From owner-freebsd-questions  Mon Nov 13 23:49:57 1995
Return-Path: owner-questions
Received: (from root@localhost)
          by freefall.freebsd.org (8.6.12/8.6.6) id XAA06465
          for questions-outgoing; Mon, 13 Nov 1995 23:49:57 -0800
Received: from irz301.inf.tu-dresden.de (irz301.inf.tu-dresden.de [141.76.1.11])
          by freefall.freebsd.org (8.6.12/8.6.6) with ESMTP id XAA06401
          ; Mon, 13 Nov 1995 23:49:10 -0800
Received: from sax.sax.de by irz301.inf.tu-dresden.de (8.6.12/8.6.12-s1) with ESMTP id IAA29034; Tue, 14 Nov 1995 08:48:24 +0100
Received: by sax.sax.de (8.6.11/8.6.12-s1) with UUCP
	id IAA25537; Tue, 14 Nov 1995 08:48:05 +0100
Received: (from j@localhost) by uriah.heep.sax.de (8.6.12/8.6.9) id IAA00168; Tue, 14 Nov 1995 08:27:35 +0100
From: J Wunsch <j@uriah.heep.sax.de>
Message-Id: <199511140727.IAA00168@uriah.heep.sax.de>
Subject: Re: elm problem :)
To: bmk@dtr.com
Date: Tue, 14 Nov 1995 08:27:34 +0100 (MET)
Cc: roberto@keltia.freenix.fr, geoff@ginsu.com, bsd@ee.petra.ac.id,
        questions@freebsd.org, freebsd-hackers@freebsd.org
Reply-To: joerg_wunsch@uriah.heep.sax.de (Joerg Wunsch)
In-Reply-To: <199511132326.PAA07428@dtr.com> from "bmk@dtr.com" at Nov 13, 95 03:26:12 pm
X-Phone: +49-351-2012 669
X-Mailer: ELM [version 2.4 PL23]
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
Content-Length: 1541      
Sender: owner-questions@freebsd.org
Precedence: bulk

As bmk@dtr.com wrote:
> 
>   I've always had to use flock style locking
> - fcntl always seemed to cause the symptoms the original complaint
> described.

That's quite surprising since both functions share a rather large part
of their implementation inside the kernel.  flock() is basically an
fcntl-style lock spanning the entire file:

/* ARGSUSED */
int
flock(p, uap, retval)
        struct proc *p;
        register struct flock_args *uap;
        int *retval;
{
	...

        lf.l_whence = SEEK_SET;
        lf.l_start = 0;
        lf.l_len = 0;
        if (uap->how & LOCK_UN) {
		...
                return (VOP_ADVLOCK(vp, (caddr_t)fp, F_UNLCK, &lf, F_FLOCK));
        }
	...
        return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf, F_FLOCK|F_WAIT));
}

and:

/* ARGSUSED */
int
fcntl(p, uap, retval)
        struct proc *p;
        register struct fcntl_args *uap;
        int *retval;
{
	...

        switch (uap->cmd) {

	...
        case F_SETLK:
		...
                if (fl.l_whence == SEEK_CUR)
                        fl.l_start += fp->f_offset;
                switch (fl.l_type) {

                case F_RDLCK:
                        if ((fp->f_flag & FREAD) == 0)
                                return (EBADF);
                        p->p_flag |= P_ADVLOCK;
                        return (VOP_ADVLOCK(vp, (caddr_t)p, F_SETLK, &fl, flg));

			...
	...
}

-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)