How to detect if a serial port is being used by another application?

[sunbearc22]

How do I detect if a serial port is being used by another application?

E.g. on a terminal, I have picocom /dev/ttyUSB0 -b 115200 running. Separately, I have an application running this script.

port='/dev/ttyUSB0'
try:
    serial_port = serial.Serial(port=port, baudrate=115200, timeout=1)
except serial.SerialException as e:
    print("could not open serial port '{}': {}".format(port, e))
else:
    print("serial port is not busy.")
    serial_port.close()

Yet this script returns serial port not busy..

[sunbearc22]

According to serialposix.py#L523, the serial.Serial.read() method will throw a SerialException relating to device disconnected or multiple access on port?. I tried it but could not get the desired response. What am I doing wrong here? I think I have corrected my except expression here.

import serial

port='/dev/ttyUSB0'
try:
    serial_port = serial.Serial(port=port, baudrate=115200, timeout=5 )
    read_data = serial_port.read()
except serial.serialutil.SerialException as err:
    raise err
else:
    print('serial_port = ', serial_port.__dict__)
    print('read_data = ', read_data)
    print("0 serial port is not busy.")
    serial_port.close()

if not read_data:
    print("1 serial port is busy.")

Output:

serial_port =  {'is_open': True, 'portstr': '/dev/ttyUSB0', 'name': '/dev/ttyUSB0', '_port': '/dev/ttyUSB0', '_baudrate': 115200, '_bytesize': 8, '_parity': 'N', '_stopbits': 1, '_timeout': 5, '_write_timeout': None, '_xonxoff': False, '_rtscts': False, '_dsrdtr': False, '_inter_byte_timeout': None, '_rs485_mode': None, '_rts_state': True, '_dtr_state': True, '_break_state': False, '_exclusive': None, 'fd': 6, 'pipe_abort_read_r': 7, 'pipe_abort_read_w': 8, 'pipe_abort_write_r': 9, 'pipe_abort_write_w': 10}
read_data =  b''
0 serial port is not busy.
1 serial port is busy.

[deepsatflow]

Trying the same code. Getting the expected results.

import serial 
def check_port_status(port_name: str, baudrate: int = 115200):

    if port_name is  None: 
        return 
    if baudrate is None:
        return 

    ## config 
    # baudrate = 115200 
    # port_name = "/dev/tty.SLAB_USBtoUART"

    try:
        # print("port name: ", port_name)
        # print("baudrate: ", baudrate)
        serial_port = serial.Serial(port=port_name, baudrate=baudrate, timeout=5)
        read_data = serial_port.read()
    except serial.serialutil.SerialException as err:
        ## get the error arguments and print 
        code, context = err.args # tuple 
        if(code == 16): 
            print(f"port {port_name} is busy, please close and try again.")
            pass 
        elif(code == 2): 
            print(f"port name: {port_name} is not valid.")
            pass 
        
        return (False, code) 

    else:
        print('serial_port = ', serial_port.__dict__)
        print('read_data = ', read_data)
        print("serial port is not busy.")
        serial_port.close()
        return (True, 1) 


if __name__ == "__main__":
    # main() 
    port_name = '/dev/tty.SLAB_USBtoUART'
    baudrate = 115200 
    check_port_status(port_name=port_name, baudrate=baudrate)

Output Instance 1 (Port is not busy):

serial_port =  {'is_open': True, 'portstr': '/dev/tty.SLAB_USBtoUART', 'name': '/dev/tty.SLAB_USBtoUART', '_port': '/dev/tty.SLAB_USBtoUART', '_baudrate': 115200, '_bytesize': 8, '_parity': 'N', '_stopbits': 1, '_timeout': 5, '_write_timeout': None, '_xonxoff': False, '_rtscts': False, '_dsrdtr': False, '_inter_byte_timeout': None, '_rs485_mode': None, '_rts_state': True, '_dtr_state': True, '_break_state': False, '_exclusive': None, 'fd': 3, 'pipe_abort_read_r': 4, 'pipe_abort_read_w': 5, 'pipe_abort_write_r': 6, 'pipe_abort_write_w': 7}
read_data =  b'x'
serial port is not busy.

Output Instance 2 (Port is busy): port /dev/tty.SLAB_USBtoUART is busy, please close and try again.

Output Instance 3 (Port is not active): port name: /dev/tty.SLAB_USBtoUART is not valid.

To reproduce: run: python3 <file_name.py>

[praveena2s]

How do I detect if a serial port is being used by another application?

E.g. on a terminal, I have picocom /dev/ttyUSB0 -b 115200 running. Separately, I have an application running this script.

port='/dev/ttyUSB0'
try:
    serial_port = serial.Serial(port=port, baudrate=115200, timeout=1)
except serial.SerialException as e:
    print("could not open serial port '{}': {}".format(port, e))
else:
    print("serial port is not busy.")
    serial_port.close()

Yet this script returns serial port not busy..

Is this issue resolved?

[praveena2s]

Trying the same code. Getting the expected results.

import serial 
def check_port_status(port_name: str, baudrate: int = 115200):

    if port_name is  None: 
        return 
    if baudrate is None:
        return 

    ## config 
    # baudrate = 115200 
    # port_name = "/dev/tty.SLAB_USBtoUART"

    try:
        # print("port name: ", port_name)
        # print("baudrate: ", baudrate)
        serial_port = serial.Serial(port=port_name, baudrate=baudrate, timeout=5)
        read_data = serial_port.read()
    except serial.serialutil.SerialException as err:
        ## get the error arguments and print 
        code, context = err.args # tuple 
        if(code == 16): 
            print(f"port {port_name} is busy, please close and try again.")
            pass 
        elif(code == 2): 
            print(f"port name: {port_name} is not valid.")
            pass 
        
        return (False, code) 

    else:
        print('serial_port = ', serial_port.__dict__)
        print('read_data = ', read_data)
        print("serial port is not busy.")
        serial_port.close()
        return (True, 1) 


if __name__ == "__main__":
    # main() 
    port_name = '/dev/tty.SLAB_USBtoUART'
    baudrate = 115200 
    check_port_status(port_name=port_name, baudrate=baudrate)

Output Instance 1 (Port is not busy):

serial_port =  {'is_open': True, 'portstr': '/dev/tty.SLAB_USBtoUART', 'name': '/dev/tty.SLAB_USBtoUART', '_port': '/dev/tty.SLAB_USBtoUART', '_baudrate': 115200, '_bytesize': 8, '_parity': 'N', '_stopbits': 1, '_timeout': 5, '_write_timeout': None, '_xonxoff': False, '_rtscts': False, '_dsrdtr': False, '_inter_byte_timeout': None, '_rs485_mode': None, '_rts_state': True, '_dtr_state': True, '_break_state': False, '_exclusive': None, 'fd': 3, 'pipe_abort_read_r': 4, 'pipe_abort_read_w': 5, 'pipe_abort_write_r': 6, 'pipe_abort_write_w': 7}
read_data =  b'x'
serial port is not busy.

Output Instance 2 (Port is busy): port /dev/tty.SLAB_USBtoUART is busy, please close and try again.

Output Instance 3 (Port is not active): port name: /dev/tty.SLAB_USBtoUART is not valid.

To reproduce: run: python3 <file_name.py>

Hi Satish, Thanks for the write up. (Did you validate your code when the port was occupied by picocom application?) I was also facing the same issue as @sunbearc22 was facing. I have installed latest pyserial v3.5 on Ubuntu v22.04.

I have re-used your code and tried against 3 different scenarios

Scen-1. Port /dev/ttyACM4 was fee (i.e., port is available/ready to use) Verification-1: Cross verification using lsof command which gives no output $ lsof /dev/ttyACM4

Verification-2: Using your code,

$ python3 serial_port_busy_check.py
serial_port =  {'is_open': True, 'portstr': '/dev/ttyACM4', 'name': '/dev/ttyACM4', '_port': '/dev/ttyACM4', '_baudrate': 115200, '_bytesize': 8, '_parity': 'N', '_stopbits': 1, '_timeout': 5, '_write_timeout': None, '_xonxoff': False, '_rtscts': False, '_dsrdtr': False, '_inter_byte_timeout': None, '_rs485_mode': None, '_rts_state': True, '_dtr_state': True, '_break_state': False, '_exclusive': None, 'fd': 3, 'pipe_abort_read_r': 4, 'pipe_abort_read_w': 5, 'pipe_abort_write_r': 6, 'pipe_abort_write_w': 7}
read_data =  b'E'
serial port is not busy.

---

Scen-2. Port /dev/ttyACM4 not present in the system (i.e., hardware is not connected to the system) Verification-1: Cross verification using lsof command which gives error

$ lsof /dev/ttyACM4
lsof: status error on /dev/ttyACM4: No such file or directory
;
;

Verification-2: Using your code,

$ python3 serial_port_busy_check.py
port name: /dev/ttyACM4 is not valid.

---

Scen-3. Port /dev/ttyACM4 was used by picocom application (i.e., port is already busy with other process) Verification-1: Cross verification using lsof command which gives port details in the output

$ lsof /dev/ttyACM4
COMMAND    PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
picocom 383023 xyz_user    3uW  CHR  166,4      0t0 1398 /dev/ttyACM4

Verification-2: Using your code,

$ python3 serial_port_busy_check.py
serial_port =  {'is_open': True, 'portstr': '/dev/ttyACM4', 'name': '/dev/ttyACM4', '_port': '/dev/ttyACM4', '_baudrate': 115200, '_bytesize': 8, '_parity': 'N', '_stopbits': 1, '_timeout': 5, '_write_timeout': None, '_xonxoff': False, '_rtscts': False, '_dsrdtr': False, '_inter_byte_timeout': None, '_rs485_mode': None, '_rts_state': True, '_dtr_state': True, '_break_state': False, '_exclusive': None, 'fd': 3, 'pipe_abort_read_r': 4, 'pipe_abort_read_w': 5, 'pipe_abort_write_r': 6, 'pipe_abort_write_w': 7}
read_data =  b''
serial port is not busy.

my concern is Scen-3 where code logic shows 'serial port is not busy' even the port was occupied by other application (in my case picocom application.)

Wondering why read_data = serial_port.read() instruction is not detecting the busy port, where lsof command is able to....

If anybody has resolved this issue, feel free to add the comments.

Thanks

[harryberlin]

i'm using this method to fill tkinter combobox.

def serial_ports():
    """ Lists serial port names

        :raises EnvironmentError:
            On unsupported or unknown platforms
        :returns:
            A list of the serial ports available on the system
    """
    if sys.platform.startswith('win'):
        ports = ['COM%s' % (i + 1) for i in range(256)]
    elif sys.platform.startswith('linux') or sys.platform.startswith('cygwin'):
        # this excludes your current terminal "/dev/tty"
        ports = glob.glob('/dev/tty[A-Za-z]*')
    elif sys.platform.startswith('darwin'):
        ports = glob.glob('/dev/tty.*')
    else:
        raise EnvironmentError('Unsupported platform')

    result = []
    for port in ports:
        try:
            s = serial.Serial(port)
            s.close()
            result.append(port)
        except (OSError, serial.SerialException):
            pass
    return result

[Frank-Buss]

@harryberlin This has nothing to do with the issue, and is also pretty bad to return 256 COM ports on Windows. You can just use "import serial.tools.list_ports", and then use this:

        ports = [port.device for port in serial.tools.list_ports.comports()]

to get only the usable serial ports. This should work on all platforms (tested on Linux).

[harryberlin]

ok, did add extra sorted() ports = [port.device for port in sorted(serial.tools.list_ports.comports())]