source: CIVL/examples/mpi/collective/c_ex08.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 *will_use;
        int *myray,*displacements=NULL,*counts=NULL,*allray=NULL;
        int size,mysize,i;
        
        init_it(&argc,&argv);
        mysize=myid+1;
        myray=(int*)malloc(mysize*sizeof(int));
        for(i=0;i<mysize;i++)
                myray[i]=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((void*)myray,1,MPI_INT, 
                                             (void*)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];
                allray=(int*)malloc(size*sizeof(int));
        }
/* different amounts of data from each processor  */
/* is gathered to the root */
        mpi_err = MPI_Gatherv(myray, mysize,         MPI_INT, 
                         allray,counts,displacements,MPI_INT, 
                         mpi_root,                                
                         MPI_COMM_WORLD);
                        
        if(myid == mpi_root){
                for(i=0;i<size;i++)
                        printf("%d ",allray[i]);
                printf("\n");
#ifdef _CIVL
                for(int j=0, k=1, t=0; j<size && t<=k; j++,t++){
                  if(t==k){
                      t=0;
                      k++;
                    }
                  $assert(allray[j] == k);
                }         
#endif
        }
#ifdef _CIVL
        free(myray);
        free(displacements);
        free(counts);
        free(allray);
#endif
    mpi_err = MPI_Finalize();
}