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Date: Wed, 15 Jan 2014 08:46:42 +0100
From: Peter Rosin <peda AT lysator DOT liu DOT se>
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To: cygwin AT cygwin DOT com
Subject: Re: fork() + file descriptor bug in 1.7.27(0.271/5/3) 2013-12-09 11:54
References: <831845 DOT 98759 DOT bm AT smtp116 DOT sbc DOT mail DOT ne1 DOT yahoo DOT com>	<52D55D96 DOT 8070407 AT redhat DOT com>	<946338 DOT 89157 DOT bm AT smtp116 DOT sbc DOT mail DOT ne1 DOT yahoo DOT com> <CAG9p0OR8JYgC1rzBCn1bvXe5ffpJS3vpmnnxw-7brJi8E8+uhQ AT mail DOT gmail DOT com>
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On 2014-01-15 05:53, Lord Laraby wrote:
> On Tue, Jan 14, 2014 at 10:50 AM, Ted Nolan wrote:
>> In message <52D55D96 DOT 8070407 AT redhat DOT com> you write:
>>>
>>> Your program may be violating POSIX, which would trigger undefined behavior.
>>>
>>> Quoting POSIX:
>>> pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05
>>>
>>
>> [long quote elided]
>>
>> Yikes!  That's pretty impenatrable.  And if it says what I think it says,
>> it seems to violate the way I've understood Unix fork() and how fds
>> (and stdio buffers) are inherited since forever.
>>
>> However..
>>
>> Do I understand that to say that if the first thing my child does is
>>
>>         fclose(fp);
>>
>> everything should be hunky-dory?
>>
>> Because I just tried that, and it's still not.
> 
> My two cents say, since the child is not referencing 'fp' at all,
> there is no violation of the POSIX semantics in this situation. It
> actually does seem, however, that the fork is closing, or at least
> forgetting the stdio file position of, fp when it forks. A possible
> memory corruption during fork from which fgets can not recover?

Let me requote one little bit quoted by Eric:

	                           (If the only action performed by one of the
	processes is one of the exec functions or _exit() (not exit()), the
	handle is never accessed in that process.)

Ted is using exit() in the children, not _exit(), and the above indicates
that exit() in fact "accesses the handle". My guess would be that fclose(3)
also "accesses the handle".

But, reading about _exit(), it seems that handle accesses are implementation
defined, so I'm not sure it will help in all situations.

Cheers,
Peter


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