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Subject: Re: [OMPI devel] Fortran 90 Interface
From: N.M. Maclaren (nmm1_at_[hidden])
Date: 2009-01-23 10:07:49


On Jan 23 2009, Jeff Squyres wrote:
>
>>> No. Open MPI's Fortran MPI_COMM_WORLD is pretty much hard-wired to 0.
>>
>> That's a mistake. But probably non-trivial to fix.
>
>Could you explain what you meant by that? There is no "fix"; Open
>MPI's Fortran MPI_COMM_WORLD has always been 0. More specifically,
>there is no coordination of the value of MPI_COMM_WORLD between MPI
>implementations, unless some choose to emulate MPICH (or MPICH2 --
>because they're different).
>
>I'm assuming what you're really referring to is the fact that there is
>no currently binary compatibility between different MPI
>implementations (forgive me if my assumption is wrong). ...

Good Lord, no!!!!! You don't know me, but that's NOT the sort of thing
I would refer to.

The reason is that it is extremely likely to be supplied by an incorrect
program (e.g. by passing an uninitialised integer that should have been
set to MPI_COMM_WORLD). When such a program is ported elsewhere (or even
when a new version of a compile moves code around), a program that has
been 'working' for ages breaks.

I could give you some amusing stories of the contents of location zero,
and how bitterly the owners of one system regretted having architected
that page zero was a page of zeroes, a decade down the line :-)

Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email: nmm1_at_[hidden]
Tel.: +44 1223 334761 Fax: +44 1223 334679