On Wed, Jan 24, 2018 at 6:55 PM, Zack Weinberg <za...@panix.com> wrote: > On Wed, Jan 24, 2018 at 4:36 PM, Ferenc Wágner <wf...@niif.hu> wrote: >> Hi, >> >> Since ceil() can be inlined when optimization is enabled, I use >> >> AC_SEARCH_LIBS([ceil], [m], , [AC_MSG_ERROR([ceil not found])]) >> AC_SUBST([m_LIBS], [$LIBS]) >> >> to link against libm only when necessary (that is, when ceil is not >> inlined). However, it does not work because GCC says in config.log: >> >> conftest.c:76:6: warning: conflicting types for built-in function 'ceil' >> char ceil (); >> ~~~~ >> >> and inlining is disabled, thus ceil becomes an undefined reference >> without -lm. However, in the actual code where math.h is #included, the >> above warning does not happen, ceil is inlined and I unnecessarily link >> with -lm. How could I overcome this? > > You could try setting -Wl,--as-needed (which really should be the > default) in your makefiles.
That was unnecessarily cryptic, sorry. The GNU linker accepts an option --as-needed which will make it actually check whether shared libraries (such as libm) satisfy any undefined symbols before adding them to the executable's list of needed shared libraries. (This is not the default only for silly historical reasons which let's not get into here.) If you add that to your link commands in your Makefiles, libm will get pulled in only if it's actually necessary, which is better than trying to probe for whether it will actually be necessary in the configure script, since inlining is somewhat unpredictable.. The compiler driver doesn't know about it, though, so you have to spell it -Wl,--as-needed on your actual command lines. If you are using Automake, I believe you can just say AM_LDFLAGS = -Wl,--as-needed and get this applied to all of your executables. zw _______________________________________________ Autoconf mailing list Autoconf@gnu.org https://lists.gnu.org/mailman/listinfo/autoconf
