-void ctl_stack_push (struct ctl_class *, void *private);
-void *ctl_stack_top (struct ctl_class *);
-void *ctl_stack_search (struct ctl_class *);
+void ctl_stack_push (const struct ctl_class *, void *private);
+void *ctl_stack_top (const struct ctl_class *);
+void *ctl_stack_search (const struct ctl_class *);