Abstract

The Serial.availableForWrite()/HardwareSerial::availableForWrite() returns how many bytes can be written to transmit buffer of serial communication.

Source Code

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

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int HardwareSerial::availableForWrite(void)
{
  tx_buffer_index_t head;
  tx_buffer_index_t tail;

  TX_BUFFER_ATOMIC {
    head = _tx_buffer_head;
    tail = _tx_buffer_tail;
  }
  if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
  return tail - head - 1;
}

No input, returns int.

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int HardwareSerial::availableForWrite(void)
{

Calculate the number of available bytes in the buffer.

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  tx_buffer_index_t head;
  tx_buffer_index_t tail;
 
  TX_BUFFER_ATOMIC {
    head = _tx_buffer_head;
    tail = _tx_buffer_tail;
  }
  if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
  return tail - head - 1;
}

In case of tx_buffer_tail ≦ tx_buffer_head, tx_buffer_tail + SERIAL_TX_BUFFER_SIZE - tx_buffer_head - 1 is the available bytes in the buffer.

In case of tx_buffer_tail > tx_buffer_head, tx_buffer_tail - tx_buffer_head - 1 is the available bytes in the buffer.

Version

Arduino AVR Boards 1.8.6

Last Update

August 25, 2019