Changing text in Statusbar pane from timer event

Hi,

I have a timer (WM_TIMER) which triggers the changing of text in a statusbar pane at regular intervals (eg 1 second). I'm noticing that the application crashes intermittently, not when setting the text, but when retrieving the text while the statusbar is being redrawn. Sometimes the crash occurs minutes after starting the app, sometimes much longer.

So far, I'm only noticing this issue in the debug build.

After the crash, the debugger leaves me at the following line in xtpstatusbar.cpp, and the variable m_strText always contains the correct current text that's displayed in the pane:

CString CXTPStatusBarPane::GetText() const
{
return m_strText;
}


Below is the stack trace which leads up to the crash:

     mfc71d.dll!ATL::CSimpleStringT<char,1>::CloneData(ATL::CStringData * pData=0x022a3c60)  Line 804 + 0xa     C++

      mfc71d.dll!ATL::CSimpleStringT<char,1>::CSimpleStringT<char,1>(const ATL::CSimpleStringT<char,0> & strSrc={...})  Line 220 + 0x9     C++

      mfc71d.dll!ATL::CStringT<char,StrTraitMFC_DLL<char,ATL::ChTraitsCRT<char> > >::CStringT<char,StrTraitMFC_DLL<char,ATL::ChTraitsCRT<char> > >(const ATL::CStringT<char,StrTraitMFC_DLL<char,ATL::ChTraitsCRT<char> > > & strSrc={...})  Line 815     C++

>     MyApp.exe!CXTPStatusBarPane::GetText()  Line 184 + 0x12     C++

      MyApp.exe!CXTPPaintManager::DrawStatusBarPaneEntry(CDC * pDC=0x0012f924, CRect rcItem={...}, CXTPStatusBarPane * pPane=0x0388cd98)  Line 2102 + 0xc     C++

      MyApp.exe!CXTPStatusBarPane::OnDraw(CDC * pDC=0x0012f924, CRect rcItem={...})  Line 266 + 0x34     C++

      MyApp.exe!CXTPStatusBar::DrawPaneEntry(CDC * pDC=0x0012f924, int nIndex=1, CRect rcItem={...})  Line 1513 + 0x2d     C++

      MyApp.exe!CXTPStatusBar::OnDraw(CDC * pDC=0x0012f924)  Line 1579 + 0x3d     C++

      MyApp.exe!CXTPStatusBar::OnPaint()  Line 1523     C++

      mfc71d.dll!CWnd::OnWndMsg(unsigned int message=15, unsigned int wParam=0, long lParam=0, long * pResult=0x0012fbf0)  Line 2023     C++

      MyApp.exe!CXTPStatusBar::OnWndMsg(unsigned int message=15, unsigned int wParam=0, long lParam=0, long * pResult=0x0012fbf0)  Line 1849     C++

      mfc71d.dll!CWnd::WindowProc(unsigned int message=15, unsigned int wParam=0, long lParam=0)  Line 1745 + 0x1e     C++

      mfc71d.dll!CControlBar::WindowProc(unsigned int nMsg=15, unsigned int wParam=0, long lParam=0)  Line 503 + 0x14     C++

      mfc71d.dll!AfxCallWndProc(CWnd * pWnd=0x02237240, HWND__ * hWnd=0x003812f8, unsigned int nMsg=15, unsigned int wParam=0, long lParam=0)  Line 241 + 0x1a     C++

      mfc71d.dll!AfxWndProc(HWND__ * hWnd=0x003812f8, unsigned int nMsg=15, unsigned int wParam=0, long lParam=0)  Line 389     C++

      mfc71d.dll!AfxWndProcBase(HWND__ * hWnd=0x003812f8

I'm wondering if this could be a threading issue, and if there's any workaround to the problem.

Thanks!

No, I'm running xp.
So far, I'm only seeing the issue in the debug build.
Also, it mainly happens when I'm working in another application. When the application containing the status bar that get's updated has focus, the crash doesn't seem to occur. It could be that I just haven't witnessed it occurring when it has the focus, though.

I use...

CXTPStatusBarPane* pPane = pStatusBar->FindPane(myPane);
assert(pPane);
pPane->SetText(myText.c_str());


.. where myText is a std::string.

Thanks for the suggestion and info, Phillip. The timer I'm using can be used either synchronously or asynchronously, and I've been using it asynchronously. I just changed it to work synchronously (with the main UI thread) and the crash is still occuring.

I think you may be correct about the issue being related to the threads though. If the pane is redrawn in a separate thread (is this true, Oleg?), it may be trying to redraw the pane at the same time I'm updating the text in that pane.   What is puzzling is that the issue only seems to be occuring in debug mode. I haven't had a crash (yet) in the release build.

If WM_TIMER (and the call to SetText) are in a different thread than the one that paints the status pane (and calls GetText), I think you have a race condition in CString handling. Debug code takes more cycles so is sometimes more likely to cause the issue. I guess it could also include non-thread-safe code not in the release compile, but that would be weird. (SetTimer() is never asynchronous; supplying a routine simply automates calling the TimerProc from the default window proc, rather than having to handle WM_TIMER.)

Your stack trace above is from the drawing thread; check your other threads' stacks for calls to CXTPStatusBarPane methods. Also, double-click on the GetText() line and examine this->m_strText.

Also, what exception is being reported?


Another idea: change your timer to 10 to 50 ms and see if you can get it to crash quicker, and maybe in release mode. I assume you have at least a dual-core system; single-core systems hide (or make much rarer) a lot of multithreading issues.

Sorry for going on about threads, I thought it was the simple SetTimer since you were only using for updating.

APCs can cause reentrancy problems that are very similar to thread issues. Glad you got it sorted out!