Question

I am looking at the asio example in http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/example/timeouts/async_tcp_client.cpp

Here’s what I am having real trouble to understand:

1. Why does handle_read call back start_read again?
2. What happens when the timer expires? I see no callback routine provided to the timer.

void start_read()
{
// Set a deadline for the read operation.
deadline_.expires_from_now(boost::posix_time::seconds(30));

// Start an asynchronous operation to read a newline-delimited message.
boost::asio::async_read_until(socket_, input_buffer_, '\n',
    boost::bind(&client::handle_read, this, _1));   

}

void handle_read(const boost::system::error_code& ec)
{
if (stopped_)
return;

if (!ec)
{
  // Extract the newline-delimited message from the buffer.
  std::string line;
  std::istream is(&input_buffer_);
  std::getline(is, line);

  // Empty messages are heartbeats and so ignored.
  if (!line.empty())
  {
    std::cout << "Received: " << line << "\n";
  }

  start_read();
}
else
{
  std::cout << "Error on receive: " << ec.message() << "\n";

  stop();
}   

}

Answer 1

Why does handle_read call back start_read again?

If it didn’t, the client would only ever read the socket once and then never again. So on a successful read, the client wants to attempt a read again. That makes for perpetual reading of the socket.

What happens when the timer expires? I see no callback routine provided to the timer.

The code is towards the top of the source file:

deadline_.async_wait(boost::bind(&client::check_deadline, this));

The check_deadline() function will close the socket if the deadline has passed.