Is xlib still used? The helpful documentation I can find is http://tronche.com/ which looks, to me, very outdated. Who uses Xlib? Is GTK+ and QT built on it? thanks in advance, ell.
Yes, toolkits like GTK+ and Qt are still built on Xlib. Migration to xcb is in progress, but far from complete or universal. Applications though really should be writing to the toolkit API's, not Xlib.
BTW, you can also find Xlib docs directly from the source at X.Org: http://www.x.org/releases/current/doc/