GDI+ drawing with multithreading - c++

Question

Disclaimer: I'm somewhat of a noob when it comes to multi-threading. I've read stuff online, have done some simple multi-threading examples.
I've got a Win32 app that wants to draw the stuff in one thread and handle the Win32 messages in another thread. However, after the windows is created and the threads start, it hangs. I have a hunch that it may have to do with WaitForMultipleObjects(), but I don't know how to make it right. Does anyone have any idea why this is happening? Should I suspend and resume threads?
Here's my code:
WinAPI:
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int nCmdShow)
{
/* initialization blah blah */
zgE->startThreads(); // <--- starts the 2 threads
WaitForMultipleObjects(zgE->getThreadsNo(), zgE->getThreads(), TRUE, INFINITE);
return TRUE;
}
This is how I start the threads:
void zgEngine::startThreads()
{
/* allocation and stuff, blah blah blah */
m_arrThreads[m_nThreads++] = CreateThread(NULL, 0, &zgEngine::handleMsg, (void*)this, NULL, NULL);
m_arrThreads[m_nThreads++] = CreateThread(NULL, 0, &zgEngine::drawObjects, (void*)this, NULL, NULL);
assert(m_nThreads <= THREADS_NO);
}
And the 2 functions that draw & handle messages are quite simplist. A while loop in each of them.
// draw function
DWORD WINAPI zgEngine::drawObjects(LPVOID lpParam)
{
while (true)
{
/* draw stuff - valid code that if called outside this function
works as intended */
}
return TRUE;
}
// message handler function
DWORD WINAPI zgEngine::handleMsg(LPVOID lpParam)
{
MSG msg;
while (true)
{
if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
{
// Process the message
if (msg.message == WM_QUIT)
break;
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return TRUE;
}
When I don't use threads and remove the "while (true)" in drawObjects(), but leave the code (to be executed only once), don't call handleMsg() and make WinMain like in the example below, it works like a charm.
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int nCmdShow)
{
/* initialization blah blah */
MSG msg;
while (true)
{
if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
{
// Process the message
if (msg.message == WM_QUIT)
break;
TranslateMessage(&msg);
DispatchMessage(&msg);
} else {
zgEngine::DrawObjects(zgE);
}
}
return TRUE;
}
Later edit: from what I've seen, PeekMessage() always returns 0 :(

Answer 1

Quote from PeekMessage on Microsofts website:
A handle to the window whose messages are to be retrieved. The window
must belong to the current thread.
If hWnd is NULL, PeekMessage retrieves messages for any window that
belongs to the current thread, and any messages on the current
thread's message queue whose hwnd value is NULL (see the MSG
structure). Therefore if hWnd is NULL, both window messages and thread
messages are processed.
If hWnd is -1, PeekMessage retrieves only messages on the current
thread's message queue whose hwnd value is NULL, that is, thread
messages as posted by PostMessage (when the hWnd parameter is NULL) or
PostThreadMessage.
I suspect the thread doesn't have a "current window", and the "current threads message queue" isn't the one that Windows actually posts the messages onto as the default thread. This is only an assumption, since it is entirely possible that there are other problems (as well?) with your approach. But I believe this is a main portion of where the problem lies.