This chapter will discuss GDK, the underpinning of GTK+,
and some of the occasions you might have to use it. To
write custom widgets and canvas items, you will need to
understand a few of these low-level details. Like chapters
two and three, this chapter is a quick summary that doesn't
hold your hand; there is no way to cover all of GDK in a
single chapter. However, the chapter will try to cover the
important concepts and data types of GDK, and should be a
useful reference on certain topics. As details come up in
later chapters, you can use this background to understand
them. This chapter does not attempt to exhaustively catalog
GDK's API.
The X Window System comes with a low-level and thoroughly
unpleasant library called Xlib. Almost every function in
GDK is a very thin wrapper around a corresponding Xlib
function; but some of the complexity (and functionality)
of Xlib is hidden, to simplify programming and to make
GDK easier to port to other windowing systems. (There is
a port of GDK to Windows available.) The concealed Xlib
functionality will rarely be of interest to application
programmers; for example, many features used only by
window managers are not exposed in GDK. If necessary, you
can use Xlib directly in your application by including
the special gdk/gdkx.h header
file. (Check out the GDK source code to see how to
extract the low-level Xlib data structures from their GDK
wrappers.)
If you need excruciating details on a GDK function, you
can typically glance at the source to determine the Xlib
function it wraps, and then read the man page for the
Xlib function. For example, here is the implementation of
gdk_draw_point():
void
gdk_draw_point (GdkDrawable *drawable,
GdkGC *gc,
gint x,
gint y)
{
GdkWindowPrivate *drawable_private;
GdkGCPrivate *gc_private;
g_return_if_fail (drawable != NULL);
g_return_if_fail (gc != NULL);
drawable_private = (GdkWindowPrivate*) drawable;
if (drawable_private->destroyed)
return;
gc_private = (GdkGCPrivate*) gc;
XDrawPoint (drawable_private->xdisplay, drawable_private->xwindow,
gc_private->xgc, x, y);
}
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Each data structure is cast to its "private" version,
which contains information relating to the particular
window system GDK is being used on; this is to keep
window-system-specific declarations out of the gdk/gdk.h header file. The private
version of each data structure contains a wrapped Xlib
data structure, which is passed to
XDrawPoint(). So the
XDrawPoint() documentation will also apply to gdk_draw_point().
