createNotify event called too often

Question

I wrote the following code to detect when a window is created on the screen by another application (not by my code):

Display* display = XOpenDisplay(":0");
XSetWindowAttributes attributes;
attributes.event_mask = SubstructureNotifyMask | StructureNotifyMask;
Window win = XDefaultRootWindow(display);
XChangeWindowAttributes(display, win, CWEventMask, &attributes);

while (1) {
   XEvent event;
   XNextEvent(display, &event);
   if (event.type == CreateNotify)
      puts("create Notify event occured\n");
}

The code basically works, however, I noticed that, when I start an application (e.g. terminal) the CreateNotify event seems to be fired multiple times. Can anybody explain why? I would have expected that CreateNotify will only be fired once for every started application/window. How do I have to modify the code to achieve this?

Answer 1

The events are for child windows. Per the Xlib Programming Manual, "Creating any children ... generates an event." A typical X application includes numerous child windows, and you're seeing each of them.

When you get the event, check event.xcreatewindow.parent (XEvent is a union per the reference). If that is win, the root window, you have a top-level window. Otherwise, you have a child window. Try:

if ( (event.type == CreateNotify) && (event.xcreatewindow.parent == win) )
    puts("top-level create Notify event occured\n");