Calling Fortran from C using iso_c_binding

Makefile

all:
	gcc -c sum_to_n.c
	gfortran sum_to_n.f90 sum_to_n.o -o sum

sum_to_n.c

#include <stdio.h>

int f_sum_to_n(int *n);

int main ()
{
  int n;
  n = 1000;

  printf("%i\n", f_sum_to_n(&n));

  return 0;
}

sum_to_n.f90

integer function sum_to_n(n) result(sum) bind(C, name="f_sum_to_n")
  use iso_c_binding
  implicit none

  integer, intent(in) :: n
  integer :: i, s
  s = 0
  do i = 1, n
     s = s + i
  end do
  sum = s
end function sum_to_n