Xlib is a library for drawing graphical user interfaces with the X Window System on Unix-like systems. It regulates a client library for the X Window protocol in X Window System, the interaction of clients with an X server. It represents transactions of the X Window protocol with function calls in the C programming language is available. These functions create an abstraction, with the programming of programs is possible without having to worry about the details of the protocol. Use a few applications, the library directly, instead usually resort to other libraries that use the Xlib functions to ask widgets, such as the X11 versions of the widget toolkits X Toolkit ( Xt ), Qt, FLTK, XForms or GTK .

The Wayland versions of the widget toolkit Clutter, GTK , Qt, SDL and EFL ( Enlightenment Foundation Libraries) use XLib not more but libwayland -client.

The Xlib first appeared in 1985 and is still used for the graphical user interfaces of many application programs in use. An alternative is the XCB library, in which particular emphasis is placed on increased efficiency.