Example 5.22

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

typedef struct {
    double val;
    int log;
} SegScanPair;

/* the user-defined function  */
void segScan( SegScanPair *in, SegScanPair *inout, int *len, MPI_Datatype *dptr ) {
    int i;
    SegScanPair c;
for (i = 0; i < *len; ++i) {
        if (in->log == inout->log)
                c.val = in->val + inout->val;
        else
                c.val = inout->val;
        c.log = inout->log;
        *inout = c;
        in++; inout++;
    }
}

int main(int argc, char *argv[]) {
        int i, base;
    SegScanPair  a, answer;
    MPI_Op       myOp;
    MPI_Datatype type[2] = {MPI_DOUBLE, MPI_INT};
    MPI_Aint     disp[2];
    int          blocklen[2] = {1, 1};
    MPI_Datatype sspair;
        int rank, size;

        MPI_Init(&argc, &argv);
        MPI_Comm_rank(MPI_COMM_WORLD, &rank);
        MPI_Comm_size(MPI_COMM_WORLD, &size);
        if(rank <= 1) {
                a.log = 0;
                a.val = 1;
        } else if (rank <= 4 ) {
                a.log = 1;
                a.val = 1;
        } else if(rank <= 6) {
                a.log = 0;
                a.val = 1;
        } else {
        a.log = 1;
        a.val = 1;
    }
        /* explain to MPI how type SegScanPair is defined */
    MPI_Address(&a, disp);
    MPI_Address(&a.log, disp + 1);
    base = disp[0];
    for (i = 0; i < 2; ++i) disp[i] -= base;
    MPI_Type_create_struct(2, blocklen, disp, type, &sspair);
    MPI_Type_commit(&sspair);
    /* create the segmented-scan user-op   */
    MPI_Op_create((MPI_User_function*)segScan, 0, &myOp);
    MPI_Scan(&a, &answer, 1, sspair, myOp, MPI_COMM_WORLD);
        int buf;
        for(i = 0; i < size; ++i) {
                if(rank == i) {
                        if(rank > 0) {
                                MPI_Recv(&buf, 1, MPI_INT, rank - 1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
                        }
                        //printf("rank = %d\n", rank);
                        printf("%d\t%d %f\n", rank, answer.log, answer.val);
                        if(rank + 1 < size) {
                                buf = rank;
                                MPI_Send(&buf, 1, MPI_INT, rank + 1, 0, MPI_COMM_WORLD);
                        }
                }
        }
        MPI_Finalize();
}