在queue.h这个文件中，实现了：
- 单链表（Singly-linked List）
- 简单队列（Simple queue）
- 特殊队列（Tail queue）
/*
* Tail queue definitions.
*/
#define TAILQ_HEAD(name, type) \
struct name { \
struct type *tqh_first; /* first element */ \
struct type **tqh_last; /* addr of last next element */ \
}
#define TAILQ_HEAD_INITIALIZER(head) \
{ NULL, &(head).tqh_first }
#define TAILQ_ENTRY(type) \
struct { \
struct type *tqe_next; /* next element */ \
struct type **tqe_prev; /* address of previous next element */ \
}
libevent中的 Tail queue 是通过双向链表+一个头尾指针结构体来实现的。注意这两个定义：
- #define TAILQ_HEAD(name, type)。指向队列的头，尾。
- #define TAILQ_ENTRY(type)。队列的前后节点指针。
/*
* tail queue access methods
*/
#define TAILQ_FIRST(head) ((head)->tqh_first)
#define TAILQ_END(head) NULL
#define TAILQ_NEXT(elm, field) ((elm)->field.tqe_next)
#define TAILQ_LAST(head, headname) \
(*(((struct headname *)((head)->tqh_last))->tqh_last))
/* XXX */
#define TAILQ_PREV(elm, headname, field) \
(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
#define TAILQ_EMPTY(head) \
(TAILQ_FIRST(head) == TAILQ_END(head))
#define TAILQ_FOREACH(var, head, field) \
for((var) = TAILQ_FIRST(head); \
(var) != TAILQ_END(head); \
(var) = TAILQ_NEXT(var, field))
#define TAILQ_FOREACH_REVERSE(var, head, headname, field) \
for((var) = TAILQ_LAST(head, headname); \
(var) != TAILQ_END(head); \
(var) = TAILQ_PREV(var, headname, field))
这些主要是一些操作 Tail queue 的开放接口。
/*
* Tail queue functions.
*/
#define TAILQ_INIT(head) do { \
(head)->tqh_first = NULL; \
(head)->tqh_last = &(head)->tqh_first; \
} while (0)
#define TAILQ_INSERT_HEAD(head, elm, field) do { \
if (((elm)->field.tqe_next = (head)->tqh_first) != NULL) \
(head)->tqh_first->field.tqe_prev = \
&(elm)->field.tqe_next; \
else \
(head)->tqh_last = &(elm)->field.tqe_next; \
(head)->tqh_first = (elm); \
(elm)->field.tqe_prev = &(head)->tqh_first; \
} while (0)
#define TAILQ_INSERT_TAIL(head, elm, field) do { \
(elm)->field.tqe_next = NULL; \
(elm)->field.tqe_prev = (head)->tqh_last; \
*(head)->tqh_last = (elm); \
(head)->tqh_last = &(elm)->field.tqe_next; \
} while (0)
#define TAILQ_INSERT_AFTER(head, listelm, elm, field) do { \
if (((elm)->field.tqe_next = (listelm)->field.tqe_next) != NULL)\
(elm)->field.tqe_next->field.tqe_prev = \
&(elm)->field.tqe_next; \
else \
(head)->tqh_last = &(elm)->field.tqe_next; \
(listelm)->field.tqe_next = (elm); \
(elm)->field.tqe_prev = &(listelm)->field.tqe_next; \
} while (0)
#define TAILQ_INSERT_BEFORE(listelm, elm, field) do { \
(elm)->field.tqe_prev = (listelm)->field.tqe_prev; \
(elm)->field.tqe_next = (listelm); \
*(listelm)->field.tqe_prev = (elm); \
(listelm)->field.tqe_prev = &(elm)->field.tqe_next; \
} while (0)
#define TAILQ_REMOVE(head, elm, field) do { \
if (((elm)->field.tqe_next) != NULL) \
(elm)->field.tqe_next->field.tqe_prev = \
(elm)->field.tqe_prev; \
else \
(head)->tqh_last = (elm)->field.tqe_prev; \
*(elm)->field.tqe_prev = (elm)->field.tqe_next; \
} while (0)
#define TAILQ_REPLACE(head, elm, elm2, field) do { \
if (((elm2)->field.tqe_next = (elm)->field.tqe_next) != NULL) \
(elm2)->field.tqe_next->field.tqe_prev = \
&(elm2)->field.tqe_next; \
else \
(head)->tqh_last = &(elm2)->field.tqe_next; \
(elm2)->field.tqe_prev = (elm)->field.tqe_prev; \
*(elm2)->field.tqe_prev = (elm2); \
} while (0)
这些是 Tail queue 的底层实现。
整个 Tail queue 的结构如下图：
一级指针指向的是queue_entry_t这个结构体，即存储queue_entry_t这个结构体的地址值。二级指针存储的是一级地址变量的地址值。所以二级指针指向的是图中的一级指针，而非结构体。
这种实现主要是使用 二级指针来实现使链表头尾连接起来，形成一个队列。很是巧妙。
libevent中event结构是通过 Tail queue 串联起来的。因为是队列结构，所以事件的到来是具有队列的特性（先进先出），就是先来的事件是先触发的。
