Xref: utzoo comp.unix.questions:10332 comp.unix.wizards:12850
Path: utzoo!attcan!uunet!vsi!friedl
From: friedl@vsi.COM (Stephen J. Friedl)
Newsgroups: comp.unix.questions,comp.unix.wizards
Subject: Re: what is the 'l' permission?
Summary: Mandatory -vs- advisory locks
Message-ID: <951@vsi.COM>
Date: 24 Nov 88 20:17:34 GMT
References: <71@attibr.UUCP> <4594@ptsfa.PacBell.COM> <483@auspex.UUCP> <4945@b-tech.ann-arbor.mi.us>
Organization: V-Systems, Inc. -- Santa Ana, CA
Lines: 30

In article <4945@b-tech.ann-arbor.mi.us>, zeeff@b-tech.ann-arbor.mi.us (Jon Zeeff) writes:
> Can someone explain "mandatory locking" as used here.  I find that Sys V.3
> allows two processes to open and write to a file with 'l' permisssions.

There are two kinds of locks supported by most versions of UNIX
(Sys V, at least), advisory and mandatory.

An advisory lock is a Post-it(tm) put on a record saying "please
keep away", and a program is free to ignore it.  It requires that
two programs dealing with a file cooperate by always querying the
locks first, and an uncooperative or buggy program can read or
write any record.

Mandatory (aka `enforcement-mode') locks, on the other hand, live
inside the read/write mechanism.  If a file has the mandatory
lock bit set in the mode, locking a record will prevent any access
to that record by any program even if this other program has no
knowledge of locks.

I'm speculating on this part, but I guess that setting the `l' mode
is required because the vast majority of programs don't use locking,
and the overhead required on each read/write call is probably too much.
Setting the lock bit probably enables this checking.


-- 
Steve Friedl    V-Systems, Inc.  +1 714 545 6442    3B2-kind-of-guy
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