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Date: Sat, 9 Feb 2019 15:51:22 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: Tom Honermann <thomas DOT honermann AT synopsys DOT com>
Cc: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>,
        Cameron Gunnin <cameron DOT gunnin AT synopsys DOT com>,
        Marc-Andre Laverdiere <marc-andre DOT laverdiere AT synopsys DOT com>
Subject: Re: fcntl with O_APPEND fails to force append mode on stderr for native Windows programs
Message-ID: <20190209145122.GY13951@calimero.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
Mail-Followup-To: Tom Honermann <thomas DOT honermann AT synopsys DOT com>,	"cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>,	Cameron Gunnin <cameron DOT gunnin AT synopsys DOT com>,	Marc-Andre Laverdiere <marc-andre DOT laverdiere AT synopsys DOT com>
References: <9E5DE55D2546134DACFE4D3C117B36FDAD0CEE1D AT us01wembx1 DOT internal DOT synopsys DOT com>
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On Feb  8 22:25, Tom Honermann wrote:
> The following program demonstrates the problem.
> [...]
> Here is where things go bananas.  If the program is run with stdout
> initially redirected to a pipe, then the stderr output from the native
> Windows program ends up overwriting the stdout output from the two
> calls to echo:
>=20
> $ ./test | cat
> $ cat out
> The syntax of this command is:
>=20
> NET
>     [ ACCOUNTS | COMPUTER | CONFIG | CONTINUE | FILE | GROUP | HELP |
>       HELPMSG | LOCALGROUP | PAUSE | SESSION | SHARE | START |
>       STATISTICS | STOP | TIME | USE | USER | VIEW ]
>=20
> Note that the lines containing "first" and "second" are now absent in
> the file.

Thanks for the testcase.  I tracked this down to a bug when calling
native (non-Cygwin) apps.  If the descriptor is in O_APPEND mode, we
have to set the file offset to EOF before calling native apps.  However,
this only worked reliably for the first descriptor in O_APPEND mode, but
only by chance for subsequent descriptors in O_APPEND mode.  Your
example code with the extra pipe to `cat' triggered the misbehaviour.

I fixed that in git master:
https://sourceware.org/git/?p=3Dnewlib-cygwin.git;a=3Dcommitdiff;h=3D0be0b8=
f0335e

This will be in the 3.0.0 release.

I just uploaded new developer snapshots to https://cygwin.com/snapshots/
You only need to replace the Cygwin DLL for testing.  Please give it a try.


Thanks,
Corinna

--=20
Corinna Vinschen
Cygwin Maintainer

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