Re: how to use $^ automatic variable with prerequisites built on target-specific variable ?

[Ralf Wildenhues]

Hello Axel,

* Axel wrote on Fri, Jul 09, 2010 at 11:32:45AM CEST:
> >I m using a target with prerequisites based on target-specific variable,
> >but it seems that the $^ doesn't work and is empty in this use-case :
> >
> >prereq:
> >     touch prereq
> >
> >test: FILE = prereq
> >
> >test: $(FILE)
> >     echo Variable: $(FILE) Prereq: $^
> >
> >
> >"make test" displays that $^ is empty, but the prerequesites are checked
> >and built. Is this a normal behaviour ? The workaround for me is to use
> >$(FILE) rather than $^ but I assumed that $^ would have worked.

> I ll add that the prerequisites are not built when using this
> syntax, the prereq target is never processed.

This question is mainly about a GNU make feature (target-specific
variables) rather than about Automake (which doesn't understand this
feature).  So further questions are better asked on a mailing list about
GNU make such as help-make at gnu.org.  Note that Automake provides a
feature of a similar name but different semantics in that it allows for
per-target flags such as, when prog is a program to be built, you can
set
  prog_CPPFLAGS
  prog_CFLAGS
  prog_LDFLAGS
  ...

in order to override
  AM_CPPFLAGS
  AM_CFLAGS
  AM_LDFLAGS
  ...

I guess I should also note that Automake encourages using portable make.

Now to your actual question about target-specific variables: I think the
issue is that make expands makefiles in basically two stages: the list
of prerequisites is expanded before it is decided which targets to
remake.  I don't think the target-specific variables are expanded for
the prerequisite list expansion already, only for the rule commands of
some target (which you had to find out is too late for you).  I think
that `info make Target-specific' does not explain this very well, so you
might want to ask the GNU make maintainer about improving the manual a
bit.

Cheers,
Ralf