[asio-users] async_read zero bytes transferred

[Tom B. <tom...@bd...>]

Using boost asio 1.69.0 on Windows. Planning to port to Linux.

I'm trying to send a binary command to a serial device and get a binary reply. The device always receives the command and sends the reply. However, sometimes my async_read handler is called with 0 bytes transferred instead of with the reply. Inserting a small delay before the async_read works around the problem, although obviously that is not a reliable fix. Retrying the async_read is a better workaround.

It looks like the problem is in boost::asio::detail::read_op<AsyncReadStream, MutableBufferSequence, MutableBufferIterator, CompletionCondition, ReadHandler>

    void operator()(const boost::system::error_code& ec,
        std::size_t bytes_transferred, int start = 0)
    {
      std::size_t max_size;
      switch (start_ = start)
      {
        case 1:
        max_size = this->check_for_completion(ec, buffers_.total_consumed());
        do
        {
          stream_.async_read_some(buffers_.prepare(max_size),
              BOOST_ASIO_MOVE_CAST(read_op)(*this));
          return; default:
          buffers_.consume(bytes_transferred);
          if ((!ec && bytes_transferred == 0) || buffers_.empty())
            break;
          max_size = this->check_for_completion(ec, buffers_.total_consumed());
        } while (max_size > 0);

        handler_(ec, buffers_.total_consumed());
      }
    }

If (!ec && bytes_transferred == 0) the asynchronous operation terminates before any data is received. This is contrary to the async_read documentation.

In our workaround, retrying the async_read means we are basically polling for the reply. That is okay when we know a reply is coming within a few milliseconds. But we also need to handle an event from the device that it sends after a relatively long time. It would be good if we don't have to poll for that message.

Is there an asio API we can use to get a characters available event callback from a serial port?

If not, is there any chance that async_read can be changes so it will read the requested amount of data before completion?

Thanks,

  Tom



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