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+'\"
+'\" 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.
+'\" 
+'\" SCCS: @(#) fileevent.n 1.6 96/02/23 13:46:29
+'\" 
+.so man.macros
+.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
+The \fIchannelId\fR argument to \fBfileevent\fR refers to an open channel,
+such as the return value from a previous \fBopen\fR or \fBsocket\fR
+command.
+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 CREDITS
+.PP
+\fBfileevent\fR is based on the \fBaddinput\fR command created
+by Mark Diekhans.
+
+.SH "SEE ALSO"
+bgerror, fconfigure, gets, puts, read
+
+.SH KEYWORDS
+asynchronous I/O, blocking, channel, event handler, nonblocking, readable,
+script, writable.