Question about .available()

Projects Programming
10 
void SoftwareSerial::flush()
{
  if (!isListening())
    return;

  uint8_t oldSREG = SREG;
  cli();
  _receive_buffer_head = _receive_buffer_tail = 0;
  SREG = oldSREG;
}

So, yes, it clears the buffer (if it's the active one).