I want to know the available size

Hello,

Is there a function (or any method) that can tell me the left space available for an already maped top level window (parent is root window) ?

The method for new top level window are possible: set the value of width and height very high then the window manager will reduce it automatically when maping.
For instance when I set 768 as height, the window manager will reduce it to 700 (I use getgeometry). In fact the window/desktop manager reduces the size because of existence of title bar, decoration, and some menu bar.

I can not do this method for a maped window, do you know any solution ?

Thank you