Internal Program Version: libxshmfence  1.3.2

This is a tiny library that exposes a event API on top of Linux
futexes. There was some discussion about using eventfd instead of this,
but the cost of adding two FDs to the X server for every DRI application
seems excessive, and by using PresentIdleNotify events, to work around
the limitations of futexes.

http://xorg.freedesktop.org/releases/individual/lib