C23 (C standard revision)

Features

Constants

• Add nullptr| constant for nullptr_t type.
• Add ' digit separator to literal constants, such as , , .
• Add 0b and 0B binary literal constant prefixes, such as , and was previously added in the C++14 standard. This change is similar to earlier established 0x and 0X hexadecimal prefixes.
• Add wb and uwb integer literal suffixes for and types, such as yields an unsigned _BitInt, and yields a signed _BitInt which has three value bits and one sign bit.
• Add the ability to specify the underlying type of an enum.
• Allow with no fixed underlying type to store values that are not representable by.

  Keywords

• Add and keywords.
• Add,,,, thread-local storage| keywords. Previously defined keywords become alternative spellings:,,,,.
• Add keyword.
• Add and keywords.
• Add keyword.
• Add keyword.
• Add,, keywords for decimal floating-point arithmetic.

  Types

• Add nullptr_t|, a null pointer type.
• Add and types for bit-precise integers. Add macro for maximum bit width.
• Add ckd_add, ckd_sub, ckd_mul macros for checked integer operations.
• Variably-modified types become a mandatory feature.
• Better support for using const with arrays.
• Standardization of the typeof operator.
• The meaning of the 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.
• Compatibility rules for structure, union, and enumerated types were changed to allow a redeclaration of a compatible type with the same tag.
• Exact-width integer may now exceed .
• Specify that intN_t when defined must designate the same type as int_leastN_t

  Syntax

• Labels can appear before declarations and at the end of compound statements.
• Unnamed parameters in function definitions.
• Zero initialization with .
• Variadic functions no longer need a named argument before the ellipsis and the 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. In addition to C++11 attributes listed below, add new attributes:
• * unsequenced allows compiler optimizations for functions producing repeatable outputs only based on their parameters
• * reproducible, similar to unsequenced, but for functions whose call order also matters
• Add single-argument static_assert for compatibility with C++17.
• Functions with no arguments listed in the prototype are understood as taking no arguments

  Preprocessor

• Add and directives, which are essentially equivalent to and. Both directives were added to standard and GCC 12.
• Add directive for binary resource inclusion and allowing the availability of a resource to be checked by preprocessor directives.
• Add directive for diagnostics.
• Add allowing the availability of a header to be checked by preprocessor directives.
• Add allowing the availability of an attribute to be checked by preprocessor directives.
• Add functional macro for variadic macros which expands to its argument only if a variadic argument has been passed to the containing macro.

  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. For labels at the end of compound statements a corresponding change was made to C++23.
• Add C++-style attributes. Add attributes,,,, and attribute for compatibility with C++11, then deprecate,, header features introduced in C11. Duplicate attributes are allowed for compatibility with C++23. All standard attributes can also be surrounded by double underscores.
• Add u8 prefix for character literals to represent UTF-8 encoding for compatibility with C++17.
• Add and preprocessing directives for compatibility with C++23.

  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 decimal floating-point arithmetic.
• The constexpr specifier for objects but not functions, unlike C++'s equivalent.
• Add type for storing UTF-8 encoded data and change the type of u8 character constants and string literals to. 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 strings and literals shall be UTF-16 encoded, and all strings and literals shall be UTF-32 encoded, unless otherwise explicitly specified.
• Allow storage class specifiers to appear in compound literal definition.

  Standard Library

Existing functions

• Add %b binary conversion specifier to printf function family.
• Add %b binary conversion specifier to scanf function family.
• Add 0b and 0B binary conversion support to functions strtol, strtoll, strtoul, strtoull, and their wide counterparts wcstol, wcstoll, wcstoul, wcstoull. If a sign is present, the 0b or 0B follows the sign.
• Add specific width modifiers %wN and %wfN to printf function family.
• Add specific width modifiers %wN and %wfN to scanf function family.
• 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.

  New functions

• Add memset_explicit function in to erase sensitive data, where memory store must always be performed regardless of optimizations.
• Add memccpy function in to efficiently concatenate strings – similar to POSIX and SVID C extensions.
• Add strdup and strndup functions in to allocate a copy of a string – similar to POSIX and SVID C extensions.
• Add memalignment function in to determine the byte alignment of a pointer.
• Add bit utility functions / macros / types in new header 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 is an exact power of 2.
• * 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 to convert time structure into calendar time value - similar to function in glibc and musl libraries.
• New functions based on IEEE 754-2019 recommendations, such as trigonometry functions operating on units of and exp10.

  Obsolete features

• Remove trigraphs.
• Remove K&R function definitions/declarations.
• Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.
• The macros in are obsolescent features.