X Window System protocol binding library. Originally for C bindings, but now generalized to several other languages. This is a lightweight replacement for the binding portion of Xlib, featuring thread transparency, XML extensibility, and a small and straightforward interface.
The version of Xlib currently being distributed by X.Org uses XCB for its transport; this allows XCB and Xlib calls to be freely mixed for ease in porting applications and toolkits.
Most of the XCB C code is autogenerated from XML descriptions. (This may be why Ohloh complains about the degree of code commenting.)