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.

Xlib has a function called XRaiseWindow.

How well do you know your window manager? Myself I would look for solution there. If it's a simple wm maybe you could even hack its code so it can keep windows of your program's class on top. Or for a really simple way out, the wm I use, cwm, has a concept of a gap that's configurable. It's an area of the screen that's left empty for things like this. cwm's maximize window command respects this gap configuration setting so your clock and stuff don't get hidden by other windows. It let's cwm get away without needing any concept of a "keep on top" property for windows. Maybe yours has something like that? Or some kind of tray or applet API?

Thanks for your reply. I'm using GNOME 3, but I want to switch because it sucks. I cannot use the tray because the volume bar would be covered e.g. by a YouTube video running in fullscreen. I tried XRaiseWindow(), but I does not raise a window "high enough", programs running in fullscreen are still higher.

I would look at amarok code, it has a feature like this.