[MPIWG Fortran] MPI_STATUS_SIZE: Why only in Fortran?

Rolf Rabenseifner rabenseifner at hlrs.de
Tue Sep 8 10:15:19 CDT 2020

It was clearly overseen when setting up the language interoperability chapter.
Yes, we should duplicate the whole table MPI-3.1 A.1.1. p672:6-12
My apologies, because I was involved.

In C, I would recommend to use the following name scheme that we already
use for other Fortran specific constants and functions:

(in C only to specify a Fortran status array and indexes into such an array
as defined in mpif.h and the mpi module. The values of Fortran MPI_STATUS_SIZE
and C MPI_F_STATUS_SIZE are identical, whereas the indexes in C are 1 less
than in Fortran to reflect that the arrays in Fortran start by default with index 1
whereas in C with index 0) 

On MPI-3.1 p656:29, I would add

In C, such an f_status array can be defined with  MPI_Fint f_status[MPI_F_STATUS_SIZE].
Within this array, one can use in C the indexes MPI_F_SOURCE, MPI_F_TAG and MPI_F_ERROR
to access the same elements as in Fortran with MPI_SOURCE, MPI_TAG and MPI_ERROR.
The C indexes are 1 less the corresponding indexes in Fortran due to the different
default array start indexes in both languages.

And MPI_F_STATUS_SIZE must be added to the list on page 15.

Yes, and I would treat it is an Errata because of the incompletness of 
Section 17.2.5.

And yes, sizeof(MPI_Status) may be 24 (bytes in C) whereas MPI_STATUS_SIZE 
and the new MPI_F_STATUS_SIZE would both be 6 (Fortran INTEGER).

Best regards

----- Original Message -----
> From: "MPI-WG Fortran working group" <mpiwg-fortran at lists.mpi-forum.org>
> To: "MPI-WG Fortran working group" <mpiwg-fortran at lists.mpi-forum.org>
> Cc: "wgropp" <wgropp at illinois.edu>
> Sent: Tuesday, September 8, 2020 3:43:29 PM
> Subject: Re: [MPIWG Fortran] MPI_STATUS_SIZE: Why only in Fortran?

> I honestly can’t remember why we restricted MPI_STATUS_SIZE to Fortran - my best
> guess is one Jeff mentions, that we were maintaining the possibility that the C
> and Fortran statues could be a different size, and the possible confusion with
> sizeof(MPI_Status) is real. I’d go with a distinct name that included FORTRAN
> as an alternative.
> Bill
> William Gropp
> Director and Chief Scientist, NCSA
> Thomas M. Siebel Chair in Computer Science
> University of Illinois Urbana-Champaign
> On Sep 8, 2020, at 8:32 AM, Jeff Squyres (jsquyres) via mpiwg-fortran < [
> mailto:mpiwg-fortran at lists.mpi-forum.org | mpiwg-fortran at lists.mpi-forum.org ]
> > wrote:
> Question: why is MPI_STATUS_SIZE only defined in Fortran?
> MPI-3.1:2.5.4 p15:36 specifically states that MPI_STATUS_SIZE is only defined in
> Fortran. Why isn't is also defined in C?
> I ask because I was just writing a test for the MPI_Status conversion functions
> in Open MPI. The test spans both Fortran and C. But in order to test the
> MPI_Status_f2c / MPI_Status_c2f functions (which are only defined in C, mind
> you), I wanted to make a stack-allocated array of MPI_Fint's for the
> INTEGER-array-style Fortran status. ...but I couldn't, because MPI_STATUS_SIZE
> isn't defined in C. Indeed, there's no mechanism in C for knowing how long an
> INTEGER-style Fortran status array needs to be -- even though we have functions
> in C for dealing with INTEGER-style Fortran statuses.
> To be totally clear, you can't do this in C:
> MPI_Fint f_status[MPI_STATUS_SIZE];
> Is this an oversight? Shouldn't we define MPI_STATUS_SIZE in all languages?
> Or, perhaps if there's possible confusion between MPI_STATUS_SIZE in C and
> sizeof(MPI_Status) (since they could actually be different values), perhaps we
> could call it MPI_FORTRAN_STATUS_SIZE in C? (Or even
> MPI_FORTRAN_INTEGER_STATUS_SIZE to differentiate it from
> sizeof(TYPE(MPI_Status))?
> Thoughts?
> If we view this as an errata, I think there's still time to get an update in for
> MPI-4.
> --
> Jeff Squyres
> [ mailto:jsquyres at cisco.com | jsquyres at cisco.com ]
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Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de .
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 .
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 .
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner .
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .

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