Tcl_StackChannel(3) Tcl Library Procedures Tcl_StackChannel(3)______________________________________________________________________________NAME
Tcl_StackChannel, Tcl_UnstackChannel - 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
reporting - can be NULL.
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.
Tcl_StackChannel replaces an existing channel with a new channel by the
same name that was registered for channel with Tcl_RegisterChannel.
Tcl_StackChannel works by creating a new channel structure under the
existing channel and pushing the existing channel functions down to the
newly created channel. The hidden channel does no buffering, newline
translations, or character set encoding. Instead, the buffering, new‐
line translations, and encoding functions all remain at the top of the
channel stack. The top-most channel gets changed to use the I/O chan‐
nel functions defined in typePtr, and the channel functions are passed
the new clientData. The existing channel structure is modified in
place, so C applications that continue to use channel will also see the
effects of the new processing module. A pointer to a new channel
structure is returned, although this new data structure is the one that
has been pushed down below the top of the channel module stack. (This
pointer can also be obtained with the Tcl_GetStackedChannel call.) 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 .
SEE ALSONotifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).
Tcl 8.2 Tcl_StackChannel(3)