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 program pack