Date: Sat, 3 Dec 2011 15:58:06 -0500
From: Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Redirecting output from running proc doesn't modify the "last modified time" field for target file
Message-ID: <20111203205806.GA23589@ednor.casa.cgf.cx>
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On Sat, Dec 03, 2011 at 09:36:56PM +0100, Corinna Vinschen wrote:
>On Dec  2 13:04, Eric Blake wrote:
>> On 12/02/2011 11:50 AM, Jon Clugston wrote:
>> > While this loop is running, the timestamp on "x.log" doesn't change
>> > (whereas on Linux it changes every 10 seconds).  It sure looks to me
>> > that Windows just doesn't bother updating the file timestamp while it
>> > is open.  I don't know if this update is required by POSIX - I would
>> > doubt that it is.
>> 
>> POSIX requires that any write() to an open file mark it for update; the
>> update doesn't have to occur right away (so you can batch up several
>> writes, but only change the mtime metadata once at the end of the
>> batch), but it DOES require that stat() and several similar functions
>> flush all marked updates prior to exposing timestamps to the user.  So
>> yes, Windows is violating POSIX, and I have no idea whether cygwin can
>> work around it.
>
>You can change all file operations to use FILE_WRITE_THROUGH and
>FILE_NO_INTERMEDIATE_BUFFERING.  Downside:  No caching.  All file
>operations must be sector aligned.  Degraded system performance.
>Broken when a process has only write permissions.
>
>Alternatively, change write(2) so that every WriteFile call is
>accompanied by a FlushFileBuffers call.  Downside: Extremly degraded
>write performance.
>
>Alternatively:  Lie.  That's how SUA does it.  It has a background
>service running which (among other things) keeps track of write
>operations of SUA applications.  If a SUA application calls write(2)
>the write timestamp is kept up to date internally, while the metadata
>on disk is still lagging in Windows style.  A SUA application calling
>stat(2) gets a POSIX compatible timestamp.  Non-SUA apps continue to
>show the "wrong" timestamp.  If non-SUA apps write to a file, SUA apps
>also show the Windows timestamp.  Cygwin could do the same.  Downside:
>We don't have a mandatory background service running.  Quite a hoop to
>jump through to implement a usually non-critical POSIX requirement.

Since I believe that this has come up before is anyone willing to
provide a FAQ entry that we can point to in the future?

cgf

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