Re: git: 8d29d2429649 - main - libc: Restore prior C23 include guards

> On Aug 24, 2026, at 18:22, Faraz Vahedi <kfv@FreeBSD.org> wrote:
> 
>> On Aug 24, 2026, at 17:52, Dag-Erling Smørgrav <des@FreeBSD.org> wrote:
>> 
>> Warner Losh <imp@bsdimp.com> writes:
>>> Dag-Erling Smørgrav <des@freebsd.org> writes:
>>>> Shouldn't these be conditional on the visibility macros?
>>> Seems fine to me since it's in the implementation namespace.  Maybe
>>> you could share your reasoning.
>> 
>> Imagine a library which is is written to be backward compatible with C99
>> but offers additional functionality which requires a C23 compiler and
>> uses one of these __STDC_VERSION_* macros to control its visibility in
>> the header.  An application written in C99 uses this library and builds
>> with -std=c99.  Because we set __STDC_VERSION_* unconditionally, the
>> library now exposes C23-only code to the C99 program, which therefore
>> fails to build.  Therefore, these macros should only be defined if
>> __BSD_VISIBLE || __ISO_C_VISIBLE >= 2023.
> 
> Since these macros solely advertise which revision of the header our libc implements
> and are neither a language-mode nor visibility switch, I believe we’re fine.  C23-only
> identifiers are already gated on our visibility macros, and exposure of these feature
> test macros per se are not of any harm, even for libraries targeting modes prior to C23.
> For the same very reason, it is also fine to have these as include guards (which brand
> new C23 headers like <stdbit.h> and <stdckdint.h> still do).
> 
> My restoration of traditional guards was just to avoid any possible POLA violation for
> those depending on them (albeit wrong) as reported by dim for math/gmp port as an
> example.
> 
> Cheers,
> Faraz

I’d also like to add that exposing these macros is quite useful and expected.  They are
how a program can check whether or not a libc header implements a given revision,
since -std=c23, for example, only speaks for the compiler and cannot guarantee that
the libc supports or has ever shipped that revision of the header.

Yours,
Faraz