The X Toolkit API

• We have seen that the Xlib API is a low-level but very efficient library for writing X Window applications.
• The X toolkit (or intrinsics) library provides higher-level programming support for writing widgets.
• Widgets are object-oriented display objects, like data entry fields, utilities for plotting data, and so on. They are usually written in the C language, but they're object-oriented in the sense that they support inheritance and maintain private data with a public API for accessing internal widget data.
• Before an application can use the X toolkit, the following function must be called before any other toolkit functions (the type String is defined as char *, and a Cardinal is an int):

  Widget XtInitialize(String shell_name, String application_class,
                      XrmOptionDescRec *options, Cardinal num_options,
                      int * argc, char **argv)