redis源码阅读之sds

放假在家，本想偷个懒，但还是觉得应该给2018一个交代，就扯扯这个吧~
由于redis是用纯c写的，所以没有string那么高端的东西，所以它就自己实现了一个动态字符串的库，用于字符串处理，就是这个SDSLib了~~~
在sdsalloc.h文件中，使用宏定义转换了如下几个函数

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#include "zmalloc.h"
#define s_malloc zmalloc
#define s_realloc zrealloc
#define s_free zfree

在这里先定义了一堆结构体sdshdrxx，其中struct 关键字后面的`__attribute__ ((__packed__))`表示该结构体再存储时，不用考虑字节对齐。除了sdshdr5之外均有len成员表示已经占用的长度，alloc成员表示实际分配给字符串的长度。注意，这里的len和alloc的类型决定了该类sds最多可以存储多少个字符

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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[];
};

紧接着，又定义了一些宏来定义如何获取对应结构体里的相关信息，这里遵循的原则就是s指向实际使用的字符串首地址，s的前面就是对应的结构体的头部。这里面比较奇怪（或者说我暂时没看懂）的地方就是结构体里面命名定义了flags成员，但是实际取flags的时候，都是直接用s[-1]来存取，其他的变量倒是都用变量名来获取，要是有大侠知道，还请不吝赐教

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#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)

下面几个函数分别获取成员变量len、剩余空间，设置len、增加len，获取alloc，设置alloc，套路都差不多，这里仅给出获取len和获取可用空间的代码，其余的非常类似

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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;
}

当创建一个新的sds串时，会根据请求大小来分配不同类型，如果传入指针和长度均存在，则memcpy

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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;
}
sds sdsnewlen(const void *init, size_t initlen) {
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. */
sh = s_malloc(hdrlen+initlen+1);
if (!init)
memset(sh, 0, hdrlen+initlen+1);
if (sh == NULL) return NULL;
s = (char*)sh+hdrlen;
fp = ((unsigned char*)s)-1;
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);
s[initlen] = '\0';
return s;
}

当给一个已知的sds串添加可用空间时，需要考虑扩充之后的类型与原来的类型是否一致

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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;
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);
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 {
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;
}

移除空闲空间时，也需要考虑到类型的转换，这里需要注意的是它并没有严格的根据类型来进行realloc，在if里面直接realloc完成copy工作，else里才用memcpy，提升效率~~~

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sds sdsRemoveFreeSpace(sds s) {
void *sh, *newsh;
char type, oldtype = s[-1] & SDS_TYPE_MASK;
int hdrlen, oldhdrlen = sdsHdrSize(oldtype);
size_t len = sdslen(s);
sh = (char*)s-oldhdrlen;
type = sdsReqType(len);
hdrlen = sdsHdrSize(type);
if (oldtype==type || type > SDS_TYPE_8) {
newsh = s_realloc(sh, oldhdrlen+len+1);
if (newsh == NULL) return NULL;
s = (char*)newsh+oldhdrlen;
} else {
newsh = s_malloc(hdrlen+len+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, len);
return s;
}

其打印函数有两个版本，一版是借助于vsnprintf实现的，相对较慢。

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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;
}

另一个版本比较快，但是仅仅能够处理几个模式

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sds sdscatfmt(sds s, char const *fmt, ...) {
size_t initlen = sdslen(s);
const char *f = fmt;
long 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;
}

这边trim函数是用memmove实现的，cset是一个字符集

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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;
}

剩下的一堆就是通常的cat，split等的一堆函数，列出来太占地方了，就是用之前的一些基本函数来实现的，这里不一一表示了~~~

写在2018年末：

最开始要坚持写博客是因为今年上半年准备换工作的事情，看了好多资料都说要显得逼格高一点最好就要有自己的博客，后来换到帝都当码农之后，觉得有些东西还是要按照自己的理解经常记录下来，没想到，有点懒散的自己还能够坚持一周一更到现在。觉得这种事自己给自己的一种内心的认同吧，越来越觉得构建自己的知识体系是多么的重要，同样一篇文章，两个不同的人读，效果可能就差的非常大，慢慢再努力吧~~~再有就是如果想要熟练某项技能，一定要经常用到（用体育圈的话来说就是“一天不练，自己知道；三天不练，教练知道；一周不练，大家都知道。”）。最后还是要恭祝大家新年快乐~~~

StolennnXB

redis源码阅读之sds