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