What is the __STDC_VERSION__ value for C23?

Question

What is the STDC_VERSION value for C11? asks about C11, and How can I use "nullptr" for null pointers prior to C23? says that the current versions of GCC and Clang define it to a placeholder value 202000L.

GCC 14 Release Series Changes, New Features, and Fixes has now added the std=c23 flag, but doesn't mention what __STDC_VERSION__ expands to.

Answer 1

From N3220:

__STDC_VERSION__ The integer constant 202311L

popular name	edition	constant
C23	fifth	202311L
C17/18	fourth	201710L
C11	third	201112L
C99	second	199901L
C??	first, amendment 1	199409L

It was in the first edition, amendment 1, to C89/90 where the addition of the predefined __STDC_VERSION__ macro was made. I'm unsure if it goes by any popular name but C94/95 has been suggested.

Answer 2

Empirically, GCC 14.1.0 still prints the placeholder version number 202000 when compiling with -std=c23 or -std=iso9899:2024:

#include <stdio.h>
int main(void)
{
    printf("__STDC_VERSION__ = %ld\n", __STDC_VERSION__);
    return 0;
}

Compiling and executing:

$ gcc --version
gcc (GCC) 14.1.0
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -O cv67.c -o cv67 && cv67
__STDC_VERSION__ = 201710
$ gcc -O -std=c23 cv67.c -o cv67 && cv67
__STDC_VERSION__ = 202000
$ gcc -O -Wall -Werror -Wextra -std=iso9899:2024 cv67.c -o cv67 && cv67
__STDC_VERSION__ = 202000
$ gcc -O -Wall -Werror -Wextra -std=iso9899:2011 cv67.c -o cv67 && cv67
__STDC_VERSION__ = 201112
$ gcc -O -Wall -Werror -Wextra -std=iso9899:1999 cv67.c -o cv67 && cv67
__STDC_VERSION__ = 199901
$ gcc -O -Wall -Werror -Wextra -std=iso9899:199409 cv67.c -o cv67 && cv67
__STDC_VERSION__ = 199409
$ gcc -O -Wall -Werror -Wextra -std=iso9899:1990 cv67.c -o cv67 && cv67
cv67.c: In function ‘main’:
cv67.c:4:40: error: ‘__STDC_VERSION__’ undeclared (first use in this function)
    4 |     printf("__STDC_VERSION__ = %ld\n", __STDC_VERSION__);
      |                                        ^~~~~~~~~~~~~~~~
cv67.c:4:40: note: each undeclared identifier is reported only once for each function it appears in
$

So GCC 14.1.0 defaults to C17/C18, and uses __STDC_VERSION__ == 202000 for C23.