Socket error: Address already in use

Question

I have a CherryPy script that I frequently run to start a server. Today I was having to start and stop it a few times to fix some bugs in a config file, and I guess the socket didn't close all the way because when I tried to start it up again I got this issue:

[23/Mar/2015:14:08:00] ENGINE Listening for SIGHUP.
[23/Mar/2015:14:08:00] ENGINE Listening for SIGTERM.
[23/Mar/2015:14:08:00] ENGINE Listening for SIGUSR1.
[23/Mar/2015:14:08:00] ENGINE Bus STARTING
CherryPy Checker:
The Application mounted at '' has an empty config.

[23/Mar/2015:14:08:00] ENGINE Started monitor thread 'Autoreloader'.
[23/Mar/2015:14:08:00] ENGINE Started monitor thread '_TimeoutMonitor'.
[23/Mar/2015:14:08:00] ENGINE Error in HTTP server: shutting down
Traceback (most recent call last):
  File "/home/andrew/virtualenvs/mikernels/lib/python2.7/site-packages/cherrypy/process/servers.py", line 188, in _start_http_thread
    self.httpserver.start()
  File "/home/andrew/virtualenvs/mikernels/lib/python2.7/site-packages/cherrypy/wsgiserver/wsgiserver2.py", line 1848, in start
    raise socket.error(msg)
error: No socket could be created

I edited CherryPy's wsgiserver2.py to see the details of the socket.error and error.strerror was

98  (98, 'Address already in use') Address already in use

Meanwhile my socket is constructed as:

af = 2
socktype = 1
proto = 6
canonname = ''
sa = ('0.0.0.0', 2112)
self.bind(af, socktype, proto)

(that's not exact code but that's what the values are when the error is fired)

I checked netstat and didn't see anything listening on port 2112, what could be causing the problem and how can I go about diagnosing it?

Thanks!

Answer 1

You can try the following

from socket import *

sock=socket()
sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
# then bind

From the docs:

The SO_REUSEADDR flag tells the kernel to reuse a local socket in TIME_WAIT state, without waiting for its natural timeout to expire.

Here's the complete explanation:

Running an example several times with too small delay between executions, could lead to this error:

socket.error: [Errno 98] Address already in use

This is because the previous execution has left the socket in a TIME_WAIT state, and can’t be immediately reused.

There is a socket flag to set, in order to prevent this, socket.SO_REUSEADDR:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((HOST, PORT))

Answer 2

You could find the process and kill it by doing:

ps aux | grep python

, finding the process ID, and stopping it manually by doing:

sudo kill -9 PID

replacing PID with your PID.

I often have to do this while testing with Flask/CherryPy. Would be interested to see if there's an easier way (for e.g. to prevent it in the first place)