Click here to download gcdfun.c.

/******************************************
 This Program gets two positive integers
 from the user and computes their GCD.
 ******************************************/
#include <stdio.h>

int getposint();

int main() {
  int a = getposint();
  int b = getposint();

  // Compute gcd
  while(b != 0) {
    int r = a % b;
    a = b;
    b = r;
  }

  // Write out gcd
  printf("The gcd is %i\n", a);

  return 0;
}

/******************************************
 This function gets a positive integer
 from the user
 ******************************************/
int getposint() {
  int k;

  printf("Enter a positive integer: ");
  fflush(stdout);
  int check = scanf(" %i", &k);

  while(check < 1 || k <= 0) {
    printf("I said *positive*, try again: ");
    fflush(stdout);
    check = scanf(" %i", &k);
  }

  return k;
}