redis-sds复习

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redis - sds

之前本来跟着付磊团队在redis组做开发，要做技术分享，所以写了这个文章，后来发生组织架构变动去做自研缓存kcache了。但这篇sds实现还是可以沉淀一下。

总体代码逻辑清晰简单，过一遍重要结构

sds内存结构

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typedef char *sds;
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[];
};

sds内存结构

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// init为c类型字符串，initlen为其长度
sds sdsnewlen(const void *init, size_t initlen) {
    void *sh;
    sds s;
    char type = sdsReqType(initlen);
    // 算出其头部偏移量
    int hdrlen = sdsHdrSize(type);
    unsigned char *fp; /* flags pointer. */

    sh = s_malloc(hdrlen+initlen+1);
    if (sh == NULL) return NULL;
    if (init==SDS_NOINIT)
        init = NULL;
    else if (!init)
        memset(sh, 0, hdrlen+initlen+1);
    // 返回的指针为指向真正字符串的位置
    s = (char*)sh+hdrlen;
    fp = ((unsigned char*)s)-1;
    switch(type) {
        ...
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        ...
    }
    ...
    // 按c类型字符串的方法存
    s[initlen] = '\0';
    return s;
}

object - sds内存结构

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// object.c
typedef struct redisObject {
    unsigned type:4;
    unsigned encoding:4;
    unsigned lru:LRU_BITS; /* LRU time (relative to global lru_clock) or
                            * LFU data (least significant 8 bits frequency
                            * and most significant 16 bits access time). */
    int refcount;
    void *ptr;
} robj;
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// object.c
#define OBJ_ENCODING_EMBSTR_SIZE_LIMIT 44
robj *createStringObject(const char *ptr, size_t len) {
    if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT)
        return createEmbeddedStringObject(ptr,len);
    else
        return createRawStringObject(ptr,len);
}

object-sds