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From: dhesi%cirrusl@oliveb.ATC.olivetti.com (Rahul Dhesi)
Newsgroups: comp.unix.wizards
Subject: Re: Is HDB locking safe?
Message-ID: <2309@cirrusl.UUCP>
Date: 25 Aug 90 04:40:49 GMT
References: <577@oglvee.UUCP> <4024@rtifs1.UUCP> <YZ85N+6@xds13.ferranti.com> <18500@rpp386.cactus.org> <+7C5OFD@xds13.ferranti.com>
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Organization: Cirrus Logic Inc.
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>I guess creating a second lock file to be held while deleting the first
>would work.

Our in-house software does file locking, to create a lock file L,
like this:

     exlock(L);
     create  L;
     exunlock(L);

     /* critical section here */

     exlock(L);
     delete L;
     exunlock(L);

The definition of exlock(L) is:

     create a lock file called L|lock, looping with sleep(5) several
     times if necessary;  if creation fails, report a very serious
     error and exit process.

The definition of exunlock(L) is:

     delete L|lock;  if nonexistent or can't be deleted, report
     a very serious error and exit process.

Barring a system or process crash during the very brief interval
that L|lock exists, the above scheme is very robust.
--
Rahul Dhesi <dhesi%cirrusl@oliveb.ATC.olivetti.com>
UUCP:  oliveb!cirrusl!dhesi