strncpy函数实现

char* strncpy(char* dst, const char* src, int len);

当len长度大于src长度时， 用'\0'替代

函数实现：

void* memcpy(void* dst, const void* src, int len)
{
	char* p = (char*)dst;
	const char* q = (const char*)src;
	if (dst > q + len || dst < src)
	{
		while (len--)
		{
			*p++ = *q++;
		}
	}
	else
	{
		p = p + len - 1;
		q = q + len - 1;
		while (len--)
		{
			*p-- = *q--;
		}
	}
	return dst;
}

char* strncpy(char* dst, const char* src, int len)
{
	int size = strlen(src);
	char *p = dst;
	if (size > len)
	{
		memcpy(p, src, len);
	}
	else
	{
		memcpy(p, src, size);
		int offset = len - size;
		char *q = p + size;
    	while (offset--)
			*q++ = '\0';
	}
	return dst;
}