Wrapping a fortran library from FetchContent

[MuellerSeb]

Hey there,

since I struggled a lot to get this working, I'd like to share a solution with you!

Aim

Compile a fortran shared library from FetchContent and provide a python wrapper with a set of interfaces that is linked against this shared library by using f2py.

Problem

I needed to set the rpath in the f2py generated SO

Solution

include(FetchContent)
set(LIB_NAME <your_lib>)
FetchContent_Declare(
  ${LIB_NAME}
  GIT_REPOSITORY <your_lib_repo>
  GIT_TAG        <your_lib_tag>
)
FetchContent_MakeAvailable(${LIB_NAME})
install(TARGETS ${LIB_NAME}
  RUNTIME DESTINATION <your_src_folder>
  LIBRARY DESTINATION <your_src_folder>
  ARCHIVE DESTINATION <your_src_folder>
)
get_target_property(LIB_INCLUDES ${LIB_NAME} INCLUDE_DIRECTORIES)
set(f2py_module_name "wrapper")
set(fortran_src_file "${CMAKE_CURRENT_SOURCE_DIR}/${f2py_module_name}.f90")
set(generated_module_file ${CMAKE_CURRENT_BINARY_DIR}/${f2py_module_name}${PYTHON_EXTENSION_MODULE_SUFFIX})
add_custom_target(${f2py_module_name} ALL DEPENDS ${generated_module_file} ${LIB_NAME})

# rpath should be set to look locally for the needed .so file
if (APPLE)
  set(rpath "@loader_path")
else()
  set(rpath "$$ORIGIN")  # need double $ to not expand the variable
endif()

add_custom_command(
  OUTPUT ${generated_module_file}
  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
  # use Command Line Tool Mode of CMake to set LDFLAGS for f2py (rpath)
  COMMAND ${CMAKE_COMMAND} -E env 'LDFLAGS=-Wl,-rpath,${rpath}' env 'NPY_DISTUTILS_APPEND_FLAGS=1'
    ${F2PY_EXECUTABLE}
    -m ${f2py_module_name}
    -c ${fortran_src_file}
    -I${LIB_INCLUDES}
    -L${LIB_INCLUDES}
    -l${LIB_NAME}
    --f90exec=${CMAKE_Fortran_COMPILER}
)
install(FILES ${generated_module_file} DESTINATION <your_src_folder>)

Magic happens in this line:

COMMAND ${CMAKE_COMMAND} -E env 'LDFLAGS=-Wl,-rpath,${rpath}' env 'NPY_DISTUTILS_APPEND_FLAGS=1'

f2py has no option to pass linker flags other than setting the env-var LDFLAGS and to set these env-vars in a custom command, I had to wrap the f2py-call with Cmakes Command Line Tool Mode. Also the NPY_DISTUTILS_APPEND_FLAGS env-var needs to be altered so numpy respects the LDFLAGS.

Maybe this is useful for someone. The solution does not target windows systems.

Cheers,
Sebastian

[thewtex]

@MuellerSeb thank you for sharing!