#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <math.h>
#include <string.h>
#include <unistd.h>
double **allocMatrix(int dim) {
    int i;
    double **matrix;
    matrix = (double **)malloc(dim*sizeof(double *));
    for(i=0; i < dim; i++) {
        matrix[i] = (double *)malloc(dim*sizeof(double));
    }
    return matrix;
}
void printMatrix(double **values, int size) {
    int i, j;

    for (i = 0; i < size; i++) {
        for (j = 0; j < size; j++) {
            printf("%10lf ", values[i][j]);
        }
        printf("n");
    }
}
int main(int argc, char* argv[]) {
    MPI_Init(&argc, &argv);
    int size, rank, i, j;
    int dimensions = 7;
    MPI_Comm_size(MPI_COMM_WORLD, &size);//number of processes
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);//rank for each process

    double **send = allocMatrix(dimensions);
    double **recv = allocMatrix(dimensions);
    int count = 0;
    for (i=0; i<dimensions; i++) {
        for (j=0; j<dimensions; j++) {
            if (rank == 0) {
                recv[i][j] = 0;
            } else {
                send[i][j] = ++count;
            }
        }
    }

    MPI_Datatype arrType;
    MPI_Type_vector(1, dimensions, 0, MPI_DOUBLE, &arrType);
    MPI_Type_commit(&arrType);
    int recvCounts[size];
    int displs[size];
    recvCounts[0] = 0;
    displs[0] = 0;
    recvCounts[1] = 1;
    displs[1] = 0;
    MPI_Gatherv(&(send[0][0]), 1, arrType,
         &(recv[0][0]), recvCounts, displs, arrType,
         0, MPI_COMM_WORLD);

    if (rank == 0) {
        printMatrix(recv, dimensions);
    }


    MPI_Finalize();
    return 0;
}

make gatherv
mpicc -Wall -o gatherv gatherv.c && ./gather
  1.000000   2.000000   3.000000   4.000000   5.000000   6.000000   7.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000

1.000000   2.000000   3.000000   4.000000   5.000000   6.000000   7.000000
  8.000000   9.000000   10.00000   11.00000   12.00000   13.00000   14.00000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000
  0.000000   0.000000   0.000000   0.000000   0.000000   0.000000   0.000000