实现字符串对像

Simple Dynamic String

目录

实现字符串对像

Simple Dynamic String

set string

APPED 过程

因为 char* 类型的功能单一， 抽象层次低， 并且不能高效地支持一些 Redis 常用的操作（比如追加操作和长度计算操作）， 所以在 Redis 程序内部， 绝大部分情况下都会使用 SDS 而不是 char* 来表示字符串。

在源代码sds.h中定义了sds以及sdshdr结构体。

 

set string

代码块

127.0.0.1:6379> set java slow

OK

127.0.0.1:6379> get java

\"slow\"

127.0.0.1:6379> 

 

中的过程

sds 内部结构

代码块

typedef char *sds;

​

/* Note: sdshdr5 is never used, we just access the flags byte directly.

 * However is here to document the layout of type 5 SDS strings. */

struct __attribute__ ((__packed__)) sdshdr5 {

    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */

    char buf[];

};

struct __attribute__ ((__packed__)) sdshdr8 {

    uint8_t len; /* used */

    uint8_t alloc; /* excluding the header and null terminator */

    unsigned char flags; /* 3 lsb of type, 5 unused bits */

    char buf[];

};

struct __attribute__ ((__packed__)) sdshdr16 {

    uint16_t len; /* used */

    uint16_t alloc; /* excluding the header and null terminator */

    unsigned char flags; /* 3 lsb of type, 5 unused bits */

    char buf[];

};

struct __attribute__ ((__packed__)) sdshdr32 {

    uint32_t len; /* used */

    uint32_t alloc; /* excluding the header and null terminator */

    unsigned char flags; /* 3 lsb of type, 5 unused bits */

    char buf[];

};

struct __attribute__ ((__packed__)) sdshdr64 {

    uint64_t len; /* used */

    uint64_t alloc; /* excluding the header and null terminator */

    unsigned char flags; /* 3 lsb of type, 5 unused bits */

    char buf[];

};

​

 

 

 

2 通过创建函数可以看到，函数返回值是sds，在函数中返回的是sdshdr结构体中数据指向部分。

代码块

 *

 * mystring = sdsnewlen(\"abc\",3);

 *

 * You can print the string with printf() as there is an implicit \\0 at the

 * end of the string. However the string is binary safe and can contain

 * \\0 characters in the middle, as the length is stored in the sds header. */

sds sdsnewlen(const void *init, size_t initlen) {

    void *sh;

    sds s;

    char type = sdsReqType(initlen);

    /* Empty strings are usually created in order to append. Use type 8

     * since type 5 is not good at this. */

    if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;

    int hdrlen = sdsHdrSize(type);

    unsigned char *fp; /* flags pointer. */

​

    sh = s_malloc(hdrlen+initlen+1);

    if (init==SDS_NOINIT)

        init = NULL;

    else if (!init)

        memset(sh, 0, hdrlen+initlen+1);

    if (sh == NULL) return NULL;

    s = (char*)sh+hdrlen;

    fp = ((unsigned char*)s)-1;

    switch(type) {

        case SDS_TYPE_5: {

            *fp = type | (initlen << SDS_TYPE_BITS);

            break;

        }

        case SDS_TYPE_8: {

            SDS_HDR_VAR(8,s);

            sh->len = initlen;

            sh->alloc = initlen;

            *fp = type;

            break;

        }

        case SDS_TYPE_16: {

            SDS_HDR_VAR(16,s);

            sh->len = initlen;

            sh->alloc = initlen;

            *fp = type;

            break;

        }

        case SDS_TYPE_32: {

            SDS_HDR_VAR(32,s);

            sh->len = initlen;

            sh->alloc = initlen;

            *fp = type;

            break;

        }

        case SDS_TYPE_64: {

            SDS_HDR_VAR(64,s);

            sh->len = initlen;

            sh->alloc = initlen;

            *fp = type;

            break;

        }

    }

    if (initlen && init)

        memcpy(s, init, initlen);

    s[initlen] = \'\\0\';

    return s;

}

​

 

APPED 过程

代码块

127.0.0.1:6379> append java more

(integer) 8

127.0.0.1:6379> get java

\"slowmore\"

 

当调用 SET 命令创建 sdshdr 时， sdshdr 的 free 属性为 0 ， Redis 也没有为 buf 创建额外的空间，

而在执行 APPEND 之后， Redis 为 buf 创建了多于所需空间一倍的大小。

代码块

/* Enlarge the free space at the end of the sds string so that the caller

 * is sure that after calling this function can overwrite up to addlen

 * bytes after the end of the string, plus one more byte for nul term.

 *

 * Note: this does not change the *length* of the sds string as returned

 * by sdslen(), but only the free buffer space we have. */

sds sdsMakeRoomFor(sds s, size_t addlen) {

    void *sh, *newsh;

    size_t avail = sdsavail(s);

    size_t len, newlen;

    char type, oldtype = s[-1] & SDS_TYPE_MASK;

    int hdrlen;

​

    /* Return ASAP if there is enough space left. */

    if (avail >= addlen) return s;

​

    len = sdslen(s);

    sh = (char*)s-sdsHdrSize(oldtype);

    newlen = (len+addlen);

    if (newlen < SDS_MAX_PREALLOC)

        newlen *= 2;

    else

        newlen += SDS_MAX_PREALLOC;

​

    type = sdsReqType(newlen);

​

    /* Don\'t use type 5: the user is appending to the string and type 5 is

     * not able to remember empty space, so sdsMakeRoomFor() must be called

     * at every appending operation. */

    if (type == SDS_TYPE_5) type = SDS_TYPE_8;

​

    hdrlen = sdsHdrSize(type);

    if (oldtype==type) {

        newsh = s_realloc(sh, hdrlen+newlen+1);

        if (newsh == NULL) return NULL;

        s = (char*)newsh+hdrlen;

    } else {

        /* Since the header size changes, need to move the string forward,

         * and can\'t use realloc */

        newsh = s_malloc(hdrlen+newlen+1);

        if (newsh == NULL) return NULL;

        memcpy((char*)newsh+hdrlen, s, len+1);

        s_free(sh);

        s = (char*)newsh+hdrlen;

        s[-1] = type;

        sdssetlen(s, len);

    }

    sdssetalloc(s, newlen);

    return s;

}

​

 

 

• Redis 的字符串表示为 sds ，而不是 C 字符串（以 \\0 结尾的 char*）。

• 对比 C 字符串， sds 有以下特性：

  • 可以高效地执行长度计算（strlen）；

  • 可以高效地执行追加操作（append）；

  • 二进制安全；

• sds 会为追加操作进行优化：加快追加操作的速度，并降低内存分配的次数，代价是多占用了一些内存，而且这些内存不会被主动释放。

仅