一些常用字符串操作函数的内部实现

我整理了一下常用的字符串库函数的内部实现，截自linux内核中的lib/string.c文件，绝对标准的程序，供大家参考。

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.
*/
void *memset(void *s, int c, size_t count)
{
char *xs = s;

while (count--)
*xs++ = c;
return s;
}

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.
*/
void *memcpy(void *dest, const void *src, size_t count)
{
char *tmp = dest;
const char *s = src;
while (count--)
*tmp++ = *s++;
return dest;
}

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;
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:

/*
* 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;
}

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');

return tmp;
}

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
*
* 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, size_t count)
{
char *tmp = dest;

while (count) {
if ((*tmp = *src) != 0)
src++;
tmp++;
count--;
}

return dest;
}

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;
}

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++) != 0) {
if (--count == 0) {
*dest = '\0';
break;
}
}
}

return tmp;
}

strcmp:

/*
* 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:

/*
* 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)
{
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:

/*
* 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;
}

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);

return sc - s;
}

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);

return sc - s;
}

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.
*/
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;
}

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;
}

from：http://blog.chinaunix.net/uid-22030783-id-2142080.html