[MPI3 Fortran] MPI-3, the Interoperability TR and choice buffers

[N.M. Maclaren]

>In my earlier reply, I expanded on one of Nick's ideas by proposing the 
>CDESC attribute for the dummy argument declaration, probably with the 
>restriction that it is only allowed in a BIND(C) interface.

Definitely only there :-)

>  Since we 
>can define the characteristics of the C descriptor, we could, for 
>example, have a field that would identify the corresponding actual 
>argument as assumed-size, and say that if that is set then the value of 
>the last extent is not usable.  The compiler for the Fortran caller 
>would be required to generate code to correctly set this flag.

I don't see much of a problem.  It would also be useful for scalars.

>In some sense, the CDESC attribute is comparable to the VALUE attribute 
>in a BIND(C) interface - it specifies a particular passing semantic that 
>the C function will understand.   If this were implemented, 
>'assumed-rank' would not be needed for the MPI interfaces.

I didn't read your proposal in detail, and think that it is a mistake.
It would be a far better idea to have an orthogonal attribute that did
nothing else than pass by descriptor.  That would be cleanest and most
future-proof.

> I think 
>that, from the Fortran side, that would be cleaner than allowing 
>assumed-rank to correspond to an assumed-size actual.  There are many 
>details to be worked out if we go this direction.

Not really.  All one would have to say is that CDESC could be used only
in a declaration in an interface, and not one in a procedure definition!
It would then be available only for calling into C.

Regards,
Nick.