Hmm...
Code:
#include <stdio.h>

int sumGlobal;

void addGlobal (int a, int b);
int addReturn (int a, int b);
void addReferenced (int a, int b, int * c); 

int main (void)
{
 int a, b;
 int sumReturn, sumReferenced;

 printf ("Two numbers please.\n");
 scanf ("%d%d", &a, &b);
 
 addGlobal(a, b);
 sumReturn = addReturn(a, b);
 addReferenced (a, b, &sumReferenced);
 
 printf ("%d %d %d\n", sumGlobal, sumReturn, sumReferenced);
 return 0;
}

void addGlobal (int a, int b)
{
 sumGlobal = a + b;
 return;
}

int addReturn (int a, int b)
{
 int c = a + b;
 return c;
}

void addReferenced (int a, int b, int * c)
{
 *c = a + b;
 return;
}