#include <petsc/private/vecimpl.h>
#include <petscvec.h>

PetscErrorCode VecScan(Vec x,Vec b)
{
  PetscErrorCode    ierr;
  PetscInt          nlocal;
  PetscScalar       ssum;
  PetscMPIInt       rank,size,tag;
  MPI_Status        status;
  MPI_Comm          comm;
  PetscScalar       *bb;
  const PetscScalar *xx;
  PetscInt          i;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(x,VEC_CLASSID,1);
  PetscValidHeaderSpecific(b,VEC_CLASSID,2);
  PetscValidType(x,1);
  PetscValidType(b,2);
  PetscCheckSameTypeAndComm(x,1,b,2);
  VecCheckSameSize(x,1,b,1);

  ierr = PetscObjectGetComm((PetscObject)x,&comm);CHKERRQ(ierr);
  ierr = MPI_Comm_size(comm,&size);CHKERRQ(ierr);
  ierr = MPI_Comm_rank(comm,&rank);CHKERRQ(ierr);
  ierr = PetscObjectGetNewTag((PetscObject)x,&tag);CHKERRQ(ierr);
  if (rank) {
    ierr = MPI_Recv(&ssum,1,MPIU_SCALAR,rank-1,tag,comm,&status);CHKERRQ(ierr);
  } else {
    ssum = 0;
  }
  ierr = VecGetLocalSize(x,&nlocal);CHKERRQ(ierr);
  ierr = VecGetArrayRead(x,&xx);CHKERRQ(ierr);
  ierr = VecGetArray(b,&bb);CHKERRQ(ierr);
  for (i=0; i<nlocal; i++) {
    ssum  += xx[i];
    bb[i] = ssum;
  }
  ierr = VecRestoreArrayRead(x,&xx);CHKERRQ(ierr);
  ierr = VecRestoreArray(b,&bb);CHKERRQ(ierr);
  if (rank < size-1) {
    ierr = MPI_Send(&ssum,1,MPIU_SCALAR,rank+1,tag,comm);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

int main(int argc,char **args)
{
  Vec            x, b;
  PetscInt       n = 7;
  PetscErrorCode ierr;

  ierr = PetscInitialize(&argc,&args,(char*)0,NULL);if (ierr) return ierr;

  ierr = VecCreate(PETSC_COMM_WORLD,&x);CHKERRQ(ierr);
  ierr = VecSetSizes(x,PETSC_DECIDE,n);CHKERRQ(ierr);
  ierr = VecSetFromOptions(x);CHKERRQ(ierr);
  ierr = VecDuplicate(x,&b);CHKERRQ(ierr);
  ierr = VecSetRandom(x,NULL);CHKERRQ(ierr);
  ierr = VecView(x,PETSC_VIEWER_STDOUT_WORLD);CHKERRQ(ierr);
  ierr = VecScan(x,b);CHKERRQ(ierr);
  ierr = VecView(b,PETSC_VIEWER_STDOUT_WORLD);CHKERRQ(ierr);
  ierr = VecDestroy(&x);CHKERRQ(ierr);
  ierr = VecDestroy(&b);CHKERRQ(ierr);

  ierr = PetscFinalize();
  return ierr;
}