'\"
'\" Copyright (c) 2001 by ActiveState Corporation
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
.TH "Standard Channels" 3tcl 7.5 Tcl "Tcl Library Procedures"
.\" 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.
.\"
.\"	# 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 ""
..
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_StandardChannels \- How the Tcl library deals with the standard channels
.BE

.SH DESCRIPTION
.PP
This page explains the initialization and use of standard channels in
the Tcl library.
.PP
The term \fIstandard channels\fR comes out of the Unix world and
refers to the three channels automatically opened by the OS for
each new application. They are \fBstdin\fR, \fBstdout\fR and
\fBstderr\fR. The first is the standard input an application can read
from, the other two refer to writable channels, one for regular
output and the other for error messages.
.PP
Tcl generalizes this concept in a cross-platform way and
exposes standard channels to the script level.
.SS "APPLICATION PROGRAMMING INTERFACES"
.PP
The public API procedures dealing directly with standard channels are
\fBTcl_GetStdChannel\fR and \fBTcl_SetStdChannel\fR. Additional public
APIs to consider are \fBTcl_RegisterChannel\fR,
\fBTcl_CreateChannel\fR and \fBTcl_GetChannel\fR.
.SH "INITIALIZATION OF TCL STANDARD CHANNELS"
.PP
Standard channels are initialized by the Tcl library in three cases:
when explicitly requested, when implicitly required before returning
channel information, or when implicitly required during registration
of a new channel.
.PP
These cases differ in how they handle unavailable platform- specific
standard channels.  (A channel is not
.QW available
if it could not be
successfully opened; for example, in a Tcl application run as a
Windows NT service.)
.TP
1)
A single standard channel is initialized when it is explicitly
specified in a call to \fBTcl_SetStdChannel\fR.  The states of the
other standard channels are unaffected.
.RS
.PP
Missing platform-specific standard channels do not matter here. This
approach is not available at the script level.
.RE
.TP
2)
All uninitialized standard channels are initialized to
platform-specific default values:
.RS
.TP
(a)
when open channels are listed with \fBTcl_GetChannelNames\fR (or the
\fBfile channels\fR script command), or
.TP
(b)
when information about any standard channel is requested with a call
to \fBTcl_GetStdChannel\fR, or with a call to \fBTcl_GetChannel\fR
which specifies one of the standard names (\fBstdin\fR, \fBstdout\fR
and \fBstderr\fR).
.PP
In case of missing platform-specific standard channels, the Tcl
standard channels are considered as initialized and then immediately
closed. This means that the first three Tcl channels then opened by
the application are designated as the Tcl standard channels.
.RE
.TP
3)
All uninitialized standard channels are initialized to
platform-specific default values when a user-requested channel is
registered with \fBTcl_RegisterChannel\fR.
.PP
In case of unavailable platform-specific standard channels the channel
whose creation caused the initialization of the Tcl standard channels
is made a normal channel.  The next three Tcl channels opened by the
application are designated as the Tcl standard channels.  In other
words, of the first four Tcl channels opened by the application the
second to fourth are designated as the Tcl standard channels.
.SH "RE-INITIALIZATION OF TCL STANDARD CHANNELS"
.PP
Once a Tcl standard channel is initialized through one of the methods
above, closing this Tcl standard channel will cause the next call to
\fBTcl_CreateChannel\fR to make the new channel the new standard
channel, too. If more than one Tcl standard channel was closed
\fBTcl_CreateChannel\fR will fill the empty slots in the order
\fBstdin\fR, \fBstdout\fR and \fBstderr\fR.
.PP
\fBTcl_CreateChannel\fR will not try to reinitialize an empty slot if
that slot was not initialized before. It is this behavior which
enables an application to employ method 1 of initialization, i.e. to
create and designate their own Tcl standard channels.
.SH "SHELL-SPECIFIC DETAILS"
.SS tclsh
.PP
The Tcl shell (or rather the function \fBTcl_Main\fR, which forms the
core of the shell's implementation) uses method 2 to initialize
the standard channels.
.SS wish
.PP
The windowing shell (or rather the function \fBTk_MainEx\fR, which
forms the core of the shell's implementation) uses method 1 to
initialize the standard channels (See \fBTk_InitConsoleChannels\fR)
on non-Unix platforms.  On Unix platforms, \fBTk_MainEx\fR implicitly
uses method 2 to initialize the standard channels.
.SH "SEE ALSO"
Tcl_CreateChannel(3tcl), Tcl_RegisterChannel(3tcl), Tcl_GetChannel(3tcl), Tcl_GetStdChannel(3tcl), Tcl_SetStdChannel(3tcl), Tk_InitConsoleChannels(3tk), tclsh(1), wish(1), Tcl_Main(3tcl), Tk_MainEx(3tk)
.SH KEYWORDS
standard channels