C2x

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Short description: C programming language standard, future revision


C2x is an informal name for the next (after C17) major C language standard revision.[1][2] It is expected to be voted on in 2023 and would therefore become C23.[3] The most recent publicly available draft of C23 was released on January 24, 2023.[4]

The first WG14 meeting for the C2x draft was held in October 2019, then virtual remote meetings were held due to COVID-19 pandemic in March/April 2020, October/November 2020, March/April 2021, August/September 2021, November/December 2021, January/February 2022, May 2022, July 2022, January 2023.[3][4]

Features

Changes integrated into the latest C23 draft are:[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.[5]
  • add memccpy() function in <string.h> to efficiently concatenate strings - similar to POSIX and SVID C extensions.[6]
  • add strdup() and strndup() functions in <string.h> to allocate a copy of a string – similar to POSIX and SVID C extensions.[7]
  • add memalignment() function in <stdlib.h> to determine the byte alignment of a pointer.[8]
  • 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.[9]
    • In the following, replace * with uc, us, ui, ul, ull for five function names, or blank for a type-generic macro.[9]
    • add stdc_count_ones*() and stdc_count_zeros*() to count number of 1 or 0 bits in value.[9]
    • add stdc_leading_ones*() and stdc_leading_zeros*() to count leading 1 or 0 bits in value.[9]
    • add stdc_trailing_ones*() and stdc_trailing_zeros*() to count trailing 1 or 0 bits in value.[9]
    • add stdc_first_leading_one*() and stdc_first_leading_zero*() to find first leading bit with 1 or 0 in value.[9]
    • add stdc_first_trailing_one*() and stdc_first_trailing_zero*() to find first trailing bit with 1 or 0 in value.[9]
    • 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).[9]
    • add stdc_bit_floor*() to determine the largest integral power of 2 that is not greater than value.[9]
    • add stdc_bit_ceil*() to determine the smallest integral power of 2 that is not less than value.[9]
    • add stdc_bit_width*() to determine number of bits to represent a value.[9]
Standard Library (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 didn't use %B as their own extension are encouraged to implement and prepend non-zero values with 0B, similar to how %X works.[10]
  • add %b binary conversion specifier to scanf() function family.[10]
  • add 0b and 0B binary conversion support to strtol() and wcstol() function families.[10]
  • 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.[11]
Preprocessor
  • add #elifdef and #elifndef directives[12] are essentially equivalent to #elif defined and #elif !defined. Both directives were added to C++23 and GCC 12 too.[13]
  • add #embed directive for binary resource inclusion.[14]
  • add #warning directive for diagnostics.[15]
  • add __has_include allowing the availability of a header to be checked by preprocessor directives.[16]
  • add __has_c_attribute allowing the availability of an attribute to be checked by preprocessor directives.[17] (see C++ compatibility group below 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.[18]
Types
  • add nullptr_t type.[19]
  • add _BitInt(N) and unsigned _BitInt(N) types for bit-precise integers. Add BITINT_MAXWIDTH macro for maximum bit width.[20][21] Add ckd_add(), ckd_sub(), ckd_mul() macros for checked integer operations.[22]
  • Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.[23]
  • Standardization of the typeof(...) operator.[24]
  • 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.[25]
Constants
  • add nullptr constant for nullptr_t type.[19]
  • add wb and uwb integer literal suffixes for _BitInt(N) and unsigned _BitInt(N) types,[26] 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,[27] such as 0b10101010 (equating to 0xAA).
  • add ' digit separator to literal constants,[28] 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.[29]
  • allow enums with no fixed underlying type to store values that aren't representable by int.[30]
C++ compatibility
  • add C++11 style attribute syntax[31] using double square brackets '"`UNIQ--nowiki-00000048-QINU`"'. Add attributes [32] '"`UNIQ--nowiki-0000004B-QINU`"',[33] '"`UNIQ--nowiki-0000004E-QINU`"',[34] '"`UNIQ--nowiki-00000051-QINU`"',[35] '"`UNIQ--nowiki-00000054-QINU`"',[36] and '"`UNIQ--nowiki-00000057-QINU`"' attribute for compatibility with C++11, then deprecate _Noreturn, noreturn, header <stdnoreturn.h> features introduced in C11.[37] Duplicate attributes are allowed for compatibility with C++23.[38]
  • add u8 prefix for character literals to represent UTF-8 encoding for compatibility with C++17.[39][40]
  • add #elifdef and #elifndef preprocessing directives for compatibility with C++23.[12] (see preprocessor group above)
Other
  • 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.[41][42]
  • Single-argument Static assert.[43]
  • Labels can appear before declarations and at the end of compound statements.[44]
  • Unnamed parameters in function definitions.[45]
  • Better support for using const with arrays.[46]
  • Zero initialization with {} (including initialization of VLAs).[47]
  • alignas, alignof, bool, true, false, static_assert, thread local become keywords.[48][49]
  • The constexpr specifier for objects but not functions, unlike C++'s equivalent.[50]
  • 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.[51]
  • add the 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.[52]
  • allow storage class specifiers to appear in compound literal definition.[53]

Obsolete features

Some old obsolete features are either removed or deprecated:

  • Remove Trigraphs.[54]
  • Remove K&R function definitions/declarations.[55][56]
  • Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.[57]
  • The *_HAS_SUBNORM macros in <float.h> are obsolescent features.[58]

Compiler support

The GCC 9,[59] Clang 9.0,[60] and Pelles C 11.00[61] compilers implement an experimental compiler flag to support this standard.

See also

References

  1. ↑ "History of C". 2022-06-27. https://en.cppreference.com/w/c/language/history. 
  2. ↑ "WG14-N2086 : C2x Charter". 2016-09-20. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2086.htm. 
  3. ↑ 3.0 3.1 "WG14-N2984 : Revised C23 Schedule". 2022-05-05. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2984.pdf. 
  4. ↑ 4.0 4.1 4.2 "WG14-N3088 : Draft for ISO/IEC 9899:2023". January 24, 2023. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3088.pdf. 
  5. ↑ "WG14-N2897 : memset_explicit()". 2021-12-27. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2897.htm. 
  6. ↑ "WG14-N2349 : Toward more efficient string copying and concatenation". 2019-03-18. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2349.htm. 
  7. ↑ "WG14-N2353 : strdup() and strndup()". 2019-03-18. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2353.htm. 
  8. ↑ "WG14-N2974 : Queryable pointer alignment". 2022-04-15. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2974.pdf. 
  9. ↑ 9.00 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 "WG14-N3022 : Modern Bit Utilities". 2022-07-06. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3022.htm. 
  10. ↑ 10.0 10.1 10.2 "WG14-N2630 : formatted input/output of binary integer numbers". 2021-01-01. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2630.pdf. 
  11. ↑ "WG14-N3020 : Qualifier-preserving standard library functions". 2022-06-13. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3020.pdf. 
  12. ↑ 12.0 12.1 "WG14-N2645 : Add support for preprocessing directives #elifdef and #elifndef". 2020-01-25. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2645.pdf. 
  13. ↑ "GCC 12 Adds Support For New #elifdef #elifndef Directives". May 12, 2021. https://www.phoronix.com/news/GCC-12-elifdef-elifndef. 
  14. ↑ "WG14-N3017 : #embed - a scannable, tooling-friendly binary resource inclusion mechanism". 2022-06-27. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3017.htm. 
  15. ↑ "WG14-N2686 : #warning". 2022-07-22. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2686.pdf. 
  16. ↑ "WG14-N2799 : __has_include for C". 2021-08-30. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2799.pdf. 
  17. ↑ "WG14-N2553 : Querying attribute support". 2020-08-04. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2553.pdf. 
  18. ↑ "WG14-N3033 : Comma omission and comma deletion". 2022-07-20. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3033.htm. 
  19. ↑ 19.0 19.1 "WR14-N3042 : Introduce the nullptr constant". 2022-07-22. https://open-std.org/JTC1/SC22/WG14/www/docs/n3042.htm. 
  20. ↑ "WG14-N2763 : Adding a Fundamental Type for N-bit integers". 2021-06-21. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2763.pdf. 
  21. ↑ "WG14-N3035 : _BitInt Fixes". 2022-07-21. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3035.pdf. 
  22. ↑ "WG14-N2867 : Checked N-Bit Integers". 2021-11-28. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2867.pdf. 
  23. ↑ "WG14-N2778 : Variably-Modified Types". 2021-07-11. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2778.pdf. 
  24. ↑ "WG14-N2899 : Not-so-magic - typeof for C". 2022-01-21. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2899.htm. 
  25. ↑ "WG14-N3007 : Type inference for object definitions". 2022-06-10. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3007.htm. 
  26. ↑ "WG14-N2775 : Literal suffixes for bit-precise integers". 2021-07-13. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2775.pdf. 
  27. ↑ "WG14-N2549 : Allow for binary integer constants". 2020-07-30. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2549.pdf. 
  28. ↑ "WG14-N2626 : Digit separators". 2020-12-15. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2626.pdf. 
  29. ↑ "WG14-N3030 : Enhancements to Enumerations". 2022-07-19. https://open-std.org/JTC1/SC22/WG14/www/docs/n3030.htm. 
  30. ↑ "WG14-N3029 : Improved Normal Enumerations". 2022-07-19. https://open-std.org/JTC1/SC22/WG14/www/docs/n3029.htm. 
  31. ↑ "WG14-N2335 : Attributes in C". 2019-03-09. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2335.pdf. 
  32. ↑ "WG14-N2554 : Minor attribute wording cleanups". 2020-08-04. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2554.pdf. 
  33. ↑ "WG14-N2334 : The deprecated attribute". 2019-01-22. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2334.pdf. 
  34. ↑ "WG14-N2408 : The fallthrough attribute". 2019-08-11. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2408.pdf. 
  35. ↑ "WG14-N2270 : The maybe_unused attribute". 2018-07-06. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2270.pdf. 
  36. ↑ "WG14-N2267 : The nodiscard attribute". 2018-07-06. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2267.pdf. 
  37. ↑ "WG14-N2764 : The noreturn attribute". 2021-06-21. https://open-std.org/JTC1/SC22/WG14/www/docs/n2764.pdf. 
  38. ↑ "WG14-N2557 : Allow Duplicate Attributes". 2020-09-01. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2557.pdf. 
  39. ↑ "WG14-N2418 : Adding the u8 character prefix". 2019-09-02. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2418.pdf. 
  40. ↑ What is the point of the UTF-8 character literals proposed for C++17?; Stack Overflow.
  41. ↑ "WG14-N2341 : ISO/IEC TS 18661-2 - Floating-point extensions for C - Part 2: Decimal floating-point arithmetic". February 26, 2019. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2341.pdf. 
  42. ↑ "WG14-N2601 : Annex X - IEC 60559 interchange and extended types". October 15, 2020. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2601.pdf. 
  43. ↑ "Harmonizing static_assert with C++". https://open-std.org/JTC1/SC22/WG14/www/docs/n2265.pdf. 
  44. ↑ "WG14-N2508 : Free Positioning of Labels Inside Compound Statements". 2020-03-28. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2508.pdf. 
  45. ↑ "WG14-N2510 : Allowing unnamed parameters in a function definition". 2020-04-09. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2510.pdf. 
  46. ↑ "WG14-N2607 : Compatibility of Pointers to Arrays with Qualifiers". 2020-10-31. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2607.pdf. 
  47. ↑ "WG14-N2900 : Consistent, Warningless, and Intuitive Initialization with {}". 2022-01-01. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2900.htm. 
  48. ↑ "WG14-N2935 : Make false and true first-class language features". 2022-02-15. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2935.pdf. 
  49. ↑ "WG14-N2934 : Revise spelling of keywords". 2022-02-15. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2934.pdf. 
  50. ↑ "WG14-N3018 : The constexpr specifier for object definitions". 2022-07-06. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3018.htm. 
  51. ↑ "WG14-N2975 : Relax requirements for variadic parameter lists". 2022-04-15. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2975.pdf. 
  52. ↑ "char8_t: A type for UTF-8 characters and strings (Revision 1)". https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2653.htm. 
  53. ↑ "WG14-N3038 : Introduce storage-class specifiers for compound literals". 2022-07-21. https://open-std.org/JTC1/SC22/WG14/www/docs/n3038.htm. 
  54. ↑ "WG14-N2940 : Removing trigraphs??!". 2022-03-02. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2940.pdf. 
  55. ↑ "WG14-N2432 : Remove support for function definitions with identifier lists proposal". September 25, 2019. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2432.pdf. 
  56. ↑ "WG14-N2841 : No function declarators without prototypes". 2021-10-10. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2841.htm. 
  57. ↑ "WG14-N2412 : Two’s complement sign representation". August 11, 2019. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2412.pdf. 
  58. ↑ "WG14-N2993 : Make *_HAS_SUBNORM be obsolescent". 2022-06-06. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2993.htm. 
  59. ↑ "GCC 9 Release Notes". https://gcc.gnu.org/gcc-9/changes.html#c. 
  60. ↑ "Clang 9.0 - add new language mode for C2x". May 14, 2019. https://github.com/llvm/llvm-project/commit/d06f3917913d2558b771ccc48d838f8cd8993c01. 
  61. ↑ "Pelles C - major changes between 10.00 and 11.00". http://www.smorgasbordet.com/pellesc/changes_1000_1100.htm. 

External links

Preceded by
C17 		C language standards Latest




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