I have been messing with Fortran some lately... here is some code that allows you to call a Fortran function from a C program. It is very simple at this point

makefile

test: test.o mysum.o
	gcc -o test test.o mysum.o -lgfortran
test.o: test.c
	gcc -c test.c
mysum.o: mysum.f90
	gfortran -ffree-form -c mysum.f90

mysum.f90

integer function mysum(a, s)
  implicit none
  integer, intent(in) :: s
  integer, dimension(s), intent(in) :: a
  integer :: i
  integer :: result = 0
  do i=1, size(a)
     result = result + a(i)
  end do
  mysum = result
end function

test.c

#include <stdio.h>

int mysum_(int* a, int* s);

int main() {
    int arr[5] = {1,2,3,4,5};
    int arr_size = 5;
    printf("%i\n", mysum_(arr, &arr_size));
}