! simple_ex.f90
include "mpiSim.f90"
program ex1f
include "useSim.f90"

! Author unknown.
!! generats one new communicator for all MPI processes running on
!! The same host.

  implicit none

! include 'mpif.h'

  integer   ::         myrank, nprocs, n, islave, master
  integer   ::         status(MPI_STATUS_SIZE)
  integer   ::         ierr, resultlen, tag
  integer   ::         color, new_comm, new_rank, new_size, one, ssum
  character (LEN=8) :: hostname
  Double Precision  :: t1, t2

  ! Initialize MPI
  Call MPI_Init(Ierr)

  ! Get my rank id and number of MPI processes
  call MPI_Comm_rank(MPI_COMM_WORLD, myrank, ierr)
  call MPI_Comm_size(MPI_COMM_WORLD, nprocs, ierr)

  ! Get hostname
  call MPI_Get_processor_name(hostname, resultlen, ierr)

  ! Synchonization point
  call MPI_Barrier (MPI_COMM_WORLD, ierr)
  t1 = MPI_Wtime()

  !-----------------------------------------------------------
  ! Do work here ...
  Write (*,*) "Rank ", myrank, " out of ", nprocs,  &
    " processes running on ", hostname(1:index(hostname,".")-1)

  ! our hostnames contain a number, which will be used to generate new
  ! communicators containing all MPI processes running on the same host.
  ! sunc<nn>.rz.rwth-aachen.de

 !  read ( hostname(5:6),'(i2)') color
 ! $$Error: color is undefined in call to MPI_Comm_split.
   call MPI_Comm_split ( MPI_COMM_WORLD, color, myrank, new_comm, ierr )
   call MPI_Comm_rank ( new_comm, new_rank, ierr )
   call MPI_Comm_size ( new_comm, new_size, ierr )

   one = 1
   ssum = 0
   call MPI_Reduce ( one, ssum, 1, MPI_INTEGER, MPI_SUM, 0,   &
                     new_comm, ierr )
   if ( new_rank .eq. 0 ) then
      Write (*,*) "my rank in MPI_COMM_WORLD is ", myrank, " out of ", nprocs, &
        " / my rank in new_comm is ", new_rank, " out of ", new_size, &
        " / ", ssum, "processes have been taking part in the reduction"
   end if
  !-----------------------------------------------------------
  ! Synchronization point
  call MPI_Barrier (MPI_COMM_WORLD, ierr)
  t2 = MPI_Wtime()
  write (*,*) "slave ", myrank, ": ", t2-t1, " seconds"

  ! Shut Down MPI
  call MPI_Finalize(ierr )

end program ex1f