NAME

       Tcl_StackChannel,  Tcl_UnstackChannel, Tcl_GetStackedChannel, Tcl_GetTopChannel - 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)

       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