### NAME

Tcl_StackChannel, Tcl_UnstackChannel, Tcl_GetStackedChannel, Tcl_GetTopChannel - stack an I/O channel on top of another, and undo it

### SYNOPSIS

#include <tcl8.4/tcl.h>
Tcl_Channel
Tcl_StackChannel(interp, typePtr, clientData, mask, channel)
int
Tcl_UnstackChannel(interp, channel)
Tcl_Channel
Tcl_GetStackedChannel(channel)
Tcl_Channel
Tcl_GetTopChannel(channel)

### ARGUMENTS

Tcl_Interp *interp (in)
  Interpreter for error reporting.

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.

### DESCRIPTION

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.

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 ALSO

Notifier, Tcl_CreateChannel, Tcl_OpenFileChannel, vwait(n).

### KEYWORDS

channel, compression

    Copyright © 1999-2000 Ajuba Solutions.
    Copyright © 1995-1997 Roger E. Critchlow Jr.