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An Introduction to X11 User Interfaces

A tutorial prepared for SciC 8011

Grant Edwards

Spring, 1995

An HTML version of this paper is available via the at www.visi.com/users/grante/Xtut

— this page includes links to information about the various toolkits and software pack-

ages mentioned.

1 Overview

This paper provides an introductory look at the X Window System. It is intended to

provide the reader with information in two areas:



A basic understanding of what goes on behind the glass.

Anybody who uses a modern Unix based workstation for scientiﬁc visualiza-

tion has almost certainly used programs running under X. While little technical

knowledge is required to use today’s point-and-click interfaces, a little knowl-

edge about what goes on behind the scenes can make things less imposing and

mysterious.



A general idea of what must be done to implement a portable GUI for a program.

In scientiﬁc visualization, like other ﬁelds, we are often faced with the age-old

decision to “build or buy.” To make reasonable decisions, we must have a realis-

tic idea of the amount of time required to “build” a portable, graphical program.

Hopefully, this paper will provide readers with a starting point for this under-

standing, and make them go back and re-examine existing software packages

should they get the urge to write a custom X11 based program.

2 Introduction to

is a system for implementing a window-based user interface on bit-mapped dis-

plays. More technically, it is a speciﬁcation for a high level communications protocol

that is used between two programs. One of these programs is called the “server” and

The ofﬁcial name is “The X Window System.” It is considered tres uncool to call it “X Windows”

– presumably because it sounds too much like “Microsoft Windows.” The preferred moniker seems to be

simply “X.” If you want to sound like an old timer you can call it “X11” implying that you might have been

around at MIT when version 10 was in use. Most experienced users refer to it as simply “X” or as “X11R3”

if a particular version/release is to be speciﬁed.

does the actually drawing on the screen and receives the raw input from the keyboard

and mouse. The other program is called the “client” and this is where the useful work

gets done. These programs may be on different computers, or they may be on the same

computer.

It is important to notice that the location of the client and server are reversed com-

pared to many client/server situations. This often causes confusion among people

new to X. For example, you have a Sun workstation at your desk: you are running

a CAD/simulation program on a large computer somewhere else and displaying the

user interface on your desktop Sun. The Sun is running the server (it’s what talks to the

keyboard, mouse, and graphics hardware). The CAD program on the large computer is

the “client” program.

The client requests that graphical operations be performed and the server provides

those services.

The X protocol speciﬁcation deﬁnes what goes back and forth between those two

programs. The X protocol is usually implemented as a layer on top of another transport

layer such as TCP/IP.

3 History

X was developed in a joint effort by MIT and Digital Equipment Corporation based

somewhat on the “W” windowing system previously developed at Stanford. In 1986,

X10 became the ﬁrst widely used version of X, though it was quickly replaced by

version 11 a year later. Since then X11 has gone through 5 more “releases” and is now

at release 6 (referred to as X11R6). Almost all workstation vendors have adopted X11

as their windowing system, though many vendors still have proprietary extensions built

into the X servers shipped with their hardware.

4 What X isn’t

The X speciﬁcations do not specify what a program’s user interface should look like,

what a window manager should do, or how applications should behave. X is simply a

low-level method of creating windows; receiving user inputs (key-presses and mouse

events); and drawing strings, drawing lines, curves and bitmaps. The behavior of the

application and the design and style of the user interface are left entirely up to client

programs. While this allows for a great deal of ﬂexibility, it results in the lack of a

common “look and feel” for X applications.

Printing is another area where the X speciﬁcations are mute. X does not provide

any printing or hard copy services as does Microsoft Windows or the Mac OS tool box.

In the absence of any other standard, postscript has been largely adopted as a de facto

standard for hard copy output by designers of X applications.

For example, Sun Microsystems includes a display Postscript extension as well as a SunView extension

in its X servers.

5 The X Protocol

5.1 Services provided by the Server

The X server provides a set of fairly low-level services that are used by clients to im-

plement whatever user interface is desired. These services are basically the following:



Input handling

The server receives input from the keyboard and mouse. These are passed as

“events” to the appropriate client program. The method used to decide which

window should receive input events various depending on the window manager

(which is actually just another client). Typical events are key-presses, mouse

movements and mouse button press/release.



Hierarchical window services.

These allow a client to tell the server to create or destroy a “window” which is

a rectangular area on the screen. Windows may be nested hierarchically within

other windows. Various operations may be performed on windows (set or query

the size and position, for example).



Graphical operations.

The client can request that the server draw lines, rectangles, arcs, dots, and poly-

gons. The color, line-width, and ﬁll style are speciﬁed by the client. X11 also

supports various bitmap operations

that can be performed on regions of win-

dows.



Text and font operations.

The client can request that text strings be “drawn” into a window at a particular

position using a speciﬁed font. Before this can be done, the client must tell the

server to load the font to be used. The client can request information about what

fonts are available and also about sizes and parameters of loaded fonts.

That’s it. The X protocol doesn’t know anything about radio buttons, scroll bars,

sliders, text entry ﬁelds or menu bars. Those are all implemented in the client – usually

using off-the shelf libraries and toolkits.

5.2 The Client/Server Interface

As stated earlier, the X protocol deﬁnes what passes back and forth between the client

and the server. The information that travels between client and server is broken up into

“packets” at the X protocol level (which is different than Ethernet or TCP/IP packets

or frames). There are four types of packets:



Request

A request packet is sent by the client to the server to ask that the server perform

some action or return some information.

Often called “bitblt” operations (short for bit block transfer).



A reply packet is sent by the server to the client in response to a request from the

server. Not all requests generate replies.



Event

An event packet is sent by the server to the client to inform it of user input or

some other happening about which it might want to do something (for example

a window was re-sized or a previously obscured window was uncovered).



Error

An error packet is sent by the server to the client to inform it that a request was

not valid. Since requests are queued, the error might not be discovered until after

several more requests have been queued by the client.

6 Low level X programming: Xlib

It is theoretically possible to write a client program using just the standard C run-

time library routines. The client would have to open a socket-based connection to a

server, construct requests and write them to the socket and interpret the reply, event,

and error error packets. Though possible, this is unnecessary since there are libraries

available that contain routines to interface to an X server using the X protocol. The

most common such library is called “Xlib” and is intended to be called from the C

programming language. The deﬁnitive books on Xlib are probably those published by

O’Reilly and Associates: Xlib Programming Manual and Xlib Reference Manual.

Though Xlib hides the details of communicating with the server, it does little else.

The functions provided by Xlib perform only the basic tasks described in section 5.1.

Even a simple program written using nothing but Xlib can require large amounts of

code. A “hello world” program

whose output is shown in ﬁgure 1 requires 200 lines

of C code. This program does nothing other than create a window, check the window

size and the size of the root display, display the text shown, and wait for any key or

mouse button before exiting. The source code for this program is shown in Appendix

A for the morbidly curious.

As you might guess, writing an entire application using Xlib directly is not prac-

tical. The client application must specify every window to be drawn and every bit of

text or line to be drawn. The application must then read events from the the server and

process them. When a window is resized, the application must take appropriate actions

and redraw everything in the correct places. What is needed is a higher level interface

that hides much of the gritty details from the application programmer.

7 X Toolkits

Since Xlib is obviously not practical as a basis for applications programming, various

toolkits have been written that provide another layer above Xlib. Several of the more

This is the “basicwn” program shown as an example in by Adrian Nye’s Xlib Programming Manual.

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