Redis源码分析（二）——链表adlist

        从基本数据结构入手分析。首先是双向链表的实现adlist.c，借此复习链表的基本操作。分析工程中感受最深的就是函数指针的大量使用，由于很长时间没有用C，这一块正是需要熟悉的。 除此之外，见识了在C语言中迭代器的实现原来可以很简洁。

 

下面是adlist.h与adlist.c，具体分析见注释。

adlist.h：

<span style="font-size:18px;">/* adlist.h - A generic doubly linked list implementation#ifndef __ADLIST_H__#define __ADLIST_H__/* Node, List, and Iterator are the only data structures used currently. */typedef struct listNode {//节点    struct listNode *prev;    struct listNode *next;    void *value;} listNode;typedef struct listIter {//迭代器    listNode *next;    int direction;} listIter;typedef struct list {//双向链表   包括三个操作函数    listNode *head;    listNode *tail;    void *(*dup)(void *ptr);//复制函数指针    void (*free)(void *ptr);//释放函数指针    int (*match)(void *ptr, void *key);//匹配函数指针    unsigned long len;  //节点计数} list;/* Functions implemented as macros *///宏定义一些基本操作#define listLength(l) ((l)->len) //取list长度#define listFirst(l) ((l)->head) //取list头结点#define listLast(l) ((l)->tail)//取list尾节点#define listPrevNode(n) ((n)->prev)//取当前节点的prev节点#define listNextNode(n) ((n)->next)//取当前节点的next节点#define listNodeValue(n) ((n)->value)//取当前节点的value指针#define listSetDupMethod(l,m) ((l)->dup = (m)) //设置list的复制方法#define listSetFreeMethod(l,m) ((l)->free = (m))//设置list的释放方法#define listSetMatchMethod(l,m) ((l)->match = (m))//设置list的匹配方法#define listGetDupMethod(l) ((l)->dup)  //取list的复制方法#define listGetFree(l) ((l)->free)     // 取list的释放方法#define listGetMatchMethod(l) ((l)->match)//取list的匹配方法/* Prototypes *///方法原型list *listCreate(void);  //创建链表void listRelease(list *list);//释放链表list *listAddNodeHead(list *list, void *value); //添加头节点list *listAddNodeTail(list *list, void *value);//添加尾节点list *listInsertNode(list *list, listNode *old_node, void *value, int after);//插入节点void listDelNode(list *list, listNode *node); //删除节点listIter *listGetIterator(list *list, int direction);//取迭代器listNode *listNext(listIter *iter);//迭代器指向nextvoid listReleaseIterator(listIter *iter);//释放迭代器list *listDup(list *orig);//复制链表listNode *listSearchKey(list *list, void *key);//查找key对应的节点listNode *listIndex(list *list, long index);//查找索引index对应的节点void listRewind(list *list, listIter *li);//重置迭代器方向为从头开始void listRewindTail(list *list, listIter *li);//重置迭代器方向为从尾部开始void listRotate(list *list);//链表循环右移操作/* Directions for iterators *///迭代器方向#define AL_START_HEAD 0  //0为从头开始 前向          #define AL_START_TAIL 1  //1为从尾部开始   后向#endif /* __ADLIST_H__ */</span>

<span style="font-size:18px;"></span> 

adlist.c :

<span style="font-size:18px;">/* adlist.c - A generic doubly linked list implementation*/#include <stdlib.h>#include "adlist.h"#include "zmalloc.h"/* Create a new list. The created list can be freed with * AlFreeList(), but private value of every node need to be freed * by the user before to call AlFreeList(). * * On error, NULL is returned. Otherwise the pointer to the new list. */ //创建一个链表，可用函数AlFreeList()来释放整个链表，但是每个节点的私有值需要在调用函数AlFreeList()之前由调用者自己释放list *listCreate(void) //返回创建链表结构的指针{    struct list *list;    if ((list = zmalloc(sizeof(*list))) == NULL) //如果分配内存失败        return NULL;    list->head = list->tail = NULL;//空链表    list->len = 0;    list->dup = NULL;    list->free = NULL;    list->match = NULL;    return list;}/* Free the whole list. * * This function can't fail. */ //释放整个链表，该函数不能失败（逐个节点依次释放，释放节点时先释放其中的私有指针指向的内容）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);//释放当前节点的value指针指向的内容        zfree(current);  //释放当前节点        current = next;    }    zfree(list);  //释放链表结构体}/* Add a new node to the list, to head, contaning the specified 'value' * pointer as value. * * On error, NULL is returned and no operation is performed (i.e. the * list remains unaltered). * On success the 'list' pointer you pass to the function is returned. */ //在头部添加一个新节点，添加失败则返回null,原链表不改变，成功则返回返回传入的list*指针list *listAddNodeHead(list *list, void *value){    listNode *node;    if ((node = zmalloc(sizeof(*node))) == NULL)//构造新节点，分配空间        return NULL;    node->value = value;    if (list->len == 0) {//原来为空链表        list->head = list->tail = node;        node->prev = node->next = NULL;    } else {               //原来为非空        node->prev = NULL;        node->next = list->head;        list->head->prev = node;        list->head = node;    }    list->len++;  //节点计数递增    return list;}/* Add a new node to the list, to tail, containing the specified 'value' * pointer as value. * * On error, NULL is returned and no operation is performed (i.e. the * list remains unaltered). * On success the 'list' pointer you pass to the function is returned. */ //添加一个新节点到尾部，失败则返回null，原链表保持不变，成功则返回传入的list*指针list *listAddNodeTail(list *list, void *value){    listNode *node;    if ((node = zmalloc(sizeof(*node))) == NULL)//为新节点分配空间        return NULL;                            //失败则返回null    node->value = value;    if (list->len == 0) {//原来为空链表        list->head = list->tail = node;        node->prev = node->next = NULL;    } else {  //原来为非空        node->prev = list->tail;        node->next = NULL;        list->tail->next = node;        list->tail = node;    }    list->len++;//节点计数递增    return list;}//在指定节点前/后插入新节点list *listInsertNode(list *list, listNode *old_node, void *value, int after) {    listNode *node;    if ((node = zmalloc(sizeof(*node))) == NULL) //为新节点分配空间        return NULL;    node->value = value;    if (after) {   //after为非零值，则在插入到其后        node->prev = old_node;        node->next = old_node->next;        if (list->tail == old_node) { //如果old_node为尾节点，则需要更新尾指针            list->tail = node;        }    } else {  //插入到其前        node->next = old_node;        node->prev = old_node->prev;        if (list->head == old_node) {  //如果old_node为头节点，则需要更新头指针            list->head = node;        }    }    if (node->prev != NULL) {//更新old_node的next        node->prev->next = node;    }    if (node->next != NULL) {//更新old_node的prev        node->next->prev = node;    }    list->len++;   //节点计数递增    return list;}/* Remove the specified node from the specified list. * It's up to the caller to free the private value of the node. * * This function can't fail. */ //删除指定节点，删除的节点由调用者释放。 该函数不能失败void listDelNode(list *list, listNode *node){    if (node->prev)//为非头结点        node->prev->next = node->next;    else  //头结点        list->head = node->next;    if (node->next) //为非尾结点        node->next->prev = node->prev;    else//为尾结点        list->tail = node->prev;    if (list->free) list->free(node->value); //释放删除节点的私有指针    zfree(node);//释放删除节点    list->len--;//节点计数递减}/* 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. */ //获取链表的迭代器，迭代器初始化后，listNext()函数就返回链表的next节点。  以指定方向遍历链表listIter *listGetIterator(list *list, int direction){    listIter *iter;    if ((iter = zmalloc(sizeof(*iter))) == NULL) return NULL; //为迭代器分配空间    if (direction == AL_START_HEAD)//若为前向迭代器        iter->next = list->head;    else                         //后向迭代器        iter->next = list->tail;    iter->direction = direction;     return iter;}/* Release the iterator memory *///释放迭代器 （不会释放迭代器当前指向的节点）void listReleaseIterator(listIter *iter) {    zfree(iter);}/* Create an iterator in the list private iterator structure *///重置迭代器方向，从头开始void listRewind(list *list, listIter *li) {    li->next = list->head;    li->direction = AL_START_HEAD;}//重置迭代器方向，从尾部开始void listRewindTail(list *list, listIter *li) {    li->next = list->tail;    li->direction = AL_START_TAIL;}/* Return the next element of an iterator. * It's valid to remove the currently returned element using * listDelNode(), but not to remove other elements. * * The function returns a pointer to the next element of the list, * or NULL if there are no more elements, so the classical usage patter * is: * * iter = listGetIterator(list,<direction>); * while ((node = listNext(iter)) != NULL) { *     doSomethingWith(listNodeValue(node)); * } * * */ //返回迭代器的next节点的指针（注意与迭代器的方向有关！），由于返回的是指针，因此可用于删除链表中的节点。 //如果没有next节点，则返回null， //注意典型应用：：获取当前节点的迭代器，在获取其next节点指针，并判断返回值是否为null。listNode *listNext(listIter *iter){    listNode *current = iter->next;    if (current != NULL) {  //移动迭代器        if (iter->direction == AL_START_HEAD)            iter->next = current->next;        else            iter->next = current->prev;    }    return current;}/* Duplicate the whole list. On out of memory NULL is returned. * On success a copy of the original list is returned. * * The 'Dup' method set with listSetDupMethod() function is used * to copy the node value. Otherwise the same pointer value of * the original node is used as value of the copied node. * * The original list both on success or error is never modified. */ //复制整个链表。内存溢出返回null，成功则返回原链表的拷贝。 listSetDupMethod()设置的Dup方法用于拷贝节点的value值， //拷贝将共用原链表的节点值，即拷贝的节点指针指向原来的节点list *listDup(list *orig){    list *copy;    listIter *iter;    listNode *node;    if ((copy = listCreate()) == NULL)        return NULL;    copy->dup = orig->dup;//复制函数指针    copy->free = orig->free;//释放函数指针    copy->match = orig->match;//匹配函数指针    iter = listGetIterator(orig, AL_START_HEAD);//获取原链表的前向迭代器，用于遍历拷贝    while((node = listNext(iter)) != NULL) {//遍历        void *value;        if (copy->dup) {//如果设置了dup函数指针，则复制拷贝            value = copy->dup(node->value);            if (value == NULL) {//拷贝节点失败                listRelease(copy);                listReleaseIterator(iter);                return NULL;            }        } else//没有设置dup函数指针，则直接返回原节点value指针，即拷贝链表与原链表的value值指向同一值            value = node->value;        if (listAddNodeTail(copy, value) == NULL) {//将拷贝所得节点添加到拷贝链表尾部            listRelease(copy);     //添加新节点失败            listReleaseIterator(iter);            return NULL;        }    }    listReleaseIterator(iter);//释放迭代器    return copy;//返回拷贝链表指针}/* Search the list for a node matching a given key. * The match is performed using the 'match' method * set with listSetMatchMethod(). If no 'match' method * is set, the 'value' pointer of every node is directly * compared with the 'key' pointer. * * On success the first matching node pointer is returned * (search starts from head). If no matching node exists * NULL is returned. */ //从头部开始查找与给定key匹配的第一个节点。匹配时使用设置的match方法把key与直接比较每个节点的value。 //找到则返回节点指针，失败则返回nulllistNode *listSearchKey(list *list, void *key){    listIter *iter;    listNode *node;    iter = listGetIterator(list, AL_START_HEAD);    while((node = listNext(iter)) != NULL) {        if (list->match) {//如果设置了match函数指针            if (list->match(node->value, key)) {                listReleaseIterator(iter);                return node;            }        } else {            if (key == node->value) {//没有设置match函数指针，则用 == 来匹配                listReleaseIterator(iter);                return node;            }        }    }    listReleaseIterator(iter);    return NULL;}/* Return the element at the specified zero-based index * where 0 is the head, 1 is the element next to head * and so on. Negative integers are used in order to count * from the tail, -1 is the last element, -2 the penultimate * and so on. If the index is out of range NULL is returned. */ //查找指定索引的节点。 头结点索引为0，逐渐递增。 -1为尾节点，向前递减listNode *listIndex(list *list, long index) {    listNode *n;    if (index < 0) {        index = (-index)-1;        n = list->tail;        while(index-- && n) n = n->prev;//逆序查找    } else {        n = list->head;        while(index-- && n) n = n->next;//顺序查找    }    return n;}/* Rotate the list removing the tail node and inserting it to the head. *///循环右移。 尾节点移到头部void listRotate(list *list) {    listNode *tail = list->tail;    if (listLength(list) <= 1) return;    /* Detach current tail */    list->tail = tail->prev;    list->tail->next = NULL;    /* Move it as head */    list->head->prev = tail;    tail->prev = NULL;    tail->next = list->head;    list->head = tail;}</span>