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Redis的Adlist实现了数据结构中的双端链表，整个结构如下：
链表节点定义：
typedef struct listNode {
struct listNode *prev;
struct listNode *next;
void *value;
} listNode;
链表定义：
typedef struct list {
listNode *head;
listNode *tail;
void *(*dup)(void *ptr);
void (*free)(void *ptr);
int (*match)(void *ptr, void *key);
unsigned long len;
} list;
其中的三个函数指针先不用管，后面遇到了再看具体是干什么的，另外还实现了一个迭代器，有点c++的味道在里面
typedef struct listIter {
listNode *next;
int direction;
} listIter;
链表三要素，创建，插入，和删除
list *listCreate(void)
{
struct list *list;
if ((list = malloc(sizeof(*list))) == NULL)
return NULL;
list->head = list->tail = NULL;
list->len = 0;
list->dup = NULL;
list->free = NULL;
list->match = NULL;
return list;
}
插入分为从头部插入和尾部插入，源代码实现头部都有非常清晰的注释，告诉这个函数的一些细节，作者很是用心：
list *listAddNodeHead(list *list, void *value)
{
listNode *node;
if ((node = malloc(sizeof(*node))) == NULL)
return NULL;
node->value = value;
if (list->len == 0) {
list->head = list->tail = node;
node->prev = node->next = NULL;
} else {
node->prev = NULL;
node->next = list->head;
list->head->prev = node;
list->head = node;
}
list->len++;
return list;
}
释放内存
void listRelease(list *list)
{
unsigned long len;
listNode *current, *next;
current = list->head;
len = list->len;
while(len--) {
next = current->next;
if (list->free) list->free(current->value);
free(current);
current = next;
}
free(list);
}
迭代器的创建，以后可以效仿这种做法，迭代器分方向：
/* Returns a list iterator 'iter'. After the initialization every
* call to listNext() will return the next element of the list.
*
* This function can't fail. */
listIter *listGetIterator(list *list, int direction)
{
listIter *iter;
if ((iter = malloc(sizeof(*iter))) == NULL) return NULL;
if (direction == AL_START_HEAD)
iter->next = list->head;
else
iter->next = list->tail;
iter->direction = direction;
return iter;
}
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