Hi, I'm not a experiencied user/programmer in the unix world, but I start to maintaim the gnome-mag code sometime ago, so I'm still have some difficulties in understand some concepts of autoconf and how to search for information, so I think that ask it here is not so bad ;-). I need to use round () in a part of the code and I saw that in GCC I must pass the -std=c99 option to the compiler and the -lm option to the linker. I yet put it in my configure.in: AC_CHECK_LIB(m, round, [M_LIBS="-lm"], AC_MSG_ERROR([libm is required to build gnome-mag. This lib comes with GNU C libraries (glibc) and you can find the lastest version from http://www.gnu.org/software/libc/])) AC_SUBST(M_LIBS) This work for me check if the function round is present in libm, but if I call this function in my code without the option -std=c99 to the compiler, the code does not work properly. I found a middle solution that is declare round () as an extern function. But, for me this is not the best solution. If I could check if the compiler supports the -std=c99 option I can use it to compile the code and only declare round () as extern if the -std=c99 is not present but the round function is in the libm. I thinked in use AC_COMPILE_IFELSE, but don't realize how this can be used to achieve this goal. Any help will be very appreciated. Thanks, -- Carlos Eduardo Rodrigues Diógenes _______________________________________________ Autoconf mailing list Autoconf@xxxxxxx http://lists.gnu.org/mailman/listinfo/autoconf