When writing a Motif program, you will be calling upon Motif and Xt functions and data structures explicitly. In order to distinguish the various toolkits, X adopts the following convention:
Motif function and data structure names begin with Xm, such as XmStringCreateSimple and XmStringFree.
Xt Intrinsics functions and most data structures begin with Xt, such as XtVaAppInitialize and XtVaCreateManagedWidget. The widget data structure is an exception to this rule.
Xlib functions and most data structures begin with X. There are no Xlib functions used in helloworld.c. However, an example of an Xlib function call is XDrawString or XDrawLine.
Any application that uses the Motif toolkit must include a header file for each widget it uses. Every Motif widget has its own header file, so we have to include the Xm/PushB.h file for the pushbutton widget, the Xm/DrawingA.h for the drawing widget and so on. However, we do not have to explicitly include the Xt header file, since Xm/Xm.h does this automatically. Every Motif program will include Xm/Xm.h, the general header for the motif library.
We'll now analyze the helloworld.c program in detail. There are six basic steps that nearly all Motif programs have to follow. These are:
Initializing the toolkit
Setting up events and callback functions
Displaying the widget hierarchy
Entering the main event-handling loop
The first step in a Motif program is to initialize the Xt Intrinsics toolkit. Before an application creates any widget, it must initialize the toolkit. There are several ways to initialize the toolkit. The most common is XtVaAppInitialize. When the XtVaAppInitialize function is called, the following tasks are performed:
The application is connected to the X display.
The command line is parsed for the standard X command-line arguments.
Resources are created using the app-default file, if any.
The top-level window is created.
XtVaAppInitialize takes several arguments.
The Application context: the first argument to XtVaAppInitialize is the address of an application context, which is a structure that Xt uses to manage some internal data associated with an application. For the Motif program we are considering, we need not know anything about this except that we need to set it in our program.
Application class name: the second argument is a string which defines the class name of the application. It is used to reference and set resources common to the application or even to a collection of applications.
Command-line arguments: the third and fourth arguments specify a list of objects as special X command-line arguments. The third argument is the list and the fourth, the number in the list. This is advanced X use and will not be considered further in this article. Just set the third argument to NULL and the fourth to 0. The fifth and sixth arguments, &argc and argv, contain the values of any command-line arguments given. These arguments may be used to receive command-line input of data in standard C fashion (e.g., file names for the program to read). Note that the command line may be used to set certain resources in X. However, these will have been removed from the argv list if they have been correctly parsed and acted upon before being passed on to the remainder of the program.
Fallback resources provide security against errors in other setting mechanisms. They are ignored if resources are set by any other means (i.e., using the app-default file). A fallback resource is a NULL-terminated list of strings. For now, we will simply set it to NULL since no fallback resources have been specified.
Additional parameters: the eighth parameter is the start of a NULL-terminated list of resource,value pairs that are applied to the top-level widget returned by XtVaAppInitialize. For now, it's a NULL-terminated list since there is no resource setting.