# File ruby/bstream.rb, line 589
    def exec_content(out,message_type)
          @out = StringIO.new
      bpos = write_int1(message_type)
      header_size = write_int4(0) + bpos

      write(@sid)
      yield

      @out.pos = bpos
      write_int4( @out.size - header_size )
      @out.rewind
      out.write( @out.read )
      out.flush
      @out = nil
    end