8.3.9.2 Objects ‘created with both libtool and without’

Sometimes, the same source file is used both to build a libtool library and to build another non-libtool target (be it a program or another library).

Let's consider the following ‘Makefile.am’.

bin_PROGRAMS = prog
prog_SOURCES = prog.c foo.c …

lib_LTLIBRARIES = libfoo.la
libfoo_la_SOURCES = foo.c …

(In this trivial case the issue could be avoided by linking ‘libfoo.la’ with ‘prog’ instead of listing ‘foo.c’ in prog_SOURCES. But let's assume we really want to keep ‘prog’ and ‘libfoo.la’ separate.)

Technically, it means that we should build ‘foo.$(OBJEXT)’ for ‘prog’, and ‘foo.lo’ for ‘libfoo.la’. The problem is that in the course of creating ‘foo.lo’, libtool may erase (or replace) ‘foo.$(OBJEXT)’, and this cannot be avoided.

Therefore, when Automake detects this situation it will complain with a message such as

object `foo.$(OBJEXT)' created both with libtool and without

A workaround for this issue is to ensure that these two objects get different basenames. As explained in Why are object files sometimes renamed?, this happens automatically when per-targets flags are used.

bin_PROGRAMS = prog
prog_SOURCES = prog.c foo.c …
prog_CFLAGS = $(AM_CFLAGS)

lib_LTLIBRARIES = libfoo.la
libfoo_la_SOURCES = foo.c …

Adding ‘prog_CFLAGS = $(AM_CFLAGS)’ is almost a no-op, because when the prog_CFLAGS is defined, it is used instead of AM_CFLAGS. However as a side effect it will cause ‘prog.c’ and ‘foo.c’ to be compiled as ‘prog-prog.$(OBJEXT)’ and ‘prog-foo.$(OBJEXT)’, which solves the issue.