Fib in c

fib.c

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

int main(int argc, char *argv[])
{
  unsigned long int fib(unsigned long int x);
  char *ptr;
  unsigned long int input = strtoul(argv[1], &ptr, 10);
  unsigned long int result = fib(input);
  printf("result: %lu\n", result);
}

unsigned long int fib(unsigned long int x)
{
  if (x <= 1) {
    return 1;
  }

  return fib(x - 1) + fib(x - 2);
}