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4.2 Building true and false
===========================

Here is another, trickier example.  It shows how to generate two
programs (‘true’ and ‘false’) from the same source file (‘true.c’).  The
difficult part is that each compilation of ‘true.c’ requires different
‘cpp’ flags.

     bin_PROGRAMS = true false
     false_SOURCES =
     false_LDADD = false.o

     true.o: true.c
             $(COMPILE) -DEXIT_CODE=0 -c true.c

     false.o: true.c
             $(COMPILE) -DEXIT_CODE=1 -o false.o -c true.c

   Note that there is no ‘true_SOURCES’ definition.  Automake will
implicitly assume that there is a source file named ‘true.c’ (Note:
Default _SOURCES), and define rules to compile ‘true.o’ and link
‘true’.  The ‘true.o: true.c’ rule supplied by the above ‘Makefile.am’,
will override the Automake generated rule to build ‘true.o’.

   ‘false_SOURCES’ is defined to be empty—that way no implicit value is
substituted.  Because we have not listed the source of ‘false’, we have
to tell Automake how to link the program.  This is the purpose of the
‘false_LDADD’ line.  A ‘false_DEPENDENCIES’ variable, holding the
dependencies of the ‘false’ target will be automatically generated by
Automake from the content of ‘false_LDADD’.

   The above rules won’t work if your compiler doesn’t accept both ‘-c’
and ‘-o’.  The simplest fix for this is to introduce a bogus dependency
(to avoid problems with a parallel ‘make’):

     true.o: true.c false.o
             $(COMPILE) -DEXIT_CODE=0 -c true.c

     false.o: true.c
             $(COMPILE) -DEXIT_CODE=1 -c true.c && mv true.o false.o

   As it turns out, there is also a much easier way to do this same
task.  Some of the above technique is useful enough that we’ve kept the
example in the manual.  However if you were to build ‘true’ and ‘false’
in real life, you would probably use per-program compilation flags, like
so:

     bin_PROGRAMS = false true

     false_SOURCES = true.c
     false_CPPFLAGS = -DEXIT_CODE=1

     true_SOURCES = true.c
     true_CPPFLAGS = -DEXIT_CODE=0

   In this case Automake will cause ‘true.c’ to be compiled twice, with
different flags.  In this instance, the names of the object files would
be chosen by automake; they would be ‘false-true.o’ and ‘true-true.o’.
(The name of the object files rarely matters.)