[Mpi-forum] C++ types inaccessible after #281
nmm1 at cam.ac.uk
Thu Jun 28 07:06:28 CDT 2012
I am putting various responses together, for clarity.
On Jun 27 2012, Marc Snir wrote:
> The standard does not say much about datatypes that do not correspond to
> primitive datatypes of the host language. I would suggest that is we
> define an MPI_Complex type for a language that that does not have a
> primitive complex, then the implementation will decide how it defines a
> complex (e.g., two successive doubles) and perform reductions
> accordingly. A good quality implementation will make C++/C interaction
That assumes that there is only one, and there may not be. Inter alia,
complex IS a primitive datatype in Fortran, but not in C+, and only
sort-of in C.
On Jun 28 2012, Rolf Rabenseifner wrote:
>I would propose to fill the gap by
>- defining in C and Fortran MPI_CPP_BOOL for C++ bool
>- defining in C and Fortran MPI_CPP_COMPLEX,
> MPI_CPP_DOUBLE_COMPLEX, MPI_CPP_LONG_DOUBLE_COMPLEX,
Yes. That would fix the problem.
> if the C/C++ standards do not require that
> C "... _Complex" and C++ "Complex<...>" is internally
> implemented in an identical way,
>To be decided by the C/C++ specialists:
> - Is MPI_CPP_..._COMPLEX needed or is MPI_C_..._COMPLEX
> enough for C++?
> - If MPI_CPP_..._COMPLEX is needed, what are the external 32
> (I expect, it's external32 and therefore same as for C.)
> - Which is the external32 size for MPI_CPP_BOOL?
The former, certainly - as others have said, the representation
will almost always be the same. C++ bool is not required to be
one byte, and its representation is unspecified, but will almost
always be one byte and 1.
>To be decided by the whole Forum:
> - Do we make this MPI-2.2 errata to help the C++
> programmer using the MPI C language binding?
Speaking for my users, yes, please!
>> On Jun 27, 2012, at 3:10 PM, Jeff Squyres wrote:
>> > Don't forget a few points:
>> > - As Jim pointed out, all MPI datatypes are supposed to be available
>> > in all language bindings. Hence, if we have (for example)
>> > MPI_CXX_COMPLEX, it needs to be available in both C and the various
>> > Fortran bindings.
>> > - This is not as wonky as it sounds;
With Fortran's C interoperability, it's not at all wonky, and an
increasing number of Fortran+C programs are doing it.
>> > - That being said, handling reductions on MPI_CXX_COMPLEX from
>> > C-based MPI implementations may be a little problematic if the types
>> > are not equivalent between C and C++, for example...
Yes, that's the only issue that needs more than the representation.
In all cases, it may be necessary to call into the owning language to
do the reduction.
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