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'\"
'\" Copyright (c) 1999 Scriptics Corporation
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
'\" RCS: @(#) $Id: ChnlStack.3,v 1.2 2000/07/24 00:03:02 jenglish Exp $
.so man.macros
.TH Tcl_StackChannel 3 8.2 Tcl "Tcl Library Procedures"
.BS
'\" Note: do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_StackChannel, Tcl_UnstackChannel, Tcl_GetStackedChannel \- stack an I/O channel on top of another, and undo it
.SH SYNOPSIS
.nf
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Channel
\fBTcl_StackChannel\fR(\fIinterp, typePtr, clientData, mask, channel\fR)
.sp
void
\fBTcl_UnstackChannel\fR(\fIinterp, channel\fR)
.sp
Tcl_Channel
\fBTcl_GetStackedChannel\fR(\fIchannel\fR)
.sp
.SH ARGUMENTS
.AS Tcl_ChannelType
.AP Tcl_Interp *interp in
Interpreter for error reporting - can be NULL.
.AP Tcl_ChannelType *typePtr in
The new channel I/O procedures to use for \fIchannel\fP.
.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\fP.
.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
\fBTcl_StackChannel\fR
replaces an existing \fIchannel\fP with a new channel by
the same name that was registered for \fIchannel\fP
with \fBTcl_RegisterChannel\fP.
.PP
\fBTcl_StackChannel\fR
works by creating a new channel structure under the existing
\fIchannel\fP 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, newline translations, and
encoding functions all remain at the top of the channel stack.
The top-most channel gets changed to use the
I/O channel functions
defined in \fItypePtr\fP, and the channel functions are passed
the new \fIclientData\fP.
The existing \fIchannel\fP structure is modified in place,
so C applications that continue to use \fIchannel\fP 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
\fBTcl_GetStackedChannel\fP call.)
If an error occurs when stacking the channel,
NULL is returned instead.
.PP
The \fImask\fP 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\fP 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\fP
is equivalent to \fBTcl_Close\fP .
.SH "SEE ALSO"
Notifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).
.SH KEYWORDS
channel, compression
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