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For most applications, the X Windows subroutines are separated into
at least three layers.
The lowest layer of the X Windows package contains those functions that
are part of the X11 library (libX11.a or libX11.sh on most
UNIX machines). The functions in this library are the implementation of
the X protocols for communication between the X client and an X server.
Rarely are the functions at this level directly called by application
programs. All application programs must be linked to this library to
support subroutines from the higher level libraries.
The next higher layer of an X Windows application is the X Toolkit. On
most UNIX machines, this library of functions will be called "libXt.a" or
"libXt.sh". The X Toolkit, (also called Xt or toolkit for short) contains
subroutines that initialize an application and that allow the organization
of screen objects into classes of widgets.
Many applications can, and are, written using only the X11 and Xt
libraries, but in most cases developers will choose to use a standard
set of widgets to incorporate a standardized "look and feel" to the
user interface. These routines that embody the look and feel of an
application can be grouped into a separate group of functions for
creation of individual widgets and modification of their properties.
There are several standard widget sets available to application
developers. The three most widely known are
Athena,
Open Look, and
Motif.
An Example Program
Here is an example of an X Windows version of the "Hello World" program
written for the Motif widget set:
01: #include < Xm/Xm.h >
02: #include < Xm/Label.h >
03:
04: XtAppContext context;
05: XmStringCharSet char_set=XmSTRING_DEFAULT_CHARSET;
06:
07: main(argc, argv)
08: int argc;
09: char *argv[];
10: {
11: Widget toplevel, label;
12: XmString S;
14:
15: /* Create an application shell. */
16: toplevel = XtAppInitialize(&context,"",NULL,0,&argc,argv,NULL,
17: NULL,0);
18:
19: /* Create the Label widget. */
20: label = XmCreateLabel(toplevel,"label",NULL,0);
21: S = XmStringCreateLtoR("Hello World!", char_set);
22: XtVaSetValues(label,
23: XmNlabelString, S,
24: NULL);
25:
26: XtRealizeWidget(toplevel);
27: XtAppMainLoop(context);
28: }
The line numbers that appear here are not part of the original program, but
have been inserted only for reference.
LINES 1-2
These lines retrieve the definitions of data structures and macros that
will be used in the rest of the program. Both Xm.h and Label.h
are part of the Motif package. Xm.h contains include
statements that will pull in the lower level definitions that are necessary
for the Xt and X11 libraries (including stdio.h). On most systems
it is not really necessary to include Xm.h, since each of the
widget definition headers (such as Label.h) contains a statement
to include Xm.h.
LINE 4
context is declared to be a variable of type XtAppContext,
or an application context. The application context may be thought of as
the parent of all widgets in the application, since it establishes a base
for inheritance for some properties of all widgets. This structure
will be initialized later in the program.
LINE 5
This line declares that the character set for this program is the
default character set for Motif programs.
LINE 12
The variable S is declared to be a Motif string. Motif strings
are different from C strings (or character arrays) in that they
have additional properties such as a character set and a text direction.
LINE 16
This line initializes communications between the application program
and the X server, and also creates the base window for the application.
The size of the base window will automatically be sized to fit the
contents.
LINES 20-24
Line 20 creates a Label widget. In line 21, a static array
of characters is converted to a Motif string, and in lines 22-24, the
Motif string is assigned to the XmNlabelString property of the label
struct.
LINE 26
The function XtRealizeWidget forces construction of the graphic
image of the widget that is specified in the call and all widgets that
inherit from the specified widget. This causes the application to
appear on the display screen. It is at this point that a window appears
on the screen containing the words "Hello World!".
LINE 27
The function XtAppMainLoop is the main event handling of the
X toolkit. After this function is called, the program waits for an
event, such as a keypress or button click, to occur. Since no widgets
were created for user interaction in this program, the only thing that
the user can do at this point is to close the application window. The
precise method of closing an application window depends on which
window manager is in use.
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