Are ut_user and ut_host in struct utmp NUL-terminated?

[Florian Weimer]

Can ut_user be exactly UT_NAMESIZE bytes long?  Or is it necessary to 
set aside a byte with the NUL terminator?

I'm not sure what POSIX says about struct utmpx.  It never says “string” 
in the context of ut_user, so I don't think it clarifies this matter.

Thanks,
Florian