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Tcl_StackChannel(3)         Tcl Library Procedures         Tcl_StackChannel(3)


       Tcl_StackChannel,  Tcl_UnstackChannel,  Tcl_GetStackedChannel, Tcl_GetTopChannel - manipulate stacked I/O chan-

       #include <tcl.h>

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

       Tcl_UnstackChannel(interp, channel)



       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.

       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

       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  transla-
       tions,  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 process-
       ing 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

       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.

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

       channel, compression

Tcl                                   8.3                  Tcl_StackChannel(3)