libevent代码阅读（2）——尾队列的学习

libevent当中大量使用了linux内置的一个数据结构——尾队列

下面是它的使用方法：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

int main()
{
    /*
      定义一个结构体，它只是尾队列的一个元素
      它必须包含一个TAILQ_ENTRY来指向上一个和下一个元素
    */
    struct tailq_entry {
        int value;

        // 指向上一个元素和下一个元素
        TAILQ_ENTRY(tailq_entry) entries;
    };

    //定义队列的头部
    TAILQ_HEAD(MyQueType, tailq_entry) my_tailq_head;

    // 队列初始化
    TAILQ_INIT(&my_tailq_head);

    //定义一个结构体指针
    struct tailq_entry *item;
    //定义另外一个指针
    struct tailq_entry *tmp_item;

    int i;
    //在队列里添加10个元素
    for(i=0; i<10; i++)
    {
        //申请内存空间
        item = (tailq_entry*)malloc(sizeof(tailq_entry));
        if (item == NULL)
        {
            perror("malloc failed");
            exit(-1);
        }
        //设置值
        item->value = i;

        /*
               将元素加到队列尾部
               参数1：指向队列头的指针
               参数2：要添加的元素
               参数3：结构体的变量名
            */
        TAILQ_INSERT_TAIL(&my_tailq_head, item, entries);
    }

    //遍历队列
    printf("Forward traversal: ");
    TAILQ_FOREACH(item, &my_tailq_head, entries)
    {
        printf("%d ",item->value);
    }
    printf("\n");


    return 0;
}

尾队列的定义

/*
 * Tail queue definitions.
 */
/*
 * 尾队列的定义
 * TAILQ_HEAD(name, type)定义一个名字为name，元素类型为type的双向队列
 * tqh_first访问第一个元素
 * tqh_last指向最后一个TAILQ_ENTRY，最后一个TAILQ_ENTRY的tqe_next为空
 * 最后一个TAILQ_ENTRY的tqe_prev指向前一个TAILQ_ENTRY，
 * 倒数第二个TAILQ_ENTRY的tqe_next指向最后一个元素
 */
#define TAILQ_HEAD(name, type)						\
struct name {								\
	struct type *tqh_first;	/* first element */			\
	struct type **tqh_last;	/*最后一个元素的地址*//* addr of last next element */		\
}

// 尾队列头部初始化
#define TAILQ_HEAD_INITIALIZER(head)					\
	{ NULL, &(head).tqh_first }

/*
 * TAILQ_ENTRY(type)定义了尾队列元素访问相邻元素的入口点，即每一个元素中有一个字段为
 * 该结构的对象，用于访问相邻元素
 */
#define TAILQ_ENTRY(type)						\
struct {								\
	struct type *tqe_next;	/* next element */			\
	struct type **tqe_prev;	/* address of previous next element */	\
}

/*
 * tail queue access methods
 */
// 尾队列头元素
#define	TAILQ_FIRST(head)		((head)->tqh_first)
// 尾队列的尾
#define	TAILQ_END(head)			NULL
// 下一个元素，访问下一个元素使用elemt的field域的tqe_next
#define	TAILQ_NEXT(elm, field)		((elm)->field.tqe_next)
// 最后一个元素
#define TAILQ_LAST(head, headname)					\
	(*(((struct headname *)((head)->tqh_last))->tqh_last))
/* XXX */
// 当前元素的上一个元素
#define TAILQ_PREV(elm, headname, field)				\
	(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))

// 判断队列是否为空
#define	TAILQ_EMPTY(head)						\
	(TAILQ_FIRST(head) == TAILQ_END(head))

// 正向遍历
#define TAILQ_FOREACH(var, head, field)					\
	for((var) = TAILQ_FIRST(head);					\
	    (var) != TAILQ_END(head);					\
	    (var) = TAILQ_NEXT(var, field))

// 反向遍历
#define TAILQ_FOREACH_REVERSE(var, head, headname, field)		\
	for((var) = TAILQ_LAST(head, headname);				\
	    (var) != TAILQ_END(head);					\
	    (var) = TAILQ_PREV(var, headname, field))

/*
 * Tail queue functions.
 */
// 尾队列初始化
#define	TAILQ_INIT(head) do {						\
	(head)->tqh_first = NULL;					\
	(head)->tqh_last = &(head)->tqh_first;				\
} while (0)

// 在队列头插入元素
#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)

// 在队列尾插入元素
#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)

// 在指定元素的后面插入元素
#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)

// 在指定元素的前面插入元素
#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)

// 删除一个元素
#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)

// 替换一个元素
#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)