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

Tcl                                                    8.3                                Tcl_StackChannel(3tcl)