[petsc-dev] PetscSF in Fortran

[Adrian Croucher]

hi


On 19/10/17 06:45, Matthew Knepley wrote:
> On Tue, Oct 17, 2017 at 11:35 PM, Adrian Croucher 
> <a.croucher at auckland.ac.nz <mailto:a.croucher at auckland.ac.nz>> wrote:
>
>
>     So, now I'm trying to add Fortran bindings for PetscSFBcastBegin()
>     and PetscSFBcastEnd().
>
>     From the C side I have added the following into
>     src/vec/is/sf/interface/f90-custom/zsff90.c:
>
>     PETSC_EXTERN void PETSC_STDCALL petscsfbcastbegin_(PetscSF *sf,
>     MPI_Datatype *unit, F90Array1d *rptr, F90Array1d *lptr , int *ierr
>     PETSC_F90_2PTR_PROTO(rptrd) PETSC_F90_2PTR_PROTO(lptrd))
>     {
>       PetscDataType ptype;
>       const void* rootdata;
>       void* leafdata;
>
>       *ierr = PetscMPIDataTypeToPetscDataType(*unit, &ptype);if
>     (*ierr) return;
>       *ierr = F90Array1dAccess(rptr, ptype, (void**) &rootdata
>     PETSC_F90_2PTR_PARAM(rptrd));if (*ierr) return;
>       *ierr = F90Array1dAccess(lptr, ptype, (void**) &leafdata
>     PETSC_F90_2PTR_PARAM(lptrd));if (*ierr) return;
>
>       *ierr = PetscSFBcastBegin(*sf, *unit, rootdata, leafdata);
>
>     }
>
>     and similarly for petscsfbcastend_(). Does this look plausible?
>
>     Then some wrappers need to be added to
>     src/vec/f90-mod/petscis.h90. I am not sure how to do those.
>
>     The difficulty is in declaring the arrays that are passed in,
>     which can be of various types. In C they are declared as void*,
>     but I'm not sure what to do with that in Fortran. I can't seem to
>     find any other example wrappers in PETSc to model it on either.
>     Any suggestions?
>


I think this is working now by just declaring those void* C variables as 
type(*) in the Fortran interface, e.g.:

       Interface
          Subroutine PetscSFBcastBegin(sf,unit,rarray,
      &       larray,ierr)
            use petscisdef
            PetscSF :: sf
            PetscInt :: unit
            type(*) :: rarray(:)
            type(*) :: larray(:)
            PetscErrorCode :: ierr
          End Subroutine PetscSFBcastBegin
       End Interface

The only difficulty I have left with this is in the MPI_Datatype 
variable. I'd forgotten that these datatypes are all different in C and 
Fortran as well.

I amended the C interface code to the following, to convert the Fortran 
MPI datatype (actually an integer) to a C MPI_Datatype:

PETSC_EXTERN void PETSC_STDCALL petscsfbcastbegin_(PetscSF *sf, MPI_Fint 
*unit, F90Array1d *rptr, F90Array1d *lptr , int *ierr 
PETSC_F90_2PTR_PROTO(rptrd) PETSC_F90_2PTR_PROTO(lptrd))
{
   PetscDataType pdtype;
   MPI_Datatype dtype;
   const void* rootdata;
   void* leafdata;

   dtype = MPI_Type_f2c(*unit);
   *ierr = PetscMPIDataTypeToPetscDataType(dtype, &pdtype);if (*ierr) 
return;
   *ierr = F90Array1dAccess(rptr, pdtype, (void**) &rootdata 
PETSC_F90_2PTR_PARAM(rptrd));if (*ierr) return;
   *ierr = F90Array1dAccess(lptr, pdtype, (void**) &leafdata 
PETSC_F90_2PTR_PARAM(lptrd));if (*ierr) return;

   *ierr = PetscSFBcastBegin(*sf, dtype, rootdata, leafdata);

}

The problem is this only seems to work if I declare the datatype in the 
calling Fortran code to be of the appropriate C MPI datatype, e.g. 
MPI_INT, rather than the corresponding Fortran datatype, e.g. 
MPI_INTEGER (which causes PetscMPIDataTypeToPetscDataType() to fail, as 
something weird gets passed in for dtype).

I was expecting the opposite to be true. It doesn't seem right to have 
to use the C datatypes in Fortran code (confusing if the Fortran 
datatypes are used elsewhere). So I suspect I've messed something up. 
Anyone have any ideas?

- Adrian

-- 
Dr Adrian Croucher
Senior Research Fellow
Department of Engineering Science
University of Auckland, New Zealand
email: a.croucher at auckland.ac.nz
tel: +64 (0)9 923 4611

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