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：。
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 #include7 #include 8 #include 9 #include 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：