XCB

XCB, short for X protocol C-language Binding, is a library that provides a low-level interface to the X Window System protocol. It is designed as an alternative to the older Xlib library and is part of the X.Org project. XCB aims to be small, efficient, and easy to generate from the X11 protocol description, enabling more direct control over the server and lower latency for clients.

The API emphasizes asynchrony and an event-driven model. Client code issues requests through functions that correspond to X11 operations. Replies, events, and errors arrive asynchronously; the library provides a mechanism to correlate requests with replies. The implementation is largely generated from the protocol descriptions, which reduces boilerplate and helps keep the code in sync with the server.

XCB covers the core X11 protocol and many widely used extensions via modular components. Although written in C, it exposes an API suitable for use directly and exposes bindings in other languages, including Python (xcffib), Rust (via x11rb and related projects), and other ecosystems.

XCB is used by many X clients, including window managers and toolkits that communicate with the X server directly. It is part of the contemporary X.Org software stack, and several projects continue to maintain and extend it. While Wayland has emerged as a successor in some environments, XCB remains a foundational interface for legacy X environments.