Joel Coding Problem atoi

#include <stdio.h>

#define uint unsigned int

int atoi(const char* buf) {
  if (buf == NULL) {
    return -1;
  }

  int ans = 0;
  uint i = 0;
  while (buf[i] != 0x0) {
    int val = buf[i] - '0';
    if (val < 0 || val > 9) {
      return -1;
    }

    ans = ans * 10;
    ans += val;
    i++;
  }

  return ans;
}

int main() {
  printf("%d\n", atoi("130"));
  printf("%d\n", atoi("1a0"));
  return 0;
}