Provided by: tcl8.6-doc_8.6.8+dfsg-3_all 
NAME
transchan - command handler API of channel transforms
SYNOPSIS
cmdPrefix option ?arg arg ...?
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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 arguments;
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 transformation 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 supported if any of the others is or the
channel will be made non-readable.
cmdPrefix drain handle
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 binary data to push upward to the level
above this transformation (the reader or a higher-level transformation).
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 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.
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 supported 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 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.
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.
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 downward. 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
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.
SEE ALSO
chan(3tcl), refchan(3tcl)
KEYWORDS
API, channel, ensemble, prefix, transformation