c - How do I gracefully exit an X11 event loop?

Question

Almost every tutorial I find tells me to do this for my event loop:

XEvent event;

while (true)
{
    XNextEvent(display, &event);

    switch (event.type)
    {
        case Expose:
            printf("Expose
");
            break;

        default:
            break;
    }
}

However, clicking the X to close the program results in this message.

XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
after 10 requests (10 known processed) with 0 events remaining.

It is indeed strange to me that the examples suggest using an infinite loop. That doesn't sound natural, and my other X11 programs don't do that. So I searched around. I found out how to capture the window close event.

Atom wmDeleteMessage = XInternAtom(mDisplay, "WM_DELETE_WINDOW", False);
XSetWMProtocols(display, window, &wmDeleteMessage, 1);

XEvent event;
bool running = true;

while (running)
{
    XNextEvent(display, &event);

    switch (event.type)
    {
        case Expose:
            printf("Expose
");
            break;

        case ClientMessage:
            if (event.xclient.data.l[0] == wmDeleteMessage)
                running = false;
            break;

        default:
            break;
    }
}

That works. It exits without errors. ... But I refuse to believe this is the normal way to do things. I mean, is this the only way to properly exit an X11 app? It seems like a lot of work just to capture the close event. How do I make a 'proper' event loop? Why is the close event so deeply buried? What am I missing?

Answer 1

The problem lays in the communication between X Server and the Window Manager.

When you call XCreateWindow or XCreateSimpleWindow, the X Server creates your window (not showing it until you explicitly map it on the screen by calling XMapWindow), and then the Window Manager is responsible for attaching all the decorations and buttons and system menu around your window.

You can call XDestroyWindow on your own to remove the window, and this usually means it just disappears from the screen, but your program is still running and the connection to the X Server is still open, so you can send it some more requests.

The problem begins when the user clicks that little X button attached to your window by the Window Manager, because it is not created by the X Server and it is not his business to decide what to do then. Now it's all in hands of Window Manager.

If the Window Manager simply called XDestroyWindow on your window, it would cause a problem if your application wanted to capture the closing event to do something before the window gets destroyed. So the convention has been established between the X Server and the Window Managers to handle this process.

The default behavior of most Window Managers is to destroy the window and close the connection with the X server, because this is what most users of Window Managers would expect: that when they close the window, the program will end (and the connection to the X Server will close with the closed window). And then, when you try to call XCloseDisplay(display), it will cause the IO error you've mentioned, because the connection to the server is already closed and the display structure is invalid.

Here's an excerpt from the Xlib documentation which explains this:

Clients that choose not to include WM_DELETE_WINDOW in the WM_PROTOCOLS property may be disconnected from the server if the user asks for one of the client's top-level windows to be deleted.

Yeah, it would be great if they didn't hide it so deep in their docs, though :-P But when you already find it, fortunately it also hints for the solution.

If you want a different behavior (that is, to capture the closing event from the Window Manager), you need to use the WM_DESTROY_WINDOW protocol.

Another excerpt from the docs:

Clients, usually those with multiple top-level windows, whose server connection must survive the deletion of some of their top-level windows, should include the atom WM_DELETE_WINDOW in the WM_PROTOCOLS property on each such window. They will receive a ClientMessage event as described above whose data[0] field is WM_DELETE_WINDOW.

I had the same error and I wanted to know exactly what causes it and why. It took me some time to figure it out and find the proper explanation in the doc, so I put my explanation here to save the time of others uninformed.