[CMake] Fortran-C mixed code

[Bill Hoffman]

Radu Serban wrote:
> Alan W. Irwin wrote:
>> On 2007-01-12 12:42-0800 Radu Serban wrote:
>>
>>> I'm attempting to use cmake to configure a package containing both 
>>> Fortran (F77) and C source code (as an alternative to the current 
>>> build system which uses autoconf+libtool) but I couldn't figure out 
>>> how to properly do that.
>>
>> Have you considered splitting your library into a fortran-only 
>> library and a
>> C-only library?  Autotools tries to support mixing fortran- and 
>> C-compiled
>> objects in the same library (using the AC_F77_LIBRARY_LDFLAGS macro 
>> and the
>> resulting FLIBS), but we could never get that to work reliably on all
>> platforms for PLplot.  Thus, we ended up splitting our libraries into
>> fortran-only and C-only when we were still building with autotools, 
>> and we
>> have continued that approach now we have migrated to cmake.  We 
>> didn't have
>> to do anything special to get this approach to work. Just use the
>> appropriate ADD_LIBRARY command for each library, and use
>> TARGET_LINK_LIBRARIES to establish the dependencies between them, and 
>> the
>> result seems to build fine on all platforms.
>>
>> Alan
>
> Hi Alan,
>
> Splitting the library into two is unfortunately not applicable to my 
> case (see below why).
>
> In my initial post I had raised 3 issues (determining the F77 
> intrinsic and run-time libraries, using a C compiler to link an 
> executable from an object created from a Fortran source and a C 
> library, and determining the Fortran compiler's name mangling scheme).
>
> The first two issues are relevant only if the C library is actually 
> built using a C++ compiler. However, I realized that cmake will stop 
> with an error if I set the environment variable CC to, say, g++ before 
> invoking cmake on my project (#error "The CMAKE_C_COMPILER is set to a 
> C++ compiler"). As such, I can always be sure that a C++ compiler will 
> not be used to create the library from the C sources and using the 
> Fortran compiler to link will work just fine (and the F77 intrinsic 
> and run-time libraries will not be needed).
>
> But knowing the Fortran name mangling scheme is still crucial (if 
> interested, see below why). For now I rely on the user to provide the 
> case (lower or upper) and number of appended underscores (none, one, 
> or two) which determine it. The defaults I provide (lower case with 
> one underscore) is probably the most common one, but I'd be much 
> happier if I could automatically determine it at configuration time... 
> Any suggestions?
>
> Finally, I'm now wondering why cmake will not allow using a C++ 
> compiler on C sources. Sure, one must be careful in conditionally 
> including the appropriate wrappers in the header files to allow C++ 
> usage, but I don't see why cmake would absolutely forbid this. Issuing 
> a warning may be appropriate, why an error? Am I missing anything?
>
You can always use the LINKER_LANGUAGE property of a target to force 
which compiler is used to link.

Back to your original example, this should do what you want:

add_executable(foo foo.f)
target_link_libraries(foo my_c_library)
set_target_properties(foo PROPERTIES LINKER_LANGUAGE CXX)

-Bill