socketpair(2) protocol

[Jan Schaumann <jschauma%netmeister.org@localhost>]

Hello,

socketpair(2) says:

"The type and protocol argument values are described
in socket(2)."

But as best as I can tell, socketpair(2) only ever
accepts a protocol of 0, no other value.  Per POSIX, 

"Specifying a protocol of 0 causes socketpair() to use
an unspecified default protocol appropriate for the
requested socket type."

which is _not_ actually protocol 0 (HOPOPT, RFC2460,
per /etc/protocols).

Is there any scenario whereby a non-0 protocol can be
specified to socketpair(2)?

As it reads right now, a user might expect to be able
to call

socketpair(PF_LOCAL, SOCK_STREAM, 6, sv)

which will lead to EPROTONOSUPPORT.

-Jan