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automake-1.16: Per-Object Flags

 
 26.8 Per-Object Flags Emulation
 ===============================
 
      One of my source files needs to be compiled with different flags.  How
      do I do that?
 
    Automake supports per-program and per-library compilation flags (see
DONTPRINTYET  ⇒Program and Library Variables and *noteFlag Variables
DONTPRINTYET  ⇒Program and Library Variables and ⇒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 ⇒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 requires?  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,
 for instance ‘noinst_LTLIBRARIES = libfoo.la’ (⇒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’, ⇒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.
 

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