How to get X Window id from within Emacs?

[Jean Louis]

The standard shell commend `xwininfo' gives me the Window id, such as
0x2e000ac and then I can run shell command like `xdotool' to activate
the Emacs window:

$ xwininfo 

xwininfo: Please select the window about which you
          would like information by clicking the
          mouse in that window.

xwininfo: Window id: 0x2e000ac "*shell* - GNU Emacs at lco2"

after switching to other workspace, then I can still do:

$ xdotool windowactivate 0x2e000ac

and I would return to Emacs X Window.

Is there a way to get the Window ID from within Emacs?

That will be useful to return to Emacs upon some notifications and
jobs done, during the watching of the movie.

Jean