Recursive Fibonacci algorithm in C

fib.c

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

int fib(int n);

int main(int argc, char **argv) {
  // compiling: gcc fib.c -o fib
  // exec with: ./fib <n>
  int i_num;

  if (argc <= 1) {
    printf("Usage: ./fib <n>\n");
    return 1;
  }

  i_num = atoi(argv[1]);

  printf("O %io. termo de fib eh %i\n", i_num, fib(i_num - 1));
  return 0;
}

int fib(int n) {
  // 0-indexed
  if (n <= 1) return 1;
  return fib(n - 2) + fib(n - 1);
}