There is a challenge here because there is no standard type in C corresponding to REAL*16. Either the reduction operation needs to be written in Fortran or MPICH needs to figure out the compiler-dependent equivalent of REAL*16 that works in C. While GCC __float128 and Intel _Quad might be equivalent, this is not a rigorous assumption.

I recommend that you write your own user-defined reduction for REAL*16 with the reduction operation callback in Fortran.

Jeff

On Thu, Aug 9, 2018 at 2:31 AM, Jiancang Zhuang <zhuangjc@ism.ac.jp> wrote:

I have found that the fortran version mpi_reduce does not work for real*18. This can be shown by the following program. I have not test the C version of mpi_reduce.

c-----------------------Fortran code begins -----------------------------
      implicit real*8 (a-h, o-z)
      include 'mpif.h'
      real*16 h1, h

      call mpi_init(ierr)
      call mpi_comm_size(mpi_comm_world, nprocs, ierr)
      call mpi_comm_rank(mpi_comm_world, myrank, ierr)

      h1 = (myrank+4) *2.00000000000000
      write(*,'(''before reduce --'', i4,2f12.8)')myrank, h1,h

      call mpi_reduce(h1,h,1,mpi_long_double,mpi_sum,0,
     &    mpi_comm_world,ierr)
      write(*,'(''after reduce --'',i4,2f12.8)')myrank, h1,h

      call mpi_bcast(h,1,mpi_long_double,0,
     &    mpi_comm_world,ierr)

      write(*,'(''bcastvalue -- '',i4,2f12.8)')myrank, h1,h

      call mpi_finalize(ierr)
       end

c-----------------------Fortran code begins -----------------------------

$ mpif77 a.f -o a.out
$ mpirun -np 3 ./a.out
before reduce --   1 10.00000000 0.00000000
after reduce --   1 10.00000000 0.00000000
before reduce --   2 12.00000000 0.00000000
before reduce --   0 8.00000000 0.00000000
after reduce --   2 12.00000000 0.00000000
after reduce --   0 8.00000000 8.00000000
bcastvalue --    0 8.00000000 8.00000000
bcastvalue --    1 10.00000000 8.00000000
bcastvalue --    2 12.00000000 8.00000000

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Jeff Hammond
jeff.science@gmail.com
http://jeffhammond.github.io/