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Message-ID: <51EFCE28.8090904@star.sr.bham.ac.uk>
Date: Wed, 24 Jul 2013 13:52:56 +0100
From: Daniel Brown <ddb AT star DOT sr DOT bham DOT ac DOT uk>
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To: cygwin AT cygwin DOT com
Subject: Re: Win32 error in C program using openmp and fork()
References: <57302C57257EF2428CCAAF9BA83EC0448222C0EA AT mbx08 DOT adf DOT bham DOT ac DOT uk> <20130722080657 DOT GD2661 AT calimero DOT vinschen DOT de> <51EE7700 DOT 50803 AT star DOT sr DOT bham DOT ac DOT uk> <20130723130851 DOT GG9689 AT calimero DOT vinschen DOT de> <20130723141855 DOT GH9689 AT calimero DOT vinschen DOT de> <20130723143910 DOT GI9689 AT calimero DOT vinschen DOT de>
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Works perfectly! Been running my code since yesterday and no issues so far.

Thanks for the quick fix,
Daniel

On 23/07/2013 15:39, Corinna Vinschen wrote:
> On Jul 23 16:18, Corinna Vinschen wrote:
>> On Jul 23 15:08, Corinna Vinschen wrote:
>>> On Jul 23 13:28, Daniel Brown wrote:
>>>> and that still had the fork error. I have also tried running in safe
>>>> mode and
>>>> stopping all my anti-virus software just incase that was interfering
>>>> somehow.
>>>> So I get as an output now...
>>>>
>>>> Daniel AT XPS15z ~
>>>> $ uname -r
>>>> 1.7.22s(0.268/5/3)
>>>>
>>>> Daniel AT XPS15z ~
>>>> $ ./a.exe
>>>> I'm an openmp thread...
>>>> I'm an openmp thread...
>>>> I'm an openmp thread...
>>>> I'm an openmp thread...
>>>> Parent fork 1 [main] a 5832 C:\cygwin\home\Daniel\a.exe: *** fatal
>>>> error in forked process - failed
>>>> to create new win32 semaphore, currentvalue -2, Win32 error 87
>>>>
>>>> However if I reduce the number of threads from 4 to 2 with:
>>>>
>>>> #pragma omp parallel num_threads(2)
>>>> {
>>>> printf("I'm an openmp thread...\n");
>>>> }
>>> Ah, now there's something different.  If I set the number of threads to
>>> 4, I can reproduce this problem almost every try with currentvalue -2,
>>> occassionally with currentvalue -1.
>>>
>>>> Looking at the source in thread.cc _fixup_after_fork() the win32
>>>> error 87 is ERROR_INVALID_PARAMETER
>>>> which is due to currentvalue < 0.
>>>>
>>>> My only guess is that there is a race condition on the
>>>> currentvalue-- operations perhaps?
>>> Apparently.  I investigate...
>> Yes, there was a race condition in setting the value of the semaphore
>> private "currentvalue" member.  The only reason to keep track of the
>> value is to allow _fixup_after_fork to set the right value, but here the
>> race comes into play.  I redesigned this part so that it only sets
>> currentvalue once, right from fork itself, to propagate the correct
>> value to the child process.
>>
>> I'm just building new 32 and 64 bit snapshots.  They should be available
>> on http://cygwin.com/snapshots/ in an hour at the latest.  Please give
>> it a try.
> Both snapshots are up.  Test away.
>
>
> Corinna
>


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