linux内核数据结构之链表【转】

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1、前言

　　 最近写代码需用到链表结构，正好公共库有关于链表的。第一眼看时，觉得有点新鲜，和我之前见到的链表结构不一样，只有前驱和后继指针，而没有数据域。后来看代码注释发现该代码来自linux内核，在linux源代码下include/Lish.h下。这个链表具备通用性，使用非常方便。只需要在结构定义一个链表结构就可以使用。

2、链表介绍

　　链表是非常基本的数据结构，根据链个数分为单链表、双链表，根据是否循环分为单向链表和循环链表。通常定义定义链表结构如下：

typedef struct node
{
     ElemType data;      //数据域
     struct node *next;  //指针域
}node, *list;

链表中包含数据域和指针域。链表通常包含一个头结点，不存放数据，方便链表操作。单向循环链表结构如下图所示：

双向循环链表结构如下图所示：

　　这样带数据域的链表降低了链表的通用性，不容易扩展。linux内核定义的链表结构不带数据域，只需要两个指针完成链表的操作。将链表节点加入数据结构，具备非常高的扩展性，通用性。链表结构定义如下所示：

struct list_head {
    struct list_head *next, *prev;
};

链表结构如下所示：

　　需要用链表结构时，只需要在结构体中定义一个链表类型的数据即可。例如定义一个app_info链表，

1 typedef struct application_info
2 {
3     uint32_t  app_id;
4     uint32_t  up_flow;
5     uint32_t  down_flow;
6     struct    list_head app_info_head;  //链表节点
7 }app_info;

定义一个app_info链表，app_info app_info_list;通过app_info_head进行链表操作。根据C语言指针操作，通过container_of和offsetof，可以根据app_info_head的地址找出app_info的起始地址，即一个完整ap_info结构的起始地址3、linux内核链表实现

　　内核实现的是双向循环链表，提供了链表操作的基本功能。

（1）初始化链表头结点

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
    struct list_head name = LIST_HEAD_INIT(name)

static inline void INIT_LIST_HEAD(struct list_head *list)
{
    list->next = list;
    list->prev = list;
}

LIST_HEAD宏创建一个链表头结点，并用LIST_HEAD_INIT宏对头结点进行赋值，使得头结点的前驱和后继指向自己。

INIT_LIST_HEAD函数对链表进行初始化，使得前驱和后继指针指针指向头结点。

（2）插入节点

1 static inline void __list_add(struct list_head *new,
 2                   struct list_head *prev,
 3                   struct list_head *next)
 4 {
 5     next->prev = new;
 6     new->next = next;
 7     new->prev = prev;
 8     prev->next = new;
 9 }
10 
11 static inline void list_add(struct list_head *new, struct list_head *head)
12 {
13     __list_add(new, head, head->next);
14 }
15 
16 static inline void list_add_tail(struct list_head *new, struct list_head *head)
17 {
18     __list_add(new, head->prev, head);
19 }

　　插入节点分为从链表头部插入list_add和链表尾部插入list_add_tail，通过调用__list_add函数进行实现，head->next指向之一个节点，head->prev指向尾部节点。

（3）删除节点

1 static inline void __list_del(struct list_head * prev, struct list_head * next)
 2 {
 3     next->prev = prev;
 4     prev->next = next;
 5 }
 6 
 7 static inline void list_del(struct list_head *entry)
 8 {
 9     __list_del(entry->prev, entry->next);
10     entry->next = LIST_POISON1;
11     entry->prev = LIST_POISON2;
12 }

　　从链表中删除一个节点，需要改变该节点前驱节点的后继结点和后继结点的前驱节点。最后设置该节点的前驱节点和后继结点指向LIST_POSITION1和LIST_POSITION2两个特殊值，这样设置是为了保证不在链表中的节点项不可访问，对LIST_POSITION1和LIST_POSITION2的访问都将引起页故障

/*
 * These are non-NULL pointers that will result in page faults
 * under normal circumstances, used to verify that nobody uses
 * non-initialized list entries.
 */
#define LIST_POISON1  ((void *) 0x00100100 + POISON_POINTER_DELTA)
#define LIST_POISON2  ((void *) 0x00200200 + POISON_POINTER_DELTA)

（4）移动节点

1 /**
 2  * list_move - delete from one list and add as another's head
 3  * @list: the entry to move
 4  * @head: the head that will precede our entry
 5  */
 6 static inline void list_move(struct list_head *list, struct list_head *head)
 7 {
 8     __list_del(list->prev, list->next);
 9     list_add(list, head);
10 }
11 
12 /**
13  * list_move_tail - delete from one list and add as another's tail
14  * @list: the entry to move
15  * @head: the head that will follow our entry
16  */
17 static inline void list_move_tail(struct list_head *list,
18                   struct list_head *head)
19 {
20     __list_del(list->prev, list->next);
21     list_add_tail(list, head);
22 }

move将一个节点移动到头部或者尾部。

（5）判断链表

1 /**
 2  * list_is_last - tests whether @list is the last entry in list @head
 3  * @list: the entry to test
 4  * @head: the head of the list
 5  */
 6 static inline int list_is_last(const struct list_head *list,
 7                 const struct list_head *head)
 8 {
 9     return list->next == head;
10 }
11 
12 /**
13  * list_empty - tests whether a list is empty
14  * @head: the list to test.
15  */
16 static inline int list_empty(const struct list_head *head)
17 {
18     return head->next == head;
19 }

list_is_last函数判断节点是否为末尾节点，list_empty判断链表是否为空。

（6）遍历链表

1 /**
 2  * list_entry - get the struct for this entry
 3  * @ptr:    the &struct list_head pointer.
 4  * @type:    the type of the struct this is embedded in.
 5  * @member:    the name of the list_struct within the struct.
 6  */
 7 #define list_entry(ptr, type, member) \
 8     container_of(ptr, type, member)
 9 
10 /**
11  * list_first_entry - get the first element from a list
12  * @ptr:    the list head to take the element from.
13  * @type:    the type of the struct this is embedded in.
14  * @member:    the name of the list_struct within the struct.
15  *
16  * Note, that list is expected to be not empty.
17  */
18 #define list_first_entry(ptr, type, member) \
19     list_entry((ptr)->next, type, member)
20 
21 /**
22  * list_for_each    -    iterate over a list
23  * @pos:    the &struct list_head to use as a loop cursor.
24  * @head:    the head for your list.
25  */
26 #define list_for_each(pos, head) \
27     for (pos = (head)->next; prefetch(pos->next), pos != (head); \
28             pos = pos->next)

宏list_entity获取链表的结构，包括数据域。list_first_entry获取链表第一个节点，包括数据源。list_for_each宏对链表节点进行遍历。

4、测试例子

编写一个简单使用链表的程序，从而掌握链表的使用。

自定义个类似的list结构如下所示：mylist.h

1 # define POISON_POINTER_DELTA 0
 2 
 3 #define LIST_POISON1  ((void *) 0x00100100 + POISON_POINTER_DELTA)
 4 #define LIST_POISON2  ((void *) 0x00200200 + POISON_POINTER_DELTA)
 5 
 6 //计算member在type中的位置
 7 #define offsetof(type, member)  (size_t)(&((type*)0)->member)
 8 //根据member的地址获取type的起始地址
 9 #define container_of(ptr, type, member) ({          \
10         const typeof(((type *)0)->member)*__mptr = (ptr);    \
11     (type *)((char *)__mptr - offsetof(type, member)); })
12 
13 //链表结构
14 struct list_head
15 {
16     struct list_head *prev;
17     struct list_head *next;
18 };
19 
20 static inline void init_list_head(struct list_head *list)
21 {
22     list->prev = list;
23     list->next = list;
24 }
25 
26 static inline void __list_add(struct list_head *new,
27     struct list_head *prev, struct list_head *next)
28 {
29     prev->next = new;
30     new->prev = prev;
31     new->next = next;
32     next->prev = new;
33 }
34 
35 //从头部添加
36 static inline void list_add(struct list_head *new , struct list_head *head)
37 {
38     __list_add(new, head, head->next);
39 }
40 //从尾部添加
41 static inline void list_add_tail(struct list_head *new, struct list_head *head)
42 {
43     __list_add(new, head->prev, head);
44 }
45 
46 static inline  void __list_del(struct list_head *prev, struct list_head *next)
47 {
48     prev->next = next;
49     next->prev = prev;
50 }
51 
52 static inline void list_del(struct list_head *entry)
53 {
54     __list_del(entry->prev, entry->next);
55     entry->next = LIST_POISON1;
56     entry->prev = LIST_POISON2;
57 }
58 
59 static inline void list_move(struct list_head *list, struct list_head *head)
60 {
61         __list_del(list->prev, list->next);
62         list_add(list, head);
63 }
64 
65 static inline void list_move_tail(struct list_head *list,
66                       struct list_head *head)
67 {
68         __list_del(list->prev, list->next);
69         list_add_tail(list, head);
70 }
71 #define list_entry(ptr, type, member) \
72     container_of(ptr, type, member)
73 
74 #define list_first_entry(ptr, type, member) \
75     list_entry((ptr)->next, type, member)
76 
77 #define list_for_each(pos, head) \
78     for (pos = (head)->next; pos != (head); pos = pos->next)

mylist.c如下所示：

1 /**@brief 练习使用linux内核链表，功能包括：
 2  * 定义链表结构，创建链表、插入节点、删除节点、移动节点、遍历节点
 3  *
 4  *@auther Anker @date 2013-12-15
 5  **/
 6 #include <stdio.h>
 7 #include <inttypes.h>
 8 #include <stdlib.h>
 9 #include <errno.h>
10 #include "mylist.h"
11 //定义app_info链表结构
12 typedef struct application_info
13 {
14     uint32_t  app_id;
15     uint32_t  up_flow;
16     uint32_t  down_flow;
17     struct    list_head app_info_node;//链表节点
18 }app_info;
19 
20 
21 app_info* get_app_info(uint32_t app_id, uint32_t up_flow, uint32_t down_flow)
22 {
23     app_info *app = (app_info*)malloc(sizeof(app_info));
24     if (app == NULL)
25     {
26     fprintf(stderr, "Failed to malloc memory, errno:%u, reason:%s\n",
27         errno, strerror(errno));
28     return NULL;
29     }
30     app->app_id = app_id;
31     app->up_flow = up_flow;
32     app->down_flow = down_flow;
33     return app;
34 }
35 static void for_each_app(const struct list_head *head)
36 {
37     struct list_head *pos;
38     app_info *app;
39     //遍历链表
40     list_for_each(pos, head)
41     {
42     app = list_entry(pos, app_info, app_info_node);
43     printf("ap_id: %u\tup_flow: %u\tdown_flow: %u\n",
44         app->app_id, app->up_flow, app->down_flow);
45 
46     }
47 }
48 
49 void destroy_app_list(struct list_head *head)
50 {
51     struct list_head *pos = head->next;
52     struct list_head *tmp = NULL;
53     while (pos != head)
54     {
55     tmp = pos->next;
56     list_del(pos);
57     pos = tmp;
58     }
59 }
60 
61 
62 int main()
63 {
64     //创建一个app_info
65     app_info * app_info_list = (app_info*)malloc(sizeof(app_info));
66     app_info *app;
67     if (app_info_list == NULL)
68     {
69     fprintf(stderr, "Failed to malloc memory, errno:%u, reason:%s\n",
70         errno, strerror(errno));
71     return -1;
72     }
73     //初始化链表头部
74     struct list_head *head = &app_info_list->app_info_node;
75     init_list_head(head);
76     //插入三个app_info
77     app = get_app_info(1001, 100, 200);
78     list_add_tail(&app->app_info_node, head);
79     app = get_app_info(1002, 80, 100);
80     list_add_tail(&app->app_info_node, head);
81     app = get_app_info(1003, 90, 120);
82     list_add_tail(&app->app_info_node, head);
83     printf("After insert three app_info: \n");
84     for_each_app(head);
85     //将第一个节点移到末尾
86     printf("Move first node to tail:\n");
87     list_move_tail(head->next, head);
88     for_each_app(head);
89     //删除最后一个节点
90     printf("Delete the last node:\n");
91     list_del(head->prev);
92     for_each_app(head);
93     destroy_app_list(head);
94     free(app_info_list);
95     return 0;
96 }

测试结果如下所示：