Find index of a cstring in a cstring

index_of.c

int index_of(char* hay, char* needle, size_t start) {
  size_t len = strlen(hay);
  size_t needle_len = strlen(needle);
  int subfind = 0;
  for (int j = start; j < len; ++j)
  {
    if (hay[j] == needle[0]) {
      subfind = 1;
      for (int k = 1; k < needle_len; ++k)
      {
        if (hay[j + k] != needle[k]) {
          subfind = 0;
          break;
        } else {
          subfind = 1;
        }
      }
      if (subfind) return j;
    }
  }
  return -1;
}