Question

I am new to socket programming and ran into a puzzling problem:

I have a windows program which I cannot alter (proprietary software) but which tries to connect to a specific ip and port with a tcp socket.

On my linux box I wrote a little python script to serve the socket to the win prog. This works fine until I kill my prog on linux. The initial server socket doesn’t close as specified and I cannot restart my program until the socket is garbage collected.

If I try the same with a linux socket (in a seperate python script) I have no problems.

Here is a minimal code example:

import socket

server = socket.socket()
server.bind(('192.168.0.111', 50001))
server.listen(1)
conn, addr = server.accept()
print 'Connection established'

running = True
while running:
    try:
        data = conn.recv(4096)
    except KeyboardInterrupt:
        conn.close()
        running = False
    else:
        if data:
            print data
        else:
            conn.close()
            running = False
server.close()

If I kill this with Ctrl-C it exits normally. But upon restarting the script I get a socket.error stating the address is already in use. After a minute or so the program works again.

I also tried a shutdown before the close (aka conn.shutdown(2) and server.shutdown…) but that has no effect.

Is there a better ‘right’ way to close a windows socket? Do I miss something fundamental about sockets in general?

Thanks!

edit: I think I just saw the answer here:
what is the correct way to close a socket in python 2.6?

Although I’m using python 2.5 it might still work.

Answer 1

You are experiencing the TIME_WAIT state of connected sockets. Even though you’ve closed your socket, it still has lingering consequences for a couple minutes. The reasons for this, as well as a socket flag you can set to disable the behavior (SO_REUSEADDR), are explained in the UNIX guide socket FAQ.

In short,

server = socket.socket()
server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server.bind(...)
...