Tcl_StackChannel(3) Tcl Library Procedures Tcl_StackChannel(3)______________________________________________________________________________NAME
Tcl_StackChannel, Tcl_UnstackChannel, Tcl_GetStackedChannel, Tcl_Get‐
TopChannel - stack an I/O channel on top of another, and undo it
Tcl_StackChannel(interp, typePtr, clientData, mask, channel)
Tcl_Interp *interp (in) Interpreter for error
Tcl_ChannelType *typePtr (in) The new channel I/O proce‐
dures to use for channel.
ClientData clientData (in) Arbitrary one-word value to
pass to channel I/O proce‐
int mask (in) Conditions under which chan‐
nel will be used: OR-ed com‐
bination of TCL_READABLE,
TCL_WRITABLE and TCL_EXCEP‐
TION. 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_Create‐
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 encod‐
ing 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 origi‐
nal 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
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 ALSONotifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).
Tcl 8.3 Tcl_StackChannel(3)