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

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