#include <X11/Xlib.h>
#include <X11/Xmu/WinUtil.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char **argv)
{
  Display *display;
  Window root;
  XEvent event;

  if (argc != 1) {
    char *name;
    name = strrchr(argv[0], '/');
    name = name ? name+1 : argv[0];
    fprintf(stderr, "usage: %s\n", name);
    exit(1);
  }

  if ((display = XOpenDisplay(NULL)) == NULL) {
    fprintf(stderr, "cannot open display\n");
    exit(1);
  }

  root = DefaultRootWindow(display);

  if (XGrabPointer(
        display, root, False, ButtonReleaseMask,
        GrabModeAsync, GrabModeAsync, None, None, CurrentTime)
      != GrabSuccess) {
    fprintf(stderr, "cannot grab pointer\n");
    exit(1);
  }

  XNextEvent(display, &event);
  if (event.type == ButtonRelease) {
    Window target;
    target = event.xbutton.subwindow;
    if (target == None)
      target = event.xbutton.window;
    else
      target = XmuClientWindow(display, target);
    if (event.xbutton.button == 1 && target != None) {
      printf("%ld\n", target);
    }
  }

  XCloseDisplay(display);
  exit(0);
}