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Tcl_StackChannel(3)                        Tcl Library Procedures                        Tcl_StackChannel(3)



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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 combina-tion combination
                                                tion of TCL_READABLE, TCL_WRITABLE and TCL_EXCEPTION.   This
                                                can be a subset of the operations currently allowed on chan-nel. channel.
                                                nel.

       Tcl_Channel channel (in)                 An existing Tcl channel such as returned by  Tcl_CreateChan-nel. Tcl_CreateChannel.
                                                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 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 chan-nel, channel,
       nel, NULL is returned instead.

       The mask parameter specifies the operations that are allowed on the new channel.  These can be a sub-set subset
       set  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(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).


KEYWORDS
       channel, compression



Tcl                                                  8.3                                 Tcl_StackChannel(3)