CODEWITHSUNDEEP
c++boostboost-asio
Jeka
Jeka

Reputation: 1384

Boost streambuf and write weird behaviour

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?

Upvotes: 1

Views: 198

Answers (1)

Tanner Sansbury
Tanner Sansbury

Reputation: 51971

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.

Upvotes: 4

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