C23 (C standard revision)

C language revisions

C23 is the informal name for ISO/IEC 9899:2024, the next standard for the C programming language, which will replace C17 (standard ISO/IEC 9899:2018).[1] It was started in 2016 informally as C2x,[2] and expected to be published in 2024.[3] The most recent publicly available working draft of C23 was released on April 1, 2023.[4] The first WG14 meeting for the C2x draft was held in October 2019,[5] 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.[4]

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.[6]
  • Add memccpy() function in <string.h> to efficiently concatenate strings – similar to POSIX and SVID C extensions.[7]
  • Add strdup() and strndup() functions in <string.h> to allocate a copy of a string – similar to POSIX and SVID C extensions.[8]
  • Add memalignment() function in <stdlib.h> to determine the byte alignment of a pointer.[9]
  • 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.[10]
    • In the following, replace * with uc, us, ui, ul, ull for five function names, or blank for a type-generic macro.[10]
    • Add stdc_count_ones*() and stdc_count_zeros*() to count number of 1 or 0 bits in value.[10]
    • Add stdc_leading_ones*() and stdc_leading_zeros*() to count leading 1 or 0 bits in value.[10]
    • Add stdc_trailing_ones*() and stdc_trailing_zeros*() to count trailing 1 or 0 bits in value.[10]
    • Add stdc_first_leading_one*() and stdc_first_leading_zero*() to find first leading bit with 1 or 0 in value.[10]
    • Add stdc_first_trailing_one*() and stdc_first_trailing_zero*() to find first trailing bit with 1 or 0 in value.[10]
    • Add stdc_has_single_bit*() to determine if value is an exact power of 2 (return true if and only if there is a single 1 bit).[10]
    • Add stdc_bit_floor*() to determine the largest integral power of 2 that is not greater than value.[10]
    • Add stdc_bit_ceil*() to determine the smallest integral power of 2 that is not less than value.[10]
    • Add stdc_bit_width*() to determine number of bits to represent a value.[10]
  • Add timegm() function in <time.h> to convert time structure into calendar time value - similar to function in glibc and musl libraries.[11]

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.[12]
  • Add %b binary conversion specifier to scanf() function family.[12]
  • Add 0b and 0B binary conversion support to strtol() and wcstol() function families.[12]
  • 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.[13]

Preprocessor

  • Add #elifdef and #elifndef directives,[14] which are essentially equivalent to #elif defined and #elif !defined. Both directives were added to C++23 standard and GCC 12.[15]
  • Add #embed directive for binary resource inclusion and __has_embed allowing the availability of a resource to be checked by preprocessor directives.[16]
  • Add #warning directive for diagnostics.[17]
  • Add __has_include allowing the availability of a header to be checked by preprocessor directives.[18]
  • Add __has_c_attribute allowing the availability of an attribute to be checked by preprocessor directives.[19] (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.[20]

Types

  • Add nullptr_t, a null pointer type.[21]
  • Add _BitInt(N) and unsigned _BitInt(N) types for bit-precise integers. Add BITINT_MAXWIDTH macro for maximum bit width.[22][23]
  • Add ckd_add(), ckd_sub(), ckd_mul() macros for checked integer operations.[24]
  • Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.[25]
  • Better support for using const with arrays.[26]
  • Standardization of the typeof(...) operator.[27]
  • 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).[28]
  • Compatibility rules for structure, union, and enumerated types were changed to allow a redeclaration of a compatible type with the same tag.[29]

Constants

  • Add nullptr constant for nullptr_t type.[21]
  • Add wb and uwb integer literal suffixes for _BitInt(N) and unsigned _BitInt(N) types,[30] 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,[31] such as 0b10101010 (equating to 0xAA).
  • Add ' digit separator to literal constants,[32] 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.[33]
  • Allow enums with no fixed underlying type to store values that are not representable by int.[34]

Keywords

  • Add true and false keywords.[35]
  • Add alignas, alignof, bool, static_assert, thread_local keywords. Previously defined keywords become alternative spellings: _Alignas, _Alignof, _Bool, _Static_assert, _Thread_local.[36]
  • 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.[37]
  • Unnamed parameters in function definitions.[38]
  • Zero initialization with {} (including initialization of VLAs).[39]
  • 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.[40]
  • Add C++11 style attribute syntax[41] using double square brackets [[]].
  • Add single-argument _Static_assert for compatibility with C++17.[42]
  • 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.[43]
  • Add C++-style attributes (see Syntax). Add attributes [44] [[deprecated]],[45] [[fallthrough]],[46] [[maybe_unused]],[47] [[nodiscard]],[48] and [[noreturn]] attribute for compatibility with C++11, then deprecate _Noreturn, noreturn, header <stdnoreturn.h> features introduced in C11.[49] Duplicate attributes are allowed for compatibility with C++23.[50] 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.[51][52]
  • Add #elifdef and #elifndef preprocessing directives for compatibility with C++23.[14] (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.[53][54]
  • The constexpr specifier for objects but not functions, unlike C++'s equivalent.[55]
  • 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.[56]
  • 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.[57]
  • Allow storage class specifiers to appear in compound literal definition.[58]

Obsolete features

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

  • Remove trigraphs.[59]
  • Remove K&R function definitions/declarations (with no information about the function arguments).[60][61]
  • Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.[62]
  • The *_HAS_SUBNORM macros in <float.h> are obsolescent features.[63]

Compiler support

The GCC 9,[64] Clang 9.0,[65] and Pelles C 11.00[66] compilers implement an experimental compiler flag to support this standard.

See also

References

  1. ^ "History of C". cppreference.com. 2022-06-27. Archived from the original on October 19, 2022.
  2. ^ "WG14-N2086 : C2x Charter". open-std.org. 2016-09-20. Archived from the original on December 22, 2022.
  3. ^ "WG14-N3156 : Updated C23 Schedule" (PDF). open-std.org. 2023-07-19. Archived (PDF) from the original on January 29, 2024.
  4. ^ a b "WG14-N3096 : Draft for ISO/IEC 9899:2024" (PDF). open-std.org. April 1, 2023. Archived (PDF) from the original on April 2, 2023.
  5. ^ "WG14-N2437 : Agenda for October 2019". open-std.org. 2019-10-21. Archived from the original on March 5, 2021.
  6. ^ "WG14-N2897 : memset_explicit()". open-std.org. 2021-12-27. Archived from the original on October 25, 2022.
  7. ^ "WG14-N2349 : Toward more efficient string copying and concatenation". open-std.org. 2019-03-18. Archived from the original on September 30, 2022.
  8. ^ "WG14-N2353 : strdup() and strndup()". open-std.org. 2019-03-18. Archived from the original on December 24, 2022.
  9. ^ "WG14-N2974 : Queryable pointer alignment" (PDF). open-std.org. 2022-04-15. Archived (PDF) from the original on October 13, 2022.
  10. ^ a b c d e f g h i j k "WG14-N3022 : Modern Bit Utilities". open-std.org. 2022-07-06. Archived from the original on December 24, 2022.
  11. ^ "WG14-N2833 : Add timegm() as non-optional part of time.h". open-std.org. 2021-10-07. Archived from the original on December 1, 2021.
  12. ^ a b c "WG14-N2630 : formatted input/output of binary integer numbers" (PDF). open-std.org. 2021-01-01. Archived (PDF) from the original on December 14, 2022.
  13. ^ "WG14-N3020 : Qualifier-preserving standard library functions" (PDF). open-std.org. 2022-06-13. Archived (PDF) from the original on October 13, 2022.
  14. ^ a b "WG14-N2645 : Add support for preprocessing directives #elifdef and #elifndef" (PDF). open-std.org. 2020-01-25. Archived (PDF) from the original on November 28, 2022.
  15. ^ "GCC 12 Adds Support For New #elifdef #elifndef Directives". phoronix. May 12, 2021. Archived from the original on December 27, 2022.
  16. ^ "WG14-N3017 : #embed - a scannable, tooling-friendly binary resource inclusion mechanism". open-std.org. 2022-06-27. Archived from the original on December 24, 2022.
  17. ^ "WG14-N2686 : #warning" (PDF). open-std.org. 2022-07-22. Archived (PDF) from the original on November 28, 2022.
  18. ^ "WG14-N2799 : __has_include for C" (PDF). open-std.org. 2021-08-30. Archived (PDF) from the original on December 24, 2022.
  19. ^ "WG14-N2553 : Querying attribute support" (PDF). open-std.org. 2020-08-04. Archived (PDF) from the original on October 14, 2022.
  20. ^ "WG14-N3033 : Comma omission and comma deletion". open-std.org. 2022-07-20. Archived from the original on December 27, 2022.
  21. ^ a b "WR14-N3042 : Introduce the nullptr constant". open-std.org. 2022-07-22. Archived from the original on December 24, 2022.
  22. ^ "WG14-N2763 : Adding a Fundamental Type for N-bit integers" (PDF). open-std.org. 2021-06-21. Archived (PDF) from the original on December 27, 2022.
  23. ^ "WG14-N3035 : _BitInt Fixes" (PDF). open-std.org. 2022-07-21. Archived (PDF) from the original on October 13, 2022.
  24. ^ "WG14-N2867 : Checked N-Bit Integers" (PDF). open-std.org. 2021-11-28. Archived (PDF) from the original on December 14, 2022.
  25. ^ "WG14-N2778 : Variably-Modified Types" (PDF). open-std.org. 2021-07-11. Archived (PDF) from the original on December 22, 2022.
  26. ^ "WG14-N2607 : Compatibility of Pointers to Arrays with Qualifiers" (PDF). open-std.org. 2020-10-31. Archived (PDF) from the original on October 13, 2022.
  27. ^ "WG14-N2899 : Not-so-magic - typeof for C". open-std.org. 2022-01-21. Archived from the original on December 24, 2022.
  28. ^ "WG14-N3007 : Type inference for object definitions". open-std.org. 2022-06-10. Archived from the original on December 24, 2022.
  29. ^ "WG14-N3037 : Improved Rules for Tag Compatibility (updates N3032)" (PDF).
  30. ^ "WG14-N2775 : Literal suffixes for bit-precise integers" (PDF). open-std.org. 2021-07-13. Archived (PDF) from the original on December 27, 2022.
  31. ^ "WG14-N2549 : Allow for binary integer constants" (PDF). open-std.org. 2020-07-30. Archived (PDF) from the original on December 22, 2022.
  32. ^ "WG14-N2626 : Digit separators" (PDF). open-std.org. 2020-12-15. Archived (PDF) from the original on December 19, 2022.
  33. ^ "WG14-N3030 : Enhancements to Enumerations". open-std.org. 2022-07-19. Archived from the original on November 26, 2022.
  34. ^ "WG14-N3029 : Improved Normal Enumerations". open-std.org. 2022-07-19. Archived from the original on January 29, 2023.
  35. ^ "WG14-N2935 : Make false and true first-class language features" (PDF). open-std.org. 2022-02-15. Archived (PDF) from the original on November 21, 2022.
  36. ^ "WG14-N2934 : Revise spelling of keywords" (PDF). open-std.org. 2022-02-15. Archived (PDF) from the original on December 24, 2022.
  37. ^ "WG14-N2508 : Free Positioning of Labels Inside Compound Statements" (PDF). open-std.org. 2020-03-28. Archived (PDF) from the original on December 27, 2022.
  38. ^ "WG14-N2510 : Allowing unnamed parameters in a function definition" (PDF). open-std.org. 2020-04-09. Archived (PDF) from the original on December 24, 2022.
  39. ^ "WG14-N2900 : Consistent, Warningless, and Intuitive Initialization with {}". open-std.org. 2022-01-01. Archived from the original on December 27, 2022.
  40. ^ "WG14-N2975 : Relax requirements for variadic parameter lists" (PDF). open-std.org. 2022-04-15. Archived (PDF) from the original on November 28, 2022.
  41. ^ "WG14-N2335 : Attributes in C" (PDF). open-std.org. 2019-03-09. Archived (PDF) from the original on October 26, 2022.
  42. ^ "WG14-N2265 : Harmonizing static_assert with C++" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on March 28, 2023.
  43. ^ "Labels at the end of compound statements (C compatibility)" (PDF). 2022-01-13.
  44. ^ "WG14-N2554 : Minor attribute wording cleanups" (PDF). open-std.org. 2020-08-04. Archived (PDF) from the original on November 28, 2022.
  45. ^ "WG14-N2334 : The deprecated attribute" (PDF). open-std.org. 2019-01-22. Archived (PDF) from the original on October 19, 2022.
  46. ^ "WG14-N2408 : The fallthrough attribute" (PDF). open-std.org. 2019-08-11. Archived (PDF) from the original on December 25, 2022.
  47. ^ "WG14-N2270 : The maybe_unused attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on December 25, 2022.
  48. ^ "WG14-N2267 : The nodiscard attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on October 19, 2022.
  49. ^ "WG14-N2764 : The noreturn attribute" (PDF). open-std.org. 2021-06-21. Archived (PDF) from the original on December 25, 2022.
  50. ^ "WG14-N2557 : Allow Duplicate Attributes" (PDF). open-std.org. 2020-09-01. Archived (PDF) from the original on November 28, 2022.
  51. ^ "WG14-N2418 : Adding the u8 character prefix" (PDF). open-std.org. 2019-09-02. Archived (PDF) from the original on January 13, 2023.
  52. ^ What is the point of the UTF-8 character literals proposed for C++17?; Stack Overflow.
  53. ^ "WG14-N2341 : ISO/IEC TS 18661-2 - Floating-point extensions for C - Part 2: Decimal floating-point arithmetic" (PDF). open-std.org. February 26, 2019. Archived (PDF) from the original on November 21, 2022.
  54. ^ "WG14-N2601 : Annex X - IEC 60559 interchange and extended types" (PDF). open-std.org. October 15, 2020. Archived (PDF) from the original on October 14, 2022.
  55. ^ "WG14-N3018 : The constexpr specifier for object definitions". open-std.org. 2022-07-06. Archived from the original on December 24, 2022.
  56. ^ "WG14-N2653 : char8_t: A type for UTF-8 characters and strings (Revision 1)". open-std.org. 2021-06-04. Archived from the original on May 27, 2023.
  57. ^ "WG14-N2728 : char16_t & char32_t string literals shall be UTF-16 & UTF-32". open-std.org. 2021-05-15. Archived from the original on May 27, 2023.
  58. ^ "WG14-N3038 : Introduce storage-class specifiers for compound literals". open-std.org. 2022-07-21. Archived from the original on November 26, 2022.
  59. ^ "WG14-N2940 : Removing trigraphs??!" (PDF). open-std.org. 2022-03-02. Archived (PDF) from the original on October 26, 2022.
  60. ^ "WG14-N2432 : Remove support for function definitions with identifier lists proposal" (PDF). open-std.org. September 25, 2019. Archived (PDF) from the original on December 27, 2022.
  61. ^ "WG14-N2841 : No function declarators without prototypes". open-std.org. 2021-10-10. Archived from the original on November 12, 2022.
  62. ^ "WG14-N2412 : Two's complement sign representation" (PDF). open-std.org. August 11, 2019. Archived (PDF) from the original on December 27, 2022.
  63. ^ "WG14-N2993 : Make *_HAS_SUBNORM be obsolescent". open-std.org. 2022-06-06. Archived from the original on December 5, 2022.
  64. ^ "GCC 9 Release Notes". GNU Project. Archived from the original on December 27, 2022.
  65. ^ "Clang 9.0 - add new language mode for C2x". LLVM Project Repository. May 14, 2019. Archived from the original on December 27, 2022.
  66. ^ "Pelles C - major changes between 10.00 and 11.00". smorgasbordet.com. Archived from the original on December 27, 2022.

Further reading

  • N3096 (working draft of C23 standard); WG14; April 2023. (free download)
  • N3149 (working draft of C23 standard); WG14; July 2023. (not available to public)
  • N3219 (ISO/IEC 9899:2023 DIS Draft); WG14; February 2024. (ISO draft available but not free)
  • ISO/IEC 9899:2024 (official C23 standard); ISO; 2024. (planning for release in 2024)

External links

  • C Language WG14 (Working Group 14)
  • WG14 Document Repository
  • WG14 Meetings - agenda and minutes
  • WG14 Charters: C2x Charter, C23 Charter, Interpreting the C23 Charter, C Standard Charter
Preceded by C language standards Latest
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