X-Git-Url: https://git.ozlabs.org/?a=blobdiff_plain;f=ccan%2Flstack%2Flstack.h;fp=ccan%2Flstack%2Flstack.h;h=7eb880a5c2f0384cc78b163f671d2a65ee2542f1;hb=d448f12d07c5b65d5db29f207de3178490f8360d;hp=41a9f05aa8fd222c05077fd8f1f2c3be8d2b1c90;hpb=c7a7fc0a1771394534cafaa7c30aa215631efc82;p=ccan

diff --git a/ccan/lstack/lstack.h b/ccan/lstack/lstack.h
index 41a9f05a..7eb880a5 100644
--- a/ccan/lstack/lstack.h
+++ b/ccan/lstack/lstack.h
@@ -50,6 +50,33 @@ struct lstack {
 #define LSTACK(name) \
 	struct lstack name = { NULL, }
 
+/**
+ * lstack_init_from_top - initialize a stack with a given top element
+ * @s: the lstack to initialize
+ * @e: pointer to the top element of the new stack
+ * @member: member of the element containing the lstack_link
+ *
+ * USE WITH CAUTION: This is for handling unusual cases where you have
+ * a pointer to an element in a previously constructed stack but can't
+ * conveniently pass around a normal struct lstack.  Usually you
+ * should use lstack_init().
+ *
+ * Example:
+ *	LSTACK(stack1);
+ *	struct lstack stack2;
+ *	struct element {
+ *		int value;
+ *		struct lstack_link link;
+ *	} el;
+ *
+ *	lstack_push(&stack1, &el, link);
+ *
+ *	lstack_init_from_top(&stack2,
+ *	                     lstack_top(&stack1, struct element, link), link);
+ */
+#define lstack_init_from_top(s, e, member) \
+	(lstack_init_((s), &(e)->member))
+
 /**
  * lstack_init - initialize a stack
  * @h: the lstack to set to an empty stack
@@ -58,9 +85,11 @@ struct lstack {
  *	struct lstack *sp = malloc(sizeof(*sp));
  *	lstack_init(sp);
  */
-static inline void lstack_init(struct lstack *s)
+#define lstack_init(s) \
+	(lstack_init_((s), NULL))
+static inline void lstack_init_(struct lstack *s, struct lstack_link *top)
 {
-	s->top = NULL;
+	s->top = top;
 }
 
 /**