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Date:	Fri, 13 Mar 2009 08:36:03 +0100
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Subject:	Re: Problem getting flock with timeout to work
From:	Robert Klemme <shortcutter AT googlemail DOT com>
To:	cygwin AT cygwin DOT com
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Corinna,

thanks for your feedback!

2009/3/12 Corinna Vinschen <corinna-cygwin AT cygwin DOT com>:
> On Mar 12 17:44, Robert Klemme wrote:
>> The second flock does not start the command as I expect it to be.
>>
>> I am referring to the man page of flock which says this about option -w:
>>
>> Fail =A0(with an exit code of 1) if the lock cannot be acquired within
>> seconds seconds. =A0Decimal fractional values are allowed.
>>
>> So, since the second flock obviously cannot obtain the lock in time, I
>> would have expected it to fail and not execute the command given.
>> This is at least a documentation issue as flock's behavior and
>> documentation do not match IMHO.
>
> This scenario (your first testcase) works fine with Cygwin 1.7 with my
> fix from a couple of hours ago. =A0I missed the fact that BSD flock lock
> semantic allows to set both lock types (LOCK_SH/LOCK_EX) regardless of
> the read/write mode in which the file has been opened, in contrast
> to POSIX lock semantic. =A0That's fixed now and other than that glitch,
> it worked.

Hey, that's good news!

> However...

> Cygwin won't be able to support the scenario from your second testcase
> for the time being. =A0The reason is that in this scenario
>
> =A0(
> =A0 =A0flock -s 200
> =A0 =A0...
> =A0) 200> lockfile
>
> the lock will end when the flock process exits. =A0The lock can only
> persist as long as the process which created the lock, or any of the
> child applications which inherited the file descriptor, still runs. =A0We
> can't maintain a global file descriptor table with lock information like
> the OS or what a master process could do if Cygwin would be designed so.

Thank you for the explanation!

> I know that's bad news, it certainly is for me since I thought I had
> implemented flock with full BSD semantics, but we just can't do that
> for now.

For me this is good enough - I find the variant "flock -c" better anyway. :=
-)

Thank you for looking into this!  Sorry for the intermediate confusion
about your "wrong" Linux.

Kind regards

robert

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