string函数库的原型

#ifndef __HAVE_ARCH_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++) != '')
                /* nothing */;
        return tmp;
}

/**
 * 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, size_t count)
{
        char *tmp = dest;
        const char *s = src;

        while (count--)
                *tmp++ = *s++;
        return dest;
}

 strcpy和memcpy主要有以下3方面的区别。
1、复制的内容不同。strcpy只能复制字符串，而memcpy可以复制任意内容，例如字符数组、整型、结构体、类等。
2、复制的方法不同。strcpy不需要指定长度，它遇到被复制字符的串结束符""才结束，所以容易溢出。memcpy则是根据其第3个参数决定复制的长度。
3、用途不同。通常在复制字符串时用strcpy，而需要复制其他类型数据时则一般用memcpy

#ifndef __HAVE_ARCH_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 - 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;
}

/**
 * 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)
                /* nothing */;
        return sc - s;
}

/**
 * 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, size_t count)
{
        char *xs = s;

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

/**
 * 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 - 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;
}