Re: Thread Communication in between UI and working thread

Dan Bloomquist <>
Wed, 19 Mar 2008 20:12:57 GMT
asm23 wrote:

Dan Bloomquist wrote:

Richard Gran wrote:

You can pass a pointer to any object (eg. struct, class) in the
Post/SendMessage but like what you did with your CString object, it
needs to be allocated in the heap.

And by either metaphor or critical section, 'they' need to know one is
done with it before moving on.

That is where the fun starts...

Thanks to the reply, I understand a half.
do you mean that this need some critical section when I want do send
some object from working thread to UI? "It's where the fun starts..."
What do you mean? Thanks.

I put a thread in a sample dialog to see if I could break it.

Thread object:
struct TS
    CString strT;
    CDialog* pDlg;
    bool bTReady;

class CA_Dialog_AppDlg : public CDialog
    TS ts;

I'm sharing a CString that is on the dialog stack. It should have been
on the heap.

UINT Thread( LPVOID pParam )
    TS& strct= *static_cast<TS*>( pParam );

    for( long i= 0; ; ++i )
        //if( strct.bTReady )
            strct.strT.Format( _T("Test This %d"), i );
            ::PostMessage( *strct.pDlg, ID_CHECK_THREAD, 0, 0 );
            strct.bTReady= false;
        //Sleep( 5 );
        //for( long j= 0; j < 20000; ++j )
    return false;

In init dialog:
    ts.pDlg= this;
    ts.bTReady= true;
    pThread= AfxBeginThread( Thread, &ts, THREAD_PRIORITY_LOWEST, 0,
    pThread->m_bAutoDelete= FALSE;
    pThread->ResumeThread( );

and member:
LRESULT CA_Dialog_AppDlg::OnCheckThread( WPARAM, LPARAM )
    SetWindowText( ts.strT );
    ts.bTReady= true;
    return false;

With the 'if( strct.bTReady )' in the thread commented the dialog is
swamped with messages and is dead. But it would not crash and the title
bar updates! I figured that I would catch CString with an invalid
pointer and I may have if I'd kept going. But that is the point,
bTReady is used to make sure the thread is not changing data while the
app is using it. It also makes sure the thread doesn't post more
messages than the app can handle.

I also didn't stop the thread when the app quit, I just deleted it. But
to do it right it takes another semaphore that absolutely tells the
thread to exit. That means in every:

while( ! strct.bTReady && ! strct.bKill )
    ::Sleep( 5 );

In the exit:

ts.bKill= true;
WaitForSingleObject( pThread->m_hThread, INFINITE );
delete pThread;

Best, Dan.
(I think I got most of this right :)

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