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X

The X protocol is used for sharing of graphical display resources across a network. Most UNIX systems use X for their graphical user interface. The X protocol allows a process to run on one machine and open a window for graphical output on another machine. In the X protocol, an X server usually corresponds to a display, and an X client is a program that shows data on that display. The X server is typically called X, and a sample X client is xterm, which opens a terminal window. X clients usually use port numbers starting at 6000 to make connections with X servers.

Xlib

Most client programs communicate with the server via the Xlib client library. Beside Xlib, the XCB library operates more closely to X protocol. In particular, most clients use libraries such as Xaw, Motif, GTK+, or Qt which in turn use Xlib for interacting with the server.

X uses a client-server model. Furthermore, the X server program runs on a computer with a graphical display and communicates with various client programs. The X server acts as a middle man for the user and the client programs, accepting requests on TCP port 6000 for graphical output (windows) from the client programs and
  1. displaying them to the user(display), and
  2. receiving user input (keyboard, mouse) and
  3. transmitting it to the client programs.
In X, the server runs on the computer of a user, while the clients may run on remote machines. This terminology reverses the common notion of client-server systems, where the client normally runs on the local computer of the user and the server runs on the remote computer. This reversal often confuses new X users since the X Window terminology takes the perspective that the X Window program is at the centre of all activity.
For example, the X Window program accepts and responds to requests from applications, and from the user's mouse and keyboard input. Therefore, applications (on remote computers) are viewed as clients of the X Window server program.
The communication protocol between server and client runs network-transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems. A client and server can communicate securely over the Internet by tunneling the connection over an encrypted connection.