The X Window System is defined by the X Window System Protocol Specification, a detailed description of the encoding and the meaning of requests and events sent between a client and a server. This standard protocol does not depend on any particular programming language. As a result, each programming language must define its own functional interface for using the X protocol. The standard X interface used by Common Lisp programmers is called CLX. CLX is a set of data types, functions, and macros which allow a Common Lisp client program to interact with an X server to send requests and to receive input events and replies.
For the most part, CLX functions are closely tied to the underlying requests in the X protocol. Many CLX functions simply add requests to an output buffer. These requests later execute asynchronously on the X display server. However, some functions of CLX lie outside the scope of the protocol--for example, reading events and managing a clientside event queue. CLX is also responsible for important batching and caching tasks that minimize network communication.
The following paragraphs show an example of a CLX client program. All CLX functions and macros are shown in upper case. Note that some of the terms used are unique to X, while other terms that are common to other window systems have different meanings in X. It may be helpful to refer to the glossary when you are uncertain of a term's meaning in the context of the X Window System.