Question

在boost :: asio中使用fire和忘记接近udp套接字是否安全？

所以，例如，如果我有这样的代码

 socket.async_send(buffer(somedata1),write_handler);
 socket.async_send(buffer(somedata2),write_handler);
 socket.async_send(buffer(somedata3),write_handler);
 socket.async_send(buffer(somedata4),write_handler);

我保证这不会失败 - 这意味着在接收端点我将获得包含somedata1，somedata2，somedata3，somedata4的4个数据包？

Answer 1

不，它肯定不安全，asio async_ *函数的 none 被记录为“火上浇油”。

basic_datagram_socket::async_send缓冲区的boost asio引用声明：“尽管可以根据需要复制buffers对象，但调用者仍保留底层内存块的所有权，这必须保证它们在处理程序之前保持有效被称为。“

如果您需要“一劳永逸”的方法，那么您需要一个类来管理您的连接并为您缓冲数据包。以下是使用deque缓冲数据包的示例：

class Connection : public boost::enable_shared_from_this<Connection>
{
  boost::asio::ip::udp::socket socket_;
  std::deque<std::vector<char> > tx_queue_;

  /// transmit the packet at the head of the queue
  void transmit()
  {
    socket_.async_send(
      boost::asio::buffer(&tx_queue_.front()[0], tx_queue_.front().size()),
      boost::bind(&Connection::write_callback,
                  boost::weak_ptr<Connection>(shared_from_this()),
                  boost::asio::placeholders::error,
                  boost::asio::placeholders::bytes_transferred));
  }

  /// The function called whenever a write event is received.
  void write_handler(boost::system::error_code const& error,
                     size_t /* bytes_transferred */)
  {
    tx_queue_.pop_front();
    if (error)
      ; // handle the error, it may be a disconnect.
    else
      if (!tx_queue_.empty())
        transmit();
  }

  /// Static callback function.
  /// It ensures that the object still exists and the event is valid
  /// before calling the write handler.
  static void write_callback(boost::weak_ptr<Connection> ptr,
                             boost::system::error_code const& error,
                             size_t bytes_transferred)
  {
    boost::shared_ptr<Connection> pointer(ptr.lock());
    if (pointer && (boost::asio::error::operation_aborted != error))
      pointer->write_handler(error, bytes_transferred);
  }

  /// Private constructor to enusure the class is created as a shared_ptr.
  explicit Connection(boost::asio::io_service& io_service) :
    socket_(io_service),
    tx_queue_()
  {}

public:

  /// Factory method to create an instance of this class.
  static boost::shared_ptr<Connection> create(boost::asio::io_service& io_service)
  { return boost::shared_ptr<Connection>(new Connection(io_service)); }

  /// Destructor, closes the socket to cancel the write callback 
  /// (by calling it with error = boost::asio::error::operation_aborted)
  /// and free the weak_ptr held by the call to bind in the transmit function.
  ~Connection()
  { socket_.close(); }

  /// add the packet to the end of the queue and send it ASAP.
#if defined(BOOST_ASIO_HAS_MOVE)
  void send(std::vector<char>&& packet)
#else
  void send(const std::vector<char>& packet)
#endif
  {
    bool queue_empty(tx_queue_.empty());
    tx_queue_.push_back(packet);
    if (queue_empty)
      transmit();
  }
};

Answer 2

无法保证仅仅因为基础协议不能保证。

只要在socket.async_send()期间提供给缓冲区的底层内存在调用处理程序之前一直有效，并且没有其他线程正在同时调用socket，那么socket.async_send()的条件已经满意，应该是安全的。

对于实现细节，basic_datagram_socket::async_send()将使其服务创建一个非组合操作（reactive_socket_send_op或win_iocp_socket_send_op）。然后该服务将reactor启动操作。某些反应堆实施可能会尝试立即运行该操作;否则，基于操作类型（读或写）将操作添加到特定于套接字文件描述符的队列。操作队列允许对给定的文件描述符进行多次读取或写入操作。

在udp socket上增强asio多个async_send

2 个答案: