apr1-gather-gatherv.c

#include <stdio.h>
#include <mpi.h>
#include <stdlib.h>
#include <math.h>
#include "l17.h"

int main(int argc, char** argv) {
    int rc;
    rc = MPI_Init(&argc, &argv);
    L17_CHECK_ERROR(MPI_Init, rc);

    int size;
    rc = MPI_Comm_size(MPI_COMM_WORLD, &size);
    L17_CHECK_ERROR(MPI_Comm_size, rc);

    int rank;
    rc = MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    L17_CHECK_ERROR(MPI_Comm_rank, rc);

    srand(rank + 1);
    int sendcount = rand() % 7 + 1;

    double* sendbuf;
    sendbuf = (double*) malloc(sendcount * sizeof(double));
    if (sendbuf == NULL) {
        printf("Error. Couldn't allocate memory.");
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

    int i;
    for (i = 0; i < sendcount; ++i)
        sendbuf[i] = sqrt(rank);

    int* recvcounts;
    if (rank == 0) {
        recvcounts = (int*) malloc(size * sizeof(int));
        if (recvcounts == NULL) {
            printf("Error. Couldn't allocate memory.");
            MPI_Abort(MPI_COMM_WORLD, 3);
        }
    }

    rc = MPI_Gather(&sendcount, 1, MPI_INT, recvcounts, 1, MPI_INT, 0, MPI_COMM_WORLD);
    L17_CHECK_ERROR(MPI_Gather, rc);

    int* displs;
    if (rank == 0) {
        displs = (int*) malloc(size * sizeof(int));
        if (displs == NULL) {
            printf("Error. Couldn't allocate memory.");
            MPI_Abort(MPI_COMM_WORLD, 4);
        }

        displs[0] = 0;
        for (i = 1; i < size; ++i)
            displs[i] = displs[i - 1] + recvcounts[i - 1];
    }

    double* recvbuf;
    int recvbufsize;

    if (rank == 0) {
        recvbufsize = displs[size - 1] + recvcounts[size - 1];
        recvbuf = (double*) malloc(recvbufsize * sizeof(double));
        if (recvbuf == NULL) {
            printf("Error. Couldn't allocate memory.");
            MPI_Abort(MPI_COMM_WORLD, 2);
        }
    }

    rc = MPI_Gatherv(sendbuf, sendcount, MPI_DOUBLE, recvbuf, recvcounts, displs, MPI_DOUBLE, 0, MPI_COMM_WORLD);
    L17_CHECK_ERROR(MPI_Gatherv, rc);

    free(sendbuf);

    if (rank == 0) {
        FILE* outputp = fopen("apr1-gather-gatherv.dat", "w");
        if (outputp == NULL) {
            printf("Error. Couldn't open the file for output values.");
            MPI_Abort(MPI_COMM_WORLD, 5);
        }

        for (i = 0; i < size; ++i) {
            int j;
            for (j = 0; j < recvcounts[i]; ++j)
                fprintf(outputp, "%f ", recvbuf[displs[i] + j]);
            fprintf(outputp, "\n");
        }

        free(displs);
        free(recvcounts);
        fclose(outputp);
        free(recvbuf);
    }

    MPI_Finalize();

    return 0;
}