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From: Achim Gratz <Stromeko AT nexgo DOT de>
To: cygwin AT cygwin DOT com
Subject: Re: freopen/fread/popen bug
References: <54EFDEF4 DOT 4060308 AT cornell DOT edu>	<20150227084311 DOT GF11124 AT calimero DOT vinschen DOT de>	<54F065A0 DOT 9030608 AT cornell DOT edu> <54F098AA DOT 9050503 AT redhat DOT com>
Date: Fri, 27 Feb 2015 20:05:58 +0100
In-Reply-To: <54F098AA.9050503@redhat.com> (Eric Blake's message of "Fri, 27	Feb 2015 09:17:46 -0700")
Message-ID: <87bnkfgoo9.fsf@Rainer.invalid>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux)
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Content-Type: text/plain

Eric Blake writes:
>> What's really happening is that we need to peek at the first few bytes
>> of f before deciding which program to call in popen.  After peeking,
>> there's a call to fseek(f,0,0) before the popen, with the intention that
>> the child receives a file pointer set to the beginning of the file.  (I
>> left this out of my STC because it didn't affect the outcome.)  This
>> apparently works on Linux.
>
> Corinna is correct - you need to fflush() after the fseek() for it to
> affect the underlying offset.  Or, just use read()/lseek() instead of
> fread()/fseek() in the parent, to avoid stdio buffering altogether.
> That way, you'll guarantee the offset the child process will inherit
> without having to worry about flushing buffered state.

Basically this is the same category of bug that was in rcs for a while.
It isn't easy to find out what is and isn't guaranteed about these
things across a fork and since things work slightly differently on Linux
that's something you can probably expect to show up more often.


Regards,
Achim.
-- 
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