Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!ukma!tut.cis.ohio-state.edu!osu-cis!att!lzaz!hutch
From: hutch@lzaz.ATT.COM (R.HUTCHISON)
Newsgroups: comp.unix.questions
Subject: Re: System V file/record locking?
Message-ID: <408@lzaz.ATT.COM>
Date: 8 Feb 89 13:36:55 GMT
References: <477@maxim.ERBE.SE>
Organization: AT&T ISL Lincroft NJ USA
Lines: 41

From article <477@maxim.ERBE.SE>, by prc@maxim.ERBE.SE (Robert Claeson):
...
} 1. What's the difference between the lockf(3) function and the locking
}    calls to the fcntl(2) system call? Since lockf is a function in the
}    standard library, one would expect it to be implemented using fcntl.
}    If this is not so, does the NFS locking protocol work on fcntl-style
}    locks as well (it works on lockf locks)?

Yes, lockf() uses fcntl().  I believe it was added at the prompting of
/usr/group.  They wanted a standard way of locking files.
} 
} 2. lockf always uses "exclusive" locks. Does this mean that it sets
}    a write lock on the file segment?

Yes.  It's a shame that it doesn't let you place read (non-exclusive) 
locks.

} 
} 3. What is the "l_whence" field used for in the flock struct (used as
}    argument to the locking functions of fcntl)? The same thing as in
}    the lseek system call?

Yes.  Same rules apply as for lseek().

} 
} 4. The documentation mentions a "F_CHKFL" function of fcntl in the man
}    page for fcntl(5). What does this function do and how do I use it?

It doesn't do anything although the header file hints that it might.
Perhaps in file systems other than system V it might be used.

} -- 
} Robert Claeson, ERBE DATA AB, P.O. Box 77, S-175 22 Jarfalla, Sweden
} "No problems." -- Alf
} Tel: +46 758-202 50  EUnet:    rclaeson@ERBE.SE  uucp:   uunet!erbe.se!rclaeson
} Fax: +46 758-197 20  Internet: rclaeson@ERBE.SE  BITNET: rclaeson@ERBE.SE

I'm surprised that there weren't any questions on mandatory locking.

Bob Hutchison
att!lzaz!hutch