My solution to: http://www.hpc.cineca.it/content/exercise-10

ex10.c

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

#define ROOT 0

int main (int argc, char** argv) {
    int i, j, rank, size;

    MPI_Datatype diagonal;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    // Create the diagonal datatype
    MPI_Type_vector(size, 1, size + 1, MPI_INT, &diagonal);
    MPI_Type_commit(&diagonal);

    // Allocate the matrix
    int A[size][size];

    // Fill it with the appropriate values
    for (i = 0; i < size; i++) {
        for (j = 0; j < size; j++) {
            A[i][j] = (i == j) * rank;
        }
    }

    // Gather all diagonals in the ROOT
    MPI_Gather(A, 1, diagonal, A, size, MPI_INT, ROOT, MPI_COMM_WORLD);
    
    // Print the matrix
    if (rank == ROOT) {
        for (i = 0; i < size; i++) {
            for (j = 0; j < size; j++) {
                printf("%d\t", A[i][j]);
            }
            printf("\n");
        }
    }

    MPI_Type_free(&diagonal);
    MPI_Finalize();

    return 0;
}