1、前言
最近寫代碼需用到鏈表結構，正好公共庫有關於鏈表的。第一眼看時，覺得有點新鮮，和我之前見到的鏈表結構不一樣，只有前驅和后繼指針，而沒有數據域。后來看代碼注釋發現該代碼來自linux內核，在linux源代碼下include/Lish.h下。這個鏈表具備通用性，使用非常方便。只需要在結構定義一個鏈表結構就可以使用。
2、鏈表介紹
鏈表是非常基本的數據結構，根據鏈個數分為單鏈表、雙鏈表，根據是否循環分為單向鏈表和循環鏈表。通常定義定義鏈表結構如下：
typedef struct node { ElemType data; //數據域 struct node *next; //指針域 }node, *list;
鏈表中包含數據域和指針域。鏈表通常包含一個頭結點，不存放數據，方便鏈表操作。單向循環鏈表結構如下圖所示：
雙向循環鏈表結構如下圖所示：
這樣帶數據域的鏈表降低了鏈表的通用性，不容易擴展。linux內核定義的鏈表結構不帶數據域，只需要兩個指針完成鏈表的操作。將鏈表節點加入數據結構，具備非常高的擴展性，通用性。鏈表結構定義如下所示：
struct list_head { struct list_head *next, *prev; };
鏈表結構如下所示：
需要用鏈表結構時，只需要在結構體中定義一個鏈表類型的數據即可。例如定義一個app_info鏈表，
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; //鏈表節點
7 }app_info;
定義一個app_info鏈表，app_info app_info_list;通過app_info_head進行鏈表操作。根據C語言指針操作，通過container_of和offsetof，可以根據app_info_head的地址找出app_info的起始地址，即一個完整ap_info結構的起始地址。可以參考：http://www.cnblogs.com/Anker/p/3472271.html。
3、linux內核鏈表實現
內核實現的是雙向循環鏈表，提供了鏈表操作的基本功能。
（1）初始化鏈表頭結點
#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宏創建一個鏈表頭結點，並用LIST_HEAD_INIT宏對頭結點進行賦值，使得頭結點的前驅和后繼指向自己。
INIT_LIST_HEAD函數對鏈表進行初始化，使得前驅和后繼指針指針指向頭結點。
（2）插入節點
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 }
插入節點分為從鏈表頭部插入list_add和鏈表尾部插入list_add_tail，通過調用__list_add函數進行實現，head->next指向之一個節點，head->prev指向尾部節點。
（3）刪除節點
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 }
從鏈表中刪除一個節點，需要改變該節點前驅節點的后繼結點和后繼結點的前驅節點。最后設置該節點的前驅節點和后繼結點指向LIST_POSITION1和LIST_POSITION2兩個特殊值，這樣設置是為了保證不在鏈表中的節點項不可訪問，對LIST_POSITION1和LIST_POSITION2的訪問都將引起頁故障
/* * 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）移動節點
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將一個節點移動到頭部或者尾部。
（5）判斷鏈表
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函數判斷節點是否為末尾節點，list_empty判斷鏈表是否為空。
（6）遍歷鏈表
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獲取鏈表的結構，包括數據域。list_first_entry獲取鏈表第一個節點，包括數據源。list_for_each宏對鏈表節點進行遍歷。
4、測試例子
編寫一個簡單使用鏈表的程序，從而掌握鏈表的使用。
自定義個類似的list結構如下所示：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 //計算member在type中的位置
7 #define offsetof(type, member) (size_t)(&((type*)0)->member)
8 //根據member的地址獲取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 //鏈表結構
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 //從頭部添加
36 static inline void list_add(struct list_head *new , struct list_head *head) 37 { 38 __list_add(new, head, head->next); 39 } 40 //從尾部添加
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 練習使用linux內核鏈表，功能包括： 2 * 定義鏈表結構，創建鏈表、插入節點、刪除節點、移動節點、遍歷節點 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 //定義app_info鏈表結構
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;//鏈表節點
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 //遍歷鏈表
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 //創建一個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 //初始化鏈表頭部
74 struct list_head *head = &app_info_list->app_info_node; 75 init_list_head(head); 76 //插入三個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 //將第一個節點移到末尾
86 printf("Move first node to tail:\n"); 87 list_move_tail(head->next, head); 88 for_each_app(head); 89 //刪除最后一個節點
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 }
測試結果如下所示：
參考網址：