Fortran module files not found by CMake

Question

I have a split project in Fortran with a subdirectory as a library:

# ./CMakeLists.txt
cmake_minimum_required (VERSION 2.8)
project (Simulation Fortran)
enable_language(Fortran)

add_subdirectory(lib)

add_executable(Simulation main.f90)
include_directories(lib)
add_dependencies(Simulation physicalConstants)
target_link_libraries(Simulation physicalConstants)

The root directory contains only a single Fortran source code file:

! ./main.f90:

program simulation
use physicalConstants

implicit none

write(*,*) "Boltzmann constant:", k_b

end program simulation

And my subdirectory lib contains another CMakeLists.txt as well as a Fortran module source file:

# ./lib/CMakeLists.txt:

cmake_minimum_required (VERSION 2.8)
enable_language(Fortran)

project(physicalConstants)
add_library( physicalConstants SHARED physicalConstants.f90)

! ./lib/physicalConstants.f90:

module physicalConstants
implicit none
save

real, parameter :: k_B = 1.38e-23

end module physicalConstants

I tried to build those using cmake. Make generates the physicalconstants.mod in the lib directory, but this file is not found during the build process of main.f90.o:

Fatal Error: Can't open module file 'physicalconstants.mod' for reading at (1): No such file or directory

What am I missing here?

Answer 1

For target A to successfully use modules from target B, the directory where B stores module files must be among A's include directories.

Variant 1

One way to achieve that is to set the property Fortran_MODULE_DIRECTORY on target B and then add that property's contents to include directories of A.

You're claiming to support ancient CMake 2.8.0, in which you'll need to do something like this:

add_executable(Simulation main.f90)
include_directories(lib)
# note that add_dependencies call is not necessary when you're actually linking
target_link_libraries(Simulation physicalConstants)
get_property(moduleDir TARGET physicalConstants PROPERTY Fortran_MODULE_DIRECTORY)
include_directories(${moduleDir})

In more modern CMake, you could do this instead:

add_executable(Simulation main.f90)
include_directories(lib)
target_link_libraries(Simulation physicalConstants)
target_include_directories(Simulation PUBLIC $<TARGET_PROPERTY:physicalConstants,Fortran_MODULE_DIRECTORY>)

You can even create a function for it:

function(LinkFortranLibraries Target)
  target_link_libraries(Target ${ARGN})
  foreach(Lib IN LISTS ARGN)
    target_include_directories(Simulation PUBLIC $<TARGET_PROPERTY:${Lib},Fortran_MODULE_DIRECTORY>)
  endforeach()
endfunction()

and then use it like this:

add_executable(Simulation main.f90)
include_directories(lib)
LinkFortranLibraries(Simulation physicalConstants)

Variant 2

If you do not use the Fortran_MODULE_DIRECTORY property, module files are stored in the binary directory corresponding to the source directory of the target producing them. This can be retrieved from the target's property BINARY_DIR, which you could use exactly as Fortran_MODULE_DIRECTORY in variant 1.

However, CMake 2.8.0 does not support the target property BINARY_DIR, so you will have to "reconstruct" its value manually:

add_executable(Simulation main.f90)
include_directories(lib ${CMAKE_CURRENT_BINARY_DIR}/lib)
target_link_libraries(Simulation physicalConstants)