int nng_socket_id(nng_socket s);
nng_socket_id() function returns a positive identifier for the socket s,
if it is valid.
Otherwise it returns
A socket is considered valid if it was ever opened with a protocol
constructor, such as the
Sockets that are allocated on the stack or statically should be
initialized with the macro
to ensure that they cannot be confused with a valid socket before they are opened.
This function returns the positive value for the socket identifier, or
-1 if the socket is invalid.