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 on MPI_STATUS_SIZE and the indexes MPI_SOURCE, MPI_TAG, and MPI_ERROR. 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: int MPI_F_STATUS_SIZE and  MPI_F_SOURCE, MPI_F_TAG, and MPI_F_ERROR, (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 Rolf ----- Original Message -----

From: "MPI-WG Fortran working group" <[email protected]> To: "MPI-WG Fortran working group" <[email protected]> Cc: "wgropp" <[email protected]> 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:[email protected] | [email protected] ]

...

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:[email protected] | [email protected] ]

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-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] . 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) .