Redis源码剖析和注释(二)— 简单动态字符串

作者:men_wen

出处:https://blog.csdn.net/men_wen/column/info/15428


1.介绍

Redis兼容传统的C语言字符串类型,但没有直接使用 C语言的传统的字符串(以’\0’结尾的字符数组) 表示,而是自己构建了一种名为 简单动态字符串(simple dynamic string,SDS) 的对象。简单动态字符串在Redis数据库中应用很广泛,例如:键值对在底层就是由SDS实现的。

在redis种,有一种数据类型叫string类型,而string类型简单的说就是SDS实现的(简单理解),先通过几个命令来感受一下string类型:redis字符串类型命令的介绍

    127.0.0.1:6379> SET str1 Redis  //设置key:value = str1:Redis
    OK
    127.0.0.1:6379> GET str1        //获取str1的value
    "Redis"
    127.0.0.1:6379> TYPE str1       //获取key的存储类型 string类型
    string
    127.0.0.1:6379> STRLEN str1     //str1的长度为5字节
    (integer) 5

2.SDS的定义

SDS定义在redis源码根目录下的sds.h/sdshdr

    typedef char *sds;  
    //sds兼容传统C风格字符串,所以起了个别名叫sds,并且可以存放sdshdr结构buf成员的地址

SDS也有一个 表头(header) 用来存放sds的信息。

    struct sdshdr {
        int len;        //buf中已占用空间的长度
        int free;       //buf中剩余可用空间的长度
        char buf[];     //初始化sds分配的数据空间,而且是柔性数组(Flexible array member)
    };

关于柔型数组可以看陈皓的一篇文章:C语言结构体里的成员数组和指针

根据这个结构体,我们用图大概表示一下str1,如下图:

  • len为5,表示这个sds长度为5字节。
  • free为2,表示这个sds还有2个字节未使用的空间。
  • buf是一个char[]的数组,分配了(len+1+free)个字节的长度,前len个字节保存着’R’、’e’、’d’、’i’、’s’这5个字符,接下来的1个字节保存着’\0’,剩下的free个字节未使用。

3. SDS的优点

SDS本质上就是 char *,因为有了表头sdshdr结构的存在,所以SDS比传统C字符串在某些方面更加优秀,并且能够兼容传统C字符串。

3.1 兼容C的部分函数

因为SDS兼容传统的C字符串,采用以’\0’作为结尾,所以SDS就能够使用一部分

3.2 二进制安全(Binary Safe)

因为传统C字符串符合ASCII编码,这种编码的操作的特点就是:遇零则止 。即,当读一个字符串时,只要遇到’\0’结尾,就认为到达末尾,就忽略’\0’结尾以后的所有字符。 因此,如果传统字符串保存图片,视频等二进制文件,操作文件时就被截断了。

而SDS表头的buf被定义为 字节数组,因为 判断是否到达字符串结尾的依据则是表头的len成员,这意味着 它可以存放任何二进制的数据和文本数据,包括’\0’,如下图:

3.3 获得字符串长度的操作复杂度为O(1)

传统的C字符串获得长度时的做法:遍历字符串的长度,遇零则止,复杂度为O(n)。

而SDS表头的 len成员就保存着字符串长度,所以获得字符串长度的操作复杂度为O(1)。

3.4 杜绝缓冲区溢出

因为SDS表头的free成员记录着buf字符数组中未使用空间的字节数,所以,在进行APPEND命令向字符串后追加字符串时,如果不够用会 先进行内存扩展,在进行追加。

总之,正是因为表头的存在,使得redis的字符串有这么多优点。

4. SDS源码剖析

4.1 SDS内存分配策略—空间预分配

空间预分配策略用于优化SDS的字符串增长操作。

  • 如果对SDS进行修改后,SDS表头的len成员小于1MB,那么就会分配和len长度相同的未使用空间。free和len成员大小相等。
  • 如果对SDS进行修改后,SDS的长度大于等于1MB,那么就会分配1MB的未使用空间。

通过空间预分配策略,Redis可以减少连续执行字符串增长操作所需的内存重分配次数。

源代码如下:

    sds sdsMakeRoomFor(sds s, size_t addlen) {      //对 sds 中 buf 的长度进行扩展
        struct sdshdr *sh, *newsh;
        size_t free = sdsavail(s);  //获得s的未使用空间长度
        size_t len, newlen;

        //free的长度够用不用扩展直接返回
        if (free >= addlen) return s;  

        //free长度不够用,需要扩展
        len = sdslen(s);    //获得s字符串的长度
        sh = (void*) (s-(sizeof(struct sdshdr)));       //获取表头地址
        newlen = (len+addlen);  //扩展后的新长度

        //空间预分配     
        //#define SDS_MAX_PREALLOC (1024*1024)  
        //预先分配内存的最大长度为 1MB
        if (newlen < SDS_MAX_PREALLOC)  //新长度小于“最大预分配长度”,就直接将扩展的新长度乘2
            newlen *= 2;
        else
            newlen += SDS_MAX_PREALLOC; //新长度大于“最大预分配长度”,就在加上一个“最大预分配长度”
        newsh = zrealloc(sh, sizeof(struct sdshdr)+newlen+1);   //获得新的扩展空间的地址
        if (newsh == NULL) return NULL;

        newsh->free = newlen - len; //更新新空间的未使用的空间free
        return newsh->buf;
    }

4.2 SDS内存释放策略—惰性空间释放

惰性空间释放用于优化SDS的字符串缩短操作。

  • 当要缩短SDS保存的字符串时,程序并不立即使用内存充分配来回收缩短后多出来的字节,而是使用表头的free成员将这些字节记录起来,并等待将来使用。

源代码如下:

    void sdsclear(sds s) {  //重置sds的buf空间,懒惰释放
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        sh->free += sh->len;    //表头free成员+已使用空间的长度len = 新的free
        sh->len = 0;            //已使用空间变为0
        sh->buf[0] = '\0';         //字符串置空
    }

4.3 Redis源码注释

  1. 在sds.h文件中,有两个static inline的函数,分别是sdslen和sdsavail函数,你可以把它认为是一个static的函数,加上了inline的属性。而inline关键字仅仅是建议编译器做内联展开处理,而不是强制。
  2. 在sds.c中,几乎所有的函数所传的参数都是sds类型,而非表头sdshdr的地址,但是使用了通过sds指针运算从而求得表头的地址的技巧,因为sds是指向sdshdr结构buf成员的。通过sds.h/sdslen函数,来分析:

这里的关键就是sds类型是指向sdshdr结构buf成员。

  1. struct sdshdr结构共有三个变量,其中sds指向的buf成员是一个柔性数组,它仅仅起到占位符的作用,并不占用该结构体的大小,因此sizeof(sizeof(struct sdshdr))大小为8字节。
  2. 由于一个SDS类型的内存是通过动态内存分配的,所以它的内存在堆区,堆由下往上增长,因此sds指针减区sizeof(struct sdshdr)的大小就得到了表头的地址,然后就可以通过”->”访问表头的成员。如下图:
        static inline size_t sdslen(const sds s) {      //计算buf中字符串的长度

            struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr))); //s指针地址减去结构体大小就是结构体的地址

           return sh->len;
        }

通过这种技巧,将表头结构隐藏起来,只对外公开sds类型。

以下源码注释可以访问放在这里:sds.c和sds.h源码注释

  • sds.h文件注释
    #ifndef __SDS_H
    #define __SDS_H

    #define SDS_MAX_PREALLOC (1024*1024)    //预先分配内存的最大长度为1MB

    #include <sys/types.h>
    #include <stdarg.h>

    typedef char *sds;  //sds兼容传统C风格字符串,所以起了个别名叫sds,并且可以存放sdshdr结构buf成员的地址

    struct sdshdr {
        unsigned int len;   //buf中已占用空间的长度
        unsigned int free;  //buf中剩余可用空间的长度
        char buf[];         //初始化sds分配的数据空间,而且是柔性数组(Flexible array member)
    };

    static inline size_t sdslen(const sds s) {      //计算buf中字符串的长度
        struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));
        return sh->len;
    }

    static inline size_t sdsavail(const sds s) {    //计算buf中的未使用空间的长度
        struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));
        return sh->free;
    }

    sds sdsnewlen(const void *init, size_t initlen);    //创建一个长度为initlen的字符串,并保存init字符串中的值
    sds sdsnew(const char *init);       //创建一个默认长度的字符串
    sds sdsempty(void);     //建立一个只有表头,字符串为空"\0"的sds
    size_t sdslen(const sds s); //计算buf中字符串的长度
    sds sdsdup(const sds s);    //拷贝一份s的副本
    void sdsfree(sds s);     //释放s字符串和表头
    size_t sdsavail(const sds s);   //计算buf中的未使用空间的长度
    sds sdsgrowzero(sds s, size_t len); //将sds扩展制定长度并赋值为0
    sds sdscatlen(sds s, const void *t, size_t len);    //将字符串t追加到s表头的buf末尾,追加len个字节
    sds sdscat(sds s, const char *t);       //将t字符串拼接到s的末尾
    sds sdscatsds(sds s, const sds t);      //将sds追加到s末尾
    sds sdscpylen(sds s, const char *t, size_t len);    //将字符串t覆盖到s表头的buf中,拷贝len个字节
    sds sdscpy(sds s, const char *t);       //将字符串覆盖到s表头的buf中

    sds sdscatvprintf(sds s, const char *fmt, va_list ap);  //打印函数,被 sdscatprintf 所调用
    #ifdef __GNUC__
    sds sdscatprintf(sds s, const char *fmt, ...)   //打印任意数量个字符串,并将这些字符串追加到给定 sds 的末尾
        __attribute__((format(printf, 2, 3)));
    #else
    sds sdscatprintf(sds s, const char *fmt, ...);  //打印任意数量个字符串,并将这些字符串追加到给定 sds 的末尾
    #endif

    sds sdscatfmt(sds s, char const *fmt, ...); //格式化打印多个字符串,并将这些字符串追加到给定 sds 的末尾
    sds sdstrim(sds s, const char *cset);   //去除sds中包含有 cset字符串出现字符 的字符
    void sdsrange(sds s, int start, int end);   //根据start和end区间截取字符串
    void sdsupdatelen(sds s);           //更新字符串s的长度
    void sdsclear(sds s);               //将字符串重置保存空间,懒惰释放
    int sdscmp(const sds s1, const sds s2);     //比较两个sds的大小,相等返回0
    sds *sdssplitlen(const char *s, int len, const char *sep, int seplen, int *count);  //使用长度为seplen的sep分隔符对长度为len的s进行分割,返回一个sds数组的地址,*count被设置为数组元素数量
    void sdsfreesplitres(sds *tokens, int count); //释放tokens中的count个sds元素
    void sdstolower(sds s);     //将sds字符串所有字符转换为小写
    void sdstoupper(sds s);     //将sds字符串所有字符转换为大写
    sds sdsfromlonglong(long long value);       //根据long long value创建一个SDS
    sds sdscatrepr(sds s, const char *p, size_t len);   //将长度为len的字符串p以带引号""的格式追加到s末尾
    sds *sdssplitargs(const char *line, int *argc); //参数拆分,主要用于 config.c 中对配置文件进行分析。
    sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);    //将s中所有在 from 中的字符串,替换成 to 中的字符串
    sds sdsjoin(char argv, int argc, char *sep);  //以分隔符连接字符串子数组构成新的字符串

    /* Low level functions exposed to the user API */
    sds sdsMakeRoomFor(sds s, size_t addlen);   //对 sds 中 buf 的长度进行扩展
    void sdsIncrLen(sds s, int incr);       //根据incr的正负,移动字符串末尾的'\0'标志
    sds sdsRemoveFreeSpace(sds s);      //回收sds中的未使用空间
    size_t sdsAllocSize(sds s);      //获得sds所有分配的空间

    #endif
  • sds.c文件注释
    /* Create a new sds string with the content specified by the 'init' pointer
     * and 'initlen'.
     * If NULL is used for 'init' the string is initialized with zero bytes.
     *
     * The string is always null-termined (all the sds strings are, always) so
     * even if you create an sds string with:
     *
     * 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) { //创建一个长度为initlen的字符串,并保存init字符串中的值
        struct sdshdr *sh;

        if (init) {
            sh = zmalloc(sizeof(struct sdshdr)+initlen+1);  //申请空间:表头+initlen长度+'\0'
        } else {
            sh = zcalloc(sizeof(struct sdshdr)+initlen+1);  //如果init为空,则将申请的空间初始化为0
        }
        if (sh == NULL) return NULL;
        sh->len = initlen;      //设置表头的len成员
        sh->free = 0;           //设置free,新的sds不预留任何空间
        if (initlen && init)
            memcpy(sh->buf, init, initlen); //将指定的字符串init拷贝到表头的buf中
        sh->buf[initlen] = '\0';    //以'\0'结尾
        return (char*)sh->buf;
    }

    /* Create an empty (zero length) sds string. Even in this case the string
     * always has an implicit null term. */
    sds sdsempty(void) {        //建立一个只有表头,字符串为空"\0"的sds
        return sdsnewlen("",0);
    }

    /* Create a new sds string starting from a null terminated C string. */
    sds sdsnew(const char *init) {  //根据字符串init,创建一个与init一样长度字符串的sds(表头+buf)
        size_t initlen = (init == NULL) ? 0 : strlen(init);
        return sdsnewlen(init, initlen);
    }

    /* Duplicate an sds string. */
    sds sdsdup(const sds s) {       //拷贝一份s的副本
        return sdsnewlen(s, sdslen(s));
    }

    /* Free an sds string. No operation is performed if 's' is NULL. */
    void sdsfree(sds s) {       //释放s字符串和表头
        if (s == NULL) return;
        zfree(s-sizeof(struct sdshdr));
    }

    /* Set the sds string length to the length as obtained with strlen(), so
     * considering as content only up to the first null term character.
     *
     * This function is useful when the sds string is hacked manually in some
     * way, like in the following example:
     *
     * s = sdsnew("foobar");
     * s[2] = '\0';
     * sdsupdatelen(s);
     * printf("%d\n", sdslen(s));
     *
     * The output will be "2", but if we comment out the call to sdsupdatelen()
     * the output will be "6" as the string was modified but the logical length
     * remains 6 bytes. */
    void sdsupdatelen(sds s) {      //更新字符串s的长度
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        int reallen = strlen(s);
        sh->free += (sh->len-reallen);
        sh->len = reallen;
    }

    /* Modify an sds string in-place to make it empty (zero length).
     * However all the existing buffer is not discarded but set as free space
     * so that next append operations will not require allocations up to the
     * number of bytes previously available. */
    void sdsclear(sds s) {  //将字符串重置保存空间,懒惰释放
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        sh->free += sh->len;    //表头free成员+已使用空间的长度len = 新的free
        sh->len = 0;            //已使用空间变为0
        sh->buf[0] = '\0';         //字符串置空
    }

    /* 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) {      //对 sds 中 buf 的长度进行扩展
        struct sdshdr *sh, *newsh;
        size_t free = sdsavail(s);  //获得s的未使用空间长度
        size_t len, newlen;

        if (free >= addlen) return s;   //free的长度够用不用扩展直接返回

        //free长度不够用,需要扩展
        len = sdslen(s);    //获得s字符串的长度
        sh = (void*) (s-(sizeof(struct sdshdr)));
        newlen = (len+addlen);  //扩展后的新长度
        if (newlen < SDS_MAX_PREALLOC)  //新长度小于“最大预分配长度”,就直接将扩展的新长度乘2
            newlen *= 2;
        else
            newlen += SDS_MAX_PREALLOC; //新长度大于“最大预分配长度”,就在加上一个“最大预分配长度”
        newsh = zrealloc(sh, sizeof(struct sdshdr)+newlen+1);   //获得新的扩展空间的地址
        if (newsh == NULL) return NULL;

        newsh->free = newlen - len; //更新新空间的未使用的空间free
        return newsh->buf;
    }

    /* Reallocate the sds string so that it has no free space at the end. The
     * contained string remains not altered, but next concatenation operations
     * will require a reallocation.
     *
     * After the call, the passed sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call. */
    sds sdsRemoveFreeSpace(sds s) { //回收sds中的未使用空间
        struct sdshdr *sh;

        sh = (void*) (s-(sizeof(struct sdshdr)));   //获得s表头的地址
        sh = zrealloc(sh, sizeof(struct sdshdr)+sh->len+1); //只分配表头len成员大小的空间
        sh->free = 0;       //更新free成员
        return sh->buf;
    }

    /* Return the total size of the allocation of the specifed sds string,
     * including:
     * 1) The sds header before the pointer.
     * 2) The string.
     * 3) The free buffer at the end if any.
     * 4) The implicit null term.
     */
    size_t sdsAllocSize(sds s) {    //获得sds所分配的空间
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));

        return sizeof(*sh)+sh->len+sh->free+1;  //表头空间+len+free+'\0'
    }

    /* Increment the sds length and decrements the left free space at the
     * end of the string according to 'incr'. Also set the null term
     * in the new end of the string.
     *
     * This function is used in order to fix the string length after the
     * user calls sdsMakeRoomFor(), writes something after the end of
     * the current string, and finally needs to set the new length.
     *
     * Note: it is possible to use a negative increment in order to
     * right-trim the string.
     *
     * Usage example:
     *
     * Using sdsIncrLen() and sdsMakeRoomFor() it is possible to mount the
     * following schema, to cat bytes coming from the kernel to the end of an
     * sds string without copying into an intermediate buffer:
     *
     * oldlen = sdslen(s);
     * s = sdsMakeRoomFor(s, BUFFER_SIZE);
     * nread = read(fd, s+oldlen, BUFFER_SIZE);
     * ... check for nread <= 0 and handle it ...
     * sdsIncrLen(s, nread);
     */
    void sdsIncrLen(sds s, int incr) {  //根据incr的正负,移动字符串末尾的'\0'标志
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));

        if (incr >= 0)
            assert(sh->free >= (unsigned int)incr); //保证free的空间大于等于要扩展的空间,否则直接终止程序
        else
            assert(sh->len >= (unsigned int)(-incr));   //保证len的空间长度大于incr绝对值的长度
        sh->len += incr;    //更新len、free和'\0'的位置
        sh->free -= incr;
        s[sh->len] = '\0';
    }

    /* Grow the sds to have the specified length. Bytes that were not part of
     * the original length of the sds will be set to zero.
     *
     * if the specified length is smaller than the current length, no operation
     * is performed. */
    sds sdsgrowzero(sds s, size_t len) {    //将sds扩展制定长度并赋值为0
        struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));
        size_t totlen, curlen = sh->len;

        if (len <= curlen) return s;
        s = sdsMakeRoomFor(s,len-curlen);   //扩展字符串sds
        if (s == NULL) return NULL;

        /* Make sure added region doesn't contain garbage */
        sh = (void*)(s-(sizeof(struct sdshdr)));    //获得表头地址
        memset(s+curlen,0,(len-curlen+1)); /* also set trailing \0 byte */  //用0来填充扩展的空间

        //更新表头
        totlen = sh->len+sh->free;  //总长度
        sh->len = len;  //使用空间的长度
        sh->free = totlen-sh->len;  //总长度-使用空间的长度 = 为使用空间的长度
        return s;
    }

    /* Append the specified binary-safe string pointed by 't' of 'len' bytes to the
     * end of the specified sds string 's'.
     *
     * After the call, the passed sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call. */
    sds sdscatlen(sds s, const void *t, size_t len) {   //将字符串t追加到s表头的buf末尾,追加len个字节
        struct sdshdr *sh;
        size_t curlen = sdslen(s);  //原有的长度

        s = sdsMakeRoomFor(s,len);  //扩展空间
        if (s == NULL) return NULL;
        sh = (void*) (s-(sizeof(struct sdshdr)));
        memcpy(s+curlen, t, len);   //字符串拼接

        //更新属性
        sh->len = curlen+len;
        sh->free = sh->free-len;
        s[curlen+len] = '\0';
        return s;
    }

    /* Append the specified null termianted C string to the sds string 's'.
     *
     * After the call, the passed sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call. */
    sds sdscat(sds s, const char *t) {  //将t字符串拼接到s的末尾
        return sdscatlen(s, t, strlen(t));
    }

    /* Append the specified sds 't' to the existing sds 's'.
     *
     * After the call, the modified sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call. */
    sds sdscatsds(sds s, const sds t) {     //将sds追加到s末尾
        return sdscatlen(s, t, sdslen(t));
    }

    /* Destructively modify the sds string 's' to hold the specified binary
     * safe string pointed by 't' of length 'len' bytes. */
    sds sdscpylen(sds s, const char *t, size_t len) {   //将字符串t覆盖到s表头的buf中,拷贝len个字节
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        size_t totlen = sh->free+sh->len;   //获得总长度

        if (totlen < len) {         //总长度小于len
            s = sdsMakeRoomFor(s,len-sh->len);  //扩展l空间
            if (s == NULL) return NULL;
            sh = (void*) (s-(sizeof(struct sdshdr)));
            totlen = sh->free+sh->len;  //更新总长度
        }
        memcpy(s, t, len);  //拷贝字符串t覆盖s原有的字符串

        //更新表头
        s[len] = '\0';
        sh->len = len;
        sh->free = totlen-len;
        return s;
    }

    /* Like sdscpylen() but 't' must be a null-termined string so that the length
     * of the string is obtained with strlen(). */
    sds sdscpy(sds s, const char *t) {      //将字符串覆盖到s表头的buf中
        return sdscpylen(s, t, strlen(t));
    }

    /* Helper for sdscatlonglong() doing the actual number -> string
     * conversion. 's' must point to a string with room for at least
     * SDS_LLSTR_SIZE bytes.
     *
     * The function returns the length of the null-terminated string
     * representation stored at 's'. */
    #define SDS_LLSTR_SIZE 21
    int sdsll2str(char *s, long long value) {   //将一个long long类型的value转化为字符串,返回字符串长度
        char *p, aux;
        unsigned long long v;
        size_t l;

        /* Generate the string representation, this method produces
         * an reversed string. */
        v = (value < 0) ? -value : value;
        p = s;
        do {
            *p++ = '0'+(v%10);
            v /= 10;
        } while(v);
        if (value < 0) *p++ = '-';

        /* Compute length and add null term. */
        l = p-s;
        *p = '\0';

        /* Reverse the string. */
        p--;
        while(s < p) {
            aux = *s;
            *s = *p;
            *p = aux;
            s++;
            p--;
        }
        return l;
    }

    /* Identical sdsll2str(), but for unsigned long long type. */
    int sdsull2str(char *s, unsigned long long v) {//将一个unsigned long long类型的value转化为字符串,返回字符串长度
        char *p, aux;
        size_t l;

        /* Generate the string representation, this method produces
         * an reversed string. */
        p = s;
        do {
            *p++ = '0'+(v%10);
            v /= 10;
        } while(v);

        /* Compute length and add null term. */
        l = p-s;
        *p = '\0';

        /* Reverse the string. */
        p--;
        while(s < p) {
            aux = *s;
            *s = *p;
            *p = aux;
            s++;
            p--;
        }
        return l;
    }

    /* Create an sds string from a long long value. It is much faster than:
     *
     * sdscatprintf(sdsempty(),"%lld\n", value);
     */
    sds sdsfromlonglong(long long value) {  //根据long long value创建一个SDS
        char buf[SDS_LLSTR_SIZE];
        int len = sdsll2str(buf,value); //返回装换成字符串的长度

        return sdsnewlen(buf,len);      //创建SDS
    }

    /* Like sdscatprintf() but gets va_list instead of being variadic. */
    sds sdscatvprintf(sds s, const char *fmt, va_list ap) {     //打印函数,被 sdscatprintf 所调用
        va_list cpy;
        char staticbuf[1024], *buf = staticbuf, *t;
        size_t buflen = strlen(fmt)*2;

        /* We try to start using a static buffer for speed.
         * If not possible we revert to heap allocation. */
        if (buflen > sizeof(staticbuf)) {
            buf = zmalloc(buflen);
            if (buf == NULL) return NULL;
        } else {
            buflen = sizeof(staticbuf);
        }

        /* Try with buffers two times bigger every time we fail to
         * fit the string in the current buffer size. */
        while(1) {
            buf[buflen-2] = '\0';
            va_copy(cpy,ap);
            vsnprintf(buf, buflen, fmt, cpy);
            va_end(cpy);
            if (buf[buflen-2] != '\0') {
                if (buf != staticbuf) zfree(buf);
                buflen *= 2;
                buf = zmalloc(buflen);
                if (buf == NULL) return NULL;
                continue;
            }
            break;
        }

        /* Finally concat the obtained string to the SDS string and return it. */
        t = sdscat(s, buf);
        if (buf != staticbuf) zfree(buf);
        return t;
    }

    /* Append to the sds string 's' a string obtained using printf-alike format
     * specifier.
     *
     * After the call, the modified sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call.
     *
     * Example:
     *
     * s = sdsnew("Sum is: ");
     * s = sdscatprintf(s,"%d+%d = %d",a,b,a+b).
     *
     * Often you need to create a string from scratch with the printf-alike
     * format. When this is the need, just use sdsempty() as the target string:
     *
     * s = sdscatprintf(sdsempty(), "... your format ...", args);
     */
    sds sdscatprintf(sds s, const char *fmt, ...) {     //打印任意数量个字符串,并将这些字符串追加到给定 sds 的末尾
        va_list ap;
        char *t;
        va_start(ap, fmt);
        t = sdscatvprintf(s,fmt,ap);
        va_end(ap);
        return t;
    }

    /* This function is similar to sdscatprintf, but much faster as it does
     * not rely on sprintf() family functions implemented by the libc that
     * are often very slow. Moreover directly handling the sds string as
     * new data is concatenated provides a performance improvement.
     *
     * However this function only handles an incompatible subset of printf-alike
     * format specifiers:
     *
     * %s - C String
     * %S - SDS string
     * %i - signed int
     * %I - 64 bit signed integer (long long, int64_t)
     * %u - unsigned int
     * %U - 64 bit unsigned integer (unsigned long long, uint64_t)
     * %% - Verbatim "%" character.
     */
    sds sdscatfmt(sds s, char const *fmt, ...) {    //格式化打印多个字符串,并将这些字符串追加到给定 sds 的末尾
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        size_t initlen = sdslen(s);
        const char *f = fmt;
        int i;
        va_list ap;

        va_start(ap,fmt);
        f = fmt;    /* Next format specifier byte to process. */
        i = initlen; /* Position of the next byte to write to dest str. */
        while(*f) {
            char next, *str;
            unsigned int l;
            long long num;
            unsigned long long unum;

            /* Make sure there is always space for at least 1 char. */
            if (sh->free == 0) {
                s = sdsMakeRoomFor(s,1);
                sh = (void*) (s-(sizeof(struct sdshdr)));
            }

            switch(*f) {
            case '%':
                next = *(f+1);
                f++;
                switch(next) {
                case 's':
                case 'S':
                    str = va_arg(ap,char*);
                    l = (next == 's') ? strlen(str) : sdslen(str);
                    if (sh->free < l) {
                        s = sdsMakeRoomFor(s,l);
                        sh = (void*) (s-(sizeof(struct sdshdr)));
                    }
                    memcpy(s+i,str,l);
                    sh->len += l;
                    sh->free -= l;
                    i += l;
                    break;
                case 'i':
                case 'I':
                    if (next == 'i')
                        num = va_arg(ap,int);
                    else
                        num = va_arg(ap,long long);
                    {
                        char buf[SDS_LLSTR_SIZE];
                        l = sdsll2str(buf,num);
                        if (sh->free < l) {
                            s = sdsMakeRoomFor(s,l);
                            sh = (void*) (s-(sizeof(struct sdshdr)));
                        }
                        memcpy(s+i,buf,l);
                        sh->len += l;
                        sh->free -= l;
                        i += l;
                    }
                    break;
                case 'u':
                case 'U':
                    if (next == 'u')
                        unum = va_arg(ap,unsigned int);
                    else
                        unum = va_arg(ap,unsigned long long);
                    {
                        char buf[SDS_LLSTR_SIZE];
                        l = sdsull2str(buf,unum);
                        if (sh->free < l) {
                            s = sdsMakeRoomFor(s,l);
                            sh = (void*) (s-(sizeof(struct sdshdr)));
                        }
                        memcpy(s+i,buf,l);
                        sh->len += l;
                        sh->free -= l;
                        i += l;
                    }
                    break;
                default: /* Handle %% and generally %<unknown>. */
                    s[i++] = next;
                    sh->len += 1;
                    sh->free -= 1;
                    break;
                }
                break;
            default:
                s[i++] = *f;
                sh->len += 1;
                sh->free -= 1;
                break;
            }
            f++;
        }
        va_end(ap);

        /* Add null-term */
        s[i] = '\0';
        return s;
    }

    /* Remove the part of the string from left and from right composed just of
     * contiguous characters found in 'cset', that is a null terminted C string.
     *
     * After the call, the modified sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call.
     *
     * Example:
     *
     * s = sdsnew("AA...AA.a.aa.aHelloWorld     :::");
     * s = sdstrim(s,"Aa. :");
     * printf("%s\n", s);
     *
     * Output will be just "Hello World".
     */
    sds sdstrim(sds s, const char *cset) {  //去除sds中包含有 cset字符串出现字符 的字符
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        char *start, *end, *sp, *ep;
        size_t len;

        //设置和备份指针位置
        sp = start = s;
        ep = end = s+sdslen(s)-1;

        //strchr()函数功能:查找cset中首次出现*sp字符的位置,成功返回第一次出现的位置
        while(sp <= end && strchr(cset, *sp)) sp++;         //从左开始修剪,sp为目标串的左边界
        while(ep > start && strchr(cset, *ep)) ep--;        //从右开始修剪,ep为目标串的右边界

        len = (sp > ep) ? 0 : ((ep-sp)+1);  //目标串的长度
        if (sh->buf != sp) memmove(sh->buf, sp, len);   //将字符串的位置前移到buf开头

        //更新表头
        sh->buf[len] = '\0';
        sh->free = sh->free+(sh->len-len);
        sh->len = len;
        return s;
    }

    /* Turn the string into a smaller (or equal) string containing only the
     * substring specified by the 'start' and 'end' indexes.
     *
     * start and end can be negative, where -1 means the last character of the
     * string, -2 the penultimate character, and so forth.
     *
     * The interval is inclusive, so the start and end characters will be part
     * of the resulting string.
     *
     * The string is modified in-place.
     *
     * Example:
     *
     * s = sdsnew("Hello World");
     * sdsrange(s,1,-1); => "ello World"
     */
    void sdsrange(sds s, int start, int end) {  //根据start和end区间截取字符串
        struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
        size_t newlen, len = sdslen(s);     //获得源串的长度

        //1,start或end为负数的情况,转化为非负数
        if (len == 0) return;
        if (start < 0) {    //如果start小于0,则从字符串尾部往前开始算起始位置
            start = len+start;
            if (start < 0) start = 0;
        }
        if (end < 0) {      //如果start小于0,则从字符串尾部往前开始算结束位置
            end = len+end;
            if (end < 0) end = 0;
        }

        //2,start和end为非负数的情况
        newlen = (start > end) ? 0 : (end-start)+1;     //截取后字符串的长度
        if (newlen != 0) {
            if (start >= (signed)len) {         //起始位置大于字符串的长度,被截取的字符串长度为0
                newlen = 0;
            } else if (end >= (signed)len) {    //结束位置大于字符串的长度,只截取到字符串的末尾
                end = len-1;
                newlen = (start > end) ? 0 : (end-start)+1;     //更新截取后字符串的长度
            }
        } else {
            start = 0;
        }

        //通过以上两种情况计算出start和end位置,用memmove函数截取
        if (start && newlen) memmove(sh->buf, sh->buf+start, newlen);   //截取后,移动到buf的前面

        //更新表头
        sh->buf[newlen] = 0;
        sh->free = sh->free+(sh->len-newlen);
        sh->len = newlen;
    }

    /* Apply tolower() to every character of the sds string 's'. */
    void sdstolower(sds s) {        //将sds字符串所有字符转换为小写
        int len = sdslen(s), j;

        for (j = 0; j < len; j++) s[j] = tolower(s[j]); //tolower()函数将大写字符转换成小写
    }

    /* Apply toupper() to every character of the sds string 's'. */
    void sdstoupper(sds s) {        //将sds字符串所有字符转换为大写
        int len = sdslen(s), j;

        for (j = 0; j < len; j++) s[j] = toupper(s[j]); //toupper()函数将小字符转换成大写
    }

    /* Compare two sds strings s1 and s2 with memcmp().
     *
     * Return value:
     *
     *     positive if s1 > s2.
     *     negative if s1 < s2.
     *     0 if s1 and s2 are exactly the same binary string.
     *
     * If two strings share exactly the same prefix, but one of the two has
     * additional characters, the longer string is considered to be greater than
     * the smaller one. */
    int sdscmp(const sds s1, const sds s2) {    //比较两个sds的大小,相等返回0
        size_t l1, l2, minlen;
        int cmp;

        l1 = sdslen(s1);
        l2 = sdslen(s2);
        minlen = (l1 < l2) ? l1 : l2;   //
        cmp = memcmp(s1,s2,minlen);     //memcmp()函数比较s1和s2内存地址的前len个字节,返回s1减s2的差值
        if (cmp == 0) return l1-l2;     //s1长度大于s2返回正差值,否则返回负差值
        return cmp;
    }

    /* Split 's' with separator in 'sep'. An array
     * of sds strings is returned. *count will be set
     * by reference to the number of tokens returned.
     *
     * On out of memory, zero length string, zero length
     * separator, NULL is returned.
     *
     * Note that 'sep' is able to split a string using
     * a multi-character separator. For example
     * sdssplit("foo_-_bar","_-_"); will return two
     * elements "foo" and "bar".
     *
     * This version of the function is binary-safe but
     * requires length arguments. sdssplit() is just the
     * same function but for zero-terminated strings.
     */
    sds *sdssplitlen(const char *s, int len, const char *sep, int seplen, int *count) { //使用长度为seplen的sep分隔符对长度为len的s进行分割,返回一个sds数组的地址,*count被设置为数组元素数量
        int elements = 0, slots = 5, start = 0, j;
        sds *tokens;

        if (seplen < 1 || len < 0) return NULL;

        tokens = zmalloc(sizeof(sds)*slots);    //默认sds长度为5个
        if (tokens == NULL) return NULL;

        if (len == 0) {     //len为0,count设为0,不分割
            *count = 0;
            return tokens;
        }
        for (j = 0; j < (len-(seplen-1)); j++) {    //sep和s对比(len-(seplen-1))次,
            /* make sure there is room for the next element and the final one */
            //如果当前字符串数组数量少于当前已存在数组+2个的时候,动态添加
            if (slots < elements+2) {
                sds *newtokens;

                slots *= 2;
                newtokens = zrealloc(tokens,sizeof(sds)*slots);
                //扩展内存失败,goto语句释放内存
                if (newtokens == NULL) goto cleanup;
                tokens = newtokens;
            }
            /* search the separator */
            //分成单字符比较或字符串比较匹配
            if ((seplen == 1 && *(s+j) == sep[0]) || (memcmp(s+j,sep,seplen) == 0)) {
                tokens[elements] = sdsnewlen(s+start,j-start);  //将s分割的一段生成为SDS然后放到预先的tokens数组里
                if (tokens[elements] == NULL) goto cleanup; //创建SDS失败要释放tokens数组
                elements++;
                start = j+seplen;
                j = j+seplen-1; /* skip the separator */
            }
        }
        /* Add the final element. We are sure there is room in the tokens array. */
        //最后一个字符串添加
        tokens[elements] = sdsnewlen(s+start,len-start);
        if (tokens[elements] == NULL) goto cleanup; //创建SDS失败要释放tokens数组
        elements++;
        *count = elements;
        return tokens;

    cleanup:    //释放内存
        {
            int i;
            for (i = 0; i < elements; i++) sdsfree(tokens[i]);
            zfree(tokens);
            *count = 0;
            return NULL;
        }
    }

    /* Free the result returned by sdssplitlen(), or do nothing if 'tokens' is NULL. */
    void sdsfreesplitres(sds *tokens, int count) {      //释放tokens中的count个sds元素
        if (!tokens) return;
        while(count--)
            sdsfree(tokens[count]);
        zfree(tokens);
    }

    /* Append to the sds string "s" an escaped string representation where
     * all the non-printable characters (tested with isprint()) are turned into
     * escapes in the form "\n\r\a...." or "\x<hex-number>".
     *
     * After the call, the modified sds string is no longer valid and all the
     * references must be substituted with the new pointer returned by the call. */
    sds sdscatrepr(sds s, const char *p, size_t len) {  //将长度为len的字符串p以带引号""的格式追加到s末尾
        s = sdscatlen(s,"\"",1);    //追加左引号,\是转义字符
        while(len--) {
            switch(*p) {
            case '\\':
            case '"':
                s = sdscatprintf(s,"\\%c",*p);
                break;
            case '\n': s = sdscatlen(s,"\\n",2); break; // "\\n"是两个字符"\n"
            case '\r': s = sdscatlen(s,"\\r",2); break;
            case '\t': s = sdscatlen(s,"\\t",2); break;
            case '\a': s = sdscatlen(s,"\\a",2); break;
            case '\b': s = sdscatlen(s,"\\b",2); break;
            default:
                if (isprint(*p))
                    s = sdscatprintf(s,"%c",*p);
                else
                    s = sdscatprintf(s,"\\x%02x",(unsigned char)*p);
                break;
            }
            p++;
        }
        return sdscatlen(s,"\"",1); //追加右引号
    }

    /* Helper function for sdssplitargs() that returns non zero if 'c'
     * is a valid hex digit. */
    int is_hex_digit(char c) {      //如果c是十六进制符号中的一个返回正数
        return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') ||
               (c >= 'A' && c <= 'F');
    }

    /* Helper function for sdssplitargs() that converts a hex digit into an
     * integer from 0 to 15 */
    int hex_digit_to_int(char c) {      //16进制字符转换为10进制整数
        switch(c) {
        case '0': return 0;
        case '1': return 1;
        case '2': return 2;
        case '3': return 3;
        case '4': return 4;
        case '5': return 5;
        case '6': return 6;
        case '7': return 7;
        case '8': return 8;
        case '9': return 9;
        case 'a': case 'A': return 10;
        case 'b': case 'B': return 11;
        case 'c': case 'C': return 12;
        case 'd': case 'D': return 13;
        case 'e': case 'E': return 14;
        case 'f': case 'F': return 15;
        default: return 0;
        }
    }

    /* Split a line into arguments, where every argument can be in the
     * following programming-language REPL-alike form:
     *
     * foo bar "newline are supported\n" and "\xff\x00otherstuff"
     *
     * The number of arguments is stored into *argc, and an array
     * of sds is returned.
     *
     * The caller should free the resulting array of sds strings with
     * sdsfreesplitres().
     *
     * Note that sdscatrepr() is able to convert back a string into
     * a quoted string in the same format sdssplitargs() is able to parse.
     *
     * The function returns the allocated tokens on success, even when the
     * input string is empty, or NULL if the input contains unbalanced
     * quotes or closed quotes followed by non space characters
     * as in: "foo"bar or "foo'
     * 这个函数主要用于 config.c 中对配置文件进行分析。
     * 例子:
     *  sds *arr = sdssplitargs("timeout 10086\r\nport 123321\r\n");
     * 会得出
     *  arr[0] = "timeout"
     *  arr[1] = "10086"
     *  arr[2] = "port"
     *  arr[3] = "123321"
     */
    sds *sdssplitargs(const char *line, int *argc) {    //参数拆分,主要用于 config.c 中对配置文件进行分析。
        const char *p = line;
        char *current = NULL;
        char vector = NULL;

        *argc = 0;
        while(1) {
            /* skip blanks */
            while(*p && isspace(*p)) p++;
            if (*p) {
                /* get a token */
                int inq=0;  /* set to 1 if we are in "quotes" */
                int insq=0; /* set to 1 if we are in 'single quotes' */
                int done=0;

                if (current == NULL) current = sdsempty();
                while(!done) {
                    if (inq) {
                        if (*p == '\\' && *(p+1) == 'x' &&
                                                 is_hex_digit(*(p+2)) &&
                                                 is_hex_digit(*(p+3)))
                        {
                            unsigned char byte;

                            byte = (hex_digit_to_int(*(p+2))*16)+
                                    hex_digit_to_int(*(p+3));
                            current = sdscatlen(current,(char*)&byte,1);
                            p += 3;
                        } else if (*p == '\\' && *(p+1)) {
                            char c;

                            p++;
                            switch(*p) {
                            case 'n': c = '\n'; break;
                            case 'r': c = '\r'; break;
                            case 't': c = '\t'; break;
                            case 'b': c = '\b'; break;
                            case 'a': c = '\a'; break;
                            default: c = *p; break;
                            }
                            current = sdscatlen(current,&c,1);
                        } else if (*p == '"') {
                            /* closing quote must be followed by a space or
                             * nothing at all. */
                            if (*(p+1) && !isspace(*(p+1))) goto err;
                            done=1;
                        } else if (!*p) {
                            /* unterminated quotes */
                            goto err;
                        } else {
                            current = sdscatlen(current,p,1);
                        }
                    } else if (insq) {
                        if (*p == '\\' && *(p+1) == '\'') {
                            p++;
                            current = sdscatlen(current,"'",1);
                        } else if (*p == '\'') {
                            /* closing quote must be followed by a space or
                             * nothing at all. */
                            if (*(p+1) && !isspace(*(p+1))) goto err;
                            done=1;
                        } else if (!*p) {
                            /* unterminated quotes */
                            goto err;
                        } else {
                            current = sdscatlen(current,p,1);
                        }
                    } else {
                        switch(*p) {
                        case ' ':
                        case '\n':
                        case '\r':
                        case '\t':
                        case '\0':
                            done=1;
                            break;
                        case '"':
                            inq=1;
                            break;
                        case '\'':
                            insq=1;
                            break;
                        default:
                            current = sdscatlen(current,p,1);
                            break;
                        }
                    }
                    if (*p) p++;
                }
                /* add the token to the vector */
                vector = zrealloc(vector,((*argc)+1)*sizeof(char*));
                vector[*argc] = current;
                (*argc)++;
                current = NULL;
            } else {
                /* Even on empty input string return something not NULL. */
                if (vector == NULL) vector = zmalloc(sizeof(void*));
                return vector;
            }
        }

    err:
        while((*argc)--)
            sdsfree(vector[*argc]);
        zfree(vector);
        if (current) sdsfree(current);
        *argc = 0;
        return NULL;
    }

    /* Modify the string substituting all the occurrences of the set of
     * characters specified in the 'from' string to the corresponding character
     * in the 'to' array.
     *
     * For instance: sdsmapchars(mystring, "ho", "01", 2)
     * will have the effect of turning the string "hello" into "0ell1".
     *
     * The function returns the sds string pointer, that is always the same
     * as the input pointer since no resize is needed. */
    sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen) {   //将s中所有在 from 中的字符串,替换成 to 中的字符串
        size_t j, i, l = sdslen(s);

        for (j = 0; j < l; j++) {   //遍历s
            for (i = 0; i < setlen; i++) {  //遍历映射
                if (s[j] == from[i]) {  //替换字符串
                    s[j] = to[i];
                    break;
                }
            }
        }
        return s;
    }

    /* Join an array of C strings using the specified separator (also a C string).
     * Returns the result as an sds string. */
    sds sdsjoin(char argv, int argc, char *sep) { //以分隔符连接字符串子数组构成新的字符串
        sds join = sdsempty();  //创建一个空sds
        int j;

        for (j = 0; j < argc; j++) {
            join = sdscat(join, argv[j]);
            if (j != argc-1) join = sdscat(join,sep);   //以*sep链接
        }
        return join;
    }

参考书籍:

  • 《Redis设计与实现》——黄健翔
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