N to the power K using optimized tail recursion

main.c

#include <stdio.h>
#include <stdlib.h>

double long npowerk(double long n, double long k) {
  if (k == 0) {
    return 1;
  }

  double long total = n;

start:
  if (k <= 1) {
    return total;
  }

  total *= n;
  k--;
  goto start;
}

int main(int argc, char ** argv) {
  if (argc != 3) {
    printf("usage: $ npowerk <int n> <int k>\n");
    return 0;
  }

  double long a = atol(argv[1]);
  double long b = atol(argv[2]);
  printf("%Lf^%Lf = %Lf\n",
    a, b, npowerk(a,b));
  return 0;
}