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Event - Event loop processing


 use Event qw(loop unloop);
 # initialize application
 Event->flavor(attribute => value, ...);
 my $ret = loop();
 # and some callback will call


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.


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.


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


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.)


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.


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


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


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.


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:


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.


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.


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


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

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.


Reports whether the $watcher has been cancelled.


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


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).


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


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


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.


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.


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.


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.


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 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.


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.


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.


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->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


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


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 <>


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, 12 Jul 1999.


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


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!


  • 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



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


Joshua N. Pritikin <>


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

 Gisle Aas <>
 Uri Guttman <>
 Nick Ing-Simmons <> (Tk)
 Sarathy <>
 Jochen Stenzel <>


Copyright © 1997 Joshua Nathaniel Pritikin & Graham Barr

Copyright © 1998, 1999, 2000, 2001, 2002, 2003, 2004 Joshua Nathaniel Pritikin

All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.