redis源码分析——3简单动态字符串

Posted 2021-09-23 毛毛and西西

一、相关定义

1. sds的定义

   typedef char *sds;
   

2. sdshdr的定义

   /* 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[];
   };
   

3. 柔性数组

   柔性数组成员（flexible array member）也叫伸缩性数组成员，这种代码结构产生于对动态结构体的需求。在日常的编程中，有时候需要在结构体中存放一个长度动态的字符串，一般的做法，是在结构体中定义一个指针成员，这个指针成员指向该字符串所在的动态内存空间，例如：

   struct s_test
   {
       int a;
       double b;
       char* p;
   };
   

   p指向字符串，这种方法造成字符串与结构体是分离的，不利于操作。把字符串和结构体连在一起的话，效果会更好，可以修改如下：

   char a[] = "Hello world";
   struct s_test *ptest = (struct s_test*)malloc(sizeof(s_test)+streln(a)+1);
   strcpy(ptest+1,a);
   

   这样一来，(char*)(ptestt + 1)就是字符串“hello world”的地址。这时候p成了多余的东西，可以去掉。但是，又产生了另外一个问题：老是使用(char*)(ptestt + 1)不方便。如果能够找出一种方法，既能直接引用该字符串，又不占用结构体的空间，就完美了，符合这种条件的代码结构应该是一个非对象的符号地址，在结构体的尾部放置一个0长度的数组是一个绝妙的解决方案。不过，C/C++标准规定不能定义长度为0的数组，因此，有些编译器就把0长度的数组成员作为自己的非标准扩展，例如：

   struct s_test2
   {
       int a;
       double b;
       char c[0];
   };
   

   c就叫柔性数组成员，如果把ptest指向的动态分配内存看作一个整体，c就是一个长度可以动态变化的结构体成员，柔性一词来源于此。c的长度为0，因此它不占用test的空间，同时ptest->c就是“hello world”的首地址，不需要再使用(char*)(ptestt + 1)这么丑陋的语法了。

二、redis string的最大长度

1. 官网介绍

   Strings are the most basic kind of Redis value. Redis Strings are binary safe, this means that a Redis string can contain any kind of data, for instance a JPEG image or a serialized Ruby object.
   
   A String value can be at max 512 Megabytes in length.
   

   那么就会有一个问题，既然最大是512Mb，那为什么还定义sdshdr64结构？

2. processMultibulkBuffer中的定义

   int processMultibulkBuffer(client *c) {
       char *newline = NULL;
       int ok;
       long long ll;
       /****************省略代码**************/
        while(c->multibulklen) {
            /****************省略代码**************/
            ok = string2ll(c->querybuf+c->qb_pos+1,newline-(c->querybuf+c->qb_pos+1),&ll);
            if (!ok || ll < 0 || ll > server.proto_max_bulk_len) {  // 这里对长度做了判断
            	addReplyError(c,"Protocol error: invalid bulk length");
            	setProtocolError("invalid bulk length",c);
            	return C_ERR;
            }
            /****************省略代码**************/
        }
   }     
   

3. proto-max-bulk-len配置

   createLongLongConfig("proto-max-bulk-len", NULL, MODIFIABLE_CONFIG, 0, LLONG_MAX, server.proto_max_bulk_len, 512ll*1024*1024, MEMORY_CONFIG, NULL, NULL), /* Bulk request max size */
   

三、sds扩容策略

1. 当长度小于1Mb时，每次增长翻倍，当大于1Mb时，每次增长1Mb

   /* 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;	// 翻倍
       else
           newlen += SDS_MAX_PREALLOC; // 增长1Mb
   
       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;
   }