libevent源碼分析(三)

上一篇
本節不講具體的代碼分析，剖析一下libevent底層的鏈表結構。

一、重要性

struct event { /*...*/                  |       struct event_base {
	TAILQ_ENTRY (event) ev_next;        |           /*此處省好多...*/
	TAILQ_ENTRY (event) ev_active_next; |           /* active event management */
	TAILQ_ENTRY (event) ev_signal_next; |           struct event_list **activequeues;
};                                      |       };

雙向鏈表struct event_list在struct event_base中扮演這重要的角色，它負責管理觸發事件。由：

/*libevent-patches-1.4\compat\sys\queue.h*/
#define TAILQ_HEAD(name, type)						\
struct name {								\
	struct type *tqh_first;	/* first element */			\
	struct type **tqh_last;	/* addr of last next element */		\
}

將TAILQ_HEAD (event_list, event)宏藉助於具體的宏定義展開得event_list定義：

/*libevent-patches-1.4\event.h*/
struct event_list {								\
	struct event *tqh_first;	/* first element */			\
	struct event **tqh_last;	/* addr of last next element */		\
};

當然對於struct event來說上面所示的幾個成員展開後你將發現其和event_list具有相似結構，在以後的事件鏈接等方面它是將event事件順利掛在event_base->activequeues上的關鍵，不信你看吧！

/*libevent-patches-1.4\event.h*/
#define TAILQ_ENTRY(type)						\
struct {								\
	struct type *tqe_next;	/* next element */			\
	struct type **tqe_prev;	/* address of previous next element */	\
}

顯然將TAILQ_ENTRY (event)展開會得到一個匿名文件結構：

struct {								
	struct event *tqe_next;	/* next element */			
	struct event **tqe_prev;	/* address of previous next element */	
};

二、具體應用

1.鏈表的初始化：TAILQ_INIT

    /*libevent-patches-1.4\event.c*/
	base->activequeues = (struct event_list **)
	    calloc(base->nactivequeues, sizeof(struct event_list *)); ...
	for (i = 0; i < base->nactivequeues; ++i) {
		base->activequeues[i] = malloc(sizeof(struct event_list));
		...
		TAILQ_INIT(base->activequeues[i]);
	}
	/*libevent-patches-1.4\compat\sys\queue.h*/
	#define	TAILQ_INIT(head) do {						\
	(head)->tqh_first = NULL;					\
	(head)->tqh_last = &(head)->tqh_first;				\
    } while (0)

2.鏈表的尾部插入：TAILQ_INSERT_TAIL

示例：TAILQ_INSERT_TAIL(&base->eventqueue, ev, ev_next);

//libevent-patches-1.4\compat\sys\queue.h
#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)

宏展開得：

do{
   ev->ev_next.tqe_next = NULL; 
   ev->ev_next.tqe_prev = base->eventqueue->tqh_last;
   *(base->eventqueue)->tqh_last = ev;   //->優先級要大於*
   (base->eventqueue)->tqh_last = &(ev)->ev_next.tqe_next;
} while (0);

你會發現其構造及其巧妙，最好在紙上推一推…
這個宏可以將所有的監聽事件都加入到base->eventqueue上，該結構是一個struct event_list結構，其第一個成員tqh_first是第一個event事件的地址，而tqh_last最終將指向鏈表最後一個事件的ev->ev_next.tqe_next成員，而ev->ev_next.tqe_prev成員則永遠指向鏈表中的上一個event事件。

3.鏈表的頭部插入：TAILQ_INSERT_HEAD

示例：TAILQ_INSERT_HEAD(&base->eventqueue, ev, ev_next);

#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)

宏展開得：

do{
	if ((ev->ev_next.tqe_next = base->eventqueue->tqh_first) != NULL)	
		base->eventqueue->tqh_first->ev_next.tqe_prev =			
		    &ev->ev_next.tqe_next;				
	else								
		base->eventqueue->tqh_last = &ev->ev_next.tqe_next;		
	base->eventqueue->tqh_first = ev;					
	ev->ev_next.tqe_prev = &base->eventqueue->tqh_first;			
} while (0);

可以發現無非是初始化當前事件的ev->ev_next.tqe_next成員爲下一個event事件地址，然後更新base->eventqueue->tqh_first值使其永遠指向第一個事件，最後設置當前頭部事件的ev->ev_next.tqe_prev成員值爲自身event指針的指針。

4.鏈表的移除：TAILQ_REMOVE

示例：TAILQ_REMOVE(&base->eventqueue, ev, ev_next);

//libevent-patches-1.4\compat\sys\queue.h
#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)

宏展開得：

do{
	if ((ev->ev_next.tqe_next) != NULL)				
		ev->ev_next.tqe_next->ev_next.tqe_prev =			
		    ev->ev_next.tqe_prev;				
	else								
		base->eventqueue->tqh_last = ev->ev_next.tqe_prev;		
	*ev->ev_next.tqe_prev = ev->ev_next.tqe_next;			
} while (0);

只要掌握了鏈表的插入操作，那麼刪除操作簡單了。分兩種情況操作，若其是最後一個事件則將base->eventqueue的tqh_last 成員向前移一位；否則將後一個事件ev->ev_next.tqe_next的前一個事件tqe_prev設置爲當前事件的ev_next.tqe_prev事件。最後調整前一個事件*(ev->ev_next.tqe_pre)指向後一個事件ev->ev_next.tqe_next即可。

三、總結

一直以爲這個中的鏈表爲雙向鏈表，想當然的以爲ev->ev_next.tqe_prev成員指向前一個事件，讀完源碼才發現tqe_prev成員似乎仍舊保存的是自身event的指針的指針(因爲它保存前一個事件的ev->ev_next.tqe_next的地址)，那麼設置該成員的意義何在？總之他應該會在某處用到，待以後遇到再說。現在先來看一看某個源文件中關於鏈表的使用代碼：
#define TAILQ_FIRST(head) ((head)->tqh_first)

    //libevent-patches-1.4\event.c
    struct event_list *activeq;
	for (ev = TAILQ_FIRST(activeq); ev; ev = TAILQ_FIRST(activeq)) {
		if (ev->ev_events & EV_PERSIST)
			event_queue_remove(base, ev, EVLIST_ACTIVE);
		else
			event_del(ev);
        ...
	}

此處是每次都將頭部事件刪去，故每次都可以直接取tqh_first即可，至於使用ev->ev_next.tqe_prev的場景目前尚未遇到。
下一篇