Program in MPI to sum an array

sumArrayMPI.c

#include <mpich/mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// size of array
#define n 10

int a[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

// Temporary array for slave process
int a2[1000];

int main(int argc, char *argv[])
{

    int pid, np,
        elements_per_process,
        n_elements_recieved;
    // np -> no. of processes
    // pid -> process id

    MPI_Status status;

    // Creation of parallel processes
    MPI_Init(&argc, &argv);

    // find out process ID,
    // and how many processes were started
    MPI_Comm_rank(MPI_COMM_WORLD, &pid);
    MPI_Comm_size(MPI_COMM_WORLD, &np);

    // master process
    if (pid == 0)
    {
        int index, i;
        elements_per_process = n / np;

        // check if more than 1 processes are run
        if (np > 1)
        {
            // distributes the portion of array
            // to child processes to calculate
            // their partial sums
            for (i = 1; i < np - 1; i++)
            {
                index = i * elements_per_process;

                MPI_Send(&elements_per_process, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
                MPI_Send(&a[index], elements_per_process, MPI_INT, i, 0, MPI_COMM_WORLD);
            }

            // last process adds remaining elements
            index = i * elements_per_process;
            int elements_left = n - index;

            MPI_Send(&elements_left, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
            MPI_Send(&a[index], elements_left, MPI_INT, i, 0, MPI_COMM_WORLD);
        }

        // master process add its own sub array
        int sum = 0;
        for (i = 0; i < elements_per_process; i++)
            sum += a[i];

        // collects partial sums from other processes
        int tmp;
        for (i = 1; i < np; i++)
        {
            MPI_Recv(&tmp, 1, MPI_INT, MPI_ANY_SOURCE, 0, MPI_COMM_WORLD, &status);
            int sender = status.MPI_SOURCE;

            sum += tmp;
        }

        // prints the final sum of array
        printf("Sum of array is: %d\n", sum);
    }

    // slave processes
    else
    {
        MPI_Recv(&n_elements_recieved, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &status);

        // stores the received array segment
        // in local array a2
        MPI_Recv(&a2, n_elements_recieved, MPI_INT, 0, 0, MPI_COMM_WORLD, &status);

        // calculates its partial sum
        int partial_sum = 0;
        for (int i = 0; i < n_elements_recieved; i++)
            partial_sum += a2[i];

        // sends the partial sum to the root process
        MPI_Send(&partial_sum, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
    }

    // cleans up all MPI state before exit of process
    MPI_Finalize();

    return 0;
}