Re: How to tell Automake about dependencies between source files.

[Ralf Wildenhues]

Hello Peter,

sorry for the delay.

* Peter Breitenlohner wrote on Wed, Mar 18, 2009 at 10:35:51AM CET:
> Let me be somewhat more explicit and give you an (idealized) Makefile.am
> (fragment):
>
> if COND
> bin_PROGRAMS = prog
> endif COND
>
> prog_SOURCES = foo.c foo.h
> nodist_prog_SOURCES = bar.h
>
> bar.h: tool bar.orig
>       .tool $(srcdir)/bar.orig
>
> ===============
>
> This fails (if COND is true) because, in our case, foo.c includes bar.h, but
> that is only known to Make once foo.c has been compiled (as least with
> depmode=gcc3).
>
> What we really would need is a dependency
>
> foo.o: bar.h
>
> but then this ought to be foo.$(OBJEXT) or maybe prog-foo.$(OBJEXT) and,
> more important, would prevent Automake from generating a rule for this
> compilation.

Here's a suggestion:

  # In the following, we rely on an undocumented limitation of Automake,
  # namely that for target overrides, it does not look through variable
  # expansions.  Further, we rely on the specific naming of the object
  # {foo,prog-foo}.$(OBJEXT), which is also an Automake-internal detail.
  foo_object = foo.$(OBJEXT)
  $(foo_object): bar.h

You can stick the above into a conditional if you need.

> I have tried adding
>
> BUILT_SOURCES = bar.h
>
> but then tool will run even if COND is false
>
> $(prog_SOURCES): bar.h
>
> is somewhat better until I run 'make dist'.

Yep.  The statement is wrong: it's not prog_SOURCES that depend on bar.h
but the object file generated from those sources.

> I could use
>
> if COND
> BUILT_SOURCES = bar.h
> endif COND
>
> or
>
> if COND
> $(prog_SOURCES): bar.h
> endif COND
>
> and these two almost do what I need, unless:
>
> COND is false but prog nevertheless needs to be built because it appears as
> dependency of something else, or I might have second thoughts and say
>
>       make prog
>
> This will then fail.

Yep.

> What I really would need is a way to tell Automake: 'before you can compile
> foo.c you must first create bar.h', and I have found no way to do that
> (other than writing a compilation rule as Automake would do).

See above.

Hope that helps.

Cheers,
Ralf