bool check_user ( const uint8_t *uaddr)
{
  if(uaddr == 0)
        return false;
  if(!is_user_vaddr(uaddr))
        return false;
  ASSERT(thread_current()->pagedir != NULL);
if(pagedir_get_page(thread_current()->pagedir, uaddr) == NULL)
{
        return false;
}
  return true;
}