'\" 
'\" Copyright (c) 2008 Donal K. Fellows
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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.TH transchan n 8.6 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
transchan \- command handler API of channel transforms
.SH SYNOPSIS
\fBcmdPrefix \fIoption\fR ?\fIarg arg ...\fR?
.BE
.SH DESCRIPTION
.PP
The Tcl-level handler for a channel transformation has to be a command with
subcommands (termed an \fIensemble\fR despite not implying that it must be
created with \fBnamespace ensemble create\fR; this mechanism is not tied to
\fBnamespace ensemble\fR in any way). Note that \fIcmdPrefix\fR is whatever
was specified in the call to \fBchan push\fR, and may consist of multiple
arguments; this will be expanded to multiple words in place of the prefix.
.PP
Of all the possible subcommands, the handler \fImust\fR support
\fBinitialize\fR and \fBfinalize\fR. Transformations for writable channels
must also support \fBwrite\fR, and transformations for readable channels must
also support \fBread\fR.
.PP
Note that in the descriptions below \fIcmdPrefix\fR may be more than one word,
and \fIhandle\fR is the value returned by the \fBchan push\fR call used to
create the transformation.
.SS "GENERIC SUBCOMMANDS"
.PP
The following subcommands are relevant to all types of channel.
.TP
\fIcmdPrefix \fBclear \fIhandle\fR
.
This optional subcommand is called to signify to the transformation that any
data stored in internal buffers (either incoming or outgoing) must be
cleared. It is called when a \fBchan seek\fR is performed on the channel being
transformed.
.TP
\fIcmdPrefix \fBfinalize \fIhandle\fR
.
This mandatory subcommand is called last for the given \fIhandle\fR, and then
never again, and it exists to allow for cleaning up any Tcl-level data
structures associated with the transformation. \fIWarning!\fR Any errors
thrown by this subcommand will be ignored. It is not guaranteed to be called
if the interpreter is deleted.
.TP
\fIcmdPrefix \fBinitialize \fIhandle mode\fR
.
This mandatory subcommand is called first, and then never again (for the given
\fIhandle\fR). Its responsibility is to initialize all parts of the
transformation at the Tcl level. The \fImode\fR is a list containing any of
\fBread \fRand \fBwrite\fR.
.RS
.TP
\fBwrite\fR
.
implies that the channel is writable.
.TP
\fBread\fR
.
implies that the channel is readable.
.PP
The return value of the subcommand should be a list containing the names of
all subcommands supported by this handler. Any error thrown by the subcommand
will prevent the creation of the transformation. The thrown error will appear
as error thrown by \fBchan push\fR.
.RE
.SS "READ-RELATED SUBCOMMANDS"
.PP
These subcommands are used for handling transformations applied to readable
channels; though strictly \fBread \fRis optional, it must be supported if any
of the others is or the channel will be made non-readable.
.TP
\fIcmdPrefix \fBdrain \fIhandle\fR
.
This optional subcommand is called whenever data in the transformation input
(i.e. read) buffer has to be forced upward, i.e. towards the user or script.
The result returned by the method is taken as the \fIbinary\fR data to push
upward to the level above this transformation (the reader or a higher-level
transformation).
.RS
.PP
In other words, when this method is called the transformation cannot defer the
actual transformation operation anymore and has to transform all data waiting
in its internal read buffers and return the result of that action.
.RE
.TP
\fIcmdPrefix \fBlimit? \fIhandle\fR
.
This optional subcommand is called to allow the Tcl I/O engine to determine
how far ahead it should read. If present, it should return an integer number
greater than zero which indicates how many bytes ahead should be read, or an
integer less than zero to indicate that the I/O engine may read as far ahead
as it likes.
.TP
\fIcmdPrefix \fBread \fIhandle buffer\fR
.
This subcommand, which must be present if the transformation is to work with
readable channels, is called whenever the base channel, or a transformation
below this transformation, pushes data upward. The \fIbuffer\fR contains the
binary data which has been given to us from below. It is the responsibility of
this subcommand to actually transform the data. The result returned by the
subcommand is taken as the binary data to push further upward to the
transformation above this transformation. This can also be the user or script
that originally read from the channel.
.RS
.PP
Note that the result is allowed to be empty, or even less than the data we
received; the transformation is not required to transform everything given to
it right now. It is allowed to store incoming data in internal buffers and to
defer the actual transformation until it has more data.
.RE
.SS "WRITE-RELATED SUBCOMMANDS"
.PP
These subcommands are used for handling transformations applied to writable
channels; though strictly \fBwrite\fR is optional, it must be supported if any
of the others is or the channel will be made non-writable.
.TP
\fIcmdPrefix \fBflush \fIhandle\fR
.
This optional subcommand is called whenever data in the transformation 'write'
buffer has to be forced downward, i.e. towards the base channel. The result
returned by the subcommand is taken as the binary data to write to the
transformation below the current transformation. This can be the base channel
as well.
.RS
.PP
In other words, when this subcommand is called the transformation cannot defer
the actual transformation operation anymore and has to transform all data
waiting in its internal write buffers and return the result of that action.
.RE
.TP
\fIcmdPrefix \fBwrite \fIhandle buffer\fR
.
This subcommand, which must be present if the transformation is to work with
writable channels, is called whenever the user, or a transformation above this
transformation, writes data downward. The \fIbuffer\fR contains the binary
data which has been written to us. It is the responsibility of this subcommand
to actually transform the data.
.RS
.PP
The result returned by the subcommand is taken as the binary data to write to
the transformation below this transformation. This can be the base channel as
well. Note that the result is allowed to be empty, or less than the data we
got; the transformation is not required to transform everything which was
written to it right now. It is allowed to store this data in internal buffers
and to defer the actual transformation until it has more data.
.RE
.SH "SEE ALSO"
chan(n), refchan(n)
.SH KEYWORDS
API, channel, ensemble, prefix, transformation
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