Fortran iso_c_binding

Work with C functions in Fortran using iso_c_binding module

Suppose we have a file test_iso_c_bind.c containing three C functions:

#include <stdio.h>
void Basics (const double a, // in
             const int n,    // in
             double *b)      // out
{    
      // bala bala
}

void Array (const double c[], // in
            double d[])       // out
{    
      // bala bala
}

double Function (double (*f) (double, void(*)), // a function pointer
                 void * g) // a pointer to some type
{
     // bala bala
}

Now the question is how to call those 3 functions in a Fortran program. One of the most common solutions is to use Fortran iso_c_binding module. Next, we'll demonstrate the way to use iso_c_binding to achieve this.

• Basics

In this section, we'll show how to call Basics function in a Fortran program test.

program test    
    use iso_c_binding ! import iso_c_binding module
    implicit none
    !> Create an interface to C function
    interface
       subroutine Basics_c(a, n, b) bind(c, name='Basics')
           use iso_c_binding, only :: c_int, c_double
           real(kind=c_double), value, intent(in) :: a
           integer(kind=c_int), value, intent(in) :: n
           real(kind=c_double), intent(out) :: b
       end subroutine Basics_c
    end interface
    !> Define C-type variables
    real(kind=c_double) :: a_c, b_c
    integer(kind=c_int) :: n_c
    !> Define Fortran-type variable
    real :: a_f, b_f
    integer :: n_f

    !> Assign values to a_f and n_f
    a_f = 1.5
    n_f = 1
    !> Convert Fortran to C
    a_c = real(a_f, kind=c_double)
    n_c = integer(n_f, kind=c_int)
    !> Call C function
    call Basics(a_c, n_c, b_c)
    !> Convert C output to Fortran type
    b_f = real(b_c)

end program test

Remarks:

1. An interface block is created to allow test program to access Basics function because we cannot import a C program to a Fortran program using use statement.
2. The name statement is used because subroutine name Basics_c is different from function name Basics. It can be omitted if Fortran subroutine name is made to be the same as the C function name.
3. The only statement specifies the only types required by declaring C function Basics. This is economical way of loading iso_c_binding module. If we are lazy about this, we can simply do use iso_c_binding without only statement.
4. The value attribute in the declaration ensures we are passing actual value of a variable, rather than its reference. C program is by default "call-by-value" whereas Fortran program uses "call-by-reference" paradigm.
5. Because b in the Basics function is declared to be a pointer (i.e. reference) to double, we don't supply value attribute.
6. Use kind statement to cast a Fortran data type to its corresponding C type or vice versa. A complete list of interoperable data types between C and Fortran can be found here.

• Array as argument

• Function as argument

• References

  • Interoperability with C (GNU): https://gcc.gnu.org/onlinedocs/gfortran/Interoperability-with-C.html