X/Motif Programming

This article will introduce the basic concepts in building a graphical user interface in X and Motif. I'll go into a quick introduction to the X Window System and its programming model, then introduce Motif and illustrate some concepts with a sample program. Finally, we'll go through the basic Motif programming principles.

The X Window System, or simply X, was originally developed at the Massachusetts Institute of Technology with support from Digital Equipment Corporation. X was developed for Project Athena to fulfill the project's need for a distributed, hardware-independent user interface platform.

X is a graphics system used primarily on UNIX systems. It provides an inherently client/server-oriented framework for displaying windowed graphics. It provides a way of writing device-independent graphical and windowing software that can be ported easily from one machine to another.

X provides functionality via a vast set of subroutine libraries. These may be called from a variety of high-level languages. However, they are most easily called from C programs.

Since X is network-oriented, the applications developed for it do not need to run on the same system as the one supporting the display. The programmer doesn't need to worry much about the practicality of this, as X normally makes this transparent to the user.

The server is the program that controls the display. It acts as a bridge between user programs (i.e., clients or applications) and the resources of the local system. These run on either local or remote systems. The server performs the following duties:

The client in X usually consists of two parts. One, the graphical user interface, which is written in one or more of Xlib, Xt or Motif. Two, the algorithmic or functional part of the application where the input is received from the interface and processing tasks are defined.

In this section, I will briefly describe the main tasks of the three levels of the X/Motif programming model. The X/Motif programming model consists of Xlib, Xt Intrinsics and Motif.

Figure 1. User Interface Library Model

Xlib handles the interface between the client application and the network. It is part of the X software architecture. The main task of Xlib is to translate C data structures and procedures into the special form of X protocol messages, which are then sent off. The converse, receiving messages and converting them to C structures, is performed by Xlib as well.

X Toolkit Intrinsics (Xt Intrinsics) is a toolkit that allows programmers to create and use widgets. Toolkits, such as the Xt Intrinsics, implement a set of user interface features or application environments such as menus, buttons or scroll bars (referred to as widgets). Since Motif is built upon Xt, we'll need to call some Xt functions. However, we do not need to understand the workings of Xt, as Motif takes care of most things for us.

The client and server are connected by a communication path called the connector. This is performed by a low-level C language interface known as Xlib. It is true that many applications can be written solely using Xlib, but in general, it will be difficult and time-consuming to write complex GUI programs only in Xlib. Many higher-level subroutine libraries, called toolkits, have been developed to remedy this problem. One of these toolkits is Motif.

Motif is a widely accepted set of user interface guidelines developed by the Open Software Foundation and its member companies around 1989, and supported since. These rules specify how an X Window System application should “look and feel”. Motif includes the Motif Toolkit (Xm), which enforces a policy on top of the X Toolkit Intrinsics (Xt). Xt is really a “mechanism, not policy” layer, and Xm provides the specific look and feel. For example, Xt does not insist that windows have titlebars or menus, but it provides hooks for developers of specific toolkits (Motif, OpenLook, Athena widgets) to take advantage of. Motif also includes the Motif Style Guide document which details how a Motif user interface should look and behave to be “Motif compliant”.