[OMPI users] Fortran vs C reductions

[Brian Taylor]

I have a question about the standards compliance of OpenMPI.  Is the
following program valid according to the MPI standard?

#include <stdio.h>
#include <mpi.h>

int main(int argc, char **argv)
{
  int rank;
  double in[2], out[2];

  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  in[1] = in[0] = (double) rank;

  MPI_Reduce(in, out, 1, MPI_2DOUBLE_PRECISION, MPI_MAXLOC, 0,
MPI_COMM_WORLD);

  if (rank == 0) printf("out = %f %f\n", out[0], out[1]);

  MPI_Finalize();

  return 0;
}

Specifically, I am wondering if MPI_2DOUBLE_PRECISION can be used with
MPI_MAXLOC (or MPI_MINLOC) in a C program.  MPI_2DOUBLE_PRECISION is not
included in the list of datatypes for reduction functions in C in Appendix
A.1 of the MPI 3.1 standard, although it is included in the list of
reduction functions for Fortran.  What exactly does that mean?

The program above runs with OpenMPI 1.10.2 and gives the output one would
expect for an equivalent program written in Fortran.  Can I rely on this
being portable to other MPI implementations?

Thanks,
Brian