Question

The following code runs a socket server on a thread. The client socket sends ‘client: hello’ to the server, and the server socket receives and replies ‘server: world’.

import socket
import threading

def server():
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.bind(('127.0.0.1', 12345))
    sock.listen(1)
    req, addr = sock.accept()
    print req.recv(1024)
    req.sendall('server: world')

def client():
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect(('127.0.0.1', 12345))
    sock.sendall('client: hello')
    print sock.recv(1024)

def main():
    t = threading.Thread(target=server)
    t.start()
    client()

if __name__ == '__main__':
    main()

It runs ok as expected the first time, but from the second time, if you do not wait for a good few seconds for the server to release the socket, and if you try this on a Linux machine or Mac (Windows do not get it somehow) you will run into this error:

Traceback (most recent call last):
  File "socket_send_receive.py", line 24, in <module>
    main()
  File "socket_send_receive.py", line 21, in main
    client()
  File "socket_send_receive.py", line 14, in client
    sock.connect(('127.0.0.1', 12345))
  File "<string>", line 1, in connect
socket.error: [Errno 111] Connection refused
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/home/cxuan/python/2.6/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/home/cxuan/python/2.6/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "socket_send_receive.py", line 6, in server
    sock.bind(('127.0.0.1', 12345))
  File "<string>", line 1, in bind
error: [Errno 98] Address already in use

I am looking for some insight into why this is happening and if it is possible to be genuinely resolved or what best practice should be adopted.

I know already using this option can be a workaround thanks to the other posts here on stackoverflow.

socket.SO_REUSEADDR

Answer 1

When a socket is closed, it ends up in a state called STATE_WAIT (see this diagram). While the socket is in this state, no one else can use the same address (ip-number/port pair) unless the SO_REUSEADDR option is set on the socket.

See e.g. the Wikipedia article on TCP for more information about how TCP works and the different states.