source: CIVL/examples/mpi/collective/c_ex09.c

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

/*
! This program shows how to use MPI_Alltoallv.  Each processor
! send/rec a different and random amount of data to/from other
! processors.  
! We use MPI_Alltoall to tell how much data is going to be sent.
*/
/* globals */
int numnodes,myid,mpi_err;
#define mpi_root 0
/* end module  */

void init_it(int  *argc, char ***argv);
void seed_random(int  id);
void random_number(float *z);

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 *sray,*rray;
        int *sdisp,*scounts,*rdisp,*rcounts;
        int ssize,rsize,i,k,j;
        float z;

        init_it(&argc,&argv);
        scounts=(int*)malloc(sizeof(int)*numnodes);
        rcounts=(int*)malloc(sizeof(int)*numnodes);
        sdisp=(int*)malloc(sizeof(int)*numnodes);
        rdisp=(int*)malloc(sizeof(int)*numnodes);
/*
! seed the random number generator with a
! different number on each processor
*/
        seed_random(myid);
/* find out how much data to send */
        for(i=0;i<numnodes;i++){
                random_number(&z);
                scounts[i]=(int)(5.0*z)+1;
        }
        printf("myid= %d scounts=%d %d %d %d\n",myid,scounts[0],scounts[1],scounts[2],scounts[3]);
for(i=0;i<numnodes;i++)
                printf("%d ",scounts[i]);
        printf("\n");
/* tell the other processors how much data is coming */
        mpi_err = MPI_Alltoall( scounts,1,MPI_INT,
                                                    rcounts,1,MPI_INT,
                                    MPI_COMM_WORLD);
/*      write(*,*)"myid= ",myid," rcounts= ",rcounts */
/* calculate displacements and the size of the arrays */
        sdisp[0]=0;
        for(i=1;i<numnodes;i++){
                sdisp[i]=scounts[i-1]+sdisp[i-1];
        }
        rdisp[0]=0;
        for(i=1;i<numnodes;i++){
                rdisp[i]=rcounts[i-1]+rdisp[i-1];
        }
        ssize=0;
        rsize=0;
        for(i=0;i<numnodes;i++){
                ssize=ssize+scounts[i];
                rsize=rsize+rcounts[i];
        }
        
/* allocate send and rec arrays */
        sray=(int*)malloc(sizeof(int)*ssize);
        rray=(int*)malloc(sizeof(int)*rsize);
        for(i=0;i<ssize;i++)
                sray[i]=myid;
/* send/rec different amounts of data to/from each processor */
        mpi_err = MPI_Alltoallv(        sray,scounts,sdisp,MPI_INT,
                                                        rray,rcounts,rdisp,MPI_INT,
                                        MPI_COMM_WORLD);
                        
        printf("myid= %d rray=",myid);
        for(i=0;i<rsize;i++)
                printf("%d ",rray[i]);
        printf("\n");
    mpi_err = MPI_Finalize();
}
/*
  0:myid= 0 scounts=1 7 4
  0:myid= 0 scounts=0 1 1 1 1 1 1 2
  1:myid= 1 scounts=6 2 4
  1:myid= 1 scounts=0 0 0 0 0 0 0 1 1 2 2 2 2 2 2 2
  2:myid= 2 scounts=1 7 4
  2:myid= 2 scounts=0 0 0 0 1 1 1 1 2 2 2 2
*/

void seed_random(int  id){
        srand((unsigned int)id);
}
void random_number(float *z){
        int i;
        i=rand();
        *z=(float)i/RAND_MAX;
}