I think you have (1) and (2) backwards in this paragraph. Yes, I did it with the wrong numbers. I corrected it below.

I think JeffH and Jed want (1). I think I want (1), too.

I see the point. I have not a strong opinion and would like to know, what the other representatives in the Forum would like to see, because (2) MPI_DATATYPE_NULL is what they mainly have today. (1) "not existing" would be a real change. Rolf ----- Original Message -----

From: "Jeff Squyres" <[email protected]> To: "MPI-WG Fortran working group" <[email protected]> Sent: Thursday, February 18, 2016 10:31:23 PM Subject: Re: [MPIWG Fortran] Provide Fortran datatypes if Fortran bindings not provided?

On Feb 18, 2016, at 3:55 PM, Rolf Rabenseifner <[email protected]> wrote:

...

I would prefer a solution where an application or middlware can check at runtime whether the Fortran MPI datatypes exist, i.e., I like mpich's solution. It is more simple than all methods that People must check prior to compile and must remove or Change parts of there software.

The reason that JeffH and Jed hate this solution is because it makes it quite difficult for an application to know whether the MPI has Fortran support or not -- they basically have to compile an MPI application *and then run it* to see if the datatypes are actually MPI_DATATYPE_NULL.

Doing this inside a GNU Autoconf/configure script is very, very difficult because of the huge differences in how you launch MPI applications across different HPC environments.

Their (strong) preference is that you can have a simple compilation test in the GNU Autoconf/configure script that just tries to compile something like the following:

----- #include <mpi.h> MPI_Datatype foo = MPI_INTEGER; -----

If that compiles, then your MPI has Fortran support. If it doesn't compile, your MPI doesn't have Fortran support.

...

In the moment,in the case of a missing accompanying Fortran Compiler, an application or middleware that wants to handle MPI Fortran datatypes must handle both cases (1) mpi.h does not provide a specific MPI Fortran datatype (2) mpi.h provides this MPI Fortran datatype as MPI_DATATYPE_NULL

The additional case, that it is mapped to a real datatype is not an exception, because it defines some sort of accompanying Fortran Compiler.

I would prefer that the MPI standard defines, which of the two options should be implemented.

I think JeffH and Jed want (1).

I think I want (1), too.

As said at the beginning, I would prefer (2),

...

- as mpich is doing it in general, and - OMPI is doing it partially, whereas (1) was not done by mpich nor by OMPI.

-- Jeff Squyres [email protected] For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/

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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)