字符串函数的实现（第二波）

1 .strcmp字符串比较

功能 ：比较两个字符串涉及对两个字符串对应的字符逐个进行比较，知道发现不匹配为止。最先不匹配的字符中较小的那个字符所在字符串被认为小于另一个字符。
函数原型

返回值：如果str1小于str2，strcmp函数就返回一个小于0的值。如果str1大于str2，函数返回一个大于0的值。如果相等，则函数就返回0；
自己实现的代码：

int my_strcmp(const char *str1, const char *str2)
{
    char *s1 = str1;
    char *s2 = str2;
    while ((*s1 == *s2) && *s2)
    {
        s1++;
        s2++;
    }
    return (*(unsigned char *)s1 - *(unsigned char *)s2);
}

下面是库函数源代码：

int __cdecl strcmp (
        const char * src,
        const char * dst
        )
{
        int ret = 0 ;

        while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
                ++src, ++dst;

        if ( ret < 0 )
                ret = -1 ;
        else if ( ret > 0 )
                ret = 1 ;

        return( ret );
}

2013中返回值是1 0 或-1.

2 .strncmp

另外strncmp就不再实现了 库函数在这里对函数就行了优化，大大提高效率。

int __cdecl strncmp
(
    const char *first,
    const char *last,
    size_t      count
)
{
    size_t x = 0;

    if (!count)
    {
        return 0;
    }

    /*
     * This explicit guard needed to deal correctly with boundary
     * cases: strings shorter than 4 bytes and strings longer than
     * UINT_MAX-4 bytes .
     */
    if( count >= 4 )
    {
        /* unroll by four */
        for (; x < count-4; x+=4)
        {
            first+=4;
            last +=4;

            if (*(first-4) == 0 || *(first-4) != *(last-4))
            {
                return(*(unsigned char *)(first-4) - *(unsigned char *)(last-4));
            }

            if (*(first-3) == 0 || *(first-3) != *(last-3))
            {
                return(*(unsigned char *)(first-3) - *(unsigned char *)(last-3));
            }

            if (*(first-2) == 0 || *(first-2) != *(last-2))
            {
                return(*(unsigned char *)(first-2) - *(unsigned char *)(last-2));
            }

            if (*(first-1) == 0 || *(first-1) != *(last-1))
            {
                return(*(unsigned char *)(first-1) - *(unsigned char *)(last-1));
            }
        }
    }

    /* residual loop */
    for (; x < count; x++)
    {
        if (*first == 0 || *first != *last)
        {
            return(*(unsigned char *)first - *(unsigned char *)last);
        }
        first+=1;
        last+=1;
    }

    return 0;
}

3 .memcpy(地址拷贝)

功能：Copy one area of memory to another
用空类型void接受参数，然后再强制类型转换为char *类型，这样就可以实现字符和数组的拷贝

void *memcpy(void *dest, const void *src, size_t count)
{
    char *tmp = dest;
    const char *s = src;

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

2013VS库中源代码：

void * __cdecl memcpy (
        void * dst,
        const void * src,
        size_t count
        )
{
        void * ret = dst;

        /*
         * copy from lower addresses to higher addresses
         */
        while (count--) {
                *(char *)dst = *(char *)src;
                dst = (char *)dst + 1;
                src = (char *)src + 1;
        }

        return(ret);
}

4 .memmove
在有可能覆盖的情况下从后向前拷贝防止覆盖数据。

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;
}