source: CIVL/examples/mpi/collective/c_ex13.c

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#ifdef _CIVL
#include<civlc.cvh>
#endif
/*
! This program shows how to use MPI_Gatherv.  Each processor sends a
! different amount of data to the root processor.  We use MPI_Gather
! first to tell the root how much data is going to be sent.
*/
/* globals */
int numnodes,myid,mpi_err;
#define mpi_root 0
/* end of 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[]){
/* poe a.out -procs 3 -rmpool 1 */
        int *sray=NULL,*displacements=NULL,*counts=NULL,*allray=NULL;
        int size,mysize,i;
        
        init_it(&argc,&argv);
        mysize=myid+1;
/* counts and displacement arrays are only required on the root */
        if(myid == mpi_root){
                counts=(int*)malloc(numnodes*sizeof(int));
                displacements=(int*)malloc(numnodes*sizeof(int));
        }
/* we gather the counts to the root */
        mpi_err = MPI_Gather(&mysize,1,MPI_INT, 
                                             counts,  1,MPI_INT, 
                                             mpi_root,MPI_COMM_WORLD);
/* calculate displacements and the size of the recv array */
        if(myid == mpi_root){
                displacements[0]=0;
                for( i=1;i<numnodes;i++){
                        displacements[i]=counts[i-1]+displacements[i-1];
                }
                size=0;
                for(i=0;i< numnodes;i++)
                        size=size+counts[i];
                sray=(int*)malloc(size*sizeof(int));
                for(i=0;i<size;i++)
                    sray[i]=i+1;
        }
/* different amounts of data for each processor  */
/* is scattered from the root */
        allray=(int*)malloc(sizeof(int)*mysize);
        mpi_err = MPI_Scatterv(sray,counts,displacements,MPI_INT, 
                               allray, mysize,           MPI_INT,
                         mpi_root,                                
                         MPI_COMM_WORLD);
        for(i=0;i<mysize;i++)
                printf("%d ",allray[i]);
        printf("\n");
#ifdef _CIVL
        for(i=0;i<mysize;i++){
          $assert(allray[i] == myid*(myid+1)/2+i+1);
        }
        free(allray);
        if(myid == mpi_root){
          free(sray);
          free(displacements);
          free(counts);
        }
#endif
    mpi_err = MPI_Finalize();
}