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| ```
//创建新的字符串
sds sdsnewlen(const void *init, size_t initlen) {
void *sh;
sds s;
//根据长度获取sds的类型
char type = sdsReqType(initlen);
/* Empty strings are usually created in order to append. Use type 8
* since type 5 is not good at this. */
//sds_type_5这个类型是不会使用的啦.
if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;
//根据类型得到头部的长度，还记得前面的类型声明吗？
int hdrlen = sdsHdrSize(type);
unsigned char *fp; /* flags pointer. */
//分配空间: 头部大小+数据大小+'\0'（空终止符，为了c语言字符串兼容）
sh = s_malloc(hdrlen+initlen+1);
//如果没有初始化数据之间全部清空
if (!init)
memset(sh, 0, hdrlen+initlen+1);
if (sh == NULL) return NULL;
//s指向buf, 返回的sds就是这个s
s = (char*)sh+hdrlen;
fp = ((unsigned char*)s)-1;
switch(type) {
case SDS_TYPE_5: {
//记得吗？对于SDS_TYPE_5类型，长度是放在flags字段里的
*fp = type | (initlen << SDS_TYPE_BITS);
break;
}
case SDS_TYPE_8: {
//这里是新定义了一个相对应类型的sh
SDS_HDR_VAR(8,s);
//设置长度
sh->len = initlen;
//设置分配的大小(排查头部跟空终止符)
sh->alloc = initlen;
//设置类型
*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;
}
}
//如果有初始化值，则执行memcpy复制数据
if (initlen && init)
memcpy(s, init, initlen);
//设置终止符,这里并不会有两个'\0',因为前面只复制了initlen个字符
s[initlen] = '\0';
//返回sds，实际执行buf
return s;
}
void sdsfree(sds s) {
if (s == NULL) return;
//得到真正的头部指向，然后释放
s_free((char*)s-sdsHdrSize(s[-1]));
}
//字符串增长
sds sdsMakeRoomFor(sds s, size_t addlen) {
void *sh, *newsh;
//获取剩余的空间，其实就是alloc-len
size_t avail = sdsavail(s);
size_t len, newlen;
//保持老的类型,因为字符串空间加长后有可能类型变化
char type, oldtype = s[-1] & SDS_TYPE_MASK;
int hdrlen;
/* Return ASAP if there is enough space left. */
//如果空间足够直接返回
if (avail >= addlen) return s;
len = sdslen(s);
sh = (char*)s-sdsHdrSize(oldtype);
newlen = (len+addlen);
//如果小于1024*1024直接翻倍空间
if (newlen < SDS_MAX_PREALLOC)
newlen *= 2;
else
newlen += SDS_MAX_PREALLOC;
//根据长度获取sds的类型
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;
//获取头部长度
hdrlen = sdsHdrSize(type);
if (oldtype==type) {
//如果类型不变
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 */
//如果类型已经改变
newsh = s_malloc(hdrlen+newlen+1);
if (newsh == NULL) return NULL;
//复制数据
memcpy((char*)newsh+hdrlen, s, len+1);
//释放老的数据
s_free(sh);
s = (char*)newsh+hdrlen;
//设置类型s[-1]实际执行flags
s[-1] = type;
//设置新字符串的长度
sdssetlen(s, len);
}
//设置新字符串分配空间大小
sdssetalloc(s, newlen);
return s;
}
```
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