Scan

int MPI_Scan(void *sbuf, void *rbuf, int count, 
             MPI_Datatype dtype, MPI_Op op, MPI_Comm comm)
{
  MPI_Status status;
  int i, myrank, nprocs;
  void *tmpbuf;

  MPI_Comm_rank(comm, &myrank);
  MPI_Comm_size(comm, &nprocs);

  /* copy the send buffer into the receive buffer */
  MPI_Sendrecv(sbuf, count, dtype, myrank, IMPI_SCAN_TAG,
               rbuf, count, dtype, myrank, IMPI_SCAN_TAG, comm, &status);

  if (myrank > 0) {
    /* receive previous buffer into a temporary and reduce */

    create a temporary buffer tmpbuf large enough for count dtypes;

    MPI_Recv(tmpbuf, count, dtype, myrank-1, IMPI_SCAN_TAG, comm, &status);

    call reduction function op on tmpbuf (invec) and rbuf (inoutvec)
    
    free tmpbuf;

    if (myrank < (nprocs - 1)) {
      /* send result to next process */
      MPI_Send(rbuf, count, dtype, myrank + 1, IMPI_SCAN_TAG, comm);
    }
  }

  return(MPI_SUCCESS);
}