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Updated: Thursday, June 13, 2019
 
 

transchan (1t)

Name

transchan - command handler API of channel transforms

Synopsis

cmdPrefix option ?arg arg ...?

Description

transchan(1t)                Tcl Built-In Commands               transchan(1t)



______________________________________________________________________________

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 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.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | runtime/tcl-8    |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO
       chan(n), refchan(n)

KEYWORDS
       API, channel, ensemble, prefix, transformation



NOTES
       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-
       core8.6.7-src.tar.gz

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.



Tcl                                   8.6                        transchan(1t)