sds簡單字串(sds.h、sds.c)

部分原始碼解讀(5.0.8版本)

定義結構:

/* 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 */c
char buf[];
};
 

記憶體結構:

結構體成員變數說明：

len:代表已使用的char陣列長度(不包含結尾\0)

alloc:代表char陣列總長度(不包含\0)

剩餘可使用長度= alloc - len;

使用sds而不使用char*的原因：
1：獲取字串長度為 O(N) 級別的操作 → 因為 C 不儲存陣列的長度，每次都需要遍歷一遍整個陣列

2：字串操作不當拼接容易造成緩衝區溢位/記憶體洩露

3：C字串僅能儲存文字資料，截止\0 則會進行擷取，後邊的無法識別

主要api

簡單字串的建立

/* 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;
//根據字串長度選擇型別確定型別(sdshdr8|sdshdr16|sdshdr32|sdshdr64)
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);//頭部長度
unsigned char *fp; /* flags pointer. */
//計算需要申請的記憶體空間並申請記憶體，記憶體大小 = 頭部長度+字串長度+1,後邊+1是因為要補'\0'
sh = s_malloc(hdrlen+initlen+1);
if (sh == NULL) return NULL;//申請失敗返回NULL
if (!init)
memset(sh, 0, hdrlen+initlen+1); //將sh指標指向的地址內容設定為0
s = (char*)sh+hdrlen;//char buf[]陣列指標位置
fp = ((unsigned char*)s)-1;//flag指標
//將*sh 轉為sdhdr型別指標，並將len、alloc、flag進行初始化
switch(type) {
case SDS_TYPE_5: {
*fp = type | (initlen << SDS_TYPE_BITS);
break;
}
case SDS_TYPE_8: {
SDS_HDR_VAR(8,s)//struct sdshdr8 *sh = (struct sdshdr8 *)((s)-(sizeof(struct sdshdr8)));;
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;
}
}
// 將字串拷貝到char buf[]
if (initlen && init)
memcpy(s, init, initlen);
s[initlen] = '\0';//末尾補'\0'
return s;
}
 

計算字串長度

#define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
static inline size_t sdslen(const sds s) { //sds指向buf的位置
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;
}

說明：

redis進行簡單字串操作時，傳遞的是sds實際上是sdshdr->buf，因為sdshdr泛型出了幾種不同長度的簡單字串，所以通過flag = sds-1可以確定sdshdr的型別，進而獲得對應的sdshdr結構體物件。如果直接傳遞sdshdr不具備通用性，reids 3.0版本之前僅有sdshdr一種型別,使用泛型的好處則避免了記憶體的浪費

redis 2.X.X中簡單字串的定義

struct sdshdr {
int len;
int free;
char buf[];
};
static inline size_t sdslen(const sds s) {
struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));
return sh->len;
}
sds sdsnewlen(const void *init, size_t initlen) {
struct sdshdr *sh;

if (init) {
sh = zmalloc(sizeof(struct sdshdr)+initlen+1);
} else {
sh = zcalloc(sizeof(struct sdshdr)+initlen+1);
}
if (sh == NULL) return NULL;
sh->len = initlen;
sh->free = 0;
if (initlen && init)
memcpy(sh->buf, init, initlen);
sh->buf[initlen] = '\0';
return (char*)sh->buf;
}