Volume 7A: XView Programming Manual

1

XView and the X Window System

The XView Toolkit allows a programmer to build the interface to an application without hav-

ing to learn many of the details of the underlying windowing system. However, it is valuable

to have some understanding of X before attempting to build applications under XView. This

chapter introduces many of the most important concepts on which the X Window System is

based and describes the computing environment for X applications. It also describes the role

of the XView Toolkit in the X Window System.

For the most part, this chapter assumes that you are new to programming the X Window Sys-

tem. This chapter describes the basics of the X Window System—further details will be

described as necessary later in the manual. However, this book does not repeat the detailed

description of Xlib programming found in Volume One, Xlib Programming Manual. If you

already have some experience programming the X Window System, you might wish to begin

at Chapter 2, The XView Programmer’s Model.

1.1 The X Window System

X controls a bit-mapped display in which every pixel (dot on the screen) is individually con-

trollable. This allows drawing of pictures in addition to text. Until recently, individual con-

trol of screen pixels was widely available only on personal computers (PCs) and high-priced

technical workstations, while more general-purpose machines were limited to output on text-

only terminals. X brings the same world of graphic output to both PCs and more powerful

machines. Figure 1-1 shows an X application in comparison with a traditional text terminal.

Like other windowing systems, X divides the screen into multiple input and output areas

called windows, each of which can act as an independent virtual terminal. Using a terminal

emulator, windows can run ordinary text-based applications. However, windows can also run

applications designed to take advantage of the graphic power of the bitmapped display.

X takes user input from a pointer, which is usually a mouse but could just as well be a track-

ball or tablet. The pointer allows the user to point at certain graphics on the screen and use

the buttons on the mouse to control a program without using the keyboard. This method of

using programs is often easier to learn than traditional keyboard control, because it is more

intuitive. Figure 1-2 shows a typical pointer being used to select a menu item.

XView and the

X Window System

XView and the X Window System 3

Dear Mr. Sekino:

.LP

This letter details the book distribution agreement we

reached in our

discussions on June 9, 1989 and in subsequent

communications

The terms are as follows:

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/fBExclusivity/fP.

Toppan shall be the esclusive distributor

in Japan for all titles published by O'Reilly and

Associates, Inc. (ORA).

ORA will refer all sales inquiries

from Janpanese customers to Toppan. The only exption is

that we will

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text terminal

X window system

total 49

1 sbin/ 3 etc/ 1 preserve/

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Figure 1-1. An X application and a traditional text terminal

Workspace

Utilities

Properties...

Exit...

cmdtool(CONSOLE)-/bin/csh

total 49

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1 diag/ 1 mdc/ 1 tmp@

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Figure 1-2. Selecting a menu item with the pointer

4 XView Programming Manual

Of course, X also handles keyboard input. The pointer is used to direct the input focus of the

keyboard (often called the keyboard focus) from window to window with only one applica-

tion at a time able to receive keyboard input.

In X, as in many other window systems, each application need not (and in fact usually does

not) consist of only a single window. Any part of an application can have its own separate

window, simplifying the management of input and output within the application code. Such

child windows are only visible within the confines of their parent window. Each window has

its own coordinate system where the origin is the upper-left corner of the window inside its

border. In basic X, windows are rectangular and oriented along the same axes as the edges of

the display. The application or the user can change the dimensions of windows.

Many of the above characteristics are also true of several other window systems. What has

made X a standard is that X is based on a network protocol—a predefined set of requests and

replies—instead of system-specific procedure calls. The X Protocol can be implemented for

different computer architectures and operating systems, making X device-independent.

Another advantage of a network-based windowing system is that programs can run on one

architecture while displaying on another. Because of this unique design, the X Window Sys-

tem can make a network of different computers cooperate. For example, a computationally-

intensive application might run on a supercomputer but take input from and display output on

a workstation across a network.

1.1.1 The Server and Client

To allow programs to be run on one machine and display on another, X was designed as a net-

work protocol between two processes, one of which is an application program called a client,

and the other, the server. The server is a resource server, controlling a user’s resources (such

as the display hardware, keyboard, and pointer) and making these resources available to user

applications. In other words, the X server isolates the device-specific code from the applica-

tion.

The server performs the following tasks:

• Allows access to the display by multiple clients. The server can deny access from clients

running on certain machines.

• Interprets network messages from clients and acts on them. These messages are known as

requests. Some requests command the server to do two-dimensional drawing or move

windows, while others ask the server for information.

• Passes user input to clients sending network messages known as events, which represent

key or button presses, pointer motion, and so forth. Events are generated asynchronously,

and events from different devices might be intermingled. The server must de-multiplex

the event stream and pass the appropriate events to each client.

• Maintains complex data structures, including windows and fonts, so that the server can

perform its tasks efficiently. Clients refer to these abstractions by ID numbers. Server-

maintained abstractions reduce the amount of data that has to be maintained by each cli-

ent and the amount of data that has to be transferred over the network.

XView and the

X Window System

XView and the X Window System 5

In X, the term display is often used as a synonym for server, as is the combined term display

server. However, the terms display and screen are not synonymous. A screen is the actual

piece of hardware on which the graphics are drawn. Both color and monochrome displays

are supported. A server might control more than one screen. For example, one server might

control a color screen and a monochrome screen for a user who wants to be able to debug an

application on both types of screens without leaving his or her seat.

The communication path between a client and the server is called a connection. Several cli-

ents may be connected to a single server. Clients may run on the same machine as the server

if that machine supports multitasking, or clients may run on other machines in the network.

In either case, the X Protocol is used by the client to send requests to draw graphics or to

query the server for information; it is used by the server to send user input or replies to

requests back to the client. All communication from the client to the server and from the

server to the client takes place using the X Protocol.*

It is common for a user to have programs running on several different hosts in the network,

all invoked from and displaying their windows on a single screen (see Figure 1-3). Clients

running remotely can be started from the remote machine or from the local machine using the

network utilities rlogin or rsh.

Personal Computer

Supercomputer

Large Minicomputer

Display Server

Local

Client

Figure 1-3. Applications can run on any system across the network

*The X Protocol is independent of the networking hardware and runs on top of any network that provides point-to-

point packet communication. TCP/IP and DECnet are the only networks currently supported. For more information

about the X Protocol, see Volume Zero, The X Protocol Reference Manual.

6 XView Programming Manual

This use of the network is known as distributed processing. The most important application

of this concept is to provide graphic output for powerful systems that cannot have built-in

graphics capabilities. However, distributed processing can also help solve the problem of

unbalanced system loads. When one host machine is overloaded, the users running clients on

that machine can arrange for programs to run on other hosts.

1.2 The Software Hierarchy

There are many different ways to write X applications because X is not restricted to a single

language, operating system, or user interface. The only requirement of an X application is

that it generate and receive X Protocol messages.

Figure 1-4 shows the layering of software in an X application. Xlib is the lowest-level C lan-

guage interface to X. The main task of Xlib is to translate C data structures and procedures

into X Protocol events; it then sends them off and receives protocol packets in return that are

unpacked into C data structures. Xlib provides full access to the capabilities of the X Proto-

col but does little to make programming easier. It handles the interface between an applica-

tion and the network and includes some optimizations that encourage efficient network

usage. The list of functions that Xlib performs is so extensive that if the programmer were

responsible for handling all these pieces directly, application programs would be too large

and prone to performance degradation and potential bugginess. For this reason, toolkits are

used to modularize the more common functions that handle the user interface portion of an

application.

XView is one of a half-dozen or so toolkits available for the X Window System. If you are

familiar with other toolkits, you will recognize that the XView Toolkit is equivalent to the Xt

Intrinsics and a widget set. Like the Intrinsics, XView is built upon Xlib. It is an object-

oriented toolkit that provides reusable, configurable user interface components, equivalent to

widgets.*

Toolkits handle many things for the programmer. They provide a framework for combining

pre-built user interface components with application-specific code. For example, if the appli-

cation needs to prompt the user for a filename, a toolkit should provide a component (a com-

mand frame) that is functionally capable of displaying the query to the user and providing the

user’s response to the application.

Any user interface component also needs to manage the interpretation of events delivered

from the window system. When events are generated, the toolkit decides whether or not to

propagate the event to the application or to use it for its own internal purposes. To continue

the example, when the user types a filename in the command frame, events are generated in

which the interface must decide whether that object should interpret the input or whether it

should be sent to the application. A toolkit thus comprises a mechanism to dispatch events

and a set of prebuilt interface objects that define the look and feel of an application.

*Widget sets are sometimes loosely referred to as toolkits. However, a toolkit comprises the functions of the Xt In-

trinsics layer and one widget set (e.g., the Athena widget set). There are several different widget sets from various

vendors that are designed to work with Xt. For more information on Xt Intrinsics-based toolkits, see Volume Four, X

Toolkit Intrinsics Programming Manual.

XView and the

X Window System

XView and the X Window System 7