char *urlencode (char *str) {
  char *ret, *ptr;
  int len = strlen (str), i;

  if ((ret = calloc (3 * len + 1, sizeof (char *))) == NULL)
    return NULL;
  memset (ret, '\0', len + 1);
  for (i = 0, ptr = ret; i < len; i++) {
    sprintf ((char *) ptr, "%%%x", str[i]);
    ptr += 3;
  }
  return ret;
}