Tcl_StackChannel(), Tcl_UnstackChannel(), Tcl_GetStackedChannel()

stack an I/O channel on top of another, and undo it 

Tcl Library Procedures


#include <tcl.h>

Tcl_Channel Tcl_StackChannel(interp, typePtr, clientData, mask, channel)

int Tcl_UnstackChannel(interp, channel)

Tcl_Channel Tcl_GetStackedChannel(channel)



Tcl_Interp *interp (in) 

Interpreter for error reporting - can be NULL.

Tcl_ChannelType *typePtr (in) 

The new channel I/O procedures to use for channel.

ClientData clientData (in) 

Arbitrary one-word value to pass to channel I/O procedures.

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 operations currently allowed on channel.

Tcl_Channel channel (in) 

An existing Tcl channel such as returned by Tcl_CreateChannel().


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 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.


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition



Notifier(), Tcl_CreateChannel(), Tcl_OpenFileChannel()

PTC MKS Toolkit 10.4 Documentation Build 39.