X/Motif Programming

As illustrated in Figure 1, the application may interact with all layers of the windowing system, the operating system and other libraries as needed. On the other hand, the user interface portion of the application should restrict itself to the Motif, Xt and Xlib libraries whenever possible.

The widget is the basic building block for the Graphical User Interface (GUI). It is common and beneficial for most GUIs assembled in Motif to look and behave in a similar fashion. Each widget in Motif is provided by default actions. Motif also prescribes certain other actions that should, whenever possible, be adhered to. Information regarding Motif GUI design is provided in the Motif Style Guide.

Each widget is defined to be of a certain class. All widgets of that class inherit the same set of resources and callback functions. Motif also defines a whole hierarchy of widget classes. There are two broad Motif widget classes that concern us. The Primitive widget class contains actual GUI components, such as buttons and text widgets. The Manager widget class defines widgets that hold other widgets.

A Motif widget may be regarded as a general abstraction for user-interface components. Motif provides widgets for almost every common GUI component, including buttons, menus and scroll bars. Motif also provides widgets whose only function is to control the layout of other widgets, enabling fairly advanced GUIs to be easily designed and assembled.

Widgets are designed to operate independently of the application except through well-defined interactions, known as callback functions. This takes much of the mundane GUI control and maintenance away from the application programmer. Widgets know how to redraw and highlight themselves and how to respond to certain events such as a mouse click. Some widgets go further than this; the Text widget, for example, is a fully functional text editor with built-in cut and paste as well as other common text-editing facilities.

Widgets are very useful because they simplify the X programming process and help preserve the look and feel of the application so it is easier to use.

The Motif Reference Manual provides definitions on all aspects of widget behavior and interaction. Basically, each widget is defined as a C data structure whose elements define a widget's data attributes, or resources and pointers to functions, such as callbacks.

The general behavior of each widget is defined as part of the Motif (Xm) library. In fact, Xt defines certain base classes of widgets which form a common foundation for nearly all Xt-based widget sets. Motif provides a widget set, the Xm library, which defines a complete set of widget classes for most GUI requirements on top of Xt.

The helloworld.c program explains the steps to follow in writing Motif programs.

helloworld.c creates a window with a single pushbutton in it. The button contains the string “Hello World!”. When the button is pressed, a string (Hello to you too!) is printed to standard output. This program illustrates a simple interface between the Motif GUI and the application code.

The program also runs forever! This is a key feature of event-driven processing. For now, we will have to quit our programs using the operating system:

The complete program listing for the helloworld.c program is in Listing 1. The display of helloworld.c on screen will look like Figure 2.

Figure 2. helloworld.c Display

Listing 1

To compile a Motif program, we have to link it with the Motif, Xt and Xlib libraries. The compile command I used for helloworld.c is

gcc helloworld.c -o helloworld -lXm -lXt
-lXext\
-lICE -lSM -lX11

Note that this compile line is required in my environment. It may be different in yours. You should check your local system documentation for the exact compilation directives.

Having successfully compiled your Motif program, the command

 ./helloworld &

will run it and display the PushButton on the screen as shown in Figure 2.