When a user clicks the close button [x] on our X11 application we want it to pop a a dialog asking “do you really want to quit?”. This is a plain X app. No fancy GTK or QT widgets here. So how to catch the “window is being closed” message?

The answer is to tell the Window Manager we are interested in these event by calling XSetWMProtocols and registering a WM_DELETE_WINDOW message with it. Then we’ll get a client message from the Window Manager if someone tries to close the window, and it won’t close it, it’ll leave that us up to us. Here’s an example….

// example.cpp
#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <iostream>

int main()
{
   Display* display = XOpenDisplay(NULL);
   Window window = XCreateSimpleWindow(display,
                                       DefaultRootWindow(display),
                                       0, 0,
                                       500, 400,
                                       0,
                                       0, 0);

   // register interest in the delete window message
   Atom wmDeleteMessage = XInternAtom(display, "WM_DELETE_WINDOW", False);
   XSetWMProtocols(display, window, &wmDeleteMessage, 1);

   std::cout << "Starting up..." << std::endl;
   XMapWindow(display, window);

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

      if (event.type == ClientMessage &&
          event.xclient.data.l[0] == wmDeleteMessage) {
         std::cout << "Shutting down now!!!" << std::endl;
         break;
      }
   }

   XCloseDisplay(display);
   return 0;
}

Compile with g++ example.cpp -o example -lX11