[MPI3 Fortran] Specify IN/OUT/INOUT in MPI spec?

Hubert Ritzdorf ritzdorf at it.neclab.eu
Thu May 28 12:10:23 CDT 2009

Jeff Squyres wrote:
> On May 26, 2009, at 3:10 PM, Bill Long wrote:
>> > "I note that none of the current Fortran API interface specifications
>> > in MPI-2.1 list the intent of the parameters.  Do we want to update
>> > all of MPI in 3.0 to list the INTENT, and/or use "more modern Fortran
>> > notation""
>> >
>> > Yes, please!
>> I'll second that.  Intents are a good idea in general.  One caution: One
> Hubert / Rolf / Craig -- do you guys agree that this is a Good Thing 
> to do for MPI-3?
I agree to this. But you cannot simply use the current MPI 
specifications IN, OUT, INOUT
for Fortran buffers. For example, the receive buffer argument in 
MPI_Recv() must be Intent (INOUT)
since it may contain data to be preserved such as Steve Lionel wrote.
>> > We should also consider whether BIND(C) and other C 
>> interoperability features should be used (especially the KIND values 
>> from ISO_C_BINDING that correspond to C types.)
> There's been extensive discussion about this (e.g., use of BIND(C)) -- 
> I'll let the other more-Fortran-knowledgeable MPI guys comment on 
> these ideas...
>> Using bind(c) and some other modern features - the VALUE attribute could
>> be used beneficially in many instances - does have the the side effect
>> that having the interface visible (i.e. including the USE MPI statement)
>> is required rather than optional.  That would involve a level of user
>> awareness that is greater than if you only use features that do not
>> require explicit interfaces.  Some compilers have an "automatic use"
>> option that would avoid having to modify existing user sources, but I
>> don't think this is universally available.  I'm all in favor of using
>> the modern stuff. I just want to make sure we have 'full disclosure' on
>> what that involves.
>> Another side effect of users including USE MPI with interfaces that
>> include intent specs:  some codes that start to experience compile-time
>> errors.  Granted, the code was wrong all along, and the compiler is now
>> able to detect that.  In the end this is a good thing, but the initial
>> response is "but it worked before" (or so they thought).
> With MPI-3 being a major revision number change, we're open to at 
> least some of this kind of stuff.
> FWIW, Open MPI has an "mpi" module with INTENT specifications.  I 
> don't know what the other MPI implementations do in their "mpi" 
> module.  To be clear: the MPI spec currently *allows* you to use 
> INTENT in your "mpi" module; it just doesn't *require* it.
Also NEC MPI has an "mpi" module with INTENT specifications.

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