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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>