[CMake] How to build 2 targets from the same source, differing in -D_SWITCHES_ only

[Marcel Loose]

Hi Eike,

I think the only safe and reliable way to do this is create several
build directories, e.g. build/type_1 and build/type_2.
When running cmake in build/type_1, you should add -D_TYPE1_ to the
preprocessor flags; same for build/type_2.

Best regards,
Marcel Loose.

On Thu, 2009-08-27 at 12:28 +0200, Eike Kroemer wrote:
> Hi there,
> 
> in the hope of achieving platform independence of my plain-C project
> I have started looking into CMake, trying to duplicate what my
> manually created makefiles do.
> 
> A minimalistic description of my problem is the following:
> 
> * I have 2 executable targets
> * both having the same main program source MAIN/main.c
> * both linking to a library libvehicle.a that is built from the same
>   set of sources, e.g. VEHICLE/read_hydrodynamic_coeffs.c
> 
> The difference between both targets is a compiler definition
> -D_TYPE1_ or -D_TYPE2_ that is used in the sourcecode, e.g.
> 
>   hydro_par_t read_hydrodynamic_coeffs(sys_par_t sys_par)
>   {
>   #if defined(_TYPE1_)
>   return(read_hydrodynamic_coeffs_type1(sys_par));
>   #elif defined(_TYPE2_)
>   return(read_hydrodynamic_coeffs_type2(sys_par));
>   #endif
>   }
> 
> So the in both cases the library libvehicle.a will contain a
> function read_hydrodynamic_coeffs but the actual content of this
> function will be different.
> 
> [
>  In realiter it's more complicated because there are more
>  executables and combinations of several switches, so I really don't
>  want to change the sources.
> ]
> 
> What I've done so far is
> 
> * in the main CMakeLists.txt:
>   add_subdirectory(CMakeTargets/sim_type1)
> 
> * in CMakeTargets/sim_type1/CMakeLists.txt:
>   add_definitions(-D_TYPE1_)
>   include_directories("../../VEHICLE/INCLUDE")
>   add_subdirectory(../../VEHICLE VEHICLE)
>   add_executable(sim_type1 ../../MAIN/main)
>   target_link_libraries(sim_type1 vehicle)
> 
> * in CMakeTargets/VEHICLE/CMakeLists.txt:
>   set(SRC "read_hydrodynamic_coeffs")
>   set(SRC "${SRC} read_hydrodynamic_coeffs_type1")
>   set(SRC "${SRC} read_hydrodynamic_coeffs_type2")
>   add_library(vehicle ${SRC})
> 
> For one target only this gives a executable
> CMakeTargets/sim_type1/sim_type1 that is working as expected.
> 
> As specified by
>   add_subdirectory(../../VEHICLE VEHICLE),
> object file and library are created locally:
> ./CMakeTargets/sim_type1/VEHICLE/CMakeFiles/vehicle.dir/read_hydrodynamic_coeffs.c.o
> ./CMakeTargets/sim_type1/VEHICLE/libvehicle.a
> 
> But as soon as I try to
>  add_subdirectory(CMakeTargets/sim_type2)
>  [rest as above, only 1->2]
> I get an error message
>   add_library cannot create target "vehicle" because another target with the
>   same name already exists.  The existing target is a static library created
>   in source directory "./VEHICLE".  See documentation
>   for policy CMP0002 for more details.
> 
> Trying to cheat with cmake_policy(SET CMP0002 OLD) I can compile
> both executables but they are messed up, obviously trying to execute
> read_hydrodynamic_coeffs for the wrong type.
> 
> So, how do I define a set of executable make targets, differing only
> in the -D_defines_ that are used to build all of the source needed?
> 
> 
> Thanks in advance,
>    Eike
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