From owner-freebsd-hackers  Thu Nov 27 21:44:01 1997
Return-Path: <owner-freebsd-hackers>
Received: (from root@localhost)
          by hub.freebsd.org (8.8.7/8.8.7) id VAA21907
          for hackers-outgoing; Thu, 27 Nov 1997 21:44:01 -0800 (PST)
          (envelope-from owner-freebsd-hackers)
Received: from anlsun.ebr.anlw.anl.gov (anlsun.ebr.anlw.anl.gov [141.221.1.2])
          by hub.freebsd.org (8.8.7/8.8.7) with SMTP id VAA21870
          for <hackers@freebsd.org>; Thu, 27 Nov 1997 21:43:57 -0800 (PST)
          (envelope-from cmott@srv.net)
Received: from darkstar.home (tc-if3-24.ida.net [208.141.171.129]) by anlsun.ebr.anlw.anl.gov (8.6.11/8.6.11) with SMTP id WAA10339 for <hackers@freebsd.org>; Thu, 27 Nov 1997 22:43:48 -0700
Date: Thu, 27 Nov 1997 22:43:01 -0700 (MST)
From: Charles Mott <cmott@srv.net>
X-Sender: cmott@darkstar.home
To: hackers@freebsd.org
Subject: flock() question
Message-ID: <Pine.BSF.3.96.971127223541.29176B-100000@darkstar.home>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: owner-freebsd-hackers@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk

Below are two test programs.  What I observe is that if a file descriptor
is opened and a process forks, then advisory locking does not appear to
work.  On the other hand, if the process forks, and then the file is
opened independently by each process, advisory locking works perfectly
well.

Is this correct?  From reading the man page on flock(2), I would have
guessed that locking would have worked in both situations.  I am running
2.2.2-R.

  -- Charles Mott

*** Test 1: open() before fork() ***
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>

main()
{
    int fd;

    fd = open("locktest", O_WRONLY | O_CREAT, 0644);

    fork();

    for (;;)
    {
        printf("(1) pid=%d\n", getpid());
        flock(fd, LOCK_EX);
        printf("(2) pid=%d\n", getpid());
        sleep(5);
        printf("(3) pid=%d\n", getpid());
        flock(fd, LOCK_UN);
    }
}


*** Test 2: open() after fork() ***
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>

main()
{
    int fd;

    fork();

    fd = open("locktest", O_WRONLY | O_CREAT, 0644);

    for (;;)
    {
        printf("(1) pid=%d\n", getpid());
        flock(fd, LOCK_EX);
        printf("(2) pid=%d\n", getpid());
        sleep(5);
        printf("(3) pid=%d\n", getpid());
        flock(fd, LOCK_UN);
    }
}