Linux queue.h之TAILQ队列分析

libevent中的例子中使用的是FreeBSD下的queue.h,在linux的/usr/include/sys/queue.h也有该头文件,但是是一个缩减版本,而且没有看到queue 的access method,不知道是不是跟我们的linux服务器版本有关,没办法google了一下,找到了FreeBSD 下queue.h的定义,我们看一下tail
queue的定义

C代码


#define TAILQ_HEAD(name, type) \

struct name { \

struct type *tqh_first; /* first element */ \

struct type **tqh_last; /* addr of last next element */\

}



#define TAILQ_ENTRY(type) \

struct { \

struct type *tqe_next; /* next element */ \

struct type **tqe_prev;/* addr of previous next element*/ \

}



#define TAILQ_INIT(head) do { \

(head)->tqh_first = NULL; \

(head)->tqh_last = &(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_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_FIRST(head) ((head)->tqh_first)



#define TAILQ_NEXT(elm, field) ((elm)->field.tqe_next)

....

我们就先分析上面的这些定义,先看个应用的例子

C代码


#include <stdio.h>

#include <stdlib.h>

#include "queue.h"



struct QUEUE_ITEM{

int value;

TAILQ_ENTRY(QUEUE_ITEM) entries;

};

TAILQ_HEAD(,QUEUE_ITEM) queue_head;

int main(int argc,char **argv){

struct QUEUE_ITEM *item;

struct QUEUE_ITEM *tmp_item;



TAILQ_INIT(&queue_head);

int i=0;

for(i=5;i<10;i+=2){

item=malloc(sizeof(item));

item->value=i;

TAILQ_INSERT_TAIL(&queue_head, item, entries);

}



struct QUEUE_ITEM *ins_item;

ins_item=malloc(sizeof(ins_item));



ins_item->value=100;

TAILQ_INSERT_BEFORE(item,ins_item,entries);





tmp_item=TAILQ_FIRST(&queue_head);

printf("first element is %d\n",tmp_item->value);



tmp_item=TAILQ_NEXT(tmp_item,entries);

printf("next element is %d\n",tmp_item->value);



tmp_item=TAILQ_NEXT(tmp_item,entries);

printf("next element is %d\n",tmp_item->value);



tmp_item=TAILQ_NEXT(tmp_item,entries);

printf("next element is %d\n",tmp_item->value);



}

结果:

Java代码


first element is 5

next element is 7

next element is 100

next element is 9

分析:

QUEUE_ITEM 是我们定义的存放在队列里的东东,简单起见只包括一个int值

TAILQ_ENTRY(QUEUE_ITEM) entries 主要是存放下一个对象和前一个对象的指针,具体见 header



根据头文件进行宏替换后,实际我们声明的是这样的结构:

C代码


struct QUEUE_ITEM{

int value;

struct {

struct QUEUE_ITEM *tqe_next;

struct QUEUE_ITEM **tqe_prev;

}entries;

};



TAILQ_HEAD(,QUEUE_ITEM) queue_head; 实际是

C代码


struct {

struct QUEUE_ITEM *tqh_first;

struct QUEUE_ITEM **tqh_last;

}queue_head;

接着我们定义了QUEUE_ITEM的两个指针变量item和tmp_item

TAILQ_INIT(&queue_head); 相当于是

C代码


do {

(&queue_head)->tqh_first = NULL;

(&queue_head)->tqh_last = &(&queue_head)->tqh_first;

} while (0);

head的初始化如 下图1

接着我们通过循环分配了几个元素,并赋值

C代码


TAILQ_INSERT_TAIL(&queue_head, item, entries); 相当于执行



do {

(item)->entries.tqe_next = NULL;

(item)->entries.tqe_prev = (&queue_head)->tqh_last;

*(&queue_head)->tqh_last = (item);

(&queue_head)->tqh_last = &(item)->entries.tqe_next;

} while (0);

也就是我们的循环执行下面代码段,结果分析见图2,3

C代码


for(i=5;i<10;i+=2){

item=malloc(sizeof(item));

item->value=i;

do {

(item)->entries.tqe_next = NULL;

//首次执行相当于item->entries.tqe_prev=&(&queue_head)->tqh_first

//以后执行相当于是(item)->entries.tqe_prev=&(前一个item)->entries.tqe_next;

(item)->entries.tqe_prev = (&queue_head)->tqh_last;

//首次执行相当于(&queue_head)->tqh_first=item

//以后执行相当于是(前一个item)->entries.tqe_next=当前item

*(&queue_head)->tqh_last = (item);

(&queue_head)->tqh_last = &(item)->entries.tqe_next;

} while (0);

}

C代码


最终建立的链表结构如图,下面看一下insert操作,经过宏替换后代码如下



struct QUEUE_ITEM *ins_item;

ins_item=malloc(sizeof(ins_item));

ins_item->value=100;



do {

(ins_item)->entries.tqe_prev = (item)->entries.tqe_prev;

(ins_item)->entries.tqe_next = (item);

//这句话体现了TAILQ的特色,tqe_prev是前一个元素的下个元素地址,

//所以正好应该是当前插入item的地址

*(item)->entries.tqe_prev = (ins_item);

(item)->entries.tqe_prev = &(ins_item)->entries.tqe_next;

} while (0);



总结:TAILQ的最大特点就是每个entry的二级指针tqe_prev其存放的是前一个元素的下个元素地址,呵呵,听起来都很拗口

我现在就是不知道为什么linux的queue.h只有建立tailq的宏定义而缺少所有的access method,初涉linux c编程,请大家指教

附经过宏替换后的所有代码

C代码


#include "stdio.h"

#include "stdlib.h"

struct QUEUE_ITEM{

int value;

struct {

struct QUEUE_ITEM *tqe_next;

struct QUEUE_ITEM **tqe_prev;

}entries;

};

struct {

struct QUEUE_ITEM *tqh_first;

struct QUEUE_ITEM **tqh_last;

}queue_head;



int main(int argc,char **argv){

struct QUEUE_ITEM *item;

struct QUEUE_ITEM *tmp_item;



do {

(&queue_head)->tqh_first = NULL;

(&queue_head)->tqh_last = &(&queue_head)->tqh_first;

} while (0);



int i=0;

for(i=5;i<10;i+=2){

item=malloc(sizeof(item));

item->value=i;

do {

(item)->entries.tqe_next = NULL;

//首次执行相当于item->entries.tqe_prev=&(&queue_head)->tqh_first

//以后执行相当于是(item)->entries.tqe_prev=&(前一个item)->entries.tqe_next;

(item)->entries.tqe_prev = (&queue_head)->tqh_last;

//首次执行相当于(&queue_head)->tqh_first=item

//以后执行相当于是(前一个item)->entries.tqe_next=当前item

*(&queue_head)->tqh_last = (item);

(&queue_head)->tqh_last = &(item)->entries.tqe_next;

} while (0);

}



struct QUEUE_ITEM *ins_item;

ins_item=malloc(sizeof(ins_item));



ins_item->value=100;

do {

(ins_item)->entries.tqe_prev = (item)->entries.tqe_prev;

(ins_item)->entries.tqe_next = (item);

*(item)->entries.tqe_prev = (ins_item);

(item)->entries.tqe_prev = &(ins_item)->entries.tqe_next;

} while (0);



tmp_item=((&queue_head)->tqh_first);

printf("first element is %d\n",tmp_item->value);



tmp_item=((tmp_item)->entries.tqe_next);

printf("next element is %d\n",tmp_item->value);



tmp_item=((tmp_item)->entries.tqe_next);

printf("next element is %d\n",tmp_item->value);



tmp_item=((tmp_item)->entries.tqe_next);

printf("next element is %d\n",tmp_item->value);



}