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transchan(n) Tcl Built-In Commands transchan(n) 





NAME


transchan - 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 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.



  

  
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.

  

  
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.

  

  
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.







  

  
implies that the channel is writable.

  

  
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.



  

  
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.





  

  
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.

  

  
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.



  

  
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.





  

  
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(n), refchan(n)






KEYWORDS


API, channel, ensemble, prefix, transformation







  

    8.6
    Tcl