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'\" Copyright (c) 1994 The Regents of the University of California.

'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.

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'\" RCS: @(#) $Id: fileevent.n,v 1.1.1.1 2002-01-16 10:25:24 markom 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

.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.