transform(n)transform(n)______________________________________________________________________________NAMEtransform - Tcl level transformations
SYNOPSIS
package require Tcl ?8.2?
package require Trf ?2.1.4?
transform ?options...? ?data?
callback operation data
_________________________________________________________________DESCRIPTION
The command transform reflects the API for a stack channel transforma‐
tion 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 abbre‐
viations 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 ver‐
sus ATTACHED.
If the option is absent the transformation is used in
immediate mode and the options listed below are recog‐
nized. 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 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 \0 characters embedded into either input or out‐
put.
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 oper‐
ation code followed by data. The latter will be empty for some
operations.
The known operations are listed below, together with an explana‐
tion 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 transforma‐
tion.
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 chan‐
nel.
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 transforma‐
tion.
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 decom‐
pressors for zip and bz2. They use the C-level equivalent
of this operation to prevent the overshooting.
SEE ALSO
trf-intro
KEYWORDS
general transformCOPYRIGHT
Copyright (c) 1996-2003, Andreas Kupries <andreas_kupries@users.sourceforge.net>
Trf transformer commands 2.1.4 transform(n)
