深入分析 Linux 內核鏈表

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：

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