🚀 go-pugleaf

In article <1990Nov16.023110.1305@csun.edu> aeusejvn@csunb.csun.edu (Jon Nadelberg) writes:
>Is there a way to check if a file is currently open and being used by
>another process?  Is there a way to "lock" a file so that other processes
>can not access it?  

The things that come immedately to mind are:

Use the exclusive flag when calling open(2) -- must be done every time.

Allow multiple opens and have the processes do read and write locks
as approprate, being shure to always lock before read (you shouldn't
need manditory locks in this case, just be consistant).  If the file
is locked the file is busy.  fnctl(2) or ioctl(2) I dont remember
which right now.

Do the "put process number in lock file" thing as in uucp.

Any sutble differences between these thecniques should be considered
based on the exact internals of the app.  And then combine as many of
these as is reasonable.

Hope it was some help,
Rob.

>Use the exclusive flag when calling open(2) -- must be done every time.

You mean O_EXCL?  That flag isn't an exclusive-use open flag; it's an
"exclusive create" flag.  It only causes an error if O_CREAT is also set
and the file already exists.

According to rwhite@nusdecs.uucp (0257014-Robert White(140)):
>Do the "put process number in lock file" thing as in uucp.

This approach cannot be used reliably without either (1) having a lock
held forever by a dead process, which I find unacceptable, or (2)
using a kernel locking call, which renders the pid file redundant.

For you doubters: Once you've determined that a lock file is stale and
you want to unlink() it, how do you know the lock file hasn't been
replaced in between the decision to unlink() and the unlink() itself?

I really like kernel record locks for lots of reasons, but the primary
one is that locks disappear automatically when the locking process
dies, even if the death is abnormal (SIGKILL, core dump, anything).
-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.uucp>, <uunet!pdn!tct!chip>
    "I've been cranky ever since my comp.unix.wizards was removed
         by that evil Chip Salzenberg."   -- John F. Haugh II

Thanks to everyone who answered my question.  We'll probably be able to
get something functioning now.  Much appreciated.



--
------------------------------------------------------------------------
-   Jon Nadelberg                                                      -
-   aeusejvn@csunb.csun.edu                                            -
------------------------------------------------------------------------

In article <274975C4.21C@tct.uucp> chip@tct.uucp (Chip Salzenberg) writes:
> According to rwhite@nusdecs.uucp (0257014-Robert White(140)):
> >Do the "put process number in lock file" thing as in uucp.
> This approach cannot be used reliably without either (1) having a lock
> held forever by a dead process, which I find unacceptable, or (2)
> using a kernel locking call, which renders the pid file redundant.

It can be used reliably if you put information like pid and date into
the name of the lock file.

> For you doubters: Once you've determined that a lock file is stale and
> you want to unlink() it, how do you know the lock file hasn't been
> replaced in between the decision to unlink() and the unlink() itself?

The old name will never be reused, so there's no danger of replacement.

---Dan

	Grammar Police, Ma'am! Open Up!

	Ahem ...

	The word y'all are fseeking and not ffinding is:

		extant

	There *is* and English word "existence", but if "existant"
is a relative thereof, it's from the wrong side of the bedsheets.

		Dank U.

-- 
 <jax@well.{UUCP,sf.ca.us} ><  Member, >        /// ///\\\    \\\  ///
 <well!jax@lll-winken.arpa >< X3J14 TC >       /// ///  \\\    \\\/// 
 <JAX on GEnie             >< for ANS  > \\\  /// ///====\\\   ///\\\ 
 <SYSOP RCFB (303) 278-0364><  Forth   >  \\\/// ///      \\\ ///  \\\

In article <21795@well.sf.ca.us> jax@well.sf.ca.us (Jack J. Woehr) writes:
>
>	Grammar Police, Ma'am! Open Up!
>
>	Ahem ...
>
>	The word y'all are fseeking and not ffinding is:
>
>		extant
>
>	There *is* and English word "existence", but if "existant"
>is a relative thereof, it's from the wrong side of the bedsheets.
>
>		Dank U.
>

 You are only partly right since you can't use extant in the context of
 "if an extant...". Therefore the word is:

	   existing
-- 
Pat Hertel                 Canadian Meteorological Centre
Analyst/Programmer         2121 N. Service Rd.        % rm God
phertel@cmc.aes.doe.ca     Dorval,Quebec              rm: God non-existent
Environment Canada         CANADA           H9P1J3

afsipmh@cid.aes.doe.CA (Patrick Hertel) writes:

> You are only partly right since you can't use extant in the context of
> "if an extant...". Therefore the word is:
>	   existing

Huh?  In my dictionary, "extant" is billed as an adjective.  Since it is
modifying the noun "file", you should be able to use it in this context.
Substitute another adjective, like "huge" or "red":

	if a huge file is open
	if a red file is open
	if an extant file is open

Same thing.

derek
-- 
Derek Terveer						det@hawkmoon.MN.ORG
Minnesota Field Hockey Association,  North Central Section
University of Minnesota Women's Lacrosse,  Midwest District