数据结构——算法之（012）（ linux C 所有字符串操作函数实现）

题目：实现linux C下常用的字符串操作函数

题目分析：

一、面试中可能经常遇到这样的问题：比如strcpy、memcpy、strstr

二、参考了linux 内核代码，对linux大神表示感谢，代码写得相当精致，这里拿来与大家分享吧

算法实现：

/* *  linux/lib/string.c * *  Copyright (C) 1991, 1992  Linus Torvalds *//* * stupid library routines.. The optimized versions should generally be found * as inline code in <asm-xx/string.h> * * These are buggy as well.. * * * Fri Jun 25 1999, Ingo Oeser <[e-mail]> * -  Added strsep() which will replace strtok() soon (because strsep() is *    reentrant and should be faster). Use only strsep() in new code, please. * * * Sat Feb 09 2002, Jason Thomas <[e-mail]>, *                    Matthew Hawkins <[e-mail]> * -  Kissed strtok() goodbye */#include "string_fun.h"/** * 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, unsigned int len){/* Yes, Virginia, it had better be unsigned */unsigned char c1, c2;if (!len)return 0;do {c1 = *s1++;c2 = *s2++;if (!c1 || !c2)break;if (c1 == c2)continue;c1 = tolower(c1);c2 = tolower(c2);if (c1 != c2)break;} while (--len);return (int)c1 - (int)c2;}int strcasecmp(const char *s1, const char *s2){int c1, c2;do {c1 = tolower(*s1++);c2 = tolower(*s2++);} while (c1 == c2 && c1 != 0);return c1 - c2;}int strncasecmp(const char *s1, const char *s2, unsigned int n){int c1, c2;do {c1 = tolower(*s1++);c2 = tolower(*s2++);} while ((--n > 0) && c1 == c2 && c1 != 0);return c1 - c2;}/** * 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;}/** * 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 * * The result is not %NUL-terminated if the source exceeds * @count bytes. * * In the case where the length of @src is less than  that  of * count, the remainder of @dest will be padded with %NUL. * */char *strncpy(char *dest, const char *src, unsigned int count){char *tmp = dest;while (count)     {if ((*tmp = *src) != 0)src++;tmp++;count--;}return dest;}/** * 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;}/** * 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, unsigned int count){char *tmp = dest;if (count)    {while (*dest)dest++;while ((*dest++ = *src++) != 0)         {if (--count == 0)            {*dest = '\0';break;}}}return tmp;}/** * strcmp - Compare two strings * @cs: One string * @ct: Another string */int strcmp(const char *cs, const char *ct){unsigned char c1, c2;while (1)     {c1 = *cs++;c2 = *ct++;if (c1 != c2)return c1 < c2 ? -1 : 1;if (!c1)break;}return 0;}/** * 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, unsigned int count){unsigned char c1, c2;while (count)     {c1 = *cs++;c2 = *ct++;if (c1 != c2)return c1 < c2 ? -1 : 1;if (!c1)break;count--;}return 0;}/** * 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;}/** * 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;}/** * strnchr - Find a character in a length limited string * @s: The string to be searched * @count: The number of characters to be searched * @c: The character to search for */char *strnchr(const char *s, unsigned int count, int c){for (; count-- && *s != '\0'; ++s)if (*s == (char)c)return (char *)s;return NULL;}/** * skip_spaces - Removes leading whitespace from @str. * @str: The string to be stripped. * * Returns a pointer to the first non-whitespace character in @str. */ char *skip_spaces(const char *str){while (isspace(*str))++str;return (char *)str;}/** * strim - Removes leading and trailing whitespace from @s. * @s: The string to be stripped. * * Note that the first trailing whitespace is replaced with a %NUL-terminator * in the given string @s. Returns a pointer to the first non-whitespace * character in @s. */char *strim(char *s){unsigned int size;char *end;size = strlen(s);if (!size)return s;end = s + size - 1;while (end >= s && isspace(*end))end--;*(end + 1) = '\0';return skip_spaces(s);}/** * strlen - Find the length of a string * @s: The string to be sized */unsigned int strlen(const char *s){const char *sc;for (sc = s; *sc != '\0'; ++sc)/* nothing */;return sc - s;}/** * 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 */unsigned int strspn(const char *s, const char *accept){const char *p;const char *a;unsigned int 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;}/** * strcspn - Calculate the length of the initial substring of @s which does not contain letters in @reject * @s: The string to be searched * @reject: The string to avoid */unsigned int strcspn(const char *s, const char *reject){const char *p;const char *r;unsigned int count = 0;for (p = s; *p != '\0'; ++p)     {for (r = reject; *r != '\0'; ++r)         {if (*p == *r)return count;}++count;}return count;}/** * 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;}/** * 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;char *end;if (sbegin == NULL)return NULL;end = strpbrk(sbegin, ct);if (end)*end++ = '\0';*s = end;return sbegin;}/** * 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, unsigned int count){char *xs = s;while (count--)*xs++ = c;return s;}/** * 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, unsigned int count){char *tmp = dest;const char *s = src;while (count--)*tmp++ = *s++;return dest;}/** * 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, unsigned int count){char *tmp;const char *s;if (dest <= src)    {tmp = dest;s = src;while (count--)*tmp++ = *s++;}     else     {tmp = dest;tmp += count;s = src;s += count;while (count--)*--tmp = *--s;}return dest;}/** * 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, unsigned int 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;}/** * 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, unsigned int size){unsigned char *p = addr;while (size)     {if (*p == c)return (void *)p;p++;size--;}  return (void *)p;}/** * 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){unsigned 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;}/** * strnstr - Find the first substring in a length-limited string * @s1: The string to be searched * @s2: The string to search for * @len: the maximum number of characters to search */char *strnstr(const char *s1, const char *s2, unsigned int len){unsigned int l2;l2 = strlen(s2);if (!l2)return (char *)s1;while (len >= l2)     {len--;if (!memcmp(s1, s2, l2))return (char *)s1;s1++;}return NULL;}/** * 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, unsigned int n){const unsigned char *p = s;while (n-- != 0)     {        if ((unsigned char)c == *p++)return (void *)(p - 1);}return NULL;}/** * memchr - Put the string into an integer. * @s: The string * * returns int which come from @s */int atoi(const char *s){    if(!s || ((*s != '-') && !isdigit(*s)))        return 0;    int value = 0;const char *t = s;    char c = *s;    do{        if(isdigit(c))            value = value*10 + c-'0';        if(s-t>0 && !isdigit(*s))break;++s;    }while((c = *s) != '\0');        return (*t == '-'? -value: value);}/** * memchr - Put the string into an long int. * @s: The string * * returns long int which come from @s */long atol(const char *s){const char *p = skip_spaces(s);    if(!p || ((*p != '-') && !isdigit(*p)))        return 0;    long value = 0;const char *t = p;    char c = *p;    do{        if(isdigit(c))            value = value*10 + c-'0';        if(p-t>0 && !isdigit(*p))break;++p;    }while((c = *p) != '\0');        return (*t == '-'? -value: value);}