I notice an uninitialized MPI_Status object can make MPI_Get_count return a wrong result,
when the data amount in the MPI operation is zero (although I only checked MPI-IO).
Attached is a test program that uses MPI collective read where only root process has
non-zero length data to read. The expected result from MPI_Get_count is 0 for all
non-root process. To mimic an uninitialized MPI_Status object, I call memset to make
the object non-zero.

Here is the code fragment.

    if (rank == 0) len = 10;
    else len = 0;

    MPI_File_read_all(fh, buf, len, MPI_BYTE, &status);

    MPI_Get_count(&status, MPI_BYTE, &get_size);

For process rank > 0, get_size may not be 0.


Wei-keng


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