X-WindowsP.K.K.Thambi The X Window System Module 5

X-WindowsP.K.K.Thambi 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.

X-WindowsP.K.K.Thambi 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-WindowsP.K.K.Thambi X-window Screen

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

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

X-WindowsP.K.K.Thambi Client-Server Window System Application 1Application 2Application n Client Application Programs Virtual display 1 Virtual display n Resource allocator Device driver Window 2 Window n Window 1 MouseKeyboard (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-WindowsP.K.K.Thambi 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-WindowsP.K.K.Thambi 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-WindowsP.K.K.Thambi X Client sends requests to the server. receives events from server. receives errors from the server

X-WindowsP.K.K.Thambi 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-WindowsP.K.K.Thambi 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

X-WindowsP.K.K.Thambi 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.

X-WindowsP.K.K.Thambi 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.

X-WindowsP.K.K.Thambi 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

X-WindowsP.K.K.Thambi 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.

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

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