--- a/src/drilbo/drilbo-x11.c
+++ b/src/drilbo/drilbo-x11.c
@@ -761,7 +761,7 @@
 
 
 x11_image_window_id display_zimage_on_X11(Window *parent_window,
-    z_image *zimage, void (*callback_func)())
+    z_image *zimage, void (*callback_func)(x11_image_window_id window_id, int event))
 {
   Window window;
   Hints hints;
--- a/src/drilbo/drilbo-x11.h
+++ b/src/drilbo/drilbo-x11.h
@@ -83,7 +83,7 @@
 int init_x11_display();
 int end_x11_display();
 x11_image_window_id display_zimage_on_X11(Window *parent_window,
-    z_image *image, void (*callback_func)());
+    z_image *image, void (*callback_func)(int,  int));
 int close_image_window(x11_image_window_id window_id);