MPI_ORDER_C

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

/**
 * @brief Illustrates how to indicate a C-like memory ordering.
 * @details This program is meant to be run with 2 processes: a sender and a
 * receiver. These two MPI processes will exchange a message made of six
 * integers. These integers turn out to be a subarray of a 2D array held on the
 * sender. An MPI subarray type will be created to extract that subarray and
 * send it, and the memory layout specified will be that of C.
 *
 * In the visualisation below, the rightmost dim (dim[1]) has been chosen as
 * being the one having consecutive elements in memory.
 *
 *                                                 The subarray we
 *               The full array                      want to send
 *
 *       +---------- dim[1] ------->         +--------- dim[1] ------->
 *       | +-----+-----+-----+-----+         | +-----+-----+-----+-----+ 
 *       | |  0  |  1  |  2  |  3  |         | |  -  |  -  |  -  |  -  | ^ Start point in
 *       | +-----+-----+-----+-----+         | +-----+-----+-----+-----+ | dim[0] = 2
 *       | |  4  |  5  |  6  |  7  |         | |  -  |  -  |  -  |  -  | V
 * dim[0]| +-----+-----+-----+-----+   dim[0]| +-----+-----+-----+-----+
 *       | |  8  |  9  |  10 |  11 |         | |  -  |  9  |  10 |  11 | ^ Element count
 *       | +-----+-----+-----+-----+         | +-----+-----+-----+-----+ | in dim[0] = 2
 *       | |  12 |  13 |  14 |  15 |         | |  -  |  13 |  14 |  15 | V
 *       V +-----+-----+-----+-----+         V +-----+-----+-----+-----+
 *                                              <---> <--------------->
 *                                        Start point   Element count
 *                                      in dim[1] = 1   in dim[1] = 3
 *
 * In brief; the 2x3 subarray to send starts at [2;1] in the 4x4 full array.
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get the number of processes and check only 2 processes are used
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if(size != 2)
    {
        printf("This application is meant to be run with 2 processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    // Get my rank and do the corresponding job
    enum role_ranks { SENDER, RECEIVER };
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
    switch(my_rank)
    {
        case SENDER:
        {
            // Declare the full array
            int full_array[4][4];
            for(int i = 0; i < 4; i++)
            {
                for(int j = 0; j < 4; j++)
                {
                    full_array[i][j] = i * 4 + j;
                }
            }

            // Create the subarray datatype
            MPI_Datatype subarray_type;
            int dimensions_full_array[2] = { 4, 4 };
            int dimensions_subarray[2] = { 2, 3 };
            int start_coordinates[2] = { 2, 1 };
            MPI_Type_create_subarray(2,  dimensions_full_array, dimensions_subarray, start_coordinates, MPI_ORDER_C, MPI_INT, &subarray_type);
            MPI_Type_commit(&subarray_type);

            // Send the message
            printf("MPI process %d sends:\n-  -  -  -\n-  -  -  -\n-  %d %d %d\n- %d %d %d\n", my_rank, full_array[2][1], full_array[2][2], full_array[2][3], full_array[3][1], full_array[3][2], full_array[3][3]);
            MPI_Send(full_array, 1, subarray_type, RECEIVER, 0, MPI_COMM_WORLD);
            break;
        }
        case RECEIVER:
        {
            // Receive the message
            int received[6];
            MPI_Recv(&received, 6, MPI_INT, SENDER, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("MPI process %d receives:\n%d %d %d %d %d %d\n", my_rank, received[0], received[1], received[2], received[3], received[4], received[5]);
            break;
        }
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}