Xlib

Xlib is an X Window System protocol client library written in the C programming language. It contains functions for interacting with an X server. These functions allow programmers to write programs without knowing the details of the protocol. Few applications use Xlib directly; rather, they employ other libraries that use Xlib functions to provide widget toolkits:

• Intrinsics (Xt)
• Athena widget set (Xaw)
• Motif
• FLTK
• GTK+
• Qt (X11 version)
• Tk

Xlib appeared around 1985, and is currently used in GUIs for many Unix-like operating systems. The XCB library is an attempt to replace Xlib. While currently Xlib is still in wide use, Xlib's implementation nowadays can use XCB as a low-level transport layer.