program pack
*
*     This simple pack program packs everything in a
*     double precision vector. One has to be a bit careful with
*     integers. The program converts them to double precision 
*     (thereby wasting storage). To avoid possible rounding
*     errors we add 0.1 before the conversion and when we
*     unpack we truncate. The MPI_Pack/MPI_Unpack stores data
*     on a byte-level, but the principle is roughly the same.
*
      implicit    none
      include    "mpif.h"
      integer     my_rank, err, k, n, status(MPI_STATUS_SIZE)
      integer     buf_len
      parameter ( buf_len = 100 )
      double precision  buffer(buf_len), vec(5)


      call MPI_Init(err)
      call MPI_Comm_rank(MPI_COMM_WORLD, my_rank, err)
 
      if ( my_rank .eq. 0 ) then 
        n = 123
        do k = 1, 5
          vec(k) = k + 1.0d0 / k
        end do
*
*       Pack
*
        buffer(1) = n + 0.1
        do k = 1, 5
          buffer(k + 1) = vec(k)
        end do

        call MPI_Send(buffer, 6, MPI_DOUBLE_PRECISION, 1, 1,
     +                MPI_COMM_WORLD, err)
      
      else
*
*       Receive message from master
*
        call MPI_Recv(buffer, 6, MPI_DOUBLE_PRECISION, 0, 1,
     +                MPI_COMM_WORLD, status, err)
*
*       Unpack
*
       n = buffer(1) ! truncation of decimals
       do k = 1, 5
         vec(k) = buffer(k + 1)
       end do

       print*, 'In slave: n   = ', n
       print*, 'In slave: vec = ', vec

      end if
      
      call MPI_Finalize(err)

      end