Event - Event loop processing


Event - Event loop processing


 use Event qw(loop unloop);

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

 my $ret = loop();

 # and some callback will call


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 (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
$result = loop([$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).

  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.

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.

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. All events support: hits, prio, and w. 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.

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. 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.

Returns whether this watcher has any events pending in the event queue.

Watcher Types

Extra attributes: min => $seconds, max => $seconds

The callback is invoked only when no events are pending. If there is never a chance to idle, an event will be generated at least every max seconds and not more often than min seconds.

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

Var watchers generate events when the given variable is read from or written to. 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 for writes unless you know what you are doing.

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

The $time and $sec are in seconds. Fractional seconds may be used if Time::HiRes is available. The constructor also accepts an after attribute for easier initialization. It might be useful to know the time at the start of today. You can find it with:

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

If interval is set then the watcher will automatically repeat. Be aware 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 invoke. However, if hard is false the new timeout will be calculated relative to the current time (this is the default).

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).

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_cb is set then timeouts use this alternate callback instead of the main callback.

Extra attribute: signal => $str


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.

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
A watcher increments hits every time it registers an event. Signals in quick succession can be clumped into a single event.

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 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 <Mark.Mielke.markm@nt.com>


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@vogon-solutions.com, 12 Jul 1999.


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


This section is concerned with advanced kernel architecture.

A scalable and explicit event delivery mechanism for UNIX

Abstract: UNIX applications not wishing to block when doing I/O often use the select() system call, to wait for events on multiple file descriptors. The select() mechanism works well for small-scale applications, but scales poorly as the number of file descriptors increases. Many modern applications, such as Internet servers, use hundreds or thousands of file descriptors, and suffer greatly from the poor scalability of select(). Previous work has shown that while the traditional implementation of select() can be improved, the poor scalability is inherent in the design. We present a new event-delivery mechanism, which allows the application to register interest in one or more sources of events, and to efficiently dequeue new events. We show that this mechanism, which requires only minor changes to applications, performs independently of the number of file descriptors.



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!



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



Joshua N. Pritikin <jpritikin@pobox.com>

For my motivation, see http://why-compete.org


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

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


Copyright 1997 Joshua Nathaniel Pritikin & Graham Barr

Copyright 1998, 1999, 2000, 2001, 2002 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.

 Event - Event loop processing