Question

For reasons that are not relevant, I need to pass a C/C++ function name into a Fortran subroutine, which, in turn, calls that C function. What I have found is that I can succesfully pass the function name into the Fortran subroutine. In that subroutine I can call the correct C function. However, the arguments of the C function get broken on this call (when called directly from C it works fine). I have used ISO C Binding to try and get this to work, to no avail.

Here is a MWE:

fortranRoutine.h:

extern "C" {
    void fortranRoutine_(void(int status));
};

---

calledfromFortran.h:

void calledfromFortran(int status);

---

main.cpp:

#include "fortranRoutine.h"
#include "calledfromFortran.h" 

using namespace std;

int main(int argc, char** argv) {
    calledfromFortran(12);
    fortranRoutine_(calledfromFortran);
    return 0;
}

---

fortranRoutine.f90:

subroutine fortranRoutine(calledfromFortran)

    use iso_c_binding

    implicit none

    interface
        subroutine calledfromFortran(status) bind (c)
            use iso_c_binding
            integer(kind = c_int), intent(in) :: status
        end subroutine calledfromFortran
    end interface

    integer(kind = c_int) :: one

    one = 2
    call calledfromFortran(one)
    return

end subroutine fortranRoutine

---

calledfromFortran.cpp:

#include <iostream>
#include "stdlib.h"

using namespace std;

void calledfromFortran(int status) {
    cout << "In calledfromFortran:" << endl;
    cout << " status: " << status << endl;
}

---

Current results

Running this currently gives:

In calledfromFortran:
 status: 12
In calledfromFortran:
 status: -1641758848

The first call to calledfromFortran from main works correctly, but when it’s called from fortranRoutine the value is broken. Note, each time it’s run that latter value changes. What am I doing wrong?

Answer 1

When passing scalar values between Fortran and C, you have basically two options:

• You pass them by reference: On the C-side you have to make sure, that you use pointers to those scalars.

• You pass them by value: On the Fortran side you have to make sure that you use the VALUE attribute, as already suggested by other posts.

As for the intent(in) attribute, it can stay there, as it does not affect, whether the variable is passed by value or reference. In Fortran, arguments are always passed by reference unless you specify the VALUE attribute. The attribute intent(in) only tells the compiler to prevent a usage of that dummy argument in the routine, which would change its value.

Additional note on the naming: You should specify your Fortran routine fortranRoutine also with bind(c). This way you can specify its name as seen from C, even if it is inside a module:

module my_interface
  use iso_c_binding

contains

  subroutine fortranRoutine(calledFromFortran) bind(c, name='fortranroutine')
  ...
  end subroutine fortranRoutine

end module my_interface

This way you can be sure, the name of the function to be called from C is fortranroutine, independent of the convention of the compiler to append underscores, prepend module names or convert names to lower case. Consequently, you would have a a header file in C, which should work compiler independently:

extern "C" {
  void fortranroutine(void(int status));
};