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Date: Wed, 23 Mar 2011 13:40:31 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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To: cygwin AT cygwin DOT com
Subject: Re: Broken autoconf mmap test
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On 3/23/2011 11:44 AM, Corinna Vinschen wrote:
> On Mar 23 10:08, Ken Brown wrote:
>> On 3/23/2011 9:26 AM, Corinna Vinschen wrote:
>>> Btw., I just experimented with another flag which has not been exposed
>>> to the Win32 API, and which is not officially documented.  It's called
>>> AT_EXTENDABLE_FILE and allows to specify a view which is bigger than
>>> the filesize.  I tried this on W7-64, and it works.  Just not exactly
>>> as we need it.  The trailing pages are not commited but only reserved,
>>> so accessing them still results in a SEGV.  And while commiting them
>>> allows to run the autoconf testcase, it also changes the filesize of
>>> the file on disk.  Oh well.
>>
>> Is the scenario in which the autoconf test fails likely to occur in
>> practice?  I wonder if it could be responsible for some of the
>> mysterious crashes that people have reported on 64-bit systems.
>
> Usually it shouldn't.  While you can write into a mmaped area beyond
> EOF up to the end of the last page, it doesn't make a lot of sense.
> The data will never show up in the mapped file.
>
> I have no idea if there are applications out there which rely on this
> crude behaviour.

I was mainly thinking of this from the point of view of a Cygwin package 
maintainer.  In view of what you said, it seems that it's probably safe 
to continue using the workaround

   export ac_cv_func_mmap_fixed_mapped=yes

that's built into cygport.  Maintainers who don't use cygport and who 
build on a 64-bit system will have to manually use a similar workaround, 
or else their builds won't use mmap.

It doesn't look like there's going to be an autoconf change to resolve this:

   http://lists.gnu.org/archive/html/autoconf/2011-03/msg00041.html

Ken

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