redis原始碼解析(二)動態字串sds基本功能函式
阿新 • • 發佈:2018-12-12
1. 簡介
本文繼上文基礎上,分析動態字串的功能函式,位於sds.c。由於函式較多,本篇介紹實現動態變化的基本增刪新建釋放函式。
2. 原始碼分析
sdsHdrSize()函式用於返回sdshdr的大小,主要使用sizeof()函式實現。
/*返回sdshdr大小,主要使用sizeof()函式*/
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;
}
sdsReqType()根據size返回sdsHdr型別
/*根據size返回sds的型別*/
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;
}
sdsnewlen()函式建立新的sds動態陣列
/* 初始化函式
* 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initlen返回type*/
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;
/*hdrlen賦值為sds的size*/
int hdrlen = sdsHdrSize(type);
unsigned char *fp; /* flags pointer. */
/*分配空間*/
sh = s_malloc(hdrlen+initlen+1);
if (!init)
memset(sh, 0, hdrlen+initlen+1);
/*錯誤返回*/
if (sh == NULL) return NULL;
/*s指向資料,fp為首指標前一位,指向flag,儲存type*/
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;
}
sdsempty()函式利用sdsnewlen建立空sds動態陣列
/* 建立空sds
* Create an empty (zero length) sds string. Even in this case the string
* always has an implicit null term. */
sds sdsempty(void) {
return sdsnewlen("",0);
}
sdsnew()利用sdsnewlen()建立sds動態陣列
/* 建立新sds
* 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);
}
sdsdup()函式使用sdsnewlen()複製已有的sds
/* 複製sds
* Duplicate an sds string.
*/
sds sdsdup(const sds s) {
return sdsnewlen(s, sdslen(s));
}
sdsfree()函式釋放sds的空間
/* 釋放sds
* Free an sds string. No operation is performed if 's' is NULL.
*/
void sdsfree(sds s) {
if (s == NULL) return;
s_free((char*)s-sdsHdrSize(s[-1]));
}
sdsupdatelen()函式更新sdsHdr的長度
/* 當sds資料變化後,呼叫該函式更新sds長度
* Set the sds string length to the length as obtained with strlen(), so
* considering as content only up to the first null term character.
*
* This function is useful when the sds string is hacked manually in some
* way, like in the following example:
*
* s = sdsnew("foobar");
* s[2] = '\0';
* sdsupdatelen(s);
* printf("%d\n", sdslen(s));
*
* The output will be "2", but if we comment out the call to sdsupdatelen()
* the output will be "6" as the string was modified but the logical length
* remains 6 bytes. */
void sdsupdatelen(sds s) {
size_t reallen = strlen(s);
sdssetlen(s, reallen);
}
sdsclear()函式將sds長度和內容置零
/* 清空sds
* Modify an sds string in-place to make it empty (zero length).
* However all the existing buffer is not discarded but set as free space
* so that next append operations will not require allocations up to the
* number of bytes previously available. */
void sdsclear(sds s) {
sdssetlen(s, 0);
s[0] = '\0';
}
動態字串之擴容:sdsMakeRoomFor()函式。根據增加的addlen來調整sdsHdr的型別。
/* sds擴容
* 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;
char type, oldtype = s[-1] & SDS_TYPE_MASK;
int hdrlen;
/* 可用空間夠大,則不擴容了
* Return ASAP if there is enough space left.
*/
if (avail >= addlen) return s;
len = sdslen(s);
sh = (char*)s-sdsHdrSize(oldtype);
newlen = (len+addlen);
/*這裡的翻倍規則和動態陣列vector是一樣的:便於下次擴容*/
if (newlen < SDS_MAX_PREALLOC)
newlen *= 2;
else
newlen += SDS_MAX_PREALLOC;
type = sdsReqType(newlen);
/* 不適用type 5
* 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 {
/* 頭部size變化了
* 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;
}
壓縮空間的函式sdsRemoveFreeSpace(),功能和擴充套件空間相反。
/* 用於節省空間,和擴容相反
* 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;
int hdrlen, oldhdrlen = sdsHdrSize(oldtype);
size_t len = sdslen(s);
sh = (char*)s-oldhdrlen;
/* Check what would be the minimum SDS header that is just good enough to
* fit this string. */
type = sdsReqType(len);
hdrlen = sdsHdrSize(type);
/* If the type is the same, or at least a large enough type is still
* required, we just realloc(), letting the allocator to do the copy
* only if really needed. Otherwise if the change is huge, we manually
* reallocate the string to use the different header 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;
}
sdsAllocSize()函式返回sds的總大小
/* 返回sds對應的總大小
* Return the total size of the allocation of the specifed sds string,
* including:
* 1) The sds header before the pointer.
* 2) The string.
* 3) The free buffer at the end if any.
* 4) The implicit null term.
*/
size_t sdsAllocSize(sds s) {
size_t alloc = sdsalloc(s);
return sdsHdrSize(s[-1])+alloc+1;
}
sdsAllocPtr()函式返回sds位置的指標
/* Return the pointer of the actual SDS allocation (normally SDS strings
* are referenced by the start of the string buffer). */
void *sdsAllocPtr(sds s) {
return (void*) (s-sdsHdrSize(s[-1]));
}
sdsIncrLen()函式:增加sds的長度,減少剩餘空間的大小
/* 增加sds長度,並減少剩餘空間大小
* Increment the sds length and decrements the left free space at the
* end of the string according to 'incr'. Also set the null term
* in the new end of the string.
*
* This function is used in order to fix the string length after the
* user calls sdsMakeRoomFor(), writes something after the end of
* the current string, and finally needs to set the new length.
*
* Note: it is possible to use a negative increment in order to
* right-trim the string.
*
* Usage example:
*
* Using sdsIncrLen() and sdsMakeRoomFor() it is possible to mount the
* following schema, to cat bytes coming from the kernel to the end of an
* sds string without copying into an intermediate buffer:
*
* oldlen = sdslen(s);
* s = sdsMakeRoomFor(s, BUFFER_SIZE);
* nread = read(fd, s+oldlen, BUFFER_SIZE);
* ... check for nread <= 0 and handle it ...
* sdsIncrLen(s, nread);
*/
void sdsIncrLen(sds s, ssize_t incr) {
unsigned char flags = s[-1];
size_t len;
switch(flags&SDS_TYPE_MASK) {
case SDS_TYPE_5: {
unsigned char *fp = ((unsigned char*)s)-1;
unsigned char oldlen = SDS_TYPE_5_LEN(flags);
assert((incr > 0 && oldlen+incr < 32) || (incr < 0 && oldlen >= (unsigned int)(-incr)));
*fp = SDS_TYPE_5 | ((oldlen+incr) << SDS_TYPE_BITS);
len = oldlen+incr;
break;
}
case SDS_TYPE_8: {
SDS_HDR_VAR(8,s);
assert((incr >= 0 && sh->alloc-sh->len >= incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
len = (sh->len += incr);
break;
}
case SDS_TYPE_16: {
SDS_HDR_VAR(16,s);
assert((incr >= 0 && sh->alloc-sh->len >= incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
len = (sh->len += incr);
break;
}
case SDS_TYPE_32: {
SDS_HDR_VAR(32,s);
assert((incr >= 0 && sh->alloc-sh->len >= (unsigned int)incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
len = (sh->len += incr);
break;
}
case SDS_TYPE_64: {
SDS_HDR_VAR(64,s);
assert((incr >= 0 && sh->alloc-sh->len >= (uint64_t)incr) || (incr < 0 && sh->len >= (uint64_t)(-incr)));
len = (sh->len += incr);
break;
}
default: len = 0; /* Just to avoid compilation warnings. */
}
s[len] = '\0';
}
sdsgrowzero()函式將sds增長len,正常的部分填充0.
/* 將sds增長至某值,增長部分填充0
* 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;
}
sdscatlen()函式將字串t增加在sds之後。
/* 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;
}
sdscat()函式和上面函式的區別在於不輸入長度len,而使用strlen獲取長度
/* 在sds之後增加一個字串t
* Append the specified null termianted 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));
}
sdscatsds()函式在已有的sds之後加上另一個sds
/* 在已有的sds之後新增另一個sds
* Append the specified sds 't' to the existing sds 's'.
*
* 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. */
sds sdscatsds(sds s, const sds t) {
return sdscatlen(s, t, sdslen(t));
}
sdscpylen()使用t代替已有的s
/* 將已有的s內容用t代替
* Destructively modify the sds string 's' to hold the specified binary
* safe string pointed by 't' of length 'len' bytes. */
sds sdscpylen(sds s, const char *t, size_t len) {
if (sdsalloc(s) < len) {
s = sdsMakeRoomFor(s,len-sdslen(s));
if (s == NULL) return NULL;
}
memcpy(s, t, len);
s[len] = '\0';
sdssetlen(s, len);
return s;
}
sdscpy()函式