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1	=head1 NAME	1	=head1 NAME
2		2
3	AnyEvent - ???	3	AnyEvent - provide framework for multiple event loops
		4
		5	EV, Event, Glib, Tk, Perl, Event::Lib, Qt, POE - various supported event loops
4		6
5	=head1 SYNOPSIS	7	=head1 SYNOPSIS
6		8
		9	use AnyEvent;
		10
		11	my $w = AnyEvent->io (fh => $fh, poll => "r|w", cb => sub { ... });
		12
		13	my $w = AnyEvent->timer (after => $seconds, cb => sub { ... });
		14	my $w = AnyEvent->timer (after => $seconds, interval => $seconds, cb => ...
		15
		16	print AnyEvent->now; # prints current event loop time
		17	print AnyEvent->time; # think Time::HiRes::time or simply CORE::time.
		18
		19	my $w = AnyEvent->signal (signal => "TERM", cb => sub { ... });
		20
		21	my $w = AnyEvent->child (pid => $pid, cb => sub {
		22	my ($pid, $status) = @_;
		23	...
		24	});
		25
		26	my $w = AnyEvent->condvar; # stores whether a condition was flagged
		27	$w->send; # wake up current and all future recv's
		28	$w->recv; # enters "main loop" till $condvar gets ->send
		29	# use a condvar in callback mode:
		30	$w->cb (sub { $_[0]->recv });
		31
		32	=head1 INTRODUCTION/TUTORIAL
		33
		34	This manpage is mainly a reference manual. If you are interested
		35	in a tutorial or some gentle introduction, have a look at the
		36	L<AnyEvent::Intro> manpage.
		37
		38	=head1 WHY YOU SHOULD USE THIS MODULE (OR NOT)
		39
		40	Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
		41	nowadays. So what is different about AnyEvent?
		42
		43	Executive Summary: AnyEvent is I<compatible>, AnyEvent is I<free of
		44	policy> and AnyEvent is I<small and efficient>.
		45
		46	First and foremost, I<AnyEvent is not an event model> itself, it only
		47	interfaces to whatever event model the main program happens to use, in a
		48	pragmatic way. For event models and certain classes of immortals alike,
		49	the statement "there can only be one" is a bitter reality: In general,
		50	only one event loop can be active at the same time in a process. AnyEvent
		51	cannot change this, but it can hide the differences between those event
		52	loops.
		53
		54	The goal of AnyEvent is to offer module authors the ability to do event
		55	programming (waiting for I/O or timer events) without subscribing to a
		56	religion, a way of living, and most importantly: without forcing your
		57	module users into the same thing by forcing them to use the same event
		58	model you use.
		59
		60	For modules like POE or IO::Async (which is a total misnomer as it is
		61	actually doing all I/O I<synchronously>...), using them in your module is
		62	like joining a cult: After you joined, you are dependent on them and you
		63	cannot use anything else, as they are simply incompatible to everything
		64	that isn't them. What's worse, all the potential users of your
		65	module are I<also> forced to use the same event loop you use.
		66
		67	AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works
		68	fine. AnyEvent + Tk works fine etc. etc. but none of these work together
		69	with the rest: POE + IO::Async? No go. Tk + Event? No go. Again: if
		70	your module uses one of those, every user of your module has to use it,
		71	too. But if your module uses AnyEvent, it works transparently with all
		72	event models it supports (including stuff like IO::Async, as long as those
		73	use one of the supported event loops. It is trivial to add new event loops
		74	to AnyEvent, too, so it is future-proof).
		75
		76	In addition to being free of having to use I<the one and only true event
		77	model>, AnyEvent also is free of bloat and policy: with POE or similar
		78	modules, you get an enormous amount of code and strict rules you have to
		79	follow. AnyEvent, on the other hand, is lean and up to the point, by only
		80	offering the functionality that is necessary, in as thin as a wrapper as
		81	technically possible.
		82
		83	Of course, AnyEvent comes with a big (and fully optional!) toolbox
		84	of useful functionality, such as an asynchronous DNS resolver, 100%
		85	non-blocking connects (even with TLS/SSL, IPv6 and on broken platforms
		86	such as Windows) and lots of real-world knowledge and workarounds for
		87	platform bugs and differences.
		88
		89	Now, if you I<do want> lots of policy (this can arguably be somewhat
		90	useful) and you want to force your users to use the one and only event
		91	model, you should I<not> use this module.
		92
7	=head1 DESCRIPTION	93	=head1 DESCRIPTION
8		94
		95	L<AnyEvent> provides an identical interface to multiple event loops. This
		96	allows module authors to utilise an event loop without forcing module
		97	users to use the same event loop (as only a single event loop can coexist
		98	peacefully at any one time).
		99
		100	The interface itself is vaguely similar, but not identical to the L<Event>
		101	module.
		102
		103	During the first call of any watcher-creation method, the module tries
		104	to detect the currently loaded event loop by probing whether one of the
		105	following modules is already loaded: L<EV>,
		106	L<Event>, L<Glib>, L<AnyEvent::Impl::Perl>, L<Tk>, L<Event::Lib>, L<Qt>,
		107	L<POE>. The first one found is used. If none are found, the module tries
		108	to load these modules (excluding Tk, Event::Lib, Qt and POE as the pure perl
		109	adaptor should always succeed) in the order given. The first one that can
		110	be successfully loaded will be used. If, after this, still none could be
		111	found, AnyEvent will fall back to a pure-perl event loop, which is not
		112	very efficient, but should work everywhere.
		113
		114	Because AnyEvent first checks for modules that are already loaded, loading
		115	an event model explicitly before first using AnyEvent will likely make
		116	that model the default. For example:
		117
		118	use Tk;
		119	use AnyEvent;
		120
		121	# .. AnyEvent will likely default to Tk
		122
		123	The I<likely> means that, if any module loads another event model and
		124	starts using it, all bets are off. Maybe you should tell their authors to
		125	use AnyEvent so their modules work together with others seamlessly...
		126
		127	The pure-perl implementation of AnyEvent is called
		128	C<AnyEvent::Impl::Perl>. Like other event modules you can load it
		129	explicitly and enjoy the high availability of that event loop :)
		130
		131	=head1 WATCHERS
		132
		133	AnyEvent has the central concept of a I<watcher>, which is an object that
		134	stores relevant data for each kind of event you are waiting for, such as
		135	the callback to call, the file handle to watch, etc.
		136
		137	These watchers are normal Perl objects with normal Perl lifetime. After
		138	creating a watcher it will immediately "watch" for events and invoke the
		139	callback when the event occurs (of course, only when the event model
		140	is in control).
		141
		142	To disable the watcher you have to destroy it (e.g. by setting the
		143	variable you store it in to C<undef> or otherwise deleting all references
		144	to it).
		145
		146	All watchers are created by calling a method on the C<AnyEvent> class.
		147
		148	Many watchers either are used with "recursion" (repeating timers for
		149	example), or need to refer to their watcher object in other ways.
		150
		151	An any way to achieve that is this pattern:
		152
		153	my $w; $w = AnyEvent->type (arg => value ..., cb => sub {
		154	# you can use $w here, for example to undef it
		155	undef $w;
		156	});
		157
		158	Note that C<my $w; $w => combination. This is necessary because in Perl,
		159	my variables are only visible after the statement in which they are
		160	declared.
		161
		162	=head2 I/O WATCHERS
		163
		164	You can create an I/O watcher by calling the C<< AnyEvent->io >> method
		165	with the following mandatory key-value pairs as arguments:
		166
		167	C<fh> the Perl I<file handle> (I<not> file descriptor) to watch for events
		168	(AnyEvent might or might not keep a reference to this file handle). C<poll>
		169	must be a string that is either C<r> or C<w>, which creates a watcher
		170	waiting for "r"eadable or "w"ritable events, respectively. C<cb> is the
		171	callback to invoke each time the file handle becomes ready.
		172
		173	Although the callback might get passed parameters, their value and
		174	presence is undefined and you cannot rely on them. Portable AnyEvent
		175	callbacks cannot use arguments passed to I/O watcher callbacks.
		176
		177	The I/O watcher might use the underlying file descriptor or a copy of it.
		178	You must not close a file handle as long as any watcher is active on the
		179	underlying file descriptor.
		180
		181	Some event loops issue spurious readyness notifications, so you should
		182	always use non-blocking calls when reading/writing from/to your file
		183	handles.
		184
		185	Example: wait for readability of STDIN, then read a line and disable the
		186	watcher.
		187
		188	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
		189	chomp (my $input = <STDIN>);
		190	warn "read: $input\n";
		191	undef $w;
		192	});
		193
		194	=head2 TIME WATCHERS
		195
		196	You can create a time watcher by calling the C<< AnyEvent->timer >>
		197	method with the following mandatory arguments:
		198
		199	C<after> specifies after how many seconds (fractional values are
		200	supported) the callback should be invoked. C<cb> is the callback to invoke
		201	in that case.
		202
		203	Although the callback might get passed parameters, their value and
		204	presence is undefined and you cannot rely on them. Portable AnyEvent
		205	callbacks cannot use arguments passed to time watcher callbacks.
		206
		207	The callback will normally be invoked once only. If you specify another
		208	parameter, C<interval>, as a strictly positive number (> 0), then the
		209	callback will be invoked regularly at that interval (in fractional
		210	seconds) after the first invocation. If C<interval> is specified with a
		211	false value, then it is treated as if it were missing.
		212
		213	The callback will be rescheduled before invoking the callback, but no
		214	attempt is done to avoid timer drift in most backends, so the interval is
		215	only approximate.
		216
		217	Example: fire an event after 7.7 seconds.
		218
		219	my $w = AnyEvent->timer (after => 7.7, cb => sub {
		220	warn "timeout\n";
		221	});
		222
		223	# to cancel the timer:
		224	undef $w;
		225
		226	Example 2: fire an event after 0.5 seconds, then roughly every second.
		227
		228	my $w = AnyEvent->timer (after => 0.5, interval => 1, cb => sub {
		229	warn "timeout\n";
		230	};
		231
		232	=head3 TIMING ISSUES
		233
		234	There are two ways to handle timers: based on real time (relative, "fire
		235	in 10 seconds") and based on wallclock time (absolute, "fire at 12
		236	o'clock").
		237
		238	While most event loops expect timers to specified in a relative way, they
		239	use absolute time internally. This makes a difference when your clock
		240	"jumps", for example, when ntp decides to set your clock backwards from
		241	the wrong date of 2014-01-01 to 2008-01-01, a watcher that is supposed to
		242	fire "after" a second might actually take six years to finally fire.
		243
		244	AnyEvent cannot compensate for this. The only event loop that is conscious
		245	about these issues is L<EV>, which offers both relative (ev_timer, based
		246	on true relative time) and absolute (ev_periodic, based on wallclock time)
		247	timers.
		248
		249	AnyEvent always prefers relative timers, if available, matching the
		250	AnyEvent API.
		251
		252	AnyEvent has two additional methods that return the "current time":
		253
9	=over 4	254	=over 4
10		255
		256	=item AnyEvent->time
		257
		258	This returns the "current wallclock time" as a fractional number of
		259	seconds since the Epoch (the same thing as C<time> or C<Time::HiRes::time>
		260	return, and the result is guaranteed to be compatible with those).
		261
		262	It progresses independently of any event loop processing, i.e. each call
		263	will check the system clock, which usually gets updated frequently.
		264
		265	=item AnyEvent->now
		266
		267	This also returns the "current wallclock time", but unlike C<time>, above,
		268	this value might change only once per event loop iteration, depending on
		269	the event loop (most return the same time as C<time>, above). This is the
		270	time that AnyEvent's timers get scheduled against.
		271
		272	I<In almost all cases (in all cases if you don't care), this is the
		273	function to call when you want to know the current time.>
		274
		275	This function is also often faster then C<< AnyEvent->time >>, and
		276	thus the preferred method if you want some timestamp (for example,
		277	L<AnyEvent::Handle> uses this to update it's activity timeouts).
		278
		279	The rest of this section is only of relevance if you try to be very exact
		280	with your timing, you can skip it without bad conscience.
		281
		282	For a practical example of when these times differ, consider L<Event::Lib>
		283	and L<EV> and the following set-up:
		284
		285	The event loop is running and has just invoked one of your callback at
		286	time=500 (assume no other callbacks delay processing). In your callback,
		287	you wait a second by executing C<sleep 1> (blocking the process for a
		288	second) and then (at time=501) you create a relative timer that fires
		289	after three seconds.
		290
		291	With L<Event::Lib>, C<< AnyEvent->time >> and C<< AnyEvent->now >> will
		292	both return C<501>, because that is the current time, and the timer will
		293	be scheduled to fire at time=504 (C<501> + C<3>).
		294
		295	With L<EV>, C<< AnyEvent->time >> returns C<501> (as that is the current
		296	time), but C<< AnyEvent->now >> returns C<500>, as that is the time the
		297	last event processing phase started. With L<EV>, your timer gets scheduled
		298	to run at time=503 (C<500> + C<3>).
		299
		300	In one sense, L<Event::Lib> is more exact, as it uses the current time
		301	regardless of any delays introduced by event processing. However, most
		302	callbacks do not expect large delays in processing, so this causes a
		303	higher drift (and a lot more system calls to get the current time).
		304
		305	In another sense, L<EV> is more exact, as your timer will be scheduled at
		306	the same time, regardless of how long event processing actually took.
		307
		308	In either case, if you care (and in most cases, you don't), then you
		309	can get whatever behaviour you want with any event loop, by taking the
		310	difference between C<< AnyEvent->time >> and C<< AnyEvent->now >> into
		311	account.
		312
		313	=back
		314
		315	=head2 SIGNAL WATCHERS
		316
		317	You can watch for signals using a signal watcher, C<signal> is the signal
		318	I<name> in uppercase and without any C<SIG> prefix, C<cb> is the Perl
		319	callback to be invoked whenever a signal occurs.
		320
		321	Although the callback might get passed parameters, their value and
		322	presence is undefined and you cannot rely on them. Portable AnyEvent
		323	callbacks cannot use arguments passed to signal watcher callbacks.
		324
		325	Multiple signal occurrences can be clumped together into one callback
		326	invocation, and callback invocation will be synchronous. Synchronous means
		327	that it might take a while until the signal gets handled by the process,
		328	but it is guaranteed not to interrupt any other callbacks.
		329
		330	The main advantage of using these watchers is that you can share a signal
		331	between multiple watchers.
		332
		333	This watcher might use C<%SIG>, so programs overwriting those signals
		334	directly will likely not work correctly.
		335
		336	Example: exit on SIGINT
		337
		338	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
		339
		340	=head2 CHILD PROCESS WATCHERS
		341
		342	You can also watch on a child process exit and catch its exit status.
		343
		344	The child process is specified by the C<pid> argument (if set to C<0>, it
		345	watches for any child process exit). The watcher will triggered only when
		346	the child process has finished and an exit status is available, not on
		347	any trace events (stopped/continued).
		348
		349	The callback will be called with the pid and exit status (as returned by
		350	waitpid), so unlike other watcher types, you I<can> rely on child watcher
		351	callback arguments.
		352
		353	This watcher type works by installing a signal handler for C<SIGCHLD>,
		354	and since it cannot be shared, nothing else should use SIGCHLD or reap
		355	random child processes (waiting for specific child processes, e.g. inside
		356	C<system>, is just fine).
		357
		358	There is a slight catch to child watchers, however: you usually start them
		359	I<after> the child process was created, and this means the process could
		360	have exited already (and no SIGCHLD will be sent anymore).
		361
		362	Not all event models handle this correctly (POE doesn't), but even for
		363	event models that I<do> handle this correctly, they usually need to be
		364	loaded before the process exits (i.e. before you fork in the first place).
		365
		366	This means you cannot create a child watcher as the very first thing in an
		367	AnyEvent program, you I<have> to create at least one watcher before you
		368	C<fork> the child (alternatively, you can call C<AnyEvent::detect>).
		369
		370	Example: fork a process and wait for it
		371
		372	my $done = AnyEvent->condvar;
		373
		374	my $pid = fork or exit 5;
		375
		376	my $w = AnyEvent->child (
		377	pid => $pid,
		378	cb => sub {
		379	my ($pid, $status) = @_;
		380	warn "pid $pid exited with status $status";
		381	$done->send;
		382	},
		383	);
		384
		385	# do something else, then wait for process exit
		386	$done->recv;
		387
		388	=head2 CONDITION VARIABLES
		389
		390	If you are familiar with some event loops you will know that all of them
		391	require you to run some blocking "loop", "run" or similar function that
		392	will actively watch for new events and call your callbacks.
		393
		394	AnyEvent is different, it expects somebody else to run the event loop and
		395	will only block when necessary (usually when told by the user).
		396
		397	The instrument to do that is called a "condition variable", so called
		398	because they represent a condition that must become true.
		399
		400	Condition variables can be created by calling the C<< AnyEvent->condvar
		401	>> method, usually without arguments. The only argument pair allowed is
		402
		403	C<cb>, which specifies a callback to be called when the condition variable
		404	becomes true, with the condition variable as the first argument (but not
		405	the results).
		406
		407	After creation, the condition variable is "false" until it becomes "true"
		408	by calling the C<send> method (or calling the condition variable as if it
		409	were a callback, read about the caveats in the description for the C<<
		410	->send >> method).
		411
		412	Condition variables are similar to callbacks, except that you can
		413	optionally wait for them. They can also be called merge points - points
		414	in time where multiple outstanding events have been processed. And yet
		415	another way to call them is transactions - each condition variable can be
		416	used to represent a transaction, which finishes at some point and delivers
		417	a result.
		418
		419	Condition variables are very useful to signal that something has finished,
		420	for example, if you write a module that does asynchronous http requests,
		421	then a condition variable would be the ideal candidate to signal the
		422	availability of results. The user can either act when the callback is
		423	called or can synchronously C<< ->recv >> for the results.
		424
		425	You can also use them to simulate traditional event loops - for example,
		426	you can block your main program until an event occurs - for example, you
		427	could C<< ->recv >> in your main program until the user clicks the Quit
		428	button of your app, which would C<< ->send >> the "quit" event.
		429
		430	Note that condition variables recurse into the event loop - if you have
		431	two pieces of code that call C<< ->recv >> in a round-robin fashion, you
		432	lose. Therefore, condition variables are good to export to your caller, but
		433	you should avoid making a blocking wait yourself, at least in callbacks,
		434	as this asks for trouble.
		435
		436	Condition variables are represented by hash refs in perl, and the keys
		437	used by AnyEvent itself are all named C<_ae_XXX> to make subclassing
		438	easy (it is often useful to build your own transaction class on top of
		439	AnyEvent). To subclass, use C<AnyEvent::CondVar> as base class and call
		440	it's C<new> method in your own C<new> method.
		441
		442	There are two "sides" to a condition variable - the "producer side" which
		443	eventually calls C<< -> send >>, and the "consumer side", which waits
		444	for the send to occur.
		445
		446	Example: wait for a timer.
		447
		448	# wait till the result is ready
		449	my $result_ready = AnyEvent->condvar;
		450
		451	# do something such as adding a timer
		452	# or socket watcher the calls $result_ready->send
		453	# when the "result" is ready.
		454	# in this case, we simply use a timer:
		455	my $w = AnyEvent->timer (
		456	after => 1,
		457	cb => sub { $result_ready->send },
		458	);
		459
		460	# this "blocks" (while handling events) till the callback
		461	# calls send
		462	$result_ready->recv;
		463
		464	Example: wait for a timer, but take advantage of the fact that
		465	condition variables are also code references.
		466
		467	my $done = AnyEvent->condvar;
		468	my $delay = AnyEvent->timer (after => 5, cb => $done);
		469	$done->recv;
		470
		471	Example: Imagine an API that returns a condvar and doesn't support
		472	callbacks. This is how you make a synchronous call, for example from
		473	the main program:
		474
		475	use AnyEvent::CouchDB;
		476
		477	...
		478
		479	my @info = $couchdb->info->recv;
		480
		481	And this is how you would just ste a callback to be called whenever the
		482	results are available:
		483
		484	$couchdb->info->cb (sub {
		485	my @info = $_[0]->recv;
		486	});
		487
		488	=head3 METHODS FOR PRODUCERS
		489
		490	These methods should only be used by the producing side, i.e. the
		491	code/module that eventually sends the signal. Note that it is also
		492	the producer side which creates the condvar in most cases, but it isn't
		493	uncommon for the consumer to create it as well.
		494
		495	=over 4
		496
		497	=item $cv->send (...)
		498
		499	Flag the condition as ready - a running C<< ->recv >> and all further
		500	calls to C<recv> will (eventually) return after this method has been
		501	called. If nobody is waiting the send will be remembered.
		502
		503	If a callback has been set on the condition variable, it is called
		504	immediately from within send.
		505
		506	Any arguments passed to the C<send> call will be returned by all
		507	future C<< ->recv >> calls.
		508
		509	Condition variables are overloaded so one can call them directly
		510	(as a code reference). Calling them directly is the same as calling
		511	C<send>. Note, however, that many C-based event loops do not handle
		512	overloading, so as tempting as it may be, passing a condition variable
		513	instead of a callback does not work. Both the pure perl and EV loops
		514	support overloading, however, as well as all functions that use perl to
		515	invoke a callback (as in L<AnyEvent::Socket> and L<AnyEvent::DNS> for
		516	example).
		517
		518	=item $cv->croak ($error)
		519
		520	Similar to send, but causes all call's to C<< ->recv >> to invoke
		521	C<Carp::croak> with the given error message/object/scalar.
		522
		523	This can be used to signal any errors to the condition variable
		524	user/consumer.
		525
		526	=item $cv->begin ([group callback])
		527
		528	=item $cv->end
		529
		530	These two methods are EXPERIMENTAL and MIGHT CHANGE.
		531
		532	These two methods can be used to combine many transactions/events into
		533	one. For example, a function that pings many hosts in parallel might want
		534	to use a condition variable for the whole process.
		535
		536	Every call to C<< ->begin >> will increment a counter, and every call to
		537	C<< ->end >> will decrement it. If the counter reaches C<0> in C<< ->end
		538	>>, the (last) callback passed to C<begin> will be executed. That callback
		539	is I<supposed> to call C<< ->send >>, but that is not required. If no
		540	callback was set, C<send> will be called without any arguments.
		541
		542	Let's clarify this with the ping example:
		543
		544	my $cv = AnyEvent->condvar;
		545
		546	my %result;
		547	$cv->begin (sub { $cv->send (\%result) });
		548
		549	for my $host (@list_of_hosts) {
		550	$cv->begin;
		551	ping_host_then_call_callback $host, sub {
		552	$result{$host} = ...;
		553	$cv->end;
		554	};
		555	}
		556
		557	$cv->end;
		558
		559	This code fragment supposedly pings a number of hosts and calls
		560	C<send> after results for all then have have been gathered - in any
		561	order. To achieve this, the code issues a call to C<begin> when it starts
		562	each ping request and calls C<end> when it has received some result for
		563	it. Since C<begin> and C<end> only maintain a counter, the order in which
		564	results arrive is not relevant.
		565
		566	There is an additional bracketing call to C<begin> and C<end> outside the
		567	loop, which serves two important purposes: first, it sets the callback
		568	to be called once the counter reaches C<0>, and second, it ensures that
		569	C<send> is called even when C<no> hosts are being pinged (the loop
		570	doesn't execute once).
		571
		572	This is the general pattern when you "fan out" into multiple subrequests:
		573	use an outer C<begin>/C<end> pair to set the callback and ensure C<end>
		574	is called at least once, and then, for each subrequest you start, call
		575	C<begin> and for each subrequest you finish, call C<end>.
		576
		577	=back
		578
		579	=head3 METHODS FOR CONSUMERS
		580
		581	These methods should only be used by the consuming side, i.e. the
		582	code awaits the condition.
		583
		584	=over 4
		585
		586	=item $cv->recv
		587
		588	Wait (blocking if necessary) until the C<< ->send >> or C<< ->croak
		589	>> methods have been called on c<$cv>, while servicing other watchers
		590	normally.
		591
		592	You can only wait once on a condition - additional calls are valid but
		593	will return immediately.
		594
		595	If an error condition has been set by calling C<< ->croak >>, then this
		596	function will call C<croak>.
		597
		598	In list context, all parameters passed to C<send> will be returned,
		599	in scalar context only the first one will be returned.
		600
		601	Not all event models support a blocking wait - some die in that case
		602	(programs might want to do that to stay interactive), so I<if you are
		603	using this from a module, never require a blocking wait>, but let the
		604	caller decide whether the call will block or not (for example, by coupling
		605	condition variables with some kind of request results and supporting
		606	callbacks so the caller knows that getting the result will not block,
		607	while still supporting blocking waits if the caller so desires).
		608
		609	Another reason I<never> to C<< ->recv >> in a module is that you cannot
		610	sensibly have two C<< ->recv >>'s in parallel, as that would require
		611	multiple interpreters or coroutines/threads, none of which C<AnyEvent>
		612	can supply.
		613
		614	The L<Coro> module, however, I<can> and I<does> supply coroutines and, in
		615	fact, L<Coro::AnyEvent> replaces AnyEvent's condvars by coroutine-safe
		616	versions and also integrates coroutines into AnyEvent, making blocking
		617	C<< ->recv >> calls perfectly safe as long as they are done from another
		618	coroutine (one that doesn't run the event loop).
		619
		620	You can ensure that C<< -recv >> never blocks by setting a callback and
		621	only calling C<< ->recv >> from within that callback (or at a later
		622	time). This will work even when the event loop does not support blocking
		623	waits otherwise.
		624
		625	=item $bool = $cv->ready
		626
		627	Returns true when the condition is "true", i.e. whether C<send> or
		628	C<croak> have been called.
		629
		630	=item $cb = $cv->cb ($cb->($cv))
		631
		632	This is a mutator function that returns the callback set and optionally
		633	replaces it before doing so.
		634
		635	The callback will be called when the condition becomes "true", i.e. when
		636	C<send> or C<croak> are called, with the only argument being the condition
		637	variable itself. Calling C<recv> inside the callback or at any later time
		638	is guaranteed not to block.
		639
		640	=back
		641
		642	=head1 GLOBAL VARIABLES AND FUNCTIONS
		643
		644	=over 4
		645
		646	=item $AnyEvent::MODEL
		647
		648	Contains C<undef> until the first watcher is being created. Then it
		649	contains the event model that is being used, which is the name of the
		650	Perl class implementing the model. This class is usually one of the
		651	C<AnyEvent::Impl:xxx> modules, but can be any other class in the case
		652	AnyEvent has been extended at runtime (e.g. in I<rxvt-unicode>).
		653
		654	The known classes so far are:
		655
		656	AnyEvent::Impl::EV based on EV (an interface to libev, best choice).
		657	AnyEvent::Impl::Event based on Event, second best choice.
		658	AnyEvent::Impl::Perl pure-perl implementation, fast and portable.
		659	AnyEvent::Impl::Glib based on Glib, third-best choice.
		660	AnyEvent::Impl::Tk based on Tk, very bad choice.
		661	AnyEvent::Impl::Qt based on Qt, cannot be autoprobed (see its docs).
		662	AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
		663	AnyEvent::Impl::POE based on POE, not generic enough for full support.
		664
		665	There is no support for WxWidgets, as WxWidgets has no support for
		666	watching file handles. However, you can use WxWidgets through the
		667	POE Adaptor, as POE has a Wx backend that simply polls 20 times per
		668	second, which was considered to be too horrible to even consider for
		669	AnyEvent. Likewise, other POE backends can be used by AnyEvent by using
		670	it's adaptor.
		671
		672	AnyEvent knows about L<Prima> and L<Wx> and will try to use L<POE> when
		673	autodetecting them.
		674
		675	=item AnyEvent::detect
		676
		677	Returns C<$AnyEvent::MODEL>, forcing autodetection of the event model
		678	if necessary. You should only call this function right before you would
		679	have created an AnyEvent watcher anyway, that is, as late as possible at
		680	runtime.
		681
		682	=item $guard = AnyEvent::post_detect { BLOCK }
		683
		684	Arranges for the code block to be executed as soon as the event model is
		685	autodetected (or immediately if this has already happened).
		686
		687	If called in scalar or list context, then it creates and returns an object
		688	that automatically removes the callback again when it is destroyed. See
		689	L<Coro::BDB> for a case where this is useful.
		690
		691	=item @AnyEvent::post_detect
		692
		693	If there are any code references in this array (you can C<push> to it
		694	before or after loading AnyEvent), then they will called directly after
		695	the event loop has been chosen.
		696
		697	You should check C<$AnyEvent::MODEL> before adding to this array, though:
		698	if it contains a true value then the event loop has already been detected,
		699	and the array will be ignored.
		700
		701	Best use C<AnyEvent::post_detect { BLOCK }> instead.
		702
		703	=back
		704
		705	=head1 WHAT TO DO IN A MODULE
		706
		707	As a module author, you should C<use AnyEvent> and call AnyEvent methods
		708	freely, but you should not load a specific event module or rely on it.
		709
		710	Be careful when you create watchers in the module body - AnyEvent will
		711	decide which event module to use as soon as the first method is called, so
		712	by calling AnyEvent in your module body you force the user of your module
		713	to load the event module first.
		714
		715	Never call C<< ->recv >> on a condition variable unless you I<know> that
		716	the C<< ->send >> method has been called on it already. This is
		717	because it will stall the whole program, and the whole point of using
		718	events is to stay interactive.
		719
		720	It is fine, however, to call C<< ->recv >> when the user of your module
		721	requests it (i.e. if you create a http request object ad have a method
		722	called C<results> that returns the results, it should call C<< ->recv >>
		723	freely, as the user of your module knows what she is doing. always).
		724
		725	=head1 WHAT TO DO IN THE MAIN PROGRAM
		726
		727	There will always be a single main program - the only place that should
		728	dictate which event model to use.
		729
		730	If it doesn't care, it can just "use AnyEvent" and use it itself, or not
		731	do anything special (it does not need to be event-based) and let AnyEvent
		732	decide which implementation to chose if some module relies on it.
		733
		734	If the main program relies on a specific event model - for example, in
		735	Gtk2 programs you have to rely on the Glib module - you should load the
		736	event module before loading AnyEvent or any module that uses it: generally
		737	speaking, you should load it as early as possible. The reason is that
		738	modules might create watchers when they are loaded, and AnyEvent will
		739	decide on the event model to use as soon as it creates watchers, and it
		740	might chose the wrong one unless you load the correct one yourself.
		741
		742	You can chose to use a pure-perl implementation by loading the
		743	C<AnyEvent::Impl::Perl> module, which gives you similar behaviour
		744	everywhere, but letting AnyEvent chose the model is generally better.
		745
		746	=head2 MAINLOOP EMULATION
		747
		748	Sometimes (often for short test scripts, or even standalone programs who
		749	only want to use AnyEvent), you do not want to run a specific event loop.
		750
		751	In that case, you can use a condition variable like this:
		752
		753	AnyEvent->condvar->recv;
		754
		755	This has the effect of entering the event loop and looping forever.
		756
		757	Note that usually your program has some exit condition, in which case
		758	it is better to use the "traditional" approach of storing a condition
		759	variable somewhere, waiting for it, and sending it when the program should
		760	exit cleanly.
		761
		762
		763	=head1 OTHER MODULES
		764
		765	The following is a non-exhaustive list of additional modules that use
		766	AnyEvent and can therefore be mixed easily with other AnyEvent modules
		767	in the same program. Some of the modules come with AnyEvent, some are
		768	available via CPAN.
		769
		770	=over 4
		771
		772	=item L<AnyEvent::Util>
		773
		774	Contains various utility functions that replace often-used but blocking
		775	functions such as C<inet_aton> by event-/callback-based versions.
		776
		777	=item L<AnyEvent::Socket>
		778
		779	Provides various utility functions for (internet protocol) sockets,
		780	addresses and name resolution. Also functions to create non-blocking tcp
		781	connections or tcp servers, with IPv6 and SRV record support and more.
		782
		783	=item L<AnyEvent::Handle>
		784
		785	Provide read and write buffers, manages watchers for reads and writes,
		786	supports raw and formatted I/O, I/O queued and fully transparent and
		787	non-blocking SSL/TLS.
		788
		789	=item L<AnyEvent::DNS>
		790
		791	Provides rich asynchronous DNS resolver capabilities.
		792
		793	=item L<AnyEvent::HTTP>
		794
		795	A simple-to-use HTTP library that is capable of making a lot of concurrent
		796	HTTP requests.
		797
		798	=item L<AnyEvent::HTTPD>
		799
		800	Provides a simple web application server framework.
		801
		802	=item L<AnyEvent::FastPing>
		803
		804	The fastest ping in the west.
		805
		806	=item L<AnyEvent::DBI>
		807
		808	Executes L<DBI> requests asynchronously in a proxy process.
		809
		810	=item L<AnyEvent::AIO>
		811
		812	Truly asynchronous I/O, should be in the toolbox of every event
		813	programmer. AnyEvent::AIO transparently fuses L<IO::AIO> and AnyEvent
		814	together.
		815
		816	=item L<AnyEvent::BDB>
		817
		818	Truly asynchronous Berkeley DB access. AnyEvent::BDB transparently fuses
		819	L<BDB> and AnyEvent together.
		820
		821	=item L<AnyEvent::GPSD>
		822
		823	A non-blocking interface to gpsd, a daemon delivering GPS information.
		824
		825	=item L<AnyEvent::IGS>
		826
		827	A non-blocking interface to the Internet Go Server protocol (used by
		828	L<App::IGS>).
		829
		830	=item L<AnyEvent::IRC>
		831
		832	AnyEvent based IRC client module family (replacing the older Net::IRC3).
		833
		834	=item L<Net::XMPP2>
		835
		836	AnyEvent based XMPP (Jabber protocol) module family.
		837
		838	=item L<Net::FCP>
		839
		840	AnyEvent-based implementation of the Freenet Client Protocol, birthplace
		841	of AnyEvent.
		842
		843	=item L<Event::ExecFlow>
		844
		845	High level API for event-based execution flow control.
		846
		847	=item L<Coro>
		848
		849	Has special support for AnyEvent via L<Coro::AnyEvent>.
		850
		851	=item L<IO::Lambda>
		852
		853	The lambda approach to I/O - don't ask, look there. Can use AnyEvent.
		854
		855	=back
		856
11	=cut	857	=cut
12		858
13	package AnyEvent;	859	package AnyEvent;
14		860
		861	no warnings;
		862	use strict qw(vars subs);
		863
15	use Carp;	864	use Carp;
16		865
17	$VERSION = 0.1;	866	our $VERSION = 4.341;
		867	our $MODEL;
18		868
19	no warnings;	869	our $AUTOLOAD;
		870	our @ISA;
		871
		872	our @REGISTRY;
		873
		874	our $WIN32;
		875
		876	BEGIN {
		877	my $win32 = ! ! ($^O =~ /mswin32/i);
		878	eval "sub WIN32(){ $win32 }";
		879	}
		880
		881	our $verbose = $ENV{PERL_ANYEVENT_VERBOSE}*1;
		882
		883	our %PROTOCOL; # (ipv4|ipv6) => (1|2), higher numbers are preferred
		884
		885	{
		886	my $idx;
		887	$PROTOCOL{$_} = ++$idx
		888	for reverse split /\s*,\s*/,
		889	$ENV{PERL_ANYEVENT_PROTOCOLS} || "ipv4,ipv6";
		890	}
20		891
21	my @models = (	892	my @models = (
22	[Coro => Coro::Event::],	893	[EV:: => AnyEvent::Impl::EV::],
23	[Event => Event::],	894	[Event:: => AnyEvent::Impl::Event::],
24	[Glib => Glib::],	895	[AnyEvent::Impl::Perl:: => AnyEvent::Impl::Perl::],
25	[Tk => Tk::],	896	# everything below here will not be autoprobed
		897	# as the pureperl backend should work everywhere
		898	# and is usually faster
		899	[Tk:: => AnyEvent::Impl::Tk::], # crashes with many handles
		900	[Glib:: => AnyEvent::Impl::Glib::], # becomes extremely slow with many watchers
		901	[Event::Lib:: => AnyEvent::Impl::EventLib::], # too buggy
		902	[Qt:: => AnyEvent::Impl::Qt::], # requires special main program
		903	[POE::Kernel:: => AnyEvent::Impl::POE::], # lasciate ogni speranza
		904	[Wx:: => AnyEvent::Impl::POE::],
		905	[Prima:: => AnyEvent::Impl::POE::],
26	);	906	);
27		907
		908	our %method = map +($_ => 1), qw(io timer time now signal child condvar one_event DESTROY);
		909
		910	our @post_detect;
		911
		912	sub post_detect(&) {
		913	my ($cb) = @_;
		914
		915	if ($MODEL) {
		916	$cb->();
		917
		918	1
		919	} else {
		920	push @post_detect, $cb;
		921
		922	defined wantarray
		923	? bless \$cb, "AnyEvent::Util::PostDetect"
		924	: ()
		925	}
		926	}
		927
		928	sub AnyEvent::Util::PostDetect::DESTROY {
		929	@post_detect = grep $_ != ${$_[0]}, @post_detect;
		930	}
		931
		932	sub detect() {
		933	unless ($MODEL) {
		934	no strict 'refs';
		935	local $SIG{__DIE__};
		936
		937	if ($ENV{PERL_ANYEVENT_MODEL} =~ /^([a-zA-Z]+)$/) {
		938	my $model = "AnyEvent::Impl::$1";
		939	if (eval "require $model") {
		940	$MODEL = $model;
		941	warn "AnyEvent: loaded model '$model' (forced by \$PERL_ANYEVENT_MODEL), using it.\n" if $verbose > 1;
		942	} else {
		943	warn "AnyEvent: unable to load model '$model' (from \$PERL_ANYEVENT_MODEL):\n$@" if $verbose;
		944	}
		945	}
		946
		947	# check for already loaded models
		948	unless ($MODEL) {
		949	for (@REGISTRY, @models) {
		950	my ($package, $model) = @$_;
		951	if (${"$package\::VERSION"} > 0) {
		952	if (eval "require $model") {
		953	$MODEL = $model;
		954	warn "AnyEvent: autodetected model '$model', using it.\n" if $verbose > 1;
		955	last;
		956	}
		957	}
		958	}
		959
		960	unless ($MODEL) {
		961	# try to load a model
		962
		963	for (@REGISTRY, @models) {
		964	my ($package, $model) = @$_;
		965	if (eval "require $package"
		966	and ${"$package\::VERSION"} > 0
		967	and eval "require $model") {
		968	$MODEL = $model;
		969	warn "AnyEvent: autoprobed model '$model', using it.\n" if $verbose > 1;
		970	last;
		971	}
		972	}
		973
		974	$MODEL
		975	or die "No event module selected for AnyEvent and autodetect failed. Install any one of these modules: EV, Event or Glib.";
		976	}
		977	}
		978
		979	push @{"$MODEL\::ISA"}, "AnyEvent::Base";
		980
		981	unshift @ISA, $MODEL;
		982
		983	require AnyEvent::Strict if $ENV{PERL_ANYEVENT_STRICT};
		984
		985	(shift @post_detect)->() while @post_detect;
		986	}
		987
		988	$MODEL
		989	}
		990
28	sub AUTOLOAD {	991	sub AUTOLOAD {
29	$AUTOLOAD =~ s/.*://;	992	(my $func = $AUTOLOAD) =~ s/.*://;
30		993
31	for (@models) {	994	$method{$func}
32	my ($model, $package) = @$_;	995	or croak "$func: not a valid method for AnyEvent objects";
33	if (defined ${"$package\::VERSION"}) {	996
34	$EVENT = "AnyEvent::Impl::$model";	997	detect unless $MODEL;
35	eval "require $EVENT"; die if $@;	998
36	goto &{"$EVENT\::$AUTOLOAD"};	999	my $class = shift;
		1000	$class->$func (@_);
		1001	}
		1002
		1003	# utility function to dup a filehandle. this is used by many backends
		1004	# to support binding more than one watcher per filehandle (they usually
		1005	# allow only one watcher per fd, so we dup it to get a different one).
		1006	sub _dupfh($$$$) {
		1007	my ($poll, $fh, $r, $w) = @_;
		1008
		1009	# cygwin requires the fh mode to be matching, unix doesn't
		1010	my ($rw, $mode) = $poll eq "r" ? ($r, "<")
		1011	: $poll eq "w" ? ($w, ">")
		1012	: Carp::croak "AnyEvent->io requires poll set to either 'r' or 'w'";
		1013
		1014	open my $fh2, "$mode&" . fileno $fh
		1015	or die "cannot dup() filehandle: $!";
		1016
		1017	# we assume CLOEXEC is already set by perl in all important cases
		1018
		1019	($fh2, $rw)
		1020	}
		1021
		1022	package AnyEvent::Base;
		1023
		1024	# default implementation for now and time
		1025
		1026	BEGIN {
		1027	if (eval "use Time::HiRes (); time (); 1") {
		1028	*_time = \&Time::HiRes::time;
		1029	# if (eval "use POSIX (); (POSIX::times())...
		1030	} else {
		1031	*_time = sub { time }; # epic fail
		1032	}
		1033	}
		1034
		1035	sub time { _time }
		1036	sub now { _time }
		1037
		1038	# default implementation for ->condvar
		1039
		1040	sub condvar {
		1041	bless { @_ == 3 ? (_ae_cb => $_[2]) : () }, AnyEvent::CondVar::
		1042	}
		1043
		1044	# default implementation for ->signal
		1045
		1046	our ($SIGPIPE_R, $SIGPIPE_W, %SIG_CB, %SIG_EV, $SIG_IO);
		1047
		1048	sub _signal_exec {
		1049	while (%SIG_EV) {
		1050	sysread $SIGPIPE_R, my $dummy, 4;
		1051	for (keys %SIG_EV) {
		1052	delete $SIG_EV{$_};
		1053	$_->() for values %{ $SIG_CB{$_} || {} };
37	}	1054	}
38	}	1055	}
		1056	}
39		1057
40	for (@models) {	1058	sub signal {
41	my ($model, $package) = @$_;	1059	my (undef, %arg) = @_;
42	$EVENT = "AnyEvent::Impl::$model";	1060
43	if (eval "require $EVENT") {	1061	unless ($SIGPIPE_R) {
44	goto &{"$EVENT\::$AUTOLOAD"};	1062	if (AnyEvent::WIN32) {
		1063	($SIGPIPE_R, $SIGPIPE_W) = AnyEvent::Util::portable_pipe ();
		1064	AnyEvent::Util::fh_nonblocking ($SIGPIPE_R) if $SIGPIPE_R;
		1065	AnyEvent::Util::fh_nonblocking ($SIGPIPE_W) if $SIGPIPE_W; # just in case
		1066	} else {
		1067	pipe $SIGPIPE_R, $SIGPIPE_W;
		1068	require Fcntl;
		1069	fcntl $SIGPIPE_R, &Fcntl::F_SETFL, &Fcntl::O_NONBLOCK if $SIGPIPE_R;
		1070	fcntl $SIGPIPE_W, &Fcntl::F_SETFL, &Fcntl::O_NONBLOCK if $SIGPIPE_W; # just in case
45	}	1071	}
		1072
		1073	$SIGPIPE_R
		1074	or Carp::croak "AnyEvent: unable to create a signal reporting pipe: $!\n";
		1075
		1076	$SIG_IO = AnyEvent->io (fh => $SIGPIPE_R, poll => "r", cb => \&_signal_exec);
46	}	1077	}
47		1078
48	die "No event module selected for AnyEvent and autodetect failed. Install any of these: Coro, Event, Glib or Tk.";	1079	my $signal = uc $arg{signal}
49	}	1080	or Carp::croak "required option 'signal' is missing";
50		1081
51	1;	1082	$SIG_CB{$signal}{$arg{cb}} = $arg{cb};
		1083	$SIG{$signal} ||= sub {
		1084	syswrite $SIGPIPE_W, "\x00", 1 unless %SIG_EV;
		1085	undef $SIG_EV{$signal};
		1086	};
52		1087
		1088	bless [$signal, $arg{cb}], "AnyEvent::Base::Signal"
		1089	}
		1090
		1091	sub AnyEvent::Base::Signal::DESTROY {
		1092	my ($signal, $cb) = @{$_[0]};
		1093
		1094	delete $SIG_CB{$signal}{$cb};
		1095
		1096	delete $SIG{$signal} unless keys %{ $SIG_CB{$signal} };
		1097	}
		1098
		1099	# default implementation for ->child
		1100
		1101	our %PID_CB;
		1102	our $CHLD_W;
		1103	our $CHLD_DELAY_W;
		1104	our $PID_IDLE;
		1105	our $WNOHANG;
		1106
		1107	sub _child_wait {
		1108	while (0 < (my $pid = waitpid -1, $WNOHANG)) {
		1109	$_->($pid, $?) for (values %{ $PID_CB{$pid} || {} }),
		1110	(values %{ $PID_CB{0} || {} });
		1111	}
		1112
		1113	undef $PID_IDLE;
		1114	}
		1115
		1116	sub _sigchld {
		1117	# make sure we deliver these changes "synchronous" with the event loop.
		1118	$CHLD_DELAY_W ||= AnyEvent->timer (after => 0, cb => sub {
		1119	undef $CHLD_DELAY_W;
		1120	&_child_wait;
		1121	});
		1122	}
		1123
		1124	sub child {
		1125	my (undef, %arg) = @_;
		1126
		1127	defined (my $pid = $arg{pid} + 0)
		1128	or Carp::croak "required option 'pid' is missing";
		1129
		1130	$PID_CB{$pid}{$arg{cb}} = $arg{cb};
		1131
		1132	unless ($WNOHANG) {
		1133	$WNOHANG = eval { local $SIG{__DIE__}; require POSIX; &POSIX::WNOHANG } || 1;
		1134	}
		1135
		1136	unless ($CHLD_W) {
		1137	$CHLD_W = AnyEvent->signal (signal => 'CHLD', cb => \&_sigchld);
		1138	# child could be a zombie already, so make at least one round
		1139	&_sigchld;
		1140	}
		1141
		1142	bless [$pid, $arg{cb}], "AnyEvent::Base::Child"
		1143	}
		1144
		1145	sub AnyEvent::Base::Child::DESTROY {
		1146	my ($pid, $cb) = @{$_[0]};
		1147
		1148	delete $PID_CB{$pid}{$cb};
		1149	delete $PID_CB{$pid} unless keys %{ $PID_CB{$pid} };
		1150
		1151	undef $CHLD_W unless keys %PID_CB;
		1152	}
		1153
		1154	package AnyEvent::CondVar;
		1155
		1156	our @ISA = AnyEvent::CondVar::Base::;
		1157
		1158	package AnyEvent::CondVar::Base;
		1159
		1160	use overload
		1161	'&{}' => sub { my $self = shift; sub { $self->send (@_) } },
		1162	fallback => 1;
		1163
		1164	sub _send {
		1165	# nop
		1166	}
		1167
		1168	sub send {
		1169	my $cv = shift;
		1170	$cv->{_ae_sent} = [@_];
		1171	(delete $cv->{_ae_cb})->($cv) if $cv->{_ae_cb};
		1172	$cv->_send;
		1173	}
		1174
		1175	sub croak {
		1176	$_[0]{_ae_croak} = $_[1];
		1177	$_[0]->send;
		1178	}
		1179
		1180	sub ready {
		1181	$_[0]{_ae_sent}
		1182	}
		1183
		1184	sub _wait {
		1185	AnyEvent->one_event while !$_[0]{_ae_sent};
		1186	}
		1187
		1188	sub recv {
		1189	$_[0]->_wait;
		1190
		1191	Carp::croak $_[0]{_ae_croak} if $_[0]{_ae_croak};
		1192	wantarray ? @{ $_[0]{_ae_sent} } : $_[0]{_ae_sent}[0]
		1193	}
		1194
		1195	sub cb {
		1196	$_[0]{_ae_cb} = $_[1] if @_ > 1;
		1197	$_[0]{_ae_cb}
		1198	}
		1199
		1200	sub begin {
		1201	++$_[0]{_ae_counter};
		1202	$_[0]{_ae_end_cb} = $_[1] if @_ > 1;
		1203	}
		1204
		1205	sub end {
		1206	return if --$_[0]{_ae_counter};
		1207	&{ $_[0]{_ae_end_cb} || sub { $_[0]->send } };
		1208	}
		1209
		1210	# undocumented/compatibility with pre-3.4
		1211	*broadcast = \&send;
		1212	*wait = \&_wait;
		1213
		1214	=head1 ERROR AND EXCEPTION HANDLING
		1215
		1216	In general, AnyEvent does not do any error handling - it relies on the
		1217	caller to do that if required. The L<AnyEvent::Strict> module (see also
		1218	the C<PERL_ANYEVENT_STRICT> environment variable, below) provides strict
		1219	checking of all AnyEvent methods, however, which is highly useful during
		1220	development.
		1221
		1222	As for exception handling (i.e. runtime errors and exceptions thrown while
		1223	executing a callback), this is not only highly event-loop specific, but
		1224	also not in any way wrapped by this module, as this is the job of the main
		1225	program.
		1226
		1227	The pure perl event loop simply re-throws the exception (usually
		1228	within C<< condvar->recv >>), the L<Event> and L<EV> modules call C<<
		1229	$Event/EV::DIED->() >>, L<Glib> uses C<< install_exception_handler >> and
		1230	so on.
		1231
		1232	=head1 ENVIRONMENT VARIABLES
		1233
		1234	The following environment variables are used by this module or its
		1235	submodules:
		1236
		1237	=over 4
		1238
		1239	=item C<PERL_ANYEVENT_VERBOSE>
		1240
		1241	By default, AnyEvent will be completely silent except in fatal
		1242	conditions. You can set this environment variable to make AnyEvent more
		1243	talkative.
		1244
		1245	When set to C<1> or higher, causes AnyEvent to warn about unexpected
		1246	conditions, such as not being able to load the event model specified by
		1247	C<PERL_ANYEVENT_MODEL>.
		1248
		1249	When set to C<2> or higher, cause AnyEvent to report to STDERR which event
		1250	model it chooses.
		1251
		1252	=item C<PERL_ANYEVENT_STRICT>
		1253
		1254	AnyEvent does not do much argument checking by default, as thorough
		1255	argument checking is very costly. Setting this variable to a true value
		1256	will cause AnyEvent to load C<AnyEvent::Strict> and then to thoroughly
		1257	check the arguments passed to most method calls. If it finds any problems
		1258	it will croak.
		1259
		1260	In other words, enables "strict" mode.
		1261
		1262	Unlike C<use strict>, it is definitely recommended ot keep it off in
		1263	production. Keeping C<PERL_ANYEVENT_STRICT=1> in your environment while
		1264	developing programs can be very useful, however.
		1265
		1266	=item C<PERL_ANYEVENT_MODEL>
		1267
		1268	This can be used to specify the event model to be used by AnyEvent, before
		1269	auto detection and -probing kicks in. It must be a string consisting
		1270	entirely of ASCII letters. The string C<AnyEvent::Impl::> gets prepended
		1271	and the resulting module name is loaded and if the load was successful,
		1272	used as event model. If it fails to load AnyEvent will proceed with
		1273	auto detection and -probing.
		1274
		1275	This functionality might change in future versions.
		1276
		1277	For example, to force the pure perl model (L<AnyEvent::Impl::Perl>) you
		1278	could start your program like this:
		1279
		1280	PERL_ANYEVENT_MODEL=Perl perl ...
		1281
		1282	=item C<PERL_ANYEVENT_PROTOCOLS>
		1283
		1284	Used by both L<AnyEvent::DNS> and L<AnyEvent::Socket> to determine preferences
		1285	for IPv4 or IPv6. The default is unspecified (and might change, or be the result
		1286	of auto probing).
		1287
		1288	Must be set to a comma-separated list of protocols or address families,
		1289	current supported: C<ipv4> and C<ipv6>. Only protocols mentioned will be
		1290	used, and preference will be given to protocols mentioned earlier in the
		1291	list.
		1292
		1293	This variable can effectively be used for denial-of-service attacks
		1294	against local programs (e.g. when setuid), although the impact is likely
		1295	small, as the program has to handle conenction and other failures anyways.
		1296
		1297	Examples: C<PERL_ANYEVENT_PROTOCOLS=ipv4,ipv6> - prefer IPv4 over IPv6,
		1298	but support both and try to use both. C<PERL_ANYEVENT_PROTOCOLS=ipv4>
		1299	- only support IPv4, never try to resolve or contact IPv6
		1300	addresses. C<PERL_ANYEVENT_PROTOCOLS=ipv6,ipv4> support either IPv4 or
		1301	IPv6, but prefer IPv6 over IPv4.
		1302
		1303	=item C<PERL_ANYEVENT_EDNS0>
		1304
		1305	Used by L<AnyEvent::DNS> to decide whether to use the EDNS0 extension
		1306	for DNS. This extension is generally useful to reduce DNS traffic, but
		1307	some (broken) firewalls drop such DNS packets, which is why it is off by
		1308	default.
		1309
		1310	Setting this variable to C<1> will cause L<AnyEvent::DNS> to announce
		1311	EDNS0 in its DNS requests.
		1312
		1313	=item C<PERL_ANYEVENT_MAX_FORKS>
		1314
		1315	The maximum number of child processes that C<AnyEvent::Util::fork_call>
		1316	will create in parallel.
		1317
		1318	=back
		1319
		1320	=head1 SUPPLYING YOUR OWN EVENT MODEL INTERFACE
		1321
		1322	This is an advanced topic that you do not normally need to use AnyEvent in
		1323	a module. This section is only of use to event loop authors who want to
		1324	provide AnyEvent compatibility.
		1325
		1326	If you need to support another event library which isn't directly
		1327	supported by AnyEvent, you can supply your own interface to it by
		1328	pushing, before the first watcher gets created, the package name of
		1329	the event module and the package name of the interface to use onto
		1330	C<@AnyEvent::REGISTRY>. You can do that before and even without loading
		1331	AnyEvent, so it is reasonably cheap.
		1332
		1333	Example:
		1334
		1335	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
		1336
		1337	This tells AnyEvent to (literally) use the C<urxvt::anyevent::>
		1338	package/class when it finds the C<urxvt> package/module is already loaded.
		1339
		1340	When AnyEvent is loaded and asked to find a suitable event model, it
		1341	will first check for the presence of urxvt by trying to C<use> the
		1342	C<urxvt::anyevent> module.
		1343
		1344	The class should provide implementations for all watcher types. See
		1345	L<AnyEvent::Impl::EV> (source code), L<AnyEvent::Impl::Glib> (Source code)
		1346	and so on for actual examples. Use C<perldoc -m AnyEvent::Impl::Glib> to
		1347	see the sources.
		1348
		1349	If you don't provide C<signal> and C<child> watchers than AnyEvent will
		1350	provide suitable (hopefully) replacements.
		1351
		1352	The above example isn't fictitious, the I<rxvt-unicode> (a.k.a. urxvt)
		1353	terminal emulator uses the above line as-is. An interface isn't included
		1354	in AnyEvent because it doesn't make sense outside the embedded interpreter
		1355	inside I<rxvt-unicode>, and it is updated and maintained as part of the
		1356	I<rxvt-unicode> distribution.
		1357
		1358	I<rxvt-unicode> also cheats a bit by not providing blocking access to
		1359	condition variables: code blocking while waiting for a condition will
		1360	C<die>. This still works with most modules/usages, and blocking calls must
		1361	not be done in an interactive application, so it makes sense.
		1362
		1363	=head1 EXAMPLE PROGRAM
		1364
		1365	The following program uses an I/O watcher to read data from STDIN, a timer
		1366	to display a message once per second, and a condition variable to quit the
		1367	program when the user enters quit:
		1368
		1369	use AnyEvent;
		1370
		1371	my $cv = AnyEvent->condvar;
		1372
		1373	my $io_watcher = AnyEvent->io (
		1374	fh => \*STDIN,
		1375	poll => 'r',
		1376	cb => sub {
		1377	warn "io event <$_[0]>\n"; # will always output <r>
		1378	chomp (my $input = <STDIN>); # read a line
		1379	warn "read: $input\n"; # output what has been read
		1380	$cv->send if $input =~ /^q/i; # quit program if /^q/i
		1381	},
		1382	);
		1383
		1384	my $time_watcher; # can only be used once
		1385
		1386	sub new_timer {
		1387	$timer = AnyEvent->timer (after => 1, cb => sub {
		1388	warn "timeout\n"; # print 'timeout' about every second
		1389	&new_timer; # and restart the time
		1390	});
		1391	}
		1392
		1393	new_timer; # create first timer
		1394
		1395	$cv->recv; # wait until user enters /^q/i
		1396
		1397	=head1 REAL-WORLD EXAMPLE
		1398
		1399	Consider the L<Net::FCP> module. It features (among others) the following
		1400	API calls, which are to freenet what HTTP GET requests are to http:
		1401
		1402	my $data = $fcp->client_get ($url); # blocks
		1403
		1404	my $transaction = $fcp->txn_client_get ($url); # does not block
		1405	$transaction->cb ( sub { ... } ); # set optional result callback
		1406	my $data = $transaction->result; # possibly blocks
		1407
		1408	The C<client_get> method works like C<LWP::Simple::get>: it requests the
		1409	given URL and waits till the data has arrived. It is defined to be:
		1410
		1411	sub client_get { $_[0]->txn_client_get ($_[1])->result }
		1412
		1413	And in fact is automatically generated. This is the blocking API of
		1414	L<Net::FCP>, and it works as simple as in any other, similar, module.
		1415
		1416	More complicated is C<txn_client_get>: It only creates a transaction
		1417	(completion, result, ...) object and initiates the transaction.
		1418
		1419	my $txn = bless { }, Net::FCP::Txn::;
		1420
		1421	It also creates a condition variable that is used to signal the completion
		1422	of the request:
		1423
		1424	$txn->{finished} = AnyAvent->condvar;
		1425
		1426	It then creates a socket in non-blocking mode.
		1427
		1428	socket $txn->{fh}, ...;
		1429	fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
		1430	connect $txn->{fh}, ...
		1431	and !$!{EWOULDBLOCK}
		1432	and !$!{EINPROGRESS}
		1433	and Carp::croak "unable to connect: $!\n";
		1434
		1435	Then it creates a write-watcher which gets called whenever an error occurs
		1436	or the connection succeeds:
		1437
		1438	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });
		1439
		1440	And returns this transaction object. The C<fh_ready_w> callback gets
		1441	called as soon as the event loop detects that the socket is ready for
		1442	writing.
		1443
		1444	The C<fh_ready_w> method makes the socket blocking again, writes the
		1445	request data and replaces the watcher by a read watcher (waiting for reply
		1446	data). The actual code is more complicated, but that doesn't matter for
		1447	this example:
		1448
		1449	fcntl $txn->{fh}, F_SETFL, 0;
		1450	syswrite $txn->{fh}, $txn->{request}
		1451	or die "connection or write error";
		1452	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
		1453
		1454	Again, C<fh_ready_r> waits till all data has arrived, and then stores the
		1455	result and signals any possible waiters that the request has finished:
		1456
		1457	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
		1458
		1459	if (end-of-file or data complete) {
		1460	$txn->{result} = $txn->{buf};
		1461	$txn->{finished}->send;
		1462	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
		1463	}
		1464
		1465	The C<result> method, finally, just waits for the finished signal (if the
		1466	request was already finished, it doesn't wait, of course, and returns the
		1467	data:
		1468
		1469	$txn->{finished}->recv;
		1470	return $txn->{result};
		1471
		1472	The actual code goes further and collects all errors (C<die>s, exceptions)
		1473	that occurred during request processing. The C<result> method detects
		1474	whether an exception as thrown (it is stored inside the $txn object)
		1475	and just throws the exception, which means connection errors and other
		1476	problems get reported tot he code that tries to use the result, not in a
		1477	random callback.
		1478
		1479	All of this enables the following usage styles:
		1480
		1481	1. Blocking:
		1482
		1483	my $data = $fcp->client_get ($url);
		1484
		1485	2. Blocking, but running in parallel:
		1486
		1487	my @datas = map $_->result,
		1488	map $fcp->txn_client_get ($_),
		1489	@urls;
		1490
		1491	Both blocking examples work without the module user having to know
		1492	anything about events.
		1493
		1494	3a. Event-based in a main program, using any supported event module:
		1495
		1496	use EV;
		1497
		1498	$fcp->txn_client_get ($url)->cb (sub {
		1499	my $txn = shift;
		1500	my $data = $txn->result;
		1501	...
		1502	});
		1503
		1504	EV::loop;
		1505
		1506	3b. The module user could use AnyEvent, too:
		1507
		1508	use AnyEvent;
		1509
		1510	my $quit = AnyEvent->condvar;
		1511
		1512	$fcp->txn_client_get ($url)->cb (sub {
		1513	...
		1514	$quit->send;
		1515	});
		1516
		1517	$quit->recv;
		1518
		1519
		1520	=head1 BENCHMARKS
		1521
		1522	To give you an idea of the performance and overheads that AnyEvent adds
		1523	over the event loops themselves and to give you an impression of the speed
		1524	of various event loops I prepared some benchmarks.
		1525
		1526	=head2 BENCHMARKING ANYEVENT OVERHEAD
		1527
		1528	Here is a benchmark of various supported event models used natively and
		1529	through AnyEvent. The benchmark creates a lot of timers (with a zero
		1530	timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
		1531	which it is), lets them fire exactly once and destroys them again.
		1532
		1533	Source code for this benchmark is found as F<eg/bench> in the AnyEvent
		1534	distribution.
		1535
		1536	=head3 Explanation of the columns
		1537
		1538	I<watcher> is the number of event watchers created/destroyed. Since
		1539	different event models feature vastly different performances, each event
		1540	loop was given a number of watchers so that overall runtime is acceptable
		1541	and similar between tested event loop (and keep them from crashing): Glib
		1542	would probably take thousands of years if asked to process the same number
		1543	of watchers as EV in this benchmark.
		1544
		1545	I<bytes> is the number of bytes (as measured by the resident set size,
		1546	RSS) consumed by each watcher. This method of measuring captures both C
		1547	and Perl-based overheads.
		1548
		1549	I<create> is the time, in microseconds (millionths of seconds), that it
		1550	takes to create a single watcher. The callback is a closure shared between
		1551	all watchers, to avoid adding memory overhead. That means closure creation
		1552	and memory usage is not included in the figures.
		1553
		1554	I<invoke> is the time, in microseconds, used to invoke a simple
		1555	callback. The callback simply counts down a Perl variable and after it was
		1556	invoked "watcher" times, it would C<< ->send >> a condvar once to
		1557	signal the end of this phase.
		1558
		1559	I<destroy> is the time, in microseconds, that it takes to destroy a single
		1560	watcher.
		1561
		1562	=head3 Results
		1563
		1564	name watchers bytes create invoke destroy comment
		1565	EV/EV 400000 224 0.47 0.35 0.27 EV native interface
		1566	EV/Any 100000 224 2.88 0.34 0.27 EV + AnyEvent watchers
		1567	CoroEV/Any 100000 224 2.85 0.35 0.28 coroutines + Coro::Signal
		1568	Perl/Any 100000 452 4.13 0.73 0.95 pure perl implementation
		1569	Event/Event 16000 517 32.20 31.80 0.81 Event native interface
		1570	Event/Any 16000 590 35.85 31.55 1.06 Event + AnyEvent watchers
		1571	Glib/Any 16000 1357 102.33 12.31 51.00 quadratic behaviour
		1572	Tk/Any 2000 1860 27.20 66.31 14.00 SEGV with >> 2000 watchers
		1573	POE/Event 2000 6328 109.99 751.67 14.02 via POE::Loop::Event
		1574	POE/Select 2000 6027 94.54 809.13 579.80 via POE::Loop::Select
		1575
		1576	=head3 Discussion
		1577
		1578	The benchmark does I<not> measure scalability of the event loop very
		1579	well. For example, a select-based event loop (such as the pure perl one)
		1580	can never compete with an event loop that uses epoll when the number of
		1581	file descriptors grows high. In this benchmark, all events become ready at
		1582	the same time, so select/poll-based implementations get an unnatural speed
		1583	boost.
		1584
		1585	Also, note that the number of watchers usually has a nonlinear effect on
		1586	overall speed, that is, creating twice as many watchers doesn't take twice
		1587	the time - usually it takes longer. This puts event loops tested with a
		1588	higher number of watchers at a disadvantage.
		1589
		1590	To put the range of results into perspective, consider that on the
		1591	benchmark machine, handling an event takes roughly 1600 CPU cycles with
		1592	EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000 CPU
		1593	cycles with POE.
		1594
		1595	C<EV> is the sole leader regarding speed and memory use, which are both
		1596	maximal/minimal, respectively. Even when going through AnyEvent, it uses
		1597	far less memory than any other event loop and is still faster than Event
		1598	natively.
		1599
		1600	The pure perl implementation is hit in a few sweet spots (both the
		1601	constant timeout and the use of a single fd hit optimisations in the perl
		1602	interpreter and the backend itself). Nevertheless this shows that it
		1603	adds very little overhead in itself. Like any select-based backend its
		1604	performance becomes really bad with lots of file descriptors (and few of
		1605	them active), of course, but this was not subject of this benchmark.
		1606
		1607	The C<Event> module has a relatively high setup and callback invocation
		1608	cost, but overall scores in on the third place.
		1609
		1610	C<Glib>'s memory usage is quite a bit higher, but it features a
		1611	faster callback invocation and overall ends up in the same class as
		1612	C<Event>. However, Glib scales extremely badly, doubling the number of
		1613	watchers increases the processing time by more than a factor of four,
		1614	making it completely unusable when using larger numbers of watchers
		1615	(note that only a single file descriptor was used in the benchmark, so
		1616	inefficiencies of C<poll> do not account for this).
		1617
		1618	The C<Tk> adaptor works relatively well. The fact that it crashes with
		1619	more than 2000 watchers is a big setback, however, as correctness takes
		1620	precedence over speed. Nevertheless, its performance is surprising, as the
		1621	file descriptor is dup()ed for each watcher. This shows that the dup()
		1622	employed by some adaptors is not a big performance issue (it does incur a
		1623	hidden memory cost inside the kernel which is not reflected in the figures
		1624	above).
		1625
		1626	C<POE>, regardless of underlying event loop (whether using its pure perl
		1627	select-based backend or the Event module, the POE-EV backend couldn't
		1628	be tested because it wasn't working) shows abysmal performance and
		1629	memory usage with AnyEvent: Watchers use almost 30 times as much memory
		1630	as EV watchers, and 10 times as much memory as Event (the high memory
		1631	requirements are caused by requiring a session for each watcher). Watcher
		1632	invocation speed is almost 900 times slower than with AnyEvent's pure perl
		1633	implementation.
		1634
		1635	The design of the POE adaptor class in AnyEvent can not really account
		1636	for the performance issues, though, as session creation overhead is
		1637	small compared to execution of the state machine, which is coded pretty
		1638	optimally within L<AnyEvent::Impl::POE> (and while everybody agrees that
		1639	using multiple sessions is not a good approach, especially regarding
		1640	memory usage, even the author of POE could not come up with a faster
		1641	design).
		1642
		1643	=head3 Summary
		1644
		1645	=over 4
		1646
		1647	=item * Using EV through AnyEvent is faster than any other event loop
		1648	(even when used without AnyEvent), but most event loops have acceptable
		1649	performance with or without AnyEvent.
		1650
		1651	=item * The overhead AnyEvent adds is usually much smaller than the overhead of
		1652	the actual event loop, only with extremely fast event loops such as EV
		1653	adds AnyEvent significant overhead.
		1654
		1655	=item * You should avoid POE like the plague if you want performance or
		1656	reasonable memory usage.
		1657
		1658	=back
		1659
		1660	=head2 BENCHMARKING THE LARGE SERVER CASE
		1661
		1662	This benchmark actually benchmarks the event loop itself. It works by
		1663	creating a number of "servers": each server consists of a socket pair, a
		1664	timeout watcher that gets reset on activity (but never fires), and an I/O
		1665	watcher waiting for input on one side of the socket. Each time the socket
		1666	watcher reads a byte it will write that byte to a random other "server".
		1667
		1668	The effect is that there will be a lot of I/O watchers, only part of which
		1669	are active at any one point (so there is a constant number of active
		1670	fds for each loop iteration, but which fds these are is random). The
		1671	timeout is reset each time something is read because that reflects how
		1672	most timeouts work (and puts extra pressure on the event loops).
		1673
		1674	In this benchmark, we use 10000 socket pairs (20000 sockets), of which 100
		1675	(1%) are active. This mirrors the activity of large servers with many
		1676	connections, most of which are idle at any one point in time.
		1677
		1678	Source code for this benchmark is found as F<eg/bench2> in the AnyEvent
		1679	distribution.
		1680
		1681	=head3 Explanation of the columns
		1682
		1683	I<sockets> is the number of sockets, and twice the number of "servers" (as
		1684	each server has a read and write socket end).
		1685
		1686	I<create> is the time it takes to create a socket pair (which is
		1687	nontrivial) and two watchers: an I/O watcher and a timeout watcher.
		1688
		1689	I<request>, the most important value, is the time it takes to handle a
		1690	single "request", that is, reading the token from the pipe and forwarding
		1691	it to another server. This includes deleting the old timeout and creating
		1692	a new one that moves the timeout into the future.
		1693
		1694	=head3 Results
		1695
		1696	name sockets create request
		1697	EV 20000 69.01 11.16
		1698	Perl 20000 73.32 35.87
		1699	Event 20000 212.62 257.32
		1700	Glib 20000 651.16 1896.30
		1701	POE 20000 349.67 12317.24 uses POE::Loop::Event
		1702
		1703	=head3 Discussion
		1704
		1705	This benchmark I<does> measure scalability and overall performance of the
		1706	particular event loop.
		1707
		1708	EV is again fastest. Since it is using epoll on my system, the setup time
		1709	is relatively high, though.
		1710
		1711	Perl surprisingly comes second. It is much faster than the C-based event
		1712	loops Event and Glib.
		1713
		1714	Event suffers from high setup time as well (look at its code and you will
		1715	understand why). Callback invocation also has a high overhead compared to
		1716	the C<< $_->() for .. >>-style loop that the Perl event loop uses. Event
		1717	uses select or poll in basically all documented configurations.
		1718
		1719	Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
		1720	clearly fails to perform with many filehandles or in busy servers.
		1721
		1722	POE is still completely out of the picture, taking over 1000 times as long
		1723	as EV, and over 100 times as long as the Perl implementation, even though
		1724	it uses a C-based event loop in this case.
		1725
		1726	=head3 Summary
		1727
		1728	=over 4
		1729
		1730	=item * The pure perl implementation performs extremely well.
		1731
		1732	=item * Avoid Glib or POE in large projects where performance matters.
		1733
		1734	=back
		1735
		1736	=head2 BENCHMARKING SMALL SERVERS
		1737
		1738	While event loops should scale (and select-based ones do not...) even to
		1739	large servers, most programs we (or I :) actually write have only a few
		1740	I/O watchers.
		1741
		1742	In this benchmark, I use the same benchmark program as in the large server
		1743	case, but it uses only eight "servers", of which three are active at any
		1744	one time. This should reflect performance for a small server relatively
		1745	well.
		1746
		1747	The columns are identical to the previous table.
		1748
		1749	=head3 Results
		1750
		1751	name sockets create request
		1752	EV 16 20.00 6.54
		1753	Perl 16 25.75 12.62
		1754	Event 16 81.27 35.86
		1755	Glib 16 32.63 15.48
		1756	POE 16 261.87 276.28 uses POE::Loop::Event
		1757
		1758	=head3 Discussion
		1759
		1760	The benchmark tries to test the performance of a typical small
		1761	server. While knowing how various event loops perform is interesting, keep
		1762	in mind that their overhead in this case is usually not as important, due
		1763	to the small absolute number of watchers (that is, you need efficiency and
		1764	speed most when you have lots of watchers, not when you only have a few of
		1765	them).
		1766
		1767	EV is again fastest.
		1768
		1769	Perl again comes second. It is noticeably faster than the C-based event
		1770	loops Event and Glib, although the difference is too small to really
		1771	matter.
		1772
		1773	POE also performs much better in this case, but is is still far behind the
		1774	others.
		1775
		1776	=head3 Summary
		1777
		1778	=over 4
		1779
		1780	=item * C-based event loops perform very well with small number of
		1781	watchers, as the management overhead dominates.
		1782
		1783	=back
		1784
		1785
		1786	=head1 SIGNALS
		1787
		1788	AnyEvent currently installs handlers for these signals:
		1789
		1790	=over 4
		1791
		1792	=item SIGCHLD
		1793
		1794	A handler for C<SIGCHLD> is installed by AnyEvent's child watcher
		1795	emulation for event loops that do not support them natively. Also, some
		1796	event loops install a similar handler.
		1797
		1798	=item SIGPIPE
		1799
		1800	A no-op handler is installed for C<SIGPIPE> when C<$SIG{PIPE}> is C<undef>
		1801	when AnyEvent gets loaded.
		1802
		1803	The rationale for this is that AnyEvent users usually do not really depend
		1804	on SIGPIPE delivery (which is purely an optimisation for shell use, or
		1805	badly-written programs), but C<SIGPIPE> can cause spurious and rare
		1806	program exits as a lot of people do not expect C<SIGPIPE> when writing to
		1807	some random socket.
		1808
		1809	The rationale for installing a no-op handler as opposed to ignoring it is
		1810	that this way, the handler will be restored to defaults on exec.
		1811
		1812	Feel free to install your own handler, or reset it to defaults.
		1813
		1814	=back
		1815
		1816	=cut
		1817
		1818	$SIG{PIPE} = sub { }
		1819	unless defined $SIG{PIPE};
		1820
		1821
		1822	=head1 FORK
		1823
		1824	Most event libraries are not fork-safe. The ones who are usually are
		1825	because they rely on inefficient but fork-safe C<select> or C<poll>
		1826	calls. Only L<EV> is fully fork-aware.
		1827
		1828	If you have to fork, you must either do so I<before> creating your first
		1829	watcher OR you must not use AnyEvent at all in the child.
		1830
		1831
		1832	=head1 SECURITY CONSIDERATIONS
		1833
		1834	AnyEvent can be forced to load any event model via
		1835	$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used to
		1836	execute arbitrary code or directly gain access, it can easily be used to
		1837	make the program hang or malfunction in subtle ways, as AnyEvent watchers
		1838	will not be active when the program uses a different event model than
		1839	specified in the variable.
		1840
		1841	You can make AnyEvent completely ignore this variable by deleting it
		1842	before the first watcher gets created, e.g. with a C<BEGIN> block:
		1843
		1844	BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }
		1845
		1846	use AnyEvent;
		1847
		1848	Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can
		1849	be used to probe what backend is used and gain other information (which is
		1850	probably even less useful to an attacker than PERL_ANYEVENT_MODEL), and
		1851	$ENV{PERL_ANYEGENT_STRICT}.
		1852
		1853
		1854	=head1 BUGS
		1855
		1856	Perl 5.8 has numerous memleaks that sometimes hit this module and are hard
		1857	to work around. If you suffer from memleaks, first upgrade to Perl 5.10
		1858	and check wether the leaks still show up. (Perl 5.10.0 has other annoying
		1859	mamleaks, such as leaking on C<map> and C<grep> but it is usually not as
		1860	pronounced).
		1861
		1862
		1863	=head1 SEE ALSO
		1864
		1865	Utility functions: L<AnyEvent::Util>.
		1866
		1867	Event modules: L<EV>, L<EV::Glib>, L<Glib::EV>, L<Event>, L<Glib::Event>,
		1868	L<Glib>, L<Tk>, L<Event::Lib>, L<Qt>, L<POE>.
		1869
		1870	Implementations: L<AnyEvent::Impl::EV>, L<AnyEvent::Impl::Event>,
		1871	L<AnyEvent::Impl::Glib>, L<AnyEvent::Impl::Tk>, L<AnyEvent::Impl::Perl>,
		1872	L<AnyEvent::Impl::EventLib>, L<AnyEvent::Impl::Qt>,
		1873	L<AnyEvent::Impl::POE>.
		1874
		1875	Non-blocking file handles, sockets, TCP clients and
		1876	servers: L<AnyEvent::Handle>, L<AnyEvent::Socket>.
		1877
		1878	Asynchronous DNS: L<AnyEvent::DNS>.
		1879
		1880	Coroutine support: L<Coro>, L<Coro::AnyEvent>, L<Coro::EV>, L<Coro::Event>,
		1881
		1882	Nontrivial usage examples: L<Net::FCP>, L<Net::XMPP2>, L<AnyEvent::DNS>.
		1883
		1884
		1885	=head1 AUTHOR
		1886
		1887	Marc Lehmann <schmorp@schmorp.de>
		1888	http://home.schmorp.de/
		1889
		1890	=cut
		1891
		1892	1
		1893

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