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The X Window System core protocol〔Robert W. Scheifler and James Gettys: ''X Window System: Core and extension protocols, X version 11, releases 6 and 6.1'', Digital Press 1996, ISBN 1-55558-148-X〕〔RFC 1013〕〔Grant Edwards. (An Introduction to X11 User Interfaces )〕 is the base protocol of the X Window System, which is a networked windowing system for bitmap displays used to build graphical user interfaces on Unix, Unix-like, and other operating systems. The X Window System is based on a client–server model: a single server controls the input/output hardware, such as the screen, the keyboard, and the mouse; all application programs act as clients, interacting with the user and with the other clients via the server. This interaction is regulated by the X Window System core protocol. Other protocols related to the X Window System exist, both built at the top of the X Window System core protocol or as separate protocols. In the X Window System core protocol, only four kinds of packets are sent, asynchronously, over the network: requests, replies, events, and errors. ''Requests'' are sent by a client to the server to ask it to perform some operation (for example, create a new window) and to send back data it holds. ''Replies'' are sent by the server to provide such data. ''Events'' are sent by the server to notify clients of user activity or other occurrences they are interested in. ''Errors'' are packet sent by the server to notify a client of errors occurred during processing of its requests. Requests may generate replies, events, and errors; other than this, the protocol does not mandate over a specific order in which packets are sent over the network. Some extensions to the core protocol exist, each one having its own requests, replies, events, and errors. X originated at MIT in 1984 (its release X11 appeared in September 1987). Its designers Bob Scheifler and Jim Gettys set as an early principle that its core protocol was to "create mechanism, not policy". As a result, the core protocol does not specify the interaction between clients and between a client and the user. These interactions are the subject of separate specifications,〔Jim Gettys. (Open Source Desktop Technology Road Map )〕 such as the ICCCM and the freedesktop.org specifications, and are typically enforced automatically by using a given widget set. == Overview== Communication between server and clients is done by exchanging packets over a channel. The connection is established by the client (how the client is started is not specified in the protocol). The client also sends the first packet, containing the byte order to be used and information about the version of the protocol and the kind of authentication the client expects the server to use. The server answers by sending back a packet stating the acceptance or refusal of the connection, or with a request for a further authentication. If the connection is accepted, the acceptance packet contains data for the client to use in the subsequent interaction with the server. After connection is established, four types of packets are exchanged between client and server over the channel: #''Request:'' The client requests information from the server or requests it to perform an action. #''Reply:'' The server responds to a request. Not all requests generate replies. #''Event:'' The server informs the client of an event, such as keyboard or mouse input, a window being moved, resized or exposed, etc. #''Error:'' The server sends an error packet if a request is invalid. Since requests are queued, error packets generated by a request may not be sent immediately. Request and reply packets have varying length, while event and error packets have a fixed length of 32 bytes. Request packets are numbered sequentially by the server as soon as it receives them: the first request from a client is numbered 1, the second 2, etc. The least significant 16 bits of the sequential number of a request is included in the reply and error packets generated by the request, if any. They are also included in event packets to indicate the sequential number of the request that the server is currently processing or has just finished processing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「X Window System core protocol」の詳細全文を読む スポンサード リンク
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