Greatest Common Factor

/***************************************************************************************************
*  cfact
*
*  Author: Stefan Willmert
*
*  Calculates the greatest common factor between two integers
*
*  Build: gcc -o cfact cfact.c
*
*
***************************************************************************************************/
#include <stdio.h>
#include <stdlib.h>

int gcd(long, long);
void Usage();

int main(int argc, char *argv[])
{
    long input = 0;
    long input2 = 0;

    if (argc != 3)
    {
        Usage();
        return 0;
    }

    input = atol(argv[1]);
    input2 = atol(argv[2]);

    if (input > input2)
        printf("%d\n", gcd(input, input2));
    else 
        printf("%d\n", gcd (input2, input));

    return 0;
}

void Usage()
{
    printf("\n");
    printf("Returns the greatest common factor between two integer numbers\n\n");
    printf("Usage: cfact number1 number2\n\n");
    printf("    number1       first number to get a factor\n");
    printf("    number2       second number to get a factor\n");
    printf("\n");
}

int gcd(long x, long y)
{
    return y ? gcd(y, x % y) : x;
}