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NAME
     Tcl_StackChannel, Tcl_UnstackChannel - stack an I/O  channel
     on top of another, and undo it

SYNOPSIS
     #include <tcl.h>

     Tcl_Channel
     Tcl_StackChannel(interp, typePtr, clientData, mask, channel)

     void
     Tcl_UnstackChannel(interp, channel)

     Tcl_Channel
     Tcl_GetStackedChannel(channel)


ARGUMENTS
     Tcl_Interp        *interp        (in)      Interpreter   for
                                                error reporting -
                                                can be NULL.

     Tcl_ChannelType   *typePtr       (in)      The  new  channel
                                                I/O procedures to
                                                use for channel.

     ClientData        clientData     (in)      Arbitrary    one-
                                                word   value   to
                                                pass  to  channel
                                                I/O procedures.

     int               mask           (in)      Conditions  under
                                                which     channel
                                                will   be   used:
                                                OR-ed combination
                                                of  TCL_READABLE,
                                                TCL_WRITABLE  and
                                                TCL_EXCEPTION.
                                                This   can  be  a
                                                subset   of   the
                                                operations
                                                currently allowed
                                                on channel.

     Tcl_Channel       channel        (in)      An  existing  Tcl
                                                channel  such  as
                                                returned       by
                                                Tcl_CreateChannel.
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DESCRIPTION
     These functions are for use by extensions that add  process-
     ing  layers  to Tcl I/O channels.  Examples include compres-
     sion and encryption modules.  These functions  transparently
     stack  and  unstack a new channel on top of an existing one.
     Any number of channels can be stacked together.

     Tcl_StackChannel replaces an existing  channel  with  a  new
     channel  by  the  same  name that was registered for channel
     with Tcl_RegisterChannel.

     Tcl_StackChannel works by creating a new  channel  structure
     under  the existing channel and pushing the existing channel
     functions down to the newly  created  channel.   The  hidden
     channel  does no buffering, newline translations, or charac-
     ter set encoding.  Instead, the buffering, newline  transla-
     tions,  and  encoding functions all remain at the top of the
     channel stack.  The top-most channel gets changed to use the
     I/O  channel  functions  defined in typePtr, and the channel
     functions are passed the new clientData.  The existing chan-
     nel  structure  is modified in place, so C applications that
     continue to use channel will also see the effects of the new
     processing  module.  A pointer to a new channel structure is
     returned, although this new data structure is the  one  that
     has  been  pushed  down  below the top of the channel module
     stack.   (This  pointer  can  also  be  obtained  with   the
     Tcl_GetStackedChannel call.)  If an error occurs when stack-
     ing the channel, NULL is returned instead.

     The mask parameter specifies the operations that are allowed
     on the new channel.  These can be a subset of the operations
     allowed on the original channel.  For example, a  read-write
     channel  may  become  read-only  after  the Tcl_StackChannel
     call.

     Closing a channel closes the channels stacked below it.  The
     close  of  stacked channels is executed in a way that allows
     buffered data to be properly flushed.

     Tcl_UnstackChannel reverses the process.  The old channel is
     associated  with the channel name, and the processing module
     added by Tcl_StackChannel is destroyed.  If there is no  old
     channel,  then Tcl_UnstackChannel is equivalent to Tcl_Close
     .


SEE ALSO
     Notifier(3),  Tcl_CreateChannel(3),  Tcl_OpenFileChannel(3),
     vwait(n).



KEYWORDS
     channel, compression

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