fifo(3tcl)                                                            fifo(3tcl)


       fifo - Create and manipulate u-turn fifo channels

       package require Tcl

       package require memchan



       fifo   creates a stream-oriented in-memory channel and returns its
              handle. There is no restriction on the ultimate size of the
              channel, it will always grow as much as is necessary to
              accommodate the data written into it.

              In contrast to the channels generated by memchan a channel created
              here effectively represents an U-turn. All data written into it
              can be read out, but only in the same order. This also means that
              a fifo channel is not seekable.

              The channels created here can be transferred between interpreters
              in the same thread and between threads, but only as a whole. It is
              not possible to use them to create a bi- or unidirectional
              connection between two interpreters.

       Memory channels created by fifo provide two read-only options which can
       be queried via the standard fconfigure command. These are

              The value of this option is the number of bytes currently stored
              in the queried memory channel.

              The value of this option is the number of bytes currently
              allocated by the queried memory channel. This number is at least
              as big as the value of -length.

       As the channels generated by fifo grow as necessary they are always
       writable. This means that a writable fileevent-handler will fire

       The channels are also readable if they contain more than zero bytes.
       Under this conditions a readable fileevent-handler will fire

       One possible application of memory channels created by memchan or fifo is
       as temporary storage device to collect data coming in over a pipe or a
       socket. If part of the processing of the incoming data is to read and
       process header bytes or similar fifo are easier to use as they do not
       require seeking back and forth to switch between the assimilation of
       headers at the beginning and writing new data at the end.

       fifo2, memchan, null

       channel, fifo, i/o, in-memory channel, memchan, stream

       Copyright (c) 1996-2003 Andreas Kupries <>

Memory channels                        2.1                            fifo(3tcl)