用ACE的Reactor模式实现网络通讯时，ACE内部用 WSAEventSelect 函数把网络事件与一个事件对象关联起来，目的是为了后面用WaitForMultipleObjects函数统一处理。

下面是ACE 的 ACE_WFMO_Reactor::register_handler_i 函数的代码，里面调用了WSAEventSelect 函数。

int
ACE_WFMO_Reactor::register_handler_i (ACE_HANDLE event_handle,ACE_HANDLE io_handle,ACE_Event_Handler *event_handler,ACE_Reactor_Mask new_masks)
{
  // If this is a Winsock 1 system,the underlying event assignment will
  // not work,so don't try. Winsock 1 must use ACE_Select_Reactor for
  // reacting to socket activity.

#if !defined (ACE_HAS_WINSOCK2) || (ACE_HAS_WINSOCK2 == 0)

  ACE_UNUSED_ARG (event_handle);
  ACE_UNUSED_ARG (io_handle);
  ACE_UNUSED_ARG (event_handler);
  ACE_UNUSED_ARG (new_masks);
  ACE_NOTSUP_RETURN (-1);

#else

  // Make sure that the <handle> is valid
  if (io_handle == ACE_INVALID_HANDLE)
    io_handle = event_handler->get_handle ();

  if (this->handler_rep_.invalid_handle (io_handle))
    {
      errno = ERROR_INVALID_HANDLE;
      return -1;
    }

  long new_network_events = 0;
  bool delete_event = false;
  auto_ptr <ACE_Auto_Event> event;

  // Look up the repository to see if the <event_handler> is already
  // there.
  ACE_Reactor_Mask old_masks;
  int found = this->handler_rep_.modify_network_events_i (io_handle,new_masks,old_masks,new_network_events,event_handle,delete_event,ACE_Reactor::ADD_MASK);

  // Check to see if the user passed us a valid event; If not then we
  // need to create one
  if (event_handle == ACE_INVALID_HANDLE)
    {
      // Note: don't change this since some C++ compilers have
      // <auto_ptr>s that don't work properly...
      auto_ptr<ACE_Auto_Event> tmp (new ACE_Auto_Event);
      event = tmp;
      event_handle = event->handle ();
      delete_event = true;
    }

  int result = ::WSAEventSelect ((SOCKET) io_handle,new_network_events);
  // If we had found the <Event_Handler> there is nothing more to do
  if (found)
    return result;
  else if (result != SOCKET_ERROR &&
           this->handler_rep_.bind_i (1,event_handler,io_handle,delete_event) != -1)
    {
      // The <event_handler> was not found in the repository,add to
      // the repository.
      if (delete_event)
        {
          // Clear out the handle in the ACE_Auto_Event so that when
          // it is destroyed,the handle isn't closed out from under
          // the reactor. After setting it,running down the event
          // (via auto_ptr<> event,above) at function return will
          // cause an error because it'll try to close an invalid handle.
          // To avoid that smashing the errno value,save the errno
          // here,explicitly remove the event so the dtor won't do it
          // again,then restore errno.
          ACE_Errno_Guard guard (errno);
          event->handle (ACE_INVALID_HANDLE);
          event->remove ();
        }
      return 0;
    }
  else
    return -1;

#endif /* ACE_HAS_WINSOCK2 || ACE_HAS_WINSOCK2 == 0 */

}