transchan

transchan(3tcl)               Tcl Built-In Commands              transchan(3tcl)



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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



Tcl                                    8.6                       transchan(3tcl)