As in title I want to add/remove items to a class derived from the WTL CListViewCtrl class from worker threads, but always get "Unhandled exception thrown: read access violation."
I tried Win32 API PostMessage and SendMessage but once the worker thread touches the HWND of CListViewCtrl I get the same exception.
// CListCtrl member function, calling from worker thread
HWND GetHwnd()
{
return hwndListCtrl; // exception here
}
I tried this SafeQueue but once worker thread touches the mutex or queue then exception again.
// SafeQueue is member variable in CListViewCtrl, created in GUI thread
SafeQueue<T> m_SafeQueue;
. . .
// member function in SafeQueue class, calling from worker thread
void enqueue(T t)
{
std::lock_guard<std::mutex> lock(m); // exception here
q->push(t);
}
I tried to create the mutex and queue with new and HeapAlloc/LocalAlloc but same exception again.
I tried Win32 API CreateMutex but no luck, same exception when accessing mutex handle from worker thread.
It works fine when I add items from the GUI thread.
Only way it works from worker threads if I declare HWND or mutex and queue as static/global but I would avoid this since I want to use more than one instance from this listcontrol and I prefer any more elegant way than global variable.
I want to make this class reusable since I want to use it many times with a few modifications (more columns, different colors).
I appreciate any help and idea how I can make this work.
Environment: VS2015 Community, WTL/C++ and Win10 Pro 64bit
I found the problem that causes access violation exception: I declared ThreadProc callback function as static member function in CListViewCtrl class.
// DO NOT USE
// in CListViewCtrl
**static** DWORD WINAPI ThreadProc(LPVOID lp)
{
. . .
}
LRESULT OnStartWorkerThread(WORD /*wNotifyCode*/, WORD /*wID*/, HWND . ..)
{
DWORD dw;
::CreateThread(NULL, 0, this->ThreadProc, NULL, 0, &dw);
}
A working solution:
class CListViewCtrl ...
{
// thread-safe queue to store listctrl items to be added later in GUI thread
SafeQueue<CListCtrlItem<nCols> > m_SafeQueue;
// thread ID of the thread in which listctrl was created, saved in OnCreate
DWORD m_dwGuiTid;
// . . .
Check if SafeAddItem function called from GUI or any other threads
BOOL InvokeRequired()
{
if (m_GuiTid == ::GetCurrentThreadId())
return false;
return true;
}
// ...
SafeAddItem member function can be called from GUI and worker threads
void SafeAddItem(CListCtrlItem<nCols> item)
{
if (!InvokeRequired())
{
// we are in GUI thread so just add listctrl item "normal" way
AddItem(item);
return;
}
// we are in other thread so enqueue listctrl item and post a message to GUI
m_SafeQueue.Enqueue(item);
::PostMessage(m_hWnd, WM_ADD_ITEM, 0, 0);
}
// . . .
Message handler of PostMessage, we are in GUI thread
LRESULT OnAddItem(UINT /*uMsg*/, WPARAM /*wParam*/, LPARAM lParam, BOOL& bHandled)
{
CListCtrlItem<nCols> item;
while (!m_SafeQueue.Empty())
{
item = m_SafeQueue.Dequeue();
// we are in GUI thread so we can add list ctrl items normal way
AddItem(item);
}
return 1;
}
// ...
}
And now we can add listctrl items from any threads this way. I pass this pointer to ThreadProc in _beginthreadex
m_ListCtrl.SafeAddItem(item);
