C 语言的字符串函数

　　C 语言 string 函数 ，在 C 语言中可以使用 char* 字符数组实现字符串，C 语言标准库 string.h 也定义了多种字符串操作函数。

　　字符串使用广泛，需要满足：

　　高效的字符串操作，比如追加、拷贝、比较、获取长度能保存任意的二进制数据，比如图片尽可能省内存为什么 Redis 不直接使用 C 语言的字符串？

　　C 语言 char* 以 '\0'标识字符串的结束，则中间含有'\0'的字符串无法被正确表示；也正因为如此，没有办法保存图像等二进制数据。C 语言 char* 获取字符串长度的时间复杂度是 O(N)；追加字符串的时间复杂度也是 O(N)，同时可能由于可用空间不足，无法追加。下面代码展示了 C 语言中 '\0' 结束字符对字符串的影响。下图展示了一个值为 "Redis" 的 C 字符串：

　　#include "stdio.h"#include "string.h"int main(void) { char *a = "red\0is"; char *b = "redis\0"; printf("%lu\n", strlen(a)); printf("%lu\n", strlen(b));}输出结果是 3 和 5。

　　SDS 定义

　　SDS（简单动态字符串） 是 simple dynamic string 的简称，Redis 使用 SDS 作为字符串的数据结构。Redis 中所有的键（key）底层都是 SDS 实现的。

　　比如：

　　redis> SET msg "hello world"OKredis> RPUSH fruits "apple" "banana" "cherry"(integer) 3Redis sds 源码主要在 sds.h 和 sds.c 中。其中可以发现 Redis 给 char* 起了别名：

　　typedef char *sds;SDS 内部结构

　　SDS 结构中有一个元数据 flags，表示的是 SDS 类型（最低 3 位）。事实上，SDS 一共设计了 5 种类型，分别是 sdshdr5、sdshdr8、sdshdr16、sdshdr32 和 sdshdr64。这 5 种类型的主要区别就在于，它们数据结构中的字符数组现有长度 len 和分配空间长度 alloc，这两个元数据的数据类型不同。

　　/* 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[];};

　　static inline size_t sdslen(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)->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;}获取剩余容量：sdsavail 函数，总容量 alloc - 已使用长度 len，时间复杂度是 O(1)。

　　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;}SDS 的主要操作 API

　　基础方法有：

　　sds sdsnewlen(const void *init, size_t initlen);sds sdstrynewlen(const void *init, size_t initlen);sds sdsnew(const char *init);sds sdsempty(void);sds sdsdup(const sds s);void sdsfree(sds s);sds sdsgrowzero(sds s, size_t len);sds sdscatlen(sds s, const void *t, size_t len);sds sdscat(sds s, const char *t);sds sdscatsds(sds s, const sds t);sds sdscpylen(sds s, const char *t, size_t len);sds sdscpy(sds s, const char *t);sds sdscatvprintf(sds s, const char *fmt, va_list ap);#ifdef __GNUC__sds sdscatprintf(sds s, const char *fmt, ...) __attribute__((format(printf, 2, 3)));#elsesds sdscatprintf(sds s, const char *fmt, ...);#ifsds sdscatfmt(sds s, char const *fmt, ...);sds sdstrim(sds s, const char *cset);void sdssubstr(sds s, size_t start, size_t len);void sdsrange(sds s, ssize_t start, ssize_t );void sdsupdatelen(sds s);void sdsclear(sds s);int sdscmp(const sds s1, const sds s2);sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count);void sdsfreesplitres(sds *tokens, int count);void sdstolower(sds s);void sdstoupper(sds s);sds sdsfromlonglong(long long value);sds sdscatrepr(sds s, const char *p, size_t len);sds *sdssplitargs(const char *line, int *argc);sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);sds sdsjoin(char **argv, int argc, char *sep);sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen);/* Callback for sdstemplate. The function gets called by sdstemplate * every time a variable needs to be expanded. The variable name is * provided as variable, and the callback is expected to return a * substitution value. Returning a NULL indicates an error. */typedef sds (*sdstemplate_callback_t)(const sds variable, void *arg);sds sdstemplate(const char *template, sdstemplate_callback_t cb_func, void *cb_arg);/* Low level functions exposed to the user API */sds sdsMakeRoomFor(sds s, size_t addlen);void sdsIncrLen(sds s, ssize_t incr);sds sdsRemoveFreeSpace(sds s);size_t sdsAllocSize(sds s);void *sdsAllocPtr(sds s);/* Export the allocator used by SDS to the program using SDS. * Sometimes the program SDS is linked to, may use a different set of * allocators, but may want to allocate or free things that SDS will * respectively free or allocate. */void *sds_malloc(size_t size);void *sds_realloc(void *ptr, size_t size);void sds_free(void *ptr);字符串初始化

　　整体和 Java 的 StringBuilder 很像了 O_o

　　/* Create a new sds string starting from a null terminated C string. */sds sdsnew(const char *init) { size_t initlen = (init == NULL) ? 0 : strlen(init); return sdsnewlen(init, initlen);}首先是判断输入的 init 字符串的长度，接着调用 sdsnewlen 分配内存空间并赋值。

　　sds sdsnewlen(const void *init, size_t initlen) { return _sdsnewlen(init, initlen, 0);}核心函数_sdsnewlen 如下，主要就是先确保空间是否足够、分配空间，然后再调用 memcpy 将 *init 复制到对应的内存空间。

　　/* 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. * If SDS_NOINIT is used, the buffer is left uninitialized; * * 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 \0 at the * of the string. However the string is binary safe and can contain * \0 characters in the middle, as the length is stored in the sds header. */sds _sdsnewlen(const void *init, size_t initlen, int trymalloc) { void *sh; sds s; char type = sdsReqType(initlen); /* Empty strings are usually created in order to app. Use type 8 * since type 5 is not good at this. */ if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8; int hdrlen = sdsHdrSize(type); unsigned char *fp; /* flags pointer. */ size_t usable; assert(initlen + hdrlen + 1 > initlen); /* Catch size_t overflow */ sh = trymalloc? s_trymalloc_usable(hdrlen+initlen+1, &usable) : s_malloc_usable(hdrlen+initlen+1, &usable); if (sh == NULL) return NULL; if (init==SDS_NOINIT) init = NULL; else if (!init) memset(sh, 0, hdrlen+initlen+1); s = (char*)sh+hdrlen; fp = ((unsigned char*)s)-1; usable = usable-hdrlen-1; if (usable > sdsTypeMaxSize(type)) usable = sdsTypeMaxSize(type); 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 = usable; *fp = type; break; } case SDS_TYPE_16: { SDS_HDR_VAR(16,s); sh->len = initlen; sh->alloc = usable; *fp = type; break; } case SDS_TYPE_32: { SDS_HDR_VAR(32,s); sh->len = initlen; sh->alloc = usable; *fp = type; break; } case SDS_TYPE_64: { SDS_HDR_VAR(64,s); sh->len = initlen; sh->alloc = usable; *fp = type; break; } } if (initlen && init) memcpy(s, init, initlen); s[initlen] = '\0'; return s;}原文 http://www.cnblogs.com/510602159-Yano/p/15603479.html