GitHub/pbot
3
0
Fork 0
mirror of https://github.com/pragma-/pbot.git synced 2024-12-25 04:02:37 +01:00
Code Issues Projects Releases Wiki Activity
36f2f91ba0
pbot/modules/n1570-cfact.txt
Pragmatic Software a5ede3307e Update C facts module
2014-08-31 20:23:27 +00:00

18596 lines
1.1 MiB
Executable File
Raw Blame History

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
value representable by an object of type wchar_t, capable of representing any character
in the current locale
3.8
1 constraint
restriction, either syntactic or semantic, by which the exposition of language elements is
to be interpreted
3.9
1 correctly rounded result
representation in the result format that is nearest in value, subject to the current rounding
mode, to what the result would be given unlimited range and precision
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 memory location
either an object of scalar type, or a maximal sequence of adjacent bit-fields all having
nonzero width
2 NOTE 1 Two threads of execution can update and access separate memory locations without interfering
with each other.
3 NOTE 2 A bit-field and an adjacent non-bit-field member are in separate memory locations. The same
applies to two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the
two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field member
declaration. It is not safe to concurrently update two non-atomic bit-fields in the same structure if all
members declared between them are also (non-zero-length) bit-fields, no matter what the sizes of those
intervening bit-fields happen to be.
4 EXAMPLE A structure declared as
struct {
char a;
int b:5, c:11, :0, d:8;
struct { int ee:8; } e;
}
contains four separate memory locations: The member a, and bit-fields d and e.ee are each separate
memory locations, and can be modified concurrently without interfering with each other. The bit-fields b
and c together constitute the fourth memory location. The bit-fields b and c cannot be concurrently
modified, but b and a, for example, can be.
3.15
1 object
region of data storage in the execution environment, the contents of which can represent
values
2 NOTE When referenced, an object may be interpreted as having a particular type; see 6.3.2.1.
3.16
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
3.17
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
3.18
1 runtime-constraint
requirement on a program when calling a library function
2 NOTE 1 Despite the similar terms, a runtime-constraint is not a kind of constraint as defined by 3.8, and
need not be diagnosed at translation time.
3 NOTE 2 Implementations that support the extensions in annex K are required to verify that the runtime-
constraints for a library function are not violated by the program; see K.3.1.4.
3.19
1 value
precise meaning of the contents of an object when interpreted as having a specific type
3.19.1
1 implementation-defined value
unspecified value where each implementation documents how the choice is made
3.19.2
1 indeterminate value
either an unspecified value or a trap representation
3.19.3
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.
3.19.4
1 trap representation
an object representation that need not represent a value of the object type
3.19.5
1 perform a trap
interrupt execution of the program such that no further operations are performed
2 NOTE In this International Standard, when the word ``trap'' is not immediately followed by
``representation'', this is the intended usage.2)
3.20
1 x
ceiling of x : the least integer greater than or equal to x
2 EXAMPLE 2. 4 is 3, -2. 4 is -2.
3.21
1 x
floor of x : the greatest integer less than or equal to x
2 EXAMPLE 2. 4 is 2, -2. 4 is -3.
2) For example, ``Trapping or stopping (if supported) is disabled...'' (F.8.2). Note that fetching a trap
representation might perform a trap but is not required to (see 6.2.6.1).
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 or runtime-
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.3) It shall not produce output dependent on any
unspecified, undefined, or implementation-defined behavior, and shall not exceed any
minimum implementation limit.
6 The two forms of conforming implementation are hosted and freestanding. A conforming
hosted implementation shall accept any strictly conforming program. A conforming
freestanding implementation shall accept any strictly conforming program in which the
use of the features specified in the library clause (clause 7) is confined to the contents of
the standard headers <float.h>, <iso646.h>, <limits.h>, <stdalign.h>,
<stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>, and
<stdnoreturn.h>. A conforming implementation may have extensions (including
additional library functions), provided they do not alter the behavior of any strictly
conforming program.4)
3) A strictly conforming program can use conditional features (see 6.10.8.3) provided the use is guarded
by an appropriate conditional inclusion preprocessing directive using the related macro. For example:
#ifdef _ _STDC_IEC_559_ _ /* FE_UPWARD defined */
/* ... */
fesetround(FE_UPWARD);
/* ... */
#endif
4) 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.5)
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), alignment <stdalign.h> (7.15),
variable arguments <stdarg.h> (7.16), boolean type and values <stdbool.h>
(7.18), common definitions <stddef.h> (7.19), integer types <stdint.h> (7.20),
<stdnoreturn.h> (7.23).
5) 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.6)
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.
6) 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 tokens7) and sequences of
white-space characters (including comments). A source file shall not end in a
partial preprocessing token or in a partial comment. Each comment is replaced by
one space character. New-line characters are retained. Whether each nonempty
sequence of white-space characters other than new-line is retained or replaced by
one space character is implementation-defined.
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.8)
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).
7) As described in 6.4, the process of dividing a source file's characters into preprocessing tokens is
context-dependent. For example, see the handling of < within a #include preprocessing directive.
8) 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.9)
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 (6.7.9).
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.
9) 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;10) 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).
10) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as
char ** argv, and so on.
5.1.2.2.3 Program termination
1 If the return type of the main function is a type compatible with int, a return from the
initial call to the main function is equivalent to calling the exit function with the value
returned by the main function as its argument;11) reaching the } that terminates the
main function returns a value of 0. If the return type is not compatible with int, the
termination status returned to the host environment is unspecified.
Forward references: definition of terms (7.1.1), the exit function (7.22.4.4).
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,12) which are changes in the state of
the execution environment. Evaluation of an expression in general includes both value
computations and initiation of side effects. Value computation for an lvalue expression
includes determining the identity of the designated object.
3 Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations
executed by a single thread, which induces a partial order among those evaluations.
Given any two evaluations A and B, if A is sequenced before B, then the execution of A
shall precede the execution of B. (Conversely, if A is sequenced before B, then B is
sequenced after A.) If A is not sequenced before or after B, then A and B are
unsequenced . Evaluations A and B are indeterminately sequenced when A is sequenced
either before or after B, but it is unspecified which.13) The presence of a sequence point
between the evaluation of expressions A and B implies that every value computation and
side effect associated with A is sequenced before every value computation and side effect
associated with B. (A summary of the sequence points is given in annex C.)
4 In the abstract machine, all expressions are evaluated as specified by the semantics. An
actual implementation need not evaluate part of an expression if it can deduce that its
value is not used and that no needed side effects are produced (including any caused by
11) In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main
will have ended in the former case, even where they would not have in the latter.
12) The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status
flags and control modes. Floating-point operations implicitly set the status flags; modes affect result
values of floating-point operations. Implementations that support such floating-point state are
required to regard changes to it as side effects -- see annex F for details. The floating-point
environment library <fenv.h> provides a programming facility for indicating when these side
effects matter, freeing the implementations in other cases.
13) The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations
cannot interleave, but can be executed in any order.
calling a function or accessing a volatile object).
5 When the processing of the abstract machine is interrupted by receipt of a signal, the
values of objects that are neither lock-free atomic objects nor of type volatile
sig_atomic_t are unspecified, as is the state of the floating-point environment. The
value of any object modified by the handler that is neither a lock-free atomic object nor of
type volatile sig_atomic_t becomes indeterminate when the handler exits, as
does the state of the floating-point environment if it is modified by the handler and not
restored to its original state.
6 The least requirements on a conforming implementation are:
-- Accesses to volatile objects are evaluated strictly according to the rules of the abstract
machine.
-- At program termination, all data written into files shall be identical to the result that
execution of the program according to the abstract semantics would have produced.
-- The input and output dynamics of interactive devices shall take place as specified in
7.21.3. The intent of these requirements is that unbuffered or line-buffered output
appear as soon as possible, to ensure that prompting messages actually appear prior to
a program waiting for input.
This is the observable behavior of the program.
7 What constitutes an interactive device is implementation-defined.
8 More stringent correspondences between abstract and actual semantics may be defined by
each implementation.
9 EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual
semantics: at every sequence point, the values of the actual objects would agree with those specified by the
abstract semantics. The keyword volatile would then be redundant.
10 Alternatively, an implementation might perform various optimizations within each translation unit, such
that the actual semantics would agree with the abstract semantics only when making function calls across
translation unit boundaries. In such an implementation, at the time of each function entry and function
return where the calling function and the called function are in different translation units, the values of all
externally linked objects and of all objects accessible via pointers therein would agree with the abstract
semantics. Furthermore, at the time of each such function entry the values of the parameters of the called
function and of all objects accessible via pointers therein would agree with the abstract semantics. In this
type of implementation, objects referred to by interrupt service routines activated by the signal function
would require explicit specification of volatile storage, as well as other implementation-defined
restrictions.
11 EXAMPLE 2 In executing the fragment
char c1, c2;
/* ... */
c1 = c1 + c2;
the ``integer promotions'' require that the abstract machine promote the value of each variable to int size
and then add the two ints and truncate the sum. Provided the addition of two chars can be done without
overflow, or with overflow wrapping silently to produce the correct result, the actual execution need only
produce the same result, possibly omitting the promotions.
12 EXAMPLE 3 Similarly, in the fragment
float f1, f2;
double d;
/* ... */
f1 = f2 * d;
the multiplication 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).
13 EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate
semantics. Values are independent of whether they are represented in a register or in memory. For
example, an implicit spilling of a register is not permitted to alter the value. Also, an explicit store and load
is required to round to the precision of the storage type. In particular, casts and assignments are required to
perform their specified conversion. For the fragment
double d1, d2;
float f;
d1 = f = expression;
d2 = (float) expression;
the values assigned to d1 and d2 are required to have been converted to float.
14 EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in
precision as well as range. The implementation cannot generally apply the mathematical associative rules
for addition or multiplication, nor the distributive rule, because of roundoff error, even in the absence of
overflow and underflow. Likewise, implementations cannot generally replace decimal constants in order to
rearrange expressions. In the following fragment, rearrangements suggested by mathematical rules for real
numbers are often not valid (see F.9).
double x, y, z;
/* ... */
x = (x * y) * z; // not equivalent to x *= y * z;
z = (x - y) + y ; // not equivalent to z = x;
z = x + x * y; // not equivalent to z = x * (1.0 + y);
y = x / 5.0; // not equivalent to y = x * 0.2;
15 EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment
int a, b;
/* ... */
a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is
next added to b, and that result is then added to 5 which results in the value assigned to a. On a machine in
which 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.
16 EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the
following fragment
#include <stdio.h>
int sum;
char *p;
/* ... */
sum = sum * 10 - '0' + (*p++ = getchar());
the expression statement is grouped as if it were written as
sum = (((sum * 10) - '0') + ((*(p++)) = (getchar())));
but the actual increment of p can occur at any time between the previous sequence point and the next
sequence point (the ;), and the call to getchar can occur at any point prior to the need of its returned
value.
Forward references: expressions (6.5), type qualifiers (6.7.3), statements (6.8), floating-
point environment <fenv.h> (7.6), the signal function (7.14), files (7.21.3).
5.1.2.4 Multi-threaded executions and data races
1 Under a hosted implementation, a program can have more than one thread of execution
(or thread ) running concurrently. The execution of each thread proceeds as defined by
the remainder of this standard. The execution of the entire program consists of an
execution of all of its threads.14) Under a freestanding implementation, it is
implementation-defined whether a program can have more than one thread of execution.
2 The value of an object visible to a thread T at a particular point is the initial value of the
object, a value stored in the object by T , or a value stored in the object by another thread,
according to the rules below.
3 NOTE 1 In some cases, there may instead be undefined behavior. Much of this section is motivated by
the desire to support atomic operations with explicit and detailed visibility constraints. However, it also
implicitly supports a simpler view for more restricted programs.
4 Two expression evaluations conflict if one of them modifies a memory location and the
other one reads or modifies the same memory location.
14) The execution can usually be viewed as an interleaving of all of the threads. However, some kinds of
atomic operations, for example, allow executions inconsistent with a simple interleaving as described
below.
5 The library defines a number of atomic operations (7.17) and operations on mutexes
(7.26.4) that are specially identified as synchronization operations. These operations play
a special role in making assignments in one thread visible to another. A synchronization
operation on one or more memory locations is either an acquire operation, a release
operation, both an acquire and release operation, or a consume operation. A
synchronization operation without an associated memory location is a fence and can be
either an acquire fence, a release fence, or both an acquire and release fence. In addition,
there are relaxed atomic operations, which are not synchronization operations, and
atomic read-modify-write operations, which have special characteristics.
6 NOTE 2 For example, a call that acquires a mutex will perform an acquire operation on the locations
composing the mutex. Correspondingly, a call that releases the same mutex will perform a release
operation on those same locations. Informally, performing a release operation on A forces prior side effects
on other memory locations to become visible to other threads that later perform an acquire or consume
operation on A. We do not include relaxed atomic operations as synchronization operations although, like
synchronization operations, they cannot contribute to data races.
7 All modifications to a particular atomic object M occur in some particular total order,
called the modification order of M . If A and B are modifications of an atomic object M ,
and A happens before B, then A shall precede B in the modification order of M , which is
defined below.
8 NOTE 3 This states that the modification orders must respect the ``happens before'' relation.
9 NOTE 4 There is a separate order for each atomic object. There is no requirement that these can be
combined into a single total order for all objects. In general this will be impossible since different threads
may observe modifications to different variables in inconsistent orders.
10 A release sequence headed by a release operation A on an atomic object M is a maximal
contiguous sub-sequence of side effects in the modification order of M , where the first
operation is A and every subsequent operation either is performed by the same thread that
performed the release or is an atomic read-modify-write operation.
11 Certain library calls synchronize with other library calls performed by another thread. In
particular, an atomic operation A that performs a release operation on an object M
synchronizes with an atomic operation B that performs an acquire operation on M and
reads a value written by any side effect in the release sequence headed by A.
12 NOTE 5 Except in the specified cases, reading a later value does not necessarily ensure visibility as
described below. Such a requirement would sometimes interfere with efficient implementation.
13 NOTE 6 The specifications of the synchronization operations define when one reads the value written by
another. For atomic variables, the definition is clear. All operations on a given mutex occur in a single total
order. Each mutex acquisition ``reads the value written'' by the last mutex release.
14 An evaluation A carries a dependency 15) to an evaluation B if:
15) The ``carries a dependency'' relation is a subset of the ``sequenced before'' relation, and is similarly
strictly intra-thread.
-- the value of A is used as an operand of B, unless:
<20> B is an invocation of the kill_dependency macro,
<20> A is the left operand of a && or || operator,
<20> A is the left operand of a ? : operator, or
<20> A is the left operand of a , operator;
or
-- A writes a scalar object or bit-field M , B reads from M the value written by A, and A
is sequenced before B, or
-- for some evaluation X , A carries a dependency to X and X carries a dependency to B.
15 An evaluation A is dependency-ordered before16) an evaluation B if:
-- A performs a release operation on an atomic object M , and, in another thread, B
performs a consume operation on M and reads a value written by any side effect in
the release sequence headed by A, or
-- for some evaluation X , A is dependency-ordered before X and X carries a
dependency to B.
16 An evaluation A inter-thread happens before an evaluation B if A synchronizes with B, A
is dependency-ordered before B, or, for some evaluation X :
-- A synchronizes with X and X is sequenced before B,
-- A is sequenced before X and X inter-thread happens before B, or
-- A inter-thread happens before X and X inter-thread happens before B.
17 NOTE 7 The ``inter-thread happens before'' relation describes arbitrary concatenations of ``sequenced
before'', ``synchronizes with'', and ``dependency-ordered before'' relationships, with two exceptions. The
first exception is that a concatenation is not permitted to end with ``dependency-ordered before'' followed
by ``sequenced before''. The reason for this limitation is that a consume operation participating in a
``dependency-ordered before'' relationship provides ordering only with respect to operations to which this
consume operation actually carries a dependency. The reason that this limitation applies only to the end of
such a concatenation is that any subsequent release operation will provide the required ordering for a prior
consume operation. The second exception is that a concatenation is not permitted to consist entirely of
``sequenced before''. The reasons for this limitation are (1) to permit ``inter-thread happens before'' to be
transitively closed and (2) the ``happens before'' relation, defined below, provides for relationships
consisting entirely of ``sequenced before''.
18 An evaluation A happens before an evaluation B if A is sequenced before B or A inter-
thread happens before B.
16) The ``dependency-ordered before'' relation is analogous to the ``synchronizes with'' relation, but uses
release/consume in place of release/acquire.
19 A visible side effect A on an object M with respect to a value computation B of M
satisfies the conditions:
-- A happens before B, and
-- there is no other side effect X to M such that A happens before X and X happens
before B.
The value of a non-atomic scalar object M , as determined by evaluation B, shall be the
value stored by the visible side effect A.
20 NOTE 8 If there is ambiguity about which side effect to a non-atomic object is visible, then there is a data
race and the behavior is undefined.
21 NOTE 9 This states that operations on ordinary variables are not visibly reordered. This is not actually
detectable without data races, but it is necessary to ensure that data races, as defined here, and with suitable
restrictions on the use of atomics, correspond to data races in a simple interleaved (sequentially consistent)
execution.
22 The visible sequence of side effects on an atomic object M , with respect to a value
computation B of M , is a maximal contiguous sub-sequence of side effects in the
modification order of M , where the first side effect is visible with respect to B, and for
every subsequent side effect, it is not the case that B happens before it. The value of an
atomic object M , as determined by evaluation B, shall be the value stored by some
operation in the visible sequence of M with respect to B. Furthermore, if a value
computation A of an atomic object M happens before a value computation B of M , and
the value computed by A corresponds to the value stored by side effect X , then the value
computed by B shall either equal the value computed by A, or be the value stored by side
effect Y , where Y follows X in the modification order of M .
23 NOTE 10 This effectively disallows compiler reordering of atomic operations to a single object, even if
both operations are ``relaxed'' loads. By doing so, we effectively make the ``cache coherence'' guarantee
provided by most hardware available to C atomic operations.
24 NOTE 11 The visible sequence depends on the ``happens before'' relation, which in turn depends on the
values observed by loads of atomics, which we are restricting here. The intended reading is that there must
exist an association of atomic loads with modifications they observe that, together with suitably chosen
modification orders and the ``happens before'' relation derived as described above, satisfy the resulting
constraints as imposed here.
25 The execution of a program contains a data race if it contains two conflicting actions in
different threads, at least one of which is not atomic, and neither happens before the
other. Any such data race results in undefined behavior.
26 NOTE 12 It can be shown that programs that correctly use simple mutexes and
memory_order_seq_cst operations to prevent all data races, and use no other synchronization
operations, behave as though the operations executed by their constituent threads were simply interleaved,
with each value computation of an object being the last value stored in that interleaving. This is normally
referred to as ``sequential consistency''. However, this applies only to data-race-free programs, and data-
race-free programs cannot observe most program transformations that do not change single-threaded
program semantics. In fact, most single-threaded program transformations continue to be allowed, since
any program that behaves differently as a result must contain undefined behavior.
27 NOTE 13 Compiler transformations that introduce assignments to a potentially shared memory location
that would not be modified by the abstract machine are generally precluded by this standard, since such an
assignment might overwrite another assignment by a different thread in cases in which an abstract machine
execution would not have encountered a data race. This includes implementations of data member
assignment that overwrite adjacent members in separate memory locations. We also generally preclude
reordering of atomic loads in cases in which the atomics in question may alias, since this may violate the
"visible sequence" rules.
28 NOTE 14 Transformations that introduce a speculative read of a potentially shared memory location may
not preserve the semantics of the program as defined in this standard, since they potentially introduce a data
race. However, they are typically valid in the context of an optimizing compiler that targets a specific
machine with well-defined semantics for data races. They would be invalid for a hypothetical machine that
is not tolerant of races or provides hardware race detection.
5.2 Environmental considerations
5.2.1 Character sets
1 Two sets of characters and their associated collating sequences shall be defined: the set in
which source files are written (the source character set ), and the set interpreted in the
execution environment (the execution character set ). Each set is further divided into a
basic character set , whose contents are given by this subclause, and a set of zero or more
locale-specific members (which are not members of the basic character set) called
extended characters. The combined set is also called the extended character set . The
values of the members of the execution character set are implementation-defined.
2 In a character constant or string literal, members of the execution character set shall be
represented by corresponding members of the source character set or by escape
sequences consisting of the backslash \ followed by one or more characters. A byte with
all bits set to 0, called the null character , shall exist in the basic execution character set; it
is used to terminate a character string.
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 sequences17)) 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.
17) 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 (7.21.7.3).
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:18)
-- 127 nesting levels of blocks
-- 63 nesting levels of conditional inclusion
-- 12 pointer, array, and function declarators (in any combinations) modifying an
arithmetic, structure, union, or void type in a declaration
-- 63 nesting levels of parenthesized declarators within a full declarator
-- 63 nesting levels of parenthesized expressions within a full expression
-- 63 significant initial characters in an internal identifier or a macro name (each
universal 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
18) 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)19)
-- 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 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> (7.20).
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
19) 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.20) 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.21) 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
k =1
f k b-k , 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
20) See 6.2.5.
21) The floating-point model is intended to clarify the description of each floating-point characteristic and
does not require the floating-point arithmetic of the implementation to be identical.
arithmetic operand.22)
4 An implementation may give zero and values that are not floating-point numbers (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 minimum range of representable values for a floating type is the most negative finite
floating-point number representable in that type through the most positive finite floating-
point number representable in that type. In addition, if negative infinity is representable
in a type, the range of that type is extended to all negative real numbers; likewise, if
positive infinity is representable in a type, the range of that type is extended to all positive
real numbers.
6 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.
7 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.
8 The rounding mode for floating-point addition is characterized by the implementation-
defined value of FLT_ROUNDS:23)
-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.
22) 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.
23) Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through
the function fesetround in <fenv.h>.
9 Except for assignment and cast (which remove all extra range and precision), the values
yielded by operators 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:24)
-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.
10 The presence or absence of subnormal numbers is characterized by the implementation-
defined values of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and
LDBL_HAS_SUBNORM:
-1 indeterminable25)
0 absent26) (type does not support subnormal numbers)
1 present (type does support subnormal numbers)
11 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
24) 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.
25) Characterization as indeterminable is intended if floating-point operations do not consistently interpret
subnormal representations as zero, nor as nonzero.
26) Characterization as absent is intended if no floating-point operations produce subnormal results from
non-subnormal inputs, even if the type format includes representations of subnormal numbers.
-- number of base-FLT_RADIX digits in the floating-point significand, p
FLT_MANT_DIG
DBL_MANT_DIG
LDBL_MANT_DIG
-- number of decimal digits, n, such that any floating-point number with p radix b digits
can be rounded to a floating-point number with n decimal digits and back again
without change to the value,
p log10 b if b is a power of 10
1 + p log10 b otherwise
FLT_DECIMAL_DIG 6
DBL_DECIMAL_DIG 10
LDBL_DECIMAL_DIG 10
-- number of decimal digits, n, such that any floating-point number in the widest
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,
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
12 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
13 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
-- minimum positive floating-point number27)
FLT_TRUE_MIN 1E-37
DBL_TRUE_MIN 1E-37
LDBL_TRUE_MIN 1E-37
Recommended practice
14 Conversion from (at least) double to decimal with DECIMAL_DIG digits and back
should be the identity function.
15 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
k =1
f k 16-k , -31 e +32
FLT_RADIX 16
FLT_MANT_DIG 6
FLT_EPSILON 9.53674316E-07F
FLT_DECIMAL_DIG 9
FLT_DIG 6
FLT_MIN_EXP -31
FLT_MIN 2.93873588E-39F
FLT_MIN_10_EXP -38
FLT_MAX_EXP +32
FLT_MAX 3.40282347E+38F
FLT_MAX_10_EXP +38
16 EXAMPLE 2 The following describes floating-point representations that also meet the requirements for
single-precision and double-precision numbers in IEC 60559,28) and the appropriate values in a
<float.h> header for types float and double:
24
x f = s 2e
k =1
f k 2-k , -125 e +128
53
x d = s 2e
k =1
f k 2-k , -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
FLT_DECIMAL_DIG 9
27) If the presence or absence of subnormal numbers is indeterminable, then the value is intended to be a
positive number no greater than the minimum normalized positive number for the type.
28) 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_TRUE_MIN 1.40129846E-45F // decimal constant
FLT_TRUE_MIN 0X1P-149F // hex constant
FLT_HAS_SUBNORM 1
FLT_MIN_10_EXP -37
FLT_MAX_EXP +128
FLT_MAX 3.40282347E+38F // decimal constant
FLT_MAX 0X1.fffffeP127F // hex constant
FLT_MAX_10_EXP +38
DBL_MANT_DIG 53
DBL_EPSILON 2.2204460492503131E-16 // decimal constant
DBL_EPSILON 0X1P-52 // hex constant
DBL_DECIMAL_DIG 17
DBL_DIG 15
DBL_MIN_EXP -1021
DBL_MIN 2.2250738585072014E-308 // decimal constant
DBL_MIN 0X1P-1022 // hex constant
DBL_TRUE_MIN 4.9406564584124654E-324 // decimal constant
DBL_TRUE_MIN 0X1P-1074 // hex constant
DBL_HAS_SUBNORM 1
DBL_MIN_10_EXP -307
DBL_MAX_EXP +1024
DBL_MAX 1.7976931348623157E+308 // decimal constant
DBL_MAX 0X1.fffffffffffffP1023 // hex constant
DBL_MAX_10_EXP +308
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>
(7.29), floating-point environment <fenv.h> (7.6), general utilities <stdlib.h>
(7.22), input/output <stdio.h> (7.21), 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 end
strictly before the scope of the other entity (the outer scope). Within the inner scope, the
identifier designates the entity declared in the inner scope; the entity declared in the outer
scope is hidden (and not visible) within the inner scope.
5 Unless explicitly stated otherwise, where this 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.
8 As a special case, a type name (which is not a declaration of an identifier) is considered to
have a scope that begins just after the place within the type name where the omitted
identifier would appear were it not omitted.
Forward references: declarations (6.7), function calls (6.5.2.2), function definitions
(6.9.1), identifiers (6.4.2), macro replacement (6.10.3), name spaces of identifiers (6.2.3),
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.29) 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.30)
29) There is no linkage between different identifiers.
4 For an identifier declared with the storage-class specifier extern in a scope in which a
prior declaration of that identifier is visible,31) 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 any32)
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).
30) A function declaration can contain the storage-class specifier static only if it is at file scope; see
6.7.1.
31) As specified in 6.2.1, the later declaration might hide the prior declaration.
32) 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 four storage
durations: static, thread, automatic, and allocated. Allocated storage is described in 7.22.3.
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,33) and retains
its last-stored value throughout its lifetime.34) 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 (or just past) reaches the end of its lifetime.
3 An object whose identifier is declared without the storage-class specifier
_Thread_local, and either with external or internal linkage or with the storage-class
specifier static, has static storage duration. Its lifetime is the entire execution of the
program and its stored value is initialized only once, prior to program startup.
4 An object whose identifier is declared with the storage-class specifier _Thread_local
has thread storage duration. Its lifetime is the entire execution of the thread for which it
is created, and its stored value is initialized when the thread is started. There is a distinct
object per thread, and use of the declared name in an expression refers to the object
associated with the thread evaluating the expression. The result of attempting to
indirectly access an object with thread storage duration from a thread other than the one
with which the object is associated is implementation-defined.
5 An object whose identifier is declared with no linkage and without the storage-class
specifier static has automatic storage duration, as do some compound literals. The
result of attempting to indirectly access an object with automatic storage duration from a
thread other than the one with which the object is associated is implementation-defined.
6 For such an object that does not have a variable length array type, its lifetime extends
from entry into the block with which it is associated until execution of that block ends in
any way. (Entering an enclosed block or calling a function suspends, but does not end,
execution of the current block.) If the block is entered recursively, a new instance of the
object is created each time. The initial value of the object is indeterminate. If an
initialization is specified for the object, it is performed each time the declaration or
compound literal is reached in the execution of the block; otherwise, the value becomes
indeterminate each time the declaration is reached.
33) 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.
34) In the case of a volatile object, the last store need not be explicit in the program.
7 For such an object that does have a variable length array type, its lifetime extends from
the declaration of the object until execution of the program leaves the scope of the
declaration.35) If the scope is entered recursively, a new instance of the object is created
each time. The initial value of the object is indeterminate.
8 A non-lvalue expression with structure or union type, where the structure or union
contains a member with array type (including, recursively, members of all contained
structures and unions) refers to an object with automatic storage duration and temporary
lifetime.36) Its lifetime begins when the expression is evaluated and its initial value is the
value of the expression. Its lifetime ends when the evaluation of the containing full
expression or full declarator ends. Any attempt to modify an object with temporary
lifetime results in undefined behavior.
Forward references: array declarators (6.7.6.2), compound literals (6.5.2.5), declarators
(6.7.6), function calls (6.5.2.2), initialization (6.7.9), statements (6.8).
6.2.5 Types
1 The meaning of a value stored in an object or returned by a function is determined by the
type of the expression used to access it. (An identifier declared to be an object is the
simplest such expression; the type is specified in the declaration of the identifier.) Types
are partitioned into object types (types that describe objects) and function types (types
that describe functions). At various points within a translation unit an object type may be
incomplete (lacking sufficient information to determine the size of objects of that type) or
complete (having sufficient information).37)
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.38) The standard and extended
signed integer types are collectively called signed integer types.39)
35) 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.
36) The address of such an object is taken implicitly when an array member is accessed.
37) A type may be incomplete or complete throughout an entire translation unit, or it may change states at
different points within a translation unit.
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.40)
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.41) 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.42) 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.
38) Implementation-defined keywords shall have the form of an identifier reserved for any use as
described in 7.1.3.
39) Therefore, any statement in this Standard about signed integer types also applies to the extended
signed integer types.
40) Therefore, any statement in this Standard about unsigned integer types also applies to the extended
unsigned integer types.
41) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
42) See ``future language directions'' (6.11.1).
11 There are three complex types, designated as float _Complex, double
_Complex, and long double _Complex.43) (Complex types are a conditional
feature that implementations need not support; see 6.10.8.3.) 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. The basic types are complete object types. Even if the
implementation defines two or more basic types to have the same representation, they are
nevertheless different types.44)
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.45)
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 object type that
cannot be completed.
43) A specification for imaginary types is in annex G.
44) 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.
45) 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.
20 Any number of derived types can be constructed from the object and function types, as
follows:
-- An array type describes a contiguously allocated nonempty set of objects with a
particular member object type, called the element type. The element type shall be
complete whenever the array type is specified. Array types are characterized by their
element type and by the number of elements in the array. An array type is said to be
derived from its element type, and if its element type is T , the array type is sometimes
called ``array of T ''. The construction of an array type from an element type is called
``array type derivation''.
-- A structure type describes a sequentially allocated nonempty set of member objects
(and, in certain circumstances, an incomplete array), each of which has an optionally
specified name and possibly distinct type.
-- A union type describes an overlapping nonempty set of member objects, each of
which has an optionally specified name and possibly distinct type.
-- A function type describes a function with specified return type. A function type is
characterized by its return type and the number and types of its parameters. A
function type is said to be derived from its return type, and if its return type is T , the
function type is sometimes called ``function returning T ''. The construction of a
function type from a return type is called ``function type derivation''.
-- A pointer type may be derived from a function type or an object type, called the
referenced type. A pointer type describes an object whose value provides a reference
to an entity of the referenced type. A pointer type derived from the referenced type T
is sometimes called ``pointer to T ''. The construction of a pointer type from a
referenced type is called ``pointer type derivation''. A pointer type is a complete
object type.
-- An atomic type describes the type designated by the construct _Atomic ( type-
name ). (Atomic types are a conditional feature that implementations need not
support; see 6.10.8.3.)
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.46)
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
46) Note that aggregate type does not include union type because an object with union type can only
contain one member at a time.
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,47) 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.48) A derived type is not qualified by the
qualifiers (if any) of the type from which it is derived.
27 Further, there is the _Atomic qualifier. The presence of the _Atomic qualifier
designates an atomic type. The size, representation, and alignment of an atomic type
need not be the same as those of the corresponding unqualified type. Therefore, this
Standard explicitly uses the phrase ``atomic, qualified or unqualified type'' whenever the
atomic version of a type is permitted along with the other qualified versions of a type.
The phrase ``qualified or unqualified type'', without specific mention of atomic, does not
include the atomic types.
28 A pointer to void shall have the same representation and alignment requirements as a
pointer to a character type.48) Similarly, pointers to qualified or unqualified versions of
compatible types shall have the same representation and alignment requirements. All
pointers to structure types shall have the same representation and alignment requirements
as each other. All pointers to union types shall have the same representation and
alignment requirements as each other. Pointers to other types need not have the same
representation or alignment requirements.
29 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-
47) See 6.7.3 regarding qualified array and function types.
48) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
qualified float'' and is a pointer to a qualified type.
30 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.49)
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.50) Such a representation is called
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.51) The value of a structure or union object is never a trap
49) 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.
50) 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.
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.52) 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.
9 Loads and stores of objects with atomic types are done with
memory_order_seq_cst semantics.
Forward references: declarations (6.7), expressions (6.5), lvalues, arrays, and function
designators (6.3.2.1), order and consistency (7.17.3).
6.2.6.2 Integer types
1 For unsigned integer types 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.53)
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;
signed char shall not have any padding 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
51) Thus, for example, structure assignment need not copy any padding bits.
52) 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.
53) 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.
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 M ) (two's complement );
-- the sign bit has the value -(2 M - 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 operands that produce such a value;
-- the +, -, *, /, and % operators where one operand 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 operands that would produce such a value is undefined.
5 The values of any padding bits are unspecified.54) 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
for signed integer types the width is one greater than the precision.
54) 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.
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 6.7.6 for declarators.55) 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 completed anywhere within their
respective translation units, 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; if one member of the pair is
declared with an alignment specifier, the other is declared with an equivalent alignment
specifier; and if one member of the pair is declared with a name, the other is declared
with the same name. For two structures, corresponding members shall be declared in the
same order. For two structures or unions, corresponding bit-fields shall have the same
widths. For two enumerations, corresponding members shall have the same values.
2 All declarations that refer to the same object or function shall have compatible type;
otherwise, the behavior is undefined.
3 A composite type can be constructed from two types that are compatible; it is a type that
is compatible with both of the two types and satisfies the following conditions:
-- If both types are array types, the following rules are applied:
<20> If one type is an array of known constant size, the composite type is an array of
that size.
<20> Otherwise, if one type is a variable length array whose size is specified by an
expression that is not evaluated, the behavior is undefined.
<20> Otherwise, if one type is a variable length array whose size is specified, the
composite type is a variable length array of that size.
<20> Otherwise, if one type is a variable length array of unspecified size, the composite
type is a variable length array of unspecified size.
<20> Otherwise, both types are arrays of unknown size and the composite type is an
array of unknown size.
The element type of the composite type is the composite type of the two element
types.
-- If 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.
55) Two types need not be identical to be compatible.
-- 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,56) if the prior declaration specifies internal or
external linkage, the type of the identifier at the later declaration becomes the composite
type.
Forward references: array declarators (6.7.6.2).
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]);
6.2.8 Alignment of objects
1 Complete object types have alignment requirements which place restrictions on the
addresses at which objects of that type may be allocated. An alignment is an
implementation-defined integer value representing the number of bytes between
successive addresses at which a given object can be allocated. An object type imposes an
alignment requirement on every object of that type: stricter alignment can be requested
using the _Alignas keyword.
2 A fundamental alignment is represented by an alignment less than or equal to the greatest
alignment supported by the implementation in all contexts, which is equal to
_Alignof (max_align_t).
3 An extended alignment is represented by an alignment greater than
_Alignof (max_align_t). It is implementation-defined whether any extended
alignments are supported and the contexts in which they are supported. A type having an
extended alignment requirement is an over-aligned type.57)
4 Alignments are represented as values of the type size_t. Valid alignments include only
those values returned by an _Alignof expression for fundamental types, plus an
additional implementation-defined set of values, which may be empty. Every valid
alignment value shall be a nonnegative integral power of two.
56) As specified in 6.2.1, the later declaration might hide the prior declaration.
57) Every over-aligned type is, or contains, a structure or union type with a member to which an extended
alignment has been applied.
5 Alignments have an order from weaker to stronger or stricter alignments. Stricter
alignments have larger alignment values. An address that satisfies an alignment
requirement also satisfies any weaker valid alignment requirement.
6 The alignment requirement of a complete type can be queried using an _Alignof
expression. The types char, signed char, and unsigned char shall have the
weakest alignment requirement.
7 Comparing alignments is meaningful and provides the obvious results:
-- Two alignments are equal when their numeric values are equal.
-- Two alignments are different when their numeric values are not equal.
-- When an alignment is larger than another it represents a stricter alignment.
6.3 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 (other than int or unsigned int)
whose integer conversion rank is less than or equal to the rank of int and
unsigned int.
-- A bit-field of type _Bool, int, signed int, or unsigned int.
If an int can represent all values of the original type (as restricted by the width, for a
bit-field), the value is converted to an int; otherwise, it is converted to an unsigned
int. These are called the integer promotions.58) 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 1.59)
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.60)
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.61)
58) 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.
59) NaNs do not compare equal to 0 and thus convert to 1.
60) The rules describe arithmetic on the mathematical value, not the value of a given type of expression.
61) 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).
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
exactly, the result is either the nearest higher or nearest lower representable value, chosen
in an implementation-defined manner. If the value being converted is outside the range of
values that can be represented, the behavior is undefined. Results of some implicit
conversions may be represented in greater range and precision than that required by the
new type (see 6.3.1.8 and 6.8.6.4).
6.3.1.5 Real floating types
1 When a value of real floating type is converted to a real floating type, if the value being
converted can be represented exactly in the new type, it is unchanged. If the value being
converted is in the range of values that can be represented but cannot be represented
exactly, the result is either the nearest higher or nearest lower representable value, chosen
in an implementation-defined manner. If the value being converted is outside the range of
values that can be represented, the behavior is undefined. Results of some implicit
conversions may be represented in greater range and precision than that required by the
new type (see 6.3.1.8 and 6.8.6.4).
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.62)
Otherwise, the integer promotions are performed on both operands. Then the
following rules are applied to the promoted operands:
If both operands have the same type, then no further conversion is needed.
Otherwise, if both operands have signed integer types or both have unsigned
integer types, the operand with the type of lesser integer conversion rank is
converted to the type of the operand with greater rank.
Otherwise, if the operand that has unsigned integer type has rank greater or
equal to the rank of the type of the other operand, then the operand with
signed integer type is converted to the type of the operand with unsigned
integer type.
Otherwise, if the type of the operand with signed integer type can represent
all of the values of the type of the operand with unsigned integer type, then
the operand with unsigned integer type is converted to the type of the
operand with signed integer type.
Otherwise, both operands are converted to the unsigned integer type
corresponding to the type of the operand with signed integer type.
2 The values of floating operands and of the results of floating expressions may be
represented in greater range and precision than that required by the type; the types are not
changed thereby.63)
62) 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).
63) The cast and assignment operators are still required to remove extra range and precision.
6.3.2 Other operands
6.3.2.1 Lvalues, arrays, and function designators
1 An lvalue is an expression (with an object type other than void) that potentially
designates an object;64) 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 _Alignof 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); this is called lvalue
conversion. If the lvalue has qualified type, the value has the unqualified version of the
type of the lvalue; additionally, if the lvalue has atomic type, the value has the non-atomic
version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the
lvalue has an incomplete type and does not have array type, the behavior is undefined. If
the lvalue designates an object of automatic storage duration that could have been
declared with the register storage class (never had its address taken), and that object
is uninitialized (not declared with an initializer and no assignment to it has been
performed prior to use), the behavior is undefined.
3 Except when it is the operand of the sizeof operator, the _Alignof 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 operator, the _Alignof operator,65) or the unary & operator, a
function designator with type ``function returning type'' is converted to an expression that
64) 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.
65) Because this conversion does not occur, the operand of the sizeof or _Alignof operator remains
a function designator and violates the constraints in 6.5.3.4.
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> (7.19), initialization (6.7.9), postfix
increment and decrement operators (6.5.2.4), prefix increment and decrement operators
(6.5.3.1), the sizeof and _Alignof operators (6.5.3.4), structure and union members
(6.5.2.3).
6.3.2.2 void
1 The (nonexistent) value of a void expression (an expression that has type void) shall not
be used in any way, and implicit or explicit conversions (except to void) shall not be
applied to such an expression. If an expression of any other type is evaluated as a void
expression, its value or designator is discarded. (A void expression is evaluated for its
side effects.)
6.3.2.3 Pointers
1 A pointer to void may be converted to or from a pointer to any object type. A pointer to
any object type may be converted to a pointer to void and back again; the result shall
compare equal to the original pointer.
2 For any qualifier q , a pointer to a non-q -qualified type may be converted to a pointer to
the q -qualified version of the type; the values stored in the original and converted pointers
shall compare equal.
3 An integer constant expression with the value 0, or such an expression cast to type
void *, is called a null pointer constant .66) 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.67)
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.
66) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19.
67) 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.
7 A pointer to an object type may be converted to a pointer to a different object type. If the
resulting pointer is not correctly aligned68) for the referenced type, the behavior is
undefined. Otherwise, when converted back again, the result shall compare equal to the
original pointer. When a pointer to an object is converted to a pointer to a character type,
the result points to the lowest addressed byte of the object. Successive increments of the
result, up to the size of the object, yield pointers to the remaining bytes of the object.
8 A pointer to a function of one type may be converted to a pointer to a function of another
type and back again; the result shall compare equal to the original pointer. If a converted
pointer is used to call a function whose type is not compatible with the referenced type,
the behavior is undefined.
Forward references: cast operators (6.5.4), equality operators (6.5.9), integer types
capable of holding object pointers (7.20.1.4), simple assignment (6.5.16.1).
68) In general, the concept ``correctly aligned'' is transitive: if a pointer to type A is correctly aligned for a
pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is
correctly aligned for a pointer to type C.
6.4 Lexical elements
Syntax
1 token:
keyword
identifier
constant
string-literal
punctuator
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each 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.69) 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.
69) 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 if unsigned
break inline void
case int volatile
char long while
const register _Alignas
continue restrict _Alignof
default return _Atomic
do short _Bool
double signed _Complex
else sizeof _Generic
enum static _Imaginary
extern struct _Noreturn
float switch _Static_assert
for typedef _Thread_local
goto 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.70)
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 D.1.71) The initial character
shall not be a universal character name designating a character whose encoding falls into
one of the ranges specified in D.2. 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.
70) One possible specification for imaginary types appears in annex G.
71) 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.
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.
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.72)
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).
72) 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.73)
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.74) Similarly, the universal
character name \unnnn designates the character whose four-digit short identifier is nnnn
(and whose eight-digit short identifier is 0000 nnnn).
73) 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).
74) 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. All floating constants of the same source form75) shall
convert to the same internal format with the same value.
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.76)
75) 1.23, 1.230, 123e-2, 123e-02, and 1.23L are all different source forms and thus need not
convert to the same internal format and value.
76) The specification for the library functions recommends more accurate conversion than required for
floating constants (see 7.22.1.3).
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 '
u' c-char-sequence '
U' 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, u, or U. 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.77)
Constraints
9 The value of an octal or hexadecimal escape sequence shall be in the range of
representable values for the corresponding type:
Prefix Corresponding Type
none unsigned char
L the unsigned type corresponding to wchar_t
u char16_t
U char32_t
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 prefixed by the letter L has type wchar_t, an integer type
defined in the <stddef.h> header; a wide character constant prefixed by the letter u or
U has type char16_t or char32_t, respectively, unsigned integer types defined in the
<uchar.h> header. The value of a wide character constant containing a single
multibyte character that maps to a single member of the extended execution character set
is the wide character corresponding to that multibyte character, as defined by the
mbtowc, mbrtoc16, or mbrtoc32 function as appropriate for its type, with an
implementation-defined current locale. The value of a wide character constant containing
more than one multibyte character or a single multibyte character that maps to multiple
members of the extended execution character set, or containing a multibyte character or
escape sequence not represented in the extended execution character set, 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.
77) 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).
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> (7.19), the mbtowc function
(7.22.7.2), Unicode utilities <uchar.h> (7.28).
6.4.5 String literals
Syntax
1 string-literal:
encoding-prefixopt " s-char-sequenceopt "
encoding-prefix:
u8
u
U
L
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
Constraints
2 A sequence of adjacent string literal tokens shall not include both a wide string literal and
a UTF-8 string literal.
Description
3 A character string literal is a sequence of zero or more multibyte characters enclosed in
double-quotes, as in "xyz". A UTF-8 string literal is the same, except prefixed by u8.
A wide string literal is the same, except prefixed by the letter L, u, or U.
4 The same considerations apply to each element of the sequence in a string literal as if it
were in an integer character constant (for a character or UTF-8 string literal) or a wide
character constant (for a wide string literal), except that the single-quote ' is
representable either by itself or by the escape sequence \', but the double-quote " shall
be represented by the escape sequence \".
Semantics
5 In translation phase 6, the multibyte character sequences specified by any sequence of
adjacent character and identically-prefixed string literal tokens are concatenated into a
single multibyte character sequence. If any of the tokens has an encoding prefix, the
resulting multibyte character sequence is treated as having the same prefix; otherwise, it
is treated as a character string literal. Whether differently-prefixed wide string literal
tokens can be concatenated and, if so, the treatment of the resulting multibyte character
sequence are implementation-defined.
6 In translation phase 7, a byte or code of value zero is appended to each multibyte
character sequence that results from a string literal or literals.78) 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 UTF-8 string literals, the array elements have type char, and are
initialized with the characters of the multibyte character sequence, as encoded in UTF-8.
For wide string literals prefixed by the letter L, the array elements have type wchar_t
and are initialized with the sequence of wide characters corresponding to the multibyte
character sequence, as defined by the mbstowcs function with an implementation-
defined current locale. For wide string literals prefixed by the letter u or U, the array
elements have type char16_t or char32_t, respectively, and are initialized with the
sequence of wide characters corresponding to the multibyte character sequence, as
defined by successive calls to the mbrtoc16, or mbrtoc32 function as appropriate for
its type, 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.
7 It is unspecified whether these arrays are distinct provided their elements have the
appropriate values. If the program attempts to modify such an array, the behavior is
undefined.
8 EXAMPLE 1 This pair of adjacent character string literals
"\x12" "3"
produces a single character string literal containing the two characters whose values are '\x12' and '3',
because escape sequences are converted into single members of the execution character set just prior to
adjacent string literal concatenation.
9 EXAMPLE 2 Each of the sequences of adjacent string literal tokens
78) A 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.
"a" "b" L"c"
"a" L"b" "c"
L"a" "b" L"c"
L"a" L"b" L"c"
is equivalent to the string literal
L"abc"
Likewise, each of the sequences
"a" "b" u"c"
"a" u"b" "c"
u"a" "b" u"c"
u"a" u"b" u"c"
is equivalent to
u"abc"
Forward references: common definitions <stddef.h> (7.19), the mbstowcs
function (7.22.8.1), Unicode utilities <uchar.h> (7.28).
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 tokens79)
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens
[ ] { } # ##
except for their spelling.80)
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
79) These tokens are sometimes called ``digraphs''.
80) 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.81) Header name
preprocessing tokens are recognized only within #include preprocessing directives and
in implementation-defined locations within #pragma directives.82)
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.
81) Thus, sequences of characters that resemble escape sequences cause undefined behavior.
82) 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.83)
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;
83) Thus, /* ... */ comments do not nest.
6.5 Expressions
1 An expression is a sequence of operators and operands that specifies computation of a
value, or that designates an object or a function, or that generates side effects, or that
performs a combination thereof. The value computations of the operands of an operator
are sequenced before the value computation of the result of the operator.
2 If a side effect on a scalar object is unsequenced relative to either a different side effect
on the same scalar object or a value computation using the value of the same scalar
object, the behavior is undefined. If there are multiple allowable orderings of the
subexpressions of an expression, the behavior is undefined if such an unsequenced side
effect occurs in any of the orderings.84)
3 The grouping of operators and operands is indicated by the syntax.85) Except as specified
later, side effects and value computations of subexpressions are unsequenced.86)
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.
84) This paragraph renders undefined statement expressions such as
i = ++i + 1;
a[i++] = i;
while allowing
i = i + 1;
a[i] = i;
85) 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.
86) In an expression that is evaluated more than once during the execution of a program, unsequenced and
indeterminately sequenced evaluations of its subexpressions need not be performed consistently in
different evaluations.
6 The effective type of an object for an access to its stored value is the declared type of the
object, if any.87) 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
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:88)
-- 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 a single
operation, thereby omitting rounding errors implied by the source code and the
expression evaluation method.89) The FP_CONTRACT pragma in <math.h> provides a
way to disallow contracted expressions. Otherwise, whether and how expressions are
contracted is implementation-defined.90)
Forward references: the FP_CONTRACT pragma (7.12.2), copying functions (7.24.2).
87) Allocated objects have no declared type.
88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.
89) The intermediate operations in the contracted expression are evaluated as if to infinite range and
precision, while the final operation is rounded to the format determined by the expression evaluation
method. A contracted expression might also omit the raising of floating-point exceptions.
90) 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 )
generic-selection
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).91)
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.
6 A generic selection is a primary expression. Its type and value depend on the selected
generic association, as detailed in the following subclause.
Forward references: declarations (6.7).
6.5.1.1 Generic selection
Syntax
1 generic-selection:
_Generic ( assignment-expression , generic-assoc-list )
generic-assoc-list:
generic-association
generic-assoc-list , generic-association
generic-association:
type-name : assignment-expression
default : assignment-expression
91) Thus, an undeclared identifier is a violation of the syntax.
Constraints
2 A generic selection shall have no more than one default generic association. The type
name in a generic association shall specify a complete object type other than a variably
modified type. No two generic associations in the same generic selection shall specify
compatible types. The controlling expression of a generic selection shall have type
compatible with at most one of the types named in its generic association list. If a
generic selection has no default generic association, its controlling expression shall
have type compatible with exactly one of the types named in its generic association list.
Semantics
3 The controlling expression of a generic selection is not evaluated. If a generic selection
has a generic association with a type name that is compatible with the type of the
controlling expression, then the result expression of the generic selection is the
expression in that generic association. Otherwise, the result expression of the generic
selection is the expression in the default generic association. None of the expressions
from any other generic association of the generic selection is evaluated.
4 The type and value of a generic selection are identical to those of its result expression. It
is an lvalue, a function designator, or a void expression if its result expression is,
respectively, an lvalue, a function designator, or a void expression.
5 EXAMPLE The cbrt type-generic macro could be implemented as follows:
#define cbrt(X) _Generic((X), \
long double: cbrtl, \
default: cbrt, \
float: cbrtf \
)(X)
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 , }
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 complete object type'', the other
expression shall have integer type, and the result has type ``type''.
Semantics
2 A postfix expression followed by an expression in square brackets [] is a subscripted
designation of an element of an array object. The definition of the subscript operator []
is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that
apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the
initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th
element of E1 (counting from zero).
3 Successive subscript operators designate an element of a multidimensional array object.
If E is an n-dimensional array ( n 2) with dimensions i <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 referenced ( n - 1)-dimensional
array, which itself is converted into a pointer if used as other than an lvalue. It follows
from this that arrays are stored in row-major order (last subscript varies fastest).
4 EXAMPLE Consider the array object defined by the declaration
int x[3][5];
Here x is a 3 <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 (6.7.6.2).
6.5.2.2 Function calls
Constraints
1 The expression that denotes the called function92) shall have type pointer to function
returning void or returning a complete 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 complete object type. In preparing for the call
to a function, the arguments are evaluated, and each parameter is assigned the value of the
corresponding argument.93)
5 If the expression that denotes the called function has type pointer to function returning an
object type, the function call expression has the same type as that object type, and has the
value determined as specified in 6.8.6.4. Otherwise, the function call has type void.
6 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:
-- 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;
92) Most often, this is the result of converting an identifier that is a function designator.
93) 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.
-- 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 There is a sequence point after the evaluations of the function designator and the actual
arguments but before the actual call. Every evaluation in the calling function (including
other function calls) that is not otherwise specifically sequenced before or after the
execution of the body of the called function is indeterminately sequenced with respect to
the execution of the called function.94)
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) (6.7.6.3), function
definitions (6.9.1), the return statement (6.8.6.4), simple assignment (6.5.16.1).
6.5.2.3 Structure and union members
Constraints
1 The first operand of the . operator shall have an atomic, qualified, or unqualified
structure or union type, and the second operand shall name a member of that type.
2 The first operand of the -> operator shall have type ``pointer to atomic, qualified, or
unqualified structure'' or ``pointer to atomic, qualified, or unqualified union'', and the
second operand shall name a member of the type pointed to.
94) In other words, function executions do not ``interleave'' with each other.
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,95) 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.96) If the first expression is a pointer to
a qualified type, the result has the so-qualified version of the type of the designated
member.
5 Accessing a member of an atomic structure or union object results in undefined
behavior.97)
6 One special guarantee is made in order to simplify the use of unions: if a union contains
several structures that share a common initial sequence (see below), and if the union
object currently contains one of these structures, it is permitted to inspect the common
initial part of any of them anywhere that a declaration of the completed type of the union
is visible. Two structures share a common initial sequence if corresponding members
have compatible types (and, for bit-fields, the same widths) for a sequence of one or more
initial members.
7 EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or
union, f().x is a valid postfix expression but is not an lvalue.
8 EXAMPLE 2 In:
struct s { int i; const int ci; };
struct s s;
const struct s cs;
volatile struct s vs;
the various members have the types:
95) If the member used to read the contents of a union object is not the same as the member last used to
store a value in the object, the appropriate part of the object representation of the value is reinterpreted
as an object representation in the new type as described in 6.2.6 (a process sometimes called ``type
punning''). This might be a trap representation.
96) 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.
97) For example, a data race would occur if access to the entire structure or union in one thread conflicts
with access to a member from another thread, where at least one access is a modification. Members
can be safely accessed using a non-atomic object which is assigned to or from the atomic object.
s.i int
s.ci const int
cs.i const int
cs.ci const int
vs.i volatile int
vs.ci volatile const int
9 EXAMPLE 3 The following is a valid fragment:
union {
struct {
int alltypes;
} n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
} u;
u.nf.type = 1;
u.nf.doublenode = 3.14;
/* ... */
if (u.n.alltypes == 1)
if (sin(u.nf.doublenode) == 0.0)
/* ... */
The following is not a valid fragment (because the union type is not visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f(struct t1 *p1, struct t2 *p2)
{
if (p1->m < 0)
p2->m = -p2->m;
return p1->m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
} u;
/* ... */
return f(&u.s1, &u.s2);
}
Forward references: address and indirection operators (6.5.3.2), structure and union
specifiers (6.7.2.1).
6.5.2.4 Postfix increment and decrement operators
Constraints
1 The operand of the postfix increment or decrement operator shall have atomic, qualified,
or 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. As a side effect, the
value of the operand object is incremented (that is, the value 1 of the appropriate type is
added to it). See the discussions of additive operators and compound assignment for
information on constraints, types, and conversions and the effects of operations on
pointers. The value computation of the result is sequenced before the side effect of
updating the stored value of the operand. With respect to an indeterminately-sequenced
function call, the operation of postfix ++ is a single evaluation. Postfix ++ on an object
with atomic type is a read-modify-write operation with memory_order_seq_cst
memory order semantics.98)
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 a complete object type or an array of unknown size, but not a
variable length array type.
2 All the constraints for initializer lists in 6.7.9 also apply to compound literals.
Semantics
3 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
98) Where a pointer to an atomic object can be formed and E has integer type, E++ is equivalent to the
following code sequence where T is the type of E:
T *addr = &E;
T old = *addr;
T new;
do {
new = old + 1;
} while (!atomic_compare_exchange_strong(addr, &old, new));
with old being the result of the operation.
Special care must be taken if E has floating type; see 6.5.16.2.
value is given by the initializer list.99)
4 If the type name specifies an array of unknown size, the size is determined by the
initializer list as specified in 6.7.9, 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.
5 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.
6 All the semantic rules for initializer lists in 6.7.9 also apply to compound literals.100)
7 String literals, and compound literals with const-qualified types, need not designate
distinct objects.101)
8 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.
9 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.
10 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});
99) 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.
100) For example, subobjects without explicit initializers are initialized to zero.
101) This allows implementations to share storage for string literals and constant compound literals with
the same or overlapping representations.
Or, if drawline instead expected pointers to struct point:
drawline(&(struct point){.x=1, .y=1},
&(struct point){.x=3, .y=4});
11 EXAMPLE 4 A read-only compound literal can be specified through constructions like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
12 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.
13 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.
14 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);
15 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.
16 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 (6.7.7), initialization (6.7.9).
6.5.3 Unary operators
Syntax
1 unary-expression:
postfix-expression
++ unary-expression
-- unary-expression
unary-operator cast-expression
sizeof unary-expression
sizeof ( type-name )
_Alignof ( type-name )
unary-operator: one of
& * + - ~ !
6.5.3.1 Prefix increment and decrement operators
Constraints
1 The operand of the prefix increment or decrement operator shall have atomic, qualified,
or 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.102)
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).
102) 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 and _Alignof operators
Constraints
1 The sizeof operator shall not be applied to an expression that has function type or an
incomplete type, to the parenthesized name of such a type, or to an expression that
designates a bit-field member. The _Alignof operator shall not be applied to a
function type or an incomplete type.
Semantics
2 The sizeof operator yields the size (in bytes) of its operand, which may be an
expression or the parenthesized name of a type. The size is determined from the type of
the operand. The result is an integer. If the type of the operand is a variable length array
type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an
integer constant.
3 The _Alignof operator yields the alignment requirement of its operand type. The
operand is not evaluated and the result is an integer constant. When applied to an array
type, the result is the alignment requirement of the element type.
4 When sizeof is applied to an operand that has type char, unsigned char, or
signed char, (or a qualified version thereof) the result is 1. When applied to an
operand that has array type, the result is the total number of bytes in the array.103) When
applied to an operand that has structure or union type, the result is the total number of
bytes in such an object, including internal and trailing padding.
5 The value of the result of both operators is implementation-defined, and its type (an
unsigned integer type) is size_t, defined in <stddef.h> (and other headers).
6 EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage
allocators and I/O systems. A storage-allocation function might accept a size (in bytes) of an object to
allocate and return a pointer to void. For example:
extern void *alloc(size_t);
double *dp = alloc(sizeof *dp);
The implementation of the alloc function should ensure that its return value is aligned suitably for
conversion to a pointer to double.
7 EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:
sizeof array / sizeof array[0]
8 EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a
function:
#include <stddef.h>
103) 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).
size_t fsize3(int n)
{
char b[n+3]; // variable length array
return sizeof b; // execution time sizeof
}
int main()
{
size_t size;
size = fsize3(10); // fsize3 returns 13
return 0;
}
Forward references: common definitions <stddef.h> (7.19), declarations (6.7),
structure and union specifiers (6.7.2.1), type names (6.7.7), array declarators (6.7.6.2).
6.5.4 Cast operators
Syntax
1 cast-expression:
unary-expression
( type-name ) cast-expression
Constraints
2 Unless the type name specifies a void type, the type name shall specify atomic, qualified,
or unqualified scalar type, and the operand shall have scalar type.
3 Conversions that involve pointers, other than where permitted by the constraints of 6.5.16.1,
shall be specified by means of an explicit cast.
4 A pointer type shall not be converted to any floating type. A floating type shall not be
converted to any pointer type.
Semantics
5 Preceding an expression by a parenthesized type name converts the value of the
expression to the named type. This construction is called a cast .104) A cast that specifies
no conversion has no effect on the type or value of an expression.
6 If the value of the expression is represented with greater range or precision than required
by the type named by the cast (6.3.1.8), then the cast specifies a conversion even if the
type of the expression is the same as the named type and removes any extra range and
precision.
Forward references: equality operators (6.5.9), function declarators (including
prototypes) (6.7.6.3), simple assignment (6.5.16.1), type names (6.7.7).
104) 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.105) If the quotient a/b is representable, the expression
(a/b)*b + a%b shall equal a; otherwise, the behavior of both a/b and a%b is
undefined.
6.5.6 Additive operators
Syntax
1 additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression - multiplicative-expression
Constraints
2 For addition, either both operands shall have arithmetic type, or one operand shall be a
pointer to a complete object type and the other shall have integer type. (Incrementing is
equivalent to adding 1.)
3 For subtraction, one of the following shall hold:
105) This is often called ``truncation toward zero''.
-- both operands have arithmetic type;
-- both operands are pointers to qualified or unqualified versions of compatible complete
object types; or
-- the left operand is a pointer to a complete object type and the right operand has
integer type.
(Decrementing is equivalent to subtracting 1.)
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.106)
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 (6.7.6.2), common definitions <stddef.h>
(7.19).
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
106) 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.
greater than or equal to the width of the promoted left operand, the behavior is undefined.
4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with
zeros. If E1 has an unsigned type, the value of the result is E1 <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; or
-- both operands are pointers to qualified or unqualified versions of compatible object
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 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.107) 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 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.108) 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.
107) 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''.
108) 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 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.109)
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).
109) 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;
if the second operand is evaluated, there is a sequence point between the evaluations of
the first and second operands. If the first operand compares equal to 0, the second
operand is not evaluated.
6.5.14 Logical OR operator
Syntax
1 logical-OR-expression:
logical-AND-expression
logical-OR-expression || logical-AND-expression
Constraints
2 Each of the operands shall have scalar type.
Semantics
3 The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it
yields 0. The result has type int.
4 Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the
second operand is evaluated, there is a sequence point between the evaluations of the first
and second operands. If the first operand compares unequal to 0, the second operand is
not evaluated.
6.5.15 Conditional operator
Syntax
1 conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
Constraints
2 The first operand shall have scalar type.
3 One of the following shall hold for the second and third 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 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 between its evaluation and the
evaluation of the second or third operand (whichever is evaluated). The second operand
is evaluated only if the first compares unequal to 0; the third operand is evaluated only if
the first compares equal to 0; the result is the value of the second or third operand
(whichever is evaluated), converted to the type described below.110)
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 referenced 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
pointer to an appropriately qualified version of void.
110) A conditional expression does not yield an lvalue.
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,111) but is not
an lvalue. The type of an assignment expression is the type the left operand would have
after lvalue conversion. The side effect of updating the stored value of the left operand is
sequenced after the value computations of the left and right operands. The evaluations of
the operands are unsequenced.
111) The implementation is permitted to read the object to determine the value but is not required to, even
when the object has volatile-qualified type.
6.5.16.1 Simple assignment
Constraints
1 One of the following shall hold:112)
-- the left operand has atomic, qualified, or unqualified arithmetic type, and the right has
arithmetic type;
-- the left operand has an atomic, qualified, or unqualified version of a structure or union
type compatible with the type of the right;
-- the left operand has atomic, qualified, or unqualified pointer type, and (considering
the type the left operand would have after lvalue conversion) both operands are
pointers to qualified or unqualified versions of compatible types, and the type pointed
to by the left has all the qualifiers of the type pointed to by the right;
-- the left operand has atomic, qualified, or unqualified pointer type, and (considering
the type the left operand would have after lvalue conversion) one operand is a pointer
to an object type, and the other is a pointer to a qualified or unqualified version of
void, and the type pointed to by the left has all the qualifiers of the type pointed to
by the right;
-- the left operand is an atomic, qualified, or unqualified pointer, and the right is a null
pointer constant; or
-- the left operand has type atomic, qualified, or unqualified _Bool, and the right is a
pointer.
Semantics
2 In simple assignment (=), the value of the right operand is converted to the type of the
assignment expression and replaces the value stored in the object designated by the left
operand.
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
112) 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).
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
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 an atomic, qualified, or
unqualified pointer to a complete object type, and the right shall have integer type; or the
left operand shall have atomic, qualified, or unqualified arithmetic type, and the right
shall have arithmetic type.
2 For the other operators, the left operand shall have atomic, qualified, or unqualified
arithmetic type, and (considering the type the left operand would have after lvalue
conversion) each operand shall have arithmetic type consistent with those allowed by the
corresponding binary operator.
Semantics
3 A compound assignment of the form E1 op = E2 is equivalent to the simple assignment
expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with
respect to an indeterminately-sequenced function call, the operation of a compound
assignment is a single evaluation. If E1 has an atomic type, compound assignment is a
read-modify-write operation with memory_order_seq_cst memory order
semantics.113)
113) Where a pointer to an atomic object can be formed and E1 and E2 have integer type, this is equivalent
to the following code sequence where T1 is the type of E1 and T2 is the type of E2:
T1 *addr = &E1;
T2 val = (E2);
T1 old = *addr;
T1 new;
do {
new = old op val;
} while (!atomic_compare_exchange_strong(addr, &old, new));
with new being the result of the operation.
If E1 or E2 has floating type, then exceptional conditions or floating-point exceptions encountered
during discarded evaluations of new should also be discarded in order to satisfy the equivalence of E1
op = E2 and E1 = E1 op (E2). For example, if annex F is in effect, the floating types involved have
IEC 60559 formats, and FLT_EVAL_METHOD is 0, the equivalent code would be:
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
/* ... */
fenv_t fenv;
T1 *addr = &E1;
T2 val = E2;
T1 old = *addr;
T1 new;
feholdexcept(&fenv);
for (;;) {
new = old op val;
if (atomic_compare_exchange_strong(addr, &old, new))
break;
feclearexcept(FE_ALL_EXCEPT);
}
feupdateenv(&fenv);
If FLT_EVAL_METHOD is not 0, then T2 must be a type with the range and precision to which E2 is
evaluated in order to satisfy the equivalence.
6.5.17 Comma operator
Syntax
1 expression:
assignment-expression
expression , assignment-expression
Semantics
2 The left operand of a comma operator is evaluated as a void expression; there is a
sequence point between its evaluation and that of the right operand. Then the right
operand is evaluated; the result has its type and value.114)
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 (6.7.9).
114) 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.115)
4 Each constant expression shall evaluate to a constant that is in the range of representable
values for its type.
Semantics
5 An expression that evaluates to a constant is required in several contexts. If a floating
expression is evaluated in the translation environment, the arithmetic range and precision
shall be at least as great as if the expression were being evaluated in the execution
environment.116)
6 An integer constant expression117) shall have integer type and shall only have operands
that are integer constants, enumeration constants, character constants, sizeof
expressions whose results are integer constants, _Alignof expressions, 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 or _Alignof 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,
115) The operand of a sizeof or _Alignof operator is usually not evaluated (6.5.3.4).
116) The use of evaluation formats as characterized by FLT_EVAL_METHOD also applies to evaluation in
the translation environment.
117) An integer constant expression is required in a number of contexts such as the size of a bit-field
member of a structure, the value of an enumeration constant, and the size of a non-variable length
array. Further constraints that apply to the integer constant expressions used in conditional-inclusion
preprocessing directives are discussed in 6.10.1.
-- a null pointer constant,
-- an address constant, or
-- an address constant for a complete object type plus or minus an integer constant
expression.
8 An arithmetic constant expression shall have arithmetic type and shall only have
operands that are integer constants, floating constants, enumeration constants, character
constants, sizeof expressions whose results are integer constants, and _Alignof
expressions. Cast operators in an arithmetic constant expression shall only convert
arithmetic types to arithmetic types, except as part of an operand to a sizeof or
_Alignof operator.
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.118)
Forward references: array declarators (6.7.6.2), initialization (6.7.9).
118) 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 ;
static_assert-declaration
declaration-specifiers:
storage-class-specifier declaration-specifiersopt
type-specifier declaration-specifiersopt
type-qualifier declaration-specifiersopt
function-specifier declaration-specifiersopt
alignment-specifier declaration-specifiersopt
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
Constraints
2 A declaration other than a static_assert declaration shall declare at least a declarator
(other than the parameters of a function or the members of a structure or union), a tag, or
the members of an enumeration.
3 If an identifier has no linkage, there shall be no more than one declaration of the identifier
(in a declarator or type specifier) with the same scope and in the same name space, except
that:
-- a typedef name may be redefined to denote the same type as it currently does,
provided that type is not a variably modified type;
-- tags may be redeclared 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;119)
-- for an enumeration constant, is the (only) declaration of the identifier;
-- for a typedef name, is the first (or 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
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 6.7.6.3) that is required to be complete.
Forward references: declarators (6.7.6), enumeration specifiers (6.7.2.2), initialization
(6.7.9), type names (6.7.7), type qualifiers (6.7.3).
6.7.1 Storage-class specifiers
Syntax
1 storage-class-specifier:
typedef
extern
static
_Thread_local
auto
register
Constraints
2 At most, one storage-class specifier may be given in the declaration specifiers in a
declaration, except that _Thread_local may appear with static or extern.120)
3 In the declaration of an object with block scope, if the declaration specifiers include
_Thread_local, they shall also include either static or extern. If
_Thread_local appears in any declaration of an object, it shall be present in every
declaration of that object.
4 _Thread_local shall not appear in the declaration specifiers of a function declaration.
119) Function definitions have a different syntax, described in 6.9.1.
120) See ``future language directions'' (6.11.5).
Semantics
5 The typedef specifier is called a ``storage-class specifier'' for syntactic convenience
only; it is discussed in 6.7.8. The meanings of the various linkages and storage durations
were discussed in 6.2.2 and 6.2.4.
6 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.121)
7 The declaration of an identifier for a function that has block scope shall have no explicit
storage-class specifier other than extern.
8 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 (6.7.8).
121) 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 operators that can be applied to an array declared with storage-class specifier
register are sizeof and _Alignof.
6.7.2 Type specifiers
Syntax
1 type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_Bool
_Complex
atomic-type-specifier
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 multisets (delimited by commas, when there
is more than one multiset per item); the type specifiers may occur in any order, possibly
intermixed with the other declaration specifiers.
-- void
-- char
-- signed char
-- unsigned char
-- short, signed short, short int, or signed short int
-- unsigned short, or unsigned short int
-- int, signed, or signed int
-- unsigned, or unsigned int
-- long, signed long, long int, or signed long int
-- unsigned long, or unsigned long int
-- long long, signed long long, long long int, or
signed long long int
-- unsigned long long, or unsigned long long int
-- float
-- double
-- long double
-- _Bool
-- float _Complex
-- double _Complex
-- long double _Complex
-- atomic type specifier
-- struct or union specifier
-- enum specifier
-- typedef name
3 The type specifier _Complex shall not be used if the implementation does not support
complex types (see 6.10.8.3).
Semantics
4 Specifiers for structures, unions, enumerations, and atomic types are discussed in 6.7.2.1
through 6.7.2.4. Declarations of typedef names are discussed in 6.7.8. The
characteristics of the other types are discussed in 6.2.5.
5 Each of the comma-separated multisets designates the same type, except that for bit-
fields, it is implementation-defined whether the specifier int designates the same type as
signed int or the same type as unsigned int.
Forward references: atomic type specifiers (6.7.2.4), enumeration specifiers (6.7.2.2),
structure and union specifiers (6.7.2.1), tags (6.7.2.3), type definitions (6.7.8).
6.7.2.1 Structure and union specifiers
Syntax
1 struct-or-union-specifier:
struct-or-union 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-listopt ;
static_assert-declaration
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 struct-declaration that does not declare an anonymous structure or anonymous union
shall contain a struct-declarator-list.
3 A structure or union shall not contain a member with incomplete or function type (hence,
a structure shall not contain an instance of itself, but may contain a pointer to an instance
of itself), except that the last member of a structure with more than one named member
may have incomplete array type; such a structure (and any union containing, possibly
recursively, a member that is such a structure) shall not be a member of a structure or an
element of an array.
4 The expression that specifies the width of a bit-field shall be an integer constant
expression with a nonnegative value that does not exceed the width of an object of the
type that would be specified were the colon and expression omitted.122) If the value is
zero, the declaration shall have no declarator.
5 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed
int, unsigned int, or some other implementation-defined type. It is
implementation-defined whether atomic types are permitted.
122) While the number of bits in a _Bool object is at least CHAR_BIT, the width (number of sign and
value bits) of a _Bool may be just 1 bit.
Semantics
6 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.
7 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.
8 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 does not contain any
named members, either directly or via an anonymous structure or anonymous union, the
behavior is undefined. The type is incomplete until immediately after the } that
terminates the list, and complete thereafter.
9 A member of a structure or union may have any complete object type other than a
variably modified type.123) 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 ;124) its width is preceded by a colon.
10 A bit-field is interpreted as having a signed or unsigned integer type consisting of the
specified number of bits.125) If the value 0 or 1 is stored into a nonzero-width bit-field of
type _Bool, the value of the bit-field shall compare equal to the value stored; a _Bool
bit-field has the semantics of a _Bool.
11 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.
12 A bit-field declaration with no declarator, but only a colon and a width, indicates an
unnamed bit-field.126) As a special case, a bit-field structure member with a width of 0
123) A structure or union cannot contain a member with a variably modified type because member names
are not ordinary identifiers as defined in 6.2.3.
124) 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.
125) 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.
126) An unnamed bit-field structure member is useful for padding to conform to externally imposed
layouts.
indicates that no further bit-field is to be packed into the unit in which the previous bit-
field, if any, was placed.
13 An unnamed member whose type specifier is a structure specifier with no tag is called an
anonymous structure; an unnamed member whose type specifier is a union specifier with
no tag is called an anonymous union. The members of an anonymous structure or union
are considered to be members of the containing structure or union. This applies
recursively if the containing structure or union is also anonymous.
14 Each non-bit-field member of a structure or union object is aligned in an implementation-
defined manner appropriate to its type.
15 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.
16 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.
17 There may be unnamed padding at the end of a structure or union.
18 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.
19 EXAMPLE 1 The following illustrates anonymous structures and unions:
struct v {
union { // anonymous union
struct { int i, j; }; // anonymous structure
struct { long k, l; } w;
};
int m;
} v1;
v1.i = 2; // valid
v1.k = 3; // invalid: inner structure is not anonymous
v1.w.k = 5; // valid
20 EXAMPLE 2 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).
21 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.
22 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.
23 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;
24 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
25 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.
26 EXAMPLE 3 Because members of anonymous structures and unions are considered to be members of the
containing structure or union, struct s in the following example has more than one named member and
thus the use of a flexible array member is valid:
struct s {
struct { int i; };
int a[];
};
Forward references: declarators (6.7.6), tags (6.7.2.3).
6.7.2.2 Enumeration specifiers
Syntax
1 enum-specifier:
enum 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.127) 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,128) but shall be
capable of representing the values of all the members of the enumeration. The
enumerated type is incomplete until immediately after the } that terminates the list of
enumerator declarations, and complete thereafter.
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.
127) 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.
128) An implementation may delay the choice of which integer type until all enumeration constants have
been seen.
Semantics
4 All declarations of structure, union, or enumerated types that have the same scope and
use the same tag declare the same type. Irrespective of whether there is a tag or what
other declarations of the type are in the same translation unit, the type is incomplete129)
until immediately after 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 identifieropt { enumerator-list }
or
enum identifieropt { enumerator-list , }
declares a structure, union, or enumerated type. The list defines the structure content ,
union content , or enumeration content . If an identifier is provided,130) 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.131)
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.131)
129) 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.
130) 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.
131) A similar construction with enum does not exist.
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:
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 (6.7.6), type definitions (6.7.8).
6.7.2.4 Atomic type specifiers
Syntax
1 atomic-type-specifier:
_Atomic ( type-name )
Constraints
2 Atomic type specifiers shall not be used if the implementation does not support atomic
types (see 6.10.8.3).
3 The type name in an atomic type specifier shall not refer to an array type, a function type,
an atomic type, or a qualified type.
Semantics
4 The properties associated with atomic types are meaningful only for expressions that are
lvalues. If the _Atomic keyword is immediately followed by a left parenthesis, it is
interpreted as a type specifier (with a type name), not as a type qualifier.
6.7.3 Type qualifiers
Syntax
1 type-qualifier:
const
restrict
volatile
_Atomic
Constraints
2 Types other than pointer types whose referenced type is an object type shall not be
restrict-qualified.
3 The type modified by the _Atomic qualifier shall not be an array type or a function
type.
Semantics
4 The properties associated with qualified types are meaningful only for expressions that
are lvalues.132)
5 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. If other qualifiers appear along with the _Atomic qualifier in a specifier-qualifier-
132) 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.
list , the resulting type is the so-qualified atomic type.
6 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.133)
7 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.134) What constitutes an access to an object that
has volatile-qualified type is implementation-defined.
8 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.135) 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).
9 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.136)
10 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.
11 EXAMPLE 1 An object declared
extern const volatile int real_time_clock;
133) 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).
134) 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.
135) 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.
136) Both of these can occur through the use of typedefs.
may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.
12 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 *''
13 EXAMPLE 3 The declaration
_Atomic volatile int *p;
specifies that p has the type ``pointer to volatile atomic int'', a pointer to a volatile-qualified atomic type.
6.7.3.1 Formal definition of restrict
1 Let D be a declaration of an ordinary identifier that provides a means of designating an
object P as a restrict-qualified pointer to type T.
2 If D appears inside a block and does not have storage class extern, let B denote the
block. If D appears in the list of parameter declarations of a function definition, let B
denote the associated block. Otherwise, let B denote the block of main (or the block of
whatever function is called at program startup in a freestanding environment).
3 In what follows, a pointer expression E is said to be based on object P if (at some
sequence point in the execution of B prior to the evaluation of E) modifying P to point to
a copy of the array object into which it formerly pointed would change the value of E.137)
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
137) 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.
object P2, associated with block B2, then either the execution of B2 shall begin before
the execution of B, or the execution of B2 shall end prior to the assignment. If these
requirements are not met, then the behavior is undefined.
5 Here an execution of B means that portion of the execution of the program that would
correspond to the lifetime of an object with scalar type and automatic storage duration
associated with B.
6 A translator is free to ignore any or all aliasing implications of uses of restrict.
7 EXAMPLE 1 The file scope declarations
int * restrict a;
int * restrict b;
extern int c[];
assert that if an object is accessed using one of a, b, or c, and that object is modified anywhere in the
program, then it is never accessed using either of the other two.
8 EXAMPLE 2 The function parameter declarations in the following example
void f(int n, int * restrict p, int * restrict q)
{
while (n-- > 0)
*p++ = *q++;
}
assert that, during each execution of the function, if an object is accessed through one of the pointer
parameters, then it is not also accessed through the other.
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
_Noreturn
Constraints
2 Function specifiers shall be used only in the declaration of an identifier for a function.
3 An inline definition of a function with external linkage shall not contain a definition of a
modifiable object with static or thread storage duration, and shall not contain a reference
to an identifier with internal linkage.
4 In a hosted environment, no function specifier(s) shall appear in a declaration of main.
Semantics
5 A function specifier may appear more than once; the behavior is the same as if it
appeared only once.
6 A function declared with an inline function specifier is an inline function. Making a
function an inline function suggests that calls to the function be as fast as possible.138)
The extent to which such suggestions are effective is implementation-defined.139)
7 Any function with internal linkage can be an inline function. For a function with external
linkage, the following restrictions apply: If a function is declared with an inline
function specifier, then it shall also be defined in the same translation unit. If all of the
file scope declarations for a function in a translation unit include the inline function
specifier without extern, then the definition in that translation unit is an inline
definition. An inline definition does not provide an external definition for the function,
and does not forbid an external definition in another translation unit. An inline definition
provides an alternative to an external definition, which a translator may use to implement
any call to the function in the same translation unit. It is unspecified whether a call to the
function uses the inline definition or the external definition.140)
8 A function declared with a _Noreturn function specifier shall not return to its caller.
Recommended practice
9 The implementation should produce a diagnostic message for a function declared with a
_Noreturn function specifier that appears to be capable of returning to its caller.
10 EXAMPLE 1 The declaration of an inline function with external linkage can result in either an external
definition, or a definition available for use only within the translation unit. A file scope declaration with
extern creates an external definition. The following example shows an entire translation unit.
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
138) 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.
139) For example, an implementation might never perform inline substitution, or might only perform inline
substitutions to calls in the scope of an inline declaration.
140) 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.
double convert(int is_fahr, double temp)
{
/* A translator may perform inline substitutions */
return is_fahr ? cels(temp) : fahr(temp);
}
11 Note that the definition of fahr is an external definition because fahr is also declared with extern, but
the definition of cels is an inline definition. Because cels has external linkage and is referenced, an
external definition has to appear in another translation unit (see 6.9); the inline definition and the external
definition are distinct and either may be used for the call.
12 EXAMPLE 2
_Noreturn void f () {
abort(); // ok
}
_Noreturn void g (int i) { // causes undefined behavior if i <= 0
if (i > 0) abort();
}
Forward references: function definitions (6.9.1).
6.7.5 Alignment specifier
Syntax
1 alignment-specifier:
_Alignas ( type-name )
_Alignas ( constant-expression )
Constraints
2 An alignment attribute shall not be specified in a declaration of a typedef, or a bit-field, or
a function, or a parameter, or an object declared with the register storage-class
specifier.
3 The constant expression shall be an integer constant expression. It shall evaluate to a
valid fundamental alignment, or to a valid extended alignment supported by the
implementation in the context in which it appears, or to zero.
4 The combined effect of all alignment attributes in a declaration shall not specify an
alignment that is less strict than the alignment that would otherwise be required for the
type of the object or member being declared.
Semantics
5 The first form is equivalent to _Alignas (_Alignof (type-name)).
6 The alignment requirement of the declared object or member is taken to be the specified
alignment. An alignment specification of zero has no effect.141) When multiple
alignment specifiers occur in a declaration, the effective alignment requirement is the
strictest specified alignment.
7 If the definition of an object has an alignment specifier, any other declaration of that
object shall either specify equivalent alignment or have no alignment specifier. If the
definition of an object does not have an alignment specifier, any other declaration of that
object shall also have no alignment specifier. If declarations of an object in different
translation units have different alignment specifiers, the behavior is undefined.
6.7.6 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
141) An alignment specification of zero also does not affect other alignment specifications in the same
declaration.
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 void type, either
directly or via one or more typedefs.
Forward references: array declarators (6.7.6.2), type definitions (6.7.8).
6.7.6.1 Pointer declarators
Semantics
1 If, in the declaration ``T D1'', D1 has the form
* 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''.
6.7.6.2 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 If an identifier is declared as having a variably modified type, it shall be an ordinary
identifier (as defined in 6.2.3), have no linkage, and have either block scope or function
prototype scope. If an identifier is declared to be an object with static or thread storage
duration, it shall not have a variable length array type.
Semantics
3 If, in the declaration ``T D1'', D1 has one of the forms:
D[ type-qualifier-listopt assignment-expressionopt ]
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 ''.142)
(See 6.7.6.3 for the meaning of the optional type qualifiers and the keyword static.)
4 If the size is not present, the array type is an incomplete type. If the size is * instead of
being an expression, the array type is a variable length array type of unspecified size,
which can only be used in declarations or type names with function prototype scope;143)
such arrays are nonetheless complete types. If the size is an integer constant expression
and the element type has a known constant size, the array type is not a variable length
array type; otherwise, the array type is a variable length array type. (Variable length
arrays are a conditional feature that implementations need not support; see 6.10.8.3.)
5 If the size is an expression that is not an integer constant expression: if it occurs in a
declaration at function prototype scope, it is treated as if it were replaced by *; otherwise,
each time it is evaluated it shall have a value greater than zero. The size of each instance
of a variable length array type does not change during its lifetime. Where a size
expression is part of the operand of a 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
142) When several ``array of'' specifications are adjacent, a multidimensional array is declared.
143) Thus, * can be used only in function declarations that are not definitions (see 6.7.6.3).
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
}
10 EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or
function prototype scope. Array objects declared with the _Thread_local, static, or extern
storage-class specifier cannot have a variable length array (VLA) type. However, an object declared with
the static storage-class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all
identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be members of
structures or unions.
extern int n;
int A[n]; // invalid: file scope VLA
extern int (*p2)[n]; // invalid: file scope VM
int B[100]; // valid: file scope but not VM
void fvla(int m, int C[m][m]); // valid: VLA with prototype scope
void fvla(int m, int C[m][m]) // valid: adjusted to auto pointer to VLA
{
typedef int VLA[m][m]; // valid: block scope typedef VLA
struct tag {
int (*y)[n]; // invalid: y not ordinary identifier
int z[n]; // invalid: z not ordinary identifier
};
int D[m]; // valid: auto VLA
static int E[m]; // invalid: static block scope VLA
extern int F[m]; // invalid: F has linkage and is VLA
int (*s)[m]; // valid: auto pointer to VLA
extern int (*r)[m]; // invalid: r has linkage and points to VLA
static int (*q)[m] = &B; // valid: q is a static block pointer to VLA
}
Forward references: function declarators (6.7.6.3), function definitions (6.9.1),
initialization (6.7.9).
6.7.6.3 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.144)
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.
144) The macros defined in the <stdarg.h> header (7.16) may be used to access arguments that
correspond to the ellipsis.
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.145)
15 For two function types to be compatible, both shall specify compatible return types.146)
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
145) See ``future language directions'' (6.11.6).
146) If both function types are ``old style'', parameter types are not compared.
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 (6.7.7).
6.7.7 Type names
Syntax
1 type-name:
specifier-qualifier-list abstract-declaratoropt
abstract-declarator:
pointer
pointeropt direct-abstract-declarator
direct-abstract-declarator:
( abstract-declarator )
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.147)
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
147) 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.
returning an int, and (h) array of an unspecified number of constant pointers to functions, each with one
parameter that has type unsigned int and an unspecified number of other parameters, returning an
int.
6.7.8 Type definitions
Syntax
1 typedef-name:
identifier
Constraints
2 If a typedef name specifies a variably modified type then it shall have block scope.
Semantics
3 In a declaration whose storage-class specifier is typedef, each declarator defines an
identifier to be a typedef name that denotes the type specified for the identifier in the way
described in 6.7.6. Any array size expressions associated with variable length array
declarators are evaluated each time the declaration of the typedef name is reached in the
order of execution. A typedef declaration does not introduce a new type, only a
synonym for the type so specified. That is, in the following declarations:
typedef T type_ident;
type_ident D;
type_ident is defined as a typedef name with the type specified by the declaration
specifiers in T (known as T ), and the identifier in D has the type `` derived-declarator-
type-list T '' where the derived-declarator-type-list is specified by the declarators of D. A
typedef name shares the same name space as other identifiers declared in ordinary
declarators.
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];
}
6.7.9 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 a complete
object type that is not a variable length array type.
4 All the expressions in an initializer for an object that has static or thread 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 or thread 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,
and any padding is initialized to zero bits;
-- if it is a union, the first named member is initialized (recursively) according to these
rules, and any padding is initialized to zero bits;
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 or UTF-8 string
literal, optionally enclosed in braces. Successive bytes of the string literal (including the
terminating null character if there is room or if the array is of unknown size) initialize the
elements of the array.
15 An array with element type compatible with a qualified or unqualified version of
wchar_t, char16_t, or char32_t may be initialized by a wide string literal with
the corresponding encoding prefix (L, u, or U, respectively), optionally enclosed in
braces. Successive wide characters of the wide string literal (including the terminating
null wide character if there is room or if the array is of unknown size) initialize the
elements of the array.
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.148) 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.149)
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.150) 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;151)
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.
148) 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.
149) 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.
150) 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.
151) Any initializer for the subobject which is overridden and so not used to initialize that subobject might
not be evaluated at all.
22 If an array of unknown size is initialized, its size is determined by the largest indexed
element with an explicit initializer. The array type is completed at the end of its
initializer list.
23 The evaluations of the initialization list expressions are indeterminately sequenced with
respect to one another and thus the order in which any side effects occur is
unspecified.152)
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 + i 3. 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
152) In particular, the evaluation order need not be the same as the order of subobject initialization.
structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero.
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> (7.19).
6.7.10 Static assertions
Syntax
1 static_assert-declaration:
_Static_assert ( constant-expression , string-literal ) ;
Constraints
2 The constant expression shall compare unequal to 0.
Semantics
3 The constant expression shall be an integer constant expression. If the value of the
constant expression compares unequal to 0, the declaration has no effect. Otherwise, the
constraint is violated and the implementation shall produce a diagnostic message that
includes the text of the string literal, except that characters not in the basic source
character set are not required to appear in the message.
Forward references: diagnostics (7.2).
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 that is not part of a compound
literal; the expression in an expression statement; the controlling expression of a selection
statement (if or switch); the controlling expression of a while or do statement; each
of the (optional) expressions of a for statement; the (optional) expression in a return
statement. There is a sequence point between the evaluation of a full expression and the
evaluation of the next full expression to be evaluated.
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.153)
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);
153) 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.154)
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.
154) 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.155)
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.
6 An iteration statement whose controlling expression is not a constant expression,156) that
performs no input/output operations, does not access volatile objects, and performs no
synchronization or atomic operations in its body, controlling expression, or (in the case of
155) 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.
156) An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.
a for statement) its expression-3, may be assumed by the implementation to
terminate.157)
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.158)
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 ;
157) This is intended to allow compiler transformations such as removal of empty loops even when
termination cannot be proven.
158) 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.
Semantics
2 A jump statement causes an unconditional jump to another place.
6.8.6.1 The goto statement
Constraints
1 The identifier in a goto statement shall name a label located somewhere in the enclosing
function. A goto statement shall not jump from outside the scope of an identifier having
a variably modified type to inside the scope of that identifier.
Semantics
2 A goto statement causes an unconditional jump to the statement prefixed by the named
label in the enclosing function.
3 EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The
following outline presents one possible approach to a problem based on these three assumptions:
1. The general initialization code accesses objects only visible to the current function.
2. The general initialization code is too large to warrant duplication.
3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by
continue statements, for example.)
/* ... */
goto first_time;
for (;;) {
// determine next operation
/* ... */
if (need to reinitialize) {
// reinitialize-only code
/* ... */
first_time:
// general initialization code
/* ... */
continue;
}
// 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;.159)
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.
159) 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.160)
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).
160) 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 than implied by the type; 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 or
_Alignof 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 or
_Alignof 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.161)
161) 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.162)
3 The return type of a function shall be void or a complete object type other than array
type.
4 The storage-class specifier, if any, in the declaration specifiers shall be either extern or
static.
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.
162) 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,163) 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 6.7.6.3 for a parameter type list; the resulting type shall be a complete 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.164) 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:
163) See ``future language directions'' (6.11.7).
164) A parameter identifier cannot be redeclared in the function body except in an enclosed block.
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.165) A new-line character ends
the preprocessing directive even if it occurs within what would otherwise be an
165) 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: identifiers (including those lexically identical to keywords) are interpreted as
described below;166) and it may contain unary operator expressions of the form
defined identifier
or
defined ( identifier )
which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is
166) 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.
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>.167) 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.168) Also, whether a
single-character character constant may have a negative value is implementation-defined.
167) 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.
168) 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)
5 Preprocessing directives of the forms
# 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.169)
Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest
integer types (7.20.1.5).
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
# 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
169) 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.
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.170) 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"
170) 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.
8 EXAMPLE 2 This illustrates macro-replaced #include directives:
#if VERSION == 1
#define INCFILE "vers1.h"
#elif VERSION == 2
#define INCFILE "vers2.h" // and so on
#else
#define INCFILE "versN.h"
#endif
#include INCFILE
Forward references: macro replacement (6.10.3).
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 name171)
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,172) 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:
171) 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.
172) Despite the name, a non-directive is a preprocessing directive.
the variable arguments. The number of arguments so combined is such that, following
merger, the number of arguments is one more than the number of parameters in the macro
definition (excluding the ...).
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.173)
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.
173) 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. The
resulting preprocessing token sequence is then rescanned, along with all subsequent
preprocessing tokens of the source file, for more macro names to replace.
2 If the name of the macro being replaced is found during this scan of the replacement list
(not including the rest of the source file's preprocessing tokens), it is not replaced.
Furthermore, if any nested replacements encounter the name of the macro being replaced,
it is not replaced. These nonreplaced macro name preprocessing tokens are no longer
available for further replacement even if they are later (re)examined in contexts in which
that macro name preprocessing token would otherwise have been replaced.
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.
4 EXAMPLE There are cases where it is not clear whether a replacement is nested or not. For example,
given the following macro definitions:
#define f(a) a*g
#define g(a) f(a)
the invocation
f(2)(9)
may expand to either
2*f(9)
or
2*9*g
Strictly conforming programs are not permitted to depend on such unspecified behavior.
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)174) 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 forms175) 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).
174) 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.
175) 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 values of the predefined macros listed in the following subclauses176) (except for
_ _FILE_ _ and _ _LINE_ _) remain constant throughout the translation unit.
2 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.
3 The implementation shall not predefine the macro _ _cplusplus, nor shall it define it
in any standard header.
Forward references: standard headers (7.1.2).
6.10.8.1 Mandatory macros
1 The following macro names shall be defined by the implementation:
_ _DATE_ _ The date of translation of the preprocessing translation unit: a character
string literal of the form "Mmm dd yyyy", where the names of the
months are the same as those generated by the asctime function, and the
first character of dd is a space character if the value is less than 10. If the
date of translation is not available, an implementation-defined valid date
shall be supplied.
_ _FILE_ _ The presumed name of the current source file (a character string literal).177)
_ _LINE_ _ The presumed line number (within the current source file) of the current
source line (an integer constant).177)
_ _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.
176) See ``future language directions'' (6.11.9).
177) The presumed source file name and line number can be changed by the #line directive.
_ _STDC_VERSION_ _ The integer constant 201ymmL.178)
_ _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.
Forward references: the asctime function (7.27.3.1).
6.10.8.2 Environment macros
1 The following macro names are conditionally defined by the implementation:
_ _STDC_ISO_10646_ _ An integer constant of the form yyyymmL (for example,
199712L). If this symbol is defined, then every character in the Unicode
required set, when stored in an object of type wchar_t, has the same
value as the short identifier of that character. The Unicode required set
consists of all the characters that are defined by ISO/IEC 10646, along with
all amendments and technical corrigenda, as of the specified year and
month. If some other encoding is used, the macro shall not be defined and
the actual encoding used is implementation-defined.
_ _STDC_MB_MIGHT_NEQ_WC_ _ The integer constant 1, intended to indicate that, in
the encoding for wchar_t, a member of the basic character set need not
have a code value equal to its value when used as the lone character in an
integer character constant.
_ _STDC_UTF_16_ _ The integer constant 1, intended to indicate that values of type
char16_t are UTF-16 encoded. If some other encoding is used, the
macro shall not be defined and the actual encoding used is implementation-
defined.
_ _STDC_UTF_32_ _ The integer constant 1, intended to indicate that values of type
char32_t are UTF-32 encoded. If some other encoding is used, the
macro shall not be defined and the actual encoding used is implementation-
defined.
Forward references: common definitions (7.19), unicode utilities (7.28).
178) This macro was not specified in ISO/IEC 9899:1990 and was specified as 199409L in
ISO/IEC 9899/AMD1:1995 and as 199901L in ISO/IEC 9899:1999. The intention is that this will
remain an integer constant of type long int that is increased with each revision of this International
Standard.
6.10.8.3 Conditional feature macros
1 The following macro names are conditionally defined by the implementation:
_ _STDC_ANALYZABLE_ _ The integer constant 1, intended to indicate conformance to
the specifications in annex L (Analyzability).
_ _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 annex G (IEC 60559 compatible complex
arithmetic).
_ _STDC_LIB_EXT1_ _ The integer constant 201ymmL, intended to indicate support
for the extensions defined in annex K (Bounds-checking interfaces).179)
_ _STDC_NO_ATOMICS_ _ The integer constant 1, intended to indicate that the
implementation does not support atomic types (including the _Atomic
type qualifier) and the <stdatomic.h> header.
_ _STDC_NO_COMPLEX_ _ The integer constant 1, intended to indicate that the
implementation does not support complex types or the <complex.h>
header.
_ _STDC_NO_THREADS_ _ The integer constant 1, intended to indicate that the
implementation does not support the <threads.h> header.
_ _STDC_NO_VLA_ _ The integer constant 1, intended to indicate that the
implementation does not support variable length arrays or variably
modified types.
2 An implementation that defines _ _STDC_NO_COMPLEX_ _ shall not define
_ _STDC_IEC_559_COMPLEX_ _.
179) The intention is that this will remain an integer constant of type long int that is increased with
each revision of this International Standard.
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 any encoding prefix,
deleting the leading and trailing double-quotes, replacing each escape sequence \" by a
double-quote, and replacing each escape sequence \\ by a single backslash. The
resulting sequence of characters is processed through translation phase 3 to produce
preprocessing tokens that are executed as if they were the pp-tokens in a pragma
directive. The original four preprocessing tokens in the unary operator expression are
removed.
2 EXAMPLE A directive of the form:
#pragma listing on "..\listing.dir"
can also be expressed as:
_Pragma ( "listing on \"..\\listing.dir\"" )
The latter form is processed in the same way whether it appears literally as shown, or results from macro
replacement, as in:
#define LISTING(x) PRAGMA(listing on #x)
#define PRAGMA(x) _Pragma(#x)
LISTING ( ..\listing.dir )
6.11 Future language directions
6.11.1 Floating types
1 Future standardization may include additional floating-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.180) 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.181)
Forward references: character handling (7.4), the setlocale function (7.11.1.1).
180) The functions that make use of the decimal-point character are the numeric conversion functions
(7.22.1, 7.29.4.1) and the formatted input/output functions (7.21.6, 7.29.2).
181) 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 ,182)
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 are183)
<assert.h> <math.h> <stdlib.h>
<complex.h> <setjmp.h> <stdnoreturn.h>
<ctype.h> <signal.h> <string.h>
<errno.h> <stdalign.h> <tgmath.h>
<fenv.h> <stdarg.h> <threads.h>
<float.h> <stdatomic.h> <time.h>
<inttypes.h> <stdbool.h> <uchar.h>
<iso646.h> <stddef.h> <wchar.h>
<limits.h> <stdint.h> <wctype.h>
<locale.h> <stdio.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
of the header or when any macro defined in the header is expanded.
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.
182) A header is not necessarily a source file, nor are the < and > delimited sequences in header names
necessarily valid source file names.
183) The headers <complex.h>, <stdatomic.h>, and <threads.h> are conditional features that
implementations need not support; see 6.10.8.3.
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).
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) and errno are always reserved for use as identifiers with
external linkage.184)
-- 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.
184) The list of reserved identifiers with external linkage includes math_errhandling, setjmp,
va_copy, and va_end.
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
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.185) 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.186) 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.187) All object-like macros listed as expanding to
185) This means that an implementation shall provide an actual function for each library function, even if it
also provides a macro for that function.
186) Such macros might not contain the sequence points that the corresponding function calls do.
187) 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.
integer constant expressions shall additionally be suitable for use in #if preprocessing
directives.
2 Provided that a library function can be declared without reference to any type defined in a
header, it is also permissible to declare the function and use it without including its
associated header.
3 There is a sequence point immediately before a library function returns.
4 The functions in the standard library are not guaranteed to be reentrant and may modify
objects with static or thread storage duration.188)
5 Unless explicitly stated otherwise in the detailed descriptions that follow, library
functions shall prevent data races as follows: A library function shall not directly or
indirectly access objects accessible by threads other than the current thread unless the
objects are accessed directly or indirectly via the function's arguments. A library
function shall not directly or indirectly modify objects accessible by threads other than
the current thread unless the objects are accessed directly or indirectly via the function's
non-const arguments.189) Implementations may share their own internal objects between
threads if the objects are not visible to users and are protected against data races.
6 Unless otherwise specified, library functions shall perform all operations solely within the
current thread if those operations have effects that are visible to users.190)
7 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)
188) Thus, a signal handler cannot, in general, call standard library functions.
189) This means, for example, that an implementation is not permitted to use a static object for internal
purposes without synchronization because it could cause a data race even in programs that do not
explicitly share objects between threads. Similarly, an implementation of memcpy is not permitted to
copy bytes beyond the specified length of the destination object and then restore the original values
because it could cause a data race if the program shared those bytes between threads.
190) This allows implementations to parallelize operations if there are no visible side effects.
#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 and static_assert macros and
refers to another macro,
NDEBUG
which is not defined by <assert.h>. If NDEBUG is defined as a macro name at the
point in the source file where <assert.h> is included, the assert macro is defined
simply as
#define assert(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.
3 The macro
static_assert
expands to _Static_assert.
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.191) It
then calls the abort function.
191) The message written might be of the form:
Assertion failed: expression, function abc, file xyz, line nnn.
Returns
3 The assert macro returns no value.
Forward references: the abort function (7.22.4.1).
7.3 Complex arithmetic <complex.h>
7.3.1 Introduction
1 The header <complex.h> defines macros and declares functions that support complex
arithmetic.192)
2 Implementations that define the macro _ _STDC_NO_COMPLEX_ _ need not provide
this header nor support any of its facilities.
3 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.
4 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.193)
5 The macros
imaginary
and
_Imaginary_I
are defined if and only if the implementation supports imaginary types;194) if defined,
they expand to _Imaginary and a constant expression of type const float
_Imaginary with the value of the imaginary unit.
6 The macro
I
expands to either _Imaginary_I or _Complex_I. If _Imaginary_I is not
defined, I shall expand to _Complex_I.
7 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then
redefine the macros complex, imaginary, and I.
192) See ``future library directions'' (7.31.1).
193) The imaginary unit is a number i such that i 2 = -1.
194) A specification for imaginary types is in informative annex G.
Forward references: IEC 60559-compatible complex arithmetic (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.195) The pragma can occur either outside external declarations or preceding all
explicit declarations and statements inside a compound statement. When outside external
declarations, the pragma takes effect from its occurrence until another
CX_LIMITED_RANGE pragma is encountered, or until the end of the translation unit.
When inside a compound statement, the pragma takes effect from its occurrence until
another CX_LIMITED_RANGE pragma is encountered (including within a nested
compound statement), or until the end of the compound statement; at the end of a
compound statement the state for the pragma is restored to its condition just before the
compound statement. If this pragma is used in any other context, the behavior is
undefined. The default state for the pragma is ``off''.
7.3.5 Trigonometric functions
7.3.5.1 The cacos functions
Synopsis
1 #include <complex.h>
double complex cacos(double complex z);
float complex cacosf(float complex z);
long double complex cacosl(long double complex z);
Description
2 The cacos functions compute the complex arc cosine of z, with branch cuts outside the
interval [-1, +1] along the real axis.
Returns
3 The cacos functions return the complex arc cosine value, in the range of a strip
mathematically unbounded along the imaginary axis and in the interval [0, ] 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]
195) 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 + y2
where the programmer can determine they are safe.
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 nonnegative 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.196)
Returns
3 The cimag functions return the imaginary part value (as a real).
7.3.9.3 The CMPLX macros
Synopsis
1 #include <complex.h>
double complex CMPLX(double x, double y);
float complex CMPLXF(float x, float y);
long double complex CMPLXL(long double x, long double y);
Description
2 The CMPLX macros expand to an expression of the specified complex type, with the real
part having the (converted) value of x and the imaginary part having the (converted)
value of y. The resulting expression shall be suitable for use as an initializer for an object
with static or thread storage duration, provided both arguments are likewise suitable.
Returns
3 The CMPLX macros return the complex value x + i y.
4 NOTE These macros act as if the implementation supported imaginary types and the definitions were:
#define CMPLX(x, y) ((double complex)((double)(x) + \
_Imaginary_I * (double)(y)))
#define CMPLXF(x, y) ((float complex)((float)(x) + \
_Imaginary_I * (float)(y)))
#define CMPLXL(x, y) ((long double complex)((long double)(x) + \
_Imaginary_I * (long double)(y)))
196) For a variable z of complex type, z == creal(z) + cimag(z)*I.
7.3.9.4 The conj functions
Synopsis
1 #include <complex.h>
double complex conj(double complex z);
float complex conjf(float complex z);
long double complex conjl(long double complex z);
Description
2 The conj functions compute the complex conjugate of z, by reversing the sign of its
imaginary part.
Returns
3 The conj functions return the complex conjugate value.
7.3.9.5 The cproj functions
Synopsis
1 #include <complex.h>
double complex cproj(double complex z);
float complex cprojf(float complex z);
long double complex cprojl(long double complex z);
Description
2 The cproj functions compute a projection of z onto the Riemann sphere: z projects to
z except that all complex infinities (even those with one infinite part and one NaN part)
project to positive infinity on the real axis. If z has an infinite part, then cproj(z) is
equivalent to
INFINITY + I * copysign(0.0, cimag(z))
Returns
3 The cproj functions return the value of the projection onto the Riemann sphere.
7.3.9.6 The creal functions
Synopsis
1 #include <complex.h>
double creal(double complex z);
float crealf(float complex z);
long double creall(long double complex z);
Description
2 The creal functions compute the real part of z.197)
Returns
3 The creal functions return the real part value.
197) 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.198) 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.199) All letters and digits are printing characters.
Forward references: EOF (7.21.1), 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
198) See ``future library directions'' (7.31.2).
199) 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.200) 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);
200) 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 lvalue201) that has type int and thread local storage
duration, the value of which is set to a positive error number by several library functions.
If a macro definition is suppressed in order to access an actual object, or a program
defines an identifier with the name errno, the behavior is undefined.
3 The value of errno in the initial thread is zero at program startup (the initial value of
errno in other threads is an indeterminate value), but is never set to zero by any library
function.202) 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,203) may also be specified by the implementation.
201) 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()).
202) 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.
203) See ``future library directions'' (7.31.3).
7.6 Floating-point environment <fenv.h>
1 The header <fenv.h> defines several macros, and declares types and functions that
provide access to the floating-point environment. The floating-point environment refers
collectively to any floating-point status flags and control modes supported by the
implementation.204) 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.205) 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 The floating-point environment has thread storage duration. The initial state for a
thread's floating-point environment is the current state of the floating-point environment
of the thread that creates it at the time of creation.
3 Certain programming conventions support the intended model of use for the floating-
point environment:206)
-- 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.
4 The type
fenv_t
represents the entire floating-point environment.
5 The type
fexcept_t
represents the floating-point status flags collectively, including any status the
implementation associates with the flags.
204) 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. It is also
designed to facilitate code portability among all systems.
205) A floating-point status flag is not an object and can be set more than once within an expression.
206) 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.
6 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 7.6.2.207) Additional implementation-defined floating-point
exceptions, with macro definitions beginning with FE_ and an uppercase letter,208) 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.209)
7 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.
8 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,210) may also be specified by the implementation. The
defined macros expand to integer constant expressions whose values are distinct
nonnegative values.211)
207) The implementation supports a floating-point 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.
208) See ``future library directions'' (7.31.4).
209) The macros should be distinct powers of two.
210) See ``future library directions'' (7.31.4).
211) Even though the rounding direction macros may expand to constants corresponding to the values of
FLT_ROUNDS, they are not required to do so.
9 The macro
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.
10 Additional implementation-defined environments, with macro definitions beginning with
FE_ and an uppercase letter,212) 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.213) 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.)
212) See ``future library directions'' (7.31.4).
213) 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.214)
7.6.2 Floating-point exceptions
1 The following functions provide access to the floating-point status flags.215) 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.
214) 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.
215) 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.216) The order in which these floating-point exceptions are
raised is unspecified, except as stated in F.8.6. Whether the feraiseexcept function
additionally raises the ``inexact'' floating-point exception whenever it raises the
``overflow'' or ``underflow'' floating-point exception is implementation-defined.
Returns
3 The feraiseexcept function returns zero if the excepts argument is zero or if all
the specified exceptions were successfully raised. Otherwise, it returns a nonzero value.
216) 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.8.6 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.217)
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:
217) 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.218)
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.
218) 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.219)
Forward references: integer types <stdint.h> (7.20), formatted input/output
functions (7.21.6), formatted wide character input/output functions (7.29.2).
7.8.1 Macros for format specifiers
1 Each of the following object-like macros expands to a character string literal containing a
conversion specifier, possibly modified by a length modifier, suitable for use within the
format argument of a formatted input/output function when converting the corresponding
integer type. These macro names have the general form of PRI (character string literals
for the fprintf and fwprintf family) or SCN (character string literals for the
fscanf and fwscanf family),220) followed by the conversion specifier, followed by a
name corresponding to a similar type name in 7.20.1. In these names, N represents the
width of the type as described in 7.20.1. For example, PRIdFAST32 can be used in a
format string to print the value of an integer of type int_fast32_t.
2 The fprintf macros for signed integers are:
PRIdN PRIdLEASTN PRIdFASTN PRIdMAX PRIdPTR
PRIiN PRIiLEASTN PRIiFASTN PRIiMAX PRIiPTR
3 The fprintf macros for unsigned integers are:
PRIoN PRIoLEASTN PRIoFASTN PRIoMAX PRIoPTR
PRIuN PRIuLEASTN PRIuFASTN PRIuMAX PRIuPTR
PRIxN PRIxLEASTN PRIxFASTN PRIxMAX PRIxPTR
PRIXN PRIXLEASTN PRIXFASTN PRIXMAX PRIXPTR
4 The fscanf macros for signed integers are:
219) See ``future library directions'' (7.31.5).
220) 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.
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.221)
Returns
3 The imaxabs function returns the absolute value.
221) The absolute value of the most negative number cannot be represented in two's complement.
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
(7.22.1.4).
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
(7.29.4.1.2).
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 7.19); 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.222) Additional macro definitions, beginning
with the characters LC_ and an uppercase letter,223) 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 functions224) 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.
222) ISO/IEC 9945-2 specifies locale and charmap formats that may be used to specify locales for C.
223) See ``future library directions'' (7.31.6).
224) 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 A call to the setlocale function may introduce a data race with other calls to the
setlocale function or with calls to functions that are affected by the current locale.
The implementation shall behave as if no library function calls the setlocale function.
Returns
6 If a pointer to a string is given for locale and the selection can be honored, the
setlocale function returns a pointer to the string associated with the specified
category for the new locale. If the selection cannot be honored, the setlocale
function returns a null pointer and the program's locale is not changed.
7 A null pointer for locale causes the setlocale function to return a pointer to the
string associated with the category for the program's current locale; the program's
locale is not changed.225)
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 (7.21.6), multibyte/wide
character conversion functions (7.22.7), multibyte/wide string conversion functions
(7.22.8), numeric conversion functions (7.22.1), the strcoll function (7.24.4.3), the
strftime function (7.27.3.5), the strxfrm function (7.24.4.5).
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.
225) The implementation shall arrange to encode in a string the various categories due to a heterogeneous
locale when category has the value LC_ALL.
3 The members of the structure with type char * are pointers to strings, any of which
(except decimal_point) can point to "", to indicate that the value is not available in
the current locale or is of zero length. Apart from grouping and mon_grouping, the
strings shall start and end in the initial shift state. The members with type char are
nonnegative numbers, any of which can be CHAR_MAX to indicate that the value is not
available in the current locale. The members include the following:
char *decimal_point
The decimal-point character used to format nonmonetary quantities.
char *thousands_sep
The character used to separate groups of digits before the decimal-point
character in formatted nonmonetary quantities.
char *grouping
A string whose elements indicate the size of each group of digits in
formatted 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.226)
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.227)
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.228)
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
226) 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.
227) 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.
228) HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that
supports infinities.
translation time.229)
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.230) 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.
229) In this case, using INFINITY will violate the constraint in 6.4.4 and thus require a diagnostic.
230) 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 raising SIGFPE and without generating any of the
floating-point exceptions ``invalid'', ``divide-by-zero'', or ``overflow'' except to reflect
the result of the function.
2 For all functions, a domain error occurs if 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.231) On a
domain error, the function returns an implementation-defined value; if the integer
expression math_errhandling & MATH_ERRNO is nonzero, the integer expression
errno acquires the value EDOM; if the integer expression math_errhandling &
MATH_ERREXCEPT is nonzero, the ``invalid'' floating-point exception is raised.
3 Similarly, a pole error (also known as a singularity or infinitary) occurs if the
mathematical function has an exact infinite result as the finite input argument(s) are
approached in the limit (for example, log(0.0)). The description of each function lists
any required pole errors; an implementation may define additional pole errors, provided
that such errors are consistent with the mathematical definition of the function. On a pole
error, the function returns an implementation-defined value; if the integer expression
231) 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.
math_errhandling & MATH_ERRNO is nonzero, the integer expression errno
acquires the value ERANGE; if the integer expression math_errhandling &
MATH_ERREXCEPT is nonzero, the ``divide-by-zero'' floating-point exception is raised.
4 Likewise, a range error occurs if the mathematical result of the function cannot be
represented in an object of the specified type, due to extreme magnitude.
5 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, then the function returns the value of the macro HUGE_VAL, HUGE_VALF, or
HUGE_VALL according to the return type, with the same sign as the correct value of the
function; 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 ``overflow'' floating-
point exception is raised.
6 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.232) 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 If a domain, pole, or range error occurs and the integer expression
math_errhandling & MATH_ERRNO is zero,233) then errno shall either be set to
the value corresponding to the error or left unmodified. If no such error occurs, errno
shall be left unmodified regardless of the setting of math_errhandling.
232) The term underflow here is intended to encompass both ``gradual underflow'' as in IEC 60559 and
also ``flush-to-zero'' underflow.
233) Math errors are being indicated by the floating-point exception flags rather than by errno.
7.12.2 The FP_CONTRACT pragma
Synopsis
1 #include <math.h>
#pragma STDC FP_CONTRACT on-off-switch
Description
2 The FP_CONTRACT pragma can be used to allow (if the state is ``on'') or disallow (if the
state is ``off'') the implementation to contract expressions (6.5). Each pragma can occur
either outside external declarations or preceding all explicit declarations and statements
inside a compound statement. When outside external declarations, the pragma takes
effect from its occurrence until another FP_CONTRACT pragma is encountered, or until
the end of the translation unit. When inside a compound statement, the pragma takes
effect from its occurrence until another FP_CONTRACT pragma is encountered
(including within a nested compound statement), or until the end of the compound
statement; at the end of a compound statement the state for the pragma is restored to its
condition just before the compound statement. If this pragma is used in any other
context, the behavior is undefined. The default state (``on'' or ``off'') for the pragma is
implementation-defined.
7.12.3 Classification macros
1 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.234)
Returns
3 The fpclassify macro returns the value of the number classification macro
appropriate to the value of its argument.
234) 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.
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.
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.235)
235) 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.
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);
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.236)
Returns
3 The signbit macro returns a nonzero value if and only if the sign of its argument value
is negative.
236) 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 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.
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 pole 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.237)
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 or if the integral power of 2 is outside the range
of int, the results are unspecified. Otherwise, the frexp functions return the value x,
such that x has a magnitude in the interval [1/2, 1) or zero, and value equals x <20> 2*exp .
If value is zero, both parts of the result are zero.
237) For small magnitude x, expm1(x) is expected to be more accurate than exp(x) - 1.
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.
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 pole 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 pole 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.238)
A domain error occurs if the argument is less than -1. A pole error may occur if the
argument equals -1.
Returns
3 The log1p functions return loge (1 + x).
238) For small magnitude x, log1p(x) is expected to be more accurate than log(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 pole error may occur if the argument is zero.
Returns
3 The log2 functions return log2 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 pole 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 + y 2 .
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 pole 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
3 2 x
e-t dt .
2
The erf functions return erf x =
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
3 2
e-t dt .
2
The erfc functions return erfc x = 1 - erf x =
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 pole 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 pole error
may occur if x is a negative integer or zero. A range error occurs if the magnitude of x is
too large and may occur if the magnitude of x is 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 60559.239)
239) ``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. If r = 0, its sign shall be that of x .'' This definition is applicable for
all implementations.
Returns
3 The remainder functions return x REM y. If y is zero, whether a domain error occurs
or the functions return zero is implementation defined.
7.12.10.3 The remquo functions
Synopsis
1 #include <math.h>
double remquo(double x, double y, int *quo);
float remquof(float x, float y, int *quo);
long double remquol(long double x, long double y,
int *quo);
Description
2 The remquo functions compute the same remainder as the remainder functions. In
the object pointed to by quo they store a value whose sign is the sign of x/y and whose
magnitude is congruent modulo 2n to the magnitude of the integral quotient of x/y, where
n is an implementation-defined integer greater than or equal to 3.
Returns
3 The remquo functions return x REM y. If y is zero, the value stored in the object
pointed to by quo is unspecified and whether a domain error occurs or the functions
return zero is implementation defined.
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 (7.22.1.3).
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.240) 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.
240) 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.241)
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);
241) 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.242)
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.243)
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.
242) 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.10.9.2.
243) 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.244) 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 type245) (both arguments need not have the same type).246)
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);
244) 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.
245) If any argument is of integer type, or any other type that is not a real floating type, the behavior is
undefined.
246) Whether an argument represented in a format wider than its semantic type is converted to the semantic
type is unspecified.
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.
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.247)
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
247) 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>
_Noreturn 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 invocation was from
another thread of execution, or if the function containing the invocation of the setjmp
macro has terminated execution248) 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 machine249)
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, thread execution continues as if the corresponding
invocation of the setjmp macro had just returned the value specified by val. The
longjmp function cannot cause the setjmp macro to return the value 0; if val is 0,
the setjmp macro returns the value 1.
5 EXAMPLE The longjmp function that returns control back to the point of the setjmp invocation
might cause memory associated with a variable length array object to be squandered.
248) 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.
249) 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,250) 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.
250) See ``future library directions'' (7.31.7). 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),251) 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 or thread
storage duration that is not a lock-free atomic object other than by assigning a value to an
object declared as volatile sig_atomic_t, or the signal handler calls any function
in the standard library other than the abort function, the _Exit function, the
quick_exit function, 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.252)
251) This includes functions called indirectly via standard library functions (e.g., a SIGABRT handler
called via the abort function).
252) If any signal is generated by an asynchronous signal handler, the behavior is undefined.
6 At program startup, the equivalent of
signal(sig, SIG_IGN);
may be executed for some signals selected in an implementation-defined manner; the
equivalent of
signal(sig, SIG_DFL);
is executed for all other signals defined by the implementation.
7 Use of this function in a multi-threaded program results in undefined behavior. The
implementation shall behave as if no library function calls the signal function.
Returns
8 If the request can be honored, the signal function returns the value of func for the
most recent successful call to signal for the specified signal sig. Otherwise, a value of
SIG_ERR is returned and a positive value is stored in errno.
Forward references: the abort function (7.22.4.1), the exit function (7.22.4.4), the
_Exit function (7.22.4.5), the quick_exit function (7.22.4.7).
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 Alignment <stdalign.h>
1 The header <stdalign.h> defines four macros.
2 The macro
alignas
expands to _Alignas; the macro
alignof
expands to _Alignof.
3 The remaining macros are suitable for use in #if preprocessing directives. They are
_ _alignas_is_defined
and
_ _alignof_is_defined
which both expand to the integer constant 1.
7.16 Variable arguments <stdarg.h>
1 The header <stdarg.h> declares a type and defines four macros, for advancing
through a list of arguments whose number and types are not known to the called function
when it is translated.
2 A function may be called with a variable number of arguments of varying types. 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 a complete object type suitable for holding information needed by the macros
va_start, va_arg, va_end, and va_copy. If access to the varying arguments is
desired, the called function shall declare an object (generally referred to as ap in this
subclause) having type va_list. The object ap may be passed as an argument to
another function; if that function invokes the va_arg macro with parameter ap, the
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.253)
7.16.1 Variable argument list access macros
1 The va_start and va_arg macros described in this subclause shall be implemented
as macros, not functions. It is unspecified whether va_copy and va_end are macros or
identifiers declared with external linkage. If a macro definition is suppressed in order to
access an actual function, or a program defines an external identifier with the same name,
the behavior is undefined. Each invocation of the va_start and va_copy macros
shall be matched by a corresponding invocation of the va_end macro in the same
function.
7.16.1.1 The va_arg macro
Synopsis
1 #include <stdarg.h>
type va_arg(va_list ap, type);
Description
2 The va_arg macro expands to an expression that has the specified type and the value of
the next argument in the call. The parameter ap shall have been initialized by the
va_start or va_copy macro (without an intervening invocation of the va_end
253) 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.
7.16.1.2 The va_copy macro
Synopsis
1 #include <stdarg.h>
void va_copy(va_list dest, va_list src);
Description
2 The va_copy macro initializes dest as a copy of src, as if the va_start macro had
been applied to dest followed by the same sequence of uses of the va_arg macro as
had previously been used to reach the present state of src. Neither the va_copy nor
va_start macro shall be invoked to reinitialize dest without an intervening
invocation of the va_end macro for the same dest.
Returns
3 The va_copy macro returns no value.
7.16.1.3 The va_end macro
Synopsis
1 #include <stdarg.h>
void va_end(va_list ap);
Description
2 The va_end macro facilitates a normal return from the function whose variable
argument list was referred to by the expansion of the va_start macro, or the function
containing the expansion of the va_copy macro, that initialized the va_list ap. The
va_end macro may modify ap so that it is no longer usable (without being reinitialized
by the va_start or va_copy macro). If there is no corresponding invocation of the
va_start or va_copy macro, or if the va_end macro is not invoked before the
return, the behavior is undefined.
Returns
3 The va_end macro returns no value.
7.16.1.4 The va_start macro
Synopsis
1 #include <stdarg.h>
void va_start(va_list ap, 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);
}
7.17 Atomics <stdatomic.h>
7.17.1 Introduction
1 The header <stdatomic.h> defines several macros and declares several types and
functions for performing atomic operations on data shared between threads.254)
2 Implementations that define the macro _ _STDC_NO_ATOMICS_ _ need not provide
this header nor support any of its facilities.
3 The macros defined are the atomic lock-free macros
ATOMIC_BOOL_LOCK_FREE
ATOMIC_CHAR_LOCK_FREE
ATOMIC_CHAR16_T_LOCK_FREE
ATOMIC_CHAR32_T_LOCK_FREE
ATOMIC_WCHAR_T_LOCK_FREE
ATOMIC_SHORT_LOCK_FREE
ATOMIC_INT_LOCK_FREE
ATOMIC_LONG_LOCK_FREE
ATOMIC_LLONG_LOCK_FREE
ATOMIC_POINTER_LOCK_FREE
which indicate the lock-free property of the corresponding atomic types (both signed and
unsigned); and
ATOMIC_FLAG_INIT
which expands to an initializer for an object of type atomic_flag.
4 The types include
memory_order
which is an enumerated type whose enumerators identify memory ordering constraints;
atomic_flag
which is a structure type representing a lock-free, primitive atomic flag; and several
atomic analogs of integer types.
5 In the following synopses:
-- An A refers to one of the atomic types.
-- A C refers to its corresponding non-atomic type.
-- An M refers to the type of the other argument for arithmetic operations. For atomic
integer types, M is C . For atomic pointer types, M is ptrdiff_t.
254) See ``future library directions'' (7.31.8).
-- The functions not ending in _explicit have the same semantics as the
corresponding _explicit function with memory_order_seq_cst for the
memory_order argument.
6 NOTE Many operations are volatile-qualified. The ``volatile as device register'' semantics have not
changed in the standard. This qualification means that volatility is preserved when applying these
operations to volatile objects.
7.17.2 Initialization
7.17.2.1 The ATOMIC_VAR_INIT macro
Synopsis
1 #include <stdatomic.h>
#define ATOMIC_VAR_INIT(C value)
Description
2 The ATOMIC_VAR_INIT macro expands to a token sequence suitable for initializing an
atomic object of a type that is initialization-compatible with value. An atomic object
with automatic storage duration that is not explicitly initialized using
ATOMIC_VAR_INIT is initially in an indeterminate state; however, the default (zero)
initialization for objects with static or thread-local storage duration is guaranteed to
produce a valid state.
3 Concurrent access to the variable being initialized, even via an atomic operation,
constitutes a data race.
4 EXAMPLE
atomic_int guide = ATOMIC_VAR_INIT(42);
7.17.2.2 The atomic_init generic function
Synopsis
1 #include <stdatomic.h>
void atomic_init(volatile A *obj, C value);
Description
2 The atomic_init generic function initializes the atomic object pointed to by obj to
the value value, while also initializing any additional state that the implementation
might need to carry for the atomic object.
3 Although this function initializes an atomic object, it does not avoid data races;
concurrent access to the variable being initialized, even via an atomic operation,
constitutes a data race.
Returns
4 The atomic_init generic function returns no value.
5 EXAMPLE
atomic_int guide;
atomic_init(&guide, 42);
7.17.3 Order and consistency
1 The enumerated type memory_order specifies the detailed regular (non-atomic)
memory synchronization operations as defined in 5.1.2.4 and may provide for operation
ordering. Its enumeration constants are as follows:255)
memory_order_relaxed
memory_order_consume
memory_order_acquire
memory_order_release
memory_order_acq_rel
memory_order_seq_cst
2 For memory_order_relaxed, no operation orders memory.
3 For memory_order_release, memory_order_acq_rel, and
memory_order_seq_cst, a store operation performs a release operation on the
affected memory location.
4 For memory_order_acquire, memory_order_acq_rel, and
memory_order_seq_cst, a load operation performs an acquire operation on the
affected memory location.
5 For memory_order_consume, a load operation performs a consume operation on the
affected memory location.
6 There shall be a single total order S on all memory_order_seq_cst operations,
consistent with the ``happens before'' order and modification orders for all affected
locations, such that each memory_order_seq_cst operation B that loads a value
from an atomic object M observes one of the following values:
-- the result of the last modification A of M that precedes B in S , if it exists, or
-- if A exists, the result of some modification of M in the visible sequence of side
effects with respect to B that is not memory_order_seq_cst and that does not
happen before A, or
255) See ``future library directions'' (7.31.8).
-- if A does not exist, the result of some modification of M in the visible sequence of
side effects with respect to B that is not memory_order_seq_cst.
7 NOTE 1 Although it is not explicitly required that S include lock operations, it can always be extended to
an order that does include lock and unlock operations, since the ordering between those is already included
in the ``happens before'' ordering.
8 NOTE 2 Atomic operations specifying memory_order_relaxed are relaxed only with respect to
memory ordering. Implementations must still guarantee that any given atomic access to a particular atomic
object be indivisible with respect to all other atomic accesses to that object.
9 For an atomic operation B that reads the value of an atomic object M , if there is a
memory_order_seq_cst fence X sequenced before B, then B observes either the
last memory_order_seq_cst modification of M preceding X in the total order S or
a later modification of M in its modification order.
10 For atomic operations A and B on an atomic object M , where A modifies M and B takes
its value, if there is a memory_order_seq_cst fence X such that A is sequenced
before X and B follows X in S , then B observes either the effects of A or a later
modification of M in its modification order.
11 For atomic operations A and B on an atomic object M , where A modifies M and B takes
its value, if there are memory_order_seq_cst fences X and Y such that A is
sequenced before X , Y is sequenced before B, and X precedes Y in S , then B observes
either the effects of A or a later modification of M in its modification order.
12 Atomic read-modify-write operations shall always read the last value (in the modification
order) stored before the write associated with the read-modify-write operation.
13 An atomic store shall only store a value that has been computed from constants and
program input values by a finite sequence of program evaluations, such that each
evaluation observes the values of variables as computed by the last prior assignment in
the sequence.256) The ordering of evaluations in this sequence shall be such that
-- If an evaluation B observes a value computed by A in a different thread, then B does
not happen before A.
-- If an evaluation A is included in the sequence, then all evaluations that assign to the
same variable and happen before A are also included.
14 NOTE 3 The second requirement disallows ``out-of-thin-air'', or ``speculative'' stores of atomics when
relaxed atomics are used. Since unordered operations are involved, evaluations may appear in this
sequence out of thread order. For example, with x and y initially zero,
256) Among other implications, atomic variables shall not decay.
// Thread 1:
r1 = atomic_load_explicit(&y, memory_order_relaxed);
atomic_store_explicit(&x, r1, memory_order_relaxed);
// Thread 2:
r2 = atomic_load_explicit(&x, memory_order_relaxed);
atomic_store_explicit(&y, 42, memory_order_relaxed);
is allowed to produce r1 == 42 && r2 == 42. The sequence of evaluations justifying this consists of:
atomic_store_explicit(&y, 42, memory_order_relaxed);
r1 = atomic_load_explicit(&y, memory_order_relaxed);
atomic_store_explicit(&x, r1, memory_order_relaxed);
r2 = atomic_load_explicit(&x, memory_order_relaxed);
On the other hand,
// Thread 1:
r1 = atomic_load_explicit(&y, memory_order_relaxed);
atomic_store_explicit(&x, r1, memory_order_relaxed);
// Thread 2:
r2 = atomic_load_explicit(&x, memory_order_relaxed);
atomic_store_explicit(&y, r2, memory_order_relaxed);
is not allowed to produce r1 == 42 && r2 = 42, since there is no sequence of evaluations that results
in the computation of 42. In the absence of ``relaxed'' operations and read-modify-write operations with
weaker than memory_order_acq_rel ordering, the second requirement has no impact.
Recommended practice
15 The requirements do not forbid r1 == 42 && r2 == 42 in the following example,
with x and y initially zero:
// Thread 1:
r1 = atomic_load_explicit(&x, memory_order_relaxed);
if (r1 == 42)
atomic_store_explicit(&y, r1, memory_order_relaxed);
// Thread 2:
r2 = atomic_load_explicit(&y, memory_order_relaxed);
if (r2 == 42)
atomic_store_explicit(&x, 42, memory_order_relaxed);
However, this is not useful behavior, and implementations should not allow it.
16 Implementations should make atomic stores visible to atomic loads within a reasonable
amount of time.
7.17.3.1 The kill_dependency macro
Synopsis
1 #include <stdatomic.h>
type kill_dependency(type y);
Description
2 The kill_dependency macro terminates a dependency chain; the argument does not
carry a dependency to the return value.
Returns
3 The kill_dependency macro returns the value of y.
7.17.4 Fences
1 This subclause introduces synchronization primitives called fences. Fences can have
acquire semantics, release semantics, or both. A fence with acquire semantics is called
an acquire fence; a fence with release semantics is called a release fence.
2 A release fence A synchronizes with an acquire fence B if there exist atomic operations
X and Y , both operating on some atomic object M , such that A is sequenced before X , X
modifies M , Y is sequenced before B, and Y reads the value written by X or a value
written by any side effect in the hypothetical release sequence X would head if it were a
release operation.
3 A release fence A synchronizes with an atomic operation B that performs an acquire
operation on an atomic object M if there exists an atomic operation X such that A is
sequenced before X , X modifies M , and B reads the value written by X or a value written
by any side effect in the hypothetical release sequence X would head if it were a release
operation.
4 An atomic operation A that is a release operation on an atomic object M synchronizes
with an acquire fence B if there exists some atomic operation X on M such that X is
sequenced before B and reads the value written by A or a value written by any side effect
in the release sequence headed by A.
7.17.4.1 The atomic_thread_fence function
Synopsis
1 #include <stdatomic.h>
void atomic_thread_fence(memory_order order);
Description
2 Depending on the value of order, this operation:
-- has no effects, if order == memory_order_relaxed;
-- is an acquire fence, if order == memory_order_acquire or order ==
memory_order_consume;
-- is a release fence, if order == memory_order_release;
-- is both an acquire fence and a release fence, if order ==
memory_order_acq_rel;
-- is a sequentially consistent acquire and release fence, if order ==
memory_order_seq_cst.
Returns
3 The atomic_thread_fence function returns no value.
7.17.4.2 The atomic_signal_fence function
Synopsis
1 #include <stdatomic.h>
void atomic_signal_fence(memory_order order);
Description
2 Equivalent to atomic_thread_fence(order), except that the resulting ordering
constraints are established only between a thread and a signal handler executed in the
same thread.
3 NOTE 1 The atomic_signal_fence function can be used to specify the order in which actions
performed by the thread become visible to the signal handler.
4 NOTE 2 Compiler optimizations and reorderings of loads and stores are inhibited in the same way as with
atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would
have inserted are not emitted.
Returns
5 The atomic_signal_fence function returns no value.
7.17.5 Lock-free property
1 The atomic lock-free macros indicate the lock-free property of integer and address atomic
types. A value of 0 indicates that the type is never lock-free; a value of 1 indicates that
the type is sometimes lock-free; a value of 2 indicates that the type is always lock-free.
2 NOTE Operations that are lock-free should also be address-free. That is, atomic operations on the same
memory location via two different addresses will communicate atomically. The implementation should not
depend on any per-process state. This restriction enables communication via memory mapped into a
process more than once and memory shared between two processes.
7.17.5.1 The atomic_is_lock_free generic function
Synopsis
1 #include <stdatomic.h>
_Bool atomic_is_lock_free(const volatile A *obj);
Description
2 The atomic_is_lock_free generic function indicates whether or not the object
pointed to by obj is lock-free.
Returns
3 The atomic_is_lock_free generic function returns nonzero (true) if and only if the
object's operations are lock-free. The result of a lock-free query on one object cannot be
inferred from the result of a lock-free query on another object.
7.17.6 Atomic integer types
1 For each line in the following table,257) the atomic type name is declared as a type that
has the same representation and alignment requirements as the corresponding direct
type.258)
257) See ``future library directions'' (7.31.8).
258) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
Atomic type name Direct type
atomic_bool _Atomic _Bool
atomic_char _Atomic char
atomic_schar _Atomic signed char
atomic_uchar _Atomic unsigned char
atomic_short _Atomic short
atomic_ushort _Atomic unsigned short
atomic_int _Atomic int
atomic_uint _Atomic unsigned int
atomic_long _Atomic long
atomic_ulong _Atomic unsigned long
atomic_llong _Atomic long long
atomic_ullong _Atomic unsigned long long
atomic_char16_t _Atomic char16_t
atomic_char32_t _Atomic char32_t
atomic_wchar_t _Atomic wchar_t
atomic_int_least8_t _Atomic int_least8_t
atomic_uint_least8_t _Atomic uint_least8_t
atomic_int_least16_t _Atomic int_least16_t
atomic_uint_least16_t _Atomic uint_least16_t
atomic_int_least32_t _Atomic int_least32_t
atomic_uint_least32_t _Atomic uint_least32_t
atomic_int_least64_t _Atomic int_least64_t
atomic_uint_least64_t _Atomic uint_least64_t
atomic_int_fast8_t _Atomic int_fast8_t
atomic_uint_fast8_t _Atomic uint_fast8_t
atomic_int_fast16_t _Atomic int_fast16_t
atomic_uint_fast16_t _Atomic uint_fast16_t
atomic_int_fast32_t _Atomic int_fast32_t
atomic_uint_fast32_t _Atomic uint_fast32_t
atomic_int_fast64_t _Atomic int_fast64_t
atomic_uint_fast64_t _Atomic uint_fast64_t
atomic_intptr_t _Atomic intptr_t
atomic_uintptr_t _Atomic uintptr_t
atomic_size_t _Atomic size_t
atomic_ptrdiff_t _Atomic ptrdiff_t
atomic_intmax_t _Atomic intmax_t
atomic_uintmax_t _Atomic uintmax_t
2 The semantics of the operations on these types are defined in 7.17.7.
3 NOTE The representation of atomic integer types need not have the same size as their corresponding
regular types. They should have the same size whenever possible, as it eases effort required to port existing
code.
7.17.7 Operations on atomic types
1 There are only a few kinds of operations on atomic types, though there are many
instances of those kinds. This subclause specifies each general kind.
7.17.7.1 The atomic_store generic functions
Synopsis
1 #include <stdatomic.h>
void atomic_store(volatile A *object, C desired);
void atomic_store_explicit(volatile A *object,
C desired, memory_order order);
Description
2 The order argument shall not be memory_order_acquire,
memory_order_consume, nor memory_order_acq_rel. Atomically replace the
value pointed to by object with the value of desired. Memory is affected according
to the value of order.
Returns
3 The atomic_store generic functions return no value.
7.17.7.2 The atomic_load generic functions
Synopsis
1 #include <stdatomic.h>
C atomic_load(volatile A *object);
C atomic_load_explicit(volatile A *object,
memory_order order);
Description
2 The order argument shall not be memory_order_release nor
memory_order_acq_rel. Memory is affected according to the value of order.
Returns
Atomically returns the value pointed to by object.
7.17.7.3 The atomic_exchange generic functions
Synopsis
1 #include <stdatomic.h>
C atomic_exchange(volatile A *object, C desired);
C atomic_exchange_explicit(volatile A *object,
C desired, memory_order order);
Description
2 Atomically replace the value pointed to by object with desired. Memory is affected
according to the value of order. These operations are read-modify-write operations
(5.1.2.4).
Returns
3 Atomically returns the value pointed to by object immediately before the effects.
7.17.7.4 The atomic_compare_exchange generic functions
Synopsis
1 #include <stdatomic.h>
_Bool atomic_compare_exchange_strong(volatile A *object,
C *expected, C desired);
_Bool atomic_compare_exchange_strong_explicit(
volatile A *object, C *expected, C desired,
memory_order success, memory_order failure);
_Bool atomic_compare_exchange_weak(volatile A *object,
C *expected, C desired);
_Bool atomic_compare_exchange_weak_explicit(
volatile A *object, C *expected, C desired,
memory_order success, memory_order failure);
Description
2 The failure argument shall not be memory_order_release nor
memory_order_acq_rel. The failure argument shall be no stronger than the
success argument. Atomically, compares the value pointed to by object for equality
with that in expected, and if true, replaces the value pointed to by object with
desired, and if false, updates the value in expected with the value pointed to by
object. Further, if the comparison is true, memory is affected according to the value of
success, and if the comparison is false, memory is affected according to the value of
failure. These operations are atomic read-modify-write operations (5.1.2.4).
3 NOTE 1 For example, the effect of atomic_compare_exchange_strong is
if (memcmp(object, expected, sizeof (*object)) == 0)
memcpy(object, &desired, sizeof (*object));
else
memcpy(expected, object, sizeof (*object));
4 A weak compare-and-exchange operation may fail spuriously. That is, even when the
contents of memory referred to by expected and object are equal, it may return zero
and store back to expected the same memory contents that were originally there.
5 NOTE 2 This spurious failure enables implementation of compare-and-exchange on a broader class of
machines, e.g. load-locked store-conditional machines.
6 EXAMPLE A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will
be in a loop.
exp = atomic_load(&cur);
do {
des = function(exp);
} while (!atomic_compare_exchange_weak(&cur, &exp, des));
When a compare-and-exchange is in a loop, the weak version will yield better performance on some
platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the
strong one is preferable.
Returns
7 The result of the comparison.
7.17.7.5 The atomic_fetch and modify generic functions
1 The following operations perform arithmetic and bitwise computations. All of these
operations are applicable to an object of any atomic integer type. None of these
operations is applicable to atomic_bool. The key, operator, and computation
correspondence is:
key op computation
add + addition
sub - subtraction
or | bitwise inclusive or
xor ^ bitwise exclusive or
and & bitwise and
Synopsis
2 #include <stdatomic.h>
C atomic_fetch_key(volatile A *object, M operand);
C atomic_fetch_key_explicit(volatile A *object,
M operand, memory_order order);
Description
3 Atomically replaces the value pointed to by object with the result of the computation
applied to the value pointed to by object and the given operand. Memory is affected
according to the value of order. These operations are atomic read-modify-write
operations (5.1.2.4). For signed integer types, arithmetic is defined to use two's
complement representation with silent wrap-around on overflow; there are no undefined
results. For address types, the result may be an undefined address, but the operations
otherwise have no undefined behavior.
Returns
4 Atomically, the value pointed to by object immediately before the effects.
5 NOTE The operation of the atomic_fetch and modify generic functions are nearly equivalent to the
operation of the corresponding op= compound assignment operators. The only differences are that the
compound assignment operators are not guaranteed to operate atomically, and the value yielded by a
compound assignment operator is the updated value of the object, whereas the value returned by the
atomic_fetch and modify generic functions is the previous value of the atomic object.
7.17.8 Atomic flag type and operations
1 The atomic_flag type provides the classic test-and-set functionality. It has two
states, set and clear.
2 Operations on an object of type atomic_flag shall be lock free.
3 NOTE Hence the operations should also be address-free. No other type requires lock-free operations, so
the atomic_flag type is the minimum hardware-implemented type needed to conform to this
International standard. The remaining types can be emulated with atomic_flag, though with less than
ideal properties.
4 The macro ATOMIC_FLAG_INIT may be used to initialize an atomic_flag to the
clear state. An atomic_flag that is not explicitly initialized with
ATOMIC_FLAG_INIT is initially in an indeterminate state.
5 EXAMPLE
atomic_flag guard = ATOMIC_FLAG_INIT;
7.17.8.1 The atomic_flag_test_and_set functions
Synopsis
1 #include <stdatomic.h>
_Bool atomic_flag_test_and_set(
volatile atomic_flag *object);
_Bool atomic_flag_test_and_set_explicit(
volatile atomic_flag *object, memory_order order);
Description
2 Atomically sets the value pointed to by object to true. Memory is affected according
to the value of order. These operations are atomic read-modify-write operations
(5.1.2.4).
Returns
3 Atomically, the value of the object immediately before the effects.
7.17.8.2 The atomic_flag_clear functions
Synopsis
1 #include <stdatomic.h>
void atomic_flag_clear(volatile atomic_flag *object);
void atomic_flag_clear_explicit(
volatile atomic_flag *object, memory_order order);
Description
2 The order argument shall not be memory_order_acquire nor
memory_order_acq_rel. Atomically sets the value pointed to by object to false.
Memory is affected according to the value of order.
Returns
3 The atomic_flag_clear functions return no value.
7.18 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.259)
259) See ``future library directions'' (7.31.9).
7.19 Common definitions <stddef.h>
1 The header <stddef.h> defines the following macros and declares the following types.
Some are also defined in other headers, as noted in their respective subclauses.
2 The types are
ptrdiff_t
which is the signed integer type of the result of subtracting two pointers;
size_t
which is the unsigned integer type of the result of the sizeof operator;
max_align_t
which is an object type whose alignment is as great as is supported by the implementation
in all contexts; 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.
7.20 Integer types <stdint.h>
1 The header <stdint.h> declares sets of integer types having specified widths, and
defines corresponding sets of macros.260) 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,261) <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'').
7.20.1 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).
260) See ``future library directions'' (7.31.10).
261) Some of these types may denote implementation-defined extended integer types.
7.20.1.1 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 such a signed
integer type with a width of exactly 8 bits.
2 The typedef name uintN_t designates an unsigned integer type with width N and no
padding bits. Thus, uint24_t denotes such an unsigned integer type with a width of
exactly 24 bits.
3 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.
7.20.1.2 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.
7.20.1.3 Fastest minimum-width integer types
1 Each of the following types designates an integer type that is usually fastest262) 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 .
262) 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.
7.20.1.4 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.
7.20.1.5 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.
7.20.2 Limits of specified-width integer types
1 The following object-like macros specify the minimum and maximum limits of the types
declared in <stdint.h>. Each macro name corresponds to a similar type name in 7.20.1.
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
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.
7.20.2.1 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
7.20.2.2 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
7.20.2.3 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
7.20.2.4 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
7.20.2.5 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
7.20.3 Limits of other integer types
1 The following object-like macros 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.263)
-- 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
WCHAR_MIN see below
WCHAR_MAX see below
-- limits of wint_t
263) A freestanding implementation need not provide all of these types.
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 7.19) 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 255.264)
5 If wint_t (see 7.29) 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.
7.20.4 Macros for integer constants
1 The following function-like macros expand to integer constants suitable for initializing
objects that have integer types corresponding to types defined in <stdint.h>. Each
macro name corresponds to a similar type name in 7.20.1.2 or 7.20.1.5.
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.
7.20.4.1 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.
264) The values WCHAR_MIN and WCHAR_MAX do not necessarily correspond to members of the extended
character set.
7.20.4.2 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)
7.21 Input/output <stdio.h>
7.21.1 Introduction
1 The header <stdio.h> defines several macros, and declares three types and many
functions for performing input and output.
2 The types declared are size_t (described in 7.19);
FILE
which is an object type capable of recording all the information needed to control a
stream, including its file position indicator, a pointer to its associated buffer (if any), an
error indicator that records whether a read/write error has occurred, and an end-of-file
indicator that records whether the end of the file has been reached; and
fpos_t
which is a complete object type other than an array type capable of recording all the
information needed to specify uniquely every position within a file.
3 The macros are NULL (described in 7.19);
_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;265)
L_tmpnam
which expands to an integer constant expression that is the size needed for an array of
char large enough to hold a temporary file name string generated by the tmpnam
function;
SEEK_CUR
SEEK_END
SEEK_SET
which expand to integer constant expressions with distinct values, suitable for use as the
third argument to the fseek function;
TMP_MAX
which expands to an integer constant expression that is the minimum number of unique
file names that can be generated by the tmpnam function;
stderr
stdin
stdout
which are expressions of type ``pointer to FILE'' that point to the FILE objects
associated, respectively, with the standard error, input, and output streams.
4 The header <wchar.h> declares a number of functions useful for wide character input
and output. The wide character input/output functions described in that subclause
provide operations analogous to most of those described here, except that the
fundamental units internal to the program are wide characters. The external
representation (in the file) is a sequence of ``generalized'' multibyte characters, as
described further in 7.21.3.
5 The input/output functions are given the following collective terms:
-- The wide character input functions -- those functions described in 7.29 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 7.29 that perform
output from wide characters and wide strings: fputwc, fputws, putwc,
putwchar, fwprintf, wprintf, vfwprintf, and vwprintf.
265) 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, printf, putc, putchar, puts, scanf,
ungetc, vfprintf, vfscanf, vprintf, and vscanf.
Forward references: files (7.21.3), the fseek function (7.21.9.2), streams (7.21.2), the
tmpnam function (7.21.4.4), <wchar.h> (7.29).
7.21.2 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.266)
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
266) 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.)267)
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.
7 Each stream has an associated lock that is used to prevent data races when multiple
threads of execution access a stream, and to restrict the interleaving of stream operations
performed by multiple threads. Only one thread may hold this lock at a time. The lock is
reentrant: a single thread may hold the lock multiple times at a given time.
8 All functions that read, write, position, or query the position of a stream lock the stream
before accessing it. They release the lock associated with the stream when the access is
complete.
Environmental limits
9 An implementation shall support text files with lines containing at least 254 characters,
including the terminating new-line character. The value of the macro BUFSIZ shall be at
least 256.
Forward references: the freopen function (7.21.5.4), the fwide function (7.29.3.5),
mbstate_t (7.30.1), the fgetpos function (7.21.9.1), the fsetpos function
(7.21.9.3).
267) The three predefined streams stdin, stdout, and stderr are unoriented at program startup.
7.21.3 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 7.21.5.3. 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.268)
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)
268) 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 (7.22.4.4), the fgetc function (7.21.7.1), the
fopen function (7.21.5.3), the fputc function (7.21.7.3), the setbuf function
(7.21.5.5), the setvbuf function (7.21.5.6), the fgetwc function (7.29.3.1), the
fputwc function (7.29.3.3), conversion state (7.29.6), the mbrtowc function
(7.29.6.3.2), the wcrtomb function (7.29.6.3.3).
7.21.4 Operations on files
7.21.4.1 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.
7.21.4.2 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,269) in
which case if the file existed previously it is still known by its original name.
7.21.4.3 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 (7.21.5.3).
7.21.4.4 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.270) The function is potentially capable of generating at
269) 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.
270) 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.
least TMP_MAX different strings, but any or all of them may already be in use by existing
files and thus not be suitable return values.
3 The tmpnam function generates a different string each time it is called.
4 Calls to the tmpnam function with a null pointer argument may introduce data races with
each other. The implementation shall behave as if no library function calls the tmpnam
function.
Returns
5 If no suitable string can be generated, the tmpnam function returns a null pointer.
Otherwise, if the argument is a null pointer, the tmpnam function leaves its result in an
internal static object and returns a pointer to that object (subsequent calls to the tmpnam
function may modify the same object). 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.
7.21.5 File access functions
7.21.5.1 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.
7.21.5.2 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 (7.21.5.3).
7.21.5.3 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.271)
r open text file for reading
w truncate to zero length or create text file for writing
wx create text file for writing
a append; open or create text file for writing at end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for writing
271) 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 7.21.2).
wbx create binary file for writing
ab append; open or create binary file for writing at end-of-file
r+ open text file for update (reading and writing)
w+ truncate to zero length or create text file for update
w+x create text file for update
a+ append; open or create text file for update, writing at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for update
w+bx or wb+x create binary file for update
a+b or ab+ append; open or create binary file for update, writing at end-of-file
4 Opening a file with read mode ('r' as the first character in the mode argument) fails if
the file does not exist or cannot be read.
5 Opening a file with exclusive mode ('x' as the last character in the mode argument)
fails if the file already exists or cannot be created. Otherwise, the file is created with
exclusive (also known as non-shared) access to the extent that the underlying system
supports exclusive access.
6 Opening a file with append mode ('a' as the first character in the mode argument)
causes all subsequent writes to the file to be forced to the then current end-of-file,
regardless of intervening calls to the fseek function. In some implementations, opening
a binary file with append mode ('b' as the second or third character in the above list of
mode argument values) may initially position the file position indicator for the stream
beyond the last data written, because of null character padding.
7 When a file is opened with update mode ('+' as the second or third character in the
above list of mode argument values), both input and output may be performed on the
associated stream. However, output shall not be directly followed by input without an
intervening call to the fflush function or to a file positioning function (fseek,
fsetpos, or rewind), and input shall not be directly followed by output without an
intervening call to a file positioning function, unless the input operation encounters end-
of-file. Opening (or creating) a text file with update mode may instead open (or create) a
binary stream in some implementations.
8 When opened, a stream is fully buffered if and only if it can be determined not to refer to
an interactive device. The error and end-of-file indicators for the stream are cleared.
Returns
9 The fopen function returns a pointer to the object controlling the stream. If the open
operation fails, fopen returns a null pointer.
Forward references: file positioning functions (7.21.9).
7.21.5.4 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.272)
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.
7.21.5.5 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.
272) 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.
Returns
3 The setbuf function returns no value.
Forward references: the setvbuf function (7.21.5.6).
7.21.5.6 The setvbuf function
Synopsis
1 #include <stdio.h>
int setvbuf(FILE * restrict stream,
char * restrict buf,
int mode, size_t size);
Description
2 The setvbuf function may be used only after the stream pointed to by stream has
been associated with an open file and before any other operation (other than an
unsuccessful call to setvbuf) is performed on the stream. The argument mode
determines how stream will be buffered, as follows: _IOFBF causes input/output to be
fully buffered; _IOLBF causes input/output to be line buffered; _IONBF causes
input/output to be unbuffered. If buf is not a null pointer, the array it points to may be
used instead of a buffer allocated by the setvbuf function273) 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.
273) 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.
7.21.6 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.274)
7.21.6.1 The fprintf function
Synopsis
1 #include <stdio.h>
int fprintf(FILE * restrict stream,
const char * restrict format, ...);
Description
2 The fprintf function writes output to the stream pointed to by stream, under control
of the string pointed to by format that specifies how subsequent arguments are
converted for output. If there are insufficient arguments for the format, the behavior is
undefined. If the format is exhausted while arguments remain, the excess arguments are
evaluated (as always) but are otherwise ignored. The fprintf function returns when
the end of the format string is encountered.
3 The format shall be a multibyte character sequence, beginning and ending in its initial
shift state. The format is composed of zero or more directives: ordinary multibyte
characters (not %), which are copied unchanged to the output stream; and conversion
specifications, each of which results in fetching zero or more subsequent arguments,
converting them, if applicable, according to the corresponding conversion specifier, and
then writing the result to the output stream.
4 Each conversion specification is introduced by the character %. After the %, the following
appear in sequence:
-- Zero or more flags (in any order) that modify the meaning of the conversion
specification.
-- An optional minimum field width. If the converted value has fewer characters than the
field width, it is padded with spaces (by default) on the left (or right, if the left
adjustment flag, described later, has been given) to the field width. The field width
takes the form of an asterisk * (described later) or a nonnegative decimal integer.275)
-- 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
274) The fprintf functions perform writes to memory for the %n specifier.
275) Note that 0 is taken as a flag, not as the beginning of a field width.
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
specified.)276)
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
276) The results of all floating conversions of a negative zero, and of negative values that round to zero,
include a minus sign.
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 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.277)
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
277) When applied to infinite and NaN values, the -, +, and space flag characters have their usual meaning;
the # and 0 flag characters have no effect.
6; if the precision is zero and the # flag is not specified, no decimal-point
character appears. The value is rounded to the appropriate number of digits.
The E conversion specifier produces a number with E instead of e
introducing the exponent. The exponent always contains at least two digits,
and only as many more digits as necessary to represent the exponent. If the
value is zero, the exponent is zero.
A double argument representing an infinity or NaN is converted in the style
of an f or F conversion specifier.
g,G A double argument representing a floating-point number is converted in
style f or e (or in style F or E in the case of a G conversion specifier),
depending on the value converted and the precision. Let P equal the
precision if nonzero, 6 if the precision is omitted, or 1 if the precision is zero.
Then, if a conversion with style E would have an exponent of X :
-- if P > X -4, the conversion is with style f (or F) and precision
P - ( X + 1).
-- otherwise, the conversion is with style e (or E) and precision P - 1.
Finally, unless the # flag is used, any trailing zeros are removed from the
fractional portion of the result and the decimal-point character is removed if
there is no fractional portion remaining.
A double argument representing an infinity or NaN is converted in the style
of an f or F conversion specifier.
a,A A double argument representing a floating-point number is converted in the
style [-]0xh.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 character278) 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
278) 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.
distinguish279) 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.280) Characters from the array are
written up to (but not including) the terminating null character. If the
precision is specified, no more than that many bytes are written. If the
precision is not specified or is greater than the size of the 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
279) 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.
280) No special provisions are made for multibyte characters.
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.281)
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.282) 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.283) 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
281) Redundant shift sequences may result if multibyte characters have a state-dependent encoding.
282) See ``future library directions'' (7.31.11).
283) 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.
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.
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 (7.29.6), the wcrtomb function (7.29.6.3.3).
7.21.6.2 The fscanf function
Synopsis
1 #include <stdio.h>
int fscanf(FILE * restrict stream,
const char * restrict format, ...);
Description
2 The fscanf function reads input from the stream pointed to by stream, under control
of the string pointed to by format that specifies the admissible input sequences and how
they are to be converted for assignment, using subsequent arguments as pointers to the
objects to receive the converted input. If there are insufficient arguments for the format,
the behavior is undefined. If the format is exhausted while arguments remain, the excess
arguments are evaluated (as always) but are otherwise ignored.
3 The format shall be a multibyte character sequence, beginning and ending in its initial
shift state. The format is composed of zero or more directives: one or more white-space
characters, an ordinary multibyte character (neither % nor a white-space character), or a
conversion specification. Each conversion specification is introduced by the character %.
After the %, the following appear in sequence:
-- An optional assignment-suppressing character *.
-- An optional decimal integer greater than zero that specifies the maximum field width
(in characters).
-- An optional length modifier that specifies the size of the receiving object.
-- A conversion specifier character that specifies the type of conversion to be applied.
4 The fscanf function executes each directive of the format in turn. When all directives
have been executed, or if a directive fails (as detailed below), the function returns.
Failures are described as input failures (due to the occurrence of an encoding error or the
unavailability of input characters), or matching failures (due to inappropriate input).
5 A directive composed of white-space character(s) is executed by reading input up to the
first non-white-space character (which remains unread), or until no more characters can
be read. The directive never fails.
6 A directive that is an ordinary multibyte character is executed by reading the next
characters of the stream. If any of those characters differ from the ones composing the
directive, the directive fails and the differing and subsequent characters remain unread.
Similarly, if end-of-file, an encoding error, or a read error prevents a character from being
read, the directive fails.
7 A directive that is a conversion specification defines a set of matching input sequences, as
described below for each specifier. A conversion specification is executed in the
following steps:
8 Input white-space characters (as specified by the isspace function) are skipped, unless
the specification includes a [, c, or n specifier.284)
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.285)
The first character, if any, after the input item remains unread. If the length of the input
item is zero, the execution of the directive fails; this condition is a matching failure unless
end-of-file, an encoding error, or a read error prevented input from the stream, in which
case it is an input failure.
10 Except in the case of a % specifier, the input item (or, in the case of a %n directive, the
count of input characters) is converted to a type appropriate to the conversion specifier. If
the input item is not a matching sequence, the execution of the directive fails: this
condition is a matching failure. Unless assignment suppression was indicated by a *, the
result of the conversion is placed in the object pointed to by the first argument following
the format argument that has not already received a conversion result. If this object
does not have an appropriate type, or if the result of the conversion cannot be represented
in the object, the behavior is undefined.
11 The length modifiers and their meanings are:
hh Specifies that a following 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.
284) These white-space characters are not counted against a specified field width.
285) 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.
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).286)
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.286)
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.
[ Matches a nonempty sequence of characters from a set of expected characters
(the scanset ).286)
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
286) 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.
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.287)
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.
287) See ``future library directions'' (7.31.11).
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 the first conversion (if any) has completed. Otherwise, the function returns the
number of input items assigned, which can be fewer than provided for, or even zero, in
the event of an early matching failure.
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:
56789 0123 56a72
will assign to i the value 56 and to x the value 789.0, will skip 0123, and will assign to name the
sequence 56\0. The next character read from the input stream will be a.
19 EXAMPLE 3 To accept repeatedly from stdin a quantity, a unit of measure, and an item name:
#include <stdio.h>
/* ... */
int count; float quant; char units[21], item[21];
do {
count = fscanf(stdin, "%f%20s of %20s", &quant, units, item);
fscanf(stdin,"%*[^\n]");
} while (!feof(stdin) && !ferror(stdin));
20 If the stdin stream contains the following lines:
2 quarts of oil
-12.8degrees Celsius
lots of luck
10.0LBS of
dirt
100ergs of energy
the execution of the above example will be analogous to the following assignments:
quant = 2; strcpy(units, "quarts"); strcpy(item, "oil");
count = 3;
quant = -12.8; strcpy(units, "degrees");
count = 2; // "C" fails to match "o"
count = 0; // "l" fails to match "%f"
quant = 10.0; strcpy(units, "LBS"); strcpy(item, "dirt");
count = 3;
count = 0; // "100e" fails to match "%f"
count = EOF;
21 EXAMPLE 4 In:
#include <stdio.h>
/* ... */
int d1, d2, n1, n2, i;
i = sscanf("123", "%d%n%n%d", &d1, &n1, &n2, &d2);
the value 123 is assigned to d1 and the value 3 to n1. Because %n can never get an input failure, the value
of 3 is also assigned to n2. The value of d2 is not affected. The value 1 is assigned to i.
22 EXAMPLE 5 The call:
#include <stdio.h>
/* ... */
int n, i;
n = sscanf("foo % bar 42", "foo%%bar%d", &i);
will assign to n the value 1 and to i the value 42 because input white-space characters are skipped for both
the % and d conversion specifiers.
23 EXAMPLE 6 In these examples, multibyte characters do have a state-dependent encoding, and the
members of the extended character set that consist of more than one byte each consist of exactly two bytes,
the first of which is denoted here by a and the second by an uppercase letter, but are only recognized as
such when in the alternate shift state. The shift sequences are denoted by and , in which the first causes
entry into the alternate shift state.
24 After the call:
#include <stdio.h>
/* ... */
char str[50];
fscanf(stdin, "a%s", str);
with the input line:
a X Y bc
str will contain X Y\0 assuming that none of the bytes of the shift sequences (or of the multibyte
characters, in the more general case) appears to be a single-byte white-space character.
25 In contrast, after the call:
#include <stdio.h>
#include <stddef.h>
/* ... */
wchar_t wstr[50];
fscanf(stdin, "a%ls", wstr);
with the same input line, wstr will contain the two wide characters that correspond to X and Y and a
terminating null wide character.
26 However, the call:
#include <stdio.h>
#include <stddef.h>
/* ... */
wchar_t wstr[50];
fscanf(stdin, "a X%ls", wstr);
with the same input line will return zero due to a matching failure against the sequence in the format
string.
27 Assuming that the first byte of the multibyte character X is the same as the first byte of the multibyte
character Y, after the call:
#include <stdio.h>
#include <stddef.h>
/* ... */
wchar_t wstr[50];
fscanf(stdin, "a Y%ls", wstr);
with the same input line, zero will again be returned, but stdin will be left with a partially consumed
multibyte character.
Forward references: the strtod, strtof, and strtold functions (7.22.1.3), the
strtol, strtoll, strtoul, and strtoull functions (7.22.1.4), conversion state
(7.29.6), the wcrtomb function (7.29.6.3.3).
7.21.6.3 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.
7.21.6.4 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
the first conversion (if any) has completed. Otherwise, the scanf function returns the
number of input items assigned, which can be fewer than provided for, or even zero, in
the event of an early matching failure.
7.21.6.5 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.
7.21.6.6 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.
7.21.6.7 The sscanf function
Synopsis
1 #include <stdio.h>
int sscanf(const char * restrict s,
const char * restrict format, ...);
Description
2 The sscanf function is equivalent to fscanf, except that input is obtained from a
string (specified by the argument s) rather than from a stream. Reaching the end of the
string is equivalent to encountering end-of-file for the fscanf function. If copying
takes place between objects that overlap, the behavior is undefined.
Returns
3 The sscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the sscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.21.6.8 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.288)
Returns
3 The vfprintf function returns the number of characters transmitted, or a negative
value if an output or encoding error occurred.
4 EXAMPLE The following shows the use of the vfprintf function in a general error-reporting routine.
#include <stdarg.h>
#include <stdio.h>
void error(char *function_name, char *format, ...)
{
va_list args;
va_start(args, format);
// print out name of function causing error
fprintf(stderr, "ERROR in %s: ", function_name);
// print out remainder of message
vfprintf(stderr, format, args);
va_end(args);
}
7.21.6.9 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.288)
Returns
3 The vfscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the vfscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
288) 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.
7.21.6.10 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.288)
Returns
3 The vprintf function returns the number of characters transmitted, or a negative value
if an output or encoding error occurred.
7.21.6.11 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.288)
Returns
3 The vscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the vscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.21.6.12 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.288) 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.
7.21.6.13 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.288) 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.
7.21.6.14 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.288)
Returns
3 The vsscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the vsscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.21.7 Character input/output functions
7.21.7.1 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.289)
289) An end-of-file and a read error can be distinguished by use of the feof and ferror functions.
7.21.7.2 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.
7.21.7.3 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.
7.21.7.4 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.
7.21.7.5 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.
7.21.7.6 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.
7.21.7.7 The putc function
Synopsis
1 #include <stdio.h>
int putc(int c, FILE *stream);
Description
2 The putc function is equivalent to fputc, except that if it is implemented as a macro, it
may evaluate stream more than once, so that argument should never be an expression
with side effects.
Returns
3 The putc function returns the character written. If a write error occurs, the error
indicator for the stream is set and putc returns EOF.
7.21.7.8 The putchar function
Synopsis
1 #include <stdio.h>
int putchar(int c);
Description
2 The putchar function is equivalent to putc with the second argument stdout.
Returns
3 The putchar function returns the character written. If a write error occurs, the error
indicator for the stream is set and putchar returns EOF.
7.21.7.9 The puts function
Synopsis
1 #include <stdio.h>
int puts(const char *s);
Description
2 The puts function writes the string pointed to by s to the stream pointed to by stdout,
and appends a new-line character to the output. The terminating null character is not
written.
Returns
3 The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative
value.
7.21.7.10 The ungetc function
Synopsis
1 #include <stdio.h>
int ungetc(int c, FILE *stream);
Description
2 The ungetc function pushes the character specified by c (converted to an unsigned
char) back onto the input stream pointed to by stream. Pushed-back characters will be
returned by subsequent reads on that stream in the reverse order of their pushing. A
successful intervening call (with the stream pointed to by stream) to a file positioning
function (fseek, fsetpos, or rewind) discards any pushed-back characters for the
stream. The external storage corresponding to the stream is unchanged.
3 One character of pushback is guaranteed. If the ungetc function is called too many
times on the same stream without an intervening read or file positioning operation on that
stream, the operation may fail.
4 If the value of c equals that of the macro EOF, the operation fails and the input stream is
unchanged.
5 A successful call to the ungetc function clears the end-of-file indicator for the stream.
The value of the file position indicator for the stream after reading or discarding all
pushed-back characters shall be the same as it was before the characters were pushed
back. 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.290)
Returns
6 The ungetc function returns the character pushed back after conversion, or EOF if the
operation fails.
Forward references: file positioning functions (7.21.9).
290) See ``future library directions'' (7.31.11).
7.21.8 Direct input/output functions
7.21.8.1 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.
7.21.8.2 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.
7.21.9 File positioning functions
7.21.9.1 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 (7.21.9.3).
7.21.9.2 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 (7.21.9.4).
7.21.9.3 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.
7.21.9.4 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.
7.21.9.5 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.
7.21.10 Error-handling functions
7.21.10.1 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.
7.21.10.2 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.
7.21.10.3 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.
7.21.10.4 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 (7.24.6.2).
7.22 General utilities <stdlib.h>
1 The header <stdlib.h> declares five types and several functions of general utility, and
defines several macros.291)
2 The types declared are size_t and wchar_t (both described in 7.19),
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 7.19);
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.
291) See ``future library directions'' (7.31.12).
7.22.1 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.
7.22.1.1 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 (7.22.1.3).
7.22.1.2 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
(7.22.1.4).
7.22.1.3 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.292)
A character sequence INF or INFINITY is interpreted as an infinity, if representable in
the return type, else like a floating constant that is too large for the range of the return
type. A character sequence NAN or NAN(n-char-sequenceopt) is interpreted as a quiet
NaN, if supported in the return type, else like a subject sequence part that does not have
the expected form; the meaning of the n-char sequence is implementation-defined.293) 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
292) 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.
293) 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.294)
Returns
10 The functions return the converted value, if any. If no conversion could be performed,
zero is returned. If the correct value overflows and default rounding is in effect (7.12.1),
plus or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the
return type and sign of the value), 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.
7.22.1.4 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
294) 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.
7.22.2 Pseudo-random sequence generation functions
7.22.2.1 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.295)
3 The rand function is not required to avoid data races with other calls to pseudo-random
sequence generation functions. The implementation shall behave as if no library function
calls the rand function.
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.
7.22.2.2 The srand function
Synopsis
1 #include <stdlib.h>
void srand(unsigned int seed);
Description
2 The srand function uses the argument as a seed for a new sequence of pseudo-random
numbers to be returned by subsequent calls to rand. If srand is then called with the
same seed value, the sequence of pseudo-random numbers shall be repeated. If rand is
called before any calls to srand have been made, the same sequence shall be generated
as when srand is first called with a seed value of 1.
3 The srand function is not required to avoid data races with other calls to pseudo-
random sequence generation functions. The implementation shall behave as if no library
function calls the srand function.
295) There are no guarantees as to the quality of the random sequence produced and some implementations
are known to produce sequences with distressingly non-random low-order bits. Applications with
particular requirements should use a generator that is known to be sufficient for their needs.
Returns
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;
}
7.22.3 Memory management functions
1 The order and contiguity of storage allocated by successive calls to the
aligned_alloc, calloc, malloc, and realloc functions is unspecified. The
pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to
a pointer to any type of object with a fundamental alignment requirement and 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.
2 For purposes of determining the existence of a data race, memory allocation functions
behave as though they accessed only memory locations accessible through their
arguments and not other static duration storage. These functions may, however, visibly
modify the storage that they allocate or deallocate. A call to free or realloc that
deallocates a region p of memory synchronizes with any allocation call that allocates all
or part of the region p. This synchronization occurs after any access of p by the
deallocating function, and before any such access by the allocating function.
7.22.3.1 The aligned_alloc function
Synopsis
1 #include <stdlib.h>
void *aligned_alloc(size_t alignment, size_t size);
Description
2 The aligned_alloc function allocates space for an object whose alignment is
specified by alignment, whose size is specified by size, and whose value is
indeterminate. The value of alignment shall be a valid alignment supported by the
implementation and the value of size shall be an integral multiple of alignment.
Returns
3 The aligned_alloc function returns either a null pointer or a pointer to the allocated
space.
7.22.3.2 The calloc function
Synopsis
1 #include <stdlib.h>
void *calloc(size_t nmemb, size_t size);
Description
2 The calloc function allocates space for an array of nmemb objects, each of whose size
is size. The space is initialized to all bits zero.296)
Returns
3 The calloc function returns either a null pointer or a pointer to the allocated space.
7.22.3.3 The free function
Synopsis
1 #include <stdlib.h>
void free(void *ptr);
Description
2 The free function causes the space pointed to by ptr to be deallocated, that is, made
available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if
the argument does not match a pointer earlier returned by a memory management
function, or if the space has been deallocated by a call to free or realloc, the
behavior is undefined.
Returns
3 The free function returns no value.
296) Note that this need not be the same as the representation of floating-point zero or a null pointer
constant.
7.22.3.4 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.
7.22.3.5 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 a memory
management 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.
7.22.4 Communication with the environment
7.22.4.1 The abort function
Synopsis
1 #include <stdlib.h>
_Noreturn void abort(void);
Description
2 The abort function causes abnormal program termination to occur, unless the signal
SIGABRT is being caught and the signal handler does not return. Whether open streams
with unwritten buffered data are flushed, open streams are closed, or temporary files are
removed is implementation-defined. An implementation-defined form of the status
unsuccessful termination is returned to the host environment by means of the function
call raise(SIGABRT).
Returns
3 The abort function does not return to its caller.
7.22.4.2 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.297) It is unspecified whether a call to the
atexit function that does not happen before the exit function is called will succeed.
Environmental limits
3 The implementation shall support the registration of at least 32 functions.
Returns
4 The atexit function returns zero if the registration succeeds, nonzero if it fails.
Forward references: the at_quick_exit function (7.22.4.3), the exit function
(7.22.4.4).
297) The atexit function registrations are distinct from the at_quick_exit registrations, so
applications may need to call both registration functions with the same argument.
7.22.4.3 The at_quick_exit function
Synopsis
1 #include <stdlib.h>
int at_quick_exit(void (*func)(void));
Description
2 The at_quick_exit function registers the function pointed to by func, to be called
without arguments should quick_exit be called.298) It is unspecified whether a call to
the at_quick_exit function that does not happen before the quick_exit function
is called will succeed.
Environmental limits
3 The implementation shall support the registration of at least 32 functions.
Returns
4 The at_quick_exit function returns zero if the registration succeeds, nonzero if it
fails.
Forward references: the quick_exit function (7.22.4.7).
7.22.4.4 The exit function
Synopsis
1 #include <stdlib.h>
_Noreturn void exit(int status);
Description
2 The exit function causes normal program termination to occur. No functions registered
by the at_quick_exit function are called. If a program calls the exit function
more than once, or calls the quick_exit function in addition to the exit function, the
behavior is undefined.
3 First, all functions registered by the atexit function are called, in the reverse order of
their registration,299) 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.
298) The at_quick_exit function registrations are distinct from the atexit registrations, so
applications may need to call both registration functions with the same argument.
299) Each function is called as many times as it was registered, and in the correct order with respect to
other registered functions.
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.
7.22.4.5 The _Exit function
Synopsis
1 #include <stdlib.h>
_Noreturn void _Exit(int status);
Description
2 The _Exit function causes normal program termination to occur and control to be
returned to the host environment. No functions registered by the atexit function, the
at_quick_exit function, or signal handlers registered by the signal function are
called. The status returned to the host environment is determined in the same way as for
the exit function (7.22.4.4). Whether open streams with unwritten buffered data are
flushed, open streams are closed, or temporary files are removed is implementation-
defined.
Returns
3 The _Exit function cannot return to its caller.
7.22.4.6 The getenv function
Synopsis
1 #include <stdlib.h>
char *getenv(const char *name);
Description
2 The getenv function searches an environment list , provided by the host environment,
for a string that matches the string pointed to by name. The set of environment names
and the method for altering the environment list are implementation-defined. The
getenv function need not avoid data races with other threads of execution that modify
the environment list.300)
300) Many implementations provide non-standard functions that modify the environment list.
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.
7.22.4.7 The quick_exit function
Synopsis
1 #include <stdlib.h>
_Noreturn void quick_exit(int status);
Description
2 The quick_exit function causes normal program termination to occur. No functions
registered by the atexit function or signal handlers registered by the signal function
are called. If a program calls the quick_exit function more than once, or calls the
exit function in addition to the quick_exit function, the behavior is undefined. If a
signal is raised while the quick_exit function is executing, the behavior is undefined.
3 The quick_exit function first calls all functions registered by the at_quick_exit
function, in the reverse order of their registration,301) except that a function is called after
any previously registered functions that had already been called at the time it was
registered. If, during the call to any such function, a call to the longjmp function is
made that would terminate the call to the registered function, the behavior is undefined.
4 Then control is returned to the host environment by means of the function call
_Exit(status).
Returns
5 The quick_exit function cannot return to its caller.
7.22.4.8 The system function
Synopsis
1 #include <stdlib.h>
int system(const char *string);
Description
2 If string is a null pointer, the system function determines whether the host
environment has a command processor . If string is not a null pointer, the system
301) Each function is called as many times as it was registered, and in the correct order with respect to
other registered functions.
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.
7.22.5 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.302) 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.
302) 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
7.22.5.1 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
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.303)
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.
7.22.5.2 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.
303) In practice, the entire array is sorted according to the comparison function.
4 If two elements compare as equal, their order in the resulting sorted array is unspecified.
Returns
5 The qsort function returns no value.
7.22.6 Integer arithmetic functions
7.22.6.1 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.304)
Returns
3 The abs, labs, and llabs, functions return the absolute value.
7.22.6.2 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.
304) The absolute value of the most negative number cannot be represented in two's complement.
7.22.7 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 at program startup and can be returned to that state by a call for
which its character pointer argument, s, is a null pointer. Subsequent calls with s as
other than a null pointer cause the internal conversion state of the function to be altered as
necessary. A call with s as a null pointer causes these functions to return a nonzero value
if encodings have state dependency, and zero otherwise.305) Changing the LC_CTYPE
category causes the conversion state of these functions to be indeterminate.
7.22.7.1 The mblen function
Synopsis
1 #include <stdlib.h>
int mblen(const char *s, size_t n);
Description
2 If s is not a null pointer, the mblen function determines the number of bytes contained
in the multibyte character pointed to by s. Except that the conversion state of the
mbtowc function is not affected, it is equivalent to
mbtowc((wchar_t *)0, (const char *)0, 0);
mbtowc((wchar_t *)0, s, n);
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 (7.22.7.2).
305) 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.
7.22.7.2 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.
7.22.7.3 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.
7.22.8 Multibyte/wide string conversion functions
1 The behavior of the multibyte string functions is affected by the LC_CTYPE category of
the current locale.
7.22.8.1 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.306)
306) The array will not be null-terminated if the value returned is n.
7.22.8.2 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.306)
7.23 _Noreturn <stdnoreturn.h>
1 The header <stdnoreturn.h> defines the macro
noreturn
which expands to _Noreturn.
7.24 String handling <string.h>
7.24.1 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.307) The type is size_t and the macro is NULL (both described in
7.19). 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).
7.24.2 Copying functions
7.24.2.1 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.
307) See ``future library directions'' (7.31.13).
7.24.2.2 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.
7.24.2.3 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.
7.24.2.4 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
s1.308) 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.
7.24.3 Concatenation functions
7.24.3.1 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.
7.24.3.2 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.309) If copying
308) 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.
309) 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 (7.24.6.3).
7.24.4 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.
7.24.4.1 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 s2.310)
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.
7.24.4.2 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
310) 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.
7.24.4.3 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.
7.24.4.4 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.
7.24.4.5 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)
7.24.5 Search functions
7.24.5.1 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. The implementation shall behave as if it reads the characters sequentially
and stops as soon as a matching character is found.
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.
7.24.5.2 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.
7.24.5.3 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.
7.24.5.4 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.
7.24.5.5 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.
7.24.5.6 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.
7.24.5.7 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.
7.24.5.8 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 strtok function is not required to avoid data races with other calls to the strtok
function.311) The implementation shall behave as if no library function calls the strtok
function.
Returns
7 The strtok function returns a pointer to the first character of a token, or a null pointer
if there is no token.
8 EXAMPLE
#include <string.h>
static char str[] = "?a???b,,,#c";
char *t;
t = strtok(str, "?"); // t points to the token "a"
t = strtok(NULL, ","); // t points to the token "??b"
t = strtok(NULL, "#,"); // t points to the token "c"
t = strtok(NULL, "?"); // t is a null pointer
Forward references: The strtok_s function (K.3.7.3.1).
311) The strtok_s function can be used instead to avoid data races.
7.24.6 Miscellaneous functions
7.24.6.1 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.
7.24.6.2 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 strerror function is not required to avoid data races with other calls to the
strerror function.312) 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.
Forward references: The strerror_s function (K.3.7.4.2).
312) The strerror_s function can be used instead to avoid data races.
7.24.6.3 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.
7.25 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.313) 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.314)
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.
313) Like other function-like macros in Standard libraries, each type-generic macro can be suppressed to
make available the corresponding ordinary function.
314) 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)
7.26 Threads <threads.h>
7.26.1 Introduction
1 The header <threads.h> includes the header <time.h>, defines macros, and
declares types, enumeration constants, and functions that support multiple threads of
execution.315)
2 Implementations that define the macro _ _STDC_NO_THREADS_ _ need not provide
this header nor support any of its facilities.
3 The macros are
thread_local
which expands to _Thread_local;
ONCE_FLAG_INIT
which expands to a value that can be used to initialize an object of type once_flag;
and
TSS_DTOR_ITERATIONS
which expands to an integer constant expression representing the maximum number of
times that destructors will be called when a thread terminates.
4 The types are
cnd_t
which is a complete object type that holds an identifier for a condition variable;
thrd_t
which is a complete object type that holds an identifier for a thread;
tss_t
which is a complete object type that holds an identifier for a thread-specific storage
pointer;
mtx_t
which is a complete object type that holds an identifier for a mutex;
tss_dtor_t
which is the function pointer type void (*)(void*), used for a destructor for a
thread-specific storage pointer;
315) See ``future library directions'' (7.31.15).
thrd_start_t
which is the function pointer type int (*)(void*) that is passed to thrd_create
to create a new thread; and
once_flag
which is a complete object type that holds a flag for use by call_once.
5 The enumeration constants are
mtx_plain
which is passed to mtx_init to create a mutex object that supports neither timeout nor
test and return;
mtx_recursive
which is passed to mtx_init to create a mutex object that supports recursive locking;
mtx_timed
which is passed to mtx_init to create a mutex object that supports timeout;
thrd_timedout
which is returned by a timed wait function to indicate that the time specified in the call
was reached without acquiring the requested resource;
thrd_success
which is returned by a function to indicate that the requested operation succeeded;
thrd_busy
which is returned by a function to indicate that the requested operation failed because a
resource requested by a test and return function is already in use;
thrd_error
which is returned by a function to indicate that the requested operation failed; and
thrd_nomem
which is returned by a function to indicate that the requested operation failed because it
was unable to allocate memory.
Forward references: date and time (7.27).
7.26.2 Initialization functions
7.26.2.1 The call_once function
Synopsis
1 #include <threads.h>
void call_once(once_flag *flag, void (*func)(void));
Description
2 The call_once function uses the once_flag pointed to by flag to ensure that
func is called exactly once, the first time the call_once function is called with that
value of flag. Completion of an effective call to the call_once function synchronizes
with all subsequent calls to the call_once function with the same value of flag.
Returns
3 The call_once function returns no value.
7.26.3 Condition variable functions
7.26.3.1 The cnd_broadcast function
Synopsis
1 #include <threads.h>
int cnd_broadcast(cnd_t *cond);
Description
2 The cnd_broadcast function unblocks all of the threads that are blocked on the
condition variable pointed to by cond at the time of the call. If no threads are blocked
on the condition variable pointed to by cond at the time of the call, the function does
nothing.
Returns
3 The cnd_broadcast function returns thrd_success on success, or thrd_error
if the request could not be honored.
7.26.3.2 The cnd_destroy function
Synopsis
1 #include <threads.h>
void cnd_destroy(cnd_t *cond);
Description
2 The cnd_destroy function releases all resources used by the condition variable
pointed to by cond. The cnd_destroy function requires that no threads be blocked
waiting for the condition variable pointed to by cond.
Returns
3 The cnd_destroy function returns no value.
7.26.3.3 The cnd_init function
Synopsis
1 #include <threads.h>
int cnd_init(cnd_t *cond);
Description
2 The cnd_init function creates a condition variable. If it succeeds it sets the variable
pointed to by cond to a value that uniquely identifies the newly created condition
variable. A thread that calls cnd_wait on a newly created condition variable will
block.
Returns
3 The cnd_init function returns thrd_success on success, or thrd_nomem if no
memory could be allocated for the newly created condition, or thrd_error if the
request could not be honored.
7.26.3.4 The cnd_signal function
Synopsis
1 #include <threads.h>
int cnd_signal(cnd_t *cond);
Description
2 The cnd_signal function unblocks one of the threads that are blocked on the
condition variable pointed to by cond at the time of the call. If no threads are blocked
on the condition variable at the time of the call, the function does nothing and return
success.
Returns
3 The cnd_signal function returns thrd_success on success or thrd_error if
the request could not be honored.
7.26.3.5 The cnd_timedwait function
Synopsis
1 #include <threads.h>
int cnd_timedwait(cnd_t *restrict cond,
mtx_t *restrict mtx,
const struct timespec *restrict ts);
Description
2 The cnd_timedwait function atomically unlocks the mutex pointed to by mtx and
endeavors to block until the condition variable pointed to by cond is signaled by a call to
cnd_signal or to cnd_broadcast, or until after the TIME_UTC-based calendar
time pointed to by ts. When the calling thread becomes unblocked it locks the variable
pointed to by mtx before it returns. The cnd_timedwait function requires that the
mutex pointed to by mtx be locked by the calling thread.
Returns
3 The cnd_timedwait function returns thrd_success upon success, or
thrd_timedout if the time specified in the call was reached without acquiring the
requested resource, or thrd_error if the request could not be honored.
7.26.3.6 The cnd_wait function
Synopsis
1 #include <threads.h>
int cnd_wait(cnd_t *cond, mtx_t *mtx);
Description
2 The cnd_wait function atomically unlocks the mutex pointed to by mtx and endeavors
to block until the condition variable pointed to by cond is signaled by a call to
cnd_signal or to cnd_broadcast. When the calling thread becomes unblocked it
locks the mutex pointed to by mtx before it returns. The cnd_wait function requires
that the mutex pointed to by mtx be locked by the calling thread.
Returns
3 The cnd_wait function returns thrd_success on success or thrd_error if the
request could not be honored.
7.26.4 Mutex functions
7.26.4.1 The mtx_destroy function
Synopsis
1 #include <threads.h>
void mtx_destroy(mtx_t *mtx);
Description
2 The mtx_destroy function releases any resources used by the mutex pointed to by
mtx. No threads can be blocked waiting for the mutex pointed to by mtx.
Returns
3 The mtx_destroy function returns no value.
7.26.4.2 The mtx_init function
Synopsis
1 #include <threads.h>
int mtx_init(mtx_t *mtx, int type);
Description
2 The mtx_init function creates a mutex object with properties indicated by type,
which must have one of the six values:
mtx_plain for a simple non-recursive mutex,
mtx_timed for a non-recursive mutex that supports timeout,
mtx_plain | mtx_recursive for a simple recursive mutex, or
mtx_timed | mtx_recursive for a recursive mutex that supports timeout.
3 If the mtx_init function succeeds, it sets the mutex pointed to by mtx to a value that
uniquely identifies the newly created mutex.
Returns
4 The mtx_init function returns thrd_success on success, or thrd_error if the
request could not be honored.
7.26.4.3 The mtx_lock function
Synopsis
1 #include <threads.h>
int mtx_lock(mtx_t *mtx);
Description
2 The mtx_lock function blocks until it locks the mutex pointed to by mtx. If the mutex
is non-recursive, it shall not be locked by the calling thread. Prior calls to mtx_unlock
on the same mutex shall synchronize with this operation.
Returns
3 The mtx_lock function returns thrd_success on success, or thrd_error if the
request could not be honored.
7.26.4.4 The mtx_timedlock function
Synopsis
1 #include <threads.h>
int mtx_timedlock(mtx_t *restrict mtx,
const struct timespec *restrict ts);
Description
2 The mtx_timedlock function endeavors to block until it locks the mutex pointed to by
mtx or until after the TIME_UTC-based calendar time pointed to by ts. The specified
mutex shall support timeout. If the operation succeeds, prior calls to mtx_unlock on
the same mutex shall synchronize with this operation.
Returns
3 The mtx_timedlock function returns thrd_success on success, or
thrd_timedout if the time specified was reached without acquiring the requested
resource, or thrd_error if the request could not be honored.
7.26.4.5 The mtx_trylock function
Synopsis
1 #include <threads.h>
int mtx_trylock(mtx_t *mtx);
Description
2 The mtx_trylock function endeavors to lock the mutex pointed to by mtx. If the
mutex is already locked, the function returns without blocking. If the operation succeeds,
prior calls to mtx_unlock on the same mutex shall synchronize with this operation.
Returns
3 The mtx_trylock function returns thrd_success on success, or thrd_busy if
the resource requested is already in use, or thrd_error if the request could not be
honored.
7.26.4.6 The mtx_unlock function
Synopsis
1 #include <threads.h>
int mtx_unlock(mtx_t *mtx);
Description
2 The mtx_unlock function unlocks the mutex pointed to by mtx. The mutex pointed to
by mtx shall be locked by the calling thread.
Returns
3 The mtx_unlock function returns thrd_success on success or thrd_error if
the request could not be honored.
7.26.5 Thread functions
7.26.5.1 The thrd_create function
Synopsis
1 #include <threads.h>
int thrd_create(thrd_t *thr, thrd_start_t func,
void *arg);
Description
2 The thrd_create function creates a new thread executing func(arg). If the
thrd_create function succeeds, it sets the object pointed to by thr to the identifier of
the newly created thread. (A thread's identifier may be reused for a different thread once
the original thread has exited and either been detached or joined to another thread.) The
completion of the thrd_create function synchronizes with the beginning of the
execution of the new thread.
Returns
3 The thrd_create function returns thrd_success on success, or thrd_nomem if
no memory could be allocated for the thread requested, or thrd_error if the request
could not be honored.
7.26.5.2 The thrd_current function
Synopsis
1 #include <threads.h>
thrd_t thrd_current(void);
Description
2 The thrd_current function identifies the thread that called it.
Returns
3 The thrd_current function returns the identifier of the thread that called it.
7.26.5.3 The thrd_detach function
Synopsis
1 #include <threads.h>
int thrd_detach(thrd_t thr);
Description
2 The thrd_detach function tells the operating system to dispose of any resources
allocated to the thread identified by thr when that thread terminates. The thread
identified by thr shall not have been previously detached or joined with another thread.
Returns
3 The thrd_detach function returns thrd_success on success or thrd_error if
the request could not be honored.
7.26.5.4 The thrd_equal function
Synopsis
1 #include <threads.h>
int thrd_equal(thrd_t thr0, thrd_t thr1);
Description
2 The thrd_equal function will determine whether the thread identified by thr0 refers
to the thread identified by thr1.
Returns
3 The thrd_equal function returns zero if the thread thr0 and the thread thr1 refer to
different threads. Otherwise the thrd_equal function returns a nonzero value.
7.26.5.5 The thrd_exit function
Synopsis
1 #include <threads.h>
_Noreturn void thrd_exit(int res);
Description
2 The thrd_exit function terminates execution of the calling thread and sets its result
code to res.
3 The program shall terminate normally after the last thread has been terminated. The
behavior shall be as if the program called the exit function with the status
EXIT_SUCCESS at thread termination time.
Returns
4 The thrd_exit function returns no value.
7.26.5.6 The thrd_join function
Synopsis
1 #include <threads.h>
int thrd_join(thrd_t thr, int *res);
Description
2 The thrd_join function joins the thread identified by thr with the current thread by
blocking until the other thread has terminated. If the parameter res is not a null pointer,
it stores the thread's result code in the integer pointed to by res. The termination of the
other thread synchronizes with the completion of the thrd_join function. The thread
identified by thr shall not have been previously detached or joined with another thread.
Returns
3 The thrd_join function returns thrd_success on success or thrd_error if the
request could not be honored.
7.26.5.7 The thrd_sleep function
Synopsis
1 #include <threads.h>
int thrd_sleep(const struct timespec *duration,
struct timespec *remaining);
Description
2 The thrd_sleep function suspends execution of the calling thread until either the
interval specified by duration has elapsed or a signal which is not being ignored is
received. If interrupted by a signal and the remaining argument is not null, the
amount of time remaining (the requested interval minus the time actually slept) is stored
in the interval it points to. The duration and remaining arguments may point to the
same object.
3 The suspension time may be longer than requested because the interval is rounded up to
an integer multiple of the sleep resolution or because of the scheduling of other activity
by the system. But, except for the case of being interrupted by a signal, the suspension
time shall not be less than that specified, as measured by the system clock TIME_UTC.
Returns
4 The thrd_sleep function returns zero if the requested time has elapsed, -1 if it has
been interrupted by a signal, or a negative value if it fails.
7.26.5.8 The thrd_yield function
Synopsis
1 #include <threads.h>
void thrd_yield(void);
Description
2 The thrd_yield function endeavors to permit other threads to run, even if the current
thread would ordinarily continue to run.
Returns
3 The thrd_yield function returns no value.
7.26.6 Thread-specific storage functions
7.26.6.1 The tss_create function
Synopsis
1 #include <threads.h>
int tss_create(tss_t *key, tss_dtor_t dtor);
Description
2 The tss_create function creates a thread-specific storage pointer with destructor
dtor, which may be null.
Returns
3 If the tss_create function is successful, it sets the thread-specific storage pointed to
by key to a value that uniquely identifies the newly created pointer and returns
thrd_success; otherwise, thrd_error is returned and the thread-specific storage
pointed to by key is set to an undefined value.
7.26.6.2 The tss_delete function
Synopsis
1 #include <threads.h>
void tss_delete(tss_t key);
Description
2 The tss_delete function releases any resources used by the thread-specific storage
identified by key.
Returns
3 The tss_delete function returns no value.
7.26.6.3 The tss_get function
Synopsis
1 #include <threads.h>
void *tss_get(tss_t key);
Description
2 The tss_get function returns the value for the current thread held in the thread-specific
storage identified by key.
Returns
3 The tss_get function returns the value for the current thread if successful, or zero if
unsuccessful.
7.26.6.4 The tss_set function
Synopsis
1 #include <threads.h>
int tss_set(tss_t key, void *val);
Description
2 The tss_set function sets the value for the current thread held in the thread-specific
storage identified by key to val.
Returns
3 The tss_set function returns thrd_success on success or thrd_error if the
request could not be honored.
7.27 Date and time <time.h>
7.27.1 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 7.19);
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; and
TIME_UTC
which expands to an integer constant greater than 0 that designates the UTC time
base.316)
3 The types declared are size_t (described in 7.19);
clock_t
and
time_t
which are real types capable of representing times;
struct timespec
which holds an interval specified in seconds and nanoseconds (which may represent a
calendar time based on a particular epoch); and
struct tm
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 timespec structure shall contain at least the following
members, in any order.317)
316) Implementations may define additional time bases, but are only required to support a real time clock
based on UTC.
317) The tv_sec member is a linear count of seconds and may not have the normal semantics of a
time_t. The semantics of the members and their normal ranges are expressed in the comments.
time_t tv_sec; // whole seconds -- 0
long tv_nsec; // nanoseconds -- [0, 999999999]
The tm structure shall contain at least the following members, in any order. The
semantics of the members and their normal ranges are expressed in the comments.318)
int tm_sec; // seconds after the minute -- [0, 60]
int tm_min; // minutes after the hour -- [0, 59]
int tm_hour; // hours since midnight -- [0, 23]
int tm_mday; // day of the month -- [1, 31]
int tm_mon; // months since January -- [0, 11]
int tm_year; // years since 1900
int tm_wday; // days since Sunday -- [0, 6]
int tm_yday; // days since January 1 -- [0, 365]
int tm_isdst; // Daylight Saving Time flag
The value of tm_isdst is positive if Daylight Saving Time is in effect, zero if Daylight
Saving Time is not in effect, and negative if the information is not available.
7.27.2 Time manipulation functions
7.27.2.1 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).319)
318) The range [0, 60] for tm_sec allows for a positive leap second.
319) 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.
7.27.2.2 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.
7.27.2.3 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.320) 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?
320) 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.
#include <stdio.h>
#include <time.h>
static const char *const wday[] = {
"Sunday", "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "-unknown-"
};
struct tm time_str;
/* ... */
time_str.tm_year = 2001 - 1900;
time_str.tm_mon = 7 - 1;
time_str.tm_mday = 4;
time_str.tm_hour = 0;
time_str.tm_min = 0;
time_str.tm_sec = 1;
time_str.tm_isdst = -1;
if (mktime(&time_str) == (time_t)(-1))
time_str.tm_wday = 7;
printf("%s\n", wday[time_str.tm_wday]);
7.27.2.4 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.
7.27.2.5 The timespec_get function
Synopsis
1 #include <time.h>
int timespec_get(struct timespec *ts, int base);
Description
2 The timespec_get function sets the interval pointed to by ts to hold the current
calendar time based on the specified time base.
3 If base is TIME_UTC, the tv_sec member is set to the number of seconds since an
implementation defined epoch, truncated to a whole value and the tv_nsec member is
set to the integral number of nanoseconds, rounded to the resolution of the system
clock.321)
Returns
4 If the timespec_get function is successful it returns the nonzero value base;
otherwise, it returns zero.
7.27.3 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 and the functions are not required to avoid data races with
each other.322) The implementation shall behave as if no other library functions call these
functions.
7.27.3.1 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];
321) Although a struct timespec object describes times with nanosecond resolution, the available
resolution is system dependent and may even be greater than 1 second.
322) Alternative time conversion functions that do avoid data races are specified in K.3.8.2.
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;
}
3 If any of the members of the broken-down time contain values that are outside their
normal ranges,323) the behavior of the asctime function is undefined. Likewise, if the
calculated year exceeds four digits or is less than the year 1000, the behavior is
undefined.
Returns
4 The asctime function returns a pointer to the string.
7.27.3.2 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 (7.27.3.4).
7.27.3.3 The gmtime function
Synopsis
1 #include <time.h>
struct tm *gmtime(const time_t *timer);
323) See 7.27.1.
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.
7.27.3.4 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.
7.27.3.5 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 7.27.1]
%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 7.27.1]
%X is replaced by the locale's appropriate time representation. [all specified in 7.27.1]
%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.
7.28 Unicode utilities <uchar.h>
1 The header <uchar.h> declares types and functions for manipulating Unicode
characters.
2 The types declared are mbstate_t (described in 7.30.1) and size_t (described in 7.19);
char16_t
which is an unsigned integer type used for 16-bit characters and is the same type as
uint_least16_t (described in 7.20.1.2); and
char32_t
which is an unsigned integer type used for 32-bit characters and is the same type as
uint_least32_t (also described in 7.20.1.2).
7.28.1 Restartable multibyte/wide character conversion functions
1 These functions have a parameter, ps, of type pointer to mbstate_t that points to an
object that can completely describe the current conversion state of the associated
multibyte character sequence, which the functions alter as necessary. If ps is a null
pointer, each function uses its own internal mbstate_t object instead, which is
initialized at program startup to the initial conversion state; the functions are not required
to avoid data races with other calls to the same function in this case. The implementation
behaves as if no library function calls these functions with a null pointer for ps.
7.28.1.1 The mbrtoc16 function
Synopsis
1 #include <uchar.h>
size_t mbrtoc16(char16_t * restrict pc16,
const char * restrict s, size_t n,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the mbrtoc16 function is equivalent to the call:
mbrtoc16(NULL, "", 1, ps)
In this case, the values of the parameters pc16 and n are ignored.
3 If s is not a null pointer, the mbrtoc16 function inspects at most n bytes beginning with
the byte pointed to by s to determine the number of bytes needed to complete the next
multibyte character (including any shift sequences). If the function determines that the
next multibyte character is complete and valid, it determines the values of the
corresponding wide characters and then, if pc16 is not a null pointer, stores the value of
the first (or only) such character in the object pointed to by pc16. Subsequent calls will
store successive wide characters without consuming any additional input until all the
characters have been stored. If the corresponding wide character is the null wide
character, the resulting state described is the initial conversion state.
Returns
4 The mbrtoc16 function returns the first of the following that applies (given the current
conversion state):
0 if the next n or fewer bytes complete the multibyte character that
corresponds to the null wide character (which is the value stored).
between 1 and n inclusive if the next n or fewer bytes complete a valid multibyte
character (which is the value stored); the value returned is the number
of bytes that complete the multibyte character.
(size_t)(-3) if the next character resulting from a previous call has been stored (no
bytes from the input have been consumed by this call).
(size_t)(-2) if the next n bytes contribute to an incomplete (but potentially valid)
multibyte character, and all n bytes have been processed (no value is
stored).324)
(size_t)(-1) if an encoding error occurs, in which case the next n or fewer bytes
do not contribute to a complete and valid multibyte character (no
value is stored); the value of the macro EILSEQ is stored in errno,
and the conversion state is unspecified.
7.28.1.2 The c16rtomb function
Synopsis
1 #include <uchar.h>
size_t c16rtomb(char * restrict s, char16_t c16,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the c16rtomb function is equivalent to the call
c16rtomb(buf, L'\0', ps)
where buf is an internal buffer.
3 If s is not a null pointer, the c16rtomb function determines the number of bytes needed
to represent the multibyte character that corresponds to the wide character given by c16
(including any shift sequences), and stores the multibyte character representation in the
324) 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).
array whose first element is pointed to by s. At most MB_CUR_MAX bytes are stored. If
c16 is a null wide character, a null byte is stored, preceded by any shift sequence needed
to restore the initial shift state; the resulting state described is the initial conversion state.
Returns
4 The c16rtomb function returns the number of bytes stored in the array object (including
any shift sequences). When c16 is not a valid wide character, an encoding error occurs:
the function stores the value of the macro EILSEQ in errno and returns
(size_t)(-1); the conversion state is unspecified.
7.28.1.3 The mbrtoc32 function
Synopsis
1 #include <uchar.h>
size_t mbrtoc32(char32_t * restrict pc32,
const char * restrict s, size_t n,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the mbrtoc32 function is equivalent to the call:
mbrtoc32(NULL, "", 1, ps)
In this case, the values of the parameters pc32 and n are ignored.
3 If s is not a null pointer, the mbrtoc32 function inspects at most n bytes beginning with
the byte pointed to by s to determine the number of bytes needed to complete the next
multibyte character (including any shift sequences). If the function determines that the
next multibyte character is complete and valid, it determines the values of the
corresponding wide characters and then, if pc32 is not a null pointer, stores the value of
the first (or only) such character in the object pointed to by pc32. Subsequent calls will
store successive wide characters without consuming any additional input until all the
characters have been stored. If the corresponding wide character is the null wide
character, the resulting state described is the initial conversion state.
Returns
4 The mbrtoc32 function returns the first of the following that applies (given the current
conversion state):
0 if the next n or fewer bytes complete the multibyte character that
corresponds to the null wide character (which is the value stored).
between 1 and n inclusive if the next n or fewer bytes complete a valid multibyte
character (which is the value stored); the value returned is the number
of bytes that complete the multibyte character.
(size_t)(-3) if the next character resulting from a previous call has been stored (no
bytes from the input have been consumed by this call).
(size_t)(-2) if the next n bytes contribute to an incomplete (but potentially valid)
multibyte character, and all n bytes have been processed (no value is
stored).325)
(size_t)(-1) if an encoding error occurs, in which case the next n or fewer bytes
do not contribute to a complete and valid multibyte character (no
value is stored); the value of the macro EILSEQ is stored in errno,
and the conversion state is unspecified.
7.28.1.4 The c32rtomb function
Synopsis
1 #include <uchar.h>
size_t c32rtomb(char * restrict s, char32_t c32,
mbstate_t * restrict ps);
Description
2 If s is a null pointer, the c32rtomb function is equivalent to the call
c32rtomb(buf, L'\0', ps)
where buf is an internal buffer.
3 If s is not a null pointer, the c32rtomb function determines the number of bytes needed
to represent the multibyte character that corresponds to the wide character given by c32
(including any shift sequences), and stores the multibyte character representation in the
array whose first element is pointed to by s. At most MB_CUR_MAX bytes are stored. If
c32 is a null wide character, a null byte is stored, preceded by any shift sequence needed
to restore the initial shift state; the resulting state described is the initial conversion state.
Returns
4 The c32rtomb function returns the number of bytes stored in the array object (including
any shift sequences). When c32 is not a valid wide character, an encoding error occurs:
the function stores the value of the macro EILSEQ in errno and returns
(size_t)(-1); the conversion state is unspecified.
325) 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).
7.29 Extended multibyte and wide character utilities <wchar.h>
7.29.1 Introduction
1 The header <wchar.h> defines four macros, and declares four data types, one tag, and
many functions.326)
2 The types declared are wchar_t and size_t (both described in 7.19);
mbstate_t
which is a complete 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);327) and
struct tm
which is declared as an incomplete structure type (the contents are described in 7.27.1).
3 The macros defined are NULL (described in 7.19); WCHAR_MIN and WCHAR_MAX
(described in 7.20.3); 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.328) 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;
326) See ``future library directions'' (7.31.16).
327) wchar_t and wint_t can be the same integer type.
328) 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 Arguments to the functions in this subclause may point to arrays containing wchar_t
values that do not correspond to members of the extended character set. Such values
shall be processed according to the specified semantics, except that it is unspecified
whether an encoding error occurs if such a value appears in the format string for a
function in 7.29.2 or 7.29.5 and the specified semantics do not require that value to be
processed by wcrtomb.
6 Unless explicitly stated otherwise, if the execution of a function described in this
subclause causes copying to take place between objects that overlap, the behavior is
undefined.
7.29.2 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.329)
7.29.2.1 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.
329) The fwprintf functions perform writes to memory for the %n specifier.
4 Each conversion specification is introduced by the wide character %. After the %, the
following appear in sequence:
-- Zero or more flags (in any order) that modify the meaning of the conversion
specification.
-- An optional minimum field width. If the converted value has fewer wide characters
than the field width, it is padded with spaces (by default) on the left (or right, if the
left adjustment flag, described later, has been given) to the field width. The field
width takes the form of an asterisk * (described later) or a nonnegative decimal
integer.330)
-- 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
330) Note that 0 is taken as a flag, not as the beginning of a field width.
specified.)331)
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,
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
331) 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 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.332)
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
332) 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.
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 character333) 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
distinguish334) 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
333) 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.
334) 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.
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-
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.335) 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.336) If the number of
335) See ``future library directions'' (7.31.16).
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.
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 (7.29.6.1.1), the mbrtowc function
(7.29.6.3.2).
7.29.2.2 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
336) 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.
as pointers to the objects to receive the converted input. If there are insufficient
arguments for the format, the behavior is undefined. If the format is exhausted while
arguments remain, the excess arguments are evaluated (as always) but are otherwise
ignored.
3 The format is composed of zero or more directives: one or more white-space wide
characters, an ordinary wide character (neither % nor a white-space wide character), or a
conversion specification. Each conversion specification is introduced by the wide
character %. After the %, the following appear in sequence:
-- An optional assignment-suppressing wide character *.
-- An optional decimal integer greater than zero that specifies the maximum field width
(in wide characters).
-- An optional length modifier that specifies the size of the receiving object.
-- A conversion specifier wide character that specifies the type of conversion to be
applied.
4 The fwscanf function executes each directive of the format in turn. When all directives
have been executed, or if a directive fails (as detailed below), the function returns.
Failures are described as input failures (due to the occurrence of an encoding error or the
unavailability of input characters), or matching failures (due to inappropriate input).
5 A directive composed of white-space wide character(s) is executed by reading input up to
the first non-white-space wide character (which remains unread), or until no more wide
characters can be read. The directive never fails.
6 A directive that is an ordinary wide character is executed by reading the next wide
character of the stream. If that wide character differs from the directive, the directive
fails and the differing and subsequent wide characters remain unread. Similarly, if end-
of-file, an encoding error, or a read error prevents a wide character from being read, the
directive fails.
7 A directive that is a conversion specification defines a set of matching input sequences, as
described below for each specifier. A conversion specification is executed in the
following steps:
8 Input white-space wide characters (as specified by the iswspace function) are skipped,
unless the specification includes a [, c, or n specifier.337)
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
337) These white-space wide characters are not counted against a specified field width.
sequence.338) The first wide character, if any, after the input item remains unread. If the
length of the input item is zero, the execution of the directive fails; this condition is a
matching failure unless end-of-file, an encoding error, or a read error prevented input
from the stream, in which case it is an input failure.
10 Except in the case of a % specifier, the input item (or, in the case of a %n directive, the
count of input wide characters) is converted to a type appropriate to the conversion
specifier. If the input item is not a matching sequence, the execution of the directive fails:
this condition is a matching failure. Unless assignment suppression was indicated by a *,
the result of the conversion is placed in the object pointed to by the first argument
following the format argument that has not already received a conversion result. If this
object does not have an appropriate type, or if the result of the conversion cannot be
represented in the object, the behavior is undefined.
11 The length modifiers and their meanings are:
hh Specifies that a following 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.
338) 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.
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.339)
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 the first conversion (if any) has completed. Otherwise, the function returns the
number of input items assigned, which can be fewer than provided for, or even zero, in
the event of an early matching failure.
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);
339) See ``future library directions'' (7.31.16).
with the input line:
25 54.32E-1 thompson
will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence
thompson\0.
18 EXAMPLE 2 The call:
#include <stdio.h>
#include <wchar.h>
/* ... */
int i; float x; double y;
fwscanf(stdin, L"%2d%f%*d %lf", &i, &x, &y);
with input:
56789 0123 56a72
will assign to i the value 56 and to x the value 789.0, will skip past 0123, and will assign to y the value
56.0. The next wide character read from the input stream will be a.
Forward references: the wcstod, wcstof, and wcstold functions (7.29.4.1.1), the
wcstol, wcstoll, wcstoul, and wcstoull functions (7.29.4.1.2), the wcrtomb
function (7.29.6.3.3).
7.29.2.3 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.
7.29.2.4 The swscanf function
Synopsis
1 #include <wchar.h>
int swscanf(const wchar_t * restrict s,
const wchar_t * restrict format, ...);
Description
2 The swscanf function is equivalent to fwscanf, except that the argument s specifies a
wide string from which the input is to be obtained, rather than from a stream. Reaching
the end of the wide string is equivalent to encountering end-of-file for the fwscanf
function.
Returns
3 The swscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the swscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.29.2.5 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.340)
Returns
3 The vfwprintf function returns the number of wide characters transmitted, or a
negative value if an output or encoding error occurred.
340) As the functions vfwprintf, vswprintf, vfwscanf, vwprintf, vwscanf, and vswscanf
invoke the va_arg macro, the value of arg after the return is indeterminate.
4 EXAMPLE The following shows the use of the vfwprintf function in a general error-reporting
routine.
#include <stdarg.h>
#include <stdio.h>
#include <wchar.h>
void error(char *function_name, wchar_t *format, ...)
{
va_list args;
va_start(args, format);
// print out name of function causing error
fwprintf(stderr, L"ERROR in %s: ", function_name);
// print out remainder of message
vfwprintf(stderr, format, args);
va_end(args);
}
7.29.2.6 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.340)
Returns
3 The vfwscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the vfwscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.29.2.7 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.340)
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.
7.29.2.8 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.340)
Returns
3 The vswscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the vswscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.29.2.9 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.340)
Returns
3 The vwprintf function returns the number of wide characters transmitted, or a negative
value if an output or encoding error occurred.
7.29.2.10 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.340)
Returns
3 The vwscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the vwscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.29.2.11 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.
7.29.2.12 The wscanf function
Synopsis
1 #include <wchar.h>
int wscanf(const wchar_t * restrict format, ...);
Description
2 The wscanf function is equivalent to fwscanf with the argument stdin interposed
before the arguments to wscanf.
Returns
3 The wscanf function returns the value of the macro EOF if an input failure occurs
before the first conversion (if any) has completed. Otherwise, the wscanf function
returns the number of input items assigned, which can be fewer than provided for, or even
zero, in the event of an early matching failure.
7.29.3 Wide character input/output functions
7.29.3.1 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.341)
7.29.3.2 The fgetws function
Synopsis
1 #include <stdio.h>
#include <wchar.h>
wchar_t *fgetws(wchar_t * restrict s,
int n, FILE * restrict stream);
Description
2 The fgetws function reads at most one less than the number of wide characters
specified by n from the stream pointed to by stream into the array pointed to by s. No
additional wide characters are read after a new-line wide character (which is retained) or
after end-of-file. A null wide character is written immediately after the last wide
character read into the array.
Returns
3 The fgetws function returns s if successful. If end-of-file is encountered and no
characters have been read into the array, the contents of the array remain unchanged and a
null pointer is returned. If a read or encoding error occurs during the operation, the array
contents are indeterminate and a null pointer is returned.
7.29.3.3 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
341) 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.
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.
7.29.3.4 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.
7.29.3.5 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.342)
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.
342) If the orientation of the stream has already been determined, fwide does not change it.
7.29.3.6 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.
7.29.3.7 The getwchar function
Synopsis
1 #include <wchar.h>
wint_t getwchar(void);
Description
2 The getwchar function is equivalent to getwc with the argument stdin.
Returns
3 The getwchar function returns the next wide character from the input stream pointed to
by stdin, or WEOF.
7.29.3.8 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.
7.29.3.9 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.
7.29.3.10 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.
7.29.4 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.
7.29.4.1 Wide string numeric conversion functions
7.29.4.1.1 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.343) A wide character sequence INF or
INFINITY is interpreted as an infinity, if representable in the return type, else like a
floating constant that is too large for the range of the return type. A wide character
sequence NAN or NAN(n-wchar-sequenceopt) is interpreted as a quiet NaN, if supported
in the return type, else like a subject sequence part that does not have the expected form;
the meaning of the n-wchar sequence is implementation-defined.344) A pointer to the
343) 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.
344) An implementation may use the n-wchar sequence to determine extra information to be represented in
the NaN's significand.
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.
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.345)
Returns
10 The functions return the converted value, if any. If no conversion could be performed,
zero is returned. If the correct value overflows and default rounding is in effect (7.12.1),
plus or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the
return type and sign of the value), 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.
345) 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.
7.29.4.1.2 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.
7.29.4.2 Wide string copying functions
7.29.4.2.1 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.
7.29.4.2.2 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
s1.346)
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.
7.29.4.2.3 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.
346) 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.
7.29.4.2.4 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.
7.29.4.3 Wide string concatenation functions
7.29.4.3.1 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.
7.29.4.3.2 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.347)
Returns
3 The wcsncat function returns the value of s1.
7.29.4.4 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.
7.29.4.4.1 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.
7.29.4.4.2 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
347) 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.
7.29.4.4.3 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.
7.29.4.4.4 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)
7.29.4.4.5 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.
7.29.4.5 Wide string search functions
7.29.4.5.1 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.
7.29.4.5.2 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.
7.29.4.5.3 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.
7.29.4.5.4 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.
7.29.4.5.5 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.
7.29.4.5.6 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.
7.29.4.5.7 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
7.29.4.5.8 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.
7.29.4.6 Miscellaneous functions
7.29.4.6.1 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.
7.29.4.6.2 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.
7.29.5 Wide character time conversion functions
7.29.5.1 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.
7.29.6 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'', 7.29.6.3 and
7.29.6.4 -- 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.348)
4 On entry, each function takes the described conversion state (either internal or pointed to
by an argument) as current. The conversion state described by the referenced object is
altered as needed to track the shift state, and the position within a multibyte character, for
the associated multibyte character sequence.
348) 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.
7.29.6.1 Single-byte/wide character conversion functions
7.29.6.1.1 The btowc function
Synopsis
1 #include <wchar.h>
wint_t btowc(int c);
Description
2 The btowc function determines whether c constitutes a valid single-byte character in the
initial shift state.
Returns
3 The btowc function returns WEOF if c has the value EOF or if (unsigned char)c
does not constitute a valid single-byte character in the initial shift state. Otherwise, it
returns the wide character representation of that character.
7.29.6.1.2 The wctob function
Synopsis
1 #include <wchar.h>
int wctob(wint_t c);
Description
2 The wctob function determines whether c corresponds to a member of the extended
character set whose multibyte character representation is a single byte when in the initial
shift state.
Returns
3 The wctob function returns EOF if c does not correspond to a multibyte character with
length one in the initial shift state. Otherwise, it returns the single-byte representation of
that character as an unsigned char converted to an int.
7.29.6.2 Conversion state functions
7.29.6.2.1 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 referenced
mbstate_t object describes an initial conversion state.
Returns
3 The mbsinit function returns nonzero if ps is a null pointer or if the referenced object
describes an initial conversion state; otherwise, it returns zero.
7.29.6.3 Restartable multibyte/wide character conversion functions
1 These functions differ from the corresponding multibyte character functions of 7.22.7
(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 functions are not required to avoid data
races with other calls to the same function in this case. 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.
7.29.6.3.1 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 (7.29.6.3.2).
7.29.6.3.2 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).349)
(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.
349) 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).
7.29.6.3.3 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.
7.29.6.4 Restartable multibyte/wide string conversion functions
1 These functions differ from the corresponding multibyte string functions of 7.22.8
(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 functions are not required to avoid data races with other
calls to the same function in this case. 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.
7.29.6.4.1 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.350) 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).
350) Thus, the value of len is ignored if dst is a null pointer.
7.29.6.4.2 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.351)
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).
351) 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.
7.30 Wide character classification and mapping utilities <wctype.h>
7.30.1 Introduction
1 The header <wctype.h> defines one macro, and declares three data types and many
functions.352)
2 The types declared are
wint_t
described in 7.29.1;
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 7.29.1).
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.
352) See ``future library directions'' (7.31.17).
7.30.2 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.
7.30.2.1 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.353)
Forward references: the wctob function (7.29.6.1.2).
7.30.2.1.1 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.
7.30.2.1.2 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
353) 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.354)
7.30.2.1.3 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.
7.30.2.1.4 The iswcntrl function
Synopsis
1 #include <wctype.h>
int iswcntrl(wint_t wc);
Description
2 The iswcntrl function tests for any control wide character.
7.30.2.1.5 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).
7.30.2.1.6 The iswgraph function
Synopsis
1 #include <wctype.h>
int iswgraph(wint_t wc);
354) 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.355)
7.30.2.1.7 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.
7.30.2.1.8 The iswprint function
Synopsis
1 #include <wctype.h>
int iswprint(wint_t wc);
Description
2 The iswprint function tests for any printing wide character.
7.30.2.1.9 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.355)
7.30.2.1.10 The iswspace function
Synopsis
1 #include <wctype.h>
int iswspace(wint_t wc);
355) 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.
7.30.2.1.11 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.
7.30.2.1.12 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).
7.30.2.2 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 (7.30.2.1).
7.30.2.2.1 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 (7.30.2.1) in the comment that follows the expression:
iswctype(wc, wctype("alnum")) // iswalnum(wc)
iswctype(wc, wctype("alpha")) // iswalpha(wc)
iswctype(wc, wctype("blank")) // iswblank(wc)
iswctype(wc, wctype("cntrl")) // iswcntrl(wc)
iswctype(wc, wctype("digit")) // iswdigit(wc)
iswctype(wc, wctype("graph")) // iswgraph(wc)
iswctype(wc, wctype("lower")) // iswlower(wc)
iswctype(wc, wctype("print")) // iswprint(wc)
iswctype(wc, wctype("punct")) // iswpunct(wc)
iswctype(wc, wctype("space")) // iswspace(wc)
iswctype(wc, wctype("upper")) // iswupper(wc)
iswctype(wc, wctype("xdigit")) // iswxdigit(wc)
Returns
4 The iswctype function returns nonzero (true) if and only if the value of the wide
character wc has the property described by desc. If desc is zero, the iswctype
function returns zero (false).
Forward references: the wctype function (7.30.2.2.2).
7.30.2.2.2 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.
7.30.3 Wide character case mapping utilities
1 The header <wctype.h> declares several functions useful for mapping wide characters.
7.30.3.1 Wide character case mapping functions
7.30.3.1.1 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.
7.30.3.1.2 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.
7.30.3.2 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 (7.30.3.1).
7.30.3.2.1 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 (7.30.3.1) in the comment that follows the expression:
towctrans(wc, wctrans("tolower")) // towlower(wc)
towctrans(wc, wctrans("toupper")) // towupper(wc)
Returns
4 The towctrans function returns the mapped value of wc using the mapping described
by desc. If desc is zero, the towctrans function returns the value of wc.
7.30.3.2.2 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.
7.31 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.
7.31.1 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.
7.31.2 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.
7.31.3 Errors <errno.h>
1 Macros that begin with E and a digit or E and an uppercase letter may be added to the
macros defined in the <errno.h> header.
7.31.4 Floating-point environment <fenv.h>
1 Macros that begin with FE_ and an uppercase letter may be added to the macros defined
in the <fenv.h> header.
7.31.5 Format conversion of integer types <inttypes.h>
1 Macros that begin with either PRI or SCN, and either a lowercase letter or X may be
added to the macros defined in the <inttypes.h> header.
7.31.6 Localization <locale.h>
1 Macros that begin with LC_ and an uppercase letter may be added to the macros defined
in the <locale.h> header.
7.31.7 Signal handling <signal.h>
1 Macros that begin with either SIG and an uppercase letter or SIG_ and an uppercase
letter may be added to the macros defined in the <signal.h> header.
7.31.8 Atomics <stdatomic.h>
1 Macros that begin with ATOMIC_ and an uppercase letter may be added to the macros
defined in the <stdatomic.h> header. Typedef names that begin with either
atomic_ or memory_, and a lowercase letter may be added to the declarations in the
<stdatomic.h> header. Enumeration constants that begin with memory_order_
and a lowercase letter may be added to the definition of the memory_order type in the
<stdatomic.h> header. Function names that begin with atomic_ and a lowercase
letter may be added to the declarations in the <stdatomic.h> header.
7.31.9 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.
7.31.10 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.
7.31.11 Input/output <stdio.h>
1 Lowercase letters may be added to the conversion specifiers and length modifiers in
fprintf and fscanf. Other characters may be used in extensions.
2 The use of ungetc on a binary stream where the file position indicator is zero prior to
the call is an obsolescent feature.
7.31.12 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.
7.31.13 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.
7.31.14 Date and time <time.h>
Macros beginning with TIME_ and an uppercase letter may be added to the macros in the
<time.h> header.
7.31.15 Threads <threads.h>
1 Function names, type names, and enumeration constants that begin with either cnd_,
mtx_, thrd_, or tss_, and a lowercase letter may be added to the declarations in the
<threads.h> header.
7.31.16 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.
7.31.17 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.
Annex A
(informative)
A. Language syntax summary
1 NOTE The notation is described in 6.1.
A.1 Lexical grammar
A.1.1 Lexical elements
(6.4) token:
keyword
identifier
constant
string-literal
punctuator
(6.4) preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each non-white-space character that cannot be one of the above
A.1.2 Keywords
(6.4.1) keyword: one of
auto if unsigned
break inline void
case int volatile
char long while
const register _Alignas
continue restrict _Alignof
default return _Atomic
do short _Bool
double signed _Complex
else sizeof _Generic
enum static _Imaginary
extern struct _Noreturn
float switch _Static_assert
for typedef _Thread_local
goto union
A.1.3 Identifiers
(6.4.2.1) identifier:
identifier-nondigit
identifier identifier-nondigit
identifier digit
(6.4.2.1) identifier-nondigit:
nondigit
universal-character-name
other implementation-defined characters
(6.4.2.1) nondigit: one of
_ a b c d e f g h i j k l m
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
(6.4.2.1) digit: one of
0 1 2 3 4 5 6 7 8 9
A.1.4 Universal character names
(6.4.3) universal-character-name:
\u hex-quad
\U hex-quad hex-quad
(6.4.3) hex-quad:
hexadecimal-digit hexadecimal-digit
hexadecimal-digit hexadecimal-digit
A.1.5 Constants
(6.4.4) constant:
integer-constant
floating-constant
enumeration-constant
character-constant
(6.4.4.1) integer-constant:
decimal-constant integer-suffixopt
octal-constant integer-suffixopt
hexadecimal-constant integer-suffixopt
(6.4.4.1) decimal-constant:
nonzero-digit
decimal-constant digit
(6.4.4.1) octal-constant:
0
octal-constant octal-digit
(6.4.4.1) hexadecimal-constant:
hexadecimal-prefix hexadecimal-digit
hexadecimal-constant hexadecimal-digit
(6.4.4.1) hexadecimal-prefix: one of
0x 0X
(6.4.4.1) nonzero-digit: one of
1 2 3 4 5 6 7 8 9
(6.4.4.1) octal-digit: one of
0 1 2 3 4 5 6 7
(6.4.4.1) hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
(6.4.4.1) integer-suffix:
unsigned-suffix long-suffixopt
unsigned-suffix long-long-suffix
long-suffix unsigned-suffixopt
long-long-suffix unsigned-suffixopt
(6.4.4.1) unsigned-suffix: one of
u U
(6.4.4.1) long-suffix: one of
l L
(6.4.4.1) long-long-suffix: one of
ll LL
(6.4.4.2) floating-constant:
decimal-floating-constant
hexadecimal-floating-constant
(6.4.4.2) decimal-floating-constant:
fractional-constant exponent-partopt floating-suffixopt
digit-sequence exponent-part floating-suffixopt
(6.4.4.2) hexadecimal-floating-constant:
hexadecimal-prefix hexadecimal-fractional-constant
binary-exponent-part floating-suffixopt
hexadecimal-prefix hexadecimal-digit-sequence
binary-exponent-part floating-suffixopt
(6.4.4.2) fractional-constant:
digit-sequenceopt . digit-sequence
digit-sequence .
(6.4.4.2) exponent-part:
e signopt digit-sequence
E signopt digit-sequence
(6.4.4.2) sign: one of
+ -
(6.4.4.2) digit-sequence:
digit
digit-sequence digit
(6.4.4.2) hexadecimal-fractional-constant:
hexadecimal-digit-sequenceopt .
hexadecimal-digit-sequence
hexadecimal-digit-sequence .
(6.4.4.2) binary-exponent-part:
p signopt digit-sequence
P signopt digit-sequence
(6.4.4.2) hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence hexadecimal-digit
(6.4.4.2) floating-suffix: one of
f l F L
(6.4.4.3) enumeration-constant:
identifier
(6.4.4.4) character-constant:
' c-char-sequence '
L' c-char-sequence '
u' c-char-sequence '
U' c-char-sequence '
(6.4.4.4) c-char-sequence:
c-char
c-char-sequence c-char
(6.4.4.4) c-char:
any member of the source character set except
the single-quote ', backslash \, or new-line character
escape-sequence
(6.4.4.4) escape-sequence:
simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence
universal-character-name
(6.4.4.4) simple-escape-sequence: one of
\' \" \? \\
\a \b \f \n \r \t \v
(6.4.4.4) octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
(6.4.4.4) hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit
A.1.6 String literals
(6.4.5) string-literal:
encoding-prefixopt " s-char-sequenceopt "
(6.4.5) encoding-prefix:
u8
u
U
L
(6.4.5) s-char-sequence:
s-char
s-char-sequence s-char
(6.4.5) s-char:
any member of the source character set except
the double-quote ", backslash \, or new-line character
escape-sequence
A.1.7 Punctuators
(6.4.6) punctuator: one of
[ ] ( ) { } . ->
++ -- & * + - ~ !
/ % << >> < > <= >= == != ^ | && ||
? : ; ...
= *= /= %= += -= <<= >>= &= ^= |=
, # ##
<: :> <% %> %: %:%:
A.1.8 Header names
(6.4.7) header-name:
< h-char-sequence >
" q-char-sequence "
(6.4.7) h-char-sequence:
h-char
h-char-sequence h-char
(6.4.7) h-char:
any member of the source character set except
the new-line character and >
(6.4.7) q-char-sequence:
q-char
q-char-sequence q-char
(6.4.7) q-char:
any member of the source character set except
the new-line character and "
A.1.9 Preprocessing numbers
(6.4.8) pp-number:
digit
. digit
pp-number digit
pp-number identifier-nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
A.2 Phrase structure grammar
A.2.1 Expressions
(6.5.1) primary-expression:
identifier
constant
string-literal
( expression )
generic-selection
(6.5.1.1) generic-selection:
_Generic ( assignment-expression , generic-assoc-list )
(6.5.1.1) generic-assoc-list:
generic-association
generic-assoc-list , generic-association
(6.5.1.1) generic-association:
type-name : assignment-expression
default : assignment-expression
(6.5.2) postfix-expression:
primary-expression
postfix-expression [ expression ]
postfix-expression ( argument-expression-listopt )
postfix-expression . identifier
postfix-expression -> identifier
postfix-expression ++
postfix-expression --
( type-name ) { initializer-list }
( type-name ) { initializer-list , }
(6.5.2) argument-expression-list:
assignment-expression
argument-expression-list , assignment-expression
(6.5.3) unary-expression:
postfix-expression
++ unary-expression
-- unary-expression
unary-operator cast-expression
sizeof unary-expression
sizeof ( type-name )
_Alignof ( type-name )
(6.5.3) unary-operator: one of
& * + - ~ !
(6.5.4) cast-expression:
unary-expression
( type-name ) cast-expression
(6.5.5) multiplicative-expression:
cast-expression
multiplicative-expression * cast-expression
multiplicative-expression / cast-expression
multiplicative-expression % cast-expression
(6.5.6) additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression - multiplicative-expression
(6.5.7) shift-expression:
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression
(6.5.8) relational-expression:
shift-expression
relational-expression < shift-expression
relational-expression > shift-expression
relational-expression <= shift-expression
relational-expression >= shift-expression
(6.5.9) equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression
(6.5.10) AND-expression:
equality-expression
AND-expression & equality-expression
(6.5.11) exclusive-OR-expression:
AND-expression
exclusive-OR-expression ^ AND-expression
(6.5.12) inclusive-OR-expression:
exclusive-OR-expression
inclusive-OR-expression | exclusive-OR-expression
(6.5.13) logical-AND-expression:
inclusive-OR-expression
logical-AND-expression && inclusive-OR-expression
(6.5.14) logical-OR-expression:
logical-AND-expression
logical-OR-expression || logical-AND-expression
(6.5.15) conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
(6.5.16) assignment-expression:
conditional-expression
unary-expression assignment-operator assignment-expression
(6.5.16) assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
(6.5.17) expression:
assignment-expression
expression , assignment-expression
(6.6) constant-expression:
conditional-expression
A.2.2 Declarations
(6.7) declaration:
declaration-specifiers init-declarator-listopt ;
static_assert-declaration
(6.7) declaration-specifiers:
storage-class-specifier declaration-specifiersopt
type-specifier declaration-specifiersopt
type-qualifier declaration-specifiersopt
function-specifier declaration-specifiersopt
alignment-specifier declaration-specifiersopt
(6.7) init-declarator-list:
init-declarator
init-declarator-list , init-declarator
(6.7) init-declarator:
declarator
declarator = initializer
(6.7.1) storage-class-specifier:
typedef
extern
static
_Thread_local
auto
register
(6.7.2) type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_Bool
_Complex
atomic-type-specifier
struct-or-union-specifier
enum-specifier
typedef-name
(6.7.2.1) struct-or-union-specifier:
struct-or-union identifieropt { struct-declaration-list }
struct-or-union identifier
(6.7.2.1) struct-or-union:
struct
union
(6.7.2.1) struct-declaration-list:
struct-declaration
struct-declaration-list struct-declaration
(6.7.2.1) struct-declaration:
specifier-qualifier-list struct-declarator-listopt ;
static_assert-declaration
(6.7.2.1) specifier-qualifier-list:
type-specifier specifier-qualifier-listopt
type-qualifier specifier-qualifier-listopt
(6.7.2.1) struct-declarator-list:
struct-declarator
struct-declarator-list , struct-declarator
(6.7.2.1) struct-declarator:
declarator
declaratoropt : constant-expression
(6.7.2.2) enum-specifier:
enum identifieropt { enumerator-list }
enum identifieropt { enumerator-list , }
enum identifier
(6.7.2.2) enumerator-list:
enumerator
enumerator-list , enumerator
(6.7.2.2) enumerator:
enumeration-constant
enumeration-constant = constant-expression
(6.7.2.4) atomic-type-specifier:
_Atomic ( type-name )
(6.7.3) type-qualifier:
const
restrict
volatile
_Atomic
(6.7.4) function-specifier:
inline
_Noreturn
(6.7.5) alignment-specifier:
_Alignas ( type-name )
_Alignas ( constant-expression )
(6.7.6) declarator:
pointeropt direct-declarator
(6.7.6) direct-declarator:
identifier
( 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 )
(6.7.6) pointer:
* type-qualifier-listopt
* type-qualifier-listopt pointer
(6.7.6) type-qualifier-list:
type-qualifier
type-qualifier-list type-qualifier
(6.7.6) parameter-type-list:
parameter-list
parameter-list , ...
(6.7.6) parameter-list:
parameter-declaration
parameter-list , parameter-declaration
(6.7.6) parameter-declaration:
declaration-specifiers declarator
declaration-specifiers abstract-declaratoropt
(6.7.6) identifier-list:
identifier
identifier-list , identifier
(6.7.7) type-name:
specifier-qualifier-list abstract-declaratoropt
(6.7.7) abstract-declarator:
pointer
pointeropt direct-abstract-declarator
(6.7.7) direct-abstract-declarator:
( abstract-declarator )
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 )
(6.7.8) typedef-name:
identifier
(6.7.9) initializer:
assignment-expression
{ initializer-list }
{ initializer-list , }
(6.7.9) initializer-list:
designationopt initializer
initializer-list , designationopt initializer
(6.7.9) designation:
designator-list =
(6.7.9) designator-list:
designator
designator-list designator
(6.7.9) designator:
[ constant-expression ]
. identifier
(6.7.10) static_assert-declaration:
_Static_assert ( constant-expression , string-literal ) ;
A.2.3 Statements
(6.8) statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
(6.8.1) labeled-statement:
identifier : statement
case constant-expression : statement
default : statement
(6.8.2) compound-statement:
{ block-item-listopt }
(6.8.2) block-item-list:
block-item
block-item-list block-item
(6.8.2) block-item:
declaration
statement
(6.8.3) expression-statement:
expressionopt ;
(6.8.4) selection-statement:
if ( expression ) statement
if ( expression ) statement else statement
switch ( expression ) statement
(6.8.5) iteration-statement:
while ( expression ) statement
do statement while ( expression ) ;
for ( expressionopt ; expressionopt ; expressionopt ) statement
for ( declaration expressionopt ; expressionopt ) statement
(6.8.6) jump-statement:
goto identifier ;
continue ;
break ;
return expressionopt ;
A.2.4 External definitions
(6.9) translation-unit:
external-declaration
translation-unit external-declaration
(6.9) external-declaration:
function-definition
declaration
(6.9.1) function-definition:
declaration-specifiers declarator declaration-listopt compound-statement
(6.9.1) declaration-list:
declaration
declaration-list declaration
A.3 Preprocessing directives
(6.10) preprocessing-file:
groupopt
(6.10) group:
group-part
group group-part
(6.10) group-part:
if-section
control-line
text-line
# non-directive
(6.10) if-section:
if-group elif-groupsopt else-groupopt endif-line
(6.10) if-group:
# if constant-expression new-line groupopt
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt
(6.10) elif-groups:
elif-group
elif-groups elif-group
(6.10) elif-group:
# elif constant-expression new-line groupopt
(6.10) else-group:
# else new-line groupopt
(6.10) endif-line:
# endif new-line
(6.10) 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
(6.10) text-line:
pp-tokensopt new-line
(6.10) non-directive:
pp-tokens new-line
(6.10) lparen:
a ( character not immediately preceded by white-space
(6.10) replacement-list:
pp-tokensopt
(6.10) pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
(6.10) new-line:
the new-line character
Annex B
(informative)
B. Library summary
B.1 Diagnostics <assert.h>
NDEBUG
static_assert
void assert(scalar expression);
B.2 Complex <complex.h>
_ _STDC_NO_COMPLEX_ _ imaginary
complex _Imaginary_I
_Complex_I I
#pragma STDC CX_LIMITED_RANGE on-off-switch
double complex cacos(double complex z);
float complex cacosf(float complex z);
long double complex cacosl(long double complex z);
double complex casin(double complex z);
float complex casinf(float complex z);
long double complex casinl(long double complex z);
double complex catan(double complex z);
float complex catanf(float complex z);
long double complex catanl(long double complex z);
double complex ccos(double complex z);
float complex ccosf(float complex z);
long double complex ccosl(long double complex z);
double complex csin(double complex z);
float complex csinf(float complex z);
long double complex csinl(long double complex z);
double complex ctan(double complex z);
float complex ctanf(float complex z);
long double complex ctanl(long double complex z);
double complex cacosh(double complex z);
float complex cacoshf(float complex z);
long double complex cacoshl(long double complex z);
double complex casinh(double complex z);
float complex casinhf(float complex z);
long double complex casinhl(long double complex z);
double complex catanh(double complex z);
float complex catanhf(float complex z);
long double complex catanhl(long double complex z);
double complex ccosh(double complex z);
float complex ccoshf(float complex z);
long double complex ccoshl(long double complex z);
double complex csinh(double complex z);
float complex csinhf(float complex z);
long double complex csinhl(long double complex z);
double complex ctanh(double complex z);
float complex ctanhf(float complex z);
long double complex ctanhl(long double complex z);
double complex cexp(double complex z);
float complex cexpf(float complex z);
long double complex cexpl(long double complex z);
double complex clog(double complex z);
float complex clogf(float complex z);
long double complex clogl(long double complex z);
double cabs(double complex z);
float cabsf(float complex z);
long double cabsl(long double complex z);
double complex cpow(double complex x, double complex y);
float complex cpowf(float complex x, float complex y);
long double complex cpowl(long double complex x,
long double complex y);
double complex csqrt(double complex z);
float complex csqrtf(float complex z);
long double complex csqrtl(long double complex z);
double carg(double complex z);
float cargf(float complex z);
long double cargl(long double complex z);
double cimag(double complex z);
float cimagf(float complex z);
long double cimagl(long double complex z);
double complex CMPLX(double x, double y);
float complex CMPLXF(float x, float y);
long double complex CMPLXL(long double x, long double y);
double complex conj(double complex z);
float complex conjf(float complex z);
long double complex conjl(long double complex z);
double complex cproj(double complex z);
float complex cprojf(float complex z);
long double complex cprojl(long double complex z);
double creal(double complex z);
float crealf(float complex z);
long double creall(long double complex z);
B.3 Character handling <ctype.h>
int isalnum(int c);
int isalpha(int c);
int isblank(int c);
int iscntrl(int c);
int isdigit(int c);
int isgraph(int c);
int islower(int c);
int isprint(int c);
int ispunct(int c);
int isspace(int c);
int isupper(int c);
int isxdigit(int c);
int tolower(int c);
int toupper(int c);
B.4 Errors <errno.h>
EDOM EILSEQ ERANGE errno
_ _STDC_WANT_LIB_EXT1_ _
errno_t
B.5 Floating-point environment <fenv.h>
fenv_t FE_OVERFLOW FE_TOWARDZERO
fexcept_t FE_UNDERFLOW FE_UPWARD
FE_DIVBYZERO FE_ALL_EXCEPT FE_DFL_ENV
FE_INEXACT FE_DOWNWARD
FE_INVALID FE_TONEAREST
#pragma STDC FENV_ACCESS on-off-switch
int feclearexcept(int excepts);
int fegetexceptflag(fexcept_t *flagp, int excepts);
int feraiseexcept(int excepts);
int fesetexceptflag(const fexcept_t *flagp,
int excepts);
int fetestexcept(int excepts);
int fegetround(void);
int fesetround(int round);
int fegetenv(fenv_t *envp);
int feholdexcept(fenv_t *envp);
int fesetenv(const fenv_t *envp);
int feupdateenv(const fenv_t *envp);
B.6 Characteristics of floating types <float.h>
FLT_ROUNDS DBL_DIG FLT_MAX
FLT_EVAL_METHOD LDBL_DIG DBL_MAX
FLT_HAS_SUBNORM FLT_MIN_EXP LDBL_MAX
DBL_HAS_SUBNORM DBL_MIN_EXP FLT_EPSILON
LDBL_HAS_SUBNORM LDBL_MIN_EXP DBL_EPSILON
FLT_RADIX FLT_MIN_10_EXP LDBL_EPSILON
FLT_MANT_DIG DBL_MIN_10_EXP FLT_MIN
DBL_MANT_DIG LDBL_MIN_10_EXP DBL_MIN
LDBL_MANT_DIG FLT_MAX_EXP LDBL_MIN
FLT_DECIMAL_DIG DBL_MAX_EXP FLT_TRUE_MIN
DBL_DECIMAL_DIG LDBL_MAX_EXP DBL_TRUE_MIN
LDBL_DECIMAL_DIG FLT_MAX_10_EXP LDBL_TRUE_MIN
DECIMAL_DIG DBL_MAX_10_EXP
FLT_DIG LDBL_MAX_10_EXP
B.7 Format conversion of integer types <inttypes.h>
imaxdiv_t
PRIdN PRIdLEASTN PRIdFASTN PRIdMAX PRIdPTR
PRIiN PRIiLEASTN PRIiFASTN PRIiMAX PRIiPTR
PRIoN PRIoLEASTN PRIoFASTN PRIoMAX PRIoPTR
PRIuN PRIuLEASTN PRIuFASTN PRIuMAX PRIuPTR
PRIxN PRIxLEASTN PRIxFASTN PRIxMAX PRIxPTR
PRIXN PRIXLEASTN PRIXFASTN PRIXMAX PRIXPTR
SCNdN SCNdLEASTN SCNdFASTN SCNdMAX SCNdPTR
SCNiN SCNiLEASTN SCNiFASTN SCNiMAX SCNiPTR
SCNoN SCNoLEASTN SCNoFASTN SCNoMAX SCNoPTR
SCNuN SCNuLEASTN SCNuFASTN SCNuMAX SCNuPTR
SCNxN SCNxLEASTN SCNxFASTN SCNxMAX SCNxPTR
intmax_t imaxabs(intmax_t j);
imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);
intmax_t strtoimax(const char * restrict nptr,
char ** restrict endptr, int base);
uintmax_t strtoumax(const char * restrict nptr,
char ** restrict endptr, int base);
intmax_t wcstoimax(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
uintmax_t wcstoumax(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
B.8 Alternative spellings <iso646.h>
and bitor not_eq xor
and_eq compl or xor_eq
bitand not or_eq
B.9 Sizes of integer types <limits.h>
CHAR_BIT CHAR_MAX INT_MIN ULONG_MAX
SCHAR_MIN MB_LEN_MAX INT_MAX LLONG_MIN
SCHAR_MAX SHRT_MIN UINT_MAX LLONG_MAX
UCHAR_MAX SHRT_MAX LONG_MIN ULLONG_MAX
CHAR_MIN USHRT_MAX LONG_MAX
B.10 Localization <locale.h>
struct lconv LC_ALL LC_CTYPE LC_NUMERIC
NULL LC_COLLATE LC_MONETARY LC_TIME
char *setlocale(int category, const char *locale);
struct lconv *localeconv(void);
B.11 Mathematics <math.h>
float_t FP_INFINITE FP_FAST_FMAL
double_t FP_NAN FP_ILOGB0
HUGE_VAL FP_NORMAL FP_ILOGBNAN
HUGE_VALF FP_SUBNORMAL MATH_ERRNO
HUGE_VALL FP_ZERO MATH_ERREXCEPT
INFINITY FP_FAST_FMA math_errhandling
NAN FP_FAST_FMAF
#pragma STDC FP_CONTRACT on-off-switch
int fpclassify(real-floating x);
int isfinite(real-floating x);
int isinf(real-floating x);
int isnan(real-floating x);
int isnormal(real-floating x);
int signbit(real-floating x);
double acos(double x);
float acosf(float x);
long double acosl(long double x);
double asin(double x);
float asinf(float x);
long double asinl(long double x);
double atan(double x);
float atanf(float x);
long double atanl(long double x);
double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);
double cos(double x);
float cosf(float x);
long double cosl(long double x);
double sin(double x);
float sinf(float x);
long double sinl(long double x);
double tan(double x);
float tanf(float x);
long double tanl(long double x);
double acosh(double x);
float acoshf(float x);
long double acoshl(long double x);
double asinh(double x);
float asinhf(float x);
long double asinhl(long double x);
double atanh(double x);
float atanhf(float x);
long double atanhl(long double x);
double cosh(double x);
float coshf(float x);
long double coshl(long double x);
double sinh(double x);
float sinhf(float x);
long double sinhl(long double x);
double tanh(double x);
float tanhf(float x);
long double tanhl(long double x);
double exp(double x);
float expf(float x);
long double expl(long double x);
double exp2(double x);
float exp2f(float x);
long double exp2l(long double x);
double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);
double frexp(double value, int *exp);
float frexpf(float value, int *exp);
long double frexpl(long double value, int *exp);
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
double ldexp(double x, int exp);
float ldexpf(float x, int exp);
long double ldexpl(long double x, int exp);
double log(double x);
float logf(float x);
long double logl(long double x);
double log10(double x);
float log10f(float x);
long double log10l(long double x);
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
double log2(double x);
float log2f(float x);
long double log2l(long double x);
double logb(double x);
float logbf(float x);
long double logbl(long double x);
double modf(double value, double *iptr);
float modff(float value, float *iptr);
long double modfl(long double value, long double *iptr);
double scalbn(double x, int n);
float scalbnf(float x, int n);
long double scalbnl(long double x, int n);
double scalbln(double x, long int n);
float scalblnf(float x, long int n);
long double scalblnl(long double x, long int n);
double cbrt(double x);
float cbrtf(float x);
long double cbrtl(long double x);
double fabs(double x);
float fabsf(float x);
long double fabsl(long double x);
double hypot(double x, double y);
float hypotf(float x, float y);
long double hypotl(long double x, long double y);
double pow(double x, double y);
float powf(float x, float y);
long double powl(long double x, long double y);
double sqrt(double x);
float sqrtf(float x);
long double sqrtl(long double x);
double erf(double x);
float erff(float x);
long double erfl(long double x);
double erfc(double x);
float erfcf(float x);
long double erfcl(long double x);
double lgamma(double x);
float lgammaf(float x);
long double lgammal(long double x);
double tgamma(double x);
float tgammaf(float x);
long double tgammal(long double x);
double ceil(double x);
float ceilf(float x);
long double ceill(long double x);
double floor(double x);
float floorf(float x);
long double floorl(long double x);
double nearbyint(double x);
float nearbyintf(float x);
long double nearbyintl(long double x);
double rint(double x);
float rintf(float x);
long double rintl(long double x);
long int lrint(double x);
long int lrintf(float x);
long int lrintl(long double x);
long long int llrint(double x);
long long int llrintf(float x);
long long int llrintl(long double x);
double round(double x);
float roundf(float x);
long double roundl(long double x);
long int lround(double x);
long int lroundf(float x);
long int lroundl(long double x);
long long int llround(double x);
long long int llroundf(float x);
long long int llroundl(long double x);
double trunc(double x);
float truncf(float x);
long double truncl(long double x);
double fmod(double x, double y);
float fmodf(float x, float y);
long double fmodl(long double x, long double y);
double remainder(double x, double y);
float remainderf(float x, float y);
long double remainderl(long double x, long double y);
double remquo(double x, double y, int *quo);
float remquof(float x, float y, int *quo);
long double remquol(long double x, long double y,
int *quo);
double copysign(double x, double y);
float copysignf(float x, float y);
long double copysignl(long double x, long double y);
double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);
double nextafter(double x, double y);
float nextafterf(float x, float y);
long double nextafterl(long double x, long double y);
double nexttoward(double x, long double y);
float nexttowardf(float x, long double y);
long double nexttowardl(long double x, long double y);
double fdim(double x, double y);
float fdimf(float x, float y);
long double fdiml(long double x, long double y);
double fmax(double x, double y);
float fmaxf(float x, float y);
long double fmaxl(long double x, long double y);
double fmin(double x, double y);
float fminf(float x, float y);
long double fminl(long double x, long double y);
double 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);
int isgreater(real-floating x, real-floating y);
int isgreaterequal(real-floating x, real-floating y);
int isless(real-floating x, real-floating y);
int islessequal(real-floating x, real-floating y);
int islessgreater(real-floating x, real-floating y);
int isunordered(real-floating x, real-floating y);
B.12 Nonlocal jumps <setjmp.h>
jmp_buf
int setjmp(jmp_buf env);
_Noreturn void longjmp(jmp_buf env, int val);
B.13 Signal handling <signal.h>
sig_atomic_t SIG_IGN SIGILL SIGTERM
SIG_DFL SIGABRT SIGINT
SIG_ERR SIGFPE SIGSEGV
void (*signal(int sig, void (*func)(int)))(int);
int raise(int sig);
B.14 Alignment <stdalign.h>
alignas
_ _alignas_is_defined
B.15 Variable arguments <stdarg.h>
va_list
type va_arg(va_list ap, type);
void va_copy(va_list dest, va_list src);
void va_end(va_list ap);
void va_start(va_list ap, parmN);
B.16 Atomics <stdatomic.h>
ATOMIC_BOOL_LOCK_FREE atomic_uint
ATOMIC_CHAR_LOCK_FREE atomic_long
ATOMIC_CHAR16_T_LOCK_FREE atomic_ulong
ATOMIC_CHAR32_T_LOCK_FREE atomic_llong
ATOMIC_WCHAR_T_LOCK_FREE atomic_ullong
ATOMIC_SHORT_LOCK_FREE atomic_char16_t
ATOMIC_INT_LOCK_FREE atomic_char32_t
ATOMIC_LONG_LOCK_FREE atomic_wchar_t
ATOMIC_LLONG_LOCK_FREE atomic_int_least8_t
ATOMIC_POINTER_LOCK_FREE atomic_uint_least8_t
ATOMIC_FLAG_INIT atomic_int_least16_t
memory_order atomic_uint_least16_t
atomic_flag atomic_int_least32_t
memory_order_relaxed atomic_uint_least32_t
memory_order_consume atomic_int_least64_t
memory_order_acquire atomic_uint_least64_t
memory_order_release atomic_int_fast8_t
memory_order_acq_rel atomic_uint_fast8_t
memory_order_seq_cst atomic_int_fast16_t
atomic_bool atomic_uint_fast16_t
atomic_char atomic_int_fast32_t
atomic_schar atomic_uint_fast32_t
atomic_uchar atomic_int_fast64_t
atomic_short atomic_uint_fast64_t
atomic_ushort atomic_intptr_t
atomic_int atomic_uintptr_t
atomic_size_t atomic_intmax_t
atomic_ptrdiff_t atomic_uintmax_t
#define ATOMIC_VAR_INIT(C value)
void atomic_init(volatile A *obj, C value);
type kill_dependency(type y);
void atomic_thread_fence(memory_order order);
void atomic_signal_fence(memory_order order);
_Bool atomic_is_lock_free(const volatile A *obj);
void atomic_store(volatile A *object, C desired);
void atomic_store_explicit(volatile A *object,
C desired, memory_order order);
C atomic_load(volatile A *object);
C atomic_load_explicit(volatile A *object,
memory_order order);
C atomic_exchange(volatile A *object, C desired);
C atomic_exchange_explicit(volatile A *object,
C desired, memory_order order);
_Bool atomic_compare_exchange_strong(volatile A *object,
C *expected, C desired);
_Bool atomic_compare_exchange_strong_explicit(
volatile A *object, C *expected, C desired,
memory_order success, memory_order failure);
_Bool atomic_compare_exchange_weak(volatile A *object,
C *expected, C desired);
_Bool atomic_compare_exchange_weak_explicit(
volatile A *object, C *expected, C desired,
memory_order success, memory_order failure);
C atomic_fetch_key(volatile A *object, M operand);
C atomic_fetch_key_explicit(volatile A *object,
M operand, memory_order order);
_Bool atomic_flag_test_and_set(
volatile atomic_flag *object);
_Bool atomic_flag_test_and_set_explicit(
volatile atomic_flag *object, memory_order order);
void atomic_flag_clear(volatile atomic_flag *object);
void atomic_flag_clear_explicit(
volatile atomic_flag *object, memory_order order);
B.17 Boolean type and values <stdbool.h>
bool
true
false
_ _bool_true_false_are_defined
B.18 Common definitions <stddef.h>
ptrdiff_t max_align_t NULL
size_t wchar_t
offsetof(type, member-designator)
_ _STDC_WANT_LIB_EXT1_ _
rsize_t
B.19 Integer types <stdint.h>
intN_t INT_LEASTN_MIN PTRDIFF_MAX
uintN_t INT_LEASTN_MAX SIG_ATOMIC_MIN
int_leastN_t UINT_LEASTN_MAX SIG_ATOMIC_MAX
uint_leastN_t INT_FASTN_MIN SIZE_MAX
int_fastN_t INT_FASTN_MAX WCHAR_MIN
uint_fastN_t UINT_FASTN_MAX WCHAR_MAX
intptr_t INTPTR_MIN WINT_MIN
uintptr_t INTPTR_MAX WINT_MAX
intmax_t UINTPTR_MAX INTN_C(value)
uintmax_t INTMAX_MIN UINTN_C(value)
INTN_MIN INTMAX_MAX INTMAX_C(value)
INTN_MAX UINTMAX_MAX UINTMAX_C(value)
UINTN_MAX PTRDIFF_MIN
_ _STDC_WANT_LIB_EXT1_ _
RSIZE_MAX
B.20 Input/output <stdio.h>
size_t _IOLBF FILENAME_MAX TMP_MAX
FILE _IONBF L_tmpnam stderr
fpos_t BUFSIZ SEEK_CUR stdin
NULL EOF SEEK_END stdout
_IOFBF FOPEN_MAX SEEK_SET
int remove(const char *filename);
int rename(const char *old, const char *new);
FILE *tmpfile(void);
char *tmpnam(char *s);
int fclose(FILE *stream);
int fflush(FILE *stream);
FILE *fopen(const char * restrict filename,
const char * restrict mode);
FILE *freopen(const char * restrict filename,
const char * restrict mode,
FILE * restrict stream);
void setbuf(FILE * restrict stream,
char * restrict buf);
int setvbuf(FILE * restrict stream,
char * restrict buf,
int mode, size_t size);
int fprintf(FILE * restrict stream,
const char * restrict format, ...);
int fscanf(FILE * restrict stream,
const char * restrict format, ...);
int printf(const char * restrict format, ...);
int scanf(const char * restrict format, ...);
int snprintf(char * restrict s, size_t n,
const char * restrict format, ...);
int sprintf(char * restrict s,
const char * restrict format, ...);
int sscanf(const char * restrict s,
const char * restrict format, ...);
int vfprintf(FILE * restrict stream,
const char * restrict format, va_list arg);
int vfscanf(FILE * restrict stream,
const char * restrict format, va_list arg);
int vprintf(const char * restrict format, va_list arg);
int vscanf(const char * restrict format, va_list arg);
int vsnprintf(char * restrict s, size_t n,
const char * restrict format, va_list arg);
int vsprintf(char * restrict s,
const char * restrict format, va_list arg);
int vsscanf(const char * restrict s,
const char * restrict format, va_list arg);
int fgetc(FILE *stream);
char *fgets(char * restrict s, int n,
FILE * restrict stream);
int fputc(int c, FILE *stream);
int fputs(const char * restrict s,
FILE * restrict stream);
int getc(FILE *stream);
int getchar(void);
int putc(int c, FILE *stream);
int putchar(int c);
int puts(const char *s);
int ungetc(int c, FILE *stream);
size_t fread(void * restrict ptr,
size_t size, size_t nmemb,
FILE * restrict stream);
size_t fwrite(const void * restrict ptr,
size_t size, size_t nmemb,
FILE * restrict stream);
int fgetpos(FILE * restrict stream,
fpos_t * restrict pos);
int fseek(FILE *stream, long int offset, int whence);
int fsetpos(FILE *stream, const fpos_t *pos);
long int ftell(FILE *stream);
void rewind(FILE *stream);
void clearerr(FILE *stream);
int feof(FILE *stream);
int ferror(FILE *stream);
void perror(const char *s);
_ _STDC_WANT_LIB_EXT1_ _
L_tmpnam_s TMP_MAX_S errno_t rsize_t
errno_t tmpfile_s(FILE * restrict * restrict streamptr);
errno_t tmpnam_s(char *s, rsize_t maxsize);
errno_t fopen_s(FILE * restrict * restrict streamptr,
const char * restrict filename,
const char * restrict mode);
errno_t freopen_s(FILE * restrict * restrict newstreamptr,
const char * restrict filename,
const char * restrict mode,
FILE * restrict stream);
int fprintf_s(FILE * restrict stream,
const char * restrict format, ...);
int fscanf_s(FILE * restrict stream,
const char * restrict format, ...);
int printf_s(const char * restrict format, ...);
int scanf_s(const char * restrict format, ...);
int snprintf_s(char * restrict s, rsize_t n,
const char * restrict format, ...);
int sprintf_s(char * restrict s, rsize_t n,
const char * restrict format, ...);
int sscanf_s(const char * restrict s,
const char * restrict format, ...);
int vfprintf_s(FILE * restrict stream,
const char * restrict format,
va_list arg);
int vfscanf_s(FILE * restrict stream,
const char * restrict format,
va_list arg);
int vprintf_s(const char * restrict format,
va_list arg);
int vscanf_s(const char * restrict format,
va_list arg);
int vsnprintf_s(char * restrict s, rsize_t n,
const char * restrict format,
va_list arg);
int vsprintf_s(char * restrict s, rsize_t n,
const char * restrict format,
va_list arg);
int vsscanf_s(const char * restrict s,
const char * restrict format,
va_list arg);
char *gets_s(char *s, rsize_t n);
B.21 General utilities <stdlib.h>
size_t ldiv_t EXIT_FAILURE MB_CUR_MAX
wchar_t lldiv_t EXIT_SUCCESS
div_t NULL RAND_MAX
double atof(const char *nptr);
int atoi(const char *nptr);
long int atol(const char *nptr);
long long int atoll(const char *nptr);
double strtod(const char * restrict nptr,
char ** restrict endptr);
float strtof(const char * restrict nptr,
char ** restrict endptr);
long double strtold(const char * restrict nptr,
char ** restrict endptr);
long int strtol(const char * restrict nptr,
char ** restrict endptr, int base);
long long int strtoll(const char * restrict nptr,
char ** restrict endptr, int base);
unsigned long int strtoul(
const char * restrict nptr,
char ** restrict endptr, int base);
unsigned long long int strtoull(
const char * restrict nptr,
char ** restrict endptr, int base);
int rand(void);
void srand(unsigned int seed);
void *aligned_alloc(size_t alignment, size_t size);
void *calloc(size_t nmemb, size_t size);
void free(void *ptr);
void *malloc(size_t size);
void *realloc(void *ptr, size_t size);
_Noreturn void abort(void);
int atexit(void (*func)(void));
int at_quick_exit(void (*func)(void));
_Noreturn void exit(int status);
_Noreturn void _Exit(int status);
char *getenv(const char *name);
_Noreturn void quick_exit(int status);
int system(const char *string);
void *bsearch(const void *key, const void *base,
size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
int abs(int j);
long int labs(long int j);
long long int llabs(long long int j);
div_t div(int numer, int denom);
ldiv_t ldiv(long int numer, long int denom);
lldiv_t lldiv(long long int numer,
long long int denom);
int mblen(const char *s, size_t n);
int mbtowc(wchar_t * restrict pwc,
const char * restrict s, size_t n);
int wctomb(char *s, wchar_t wchar);
size_t mbstowcs(wchar_t * restrict pwcs,
const char * restrict s, size_t n);
size_t wcstombs(char * restrict s,
const wchar_t * restrict pwcs, size_t n);
_ _STDC_WANT_LIB_EXT1_ _
errno_t
rsize_t
constraint_handler_t
constraint_handler_t set_constraint_handler_s(
constraint_handler_t handler);
void abort_handler_s(
const char * restrict msg,
void * restrict ptr,
errno_t error);
void ignore_handler_s(
const char * restrict msg,
void * restrict ptr,
errno_t error);
errno_t getenv_s(size_t * restrict len,
char * restrict value, rsize_t maxsize,
const char * restrict name);
void *bsearch_s(const void *key, const void *base,
rsize_t nmemb, rsize_t size,
int (*compar)(const void *k, const void *y,
void *context),
void *context);
errno_t qsort_s(void *base, rsize_t nmemb, rsize_t size,
int (*compar)(const void *x, const void *y,
void *context),
void *context);
errno_t wctomb_s(int * restrict status,
char * restrict s,
rsize_t smax,
wchar_t wc);
errno_t mbstowcs_s(size_t * restrict retval,
wchar_t * restrict dst, rsize_t dstmax,
const char * restrict src, rsize_t len);
errno_t wcstombs_s(size_t * restrict retval,
char * restrict dst, rsize_t dstmax,
const wchar_t * restrict src, rsize_t len);
B.22 _Noreturn <stdnoreturn.h>
noreturn
B.23 String handling <string.h>
size_t
NULL
void *memcpy(void * restrict s1,
const void * restrict s2, size_t n);
void *memmove(void *s1, const void *s2, size_t n);
char *strcpy(char * restrict s1,
const char * restrict s2);
char *strncpy(char * restrict s1,
const char * restrict s2, size_t n);
char *strcat(char * restrict s1,
const char * restrict s2);
char *strncat(char * restrict s1,
const char * restrict s2, size_t n);
int memcmp(const void *s1, const void *s2, size_t n);
int strcmp(const char *s1, const char *s2);
int strcoll(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
size_t strxfrm(char * restrict s1,
const char * restrict s2, size_t n);
void *memchr(const void *s, int c, size_t n);
char *strchr(const char *s, int c);
size_t strcspn(const char *s1, const char *s2);
char *strpbrk(const char *s1, const char *s2);
char *strrchr(const char *s, int c);
size_t strspn(const char *s1, const char *s2);
char *strstr(const char *s1, const char *s2);
char *strtok(char * restrict s1,
const char * restrict s2);
void *memset(void *s, int c, size_t n);
char *strerror(int errnum);
size_t strlen(const char *s);
_ _STDC_WANT_LIB_EXT1_ _
errno_t
rsize_t
errno_t memcpy_s(void * restrict s1, rsize_t s1max,
const void * restrict s2, rsize_t n);
errno_t memmove_s(void *s1, rsize_t s1max,
const void *s2, rsize_t n);
errno_t strcpy_s(char * restrict s1,
rsize_t s1max,
const char * restrict s2);
errno_t strncpy_s(char * restrict s1,
rsize_t s1max,
const char * restrict s2,
rsize_t n);
errno_t strcat_s(char * restrict s1,
rsize_t s1max,
const char * restrict s2);
errno_t strncat_s(char * restrict s1,
rsize_t s1max,
const char * restrict s2,
rsize_t n);
char *strtok_s(char * restrict s1,
rsize_t * restrict s1max,
const char * restrict s2,
char ** restrict ptr);
errno_t memset_s(void *s, rsize_t smax, int c, rsize_t n)
errno_t strerror_s(char *s, rsize_t maxsize,
errno_t errnum);
size_t strerrorlen_s(errno_t errnum);
size_t strnlen_s(const char *s, size_t maxsize);
B.24 Type-generic math <tgmath.h>
acos sqrt fmod nextafter
asin fabs frexp nexttoward
atan atan2 hypot remainder
acosh cbrt ilogb remquo
asinh ceil ldexp rint
atanh copysign lgamma round
cos erf llrint scalbn
sin erfc llround scalbln
tan exp2 log10 tgamma
cosh expm1 log1p trunc
sinh fdim log2 carg
tanh floor logb cimag
exp fma lrint conj
log fmax lround cproj
pow fmin nearbyint creal
B.25 Threads <threads.h>
thread_local once_flag
ONCE_FLAG_INIT mtx_plain
TSS_DTOR_ITERATIONS mtx_recursive
cnd_t mtx_timed
thrd_t thrd_timedout
tss_t thrd_success
mtx_t thrd_busy
tss_dtor_t thrd_error
thrd_start_t thrd_nomem
void call_once(once_flag *flag, void (*func)(void));
int cnd_broadcast(cnd_t *cond);
void cnd_destroy(cnd_t *cond);
int cnd_init(cnd_t *cond);
int cnd_signal(cnd_t *cond);
int cnd_timedwait(cnd_t *restrict cond,
mtx_t *restrict mtx,
const struct timespec *restrict ts);
int cnd_wait(cnd_t *cond, mtx_t *mtx);
void mtx_destroy(mtx_t *mtx);
int mtx_init(mtx_t *mtx, int type);
int mtx_lock(mtx_t *mtx);
int mtx_timedlock(mtx_t *restrict mtx,
const struct timespec *restrict ts);
int mtx_trylock(mtx_t *mtx);
int mtx_unlock(mtx_t *mtx);
int thrd_create(thrd_t *thr, thrd_start_t func,
void *arg);
thrd_t thrd_current(void);
int thrd_detach(thrd_t thr);
int thrd_equal(thrd_t thr0, thrd_t thr1);
_Noreturn void thrd_exit(int res);
int thrd_join(thrd_t thr, int *res);
int thrd_sleep(const struct timespec *duration,
struct timespec *remaining);
void thrd_yield(void);
int tss_create(tss_t *key, tss_dtor_t dtor);
void tss_delete(tss_t key);
void *tss_get(tss_t key);
int tss_set(tss_t key, void *val);
B.26 Date and time <time.h>
NULL size_t struct timespec
CLOCKS_PER_SEC clock_t struct tm
TIME_UTC time_t
clock_t clock(void);
double difftime(time_t time1, time_t time0);
time_t mktime(struct tm *timeptr);
time_t time(time_t *timer);
int timespec_get(timespec *ts, int base);
char *asctime(const struct tm *timeptr);
char *ctime(const time_t *timer);
struct tm *gmtime(const time_t *timer);
struct tm *localtime(const time_t *timer);
size_t strftime(char * restrict s,
size_t maxsize,
const char * restrict format,
const struct tm * restrict timeptr);
_ _STDC_WANT_LIB_EXT1_ _
errno_t
rsize_t
errno_t asctime_s(char *s, rsize_t maxsize,
const struct tm *timeptr);
errno_t ctime_s(char *s, rsize_t maxsize,
const time_t *timer);
struct tm *gmtime_s(const time_t * restrict timer,
struct tm * restrict result);
struct tm *localtime_s(const time_t * restrict timer,
struct tm * restrict result);
B.27 Unicode utilities <uchar.h>
mbstate_t size_t char16_t char32_t
size_t mbrtoc16(char16_t * restrict pc16,
const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t c16rtomb(char * restrict s, char16_t c16,
mbstate_t * restrict ps);
size_t mbrtoc32(char32_t * restrict pc32,
const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t c32rtomb(char * restrict s, char32_t c32,
mbstate_t * restrict ps);
B.28 Extended multibyte/wide character utilities <wchar.h>
wchar_t wint_t WCHAR_MAX
size_t struct tm WCHAR_MIN
mbstate_t NULL WEOF
int fwprintf(FILE * restrict stream,
const wchar_t * restrict format, ...);
int fwscanf(FILE * restrict stream,
const wchar_t * restrict format, ...);
int swprintf(wchar_t * restrict s, size_t n,
const wchar_t * restrict format, ...);
int swscanf(const wchar_t * restrict s,
const wchar_t * restrict format, ...);
int vfwprintf(FILE * restrict stream,
const wchar_t * restrict format, va_list arg);
int vfwscanf(FILE * restrict stream,
const wchar_t * restrict format, va_list arg);
int vswprintf(wchar_t * restrict s, size_t n,
const wchar_t * restrict format, va_list arg);
int vswscanf(const wchar_t * restrict s,
const wchar_t * restrict format, va_list arg);
int vwprintf(const wchar_t * restrict format,
va_list arg);
int vwscanf(const wchar_t * restrict format,
va_list arg);
int wprintf(const wchar_t * restrict format, ...);
int wscanf(const wchar_t * restrict format, ...);
wint_t fgetwc(FILE *stream);
wchar_t *fgetws(wchar_t * restrict s, int n,
FILE * restrict stream);
wint_t fputwc(wchar_t c, FILE *stream);
int fputws(const wchar_t * restrict s,
FILE * restrict stream);
int fwide(FILE *stream, int mode);
wint_t getwc(FILE *stream);
wint_t getwchar(void);
wint_t putwc(wchar_t c, FILE *stream);
wint_t putwchar(wchar_t c);
wint_t ungetwc(wint_t c, FILE *stream);
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);
long int wcstol(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
long long int wcstoll(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
unsigned long int wcstoul(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
unsigned long long int wcstoull(
const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
wchar_t *wcscpy(wchar_t * restrict s1,
const wchar_t * restrict s2);
wchar_t *wcsncpy(wchar_t * restrict s1,
const wchar_t * restrict s2, size_t n);
wchar_t *wmemcpy(wchar_t * restrict s1,
const wchar_t * restrict s2, size_t n);
wchar_t *wmemmove(wchar_t *s1, const wchar_t *s2,
size_t n);
wchar_t *wcscat(wchar_t * restrict s1,
const wchar_t * restrict s2);
wchar_t *wcsncat(wchar_t * restrict s1,
const wchar_t * restrict s2, size_t n);
int wcscmp(const wchar_t *s1, const wchar_t *s2);
int wcscoll(const wchar_t *s1, const wchar_t *s2);
int wcsncmp(const wchar_t *s1, const wchar_t *s2,
size_t n);
size_t wcsxfrm(wchar_t * restrict s1,
const wchar_t * restrict s2, size_t n);
int wmemcmp(const wchar_t *s1, const wchar_t *s2,
size_t n);
wchar_t *wcschr(const wchar_t *s, wchar_t c);
size_t wcscspn(const wchar_t *s1, const wchar_t *s2);
wchar_t *wcspbrk(const wchar_t *s1, const wchar_t *s2);
wchar_t *wcsrchr(const wchar_t *s, wchar_t c);
size_t wcsspn(const wchar_t *s1, const wchar_t *s2);
wchar_t *wcsstr(const wchar_t *s1, const wchar_t *s2);
wchar_t *wcstok(wchar_t * restrict s1,
const wchar_t * restrict s2,
wchar_t ** restrict ptr);
wchar_t *wmemchr(const wchar_t *s, wchar_t c, size_t n);
size_t wcslen(const wchar_t *s);
wchar_t *wmemset(wchar_t *s, wchar_t c, size_t n);
size_t wcsftime(wchar_t * restrict s, size_t maxsize,
const wchar_t * restrict format,
const struct tm * restrict timeptr);
wint_t btowc(int c);
int wctob(wint_t c);
int mbsinit(const mbstate_t *ps);
size_t mbrlen(const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t mbrtowc(wchar_t * restrict pwc,
const char * restrict s, size_t n,
mbstate_t * restrict ps);
size_t wcrtomb(char * restrict s, wchar_t wc,
mbstate_t * restrict ps);
size_t mbsrtowcs(wchar_t * restrict dst,
const char ** restrict src, size_t len,
mbstate_t * restrict ps);
size_t wcsrtombs(char * restrict dst,
const wchar_t ** restrict src, size_t len,
mbstate_t * restrict ps);
_ _STDC_WANT_LIB_EXT1_ _
errno_t
rsize_t
int fwprintf_s(FILE * restrict stream,
const wchar_t * restrict format, ...);
int fwscanf_s(FILE * restrict stream,
const wchar_t * restrict format, ...);
int snwprintf_s(wchar_t * restrict s,
rsize_t n,
const wchar_t * restrict format, ...);
int swprintf_s(wchar_t * restrict s, rsize_t n,
const wchar_t * restrict format, ...);
int swscanf_s(const wchar_t * restrict s,
const wchar_t * restrict format, ...);
int vfwprintf_s(FILE * restrict stream,
const wchar_t * restrict format,
va_list arg);
int vfwscanf_s(FILE * restrict stream,
const wchar_t * restrict format, va_list arg);
int vsnwprintf_s(wchar_t * restrict s,
rsize_t n,
const wchar_t * restrict format,
va_list arg);
int vswprintf_s(wchar_t * restrict s,
rsize_t n,
const wchar_t * restrict format,
va_list arg);
int vswscanf_s(const wchar_t * restrict s,
const wchar_t * restrict format,
va_list arg);
int vwprintf_s(const wchar_t * restrict format,
va_list arg);
int vwscanf_s(const wchar_t * restrict format,
va_list arg);
int wprintf_s(const wchar_t * restrict format, ...);
int wscanf_s(const wchar_t * restrict format, ...);
errno_t wcscpy_s(wchar_t * restrict s1,
rsize_t s1max,
const wchar_t * restrict s2);
errno_t wcsncpy_s(wchar_t * restrict s1,
rsize_t s1max,
const wchar_t * restrict s2,
rsize_t n);
errno_t wmemcpy_s(wchar_t * restrict s1,
rsize_t s1max,
const wchar_t * restrict s2,
rsize_t n);
errno_t wmemmove_s(wchar_t *s1, rsize_t s1max,
const wchar_t *s2, rsize_t n);
errno_t wcscat_s(wchar_t * restrict s1,
rsize_t s1max,
const wchar_t * restrict s2);
errno_t wcsncat_s(wchar_t * restrict s1,
rsize_t s1max,
const wchar_t * restrict s2,
rsize_t n);
wchar_t *wcstok_s(wchar_t * restrict s1,
rsize_t * restrict s1max,
const wchar_t * restrict s2,
wchar_t ** restrict ptr);
size_t wcsnlen_s(const wchar_t *s, size_t maxsize);
errno_t wcrtomb_s(size_t * restrict retval,
char * restrict s, rsize_t smax,
wchar_t wc, mbstate_t * restrict ps);
errno_t mbsrtowcs_s(size_t * restrict retval,
wchar_t * restrict dst, rsize_t dstmax,
const char ** restrict src, rsize_t len,
mbstate_t * restrict ps);
errno_t wcsrtombs_s(size_t * restrict retval,
char * restrict dst, rsize_t dstmax,
const wchar_t ** restrict src, rsize_t len,
mbstate_t * restrict ps);
B.29 Wide character classification and mapping utilities <wctype.h>
wint_t wctrans_t wctype_t WEOF
int iswalnum(wint_t wc);
int iswalpha(wint_t wc);
int iswblank(wint_t wc);
int iswcntrl(wint_t wc);
int iswdigit(wint_t wc);
int iswgraph(wint_t wc);
int iswlower(wint_t wc);
int iswprint(wint_t wc);
int iswpunct(wint_t wc);
int iswspace(wint_t wc);
int iswupper(wint_t wc);
int iswxdigit(wint_t wc);
int iswctype(wint_t wc, wctype_t desc);
wctype_t wctype(const char *property);
wint_t towlower(wint_t wc);
wint_t towupper(wint_t wc);
wint_t towctrans(wint_t wc, wctrans_t desc);
wctrans_t wctrans(const char *property);
Annex C
(informative)
C. Sequence points
1 The following are the sequence points described in 5.1.2.3:
-- Between the evaluations of the function designator and actual arguments in a function
call and the actual call. (6.5.2.2).
-- Between the evaluations of the first and second operands of the following operators:
logical AND && (6.5.13); logical OR || (6.5.14); comma , (6.5.17).
-- Between the evaluations of the first operand of the conditional ? : operator and
whichever of the second and third operands is evaluated (6.5.15).
-- The end of a full declarator: declarators (6.7.6);
-- Between the evaluation of a full expression and the next full expression to be
evaluated. The following are full expressions: an initializer that is not part of a
compound literal (6.7.9); the expression in an expression statement (6.8.3); the
controlling expression of a selection statement (if or switch) (6.8.4); the
controlling expression of a while or do statement (6.8.5); each of the (optional)
expressions of a for statement (6.8.5.3); the (optional) expression in a return
statement (6.8.6.4).
-- Immediately before a library function returns (7.1.4).
-- After the actions associated with each formatted input/output function conversion
specifier (7.21.6, 7.29.2).
-- Immediately before and immediately after each call to a comparison function, and
also between any call to a comparison function and any movement of the objects
passed as arguments to that call (7.22.5).
Annex D
(normative)
D. Universal character names for identifiers
1 This clause lists the hexadecimal code values that are valid in universal character names
in identifiers.
D.1 Ranges of characters allowed
1 00A8, 00AA, 00AD, 00AF, 00B2-00B5, 00B7-00BA, 00BC-00BE, 00C0-00D6,
00D8-00F6, 00F8-00FF
2 0100-167F, 1681-180D, 180F-1FFF
3 200B-200D, 202A-202E, 203F-2040, 2054, 2060-206F
4 2070-218F, 2460-24FF, 2776-2793, 2C00-2DFF, 2E80-2FFF
5 3004-3007, 3021-302F, 3031-303F
6 3040-D7FF
7 F900-FD3D, FD40-FDCF, FDF0-FE44, FE47-FFFD
8 10000-1FFFD, 20000-2FFFD, 30000-3FFFD, 40000-4FFFD, 50000-5FFFD,
60000-6FFFD, 70000-7FFFD, 80000-8FFFD, 90000-9FFFD, A0000-AFFFD,
B0000-BFFFD, C0000-CFFFD, D0000-DFFFD, E0000-EFFFD
D.2 Ranges of characters disallowed initially
1 0300-036F, 1DC0-1DFF, 20D0-20FF, FE20-FE2F
Annex E
(informative)
E. Implementation limits
1 The contents of the header <limits.h> are given below, in alphabetical order. The
minimum magnitudes shown shall be replaced by implementation-defined magnitudes
with the same sign. The values shall all be constant expressions suitable for use in #if
preprocessing directives. The components are described further in 5.2.4.2.1.
#define CHAR_BIT 8
#define CHAR_MAX UCHAR_MAX or SCHAR_MAX
#define CHAR_MIN 0 or SCHAR_MIN
#define INT_MAX +32767
#define INT_MIN -32767
#define LONG_MAX +2147483647
#define LONG_MIN -2147483647
#define LLONG_MAX +9223372036854775807
#define LLONG_MIN -9223372036854775807
#define MB_LEN_MAX 1
#define SCHAR_MAX +127
#define SCHAR_MIN -127
#define SHRT_MAX +32767
#define SHRT_MIN -32767
#define UCHAR_MAX 255
#define USHRT_MAX 65535
#define UINT_MAX 65535
#define ULONG_MAX 4294967295
#define ULLONG_MAX 18446744073709551615
2 The contents of the header <float.h> are given below. All integer values, except
FLT_ROUNDS, shall be constant expressions suitable for use in #if preprocessing
directives; all floating values shall be constant expressions. The components are
described further in 5.2.4.2.2.
3 The values given in the following list shall be replaced by implementation-defined
expressions:
#define FLT_EVAL_METHOD
#define FLT_ROUNDS
4 The values given in the following list shall be replaced by implementation-defined
constant expressions that are greater or equal in magnitude (absolute value) to those
shown, with the same sign:
#define DLB_DECIMAL_DIG 10
#define DBL_DIG 10
#define DBL_MANT_DIG
#define DBL_MAX_10_EXP +37
#define DBL_MAX_EXP
#define DBL_MIN_10_EXP -37
#define DBL_MIN_EXP
#define DECIMAL_DIG 10
#define FLT_DECIMAL_DIG 6
#define FLT_DIG 6
#define FLT_MANT_DIG
#define FLT_MAX_10_EXP +37
#define FLT_MAX_EXP
#define FLT_MIN_10_EXP -37
#define FLT_MIN_EXP
#define FLT_RADIX 2
#define LDLB_DECIMAL_DIG 10
#define LDBL_DIG 10
#define LDBL_MANT_DIG
#define LDBL_MAX_10_EXP +37
#define LDBL_MAX_EXP
#define LDBL_MIN_10_EXP -37
#define LDBL_MIN_EXP
5 The values given in the following list shall be replaced by implementation-defined
constant expressions with values that are greater than or equal to those shown:
#define DBL_MAX 1E+37
#define FLT_MAX 1E+37
#define LDBL_MAX 1E+37
6 The values given in the following list shall be replaced by implementation-defined
constant expressions with (positive) values that are less than or equal to those shown:
#define DBL_EPSILON 1E-9
#define DBL_MIN 1E-37
#define FLT_EPSILON 1E-5
#define FLT_MIN 1E-37
#define LDBL_EPSILON 1E-9
#define LDBL_MIN 1E-37
Annex F
(normative)
F. IEC 60559 floating-point arithmetic
F.1 Introduction
1 This annex specifies C language support for the IEC 60559 floating-point standard. The
IEC 60559 floating-point standard is specifically Binary floating-point arithmetic for
microprocessor systems, second edition (IEC 60559:1989), previously designated
IEC 559:1989 and as IEEE Standard for Binary Floating-Point Arithmetic
(ANSI/IEEE 754-1985). IEEE Standard for Radix-Independent Floating-Point
Arithmetic (ANSI/IEEE 854-1987) generalizes the binary standard to remove
dependencies on radix and word length. IEC 60559 generally refers to the floating-point
standard, as in IEC 60559 operation, IEC 60559 format, etc. An implementation that
defines _ _STDC_IEC_559_ _ shall conform to the specifications in this annex.356)
Where a binding between the C language and IEC 60559 is indicated, the
IEC 60559-specified behavior is adopted by reference, unless stated otherwise. Since
negative and positive infinity are representable in IEC 60559 formats, all real numbers lie
within the range of representable values.
F.2 Types
1 The C floating types match the IEC 60559 formats as follows:
-- The float type matches the IEC 60559 single format.
-- The double type matches the IEC 60559 double format.
-- The long double type matches an IEC 60559 extended format,357) else a
non-IEC 60559 extended format, else the IEC 60559 double format.
Any non-IEC 60559 extended format used for the long double type shall have more
precision than IEC 60559 double and at least the range of IEC 60559 double.358)
356) Implementations that do not define _ _STDC_IEC_559_ _ are not required to conform to these
specifications.
357) ``Extended'' is IEC 60559's double-extended data format. Extended refers to both the common 80-bit
and quadruple 128-bit IEC 60559 formats.
358) A non-IEC 60559 long double type is required to provide infinity and NaNs, as its values include
all double values.
Recommended practice
2 The long double type should match an IEC 60559 extended format.
F.2.1 Infinities, signed zeros, and NaNs
1 This specification does not define the behavior of signaling NaNs.359) It generally uses
the term NaN to denote quiet NaNs. The NAN and INFINITY macros and the nan
functions in <math.h> provide designations for IEC 60559 NaNs and infinities.
F.3 Operators and functions
1 C operators and functions provide IEC 60559 required and recommended facilities as
listed below.
-- The +, -, *, and / operators provide the IEC 60559 add, subtract, multiply, and
divide operations.
-- The sqrt functions in <math.h> provide the IEC 60559 square root operation.
-- The remainder functions in <math.h> provide the IEC 60559 remainder
operation. The remquo functions in <math.h> provide the same operation but
with additional information.
-- The rint functions in <math.h> provide the IEC 60559 operation that rounds a
floating-point number to an integer value (in the same precision). The nearbyint
functions in <math.h> provide the nearbyinteger function recommended in the
Appendix to ANSI/IEEE 854.
-- The conversions for floating types provide the IEC 60559 conversions between
floating-point precisions.
-- The conversions from integer to floating types provide the IEC 60559 conversions
from integer to floating point.
-- The conversions from floating to integer types provide IEC 60559-like conversions
but always round toward zero.
-- The lrint and llrint functions in <math.h> provide the IEC 60559
conversions, which honor the directed rounding mode, from floating point to the
long int and long long int integer formats. The lrint and llrint
functions can be used to implement IEC 60559 conversions from floating to other
integer formats.
-- The translation time conversion of floating constants and the strtod, strtof,
strtold, fprintf, fscanf, and related library functions in <stdlib.h>,
359) Since NaNs created by IEC 60559 operations are always quiet, quiet NaNs (along with infinities) are
sufficient for closure of the arithmetic.
<stdio.h>, and <wchar.h> provide IEC 60559 binary-decimal conversions. The
strtold function in <stdlib.h> provides the conv function recommended in the
Appendix to ANSI/IEEE 854.
-- The relational and equality operators provide IEC 60559 comparisons. IEC 60559
identifies a need for additional comparison predicates to facilitate writing code that
accounts for NaNs. The comparison macros (isgreater, isgreaterequal,
isless, islessequal, islessgreater, and isunordered) in <math.h>
supplement the language operators to address this need. The islessgreater and
isunordered macros provide respectively a quiet version of the <> predicate and
the unordered predicate recommended in the Appendix to IEC 60559.
-- The feclearexcept, feraiseexcept, and fetestexcept functions in
<fenv.h> provide the facility to test and alter the IEC 60559 floating-point
exception status flags. The fegetexceptflag and fesetexceptflag
functions in <fenv.h> provide the facility to save and restore all five status flags at
one time. These functions are used in conjunction with the type fexcept_t and the
floating-point exception macros (FE_INEXACT, FE_DIVBYZERO,
FE_UNDERFLOW, FE_OVERFLOW, FE_INVALID) also in <fenv.h>.
-- The fegetround and fesetround functions in <fenv.h> provide the facility
to select among the IEC 60559 directed rounding modes represented by the rounding
direction macros in <fenv.h> (FE_TONEAREST, FE_UPWARD, FE_DOWNWARD,
FE_TOWARDZERO) and the values 0, 1, 2, and 3 of FLT_ROUNDS are the
IEC 60559 directed rounding modes.
-- The fegetenv, feholdexcept, fesetenv, and feupdateenv functions in
<fenv.h> provide a facility to manage the floating-point environment, comprising
the IEC 60559 status flags and control modes.
-- The copysign functions in <math.h> provide the copysign function
recommended in the Appendix to IEC 60559.
-- The fabs functions in <math.h> provide the abs function recommended in the
Appendix to IEC 60559.
-- The unary minus (-) operator provides the unary minus (-) operation recommended
in the Appendix to IEC 60559.
-- The scalbn and scalbln functions in <math.h> provide the scalb function
recommended in the Appendix to IEC 60559.
-- The logb functions in <math.h> provide the logb function recommended in the
Appendix to IEC 60559, but following the newer specifications in ANSI/IEEE 854.
-- The nextafter and nexttoward functions in <math.h> provide the nextafter
function recommended in the Appendix to IEC 60559 (but with a minor change to
better handle signed zeros).
-- The isfinite macro in <math.h> provides the finite function recommended in
the Appendix to IEC 60559.
-- The isnan macro in <math.h> provides the isnan function recommended in the
Appendix to IEC 60559.
-- The signbit macro and the fpclassify macro in <math.h>, used in
conjunction with the number classification macros (FP_NAN, FP_INFINITE,
FP_NORMAL, FP_SUBNORMAL, FP_ZERO), provide the facility of the class
function recommended in the Appendix to IEC 60559 (except that the classification
macros defined in 7.12.3 do not distinguish signaling from quiet NaNs).
F.4 Floating to integer conversion
1 If the integer type is _Bool, 6.3.1.2 applies and no floating-point exceptions are raised
(even for NaN). Otherwise, if the floating value is infinite or NaN or if the integral part
of the floating value exceeds the range of the integer type, then the ``invalid'' floating-
point exception is raised and the resulting value is unspecified. Otherwise, the resulting
value is determined by 6.3.1.4. Conversion of an integral floating value that does not
exceed the range of the integer type raises no floating-point exceptions; whether
conversion of a non-integral floating value raises the ``inexact'' floating-point exception is
unspecified.360)
F.5 Binary-decimal conversion
1 Conversion from the widest supported IEC 60559 format to decimal with
DECIMAL_DIG digits and back is the identity function.361)
2 Conversions involving IEC 60559 formats follow all pertinent recommended practice. In
particular, conversion between any supported IEC 60559 format and decimal with
DECIMAL_DIG or fewer significant digits is correctly rounded (honoring the current
rounding mode), which assures that conversion from the widest supported IEC 60559
format to decimal with DECIMAL_DIG digits and back is the identity function.
360) ANSI/IEEE 854, but not IEC 60559 (ANSI/IEEE 754), directly specifies that floating-to-integer
conversions raise the ``inexact'' floating-point exception for non-integer in-range values. In those
cases where it matters, library functions can be used to effect such conversions with or without raising
the ``inexact'' floating-point exception. See rint, lrint, llrint, and nearbyint in
<math.h>.
361) If the minimum-width IEC 60559 extended format (64 bits of precision) is supported,
DECIMAL_DIG shall be at least 21. If IEC 60559 double (53 bits of precision) is the widest
IEC 60559 format supported, then DECIMAL_DIG shall be at least 17. (By contrast, LDBL_DIG and
DBL_DIG are 18 and 15, respectively, for these formats.)
3 Functions such as strtod that convert character sequences to floating types honor the
rounding direction. Hence, if the rounding direction might be upward or downward, the
implementation cannot convert a minus-signed sequence by negating the converted
unsigned sequence.
F.6 The return statement
If the return expression is evaluated in a floating-point format different from the return
type, the expression is converted as if by assignment362) to the return type of the function
and the resulting value is returned to the caller.
F.7 Contracted expressions
1 A contracted expression is correctly rounded (once) and treats infinities, NaNs, signed
zeros, subnormals, and the rounding directions in a manner consistent with the basic
arithmetic operations covered by IEC 60559.
Recommended practice
2 A contracted expression should raise floating-point exceptions in a manner generally
consistent with the basic arithmetic operations.
F.8 Floating-point environment
1 The floating-point environment defined in <fenv.h> includes the IEC 60559 floating-
point exception status flags and directed-rounding control modes. It includes also
IEC 60559 dynamic rounding precision and trap enablement modes, if the
implementation supports them.363)
F.8.1 Environment management
1 IEC 60559 requires that floating-point operations implicitly raise floating-point exception
status flags, and that rounding control modes can be set explicitly to affect result values of
floating-point operations. When the state for the FENV_ACCESS pragma (defined in
<fenv.h>) is ``on'', these changes to the floating-point state are treated as side effects
which respect sequence points.364)
362) Assignment removes any extra range and precision.
363) This specification does not require dynamic rounding precision nor trap enablement modes.
364) If the state for the FENV_ACCESS pragma is ``off'', the implementation is free to assume the floating-
point control modes will be the default ones and the floating-point status flags will not be tested,
which allows certain optimizations (see F.9).
F.8.2 Translation
1 During translation the IEC 60559 default modes are in effect:
-- The rounding direction mode is rounding to nearest.
-- The rounding precision mode (if supported) is set so that results are not shortened.
-- Trapping or stopping (if supported) is disabled on all floating-point exceptions.
Recommended practice
2 The implementation should produce a diagnostic message for each translation-time
floating-point exception, other than ``inexact'';365) the implementation should then
proceed with the translation of the program.
F.8.3 Execution
1 At program startup the floating-point environment is initialized as prescribed by
IEC 60559:
-- All floating-point exception status flags are cleared.
-- The rounding direction mode is rounding to nearest.
-- The dynamic rounding precision mode (if supported) is set so that results are not
shortened.
-- Trapping or stopping (if supported) is disabled on all floating-point exceptions.
F.8.4 Constant expressions
1 An arithmetic constant expression of floating type, other than one in an initializer for an
object that has static or thread storage duration, is evaluated (as if) during execution; thus,
it is affected by any operative floating-point control modes and raises floating-point
exceptions as required by IEC 60559 (provided the state for the FENV_ACCESS pragma
is ``on'').366)
2 EXAMPLE
365) As floating constants are converted to appropriate internal representations at translation time, their
conversion is subject to default rounding modes and raises no execution-time floating-point exceptions
(even where the state of the FENV_ACCESS pragma is ``on''). Library functions, for example
strtod, provide execution-time conversion of numeric strings.
366) Where the state for the FENV_ACCESS pragma is ``on'', results of inexact expressions like 1.0/3.0
are affected by rounding modes set at execution time, and expressions such as 0.0/0.0 and
1.0/0.0 generate execution-time floating-point exceptions. The programmer can achieve the
efficiency of translation-time evaluation through static initialization, such as
const static double one_third = 1.0/3.0;
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(void)
{
float w[] = { 0.0/0.0 }; // raises an exception
static float x = 0.0/0.0; // does not raise an exception
float y = 0.0/0.0; // raises an exception
double z = 0.0/0.0; // raises an exception
/* ... */
}
3 For the static initialization, the division is done at translation time, raising no (execution-time) floating-
point exceptions. On the other hand, for the three automatic initializations the invalid division occurs at
execution time.
F.8.5 Initialization
1 All computation for automatic initialization is done (as if) at execution time; thus, it is
affected by any operative modes and raises floating-point exceptions as required by
IEC 60559 (provided the state for the FENV_ACCESS pragma is ``on''). All computation
for initialization of objects that have static or thread storage duration is done (as if) at
translation time.
2 EXAMPLE
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(void)
{
float u[] = { 1.1e75 }; // raises exceptions
static float v = 1.1e75; // does not raise exceptions
float w = 1.1e75; // raises exceptions
double x = 1.1e75; // may raise exceptions
float y = 1.1e75f; // may raise exceptions
long double z = 1.1e75; // does not raise exceptions
/* ... */
}
3 The static initialization of v raises no (execution-time) floating-point exceptions because its computation is
done at translation time. The automatic initialization of u and w require an execution-time conversion to
float of the wider value 1.1e75, which raises floating-point exceptions. The automatic initializations
of x and y entail execution-time conversion; however, in some expression evaluation methods, the
conversions is not to a narrower format, in which case no floating-point exception is raised.367) The
automatic initialization of z entails execution-time conversion, but not to a narrower format, so no floating-
point exception is raised. Note that the conversions of the floating constants 1.1e75 and 1.1e75f to
367) Use of float_t and double_t variables increases the likelihood of translation-time computation.
For example, the automatic initialization
double_t x = 1.1e75;
could be done at translation time, regardless of the expression evaluation method.
their internal representations occur at translation time in all cases.
F.8.6 Changing the environment
1 Operations defined in 6.5 and functions and macros defined for the standard libraries
change floating-point status flags and control modes just as indicated by their
specifications (including conformance to IEC 60559). They do not change flags or modes
(so as to be detectable by the user) in any other cases.
2 If the argument to the feraiseexcept function in <fenv.h> represents IEC 60559
valid coincident floating-point exceptions for atomic operations (namely ``overflow'' and
``inexact'', or ``underflow'' and ``inexact''), then ``overflow'' or ``underflow'' is raised
before ``inexact''.
F.9 Optimization
1 This section identifies code transformations that might subvert IEC 60559-specified
behavior, and others that do not.
F.9.1 Global transformations
1 Floating-point arithmetic operations and external function calls may entail side effects
which optimization shall honor, at least where the state of the FENV_ACCESS pragma is
``on''. The flags and modes in the floating-point environment may be regarded as global
variables; floating-point operations (+, *, etc.) implicitly read the modes and write the
flags.
2 Concern about side effects may inhibit code motion and removal of seemingly useless
code. For example, in
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(double x)
{
/* ... */
for (i = 0; i < n; i++) x + 1;
/* ... */
}
x + 1 might raise floating-point exceptions, so cannot be removed. And since the loop
body might not execute (maybe 0 n), x + 1 cannot be moved out of the loop. (Of
course these optimizations are valid if the implementation can rule out the nettlesome
cases.)
3 This specification does not require support for trap handlers that maintain information
about the order or count of floating-point exceptions. Therefore, between function calls,
floating-point exceptions need not be precise: the actual order and number of occurrences
of floating-point exceptions (> 1) may vary from what the source code expresses. Thus,
the preceding loop could be treated as
if (0 < n) x + 1;
F.9.2 Expression transformations
1 x /2 x <20> 0.5 Although similar transformations involving inexact constants
generally do not yield numerically equivalent expressions, if the
constants are exact then such transformations can be made on
IEC 60559 machines and others that round perfectly.
1 <20> x and x /1 x The expressions 1 <20> x , x /1, and x are equivalent (on IEC 60559
machines, among others).368)
x / x 1. 0 The expressions x / x and 1. 0 are not equivalent if x can be zero,
infinite, or NaN.
x - y x + (- y ) The expressions x - y , x + (- y ), and (- y ) + x are equivalent (on
IEC 60559 machines, among others).
x - y -( y - x ) The expressions x - y and -( y - x ) are not equivalent because 1 - 1
is +0 but -(1 - 1) is -0 (in the default rounding direction).369)
x - x 0. 0 The expressions x - x and 0. 0 are not equivalent if x is a NaN or
infinite.
0 <20> x 0. 0 The expressions 0 <20> x and 0. 0 are not equivalent if x is a NaN,
infinite, or -0.
x+0 x The expressions x + 0 and x are not equivalent if x is -0, because
(-0) + (+0) yields +0 (in the default rounding direction), not -0.
x-0 x (+0) - (+0) yields -0 when rounding is downward (toward -), but
+0 otherwise, and (-0) - (+0) always yields -0; so, if the state of the
FENV_ACCESS pragma is ``off'', promising default rounding, then
the implementation can replace x - 0 by x , even if x might be zero.
-x 0 - x The expressions - x and 0 - x are not equivalent if x is +0, because
-(+0) yields -0, but 0 - (+0) yields +0 (unless rounding is
downward).
368) Strict support for signaling NaNs -- not required by this specification -- would invalidate these and
other transformations that remove arithmetic operators.
369) IEC 60559 prescribes a signed zero to preserve mathematical identities across certain discontinuities.
Examples include:
1/(1/ <20> ) is <20>
and
conj(csqrt( z )) is csqrt(conj( z )),
for complex z .
F.9.3 Relational operators
1 x x false The expression x x is true if x is a NaN.
x = x true The expression x = x is false if x is a NaN.
x < y isless( x , y ) (and similarly for , >, ) Though numerically equal, these
expressions are not equivalent because of side effects when x or y is a
NaN and the state of the FENV_ACCESS pragma is ``on''. This
transformation, which would be desirable if extra code were required
to cause the ``invalid'' floating-point exception for unordered cases,
could be performed provided the state of the FENV_ACCESS pragma
is ``off''.
The sense of relational operators shall be maintained. This includes handling unordered
cases as expressed by the source code.
2 EXAMPLE
// calls g and raises ``invalid'' if a and b are unordered
if (a < b)
f();
else
g();
is not equivalent to
// calls f and raises ``invalid'' if a and b are unordered
if (a >= b)
g();
else
f();
nor to
// calls f without raising ``invalid'' if a and b are unordered
if (isgreaterequal(a,b))
g();
else
f();
nor, unless the state of the FENV_ACCESS pragma is ``off'', to
// calls g without raising ``invalid'' if a and b are unordered
if (isless(a,b))
f();
else
g();
but is equivalent to
if (!(a < b))
g();
else
f();
F.9.4 Constant arithmetic
1 The implementation shall honor floating-point exceptions raised by execution-time
constant arithmetic wherever the state of the FENV_ACCESS pragma is ``on''. (See F.8.4
and F.8.5.) An operation on constants that raises no floating-point exception can be
folded during translation, except, if the state of the FENV_ACCESS pragma is ``on'', a
further check is required to assure that changing the rounding direction to downward does
not alter the sign of the result,370) and implementations that support dynamic rounding
precision modes shall assure further that the result of the operation raises no floating-
point exception when converted to the semantic type of the operation.
F.10 Mathematics <math.h>
1 This subclause contains specifications of <math.h> facilities that are particularly suited
for IEC 60559 implementations.
2 The Standard C macro HUGE_VAL and its float and long double analogs,
HUGE_VALF and HUGE_VALL, expand to expressions whose values are positive
infinities.
3 Special cases for functions in <math.h> are covered directly or indirectly by
IEC 60559. The functions that IEC 60559 specifies directly are identified in F.3. The
other functions in <math.h> treat infinities, NaNs, signed zeros, subnormals, and
(provided the state of the FENV_ACCESS pragma is ``on'') the floating-point status flags
in a manner consistent with the basic arithmetic operations covered by IEC 60559.
4 The expression math_errhandling & MATH_ERREXCEPT shall evaluate to a
nonzero value.
5 The ``invalid'' and ``divide-by-zero'' floating-point exceptions are raised as specified in
subsequent subclauses of this annex.
6 The ``overflow'' floating-point exception is raised whenever an infinity -- or, because of
rounding direction, a maximal-magnitude finite number -- is returned in lieu of a value
whose magnitude is too large.
7 The ``underflow'' floating-point exception is raised whenever a result is tiny (essentially
subnormal or zero) and suffers loss of accuracy.371)
370) 0 - 0 yields -0 instead of +0 just when the rounding direction is downward.
371) IEC 60559 allows different definitions of underflow. They all result in the same values, but differ on
when the floating-point exception is raised.
8 Whether or when library functions raise the ``inexact'' floating-point exception is
unspecified, unless explicitly specified otherwise.
9 Whether or when library functions raise an undeserved ``underflow'' floating-point
exception is unspecified.372) Otherwise, as implied by F.8.6, the <math.h> functions do
not raise spurious floating-point exceptions (detectable by the user), other than the
``inexact'' floating-point exception.
10 Whether the functions honor the rounding direction mode is implementation-defined,
unless explicitly specified otherwise.
11 Functions with a NaN argument return a NaN result and raise no floating-point exception,
except where stated otherwise.
12 The specifications in the following subclauses append to the definitions in <math.h>.
For families of functions, the specifications apply to all of the functions even though only
the principal function is shown. Unless otherwise specified, where the symbol ``<60>''
occurs in both an argument and the result, the result has the same sign as the argument.
Recommended practice
13 If a function with one or more NaN arguments returns a NaN result, the result should be
the same as one of the NaN arguments (after possible type conversion), except perhaps
for the sign.
F.10.1 Trigonometric functions
F.10.1.1 The acos functions
1 -- acos(1) returns +0.
-- acos( x ) returns a NaN and raises the ``invalid'' floating-point exception for
| x | > 1.
F.10.1.2 The asin functions
1 -- asin(<28>0) returns <20>0.
-- asin( x ) returns a NaN and raises the ``invalid'' floating-point exception for
| x | > 1.
372) It is intended that undeserved ``underflow'' and ``inexact'' floating-point exceptions are raised only if
avoiding them would be too costly.
F.10.1.3 The atan functions
1 -- atan(<28>0) returns <20>0.
-- atan(<28>) returns <20> /2.
F.10.1.4 The atan2 functions
1 -- atan2(<28>0, -0) returns <20> .373)
-- atan2(<28>0, +0) returns <20>0.
-- atan2(<28>0, x ) returns <20> for x < 0.
-- atan2(<28>0, x ) returns <20>0 for x > 0.
-- atan2( y , <20>0) returns - /2 for y < 0.
-- atan2( y , <20>0) returns /2 for y > 0.
-- atan2(<28> y , -) returns <20> for finite y > 0.
-- atan2(<28> y , +) returns <20>0 for finite y > 0.
-- atan2(<28>, x ) returns <20> /2 for finite x .
-- atan2(<28>, -) returns <20>3 /4.
-- atan2(<28>, +) returns <20> /4.
F.10.1.5 The cos functions
1 -- cos(<28>0) returns 1.
-- cos(<28>) returns a NaN and raises the ``invalid'' floating-point exception.
F.10.1.6 The sin functions
1 -- sin(<28>0) returns <20>0.
-- sin(<28>) returns a NaN and raises the ``invalid'' floating-point exception.
F.10.1.7 The tan functions
1 -- tan(<28>0) returns <20>0.
-- tan(<28>) returns a NaN and raises the ``invalid'' floating-point exception.
373) atan2(0, 0) does not raise the ``invalid'' floating-point exception, nor does atan2( y , 0) raise
the ``divide-by-zero'' floating-point exception.
F.10.2 Hyperbolic functions
F.10.2.1 The acosh functions
1 -- acosh(1) returns +0.
-- acosh( x ) returns a NaN and raises the ``invalid'' floating-point exception for x < 1.
-- acosh(+) returns +.
F.10.2.2 The asinh functions
1 -- asinh(<28>0) returns <20>0.
-- asinh(<28>) returns <20>.
F.10.2.3 The atanh functions
1 -- atanh(<28>0) returns <20>0.
-- atanh(<28>1) returns <20> and raises the ``divide-by-zero'' floating-point exception.
-- atanh( x ) returns a NaN and raises the ``invalid'' floating-point exception for
| x | > 1.
F.10.2.4 The cosh functions
1 -- cosh(<28>0) returns 1.
-- cosh(<28>) returns +.
F.10.2.5 The sinh functions
1 -- sinh(<28>0) returns <20>0.
-- sinh(<28>) returns <20>.
F.10.2.6 The tanh functions
1 -- tanh(<28>0) returns <20>0.
-- tanh(<28>) returns <20>1.
F.10.3 Exponential and logarithmic functions
F.10.3.1 The exp functions
1 -- exp(<28>0) returns 1.
-- exp(-) returns +0.
-- exp(+) returns +.
F.10.3.2 The exp2 functions
1 -- exp2(<28>0) returns 1.
-- exp2(-) returns +0.
-- exp2(+) returns +.
F.10.3.3 The expm1 functions
1 -- expm1(<28>0) returns <20>0.
-- expm1(-) returns -1.
-- expm1(+) returns +.
F.10.3.4 The frexp functions
1 -- frexp(<28>0, exp) returns <20>0, and stores 0 in the object pointed to by exp.
-- frexp(<28>, exp) returns <20>, and stores an unspecified value in the object
pointed to by exp.
-- frexp(NaN, exp) stores an unspecified value in the object pointed to by exp
(and returns a NaN).
2 frexp raises no floating-point exceptions.
3 When the radix of the argument is a power of 2, the returned value is exact and is
independent of the current rounding direction mode.
4 On a binary system, the body of the frexp function might be
{
*exp = (value == 0) ? 0 : (int)(1 + logb(value));
return scalbn(value, -(*exp));
}
F.10.3.5 The ilogb functions
1 When the correct result is representable in the range of the return type, the returned value
is exact and is independent of the current rounding direction mode.
2 If the correct result is outside the range of the return type, the numeric result is
unspecified and the ``invalid'' floating-point exception is raised.
3 ilogb( x ), for x zero, infinite, or NaN, raises the ``invalid'' floating-point exception and
returns the value specified in 7.12.6.5.
F.10.3.6 The ldexp functions
1 On a binary system, ldexp(x, exp) is equivalent to scalbn(x, exp).
F.10.3.7 The log functions
1 -- log(<28>0) returns - and raises the ``divide-by-zero'' floating-point exception.
-- log(1) returns +0.
-- log( x ) returns a NaN and raises the ``invalid'' floating-point exception for x < 0.
-- log(+) returns +.
F.10.3.8 The log10 functions
1 -- log10(<28>0) returns - and raises the ``divide-by-zero'' floating-point exception.
-- log10(1) returns +0.
-- log10( x ) returns a NaN and raises the ``invalid'' floating-point exception for x < 0.
-- log10(+) returns +.
F.10.3.9 The log1p functions
1 -- log1p(<28>0) returns <20>0.
-- log1p(-1) returns - and raises the ``divide-by-zero'' floating-point exception.
-- log1p( x ) returns a NaN and raises the ``invalid'' floating-point exception for
x < -1.
-- log1p(+) returns +.
F.10.3.10 The log2 functions
1 -- log2(<28>0) returns - and raises the ``divide-by-zero'' floating-point exception.
-- log2(1) returns +0.
-- log2( x ) returns a NaN and raises the ``invalid'' floating-point exception for x < 0.
-- log2(+) returns +.
F.10.3.11 The logb functions
1 -- logb(<28>0) returns - and raises the ``divide-by-zero'' floating-point exception.
-- logb(<28>) returns +.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.3.12 The modf functions
1 -- modf(<28> x , iptr) returns a result with the same sign as x .
-- modf(<28>, iptr) returns <20>0 and stores <20> in the object pointed to by iptr.
-- modf(NaN, iptr) stores a NaN in the object pointed to by iptr (and returns a
NaN).
2 The returned values are exact and are independent of the current rounding direction
mode.
3 modf behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double modf(double value, double *iptr)
{
int save_round = fegetround();
fesetround(FE_TOWARDZERO);
*iptr = nearbyint(value);
fesetround(save_round);
return copysign(
isinf(value) ? 0.0 :
value - (*iptr), value);
}
F.10.3.13 The scalbn and scalbln functions
1 -- scalbn(<28>0, n) returns <20>0.
-- scalbn( x , 0) returns x .
-- scalbn(<28>, n) returns <20>.
2 If the calculation does not overflow or underflow, the returned value is exact and
independent of the current rounding direction mode.
F.10.4 Power and absolute value functions
F.10.4.1 The cbrt functions
1 -- cbrt(<28>0) returns <20>0.
-- cbrt(<28>) returns <20>.
F.10.4.2 The fabs functions
1 -- fabs(<28>0) returns +0.
-- fabs(<28>) returns +.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.4.3 The hypot functions
1 -- hypot( x , y ), hypot( y , x ), and hypot( x , - y ) are equivalent.
-- hypot( x , <20>0) is equivalent to fabs( x ).
-- hypot(<28>, y ) returns +, even if y is a NaN.
F.10.4.4 The pow functions
1 -- pow(<28>0, y ) returns <20> and raises the ``divide-by-zero'' floating-point exception
for y an odd integer < 0.
-- pow(<28>0, y ) returns + and raises the ``divide-by-zero'' floating-point exception
for y < 0, finite, and not an odd integer.
-- pow(<28>0, -) returns + and may raise the ``divide-by-zero'' floating-point
exception.
-- pow(<28>0, y ) returns <20>0 for y an odd integer > 0.
-- pow(<28>0, y ) returns +0 for y > 0 and not an odd integer.
-- pow(-1, <20>) returns 1.
-- pow(+1, y ) returns 1 for any y , even a NaN.
-- pow( x , <20>0) returns 1 for any x , even a NaN.
-- pow( x , y ) returns a NaN and raises the ``invalid'' floating-point exception for
finite x < 0 and finite non-integer y .
-- pow( x , -) returns + for | x | < 1.
-- pow( x , -) returns +0 for | x | > 1.
-- pow( x , +) returns +0 for | x | < 1.
-- pow( x , +) returns + for | x | > 1.
-- pow(-, y ) returns -0 for y an odd integer < 0.
-- pow(-, y ) returns +0 for y < 0 and not an odd integer.
-- pow(-, y ) returns - for y an odd integer > 0.
-- pow(-, y ) returns + for y > 0 and not an odd integer.
-- pow(+, y ) returns +0 for y < 0.
-- pow(+, y ) returns + for y > 0.
F.10.4.5 The sqrt functions
1 sqrt is fully specified as a basic arithmetic operation in IEC 60559. The returned value
is dependent on the current rounding direction mode.
F.10.5 Error and gamma functions
F.10.5.1 The erf functions
1 -- erf(<28>0) returns <20>0.
-- erf(<28>) returns <20>1.
F.10.5.2 The erfc functions
1 -- erfc(-) returns 2.
-- erfc(+) returns +0.
F.10.5.3 The lgamma functions
1 -- lgamma(1) returns +0.
-- lgamma(2) returns +0.
-- lgamma( x ) returns + and raises the ``divide-by-zero'' floating-point exception for
x a negative integer or zero.
-- lgamma(-) returns +.
-- lgamma(+) returns +.
F.10.5.4 The tgamma functions
1 -- tgamma(<28>0) returns <20> and raises the ``divide-by-zero'' floating-point exception.
-- tgamma( x ) returns a NaN and raises the ``invalid'' floating-point exception for x a
negative integer.
-- tgamma(-) returns a NaN and raises the ``invalid'' floating-point exception.
-- tgamma(+) returns +.
F.10.6 Nearest integer functions
F.10.6.1 The ceil functions
1 -- ceil(<28>0) returns <20>0.
-- ceil(<28>) returns <20>.
2 The returned value is independent of the current rounding direction mode.
3 The double version of ceil behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double ceil(double x)
{
double result;
int save_round = fegetround();
fesetround(FE_UPWARD);
result = rint(x); // or nearbyint instead of rint
fesetround(save_round);
return result;
}
4 The ceil functions may, but are not required to, raise the ``inexact'' floating-point
exception for finite non-integer arguments, as this implementation does.
F.10.6.2 The floor functions
1 -- floor(<28>0) returns <20>0.
-- floor(<28>) returns <20>.
2 The returned value and is independent of the current rounding direction mode.
3 See the sample implementation for ceil in F.10.6.1. The floor functions may, but are
not required to, raise the ``inexact'' floating-point exception for finite non-integer
arguments, as that implementation does.
F.10.6.3 The nearbyint functions
1 The nearbyint functions use IEC 60559 rounding according to the current rounding
direction. They do not raise the ``inexact'' floating-point exception if the result differs in
value from the argument.
-- nearbyint(<28>0) returns <20>0 (for all rounding directions).
-- nearbyint(<28>) returns <20> (for all rounding directions).
F.10.6.4 The rint functions
1 The rint functions differ from the nearbyint functions only in that they do raise the
``inexact'' floating-point exception if the result differs in value from the argument.
F.10.6.5 The lrint and llrint functions
1 The lrint and llrint functions provide floating-to-integer conversion as prescribed
by IEC 60559. They round according to the current rounding direction. If the rounded
value is outside the range of the return type, the numeric result is unspecified and the
``invalid'' floating-point exception is raised. When they raise no other floating-point
exception and the result differs from the argument, they raise the ``inexact'' floating-point
exception.
F.10.6.6 The round functions
1 -- round(<28>0) returns <20>0.
-- round(<28>) returns <20>.
2 The returned value is independent of the current rounding direction mode.
3 The double version of round behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double round(double x)
{
double result;
fenv_t save_env;
feholdexcept(&save_env);
result = rint(x);
if (fetestexcept(FE_INEXACT)) {
fesetround(FE_TOWARDZERO);
result = rint(copysign(0.5 + fabs(x), x));
}
feupdateenv(&save_env);
return result;
}
The round functions may, but are not required to, raise the ``inexact'' floating-point
exception for finite non-integer numeric arguments, as this implementation does.
F.10.6.7 The lround and llround functions
1 The lround and llround functions differ from the lrint and llrint functions
with the default rounding direction just in that the lround and llround functions
round halfway cases away from zero and need not raise the ``inexact'' floating-point
exception for non-integer arguments that round to within the range of the return type.
F.10.6.8 The trunc functions
1 The trunc functions use IEC 60559 rounding toward zero (regardless of the current
rounding direction). The returned value is exact.
-- trunc(<28>0) returns <20>0.
-- trunc(<28>) returns <20>.
2 The returned value is independent of the current rounding direction mode. The trunc
functions may, but are not required to, raise the ``inexact'' floating-point exception for
finite non-integer arguments.
F.10.7 Remainder functions
F.10.7.1 The fmod functions
1 -- fmod(<28>0, y ) returns <20>0 for y not zero.
-- fmod( x , y ) returns a NaN and raises the ``invalid'' floating-point exception for x
infinite or y zero (and neither is a NaN).
-- fmod( x , <20>) returns x for x not infinite.
2 When subnormal results are supported, the returned value is exact and is independent of
the current rounding direction mode.
3 The double version of fmod behaves as though implemented by
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
double fmod(double x, double y)
{
double result;
result = remainder(fabs(x), (y = fabs(y)));
if (signbit(result)) result += y;
return copysign(result, x);
}
F.10.7.2 The remainder functions
1 The remainder functions are fully specified as a basic arithmetic operation in
IEC 60559.
2 When subnormal results are supported, the returned value is exact and is independent of
the current rounding direction mode.
F.10.7.3 The remquo functions
1 The remquo functions follow the specifications for the remainder functions. They
have no further specifications special to IEC 60559 implementations.
2 When subnormal results are supported, the returned value is exact and is independent of
the current rounding direction mode.
F.10.8 Manipulation functions
F.10.8.1 The copysign functions
1 copysign is specified in the Appendix to IEC 60559.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.8.2 The nan functions
1 All IEC 60559 implementations support quiet NaNs, in all floating formats.
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.8.3 The nextafter functions
1 -- nextafter( x , y ) raises the ``overflow'' and ``inexact'' floating-point exceptions
for x finite and the function value infinite.
-- nextafter( x , y ) raises the ``underflow'' and ``inexact'' floating-point
exceptions for the function value subnormal or zero and x y .
2 Even though underflow or overflow can occur, the returned value is independent of the
current rounding direction mode.
F.10.8.4 The nexttoward functions
1 No additional requirements beyond those on nextafter.
2 Even though underflow or overflow can occur, the returned value is independent of the
current rounding direction mode.
F.10.9 Maximum, minimum, and positive difference functions
F.10.9.1 The fdim functions
1 No additional requirements.
F.10.9.2 The fmax functions
1 If just one argument is a NaN, the fmax functions return the other argument (if both
arguments are NaNs, the functions return a NaN).
2 The returned value is exact and is independent of the current rounding direction mode.
3 The body of the fmax function might be374)
{ return (isgreaterequal(x, y) ||
isnan(y)) ? x : y; }
F.10.9.3 The fmin functions
1 The fmin functions are analogous to the fmax functions (see F.10.9.2).
2 The returned value is exact and is independent of the current rounding direction mode.
F.10.10 Floating multiply-add
F.10.10.1 The fma functions
1 -- fma( x , y , z ) computes xy + z , correctly rounded once.
-- fma( x , y , z ) returns a NaN and optionally raises the ``invalid'' floating-point
exception if one of x and y is infinite, the other is zero, and z is a NaN.
-- fma( x , y , z ) returns a NaN and raises the ``invalid'' floating-point exception if
one of x and y is infinite, the other is zero, and z is not a NaN.
-- fma( x , y , z ) returns a NaN and raises the ``invalid'' floating-point exception if x
times y is an exact infinity and z is also an infinity but with the opposite sign.
374) Ideally, fmax would be sensitive to the sign of zero, for example fmax(-0. 0, +0. 0) would
return +0; however, implementation in software might be impractical.
F.10.11 Comparison macros
1 Relational operators and their corresponding comparison macros (7.12.14) produce
equivalent result values, even if argument values are represented in wider formats. Thus,
comparison macro arguments represented in formats wider than their semantic types are
not converted to the semantic types, unless the wide evaluation method converts operands
of relational operators to their semantic types. The standard wide evaluation methods
characterized by FLT_EVAL_METHOD equal to 1 or 2 (5.2.4.2.2), do not convert
operands of relational operators to their semantic types.
Annex G
(normative)
G. IEC 60559-compatible complex arithmetic
G.1 Introduction
1 This annex supplements annex F to specify complex arithmetic for compatibility with
IEC 60559 real floating-point arithmetic. An implementation that defines
_ _STDC_IEC_559_COMPLEX_ _ shall conform to the specifications in this annex.375)
G.2 Types
1 There is a new keyword _Imaginary, which is used to specify imaginary types. It is
used as a type specifier within declaration specifiers in the same way as _Complex is
(thus, _Imaginary float is a valid type name).
2 There are three imaginary types, designated as float _Imaginary, double
_Imaginary, and long double _Imaginary. The imaginary types (along with
the real floating and complex types) are floating types.
3 For imaginary types, the corresponding real type is given by deleting the keyword
_Imaginary from the type name.
4 Each imaginary type has the same representation and alignment requirements as the
corresponding real type. The value of an object of imaginary type is the value of the real
representation times the imaginary unit.
5 The imaginary type domain comprises the imaginary types.
G.3 Conventions
1 A complex or imaginary value with at least one infinite part is regarded as an infinity
(even if its other part is a NaN). A complex or imaginary value is a finite number if each
of its parts is a finite number (neither infinite nor NaN). A complex or imaginary value is
a zero if each of its parts is a zero.
375) Implementations that do not define _ _STDC_IEC_559_COMPLEX_ _ are not required to conform
to these specifications.
G.4 Conversions
G.4.1 Imaginary types
1 Conversions among imaginary types follow rules analogous to those for real floating
types.
G.4.2 Real and imaginary
1 When a value of imaginary type is converted to a real type other than _Bool,376) the
result is a positive zero.
2 When a value of real type is converted to an imaginary type, the result is a positive
imaginary zero.
G.4.3 Imaginary and complex
1 When a value of imaginary type is converted to a complex type, the real part of the
complex result value is a positive zero and the imaginary part of the complex result value
is determined by the conversion rules for the corresponding real types.
2 When a value of complex type is converted to an imaginary type, the real part of the
complex value is discarded and the value of the imaginary part is converted according to
the conversion rules for the corresponding real types.
G.5 Binary operators
1 The following subclauses supplement 6.5 in order to specify the type of the result for an
operation with an imaginary operand.
2 For most operand types, the value of the result of a binary operator with an imaginary or
complex operand is completely determined, with reference to real arithmetic, by the usual
mathematical formula. For some operand types, the usual mathematical formula is
problematic because of its treatment of infinities and because of undue overflow or
underflow; in these cases the result satisfies certain properties (specified in G.5.1), but is
not completely determined.
376) See 6.3.1.2.
G.5.1 Multiplicative operators
Semantics
1 If one operand has real type and the other operand has imaginary type, then the result has
imaginary type. If both operands have imaginary type, then the result has real type. (If
either operand has complex type, then the result has complex type.)
2 If the operands are not both complex, then the result and floating-point exception
behavior of the * operator is defined by the usual mathematical formula:
* u iv u + iv
x xu i ( xv ) ( xu) + i ( xv )
iy i ( yu) - yv (- yv ) + i ( yu)
x + iy ( xu) + i ( yu) (- yv ) + i ( xv )
3 If the second operand is not complex, then the result and floating-point exception
behavior of the / operator is defined by the usual mathematical formula:
/ u iv
x x /u i (- x /v )
iy i ( y /u ) y/v
x + iy ( x /u ) + i ( y /u ) ( y /v ) + i (- x /v )
4 The * and / operators satisfy the following infinity properties for all real, imaginary, and
complex operands:377)
-- if one operand is an infinity and the other operand is a nonzero finite number or an
infinity, then the result of the * operator is an infinity;
-- if the first operand is an infinity and the second operand is a finite number, then the
result of the / operator is an infinity;
-- if the first operand is a finite number and the second operand is an infinity, then the
result of the / operator is a zero;
377) These properties are already implied for those cases covered in the tables, but are required for all cases
(at least where the state for CX_LIMITED_RANGE is ``off'').
-- if the first operand is a nonzero finite number or an infinity and the second operand is
a zero, then the result of the / operator is an infinity.
5 If both operands of the * operator are complex or if the second operand of the / operator
is complex, the operator raises floating-point exceptions if appropriate for the calculation
of the parts of the result, and may raise spurious floating-point exceptions.
6 EXAMPLE 1 Multiplication of double _Complex operands could be implemented as follows. Note
that the imaginary unit I has imaginary type (see G.6).
#include <math.h>
#include <complex.h>
/* Multiply z * w ... */
double complex _Cmultd(double complex z, double complex w)
{
#pragma STDC FP_CONTRACT OFF
double a, b, c, d, ac, bd, ad, bc, x, y;
a = creal(z); b = cimag(z);
c = creal(w); d = cimag(w);
ac = a * c; bd = b * d;
ad = a * d; bc = b * c;
x = ac - bd; y = ad + bc;
if (isnan(x) && isnan(y)) {
/* Recover infinities that computed as NaN+iNaN ... */
int recalc = 0;
if (isinf(a) || isinf(b)) { // z is infinite
/* "Box" the infinity and change NaNs in the other factor to 0 */
a = copysign(isinf(a) ? 1.0 : 0.0, a);
b = copysign(isinf(b) ? 1.0 : 0.0, b);
if (isnan(c)) c = copysign(0.0, c);
if (isnan(d)) d = copysign(0.0, d);
recalc = 1;
}
if (isinf(c) || isinf(d)) { // w is infinite
/* "Box" the infinity and change NaNs in the other factor to 0 */
c = copysign(isinf(c) ? 1.0 : 0.0, c);
d = copysign(isinf(d) ? 1.0 : 0.0, d);
if (isnan(a)) a = copysign(0.0, a);
if (isnan(b)) b = copysign(0.0, b);
recalc = 1;
}
if (!recalc && (isinf(ac) || isinf(bd) ||
isinf(ad) || isinf(bc))) {
/* Recover infinities from overflow by changing NaNs to 0 ... */
if (isnan(a)) a = copysign(0.0, a);
if (isnan(b)) b = copysign(0.0, b);
if (isnan(c)) c = copysign(0.0, c);
if (isnan(d)) d = copysign(0.0, d);
recalc = 1;
}
if (recalc) {
x = INFINITY * ( a * c - b * d );
y = INFINITY * ( a * d + b * c );
}
}
return x + I * y;
}
7 This implementation achieves the required treatment of infinities at the cost of only one isnan test in
ordinary (finite) cases. It is less than ideal in that undue overflow and underflow may occur.
8 EXAMPLE 2 Division of two double _Complex operands could be implemented as follows.
#include <math.h>
#include <complex.h>
/* Divide z / w ... */
double complex _Cdivd(double complex z, double complex w)
{
#pragma STDC FP_CONTRACT OFF
double a, b, c, d, logbw, denom, x, y;
int ilogbw = 0;
a = creal(z); b = cimag(z);
c = creal(w); d = cimag(w);
logbw = logb(fmax(fabs(c), fabs(d)));
if (isfinite(logbw)) {
ilogbw = (int)logbw;
c = scalbn(c, -ilogbw); d = scalbn(d, -ilogbw);
}
denom = c * c + d * d;
x = scalbn((a * c + b * d) / denom, -ilogbw);
y = scalbn((b * c - a * d) / denom, -ilogbw);
/* Recover infinities and zeros that computed as NaN+iNaN; */
/* the only cases are nonzero/zero, infinite/finite, and finite/infinite, ... */
if (isnan(x) && isnan(y)) {
if ((denom == 0.0) &&
(!isnan(a) || !isnan(b))) {
x = copysign(INFINITY, c) * a;
y = copysign(INFINITY, c) * b;
}
else if ((isinf(a) || isinf(b)) &&
isfinite(c) && isfinite(d)) {
a = copysign(isinf(a) ? 1.0 : 0.0, a);
b = copysign(isinf(b) ? 1.0 : 0.0, b);
x = INFINITY * ( a * c + b * d );
y = INFINITY * ( b * c - a * d );
}
else if ((logbw == INFINITY) &&
isfinite(a) && isfinite(b)) {
c = copysign(isinf(c) ? 1.0 : 0.0, c);
d = copysign(isinf(d) ? 1.0 : 0.0, d);
x = 0.0 * ( a * c + b * d );
y = 0.0 * ( b * c - a * d );
}
}
return x + I * y;
}
9 Scaling the denominator alleviates the main overflow and underflow problem, which is more serious than
for multiplication. In the spirit of the multiplication example above, this code does not defend against
overflow and underflow in the calculation of the numerator. Scaling with the scalbn function, instead of
with division, provides better roundoff characteristics.
G.5.2 Additive operators
Semantics
1 If both operands have imaginary type, then the result has imaginary type. (If one operand
has real type and the other operand has imaginary type, or if either operand has complex
type, then the result has complex type.)
2 In all cases the result and floating-point exception behavior of a + or - operator is defined
by the usual mathematical formula:
+ or - u iv u + iv
x x<>u x <20> iv ( x <20> u) <20> iv
iy <20>u + iy i( y <20> v) <20>u + i ( y <20> v )
x + iy ( x <20> u) + iy x + i( y <20> v) ( x <20> u) + i ( y <20> v)
G.6 Complex arithmetic <complex.h>
1 The macros
imaginary
and
_Imaginary_I
are defined, respectively, as _Imaginary and a constant expression of type const
float _Imaginary with the value of the imaginary unit. The macro
I
is defined to be _Imaginary_I (not _Complex_I as stated in 7.3). Notwithstanding
the provisions of 7.1.3, a program may undefine and then perhaps redefine the macro
imaginary.
2 This subclause contains specifications for the <complex.h> functions that are
particularly suited to IEC 60559 implementations. For families of functions, the
specifications apply to all of the functions even though only the principal function is
shown. Unless otherwise specified, where the symbol ``<60>'' occurs in both an argument
and the result, the result has the same sign as the argument.
3 The functions are continuous onto both sides of their branch cuts, taking into account the
sign of zero. For example, csqrt(-2 <20> i 0) = <20>i 2.
4 Since complex and imaginary values are composed of real values, each function may be
regarded as computing real values from real values. Except as noted, the functions treat
real infinities, NaNs, signed zeros, subnormals, and the floating-point exception flags in a
manner consistent with the specifications for real functions in F.10.378)
5 The functions cimag, conj, cproj, and creal are fully specified for all
implementations, including IEC 60559 ones, in 7.3.9. These functions raise no floating-
point exceptions.
6 Each of the functions cabs and carg is specified by a formula in terms of a real
function (whose special cases are covered in annex F):
cabs( x + iy ) = hypot( x , y )
carg( x + iy ) = atan2( y , x )
7 Each of the functions casin, catan, ccos, csin, and ctan is specified implicitly by
a formula in terms of other complex functions (whose special cases are specified below):
casin( z ) = -i casinh(iz )
catan( z ) = -i catanh(iz )
ccos( z ) = ccosh(iz )
csin( z ) = -i csinh(iz )
ctan( z ) = -i ctanh(iz )
8 For the other functions, the following subclauses specify behavior for special cases,
including treatment of the ``invalid'' and ``divide-by-zero'' floating-point exceptions. For
families of functions, the specifications apply to all of the functions even though only the
principal function is shown. For a function f satisfying f (conj( z )) = conj( f ( z )), the
specifications for the upper half-plane imply the specifications for the lower half-plane; if
the function f is also either even, f (- z ) = f ( z ), or odd, f (- z ) = - f ( z ), then the
specifications for the first quadrant imply the specifications for the other three quadrants.
9 In the following subclauses, cis( y ) is defined as cos( y ) + i sin( y ).
378) As noted in G.3, a complex value with at least one infinite part is regarded as an infinity even if its
other part is a NaN.
G.6.1 Trigonometric functions
G.6.1.1 The cacos functions
1 -- cacos(conj( z )) = conj(cacos( z )).
-- cacos(<28>0 + i 0) returns /2 - i 0.
-- cacos(<28>0 + i NaN) returns /2 + i NaN.
-- cacos( x + i ) returns /2 - i , for finite x .
-- cacos( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for nonzero finite x .
-- cacos(- + iy ) returns - i , for positive-signed finite y .
-- cacos(+ + iy ) returns +0 - i , for positive-signed finite y .
-- cacos(- + i ) returns 3 /4 - i .
-- cacos(+ + i ) returns /4 - i .
-- cacos(<28> + i NaN) returns NaN <20> i (where the sign of the imaginary part of the
result is unspecified).
-- cacos(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite y .
-- cacos(NaN + i ) returns NaN - i .
-- cacos(NaN + i NaN) returns NaN + i NaN.
G.6.2 Hyperbolic functions
G.6.2.1 The cacosh functions
1 -- cacosh(conj( z )) = conj(cacosh( z )).
-- cacosh(<28>0 + i 0) returns +0 + i /2.
-- cacosh( x + i ) returns + + i /2, for finite x .
-- cacosh( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid''
floating-point exception, for finite x .
-- cacosh(- + iy ) returns + + i , for positive-signed finite y .
-- cacosh(+ + iy ) returns + + i 0, for positive-signed finite y .
-- cacosh(- + i ) returns + + i 3 /4.
-- cacosh(+ + i ) returns + + i /4.
-- cacosh(<28> + i NaN) returns + + i NaN.
-- cacosh(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid''
floating-point exception, for finite y .
-- cacosh(NaN + i ) returns + + i NaN.
-- cacosh(NaN + i NaN) returns NaN + i NaN.
G.6.2.2 The casinh functions
1 -- casinh(conj( z )) = conj(casinh( z )) and casinh is odd.
-- casinh(+0 + i 0) returns 0 + i 0.
-- casinh( x + i ) returns + + i /2 for positive-signed finite x .
-- casinh( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid''
floating-point exception, for finite x .
-- casinh(+ + iy ) returns + + i 0 for positive-signed finite y .
-- casinh(+ + i ) returns + + i /4.
-- casinh(+ + i NaN) returns + + i NaN.
-- casinh(NaN + i 0) returns NaN + i 0.
-- casinh(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid''
floating-point exception, for finite nonzero y .
-- casinh(NaN + i ) returns <20> + i NaN (where the sign of the real part of the result
is unspecified).
-- casinh(NaN + i NaN) returns NaN + i NaN.
G.6.2.3 The catanh functions
1 -- catanh(conj( z )) = conj(catanh( z )) and catanh is odd.
-- catanh(+0 + i 0) returns +0 + i 0.
-- catanh(+0 + i NaN) returns +0 + i NaN.
-- catanh(+1 + i 0) returns + + i 0 and raises the ``divide-by-zero'' floating-point
exception.
-- catanh( x + i ) returns +0 + i /2, for finite positive-signed x .
-- catanh( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid''
floating-point exception, for nonzero finite x .
-- catanh(+ + iy ) returns +0 + i /2, for finite positive-signed y .
-- catanh(+ + i ) returns +0 + i /2.
-- catanh(+ + i NaN) returns +0 + i NaN.
-- catanh(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid''
floating-point exception, for finite y .
-- catanh(NaN + i ) returns <20>0 + i /2 (where the sign of the real part of the result is
unspecified).
-- catanh(NaN + i NaN) returns NaN + i NaN.
G.6.2.4 The ccosh functions
1 -- ccosh(conj( z )) = conj(ccosh( z )) and ccosh is even.
-- ccosh(+0 + i 0) returns 1 + i 0.
-- ccosh(+0 + i ) returns NaN <20> i 0 (where the sign of the imaginary part of the
result is unspecified) and raises the ``invalid'' floating-point exception.
-- ccosh(+0 + i NaN) returns NaN <20> i 0 (where the sign of the imaginary part of the
result is unspecified).
-- ccosh( x + i ) returns NaN + i NaN and raises the ``invalid'' floating-point
exception, for finite nonzero x .
-- ccosh( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite nonzero x .
-- ccosh(+ + i 0) returns + + i 0.
-- ccosh(+ + iy ) returns + cis( y ), for finite nonzero y .
-- ccosh(+ + i ) returns <20> + i NaN (where the sign of the real part of the result is
unspecified) and raises the ``invalid'' floating-point exception.
-- ccosh(+ + i NaN) returns + + i NaN.
-- ccosh(NaN + i 0) returns NaN <20> i 0 (where the sign of the imaginary part of the
result is unspecified).
-- ccosh(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for all nonzero numbers y .
-- ccosh(NaN + i NaN) returns NaN + i NaN.
G.6.2.5 The csinh functions
1 -- csinh(conj( z )) = conj(csinh( z )) and csinh is odd.
-- csinh(+0 + i 0) returns +0 + i 0.
-- csinh(+0 + i ) returns <20>0 + i NaN (where the sign of the real part of the result is
unspecified) and raises the ``invalid'' floating-point exception.
-- csinh(+0 + i NaN) returns <20>0 + i NaN (where the sign of the real part of the result is
unspecified).
-- csinh( x + i ) returns NaN + i NaN and raises the ``invalid'' floating-point
exception, for positive finite x .
-- csinh( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite nonzero x .
-- csinh(+ + i 0) returns + + i 0.
-- csinh(+ + iy ) returns + cis( y ), for positive finite y .
-- csinh(+ + i ) returns <20> + i NaN (where the sign of the real part of the result is
unspecified) and raises the ``invalid'' floating-point exception.
-- csinh(+ + i NaN) returns <20> + i NaN (where the sign of the real part of the result
is unspecified).
-- csinh(NaN + i 0) returns NaN + i 0.
-- csinh(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for all nonzero numbers y .
-- csinh(NaN + i NaN) returns NaN + i NaN.
G.6.2.6 The ctanh functions
1 -- ctanh(conj( z )) = conj(ctanh( z ))and ctanh is odd.
-- ctanh(+0 + i 0) returns +0 + i 0.
-- ctanh( x + i ) returns NaN + i NaN and raises the ``invalid'' floating-point
exception, for finite x .
-- ctanh( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite x .
-- ctanh(+ + iy ) returns 1 + i 0 sin(2 y ), for positive-signed finite y .
-- ctanh(+ + i ) returns 1 <20> i 0 (where the sign of the imaginary part of the result
is unspecified).
-- ctanh(+ + i NaN) returns 1 <20> i 0 (where the sign of the imaginary part of the
result is unspecified).
-- ctanh(NaN + i 0) returns NaN + i 0.
-- ctanh(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for all nonzero numbers y .
-- ctanh(NaN + i NaN) returns NaN + i NaN.
G.6.3 Exponential and logarithmic functions
G.6.3.1 The cexp functions
1 -- cexp(conj( z )) = conj(cexp( z )).
-- cexp(<28>0 + i 0) returns 1 + i 0.
-- cexp( x + i ) returns NaN + i NaN and raises the ``invalid'' floating-point
exception, for finite x .
-- cexp( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite x .
-- cexp(+ + i 0) returns + + i 0.
-- cexp(- + iy ) returns +0 cis( y ), for finite y .
-- cexp(+ + iy ) returns + cis( y ), for finite nonzero y .
-- cexp(- + i ) returns <20>0 <20> i 0 (where the signs of the real and imaginary parts of
the result are unspecified).
-- cexp(+ + i ) returns <20> + i NaN and raises the ``invalid'' floating-point
exception (where the sign of the real part of the result is unspecified).
-- cexp(- + i NaN) returns <20>0 <20> i 0 (where the signs of the real and imaginary parts
of the result are unspecified).
-- cexp(+ + i NaN) returns <20> + i NaN (where the sign of the real part of the result
is unspecified).
-- cexp(NaN + i 0) returns NaN + i 0.
-- cexp(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for all nonzero numbers y .
-- cexp(NaN + i NaN) returns NaN + i NaN.
G.6.3.2 The clog functions
1 -- clog(conj( z )) = conj(clog( z )).
-- clog(-0 + i 0) returns - + i and raises the ``divide-by-zero'' floating-point
exception.
-- clog(+0 + i 0) returns - + i 0 and raises the ``divide-by-zero'' floating-point
exception.
-- clog( x + i ) returns + + i /2, for finite x .
-- clog( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite x .
-- clog(- + iy ) returns + + i , for finite positive-signed y .
-- clog(+ + iy ) returns + + i 0, for finite positive-signed y .
-- clog(- + i ) returns + + i 3 /4.
-- clog(+ + i ) returns + + i /4.
-- clog(<28> + i NaN) returns + + i NaN.
-- clog(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite y .
-- clog(NaN + i ) returns + + i NaN.
-- clog(NaN + i NaN) returns NaN + i NaN.
G.6.4 Power and absolute-value functions
G.6.4.1 The cpow functions
1 The cpow functions raise floating-point exceptions if appropriate for the calculation of
the parts of the result, and may also raise spurious floating-point exceptions.379)
G.6.4.2 The csqrt functions
1 -- csqrt(conj( z )) = conj(csqrt( z )).
-- csqrt(<28>0 + i 0) returns +0 + i 0.
-- csqrt( x + i ) returns + + i , for all x (including NaN).
-- csqrt( x + i NaN) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite x .
-- csqrt(- + iy ) returns +0 + i , for finite positive-signed y .
-- csqrt(+ + iy ) returns + + i 0, for finite positive-signed y .
-- csqrt(- + i NaN) returns NaN <20> i (where the sign of the imaginary part of the
result is unspecified).
-- csqrt(+ + i NaN) returns + + i NaN.
-- csqrt(NaN + iy ) returns NaN + i NaN and optionally raises the ``invalid'' floating-
point exception, for finite y .
-- csqrt(NaN + i NaN) returns NaN + i NaN.
379) This allows cpow( z , c ) to be implemented as cexp(c clog( z )) without precluding
implementations that treat special cases more carefully.
G.7 Type-generic math <tgmath.h>
1 Type-generic macros that accept complex arguments also accept imaginary arguments. If
an argument is imaginary, the macro expands to an expression whose type is real,
imaginary, or complex, as appropriate for the particular function: if the argument is
imaginary, then the types of cos, cosh, fabs, carg, cimag, and creal are real; the
types of sin, tan, sinh, tanh, asin, atan, asinh, and atanh are imaginary; and
the types of the others are complex.
2 Given an imaginary argument, each of the type-generic macros cos, sin, tan, cosh,
sinh, tanh, asin, atan, asinh, atanh is specified by a formula in terms of real
functions:
cos(iy ) = cosh( y )
sin(iy ) = i sinh( y )
tan(iy ) = i tanh( y )
cosh(iy ) = cos( y )
sinh(iy ) = i sin( y )
tanh(iy ) = i tan( y )
asin(iy ) = i asinh( y )
atan(iy ) = i atanh( y )
asinh(iy ) = i asin( y )
atanh(iy ) = i atan( y )
Annex H
(informative)
H. Language independent arithmetic
H.1 Introduction
1 This annex documents the extent to which the C language supports the ISO/IEC 10967-1
standard for language-independent arithmetic (LIA-1). LIA-1 is more general than
IEC 60559 (annex F) in that it covers integer and diverse floating-point arithmetics.
H.2 Types
1 The relevant C arithmetic types meet the requirements of LIA-1 types if an
implementation adds notification of exceptional arithmetic operations and meets the 1
unit in the last place (ULP) accuracy requirement (LIA-1 subclause 5.2.8).
H.2.1 Boolean type
1 The LIA-1 data type Boolean is implemented by the C data type bool with values of
true and false, all from <stdbool.h>.
H.2.2 Integer types
1 The signed C integer types int, long int, long long int, and the corresponding
unsigned types are compatible with LIA-1. If an implementation adds support for the
LIA-1 exceptional values ``integer_overflow'' and ``undefined'', then those types are
LIA-1 conformant types. C's unsigned integer types are ``modulo'' in the LIA-1 sense
in that overflows or out-of-bounds results silently wrap. An implementation that defines
signed integer types as also being modulo need not detect integer overflow, in which case,
only integer divide-by-zero need be detected.
2 The parameters for the integer data types can be accessed by the following:
maxint INT_MAX, LONG_MAX, LLONG_MAX, UINT_MAX, ULONG_MAX,
ULLONG_MAX
minint INT_MIN, LONG_MIN, LLONG_MIN
3 The parameter ``bounded'' is always true, and is not provided. The parameter ``minint''
is always 0 for the unsigned types, and is not provided for those types.
H.2.2.1 Integer operations
1 The integer operations on integer types are the following:
addI x + y
subI x - y
mulI x * y
divI, divtI x / y
remI, remtI x % y
negI -x
absI abs(x), labs(x), llabs(x)
eqI x == y
neqI x != y
lssI x < y
leqI x <= y
gtrI x > y
geqI x >= y
where x and y are expressions of the same integer type.
H.2.3 Floating-point types
1 The C floating-point types float, double, and long double are compatible with
LIA-1. If an implementation adds support for the LIA-1 exceptional values
``underflow'', ``floating_overflow'', and ``"undefined'', then those types are conformant
with LIA-1. An implementation that uses IEC 60559 floating-point formats and
operations (see annex F) along with IEC 60559 status flags and traps has LIA-1
conformant types.
H.2.3.1 Floating-point parameters
1 The parameters for a floating point data type can be accessed by the following:
r FLT_RADIX
p FLT_MANT_DIG, DBL_MANT_DIG, LDBL_MANT_DIG
emax FLT_MAX_EXP, DBL_MAX_EXP, LDBL_MAX_EXP
emin FLT_MIN_EXP, DBL_MIN_EXP, LDBL_MIN_EXP
2 The derived constants for the floating point types are accessed by the following:
fmax FLT_MAX, DBL_MAX, LDBL_MAX
fminN FLT_MIN, DBL_MIN, LDBL_MIN
epsilon FLT_EPSILON, DBL_EPSILON, LDBL_EPSILON
rnd_style FLT_ROUNDS
H.2.3.2 Floating-point operations
1 The floating-point operations on floating-point types are the following:
addF x + y
subF x - y
mulF x * y
divF x / y
negF -x
absF fabsf(x), fabs(x), fabsl(x)
exponentF 1.f+logbf(x), 1.0+logb(x), 1.L+logbl(x)
scaleF scalbnf(x, n), scalbn(x, n), scalbnl(x, n),
scalblnf(x, li), scalbln(x, li), scalblnl(x, li)
intpartF modff(x, &y), modf(x, &y), modfl(x, &y)
fractpartF modff(x, &y), modf(x, &y), modfl(x, &y)
eqF x == y
neqF x != y
lssF x < y
leqF x <= y
gtrF x > y
geqF x >= y
where x and y are expressions of the same floating point type, n is of type int, and li
is of type long int.
H.2.3.3 Rounding styles
1 The C Standard requires all floating types to use the same radix and rounding style, so
that only one identifier for each is provided to map to LIA-1.
2 The FLT_ROUNDS parameter can be used to indicate the LIA-1 rounding styles:
truncate FLT_ROUNDS == 0
nearest FLT_ROUNDS == 1
other FLT_ROUNDS != 0 && FLT_ROUNDS != 1
provided that an implementation extends FLT_ROUNDS to cover the rounding style used
in all relevant LIA-1 operations, not just addition as in C.
H.2.4 Type conversions
1 The LIA-1 type conversions are the following type casts:
cvtI' I (int)i, (long int)i, (long long int)i,
(unsigned int)i, (unsigned long int)i,
(unsigned long long int)i
cvtF I (int)x, (long int)x, (long long int)x,
(unsigned int)x, (unsigned long int)x,
(unsigned long long int)x
cvtI F (float)i, (double)i, (long double)i
cvtF' F (float)x, (double)x, (long double)x
2 In the above conversions from floating to integer, the use of (cast)x can be replaced with
(cast)round(x), (cast)rint(x), (cast)nearbyint(x), (cast)trunc(x),
(cast)ceil(x), or (cast)floor(x). In addition, C's floating-point to integer
conversion functions, lrint(), llrint(), lround(), and llround(), can be
used. They all meet LIA-1's requirements on floating to integer rounding for in-range
values. For out-of-range values, the conversions shall silently wrap for the modulo types.
3 The fmod() function is useful for doing silent wrapping to unsigned integer types, e.g.,
fmod( fabs(rint(x)), 65536.0 ) or (0.0 <= (y = fmod( rint(x),
65536.0 )) ? y : 65536.0 + y) will compute an integer value in the range 0.0
to 65535.0 which can then be cast to unsigned short int. But, the
remainder() function is not useful for doing silent wrapping to signed integer types,
e.g., remainder( rint(x), 65536.0 ) will compute an integer value in the
range -32767.0 to +32768.0 which is not, in general, in the range of signed short
int.
4 C's conversions (casts) from floating-point to floating-point can meet LIA-1
requirements if an implementation uses round-to-nearest (IEC 60559 default).
5 C's conversions (casts) from integer to floating-point can meet LIA-1 requirements if an
implementation uses round-to-nearest.
H.3 Notification
1 Notification is the process by which a user or program is informed that an exceptional
arithmetic operation has occurred. C's operations are compatible with LIA-1 in that C
allows an implementation to cause a notification to occur when any arithmetic operation
returns an exceptional value as defined in LIA-1 clause 5.
H.3.1 Notification alternatives
1 LIA-1 requires at least the following two alternatives for handling of notifications:
setting indicators or trap-and-terminate. LIA-1 allows a third alternative: trap-and-
resume.
2 An implementation need only support a given notification alternative for the entire
program. An implementation may support the ability to switch between notification
alternatives during execution, but is not required to do so. An implementation can
provide separate selection for each kind of notification, but this is not required.
3 C allows an implementation to provide notification. C's SIGFPE (for traps) and
FE_INVALID, FE_DIVBYZERO, FE_OVERFLOW, FE_UNDERFLOW (for indicators)
can provide LIA-1 notification.
4 C's signal handlers are compatible with LIA-1. Default handling of SIGFPE can
provide trap-and-terminate behavior, except for those LIA-1 operations implemented by
math library function calls. User-provided signal handlers for SIGFPE allow for trap-
and-resume behavior with the same constraint.
H.3.1.1 Indicators
1 C's <fenv.h> status flags are compatible with the LIA-1 indicators.
2 The following mapping is for floating-point types:
undefined FE_INVALID, FE_DIVBYZERO
floating_overflow FE_OVERFLOW
underflow FE_UNDERFLOW
3 The floating-point indicator interrogation and manipulation operations are:
set_indicators feraiseexcept(i)
clear_indicators feclearexcept(i)
test_indicators fetestexcept(i)
current_indicators fetestexcept(FE_ALL_EXCEPT)
where i is an expression of type int representing a subset of the LIA-1 indicators.
4 C allows an implementation to provide the following LIA-1 required behavior: at
program termination if any indicator is set the implementation shall send an unambiguous
and ``hard to ignore'' message (see LIA-1 subclause 6.1.2)
5 LIA-1 does not make the distinction between floating-point and integer for ``undefined''.
This documentation makes that distinction because <fenv.h> covers only the floating-
point indicators.
H.3.1.2 Traps
1 C is compatible with LIA-1's trap requirements for arithmetic operations, but not for
math library functions (which are not permitted to invoke a user's signal handler for
SIGFPE). An implementation can provide an alternative of notification through
termination with a ``hard-to-ignore'' message (see LIA-1 subclause 6.1.3).
2 LIA-1 does not require that traps be precise.
3 C does require that SIGFPE be the signal corresponding to LIA-1 arithmetic exceptions,
if there is any signal raised for them.
4 C supports signal handlers for SIGFPE and allows trapping of LIA-1 arithmetic
exceptions. When LIA-1 arithmetic exceptions do trap, C's signal-handler mechanism
allows trap-and-terminate (either default implementation behavior or user replacement for
it) or trap-and-resume, at the programmer's option.
Reference in New Issue View Git Blame Copy Permalink