HardwareSerial::read()

Abstract

The Serial.read()/HardwareSerial::read() reads data from receive buffer.

Source Code

The HardwareSerial::read() is defined in hardware/arduino/avr/cores/arduino/HardwareSerial.cpp as below.

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int HardwareSerial::read(void)
{
  // if the head isn't ahead of the tail, we don't have any characters
  if (_rx_buffer_head == _rx_buffer_tail) {
    return -1;
  } else {
    unsigned char c = _rx_buffer[_rx_buffer_tail];
    _rx_buffer_tail = (rx_buffer_index_t)(_rx_buffer_tail + 1) % SERIAL_RX_BUFFER_SIZE;
    return c;
  }
}

No inputs, output is int.

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int HardwareSerial::read(void)
{

If the head and tail of the buffer is same, there is no data in the buffer, returns -1.

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// if the head isn't ahead of the tail, we don't have any characters
if (_rx_buffer_head == _rx_buffer_tail) {
  return -1;
} else {

If the head and tail of the buffer is not same, reads the oldest data which is stored in buffer[tail], advance _rx_buffer_tail one byte, then return the data.

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} else {
    unsigned char c = _rx_buffer[_rx_buffer_tail];
    _rx_buffer_tail = (rx_buffer_index_t)(_rx_buffer_tail + 1) % SERIAL_RX_BUFFER_SIZE;
    return c;
  }
}

Because the receive buffer is a ring buffer, if the tail gets to the last of the buffer, it should go back to the head of the buffer. To do that, add 1 to tail then get reminder of division by buffer size(SERIAL_RX_BUFFER_SIZE).

To put the received data to the buffer, an interrupt handler named USART_RX_vect is used.

Version

Arduino AVR Boards 1.8.6

License

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/*
  Copyright (c) 2019 garretlab.
  Added source code explanation by garretlab.
*/

/*
  HardwareSerial.cpp - Hardware serial library for Wiring
  Copyright (c) 2006 Nicholas Zambetti.  All right reserved.

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

  Modified 23 November 2006 by David A. Mellis
  Modified 28 September 2010 by Mark Sproul
  Modified 14 August 2012 by Alarus
  Modified 3 December 2013 by Matthijs Kooijman
*/

Last Update

August 25, 2019