Motif/Lesstif Application Development

A tutorial designed to help you build your own GUI.
Debugging and Experimenting

Debugging Motif applications can be challenging, due to the heavy use of callback functions. I find the trusty printf is often the quickest way to ferret out anomalies. The situation is not improved by the way the library relies on type casting to implement the object-oriented design of the widget libraries. Refer to documentation on function signatures unless you are absolutely certain you have it right.

A program called editres provides a mechanism to quickly tweak resource settings at runtime. By pointing the program to your running application, you can retrieve the widget tree and examine or change resources for nearly every widget instance. This is a great way to experiment with alternative fonts, colors, attachments and labels. Applications that are properly linked can speak the editres protocol, hosting a two-way communications link to the outside world without changing a single line of code.

In order to add support for editres to your application, add the following two lines:

#include <X11/Xmu/Editres.h>
XtAddEventHandler(g_topshell, (EventMask) 0,\
True,\
(XtEventHandler) _XEditResCheckMessages, 0);

and link against the Xmu library—place it after the Motif library on the link command. Kenton Lee has a good article on using editres at www.rahul.net/kenton/editres.html. The source for editres is included with X distributions, so any machine running X should have the utility.

Alternative Approaches

Coding an application using the technique described here can be a bit tedious at times, although it yields the most flexibility and control over the interface. There are many different approaches to developing Motif applications, including wrapper libraries and GUI builders. Two of the most common are the User Interface Language (UIL) and the Motif Tools library. Both reduce the amount of energy that goes into coding, and each has different advantages.

UIL is a C-like description language used to create an interface implemented via Motif and other Xt widgets. The UIL files are compiled and fed to the Motif resource manager (MRM) at runtime; the MRM then renders the interface, installs callbacks, etc. The chief advantages of UIL are rapid layout of screens, extensive error checking, easier internationalization and less complex code. The main disadvantages of UIL are reduced flexibility (due to the language's simplicity), which could require additional coding in Motif, yet another language to learn, and instability problems. Another issue for the aspiring Lesstif developer is that UIL/MRM is not yet fully supported.

Motif Tools (Xmt) is a library that came with the O'Reilly book, Motif Tools by David Flanagan. This library provides a few additional widgets and a significant number of support functions, letting you program using resource files. This approach has the advantage of providing more runtime flexibility, since the resource files are read at runtime. Minor changes to the GUI may not require you to recompile, and end users can edit the resource files to alter the interface to their tastes. In addition, application code can be significantly simpler, since you rely on Xmt to do the detailed implementation work. Xmt does have a fairly steep learning curve and may still lack some of the flexibility provided by coding directly with the Motif API.

I stuck to C in this article, since there was so much I wanted to cover; however, C++ is also a good fit for Motif development. The biggest issue is that callback functions must be static member functions or global functions. In fact, user interfaces are particularly well-suited for GUI development, since you have visual objects that can be represented by the C++ objects. Also, the encapsulation C++ brings to the table can make the programs more modular and easier to maintain.

Resources

Glen Wiley is a Senior Software Engineer for 3Com Corporation's Carrier Systems—Network Management Systems Research and Development. He began programming around 1985 and was first introduced to UNIX in 1987 via an AT&T 3B2 running System V. When not trying to cram more stuff into his overtaxed brain, he spends time as a human trampoline for his two sons Stephen and Richard. He can be reached at gwiley@ieee.org.

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