CMake/Modules/CMakeAddFortranSubdirectory.cmake

# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
# file LICENSE.rst or https://cmake.org/licensing for details.

#[=======================================================================[.rst:
CMakeAddFortranSubdirectory
---------------------------

This module provides a command to add a Fortran project located in a
subdirectory.

Load this module in a CMake project with:

.. code-block:: cmake

  include(CMakeAddFortranSubdirectory)

Commands
^^^^^^^^

This module provides the following command:

.. command:: cmake_add_fortran_subdirectory

  Adds a Fortran-only subproject from subdirectory to the current project:

  .. code-block:: cmake

    cmake_add_fortran_subdirectory(
      <subdir>
      PROJECT <project-name>
      ARCHIVE_DIR <dir>
      RUNTIME_DIR <dir>
      LIBRARIES <libs>...
      LINK_LIBRARIES
        [LINK_LIBS <lib> <deps>...]...
      [CMAKE_COMMAND_LINE <flags>...]
      NO_EXTERNAL_INSTALL
    )

  This command checks whether the current compiler supports Fortran or attempts
  to locate a Fortran compiler.  If a compatible Fortran compiler is found, the
  Fortran project located in ``<subdir>`` is added as a subdirectory to the
  current project.

  If no Fortran compiler is found and the compiler is ``MSVC``, it searches for
  the MinGW ``gfortran`` compiler.  In this case, the Fortran project is built
  as an external project using MinGW tools, and Fortran-related imported targets
  are created.  This setup works only if the Fortran code is built as a shared
  DLL library, so the :variable:`BUILD_SHARED_LIBS` variable is enabled in the
  external project.  Additionally, the :variable:`CMAKE_GNUtoMS` variable is set
  to ``ON`` to ensure that Microsoft-compatible ``.lib`` files are created.

  The options are:

  ``PROJECT``
    The name of the Fortran project as defined in the top-level
    ``CMakeLists.txt`` located in ``<subdir>``.

  ``ARCHIVE_DIR``
    Directory where the project places ``.lib`` archive files.  A relative path
    is interpreted as relative to :variable:`CMAKE_CURRENT_BINARY_DIR`.

  ``RUNTIME_DIR``
    Directory where the project places ``.dll`` runtime files.  A relative path
    is interpreted as relative to :variable:`CMAKE_CURRENT_BINARY_DIR`.

  ``LIBRARIES``
    Names of library targets to create or import into the current project.

  ``LINK_LIBRARIES``
    Specifies link interface libraries for ``LIBRARIES``.  This option expects a
    list of ``LINK_LIBS <lib> <deps>...`` items, where:

    * ``LINK_LIBS`` marks the start of a new pair
    * ``<lib>`` is a library target.
    * ``<deps>...`` represents one or more dependencies required by ``<lib>``.

  ``CMAKE_COMMAND_LINE``
    Additional command-line flags passed to :manual:`cmake(1)` command when
    configuring the Fortran subproject.

  ``NO_EXTERNAL_INSTALL``
    Prevents installation of the external project.

    .. note::

      The ``NO_EXTERNAL_INSTALL`` option is required for forward compatibility
      with a future version that supports installation of the external project
      binaries during ``make install``.

Examples
^^^^^^^^

Adding a Fortran subdirectory to a project can be done by including this module
and calling the ``cmake_add_fortran_subdirectory()`` command.  In the following
example, a Fortran project provides the ``hello`` library and its dependent
``world`` library:

.. code-block:: cmake

  include(CMakeAddFortranSubdirectory)

  cmake_add_fortran_subdirectory(
    fortran-subdir
    PROJECT FortranHelloWorld
    ARCHIVE_DIR lib
    RUNTIME_DIR bin
    LIBRARIES hello world
    LINK_LIBRARIES
      LINK_LIBS hello world # hello library depends on the world library
    NO_EXTERNAL_INSTALL
  )

  # The Fortran target can be then linked to the main project target.
  add_executable(main main.c)
  target_link_libraries(main PRIVATE hello)

See Also
^^^^^^^^

There are multiple ways to integrate Fortran libraries.  Alternative approaches
include:

* The :command:`add_subdirectory` command to add the subdirectory directly to
  the build.
* The :command:`export` command can be used in the subproject to provide
  :ref:`Imported Targets` or similar for integration with other projects.
* The :module:`FetchContent` or :module:`ExternalProject` modules when working
  with external dependencies.
#]=======================================================================]

include(CheckLanguage)
include(ExternalProject)

function(_setup_mingw_config_and_build source_dir build_dir)
  # Look for a MinGW gfortran.
  find_program(MINGW_GFORTRAN
    NAMES gfortran
    PATHS
      c:/MinGW/bin
      "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\MinGW;InstallLocation]/bin"
    )
  if(NOT MINGW_GFORTRAN)
    message(FATAL_ERROR
      "gfortran not found, please install MinGW with the gfortran option."
      "Or set the cache variable MINGW_GFORTRAN to the full path. "
      " This is required to build")
  endif()

  # Validate the MinGW gfortran we found.
  if(CMAKE_SIZEOF_VOID_P EQUAL 8)
    set(_mingw_target "Target:.*64.*mingw")
  else()
    set(_mingw_target "Target:.*mingw32")
  endif()
  execute_process(COMMAND "${MINGW_GFORTRAN}" -v
    ERROR_VARIABLE out ERROR_STRIP_TRAILING_WHITESPACE)
  if(NOT "${out}" MATCHES "${_mingw_target}")
    string(REPLACE "\n" "\n  " out "  ${out}")
    message(FATAL_ERROR
      "MINGW_GFORTRAN is set to\n"
      "  ${MINGW_GFORTRAN}\n"
      "which is not a MinGW gfortran for this architecture.  "
      "The output from -v does not match \"${_mingw_target}\":\n"
      "${out}\n"
      "Set MINGW_GFORTRAN to a proper MinGW gfortran for this architecture."
      )
  endif()

  # Configure scripts to run MinGW tools with the proper PATH.
  get_filename_component(MINGW_PATH ${MINGW_GFORTRAN} PATH)
  file(TO_NATIVE_PATH "${MINGW_PATH}" MINGW_PATH)
  string(REPLACE "\\" "\\\\" MINGW_PATH "${MINGW_PATH}")
  configure_file(
    ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/CMakeAddFortranSubdirectory/config_mingw.cmake.in
    ${build_dir}/config_mingw.cmake
    @ONLY)
  configure_file(
    ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/CMakeAddFortranSubdirectory/build_mingw.cmake.in
    ${build_dir}/build_mingw.cmake
    @ONLY)
endfunction()

function(_add_fortran_library_link_interface library depend_library)
  set_target_properties(${library} PROPERTIES
    IMPORTED_LINK_INTERFACE_LIBRARIES_NOCONFIG "${depend_library}")
endfunction()


function(cmake_add_fortran_subdirectory subdir)
  # Parse arguments to function
  set(options NO_EXTERNAL_INSTALL)
  set(oneValueArgs PROJECT ARCHIVE_DIR RUNTIME_DIR)
  set(multiValueArgs LIBRARIES LINK_LIBRARIES CMAKE_COMMAND_LINE)
  cmake_parse_arguments(ARGS "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
  if(NOT ARGS_NO_EXTERNAL_INSTALL)
    message(FATAL_ERROR
      "Option NO_EXTERNAL_INSTALL is required (for forward compatibility) "
      "but was not given."
      )
  endif()

  # if we are not using MSVC without fortran support
  # then just use the usual add_subdirectory to build
  # the fortran library
  check_language(Fortran)
  if(NOT (MSVC AND (NOT CMAKE_Fortran_COMPILER)))
    add_subdirectory(${subdir})
    return()
  endif()

  # if we have MSVC without Intel fortran then setup
  # external projects to build with mingw fortran

  set(source_dir "${CMAKE_CURRENT_SOURCE_DIR}/${subdir}")
  set(project_name "${ARGS_PROJECT}")
  set(library_dir "${ARGS_ARCHIVE_DIR}")
  set(binary_dir "${ARGS_RUNTIME_DIR}")
  set(libraries ${ARGS_LIBRARIES})
  # use the same directory that add_subdirectory would have used
  set(build_dir "${CMAKE_CURRENT_BINARY_DIR}/${subdir}")
  foreach(dir_var library_dir binary_dir)
    if(NOT IS_ABSOLUTE "${${dir_var}}")
      get_filename_component(${dir_var}
        "${CMAKE_CURRENT_BINARY_DIR}/${${dir_var}}" ABSOLUTE)
    endif()
  endforeach()
  # create build and configure wrapper scripts
  _setup_mingw_config_and_build("${source_dir}" "${build_dir}")
  # create the external project
  externalproject_add(${project_name}_build
    SOURCE_DIR ${source_dir}
    BINARY_DIR ${build_dir}
    CONFIGURE_COMMAND ${CMAKE_COMMAND}
    -P ${build_dir}/config_mingw.cmake
    BUILD_COMMAND ${CMAKE_COMMAND}
    -P ${build_dir}/build_mingw.cmake
    BUILD_ALWAYS 1
    INSTALL_COMMAND ""
    )
  # create imported targets for all libraries
  foreach(lib ${libraries})
    add_library(${lib} SHARED IMPORTED GLOBAL)
    set_property(TARGET ${lib} APPEND PROPERTY IMPORTED_CONFIGURATIONS NOCONFIG)
    set_target_properties(${lib} PROPERTIES
      IMPORTED_IMPLIB_NOCONFIG   "${library_dir}/lib${lib}.lib"
      IMPORTED_LOCATION_NOCONFIG "${binary_dir}/lib${lib}.dll"
      )
    add_dependencies(${lib} ${project_name}_build)
  endforeach()

  # now setup link libraries for targets
  set(start FALSE)
  set(target)
  foreach(lib ${ARGS_LINK_LIBRARIES})
    if("${lib}" STREQUAL "LINK_LIBS")
      set(start TRUE)
    else()
      if(start)
        if(DEFINED target)
          # process current target and target_libs
          _add_fortran_library_link_interface(${target} "${target_libs}")
          # zero out target and target_libs
          set(target)
          set(target_libs)
        endif()
        # save the current target and set start to FALSE
        set(target ${lib})
        set(start FALSE)
      else()
        # append the lib to target_libs
        list(APPEND target_libs "${lib}")
      endif()
    endif()
  endforeach()
  # process anything that is left in target and target_libs
  if(DEFINED target)
    _add_fortran_library_link_interface(${target} "${target_libs}")
  endif()
endfunction()