It writes in Section 9.3 in Unix Network programming about SCTP:
"The sctp_bindx call can be used on a bound or unbound socket."
And then it writes:
"The port number in all the socket address structures must be the same and must match any port number that is already bound; if it doesn't, then sctp_bindx will fail, returning the error code EINVAL."
It is easy to understand that the port number must be the same. What confuses me is that the port number must match any port number that is already bound. But previously it says that the sctp_bindx can be used on a unbound socket. I think that if the socket is unbound, the port number the socket will use must be unused as well. For example, assuming that a SCTP server wants to use port 123 to receive requests, the unbound SCTP socket will call sctp_bindx to bind itself to the port 123. Can it be done?