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Re: adding specific C flags for a SINGLE source file

From: Bruce Allen
Subject: Re: adding specific C flags for a SINGLE source file
Date: Thu, 9 Dec 2004 18:20:06 -0600 (CST)


this comes *really* close to working.  Problem is that in the Makefile one
ends up with:

$(libboincbenchmark_a_CPPFLAGS) $(CPPFLAGS) $(libboincbenchmark_a_CXXFLAGS) 

Notice (grrrr!!) that user's $(CXXFLAGS) is *last*.  So I can't over-ride
it with libboincbenchmark_a_CXXFLAGS.

In other words:
and I end up with
  -O3 -g -O2.

Any idea how to get around this?


On Fri, 10 Dec 2004, Alexandre Duret-Lutz wrote:

> How about this proposed new entry for the FAQ?
> 26.7 Per-Object Flags Emulation
> ===============================
>      One of my source files needs to be compiled with different flags.  How
>      do I do?
>    Automake supports per-program and per-library compilation flags (see
> *Note Program and Library Variables:: and *Note Flag Variables
> Ordering::).  With this you can define compilation flags that apply to
> all files compiled for a target.  For instance in
>      bin_PROGRAMS = foo
>      foo_SOURCES = foo.c foo.h bar.c bar.h main.c
>      foo_CFLAGS = -some -flags
> `foo-foo.o', `foo-bar.o', and `foo-main.o' will all be compiled with
> `-some -flags'.  (If you wonder about the names of these object files,
> see *Note renamed objects::.)  Note that `foo_CFLAGS' gives the flags
> to use when compiling all the C sources of the _program_ `foo', it has
> nothing to do with `foo.c' or `foo-foo.o' specifically.
>    What if `foo.c' needs to be compiled into `foo.o' using some
> specific flags, that none of the other files require?  Obviously
> per-program flags are not directly applicable here.  Something like
> per-object flags are expected, i.e., flags that would be used only when
> creating `foo-foo.o'.  Automake does not support that, however this is
> easy to simulate using a library that contains only that object, and
> compiling this library with per-library flags.
>      bin_PROGRAMS = foo
>      foo_SOURCES = bar.c bar.h main.c
>      foo_CFLAGS = -some -flags
>      foo_LDADD = libfoo.a
>      noinst_LIBRARIES = libfoo.a
>      libfoo_a_SOURCES = foo.c foo.h
>      libfoo_a_CFLAGS = -some -other -flags
>    Here `foo-bar.o' and `foo-main.o' will all be compiled with `-some
> -flags', while `libfoo_a-foo.o' will be compiled using `-some -other
> -flags'.  Eventually, all three objects will be linked to form `foo'.
>    This trick can also be achieved using Libtool convenience libraries,
> i.e., `noinst_LTLIBRARIES =' (*note Libtool Convenience
> Libraries::).
>    Another tempting idea to implement per-object flags is to override
> the compile rules `automake' would output for these files.  Automake
> will not define a rule for a target you have defined, so you could
> think about defining the `foo-foo.o: foo.c' rule yourself.  We
> recommend against this, because this is error prone.  For instance if
> you add such a rule to the first example, it will break the day you
> decide to remove `foo_CFLAGS' (because `foo.c' will then be compiled as
> `foo.o' instead of `foo-foo.o', see *Note renamed objects::).  Also in
> order to support dependency tracking, the two `.o'/`.obj' extensions,
> and all the other flags variables involved in a compilation, you will
> end up modifying a copy of the rule previously output by `automake' for
> this file.  If a new release of Automake generates a different rule,
> your copy will need to be updated by hand.
> -- 
> Alexandre Duret-Lutz

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