Tcl_StackChannel(3) Tcl Library Procedures Tcl_StackChannel(3)
Tcl_StackChannel, Tcl_UnstackChannel, Tcl_GetStackedChannel, Tcl_GetTopChannel - manipu-
late stacked I/O channels
Tcl_StackChannel(interp, typePtr, clientData, mask, channel)
Tcl_Interp *interp (in) Interpreter for error reporting.
Tcl_ChannelType *typePtr (in) The new channel I/O procedures to use for chan-
ClientData clientData (in) Arbitrary one-word value to pass to channel I/O
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 oper-
ations currently allowed on channel.
Tcl_Channel channel (in) An existing Tcl channel such as returned by
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
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_VER-
SION_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 func-
tions all remain at the top of the channel stack. A pointer to the new top channel struc-
ture 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
Tcl_GetStackedChannel returns the channel in the stack of channels which is just below the
Notifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).
Tcl 8.3 Tcl_StackChannel(3)