Question

If I connect to an inexistent socket with pyzmq I need to hit CTRL_C to stop the program. Could someone explay why this happens?

import zmq

INVALID_ADDR = 'ipc:///tmp/idontexist.socket'

context = zmq.Context()
socket = context.socket(zmq.REQ)

socket.connect(INVALID_ADDR)
socket.send('hello')

poller = zmq.Poller()
poller.register(socket, zmq.POLLIN)
conn = dict(poller.poll(1000))
if conn:
    if conn.get(socket) == zmq.POLLIN:
        print "got result: ", socket.recv(zmq.NOBLOCK)
else:
    print 'got no result'

Answer 1

This question was also posted as a pyzmq Issue on GitHub. I will paraphrase my explanation here (I hope that is appropriate, I am fairly new to SO):

A general rule: When in doubt, hangs at the end of your zeromq program are due to LINGER.

The hang here is caused by the LINGER socket option, and happens in the context.term() method called during garbage collection at the very end of the script. The LINGER behavior is described in the zeromq docs, but to put it simply, it is a timeout (in milliseconds) to wait for any pending messages in the queue to be handled after closing the socket before dropping the messages. The default behavior is LINGER=-1, which means to wait forever.

In this case, since no peer was ever started, the ‘hello’ message that you tried to send is still waiting in the send queue when the socket tries to close. With LINGER=-1, ZeroMQ will wait until a peer is ready to receive that message before shutting down. If you bind a REP socket to ‘ipc:///tmp/idontexist.socket’ while this script is apparently hanging, the message will be delivered and the script will finish exiting cleanly.

If you do not want your script to wait (as indicated by your print statements that you have already given up on getting a reply), set LINGER to any non-negative value (e.g. socket.linger = 0), and context.term() will return after waiting the specified number of milliseconds.

I should note that the INVALID_ADDR variable name suggests an understanding that connection to an interface that does not yet have a listener is not valid – this is incorrect. zeromq allows bind/connect events to happen in any order, as illustrated by the behavior described above, of binding a REP socket to the interface while the sending script is blocking on term().