redis string

一阶冈、什么是 SDS

Redis是使用C語(yǔ)言開(kāi)發(fā)的润绵,所以為了保證能盡量復(fù)用C標(biāo)準(zhǔn)庫(kù)中的字符串操作函數(shù),Redis保留了使用字符數(shù)組來(lái)保存實(shí)際的數(shù)據(jù)岗钩。但是纽窟,和C語(yǔ)言僅用字符數(shù)組不同,Redis還專(zhuān)門(mén)設(shè)計(jì)了SDS(即簡(jiǎn)單動(dòng)態(tài)字符串兼吓,Simple Dynamic String)的數(shù)據(jù)結(jié)構(gòu)臂港。
源碼文件:src/sds.h、src/sds.c视搏、src/sdsalloc.h

二审孽、SDS 做了哪些優(yōu)化

2.1 SDS 數(shù)據(jù)結(jié)構(gòu)及內(nèi)存優(yōu)化

SDS中是通過(guò)設(shè)計(jì)不同SDS類(lèi)型來(lái)表示不同大小的字符串,并使用attribute ((packed))這個(gè)編程小技巧浑娜,來(lái)實(shí)現(xiàn)緊湊型內(nèi)存布局瓷胧,達(dá)到節(jié)省內(nèi)存的目的。

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[];
};
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[];
};

#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
#define SDS_TYPE_MASK 7
#define SDS_TYPE_BITS 3
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
#define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
#define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)

static inline size_t sdslen(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->len;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->len;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->len;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->len;
    }
    return 0;
}

static inline size_t sdsavail(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5: {
            return 0;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            return sh->alloc - sh->len;
        }
    }
    return 0;
}

static inline void sdssetlen(sds s, size_t newlen) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            {
                unsigned char *fp = ((unsigned char*)s)-1;
                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
            }
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->len = newlen;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->len = newlen;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->len = newlen;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->len = newlen;
            break;
    }
}

static inline void sdsinclen(sds s, size_t inc) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            {
                unsigned char *fp = ((unsigned char*)s)-1;
                unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc;
                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
            }
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->len += inc;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->len += inc;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->len += inc;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->len += inc;
            break;
    }
}

/* sdsalloc() = sdsavail() + sdslen() */
static inline size_t sdsalloc(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->alloc;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->alloc;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->alloc;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->alloc;
    }
    return 0;
}

static inline void sdssetalloc(sds s, size_t newlen) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            /* Nothing to do, this type has no total allocation info. */
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->alloc = newlen;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->alloc = newlen;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->alloc = newlen;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->alloc = newlen;
            break;
    }
}

2.2 存儲(chǔ)任意二進(jìn)制數(shù)據(jù)

C語(yǔ)言使用 char*實(shí)現(xiàn)字符串棚愤,'0'表示字符串結(jié)束搓萧,不滿足Redis希望能保存任意二進(jìn)制數(shù)據(jù)的訴求。而SDS不通過(guò)字符串中的“0”字符判斷字符串結(jié)束宛畦,而是直接將其作為二進(jìn)制數(shù)據(jù)處理瘸洛,可以用來(lái)保存圖片等二進(jìn)制數(shù)據(jù)。

/* Create a new sds string starting from a null terminated C string. */
sds sdsnew(const char *init) {
    size_t initlen = (init == NULL) ? 0 : strlen(init);
    return sdsnewlen(init, initlen);
}

sds sdsnewlen(const void *init, size_t initlen) {
    return _sdsnewlen(init, initlen, 0);
}

/* 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.
 * If SDS_NOINIT is used, the buffer is left uninitialized;
 *
 * 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, int trymalloc) {
    void *sh;
    sds s;
    char type = sdsReqType(initlen);
    /* 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);
    unsigned char *fp; /* flags pointer. */
    size_t usable;

    assert(initlen + hdrlen + 1 > initlen); /* Catch size_t overflow */
    sh = trymalloc?
        s_trymalloc_usable(hdrlen+initlen+1, &usable) :
        s_malloc_usable(hdrlen+initlen+1, &usable);
    if (sh == NULL) return NULL;
    if (init==SDS_NOINIT)
        init = NULL;
    else if (!init)
        memset(sh, 0, hdrlen+initlen+1);
    s = (char*)sh+hdrlen;
    fp = ((unsigned char*)s)-1;
    usable = usable-hdrlen-1;
    if (usable > sdsTypeMaxSize(type))
        usable = sdsTypeMaxSize(type);
    switch(type) {
        case SDS_TYPE_5: {
            *fp = type | (initlen << SDS_TYPE_BITS);
            break;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            sh->len = initlen;
            sh->alloc = usable;
            *fp = type;
            break;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            sh->len = initlen;
            sh->alloc = usable;
            *fp = type;
            break;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            sh->len = initlen;
            sh->alloc = usable;
            *fp = type;
            break;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            sh->len = initlen;
            sh->alloc = usable;
            *fp = type;
            break;
        }
    }
    if (initlen && init)
        memcpy(s, init, initlen);
    s[initlen] = '\0';
    return s;
}

2.3 降低了 string 操作的復(fù)雜度

SDS增加了字符數(shù)組長(zhǎng)度和分配空間大小等元數(shù)據(jù)次和,抽象出sdsMakeRoomFor通用方法反肋,降低字符串操作的復(fù)雜度(Java list也有類(lèi)似實(shí)現(xiàn))

/* Append the specified null terminated 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) {
    size_t curlen = sdslen(s);

    s = sdsMakeRoomFor(s,len);
    if (s == NULL) return NULL;
    memcpy(s+curlen, t, len);
    sdssetlen(s, curlen+len);
    s[curlen+len] = '\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) {
    void *sh, *newsh;
    size_t avail = sdsavail(s);
    size_t len, newlen, reqlen;
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen;
    size_t usable;

    /* Return ASAP if there is enough space left. */
    if (avail >= addlen) return s;

    len = sdslen(s);
    sh = (char*)s-sdsHdrSize(oldtype);
    reqlen = newlen = (len+addlen);
    assert(newlen > len);   /* Catch size_t overflow */
    if (newlen < SDS_MAX_PREALLOC)
        newlen *= 2;
    else
        newlen += SDS_MAX_PREALLOC;

    type = sdsReqType(newlen);

    /* Don't use type 5: the user is appending to the string and type 5 is
     * not able to remember empty space, so sdsMakeRoomFor() must be called
     * at every appending operation. */
    if (type == SDS_TYPE_5) type = SDS_TYPE_8;

    hdrlen = sdsHdrSize(type);
    assert(hdrlen + newlen + 1 > reqlen);  /* Catch size_t overflow */
    if (oldtype==type) {
        newsh = s_realloc_usable(sh, hdrlen+newlen+1, &usable);
        if (newsh == NULL) return NULL;
        s = (char*)newsh+hdrlen;
    } else {
        /* Since the header size changes, need to move the string forward,
         * and can't use realloc */
        newsh = s_malloc_usable(hdrlen+newlen+1, &usable);
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1);
        s_free(sh);
        s = (char*)newsh+hdrlen;
        s[-1] = type;
        sdssetlen(s, len);
    }
    usable = usable-hdrlen-1;
    if (usable > sdsTypeMaxSize(type))
        usable = sdsTypeMaxSize(type);
    sdssetalloc(s, usable);
    return s;
}

static inline size_t sdslen(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->len;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->len;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->len;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->len;
    }
    return 0;
}
?著作權(quán)歸作者所有,轉(zhuǎn)載或內(nèi)容合作請(qǐng)聯(lián)系作者
  • 序言:七十年代末,一起剝皮案震驚了整個(gè)濱河市踏施,隨后出現(xiàn)的幾起案子石蔗,更是在濱河造成了極大的恐慌,老刑警劉巖畅形,帶你破解...
    沈念sama閱讀 216,402評(píng)論 6 499
  • 序言:濱河連續(xù)發(fā)生了三起死亡事件养距,死亡現(xiàn)場(chǎng)離奇詭異,居然都是意外死亡日熬,警方通過(guò)查閱死者的電腦和手機(jī)棍厌,發(fā)現(xiàn)死者居然都...
    沈念sama閱讀 92,377評(píng)論 3 392
  • 文/潘曉璐 我一進(jìn)店門(mén),熙熙樓的掌柜王于貴愁眉苦臉地迎上來(lái)竖席,“玉大人耘纱,你說(shuō)我怎么就攤上這事”霞觯” “怎么了束析?”我有些...
    開(kāi)封第一講書(shū)人閱讀 162,483評(píng)論 0 353
  • 文/不壞的土叔 我叫張陵,是天一觀的道長(zhǎng)憎亚。 經(jīng)常有香客問(wèn)我员寇,道長(zhǎng)弄慰,這世上最難降的妖魔是什么? 我笑而不...
    開(kāi)封第一講書(shū)人閱讀 58,165評(píng)論 1 292
  • 正文 為了忘掉前任丁恭,我火速辦了婚禮,結(jié)果婚禮上斋日,老公的妹妹穿的比我還像新娘牲览。我一直安慰自己,他們只是感情好恶守,可當(dāng)我...
    茶點(diǎn)故事閱讀 67,176評(píng)論 6 388
  • 文/花漫 我一把揭開(kāi)白布第献。 她就那樣靜靜地躺著,像睡著了一般兔港。 火紅的嫁衣襯著肌膚如雪庸毫。 梳的紋絲不亂的頭發(fā)上,一...
    開(kāi)封第一講書(shū)人閱讀 51,146評(píng)論 1 297
  • 那天衫樊,我揣著相機(jī)與錄音飒赃,去河邊找鬼。 笑死科侈,一個(gè)胖子當(dāng)著我的面吹牛载佳,可吹牛的內(nèi)容都是我干的。 我是一名探鬼主播臀栈,決...
    沈念sama閱讀 40,032評(píng)論 3 417
  • 文/蒼蘭香墨 我猛地睜開(kāi)眼蔫慧,長(zhǎng)吁一口氣:“原來(lái)是場(chǎng)噩夢(mèng)啊……” “哼!你這毒婦竟也來(lái)了权薯?” 一聲冷哼從身側(cè)響起姑躲,我...
    開(kāi)封第一講書(shū)人閱讀 38,896評(píng)論 0 274
  • 序言:老撾萬(wàn)榮一對(duì)情侶失蹤,失蹤者是張志新(化名)和其女友劉穎盟蚣,沒(méi)想到半個(gè)月后黍析,有當(dāng)?shù)厝嗽跇?shù)林里發(fā)現(xiàn)了一具尸體,經(jīng)...
    沈念sama閱讀 45,311評(píng)論 1 310
  • 正文 獨(dú)居荒郊野嶺守林人離奇死亡屎开,尸身上長(zhǎng)有42處帶血的膿包…… 初始之章·張勛 以下內(nèi)容為張勛視角 年9月15日...
    茶點(diǎn)故事閱讀 37,536評(píng)論 2 332
  • 正文 我和宋清朗相戀三年橄仍,在試婚紗的時(shí)候發(fā)現(xiàn)自己被綠了。 大學(xué)時(shí)的朋友給我發(fā)了我未婚夫和他白月光在一起吃飯的照片牍戚。...
    茶點(diǎn)故事閱讀 39,696評(píng)論 1 348
  • 序言:一個(gè)原本活蹦亂跳的男人離奇死亡侮繁,死狀恐怖,靈堂內(nèi)的尸體忽然破棺而出如孝,到底是詐尸還是另有隱情宪哩,我是刑警寧澤,帶...
    沈念sama閱讀 35,413評(píng)論 5 343
  • 正文 年R本政府宣布第晰,位于F島的核電站锁孟,受9級(jí)特大地震影響彬祖,放射性物質(zhì)發(fā)生泄漏。R本人自食惡果不足惜品抽,卻給世界環(huán)境...
    茶點(diǎn)故事閱讀 41,008評(píng)論 3 325
  • 文/蒙蒙 一储笑、第九天 我趴在偏房一處隱蔽的房頂上張望。 院中可真熱鬧圆恤,春花似錦突倍、人聲如沸。這莊子的主人今日做“春日...
    開(kāi)封第一講書(shū)人閱讀 31,659評(píng)論 0 22
  • 文/蒼蘭香墨 我抬頭看了看天上的太陽(yáng)。三九已至淡喜,卻和暖如春秕磷,著一層夾襖步出監(jiān)牢的瞬間,已是汗流浹背炼团。 一陣腳步聲響...
    開(kāi)封第一講書(shū)人閱讀 32,815評(píng)論 1 269
  • 我被黑心中介騙來(lái)泰國(guó)打工澎嚣, 沒(méi)想到剛下飛機(jī)就差點(diǎn)兒被人妖公主榨干…… 1. 我叫王不留,地道東北人瘟芝。 一個(gè)月前我還...
    沈念sama閱讀 47,698評(píng)論 2 368
  • 正文 我出身青樓币叹,卻偏偏與公主長(zhǎng)得像,于是被迫代替她去往敵國(guó)和親模狭。 傳聞我的和親對(duì)象是個(gè)殘疾皇子颈抚,可洞房花燭夜當(dāng)晚...
    茶點(diǎn)故事閱讀 44,592評(píng)論 2 353

推薦閱讀更多精彩內(nèi)容