Redis设计与实现读书笔记（一） SDS

typedef char *sds;

struct sdshdr {
    unsigned int len;    //定义当前字符串长度（不包含‘\0‘）
    unsigned intfree;   //定义当前对象可容纳的空间数
    char buf[];          
};

            /*
             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 * After the call, the passed sds string is no longer valid and all the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) {
    return sdscatlen(s, t, strlen(t));
}

            /*
             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 * After the call, the passed sds string is no longer valid and all the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);    //当前长度

    s = sdsMakeRoomFor(s,len);    //进行空间分配，如果空间足够是不需要进行分配的
    if (s == NULL) return NULL;   //分配失败
    sh = (void*) (s-(sizeof(struct sdshdr))); //s的结构体指针首部
    memcpy(s+curlen, t, len);                 //直接进行了字节拷贝，因为已经将需要的内存分配好了
    sh->len = curlen+len;                     //计算使用长度，长度就是当前字符串长度加上函数参数len
    sh->free = sh->free-len;                  //分配完空间后会有一个free，这时用掉了len个空间所以要减去
    s[curlen+len] = ‘\0‘;                     //s的末尾添加‘\0’
    return s;
}

            /*
             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;    //空间充足
    len = sdslen(s);
    sh = (void*) (s-(sizeof(struct sdshdr)));  //新的使用空间
    newlen = (len+addlen);
    if (newlen < SDS_MAX_PREALLOC)  
        newlen *= 2;                          //分配两倍空间（1份多余空间）
    else
        newlen += SDS_MAX_PREALLOC;         
    newsh = zrealloc(sh, sizeof(struct sdshdr)+newlen+1);  //分配空间
    if (newsh == NULL) return NULL;

    newsh->free = newlen - len;                           //只修改free属性，len需要自行修改
    return newsh->buf;
}