Terminating Threads! [modified]

Caslen

I have an MFC MDI app in which I have included a docking control bar (Cristi Posea - CSizingControlBar - a resizable control bar) The controlbar has a dialog associated with it which, amongst other things, continuously grabs data from a PCI A/D card using a worker thread. I have the following code, first initialise the control bar and associated dialog:- in MainFrm.h:-

class CMainFrame : public CMDIFrameWnd
{
DECLARE_DYNAMIC(CMainFrame)
public:
CMainFrame();

CMainControlBar	m\_MCB;			//the control bar
CDataControls	m\_GrabDataDialog; 	//the dialog attached to the control bar

//other stuff...
.
.
.
}

then in MainFrm.cpp:-

int CMainFrame::EnableControlBar()
{

if (!m\_MCB.Create(\_T("Data Window"), this, 50))
{
	TRACE0("Failed to create Data Window\\n");
	return -1;
}
m\_MCB.SetBarStyle(m\_MCB.GetBarStyle() | CBRS\_TOOLTIPS | CBRS\_FLYBY | CBRS\_SIZE\_DYNAMIC );
m\_MCB.EnableDocking(CBRS\_ALIGN\_ANY);

#ifdef _SCB_REPLACE_MINIFRAME
m_pFloatingFrameClass = RUNTIME_CLASS(CSCBMiniDockFrameWnd);
#endif

DockControlBar(&m\_MCB, AFX\_IDW\_DOCKBAR\_LEFT);

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//~~~~  Attach Controls Dialog  
//~~~~  to  Window if possible
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
if (!m\_DataDialog.Create(IDD\_MAINCONTROLS,&m\_MCB))
{
	TRACE0("Failed to create instant bar child\\n");
	return -1;		// fail to create
}

//show the bar!
m\_DataDialog.ShowWindow(SW\_SHOW);

return 0;

}

Next, set up the thread:-

class CDataControls : public CDialog
{
// Construction
public:
virtual ~CDataControls();
CDataControls(CWnd* pParent = NULL);

CGraphDraw m\_Graph; //the custom control
void DoDataDraw(); //passes data to the custom control
void DoDataGrab(); //grabs data from an A/D converter

BOOL m\_TerminateThreads; 	//flag to terminate the thread
CWinThread \*pDataGrabThread; 	//thread pointer

double	Sig\[1024\];

//other stuff...
.
.
.
};

and:-

BOOL CDataControls::OnInitDialog()
{
CDialog::OnInitDialog();

//Initialise Custom Control
m\_Graph.Initialise();

//Setup Grab Thread
pDataGrabThread = AfxBeginThread(DataGrabThread,this,THREAD\_PRIORITY\_NORMAL, 0, CREATE\_SUSPENDED);		
    if(pDataGrabThread==NULL)
	AfxMessageBox(L"Crap Thread");

pDataGrabThread->m\_bAutoDelete=FALSE;
pDataGrabThread->ResumeThread();


return TRUE;

}

Now the actual thread:-

Cool_Dev

i think its not a safe practice to access the m_TerminateThread variable in main UI thread from another thread, without using any locking mechanisms. Better to use an Event object, which is initially non-signalled, set it to signaled in CDataControls::OnClose(), and wait on thread handle. And in Thread function,

//m_hEvent is the Event handle in CDataControls
while(WAIT_OBJECT_0 != WaitForSingleObject(self->m_hEvent,0))
{
self->DoDataGrab(); //go and grab the data
Sleep(100); //don't hog the CPU
}

or,

while(WAIT_OBJECT_0 != WaitForSingleObjectself->m_hEvent,100))
{
self->DoDataGrab(); //go and grab the data
}

Roger Allen

Actually it is safe in this particular case. As the worker thread only ever reads the state of the variable, but he has to make sure it is reading the changes on each loop, i.e. it must be declared [code]mutable[/code] elase a compiler optimisation step may remove the follow on reads. As long as multiple threads are not read/writing it should be fine.

If you vote me down, my score will only get lower

David Crow

Caslen wrote:

::SendMessage(m_ControlsDialog,WM_CLOSE,0,0); //send a message to close the dialog and thread

You've not shown how m_ControlsDialog was declared. I assume it's safe to send a message to. If it's a modeless dialog, why not just call DestroyWindow()?

Caslen wrote:

while(self->m_TerminateThreads!=0)

This loop is suspect if m_TerminateThreads is not declared as volatile. Otherwise, it may or may not execute, or it may fail to end. Because m_TerminateThreads is not being changed within the loop, the compiler would likely optimize it out. A better way of having the secondary thread end is to have the primary thread signal an event.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Man who follows car will be exhausted." - Confucius

Roger Allen

Note that you should declare m_TerminateThreads as mutable volatile as a compiler optimisation could break your code. These lines

while(self->m_TerminateThreads!=0)
{
self->DoDataGrab(); //go and grab the data

could be interpreted as the compiler as:

if (self->m_TerminateThreads!=0)
{
for (;;)
{
self->DoDataGrab();

If you vote me down, my score will only get lower

Caslen

Cool_Dev wrote:

Better to use an Event object,

Thanks, I tried this - it works ok but I still need the WaitForSingleObject() in the CDataControls::OnClose() (see below) Why is this?

void CDataControls::OnClose()
{
SetEvent(g_Event);
WaitForSingleObject(pDataGrabThread>m_hThread,1000); //give it chance to stop delete pDataGrabThread; //then delete it

CDialog::OnClose();

}

Chris Losinger

because you need to give the thread a chance to see the event and react to it.

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Caslen

I already figured out the 'volatile' thing - I was more concerned about why I had to 'SendMessage' from CMainFrame::OnClose() - why wasn't CDataControls::OnClose() automatically closed when the App is closed?

Caslen

Thanks! Sometimes it's good to know why, not just that it works :)

Luc Pattyn

IMO it needs to be no wider than what can be handled atomically (hence 32-bit) AND declared volatile. Then it does not matter how many readers there are, as long as there is only one writer all is fine. With several writers, it most often needs a lock of some kind. :)

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David Crow

Caslen wrote:

...why wasn't CDataControls::OnClose() automatically closed when the App is closed?

Because it was in the secondary thread, which was not terminating, perhaps? To test this, change your code to:

WaitForSingleObject(pDataGrabThread->m_hThread, INFINITE);

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Man who follows car will be exhausted." - Confucius

Caslen

Ok, that is probably it - but it's correct to use SendMessage() from CMainframe to close it though?

David Crow

Caslen wrote:

...but it's correct to use SendMessage() from CMainframe to close it though?

Is the dialog modeless?

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Man who follows car will be exhausted." - Confucius

Caslen

Yes it is.

David Crow

Then just call DestroyWindow().

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Man who follows car will be exhausted." - Confucius