The X Window System

The public-domain X Window System (or just X) implements a hierarchy of resizable windows and supports high-performance device-independent graphics in a client-server paradigm. X is based on an asynchronous network protocol and not on procedure or system calls. This allows many advantages:

This last advantage is because X is designed to be efficient on the network; this was achieved via the client-server paradigm discussed below. The information necessary to set up a new window is minimal; the size of the window, background color, font style and size, ... If an intelligent ``server'' -- the X server -- receives this minimal information, it can act on it and ``serve'' a window with the appropriate characteristics. The amount of information that needs to be carried over the network is small compared to, say, a bitmap description of the same window. This renders X an efficient system for maintaining windows at a distance.

The Client-Server Paradigm

The client is the computer requiring a window to be opened. It can be local, meaning being the same CPU as the X server, or on a local network, or on a World Wide network, such as the Internet. The server is implemented on the local, hopefully fast, CPU and display. It carries out the operations that the client requires. This division of labor greatly enhances the speed of graphics display. But the most important important advantage is that the window required by the client computer can be located anywhere on the network; X requests are then sent asynchronously over the network and fulfilled by the local server. This allows full interactive sessions, with all the sophisticated graphics and image manipulations that may be required, to be conducted from a distance.

This raises the question of security. X can open a window on any server on the network; would you like a person in Japan, say, to be able to open a window on your computer without your permission? To guard against this, X supports a security complement. Before a client can open a window on any display, the server must be authorized to accept X requests from this particular client. To do this, you must type on the server


	xhost client_hostname

where ``client_hostname'' is the network name of the client you want to authorize on the server. Then, the client must be told where to send the X requests; type on it 


	setenv DISPLAY server_hostname:0

where ``server_hostname'' is the server hostname.

A server can serve more than one client at once. These various clients need only be authorized to use the server. Consequently, it is common practice to have numerous windows talking to different computers, either locally or on the net. Also, a client can communicate with many servers simultaneously; this is the feature used in ``shared application'' environments, where applications simulaneously display results on many servers, or in teleconferencing software, where voice and video images are shared by different displays.

X Window Managers

Unlike many other window systems which implement a particular user interface, X is a substrate on which many different user interfaces can be built. The application which controls the user interface is the window manager. This a client which has authority over the layout of the windows on the screen, and to some extent over the properties of the windows. It is this client which defines the ``look and feel'' of the window on the screen. Among others, there are

These different window managers will present very different looks and feels to the user; however, applications running under the X protocol will work on all X servers, regardless of the window manager used.

X Tools

Many utilities (clients) have been written to run under X; these are typically available as ``contributions'' in the X distribution kits. They are often written in C and make use of the standard X libraries, Xlib and Xt Intrinsics. Some have been written at MIT; many others come from other sources.

Some of the most useful utilities are:

xv
An image-handling tool for Gif, Pict,... files
display
Another image-handling tool for Gif, Pict,... files
ghostview
A postscript previewer
ximage
A NCSA program to display data in image form
xdvi
A TeX (word processor) previewer
xwd
Dump an image of an X window
xpr
Print an X window dump
xpaint
A MacPaint-like program
mpeg_play
To ``play'' mpeg ``movies''

These utilities are often intuitive to use; a little trial & error is all that is needed. You can get general descriptons of these commands, and some help concerning their use, in the ``man'' pages.

References

The interested reader can find the best references to the X window system in a series of authoritative monographs published by O'Reilly & Associates, Inc. , with titles such as

Note that these monographs are very detailed and may contain far more information than you ever really wanted to know.