man/mann/fileevent.n - toolchains/skids - Git at Google

 '\"
 '\" Copyright (c) 1994 The Regents of the University of California.
 '\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
 '\"
 '\" See the file "license.terms" for information on usage and redistribution
 '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 '\"
 '\" RCS: @(#) $Id: fileevent.n,v 1.5 2001/09/27 05:50:56 andreas_kupries Exp $
 '\"
 '\" The 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
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 '\"	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
 '\"	Start of list of standard options for a Tk widget.  The
 '\"	options follow on successive lines, in four 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.
 '\"
 '\" RCS: @(#) $Id: man.macros,v 1.4 2000/08/25 06:18:32 ericm 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
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 .br
 .mk ^y
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 .if n .fi
 ..
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 .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
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 .el .nr ^v 1u
 ..
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 .de VE
 .ie n 'mc
 .el \{\
 .ev 2
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 .mk ^t
 \h'|\\n(^lu+3n'\L'|\\n(^Yu-1v\(bv'\v'\\n(^tu+1v-\\n(^Yu'\h'-|\\n(^lu+3n'
 .sp -1
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 .\}
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 '\"	# Special macro to handle page bottom:  finish off current
 '\"	# box/sidebar if in box/sidebar mode, then invoked standard
 '\"	# page bottom macro.
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 .ev 2
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 .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
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 .\}
 .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
 .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 \\fBoptions\\fR 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
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 .ta .25i .5i .75i 1i
 ..
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 .fi
 .RE
 ..
 .de UL
 \\$1\l'|0\(ul'\\$2
 ..
 .TH fileevent n 7.5 Tcl "Tcl Built-In Commands"
 .BS
 '\" Note:  do not modify the .SH NAME line immediately below!
 .SH NAME
 fileevent \- Execute a script when a channel becomes readable or writable
 .SH SYNOPSIS
 \fBfileevent \fIchannelId \fBreadable \fR?\fIscript\fR?
 .sp
 \fBfileevent \fIchannelId \fBwritable \fR?\fIscript\fR?
 .BE

 .SH DESCRIPTION
 .PP
 This command is used to create \fIfile event handlers\fR.  A file event
 handler is a binding between a channel and a script, such that the script
 is evaluated whenever the channel becomes readable or writable.  File event
 handlers are most commonly used to allow data to be received from another
 process on an event-driven basis, so that the receiver can continue to
 interact with the user while waiting for the data to arrive.  If an
 application invokes \fBgets\fR or \fBread\fR on a blocking channel when
 there is no input data available, the process will block; until the input
 data arrives, it will not be able to service other events, so it will
 appear to the user to ``freeze up''.  With \fBfileevent\fR, the process can
 tell when data is present and only invoke \fBgets\fR or \fBread\fR when
 they won't block.
 .PP
 .VS
 The \fIchannelId\fR argument to \fBfileevent\fR refers to an open
 channel such as a Tcl standard channel (\fBstdin\fR, \fBstdout\fR,
 or \fBstderr\fR), the return value from an invocation of \fBopen\fR
 or \fBsocket\fR, or the result of a channel creation command provided
 by a Tcl extension.
 .VE
 .PP
 If the \fIscript\fR argument is specified, then \fBfileevent\fR
 creates a new event handler:  \fIscript\fR will be evaluated
 whenever the channel becomes readable or writable (depending on the
 second argument to \fBfileevent\fR).
 In this case \fBfileevent\fR returns an empty string.
 The \fBreadable\fR and \fBwritable\fR event handlers for a file
 are independent, and may be created and deleted separately.
 However, there may be at most one \fBreadable\fR and one \fBwritable\fR
 handler for a file at a given time in a given interpreter.
 If \fBfileevent\fR is called when the specified handler already
 exists in the invoking interpreter, the new script replaces the old one.
 .PP
 If the \fIscript\fR argument is not specified, \fBfileevent\fR
 returns the current script for \fIchannelId\fR, or an empty string
 if there is none.
 If the \fIscript\fR argument is specified as an empty string
 then the event handler is deleted, so that no script will be invoked.
 A file event handler is also deleted automatically whenever
 its channel is closed or its interpreter is deleted.
 .PP
 A channel is considered to be readable if there is unread data
 available on the underlying device.
 A channel is also considered to be readable if there is unread
 data in an input buffer, except in the special case where the
 most recent attempt to read from the channel was a \fBgets\fR
 call that could not find a complete line in the input buffer.
 This feature allows a file to be read a line at a time in nonblocking mode
 using events.
 A channel is also considered to be readable if an end of file or
 error condition is present on the underlying file or device.
 It is important for \fIscript\fR to check for these conditions
 and handle them appropriately;  for example, if there is no special
 check for end of file, an infinite loop may occur where \fIscript\fR
 reads no data, returns, and is immediately invoked again.
 .PP
 A channel is considered to be writable if at least one byte of data
 can be written to the underlying file or device without blocking,
 or if an error condition is present on the underlying file or device.
 .PP
 Event-driven I/O works best for channels that have been
 placed into nonblocking mode with the \fBfconfigure\fR command.
 In blocking mode, a \fBputs\fR command may block if you give it
 more data than the underlying file or device can accept, and a
 \fBgets\fR or \fBread\fR command will block if you attempt to read
 more data than is ready;  no events will be processed while the
 commands block.
 In nonblocking mode \fBputs\fR, \fBread\fR, and \fBgets\fR never block.
 See the documentation for the individual commands for information
 on how they handle blocking and nonblocking channels.
 .PP
 The script for a file event is executed at global level (outside the
 context of any Tcl procedure) in the interpreter in which the
 \fBfileevent\fR command was invoked.
 If an error occurs while executing the script then the
 \fBbgerror\fR mechanism is used to report the error.
 In addition, the file event handler is deleted if it ever returns
 an error;  this is done in order to prevent infinite loops due to
 buggy handlers.

 .SH EXAMPLE
 .PP
 .CS
  proc GetData {chan} {
     if {![eof $chan]} {
         puts [gets $chan]
     }
  }

  fileevent $chan readable [list GetData $chan]

 .CE
 In this setup \fBGetData\fR will be called with the channel as an
 argument whenever $chan becomes readable.

 .SH CREDITS
 .PP
 \fBfileevent\fR is based on the \fBaddinput\fR command created
 by Mark Diekhans.

 .SH "SEE ALSO"
 bgerror(n), fconfigure(n), gets(n), puts(n), read(n), Tcl_StandardChannels(3)

 .SH KEYWORDS
 asynchronous I/O, blocking, channel, event handler, nonblocking, readable,
 script, writable.
	'\"
	'\" Copyright (c) 1994 The Regents of the University of California.
	'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
	'\"
	'\" See the file "license.terms" for information on usage and redistribution
	'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
	'\"
	'\" RCS: @(#) $Id: fileevent.n,v 1.5 2001/09/27 05:50:56 andreas_kupries Exp $
	'\"
	'\" The 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
	'\" Start of list of standard options for a Tk widget. The
	'\" options follow on successive lines, in four 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.
	'\"
	'\" RCS: @(#) $Id: man.macros,v 1.4 2000/08/25 06:18:32 ericm 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
	.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 \\fBoptions\\fR 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
	..
	.de UL
	\\$1\l'\|0\(ul'\\$2
	..
	.TH fileevent n 7.5 Tcl "Tcl Built-In Commands"
	.BS
	'\" Note: do not modify the .SH NAME line immediately below!
	.SH NAME
	fileevent \- Execute a script when a channel becomes readable or writable
	.SH SYNOPSIS
	\fBfileevent \fIchannelId \fBreadable \fR?\fIscript\fR?
	.sp
	\fBfileevent \fIchannelId \fBwritable \fR?\fIscript\fR?
	.BE

	.SH DESCRIPTION
	.PP
	This command is used to create \fIfile event handlers\fR. A file event
	handler is a binding between a channel and a script, such that the script
	is evaluated whenever the channel becomes readable or writable. File event
	handlers are most commonly used to allow data to be received from another
	process on an event-driven basis, so that the receiver can continue to
	interact with the user while waiting for the data to arrive. If an
	application invokes \fBgets\fR or \fBread\fR on a blocking channel when
	there is no input data available, the process will block; until the input
	data arrives, it will not be able to service other events, so it will
	appear to the user to ``freeze up''. With \fBfileevent\fR, the process can
	tell when data is present and only invoke \fBgets\fR or \fBread\fR when
	they won't block.
	.PP
	.VS
	The \fIchannelId\fR argument to \fBfileevent\fR refers to an open
	channel such as a Tcl standard channel (\fBstdin\fR, \fBstdout\fR,
	or \fBstderr\fR), the return value from an invocation of \fBopen\fR
	or \fBsocket\fR, or the result of a channel creation command provided
	by a Tcl extension.
	.VE
	.PP
	If the \fIscript\fR argument is specified, then \fBfileevent\fR
	creates a new event handler: \fIscript\fR will be evaluated
	whenever the channel becomes readable or writable (depending on the
	second argument to \fBfileevent\fR).
	In this case \fBfileevent\fR returns an empty string.
	The \fBreadable\fR and \fBwritable\fR event handlers for a file
	are independent, and may be created and deleted separately.
	However, there may be at most one \fBreadable\fR and one \fBwritable\fR
	handler for a file at a given time in a given interpreter.
	If \fBfileevent\fR is called when the specified handler already
	exists in the invoking interpreter, the new script replaces the old one.
	.PP
	If the \fIscript\fR argument is not specified, \fBfileevent\fR
	returns the current script for \fIchannelId\fR, or an empty string
	if there is none.
	If the \fIscript\fR argument is specified as an empty string
	then the event handler is deleted, so that no script will be invoked.
	A file event handler is also deleted automatically whenever
	its channel is closed or its interpreter is deleted.
	.PP
	A channel is considered to be readable if there is unread data
	available on the underlying device.
	A channel is also considered to be readable if there is unread
	data in an input buffer, except in the special case where the
	most recent attempt to read from the channel was a \fBgets\fR
	call that could not find a complete line in the input buffer.
	This feature allows a file to be read a line at a time in nonblocking mode
	using events.
	A channel is also considered to be readable if an end of file or
	error condition is present on the underlying file or device.
	It is important for \fIscript\fR to check for these conditions
	and handle them appropriately; for example, if there is no special
	check for end of file, an infinite loop may occur where \fIscript\fR
	reads no data, returns, and is immediately invoked again.
	.PP
	A channel is considered to be writable if at least one byte of data
	can be written to the underlying file or device without blocking,
	or if an error condition is present on the underlying file or device.
	.PP
	Event-driven I/O works best for channels that have been
	placed into nonblocking mode with the \fBfconfigure\fR command.
	In blocking mode, a \fBputs\fR command may block if you give it
	more data than the underlying file or device can accept, and a
	\fBgets\fR or \fBread\fR command will block if you attempt to read
	more data than is ready; no events will be processed while the
	commands block.
	In nonblocking mode \fBputs\fR, \fBread\fR, and \fBgets\fR never block.
	See the documentation for the individual commands for information
	on how they handle blocking and nonblocking channels.
	.PP
	The script for a file event is executed at global level (outside the
	context of any Tcl procedure) in the interpreter in which the
	\fBfileevent\fR command was invoked.
	If an error occurs while executing the script then the
	\fBbgerror\fR mechanism is used to report the error.
	In addition, the file event handler is deleted if it ever returns
	an error; this is done in order to prevent infinite loops due to
	buggy handlers.

	.SH EXAMPLE
	.PP
	.CS
	proc GetData {chan} {
	if {![eof $chan]} {
	puts [gets $chan]
	}
	}

	fileevent $chan readable [list GetData $chan]

	.CE
	In this setup \fBGetData\fR will be called with the channel as an
	argument whenever $chan becomes readable.

	.SH CREDITS
	.PP
	\fBfileevent\fR is based on the \fBaddinput\fR command created
	by Mark Diekhans.

	.SH "SEE ALSO"
	bgerror(n), fconfigure(n), gets(n), puts(n), read(n), Tcl_StandardChannels(3)

	.SH KEYWORDS
	asynchronous I/O, blocking, channel, event handler, nonblocking, readable,
	script, writable.