On 2024-02-25 02:28, Jeffrey Walton wrote: > On Sun, Feb 25, 2024 at 2:09 AM Jeffrey Walton <noloader@xxxxxxxxx> wrote: >> The page <https://www.gnu.org/software/autoconf-archive/ax_gcc_builtin.html> >> does not list __builtin_offsetof in the list of documented builtins. >> But the page says "Unsupported built-ins will be tested with an empty >> parameter set and the result of the check might be wrong or >> meaningless so use with care." >> >> Is it safe to use ax_gcc_builtin to detect __builtin_offsetof? This is the Autoconf list. Despite the similar name, Autoconf Archive is a separate project, and the author(s) of that macro (who would be the people who can actually answer this question) might not be on this list. Personally, if I really needed to probe the whether __builtin_offsetof works then the Autoconf-provided AC_COMPUTE_INT macro should be more than good enough to do this. Something like (untested): AC_CACHE_CHECK([for __builtin_offsetof], [my_cv_builtin_offsetof], [AC_COMPUTE_INT([testval], [__builtin_offsetof(struct foo, b) == (char *)&bar.b - (char *)&bar], [ struct foo { char a; int b; } bar;], [testval=0]) AS_CASE([$testval], [1], [my_cv_builtin_offsetof=yes], [my_cv_builtin_offsetof=no])]) AS_CASE([$my_cv_builtin_offsetof], [yes], [AC_DEFINE([HAVE___BUILTIN_OFFSETOF], [1], [Define to 1 if __builtin_offsetof works])]) Test on both a system that supports __builtin_offsetof and one that doesn't, this will probably be good enough unless you have specific knowledge of systems that are buggy or different in some way. Hope that helps, Nick
