110 lines
4.9 KiB
Plaintext
110 lines
4.9 KiB
Plaintext
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'\"
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'\" Copyright (c) 1994 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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'\" SCCS: @(#) fileevent.n 1.6 96/02/23 13:46:29
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'\"
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.so man.macros
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.TH fileevent n 7.5 Tcl "Tcl Built-In Commands"
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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fileevent \- Execute a script when a channel becomes readable or writable
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.SH SYNOPSIS
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\fBfileevent \fIchannelId \fBreadable \fR?\fIscript\fR?
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.sp
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\fBfileevent \fIchannelId \fBwritable \fR?\fIscript\fR?
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.BE
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.SH DESCRIPTION
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.PP
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This command is used to create \fIfile event handlers\fR. A file event
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handler is a binding between a channel and a script, such that the script
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is evaluated whenever the channel becomes readable or writable. File event
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handlers are most commonly used to allow data to be received from another
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process on an event-driven basis, so that the receiver can continue to
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interact with the user while waiting for the data to arrive. If an
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application invokes \fBgets\fR or \fBread\fR on a blocking channel when
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there is no input data available, the process will block; until the input
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data arrives, it will not be able to service other events, so it will
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appear to the user to ``freeze up''. With \fBfileevent\fR, the process can
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tell when data is present and only invoke \fBgets\fR or \fBread\fR when
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they won't block.
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.PP
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The \fIchannelId\fR argument to \fBfileevent\fR refers to an open channel,
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such as the return value from a previous \fBopen\fR or \fBsocket\fR
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command.
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If the \fIscript\fR argument is specified, then \fBfileevent\fR
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creates a new event handler: \fIscript\fR will be evaluated
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whenever the channel becomes readable or writable (depending on the
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second argument to \fBfileevent\fR).
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In this case \fBfileevent\fR returns an empty string.
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The \fBreadable\fR and \fBwritable\fR event handlers for a file
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are independent, and may be created and deleted separately.
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However, there may be at most one \fBreadable\fR and one \fBwritable\fR
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handler for a file at a given time in a given interpreter.
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If \fBfileevent\fR is called when the specified handler already
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exists in the invoking interpreter, the new script replaces the old one.
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.PP
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If the \fIscript\fR argument is not specified, \fBfileevent\fR
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returns the current script for \fIchannelId\fR, or an empty string
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if there is none.
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If the \fIscript\fR argument is specified as an empty string
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then the event handler is deleted, so that no script will be invoked.
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A file event handler is also deleted automatically whenever
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its channel is closed or its interpreter is deleted.
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.PP
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A channel is considered to be readable if there is unread data
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available on the underlying device.
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A channel is also considered to be readable if there is unread
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data in an input buffer, except in the special case where the
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most recent attempt to read from the channel was a \fBgets\fR
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call that could not find a complete line in the input buffer.
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This feature allows a file to be read a line at a time in nonblocking mode
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using events.
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A channel is also considered to be readable if an end of file or
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error condition is present on the underlying file or device.
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It is important for \fIscript\fR to check for these conditions
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and handle them appropriately; for example, if there is no special
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check for end of file, an infinite loop may occur where \fIscript\fR
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reads no data, returns, and is immediately invoked again.
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.PP
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A channel is considered to be writable if at least one byte of data
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can be written to the underlying file or device without blocking,
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or if an error condition is present on the underlying file or device.
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.PP
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Event-driven I/O works best for channels that have been
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placed into nonblocking mode with the \fBfconfigure\fR command.
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In blocking mode, a \fBputs\fR command may block if you give it
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more data than the underlying file or device can accept, and a
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\fBgets\fR or \fBread\fR command will block if you attempt to read
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more data than is ready; no events will be processed while the
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commands block.
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In nonblocking mode \fBputs\fR, \fBread\fR, and \fBgets\fR never block.
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See the documentation for the individual commands for information
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on how they handle blocking and nonblocking channels.
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.PP
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The script for a file event is executed at global level (outside the
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context of any Tcl procedure) in the interpreter in which the
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\fBfileevent\fR command was invoked.
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If an error occurs while executing the script then the
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\fBbgerror\fR mechanism is used to report the error.
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In addition, the file event handler is deleted if it ever returns
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an error; this is done in order to prevent infinite loops due to
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buggy handlers.
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.SH CREDITS
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.PP
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\fBfileevent\fR is based on the \fBaddinput\fR command created
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by Mark Diekhans.
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.SH "SEE ALSO"
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bgerror, fconfigure, gets, puts, read
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.SH KEYWORDS
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asynchronous I/O, blocking, channel, event handler, nonblocking, readable,
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script, writable.
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