Allreduce

int MPI_Allreduce(void *sbuf, void *rbuf, int count, MPI_Datatype dtype, 
                  MPI_op op, MPI_Comm comm)
{
  MPI_Reduce(sbuf, rbuf, count, dtype, op, 0, comm);
  MPI_Bcast(rbuf, count, dtype, 0, comm);
  return(MPI_SUCCESS);
}