What does an Arduino do if the serial connection is closed on the other end?

Question

I have a simple method that simply waits for an input, computes a response, and sends it:

void loop() { 
  if (Serial.available() > 0){
    input = Serial.read();

    if (input == 10){
      digitalWrite(trigPin, LOW);
      delayMicroseconds(2);
    
      digitalWrite(trigPin, HIGH);
      delayMicroseconds(10);
      digitalWrite(trigPin, LOW);
    
      duration = PulseDuration(echoPin, HIGH, 6000);
      distance = duration * 0.00034/2;  // 0.00034 m/us is speed of sound

      Serial.println(distance, 4);
      }
  }
}

My question: What happens to the arduino if I close the serial connection from the receiving end while it's printing out data (in the last line of the loop)?

Context and motivation:

The program works well most times. I can open a python program that sends the requests and prints out the reply using pyserial. After I close the python side and reopen it, connection is set and I can start listening on the side nicely just like the first time. Sometimes, however, when I try to restart the python program, I get a connection error saying that the port is already opened (I'm running in Windows.. there are tons of posts about this problem, but none of them have been helpful). Some other times, when connecting with the python program (which connects with connection = serial.Serial(self.arduino_port, 9600, timeout=0) it just blocks and doesn't connect, it doesn't give the "port already opened" error, and it doesn't advance to the next statement. I am confident I did close the previous python run before attempting a new one (process explorer shows no active python programs running, it also shows no open processes using a serial handler). I'm not here to solve that problem directly, though, just to ask a related question.

I'm also confident running connection.close() does not terminate the arduino program, as I have been able to restart my python program without having to reset the arduino or anything, suggesting the arduino kept running its loop after I closed the connection on the python side.

All that to say that my latest hypothesis is that these issues arise when I close the serial connection connection.close() while the arduino is printing, but because the port is closed, that Arduino is stuck and cannot do the Serial.println(), as the USB never asks to flush the data in its buffer. Is this possible/does it make sense? What happens to the arduino if I close the serial connection while it's printing out data?

I'm not posting my python program here as it's not directly relevant to the question and I can't seem to make a minimum reproducible program that doesn't expose confidential code.

Answer 1

That's some strange Intel monstrosity. It's impossible to know what it does or how. Intel's foray into the Arduinosphere was brief and somewhat abortive.

In general though (for most cores) the DTR signal (and sometimes the RTS signal too) of the CDC/ACM protocol is used to "gate" the serial data - if it's asserted then serial data gets sent through to the host PC. If it's not asserted then it just gets thrown away.

I have seen cores where if DTR is not asserted the data gets buffered until the TX buffer is full and then blocks, but that is rare.

You might find your answer if you delve into the core's source code, but you may find that big chunks of it are hidden away in closed source pre-compiled libraries, and the fact it seems to run some custom RTOS means it'll be even more obfuscated. YMMV...