> It's a unix domain socket. . .

I see.  I recalled that I've surveyed the sockname behavior for unix
domain socket some years ago to emulate it on SysV, though I cannot
find the results any more, unfortunately.

Yes, that would be the problem since 4.3BSD.  Returning garbage is bug
and should be filed as PR.
I think the FreeBSD behavior, returning zero size sockaddr, is not the
right thing.  The sa_family and sa_len member should be filled.

Atsushi Onoe