redis数据结构之SDS

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redis数据结构之SDS

SDS 数据结构

关于sds数据结构的定义在sds.h里面，sds根据string的长度来选择使用不同的struct，后面会介绍。

typedef char *sds;

/* Note: sdshdr5 is never used, we just access the flags byte directly.
 * However is here to document the layout of type 5 SDS strings. */
struct __attribute__ ((__packed__)) sdshdr5 {
    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
    char buf[];
};
/**
 * water
 * __attribute__ ((__packed__)) 告诉编译器取消结构在编译过程中的优化对齐,按照实际占用字节数进行对齐，是GCC特有的语法
 * 比如 struct test{ char c;int b} sizeof(char)=1 sizeof(int)=2 紧凑sizeof(test)=3 非紧凑sizeof(test)=4
 */
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[];
};
struct __attribute__ ((__packed__)) sdshdr16 {
    uint16_t len; /* used */
    uint16_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr32 {
    uint32_t len; /* used */
    uint32_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr64 {
    uint64_t len; /* used */
    uint64_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};

• len 使用的长度，不包含结尾’\0’
• alloc 分配的长度，不包含头和结尾’\0’
• buf[]存放字符
• flags 存放结构体类型

sdsReqType根据string长度，返回对应宏

#define SDS_TYPE_5  0
#define SDS_TYPE_8  1
#define SDS_TYPE_16 2
#define SDS_TYPE_32 3
#define SDS_TYPE_64 4
static inline char sdsReqType(size_t string_size) {
    if (string_size < 1<<5)
        return SDS_TYPE_5;
    if (string_size < 1<<8)
        return SDS_TYPE_8;
    if (string_size < 1<<16)
        return SDS_TYPE_16;
#if (LONG_MAX == LLONG_MAX)
    if (string_size < 1ll<<32)
        return SDS_TYPE_32;
#endif
    return SDS_TYPE_64;
}

sdsHdrSize根据type类型，返回对应结构体的size

static inline int sdsHdrSize(char type) {
    switch(type&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return sizeof(struct sdshdr5);
        case SDS_TYPE_8:
            return sizeof(struct sdshdr8);
        case SDS_TYPE_16:
            return sizeof(struct sdshdr16);
        case SDS_TYPE_32:
            return sizeof(struct sdshdr32);
        case SDS_TYPE_64:
            return sizeof(struct sdshdr64);
    }
    return 0;
}

sdsnew 创建一个sds string结构

#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
sds sdsnew(const char *init) {
    size_t initlen = (init == NULL) ? 0 : strlen(init);//不包含空字符
    return sdsnewlen(init, initlen);
}
sds sdsnewlen(const void *init, size_t initlen) {
    void *sh;
    sds s;
    char type = sdsReqType(initlen);//type标示了对应的宏
    /* Empty strings are usually created in order to append. Use type 8
     * since type 5 is not good at this. */
    if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;
    int hdrlen = sdsHdrSize(type);//对应结构体的size
    unsigned char *fp; /* flags pointer. */

    sh = s_malloc(hdrlen+initlen+1);//分配结构体的size（因为是非紧凑型的，所以是size(len)+size(alloc)+size(flags) + 字符串的size + 结尾空字符'\0'）
    if (!init)
        memset(sh, 0, hdrlen+initlen+1);
    if (sh == NULL) return NULL;
    s = (char*)sh+hdrlen;//s是buf的起始位置
    fp = ((unsigned char*)s)-1;//fp是type的地址
    switch(type) {//根据type的类型，创建对应
        case SDS_TYPE_5: {
            *fp = type | (initlen << SDS_TYPE_BITS);
            break;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);//宏替换，sh指向sdshdr8的指针
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
    }
    if (initlen && init)
        memcpy(s, init, initlen);//init所指向的区域拷贝initlen个字节到s所指向的区域，字符串拷贝
    s[initlen] = '\0';
    return s;
}