[mpich-discuss] MPI user-defined datatype size

[Mccall, Kurt E. (MSFC-EV41)]

Hi,

I'm running MPICH 4.1.2.

Should a MPI user-defined datatype created by MPI_Type_create_struct have the same size as the C++ object on which it is based?   Comparing sizeof(cpp_obj) and the output
>From MPI_Type_size(mpi_data_type_, &size1), the C++ object is four bytes larger than the MPI datatype.   I don't know if this is because I have to do something special to take
padding into  account when I create the MPI user datatype.

Here is my C++ message class:

struct MsgTupleSeq
{
    static const int N_INDICES_MAX_ = 4;   // culprit?

    int n_vars_;
    int n_ndxs_;
   short start_[N_INDICES_MAX_];
    short end_[N_INDICES_MAX_];
    short current_[N_INDICES_MAX_];
    long long n_tuples_;
    int unique_id_;
    int subordinate_id_;
    int opcode_;
    char error_msg_[512];
}

That is 564 bytes for the non-static members, but such an object is 568 bytes long with the 4 bytes of padding that the compiler inserts.

I define the MPI user datatype with the code below, using the MsgTupleSeq object to compute the offsets.  MPI_Type_size returns 564 bytes for the user datatype,
so it seems like it isn't seeing the padding.

    MsgTupleSeq obj;
   MPI_Datatype mpi_data_type;
    int struct_len = 10, i = 0;

    int block_len[struct_len];
    MPI_Datatype types[struct_len];
    MPI_Aint displacements[struct_len];

    // the integer "n_vars_" member
    block_len[i] = 1;
    types[i] = MPI_INT;
    displacements[i] = (size_t) &obj.n_vars_ - (size_t) &obj;

    // the integer "n_ndxs_" member
    ++i;
    block_len[i] = 1;
    types[i] = MPI_INT;
    displacements[i] = (size_t) &obj.n_ndxs_ - (size_t) &obj;

    // the short integer "start_" array member
    ++i;
    block_len[i] = N_INDICES_MAX_;
    types[i] = MPI_SHORT;
    displacements[i] = (size_t) &obj.start_ - (size_t) &obj;

    // the short integer "end_" array member
    ++i;
    block_len[i] = N_INDICES_MAX_;
    types[i] = MPI_SHORT;
    displacements[i] = (size_t) &obj.end_ - (size_t) &obj;

    // the short integer "current_" array member
    ++i;
    block_len[i] = N_INDICES_MAX_;
    types[i] = MPI_SHORT;
    displacements[i] = (size_t) &obj.current_ - (size_t) &obj;

    // the long long integer "n_tuples_" member
    ++i;
    block_len[i] = 1;
    types[i] = MPI_LONG_LONG;
    displacements[i] = (size_t) &obj.n_tuples_ - (size_t) &obj;

    // the integer "unique_id_" member
    ++i;
    block_len[i] = 1;
    types[i] = MPI_INT;
    displacements[i] = (size_t) &obj.unique_id_ - (size_t) &obj;

   // the integer "subordinate_id_" member
    ++i;
    block_len[i] = 1;
    types[i] = MPI_INT;
    displacements[i] = (size_t) &obj.subordinate_id_ - (size_t) &obj;

    // the integer "opcode_" member
    ++i;
    block_len[i] = 1;
    types[i] = MPI_INT;
    displacements[i] = (size_t) &obj.opcode_  - (size_t) &obj;

    // the char array "error_msg_" member
    ++i;
    block_len[i] = 512;
    types[i] = MPI_CHAR;
    displacements[i] = (size_t) &obj.error_msg_[0] - (size_t) &obj;

    MPI_Type_create_struct(struct_len, block_len, displacements, types, &mpi_data_type);
    MPI_Type_commit(&mpi_data_type);




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