MPI_Send works only with statically allocated buffer

Question

If I want to define my own type, and use it as a datatype with MPI_Send to take only even rows from a matrix, does that matrix (send buffer) have to be allocated statically?

I seem to have problems when I allocate it dynamically. Is this because addresses need to be successive for data to be sent?

Answer 1

No, memory to be sent with MPI_Send does not have to be statically allocated.

To send array subsets, you likely want to use MPI_Type_indexed. Here is a slightly modified version of the example from the mpi.deino.net article on MPI_Type_indexed, where I have replaced the statically allocated buffer

int buffer[27];

to a dynamically allocated buffer

int* buffer = (int*)malloc(27 * sizeof(int));

I hope it helps:

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

int main(int argc, char *argv[])
{
    int rank, size, i;
    MPI_Datatype type, type2;
    int blocklen[3] = { 2, 3, 1 };
    int displacement[3] = { 0, 3, 8 };
    int* buffer = (int*)malloc(27 * sizeof(int)); //int buffer[27];
    MPI_Status status;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if (size < 2)
    {
        printf("Please run with 2 processes.\n");
        MPI_Finalize();
        return 1;
    }
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    MPI_Type_contiguous(3, MPI_INT, &type2);
    MPI_Type_commit(&type2);
    MPI_Type_indexed(3, blocklen, displacement, type2, &type);
    MPI_Type_commit(&type);

    if (rank == 0)
    {
        for (i=0; i<27; i++)
            buffer[i] = i;
        MPI_Send(buffer, 1, type, 1, 123, MPI_COMM_WORLD);
    }

    if (rank == 1)
    {
        for (i=0; i<27; i++)
            buffer[i] = -1;
        MPI_Recv(buffer, 1, type, 0, 123, MPI_COMM_WORLD, &status);
        for (i=0; i<27; i++)
            printf("buffer[%d] = %d\n", i, buffer[i]);
        fflush(stdout);
    }

    MPI_Finalize();
    free(buffer);
    return 0;
}