【Redis源码剖析】

今天我们主要讲述Redis中双向链表的实现，源代码主要在adlist.h和adlist.c文件。

学习过《数据结构》的童鞋应该不会对双向链表感到陌生，这是一种比较简单的数据结构。与我们以前接触到的“常规”的链表结构相比，Redis中的双向链表主要有以下两点不同：

1、双向列表list的节点结构

双向链表作为一个容器，必然需要支持不同类型数据的存储。由于C++中有“模板”的概念，这一点在C++中可以很容易实现。C++ STL中也有一个双向循环列表，它的节点定义是这样的：

template<class T>struct __list_node {    typedef void* void_pointer;    void_pointer prev;    void_pointer next;    T data;};

在C语言中，并没有模板、泛型这样的概念，为了实现通用性，支持不同类型数据的存储，Redis将节点类型的数据域定义为void *类型，从而模拟了“泛型”。

typedef struct listNode {    struct listNode *prev;    struct listNode *next;    void *value;} listNode;

更巧妙的是，由于一个链表中的所有listNode都是同一种类型的数据，作者定义链表结构时引入了三个函数指针：

/* 双向链表的定义 */typedef struct list {    listNode *head;    listNode *tail;    /* 定义三个函数指针 */    void *(*dup)(void *ptr);    //  复制    void (*free)(void *ptr);    //  释放    int (*match)(void *ptr, void *key); // 匹配查找    unsigned long len;} list;

这三个函数指针用来操作listNode结构的数据value。

2、简单的迭代器

在C++或Java中，迭代器是一种很常见的类型，c语言中并没有现成的迭代器。在Redis中，作者用很简洁的方法实现了链表的迭代器并支持正向迭代和反向迭代。这一点可以在后面的源码中看到。

Redis双向链表的实现简洁易懂，对于里面大多数函数我都做了注释，具体如下。

adlist.h头文件：

/* adlist.h - A generic doubly linked list implementation * * Copyright (c) 2006-2012, Salvatore Sanfilippo <antirez at gmail dot com> * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are met: * *   * Redistributions of source code must retain the above copyright notice, *     this list of conditions and the following disclaimer. *   * Redistributions in binary form must reproduce the above copyright *     notice, this list of conditions and the following disclaimer in the *     documentation and/or other materials provided with the distribution. *   * Neither the name of Redis nor the names of its contributors may be used *     to endorse or promote products derived from this software without *     specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE * POSSIBILITY OF SUCH DAMAGE. */#ifndef __ADLIST_H__#define __ADLIST_H__/* Node, List, and Iterator are the only data structures used currently. *//* 定义双向链表的节点 */typedef struct listNode {    // 前一个节点    struct listNode *prev;    // 下一个节点    struct listNode *next;    // 指向节点存储的数据。C语言中没有泛型的概念，利用void*指针来“模拟”泛型操作    void *value;} listNode;/* 双向链表的迭代器 */typedef struct listIter {    // 指向当前节点    listNode *next;    // 迭代器方向    int direction;} listIter;/* 双向链表的定义 */typedef struct list {    // 链表头节点    listNode *head;    // 链表为节点    listNode *tail;    /* 定义三个函数指针         为什么要定义这三个函数指针？因为listnode中的数据区域为一个void类型的指针，        所指向的结构可能千差万别，而且这些内存需要手动释放。将常用的这几个函数定义        在这里，可以在需要的时候直接回调。    */    void *(*dup)(void *ptr);    //  复制    void (*free)(void *ptr);    //  释放    int (*match)(void *ptr, void *key); // 匹配查找    // 链表长度    unsigned long len;} list;/* Functions implemented as macros *//* 宏定义，主要是链表的一些基本操作 */// 获取长度#define listLength(l) ((l)->len)// 获取头结点#define listFirst(l) ((l)->head)// 获取尾节点#define listLast(l) ((l)->tail)// 获取前一个节点#define listPrevNode(n) ((n)->prev)// 获取下一个节点#define listNextNode(n) ((n)->next)// 获取节点的值，是一个void类型的指针#define listNodeValue(n) ((n)->value)/* 下面三个宏定义主要用来设置list结构中的三个函数指针, 参数m为method的意思 */#define listSetDupMethod(l,m) ((l)->dup = (m))#define listSetFreeMethod(l,m) ((l)->free = (m))#define listSetMatchMethod(l,m) ((l)->match = (m))/* 下面三个宏定义主要用来获取list结构的单个函数指针 */#define listGetDupMethod(l) ((l)->dup)#define listGetFree(l) ((l)->free)#define listGetMatchMethod(l) ((l)->match)/* Prototypes *//* 操作list相关的函数原型声明 */// 创建一个listlist *listCreate(void);// 销毁一个给定的list结构void listRelease(list *list);// 在表头插入一个节点list *listAddNodeHead(list *list, void *value);// 在表尾插入一个节点list *listAddNodeTail(list *list, void *value);// 在指定的位置上插入一个节点list *listInsertNode(list *list, listNode *old_node, void *value, int after);// 删除一个节点void listDelNode(list *list, listNode *node);// 获取一个给定方向上的迭代器listIter *listGetIterator(list *list, int direction);// 获取给定迭代器的下一个节点listNode *listNext(listIter *iter);// 释放迭代器void listReleaseIterator(listIter *iter);// 复制一个listlist *listDup(list *orig);// 根据关键字查找listlistNode *listSearchKey(list *list, void *key);// 根据下标索引查找listlistNode *listIndex(list *list, long index);// 重置迭代器为链表头结点void listRewind(list *list, listIter *li);// 重置迭代器为链表尾节点void listRewindTail(list *list, listIter *li);// 从函数名看不出该函数的作用，后面我们看看源码就明白这个函数只是将最后一个节点移动到头部void listRotate(list *list);/* Directions for iterators *//* 迭代器的迭代方向，AL_START_HEAD从前往后，AL_START_TAIL从后往前 */#define AL_START_HEAD 0#define AL_START_TAIL 1#endif /* __ADLIST_H__ */

adlist.c源文件

/* adlist.c - A generic doubly linked list implementation * * Copyright (c) 2006-2010, Salvatore Sanfilippo <antirez at gmail dot com> * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are met: * *   * Redistributions of source code must retain the above copyright notice, *     this list of conditions and the following disclaimer. *   * Redistributions in binary form must reproduce the above copyright *     notice, this list of conditions and the following disclaimer in the *     documentation and/or other materials provided with the distribution. *   * Neither the name of Redis nor the names of its contributors may be used *     to endorse or promote products derived from this software without *     specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE * POSSIBILITY OF SUCH DAMAGE. */#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listlist *listCreate(void){    struct list *list;    // 申请空间，zmalloc函数是redis定义的空间配置函数，我们后面会分析其原理，现在就暂时把它等同于malloc    if ((list = zmalloc(sizeof(*list))) == NULL)        return NULL;    // 进行“常规”的初始化，这些跟我们在《数据结构》中学习的过程基本一致，注意这里还把三个函数指针设为null    list->head = list->tail = NULL;    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void listRelease(list *list){    // 先销毁链表中的所有节点    unsigned long len;    listNode *current, *next;    current = list->head;    len = list->len;    while(len--) {        next = current->next;        // 通过list中的回调函数来释放每一节点数据域的内存空间        if (list->free) list->free(current->value);        // 销毁节点空间，zfree是redis定义的空间释放函数，和zmalloc对应        zfree(current);        current = next;    }    // 最后把链表也销毁    zfree(list);}/* Add a new node to the list, to head, 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. */// 在表头插入一个节点list *listAddNodeHead(list *list, void *value){    // 定义一个新的节点并为其申请空间    listNode *node;    if ((node = zmalloc(sizeof(*node))) == NULL)        return NULL;    // 设置节点的数据域    node->value = value;    // 往链表头部插入一个新节点需要考虑链表为空的情况    if (list->len == 0) {   // 链表为空        list->head = list->tail = node;        node->prev = node->next = NULL;    } else {        // 链表不为空时的插入过程，主要是指针操作，最后新节点成为头结点。        node->prev = NULL;        node->next = list->head;        list->head->prev = node;        list->head = node;    }    // 最后别忘了更新链表长度    list->len++;    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. */// 在表尾插入一个节点list *listAddNodeTail(list *list, void *value){    // 定义一个新的节点并为其申请空间    listNode *node;    if ((node = zmalloc(sizeof(*node))) == NULL)        return NULL;    // 设置节点的数据域    node->value = value;    // 往链表尾部插入一个新节点需要考虑链表为空的情况    if (list->len == 0) {        list->head = list->tail = node;        node->prev = node->next = NULL;    } else {        // 链表不为空时的插入过程，主要是指针操作，最后新节点成为尾节点        node->prev = list->tail;        node->next = NULL;        list->tail->next = node;        list->tail = node;    }    // 最后更新链表长度    list->len++;    return list;}// 在old_value元素的前面或后面（由after参数决定）插入一个新节点list *listInsertNode(list *list, listNode *old_node, void *value, int after) {    // 定义一个新的节点并为其申请空间    listNode *node;    if ((node = zmalloc(sizeof(*node))) == NULL)        return NULL;    // 设置节点的数据域    node->value = value;    if (after) {        // 在目标节点的后面插入一个新节点，需要考虑目标节点为尾节点的情况        node->prev = old_node;        node->next = old_node->next;        if (list->tail == old_node) {            list->tail = node;        }    } else {        // 在目标节点的前面插入一个新节点，需要考虑目标节点为头结点的情况        node->next = old_node;        node->prev = old_node->prev;        if (list->head == old_node) {            list->head = node;        }    }    // 将插入节点前后的节点链接起来    if (node->prev != NULL) {        node->prev->next = node;    }    if (node->next != NULL) {        node->next->prev = node;    }    // 更新链表长度    list->len++;    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. */ // 删除一个节点void listDelNode(list *list, listNode *node){    // 删除链表中的一个节点要考虑被删除节点是否为头结点或尾节点    if (node->prev)        //  如果node->prev存在，说明该节点不是头结点，直接将prev节点的next指向被删除节点的下一个        node->prev->next = node->next;    else        // 被删除节点是头结点，让被删除节点的下一个节点成为头结点        list->head = node->next;    if (node->next)        // 如果node->next存在，说明该节点不是尾节点        node->next->prev = node->prev;    else        // 被删除节点是尾节点        list->tail = node->prev;    // 回调free函数删除节点的数据域    if (list->free) list->free(node->value);    // 释放节点空间    zfree(node);    // 最后更新链表长度    list->len--;}/* 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. */ // 获取一个给定方向上的迭代器listIter *listGetIterator(list *list, int direction){    // 定义一个迭代器并为其分配空间    listIter *iter;    // 这里需要注意，调用该函数时分配了动态内存，使用完需要手动释放。这一点我们可以在后面代码中看到    // listGetIterator和listReleaseIterator是成对出现的    if ((iter = zmalloc(sizeof(*iter))) == NULL) return NULL;    if (direction == AL_START_HEAD)        // 如果该迭代器的方向是从前往后，则迭代器的next指针指向头结点        iter->next = list->head;    else        // 如果该迭代器的方向是从后往前，则迭代器的next指针指向尾节点        iter->next = list->tail;    // 设置迭代器方向    iter->direction = direction;    return iter;}/* Release the iterator memory */// 释放迭代器void listReleaseIterator(listIter *iter) {    zfree(iter);}/* Create an iterator in the list private iterator structure */// 重置迭代器为链表头结点void listRewind(list *list, listIter *li) {    li->next = list->head;    li->direction = AL_START_HEAD;}// 重置迭代器为链表尾结点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)); * } * * */ // 获取给定迭代器的下一个节点listNode *listNext(listIter *iter){    // 获取当前迭代器所指向的节点（不要被iter->next中的next迷惑，listIter中的next字段指向一个节点）    listNode *current = iter->next;    // 根据迭代器的方向将下一个节点设置为当前节点。对于双向链表来说向前和向后移动都很方便    if (current != NULL) {        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. */// 复制一个listlist *listDup(list *orig){    list *copy;     // 复制后的新链表    listIter *iter; // 迭代器    listNode *node; //  链表节点    if ((copy = listCreate()) == NULL)        return NULL;    // 设置新链表的三个函数指针    copy->dup = orig->dup;    copy->free = orig->free;    copy->match = orig->match;    // 获取从前往后的迭代器，需要手动释放    iter = listGetIterator(orig, AL_START_HEAD);    // 遍历旧链表，将每个节点复制一份放入新链表    while((node = listNext(iter)) != NULL) {        // 复制一个节点的时候要考虑链表中的函数指针dup有没有设置        void *value;        if (copy->dup) {            // 如果设置了节点数据域的复制方法dup，直接调用该方法即可            value = copy->dup(node->value);            if (value == NULL) {    // 出错处理                listRelease(copy);                listReleaseIterator(iter);                return NULL;            }        } else            // 如果没有定义dup方法，则直接复制指针，这样两者指向统一内存区域            value = node->value;            // 调用listAddNodeTail函数从链表尾部插入节点        if (listAddNodeTail(copy, value) == NULL) {            // 出错处理            listRelease(copy);            listReleaseIterator(iter);            return NULL;        }    }    // 最后释放迭代器    listReleaseIterator(iter);    return copy;}/* 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. */ // 根据关键字查找listlistNode *listSearchKey(list *list, void *key){    listIter *iter; // 链表迭代器    listNode *node; // 链表节点    // 获取从前往后的迭代器，需要手动释放    iter = listGetIterator(list, AL_START_HEAD);    while((node = listNext(iter)) != NULL) {        // 从前往后遍历遍历链表，直到找到目标节点        if (list->match) {            // 如果链表list设置了match函数，则用match函数判断当前节点是否和给点节点相同            if (list->match(node->value, key)) {                listReleaseIterator(iter);  // 释放迭代器                return node;            }        } else {            // 如果链表list没有设置了match函数，则比较指针是否相等            if (key == node->value) {                listReleaseIterator(iter);  // // 释放迭代器                return node;            }        }    }    listReleaseIterator(iter);  // 释放迭代器    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. */ // 根据下标索引查找listlistNode *listIndex(list *list, long index) {    listNode *n;    // redis在这点上支持正向下标和反向下标，这点从其命令操作就可以知道    if (index < 0) {        // 如果index为负数，则从后往前数，最后一个节点的下标为-1        index = (-index)-1;        n = list->tail;        while(index-- && n) n = n->prev;    } else {        // 如果index为正数数，则从前往后数，第一个节点的下标为0        n = list->head;        while(index-- && n) n = n->next;    }    return n;}/* Rotate the list removing the tail node and inserting it to the head. */// 将最后一个节点移动到头部void listRotate(list *list) {    listNode *tail = list->tail;    // 如果链表中的节点数目小于2，则直接返回    if (listLength(list) <= 1) return;    // 设置新的尾部节点（注意指针的操作顺序）    /* Detach current tail */    list->tail = tail->prev;    list->tail->next = NULL;    /* Move it as head */    // 调整指针，设置新的头结点    list->head->prev = tail;    tail->prev = NULL;    tail->next = list->head;    list->head = tail;}