[CMake] How to have generated source compiled multiple ways?

[Paul Smith]

On Mon, 2013-08-12 at 22:35 +0200, Eric Noulard wrote:
> 2013/8/12 Paul Smith <paul at mad-scientist.net>:
> >
> > So I have tried using add_custom_command() with OUTPUT, but the
> > documentation says:
> >
> > "Do not list the output in more than one independent target that may build
> > in parallel or the two instances of the rule may conflict (instead use
> > add_custom_target to drive the command and make the other targets depend on
> > that one)."
> >
> > Sure enough, it seems that if I have my different libraries depend on these
> > outputs I get problems during parallel builds.  So then I tried to use
> > add_custom_target(), but that says:
> >
> > "The target has no output file and is ALWAYS CONSIDERED OUT OF DATE"
> >
> > and, sure enough, if I do it this way my source files (and everything that
> > depends on them) rebuild every time I run the build, even if nothing has
> > changed.  This is a big bummer.
> 
> I did never tried befoire but think you should "simply" keep your
> current add_custom_command and create a new target with
> add_custom_target whose only purpose is to "serialize" the dependency
> for the 2 (or more) independent libraries which use the output of your
> custom command concurrently.

> add_custom_target(SerializeTarget
>   DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/generated.c)
> 
> add_library(MyLib1 ${CMAKE_CURRENT_BINARY_DIR}/generated.c lib1.c)
> set_target_properties(MyLib1 PROPERTIES COMPILE_FLAGS -DDEFGEN=3)
> add_dependencies(MyLib1 SerializeTarget)
> 
> add_library(MyLib2 ${CMAKE_CURRENT_BINARY_DIR}/generated.c lib2.c)
> set_target_properties(MyLib2 PROPERTIES COMPILE_FLAGS -DDEFGEN=4)
> add_dependencies(MyLib2 SerializeTarget)

OK, maybe I'm misunderstanding but if the SerializeTarget is considered
"always out of date" because it was defined with add_custom_target()
then won't that cause the two libraries to always rebuild, since
something they depend on is always out of date?  Maybe I'm thinking of
it too much like make.

I'll look at the stuff you've sent and try to adjust it for my
situation.

Thanks!