redis 5.0.7 源码阅读——字典dict
作者：互联网
redis中字典相关的文件为：dict.h与dict.c
与其说是一个字典，道不如说是一个哈希表。
一、数据结构
dictEntry
1 typedef struct dictEntry {
2 void *key;
3 union {
4 void *val;
5 uint64_t u64;
6 int64_t s64;
7 double d;
8 } v;
9 struct dictEntry *next;
10 } dictEntry;
dictEntry是一个kv对的单向链表，其中v是一个联合体，支持数字，或者是指向一块内存的指针。
1 /*
2 +---------------+
3 |void *key |
4 +---------------+
5 |union{...} v |
6 +---------------+
7 |dictEntry *next|---+
8 +---------------+ |
9 |
10 +---------------+ <-+
11 |void *key |
12 +---------------+
13 |union{...} v |
14 +---------------+
15 |dictEntry *next|
16 +---------------+
17 */
为了节约篇幅，后续用以下结构表示：
1 /* 2 +---+ +---+ 3 |K|V|->|K|V|->NULL 4 +---+ +---+ 5 */
dictht
1 typedef struct dictht {
2 dictEntry **table;
3 unsigned long size;
4 /*
5 这样写可能更容易理解
6 const unsigned long size = 4;
7 dictEntry *table[size];
8 */
9
10
11 unsigned long sizemask;
12 //sizemask，始终为size-1
13
14 unsigned long used;
15 //当前总dictEntry数量
16 } dictht;
dictht是一个hash table，整体结构大致为：
1 /* 2 +----------------------+ +---> +-----------------+ +---+ 3 |dictEntry **table |---+ |dictEntry *bucket|->|K|V|->NULL 4 +----------------------+ +-----------------+ +---+ 5 |unsigned long size = 4| |dictEntry *bucket|->NULL 6 +----------------------+ +-----------------+ 7 |unsigned long sizemask| |dictEntry *bucket|->NULL 8 +----------------------+ +-----------------+ 9 |unsigned long used | |dictEntry *bucket|->NULL 10 +----------------------+ +-----------------+ 11 */
其中，table指向大小为sizeof(dictEntry*) * size的一片内存空间，每个dictEntry*可以视为一个bucket，每个bucket下挂着一个dictEntry单向链表。
size的值始终为2的位数，而sizemask的值始终为size-1，其作用是决定kv对要挂在哪个bucket上。
举个例子，size=4时，sizemask=3，其二进制为 0011，若通过hash函数计算出来key对应的hash值hash_value为5，二进制为0101，则通过位运算 sizemask & hash_value = 0011 & 0101 = 0001，十进制为1，则将会挂在idx = 1的bucket上。
dictType
1 typedef struct dictType {
2 uint64_t (*hashFunction)(const void *key);
3 void *(*keyDup)(void *privdata, const void *key);
4 void *(*valDup)(void *privdata, const void *obj);
5 int (*keyCompare)(void *privdata, const void *key1, const void *key2);
6 void (*keyDestructor)(void *privdata, void *key);
7 void (*valDestructor)(void *privdata, void *obj);
8 } dictType;
dictType用于自定义一些操作的方法，如拷贝key、拷贝value、销毁key、销毁value，比较key，以及hash函数。
dict
1 typedef struct dict {
2 dictType *type;
3 void *privdata;
4 dictht ht[2];
5 long rehashidx; /* rehashing not in progress if rehashidx == -1 */
6 unsigned long iterators; /* number of iterators currently running */
7 } dict;
之前提到的dictType与dictht都是dict的成员变量。除此之外，还有privdata，是在创建dict的时候调用者传入，用于特定操作时回传给函数的。如：
1 #define dictFreeVal(d, entry) \
2 if ((d)->type->valDestructor) \
3 (d)->type->valDestructor((d)->privdata, (entry)->v.val)
4
5 #define dictSetVal(d, entry, _val_) do { \
6 if ((d)->type->valDup) \
7 (entry)->v.val = (d)->type->valDup((d)->privdata, _val_); \
8 else \
9 (entry)->v.val = (_val_); \
10 } while(0)
11
12 #define dictSetSignedIntegerVal(entry, _val_) \
13 do { (entry)->v.s64 = _val_; } while(0)
14
15 #define dictSetUnsignedIntegerVal(entry, _val_) \
16 do { (entry)->v.u64 = _val_; } while(0)
17
18 #define dictSetDoubleVal(entry, _val_) \
19 do { (entry)->v.d = _val_; } while(0)
20
21 #define dictFreeKey(d, entry) \
22 if ((d)->type->keyDestructor) \
23 (d)->type->keyDestructor((d)->privdata, (entry)->key)
24
25 #define dictSetKey(d, entry, _key_) do { \
26 if ((d)->type->keyDup) \
27 (entry)->key = (d)->type->keyDup((d)->privdata, _key_); \
28 else \
29 (entry)->key = (_key_); \
30 } while(0)
31
32 #define dictCompareKeys(d, key1, key2) \
33 (((d)->type->keyCompare) ? \
34 (d)->type->keyCompare((d)->privdata, key1, key2) : \
35 (key1) == (key2))
rehashidx，是与ht[2]配合实现渐进式rehash操作的。若使用一步到位的方式，当key的数量非常大的时候，rehashing期间，是会卡死所有操作的。
iterators，是用于记录当前使用的安全迭代器数量，与rehashing操作有关。 整体结构如下：1 /* 2 +---------+ /+-----------+ +-->+----------+ +---+ 3 |dictType*| / |dictEntry**|---+ |dictEntry*|->|K|V|->NULL 4 +---------+ / +-----------+ +----------+ +---+ 5 |privdata | / |size | |dictEntry*|->NULL 6 +---------+/ +-----------+ +----------+ 7 |ht[2] | |sizemask | |dictEntry*|->NULL 8 +---------+\ +-----------+ +----------+ 9 |rehashidx| \ |used | |dictEntry*|->NULL 10 +---------+ \ +-----------+ +----------+ 11 |iterators| \ 12 +---------+ \+-----------+ 13 |dictEntry**|-->NULL 14 +-----------+ 15 |size | 16 +-----------+ 17 |sizemask | 18 +-----------+ 19 |used | 20 +-----------+ 21 */
二、创建
1 static void _dictReset(dictht *ht)
2 {
3 ht->table = NULL;
4 ht->size = 0;
5 ht->sizemask = 0;
6 ht->used = 0;
7 }
8
9 int _dictInit(dict *d, dictType *type,
10 void *privDataPtr)
11 {
12 _dictReset(&d->ht[0]);
13 _dictReset(&d->ht[1]);
14 d->type = type;
15 d->privdata = privDataPtr;
16 d->rehashidx = -1;
17 d->iterators = 0;
18 return DICT_OK;
19 }
20
21 dict *dictCreate(dictType *type,
22 void *privDataPtr)
23 {
24 dict *d = zmalloc(sizeof(*d));
25
26 _dictInit(d,type,privDataPtr);
27 return d;
28 }
可以调用dictCreate创建一个空的dict，它会分配好dict的空间，并初始化所有成员变量。在这里把privdata传入并保存。搜了一下整个redis源码的dictCreate调用，看到传入的值全为NULL。目前的理解暂时不清楚这个变量是什么时候赋值的。初始化后的dict结构如下：
1 /* 2 +------------+ /+-----------+ 3 |dictType* | / |dictEntry**|-->NULL 4 +------------+ / +-----------+ 5 |privdata | / |size=0 | 6 +------------+/ +-----------+ 7 |ht[2] | |sizemask=0 | 8 +------------+\ +-----------+ 9 |rehashidx=-1| \ |used=0 | 10 +------------+ \ +-----------+ 11 |iterators=0 | \ 12 +------------+ \+-----------+ 13 |dictEntry**|-->NULL 14 +-----------+ 15 |size=0 | 16 +-----------+ 17 |sizemask=0 | 18 +-----------+ 19 |used=0 | 20 +-----------+ 21 */
刚创建好的dict是存不了任何数据的，其两个hash table的size都为0。这里先说明一下resize操作：
1 #define DICT_HT_INITIAL_SIZE 4
2
3 static unsigned long _dictNextPower(unsigned long size)
4 {
5 unsigned long i = DICT_HT_INITIAL_SIZE;
6
7 if (size >= LONG_MAX) return LONG_MAX + 1LU;
8 while(1) {
9 if (i >= size)
10 return i;
11 i *= 2;
12 }
13 }
14
15 /* Expand or create the hash table */
16 int dictExpand(dict *d, unsigned long size)
17 {
18 /* the size is invalid if it is smaller than the number of
19 * elements already inside the hash table */
20 if (dictIsRehashing(d) || d->ht[0].used > size)
21 return DICT_ERR;
22
23 dictht n; /* the new hash table */
24 unsigned long realsize = _dictNextPower(size);
25
26 /* Rehashing to the same table size is not useful. */
27 if (realsize == d->ht[0].size) return DICT_ERR;
28
29 /* Allocate the new hash table and initialize all pointers to NULL */
30 n.size = realsize;
31 n.sizemask = realsize-1;
32 n.table = zcalloc(realsize*sizeof(dictEntry*));
33 n.used = 0;
34
35 /* Is this the first initialization? If so it's not really a rehashing
36 * we just set the first hash table so that it can accept keys. */
37 if (d->ht[0].table == NULL) {
38 d->ht[0] = n;
39 return DICT_OK;
40 }
41
42 /* Prepare a second hash table for incremental rehashing */
43 d->ht[1] = n;
44 d->rehashidx = 0;
45 return DICT_OK;
46 }
47
48 int dictResize(dict *d)
49 {
50 int minimal;
51
52 if (!dict_can_resize || dictIsRehashing(d)) return DICT_ERR;
53 minimal = d->ht[0].used;
54 if (minimal < DICT_HT_INITIAL_SIZE)
55 minimal = DICT_HT_INITIAL_SIZE;
56 return dictExpand(d, minimal);
57 }
_dictNextPower用于获取当前要分配给hash table的size，得到的值一定是2的倍数，初始值为4。
dictExpand，从源码注释上看，它是为了扩容hash table，或者创建一个。它不允许与rehashing操作同时进行，也不能强制缩容。在使用_dictNextPower得到需要的size之后，它先是使用一个临时变量n去分配空间，然后进行判断，若ht[0].table的值为NULL，则认为是刚create出来的dict，直接把n赋值给ht[0]，否则给ht[1]，并开始rehashing操作。
dictResize操作就不用多说了。
三、rehashing操作
若有这样一个dict，假设K1、K2、K3、K4计算出来的hash值分别为0、5、2、7，使用sizemask计算出来的idx分别为0、1、2、3
1 /* 2 +----+ 3 +->|K1|V|->NULL 4 +------------+ /+-----------+ +->+----------+ / +----+ 5 |dictType* | / |dictEntry**|--+ |dictEntry*|/ +----+ 6 +------------+ / +-----------+ +----------+ +-->|K2|V|->NULL 7 |privdata | / |size=4 | |dictEntry*|/ +----+ 8 +------------+/ +-----------+ +----------+ 9 |ht[2] | |sizemask=3 | |dictEntry*|\ +----+ 10 +------------+\ +-----------+ +----------+ +-->|K3|V|->NULL 11 |rehashidx=-1| \ |used=4 | |dictEntry*|\ +----+ 12 +------------+ \ +-----------+ +----------+ \ +----+ 13 |iterators=0 | \ +->|K4|V|->NULL 14 +------------+ \+-----------+ +----+ 15 |dictEntry**|-->NULL 16 +-----------+ 17 |size=0 | 18 +-----------+ 19 |sizemask=0 | 20 +-----------+ 21 |used=0 | 22 +-----------+ 23 */
1 static int _dictExpandIfNeeded(dict *d)
2 {
3 /* Incremental rehashing already in progress. Return. */
4 if (dictIsRehashing(d)) return DICT_OK;
5
6 /* If the hash table is empty expand it to the initial size. */
7 if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE);
8
9 /* If we reached the 1:1 ratio, and we are allowed to resize the hash
10 * table (global setting) or we should avoid it but the ratio between
11 * elements/buckets is over the "safe" threshold, we resize doubling
12 * the number of buckets. */
13 if (d->ht[0].used >= d->ht[0].size &&
14 (dict_can_resize ||
15 d->ht[0].used/d->ht[0].size > dict_force_resize_ratio))
16 {
17 return dictExpand(d, d->ht[0].used*2);
18 }
19 return DICT_OK;
20 }
通过函数_dictExpandIfNeeded，可知当used >= size且dict_can_resize == TRUE的时候，需要调用dictExpand进入rehashing状态。dict_can_resize默认为1
1 static int dict_can_resize = 1; 2 static unsigned int dict_force_resize_ratio = 5;
需要的size为当前used * 2，即为8。调用dictExpand之后的结构：
1 /* 2 +----+ 3 +->|K1|V|->NULL 4 +->+----------+ / +----+ 5 | |dictEntry*|/ +----+ 6 | +----------+ +-->|K2|V|->NULL 7 | |dictEntry*|/ +----+ 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 10 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 11 |privdata | / |size=4 | |dictEntry*|\ +----+ 12 +------------+/ +-----------+ +----------+ \ +----+ 13 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 14 +------------+\ +-----------+ +----+ 15 |rehashidx=0 | \ |used=4 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ 19 |dictEntry**|--+ |dictEntry*|->NULL 20 +-----------+ +----------+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ 23 |sizemask=7 | |dictEntry*|->NULL 24 +-----------+ +----------+ 25 |used=0 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ 29 |dictEntry*|->NULL 30 +----------+ 31 |dictEntry*|->NULL 32 +----------+ 33 |dictEntry*|->NULL 34 +----------+ 35 */
根据rehashing操作
1 int dictRehash(dict *d, int n) {
2 int empty_visits = n*10; /* Max number of empty buckets to visit. */
3 if (!dictIsRehashing(d)) return 0;
4
5 while(n-- && d->ht[0].used != 0) {
6 dictEntry *de, *nextde;
7
8 /* Note that rehashidx can't overflow as we are sure there are more
9 * elements because ht[0].used != 0 */
10 assert(d->ht[0].size > (unsigned long)d->rehashidx);
11 while(d->ht[0].table[d->rehashidx] == NULL) {
12 d->rehashidx++;
13 if (--empty_visits == 0) return 1;
14 }
15 de = d->ht[0].table[d->rehashidx];
16 /* Move all the keys in this bucket from the old to the new hash HT */
17 while(de) {
18 uint64_t h;
19
20 nextde = de->next;
21 /* Get the index in the new hash table */
22 h = dictHashKey(d, de->key) & d->ht[1].sizemask;
23 de->next = d->ht[1].table[h];
24 d->ht[1].table[h] = de;
25 d->ht[0].used--;
26 d->ht[1].used++;
27 de = nextde;
28 }
29 d->ht[0].table[d->rehashidx] = NULL;
30 d->rehashidx++;
31 }
32
33 /* Check if we already rehashed the whole table... */
34 if (d->ht[0].used == 0) {
35 zfree(d->ht[0].table);
36 d->ht[0] = d->ht[1];
37 _dictReset(&d->ht[1]);
38 d->rehashidx = -1;
39 return 0;
40 }
41
42 /* More to rehash... */
43 return 1;
44 }
rehashing操作将会把ht[0]里，rehashidx的值对应的bucket下的所有dictEntry，移至ht[1]，之后对rehashidx进行自增处理。当ht[0]->used为0时，认为ht[0]的所有dictEntry已经移至ht[1]，此时return 0，否则 return 1，告诉调用者，还需要继续进行rehashing操作。同时，rehashing时允许最多跳过10n的空bucket，就要退出流程。假设传入的n=1，即只进行一次rehashing操作，转换至完成之后的结构：
1 /* 2 3 +->NULL 4 +->+----------+ / 5 | |dictEntry*|/ +----+ 6 | +----------+ +-->|K2|V|->NULL 7 | |dictEntry*|/ +----+ 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 10 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 11 |privdata | / |size=4 | |dictEntry*|\ +----+ 12 +------------+/ +-----------+ +----------+ \ +----+ 13 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 14 +------------+\ +-----------+ +----+ 15 |rehashidx=1 | \ |used=3 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ +----+ 19 |dictEntry**|--+ |dictEntry*|-->|K1|V|->NULL 20 +-----------+ +----------+ +----+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ 23 |sizemask=7 | |dictEntry*|->NULL 24 +-----------+ +----------+ 25 |used=1 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ 29 |dictEntry*|->NULL 30 +----------+ 31 |dictEntry*|->NULL 32 +----------+ 33 |dictEntry*|->NULL 34 +----------+ 35 */
所有节点移完时：
1 /* 2 3 4 +->+----------+ 5 | |dictEntry*|->NULL 6 | +----------+ 7 | |dictEntry*|->NULL 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|->NULL 10 +------------+ / +-----------+ +----------+ 11 |privdata | / |size=4 | |dictEntry*|->NULL 12 +------------+/ +-----------+ +----------+ 13 |ht[2] | |sizemask=3 | 14 +------------+\ +-----------+ 15 |rehashidx=4 | \ |used=0 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ +----+ 19 |dictEntry**|--+ |dictEntry*|-->|K1|V|->NULL 20 +-----------+ +----------+ +----+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ +----+ 23 |sizemask=7 | |dictEntry*|-->|K3|V|->NULL 24 +-----------+ +----------+ +----+ 25 |used=4 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ +----+ 29 |dictEntry*|-->|K2|V|->NULL 30 +----------+ +----+ 31 |dictEntry*|->NULL 32 +----------+ +----+ 33 |dictEntry*|-->|K4|V|->NULL 34 +----------+ +----+ 35 */
此时ht[0]->used为0，释放原ht[0]的hash table，把ht[1]赋值给ht[0]，并设置ht[1] = NULL，最后重置rehashidx=-1，rehashing操作结束
1 /* 2 +------------+ /+-----------+ +-->+----------+ +----+ 3 |dictType* | / |dictEntry**|---+ |dictEntry*|-->|K1|V|->NULL 4 +------------+ / +-----------+ +----------+ +----+ 5 |privdata | / |size=8 | |dictEntry*|->NULL 6 +------------+/ +-----------+ +----------+ +----+ 7 |ht[2] | |sizemask=7 | |dictEntry*|-->|K3|V|->NULL 8 +------------+\ +-----------+ +----------+ +----+ 9 |rehashidx=-1| \ |used=4 | |dictEntry*|->NULL 10 +------------+ \ +-----------+ +----------+ 11 |iterators=0 | \ |dictEntry*|->NULL 12 +------------+ \+-----------+ +----------+ +----+ 13 |dictEntry**|->NULL |dictEntry*|-->|K2|V|->NULL 14 +-----------+ +----------+ +----+ 15 |size=0 | |dictEntry*|->NULL 16 +-----------+ +----------+ +----+ 17 |sizemask=0 | |dictEntry*|-->|K4|V|->NULL 18 +-----------+ +----------+ +----+ 19 |used=0 | 20 +-----------+ 21 */
rehashing操作的触发共有两种方式
1、定时操作
1 long long timeInMilliseconds(void) {
2 struct timeval tv;
3
4 gettimeofday(&tv,NULL);
5 return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);
6 }
7
8 /* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds */
9 int dictRehashMilliseconds(dict *d, int ms) {
10 long long start = timeInMilliseconds();
11 int rehashes = 0;
12
13 while(dictRehash(d,100)) {
14 rehashes += 100;
15 if (timeInMilliseconds()-start > ms) break;
16 }
17 return rehashes;
18 }
外部传入一个毫秒时间，在这时间内循环执行rehashing，每次执行100次。
2、操作时触发
1 static void _dictRehashStep(dict *d) {
2 if (d->iterators == 0) dictRehash(d,1);
3 }
在插入、删除、查找等操作时，顺带执行一次rehashing操作。值得注意的是，如果存在安全的迭代器，即d->iterators != 0，则不会进行rehashing操作
三、插入
获取可插入新节点的bucket idx的方法：
1 static long _dictKeyIndex(dict *d, const void *key, uint64_t hash, dictEntry **existing)
2 {
3 unsigned long idx, table;
4 dictEntry *he;
5 if (existing) *existing = NULL;
6
7 /* Expand the hash table if needed */
8 if (_dictExpandIfNeeded(d) == DICT_ERR)
9 return -1;
10 for (table = 0; table <= 1; table++) {
11 idx = hash & d->ht[table].sizemask;
12 /* Search if this slot does not already contain the given key */
13 he = d->ht[table].table[idx];
14 while(he) {
15 if (key==he->key || dictCompareKeys(d, key, he->key)) {
16 if (existing) *existing = he;
17 return -1;
18 }
19 he = he->next;
20 }
21 if (!dictIsRehashing(d)) break;
22 }
23 return idx;
24 }
此方法在进行查找idx之前，先进行一次判断，是否需要rehashing操作。而后进行查找。idx的值就是通过hash函数计算出来的hash_value与sizemask做位运算的结果，然后遍历此idx对应的bucket，若已存在相同的key，则认为不可插入，并把对应的dictEntry用传入的二级指针的方式传出，供调用者使用。若不存在，则需要判断是否正在进行rehashing操作。若在，则会对ht[1]做一次相同的操作。最终可以得到一个idx值，或传出一个dictEntry。
由于rehashing期间，将会把ht[0]的所有dictEntry依次转移至ht[1]，为了防止新插入的dictEntry落到ht[0]已完成rehashing操作的bucket上，在rehashing期间，返回的可插入的idx一定是属于ht[1]的。
插入方法：
1 dictEntry *dictAddRaw(dict *d, void *key, dictEntry **existing)
2 {
3 long index;
4 dictEntry *entry;
5 dictht *ht;
6
7 if (dictIsRehashing(d)) _dictRehashStep(d);
8
9 /* Get the index of the new element, or -1 if
10 * the element already exists. */
11 if ((index = _dictKeyIndex(d, key, dictHashKey(d,key), existing)) == -1)
12 return NULL;
13
14 /* Allocate the memory and store the new entry.
15 * Insert the element in top, with the assumption that in a database
16 * system it is more likely that recently added entries are accessed
17 * more frequently. */
18 ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];
19 entry = zmalloc(sizeof(*entry));
20 entry->next = ht->table[index];
21 ht->table[index] = entry;
22 ht->used++;
23
24 /* Set the hash entry fields. */
25 dictSetKey(d, entry, key);
26 return entry;
27 }
若不存在相同key，则插入，否则，传出dictEntry的指针。插入时，由于没有记录每个dictEntry链表的尾指针，所以使用头插法，可以节约插入时的时间消耗。
dictAddRaw做为最终插入的方法，被多个方法所调用：
1 //若不存在，则插入，否则报错
2 int dictAdd(dict *d, void *key, void *val)
3 {
4 dictEntry *entry = dictAddRaw(d,key,NULL);
5
6 if (!entry) return DICT_ERR;
7 dictSetVal(d, entry, val);
8 return DICT_OK;
9 }
10
11 //若存在，则替换value，否则插入
12 int dictReplace(dict *d, void *key, void *val)
13 {
14 dictEntry *entry, *existing, auxentry;
15 entry = dictAddRaw(d,key,&existing);
16 if (entry) {
17 dictSetVal(d, entry, val);
18 return 1;
19 }
20 auxentry = *existing;
21 dictSetVal(d, existing, val);
22 dictFreeVal(d, &auxentry);
23 return 0;
24 }
25
26 //若存在，则返回对应dictEntry，否则插入后返回新的dictEntry
27 dictEntry *dictAddOrFind(dict *d, void *key) {
28 dictEntry *entry, *existing;
29 entry = dictAddRaw(d,key,&existing);
30 return entry ? entry : existing;
31 }
对于一个刚刚create的dict：
1 /* 2 3 +------------+ /+-----------+ 4 |dictType* | / |dictEntry**|-->NULL 5 +------------+ / +-----------+ 6 |privdata | / |size=0 | 7 +------------+/ +-----------+ 8 |ht[2] | |sizemask=0 | 9 +------------+\ +-----------+ 10 |rehashidx=-1| \ |used=0 | 11 +------------+ \ +-----------+ 12 |iterators=0 | \ 13 +------------+ \+-----------+ 14 |dictEntry**|-->NULL 15 +-----------+ 16 |size=0 | 17 +-----------+ 18 |sizemask=0 | 19 +-----------+ 20 |used=0 | 21 +-----------+ 22 */
假设K1、K2、K3、K4计算出来的hash值分别为0、5、2、7，使用sizemask计算出来的idx分别为0、1、2、3
现调用dictAdd方法进行插入
1、插入K1
执行完dictAddRaw中的_dictKeyIndex里的_dictExpandIfNeeded：
1 /* 2 3 +-->NULL 4 +------------+ /+-----------+ +->+----------+ / 5 |dictType* | / |dictEntry**|--+ |dictEntry*|/ 6 +------------+ / +-----------+ +----------+ +--->NULL 7 |privdata | / |size=4 | |dictEntry*|/ 8 +------------+/ +-----------+ +----------+ 9 |ht[2] | |sizemask=3 | |dictEntry*|\ 10 +------------+\ +-----------+ +----------+ +--->NULL 11 |rehashidx=-1| \ |used=0 | |dictEntry*|\ 12 +------------+ \ +-----------+ +----------+ \ 13 |iterators=0 | \ +-->NULL 14 +------------+ \+-----------+ 15 |dictEntry**|-->NULL 16 +-----------+ 17 |size=0 | 18 +-----------+ 19 |sizemask=0 | 20 +-----------+ 21 |used=0 | 22 +-----------+ 23 */
同时得到其在ht[0]的idx = 0，且不在rehashing操作中，于是直接插入
1 /* 2 +----+ 3 +->|K1|V|->NULL 4 +------------+ /+-----------+ +->+----------+ / +----+ 5 |dictType* | / |dictEntry**|--+ |dictEntry*|/ 6 +------------+ / +-----------+ +----------+ +--->NULL 7 |privdata | / |size=4 | |dictEntry*|/ 8 +------------+/ +-----------+ +----------+ 9 |ht[2] | |sizemask=3 | |dictEntry*|\ 10 +------------+\ +-----------+ +----------+ +--->NULL 11 |rehashidx=-1| \ |used=1 | |dictEntry*|\ 12 +------------+ \ +-----------+ +----------+ \ 13 |iterators=0 | \ +-->NULL 14 +------------+ \+-----------+ 15 |dictEntry**|-->NULL 16 +-----------+ 17 |size=0 | 18 +-----------+ 19 |sizemask=0 | 20 +-----------+ 21 |used=0 | 22 +-----------+ 23 */
2、插入K2、K3、K4后：
1 /* 2 +----+ 3 +->|K1|V|->NULL 4 +------------+ /+-----------+ +->+----------+ / +----+ 5 |dictType* | / |dictEntry**|--+ |dictEntry*|/ +----- 6 +------------+ / +-----------+ +----------+ +-->|K2|V|->NULL 7 |privdata | / |size=4 | |dictEntry*|/ +----+ 8 +------------+/ +-----------+ +----------+ 9 |ht[2] | |sizemask=3 | |dictEntry*|\ +----+ 10 +------------+\ +-----------+ +----------+ +-->|K3|V|->NULL 11 |rehashidx=-1| \ |used=4 | |dictEntry*|\ +----+ 12 +------------+ \ +-----------+ +----------+ \ +----+ 13 |iterators=0 | \ +->|K4|V|->NULL 14 +------------+ \+-----------+ +----+ 15 |dictEntry**|-->NULL 16 +-----------+ 17 |size=0 | 18 +-----------+ 19 |sizemask=0 | 20 +-----------+ 21 |used=0 | 22 +-----------+ 23 */
3、此时若有一个K5，计算出来的hash值为8，则：
i.因此刻不在rehashing操作，所以不用做处理
ii.执行完dictAddRaw中的_dictKeyIndex里的_dictExpandIfNeeded：
1 /* 2 +----+ 3 +->|K1|V|->NULL 4 +->+----------+ / +----+ 5 | |dictEntry*|/ +----+ 6 | +----------+ +-->|K2|V|->NULL 7 | |dictEntry*|/ +----+ 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 10 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 11 |privdata | / |size=4 | |dictEntry*|\ +----+ 12 +------------+/ +-----------+ +----------+ \ +----+ 13 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 14 +------------+\ +-----------+ +----+ 15 |rehashidx=0 | \ |used=4 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ 19 |dictEntry**|--+ |dictEntry*|->NULL 20 +-----------+ +----------+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ 23 |sizemask=7 | |dictEntry*|->NULL 24 +-----------+ +----------+ 25 |used=0 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ 29 |dictEntry*|->NULL 30 +----------+ 31 |dictEntry*|->NULL 32 +----------+ 33 |dictEntry*|->NULL 34 +----------+ 35 */
同时得到其在ht[1]的idx=0
iii.插入
1 /* 2 +----+ 3 +->|K1|V|->NULL 4 +->+----------+ / +----+ 5 | |dictEntry*|/ +----+ 6 | +----------+ +-->|K2|V|->NULL 7 | |dictEntry*|/ +----+ 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 10 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 11 |privdata | / |size=4 | |dictEntry*|\ +----+ 12 +------------+/ +-----------+ +----------+ \ +----+ 13 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 14 +------------+\ +-----------+ +----+ 15 |rehashidx=0 | \ |used=4 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ +----+ 19 |dictEntry**|--+ |dictEntry*|-->|K5|V|->NULL 20 +-----------+ +----------+ +----+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ 23 |sizemask=7 | |dictEntry*|->NULL 24 +-----------+ +----------+ 25 |used=1 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ 29 |dictEntry*|->NULL 30 +----------+ 31 |dictEntry*|->NULL 32 +----------+ 33 |dictEntry*|->NULL 34 +----------+ 35 */
4、此时若有一个K6，计算出来的hash值为16，则：
i.此时已处理rehashing操作，执行一步：
1 /* 2 3 +-->NULL 4 +->+----------+ / 5 | |dictEntry*|/ +----+ 6 | +----------+ +-->|K2|V|->NULL 7 | |dictEntry*|/ +----+ 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 10 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 11 |privdata | / |size=4 | |dictEntry*|\ +----+ 12 +------------+/ +-----------+ +----------+ \ +----+ 13 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 14 +------------+\ +-----------+ +----+ 15 |rehashidx=1 | \ |used=3 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ +----+ +----+ 19 |dictEntry**|--+ |dictEntry*|-->|K1|V|->|K5|V|->NULL 20 +-----------+ +----------+ +----+ +----+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ 23 |sizemask=7 | |dictEntry*|->NULL 24 +-----------+ +----------+ 25 |used=2 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ 29 |dictEntry*|->NULL 30 +----------+ 31 |dictEntry*|->NULL 32 +----------+ 33 |dictEntry*|->NULL 34 +----------+ 35 */
ii.执行完dictAddRaw中的_dictKeyIndex里的_dictExpandIfNeeded，因已在进行rehashing，所以不做任何处理，只返回其在ht[1]的idx 0
iii.头插法将K6插入
1 /* 2 3 +-->NULL 4 +->+----------+ / 5 | |dictEntry*|/ +----+ 6 | +----------+ +-->|K2|V|->NULL 7 | |dictEntry*|/ +----+ 8 +------------+ /+-----------+ | +----------+ 9 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 10 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 11 |privdata | / |size=4 | |dictEntry*|\ +----+ 12 +------------+/ +-----------+ +----------+ \ +----+ 13 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 14 +------------+\ +-----------+ +----+ 15 |rehashidx=1 | \ |used=3 | 16 +------------+ \ +-----------+ 17 |iterators=0 | \ 18 +------------+ \+-----------+ +->+----------+ +----+ +----+ +----+ 19 |dictEntry**|--+ |dictEntry*|-->|K6|V|->|K1|V|->|K5|V|->NULL 20 +-----------+ +----------+ +----+ +----+ +----+ 21 |size=8 | |dictEntry*|->NULL 22 +-----------+ +----------+ 23 |sizemask=7 | |dictEntry*|->NULL 24 +-----------+ +----------+ 25 |used=3 | |dictEntry*|->NULL 26 +-----------+ +----------+ 27 |dictEntry*|->NULL 28 +----------+ 29 |dictEntry*|->NULL 30 +----------+ 31 |dictEntry*|->NULL 32 +----------+ 33 |dictEntry*|->NULL 34 +----------+ 35 */
以上为正常插入时的情况，key已存在，或是调用另外两个方法的情况与之大同小异，不再做过多叙述。
四、查找
1 dictEntry *dictFind(dict *d, const void *key)
2 {
3 dictEntry *he;
4 uint64_t h, idx, table;
5
6 if (d->ht[0].used + d->ht[1].used == 0) return NULL; /* dict is empty */
7 if (dictIsRehashing(d)) _dictRehashStep(d);
8 h = dictHashKey(d, key);
9 for (table = 0; table <= 1; table++) {
10 idx = h & d->ht[table].sizemask;
11 he = d->ht[table].table[idx];
12 while(he) {
13 if (key==he->key || dictCompareKeys(d, key, he->key))
14 return he;
15 he = he->next;
16 }
17 if (!dictIsRehashing(d)) return NULL;
18 }
19 return NULL;
20 }
同样，若在rehashing期间，则执行一次。首先在ht[0]里查找，计算出hash值对应ht[0]的idx，取得其bucket，然后遍历之，找到与指定key相同的dictEntry。若ht[0]中找不到指定的key，且正在进行rehashing操作，则去ht[1]以相同方式也查找一次。
redis额外提供一个，根据key只获取其value的方法：
1 void *dictFetchValue(dict *d, const void *key) {
2 dictEntry *he;
3
4 he = dictFind(d,key);
5 return he ? dictGetVal(he) : NULL;
6 }
key不存在时返回NULL
五、删除
删除方法：
1 static dictEntry *dictGenericDelete(dict *d, const void *key, int nofree) {
2 uint64_t h, idx;
3 dictEntry *he, *prevHe;
4 int table;
5
6 if (d->ht[0].used == 0 && d->ht[1].used == 0) return NULL;
7
8 if (dictIsRehashing(d)) _dictRehashStep(d);
9 h = dictHashKey(d, key);
10
11 for (table = 0; table <= 1; table++) {
12 idx = h & d->ht[table].sizemask;
13 he = d->ht[table].table[idx];
14 prevHe = NULL;
15 while(he) {
16 if (key==he->key || dictCompareKeys(d, key, he->key)) {
17 /* Unlink the element from the list */
18 if (prevHe)
19 prevHe->next = he->next;
20 else
21 d->ht[table].table[idx] = he->next;
22 if (!nofree) {
23 dictFreeKey(d, he);
24 dictFreeVal(d, he);
25 zfree(he);
26 }
27 d->ht[table].used--;
28 return he;
29 }
30 prevHe = he;
31 he = he->next;
32 }
33 if (!dictIsRehashing(d)) break;
34 }
35 return NULL; /* not found */
36 }
查找方式与dictFind相同。找到之后，由调用者指定是否要销毁此dictEntry，若不销毁，则要把对应指针传出来，给外部使用。
此方法被两个接口所调用：
1 int dictDelete(dict *ht, const void *key) {
2 return dictGenericDelete(ht,key,0) ? DICT_OK : DICT_ERR;
3 }
4
5 dictEntry *dictUnlink(dict *ht, const void *key) {
6 return dictGenericDelete(ht,key,1);
7 }
dictDelete就不用多说了，直接删除对应dictEntry。关于为什么需要dictUnlink，源码的注释上写道，如果有某种操作，需要先查找指定key对应的dictEntry，然后对其做点操作，接着就直接删除，在没有dictUnlink的时候，需要这样：
1 entry = dictFind(...); 2 // Do something with entry 3 dictDelete(dictionary,entry);
实际需要查找两次。而在有dictUnlink的情况下：
1 entry = dictUnlink(dictionary,entry); 2 // Do something with entry 3 dictFreeUnlinkedEntry(entry);
只需要一次查找，配合专门的删除操作，即可。
1 void dictFreeUnlinkedEntry(dict *d, dictEntry *he) {
2 if (he == NULL) return;
3 dictFreeKey(d, he);
4 dictFreeVal(d, he);
5 zfree(he);
6 }
六、销毁
清空一个hash table的方法
1 int _dictClear(dict *d, dictht *ht, void(callback)(void *)) {
2 unsigned long i;
3
4 /* Free all the elements */
5 for (i = 0; i < ht->size && ht->used > 0; i++) {
6 dictEntry *he, *nextHe;
7
8 if (callback && (i & 65535) == 0) callback(d->privdata);
9
10 if ((he = ht->table[i]) == NULL) continue;
11 while(he) {
12 nextHe = he->next;
13 dictFreeKey(d, he);
14 dictFreeVal(d, he);
15 zfree(he);
16 ht->used--;
17 he = nextHe;
18 }
19 }
20 /* Free the table and the allocated cache structure */
21 zfree(ht->table);
22 /* Re-initialize the table */
23 _dictReset(ht);
24 return DICT_OK; /* never fails */
25 }
两层循环，分别遍历所有bucket与单bucket里所有dictEntry进行释放。关于这里的 (i&65535) == 0的判断，_dictClear方法仅在相同文件的方法dictEmpty与dictRelease调用
1 void dictRelease(dict *d)
2 {
3 _dictClear(d,&d->ht[0],NULL);
4 _dictClear(d,&d->ht[1],NULL);
5 zfree(d);
6 }
7
8 void dictEmpty(dict *d, void(callback)(void*)) {
9 _dictClear(d,&d->ht[0],callback);
10 _dictClear(d,&d->ht[1],callback);
11 d->rehashidx = -1;
12 d->iterators = 0;
13 }
dictRelease不用多说，传入的callback为NULL。而dictEmpty，搜索redis源码所有文件的调用，
ccx@ccx:~/Desktop/redis-5.0.7/redis-5.0.7/src$ grep dictEmpty * -r
blocked.c: dictEmpty(c->bpop.keys,NULL);
db.c: dictEmpty(server.db[j].dict,callback);
db.c: dictEmpty(server.db[j].expires,callback);
dict.c:void dictEmpty(dict *d, void(callback)(void*)) {
dict.h:void dictEmpty(dict *d, void(callback)(void*));
replication.c: dictEmpty(server.repl_scriptcache_dict,NULL);
sentinel.c: dictEmpty(server.commands,NULL);
仅db.c里传了callback进来，对应的方法为
1 long long emptyDb(int dbnum, int flags, void(callback)(void*));
继续搜索：
ccx@ccx:~/Desktop/redis-5.0.7/redis-5.0.7/src$ grep emptyDb * -r
cluster.c: emptyDb(-1,EMPTYDB_NO_FLAGS,NULL);
db.c:long long emptyDb(int dbnum, int flags, void(callback)(void*)) {
db.c: emptyDbAsync(&server.db[j]);
db.c:/* Return the set of flags to use for the emptyDb() call for FLUSHALL*/
db.c: server.dirty += emptyDb(c->db->id,flags,NULL);
db.c: server.dirty += emptyDb(-1,flags,NULL);
debug.c: emptyDb(-1,EMPTYDB_NO_FLAGS,NULL);
debug.c: emptyDb(-1,EMPTYDB_NO_FLAGS,NULL);
lazyfree.c:void emptyDbAsync(redisDb *db) {
replication.c: * data with emptyDb(), and while we load the new data received as an
replication.c:/* Callback used by emptyDb() while flushing away old data to load*/
replication.c: emptyDb(
server.h:long long emptyDb(int dbnum, int flags, void(callback)(void*));
server.h:void emptyDbAsync(redisDb *db);
真正调用的地方传入的也是NULL，并不知道是拿来做什么用的...
ps：用grep查找只是方便cv过来....
七、迭代器操作
数据结构：
1 typedef struct dictIterator {
2 dict *d;
3 long index;
4 int table, safe;
5 dictEntry *entry, *nextEntry;
6 /* unsafe iterator fingerprint for misuse detection. */
7 long long fingerprint;
8 } dictIterator;
根据源码注释可知，如果是个安全的迭代器，即safe == 1，则在迭代中可以调用dictAdd、dictFind等方法，否则只能调用dictNext。
index表示，ht[table]对应的bucket的idx。
获取迭代器：
1 dictIterator *dictGetIterator(dict *d)
2 {
3 dictIterator *iter = zmalloc(sizeof(*iter));
4
5 iter->d = d;
6 iter->table = 0;
7 iter->index = -1;
8 iter->safe = 0;
9 iter->entry = NULL;
10 iter->nextEntry = NULL;
11 return iter;
12 }
13
14 dictIterator *dictGetSafeIterator(dict *d) {
15 dictIterator *i = dictGetIterator(d);
16
17 i->safe = 1;
18 return i;
19 }
刚获取的迭代器并不指向具体哪个dictEntry
next操作：
1 dictEntry *dictNext(dictIterator *iter)
2 {
3 while (1) {
4 if (iter->entry == NULL) {
5 dictht *ht = &iter->d->ht[iter->table];
6 if (iter->index == -1 && iter->table == 0) {
7 if (iter->safe)
8 iter->d->iterators++;
9 else
10 iter->fingerprint = dictFingerprint(iter->d);
11 }
12 iter->index++;
13 if (iter->index >= (long) ht->size) {
14 if (dictIsRehashing(iter->d) && iter->table == 0) {
15 iter->table++;
16 iter->index = 0;
17 ht = &iter->d->ht[1];
18 } else {
19 break;
20 }
21 }
22 iter->entry = ht->table[iter->index];
23 } else {
24 iter->entry = iter->nextEntry;
25 }
26 if (iter->entry) {
27 /* We need to save the 'next' here, the iterator user
28 * may delete the entry we are returning. */
29 iter->nextEntry = iter->entry->next;
30 return iter->entry;
31 }
32 }
33 return NULL;
34 }
对于一个新的迭代器，首次调用时，会根据是否安全，做不同操作。安全的迭代器会给dict里的计数器+1，不安全的将会记录本字典的指纹。之后会遍历ht[0]，取到第一个非NULL的dictEntry。当ht[0]遍历完且取不到非NULL的dictEntry时，如果正在进行rehashing操作，则会去ht[1]里找。
如：
1 /* 2 3 +-------------------------+ 4 +----|dict * | 5 | +-------------------------+ 6 | |long index | 7 | +-------------------------+ 8 | |int table | 9 | +-------------------------+ 10 | |int safe | 11 | +-------------------------+ 12 | |dictEntry *entry |->NULL 13 | +-------------------------+ 14 | |dictEntry *entrynextEntry|->NULL 15 | +-------------------------+ 16 | |long long fingerprint | 17 | +-------------------------+ 18 | 19 | 20 | 21 | +-->NULL 22 | +->+----------+ / 23 | | |dictEntry*|/ +----+ 24 | | +----------+ +-->|K2|V|->NULL 25 | | |dictEntry*|/ +----+ 26 +--->+------------+ /+-----------+ | +----------+ 27 |dictType* | / |dictEntry**|--+ |dictEntry*|\ +----+ 28 +------------+ / +-----------+ +----------+ +-->|K3|V|->NULL 29 |privdata | / |size=4 | |dictEntry*|\ +----+ 30 +------------+/ +-----------+ +----------+ \ +----+ 31 |ht[2] | |sizemask=3 | +->|K4|V|->NULL 32 +------------+\ +-----------+ +----+ 33 |rehashidx=1 | \ |used=3 | 34 +------------+ \ +-----------+ 35 |iterators=0 | \ 36 +------------+ \+-----------+ +->+----------+ +----+ +----+ 37 |dictEntry**|--+ |dictEntry*|-->|K1|V|->|K5|V|->NULL 38 +-----------+ +----------+ +----+ +----+ 39 |size=8 | |dictEntry*|->NULL 40 +-----------+ +----------+ 41 |sizemask=7 | |dictEntry*|->NULL 42 +-----------+ +----------+ 43 |used=3 | |dictEntry*|->NULL 44 +-----------+ +----------+ 45 |dictEntry*|->NULL 46 +----------+ 47 |dictEntry*|->NULL 48 +----------+ 49 |dictEntry*|->NULL 50 +----------+ 51 |dictEntry*|->NULL 52 +----------+ 53 */
遍历顺序为，K2，K3，K4，K1，K5。
迭代器销毁：
1 void dictReleaseIterator(dictIterator *iter)
2 {
3 if (!(iter->index == -1 && iter->table == 0)) {
4 if (iter->safe)
5 iter->d->iterators--;
6 else
7 assert(iter->fingerprint == dictFingerprint(iter->d));
8 }
9 zfree(iter);
10 }
与首次执行next操作相对应，若为safe的迭代器，要给dict的计算减1，否则要校验期间dict的指纹是否发生了变化 。
指纹的计算：
1 long long dictFingerprint(dict *d) {
2 long long integers[6], hash = 0;
3 int j;
4
5 integers[0] = (long) d->ht[0].table;
6 integers[1] = d->ht[0].size;
7 integers[2] = d->ht[0].used;
8 integers[3] = (long) d->ht[1].table;
9 integers[4] = d->ht[1].size;
10 integers[5] = d->ht[1].used;
11
12 /* We hash N integers by summing every successive integer with the integer
13 * hashing of the previous sum. Basically:
14 *
15 * Result = hash(hash(hash(int1)+int2)+int3) ...
16 *
17 * This way the same set of integers in a different order will (likely) hash
18 * to a different number. */
19 for (j = 0; j < 6; j++) {
20 hash += integers[j];
21 /* For the hashing step we use Tomas Wang's 64 bit integer hash. */
22 hash = (~hash) + (hash << 21); // hash = (hash << 21) - hash - 1;
23 hash = hash ^ (hash >> 24);
24 hash = (hash + (hash << 3)) + (hash << 8); // hash * 265
25 hash = hash ^ (hash >> 14);
26 hash = (hash + (hash << 2)) + (hash << 4); // hash * 21
27 hash = hash ^ (hash >> 28);
28 hash = hash + (hash << 31);
29 }
30 return hash;
31 }
对于不安全的迭代器，在迭代过程中，不允许操作任何修改dict的操作，是只读的，不会发生迭代器失效的问题。对于安全的迭代器，在进行操作本节点的时候，redis中记录了当前迭代的bucket idx，以及当前dictEntry的next节点。如果只是add操作，即使是用了头插法把新dictEntry插在本节点之前，对迭代器本身并没有影响。如果是delete了本节点，迭代器中还记录了next节点的位置，调用next时直接取就好。如果next为空，则可以认为当前bucket遍历完了，取下一个bucket就行了。当然，如果在add/delete等操作的时候，进行了rehashing操作，那么当前迭代器里记录的next，在rehashing之后，可能就不是当前节点新位置的next了。所以在使用安全迭代器的时候，禁止了rehashing操作。
八、其它操作
dict还支持其它的一些操作。
1、随机获取一个key，可以用于一些随机操作的dictGetRandomKey
2、随机获取n个key：dictGetSomeKeys
3、scan操作
关于scan操作，redis采用了一个很巧妙的方法，保证了在开始scan时未删除的元素一定能遍历到，又能保证尽量少地重复遍历。
这里直接给个传送门，这里讲得很好：
https://blog.csdn.net/gqtcgq/article/details/50533336
redis 5.0.7 下载链接
http://download.redis.io/releases/redis-5.0.7.tar.gz
源码阅读顺序参考：
https://github.com/huangz1990/blog/blob/master/diary/2014/how-to-read-redis-source-code.rst
其它参考：
https://zhuanlan.zhihu.com/p/42156903
标签：5.0,+----+,dictEntry,redis,ht,源码,+----------+,NULL,+-----------+ 来源： https://www.cnblogs.com/mtxcat/p/14033844.html