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Tcl_StackChannel(3)	    Tcl Library Procedures	   Tcl_StackChannel(3)

______________________________________________________________________________

NAME
       Tcl_StackChannel,  Tcl_UnstackChannel,  Tcl_GetStackedChannel, Tcl_Get‐
       TopChannel - manipulate stacked I/O channels

SYNOPSIS
       #include <tcl.h>

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

       int
       Tcl_UnstackChannel(interp, channel)

       Tcl_Channel
       Tcl_GetStackedChannel(channel)

       Tcl_Channel
       Tcl_GetTopChannel(channel)

ARGUMENTS
       Tcl_Interp *interp (in)			Interpreter for error  report‐
						ing.

       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	proce‐
						dures.

       int mask (in)				Conditions under which channel
						will  be  used: OR-ed combina‐
						tion	 of	 TCL_READABLE,
						TCL_WRITABLE   and  TCL_EXCEP‐
						TION.  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_CreateChan‐
						nel.
_________________________________________________________________

DESCRIPTION
       These functions are for use by extensions that add processing layers to
       Tcl I/O channels.  Examples include compression 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.

       The  implementation  of	the Tcl channel code was rewritten in 8.3.2 to
       correct some problems with the previous implementation with  regard  to
       stacked	channels.   Anyone  using stacked channels or creating stacked
       channel	drivers	 should	 update	 to  the   new	 TCL_CHANNEL_VERSION_2
       Tcl_ChannelType structure.  See Tcl_CreateChannel for details.

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

       Tcl_StackChannel works by creating a new channel structure and  placing
       itself  on  top	of  the	 channel stack.	 EOL translation, encoding and
       buffering options are shared between all channels in  the  stack.   The
       hidden  channel	does  no buffering, newline translations, or character
       set encoding.  Instead, the buffering, newline translations, and encod‐
       ing functions all remain at the top of the channel stack.  A pointer to
       the new top channel structure is returned.  If  an  error  occurs  when
       stacking 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 origi‐
       nal  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.   If an error occurs
       unstacking the channel, TCL_ERROR  is  returned,	 otherwise  TCL_OK  is
       returned.

       Tcl_GetTopChannel  returns the top channel in the stack of channels the
       supplied channel is part of.

       Tcl_GetStackedChannel returns the channel  in  the  stack  of  channels
       which is just below the supplied channel.

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

KEYWORDS
       channel, compression

Tcl				      8.3		   Tcl_StackChannel(3)
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