MPI_Scatterv example

scatterv.c

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

#define SIZE 4

// mpic++ -o prog.exe hello.cpp
// mprin -np prog.exe

int main(int argc, char *argv[])
{
    int rank, size; // for storing this process' rank, and the number of processes

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

    int *sendcounts;            // array describing how many elements to send to each process
    int *displs;                // array describing the displacements where each segment begins
    char data[SIZE][SIZE];      // array descrbing data to be distributed.
    int rem = (SIZE*SIZE)%size; // elements remaining after division among processes
    int sum = 0;                // Sum of counts. Used to calculate displacements
    char rec_buf[100];          // buffer where the received data should be storedto
    int recv_count;

    // the data to be distribute

    

    // define info only relevant to rank 0.
    if(rank == 0) {
        // init data.
        char in='a';
        for(int i=0; i < SIZE; i++) {
            for(int j = 0; j <  SIZE; j++) {
                data[i][j] = in++;
            }
        }
        sendcounts = malloc(sizeof(int)*size);
        displs = malloc(sizeof(int)*size);
        // calculate send counts and displacements
        for (int i = 0; i < size; i++) {
            sendcounts[i] = (SIZE*SIZE)/size;
            if (rem > 0) {
                sendcounts[i]++;
                rem--;
            }

            displs[i] = sum;
            sum += sendcounts[i];
        }
    }

    // print calculated send counts and displacements for each process
    if (rank == 0) {
        for (int i = 0; i < size; i++) {
            printf("sendcounts[%d] = %d\tdispls[%d] = %d\n", i, sendcounts[i], i, displs[i]);
        }
    }

    // tell every rank how much data they are about to receive.
    MPI_Scatter(sendcounts, 1,  MPI_INT, &recv_count,  1, MPI_INT, 0, MPI_COMM_WORLD);

    // divide the data among processes as described by sendcounts and displs
    MPI_Scatterv(&data, sendcounts, displs, MPI_CHAR, &rec_buf, recv_count, MPI_CHAR, 0, MPI_COMM_WORLD);

    // print what each process received
    printf("%d: ", rank);
    for (int i = 0; i < recv_count; i++) {
        printf("%c\t", rec_buf[i]);
    }
    printf("\n");

    MPI_Finalize();

    if(rank == 0) {
        free(sendcounts);
        free(displs);
    }

    return 0;
}