Interoperability: Fortran to C++

Question

I would like to develop a Fortran Static Library that include a custom subroutine that takes a long time to complete. This static library will be linked in my C++ application statically too.

My goal is monitoring the current status of this subroutine in my C++ application in real-time.

So, for each step of a "fortran loop", I would like to send the loop index to my C++ application.

I'm new in Fortran world, so I was thinking that this task could be something like this:

My C++ header:

extern "C" void fortran_status(int* value);

My Fortran-90 file:

module my_interfaces
    use iso_c_binding
        interface
            subroutine fortran_status(progress_value) bind(C, name = 'fortran_status')
                use, intrinsic :: iso_c_binding
                integer(c_int), intent(out) :: progress_value
            end subroutine fortran_status
        end interface
end module my_interfaces


! My long calc subroutine
subroutine my_long_calc(progress_value) BIND(C, name = 'my_long_calc')
    use, intrinsic :: ISO_C_BINDING
    use my_interfaces
    implicit none

    EXTERNAL fortran_status

    integer (C_INT), INTENT(INOUT) :: progress_value
    integer (C_INT) :: count


    do count = 0, 5    
        progress_value = progress_value + 1

        ! Send 'progress_value' to a C++ function:
        call fortran_status(progress_value)

        ! Wait 1 second:
        call sleep(1)
    end do

end subroutine my_long_calc

This Fortran code gives me a compile-time error:

error #6406: Conflicting attributes or multiple declaration of name.   [FORTRAN_STATUS]       C:\Users\lamar\Desktop\Lib1_interface\Lib1.f90    18 

I'm using Microsoft Visual Studio 2019 with Intel Visual Fortran (Windows 10 x64).

How could I monitoring that subroutine status? Or get the fortran loop index in my C++ application?

UPDATE 1:

I removed the EXTERNAL fortran_status and now I got no errors in compile-time of my Fortran code.

Now, I would like to link this static Lib (x86) with my C++ code:

#include <iostream>

extern "C" {
  void my_long_calc(void);
  void fortran_status(int* value);
}

void fortran_status(int* value)
{
  std::cout << "Fortran current status = " << *value << std::endl;
}

int main (void)
{
  std::cout << "Monitoring Fortran subroutine..." << std::endl;

  my_long_calc();

  return 0;
}

I'm trying to compile and link it using MingW:

g++ -Wall -L. .\Lib2.lib -lgfortran .\main.cpp -o app.exe

And I got this linker error: undefined reference to my_long_calc

How can I link it?

I would like to see in my terminal output:

Monitoring Fortran subroutine...

Fortran current status = 0
Fortran current status = 1
Fortran current status = 2
​​​​​​​Fortran current status = 3
​​​​​​​Fortran current status = 4
​​​​​​​Fortran current status = 5

UPDATE 2

Now this changed Fortran code compiles and it work well ONLY if I'm using MingW g++ (x86).

Fortran code:

module my_interfaces
  use iso_c_binding
  interface
    subroutine fortran_status(progress_value) bind(C, name = 'fortran_status')
      use, intrinsic :: iso_c_binding
      integer(c_int), intent(out) :: progress_value
    end subroutine fortran_status
  end interface
end module my_interfaces


! My long calc subroutine
subroutine my_long_calc() BIND(C, name = 'my_long_calc')
  use, intrinsic :: ISO_C_BINDING
  use my_interfaces
  implicit none

  integer (C_INT) :: progress_value
  integer (C_INT) :: count

  progress_value = 0

  do count = 0, 5
    progress_value = count

    ! Send 'progress_value' to a C++ function:
    call fortran_status(progress_value)

    ! Wait 1 second:
    call sleep(1)
  end do
end subroutine my_long_calc

C++ Code:

#include <iostream>

extern "C" {
    void my_long_calc(void);
    void fortran_status(int* value);
}

void fortran_status(int* value)
{
    std::cout << "Fortran current status = " << *value << std::endl;
}

int main (void)
{
    std::cout << "Monitoring Fortran subroutine..." << std::endl;

    my_long_calc();

    return 0;
}

Compile c++: g++ -Wall -c .\main.cpp

Compile fortran: gfortran -c .\gfortran.f90

Link all together: g++ .\main.o .\gfortran.o -o app.exe -lgfortran

The problem now is that I need to use Visual Studio 2019 and Visual Fortran to develop my Fortran Code. And I would like to compile all Fortran code to a single static library (*.lib) file using Visual Studio.

What I need to change to get to link the *.lib file (from Visual Fortran) using the MingW g++ command?

I was thinking to use something like this: g++ main.cpp my_lib.lib -o app.exe

But I got this linker error: undefined reference to my_long_calc

What I need to do?

Answer 1

The code in your "Update 2" is perfectly fine and works with Intel Visual Fortran and Microsoft Visual C++. See also the discussion you started in https://community.intel.com/t5/Intel-Fortran-Compiler/Interoperability-Fortran-to-C/m-p/1144147

I do not recommend mixing Intel Visual Fortran compiled objects/libraries with g++ on Windows. You can get Microsoft Visual Studio Community Edition free if you meet the rather liberal license terms. If you insist on using g++, mix it with gfortran.