Redis 数据结构-简单动态字符串

Redis 数据结构-简单动态字符串

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1、简介

Redis 之所以快主要得益于它的数据结构、操作内存数据库、单线程和多路 I/O 复用模型，进一步窥探下它常见的五种基本数据的底层数据结构。

Redis 常见数据类型对应的的底层数据结构。

Redis 数据结构-简单动态字符串

2、简单动态字符串

String是Redis 最基本的类型，最大能存储 512MB 的数据，String类型是二进制安全的，它可以存储任何数据包括数字、图片、序列化对象等。虽然Redis 是C 语言写的，但Redis 中并没有使用 C 中 char 来表示字符串，而是自定义了一种新的字符串结构 简单动态字符串（Simple Dynamic Strings，SDS）来存储字符串和整型数据。

C 语言字符串有以下几个问题：

SDS 结构

例如执行以下命令

set name "tjt"

Redis 数据结构-简单动态字符串

set命令执行后，Redis将在底层创建两个SDS，一个是包含name 的SDS，另一个是包含“tjt”的SDS。

从Redis 源码的sds.h 文件中可以看到SDS 的结构体。

Redis 数据结构-简单动态字符串

从sds.h源码中截取出sdshdr8 如下。

1 struct __attribute__ ((__packed__)) sdshdr8 {
2     uint8_t len; /* used */
3     uint8_t alloc; /* excluding the header and null terminator */
4     unsigned char flags; /* 3 lsb of type, 5 unused bits */
5     char buf[];
6 };

Redis 数据结构-简单动态字符串

例如，一个包含字符串“tjt"的SDS 结构如下:

Redis 数据结构-简单动态字符串

动态字符串SDS 具备动态扩容的能力，例如给SDS 'tjt' 追加一段字符串 ",go”，这里首先会申请新内存空间。

Redis 数据结构-简单动态字符串

3、Redis 使用SDS 的优势

1、SDS可以通过常数级别获取字符串的长度

redis的结构中存储了字符串的长度，所以获取字符串的长度复杂度为O(1)，c 中字符串没记录长度，需要遍历整个长度，复杂度为O(N)。

2、杜绝缓冲区溢出

3、减少修改字符串时的内存分配次数

4、二进制安全

5、Int、Raw和 embstr 动态存储

简单动态字符串结构在数据存储过程中，字符串对象会根据保存值的类型、长度不同，动态匹配三种存储结构：Int、Raw和 embstr 。

Redis 数据结构-简单动态字符串

sds.h 文件完整源码

Redis 数据结构-简单动态字符串Redis 数据结构-简单动态字符串

  1 /* SDSLib 2.0 -- A C dynamic strings library
  2  *
  3  * Copyright (c) 2006-2015, Salvatore Sanfilippo <antirez at gmail dot com>
  4  * Copyright (c) 2015, Oran Agra
  5  * Copyright (c) 2015, Redis Labs, Inc
  6  * All rights reserved.
  7  *
  8  * Redistribution and use in source and binary forms, with or without
  9  * modification, are permitted provided that the following conditions are met:
 10  *
 11  *   * Redistributions of source code must retain the above copyright notice,
 12  *     this list of conditions and the following disclaimer.
 13  *   * Redistributions in binary form must reproduce the above copyright
 14  *     notice, this list of conditions and the following disclaimer in the
 15  *     documentation and/or other materials provided with the distribution.
 16  *   * Neither the name of Redis nor the names of its contributors may be used
 17  *     to endorse or promote products derived from this software without
 18  *     specific prior written permission.
 19  *
 20  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 21  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 23  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 24  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 25  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 26  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 27  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 28  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 29  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 30  * POSSIBILITY OF SUCH DAMAGE.
 31  */
 32 
 33 #ifndef __SDS_H
 34 #define __SDS_H
 35 
 36 #define SDS_MAX_PREALLOC (1024*1024)
 37 const char *SDS_NOINIT;
 38 
 39 #include <sys/types.h>
 40 #include <stdarg.h>
 41 #include <stdint.h>
 42 
 43 typedef char *sds;
 44 
 45 /* Note: sdshdr5 is never used, we just access the flags byte directly.
 46  * However is here to document the layout of type 5 SDS strings. */
 47 struct __attribute__ ((__packed__)) sdshdr5 {
 48     unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
 49     char buf[];
 50 };
 51 struct __attribute__ ((__packed__)) sdshdr8 {
 52     uint8_t len; /* used */
 53     uint8_t alloc; /* excluding the header and null terminator */
 54     unsigned char flags; /* 3 lsb of type, 5 unused bits */
 55     char buf[];
 56 };
 57 struct __attribute__ ((__packed__)) sdshdr16 {
 58     uint16_t len; /* used */
 59     uint16_t alloc; /* excluding the header and null terminator */
 60     unsigned char flags; /* 3 lsb of type, 5 unused bits */
 61     char buf[];
 62 };
 63 struct __attribute__ ((__packed__)) sdshdr32 {
 64     uint32_t len; /* used */
 65     uint32_t alloc; /* excluding the header and null terminator */
 66     unsigned char flags; /* 3 lsb of type, 5 unused bits */
 67     char buf[];
 68 };
 69 struct __attribute__ ((__packed__)) sdshdr64 {
 70     uint64_t len; /* used */
 71     uint64_t alloc; /* excluding the header and null terminator */
 72     unsigned char flags; /* 3 lsb of type, 5 unused bits */
 73     char buf[];
 74 };
 75 
 76 #define SDS_TYPE_5  0
 77 #define SDS_TYPE_8  1
 78 #define SDS_TYPE_16 2
 79 #define SDS_TYPE_32 3
 80 #define SDS_TYPE_64 4
 81 #define SDS_TYPE_MASK 7
 82 #define SDS_TYPE_BITS 3
 83 #define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
 84 #define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
 85 #define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)
 86 
 87 static inline size_t sdslen(const sds s) {
 88     unsigned char flags = s[-1];
 89     switch(flags&SDS_TYPE_MASK) {
 90         case SDS_TYPE_5:
 91             return SDS_TYPE_5_LEN(flags);
 92         case SDS_TYPE_8:
 93             return SDS_HDR(8,s)->len;
 94         case SDS_TYPE_16:
 95             return SDS_HDR(16,s)->len;
 96         case SDS_TYPE_32:
 97             return SDS_HDR(32,s)->len;
 98         case SDS_TYPE_64:
 99             return SDS_HDR(64,s)->len;
100     }
101     return 0;
102 }
103 
104 static inline size_t sdsavail(const sds s) {
105     unsigned char flags = s[-1];
106     switch(flags&SDS_TYPE_MASK) {
107         case SDS_TYPE_5: {
108             return 0;
109         }
110         case SDS_TYPE_8: {
111             SDS_HDR_VAR(8,s);
112             return sh->alloc - sh->len;
113         }
114         case SDS_TYPE_16: {
115             SDS_HDR_VAR(16,s);
116             return sh->alloc - sh->len;
117         }
118         case SDS_TYPE_32: {
119             SDS_HDR_VAR(32,s);
120             return sh->alloc - sh->len;
121         }
122         case SDS_TYPE_64: {
123             SDS_HDR_VAR(64,s);
124             return sh->alloc - sh->len;
125         }
126     }
127     return 0;
128 }
129 
130 static inline void sdssetlen(sds s, size_t newlen) {
131     unsigned char flags = s[-1];
132     switch(flags&SDS_TYPE_MASK) {
133         case SDS_TYPE_5:
134             {
135                 unsigned char *fp = ((unsigned char*)s)-1;
136                 *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
137             }
138             break;
139         case SDS_TYPE_8:
140             SDS_HDR(8,s)->len = newlen;
141             break;
142         case SDS_TYPE_16:
143             SDS_HDR(16,s)->len = newlen;
144             break;
145         case SDS_TYPE_32:
146             SDS_HDR(32,s)->len = newlen;
147             break;
148         case SDS_TYPE_64:
149             SDS_HDR(64,s)->len = newlen;
150             break;
151     }
152 }
153 
154 static inline void sdsinclen(sds s, size_t inc) {
155     unsigned char flags = s[-1];
156     switch(flags&SDS_TYPE_MASK) {
157         case SDS_TYPE_5:
158             {
159                 unsigned char *fp = ((unsigned char*)s)-1;
160                 unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc;
161                 *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
162             }
163             break;
164         case SDS_TYPE_8:
165             SDS_HDR(8,s)->len += inc;
166             break;
167         case SDS_TYPE_16:
168             SDS_HDR(16,s)->len += inc;
169             break;
170         case SDS_TYPE_32:
171             SDS_HDR(32,s)->len += inc;
172             break;
173         case SDS_TYPE_64:
174             SDS_HDR(64,s)->len += inc;
175             break;
176     }
177 }
178 
179 /* sdsalloc() = sdsavail() + sdslen() */
180 static inline size_t sdsalloc(const sds s) {
181     unsigned char flags = s[-1];
182     switch(flags&SDS_TYPE_MASK) {
183         case SDS_TYPE_5:
184             return SDS_TYPE_5_LEN(flags);
185         case SDS_TYPE_8:
186             return SDS_HDR(8,s)->alloc;
187         case SDS_TYPE_16:
188             return SDS_HDR(16,s)->alloc;
189         case SDS_TYPE_32:
190             return SDS_HDR(32,s)->alloc;
191         case SDS_TYPE_64:
192             return SDS_HDR(64,s)->alloc;
193     }
194     return 0;
195 }
196 
197 static inline void sdssetalloc(sds s, size_t newlen) {
198     unsigned char flags = s[-1];
199     switch(flags&SDS_TYPE_MASK) {
200         case SDS_TYPE_5:
201             /* Nothing to do, this type has no total allocation info. */
202             break;
203         case SDS_TYPE_8:
204             SDS_HDR(8,s)->alloc = newlen;
205             break;
206         case SDS_TYPE_16:
207             SDS_HDR(16,s)->alloc = newlen;
208             break;
209         case SDS_TYPE_32:
210             SDS_HDR(32,s)->alloc = newlen;
211             break;
212         case SDS_TYPE_64:
213             SDS_HDR(64,s)->alloc = newlen;
214             break;
215     }
216 }
217 
218 sds sdsnewlen(const void *init, size_t initlen);
219 sds sdsnew(const char *init);
220 sds sdsempty(void);
221 sds sdsdup(const sds s);
222 void sdsfree(sds s);
223 sds sdsgrowzero(sds s, size_t len);
224 sds sdscatlen(sds s, const void *t, size_t len);
225 sds sdscat(sds s, const char *t);
226 sds sdscatsds(sds s, const sds t);
227 sds sdscpylen(sds s, const char *t, size_t len);
228 sds sdscpy(sds s, const char *t);
229 
230 sds sdscatvprintf(sds s, const char *fmt, va_list ap);
231 #ifdef __GNUC__
232 sds sdscatprintf(sds s, const char *fmt, ...)
233     __attribute__((format(printf, 2, 3)));
234 #else
235 sds sdscatprintf(sds s, const char *fmt, ...);
236 #endif
237 
238 sds sdscatfmt(sds s, char const *fmt, ...);
239 sds sdstrim(sds s, const char *cset);
240 void sdsrange(sds s, ssize_t start, ssize_t end);
241 void sdsupdatelen(sds s);
242 void sdsclear(sds s);
243 int sdscmp(const sds s1, const sds s2);
244 sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count);
245 void sdsfreesplitres(sds *tokens, int count);
246 void sdstolower(sds s);
247 void sdstoupper(sds s);
248 sds sdsfromlonglong(long long value);
249 sds sdscatrepr(sds s, const char *p, size_t len);
250 sds *sdssplitargs(const char *line, int *argc);
251 sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);
252 sds sdsjoin(char **argv, int argc, char *sep);
253 sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen);
254 
255 /* Low level functions exposed to the user API */
256 sds sdsMakeRoomFor(sds s, size_t addlen);
257 void sdsIncrLen(sds s, ssize_t incr);
258 sds sdsRemoveFreeSpace(sds s);
259 size_t sdsAllocSize(sds s);
260 void *sdsAllocPtr(sds s);
261 
262 /* Export the allocator used by SDS to the program using SDS.
263  * Sometimes the program SDS is linked to, may use a different set of
264  * allocators, but may want to allocate or free things that SDS will
265  * respectively free or allocate. */
266 void *sds_malloc(size_t size);
267 void *sds_realloc(void *ptr, size_t size);
268 void sds_free(void *ptr);
269 
270 #ifdef REDIS_TEST
271 int sdsTest(int argc, char *argv[]);
272 #endif
273 
274 #endif

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