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

*

* These are buggy as well..

*

* * Fri Jun 25 1999, Ingo Oeser

* - 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 ,

* Matthew Hawkins

* - 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);

}