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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.

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


KEYWORDS

       channel, compression

Tcl                                   8.3                  Tcl_StackChannel(3)

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