redis 5.0.7 源碼閱讀——雙向鏈表

redis中雙向鏈表相關的文件爲:adlist.h與adlist.cgit

1、數據結構github

redis裏定義的雙向鏈表,與普通雙向鏈表大體相同redis

單個節點:數據結構

1 typedef struct listNode {
2     struct listNode *prev;
3     struct listNode *next;
4     void *value;
5 } listNode;

鏈表:函數

1 typedef struct list {
2     listNode *head;
3     listNode *tail;
4     void *(*dup)(void *ptr);
5     void (*free)(void *ptr);
6     int (*match)(void *ptr, void *key);
7     unsigned long len;
8 } list;

鏈表以函數指針的方式,實現了複製、銷燬與比較的方法的多態。spa

迭代器:指針

1 typedef struct listIter {
2     listNode *next;
3     int direction;
4 } listIter;

迭代器中有個成員變量direction,用於表示當前遍歷的方向。code

大體結構:blog

 1 /*
 2 +-------------------+        +----------------> +--------------+ <-------+
 3 |listNode *head     |--------+                  |listNode *prev|-->NULL  |
 4 +-------------------+                           +--------------+         |
 5 |listNode *tail     |--------+                  |listNode *next|----+    |
 6 +-------------------+        |                  +--------------+    |    |
 7 |void *(*dup)(...)  |        |                  |void *value   |    |    |
 8 +-------------------+        |                  +--------------+    |    |
 9 |void (*free)(...)  |        |                                      |    |
10 +-------------------+        |                                      |    |
11 |int (*match)(...)  |        |                                      |    |
12 +-------------------+        +----------------> +--------------+ <--+    |
13 |unsigned long len  |                           |listNode *prev|---------+
14 +-------------------+                           +--------------+
15                                                 |listNode *next|-->NULL
16                                                 +--------------+
17                                                 |void *value   |
18                                                 +--------------+    
19 */                                            

2、建立接口

redis中建立一個初始雙向鏈表比較簡單,只要分配好內存,並給成員變量賦初值就能夠了

 1 list *listCreate(void)
 2 {
 3     struct list *list;
 4 
 5     if ((list = zmalloc(sizeof(*list))) == NULL)
 6         return NULL;
 7     list->head = list->tail = NULL;
 8     list->len = 0;
 9     list->dup = NULL;
10     list->free = NULL;
11     list->match = NULL;
12     return list;
13 }

 

redis中提供了頭插法、尾插法以及指定位置插入節點三種方式向鏈表中添加節點,與普通雙向鏈表無異,此處不作詳細敘述。

3、銷燬

因鏈表中每一個節點的value可能指向堆空間,故不能直接把list結構體free,這樣會形成內存泄露。須要先將每一個節點的value釋放,才能夠free結構體

清空全部節點:

 1 void listEmpty(list *list)
 2 {
 3     unsigned long len;
 4     listNode *current, *next;
 5 
 6     current = list->head;
 7     len = list->len;
 8     while(len--) {
 9         next = current->next;
10         //若指定了銷燬的函數,則使用指定的函數進行銷燬value
11         if (list->free) list->free(current->value);
12         zfree(current);
13         current = next;
14     }
15     list->head = list->tail = NULL;
16     list->len = 0;
17 }

銷燬鏈表:

1 void listRelease(list *list)
2 {
3     listEmpty(list);
4     zfree(list);
5 }

一樣,redis的鏈表提供了與普通鏈表相同的刪除單個節點的操做,此處也不作敘述。

4、迭代器操做

redis中提供了獲取迭代器的接口

 1 listIter *listGetIterator(list *list, int direction)
 2 {
 3     listIter *iter;
 4 
 5     if ((iter = zmalloc(sizeof(*iter))) == NULL) return NULL;
 6     if (direction == AL_START_HEAD)
 7         iter->next = list->head;
 8     else
 9         iter->next = list->tail;
10     iter->direction = direction;
11     return iter;
12 }

以AL_START_HEAD爲例,生成好的迭代器結構以下:

 

 1 /*
 2 +-------------------+    +---> +--------------+ <-------+----+
 3 |listNode *head     |----+     |listNode *prev|-->NULL  |    |  
 4 +-------------------+          +--------------+         |    |  +--------------+
 5 |listNode *tail     |----+     |listNode *next|----+    |    +--|listNode *next|
 6 +-------------------+    |     +--------------+    |    |       +--------------+
 7 |void *(*dup)(...)  |    |     |void *value   |    |    |       |int direction |
 8 +-------------------+    |     +--------------+    |    |       +--------------+
 9 |void (*free)(...)  |    |                         |    |
10 +-------------------+    |                         |    |
11 |int (*match)(...)  |    |                         |    |
12 +-------------------+    +---> +--------------+ <--+    |
13 |unsigned long len  |          |listNode *prev|---------+
14 +-------------------+          +--------------+
15                                |listNode *next|-->NULL
16                                +--------------+
17                                |void *value   |
18                                +--------------+    
19 */                                                                                

迭代器的next方法:

 1 listNode *listNext(listIter *iter)
 2 {
 3     listNode *current = iter->next;
 4 
 5     if (current != NULL) {
 6         if (iter->direction == AL_START_HEAD)
 7             iter->next = current->next;
 8         else
 9             iter->next = current->prev;
10     }
11     return current;
12 }

調用一次以後的結構:

 1 /*
 2 +-------------------+    +---> +--------------+ <-------+
 3 |listNode *head     |----+     |listNode *prev|-->NULL  |      
 4 +-------------------+          +--------------+         |       +--------------+
 5 |listNode *tail     |----+     |listNode *next|----+    |    +--|listNode *next|
 6 +-------------------+    |     +--------------+    |    |    |  +--------------+
 7 |void *(*dup)(...)  |    |     |void *value   |    |    |    |  |int direction |
 8 +-------------------+    |     +--------------+    |    |    |  +--------------+
 9 |void (*free)(...)  |    |                         |    |    |
10 +-------------------+    |                         |    |    |
11 |int (*match)(...)  |    |                         |    |    |
12 +-------------------+    +---> +--------------+ <--+----|----+    
13 |unsigned long len  |          |listNode *prev|---------+
14 +-------------------+          +--------------+
15                                |listNode *next|-->NULL
16                                +--------------+
17                                |void *value   |
18                                +--------------+    
19 */                                                                              

再次調用:

 1 /*
 2 +-------------------+    +---> +--------------+ <-------+
 3 |listNode *head     |----+     |listNode *prev|-->NULL  |      
 4 +-------------------+          +--------------+         |       +--------------+
 5 |listNode *tail     |----+     |listNode *next|----+    |    +--|listNode *next|
 6 +-------------------+    |     +--------------+    |    |    |  +--------------+
 7 |void *(*dup)(...)  |    |     |void *value   |    |    |    |  |int direction |
 8 +-------------------+    |     +--------------+    |    |    |  +--------------+
 9 |void (*free)(...)  |    |                         |    |    |
10 +-------------------+    |                         |    |    |
11 |int (*match)(...)  |    |                         |    |    |
12 +-------------------+    +---> +--------------+ <--+    |    +-->NULL    
13 |unsigned long len  |          |listNode *prev|---------+
14 +-------------------+          +--------------+
15                                |listNode *next|-->NULL
16                                +--------------+
17                                |void *value   |
18                                +--------------+    
19 */                                                                            

調用next函數的返回值爲調用以前的listNode首地址

5、其它操做

redis的雙向鏈表還提供了其它操做。其中,查找指定的key與複製整個list依賴於迭代器的使用,並使用到自定義的比較/複製方法。

除此以外,還提供了相似隨機讀取的方式,其內部實現爲遍歷,且「越界」時返回NULL。同時,它支持index爲負數,表示從尾開始。相似旋轉的操做,把尾節點移至原頭節點以前,成爲新的頭節點。固然,還有拼接兩個鏈表的操做。

 

 

redis 5.0.7 下載連接

http://download.redis.io/releases/redis-5.0.7.tar.gz

源碼閱讀順序參考:

https://github.com/huangz1990/blog/blob/master/diary/2014/how-to-read-redis-source-code.rst

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