For MPI operation, the "reading node" has to call the fdf initialization routine, serialize the fdf data structure, and broadcast it. Other nodes receive the structure and de-serialize it. In the following snippet we assume that 0 is the reading node:
if (Node .eq. 0) then
call fdf_init(filein, fileout)
else
!
endif
#ifdef MPI
call broadcast_fdf_struct(0,mpi_comm_world)
!
! Required in MPI operation. Choose one of the two, according
! to whether the default unit handler is fine or not
!! call fdf_set_unit_handler( custom_unit_handler )
!! call fdf_set_unit_handler( fdf_legacy_unit_handler )
#endif
The routine broadcast_fdf_struct can be found in file broadcast_fdf_struct.F90
in the doc directory of the fdf distribution. It should be general enough for most codes.