> On May 2, 2022, at 2:02 PM, Bob Friesenhahn <bfriesen@xxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, 2 May 2022, Philip Prindeville wrote: > >> Hi, >> >> I was wondering how to do discovery of functions like open_memstream() which is only exposed by <features.h> when compilation used -D_GNU_SOURCE or -D_POSIX_C_SOURCE=200809L. >> >> It might be the case that I compile everything with a certain flag, or I might just compile one-off modules such as: >> >> time.o: _ASTCFLAGS+=-D_XOPEN_SOURCE=700 >> >> What's best practices for handling situations like this? >> >> How do I bracket a particular AC_CHECK_FUNCS() invocation with defines that might be critical? >> >> I tried using: >> >> save_CFLAGS="$CFLAGS" >> CFLAGS="CFLAGS -D_POSIX_C_SOURCE=200809L" >> AC_CHECK_FUNCS([open_memstream]) >> CFLAGS="$save_CFLAGS" > > Normally one should use CPPFLAGS rather than CFLAGS for options pertaining to the C pre-processor. Noted... and the comment about the missing '$' in the other message... Turns out that I don't need this... AC_USE_SYSTEM_EXTENSIONS is causing it to be exposed. > When library function availability is checked, it is normally done using the linker. > > When in doubt, check config.log, which will show the test program and steps which were used while performing the test. > > It is not wise to mix C/OS standard influencing pre-processor options when compiling since they may require a different ABI, which could be provided by alternate libraries. It might not be possible to support more than one ABI at a time. > > Bob Hmm... good point. -Philip
