Posted by jdp on Jan 11, 2011; 11:44pm
URL: https://forum.jogamp.org/Root-win-tp2219141p2237909.html

well, i can present this bit of a review for X11 a classic example is xearth. it demonstrates what the wm_spec calls virtual roots. from xearth//x11.c...

/* 
 * (taken nearly verbatim from the june 1993 comp.windows.x FAQ, item 148)
 */
static Window GetVRoot(dpy)
     Display *dpy;
{
  int          i;
  Window       rootReturn, parentReturn, *children;
  unsigned int numChildren;
  Atom         __SWM_VROOT = None;
  Window       rslt = root;

  __SWM_VROOT = XInternAtom(dpy, "__SWM_VROOT", False);
  XQueryTree(dpy, root, &rootReturn, &parentReturn, &children, &numChildren);
  for (i=0; i<numChildren; i++)
  {
    Atom          actual_type;
    int           actual_format;
    unsigned long nitems, bytesafter;
    Window       *newRoot = NULL;

    /* item 148 in the FAQ neglects to mention that there is a race
     * condition here; consider a child of the root window that
     * existed when XQueryTree() was called, but has disappeared
     * before XGetWindowProperty() gets called for that window ...
     */
    if ((XGetWindowProperty(dpy, children[i], __SWM_VROOT, 0, 1,
                            False, XA_WINDOW, &actual_type,
                            &actual_format, &nitems, &bytesafter,
                            (unsigned char **) &newRoot) == Success)
        && newRoot)
    {
      rslt = *newRoot;
      break;
    }
  }

  /* item 148 in the FAQ also neglects to mention that we probably
   * want to free the list of children after we're done with it ...
   */
  XFree((void *) children);

  return rslt;
}

it's using a funny atom, __SWM_VROOT that differs from the expected _NET_VIRTUAL_ROOTS. however, we see the same elsewhere as well

  //www.mail-archive.com/xorg-devel@lists.x.org/msg06346.html

from vroot.h...

static Window VirtualRootWindowOfScreen(Screen *screen)
{
       static Screen *save_screen = (Screen *)0;
       static Window root = (Window)0;

       if (screen != save_screen) {
               Display *dpy = DisplayOfScreen(screen);
               Atom __SWM_VROOT = None;
               int i;
               Window rootReturn, parentReturn, *children;
               unsigned int numChildren;

    /* first check for a hex or decimal window ID in the environment */
    const char *xss_id = getenv("XSCREENSAVER_WINDOW");
    if (xss_id && *xss_id) {
      unsigned long id = 0;
      char c;
      if (1 == sscanf (xss_id, " 0x%lx %c", &id, &c) ||
          1 == sscanf (xss_id, " %lu %c",   &id, &c)) {
        root = (Window) id;
        save_screen = screen;
        return root;
      }
    }

               root = RootWindowOfScreen(screen);

               /* go look for a virtual root */
               __SWM_VROOT = XInternAtom(dpy, "__SWM_VROOT", False);
               if (XQueryTree(dpy, root, &rootReturn, &parentReturn,
                                &children, &numChildren)) {
                       for (i = 0; i < numChildren; i++) {
                               Atom actual_type;
                               int actual_format;
                               unsigned long nitems, bytesafter;
                               unsigned char *newRoot = 0;

                               if (XGetWindowProperty(dpy, children[i],
                                       __SWM_VROOT, 0, 1, False, XA_WINDOW,
                                       &actual_type, &actual_format,
                                       &nitems, &bytesafter,
                                       &newRoot) == Success
                                   && newRoot) {
                                   root = *((Window *) newRoot);
                                   break;
                               }
                       }
                       if (children)
                               XFree((char *)children);
               }

               save_screen = screen;
       }

       return root;
}