Re: CWinThread, CFrameWnd and memory leak???

"Ian Semmel" <>
Sun, 10 Feb 2008 19:15:04 +0000
"" <> wrote in message

On Feb 10, 6:25 am, Joseph M. Newcomer <> wrote:

I had missed that the CFrameWnd is created in the secondary thread.

I agree with all the posts: bottom line, don't create windows in

secondary threads,

especially not windows that are part of a critical framework like


Not sure why anyone would want to use MSCOMM for anything; it is so

trivial to write good

serial port code without it that I have no idea why anyone would want

to use it.


On Sat, 09 Feb 2008 10:50:17 -0600, "Doug Harrison [MVP]"

<> wrote:

On Sat, 9 Feb 2008 07:21:14 -0800 (PST),



I guess it's just me being stupid, but I been googling and trying


hours now and can't solve this problem myself.

The Problem is I have created a class (CmyUIthread) derived from
CWinThread, but when my test-app exit the compiler reports a memory
leak from a CFrameWnd object.

My test-app is just a dialog-app with 2 buttons. The first button


create and starts the ui-thread:

       m_p_thread = new CmyUIthread;

And the tread starts perfect :)

The second button stops the thread:

       m_p_thread->PostThreadMessage( WM_QUIT,0,0);

And the thread stops :)

Use PostQuitMessage.

But when i quit my dlg-test-app the compiler reports the memory

I tried adding a third button that:

       delete m_p_thread;

but this results in access violations.

The access violation occurs because you're relying on the auto-delete
behavior of CWinThread, which is a mistake. Go here for more:

CmyUIthread looks like this:



BOOL CmyUIthread::InitInstance()
       // TODO: perform and per-thread initialization here
       CFrameWnd *wnd = new CFrameWnd;
       wnd->Create( NULL, "myUIthread Window");
       wnd->ShowWindow( SW_SHOW);
       m_pMainWnd = wnd;

       return TRUE;

int CmyUIthread::ExitInstance()
       // TODO: perform any per-thread cleanup here
       return CWinThread::ExitInstance();

BEGIN_MESSAGE_MAP(CmyUIthread, CWinThread)

What is wrong??? do I miss some cleanup???

Why are you creating windows in a secondary thread? It's preferable

to keep

all the windows in the primary thread, where issues like this don't


If you are leaking a CFrameWnd object, it is due to the window not
receiving the WM_NCDESTROY message or you overriding it and not

calling the

base version, which does "delete this". You should be able to set a
breakpoint in CFrameWnd::OnNcDestroy and verify it isn't being

called. It

may be that you need to call DestroyWindow on it before exiting your
secondary thread. The tidier way would be to post WM_CLOSE, but it is
possible the frame window will decline to close, so you'd have to


this before using PostQuitMessage.

Joseph M. Newcomer [MVP]
MVP Tips:

Thank you all for the good feedback, I really appreciate it :)

I was wondering if I had to delete the "net CFrameWnd" but after
comparing it to a SDI application I could not find anywhere it was
deleted... guess that CWinApp deletes it???

My intention was to start a UI-thread with CFrameWnd and a CChildView
with no GUI (hidden anyway) and use CMCComm in that thread. The reason
for CMSComm for serial communication was just a habit... maybe it is
not a good way to do serial communication, but I had used it before in
single-thread app with satisfaction..

So maybe I should try something new :) anyway, I am still happy
that I learned a little about UI-thread :)

With kind regards, Ole :)

As mentioned previously, if you just need a window as a vehicle for
posting messages to, you can embed a CWnd-derived class into your thread
class and use that (see the code for CAsyncSocket and the 'Event Sink').

eg I use this in my UI threads for inter-thread communication rather
than using PostThreadMessage (which appears to drop messages sometimes)

// Thread class with embedded window class (outline of)
class CProtocol : public CWinThread

    class CProtocolWnd : public CWnd
    // Construction
        void AttachToProtocol ( CProtocol* pProtocol );

    afx_msg LRESULT OnStartProcessing (WPARAM wParam, LPARAM lParam);

        CProtocol* m_pProtocol;
    CWnd* GetProtocolWnd ();
    CProtocolWnd* m_ppWnd;

and in the .cpp file

// Embedded window class
CProtocol::CProtocolWnd::CProtocolWnd ()
    m_pProtocol = 0;

void CProtocol::CProtocolWnd::AttachToProtocol ( CProtocol* pProtocol )
    m_pProtocol = pProtocol;

BEGIN_MESSAGE_MAP(CProtocol::CProtocolWnd, CWnd)
ON_MESSAGE(TM_START_THREAD, CProtocolWnd::OnStartProcessing ) // user

LRESULT CProtocol::CProtocolWnd::OnStartProcessing (WPARAM wParam,
LPARAM lParam)
    m_pProtocol ->OnStartProcessing ( wParam, lParam ); // whatever
    return 1;

// Protocol Class - I do this in a Create function rather than the
    m_ppWnd = new CProtocolWnd ();
    if ( ! m_ppWnd->CreateEx (0,
                             _T("Protocol Notification Sink"),
                             WS_OVERLAPPED, 0, 0, 0, 0, NULL, NULL))
        ... error

    m_ppWnd -> AttachToProtocol ( this );
    if ( m_ppWnd != 0 )
        m_ppWnd -> DestroyWindow ();
        delete m_ppWnd;
        m_ppWnd = 0;

To communicate with the thread, other threads can use

CWnd* pWnd = pointerToCProtocol -> GetProtocolWnd();

pWnd -> PostMessage ( ... );

etc. It's a bit simpler if all your posting is internal.

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