Re: How to current close window while messages still in queue

"Doug Harrison [MVP]" <>
Tue, 04 Nov 2008 14:11:38 -0600
On Tue, 4 Nov 2008 19:51:35 +0200, "vvf" <> wrote:

Hi All,

Here is my scenario:

I have an MFC application. In one of the dialogs, I have a worker thread.

Here's how I start the thread:

 m_pThread = AfxBeginThread((AFX_THREADPROC)LoadThread, this, 0, 0,

 m_pThread->m_bAutoDelete = FALSE;

The thread reads some rows from a database (SELECT * ..), and then iterates
through the result set and sends the data rows one by one to the GUI to
update a GUI control:

 for (int i = 0; i <= iUpperBound && (WaitForSingleObject(m_hQuitEvent, 0)
== WAIT_TIMEOUT); ++i)
         // Construct data, etc...
         CData* pData = new CData; // pseudocode
         PostMessage(UpdateMessage, (WPARAM)pData, ... ) // Update the GUI
and delete pData

After the for loop is over, the thread function just finishes and this
thread ends. The m_hQuitEvent is there so that I can set it if I want to
abort the processing.

Here is how the message handler looks like:

LRESULT CWhateverDialog::OnUpdateMessage(WPARAM wParam, ...)
 CData* pData = (CData*)wParam;
 // update GUI control w/ pData information

 delete pData;
  return 0;

The problem is that when I close the dialog, even though I wait for the
thread to complete (WaitForSingleObject on its handle -
CWinThread::m_hThread), there are still some messages in the message queue
that are not yet processed and so I end up having some memory leaks (because
the handler never gets a chance to be called to de-allocate the objects
allocated and postmessage'ed to the GUI.)

What is the correct way to handle this situation ?

You have to process the pending messages before destroying their target,
namely, the dialog window. So just run a local message loop in OnOK or
OnCancel to drain the message queue of those specific messages.

It sounds like you have the CWinThread lifetime cycle pretty much well in
hand as I described here:

It sounds like it would be nice for the thread to post an "I'm finished"
message to indicate a normal exit, at which point, the dialog would
WaitForSingleObject on the thread handle and delete the CWinThread object.
If you're interested in the full results, until you get this message, you
must keep the dialog alive, hidden if appropriate, and if there are other
things that cannot be done until this processing has completed, you'll have
to account for that, too.

Concerning the early thread shutdown case, it looks like it's designed to
terminate quickly once you set the m_hQuitEvent object, and if so, it's
correct to WFSO on the thread handle after setting the event. You'll still
have to process the pending messages as described above, though.

Doug Harrison
Visual C++ MVP

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