串

之前做过一些项目，用C++或B写代码，都是直接用自带的字符串函数，很少涉及到底层；

关于串，我参考linux下的string.c文件，具体了解串的基本函数的操作。

下面是linux下string.c的源代码，虽然简单，但是个人感觉很多细节，大神处理的就是好：

#include <linux/types.h>#include <linux/string.h>#include <linux/ctype.h>#ifndef __HAVE_ARCH_STRNICMP/** * strnicmp - Case insensitive, length-limited string comparison * @s1: One string * @s2: The other string * @len: the maximum number of characters to compare */int strnicmp(const char *s1, const char *s2, size_t len){/* Yes, Virginia, it had better be unsigned */unsigned char c1, c2;c1 = 0;c2 = 0;if (len) {do {c1 = *s1; c2 = *s2;s1++; s2++;if (!c1)break;if (!c2)break;if (c1 == c2)continue;c1 = tolower(c1);c2 = tolower(c2);if (c1 != c2)break;} while (--len);}return (int)c1 - (int)c2;}#endifchar * ___strtok;#ifndef __HAVE_ARCH_STRCPY/** * strcpy - Copy a %NUL terminated string * @dest: Where to copy the string to * @src: Where to copy the string from */char * strcpy(char * dest,const char *src){char *tmp = dest;while ((*dest++ = *src++) != '\0')/* nothing */;return tmp;}#endif#ifndef __HAVE_ARCH_STRNCPY/** * strncpy - Copy a length-limited, %NUL-terminated string * @dest: Where to copy the string to * @src: Where to copy the string from * @count: The maximum number of bytes to copy * * Note that unlike userspace strncpy, this does not %NUL-pad the buffer. * However, the result is not %NUL-terminated if the source exceeds * @count bytes. */char * strncpy(char * dest,const char *src,size_t count){char *tmp = dest;while (count-- && (*dest++ = *src++) != '\0')/* nothing */;return tmp;}#endif#ifndef __HAVE_ARCH_STRCAT/** * strcat - Append one %NUL-terminated string to another * @dest: The string to be appended to * @src: The string to append to it */char * strcat(char * dest, const char * src){char *tmp = dest;while (*dest)dest++;while ((*dest++ = *src++) != '\0');return tmp;}#endif#ifndef __HAVE_ARCH_STRNCAT/** * strncat - Append a length-limited, %NUL-terminated string to another * @dest: The string to be appended to * @src: The string to append to it * @count: The maximum numbers of bytes to copy * * Note that in contrast to strncpy, strncat ensures the result is * terminated. */char * strncat(char *dest, const char *src, size_t count){char *tmp = dest;if (count) {while (*dest)dest++;while ((*dest++ = *src++)) {if (--count == 0) {*dest = '\0';break;}}}return tmp;}#endif#ifndef __HAVE_ARCH_STRCMP/** * strcmp - Compare two strings * @cs: One string * @ct: Another string */int strcmp(const char * cs,const char * ct){register signed char __res;while (1) {if ((__res = *cs - *ct++) != 0 || !*cs++)break;}return __res;}#endif#ifndef __HAVE_ARCH_STRNCMP/** * strncmp - Compare two length-limited strings * @cs: One string * @ct: Another string * @count: The maximum number of bytes to compare */int strncmp(const char * cs,const char * ct,size_t count){register signed char __res = 0;while (count) {if ((__res = *cs - *ct++) != 0 || !*cs++)break;count--;}return __res;}#endif#ifndef __HAVE_ARCH_STRCHR/** * strchr - Find the first occurrence of a character in a string * @s: The string to be searched * @c: The character to search for */char * strchr(const char * s, int c){for(; *s != (char) c; ++s)if (*s == '\0')return NULL;return (char *) s;}#endif#ifndef __HAVE_ARCH_STRRCHR/** * strrchr - Find the last occurrence of a character in a string * @s: The string to be searched * @c: The character to search for */char * strrchr(const char * s, int c){       const char *p = s + strlen(s);       do {           if (*p == (char)c)               return (char *)p;       } while (--p >= s);       return NULL;}#endif#ifndef __HAVE_ARCH_STRLEN/** * strlen - Find the length of a string * @s: The string to be sized */size_t strlen(const char * s){const char *sc;for (sc = s; *sc != '\0'; ++sc)/* nothing */;return sc - s;}#endif#ifndef __HAVE_ARCH_STRNLEN/** * strnlen - Find the length of a length-limited string * @s: The string to be sized * @count: The maximum number of bytes to search */size_t strnlen(const char * s, size_t count){const char *sc;for (sc = s; count-- && *sc != '\0'; ++sc)/* nothing */;return sc - s;}#endif#ifndef __HAVE_ARCH_STRSPN/** * strspn - Calculate the length of the initial substring of @s which only * contain letters in @accept * @s: The string to be searched * @accept: The string to search for */size_t strspn(const char *s, const char *accept){const char *p;const char *a;size_t count = 0;for (p = s; *p != '\0'; ++p) {for (a = accept; *a != '\0'; ++a) {if (*p == *a)break;}if (*a == '\0')return count;++count;}return count;}#endif#ifndef __HAVE_ARCH_STRPBRK/** * strpbrk - Find the first occurrence of a set of characters * @cs: The string to be searched * @ct: The characters to search for */char * strpbrk(const char * cs,const char * ct){const char *sc1,*sc2;for( sc1 = cs; *sc1 != '\0'; ++sc1) {for( sc2 = ct; *sc2 != '\0'; ++sc2) {if (*sc1 == *sc2)return (char *) sc1;}}return NULL;}#endif#ifndef __HAVE_ARCH_STRTOK/** * strtok - Split a string into tokens * @s: The string to be searched * @ct: The characters to search for * * WARNING: strtok is deprecated, use strsep instead. */char * strtok(char * s,const char * ct){char *sbegin, *send;sbegin  = s ? s : ___strtok;if (!sbegin) {return NULL;}sbegin += strspn(sbegin,ct);if (*sbegin == '\0') {___strtok = NULL;return( NULL );}send = strpbrk( sbegin, ct);if (send && *send != '\0')*send++ = '\0';___strtok = send;return (sbegin);}#endif#ifndef __HAVE_ARCH_STRSEP/** * strsep - Split a string into tokens * @s: The string to be searched * @ct: The characters to search for * * strsep() updates @s to point after the token, ready for the next call. * * It returns empty tokens, too, behaving exactly like the libc function * of that name. In fact, it was stolen from glibc2 and de-fancy-fied. * Same semantics, slimmer shape. ;) */char * strsep(char **s, const char *ct){char *sbegin = *s, *end;if (sbegin == NULL)return NULL;end = strpbrk(sbegin, ct);if (end)*end++ = '\0';*s = end;return sbegin;}#endif#ifndef __HAVE_ARCH_MEMSET/** * memset - Fill a region of memory with the given value * @s: Pointer to the start of the area. * @c: The byte to fill the area with * @count: The size of the area. * * Do not use memset() to access IO space, use memset_io() instead. */void * memset(void * s,int c,size_t count){char *xs = (char *) s;while (count--)*xs++ = c;return s;}#endif#ifndef __HAVE_ARCH_BCOPY/** * bcopy - Copy one area of memory to another * @src: Where to copy from * @dest: Where to copy to * @count: The size of the area. * * Note that this is the same as memcpy(), with the arguments reversed. * memcpy() is the standard, bcopy() is a legacy BSD function. * * You should not use this function to access IO space, use memcpy_toio() * or memcpy_fromio() instead. */void bcopy(const void * srcp, void * destp, size_t count){const char *src = srcp;char *dest = destp;while (count--)*dest++ = *src++;}#endif#ifndef __HAVE_ARCH_MEMCPY/** * memcpy - Copy one area of memory to another * @dest: Where to copy to * @src: Where to copy from * @count: The size of the area. * * You should not use this function to access IO space, use memcpy_toio() * or memcpy_fromio() instead. */void * memcpy(void * dest,const void *src,size_t count){char *tmp = (char *) dest, *s = (char *) src;while (count--)*tmp++ = *s++;return dest;}#endif#ifndef __HAVE_ARCH_MEMMOVE/** * memmove - Copy one area of memory to another * @dest: Where to copy to * @src: Where to copy from * @count: The size of the area. * * Unlike memcpy(), memmove() copes with overlapping areas. */void * memmove(void * dest,const void *src,size_t count){char *tmp, *s;if (dest <= src) {tmp = (char *) dest;s = (char *) src;while (count--)*tmp++ = *s++;}else {tmp = (char *) dest + count;s = (char *) src + count;while (count--)*--tmp = *--s;}return dest;}#endif#ifndef __HAVE_ARCH_MEMCMP/** * memcmp - Compare two areas of memory * @cs: One area of memory * @ct: Another area of memory * @count: The size of the area. */int memcmp(const void * cs,const void * ct,size_t count){const unsigned char *su1, *su2;int res = 0;for( su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)if ((res = *su1 - *su2) != 0)break;return res;}#endif#ifndef __HAVE_ARCH_MEMSCAN/** * memscan - Find a character in an area of memory. * @addr: The memory area * @c: The byte to search for * @size: The size of the area. * * returns the address of the first occurrence of @c, or 1 byte past * the area if @c is not found */void * memscan(void * addr, int c, size_t size){unsigned char * p = (unsigned char *) addr;while (size) {if (*p == c)return (void *) p;p++;size--;}  return (void *) p;}#endif#ifndef __HAVE_ARCH_STRSTR/** * strstr - Find the first substring in a %NUL terminated string * @s1: The string to be searched * @s2: The string to search for */char * strstr(const char * s1,const char * s2){int l1, l2;l2 = strlen(s2);if (!l2)return (char *) s1;l1 = strlen(s1);while (l1 >= l2) {l1--;if (!memcmp(s1,s2,l2))return (char *) s1;s1++;}return NULL;}#endif#ifndef __HAVE_ARCH_MEMCHR/** * memchr - Find a character in an area of memory. * @s: The memory area * @c: The byte to search for * @n: The size of the area. * * returns the address of the first occurrence of @c, or %NULL * if @c is not found */void *memchr(const void *s, int c, size_t n){const unsigned char *p = s;while (n-- != 0) {        if ((unsigned char)c == *p++) {return (void *)(p-1);}}return NULL;}#endif