how to get any available port

[Paul Rubin]

Grant Edwards <grante at visi.com> writes:
> > In the nomenclature of some of these applications, that kind
> > of transfer is called a client to client connection.  Both
> > ends are called clients.
> 
> IIRC, we were talking about TCP sockets.

Yes, but if the person was talking about using TCPs sockets in a chat
application, they may have been using chat terminology rather than TCP
terminology.  In a typical chat app, a zillion clients all talk
through TCP to a central server, which is like a phone switch.  That
lets Alice connect to the server and immediately see if Bob is online
and what Bob's (possibly dynamic) IP address is.  If Alice wants to
send Bob a file, she opens a TCP connection directly to Bob.  That's
called a client to client connection because Alice and Bob are both
clients.

So, there's a reasonable application for wanting to open a listener
port without binding any specific port number.  Alice would just ask
her OS to assign her a port (say it assigns 23789) and listen on it,
instead of having to contend with other apps on the same IP for some
specific port number.  Then she'd send a message through the chat
server asking Bob to connect to port 23789 on her machine.

I'm not sure if there's a way to do this.  Do you happen to know?