Linux queue.h之TAILQ队列分析

这两天想看看memcached的实现,所以先学习了libevent,使用起来还是比较简单的,其实是对select/poll/kqueue等的封装,学习libevent过程中又遇到了linux下队列的使用,简单分析如下,权当做记录:

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

#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

#include
#include
#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:

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:

 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

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

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

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);

队列初始状态如下：

向空队列中插入第一个元素过程如下：

也就是我们的循环执行下面代码段,结果分析见图2,3

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

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

#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);

}