transchan(n) Tcl Built-In Commands transchan(n)______________________________________________________________________________NAMEtranschan - command handler API of channel transforms
SYNOPSIS
cmdPrefix option ?arg arg ...?
_________________________________________________________________DESCRIPTION
The Tcl-level handler for a channel transformation has to be a command
with subcommands (termed an ensemble despite not implying that it must
be created with namespace ensemble create; this mechanism is not tied
to namespace ensemble in any way). Note that cmdPrefix is whatever was
specified in the call to chan push, and may consist of multiple argu‐
ments; this will be expanded to multiple words in place of the prefix.
Of all the possible subcommands, the handler must support initialize
and finalize. Transformations for writable channels must also support
write, and transformations for readable channels must also support
read.
Note that in the descriptions below cmdPrefix may be more than one
word, and handle is the value returned by the chan push call used to
create the transformation.
GENERIC SUBCOMMANDS
The following subcommands are relevant to all types of channel.
cmdPrefix clear handle
This optional subcommand is called to signify to the transforma‐
tion that any data stored in internal buffers (either incoming
or outgoing) must be cleared. It is called when a chan seek is
performed on the channel being transformed.
cmdPrefix finalize handle
This mandatory subcommand is called last for the given handle,
and then never again, and it exists to allow for cleaning up any
Tcl-level data structures associated with the transformation.
Warning! Any errors thrown by this subcommand will be ignored.
It is not guaranteed to be called if the interpreter is deleted.
cmdPrefix initialize handle mode
This mandatory subcommand is called first, and then never again
(for the given handle). Its responsibility is to initialize all
parts of the transformation at the Tcl level. The mode is a list
containing any of read and write.
write implies that the channel is writable.
read implies that the channel is readable.
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
chan push.
READ-RELATED SUBCOMMANDS
These subcommands are used for handling transformations applied to
readable channels; though strictly read is optional, it must be sup‐
ported if any of the others is or the channel will be made non-read‐
able.
cmdPrefix drain handle
This optional subcommand is called whenever data in the trans‐
formation 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 binary data to push upward to the level above
this transformation (the reader or a higher-level transforma‐
tion).
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.
cmdPrefix limit? handle
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.
cmdPrefix read handle buffer
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 buffer contains the binary data which has been
given to us from below. It is the responsibility of this subcom‐
mand 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.
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.
WRITE-RELATED SUBCOMMANDS
These subcommands are used for handling transformations applied to
writable channels; though strictly write is optional, it must be sup‐
ported if any of the others is or the channel will be made non-
writable.
cmdPrefix flush handle
This optional subcommand is called whenever data in the trans‐
formation '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 cur‐
rent transformation. This can be the base channel as well.
In other words, when this subcommand is called the transforma‐
tion cannot defer the actual transformation operation anymore
and has to transform all data waiting in its internal write buf‐
fers and return the result of that action.
cmdPrefix write handle buffer
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 down‐
ward. The buffer contains the binary data which has been written
to us. 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 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 transfor‐
mation 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.
SEE ALSOchan(n), refchan(n)KEYWORDS
API, channel, ensemble, prefix, transformation
Tcl 8.6 transchan(n)
