[MPI3 Fortran] [Mpi-comments] MPI 3.0:Fortran 2008 interface - issue with the LOGICAL kind

Bill Long longb at cray.com
Mon Mar 11 08:01:37 CDT 2013

Hi Martin,

Ultimately, the problem is that some Fortran compilers (one out of five 
that I tried) will give an error for a dummy argument declared default 
LOGICAL in a BIND(C) interface.  This is because there is not really a 
good corresponding type in C for all Fortran compilers, and the internal 
representation of LOGICAL in Fortran might not even match the _Bool type 
in C.

One solution to the problem, at least for the "normal" MPI functions, is 
the write the Fortran wrapper in Fortran, with no BIND(C) [gets around 
the compiler error message] and within the wrapper, convert the LOGICAL 
value to an int value that is passed in a call to the actual C function 
[since the corresponding  C argument is an int].   If the argument is 
intent(out) then you convert it back from int to LOGICAL after the C 
function call, before returning from the wrapper.  These conversions are 
each single lines of Fortran code, and (if the wrapper is in a module) 
will probably get inlined, and possibly optimized away for some Fortran 
implementations.  The goal is that C should never have to be confronted 
with the foreign Fortran default LOGICAL directly.

I believe that the current wording of the MPI 3 spec for using mpif.h 
still requires dealing with the no/single/double underscore issue, and 
that is assuming one of those is the choice for default linker names in 
Fortran for a particular implementation.  There is nothing in the 
Fortran standard that requires any of these three options.

I noted that Nick Maclaren replied to this message as well.  For those 
not aware, Nick is in a rather unique position of having participated on 
both the C and Fortran standardization committees.   He pretty much 
knows where all the  [Fortran-C interoperability] dead bodies are buried.


On 3/11/13 12:18 AM, Schulz, Martin wrote:
> Hi all,
> I am trying to catch up with the various threads on this (sorry, last week was hectic and I didn't get to all emails). I have to admit that I am still confused why there is a problem, but we can clarify this during a phone call or during the march meeting. However, doesn't not doing BINDC create a problem, since I now have to again deal with the no/one/two underscore issue? This sounds like a bad interface design to me, especially when we starting mixing  things.
> Martin
> On Mar 4, 2013, at 9:47 AM, Rolf Rabenseifner <rabenseifner at hlrs.de> wrote:
>> Dear all,
>> I forget to say thank you to all of you who helped to solve
>> this hard problem. It was a really helpful discussion.
>> It is a pity that we could not keep the BIND(C) for
>> all routines in mpi_f08, which was one goal with mpi_f08 and TS 29113.
>> In my proposal #364, we have to remove BIND(C) from
>> - the 30 normal MPI routines with LOGICAL arguments,
>> - the  9 MPI routines with callbacks in the argument list,
>> - and 10 predefined callbacks,
>> i.e., from 49 out of 357 MPI routines.
>> I do not see a better solution that can be done by the MPI Forum.
>> Better solutions are still possible for the 30 normal routines
>> and/or for the 9 routines with callback-arguments as soon as
>> the Fortran standard will include some additions and
>> those additions are provided by the compilers - but if this
>> will happen then it will not happen soon.
>> It was luck that all routines with buffer arguments do not
>> have also a LOGICAL argument.
>> Best regards
>> Rolf
>> ------------------
>> Malcolm and all MPI implementors,
>> and TOOLS implementors,
>> First Malcolm, thank you for your helpful answer.
>> After all the discussion, the MPI-3.0 errata decision is
>> very simple: Remove BIND(C) everywhere where LOGICAL is around.
>> I did it in the ticket #364
>> https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/364
>> I hope that one of the implementors with Fortran knowledge goes
>> through my proposal and returns an "okay" as a review on the ticket.
>> The major changes:
>> - BIND(C) removed on all callback abstract interfaces.
>> - BIND(C) removed on all predefined callbacks.
>> - Creating a new routine group "MPI_TEST" for all MPI routines
>>    with LOGICAL arguments, except MPI_OP_CREATE, which is already
>>    in an own group.
>> - Telling that the routines in the groups MPI_TEST,
>>    cannot be implemented with BIND(C) in Fortran 2008 + TS 29113 and earlier.
>> TOOLS implementors should be aware that I added this new routine group.
>> Best regards
>> Rolf
>> ----- Original Message -----
>>> From: "Malcolm Cohen" <malcolm at nag-j.co.jp>
>>> To: "MPI-3 Fortran working group" <mpi3-fortran at lists.mpi-forum.org>
>>> Sent: Monday, March 4, 2013 3:56:50 AM
>>> Subject: Re: [MPI3 Fortran] Fwd: [Mpi-comments] MPI 3.0:Fortran 2008 interface - issue with the LOGICAL kind
>>>>    (B1) Is it possible, that a BIND(C) routine
>>>>         has a dummy argument that is a non-BIND(C)
>>>>         ABSTRACT INTERFACE procedure argument?
>>> That is not possible: all dummy arguments of a BIND(C) routine must be
>>> interoperable - for procedures that requires BIND(C).
>>> It would be reasonable to extend C_FUNLOC and C_F_PROCPOINTER to
>>> permit
>>> non-interoperable procedures, similarly to how we permit
>>> non-interoperable
>>> variables in C_LOC and C_F_POINTER. Then the argument could be
>>> C_FUNPTR (by
>>> value). I don't know whether this would help you though.
>>>>    (B2) Do we have any problem with
>>>>         the TYPE(C_PTR), VALUE dummy arguments
>>>>         in the ABSTRACT INTERFACE MPI_User_function
>>>>         on MPI-3.0 page 183 line 45 - page 184 line 3
>>>>         if the we remove the BIND(C) from this
>>>>         ABSTRACT INTERFACE (i.e. on page 183, line 47).
>>> No problem.
>>> Cheers,
>>> --
>>> ................................Malcolm Cohen, Nihon NAG, Tokyo.
>>> _______________________________________________
>>> mpi3-fortran mailing list
>>> mpi3-fortran at lists.mpi-forum.org
>>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-fortran
>> --
>> 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)
> ________________________________________________________________________
> Martin Schulz, schulzm at llnl.gov, http://people.llnl.gov/schulzm
> CASC @ Lawrence Livermore National Laboratory, Livermore, USA
> _______________________________________________
> mpi3-fortran mailing list
> mpi3-fortran at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-fortran

Bill Long                                           longb at cray.com
Fortran Technical Support    &                 voice: 651-605-9024
Bioinformatics Software Development            fax:   651-605-9142
Cray Inc./Cray Plaza, Suite 210/380 Jackson St./St. Paul, MN 55101

More information about the mpiwg-fortran mailing list