transform(n)															      transform(n)

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NAME

       transform - Tcl level transformations

SYNOPSIS

       package require Tcl  ?8.2?

       package require Trf  ?2.1.4?

       transform ?options...? ?data?

       callback operation data

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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 com-
		     mand 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 com-
       mand 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  com-
       mandline  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  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 opera-
	      tions.

	      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  trans-
		     formation 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 transfor-
		     mation 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 over-
		     shoot 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>

Trf transformer commands					       2.1.4							      transform(n)