MsgWaitForMultipleObjects and the queue state

Raymond Chen

One danger of the MsgWaitForMultipleObjects function is calling it when there are already messages waiting to be processed, because MsgWaitForMultipleObjects returns only when there is a new event in the queue.

In other words, consider the following scenario:

• PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE) returns TRUE indicating that there is a message.
• Instead of processing the message, you ignore it and call MsgWaitForMultipleObjects.

This wait will not return immediately, even though there is a message in the queue. That’s because the call to PeekMessage told you that a message was ready, and you willfully ignored it. The MsgWaitForMultipleObjects message tells you only when there are new messages; any message that you already knew about doesn’t count.

A common variation on this is the following:

• MsgWaitForMultipleObjects returns that there is a message.
• You call PeekMessage(&msg, NULL, 0, 0, PM_REMOVE) and process that message.
• You call MsgWaitForMultipleObjects to wait for more messages.

If it so happens that there were two messages in your queue, the MsgWaitForMultipleObjects does not return immediately, because there are no new messages; there is an old message you willfully ignored, however.

When MsgWaitForMultipleObjects tells you that there is a message in your message queue, you have to process all of the messages until PeekMessage returns FALSE, indicating that there are no more messages.

Note, however, that this sequence is not a problem:

• PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE) returns FALSE indicating that there is no message.
• A message is posted into your queue.
• You call MsgWaitForMultipleObjects and include the QS_ALLPOSTMESSAGE flag.

This wait does return immediately, because the incoming posted message sets the “There is a new message in the queue that nobody knows about” flag, which QS_ALLPOSTMESSAGE matches and therefore causes MsgWaitForMultipleObjects to return immediately.

The MsgWaitForMultipleObjectsEx function lets you pass the MWMO_INPUTAVAILABLE flag to indicate that it should check for previously-ignored input.

Armed with this knowledge, explain why the observed behavior with the following code is “Sometimes my program gets stuck and reports one fewer record than it should. I have to jiggle the mouse to get the value to update. After a while longer, it falls two behind, then three…”

// Assume that there is a worker thread that processes records and
// posts a WM_NEWRECORD message for each new record.
BOOL WaitForNRecords(HANDLE h, UINT cRecordsExpected)
{
  MSG msg;
  UINT cRecords = 0;
  while (true) {
    switch (MsgWaitForMultipleObjects(1, &h,
                         FALSE, INFINITE, QS_ALLINPUT)) {
    case WAIT_OBJECT_0:
      DoSomethingWith(h); // event has been signalled
      break;
    case WAIT_OBJECT_1:
      // we have a message - peek and dispatch it
      if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
      }
      if (SendMessage(hwndNotify, WM_GETRECORDCOUNT,
                      0, 0) >= cRecordsExpected) {
        return TRUE; // we got enough records
      }
      break;
    default:
      return FALSE; // unexpected failure
    }
  }
}

Raymond Chen

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