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))