1.sds介绍
- sds:Simple Dynamic String即简单动态字符串，为redis的默认字符串表示。
2.sds.h源码分析
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sds结构体的内存布局
/* 0长度数组的妙用 * ---------------- * | len | * |——————————————| * | alloc | * |——————————————| * | flags | * |——————————————|<----buf * | buf[0~n] | * | . | * | . | | . | * |——————————————| */ -
sds结构体的定义
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sds结构体定义在src/sds.h，其中根据buf[]中字符串的长度来选择不同的sds结构体
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. */ //sdshdr5与其他sds结构体的区别是：没有alloc字段，即无法获得free=alloc-len //sdshdr5中flags字段的低3bits表示sds类型，高5bits位buf的length字段 struct __attribute__ ((__packed__)) sdshdr5 { unsigned char flags; /* 3 lsb of type, and 5 msb of string length */ char buf[];//等价于char buf[0]; }; 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 */ //0长度数组 char buf[]等价于char buf[0] 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 */ //sdsnhdr的类型 unsigned char flags; /* 3 lsb of type, 5 unused bits */ char buf[]; }; -
sds.h中相关函数
//返回sds的已使用空间字节数即：sds结构体中的len字段 //函数的参数s为sds结构体中buf[]的首地址 static inline size_t sdslen(const sds s) { //s[-1]即:*(s-1) unsigned char flags = s[-1]; //判断结构体的类型 switch(flags&SDS_TYPE_MASK) { case SDS_TYPE_5: return SDS_TYPE_5_LEN(flags);//高5bits表示buf len case SDS_TYPE_8: return __SDS_HR(8,s)->len; case SDS_TYPE_16: return SDS_HDR(16,s)->len; case SDS_TYPE_32: return SDS_HDR(32,s)->len; case SDS_TYPE_64: return SDS_HDR(64,s)->len; } return 0; } //获取可用的buf空间大小，即：free = alloc-len static inline size_t sdsavail(const sds s) { unsigned char flags = s[-1]; switch(flags&SDS_TYPE_MASK) { case SDS_TYPE_5: { return 0; } case SDS_TYPE_8: { SDS_HDR_VAR(8,s); return sh->alloc - sh->len; } case SDS_TYPE_16: { SDS_HDR_VAR(16,s); return sh->alloc - sh->len; } case SDS_TYPE_32: { SDS_HDR_VAR(32,s); return sh->alloc - sh->len; } case SDS_TYPE_64: { SDS_HDR_VAR(64,s); return sh->alloc - sh->len; } } return 0; } //设置sds结构体len static inline void sdssetlen(sds s, size_t newlen) { unsigned char flags = s[-1]; switch(flags&SDS_TYPE_MASK) { case SDS_TYPE_5: { unsigned char *fp = ((unsigned char*)s)-1; *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS); } break; case SDS_TYPE_8: SDS_HDR(8,s)->len = newlen; break; case SDS_TYPE_16: SDS_HDR(16,s)->len = newlen; break; case SDS_TYPE_32: SDS_HDR(32,s)->len = newlen; break; case SDS_TYPE_64: SDS_HDR(64,s)->len = newlen; break; } } static inline void sdsinclen(sds s, size_t inc) { unsigned char flags = s[-1]; switch(flags&SDS_TYPE_MASK) { case SDS_TYPE_5: { unsigned char *fp = ((unsigned char*)s)-1; unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc; *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS); } break; case SDS_TYPE_8: SDS_HDR(8,s)->len += inc; break; case SDS_TYPE_16: SDS_HDR(16,s)->len += inc; break; case SDS_TYPE_32: SDS_HDR(32,s)->len += inc; break; case SDS_TYPE_64: SDS_HDR(64,s)->len += inc; break; } } //alloc不包括\0 /* sdsalloc() = sdsavail() + sdslen() */ static inline size_t sdsalloc(const sds s) { unsigned char flags = s[-1]; switch(flags&SDS_TYPE_MASK) { case SDS_TYPE_5: return SDS_TYPE_5_LEN(flags); case SDS_TYPE_8: return SDS_HDR(8,s)->alloc; case SDS_TYPE_16: return SDS_HDR(16,s)->alloc; case SDS_TYPE_32: return SDS_HDR(32,s)->alloc; case SDS_TYPE_64: return SDS_HDR(64,s)->alloc; } return 0; } //设置sds中alloc字段为newlen static inline void sdssetalloc(sds s, size_t newlen) { unsigned char flags = s[-1]; switch(flags&SDS_TYPE_MASK) { case SDS_TYPE_5: /* Nothing to do, this type has no total allocation info. */ break; case SDS_TYPE_8: SDS_HDR(8,s)->alloc = newlen; break; case SDS_TYPE_16: SDS_HDR(16,s)->alloc = newlen; break; case SDS_TYPE_32: SDS_HDR(32,s)->alloc = newlen; break; case SDS_TYPE_64: SDS_HDR(64,s)->alloc = newlen; break; } }
3.sds.c源码分析
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sdsnew:创建一个包含给定C字符串的sds
ds sdsnewlen(const void *init, size_t initlen) { void *sh; sds s; char type = sdsReqType(initlen); /* Empty strings are usually created in order to append. Use type 8 * since type 5 is not good at this. */ //因为type 5没有free=alloc-len字段，只有len字段 if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8; int hdrlen = sdsHdrSize(type); unsigned char *fp; /* flags pointer. */ sh = s_malloc(hdrlen+initlen+1); //如果init为NULL，则buff内容为0 if (!init) memset(sh, 0, hdrlen+initlen+1); if (sh == NULL) return NULL; s = (char*)sh+hdrlen; //flags字段 fp = ((unsigned char*)s)-1; switch(type) { case SDS_TYPE_5: { //低3bits位type，高5bits位buff的length *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; } } if (initlen && init) memcpy(s, init, initlen); //buff以0结尾 s[initlen] = '\0'; return s; } -
sdsfree:释放给定的sds
void sdsfree(sds s) { if (s == NULL) return; s_free((char*)s-sdsHdrSize(s[-1])); } -
//保证alloc-len >= addlen sds sdsMakeRoomFor(sds s, size_t addlen) { void *sh, *newsh; //avail=alloc-len 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); //如果newlen小于1M,则分配和newlen同样大小的未使用空间 if (newlen < SDS_MAX_PREALLOC) newlen *= 2; else newlen += SDS_MAX_PREALLOC;//如果newlen的长度大于1M，则分配1M的未使用空间. 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); //如果新的sds结构体类型与old sds结构体类型相同 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; } -
sdscat:将给定的C字符串拼接到sds字符串的末尾
ds sdscatlen(sds s, const void *t, size_t len) { size_t curlen = sdslen(s); //确保s所指向的buf[]有足够空间 s = sdsMakeRoomFor(s,len); if (s == NULL) return NULL; memcpy(s+curlen, t, len); sdssetlen(s, curlen+len); s[curlen+len] = '\0'; return s; } -
其他函数注释在参考文献2指向的github仓库查看相关文件。
4.参考文献
- 《Redis设计与实现》
- 《redis_reading》
