Boost streambuf and write weird behaviour

Question

I am using streambuf as a storage for serial port communication

io_service io;
serial_port sp(io);
sp.open("COM4");
sp.set_option( serial_port_base::baud_rate( 115200 ) );
sp.set_option( serial_port_base::character_size( 8 ) );
sp.set_option( serial_port_base::stop_bits( serial_port_base::stop_bits::one )  );
sp.set_option( serial_port_base::parity( serial_port_base::parity::none ) );
sp.set_option( serial_port_base::flow_control( serial_port_base::flow_control::none )   );
boost::asio::streambuf buffer;
// Fill buffer here
auto size = buffer.size();
std::cout << boost::format("Buffer size: %d") % size << std::endl;
boost::asio::write( sp, buffer );
size = buffer.size();
std::cout << boost::format("Buffer size: %d" ) % size << std::endl;

Output:

Buffer size: 8
Buffer size: 0

Why method size() return different values?

Can you elaborate on why, after you've written out the buffer, you expect it to have a nonzero size? — Mark B, May 27 '16 at 16:39
@MarkB I am expecting that write operation on serial port or socket will not modify my storage buffer. Usually any write methods do not modify source. Maybe I am interpret sreambuf incorrect? — Jeka, May 27 '16 at 16:48

score 4 · Accepted Answer · edited May 23 '17 at 12:16

The boost::asio::write() operation consumes from the streambuf's input sequence, and streambuf::size() returns the size of the input sequence.

In this particular case, the streambuf's input sequence contained 8 bytes before the write() operation. The write() operation blocks until all data from the input sequence has been written or until an error occurs. The write() operation succeeds, consuming 8 bytes from the input sequence, and returns a value to indicate 8 bytes have been transferred. As all of the input sequence has been consumed, streambuf.size() returns 0.

For more information on streambuf usage, consider reading this answer.

Boost streambuf and write weird behaviour

1 Answers1