1transchan(n) Tcl Built-In Commands transchan(n)
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8 transchan - command handler API of channel transforms
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11 cmdPrefix option ?arg arg ...?
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15 The Tcl-level handler for a channel transformation has to be a command
16 with subcommands (termed an ensemble despite not implying that it must
17 be created with namespace ensemble create; this mechanism is not tied
18 to namespace ensemble in any way). Note that cmdPrefix is whatever was
19 specified in the call to chan push, and may consist of multiple argu‐
20 ments; this will be expanded to multiple words in place of the prefix.
21 Of all the possible subcommands, the handler must support initialize
23 and finalize. Transformations for writable channels must also support
24 write, and transformations for readable channels must also support
25 read.
26 Note that in the descriptions below cmdPrefix may be more than one
28 word, and handle is the value returned by the chan push call used to
29 create the transformation.
30 GENERIC SUBCOMMANDS
32 The following subcommands are relevant to all types of channel.
33 cmdPrefix clear handle
35 This optional subcommand is called to signify to the transforma‐
36 tion that any data stored in internal buffers (either incoming
37 or outgoing) must be cleared. It is called when a chan seek is
38 performed on the channel being transformed.
39 cmdPrefix finalize handle
41 This mandatory subcommand is called last for the given handle,
42 and then never again, and it exists to allow for cleaning up any
43 Tcl-level data structures associated with the transformation.
44 Warning! Any errors thrown by this subcommand will be ignored.
45 It is not guaranteed to be called if the interpreter is deleted.
46 cmdPrefix initialize handle mode
48 This mandatory subcommand is called first, and then never again
49 (for the given handle). Its responsibility is to initialize all
50 parts of the transformation at the Tcl level. The mode is a list
51 containing any of read and write.
52 write implies that the channel is writable.
54 read implies that the channel is readable.
56 The return value of the subcommand should be a list containing
58 the names of all subcommands supported by this handler. Any
59 error thrown by the subcommand will prevent the creation of the
60 transformation. The thrown error will appear as error thrown by
61 chan push.
62 READ-RELATED SUBCOMMANDS
64 These subcommands are used for handling transformations applied to
65 readable channels; though strictly read is optional, it must be sup‐
66 ported if any of the others is or the channel will be made non-read‐
67 able.
68 cmdPrefix drain handle
70 This optional subcommand is called whenever data in the trans‐
71 formation input (i.e. read) buffer has to be forced upward, i.e.
72 towards the user or script. The result returned by the method
73 is taken as the binary data to push upward to the level above
74 this transformation (the reader or a higher-level transforma‐
75 tion).
76 In other words, when this method is called the transformation
78 cannot defer the actual transformation operation anymore and has
79 to transform all data waiting in its internal read buffers and
80 return the result of that action.
81 cmdPrefix limit? handle
83 This optional subcommand is called to allow the Tcl I/O engine
84 to determine how far ahead it should read. If present, it should
85 return an integer number greater than zero which indicates how
86 many bytes ahead should be read, or an integer less than zero to
87 indicate that the I/O engine may read as far ahead as it likes.
88 cmdPrefix read handle buffer
90 This subcommand, which must be present if the transformation is
91 to work with readable channels, is called whenever the base
92 channel, or a transformation below this transformation, pushes
93 data upward. The buffer contains the binary data which has been
94 given to us from below. It is the responsibility of this subcom‐
95 mand to actually transform the data. The result returned by the
96 subcommand is taken as the binary data to push further upward to
97 the transformation above this transformation. This can also be
98 the user or script that originally read from the channel.
99 Note that the result is allowed to be empty, or even less than
101 the data we received; the transformation is not required to
102 transform everything given to it right now. It is allowed to
103 store incoming data in internal buffers and to defer the actual
104 transformation until it has more data.
105 WRITE-RELATED SUBCOMMANDS
107 These subcommands are used for handling transformations applied to
108 writable channels; though strictly write is optional, it must be sup‐
109 ported if any of the others is or the channel will be made non-
110 writable.
111 cmdPrefix flush handle
113 This optional subcommand is called whenever data in the trans‐
114 formation 'write' buffer has to be forced downward, i.e. towards
115 the base channel. The result returned by the subcommand is taken
116 as the binary data to write to the transformation below the cur‐
117 rent transformation. This can be the base channel as well.
118 In other words, when this subcommand is called the transforma‐
120 tion cannot defer the actual transformation operation anymore
121 and has to transform all data waiting in its internal write buf‐
122 fers and return the result of that action.
123 cmdPrefix write handle buffer
125 This subcommand, which must be present if the transformation is
126 to work with writable channels, is called whenever the user, or
127 a transformation above this transformation, writes data down‐
128 ward. The buffer contains the binary data which has been written
129 to us. It is the responsibility of this subcommand to actually
130 transform the data.
131 The result returned by the subcommand is taken as the binary
133 data to write to the transformation below this transformation.
134 This can be the base channel as well. Note that the result is
135 allowed to be empty, or less than the data we got; the transfor‐
136 mation is not required to transform everything which was written
137 to it right now. It is allowed to store this data in internal
138 buffers and to defer the actual transformation until it has more
139 data.
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142 chan(n), refchan(n)
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145 API, channel, ensemble, prefix, transformation
146Tcl 8.6 transchan(n)