C23

ISO/IEC 9899:2024 , a.k.a. C23 , is the current revision of the C standard.

• The latest WD of C23 (n3149, 2023-07-09, a password-protected zip archive, see n3150 ).
• The closest publicly accessible draft to the C23 is the first C2Y draft (n3220, 2024-02-22, which differs from C23 only in editorial changes, see n3221 ).

This section is incomplete Reason: accepted changes included in the final draft

C23 bumps the predefined macro __STDC_VERSION__ to 202311L .

Contents 1 New language features 1.1 Feature test macros for optional features 2 New library features 2.1 New headers 2.2 Library features 3 Obsolete features 3.1 Removed features 3.2 Deprecated features 4 Defect reports 5 Compiler support 5.1 C23 core language features 5.2 C23 library features

New language features

• Decimal floating-point types ( _Decimal32 , _Decimal64 , and _Decimal128 )
• Bit-precise integers ( _BitInt )
• Binary integer constants
• u8 character constants
• Type change of u8 string literals
• Digit separator '
• Empty initializer = { }
• Attributes
  • [[ deprecated ]]
  • [[ fallthrough ]]
  • [[ maybe_unused ]]
  • [[ nodiscard ]]
  • [[ noreturn ]]
  • [[ reproducible ]]
  • [[ unsequenced ]]
• Unnamed parameters in function definitions
• Identical cvr-qualifications for array types and their element types
• Single-argument static_assert
• alignas and alignof becomes keywords (may be predefined macros for compatibility reasons)
• static_assert becomes a keyword (may be a predefined macro for compatibility reasons)
• thread_local becomes a keyword (may be a predefined macro for compatibility reasons)
• Labels followed by declarations and }
• nullptr constant and the associated nullptr_t type
• true and false become keywords (may be predefined macros for compatibility reasons)
• New preprocessor directives
  • #elifdef
  • #elifndef
  • #warning
  • #embed
• Pragmas for rounding direction
  • STDC FENV_ROUND
  • STDC FENV_DEC_ROUND

Feature test macros for optional features

• __STDC_IEC_60559_BFP__
  • Indicates IEEE-754 binary floating-point arithmetic and required math functions are supported.

  This macro supersedes __STDC_IEC_559__ .

• __STDC_IEC_60559_DFP__
  • Indicates IEEE-754 decimal floating-point arithmetic and required math functions are supported.
• __STDC_IEC_60559_COMPLEX__
  • Indicates IEEE-754 complex arithmetic and required math functions are supported.

  This macro supersedes __STDC_IEC_559_COMPLEX__ .

New library features

New headers

• <stdbit.h>
• <stdckdint.h>

Library features

• Extended binary floating-point math functions

  • This section is incomplete Reason: List TBD

• Decimal floating-point math functions
  • - d N variants for existing and new floating-point math functions
  • quantized N ()
  • samequantumd N ()
  • quantumd N ()
  • llquantexpd N ()
  • encodedecd N ()
  • decodedecd N ()
  • encodebind N ()
  • decodebind N ()
• Floating-point formatting functions
• Library support for UTF-8
  • char8_t type alias
  • mbrtoc8()
  • c8rtomb()
  • atomic_char8_t type alias
  • ATOMIC_CHAR8_T_LOCK_FREE test macro
• memset_explicit()
• POSIX functions
  • memccpy()
  • strdup()
  • strndup()
  • gmtime_r()
  • localtime_r()
  • Extensions for strftime() and wcsftime()
• Extensions for fscanf() and fprintf() function families
  • w N and wf N length modifiers for [u]int N _t and [u]int_fast N _t respectively
  • H , D , and DD length modifiers for _Decimal32 , _Decimal64 , and _Decimal128 respectively
  • b conversion specifier for unsigned integer types
• timespec_getres()
• Macro constants for width of integer types
• Additional numeric limit macros for floating-point types
• Library version-test macros
  • __STDC_VERSION_FENV_H__
  • __STDC_VERSION_MATH_H__
  • __STDC_VERSION_STDINT_H__
  • __STDC_VERSION_STDLIB_H__
  • __STDC_VERSION_TGMATH_H__
  • __STDC_VERSION_TIME_H__
  • __STDC_VERSION_STDCKDINT_H__
  • __STDC_VERSION_STDBIT_H__

Obsolete features

Removed features

• Old-style function declarations and function definitions
• Representations for signed integers other than two's complement
• Permission that u / U -prefixed character constants and string literals may be not UTF-16/32
• Mixed wide string literal concatenation
• Support for calling realloc() with zero size (the behavior becomes undefined)
• alignas and alignof are provided as macros defined in <stdalign.h> (becomes keywords)
• static_assert is provided as a macro defined in <assert.h> (becomes a keyword)
• thread_local is provided as a macro defined in <threads.h> (becomes a keyword)

Deprecated features

• <stdnoreturn.h>
• <stdalign.h>
• <stdbool.h>
• Old feature-test macros
  • __STDC_IEC_559__
  • __STDC_IEC_559_COMPLEX__
• _Noreturn function specifier
• _Noreturn attribute token
• asctime()
• ctime()
• DECIMAL_DIG (use the appropriate type-specific macro ( FLT_DECIMAL_DIG , etc) instead)
• Definition of following numeric limit macros in <math.h> (they should be used via <float.h> )
  • INFINITY
  • DEC_INFINITY
  • NAN
  • DEC_NAN
• __bool_true_false_are_defined

Defect reports

Compiler support

C23 core language features

This section is incomplete Reason: status for Apple Clang and other compilers supporting C2x

C23 feature	Paper(s)	GCC	Clang	MSVC	Apple Clang	EDG eccp	Intel C++	Nvidia HPC C++ (ex PGI)*	Nvidia nvcc	Cray
static_assert with no message	N2265	9	9	Yes	Yes	6.5	2021.1.2 (clang based)
[[ nodiscard ]]	N2267	10	9		Yes	6.4	2021.1.2 (clang based)
[[ maybe_unused ]]	N2270	10	9		Yes	6.4	2021.1.2 (clang based)
[[ deprecated ]]	N2334	10	9		Yes	6.4	2021.1.2 (clang based)
Attributes	N2335 N2554	10	9		Yes	6.4	2021.1.2 (clang based)
IEEE 754 decimal floating-point types	N2341	4.2 (partial)* 12					13.0 (partial)*
[[ fallthrough ]]	N2408	10	9		Yes	6.4	2021.1.2 (clang based)
u8 character constants	N2418	10	15			6.5	2022.2
Removal of function definitions without prototype	N2432	10	15				2022.2
[[ nodiscard ]] with message	N2448	11	10		Yes	6.4	2021.1.2 (clang based)
Unnamed parameters in function definitions	N2480	11	11		Yes	6.4	2021.1.2 (clang based)
Labels before declarations and end of blocks	N2508	11	16	Partial*		6.5	17.0 *
Binary integer constants	N2549	4.3 * 11	2.9 * 9	19.0 (2015)* *	Yes	6.5	11.0 *
__has_c_attribute in preprocessor conditionals	N2553	11	9		Yes	6.5	2021.1.2 (clang based)
Allow duplicate attributes	N2557	11	13		Yes	6.5	2021.4 (clang-based
IEEE 754 interchange and extended types	N2601	7 (partial)* 14	6 (partial)*		Partial*
Digit separators	N2626	12	13	19.0 (2015)* *	Yes	6.5	18.0 *
#elifdef and #elifndef	N2645	12	13	19.40*	13.1.6*	6.5	2021.4
Type change of u8 string literals	N2653	13
[[ maybe_unused ]] for labels	N2662	11	16			6.5	2022.2
#warning	N2686	Yes	Yes		Yes	6.5	Yes
Bit-precise integer types ( _BitInt )	N2763	14 (partial)*	15			6.5	2022.2
[[ noreturn ]]	N2764	13	15			6.5	2022.2
Suffixes for bit-precise integer constants	N2775	14	15				2022.2
__has_include in preprocessor conditionals	N2799	5	Yes	19.11*	Yes	6.5	18.0
Identifier Syntax using Unicode Standard Annex 31	N2836	13	15			6.5	2022.2
Removal of function declarations without prototype	N2841	13	15				2022.2
Empty initializers	N2900	Partial* 13	Partial*		Partial*	Partial*	Partial*
typeof and typeof_unqual	N2927 N2930	Partial* 13	Partial* 16	19.39*	Partial*	Partial*	Partial*			Partial*
New spelling of keywords	N2934	13	16			6.5
Predefined true and false	N2935	13	15				2022.2
[[ unsequenced ]] and [[ reproducible ]]	N2956	15
Relax requirements for variadic parameter list	N2975	13	16			6.5	2023.1
Type inference in object definitions	N3007	13	18
#embed	N3017	15	19
constexpr objects	N3018	13	19
Improved Normal Enumerations	N3029	13	20 *
Enumerations with fixed underlying types	N3030	13	20 *
__VA_OPT__	N3033	8 13	12	19.39*		6.5
Storage-class specifiers for compound literals	N3038	13
nullptr	N3042	13	16
C23 feature	Paper(s)	GCC	Clang	MSVC	Apple Clang	EDG eccp	Intel C++	Nvidia HPC C++ (ex PGI)*	Nvidia nvcc	Cray

C23 library features

This section is incomplete Reason: a different list for C standard libraries