C23 (C standard revision)

Short description: C programming language standard, future revision

C23 is the informal name for what will likely become ISO/IEC 9899:2024, the next standard for the C programming language, which will replace C17 (standard ISO/IEC 9899:2018). It was started in 2016 informally as C2x, and expected to be published in 2024. The most recent publicly available working draft of C23 was released on April 1, 2023. The first WG14 meeting for the C2x draft was held in October 2019, virtual remote meetings were held in 2020 due to the COVID-19 pandemic, then various teleconference meetings continued to occur through 2024.

Features

Changes integrated into the latest working draft of C23 are listed below.

Standard Library

New functions

• Add memset_explicit() function in <string.h> to erase sensitive data, where memory store must always be performed regardless of optimizations.
• Add memccpy() function in <string.h> to efficiently concatenate strings – similar to POSIX and SVID C extensions.
• Add strdup() and strndup() functions in <string.h> to allocate a copy of a string – similar to POSIX and SVID C extensions.
• Add memalignment() function in <stdlib.h> to determine the byte alignment of a pointer.
• Add bit utility functions / macros / types in new header <stdbit.h> to examine many integer types. All start with stdc_ to minimize conflict with legacy code and 3rd party libraries.
  • In the following, replace * with uc, us, ui, ul, ull for five function names, or blank for a type-generic macro.
  • Add stdc_count_ones*() and stdc_count_zeros*() to count number of 1 or 0 bits in value.
  • Add stdc_leading_ones*() and stdc_leading_zeros*() to count leading 1 or 0 bits in value.
  • Add stdc_trailing_ones*() and stdc_trailing_zeros*() to count trailing 1 or 0 bits in value.
  • Add stdc_first_leading_one*() and stdc_first_leading_zero*() to find first leading bit with 1 or 0 in value.
  • Add stdc_first_trailing_one*() and stdc_first_trailing_zero*() to find first trailing bit with 1 or 0 in value.
  • Add stdc_has_single_bit*() to determine if value an exact power of 2 (return true if and only if there is a single 1 bit).
  • Add stdc_bit_floor*() to determine the largest integral power of 2 that is not greater than value.
  • Add stdc_bit_ceil*() to determine the smallest integral power of 2 that is not less than value.
  • Add stdc_bit_width*() to determine number of bits to represent a value.
• Add timegm() function in <time.h> to convert time structure into calendar time value - similar to function in glibc and musl libraries.

Existing functions

• Add %b binary conversion specifier to printf() function family, prepending non-zero values with 0b, similar to how %x works. Implementations that previously did not use %B as their own extension are encouraged to implement and prepend non-zero values with 0B, similar to how %X works.
• Add %b binary conversion specifier to scanf() function family.
• Add 0b and 0B binary conversion support to strtol() and wcstol() function families.
• Make the functions bsearch(), bsearch s(), memchr(), strchr(), strpbrk(), strrchr(), strstr(), and their wide counterparts wmemchr(), wcschr(), wcspbrk(), wcsrchr(), wcsstr() return a const qualified object if one was passed to them.

Preprocessor

• Add

  #elifdef

  and

  #elifndef

  directives, which are essentially equivalent to

  #elif defined

  and

  #elif !defined

  . Both directives were added to C++23 standard and GCC 12.
• Add

  #embed

  directive for binary resource inclusion and

  __has_embed

  allowing the availability of a resource to be checked by preprocessor directives.
• Add

  #warning

  directive for diagnostics.
• Add

  __has_include

  allowing the availability of a header to be checked by preprocessor directives.
• Add

  __has_c_attribute

  allowing the availability of an attribute to be checked by preprocessor directives. (see "C++ compatibility" group for new attribute feature)
• Add

  __VA_OPT__

  functional macro for variadic macros which expands to its argument only if a variadic argument has been passed to the containing macro.

Types

• Add nullptr t, a null pointer type.
• Add

  _BitInt(N)

  and

  unsigned _BitInt(N)

  types for bit-precise integers. Add

  BITINT_MAXWIDTH

  macro for maximum bit width.
• Add ckd_add(), ckd_sub(), ckd_mul() macros for checked integer operations.
• Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.
• Better support for using const with arrays.
• Standardization of the typeof(...) operator.
• The meaning of the

  auto

  keyword was changed to cause type inference while also retaining its old meaning of a storage class specifier if used alongside a type. Unlike C++, C23 allows type inference only for object definitions (no inferring function return type or function parameter type).
• Compatibility rules for structure, union, and enumerated types were changed to allow a redeclaration of a compatible type with the same tag.

Constants

• Add nullptr constant for nullptr_t type.
• Add wb and uwb integer literal suffixes for

  _BitInt(N)

  and

  unsigned _BitInt(N)

  types, such as

  6uwb

  yields an unsigned _BitInt(3), and

  -6wb

  yields a signed _BitInt(4) which has three value bits and one sign bit.
• Add 0b and 0B binary literal constant prefixes, such as

  0b10101010

  (equating to 0xAA).
• Add ' digit separator to literal constants, such as

  0xFE'DC'BA'98

  (equating to 0xFEDCBA98),

  299'792'458

  (equating to 299792458),

  1.414'213'562

  (equating to 1.414213562).
• Add the ability to specify the underlying type of an enum.
• Allow

  enum

  s with no fixed underlying type to store values that are not representable by

  int

  .

Keywords

• Add

  true

  and

  false

  keywords.
• Add

  alignas

  ,

  alignof

  ,

  bool

  ,

  static_assert

  , thread local keywords. Previously defined keywords become alternative spellings:

  _Alignas

  ,

  _Alignof

  ,

  _Bool

  ,

  _Static_assert

  ,

  _Thread_local

  .
• Add

  _BitInt

  keyword (see "types" group)
• Add

  typeof

  and

  typeof_unqual

  keywords (see "types" group)
• Add

  nullptr

  keyword (see "constants" group)
• Add

  constexpr

  keyword (see "other" group)
• Add

  _Decimal32

  ,

  _Decimal64

  ,

  _Decimal128

  keywords for (optional) decimal floating-point arithmetic (see "other" group)

Syntax

• Labels can appear before declarations and at the end of compound statements.
• Unnamed parameters in function definitions.
• Zero initialization with {} (including initialization of VLAs).
• Variadic functions no longer need a named argument before the ellipsis and the

  va_start

  macro no longer needs a second argument nor does it evaluate any argument after the first one if present.
• Add C++11 style attribute syntax using double square brackets

  [[]]

  .
• Add single-argument Static assert for compatibility with C++17.
• Functions with no arguments listed in the prototype void foo() are understood as taking no arguments (see removal of K&R function declarations)

C++ compatibility

• Various syntax changes improve compatibility with C++, such as labels before declarations, unnamed function arguments, zero initialization with {}, variadic functions without named argument, C++11 style attributes, Static assert (see Syntax). For labels at the end of compound statements a corresponding change was made to C++23.
• Add C++-style attributes (see Syntax). Add attributes

  deprecated

  ,

  fallthrough

  ,

  maybe unused

  ,

  nodiscard

  , and

  noreturn

  attribute for compatibility with C++11, then deprecate

  _Noreturn

  ,

  noreturn

  , header <stdnoreturn.h> features introduced in C11. Duplicate attributes are allowed for compatibility with C++23. All standard attributes can also be surrounded by double underscores (e.g

  deprecated

  is equivalent to

  deprecated

  ).
• Add u8 prefix for character literals to represent UTF-8 encoding for compatibility with C++17.
• Add

  #elifdef

  and

  #elifndef

  preprocessing directives for compatibility with C++23. (see "preprocessor" group)

Other features

• Support for the ISO/IEC 60559:2020, the current version of the IEEE 754 standard for floating-point arithmetic, with extended binary floating-point arithmetic and (optional) decimal floating-point arithmetic.

• The constexpr specifier for objects but not functions, unlike C++'s equivalent.
• Add

  char8_t

  type for storing UTF-8 encoded data and change the type of u8 character constants and string literals to

  char8_t

  . Also, the functions mbrtoc8() and c8rtomb() to convert a narrow multibyte character to UTF-8 encoding and a single code point from UTF-8 to a narrow multibyte character representation respectively.
• Clarify that all

  char16_t

  strings and literals shall be UTF-16 encoded, and all

  char32_t

  strings and literals shall be UTF-32 encoded, unless otherwise explicitly specified.
• Allow storage class specifiers to appear in compound literal definition.

Obsolete features

Some old obsolete features are either removed or deprecated from the working draft of C23:

• Remove Trigraphs.
• Remove K&R function definitions/declarations (with no information about the function arguments).
• Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.
• The

  *_HAS_SUBNORM

  macros in <float.h> are obsolescent features.

Compiler support

The GCC 9, Clang 9.0, and Pelles C 11.00 compilers implement an experimental compiler flag to support this standard.

See also

• C++23, C++20, C++17, C++14, C++11, C++03, C++98, versions of the C++ programming language standard
• Compatibility of C and C++

External links

• C Language Working Group WG14 Documents
  • N3096 - working draft of C23 standard - dated April 1, 2023

Preceded by C17

C language standards

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