Different processes, same PID | Python

Question

I'm trying to spawn few processes using the multiprocessing module. However when I call os.getpid() each process returns the same PID as the main calling process.

Why? I'm using Python 2.7 on Ubuntu 14.04.

class Listener(multiprocessing.Process):
    def __init__(self, _ttl):
        super(Listener, self).__init__()
        self.ttl = _ttl
        self.socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.socket.bind(('localhost', 0))

    def get_pid(self):
        return os.getpid()  

    def get_name(self):
        return self.socket.getsockname()

    def run(self):
        self.socket.listen(1)
        time.sleep(self.ttl)

    def listen(self):
        self.start()

class TestEx1(unittest.TestCase):
    def test_address_to_pid(self):
        listener1 = Listener(12)
        listener2 = Listener(12)
        listener3 = Listener(12)
        listener1.listen()
        listener2.listen()
        listener3.listen()
        address1 = str(str(listener1.get_name()[0])) + ":" + str(listener1.get_name()[1])
        address2 = str(str(listener2.get_name()[0])) + ":" + str(listener2.get_name()[1])
        address3 = str(str(listener3.get_name()[0])) + ":" + str(listener3.get_name()[1])

        print listener1.get_pid()
        print ex1.address_to_pid(address1)

        print listener2.get_pid()
        print ex1.address_to_pid(address2)

        print listener3.get_pid()
        print ex1.address_to_pid(address3)


        assert(str(ex1.address_to_pid(address1)) == str(listener1.get_pid()))
        assert(str(ex1.address_to_pid(address2)) == str(listener2.get_pid()))
        assert(str(ex1.address_to_pid(address3)) == str(listener3.get_pid()))

        listener2.join()
        listener2.join()
        listener3.join()

suztomo · Accepted Answer · 2015-07-29T23:44:40.580

1

It is because os.getpid() is called from the original process that runs the test.

The Listener instances are Python objects accessible in the original processes. Because these objects are instances of multipricessing.Process they may help the original process to spawn new processes. In original process of test, however, TestEx1 class is calling os.getpid() via these objects (in the original process). That's why they return same pid.

In order to get pid of the Listener processes, call listener1.pid attribute after the process starts. Documentation explains

pid

Return the process ID. Before the process is spawned, this will be None.

edited Jul 29 '15 at 23:44

answered Jul 29 '15 at 23:27

suztomo

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How can I fix that problem? – triple fault Jul 29 '15 at 23:29
How about call ing `print 'process id:', os.getpid()` at Listener's `run ` method? – suztomo Jul 29 '15 at 23:37
1

Also I found Process instance has `pid` attribute. – suztomo Jul 29 '15 at 23:41
One more thing I dont really understand: When I use netstat -nptl, each listener listen under the same PID. Why? – triple fault Jul 29 '15 at 23:46
That's interesting. Can you create another question with the latest code, netstat version and Linux version (Ubuntu 14.04) in it? – suztomo Jul 29 '15 at 23:53
http://stackoverflow.com/questions/31713233/different-processes-showed-as-same-pid-within-netstat – triple fault Jul 30 '15 at 00:11
@Doppelganger I appreciate if you are satisfied with `pid` attribute and you can "accept" my answer for this question. – suztomo Jul 30 '15 at 01:39

Different processes, same PID | Python

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