Re: [PATCH v2 1/3] compat/posix.h: track SA_RESTART fallback

> Carlo Marcelo Arenas Belón <carenas@gmail.com> writes:
> 
> > On Wed, Jun 25, 2025 at 09:07:15AM -0800, Junio C Hamano wrote:
> >> "Carlo Marcelo Arenas Belón via GitGitGadget"
> >> <gitgitgadget@gmail.com> writes:
> >> 
> >> > +# Define USE_NON_POSIX_SIGNAL if don't have support for SA_RESTART or
> >> > +# prefer using ANSI C signal() over POSIX sigaction()
> >> > +
> >> > +AC_CACHE_CHECK([whether SA_RESTART is supported], [ac_cv_siginterrupt], [
> >> > +	AC_COMPILE_IFELSE(
> >> > +		[AC_LANG_PROGRAM([#include <signal.h>], [[
> >> > +		#ifdef SA_RESTART
> >> > +		#endif
> >> > +		siginterrupt(SIGCHLD, 1)
> >> 
> >> This is curious.  What is this #ifdef/#endif doing that does not
> >> have anything in it?
> >
> > It checks that `SA_RESTART` is defined in `signal.h`, which should
> > fail at least in QNX, NonStop and Windows.
> 
> The above roughly expands to
> 
>         #include <signal.h>
>         int main(void)
>         {
>                 #ifdef SA_RESTART
>                 #endif
>                 siginterrupt(SIGCHLD, 1);
>                 return 0;
>         }
> 
> Are you saying that a preprocessor macro SA_RESTART, which may or
> may not be defined, when asked by "#ifdef", causes what is left in
> the preprocessed source change in any meaningful way to cause the
> compilation to fail?