Re: Change a flag for a specific file

[Ralf Wildenhues]

Hello Sylvestre,

* Sylvestre Ledru wrote on Wed, Oct 11, 2006 at 04:35:49PM CEST:
> 
> Here is what I did :
> noinst_LTLIBRARIES = libdummy-lapack.la
> libdummy_lapack_la_SOURCES = dlamch.f
> libdummy_lapack_la_FFLAGS = -O0
> 
> This solution gives me : 
> /bin/sh ../../libtool --tag=F77 --mode=compile gfortran -O0 -O2 -g -Wall
> -I/home/sylvestre/dev/scilab5/modules/core/includes -c -o
> libdummy_lapack_la-dlamch.lo `test -f 'dlamch.f' || echo './'`dlamch.f
> libtool: compile:  gfortran -O0 -O2 -g -Wall
> -I/home/sylvestre/dev/scilab5/modules/core/includes -c dlamch.f  -fPIC
> -o .libs/libdummy_lapack_la-dlamch.o
> 
> As you can see, there are to -O option with different values.

Yep, the one from $(libdummy_lapack_la_FFLAGS) and the one from
$(FFLAGS).

A good idea is to view the per-target flags as an override for the AM_*
flags, and view the unadorned flags as user-owned ("you" are the
developer, the person issuing ./configure is the user in this case).
You should never change the latter.

So, as a general rule, do not change
  CFLAGS, FFLAGS, FCFLAGS, CXXFLAGS, ...

in a Makefile.am, but instead use
  AM_CFLAGS, AM_FFLAGS, AM_FCFLAGS, AM_CXXFLAGS, ...

so that the user has a chance to override your choice.  This is done
because usually there are lots of systems the developer cannot test on,
so the user needs a way to have the last word.

Whenever you need to override AM_*FLAGS, do that with per-target flags.

I should add that there is a bug in Automake 1.9.6 in that for linking,
these above-mentioned semantics are not coherent.  This has been fixed
in 1.9b and will be fixed in Automake 1.10:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/automake/NEWS?rev=1.295&content-type=text/x-cvsweb-markup&cvsroot=automake

(If that bothers you in what you're trying to do, please report back.)


Further note that it often comes handy to compute some sensible value
for AM_FFLAGS from some configure tests.  For that, you can do something
like

  AM_FFLAGS=$some_value
  AC_SUBST([AM_FFLAGS])

in the configure.ac file.

Cheers,
Ralf