blocking_reader.h

/**
--------------------------------------------------------------------------------
-   Module      :   blocking_reader.h
-   Description :   A class that provides basic support for  
-                   blocking & time-outable single character reads from 
-                   boost::asio::serial_port.
-   Author      :   Unknown
-                   Tim Zaman, 18-FEB-2016
--------------------------------------------------------------------------------
*/

/*

Copyright and licence unknown.
 
*/

/*

Usage:

blocking_reader reader(port, 500);
char c;
if (!reader.read_char(c)){
	return false;
}

*/

#ifndef BLOCKING_READER_H
#define BLOCKING_READER_H

#include <boost/asio/serial_port.hpp> 
#include <boost/bind.hpp>

class blocking_reader
{
	boost::asio::serial_port& port;
	size_t timeout;
	char c;
	boost::asio::deadline_timer timer;
	bool read_error;

	// Called when an async read completes or has been cancelled
	void read_complete(const boost::system::error_code& error,
						size_t bytes_transferred) {		

		read_error = (error || bytes_transferred == 0);
		
		// Read has finished, so cancel the
		// timer.
		timer.cancel();
	}

	// Called when the timer's deadline expires.
	void time_out(const boost::system::error_code& error) {

		// Was the timeout was cancelled?
		if (error) {
			// yes
			return;
		}

		// no, we have timed out, so kill
		// the read operation
		// The read callback will be called
		// with an error
		port.cancel();
	}

public:

	blocking_reader(boost::asio::serial_port& port, size_t timeout) :
												port(port), timeout(timeout),
												timer(port.get_io_service()),
												read_error(true) {
		 
	}

	// Reads a character or times out
	bool read_char(char& val) {
		
		val = c = '\0';

		// After a timeout & cancel it seems we need
		// to do a reset for subsequent reads to work.
		port.get_io_service().reset();

		// Asynchronously read 1 character.
		boost::asio::async_read(port, boost::asio::buffer(&c, 1), 
				boost::bind(&blocking_reader::read_complete, 
						this, 
						boost::asio::placeholders::error, 
						boost::asio::placeholders::bytes_transferred)); 

		// Setup a deadline time to implement our timeout.
		timer.expires_from_now(boost::posix_time::milliseconds(timeout));
		timer.async_wait(boost::bind(&blocking_reader::time_out,
								this, boost::asio::placeholders::error));

		// This will block until a character is read
		// or until the it is cancelled.
		port.get_io_service().run();

		if (!read_error)
			val = c;

		return !read_error;
	}
};


#endif // BLOCKING_READER_H