redis

数据结构，源码：

// 最大预分配长度#define SDS_MAX_PREALLOC (1024*1024)#include <sys/types.h>#include <stdarg.h>// Redis没有直接使用sdshdr结构，而是定义了sds类型来操作sdshdr结构typedef char *sds;// redis 自定义字符串结构struct sdshdr {// 当前字符串长度    unsigned int len;// 剩余可用长度    unsigned int free;// 字符数组，实际存放地方    char buf[];};static inline size_t sdslen(const sds s) {    // 其中sds指向sdshdr结构的buf[]，通过如下操作来获得整个sdshdr结构：//  sizeof(struct sdshdr)的值为8/*  这里为什么用s-(sizeof(struct sdshdr))就得到sdshdr *指针？    从后面我们可以看到sds指向sdshdr结构的buf[]字符数组，所以    s-(sizeof(struct sdshdr))就是sdshdr结构的地址。*/    struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));    return sh->len;}static inline size_t sdsavail(const sds s) {    struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));    return sh->free;}

SDS和C字符串的区别

（1）sds 的sdslen的时间复杂度是O(1)，长度保存在len字段中；C语言的strlen函数的时间复杂度是O(N)，需要遍历完整字符串，直到最后的空字符串 ‘\0’。

（2）C字符串不记录自身长度，容易导致缓冲区溢出（buffer overflow）杜绝缓冲区溢出。SDS需要增加长度时，先检查buf[]数字空间是否足够，如果不够的话自动将SDS的buf空间扩展至足够大，然后再执行修改操作。

（3）减少内存重新分配时带来的内存重新分配次数。使用free字段标记还可以使用的空闲空间。不必每次减少或者增加长度都重新申请内存空间。

      对于C语言字符串，比如append操作，操作前通过内存重新分配数组的大小--如果忘记了会产生缓冲区溢出。对于缩短字符串操作，比如trim程序通过重新分配内存释放不再使用的空间，否则或内存泄漏。

     SDS中，buf数组的长度不一定就是字符串数量 + 1，数组中可以包含未被使用的空间，这些字节数量使用 free标记的。

stdnewlen

/* 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. *//** 新建sds字符串，根据ini指针和initlen指定的长度。*/sds sdsnewlen(const void *init, size_t initlen) {    struct sdshdr *sh;    if (init) {        sh = zmalloc(sizeof(struct sdshdr)+initlen+1);    } else {    //当init参数为NULL，使用zcalloc的方法，使用'\0' 填充        sh = zcalloc(sizeof(struct sdshdr)+initlen+1);    }    if (sh == NULL) return NULL;    sh->len = initlen;    sh->free = 0;    if (initlen && init)// 拷贝init 源的字符串到sds的buf        memcpy(sh->buf, init, initlen);// 标记C字符串空结尾    sh->buf[initlen] = '\0';// 返回sds    return (char*)sh->buf;}

        接着看其他方法：

/* Create an empty (zero length) sds string. Even in this case the string * always has an implicit null term. */// 创建一个空sds字符串（长度是0）；同样会创建sdshdr结构，buf有一个字符 '\0'sds sdsempty(void) {    return sdsnewlen("",0);}/* Create a new sds string starting from a null terminated C string. */// 创建一个sds 字符串，从C语言字符串（以null结尾）sds sdsnew(const char *init) {    size_t initlen = (init == NULL) ? 0 : strlen(init);    return sdsnewlen(init, initlen);}/* Duplicate an sds string. */// 复制一个sdssds sdsdup(const sds s) {    return sdsnewlen(s, sdslen(s));}/* Free an sds string. No operation is performed if 's' is NULL. */// 释放sds空间void sdsfree(sds s) {    if (s == NULL) return;    zfree(s-sizeof(struct sdshdr));}

           sdsupdatelen函数作用

/* 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. */ /**   设置sds 字符串的长度为strlen方法获取的长度；所以sds字符串内容是遇到第一个null前面的部分。      当sds string被hack方式手工修改了，例如：   s = sdsnew("foobar");   s[2] = '\0';   sdsupdatelen(s);   printf("%d\n", sdslen(s));      上面的执行结果是2，   如果注释掉sdsupdatelen 这一行代码，输出结果是6，虽然字符串被修改了，但是len字段没有改    */void sdsupdatelen(sds s) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    int reallen = strlen(s);    sh->free += (sh->len-reallen);    sh->len = reallen;}

      清空sds字符串

/* 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. */ /**   在原位置修改sds字符串成空字符串（长度为0）。   但是原来已经存在的缓冲区不会释放掉，所以后续的append操作不需要分配内存空间如果前一次申请的空间足够。 */void sdsclear(sds s) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    sh->free += sh->len;    sh->len = 0;    sh->buf[0] = '\0';}

数据结构，源码：

// 最大预分配长度#define SDS_MAX_PREALLOC (1024*1024)#include <sys/types.h>#include <stdarg.h>// Redis没有直接使用sdshdr结构，而是定义了sds类型来操作sdshdr结构typedef char *sds;// redis 自定义字符串结构struct sdshdr {// 当前字符串长度    unsigned int len;// 剩余可用长度    unsigned int free;// 字符数组，实际存放地方    char buf[];};static inline size_t sdslen(const sds s) {    // 其中sds指向sdshdr结构的buf[]，通过如下操作来获得整个sdshdr结构：//  sizeof(struct sdshdr)的值为8/*  这里为什么用s-(sizeof(struct sdshdr))就得到sdshdr *指针？    从后面我们可以看到sds指向sdshdr结构的buf[]字符数组，所以    s-(sizeof(struct sdshdr))就是sdshdr结构的地址。*/    struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));    return sh->len;}static inline size_t sdsavail(const sds s) {    struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));    return sh->free;}

SDS和C字符串的区别

（1）sds 的sdslen的时间复杂度是O(1)，长度保存在len字段中；C语言的strlen函数的时间复杂度是O(N)，需要遍历完整字符串，直到最后的空字符串 ‘\0’。

（2）C字符串不记录自身长度，容易导致缓冲区溢出（buffer overflow）杜绝缓冲区溢出。SDS需要增加长度时，先检查buf[]数字空间是否足够，如果不够的话自动将SDS的buf空间扩展至足够大，然后再执行修改操作。

（3）减少内存重新分配时带来的内存重新分配次数。使用free字段标记还可以使用的空闲空间。不必每次减少或者增加长度都重新申请内存空间。

      对于C语言字符串，比如append操作，操作前通过内存重新分配数组的大小--如果忘记了会产生缓冲区溢出。对于缩短字符串操作，比如trim程序通过重新分配内存释放不再使用的空间，否则或内存泄漏。

     SDS中，buf数组的长度不一定就是字符串数量 + 1，数组中可以包含未被使用的空间，这些字节数量使用 free标记的。

stdnewlen

/* 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. *//** 新建sds字符串，根据ini指针和initlen指定的长度。*/sds sdsnewlen(const void *init, size_t initlen) {    struct sdshdr *sh;    if (init) {        sh = zmalloc(sizeof(struct sdshdr)+initlen+1);    } else {    //当init参数为NULL，使用zcalloc的方法，使用'\0' 填充        sh = zcalloc(sizeof(struct sdshdr)+initlen+1);    }    if (sh == NULL) return NULL;    sh->len = initlen;    sh->free = 0;    if (initlen && init)// 拷贝init 源的字符串到sds的buf        memcpy(sh->buf, init, initlen);// 标记C字符串空结尾    sh->buf[initlen] = '\0';// 返回sds    return (char*)sh->buf;}

        接着看其他方法：

/* Create an empty (zero length) sds string. Even in this case the string * always has an implicit null term. */// 创建一个空sds字符串（长度是0）；同样会创建sdshdr结构，buf有一个字符 '\0'sds sdsempty(void) {    return sdsnewlen("",0);}/* Create a new sds string starting from a null terminated C string. */// 创建一个sds 字符串，从C语言字符串（以null结尾）sds sdsnew(const char *init) {    size_t initlen = (init == NULL) ? 0 : strlen(init);    return sdsnewlen(init, initlen);}/* Duplicate an sds string. */// 复制一个sdssds sdsdup(const sds s) {    return sdsnewlen(s, sdslen(s));}/* Free an sds string. No operation is performed if 's' is NULL. */// 释放sds空间void sdsfree(sds s) {    if (s == NULL) return;    zfree(s-sizeof(struct sdshdr));}

           sdsupdatelen函数作用

/* 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. */ /**   设置sds 字符串的长度为strlen方法获取的长度；所以sds字符串内容是遇到第一个null前面的部分。      当sds string被hack方式手工修改了，例如：   s = sdsnew("foobar");   s[2] = '\0';   sdsupdatelen(s);   printf("%d\n", sdslen(s));      上面的执行结果是2，   如果注释掉sdsupdatelen 这一行代码，输出结果是6，虽然字符串被修改了，但是len字段没有改    */void sdsupdatelen(sds s) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    int reallen = strlen(s);    sh->free += (sh->len-reallen);    sh->len = reallen;}

      清空sds字符串

/* 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. */ /**   在原位置修改sds字符串成空字符串（长度为0）。   但是原来已经存在的缓冲区不会释放掉，所以后续的append操作不需要分配内存空间如果前一次申请的空间足够。 */void sdsclear(sds s) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    sh->free += sh->len;    sh->len = 0;    sh->buf[0] = '\0';}

       sdsMakeRoomFor

       在sds.h 文件定义SDS_MAX_PREALLOC大小1M，如果小于1M，新的空间大小是 2 * (申请空间 + len)。#define SDS_MAX_PREALLOC (1024*1024) 

/* 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. */ /**   在sds的后面增加空闲空间，增加addlen个字节，另外还有空字符'\0';   注意：这个方法不会修该sds的使用slslen方法计算的长度, 只是增加free buffer 孔家 */sds sdsMakeRoomFor(sds s, size_t addlen) {    struct sdshdr *sh, *newsh;    size_t free = sdsavail(s);    size_t len, newlen;    if (free >= addlen) return s;    len = sdslen(s);    sh = (void*) (s-(sizeof(struct sdshdr)));    newlen = (len+addlen);    if (newlen < SDS_MAX_PREALLOC)        newlen *= 2;    else        newlen += SDS_MAX_PREALLOC;    newsh = zrealloc(sh, sizeof(struct sdshdr)+newlen+1);    if (newsh == NULL) return NULL;    newsh->free = newlen - len;    return newsh->buf;}

     释放空闲空间sdsRemoveFreeSpace

     注意这个方法调用后，要是用新的sds指针了

/* 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) {    struct sdshdr *sh;    sh = (void*) (s-(sizeof(struct sdshdr)));    sh = zrealloc(sh, sizeof(struct sdshdr)+sh->len+1);    sh->free = 0;    return sh->buf;}

       sdsAllocSize

/* 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. */ /**    返回sds总长度，包含以下4部分1）指针前面的 shd header2）字符串3）空闲字节4）隐含的 null 空字符 */size_t sdsAllocSize(sds s) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    return sizeof(*sh)+sh->len+sh->free+1;}

      sdsIncrLen 增加指定长度，可以是负数

/* 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); */ /**   使用场景：在系统内核读入到buffer时候不经过中间buffer，具体的遇到可以关注 */void sdsIncrLen(sds s, int incr) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    if (incr >= 0)        assert(sh->free >= (unsigned int)incr);    else        assert(sh->len >= (unsigned int)(-incr));    sh->len += incr;    sh->free -= incr;    s[sh->len] = '\0';}

       sdsgrowzero

/* 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. *//**  增加sds长度到指定len，新增字符设置成0.  如果指定的长度小于原来长度，什么也不做*/sds sdsgrowzero(sds s, size_t len) {    struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));    size_t totlen, curlen = sh->len;    if (len <= curlen) return s;    s = sdsMakeRoomFor(s,len-curlen);    if (s == NULL) return NULL;    /* Make sure added region doesn't contain garbage */    sh = (void*)(s-(sizeof(struct sdshdr)));    memset(s+curlen,0,(len-curlen+1)); /* also set trailing \0 byte */    totlen = sh->len+sh->free;    sh->len = len;    sh->free = totlen-sh->len;    return s;}

       字符串拼接 sdscatlen

/* 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. */ /**   字符换拼接（二进制安全--不以null作为判断结束的标志） */sds sdscatlen(sds s, const void *t, size_t len) {    struct sdshdr *sh;    size_t curlen = sdslen(s);// free 足够，直接使用；free不够，重新分配内存空间    s = sdsMakeRoomFor(s,len);    if (s == NULL) return NULL;    sh = (void*) (s-(sizeof(struct sdshdr)));    memcpy(s+curlen, t, len);    sh->len = curlen+len;    sh->free = sh->free-len;    s[curlen+len] = '\0';    return s;}

     字符串拷贝sdscpylen

/* Destructively modify the sds string 's' to hold the specified binary * safe string pointed by 't' of length 'len' bytes. *//**  原来的指向s的指针已经不能用了*/ sds sdscpylen(sds s, const char *t, size_t len) {    struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));    size_t totlen = sh->free+sh->len;    if (totlen < len) {        s = sdsMakeRoomFor(s,len-sh->len);        if (s == NULL) return NULL;        sh = (void*) (s-(sizeof(struct sdshdr)));        totlen = sh->free+sh->len;    }    memcpy(s, t, len);    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(). *//**  t指针指向的目标必须是C字符串（'\0'结尾）*/ sds sdscpy(sds s, const char *t) {    return sdscpylen(s, t, strlen(t));}