-void avl_traverse_lock(avl_tree_lock *t, void (*callback)(void *));
-void avl_traverse(avl_tree *t, void (*callback)(void *));
+int avl_traverse_lock(avl_tree_lock *t, int (*callback)(void *entry, void *data), void *data);
+int avl_traverse(avl_tree *t, int (*callback)(void *entry, void *data), void *data);