标签：des style class blog c code
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 <ioe@informatik.tu-chemnitz.de>
* - 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 <jason@topic.com.au>,
* Matthew Hawkins <matt@mh.dropbear.id.au>
* - 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);
}
数据结构——算法之（012）（ linux C 所有字符串操作函数实现）,布布扣,bubuko.com
数据结构——算法之（012）（ linux C 所有字符串操作函数实现）
标签：des style class blog c code
原文地址：http://blog.csdn.net/mr_chenping/article/details/26703281
