Tail queue位于/usr/include/x86_64-linux-gnu/sys/queue.h
queue.h包含以下几种数据结构：
- 双链表（List）
- 单链表（Singly-linked List）
- 单链尾队列（Singly-linked Tail queue）
- 简单队列（Simple queue）
- 双链尾队列（Tail queue）
- 循环队列（Circular queue）
Tail queue几个参数含义
HEADNAME
链表头结点的名字，用TAILQ_HEAD(HEADNAME, TYPE);定义
TYPE elm
用户定义结构体，该结构体必须用TAILQ_ENTRY(TYPE);定义一个变量field
head
头结点指针，即HEADNAME*
field
用TAILQ_ENTRY声明的变量名
对于上面demo，
HEADNAME是tailhead
TYPE elm是entry
head是tailhead*
field是entries
Tail queue代码
/*
* Tail queue declarations.
*/
#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 */ \
}
/*
* Tail queue functions.
*/
#define TAILQ_EMPTY(head) ((head)->tqh_first == NULL)
#define TAILQ_FIRST(head) ((head)->tqh_first)
#define TAILQ_FOREACH(var, head, field) \
for ((var) = TAILQ_FIRST((head)); \
(var); \
(var) = TAILQ_NEXT((var), field))
#define TAILQ_FOREACH_REVERSE(var, head, headname, field) \
for ((var) = TAILQ_LAST((head), headname); \
(var); \
(var) = TAILQ_PREV((var), headname, field))
#define TAILQ_INIT(head) do { \
TAILQ_FIRST((head)) = NULL; \
(head)->tqh_last = &TAILQ_FIRST((head)); \
} while (0)
#define TAILQ_INSERT_AFTER(head, listelm, elm, field) do { \
if ((TAILQ_NEXT((elm), field) = TAILQ_NEXT((listelm), field)) != NULL)\
TAILQ_NEXT((elm), field)->field.tqe_prev = \
&TAILQ_NEXT((elm), field); \
else \
(head)->tqh_last = &TAILQ_NEXT((elm), field); \
TAILQ_NEXT((listelm), field) = (elm); \
(elm)->field.tqe_prev = &TAILQ_NEXT((listelm), field); \
} while (0)
#define TAILQ_INSERT_BEFORE(listelm, elm, field) do { \
(elm)->field.tqe_prev = (listelm)->field.tqe_prev; \
TAILQ_NEXT((elm), field) = (listelm); \
*(listelm)->field.tqe_prev = (elm); \
(listelm)->field.tqe_prev = &TAILQ_NEXT((elm), field); \
} while (0)
#define TAILQ_INSERT_HEAD(head, elm, field) do { \
if ((TAILQ_NEXT((elm), field) = TAILQ_FIRST((head))) != NULL) \
TAILQ_FIRST((head))->field.tqe_prev = \
&TAILQ_NEXT((elm), field); \
else \
(head)->tqh_last = &TAILQ_NEXT((elm), field); \
TAILQ_FIRST((head)) = (elm); \
(elm)->field.tqe_prev = &TAILQ_FIRST((head)); \
} while (0)
#define TAILQ_INSERT_TAIL(head, elm, field) do { \
TAILQ_NEXT((elm), field) = NULL; \
(elm)->field.tqe_prev = (head)->tqh_last; \
*(head)->tqh_last = (elm); \
(head)->tqh_last = &TAILQ_NEXT((elm), field); \
} while (0)
#define TAILQ_LAST(head, headname) \
(*(((struct headname *)((head)->tqh_last))->tqh_last))
#define TAILQ_NEXT(elm, field) ((elm)->field.tqe_next)
#define TAILQ_PREV(elm, headname, field) \
(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
#define TAILQ_REMOVE(head, elm, field) do { \
if ((TAILQ_NEXT((elm), field)) != NULL) \
TAILQ_NEXT((elm), field)->field.tqe_prev = \
(elm)->field.tqe_prev; \
else \
(head)->tqh_last = (elm)->field.tqe_prev; \
*(elm)->field.tqe_prev = TAILQ_NEXT((elm), field); \
} while (0)其中TAILQ_HEAD是头结点，TAILQ_ENTRY是非头结点
代码实现中比较难理解的是TAILQ_LAST和TAILQ_PREV。二级指针为啥要转一级指针，它又是如何得到尾元素和上一个元素的。
接下来通过一个demo来打印出Tail queue各个结点的地址和值。
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
#define ITEM_NUM 4
// #pragma pack(1)
// 由于字节对齐，一共占用24字节
typedef struct _QUEUE_ITEM {
int value;
TAILQ_ENTRY(_QUEUE_ITEM) entries;
}QUEUE_ITEM;
// #pragma pack()
TAILQ_HEAD(TAIL_QUEUE, _QUEUE_ITEM) queue_head;
int main(int argc, char **argv) {
QUEUE_ITEM *item[ITEM_NUM];
TAILQ_INIT(&queue_head);
int i = 0;
for (i = 0; i < ITEM_NUM; ++i) {
item[i] = (QUEUE_ITEM*)malloc(sizeof(QUEUE_ITEM));
item[i]->value = i;
TAILQ_INSERT_TAIL(&queue_head, item[i], entries);
}
printf("queue_head:%p, first:%p, last:%p\n", (void*)&queue_head, (void*)queue_head.tqh_first, (void*)queue_head.tqh_last);
for(i = 0; i < ITEM_NUM; ++i)
{
printf("item[%d]: item:%p, next:%p, &next:%p, prev:%p, *prev:%p\n",
i, (void*)item[i], (void*)item[i]->entries.tqe_next,
(void*)&(item[i]->entries.tqe_next), (void*)item[i]->entries.tqe_prev, (void*)(*item[i]->entries.tqe_prev));
}
printf("last item:%p\n", TAILQ_LAST(&queue_head, TAIL_QUEUE));
printf("sizeof(QUEUE_ITEM): %lu\n", sizeof(QUEUE_ITEM));
return 0;
}
运行输出：
通过上面输出可以得到Tail queue的内存布局如下：
粉红色的是域的地址，绿色是域值，0 1 2 3是data
注意上图标错了，entry指的tqe_next和tqe_prev，不包括data。
现在再来看TAILQ_LAST
#define TAILQ_LAST(head, headname) \
(*(((struct headname *)((head)->tqh_last))->tqh_last))((head)->tqh_last)指向尾元素的tqe_next域的地址，即获取到尾元素的tqe_next域的地址0x1f35078
((struct headname *)((head)->tqh_last))将0x1f35078强转为头结点指针。为什么能强转呢？因为头结点和非头结点的内存布局是一样的（都是两个指针）。
(((struct headname *)((head)->tqh_last))->tqh_last)的意思是获取尾元素tqe_prev域的值0x1f35058，可以理解为0x1f35078+8后解引用。0x1f35058即倒数第二个结点的tqe_next域的地址。
(*(((struct headname *)((head)->tqh_last))->tqh_last)) 最后对0x1f35058解引用，得到0x1f35070，即尾元素的首地址，此时拿到尾元素指针。
理解了TAILQ_LAST再看TAILQ_PREV就比较简单
#define TAILQ_PREV(elm, headname, field) \
(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))假设当前节点是节点2
((elm)->field.tqe_prev)拿到节点1的next域的地址0x1f35038
(((struct headname *)((elm)->field.tqe_prev))->tqh_last)拿到结点0的next域的地址0x1f35018
(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))对0x1f35018解引用得到节点1的首地址0x1f35030，即指向节点1的指针。