'\"
'\" 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 1.7 2004/11/20 00:17:32 dgp Exp $
'\" RCS: @(#) $Id: fileevent.n,v 1.8 2005/04/06 20:55:22 dkf Exp $
'\" 
.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

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