'\"
'\" Copyright (c) 1999-2000 Ajuba Solutions.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
.TH Tcl_StackChannel 3 8.3 Tcl "Tcl Library Procedures"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_StackChannel, Tcl_UnstackChannel, Tcl_GetStackedChannel, Tcl_GetTopChannel \- manipulate stacked I/O channels
.SH SYNOPSIS
.nf
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Channel
\fBTcl_StackChannel\fR(\fIinterp, typePtr, clientData, mask, channel\fR)
.sp
int
\fBTcl_UnstackChannel\fR(\fIinterp, channel\fR)
.sp
Tcl_Channel
\fBTcl_GetStackedChannel\fR(\fIchannel\fR)
.sp
Tcl_Channel
\fBTcl_GetTopChannel\fR(\fIchannel\fR)
.sp
.SH ARGUMENTS
.AS Tcl_ChannelType clientData
.AP Tcl_Interp *interp in
Interpreter for error reporting.
.AP "const Tcl_ChannelType" *typePtr in
The new channel I/O procedures to use for \fIchannel\fR.
.AP ClientData clientData in
Arbitrary one-word value to pass to channel I/O procedures.
.AP int mask in
Conditions under which \fIchannel\fR will be used: OR-ed combination of
\fBTCL_READABLE\fR, \fBTCL_WRITABLE\fR and \fBTCL_EXCEPTION\fR.
This can be a subset of the operations currently allowed on \fIchannel\fR.
.AP Tcl_Channel channel in
An existing Tcl channel such as returned by \fBTcl_CreateChannel\fR.
.BE

.SH DESCRIPTION
.PP
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.
.PP
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 \fBTCL_CHANNEL_VERSION_2\fR
\fBTcl_ChannelType\fR structure.  See \fBTcl_CreateChannel\fR for details.
.PP
\fBTcl_StackChannel\fR stacks a new \fIchannel\fR on an existing channel
with the same name that was registered for \fIchannel\fR by
\fBTcl_RegisterChannel\fR.
.PP
\fBTcl_StackChannel\fR 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.
.PP
The \fImask\fR 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 \fBTcl_StackChannel\fR call.
.PP
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.
.PP
\fBTcl_UnstackChannel\fR reverses the process.  The old channel is
associated with the channel name, and the processing module added by
\fBTcl_StackChannel\fR is destroyed.  If there is no old channel, then
\fBTcl_UnstackChannel\fR is equivalent to \fBTcl_Close\fR.  If an error
occurs unstacking the channel, \fBTCL_ERROR\fR is returned, otherwise
\fBTCL_OK\fR is returned.
.PP
\fBTcl_GetTopChannel\fR returns the top channel in the stack of
channels the supplied channel is part of.
.PP
\fBTcl_GetStackedChannel\fR returns the channel in the stack of
channels which is just below the supplied channel.

.SH "SEE ALSO"
Notifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n)

.SH KEYWORDS
channel, compression