Is it possible to unlisten on a socket?

After closing the socket, your programs may still tell you that the socket is "in use", this is because of some weirdiness I don't know exactly about. But the manpage about sockets shows you there is a flag to re-use the same socket, lazily called: "SO_REUSEADDR". Set it using "setsockopt()".

Close it. As I recall;

close(fd);

Based on your edited version of the question, I'm not sure you have to "unlisten" or close(). Two options come to mind:

1) After you invoke listen(), connections are not actually accepted until (logically enough) you call accept(). You can "unlisten" by simply ignoring socket activity and deferring any accept()'s until you are ready for them. Any inbound connection attempts backlog onto the queue that was created when the port was opened in listen mode. Once the backlog queue is full in the stack, further connection attempts are simply dropped on the floor. When you resume with accepts(), you'll quickly dequeue the backlog and be ready for more connections.

2) If you really want the port to appear completely closed temporarily, you might dynamically apply the kernel level packet filter to the port to prevent the inbound connection attempts from reaching the network stack. For example, you could use Berkeley Packet Filter (BPF) on most *nix platforms. That is you want to drop inbound packets coming in to the port of interest using the platform's firewall features. This, of course, varies by platform, but is a possible approach.