transform(n)

NAME
transform - Tcl level transformations

SYNOPSIS
package require Tcl ?8.2?

package require Trf ?2.1.3?

transform ?options...? ?data?

callback operation data

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.

        The argument value specifies whether to run the read or the write part of the transformation specified via option -command on the immediate data.

        Beyond the argument values listed above all unique abbreviations are recognized too.

    -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.

        The exact nature of the various possible calls and their expected results is described later, in section CALLBACK API.

    -attach channel
        The presence/absence of this option determines the main operation mode of the transformation.

        If present the transformation will be stacked onto the channel whose handle was given to the option and run in attached mode. More about this in section IMMEDIATE versus ATTACHED.

        If the option is absent the transformation is used in immediate mode and the options listed below are recognized. More about this in section IMMEDIATE versus ATTACHED.

    -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.

        If the transformation is in immediate mode and this option is absent the data to transform is expected as the last argument to the transformation.

    -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.

        If the transformation is in immediate mode and this option is absent the generated data is returned as the result of the command itself.

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.

    The known operations are listed below, together with an explanation of the arguments, what is expected of them, and how their results are handled.

    create/write
        When called data is empty. The result of the call is ignored.

        This is the first operation executed for the write side of the transformation. It has to initialize the internals of this part of the transformation and ready it for future calls.

    delete/write
        When called data is empty. The result of the call is ignored.

        This is the last operation executed for the write side of the transformation. It has to shutdown the internals of this part of the transformation and release any resources which were acquired over the lifetime of the transformation.

    write
        The operation is called whenever data is written to the channel.

        At the time of the call the argument data will contain the bytes to transform. 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.

        This operation has to transform the contents of data, using whatever data was left over from the last call of the operation. The transformation is allowed to buffer incomplete data.

    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.

        The write side of the transformation has to clear its internal buffers. This operation is called when the user seeks on the channel, thus invalidating any incomplete transformation.

    create/read
        When called data is empty. The result of the call is ignored.

        This is the first operation executed for the read side of the transformation. It has to initialize the internals of this part of the transformation and ready it for future calls.

    delete/read
        When called data is empty. The result of the call is ignored.

        This is the last operation executed for the write side of the transformation. It has to shutdown the internals of this part of the transformation and release any resources which were acquired over the lifetime of the transformation.

    read
        The operation is called whenever data is read from the channel.

        At the time of the call the argument data will contain the bytes to transform. 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.

        This operation has to transform the contents of data, using whatever data was left over from the last call of the operation. The transformation is allowed to buffer incomplete data.

    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.

        The read side of the transformation has to clear its internal buffers. This operation is called when the user seeks on the channel, thus invalidating any incomplete transformation.

    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.

        This has to be used if a transformation employs some kind of end-of-data marker. We cannot allow the generic layer to overshoot this marker because any data read after it cannot be stuffed back into the core buffers, causing the I/O system to loose data if the transformation is unstacked after it recognized the end of its data. This is a limitation of the I/O system in the tcl core.

        Returning a positive value will cause the I/O system to slow down, but also ensures that no data is lost.

        Two examples for such transformations are the data decompressors for zip and bz2. They use the C-level equivalent of this operation to prevent the overshooting.

SEE ALSO
trf-intro

KEYWORDS
general transform

COPYRIGHT
Copyright (c) 1996-2003, Andreas Kupries <andreas_kupries@users.sourceforge.net>