Question

In one C++ application I have a window whose message loop runs in a separate thread because the main thread is busy calculating and rendering a simulation. The window acts as a log window for the simulation. When the simulation throws an exception, the simulation closes and the log window displays the details.

Now the main thread should wait until the log window is closed. Since the message loop runs on a separate thread, I tried to

WaitForSingleObject(logwindow->thread, INFINITE);

from the main thread.

However, this seems to block the message pump and the log window freezes. So how do I correctly wait until a window is closed or a thread ends?

ED: The window is created on the main thread but runs on a different thread. I’ll go ahead and change it so it creates on the message loop thread too.

Answer 1

You have a few options.

1. Run all the UI out of the main thread and get the worker thread to synchronize reporting back to the main thread to display, for example via PostMessage or SendMessage.
2. Don’t wait at all and get the worker thread to post a message to the main thread when the worker is done.
3. Use MsgWaitForMultipleObjects for your wait.

To elaborate on MsgWaitForMultipleObjects, it’s a wait function that can be configured to return when messages arrive in the queue. Thus you can keep your message pump alive whilst also using a blocking wait in between processing queued messages.

In pseudo-code you would write it like this:

do
{    
    WaitResult = MsgWaitForMultipleObjects(1, hThread, TRUE, INFINITE, QS_ALLEVENTS);
    if (WaitResult == MessageArrivedOnQueue) 
        PumpMessageQueue();
} while (WaitResult != WaitHandlesSignaled)