1fifo(n) fifo(n)
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8 fifo - Create and manipulate u-turn fifo channels
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11 package require Tcl
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13 package require memchan
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15 fifo
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20 fifo creates a stream-oriented in-memory channel and returns its han‐
21 dle. There is no restriction on the ultimate size of the chan‐
22 nel, it will always grow as much as is necessary to accommodate
23 the data written into it.
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25 In contrast to the channels generated by memchan a channel cre‐
26 ated here effectively represents an U-turn. All data written
27 into it can be read out, but only in the same order. This also
28 means that a fifo channel is not seekable.
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30 The channels created here can be transferred between inter‐
31 preters in the same thread and between threads, but only as a
32 whole. It is not possible to use them to create a bi- or unidi‐
33 rectional connection between two interpreters.
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35 Memory channels created by fifo provide two read-only options which can
36 be queried via the standard fconfigure command. These are
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38 -length
39 The value of this option is the number of bytes currently stored
40 in the queried memory channel.
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42 -allocated
43 The value of this option is the number of bytes currently allo‐
44 cated by the queried memory channel. This number is at least as
45 big as the value of -length.
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47 As the channels generated by fifo grow as necessary they are always
48 writable. This means that a writable fileevent-handler will fire con‐
49 tinuously.
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51 The channels are also readable if they contain more than zero bytes.
52 Under this conditions a readable fileevent-handler will fire continu‐
53 ously.
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56 One possible application of memory channels created by memchan or fifo
57 is as temporary storage device to collect data coming in over a pipe or
58 a socket. If part of the processing of the incoming data is to read and
59 process header bytes or similar fifo are easier to use as they do not
60 require seeking back and forth to switch between the assimilation of
61 headers at the beginning and writing new data at the end.
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64 fifo2, memchan, null
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67 channel, fifo, i/o, in-memory channel, memchan, stream
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70 Copyright (c) 1996-2003 Andreas Kupries <andreas_kupries@users.sourceforge.net>
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75Memory channels 2.1 fifo(n)