code: def fn2lun(fname):
      
      fnameP    = stypes.stringToCharP(fname)
      unit_out  = ctypes.c_int()
      fname_len = ctypes.c_int(len(fname)+1)
      libspice.fn2lun_(fnameP,ctypes.byref(unit_out),fname_len)
      return unit_out.value
docs: Internal undocumented command for mapping name of open file to
      its FORTRAN (F2C) logical unit.
  
      https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/fn2lun.html
  
      :param fname: name of the file to be mapped to its logical unit.
      :type fname: str
      :return: the FORTRAN (F2C) logical unit associated with the filename.
      :rtype: int