How to discard data as it is sent with boost::asio?

Question

I'm writing some code that reads and writes to serial device using boost::asio class. However, when sending several strings between programs, I've noticed that on the receiving program the data is read in the order as it was written to the serial port, and not as the data is sent from the other program - If I start reading data some seconds later, I don't get the values that I am sending at the moment but those that were sent previously. I'm assuming this is caused by how I am setting up my boost::asio::serial_port:

int main(int argc, char const *argv[]){

    int baud=atoi(argv[1]);
    std::string pty=argv[2];

    printf("Virtual device: %s\n",pty.data());
    printf("Baud rate: %d\n",baud);

    boost::asio::io_service io;
    boost::asio::serial_port port(io, pty);
    port.set_option(boost::asio::serial_port_base::baud_rate(baud));

    // counter that writes to serial port in 1s intervals
    int val=0;
    while (1){
        std::string data=std::to_string(val);
        data+='\n';
        std::cout << data;
        write(port,boost::asio::buffer(data.c_str(),data.size()));
        sleep(1); 
        val++;
        data.clear();
    }

    port.close(); 
    return 0;
}

Is there a way to force past data to be discarded as soon as a new value is sent to the serial port (which I assume should be done on the write() part of the code)?

Answer 1

Boost.Asio does not provide a higher-level abstraction for flushing a serial port's buffers. However, this can often be accomplished by having platform specific calls, such as tcflush() or PurgeComm(), operate on a serial port's native_handle().

Each serial port has a receive and transmit buffer, and flushing operates on one or both of the buffers. For example, if two serial ports are connected (/dev/pts/3 and /dev/pts/4), and program A opens and writes to /dev/pts/3, then it can only flush the buffers associated with /dev/pts/3 (data received on /dev/pts/3 but not read, and data written to /dev/pts/3 but not transmitted). Therefore, if program B starts, opens /dev/pts/4, and wants to read non-stale data, then program B needs to flush the receive buffer for /dev/pts/4 after opening the serial port.

---

Here is a complete example running on CentOs. When the example runs as a writer, it will write a sequentially increasing number to the serial port once a second. When the example runs as a writer, it will read five numbers, sleep for 5 seconds and flush its read buffer every other iteration:

#include <iostream>
#include <vector>
#include <boost/asio.hpp>
#include <boost/thread.hpp>

/// @brief Different ways a serial port may be flushed.
enum flush_type
{
  flush_receive = TCIFLUSH,
  flush_send = TCOFLUSH,
  flush_both = TCIOFLUSH
};

/// @brief Flush a serial port's buffers.
///
/// @param serial_port Port to flush.
/// @param what Determines the buffers to flush.
/// @param error Set to indicate what error occurred, if any.
void flush_serial_port(
  boost::asio::serial_port& serial_port,
  flush_type what,
  boost::system::error_code& error)
{
  if (0 == ::tcflush(serial_port.lowest_layer().native_handle(), what))
  {
    error = boost::system::error_code();
  }
  else
  {
    error = boost::system::error_code(errno,
        boost::asio::error::get_system_category());
  }
}

/// @brief Reads 5 numbers from the serial port, then sleeps for 5 seconds,
///        flushing its read buffer every other iteration.
void read_main(boost::asio::serial_port& serial_port)
{
  std::vector<unsigned char> buffer(5);
  for (bool flush = false;; flush = !flush)
  {
    std::size_t bytes_transferred =
        read(serial_port, boost::asio::buffer(buffer));
    for (std::size_t i = 0; i < bytes_transferred; ++i)
      std::cout << static_cast<unsigned int>(buffer[i]) << " ";
    boost::this_thread::sleep_for(boost::chrono::seconds(5));
    if (flush)
    {
      boost::system::error_code error;
      flush_serial_port(serial_port, flush_receive, error);
      std::cout << "flush: " << error.message() << std::endl;
    }
    else
    {
      std::cout << "noflush" << std::endl;
    }
  }
}

/// @brief Write a sequentially increasing number to the serial port
///        every second.
void write_main(boost::asio::serial_port& serial_port)
{
  for (unsigned char i = 0; ; ++i)
  {
    write(serial_port, boost::asio::buffer(&i, sizeof i));
    boost::this_thread::sleep_for(boost::chrono::seconds(1));
  }
}

int main(int argc, char* argv[])
{
  boost::asio::io_service io_service;
  boost::asio::serial_port serial_port(io_service, argv[2]);
  if (!strcmp(argv[1], "read"))
    read_main(serial_port);
  else if (!strcmp(argv[1], "write"))
    write_main(serial_port);
}

Create virtual serial ports with socat:

$ socat -d -d PTY: PTY:
2014/03/23 16:22:22 socat[12056] N PTY is /dev/pts/3
2014/03/23 16:22:22 socat[12056] N PTY is /dev/pts/4
2014/03/23 16:22:22 socat[12056] N starting data transfer loop with
                                 FDs [3,3] and [5,5]

Starting both the read and write examples:

$ ./a.out read /dev/pts/3 & ./a.out write /dev/pts/4
[1] 12238
0 1 2 3 4 noflush
5 6 7 8 9 flush: Success
14 15 16 17 18 noflush
19 20 21 22 23 flush: Success
28 29 30 31 32 noflush
33 34 35 36 37 flush: Success

As demonstrating in the output, numbers are only skipped in the sequence when the reader flushes its read buffer: 3 4 noflush 5 6 7 8 9 flush 14 15.