FIX: CSocket Operation Hangs if a Timer is Active

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, version 4.2

SYMPTOMS

A call to CSocket Connect, Accept, Send, or Receive hangs if a timer is active.

CAUSE

CSocket operations are written using "pseudo-blocking." The WinSock API call is not a true blocking call, but if it doesn't return immediately then CSocket will call PumpMessages. PumpMessages looks for the FD_xxx notification and the CSocket function will not return until the FD_xxx notification arrives.

PumpMessages will not detect the socket notification if a timer is active because it looks for WM_TIMER messages first. When a WM_TIMER message arrives, the function doesn't look any further for other messages.

The PumpMessages function is looking for WM_TIMER messages because it sets up its own timer to cause the application to retry its socket calls and do periodic idle-time processing while waiting for the socket notification.

RESOLUTION

There are two possible resolutions:

1. Disable all timers while making CSocket calls.

2. Override the PumpMessages function so that it only looks for its own WM_TIMER messages and does not pick up other timer messages. This can be done by borrowing the CSocket::PumpMessages function from SOCKCORE.CPP and changing it as follows:

   #if _MFC_VER == 0x0420

      #define _AFX_SOCK_THREAD_STATE AFX_MODULE_THREAD_STATE
      #define WM_SOCKET_NOTIFY    0x0373
      #define WM_SOCKET_DEAD      0x0374
   
      #define _afxSockThreadState AfxGetModuleThreadState()
   
      #endif
   
      BOOL CMySocket::PumpMessages(UINT uStopFlag)
      {
      #if _MFC_VER != 0x0420
        return CSocket::PumpMessages(uStopFlag);
      #else
   
        // The same socket better not be blocking in more than one place.
        ASSERT(m_pbBlocking == NULL);
   
        _AFX_SOCK_THREAD_STATE* pState = _afxSockThreadState;
   
        ASSERT(pState->m_hSocketWindow != NULL);
   
        BOOL bBlocking = TRUE;
        m_pbBlocking = &bBlocking;
        CWinThread* pThread = AfxGetThread();
   
        // This is not a timeout in the WinSock sense, but more
        // like a WM_KICKIDLE to keep message pumping alive
        UINT nTimerID = ::SetTimer(pState->m_hSocketWindow, 0,
                                 m_nTimeOut, NULL);
   
        if (nTimerID == 0)
          AfxThrowResourceException();
   
        BOOL bPeek = TRUE;
   
        while (bBlocking)
        {
          TRY
          {
            MSG msg;
            if (::PeekMessage(&msg, pState->m_hSocketWindow,
              WM_SOCKET_NOTIFY, WM_SOCKET_DEAD, PM_REMOVE))
            {
              if (msg.message == WM_SOCKET_NOTIFY &&
                 (SOCKET)msg.wParam == m_hSocket)
              {
                if (WSAGETSELECTEVENT(msg.lParam) == FD_CLOSE)
                {
                  break;
                }
                if (WSAGETSELECTEVENT(msg.lParam) == uStopFlag)
                {
                  if (uStopFlag == FD_CONNECT)
                    m_nConnectError = WSAGETSELECTERROR(msg.lParam);
                  break;
                }
              }
              if (msg.wParam != 0 || msg.lParam != 0)
                CSocket::AuxQueueAdd(msg.message, msg.wParam, msg.lParam);
   
              bPeek = TRUE;
            }
            else if (::PeekMessage(&msg, pState->m_hSocketWindow,
              WM_TIMER, WM_TIMER, PM_REMOVE))
            {
                break;
            }
   
            if (bPeek && ::PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE))
            {
              if (OnMessagePending())
              {
                // allow user-interface updates
                pThread->OnIdle(-1);
              }
              else
              {
                bPeek = FALSE;
              }
            }
            else
            {
              // no work to do -- allow CPU to sleep
              WaitMessage();
              bPeek = TRUE;
            }
          }
          CATCH_ALL(e)
          {
            TRACE0("Error: caught exception in PumpMessage - continuing.\n");
            bPeek = TRUE;
          }
          END_CATCH_ALL
        }
   
        ::KillTimer(pState->m_hSocketWindow, nTimerID);
   
        if (!bBlocking)
        {
          WSASetLastError(WSAEINTR);
          return FALSE;
        }
        m_pbBlocking = NULL;
   
        ::PostMessage(pState->m_hSocketWindow,WM_SOCKET_NOTIFY,0,0);
   
        return TRUE;
   
      #endif // _MFC_VER
      }
   
   

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been fixed with the Visual C++ 4.2b patch. For more information on this patch, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q156934
   TITLE     : PATCH: Visual C++ 4.2b Patch

   ARTICLE-ID: Q160491
   TITLE     : Information on Files Modified by VC42b Patch: Part 1 of 4

   ARTICLE-ID: Q160496
   TITLE     : Information on Files Modified by VC42b Patch: Part 2 of 4

   ARTICLE-ID: Q160505
   TITLE     : Information on Files Modified by VC42b Patch: Part 3 of 4

   ARTICLE-ID: Q160506
   TITLE     : Information on Files Modified by VC42b Patch: Part 4 of 4