 setpayloadsigd
qsort roundevend setpayloadsigf
quantized128 roundevenf setpayloadsigl
quantized32 roundevenl setpayloadsig
quantized64 roundeven setpayload
quantized roundf setvbuf
quantize roundl set_constraint_handler_s
quantumd128 round short
quantumd32 RSIZE_MAX SHRT_MAX
quantumd64 rsize_t SHRT_MIN
quantumd rsqrtd128 SHRT_WIDTH
quantum rsqrtd32 signal
quick_exit rsqrtd64 signbit
QVoid rsqrtd signed
sig_atomic_t tand128 truncf
sind128 tand32 truncl
sind32 tand64 trunc
sind64 tand TSS_DTOR_ITERATIONS
sind tanf tv_nsec
sinf tanhd128 tv_sec
sinhd128 tanhd32 typedef
sinhd32 tanhd64 typeof_unqual
sinhd64 tanhd typeof
sinhd tanhf UCHAR_MAX
sinhf tanhl UCHAR_WIDTH
sinhl tanh ufromfpd128
sinh tanl ufromfpd32
sinl tanpid128 ufromfpd64
sinpid128 tanpid32 ufromfpd
sinpid32 tanpid64 ufromfpf
sinpid64 tanpid ufromfpl
sinpid tanpif ufromfpxd128
sinpif tanpil ufromfpxd32
sinpil tanpi ufromfpxd64
sinpi tan ufromfpxd
sin tgammad128 ufromfpxf
sizeof tgammad32 ufromfpxl
SIZE_MAX tgammad64 ufromfpx
size_t tgammad ufromfp
SIZE_WIDTH tgammaf ULLONG_MAX
snprintf_s tgammal ULLONG_WIDTH
snprintf tgamma ULONG_MAX
snwprintf_s thousands_sep ULONG_WIDTH
sprintf_s thread_local undef
sprintf timespec_getres ungetc
sqrtd128 timespec_get ungetwc
sqrtd32 timespec union
sqrtd64 time_t unreachable
sqrtd time unsequenced
sqrtf tmpfile_s unsigned
sqrtl tmpfile USHRT_MAX
sqrt tmpnam_s USHRT_WIDTH
srand tmpnam va_arg
sscanf_s TMP_MAX_S va_copy
sscanf TMP_MAX va_end
static_assert tm_hour va_list
static tm_isdst va_start
STDC tm_mday vfprintf_s
stderr tm_min vfprintf
stdin tm_mon vfscanf_s
stdout tm_sec vfscanf
subd tm_wday vfwprintf_s
subf tm_yday vfwprintf
suffix tm_year vfwscanf_s
switch tm vfwscanf
swprintf_s true void
swprintf truncd128 volatile
swscanf_s truncd32 vprintf_s
swscanf truncd64 vprintf
system truncd vscanf_s
vscanf wctomb xdivf
vsnprintf_s wctrans_t xfmad
vsnprintf wctrans xfmaf
vsnwprintf_s wctype_t xmuld
vsprintf_s wctype xmulf
vsprintf WEOF xor_eq
vsscanf_s while xor
vsscanf WINT_MAX xsqrtd
vswprintf_s WINT_MIN xsqrtf
vswprintf wint_t xsubd
vswscanf_s WINT_WIDTH xsubf
vswscanf wmemchr X_DECIMAL_DIG
vwprintf_s wmemcmp X_DIG
vwprintf wmemcpy_s X_EPSILON
vwscanf_s wmemcpy X_MANT_DIG
vwscanf wmemmove_s X_MAX_10_EXP
warning wmemmove X_MAX_EXP
WCHAR_MAX wmemset X_MAX
WCHAR_MIN wprintf_s X_MIN_10_EXP
wchar_t wprintf X_MIN_EXP
WCHAR_WIDTH wscanf_s X_MIN
wcrtomb_s wscanf X_SNAN
wcrtomb xaddd X_TRUE_MIN
wctob xaddf X_
wctomb_s xdivd
J.6.3 Type inference
1 A declaration for which a type is inferred (6.7.9) may additionally accept pointer declarators, function
declarators, and may have more than one declarator.
K. Annex K (normative) Bounds-checking interfaces
K.1 Background
1 Traditionally, the C Library has contained many functions that trust the programmer to provide
output character arrays big enough to hold the result being produced. Not only do these functions
not check that the arrays are big enough, they frequently lack the information needed to perform
such checks. While it is possible to write safe, robust, and error-free code using the existing library,
the library tends to promote programming styles that lead to mysterious failures if a result is too big
for the provided array.
2 A common programming style is to declare character arrays large enough to handle most practical
cases. However, if these arrays are not large enough to handle the resulting strings, data can be
written past the end of the array overwriting other data and program structures. The program never
gets any indication that a problem exists, and so never has a chance to recover or to fail gracefully.
3 Worse, this style of programming has compromised the security of computers and networks. Buffer
overflows can often be exploited to run arbitrary code with the permissions of the vulnerable
(defective) program.
4 If the programmer writes runtime checks to verify lengths before calling library functions, then
those runtime checks frequently duplicate work done inside the library functions, which discover
string lengths as a side effect of doing their job.
5 This annex provides alternative library functions that promote safer, more secure programming. The
alternative functions verify that output buffers are large enough for the intended result and return a
failure indicator if they are not. Data is never written past the end of an array. All string results are
null terminated.
6 This annex also addresses another problem that complicates writing robust code: functions that are
not reentrant because they return pointers to static objects owned by the function. Such functions
can be troublesome since a previously returned result can change if the function is called again,
perhaps by another thread.
K.2 Scope
1 This annex specifies a series of optional extensions that can be useful in the mitigation of security
vulnerabilities in programs, and comprise new functions, macros, and types declared or defined in
existing standard headers.
2 An implementation that defines __STDC_LIB_EXT1__ shall conform to the specifications in this
annex.465)
3 Subclause K.3 should be read as if it were merged into the parallel structure of named subclauses of
Clause 7.
K.3 Library
K.3.1 Introduction
K.3.1.1 Standard headers
1 The functions, macros, and types declared or defined in K.3 and its subclauses are not declared
or defined by their respective headers if __STDC_WANT_LIB_EXT1__ is defined as a macro which
expands to the integer constant 0 at the point in the source file where the appropriate header is first
included.
2 The functions, macros, and types declared or defined in K.3 and its subclauses are declared and
defined by their respective headers if __STDC_WANT_LIB_EXT1__ is defined as a macro which ex-
pands to the integer constant 1 at the point in the source file where the appropriate header is first
465) Implementations that do not define __STDC_LIB_EXT1__ are not required to conform to these specifications.
included.466)
3 It is implementation-defined whether the functions, macros, and types declared or defined in K.3 and
its subclauses are declared or defined by their respective headers if __STDC_WANT_LIB_EXT1__ is not
defined as a macro at the point in the source file where the appropriate header is first included.467)
4 Within a preprocessing translation unit, __STDC_WANT_LIB_EXT1__ shall be defined identically for
all inclusions of any headers from Subclause K.3. If __STDC_WANT_LIB_EXT1__ is defined differently
for any such inclusion, the implementation shall issue a diagnostic as if a preprocessor error directive
were used.
K.3.1.2 Reserved identifiers
1 Each macro name in any of the following subclauses is reserved for use as specified if it is defined
by any of its associated headers when included; unless explicitly stated otherwise (see 7.1.4).
2 All identifiers with external linkage in any of the following subclauses are reserved for use as
identifiers with external linkage if any of them are used by the program. None of them are reserved
if none of them are used.
3 Each identifier with file scope listed in any of the following subclauses is reserved for use as a
macro name and as an identifier with file scope in the same name space if it is defined by any of its
associated headers when included.
K.3.1.3 Use of errno
1 An implementation may set errno for the functions defined in this annex, but is not required to.
K.3.1.4 Runtime-constraint violations
1 Most functions in this annex include as part of their specification a list of runtime-constraints. These
runtime-constraints are requirements on the program using the library.468)
2 Implementations shall verify that the runtime-constraints for a function are not violated by the
program. If a runtime-constraint is violated, the implementation shall call the currently registered
runtime-constraint handler (see set_constraint_handler_s in <stdlib.h>). Multiple runtime-
constraint violations in the same call to a library function result in only one call to the runtime-
constraint handler. It is unspecified which one of the multiple runtime-constraint violations cause
the handler to be called.
3 If the runtime-constraints section for a function states an action to be performed when a runtime-
constraint violation occurs, the function shall perform the action before calling the runtime-constraint
handler. If the runtime-constraints section lists actions that are prohibited when a runtime-constraint
violation occurs, then such actions are prohibited to the function both before calling the handler and
after the handler returns.
4 The runtime-constraint handler might not return. If the handler does return, the library function
whose runtime-constraint was violated shall return some indication of failure as given by the returns
section in the function’s specification.
K.3.2 Errors <errno.h>
1 The header <errno.h> defines a type.
2 The type is
errno_t
466) Future revisions of this document might define meanings for other values of __STDC_WANT_LIB_EXT1__ .
467) Subclause 7.1.3 reserves certain names and patterns of names that an implementation can use in headers. All other names
are not reserved, and a conforming implementation is not permitted to use them. While some of the names defined in K.3 and
its subclauses are reserved, others are not. If an unreserved name is defined in a header when __STDC_WANT_LIB_EXT1__ is
defined as 0, the implementation is not conforming.
468) Although runtime-constraints replace many cases of undefined behavior, undefined behavior still exists in this annex.
Implementations are free to detect any case of undefined behavior and treat it as a runtime-constraint violation by calling the
runtime-constraint handler. This license comes directly from the definition of undefined behavior.
which is type int.469)
K.3.3 Common definitions <stddef.h>
1 The header <stddef.h> defines a type.
2 The type is
rsize_t
which is the type size_t.470)
K.3.4 Integer types <stdint.h>
1 The header <stdint.h> defines a macro.
2 The macro is
RSIZE_MAX
which expands to a value471) of type size_t. Functions that have parameters of type rsize_t con-
sider it a runtime-constraint violation if the values of those parameters are greater than RSIZE_MAX.
Recommended practice
3 Extremely large object sizes are frequently a sign that an object’s size was calculated incorrectly. For
example, negative numbers appear as very large positive numbers when converted to an unsigned
type like size_t. Also, some implementations do not support objects as large as the maximum
value that can be represented by type size_t.
4 For those reasons, it is sometimes beneficial to restrict the range of object sizes to detect programming
errors. For implementations targeting machines with large address spaces, it is recommended that
RSIZE_MAX be defined as the smaller of the size of the largest object supported or (SIZE_MAX >> 1) ,
even if this limit is smaller than the size of some legitimate, but very large, objects. Implementations
targeting machines with small address spaces may wish to define RSIZE_MAX as SIZE_MAX, which
means that there is no object size that is considered a runtime-constraint violation.
K.3.5 Input/output <stdio.h>
1 The header <stdio.h> defines several macros and two types.
2 The macros are
L_tmpnam_s
which expands to an integer constant expression that is the size needed for an array of char large
enough to hold a temporary file name string generated by the tmpnam_s function;
TMP_MAX_S
which expands to an integer constant expression that is the maximum number of unique file names
that can be generated by the tmpnam_s function.
3 The types are
errno_t
which is type int; and
469) As a matter of programming style, errno_t can be used as the type of something that deals only with the values that
might be found in errno. For example, a function which returns the value of errno could be declared as having the return
type errno_t.
470) See the description of the RSIZE_MAX macro in <stdint.h>.
471) The macro RSIZE_MAX need not expand to a constant expression.
rsize_t
which is the type size_t.
K.3.5.1 Operations on files
K.3.5.1.1 The tmpfile_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
errno_t tmpfile_s(FILE * restrict * restrict streamptr);
Runtime-constraints
2 streamptr shall not be a null pointer.
3 If there is a runtime-constraint violation, tmpfile_s does not attempt to create a file.
Description
4 The tmpfile_s function creates a temporary binary file that is different from any other existing file
and that will automatically be removed when it is closed or at program termination. If the program
terminates abnormally, whether an open temporary file is removed is implementation-defined. The
file is opened for update with "wb+" mode with the meaning that mode has in the fopen_s function
(including the mode’s effect on exclusive access and file permissions).
5 If the file was created successfully, then the pointer to FILE pointed to by streamptr will be set to
the pointer to the object controlling the opened file. Otherwise, the pointer to FILE pointed to by
streamptr will be set to a null pointer.
Recommended practice
It should be possible to open at least TMP_MAX_S temporary files during the lifetime of the program
(this limit may be shared with tmpnam_s) and there should be no limit on the number simultaneously
open other than this limit and any limit on the number of open files (FOPEN_MAX).
Returns
6 The tmpfile_s function returns zero if it created the file. If it did not create the file or there was a
runtime-constraint violation, tmpfile_s returns a nonzero value.
K.3.5.1.2 The tmpnam_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
errno_t tmpnam_s(char *s, rsize_t maxsize);
Runtime-constraints
2 s shall not be a null pointer. maxsize shall be less than or equal to RSIZE_MAX. maxsize shall be
greater than the length of the generated file name string.
Description
3 The tmpnam_s function generates a string that is a valid file name and that is not the same as the
name of an existing file.472) The function is potentially capable of generating TMP_MAX_S different
strings, but any or all of them may already be in use by existing files and thus not be suitable return
values. The lengths of these strings shall be less than the value of the L_tmpnam_s macro.
4 The tmpnam_s function generates a different string each time it is called.
472) Files created using strings generated by the tmpnam_s function are temporary only in the sense that their names are not
expected to collide with those generated by conventional naming rules for the implementation. It is still necessary to use the
remove function to remove such files when their use is ended, and before program termination.
5 It is assumed that s points to an array of at least maxsize characters. This array will be set to
generated string, as specified below.
6 The implementation shall behave as if no library function except tmpnam calls the tmpnam_s func-
tion.473)
Recommended practice
7 After a program obtains a file name using the tmpnam_s function and before the program creates a
file with that name, the possibility exists that someone else may create a file with that same name.
To avoid this race condition, the tmpfile_s function should be used instead of tmpnam_s when
possible. One situation that requires the use of the tmpnam_s function is when the program needs to
create a temporary directory rather than a temporary file.
8 Implementations should take care in choosing the patterns used for names returned by tmpnam_s.
For example, making a thread ID part of the names avoids the race condition and possible conflict
when multiple programs run simultaneously by the same user generate the same temporary file
names.
Returns
9 If no suitable string can be generated, or if there is a runtime-constraint violation, the tmpnam_s
function:
— if s is not null and maxsize is both greater than zero and not greater than RSIZE_MAX, writes a
null character to s[0]
— returns a nonzero value.
10 Otherwise, the tmpnam_s function writes the string in the array pointed to by s and returns zero.
Environmental limits
11 The value of the macro TMP_MAX_S shall be at least 25.
K.3.5.2 File access functions
K.3.5.2.1 The fopen_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
errno_t fopen_s(FILE * restrict * restrict streamptr,
const char * restrict filename, const char * restrict mode);
Runtime-constraints
2 None of streamptr, filename, or mode shall be a null pointer.
3 If there is a runtime-constraint violation, fopen_s does not attempt to open a file. Furthermore, if
streamptr is not a null pointer, fopen_s sets *streamptr to the null pointer.
Description
4 The fopen_s function opens the file whose name is the string pointed to by filename, and associates
a stream with it.
5 The mode string shall be as described for fopen, with the addition that modes starting with the
character ’w’ or ’a’ may be preceded by the character ’u’ , see below:
uw truncate to zero length or create text file for writing, default permissions
uwx create text file for writing, default permissions
ua append; open or create text file for writing at end-of-file, default permissions
473) An implementation can have tmpnam call tmpnam_s (perhaps so there is only one naming convention for temporary files),
but this is not required.
uwb truncate to zero length or create binary file for writing, default permissions
uwbx create binary file for writing, default permissions
uab append; open or create binary file for writing at end-of-file, default permissions
uw+ truncate to zero length or create text file for update, default permissions
uw+x create text file for update, default permissions
ua+ append; open or create text file for update, writing at end-of-file, default permis-
sions
uw+b or uwb+ truncate to zero length or create binary file for update, default permissions
uw+bx or uwb+x create binary file for update, default permissions
ua+b or uab+ append; open or create binary file for update, writing at end-of-file, default permis-
sions
6 Opening a file with exclusive mode (’x’ as the last character in the mode argument) fails if the file
already exists or cannot be created.
7 To the extent that the underlying system supports the concepts, files opened for writing shall be
opened with exclusive (also known as non-shared) access. If the file is being created, and the first
character of the mode string is not ’u’ , to the extent that the underlying system supports it, the file
shall have a file permission that prevents other users on the system from accessing the file. If the
file is being created and first character of the mode string is ’u’ , then by the time the file has been
closed, it shall have the system default file access permissions.474)
8 If the file was opened successfully, then the pointer to FILE pointed to by streamptr will be set to
the pointer to the object controlling the opened file. Otherwise, the pointer to FILE pointed to by
streamptr will be set to a null pointer.
Returns
9 The fopen_s function returns zero if it opened the file. If it did not open the file or if there was a
runtime-constraint violation, fopen_s returns a nonzero value.
K.3.5.2.2 The freopen_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
errno_t freopen_s(FILE * restrict * restrict newstreamptr,
const char * restrict filename, const char * restrict mode,
FILE * restrict stream);
Runtime-constraints
2 None of newstreamptr, mode, and stream shall be a null pointer.
3 If there is a runtime-constraint violation, freopen_s neither attempts to close any file associated with
stream nor attempts to open a file. Furthermore, if newstreamptr is not a null pointer, fopen_s
sets *newstreamptr to the null pointer.
Description
4 The freopen_s function opens the file whose name is the string pointed to by filename and
associates the stream pointed to by stream with it. The mode argument has the same meaning as in
the fopen_s function (including the mode’s effect on exclusive access and file permissions).
5 If filename is a null pointer, the freopen_s function attempts to change the mode of the stream
to that specified by mode, as if the name of the file currently associated with the stream had been
474) These are the same permissions that the file would have been created with by fopen.
used. It is implementation-defined which changes of mode are permitted (if any), and under what
circumstances.
6 The freopen_s function first attempts to close any file that is associated with stream. Failure to
close the file is ignored. The error and end-of-file indicators for the stream are cleared.
7 If the file was opened successfully, then the pointer to FILE pointed to by newstreamptr will be set
to the value of stream. Otherwise, the pointer to FILE pointed to by newstreamptr will be set to a
null pointer.
Returns
8 The freopen_s function returns zero if it opened the file. If it did not open the file or there was a
runtime-constraint violation, freopen_s returns a nonzero value.
K.3.5.3 Formatted input/output functions
1 Unless explicitly stated otherwise, if the execution of a function described in this subclause causes
copying to take place between objects that overlap, the objects take on unspecified values.
K.3.5.3.1 The fprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int fprintf_s(FILE * restrict stream, const char * restrict format, ...);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. The %n specifier475) (modified or not by flags,
field width, or precision) shall not appear in the string pointed to by format. Any argument to
fprintf_s corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the476) fprintf_s function does not attempt to produce
further output, and it is unspecified to what extent fprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The fprintf_s function is equivalent to the fprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The fprintf_s function returns the number of characters transmitted, or a negative value if an
output error, encoding error, or runtime-constraint violation occurred.
K.3.5.3.2 The fscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int fscanf_s(FILE * restrict stream, const char * restrict format, ...);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. Any argument indirected though in order to
store converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the477) fscanf_s function does not attempt to perform
further input, and it is unspecified to what extent fscanf_s performed input before discovering the
runtime-constraint violation.
475) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
476) Because an implementation can treat any undefined behavior as a runtime-constraint violation, an implementation can
treat any unsupported specifiers in the string pointed to by format as a runtime-constraint violation.
477) Because an implementation can treat any undefined behavior as a runtime-constraint violation, an implementation can
treat any unsupported specifiers in the string pointed to by format as a runtime-constraint violation.
Description
4 The fscanf_s function is equivalent to fscanf except that the c, s, and [ conversion specifiers
apply to a pair of arguments (unless assignment suppression is indicated by a *). The first of these
arguments is the same as for fscanf. That argument is immediately followed in the argument list
by the second argument, which has type rsize_t and gives the number of elements in the array
pointed to by the first argument of the pair. If the first argument points to a scalar object, it is
considered to be an array of one element.478)
5 A matching failure occurs if the number of elements in a receiving object is insufficient to hold the
converted input (including any trailing null character).
Returns
6 The fscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the fscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
7 EXAMPLE 1 The call:
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
/* ... */
int n, i; float x; char name[50];
n = fscanf_s(stdin, "%d%f%s", &i, &x, name, (rsize_t) 50);
with the input line:
25 54.32E-1 thompson
will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.
8 EXAMPLE 2 The call:
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
/* ... */
int n; char s[5];
n = fscanf_s(stdin, "%s", s, sizeof s);
with the input line:
hello
will assign to n the value 0 since a matching failure occurred because the sequence hello\0 requires an array of six characters
to store it.
K.3.5.3.3 The printf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int printf_s(const char * restrict format, ...);
478) If the format is known at translation time, an implementation can issue a diagnostic for any argument used to store
the result from a c, s, or [ conversion specifier if that argument is not followed by an argument of a type compatible with
rsize_t. A limited amount of checking can be done if even if the format is not known at translation time. For example, an
implementation could issue a diagnostic for each argument after format that has of type pointer to one of char, signed char,
unsigned char, or void that is not followed by an argument of a type compatible with rsize_t. The diagnostic could warn
that unless the pointer is being used with a conversion specifier using the hh length modifier, a length argument is expected
to follow the pointer argument. Another useful diagnostic could flag any non-pointer argument following format that did
not have a type compatible with rsize_t.
Runtime-constraints
2 format shall not be a null pointer. The %n specifier479) (modified or not by flags, field width,
or precision) shall not appear in the string pointed to by format. Any argument to printf_s
corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the printf_s function does not attempt to produce further
output, and it is unspecified to what extent printf_s produced output before discovering the
runtime-constraint violation.
Description
4 The printf_s function is equivalent to the printf function except for the explicit runtime-
constraints listed above.
Returns
5 The printf_s function returns the number of characters transmitted, or a negative value if an
output error, encoding error, or runtime-constraint violation occurred.
K.3.5.3.4 The scanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int scanf_s(const char * restrict format, ...);
Runtime-constraints
2 format shall not be a null pointer. Any argument indirected though in order to store converted
input shall not be a null pointer.
3 If there is a runtime-constraint violation, the scanf_s function does not attempt to perform further
input, and it is unspecified to what extent scanf_s performed input before discovering the runtime-
constraint violation.
Description
4 The scanf_s function is equivalent to fscanf_s with the argument stdin interposed before the
arguments to scanf_s.
Returns
5 The scanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the scanf_s function returns the
number of input items assigned, which can be fewer than provided for, or even zero, in the event of
an early matching failure.
K.3.5.3.5 The snprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int snprintf_s(char * restrict s, rsize_t n, const char * restrict format, ...);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than RSIZE_MAX.
The %n specifier480) (modified or not by flags, field width, or precision) shall not appear in the string
pointed to by format. Any argument to snprintf_s corresponding to a %s specifier shall not be a
null pointer. No encoding error shall occur.
479) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
480) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero and
not greater than RSIZE_MAX, then the snprintf_s function sets s[0] to the null character.
Description
4 The snprintf_s function is equivalent to the snprintf function except for the explicit runtime-
constraints listed above.
5 The snprintf_s function, unlike sprintf_s, will truncate the result to fit within the array pointed
to by s.
Returns
6 The snprintf_s function returns the number of characters that would have been written had n
been sufficiently large, not counting the terminating null character, or a negative value if a runtime-
constraint violation occurred. Thus, the null-terminated output has been completely written if and
only if the returned value is both nonnegative and less than n.
K.3.5.3.6 The sprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int sprintf_s(char * restrict s, rsize_t n, const char * restrict format, ...);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than RSIZE_MAX.
The number of characters (including the trailing null) required for the result to be written to the
array pointed to by s shall not be greater than n. The %n specifier481) (modified or not by flags,
field width, or precision) shall not appear in the string pointed to by format. Any argument to
sprintf_s corresponding to a %s specifier shall not be a null pointer. No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero and
not greater than RSIZE_MAX, then the sprintf_s function sets s[0] to the null character.
Description
4 The sprintf_s function is equivalent to the sprintf function except for the parameter n and the
explicit runtime-constraints listed above.
5 The sprintf_s function, unlike snprintf_s, treats a result too big for the array pointed to by s as a
runtime-constraint violation.
Returns
6 If no runtime-constraint violation occurred, the sprintf_s function returns the number of characters
written in the array, not counting the terminating null character. If an encoding error occurred,
sprintf_s returns a negative value. If any other runtime-constraint violation occurred, sprintf_s
returns zero.
481) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
K.3.5.3.7 The sscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int sscanf_s(const char * restrict s, const char * restrict format, ...);
Runtime-constraints
2 Neither s nor format shall be a null pointer. Any argument indirected though in order to store
converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the sscanf_s function does not attempt to perform further
input, and it is unspecified to what extent sscanf_s performed input before discovering the runtime-
constraint violation.
Description
4 The sscanf_s function is equivalent to fscanf_s, except that input is obtained from a string
(specified by the argument s) rather than from a stream. Reaching the end of the string is equivalent
to encountering end-of-file for the fscanf_s function. If copying takes place between objects that
overlap, the objects take on unspecified values.
Returns
5 The sscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the sscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.5.3.8 The vfprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vfprintf_s(FILE *restrict stream, const char *restrict format, va_list arg);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. The %n specifier482) (modified or not by flags,
field width, or precision) shall not appear in the string pointed to by format. Any argument to
vfprintf_s corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the vfprintf_s function does not attempt to produce
further output, and it is unspecified to what extent vfprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The vfprintf_s function is equivalent to the vfprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The vfprintf_s function returns the number of characters transmitted, or a negative value if an
output error, encoding error, or runtime-constraint violation occurred.
482) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
K.3.5.3.9 The vfscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vfscanf_s(FILE *restrict stream, const char *restrict format, va_list arg);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. Any argument indirected though in order to
store converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the vfscanf_s function does not attempt to perform
further input, and it is unspecified to what extent vfscanf_s performed input before discovering
the runtime-constraint violation.
Description
4 The vfscanf_s function is equivalent to fscanf_s, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfscanf_s function does not invoke the va_end macro.483)
Returns
5 The vfscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the vfscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.5.3.10 The vprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vprintf_s(const char * restrict format, va_list arg);
Runtime-constraints
2 format shall not be a null pointer. The %n specifier484) (modified or not by flags, field width,
or precision) shall not appear in the string pointed to by format. Any argument to vprintf_s
corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the vprintf_s function does not attempt to produce
further output, and it is unspecified to what extent vprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The vprintf_s function is equivalent to the vprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The vprintf_s function returns the number of characters transmitted, or a negative value if an
output error, encoding error, or runtime-constraint violation occurred.
K.3.5.3.11 The vscanf_s function
483) As the functions vfprintf_s , vfscanf_s , vprintf_s , vscanf_s , vsnprintf_s , vsprintf_s , and vsscanf_s invoke
the va_arg macro, the representation of arg after the return is indeterminate.
484) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vscanf_s(const char * restrict format, va_list arg);
Runtime-constraints
2 format shall not be a null pointer. Any argument indirected though in order to store converted
input shall not be a null pointer.
3 If there is a runtime-constraint violation, the vscanf_s function does not attempt to perform further
input, and it is unspecified to what extent vscanf_s performed input before discovering the runtime-
constraint violation.
Description
4 The vscanf_s function is equivalent to scanf_s, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vscanf_s function does not invoke the va_end macro485) .
Returns
5 The vscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the vscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.5.3.12 The vsnprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vsnprintf_s(char *restrict s, rsize_t n, const char *restrict format,
va_list arg);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than RSIZE_MAX.
The %n specifier486) (modified or not by flags, field width, or precision) shall not appear in the string
pointed to by format. Any argument to vsnprintf_s corresponding to a %s specifier shall not be a
null pointer. No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero and
not greater than RSIZE_MAX, then the vsnprintf_s function sets s[0] to the null character.
Description
4 The vsnprintf_s function is equivalent to the vsnprintf function except for the explicit runtime-
constraints listed above.
5 The vsnprintf_s function, unlike vsprintf_s, will truncate the result to fit within the array pointed
to by s.
Returns
6 The vsnprintf_s function returns the number of characters that would have been written had n
been sufficiently large, not counting the terminating null character, or a negative value if a runtime-
constraint violation occurred. Thus, the null-terminated output has been completely written if and
485) As the functions vfprintf_s , vfscanf_s , vprintf_s , vscanf_s , vsnprintf_s , vsprintf_s , and vsscanf_s invoke
the va_arg macro, the representation of arg after the return is indeterminate.
486) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
only if the returned value is both nonnegative and less than n.
K.3.5.3.13 The vsprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vsprintf_s(char * restrict s, rsize_t n, const char * restrict format,
va_list arg);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than RSIZE_MAX.
The number of characters (including the trailing null) required for the result to be written to the array
pointed to by s shall not be greater than n. The %n specifier487) (modified or not by flags, field width,
or precision) shall not appear in the string pointed to by format. Any argument to vsprintf_s
corresponding to a %s specifier shall not be a null pointer. No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero and
not greater than RSIZE_MAX, then the vsprintf_s function sets s[0] to the null character.
Description
4 The vsprintf_s function is equivalent to the vsprintf function except for the parameter n and the
explicit runtime-constraints listed above.
5 The vsprintf_s function, unlike vsnprintf_s, treats a result too big for the array pointed to by s
as a runtime-constraint violation.
Returns
6 If no runtime-constraint violation occurred, the vsprintf_s function returns the number of char-
acters written in the array, not counting the terminating null character. If an encoding error oc-
curred, vsprintf_s returns a negative value. If any other runtime-constraint violation occurred,
vsprintf_s returns zero.
K.3.5.3.14 The vsscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
int vsscanf_s(const char *restrict s, const char *restrict format, va_list arg);
Runtime-constraints
2 Neither s nor format shall be a null pointer. Any argument indirected though in order to store
converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the vsscanf_s function does not attempt to perform
further input, and it is unspecified to what extent vsscanf_s performed input before discovering
the runtime-constraint violation.
Description
4 The vsscanf_s function is equivalent to sscanf_s, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsscanf_s function does not invoke the va_end macro.488)
487) It is not a runtime-constraint violation for the characters %n to appear in sequence in the string pointed at by format
when those characters are not a interpreted as a %n specifier. For example, if the entire format string was %%n.
488) As the functions vfprintf_s , vfscanf_s , vprintf_s , vscanf_s , vsnprintf_s , vsprintf_s , and vsscanf_s invoke
the va_arg macro, the value of arg after the return is indeterminate.
Returns
5 The vsscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the vscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.5.4 Character input/output functions
K.3.5.4.1 The gets_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
char *gets_s(char *s, rsize_t n);
Runtime-constraints
2 s shall not be a null pointer. n shall neither be equal to zero nor be greater than RSIZE_MAX. A new-
line character, end-of-file, or read error shall occur within reading n-1 characters from stdin.489)
3 If there is a runtime-constraint violation, characters are read and discarded from stdin until a
new-line character is read, or end-of-file or a read error occurs, and if s is not a null pointer, s[0] is
set to the null character.
Description
4 The gets_s function reads at most one less than the number of characters specified by n from the
stream pointed to by stdin, into the array pointed to by s. No additional characters are read after a
new-line character (which is discarded) or after end-of-file. The discarded new-line character does
not count towards number of characters read. A null character is written immediately after the last
character read into the array.
5 If end-of-file is encountered and no characters have been read into the array, or if a read error
occurs during the operation, then s[0] is set to the null character, and the other elements of s take
unspecified values.
Recommended practice
6 The fgets function allows properly-written programs to safely process input lines too long to store
in the result array. In general this requires that callers of fgets pay attention to the presence or
absence of a new-line character in the result array. Consider using fgets (along with any needed
processing based on new-line characters) instead of gets_s.
Returns
7 The gets_s function returns s if successful. If there was a runtime-constraint violation, or if end-of-
file is encountered and no characters have been read into the array, or if a read error occurs during
the operation, then a null pointer is returned.
489) The gets_s function, unlike the historical gets function, makes it a runtime-constraint violation for a line of input to
overflow the buffer to store it. Unlike the fgets function, gets_s maintains a one-to-one relationship between input lines
and successful calls to gets_s. Programs that use gets expect such a relationship.
K.3.6 General utilities <stdlib.h>
1 The header <stdlib.h> defines three types.
2 The types are
errno_t
which is type int; and
rsize_t
which is the type size_t; and
constraint_handler_t
which has the following definition
typedef void (*constraint_handler_t)(
const char * restrict msg,
void * restrict ptr,
errno_t error);
K.3.6.1 Runtime-constraint handling
K.3.6.1.1 The set_constraint_handler_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
constraint_handler_t set_constraint_handler_s(constraint_handler_t handler);
Description
2 The set_constraint_handler_s function sets the runtime-constraint handler to be handler. The
runtime-constraint handler is the function to be called when a library function detects a runtime-
constraint violation. Only the most recent handler registered with set_constraint_handler_s is
called when a runtime-constraint violation occurs.
3 When the handler is called, it is passed the following arguments in the following order:
1. A pointer to a character string describing the runtime-constraint violation.
2. A null pointer or a pointer to an implementation-defined object.
3. If the function calling the handler has a return type declared as errno_t, the return value of
the function is passed. Otherwise, a positive value of type errno_t is passed.
4 The implementation has a default constraint handler that is used if no calls to the
set_constraint_handler_s function have been made. The behavior of the default handler is
implementation-defined, and it may cause the program to exit or abort.
5 If the handler argument to set_constraint_handler_s is a null pointer, the implementation
default handler becomes the current constraint handler.
Returns
6 The set_constraint_handler_s function returns a pointer to the previously registered handler.490)
490) If the previous handler was registered by calling set_constraint_handler_s with a null pointer argument, a pointer to
the implementation default handler is returned (not NULL).
K.3.6.1.2 The abort_handler_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
void abort_handler_s(const char * restrict msg, void * restrict ptr,
errno_t error);
Description
2 A pointer to the abort_handler_s function shall be a suitable argument to the
set_constraint_handler_s function.
3 The abort_handler_s function writes a message on the standard error stream in an implementation-
defined format. The message shall include the string pointed to by msg. The abort_handler_s
function then calls the abort function.491)
Returns
4 The abort_handler_s function does not return to its caller.
K.3.6.1.3 The ignore_handler_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
void ignore_handler_s(const char * restrict msg, void * restrict ptr,
errno_t error);
Description
2 A pointer to the ignore_handler_s function shall be a suitable argument to the
set_constraint_handler_s function.
3 The ignore_handler_s function simply returns to its caller.492)
Returns
4 The ignore_handler_s function returns no value.
K.3.6.2 Communication with the environment
K.3.6.2.1 The getenv_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
errno_t getenv_s(size_t * restrict len, char * restrict value, rsize_t maxsize,
const char * restrict name);
Runtime-constraints
2 name shall not be a null pointer. maxsize shall not be greater than RSIZE_MAX. If maxsize is not
equal to zero, then value shall not be a null pointer.
491) Many implementations invoke a debugger when the abort function is called.
492) If the runtime-constraint handler is set to the ignore_handler_s function, any library function in which a runtime-
constraint violation occurs will return to its caller. The caller can determine whether a runtime-constraint violation occurred
based on the library function’s specification (usually, the library function returns a nonzero errno_t).
3 If there is a runtime-constraint violation, the integer pointed to by len is set to 0 (if len is not null),
and the environment list is not searched.
Description
4 The getenv_s function searches an environment list, provided by the host environment, for a string
that matches the string pointed to by name.
5 If that name is found then getenv_s performs the following actions. If len is not a null pointer, the
length of the string associated with the matched list member is stored in the integer pointed to by
len. If the length of the associated string is less than maxsize, then the associated string is copied to
the array pointed to by value.
6 If that name is not found then getenv_s performs the following actions. If len is not a null pointer,
zero is stored in the integer pointed to by len. If maxsize is greater than zero, then value[0] is set
to the null character.
7 The set of environment names and the method for altering the environment list are implementation-
defined. The getenv_s function need not avoid data races with other threads of execution that
modify the environment list.493)
Returns
8 The getenv_s function returns zero if the specified name is found and the associated string was
successfully stored in value. Otherwise, a nonzero value is returned.
K.3.6.3 Searching and sorting utilities
1 These utilities make use of a comparison function to search or sort arrays of unspecified type. Where
an argument declared as size_t nmemb specifies the length of the array for a function, if nmemb has
the value zero on a call to that function, then the comparison function is not called, a search finds no
matching element, sorting performs no rearrangement, and the pointer to the array may be null.
2 The implementation shall ensure that the second argument of the comparison function (when called
from bsearch_s), or both arguments (when called from qsort_s), are pointers to elements of the
array.494) The first argument when called from bsearch_s shall equal key.
3 The comparison function shall not alter the contents of either the array or search key. The implemen-
tation may reorder elements of the array between calls to the comparison function, but shall not
otherwise alter the contents of any individual element.
4 When the same objects (consisting of size bytes, irrespective of their current positions in the array)
are passed more than once to the comparison function, the results shall be consistent with one
another. That is, for qsort_s they shall define a total ordering on the array, and for bsearch_s the
same object shall always compare the same way with the key.
5 A sequence point occurs immediately before and immediately after each call to the comparison
function, and also between any call to the comparison function and any movement of the objects
passed as arguments to that call.
K.3.6.3.1 The bsearch_s generic function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
void *bsearch_s(const void *key, QVoid *base, rsize_t nmemb, rsize_t size,
int (*compar)(const void *k, const void *y, void *context),
void *context);
493) Many implementations provide non-standard functions that modify the environment list.
494) That is, if the value passed is p, then the following expressions are always valid and nonzero:
((char *)p - (char *)base) % size == 0
(char *)p >= (char *)base
(char *)p < (char *)base + nmemb * size
Runtime-constraints
2 Neither nmemb nor size shall be greater than RSIZE_MAX. If nmemb is not equal to zero, then none of
key, base, or compar shall be a null pointer.
3 If there is a runtime-constraint violation, the bsearch_s function does not search the array.
Description
4 The bsearch_s function searches an array of nmemb objects, the initial element of which is pointed
to by base, for an element that matches the object pointed to by key. The size of each element of the
array is specified by size.
5 The comparison function pointed to by compar is called with three arguments. The first two point
to the key object and to an array element, in that order. The function shall return an integer less
than, equal to, or greater than zero if the key object is considered, respectively, to be less than,
to match, or to be greater than the array element. The array shall consist of: all the elements
that compare less than, all the elements that compare equal to, and all the elements that compare
greater than the key object, in that order.495) The third argument to the comparison function is the
context argument passed to bsearch_s. The sole use of context by bsearch_s is to pass it to the
comparison function.496)
Returns
6 The bsearch_s function returns a pointer to a matching element of the array, or a null pointer if no
match is found or there is a runtime-constraint violation. If two elements compare as equal, which
element is matched is unspecified.
7 The bsearch_s function is generic in the qualification of the type pointed to by the argument to
base. If this argument is a pointer to a const-qualified object type, the returned pointer will be a
pointer to const-qualified void. Otherwise, the argument shall be a pointer to an unqualified object
type or a null pointer constant497) , and the returned pointer will be a pointer to unqualified void.
8 The external declaration of bsearch_s has the concrete type:
void * (const void *, const void *, rsize_t, rsize_t, int (*) (const void *,
const void *), void *)
, which supports all correct uses. If a macro definition of the generic function is suppressed in order
to access an actual function, the external declaration with this concrete type is visible498) .
K.3.6.3.2 The qsort_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
errno_t qsort_s(void *base, rsize_t nmemb, rsize_t size,
int (*compar)(const void *x, const void *y, void *context),
void *context);
Runtime-constraints
2 Neither nmemb nor size shall be greater than RSIZE_MAX. If nmemb is not equal to zero, then neither
base nor compar shall be a null pointer.
3 If there is a runtime-constraint violation, the qsort_s function does not sort the array.
495) In practice, this means that the entire array has been sorted according to the comparison function.
496) The context argument is for the use of the comparison function in performing its duties. For example, it might specify a
collating sequence used by the comparison function.
497) If the argument is a null pointer and the call is executed, the behavior is undefined.
498) This is an obsolescent feature.
Description
4 The qsort_s function sorts an array of nmemb objects, the initial element of which is pointed to by
base. The size of each object is specified by size.
5 The contents of the array are sorted into ascending order according to a comparison function pointed
to by compar, which is called with three arguments. The first two point to the objects being compared.
The function shall return an integer less than, equal to, or greater than zero if the first argument is
considered to be respectively less than, equal to, or greater than the second. The third argument to
the comparison function is the context argument passed to qsort_s. The sole use of context by
qsort_s is to pass it to the comparison function499) .
6 If two elements compare as equal, their relative order in the resulting sorted array is unspecified.
Returns
7 The qsort_s function returns zero if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.6.4 Multibyte/wide character conversion functions
1 The behavior of the multibyte character functions is affected by the LC_CTYPE category of the current
locale. For a state-dependent encoding, each function is placed into its initial conversion state by a
call for which its character pointer argument, s, is a null pointer. Subsequent calls with s as other
than a null pointer cause the internal conversion state of the function to be altered as necessary. A
call with s as a null pointer causes these functions to set the int pointed to by their status argument
to a nonzero value if encodings have state dependency, and zero otherwise. 500)
Changing the LC_CTYPE category causes the internal object describing the conversion state of these
functions to have an indeterminate representation.
K.3.6.4.1 The wctomb_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
errno_t wctomb_s(int *restrict status, char *restrict s, rsize_t smax,
wchar_t wc);
Runtime-constraints
2 Let n denote the number of bytes needed to represent the multibyte character corresponding to the
wide character given by wc (including any shift sequences).
3 If s is not a null pointer, then smax shall not be less than n, and smax shall not be greater than
RSIZE_MAX. If s is a null pointer, then smax shall equal zero.
4 If there is a runtime-constraint violation, wctomb_s does not modify the int pointed to by status,
and if s is not a null pointer, no more than smax elements in the array pointed to by s will be
accessed.
Description
5 The wctomb_s function determines n and stores the multibyte character representation of wc in the
array whose first element is pointed to by s (if s is not a null pointer). The number of characters
stored never exceeds MB_CUR_MAX or smax. If wc is a null wide character, a null byte is stored,
preceded by any shift sequence needed to restore the initial shift state, and the function is left in the
initial conversion state.
6 The implementation shall behave as if no library function calls the wctomb_s function.
7 If s is a null pointer, the wctomb_s function stores into the int pointed to by status a nonzero or zero
499) The context argument is for the use of the comparison function in performing its duties. For example, it might specify a
collating sequence used by the comparison function.
500) If the locale employs special bytes to change the shift state, these bytes do not produce separate wide character codes, but
are grouped with an adjacent multibyte character.
value, if multibyte character encodings, respectively, do or do not have state-dependent encodings.
8 If s is not a null pointer, the wctomb_s function stores into the int pointed to by status either n or
−1 if wc, respectively, does or does not correspond to a valid multibyte character.
9 In no case will the int pointed to by status be set to a value greater than the MB_CUR_MAX macro.
Returns
10 The wctomb_s function returns zero if successful, and a nonzero value if there was a runtime-
constraint violation or wc did not correspond to a valid multibyte character.
K.3.6.5 Multibyte/wide string conversion functions
1 The behavior of the multibyte string functions is affected by the LC_CTYPE category of the current
locale.
K.3.6.5.1 The mbstowcs_s function
Synopsis
1 #include <stdlib.h>
errno_t mbstowcs_s(size_t *restrict retval, wchar_t *restrict dst,
rsize_t dstmax, const char * restrict src, rsize_t len);
Runtime-constraints
2 Neither retval nor src shall be a null pointer. If dst is not a null pointer, then neither len nor
dstmax shall be greater than RSIZE_MAX/sizeof(wchar_t). If dst is a null pointer, then dstmax
shall equal zero. If dst is not a null pointer, then dstmax shall not equal zero. If dst is not a null
pointer and len is not less than dstmax, then a null character shall occur within the first dstmax
multibyte characters of the array pointed to by src.
3 If there is a runtime-constraint violation, then mbstowcs_s does the following. If retval is not
a null pointer, then mbstowcs_s sets *retval to (size_t)(-1) . If dst is not a null pointer and
dstmax is greater than zero and not greater than RSIZE_MAX/sizeof(wchar_t), then mbstowcs_s
sets dst[0] to the null wide character.
Description
4 The mbstowcs_s function converts a sequence of multibyte characters that begins in the initial shift
state from the array pointed to by src into a sequence of corresponding wide characters. If dst is
not a null pointer, the converted characters are stored into the array pointed to by dst. Conversion
continues up to and including a terminating null character, which is also stored. Conversion stops
earlier in two cases: when a sequence of bytes is encountered that does not form a valid multibyte
character, or (if dst is not a null pointer) when len wide characters have been stored into the array
pointed to by dst.501) If dst is not a null pointer and no null wide character was stored into the
array pointed to by dst, then dst[len] is set to the null wide character. Each conversion takes place
as if by a call to the mbrtowc function.
5 Regardless of whether dst is or is not a null pointer, if the input conversion encounters a sequence of
bytes that do not form a valid multibyte character, an encoding error occurs: the mbstowcs_s func-
tion stores the value (size_t)(-1) into *retval . Otherwise, the mbstowcs_s function stores into
*retval the number of multibyte characters successfully converted, not including the terminating
null character (if any).
6 All elements following the terminating null wide character (if any) written by mbstowcs_s in the
array of dstmax wide characters pointed to by dst take unspecified values when mbstowcs_s
returns.502)
7 If copying takes place between objects that overlap, the objects take on unspecified values.
501) Thus, the value of len is ignored if dst is a null pointer.
502) This allows an implementation to attempt converting the multibyte string before discovering a terminating null character
did not occur where required.
Returns
8 The mbstowcs_s function returns zero if no runtime-constraint violation and no encoding error
occurred. Otherwise, a nonzero value is returned.
K.3.6.5.2 The wcstombs_s function
Synopsis
1 #include <stdlib.h>
errno_t wcstombs_s(size_t * restrict retval, char * restrict dst, rsize_t dstmax,
const wchar_t * restrict src, rsize_t len);
Runtime-constraints
2 Neither retval nor src shall be a null pointer. If dst is not a null pointer, then len shall not
be greater than RSIZE_MAX/sizeof(wchar_t) and dstmax shall be nonzero and not greater than
RSIZE_MAX. If dst is a null pointer, then dstmax shall equal zero. If dst is not a null pointer and
len is not less than dstmax, then the conversion shall have been stopped (see below) because a
terminating null wide character was reached or because an encoding error occurred.
3 If there is a runtime-constraint violation, then wcstombs_s does the following. If retval is not
a null pointer, then wcstombs_s sets *retval to (size_t)(-1) . If dst is not a null pointer and
dstmax is greater than zero and not greater than RSIZE_MAX, then wcstombs_s sets dst[0] to the
null character.
Description
4 The wcstombs_s function converts a sequence of wide characters from the array pointed to by
src into a sequence of corresponding multibyte characters that begins in the initial shift state. If
dst is not a null pointer, the converted characters are then stored into the array pointed to by dst.
Conversion continues up to and including a terminating null wide character, which is also stored.
Conversion stops earlier in two cases:
— when a wide character is reached that does not correspond to a valid multibyte character;
— (if dst is not a null pointer) when the next multibyte character would exceed the limit of n
total bytes to be stored into the array pointed to by dst. If the wide character being converted
is the null wide character, then n is the lesser of len or dstmax. Otherwise, n is the lesser of
len or dstmax-1.
If the conversion stops without converting a null wide character and dst is not a null pointer, then
a null character is stored into the array pointed to by dst immediately following any multibyte
characters already stored. Each conversion takes place as if by a call to the wcrtomb function.503)
5 Regardless of whether dst is or is not a null pointer, if the input conversion encounters a wide
character that does not correspond to a valid multibyte character, an encoding error occurs: the
wcstombs_s function stores the value (size_t)(-1) into *retval . Otherwise, the wcstombs_s
function stores into *retval the number of bytes in the resulting multibyte character sequence, not
including the terminating null character (if any).
6 All elements following the terminating null character (if any) written by wcstombs_s in the array of
dstmax elements pointed to by dst take unspecified values when wcstombs_s returns.504)
7 If copying takes place between objects that overlap, the objects take on unspecified values.
Returns
8 The wcstombs_s function returns zero if no runtime-constraint violation and no encoding error
occurred. Otherwise, a nonzero value is returned.
503) If conversion stops because a terminating null wide character has been reached, the bytes stored include those necessary
to reach the initial shift state immediately before the null byte. However, if the conversion stops before a terminating null
wide character has been reached, the result will be null terminated, but might not end in the initial shift state.
504) When len is not less than dstmax, the implementation might fill the array before discovering a runtime-constraint
violation.
K.3.7 String handling <string.h>
1 The header <string.h> defines two types.
2 The types are
errno_t
which is type int; and
rsize_t
which is the type size_t.
K.3.7.1 Copying functions
K.3.7.1.1 The memcpy_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t memcpy_s(void * restrict s1, rsize_t s1max, const void * restrict s2,
rsize_t n);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than RSIZE_MAX. n
shall not be greater than s1max. Copying shall not take place between objects that overlap.
3 If there is a runtime-constraint violation, the memcpy_s function stores zeros in the first s1max
characters of the object pointed to by s1 if s1 is not a null pointer and s1max is not greater than
RSIZE_MAX.
Description
4 The memcpy_s function copies n characters from the object pointed to by s2 into the object pointed
to by s1.
Returns
5 The memcpy_s function returns zero if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.7.1.2 The memmove_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t memmove_s(void *s1, rsize_t s1max, const void *s2, rsize_t n);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than RSIZE_MAX. n
shall not be greater than s1max.
3 If there is a runtime-constraint violation, the memmove_s function stores zeros in the first s1max
characters of the object pointed to by s1 if s1 is not a null pointer and s1max is not greater than
RSIZE_MAX.
Description
4 The memmove_s function copies n characters from the object pointed to by s2 into the object pointed
to by s1. This copying takes place as if the n characters from the object pointed to by s2 are first
copied into a temporary array of n characters that does not overlap the objects pointed to by s1 or
s2, and then the n characters from the temporary array are copied into the object pointed to by s1.
Returns
5 The memmove_s function returns zero if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.7.1.3 The strcpy_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t strcpy_s(char * restrict s1, rsize_t s1max, const char * restrict s2);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. s1max shall not be greater than RSIZE_MAX. s1max shall
not equal zero. s1max shall be greater than strnlen_s(s2, s1max). Copying shall not take place
between objects that overlap.
3 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX, then strcpy_s sets s1[0] to the null character.
Description
4 The strcpy_s function copies the string pointed to by s2 (including the terminating null character)
into the array pointed to by s1.
5 All elements following the terminating null character (if any) written by strcpy_s in the array of
s1max characters pointed to by s1 take unspecified values when strcpy_s returns.505)
Returns
6 The strcpy_s function returns zero506) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.7.1.4 The strncpy_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t strncpy_s(char * restrict s1, rsize_t s1max, const char * restrict s2,
rsize_t n);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than RSIZE_MAX.
s1max shall not equal zero. If n is not less than s1max, then s1max shall be greater than
strnlen_s(s2, s1max). Copying shall not take place between objects that overlap.
3 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX, then strncpy_s sets s1[0] to the null character.
Description
4 The strncpy_s function copies not more than n successive characters (characters that follow a null
character are not copied) from the array pointed to by s2 to the array pointed to by s1. If no null
character was copied from s2, then s1[n] is set to a null character.
5 All elements following the terminating null character (if any) written by strncpy_s in the array
of s1max characters pointed to by s1 take unspecified values when strncpy_s returns a nonzero
value.507)
505) This allows an implementation to copy characters from s2 to s1 while simultaneously checking if any of those characters
are null. Such an approach might write a character to every element of s1 before discovering that the first element was set to
the null character.
506) A zero return value implies that all of the requested characters from the string pointed to by s2 fit within the array
pointed to by s1 and that the result in s1 is null terminated.
507) This allows an implementation to copy characters from s2 to s1 while simultaneously checking if any of those characters
are null. Such an approach might write a character to every element of s1 before discovering that the first element was set to
the null character.
Returns
6 The strncpy_s function returns zero508) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
7 EXAMPLE 1 The strncpy_s function can be used to copy a string without the danger that the result will not be null
terminated or that characters will be written past the end of the destination array.
#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
/* ... */
char src1[100] = "hello";
char src2[7] = {’g’, ’o’, ’o’, ’d’, ’b’, ’y’, ’e’};
char dst1[6], dst2[5], dst3[5];
int r1, r2, r3;
r1 = strncpy_s(dst1, 6, src1, 100);
r2 = strncpy_s(dst2, 5, src2, 7);
r3 = strncpy_s(dst3, 5, src2, 4);
The first call will assign to r1 the value zero and to dst1 the sequence hello\0.
The second call will assign to r2 a nonzero value and to dst2 the sequence \0.
The third call will assign to r3 the value zero and to dst3 the sequence good\0.
K.3.7.2 Concatenation functions
K.3.7.2.1 The strcat_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t strcat_s(char * restrict s1, rsize_t s1max, const char * restrict s2);
Runtime-constraints
2 Let m denote the value s1max - strnlen_s(s1, s1max) upon entry to strcat_s.
3 Neither s1 nor s2 shall be a null pointer. s1max shall not be greater than RSIZE_MAX. s1max shall
not equal zero. m shall not equal zero.509) m shall be greater than strnlen_s(s2, m). Copying shall
not take place between objects that overlap.
4 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX, then strcat_s sets s1[0] to the null character.
Description
5 The strcat_s function appends a copy of the string pointed to by s2 (including the terminating
null character) to the end of the string pointed to by s1. The initial character from s2 overwrites the
null character at the end of s1.
6 All elements following the terminating null character (if any) written by strcat_s in the array of
s1max characters pointed to by s1 take unspecified values when strcat_s returns.510)
Returns
7 The strcat_s function returns zero511) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.7.2.2 The strncat_s function
508) A zero return value implies that all of the requested characters from the string pointed to by s2 fit within the array
pointed to by s1 and that the result in s1 is null terminated.
509) Zero means that s1 was not null terminated upon entry to strcat_s.
510) This allows an implementation to append characters from s2 to s1 while simultaneously checking if any of those
characters are null. Such an approach might write a character to every element of s1 before discovering that the first element
was set to the null character.
511) A zero return value implies that all of the requested characters from the string pointed to by s2 were appended to the
string pointed to by s1 and that the result in s1 is null terminated.
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t strncat_s(char * restrict s1, rsize_t s1max, const char * restrict s2,
rsize_t n);
Runtime-constraints
2 Let m denote the value s1max - strnlen_s(s1, s1max) upon entry to strncat_s.
3 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than RSIZE_MAX.
s1max shall not equal zero. m shall not equal zero.512) If n is not less than m, then m shall be greater
than strnlen_s(s2, m). Copying shall not take place between objects that overlap.
4 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX, then strncat_s sets s1[0] to the null character.
Description
5 The strncat_s function appends not more than n successive characters (characters that follow a
null character are not copied) from the array pointed to by s2 to the end of the string pointed to by
s1. The initial character from s2 overwrites the null character at the end of s1. If no null character
was copied from s2, then s1[s1max- m +n] is set to a null character.
6 All elements following the terminating null character (if any) written by strncat_s in the array of
s1max characters pointed to by s1 take unspecified values when strncat_s returns.513)
Returns
7 The strncat_s function returns zero514) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
8 EXAMPLE 1 The strncat_s function can be used to copy a string without the danger that the result will not be null
terminated or that characters will be written past the end of the destination array.
#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
/* ... */
char s1[100] = "good";
char s2[6] = "hello";
char s3[6] = "hello";
char s4[7] = "abc";
char s5[1000] = "bye";
int r1, r2, r3, r4;
r1 = strncat_s(s1, 100, s5, 1000);
r2 = strncat_s(s2, 6, "", 1);
r3 = strncat_s(s3, 6, "X", 2);
r4 = strncat_s(s4, 7, "defghijklmn", 3);
After the first call r1 will have the value zero and s1 will contain the sequence goodbye\0.
After the second call r2 will have the value zero and s2 will contain the sequence hello\0.
After the third call r3 will have a nonzero value and s3 will contain the sequence \0.
After the fourth call r4 will have the value zero and s4 will contain the sequence abcdef\0.
K.3.7.3 Search functions
K.3.7.3.1 The strtok_s function
512) Zero means that s1 was not null terminated upon entry to strncat_s.
513) This allows an implementation to append characters from s2 to s1 while simultaneously checking if any of those
characters are null. Such an approach might write a character to every element of s1 before discovering that the first element
was set to the null character.
514) A zero return value implies that all of the requested characters from the string pointed to by s2 were appended to the
string pointed to by s1 and that the result in s1 is null terminated.
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
char *strtok_s(char * restrict s1, rsize_t * restrict s1max,
const char * restrict s2, char ** restrict ptr);
Runtime-constraints
2 None of s1max, s2, or ptr shall be a null pointer. If s1 is a null pointer, then *ptr shall not be a
null pointer. The value of *s1max shall not be greater than RSIZE_MAX. The end of the token found
shall occur within the first *s1max characters of s1 for the first call, and shall occur within the first
*s1max characters of where searching resumes on subsequent calls.
3 If there is a runtime-constraint violation, the strtok_s function does not indirect through the s1 or
s2 pointers, and does not store a value in the object pointed to by ptr.
Description
4 A sequence of calls to the strtok_s function breaks the string pointed to by s1 into a sequence of
tokens, each of which is delimited by a character from the string pointed to by s2. The fourth argu-
ment points to a caller-provided char pointer into which the strtok_s function stores information
necessary for it to continue scanning the same string.
5 The first call in a sequence has a non-null first argument and s1max points to an object whose value
is the number of elements in the character array pointed to by the first argument. The first call stores
an initial value in the object pointed to by ptr and updates the value pointed to by s1max to reflect
the number of elements that remain in relation to ptr. Subsequent calls in the sequence have a null
first argument and the objects pointed to by s1max and ptr are required to have the values stored
by the previous call in the sequence, which are then updated. The separator string pointed to by s2
may be different from call to call.
6 The first call in the sequence searches the string pointed to by s1 for the first character that is not
contained in the current separator string pointed to by s2. If no such character is found, then there
are no tokens in the string pointed to by s1 and the strtok_s function returns a null pointer. If such
a character is found, it is the start of the first token.
7 The strtok_s function then searches from there for the first character in s1 that is contained in the
current separator string. If no such character is found, the current token extends to the end of the
string pointed to by s1, and subsequent searches in the same string for a token return a null pointer.
If such a character is found, it is overwritten by a null character, which terminates the current token.
8 In all cases, the strtok_s function stores sufficient information in the pointer pointed to by ptr so
that subsequent calls, with a null pointer for s1 and the unmodified pointer value for ptr, shall start
searching just past the element overwritten by a null character (if any).
Returns
9 The strtok_s function returns a pointer to the first character of a token, or a null pointer if there is
no token or there is a runtime-constraint violation.
10 EXAMPLE
#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
static char str1[] = "?a???b,,,#c";
static char str2[] = "\t \t";
char *t, *ptr1, *ptr2;
rsize_t max1 = sizeof (str1);
rsize_t max2 = sizeof (str2);
t = strtok_s(str1, &max1, "?", &ptr1); // t points to the token "a"
t = strtok_s(NULL, &max1, ",", &ptr1); // t points to the token "??b"
t = strtok_s(str2, &max2, " \t", &ptr2); // t is a null pointer
t = strtok_s(NULL, &max1, "#,", &ptr1); // t points to the token "c"
t = strtok_s(NULL, &max1, "?", &ptr1); // t is a null pointer
K.3.7.4 Miscellaneous functions
K.3.7.4.1 The memset_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t memset_s(void *s, rsize_t smax, int c, rsize_t n)
Runtime-constraints
2 s shall not be a null pointer. Neither smax nor n shall be greater than RSIZE_MAX. n shall not be
greater than smax.
3 If there is a runtime-constraint violation, then if s is not a null pointer and smax is not greater than
RSIZE_MAX, the memset_s function stores the value of c (converted to an unsigned char) into each
of the first smax characters of the object pointed to by s.
Description
4 The memset_s function copies the value of c (converted to an unsigned char) into each of the first
n characters of the object pointed to by s. Unlike memset, any call to the memset_s function shall be
evaluated strictly according to the rules of the abstract machine as described in (5.1.2.3). That is, any
call to the memset_s function shall assume that the memory indicated by s and n may be accessible
in the future and thus contains the values indicated by c.
Returns
5 The memset_s function returns zero if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.7.4.2 The strerror_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
errno_t strerror_s(char *s, rsize_t maxsize, errno_t errnum);
Runtime-constraints
2 s shall not be a null pointer. maxsize shall not be greater than RSIZE_MAX. maxsize shall not equal
zero.
3 If there is a runtime-constraint violation, then the array (if any) pointed to by s is not modified.
Description
4 The strerror_s function maps the number in errnum to a locale-specific message string. Typically,
the values for errnum come from errno, but strerror_s shall map any value of type int to a
message.
5 If the length of the desired string is less than maxsize, then the string is copied to the array pointed
to by s.
6 Otherwise, if maxsize is greater than zero, then maxsize-1 characters are copied from the string
to the array pointed to by s and then s[maxsize-1] is set to the null character. Then, if maxsize
is greater than 3, then s[maxsize-2], s[maxsize-3], and s[maxsize-4] are set to the character
period (.).
Returns
7 The strerror_s function returns zero if the length of the desired string was less than maxsize and
there was no runtime-constraint violation. Otherwise, the strerror_s function returns a nonzero
value.
K.3.7.4.3 The strerrorlen_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
size_t strerrorlen_s(errno_t errnum);
Description
2 The strerrorlen_s function calculates the length of the (untruncated) locale-specific message
string that the strerror_s function maps to errnum.
Returns
3 The strerrorlen_s function returns the number of characters (not including the null character) in
the full message string.
K.3.7.4.4 The strnlen_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
size_t strnlen_s(const char *s, size_t maxsize);
Description
2 The strnlen_s function computes the length of the string pointed to by s.
Returns
3 If s is a null pointer,515) then the strnlen_s function returns zero.
4 Otherwise, the strnlen_s function returns the number of characters that precede the terminating
null character. If there is no null character in the first maxsize characters of s then strnlen_s
returns maxsize. At most the first maxsize characters of s shall be accessed by strnlen_s.
K.3.8 Date and time <time.h>
1 The header <time.h> defines two types.
2 The types are
errno_t
which is type int; and
rsize_t
which is the type size_t.
K.3.8.1 Components of time
1 A broken-down time is normalized if the values of the members of the tm structure are in their normal
rages.516)
K.3.8.2 Time conversion functions
1 Like the strftime function, the asctime_s and ctime_s functions do not return a pointer to a static
object, and other library functions are permitted to call them.
K.3.8.2.1 The asctime_s function
515) Note that the strnlen_s function has no runtime-constraints. This lack of runtime-constraints along with the values
returned for a null pointer or an unterminated string argument make strnlen_s useful in algorithms that gracefully handle
such exceptional data.
516) The normal ranges are defined in 7.29.1.
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
errno_t asctime_s(char *s, rsize_t maxsize, const struct tm *timeptr);
Runtime-constraints
2 Neither s nor timeptr shall be a null pointer. maxsize shall not be less than 26 and shall not be
greater than RSIZE_MAX. The broken-down time pointed to by timeptr shall be normalized. The
calendar year represented by the broken-down time pointed to by timeptr shall not be less than
calendar year 0 and shall not be greater than calendar year 9999.
3 If there is a runtime-constraint violation, there is no attempt to convert the time, and s[0] is set to a
null character if s is not a null pointer and maxsize is not zero and is not greater than RSIZE_MAX.
Description
4 The asctime_s function converts the normalized broken-down time in the structure pointed to by
timeptr into a 26 character (including the null character) string in the form
Sun Sep 16 01:03:52 1973\n\0
The fields making up this string are (in order):
1. The name of the day of the week represented by timeptr->tm_wday using the following three
character weekday names: Sun, Mon, Tue, Wed, Thu, Fri, and Sat.
2. The character space.
3. The name of the month represented by timeptr->tm_mon using the following three character
month names: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, and Dec.
4. The character space.
5. The value of timeptr->tm_mday as if printed using the fprintf format "%2d".
6. The character space.
7. The value of timeptr->tm_hour as if printed using the fprintf format "%.2d".
8. The character colon.
9. The value of timeptr->tm_min as if printed using the fprintf format "%.2d".
10. The character colon.
11. The value of timeptr->tm_sec as if printed using the fprintf format "%.2d".
12. The character space.
13. The value of timeptr->tm_year + 1900 as if printed using the fprintf format "%4d".
14. The character new line.
15. The null character.
Recommended practice
The strftime function allows more flexible formatting and supports locale-specific behavior. If you
do not require the exact form of the result string produced by the asctime_s function, consider
using the strftime function instead.
Returns
5 The asctime_s function returns zero if the time was successfully converted and stored into the
array pointed to by s. Otherwise, it returns a nonzero value.
K.3.8.2.2 The ctime_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
errno_t ctime_s(char *s, rsize_t maxsize, const time_t *timer);
Runtime-constraints
2 Neither s nor timer shall be a null pointer. maxsize shall not be less than 26 and shall not be greater
than RSIZE_MAX.
3 If there is a runtime-constraint violation, s[0] is set to a null character if s is not a null pointer and
maxsize is not equal zero and is not greater than RSIZE_MAX.
Description
4 The ctime_s function converts the calendar time pointed to by timer to local time in the form of a
string. It is equivalent to
asctime_s(s, maxsize, localtime_s(timer, &(struct tm){ 0 }))
Recommended practice
The strftime function allows more flexible formatting and supports locale-specific behavior. If you
do not require the exact form of the result string produced by the ctime_s function, consider using
the strftime function instead.
Returns
5 The ctime_s function returns zero if the time was successfully converted and stored into the array
pointed to by s. Otherwise, it returns a nonzero value.
K.3.8.2.3 The gmtime_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
struct tm *gmtime_s(const time_t * restrict timer, struct tm * restrict result);
Runtime-constraints
2 Neither timer nor result shall be a null pointer.
3 If there is a runtime-constraint violation, there is no attempt to convert the time.
Description
4 The gmtime_s function converts the calendar time pointed to by timer into a broken-down time,
expressed as UTC. The broken-down time is stored in the structure pointed to by result.
Returns
5 The gmtime_s function returns result, or a null pointer if the specified time cannot be converted to
UTC or there is a runtime-constraint violation.
K.3.8.2.4 The localtime_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
struct tm *localtime_s(const time_t *restrict timer, struct tm *restrict result);
Runtime-constraints
2 Neither timer nor result shall be a null pointer.
3 If there is a runtime-constraint violation, there is no attempt to convert the time.
Description
4 The localtime_s function converts the calendar time pointed to by timer into a broken-down time,
expressed as local time. The broken-down time is stored in the structure pointed to by result.
Returns
5 The localtime_s function returns result, or a null pointer if the specified time cannot be converted
to local time or there is a runtime-constraint violation.
K.3.9 Extended multibyte and wide character utilities <wchar.h>
1 The header <wchar.h> defines two types.
2 The types are
errno_t
which is type int; and
rsize_t
which is the type size_t.
3 Unless explicitly stated otherwise, if the execution of a function described in this subclause causes
copying to take place between objects that overlap, the objects take on unspecified values.
K.3.9.1 Formatted wide character input/output functions
K.3.9.1.1 The fwprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
int fwprintf_s(FILE * restrict stream, const wchar_t * restrict format, ...);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. The %n specifier517) (modified or not by flags,
field width, or precision) shall not appear in the wide string pointed to by format. Any argument to
fwprintf_s corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the fwprintf_s function does not attempt to produce
further output, and it is unspecified to what extent fwprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The fwprintf_s function is equivalent to the fwprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The fwprintf_s function returns the number of wide characters transmitted, or a negative value if
an output error, encoding error, or runtime-constraint violation occurred.
K.3.9.1.2 The fwscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
517) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
#include <wchar.h>
int fwscanf_s(FILE * restrict stream, const wchar_t * restrict format, ...);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. Any argument indirected though in order to
store converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the fwscanf_s function does not attempt to perform
further input, and it is unspecified to what extent fwscanf_s performed input before discovering
the runtime-constraint violation.
Description
4 The fwscanf_s function is equivalent to fwscanf except that the c, s, and [ conversion specifiers
apply to a pair of arguments (unless assignment suppression is indicated by a *). The first of these
arguments is the same as for fwscanf. That argument is immediately followed in the argument
list by the second argument, which has type size_t and gives the number of elements in the array
pointed to by the first argument of the pair. If the first argument points to a scalar object, it is
considered to be an array of one element.518)
5 A matching failure occurs if the number of elements in a receiving object is insufficient to hold the
converted input (including any trailing null character).
Returns
6 The fwscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the fwscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.9.1.3 The snwprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
int snwprintf_s(wchar_t * restrict s, rsize_t n, const wchar_t * restrict format,
...);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than
RSIZE_MAX/sizeof(wchar_t). The %n specifier519) (modified or not by flags, field width, or pre-
cision) shall not appear in the wide string pointed to by format. Any argument to snwprintf_s
corresponding to a %s specifier shall not be a null pointer. No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero
and not greater than RSIZE_MAX/sizeof(wchar_t), then the snwprintf_s function sets s[0] to
the null wide character.
Description
4 The snwprintf_s function is equivalent to the swprintf function except for the explicit runtime-
constraints listed above.
518) If the format is known at translation time, an implementation can issue a diagnostic for any argument used to store
the result from a c, s, or [ conversion specifier if that argument is not followed by an argument of a type compatible with
rsize_t. A limited amount of checking can be done if even if the format is not known at translation time. For example, an
implementation could issue a diagnostic for each argument after format that has of type pointer to one of char, signed char,
unsigned char, or void that is not followed by an argument of a type compatible with rsize_t. The diagnostic could warn
that unless the pointer is being used with a conversion specifier using the hh length modifier, a length argument is expected
to follow the pointer argument. Another useful diagnostic could flag any non-pointer argument following format that did
not have a type compatible with rsize_t.
519) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
5 The snwprintf_s function, unlike swprintf_s, will truncate the result to fit within the array pointed
to by s.
Returns
6 The snwprintf_s function returns the number of wide characters that would have been written
had n been sufficiently large, not counting the terminating wide null character, or a negative value
if a runtime-constraint violation occurred. Thus, the null-terminated output has been completely
written if and only if the returned value is both nonnegative and less than n.
K.3.9.1.4 The swprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
int swprintf_s(wchar_t * restrict s, rsize_t n, const wchar_t * restrict format,
...);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than
RSIZE_MAX/sizeof(wchar_t). The number of wide characters (including the trailing null) required
for the result to be written to the array pointed to by s shall not be greater than n. The %n specifier520)
(modified or not by flags, field width, or precision) shall not appear in the wide string pointed to by
format. Any argument to swprintf_s corresponding to a %s specifier shall not be a null pointer.
No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero and
not greater than RSIZE_MAX/sizeof(wchar_t), then the swprintf_s function sets s[0] to the null
wide character.
Description
4 The swprintf_s function is equivalent to the swprintf function except for the explicit runtime-
constraints listed above.
5 The swprintf_s function, unlike snwprintf_s, treats a result too big for the array pointed to by s
as a runtime-constraint violation.
Returns
6 If no runtime-constraint violation occurred, the swprintf_s function returns the number of wide
characters written in the array, not counting the terminating null wide character. If an encoding
error occurred or if n or more wide characters are requested to be written, swprintf_s returns a
negative value. If any other runtime-constraint violation occurred, swprintf_s returns zero.
K.3.9.1.5 The swscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
int swscanf_s(const wchar_t * restrict s, const wchar_t * restrict format, ...);
Runtime-constraints
2 Neither s nor format shall be a null pointer. Any argument indirected though in order to store
converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the swscanf_s function does not attempt to perform
further input, and it is unspecified to what extent swscanf_s performed input before discovering
the runtime-constraint violation.
520) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
Description
4 The swscanf_s function is equivalent to fwscanf_s, except that the argument s specifies a wide
string from which the input is to be obtained, rather than from a stream. Reaching the end of the
wide string is equivalent to encountering end-of-file for the fwscanf_s function.
Returns
5 The swscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the swscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.9.1.6 The vfwprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
#include <wchar.h>
int vfwprintf_s(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. The %n specifier521) (modified or not by flags,
field width, or precision) shall not appear in the wide string pointed to by format. Any argument to
vfwprintf_s corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the vfwprintf_s function does not attempt to produce
further output, and it is unspecified to what extent vfwprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The vfwprintf_s function is equivalent to the vfwprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The vfwprintf_s function returns the number of wide characters transmitted, or a negative value if
an output error, encoding error, or runtime-constraint violation occurred.
K.3.9.1.7 The vfwscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <stdio.h>
#include <wchar.h>
int vfwscanf_s(FILE * restrict stream, const wchar_t * restrict format,
va_list arg);
Runtime-constraints
2 Neither stream nor format shall be a null pointer. Any argument indirected though in order to
store converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the vfwscanf_s function does not attempt to perform
further input, and it is unspecified to what extent vfwscanf_s performed input before discovering
the runtime-constraint violation.
521) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
Description
4 The vfwscanf_s function is equivalent to fwscanf_s, with the variable argument list replaced by
arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg
calls). The vfwscanf_s function does not invoke the va_end macro522) .
Returns
5 The vfwscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the vfwscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.9.1.8 The vsnwprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <wchar.h>
int vsnwprintf_s(wchar_t *restrict s, rsize_t n, const wchar_t *restrict format,
va_list arg);
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than
RSIZE_MAX/sizeof(wchar_t). The %n specifier523) (modified or not by flags, field width, or preci-
sion) shall not appear in the wide string pointed to by format. Any argument to vsnwprintf_s
corresponding to a %s specifier shall not be a null pointer. No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero and
not greater than RSIZE_MAX/sizeof(wchar_t), then the vsnwprintf_s function sets s[0] to the
null wide character.
Description
4 The vsnwprintf_s function is equivalent to the vswprintf function except for the explicit runtime-
constraints listed above.
5 The vsnwprintf_s function, unlike vswprintf_s, will truncate the result to fit within the array
pointed to by s.
Returns
6 The vsnwprintf_s function returns the number of wide characters that would have been written
had n been sufficiently large, not counting the terminating null character, or a negative value if
a runtime-constraint violation occurred. Thus, the null-terminated output has been completely
written if and only if the returned value is both nonnegative and less than n.
K.3.9.1.9 The vswprintf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <wchar.h>
int vswprintf_s(wchar_t *restrict s, rsize_t n, const wchar_t *restrict format,
va_list arg);
522) As the functions vfwscanf_s , vwscanf_s , and vswscanf_s invoke the va_arg macro, the representation of arg after the
return is indeterminate.
523) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
Runtime-constraints
2 Neither s nor format shall be a null pointer. n shall neither equal zero nor be greater than
RSIZE_MAX/sizeof(wchar_t). The number of wide characters (including the trailing null) required
for the result to be written to the array pointed to by s shall not be greater than n. The %n specifier524)
(modified or not by flags, field width, or precision) shall not appear in the wide string pointed to by
format. Any argument to vswprintf_s corresponding to a %s specifier shall not be a null pointer.
No encoding error shall occur.
3 If there is a runtime-constraint violation, then if s is not a null pointer and n is greater than zero
and not greater than RSIZE_MAX/sizeof(wchar_t), then the vswprintf_s function sets s[0] to
the null wide character.
Description
4 The vswprintf_s function is equivalent to the vswprintf function except for the explicit runtime-
constraints listed above.
5 The vswprintf_s function, unlike vsnwprintf_s, treats a result too big for the array pointed to by
s as a runtime-constraint violation.
Returns
6 If no runtime-constraint violation occurred, the vswprintf_s function returns the number of wide
characters written in the array, not counting the terminating null wide character. If an encoding
error occurred or if n or more wide characters are requested to be written, vswprintf_s returns a
negative value. If any other runtime-constraint violation occurred, vswprintf_s returns zero.
K.3.9.1.10 The vswscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <wchar.h>
int vswscanf_s(const wchar_t * restrict s, const wchar_t * restrict format,
va_list arg);
Runtime-constraints
2 Neither s nor format shall be a null pointer. Any argument indirected though in order to store
converted input shall not be a null pointer.
3 If there is a runtime-constraint violation, the vswscanf_s function does not attempt to perform
further input, and it is unspecified to what extent vswscanf_s performed input before discovering
the runtime-constraint violation.
Description
4 The vswscanf_s function is equivalent to swscanf_s, with the variable argument list replaced by
arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg
calls). The vswscanf_s function does not invoke the va_end macro525) .
Returns
5 The vswscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the vswscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.9.1.11 The vwprintf_s function
524) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
525) As the functions vfwscanf_s , vwscanf_s , and vswscanf_s invoke the va_arg macro, the representation of arg after the
return is indeterminate.
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <wchar.h>
int vwprintf_s(const wchar_t * restrict format, va_list arg);
Runtime-constraints
2 format shall not be a null pointer. The %n specifier526) (modified or not by flags, field width, or
precision) shall not appear in the wide string pointed to by format. Any argument to vwprintf_s
corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the vwprintf_s function does not attempt to produce
further output, and it is unspecified to what extent vwprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The vwprintf_s function is equivalent to the vwprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The vwprintf_s function returns the number of wide characters transmitted, or a negative value if
an output error, encoding error, or runtime-constraint violation occurred.
K.3.9.1.12 The vwscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <stdarg.h>
#include <wchar.h>
int vwscanf_s(const wchar_t * restrict format, va_list arg);
Runtime-constraints
2 format shall not be a null pointer. Any argument indirected though in order to store converted
input shall not be a null pointer.
3 If there is a runtime-constraint violation, the vwscanf_s function does not attempt to perform
further input, and it is unspecified to what extent vwscanf_s performed input before discovering
the runtime-constraint violation.
Description
4 The vwscanf_s function is equivalent to wscanf_s, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vwscanf_s function does not invoke the va_end macro527) .
Returns
5 The vwscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the vwscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.9.1.13 The wprintf_s function
Synopsis
1
526) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
527) As the functions vfwscanf_s , vwscanf_s , and vswscanf_s invoke the va_arg macro, the representation of arg after the
return is indeterminate.
#define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
int wprintf_s(const wchar_t * restrict format, ...);
Runtime-constraints
2 format shall not be a null pointer. The %n specifier528) (modified or not by flags, field width, or
precision) shall not appear in the wide string pointed to by format. Any argument to wprintf_s
corresponding to a %s specifier shall not be a null pointer.
3 If there is a runtime-constraint violation, the wprintf_s function does not attempt to produce
further output, and it is unspecified to what extent wprintf_s produced output before discovering
the runtime-constraint violation.
Description
4 The wprintf_s function is equivalent to the wprintf function except for the explicit runtime-
constraints listed above.
Returns
5 The wprintf_s function returns the number of wide characters transmitted, or a negative value if
an output error, encoding error, or runtime-constraint violation occurred.
K.3.9.1.14 The wscanf_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
int wscanf_s(const wchar_t * restrict format, ...);
Runtime-constraints
2 format shall not be a null pointer. Any argument indirected though in order to store converted
input shall not be a null pointer.
3 If there is a runtime-constraint violation, the wscanf_s function does not attempt to perform further
input, and it is unspecified to what extent wscanf_s performed input before discovering the runtime-
constraint violation.
Description
4 The wscanf_s function is equivalent to fwscanf_s with the argument stdin interposed before the
arguments to wscanf_s.
Returns
5 The wscanf_s function returns the value of the macro EOF if an input failure occurs before any
conversion or if there is a runtime-constraint violation. Otherwise, the wscanf_s function returns
the number of input items assigned, which can be fewer than provided for, or even zero, in the event
of an early matching failure.
K.3.9.2 General wide string utilities
K.3.9.2.1 Wide string copying functions
K.3.9.2.1.1 The wcscpy_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
errno_t wcscpy_s(wchar_t *restrict s1, rsize_t s1max,
const wchar_t *restrict s2);
528) It is not a runtime-constraint violation for the wide characters %n to appear in sequence in the wide string pointed at
by format when those wide characters are not a interpreted as a %n specifier. For example, if the entire format string was
L"%%n".
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. s1max shall not be greater than
RSIZE_MAX/sizeof(wchar_t). s1max shall not equal zero. s1max shall be greater than
wcsnlen_s(s2, s1max) . Copying shall not take place between objects that overlap.
3 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX/sizeof(wchar_t), then wcscpy_s sets s1[0] to the null wide
character.
Description
4 The wcscpy_s function copies the wide string pointed to by s2 (including the terminating null wide
character) into the array pointed to by s1.
5 All elements following the terminating null wide character (if any) written by wcscpy_s in the array
of s1max wide characters pointed to by s1 take unspecified values when wcscpy_s returns.529)
Returns
6 The wcscpy_s function returns zero530) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.9.2.1.2 The wcsncpy_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
errno_t wcsncpy_s(wchar_t * restrict s1, rsize_t s1max,
const wchar_t * restrict s2, rsize_t n);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than
RSIZE_MAX/sizeof(wchar_t). s1max shall not equal zero. If n is not less than s1max, then
s1max shall be greater than wcsnlen_s(s2, s1max) . Copying shall not take place between objects
that overlap.
3 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX/sizeof(wchar_t), then wcsncpy_s sets s1[0] to the null
wide character.
Description
4 The wcsncpy_s function copies not more than n successive wide characters (wide characters that
follow a null wide character are not copied) from the array pointed to by s2 to the array pointed to
by s1. If no null wide character was copied from s2, then s1[n] is set to a null wide character.
5 All elements following the terminating null wide character (if any) written by wcsncpy_s in the array
of s1max wide characters pointed to by s1 take unspecified values when wcsncpy_s returns.531)
Returns
6 The wcsncpy_s function returns zero532) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
7 EXAMPLE 1 The wcsncpy_s function can be used to copy a wide string without the danger that the result will not be null
terminated or that wide characters will be written past the end of the destination array.
529) This allows an implementation to copy wide characters from s2 to s1 while simultaneously checking if any of those wide
characters are null. Such an approach might write a wide character to every element of s1 before discovering that the first
element was set to the null wide character.
530) A zero return value implies that all of the requested wide characters from the string pointed to by s2 fit within the array
pointed to by s1 and that the result in s1 is null terminated.
531) This allows an implementation to copy wide characters from s2 to s1 while simultaneously checking if any of those wide
characters are null. Such an approach might write a wide character to every element of s1 before discovering that the first
element was set to the null wide character.
532) A zero return value implies that all of the requested wide characters from the string pointed to by s2 fit within the array
pointed to by s1 and that the result in s1 is null terminated.
#define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
/* ... */
wchar_t src1[100] = L"hello";
wchar_t src2[7] = {L’g’, L’o’, L’o’, L’d’, L’b’, L’y’, L’e’};
wchar_t dst1[6], dst2[5], dst3[5];
int r1, r2, r3;
r1 = wcsncpy_s(dst1, 6, src1, 100);
r2 = wcsncpy_s(dst2, 5, src2, 7);
r3 = wcsncpy_s(dst3, 5, src2, 4);
The first call will assign to r1 the value zero and to dst1 the sequence of wide characters hello\0.
The second call will assign to r2 a nonzero value and to dst2 the sequence of wide characters \0.
The third call will assign to r3 the value zero and to dst3 the sequence of wide characters good\0.
K.3.9.2.1.3 The wmemcpy_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
errno_t wmemcpy_s(wchar_t *restrict s1, rsize_t s1max,
const wchar_t *restrict s2, rsize_t n);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than
RSIZE_MAX/sizeof(wchar_t). n shall not be greater than s1max. Copying shall not take
place between objects that overlap.
3 If there is a runtime-constraint violation, the wmemcpy_s function stores zeros in the first s1max wide
characters of the object pointed to by s1 if s1 is not a null pointer and s1max is not greater than
RSIZE_MAX/sizeof(wchar_t).
Description
4 The wmemcpy_s function copies n successive wide characters from the object pointed to by s2 into
the object pointed to by s1.
Returns
5 The wmemcpy_s function returns zero if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.9.2.1.4 The wmemmove_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
errno_t wmemmove_s(wchar_t *s1, rsize_t s1max, const wchar_t *s2, rsize_t n);
Runtime-constraints
2 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than
RSIZE_MAX/sizeof(wchar_t). n shall not be greater than s1max.
3 If there is a runtime-constraint violation, the wmemmove_s function stores zeros in the first s1max
wide characters of the object pointed to by s1 if s1 is not a null pointer and s1max is not greater than
RSIZE_MAX/sizeof(wchar_t).
Description
4 The wmemmove_s function copies n successive wide characters from the object pointed to by s2 into
the object pointed to by s1. This copying takes place as if the n wide characters from the object
pointed to by s2 are first copied into a temporary array of n wide characters that does not overlap
the objects pointed to by s1 or s2, and then the n wide characters from the temporary array are
copied into the object pointed to by s1.
Returns
5 The wmemmove_s function returns zero if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.9.2.2 Wide string concatenation functions
K.3.9.2.2.1 The wcscat_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
errno_t wcscat_s(wchar_t * restrict s1, rsize_t s1max,
const wchar_t * restrict s2);
Runtime-constraints
2 Let m denote the value s1max - wcsnlen_s(s1, s1max) upon entry to wcscat_s.
3 Neither s1 nor s2 shall be a null pointer. s1max shall not be greater than
RSIZE_MAX/sizeof(wchar_t). s1max shall not equal zero. m shall not equal zero.533) m
shall be greater than wcsnlen_s(s2, m). Copying shall not take place between objects that overlap.
4 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX/sizeof(wchar_t), then wcscat_s sets s1[0] to the null wide
character.
Description
5 The wcscat_s function appends a copy of the wide string pointed to by s2 (including the terminating
null wide character) to the end of the wide string pointed to by s1. The initial wide character from
s2 overwrites the null wide character at the end of s1.
6 All elements following the terminating null wide character (if any) written by wcscat_s in the array
of s1max wide characters pointed to by s1 take unspecified values when wcscat_s returns.534)
Returns
7 The wcscat_s function returns zero535) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
K.3.9.2.2.2 The wcsncat_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
errno_t wcsncat_s(wchar_t * restrict s1, rsize_t s1max,
const wchar_t * restrict s2, rsize_t n);
Runtime-constraints
2 Let m denote the value s1max - wcsnlen_s(s1, s1max) upon entry to wcsncat_s.
3 Neither s1 nor s2 shall be a null pointer. Neither s1max nor n shall be greater than
RSIZE_MAX/sizeof(wchar_t). s1max shall not equal zero. m shall not equal zero.536) If n
533) Zero means that s1 was not null terminated upon entry to wcscat_s .
534) This allows an implementation to append wide characters from s2 to s1 while simultaneously checking if any of those
wide characters are null. Such an approach might write a wide character to every element of s1 before discovering that the
first element was set to the null wide character.
535) A zero return value implies that all of the requested wide characters from the wide string pointed to by s2 were appended
to the wide string pointed to by s1 and that the result in s1 is null terminated.
536) Zero means that s1 was not null terminated upon entry to wcsncat_s .
is not less than m, then m shall be greater than wcsnlen_s(s2, m). Copying shall not take place
between objects that overlap.
4 If there is a runtime-constraint violation, then if s1 is not a null pointer and s1max is greater than
zero and not greater than RSIZE_MAX/sizeof(wchar_t), then wcsncat_s sets s1[0] to the null
wide character.
Description
5 The wcsncat_s function appends not more than n successive wide characters (wide characters that
follow a null wide character are not copied) from the array pointed to by s2 to the end of the wide
string pointed to by s1. The initial wide character from s2 overwrites the null wide character at the
end of s1. If no null wide character was copied from s2, then s1[s1max- m +n] is set to a null wide
character.
6 All elements following the terminating null wide character (if any) written by wcsncat_s in the array
of s1max wide characters pointed to by s1 take unspecified values when wcsncat_s returns.537)
Returns
7 The wcsncat_s function returns zero538) if there was no runtime-constraint violation. Otherwise, a
nonzero value is returned.
8 EXAMPLE 1 The wcsncat_s function can be used to copy a wide string without the danger that the result will not be null
terminated or that wide characters will be written past the end of the destination array.
#define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
/* ... */
wchar_t s1[100] = L"good";
wchar_t s2[6] = L"hello";
wchar_t s3[6] = L"hello";
wchar_t s4[7] = L"abc";
wchar_t s5[1000] = L"bye";
int r1, r2, r3, r4;
r1 = wcsncat_s(s1, 100, s5, 1000);
r2 = wcsncat_s(s2, 6, L"", 1);
r3 = wcsncat_s(s3, 6, L"X", 2);
r4 = wcsncat_s(s4, 7, L"defghijklmn", 3);
After the first call r1 will have the value zero and s1 will be the wide character sequence goodbye\0.
After the second call r2 will have the value zero and s2 will be the wide character sequence hello\0.
After the third call r3 will have a nonzero value and s3 will be the wide character sequence \0.
After the fourth call r4 will have the value zero and s4 will be the wide character sequence abcdef\0.
K.3.9.2.3 Wide string search functions
K.3.9.2.3.1 The wcstok_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
wchar_t *wcstok_s(wchar_t * restrict s1, rsize_t * restrict s1max,
const wchar_t * restrict s2, wchar_t ** restrict ptr);
Runtime-constraints
2 None of s1max, s2, or ptr shall be a null pointer. If s1 is a null pointer, then *ptr shall not be a null
pointer. The value of *s1max shall not be greater than RSIZE_MAX/sizeof(wchar_t). The end of
537) This allows an implementation to append wide characters from s2 to s1 while simultaneously checking if any of those
wide characters are null. Such an approach might write a wide character to every element of s1 before discovering that the
first element was set to the null wide character.
538) A zero return value implies that all of the requested wide characters from the wide string pointed to by s2 were appended
to the wide string pointed to by s1 and that the result in s1 is null terminated.
the token found shall occur within the first *s1max wide characters of s1 for the first call, and shall
occur within the first *s1max wide characters of where searching resumes on subsequent calls.
3 If there is a runtime-constraint violation, the wcstok_s function does not indirect through the s1 or
s2 pointers, and does not store a value in the object pointed to by ptr.
Description
4 A sequence of calls to the wcstok_s function breaks the wide string pointed to by s1 into a sequence
of tokens, each of which is delimited by a wide character from the wide string pointed to by s2.
The fourth argument points to a caller-provided wchar_t pointer into which the wcstok_s function
stores information necessary for it to continue scanning the same wide string.
5 The first call in a sequence has a non-null first argument and s1max points to an object whose value
is the number of elements in the wide character array pointed to by the first argument. The first call
stores an initial value in the object pointed to by ptr and updates the value pointed to by s1max
to reflect the number of elements that remain in relation to ptr. Subsequent calls in the sequence
have a null first argument and the objects pointed to by s1max and ptr are required to have the
values stored by the previous call in the sequence, which are then updated. The separator wide
string pointed to by s2 may be different from call to call.
6 The first call in the sequence searches the wide string pointed to by s1 for the first wide character
that is not contained in the current separator wide string pointed to by s2. If no such wide character
is found, then there are no tokens in the wide string pointed to by s1 and the wcstok_s function
returns a null pointer. If such a wide character is found, it is the start of the first token.
7 The wcstok_s function then searches from there for the first wide character in s1 that is contained
in the current separator wide string. If no such wide character is found, the current token extends
to the end of the wide string pointed to by s1, and subsequent searches in the same wide string
for a token return a null pointer. If such a wide character is found, it is overwritten by a null wide
character, which terminates the current token.
8 In all cases, the wcstok_s function stores sufficient information in the pointer pointed to by ptr so
that subsequent calls, with a null pointer for s1 and the unmodified pointer value for ptr, shall start
searching just past the element overwritten by a null wide character (if any).
Returns
9 The wcstok_s function returns a pointer to the first wide character of a token, or a null pointer if
there is no token or there is a runtime-constraint violation.
10 EXAMPLE
#define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
static wchar_t str1[] = L"?a???b,,,#c";
static wchar_t str2[] = L"\t \t";
wchar_t *t, *ptr1, *ptr2;
rsize_t max1 = wcslen(str1)+1;
rsize_t max2 = wcslen(str2)+1;
t = wcstok_s(str1, &max1, "?", &ptr1); // t points to the token "a"
t = wcstok_s(NULL, &max1, ",", &ptr1); // t points to the token "??b"
t = wcstok_s(str2, &max2, " \t", &ptr2); // t is a null pointer
t = wcstok_s(NULL, &max1, "#,", &ptr1); // t points to the token "c"
t = wcstok_s(NULL, &max1, "?", &ptr1); // t is a null pointer
K.3.9.2.4 Miscellaneous functions
K.3.9.2.4.1 The wcsnlen_s function
Synopsis
1 #define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
size_t wcsnlen_s(const wchar_t *s, size_t maxsize);
Description
2 The wcsnlen_s function computes the length of the wide string pointed to by s.
Returns
3 If s is a null pointer,539) then the wcsnlen_s function returns zero.
4 Otherwise, the wcsnlen_s function returns the number of wide characters that precede the termi-
nating null wide character. If there is no null wide character in the first maxsize wide characters of
s then wcsnlen_s returns maxsize. At most the first maxsize wide characters of s shall be accessed
by wcsnlen_s.
K.3.9.3 Extended multibyte/wide character conversion utilities
K.3.9.3.1 Restartable multibyte/wide character conversion functions
1 Unlike wcrtomb, wcrtomb_s does not permit the ps parameter (the pointer to the conversion state)
to be a null pointer.
K.3.9.3.1.1 The wcrtomb_s function
Synopsis
1 #include <wchar.h>
errno_t wcrtomb_s(size_t * restrict retval, char * restrict s, rsize_t smax,
wchar_t wc, mbstate_t * restrict ps);
Runtime-constraints
2 Neither retval nor ps shall be a null pointer. If s is not a null pointer, then smax shall not equal
zero and shall not be greater than RSIZE_MAX. If s is not a null pointer, then smax shall be not be less
than the number of bytes to be stored in the array pointed to by s. If s is a null pointer, then smax
shall equal zero.
3 If there is a runtime-constraint violation, then wcrtomb_s does the following. If s is not a null pointer
and smax is greater than zero and not greater than RSIZE_MAX, then wcrtomb_s sets s[0] to the null
character. If retval is not a null pointer, then wcrtomb_s sets *retval to (size_t)(-1) .
Description
4 If s is a null pointer, the wcrtomb_s function is equivalent to the call
wcrtomb_s(&retval, buf, sizeof buf, L’\0’, ps)
where retval and buf are internal variables of the appropriate types, and the size of buf is greater
than MB_CUR_MAX.
5 If s is not a null pointer, the wcrtomb_s function determines the number of bytes needed to represent
the multibyte character that corresponds to the wide character given by wc (including any shift
sequences), and stores the multibyte character representation in the array whose first element is
pointed to by s. At most MB_CUR_MAX bytes are stored. If wc is a null wide character, a null byte is
stored, preceded by any shift sequence needed to restore the initial shift state; the resulting state
described is the initial conversion state.
6 If wc does not correspond to a valid multibyte character, an encoding error occurs: the wcrtomb_s
function stores the value (size_t)(-1) into *retval and the conversion state is unspecified.
Otherwise, the wcrtomb_s function stores into *retval the number of bytes (including any shift
sequences) stored in the array pointed to by s.
539) Note that the wcsnlen_s function has no runtime-constraints. This lack of runtime-constraints along with the values
returned for a null pointer or an unterminated wide string argument make wcsnlen_s useful in algorithms that gracefully
handle such exceptional data.
Returns
7 The wcrtomb_s function returns zero if no runtime-constraint violation and no encoding error
occurred. Otherwise, a nonzero value is returned.
K.3.9.3.2 Restartable multibyte/wide string conversion functions
1 Unlike mbsrtowcs and wcsrtombs, mbsrtowcs_s and wcsrtombs_s do not permit the ps parameter
(the pointer to the conversion state) to be a null pointer.
K.3.9.3.2.1 The mbsrtowcs_s function
Synopsis
1 #include <wchar.h>
errno_t mbsrtowcs_s(size_t * restrict retval, wchar_t * restrict dst,
rsize_t dstmax, const char ** restrict src, rsize_t len,
mbstate_t * restrict ps);
Runtime-constraints
2 None of retval, src, *src , or ps shall be null pointers. If dst is not a null pointer, then neither
len nor dstmax shall be greater than RSIZE_MAX/sizeof(wchar_t). If dst is a null pointer, then
dstmax shall equal zero. If dst is not a null pointer, then dstmax shall not equal zero. If dst is not a
null pointer and len is not less than dstmax, then a null character shall occur within the first dstmax
multibyte characters of the array pointed to by *src .
3 If there is a runtime-constraint violation, then mbsrtowcs_s does the following. If retval is not
a null pointer, then mbsrtowcs_s sets *retval to (size_t)(-1) . If dst is not a null pointer and
dstmax is greater than zero and not greater than RSIZE_MAX/sizeof(wchar_t), then mbsrtowcs_s
sets dst[0] to the null wide character.
Description
4 The mbsrtowcs_s function converts a sequence of multibyte characters that begins in the conversion
state described by the object pointed to by ps, from the array indirectly pointed to by src into a
sequence of corresponding wide characters. If dst is not a null pointer, the converted characters are
stored into the array pointed to by dst. Conversion continues up to and including a terminating null
character, which is also stored. Conversion stops earlier in two cases: when a sequence of bytes is
encountered that does not form a valid multibyte character, or (if dst is not a null pointer) when len
wide characters have been stored into the array pointed to by dst.540) If dst is not a null pointer
and no null wide character was stored into the array pointed to by dst, then dst[len] is set to the
null wide character. Each conversion takes place as if by a call to the mbrtowc function.
5 If dst is not a null pointer, the pointer object pointed to by src is assigned either a null pointer (if
conversion stopped due to reaching a terminating null character) or the address just past the last
multibyte character converted (if any). If conversion stopped due to reaching a terminating null
character and if dst is not a null pointer, the resulting state described is the initial conversion state.
6 Regardless of whether dst is or is not a null pointer, if the input conversion encounters a sequence
of bytes that do not form a valid multibyte character, an encoding error occurs: the mbsrtowcs_s
function stores the value (size_t)(-1) into *retval and the conversion state is unspecified.
Otherwise, the mbsrtowcs_s function stores into *retval the number of multibyte characters
successfully converted, not including the terminating null character (if any).
7 All elements following the terminating null wide character (if any) written by mbsrtowcs_s in the
array of dstmax wide characters pointed to by dst take unspecified values when mbsrtowcs_s
returns.541)
8 If copying takes place between objects that overlap, the objects take on unspecified values.
540) Thus, the value of len is ignored if dst is a null pointer.
541) This allows an implementation to attempt converting the multibyte string before discovering a terminating null character
did not occur where required.
Returns
9 The mbsrtowcs_s function returns zero if no runtime-constraint violation and no encoding error
occurred. Otherwise, a nonzero value is returned.
K.3.9.3.2.2 The wcsrtombs_s function
Synopsis
1 #include <wchar.h>
errno_t wcsrtombs_s(size_t * restrict retval, char * restrict dst,
rsize_t dstmax, const wchar_t ** restrict src, rsize_t len,
mbstate_t * restrict ps);
Runtime-constraints
2 None of retval, src, *src , or ps shall be null pointers. If dst is not a null pointer, then neither len
shall be greater than RSIZE_MAX/sizeof(wchar_t) nor dstmax shall be greater than RSIZE_MAX. If
dst is a null pointer, then dstmax shall equal zero. If dst is not a null pointer, then dstmax shall not
equal zero. If dst is not a null pointer and len is not less than dstmax, then the conversion shall
have been stopped (see below) because a terminating null wide character was reached or because an
encoding error occurred.
3 If there is a runtime-constraint violation, then wcsrtombs_s does the following. If retval is not
a null pointer, then wcsrtombs_s sets *retval to (size_t)(-1) . If dst is not a null pointer and
dstmax is greater than zero and not greater than RSIZE_MAX, then wcsrtombs_s sets dst[0] to the
null character.
Description
4 The wcsrtombs_s function converts a sequence of wide characters from the array indirectly pointed
to by src into a sequence of corresponding multibyte characters that begins in the conversion state
described by the object pointed to by ps. If dst is not a null pointer, the converted characters are then
stored into the array pointed to by dst. Conversion continues up to and including a terminating
null wide character, which is also stored. Conversion stops earlier in two cases:
— when a wide character is reached that does not correspond to a valid multibyte character;
— (if dst is not a null pointer) when the next multibyte character would exceed the limit of n
total bytes to be stored into the array pointed to by dst. If the wide character being converted
is the null wide character, then n is the lesser of len or dstmax. Otherwise, n is the lesser of
len or dstmax-1.
If the conversion stops without converting a null wide character and dst is not a null pointer, then
a null character is stored into the array pointed to by dst immediately following any multibyte
characters already stored. Each conversion takes place as if by a call to the wcrtomb function.542)
5 If dst is not a null pointer, the pointer object pointed to by src is assigned either a null pointer (if
conversion stopped due to reaching a terminating null wide character) or the address just past the
last wide character converted (if any). If conversion stopped due to reaching a terminating null wide
character, the resulting state described is the initial conversion state.
6 Regardless of whether dst is or is not a null pointer, if the input conversion encounters a wide
character that does not correspond to a valid multibyte character, an encoding error occurs: the
wcsrtombs_s function stores the value (size_t)(-1) into *retval and the conversion state is
unspecified. Otherwise, the wcsrtombs_s function stores into *retval the number of bytes in the
resulting multibyte character sequence, not including the terminating null character (if any).
7 All elements following the terminating null character (if any) written by wcsrtombs_s in the array
542) If conversion stops because a terminating null wide character has been reached, the bytes stored include those necessary
to reach the initial shift state immediately before the null byte. However, if the conversion stops before a terminating null
wide character has been reached, the result will be null terminated, but might not end in the initial shift state.
of dstmax elements pointed to by dst take unspecified values when wcsrtombs_s returns.543)
8 If copying takes place between objects that overlap, the objects take on unspecified values.
Returns
9 The wcsrtombs_s function returns zero if no runtime-constraint violation and no encoding error
occurred. Otherwise, a nonzero value is returned.
543) When len is not less than dstmax, the implementation might fill the array before discovering a runtime-constraint
violation.
L. Annex L (normative) Analyzability
L.1 Scope
1 This annex specifies optional behavior that can aid in the analyzability of C programs.
2 An implementation that defines __STDC_ANALYZABLE__ shall conform to the specifications in this
annex.544)
L.2 Definitions
L.2.1
1 out-of-bounds store
an (attempted) access (3.1) that, at run time, for a given computational state, would modify (or, for
an object declared volatile, fetch) one or more bytes that lie outside the bounds permitted by this
document.
L.2.2
1 bounded undefined behavior
undefined behavior (3.4.3) that does not perform an out-of-bounds store.
2 Note 1 to entry: The behavior might perform a trap.
3 Note 2 to entry: Any values produced might be unspecified values, and the representation of objects that are written to
might become indeterminate.
L.2.3
1 critical undefined behavior
undefined behavior that is not bounded undefined behavior.
2 Note 1 to entry: The behavior might perform an out-of-bounds store or perform a trap.
L.3 Requirements
1 If the program performs a trap (3.19.5), the implementation is permitted to invoke a runtime-
constraint handler. Any such semantics are implementation-defined.
2 All undefined behavior shall be limited to bounded undefined behavior, except for the following
which are permitted to result in critical undefined behavior:
— An object is referred to outside of its lifetime (6.2.4).
— A store is performed to an object that has two incompatible declarations (6.2.7),
— A pointer is used to call a function whose type is not compatible with the referenced type
(6.2.7, 6.3.2.3, 6.5.2.2).
— An lvalue does not designate an object when evaluated (6.3.2.1).
— The program attempts to modify a string literal (6.4.5).
— The operand of the unary * operator has an invalid value (6.5.3.2).
— Addition or subtraction of a pointer into, or just beyond, an array object and an integer type
produces a result that points just beyond the array object and is used as the operand of a unary
* operator that is evaluated (6.5.6).
— An attempt is made to modify an object defined with a const-qualified type through use of an
lvalue with non-const-qualified type (6.7.3).
544) Implementations that do not define __STDC_ANALYZABLE__ are not required to conform to these specifications.
— An argument to a function or macro defined in the standard library has an invalid value or a
type not expected by a function with variable number of arguments (7.1.4).
— The longjmp function is called with a jmp_buf argument where the most recent invocation
of the setjmp macro in the same invocation of the program with the corresponding jmp_buf
argument is nonexistent, or the invocation was from another thread of execution, or the
function containing the invocation has terminated execution in the interim, or the invocation
was within the scope of an identifier with variably modified type and execution has left that
scope in the interim (7.13.2.1).
— The value of a pointer that refers to space deallocated by a call to the free or realloc function is
used (7.24.3).
— A string or wide string utility function accesses an array beyond the end of an object (7.26.1,
and 7.31.4).
M. Annex M (informative) Change History
M.1 Fifth Edition
1 Major changes in this fifth edition (__STDC_VERSION__ 202311L) include:
— allowed for implementations to provide keywords such as bool, static_assert, true, false,
and others with additional support to define them as macros and enable transition of programs
easily;
— removed obsolete sign representations and integer width constraints (so-called "2’s comple-
ment");
— added a one-argument version of static_assert;
— removed support for function definitions with identifier lists;
— mandated function declarations whose parameter list is empty by the same as parameter list
which only contain a single void;
— harmonization with ISO/IEC 9945 (POSIX):
• extended month name formats for strftime
• integration of functions: gmtime_r, localtime_r, memccpy, strdup, strndup
— harmonization with floating point standard IEC 60559:
• integration of binary floating-point technical specification TS 18661-1
• integration of decimal floating-point technical specification TS 18661-2
• integration of decimal floating-point technical specification TS 18661-4a
— made the DECIMAL_DIG macro obsolescent;
— added version test macros to certain library headers to aid in upgrading and portability to be
used alongside the __STDC_VERSION__ macro;
— added the attributes feature, which includes the attributes:
• deprecated, for marking entites as discouraged for future use;
• fallthrough, for explicitly marking cases where fallthrough in switches or labels is
intended rather than accidental;
• maybe_unused, for marking entities which may end up not being used;
• nodiscard, for marking entities which, when used, should have their value handled in
some way by a program;
• reproducible, for marking function types for which inputs may always produce pre-
dictable output if given the same input (e.g., cached data) but for which the order of such
calls still matter;
• unsequenced, for marking function types which always produce predictable output and
have no dependencies upon other data (and other relevant caveats), and,
• _Noreturn , for indicating a function shall never return;
— added the u8 character prefix to match the u8 string prefix;
— mandated all u8, u, and U strings be UTF-8, UTF-16, and UTF-32, respectively, as defined by
ISO/IEC 10646;
— separated the literal, wide literal, and UTF-8 literal, UTF-16 literal, and UTF-32 literal encodings
for strings and characters and now have a solely execution-based version of these, particularly
execution and wide execution encodings;
— added mbrtoc8 and c8rtomb functions missing from <uchar.h>;
— compound literals may also include storage-class specifiers as part of the type to change the
lifetime of the compound literal (and possibly turn it into a constant expression)
— added the constexpr specifier for object definitions and improved what is recognized as a
constant expression in conjunction with the constexpr storage-class specifier;
— added the typeof and typeof_unqual operations for computing the type of an expression;
— improved tag compatibility rules, enabling more types to be compatible with other types;
— added the _BitInt the bit-precise integer types;
— improved rules for handling enumerations without underlying types;
— added a new colon-delimited type specifier for enumerations to specify a fixed underlying
type;
— added a new header <stdbit.h> and a suite of bit and byte-handling utilities for portable
access to many implementation’s most efficiency functionality;
— modified existing functions to preserve the const-ness of the type placed into the function;
— added a feature to embed binary data as faithfully as possible with a new preprocessor directive
#embed;
— added a nullptr constant and a nullptr_t type with a well-defined underlying representa-
tion identical to a pointer to void;
— added the __VA_OPT__ specifier and clarified language in the handling of macro invocation
and arguments;
— mandated support for variably-modified types (but not variable-length arrays themselves);
— ellipses on functions may appear without a preceding parameter in the parameter list of
functions and va_start no longer requires such an argument to be passed to it;
— unicode identifiers allowed in syntax;
— memset_explicit function for writing data;
— certain type definitions, bit-precise integer types, and extended integer types may exceed
the normal boundaries of intmax_t and uintmax_t for signed and unsigned integer types,
respectively;
— names of functions, macros, and variables in this document, where clarified, are potentially
reserved rather than reserved to avoid undefined behavior for a large class of identifiers used
by programs existing and to be created;
— mandated support for call_once;
— allowed ptrdiff_t to be an integer type of at least 16, rather than requiring an integer type
with a width of at least 17;
— added the __has_include feature;
— changed the type qualifiers of the _Imaginary_I and _Complex_I macros;
— added $ and $ into the source and execution character set;
— added the auto type specifier for single object definitions using type inference;
— added the #elifdef and #elifndef conditional inclusion preprocessor directives;
— added the #warning directive;
— binary integer literals and appropriate formatting for input/output of binary integer numbers;
— digit seperators with ’ ;
— removed conditional support for mixed wide and narrow string literal concatenation;
— added support for additional time bases in time.h;
— zero-sized reallocations with realloc are undefined behavior;
— added an unreachable feature which invokes undefined behavior if reached during program
execution;
M.2 Fourth Edition
1 There were no major changes in the fourth edition (__STDC_VERSION__ 201710L), only technical
corrections and clarifications.
M.3 Third Edition
1 Major changes in the third edition (__STDC_VERSION__ 201112L) included:
— conditional (optional) features (including some that were previously mandatory)
— support for multiple threads of execution including an improved memory sequencing model,
atomic objects, and thread storage (<stdatomic.h> and <threads.h>)
— additional floating-point characteristic macros (<float.h>)
— querying and specifying alignment of objects (<stdalign.h>, <stdlib.h>)
— Unicode characters and strings (<uchar.h>) (originally specified in ISO/IEC TR 19769:2004)
— type-generic expressions
— static assertions
— anonymous structures and unions
— no-return functions
— macros to create complex numbers (<complex.h>)
— support for opening files for exclusive access
— removed the gets function (<stdio.h>)
— added the aligned_alloc, at_quick_exit, and quick_exit functions (<stdlib.h>)
— (conditional) support for bounds-checking interfaces (originally specified in ISO/IEC TR 24731–
1:2007)
— (conditional) support for analyzability
M.4 Second Edition
1 Major changes in the second edition (__STDC_VERSION__ 199901L) included:
— restricted character set support via digraphs and <iso646.h> (originally specified in
ISO/IEC 9899:1990/Amd 1:1995)
— wide character library support in <wchar.h> and <wctype.h> (originally specified in
ISO/IEC 9899:1990/Amd 1:1995)
— more precise aliasing rules via effective type
— restricted pointers
— variable length arrays
— flexible array members
— static and type qualifiers in parameter array declarators
— complex (and imaginary) support in <complex.h>
— type-generic math macros in <tgmath.h>
— the long long int type and library functions
— extended integer types
— increased minimum translation limits
— additional floating-point characteristics in <float.h>
— remove implicit int
— reliable integer division
— universal character names (\u and \U)
— extended identifiers
— hexadecimal floating constants and %a and %A printf/scanf conversion specifiers
— compound literals
— designated initializers
— // comments
— specified width integer types and corresponding library functions in <inttypes.h> and
<stdint.h>
— remove implicit function declaration
— preprocessor arithmetic done in intmax_t/uintmax_t
— mixed declarations and statements
— new block scopes for selection and iteration statements
— integer constant type rules
— integer promotion rules
— macros with a variable number of arguments (__VA_ARGS__ )
— the vscanf family of functions in <stdio.h> and <wchar.h>
— additional math library functions in <math.h>
— treatment of error conditions by math library functions (math_errhandling)
— floating-point environment access in <fenv.h>
— IEC 60559 (also known as IEC 559 or IEEE arithmetic) support
— trailing comma allowed in enum declaration
— %lf conversion specifier allowed in printf
— inline functions
— the snprintf family of functions in <stdio.h>
— boolean type in <stdbool.h>
— idempotent type qualifiers
— empty macro arguments
— new structure type compatibility rules (tag compatibility)
— additional predefined macro names
— _Pragma preprocessing operator
— standard pragmas
— __func__ predefined identifier
— va_copy macro
— additional strftime conversion specifiers
— LIA compatibility annex
— deprecate ungetc at the beginning of a binary file
— remove deprecation of aliased array parameters
— conversion of array to pointer not limited to lvalues
— relaxed constraints on aggregate and union initialization
— relaxed restrictions on portable header names
— return without expression not permitted in function that returns a value (and vice versa)
M.5 First Edition, Amendment 1
1 Major changes in the amendment to the first edition (__STDC_VERSION__ 199409L) included:
— addition of the predefined __STDC_VERSION__ macro
— restricted character set support via digraphs and <iso646.h>
— wide character library support in <wchar.h> and <wctype.h>
N.
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