Draw to X screen on top of all windows

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http://www.linuxquestions.org – I'm trying to write a little volume bar application for my desktop. I want the volume bar to be displayed on top of all other windows *and* on top of any application running in fullscreen mode. I was thinking about using Xlib to draw a simple "pop-up" window that displays the volume bar and disappears after 2-3 seconds. The problem is that the window will not be visible if a program is occupying the full screen. Any ideas? Btw, I don't need a window around by volume bar, so if there's a method to draw to the screen directly I'll be fine with that. (HowTos)