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.