[cairo] clear operator

[control H]

Hi,

I'm trying to use the CLEAR operator, but it doesn't give the expected
results. I'm using gtk, the configure event I made looks like this:

static cairo_surface_t *surface = NULL;

static gboolean
configure_event (GtkWidget         *widget,
		  GdkEventConfigure *event,
		  gpointer           data)
{
  GtkAllocation allocation;
  cairo_t *cr,*cr1,*cr2;
  cairo_surface_type_t st;
  cairo_surface_t *surf1,*surf2;

  int w,h,r;

  if (surface)
    cairo_surface_destroy (surface);

  gtk_widget_get_allocation (widget, &allocation);
  w=allocation.width;
  h=allocation.height;
  r=h/2;

  surface = gdk_window_create_similar_surface (gtk_widget_get_window (widget),
					       CAIRO_CONTENT_COLOR,
                                               allocation.width,
                                               allocation.height);
  /* Initialize the surface to white */
  cr=cairo_create (surface);
  cairo_set_source_rgb (cr, 1, 1, 1);
  cairo_paint (cr);

  surf1=cairo_surface_create_similar(cairo_get_target(cr),
				     CAIRO_CONTENT_COLOR_ALPHA,
				     w,h);
  cr1=cairo_create(surf1);
  cairo_set_source_rgba(cr1,0.,1.,0.,.8);
  cairo_move_to(cr1,w/2,h/2);
  cairo_arc(cr1,w/2,h/2,MIN(w,h)/3,0,2*G_PI);
  cairo_fill(cr1);

  surf2=cairo_surface_create_similar(cairo_get_target(cr),
				     CAIRO_CONTENT_COLOR_ALPHA,
				     w,h);
  cr2=cairo_create(surf2);
  cairo_set_source_rgba(cr2,1.,1.,0.,.7);
  cairo_arc(cr2,w/2.5,h/2,MIN(w,h)/4,0,2*G_PI);
  cairo_fill(cr2);

  cairo_set_operator(cr1,CAIRO_OPERATOR_CLEAR);
  //cairo_set_operator(cr1,CAIRO_OPERATOR_DEST_OUT);
  cairo_set_source_surface(cr1,surf2,0,0);
  cairo_paint(cr1);

  cairo_set_source_surface(cr,surf1,0,0);
  cairo_set_operator(cr,CAIRO_OPERATOR_OVER);
  cairo_paint(cr);

  cairo_surface_destroy(surf1);
  cairo_surface_destroy(surf2);
  cairo_destroy(cr1);
  cairo_destroy(cr2);

  cairo_destroy (cr);

  return TRUE;
}

The result is just a white screen. However, if I change
cairo_set_operator(cr1,CAIRO_OPERATOR_CLEAR);
into
cairo_set_operator(cr1,CAIRO_OPERATOR_DEST_OUT);
or any other operator, it gives the expected result. What am I missing here?