COMP 121-401: UNIX Programming
X Windows
Andrew Nashel
nashel@cs.unc.edu
Department of Computer Science
February 4, 2004
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Course topics
• X Windows
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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 cont’d
X was developed in the mid 80’s 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 cont’d
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 X11based 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 cont’d
Fancy xterm:
OS X 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 didn’t 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 DISPLAY=server:display
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