27.8 Per-Object Flags Emulation

One of my source files needs to be compiled with different flags.  How
do I do?

Automake supports per-program and per-library compilation flags (see Program and Library Variables and Flag Variables Ordering). With this you can define compilation flags that apply to all files compiled for a target. For instance, in

bin_PROGRAMS = foo
foo_SOURCES = foo.c foo.h bar.c bar.h main.c
foo_CFLAGS = -some -flags

foo-foo.o, foo-bar.o, and foo-main.o will all be compiled with ‘-some -flags’. (If you wonder about the names of these object files, see Why are object files sometimes renamed?.) Note that foo_CFLAGS gives the flags to use when compiling all the C sources of the program foo, it has nothing to do with foo.c or foo-foo.o specifically.

What if foo.c needs to be compiled into foo.o using some specific flags, that none of the other files require? Obviously per-program flags are not directly applicable here. Something like per-object flags are expected, i.e., flags that would be used only when creating foo-foo.o. Automake does not support that, however this is easy to simulate using a library that contains only that object, and compiling this library with per-library flags.

bin_PROGRAMS = foo
foo_SOURCES = bar.c bar.h main.c
foo_CFLAGS = -some -flags
foo_LDADD = libfoo.a
noinst_LIBRARIES = libfoo.a
libfoo_a_SOURCES = foo.c foo.h
libfoo_a_CFLAGS = -some -other -flags

Here foo-bar.o and foo-main.o will all be compiled with ‘-some -flags’, while libfoo_a-foo.o will be compiled using ‘-some -other -flags’. Eventually, all three objects will be linked to form foo.

This trick can also be achieved using Libtool convenience libraries, for instance ‘noinst_LTLIBRARIES = libfoo.la’ (see Libtool Convenience Libraries).

Another tempting idea to implement per-object flags is to override the compile rules automake would output for these files. Automake will not define a rule for a target you have defined, so you could think about defining the ‘foo-foo.o: foo.c’ rule yourself. We recommend against this, because this is error prone. For instance, if you add such a rule to the first example, it will break the day you decide to remove foo_CFLAGS (because foo.c will then be compiled as foo.o instead of foo-foo.o, see Why are object files sometimes renamed?). Also in order to support dependency tracking, the two .o/.obj extensions, and all the other flags variables involved in a compilation, you will end up modifying a copy of the rule previously output by automake for this file. If a new release of Automake generates a different rule, your copy will need to be updated by hand.