Redis 源码分析-字符串

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动态长度字符串
本质就是typedef char *sds;
但是redis在sds的前面又增加了一个数据结构(不同长度数据结构不一样)
和c++的string不一样,这里的长度,容量和内存申请都是完全独立的
需要非常小心的手动维护
否则会出现数值和实际申请的内存不一致的情况

相关文件

  • sds.h
  • sds.c

相关概念

  • 内存对齐
    gcc的对齐是有优化的,按照结构体的最大位数进行对齐
    __attribute__ ((__packed__))的含义是取消gcc的对齐,数据实际占用多少就用多少内存
    没有空空间
  • 宏中#的含义
    • #,将参数按字面表现转化为字符串
    • ##,字符串拼接

主要函数

外部调用

  • 创建字符串
    sds sdsnew(const char *init);
  • 创建空字符串
    sds sdsempty(void);
  • 复制字符串
    sds sdsdup(const sds s);
  • 释放字符串
    void sdsfree(sds s);
  • 连接字符串
    sds sdscat(sds s, const char *t);
  • 复制字符串到已有字符串
    sds sdscpy(sds s, const char *t);

内部调用

  • 扩展字符串容量
    sds sdsMakeRoomFor(sds s, size_t addlen);
  • 维护字符串新增长度
    void sdsIncrLen(sds s, int incr);
  • 将容量设置为长度,节省内存
    sds sdsRemoveFreeSpace(sds s);
  • 获取字符串容量(包含结构体长度)
    size_t sdsAllocSize(sds s);
  • 获取字符串实际结构指针
    void *sdsAllocPtr(sds s);
  • 获取字符串长度
    static inline size_t sdslen(const sds s)
  • 获取字符串剩余空间
    static inline size_t sdsavail(const sds s)
  • 变更字符传长度记录
    static inline void sdssetlen(sds s, size_t newlen)
  • 增加字符串长度记录
    static inline void sdsinclen(sds s, size_t inc)
  • 获取字符串容量
    static inline size_t sdsalloc(const sds s)
  • 变更字符串容量记录
    static inline void sdssetalloc(sds s, size_t newlen)

数据结构

  • 数据记录是使用char*
  • 所有的sdshdr结尾都使用char buf[]动态数组做结尾,不占用内存
  • flags字段的低3位是记录改字符串的数据结构类型
  • sdshdr5的高5位记录字符串长度
  • 非sdshdr5的数据结构len是长度(实际使用的连续可用内存的大小)
  • alloc是容量(实际申请的可用连续内存的大小)
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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[];
};

函数分析

#define SDS_HDR_VAR(T,s)

功能

获取sds字符串的原始结构体指针sh

源码

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#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));

#define SDS_HDR(T,s)

功能

将sds字符串转换为原始结构体指针

源码

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#define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))

sds sdsnewlen(const void *init, size_t initlen);

功能

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/* 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) {
    void *sh;
    sds s;
    char type = sdsReqType(initlen); // 根据长度判断出需要存储的数据类型
    /* 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; // 使用SDS_TYPE_8来做空字符串
    int hdrlen = sdsHdrSize(type); // 获取结构体本身长度
    unsigned char *fp; /* flags pointer. */

    sh = s_malloc(hdrlen+initlen+1); // 申请内存(结构体长度+字符串长度+结尾终止符)
    if (!init)
        memset(sh, 0, hdrlen+initlen+1);
    if (sh == NULL) return NULL;
    s = (char*)sh+hdrlen;
    fp = ((unsigned char*)s)-1; // 根据内存排序,这里的[-1]就是结构体中的`flags`
    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 = 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); // 赋值
    s[initlen] = '\0'; // 标记结束符
    return s;
}

sds sdsempty(void);

功能

创建空字符串
常用于复制,连接字符串

源码

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sds sdsempty(void) {
    return sdsnewlen("",0); // 这里即使长度是0,结尾也有结束符
}

sds sdsnew(const char *init);

功能

根据字符串创建一个sds

源码

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sds sdsnew(const char *init) {
    size_t initlen = (init == NULL) ? 0 : strlen(init);
    return sdsnewlen(init, initlen);
}

sds sdsdup(const sds s);

功能

复制字符串

源码

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sds sdsdup(const sds s) {
    return sdsnewlen(s, sdslen(s));
}

void sdsfree(sds s);

功能

释放字符串空间

源码

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void sdsfree(sds s) {
    if (s == NULL) return;
    s_free((char*)s-sdsHdrSize(s[-1]));
}

void sdsclear(sds s);

功能

清空字符串内容
申请的内存和结构不动
后续使用这个字符串不需要重新申请内存

源码

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void sdsclear(sds s) {
    sdssetlen(s, 0);
    s[0] = '\0';
}

sds sdsMakeRoomFor(sds s, size_t addlen);

功能

扩大字符串的容量
为连接/扩展字符串做准备

源码

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sds sdsMakeRoomFor(sds s, size_t addlen) {
    void *sh, *newsh;
    size_t avail = sdsavail(s);
    size_t len, newlen;
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen;

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

    len = sdslen(s);
    sh = (char*)s-sdsHdrSize(oldtype);
    newlen = (len+addlen);
    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);
    if (oldtype==type) {
        newsh = s_realloc(sh, hdrlen+newlen+1);
        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(hdrlen+newlen+1);
        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);
    }
    sdssetalloc(s, newlen);
    return s;
}

sds sdsRemoveFreeSpace(sds s);

功能

清空字符串结构的容量冗余
使容量等于字符串长度
节省内存使用

源码

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/* Reallocate the sds string so that it has no free space at the end. The
 * contained string remains not altered, but next concatenation operations
 * will require a reallocation.
 *
 * 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 sdsRemoveFreeSpace(sds s) {
    void *sh, *newsh;
    char type, oldtype = s[-1] & SDS_TYPE_MASK; // 获取字符串结构类型
    int hdrlen;
    size_t len = sdslen(s); // 获取字符串长度
    sh = (char*)s-sdsHdrSize(oldtype); // 获取字符串原始结构指针

    type = sdsReqType(len); // 获取最终转化的类型
    hdrlen = sdsHdrSize(type); // 获取最终转化的结构头长度
    if (oldtype==type) { // 类型没有变化
        newsh = s_realloc(sh, hdrlen+len+1); // 重新分配申请内存的大小(截断以前的剩余空间)
        if (newsh == NULL) return NULL;
        s = (char*)newsh+hdrlen; // 获取新的字符串首地址
    } else {
        newsh = s_malloc(hdrlen+len+1); // 申请新结构体空间
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1); // 复制原始字符串数据
        s_free(sh); // 释放原始指针
        s = (char*)newsh+hdrlen; // 获取新的字符串首地址
        s[-1] = type; // 设置新类型
        sdssetlen(s, len); // 标记字符串长度
    }
    sdssetalloc(s, len); // 标记字符串容量
    return s;
}

sds sdsgrowzero(sds s, size_t len);

功能

从有效字符串结尾开始,直到len位置
初始化所有数据为0

源码

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/* Grow the sds to have the specified length. Bytes that were not part of
 * the original length of the sds will be set to zero.
 *
 * if the specified length is smaller than the current length, no operation
 * is performed. */
sds sdsgrowzero(sds s, size_t len) {
    size_t curlen = sdslen(s);

    if (len <= curlen) return s;
    s = sdsMakeRoomFor(s,len-curlen);
    if (s == NULL) return NULL;

    /* Make sure added region doesn't contain garbage */
    memset(s+curlen,0,(len-curlen+1)); /* also set trailing \0 byte */
    sdssetlen(s, len);
    return s;
}

sds sdscatlen(sds s, const void *t, size_t len);

功能

字符串连接

源码

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/* 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;
}

int sdsll2str(char *s, long long value);

功能

将数字按字面值转化为字符串

源码

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/* Helper for sdscatlonglong() doing the actual number -> string
 * conversion. 's' must point to a string with room for at least
 * SDS_LLSTR_SIZE bytes.
 *
 * The function returns the length of the null-terminated string
 * representation stored at 's'. */
#define SDS_LLSTR_SIZE 21
int sdsll2str(char *s, long long value) {
    char *p, aux;
    unsigned long long v;
    size_t l;

    /* Generate the string representation, this method produces
     * an reversed string. */
    v = (value < 0) ? -value : value;
    p = s;
    do {
        *p++ = '0'+(v%10);
        v /= 10;
    } while(v);
    if (value < 0) *p++ = '-';

    /* Compute length and add null term. */
    l = p-s;
    *p = '\0';

    /* Reverse the string. */
    p--;
    while(s < p) {
        aux = *s;
        *s = *p;
        *p = aux;
        s++;
        p--;
    }
    return l;
}

sds sdsfromlonglong(long long value);

功能

将数字转化为sds

源码

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sds sdsfromlonglong(long long value) {
    char buf[SDS_LLSTR_SIZE];
    int len = sdsll2str(buf,value);

    return sdsnewlen(buf,len);
}

sds sdscatvprintf(sds s, const char *fmt, va_list ap);

功能

字符串格式化连接

源码

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/* Like sdscatprintf() but gets va_list instead of being variadic. */
sds sdscatvprintf(sds s, const char *fmt, va_list ap) {
    va_list cpy;
    char staticbuf[1024], *buf = staticbuf, *t;
    size_t buflen = strlen(fmt)*2;

    /* We try to start using a static buffer for speed.
     * If not possible we revert to heap allocation. */
    if (buflen > sizeof(staticbuf)) {
        buf = s_malloc(buflen);
        if (buf == NULL) return NULL;
    } else {
        buflen = sizeof(staticbuf);
    }

    /* Try with buffers two times bigger every time we fail to
     * fit the string in the current buffer size. */
    while(1) {
        buf[buflen-2] = '\0';
        va_copy(cpy,ap);
        vsnprintf(buf, buflen, fmt, cpy);
        va_end(cpy);
        if (buf[buflen-2] != '\0') {
            if (buf != staticbuf) s_free(buf);
            buflen *= 2;
            buf = s_malloc(buflen);
            if (buf == NULL) return NULL;
            continue;
        }
        break;
    }

    /* Finally concat the obtained string to the SDS string and return it. */
    t = sdscat(s, buf);
    if (buf != staticbuf) s_free(buf);
    return t;
}

sds sdscatprintf(sds s, const char *fmt, ...);

功能

字符串格式化连接

源码

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/* Append to the sds string 's' a string obtained using printf-alike format
 * specifier.
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call.
 *
 * Example:
 *
 * s = sdsnew("Sum is: ");
 * s = sdscatprintf(s,"%d+%d = %d",a,b,a+b).
 *
 * Often you need to create a string from scratch with the printf-alike
 * format. When this is the need, just use sdsempty() as the target string:
 *
 * s = sdscatprintf(sdsempty(), "... your format ...", args);
 */
sds sdscatprintf(sds s, const char *fmt, ...) {
    va_list ap;
    char *t;
    va_start(ap, fmt);
    t = sdscatvprintf(s,fmt,ap);
    va_end(ap);
    return t;
}

sds sdscatfmt(sds s, char const *fmt, ...);

功能

字符串格式化连接(高效简略版)

源码

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/* This function is similar to sdscatprintf, but much faster as it does
 * not rely on sprintf() family functions implemented by the libc that
 * are often very slow. Moreover directly handling the sds string as
 * new data is concatenated provides a performance improvement.
 *
 * However this function only handles an incompatible subset of printf-alike
 * format specifiers:
 *
 * %s - C String
 * %S - SDS string
 * %i - signed int
 * %I - 64 bit signed integer (long long, int64_t)
 * %u - unsigned int
 * %U - 64 bit unsigned integer (unsigned long long, uint64_t)
 * %% - Verbatim "%" character.
 */
sds sdscatfmt(sds s, char const *fmt, ...) {
    size_t initlen = sdslen(s);
    const char *f = fmt;
    int i;
    va_list ap;

    va_start(ap,fmt);
    f = fmt;    /* Next format specifier byte to process. */
    i = initlen; /* Position of the next byte to write to dest str. */
    while(*f) {
        char next, *str;
        size_t l;
        long long num;
        unsigned long long unum;

        /* Make sure there is always space for at least 1 char. */
        if (sdsavail(s)==0) {
            s = sdsMakeRoomFor(s,1);
        }

        switch(*f) {
        case '%':
            next = *(f+1);
            f++;
            switch(next) {
            case 's':
            case 'S':
                str = va_arg(ap,char*);
                l = (next == 's') ? strlen(str) : sdslen(str);
                if (sdsavail(s) < l) {
                    s = sdsMakeRoomFor(s,l);
                }
                memcpy(s+i,str,l);
                sdsinclen(s,l);
                i += l;
                break;
            case 'i':
            case 'I':
                if (next == 'i')
                    num = va_arg(ap,int);
                else
                    num = va_arg(ap,long long);
                {
                    char buf[SDS_LLSTR_SIZE];
                    l = sdsll2str(buf,num);
                    if (sdsavail(s) < l) {
                        s = sdsMakeRoomFor(s,l);
                    }
                    memcpy(s+i,buf,l);
                    sdsinclen(s,l);
                    i += l;
                }
                break;
            case 'u':
            case 'U':
                if (next == 'u')
                    unum = va_arg(ap,unsigned int);
                else
                    unum = va_arg(ap,unsigned long long);
                {
                    char buf[SDS_LLSTR_SIZE];
                    l = sdsull2str(buf,unum);
                    if (sdsavail(s) < l) {
                        s = sdsMakeRoomFor(s,l);
                    }
                    memcpy(s+i,buf,l);
                    sdsinclen(s,l);
                    i += l;
                }
                break;
            default: /* Handle %% and generally %<unknown>. */
                s[i++] = next;
                sdsinclen(s,1);
                break;
            }
            break;
        default:
            s[i++] = *f;
            sdsinclen(s,1);
            break;
        }
        f++;
    }
    va_end(ap);

    /* Add null-term */
    s[i] = '\0';
    return s;
}

sds sdstrim(sds s, const char *cset);

功能

去除字符串两边需要过滤的字符集

源码

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/* Remove the part of the string from left and from right composed just of
 * contiguous characters found in 'cset', that is a null terminted C string.
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call.
 *
 * Example:
 *
 * s = sdsnew("AA...AA.a.aa.aHelloWorld     :::");
 * s = sdstrim(s,"Aa. :");
 * printf("%s\n", s);
 *
 * Output will be just "Hello World".
 */
sds sdstrim(sds s, const char *cset) {
    char *start, *end, *sp, *ep;
    size_t len;

    sp = start = s;
    ep = end = s+sdslen(s)-1;
    while(sp <= end && strchr(cset, *sp)) sp++;
    while(ep > sp && strchr(cset, *ep)) ep--;
    len = (sp > ep) ? 0 : ((ep-sp)+1);
    if (s != sp) memmove(s, sp, len);
    s[len] = '\0';
    sdssetlen(s,len);
    return s;
}

void sdsrange(sds s, int start, int end)

功能

截取字符串
支持负数倒叙定位

源码

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/* Turn the string into a smaller (or equal) string containing only the
 * substring specified by the 'start' and 'end' indexes.
 *
 * start and end can be negative, where -1 means the last character of the
 * string, -2 the penultimate character, and so forth.
 *
 * The interval is inclusive, so the start and end characters will be part
 * of the resulting string.
 *
 * The string is modified in-place.
 *
 * Example:
 *
 * s = sdsnew("Hello World");
 * sdsrange(s,1,-1); => "ello World"
 */
void sdsrange(sds s, int start, int end) {
    size_t newlen, len = sdslen(s);

    if (len == 0) return;
    if (start < 0) {
        start = len+start;
        if (start < 0) start = 0;
    }
    if (end < 0) {
        end = len+end;
        if (end < 0) end = 0;
    }
    newlen = (start > end) ? 0 : (end-start)+1;
    if (newlen != 0) {
        if (start >= (signed)len) {
            newlen = 0;
        } else if (end >= (signed)len) {
            end = len-1;
            newlen = (start > end) ? 0 : (end-start)+1;
        }
    } else {
        start = 0;
    }
    if (start && newlen) memmove(s, s+start, newlen);
    s[newlen] = 0;
    sdssetlen(s,newlen);
}

void sdstolower(sds s);

功能

所有字符变为小写

源码

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/* Apply tolower() to every character of the sds string 's'. */
void sdstolower(sds s) {
    int len = sdslen(s), j;

    for (j = 0; j < len; j++) s[j] = tolower(s[j]);
}

void sdstoupper(sds s);

功能

所有字符变为大写

源码

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/* Apply toupper() to every character of the sds string 's'. */
void sdstoupper(sds s) {
    int len = sdslen(s), j;

    for (j = 0; j < len; j++) s[j] = toupper(s[j]);
}

int sdscmp(const sds s1, const sds s2);

功能

比较两个字符串的大小

源码

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/* Compare two sds strings s1 and s2 with memcmp().
 *
 * Return value:
 *
 *     positive if s1 > s2.
 *     negative if s1 < s2.
 *     0 if s1 and s2 are exactly the same binary string.
 *
 * If two strings share exactly the same prefix, but one of the two has
 * additional characters, the longer string is considered to be greater than
 * the smaller one. */
int sdscmp(const sds s1, const sds s2) {
    size_t l1, l2, minlen;
    int cmp;

    l1 = sdslen(s1);
    l2 = sdslen(s2);
    minlen = (l1 < l2) ? l1 : l2;
    cmp = memcmp(s1,s2,minlen);
    if (cmp == 0) return l1-l2;
    return cmp;
}

sds *sdssplitlen(const char *s, int len, const char *sep, int seplen, int *count);

功能

将字符串按照sep拆分为字符数组

源码

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/* Split 's' with separator in 'sep'. An array
 * of sds strings is returned. *count will be set
 * by reference to the number of tokens returned.
 *
 * On out of memory, zero length string, zero length
 * separator, NULL is returned.
 *
 * Note that 'sep' is able to split a string using
 * a multi-character separator. For example
 * sdssplit("foo_-_bar","_-_"); will return two
 * elements "foo" and "bar".
 *
 * This version of the function is binary-safe but
 * requires length arguments. sdssplit() is just the
 * same function but for zero-terminated strings.
 */
sds *sdssplitlen(const char *s, int len, const char *sep, int seplen, int *count) {
    int elements = 0, slots = 5, start = 0, j;
    sds *tokens;

    if (seplen < 1 || len < 0) return NULL;

    tokens = s_malloc(sizeof(sds)*slots);
    if (tokens == NULL) return NULL;

    if (len == 0) {
        *count = 0;
        return tokens;
    }
    for (j = 0; j < (len-(seplen-1)); j++) {
        /* make sure there is room for the next element and the final one */
        if (slots < elements+2) {
            sds *newtokens;

            slots *= 2;
            newtokens = s_realloc(tokens,sizeof(sds)*slots);
            if (newtokens == NULL) goto cleanup;
            tokens = newtokens;
        }
        /* search the separator */
        if ((seplen == 1 && *(s+j) == sep[0]) || (memcmp(s+j,sep,seplen) == 0)) {
            tokens[elements] = sdsnewlen(s+start,j-start);
            if (tokens[elements] == NULL) goto cleanup;
            elements++;
            start = j+seplen;
            j = j+seplen-1; /* skip the separator */
        }
    }
    /* Add the final element. We are sure there is room in the tokens array. */
    tokens[elements] = sdsnewlen(s+start,len-start);
    if (tokens[elements] == NULL) goto cleanup;
    elements++;
    *count = elements;
    return tokens;

cleanup:
    {
        int i;
        for (i = 0; i < elements; i++) sdsfree(tokens[i]);
        s_free(tokens);
        *count = 0;
        return NULL;
    }
}

void sdsfreesplitres(sds *tokens, int count);

功能

整体释放字符数组

源码

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/* Free the result returned by sdssplitlen(), or do nothing if 'tokens' is NULL. */
void sdsfreesplitres(sds *tokens, int count) {
    if (!tokens) return;
    while(count--)
        sdsfree(tokens[count]);
    s_free(tokens);
}

sds sdsjoin(char **argv, int argc, char *sep);

功能

字符数组连接为一个字符串
连接符为sep

源码

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/* Join an array of C strings using the specified separator (also a C string).
 * Returns the result as an sds string. */
sds sdsjoin(char **argv, int argc, char *sep) {
    sds join = sdsempty();
    int j;

    for (j = 0; j < argc; j++) {
        join = sdscat(join, argv[j]);
        if (j != argc-1) join = sdscat(join,sep);
    }
    return join;
}