How do I obtain, and synchronize, a complete list of all X11 windows?

Question

I want to monitor all the open windows under X11. Currently, I'm doing this as follows:

1. Initially walking the whole tree by recursively calling XQueryTree from the root window
2. Listening for substructure changes on the whole desktop: XSelectInput( display, root_window, SubstructureNotifyMask | PropertyChangeMask )
3. Handling all MapNotify, UnmapNotify and DestroyNotify events, updating my own list of windows in the process

I'm mainly worried about point 1. During the recursion, XQueryTree will be called multiple times. Is there any way to ensure that the tree does not change in the meantime? In other words, to get a 'snapshot' of the whole tree at one point in time?

Also, I've noticed that under some X11 systems, not all events arrive correctly. For example, when opening a new window on the desktop, MapNotify for that window may never arrive at my monitoring application. How can this be? Is it possible that it is thrown away before arriving?

Update:

I've written a small program that will monitor X events on the root window (see below). Now, when I run this program and start and quit xcalc, I get the following output:

Reparented: 0x4a0005b to 0x1001e40
Mapped    : 0x1001e40
Destroyed : 0x1001e40

That's it. I'm never notified of the real window (0x4a0005b) being destroyed. Not even of it being mapped! Can anyone tell me why not? Does SubStructureNotifyMask only cause events of direct subwindows to be sent instead of the whole subtree?

By the way, this apparently does not happen when Compiz is running. Then no reparenting is done:

Mapped    : 0x4a0005b
Mapped    : 0x4e00233
Destroyed : 0x4a0005b
Destroyed : 0x4e00233

Monitoring program source:

#include <X11/Xlib.h>
#include <cstdio>

int main()
{
    Display *display;
    Window rootwin;

    display = XOpenDisplay( NULL );
    rootwin = DefaultRootWindow( display );
    XSelectInput( display, rootwin, SubstructureNotifyMask );

    XEvent event;

    while ( 1 ) {
        XNextEvent( display, &event );
        if ( event.type == MapNotify ) {
            XMapEvent *mapevent = (XMapEvent *)&event;
            printf( "Mapped    : 0x%x\n", (unsigned int)(mapevent->window) );
        }
        if ( event.type == DestroyNotify ) {
            XDestroyWindowEvent *destroywindowevent = (XDestroyWindowEvent *)&event;
            printf( "Destroyed : 0x%x\n", (unsigned int)(destroywindowevent->window) );
        }
        if ( event.type == ReparentNotify ) {
            XReparentEvent *reparentevent = (XReparentEvent *)&event;
            printf( "Reparented: 0x%x to 0x%x\n", (unsigned int)(reparentevent->window), (unsigned int)(reparentevent->parent) );
        }
    }

    return 0;
}

Answer 1

X11 is a remote protocol. This means when you query the X server for any information, you always get your own copy. Your copy never changes when the X server updates its internal data structures.

This means the tree won't suddenly change while you traverse it but when you use the information in it (like examining a window), that information might be stale (someone might have closed the window). That is why you need to do proper error handling.