DLL crashes app after reuse - c++

Question

I use an application that uses mdi and a script can be attached to, and detached from, a mdi window to be run/stopped on demand; this script loads my dll that does some work; it does fine so; however, when I detach the script still all is fine and the application should unload the dll (and it calls dllmain with the appropriate thread_attach/detach and process_attach/detach operations). Now if I try to re-attach the script to the winow, or to attach it to another window, after the dll has been in use once - the main application crashes. I have isloated the problem to a thread that is created by the dll; the tread crates a window; so, I create the thread like so:
if (!hThread) hThread = CreateThread(NULL, 0, ThreadProc, NULL, 0, NULL);
and, when the script is detached it shuts down the thread like so (no matter if the commented-out lines are uncommented-out):
SendMessage(hWnd, WM_DESTROY, 0, 0);
//TerminateThread(hThread, 0);
//WaitForSingleObject(hWndThread, INFINITE);
CloseHandle(hThread);
hThread = NULL;
I'm at a loss here as to why the main app crashes. A different thread (i.e. one that would simply sleep for a second and loop, will do no harm. What gives?

Answer 1

Ok, here are some ideas:
You said your thread opens a window. Do you run a message loop in the thread function, or you expect your window to be serviced by some other message loop?
If you are running your own message loop in the thread, then exiting the loop may or may not happen, depending on how you have written it. If you use something like:
while(GetMessage(&msg, ...) // msg loop in the thread function
{
....
}
DestroyWindow(hWnd); // see comment below
then this requires a WM_QUIT and not WM_DESTROY to exit. Anyway, the best is to send your window a WM_QUIT and after exiting the message loop then call DestroyWindow() to destroy it properly.
Quoting from MSDN:
DestroyWindow function
Destroys the specified window. The function sends WM_DESTROY and WM_NCDESTROY messages to the window to deactivate it and remove the keyboard focus from it. The function also destroys the window's menu, flushes the thread message queue, destroys timers, removes clipboard ownership, and breaks the clipboard viewer chain (if the window is at the top of the viewer chain
After posting WM_QUIT message to your window, your main thread should wait for the window thread to exit. Here is some relevant code:
SendMessage(hWnd, WM_QUIT, 0, 0); // send your quit message to exit the msg loop
if (WaitForSingleObject(hThread, 5000) != WAIT_OBJECT_0) // wait up to 5 seconds
{
TerminateThread(hThread, -1); // bad! try to never end here
}
I hope this helps. I use this in a threaded log viewer that uses a window to display log messages.