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

void collatz(int n) {
   ...
}

int main(int argc, char *argv[]) {
   if (argc != 2) {
      printf("Usage: %s number\n", argv[0]);
      return 1;
   }
   int n = atoi(argv[1]);
   if (n > 0)
      collatz(n);
   return 0;
}