typedef struct redisObject {
 unsigned type:4;
 unsigned encoding:4;
 // 当内存超限时采用LRU算法清除内存中的对象
 unsigned lru:LRU_BITS; /* LRU time (relative to global lru_clock) or
       * LFU data (least significant 8 bits frequency
       * and most significant 16 bits access time). */
 // 该对象被引用数
 int refcount;
 // 对象的值指针
 void *ptr;
} robj;

/* A redis object, that is a type able to hold a string / list / set */

/* The actual Redis Object */
#define OBJ_STRING 0 /* 字符串对象. */
#define OBJ_LIST 1  /* 列表对象. */
#define OBJ_SET 2  /* 集合对象. */
#define OBJ_ZSET 3  /* 有序集合对象. */
#define OBJ_HASH 4  /* 哈希对象. */

/* The "module" object type is a special one that signals that the object
 * is one directly managed by a Redis module. In this case the value points
 * to a moduleValue struct, which contains the object value (which is only
 * handled by the module itself) and the RedisModuleType struct which lists
 * function pointers in order to serialize, deserialize, AOF-rewrite and
 * free the object.
 *
 * Inside the RDB file, module types are encoded as OBJ_MODULE followed
 * by a 64 bit module type ID, which has a 54 bits module-specific signature
 * in order to dispatch the loading to the right module, plus a 10 bits
 * encoding version. */
#define OBJ_MODULE 5 /* 模块对象. */
#define OBJ_STREAM 6 /* 流对象. */

/* Objects encoding. Some kind of objects like Strings and Hashes can be
 * internally represented in multiple ways. The 'encoding' field of the object
 * is set to one of this fields for this object. */
#define OBJ_ENCODING_RAW 0  /* 简单动态字符串 */
#define OBJ_ENCODING_INT 1  /* long类型的整数 */
#define OBJ_ENCODING_HT 2  /* 字典 */
#define OBJ_ENCODING_ZIPMAP 3 /* 压缩字典 */
#define OBJ_ENCODING_LINKEDLIST 4 /* 不再使用的旧列表，使用双端链表. */
#define OBJ_ENCODING_ZIPLIST 5 /* 压缩列表 */
#define OBJ_ENCODING_INTSET 6 /* 整数集合 */
#define OBJ_ENCODING_SKIPLIST 7 /* 跳跃表和字典 */
#define OBJ_ENCODING_EMBSTR 8 /* embstr编码的简单动态字符串 */
#define OBJ_ENCODING_QUICKLIST 9 /* 编码为ziplist的列表 */
#define OBJ_ENCODING_STREAM 10 /* 编码为listpacks的基数树 */

/* Create a string object with EMBSTR encoding if it is smaller than
 * OBJ_ENCODING_EMBSTR_SIZE_LIMIT, otherwise the RAW encoding is
 * used.
 *
 * The current limit of 44 is chosen so that the biggest string object
 * we allocate as EMBSTR will still fit into the 64 byte arena of jemalloc. */
#define OBJ_ENCODING_EMBSTR_SIZE_LIMIT 44
robj *createStringObject(const char *ptr, size_t len) {
 if (len = OBJ_ENCODING_EMBSTR_SIZE_LIMIT)
  return createEmbeddedStringObject(ptr,len);
 else
  return createRawStringObject(ptr,len);
}

struct sdshdr {
 // 字符串长度
 int len;
 // 字符数组未使用的字符数量
 int free;
 // 保存字符串的字符数组
 char buf[];
}