Provided by: libevent-perl_1.27-1build1_amd64 
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@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@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@perl.org
AUTHOR
Joshua N. Pritikin <jpritikin@pobox.com>
ACKNOWLEDGMENT
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
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.
perl v5.30.0 2019-10-18 Event(3pm)