From owner-freebsd-current  Sat Nov  6  5:45:52 1999
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Date: Sat, 6 Nov 1999 13:45:48 +0000
From: David Malone <dwmalone@maths.tcd.ie>
To: Ollivier Robert <roberto@keltia.freenix.fr>
Cc: FreeBSD Current Users' list <freebsd-current@FreeBSD.ORG>
Subject: Re: Serious locking problem in CURRENT
Message-ID: <19991106134548.A2921@walton.maths.tcd.ie>
References: <19991105225916.A14961@keltia.freenix.fr> <19991106005016.A865@keltia.freenix.fr>
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On Sat, Nov 06, 1999 at 12:50:16AM +0100, Ollivier Robert wrote:

A child process seems to be able to let go of a parent's lock on
4.0 by closing a file discriptor, the same doesn't seem to be true
on 3.3.

For example, if you try to run two copies of the included program
as on 3.3, you get a resource unavailable message.

	12:57:walton 6% ./lockf_test /tmp/blah 5 &
	[1] 2453
	13:01:walton 7% ./lockf_test /tmp/blah 5
	Unable to open file: Resource temporarily unavailable
	13:01:walton 8%
	[1]    Done                          ./lockf_test /tmp/blah 5

On 4.0 two run concurrently quite happily:

	12:54:gonzo 65% ./lockf_test /tmp/blah 5 &
	[1] 943
	13:02:gonzo 66% ./lockf_test /tmp/blah 5
	[1]    Done                          ./lockf_test /tmp/blah 5
	13:02:gonzo 67%

If you remove the "if(fork==0){close(fd);}" it two copies will not
run concurrently on 4.0. I'm not sure which behavior is correct, but
I suspect the 3.0 behavior is, 'cos /usr/bin/lockf is broken by
this on 4.0.

	David.

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc,char **argv)
{
	int fd;

	if( argc != 3 ) {
		fprintf(stderr, "Usage: %s filename seconds\n", argv[0]);
		exit(1);
	}

	if( (fd = open(argv[1], O_RDONLY|O_CREAT|O_NONBLOCK|O_EXLOCK,0666)) == -1 ) {
		perror("Unable to open file");
		exit(2);
	}

	if( fork() == 0 ) {
		close(fd);
	}

	sleep(atoi(argv[2]));

	exit(0);
}


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