source: CIVL/examples/mpi/collective/c_ex06.c

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

/*
! This program shows how to use MPI_Scatter and MPI_Reduce
! Each processor gets different data from the root processor
! by way of mpi_scatter.  The data is summed and then sent back
! to the root processor using MPI_Reduce.  The root processor
! then prints the global sum. 
*/
/*  globals */
int numnodes,myid,mpi_err;
#define mpi_root 0
#ifdef _CIVL
$input int count=4;
#endif
/* end globals  */

void init_it(int  *argc, char ***argv);

void init_it(int  *argc, char ***argv) {
        mpi_err = MPI_Init(argc,argv);
    mpi_err = MPI_Comm_size( MPI_COMM_WORLD, &numnodes );
    mpi_err = MPI_Comm_rank(MPI_COMM_WORLD, &myid);
}

int main(int argc,char *argv[]){
        int *myray=NULL,*send_ray=NULL,*back_ray=NULL;
#ifndef _CIVL
        int count;
#endif
        int size,mysize,i,k,j,total,gtotal;
        
        init_it(&argc,&argv);
/* each processor will get count elements from the root */
#ifndef _CIVL
        count=4;
#endif
        myray=(int*)malloc(count*sizeof(int));
/* create the data to be sent on the root */
        if(myid == mpi_root){
            size=count*numnodes;
                send_ray=(int*)malloc(size*sizeof(int));
                back_ray=(int*)malloc(numnodes*sizeof(int));
                for(i=0;i<size;i++)
                        send_ray[i]=i;
                }
/* send different data to each processor */
        mpi_err = MPI_Scatter(  send_ray, count,   MPI_INT,
                                                    myray,    count,   MPI_INT,
                                    mpi_root,
                                    MPI_COMM_WORLD);                    
/* each processor does a local sum */
        total=0;
        for(i=0;i<count;i++)
            total=total+myray[i];
        printf("myid= %d total= %d\n",myid,total);
/* send the local sums back to the root */
    mpi_err = MPI_Reduce(&total,    &gtotal, 1,  MPI_INT, 
                                                MPI_SUM, 
                                mpi_root,                  
                                MPI_COMM_WORLD);
/* the root prints the global sum */
        if(myid == mpi_root){
          printf("results from all processors= %d \n",gtotal);
        }
#ifdef _CIVL
        free(myray);
        free(send_ray);
        free(back_ray);
#endif
    mpi_err = MPI_Finalize();
}