Linux queue.h之TAILQ队列分析
2016-08-21 15:08
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from http://blog.chinaunix.net/uid-26657936-id-3129789.html
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
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
#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:
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);
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
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
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);
}
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
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
#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:
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);
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
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
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);
}
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