MFC UI Threads, OnIdle, and Temp Map Headaches

"Jay Daniel" <>
12 May 2006 06:18:36 -0700
  I'm hoping someone can shed some light on some difficulties I've been
having with an MFC application I'm writing. I think I've solved my
problem, but I'm wary since it's one of those "I'm not sure why I have
to do this" solutions that generally mean one doesn't fully understand
the problem.

  My application is written using VS2005 and MFC 8.0 and can be
compiled either for the desktop our (our ultimate target) WinCE 5.0.
In addition to the "main" application thread (which is just responsible
for starting/ending threads in the application), we have several other
CWinThread-derived threads--one of which is responsible for creating
the UI for the application. I started wondering where I'd gone wrong
when I noticed that over time as our application ran, it would slow
down until it eventually hung (although it would often recover if given
about 5 minutes to get its whits about it). Eventually, I came up with
the theory that it was somehow my GDI calls that were slowing things
down. It seems I was pretty close, but it's actually the temporary
objects being created with a smattering of whatever::FromHandle() calls
that were growing the threads tempMap to huge sizes.

  The upshot of all this is that I noticed that the section of
CWinThread::OnIdle that would be responsible for deleting the temporary
objects was never being called. That noticed, I "solved" the problem
by adding a call to OnIdle(1) every few minutes in the application.
This is just a temporay kludge to prove my point, but I'd like to know
how I should implement a proper solution. Taking a look at MFC's code,
however, I wonder if there's a bug. Let me explain.

  Here's what CWinThread::Run() looks like:

    for (;;)
        // phase1: check to see if we can do idle work
        while (bIdle &&
            !::PeekMessage(&(pState->m_msgCur), NULL, NULL, NULL, PM_NOREMOVE))
            // call OnIdle while in bIdle state
            if (!OnIdle(lIdleCount++))
                bIdle = FALSE; // assume "no idle" state

        // phase2: pump messages while available
            // pump message, but quit on WM_QUIT
            if (!PumpMessage())
                return ExitInstance();

            // reset "no idle" state after pumping "normal" message
            //if (IsIdleMessage(&m_msgCur))
            if (IsIdleMessage(&(pState->m_msgCur)))
                bIdle = TRUE;
                lIdleCount = 0;

        } while (::PeekMessage(&(pState->m_msgCur), NULL, NULL, NULL,

  So if I'm understanding things correctly, it'll call OnIdle(0) to
start, then increment lIdleCount to 1. Then, if CWinThread::OnIdle had
indicated there was more to be done (and this is the stick point),
it'll call it again with OnIdle(1). HOWEVER, taking a look at
CWinThread::OnIdle() I see this:

    if (lCount <= 0)
        /* Send WM_IDLEUPDATECMDUI to pMainWnd and all it's children as well
as all registered Frame Windows */
    else if (lCount >= 0)
        if (pState->m_nTempMapLock == 0)
            // free temp maps, OLE DLLs, etc.

    return lCount < 0; // nothing more to do if lCount >= 0

SO... CWinThread::OnIdle(0) will ALWAYS return FALSE to indicate that
it has nothing more to do, despite the fact that the second portion
(which cleans up the TempMaps) has never been executed. Is this a bug?
 I ask only because the construct is:

if (lCount <= 0)
else if (lCount >=0)

which doesn't make any sense. Logically, they could just have "else
(lCount > 0)" since the mere presence of "else" and the preceding block
will preclude hitting that section if lCount == 0. Another possibility
for this being a bug would have been if they had intended to NOT have
the else at all (thus an lCount < 0 executes block 1, lCount == 0
executes both, and lCount > 0 only executes block 2). I've digressed a
bit, but I do wonder whether this is a mistake in MFC.

Moving on, I can see that CWinApp rectifies this situation by
overriding OnIdle as follows:

    if (lCount <= 0)

        // call doc-template idle hook
        POSITION pos = NULL;
        if (m_pDocManager != NULL)
            pos = m_pDocManager->GetFirstDocTemplatePosition();

        while (pos != NULL)
            CDocTemplate* pTemplate = m_pDocManager->GetNextDocTemplate(pos);
            ASSERT_KINDOF(CDocTemplate, pTemplate);
    else if (lCount == 1)
    return lCount < 1; // more to do if lCount < 1

So, CWinApp's version calls OnIdle for any registered Document
templates, and then says "I have more to do" so that it can call
CWinThread::OnIdle(1) and clean up the temp maps. I'm not sure what
this discovery tells me about how MFC is supposed to be used, but maybe
someone could clarify for me. The following are the options I'm able
to come up with:

1. Applications should always use the main CWinApp-derived thread as
the one that creates any UI elements.

2. CWinThread-derived UI threads that create windows objects are
responsible for making sure the tempMaps get cleaned up on their own.

Could someone tell me if I've completely missed the mark or if any of
this makes sense?

Best Regards,

Jay Daniel

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