-extern void register_font_driver (struct font_driver *driver, FRAME_PTR f);
-extern void free_font_driver_list (FRAME_PTR f);
-extern Lisp_Object font_update_drivers (FRAME_PTR f, Lisp_Object list);
-extern Lisp_Object font_range (ptrdiff_t, ptrdiff_t *,
+extern void register_font_driver (struct font_driver *driver, struct frame *f);
+extern void free_font_driver_list (struct frame *f);
+#ifdef ENABLE_CHECKING
+extern bool valid_font_driver (struct font_driver *);
+#else
+INLINE bool
+valid_font_driver (struct font_driver *d)
+{
+ return true;
+}
+#endif
+extern Lisp_Object font_update_drivers (struct frame *f, Lisp_Object list);
+extern Lisp_Object font_range (ptrdiff_t, ptrdiff_t, ptrdiff_t *,