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Event(3pm) User Contributed Perl Documentation Event(3pm)

NAME
Event - Event loop processing

SYNOPSIS
use Event qw(loop unloop);

# initialize application
Event->flavor(attribute => value, ...);

my $ret = loop();

# and some callback will call
unloop('ok');

DESCRIPTION
ALERT: Marc Lehmann may have taken over the future of event loops in Perl. Check out his
libev library and EV Perl module. 25 Aug 2009

The Event module provide a central facility to watch for various types of events and invoke a
callback when these events occur. The idea is to delay the handling of events so that they
may be dispatched in priority order when it is safe for callbacks to execute.

Events (in the ordinary sense of the word) are detected by watchers, which reify them as
events (in the special Event module sense). For clarity, the former type of events may be
called "source events", and the latter "target events". Source events, such as signals
arriving, happen whether or not they are being watched. If a source event occurs which a
watcher is actively watching then the watcher generates a corresponding target event. Target
events are only created by watchers. If several watchers are interested in the same source
event then each will generate their own target event. Hence, any particular source event may
result in zero, one, two, or any number of target events: the same as the number of watchers
which were actively watching for it.

Target events are queued to be processed in priority order (priority being determined by the
creating watcher) and in FIFO order among events of the same priority. Queued ("pending")
events can, in some cases, be cancelled before being processed. A queued event is processed
by being passed to the callback function (or method on a particular object or class) which
was specified to the watcher.

A watcher, once created, operates autonomously without the Event user having to retain any
reference to it. However, keeping a reference makes it possible to modify most of the
watcher's characteristics. A watcher can be switched between active and inactive states.
When inactive, it does not generate target events.

Some types of source event are not reified as target events immediately. Signals received,
for example, are counted initially. The counted signals are reified at certain execution
points. Hence, signal events may be processed out of order, and if handled carelessly, on
the wrong side of a state change in event handling. A useful way to view this is that
occurrence of the source event is not actually the arrival of the signal but is triggered by
the counting of the signal.

Reification can be forced when necessary. The schedule on which some other events are
created is non-obvious. This is especially the case with watchers that watch for a condition
rather than an event. In some cases, target events are generated on a schedule that depends
on the operation of the event loop.

PERL API
Events (the occurrence of such) are noticed and queued by 'event watchers'. The creation and
configuration of event watchers is the primary topic of the rest of this document.

The following functions control or interrogate the event loop as a whole:

$result = loop([$timeout])
Will enter a loop that calls one_event() until unloop() is called. The argument passed
to unloop() is the return value of loop(). Loops can be nested.

unloop($result)
Make the inner-most loop() return with $result.

unloop_all($result)
Cause all pending loop()s to return immediately. This is not implemented with "die". It
is works as if unloop($result) were called for all nested loops.

sweep([$max_prio])
Queue all pending events and dispatch any with priority strictly less than $max_prio (the
highest priority is 0). The default is to process all events except idle events. (While
idle events are ignored by sweep, idle watchers are not ignored. If you want to avoid
triggering an idle watcher then set "max" to "undef" or stop() it.)

one_event([$timeout])
If any events are outstanding then invoke the corresponding callback of the highest
priority event. If there are no events available, block forever or until $timeout. Use
of this API is not recommended because it is not efficient and does not trap exceptions.
However, you might wish to understand how it works:

1. Queue asyncronous events (signals, etc). That is, previously recorded events are
reified.

2. If there are any events with priority 5 or less (see StarvePrio) then service the
next one and return.

3. Calculate the maximum wait time (minimum time till the next timer expiration) and
pass control to the poll/select system call. Upon return, queue all pending events.

4. Queue asyncronous events again.

5. If there are any events then service the next one and return.

6. Service the next idle watcher.

StarvePrio is the priority level for which events are dispatched during step 2. It
cannot be changed without a recompile. In the rare case that an event is always pending
at step 2 then I/O watchers will starve. However, this is highly unlikely since async
watchers should never queue events so rapidly.

all_watchers()
Returns a list of all watchers (including stopped watchers).

all_running()
Returns a list of all watchers with actively running callbacks. Watchers are returned in
order of most recent to least recent.

all_idle()
Returns a list of all the idle watchers. If the event queue is very busy, all the idle
watchers will sit on the idle queue waiting to run. However, be aware that if an idle
watcher has the "max" attribute set then it will queue a normal event when its "max" wait
time is exceeded.

queue_pending()
Examines asynchronous source events (timers & signals) and reifies them as target events.
queue_pending() is only called implicitly by sweep() and one_event(). Otherwise,
queue_pending() is not called implicitly.

NOTE: Signal watchers generate target events according to which watchers are active at
the time that queue_pending() is called rather than according to the time the signal is
received. This is best explained by example. See the file "demo/queue_pending.t".

Event Watcher Constructors
All watchers are constructed in one of the following ways:

$w = Event->flavor( [attr1 => $value,]... );

$w = Event::flavor($Class, [attr1 => $value,]...);

$w = Event::flavor->new([attr1 => $value,]...);

Where flavor is substituted with the kind of watcher. Built-in types include idle, io,
signal, timer, and var.

New watchers (hopefully) have reasonable defaults and can also be customized by passing extra
attributes to the constructor. When created, watcher objects are "started" and are waiting
for events (see "$event->start" below).

NetServer::Portal can display watchers in real-time, formatted similarly to the popular "top"
program. You may find this a useful aide for debugging.

Shared Watcher Attributes
Watchers are configured with attributes (also known as properties). For example:

$watcher->cb(\&some_code); # set callback

warn $event->w->desc.": ".$event->hits." events happened; Wow!";

All watchers support the following attributes: cb, cbtime, debug, desc, prio, max_cb_tm,
reentrant, and repeat. Watcher constructors accept the preceding and additionally: async and
nice. Moreover, watchers also offer extra attributes according to their specialty.

Shared Watcher Methods
The following methods are available for all watchers:

$watcher->start
Activate the watcher. Watchers refuse to start() without sufficient configuration
information to generate events. Constructors always invoke start() unless the
"parked=>1" option is requested. You will need to set the parked option if you
preallocate unconfigured watchers.

Note: If there are any unreified asynchronous events that are of interest to the watcher,
it will see these events even though they happened before it was started. This affects
signal watchers, but there will only be existing unreified signal events if Event was
already handling the signal, which it would only do if there were another active watcher
for the same signal. If this situation might occur, and it would be a problem for the
new watcher to see older events, call queue_pending() immediately before starting the new
watcher in order to reify any outstanding events. This explanation may be more clear if
read along with "demo/queue_pending.t".

$watcher->again
This is the same as the "start" except if a watcher has special repeat behavior. For
example, repeating timers recalculate their alarm time using the "interval" parameter.

$watcher->now
Cause the watcher to generate an event, even if it is stopped. The callback may or may
not run immediately depending upon the event's priority. If you must unconditionally
invoke the callback, consider something like

$w->cb->($w);

$watcher->stop
Don't look for events any more. Running events are allowed to complete but pending
events are cancelled. Note that a stopped watcher can be reactivated by calling the
"start" or "again" methods.

Watchers are stopped implicitly if their new configuration deprives them of the ability
to generate events. For instance:

my $io_watcher = Event->io(timeout => 1); # started
$io_watcher->timeout(undef); # stopped implicitly
$io_watcher->timeout(1); # still stopped
$io_watcher->start; # restarted

$watcher->cancel
Stop and destroy $watcher. Running events are allowed to complete but pending events are
cancelled. Cancelled watchers are no longer valid except for read-only operations. For
example, prio() can return the watcher's priority, but start() will fail.

$watcher->is_cancelled
Reports whether the $watcher has been cancelled.

$watcher->is_active
Reports whether the $watcher has been started. The return value is not affected by
suspend.

$watcher->is_running
Zero if the callback is not running. Otherwise, the number of levels that the callback
has been entered. This can be greater than one if a "reentrant" callback invokes "loop"
(or "sweep", with lesser probability).

$watcher->is_suspended
Reports whether the $watcher is suspended. Suspension is a debugging feature; see the
discussion of the "suspend" attribute below.

$watcher->pending
In scalar context, returns a boolean indicating whether this watcher has any events
pending in the event queue. In array context, returns a list of all the watcher's
pending events.

Note that this does not check for unreified asynchronous events. Call queue_pending()
first if you want to see signals received since the last operation of the event loop.

Watcher Types
idle
Extra attributes: min => $seconds, max => $seconds

Watches for the Event system to be idle, i.e., to have no events pending. If the system
is never idle, an event will be generated at least every "max" seconds. While Event is
idle, events will be generated not more often than "min" seconds.

If neither "min" nor "max" is specified, the watcher defaults to one-shot behaviour
("repeat" false), otherwise it defaults to repeating. In either case, the default can be
overridden by specifying a "repeat" attribute. "min" defaults to 0.01, and "max"
defaults to infinity.

var Extra attributes: var => \$var, poll => 'rw'

Var watchers generate events when the given variable is read from or written to, as
specified by "poll". "poll" defaults to "w".

As perl is a concise language, it is often difficult to predict when a variable will be
read. For this reason, variable watchers should poll only for writes unless you know
what you are doing.

timer
Extra attributes: at => $time, after => $sec, interval => $sec, hard => $bool

Generate events at particular times. The $time and $sec are in seconds. Fractional
seconds may be used if Time::HiRes is available. "at" and "after" are mutually
exclusive.

"at" or "after" specify the initial time that the event will trigger. Subsequent timer
events occur at intervals specified by "interval" or "after" (in that order of
preference) if either was supplied. The timer defaults to one-shot behaviour if
"interval" was not specified, or repeating behaviour if "interval" was specified; in
either case this can be overridden by providing "repeat" explicitly.

You need to know the time at the start of today if you are trying to set timers to
trigger at day relative times. You can find it with:

use Time::Local;
my $TodaySeconds = int timelocal(0,0,0,(localtime)[3,4,5]);

This calculation may seem a little heavy weight. If you want to use UTC rather than
local time then you can use this instead:

my $TodaySeconds = time - time % 86400;

Beware that, due to lags in the event loop, the "interval" timeout may already be in the
past. If the "hard" flag is set, the event will be queued for execution relative to the
last time the callback was invoked. However, if "hard" is false the new timeout will be
calculated relative to the current time. "hard" defaults to false.

io Extra attributes: fd => $fd, poll => 'rwe' [timeout => $seconds, hard => $bool,
timeout_cb => \&code]

The callback is invoked when the file descriptor, "fd", has data to be read, written, or
pending exceptions. "fd" can be a GLOB, an IO::Handle object, or a file number (file
descriptor). "poll" defaults to "r".

Note that it is your option whether to have multiple watchers per file handle or to use a
single watcher for all event conditions.

If "timeout" is set, events are also generated regularly if no actual I/O event occurs.
If "timeout_cb" is set then timeouts use this alternate callback instead of the main
callback.

signal
Extra attribute: signal => $str

Generates events based on signal arrival. The events are not actually generated
immediately when the signal arrives: signals received are counted and reified by
queue_pending() or implicitly by one_event(). Several signals of the same type may be
merged into a single event. In such cases, the number of signals represented by a single
event is stored in the "hits" attribute.

PRIORITY
Priority is used to sort the event queue. Meaningful priorities range from -1 to 6
inclusive. Lower numbers mean higher priority (-1 is the highest priority and 6 is the
lowest). If multiple events get queued, the ones with the highest priority are serviced
first. Events with equal priority are serviced in first-in-first-out order.

use Event qw(PRIO_HIGH PRIO_NORMAL); # some constants

LEVELS: -1 0 1 2 3 4 5 6
----------------------+-------------+---------------
PRIO_HIGH PRIO_NORMAL

A negative priority causes the callback to be invoked immediately upon event occurrence. Use
this with caution. While it may seem advantageous to use negative priorities, they bypass
the whole point of having an event queue.

Each watcher has a default priority, assigned by its constructor:

io PRIO_NORMAL
signal PRIO_HIGH
timer PRIO_NORMAL
var PRIO_NORMAL

Default priorities are stored in ${"Event::${type}::DefaultPriority"}. If the default
priority is not satisfactory for your purposes, the constructor options "nice", "async", or
"prio" can be used to adjust it. "nice" specifies an offset from the default priority;
"async" forces the priority to -1; and "prio" assigns a given priority of your choice. If
more than one of these options are given then "prio" overrides "async" overrides "nice".

WATCHER CONSTRUCTOR ATTRIBUTES
These options are only supported as constructor arguments.

after => $seconds
See the discussion of the timer watcher.

async => $bool
If $bool then the watcher priority is set to -1.

nice => $offset
Offset from the default priority.

parked => $yes
By default, watcher constructors automatically invoke the start() method. If you don't
want the watcher started then request "parked=>1".

WATCHER ATTRIBUTES
at => $time
The expiration time in the same units as the system clock. For a timer, "at" will
usually be in the future.

cb => \&code
cb => [$class_or_object, $method_name]
The function or method to call when an event is dispatched. The callback is invoked with
$event as its only argument.

Perhaps you are wondering what happens if something goes wrong and an untrapped "die"
occurs within your callback? $Event::DIED is just for this purpose. See the full
description of "DIED" below.

cbtime => $time
When the callback was invoked most recently.

data => $anything
The data() method associates arbitrary data with a watcher.

This method is not intended for implementers of watchers. If you are subclassing or
implementing a watcher, consider the private() method.

debug => $bool
Debugging can be activated globally or per watcher. When debugging is enabled for a
particular watcher, $Event::DebugLevel is treated as two levels higher. Levels of 1, 2,
3, or 4 give progressively more diagnostics on STDERR.

desc => $string
An identifying name. If this is not passed explicitly to the constructor, it will be
initialized with a string that attempts to identify the location in the source code where
the watcher was constructed.

fd => $filehandle
This attribute can accept either a perl-esque filehandle or a system call derived file
descriptor number.

hard => $bool
Determines how repeating timers (or timeouts) are recalculated. The timer is restarted
either before or after the callback depending on whether it is true or false,
respectively. In long-running callbacks this can make a significant difference.

interval => $seconds
How long between repeating timeouts. The "at" attribute is recalculated using "interval"
upon callback return.

max => $seconds
The maximum number of seconds to wait before triggering the callback. Similar to a
"timeout".

max_cb_tm => $seconds
The maximum number of seconds to spend in a callback. If a callback uses more time then
it is aborted. Defaults to 1 sec. This feature is normally disabled. See Event::Stats.

min => $seconds
Enforce a minimum number of seconds between triggering events.

poll => $bits
Determines which kinds of events are of interest. This attribute can be set with either
strings or bit constants. The bit constants are available via 'use Event::Watcher qw(R W
E T);'.

string constant description
------ -------- ---------------
'r' R read
'w' W write
'e' E exception
't' T timeout

Thus, both of these statements enable interest in read:

$w->poll($w->poll . 'r');
$w->poll($w->poll | R);

A given type of watcher may support all or a subset of the available events.

prio => $level
Changes the watcher's priority to the given level. Events generated by a watcher usually
inherit the priority of the watcher.

private => $anything
Use the private() method to associate arbitrary data with a watcher. This method is
intended for implementers of watchers or watcher subclasses. Each caller's package
accesses its own private attribute.

reentrant => $bool
By default, callbacks are allowed to invoke "sweep" or "loop" which in turn may invoke
the same callback again recursively. This can be useful but can also be confusing.
Moreover, if you keep reentering callbacks you will quickly run out of stack space.
Disable this feature per watcher by setting reentrant to false. This will cause the
watcher to be suspended during recursive calls to "sweep" or "loop".

repeat => $bool
The repeat flag controls whether the callback should either be one-shot or continue
waiting for new events. The default setting depends on the type of watcher. io, signal,
and var default to true.

signal => $str
The callback is invoked after the specified signal is received. The $str string should
be something like 'INT' or 'QUIT'. Also see the documentation for %SIG.

A given signal can be handled by %SIG or Event, but not both at the same time. Event
handles the signal as long as there is at least one active watcher. If all watchers for
the signal are cancelled or stopped then Event sets the signal handler to SIG_DFL.

suspend => $bool
Stop looking for events. Running events are allowed to complete, but queued events are
cancelled.

Suspend is for debugging. If you suspend all watchers in an application then you can
examine the complete state unchanged for as long as you like without worrying about timer
expirations. If you actually wish to stop a watcher then use the stop() method.

timeout => $seconds
The number of seconds before a watcher times out.

timeout_cb => \&code
timeout_cb => [$class_or_object, $method_name]
This is an optional attribute for use when it is desired that timeouts be serviced in a
separate code path than normal events. When this attribute is unset, timeouts are
serviced by "cb".

var => $ref
A reference to the variable being watched.

EVENT ATTRIBUTES
got => $bits
"got" is available in the callback of watchers with "poll". "got" is in the same format
as "poll" except that it gives what kind of event actually happened. In contrast, "poll"
is just an indication of interest.

hits => $int
Signals in quick succession can be clumped into a single event. The number of signals
clumped together is indicated by this attribute. This is always one for event types
which don't clump.

prio => $level
Be aware that this priority can differ from the watcher's priority. For instance, the
watcher's priority may have changed since the event was generated. Moreover, the C
extension API offers the freedom to queue events of arbitrary priority.

w => $watcher
This method return the event's watcher. It is read-only.

Customization and Exceptions
• $Event::DebugLevel

Enables progressively more debugging output. Meaningful levels range from 1 (least
output) to 5 (most output). Also see "debug".

• $Event::DIED

When "loop" or "sweep" is called, an exception context is established for the duration of
event processing. If an exception is detected then $Event::DIED is invoked. The default
hook uses "warn" to output the exception. After the DIED handler completes, event
processing continues as if nothing happened.

If you'd like more detailed output you can install the verbose handler:

$Event::DIED = \&Event::verbose_exception_handler;

Or you can write your own. The handler is invoked like this:

$Event::DIED->($event, $@);

If you do not want to continue looping after an error, you can do something like this:

$Event::DIED = sub {
Event::verbose_exception_handler(@_);
Event::unloop_all();
};

• Event->add_hooks(key => sub { ... }, ...);

The bulk of Event's implementation is in C for maximum performance. The "add_hooks"
method allows insertion of perl code at key points in the optimized event processing
core. While flexible, this can hurt performance *significantly*. If you want
customization *and* performance, please see the C API.

Currently support hooks are detailed as follows:

hook purpose
------------- ----------------------------------------------
prepare returns minimum time to block (timeable)
check assess state after normal return from select/poll
asynccheck check for signals, etc
callback invoked before each event callback

C API
Event also has a direct API for callbacks written exclusively in C. See Event::MakeMaker.

WHAT ABOUT THREADS?
Event loops and threads are two different solutions to the same problem: asynchronous
processing. Event loops have been around since the beginning of computing. They are well
understood and proven to be a good solution for many applications.

While event loops make use of basic operating system services, the bulk of their
implementation is usually outside the kernel. While an event loop may appear to do many
things in parallel, it does not, even on multiprocessor hardware. Actions are always
dispatched sequentially. This implies that long running callbacks must be avoided because
otherwise event processing is halted.

Event loops work well when actions are short and to the point. Long-running tasks must be
broken into short steps and scheduled for execution. Some sort of a state machine is usually
required. While a big, complex application server is usually simpler to implement in a
multithreaded fashion, a web browser can easily get by without threads. Consider a JPEG file
download and render. When some new bytes are available they are sorted to the right place on
the screen. Only a little state must be kept to keep track of how much has been rendered and
to process subsequent incoming bytes.

Threads can either substitute for an event loop or complement it. Threads are similar to
processes in that the operating system manages task switching for you. However, the
difference is that all threads share the same address space. This is good and bad. Higher
performance can be achieved but since data is shared between threads, extreme care must be
taken to access or modify global data. The operating system can switch threads at any moment
or can execute multiple threads simultaneously. I hope this sounds dangerous! It is!
Threads can introduce maddeningly complicated and hard to debug synchronization problems.

Threads are like rocket fuel. They are essential when you really need them but most
applications would be better off with a simple event loop. Even if threads are genuinely
needed, consider confining them to the parts of an application where truly scalable
performance is really worth the difficulty of a multithreaded implementation. For example,
most GUIs applications do not need threads and most scientific compute intensive problems can
be isolated from event dispatching. On the other hand, high performance transaction servers
generally do mandate a truly multithreaded approach.

Another consideration is that threads are not quite as widely available as event loops.
While a few forward-thinking operating systems have offered threads since the beginning,
their addition to many popular operating systems is much more recent and some still offer no
threads support. If portability is a requirement, one must check that threads support is
available and also carefully test a particular threads implementation to see whether it
supports the features you need. It is likely that all platforms will have a solid
implementation soon but at this point in history it is best to double check.

Many suggestions by Mark Mielke <Mark.Mielke.markm AT nt.com>

WHAT ABOUT NON-PREEMPTIVE THREADS?
The Java language is oriented to use non-preemptive threads, yet even Java uses an event-loop
for Swing (AFAIK). That is one of the reasons I don't use Java for network-centric
applications. My belief is that the benefit of multi-threading is the gain in performance on
SMP hardware. In my view, non-preemptive threads (java green-threads) are usually poor
design. I find them harder to work with, harder to debug, and slower for a rather marginal
gain in readability. I really like working with a state machine. I find it leads to more
stable and better code. It also has the benefit of abstracting away how concurrency is
achieved.

Contributed by artur AT vogon-solutions.com, 12 Jul 1999.

BUGS
No support for epoll, or better, libevent.

The scope of events is pretty strange compared to most other perl objects. I'm not sure if
this is a bug or a feature (OK, probably it was a mistake). We'll probably want to re-work
things for Perl6.

The meaning of $io->timeout(0) might change. Use "undef" to unset the timeout.

There seems to be some sort of bug in the global destruction phase:

Attempt to free unreferenced scalar during global destruction.
Use of uninitialized value during global destruction.
Explicit blessing to '' (assuming package main) during global
destruction.

THE FUTURE
Even if this module does not end up being the One and True Event Loop, the author will insure
that it is source compatible with its successor, or arrange for gradual migration. Back in
the early days, the Event programming API was changing at every release. Care was taken to
allow the old API to continue to work, and the transition was eased by printing out lots of
warnings about the new usage. So you shouldn't sit on your hands in anticipation of the One
and True Event Loop. Just start coding!

ALSO SEE
• Useful and Fun

Time::HiRes, NetServer::Portal, Time::Warp

• Message Passing

COPE, IPC::LDT, Event-tcp

• GUI

While Tk does not yet support Event, PerlQt does.

• C API

Inline

SUPPORT
If you have insights or complaints then please subscribe to the mailing list! Send email to:

perl-loop-subscribe AT perl.org

AUTHOR
Joshua N. Pritikin <jpritikin AT pobox.com>

ACKNOWLEDGMENT
Initial 0.01 implementation by Graham Barr <gbarr AT pobox.com>. Other contributors include at
least those lists below and folks mentioned in the ChangeLog.

Gisle Aas <gisle AT aas.no>
Uri Guttman <uri AT sysarch.com>
Nick Ing-Simmons <nick AT ni-s.com> (Tk)
Sarathy <gsar AT engin.edu>
Jochen Stenzel <perl AT jochen-stenzel.de>

perl v5.38.2 2024-04-01 Event(3pm)

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