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++) != '');

        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++) != '');

        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 = '';
                    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 == '')
                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 != ''; ++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 != ''; ++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++ = '';
        *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