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Message-ID: | <4D8A308F.508@cornell.edu> |
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 -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
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