Doc/View : have problem in passing a Doc pointer to thread funcion.

Question

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Hi, I've got a problem in passing a Doc pointer to my thread function.It worked like this:

C++

CMegaDoc::OnStartProcess()
{
  AfxBeginThread(ThreadProcess,(LPVOID)this);
}
 CMegaDoc::OnOpenDocument(filename)
{ 
  ReadFile(filename);// I'm quite sure there is no problem in this function.
}
ThreadProcess(LPVOID pParam)
{
  CMegaDoc *pDoc = (CMegaDoc *)pParam;
  pDoc->OnOpenDocument(filename);//it's hihgly probable that something goes wrong when calling this method.
  do someting...
  pDoc->UpDateAllViews(NULL);
}

C++

CMegaView::DrawData(CDC* pDC)
{
  CMegaDoc pDoc = GetDocument();
  int nCount = pDoc->m_CenterCoord.GetSize();
  if(nCount>=2)
 {
     for(int i=0;i<nCount;i++)
     {
        int x1 = pDoc->m_CenterCoord.GetAt(i).x;
        int y1 = pDoc->m_CenterCoord.GetAt(i).y;
        int x2 = pDoc->m_CenterCoord.GetAt(i+1).x;
        int y2 = pDoc->m_CenterCoord.GetAt(i+1).x;
        pDC->MoveTo(x1,y1);
        pDC->LineTo(x2,y2);

      }
 }
  
}

CMegaView::OnDraw(CDC*pDC)
{
   DrawData(CDC* pDC);
}

The problem here is that when running Release version,it works all right,while the Debug version encounters a Debug Assertion Failure.Debugger says there is a access violation,but I can't see why and how to solve it.

Posted 2-Mar-13 19:08pm

louisejackie

Updated 3-Mar-13 0:02am

v2

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Jochen Arndt · Accepted Answer · 2013-03-02T22:18:00

Solution 1

See Multithreading: Programming Tips[^] in the MSDN.

You did not tell us about your CMegaDoc class. If it is derived from CDocument or contains window handles, and calling member functions from the thread that access window handles, there will be assertions:

Access to a window from a thread that did not create the window is not supported by MFC. The debug assertions occur probably in CWnd::AssertValid(). See the source code in wincore.cpp and the comments about multithreaded applications.

If this happens with your application, you must change the design.

Posted 2-Mar-13 22:18pm

Jochen Arndt

Comments

louisejackie 3-Mar-13 4:48am

Hi Jochen,thanks for your attention!The CMegaDoc is derived from CDocument. This is a SDI application. My intention is to open a series of bmp files and do some processing at the same time.The processing result is a sery of circle's center coordinate,which I reserve in a CArray<cpoint,cpoint>ptCenterArray, so that I could draw a chart in CMegaView's member function OnDraw().The problem is it didn't draw anything. How come?

Jochen Arndt 3-Mar-13 5:29am

This can't be done this way because MFC classes are not thread safe.

I suggest to start without using threads to ensure that your code works. Then you can move the processing code to worker threads. With images, create a copy of them in memory and pass this to the thread for processing. When processing is finished, signal this to your main thread (e.g. by posting a user defined message to your applications main window). When using a CArray to pass the processing result, you must ensure that memory is only allocated by the thread that owns the array. So you might add an array to your worker thread class and provide a function to read or copy the content when processing has finished.

louisejackie 3-Mar-13 5:43am

If I don't start this way ,the application will behave like dead when running. I can't even click on other menu items!

Jochen Arndt 3-Mar-13 5:59am

While the image is processed, your applications main thread is of course blocked. My suggestion to use a non-threaded version was to test your image processing code only (use simple images to reduce the processing time). Because it is easier to find problems this way rather than inside a worker thread. The final application must use a thread if processing needs some time.

Read the link from my answer and follow the contained links. Then think about how to implement the worker thread for your processing (especially how to pass data between the threads).

louisejackie 3-Mar-13 6:11am

You have it right. I tried it in a member function of CMegaDoc, and there is no problem.But there is still no effect when drawing the data.

Jochen Arndt 3-Mar-13 6:20am

That is probably another problem. Therefore I suggested to test the code before using a thread. You may post a new question regarding the draw problem showing relevant code snippets from your OnDraw() (or try first to find the problem yourself).

Jochen Arndt 3-Mar-13 7:04am

I missed that you updated your question.

The code looks fine and something should be drawn when the coordinates are not identical. Maybe drawing is not triggered. Did you call InvalidateRect() from CMegaView::OnUpdate()?

louisejackie 3-Mar-13 7:10am

Yep, I tried that,but nothing is drawn. As you've pointed out, MFC is not thread safe,and I can't call a member funcion with a work thread function.If so,I didn't see any other ways of solving the problem of calling a member function using a thread function.In my case, I need to refresh data in my thread function so that the data can be dranw in time.Can you enlighten me on how you solve similar problem?

Jochen Arndt 3-Mar-13 7:36am

When the thread has finished processing, it can post a user defined message to the main frame window of your application (set the CWinApp member m_pMainWnd to the main frame window in InitInstance()) and pass m_pMainWnd->m_hWnd to the thread class which stores it in a member variable (e.g. m_MainHWnd):

// Thread class
#define MEGA_UPDATE_VIEW (WM_APP + 1)

// When processing has been finished
::PostMessage(m_MainHWnd, MEGA_UPDATE_VIEW , 0, 0);

Main frame class:
ON_MESSAGE(MEGA_UPDATE_VIEW, OnUpdateView)

LRESULT CMainFrame::OnUpdateView(WPARAM wParam, LPARAM lParam)
{
// get ptr to document class
// function must be implemented in your app class
CMegaDoc *pDoc = theApp.GetDoc();
// copy worker thread results into CArray
m_pThread->CopyResult(&pDoc->m_CenterCoord);
// trigger update of view
pDoc->UpdateAllViews(NULL);
return 0;
}
You may use the WPARAM and LPARAM parameters to pass additional information.

louisejackie 11-Mar-13 5:54am

Hi Jochen,I had read some articles on multithread programming. But still I have problem in solving my case.The ThreadProcess(LPVOID pParam) uses Doc's member function to refresh processed data, so I have to get a Doc pointer to call it.If so,how can I get the handle to View?

Jochen Arndt 11-Mar-13 10:40am

Use CDocument::UpdateAllViews() as also suggested by nv3. This will call CView::OnUpdate() of all views (except the one passed as first parameter if not NULL). This is the recommended method that does not require to get the view pointers from your document.

louisejackie 12-Mar-13 0:58am

I tried CDocument::UpdateAllViews() in my thread function.And that's why my program cannot run under Debug version.Thread function is discouraged in updating views?

Jochen Arndt 12-Mar-13 5:08am

CDocument is a MFC class! You can't call member functions from another thread. You must use something like I have posted in my above comment: The thread sends a user defined message to your main frame class and there you can call UpdateAllViews().

Once again: MFC classes are not thread safe. So you can't pass MFC classes such as CDocument to another thread and call member functions.

nv3 3-Mar-13 6:51am

Pretty good diagnosis and hints, Jochen.

Jochen Arndt 3-Mar-13 6:56am

Thank you.

louisejackie 3-Mar-13 7:15am

Hi nv3! It's nice to see you here. The problem here is how to make Doc data interact with View. Data in doc updated,view displays them immediately.

nv3 3-Mar-13 8:34am

That's what UpdateAllViews is for. Call it from your document when some data has changed and all views that show this document will be notified by calling their OnUpdate function. And read the documentation on UpdateAllViews. It contains some valuable hints on how to use it correctly.