The X Window System Graphical User Interface

X-Windows P.K.K.Thambi
The X Window System
Module 5

X Window System
 The X Window system was developed as part of
Project Athena at MIT. In 1987, X Version 11 is
released. X is now controlled and maintained by the
Open Group.
 The X Windows System, also referred to as ‘X’ or
“X11”, is the standard graphical engine for Unix ,
Linux and Sun workstations.
 It is largely OS and hardware independent, it is
network-transparent, and it supports many different
desktops.

The Graphic User Interface in X
 X Window uses a bit-mapped display where each
pixel can be manipulated individually.
 The entire display is known as the root window, and
individual applications are displayed as windows on
this root window.
 X is started with the startx or xinit commands.
– X can also be invoked during system startup

X-window Screen

Client / Server Architecture
Separate programs that talks together for
a specific aim.
Server will be the program that supplies
the services and the client is who make
the requests.

X Window Clients and Servers
 Although you can easily use the X Window system to run
programs stored on your local computer, you can also run
applications over the network
 X Window uses a client/server model in which a program can run
on one computer but display its output on another
 The desktop system from which you run a program is called the
X server, the system that hosts and executes the program is called
the X client (this is opposite of normal networking)

X Protocol
Based on TCP/IP stack
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.

X Protocol cont’d
The X Protocol is also divided into device dependent and device
independent layers.

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

Client-Server Window System
Application 1 Application 2 Application n
Client Application
Programs
Virtual
display 1
Virtual
display n
Resource
allocator
Device
driver
Window
2 Window
n
Window
1
Mouse
Keyboard
(After Fig 10.2, Dix, Finlay,
Abowd and Beale)
Virtual
display 2
Server
Devices
Device-independent abstraction level
Translates abstraction into reality:
one per terminal type
Could be
Window
Manager

X Server Design
 Device Dependent Layer
– It is this layer that is responsible for localizing the X server
to the native environment, be it Windows NT or Solaris.
– This layer swaps bytes of data from machines with differing
byte ordering. Byte ordering (MSB and LSB) is noted in
each X request.
– This layer hides the architectural differences in hardware
and operating systems.
– Maintains device driver dependencies for keyboard, mouse
and video.

X Server
The X server therefore:
 displays drawing requests on the screen.
 replies to information requests.
 reports an error in a request.
 Manages the keyboard, mouse and display device.
– Multiplexes keyboard and mouse input onto the network (or
via local IPC) to the respective X clients. (X events)
 creates, maps and destroys windows.
– writes and draws in windows

X Client
• sends requests to the server.
• receives events from server.
• receives errors from the server

X Protocol
 X client communicate with X server using the X protocol.
 Data is exchanged in an asynchronous manner over a two-way
communication path that enables transmission of a stream of 8
–bit bytes.
 X protocol is the machine language for the X Window system

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

Protocol Messages - Requests
Requests
 X clients make requests to the X server for a certain action to
take place. i.e.: Create Window
 To enhance performance, the X client normally does not
expect nor wait for a response. The request is typically left to
the reliable network layer to deliver.

Protocol Messages - Replies
Replies
 The X Server will respond to certain X client requests that
require a reply. As noted, not all requests require a reply.

Protocol Messages - Events
Events
 The X server will forward to the X client an event that the
application is expecting. This could include keyboard or
mouse input. To minimize network traffic, only expected
events are sent to X clients.
 X events are 32 bytes

Protocol Messages - Errors
Errors
 The X server will report errors in requests to the X client.
Errors are like an event but are handled differently.
 X errors are the same size as events to simplify their handling.
They are sent to the error handling routine of the X client.

Xlib: The Assemble language of X
 A set of C library of X window system
 Xlib gives you access to the X protocol through more than 300
utility routines.
 It is the The Assemble language of X Window System.

X Toolkit: The High level language of X
 XToolkit Intrinsic (Xt Intrinsic) – an object- oriented
approach to implement the basic building blocks called
widgets

Structure of an X application
X application
X toolkit
Xt intrinsic
Xlib
X protocol
X Server
Device- dependent Layer
X protocol
Network
Interface
User

The X Window System Graphical User Interface

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