How to keep a boost asio async read active continuously

Question

I'm building a tcp client that must send requests and read responses, but also must be able to detect incoming data from the tcp server that is not a response to a request - the server can originate a tx/rx sequence.

What is the best way to keep an async read active all the time. I tried the following:

In my "handle_connect" method I start an async read and also an async write. The async read looks like this:


    size_t bytes_transferred = BUFFER_SIZE;
    boost::asio::async_read(m_socket,
        boost::asio::buffer(rcvbuf, bytes_transferred),
        boost::bind(&CClientSock::handle_read, 
        this,
        boost::asio::placeholders::error,
        boost::asio::placeholders::bytes_transferred));

The async write fills a buffer and then starts the write:


    boost::asio::async_write(m_socket,
        boost::asio::buffer(sndbuf, request_length),
        boost::bind(&CClientSock::handle_write, 
        this,
        boost::asio::placeholders::error,
        boost::asio::placeholders::bytes_transferred));

In the async write handler - it does nothing since the async read is already started.

This doesn't work. The data that should be read from the server's response to the client's write never triggers the async read handler. I'm not actually certain the write ever executes...

Can this be done since the reads and writes are not queued in the normal order? Would I have to start a read, then cancel it if I needed to start a write?

Could you show us more relevant code? It's unclear how all the handlers are implemented. — sehe

sehe sehe · Accepted Answer · 2014-09-18T12:56:37

In absense of a clear SSCCE/sample, I'll highlight a few ideas:

to accept "unsollicited" incoming data/requests, you typically use a listener to accept new incoming connections as opposed to awaiting data indefinitely on the same connection
if you need to sequence different asynchronous tasks on the same resource, use a Boost Asio strand: Why do I need strand per connection when using boost::asio?

to keep reading, in Actor Based asynchrony, just post the read operation from within the completion handler for the previous read operation:

 void handle_read(boost::system::error_code ec, size_t bytes_received) {
     if (!ec)
     {

         /* do your usual handling on the incoming data */


         boost::asio::async_read(m_socket,
            boost::asio::buffer(rcvbuf, bytes_transferred),
            boost::bind(&CClientSock::handle_read, 
            this,
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));
     }

 }

How to keep a boost asio async read active continuously

2 Answers