On 03/09/2013 06:26 PM, Michele Martone wrote: > On 20130309@17:58, Stefano Lattarini wrote: >> On 03/09/2013 04:43 PM, Michele Martone wrote: >>> Hi, >>> >>> I'm using the "Per-Object Flags Emulation" functionality of automake >>> to build a single library (.a) file from two .a files, each made of .o >>> files compiled with different flags. >>> >>> In order to apply the example of >>> >>> http://www.gnu.org/software/automake/manual/automake.html#Per_002dObject-Flags >>> >>> in my Makefile.am, I declare something similar to >>> libfoo_a_CFLAGS = -some -other -flags >>> expecting that libfoo.a will be compiled with -some -other -flags >>> rather than CFLAGS. >>> >>> However in the resulting Makefile I see that something like the >>> following: >>> libfoo_a-foo.o: foo.c >>> $(CC) ... $(libfoo_a_CFLAGS) $(CFLAGS) ... >>> is being generated instead. >>> In other words, CFLAGS is not being replaced by libfoo_a_CFLAGS: only >>> prepended: >>> >> This is correct: CFLAGS is a user-reserved variable, so it should always >> be honoured and should never be overridden by Automake rules (nor by >> package developers). The correct developer-reserved variable to specify >> extra C compilation flags is AM_CFLAGS -- and *that* is being correctly >> overwritten. > > Thanks Stefano -- makes sense to me, although this was not clear from the > manual. > (maybe it needs a correction ? I would suggest using > "will be compiled using also" instead of "will be compiled using"). > No, the "also" would suggest that '-some -flags' are used too, while in fact, only '-some -other -flags' are. Still, to reduce the chance for confusion, we could explicitly state that $(CFLAGS) is still used in both cases; while that is already implied by other parts of the documentation, this might be one of the cases where "repetita juvant". Patches welcome.
> But, is there a clean way for preventing CFLAGS being used for some > library files ? > Not that I know of; and it's by design: doing that would prevent user customization, which is a Bad Thing. > In a way to get in Makefile rules like: > libfoo_a-foo.o: foo.c > $(CC) ... $(libfoo_a_CFLAGS) ... > instead of: > libfoo_a-foo.o: foo.c > $(CC) ... $(libfoo_a_CFLAGS) $(CFLAGS) ... > > or at least to get a custom "AM_POST_CFLAGS" appended *after* CFLAGS: > libfoo_a-foo.o: foo.c > $(CC) ... $(libfoo_a_CFLAGS) $(CFLAGS) $(AM_POST_CFLAGS) ... > > ? > > In such a way, I (developer) may use AM_POST_CFLAGS to introduce some > flags that may invalidate some undesired CFLAGS (e.g.: invalidate > loop unrolling flags that may be implied by CFLAGS). > I'd say, don't do that either; just state in your README or INSTALL file (or similar documentation) that such flags are unsupported or discouraged. If a user wants to shoot himself in the foot by ignoring your advice, let him. This is UNIX :-) HTH, Stefano
