[CMake] CMake thinks Fortran is C++ when linking statically

[Michael Hertling]

On 05/10/2011 12:19 PM, Erlend Pedersen wrote:
> I am having problems with Fortran linking that doesn't work when
> building static libraries, only when building shared libraries. I am
> using cmake from Ubuntu 10.04 (2.8.0-5ubuntu1).
> 
> I use CMake to build a project, including project-internal shared
> libraries. All executables are Fortran, but one of the internal
> libraries contain minimal sprinklings of C and C++. All works well when
> building shared libraries, but when I switch to building static
> libraries, CMake decides to try to link all Fortran executables as if
> they were C++. Unfortunately, this means that the main function is not
> properly located, and linking fails.
> 
> I tried forcing Fortran linkage on libraries only (hoping this would
> propagate into the executables that link to them), using 
> 
>   SET_TARGET_PROPERTIES(<libname> PROPERTIES LINKER_LANGUAGE Fortran)
> 
> and when building the libraries, output shows that they are indeed
> treated as Fortran static libraries. But the executables (typically
> conisting of a single .f90 file, and linked to one or more of the
> internal libraries) are still treated as C++.
> 
> Only when I force the LINKER_LANGUAGE by using SET_TARGET_PROPERTIES on
> both libraries and executables does things build properly. But that
> seems needlessly redundant and noisy and redundant.
> 
> Is there some easier way I can convince CMake to link the executables
> correctly? Is this a bug?

No, this isn't a bug. The behaviour you observe occurs because of

CMAKE_CXX_LINKER_PREFERENCE_PROPAGATES==TRUE
CMAKE_CXX_LINKER_PREFERENCE==30 > 20==CMAKE_Fortran_LINKER_PREFERENCE

i.e. when a static library with C++ code is mixed with Fortran code,
the C++ linker is taken into account along with the Fortran linker,
and C++ will win; this does not apply to _shared_ libraries. Hence,
setting CMAKE_CXX_LINKER_PREFERENCE_PROPAGATES to FALSE is probably
the appropriate solution in your case, i.e. Fortran code with one,
say, C++-contaminated _static_ library. Alternatively, you might
raise the Fortran linker preference above the C++ linker's one.

'hope that helps.

Regards,

Michael