Re: adding specific C flags for a SINGLE source file

[Sander Niemeijer]

On vrijdag, dec 10, 2004, at 21:35 Europe/Amsterdam, Alexandre 
Duret-Lutz wrote:

> user_CFLAGS=$CFLAGS
> AC_PROG_CC
> if test "x$user_CFLAGS" = x; then
>   # If the user didn't specify CFLAGS, then CFLAGS contains
>   # a subset of -g -O2 selected by AC_PROG_CC.  This is not
>   # a user setting, and we want to be able to override this
>   # locally in our rules, so put these flags in a separate
>   # variable and empty CFLAGS.
>   AC_SUBST([DEFAULTFLAGS], [$CFLAGS])
>   CFLAGS=
> fi
>
> and in Makefile.am use
>   foo_CFLAGS = $(DEFAULTFLAGS)
> and
>   libfoo_a_CFLAGS = $(DEFAULTFLAGS) $(O3)
> as appropriate.
>
> ($(O3) being the Makefile variable that contains -O3 if the compiler
> support it).
>
> Does that sound sensible?

This is indeed a solution I hadn't thought about yet. But unfortunately 
it has some downsides. If you have a project with a lot of targets (as 
we have) and only one target needs the $(O3) override, then this 
approach would force you to create per target CFLAGS entries for all 
other targets (e.g. 'bar_CFLAGS = $(DEFAULTFLAGS)'). Furthermore, using 
a 'make CFLAGS=<some flags>' will not work anymore (you will now need 
to call 'make DEFAULTFLAGS=<some flags>'.

By the way, I totally agree with the argument that the user should 
always have the last say, which is indeed possible by using a variable 
for the flags you are appending (in your case the $(O3) and in my case 
I have called it $(DISOPT_FLAG)).

After some more pondering I think I might know an even cleaner solution 
to the problem (which would require a change to automake however): How 
about omitting CFLAGS when target_CFLAGS are provided?

This means one would be able to write:

libfoo_a_CFLAGS = $(AM_CFLAGS) $(CFLAGS) $(O3)

and if libfoo_a_CFLAGS is defined empty no CFLAGS will be used at all.

Of course, introducing this would break backward compatibility. So how 
about enabling this behavior by using some automake flag in 
AM_INIT_AUTOMAKE? By default a target_CFLAGS COMPILE target will 
contain '$(target_CFLAGS) $(CFLAGS)', but if this special automake flag 
is provided it will only be '$(target_CFLAGS)' (but only when using 
target specific flags, the '$(AM_CFLAGS) $(CFLAGS)' combination for the 
default COMPILE rule will _not_ be modified). Does this sound sensible? 
The implementation effort should at least be quite low as far as I can 
guess.

Best regards,
Sander