clistctrl threadsafe access

[mwong7025]

I have clist control in my CListView based class. I also defined a function AddMsg to add item to the clist control. This function will be called from many places (threads) in the problem. I wan to have a threadsafe access to the control. I tried to use CCriticalSection for it, but the program will lock up, I think some deadlock is happening. How can I solve it.


CCriticalSection m_CriticalSection;

void CMyView::AddMsg(CString &msg)
{
m_CriticalSection.Lock();

do list control access...
m_CriticalSection.Unlock();
}

[Alin]

how are you accessing this function from the thread(s)? it is not advisible to directly access MFC objects from a thread, you should rather PostMessage() from the thread. In the message handler do whatever you have to. This site has a very good FAQ section on threads, which I recommend checking out.

[Marc G]

[ moved thread ]

[mwong7025]

Thanks for the reply, but if I use PostMessage(), since I will pass the local CString pointer, the pointer will not be valid by the time the message is being pocessed. What should I do?

[Alin]

Thanks for the reply, but if I use PostMessage(), since I will pass the local CString pointer, the pointer will not be valid by the time the message is being pocessed. What should I do?

you can try sommething like this:

in the thread:

CString strToSend = new CString("From thread");

::PostMessage(m_handleToYourClass, WM_YOUR_CUSTOM_MESSAGE, 0, reinterpret_cast<LPARAM>strToSend);

in the message handler:

CYourClass::OnMessage(WPARAM wParam, LPARAM lParam)
{     
	std::auto_ptr <CString> strFromThread(reinterpret_cast <CString *>(lParam));

	// do something with strFromThread
	// additionally there is no need to delete [] anything
}

[MikeAThon]

I'm not so familiar with auto-pointer, but shouldn't this line

::PostMessage(m_handleToYourClass, WM_YOUR_CUSTOM_MESSAGE, 0, 
     reinterpret_cast<LPARAM>strToSend);

be

::PostMessage(m_handleToYourClass, WM_YOUR_CUSTOM_MESSAGE, 0,
     reinterpret_cast<LPARAM>&strToSend);

Mike

[Oliver M.]

You cannot use the CListCtrl class from multiple threads because MFC uses thread-local storage to map window handles to MFC objects.

Try to use the window handle of the list control instead of the CListCtrl class. You may use the ListView_* macros defined in commctrl.h that makes most of the work almost painless.

Ah, and by the way: Consider using a CSingleLock with your critical section to wrap syncronization and support exception safety, e.g. like this:

{
	// Lock critical section.
	CSingleLock sl(&m_CriticalSection, TRUE);


	// Insert new item into list control.
	LVITEM lvItem;
	memset(&lvItem,0,sizeof(LVITEM));
	lvItem.iItem = 0x0fffffff;
	const int ciItem(ListView_InsertItem(m_hListCtrl, &lvItem));
}