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 accomodate 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 transfered 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

-length

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

-allocated

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 continuously.

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

One possible application of memory channels created by memchan or fifo is as temporay 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.