为什么sizeof char返回4？

Question

我想将断开连接类型的断开数据包作为一个字节发送。 我做了结构等等。

struct NET_PROTO_HEADER
{
    unsigned int mLength;
    unsigned short mPacketGroup;
    unsigned short mPacketType;
};

struct NET3_SERVER_DISCONNECT : public NET_PROTO_HEADER
{
    unsigned char mType;
};

但是当我尝试发送它时，sizeof方法返回大小为12，并且它在末尾以addonitional nulls发送。

NET3_SERVER_DISCONNECT data;
    data.mLength = 9;
    data.mPacketGroup = 3;
    data.mPacketType = 100;
    data.mType = 0xcb;
                    boost::asio::async_write(socket_, boost::asio::buffer((char*)&data, sizeof(data)),
                        boost::bind(&Connection::handle_write, shared_from_this(),
                        boost::asio::placeholders::error,
                        boost::asio::placeholders::bytes_transferred));

我在这里做错了什么？ 感谢。