Need capital suffix ".F" or ".F90" for preprocessed Fortran source files

.gitignore

.DUMMY
.dummy

Readme.md

CMake Fortran Introspection

Modern build systems like CMake and Meson introspect Fortran source files to determine the build order of source files. This is vital for Fortran source files using modules and submodules.

Fortran module .mod file files are generated when compiling a Fortran source file. The order of compiling Fortran source files with modules is vital to avoiding build failure due to missing module files.

Fails due to improper build order:

gfortran main.f90 m1.f90 m2.F90

Proper build order (success):

gfortran m1.f90 m2.F90 main.f90

If you change the file extension to m2.f90, the build fails due to the preprocessing not being enabled:

mv m2.F90 m2.f90

gfortran m1.f90 m2.f90 main.f90

This is true on both case sensitive and case insensitive file systems.

Detect case sensitivity of filesystem

c++ -std=c++17 case_sensitive.cpp -o case_sensitive

./case_sensitive

case_sensitive.cpp

#include <filesystem>
#include <fstream>
#include <iostream>
#include <stdexcept>
#include <cstdlib>


void create_empty_file(const std::filesystem::path& path)
{
    std::ofstream file(path);
    file.close();
}


bool case_sensitive(const std::filesystem::path& path)
{
    if (!std::filesystem::is_directory(path))
      throw std::invalid_argument("path is not a directory");

    const auto p1 = path / ".dummy";
    const auto p2 = path / ".DUMMY";

    for (const auto& p : {p1, p2})
    {
        if (!std::filesystem::exists(p))
            create_empty_file(p);
    }

    return !std::filesystem::equivalent(p1, p2);
}


int main(int argc, char* argv[])
{
    const std::string in = (argc > 1) ? argv[1] : ".";

    const auto p = std::filesystem::weakly_canonical(in);

    const auto s = case_sensitive(p);

    std::cout << "filesystem under " << p.root_path() << " is case " << (s ? "sensitive" : "insensitive") <<  "\n";

    return EXIT_SUCCESS;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.19)

project(mod LANGUAGES CXX Fortran)

message(STATUS "CMake ${CMAKE_VERSION} ${CMAKE_GENERATOR}")

add_executable(main main.f90 m1.f90 m2.F90)

if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.1)
  set(cxx_ldflags "-lstdc++fs")
endif()

add_executable(case_sensitive case_sensitive.cpp)
target_compile_features(case_sensitive PRIVATE cxx_std_17)
target_link_libraries(case_sensitive PRIVATE ${cxx_ldflags})

file(GENERATE OUTPUT .gitignore CONTENT "*")

m1.f90

module m1

implicit none

contains

pure real function pi()
pi = 4*atan(1.)
end function

end module m1

m2.F90

module m2

#if 0
use, intrinsic :: fake
#endif

use m1, only : pi

implicit none

contains

pure real function twopi()
twopi = 2*pi()
end function

end module

main.f90

program test

use m2

implicit none

print *, "2pi = ", twopi()

end program