C / C++ / MFC Discussion Boards

Re: Help with threads

Richard MacCutchan4-Sep-12 22:06

Richard MacCutchan

4-Sep-12 22:06

Try some better spacing, and use paragraphs; this is very hard on the eyes.

One of these days I'm going to think of a really clever signature.

Re: Help with threads

David Crow5-Sep-12 3:15

David Crow

5-Sep-12 3:15

pasztorpisti wrote:
The less hassle free solution is sending a message from the worker thread to the ui thread by using SendMessage()...

What happens when the primary (UI) thread is blocked and the worker thread sends it a message via SendMessage()?

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Re: Help with threads

pasztorpisti5-Sep-12 3:31

pasztorpisti

5-Sep-12 3:31

Then the worker thread is waiting until the message is processed on the gui thread. Exploiting this you can for example pop up a messagebox from the worker thread by sending a WM_APP+X message to the gui thread that processes WM_APP+X and "blocks" until the message box is closed. The worker thread continues running only after the message box is closed and the processing of WM_APP+X is finished on the gui thread. I quoted "blocks" because a MessageBox call doesn't really block the gui thread, it just blocks the processing of a single message an it runs an inner messageloop of its own so processing other messages from the queue of the gui thread goes on. For example your windows are still drawn (by WM_PAINT messages) while a messagebox is active.
PS: The ui thread should never be blocked, that causes unresponsive UI. Often thats the reason for multithreading and not gaining performance. If your UI thread is blocked then your program is either buggy or poorly designed.

Re: Help with threads

David Crow5-Sep-12 3:46

David Crow

5-Sep-12 3:46

pasztorpisti wrote:
Then the worker thread is waiting until the message is processed on the gui thread.

I'm not talking about the primary thread being busy for a few nanoseconds. I'm talking about deadlock. For example, the secondary thread sends a "add item to the control" message (e.g., LB_ADDSTRING) to the primary thread, which is blocked waiting on the secondary thread to complete. Now the secondary thread cannot complete because it is no longer running (waiting on SendMessage() to return). This is the primary reason why SendMessage() should rarely, if ever, be used to communicate between primary and secondary threads.

Re: Help with threads

pasztorpisti5-Sep-12 3:55

pasztorpisti

5-Sep-12 3:55

The gui thread should never block, as a consequence it should never wait for the worker thread to complete. When the worker thread completes it can send/post a message to the gui thread to signal. Why would you want a blocking-wait on the gui thread???

Re: Help with threads

Albert Holguin5-Sep-12 12:26

Albert Holguin

5-Sep-12 12:26

SendMessage() is a blocking call... that's what he was pointing out.

Re: Help with threads

Chuck O'Toole5-Sep-12 3:57

Chuck O'Toole

5-Sep-12 3:57

DavidCrow wrote:
For example, the secondary thread sends a "add item to the control" message
(e.g., LB_ADDSTRING) to the primary thread

This is the scenario that's confused me from time to time. It is really legal for a thread to do a SendMessage() to a control (the target of the LB_ADDSTRING) that is owned by another thread? I thought that Windows checked "thread ownership" of the control and returned an error on the SendMessage(). If the "primary thread" is, as is implied in your reply, the UI thread, then the "secondary thread" should not be allowed access to the control.

I've always used "PostMessage()" and used "user defined messages" to have secondary threads pass messages / commands to the primary thread for action. Since "PostMessage()" is just a queueing action, there is no deadlock (although you might not get the immediate feedback of a screen update).

Does anyone have the definitive answer on "SendMessage and Control Owner Thread" question?

Re: Help with threads

Albert Holguin5-Sep-12 12:25

Albert Holguin

5-Sep-12 12:25

That's a good question... I do believe the framework allows you to do it though... although now that you're questioning it, you're making me question whether I remember that correctly.

Re: Help with threads

Chuck O'Toole6-Sep-12 15:46

Chuck O'Toole

6-Sep-12 15:46

You know, some days I don't remember where my head is. Of course you can do it and, in fact, I do it all the time in certain apps. For example, if I have a "DO IT!" button, that usually creates a worker thread that does some stuff and updates a "log view" edit control in the main dialog. The GUI thread continues on and watches for the user clicking on "STOP DOING IT!".

Meanwhile, the worker thread does a

TheDialog->m_editcontrol.SetText(logbuffer);

or something equivalent to update the subclassed edit window. Works just fine. I guess I forget this because the "SendMessage()" is buried in the SetText() function and not something I explicitly do but it is still "SendMessage()" from a thread that did not "own" or "create" the control.

Oh well, must be getting old.

Re: Help with threads

Albert Holguin7-Sep-12 3:54

Albert Holguin

7-Sep-12 3:54

Chuck O'Toole wrote:

Oh well, must be getting old.

Happens...

Yeah, as a rule of thumb though, I like to tell people to PostMessage() when doing things between threads (since they're supposed to be independent anyway). As you probably know, the synchronicity of SendMessage() has the potential for deadlocks so it should only be used when you truly need it.

Re: Help with threads

AndrewG12315-Sep-12 9:55

AndrewG1231

5-Sep-12 9:55

Ok, instead of modifying my original question I will continue the thread since it has been really helpful. So, my program uses the following code to create a new thread.

m_Thread = AfxBeginThread(Acq_Data,this,THREAD_PRIORITY_HIGHEST);

This thread is told to look at the current class and the controlling function is Acq_Data. In Acq_Data there is the following code.

while(dlg->m_Continue && ! dlg->m_inst->m_Flags.Halt)
	{
		dlg->m_inst->ADCdbInquire(dlg->m_halfdata,hdlg,MSG_DRAW_SPECTRUM);
	}

The ADCdbInquire()function contains a call to PostMessage()

bool CPXI::ADCdbInquire(short *halfbuffer, HWND hwnd ,int Message)
{
    short iStatus, iHalfReady, iDAQstopped = 0;
    unsigned long ulPtsTfr=1500; // number of points transferred

    // Are you ready?
    iStatus = DAQ_DB_HalfReady(m_6052_Device, &iHalfReady,
         &iDAQstopped);

    if ((iHalfReady == 1) && (iDAQstopped == 0))
    {
        iStatus = DAQ_DB_Transfer(m_6052_Device, halfbuffer,
        &ulPtsTfr, &iDAQstopped);

        NIDAQErrorHandler(iStatus, "DAQ_DB_Transfer",m_SuppressErrors);

        if(Message != -1)
            PostMessage(hwnd,WM_COMMAND,Message,NULL);

        return true;
    }
    return false;

}

and is sending a custom message defined in the header file.

#define MSG_DRAW_SPECTRUM			(WM_APP + 5)

This custom messages refers to member function OnDrawSpectrum() where the following code exists to update the window.

void CDlg_SpectrumAnalyzer::OnDrawSpectrum()
{

    int starti,finishi;
    starti = m_datacount * m_BuffSize / 2;
    finishi = (m_datacount + 1)* m_BuffSize / 2;

    if(finishi > m_AcqNPts)
        finishi = m_AcqNPts;

    int j = 0;
    for(int i = starti; i < finishi; i++)
    {
        m_fftdata[i].re = m_halfdata[j++];
        m_fftdata[i].im = 0.;
    }



    m_datacount++;

    // if we are done collecting draw the graph
    if((m_datacount == m_NBuffs))
    {

        OnSpectrumStart();
        float freq = 0;
        float stepsize = ((float)(m_MaxFreq)) / ((float)(m_AcqNPts-1)/2.0);
        m_datacount = 0;

        m_Graph->ClearGraph(m_graphNum,false);

        // do fft
        fftw_one(m_the_plan, m_fftdata, NULL);

        //m_AcqNPts should be 2^n therefore /2 should be integer
        for(int i = m_AcqNPts-1; i >= m_AcqNPts/2; i-= 1)
        {
            // calculate power spectrum
            m_fftdata[i].re = sqrt(m_fftdata[i].re * m_fftdata[i].re +
                                   m_fftdata[i].im * m_fftdata[i].im);
            if(m_LogY)
                m_fftdata[i].re = log(m_fftdata[i].re);

            if(m_RtHz)
                m_Graph->AddPoint(m_graphNum,sqrt(freq),m_fftdata[i].re,false);
            else
                m_Graph->AddPoint(m_graphNum,freq,m_fftdata[i].re,false);

            freq += stepsize;
        }


        if(!m_LogY)
            m_Graph->SetAxisProps(_T("Intensity"), _T("I"), 2, GRAPH_Y_AXIS,false);
        else
            m_Graph->SetAxisProps(_T("log Intensity"), _T("log(I)"), 2, GRAPH_Y_AXIS,false);


        if(m_RtHz)
            m_Graph->SetAxisProps(_T("Rt Frequency"), _T("Rt Hz"), 2, GRAPH_X_AXIS, false);
        else
            m_Graph->SetAxisProps(_T("Frequency"), _T("Hz"), 2, GRAPH_X_AXIS, false);

        EnableItems(true);
        m_Graph->UpdateWindows(GRAPH_WUV_ALL);

    }


}

So, my question...in order to get this sequence to run several times, say 5 times, and update the window how would I use PostMessage()? Or if I have misunderstood something please let me know.

Thanks again for all the help.

Re: Help with threads

David Crow6-Sep-12 5:41

David Crow

6-Sep-12 5:41

AndrewG1231 wrote:

So, my question...in order to get this sequence to run several times, say 5 times, and update the window how would I use PostMessage()? Or if I have misunderstood something please let me know.

See here for more.

Re: Help with threads

AndrewG123111-Sep-12 11:33

AndrewG1231

11-Sep-12 11:33

Thanks for the link to the article and I have tried to implement the PostMessage() in my code.

However, I am still having problems trying to get it to execute more than once and update the window (looping the code is not working). After reading your suggested article, I am still not sure how to do this correctly, can you help?

My code is as follows.

void CDlg_SpectrumAnalyzer::OnBtnClickedRun()
{          
PostMessage(WM_COMMAND,IDC_SPECTRUM_START,0);
}

Yes, this is a "Run" button that sends the message and it executes correctly if called only a single time.

Re: Help with threads

David Crow12-Sep-12 2:45

David Crow

12-Sep-12 2:45

AndrewG1231 wrote:
...it executes correctly if called only a single time.

What happens when the handler is called a second time?

Re: Help with threads

AndrewG123112-Sep-12 6:21

AndrewG1231

12-Sep-12 6:21

When it is called the second time I encounter some code for checking the state of the instrument my program is controlling. You can see the whole block of code this belongs to in the first post of this thread.

if(!m_inst->StartOperation())
        {
            MessageBox("The Instrument is Currently in use by another function");
            return;
        }

I thought this indicated that the previous thread wasn't terminating before the message posted again and I tried to use Sleep() to delay the call, but this did not work. I encountered the same situation or the window did not update. I also tried to use the WaitForSingleObject(), but this failed as well.

Re: Help with threads

David Crow12-Sep-12 8:17

David Crow

12-Sep-12 8:17

Look this over. It's from memory and completly untested so there may be a few small details left out, but it should give you good idea of how primary and secondary threads communicate.

BEGIN_MESSAGE_MAP(CMyDialog, CDiaog)
    ON_BN_CLICKED(IDC_BUTTON, OnClick)
END_MESSAGE_MAP()

void CMyDialog::Acq_Data( void )
{
    while (1)
    {
        // if a STOP event has been signaled [m_pEventStopRequested.SetEvent()]
        if (WaitForSingleObject(m_pEventStopRequested->m_hObject, 0U) == WAIT_OBJECT_0)
        {
            // let OnClick() know that the secondary thread is not running
            m_pEventThreadDone->SetEvent();
            
            return;
        }
        else
            ADCdbInquire(...); // this functon can post messages back to the primary thread
    }
}

/* static */ UINT CMyDialog::Acq_Data( LPVOID lpVoid )
{
    CMyDialog *pDlg = (CMyDialog *) lpVoid;

    pDlg->Acq_Data();

    // once here, the secondary thread is done so post a message
    // back to the primary thread to tell it to enable the button

    return 0;
}

// put these in CMyDialog
CWinThread *m_pThread = NULL;
CEvent *m_pEventStopRequested = new CEvent(FALSE, TRUE); // stop requested (initially no)?
CEvent *m_pEventThreadDone = new CEvent(TRUE, TRUE);     // thread done (initially yes)?

void CMyDialog::OnClick() 
{
    // if the secondary thread is not running
    if (WaitForSingleObject(m_pEventThreadDone->m_hObject, 0U) == WAIT_OBJECT_0)
    {
        // at this point, you could disable the button that
        // started this so that it could not be clicked again

        // set the STOP and DONE events to nonsignaled (haven't happened yet)
        m_pEventStopRequested->ResetEvent();
        m_pEventThreadDone->ResetEvent();

        // start the secondary thread
        m_pThread = AfxBeginThread(Acq_Data, this, THREAD_PRIORITY_HIGHEST);
    }
}

modified 13-Sep-12 9:44am.

Re: Help with threads

AndrewG123112-Sep-12 14:21

AndrewG1231

12-Sep-12 14:21

Thanks for the suggestions! However, this piece of code is causing me to get the error, "A nonstatic member reference must be relative to a specific object". I tried to use this-> before m_pEventStopRequested->m_hObject but it did not solve the problem. I have the declaration for the CEvent in the constructor, but I am not sure to what object it is referring.

if (WaitForSingleObject(m_pEventStopRequested->m_hObject, 0U) == WAIT_OBJECT_0)
       {
           // let OnClick() know that the secondary thread is not running
           m_pEventThreadDone.SetEvent();
           return;
       }

Would you happen to know what object it is referring to?

Re: Help with threads

David Crow13-Sep-12 3:47

David Crow

13-Sep-12 3:47

AndrewG1231 wrote:
However, this piece of code is causing me to get the error, "A nonstatic member reference must be relative to a specific object".

What is the exact error number?

AndrewG1231 wrote:
m_pEventThreadDone.SetEvent();

Should be:

m_pEventThreadDone->SetEvent();

That, and adding a return value, is all I had to do to get the code to compile.

Re: Help with threads

AndrewG123113-Sep-12 7:46

AndrewG1231

13-Sep-12 7:46

Here is the exact compiler message of the error.

Dlg_SpectrumAnalyzer.cpp(150): error C2227: left of '->m_hObject' must point to class/struct/union/generic type
1>Dlg_SpectrumAnalyzer.cpp(153): error C2227: left of '->SetEvent' must point to class/struct/union/generic type
1>Dlg_SpectrumAnalyzer.cpp(154): error C2561: 'CDlg_SpectrumAnalyzer::Acq_Data' : function must return a value

However, I implemented you suggestion a bit differently. I did not use void Acq_DAta(void) instead I have incorporated everything in the following member.

UINT CDlg_SpectrumAnalyzer::Acq_Data(LPVOID pParam)
{
	
	CDlg_SpectrumAnalyzer *dlg =(CDlg_SpectrumAnalyzer*) pParam;
	HWND hdlg = dlg->GetSafeHwnd();
	// start data ACQ
	while(1)
	{
	     if (WaitForSingleObject(m_pEventStopRequested->m_hObject, 0U) == WAIT_OBJECT_0)
             {
            // let OnClick() know that the secondary thread is not running
            m_pEventThreadDone->SetEvent();
            return;
	     }
	     else
	     {
			
		dlg->m_inst->ADCdbRead(dlg->m_AcqChan,dlg->m_MaxFreq*2,dlg->m_BuffSize,dlg->m_fulldata,dlg->m_halfdata);
		while(dlg->m_Continue && ! dlg->m_inst->m_Flags.Halt)
			dlg->m_inst->ADCdbInquire(dlg-   >m_halfdata,hdlg,MSG_DRAW_SPECTRUM);

			dlg->m_inst->ADCdbStop();
			//release instrument
			dlg->m_inst->EndOperation();
			
			return 1;
	     }
		 
	}

}

Here is the button control code.

void CDlg_SpectrumAnalyzer::OnBnClickedRun()
{
	// TODO: Add your control notification handler code here
	 if (WaitForSingleObject(m_pEventThreadDone->m_hObject, 0U) == WAIT_OBJECT_0)
    {
        // at this point, you could disable the button that
        // started this so that it could not be clicked again

        // set the STOP and DONE events to nonsignaled (haven't happened yet)
        m_pEventStopRequested->ResetEvent();
        m_pEventThreadDone->ResetEvent();
 
        // start the secondary thread
        m_pThread = AfxBeginThread(Acq_Data, this, THREAD_PRIORITY_HIGHEST);
    }
		
}

Have I done something foolish in the way I was trying to use your suggestion?

modified 13-Sep-12 13:54pm.

Re: Help with threads

David Crow13-Sep-12 7:55

David Crow

13-Sep-12 7:55

AndrewG1231 wrote:
Have I done something foolish in the way I was trying to use your suggestion?

Yes, you are trying to access non-static members from within a static function. Check my code again. Note there are two Acq_Data() functions.

Re: Help with threads

AndrewG123113-Sep-12 8:54

AndrewG1231

13-Sep-12 8:54

So, I saw the two Acq_Data() functions and I ran into the following problem with the use of the dlg pointer and the hdlg entry in the ADCdbInquire(...) member which is of the type HWND.

dlg->m_inst->ADCdbRead(dlg->m_AcqChan,dlg->m_MaxFreq*2,dlg->m_BuffSize,dlg->m_fulldata,dlg->m_halfdata);

	while(dlg->m_Continue && ! dlg->m_inst->m_Flags.Halt)
              dlg->m_inst->ADCdbInquire(dlg->m_halfdata,hdlg,MSG_DRAW_SPECTRUM);

dlg->m_inst->ADCdbStop();
//release instrument
dlg->m_inst->EndOperation();

I tried to combine the call to function and the actual code to enable the process, apparently, this leads to the compiler issue. How can I get the handle? I did try just using HWND hdlg = GetSafeHwnd(); and eliminating all the dlg code which compiled fine but led to the following runtime error.

Unhandled exception at 0x012d2ba9 in WinSTM.exe: 0xC0000005: Access violation reading location 0x000002af.

Your patience has been amazing..usually by now I have received some type of comment about my newbie-ness. Thanks! Big Grin | :-D

Re: Help with threads

David Crow13-Sep-12 9:32

David Crow

13-Sep-12 9:32

AndrewG1231 wrote:
...I ran into the following problem with the use of the dlg pointer...

From whence comes the dlg pointer?

AndrewG1231 wrote:
which compiled fine but led to the following runtime error.
Unhandled exception at 0x012d2ba9 in WinSTM.exe: 0xC0000005: Access violation reading location 0x000002af.

Sounds like a NULL pointer. Use the debugger, set a breakpoint on the call to ADCdbRead(), and check the values of dlg, m_inst, and m_Flags.

Re: Help with threads

AndrewG123127-Sep-12 12:02

AndrewG1231

27-Sep-12 12:02

Hey, it has been a little while, but your suggestions were very helpful in diagnosing my problem. I am currently working on a solution, but wanted to share my thanks. Smile | :)

Re: Help with threads

David Crow28-Sep-12 2:13

David Crow

28-Sep-12 2:13

Glad you're progressing.

Re: Help with threads

Albert Holguin5-Sep-12 12:22

Albert Holguin

5-Sep-12 12:22

Actually, SendMessage is synchronous... so if it's threaded, it really should be a PostMessage, otherwise you'll tie up the worker thread waiting on the GUI update.

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.