8.1.3 Conditional compilation of sources

You can't put a configure substitution (e.g., ‘@FOO@’ or ‘$(FOO)’ where FOO is defined via AC_SUBST) into a _SOURCES variable. The reason for this is a bit hard to explain, but suffice to say that it simply won't work. Automake will give an error if you try to do this.

Fortunately there are two other ways to achieve the same result. One is to use configure substitutions in _LDADD variables, the other is to use an Automake conditional.

Conditional Compilation using _LDADD Substitutions

Automake must know all the source files that could possibly go into a program, even if not all the files are built in every circumstance. Any files that are only conditionally built should be listed in the appropriate EXTRA_ variable. For instance, if ‘hello-linux.c’ or ‘hello-generic.c’ were conditionally included in hello, the ‘Makefile.am’ would contain:

bin_PROGRAMS = hello
hello_SOURCES = hello-common.c
EXTRA_hello_SOURCES = hello-linux.c hello-generic.c
hello_LDADD = $(HELLO_SYSTEM)
hello_DEPENDENCIES = $(HELLO_SYSTEM)

You can then setup the ‘$(HELLO_SYSTEM)’ substitution from ‘configure.ac’:

…
case $host in
  *linux*) HELLO_SYSTEM='hello-linux.$(OBJEXT)' ;;
  *)       HELLO_SYSTEM='hello-generic.$(OBJEXT)' ;;
esac
AC_SUBST([HELLO_SYSTEM])
…

In this case, the variable HELLO_SYSTEM should be replaced by either ‘hello-linux.o’ or ‘hello-generic.o’, and added to both hello_DEPENDENCIES and hello_LDADD in order to be built and linked in.

Conditional Compilation using Automake Conditionals

An often simpler way to compile source files conditionally is to use Automake conditionals. For instance, you could use this ‘Makefile.am’ construct to build the same ‘hello’ example:

bin_PROGRAMS = hello
if LINUX
hello_SOURCES = hello-linux.c hello-common.c
else
hello_SOURCES = hello-generic.c hello-common.c
endif

In this case, ‘configure.ac’ should setup the LINUX conditional using AM_CONDITIONAL (see section Conditionals).

When using conditionals like this you don't need to use the EXTRA_ variable, because Automake will examine the contents of each variable to construct the complete list of source files.

If your program uses a lot of files, you will probably prefer a conditional ‘+=’.

bin_PROGRAMS = hello
hello_SOURCES = hello-common.c
if LINUX
hello_SOURCES += hello-linux.c
else
hello_SOURCES += hello-generic.c
endif