Xlib

Xlib is a C language client library for the X Window System. It provides an interface to the X11 protocol that allows applications to open a connection to an X server, create windows and other drawing surfaces, handle events, and issue drawing commands. The API exposes opaque data types such as Display, Window, GC, and XID, and communicates with the server in a network-transparent fashion.

As part of the X.Org ecosystem, Xlib has been the traditional foundation for X client development. It forms the basis for many older toolkits and applications (such as Motif, Xaw, and early GTK versions). It has been complemented by lower-level bindings like XCB and by newer window systems in some environments, though Xlib remains widely available on classic X installations.

Core concepts include the connection object (Display), screens and windows, graphics context (GC), and drawing primitives. Typical usage begins with XOpenDisplay, acquiring a Window (via XCreateSimpleWindow), selecting input events, mapping the window, and then processing events in an event loop with XNextEvent. Rendering is performed through functions like XDrawLine and XFillRectangle using a GC.

Resource management is supported through the X Resource Database (Xrm) and color and font resources. Xlib also provides XImage and Pixmap for offscreen or image data, and handles colormaps and visuals for color management. Threading support exists but requires explicit initialization with XInitThreads and careful synchronization; most applications use a single-threaded approach with the event loop.

Xlib is increasingly complemented or replaced by XCB and, in many environments, by Wayland-based toolkits. It remains in use primarily by legacy applications and environments that depend on established X11 infrastructure.