Tail queue位于/usr/include/x86 _ 64-Linux-GNU/sys/queue.h
queue.h包含以下数据结构：
双链表(List )单链表(单链尾队列(单链尾队列)单链尾队列(Tail queue )
头名
链表开头节点的名称为Tailq_head(headname，TYPE )； 定义
类型elm
用户定义必须使用Tailq_entry(type )的结构； 定义变量字段
头戴
头节点指针，即头名称*
字段
用TAILQ_ENTRY声明的变量名称
对于上面的demo，
头名是tailhead
类型elm是条目
头是tailhead*
字段为entries
Tail queue代码/* * tailqueuedeclarations.*/# definetailq _ head (name，type ) structname(structtype ) tqh _ fir sion/* addroflastnextelement *//} # definetailq _ head _ initializer (head ){ NULL，) head ).tqh _ first } # defifiniad/* addressofpreviousnextelement *//}/* * tailqueuefunctions.*/# definetailq _ empty (head ) )-tqh _ first=(var )=tailq_next () var ()，field ) ) definetailq_foreach_reverse ) var，head，headname，field ) ) for (var (VAD (var )=Tailq_prev )、headname、field ) )、definetailq _ init (head ) do{Tailq_first ) ) head } (((}while(0) #definetailq_insert_after ) head，listelm，elm，field ) do(if ) ) t
AILQ_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)#defineTAILQ_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)#defineTAILQ_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)#defineTAILQ_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)#defineTAILQ_LAST(head, headname)(*(((struct headname *)((head)->tqh_last))->tqh_last))#defineTAILQ_NEXT(elm, field) ((elm)->field.tqe_next)#defineTAILQ_PREV(elm, headname, field)(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))#defineTAILQ_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:%pn", (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:%pn", 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:%pn", TAILQ_LAST(&queue_head, TAIL_QUEUE)); printf("sizeof(QUEUE_ITEM): %lun", 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
#defineTAILQ_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
#defineTAILQ_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。