First of all I have to admit that I'm a newbie, but while i was studying gtk2 from the tutorials i found this page 
https://developer.gnome.org/gtk-tutorial/2.22/x2470.html

In this page is written:
--->To create an offscreen pixmap, we call the function:

GdkPixmap* gdk_pixmap_new               (GdkWindow  *window,
					 gint        width,
					 gint        height,
					 gint        depth);

The window parameter specifies a GDK window that this pixmap takes some of its properties from. width and height specify the size of the pixmap. depth specifies the color depth, that is the number of bits per pixel, for the new window. If the depth is specified as -1, it will match the depth of window.
<---

Instead in my header file /usr/include/gtk-2.0/gdk/gdkpixmap.h the prototype for gdk_pixmap_new is

GdkPixmap* gdk_pixmap_new		(GdkDrawable *drawable,
					 gint	      width,
					 gint	      height,
					 gint	      depth);


with the first argument of type GdkDrawable. Am I missing something or something is wrong with the tutorial?
(Just in case you know another tutorial on gdk* can you suggest it to me?)