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Introduction to Linux

By Machtelt Garrels

7.3.2. The X Window System

The X Window System is a network-transparent window system which runs on a wide range of computing and graphics machines. X Window System servers run on computers with bitmap displays. The X server distributes user input to and accepts output requests from several client programs through a variety of different interprocess communication channels. Although the most common case is for the client programs to be running on the same machine as the server, clients can be run transparently from other machines (including machines with different architectures and operating systems) as well. We will learn how to do this in Chapter 10 on networking and remote applications.

X supports overlapping hierarchical sub-windows and text and graphics operations, on both monochrome and color displays. The number of X client programs that use the X server is quite large. Some of the programs provided in the core X Consortium distribution include:

  • xterm : a terminal emulator

  • twm : a minimalistic window manager

  • xdm : a display manager

  • xconsole : a console redirect program

  • bitmap : a bitmap editor

  • xauth , xhost and iceauth : access control programs

  • xset , xmodmap and many others: user preference setting programs

  • xclock : a clock

  • xlsfonts and others: a font displayer, utilities for listing information about fonts, windows and displays

  • xfs : a font server

  • ...

We refer again to the man pages of these commands for detailed information. More explanations on available functions can be found in the Xlib - C language X Interface manual that comes with your X distribution, the X Window System Protocol specification, and the various manuals and documentation of X toolkits. The /usr/share/doc directory contains references to these documents and many others.

Many other utilities, window managers, games, toolkits and gadgets are included as user-contributed software in the X Consortium distribution, or are available using anonymous FTP on the Internet. Good places to start are http://www.x.org and http://www.xfree.org .

Furthermore, all your graphical applications, such as your browser, your E-mail program, your image viewing programs, sound playing tools and so on, are all clients to your X server. Note that in normal operation, that is in graphical mode, X clients and the X server on Linux run on the same machine.

7.3.2.1. Display names

From the user's perspective, every X server has a display name in the form of:

hostname :displaynumber .screennumber

This information is used by the application to determine how it should connect to the X server and which screen it should use by default (on displays with multiple monitors):

  • hostname : The host name specifies the name of the client machine to which the display is physically connected. If the host name is not given, the most efficient way of communicating to a server on the same machine will be used.

  • displaynumber : The phrase "display" is usually used to refer to a collection of monitors that share a common key board and pointer (mouse, tablet, etc.). Most workstations tend to only have one keyboard, and therefore, only one display. Larger, multi-user systems, however, frequently have several displays so that more than one person can be doing graphics work at once. To avoid confusion, each display on a machine is assigned a display number (beginning at 0) when the X server for that display is started. The display number must always be given in a display name.

  • screen number : Some displays share a single keyboard and pointer among two or more monitors. Since each monitor has its own set of windows, each screen is assigned a screen number (beginning at 0) when the X server for that display is started. If the screen number is not

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