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Comparing AnyEvent/README (file contents):
Revision 1.15 by root, Wed Apr 16 15:10:10 2008 UTC vs.
Revision 1.64 by root, Fri Dec 31 04:50:44 2010 UTC

…		…
1	NAME	1	NAME
2	AnyEvent - provide framework for multiple event loops	2	AnyEvent - the DBI of event loop programming
3		3
4	EV, Event, Coro::EV, Coro::Event, Glib, Tk, Perl - various supported	4	EV, Event, Glib, Tk, Perl, Event::Lib, Irssi, rxvt-unicode, IO::Async,
5	event loops	5	Qt and POE are various supported event loops/environments.
6		6
7	SYNOPSIS	7	SYNOPSIS
8	use AnyEvent;	8	use AnyEvent;
9		9
		10	# if you prefer function calls, look at the AE manpage for
		11	# an alternative API.
		12
		13	# file handle or descriptor readable
10	my $w = AnyEvent->io (fh => $fh, poll => "r\|w", cb => sub {	14	my $w = AnyEvent->io (fh => $fh, poll => "r", cb => sub { ... });
		15
		16	# one-shot or repeating timers
		17	my $w = AnyEvent->timer (after => $seconds, cb => sub { ... });
		18	my $w = AnyEvent->timer (after => $seconds, interval => $seconds, cb => ...);
		19
		20	print AnyEvent->now; # prints current event loop time
		21	print AnyEvent->time; # think Time::HiRes::time or simply CORE::time.
		22
		23	# POSIX signal
		24	my $w = AnyEvent->signal (signal => "TERM", cb => sub { ... });
		25
		26	# child process exit
		27	my $w = AnyEvent->child (pid => $pid, cb => sub {
		28	my ($pid, $status) = @_;
11	...	29	...
12	});	30	});
13		31
14	my $w = AnyEvent->timer (after => $seconds, cb => sub {	32	# called when event loop idle (if applicable)
15	...	33	my $w = AnyEvent->idle (cb => sub { ... });
16	});
17		34
18	my $w = AnyEvent->condvar; # stores wether a condition was flagged	35	my $w = AnyEvent->condvar; # stores whether a condition was flagged
		36	$w->send; # wake up current and all future recv's
19	$w->wait; # enters "main loop" till $condvar gets ->broadcast	37	$w->recv; # enters "main loop" till $condvar gets ->send
20	$w->broadcast; # wake up current and all future wait's	38	# use a condvar in callback mode:
		39	$w->cb (sub { $_[0]->recv });
		40
		41	INTRODUCTION/TUTORIAL
		42	This manpage is mainly a reference manual. If you are interested in a
		43	tutorial or some gentle introduction, have a look at the AnyEvent::Intro
		44	manpage.
		45
		46	SUPPORT
		47	An FAQ document is available as AnyEvent::FAQ.
		48
		49	There also is a mailinglist for discussing all things AnyEvent, and an
		50	IRC channel, too.
		51
		52	See the AnyEvent project page at the Schmorpforge Ta-Sa Software
		53	Repository, at <http://anyevent.schmorp.de>, for more info.
21		54
22	WHY YOU SHOULD USE THIS MODULE (OR NOT)	55	WHY YOU SHOULD USE THIS MODULE (OR NOT)
23	Glib, POE, IO::Async, Event... CPAN offers event models by the dozen	56	Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
24	nowadays. So what is different about AnyEvent?	57	nowadays. So what is different about AnyEvent?
25		58
26	Executive Summary: AnyEvent is compatible, AnyEvent is *free of	59	Executive Summary: AnyEvent is compatible, AnyEvent is *free of
27	policy* and AnyEvent is small and efficient.	60	policy* and AnyEvent is small and efficient.
28		61
29	First and foremost, AnyEvent is not an event model itself, it only	62	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	63	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,	64	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	65	the statement "there can only be one" is a bitter reality: In general,
33	helps hiding the differences.	66	only one event loop can be active at the same time in a process.
		67	AnyEvent cannot change this, but it can hide the differences between
		68	those event loops.
34		69
35	The goal of AnyEvent is to offer module authors the ability to do event	70	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	71	programming (waiting for I/O or timer events) without subscribing to a
37	religion, a way of living, and most importantly: without forcing your	72	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	73	module users into the same thing by forcing them to use the same event
39	model you use.	74	model you use.
40		75
41	For modules like POE or IO::Async (which is actually doing all I/O	76	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:	77	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	78	like joining a cult: After you join, you are dependent on them and you
44	else, as it is simply incompatible to everything that isn't itself.	79	cannot use anything else, as they are simply incompatible to everything
		80	that isn't them. What's worse, all the potential users of your module
		81	are also forced to use the same event loop you use.
45		82
46	AnyEvent + POE works fine. AnyEvent + Glib works fine. AnyEvent + Tk	83	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	84	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,	85	with the rest: POE + EV? No go. Tk + Event? No go. Again: if your module
49	every user of your module has to use it, too. If your module uses	86	uses one of those, every user of your module has to use it, too. But if
50	AnyEvent, it works transparently with all event models it supports	87	your module uses AnyEvent, it works transparently with all event models
51	(including stuff like POE and IO::Async).	88	it supports (including stuff like IO::Async, as long as those use one of
		89	the supported event loops. It is easy to add new event loops to
		90	AnyEvent, too, so it is future-proof).
52		91
53	In addition of being free of having to use *the one and only true event	92	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	93	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	94	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	95	follow. AnyEvent, on the other hand, is lean and to the point, by only
57	offering the functionality that is useful, in as thin as a wrapper as	96	offering the functionality that is necessary, in as thin as a wrapper as
58	technically possible.	97	technically possible.
59		98
		99	Of course, AnyEvent comes with a big (and fully optional!) toolbox of
		100	useful functionality, such as an asynchronous DNS resolver, 100%
		101	non-blocking connects (even with TLS/SSL, IPv6 and on broken platforms
		102	such as Windows) and lots of real-world knowledge and workarounds for
		103	platform bugs and differences.
		104
60	Of course, if you want lots of policy (this can arguably be somewhat	105	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	106	useful) and you want to force your users to use the one and only event
62	model, you should not use this module.	107	model, you should not use this module.
63		108
64	DESCRIPTION	109	DESCRIPTION
65	AnyEvent provides an identical interface to multiple event loops. This	110	AnyEvent provides a uniform interface to various event loops. This
66	allows module authors to utilise an event loop without forcing module	111	allows module authors to use event loop functionality without forcing
67	users to use the same event loop (as only a single event loop can	112	module users to use a specific event loop implementation (since more
68	coexist peacefully at any one time).	113	than one event loop cannot coexist peacefully).
69		114
70	The interface itself is vaguely similar but not identical to the Event	115	The interface itself is vaguely similar, but not identical to the Event
71	module.	116	module.
72		117
73	On the first call of any method, the module tries to detect the	118	During the first call of any watcher-creation method, the module tries
74	currently loaded event loop by probing wether any of the following	119	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	120	following modules is already loaded: EV, AnyEvent::Impl::Perl, Event,
76	one found is used. If none are found, the module tries to load these	121	Glib, Tk, Event::Lib, Qt, POE. The first one found is used. If none are
77	modules in the order given. The first one that could be successfully	122	detected, the module tries to load the first four modules in the order
78	loaded will be used. If still none could be found, AnyEvent will fall	123	given; but note that if EV is not available, the pure-perl
79	back to a pure-perl event loop, which is also not very efficient.	124	AnyEvent::Impl::Perl should always work, so the other two are not
		125	normally tried.
80		126
81	Because AnyEvent first checks for modules that are already loaded,	127	Because AnyEvent first checks for modules that are already loaded,
82	loading an Event model explicitly before first using AnyEvent will	128	loading an event model explicitly before first using AnyEvent will
83	likely make that model the default. For example:	129	likely make that model the default. For example:
84		130
85	use Tk;	131	use Tk;
86	use AnyEvent;	132	use AnyEvent;
87		133
88	# .. AnyEvent will likely default to Tk	134	# .. AnyEvent will likely default to Tk
89		135
		136	The likely means that, if any module loads another event model and
		137	starts using it, all bets are off - this case should be very rare
		138	though, as very few modules hardcode event loops without announcing this
		139	very loudly.
		140
90	The pure-perl implementation of AnyEvent is called	141	The pure-perl implementation of AnyEvent is called
91	"AnyEvent::Impl::Perl". Like other event modules you can load it	142	"AnyEvent::Impl::Perl". Like other event modules you can load it
92	explicitly.	143	explicitly and enjoy the high availability of that event loop :)
93		144
94	WATCHERS	145	WATCHERS
95	AnyEvent has the central concept of a watcher, which is an object that	146	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	147	stores relevant data for each kind of event you are waiting for, such as
97	the callback to call, the filehandle to watch, etc.	148	the callback to call, the file handle to watch, etc.
98		149
99	These watchers are normal Perl objects with normal Perl lifetime. After	150	These watchers are normal Perl objects with normal Perl lifetime. After
100	creating a watcher it will immediately "watch" for events and invoke the	151	creating a watcher it will immediately "watch" for events and invoke the
		152	callback when the event occurs (of course, only when the event model is
		153	in control).
		154
		155	Note that callbacks must not permanently change global variables
		156	potentially in use by the event loop (such as $_ or $[) and that
		157	callbacks must not "die". The former is good programming practice in
		158	Perl and the latter stems from the fact that exception handling differs
		159	widely between event loops.
		160
101	callback. To disable the watcher you have to destroy it (e.g. by setting	161	To disable a watcher you have to destroy it (e.g. by setting the
102	the variable that stores it to "undef" or otherwise deleting all	162	variable you store it in to "undef" or otherwise deleting all references
103	references to it).	163	to it).
104		164
105	All watchers are created by calling a method on the "AnyEvent" class.	165	All watchers are created by calling a method on the "AnyEvent" class.
106		166
		167	Many watchers either are used with "recursion" (repeating timers for
		168	example), or need to refer to their watcher object in other ways.
		169
		170	One way to achieve that is this pattern:
		171
		172	my $w; $w = AnyEvent->type (arg => value ..., cb => sub {
		173	# you can use $w here, for example to undef it
		174	undef $w;
		175	});
		176
		177	Note that "my $w; $w =" combination. This is necessary because in Perl,
		178	my variables are only visible after the statement in which they are
		179	declared.
		180
107	IO WATCHERS	181	I/O WATCHERS
		182	$w = AnyEvent->io (
		183	fh => <filehandle_or_fileno>,
		184	poll => <"r" or "w">,
		185	cb => <callback>,
		186	);
		187
108	You can create I/O watcher by calling the "AnyEvent->io" method with the	188	You can create an I/O watcher by calling the "AnyEvent->io" method with
109	following mandatory arguments:	189	the following mandatory key-value pairs as arguments:
110		190
111	"fh" the Perl filehandle (not filedescriptor) to watch for events.	191	"fh" is the Perl file handle (or a naked file descriptor) to watch for
		192	events (AnyEvent might or might not keep a reference to this file
		193	handle). Note that only file handles pointing to things for which
		194	non-blocking operation makes sense are allowed. This includes sockets,
		195	most character devices, pipes, fifos and so on, but not for example
		196	files or block devices.
		197
112	"poll" must be a string that is either "r" or "w", that creates a	198	"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	199	watcher waiting for "r"eadable or "w"ritable events, respectively.
		200
114	to invoke everytime the filehandle becomes ready.	201	"cb" is the callback to invoke each time the file handle becomes ready.
115		202
116	Filehandles will be kept alive, so as long as the watcher exists, the	203	Although the callback might get passed parameters, their value and
117	filehandle exists, too.	204	presence is undefined and you cannot rely on them. Portable AnyEvent
		205	callbacks cannot use arguments passed to I/O watcher callbacks.
118		206
119	Example:	207	The I/O watcher might use the underlying file descriptor or a copy of
		208	it. You must not close a file handle as long as any watcher is active on
		209	the underlying file descriptor.
120		210
		211	Some event loops issue spurious readiness notifications, so you should
		212	always use non-blocking calls when reading/writing from/to your file
		213	handles.
		214
121	# wait for readability of STDIN, then read a line and disable the watcher	215	Example: wait for readability of STDIN, then read a line and disable the
		216	watcher.
		217
122	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {	218	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
123	chomp (my $input = <STDIN>);	219	chomp (my $input = <STDIN>);
124	warn "read: $input\n";	220	warn "read: $input\n";
125	undef $w;	221	undef $w;
126	});	222	});
127		223
128	TIME WATCHERS	224	TIME WATCHERS
		225	$w = AnyEvent->timer (after => <seconds>, cb => <callback>);
		226
		227	$w = AnyEvent->timer (
		228	after => <fractional_seconds>,
		229	interval => <fractional_seconds>,
		230	cb => <callback>,
		231	);
		232
129	You can create a time watcher by calling the "AnyEvent->timer" method	233	You can create a time watcher by calling the "AnyEvent->timer" method
130	with the following mandatory arguments:	234	with the following mandatory arguments:
131		235
132	"after" after how many seconds (fractions are supported) should the	236	"after" specifies after how many seconds (fractional values are
133	timer activate. "cb" the callback to invoke.	237	supported) the callback should be invoked. "cb" is the callback to
		238	invoke in that case.
134		239
135	The timer callback will be invoked at most once: if you want a repeating	240	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	241	presence is undefined and you cannot rely on them. Portable AnyEvent
137	and Glib).	242	callbacks cannot use arguments passed to time watcher callbacks.
138		243
139	Example:	244	The callback will normally be invoked only once. If you specify another
		245	parameter, "interval", as a strictly positive number (> 0), then the
		246	callback will be invoked regularly at that interval (in fractional
		247	seconds) after the first invocation. If "interval" is specified with a
		248	false value, then it is treated as if it were not specified at all.
140		249
		250	The callback will be rescheduled before invoking the callback, but no
		251	attempt is made to avoid timer drift in most backends, so the interval
		252	is only approximate.
		253
141	# fire an event after 7.7 seconds	254	Example: fire an event after 7.7 seconds.
		255
142	my $w = AnyEvent->timer (after => 7.7, cb => sub {	256	my $w = AnyEvent->timer (after => 7.7, cb => sub {
143	warn "timeout\n";	257	warn "timeout\n";
144	});	258	});
145		259
146	# to cancel the timer:	260	# to cancel the timer:
147	undef $w;	261	undef $w;
148		262
149	CONDITION WATCHERS	263	Example 2: fire an event after 0.5 seconds, then roughly every second.
		264
		265	my $w = AnyEvent->timer (after => 0.5, interval => 1, cb => sub {
		266	warn "timeout\n";
		267	};
		268
		269	TIMING ISSUES
		270	There are two ways to handle timers: based on real time (relative, "fire
		271	in 10 seconds") and based on wallclock time (absolute, "fire at 12
		272	o'clock").
		273
		274	While most event loops expect timers to specified in a relative way,
		275	they use absolute time internally. This makes a difference when your
		276	clock "jumps", for example, when ntp decides to set your clock backwards
		277	from the wrong date of 2014-01-01 to 2008-01-01, a watcher that is
		278	supposed to fire "after a second" might actually take six years to
		279	finally fire.
		280
		281	AnyEvent cannot compensate for this. The only event loop that is
		282	conscious of these issues is EV, which offers both relative (ev_timer,
		283	based on true relative time) and absolute (ev_periodic, based on
		284	wallclock time) timers.
		285
		286	AnyEvent always prefers relative timers, if available, matching the
		287	AnyEvent API.
		288
		289	AnyEvent has two additional methods that return the "current time":
		290
		291	AnyEvent->time
		292	This returns the "current wallclock time" as a fractional number of
		293	seconds since the Epoch (the same thing as "time" or
		294	"Time::HiRes::time" return, and the result is guaranteed to be
		295	compatible with those).
		296
		297	It progresses independently of any event loop processing, i.e. each
		298	call will check the system clock, which usually gets updated
		299	frequently.
		300
		301	AnyEvent->now
		302	This also returns the "current wallclock time", but unlike "time",
		303	above, this value might change only once per event loop iteration,
		304	depending on the event loop (most return the same time as "time",
		305	above). This is the time that AnyEvent's timers get scheduled
		306	against.
		307
		308	*In almost all cases (in all cases if you don't care), this is the
		309	function to call when you want to know the current time.*
		310
		311	This function is also often faster then "AnyEvent->time", and thus
		312	the preferred method if you want some timestamp (for example,
		313	AnyEvent::Handle uses this to update its activity timeouts).
		314
		315	The rest of this section is only of relevance if you try to be very
		316	exact with your timing; you can skip it without a bad conscience.
		317
		318	For a practical example of when these times differ, consider
		319	Event::Lib and EV and the following set-up:
		320
		321	The event loop is running and has just invoked one of your callbacks
		322	at time=500 (assume no other callbacks delay processing). In your
		323	callback, you wait a second by executing "sleep 1" (blocking the
		324	process for a second) and then (at time=501) you create a relative
		325	timer that fires after three seconds.
		326
		327	With Event::Lib, "AnyEvent->time" and "AnyEvent->now" will both
		328	return 501, because that is the current time, and the timer will be
		329	scheduled to fire at time=504 (501 + 3).
		330
		331	With EV, "AnyEvent->time" returns 501 (as that is the current time),
		332	but "AnyEvent->now" returns 500, as that is the time the last event
		333	processing phase started. With EV, your timer gets scheduled to run
		334	at time=503 (500 + 3).
		335
		336	In one sense, Event::Lib is more exact, as it uses the current time
		337	regardless of any delays introduced by event processing. However,
		338	most callbacks do not expect large delays in processing, so this
		339	causes a higher drift (and a lot more system calls to get the
		340	current time).
		341
		342	In another sense, EV is more exact, as your timer will be scheduled
		343	at the same time, regardless of how long event processing actually
		344	took.
		345
		346	In either case, if you care (and in most cases, you don't), then you
		347	can get whatever behaviour you want with any event loop, by taking
		348	the difference between "AnyEvent->time" and "AnyEvent->now" into
		349	account.
		350
		351	AnyEvent->now_update
		352	Some event loops (such as EV or AnyEvent::Impl::Perl) cache the
		353	current time for each loop iteration (see the discussion of
		354	AnyEvent->now, above).
		355
		356	When a callback runs for a long time (or when the process sleeps),
		357	then this "current" time will differ substantially from the real
		358	time, which might affect timers and time-outs.
		359
		360	When this is the case, you can call this method, which will update
		361	the event loop's idea of "current time".
		362
		363	A typical example would be a script in a web server (e.g.
		364	"mod_perl") - when mod_perl executes the script, then the event loop
		365	will have the wrong idea about the "current time" (being potentially
		366	far in the past, when the script ran the last time). In that case
		367	you should arrange a call to "AnyEvent->now_update" each time the
		368	web server process wakes up again (e.g. at the start of your script,
		369	or in a handler).
		370
		371	Note that updating the time might cause some events to be handled.
		372
		373	SIGNAL WATCHERS
		374	$w = AnyEvent->signal (signal => <uppercase_signal_name>, cb => <callback>);
		375
		376	You can watch for signals using a signal watcher, "signal" is the signal
		377	name in uppercase and without any "SIG" prefix, "cb" is the Perl
		378	callback to be invoked whenever a signal occurs.
		379
		380	Although the callback might get passed parameters, their value and
		381	presence is undefined and you cannot rely on them. Portable AnyEvent
		382	callbacks cannot use arguments passed to signal watcher callbacks.
		383
		384	Multiple signal occurrences can be clumped together into one callback
		385	invocation, and callback invocation will be synchronous. Synchronous
		386	means that it might take a while until the signal gets handled by the
		387	process, but it is guaranteed not to interrupt any other callbacks.
		388
		389	The main advantage of using these watchers is that you can share a
		390	signal between multiple watchers, and AnyEvent will ensure that signals
		391	will not interrupt your program at bad times.
		392
		393	This watcher might use %SIG (depending on the event loop used), so
		394	programs overwriting those signals directly will likely not work
		395	correctly.
		396
		397	Example: exit on SIGINT
		398
		399	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
		400
		401	Restart Behaviour
		402	While restart behaviour is up to the event loop implementation, most
		403	will not restart syscalls (that includes Async::Interrupt and AnyEvent's
		404	pure perl implementation).
		405
		406	Safe/Unsafe Signals
		407	Perl signals can be either "safe" (synchronous to opcode handling) or
		408	"unsafe" (asynchronous) - the former might get delayed indefinitely, the
		409	latter might corrupt your memory.
		410
		411	AnyEvent signal handlers are, in addition, synchronous to the event
		412	loop, i.e. they will not interrupt your running perl program but will
		413	only be called as part of the normal event handling (just like timer,
		414	I/O etc. callbacks, too).
		415
		416	Signal Races, Delays and Workarounds
		417	Many event loops (e.g. Glib, Tk, Qt, IO::Async) do not support attaching
		418	callbacks to signals in a generic way, which is a pity, as you cannot do
		419	race-free signal handling in perl, requiring C libraries for this.
		420	AnyEvent will try to do its best, which means in some cases, signals
		421	will be delayed. The maximum time a signal might be delayed is specified
		422	in $AnyEvent::MAX_SIGNAL_LATENCY (default: 10 seconds). This variable
		423	can be changed only before the first signal watcher is created, and
		424	should be left alone otherwise. This variable determines how often
		425	AnyEvent polls for signals (in case a wake-up was missed). Higher values
		426	will cause fewer spurious wake-ups, which is better for power and CPU
		427	saving.
		428
		429	All these problems can be avoided by installing the optional
		430	Async::Interrupt module, which works with most event loops. It will not
		431	work with inherently broken event loops such as Event or Event::Lib (and
		432	not with POE currently, as POE does its own workaround with one-second
		433	latency). For those, you just have to suffer the delays.
		434
		435	CHILD PROCESS WATCHERS
		436	$w = AnyEvent->child (pid => <process id>, cb => <callback>);
		437
		438	You can also watch for a child process exit and catch its exit status.
		439
		440	The child process is specified by the "pid" argument (on some backends,
		441	using 0 watches for any child process exit, on others this will croak).
		442	The watcher will be triggered only when the child process has finished
		443	and an exit status is available, not on any trace events
		444	(stopped/continued).
		445
		446	The callback will be called with the pid and exit status (as returned by
		447	waitpid), so unlike other watcher types, you can rely on child watcher
		448	callback arguments.
		449
		450	This watcher type works by installing a signal handler for "SIGCHLD",
		451	and since it cannot be shared, nothing else should use SIGCHLD or reap
		452	random child processes (waiting for specific child processes, e.g.
		453	inside "system", is just fine).
		454
		455	There is a slight catch to child watchers, however: you usually start
		456	them after the child process was created, and this means the process
		457	could have exited already (and no SIGCHLD will be sent anymore).
		458
		459	Not all event models handle this correctly (neither POE nor IO::Async
		460	do, see their AnyEvent::Impl manpages for details), but even for event
		461	models that do handle this correctly, they usually need to be loaded
		462	before the process exits (i.e. before you fork in the first place).
		463	AnyEvent's pure perl event loop handles all cases correctly regardless
		464	of when you start the watcher.
		465
		466	This means you cannot create a child watcher as the very first thing in
		467	an AnyEvent program, you have to create at least one watcher before
		468	you "fork" the child (alternatively, you can call "AnyEvent::detect").
		469
		470	As most event loops do not support waiting for child events, they will
		471	be emulated by AnyEvent in most cases, in which the latency and race
		472	problems mentioned in the description of signal watchers apply.
		473
		474	Example: fork a process and wait for it
		475
		476	my $done = AnyEvent->condvar;
		477
		478	my $pid = fork or exit 5;
		479
		480	my $w = AnyEvent->child (
		481	pid => $pid,
		482	cb => sub {
		483	my ($pid, $status) = @_;
		484	warn "pid $pid exited with status $status";
		485	$done->send;
		486	},
		487	);
		488
		489	# do something else, then wait for process exit
		490	$done->recv;
		491
		492	IDLE WATCHERS
		493	$w = AnyEvent->idle (cb => <callback>);
		494
		495	This will repeatedly invoke the callback after the process becomes idle,
		496	until either the watcher is destroyed or new events have been detected.
		497
		498	Idle watchers are useful when there is a need to do something, but it is
		499	not so important (or wise) to do it instantly. The callback will be
		500	invoked only when there is "nothing better to do", which is usually
		501	defined as "all outstanding events have been handled and no new events
		502	have been detected". That means that idle watchers ideally get invoked
		503	when the event loop has just polled for new events but none have been
		504	detected. Instead of blocking to wait for more events, the idle watchers
		505	will be invoked.
		506
		507	Unfortunately, most event loops do not really support idle watchers
		508	(only EV, Event and Glib do it in a usable fashion) - for the rest,
		509	AnyEvent will simply call the callback "from time to time".
		510
		511	Example: read lines from STDIN, but only process them when the program
		512	is otherwise idle:
		513
		514	my @lines; # read data
		515	my $idle_w;
		516	my $io_w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
		517	push @lines, scalar <STDIN>;
		518
		519	# start an idle watcher, if not already done
		520	$idle_w \|\|= AnyEvent->idle (cb => sub {
		521	# handle only one line, when there are lines left
		522	if (my $line = shift @lines) {
		523	print "handled when idle: $line";
		524	} else {
		525	# otherwise disable the idle watcher again
		526	undef $idle_w;
		527	}
		528	});
		529	});
		530
		531	CONDITION VARIABLES
		532	$cv = AnyEvent->condvar;
		533
		534	$cv->send (<list>);
		535	my @res = $cv->recv;
		536
		537	If you are familiar with some event loops you will know that all of them
		538	require you to run some blocking "loop", "run" or similar function that
		539	will actively watch for new events and call your callbacks.
		540
		541	AnyEvent is slightly different: it expects somebody else to run the
		542	event loop and will only block when necessary (usually when told by the
		543	user).
		544
		545	The tool to do that is called a "condition variable", so called because
		546	they represent a condition that must become true.
		547
		548	Now is probably a good time to look at the examples further below.
		549
150	Condition watchers can be created by calling the "AnyEvent->condvar"	550	Condition variables can be created by calling the "AnyEvent->condvar"
151	method without any arguments.	551	method, usually without arguments. The only argument pair allowed is
		552	"cb", which specifies a callback to be called when the condition
		553	variable becomes true, with the condition variable as the first argument
		554	(but not the results).
152		555
153	A condition watcher watches for a condition - precisely that the	556	After creation, the condition variable is "false" until it becomes
154	"->broadcast" method has been called.	557	"true" by calling the "send" method (or calling the condition variable
		558	as if it were a callback, read about the caveats in the description for
		559	the "->send" method).
155		560
		561	Since condition variables are the most complex part of the AnyEvent API,
		562	here are some different mental models of what they are - pick the ones
		563	you can connect to:
		564
		565	* Condition variables are like callbacks - you can call them (and pass
		566	them instead of callbacks). Unlike callbacks however, you can also
		567	wait for them to be called.
		568
		569	* Condition variables are signals - one side can emit or send them,
		570	the other side can wait for them, or install a handler that is
		571	called when the signal fires.
		572
		573	* Condition variables are like "Merge Points" - points in your program
		574	where you merge multiple independent results/control flows into one.
		575
		576	* Condition variables represent a transaction - functions that start
		577	some kind of transaction can return them, leaving the caller the
		578	choice between waiting in a blocking fashion, or setting a callback.
		579
		580	* Condition variables represent future values, or promises to deliver
		581	some result, long before the result is available.
		582
		583	Condition variables are very useful to signal that something has
		584	finished, for example, if you write a module that does asynchronous http
		585	requests, then a condition variable would be the ideal candidate to
		586	signal the availability of results. The user can either act when the
		587	callback is called or can synchronously "->recv" for the results.
		588
		589	You can also use them to simulate traditional event loops - for example,
		590	you can block your main program until an event occurs - for example, you
		591	could "->recv" in your main program until the user clicks the Quit
		592	button of your app, which would "->send" the "quit" event.
		593
156	Note that condition watchers recurse into the event loop - if you have	594	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.	595	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	596	lose. Therefore, condition variables are good to export to your caller,
159	should avoid making a blocking wait, at least in callbacks, as this	597	but you should avoid making a blocking wait yourself, at least in
160	usually asks for trouble.	598	callbacks, as this asks for trouble.
161		599
162	The watcher has only two methods:	600	Condition variables are represented by hash refs in perl, and the keys
		601	used by AnyEvent itself are all named "_ae_XXX" to make subclassing easy
		602	(it is often useful to build your own transaction class on top of
		603	AnyEvent). To subclass, use "AnyEvent::CondVar" as base class and call
		604	its "new" method in your own "new" method.
163		605
164	$cv->wait	606	There are two "sides" to a condition variable - the "producer side"
		607	which eventually calls "-> send", and the "consumer side", which waits
		608	for the send to occur.
		609
		610	Example: wait for a timer.
		611
		612	# condition: "wait till the timer is fired"
		613	my $timer_fired = AnyEvent->condvar;
		614
		615	# create the timer - we could wait for, say
		616	# a handle becomign ready, or even an
		617	# AnyEvent::HTTP request to finish, but
		618	# in this case, we simply use a timer:
		619	my $w = AnyEvent->timer (
		620	after => 1,
		621	cb => sub { $timer_fired->send },
		622	);
		623
		624	# this "blocks" (while handling events) till the callback
		625	# calls ->send
		626	$timer_fired->recv;
		627
		628	Example: wait for a timer, but take advantage of the fact that condition
		629	variables are also callable directly.
		630
		631	my $done = AnyEvent->condvar;
		632	my $delay = AnyEvent->timer (after => 5, cb => $done);
		633	$done->recv;
		634
		635	Example: Imagine an API that returns a condvar and doesn't support
		636	callbacks. This is how you make a synchronous call, for example from the
		637	main program:
		638
		639	use AnyEvent::CouchDB;
		640
		641	...
		642
		643	my @info = $couchdb->info->recv;
		644
		645	And this is how you would just set a callback to be called whenever the
		646	results are available:
		647
		648	$couchdb->info->cb (sub {
		649	my @info = $_[0]->recv;
		650	});
		651
		652	METHODS FOR PRODUCERS
		653	These methods should only be used by the producing side, i.e. the
		654	code/module that eventually sends the signal. Note that it is also the
		655	producer side which creates the condvar in most cases, but it isn't
		656	uncommon for the consumer to create it as well.
		657
		658	$cv->send (...)
		659	Flag the condition as ready - a running "->recv" and all further
		660	calls to "recv" will (eventually) return after this method has been
		661	called. If nobody is waiting the send will be remembered.
		662
		663	If a callback has been set on the condition variable, it is called
		664	immediately from within send.
		665
		666	Any arguments passed to the "send" call will be returned by all
		667	future "->recv" calls.
		668
		669	Condition variables are overloaded so one can call them directly (as
		670	if they were a code reference). Calling them directly is the same as
		671	calling "send".
		672
		673	$cv->croak ($error)
		674	Similar to send, but causes all calls to "->recv" to invoke
		675	"Carp::croak" with the given error message/object/scalar.
		676
		677	This can be used to signal any errors to the condition variable
		678	user/consumer. Doing it this way instead of calling "croak" directly
		679	delays the error detection, but has the overwhelming advantage that
		680	it diagnoses the error at the place where the result is expected,
		681	and not deep in some event callback with no connection to the actual
		682	code causing the problem.
		683
		684	$cv->begin ([group callback])
		685	$cv->end
		686	These two methods can be used to combine many transactions/events
		687	into one. For example, a function that pings many hosts in parallel
		688	might want to use a condition variable for the whole process.
		689
		690	Every call to "->begin" will increment a counter, and every call to
		691	"->end" will decrement it. If the counter reaches 0 in "->end", the
		692	(last) callback passed to "begin" will be executed, passing the
		693	condvar as first argument. That callback is supposed to call
		694	"->send", but that is not required. If no group callback was set,
		695	"send" will be called without any arguments.
		696
		697	You can think of "$cv->send" giving you an OR condition (one call
		698	sends), while "$cv->begin" and "$cv->end" giving you an AND
		699	condition (all "begin" calls must be "end"'ed before the condvar
		700	sends).
		701
		702	Let's start with a simple example: you have two I/O watchers (for
		703	example, STDOUT and STDERR for a program), and you want to wait for
		704	both streams to close before activating a condvar:
		705
		706	my $cv = AnyEvent->condvar;
		707
		708	$cv->begin; # first watcher
		709	my $w1 = AnyEvent->io (fh => $fh1, cb => sub {
		710	defined sysread $fh1, my $buf, 4096
		711	or $cv->end;
		712	});
		713
		714	$cv->begin; # second watcher
		715	my $w2 = AnyEvent->io (fh => $fh2, cb => sub {
		716	defined sysread $fh2, my $buf, 4096
		717	or $cv->end;
		718	});
		719
		720	$cv->recv;
		721
		722	This works because for every event source (EOF on file handle),
		723	there is one call to "begin", so the condvar waits for all calls to
		724	"end" before sending.
		725
		726	The ping example mentioned above is slightly more complicated, as
		727	the there are results to be passwd back, and the number of tasks
		728	that are begun can potentially be zero:
		729
		730	my $cv = AnyEvent->condvar;
		731
		732	my %result;
		733	$cv->begin (sub { shift->send (\%result) });
		734
		735	for my $host (@list_of_hosts) {
		736	$cv->begin;
		737	ping_host_then_call_callback $host, sub {
		738	$result{$host} = ...;
		739	$cv->end;
		740	};
		741	}
		742
		743	$cv->end;
		744
		745	This code fragment supposedly pings a number of hosts and calls
		746	"send" after results for all then have have been gathered - in any
		747	order. To achieve this, the code issues a call to "begin" when it
		748	starts each ping request and calls "end" when it has received some
		749	result for it. Since "begin" and "end" only maintain a counter, the
		750	order in which results arrive is not relevant.
		751
		752	There is an additional bracketing call to "begin" and "end" outside
		753	the loop, which serves two important purposes: first, it sets the
		754	callback to be called once the counter reaches 0, and second, it
		755	ensures that "send" is called even when "no" hosts are being pinged
		756	(the loop doesn't execute once).
		757
		758	This is the general pattern when you "fan out" into multiple (but
		759	potentially zero) subrequests: use an outer "begin"/"end" pair to
		760	set the callback and ensure "end" is called at least once, and then,
		761	for each subrequest you start, call "begin" and for each subrequest
		762	you finish, call "end".
		763
		764	METHODS FOR CONSUMERS
		765	These methods should only be used by the consuming side, i.e. the code
		766	awaits the condition.
		767
		768	$cv->recv
165	Wait (blocking if necessary) until the "->broadcast" method has been	769	Wait (blocking if necessary) until the "->send" or "->croak" methods
166	called on c<$cv>, while servicing other watchers normally.	770	have been called on $cv, while servicing other watchers normally.
167		771
168	You can only wait once on a condition - additional calls will return	772	You can only wait once on a condition - additional calls are valid
169	immediately.	773	but will return immediately.
		774
		775	If an error condition has been set by calling "->croak", then this
		776	function will call "croak".
		777
		778	In list context, all parameters passed to "send" will be returned,
		779	in scalar context only the first one will be returned.
		780
		781	Note that doing a blocking wait in a callback is not supported by
		782	any event loop, that is, recursive invocation of a blocking "->recv"
		783	is not allowed, and the "recv" call will "croak" if such a condition
		784	is detected. This condition can be slightly loosened by using
		785	Coro::AnyEvent, which allows you to do a blocking "->recv" from any
		786	thread that doesn't run the event loop itself.
170		787
171	Not all event models support a blocking wait - some die in that case	788	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	789	(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*,	790	using this from a module, never require a blocking wait*. Instead,
174	but let the caller decide wether the call will block or not (for	791	let the caller decide whether the call will block or not (for
175	example, by coupling condition variables with some kind of request	792	example, by coupling condition variables with some kind of request
176	results and supporting callbacks so the caller knows that getting	793	results and supporting callbacks so the caller knows that getting
177	the result will not block, while still suppporting blocking waits if	794	the result will not block, while still supporting blocking waits if
178	the caller so desires).	795	the caller so desires).
179		796
180	Another reason never to "->wait" in a module is that you cannot	797	You can ensure that "->recv" never blocks by setting a callback and
181	sensibly have two "->wait"'s in parallel, as that would require	798	only calling "->recv" from within that callback (or at a later
182	multiple interpreters or coroutines/threads, none of which	799	time). This will work even when the event loop does not support
183	"AnyEvent" can supply (the coroutine-aware backends "Coro::EV" and	800	blocking waits otherwise.
184	"Coro::Event" explicitly support concurrent "->wait"'s from
185	different coroutines, however).
186		801
187	$cv->broadcast	802	$bool = $cv->ready
188	Flag the condition as ready - a running "->wait" and all further	803	Returns true when the condition is "true", i.e. whether "send" or
189	calls to "wait" will return after this method has been called. If	804	"croak" have been called.
190	nobody is waiting the broadcast will be remembered..
191		805
192	Example:	806	$cb = $cv->cb ($cb->($cv))
		807	This is a mutator function that returns the callback set and
		808	optionally replaces it before doing so.
193		809
194	# wait till the result is ready	810	The callback will be called when the condition becomes "true", i.e.
195	my $result_ready = AnyEvent->condvar;	811	when "send" or "croak" are called, with the only argument being the
		812	condition variable itself. If the condition is already true, the
		813	callback is called immediately when it is set. Calling "recv" inside
		814	the callback or at any later time is guaranteed not to block.
196		815
197	# do something such as adding a timer	816	SUPPORTED EVENT LOOPS/BACKENDS
198	# or socket watcher the calls $result_ready->broadcast	817	The available backend classes are (every class has its own manpage):
199	# when the "result" is ready.
200		818
201	$result_ready->wait;	819	Backends that are autoprobed when no other event loop can be found.
		820	EV is the preferred backend when no other event loop seems to be in
		821	use. If EV is not installed, then AnyEvent will fall back to its own
		822	pure-perl implementation, which is available everywhere as it comes
		823	with AnyEvent itself.
202		824
203	SIGNAL WATCHERS	825	AnyEvent::Impl::EV based on EV (interface to libev, best choice).
204	You can listen for signals using a signal watcher, "signal" is the	826	AnyEvent::Impl::Perl pure-perl implementation, fast and portable.
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		827
209	These watchers might use %SIG, so programs overwriting those signals	828	Backends that are transparently being picked up when they are used.
210	directly will likely not work correctly.	829	These will be used if they are already loaded when the first watcher
		830	is created, in which case it is assumed that the application is
		831	using them. This means that AnyEvent will automatically pick the
		832	right backend when the main program loads an event module before
		833	anything starts to create watchers. Nothing special needs to be done
		834	by the main program.
211		835
212	Example: exit on SIGINT	836	AnyEvent::Impl::Event based on Event, very stable, few glitches.
		837	AnyEvent::Impl::Glib based on Glib, slow but very stable.
		838	AnyEvent::Impl::Tk based on Tk, very broken.
		839	AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
		840	AnyEvent::Impl::POE based on POE, very slow, some limitations.
		841	AnyEvent::Impl::Irssi used when running within irssi.
		842	AnyEvent::Impl::IOAsync based on IO::Async.
		843	AnyEvent::Impl::Cocoa based on Cocoa::EventLoop.
213		844
214	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });	845	Backends with special needs.
		846	Qt requires the Qt::Application to be instantiated first, but will
		847	otherwise be picked up automatically. As long as the main program
		848	instantiates the application before any AnyEvent watchers are
		849	created, everything should just work.
215		850
216	CHILD PROCESS WATCHERS	851	AnyEvent::Impl::Qt based on Qt.
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		852
223	Example: wait for pid 1333	853	Event loops that are indirectly supported via other backends.
		854	Some event loops can be supported via other modules:
224		855
225	my $w = AnyEvent->child (pid => 1333, cb => sub { warn "exit status $?" });	856	There is no direct support for WxWidgets (Wx) or Prima.
226		857
227	GLOBALS	858	WxWidgets has no support for watching file handles. However, you can
		859	use WxWidgets through the POE adaptor, as POE has a Wx backend that
		860	simply polls 20 times per second, which was considered to be too
		861	horrible to even consider for AnyEvent.
		862
		863	Prima is not supported as nobody seems to be using it, but it has a
		864	POE backend, so it can be supported through POE.
		865
		866	AnyEvent knows about both Prima and Wx, however, and will try to
		867	load POE when detecting them, in the hope that POE will pick them
		868	up, in which case everything will be automatic.
		869
		870	GLOBAL VARIABLES AND FUNCTIONS
		871	These are not normally required to use AnyEvent, but can be useful to
		872	write AnyEvent extension modules.
		873
228	$AnyEvent::MODEL	874	$AnyEvent::MODEL
229	Contains "undef" until the first watcher is being created. Then it	875	Contains "undef" until the first watcher is being created, before
		876	the backend has been autodetected.
		877
230	contains the event model that is being used, which is the name of	878	Afterwards it contains the event model that is being used, which is
231	the Perl class implementing the model. This class is usually one of	879	the name of the Perl class implementing the model. This class is
232	the "AnyEvent::Impl:xxx" modules, but can be any other class in the	880	usually one of the "AnyEvent::Impl::xxx" modules, but can be any
233	case AnyEvent has been extended at runtime (e.g. in rxvt-unicode).	881	other class in the case AnyEvent has been extended at runtime (e.g.
234		882	in rxvt-unicode it will be "urxvt::anyevent").
235	The known classes so far are:
236
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).
240	AnyEvent::Impl::Event based on Event, also second best choice :)
241	AnyEvent::Impl::Glib based on Glib, third-best choice.
242	AnyEvent::Impl::Tk based on Tk, very bad choice.
243	AnyEvent::Impl::Perl pure-perl implementation, inefficient but portable.
244		883
245	AnyEvent::detect	884	AnyEvent::detect
246	Returns $AnyEvent::MODEL, forcing autodetection of the event model	885	Returns $AnyEvent::MODEL, forcing autodetection of the event model
247	if necessary. You should only call this function right before you	886	if necessary. You should only call this function right before you
248	would have created an AnyEvent watcher anyway, that is, very late at	887	would have created an AnyEvent watcher anyway, that is, as late as
249	runtime.	888	possible at runtime, and not e.g. during initialisation of your
		889	module.
		890
		891	If you need to do some initialisation before AnyEvent watchers are
		892	created, use "post_detect".
		893
		894	$guard = AnyEvent::post_detect { BLOCK }
		895	Arranges for the code block to be executed as soon as the event
		896	model is autodetected (or immediately if that has already happened).
		897
		898	The block will be executed after the actual backend has been
		899	detected ($AnyEvent::MODEL is set), but before any watchers have
		900	been created, so it is possible to e.g. patch @AnyEvent::ISA or do
		901	other initialisations - see the sources of AnyEvent::Strict or
		902	AnyEvent::AIO to see how this is used.
		903
		904	The most common usage is to create some global watchers, without
		905	forcing event module detection too early, for example, AnyEvent::AIO
		906	creates and installs the global IO::AIO watcher in a "post_detect"
		907	block to avoid autodetecting the event module at load time.
		908
		909	If called in scalar or list context, then it creates and returns an
		910	object that automatically removes the callback again when it is
		911	destroyed (or "undef" when the hook was immediately executed). See
		912	AnyEvent::AIO for a case where this is useful.
		913
		914	Example: Create a watcher for the IO::AIO module and store it in
		915	$WATCHER, but do so only do so after the event loop is initialised.
		916
		917	our WATCHER;
		918
		919	my $guard = AnyEvent::post_detect {
		920	$WATCHER = AnyEvent->io (fh => IO::AIO::poll_fileno, poll => 'r', cb => \&IO::AIO::poll_cb);
		921	};
		922
		923	# the \|\|= is important in case post_detect immediately runs the block,
		924	# as to not clobber the newly-created watcher. assigning both watcher and
		925	# post_detect guard to the same variable has the advantage of users being
		926	# able to just C<undef $WATCHER> if the watcher causes them grief.
		927
		928	$WATCHER \|\|= $guard;
		929
		930	@AnyEvent::post_detect
		931	If there are any code references in this array (you can "push" to it
		932	before or after loading AnyEvent), then they will be called directly
		933	after the event loop has been chosen.
		934
		935	You should check $AnyEvent::MODEL before adding to this array,
		936	though: if it is defined then the event loop has already been
		937	detected, and the array will be ignored.
		938
		939	Best use "AnyEvent::post_detect { BLOCK }" when your application
		940	allows it, as it takes care of these details.
		941
		942	This variable is mainly useful for modules that can do something
		943	useful when AnyEvent is used and thus want to know when it is
		944	initialised, but do not need to even load it by default. This array
		945	provides the means to hook into AnyEvent passively, without loading
		946	it.
		947
		948	Example: To load Coro::AnyEvent whenever Coro and AnyEvent are used
		949	together, you could put this into Coro (this is the actual code used
		950	by Coro to accomplish this):
		951
		952	if (defined $AnyEvent::MODEL) {
		953	# AnyEvent already initialised, so load Coro::AnyEvent
		954	require Coro::AnyEvent;
		955	} else {
		956	# AnyEvent not yet initialised, so make sure to load Coro::AnyEvent
		957	# as soon as it is
		958	push @AnyEvent::post_detect, sub { require Coro::AnyEvent };
		959	}
250		960
251	WHAT TO DO IN A MODULE	961	WHAT TO DO IN A MODULE
252	As a module author, you should "use AnyEvent" and call AnyEvent methods	962	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.	963	freely, but you should not load a specific event module or rely on it.
254		964
255	Be careful when you create watchers in the module body - Anyevent will	965	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,	966	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	967	so by calling AnyEvent in your module body you force the user of your
258	module to load the event module first.	968	module to load the event module first.
259		969
		970	Never call "->recv" on a condition variable unless you know that the
		971	"->send" method has been called on it already. This is because it will
		972	stall the whole program, and the whole point of using events is to stay
		973	interactive.
		974
		975	It is fine, however, to call "->recv" when the user of your module
		976	requests it (i.e. if you create a http request object ad have a method
		977	called "results" that returns the results, it may call "->recv" freely,
		978	as the user of your module knows what she is doing. Always).
		979
260	WHAT TO DO IN THE MAIN PROGRAM	980	WHAT TO DO IN THE MAIN PROGRAM
261	There will always be a single main program - the only place that should	981	There will always be a single main program - the only place that should
262	dictate which event model to use.	982	dictate which event model to use.
263		983
264	If it doesn't care, it can just "use AnyEvent" and use it itself, or not	984	If the program is not event-based, it need not do anything special, even
265	do anything special and let AnyEvent decide which implementation to	985	when it depends on a module that uses an AnyEvent. If the program itself
266	chose.	986	uses AnyEvent, but does not care which event loop is used, all it needs
		987	to do is "use AnyEvent". In either case, AnyEvent will choose the best
		988	available loop implementation.
267		989
268	If the main program relies on a specific event model (for example, in	990	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	991	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,	992	event module before loading AnyEvent or any module that uses it:
271	generally, as early as possible. The reason is that modules might create	993	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	994	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	995	will decide on the event model to use as soon as it creates watchers,
274	wrong one unless you load the correct one yourself.	996	and it might choose the wrong one unless you load the correct one
		997	yourself.
275		998
276	You can chose to use a rather inefficient pure-perl implementation by	999	You can chose to use a pure-perl implementation by loading the
277	loading the "AnyEvent::Impl::Perl" module, but letting AnyEvent chose is	1000	"AnyEvent::Impl::Perl" module, which gives you similar behaviour
278	generally better.	1001	everywhere, but letting AnyEvent chose the model is generally better.
		1002
		1003	MAINLOOP EMULATION
		1004	Sometimes (often for short test scripts, or even standalone programs who
		1005	only want to use AnyEvent), you do not want to run a specific event
		1006	loop.
		1007
		1008	In that case, you can use a condition variable like this:
		1009
		1010	AnyEvent->condvar->recv;
		1011
		1012	This has the effect of entering the event loop and looping forever.
		1013
		1014	Note that usually your program has some exit condition, in which case it
		1015	is better to use the "traditional" approach of storing a condition
		1016	variable somewhere, waiting for it, and sending it when the program
		1017	should exit cleanly.
		1018
		1019	OTHER MODULES
		1020	The following is a non-exhaustive list of additional modules that use
		1021	AnyEvent as a client and can therefore be mixed easily with other
		1022	AnyEvent modules and other event loops in the same program. Some of the
		1023	modules come as part of AnyEvent, the others are available via CPAN.
		1024
		1025	AnyEvent::Util
		1026	Contains various utility functions that replace often-used blocking
		1027	functions such as "inet_aton" with event/callback-based versions.
		1028
		1029	AnyEvent::Socket
		1030	Provides various utility functions for (internet protocol) sockets,
		1031	addresses and name resolution. Also functions to create non-blocking
		1032	tcp connections or tcp servers, with IPv6 and SRV record support and
		1033	more.
		1034
		1035	AnyEvent::Handle
		1036	Provide read and write buffers, manages watchers for reads and
		1037	writes, supports raw and formatted I/O, I/O queued and fully
		1038	transparent and non-blocking SSL/TLS (via AnyEvent::TLS).
		1039
		1040	AnyEvent::DNS
		1041	Provides rich asynchronous DNS resolver capabilities.
		1042
		1043	AnyEvent::HTTP, AnyEvent::IRC, AnyEvent::XMPP, AnyEvent::GPSD,
		1044	AnyEvent::IGS, AnyEvent::FCP
		1045	Implement event-based interfaces to the protocols of the same name
		1046	(for the curious, IGS is the International Go Server and FCP is the
		1047	Freenet Client Protocol).
		1048
		1049	AnyEvent::Handle::UDP
		1050	Here be danger!
		1051
		1052	As Pauli would put it, "Not only is it not right, it's not even
		1053	wrong!" - there are so many things wrong with AnyEvent::Handle::UDP,
		1054	most notably its use of a stream-based API with a protocol that
		1055	isn't streamable, that the only way to improve it is to delete it.
		1056
		1057	It features data corruption (but typically only under load) and
		1058	general confusion. On top, the author is not only clueless about UDP
		1059	but also fact-resistant - some gems of his understanding: "connect
		1060	doesn't work with UDP", "UDP packets are not IP packets", "UDP only
		1061	has datagrams, not packets", "I don't need to implement proper error
		1062	checking as UDP doesn't support error checking" and so on - he
		1063	doesn't even understand what's wrong with his module when it is
		1064	explained to him.
		1065
		1066	AnyEvent::DBI
		1067	Executes DBI requests asynchronously in a proxy process for you,
		1068	notifying you in an event-based way when the operation is finished.
		1069
		1070	AnyEvent::AIO
		1071	Truly asynchronous (as opposed to non-blocking) I/O, should be in
		1072	the toolbox of every event programmer. AnyEvent::AIO transparently
		1073	fuses IO::AIO and AnyEvent together, giving AnyEvent access to
		1074	event-based file I/O, and much more.
		1075
		1076	AnyEvent::HTTPD
		1077	A simple embedded webserver.
		1078
		1079	AnyEvent::FastPing
		1080	The fastest ping in the west.
		1081
		1082	Coro
		1083	Has special support for AnyEvent via Coro::AnyEvent.
		1084
		1085	SIMPLIFIED AE API
		1086	Starting with version 5.0, AnyEvent officially supports a second, much
		1087	simpler, API that is designed to reduce the calling, typing and memory
		1088	overhead by using function call syntax and a fixed number of parameters.
		1089
		1090	See the AE manpage for details.
		1091
		1092	ERROR AND EXCEPTION HANDLING
		1093	In general, AnyEvent does not do any error handling - it relies on the
		1094	caller to do that if required. The AnyEvent::Strict module (see also the
		1095	"PERL_ANYEVENT_STRICT" environment variable, below) provides strict
		1096	checking of all AnyEvent methods, however, which is highly useful during
		1097	development.
		1098
		1099	As for exception handling (i.e. runtime errors and exceptions thrown
		1100	while executing a callback), this is not only highly event-loop
		1101	specific, but also not in any way wrapped by this module, as this is the
		1102	job of the main program.
		1103
		1104	The pure perl event loop simply re-throws the exception (usually within
		1105	"condvar->recv"), the Event and EV modules call "$Event/EV::DIED->()",
		1106	Glib uses "install_exception_handler" and so on.
		1107
		1108	ENVIRONMENT VARIABLES
		1109	The following environment variables are used by this module or its
		1110	submodules.
		1111
		1112	Note that AnyEvent will remove all environment variables starting with
		1113	"PERL_ANYEVENT_" from %ENV when it is loaded while taint mode is
		1114	enabled.
		1115
		1116	"PERL_ANYEVENT_VERBOSE"
		1117	By default, AnyEvent will be completely silent except in fatal
		1118	conditions. You can set this environment variable to make AnyEvent
		1119	more talkative.
		1120
		1121	When set to 1 or higher, causes AnyEvent to warn about unexpected
		1122	conditions, such as not being able to load the event model specified
		1123	by "PERL_ANYEVENT_MODEL".
		1124
		1125	When set to 2 or higher, cause AnyEvent to report to STDERR which
		1126	event model it chooses.
		1127
		1128	When set to 8 or higher, then AnyEvent will report extra information
		1129	on which optional modules it loads and how it implements certain
		1130	features.
		1131
		1132	"PERL_ANYEVENT_STRICT"
		1133	AnyEvent does not do much argument checking by default, as thorough
		1134	argument checking is very costly. Setting this variable to a true
		1135	value will cause AnyEvent to load "AnyEvent::Strict" and then to
		1136	thoroughly check the arguments passed to most method calls. If it
		1137	finds any problems, it will croak.
		1138
		1139	In other words, enables "strict" mode.
		1140
		1141	Unlike "use strict" (or its modern cousin, "use common::sense", it
		1142	is definitely recommended to keep it off in production. Keeping
		1143	"PERL_ANYEVENT_STRICT=1" in your environment while developing
		1144	programs can be very useful, however.
		1145
		1146	"PERL_ANYEVENT_MODEL"
		1147	This can be used to specify the event model to be used by AnyEvent,
		1148	before auto detection and -probing kicks in. It must be a string
		1149	consisting entirely of ASCII letters. The string "AnyEvent::Impl::"
		1150	gets prepended and the resulting module name is loaded and if the
		1151	load was successful, used as event model. If it fails to load
		1152	AnyEvent will proceed with auto detection and -probing.
		1153
		1154	This functionality might change in future versions.
		1155
		1156	For example, to force the pure perl model (AnyEvent::Impl::Perl) you
		1157	could start your program like this:
		1158
		1159	PERL_ANYEVENT_MODEL=Perl perl ...
		1160
		1161	"PERL_ANYEVENT_PROTOCOLS"
		1162	Used by both AnyEvent::DNS and AnyEvent::Socket to determine
		1163	preferences for IPv4 or IPv6. The default is unspecified (and might
		1164	change, or be the result of auto probing).
		1165
		1166	Must be set to a comma-separated list of protocols or address
		1167	families, current supported: "ipv4" and "ipv6". Only protocols
		1168	mentioned will be used, and preference will be given to protocols
		1169	mentioned earlier in the list.
		1170
		1171	This variable can effectively be used for denial-of-service attacks
		1172	against local programs (e.g. when setuid), although the impact is
		1173	likely small, as the program has to handle conenction and other
		1174	failures anyways.
		1175
		1176	Examples: "PERL_ANYEVENT_PROTOCOLS=ipv4,ipv6" - prefer IPv4 over
		1177	IPv6, but support both and try to use both.
		1178	"PERL_ANYEVENT_PROTOCOLS=ipv4" - only support IPv4, never try to
		1179	resolve or contact IPv6 addresses.
		1180	"PERL_ANYEVENT_PROTOCOLS=ipv6,ipv4" support either IPv4 or IPv6, but
		1181	prefer IPv6 over IPv4.
		1182
		1183	"PERL_ANYEVENT_EDNS0"
		1184	Used by AnyEvent::DNS to decide whether to use the EDNS0 extension
		1185	for DNS. This extension is generally useful to reduce DNS traffic,
		1186	but some (broken) firewalls drop such DNS packets, which is why it
		1187	is off by default.
		1188
		1189	Setting this variable to 1 will cause AnyEvent::DNS to announce
		1190	EDNS0 in its DNS requests.
		1191
		1192	"PERL_ANYEVENT_MAX_FORKS"
		1193	The maximum number of child processes that
		1194	"AnyEvent::Util::fork_call" will create in parallel.
		1195
		1196	"PERL_ANYEVENT_MAX_OUTSTANDING_DNS"
		1197	The default value for the "max_outstanding" parameter for the
		1198	default DNS resolver - this is the maximum number of parallel DNS
		1199	requests that are sent to the DNS server.
		1200
		1201	"PERL_ANYEVENT_RESOLV_CONF"
		1202	The file to use instead of /etc/resolv.conf (or OS-specific
		1203	configuration) in the default resolver. When set to the empty
		1204	string, no default config will be used.
		1205
		1206	"PERL_ANYEVENT_CA_FILE", "PERL_ANYEVENT_CA_PATH".
		1207	When neither "ca_file" nor "ca_path" was specified during
		1208	AnyEvent::TLS context creation, and either of these environment
		1209	variables exist, they will be used to specify CA certificate
		1210	locations instead of a system-dependent default.
		1211
		1212	"PERL_ANYEVENT_AVOID_GUARD" and "PERL_ANYEVENT_AVOID_ASYNC_INTERRUPT"
		1213	When these are set to 1, then the respective modules are not loaded.
		1214	Mostly good for testing AnyEvent itself.
279		1215
280	SUPPLYING YOUR OWN EVENT MODEL INTERFACE	1216	SUPPLYING YOUR OWN EVENT MODEL INTERFACE
		1217	This is an advanced topic that you do not normally need to use AnyEvent
		1218	in a module. This section is only of use to event loop authors who want
		1219	to provide AnyEvent compatibility.
		1220
281	If you need to support another event library which isn't directly	1221	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	1222	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	1223	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	1224	event module and the package name of the interface to use onto
285	@AnyEvent::REGISTRY. You can do that before and even without loading	1225	@AnyEvent::REGISTRY. You can do that before and even without loading
286	AnyEvent.	1226	AnyEvent, so it is reasonably cheap.
287		1227
288	Example:	1228	Example:
289		1229
290	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];	1230	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
291		1231
292	This tells AnyEvent to (literally) use the "urxvt::anyevent::"	1232	This tells AnyEvent to (literally) use the "urxvt::anyevent::"
293	package/class when it finds the "urxvt" package/module is loaded. When	1233	package/class when it finds the "urxvt" package/module is already
		1234	loaded.
		1235
294	AnyEvent is loaded and asked to find a suitable event model, it will	1236	When AnyEvent is loaded and asked to find a suitable event model, it
295	first check for the presence of urxvt.	1237	will first check for the presence of urxvt by trying to "use" the
		1238	"urxvt::anyevent" module.
296		1239
297	The class should provide implementations for all watcher types (see	1240	The class should provide implementations for all watcher types. See
298	AnyEvent::Impl::Event (source code), AnyEvent::Impl::Glib (Source code)	1241	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	1242	so on for actual examples. Use "perldoc -m AnyEvent::Impl::Glib" to see
300	see the sources).	1243	the sources.
301		1244
		1245	If you don't provide "signal" and "child" watchers than AnyEvent will
		1246	provide suitable (hopefully) replacements.
		1247
302	The above isn't fictitious, the rxvt-unicode (a.k.a. urxvt) uses the	1248	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	1249	terminal emulator uses the above line as-is. An interface isn't included
304	doesn't make sense outside the embedded interpreter inside	1250	in AnyEvent because it doesn't make sense outside the embedded
305	rxvt-unicode, and it is updated and maintained as part of the	1251	interpreter inside rxvt-unicode, and it is updated and maintained as
306	rxvt-unicode distribution.	1252	part of the rxvt-unicode distribution.
307		1253
308	rxvt-unicode also cheats a bit by not providing blocking access to	1254	rxvt-unicode also cheats a bit by not providing blocking access to
309	condition variables: code blocking while waiting for a condition will	1255	condition variables: code blocking while waiting for a condition will
310	"die". This still works with most modules/usages, and blocking calls	1256	"die". This still works with most modules/usages, and blocking calls
311	must not be in an interactive application, so it makes sense.	1257	must not be done in an interactive application, so it makes sense.
312		1258
313	ENVIRONMENT VARIABLES	1259	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	1260	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	1261	timer to display a message once per second, and a condition variable to
322	program when the user enters quit:	1262	quit the program when the user enters quit:
323		1263
324	use AnyEvent;	1264	use AnyEvent;
325		1265
326	my $cv = AnyEvent->condvar;	1266	my $cv = AnyEvent->condvar;
327		1267
328	my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {	1268	my $io_watcher = AnyEvent->io (
		1269	fh => \*STDIN,
		1270	poll => 'r',
		1271	cb => sub {
329	warn "io event <$_[0]>\n"; # will always output <r>	1272	warn "io event <$_[0]>\n"; # will always output <r>
330	chomp (my $input = <STDIN>); # read a line	1273	chomp (my $input = <STDIN>); # read a line
331	warn "read: $input\n"; # output what has been read	1274	warn "read: $input\n"; # output what has been read
332	$cv->broadcast if $input =~ /^q/i; # quit program if /^q/i	1275	$cv->send if $input =~ /^q/i; # quit program if /^q/i
		1276	},
		1277	);
		1278
		1279	my $time_watcher = AnyEvent->timer (after => 1, interval => 1, cb => sub {
		1280	warn "timeout\n"; # print 'timeout' at most every second
333	});	1281	});
334		1282
335	my $time_watcher; # can only be used once
336
337	sub new_timer {
338	$timer = AnyEvent->timer (after => 1, cb => sub {
339	warn "timeout\n"; # print 'timeout' about every second
340	&new_timer; # and restart the time
341	});
342	}
343
344	new_timer; # create first timer
345
346	$cv->wait; # wait until user enters /^q/i	1283	$cv->recv; # wait until user enters /^q/i
347		1284
348	REAL-WORLD EXAMPLE	1285	REAL-WORLD EXAMPLE
349	Consider the Net::FCP module. It features (among others) the following	1286	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:	1287	API calls, which are to freenet what HTTP GET requests are to http:
351		1288
400	syswrite $txn->{fh}, $txn->{request}	1337	syswrite $txn->{fh}, $txn->{request}
401	or die "connection or write error";	1338	or die "connection or write error";
402	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });	1339	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
403		1340
404	Again, "fh_ready_r" waits till all data has arrived, and then stores the	1341	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:	1342	result and signals any possible waiters that the request has finished:
406		1343
407	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};	1344	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
408		1345
409	if (end-of-file or data complete) {	1346	if (end-of-file or data complete) {
410	$txn->{result} = $txn->{buf};	1347	$txn->{result} = $txn->{buf};
411	$txn->{finished}->broadcast;	1348	$txn->{finished}->send;
412	$txb->{cb}->($txn) of $txn->{cb}; # also call callback	1349	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
413	}	1350	}
414		1351
415	The "result" method, finally, just waits for the finished signal (if the	1352	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	1353	request was already finished, it doesn't wait, of course, and returns
417	the data:	1354	the data:
418		1355
419	$txn->{finished}->wait;	1356	$txn->{finished}->recv;
420	return $txn->{result};	1357	return $txn->{result};
421		1358
422	The actual code goes further and collects all errors ("die"s,	1359	The actual code goes further and collects all errors ("die"s,
423	exceptions) that occured during request processing. The "result" method	1360	exceptions) that occurred during request processing. The "result" method
424	detects wether an exception as thrown (it is stored inside the $txn	1361	detects whether an exception as thrown (it is stored inside the $txn
425	object) and just throws the exception, which means connection errors and	1362	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,	1363	other problems get reported to the code that tries to use the result,
427	not in a random callback.	1364	not in a random callback.
428		1365
429	All of this enables the following usage styles:	1366	All of this enables the following usage styles:
430		1367
431	1. Blocking:	1368	1. Blocking:
…		…
459		1396
460	my $quit = AnyEvent->condvar;	1397	my $quit = AnyEvent->condvar;
461		1398
462	$fcp->txn_client_get ($url)->cb (sub {	1399	$fcp->txn_client_get ($url)->cb (sub {
463	...	1400	...
464	$quit->broadcast;	1401	$quit->send;
465	});	1402	});
466		1403
467	$quit->wait;	1404	$quit->recv;
		1405
		1406	BENCHMARKS
		1407	To give you an idea of the performance and overheads that AnyEvent adds
		1408	over the event loops themselves and to give you an impression of the
		1409	speed of various event loops I prepared some benchmarks.
		1410
		1411	BENCHMARKING ANYEVENT OVERHEAD
		1412	Here is a benchmark of various supported event models used natively and
		1413	through AnyEvent. The benchmark creates a lot of timers (with a zero
		1414	timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
		1415	which it is), lets them fire exactly once and destroys them again.
		1416
		1417	Source code for this benchmark is found as eg/bench in the AnyEvent
		1418	distribution. It uses the AE interface, which makes a real difference
		1419	for the EV and Perl backends only.
		1420
		1421	Explanation of the columns
		1422	watcher is the number of event watchers created/destroyed. Since
		1423	different event models feature vastly different performances, each event
		1424	loop was given a number of watchers so that overall runtime is
		1425	acceptable and similar between tested event loop (and keep them from
		1426	crashing): Glib would probably take thousands of years if asked to
		1427	process the same number of watchers as EV in this benchmark.
		1428
		1429	bytes is the number of bytes (as measured by the resident set size,
		1430	RSS) consumed by each watcher. This method of measuring captures both C
		1431	and Perl-based overheads.
		1432
		1433	create is the time, in microseconds (millionths of seconds), that it
		1434	takes to create a single watcher. The callback is a closure shared
		1435	between all watchers, to avoid adding memory overhead. That means
		1436	closure creation and memory usage is not included in the figures.
		1437
		1438	invoke is the time, in microseconds, used to invoke a simple callback.
		1439	The callback simply counts down a Perl variable and after it was invoked
		1440	"watcher" times, it would "->send" a condvar once to signal the end of
		1441	this phase.
		1442
		1443	destroy is the time, in microseconds, that it takes to destroy a
		1444	single watcher.
		1445
		1446	Results
		1447	name watchers bytes create invoke destroy comment
		1448	EV/EV 100000 223 0.47 0.43 0.27 EV native interface
		1449	EV/Any 100000 223 0.48 0.42 0.26 EV + AnyEvent watchers
		1450	Coro::EV/Any 100000 223 0.47 0.42 0.26 coroutines + Coro::Signal
		1451	Perl/Any 100000 431 2.70 0.74 0.92 pure perl implementation
		1452	Event/Event 16000 516 31.16 31.84 0.82 Event native interface
		1453	Event/Any 16000 1203 42.61 34.79 1.80 Event + AnyEvent watchers
		1454	IOAsync/Any 16000 1911 41.92 27.45 16.81 via IO::Async::Loop::IO_Poll
		1455	IOAsync/Any 16000 1726 40.69 26.37 15.25 via IO::Async::Loop::Epoll
		1456	Glib/Any 16000 1118 89.00 12.57 51.17 quadratic behaviour
		1457	Tk/Any 2000 1346 20.96 10.75 8.00 SEGV with >> 2000 watchers
		1458	POE/Any 2000 6951 108.97 795.32 14.24 via POE::Loop::Event
		1459	POE/Any 2000 6648 94.79 774.40 575.51 via POE::Loop::Select
		1460
		1461	Discussion
		1462	The benchmark does not measure scalability of the event loop very
		1463	well. For example, a select-based event loop (such as the pure perl one)
		1464	can never compete with an event loop that uses epoll when the number of
		1465	file descriptors grows high. In this benchmark, all events become ready
		1466	at the same time, so select/poll-based implementations get an unnatural
		1467	speed boost.
		1468
		1469	Also, note that the number of watchers usually has a nonlinear effect on
		1470	overall speed, that is, creating twice as many watchers doesn't take
		1471	twice the time - usually it takes longer. This puts event loops tested
		1472	with a higher number of watchers at a disadvantage.
		1473
		1474	To put the range of results into perspective, consider that on the
		1475	benchmark machine, handling an event takes roughly 1600 CPU cycles with
		1476	EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000
		1477	CPU cycles with POE.
		1478
		1479	"EV" is the sole leader regarding speed and memory use, which are both
		1480	maximal/minimal, respectively. When using the AE API there is zero
		1481	overhead (when going through the AnyEvent API create is about 5-6 times
		1482	slower, with other times being equal, so still uses far less memory than
		1483	any other event loop and is still faster than Event natively).
		1484
		1485	The pure perl implementation is hit in a few sweet spots (both the
		1486	constant timeout and the use of a single fd hit optimisations in the
		1487	perl interpreter and the backend itself). Nevertheless this shows that
		1488	it adds very little overhead in itself. Like any select-based backend
		1489	its performance becomes really bad with lots of file descriptors (and
		1490	few of them active), of course, but this was not subject of this
		1491	benchmark.
		1492
		1493	The "Event" module has a relatively high setup and callback invocation
		1494	cost, but overall scores in on the third place.
		1495
		1496	"IO::Async" performs admirably well, about on par with "Event", even
		1497	when using its pure perl backend.
		1498
		1499	"Glib"'s memory usage is quite a bit higher, but it features a faster
		1500	callback invocation and overall ends up in the same class as "Event".
		1501	However, Glib scales extremely badly, doubling the number of watchers
		1502	increases the processing time by more than a factor of four, making it
		1503	completely unusable when using larger numbers of watchers (note that
		1504	only a single file descriptor was used in the benchmark, so
		1505	inefficiencies of "poll" do not account for this).
		1506
		1507	The "Tk" adaptor works relatively well. The fact that it crashes with
		1508	more than 2000 watchers is a big setback, however, as correctness takes
		1509	precedence over speed. Nevertheless, its performance is surprising, as
		1510	the file descriptor is dup()ed for each watcher. This shows that the
		1511	dup() employed by some adaptors is not a big performance issue (it does
		1512	incur a hidden memory cost inside the kernel which is not reflected in
		1513	the figures above).
		1514
		1515	"POE", regardless of underlying event loop (whether using its pure perl
		1516	select-based backend or the Event module, the POE-EV backend couldn't be
		1517	tested because it wasn't working) shows abysmal performance and memory
		1518	usage with AnyEvent: Watchers use almost 30 times as much memory as EV
		1519	watchers, and 10 times as much memory as Event (the high memory
		1520	requirements are caused by requiring a session for each watcher).
		1521	Watcher invocation speed is almost 900 times slower than with AnyEvent's
		1522	pure perl implementation.
		1523
		1524	The design of the POE adaptor class in AnyEvent can not really account
		1525	for the performance issues, though, as session creation overhead is
		1526	small compared to execution of the state machine, which is coded pretty
		1527	optimally within AnyEvent::Impl::POE (and while everybody agrees that
		1528	using multiple sessions is not a good approach, especially regarding
		1529	memory usage, even the author of POE could not come up with a faster
		1530	design).
		1531
		1532	Summary
		1533	* Using EV through AnyEvent is faster than any other event loop (even
		1534	when used without AnyEvent), but most event loops have acceptable
		1535	performance with or without AnyEvent.
		1536
		1537	* The overhead AnyEvent adds is usually much smaller than the overhead
		1538	of the actual event loop, only with extremely fast event loops such
		1539	as EV adds AnyEvent significant overhead.
		1540
		1541	* You should avoid POE like the plague if you want performance or
		1542	reasonable memory usage.
		1543
		1544	BENCHMARKING THE LARGE SERVER CASE
		1545	This benchmark actually benchmarks the event loop itself. It works by
		1546	creating a number of "servers": each server consists of a socket pair, a
		1547	timeout watcher that gets reset on activity (but never fires), and an
		1548	I/O watcher waiting for input on one side of the socket. Each time the
		1549	socket watcher reads a byte it will write that byte to a random other
		1550	"server".
		1551
		1552	The effect is that there will be a lot of I/O watchers, only part of
		1553	which are active at any one point (so there is a constant number of
		1554	active fds for each loop iteration, but which fds these are is random).
		1555	The timeout is reset each time something is read because that reflects
		1556	how most timeouts work (and puts extra pressure on the event loops).
		1557
		1558	In this benchmark, we use 10000 socket pairs (20000 sockets), of which
		1559	100 (1%) are active. This mirrors the activity of large servers with
		1560	many connections, most of which are idle at any one point in time.
		1561
		1562	Source code for this benchmark is found as eg/bench2 in the AnyEvent
		1563	distribution. It uses the AE interface, which makes a real difference
		1564	for the EV and Perl backends only.
		1565
		1566	Explanation of the columns
		1567	sockets is the number of sockets, and twice the number of "servers"
		1568	(as each server has a read and write socket end).
		1569
		1570	create is the time it takes to create a socket pair (which is
		1571	nontrivial) and two watchers: an I/O watcher and a timeout watcher.
		1572
		1573	request, the most important value, is the time it takes to handle a
		1574	single "request", that is, reading the token from the pipe and
		1575	forwarding it to another server. This includes deleting the old timeout
		1576	and creating a new one that moves the timeout into the future.
		1577
		1578	Results
		1579	name sockets create request
		1580	EV 20000 62.66 7.99
		1581	Perl 20000 68.32 32.64
		1582	IOAsync 20000 174.06 101.15 epoll
		1583	IOAsync 20000 174.67 610.84 poll
		1584	Event 20000 202.69 242.91
		1585	Glib 20000 557.01 1689.52
		1586	POE 20000 341.54 12086.32 uses POE::Loop::Event
		1587
		1588	Discussion
		1589	This benchmark does measure scalability and overall performance of the
		1590	particular event loop.
		1591
		1592	EV is again fastest. Since it is using epoll on my system, the setup
		1593	time is relatively high, though.
		1594
		1595	Perl surprisingly comes second. It is much faster than the C-based event
		1596	loops Event and Glib.
		1597
		1598	IO::Async performs very well when using its epoll backend, and still
		1599	quite good compared to Glib when using its pure perl backend.
		1600
		1601	Event suffers from high setup time as well (look at its code and you
		1602	will understand why). Callback invocation also has a high overhead
		1603	compared to the "$_->() for .."-style loop that the Perl event loop
		1604	uses. Event uses select or poll in basically all documented
		1605	configurations.
		1606
		1607	Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
		1608	clearly fails to perform with many filehandles or in busy servers.
		1609
		1610	POE is still completely out of the picture, taking over 1000 times as
		1611	long as EV, and over 100 times as long as the Perl implementation, even
		1612	though it uses a C-based event loop in this case.
		1613
		1614	Summary
		1615	* The pure perl implementation performs extremely well.
		1616
		1617	* Avoid Glib or POE in large projects where performance matters.
		1618
		1619	BENCHMARKING SMALL SERVERS
		1620	While event loops should scale (and select-based ones do not...) even to
		1621	large servers, most programs we (or I :) actually write have only a few
		1622	I/O watchers.
		1623
		1624	In this benchmark, I use the same benchmark program as in the large
		1625	server case, but it uses only eight "servers", of which three are active
		1626	at any one time. This should reflect performance for a small server
		1627	relatively well.
		1628
		1629	The columns are identical to the previous table.
		1630
		1631	Results
		1632	name sockets create request
		1633	EV 16 20.00 6.54
		1634	Perl 16 25.75 12.62
		1635	Event 16 81.27 35.86
		1636	Glib 16 32.63 15.48
		1637	POE 16 261.87 276.28 uses POE::Loop::Event
		1638
		1639	Discussion
		1640	The benchmark tries to test the performance of a typical small server.
		1641	While knowing how various event loops perform is interesting, keep in
		1642	mind that their overhead in this case is usually not as important, due
		1643	to the small absolute number of watchers (that is, you need efficiency
		1644	and speed most when you have lots of watchers, not when you only have a
		1645	few of them).
		1646
		1647	EV is again fastest.
		1648
		1649	Perl again comes second. It is noticeably faster than the C-based event
		1650	loops Event and Glib, although the difference is too small to really
		1651	matter.
		1652
		1653	POE also performs much better in this case, but is is still far behind
		1654	the others.
		1655
		1656	Summary
		1657	* C-based event loops perform very well with small number of watchers,
		1658	as the management overhead dominates.
		1659
		1660	THE IO::Lambda BENCHMARK
		1661	Recently I was told about the benchmark in the IO::Lambda manpage, which
		1662	could be misinterpreted to make AnyEvent look bad. In fact, the
		1663	benchmark simply compares IO::Lambda with POE, and IO::Lambda looks
		1664	better (which shouldn't come as a surprise to anybody). As such, the
		1665	benchmark is fine, and mostly shows that the AnyEvent backend from
		1666	IO::Lambda isn't very optimal. But how would AnyEvent compare when used
		1667	without the extra baggage? To explore this, I wrote the equivalent
		1668	benchmark for AnyEvent.
		1669
		1670	The benchmark itself creates an echo-server, and then, for 500 times,
		1671	connects to the echo server, sends a line, waits for the reply, and then
		1672	creates the next connection. This is a rather bad benchmark, as it
		1673	doesn't test the efficiency of the framework or much non-blocking I/O,
		1674	but it is a benchmark nevertheless.
		1675
		1676	name runtime
		1677	Lambda/select 0.330 sec
		1678	+ optimized 0.122 sec
		1679	Lambda/AnyEvent 0.327 sec
		1680	+ optimized 0.138 sec
		1681	Raw sockets/select 0.077 sec
		1682	POE/select, components 0.662 sec
		1683	POE/select, raw sockets 0.226 sec
		1684	POE/select, optimized 0.404 sec
		1685
		1686	AnyEvent/select/nb 0.085 sec
		1687	AnyEvent/EV/nb 0.068 sec
		1688	+state machine 0.134 sec
		1689
		1690	The benchmark is also a bit unfair (my fault): the IO::Lambda/POE
		1691	benchmarks actually make blocking connects and use 100% blocking I/O,
		1692	defeating the purpose of an event-based solution. All of the newly
		1693	written AnyEvent benchmarks use 100% non-blocking connects (using
		1694	AnyEvent::Socket::tcp_connect and the asynchronous pure perl DNS
		1695	resolver), so AnyEvent is at a disadvantage here, as non-blocking
		1696	connects generally require a lot more bookkeeping and event handling
		1697	than blocking connects (which involve a single syscall only).
		1698
		1699	The last AnyEvent benchmark additionally uses AnyEvent::Handle, which
		1700	offers similar expressive power as POE and IO::Lambda, using
		1701	conventional Perl syntax. This means that both the echo server and the
		1702	client are 100% non-blocking, further placing it at a disadvantage.
		1703
		1704	As you can see, the AnyEvent + EV combination even beats the
		1705	hand-optimised "raw sockets benchmark", while AnyEvent + its pure perl
		1706	backend easily beats IO::Lambda and POE.
		1707
		1708	And even the 100% non-blocking version written using the high-level (and
		1709	slow :) AnyEvent::Handle abstraction beats both POE and IO::Lambda
		1710	higher level ("unoptimised") abstractions by a large margin, even though
		1711	it does all of DNS, tcp-connect and socket I/O in a non-blocking way.
		1712
		1713	The two AnyEvent benchmarks programs can be found as eg/ae0.pl and
		1714	eg/ae2.pl in the AnyEvent distribution, the remaining benchmarks are
		1715	part of the IO::Lambda distribution and were used without any changes.
		1716
		1717	SIGNALS
		1718	AnyEvent currently installs handlers for these signals:
		1719
		1720	SIGCHLD
		1721	A handler for "SIGCHLD" is installed by AnyEvent's child watcher
		1722	emulation for event loops that do not support them natively. Also,
		1723	some event loops install a similar handler.
		1724
		1725	Additionally, when AnyEvent is loaded and SIGCHLD is set to IGNORE,
		1726	then AnyEvent will reset it to default, to avoid losing child exit
		1727	statuses.
		1728
		1729	SIGPIPE
		1730	A no-op handler is installed for "SIGPIPE" when $SIG{PIPE} is
		1731	"undef" when AnyEvent gets loaded.
		1732
		1733	The rationale for this is that AnyEvent users usually do not really
		1734	depend on SIGPIPE delivery (which is purely an optimisation for
		1735	shell use, or badly-written programs), but "SIGPIPE" can cause
		1736	spurious and rare program exits as a lot of people do not expect
		1737	"SIGPIPE" when writing to some random socket.
		1738
		1739	The rationale for installing a no-op handler as opposed to ignoring
		1740	it is that this way, the handler will be restored to defaults on
		1741	exec.
		1742
		1743	Feel free to install your own handler, or reset it to defaults.
		1744
		1745	RECOMMENDED/OPTIONAL MODULES
		1746	One of AnyEvent's main goals is to be 100% Pure-Perl(tm): only perl (and
		1747	its built-in modules) are required to use it.
		1748
		1749	That does not mean that AnyEvent won't take advantage of some additional
		1750	modules if they are installed.
		1751
		1752	This section explains which additional modules will be used, and how
		1753	they affect AnyEvent's operation.
		1754
		1755	Async::Interrupt
		1756	This slightly arcane module is used to implement fast signal
		1757	handling: To my knowledge, there is no way to do completely
		1758	race-free and quick signal handling in pure perl. To ensure that
		1759	signals still get delivered, AnyEvent will start an interval timer
		1760	to wake up perl (and catch the signals) with some delay (default is
		1761	10 seconds, look for $AnyEvent::MAX_SIGNAL_LATENCY).
		1762
		1763	If this module is available, then it will be used to implement
		1764	signal catching, which means that signals will not be delayed, and
		1765	the event loop will not be interrupted regularly, which is more
		1766	efficient (and good for battery life on laptops).
		1767
		1768	This affects not just the pure-perl event loop, but also other event
		1769	loops that have no signal handling on their own (e.g. Glib, Tk, Qt).
		1770
		1771	Some event loops (POE, Event, Event::Lib) offer signal watchers
		1772	natively, and either employ their own workarounds (POE) or use
		1773	AnyEvent's workaround (using $AnyEvent::MAX_SIGNAL_LATENCY).
		1774	Installing Async::Interrupt does nothing for those backends.
		1775
		1776	EV This module isn't really "optional", as it is simply one of the
		1777	backend event loops that AnyEvent can use. However, it is simply the
		1778	best event loop available in terms of features, speed and stability:
		1779	It supports the AnyEvent API optimally, implements all the watcher
		1780	types in XS, does automatic timer adjustments even when no monotonic
		1781	clock is available, can take avdantage of advanced kernel interfaces
		1782	such as "epoll" and "kqueue", and is the fastest backend by far.
		1783	You can even embed Glib/Gtk2 in it (or vice versa, see EV::Glib and
		1784	Glib::EV).
		1785
		1786	If you only use backends that rely on another event loop (e.g.
		1787	"Tk"), then this module will do nothing for you.
		1788
		1789	Guard
		1790	The guard module, when used, will be used to implement
		1791	"AnyEvent::Util::guard". This speeds up guards considerably (and
		1792	uses a lot less memory), but otherwise doesn't affect guard
		1793	operation much. It is purely used for performance.
		1794
		1795	JSON and JSON::XS
		1796	One of these modules is required when you want to read or write JSON
		1797	data via AnyEvent::Handle. JSON is also written in pure-perl, but
		1798	can take advantage of the ultra-high-speed JSON::XS module when it
		1799	is installed.
		1800
		1801	Net::SSLeay
		1802	Implementing TLS/SSL in Perl is certainly interesting, but not very
		1803	worthwhile: If this module is installed, then AnyEvent::Handle (with
		1804	the help of AnyEvent::TLS), gains the ability to do TLS/SSL.
		1805
		1806	Time::HiRes
		1807	This module is part of perl since release 5.008. It will be used
		1808	when the chosen event library does not come with a timing source of
		1809	its own. The pure-perl event loop (AnyEvent::Impl::Perl) will
		1810	additionally use it to try to use a monotonic clock for timing
		1811	stability.
		1812
		1813	FORK
		1814	Most event libraries are not fork-safe. The ones who are usually are
		1815	because they rely on inefficient but fork-safe "select" or "poll" calls
		1816	- higher performance APIs such as BSD's kqueue or the dreaded Linux
		1817	epoll are usually badly thought-out hacks that are incompatible with
		1818	fork in one way or another. Only EV is fully fork-aware and ensures that
		1819	you continue event-processing in both parent and child (or both, if you
		1820	know what you are doing).
		1821
		1822	This means that, in general, you cannot fork and do event processing in
		1823	the child if the event library was initialised before the fork (which
		1824	usually happens when the first AnyEvent watcher is created, or the
		1825	library is loaded).
		1826
		1827	If you have to fork, you must either do so before creating your first
		1828	watcher OR you must not use AnyEvent at all in the child OR you must do
		1829	something completely out of the scope of AnyEvent.
		1830
		1831	The problem of doing event processing in the parent and the child is
		1832	much more complicated: even for backends that are fork-aware or
		1833	fork-safe, their behaviour is not usually what you want: fork clones all
		1834	watchers, that means all timers, I/O watchers etc. are active in both
		1835	parent and child, which is almost never what you want. USing "exec" to
		1836	start worker children from some kind of manage rprocess is usually
		1837	preferred, because it is much easier and cleaner, at the expense of
		1838	having to have another binary.
		1839
		1840	SECURITY CONSIDERATIONS
		1841	AnyEvent can be forced to load any event model via
		1842	$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used
		1843	to execute arbitrary code or directly gain access, it can easily be used
		1844	to make the program hang or malfunction in subtle ways, as AnyEvent
		1845	watchers will not be active when the program uses a different event
		1846	model than specified in the variable.
		1847
		1848	You can make AnyEvent completely ignore this variable by deleting it
		1849	before the first watcher gets created, e.g. with a "BEGIN" block:
		1850
		1851	BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }
		1852
		1853	use AnyEvent;
		1854
		1855	Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can
		1856	be used to probe what backend is used and gain other information (which
		1857	is probably even less useful to an attacker than PERL_ANYEVENT_MODEL),
		1858	and $ENV{PERL_ANYEVENT_STRICT}.
		1859
		1860	Note that AnyEvent will remove all environment variables starting with
		1861	"PERL_ANYEVENT_" from %ENV when it is loaded while taint mode is
		1862	enabled.
		1863
		1864	BUGS
		1865	Perl 5.8 has numerous memleaks that sometimes hit this module and are
		1866	hard to work around. If you suffer from memleaks, first upgrade to Perl
		1867	5.10 and check wether the leaks still show up. (Perl 5.10.0 has other
		1868	annoying memleaks, such as leaking on "map" and "grep" but it is usually
		1869	not as pronounced).
468		1870
469	SEE ALSO	1871	SEE ALSO
470	Event modules: Coro::EV, EV, EV::Glib, Glib::EV, Coro::Event, Event,	1872	Tutorial/Introduction: AnyEvent::Intro.
471	Glib::Event, Glib, Coro, Tk.
472		1873
473	Implementations: AnyEvent::Impl::CoroEV, AnyEvent::Impl::EV,	1874	FAQ: AnyEvent::FAQ.
474	AnyEvent::Impl::CoroEvent, AnyEvent::Impl::Event, AnyEvent::Impl::Glib,	1875
		1876	Utility functions: AnyEvent::Util.
		1877
		1878	Event modules: EV, EV::Glib, Glib::EV, Event, Glib::Event, Glib, Tk,
		1879	Event::Lib, Qt, POE.
		1880
		1881	Implementations: AnyEvent::Impl::EV, AnyEvent::Impl::Event,
		1882	AnyEvent::Impl::Glib, AnyEvent::Impl::Tk, AnyEvent::Impl::Perl,
		1883	AnyEvent::Impl::EventLib, AnyEvent::Impl::Qt, AnyEvent::Impl::POE,
475	AnyEvent::Impl::Tk, AnyEvent::Impl::Perl.	1884	AnyEvent::Impl::IOAsync, Anyevent::Impl::Irssi.
476		1885
477	Nontrivial usage examples: Net::FCP, Net::XMPP2.	1886	Non-blocking file handles, sockets, TCP clients and servers:
		1887	AnyEvent::Handle, AnyEvent::Socket, AnyEvent::TLS.
478		1888
		1889	Asynchronous DNS: AnyEvent::DNS.
479		1890
		1891	Thread support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event.
		1892
		1893	Nontrivial usage examples: AnyEvent::GPSD, AnyEvent::IRC,
		1894	AnyEvent::HTTP.
		1895
		1896	AUTHOR
		1897	Marc Lehmann <schmorp@schmorp.de>
		1898	http://home.schmorp.de/
		1899

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Comparing AnyEvent/README (file contents): Revision 1.15 by root, Wed Apr 16 15:10:10 2008 UTC vs. Revision 1.64 by root, Fri Dec 31 04:50:44 2010 UTC

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Comparing AnyEvent/README (file contents):
Revision 1.15 by root, Wed Apr 16 15:10:10 2008 UTC vs.
Revision 1.64 by root, Fri Dec 31 04:50:44 2010 UTC