Re: A _very_ basic question about WM_TIMER and threading

From:

Norbert Unterberg <nunterberg@newsgroups.nospam>

Newsgroups:

microsoft.public.vc.mfc

Date:

Wed, 03 Sep 2008 16:11:28 +0200

Message-ID:

<#Jv$27cDJHA.4724@TK2MSFTNGP05.phx.gbl>

andrew schrieb:

What makes you believe that your handler is being executed more than once?

--
David Wilkinson
Visual C++ MVP

My OnTimer handler code:

void CMyView::OnTimer(UINT nIDEvent)
{
    static volatile bool bEntered = false;
    ASSERT(!bEntered);
    if (!bEntered) {
       bEntered = true;
       .... // handler code
       bEntered = false;
    }
}

While the app is running in the debugger I get ASSERT() raised occasionally.

There is no need to deal with re-entrancy if your handler does not
contain a message loop. It can be re-entered if you allow your handler
to process messages.

Note that message processing can happen even if you do not intend to:
Opening a dialog or displaying a message box does start message
processing (the windows dialog handler and message box handler contain
internal message pumps).

Another way to stop the handler from being called multiple times is to
stop the timer when you do your (lengthy?) processing in the handler,
and re-start the timer when you leave the handler.

There is no need to make the variable volatile since it is not changed
anywhere except in this single function. And if you have a document/view
application that allows more than one view than you should make bEntered
a non-static member of the CMyView class otherwise it would also block
unrelated views in the same application.

Norbert