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1.7.9: Problem with line endings of Perl output redirected to a file with textmode mounting

Hello all,

let me report a strange behaviour with Cygwin Perl (I'm using cygwin1.dll
1.7.9-1, full installation 2 weeks ago).

File foo.h is an ordinary text file, all lines are terminated with DOS
style line endings <cr> <lf> (hex: 0d 0a).
It is located in a directory with textmode mounting in cygwin.
One <cr> <lf> sequence of foo.h is split by a 4096 byte boundary within
the file: "od -c -Ax foo.h" shows a <cr> (='\r') at byte offset 4095
and a <lf> (='\n') at offset 4096 (0x1000):
000ff0   /   /   /   /   /   /  \r  \n   /   /   X   X   X   X   X  \r
001000  \n   /   /  \r  \n   /   /  \r  \n

Now I issued the command "perl -pe 's/12345/54321/' foo.h >foomod.h"
to produce foomod.h, located in the same directory as foo.h, thus with
textmode mounting too.
When I examined the result, I noticed that foomod.h was one byte bigger
then foo.h. I expected identical size, and "od -c -Ax foomod.h" reports:
000ff0   /   /   /   /   /   /  \r  \n   /   /   X   X   X   X   X  \r
001000  \r  \n   /   /  \r  \n   /   /  \r  \n

Ups! The original <cr> <lf> sequence starting at offset 4095 (0xfff)
became a three character sequence <cr> <cr> <lf>! The <cr> is duplicated!

In other files created by Perl with output redirection I observed this
behaviour with every <cr> <lf> line ending, that is split by a 4096 byte
boundary (even multiple times in one output file). Line endings not
split by a 4096 byte boundary do not show this behaviour.

The behaviour does not occur, when the destination file is located
in a directory with binmode mounting. It does not occur either, when
I use sed instead of Perl ("sed -e 's/12345/54321/' foo.h >foomod.h"),
so I think the problem is specific to Cygwin Perl, not to Cygwin in

I this a bug of the output buffering mechanism of Cygwin Perl?
Or do I anything wrong?
Any answer is highly appreciated. Thanks in advance.

Best regards

Mit freundlichen Grüßen

Dipl. Inform. Sven Severus
HIMA Paul Hildebrandt GmbH + CO KG
Abt: Entwicklung Software
Albert-Bassermann-Strasse 28
68782 Bruehl

Tel: +49 6202 709-289
Fax: +49 6202 709-299

HIMA Paul Hildebrandt GmbH + Co KG, Albert-Bassermann-Str. 28, 68782 Bruehl bei Mannheim
Kommanditgesellschaft, Sitz Bruehl, Deutschland - Registergericht Mannheim HRA 421017
Ust-ID: DE 144286400, St.Nr: 43038 00190

Persoenlich haftende Gesellschafterin Paul Hildebrandt Verwaltungsgesellschaft mbH,
Sitz Bruehl, Deutschland - Registergericht Mannheim HRB 420588

Geschaeftsfuehrer: Dipl.-Betriebswirt Steffen Philipp

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