sds(Simple Dynamic String): 它其实就是普通的字符串，只是在每个字符串的前面加了一个管理用的头部，相关类型结构的定义如下
typedef char *sds; /* 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[]; };
从定义上看，5种定义对应了最大长度不同的字符串，定义这5种不同的类型可能是为了尽量减少sdshdr占用的空间。
为了分sdshdr是哪一种类型，它在每一种定义中都加上了一个8bit的flags字段，用其中的低3位标识sdshdr的类型。
为了能从buf直接找到到flags，所以在定义时必须用((__packed__))强制不要按字节对齐，这样不管是哪种类型的hdr，都可以用buf[-1]找到对应的flags
相应地，sds内部实现了一些基本的操作，主要看看sdsnewlen和sdsMakeRoomFor这两个函数，大概就能知道sds的内部操作方式以及内存分配的方式。
sdsnewlen是根据指定内容和指定长度创建一个新的sds。这段代码有个bug，见下面的注释
/* 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 at the * end of the string. However the string is binary safe and can contain * 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; /* 根据要求的长度返回合适的类型 */ char type = sdsReqType(initlen); /* Empty strings are usually created in order to append. Use type 8 * since type 5 is not good at this. */ /* type 5 不太适合动态扩展，默认用8 */ if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8; /* 不同的type对应的sdshdr的大小不一样 */ int hdrlen = sdsHdrSize(type); unsigned char *fp; /* flags pointer. */ sh = s_malloc(hdrlen+initlen+1); /* 如果没有指定初始内容，则将新分配的空间清0 */ if (!init) memset(sh, 0, hdrlen+initlen+1); /* 这个判断应该放到前面吧，不然上面memset会导致程序挂掉 */ if (sh == NULL) return NULL; /* 找到buf的位置，也是要返回给caller的 */ s = (char*)sh+hdrlen; /* 填充对应sdshdr的flags字段 */ 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; } } /* 指定了初始内容并且内容非空，填充到新分配的空间当中 */ if (initlen && init) memcpy(s, init, initlen); s[initlen] = ' '; return s; }
sdsMakeRoomFor是在已存在的sds上动态扩展其空间，可以很清楚地看到扩展时使用的策略。
/* 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); /* 新长度小于1M就double，否则就增加1M */ if (newlen < SDS_MAX_PREALLOC) newlen *= 2; else newlen += SDS_MAX_PREALLOC; /* newlen对应的sdshdr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. */ /* type 5不适合做扩展 */ if (type == SDS_TYPE_5) type = SDS_TYPE_8; hdrlen = sdsHdrSize(type); /* 如果前后type不需要变化，则直接使用realloc * 否则，分配新的空间，拷贝原来的内容，释放原来的空间 */ 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); } /* 设置sdshdr的alloc字段(type 5除外) */ sdssetalloc(s, newlen); return s; }