C ++：boost :: asio：write（）不支持UDP套接字吗？

Question

Trying to use boost::asio::write() to write to a boost::asio::ip::udp::socket object. While I'm busy re-reading the docs to see if I've gone wrong somewhere, can someone confirm if maybe this is not supported? I'm now thinking boost::asio::write() only supports tcp::socket objects, not udp::socket .

This is what I get when I try to compile:

/usr/include/boost/asio/impl/write.ipp: In function ‘size_t
boost::asio::write(SyncWriteStream&, const ConstBufferSequence&, [...cut...]
test/test.cpp:76:   instantiated from here
/usr/include/boost/asio/impl/write.ipp:44: error: ‘class
boost::asio::basic_datagram_socket<boost::asio::ip::udp,
boost::asio::datagram_socket_service<boost::asio::ip::udp> >’ has no member named
‘write_some’

I think I decided to try boost::asio::write() when I read this in the docs:

The send operation may not transmit all of the data to the peer. Consider using the write function if you need to ensure that all data is written before the blocking operation completes.`

...but in going back, I see that text is only in boost::asio::ip::tcp::socket::send() and not in the UDP version. Source.

Answer 1

Assuming you want synchronous behavior, you need to use the send or send_to member methods of the boost::asio::ip::udp::socket class. The boost::asio::write free function uses the SyncWriteStream type requirement, which a UDP socket does not meet.

Answer 2

I use async_send_to instead. Perhaps that's possible for you too?

void MyUdpServer::sendMessage(Message& message)
{
    m_message = message;

    m_socket.async_send_to(
        asio::buffer(&m_message, sizeof(m_message)), m_endpoint,
        boost::bind(&MyUdpServer::handle_send_to, shared_from_this(), 
        asio::placeholders::error));
} 

where m_socket is an asio::ip::udp::socket .

Answer 3

Seems to me that asio::write requires the stream class to have write_some implemented. IIRC, When writing to UDP sockets you want to write everything at one time in order to reduce fragmentation (unless your data + UDP header + ip header size is > PMTU, then you've got some work ahead of you).

Do you have code on the receiving side that reorders/reassembles the UDP packets? You will get UDP packets out of order and fragmented no matter how good your network is.