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1. Scope
1 This International Standard 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 International Standard 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;
1) This International Standard 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.
-- all minimal requirements of a data-processing system that is capable of supporting a
conforming implementation.
2. Normative references
1 The following normative documents contain provisions which, through reference in this
text, constitute provisions of this International Standard. For dated references,
subsequent amendments to, or revisions of, any of these publications do not apply.
However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative
documents indicated below. For undated references, the latest edition of the normative
document referred to applies. Members of ISO and IEC maintain registers of currently
valid International Standards.
2 ISO 31-11:1992, Quantities and units -- Part 11: Mathematical signs and symbols for
use in the physical sciences and technology.
3 ISO/IEC 646, Information technology -- ISO 7-bit coded character set for information
interchange.
4 ISO/IEC 2382-1:1993, Information technology -- Vocabulary -- Part 1: Fundamental
terms.
5 ISO 4217, Codes for the representation of currencies and funds.
6 ISO 8601, Data elements and interchange formats -- Information interchange --
Representation of dates and times.
7 ISO/IEC 10646 (all parts), Information technology -- Universal Multiple-Octet Coded
Character Set (UCS).
8 IEC 60559:1989, Binary floating-point arithmetic for microprocessor systems (previously
designated IEC 559:1989).
3. Terms, definitions, and symbols
1 For the purposes of this International Standard, the following definitions apply. Other
terms are defined where they appear in italic type or on the left side of a syntax rule.
Terms explicitly defined in this International Standard are not to be presumed to refer
implicitly to similar terms defined elsewhere. Terms not defined in this International
Standard are to be interpreted according to ISO/IEC 2382-1. Mathematical symbols not
defined in this International Standard are to be interpreted according to ISO 31-11.
3.1
1 access
execution-time action to read or modify the value of an object
2 NOTE 1 Where only one of these two actions is meant, ``read'' or ``modify'' is used.
3 NOTE 2 "Modify'' includes the case where the new value being stored is the same as the previous value.
4 NOTE 3 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
actual parameter (deprecated)
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 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
implementation documents
2 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 International Standard imposes no requirements
2 NOTE 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 EXAMPLE An example of undefined behavior is the behavior on integer overflow.
3.4.4
1 unspecified behavior
use of an unspecified value, or other behavior where this International Standard provides
two or more possibilities and imposes no further requirements on which is chosen in any
instance
2 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 may
have one of two values
2 NOTE 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 It is possible to express the address of each individual byte of an object uniquely.
3 NOTE 2 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 The extended character set is a superset of the basic character set.
3.7.3
1 wide character
bit representation that fits in 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
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 International Standard that contains additional
information relevant to this subclause
3.12
1 implementation
particular set of software, running in a particular translation environment under particular
control 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 object
region of data storage in the execution environment, the contents of which can represent
values
2 NOTE When referenced, an object may be interpreted as having a particular type; see 6.3.2.1.
3.15
1 parameter
formal parameter
formal argument (deprecated)
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
3.16
1 recommended practice
specification that is strongly recommended as being in keeping with the intent of the
standard, but that may be impractical for some implementations
3.17
1 value
precise meaning of the contents of an object when interpreted as having a specific type
3.17.1
1 implementation-defined value
unspecified value where each implementation documents how the choice is made
3.17.2
1 indeterminate value
either an unspecified value or a trap representation
3.17.3
1 unspecified value
valid value of the relevant type where this International Standard imposes no
requirements on which value is chosen in any instance
2 NOTE An unspecified value cannot be a trap representation.
3.18
1 x
ceiling of x: the least integer greater than or equal to x
2 EXAMPLE 2. 4 is 3, -2. 4 is -2.
3.19
1 x
floor of x: the greatest integer less than or equal to x
2 EXAMPLE 2. 4 is 2, -2. 4 is -3.
4. Conformance
1 In this International Standard, ``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 is violated, the
behavior is undefined. Undefined behavior is otherwise indicated in this International
Standard 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 International Standard.2) 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 that does not
use complex types and 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>, <stdarg.h>, <stdbool.h>, <stddef.h>, and
<stdint.h>. A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behavior of any strictly conforming
program.3)
2) A strictly conforming program can use conditional features (such as those in annex F) provided the
use is guarded by a #ifdef directive with the appropriate macro. For example:
#ifdef _ _STDC_IEC_559_ _ /* FE_UPWARD defined */
/* ... */
fesetround(FE_UPWARD);
/* ... */
#endif
3) This implies that a conforming implementation reserves no identifiers other than those explicitly
reserved in this International Standard.
7 A conforming program is one that is acceptable to a conforming implementation.4)
8 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 (6.10.5),
characteristics of floating types <float.h> (7.7), alternative spellings <iso646.h>
(7.9), sizes of integer types <limits.h> (7.10), variable arguments <stdarg.h>
(7.15), boolean type and values <stdbool.h> (7.16), common definitions
<stddef.h> (7.17), integer types <stdint.h> (7.18).
4) Strictly conforming programs are intended to be maximally portable among conforming
implementations. Conforming programs may depend upon nonportable features of a conforming
implementation.
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 International Standard. 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 International Standard. 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 directives (6.10).
5.1.1.2 Translation phases
1 The precedence among the syntax rules of translation is specified by the following
phases.5)
1. 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. Trigraph sequences are replaced by
corresponding single-character internal representations.
5) Implementations shall 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.
2. 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.
3. The source file is decomposed into preprocessing tokens6) 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.
4. 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 universal character name is produced by token
concatenation (6.10.3.3), 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.
5. 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; if there is no corresponding member, it is converted to an implementation-
defined member other than the null (wide) character.7)
6. Adjacent string literal tokens are concatenated.
7. 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.
8. 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 directives (6.10), trigraph sequences (5.2.1.1), external definitions (6.9).
6) 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.
7) An implementation need not convert all non-corresponding source characters to the same execution
character.
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.8)
2 EXAMPLE An implementation shall issue a diagnostic for the translation unit:
char i;
int i;
because in those cases where wording in this International Standard describes the behavior for a construct
as being both a constraint error and resulting in undefined behavior, the constraint error shall 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 (6.7.8).
5.1.2.1 Freestanding environment
1 In a freestanding environment (in which C program execution may take place without any
benefit 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) The intent is that an implementation should identify the nature of, and where possible localize, each
violation. Of course, an implementation is free to produce any number of diagnostics as long as a
valid program is still correctly translated. It may also successfully translate an invalid program.
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 equivalent;9) 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).
9) 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.
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;10) 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 (7.20.4.3).
5.1.2.3 Program execution
1 The semantic descriptions in this International Standard describe the behavior of an
abstract machine in which issues of optimization are irrelevant.
2 Accessing a volatile object, modifying an object, modifying a file, or calling a function
that does any of those operations are all side effects,11) which are changes in the state of
the execution environment. Evaluation of an expression may produce side effects. At
certain specified points in the execution sequence called sequence points, all side effects
of previous evaluations shall be complete and no side effects of subsequent evaluations
shall have taken place. (A summary of the sequence points is given in annex C.)
3 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 accessing a volatile object).
4 When the processing of the abstract machine is interrupted by receipt of a signal, only the
values of objects as of the previous sequence point may be relied on. Objects that may be
modified between the previous sequence point and the next sequence point need not have
received their correct values yet.
5 The least requirements on a conforming implementation are:
-- At sequence points, volatile objects are stable in the sense that previous accesses are
complete and subsequent accesses have not yet occurred.
10) 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.
11) 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.
-- 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
7.19.3. 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.
6 What constitutes an interactive device is implementation-defined.
7 More stringent correspondences between abstract and actual semantics may be defined by
each implementation.
8 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.
9 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.
10 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
overflow, or with overflow wrapping silently to produce the correct result, the actual execution need only
produce the same result, possibly omitting the promotions.
11 EXAMPLE 3 Similarly, in the fragment
float f1, f2;
double d;
/* ... */
f1 = f2 * d;
the multiplication may 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).
12 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.
13 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.8).
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;
14 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 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 overflow silently generates some value and where positive and negative overflows cancel, the
above expression statement can be rewritten by the implementation in any of the above ways because the
same result will occur.
15 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), the
signal function (7.14), files (7.19.3).
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.
3 Both the basic source and basic execution character sets shall have the following
members: the 26 uppercase 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 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 International Standard 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 International
Standard 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 Trigraph sequences
1 Before any other processing takes place, each occurrence of one of the following
sequences of three characters (called trigraph sequences12)) is replaced with the
corresponding single character.
??= # ??) ] ??! |
??( [ ??' ^ ??> }
??/ \ ??< { ??- ~
No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed
above is not changed.
2 EXAMPLE 1
??=define arraycheck(a, b) a??(b??) ??!??! b??(a??)
becomes
#define arraycheck(a, b) a[b] || b[a]
3 EXAMPLE 2 The following source line
printf("Eh???/n");
becomes (after replacement of the trigraph sequence ??/)
printf("Eh?\n");
5.2.1.2 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 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.
12) The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as
described in ISO/IEC 646, which is a subset of the seven-bit US ASCII code set.
-- 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 nongraphic 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
International Standard.
Forward references: the isprint function (7.4.1.8), the fputc function (7.19.7.3).
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 translators 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 at least one program that
contains at least one instance of every one of the following limits:13)
-- 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 incomplete 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 character name or extended source character is considered a single
character)
-- 31 significant initial characters in an external identifier (each universal character name
specifying a short identifier of 0000FFFF or less is considered 6 characters, each
13) Implementations should avoid imposing fixed translation limits whenever possible.
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)14)
-- 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
-- 4095 characters in a logical source line
-- 4095 characters in a character string literal or wide string literal (after concatenation)
-- 65535 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 struct-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> (7.18).
5.2.4.2.1 Sizes of integer types <limits.h>
1 The values given below shall be replaced by constant expressions suitable for use in #if
preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the
following shall be replaced by expressions that have the same type as would an
expression that is an object of the corresponding type converted according to the integer
promotions. Their implementation-defined values shall be equal or greater in magnitude
14) See ``future language directions'' (6.11.3).
(absolute value) to those shown, with the same sign.
-- number of bits for smallest object that is not a bit-field (byte)
CHAR_BIT 8
-- minimum value for an object of type signed char
SCHAR_MIN -127 // -(27 - 1)
-- maximum value for an object of type signed char
SCHAR_MAX +127 // 27 - 1
-- maximum value for an object of type unsigned char
UCHAR_MAX 255 // 28 - 1
-- minimum value for an object of type char
CHAR_MIN see below
-- maximum value for an object of type char
CHAR_MAX see below
-- maximum number of bytes in a multibyte character, for any supported locale
MB_LEN_MAX 1
-- minimum value for an object of type short int
SHRT_MIN -32767 // -(215 - 1)
-- maximum value for an object of type short int
SHRT_MAX +32767 // 215 - 1
-- maximum value for an object of type unsigned short int
USHRT_MAX 65535 // 216 - 1
-- minimum value for an object of type int
INT_MIN -32767 // -(215 - 1)
-- maximum value for an object of type int
INT_MAX +32767 // 215 - 1
-- maximum value for an object of type unsigned int
UINT_MAX 65535 // 216 - 1
-- minimum value for an object of type long int
LONG_MIN -2147483647 // -(231 - 1)
-- maximum value for an object of type long int
LONG_MAX +2147483647 // 231 - 1
-- maximum value for an object of type unsigned long int
ULONG_MAX 4294967295 // 232 - 1
-- minimum value for an object of type long long int
LLONG_MIN -9223372036854775807 // -(263 - 1)
-- maximum value for an object of type long long int
LLONG_MAX +9223372036854775807 // 263 - 1
-- maximum value for an object of type unsigned long long int
ULLONG_MAX 18446744073709551615 // 264 - 1
2 If the value of an object of type char is treated as a signed integer when used in an
expression, 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.15) The value UCHAR_MAX shall equal 2CHAR_BIT - 1.
Forward references: representations of types (6.2.6), conditional inclusion (6.10.1).
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
representation of floating-point numbers and values that provide information about an
implementation's floating-point arithmetic.16) The following parameters are used to
define the model for each floating-point type:
s sign (<28>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)
2 A floating-point number (x) is defined by the following model:
p
x = sb e f k b-k ,
k=1
emin e emax
3 In addition to normalized floating-point numbers ( f 1 > 0 if x 0), floating types may be
able to contain other kinds of floating-point numbers, such as subnormal floating-point
numbers (x 0, e = emin , f 1 = 0) and unnormalized floating-point numbers (x 0,
e > emin , f 1 = 0), and values that are not floating-point numbers, such as infinities and
NaNs. A NaN is an encoding 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
15) See 6.2.5.
16) 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.
arithmetic operand.17)
4 An implementation may give zero and non-numeric values (such as infinities and NaNs) a
sign or may leave them unsigned. Wherever such values are unsigned, any requirement
in this International Standard to retrieve the sign shall produce an unspecified sign, and
any requirement to set the sign shall be ignored.
5 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.
6 All integer values in the <float.h> header, except FLT_ROUNDS, shall be constant
expressions suitable for use in #if preprocessing directives; all floating values shall be
constant expressions. All except DECIMAL_DIG, FLT_EVAL_METHOD, FLT_RADIX,
and FLT_ROUNDS have separate names for all three floating-point types. The floating-
point model representation is provided for all values except FLT_EVAL_METHOD and
FLT_ROUNDS.
7 The rounding mode for floating-point addition is characterized by the implementation-
defined value of FLT_ROUNDS:18)
-1 indeterminable
0 toward zero
1 to nearest
2 toward positive infinity
3 toward negative infinity
All other values for FLT_ROUNDS characterize implementation-defined rounding
behavior.
8 Except for assignment and cast (which remove all extra range and precision), the values
of operations with floating operands and values subject to the usual arithmetic
conversions and of floating constants are evaluated to a format whose range and precision
may be greater than required by the type. The use of evaluation formats is characterized
by the implementation-defined value of FLT_EVAL_METHOD:19)
17) IEC 60559:1989 specifies quiet and signaling NaNs. For implementations that do not support
IEC 60559:1989, the terms quiet NaN and signaling NaN are intended to apply to encodings with
similar behavior.
18) Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through
the function fesetround in <fenv.h>.
-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.
9 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 in the widest
supported floating type 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
-- 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,
19) 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.
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 normalized floating-point number, emin
FLT_MIN_EXP
DBL_MIN_EXP
LDBL_MIN_EXP
-- minimum negative integer such that 10 raised to that power is in the range of
normalized floating-point numbers, log10 b emin -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, 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 )b emax )
FLT_MAX_10_EXP +37
DBL_MAX_10_EXP +37
LDBL_MAX_10_EXP +37
10 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, (1 - b- p )b emax
FLT_MAX 1E+37
DBL_MAX 1E+37
LDBL_MAX 1E+37
11 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 value greater than 1 that is representable in the
given floating point type, b1- p
FLT_EPSILON 1E-5
DBL_EPSILON 1E-9
LDBL_EPSILON 1E-9
-- minimum normalized positive floating-point number, b emin -1
FLT_MIN 1E-37
DBL_MIN 1E-37
LDBL_MIN 1E-37
Recommended practice
12 Conversion from (at least) double to decimal with DECIMAL_DIG digits and back
should be the identity function.
13 EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum
requirements of this International Standard, and the appropriate values in a <float.h> header for type
float:
6
x = s16e f k 16-k ,
k=1
-31 e +32
FLT_RADIX 16
FLT_MANT_DIG 6
FLT_EPSILON 9.53674316E-07F
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
14 EXAMPLE 2 The following describes floating-point representations that also meet the requirements for
single-precision and double-precision normalized numbers in IEC 60559,20) and the appropriate values in a
<float.h> header for types float and double:
24
x f = s2e f k 2-k ,
k=1
-125 e +128
53
x d = s2e f k 2-k ,
k=1
-1021 e +1024
FLT_RADIX 2
DECIMAL_DIG 17
FLT_MANT_DIG 24
FLT_EPSILON 1.19209290E-07F // decimal constant
FLT_EPSILON 0X1P-23F // hex constant
20) 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.
FLT_DIG 6
FLT_MIN_EXP -125
FLT_MIN 1.17549435E-38F // decimal constant
FLT_MIN 0X1P-126F // hex constant
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_EPSILON 2.2204460492503131E-16 // decimal constant
DBL_EPSILON 0X1P-52 // hex constant
DBL_DIG 15
DBL_MIN_EXP -1021
DBL_MIN 2.2250738585072014E-308 // decimal constant
DBL_MIN 0X1P-1022 // hex constant
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
If a type wider than double were supported, then DECIMAL_DIG would be greater than 17. For
example, if the widest type were to use the minimal-width IEC 60559 double-extended format (64 bits of
precision), then DECIMAL_DIG would be 21.
Forward references: conditional inclusion (6.10.1), complex arithmetic
<complex.h> (7.3), extended multibyte and wide character utilities <wchar.h>
(7.24), floating-point environment <fenv.h> (7.6), general utilities <stdlib.h>
(7.20), input/output <stdio.h> (7.19), mathematics <math.h> (7.12).
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 that declares the types of its parameters.)
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 be a
strict subset of 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 International Standard 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. Any
other identifier has scope that begins just after the completion of its declarator.
Forward references: declarations (6.7), function calls (6.5.2.2), function definitions
(6.9.1), identifiers (6.4.2), name spaces of identifiers (6.2.3), macro replacement (6.10.3),
source file inclusion (6.10.2), statements (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 linkage.21) 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 or a function contains the storage-
class specifier static, the identifier has internal linkage.22)
4 For an identifier declared with the storage-class specifier extern in a scope in which a
21) There is no linkage between different identifiers.
22) A function declaration can contain the storage-class specifier static only if it is at file scope; see
6.7.1.
prior declaration of that identifier is visible,23) 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,
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.
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 any24)
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);
-- all other identifiers, called ordinary identifiers (declared in ordinary declarators or as
enumeration 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).
23) As specified in 6.2.1, the later declaration might hide the prior declaration.
24) There is only one name space for tags even though three are possible.
6.2.4 Storage durations of objects
1 An object has a storage duration that determines its lifetime. There are three storage
durations: static, automatic, and allocated. Allocated storage is described in 7.20.3.
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 address,25) and retains
its last-stored value throughout its lifetime.26) If an object is referred to outside of its
lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when
the object it points to reaches the end of its lifetime.
3 An object whose identifier is declared 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 no linkage and without the storage-class
specifier static has automatic storage duration.
5 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 value of the object is indeterminate. If an
initialization is specified for the object, it is performed each time the declaration is
reached in the execution of the block; otherwise, the value becomes indeterminate each
time the declaration is reached.
6 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
declaration.27) If the scope is entered recursively, a new instance of the object is created
each time. The initial value of the object is indeterminate.
Forward references: statements (6.8), function calls (6.5.2.2), declarators (6.7.5), array
declarators (6.7.5.2), initialization (6.7.8).
25) The term ``constant address'' means that two pointers to the object constructed at possibly different
times will compare equal. The address may be different during two different executions of the same
program.
26) In the case of a volatile object, the last store need not be explicit in the program.
27) 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.
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 fully describe objects), function types (types
that describe functions), and incomplete types (types that describe objects but lack
information needed to determine their sizes).
2 An object declared as type _Bool is large enough to store the values 0 and 1.
3 An object declared as type char is large enough to store any member of the basic
execution character 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.) There may also be
implementation-defined extended signed integer types.28) The standard and extended
signed integer types are collectively called signed integer types.29)
5 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>).
6 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 standard and extended unsigned integer types are collectively called unsigned integer
types.30)
28) Implementation-defined keywords shall have the form of an identifier reserved for any use as
described in 7.1.3.
29) Therefore, any statement in this Standard about signed integer types also applies to the extended
signed integer types.
30) Therefore, any statement in this Standard about unsigned integer types also applies to the extended
unsigned integer types.
7 The standard signed integer types and standard unsigned integer types are collectively
called the standard integer types, the extended signed integer types and extended
unsigned integer types are collectively called the extended integer types.
8 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.
9 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.31) A computation involving unsigned operands can never overflow,
because a result that cannot be represented by the resulting unsigned integer type is
reduced modulo the number that is one greater than the largest value that can be
represented by the resulting type.
10 There are three real floating types, designated as float, double, and long
double.32) 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.
11 There are three complex types, designated as float _Complex, double
_Complex, and long double _Complex.33) The real floating and complex types
are collectively called the floating types.
12 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.
13 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.
14 The type char, the signed and unsigned integer types, and the floating types are
collectively called the basic types. Even if the implementation defines two or more basic
types to have the same representation, they are nevertheless different types.34)
31) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
32) See ``future language directions'' (6.11.1).
33) A specification for imaginary types is in informative annex G.
34) An implementation may define new keywords that provide alternative ways to designate a basic (or
any other) type; this does not violate the requirement that all basic types be different.
Implementation-defined keywords shall have the form of an identifier reserved for any use as
described in 7.1.3.
15 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.35)
16 An enumeration comprises a set of named integer constant values. Each distinct
enumeration constitutes a different enumerated type.
17 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.
18 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.
19 The void type comprises an empty set of values; it is an incomplete type that cannot be
completed.
20 Any number of derived types can be constructed from the object, function, and
incomplete types, as follows:
-- An array type describes a contiguously allocated nonempty set of objects with a
particular member object type, called the element type.36) 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''.
35) 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.
36) Since object types do not include incomplete types, an array of incomplete type cannot be constructed.
-- A pointer type may be derived from a function type, an object type, or an incomplete
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''.
These methods of constructing derived types can be applied recursively.
21 Arithmetic types and pointer types are collectively called scalar types. Array and
structure types are collectively called aggregate types.37)
22 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 declarations of that type, by declaring the same structure or
union tag with its defining content later in the same scope.
23 A type has known constant size if the type is not incomplete and is not a variable length
array type.
24 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.
25 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.
26 Any type so far mentioned is an unqualified type. Each unqualified type has several
qualified versions of its type,38) 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.39) A derived type is not qualified by the
qualifiers (if any) of the type from which it is derived.
27 A pointer to void shall have the same representation and alignment requirements as a
pointer to a character type.39) Similarly, pointers to qualified or unqualified versions of
compatible types shall have the same representation and alignment requirements. All
37) Note that aggregate type does not include union type because an object with union type can only
contain one member at a time.
38) See 6.7.3 regarding qualified array and function types.
39) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
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.
28 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.
29 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.40)
4 Values stored in non-bit-field objects of any other object type consist of n <20> CHAR_BIT
bits, where n is the size of an object of that type, in bytes. The value may be copied into
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 the stored
value of an object has such a representation and 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 undefined.41) Such a representation is called
40) 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 2
CHAR_BIT
- 1.
a trap 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 values.42) The value of a structure or union object is never a trap
representation, even though the value of a member of the structure or union object may be
a trap representation.
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.43) 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 trap representation shall
not be generated.
Forward references: declarations (6.7), expressions (6.5), lvalues, arrays, and function
designators (6.3.2.1).
6.2.6.2 Integer types
1 For unsigned integer types other than unsigned char, the bits of the object
representation shall be divided into two groups: value bits and padding bits (there need
not be any of the latter). If there are N value bits, each bit shall represent a different
power of 2 between 1 and 2 N -1 , so that objects of that type shall be capable of
representing values from 0 to 2 N - 1 using a pure binary representation; this shall be
known as the value representation. The values of any padding bits are unspecified.44)
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. There need not be any padding bits;
41) Thus, an automatic variable can be initialized to a trap representation without causing undefined
behavior, but the value of the variable cannot be used until a proper value is stored in it.
42) Thus, for example, structure assignment need not copy any padding bits.
43) 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 may distinguish between them.
44) Some combinations of padding bits might generate trap representations, for example, if one padding
bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap
representation other than as part of an exceptional condition such as an 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.
there shall be exactly one 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 there are
M value bits in the signed type and N in the unsigned type, then M N ). If the sign bit
is zero, it shall not affect the resulting value. If the sign bit is one, the value shall be
modified in one of the following ways:
-- the corresponding value with sign bit 0 is negated (sign and magnitude);
-- the sign bit has the value -(2 N ) (two's complement);
-- the sign bit has the value -(2 N - 1) (ones' complement ).
Which of these applies is implementation-defined, as is whether the value with sign bit 1
and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones'
complement), is a trap representation or a normal value. In the case of sign and
magnitude and ones' complement, if this representation is a normal value it is called a
negative zero.
3 If the implementation supports negative zeros, they shall be generated only by:
-- the &, |, ^, ~, <<, and >> operators with arguments that produce such a value;
-- the +, -, *, /, and % operators where one argument is a negative zero and the result is
zero;
-- compound assignment operators based on the above cases.
It is unspecified whether these cases actually generate a negative zero or a normal zero,
and whether a negative zero becomes a normal zero when stored in an object.
4 If the implementation does not support negative zeros, the behavior of the &, |, ^, ~, <<,
and >> operators with arguments that would produce such a value is undefined.
5 The values of any padding bits are unspecified.45) A valid (non-trap) 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.
6 The precision of an integer type is the number of bits it uses to represent values,
excluding any sign and padding bits. The width of an integer type is the same but
including any sign bit; thus for unsigned integer types the two values are the same, while
45) Some combinations of padding bits might generate trap representations, for example, if one padding
bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap
representation other than as part of an exceptional condition such as an overflow. All other
combinations of padding bits are alternative object representations of the value specified by the value
bits.
for signed integer types the width is one greater than the precision.
6.2.7 Compatible type and composite type
1 Two types have compatible type if their types 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 6.7.5 for declarators.46) Moreover, two structure,
union, or enumerated types declared in separate translation units are compatible if their
tags and members satisfy the following requirements: If one is declared with a tag, the
other shall be declared with the same tag. If both are complete types, then the following
additional requirements apply: there shall be a one-to-one correspondence between their
members such that each pair of corresponding members are declared with compatible
types, and such that if one member of a corresponding pair is declared with a name, the
other member 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.
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 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, the composite type is that type.
-- If only one type is a function type with a parameter type list (a function prototype),
the composite type is a function prototype with the parameter type list.
-- If both types are function types with parameter type lists, the type of each parameter
in the composite parameter type list is the composite type of the corresponding
parameters.
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 visible,47) if the prior declaration specifies internal or
external linkage, the type of the identifier at the later declaration becomes the composite
type.
46) Two types need not be identical to be compatible.
47) As specified in 6.2.1, the later declaration might hide the prior declaration.
5 EXAMPLE Given the following two file scope declarations:
int f(int (*)(), double (*)[3]);
int f(int (*)(char *), double (*)[]);
The resulting composite type for the function is:
int f(int (*)(char *), double (*)[3]);
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 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
representation.
-- 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 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.
-- 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 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.
If an int can represent all values of the original type, the value is converted to an int;
otherwise, it is converted to an unsigned int. These are called the integer
promotions.48) All other types are unchanged by the integer promotions.
3 The integer promotions preserve value including sign. As discussed earlier, whether a
``plain'' char is treated as signed 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 0 if the value compares equal
to 0; otherwise, the result is 1.
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.49)
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.
6.3.1.4 Real floating and integer
1 When a finite value of real 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.50)
2 When a value of integer type is converted to a real 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
48) 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.
49) The rules describe arithmetic on the mathematical value, not the value of a given type of expression.
50) 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).
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.
6.3.1.5 Real floating types
1 When a float is promoted to double or long double, or a double is promoted
to long double, its value is unchanged (if the source value is represented in the
precision and range of its type).
2 When a double is demoted to float, a long double is demoted to double or
float, or a value being represented in greater precision and range than required by its
semantic type (see 6.3.1.8) is explicitly converted (including to its own 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.
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, 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 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:
First, 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.51)
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 precision and range than that required by the type; the types are not
changed thereby.52)
51) 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).
52) The cast and assignment operators are still required to perform their specified conversions as
described in 6.3.1.4 and 6.3.1.5.
6.3.2 Other operands
6.3.2.1 Lvalues, arrays, and function designators
1 An lvalue is an expression with an object type or an incomplete type other than void;53)
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, 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). If the lvalue has qualified type, the value has the
unqualified 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.
3 Except when it is the operand of the sizeof operator 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 operator54) 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> (7.17), initialization (6.7.8), postfix
increment and decrement operators (6.5.2.4), prefix increment and decrement operators
(6.5.3.1), the sizeof operator (6.5.3.4), structure and union members (6.5.2.3).
53) 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 International Standard 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.
54) Because this conversion does not occur, the operand of the sizeof operator remains a function
designator and violates the constraint 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 incomplete or object
type. A pointer to any incomplete or 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, or such an expression cast to type
void *, is called a null pointer constant.55) If a null pointer constant 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 be a trap representation.56)
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 or incomplete type may be converted to a pointer to a different
object or incomplete type. If the resulting pointer is not correctly aligned57) for the
pointed-to 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
55) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.17.
56) 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.
57) 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.
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 pointed-to type,
the behavior is undefined.
Forward references: cast operators (6.5.4), equality operators (6.5.9), integer types
capable of holding object pointers (7.18.1.4), simple assignment (6.5.16.1).
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 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.
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
single non-white-space characters that do not lexically match the other preprocessing
token categories.58) 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.
58) An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.3.3); it cannot
occur in source files.
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 preprocessing directives and 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 constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.3), 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
auto enum restrict unsigned
break extern return void
case float short volatile
char for signed while
const goto sizeof _Bool
continue if static _Complex
default inline struct _Imaginary
do int switch
double long typedef
else register union
Semantics
2 The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as
keywords, and shall not be used otherwise. The keyword _Imaginary is reserved for
specifying imaginary types.59)
59) One possible specification for imaginary types appears in annex G.
6.4.2 Identifiers
6.4.2.1 General
Syntax
1 identifier:
identifier-nondigit
identifier identifier-nondigit
identifier digit
identifier-nondigit:
nondigit
universal-character-name
other implementation-defined characters
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 identifier is a sequence of nondigit characters (including the underscore _, the
lowercase and uppercase Latin letters, and other characters) and digits, 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 Each universal character name in an identifier shall designate a character whose encoding
in ISO/IEC 10646 falls into one of the ranges specified in annex D.60) The initial
character shall not be a universal character name designating a digit. An implementation
may allow multibyte characters that are not part of the basic source character set to
appear in identifiers; which characters and their correspondence to universal character
names is implementation-defined.
4 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.
60) On systems in which linkers cannot accept extended characters, an encoding of the universal character
name may be used in forming valid external identifiers. For example, some otherwise unused
character or sequence of characters may be used to encode the \u in a universal character name.
Extended characters may produce a long external identifier.
Implementation limits
5 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.
6 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 (6.10.3).
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.61)
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).
61) 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 specify a character whose short identifier is less than
00A0 other than 0024 ($), 0040 (@), or 0060 (`), nor one in the range D800 through
DFFF inclusive.62)
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 \Unnnnnnnn designates the character whose eight-digit
short identifier (as specified by ISO/IEC 10646) is nnnnnnnn.63) Similarly, the universal
character name \unnnn designates the character whose four-digit short identifier is nnnn
(and whose eight-digit short identifier is 0000nnnn).
62) 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, and the S-zone (reserved for use by
UTF-16).
63) Short identifiers for characters were first specified in ISO/IEC 10646-1/AMD9:1997.
6.4.4 Constants
Syntax
1 constant:
integer-constant
floating-constant
enumeration-constant
character-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
decimal-constant:
nonzero-digit
decimal-constant digit
octal-constant:
0
octal-constant octal-digit
hexadecimal-constant:
hexadecimal-prefix hexadecimal-digit
hexadecimal-constant hexadecimal-digit
hexadecimal-prefix: one of
0x 0X
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: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
integer-suffix:
unsigned-suffix long-suffixopt
unsigned-suffix long-long-suffix
long-suffix unsigned-suffixopt
long-long-suffix unsigned-suffixopt
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.
3 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.
Semantics
4 The value of a decimal constant is computed base 10; that of an octal constant, base 8;
that of a hexadecimal constant, base 16. The lexically first digit is the most significant.
5 The type of an integer constant is the first of the corresponding list in which its value can
be represented.
Octal or Hexadecimal
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
6 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.
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 digit
hexadecimal-fractional-constant:
hexadecimal-digit-sequenceopt .
hexadecimal-digit-sequence
hexadecimal-digit-sequence .
binary-exponent-part:
p signopt digit-sequence
P signopt digit-sequence
hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence hexadecimal-digit
floating-suffix: one of
f l F L
Description
2 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. 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
3 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.
4 An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has
type float. If suffixed by the letter l or L, it has type long double.
5 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.
Recommended practice
6 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.
7 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.64)
64) The specification for the library functions recommends more accurate conversion than required for
floating constants (see 7.20.1.3).
6.4.4.3 Enumeration constants
Syntax
1 enumeration-constant:
identifier
Semantics
2 An identifier declared as an enumeration constant has type int.
Forward references: enumeration specifiers (6.7.2.2).
6.4.4.4 Character constants
Syntax
1 character-constant:
' c-char-sequence '
L' c-char-sequence '
c-char-sequence:
c-char
c-char-sequence c-char
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 wide character constant is the same, except prefixed by the
letter L. 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 nongraphic characters are representable by escape sequences
consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t,
and \v.65)
65) 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).
Constraints
9 The value of an octal or hexadecimal escape sequence shall be in the range of
representable values for the type unsigned char for an integer character constant, or
the unsigned type corresponding to wchar_t for a wide character constant.
Semantics
10 An integer character constant has type int. The value of an integer character constant
containing a single character that maps to a single-byte execution character is the
numerical value of the representation of the mapped character 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-byte
execution character, 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.
11 A wide character constant has type wchar_t, an integer type defined in the
<stddef.h> header. The value of a wide character constant containing a single
multibyte character that maps to a member of the extended execution character set is the
wide character corresponding to that multibyte character, as defined by the mbtowc
function, with an implementation-defined current locale. The value of a wide character
constant containing more than one multibyte character, or containing a multibyte
character or escape sequence not represented in the extended execution character set, is
implementation-defined.
12 EXAMPLE 1 The construction '\0' is commonly used to represent the null character.
13 EXAMPLE 2 Consider implementations that use two's-complement representation for integers and 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.
14 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' may 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.)
15 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> (7.17), the mbtowc function
(7.20.7.2).
6.4.5 String literals
Syntax
1 string-literal:
" s-char-sequenceopt "
L" 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
Description
2 A character string literal is a sequence of zero or more multibyte characters enclosed in
double-quotes, as in "xyz". A wide string literal is the same, except prefixed by the
letter L.
3 The same considerations apply to each element of the sequence in a character string
literal or a wide string literal as if it were in an integer character constant or a wide
character constant, 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
4 In translation phase 6, the multibyte character sequences specified by any sequence of
adjacent character and wide string literal tokens are concatenated into a single multibyte
character sequence. If any of the tokens are wide string literal tokens, the resulting
multibyte character sequence is treated as a wide string literal; otherwise, it is treated as a
character string literal.
5 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.66) 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; for wide string literals, the array elements have type wchar_t, and are
initialized with the sequence of wide characters corresponding to the multibyte character
66) A character string literal need not be a string (see 7.1.1), because a null character may be embedded in
it by a \0 escape sequence.
sequence, as defined by the mbstowcs function with an implementation-defined current
locale. The value of a string literal containing a multibyte character or escape sequence
not represented in the execution character set is implementation-defined.
6 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.
7 EXAMPLE 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.
Forward references: common definitions <stddef.h> (7.17), the mbstowcs
function (7.20.8.1).
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 tokens67)
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens
[ ] { } # ##
except for their spelling.68)
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
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
67) These tokens are sometimes called ``digraphs''.
68) Thus [ and <: behave differently when ``stringized'' (see 6.10.3.2), but can otherwise be freely
interchanged.
sequence between the " delimiters, the behavior is undefined.69) Header name
preprocessing tokens are recognized only within #include preprocessing directives and
in implementation-defined locations within #pragma directives.70)
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 digit
pp-number identifier-nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
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.
69) Thus, sequences of characters that resemble escape sequences cause undefined behavior.
70) For an example of a header name preprocessing token used in a #pragma directive, see 6.10.9.
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.71)
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;
71) 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.
2 Between the previous and next sequence point an object shall have its stored value
modified at most once by the evaluation of an expression.72) Furthermore, the prior value
shall be read only to determine the value to be stored.73)
3 The grouping of operators and operands is indicated by the syntax.74) Except as specified
later (for the function-call (), &&, ||, ?:, and comma operators), the order of evaluation
of subexpressions and the order in which side effects take place are both unspecified.
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.75) If a value is stored into an object having no declared type through an
lvalue having a type that is not a character type, then the type of the lvalue becomes the
72) A floating-point status flag is not an object and can be set more than once within an expression.
73) This paragraph renders undefined statement expressions such as
i = ++i + 1;
a[i++] = i;
while allowing
i = i + 1;
a[i] = i;
74) 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.
75) Allocated objects have no declared type.
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:76)
-- 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,
-- 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 an atomic
operation, thereby omitting rounding errors implied by the source code and the
expression evaluation method.77) The FP_CONTRACT pragma in <math.h> provides a
way to disallow contracted expressions. Otherwise, whether and how expressions are
contracted is implementation-defined.78)
Forward references: the FP_CONTRACT pragma (7.12.2), copying functions (7.21.2).
76) The intent of this list is to specify those circumstances in which an object may or may not be aliased.
77) A contracted expression might also omit the raising of floating-point exceptions.
78) 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.
6.5.1 Primary expressions
Syntax
1 primary-expression:
identifier
constant
string-literal
( expression )
Semantics
2 An identifier is a primary expression, provided it has been declared as designating an
object (in which case it is an lvalue) or a function (in which case it is a function
designator).79)
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 and value are identical to
those of the unparenthesized expression. It is an lvalue, a function designator, or a void
expression if the unparenthesized expression is, respectively, an lvalue, a function
designator, or a void expression.
Forward references: declarations (6.7).
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 --
( type-name ) { initializer-list }
( type-name ) { initializer-list , }
79) Thus, an undeclared identifier is a violation of the syntax.
argument-expression-list:
assignment-expression
argument-expression-list , assignment-expression
6.5.2.1 Array subscripting
Constraints
1 One of the expressions shall have type ``pointer to 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 <20> j <20> . . . <20> k, then E (used as
other than an lvalue) is converted to a pointer to an (n - 1)-dimensional array with
dimensions j <20> . . . <20> k. If the unary * operator is applied to this pointer explicitly, or
implicitly as a result of subscripting, the result is the pointed-to (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 <20> 5 array of ints; more precisely, x is an array of three element objects, each of which is an
array of five ints. In the expression x[i], which is equivalent to (*((x)+(i))), x is first converted to
a pointer to the initial array of five ints. 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 ints. When used in the
expression x[i][j], that array is in turn converted to a pointer to the first of the ints, so x[i][j]
yields an int.
Forward references: additive operators (6.5.6), address and indirection operators
(6.5.3.2), array declarators (6.7.5.2).
6.5.2.2 Function calls
Constraints
1 The expression that denotes the called function80) shall have type pointer to function
returning void or returning an object type other than an array type.
2 If the expression that denotes the called function has a type that includes a prototype, 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 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.81)
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. If
an attempt is made to modify the result of a function call or to access it after the next
sequence point, the behavior is undefined.
6 If the expression that denotes the called function has a type that does not include a
prototype, the integer promotions are performed on each argument, and arguments that
have type float are promoted to double. These are called the default argument
promotions. If the number of arguments does not equal the number of parameters, the
behavior is undefined. If the function is defined with a type that includes a prototype, and
either the prototype ends with an ellipsis (, ...) or the types of the arguments after
promotion are not compatible with the types of the parameters, the behavior is undefined.
If the function is defined with a type that does not include a prototype, and the types of
the arguments after promotion are not compatible with those of the parameters after
promotion, the behavior is undefined, except for the following cases:
80) Most often, this is the result of converting an identifier that is a function designator.
81) A function may 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 may
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 6.9.1.
-- one promoted type is a signed integer type, the other promoted type is the
corresponding unsigned integer type, and the value is representable in both types;
-- both types are pointers to qualified or unqualified versions of a character type or
void.
7 If the expression that denotes the called function has a type that does include a prototype,
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 last declared parameter. The default argument
promotions are performed on trailing arguments.
8 No other conversions are performed implicitly; in particular, the number and types of
arguments are not compared with those of the parameters in a function definition that
does not include a function prototype declarator.
9 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.
10 The order of evaluation of the function designator, the actual arguments, and
subexpressions within the actual arguments is unspecified, but there is a sequence point
before the actual call.
11 Recursive function calls shall be permitted, both directly and indirectly through any chain
of other functions.
12 EXAMPLE In the function call
(*pf[f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 may 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 (including prototypes) (6.7.5.3), 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 a 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 qualified or unqualified
structure'' or ``pointer to 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,82) 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.83) 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 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 complete 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.
6 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.
7 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
82) If the member used to access 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 trap representation.
83) 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.
8 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 qualified or
unqualified real or pointer type and shall be a modifiable lvalue.
Semantics
2 The result of the postfix ++ operator is the value of the operand. After the result is
obtained, the value of the operand 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 side effect of updating the stored value of the operand shall occur between
the previous and the next sequence point.
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
Constraints
1 The type name shall specify an object type or an array of unknown size, but not a variable
length array type.
2 No initializer shall attempt to provide a value for an object not contained within the entire
unnamed object specified by the compound literal.
3 If the compound literal occurs outside the body of a function, the initializer list shall
consist of constant expressions.
Semantics
4 A postfix expression that consists of a parenthesized type name followed by a brace-
enclosed list of initializers is a compound literal. It provides an unnamed object whose
value is given by the initializer list.84)
5 If the type name specifies an array of unknown size, the size is determined by the
initializer list as specified in 6.7.8, and the type of the compound literal is that of the
completed array type. Otherwise (when the type name specifies an object type), the type
of the compound literal is that specified by the type name. In either case, the result is an
lvalue.
84) 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.
6 The value of the compound literal is that of an unnamed object initialized by the
initializer list. If the compound literal occurs outside the body of a function, the object
has static storage duration; otherwise, it has automatic storage duration associated with
the enclosing block.
7 All the semantic rules and constraints for initializer lists in 6.7.8 are applicable to
compound literals.85)
8 String literals, and compound literals with const-qualified types, need not designate
distinct objects.86)
9 EXAMPLE 1 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.
10 EXAMPLE 2 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.
11 EXAMPLE 3 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},
&(struct point){.x=3, .y=4});
12 EXAMPLE 4 A read-only compound literal can be specified through constructions like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
85) For example, subobjects without explicit initializers are initialized to zero.
86) This allows implementations to share storage for string literals and constant compound literals with
the same or overlapping representations.
13 EXAMPLE 5 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.
14 EXAMPLE 6 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.
15 EXAMPLE 7 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);
16 EXAMPLE 8 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.
17 Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the
lifetime of the unnamed object would be the body of the loop only, and on entry next time around p would
have an indeterminate value, which would result in undefined behavior.
Forward references: type names (6.7.6), initialization (6.7.8).
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 )
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 qualified or
unqualified 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.87)
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.
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).
87) 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.
6.5.3.4 The sizeof operator
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.
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 When 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.88) 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.
4 The value of the result is implementation-defined, and its type (an unsigned integer type)
is size_t, defined in <stddef.h> (and other headers).
5 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 should ensure that its return value is aligned suitably for
conversion to a pointer to double.
6 EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:
sizeof array / sizeof array[0]
7 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
}
88) 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> (7.17), declarations (6.7),
structure and union specifiers (6.7.2.1), type names (6.7.6), array declarators (6.7.5.2).
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 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.
Semantics
4 Preceding an expression by a parenthesized type name converts the value of the
expression to the named type. This construction is called a cast.89) A cast that specifies
no conversion has no effect on the type or value of an expression.
5 If the value of the expression is represented with greater precision or range 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.
Forward references: equality operators (6.5.9), function declarators (including
prototypes) (6.7.5.3), simple assignment (6.5.16.1), type names (6.7.6).
89) A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the
unqualified version of the type.
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.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the binary * operator is the product of the operands.
5 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.
6 When integers are divided, the result of the / operator is the algebraic quotient with any
fractional part discarded.90) If the quotient a/b is representable, the expression
(a/b)*b + a%b shall equal a.
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 an 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;
90) This is often called ``truncation toward zero''.
-- both operands are pointers to qualified or unqualified versions of compatible object
types; or
-- the left operand is a pointer to an object type and the right operand has integer type.
(Decrementing is equivalent to subtracting 1.)
Semantics
4 If both operands have arithmetic type, the usual arithmetic conversions are performed on
them.
5 The result of the binary + operator is the sum of the operands.
6 The result of the binary - operator is the difference resulting from the subtraction of the
second operand from the first.
7 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.
8 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 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 both the pointer
operand and the result point to elements of the same array object, or one past the last
element of the array object, the evaluation shall not produce an overflow; otherwise, 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.
9 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.91)
10 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
}
11 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 (6.7.5.2), common definitions <stddef.h>
(7.17).
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.
91) 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 may overlap
another object in the program) just after the end of the object in order to satisfy the ``one past the last
element'' requirements.
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 <20> 2E2 , reduced modulo
one more than the maximum value representable in the result type. If E1 has a signed
type and nonnegative value, and E1 <20> 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;
-- both operands are pointers to qualified or unqualified versions of compatible object
types; or
-- both operands are pointers to qualified or unqualified versions of compatible
incomplete types.
Semantics
3 If both of the operands have arithmetic type, the usual arithmetic conversions are
performed.
4 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.
5 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 or incomplete 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.
6 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.92)
The result has type int.
6.5.9 Equality operators
Syntax
1 equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression
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 or incomplete type and the other is a pointer to a
qualified or unqualified version of void; or
-- one operand is a pointer and the other is a null pointer constant.
Semantics
3 The == (equal to) and != (not equal to) operators are analogous to the relational
operators except for their lower precedence.93) 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.
4 If both of the operands have arithmetic type, the usual arithmetic conversions are
performed. 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.
92) 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''.
93) Because of the precedences, a<b == c<d is 1 whenever a<b and c<d have the same truth-value.
5 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 or incomplete 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.
6 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
space.94)
7 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.
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).
94) Two objects may 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.
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;
there is a sequence point after the evaluation of the first operand. 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; there is
a sequence point after the evaluation of the first operand. 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 operands:
-- 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;
-- one operand is a pointer and the other is a null pointer constant; or
-- one operand is a pointer to an object or incomplete type and the other is a pointer to a
qualified or unqualified version of void.
Semantics
4 The first operand is evaluated; there is a sequence point after its evaluation. 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 below.95) If an attempt is made
to modify the result of a conditional operator or to access it after the next sequence point,
the behavior is undefined.
5 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.
6 If both the second and third operands are pointers or one is a null pointer constant and the
other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers
of the types pointed-to by both operands. 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
95) A conditional expression does not yield an lvalue.
pointer to an appropriately qualified version of void.
7 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.
8 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, but is not an
lvalue. The type of an assignment expression is the type of the left operand unless the
left operand has qualified type, in which case it is the unqualified version of the type of
the left operand. The side effect of updating the stored value of the left operand shall
occur between the previous and the next sequence point.
4 The order of evaluation of the operands is unspecified. If an attempt is made to modify
the result of an assignment operator or to access it after the next sequence point, the
behavior is undefined.
6.5.16.1 Simple assignment
Constraints
1 One of the following shall hold:96)
-- the left operand has qualified or unqualified arithmetic type and the right has
arithmetic type;
-- the left operand has a qualified or unqualified version of a structure or union type
compatible with the type of the right;
-- 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;
-- one operand is a pointer to an object or incomplete 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 is a pointer and the right is a null pointer constant; or
-- the left operand has type _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.
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 may 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
96) 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).
negative, so the operands of the comparison can never compare equal. Therefore, for full portability, the
variable c should be declared as int.
5 EXAMPLE 2 In the fragment:
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 a pointer to an object
type and the right shall have integer type, or the left operand shall have qualified or
unqualified arithmetic type and the right shall have arithmetic type.
2 For the other operators, each operand shall have arithmetic type consistent with those
allowed by the corresponding binary operator.
Semantics
3 A compound assignment of the form E1 op = E2 differs from the simple assignment
expression E1 = E1 op (E2) only in that the lvalue E1 is evaluated only once.
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 after its evaluation. Then the right operand is evaluated; the result has its
type and value.97) If an attempt is made to modify the result of a comma operator or to
access it after the next sequence point, the behavior is undefined.
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 (6.7.8).
97) 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
evaluated.98)
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 precision and range
shall be at least as great as if the expression were being evaluated in the execution
environment.
6 An integer constant expression99) shall have integer type and shall only have operands
that are integer constants, enumeration constants, character constants, sizeof
expressions whose results are integer constants, and floating constants 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 sizeof
operator.
7 More latitude is permitted for constant expressions in initializers. Such a constant
expression shall be, or evaluate to, one of the following:
-- an arithmetic constant expression,
-- a null pointer constant,
98) The operand of a sizeof operator is usually not evaluated (6.5.3.4).
99) An integer constant expression is used to specify the size of a bit-field member of a structure, the
value of an enumeration constant, the size of an array, or the value of a case constant. Further
constraints that apply to the integer constant expressions used in conditional-inclusion preprocessing
directives are discussed in 6.10.1.
-- an address constant, or
-- an address constant for an object type plus or minus an integer constant expression.
8 An arithmetic constant expression shall have arithmetic type and shall only have
operands that are integer constants, floating constants, enumeration constants, character
constants, and sizeof expressions. Cast operators in an arithmetic constant expression
shall only convert arithmetic types to arithmetic types, except as part of an operand to a
sizeof operator whose result is an integer constant.
9 An address constant is a null pointer, 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. The array-subscript [] and member-access .
and -> operators, 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 operators.
10 An implementation may accept other forms of constant expressions.
11 The semantic rules for the evaluation of a constant expression are the same as for
nonconstant expressions.100)
Forward references: array declarators (6.7.5.2), initialization (6.7.8).
100) 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 ;
declaration-specifiers:
storage-class-specifier declaration-specifiersopt
type-specifier declaration-specifiersopt
type-qualifier declaration-specifiersopt
function-specifier declaration-specifiersopt
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
Constraints
2 A 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
for tags as specified in 6.7.2.3.
4 All declarations in the same scope that refer to the same object or function shall specify
compatible types.
Semantics
5 A declaration specifies the interpretation and attributes 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 body;101)
-- for an enumeration constant or typedef name, is the (only) declaration of the
identifier.
6 The declaration specifiers consist of a sequence of specifiers that indicate the linkage,
storage duration, and part of the type of the entities that the declarators denote. The init-
declarator-list is a comma-separated sequence of declarators, each of which may have
101) Function definitions have a different syntax, described in 6.9.1.
additional type information, or an initializer, or both. The declarators contain the
identifiers (if any) being declared.
7 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 (including in prototypes), it is the adjusted
type (see 6.7.5.3) that is required to be complete.
Forward references: declarators (6.7.5), enumeration specifiers (6.7.2.2), initialization
(6.7.8).
6.7.1 Storage-class specifiers
Syntax
1 storage-class-specifier:
typedef
extern
static
auto
register
Constraints
2 At most, one storage-class specifier may be given in the declaration specifiers in a
declaration.102)
Semantics
3 The typedef specifier is called a ``storage-class specifier'' for syntactic convenience
only; it is discussed in 6.7.7. The meanings of the various linkages and storage durations
were discussed in 6.2.2 and 6.2.4.
4 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-defined.103)
5 The declaration of an identifier for a function that has block scope shall have no explicit
storage-class specifier other than extern.
102) See ``future language directions'' (6.11.5).
103) The implementation may 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.
6 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.
Forward references: type definitions (6.7.7).
6.7.2 Type specifiers
Syntax
1 type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_Bool
_Complex
struct-or-union-specifier
enum-specifier
typedef-name
Constraints
2 At least one type specifier shall be given in the declaration specifiers in each declaration,
and in the specifier-qualifier list in each struct declaration and type name. Each list of
type specifiers shall be one of the following sets (delimited by commas, when there is
more than one set on a line); 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
-- float
-- double
-- long double
-- _Bool
-- float _Complex
-- double _Complex
-- long double _Complex
-- struct or union specifier
-- enum specifier
-- typedef name
3 The type specifier _Complex shall not be used if the implementation does not provide
complex types.104)
Semantics
4 Specifiers for structures, unions, and enumerations are discussed in 6.7.2.1 through
6.7.2.3. Declarations of typedef names are discussed in 6.7.7. The characteristics of the
other types are discussed in 6.2.5.
5 Each of the comma-separated sets 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: enumeration specifiers (6.7.2.2), structure and union specifiers
(6.7.2.1), tags (6.7.2.3), type definitions (6.7.7).
104) Freestanding implementations are not required to provide complex types.
6.7.2.1 Structure and union specifiers
Syntax
1 struct-or-union-specifier:
struct-or-union identifieropt { struct-declaration-list }
struct-or-union identifier
struct-or-union:
struct
union
struct-declaration-list:
struct-declaration
struct-declaration-list struct-declaration
struct-declaration:
specifier-qualifier-list struct-declarator-list ;
specifier-qualifier-list:
type-specifier specifier-qualifier-listopt
type-qualifier specifier-qualifier-listopt
struct-declarator-list:
struct-declarator
struct-declarator-list , struct-declarator
struct-declarator:
declarator
declaratoropt : constant-expression
Constraints
2 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.
3 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 omitted. If the value is zero,
the declaration shall have no declarator.
4 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed
int, unsigned int, or some other implementation-defined type.
Semantics
5 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.
6 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.
7 The presence of a struct-declaration-list in a struct-or-union-specifier declares a new type,
within a translation unit. The struct-declaration-list is a sequence of declarations for the
members of the structure or union. If the struct-declaration-list contains no named
members, the behavior is undefined. The type is incomplete until after the } that
terminates the list.
8 A member of a structure or union may have any object type other than a variably
modified type.105) 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;106) its
width is preceded by a colon.
9 A bit-field is interpreted as a signed or unsigned integer type consisting of the specified
number of bits.107) 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.
10 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.
11 A bit-field declaration with no declarator, but only a colon and a width, indicates an
unnamed bit-field.108) 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.
105) A structure or union can not contain a member with a variably modified type because member names
are not ordinary identifiers as defined in 6.2.3.
106) 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.
107) 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.
108) An unnamed bit-field structure member is useful for padding to conform to externally imposed
layouts.
12 Each non-bit-field member of a structure or union object is aligned in an implementation-
defined manner appropriate to its type.
13 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.
14 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.
15 There may be unnamed padding at the end of a structure or union.
16 As a special case, the last element 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.
17 EXAMPLE 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).
18 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.
19 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.
20 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;
21 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
22 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 might be copied or simply overwritten with indeterminate values.
Forward references: tags (6.7.2.3).
6.7.2.2 Enumeration specifiers
Syntax
1 enum-specifier:
enum identifieropt { enumerator-list }
enum identifieropt { enumerator-list , }
enum identifier
enumerator-list:
enumerator
enumerator-list , enumerator
enumerator:
enumeration-constant
enumeration-constant = constant-expression
Constraints
2 The expression that defines the value of an enumeration constant shall be an integer
constant expression that has a value representable as an int.
Semantics
3 The identifiers in an enumerator list are declared as constants that have type int and
may appear wherever such are permitted.109) 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.
4 Each enumerated type shall be compatible with char, a signed integer type, or an
unsigned integer type. The choice of type is implementation-defined,110) but shall be
capable of representing the values of all the members of the enumeration. The
enumerated type is incomplete until after the } that terminates the list of enumerator
declarations.
109) Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from
each other and from other identifiers declared in ordinary declarators.
110) An implementation may delay the choice of which integer type until all enumeration constants have
been seen.
5 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 }.
Forward references: tags (6.7.2.3).
6.7.2.3 Tags
Constraints
1 A specific type shall have its content defined at most once.
2 Where two declarations that use the same tag declare the same type, they shall both use
the same choice of struct, union, or enum.
3 A type specifier of the form
enum identifier
without an enumerator list shall only appear after the type it specifies is complete.
Semantics
4 All declarations of structure, union, or enumerated types that have the same scope and
use the same tag declare the same type. The type is incomplete111) until the closing brace
of the list defining the content, and complete thereafter.
5 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.
6 A type specifier of the form
struct-or-union identifieropt { struct-declaration-list }
or
enum identifier { enumerator-list }
or
enum identifier { enumerator-list , }
declares a structure, union, or enumerated type. The list defines the structure content,
111) An incomplete type may only by 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. (See incomplete types
in 6.2.5.) The specification has to be complete before such a function is called or defined.
union content, or enumeration content. If an identifier is provided,112) the type specifier
also declares the identifier to be the tag of that type.
7 A declaration of the form
struct-or-union identifier ;
specifies a structure or union type and declares the identifier as a tag of that type.113)
8 If a type specifier of the form
struct-or-union 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.113)
9 If a type specifier of the form
struct-or-union identifier
or
enum identifier
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.
10 EXAMPLE 1 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.
11 The following alternative formulation uses the typedef mechanism:
112) 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.
113) A similar construction with enum does not exist.
typedef struct tnode TNODE;
struct tnode {
int count;
TNODE *left, *right;
};
TNODE s, *sp;
12 EXAMPLE 2 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;
may 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 (6.7.5), array declarators (6.7.5.2), type definitions
(6.7.7).
6.7.3 Type qualifiers
Syntax
1 type-qualifier:
const
restrict
volatile
Constraints
2 Types other than pointer types derived from object or incomplete types shall not be
restrict-qualified.
Semantics
3 The properties associated with qualified types are meaningful only for expressions that
are lvalues.114)
4 If the same qualifier appears more than once in the same specifier-qualifier-list, either
directly or via one or more typedefs, the behavior is the same as if it appeared only
once.
114) The implementation may 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.
5 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 undefined.115)
6 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.116) What constitutes an access to an object that
has volatile-qualified type is implementation-defined.
7 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.117) 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).
8 If the specification of an array type includes any type qualifiers, the element type is so-
qualified, not the array type. If the specification of a function type includes any type
qualifiers, the behavior is undefined.118)
9 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.
10 EXAMPLE 1 An object declared
extern const volatile int real_time_clock;
may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.
115) 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).
116) A volatile declaration may 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 shall not be ``optimized out'' by an implementation or reordered except as
permitted by the rules for evaluating expressions.
117) 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.
118) Both of these can occur through the use of typedefs.
11 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
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 *''
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.119)
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
119) 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.
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.
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
}
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;
}
6.7.4 Function specifiers
Syntax
1 function-specifier:
inline
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 storage duration, and shall not contain a reference to an
identifier with internal linkage.
4 In a hosted environment, the inline function specifier shall not appear in a declaration
of main.
Semantics
5 A function declared with an inline function specifier is an inline function. The
function specifier may appear more than once; the behavior is the same as if it appeared
only once. Making a function an inline function suggests that calls to the function be as
fast as possible.120) The extent to which such suggestions are effective is
implementation-defined.121)
6 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.122)
7 EXAMPLE 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.
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);
}
8 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 may be used for the call.
Forward references: function definitions (6.9.1).
120) 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.
121) For example, an implementation might never perform inline substitution, or might only perform inline
substitutions to calls in the scope of an inline declaration.
122) 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.
6.7.5 Declarators
Syntax
1 declarator:
pointeropt direct-declarator
direct-declarator:
identifier
( 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 * ]
direct-declarator ( parameter-type-list )
direct-declarator ( identifier-listopt )
pointer:
* type-qualifier-listopt
* 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:
declaration-specifiers declarator
declaration-specifiers abstract-declaratoropt
identifier-list:
identifier
identifier-list , identifier
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. The end of a full
declarator is a sequence point. 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
then the type specified for ident is T .
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 incomplete type, either
directly or via one or more typedefs.
Forward references: array declarators (6.7.5.2), type definitions (6.7.7).
6.7.5.1 Pointer declarators
Semantics
1 If, in the declaration ``T D1'', D1 has the form
* 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.
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 shall not be modified through that pointer,
but ptr_to_constant itself may be changed to point to another object. Similarly, the contents of the
int pointed to by constant_ptr may be modified, but constant_ptr itself shall always point to the
same location.
4 The declaration of the constant pointer constant_ptr may 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''.
6.7.5.2 Array declarators
Constraints
1 In addition to optional type qualifiers and the keyword static, the [ and ] may delimit
an expression 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 An ordinary identifier (as defined in 6.2.3) that has a variably modified type shall have
either block scope and no linkage or function prototype scope. If an identifier is declared
to be an object with static 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 ]
D[ static type-qualifier-listopt assignment-expression ]
D[ type-qualifier-list static assignment-expression ]
D[ type-qualifier-listopt * ]
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 ''.123)
(See 6.7.5.3 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 with function prototype scope;124) such arrays are
nonetheless complete types. If the size is an integer constant expression and the element
123) When several ``array of'' specifications are adjacent, a multidimensional array is declared.
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.
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 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.
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
// n == 6 and m == n+1
}
124) Thus, * can be used only in function declarations that are not definitions (see 6.7.5.3).
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 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 (6.7.5.3), function definitions (6.9.1),
initialization (6.7.8).
6.7.5.3 Function declarators (including prototypes)
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 An identifier list in a function declarator that is not part of a definition of that function
shall be empty.
4 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
5 If, in the declaration ``T D1'', D1 has the form
D( parameter-type-list )
or
D( identifier-listopt )
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
T ''.
6 A parameter type list specifies the types of, and may declare identifiers for, the
parameters of the function.
7 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.
8 A declaration of a parameter as ``function returning type'' shall be adjusted to ``pointer to
function returning type'', as in 6.3.2.1.
9 If the list terminates with an ellipsis (, ...), no information about the number or types
of the parameters after the comma is supplied.125)
10 The special case of an unnamed parameter of type void as the only item in the list
specifies that the function has no parameters.
11 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.
12 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.
13 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.
14 An identifier list declares only the identifiers of the parameters of the function. An empty
list in a function declarator that is part of a definition of that function specifies that the
function has no parameters. The empty list in a function declarator that is not part of a
definition of that function specifies that no information about the number or types of the
parameters is supplied.126)
15 For two function types to be compatible, both shall specify compatible return types.127)
125) The macros defined in the <stdarg.h> header (7.15) may be used to access arguments that
correspond to the ellipsis.
126) See ``future language directions'' (6.11.6).
127) If both function types are ``old style'', parameter types are not compared.
Moreover, the parameter type lists, if both are present, shall agree in the number of
parameters and in use of the ellipsis terminator; corresponding parameters shall have
compatible types. If one type has a parameter type list and the other type is specified by a
function declarator that is not part of a function definition and that contains an empty
identifier list, the parameter list shall not have an ellipsis terminator and the type of each
parameter shall be compatible with the type that results from the application of the
default argument promotions. If one type has a parameter type list and the other type is
specified by a function definition that contains a (possibly empty) identifier list, both shall
agree in the number of parameters, and the type of each prototype parameter shall be
compatible with the type that results from the application of the default argument
promotions to the type of the corresponding identifier. (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.)
16 EXAMPLE 1 The declaration
int f(void), *fip(), (*pfi)();
declares a function f with no parameters returning an int, a function fip with no parameter specification
returning a pointer to an int, and a pointer pfi to a function with no parameter specification 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.
17 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.
18 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.
19 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.
20 EXAMPLE 4 The following prototype has a variably modified parameter.
void addscalar(int n, int m,
double a[n][n*m+300], double x);
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;
}
21 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 must 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 (6.7.6).
6.7.6 Type names
Syntax
1 type-name:
specifier-qualifier-list abstract-declaratoropt
abstract-declarator:
pointer
pointeropt direct-abstract-declarator
direct-abstract-declarator:
( 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 [ * ]
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.128)
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 ints, (e) pointer to a variable length array of an unspecified number of ints, (f) function
with no parameter specification 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.
128) As indicated by the syntax, empty parentheses in a type name are interpreted as ``function with no
parameter specification'', rather than redundant parentheses around the omitted identifier.
6.7.7 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 6.7.5. 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.
4 EXAMPLE 1 After
typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;
the constructions
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
parameter specification 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];
}
6.7.8 Initialization
Syntax
1 initializer:
assignment-expression
{ initializer-list }
{ initializer-list , }
initializer-list:
designationopt initializer
initializer-list , designationopt initializer
designation:
designator-list =
designator-list:
designator
designator-list designator
designator:
[ constant-expression ]
. identifier
Constraints
2 No initializer shall attempt to provide a value for an object not contained within the entity
being initialized.
3 The type of the entity to be initialized shall be an array of unknown size or an object type
that is not a variable length array type.
4 All the expressions in an initializer for an object that has static storage duration shall be
constant expressions or string literals.
5 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.
6 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.
7 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
8 An initializer specifies the initial value stored in an object.
9 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 value even after initialization.
10 If an object that has automatic storage duration is not initialized explicitly, its value is
indeterminate. If an object that has static storage duration is not initialized explicitly,
then:
-- if it has pointer type, it is initialized to a null pointer;
-- if it has arithmetic type, it is initialized to (positive or unsigned) zero;
-- if it is an aggregate, every member is initialized (recursively) according to these rules;
-- if it is a union, the first named member is initialized (recursively) according to these
rules.
11 The initializer for a scalar shall be a single expression, optionally enclosed in braces. 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.
12 The rest of this subclause deals with initializers for objects that have aggregate or union
type.
13 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.
14 An array of character type may be initialized by a character string literal, optionally
enclosed in braces. Successive characters of the character 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.
15 An array with element type compatible with wchar_t may be initialized by a wide
string literal, 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.
16 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.
17 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.129) 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.130)
18 Each designator list begins its description with the current object associated with the
closest surrounding 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.131) The current object that results at the end of the
designator list is the subobject to be initialized by the following initializer.
19 The initialization shall occur in initializer list order, each initializer provided for a
particular subobject overriding any previously listed initializer for the same subobject;132)
all subobjects that are not initialized explicitly shall be initialized implicitly the same as
objects that have static storage duration.
20 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.
21 If there are fewer initializers in a brace-enclosed list than there are elements or members
of an 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.
22 If an array of unknown size is initialized, its size is determined by the largest indexed
element with an explicit initializer. At the end of its initializer list, the array no longer
has incomplete type.
129) 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.
130) 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.
131) 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.
132) Any initializer for the subobject which is overridden and so not used to initialize that subobject might
not be evaluated at all.
23 The order in which any side effects occur among the initialization list expressions is
unspecified.133)
24 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.
25 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.
26 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].
27 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.
28 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.
133) In particular, the evaluation order need not be the same as the order of subobject initialization.
29 EXAMPLE 6 The declaration
short q[4][3][2] = {
{ 1 },
{ 2, 3 },
{ 4, 5, 6 }
};
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 may 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.
30 Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to
cause confusion.
31 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.
32 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.
33 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",
};
34 EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:
div_t answer = { .quot = 2, .rem = -1 };
35 EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists
might be misunderstood:
struct { int a[3], b; } w[] =
{ [0].a = {1}, [1].a[0] = 2 };
36 EXAMPLE 12 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
};
37 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.
38 EXAMPLE 13 Any member of a union can be initialized:
union { /* ... */ } u = { .any_member = 42 };
Forward references: common definitions <stddef.h> (7.17).
6.8 Statements and blocks
Syntax
1 statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Semantics
2 A statement specifies an action to be performed. Except as indicated, statements are
executed in sequence.
3 A block allows a set of declarations and statements to be grouped into one syntactic unit.
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 (including storing an indeterminate value in objects without an
initializer) 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 or of a declarator.
Each of the following is a full expression: an initializer; 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. The end of a full
expression is a sequence point.
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 labeled-statement:
identifier : statement
case constant-expression : statement
default : 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.
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
statement
Semantics
2 A compound statement is a block.
6.8.3 Expression and null statements
Syntax
1 expression-statement:
expressionopt ;
Semantics
2 The expression in an expression statement is evaluated as a void expression for its side
effects.134)
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 may be made explicit by converting the expression to a void expression by means of
a cast:
int p(int);
/* ... */
(void)p(0);
134) Such as assignments, and function calls which have side effects.
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.
6 EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound
statement.
while (loop1) {
/* ... */
while (loop2) {
/* ... */
if (want_out)
goto end_loop1;
/* ... */
}
/* ... */
end_loop1: ;
}
Forward references: iteration statements (6.8.5).
6.8.4 Selection statements
Syntax
1 selection-statement:
if ( expression ) statement
if ( expression ) statement else statement
switch ( expression ) statement
Semantics
2 A selection statement selects among a set of statements depending on the value of a
controlling expression.
3 A selection statement is a block whose scope is a strict subset of the scope of its
enclosing block. Each associated substatement is also a block whose scope is a strict
subset of the scope of the selection statement.
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.135)
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 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 labeled statement. 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.
135) 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.
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 indeterminate value. Similarly, the call to the function f cannot be reached.
6.8.5 Iteration statements
Syntax
1 iteration-statement:
while ( expression ) statement
do statement while ( expression ) ;
for ( expressionopt ; expressionopt ; expressionopt ) statement
for ( declaration expressionopt ; expressionopt ) statement
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 statement 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 jump.136)
5 An iteration statement is a block whose scope is a strict subset of the scope of its
enclosing block. The loop body is also a block whose scope is a strict subset of the scope
of the iteration statement.
136) 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.
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
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.137)
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.
137) 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.
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;
}
// 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;.138)
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.
138) Following the contin: label is a null 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.
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.139)
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).
139) The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not
apply to the case of function return. The representation of floating-point values may have wider range
or precision and is determined by FLT_EVAL_METHOD. A cast may 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 specifiers auto and 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 (other than as a part of the operand of a sizeof
operator whose result is an integer constant), there shall be exactly one external definition
for the identifier in the translation unit.
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 sizeof operator
whose result is an integer constant), somewhere in the entire program there shall be
exactly one external definition for the identifier; otherwise, there shall be no more than
one.140)
140) Thus, if an identifier declared with external linkage is not used in an expression, there need be no
external definition for it.
6.9.1 Function definitions
Syntax
1 function-definition:
declaration-specifiers declarator declaration-listopt compound-statement
declaration-list:
declaration
declaration-list declaration
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.141)
3 The return type of a function shall be void or an object type other than array type.
4 The storage-class specifier, if any, in the declaration specifiers shall be either extern or
static.
5 If the declarator includes a parameter type list, the declaration of each parameter shall
include an identifier, except for the special case of a parameter list consisting of a single
parameter of type void, in which case there shall not be an identifier. No declaration list
shall follow.
6 If the declarator includes an identifier list, each declaration in the declaration list shall
have at least one declarator, those declarators shall declare only identifiers from the
identifier list, and every identifier in the identifier list shall be declared. An identifier
declared as a typedef name shall not be redeclared as a parameter. The declarations in the
declaration list shall contain no storage-class specifier other than register and no
initializations.
141) The intent is that the type category in a function definition 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
Semantics
7 The declarator in a function definition specifies the name of the function being defined
and the identifiers of its parameters. If the declarator includes a parameter type list, the
list also specifies the types of all the parameters; such a declarator also serves as a
function prototype for later calls to the same function in the same translation unit. If the
declarator includes an identifier list,142) the types of the parameters shall be declared in a
following declaration list. In either case, the type of each parameter is adjusted as
described in 6.7.5.3 for a parameter type list; the resulting type shall be an object type.
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 Each parameter has automatic storage duration. Its identifier is an lvalue, which is in
effect declared at the head of the compound statement that constitutes the function body
(and therefore cannot be redeclared in the function body except in an enclosed block).
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 that constitutes the
body of the function definition is executed.
12 If the } that terminates a function is reached, and the value of the function call is used by
the caller, the behavior is undefined.
13 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. The following similar definition uses the identifier-list form for the parameter
declarations:
142) See ``future language directions'' (6.11.7).
extern int max(a, b)
int a, b;
{
return a > b ? a : b;
}
Here int a, b; is the declaration list for the parameters. The difference between these two definitions is
that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls
to the function, whereas the second form does not.
14 EXAMPLE 2 To pass one function to another, one might say
int f(void);
/* ... */
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
else-group:
# else new-line groupopt
endif-line:
# endif new-line
control-line:
# include 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
# 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
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.143) A new-line character ends
the preprocessing directive even if it occurs within what would otherwise be an
143) 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 in 6.10.3.2, for example).
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 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
5 The only white-space characters that shall appear between preprocessing tokens within a
preprocessing 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).
Semantics
6 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.
7 The preprocessing tokens within a preprocessing directive are not subject to macro
expansion unless otherwise stated.
8 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.
6.10.1 Conditional inclusion
Constraints
1 The expression that controls conditional inclusion shall be an integer constant expression
except that: it shall not contain a cast; identifiers (including those lexically identical to
keywords) are interpreted as described below;144) and it may contain unary operator
expressions of the form
144) 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.
defined identifier
or
defined ( identifier )
which evaluate 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.
2 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
3 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.
4 Prior to evaluation, macro invocations in the list of preprocessing tokens that will become
the controlling 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 the defined unary
operator have been performed, all remaining identifiers (including those lexically
identical to keywords) are replaced with the pp-number 0, 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>.145) 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-defined.146) Also, whether a
single-character character constant may have a negative value is implementation-defined.
5 Preprocessing directives of the forms
145) 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.
# ifdef identifier new-line groupopt
# ifndef 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 and #if !defined identifier
respectively.
6 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 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. 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.147)
Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest
integer types (7.18.1.5).
6.10.2 Source file inclusion
Constraints
1 A #include directive shall identify a header or source file that can be processed by the
implementation.
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
146) 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)
147) As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive
before the terminating new-line character. However, comments may appear anywhere in a source file,
including within a preprocessing directive.
# 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.148) 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 nondigits 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:
148) 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.
#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 (6.10.3).
6.10.3 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 more arguments in the invocation than there are
parameters in the macro definition (excluding the ...). There shall exist a )
preprocessing token that terminates the invocation.
5 The identifier _ _VA_ARGS_ _ 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 preprocessing 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 name149)
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,150) the behavior is undefined.
12 If there is a ... in the identifier-list in the macro definition, then the trailing arguments,
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
149) 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.
150) Despite the name, a non-directive is a preprocessing directive.
merger, the number of arguments is one more than the number of parameters in the macro
definition (excluding the ...).
6.10.3.1 Argument substitution
1 After the arguments for the invocation of a function-like macro have been identified,
argument substitution takes place. A parameter in the replacement list, unless preceded
by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is
replaced by the corresponding argument after all macros contained therein have been
expanded. Before being substituted, each argument's preprocessing tokens are
completely macro replaced as if they formed the rest of the preprocessing file; no other
preprocessing tokens are available.
2 An identifier _ _VA_ARGS_ _ that occurs in the replacement list shall be treated as if it
were a parameter, and the variable arguments shall form the preprocessing tokens used to
replace it.
6.10.3.2 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. Each occurrence of white space between the 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 argument
is deleted. Otherwise, the original spelling of each preprocessing token in the 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 argument is "". The order of evaluation of # and
## operators is unspecified.
6.10.3.3 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 preprocessing token sequence; however, if an argument consists of no
preprocessing tokens, the parameter is replaced by a placemarker preprocessing token
instead.151)
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: concatenation 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 preprocessing 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.
151) Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that
exist only within translation phase 4.
6.10.3.4 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. Then, the
resulting preprocessing token sequence is 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.
3 The resulting completely macro-replaced preprocessing token sequence is not processed
as a preprocessing directive even if it resembles one, but all pragma unary operator
expressions within it are then processed as specified in 6.10.9 below.
6.10.3.5 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));
6.10.4 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 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). The digit sequence shall not specify zero, nor a number greater than
2147483647.
4 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.
5 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.
6.10.5 Error directive
Semantics
1 A preprocessing directive of the form
# error pp-tokensopt new-line
causes the implementation to produce a diagnostic message that includes the specified
sequence of preprocessing tokens.
6.10.6 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)152) causes the implementation to behave in an
implementation-defined manner. 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 forms153) whose meanings are described
elsewhere:
#pragma STDC FP_CONTRACT on-off-switch
#pragma STDC FENV_ACCESS on-off-switch
#pragma STDC CX_LIMITED_RANGE on-off-switch
on-off-switch: one of
ON OFF DEFAULT
Forward references: the FP_CONTRACT pragma (7.12.2), the FENV_ACCESS pragma
(7.6.1), the CX_LIMITED_RANGE pragma (7.3.4).
152) 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.
153) See ``future language directions'' (6.11.8).
6.10.7 Null directive
Semantics
1 A preprocessing directive of the form
# new-line
has no effect.
6.10.8 Predefined macro names
1 The following macro names154) 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).155)
_ _LINE_ _ The presumed line number (within the current source file) of the current
source line (an integer constant).155)
_ _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_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.
_ _STDC_VERSION_ _ The integer constant 199901L.156)
_ _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 function. If the time of translation is not available, an
implementation-defined valid time shall be supplied.
154) See ``future language directions'' (6.11.9).
155) The presumed source file name and line number can be changed by the #line directive.
156) This macro was not specified in ISO/IEC 9899:1990 and was specified as 199409L in
ISO/IEC 9899/AMD1:1995. The intention is that this will remain an integer constant of type long
int that is increased with each revision of this International Standard.
2 The following macro names are conditionally defined by the implementation:
_ _STDC_IEC_559_ _ The integer constant 1, intended to indicate conformance to the
specifications in annex F (IEC 60559 floating-point arithmetic).
_ _STDC_IEC_559_COMPLEX_ _ The integer constant 1, intended to indicate
adherence to the specifications in informative annex G (IEC 60559
compatible complex arithmetic).
_ _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.
3 The values of the predefined macros (except for _ _FILE_ _ and _ _LINE_ _) remain
constant throughout the translation unit.
4 None of these macro names, nor the identifier defined, 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.
5 The implementation shall not predefine the macro _ _cplusplus, nor shall it define it
in any standard header.
Forward references: the asctime function (7.23.3.1), standard headers (7.1.2).
6.10.9 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 the L prefix, if
present, 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-point 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 Function declarators
1 The use of function declarators with empty parentheses (not prototype-format parameter
type declarators) is an obsolescent feature.
6.11.7 Function definitions
1 The use of function definitions with separate parameter identifier and declaration lists
(not prototype-format parameter type and identifier declarators) is an obsolescent feature.
6.11.8 Pragma directives
1 Pragmas whose first preprocessing token is STDC are reserved for future standardization.
6.11.9 Predefined macro names
1 Macro names beginning with _ _STDC_ are reserved for future standardization.
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.157) 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 5.2.1.2). A shift sequence shall not have a
corresponding wide character; it is instead taken to be an adjunct to an adjacent multibyte
character.158)
Forward references: character handling (7.4), the setlocale function (7.11.1.1).
157) The functions that make use of the decimal-point character are the numeric conversion functions
(7.20.1, 7.24.4.1) and the formatted input/output functions (7.19.6, 7.24.2).
158) For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX shall thus 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.
7.1.2 Standard headers
1 Each library function is declared, with a type that includes a prototype, in a header,159)
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. Declarations of types described in this clause shall not
include type qualifiers, unless explicitly stated otherwise.
2 The standard headers are
<assert.h> <inttypes.h> <signal.h> <stdlib.h>
<complex.h> <iso646.h> <stdarg.h> <string.h>
<ctype.h> <limits.h> <stdbool.h> <tgmath.h>
<errno.h> <locale.h> <stddef.h> <time.h>
<fenv.h> <math.h> <stdint.h> <wchar.h>
<float.h> <setjmp.h> <stdio.h> <wctype.h>
3 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.
4 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.
5 Any definition of an object-like macro described in this clause 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.
6 Any declaration of a library function shall have external linkage.
7 A summary of the contents of the standard headers is given in annex B.
Forward references: diagnostics (7.2).
159) A header is not necessarily a source file, nor are the < and > delimited sequences in header names
necessarily valid source file names.
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 identifiers that begin with an underscore and either an uppercase letter or another
underscore are always reserved for any use.
-- All identifiers that begin with an underscore are always reserved for use as identifiers
with file scope in both the ordinary and tag name spaces.
-- 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) are always reserved for use as identifiers with external
linkage.160)
-- Each identifier with file scope listed in any of the following subclauses (including the
future 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.
2 No other identifiers are reserved. If the program declares or defines an identifier in a
context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved
identifier as a macro name, the behavior is undefined.
3 If the program removes (with #undef) any macro definition of an identifier in the first
group listed above, the behavior is undefined.
7.1.4 Use of library functions
1 Each of the following statements applies unless explicitly stated otherwise in the detailed
descriptions 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 promotion) not expected by a function
with 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. Any function
declared in a header may be additionally implemented as a function-like macro defined in
160) The list of reserved identifiers with external linkage includes errno, math_errhandling,
setjmp, and va_end.
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.161) 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.162) 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.163) 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 storage duration.164)
161) This means that an implementation shall provide an actual function for each library function, even if it
also provides a macro for that function.
162) Such macros might not contain the sequence points that the corresponding function calls do.
163) Because external identifiers and some macro names beginning with an underscore are reserved,
implementations may 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 may 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.
164) Thus, a signal handler cannot, in general, call standard library functions.
5 EXAMPLE The function atoi may 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);
7.2 Diagnostics <assert.h>
1 The header <assert.h> defines the assert macro 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(ignore) ((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, 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.165) It
then calls the abort function.
Returns
3 The assert macro returns no value.
Forward references: the abort function (7.20.4.1).
165) 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.166) Each synopsis specifies a family of functions consisting of a principal
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.
2 The macro
complex
expands to _Complex; the macro
_Complex_I
expands to a constant expression of type const float _Complex, with the value of
the imaginary unit.167)
3 The macros
imaginary
and
_Imaginary_I
are defined if and only if the implementation supports imaginary types;168) if defined,
they expand to _Imaginary and a constant expression of type const float
_Imaginary with the value of the imaginary unit.
4 The macro
I
expands to either _Imaginary_I or _Complex_I. If _Imaginary_I is not
defined, I shall expand to _Complex_I.
5 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then
redefine the macros complex, imaginary, and I.
Forward references: IEC 60559-compatible complex arithmetic (annex G).
166) See ``future library directions'' (7.26.1).
167) The imaginary unit is a number i such that i 2 = -1.
168) A specification for imaginary types is in informative annex 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 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
problematic 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.169) The pragma can occur either outside external declarations or preceding all
explicit declarations and statements inside a compound statement. When outside external
169) The purpose of the pragma is to allow the implementation to use the formulas:
(x + iy) <20> (u + iv) = (xu - yv) + i(yu + xv)
(x + iy) / (u + iv) = [(xu + yv) + i(yu - xv)]/(u2 + v 2 )
| x + iy | = x 2 + y 2
where the programmer can determine they are safe.
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, ] along the
real axis.
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 [- /2, + /2]
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 [- /2, + /2]
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 non-negative values along the real axis and in the interval [-i , +i ] 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 mathematically unbounded along the real axis and in the interval [-i /2, +i /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 [-i /2, +i /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 [-i , +i ] 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 [- , + ].
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.170)
Returns
3 The cimag functions return the imaginary part value (as a real).
7.3.9.3 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.
7.3.9.4 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.
170) For a variable z of complex type, z == creal(z) + cimag(z)*I.
7.3.9.5 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.171)
Returns
3 The creal functions return the real part value.
171) 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.172) 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.173) All letters and digits are printing characters.
Forward references: EOF (7.19.1), 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
172) See ``future library directions'' (7.26.2).
173) 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).
none of iscntrl, isdigit, ispunct, or isspace is true.174) 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.
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);
174) The functions islower and isupper test true or false separately for each of these additional
characters; all four combinations are possible.
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 lvalue175) that has type int, the value of which is set to a
positive error number by several library functions. It is unspecified whether errno is a
macro or an identifier declared with external linkage. 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 is zero at program startup, but is never set to zero by any library
function.176) 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 International Standard.
4 Additional macro definitions, beginning with E and a digit or E and an uppercase
letter,177) may also be specified by the implementation.
175) 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()).
176) Thus, a program that uses errno for error checking should 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.
177) See ``future library directions'' (7.26.3).
7.6 Floating-point environment <fenv.h>
1 The header <fenv.h> declares two types and several macros and functions to 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.178) 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.179) 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 Certain programming conventions support the intended model of use for the floating-
point environment:180)
-- 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
documentation promises otherwise;
-- a function call is assumed to have the potential for raising floating-point exceptions,
unless its documentation promises otherwise.
3 The type
fenv_t
represents the entire floating-point environment.
4 The type
fexcept_t
represents the floating-point status flags collectively, including any status the
implementation associates with the flags.
178) This header is designed to support the floating-point exception status flags and directed-rounding
control modes required by IEC 60559, and other similar floating-point state information. Also it is
designed to facilitate code portability among all systems.
179) A floating-point status flag is not an object and can be set more than once within an expression.
180) 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.
5 Each of the macros
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 7.6.2.181) Additional implementation-defined floating-point
exceptions, with macro definitions beginning with FE_ and an uppercase letter, 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.182)
6 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.
7 Each of the macros
FE_DOWNWARD
FE_TONEAREST
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, may also be specified by the implementation. The
defined macros expand to integer constant expressions whose values are distinct
nonnegative values.183)
8 The macro
181) The implementation supports an exception if there are circumstances where a call to at least one of the
functions in 7.6.2, using the macro as the appropriate argument, will succeed. It is not necessary for
all the functions to succeed all the time.
182) The macros should be distinct powers of two.
183) Even though the rounding direction macros may expand to constants corresponding to the values of
FLT_ROUNDS, they are not required to do so.
FE_DFL_ENV
represents the default 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 floating-point environment.
9 Additional implementation-defined environments, with macro definitions beginning with
FE_ and an uppercase letter, 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.184) 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, sets floating-point control
modes, or runs under non-default mode settings, 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.)
184) 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 default
modes are in effect and the flags are not tested.
3 EXAMPLE
#include <fenv.h>
void f(double x)
{
#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 shall
contain an appropriately placed invocation of #pragma STDC FENV_ACCESS ON.185)
7.6.2 Floating-point exceptions
1 The following functions provide access to the floating-point status flags.186) 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.
7.6.2.1 The feclearexcept function
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.
185) 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.
186) The functions fetestexcept, feraiseexcept, and feclearexcept support the basic
abstraction of flags that are either set or clear. An implementation may 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.
7.6.2.2 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.
7.6.2.3 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.187) The order in which these floating-point exceptions are
raised is unspecified, except as stated in F.7.6. 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.
187) The effect is intended to be similar to that of floating-point exceptions raised by arithmetic operations.
Hence, enabled traps for floating-point exceptions raised by this function are taken. The specification
in F.7.6 is in the same spirit.
7.6.2.4 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. 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.
7.6.2.5 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 exception flags are currently set. The excepts argument specifies the floating-
point status flags to be queried.188)
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:
188) This mechanism allows testing several floating-point exceptions with just one function call.
#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();
/* ... */
}
7.6.3 Rounding
1 The fegetround and fesetround functions provide control of rounding direction
modes.
7.6.3.1 The fegetround function
Synopsis
1 #include <fenv.h>
int fegetround(void);
Description
2 The fegetround function gets the current rounding direction.
Returns
3 The fegetround function returns the value of the rounding direction macro
representing the current rounding direction or a negative value if there is no such
rounding direction macro or the current rounding direction is not determinable.
7.6.3.2 The fesetround function
Synopsis
1 #include <fenv.h>
int fesetround(int round);
Description
2 The fesetround function establishes the rounding direction represented by its
argument round. 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 requested rounding direction
was established.
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 round_dir)
{
#pragma STDC FENV_ACCESS ON
int save_round;
int setround_ok;
save_round = fegetround();
setround_ok = fesetround(round_dir);
assert(setround_ok == 0);
/* ... */
fesetround(save_round);
/* ... */
}
7.6.4 Environment
1 The functions in this section manage the floating-point environment -- status flags and
control modes -- as one entity.
7.6.4.1 The fegetenv function
Synopsis
1 #include <fenv.h>
int fegetenv(fenv_t *envp);
Description
2 The fegetenv function attempts to store the current 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.
7.6.4.2 The feholdexcept function
Synopsis
1 #include <fenv.h>
int feholdexcept(fenv_t *envp);
Description
2 The feholdexcept function saves the current 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.189)
Returns
3 The feholdexcept function returns zero if and only if non-stop floating-point
exception handling was successfully installed.
7.6.4.3 The fesetenv function
Synopsis
1 #include <fenv.h>
int fesetenv(const fenv_t *envp);
Description
2 The fesetenv function attempts to establish the 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 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.
7.6.4.4 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 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 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.
189) 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.
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 standard floating-point types.
2 The macros, their meanings, and the constraints (or restrictions) on their values are listed
in 5.2.4.2.2.
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.190)
Forward references: integer types <stdint.h> (7.18), formatted input/output
functions (7.19.6), formatted wide character input/output functions (7.24.2).
7.8.1 Macros for format specifiers
1 Each of the following object-like macros191) 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),192) followed by the conversion specifier,
followed by a name corresponding to a similar type name in 7.18.1. In these names, N
represents the width of the type as described in 7.18.1. 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
190) See ``future library directions'' (7.26.4).
191) C++ implementations should define these macros only when _ _STDC_FORMAT_MACROS is defined
before <inttypes.h> is included.
192) Separate macros are given for use with fprintf and fscanf functions because, in the general case,
different format specifiers may be required for fprintf and fscanf, even when the type is the
same.
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"
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.193)
193) The absolute value of the most negative number cannot be represented in two's complement.
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
(7.20.1.4).
7.8.2.4 The wcstoimax and wcstoumax functions
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
(7.24.4.1.2).
7.9 Alternative spellings <iso646.h>
1 The header <iso646.h> defines the following eleven macros (on the left) that expand
to the corresponding tokens (on the right):
and &&
and_eq &=
bitand &
bitor |
compl ~
not !
not_eq !=
or ||
or_eq |=
xor ^
xor_eq ^=
7.10 Sizes 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.
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 7.17); 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 argument to the setlocale function.194) Additional macro definitions, beginning
with the characters LC_ and an uppercase letter,195) 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);
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 functions196) 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.
194) ISO/IEC 9945-2 specifies locale and charmap formats that may be used to specify locales for C.
195) See ``future library directions'' (7.26.5).
196) The only functions in 7.4 whose behavior is not affected by the current locale are isdigit and
isxdigit.
4 At program startup, the equivalent of
setlocale(LC_ALL, "C");
is executed.
5 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.197)
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, but may be
overwritten by a subsequent call to the setlocale function.
Forward references: formatted input/output functions (7.19.6), multibyte/wide
character conversion functions (7.20.7), multibyte/wide string conversion functions
(7.20.8), numeric conversion functions (7.20.1), the strcoll function (7.21.4.3), the
strftime function (7.23.3.5), the strxfrm function (7.21.4.5).
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.
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
197) The implementation shall arrange to encode in a string the various categories due to a heterogeneous
locale when category has the value LC_ALL.
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 nonmonetary 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 internationally 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 interpreted 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 may 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 1.234,56 -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.198)
Integer arithmetic functions and conversion functions are discussed later.
2 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.199)
3 The macro
HUGE_VAL
expands to a positive double constant expression, not necessarily representable as a
float. The macros
HUGE_VALF
HUGE_VALL
are respectively float and long double analogs of HUGE_VAL.200)
4 The macro
INFINITY
expands to a constant expression of type float representing positive or unsigned
infinity, if available; else to a positive constant of type float that overflows at
198) 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.
199) 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.
200) HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that
supports infinities.
translation time.201)
5 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.
6 The number classification macros
FP_INFINITE
FP_NAN
FP_NORMAL
FP_SUBNORMAL
FP_ZERO
represent the 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.
7 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.202) 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.
8 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.
201) In this case, using INFINITY will violate the constraint in 6.4.4 and thus require a diagnostic.
202) 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.
9 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, or the bitwise OR of both. 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 suppressed 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 implementation
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 stated otherwise. Each function shall execute
as if it were a single operation without generating any externally visible exceptional
conditions.
2 For all functions, a domain error occurs 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.203) 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 range error occurs if the mathematical result of the function cannot be
represented in an object of the specified type, due to extreme magnitude.
4 A floating result overflows if the magnitude of the mathematical result is finite but so
large that the mathematical result cannot be represented without extraordinary roundoff
error in an object of the specified type. If a floating result overflows and default rounding
is in effect, or if the mathematical result is an exact infinity from finite arguments (for
example log(0.0)), then the function returns the value of the macro HUGE_VAL,
203) 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.
HUGE_VALF, or HUGE_VALL according to the return type, with the same sign as the
correct value of the function; 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 if the mathematical result is an exact
infinity and the ``overflow'' floating-point exception is raised otherwise.
5 The result underflows if the magnitude of the mathematical result is so small that the
mathematical result cannot be represented, without extraordinary roundoff error, in an
object of the specified type.204) If the result underflows, the function returns 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 expression
math_errhandling & MATH_ERREXCEPT is nonzero, whether the ``underflow''
floating-point exception is raised is implementation-defined.
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.
204) The term underflow here is intended to encompass both ``gradual underflow'' as in IEC 60559 and
also ``flush-to-zero'' underflow.
7.12.3 Classification macros
1 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
1 #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.205)
Returns
3 The fpclassify macro returns the value of the number classification macro
appropriate to the value of its argument.
4 EXAMPLE The fpclassify macro might be implemented in terms of ordinary functions as
#define fpclassify(x) \
((sizeof (x) == sizeof (float)) ? _ _fpclassifyf(x) : \
(sizeof (x) == sizeof (double)) ? _ _fpclassifyd(x) : \
_ _fpclassifyl(x))
7.12.3.2 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.
205) 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 isfinite macro returns a nonzero value if and only if its argument has a finite
value.
7.12.3.3 The isinf macro
Synopsis
1 #include <math.h>
int isinf(real-floating x);
Description
2 The isinf macro determines whether its argument value is an infinity (positive or
negative). 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 isinf macro returns a nonzero value if and only if its argument has an infinite
value.
7.12.3.4 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.206)
Returns
3 The isnan macro returns a nonzero value if and only if its argument has a NaN value.
7.12.3.5 The isnormal macro
Synopsis
1 #include <math.h>
int isnormal(real-floating x);
206) 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.
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.
7.12.3.6 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 negative.207)
Returns
3 The signbit macro returns a nonzero value if and only if the sign of its argument value
is negative.
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);
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, ] radians.
207) The signbit macro reports the sign of all values, including infinities, zeros, and NaNs. If zero is
unsigned, it is treated as positive.
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);
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].
Returns
3 The asin functions return arcsin x in the interval [- /2, + /2] 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);
Description
2 The atan functions compute the principal value of the arc tangent of x.
Returns
3 The atan functions return arctan x in the interval [- /2, + /2] 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);
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.
Returns
3 The atan2 functions return arctan y/x in the interval [- , + ] 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);
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);
Description
2 The sin functions compute the sine of x (measured in radians).
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);
Description
2 The tan functions return the tangent of x (measured in radians).
Returns
3 The tan functions return tan 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);
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);
Description
2 The asinh functions compute the arc hyperbolic sine of x.
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);
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 range error may occur if the argument
equals -1 or +1.
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);
Description
2 The cosh functions compute the hyperbolic cosine of x. A range error occurs if the
magnitude of 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);
Description
2 The sinh functions compute the hyperbolic sine of x. A range error occurs if the
magnitude of x is too large.
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);
Description
2 The tanh functions compute the hyperbolic tangent of x.
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);
Description
2 The exp functions compute the base-e exponential of x. A range error occurs if the
magnitude of x is too large.
Returns
3 The exp functions return ex .
7.12.6.2 The exp2 functions
Synopsis
1 #include <math.h>
double exp2(double x);
float exp2f(float x);
long double exp2l(long double x);
Description
2 The exp2 functions compute the base-2 exponential of x. A range error occurs if the
magnitude of x is too large.
Returns
3 The exp2 functions return 2x .
7.12.6.3 The expm1 functions
Synopsis
1 #include <math.h>
double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);
Description
2 The expm1 functions compute the base-e exponential of the argument, minus 1. A range
error occurs if x is too large.208)
Returns
3 The expm1 functions return ex - 1.
7.12.6.4 The frexp functions
Synopsis
1 #include <math.h>
double frexp(double value, int *exp);
float frexpf(float value, int *exp);
long double frexpl(long double value, int *exp);
Description
2 The frexp functions break a floating-point number into a normalized fraction and an
integral power of 2. They store the integer in the int object pointed to by exp.
Returns
3 If value is not a floating-point number, the results are unspecified. Otherwise, the
frexp functions return the value x, such that x has a magnitude in the interval [1/2, 1) or
zero, and value equals x <20> 2*exp . If value is zero, both parts of the result are zero.
7.12.6.5 The ilogb functions
Synopsis
1 #include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
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 casting 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.
208) For small magnitude x, expm1(x) is expected to be more accurate than exp(x) - 1.
Returns
3 The ilogb functions return the exponent of x as a signed int value.
Forward references: the logb functions (7.12.6.11).
7.12.6.6 The ldexp functions
Synopsis
1 #include <math.h>
double ldexp(double x, int exp);
float ldexpf(float x, int exp);
long double ldexpl(long double x, int exp);
Description
2 The ldexp functions multiply a floating-point number by an integral power of 2. A
range error may occur.
Returns
3 The ldexp functions return x <20> 2exp .
7.12.6.7 The log functions
Synopsis
1 #include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
Description
2 The log functions compute the base-e (natural) logarithm of x. A domain error occurs if
the argument is negative. A range error may occur if the argument is zero.
Returns
3 The log functions return loge x.
7.12.6.8 The log10 functions
Synopsis
1 #include <math.h>
double log10(double x);
float log10f(float x);
long double log10l(long double x);
Description
2 The log10 functions compute the base-10 (common) logarithm of x. A domain error
occurs if the argument is negative. A range error may occur if the argument is zero.
Returns
3 The log10 functions return log10 x.
7.12.6.9 The log1p functions
Synopsis
1 #include <math.h>
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
Description
2 The log1p functions compute the base-e (natural) logarithm of 1 plus the argument.209)
A domain error occurs if the argument is less than -1. A range error may occur if the
argument equals -1.
Returns
3 The log1p functions return loge (1 + x).
7.12.6.10 The log2 functions
Synopsis
1 #include <math.h>
double log2(double x);
float log2f(float x);
long double log2l(long double x);
Description
2 The log2 functions compute the base-2 logarithm of x. A domain error occurs if the
argument is less than zero. A range error may occur if the argument is zero.
Returns
3 The log2 functions return log2 x.
209) For small magnitude x, log1p(x) is expected to be more accurate than log(1 + x).
7.12.6.11 The logb functions
Synopsis
1 #include <math.h>
double logb(double x);
float logbf(float x);
long double logbl(long double x);
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 <20> FLT_RADIX-logb(x) < FLT_RADIX
A domain error or range error may occur if the argument is zero.
Returns
3 The logb functions return the signed exponent of x.
7.12.6.12 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);
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.
7.12.6.13 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);
Description
2 The scalbn and scalbln functions compute x <20> FLT_RADIXn efficiently, not
normally by computing FLT_RADIXn explicitly. A range error may occur.
Returns
3 The scalbn and scalbln functions return x <20> FLT_RADIXn .
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);
Description
2 The cbrt functions compute the real cube root of x.
Returns
3 The cbrt functions return x1/3 .
7.12.7.2 The fabs functions
Synopsis
1 #include <math.h>
double fabs(double x);
float fabsf(float x);
long double fabsl(long double x);
Description
2 The fabs functions compute the absolute value of a floating-point number x.
Returns
3 The fabs functions return | x |.
7.12.7.3 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);
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 may occur.
3 Returns
4 The hypot functions return x2 + y2 .
7.12.7.4 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);
Description
2 The pow functions compute x raised to the power y. A domain error occurs if x is finite
and negative and y is finite and not an integer value. A range error may occur. A domain
error may occur if x is zero and y is zero. A domain error or range error may occur if x
is zero and y is less than zero.
Returns
3 The pow functions return xy .
7.12.7.5 The sqrt functions
Synopsis
1 #include <math.h>
double sqrt(double x);
float sqrtf(float x);
long double sqrtl(long double x);
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);
Description
2 The erf functions compute the error function of x.
Returns
2 x
3
The erf functions return erf x = e-t dt.
2
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);
Description
2 The erfc functions compute the complementary error function of x. A range error
occurs if x is too large.
Returns
2
3
The erfc functions return erfc x = 1 - erf x = e-t dt.
2
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);
Description
2 The lgamma functions compute the natural logarithm of the absolute value of gamma of
x. A range error occurs if x is too large. A range error may occur if x is a negative
integer or zero.
Returns
3 The lgamma functions return loge | (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);
Description
2 The tgamma functions compute the gamma function of x. A domain error or range error
may occur if x is a negative integer or zero. A range error may occur if the magnitude of
x is too large or too small.
Returns
3 The tgamma functions return (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);
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);
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);
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);
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);
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);
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);
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 trunc functions
Synopsis
1 #include <math.h>
double trunc(double x);
float truncf(float x);
long double truncl(long double x);
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.
7.12.10 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);
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);
Description
2 The remainder functions compute the remainder x REM y required by IEC 60559.210)
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.
210) ``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 x/y; whenever
| n - x/y | = 1/2, then n is even. Thus, the remainder is always exact. If r = 0, its sign shall be that of
x.'' This definition is applicable for all implementations.
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.
7.12.11 Manipulation functions
7.12.11.1 The copysign functions
Synopsis
1 #include <math.h>
double copysign(double x, double y);
float copysignf(float x, float y);
long double copysignl(long double x, long double y);
Description
2 The copysign functions produce a value with the magnitude of x and the sign of y.
They produce a NaN (with the sign of y) if x is a NaN. On implementations that
represent a signed zero but do not treat negative zero consistently in arithmetic
operations, the copysign functions 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);
Description
2 The call nan("n-char-sequence") is equivalent to strtod("NAN(n-char-
sequence)", (char**) NULL); the call nan("") is equivalent to
strtod("NAN()", (char**) NULL). If tagp does not point to an n-char
sequence or an empty string, the call is equivalent to strtod("NAN", (char**)
NULL). 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 (7.20.1.3).
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);
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
function.211) The nextafter functions return y if x equals y. A range error may occur
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.
211) The argument values are converted to the type of the function, even by a macro implementation of the
function.
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);
Description
2 The nexttoward functions are equivalent to the nextafter functions except that the
second parameter has type long double and the functions return y converted to the
type of the function if x equals y.212)
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);
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);
212) 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.
Description
2 The fmax functions determine the maximum numeric value of their arguments.213)
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);
Description
2 The fmin functions determine the minimum numeric value of their arguments.214)
Returns
3 The fmin functions return the minimum numeric value of their arguments.
7.12.13 Floating 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);
Description
2 The fma functions compute (x <20> 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 may occur.
Returns
3 The fma functions return (x <20> y) + z, rounded as one ternary operation.
213) NaN arguments are treated as missing data: if one argument is a NaN and the other numeric, then the
fmax functions choose the numeric value. See F.9.9.2.
214) The fmin functions are analogous to the fmax functions in their treatment of NaNs.
7.12.14 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.215) The following
subclauses provide macros that are quiet (non floating-point exception raising) versions
of the relational operators, and other comparison macros that facilitate writing efficient
code that accounts for 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 type.
7.12.14.1 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
argument. 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.
Returns
3 The isgreater macro returns the value of (x) > (y).
7.12.14.2 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.
215) 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.
Returns
3 The isgreaterequal macro returns the value of (x) >= (y).
7.12.14.3 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.
Returns
3 The isless macro returns the value of (x) < (y).
7.12.14.4 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 second 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.
Returns
3 The islessequal macro returns the value of (x) <= (y).
7.12.14.5 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 unordered (nor does it evaluate x
and y twice).
Returns
3 The islessgreater macro returns the value of (x) < (y) || (x) > (y).
7.12.14.6 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.
Returns
3 The isunordered macro returns 1 if its arguments are unordered and 0 otherwise.
7.13 Nonlocal 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.216)
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
216) These functions are useful for dealing with unusual conditions encountered in a low-level function of
a program.
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>
void longjmp(jmp_buf env, int val);
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 function containing
the invocation of the setjmp macro has terminated execution217) 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 machine218)
have state, as of the time the longjmp function was called, except that the values 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 are
indeterminate.
Returns
4 After longjmp is completed, program 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.
217) 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.
218) This includes, but is not limited to, the floating-point status flags and the state of open files.
#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
}
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,219) 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.
219) See ``future library directions'' (7.26.9). The names of the signal numbers reflect the following terms
(respectively): abort, floating-point exception, illegal instruction, interrupt, segmentation violation,
and termination.
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), is called a signal
handler.
3 When a signal occurs and func points to a function, it is implementation-defined
whether the equivalent 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 storage duration
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, 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
value of errno is indeterminate.220)
6 At program startup, the equivalent of
signal(sig, SIG_IGN);
220) If any signal is generated by an asynchronous signal handler, the behavior is undefined.
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 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 (7.20.4.1), the exit function (7.20.4.3), the
_Exit function (7.20.4.4).
7.14.2 Send signal
7.14.2.1 The raise function
Synopsis
1 #include <signal.h>
int raise(int sig);
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 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. As
described in 6.9.1, its parameter list contains one or more parameters. The rightmost
parameter plays a special role in the access mechanism, and will be designated parmN in
this description.
3 The type declared is
va_list
which is an 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
value of ap in the calling function is indeterminate and shall be passed to the va_end
macro prior to any further reference to ap.221)
7.15.1 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.
7.15.1.1 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
221) It is permitted to create a pointer to a va_list and pass that pointer to another function, in which
case the original function may make further use of the original list after the other function returns.
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 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 there is no actual next argument, or 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:
-- 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 void and the other is a pointer to a character type.
Returns
3 The first invocation of the va_arg macro after that of the va_start macro returns the
value of the argument after that specified by parmN . Successive invocations return the
values of the remaining arguments in succession.
7.15.1.2 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.
7.15.1.3 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.
7.15.1.4 The va_start macro
Synopsis
1 #include <stdarg.h>
void va_start(va_list ap, parmN);
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 The parameter parmN is the identifier of the rightmost parameter in the variable
parameter list in the function definition (the one just before the , ...). If the parameter
parmN is declared with the register storage class, with a function or array type, or
with a type that is not compatible with the type that results after application of the default
argument promotions, the behavior is undefined.
Returns
5 The va_start macro returns no value.
6 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, n_ptrs);
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, ...);
7 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, f4_after);
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);
}
7.16 Boolean type and values <stdbool.h>
1 The header <stdbool.h> defines four macros.
2 The macro
bool
expands to _Bool.
3 The remaining three macros are suitable for use in #if preprocessing directives. They
are
true
which expands to the integer constant 1,
false
which expands to the integer constant 0, and
_ _bool_true_false_are_defined
which expands to the integer constant 1.
4 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then
redefine the macros bool, true, and false.222)
222) See ``future library directions'' (7.26.7).
7.17 Common definitions <stddef.h>
1 The following types and macros are defined in the standard header <stddef.h>. 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; and
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_ _.
3 The macros are
NULL
which expands to an implementation-defined null pointer constant; 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 structure member (designated by member-designator),
from the beginning of its structure (designated by 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 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.
Forward references: localization (7.11).
7.18 Integer types <stdint.h>
1 The header <stdint.h> declares sets of integer types having specified widths, and
defines corresponding sets of macros.223) 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,224) <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'').
7.18.1 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).
223) See ``future library directions'' (7.26.8).
224) Some of these types may denote implementation-defined extended integer types.
7.18.1.1 Exact-width integer types
1 The typedef name intN_t designates a signed integer type with width N , no padding
bits, and a two's complement representation. Thus, int8_t denotes 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 . Thus,
uint24_t denotes an unsigned integer type with a width of exactly 24 bits.
3 These types are optional. However, if an implementation provides integer types with
widths of 8, 16, 32, or 64 bits, no padding bits, and (for the signed types) that have a
two's complement representation, it shall define the corresponding typedef names.
7.18.1.2 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 The following types are required:
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.
7.18.1.3 Fastest minimum-width integer types
1 Each of the following types designates an integer type that is usually fastest225) 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 .
225) 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.
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.
7.18.1.4 Integer types capable of holding object pointers
1 The following type designates a signed 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 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.
7.18.1.5 Greatest-width integer types
1 The following type designates a signed integer type capable of representing any value of
any signed integer type:
intmax_t
The following type designates an unsigned integer type capable of representing any value
of any unsigned integer type:
uintmax_t
These types are required.
7.18.2 Limits of specified-width integer types
1 The following object-like macros226) specify the minimum and maximum limits of the
types declared in <stdint.h>. Each macro name corresponds to a similar type name in
7.18.1.
2 Each instance of any defined macro shall be replaced by a constant expression suitable
for use in #if preprocessing directives, and this expression shall have the same type as
would an expression that is an object of the corresponding type converted according to
226) C++ implementations should define these macros only when _ _STDC_LIMIT_MACROS is defined
before <stdint.h> is included.
the integer promotions. Its implementation-defined value shall be equal to or greater in
magnitude (absolute value) than the corresponding value given below, with the same sign,
except where stated to be exactly the given value.
7.18.2.1 Limits of exact-width integer types
1 -- minimum values of exact-width signed integer types
INTN_MIN exactly -(2 N -1 )
-- maximum values of exact-width signed integer types
INTN_MAX exactly 2 N -1 - 1
-- maximum values of exact-width unsigned integer types
UINTN_MAX exactly 2 N - 1
7.18.2.2 Limits of minimum-width integer types
1 -- minimum values of minimum-width signed integer types
INT_LEASTN_MIN -(2 N -1 - 1)
-- maximum values of minimum-width signed integer types
INT_LEASTN_MAX 2 N -1 - 1
-- maximum values of minimum-width unsigned integer types
UINT_LEASTN_MAX 2N - 1
7.18.2.3 Limits of fastest minimum-width integer types
1 -- minimum values of fastest minimum-width signed integer types
INT_FASTN_MIN -(2 N -1 - 1)
-- maximum values of fastest minimum-width signed integer types
INT_FASTN_MAX 2 N -1 - 1
-- maximum values of fastest minimum-width unsigned integer types
UINT_FASTN_MAX 2N - 1
7.18.2.4 Limits of integer types capable of holding object pointers
1 -- minimum value of pointer-holding signed integer type
INTPTR_MIN -(215 - 1)
-- maximum value of pointer-holding signed integer type
INTPTR_MAX 215 - 1
-- maximum value of pointer-holding unsigned integer type
UINTPTR_MAX 216 - 1
7.18.2.5 Limits of greatest-width integer types
1 -- minimum value of greatest-width signed integer type
INTMAX_MIN -(263 - 1)
-- maximum value of greatest-width signed integer type
INTMAX_MAX 263 - 1
-- maximum value of greatest-width unsigned integer type
UINTMAX_MAX 264 - 1
7.18.3 Limits of other integer types
1 The following object-like macros227) specify the minimum and maximum limits 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, and this expression shall have the same type as would an
expression that is an object of the corresponding type converted according to the integer
promotions. Its implementation-defined value shall be equal to or greater in magnitude
(absolute value) than the corresponding value given below, with the same sign. An
implementation shall define only the macros corresponding to those typedef names it
actually provides.228)
-- limits of ptrdiff_t
PTRDIFF_MIN -65535
PTRDIFF_MAX +65535
-- limits of sig_atomic_t
SIG_ATOMIC_MIN see below
SIG_ATOMIC_MAX see below
-- limit of size_t
SIZE_MAX 65535
-- limits of wchar_t
227) C++ implementations should define these macros only when _ _STDC_LIMIT_MACROS is defined
before <stdint.h> is included.
228) A freestanding implementation need not provide all of these types.
WCHAR_MIN see below
WCHAR_MAX see below
-- limits of wint_t
WINT_MIN see below
WINT_MAX see below
3 If sig_atomic_t (see 7.14) is defined as a signed integer type, the value of
SIG_ATOMIC_MIN shall be no greater than -127 and the value of SIG_ATOMIC_MAX
shall be no less than 127; otherwise, sig_atomic_t is defined as an unsigned integer
type, and the value of SIG_ATOMIC_MIN shall be 0 and the value of
SIG_ATOMIC_MAX shall be no less than 255.
4 If wchar_t (see 7.17) is defined as a signed integer type, the value of WCHAR_MIN
shall be no greater than -127 and the value of WCHAR_MAX shall be no less than 127;
otherwise, wchar_t is defined as an unsigned integer type, and the value of
WCHAR_MIN shall be 0 and the value of WCHAR_MAX shall be no less than 255.229)
5 If wint_t (see 7.24) is defined as a signed integer type, the value of WINT_MIN shall
be no greater than -32767 and the value of WINT_MAX shall be no less than 32767;
otherwise, wint_t is defined as an unsigned integer type, and the value of WINT_MIN
shall be 0 and the value of WINT_MAX shall be no less than 65535.
7.18.4 Macros for integer constants
1 The following function-like macros230) 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 7.18.1.2 or
7.18.1.5.
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
suitable for use in #if preprocessing directives. 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.
229) The values WCHAR_MIN and WCHAR_MAX do not necessarily correspond to members of the extended
character set.
230) C++ implementations should define these macros only when _ _STDC_CONSTANT_MACROS is
defined before <stdint.h> is included.
7.18.4.1 Macros for minimum-width integer constants
1 The macro INTN_C(value) shall expand to an integer constant expression
corresponding to the type int_leastN_t. The macro UINTN_C(value) shall expand
to an integer constant expression corresponding 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.
7.18.4.2 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)
7.19 Input/output <stdio.h>
7.19.1 Introduction
1 The header <stdio.h> declares three types, several macros, and many functions for
performing input and output.
2 The types declared are size_t (described in 7.17);
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 an 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 7.17);
_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;231)
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 maximum 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 7.19.3.
5 The input/output functions are given the following collective terms:
-- The wide character input functions -- those functions described in 7.24 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 7.24 that perform
output from wide characters and wide strings: fputwc, fputws, putwc,
putwchar, fwprintf, wprintf, vfwprintf, and vwprintf.
231) If the implementation imposes no practical limit on the length of file name strings, the value of
FILENAME_MAX should instead be the recommended size of an array intended to hold a file name
string. 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.
-- The wide character input/output functions -- the union of the ungetwc function, the
wide character 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, gets, printf, putc, putchar, puts,
scanf, ungetc, vfprintf, vfscanf, vprintf, and vscanf.
Forward references: files (7.19.3), the fseek function (7.19.9.2), streams (7.19.2), the
tmpnam function (7.19.4.4), <wchar.h> (7.24).
7.19.2 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.232)
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/output function has been applied to a stream without orientation, the
232) 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.
stream becomes a wide-oriented stream. Similarly, once a byte input/output function has
been applied to a stream without orientation, 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.)233)
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 are henceforth indeterminate.
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.
Environmental limits
7 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 (7.19.5.4), the fwide function (7.24.3.5),
mbstate_t (7.25.1), the fgetpos function (7.19.9.1), the fsetpos function
(7.19.9.3).
233) The three predefined streams stdin, stdout, and stderr are unoriented at program startup.
7.19.3 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 7.19.5.3. 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 value of a pointer to a FILE object is
indeterminate after 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
encodings valid for use internal to the program).
-- A file need not begin nor end in the initial shift state.234)
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)
234) 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.
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 (7.20.4.3), the fgetc function (7.19.7.1), the
fopen function (7.19.5.3), the fputc function (7.19.7.3), the setbuf function
(7.19.5.5), the setvbuf function (7.19.5.6), the fgetwc function (7.24.3.1), the
fputwc function (7.24.3.3), conversion state (7.24.6), the mbrtowc function
(7.24.6.3.2), the wcrtomb function (7.24.6.3.3).
7.19.4 Operations on files
7.19.4.1 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.
7.19.4.2 The rename function
Synopsis
1 #include <stdio.h>
int rename(const char *old, const char *new);
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,235) in
which case if the file existed previously it is still known by its original name.
7.19.4.3 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 (7.19.5.3).
7.19.4.4 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.236) The function is potentially capable of generating
235) Among the reasons the implementation may 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.
236) Files created using strings generated by the tmpnam function are temporary only in the sense that
their names should not 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.
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 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). 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.
7.19.5 File access functions
7.19.5.1 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.
7.19.5.2 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 (7.19.5.3).
7.19.5.3 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.237)
r open text file for reading
w truncate to zero length or 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
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
a+ append; open or create text file for update, writing at end-of-file
237) If the string begins with one of the above 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 7.19.2).
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
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 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.
6 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.
7 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
8 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 (7.19.9).
7.19.5.4 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.238)
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.
7.19.5.5 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 (7.19.5.6).
7.19.5.6 The setvbuf function
Synopsis
1 #include <stdio.h>
int setvbuf(FILE * restrict stream,
char * restrict buf,
int mode, size_t size);
238) 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 may be assigned.
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 function239) 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 contents of the array at any time are
indeterminate.
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.
7.19.6 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.240)
7.19.6.1 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
239) The buffer has to have a lifetime at least as great as the open stream, so the stream should be closed
before a buffer that has automatic storage duration is deallocated upon block exit.
240) The fprintf functions perform writes to memory for the %n specifier.
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.241)
-- An optional precision that gives the minimum number of digits to appear for the 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 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:
- 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 negative value is converted if this flag is not
241) Note that 0 is taken as a flag, not as the beginning of a field width.
specified.)242)
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 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 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 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 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 according 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 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
242) The results of all floating conversions of a negative zero, and of negative values that round to zero,
include a minus sign.
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 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 conversion specifier applies to a pointer to a long long int
argument.
j Specifies that a following 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 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 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.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier
applies to a long double 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.
o,u,x,X The unsigned int argument is converted to 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.243)
e,E A double argument representing a floating-point number is converted in the
style [-]d.ddd e<>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
243) When applied to infinite and NaN values, the -, +, and space flag characters have their usual meaning;
the # and 0 flag characters have no effect.
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.hhhh p<>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 character244) 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
distinguish245) 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.
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 the initial element of a two-element array of wchar_t, 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 the initial
element of an array of character type.246) Characters from the array are
244) 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.
245) The precision p is sufficient to distinguish values of the source type if 16 p-1 > b n where b is
FLT_RADIX and n is the number of base-b digits in the significand of the source type. A smaller p
might suffice depending on the implementation's scheme for determining the digit to the left of the
decimal-point character.
246) No special provisions are made for multibyte characters.
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 array, the array shall
contain a null character.
If an l length modifier is present, the argument shall be a pointer to the initial
element of an array of wchar_t type. Wide characters from the array 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 array 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 array 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.247)
p The argument shall be a pointer to void. The value of the pointer is
converted to a sequence of printing characters, in an implementation-defined
manner.
n The argument shall be a pointer to signed integer into which is written the
number of characters written to the output stream so far by this call to
fprintf. 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.248) If any argument is
not the correct type for the corresponding conversion specification, the behavior is
undefined.
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.
247) Redundant shift sequences may result if multibyte characters have a state-dependent encoding.
248) See ``future library directions'' (7.26.9).
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
DECIMAL_DIG, then the result should be correctly rounded.249) If the number of
significant decimal digits is more than DECIMAL_DIG but the source value is exactly
representable with DECIMAL_DIG 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 DECIMAL_DIG 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.
Returns
14 The fprintf function returns the number of characters transmitted, or a negative value
if an output or encoding error occurred.
Environmental limits
15 The number of characters that can be produced by any single conversion shall be at least
4095.
16 EXAMPLE 1 To print a date and time in the form ``Sunday, July 3, 10:02'' followed by to five decimal
places:
#include <math.h>
#include <stdio.h>
/* ... */
char *weekday, *month; // pointers to strings
int day, hour, min;
fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
weekday, month, day, hour, min);
fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
17 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.
249) 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.
18 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|
Forward references: conversion state (7.24.6), the wcrtomb function (7.24.6.3.3).
7.19.6.2 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. 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.
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 (as specified by the isspace function) are skipped, unless
the specification includes a [, c, or n specifier.250)
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.251)
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
250) These white-space characters are not counted against a specified field width.
251) 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.
in the object, the behavior is undefined.
11 The length modifiers and their meanings are:
hh Specifies that a following 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 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 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 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 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 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.
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.
If a length modifier appears with any conversion specifier other than as specified above,
the behavior is undefined.
12 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. The corresponding argument shall be a pointer to
signed integer.
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. The corresponding argument shall be a pointer to signed
integer.
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. The corresponding argument shall be a pointer to
unsigned integer.
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. The corresponding argument shall be a pointer to
unsigned integer.
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. The corresponding argument shall be a pointer to
unsigned integer.
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. The corresponding argument shall be a pointer to floating.
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).252)
If no l length modifier is present, the corresponding argument shall be a
pointer to the initial element of a character array 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 the initial element of an array 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.252)
If no l length modifier is present, the corresponding argument shall be a
pointer to the initial element of a character array 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
252) 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.
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 the initial element of an array 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).252)
If no l length modifier is present, the corresponding argument shall be a
pointer to the initial element of a character array 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 the initial element of an array 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 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 to 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 to
signed integer into which is to be written the number of characters read from
the input stream so far by this call to the fscanf function. 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.253)
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 (including new-line characters) is 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 any conversion. 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.
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:
#include <stdio.h>
/* ... */
int i; float x; char name[50];
fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name);
with input:
253) See ``future library directions'' (7.26.9).
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 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.
23 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.
24 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.
25 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.
26 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 (7.20.1.3), the
strtol, strtoll, strtoul, and strtoull functions (7.20.1.4), conversion state
(7.24.6), the wcrtomb function (7.24.6.3.3).
7.19.6.3 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.
7.19.6.4 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 arguments to scanf.
Returns
3 The scanf function returns the value of the macro EOF if an input failure occurs before
any conversion. 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.
7.19.6.5 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 nonnegative and less than n.
7.19.6.6 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.
7.19.6.7 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 any conversion. 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.
7.19.6.8 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 macro.254)
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);
}
254) As the functions vfprintf, vfscanf, vprintf, vscanf, vsnprintf, vsprintf, and
vsscanf invoke the va_arg macro, the value of arg after the return is indeterminate.
7.19.6.9 The vfscanf function
Synopsis
1 #include <stdarg.h>
#include <stdio.h>
int vfscanf(FILE * restrict stream,
const char * restrict format,
va_list arg);
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.254)
Returns
3 The vfscanf function returns the value of the macro EOF if an input failure occurs
before any conversion. 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.
7.19.6.10 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.254)
Returns
3 The vprintf function returns the number of characters transmitted, or a negative value
if an output or encoding error occurred.
7.19.6.11 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.254)
Returns
3 The vscanf function returns the value of the macro EOF if an input failure occurs
before any conversion. 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.
7.19.6.12 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.254) 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 nonnegative and less than n.
7.19.6.13 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.254) 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.
7.19.6.14 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.254)
Returns
3 The vsscanf function returns the value of the macro EOF if an input failure occurs
before any conversion. 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.
7.19.7 Character input/output functions
7.19.7.1 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.255)
7.19.7.2 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 array contents are
indeterminate and a null pointer is returned.
255) An end-of-file and a read error can be distinguished by use of the feof and ferror functions.
7.19.7.3 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.
7.19.7.4 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.
7.19.7.5 The getc function
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.
7.19.7.6 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.
7.19.7.7 The gets function
Synopsis
1 #include <stdio.h>
char *gets(char *s);
Description
2 The gets function reads characters from the input stream pointed to by stdin, into the
array pointed to by s, until end-of-file is encountered or a new-line character is read.
Any new-line character is discarded, and a null character is written immediately after the
last character read into the array.
Returns
3 The gets 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 array contents are
indeterminate and a null pointer is returned.
Forward references: future library directions (7.26.9).
7.19.7.8 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.
7.19.7.9 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.
7.19.7.10 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.
7.19.7.11 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. 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 is indeterminate after the
call.256)
Returns
6 The ungetc function returns the character pushed back after conversion, or EOF if the
operation fails.
Forward references: file positioning functions (7.19.9).
256) See ``future library directions'' (7.26.9).
7.19.8 Direct input/output functions
7.19.8.1 The fread function
Synopsis
1 #include <stdio.h>
size_t fread(void * restrict ptr,
size_t size, size_t nmemb,
FILE * restrict stream);
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 value of the file position indicator for the stream is
indeterminate. If a partial element is read, its value 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.
7.19.8.2 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 value 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.
7.19.9 File positioning functions
7.19.9.1 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 (7.19.9.3).
7.19.9.2 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 (7.19.9.4).
7.19.9.3 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.
7.19.9.4 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.
7.19.9.5 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.
7.19.10 Error-handling functions
7.19.10.1 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.
7.19.10.2 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.
7.19.10.3 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.
7.19.10.4 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 (7.21.6.2).
7.20 General utilities <stdlib.h>
1 The header <stdlib.h> declares five types and several functions of general utility, and
defines several macros.257)
2 The types declared are size_t and wchar_t (both described in 7.17),
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.
3 The macros defined are NULL (described in 7.17);
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.
257) See ``future library directions'' (7.26.10).
7.20.1 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.
7.20.1.1 The atof function
Synopsis
1 #include <stdlib.h>
double atof(const char *nptr);
Description
2 The atof function converts the initial portion of the string pointed to by nptr to
double representation. Except for the behavior on error, it is equivalent to
strtod(nptr, (char **)NULL)
Returns
3 The atof function returns the converted value.
Forward references: the strtod, strtof, and strtold functions (7.20.1.3).
7.20.1.2 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, (char **)NULL, 10)
atol: strtol(nptr, (char **)NULL, 10)
atoll: strtoll(nptr, (char **)NULL, 10)
Returns
3 The atoi, atol, and atoll functions return the converted value.
Forward references: the strtol, strtoll, strtoul, and strtoull functions
(7.20.1.4).
7.20.1.3 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 (as specified by the isspace function), a
subject sequence resembling a floating-point 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;
-- 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;
-- 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.258)
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 sequences is implementation-defined.259) 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 DECIMAL_DIG (defined in
<float.h>) significant digits, the result should be correctly rounded. If the subject
sequence D has the decimal form and more than DECIMAL_DIG significant digits,
consider the two bounding, adjacent decimal strings L and U, both having
DECIMAL_DIG 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
258) 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 may 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.
259) An implementation may use the n-char sequence to determine extra information to be represented in
the NaN's significand.
stipulation that the error with respect to D should have a correct sign for the current
rounding direction.260)
Returns
10 The 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, plus or
minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the return
type and sign of the value), and the value of the macro ERANGE is stored in errno. 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; whether errno acquires
the value ERANGE is implementation-defined.
7.20.1.4 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 (as specified by the isspace function), a subject sequence
260) DECIMAL_DIG, defined in <float.h>, should be sufficiently large that L and U will usually round
to the same internal floating value, but if not will round to adjacent values.
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. 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. 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 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, 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.
7.20.2 Pseudo-random sequence generation functions
7.20.2.1 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.
3 The implementation shall behave as if no library function calls the rand function.
Returns
4 The rand function returns a pseudo-random integer.
Environmental limits
5 The value of the RAND_MAX macro shall be at least 32767.
7.20.2.2 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 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;
}
7.20.3 Memory management functions
1 The order and contiguity of storage allocated by successive calls to the 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
and then 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, 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.
7.20.3.1 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 zero.261)
Returns
3 The calloc function returns either a null pointer or a pointer to the allocated space.
7.20.3.2 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 not match a pointer earlier returned by the calloc, malloc, or
261) Note that this need not be the same as the representation of floating-point zero or a null pointer
constant.
realloc 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.
7.20.3.3 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 value is indeterminate.
Returns
3 The malloc function returns either a null pointer or a pointer to the allocated space.
7.20.3.4 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
indeterminate 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 the
calloc, malloc, or realloc function, or if the space has been deallocated by a call
to the free or realloc function, the behavior is undefined. If memory for the new
object cannot be 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 could not be
allocated.
7.20.4 Communication with the environment
7.20.4.1 The abort function
Synopsis
1 #include <stdlib.h>
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.
7.20.4.2 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.
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 exit function (7.20.4.3).
7.20.4.3 The exit function
Synopsis
1 #include <stdlib.h>
void exit(int status);
Description
2 The exit function causes normal program termination to occur. If more than one call to
the exit function is executed by a program, the behavior is undefined.
3 First, all functions registered by the atexit function are called, in the reverse order of
their registration,262) 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.
7.20.4.4 The _Exit function
Synopsis
1 #include <stdlib.h>
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 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 (7.20.4.3).
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.
262) Each function is called as many times as it was registered, and in the correct order with respect to
other registered functions.
7.20.4.5 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.
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.
7.20.4.6 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.
7.20.5 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.263) 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.
7.20.5.1 The bsearch 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 *));
Description
2 The bsearch 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
263) 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
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.264)
Returns
4 The bsearch 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.
7.20.5.2 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.
264) In practice, the entire array is sorted according to the comparison function.
7.20.6 Integer arithmetic functions
7.20.6.1 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 undefined.265)
Returns
3 The abs, labs, and llabs, functions return the absolute value.
7.20.6.2 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, respectively, 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.
265) The absolute value of the most negative number cannot be represented in two's complement.
7.20.7 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 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 return a nonzero value if encodings have state dependency, and zero
otherwise.266) Changing the LC_CTYPE category causes the conversion state of these
functions to be indeterminate.
7.20.7.1 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, s, n);
3 The implementation shall behave as if no library function calls the mblen function.
Returns
4 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 (7.20.7.2).
266) 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.
7.20.7.2 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.
7.20.7.3 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.
7.20.8 Multibyte/wide string conversion functions
1 The behavior of the multibyte string functions is affected by the LC_CTYPE category of
the current locale.
7.20.8.1 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.267)
267) The array will not be null-terminated if the value returned is n.
7.20.8.2 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.267)
7.21 String handling <string.h>
7.21.1 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.268) The type is size_t and the macro is NULL (both described in
7.17). 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).
7.21.2 Copying functions
7.21.2.1 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.
268) See ``future library directions'' (7.26.11).
7.21.2.2 The memmove function
Synopsis
1 #include <string.h>
void *memmove(void *s1, const void *s2, size_t n);
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.
7.21.2.3 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.
7.21.2.4 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
s1.269) 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.
7.21.3 Concatenation functions
7.21.3.1 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.
7.21.3.2 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.270) If copying
269) 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.
270) Thus, the maximum number of characters that can end up in the array pointed to by s1 is
strlen(s1)+n+1.
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 (7.21.6.3).
7.21.4 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.
7.21.4.1 The memcmp function
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 s2.271)
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.
7.21.4.2 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
271) The contents of ``holes'' used as padding for purposes of alignment within structure objects are
indeterminate. Strings shorter than their allocated space and unions may also cause problems in
comparison.
pointed to by s2.
7.21.4.3 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.
7.21.4.4 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 null-terminated array pointed to by s2.
7.21.4.5 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 contents of the array
pointed to by s1 are indeterminate.
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)
7.21.5 Search functions
7.21.5.1 The memchr function
Synopsis
1 #include <string.h>
void *memchr(const void *s, int c, size_t n);
Description
2 The memchr 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.
Returns
3 The memchr function returns a pointer to the located character, or a null pointer if the
character does not occur in the object.
7.21.5.2 The strchr function
Synopsis
1 #include <string.h>
char *strchr(const char *s, int c);
Description
2 The strchr 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 function returns a pointer to the located character, or a null pointer if the
character does not occur in the string.
7.21.5.3 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.
7.21.5.4 The strpbrk function
Synopsis
1 #include <string.h>
char *strpbrk(const char *s1, const char *s2);
Description
2 The strpbrk 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 strpbrk function returns a pointer to the character, or a null pointer if no character
from s2 occurs in s1.
7.21.5.5 The strrchr function
Synopsis
1 #include <string.h>
char *strrchr(const char *s, int c);
Description
2 The strrchr 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 function returns a pointer to the character, or a null pointer if c does not
occur in the string.
7.21.5.6 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.
7.21.5.7 The strstr function
Synopsis
1 #include <string.h>
char *strstr(const char *s1, const char *s2);
Description
2 The strstr 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 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.
7.21.5.8 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. 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 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
7.21.6 Miscellaneous functions
7.21.6.1 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.
7.21.6.2 The strerror function
Synopsis
1 #include <string.h>
char *strerror(int errnum);
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 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, but may be
overwritten by a subsequent call to the strerror function.
7.21.6.3 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.
7.22 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 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 modf, there is a corresponding type-generic
macro.272) The parameters whose corresponding real type is double in the function
synopsis are generic parameters. Use of the macro invokes a function whose
corresponding real type and type domain are determined by the arguments for the generic
parameters.273)
3 Use of the macro invokes a function whose generic parameters have the corresponding
real type determined as follows:
-- First, if any argument for generic parameters has type long double, the type
determined is long double.
-- Otherwise, if any argument for generic parameters has type double or is of integer
type, the type determined is double.
-- Otherwise, the type determined is float.
4 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.
272) Like other function-like macros in Standard libraries, each type-generic macro can be suppressed to
make available the corresponding ordinary function.
273) 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.
5 For each unsuffixed function in <math.h> without a c-prefixed counterpart in
<complex.h> (except modf), the corresponding type-generic macro has the same
name as the function. These type-generic macros are:
atan2 fma llround remainder
cbrt fmax log10 remquo
ceil fmin log1p rint
copysign fmod log2 round
erf frexp logb scalbn
erfc hypot lrint scalbln
exp2 ilogb lround tgamma
expm1 ldexp nearbyint trunc
fdim lgamma nextafter
floor llrint nexttoward
If all arguments for generic parameters are real, then use of the macro invokes a real
function; otherwise, use of the macro results in undefined behavior.
6 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 conj creal
cimag cproj
Use of the macro with any real or complex argument invokes a complex function.
7 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;
functions invoked by use of type-generic macros are shown in the following table:
macro use invokes
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 behavior
rint(dc) undefined behavior
fmax(ldc, ld) undefined behavior
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)
7.23 Date and time <time.h>
7.23.1 Components of time
1 The header <time.h> defines two macros, and declares several 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 macros defined are NULL (described in 7.17); and
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.
3 The types declared are size_t (described in 7.17);
clock_t
and
time_t
which are arithmetic types capable of representing times; and
struct tm
which holds the components of a calendar time, called the broken-down time.
4 The range and precision of times representable in clock_t and time_t are
implementation-defined. 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.274)
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
274) The range [0, 60] for tm_sec allows for a positive leap second.
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.
7.23.2 Time manipulation functions
7.23.2.1 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 to the processor
time used by the program 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 or its value cannot be represented, the function
returns the value (clock_t)(-1).275)
7.23.2.2 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.
275) In order to measure the time spent in a program, the clock function should be called at the start of
the program and its return value subtracted from the value returned by subsequent calls.
7.23.2.3 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.276) 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;
/* ... */
276) 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.
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]);
7.23.2.4 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.
7.23.3 Time conversion functions
1 Except for the strftime function, these functions each return a pointer to one of two
types of static objects: a broken-down time structure or an array of char. Execution of
any of the functions that return a pointer to one of these object types may overwrite the
information in any object of the same type pointed to by the value returned from any
previous call to any of them. The implementation shall behave as if no other library
functions call these functions.
7.23.3.1 The asctime function
Synopsis
1 #include <time.h>
char *asctime(const struct tm *timeptr);
Description
2 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.
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"
};
static char result[26];
sprintf(result, "%.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;
}
Returns
3 The asctime function returns a pointer to the string.
7.23.3.2 The ctime function
Synopsis
1 #include <time.h>
char *ctime(const time_t *timer);
Description
2 The ctime function converts the calendar time pointed to by timer to local time in the
form of a string. It is equivalent to
asctime(localtime(timer))
Returns
3 The ctime function returns the pointer returned by the asctime function with that
broken-down time as argument.
Forward references: the localtime function (7.23.3.4).
7.23.3.3 The gmtime function
Synopsis
1 #include <time.h>
struct tm *gmtime(const time_t *timer);
Description
2 The gmtime function converts the calendar time pointed to by timer into a broken-
down time, expressed as UTC.
Returns
3 The gmtime function returns a pointer to the broken-down time, or a null pointer if the
specified time cannot be converted to UTC.
7.23.3.4 The localtime function
Synopsis
1 #include <time.h>
struct tm *localtime(const time_t *timer);
Description
2 The localtime function converts the calendar time pointed to by timer into a
broken-down time, expressed as local time.
Returns
3 The localtime function returns a pointer to the broken-down time, or a null pointer if
the specified time cannot be converted to local time.
7.23.3.5 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 is replaced by appropriate characters as described in the
following list. The appropriate characters are 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 7.23.1]
%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 7.23.1]
%X is replaced by the locale's appropriate time representation. [all specified in 7.23.1]
%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.
%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 contents of the array are indeterminate.
7.24 Extended multibyte and wide character utilities <wchar.h>
7.24.1 Introduction
1 The header <wchar.h> declares four data types, one tag, four macros, and many
functions.277)
2 The types declared are wchar_t and size_t (both described in 7.17);
mbstate_t
which is an object type other than an array type that can hold the conversion state
information 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);278) and
struct tm
which is declared as an incomplete structure type (the contents are described in 7.23.1).
3 The macros defined are NULL (described in 7.17); WCHAR_MIN and WCHAR_MAX
(described in 7.18.3); 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.279) 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;
277) See ``future library directions'' (7.26.12).
278) wchar_t and wint_t can be the same integer type.
279) The value of the macro WEOF may differ from that of EOF and need not be negative.
-- Functions for wide string date and time conversion; and
-- Functions that provide extended capabilities for conversion between multibyte and
wide character sequences.
5 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.
7.24.2 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.280)
7.24.2.1 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
280) The fwprintf functions perform writes to memory for the %n specifier.
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.281)
-- An optional precision that gives the minimum number of digits to appear for the 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 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 negative value is converted if this flag is not
specified.)282)
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.
# 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 x (or X)
conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g,
281) Note that 0 is taken as a flag, not as the beginning of a field width.
282) The results of all floating conversions of a negative zero, and of negative values that round to zero,
include a minus sign.
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 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 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 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 according 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 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 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 conversion specifier applies to a pointer to a long long int
argument.
j Specifies that a following 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 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 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.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier
applies to a long double 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.
o,u,x,X The unsigned int argument is converted to 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.283)
e,E A double argument representing a floating-point number is converted in the
style [-]d.ddd e<>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,
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.hhhh p<>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 character284) 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
283) 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.
284) 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.
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
distinguish285) 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.
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 the initial
element of a character array containing a multibyte character sequence
beginning in the initial shift state. Characters from the array 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 array, the converted array shall contain a
null wide character.
If an l length modifier is present, the argument shall be a pointer to the initial
element of an array of wchar_t type. Wide characters from the array 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 array
shall contain a null wide character.
p The argument shall be a pointer to void. The value of the pointer is
converted to a sequence of printing wide characters, in an implementation-
285) The precision p is sufficient to distinguish values of the source type if 16 p-1 > b n where b is
FLT_RADIX and n is the number of base-b digits in the significand of the source type. A smaller p
might suffice depending on the implementation's scheme for determining the digit to the left of the
decimal-point wide character.
defined manner.
n The argument shall be a pointer to signed integer into which is written the
number of wide characters written to the output stream so far by this call to
fwprintf. 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.286) If any argument is
not the correct type for the corresponding conversion specification, the behavior is
undefined.
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
DECIMAL_DIG, then the result should be correctly rounded.287) If the number of
significant decimal digits is more than DECIMAL_DIG but the source value is exactly
representable with DECIMAL_DIG 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 DECIMAL_DIG 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.
Returns
14 The fwprintf function returns the number of wide characters transmitted, or a negative
value if an output or encoding error occurred.
286) See ``future library directions'' (7.26.12).
287) 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.
Environmental limits
15 The number of wide characters that can be produced by any single conversion shall be at
least 4095.
16 EXAMPLE To print a date and time in the form ``Sunday, July 3, 10:02'' followed by 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));
Forward references: the btowc function (7.24.6.1.1), the mbrtowc function
(7.24.6.3.2).
7.24.2.2 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. 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.
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 (as specified by the iswspace function) are skipped,
unless the specification includes a [, c, or n specifier.288)
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.289) 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
288) These white-space wide characters are not counted against a specified field width.
289) 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.
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 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 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 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 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 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 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.
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.
If a length modifier appears with any conversion specifier other than as specified above,
the behavior is undefined.
12 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. The corresponding argument shall be a pointer to
signed integer.
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. The corresponding argument shall be a pointer to signed
integer.
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. The corresponding argument shall be a pointer to
unsigned integer.
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. The corresponding argument shall be a pointer to
unsigned integer.
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. The corresponding argument shall be a pointer to
unsigned integer.
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. The corresponding argument shall be a pointer to floating.
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 the initial element of a character array 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 the initial element of an array 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 the initial element of a character array 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 the initial element of an array 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 the initial element of a character array 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 the initial element of an array 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 to 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 to
signed integer into which is to be written the number of wide characters read
from the input stream so far by this call to the fwscanf function. 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.290)
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 (including new-line wide characters) is 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 any conversion. 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.
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.
290) See ``future library directions'' (7.26.12).
Forward references: the wcstod, wcstof, and wcstold functions (7.24.4.1.1), the
wcstol, wcstoll, wcstoul, and wcstoull functions (7.24.4.1.2), the wcrtomb
function (7.24.6.3.3).
7.24.2.3 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.
7.24.2.4 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 any conversion. 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.
7.24.2.5 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 macro.291)
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);
}
291) As the functions vfwprintf, vswprintf, vfwscanf, vwprintf, vwscanf, and vswscanf
invoke the va_arg macro, the value of arg after the return is indeterminate.
7.24.2.6 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.291)
Returns
3 The vfwscanf function returns the value of the macro EOF if an input failure occurs
before any conversion. 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.
7.24.2.7 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.291)
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.
7.24.2.8 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.291)
Returns
3 The vswscanf function returns the value of the macro EOF if an input failure occurs
before any conversion. 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.
7.24.2.9 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.291)
Returns
3 The vwprintf function returns the number of wide characters transmitted, or a negative
value if an output or encoding error occurred.
7.24.2.10 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.291)
Returns
3 The vwscanf function returns the value of the macro EOF if an input failure occurs
before any conversion. 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.
7.24.2.11 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.
7.24.2.12 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 any conversion. 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.
7.24.3 Wide character input/output functions
7.24.3.1 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 value of
the macro EILSEQ is stored in errno and the fgetwc function returns WEOF.292)
7.24.3.2 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
292) 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.
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
contents are indeterminate and a null pointer is returned.
7.24.3.3 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.
7.24.3.4 The fputws function
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 nonnegative value.
7.24.3.5 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.293)
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.
7.24.3.6 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.
7.24.3.7 The getwchar function
Synopsis
1 #include <wchar.h>
wint_t getwchar(void);
293) If the orientation of the stream has already been determined, fwide does not change it.
Description
2 The getwchar function is equivalent to getwc with the argument stdin.
Returns
3 The getwchar function returns the next wide character from the input stream pointed to
by stdin, or WEOF.
7.24.3.8 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.
7.24.3.9 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.
7.24.3.10 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. 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.
Returns
6 The ungetwc function returns the wide character pushed back, or WEOF if the operation
fails.
7.24.4 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.
7.24.4.1 Wide string numeric conversion functions
7.24.4.1.1 The wcstod, wcstof, and wcstold functions
Synopsis
1 #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
2 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 (as specified by the iswspace
function), a subject sequence resembling a floating-point 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 for the corresponding single-byte
characters in 6.4.4.2;
-- 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;
-- 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.
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, 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.294) 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 sequences is implementation-defined.295) 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 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.
294) 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 may 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.
295) An implementation may use the n-wchar sequence to determine extra information to be represented in
the NaN's significand.
9 If the subject sequence has the decimal form and at most DECIMAL_DIG (defined in
<float.h>) significant digits, the result should be correctly rounded. If the subject
sequence D has the decimal form and more than DECIMAL_DIG significant digits,
consider the two bounding, adjacent decimal strings L and U, both having
DECIMAL_DIG 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.296)
Returns
10 The 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, plus or
minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the return
type and sign of the value), and the value of the macro ERANGE is stored in errno. 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; whether errno acquires
the value ERANGE is implementation-defined.
296) DECIMAL_DIG, defined in <float.h>, should be sufficiently large that L and U will usually round
to the same internal floating value, but if not will round to adjacent values.
7.24.4.1.2 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);
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 (as specified by the iswspace
function), 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. 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.
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 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, 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.
7.24.4.2 Wide string copying functions
7.24.4.2.1 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.
7.24.4.2.2 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
s1.297)
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.
7.24.4.2.3 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.
297) 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.
7.24.4.2.4 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.
7.24.4.3 Wide string concatenation functions
7.24.4.3.1 The wcscat function
Synopsis
1 #include <wchar.h>
wchar_t *wcscat(wchar_t * restrict s1,
const wchar_t * restrict s2);
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.
7.24.4.3.2 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.298)
Returns
3 The wcsncat function returns the value of s1.
7.24.4.4 Wide string comparison functions
1 Unless explicitly stated otherwise, the functions described in this subclause order two
wide characters the same way as two integers of the underlying integer type designated
by wchar_t.
7.24.4.4.1 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.
7.24.4.4.2 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
298) Thus, the maximum number of wide characters that can end up in the array pointed to by s1 is
wcslen(s1)+n+1.
wide string pointed to by s2 when both are interpreted as appropriate to the current
locale.
7.24.4.4.3 The wcsncmp function
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.
7.24.4.4.4 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 terminating null wide character). If the value returned is n or greater, the contents of
the array pointed to by s1 are indeterminate.
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)
7.24.4.4.5 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.
7.24.4.5 Wide string search functions
7.24.4.5.1 The wcschr function
Synopsis
1 #include <wchar.h>
wchar_t *wcschr(const wchar_t *s, wchar_t c);
Description
2 The wcschr 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 function returns a pointer to the located wide character, or a null pointer if
the wide character does not occur in the wide string.
7.24.4.5.2 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.
7.24.4.5.3 The wcspbrk function
Synopsis
1 #include <wchar.h>
wchar_t *wcspbrk(const wchar_t *s1, const wchar_t *s2);
Description
2 The wcspbrk 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 function returns a pointer to the wide character in s1, or a null pointer if
no wide character from s2 occurs in s1.
7.24.4.5.4 The wcsrchr function
Synopsis
1 #include <wchar.h>
wchar_t *wcsrchr(const wchar_t *s, wchar_t c);
Description
2 The wcsrchr 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 function returns a pointer to the wide character, or a null pointer if c does
not occur in the wide string.
7.24.4.5.5 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.
7.24.4.5.6 The wcsstr function
Synopsis
1 #include <wchar.h>
wchar_t *wcsstr(const wchar_t *s1, const wchar_t *s2);
Description
2 The wcsstr 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 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.
7.24.4.5.7 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
7.24.4.5.8 The wmemchr function
Synopsis
1 #include <wchar.h>
wchar_t *wmemchr(const wchar_t *s, wchar_t c,
size_t n);
Description
2 The wmemchr function locates the first occurrence of c in the initial n wide characters of
the object pointed to by s.
Returns
3 The wmemchr function returns a pointer to the located wide character, or a null pointer if
the wide character does not occur in the object.
7.24.4.6 Miscellaneous functions
7.24.4.6.1 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.
7.24.4.6.2 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.
7.24.5 Wide character time conversion functions
7.24.5.1 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
corresponding 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 contents of the array are
indeterminate.
7.24.6 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'', 7.24.6.3 and
7.24.6.4 -- 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.299)
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 pointed-to object is
altered as needed to track the shift state, and the position within a multibyte character, for
the associated multibyte character sequence.
299) 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.
7.24.6.1 Single-byte/wide character conversion functions
7.24.6.1.1 The btowc function
Synopsis
1 #include <stdio.h>
#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.
7.24.6.1.2 The wctob function
Synopsis
1 #include <stdio.h>
#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.
7.24.6.2 Conversion state functions
7.24.6.2.1 The mbsinit function
Synopsis
1 #include <wchar.h>
int mbsinit(const mbstate_t *ps);
Description
2 If ps is not a null pointer, the mbsinit function determines whether the pointed-to
mbstate_t object describes an initial conversion state.
Returns
3 The mbsinit function returns nonzero if ps is a null pointer or if the pointed-to object
describes an initial conversion state; otherwise, it returns zero.
7.24.6.3 Restartable multibyte/wide character conversion functions
1 These functions differ from the corresponding multibyte character functions of 7.20.7
(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 at
program startup to the initial conversion state. 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.
7.24.6.3.1 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 designated 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 (7.24.6.3.2).
7.24.6.3.2 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).300)
(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.
300) 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).
7.24.6.3.3 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.
7.24.6.4 Restartable multibyte/wide string conversion functions
1 These functions differ from the corresponding multibyte string functions of 7.20.8
(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 at program startup
to the initial conversion state. 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.
7.24.6.4.1 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.301) 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).
301) Thus, the value of len is ignored if dst is a null pointer.
7.24.6.4.2 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.302)
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).
302) 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.
7.25 Wide character classification and mapping utilities <wctype.h>
7.25.1 Introduction
1 The header <wctype.h> declares three data types, one macro, and many functions.303)
2 The types declared are
wint_t
described in 7.24.1;
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 7.24.1).
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.
303) See ``future library directions'' (7.26.13).
7.25.2 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.
7.25.2.1 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.304)
Forward references: the wctob function (7.24.6.1.2).
7.25.2.1.1 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.
7.25.2.1.2 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
304) 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.
wide characters for which none of iswcntrl, iswdigit, iswpunct, or iswspace
is true.305)
7.25.2.1.3 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.
7.25.2.1.4 The iswcntrl function
Synopsis
1 #include <wctype.h>
int iswcntrl(wint_t wc);
Description
2 The iswcntrl function tests for any control wide character.
7.25.2.1.5 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).
7.25.2.1.6 The iswgraph function
Synopsis
1 #include <wctype.h>
int iswgraph(wint_t wc);
305) The functions iswlower and iswupper test true or false separately for each of these additional
wide characters; all four combinations are possible.
Description
2 The iswgraph function tests for any wide character for which iswprint is true and
iswspace is false.306)
7.25.2.1.7 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.
7.25.2.1.8 The iswprint function
Synopsis
1 #include <wctype.h>
int iswprint(wint_t wc);
Description
2 The iswprint function tests for any printing wide character.
7.25.2.1.9 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.306)
7.25.2.1.10 The iswspace function
Synopsis
1 #include <wctype.h>
int iswspace(wint_t wc);
306) Note that the behavior of the iswgraph and iswpunct functions may differ from their
corresponding functions in 7.4.1 with respect to printing, white-space, single-byte execution
characters other than ' '.
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.
7.25.2.1.11 The iswupper function
Synopsis
1 #include <wctype.h>
int iswupper(wint_t wc);
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.
7.25.2.1.12 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).
7.25.2.2 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 (7.25.2.1).
7.25.2.2.1 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 (7.25.2.1) 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.
Forward references: the wctype function (7.25.2.2.2).
7.25.2.2.2 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.
7.25.3 Wide character case mapping utilities
1 The header <wctype.h> declares several functions useful for mapping wide characters.
7.25.3.1 Wide character case mapping functions
7.25.3.1.1 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 corresponding 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.
7.25.3.1.2 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 corresponding 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.
7.25.3.2 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 (7.25.3.1).
7.25.3.2.1 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 (7.25.3.1) 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.
7.25.3.2.2 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.
7.26 Future library directions
1 The following names are grouped under individual headers for convenience. All external
names described below are reserved no matter what headers are included by the program.
7.26.1 Complex arithmetic <complex.h>
1 The function names
cerf cexpm1 clog2
cerfc clog10 clgamma
cexp2 clog1p ctgamma
and the same names suffixed with f or l may be added to the declarations in the
<complex.h> header.
7.26.2 Character handling <ctype.h>
1 Function names that begin with either is or to, and a lowercase letter may be added to
the declarations in the <ctype.h> header.
7.26.3 Errors <errno.h>
1 Macros that begin with E and a digit or E and an uppercase letter may be added to the
declarations in the <errno.h> header.
7.26.4 Format conversion of integer types <inttypes.h>
1 Macro names beginning with PRI or SCN followed by any lowercase letter or X may be
added to the macros defined in the <inttypes.h> header.
7.26.5 Localization <locale.h>
1 Macros that begin with LC_ and an uppercase letter may be added to the definitions in
the <locale.h> header.
7.26.6 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 definitions in the <signal.h> header.
7.26.7 Boolean type and values <stdbool.h>
1 The ability to undefine and perhaps then redefine the macros bool, true, and false is
an obsolescent feature.
7.26.8 Integer types <stdint.h>
1 Typedef names beginning with int or uint and ending with _t may be added to the
types defined in the <stdint.h> header. Macro names beginning with INT or UINT
and ending with _MAX, _MIN, or _C may be added to the macros defined in the
<stdint.h> header.
7.26.9 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 gets function is obsolescent, and is deprecated.
3 The use of ungetc on a binary stream where the file position indicator is zero prior to
the call is an obsolescent feature.
7.26.10 General utilities <stdlib.h>
1 Function names that begin with str and a lowercase letter may be added to the
declarations in the <stdlib.h> header.
7.26.11 String handling <string.h>
1 Function names that begin with str, mem, or wcs and a lowercase letter may be added
to the declarations in the <string.h> header.
7.26.12 Extended multibyte and wide character utilities <wchar.h>
1 Function names that begin with wcs and a lowercase letter 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.
7.26.13 Wide character classification and mapping utilities
<wctype.h>
1 Function names that begin with is or to and a lowercase letter may be added to the
declarations in the <wctype.h> header.
9
0.0.0 End
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