- NAME
- transchan — command handler API of channel transforms
- SYNOPSIS
- DESCRIPTION
- GENERIC
SUBCOMMANDS
- cmdPrefix clear handle
- cmdPrefix finalize handle
- cmdPrefix initialize handle mode
- write
- read
- READ-RELATED SUBCOMMANDS
- cmdPrefix drain handle
- cmdPrefix limit? handle
- cmdPrefix read handle buffer
- WRITE-RELATED SUBCOMMANDS
- cmdPrefix flush handle
- cmdPrefix write handle buffer
- SEE
ALSO
- KEYWORDS
transchan — command handler API of channel transforms
chan push channel cmdPrefix
cmdPrefix clear handle
cmdPrefix drain handle
cmdPrefix finalize handle
cmdPrefix flush handle
cmdPrefix initialize handle mode
cmdPrefix limit? handle
cmdPrefix read handle buffer
cmdPrefix write handle buffer
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.
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.
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.
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.
chan, refchan
API, channel, ensemble, prefix, transformation
Copyright © 2008 Donal K. Fellows