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Date:	Tue, 14 Apr 2015 09:39:37 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: Shared memory handling for mixed C/FORTRAN program
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On Apr 13 11:00, Christoph Weise wrote:
>=20
>=20
> > The problem is the call to shmget:
> >=20
> >   #undef  PAGESIZE
> >   #define PAGESIZE 512
> >   shmid =3D shmget(key, PAGESIZE, IPC_ALLOC);
> >=20
> > Since you're requesting only 512 bytes, the shared memory segment the
> > following shmat call returns is only 4K.  The shmget call should request
> > as much memory as it needs so that the OS call is called with the right
> > view size.
> >=20
> > I re-read the POSIX man page for shmget, and it doesn't mention anything
> > which would point out that Cygwin's behaviour here is wrong.  If anybody
> > has more information on this, please share them.
>=20
> Yes, you're right. I tried a workaround that uses a second shm section to=
 pass between executables either the size or more simply the shm ID of the =
main shm section. Either way this circumvented the problem.
>=20
> However with this solution there is an odd change in the base address
> of the shm section between calls from the routine that creates the
> section to the executables that use it.

You can't rely on getting the shm region allocated at the same memory
address in different executables.

> >On second thought, adjusting Cygwin's behaviour to Linux here is rather
> >trivial.  I applied a patch to the git repo and uploaded new developer
> >snapshots (2015-04-12) to https://cygwin.com/snapshots/
> >You only have to replace the DLL itself, cygserver is not affected by
> >this patch.  Please give it a try.
>=20
> That did the trick. With the new cygwin1 DLL I don't need to pass shm
> section information between executables and the overall behavior is
> consistent with that on Linux.
>=20
> Thank you for your help! I am very happy this works now on cygwin.

Thanks for the feedback.


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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