[OMPI users] Wrong values when reading file with MPI IO

[david . froger . ml]

Hello,

Here is a simple `C` program reading a file in parallel with `MPI IO`:
    #include <stdio.h>
    #include <stdlib.h>

    #include "mpi.h"

    #define N 10

    main( int argc, char **argv )
    {
        int rank, size;
        MPI_Init(&argc, &argv);
        MPI_Comm_rank( MPI_COMM_WORLD, &rank );
        MPI_Comm_size( MPI_COMM_WORLD, &size );

        int i0 = N *  rank / size;
        int i1 = N * (rank+1) / size;
        printf("rank: %d, i0: %d, i1: %d\n", rank, i0, i1);

        int i;
        double* data = malloc( (i1-i0)*sizeof(double) );
        for (i = 0 ; i < i1-i0 ; i++)
            data[i] = 123.;

        MPI_File f;
        MPI_File_open(MPI_COMM_WORLD, "data.bin", MPI_MODE_RDONLY,
                      MPI_INFO_NULL, &f);

        MPI_File_set_view(f, i0, MPI_DOUBLE, MPI_DOUBLE, "native",
                          MPI_INFO_NULL);

        MPI_Status status;
        MPI_File_read(f, data, i1-i0, MPI_DOUBLE, &status);

        int count;
        MPI_Get_count(&status, MPI_DOUBLE, &count);
        printf("rank %d, %d value read\n", rank, count);

        for (i = 0 ; i < i1-i0 ; i++) {
            printf("rank: %d index: %d value: %.2f\n", rank, i, 
data[i]);
        }

        MPI_File_close(&f);

        MPI_Finalize();

        free(data);

        return 0;
    }

With one processus:

    ./read_mpi_io

Values read are correct:

    rank: 0, i0: 0, i1: 10
    rank 0, 10 value read
    rank: 0 index: 0 value: 0.00
    rank: 0 index: 1 value: 1.00
    rank: 0 index: 2 value: 2.00
    rank: 0 index: 3 value: 3.00
    rank: 0 index: 4 value: 4.00
    rank: 0 index: 5 value: 5.00
    rank: 0 index: 6 value: 6.00
    rank: 0 index: 7 value: 7.00
    rank: 0 index: 8 value: 8.00
    rank: 0 index: 9 value: 9.00

But with two processus:

    mpirun -n 2 ./read_mpi_io

I get wrong values (zeros):

    rank: 0, i0: 0, i1: 5
    rank: 1, i0: 5, i1: 10
    rank 0, 5 value read
    rank: 0 index: 0 value: 0.00
    rank 1, 5 value read
    rank: 1 index: 0 value: 0.00
    rank: 0 index: 1 value: 1.00
    rank: 0 index: 2 value: 2.00
    rank: 1 index: 1 value: 0.00
    rank: 1 index: 2 value: 0.00
    rank: 1 index: 3 value: 0.00
    rank: 1 index: 4 value: 0.00
    rank: 0 index: 3 value: 3.00
    rank: 0 index: 4 value: 4.00


What's wrong in my C code?

Thanks,
David