Implement C23 memset_explicit (bug 32378)

[Collin Funk]

Florian Weimer <fweimer@redhat.com> writes:

> * Paul Eggert:
>
>> On 2025-09-30 08:33, Joseph Myers wrote:
>>> If the user is using GCC 15 or later (or any compiler defaulting to C23 or
>>> a later language version), C23 functions*will* be visible by default
>>
>> Yes, good point, so the question here isn't about GCC 15; it's about
>> older GCCs and non-GCCs that do not claim conformance to C23. That's
>> an important set of use cases even today, and making memset_explicit
>> visible would be useful for these cases.
>
> But this is actually a compelling reason to use __USE_MISC: An upstream
> developer using GCC 15 might not use _GNU_SOURCE because there is no
> error.  But when someone uses GCC 13 or earlier to compile the sources
> and we don't use __USE_MISC in the header, GCC will provide an implicit
> function declaration.  This could introduce subtle bugs (depending on
> the ABI for implicit function declarations).  With __USE_MISC, that
> happens only if the programmer uses _ISOC11_SOURCE or similar.
> (Typically, -std=c17 or similar would come from upstream, and encounter
> errors with GCC 14 or later in upstream, so that is covered as well.)

Ah, okay. Thanks for the explanation. I am okay with __USE_MISC now, to
avoid errors in projects where people may use different GCC versions.

I assume Joseph's V2 patch is good, but I will review it later today.

Collin