typedef char *sds;

每个sds.h/sdshdr结构表示一个SDS的值 
struct sdshdr{ 
//记录buf数组中已使用的字节的数量 
//等于sds所保存字符串的长度 
int len; 

//记录buf中未使用的数据 
int free; 

//字符数组，用于保存字符串 
} 


* free 属性的值为0,表示这个SDS没有分配任何未使用的空间 
* len 属性长度为5,表示这个SDS保存一个五字节长的字符串 
* buf 属性是一个char类型的数组,数组的前5个字节分别保存了'R','e','d','i','s'五个字符,而最后一个字节则保存了空字符串'\0' 

/* 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[];
};

/* 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;
 
 /* 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;
}
 
/* 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;
 size_t len = sdslen(s);
 sh = (char*)s-sdsHdrSize(oldtype);
 
 type = sdsReqType(len);
 hdrlen = sdsHdrSize(type);
 if (oldtype==type) {
 newsh = s_realloc(sh, hdrlen+len+1);
 if (newsh == NULL) return NULL;
 s = (char*)newsh+hdrlen;
 } 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;
}

127.0.0.1:6379> set test test
OK
127.0.0.1:6379> append test " test"
(integer) 9
127.0.0.1:6379> get test
"test test"
127.0.0.1:6379> setbit test 36 1
(integer) 0
127.0.0.1:6379> get test
"test(test"
127.0.0.1:6379> getrange test -5 -1
"(test"