Question

The following MPI program simply defines an MPI type for the message, and then sends a message from process 0 to process 1. However, it crashes with a very uninformative error message. What’s wrong with it?

#include <stdio.h>
#include <mpi.h>

struct P {
    char choices[64];
    int next_choice;
};

typedef struct P Packing;

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

    if (rank == 0) {
        /*Defines a new MPI datatype tag for Packings.*/
        int field_count = 2;
        int field_lengths[2] = {64,1};
        MPI_Aint field_offsets[2] = {0,64 * sizeof(char)};
        MPI_Datatype field_types[2] = {MPI_CHAR,MPI_INT};
        MPI_Type_struct(field_count,field_lengths,
            field_offsets,field_types,&PACKING_TYPE);
        MPI_Type_commit(&PACKING_TYPE);
    }
    MPI_Barrier(MPI_COMM_WORLD);

    if (rank == 0) {
        Packing pack;
        MPI_Status status;
        MPI_Send(&pack,1,PACKING_TYPE,1,0,MPI_COMM_WORLD);
        puts("Process 0 sent message.");
    }

    if (rank == 1) {
        Packing pack;
        MPI_Status status;
        MPI_Recv(&pack,1,PACKING_TYPE,MPI_ANY_SOURCE,
            MPI_ANY_TAG,MPI_COMM_WORLD,&status);
        puts("Process 1 recieved message.");
    }

    MPI_Barrier(MPI_COMM_WORLD);
    if(rank == 0) {
        MPI_Type_free(&PACKING_TYPE);
    }
    MPI_Finalize();
    return 0;
}

Answer 1

Neither MPI_Type_struct() nor MPI_Type_commit() do any communicating (eg, there’s no communicator as part of the argument list); they’re purely local operations. So every process that uses the type must create it. (This makes sense; any process receiving data needs to understand its layout).

So after getting rid of the if (rank == 0) test (and thus, the barriers) around the type creation and freeing operation, your code works.

By the way — it’s always safest to actually measure the offsets (eg, using offsetof or similar) rather than assume them; the compiler is free to insert (eg) padding into the structure to help with alignment.