I thought it was pretty well known that everything related to strings in C stdlib (including all str... functions) is bad. You just need to bring in your own string library.

Cant you just:

  for(int i = 0; i < len(characters); i++)
  {
    if(characters[i]-48 <= 9 && characters[i]-48 >= 0)
    {
      ret = ret \* 10 + characters[i] - 48;
    }
    else
    {
      return ERROR;
    }
  }
  return ret;

Adjust until it actually works, but you get the picture.