Changing icon of button on toolbar

[Edoardo Vigoni <vigoni aero polimi it>]

I must change icon of a button clicking it.
I found an old mail in this list in wich there was the following code,
but doesn't work. Compiler didn't accept. I don't understand
GTK_PIXMAP(widget->button->pixmapwid). I substituted it with
GTK_PIXMAP(data), but crashed. What's the solution?
Thank's

#include <gtk/gtk.h>

/* Our usual callback function */
void callback( GtkWidget *widget,
               gpointer   data )
{
  static int i=0;

  GtkWidget *pix;
  GdkPixmap *pixmap;
  GdkBitmap *mask;

  pix = GTK_WIDGET(data);

  if (i==0)
    pixmap = gdk_pixmap_create_from_xpm
(widget->window,
                                         &mask,
                                         NULL,
                                         "stop.xpm");
  else if (i==1)
    pixmap = gdk_pixmap_create_from_xpm
(widget->window,
                                         &mask,
                                         NULL,
                                         "start.xpm");
  else
    g_print("Can't get pixmap\n");

  gtk_pixmap_set
(GTK_PIXMAP(widget->button->pixmapwid), pixmap, mask);

  gtk_widget_show(widget->button->pixmapwid);

  if (i==0)
    i=1;
  else
    i=0;
}

int main( int   argc,
          char *argv[] )
{
  /* GtkWidget is the storage type for widgets */
  GtkWidget *window;
  GtkWidget *button;
  GtkWidget *pixmapwid;
  GdkPixmap *pixmap;
  GdkBitmap *mask;
  GtkStyle *style;
  
  gtk_init (&argc, &argv);
  
  /* Create a new window */
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  
  gtk_window_set_title (GTK_WINDOW (window), "Pixmap'd
Buttons!");
  
  /* It's a good idea to do this for all windows. */
  gtk_signal_connect (GTK_OBJECT (window), "destroy",
                      GTK_SIGNAL_FUNC (gtk_exit), NULL);
  
  gtk_signal_connect (GTK_OBJECT (window),
"delete_event",
                      GTK_SIGNAL_FUNC (gtk_exit), NULL);
  
  /* Sets the border width of the window. */
  gtk_container_set_border_width (GTK_CONTAINER
(window), 10);
  gtk_widget_realize(window);
  
  /* Create a new button */
  button = gtk_button_new ();
  
  /* Get the style of the button to get the
   * background color. */
  style = gtk_widget_get_style(window);
  
  /* Now on to the xpm stuff */
  pixmap = gdk_pixmap_create_from_xpm (window->window,
&mask,
                                       &style->bg[GTK_STATE_NORMAL],
                                       "start.xpm");
  pixmapwid = gtk_pixmap_new (pixmap, mask);
  
  /* Connect the "clicked" signal of the button to our
callback */
  gtk_signal_connect (GTK_OBJECT (button), "clicked",
                      GTK_SIGNAL_FUNC (callback), (gpointer)
pixmapwid);
  
  gtk_widget_show(pixmapwid);

  /* Pack and show all our widgets */
  gtk_container_add (GTK_CONTAINER (button),
pixmapwid);
  
  gtk_widget_show(button);
  
  gtk_container_add (GTK_CONTAINER (window), button);
  
  gtk_widget_show (window);
  
  /* Rest in gtk_main and wait for the fun to begin!
*/
  gtk_main ();
  
  return(0);
}