tux35(26)% cat gcd.c
int main(int argc, char *argv[]) {
  int a, b, r;
  int x, y;
  int ax = 1; //a = ax * original_a + ay * original_b
  int bx = 0;
  int ay = 0; //b = bx * original_a + by * original_b
  int by = 1;

  if(argc != 3) {
    printf("usage: gcd a b\n");
    exit(0);
  }

  for(a = atoi(argv[1]), b = atoi(argv[2]); b; r = a % b, a = b, b = r) {
    y = -a / b;

    x = ax + bx * y;
    y = ay + by * y;

    ax = bx;
    ay = by;

    bx = x;
    by = y;
  }

  printf("%i = %i*%i + %i*%i\n", a, ax, atoi(argv[1]), ay, atoi(argv[2]));
}

tux35(27)% ./gcd
usage: gcd a b

tux35(28)% ./gcd 51 69
3 = -4*51 + 3*69

tux35(29)% ./gcd 1479 323
17 = 7*1479 + -32*323

tux35(30)% ./gcd 10946 17711
1 = -6765*10946 + 4181*17711