Redis源码-数据结构之sds字符串

国庆节除了陪伴吕友，花了差不多两天时间初步了解了一下Redis，接下来一段时间，将深入阅读Redis源码，也做一个记录，看一个源码，我觉得还是先从基础的模块看起，比如，数据结构，Redis中的实现了很多的数据结构，当然，很多开源项目也是自己实现各种数据结构以满足定制需求，我首先阅读的是底层字符串模块-SDS

sds的定义：

typedef char *sds;

说白了就是字符串的别名，整个Redis的字符串用的就是这个，但是内部使用了一个结构来维护这个sds

struct sdshdr {
    unsigned int len;
    unsigned int free;
    char buf[];
}; 

之所以这样设计，是为了符合Redis的某一些特性，总的来说，sds相比于C语言的字符串具有以下几个优点：

1.获取字符串长度的复杂度为O(1)，因为内部维护了一个保存字符串长度的len

static inline size_t sdslen(const sds s) {
    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;
}

要想访问到len和free字段，还要把指针直到结构体开始出，当时看到这里还犹豫了一下，基础知识不行— —！

2.是二进制安全的

这里要解释一下什么是二进制安全，C语言中的字符串采用ASCII编码表示，但是这里我们用的是一个字符数组，怎么存进去的，拿出来还是什么，例如：test\0test\0，那么len就会是9，读出来也是test\0test\0，而不是说读到第一个结束符就结束了

    sds s = sdsnewlen("test", 5);
    printf("%s\n", s);
    size_t len = sdslen(s);
    printf("%zu\n", len);
    size_t free = sdsavail(s);
    printf("%zu\n", free);

可以得出得到len=5

源码：

/* 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) {
    struct sdshdr *sh; 

    if (init) {
        sh = malloc(sizeof(struct sdshdr)+initlen+1);
    } else {
        sh = calloc(1,sizeof(struct sdshdr)+initlen+1);
    }
    if (sh == NULL) return NULL;
    sh->len = initlen;
    sh->free = 0;
    if (initlen && init)
        memcpy(sh->buf, init, initlen);
    sh->buf[initlen] = '\0';
    return (char*)sh->buf;
}

3.不会出现缓冲区溢出，利用free字段可以减少修改字符串长度导致的内存重新分配

我们知道，C语言中的这两组函数如果使用不当，是会造成缓冲区溢出的

char *strcat(char *dest, const char *src);
char *strncat(char *dest, const char *src, size_t n);
The  strcat()  function  appends the src string to the dest string, overwriting the null byte ('\0') at the end of dest, and then adds a terminating null byte.  The strings may not overlap, and the dest string must have enough space for the result.
The strncat() function is similar, except that
*  it will use at most n characters from src; and
*  src does not need to be null terminated if it contains n or more characters.
As with strcat(), the resulting string in dest is always null terminated.

而sds中的cat实现如下：

/* 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) {
    struct sdshdr *sh;
    size_t curlen = sdslen(s);

    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;
}

那么sdsMakeRoomFor是如何实现的呢？

/* 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) {
    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 = realloc(sh, sizeof(struct sdshdr)+newlen+1);
    if (newsh == NULL) return NULL;

    newsh->free = newlen - len;
    return newsh->buf;
}

可以看出sds对于字符串长度修改有这样一个规则，如果

AddLen < free 使用free

AddLen > free && AddLen < 1M free = len

AddLen > free && AddLen > 1M free = 1M