Path: utzoo!utgpu!news-server.csri.toronto.edu!csri.toronto.edu!wayne
Newsgroups: comp.os.minix
From: wayne@csri.toronto.edu (Wayne Hayes)
Subject: Re: File locking in MINIX and UNIX
Message-ID: <1991Feb16.013731.18087@jarvis.csri.toronto.edu>
Organization: CSRI, University of Toronto
References: <39313@cup.portal.com>
Date: 16 Feb 91 06:37:31 GMT
Lines: 33

Well, some Unixes support *voluntary* file locking, that is, the OS will
tell you if a file is locked only if you ask, but it's not enforced.
Also, the first process has to explicitly lock the file, and the second
process has to explicitly ask if the file is locked.  Regardless of
whether the file is locked, the second (any any other) process(es) may
open the file and do with it as they please.  This can usually be
construed as a feature, and is *very* useful under many circumstances.
It is in fact quite rare that two processes will access the same file
for writing at the same time without each other's knowledge.  It 
requires that either the file is writable by more than one user ID,
or that one person is silly enough to start two different programs
that write to the same file when they're not supposed to.  The only
time I have ever encountered the former is with an older Unix system
that had the /usr/spool/mail/gripes file world-writable so you could
put gripes there.  Occasionally someone would put a gripe there via
mail (as they should), but later want to edit the message.  This is
why the file was world-writable.  Then under the extraordinary
circumstance that this very same urge befalls two people at the same
time, the first poor fellow to save his changes looses those changes
when the second guy saves *his* changes.  This happened only once that
I know of.

Anyway, it's very useful to be able to have one process write while
another reads, for instance when you have a log file being created
in the background and you want to read the log file with "tail -f"
(which BTW doesn't yet work in Minix, but I'll be posting some
diffs for tail in the near future to support this.)

-- 
"Dad, what should I be when I grow up?"
"Honest." -- Robert M. Pirsig, _Zen and the Art of Motorcycle Maintenence_.

Wayne Hayes	INTERNET: wayne@csri.utoronto.ca	CompuServe: 72401,3525