Using cmake to build FORTRAN projects with modules

If you are building FORTRAN projects in linux, you can use Cmake as your build system. Let’s build a very simple project consisting a module and a FORTRAN program file.

The module is defined in the file hello_mod.f90 which contains following code:

module hello
  contains 
    subroutine say_hello(name)
      implicit none
      character(*),optional,intent(in):: name

      if (present(name)) then
        write (*,10) name
      else
        write (*,10) "world"
      endif

10    format ("Hello",x,a,"!")      
    end subroutine say_hello

    subroutine say_goodbye(name)
      implicit none
      character(*),optional,intent(in):: name
      
      if (present(name)) then
        write (*,10) name
      else
        write (*,10) "world"
      endif

10    format ("Goodbye,",x,a,"!")
    end subroutine say_goodbye

end module hello

The file main.f90 contains the program which in turn uses the the hello module to display messages. The content of main.f90 is trivial as you can see.

program  main
  use hello
  
  call say_hello("Dolly")
  call say_goodbye("friends")
end program main

We need to create a CMakeLists.txt file and define FORTRAN as language.

cmake_minimum_required(VERSION 3.0)

project(test_modules LANGUAGES Fortran)

set (CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/modules)


add_executable(test 
                main.f90
                hello_mod.f90)

target_include_directories(test PRIVATE
                ${CMAKE_BINARY_DIR}/modules)

# use this if you need to link to libraries
# ---------------------------------------------------------------
#find_library(target <library>) # use to locate external libraries
#
#target_link_libraries (test PRIVATE
#                         library_1
#                         library_2)

Now when we run cmake using this file, it will create an empty directory named modules under our build directory. We then run make and it will place a module file hello.mod in this directory. It will also create the object files hello_mod-f90.o and main.f90.o in the folder CMakeFiles/test.dir and finally will link these two source files into the executable test.