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Message-ID: <21333.26515.393838.380071@compute01.cs.columbia.edu>
Date: Mon, 21 Apr 2014 14:46:43 -0400
To: cygwin AT cygwin DOT com
Subject: Re: fstat st_size on open files on Parallels filesystem is wrong
In-Reply-To: <227151856.20140421223417@yandex.ru>
References: <21333 DOT 25325 DOT 11106 DOT 958642 AT compute01 DOT cs DOT columbia DOT edu>	<227151856 DOT 20140421223417 AT yandex DOT ru>
From: lennox AT cs DOT columbia DOT edu
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On Monday, April 21 2014, "Andrey Repin" wrote to "lennox at
cs.columbia.edu, cygwin at cygwin.com" saying:

> Greetings, lennox at cs.columbia.edu!
> 
> > I’m running cygwin64 1.7.29 in a Windows 8.1 Pro virtual machine, running in
> > Parallels Desktop 9.0.24229 on Mac OS X 10.9.2.
> 
> > Parallels Desktop automatically mounts my Mac OS X home directory as a Z:
> > drive in Windows.  Cygwin mount reports this drive as being type "prlsf".
> 
> > Unfortunately, I've discovered that if I have an open file on this
> > filesystem which has been written to, the size returned by Cygwin fstat() on
> > the open file is wrong.  A stat() of the file after it's been closed is
> > correct.
> 
> > This has the consequence that emacs always thinks saved files have been
> > modified externally, since emacs looks at files' sizes (as well as their
> > modification times) to detect external changes.  This makes emacs
> > near-unusable.
> 
> > This problem does not occur for files in my Cygwin home directory, or other
> > locations mounted on my Windows C: drive.
> 
> > I've attached a simple unit test program that illustrates the problem.
> > I've also attached my cygcheck -s -v -r output.
> 
> > Any ideas?  Is this a Cygwin bug, a Parallels bug, or something else?
> > Glancing over the Cygwin code, I see that there are a few cases where fstat
> > has special cases for certain filesystem types.
> 
> You never flushing the buffer in your test code, or I'm reading it wrong?

This is using Posix APIs -- open() / write() -- not C APIs, fopen() /
fwrite(), so there shouldn't be a buffer?  Notice that the test behaves as I
expect for a file on NTFS.

Adding a call to fsync() prior to the fstat() call doesn't change anything.

I've modeled this as closely as I could on what it appears emacs is doing,
as far as I could tell.

-- 
Jonathan Lennox
lennox at cs.columbia.edu

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