strncpy&sprintf behaviour

[teklabs!ogcvax!metheus!cdi!caf]

  Is there an "official" definition of what strncpy does when the
source string meets or exceeds the specified number of characters
(are n-1 characters plus a null stored, etc)?  The SYS3 documentation
leaves something to the imagination.  Also, is sprintf always supposed
to add a trailing null?
Chuck Forsberg  ...!teklabs!ogcvax!metheus!cdi