Sending and receiving 2D array over MPI

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Just to amplify Joel’s points a bit:

This goes much easier if you allocate your arrays so that they’re contiguous (something C’s “multidimensional arrays” don’t give you automatically:)

int **alloc_2d_int(int rows, int cols) {
    int *data = (int *)malloc(rows*cols*sizeof(int));
    int **array= (int **)malloc(rows*sizeof(int*));
    for (int i=0; i<rows; i++)
        array[i] = &(data[cols*i]);

    return array;
}

/*...*/
int **A;
/*...*/
A = alloc_2d_init(N,M);

Then, you can do sends and recieves of the entire NxM array with

MPI_Send(&(A[0][0]), N*M, MPI_INT, destination, tag, MPI_COMM_WORLD);

and when you’re done, free the memory with 

free(A[0]);
free(A);

Also, MPI_Recv is a blocking recieve, and MPI_Send can be a blocking send. One thing that means, as per Joel’s point, is that you definately don’t need Barriers. Further, it means that if you have a send/recieve pattern as above, you can get yourself into a deadlock situation — everyone is sending, no one is recieving. Safer is:

if (myrank == 0) {
   MPI_Send(&(A[0][0]), N*M, MPI_INT, 1, tagA, MPI_COMM_WORLD);
   MPI_Recv(&(B[0][0]), N*M, MPI_INT, 1, tagB, MPI_COMM_WORLD, &status);
} else if (myrank == 1) {
   MPI_Recv(&(A[0][0]), N*M, MPI_INT, 0, tagA, MPI_COMM_WORLD, &status);
   MPI_Send(&(B[0][0]), N*M, MPI_INT, 0, tagB, MPI_COMM_WORLD);
}

Another, more general, approach is to use MPI_Sendrecv:

int *sendptr, *recvptr;
int neigh = MPI_PROC_NULL;

if (myrank == 0) {
   sendptr = &(A[0][0]);
   recvptr = &(B[0][0]);
   neigh = 1;
} else {
   sendptr = &(B[0][0]);
   recvptr = &(A[0][0]);
   neigh = 0;
}
MPI_Sendrecv(sendptr, N*M, MPI_INT, neigh, tagA, recvptr, N*M, MPI_INT, neigh, tagB, MPI_COMM_WORLD, &status);

or nonblocking sends and/or recieves.

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