GdkPixmap as X11 Pixmap?

[Ingo Brückl <ib wupperonline de>]

I'm trying to get an X11 Pixmap from a GDK rendering function as follows
(my_pixbuf has been created by gdk-pixbuf-csource):

  GdkPixmap *gdkIcon;
  GdkBitmap *gdkIconMask;
  Pixmap Icon;
  Pixmap IconMask;

  gtk_init(&argc, &argv);

  gdk_pixbuf_render_pixmap_and_mask_for_colormap
  (
    gdk_pixbuf_new_from_inline(-1, my_pixbuf, FALSE, NULL),
    gdk_screen_get_system_colormap(gdk_screen_get_default()),
    &gdkIcon,
    &gdkIconMask,
    128
  );

  Icon = GDK_PIXMAP_XID(gdkIcon);
  IconMask = GDK_PIXMAP_XID(gdkIconMask);

Icon and IconMask are used then to set icon_pixmap and icon_mask with
XSetWMHints() for a window created by XCreateWindow().

What I see on the X11 window is an icon that "somehow" looks like my_pixbuf.
It seems that there is some "distortion" or that it is partly destroyed,
maybe overwritten with other data?
Different calls of the program show different distortions of the icon.

What am I doing wrong? Isn't GDK_PIXMAP_XID(gdkIcon) a X11 Pixmap that I can
use in X11 functions?

Ingo