! roll_call.F  check that processors are available  

program roll_call
  include 'mpif.h'  ! use my_mpif.h not available on your system
  implicit none
  integer :: proc, numprocs
  integer :: myid           ! myid is 0 to numprocs-1 
  integer :: count = 1 
  integer :: tag = 0 
  integer :: master = 0 
  double precision :: esec 
  integer, dimension(1000) :: buf ! subscripts start at 1 
  integer :: status 
  integer :: ierr  ! "C" return as last argument

  call MPI_Init(ierr) 
  call MPI_Comm_size(MPI_COMM_WORLD, numprocs, ierr) 
  call MPI_Comm_rank(MPI_COMM_WORLD, myid, ierr) 

  if(myid == master) then
    print *, 'roll_call.F  numprocs=', numprocs 
    print *, 'printout may not be in order on multiprocessors'
    do proc=0,numprocs-1  ! rank starts at zero, last is numprocs-1
      buf(1) = 10*proc 
      call MPI_Send(buf, count, MPI_INTEGER, proc, tag, MPI_COMM_WORLD, &
                    ierr) 
      buf(1) = 1 
      call MPI_Recv(buf, count, MPI_INTEGER, proc, tag, MPI_COMM_WORLD, &
                    status, ierr) 
      print *, 'proc', proc, ' answers with ', buf(1)   
    end do
    esec = MPI_Wtime() 
    print *, 'roll_call.c took', esec, ' seconds'
  else ! slaves 
    call MPI_Recv(buf, count, MPI_INTEGER, master, tag, MPI_COMM_WORLD, &
                  status, ierr)   
    print *, 'proc', myid, ' received', buf(1) 
    buf(1) = 10*buf(1) 
    call MPI_Send(buf, count, MPI_INTEGER, master, tag, MPI_COMM_WORLD, &
                  ierr) 
  end if
  call MPI_Finalize(ierr) 
end ! roll_call.F