From 5cdc505e7ec54d144c10f12a5453b7793b891e1e Mon Sep 17 00:00:00 2001 From: David Gibson Date: Sun, 14 Jun 2015 00:24:57 +1000 Subject: [PATCH] lqueue: Allow a queueu to be initialized from an existing back element There are occasional cases where you might construct a valid queue, and retain a direct pointer to the back element, but not the struct lqueue used to build it. This patch adds a new lqueue_init_from_back() macro to reconstruct a valid struct lqueue from the element pointer for cases like this. Signed-off-by: David Gibson --- ccan/lqueue/lqueue.h | 33 +++++++++++++++++++++++++++++++-- 1 file changed, 31 insertions(+), 2 deletions(-) diff --git a/ccan/lqueue/lqueue.h b/ccan/lqueue/lqueue.h index 15d242ef..1af5847f 100644 --- a/ccan/lqueue/lqueue.h +++ b/ccan/lqueue/lqueue.h @@ -50,6 +50,33 @@ struct lqueue { #define LQUEUE(name) \ struct lqueue name = { NULL, } +/** + * lqueue_init_from_back - initialize a queue with a specific back element + * @s: the lqueue to initialize + * @e: pointer to the back element of the new queue + * @member: member of the element containing the lqueue_link + * + * USE WITH CAUTION: This is for handling unusual cases where you have + * a pointer to an element in a previously constructed queue but can't + * conveniently pass around a normal struct lqueue. Usually you + * should use lqueue_init(). + * + * Example: + * LQUEUE(queue1); + * struct lqueue queue2; + * struct element { + * int value; + * struct lqueue_link link; + * } el; + * + * lqueue_enqueue(&queue1, &el, link); + * + * lqueue_init_from_back(&queue2, + * lqueue_back(&queue1, struct element, link), link); + */ +#define lqueue_init_from_back(s, e, member) \ + (lqueue_init_((s), &(e)->member)) + /** * lqueue_init - initialize a queue * @h: the lqueue to set to an empty queue @@ -58,9 +85,11 @@ struct lqueue { * struct lqueue *qp = malloc(sizeof(*qp)); * lqueue_init(qp); */ -static inline void lqueue_init(struct lqueue *q) +#define lqueue_init(s) \ + (lqueue_init_((s), NULL)) +static inline void lqueue_init_(struct lqueue *q, struct lqueue_link *back) { - q->back = NULL; + q->back = back; } /** -- 2.39.2