Linux queue.h之TAILQ隊列分析

轉自

這兩天想看看memcached的實現,因此先學習了libevent,使用起來仍是比較簡單的,實際上是對select/poll/kqueue等的封裝,學習libevent過程當中又遇到了linux下隊列的使用,簡單分析以下,權當作記錄: 

libevent中的例子中使用的是FreeBSD下的queue.h,在linux的/usr/include/sys/queue.h也有該頭文件,可是是一個縮減版本,並且沒有看到queue 的access method,不知道是否是跟咱們的linux服務器版本有關,沒辦法google了一下,找到了FreeBSD 下queue.h的定義,咱們看一下tail queue的定義 

#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)
....

咱們就先分析上面的這些定義,先看個應用的例子 

#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);

}

結果: 

first element is 5
next element is 7
next element is 100
next element is 9

分析: 
QUEUE_ITEM 是咱們定義的存放在隊列裏的東東,簡單起見只包括一個int值 
TAILQ_ENTRY(QUEUE_ITEM) entries 主要是存放下一個對象和前一個對象的指針,具體見 header 
  
根據頭文件進行宏替換後,實際咱們聲明的是這樣的結構: 

 struct QUEUE_ITEM{
    int value;
        struct {
            struct QUEUE_ITEM *tqe_next;
        struct QUEUE_ITEM **tqe_prev;
        }entries;
};

TAILQ_HEAD(,QUEUE_ITEM) queue_head; 實際是 

struct {
    struct QUEUE_ITEM *tqh_first;
    struct QUEUE_ITEM **tqh_last;
}queue_head;

接着咱們定義了QUEUE_ITEM的兩個指針變量item和tmp_item 

TAILQ_INIT(&queue_head); 至關因而 

do {
    (&queue_head)->tqh_first = NULL;
    (&queue_head)->tqh_last = &(&queue_head)->tqh_first;
} while (0);

head的初始化如 下圖1 

接着咱們經過循環分配了幾個元素,並賦值 

TAILQ_INSERT_TAIL(&queue_head, item, entries); 至關於執行

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);

也就是咱們的循環執行下面代碼段,結果分析見圖2,3 

for(i=5;i<10;i+=2){
    item=malloc(sizeof(item));
    item->value=i;
    do {
        (item)->entries.tqe_next = NULL;
        //首次執行至關於item->entries.tqe_prev=&(&queue_head)->tqh_first
        //之後執行至關因而(item)->entries.tqe_prev=&(前一個item)->entries.tqe_next;
        (item)->entries.tqe_prev = (&queue_head)->tqh_last;
        //首次執行至關於(&queue_head)->tqh_first=item
        //之後執行至關因而(前一個item)->entries.tqe_next=當前item
        *(&queue_head)->tqh_last = (item);
        (&queue_head)->tqh_last = &(item)->entries.tqe_next;
    } while (0);
}

最終創建的鏈表結構如圖,下面看一下insert操做,通過宏替換後代碼以下

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);
    //這句話體現了TAILQ的特點,tqe_prev是前一個元素的下個元素地址,
    //因此正好應該是當前插入item的地址
    *(item)->entries.tqe_prev = (ins_item);
    (item)->entries.tqe_prev = &(ins_item)->entries.tqe_next;
} while (0);

 

總結:TAILQ的最大特色就是每一個entry的二級指針tqe_prev其存放的是前一個元素的下個元素地址,呵呵,聽起來都很拗口 
我如今就是不知道爲何linux的queue.h只有創建tailq的宏定義而缺乏全部的access method,初涉linux c編程,請你們指教 

附通過宏替換後的全部代碼 

#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;
            //首次執行至關於item->entries.tqe_prev=&(&queue_head)->tqh_first
            //之後執行至關因而(item)->entries.tqe_prev=&(前一個item)->entries.tqe_next;
            (item)->entries.tqe_prev = (&queue_head)->tqh_last;
            //首次執行至關於(&queue_head)->tqh_first=item
            //之後執行至關因而(前一個item)->entries.tqe_next=當前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);

}