/* 实现内存复制的函数 */
void* my_memcpy(void* dest, const void* src, int len)
{
    if(dest == NULL || src == NULL || len < 0)
    {
        return NULL;
    }
    void *ret = dest;
    char* p_dest = (char*)dest;
    char* p_src = (char*)src;
    /* 判断内存地址是否重合，如果内存重合就从高位开始拷贝 */
    if(p_dest > p_src && p_src+len > p_dest)
    {
        p_dest = p_dest + len - 1;
        p_src = p_src + len - 1;    
		while(len--)
        {
            *p_dest-- = *p_src--;
        }
    }
    /* 如果没有重合就从低位开始拷贝 */
    else 
    {
        while(len--)
        {
            *p_dest++ = *p_src++;
        }
    }
    return ret;
}

char *my_strcpy(char *strDest, const char *strSrc) 
{ 
    char *ptr; 
    if(strDest == NULL || strSrc == NULL) 
        exit(-1); 
    ptr = strDest; 
    while(*strSrc != '\0') 
    { 
        *strDest++ = *strSrc++; 
    } 
    *strDest = '\0'; 
    return ptr; 
}