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

: at_start: {auth_results {shxd.cx}}


> On Aug 25, 2026, at 10:34 AM, Dag-Erling Smørgrav <des@FreeBSD.org> wrote:
> 
> Devin Teske <dteske@freebsd.org> writes:
>> If someone uses __STDC_VERSION_*_H__ to inject C23 syntax of their
>> own, they are asking the advertisement to do visibility's job.  I may
>> be missing a case where that is the right test; I have not found it in
>> 7.1.2p6 yet.
> 
> Imagine a single-header library (i.e. a big collection of static inline
> functions) that uses __STDC_VERSION_FOO_H__ to choose between a C23
> solution and a backward-compatible solution.  This header is then
> included in a C99 program and fails because we are advertising C23
> features to a C99 program.  It's really not that hard to understand.
> 
>> Your predicate (__ISO_C_VISIBLE >= 2023 || __BSD_VISIBLE) would make
>> the macros mean "this compilation can see C23 semantics from this
>> header."  That is a policy we can talk about on its own.  I would
>> rather not fold it into the _FOO_H_ restore, which was only POLA after
>> gmp.
> 
> I don't know what you mean with “fold it into the _FOO_H_ restore”.  The
> “_FOO_H_ restore” has already happened and we're not going to rewrite
> history.  I am asking for this feature to be implemented correctly.
> 

(combining my response to two people into one e-mail to limit branching of the thread)

DES,

You asked what I meant by "fold it into the _FOO_H_ restore," and you
wrote that we are not going to rewrite history. I never meant that we
would. A later commit can add the #if without touching 8d29d242; one
can even reverse a change that way. I did not want the new #if on
__STDC_VERSION_*_H__ handled together with putting the old
_STDIO_H_-style names back for gmp.

DES and Warner,

stdio.h defines the macro before it includes cdefs.h. cdefs.h would
come first; 77 other headers under /usr/include already test
__BSD_VISIBLE. Then the #if as you both named:

    #include <sys/cdefs.h>
    #if __ISO_C_VISIBLE >= 2023 || __BSD_VISIBLE
    #define __STDC_VERSION_STDIO_H__	202311L
    #endif

Same pattern on the other headers. If you both agree that is the
approach, Faraz can put the Differential up.

Devin