[mpich-discuss] Value of fortran communicator handles

[Kenneth Raffenetti]

Marco,

Yes, this is expected. Communicator handles are opaque objects. No 
guarantees are made by MPI about their values to the user. To reason 
about them, you need to use MPI_COMM_COMPARE.

Ken

On 05/20/2014 09:33 AM, marco restelli wrote:
> Hi,
>     I noticed that the attached fortran code produces this output:
>
> $ mpiexec -n 4 -l ./mpitest
> [0]  color =            1 ; newcomm =  -2080374780
> [1]  color =            1 ; newcomm =  -2080374782
> [2]  color =            2 ; newcomm =  -2080374782
> [3]  color =            2 ; newcomm =  -2080374782
>
> I know that I am not supposed to use the values of the communicator
> handles, but I was surprised to see that two processors in the same
> communicator have two different values for such an handle.
>
> More precisely, I was assuming that newcomm had one value on procs 0
> and 1 and another value on 2 and 3.
>
> So my question is whether this is the expected behaviour or not. (In
> fact, I need to debug another problem in my code and I wonder whether
> this could be related or not).
>
> Thank you,
>     Marco
>
>
>
> mpich: version 1.5
> gfortran: 4.7.3
>
>
>
> program mpitest
>
>   implicit none
>   include "mpif.h"
>
>   integer :: key, color, newcomm, mpi_id, ierr
>
>    call mpi_init(ierr)
>    call mpi_comm_rank(mpi_comm_world,mpi_id,ierr)
>
>    key = 1 ! same ordering as in comm
>    select case(mpi_id)
>     case(0)
>      color = 1
>     case(1)
>      color = 1
>     case(2)
>      color = 2
>     case(3)
>      color = 2
>    end select
>    call mpi_comm_split( mpi_comm_world , color,key , newcomm , ierr)
>
>    write(*,*) "color = ",color, "; newcomm = ",newcomm
>
>    call mpi_finalize(ierr)
>
> end program mpitest
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