X Windows

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X Windows
COMP 121-401 UNIX Programming

• Andrew Nashel
• nashel_at_cs.unc.edu
• Department of Computer Science
• February 4, 2004

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Course topics

• X Windows
• X Protocol
• Window managers
• Classic X apps
• Starting X apps

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X Windows

• The X Windows System, also referred to as X or
  X11, is the standard graphical engine for Unix
  and Linux.
  
• It is largely OS and hardware independent, it is
  network-transparent, and it supports many
  different desktops.

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X Windows contd

• X was developed in the mid 80s to provide a
  standard GUI for Unix systems, similar to
  Microsoft Windows.
  
• Windows only runs locally on the machine, but X
  Windows uses the X Protocol to separate the
  processing and display for an application.

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X Protocol

• The X Protocol provides a client-server
  architecture at the application level
  
• The X client is the processing part of the
  application and often runs on a remote machine.
  
• The X server is the display and interaction
  system.

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X Protocol contd

• The X Protocol is also divided into device
  dependent and device independent layers.

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X architecture

• The client-server nature of the X Protocol allows
  a single X server to support many clients
  (applications) on several hosts.

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Client-server design

• X server
• Runs locally and accepts multiple X clients
• Displays drawing requests and errors
• Handles keyboard, video, and mouse
• Creates and destroys windows
• X client
• An application written using X libraries (e.g.
  Xlib)
  
• Sends request and receives events and errors

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X Protocol messages

• Requests client sends requests to the server
  (e.g. create window)
  
• Replies server response to client requests
• Events server forwards events (such as mouse
  clicks or keyboard entry) to the client
  
• Errors server reports errors to the client

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An aside XFree86

• What is XFree86?
• In short, XFree86 is an open source X11-based
  desktop infrastructure. xfree86.org
  
• It runs primarily on Unix and Unix-like OSes such
  as Linux (and also under Cygwin).

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Using X Windows

• First, an X server must be running
• On a local PC or workstation, you can usually
  start the X server with the startx command or
  it may start automatically, presenting a
  graphical login display.
• Historical note An X terminal is a dumb terminal
  that only runs the X server locally, and always
  connects to X clients on a remote host.

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xterm

• The most important X application is, rather
  ironically, the terminal program xterm.
  

Old school xterm
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Xterm contd
OS X xterm
Fancy xterm
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An aside Copying text

• In X Windows, the copying interaction is very
  different from the Ctrl-C/Ctrl-V of Windows.
  
• In an xterm or other X application, highlight the
  text with the left button, and then paste by
  middle clicking in any window accepting input.

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Some classic X apps

• xman manual pages app

Not this
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Some classic X apps

• xbiff mail notification
• Back when we didnt have mail servers (and we
  liked it!), mail was stored in local spool files
  and a process notified the user of new mail.
  xbiff is named after biff, which is named after
  Biff the dog, who barked at the postman.
  Seriously.

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Some classic X apps

• xeyes ok, this is really the most important

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Some classic X apps

• xclock pretty self-explanatory

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Window managers

• The window manager is a special X client that
  controls the placement and movement of
  applications, provides title bars and control
  buttons, menus and taskbars. Some support
  virtual desktops and very fancy graphics.
• Classic window managers include twm, mwm, olwm,
  fvwm
  

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Starting X applications

• To launch an X client from a remote host for
  display on the local X server, you need to set
  two things
  
• 1) Permission for the remote host to display X
  clients on the local machine.
  
• xhost remotehost
• 2) The target display for the remote
  application.
  
• setenv DISPLAYserverdisplay

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Homework for Fri., Feb 2

• Try and work out any problems you had with
  profiling and/or porting code to a Unix system.
  
• Ill be able to help on a case by case basis,
  tomorrow and Friday.