Re: Multithreaded server : Problem with WSAEventSelect

From:

Ulrich Eckhardt <eckhardt@satorlaser.com>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 06 Mar 2007 17:20:21 +0100

Message-ID:

<6vg0c4-308.ln1@satorlaser.homedns.org>

NaeiKinDus wrote:

if (WSAEventSelect(sSock, sEvent, FD_ACCEPT) == SOCKET_ERROR)
{
cerr << "WSAEventSelect did not work for the following reason:
errcode" << WSAGetLastError() << endl;
closesocket(sSock);
WSACleanup();
WSACloseEvent(sEvent);
return INIT_FAILURE;
}

Okay, I'd use exceptions for error handling but this code is correct.

while (TRUE)
{
   hEvent = WSAWaitForMultipleEvents(1, &sEvent, FALSE, WSA_INFINITE,
              FALSE);
   switch (hEvent)
   {
     // ... Creates the thread, then sends it the client's socket
   }
}

There is some overkill here, which might stem from the simplifications for
posting here: if you are waiting infinitely for a client to connect, you
could as well call WSAAccept() directly. Using an array of events only
makes sense when you e.g. wait for a new client or for a 'terminate' event.

Anyway, the thing that might cause problems is a) the call to accept and b)
the way how you forward the socket handle to the other thread. Since you
are using window messages (seen below), you can't do so in a 'normal',
type-safe way and casting things back and forth has always been cause of
trouble.

  if (PeekMessage(&msg, NULL, WM_USER, WM_USER, PM_REMOVE))
    if (msg.message == WM_USER)
    {
      clients[TotalEvent] = (SOCKET)msg.lParam;
      //if (WSAEventSelect(clients[TotalEvent], EventArray[TotalEvent],
      // 0) == SOCKET_ERROR) // Tried this from MSDN... Supposed to
      // reset the parameters used by a previous call to
      // WSAEventSelect()
      //{
      // if (WSAGetLastError() == WSAENOTSOCK)
      // cerr << "Not a socket !";
      //}
      if (WSAEventSelect(clients[TotalEvent], EventArray[TotalEvent],
          FD_READ|FD_WRITE) == SOCKET_ERROR)//Ka-boom! Here it blows up
      {
        cerr << "In Thread /*/ Socket Error ! Errcode: " <<
        WSAGetLastError() << endl;
      }
      EventArray[TotalEvent] = WSACreateEvent();
      TotalEvent++;
    }

Ahem, aren't you mixing creating the event and associating it with some
state of the socket handle? Also, there is things like structures to bundle
an event with a socket handle and there are containers like std::vector to
hold them. There are constructors to zero members so you can clearly
distinguish them from valid ones. Doing the same manually is nonsense and
error-prone.

Uli

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