Define C23 header version macros

[Paul Eggert]

Thanks for taking this on.

On 2025-11-20 13:48, Joseph Myers wrote:

> I'm not sure this is sufficiently much of a
> user-visible feature to be worth a mention in NEWS.

It's probably worth mentioning, in part to answer the questions posed in 
my last comment at the end of this email.


> --- a/assert/assert.h
> +++ b/assert/assert.h
> @@ -34,6 +34,12 @@
>   #define	_ASSERT_H	1
>   #include <features.h>
>   
> +#if __GLIBC_USE (ISOC23)
> +# ifndef __STDC_VERSION_ASSERT_H__
> +#  define __STDC_VERSION_ASSERT_H__ 202311L
> +# endif
> +#endif

assert.h is special because it can be multiply included with different 
effects each time. So shouldn't this be done like other assert macros? 
That is, define __STDC_VERSION_ASSERT_H__ without using the ifndef here, 
and near the file's start if _ASSERT_H is already defined then #undef 
__STDC_VERSION_ASSERT_H__ there. Although this sort of thing matters 
only if the user fiddles with _ISOC23_SOURCE (which the user shouldn't 
do of course) it would be a bit more robust in that fiddly situation.

Also, suppose the user compiles with --std=gnu2y? What should the value 
of __STDC_VERSION_ASSERT_H__ be then? Perhaps it should be some 
intermediate value like 202505L for the latest C2y draft that glibc 
supports? Similarly for other __STDC_VERSION_*_H__ macros.


> +# ifndef __STDC_VERSION_LIMITS_H__
> +#  define __STDC_VERSION_LIMITS_H__ 202311L
> +# endif

Why is the ifndef needed here, instead of a plain #define? Other places 
don't have such an ifndef (except for assert.h, which is special because 
it can be multiply included and which I discussed above).


> +#if __GLIBC_USE (ISOC23)
> +# define __STDC_VERSION_STDIO_H__ 202311L
> +#endif

Since glibc supports C23 <stdio.h>'s features regardless of whether 
__GLIBC_USE (ISOC23) is true, how about doing this #define 
unconditionally? Similarly for other __STDC_VERSION_*_H__ macros: the 
idea is to define each such macro if the corresponding C23 features are 
supported, regardless of whether __GLIBC_USE (ISOC23) is true.

Admittedly this could get a little tricky, e.g., <complex.h> needs at 
least __USE_ISOC11 before it can define __STDC_VERSION_COMPLEX_H__ to 
202311L, because macros like CMPLX are available only then.