[petsc-dev] PetscSF in Fortran

[Jed Brown]

Lawrence Mitchell <wencel at gmail.com> writes:

>> On 24 Oct 2017, at 06:21, Barry Smith <bsmith at mcs.anl.gov> wrote:
>> 
>>>>> - With PetscSFBcastBegin() / PetscSFBcastEnd() you currently still have to use the C MPI types in the Fortran calling code, rather than the Fortran ones. I think it is a bit confusing to have to mix the two up in the same code. If you put MPI_INTEGER instead of MPI_INT for example, it dies in F90Array1dAccess() with 'unsupported MPI_Datatype'. Could the Fortran MPI types be supported in these routines just by adding them as alternatives into the conditionals?
>>>>  Hmm, Jed will need to provide wisdom the Linux manual page for MPI_INTEGER clearly states:
>>>> 
>>>> Note that the Fortran types should only be used in Fortran programs, and the C types should only be used in C programs. For example, it is in error to use MPI_INT for a Fortran INTEGER. Datatypes are of type MPI_Datatype in C and of type INTEGER in Fortran.
>>>> 
>>>>  If this is true then there is no place we can do the change properly.
>>> 
>>> Yes, I first thought that MPI_Type_f2c() would convert Fortran MPI datatypes to the C ones (e.g. turn MPI_INTEGER into MPI_INT), so that the C side of the interface wouldn't have to worry about the Fortran MPI datatypes, but it doesn't really seem to do that.
>> 
>>  Do did I. Its existence seems to contradict the statement in the Linux manual page. Well wait for Jed.
>
> A fortran INTEGER may have a different width to a C int.  Hence the distinction.  The MPI_XXX_f2c functions convert handles on the fortran side to handles on the C side (MPI_XXX_c2f does the opposite).  Perhaps the standard sheds some light:
>
> Moving handles from Fortran to C
>
> http://mpi-forum.org/docs/mpi-3.1/mpi31-report/node446.htm#Node446

Thanks, Lawrence.  And my recollection is that the C types are
guaranteed to exist if the MPI was built to support Fortran, but may not
be defined otherwise.  So PETSc uses it in code that is only compiled
when Fortran is available, it should all work.

> Interlanguage communication
>
> http://mpi-forum.org/docs/mpi-3.1/mpi31-report/node456.htm#Node456
>
> Lawrence