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Date:	Tue, 14 Jan 2014 11:18:26 +0100
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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
Message-ID:	<20140114101826.GC2380@calimero.vinschen.de>
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On Jan 13 11:06, tednolan AT bellsouth DOT net wrote:
> Hello,
>=20
> I'm running:
>=20
> CYGWIN_NT-6.1 prog5 1.7.27(0.271/5/3) 2013-12-09 11:54 x86_64 Cygwin
> gcc (GCC) 4.8.2
>=20
> on a 64 bit Win7 system.
>=20
> I have just run into an odd bug, which I have boiled down into the program
> below (which started as a mod to tiff2ps).
>=20
> If you compile this program:
>=20
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3DCUT HERE=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> [...]
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3DEnd code=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>=20
> and run it with this data:
>=20
> 00.tif  00.eps
> 01.tif  01.eps
> 02.tif  02.eps
>=20
> It will run forever.
>=20
> However, if you uncomment the fprintf with the ftell(), it runs as
> expected.

Alternatively it runs as expected when dropping the fork() call.  I'm
totally baffled, especially because this is such a blatant misbehaviour
that it should have been found much earlier.  I ran your testcase under
Cygwin versions from 2010, and the problem showed up, too, so this
problem is not exactly new.

I tried to debug it a bit already and for some reason the *Windows* call
reading the input file always starts reading at position 0 again.

Still to debug:

- Why on earth does the OS "forget" the current file position, just
  because the process calls fork?

- Why on earth does the OS not forget the current file position, just
  because the process calls ftell before fork?

Thanks for the report and the testcase.


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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