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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 al-
						lowed 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 un-
       stacking	the channel, TCL_ERROR is returned, otherwise  TCL_OK  is  re-
       turned.

       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)

NAME | SYNOPSIS | ARGUMENTS | DESCRIPTION | SEE ALSO | KEYWORDS

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