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

Jeff Squyres (jsquyres) jsquyres at cisco.com
Tue Mar 5 08:09:27 CST 2013

The problems I have with changing to any flavor of INTEGER are twofold:

1. Backwards incompatibility.  Yes, mpi_f08 is new, but there's now nonzero number of codes out there that use it, and are therefore using default LOGICAL.

2. Violates the Law of Least Astonishment.  For 15+ years, Fortran devs have been using default LOGICAL for MPI's boolean flags, and testing with .true. and .false..  Changing that to INTEGER (any flavor) would be downright weird.

On Mar 4, 2013, at 7:57 PM, Craig Rasmussen <rasmus at cas.uoregon.edu> wrote:

> I talked with Bill briefly on the phone and he suggested something close to what we already do, namely use overloading to get around the problem (see Bill's earlier comment on two levels of wrappers).
> We could switch to LOGICAL(C_BOOL) dummy arguments in all procedures with BIND(C) interfaces.  Then in addition we could overload these functions (like MPI_Test) with thin wrappers that are required to call the regular BIND(C) version so that the tools folks are happy.  Regular users would never know the difference.  I suppose we could actually use INTEGER(C_INT) in these interfaces rather than LOGICAL(C_BOOL) as there is less chance of confusion and INTEGER(C_INT) and LOGICAL can always different.  Whereas LOGICAL(C_BOOL) and LOGICAL may be the same type.
> Bill also suggested that replacing LOGICAL dummy arguments with INTEGER(C_INT) in the callback routines would a safer way to go.
> Craig Rasmussen
> CAS Scientific Programmer
> rasmus at cas.uoregon.edu
> On Mar 4, 2013, at 12:19 PM, Bill Long wrote:
>> On 3/4/13 1:26 PM, Tobias Burnus wrote:
>>> Bill Long wrote:
>>>>> There isn't any difficulty about a Fortran wrapper using integers (as
>>>>> C does), and converting them to LOGICAL, but it can't be BIND(C).
>>>> There is also no difficulty in having the wrapper arguments being
>>>> subroutine sub (x) bind(c, name="MPI_xxx_f")
>>>> logical :: x
>>>> integer(int) :: cx
>>>> cx = merge(1, 0, x)
>>> Well, except that compilers might simply reject a default-kind LOGICAL
>>> with BIND(C), in line with the Fortran standard.
>> Ug. I tried 5 compilers.  Three (Cray, Intel, PGI) just let it pass. One gave an error, and one gave a warning.  What a mess.  I'm more convinced now that before that the only practical solution here is to get rid of the LOGICAL arguments, replacing them with INTEGER arguments, which is what the C functions expect anyway.  (Alternatively, the problem goes away if support for mpif.h is removed, which I would highly recommend anyway.)
>> I'm trying to work out how to use two layers of wrapper functions to get around the error from that one compiler.  Unattractive, to say the least.
>> Cheers,
>> Bill
>>> Your code above assumes that a Fortran compiler allows passing a
>>> default-kind LOGICAL to a Bind(C) procedure and that it then can handle
>>> that LOGICAL. I think no one really doubts the second part. But the
>>> first part is not really covered by the Fortran standard and
>>> standard-conforming compilers do exist, which reject default-kind LOGICALs.
>>> J3/WG5 could change the standard such that it supports other LOGICAL
>>> kinds besides C_Bool, including kind(.true.). However, if
>>> LOGICAL(kind=kind(.true.) / C_INT) exists, users will assume that it
>>> acts like C's "int", which will lead to problems. So far, I haven't seen
>>> a convincing proposal how to solve this.
>>> Tobias
>>> _______________________________________________
>>> 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
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Jeff Squyres
jsquyres at cisco.com
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