'\"
'\" Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
'\" RCS: @(#) $Id: SetChanErr.3,v 1.4 2007/12/13 15:22:31 dgp Exp $
.\" The -*- nroff -*- definitions below are for supplemental macros used
.\" in Tcl/Tk manual entries.
.\"
.\" .AP type name in/out ?indent?
.\"	Start paragraph describing an argument to a library procedure.
.\"	type is type of argument (int, etc.), in/out is either "in", "out",
.\"	or "in/out" to describe whether procedure reads or modifies arg,
.\"	and indent is equivalent to second arg of .IP (shouldn't ever be
.\"	needed;  use .AS below instead)
.\"
.\" .AS ?type? ?name?
.\"	Give maximum sizes of arguments for setting tab stops.  Type and
.\"	name are examples of largest possible arguments that will be passed
.\"	to .AP later.  If args are omitted, default tab stops are used.
.\"
.\" .BS
.\"	Start box enclosure.  From here until next .BE, everything will be
.\"	enclosed in one large box.
.\"
.\" .BE
.\"	End of box enclosure.
.\"
.\" .CS
.\"	Begin code excerpt.
.\"
.\" .CE
.\"	End code excerpt.
.\"
.\" .VS ?version? ?br?
.\"	Begin vertical sidebar, for use in marking newly-changed parts
.\"	of man pages.  The first argument is ignored and used for recording
.\"	the version when the .VS was added, so that the sidebars can be
.\"	found and removed when they reach a certain age.  If another argument
.\"	is present, then a line break is forced before starting the sidebar.
.\"
.\" .VE
.\"	End of vertical sidebar.
.\"
.\" .DS
.\"	Begin an indented unfilled display.
.\"
.\" .DE
.\"	End of indented unfilled display.
.\"
.\" .SO ?manpage?
.\"	Start of list of standard options for a Tk widget. The manpage
.\"	argument defines where to look up the standard options; if
.\"	omitted, defaults to "options". The options follow on successive
.\"	lines, in three columns separated by tabs.
.\"
.\" .SE
.\"	End of list of standard options for a Tk widget.
.\"
.\" .OP cmdName dbName dbClass
.\"	Start of description of a specific option.  cmdName gives the
.\"	option's name as specified in the class command, dbName gives
.\"	the option's name in the option database, and dbClass gives
.\"	the option's class in the option database.
.\"
.\" .UL arg1 arg2
.\"	Print arg1 underlined, then print arg2 normally.
.\"
.\" .QW arg1 ?arg2?
.\"	Print arg1 in quotes, then arg2 normally (for trailing punctuation).
.\"
.\" .PQ arg1 ?arg2?
.\"	Print an open parenthesis, arg1 in quotes, then arg2 normally
.\"	(for trailing punctuation) and then a closing parenthesis.
.\"
.\" RCS: @(#) $Id: man.macros,v 1.9 2008/01/29 15:32:33 dkf Exp $
.\"
.\"	# Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
.if t .wh -1.3i ^B
.nr ^l \n(.l
.ad b
.\"	# Start an argument description
.de AP
.ie !"\\$4"" .TP \\$4
.el \{\
.   ie !"\\$2"" .TP \\n()Cu
.   el          .TP 15
.\}
.ta \\n()Au \\n()Bu
.ie !"\\$3"" \{\
\&\\$1 \\fI\\$2\\fP (\\$3)
.\".b
.\}
.el \{\
.br
.ie !"\\$2"" \{\
\&\\$1	\\fI\\$2\\fP
.\}
.el \{\
\&\\fI\\$1\\fP
.\}
.\}
..
.\"	# define tabbing values for .AP
.de AS
.nr )A 10n
.if !"\\$1"" .nr )A \\w'\\$1'u+3n
.nr )B \\n()Au+15n
.\"
.if !"\\$2"" .nr )B \\w'\\$2'u+\\n()Au+3n
.nr )C \\n()Bu+\\w'(in/out)'u+2n
..
.AS Tcl_Interp Tcl_CreateInterp in/out
.\"	# BS - start boxed text
.\"	# ^y = starting y location
.\"	# ^b = 1
.de BS
.br
.mk ^y
.nr ^b 1u
.if n .nf
.if n .ti 0
.if n \l'\\n(.lu\(ul'
.if n .fi
..
.\"	# BE - end boxed text (draw box now)
.de BE
.nf
.ti 0
.mk ^t
.ie n \l'\\n(^lu\(ul'
.el \{\
.\"	Draw four-sided box normally, but don't draw top of
.\"	box if the box started on an earlier page.
.ie !\\n(^b-1 \{\
\h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.el \}\
\h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.\}
.fi
.br
.nr ^b 0
..
.\"	# VS - start vertical sidebar
.\"	# ^Y = starting y location
.\"	# ^v = 1 (for troff;  for nroff this doesn't matter)
.de VS
.if !"\\$2"" .br
.mk ^Y
.ie n 'mc \s12\(br\s0
.el .nr ^v 1u
..
.\"	# VE - end of vertical sidebar
.de VE
.ie n 'mc
.el \{\
.ev 2
.nf
.ti 0
.mk ^t
\h'|\\n(^lu+3n'\L'|\\n(^Yu-1v\(bv'\v'\\n(^tu+1v-\\n(^Yu'\h'-|\\n(^lu+3n'
.sp -1
.fi
.ev
.\}
.nr ^v 0
..
.\"	# Special macro to handle page bottom:  finish off current
.\"	# box/sidebar if in box/sidebar mode, then invoked standard
.\"	# page bottom macro.
.de ^B
.ev 2
'ti 0
'nf
.mk ^t
.if \\n(^b \{\
.\"	Draw three-sided box if this is the box's first page,
.\"	draw two sides but no top otherwise.
.ie !\\n(^b-1 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
.el \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
.\}
.if \\n(^v \{\
.nr ^x \\n(^tu+1v-\\n(^Yu
\kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c
.\}
.bp
'fi
.ev
.if \\n(^b \{\
.mk ^y
.nr ^b 2
.\}
.if \\n(^v \{\
.mk ^Y
.\}
..
.\"	# DS - begin display
.de DS
.RS
.nf
.sp
..
.\"	# DE - end display
.de DE
.fi
.RE
.sp
..
.\"	# SO - start of list of standard options
.de SO
'ie '\\$1'' .ds So \\fBoptions\\fR
'el .ds So \\fB\\$1\\fR
.SH "STANDARD OPTIONS"
.LP
.nf
.ta 5.5c 11c
.ft B
..
.\"	# SE - end of list of standard options
.de SE
.fi
.ft R
.LP
See the \\*(So manual entry for details on the standard options.
..
.\"	# OP - start of full description for a single option
.de OP
.LP
.nf
.ta 4c
Command-Line Name:	\\fB\\$1\\fR
Database Name:	\\fB\\$2\\fR
Database Class:	\\fB\\$3\\fR
.fi
.IP
..
.\"	# CS - begin code excerpt
.de CS
.RS
.nf
.ta .25i .5i .75i 1i
..
.\"	# CE - end code excerpt
.de CE
.fi
.RE
..
.\"	# UL - underline word
.de UL
\\$1\l'|0\(ul'\\$2
..
.\"	# QW - apply quotation marks to word
.de QW
.ie '\\*(lq'"' ``\\$1''\\$2
.\"" fix emacs highlighting
.el \\*(lq\\$1\\*(rq\\$2
..
.\"	# PQ - apply parens and quotation marks to word
.de PQ
.ie '\\*(lq'"' (``\\$1''\\$2)\\$3
.\"" fix emacs highlighting
.el (\\*(lq\\$1\\*(rq\\$2)\\$3
..
.\"	# QR - quoted range
.de QR
.ie '\\*(lq'"' ``\\$1''\\-``\\$2''\\$3
.\"" fix emacs highlighting
.el \\*(lq\\$1\\*(rq\\-\\*(lq\\$2\\*(rq\\$3
..
.\"	# MT - "empty" string
.de MT
.QW ""
..
.TH Tcl_SetChannelError 3 8.5 Tcl "Tcl Library Procedures"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_SetChannelError, Tcl_SetChannelErrorInterp, Tcl_GetChannelError, Tcl_GetChannelErrorInterp \- functions to create/intercept Tcl errors by channel drivers.
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
void
\fBTcl_SetChannelError\fR(\fIchan, msg\fR)
.sp
void
\fBTcl_SetChannelErrorInterp\fR(\fIinterp, msg\fR)
.sp
void
\fBTcl_GetChannelError\fR(\fIchan, msgPtr\fR)
.sp
void
\fBTcl_GetChannelErrorInterp\fR(\fIinterp, msgPtr\fR)
.sp
.SH ARGUMENTS
.AS Tcl_Channel chan
.AP Tcl_Channel chan in
Refers to the Tcl channel whose bypass area is accessed.
.AP Tcl_Interp* interp in
Refers to the Tcl interpreter whose bypass area is accessed.
.AP Tcl_Obj* msg in
Error message put into a bypass area.  A list of return options and
values, followed by a string message.  Both message and the
option/value information are optional.
.AP Tcl_Obj** msgPtr out
Reference to a place where the message stored in the accessed bypass
area can be stored in.
.BE
.SH DESCRIPTION
.PP
The current definition of a Tcl channel driver does not permit the
direct return of arbitrary error messages, except for the setting and
retrieval of channel options. All other functions are restricted to
POSIX error codes.
.PP
The functions described here overcome this limitation. Channel drivers
are allowed to use \fBTcl_SetChannelError\fR and
\fBTcl_SetChannelErrorInterp\fR to place arbitrary error messages in
\fBbypass areas\fI defined for channels and interpreters. And the
generic I/O layer uses \fBTcl_GetChannelError\fR and
\fBTcl_GetChannelErrorInterp\fR to look for messages in the bypass
areas and arrange for their return as errors. The posix error codes
set by a driver are used now if and only if no messages are present.
.PP
\fBTcl_SetChannelError\fR stores error information in the bypass area
of the specified channel. The number of references to the \fBmsg\fR
object goes up by one. Previously stored information will be
discarded, by releasing the reference held by the channel. The channel
reference must not be NULL.
.PP
\fBTcl_SetChannelErrorInterp\fR stores error information in the bypass
area of the specified interpreter. The number of references to the
\fBmsg\fR object goes up by one. Previously stored information will be
discarded, by releasing the reference held by the interpreter. The
interpreter reference must not be NULL.
.PP
\fBTcl_GetChannelError\fR places either the error message held in the
bypass area of the specified channel into \fImsgPtr\fR, or NULL; and
resets the bypass. I.e. after an invokation all following invokations
will return NULL, until an intervening invokation of
\fBTcl_SetChannelError\fR with a non-NULL message. The \fImsgPtr\fR
must not be NULL. The reference count of the message is not touched.
The reference previously held by the channel is now held by the caller
of the function and it is its responsibility to release that reference
when it is done with the object.
.PP
\fBTcl_GetChannelErrorInterp\fR places either the error message held
in the bypass area of the specified interpreter into \fImsgPtr\fR, or
NULL; and resets the bypass. I.e. after an invokation all following
invokations will return NULL, until an intervening invokation of
\fBTcl_SetChannelErrorInterp\fR with a non-NULL message. The
\fImsgPtr\fR must not be NULL. The reference count of the message is
not touched.  The reference previously held by the interpreter is now
held by the caller of the function and it is its responsibility to
release that reference when it is done with the object.
.PP
Which functions of a channel driver are allowed to use which bypass
function is listed below, as is which functions of the public channel
API may leave a messages in the bypass areas.
.PP
.IP \fBTcl_DriverCloseProc\fR
May use \fBTcl_SetChannelErrorInterp\fR, and only this function.
.IP \fBTcl_DriverInputProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverOutputProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverSeekProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverWideSeekProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverSetOptionProc\fR
Has already the ability to pass arbitrary error messages. Must
\fBnot\fR use any of the new functions.
.IP \fBTcl_DriverGetOptionProc\fR
Has already the ability to pass arbitrary error messages. Must
\fBnot\fR use any of the new functions.
.IP \fBTcl_DriverWatchProc\fR
Must \fBnot\fR use any of the new functions. Is internally called and
has no ability to return any type of error whatsoever.
.IP \fBTcl_DriverBlockModeProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverGetHandleProc\fR
Must \fBnot\fR use any of the new functions. It is only a low-level
function, and not used by Tcl commands.
.IP \fBTcl_DriverHandlerProc\fR
Must \fBnot\fR use any of the new functions. Is internally called and
has no ability to return any type of error whatsoever.
.PP
Given the information above the following public functions of the Tcl
C API are affected by these changes. I.e. when these functions are
called the channel may now contain a stored arbitrary error message
requiring processing by the caller.
.PP
.IP \fBTcl_StackChannel\fR
.IP \fBTcl_Seek\fR
.IP \fBTcl_Tell\fR
.IP \fBTcl_ReadRaw\fR
.IP \fBTcl_Read\fR
.IP \fBTcl_ReadChars\fR
.IP \fBTcl_Gets\fR
.IP \fBTcl_GetsObj\fR
.IP \fBTcl_Flush\fR
.IP \fBTcl_WriteRaw\fR
.IP \fBTcl_WriteObj\fR
.IP \fBTcl_Write\fR
.IP \fBTcl_WriteChars\fR
.PP
All other API functions are unchanged. Especially the functions below
leave all their error information in the interpreter result.
.PP
.IP \fBTcl_Close\fR
.IP \fBTcl_UnregisterChannel\fR
.IP \fBTcl_UnstackChannel\fR

.SH "SEE ALSO"
Tcl_Close(3), Tcl_OpenFileChannel(3), Tcl_SetErrno(3)

.SH KEYWORDS
channel driver, error messages, channel type
