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transform(3trf) | transform(3trf) |
NAME¶
transform - Tcl level transformationsSYNOPSIS¶
package require Tcl ?8.2?DESCRIPTION¶
The command transform reflects the API for a stack channel transformation into the tcl level, thus enabling the writing of transformations in tcl.- transform ?options...? ?data?
- -mode read|write
- This option is accepted by the command if and only if it is
used in immediate mode. See section IMMEDIATE versus
ATTACHED for an explanation of the term.
- -command cmd
- This option has to be present and is always understood. Its
argument is a command prefix. This command prefix will be called by
internally whenever some operation of the transformation has to be
executed. An empty cmd is not allowed.
- -attach channel
- The presence/absence of this option determines the main
operation mode of the transformation.
- -in channel
- This options is legal if and only if the transformation is
used in immediate mode. It provides the handle of the channel the
data to transform has to be read from.
- -out channel
- This options is legal if and only if the transformation is
used in immediate mode. It provides the handle of the channel the
generated transformation result is written to.
IMMEDIATE VERSUS ATTACHED¶
The transformation distinguishes between two main ways of using it. These are the immediate and attached operation modes. For the attached mode the option -attach is used to associate the transformation with an existing channel. During the execution of the command no transformation is performed, instead the channel is changed in such a way, that from then on all data written to or read from it passes through the transformation and is modified by it according to the definition above. This attachment can be revoked by executing the command unstack for the chosen channel. This is the only way to do this at the Tcl level. In the second mode, which can be detected by the absence of option -attach, the transformation immediately takes data from either its commandline or a channel, transforms it, and returns the result either as result of the command, or writes it into a channel. The mode is named after the immediate nature of its execution. Where the data is taken from, and delivered to, is governed by the presence and absence of the options -in and -out. It should be noted that this ability to immediately read from and/or write to a channel is an historic artifact which was introduced at the beginning of Trf's life when Tcl version 7.6 was current as this and earlier versions have trouble to deal with \0 characters embedded into either input or output.CALLBACK API¶
Here we describe the API of the callback command implementing the actual transformation.- callback operation data
- The callback is always called with two arguments, first an
operation code followed by data. The latter will be empty for some
operations.
- create/write
- When called data is empty. The result of the call is
ignored.
- delete/write
- When called data is empty. The result of the call is
ignored.
- write
- The operation is called whenever data is written to the
channel.
- flush/write
- When called data is empty. The operation has to transform any incomplete data it has buffered internally on the write side. The result of the call is taken as the result of the transformation and handed to the next stage down in the stack of transformation associated with the channel.
- clear/write
- When called data is empty. The result of the call is
ignored.
- create/read
- When called data is empty. The result of the call is
ignored.
- delete/read
- When called data is empty. The result of the call is
ignored.
- read
- The operation is called whenever data is read from the
channel.
- flush/read
- When called data is empty. The operation has to transform any incomplete data it has buffered internally on the read side. The result of the call is taken as the result of the transformation and posted to the next stage up in the stack of transformation associated with the channel.
- clear/read
- When called data is empty. The result of the call is
ignored.
- query/maxRead
- When called data is empty. The result of the call is
interpreted as integer number. This operation is used by the generic layer
to determine if the transformation establishes a limit on the number of
bytes it (the generic layer) is allowed read from the transformations
lower in the stack. A negative result unsets any limit.
SEE ALSO¶
trf-introKEYWORDS¶
general transformCOPYRIGHT¶
Copyright (c) 1996-2003, Andreas Kupries <andreas_kupries@users.sourceforge.net>
2.1.3 | Trf transformer commands |