linux内核数据结构之链表

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_info2 {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。可以参考：http://www.cnblogs.com/Anker/p/3472271.html。

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 tail14  * @list: the entry to move15  * @head: the head that will follow our entry16  */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 empty14  * @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 list12  * @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 list23  * @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_head15 {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_info13 {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_info65     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_info77     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：

参考网址：

https://www.ibm.com/developerworks/cn/linux/kernel/l-chain/

原文：http://www.cnblogs.com/Anker/p/3475643.html