Mandatory locking (was Re: the 'l' permission)

[Mike Howard]

In article <1988Nov26.220052.19423 at ateng.ateng.com> chip at ateng.ateng.com (Chip Salzenberg) writes:
>[Followups directed to comp.unix.xenix; you'll see why.]
>
>According to guy at auspex.UUCP (Guy Harris):
>>"Mandatory locking" merely means that if you use
>>"fcntl" or "lockf" to lock a region of the file, attempts to write the
> ....
>None, that's right, *none* of these locking methods provides advisory
>locking under SCO Xenix.  Even though fcntl() and lockf() MUST be
>advisory to conform to the SVID.  Instead, we get mandatory locking and,

Would someone knowledgable please explain (with examples) the distinction
between `manditory' and `advisory' locking.  (more than 25 words is ok with
me :-).   
-- 
Mike Howard
uunet!milhow1!how