[MPI3 Fortran] Start discussing new Fortran binding

Jeff Squyres jsquyres at cisco.com
Thu Feb 14 20:22:33 CST 2008

On Feb 14, 2008, at 9:03 PM, Craig E. Rasmussen wrote:

>>> ! We need to figure out the type of MPI handles.
>>> ! Currently I'm leaning toward TYPE(C_PTR) rather than
>>> ! the INTEGER(MPI_HANDLE_KIND) Jeff and I used in our paper.
>> Will this be able to handle MPI implementations that use integers as
>> MPI handles in C?
> So are we talking about global constants like MPI_COMM_WORLD?  If so  
> it
> looks like we can use the same global integers that are used in C.

That's the point, though -- some MPI implementations (e.g., Open MPI)  
use pointers for MPI handles in C, while others (e.g., MPICH) use C  
int's.  The standard doesn't specify what the type has to be, so  
implementations do what they please.

> We
> would just initialize MPI_COMM_WORLD in Fortran MPI_Init to the  
> address of
> the global C integer.  If you can grok the Fortran, I've included  
> the code
> snippets:
> ! the global integer in OMPI
>  integer, bind(C) :: ompi_mpi_comm_world
> ! the declaration of the handle in Fortran
>  type(C_PTR) :: MPI_COMM_WORLD
> ! the initialization in MPI_Init()
> MPI_COMM_WORLD = C_LOC(ompi_mpi_comm_world)

If I grok this, it looks like the Fortran binding for MPI_COMM_WORLD  
will always be a C pointer.  How would that translate automatically  
back to the C bindings if the back-end C type is an integer?  I  
thought the goal was to make the C and Fortran types be the same (or  
automatically translatable by the compiler).

(note that OMPI uses pointers in C, so ompi_mpi_comm_world is a  
pointer, not an integer.  But you could s/ompi_mpi_comm_world/ 
mpich_mpi_comm_world/ and have a valid-ish example)

Jeff Squyres
Cisco Systems

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