NAME

       Tcl_StackChannel,  Tcl_UnstackChannel,  Tcl_GetStackedChannel, Tcl_GetTopChannel - 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 reporting.

       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.
________________________________________________________________________________________________________________

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 encoding 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 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.  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(3tcl), Tcl_CreateChannel(3tcl), Tcl_OpenFileChannel(3tcl), vwait(3tcl).

KEYWORDS

       channel, compression