C23

The next generation of the C standard.

See: The current WD of C23.

Obsolete

Removed

• Old-style function declarations and 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)
• __alignof_is_defined and __alignas_is_defined
• static_assert is not provided as a macro defined in <assert.h> (becomes a keyword)
• thread_local is not provided as a macro defined in <threads.h> (becomes a keyword)

Deprecated

• <stdnoreturn.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

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
• 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
• Decimal floating-point math functions
  • -dN variants for existing and new floating-point math functions
  • quantizedN()
  • samequantumdN()
  • quantumdN()
  • llquantexpdN()
  • encodedecdN()
  • decodedecdN()
  • encodebindN()
  • decodebindN()
• 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
  • wN and wfN length modifiers for [u]intN_t and [u]int_fastN_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__

Defect reports

Compiler support

C23 core language features

C23 library features