Calling C from Fortran

Calling C from Fortran.md

main.f contains the function invocation and hello.c contains the function being invoked.

Using GNU's "gfortran" compiler and GNU's C compiler, one needs to add an underscore to the C function name to match the symbol generated by the Fortran compiler. Fortran is "call by reference" so the formal argument is a pointer to the actual argument. The calling function allocates an anonymous variable to store the literal integer 3 in. If the actual argument had been a variable, the called function would be allowed to alter it. Note also, that the caller has no way of knowing the number or types of aguments to hello. Since it is invoked with the call keyword, it is assumed to be a Fortran "procedure" which is essentially a function with no return value.

hello.c

#include <stdio.h>
#include "hello.h"

void hello(int repeatCount) {
    int i;
    for (i = 0; i < repeatCount; i++) {
        printf("hello from C\n");
    }
}

void hello_(int *repeatCount) {
    int i;
    for (i = 0; i < *repeatCount; i++) {
        printf("hello from C\n");
    }
}

main.f

      call hello(3)
      end