A C program to calculate the greatest common divisor of two numbers using the Euclidean algorithm.

gcd.c

#include <stdio.h>
#include <inttypes.h>

int gcd(int a, int b) {
  return b == 0 ? a : gcd(b, a - (((int) (a / b)) * b));
}

int main(int argc, char const *argv[]) {
  if (argc >= 3) {
    const unsigned short pairs = (argc - 1) / 2;
    int valueA, valueB, a, b, tmp;

    for (int i = 0; i < pairs; i++) {
      a = valueA = (int) strtoimax(argv[i * 2 + 1], NULL, 10);
      b = valueB = (int) strtoimax(argv[i * 2 + 2], NULL, 10);
      while (b != 0) {
        tmp = b;
        b = a - (((int) (a / b)) * b);
        a = tmp;
      }

      printf("gcd(%d, %d) = %d\n", valueA, valueB, a);
      printf("gcd(%d, %d) = %d\n", valueA, valueB, gcd(valueA, valueB));
    }
  }
  return 0;
}