Hi,

I was wondering if there is anyway to determine programmatically whether a
port is available or whether its already in use before I do a bind() to that
port. Something akin to the macro INADDR_ANY - alhtough that gives the local
address (if I am not mistaken), what I am looking for is an easy way to get
a port number thats not already in use. Are there any macros or functions
that would let me do this?

"Rookie" <dominicjoseph@rediffmail.com> writes:

> Hi,
>
> I was wondering if there is anyway to determine programmatically whether a
> port is available or whether its already in use before I do a bind() to that
> port. Something akin to the macro INADDR_ANY - alhtough that gives the local
> address (if I am not mistaken), what I am looking for is an easy way to get
> a port number thats not already in use. Are there any macros or functions
> that would let me do this?

bind() to port 0, and the kernel will assign one for you.  Use
getsockname() to find out which port you were given.

-- 
M�ns Rullg�rd
mru@mru.ath.cx

In article <cjoo54$1lb$1@gist.usc.edu>,
	"Rookie" <dominicjoseph@rediffmail.com> writes:
> Hi,
> 
> I was wondering if there is anyway to determine programmatically whether a
> port is available or whether its already in use before I do a bind() to that
> port. Something akin to the macro INADDR_ANY - alhtough that gives the local
> address (if I am not mistaken), what I am looking for is an easy way to get
> a port number thats not already in use. Are there any macros or functions
> that would let me do this?

Even if there were, would it make sense?  There'd be a window between when
you checked and when you bound to it, so something else could still get
there first, and the bind() could still fail.

As for picking a free local port, see inet(7p); you can either specify a
port number of zero, or not bind it at all before calling sendto() (UDP
only) or connect() (UDP or TCP), and let the system pick the port; unless
you want a privileged local port, in which case you have to either write
your own more complicated code to pick a port number, or use something
like rresvport(3socket) or rresvport_af(3socket) to pick the port number.

Most of the time, you probably don't want to bind the local address at
all; that lets the system pick the appropriate one in case there are
multiple interfaces.

-- 
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