Interoperability: Fortran to C++ - fortran

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.