How to close a socket left open by a killed progra

Question

I have a Python application which opens a simple TCP socket to communicate with another Python application on a separate host. Sometimes the program will either error or I will directly kill it, and in either case the socket may be left open for some unknown time.

The next time I go to run the program I get this error:

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

Now the program always tries to use the same port, so it appears as though it is still open. I checked and am quite sure the program isn't running in the background and yet my address is still in use.

SO, how can I manually (or otherwise) close a socket/address so that my program can immediately re-use it?

Update

Based on Mike's answer I checked out the socket(7) page and looked at SO_REUSEADDR:

SO_REUSEADDR
    Indicates that the rules used in validating addresses supplied in a bind(2) call should
    allow reuse of local addresses.  For AF_INET sockets this means that a socket may bind,
    except when there is an active listening socket bound to the address.  When the listen‐
    ing  socket is bound to INADDR_ANY with a specific port then it is not possible to bind
    to this port for any local address.  Argument is an integer boolean flag.

Answer 1

Assume your socket is named s... you need to set socket.SO_REUSEADDR on the server's socket before binding to an interface... this will allow you to immediately restart a TCP server...

s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((ADDR, PORT))

Answer 2

You are confusing sockets, connections, and ports. Sockets are endpoints of connections, which in turn are 5-tuples {protocol, local-ip, local-port, remote-ip, remote-port}. The killed program's socket has been closed by the OS, and ditto the connection. The only relic of the connection is the peer's socket and the corresponding port at the peer host. So what you should really be asking about is how to reuse the local port. To which the answer is SO_REUSEADDR as per the other answers.

Answer 3

You might want to try using Twisted for your networking. Mike gave the correct low-level answer, SO_REUSEADDR, but he didn't mention that this isn't a very good option to set on Windows. This is the sort of thing that Twisted takes care of for you automatically. There are many, many other examples of this kind of boring low-level detail that you have to pay attention to when using the socket module directly but which you can forget about if you use a higher level library like Twisted.