
  -spec pid2name(Pid) -> {ok, Filename} | undefined
                    when Filename :: filename_all(), Pid :: pid().

  If Pid is an I/O device, that is, a pid returned from open/2,
  this function returns the filename, or rather:

  {ok, Filename}:
    If the file server of this node is not a slave, the file was
    opened by the file server of this node (this implies that Pid
    must be a local pid) and the file is not closed. Filename is
    the filename in flat string format.

  undefined:
    In all other cases.

  Warning:
    This function is intended for debugging only.
