def add_file(name, mode)
raise BlockNeeded unless block_given?
raise ClosedIO if @closed
raise NonSeekableIO unless @io.respond_to? :pos=
name, prefix = split_name(name)
init_pos = @io.pos
@io.write "\0" * 512
yield RestrictedStream.new(@io)
size = @io.pos - init_pos - 512
remainder = (512 - (size % 512)) % 512
@io.write("\0" * remainder)
final_pos = @io.pos
@io.pos = init_pos
header = TarHeader.new(:name => name, :mode => mode,
:size => size, :prefix => prefix).to_s
@io.write header
@io.pos = final_pos
end