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		1	=encoding utf-8
		2
1	=head1 NAME	3	=head1 NAME
2		4
3	libev - a high performance full-featured event loop written in C	5	libev - a high performance full-featured event loop written in C
4		6
5	=head1 SYNOPSIS	7	=head1 SYNOPSIS
6		8
7	#include <ev.h>	9	#include <ev.h>
8		10
9	=head1 DESCRIPTION	11	=head2 EXAMPLE PROGRAM
		12
		13	// a single header file is required
		14	#include <ev.h>
		15
		16	#include <stdio.h> // for puts
		17
		18	// every watcher type has its own typedef'd struct
		19	// with the name ev_TYPE
		20	ev_io stdin_watcher;
		21	ev_timer timeout_watcher;
		22
		23	// all watcher callbacks have a similar signature
		24	// this callback is called when data is readable on stdin
		25	static void
		26	stdin_cb (EV_P_ ev_io *w, int revents)
		27	{
		28	puts ("stdin ready");
		29	// for one-shot events, one must manually stop the watcher
		30	// with its corresponding stop function.
		31	ev_io_stop (EV_A_ w);
		32
		33	// this causes all nested ev_run's to stop iterating
		34	ev_break (EV_A_ EVBREAK_ALL);
		35	}
		36
		37	// another callback, this time for a time-out
		38	static void
		39	timeout_cb (EV_P_ ev_timer *w, int revents)
		40	{
		41	puts ("timeout");
		42	// this causes the innermost ev_run to stop iterating
		43	ev_break (EV_A_ EVBREAK_ONE);
		44	}
		45
		46	int
		47	main (void)
		48	{
		49	// use the default event loop unless you have special needs
		50	struct ev_loop *loop = EV_DEFAULT;
		51
		52	// initialise an io watcher, then start it
		53	// this one will watch for stdin to become readable
		54	ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
		55	ev_io_start (loop, &stdin_watcher);
		56
		57	// initialise a timer watcher, then start it
		58	// simple non-repeating 5.5 second timeout
		59	ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
		60	ev_timer_start (loop, &timeout_watcher);
		61
		62	// now wait for events to arrive
		63	ev_run (loop, 0);
		64
		65	// break was called, so exit
		66	return 0;
		67	}
		68
		69	=head1 ABOUT THIS DOCUMENT
		70
		71	This document documents the libev software package.
		72
		73	The newest version of this document is also available as an html-formatted
		74	web page you might find easier to navigate when reading it for the first
		75	time: L<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
		76
		77	While this document tries to be as complete as possible in documenting
		78	libev, its usage and the rationale behind its design, it is not a tutorial
		79	on event-based programming, nor will it introduce event-based programming
		80	with libev.
		81
		82	Familiarity with event based programming techniques in general is assumed
		83	throughout this document.
		84
		85	=head1 WHAT TO READ WHEN IN A HURRY
		86
		87	This manual tries to be very detailed, but unfortunately, this also makes
		88	it very long. If you just want to know the basics of libev, I suggest
		89	reading L</ANATOMY OF A WATCHER>, then the L</EXAMPLE PROGRAM> above and
		90	look up the missing functions in L</GLOBAL FUNCTIONS> and the C<ev_io> and
		91	C<ev_timer> sections in L</WATCHER TYPES>.
		92
		93	=head1 ABOUT LIBEV
10		94
11	Libev is an event loop: you register interest in certain events (such as a	95	Libev is an event loop: you register interest in certain events (such as a
12	file descriptor being readable or a timeout occuring), and it will manage	96	file descriptor being readable or a timeout occurring), and it will manage
13	these event sources and provide your program with events.	97	these event sources and provide your program with events.
14		98
15	To do this, it must take more or less complete control over your process	99	To do this, it must take more or less complete control over your process
16	(or thread) by executing the I<event loop> handler, and will then	100	(or thread) by executing the I<event loop> handler, and will then
17	communicate events via a callback mechanism.	101	communicate events via a callback mechanism.
…		…
19	You register interest in certain events by registering so-called I<event	103	You register interest in certain events by registering so-called I<event
20	watchers>, which are relatively small C structures you initialise with the	104	watchers>, which are relatively small C structures you initialise with the
21	details of the event, and then hand it over to libev by I<starting> the	105	details of the event, and then hand it over to libev by I<starting> the
22	watcher.	106	watcher.
23		107
24	=head1 FEATURES	108	=head2 FEATURES
25		109
26	Libev supports select, poll, the linux-specific epoll and the bsd-specific	110	Libev supports C<select>, C<poll>, the Linux-specific aio and C<epoll>
27	kqueue mechanisms for file descriptor events, relative timers, absolute	111	interfaces, the BSD-specific C<kqueue> and the Solaris-specific event port
28	timers with customised rescheduling, signal events, process status change	112	mechanisms for file descriptor events (C<ev_io>), the Linux C<inotify>
		113	interface (for C<ev_stat>), Linux eventfd/signalfd (for faster and cleaner
		114	inter-thread wakeup (C<ev_async>)/signal handling (C<ev_signal>)) relative
		115	timers (C<ev_timer>), absolute timers with customised rescheduling
		116	(C<ev_periodic>), synchronous signals (C<ev_signal>), process status
29	events (related to SIGCHLD), and event watchers dealing with the event	117	change events (C<ev_child>), and event watchers dealing with the event
30	loop mechanism itself (idle, prepare and check watchers). It also is quite	118	loop mechanism itself (C<ev_idle>, C<ev_embed>, C<ev_prepare> and
		119	C<ev_check> watchers) as well as file watchers (C<ev_stat>) and even
		120	limited support for fork events (C<ev_fork>).
		121
		122	It also is quite fast (see this
31	fast (see this L<benchmark|http://libev.schmorp.de/bench.html> comparing	123	L<benchmark|http://libev.schmorp.de/bench.html> comparing it to libevent
32	it to libevent for example).	124	for example).
33		125
34	=head1 CONVENTIONS	126	=head2 CONVENTIONS
35		127
36	Libev is very configurable. In this manual the default configuration	128	Libev is very configurable. In this manual the default (and most common)
37	will be described, which supports multiple event loops. For more info	129	configuration will be described, which supports multiple event loops. For
38	about various configuration options please have a look at the file	130	more info about various configuration options please have a look at
39	F<README.embed> in the libev distribution. If libev was configured without	131	B<EMBED> section in this manual. If libev was configured without support
40	support for multiple event loops, then all functions taking an initial	132	for multiple event loops, then all functions taking an initial argument of
41	argument of name C<loop> (which is always of type C<struct ev_loop *>)	133	name C<loop> (which is always of type C<struct ev_loop *>) will not have
42	will not have this argument.	134	this argument.
43		135
44	=head1 TIME AND OTHER GLOBAL FUNCTIONS	136	=head2 TIME REPRESENTATION
45		137
46	Libev represents time as a single floating point number, representing the	138	Libev represents time as a single floating point number, representing
47	(fractional) number of seconds since the (POSIX) epoch (somewhere near	139	the (fractional) number of seconds since the (POSIX) epoch (in practice
48	the beginning of 1970, details are complicated, don't ask). This type is	140	somewhere near the beginning of 1970, details are complicated, don't
49	called C<ev_tstamp>, which is what you should use too. It usually aliases	141	ask). This type is called C<ev_tstamp>, which is what you should use
50	to the double type in C.	142	too. It usually aliases to the C<double> type in C. When you need to do
		143	any calculations on it, you should treat it as some floating point value.
		144
		145	Unlike the name component C<stamp> might indicate, it is also used for
		146	time differences (e.g. delays) throughout libev.
		147
		148	=head1 ERROR HANDLING
		149
		150	Libev knows three classes of errors: operating system errors, usage errors
		151	and internal errors (bugs).
		152
		153	When libev catches an operating system error it cannot handle (for example
		154	a system call indicating a condition libev cannot fix), it calls the callback
		155	set via C<ev_set_syserr_cb>, which is supposed to fix the problem or
		156	abort. The default is to print a diagnostic message and to call C<abort
		157	()>.
		158
		159	When libev detects a usage error such as a negative timer interval, then
		160	it will print a diagnostic message and abort (via the C<assert> mechanism,
		161	so C<NDEBUG> will disable this checking): these are programming errors in
		162	the libev caller and need to be fixed there.
		163
		164	Via the C<EV_FREQUENT> macro you can compile in and/or enable extensive
		165	consistency checking code inside libev that can be used to check for
		166	internal inconsistencies, suually caused by application bugs.
		167
		168	Libev also has a few internal error-checking C<assert>ions. These do not
		169	trigger under normal circumstances, as they indicate either a bug in libev
		170	or worse.
		171
		172
		173	=head1 GLOBAL FUNCTIONS
		174
		175	These functions can be called anytime, even before initialising the
		176	library in any way.
51		177
52	=over 4	178	=over 4
53		179
54	=item ev_tstamp ev_time ()	180	=item ev_tstamp ev_time ()
55		181
56	Returns the current time as libev would use it.	182	Returns the current time as libev would use it. Please note that the
		183	C<ev_now> function is usually faster and also often returns the timestamp
		184	you actually want to know. Also interesting is the combination of
		185	C<ev_now_update> and C<ev_now>.
		186
		187	=item ev_sleep (ev_tstamp interval)
		188
		189	Sleep for the given interval: The current thread will be blocked
		190	until either it is interrupted or the given time interval has
		191	passed (approximately - it might return a bit earlier even if not
		192	interrupted). Returns immediately if C<< interval <= 0 >>.
		193
		194	Basically this is a sub-second-resolution C<sleep ()>.
		195
		196	The range of the C<interval> is limited - libev only guarantees to work
		197	with sleep times of up to one day (C<< interval <= 86400 >>).
57		198
58	=item int ev_version_major ()	199	=item int ev_version_major ()
59		200
60	=item int ev_version_minor ()	201	=item int ev_version_minor ()
61		202
62	You can find out the major and minor version numbers of the library	203	You can find out the major and minor ABI version numbers of the library
63	you linked against by calling the functions C<ev_version_major> and	204	you linked against by calling the functions C<ev_version_major> and
64	C<ev_version_minor>. If you want, you can compare against the global	205	C<ev_version_minor>. If you want, you can compare against the global
65	symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the	206	symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
66	version of the library your program was compiled against.	207	version of the library your program was compiled against.
67		208
		209	These version numbers refer to the ABI version of the library, not the
		210	release version.
		211
68	Usually, its a good idea to terminate if the major versions mismatch,	212	Usually, it's a good idea to terminate if the major versions mismatch,
69	as this indicates an incompatible change. Minor versions are usually	213	as this indicates an incompatible change. Minor versions are usually
70	compatible to older versions, so a larger minor version alone is usually	214	compatible to older versions, so a larger minor version alone is usually
71	not a problem.	215	not a problem.
72		216
		217	Example: Make sure we haven't accidentally been linked against the wrong
		218	version (note, however, that this will not detect other ABI mismatches,
		219	such as LFS or reentrancy).
		220
		221	assert (("libev version mismatch",
		222	ev_version_major () == EV_VERSION_MAJOR
		223	&& ev_version_minor () >= EV_VERSION_MINOR));
		224
		225	=item unsigned int ev_supported_backends ()
		226
		227	Return the set of all backends (i.e. their corresponding C<EV_BACKEND_*>
		228	value) compiled into this binary of libev (independent of their
		229	availability on the system you are running on). See C<ev_default_loop> for
		230	a description of the set values.
		231
		232	Example: make sure we have the epoll method, because yeah this is cool and
		233	a must have and can we have a torrent of it please!!!11
		234
		235	assert (("sorry, no epoll, no sex",
		236	ev_supported_backends () & EVBACKEND_EPOLL));
		237
		238	=item unsigned int ev_recommended_backends ()
		239
		240	Return the set of all backends compiled into this binary of libev and
		241	also recommended for this platform, meaning it will work for most file
		242	descriptor types. This set is often smaller than the one returned by
		243	C<ev_supported_backends>, as for example kqueue is broken on most BSDs
		244	and will not be auto-detected unless you explicitly request it (assuming
		245	you know what you are doing). This is the set of backends that libev will
		246	probe for if you specify no backends explicitly.
		247
		248	=item unsigned int ev_embeddable_backends ()
		249
		250	Returns the set of backends that are embeddable in other event loops. This
		251	value is platform-specific but can include backends not available on the
		252	current system. To find which embeddable backends might be supported on
		253	the current system, you would need to look at C<ev_embeddable_backends ()
		254	& ev_supported_backends ()>, likewise for recommended ones.
		255
		256	See the description of C<ev_embed> watchers for more info.
		257
73	=item ev_set_allocator (void *(*cb)(void *ptr, long size))	258	=item ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())
74		259
75	Sets the allocation function to use (the prototype is similar to the	260	Sets the allocation function to use (the prototype is similar - the
76	realloc C function, the semantics are identical). It is used to allocate	261	semantics are identical to the C<realloc> C89/SuS/POSIX function). It is
77	and free memory (no surprises here). If it returns zero when memory	262	used to allocate and free memory (no surprises here). If it returns zero
78	needs to be allocated, the library might abort or take some potentially	263	when memory needs to be allocated (C<size != 0>), the library might abort
79	destructive action. The default is your system realloc function.	264	or take some potentially destructive action.
		265
		266	Since some systems (at least OpenBSD and Darwin) fail to implement
		267	correct C<realloc> semantics, libev will use a wrapper around the system
		268	C<realloc> and C<free> functions by default.
80		269
81	You could override this function in high-availability programs to, say,	270	You could override this function in high-availability programs to, say,
82	free some memory if it cannot allocate memory, to use a special allocator,	271	free some memory if it cannot allocate memory, to use a special allocator,
83	or even to sleep a while and retry until some memory is available.	272	or even to sleep a while and retry until some memory is available.
84		273
		274	Example: The following is the C<realloc> function that libev itself uses
		275	which should work with C<realloc> and C<free> functions of all kinds and
		276	is probably a good basis for your own implementation.
		277
		278	static void *
		279	ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
		280	{
		281	if (size)
		282	return realloc (ptr, size);
		283
		284	free (ptr);
		285	return 0;
		286	}
		287
		288	Example: Replace the libev allocator with one that waits a bit and then
		289	retries.
		290
		291	static void *
		292	persistent_realloc (void *ptr, size_t size)
		293	{
		294	if (!size)
		295	{
		296	free (ptr);
		297	return 0;
		298	}
		299
		300	for (;;)
		301	{
		302	void *newptr = realloc (ptr, size);
		303
		304	if (newptr)
		305	return newptr;
		306
		307	sleep (60);
		308	}
		309	}
		310
		311	...
		312	ev_set_allocator (persistent_realloc);
		313
85	=item ev_set_syserr_cb (void (*cb)(const char *msg));	314	=item ev_set_syserr_cb (void (*cb)(const char *msg) throw ())
86		315
87	Set the callback function to call on a retryable syscall error (such	316	Set the callback function to call on a retryable system call error (such
88	as failed select, poll, epoll_wait). The message is a printable string	317	as failed select, poll, epoll_wait). The message is a printable string
89	indicating the system call or subsystem causing the problem. If this	318	indicating the system call or subsystem causing the problem. If this
90	callback is set, then libev will expect it to remedy the sitution, no	319	callback is set, then libev will expect it to remedy the situation, no
91	matter what, when it returns. That is, libev will generally retry the	320	matter what, when it returns. That is, libev will generally retry the
92	requested operation, or, if the condition doesn't go away, do bad stuff	321	requested operation, or, if the condition doesn't go away, do bad stuff
93	(such as abort).	322	(such as abort).
94		323
		324	Example: This is basically the same thing that libev does internally, too.
		325
		326	static void
		327	fatal_error (const char *msg)
		328	{
		329	perror (msg);
		330	abort ();
		331	}
		332
		333	...
		334	ev_set_syserr_cb (fatal_error);
		335
		336	=item ev_feed_signal (int signum)
		337
		338	This function can be used to "simulate" a signal receive. It is completely
		339	safe to call this function at any time, from any context, including signal
		340	handlers or random threads.
		341
		342	Its main use is to customise signal handling in your process, especially
		343	in the presence of threads. For example, you could block signals
		344	by default in all threads (and specifying C<EVFLAG_NOSIGMASK> when
		345	creating any loops), and in one thread, use C<sigwait> or any other
		346	mechanism to wait for signals, then "deliver" them to libev by calling
		347	C<ev_feed_signal>.
		348
95	=back	349	=back
96		350
97	=head1 FUNCTIONS CONTROLLING THE EVENT LOOP	351	=head1 FUNCTIONS CONTROLLING EVENT LOOPS
98		352
99	An event loop is described by a C<struct ev_loop *>. The library knows two	353	An event loop is described by a C<struct ev_loop *> (the C<struct> is
100	types of such loops, the I<default> loop, which supports signals and child	354	I<not> optional in this case unless libev 3 compatibility is disabled, as
101	events, and dynamically created loops which do not.	355	libev 3 had an C<ev_loop> function colliding with the struct name).
102		356
103	If you use threads, a common model is to run the default event loop	357	The library knows two types of such loops, the I<default> loop, which
104	in your main thread (or in a separate thrad) and for each thread you	358	supports child process events, and dynamically created event loops which
105	create, you also create another event loop. Libev itself does no locking	359	do not.
106	whatsoever, so if you mix calls to the same event loop in different
107	threads, make sure you lock (this is usually a bad idea, though, even if
108	done correctly, because its hideous and inefficient).
109		360
110	=over 4	361	=over 4
111		362
112	=item struct ev_loop *ev_default_loop (unsigned int flags)	363	=item struct ev_loop *ev_default_loop (unsigned int flags)
113		364
114	This will initialise the default event loop if it hasn't been initialised	365	This returns the "default" event loop object, which is what you should
115	yet and return it. If the default loop could not be initialised, returns	366	normally use when you just need "the event loop". Event loop objects and
116	false. If it already was initialised it simply returns it (and ignores the	367	the C<flags> parameter are described in more detail in the entry for
117	flags).	368	C<ev_loop_new>.
		369
		370	If the default loop is already initialised then this function simply
		371	returns it (and ignores the flags. If that is troubling you, check
		372	C<ev_backend ()> afterwards). Otherwise it will create it with the given
		373	flags, which should almost always be C<0>, unless the caller is also the
		374	one calling C<ev_run> or otherwise qualifies as "the main program".
118		375
119	If you don't know what event loop to use, use the one returned from this	376	If you don't know what event loop to use, use the one returned from this
120	function.	377	function (or via the C<EV_DEFAULT> macro).
		378
		379	Note that this function is I<not> thread-safe, so if you want to use it
		380	from multiple threads, you have to employ some kind of mutex (note also
		381	that this case is unlikely, as loops cannot be shared easily between
		382	threads anyway).
		383
		384	The default loop is the only loop that can handle C<ev_child> watchers,
		385	and to do this, it always registers a handler for C<SIGCHLD>. If this is
		386	a problem for your application you can either create a dynamic loop with
		387	C<ev_loop_new> which doesn't do that, or you can simply overwrite the
		388	C<SIGCHLD> signal handler I<after> calling C<ev_default_init>.
		389
		390	Example: This is the most typical usage.
		391
		392	if (!ev_default_loop (0))
		393	fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
		394
		395	Example: Restrict libev to the select and poll backends, and do not allow
		396	environment settings to be taken into account:
		397
		398	ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
		399
		400	=item struct ev_loop *ev_loop_new (unsigned int flags)
		401
		402	This will create and initialise a new event loop object. If the loop
		403	could not be initialised, returns false.
		404
		405	This function is thread-safe, and one common way to use libev with
		406	threads is indeed to create one loop per thread, and using the default
		407	loop in the "main" or "initial" thread.
121		408
122	The flags argument can be used to specify special behaviour or specific	409	The flags argument can be used to specify special behaviour or specific
123	backends to use, and is usually specified as 0 (or EVFLAG_AUTO).	410	backends to use, and is usually specified as C<0> (or C<EVFLAG_AUTO>).
124		411
125	It supports the following flags:	412	The following flags are supported:
126		413
127	=over 4	414	=over 4
128		415
129	=item EVFLAG_AUTO	416	=item C<EVFLAG_AUTO>
130		417
131	The default flags value. Use this if you have no clue (its the right	418	The default flags value. Use this if you have no clue (it's the right
132	thing, believe me).	419	thing, believe me).
133		420
134	=item EVFLAG_NOENV	421	=item C<EVFLAG_NOENV>
135		422
136	If this flag bit is ored into the flag value (or the program runs setuid	423	If this flag bit is or'ed into the flag value (or the program runs setuid
137	or setgid) then libev will I<not> look at the environment variable	424	or setgid) then libev will I<not> look at the environment variable
138	C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will	425	C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will
139	override the flags completely if it is found in the environment. This is	426	override the flags completely if it is found in the environment. This is
140	useful to try out specific backends to test their performance, or to work	427	useful to try out specific backends to test their performance, to work
141	around bugs.	428	around bugs, or to make libev threadsafe (accessing environment variables
		429	cannot be done in a threadsafe way, but usually it works if no other
		430	thread modifies them).
142		431
		432	=item C<EVFLAG_FORKCHECK>
		433
		434	Instead of calling C<ev_loop_fork> manually after a fork, you can also
		435	make libev check for a fork in each iteration by enabling this flag.
		436
		437	This works by calling C<getpid ()> on every iteration of the loop,
		438	and thus this might slow down your event loop if you do a lot of loop
		439	iterations and little real work, but is usually not noticeable (on my
		440	GNU/Linux system for example, C<getpid> is actually a simple 5-insn
		441	sequence without a system call and thus I<very> fast, but my GNU/Linux
		442	system also has C<pthread_atfork> which is even faster). (Update: glibc
		443	versions 2.25 apparently removed the C<getpid> optimisation again).
		444
		445	The big advantage of this flag is that you can forget about fork (and
		446	forget about forgetting to tell libev about forking, although you still
		447	have to ignore C<SIGPIPE>) when you use this flag.
		448
		449	This flag setting cannot be overridden or specified in the C<LIBEV_FLAGS>
		450	environment variable.
		451
		452	=item C<EVFLAG_NOINOTIFY>
		453
		454	When this flag is specified, then libev will not attempt to use the
		455	I<inotify> API for its C<ev_stat> watchers. Apart from debugging and
		456	testing, this flag can be useful to conserve inotify file descriptors, as
		457	otherwise each loop using C<ev_stat> watchers consumes one inotify handle.
		458
		459	=item C<EVFLAG_SIGNALFD>
		460
		461	When this flag is specified, then libev will attempt to use the
		462	I<signalfd> API for its C<ev_signal> (and C<ev_child>) watchers. This API
		463	delivers signals synchronously, which makes it both faster and might make
		464	it possible to get the queued signal data. It can also simplify signal
		465	handling with threads, as long as you properly block signals in your
		466	threads that are not interested in handling them.
		467
		468	Signalfd will not be used by default as this changes your signal mask, and
		469	there are a lot of shoddy libraries and programs (glib's threadpool for
		470	example) that can't properly initialise their signal masks.
		471
		472	=item C<EVFLAG_NOSIGMASK>
		473
		474	When this flag is specified, then libev will avoid to modify the signal
		475	mask. Specifically, this means you have to make sure signals are unblocked
		476	when you want to receive them.
		477
		478	This behaviour is useful when you want to do your own signal handling, or
		479	want to handle signals only in specific threads and want to avoid libev
		480	unblocking the signals.
		481
		482	It's also required by POSIX in a threaded program, as libev calls
		483	C<sigprocmask>, whose behaviour is officially unspecified.
		484
		485	This flag's behaviour will become the default in future versions of libev.
		486
143	=item EVMETHOD_SELECT portable select backend	487	=item C<EVBACKEND_SELECT> (value 1, portable select backend)
144		488
145	=item EVMETHOD_POLL poll backend (everywhere except windows)	489	This is your standard select(2) backend. Not I<completely> standard, as
		490	libev tries to roll its own fd_set with no limits on the number of fds,
		491	but if that fails, expect a fairly low limit on the number of fds when
		492	using this backend. It doesn't scale too well (O(highest_fd)), but its
		493	usually the fastest backend for a low number of (low-numbered :) fds.
146		494
147	=item EVMETHOD_EPOLL linux only	495	To get good performance out of this backend you need a high amount of
		496	parallelism (most of the file descriptors should be busy). If you are
		497	writing a server, you should C<accept ()> in a loop to accept as many
		498	connections as possible during one iteration. You might also want to have
		499	a look at C<ev_set_io_collect_interval ()> to increase the amount of
		500	readiness notifications you get per iteration.
148		501
149	=item EVMETHOD_KQUEUE some bsds only	502	This backend maps C<EV_READ> to the C<readfds> set and C<EV_WRITE> to the
		503	C<writefds> set (and to work around Microsoft Windows bugs, also onto the
		504	C<exceptfds> set on that platform).
150		505
151	=item EVMETHOD_DEVPOLL solaris 8 only	506	=item C<EVBACKEND_POLL> (value 2, poll backend, available everywhere except on windows)
152		507
153	=item EVMETHOD_PORT solaris 10 only	508	And this is your standard poll(2) backend. It's more complicated
		509	than select, but handles sparse fds better and has no artificial
		510	limit on the number of fds you can use (except it will slow down
		511	considerably with a lot of inactive fds). It scales similarly to select,
		512	i.e. O(total_fds). See the entry for C<EVBACKEND_SELECT>, above, for
		513	performance tips.
154		514
155	If one or more of these are ored into the flags value, then only these	515	This backend maps C<EV_READ> to C<POLLIN | POLLERR | POLLHUP>, and
156	backends will be tried (in the reverse order as given here). If one are	516	C<EV_WRITE> to C<POLLOUT | POLLERR | POLLHUP>.
157	specified, any backend will do.	517
		518	=item C<EVBACKEND_EPOLL> (value 4, Linux)
		519
		520	Use the Linux-specific epoll(7) interface (for both pre- and post-2.6.9
		521	kernels).
		522
		523	For few fds, this backend is a bit little slower than poll and select, but
		524	it scales phenomenally better. While poll and select usually scale like
		525	O(total_fds) where total_fds is the total number of fds (or the highest
		526	fd), epoll scales either O(1) or O(active_fds).
		527
		528	The epoll mechanism deserves honorable mention as the most misdesigned
		529	of the more advanced event mechanisms: mere annoyances include silently
		530	dropping file descriptors, requiring a system call per change per file
		531	descriptor (and unnecessary guessing of parameters), problems with dup,
		532	returning before the timeout value, resulting in additional iterations
		533	(and only giving 5ms accuracy while select on the same platform gives
		534	0.1ms) and so on. The biggest issue is fork races, however - if a program
		535	forks then I<both> parent and child process have to recreate the epoll
		536	set, which can take considerable time (one syscall per file descriptor)
		537	and is of course hard to detect.
		538
		539	Epoll is also notoriously buggy - embedding epoll fds I<should> work,
		540	but of course I<doesn't>, and epoll just loves to report events for
		541	totally I<different> file descriptors (even already closed ones, so
		542	one cannot even remove them from the set) than registered in the set
		543	(especially on SMP systems). Libev tries to counter these spurious
		544	notifications by employing an additional generation counter and comparing
		545	that against the events to filter out spurious ones, recreating the set
		546	when required. Epoll also erroneously rounds down timeouts, but gives you
		547	no way to know when and by how much, so sometimes you have to busy-wait
		548	because epoll returns immediately despite a nonzero timeout. And last
		549	not least, it also refuses to work with some file descriptors which work
		550	perfectly fine with C<select> (files, many character devices...).
		551
		552	Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
		553	cobbled together in a hurry, no thought to design or interaction with
		554	others. Oh, the pain, will it ever stop...
		555
		556	While stopping, setting and starting an I/O watcher in the same iteration
		557	will result in some caching, there is still a system call per such
		558	incident (because the same I<file descriptor> could point to a different
		559	I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
		560	file descriptors might not work very well if you register events for both
		561	file descriptors.
		562
		563	Best performance from this backend is achieved by not unregistering all
		564	watchers for a file descriptor until it has been closed, if possible,
		565	i.e. keep at least one watcher active per fd at all times. Stopping and
		566	starting a watcher (without re-setting it) also usually doesn't cause
		567	extra overhead. A fork can both result in spurious notifications as well
		568	as in libev having to destroy and recreate the epoll object, which can
		569	take considerable time and thus should be avoided.
		570
		571	All this means that, in practice, C<EVBACKEND_SELECT> can be as fast or
		572	faster than epoll for maybe up to a hundred file descriptors, depending on
		573	the usage. So sad.
		574
		575	While nominally embeddable in other event loops, this feature is broken in
		576	a lot of kernel revisions, but probably(!) works in current versions.
		577
		578	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
		579	C<EVBACKEND_POLL>.
		580
		581	=item C<EVBACKEND_LINUXAIO> (value 64, Linux)
		582
		583	Use the Linux-specific Linux AIO (I<not> C<< aio(7) >> but C<<
		584	io_submit(2) >>) event interface available in post-4.18 kernels (but libev
		585	only tries to use it in 4.19+).
		586
		587	This is another Linux train wreck of an event interface.
		588
		589	If this backend works for you (as of this writing, it was very
		590	experimental), it is the best event interface available on Linux and might
		591	be well worth enabling it - if it isn't available in your kernel this will
		592	be detected and this backend will be skipped.
		593
		594	This backend can batch oneshot requests and supports a user-space ring
		595	buffer to receive events. It also doesn't suffer from most of the design
		596	problems of epoll (such as not being able to remove event sources from
		597	the epoll set), and generally sounds too good to be true. Because, this
		598	being the Linux kernel, of course it suffers from a whole new set of
		599	limitations, forcing you to fall back to epoll, inheriting all its design
		600	issues.
		601
		602	For one, it is not easily embeddable (but probably could be done using
		603	an event fd at some extra overhead). It also is subject to a system wide
		604	limit that can be configured in F</proc/sys/fs/aio-max-nr>. If no AIO
		605	requests are left, this backend will be skipped during initialisation, and
		606	will switch to epoll when the loop is active.
		607
		608	Most problematic in practice, however, is that not all file descriptors
		609	work with it. For example, in Linux 5.1, TCP sockets, pipes, event fds,
		610	files, F</dev/null> and many others are supported, but ttys do not work
		611	properly (a known bug that the kernel developers don't care about, see
		612	L<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
		613	(yet?) a generic event polling interface.
		614
		615	Overall, it seems the Linux developers just don't want it to have a
		616	generic event handling mechanism other than C<select> or C<poll>.
		617
		618	To work around all these problem, the current version of libev uses its
		619	epoll backend as a fallback for file descriptor types that do not work. Or
		620	falls back completely to epoll if the kernel acts up.
		621
		622	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
		623	C<EVBACKEND_POLL>.
		624
		625	=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)
		626
		627	Kqueue deserves special mention, as at the time this backend was
		628	implemented, it was broken on all BSDs except NetBSD (usually it doesn't
		629	work reliably with anything but sockets and pipes, except on Darwin,
		630	where of course it's completely useless). Unlike epoll, however, whose
		631	brokenness is by design, these kqueue bugs can be (and mostly have been)
		632	fixed without API changes to existing programs. For this reason it's not
		633	being "auto-detected" on all platforms unless you explicitly specify it
		634	in the flags (i.e. using C<EVBACKEND_KQUEUE>) or libev was compiled on a
		635	known-to-be-good (-enough) system like NetBSD.
		636
		637	You still can embed kqueue into a normal poll or select backend and use it
		638	only for sockets (after having made sure that sockets work with kqueue on
		639	the target platform). See C<ev_embed> watchers for more info.
		640
		641	It scales in the same way as the epoll backend, but the interface to the
		642	kernel is more efficient (which says nothing about its actual speed, of
		643	course). While stopping, setting and starting an I/O watcher does never
		644	cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to
		645	two event changes per incident. Support for C<fork ()> is very bad (you
		646	might have to leak fds on fork, but it's more sane than epoll) and it
		647	drops fds silently in similarly hard-to-detect cases.
		648
		649	This backend usually performs well under most conditions.
		650
		651	While nominally embeddable in other event loops, this doesn't work
		652	everywhere, so you might need to test for this. And since it is broken
		653	almost everywhere, you should only use it when you have a lot of sockets
		654	(for which it usually works), by embedding it into another event loop
		655	(e.g. C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> (but C<poll> is of course
		656	also broken on OS X)) and, did I mention it, using it only for sockets.
		657
		658	This backend maps C<EV_READ> into an C<EVFILT_READ> kevent with
		659	C<NOTE_EOF>, and C<EV_WRITE> into an C<EVFILT_WRITE> kevent with
		660	C<NOTE_EOF>.
		661
		662	=item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8)
		663
		664	This is not implemented yet (and might never be, unless you send me an
		665	implementation). According to reports, C</dev/poll> only supports sockets
		666	and is not embeddable, which would limit the usefulness of this backend
		667	immensely.
		668
		669	=item C<EVBACKEND_PORT> (value 32, Solaris 10)
		670
		671	This uses the Solaris 10 event port mechanism. As with everything on Solaris,
		672	it's really slow, but it still scales very well (O(active_fds)).
		673
		674	While this backend scales well, it requires one system call per active
		675	file descriptor per loop iteration. For small and medium numbers of file
		676	descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
		677	might perform better.
		678
		679	On the positive side, this backend actually performed fully to
		680	specification in all tests and is fully embeddable, which is a rare feat
		681	among the OS-specific backends (I vastly prefer correctness over speed
		682	hacks).
		683
		684	On the negative side, the interface is I<bizarre> - so bizarre that
		685	even sun itself gets it wrong in their code examples: The event polling
		686	function sometimes returns events to the caller even though an error
		687	occurred, but with no indication whether it has done so or not (yes, it's
		688	even documented that way) - deadly for edge-triggered interfaces where you
		689	absolutely have to know whether an event occurred or not because you have
		690	to re-arm the watcher.
		691
		692	Fortunately libev seems to be able to work around these idiocies.
		693
		694	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
		695	C<EVBACKEND_POLL>.
		696
		697	=item C<EVBACKEND_ALL>
		698
		699	Try all backends (even potentially broken ones that wouldn't be tried
		700	with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
		701	C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.
		702
		703	It is definitely not recommended to use this flag, use whatever
		704	C<ev_recommended_backends ()> returns, or simply do not specify a backend
		705	at all.
		706
		707	=item C<EVBACKEND_MASK>
		708
		709	Not a backend at all, but a mask to select all backend bits from a
		710	C<flags> value, in case you want to mask out any backends from a flags
		711	value (e.g. when modifying the C<LIBEV_FLAGS> environment variable).
158		712
159	=back	713	=back
160		714
161	=item struct ev_loop *ev_loop_new (unsigned int flags)	715	If one or more of the backend flags are or'ed into the flags value,
		716	then only these backends will be tried (in the reverse order as listed
		717	here). If none are specified, all backends in C<ev_recommended_backends
		718	()> will be tried.
162		719
163	Similar to C<ev_default_loop>, but always creates a new event loop that is	720	Example: Try to create a event loop that uses epoll and nothing else.
164	always distinct from the default loop. Unlike the default loop, it cannot
165	handle signal and child watchers, and attempts to do so will be greeted by
166	undefined behaviour (or a failed assertion if assertions are enabled).
167		721
168	=item ev_default_destroy ()	722	struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
		723	if (!epoller)
		724	fatal ("no epoll found here, maybe it hides under your chair");
169		725
170	Destroys the default loop again (frees all memory and kernel state	726	Example: Use whatever libev has to offer, but make sure that kqueue is
171	etc.). This stops all registered event watchers (by not touching them in	727	used if available.
172	any way whatsoever, although you cnanot rely on this :).	728
		729	struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
		730
		731	Example: Similarly, on linux, you mgiht want to take advantage of the
		732	linux aio backend if possible, but fall back to something else if that
		733	isn't available.
		734
		735	struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
173		736
174	=item ev_loop_destroy (loop)	737	=item ev_loop_destroy (loop)
175		738
176	Like C<ev_default_destroy>, but destroys an event loop created by an	739	Destroys an event loop object (frees all memory and kernel state
177	earlier call to C<ev_loop_new>.	740	etc.). None of the active event watchers will be stopped in the normal
		741	sense, so e.g. C<ev_is_active> might still return true. It is your
		742	responsibility to either stop all watchers cleanly yourself I<before>
		743	calling this function, or cope with the fact afterwards (which is usually
		744	the easiest thing, you can just ignore the watchers and/or C<free ()> them
		745	for example).
178		746
179	=item ev_default_fork ()	747	Note that certain global state, such as signal state (and installed signal
		748	handlers), will not be freed by this function, and related watchers (such
		749	as signal and child watchers) would need to be stopped manually.
180		750
181	This function reinitialises the kernel state for backends that have	751	This function is normally used on loop objects allocated by
182	one. Despite the name, you can call it anytime, but it makes most sense	752	C<ev_loop_new>, but it can also be used on the default loop returned by
183	after forking, in either the parent or child process (or both, but that	753	C<ev_default_loop>, in which case it is not thread-safe.
184	again makes little sense).
185		754
186	You I<must> call this function after forking if and only if you want to	755	Note that it is not advisable to call this function on the default loop
187	use the event library in both processes. If you just fork+exec, you don't	756	except in the rare occasion where you really need to free its resources.
188	have to call it.	757	If you need dynamically allocated loops it is better to use C<ev_loop_new>
189		758	and C<ev_loop_destroy>.
190	The function itself is quite fast and its usually not a problem to call
191	it just in case after a fork. To make this easy, the function will fit in
192	quite nicely into a call to C<pthread_atfork>:
193
194	pthread_atfork (0, 0, ev_default_fork);
195		759
196	=item ev_loop_fork (loop)	760	=item ev_loop_fork (loop)
197		761
198	Like C<ev_default_fork>, but acts on an event loop created by	762	This function sets a flag that causes subsequent C<ev_run> iterations
199	C<ev_loop_new>. Yes, you have to call this on every allocated event loop	763	to reinitialise the kernel state for backends that have one. Despite
200	after fork, and how you do this is entirely your own problem.	764	the name, you can call it anytime you are allowed to start or stop
		765	watchers (except inside an C<ev_prepare> callback), but it makes most
		766	sense after forking, in the child process. You I<must> call it (or use
		767	C<EVFLAG_FORKCHECK>) in the child before resuming or calling C<ev_run>.
201		768
		769	In addition, if you want to reuse a loop (via this function or
		770	C<EVFLAG_FORKCHECK>), you I<also> have to ignore C<SIGPIPE>.
		771
		772	Again, you I<have> to call it on I<any> loop that you want to re-use after
		773	a fork, I<even if you do not plan to use the loop in the parent>. This is
		774	because some kernel interfaces *cough* I<kqueue> *cough* do funny things
		775	during fork.
		776
		777	On the other hand, you only need to call this function in the child
		778	process if and only if you want to use the event loop in the child. If
		779	you just fork+exec or create a new loop in the child, you don't have to
		780	call it at all (in fact, C<epoll> is so badly broken that it makes a
		781	difference, but libev will usually detect this case on its own and do a
		782	costly reset of the backend).
		783
		784	The function itself is quite fast and it's usually not a problem to call
		785	it just in case after a fork.
		786
		787	Example: Automate calling C<ev_loop_fork> on the default loop when
		788	using pthreads.
		789
		790	static void
		791	post_fork_child (void)
		792	{
		793	ev_loop_fork (EV_DEFAULT);
		794	}
		795
		796	...
		797	pthread_atfork (0, 0, post_fork_child);
		798
		799	=item int ev_is_default_loop (loop)
		800
		801	Returns true when the given loop is, in fact, the default loop, and false
		802	otherwise.
		803
		804	=item unsigned int ev_iteration (loop)
		805
		806	Returns the current iteration count for the event loop, which is identical
		807	to the number of times libev did poll for new events. It starts at C<0>
		808	and happily wraps around with enough iterations.
		809
		810	This value can sometimes be useful as a generation counter of sorts (it
		811	"ticks" the number of loop iterations), as it roughly corresponds with
		812	C<ev_prepare> and C<ev_check> calls - and is incremented between the
		813	prepare and check phases.
		814
202	=item unsigned int ev_method (loop)	815	=item unsigned int ev_depth (loop)
203		816
		817	Returns the number of times C<ev_run> was entered minus the number of
		818	times C<ev_run> was exited normally, in other words, the recursion depth.
		819
		820	Outside C<ev_run>, this number is zero. In a callback, this number is
		821	C<1>, unless C<ev_run> was invoked recursively (or from another thread),
		822	in which case it is higher.
		823
		824	Leaving C<ev_run> abnormally (setjmp/longjmp, cancelling the thread,
		825	throwing an exception etc.), doesn't count as "exit" - consider this
		826	as a hint to avoid such ungentleman-like behaviour unless it's really
		827	convenient, in which case it is fully supported.
		828
		829	=item unsigned int ev_backend (loop)
		830
204	Returns one of the C<EVMETHOD_*> flags indicating the event backend in	831	Returns one of the C<EVBACKEND_*> flags indicating the event backend in
205	use.	832	use.
206		833
207	=item ev_tstamp = ev_now (loop)	834	=item ev_tstamp ev_now (loop)
208		835
209	Returns the current "event loop time", which is the time the event loop	836	Returns the current "event loop time", which is the time the event loop
210	got events and started processing them. This timestamp does not change	837	received events and started processing them. This timestamp does not
211	as long as callbacks are being processed, and this is also the base time	838	change as long as callbacks are being processed, and this is also the base
212	used for relative timers. You can treat it as the timestamp of the event	839	time used for relative timers. You can treat it as the timestamp of the
213	occuring (or more correctly, the mainloop finding out about it).	840	event occurring (or more correctly, libev finding out about it).
214		841
		842	=item ev_now_update (loop)
		843
		844	Establishes the current time by querying the kernel, updating the time
		845	returned by C<ev_now ()> in the progress. This is a costly operation and
		846	is usually done automatically within C<ev_run ()>.
		847
		848	This function is rarely useful, but when some event callback runs for a
		849	very long time without entering the event loop, updating libev's idea of
		850	the current time is a good idea.
		851
		852	See also L</The special problem of time updates> in the C<ev_timer> section.
		853
		854	=item ev_suspend (loop)
		855
		856	=item ev_resume (loop)
		857
		858	These two functions suspend and resume an event loop, for use when the
		859	loop is not used for a while and timeouts should not be processed.
		860
		861	A typical use case would be an interactive program such as a game: When
		862	the user presses C<^Z> to suspend the game and resumes it an hour later it
		863	would be best to handle timeouts as if no time had actually passed while
		864	the program was suspended. This can be achieved by calling C<ev_suspend>
		865	in your C<SIGTSTP> handler, sending yourself a C<SIGSTOP> and calling
		866	C<ev_resume> directly afterwards to resume timer processing.
		867
		868	Effectively, all C<ev_timer> watchers will be delayed by the time spend
		869	between C<ev_suspend> and C<ev_resume>, and all C<ev_periodic> watchers
		870	will be rescheduled (that is, they will lose any events that would have
		871	occurred while suspended).
		872
		873	After calling C<ev_suspend> you B<must not> call I<any> function on the
		874	given loop other than C<ev_resume>, and you B<must not> call C<ev_resume>
		875	without a previous call to C<ev_suspend>.
		876
		877	Calling C<ev_suspend>/C<ev_resume> has the side effect of updating the
		878	event loop time (see C<ev_now_update>).
		879
215	=item ev_loop (loop, int flags)	880	=item bool ev_run (loop, int flags)
216		881
217	Finally, this is it, the event handler. This function usually is called	882	Finally, this is it, the event handler. This function usually is called
218	after you initialised all your watchers and you want to start handling	883	after you have initialised all your watchers and you want to start
219	events.	884	handling events. It will ask the operating system for any new events, call
		885	the watcher callbacks, and then repeat the whole process indefinitely: This
		886	is why event loops are called I<loops>.
220		887
221	If the flags argument is specified as 0, it will not return until either	888	If the flags argument is specified as C<0>, it will keep handling events
222	no event watchers are active anymore or C<ev_unloop> was called.	889	until either no event watchers are active anymore or C<ev_break> was
		890	called.
223		891
		892	The return value is false if there are no more active watchers (which
		893	usually means "all jobs done" or "deadlock"), and true in all other cases
		894	(which usually means " you should call C<ev_run> again").
		895
		896	Please note that an explicit C<ev_break> is usually better than
		897	relying on all watchers to be stopped when deciding when a program has
		898	finished (especially in interactive programs), but having a program
		899	that automatically loops as long as it has to and no longer by virtue
		900	of relying on its watchers stopping correctly, that is truly a thing of
		901	beauty.
		902
		903	This function is I<mostly> exception-safe - you can break out of a
		904	C<ev_run> call by calling C<longjmp> in a callback, throwing a C++
		905	exception and so on. This does not decrement the C<ev_depth> value, nor
		906	will it clear any outstanding C<EVBREAK_ONE> breaks.
		907
224	A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle	908	A flags value of C<EVRUN_NOWAIT> will look for new events, will handle
225	those events and any outstanding ones, but will not block your process in	909	those events and any already outstanding ones, but will not wait and
226	case there are no events.	910	block your process in case there are no events and will return after one
		911	iteration of the loop. This is sometimes useful to poll and handle new
		912	events while doing lengthy calculations, to keep the program responsive.
227		913
228	A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if	914	A flags value of C<EVRUN_ONCE> will look for new events (waiting if
229	neccessary) and will handle those and any outstanding ones. It will block	915	necessary) and will handle those and any already outstanding ones. It
230	your process until at least one new event arrives.	916	will block your process until at least one new event arrives (which could
		917	be an event internal to libev itself, so there is no guarantee that a
		918	user-registered callback will be called), and will return after one
		919	iteration of the loop.
231		920
232	This flags value could be used to implement alternative looping	921	This is useful if you are waiting for some external event in conjunction
233	constructs, but the C<prepare> and C<check> watchers provide a better and	922	with something not expressible using other libev watchers (i.e. "roll your
234	more generic mechanism.	923	own C<ev_run>"). However, a pair of C<ev_prepare>/C<ev_check> watchers is
		924	usually a better approach for this kind of thing.
235		925
		926	Here are the gory details of what C<ev_run> does (this is for your
		927	understanding, not a guarantee that things will work exactly like this in
		928	future versions):
		929
		930	- Increment loop depth.
		931	- Reset the ev_break status.
		932	- Before the first iteration, call any pending watchers.
		933	LOOP:
		934	- If EVFLAG_FORKCHECK was used, check for a fork.
		935	- If a fork was detected (by any means), queue and call all fork watchers.
		936	- Queue and call all prepare watchers.
		937	- If ev_break was called, goto FINISH.
		938	- If we have been forked, detach and recreate the kernel state
		939	as to not disturb the other process.
		940	- Update the kernel state with all outstanding changes.
		941	- Update the "event loop time" (ev_now ()).
		942	- Calculate for how long to sleep or block, if at all
		943	(active idle watchers, EVRUN_NOWAIT or not having
		944	any active watchers at all will result in not sleeping).
		945	- Sleep if the I/O and timer collect interval say so.
		946	- Increment loop iteration counter.
		947	- Block the process, waiting for any events.
		948	- Queue all outstanding I/O (fd) events.
		949	- Update the "event loop time" (ev_now ()), and do time jump adjustments.
		950	- Queue all expired timers.
		951	- Queue all expired periodics.
		952	- Queue all idle watchers with priority higher than that of pending events.
		953	- Queue all check watchers.
		954	- Call all queued watchers in reverse order (i.e. check watchers first).
		955	Signals and child watchers are implemented as I/O watchers, and will
		956	be handled here by queueing them when their watcher gets executed.
		957	- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
		958	were used, or there are no active watchers, goto FINISH, otherwise
		959	continue with step LOOP.
		960	FINISH:
		961	- Reset the ev_break status iff it was EVBREAK_ONE.
		962	- Decrement the loop depth.
		963	- Return.
		964
		965	Example: Queue some jobs and then loop until no events are outstanding
		966	anymore.
		967
		968	... queue jobs here, make sure they register event watchers as long
		969	... as they still have work to do (even an idle watcher will do..)
		970	ev_run (my_loop, 0);
		971	... jobs done or somebody called break. yeah!
		972
236	=item ev_unloop (loop, how)	973	=item ev_break (loop, how)
237		974
238	Can be used to make a call to C<ev_loop> return early. The C<how> argument	975	Can be used to make a call to C<ev_run> return early (but only after it
239	must be either C<EVUNLOOP_ONCE>, which will make the innermost C<ev_loop>	976	has processed all outstanding events). The C<how> argument must be either
240	call return, or C<EVUNLOOP_ALL>, which will make all nested C<ev_loop>	977	C<EVBREAK_ONE>, which will make the innermost C<ev_run> call return, or
241	calls return.	978	C<EVBREAK_ALL>, which will make all nested C<ev_run> calls return.
		979
		980	This "break state" will be cleared on the next call to C<ev_run>.
		981
		982	It is safe to call C<ev_break> from outside any C<ev_run> calls, too, in
		983	which case it will have no effect.
242		984
243	=item ev_ref (loop)	985	=item ev_ref (loop)
244		986
245	=item ev_unref (loop)	987	=item ev_unref (loop)
246		988
247	Ref/unref can be used to add or remove a refcount on the event loop: Every	989	Ref/unref can be used to add or remove a reference count on the event
248	watcher keeps one reference. If you have a long-runing watcher you never	990	loop: Every watcher keeps one reference, and as long as the reference
249	unregister that should not keep ev_loop from running, ev_unref() after	991	count is nonzero, C<ev_run> will not return on its own.
250	starting, and ev_ref() before stopping it. Libev itself uses this for	992
251	example for its internal signal pipe: It is not visible to you as a user	993	This is useful when you have a watcher that you never intend to
252	and should not keep C<ev_loop> from exiting if the work is done. It is	994	unregister, but that nevertheless should not keep C<ev_run> from
		995	returning. In such a case, call C<ev_unref> after starting, and C<ev_ref>
		996	before stopping it.
		997
		998	As an example, libev itself uses this for its internal signal pipe: It
		999	is not visible to the libev user and should not keep C<ev_run> from
		1000	exiting if no event watchers registered by it are active. It is also an
253	also an excellent way to do this for generic recurring timers or from	1001	excellent way to do this for generic recurring timers or from within
254	within third-party libraries. Just remember to unref after start and ref	1002	third-party libraries. Just remember to I<unref after start> and I<ref
255	before stop.	1003	before stop> (but only if the watcher wasn't active before, or was active
		1004	before, respectively. Note also that libev might stop watchers itself
		1005	(e.g. non-repeating timers) in which case you have to C<ev_ref>
		1006	in the callback).
		1007
		1008	Example: Create a signal watcher, but keep it from keeping C<ev_run>
		1009	running when nothing else is active.
		1010
		1011	ev_signal exitsig;
		1012	ev_signal_init (&exitsig, sig_cb, SIGINT);
		1013	ev_signal_start (loop, &exitsig);
		1014	ev_unref (loop);
		1015
		1016	Example: For some weird reason, unregister the above signal handler again.
		1017
		1018	ev_ref (loop);
		1019	ev_signal_stop (loop, &exitsig);
		1020
		1021	=item ev_set_io_collect_interval (loop, ev_tstamp interval)
		1022
		1023	=item ev_set_timeout_collect_interval (loop, ev_tstamp interval)
		1024
		1025	These advanced functions influence the time that libev will spend waiting
		1026	for events. Both time intervals are by default C<0>, meaning that libev
		1027	will try to invoke timer/periodic callbacks and I/O callbacks with minimum
		1028	latency.
		1029
		1030	Setting these to a higher value (the C<interval> I<must> be >= C<0>)
		1031	allows libev to delay invocation of I/O and timer/periodic callbacks
		1032	to increase efficiency of loop iterations (or to increase power-saving
		1033	opportunities).
		1034
		1035	The idea is that sometimes your program runs just fast enough to handle
		1036	one (or very few) event(s) per loop iteration. While this makes the
		1037	program responsive, it also wastes a lot of CPU time to poll for new
		1038	events, especially with backends like C<select ()> which have a high
		1039	overhead for the actual polling but can deliver many events at once.
		1040
		1041	By setting a higher I<io collect interval> you allow libev to spend more
		1042	time collecting I/O events, so you can handle more events per iteration,
		1043	at the cost of increasing latency. Timeouts (both C<ev_periodic> and
		1044	C<ev_timer>) will not be affected. Setting this to a non-null value will
		1045	introduce an additional C<ev_sleep ()> call into most loop iterations. The
		1046	sleep time ensures that libev will not poll for I/O events more often then
		1047	once per this interval, on average (as long as the host time resolution is
		1048	good enough).
		1049
		1050	Likewise, by setting a higher I<timeout collect interval> you allow libev
		1051	to spend more time collecting timeouts, at the expense of increased
		1052	latency/jitter/inexactness (the watcher callback will be called
		1053	later). C<ev_io> watchers will not be affected. Setting this to a non-null
		1054	value will not introduce any overhead in libev.
		1055
		1056	Many (busy) programs can usually benefit by setting the I/O collect
		1057	interval to a value near C<0.1> or so, which is often enough for
		1058	interactive servers (of course not for games), likewise for timeouts. It
		1059	usually doesn't make much sense to set it to a lower value than C<0.01>,
		1060	as this approaches the timing granularity of most systems. Note that if
		1061	you do transactions with the outside world and you can't increase the
		1062	parallelity, then this setting will limit your transaction rate (if you
		1063	need to poll once per transaction and the I/O collect interval is 0.01,
		1064	then you can't do more than 100 transactions per second).
		1065
		1066	Setting the I<timeout collect interval> can improve the opportunity for
		1067	saving power, as the program will "bundle" timer callback invocations that
		1068	are "near" in time together, by delaying some, thus reducing the number of
		1069	times the process sleeps and wakes up again. Another useful technique to
		1070	reduce iterations/wake-ups is to use C<ev_periodic> watchers and make sure
		1071	they fire on, say, one-second boundaries only.
		1072
		1073	Example: we only need 0.1s timeout granularity, and we wish not to poll
		1074	more often than 100 times per second:
		1075
		1076	ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
		1077	ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
		1078
		1079	=item ev_invoke_pending (loop)
		1080
		1081	This call will simply invoke all pending watchers while resetting their
		1082	pending state. Normally, C<ev_run> does this automatically when required,
		1083	but when overriding the invoke callback this call comes handy. This
		1084	function can be invoked from a watcher - this can be useful for example
		1085	when you want to do some lengthy calculation and want to pass further
		1086	event handling to another thread (you still have to make sure only one
		1087	thread executes within C<ev_invoke_pending> or C<ev_run> of course).
		1088
		1089	=item int ev_pending_count (loop)
		1090
		1091	Returns the number of pending watchers - zero indicates that no watchers
		1092	are pending.
		1093
		1094	=item ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))
		1095
		1096	This overrides the invoke pending functionality of the loop: Instead of
		1097	invoking all pending watchers when there are any, C<ev_run> will call
		1098	this callback instead. This is useful, for example, when you want to
		1099	invoke the actual watchers inside another context (another thread etc.).
		1100
		1101	If you want to reset the callback, use C<ev_invoke_pending> as new
		1102	callback.
		1103
		1104	=item ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())
		1105
		1106	Sometimes you want to share the same loop between multiple threads. This
		1107	can be done relatively simply by putting mutex_lock/unlock calls around
		1108	each call to a libev function.
		1109
		1110	However, C<ev_run> can run an indefinite time, so it is not feasible
		1111	to wait for it to return. One way around this is to wake up the event
		1112	loop via C<ev_break> and C<ev_async_send>, another way is to set these
		1113	I<release> and I<acquire> callbacks on the loop.
		1114
		1115	When set, then C<release> will be called just before the thread is
		1116	suspended waiting for new events, and C<acquire> is called just
		1117	afterwards.
		1118
		1119	Ideally, C<release> will just call your mutex_unlock function, and
		1120	C<acquire> will just call the mutex_lock function again.
		1121
		1122	While event loop modifications are allowed between invocations of
		1123	C<release> and C<acquire> (that's their only purpose after all), no
		1124	modifications done will affect the event loop, i.e. adding watchers will
		1125	have no effect on the set of file descriptors being watched, or the time
		1126	waited. Use an C<ev_async> watcher to wake up C<ev_run> when you want it
		1127	to take note of any changes you made.
		1128
		1129	In theory, threads executing C<ev_run> will be async-cancel safe between
		1130	invocations of C<release> and C<acquire>.
		1131
		1132	See also the locking example in the C<THREADS> section later in this
		1133	document.
		1134
		1135	=item ev_set_userdata (loop, void *data)
		1136
		1137	=item void *ev_userdata (loop)
		1138
		1139	Set and retrieve a single C<void *> associated with a loop. When
		1140	C<ev_set_userdata> has never been called, then C<ev_userdata> returns
		1141	C<0>.
		1142
		1143	These two functions can be used to associate arbitrary data with a loop,
		1144	and are intended solely for the C<invoke_pending_cb>, C<release> and
		1145	C<acquire> callbacks described above, but of course can be (ab-)used for
		1146	any other purpose as well.
		1147
		1148	=item ev_verify (loop)
		1149
		1150	This function only does something when C<EV_VERIFY> support has been
		1151	compiled in, which is the default for non-minimal builds. It tries to go
		1152	through all internal structures and checks them for validity. If anything
		1153	is found to be inconsistent, it will print an error message to standard
		1154	error and call C<abort ()>.
		1155
		1156	This can be used to catch bugs inside libev itself: under normal
		1157	circumstances, this function will never abort as of course libev keeps its
		1158	data structures consistent.
256		1159
257	=back	1160	=back
258		1161
		1162
259	=head1 ANATOMY OF A WATCHER	1163	=head1 ANATOMY OF A WATCHER
260		1164
		1165	In the following description, uppercase C<TYPE> in names stands for the
		1166	watcher type, e.g. C<ev_TYPE_start> can mean C<ev_timer_start> for timer
		1167	watchers and C<ev_io_start> for I/O watchers.
		1168
261	A watcher is a structure that you create and register to record your	1169	A watcher is an opaque structure that you allocate and register to record
262	interest in some event. For instance, if you want to wait for STDIN to	1170	your interest in some event. To make a concrete example, imagine you want
263	become readable, you would create an ev_io watcher for that:	1171	to wait for STDIN to become readable, you would create an C<ev_io> watcher
		1172	for that:
264		1173
265	static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)	1174	static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
266	{	1175	{
267	ev_io_stop (w);	1176	ev_io_stop (w);
268	ev_unloop (loop, EVUNLOOP_ALL);	1177	ev_break (loop, EVBREAK_ALL);
269	}	1178	}
270		1179
271	struct ev_loop *loop = ev_default_loop (0);	1180	struct ev_loop *loop = ev_default_loop (0);
		1181
272	struct ev_io stdin_watcher;	1182	ev_io stdin_watcher;
		1183
273	ev_init (&stdin_watcher, my_cb);	1184	ev_init (&stdin_watcher, my_cb);
274	ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);	1185	ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
275	ev_io_start (loop, &stdin_watcher);	1186	ev_io_start (loop, &stdin_watcher);
		1187
276	ev_loop (loop, 0);	1188	ev_run (loop, 0);
277		1189
278	As you can see, you are responsible for allocating the memory for your	1190	As you can see, you are responsible for allocating the memory for your
279	watcher structures (and it is usually a bad idea to do this on the stack,	1191	watcher structures (and it is I<usually> a bad idea to do this on the
280	although this can sometimes be quite valid).	1192	stack).
281		1193
		1194	Each watcher has an associated watcher structure (called C<struct ev_TYPE>
		1195	or simply C<ev_TYPE>, as typedefs are provided for all watcher structs).
		1196
282	Each watcher structure must be initialised by a call to C<ev_init	1197	Each watcher structure must be initialised by a call to C<ev_init (watcher
283	(watcher *, callback)>, which expects a callback to be provided. This	1198	*, callback)>, which expects a callback to be provided. This callback is
284	callback gets invoked each time the event occurs (or, in the case of io	1199	invoked each time the event occurs (or, in the case of I/O watchers, each
285	watchers, each time the event loop detects that the file descriptor given	1200	time the event loop detects that the file descriptor given is readable
286	is readable and/or writable).	1201	and/or writable).
287		1202
288	Each watcher type has its own C<< ev_<type>_set (watcher *, ...) >> macro	1203	Each watcher type further has its own C<< ev_TYPE_set (watcher *, ...) >>
289	with arguments specific to this watcher type. There is also a macro	1204	macro to configure it, with arguments specific to the watcher type. There
290	to combine initialisation and setting in one call: C<< ev_<type>_init	1205	is also a macro to combine initialisation and setting in one call: C<<
291	(watcher *, callback, ...) >>.	1206	ev_TYPE_init (watcher *, callback, ...) >>.
292		1207
293	To make the watcher actually watch out for events, you have to start it	1208	To make the watcher actually watch out for events, you have to start it
294	with a watcher-specific start function (C<< ev_<type>_start (loop, watcher	1209	with a watcher-specific start function (C<< ev_TYPE_start (loop, watcher
295	*) >>), and you can stop watching for events at any time by calling the	1210	*) >>), and you can stop watching for events at any time by calling the
296	corresponding stop function (C<< ev_<type>_stop (loop, watcher *) >>.	1211	corresponding stop function (C<< ev_TYPE_stop (loop, watcher *) >>.
297		1212
298	As long as your watcher is active (has been started but not stopped) you	1213	As long as your watcher is active (has been started but not stopped) you
299	must not touch the values stored in it. Most specifically you must never	1214	must not touch the values stored in it. Most specifically you must never
300	reinitialise it or call its set method.	1215	reinitialise it or call its C<ev_TYPE_set> macro.
301
302	You cna check whether an event is active by calling the C<ev_is_active
303	(watcher *)> macro. To see whether an event is outstanding (but the
304	callback for it has not been called yet) you cna use the C<ev_is_pending
305	(watcher *)> macro.
306		1216
307	Each and every callback receives the event loop pointer as first, the	1217	Each and every callback receives the event loop pointer as first, the
308	registered watcher structure as second, and a bitset of received events as	1218	registered watcher structure as second, and a bitset of received events as
309	third argument.	1219	third argument.
310		1220
311	The rceeived events usually include a single bit per event type received	1221	The received events usually include a single bit per event type received
312	(you can receive multiple events at the same time). The possible bit masks	1222	(you can receive multiple events at the same time). The possible bit masks
313	are:	1223	are:
314		1224
315	=over 4	1225	=over 4
316		1226
317	=item EV_READ	1227	=item C<EV_READ>
318		1228
319	=item EV_WRITE	1229	=item C<EV_WRITE>
320		1230
321	The file descriptor in the ev_io watcher has become readable and/or	1231	The file descriptor in the C<ev_io> watcher has become readable and/or
322	writable.	1232	writable.
323		1233
324	=item EV_TIMEOUT	1234	=item C<EV_TIMER>
325		1235
326	The ev_timer watcher has timed out.	1236	The C<ev_timer> watcher has timed out.
327		1237
328	=item EV_PERIODIC	1238	=item C<EV_PERIODIC>
329		1239
330	The ev_periodic watcher has timed out.	1240	The C<ev_periodic> watcher has timed out.
331		1241
332	=item EV_SIGNAL	1242	=item C<EV_SIGNAL>
333		1243
334	The signal specified in the ev_signal watcher has been received by a thread.	1244	The signal specified in the C<ev_signal> watcher has been received by a thread.
335		1245
336	=item EV_CHILD	1246	=item C<EV_CHILD>
337		1247
338	The pid specified in the ev_child watcher has received a status change.	1248	The pid specified in the C<ev_child> watcher has received a status change.
339		1249
		1250	=item C<EV_STAT>
		1251
		1252	The path specified in the C<ev_stat> watcher changed its attributes somehow.
		1253
340	=item EV_IDLE	1254	=item C<EV_IDLE>
341		1255
342	The ev_idle watcher has determined that you have nothing better to do.	1256	The C<ev_idle> watcher has determined that you have nothing better to do.
343		1257
344	=item EV_PREPARE	1258	=item C<EV_PREPARE>
345		1259
346	=item EV_CHECK	1260	=item C<EV_CHECK>
347		1261
348	All ev_prepare watchers are invoked just I<before> C<ev_loop> starts	1262	All C<ev_prepare> watchers are invoked just I<before> C<ev_run> starts to
349	to gather new events, and all ev_check watchers are invoked just after	1263	gather new events, and all C<ev_check> watchers are queued (not invoked)
350	C<ev_loop> has gathered them, but before it invokes any callbacks for any	1264	just after C<ev_run> has gathered them, but before it queues any callbacks
		1265	for any received events. That means C<ev_prepare> watchers are the last
		1266	watchers invoked before the event loop sleeps or polls for new events, and
		1267	C<ev_check> watchers will be invoked before any other watchers of the same
		1268	or lower priority within an event loop iteration.
		1269
351	received events. Callbacks of both watcher types can start and stop as	1270	Callbacks of both watcher types can start and stop as many watchers as
352	many watchers as they want, and all of them will be taken into account	1271	they want, and all of them will be taken into account (for example, a
353	(for example, a ev_prepare watcher might start an idle watcher to keep	1272	C<ev_prepare> watcher might start an idle watcher to keep C<ev_run> from
354	C<ev_loop> from blocking).	1273	blocking).
355		1274
		1275	=item C<EV_EMBED>
		1276
		1277	The embedded event loop specified in the C<ev_embed> watcher needs attention.
		1278
		1279	=item C<EV_FORK>
		1280
		1281	The event loop has been resumed in the child process after fork (see
		1282	C<ev_fork>).
		1283
		1284	=item C<EV_CLEANUP>
		1285
		1286	The event loop is about to be destroyed (see C<ev_cleanup>).
		1287
		1288	=item C<EV_ASYNC>
		1289
		1290	The given async watcher has been asynchronously notified (see C<ev_async>).
		1291
		1292	=item C<EV_CUSTOM>
		1293
		1294	Not ever sent (or otherwise used) by libev itself, but can be freely used
		1295	by libev users to signal watchers (e.g. via C<ev_feed_event>).
		1296
356	=item EV_ERROR	1297	=item C<EV_ERROR>
357		1298
358	An unspecified error has occured, the watcher has been stopped. This might	1299	An unspecified error has occurred, the watcher has been stopped. This might
359	happen because the watcher could not be properly started because libev	1300	happen because the watcher could not be properly started because libev
360	ran out of memory, a file descriptor was found to be closed or any other	1301	ran out of memory, a file descriptor was found to be closed or any other
		1302	problem. Libev considers these application bugs.
		1303
361	problem. You best act on it by reporting the problem and somehow coping	1304	You best act on it by reporting the problem and somehow coping with the
362	with the watcher being stopped.	1305	watcher being stopped. Note that well-written programs should not receive
		1306	an error ever, so when your watcher receives it, this usually indicates a
		1307	bug in your program.
363		1308
364	Libev will usually signal a few "dummy" events together with an error,	1309	Libev will usually signal a few "dummy" events together with an error, for
365	for example it might indicate that a fd is readable or writable, and if	1310	example it might indicate that a fd is readable or writable, and if your
366	your callbacks is well-written it can just attempt the operation and cope	1311	callbacks is well-written it can just attempt the operation and cope with
367	with the error from read() or write(). This will not work in multithreaded	1312	the error from read() or write(). This will not work in multi-threaded
368	programs, though, so beware.	1313	programs, though, as the fd could already be closed and reused for another
		1314	thing, so beware.
369		1315
370	=back	1316	=back
371		1317
372	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER	1318	=head2 GENERIC WATCHER FUNCTIONS
373		1319
374	Each watcher has, by default, a member C<void *data> that you can change	1320	=over 4
375	and read at any time, libev will completely ignore it. This cna be used
376	to associate arbitrary data with your watcher. If you need more data and
377	don't want to allocate memory and store a pointer to it in that data
378	member, you can also "subclass" the watcher type and provide your own
379	data:
380		1321
381	struct my_io	1322	=item C<ev_init> (ev_TYPE *watcher, callback)
		1323
		1324	This macro initialises the generic portion of a watcher. The contents
		1325	of the watcher object can be arbitrary (so C<malloc> will do). Only
		1326	the generic parts of the watcher are initialised, you I<need> to call
		1327	the type-specific C<ev_TYPE_set> macro afterwards to initialise the
		1328	type-specific parts. For each type there is also a C<ev_TYPE_init> macro
		1329	which rolls both calls into one.
		1330
		1331	You can reinitialise a watcher at any time as long as it has been stopped
		1332	(or never started) and there are no pending events outstanding.
		1333
		1334	The callback is always of type C<void (*)(struct ev_loop *loop, ev_TYPE *watcher,
		1335	int revents)>.
		1336
		1337	Example: Initialise an C<ev_io> watcher in two steps.
		1338
		1339	ev_io w;
		1340	ev_init (&w, my_cb);
		1341	ev_io_set (&w, STDIN_FILENO, EV_READ);
		1342
		1343	=item C<ev_TYPE_set> (ev_TYPE *watcher, [args])
		1344
		1345	This macro initialises the type-specific parts of a watcher. You need to
		1346	call C<ev_init> at least once before you call this macro, but you can
		1347	call C<ev_TYPE_set> any number of times. You must not, however, call this
		1348	macro on a watcher that is active (it can be pending, however, which is a
		1349	difference to the C<ev_init> macro).
		1350
		1351	Although some watcher types do not have type-specific arguments
		1352	(e.g. C<ev_prepare>) you still need to call its C<set> macro.
		1353
		1354	See C<ev_init>, above, for an example.
		1355
		1356	=item C<ev_TYPE_init> (ev_TYPE *watcher, callback, [args])
		1357
		1358	This convenience macro rolls both C<ev_init> and C<ev_TYPE_set> macro
		1359	calls into a single call. This is the most convenient method to initialise
		1360	a watcher. The same limitations apply, of course.
		1361
		1362	Example: Initialise and set an C<ev_io> watcher in one step.
		1363
		1364	ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
		1365
		1366	=item C<ev_TYPE_start> (loop, ev_TYPE *watcher)
		1367
		1368	Starts (activates) the given watcher. Only active watchers will receive
		1369	events. If the watcher is already active nothing will happen.
		1370
		1371	Example: Start the C<ev_io> watcher that is being abused as example in this
		1372	whole section.
		1373
		1374	ev_io_start (EV_DEFAULT_UC, &w);
		1375
		1376	=item C<ev_TYPE_stop> (loop, ev_TYPE *watcher)
		1377
		1378	Stops the given watcher if active, and clears the pending status (whether
		1379	the watcher was active or not).
		1380
		1381	It is possible that stopped watchers are pending - for example,
		1382	non-repeating timers are being stopped when they become pending - but
		1383	calling C<ev_TYPE_stop> ensures that the watcher is neither active nor
		1384	pending. If you want to free or reuse the memory used by the watcher it is
		1385	therefore a good idea to always call its C<ev_TYPE_stop> function.
		1386
		1387	=item bool ev_is_active (ev_TYPE *watcher)
		1388
		1389	Returns a true value iff the watcher is active (i.e. it has been started
		1390	and not yet been stopped). As long as a watcher is active you must not modify
		1391	it.
		1392
		1393	=item bool ev_is_pending (ev_TYPE *watcher)
		1394
		1395	Returns a true value iff the watcher is pending, (i.e. it has outstanding
		1396	events but its callback has not yet been invoked). As long as a watcher
		1397	is pending (but not active) you must not call an init function on it (but
		1398	C<ev_TYPE_set> is safe), you must not change its priority, and you must
		1399	make sure the watcher is available to libev (e.g. you cannot C<free ()>
		1400	it).
		1401
		1402	=item callback ev_cb (ev_TYPE *watcher)
		1403
		1404	Returns the callback currently set on the watcher.
		1405
		1406	=item ev_set_cb (ev_TYPE *watcher, callback)
		1407
		1408	Change the callback. You can change the callback at virtually any time
		1409	(modulo threads).
		1410
		1411	=item ev_set_priority (ev_TYPE *watcher, int priority)
		1412
		1413	=item int ev_priority (ev_TYPE *watcher)
		1414
		1415	Set and query the priority of the watcher. The priority is a small
		1416	integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI>
		1417	(default: C<-2>). Pending watchers with higher priority will be invoked
		1418	before watchers with lower priority, but priority will not keep watchers
		1419	from being executed (except for C<ev_idle> watchers).
		1420
		1421	If you need to suppress invocation when higher priority events are pending
		1422	you need to look at C<ev_idle> watchers, which provide this functionality.
		1423
		1424	You I<must not> change the priority of a watcher as long as it is active or
		1425	pending.
		1426
		1427	Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is
		1428	fine, as long as you do not mind that the priority value you query might
		1429	or might not have been clamped to the valid range.
		1430
		1431	The default priority used by watchers when no priority has been set is
		1432	always C<0>, which is supposed to not be too high and not be too low :).
		1433
		1434	See L</WATCHER PRIORITY MODELS>, below, for a more thorough treatment of
		1435	priorities.
		1436
		1437	=item ev_invoke (loop, ev_TYPE *watcher, int revents)
		1438
		1439	Invoke the C<watcher> with the given C<loop> and C<revents>. Neither
		1440	C<loop> nor C<revents> need to be valid as long as the watcher callback
		1441	can deal with that fact, as both are simply passed through to the
		1442	callback.
		1443
		1444	=item int ev_clear_pending (loop, ev_TYPE *watcher)
		1445
		1446	If the watcher is pending, this function clears its pending status and
		1447	returns its C<revents> bitset (as if its callback was invoked). If the
		1448	watcher isn't pending it does nothing and returns C<0>.
		1449
		1450	Sometimes it can be useful to "poll" a watcher instead of waiting for its
		1451	callback to be invoked, which can be accomplished with this function.
		1452
		1453	=item ev_feed_event (loop, ev_TYPE *watcher, int revents)
		1454
		1455	Feeds the given event set into the event loop, as if the specified event
		1456	had happened for the specified watcher (which must be a pointer to an
		1457	initialised but not necessarily started event watcher). Obviously you must
		1458	not free the watcher as long as it has pending events.
		1459
		1460	Stopping the watcher, letting libev invoke it, or calling
		1461	C<ev_clear_pending> will clear the pending event, even if the watcher was
		1462	not started in the first place.
		1463
		1464	See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related
		1465	functions that do not need a watcher.
		1466
		1467	=back
		1468
		1469	See also the L</ASSOCIATING CUSTOM DATA WITH A WATCHER> and L</BUILDING YOUR
		1470	OWN COMPOSITE WATCHERS> idioms.
		1471
		1472	=head2 WATCHER STATES
		1473
		1474	There are various watcher states mentioned throughout this manual -
		1475	active, pending and so on. In this section these states and the rules to
		1476	transition between them will be described in more detail - and while these
		1477	rules might look complicated, they usually do "the right thing".
		1478
		1479	=over 4
		1480
		1481	=item initialised
		1482
		1483	Before a watcher can be registered with the event loop it has to be
		1484	initialised. This can be done with a call to C<ev_TYPE_init>, or calls to
		1485	C<ev_init> followed by the watcher-specific C<ev_TYPE_set> function.
		1486
		1487	In this state it is simply some block of memory that is suitable for
		1488	use in an event loop. It can be moved around, freed, reused etc. at
		1489	will - as long as you either keep the memory contents intact, or call
		1490	C<ev_TYPE_init> again.
		1491
		1492	=item started/running/active
		1493
		1494	Once a watcher has been started with a call to C<ev_TYPE_start> it becomes
		1495	property of the event loop, and is actively waiting for events. While in
		1496	this state it cannot be accessed (except in a few documented ways), moved,
		1497	freed or anything else - the only legal thing is to keep a pointer to it,
		1498	and call libev functions on it that are documented to work on active watchers.
		1499
		1500	=item pending
		1501
		1502	If a watcher is active and libev determines that an event it is interested
		1503	in has occurred (such as a timer expiring), it will become pending. It will
		1504	stay in this pending state until either it is stopped or its callback is
		1505	about to be invoked, so it is not normally pending inside the watcher
		1506	callback.
		1507
		1508	The watcher might or might not be active while it is pending (for example,
		1509	an expired non-repeating timer can be pending but no longer active). If it
		1510	is stopped, it can be freely accessed (e.g. by calling C<ev_TYPE_set>),
		1511	but it is still property of the event loop at this time, so cannot be
		1512	moved, freed or reused. And if it is active the rules described in the
		1513	previous item still apply.
		1514
		1515	It is also possible to feed an event on a watcher that is not active (e.g.
		1516	via C<ev_feed_event>), in which case it becomes pending without being
		1517	active.
		1518
		1519	=item stopped
		1520
		1521	A watcher can be stopped implicitly by libev (in which case it might still
		1522	be pending), or explicitly by calling its C<ev_TYPE_stop> function. The
		1523	latter will clear any pending state the watcher might be in, regardless
		1524	of whether it was active or not, so stopping a watcher explicitly before
		1525	freeing it is often a good idea.
		1526
		1527	While stopped (and not pending) the watcher is essentially in the
		1528	initialised state, that is, it can be reused, moved, modified in any way
		1529	you wish (but when you trash the memory block, you need to C<ev_TYPE_init>
		1530	it again).
		1531
		1532	=back
		1533
		1534	=head2 WATCHER PRIORITY MODELS
		1535
		1536	Many event loops support I<watcher priorities>, which are usually small
		1537	integers that influence the ordering of event callback invocation
		1538	between watchers in some way, all else being equal.
		1539
		1540	In libev, Watcher priorities can be set using C<ev_set_priority>. See its
		1541	description for the more technical details such as the actual priority
		1542	range.
		1543
		1544	There are two common ways how these these priorities are being interpreted
		1545	by event loops:
		1546
		1547	In the more common lock-out model, higher priorities "lock out" invocation
		1548	of lower priority watchers, which means as long as higher priority
		1549	watchers receive events, lower priority watchers are not being invoked.
		1550
		1551	The less common only-for-ordering model uses priorities solely to order
		1552	callback invocation within a single event loop iteration: Higher priority
		1553	watchers are invoked before lower priority ones, but they all get invoked
		1554	before polling for new events.
		1555
		1556	Libev uses the second (only-for-ordering) model for all its watchers
		1557	except for idle watchers (which use the lock-out model).
		1558
		1559	The rationale behind this is that implementing the lock-out model for
		1560	watchers is not well supported by most kernel interfaces, and most event
		1561	libraries will just poll for the same events again and again as long as
		1562	their callbacks have not been executed, which is very inefficient in the
		1563	common case of one high-priority watcher locking out a mass of lower
		1564	priority ones.
		1565
		1566	Static (ordering) priorities are most useful when you have two or more
		1567	watchers handling the same resource: a typical usage example is having an
		1568	C<ev_io> watcher to receive data, and an associated C<ev_timer> to handle
		1569	timeouts. Under load, data might be received while the program handles
		1570	other jobs, but since timers normally get invoked first, the timeout
		1571	handler will be executed before checking for data. In that case, giving
		1572	the timer a lower priority than the I/O watcher ensures that I/O will be
		1573	handled first even under adverse conditions (which is usually, but not
		1574	always, what you want).
		1575
		1576	Since idle watchers use the "lock-out" model, meaning that idle watchers
		1577	will only be executed when no same or higher priority watchers have
		1578	received events, they can be used to implement the "lock-out" model when
		1579	required.
		1580
		1581	For example, to emulate how many other event libraries handle priorities,
		1582	you can associate an C<ev_idle> watcher to each such watcher, and in
		1583	the normal watcher callback, you just start the idle watcher. The real
		1584	processing is done in the idle watcher callback. This causes libev to
		1585	continuously poll and process kernel event data for the watcher, but when
		1586	the lock-out case is known to be rare (which in turn is rare :), this is
		1587	workable.
		1588
		1589	Usually, however, the lock-out model implemented that way will perform
		1590	miserably under the type of load it was designed to handle. In that case,
		1591	it might be preferable to stop the real watcher before starting the
		1592	idle watcher, so the kernel will not have to process the event in case
		1593	the actual processing will be delayed for considerable time.
		1594
		1595	Here is an example of an I/O watcher that should run at a strictly lower
		1596	priority than the default, and which should only process data when no
		1597	other events are pending:
		1598
		1599	ev_idle idle; // actual processing watcher
		1600	ev_io io; // actual event watcher
		1601
		1602	static void
		1603	io_cb (EV_P_ ev_io *w, int revents)
382	{	1604	{
383	struct ev_io io;	1605	// stop the I/O watcher, we received the event, but
384	int otherfd;	1606	// are not yet ready to handle it.
385	void *somedata;	1607	ev_io_stop (EV_A_ w);
386	struct whatever *mostinteresting;	1608
		1609	// start the idle watcher to handle the actual event.
		1610	// it will not be executed as long as other watchers
		1611	// with the default priority are receiving events.
		1612	ev_idle_start (EV_A_ &idle);
387	}	1613	}
388		1614
389	And since your callback will be called with a pointer to the watcher, you	1615	static void
390	can cast it back to your own type:	1616	idle_cb (EV_P_ ev_idle *w, int revents)
391
392	static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
393	{	1617	{
394	struct my_io *w = (struct my_io *)w_;	1618	// actual processing
395	...	1619	read (STDIN_FILENO, ...);
		1620
		1621	// have to start the I/O watcher again, as
		1622	// we have handled the event
		1623	ev_io_start (EV_P_ &io);
396	}	1624	}
397		1625
398	More interesting and less C-conformant ways of catsing your callback type	1626	// initialisation
399	have been omitted....	1627	ev_idle_init (&idle, idle_cb);
		1628	ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
		1629	ev_io_start (EV_DEFAULT_ &io);
		1630
		1631	In the "real" world, it might also be beneficial to start a timer, so that
		1632	low-priority connections can not be locked out forever under load. This
		1633	enables your program to keep a lower latency for important connections
		1634	during short periods of high load, while not completely locking out less
		1635	important ones.
400		1636
401		1637
402	=head1 WATCHER TYPES	1638	=head1 WATCHER TYPES
403		1639
404	This section describes each watcher in detail, but will not repeat	1640	This section describes each watcher in detail, but will not repeat
405	information given in the last section.	1641	information given in the last section. Any initialisation/set macros,
		1642	functions and members specific to the watcher type are explained.
406		1643
		1644	Members are additionally marked with either I<[read-only]>, meaning that,
		1645	while the watcher is active, you can look at the member and expect some
		1646	sensible content, but you must not modify it (you can modify it while the
		1647	watcher is stopped to your hearts content), or I<[read-write]>, which
		1648	means you can expect it to have some sensible content while the watcher
		1649	is active, but you can also modify it. Modifying it may not do something
		1650	sensible or take immediate effect (or do anything at all), but libev will
		1651	not crash or malfunction in any way.
		1652
		1653
407	=head2 struct ev_io - is my file descriptor readable or writable	1654	=head2 C<ev_io> - is this file descriptor readable or writable?
408		1655
409	I/O watchers check whether a file descriptor is readable or writable	1656	I/O watchers check whether a file descriptor is readable or writable
410	in each iteration of the event loop (This behaviour is called	1657	in each iteration of the event loop, or, more precisely, when reading
411	level-triggering because you keep receiving events as long as the	1658	would not block the process and writing would at least be able to write
412	condition persists. Remember you cna stop the watcher if you don't want to	1659	some data. This behaviour is called level-triggering because you keep
413	act on the event and neither want to receive future events).	1660	receiving events as long as the condition persists. Remember you can stop
		1661	the watcher if you don't want to act on the event and neither want to
		1662	receive future events.
414		1663
415	In general you can register as many read and/or write event watchers oer	1664	In general you can register as many read and/or write event watchers per
416	fd as you want (as long as you don't confuse yourself). Setting all file	1665	fd as you want (as long as you don't confuse yourself). Setting all file
417	descriptors to non-blocking mode is also usually a good idea (but not	1666	descriptors to non-blocking mode is also usually a good idea (but not
418	required if you know what you are doing).	1667	required if you know what you are doing).
419		1668
420	You have to be careful with dup'ed file descriptors, though. Some backends	1669	Another thing you have to watch out for is that it is quite easy to
421	(the linux epoll backend is a notable example) cannot handle dup'ed file	1670	receive "spurious" readiness notifications, that is, your callback might
422	descriptors correctly if you register interest in two or more fds pointing	1671	be called with C<EV_READ> but a subsequent C<read>(2) will actually block
423	to the same file/socket etc. description.	1672	because there is no data. It is very easy to get into this situation even
		1673	with a relatively standard program structure. Thus it is best to always
		1674	use non-blocking I/O: An extra C<read>(2) returning C<EAGAIN> is far
		1675	preferable to a program hanging until some data arrives.
424		1676
425	If you must do this, then force the use of a known-to-be-good backend	1677	If you cannot run the fd in non-blocking mode (for example you should
426	(at the time of this writing, this includes only EVMETHOD_SELECT and	1678	not play around with an Xlib connection), then you have to separately
427	EVMETHOD_POLL).	1679	re-test whether a file descriptor is really ready with a known-to-be good
		1680	interface such as poll (fortunately in the case of Xlib, it already does
		1681	this on its own, so its quite safe to use). Some people additionally
		1682	use C<SIGALRM> and an interval timer, just to be sure you won't block
		1683	indefinitely.
		1684
		1685	But really, best use non-blocking mode.
		1686
		1687	=head3 The special problem of disappearing file descriptors
		1688
		1689	Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
		1690	a file descriptor (either due to calling C<close> explicitly or any other
		1691	means, such as C<dup2>). The reason is that you register interest in some
		1692	file descriptor, but when it goes away, the operating system will silently
		1693	drop this interest. If another file descriptor with the same number then
		1694	is registered with libev, there is no efficient way to see that this is,
		1695	in fact, a different file descriptor.
		1696
		1697	To avoid having to explicitly tell libev about such cases, libev follows
		1698	the following policy: Each time C<ev_io_set> is being called, libev
		1699	will assume that this is potentially a new file descriptor, otherwise
		1700	it is assumed that the file descriptor stays the same. That means that
		1701	you I<have> to call C<ev_io_set> (or C<ev_io_init>) when you change the
		1702	descriptor even if the file descriptor number itself did not change.
		1703
		1704	This is how one would do it normally anyway, the important point is that
		1705	the libev application should not optimise around libev but should leave
		1706	optimisations to libev.
		1707
		1708	=head3 The special problem of dup'ed file descriptors
		1709
		1710	Some backends (e.g. epoll), cannot register events for file descriptors,
		1711	but only events for the underlying file descriptions. That means when you
		1712	have C<dup ()>'ed file descriptors or weirder constellations, and register
		1713	events for them, only one file descriptor might actually receive events.
		1714
		1715	There is no workaround possible except not registering events
		1716	for potentially C<dup ()>'ed file descriptors, or to resort to
		1717	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
		1718
		1719	=head3 The special problem of files
		1720
		1721	Many people try to use C<select> (or libev) on file descriptors
		1722	representing files, and expect it to become ready when their program
		1723	doesn't block on disk accesses (which can take a long time on their own).
		1724
		1725	However, this cannot ever work in the "expected" way - you get a readiness
		1726	notification as soon as the kernel knows whether and how much data is
		1727	there, and in the case of open files, that's always the case, so you
		1728	always get a readiness notification instantly, and your read (or possibly
		1729	write) will still block on the disk I/O.
		1730
		1731	Another way to view it is that in the case of sockets, pipes, character
		1732	devices and so on, there is another party (the sender) that delivers data
		1733	on its own, but in the case of files, there is no such thing: the disk
		1734	will not send data on its own, simply because it doesn't know what you
		1735	wish to read - you would first have to request some data.
		1736
		1737	Since files are typically not-so-well supported by advanced notification
		1738	mechanism, libev tries hard to emulate POSIX behaviour with respect
		1739	to files, even though you should not use it. The reason for this is
		1740	convenience: sometimes you want to watch STDIN or STDOUT, which is
		1741	usually a tty, often a pipe, but also sometimes files or special devices
		1742	(for example, C<epoll> on Linux works with F</dev/random> but not with
		1743	F</dev/urandom>), and even though the file might better be served with
		1744	asynchronous I/O instead of with non-blocking I/O, it is still useful when
		1745	it "just works" instead of freezing.
		1746
		1747	So avoid file descriptors pointing to files when you know it (e.g. use
		1748	libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or
		1749	when you rarely read from a file instead of from a socket, and want to
		1750	reuse the same code path.
		1751
		1752	=head3 The special problem of fork
		1753
		1754	Some backends (epoll, kqueue, probably linuxaio) do not support C<fork ()>
		1755	at all or exhibit useless behaviour. Libev fully supports fork, but needs
		1756	to be told about it in the child if you want to continue to use it in the
		1757	child.
		1758
		1759	To support fork in your child processes, you have to call C<ev_loop_fork
		1760	()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to
		1761	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
		1762
		1763	=head3 The special problem of SIGPIPE
		1764
		1765	While not really specific to libev, it is easy to forget about C<SIGPIPE>:
		1766	when writing to a pipe whose other end has been closed, your program gets
		1767	sent a SIGPIPE, which, by default, aborts your program. For most programs
		1768	this is sensible behaviour, for daemons, this is usually undesirable.
		1769
		1770	So when you encounter spurious, unexplained daemon exits, make sure you
		1771	ignore SIGPIPE (and maybe make sure you log the exit status of your daemon
		1772	somewhere, as that would have given you a big clue).
		1773
		1774	=head3 The special problem of accept()ing when you can't
		1775
		1776	Many implementations of the POSIX C<accept> function (for example,
		1777	found in post-2004 Linux) have the peculiar behaviour of not removing a
		1778	connection from the pending queue in all error cases.
		1779
		1780	For example, larger servers often run out of file descriptors (because
		1781	of resource limits), causing C<accept> to fail with C<ENFILE> but not
		1782	rejecting the connection, leading to libev signalling readiness on
		1783	the next iteration again (the connection still exists after all), and
		1784	typically causing the program to loop at 100% CPU usage.
		1785
		1786	Unfortunately, the set of errors that cause this issue differs between
		1787	operating systems, there is usually little the app can do to remedy the
		1788	situation, and no known thread-safe method of removing the connection to
		1789	cope with overload is known (to me).
		1790
		1791	One of the easiest ways to handle this situation is to just ignore it
		1792	- when the program encounters an overload, it will just loop until the
		1793	situation is over. While this is a form of busy waiting, no OS offers an
		1794	event-based way to handle this situation, so it's the best one can do.
		1795
		1796	A better way to handle the situation is to log any errors other than
		1797	C<EAGAIN> and C<EWOULDBLOCK>, making sure not to flood the log with such
		1798	messages, and continue as usual, which at least gives the user an idea of
		1799	what could be wrong ("raise the ulimit!"). For extra points one could stop
		1800	the C<ev_io> watcher on the listening fd "for a while", which reduces CPU
		1801	usage.
		1802
		1803	If your program is single-threaded, then you could also keep a dummy file
		1804	descriptor for overload situations (e.g. by opening F</dev/null>), and
		1805	when you run into C<ENFILE> or C<EMFILE>, close it, run C<accept>,
		1806	close that fd, and create a new dummy fd. This will gracefully refuse
		1807	clients under typical overload conditions.
		1808
		1809	The last way to handle it is to simply log the error and C<exit>, as
		1810	is often done with C<malloc> failures, but this results in an easy
		1811	opportunity for a DoS attack.
		1812
		1813	=head3 Watcher-Specific Functions
428		1814
429	=over 4	1815	=over 4
430		1816
431	=item ev_io_init (ev_io *, callback, int fd, int events)	1817	=item ev_io_init (ev_io *, callback, int fd, int events)
432		1818
433	=item ev_io_set (ev_io *, int fd, int events)	1819	=item ev_io_set (ev_io *, int fd, int events)
434		1820
435	Configures an ev_io watcher. The fd is the file descriptor to rceeive	1821	Configures an C<ev_io> watcher. The C<fd> is the file descriptor to
436	events for and events is either C<EV_READ>, C<EV_WRITE> or C<EV_READ |	1822	receive events for and C<events> is either C<EV_READ>, C<EV_WRITE> or
437	EV_WRITE> to receive the given events.	1823	C<EV_READ | EV_WRITE>, to express the desire to receive the given events.
		1824
		1825	=item int fd [read-only]
		1826
		1827	The file descriptor being watched.
		1828
		1829	=item int events [read-only]
		1830
		1831	The events being watched.
438		1832
439	=back	1833	=back
440		1834
		1835	=head3 Examples
		1836
		1837	Example: Call C<stdin_readable_cb> when STDIN_FILENO has become, well
		1838	readable, but only once. Since it is likely line-buffered, you could
		1839	attempt to read a whole line in the callback.
		1840
		1841	static void
		1842	stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
		1843	{
		1844	ev_io_stop (loop, w);
		1845	.. read from stdin here (or from w->fd) and handle any I/O errors
		1846	}
		1847
		1848	...
		1849	struct ev_loop *loop = ev_default_init (0);
		1850	ev_io stdin_readable;
		1851	ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
		1852	ev_io_start (loop, &stdin_readable);
		1853	ev_run (loop, 0);
		1854
		1855
441	=head2 struct ev_timer - relative and optionally recurring timeouts	1856	=head2 C<ev_timer> - relative and optionally repeating timeouts
442		1857
443	Timer watchers are simple relative timers that generate an event after a	1858	Timer watchers are simple relative timers that generate an event after a
444	given time, and optionally repeating in regular intervals after that.	1859	given time, and optionally repeating in regular intervals after that.
445		1860
446	The timers are based on real time, that is, if you register an event that	1861	The timers are based on real time, that is, if you register an event that
447	times out after an hour and youreset your system clock to last years	1862	times out after an hour and you reset your system clock to January last
448	time, it will still time out after (roughly) and hour. "Roughly" because	1863	year, it will still time out after (roughly) one hour. "Roughly" because
449	detecting time jumps is hard, and soem inaccuracies are unavoidable (the	1864	detecting time jumps is hard, and some inaccuracies are unavoidable (the
450	monotonic clock option helps a lot here).	1865	monotonic clock option helps a lot here).
451		1866
		1867	The callback is guaranteed to be invoked only I<after> its timeout has
		1868	passed (not I<at>, so on systems with very low-resolution clocks this
		1869	might introduce a small delay, see "the special problem of being too
		1870	early", below). If multiple timers become ready during the same loop
		1871	iteration then the ones with earlier time-out values are invoked before
		1872	ones of the same priority with later time-out values (but this is no
		1873	longer true when a callback calls C<ev_run> recursively).
		1874
		1875	=head3 Be smart about timeouts
		1876
		1877	Many real-world problems involve some kind of timeout, usually for error
		1878	recovery. A typical example is an HTTP request - if the other side hangs,
		1879	you want to raise some error after a while.
		1880
		1881	What follows are some ways to handle this problem, from obvious and
		1882	inefficient to smart and efficient.
		1883
		1884	In the following, a 60 second activity timeout is assumed - a timeout that
		1885	gets reset to 60 seconds each time there is activity (e.g. each time some
		1886	data or other life sign was received).
		1887
452	=over 4	1888	=over 4
453		1889
		1890	=item 1. Use a timer and stop, reinitialise and start it on activity.
		1891
		1892	This is the most obvious, but not the most simple way: In the beginning,
		1893	start the watcher:
		1894
		1895	ev_timer_init (timer, callback, 60., 0.);
		1896	ev_timer_start (loop, timer);
		1897
		1898	Then, each time there is some activity, C<ev_timer_stop> it, initialise it
		1899	and start it again:
		1900
		1901	ev_timer_stop (loop, timer);
		1902	ev_timer_set (timer, 60., 0.);
		1903	ev_timer_start (loop, timer);
		1904
		1905	This is relatively simple to implement, but means that each time there is
		1906	some activity, libev will first have to remove the timer from its internal
		1907	data structure and then add it again. Libev tries to be fast, but it's
		1908	still not a constant-time operation.
		1909
		1910	=item 2. Use a timer and re-start it with C<ev_timer_again> inactivity.
		1911
		1912	This is the easiest way, and involves using C<ev_timer_again> instead of
		1913	C<ev_timer_start>.
		1914
		1915	To implement this, configure an C<ev_timer> with a C<repeat> value
		1916	of C<60> and then call C<ev_timer_again> at start and each time you
		1917	successfully read or write some data. If you go into an idle state where
		1918	you do not expect data to travel on the socket, you can C<ev_timer_stop>
		1919	the timer, and C<ev_timer_again> will automatically restart it if need be.
		1920
		1921	That means you can ignore both the C<ev_timer_start> function and the
		1922	C<after> argument to C<ev_timer_set>, and only ever use the C<repeat>
		1923	member and C<ev_timer_again>.
		1924
		1925	At start:
		1926
		1927	ev_init (timer, callback);
		1928	timer->repeat = 60.;
		1929	ev_timer_again (loop, timer);
		1930
		1931	Each time there is some activity:
		1932
		1933	ev_timer_again (loop, timer);
		1934
		1935	It is even possible to change the time-out on the fly, regardless of
		1936	whether the watcher is active or not:
		1937
		1938	timer->repeat = 30.;
		1939	ev_timer_again (loop, timer);
		1940
		1941	This is slightly more efficient then stopping/starting the timer each time
		1942	you want to modify its timeout value, as libev does not have to completely
		1943	remove and re-insert the timer from/into its internal data structure.
		1944
		1945	It is, however, even simpler than the "obvious" way to do it.
		1946
		1947	=item 3. Let the timer time out, but then re-arm it as required.
		1948
		1949	This method is more tricky, but usually most efficient: Most timeouts are
		1950	relatively long compared to the intervals between other activity - in
		1951	our example, within 60 seconds, there are usually many I/O events with
		1952	associated activity resets.
		1953
		1954	In this case, it would be more efficient to leave the C<ev_timer> alone,
		1955	but remember the time of last activity, and check for a real timeout only
		1956	within the callback:
		1957
		1958	ev_tstamp timeout = 60.;
		1959	ev_tstamp last_activity; // time of last activity
		1960	ev_timer timer;
		1961
		1962	static void
		1963	callback (EV_P_ ev_timer *w, int revents)
		1964	{
		1965	// calculate when the timeout would happen
		1966	ev_tstamp after = last_activity - ev_now (EV_A) + timeout;
		1967
		1968	// if negative, it means we the timeout already occurred
		1969	if (after < 0.)
		1970	{
		1971	// timeout occurred, take action
		1972	}
		1973	else
		1974	{
		1975	// callback was invoked, but there was some recent
		1976	// activity. simply restart the timer to time out
		1977	// after "after" seconds, which is the earliest time
		1978	// the timeout can occur.
		1979	ev_timer_set (w, after, 0.);
		1980	ev_timer_start (EV_A_ w);
		1981	}
		1982	}
		1983
		1984	To summarise the callback: first calculate in how many seconds the
		1985	timeout will occur (by calculating the absolute time when it would occur,
		1986	C<last_activity + timeout>, and subtracting the current time, C<ev_now
		1987	(EV_A)> from that).
		1988
		1989	If this value is negative, then we are already past the timeout, i.e. we
		1990	timed out, and need to do whatever is needed in this case.
		1991
		1992	Otherwise, we now the earliest time at which the timeout would trigger,
		1993	and simply start the timer with this timeout value.
		1994
		1995	In other words, each time the callback is invoked it will check whether
		1996	the timeout occurred. If not, it will simply reschedule itself to check
		1997	again at the earliest time it could time out. Rinse. Repeat.
		1998
		1999	This scheme causes more callback invocations (about one every 60 seconds
		2000	minus half the average time between activity), but virtually no calls to
		2001	libev to change the timeout.
		2002
		2003	To start the machinery, simply initialise the watcher and set
		2004	C<last_activity> to the current time (meaning there was some activity just
		2005	now), then call the callback, which will "do the right thing" and start
		2006	the timer:
		2007
		2008	last_activity = ev_now (EV_A);
		2009	ev_init (&timer, callback);
		2010	callback (EV_A_ &timer, 0);
		2011
		2012	When there is some activity, simply store the current time in
		2013	C<last_activity>, no libev calls at all:
		2014
		2015	if (activity detected)
		2016	last_activity = ev_now (EV_A);
		2017
		2018	When your timeout value changes, then the timeout can be changed by simply
		2019	providing a new value, stopping the timer and calling the callback, which
		2020	will again do the right thing (for example, time out immediately :).
		2021
		2022	timeout = new_value;
		2023	ev_timer_stop (EV_A_ &timer);
		2024	callback (EV_A_ &timer, 0);
		2025
		2026	This technique is slightly more complex, but in most cases where the
		2027	time-out is unlikely to be triggered, much more efficient.
		2028
		2029	=item 4. Wee, just use a double-linked list for your timeouts.
		2030
		2031	If there is not one request, but many thousands (millions...), all
		2032	employing some kind of timeout with the same timeout value, then one can
		2033	do even better:
		2034
		2035	When starting the timeout, calculate the timeout value and put the timeout
		2036	at the I<end> of the list.
		2037
		2038	Then use an C<ev_timer> to fire when the timeout at the I<beginning> of
		2039	the list is expected to fire (for example, using the technique #3).
		2040
		2041	When there is some activity, remove the timer from the list, recalculate
		2042	the timeout, append it to the end of the list again, and make sure to
		2043	update the C<ev_timer> if it was taken from the beginning of the list.
		2044
		2045	This way, one can manage an unlimited number of timeouts in O(1) time for
		2046	starting, stopping and updating the timers, at the expense of a major
		2047	complication, and having to use a constant timeout. The constant timeout
		2048	ensures that the list stays sorted.
		2049
		2050	=back
		2051
		2052	So which method the best?
		2053
		2054	Method #2 is a simple no-brain-required solution that is adequate in most
		2055	situations. Method #3 requires a bit more thinking, but handles many cases
		2056	better, and isn't very complicated either. In most case, choosing either
		2057	one is fine, with #3 being better in typical situations.
		2058
		2059	Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
		2060	rather complicated, but extremely efficient, something that really pays
		2061	off after the first million or so of active timers, i.e. it's usually
		2062	overkill :)
		2063
		2064	=head3 The special problem of being too early
		2065
		2066	If you ask a timer to call your callback after three seconds, then
		2067	you expect it to be invoked after three seconds - but of course, this
		2068	cannot be guaranteed to infinite precision. Less obviously, it cannot be
		2069	guaranteed to any precision by libev - imagine somebody suspending the
		2070	process with a STOP signal for a few hours for example.
		2071
		2072	So, libev tries to invoke your callback as soon as possible I<after> the
		2073	delay has occurred, but cannot guarantee this.
		2074
		2075	A less obvious failure mode is calling your callback too early: many event
		2076	loops compare timestamps with a "elapsed delay >= requested delay", but
		2077	this can cause your callback to be invoked much earlier than you would
		2078	expect.
		2079
		2080	To see why, imagine a system with a clock that only offers full second
		2081	resolution (think windows if you can't come up with a broken enough OS
		2082	yourself). If you schedule a one-second timer at the time 500.9, then the
		2083	event loop will schedule your timeout to elapse at a system time of 500
		2084	(500.9 truncated to the resolution) + 1, or 501.
		2085
		2086	If an event library looks at the timeout 0.1s later, it will see "501 >=
		2087	501" and invoke the callback 0.1s after it was started, even though a
		2088	one-second delay was requested - this is being "too early", despite best
		2089	intentions.
		2090
		2091	This is the reason why libev will never invoke the callback if the elapsed
		2092	delay equals the requested delay, but only when the elapsed delay is
		2093	larger than the requested delay. In the example above, libev would only invoke
		2094	the callback at system time 502, or 1.1s after the timer was started.
		2095
		2096	So, while libev cannot guarantee that your callback will be invoked
		2097	exactly when requested, it I<can> and I<does> guarantee that the requested
		2098	delay has actually elapsed, or in other words, it always errs on the "too
		2099	late" side of things.
		2100
		2101	=head3 The special problem of time updates
		2102
		2103	Establishing the current time is a costly operation (it usually takes
		2104	at least one system call): EV therefore updates its idea of the current
		2105	time only before and after C<ev_run> collects new events, which causes a
		2106	growing difference between C<ev_now ()> and C<ev_time ()> when handling
		2107	lots of events in one iteration.
		2108
		2109	The relative timeouts are calculated relative to the C<ev_now ()>
		2110	time. This is usually the right thing as this timestamp refers to the time
		2111	of the event triggering whatever timeout you are modifying/starting. If
		2112	you suspect event processing to be delayed and you I<need> to base the
		2113	timeout on the current time, use something like the following to adjust
		2114	for it:
		2115
		2116	ev_timer_set (&timer, after + (ev_time () - ev_now ()), 0.);
		2117
		2118	If the event loop is suspended for a long time, you can also force an
		2119	update of the time returned by C<ev_now ()> by calling C<ev_now_update
		2120	()>, although that will push the event time of all outstanding events
		2121	further into the future.
		2122
		2123	=head3 The special problem of unsynchronised clocks
		2124
		2125	Modern systems have a variety of clocks - libev itself uses the normal
		2126	"wall clock" clock and, if available, the monotonic clock (to avoid time
		2127	jumps).
		2128
		2129	Neither of these clocks is synchronised with each other or any other clock
		2130	on the system, so C<ev_time ()> might return a considerably different time
		2131	than C<gettimeofday ()> or C<time ()>. On a GNU/Linux system, for example,
		2132	a call to C<gettimeofday> might return a second count that is one higher
		2133	than a directly following call to C<time>.
		2134
		2135	The moral of this is to only compare libev-related timestamps with
		2136	C<ev_time ()> and C<ev_now ()>, at least if you want better precision than
		2137	a second or so.
		2138
		2139	One more problem arises due to this lack of synchronisation: if libev uses
		2140	the system monotonic clock and you compare timestamps from C<ev_time>
		2141	or C<ev_now> from when you started your timer and when your callback is
		2142	invoked, you will find that sometimes the callback is a bit "early".
		2143
		2144	This is because C<ev_timer>s work in real time, not wall clock time, so
		2145	libev makes sure your callback is not invoked before the delay happened,
		2146	I<measured according to the real time>, not the system clock.
		2147
		2148	If your timeouts are based on a physical timescale (e.g. "time out this
		2149	connection after 100 seconds") then this shouldn't bother you as it is
		2150	exactly the right behaviour.
		2151
		2152	If you want to compare wall clock/system timestamps to your timers, then
		2153	you need to use C<ev_periodic>s, as these are based on the wall clock
		2154	time, where your comparisons will always generate correct results.
		2155
		2156	=head3 The special problems of suspended animation
		2157
		2158	When you leave the server world it is quite customary to hit machines that
		2159	can suspend/hibernate - what happens to the clocks during such a suspend?
		2160
		2161	Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
		2162	all processes, while the clocks (C<times>, C<CLOCK_MONOTONIC>) continue
		2163	to run until the system is suspended, but they will not advance while the
		2164	system is suspended. That means, on resume, it will be as if the program
		2165	was frozen for a few seconds, but the suspend time will not be counted
		2166	towards C<ev_timer> when a monotonic clock source is used. The real time
		2167	clock advanced as expected, but if it is used as sole clocksource, then a
		2168	long suspend would be detected as a time jump by libev, and timers would
		2169	be adjusted accordingly.
		2170
		2171	I would not be surprised to see different behaviour in different between
		2172	operating systems, OS versions or even different hardware.
		2173
		2174	The other form of suspend (job control, or sending a SIGSTOP) will see a
		2175	time jump in the monotonic clocks and the realtime clock. If the program
		2176	is suspended for a very long time, and monotonic clock sources are in use,
		2177	then you can expect C<ev_timer>s to expire as the full suspension time
		2178	will be counted towards the timers. When no monotonic clock source is in
		2179	use, then libev will again assume a timejump and adjust accordingly.
		2180
		2181	It might be beneficial for this latter case to call C<ev_suspend>
		2182	and C<ev_resume> in code that handles C<SIGTSTP>, to at least get
		2183	deterministic behaviour in this case (you can do nothing against
		2184	C<SIGSTOP>).
		2185
		2186	=head3 Watcher-Specific Functions and Data Members
		2187
		2188	=over 4
		2189
454	=item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)	2190	=item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
455		2191
456	=item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)	2192	=item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
457		2193
458	Configure the timer to trigger after C<after> seconds. If C<repeat> is	2194	Configure the timer to trigger after C<after> seconds (fractional and
459	C<0.>, then it will automatically be stopped. If it is positive, then the	2195	negative values are supported). If C<repeat> is C<0.>, then it will
		2196	automatically be stopped once the timeout is reached. If it is positive,
460	timer will automatically be configured to trigger again C<repeat> seconds	2197	then the timer will automatically be configured to trigger again C<repeat>
461	later, again, and again, until stopped manually.	2198	seconds later, again, and again, until stopped manually.
462		2199
463	The timer itself will do a best-effort at avoiding drift, that is, if you	2200	The timer itself will do a best-effort at avoiding drift, that is, if
464	configure a timer to trigger every 10 seconds, then it will trigger at	2201	you configure a timer to trigger every 10 seconds, then it will normally
465	exactly 10 second intervals. If, however, your program cannot keep up with	2202	trigger at exactly 10 second intervals. If, however, your program cannot
466	the timer (ecause it takes longer than those 10 seconds to do stuff) the	2203	keep up with the timer (because it takes longer than those 10 seconds to
467	timer will not fire more than once per event loop iteration.	2204	do stuff) the timer will not fire more than once per event loop iteration.
468		2205
469	=item ev_timer_again (loop)	2206	=item ev_timer_again (loop, ev_timer *)
470		2207
471	This will act as if the timer timed out and restart it again if it is	2208	This will act as if the timer timed out, and restarts it again if it is
472	repeating. The exact semantics are:	2209	repeating. It basically works like calling C<ev_timer_stop>, updating the
		2210	timeout to the C<repeat> value and calling C<ev_timer_start>.
473		2211
		2212	The exact semantics are as in the following rules, all of which will be
		2213	applied to the watcher:
		2214
		2215	=over 4
		2216
		2217	=item If the timer is pending, the pending status is always cleared.
		2218
474	If the timer is started but nonrepeating, stop it.	2219	=item If the timer is started but non-repeating, stop it (as if it timed
		2220	out, without invoking it).
475		2221
476	If the timer is repeating, either start it if necessary (with the repeat	2222	=item If the timer is repeating, make the C<repeat> value the new timeout
477	value), or reset the running timer to the repeat value.	2223	and start the timer, if necessary.
478
479	This sounds a bit complicated, but here is a useful and typical
480	example: Imagine you have a tcp connection and you want a so-called idle
481	timeout, that is, you want to be called when there have been, say, 60
482	seconds of inactivity on the socket. The easiest way to do this is to
483	configure an ev_timer with after=repeat=60 and calling ev_timer_again each
484	time you successfully read or write some data. If you go into an idle
485	state where you do not expect data to travel on the socket, you can stop
486	the timer, and again will automatically restart it if need be.
487		2224
488	=back	2225	=back
489		2226
		2227	This sounds a bit complicated, see L</Be smart about timeouts>, above, for a
		2228	usage example.
		2229
		2230	=item ev_tstamp ev_timer_remaining (loop, ev_timer *)
		2231
		2232	Returns the remaining time until a timer fires. If the timer is active,
		2233	then this time is relative to the current event loop time, otherwise it's
		2234	the timeout value currently configured.
		2235
		2236	That is, after an C<ev_timer_set (w, 5, 7)>, C<ev_timer_remaining> returns
		2237	C<5>. When the timer is started and one second passes, C<ev_timer_remaining>
		2238	will return C<4>. When the timer expires and is restarted, it will return
		2239	roughly C<7> (likely slightly less as callback invocation takes some time,
		2240	too), and so on.
		2241
		2242	=item ev_tstamp repeat [read-write]
		2243
		2244	The current C<repeat> value. Will be used each time the watcher times out
		2245	or C<ev_timer_again> is called, and determines the next timeout (if any),
		2246	which is also when any modifications are taken into account.
		2247
		2248	=back
		2249
		2250	=head3 Examples
		2251
		2252	Example: Create a timer that fires after 60 seconds.
		2253
		2254	static void
		2255	one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
		2256	{
		2257	.. one minute over, w is actually stopped right here
		2258	}
		2259
		2260	ev_timer mytimer;
		2261	ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
		2262	ev_timer_start (loop, &mytimer);
		2263
		2264	Example: Create a timeout timer that times out after 10 seconds of
		2265	inactivity.
		2266
		2267	static void
		2268	timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
		2269	{
		2270	.. ten seconds without any activity
		2271	}
		2272
		2273	ev_timer mytimer;
		2274	ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
		2275	ev_timer_again (&mytimer); /* start timer */
		2276	ev_run (loop, 0);
		2277
		2278	// and in some piece of code that gets executed on any "activity":
		2279	// reset the timeout to start ticking again at 10 seconds
		2280	ev_timer_again (&mytimer);
		2281
		2282
490	=head2 ev_periodic - to cron or not to cron it	2283	=head2 C<ev_periodic> - to cron or not to cron?
491		2284
492	Periodic watchers are also timers of a kind, but they are very versatile	2285	Periodic watchers are also timers of a kind, but they are very versatile
493	(and unfortunately a bit complex).	2286	(and unfortunately a bit complex).
494		2287
495	Unlike ev_timer's, they are not based on real time (or relative time)	2288	Unlike C<ev_timer>, periodic watchers are not based on real time (or
496	but on wallclock time (absolute time). You can tell a periodic watcher	2289	relative time, the physical time that passes) but on wall clock time
497	to trigger "at" some specific point in time. For example, if you tell a	2290	(absolute time, the thing you can read on your calendar or clock). The
498	periodic watcher to trigger in 10 seconds (by specifiying e.g. c<ev_now ()	2291	difference is that wall clock time can run faster or slower than real
499	+ 10.>) and then reset your system clock to the last year, then it will	2292	time, and time jumps are not uncommon (e.g. when you adjust your
500	take a year to trigger the event (unlike an ev_timer, which would trigger	2293	wrist-watch).
501	roughly 10 seconds later and of course not if you reset your system time
502	again).
503		2294
504	They can also be used to implement vastly more complex timers, such as	2295	You can tell a periodic watcher to trigger after some specific point
		2296	in time: for example, if you tell a periodic watcher to trigger "in 10
		2297	seconds" (by specifying e.g. C<ev_now () + 10.>, that is, an absolute time
		2298	not a delay) and then reset your system clock to January of the previous
		2299	year, then it will take a year or more to trigger the event (unlike an
		2300	C<ev_timer>, which would still trigger roughly 10 seconds after starting
		2301	it, as it uses a relative timeout).
		2302
		2303	C<ev_periodic> watchers can also be used to implement vastly more complex
505	triggering an event on eahc midnight, local time.	2304	timers, such as triggering an event on each "midnight, local time", or
		2305	other complicated rules. This cannot easily be done with C<ev_timer>
		2306	watchers, as those cannot react to time jumps.
		2307
		2308	As with timers, the callback is guaranteed to be invoked only when the
		2309	point in time where it is supposed to trigger has passed. If multiple
		2310	timers become ready during the same loop iteration then the ones with
		2311	earlier time-out values are invoked before ones with later time-out values
		2312	(but this is no longer true when a callback calls C<ev_run> recursively).
		2313
		2314	=head3 Watcher-Specific Functions and Data Members
506		2315
507	=over 4	2316	=over 4
508		2317
509	=item ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)	2318	=item ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)
510		2319
511	=item ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)	2320	=item ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)
512		2321
513	Lots of arguments, lets sort it out... There are basically three modes of	2322	Lots of arguments, let's sort it out... There are basically three modes of
514	operation, and we will explain them from simplest to complex:	2323	operation, and we will explain them from simplest to most complex:
515
516		2324
517	=over 4	2325	=over 4
518		2326
519	=item * absolute timer (interval = reschedule_cb = 0)	2327	=item * absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
520		2328
521	In this configuration the watcher triggers an event at the wallclock time	2329	In this configuration the watcher triggers an event after the wall clock
522	C<at> and doesn't repeat. It will not adjust when a time jump occurs,	2330	time C<offset> has passed. It will not repeat and will not adjust when a
523	that is, if it is to be run at January 1st 2011 then it will run when the	2331	time jump occurs, that is, if it is to be run at January 1st 2011 then it
524	system time reaches or surpasses this time.	2332	will be stopped and invoked when the system clock reaches or surpasses
		2333	this point in time.
525		2334
526	=item * non-repeating interval timer (interval > 0, reschedule_cb = 0)	2335	=item * repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
527		2336
528	In this mode the watcher will always be scheduled to time out at the next	2337	In this mode the watcher will always be scheduled to time out at the next
529	C<at + N * interval> time (for some integer N) and then repeat, regardless	2338	C<offset + N * interval> time (for some integer N, which can also be
530	of any time jumps.	2339	negative) and then repeat, regardless of any time jumps. The C<offset>
		2340	argument is merely an offset into the C<interval> periods.
531		2341
532	This can be used to create timers that do not drift with respect to system	2342	This can be used to create timers that do not drift with respect to the
533	time:	2343	system clock, for example, here is an C<ev_periodic> that triggers each
		2344	hour, on the hour (with respect to UTC):
534		2345
535	ev_periodic_set (&periodic, 0., 3600., 0);	2346	ev_periodic_set (&periodic, 0., 3600., 0);
536		2347
537	This doesn't mean there will always be 3600 seconds in between triggers,	2348	This doesn't mean there will always be 3600 seconds in between triggers,
538	but only that the the callback will be called when the system time shows a	2349	but only that the callback will be called when the system time shows a
539	full hour (UTC), or more correct, when the system time is evenly divisible	2350	full hour (UTC), or more correctly, when the system time is evenly divisible
540	by 3600.	2351	by 3600.
541		2352
542	Another way to think about it (for the mathematically inclined) is that	2353	Another way to think about it (for the mathematically inclined) is that
543	ev_periodic will try to run the callback in this mode at the next possible	2354	C<ev_periodic> will try to run the callback in this mode at the next possible
544	time where C<time = at (mod interval)>, regardless of any time jumps.	2355	time where C<time = offset (mod interval)>, regardless of any time jumps.
545		2356
		2357	The C<interval> I<MUST> be positive, and for numerical stability, the
		2358	interval value should be higher than C<1/8192> (which is around 100
		2359	microseconds) and C<offset> should be higher than C<0> and should have
		2360	at most a similar magnitude as the current time (say, within a factor of
		2361	ten). Typical values for offset are, in fact, C<0> or something between
		2362	C<0> and C<interval>, which is also the recommended range.
		2363
		2364	Note also that there is an upper limit to how often a timer can fire (CPU
		2365	speed for example), so if C<interval> is very small then timing stability
		2366	will of course deteriorate. Libev itself tries to be exact to be about one
		2367	millisecond (if the OS supports it and the machine is fast enough).
		2368
546	=item * manual reschedule mode (reschedule_cb = callback)	2369	=item * manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
547		2370
548	In this mode the values for C<interval> and C<at> are both being	2371	In this mode the values for C<interval> and C<offset> are both being
549	ignored. Instead, each time the periodic watcher gets scheduled, the	2372	ignored. Instead, each time the periodic watcher gets scheduled, the
550	reschedule callback will be called with the watcher as first, and the	2373	reschedule callback will be called with the watcher as first, and the
551	current time as second argument.	2374	current time as second argument.
552		2375
553	NOTE: I<This callback MUST NOT stop or destroy the periodic or any other	2376	NOTE: I<This callback MUST NOT stop or destroy any periodic watcher, ever,
554	periodic watcher, ever, or make any event loop modificstions>. If you need	2377	or make ANY other event loop modifications whatsoever, unless explicitly
555	to stop it, return 1e30 (or so, fudge fudge) and stop it afterwards.	2378	allowed by documentation here>.
556		2379
		2380	If you need to stop it, return C<now + 1e30> (or so, fudge fudge) and stop
		2381	it afterwards (e.g. by starting an C<ev_prepare> watcher, which is the
		2382	only event loop modification you are allowed to do).
		2383
557	Its prototype is c<ev_tstamp (*reschedule_cb)(struct ev_periodic *w,	2384	The callback prototype is C<ev_tstamp (*reschedule_cb)(ev_periodic
558	ev_tstamp now)>, e.g.:	2385	*w, ev_tstamp now)>, e.g.:
559		2386
		2387	static ev_tstamp
560	static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)	2388	my_rescheduler (ev_periodic *w, ev_tstamp now)
561	{	2389	{
562	return now + 60.;	2390	return now + 60.;
563	}	2391	}
564		2392
565	It must return the next time to trigger, based on the passed time value	2393	It must return the next time to trigger, based on the passed time value
566	(that is, the lowest time value larger than to the second argument). It	2394	(that is, the lowest time value larger than to the second argument). It
567	will usually be called just before the callback will be triggered, but	2395	will usually be called just before the callback will be triggered, but
568	might be called at other times, too.	2396	might be called at other times, too.
569		2397
		2398	NOTE: I<< This callback must always return a time that is higher than or
		2399	equal to the passed C<now> value >>.
		2400
570	This can be used to create very complex timers, such as a timer that	2401	This can be used to create very complex timers, such as a timer that
571	triggers on each midnight, local time. To do this, you would calculate the	2402	triggers on "next midnight, local time". To do this, you would calculate
572	next midnight after C<now> and return the timestamp value for this. How you do this	2403	the next midnight after C<now> and return the timestamp value for
573	is, again, up to you (but it is not trivial).	2404	this. Here is a (completely untested, no error checking) example on how to
		2405	do this:
		2406
		2407	#include <time.h>
		2408
		2409	static ev_tstamp
		2410	my_rescheduler (ev_periodic *w, ev_tstamp now)
		2411	{
		2412	time_t tnow = (time_t)now;
		2413	struct tm tm;
		2414	localtime_r (&tnow, &tm);
		2415
		2416	tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
		2417	++tm.tm_mday; // midnight next day
		2418
		2419	return mktime (&tm);
		2420	}
		2421
		2422	Note: this code might run into trouble on days that have more then two
		2423	midnights (beginning and end).
574		2424
575	=back	2425	=back
576		2426
577	=item ev_periodic_again (loop, ev_periodic *)	2427	=item ev_periodic_again (loop, ev_periodic *)
578		2428
579	Simply stops and restarts the periodic watcher again. This is only useful	2429	Simply stops and restarts the periodic watcher again. This is only useful
580	when you changed some parameters or the reschedule callback would return	2430	when you changed some parameters or the reschedule callback would return
581	a different time than the last time it was called (e.g. in a crond like	2431	a different time than the last time it was called (e.g. in a crond like
582	program when the crontabs have changed).	2432	program when the crontabs have changed).
583		2433
		2434	=item ev_tstamp ev_periodic_at (ev_periodic *)
		2435
		2436	When active, returns the absolute time that the watcher is supposed
		2437	to trigger next. This is not the same as the C<offset> argument to
		2438	C<ev_periodic_set>, but indeed works even in interval and manual
		2439	rescheduling modes.
		2440
		2441	=item ev_tstamp offset [read-write]
		2442
		2443	When repeating, this contains the offset value, otherwise this is the
		2444	absolute point in time (the C<offset> value passed to C<ev_periodic_set>,
		2445	although libev might modify this value for better numerical stability).
		2446
		2447	Can be modified any time, but changes only take effect when the periodic
		2448	timer fires or C<ev_periodic_again> is being called.
		2449
		2450	=item ev_tstamp interval [read-write]
		2451
		2452	The current interval value. Can be modified any time, but changes only
		2453	take effect when the periodic timer fires or C<ev_periodic_again> is being
		2454	called.
		2455
		2456	=item ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]
		2457
		2458	The current reschedule callback, or C<0>, if this functionality is
		2459	switched off. Can be changed any time, but changes only take effect when
		2460	the periodic timer fires or C<ev_periodic_again> is being called.
		2461
584	=back	2462	=back
585		2463
		2464	=head3 Examples
		2465
		2466	Example: Call a callback every hour, or, more precisely, whenever the
		2467	system time is divisible by 3600. The callback invocation times have
		2468	potentially a lot of jitter, but good long-term stability.
		2469
		2470	static void
		2471	clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
		2472	{
		2473	... its now a full hour (UTC, or TAI or whatever your clock follows)
		2474	}
		2475
		2476	ev_periodic hourly_tick;
		2477	ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
		2478	ev_periodic_start (loop, &hourly_tick);
		2479
		2480	Example: The same as above, but use a reschedule callback to do it:
		2481
		2482	#include <math.h>
		2483
		2484	static ev_tstamp
		2485	my_scheduler_cb (ev_periodic *w, ev_tstamp now)
		2486	{
		2487	return now + (3600. - fmod (now, 3600.));
		2488	}
		2489
		2490	ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
		2491
		2492	Example: Call a callback every hour, starting now:
		2493
		2494	ev_periodic hourly_tick;
		2495	ev_periodic_init (&hourly_tick, clock_cb,
		2496	fmod (ev_now (loop), 3600.), 3600., 0);
		2497	ev_periodic_start (loop, &hourly_tick);
		2498
		2499
586	=head2 ev_signal - signal me when a signal gets signalled	2500	=head2 C<ev_signal> - signal me when a signal gets signalled!
587		2501
588	Signal watchers will trigger an event when the process receives a specific	2502	Signal watchers will trigger an event when the process receives a specific
589	signal one or more times. Even though signals are very asynchronous, libev	2503	signal one or more times. Even though signals are very asynchronous, libev
590	will try its best to deliver signals synchronously, i.e. as part of the	2504	will try its best to deliver signals synchronously, i.e. as part of the
591	normal event processing, like any other event.	2505	normal event processing, like any other event.
592		2506
		2507	If you want signals to be delivered truly asynchronously, just use
		2508	C<sigaction> as you would do without libev and forget about sharing
		2509	the signal. You can even use C<ev_async> from a signal handler to
		2510	synchronously wake up an event loop.
		2511
593	You cna configure as many watchers as you like per signal. Only when the	2512	You can configure as many watchers as you like for the same signal, but
594	first watcher gets started will libev actually register a signal watcher	2513	only within the same loop, i.e. you can watch for C<SIGINT> in your
595	with the kernel (thus it coexists with your own signal handlers as long	2514	default loop and for C<SIGIO> in another loop, but you cannot watch for
596	as you don't register any with libev). Similarly, when the last signal	2515	C<SIGINT> in both the default loop and another loop at the same time. At
597	watcher for a signal is stopped libev will reset the signal handler to	2516	the moment, C<SIGCHLD> is permanently tied to the default loop.
598	SIG_DFL (regardless of what it was set to before).	2517
		2518	Only after the first watcher for a signal is started will libev actually
		2519	register something with the kernel. It thus coexists with your own signal
		2520	handlers as long as you don't register any with libev for the same signal.
		2521
		2522	If possible and supported, libev will install its handlers with
		2523	C<SA_RESTART> (or equivalent) behaviour enabled, so system calls should
		2524	not be unduly interrupted. If you have a problem with system calls getting
		2525	interrupted by signals you can block all signals in an C<ev_check> watcher
		2526	and unblock them in an C<ev_prepare> watcher.
		2527
		2528	=head3 The special problem of inheritance over fork/execve/pthread_create
		2529
		2530	Both the signal mask (C<sigprocmask>) and the signal disposition
		2531	(C<sigaction>) are unspecified after starting a signal watcher (and after
		2532	stopping it again), that is, libev might or might not block the signal,
		2533	and might or might not set or restore the installed signal handler (but
		2534	see C<EVFLAG_NOSIGMASK>).
		2535
		2536	While this does not matter for the signal disposition (libev never
		2537	sets signals to C<SIG_IGN>, so handlers will be reset to C<SIG_DFL> on
		2538	C<execve>), this matters for the signal mask: many programs do not expect
		2539	certain signals to be blocked.
		2540
		2541	This means that before calling C<exec> (from the child) you should reset
		2542	the signal mask to whatever "default" you expect (all clear is a good
		2543	choice usually).
		2544
		2545	The simplest way to ensure that the signal mask is reset in the child is
		2546	to install a fork handler with C<pthread_atfork> that resets it. That will
		2547	catch fork calls done by libraries (such as the libc) as well.
		2548
		2549	In current versions of libev, the signal will not be blocked indefinitely
		2550	unless you use the C<signalfd> API (C<EV_SIGNALFD>). While this reduces
		2551	the window of opportunity for problems, it will not go away, as libev
		2552	I<has> to modify the signal mask, at least temporarily.
		2553
		2554	So I can't stress this enough: I<If you do not reset your signal mask when
		2555	you expect it to be empty, you have a race condition in your code>. This
		2556	is not a libev-specific thing, this is true for most event libraries.
		2557
		2558	=head3 The special problem of threads signal handling
		2559
		2560	POSIX threads has problematic signal handling semantics, specifically,
		2561	a lot of functionality (sigfd, sigwait etc.) only really works if all
		2562	threads in a process block signals, which is hard to achieve.
		2563
		2564	When you want to use sigwait (or mix libev signal handling with your own
		2565	for the same signals), you can tackle this problem by globally blocking
		2566	all signals before creating any threads (or creating them with a fully set
		2567	sigprocmask) and also specifying the C<EVFLAG_NOSIGMASK> when creating
		2568	loops. Then designate one thread as "signal receiver thread" which handles
		2569	these signals. You can pass on any signals that libev might be interested
		2570	in by calling C<ev_feed_signal>.
		2571
		2572	=head3 Watcher-Specific Functions and Data Members
599		2573
600	=over 4	2574	=over 4
601		2575
602	=item ev_signal_init (ev_signal *, callback, int signum)	2576	=item ev_signal_init (ev_signal *, callback, int signum)
603		2577
604	=item ev_signal_set (ev_signal *, int signum)	2578	=item ev_signal_set (ev_signal *, int signum)
605		2579
606	Configures the watcher to trigger on the given signal number (usually one	2580	Configures the watcher to trigger on the given signal number (usually one
607	of the C<SIGxxx> constants).	2581	of the C<SIGxxx> constants).
608		2582
		2583	=item int signum [read-only]
		2584
		2585	The signal the watcher watches out for.
		2586
609	=back	2587	=back
610		2588
		2589	=head3 Examples
		2590
		2591	Example: Try to exit cleanly on SIGINT.
		2592
		2593	static void
		2594	sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
		2595	{
		2596	ev_break (loop, EVBREAK_ALL);
		2597	}
		2598
		2599	ev_signal signal_watcher;
		2600	ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
		2601	ev_signal_start (loop, &signal_watcher);
		2602
		2603
611	=head2 ev_child - wait for pid status changes	2604	=head2 C<ev_child> - watch out for process status changes
612		2605
613	Child watchers trigger when your process receives a SIGCHLD in response to	2606	Child watchers trigger when your process receives a SIGCHLD in response to
614	some child status changes (most typically when a child of yours dies).	2607	some child status changes (most typically when a child of yours dies or
		2608	exits). It is permissible to install a child watcher I<after> the child
		2609	has been forked (which implies it might have already exited), as long
		2610	as the event loop isn't entered (or is continued from a watcher), i.e.,
		2611	forking and then immediately registering a watcher for the child is fine,
		2612	but forking and registering a watcher a few event loop iterations later or
		2613	in the next callback invocation is not.
		2614
		2615	Only the default event loop is capable of handling signals, and therefore
		2616	you can only register child watchers in the default event loop.
		2617
		2618	Due to some design glitches inside libev, child watchers will always be
		2619	handled at maximum priority (their priority is set to C<EV_MAXPRI> by
		2620	libev)
		2621
		2622	=head3 Process Interaction
		2623
		2624	Libev grabs C<SIGCHLD> as soon as the default event loop is
		2625	initialised. This is necessary to guarantee proper behaviour even if the
		2626	first child watcher is started after the child exits. The occurrence
		2627	of C<SIGCHLD> is recorded asynchronously, but child reaping is done
		2628	synchronously as part of the event loop processing. Libev always reaps all
		2629	children, even ones not watched.
		2630
		2631	=head3 Overriding the Built-In Processing
		2632
		2633	Libev offers no special support for overriding the built-in child
		2634	processing, but if your application collides with libev's default child
		2635	handler, you can override it easily by installing your own handler for
		2636	C<SIGCHLD> after initialising the default loop, and making sure the
		2637	default loop never gets destroyed. You are encouraged, however, to use an
		2638	event-based approach to child reaping and thus use libev's support for
		2639	that, so other libev users can use C<ev_child> watchers freely.
		2640
		2641	=head3 Stopping the Child Watcher
		2642
		2643	Currently, the child watcher never gets stopped, even when the
		2644	child terminates, so normally one needs to stop the watcher in the
		2645	callback. Future versions of libev might stop the watcher automatically
		2646	when a child exit is detected (calling C<ev_child_stop> twice is not a
		2647	problem).
		2648
		2649	=head3 Watcher-Specific Functions and Data Members
615		2650
616	=over 4	2651	=over 4
617		2652
618	=item ev_child_init (ev_child *, callback, int pid)	2653	=item ev_child_init (ev_child *, callback, int pid, int trace)
619		2654
620	=item ev_child_set (ev_child *, int pid)	2655	=item ev_child_set (ev_child *, int pid, int trace)
621		2656
622	Configures the watcher to wait for status changes of process C<pid> (or	2657	Configures the watcher to wait for status changes of process C<pid> (or
623	I<any> process if C<pid> is specified as C<0>). The callback can look	2658	I<any> process if C<pid> is specified as C<0>). The callback can look
624	at the C<rstatus> member of the C<ev_child> watcher structure to see	2659	at the C<rstatus> member of the C<ev_child> watcher structure to see
625	the status word (use the macros from C<sys/wait.h>). The C<rpid> member	2660	the status word (use the macros from C<sys/wait.h> and see your systems
626	contains the pid of the process causing the status change.	2661	C<waitpid> documentation). The C<rpid> member contains the pid of the
		2662	process causing the status change. C<trace> must be either C<0> (only
		2663	activate the watcher when the process terminates) or C<1> (additionally
		2664	activate the watcher when the process is stopped or continued).
		2665
		2666	=item int pid [read-only]
		2667
		2668	The process id this watcher watches out for, or C<0>, meaning any process id.
		2669
		2670	=item int rpid [read-write]
		2671
		2672	The process id that detected a status change.
		2673
		2674	=item int rstatus [read-write]
		2675
		2676	The process exit/trace status caused by C<rpid> (see your systems
		2677	C<waitpid> and C<sys/wait.h> documentation for details).
627		2678
628	=back	2679	=back
629		2680
		2681	=head3 Examples
		2682
		2683	Example: C<fork()> a new process and install a child handler to wait for
		2684	its completion.
		2685
		2686	ev_child cw;
		2687
		2688	static void
		2689	child_cb (EV_P_ ev_child *w, int revents)
		2690	{
		2691	ev_child_stop (EV_A_ w);
		2692	printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
		2693	}
		2694
		2695	pid_t pid = fork ();
		2696
		2697	if (pid < 0)
		2698	// error
		2699	else if (pid == 0)
		2700	{
		2701	// the forked child executes here
		2702	exit (1);
		2703	}
		2704	else
		2705	{
		2706	ev_child_init (&cw, child_cb, pid, 0);
		2707	ev_child_start (EV_DEFAULT_ &cw);
		2708	}
		2709
		2710
		2711	=head2 C<ev_stat> - did the file attributes just change?
		2712
		2713	This watches a file system path for attribute changes. That is, it calls
		2714	C<stat> on that path in regular intervals (or when the OS says it changed)
		2715	and sees if it changed compared to the last time, invoking the callback
		2716	if it did. Starting the watcher C<stat>'s the file, so only changes that
		2717	happen after the watcher has been started will be reported.
		2718
		2719	The path does not need to exist: changing from "path exists" to "path does
		2720	not exist" is a status change like any other. The condition "path does not
		2721	exist" (or more correctly "path cannot be stat'ed") is signified by the
		2722	C<st_nlink> field being zero (which is otherwise always forced to be at
		2723	least one) and all the other fields of the stat buffer having unspecified
		2724	contents.
		2725
		2726	The path I<must not> end in a slash or contain special components such as
		2727	C<.> or C<..>. The path I<should> be absolute: If it is relative and
		2728	your working directory changes, then the behaviour is undefined.
		2729
		2730	Since there is no portable change notification interface available, the
		2731	portable implementation simply calls C<stat(2)> regularly on the path
		2732	to see if it changed somehow. You can specify a recommended polling
		2733	interval for this case. If you specify a polling interval of C<0> (highly
		2734	recommended!) then a I<suitable, unspecified default> value will be used
		2735	(which you can expect to be around five seconds, although this might
		2736	change dynamically). Libev will also impose a minimum interval which is
		2737	currently around C<0.1>, but that's usually overkill.
		2738
		2739	This watcher type is not meant for massive numbers of stat watchers,
		2740	as even with OS-supported change notifications, this can be
		2741	resource-intensive.
		2742
		2743	At the time of this writing, the only OS-specific interface implemented
		2744	is the Linux inotify interface (implementing kqueue support is left as an
		2745	exercise for the reader. Note, however, that the author sees no way of
		2746	implementing C<ev_stat> semantics with kqueue, except as a hint).
		2747
		2748	=head3 ABI Issues (Largefile Support)
		2749
		2750	Libev by default (unless the user overrides this) uses the default
		2751	compilation environment, which means that on systems with large file
		2752	support disabled by default, you get the 32 bit version of the stat
		2753	structure. When using the library from programs that change the ABI to
		2754	use 64 bit file offsets the programs will fail. In that case you have to
		2755	compile libev with the same flags to get binary compatibility. This is
		2756	obviously the case with any flags that change the ABI, but the problem is
		2757	most noticeably displayed with ev_stat and large file support.
		2758
		2759	The solution for this is to lobby your distribution maker to make large
		2760	file interfaces available by default (as e.g. FreeBSD does) and not
		2761	optional. Libev cannot simply switch on large file support because it has
		2762	to exchange stat structures with application programs compiled using the
		2763	default compilation environment.
		2764
		2765	=head3 Inotify and Kqueue
		2766
		2767	When C<inotify (7)> support has been compiled into libev and present at
		2768	runtime, it will be used to speed up change detection where possible. The
		2769	inotify descriptor will be created lazily when the first C<ev_stat>
		2770	watcher is being started.
		2771
		2772	Inotify presence does not change the semantics of C<ev_stat> watchers
		2773	except that changes might be detected earlier, and in some cases, to avoid
		2774	making regular C<stat> calls. Even in the presence of inotify support
		2775	there are many cases where libev has to resort to regular C<stat> polling,
		2776	but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
		2777	many bugs), the path exists (i.e. stat succeeds), and the path resides on
		2778	a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
		2779	xfs are fully working) libev usually gets away without polling.
		2780
		2781	There is no support for kqueue, as apparently it cannot be used to
		2782	implement this functionality, due to the requirement of having a file
		2783	descriptor open on the object at all times, and detecting renames, unlinks
		2784	etc. is difficult.
		2785
		2786	=head3 C<stat ()> is a synchronous operation
		2787
		2788	Libev doesn't normally do any kind of I/O itself, and so is not blocking
		2789	the process. The exception are C<ev_stat> watchers - those call C<stat
		2790	()>, which is a synchronous operation.
		2791
		2792	For local paths, this usually doesn't matter: unless the system is very
		2793	busy or the intervals between stat's are large, a stat call will be fast,
		2794	as the path data is usually in memory already (except when starting the
		2795	watcher).
		2796
		2797	For networked file systems, calling C<stat ()> can block an indefinite
		2798	time due to network issues, and even under good conditions, a stat call
		2799	often takes multiple milliseconds.
		2800
		2801	Therefore, it is best to avoid using C<ev_stat> watchers on networked
		2802	paths, although this is fully supported by libev.
		2803
		2804	=head3 The special problem of stat time resolution
		2805
		2806	The C<stat ()> system call only supports full-second resolution portably,
		2807	and even on systems where the resolution is higher, most file systems
		2808	still only support whole seconds.
		2809
		2810	That means that, if the time is the only thing that changes, you can
		2811	easily miss updates: on the first update, C<ev_stat> detects a change and
		2812	calls your callback, which does something. When there is another update
		2813	within the same second, C<ev_stat> will be unable to detect unless the
		2814	stat data does change in other ways (e.g. file size).
		2815
		2816	The solution to this is to delay acting on a change for slightly more
		2817	than a second (or till slightly after the next full second boundary), using
		2818	a roughly one-second-delay C<ev_timer> (e.g. C<ev_timer_set (w, 0., 1.02);
		2819	ev_timer_again (loop, w)>).
		2820
		2821	The C<.02> offset is added to work around small timing inconsistencies
		2822	of some operating systems (where the second counter of the current time
		2823	might be be delayed. One such system is the Linux kernel, where a call to
		2824	C<gettimeofday> might return a timestamp with a full second later than
		2825	a subsequent C<time> call - if the equivalent of C<time ()> is used to
		2826	update file times then there will be a small window where the kernel uses
		2827	the previous second to update file times but libev might already execute
		2828	the timer callback).
		2829
		2830	=head3 Watcher-Specific Functions and Data Members
		2831
		2832	=over 4
		2833
		2834	=item ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)
		2835
		2836	=item ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)
		2837
		2838	Configures the watcher to wait for status changes of the given
		2839	C<path>. The C<interval> is a hint on how quickly a change is expected to
		2840	be detected and should normally be specified as C<0> to let libev choose
		2841	a suitable value. The memory pointed to by C<path> must point to the same
		2842	path for as long as the watcher is active.
		2843
		2844	The callback will receive an C<EV_STAT> event when a change was detected,
		2845	relative to the attributes at the time the watcher was started (or the
		2846	last change was detected).
		2847
		2848	=item ev_stat_stat (loop, ev_stat *)
		2849
		2850	Updates the stat buffer immediately with new values. If you change the
		2851	watched path in your callback, you could call this function to avoid
		2852	detecting this change (while introducing a race condition if you are not
		2853	the only one changing the path). Can also be useful simply to find out the
		2854	new values.
		2855
		2856	=item ev_statdata attr [read-only]
		2857
		2858	The most-recently detected attributes of the file. Although the type is
		2859	C<ev_statdata>, this is usually the (or one of the) C<struct stat> types
		2860	suitable for your system, but you can only rely on the POSIX-standardised
		2861	members to be present. If the C<st_nlink> member is C<0>, then there was
		2862	some error while C<stat>ing the file.
		2863
		2864	=item ev_statdata prev [read-only]
		2865
		2866	The previous attributes of the file. The callback gets invoked whenever
		2867	C<prev> != C<attr>, or, more precisely, one or more of these members
		2868	differ: C<st_dev>, C<st_ino>, C<st_mode>, C<st_nlink>, C<st_uid>,
		2869	C<st_gid>, C<st_rdev>, C<st_size>, C<st_atime>, C<st_mtime>, C<st_ctime>.
		2870
		2871	=item ev_tstamp interval [read-only]
		2872
		2873	The specified interval.
		2874
		2875	=item const char *path [read-only]
		2876
		2877	The file system path that is being watched.
		2878
		2879	=back
		2880
		2881	=head3 Examples
		2882
		2883	Example: Watch C</etc/passwd> for attribute changes.
		2884
		2885	static void
		2886	passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
		2887	{
		2888	/* /etc/passwd changed in some way */
		2889	if (w->attr.st_nlink)
		2890	{
		2891	printf ("passwd current size %ld\n", (long)w->attr.st_size);
		2892	printf ("passwd current atime %ld\n", (long)w->attr.st_mtime);
		2893	printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime);
		2894	}
		2895	else
		2896	/* you shalt not abuse printf for puts */
		2897	puts ("wow, /etc/passwd is not there, expect problems. "
		2898	"if this is windows, they already arrived\n");
		2899	}
		2900
		2901	...
		2902	ev_stat passwd;
		2903
		2904	ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
		2905	ev_stat_start (loop, &passwd);
		2906
		2907	Example: Like above, but additionally use a one-second delay so we do not
		2908	miss updates (however, frequent updates will delay processing, too, so
		2909	one might do the work both on C<ev_stat> callback invocation I<and> on
		2910	C<ev_timer> callback invocation).
		2911
		2912	static ev_stat passwd;
		2913	static ev_timer timer;
		2914
		2915	static void
		2916	timer_cb (EV_P_ ev_timer *w, int revents)
		2917	{
		2918	ev_timer_stop (EV_A_ w);
		2919
		2920	/* now it's one second after the most recent passwd change */
		2921	}
		2922
		2923	static void
		2924	stat_cb (EV_P_ ev_stat *w, int revents)
		2925	{
		2926	/* reset the one-second timer */
		2927	ev_timer_again (EV_A_ &timer);
		2928	}
		2929
		2930	...
		2931	ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
		2932	ev_stat_start (loop, &passwd);
		2933	ev_timer_init (&timer, timer_cb, 0., 1.02);
		2934
		2935
630	=head2 ev_idle - when you've got nothing better to do	2936	=head2 C<ev_idle> - when you've got nothing better to do...
631		2937
632	Idle watchers trigger events when there are no other I/O or timer (or	2938	Idle watchers trigger events when no other events of the same or higher
633	periodic) events pending. That is, as long as your process is busy	2939	priority are pending (prepare, check and other idle watchers do not count
634	handling sockets or timeouts it will not be called. But when your process	2940	as receiving "events").
635	is idle all idle watchers are being called again and again - until	2941
		2942	That is, as long as your process is busy handling sockets or timeouts
		2943	(or even signals, imagine) of the same or higher priority it will not be
		2944	triggered. But when your process is idle (or only lower-priority watchers
		2945	are pending), the idle watchers are being called once per event loop
636	stopped, that is, or your process receives more events.	2946	iteration - until stopped, that is, or your process receives more events
		2947	and becomes busy again with higher priority stuff.
637		2948
638	The most noteworthy effect is that as long as any idle watchers are	2949	The most noteworthy effect is that as long as any idle watchers are
639	active, the process will not block when waiting for new events.	2950	active, the process will not block when waiting for new events.
640		2951
641	Apart from keeping your process non-blocking (which is a useful	2952	Apart from keeping your process non-blocking (which is a useful
642	effect on its own sometimes), idle watchers are a good place to do	2953	effect on its own sometimes), idle watchers are a good place to do
643	"pseudo-background processing", or delay processing stuff to after the	2954	"pseudo-background processing", or delay processing stuff to after the
644	event loop has handled all outstanding events.	2955	event loop has handled all outstanding events.
645		2956
		2957	=head3 Abusing an C<ev_idle> watcher for its side-effect
		2958
		2959	As long as there is at least one active idle watcher, libev will never
		2960	sleep unnecessarily. Or in other words, it will loop as fast as possible.
		2961	For this to work, the idle watcher doesn't need to be invoked at all - the
		2962	lowest priority will do.
		2963
		2964	This mode of operation can be useful together with an C<ev_check> watcher,
		2965	to do something on each event loop iteration - for example to balance load
		2966	between different connections.
		2967
		2968	See L</Abusing an ev_check watcher for its side-effect> for a longer
		2969	example.
		2970
		2971	=head3 Watcher-Specific Functions and Data Members
		2972
646	=over 4	2973	=over 4
647		2974
648	=item ev_idle_init (ev_signal *, callback)	2975	=item ev_idle_init (ev_idle *, callback)
649		2976
650	Initialises and configures the idle watcher - it has no parameters of any	2977	Initialises and configures the idle watcher - it has no parameters of any
651	kind. There is a C<ev_idle_set> macro, but using it is utterly pointless,	2978	kind. There is a C<ev_idle_set> macro, but using it is utterly pointless,
652	believe me.	2979	believe me.
653		2980
654	=back	2981	=back
655		2982
656	=head2 prepare and check - your hooks into the event loop	2983	=head3 Examples
657		2984
		2985	Example: Dynamically allocate an C<ev_idle> watcher, start it, and in the
		2986	callback, free it. Also, use no error checking, as usual.
		2987
		2988	static void
		2989	idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
		2990	{
		2991	// stop the watcher
		2992	ev_idle_stop (loop, w);
		2993
		2994	// now we can free it
		2995	free (w);
		2996
		2997	// now do something you wanted to do when the program has
		2998	// no longer anything immediate to do.
		2999	}
		3000
		3001	ev_idle *idle_watcher = malloc (sizeof (ev_idle));
		3002	ev_idle_init (idle_watcher, idle_cb);
		3003	ev_idle_start (loop, idle_watcher);
		3004
		3005
		3006	=head2 C<ev_prepare> and C<ev_check> - customise your event loop!
		3007
658	Prepare and check watchers usually (but not always) are used in	3008	Prepare and check watchers are often (but not always) used in pairs:
659	tandom. Prepare watchers get invoked before the process blocks and check	3009	prepare watchers get invoked before the process blocks and check watchers
660	watchers afterwards.	3010	afterwards.
661		3011
		3012	You I<must not> call C<ev_run> (or similar functions that enter the
		3013	current event loop) or C<ev_loop_fork> from either C<ev_prepare> or
		3014	C<ev_check> watchers. Other loops than the current one are fine,
		3015	however. The rationale behind this is that you do not need to check
		3016	for recursion in those watchers, i.e. the sequence will always be
		3017	C<ev_prepare>, blocking, C<ev_check> so if you have one watcher of each
		3018	kind they will always be called in pairs bracketing the blocking call.
		3019
662	Their main purpose is to integrate other event mechanisms into libev. This	3020	Their main purpose is to integrate other event mechanisms into libev and
663	could be used, for example, to track variable changes, implement your own	3021	their use is somewhat advanced. They could be used, for example, to track
664	watchers, integrate net-snmp or a coroutine library and lots more.	3022	variable changes, implement your own watchers, integrate net-snmp or a
		3023	coroutine library and lots more. They are also occasionally useful if
		3024	you cache some data and want to flush it before blocking (for example,
		3025	in X programs you might want to do an C<XFlush ()> in an C<ev_prepare>
		3026	watcher).
665		3027
666	This is done by examining in each prepare call which file descriptors need	3028	This is done by examining in each prepare call which file descriptors
667	to be watched by the other library, registering ev_io watchers for them	3029	need to be watched by the other library, registering C<ev_io> watchers
668	and starting an ev_timer watcher for any timeouts (many libraries provide	3030	for them and starting an C<ev_timer> watcher for any timeouts (many
669	just this functionality). Then, in the check watcher you check for any	3031	libraries provide exactly this functionality). Then, in the check watcher,
670	events that occured (by making your callbacks set soem flags for example)	3032	you check for any events that occurred (by checking the pending status
671	and call back into the library.	3033	of all watchers and stopping them) and call back into the library. The
		3034	I/O and timer callbacks will never actually be called (but must be valid
		3035	nevertheless, because you never know, you know?).
672		3036
673	As another example, the perl Coro module uses these hooks to integrate	3037	As another example, the Perl Coro module uses these hooks to integrate
674	coroutines into libev programs, by yielding to other active coroutines	3038	coroutines into libev programs, by yielding to other active coroutines
675	during each prepare and only letting the process block if no coroutines	3039	during each prepare and only letting the process block if no coroutines
676	are ready to run.	3040	are ready to run (it's actually more complicated: it only runs coroutines
		3041	with priority higher than or equal to the event loop and one coroutine
		3042	of lower priority, but only once, using idle watchers to keep the event
		3043	loop from blocking if lower-priority coroutines are active, thus mapping
		3044	low-priority coroutines to idle/background tasks).
		3045
		3046	When used for this purpose, it is recommended to give C<ev_check> watchers
		3047	highest (C<EV_MAXPRI>) priority, to ensure that they are being run before
		3048	any other watchers after the poll (this doesn't matter for C<ev_prepare>
		3049	watchers).
		3050
		3051	Also, C<ev_check> watchers (and C<ev_prepare> watchers, too) should not
		3052	activate ("feed") events into libev. While libev fully supports this, they
		3053	might get executed before other C<ev_check> watchers did their job. As
		3054	C<ev_check> watchers are often used to embed other (non-libev) event
		3055	loops those other event loops might be in an unusable state until their
		3056	C<ev_check> watcher ran (always remind yourself to coexist peacefully with
		3057	others).
		3058
		3059	=head3 Abusing an C<ev_check> watcher for its side-effect
		3060
		3061	C<ev_check> (and less often also C<ev_prepare>) watchers can also be
		3062	useful because they are called once per event loop iteration. For
		3063	example, if you want to handle a large number of connections fairly, you
		3064	normally only do a bit of work for each active connection, and if there
		3065	is more work to do, you wait for the next event loop iteration, so other
		3066	connections have a chance of making progress.
		3067
		3068	Using an C<ev_check> watcher is almost enough: it will be called on the
		3069	next event loop iteration. However, that isn't as soon as possible -
		3070	without external events, your C<ev_check> watcher will not be invoked.
		3071
		3072	This is where C<ev_idle> watchers come in handy - all you need is a
		3073	single global idle watcher that is active as long as you have one active
		3074	C<ev_check> watcher. The C<ev_idle> watcher makes sure the event loop
		3075	will not sleep, and the C<ev_check> watcher makes sure a callback gets
		3076	invoked. Neither watcher alone can do that.
		3077
		3078	=head3 Watcher-Specific Functions and Data Members
677		3079
678	=over 4	3080	=over 4
679		3081
680	=item ev_prepare_init (ev_prepare *, callback)	3082	=item ev_prepare_init (ev_prepare *, callback)
681		3083
682	=item ev_check_init (ev_check *, callback)	3084	=item ev_check_init (ev_check *, callback)
683		3085
684	Initialises and configures the prepare or check watcher - they have no	3086	Initialises and configures the prepare or check watcher - they have no
685	parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set>	3087	parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set>
686	macros, but using them is utterly, utterly pointless.	3088	macros, but using them is utterly, utterly, utterly and completely
		3089	pointless.
687		3090
688	=back	3091	=back
689		3092
		3093	=head3 Examples
		3094
		3095	There are a number of principal ways to embed other event loops or modules
		3096	into libev. Here are some ideas on how to include libadns into libev
		3097	(there is a Perl module named C<EV::ADNS> that does this, which you could
		3098	use as a working example. Another Perl module named C<EV::Glib> embeds a
		3099	Glib main context into libev, and finally, C<Glib::EV> embeds EV into the
		3100	Glib event loop).
		3101
		3102	Method 1: Add IO watchers and a timeout watcher in a prepare handler,
		3103	and in a check watcher, destroy them and call into libadns. What follows
		3104	is pseudo-code only of course. This requires you to either use a low
		3105	priority for the check watcher or use C<ev_clear_pending> explicitly, as
		3106	the callbacks for the IO/timeout watchers might not have been called yet.
		3107
		3108	static ev_io iow [nfd];
		3109	static ev_timer tw;
		3110
		3111	static void
		3112	io_cb (struct ev_loop *loop, ev_io *w, int revents)
		3113	{
		3114	}
		3115
		3116	// create io watchers for each fd and a timer before blocking
		3117	static void
		3118	adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
		3119	{
		3120	int timeout = 3600000;
		3121	struct pollfd fds [nfd];
		3122	// actual code will need to loop here and realloc etc.
		3123	adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
		3124
		3125	/* the callback is illegal, but won't be called as we stop during check */
		3126	ev_timer_init (&tw, 0, timeout * 1e-3, 0.);
		3127	ev_timer_start (loop, &tw);
		3128
		3129	// create one ev_io per pollfd
		3130	for (int i = 0; i < nfd; ++i)
		3131	{
		3132	ev_io_init (iow + i, io_cb, fds [i].fd,
		3133	((fds [i].events & POLLIN ? EV_READ : 0)
		3134	| (fds [i].events & POLLOUT ? EV_WRITE : 0)));
		3135
		3136	fds [i].revents = 0;
		3137	ev_io_start (loop, iow + i);
		3138	}
		3139	}
		3140
		3141	// stop all watchers after blocking
		3142	static void
		3143	adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
		3144	{
		3145	ev_timer_stop (loop, &tw);
		3146
		3147	for (int i = 0; i < nfd; ++i)
		3148	{
		3149	// set the relevant poll flags
		3150	// could also call adns_processreadable etc. here
		3151	struct pollfd *fd = fds + i;
		3152	int revents = ev_clear_pending (iow + i);
		3153	if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
		3154	if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
		3155
		3156	// now stop the watcher
		3157	ev_io_stop (loop, iow + i);
		3158	}
		3159
		3160	adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
		3161	}
		3162
		3163	Method 2: This would be just like method 1, but you run C<adns_afterpoll>
		3164	in the prepare watcher and would dispose of the check watcher.
		3165
		3166	Method 3: If the module to be embedded supports explicit event
		3167	notification (libadns does), you can also make use of the actual watcher
		3168	callbacks, and only destroy/create the watchers in the prepare watcher.
		3169
		3170	static void
		3171	timer_cb (EV_P_ ev_timer *w, int revents)
		3172	{
		3173	adns_state ads = (adns_state)w->data;
		3174	update_now (EV_A);
		3175
		3176	adns_processtimeouts (ads, &tv_now);
		3177	}
		3178
		3179	static void
		3180	io_cb (EV_P_ ev_io *w, int revents)
		3181	{
		3182	adns_state ads = (adns_state)w->data;
		3183	update_now (EV_A);
		3184
		3185	if (revents & EV_READ ) adns_processreadable (ads, w->fd, &tv_now);
		3186	if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
		3187	}
		3188
		3189	// do not ever call adns_afterpoll
		3190
		3191	Method 4: Do not use a prepare or check watcher because the module you
		3192	want to embed is not flexible enough to support it. Instead, you can
		3193	override their poll function. The drawback with this solution is that the
		3194	main loop is now no longer controllable by EV. The C<Glib::EV> module uses
		3195	this approach, effectively embedding EV as a client into the horrible
		3196	libglib event loop.
		3197
		3198	static gint
		3199	event_poll_func (GPollFD *fds, guint nfds, gint timeout)
		3200	{
		3201	int got_events = 0;
		3202
		3203	for (n = 0; n < nfds; ++n)
		3204	// create/start io watcher that sets the relevant bits in fds[n] and increment got_events
		3205
		3206	if (timeout >= 0)
		3207	// create/start timer
		3208
		3209	// poll
		3210	ev_run (EV_A_ 0);
		3211
		3212	// stop timer again
		3213	if (timeout >= 0)
		3214	ev_timer_stop (EV_A_ &to);
		3215
		3216	// stop io watchers again - their callbacks should have set
		3217	for (n = 0; n < nfds; ++n)
		3218	ev_io_stop (EV_A_ iow [n]);
		3219
		3220	return got_events;
		3221	}
		3222
		3223
		3224	=head2 C<ev_embed> - when one backend isn't enough...
		3225
		3226	This is a rather advanced watcher type that lets you embed one event loop
		3227	into another (currently only C<ev_io> events are supported in the embedded
		3228	loop, other types of watchers might be handled in a delayed or incorrect
		3229	fashion and must not be used).
		3230
		3231	There are primarily two reasons you would want that: work around bugs and
		3232	prioritise I/O.
		3233
		3234	As an example for a bug workaround, the kqueue backend might only support
		3235	sockets on some platform, so it is unusable as generic backend, but you
		3236	still want to make use of it because you have many sockets and it scales
		3237	so nicely. In this case, you would create a kqueue-based loop and embed
		3238	it into your default loop (which might use e.g. poll). Overall operation
		3239	will be a bit slower because first libev has to call C<poll> and then
		3240	C<kevent>, but at least you can use both mechanisms for what they are
		3241	best: C<kqueue> for scalable sockets and C<poll> if you want it to work :)
		3242
		3243	As for prioritising I/O: under rare circumstances you have the case where
		3244	some fds have to be watched and handled very quickly (with low latency),
		3245	and even priorities and idle watchers might have too much overhead. In
		3246	this case you would put all the high priority stuff in one loop and all
		3247	the rest in a second one, and embed the second one in the first.
		3248
		3249	As long as the watcher is active, the callback will be invoked every
		3250	time there might be events pending in the embedded loop. The callback
		3251	must then call C<ev_embed_sweep (mainloop, watcher)> to make a single
		3252	sweep and invoke their callbacks (the callback doesn't need to invoke the
		3253	C<ev_embed_sweep> function directly, it could also start an idle watcher
		3254	to give the embedded loop strictly lower priority for example).
		3255
		3256	You can also set the callback to C<0>, in which case the embed watcher
		3257	will automatically execute the embedded loop sweep whenever necessary.
		3258
		3259	Fork detection will be handled transparently while the C<ev_embed> watcher
		3260	is active, i.e., the embedded loop will automatically be forked when the
		3261	embedding loop forks. In other cases, the user is responsible for calling
		3262	C<ev_loop_fork> on the embedded loop.
		3263
		3264	Unfortunately, not all backends are embeddable: only the ones returned by
		3265	C<ev_embeddable_backends> are, which, unfortunately, does not include any
		3266	portable one.
		3267
		3268	So when you want to use this feature you will always have to be prepared
		3269	that you cannot get an embeddable loop. The recommended way to get around
		3270	this is to have a separate variables for your embeddable loop, try to
		3271	create it, and if that fails, use the normal loop for everything.
		3272
		3273	=head3 C<ev_embed> and fork
		3274
		3275	While the C<ev_embed> watcher is running, forks in the embedding loop will
		3276	automatically be applied to the embedded loop as well, so no special
		3277	fork handling is required in that case. When the watcher is not running,
		3278	however, it is still the task of the libev user to call C<ev_loop_fork ()>
		3279	as applicable.
		3280
		3281	=head3 Watcher-Specific Functions and Data Members
		3282
		3283	=over 4
		3284
		3285	=item ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
		3286
		3287	=item ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)
		3288
		3289	Configures the watcher to embed the given loop, which must be
		3290	embeddable. If the callback is C<0>, then C<ev_embed_sweep> will be
		3291	invoked automatically, otherwise it is the responsibility of the callback
		3292	to invoke it (it will continue to be called until the sweep has been done,
		3293	if you do not want that, you need to temporarily stop the embed watcher).
		3294
		3295	=item ev_embed_sweep (loop, ev_embed *)
		3296
		3297	Make a single, non-blocking sweep over the embedded loop. This works
		3298	similarly to C<ev_run (embedded_loop, EVRUN_NOWAIT)>, but in the most
		3299	appropriate way for embedded loops.
		3300
		3301	=item struct ev_loop *other [read-only]
		3302
		3303	The embedded event loop.
		3304
		3305	=back
		3306
		3307	=head3 Examples
		3308
		3309	Example: Try to get an embeddable event loop and embed it into the default
		3310	event loop. If that is not possible, use the default loop. The default
		3311	loop is stored in C<loop_hi>, while the embeddable loop is stored in
		3312	C<loop_lo> (which is C<loop_hi> in the case no embeddable loop can be
		3313	used).
		3314
		3315	struct ev_loop *loop_hi = ev_default_init (0);
		3316	struct ev_loop *loop_lo = 0;
		3317	ev_embed embed;
		3318
		3319	// see if there is a chance of getting one that works
		3320	// (remember that a flags value of 0 means autodetection)
		3321	loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
		3322	? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
		3323	: 0;
		3324
		3325	// if we got one, then embed it, otherwise default to loop_hi
		3326	if (loop_lo)
		3327	{
		3328	ev_embed_init (&embed, 0, loop_lo);
		3329	ev_embed_start (loop_hi, &embed);
		3330	}
		3331	else
		3332	loop_lo = loop_hi;
		3333
		3334	Example: Check if kqueue is available but not recommended and create
		3335	a kqueue backend for use with sockets (which usually work with any
		3336	kqueue implementation). Store the kqueue/socket-only event loop in
		3337	C<loop_socket>. (One might optionally use C<EVFLAG_NOENV>, too).
		3338
		3339	struct ev_loop *loop = ev_default_init (0);
		3340	struct ev_loop *loop_socket = 0;
		3341	ev_embed embed;
		3342
		3343	if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
		3344	if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
		3345	{
		3346	ev_embed_init (&embed, 0, loop_socket);
		3347	ev_embed_start (loop, &embed);
		3348	}
		3349
		3350	if (!loop_socket)
		3351	loop_socket = loop;
		3352
		3353	// now use loop_socket for all sockets, and loop for everything else
		3354
		3355
		3356	=head2 C<ev_fork> - the audacity to resume the event loop after a fork
		3357
		3358	Fork watchers are called when a C<fork ()> was detected (usually because
		3359	whoever is a good citizen cared to tell libev about it by calling
		3360	C<ev_loop_fork>). The invocation is done before the event loop blocks next
		3361	and before C<ev_check> watchers are being called, and only in the child
		3362	after the fork. If whoever good citizen calling C<ev_default_fork> cheats
		3363	and calls it in the wrong process, the fork handlers will be invoked, too,
		3364	of course.
		3365
		3366	=head3 The special problem of life after fork - how is it possible?
		3367
		3368	Most uses of C<fork ()> consist of forking, then some simple calls to set
		3369	up/change the process environment, followed by a call to C<exec()>. This
		3370	sequence should be handled by libev without any problems.
		3371
		3372	This changes when the application actually wants to do event handling
		3373	in the child, or both parent in child, in effect "continuing" after the
		3374	fork.
		3375
		3376	The default mode of operation (for libev, with application help to detect
		3377	forks) is to duplicate all the state in the child, as would be expected
		3378	when I<either> the parent I<or> the child process continues.
		3379
		3380	When both processes want to continue using libev, then this is usually the
		3381	wrong result. In that case, usually one process (typically the parent) is
		3382	supposed to continue with all watchers in place as before, while the other
		3383	process typically wants to start fresh, i.e. without any active watchers.
		3384
		3385	The cleanest and most efficient way to achieve that with libev is to
		3386	simply create a new event loop, which of course will be "empty", and
		3387	use that for new watchers. This has the advantage of not touching more
		3388	memory than necessary, and thus avoiding the copy-on-write, and the
		3389	disadvantage of having to use multiple event loops (which do not support
		3390	signal watchers).
		3391
		3392	When this is not possible, or you want to use the default loop for
		3393	other reasons, then in the process that wants to start "fresh", call
		3394	C<ev_loop_destroy (EV_DEFAULT)> followed by C<ev_default_loop (...)>.
		3395	Destroying the default loop will "orphan" (not stop) all registered
		3396	watchers, so you have to be careful not to execute code that modifies
		3397	those watchers. Note also that in that case, you have to re-register any
		3398	signal watchers.
		3399
		3400	=head3 Watcher-Specific Functions and Data Members
		3401
		3402	=over 4
		3403
		3404	=item ev_fork_init (ev_fork *, callback)
		3405
		3406	Initialises and configures the fork watcher - it has no parameters of any
		3407	kind. There is a C<ev_fork_set> macro, but using it is utterly pointless,
		3408	really.
		3409
		3410	=back
		3411
		3412
		3413	=head2 C<ev_cleanup> - even the best things end
		3414
		3415	Cleanup watchers are called just before the event loop is being destroyed
		3416	by a call to C<ev_loop_destroy>.
		3417
		3418	While there is no guarantee that the event loop gets destroyed, cleanup
		3419	watchers provide a convenient method to install cleanup hooks for your
		3420	program, worker threads and so on - you just to make sure to destroy the
		3421	loop when you want them to be invoked.
		3422
		3423	Cleanup watchers are invoked in the same way as any other watcher. Unlike
		3424	all other watchers, they do not keep a reference to the event loop (which
		3425	makes a lot of sense if you think about it). Like all other watchers, you
		3426	can call libev functions in the callback, except C<ev_cleanup_start>.
		3427
		3428	=head3 Watcher-Specific Functions and Data Members
		3429
		3430	=over 4
		3431
		3432	=item ev_cleanup_init (ev_cleanup *, callback)
		3433
		3434	Initialises and configures the cleanup watcher - it has no parameters of
		3435	any kind. There is a C<ev_cleanup_set> macro, but using it is utterly
		3436	pointless, I assure you.
		3437
		3438	=back
		3439
		3440	Example: Register an atexit handler to destroy the default loop, so any
		3441	cleanup functions are called.
		3442
		3443	static void
		3444	program_exits (void)
		3445	{
		3446	ev_loop_destroy (EV_DEFAULT_UC);
		3447	}
		3448
		3449	...
		3450	atexit (program_exits);
		3451
		3452
		3453	=head2 C<ev_async> - how to wake up an event loop
		3454
		3455	In general, you cannot use an C<ev_loop> from multiple threads or other
		3456	asynchronous sources such as signal handlers (as opposed to multiple event
		3457	loops - those are of course safe to use in different threads).
		3458
		3459	Sometimes, however, you need to wake up an event loop you do not control,
		3460	for example because it belongs to another thread. This is what C<ev_async>
		3461	watchers do: as long as the C<ev_async> watcher is active, you can signal
		3462	it by calling C<ev_async_send>, which is thread- and signal safe.
		3463
		3464	This functionality is very similar to C<ev_signal> watchers, as signals,
		3465	too, are asynchronous in nature, and signals, too, will be compressed
		3466	(i.e. the number of callback invocations may be less than the number of
		3467	C<ev_async_send> calls). In fact, you could use signal watchers as a kind
		3468	of "global async watchers" by using a watcher on an otherwise unused
		3469	signal, and C<ev_feed_signal> to signal this watcher from another thread,
		3470	even without knowing which loop owns the signal.
		3471
		3472	=head3 Queueing
		3473
		3474	C<ev_async> does not support queueing of data in any way. The reason
		3475	is that the author does not know of a simple (or any) algorithm for a
		3476	multiple-writer-single-reader queue that works in all cases and doesn't
		3477	need elaborate support such as pthreads or unportable memory access
		3478	semantics.
		3479
		3480	That means that if you want to queue data, you have to provide your own
		3481	queue. But at least I can tell you how to implement locking around your
		3482	queue:
		3483
		3484	=over 4
		3485
		3486	=item queueing from a signal handler context
		3487
		3488	To implement race-free queueing, you simply add to the queue in the signal
		3489	handler but you block the signal handler in the watcher callback. Here is
		3490	an example that does that for some fictitious SIGUSR1 handler:
		3491
		3492	static ev_async mysig;
		3493
		3494	static void
		3495	sigusr1_handler (void)
		3496	{
		3497	sometype data;
		3498
		3499	// no locking etc.
		3500	queue_put (data);
		3501	ev_async_send (EV_DEFAULT_ &mysig);
		3502	}
		3503
		3504	static void
		3505	mysig_cb (EV_P_ ev_async *w, int revents)
		3506	{
		3507	sometype data;
		3508	sigset_t block, prev;
		3509
		3510	sigemptyset (&block);
		3511	sigaddset (&block, SIGUSR1);
		3512	sigprocmask (SIG_BLOCK, &block, &prev);
		3513
		3514	while (queue_get (&data))
		3515	process (data);
		3516
		3517	if (sigismember (&prev, SIGUSR1)
		3518	sigprocmask (SIG_UNBLOCK, &block, 0);
		3519	}
		3520
		3521	(Note: pthreads in theory requires you to use C<pthread_setmask>
		3522	instead of C<sigprocmask> when you use threads, but libev doesn't do it
		3523	either...).
		3524
		3525	=item queueing from a thread context
		3526
		3527	The strategy for threads is different, as you cannot (easily) block
		3528	threads but you can easily preempt them, so to queue safely you need to
		3529	employ a traditional mutex lock, such as in this pthread example:
		3530
		3531	static ev_async mysig;
		3532	static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
		3533
		3534	static void
		3535	otherthread (void)
		3536	{
		3537	// only need to lock the actual queueing operation
		3538	pthread_mutex_lock (&mymutex);
		3539	queue_put (data);
		3540	pthread_mutex_unlock (&mymutex);
		3541
		3542	ev_async_send (EV_DEFAULT_ &mysig);
		3543	}
		3544
		3545	static void
		3546	mysig_cb (EV_P_ ev_async *w, int revents)
		3547	{
		3548	pthread_mutex_lock (&mymutex);
		3549
		3550	while (queue_get (&data))
		3551	process (data);
		3552
		3553	pthread_mutex_unlock (&mymutex);
		3554	}
		3555
		3556	=back
		3557
		3558
		3559	=head3 Watcher-Specific Functions and Data Members
		3560
		3561	=over 4
		3562
		3563	=item ev_async_init (ev_async *, callback)
		3564
		3565	Initialises and configures the async watcher - it has no parameters of any
		3566	kind. There is a C<ev_async_set> macro, but using it is utterly pointless,
		3567	trust me.
		3568
		3569	=item ev_async_send (loop, ev_async *)
		3570
		3571	Sends/signals/activates the given C<ev_async> watcher, that is, feeds
		3572	an C<EV_ASYNC> event on the watcher into the event loop, and instantly
		3573	returns.
		3574
		3575	Unlike C<ev_feed_event>, this call is safe to do from other threads,
		3576	signal or similar contexts (see the discussion of C<EV_ATOMIC_T> in the
		3577	embedding section below on what exactly this means).
		3578
		3579	Note that, as with other watchers in libev, multiple events might get
		3580	compressed into a single callback invocation (another way to look at
		3581	this is that C<ev_async> watchers are level-triggered: they are set on
		3582	C<ev_async_send>, reset when the event loop detects that).
		3583
		3584	This call incurs the overhead of at most one extra system call per event
		3585	loop iteration, if the event loop is blocked, and no syscall at all if
		3586	the event loop (or your program) is processing events. That means that
		3587	repeated calls are basically free (there is no need to avoid calls for
		3588	performance reasons) and that the overhead becomes smaller (typically
		3589	zero) under load.
		3590
		3591	=item bool = ev_async_pending (ev_async *)
		3592
		3593	Returns a non-zero value when C<ev_async_send> has been called on the
		3594	watcher but the event has not yet been processed (or even noted) by the
		3595	event loop.
		3596
		3597	C<ev_async_send> sets a flag in the watcher and wakes up the loop. When
		3598	the loop iterates next and checks for the watcher to have become active,
		3599	it will reset the flag again. C<ev_async_pending> can be used to very
		3600	quickly check whether invoking the loop might be a good idea.
		3601
		3602	Not that this does I<not> check whether the watcher itself is pending,
		3603	only whether it has been requested to make this watcher pending: there
		3604	is a time window between the event loop checking and resetting the async
		3605	notification, and the callback being invoked.
		3606
		3607	=back
		3608
		3609
690	=head1 OTHER FUNCTIONS	3610	=head1 OTHER FUNCTIONS
691		3611
692	There are some other fucntions of possible interest. Described. Here. Now.	3612	There are some other functions of possible interest. Described. Here. Now.
693		3613
694	=over 4	3614	=over 4
695		3615
696	=item ev_once (loop, int fd, int events, ev_tstamp timeout, callback)	3616	=item ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)
697		3617
698	This function combines a simple timer and an I/O watcher, calls your	3618	This function combines a simple timer and an I/O watcher, calls your
699	callback on whichever event happens first and automatically stop both	3619	callback on whichever event happens first and automatically stops both
700	watchers. This is useful if you want to wait for a single event on an fd	3620	watchers. This is useful if you want to wait for a single event on an fd
701	or timeout without havign to allocate/configure/start/stop/free one or	3621	or timeout without having to allocate/configure/start/stop/free one or
702	more watchers yourself.	3622	more watchers yourself.
703		3623
704	If C<fd> is less than 0, then no I/O watcher will be started and events is	3624	If C<fd> is less than 0, then no I/O watcher will be started and the
705	ignored. Otherwise, an ev_io watcher for the given C<fd> and C<events> set	3625	C<events> argument is being ignored. Otherwise, an C<ev_io> watcher for
706	will be craeted and started.	3626	the given C<fd> and C<events> set will be created and started.
707		3627
708	If C<timeout> is less than 0, then no timeout watcher will be	3628	If C<timeout> is less than 0, then no timeout watcher will be
709	started. Otherwise an ev_timer watcher with after = C<timeout> (and repeat	3629	started. Otherwise an C<ev_timer> watcher with after = C<timeout> (and
710	= 0) will be started.	3630	repeat = 0) will be started. C<0> is a valid timeout.
711		3631
712	The callback has the type C<void (*cb)(int revents, void *arg)> and	3632	The callback has the type C<void (*cb)(int revents, void *arg)> and is
713	gets passed an events set (normally a combination of EV_ERROR, EV_READ,	3633	passed an C<revents> set like normal event callbacks (a combination of
714	EV_WRITE or EV_TIMEOUT) and the C<arg> value passed to C<ev_once>:	3634	C<EV_ERROR>, C<EV_READ>, C<EV_WRITE> or C<EV_TIMER>) and the C<arg>
		3635	value passed to C<ev_once>. Note that it is possible to receive I<both>
		3636	a timeout and an io event at the same time - you probably should give io
		3637	events precedence.
715		3638
		3639	Example: wait up to ten seconds for data to appear on STDIN_FILENO.
		3640
716	static void stdin_ready (int revents, void *arg)	3641	static void stdin_ready (int revents, void *arg)
		3642	{
		3643	if (revents & EV_READ)
		3644	/* stdin might have data for us, joy! */;
		3645	else if (revents & EV_TIMER)
		3646	/* doh, nothing entered */;
		3647	}
		3648
		3649	ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
		3650
		3651	=item ev_feed_fd_event (loop, int fd, int revents)
		3652
		3653	Feed an event on the given fd, as if a file descriptor backend detected
		3654	the given events.
		3655
		3656	=item ev_feed_signal_event (loop, int signum)
		3657
		3658	Feed an event as if the given signal occurred. See also C<ev_feed_signal>,
		3659	which is async-safe.
		3660
		3661	=back
		3662
		3663
		3664	=head1 COMMON OR USEFUL IDIOMS (OR BOTH)
		3665
		3666	This section explains some common idioms that are not immediately
		3667	obvious. Note that examples are sprinkled over the whole manual, and this
		3668	section only contains stuff that wouldn't fit anywhere else.
		3669
		3670	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
		3671
		3672	Each watcher has, by default, a C<void *data> member that you can read
		3673	or modify at any time: libev will completely ignore it. This can be used
		3674	to associate arbitrary data with your watcher. If you need more data and
		3675	don't want to allocate memory separately and store a pointer to it in that
		3676	data member, you can also "subclass" the watcher type and provide your own
		3677	data:
		3678
		3679	struct my_io
		3680	{
		3681	ev_io io;
		3682	int otherfd;
		3683	void *somedata;
		3684	struct whatever *mostinteresting;
		3685	};
		3686
		3687	...
		3688	struct my_io w;
		3689	ev_io_init (&w.io, my_cb, fd, EV_READ);
		3690
		3691	And since your callback will be called with a pointer to the watcher, you
		3692	can cast it back to your own type:
		3693
		3694	static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
		3695	{
		3696	struct my_io *w = (struct my_io *)w_;
		3697	...
		3698	}
		3699
		3700	More interesting and less C-conformant ways of casting your callback
		3701	function type instead have been omitted.
		3702
		3703	=head2 BUILDING YOUR OWN COMPOSITE WATCHERS
		3704
		3705	Another common scenario is to use some data structure with multiple
		3706	embedded watchers, in effect creating your own watcher that combines
		3707	multiple libev event sources into one "super-watcher":
		3708
		3709	struct my_biggy
		3710	{
		3711	int some_data;
		3712	ev_timer t1;
		3713	ev_timer t2;
		3714	}
		3715
		3716	In this case getting the pointer to C<my_biggy> is a bit more
		3717	complicated: Either you store the address of your C<my_biggy> struct in
		3718	the C<data> member of the watcher (for woozies or C++ coders), or you need
		3719	to use some pointer arithmetic using C<offsetof> inside your watchers (for
		3720	real programmers):
		3721
		3722	#include <stddef.h>
		3723
		3724	static void
		3725	t1_cb (EV_P_ ev_timer *w, int revents)
		3726	{
		3727	struct my_biggy big = (struct my_biggy *)
		3728	(((char *)w) - offsetof (struct my_biggy, t1));
		3729	}
		3730
		3731	static void
		3732	t2_cb (EV_P_ ev_timer *w, int revents)
		3733	{
		3734	struct my_biggy big = (struct my_biggy *)
		3735	(((char *)w) - offsetof (struct my_biggy, t2));
		3736	}
		3737
		3738	=head2 AVOIDING FINISHING BEFORE RETURNING
		3739
		3740	Often you have structures like this in event-based programs:
		3741
		3742	callback ()
717	{	3743	{
718	if (revents & EV_TIMEOUT)	3744	free (request);
719	/* doh, nothing entered */
720	else if (revents & EV_READ)
721	/* stdin might have data for us, joy! */
722	}	3745	}
723		3746
724	ev_once (STDIN_FILENO, EV_READm 10., stdin_ready, 0);	3747	request = start_new_request (..., callback);
725		3748
726	=item ev_feed_event (loop, watcher, int events)	3749	The intent is to start some "lengthy" operation. The C<request> could be
		3750	used to cancel the operation, or do other things with it.
727		3751
728	Feeds the given event set into the event loop, as if the specified event	3752	It's not uncommon to have code paths in C<start_new_request> that
729	has happened for the specified watcher (which must be a pointer to an	3753	immediately invoke the callback, for example, to report errors. Or you add
730	initialised but not necessarily active event watcher).	3754	some caching layer that finds that it can skip the lengthy aspects of the
		3755	operation and simply invoke the callback with the result.
731		3756
732	=item ev_feed_fd_event (loop, int fd, int revents)	3757	The problem here is that this will happen I<before> C<start_new_request>
		3758	has returned, so C<request> is not set.
733		3759
734	Feed an event on the given fd, as if a file descriptor backend detected it.	3760	Even if you pass the request by some safer means to the callback, you
		3761	might want to do something to the request after starting it, such as
		3762	canceling it, which probably isn't working so well when the callback has
		3763	already been invoked.
735		3764
736	=item ev_feed_signal_event (loop, int signum)	3765	A common way around all these issues is to make sure that
		3766	C<start_new_request> I<always> returns before the callback is invoked. If
		3767	C<start_new_request> immediately knows the result, it can artificially
		3768	delay invoking the callback by using a C<prepare> or C<idle> watcher for
		3769	example, or more sneakily, by reusing an existing (stopped) watcher and
		3770	pushing it into the pending queue:
737		3771
738	Feed an event as if the given signal occured (loop must be the default loop!).	3772	ev_set_cb (watcher, callback);
		3773	ev_feed_event (EV_A_ watcher, 0);
		3774
		3775	This way, C<start_new_request> can safely return before the callback is
		3776	invoked, while not delaying callback invocation too much.
		3777
		3778	=head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS
		3779
		3780	Often (especially in GUI toolkits) there are places where you have
		3781	I<modal> interaction, which is most easily implemented by recursively
		3782	invoking C<ev_run>.
		3783
		3784	This brings the problem of exiting - a callback might want to finish the
		3785	main C<ev_run> call, but not the nested one (e.g. user clicked "Quit", but
		3786	a modal "Are you sure?" dialog is still waiting), or just the nested one
		3787	and not the main one (e.g. user clocked "Ok" in a modal dialog), or some
		3788	other combination: In these cases, a simple C<ev_break> will not work.
		3789
		3790	The solution is to maintain "break this loop" variable for each C<ev_run>
		3791	invocation, and use a loop around C<ev_run> until the condition is
		3792	triggered, using C<EVRUN_ONCE>:
		3793
		3794	// main loop
		3795	int exit_main_loop = 0;
		3796
		3797	while (!exit_main_loop)
		3798	ev_run (EV_DEFAULT_ EVRUN_ONCE);
		3799
		3800	// in a modal watcher
		3801	int exit_nested_loop = 0;
		3802
		3803	while (!exit_nested_loop)
		3804	ev_run (EV_A_ EVRUN_ONCE);
		3805
		3806	To exit from any of these loops, just set the corresponding exit variable:
		3807
		3808	// exit modal loop
		3809	exit_nested_loop = 1;
		3810
		3811	// exit main program, after modal loop is finished
		3812	exit_main_loop = 1;
		3813
		3814	// exit both
		3815	exit_main_loop = exit_nested_loop = 1;
		3816
		3817	=head2 THREAD LOCKING EXAMPLE
		3818
		3819	Here is a fictitious example of how to run an event loop in a different
		3820	thread from where callbacks are being invoked and watchers are
		3821	created/added/removed.
		3822
		3823	For a real-world example, see the C<EV::Loop::Async> perl module,
		3824	which uses exactly this technique (which is suited for many high-level
		3825	languages).
		3826
		3827	The example uses a pthread mutex to protect the loop data, a condition
		3828	variable to wait for callback invocations, an async watcher to notify the
		3829	event loop thread and an unspecified mechanism to wake up the main thread.
		3830
		3831	First, you need to associate some data with the event loop:
		3832
		3833	typedef struct {
		3834	mutex_t lock; /* global loop lock */
		3835	ev_async async_w;
		3836	thread_t tid;
		3837	cond_t invoke_cv;
		3838	} userdata;
		3839
		3840	void prepare_loop (EV_P)
		3841	{
		3842	// for simplicity, we use a static userdata struct.
		3843	static userdata u;
		3844
		3845	ev_async_init (&u->async_w, async_cb);
		3846	ev_async_start (EV_A_ &u->async_w);
		3847
		3848	pthread_mutex_init (&u->lock, 0);
		3849	pthread_cond_init (&u->invoke_cv, 0);
		3850
		3851	// now associate this with the loop
		3852	ev_set_userdata (EV_A_ u);
		3853	ev_set_invoke_pending_cb (EV_A_ l_invoke);
		3854	ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
		3855
		3856	// then create the thread running ev_run
		3857	pthread_create (&u->tid, 0, l_run, EV_A);
		3858	}
		3859
		3860	The callback for the C<ev_async> watcher does nothing: the watcher is used
		3861	solely to wake up the event loop so it takes notice of any new watchers
		3862	that might have been added:
		3863
		3864	static void
		3865	async_cb (EV_P_ ev_async *w, int revents)
		3866	{
		3867	// just used for the side effects
		3868	}
		3869
		3870	The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
		3871	protecting the loop data, respectively.
		3872
		3873	static void
		3874	l_release (EV_P)
		3875	{
		3876	userdata *u = ev_userdata (EV_A);
		3877	pthread_mutex_unlock (&u->lock);
		3878	}
		3879
		3880	static void
		3881	l_acquire (EV_P)
		3882	{
		3883	userdata *u = ev_userdata (EV_A);
		3884	pthread_mutex_lock (&u->lock);
		3885	}
		3886
		3887	The event loop thread first acquires the mutex, and then jumps straight
		3888	into C<ev_run>:
		3889
		3890	void *
		3891	l_run (void *thr_arg)
		3892	{
		3893	struct ev_loop *loop = (struct ev_loop *)thr_arg;
		3894
		3895	l_acquire (EV_A);
		3896	pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
		3897	ev_run (EV_A_ 0);
		3898	l_release (EV_A);
		3899
		3900	return 0;
		3901	}
		3902
		3903	Instead of invoking all pending watchers, the C<l_invoke> callback will
		3904	signal the main thread via some unspecified mechanism (signals? pipe
		3905	writes? C<Async::Interrupt>?) and then waits until all pending watchers
		3906	have been called (in a while loop because a) spurious wakeups are possible
		3907	and b) skipping inter-thread-communication when there are no pending
		3908	watchers is very beneficial):
		3909
		3910	static void
		3911	l_invoke (EV_P)
		3912	{
		3913	userdata *u = ev_userdata (EV_A);
		3914
		3915	while (ev_pending_count (EV_A))
		3916	{
		3917	wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
		3918	pthread_cond_wait (&u->invoke_cv, &u->lock);
		3919	}
		3920	}
		3921
		3922	Now, whenever the main thread gets told to invoke pending watchers, it
		3923	will grab the lock, call C<ev_invoke_pending> and then signal the loop
		3924	thread to continue:
		3925
		3926	static void
		3927	real_invoke_pending (EV_P)
		3928	{
		3929	userdata *u = ev_userdata (EV_A);
		3930
		3931	pthread_mutex_lock (&u->lock);
		3932	ev_invoke_pending (EV_A);
		3933	pthread_cond_signal (&u->invoke_cv);
		3934	pthread_mutex_unlock (&u->lock);
		3935	}
		3936
		3937	Whenever you want to start/stop a watcher or do other modifications to an
		3938	event loop, you will now have to lock:
		3939
		3940	ev_timer timeout_watcher;
		3941	userdata *u = ev_userdata (EV_A);
		3942
		3943	ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
		3944
		3945	pthread_mutex_lock (&u->lock);
		3946	ev_timer_start (EV_A_ &timeout_watcher);
		3947	ev_async_send (EV_A_ &u->async_w);
		3948	pthread_mutex_unlock (&u->lock);
		3949
		3950	Note that sending the C<ev_async> watcher is required because otherwise
		3951	an event loop currently blocking in the kernel will have no knowledge
		3952	about the newly added timer. By waking up the loop it will pick up any new
		3953	watchers in the next event loop iteration.
		3954
		3955	=head2 THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS
		3956
		3957	While the overhead of a callback that e.g. schedules a thread is small, it
		3958	is still an overhead. If you embed libev, and your main usage is with some
		3959	kind of threads or coroutines, you might want to customise libev so that
		3960	doesn't need callbacks anymore.
		3961
		3962	Imagine you have coroutines that you can switch to using a function
		3963	C<switch_to (coro)>, that libev runs in a coroutine called C<libev_coro>
		3964	and that due to some magic, the currently active coroutine is stored in a
		3965	global called C<current_coro>. Then you can build your own "wait for libev
		3966	event" primitive by changing C<EV_CB_DECLARE> and C<EV_CB_INVOKE> (note
		3967	the differing C<;> conventions):
		3968
		3969	#define EV_CB_DECLARE(type) struct my_coro *cb;
		3970	#define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
		3971
		3972	That means instead of having a C callback function, you store the
		3973	coroutine to switch to in each watcher, and instead of having libev call
		3974	your callback, you instead have it switch to that coroutine.
		3975
		3976	A coroutine might now wait for an event with a function called
		3977	C<wait_for_event>. (the watcher needs to be started, as always, but it doesn't
		3978	matter when, or whether the watcher is active or not when this function is
		3979	called):
		3980
		3981	void
		3982	wait_for_event (ev_watcher *w)
		3983	{
		3984	ev_set_cb (w, current_coro);
		3985	switch_to (libev_coro);
		3986	}
		3987
		3988	That basically suspends the coroutine inside C<wait_for_event> and
		3989	continues the libev coroutine, which, when appropriate, switches back to
		3990	this or any other coroutine.
		3991
		3992	You can do similar tricks if you have, say, threads with an event queue -
		3993	instead of storing a coroutine, you store the queue object and instead of
		3994	switching to a coroutine, you push the watcher onto the queue and notify
		3995	any waiters.
		3996
		3997	To embed libev, see L</EMBEDDING>, but in short, it's easiest to create two
		3998	files, F<my_ev.h> and F<my_ev.c> that include the respective libev files:
		3999
		4000	// my_ev.h
		4001	#define EV_CB_DECLARE(type) struct my_coro *cb;
		4002	#define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
		4003	#include "../libev/ev.h"
		4004
		4005	// my_ev.c
		4006	#define EV_H "my_ev.h"
		4007	#include "../libev/ev.c"
		4008
		4009	And then use F<my_ev.h> when you would normally use F<ev.h>, and compile
		4010	F<my_ev.c> into your project. When properly specifying include paths, you
		4011	can even use F<ev.h> as header file name directly.
		4012
		4013
		4014	=head1 LIBEVENT EMULATION
		4015
		4016	Libev offers a compatibility emulation layer for libevent. It cannot
		4017	emulate the internals of libevent, so here are some usage hints:
		4018
		4019	=over 4
		4020
		4021	=item * Only the libevent-1.4.1-beta API is being emulated.
		4022
		4023	This was the newest libevent version available when libev was implemented,
		4024	and is still mostly unchanged in 2010.
		4025
		4026	=item * Use it by including <event.h>, as usual.
		4027
		4028	=item * The following members are fully supported: ev_base, ev_callback,
		4029	ev_arg, ev_fd, ev_res, ev_events.
		4030
		4031	=item * Avoid using ev_flags and the EVLIST_*-macros, while it is
		4032	maintained by libev, it does not work exactly the same way as in libevent (consider
		4033	it a private API).
		4034
		4035	=item * Priorities are not currently supported. Initialising priorities
		4036	will fail and all watchers will have the same priority, even though there
		4037	is an ev_pri field.
		4038
		4039	=item * In libevent, the last base created gets the signals, in libev, the
		4040	base that registered the signal gets the signals.
		4041
		4042	=item * Other members are not supported.
		4043
		4044	=item * The libev emulation is I<not> ABI compatible to libevent, you need
		4045	to use the libev header file and library.
739		4046
740	=back	4047	=back
741		4048
		4049	=head1 C++ SUPPORT
		4050
		4051	=head2 C API
		4052
		4053	The normal C API should work fine when used from C++: both ev.h and the
		4054	libev sources can be compiled as C++. Therefore, code that uses the C API
		4055	will work fine.
		4056
		4057	Proper exception specifications might have to be added to callbacks passed
		4058	to libev: exceptions may be thrown only from watcher callbacks, all other
		4059	callbacks (allocator, syserr, loop acquire/release and periodic reschedule
		4060	callbacks) must not throw exceptions, and might need a C<noexcept>
		4061	specification. If you have code that needs to be compiled as both C and
		4062	C++ you can use the C<EV_NOEXCEPT> macro for this:
		4063
		4064	static void
		4065	fatal_error (const char *msg) EV_NOEXCEPT
		4066	{
		4067	perror (msg);
		4068	abort ();
		4069	}
		4070
		4071	...
		4072	ev_set_syserr_cb (fatal_error);
		4073
		4074	The only API functions that can currently throw exceptions are C<ev_run>,
		4075	C<ev_invoke>, C<ev_invoke_pending> and C<ev_loop_destroy> (the latter
		4076	because it runs cleanup watchers).
		4077
		4078	Throwing exceptions in watcher callbacks is only supported if libev itself
		4079	is compiled with a C++ compiler or your C and C++ environments allow
		4080	throwing exceptions through C libraries (most do).
		4081
		4082	=head2 C++ API
		4083
		4084	Libev comes with some simplistic wrapper classes for C++ that mainly allow
		4085	you to use some convenience methods to start/stop watchers and also change
		4086	the callback model to a model using method callbacks on objects.
		4087
		4088	To use it,
		4089
		4090	#include <ev++.h>
		4091
		4092	This automatically includes F<ev.h> and puts all of its definitions (many
		4093	of them macros) into the global namespace. All C++ specific things are
		4094	put into the C<ev> namespace. It should support all the same embedding
		4095	options as F<ev.h>, most notably C<EV_MULTIPLICITY>.
		4096
		4097	Care has been taken to keep the overhead low. The only data member the C++
		4098	classes add (compared to plain C-style watchers) is the event loop pointer
		4099	that the watcher is associated with (or no additional members at all if
		4100	you disable C<EV_MULTIPLICITY> when embedding libev).
		4101
		4102	Currently, functions, static and non-static member functions and classes
		4103	with C<operator ()> can be used as callbacks. Other types should be easy
		4104	to add as long as they only need one additional pointer for context. If
		4105	you need support for other types of functors please contact the author
		4106	(preferably after implementing it).
		4107
		4108	For all this to work, your C++ compiler either has to use the same calling
		4109	conventions as your C compiler (for static member functions), or you have
		4110	to embed libev and compile libev itself as C++.
		4111
		4112	Here is a list of things available in the C<ev> namespace:
		4113
		4114	=over 4
		4115
		4116	=item C<ev::READ>, C<ev::WRITE> etc.
		4117
		4118	These are just enum values with the same values as the C<EV_READ> etc.
		4119	macros from F<ev.h>.
		4120
		4121	=item C<ev::tstamp>, C<ev::now>
		4122
		4123	Aliases to the same types/functions as with the C<ev_> prefix.
		4124
		4125	=item C<ev::io>, C<ev::timer>, C<ev::periodic>, C<ev::idle>, C<ev::sig> etc.
		4126
		4127	For each C<ev_TYPE> watcher in F<ev.h> there is a corresponding class of
		4128	the same name in the C<ev> namespace, with the exception of C<ev_signal>
		4129	which is called C<ev::sig> to avoid clashes with the C<signal> macro
		4130	defined by many implementations.
		4131
		4132	All of those classes have these methods:
		4133
		4134	=over 4
		4135
		4136	=item ev::TYPE::TYPE ()
		4137
		4138	=item ev::TYPE::TYPE (loop)
		4139
		4140	=item ev::TYPE::~TYPE
		4141
		4142	The constructor (optionally) takes an event loop to associate the watcher
		4143	with. If it is omitted, it will use C<EV_DEFAULT>.
		4144
		4145	The constructor calls C<ev_init> for you, which means you have to call the
		4146	C<set> method before starting it.
		4147
		4148	It will not set a callback, however: You have to call the templated C<set>
		4149	method to set a callback before you can start the watcher.
		4150
		4151	(The reason why you have to use a method is a limitation in C++ which does
		4152	not allow explicit template arguments for constructors).
		4153
		4154	The destructor automatically stops the watcher if it is active.
		4155
		4156	=item w->set<class, &class::method> (object *)
		4157
		4158	This method sets the callback method to call. The method has to have a
		4159	signature of C<void (*)(ev_TYPE &, int)>, it receives the watcher as
		4160	first argument and the C<revents> as second. The object must be given as
		4161	parameter and is stored in the C<data> member of the watcher.
		4162
		4163	This method synthesizes efficient thunking code to call your method from
		4164	the C callback that libev requires. If your compiler can inline your
		4165	callback (i.e. it is visible to it at the place of the C<set> call and
		4166	your compiler is good :), then the method will be fully inlined into the
		4167	thunking function, making it as fast as a direct C callback.
		4168
		4169	Example: simple class declaration and watcher initialisation
		4170
		4171	struct myclass
		4172	{
		4173	void io_cb (ev::io &w, int revents) { }
		4174	}
		4175
		4176	myclass obj;
		4177	ev::io iow;
		4178	iow.set <myclass, &myclass::io_cb> (&obj);
		4179
		4180	=item w->set (object *)
		4181
		4182	This is a variation of a method callback - leaving out the method to call
		4183	will default the method to C<operator ()>, which makes it possible to use
		4184	functor objects without having to manually specify the C<operator ()> all
		4185	the time. Incidentally, you can then also leave out the template argument
		4186	list.
		4187
		4188	The C<operator ()> method prototype must be C<void operator ()(watcher &w,
		4189	int revents)>.
		4190
		4191	See the method-C<set> above for more details.
		4192
		4193	Example: use a functor object as callback.
		4194
		4195	struct myfunctor
		4196	{
		4197	void operator() (ev::io &w, int revents)
		4198	{
		4199	...
		4200	}
		4201	}
		4202
		4203	myfunctor f;
		4204
		4205	ev::io w;
		4206	w.set (&f);
		4207
		4208	=item w->set<function> (void *data = 0)
		4209
		4210	Also sets a callback, but uses a static method or plain function as
		4211	callback. The optional C<data> argument will be stored in the watcher's
		4212	C<data> member and is free for you to use.
		4213
		4214	The prototype of the C<function> must be C<void (*)(ev::TYPE &w, int)>.
		4215
		4216	See the method-C<set> above for more details.
		4217
		4218	Example: Use a plain function as callback.
		4219
		4220	static void io_cb (ev::io &w, int revents) { }
		4221	iow.set <io_cb> ();
		4222
		4223	=item w->set (loop)
		4224
		4225	Associates a different C<struct ev_loop> with this watcher. You can only
		4226	do this when the watcher is inactive (and not pending either).
		4227
		4228	=item w->set ([arguments])
		4229
		4230	Basically the same as C<ev_TYPE_set> (except for C<ev::embed> watchers>),
		4231	with the same arguments. Either this method or a suitable start method
		4232	must be called at least once. Unlike the C counterpart, an active watcher
		4233	gets automatically stopped and restarted when reconfiguring it with this
		4234	method.
		4235
		4236	For C<ev::embed> watchers this method is called C<set_embed>, to avoid
		4237	clashing with the C<set (loop)> method.
		4238
		4239	=item w->start ()
		4240
		4241	Starts the watcher. Note that there is no C<loop> argument, as the
		4242	constructor already stores the event loop.
		4243
		4244	=item w->start ([arguments])
		4245
		4246	Instead of calling C<set> and C<start> methods separately, it is often
		4247	convenient to wrap them in one call. Uses the same type of arguments as
		4248	the configure C<set> method of the watcher.
		4249
		4250	=item w->stop ()
		4251
		4252	Stops the watcher if it is active. Again, no C<loop> argument.
		4253
		4254	=item w->again () (C<ev::timer>, C<ev::periodic> only)
		4255
		4256	For C<ev::timer> and C<ev::periodic>, this invokes the corresponding
		4257	C<ev_TYPE_again> function.
		4258
		4259	=item w->sweep () (C<ev::embed> only)
		4260
		4261	Invokes C<ev_embed_sweep>.
		4262
		4263	=item w->update () (C<ev::stat> only)
		4264
		4265	Invokes C<ev_stat_stat>.
		4266
		4267	=back
		4268
		4269	=back
		4270
		4271	Example: Define a class with two I/O and idle watchers, start the I/O
		4272	watchers in the constructor.
		4273
		4274	class myclass
		4275	{
		4276	ev::io io ; void io_cb (ev::io &w, int revents);
		4277	ev::io io2 ; void io2_cb (ev::io &w, int revents);
		4278	ev::idle idle; void idle_cb (ev::idle &w, int revents);
		4279
		4280	myclass (int fd)
		4281	{
		4282	io .set <myclass, &myclass::io_cb > (this);
		4283	io2 .set <myclass, &myclass::io2_cb > (this);
		4284	idle.set <myclass, &myclass::idle_cb> (this);
		4285
		4286	io.set (fd, ev::WRITE); // configure the watcher
		4287	io.start (); // start it whenever convenient
		4288
		4289	io2.start (fd, ev::READ); // set + start in one call
		4290	}
		4291	};
		4292
		4293
		4294	=head1 OTHER LANGUAGE BINDINGS
		4295
		4296	Libev does not offer other language bindings itself, but bindings for a
		4297	number of languages exist in the form of third-party packages. If you know
		4298	any interesting language binding in addition to the ones listed here, drop
		4299	me a note.
		4300
		4301	=over 4
		4302
		4303	=item Perl
		4304
		4305	The EV module implements the full libev API and is actually used to test
		4306	libev. EV is developed together with libev. Apart from the EV core module,
		4307	there are additional modules that implement libev-compatible interfaces
		4308	to C<libadns> (C<EV::ADNS>, but C<AnyEvent::DNS> is preferred nowadays),
		4309	C<Net::SNMP> (C<Net::SNMP::EV>) and the C<libglib> event core (C<Glib::EV>
		4310	and C<EV::Glib>).
		4311
		4312	It can be found and installed via CPAN, its homepage is at
		4313	L<http://software.schmorp.de/pkg/EV>.
		4314
		4315	=item Python
		4316
		4317	Python bindings can be found at L<http://code.google.com/p/pyev/>. It
		4318	seems to be quite complete and well-documented.
		4319
		4320	=item Ruby
		4321
		4322	Tony Arcieri has written a ruby extension that offers access to a subset
		4323	of the libev API and adds file handle abstractions, asynchronous DNS and
		4324	more on top of it. It can be found via gem servers. Its homepage is at
		4325	L<http://rev.rubyforge.org/>.
		4326
		4327	Roger Pack reports that using the link order C<-lws2_32 -lmsvcrt-ruby-190>
		4328	makes rev work even on mingw.
		4329
		4330	=item Haskell
		4331
		4332	A haskell binding to libev is available at
		4333	L<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>.
		4334
		4335	=item D
		4336
		4337	Leandro Lucarella has written a D language binding (F<ev.d>) for libev, to
		4338	be found at L<http://www.llucax.com.ar/proj/ev.d/index.html>.
		4339
		4340	=item Ocaml
		4341
		4342	Erkki Seppala has written Ocaml bindings for libev, to be found at
		4343	L<http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.
		4344
		4345	=item Lua
		4346
		4347	Brian Maher has written a partial interface to libev for lua (at the
		4348	time of this writing, only C<ev_io> and C<ev_timer>), to be found at
		4349	L<http://github.com/brimworks/lua-ev>.
		4350
		4351	=item Javascript
		4352
		4353	Node.js (L<http://nodejs.org>) uses libev as the underlying event library.
		4354
		4355	=item Others
		4356
		4357	There are others, and I stopped counting.
		4358
		4359	=back
		4360
		4361
		4362	=head1 MACRO MAGIC
		4363
		4364	Libev can be compiled with a variety of options, the most fundamental
		4365	of which is C<EV_MULTIPLICITY>. This option determines whether (most)
		4366	functions and callbacks have an initial C<struct ev_loop *> argument.
		4367
		4368	To make it easier to write programs that cope with either variant, the
		4369	following macros are defined:
		4370
		4371	=over 4
		4372
		4373	=item C<EV_A>, C<EV_A_>
		4374
		4375	This provides the loop I<argument> for functions, if one is required ("ev
		4376	loop argument"). The C<EV_A> form is used when this is the sole argument,
		4377	C<EV_A_> is used when other arguments are following. Example:
		4378
		4379	ev_unref (EV_A);
		4380	ev_timer_add (EV_A_ watcher);
		4381	ev_run (EV_A_ 0);
		4382
		4383	It assumes the variable C<loop> of type C<struct ev_loop *> is in scope,
		4384	which is often provided by the following macro.
		4385
		4386	=item C<EV_P>, C<EV_P_>
		4387
		4388	This provides the loop I<parameter> for functions, if one is required ("ev
		4389	loop parameter"). The C<EV_P> form is used when this is the sole parameter,
		4390	C<EV_P_> is used when other parameters are following. Example:
		4391
		4392	// this is how ev_unref is being declared
		4393	static void ev_unref (EV_P);
		4394
		4395	// this is how you can declare your typical callback
		4396	static void cb (EV_P_ ev_timer *w, int revents)
		4397
		4398	It declares a parameter C<loop> of type C<struct ev_loop *>, quite
		4399	suitable for use with C<EV_A>.
		4400
		4401	=item C<EV_DEFAULT>, C<EV_DEFAULT_>
		4402
		4403	Similar to the other two macros, this gives you the value of the default
		4404	loop, if multiple loops are supported ("ev loop default"). The default loop
		4405	will be initialised if it isn't already initialised.
		4406
		4407	For non-multiplicity builds, these macros do nothing, so you always have
		4408	to initialise the loop somewhere.
		4409
		4410	=item C<EV_DEFAULT_UC>, C<EV_DEFAULT_UC_>
		4411
		4412	Usage identical to C<EV_DEFAULT> and C<EV_DEFAULT_>, but requires that the
		4413	default loop has been initialised (C<UC> == unchecked). Their behaviour
		4414	is undefined when the default loop has not been initialised by a previous
		4415	execution of C<EV_DEFAULT>, C<EV_DEFAULT_> or C<ev_default_init (...)>.
		4416
		4417	It is often prudent to use C<EV_DEFAULT> when initialising the first
		4418	watcher in a function but use C<EV_DEFAULT_UC> afterwards.
		4419
		4420	=back
		4421
		4422	Example: Declare and initialise a check watcher, utilising the above
		4423	macros so it will work regardless of whether multiple loops are supported
		4424	or not.
		4425
		4426	static void
		4427	check_cb (EV_P_ ev_timer *w, int revents)
		4428	{
		4429	ev_check_stop (EV_A_ w);
		4430	}
		4431
		4432	ev_check check;
		4433	ev_check_init (&check, check_cb);
		4434	ev_check_start (EV_DEFAULT_ &check);
		4435	ev_run (EV_DEFAULT_ 0);
		4436
		4437	=head1 EMBEDDING
		4438
		4439	Libev can (and often is) directly embedded into host
		4440	applications. Examples of applications that embed it include the Deliantra
		4441	Game Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe)
		4442	and rxvt-unicode.
		4443
		4444	The goal is to enable you to just copy the necessary files into your
		4445	source directory without having to change even a single line in them, so
		4446	you can easily upgrade by simply copying (or having a checked-out copy of
		4447	libev somewhere in your source tree).
		4448
		4449	=head2 FILESETS
		4450
		4451	Depending on what features you need you need to include one or more sets of files
		4452	in your application.
		4453
		4454	=head3 CORE EVENT LOOP
		4455
		4456	To include only the libev core (all the C<ev_*> functions), with manual
		4457	configuration (no autoconf):
		4458
		4459	#define EV_STANDALONE 1
		4460	#include "ev.c"
		4461
		4462	This will automatically include F<ev.h>, too, and should be done in a
		4463	single C source file only to provide the function implementations. To use
		4464	it, do the same for F<ev.h> in all files wishing to use this API (best
		4465	done by writing a wrapper around F<ev.h> that you can include instead and
		4466	where you can put other configuration options):
		4467
		4468	#define EV_STANDALONE 1
		4469	#include "ev.h"
		4470
		4471	Both header files and implementation files can be compiled with a C++
		4472	compiler (at least, that's a stated goal, and breakage will be treated
		4473	as a bug).
		4474
		4475	You need the following files in your source tree, or in a directory
		4476	in your include path (e.g. in libev/ when using -Ilibev):
		4477
		4478	ev.h
		4479	ev.c
		4480	ev_vars.h
		4481	ev_wrap.h
		4482
		4483	ev_win32.c required on win32 platforms only
		4484
		4485	ev_select.c only when select backend is enabled
		4486	ev_poll.c only when poll backend is enabled
		4487	ev_epoll.c only when the epoll backend is enabled
		4488	ev_linuxaio.c only when the linux aio backend is enabled
		4489	ev_kqueue.c only when the kqueue backend is enabled
		4490	ev_port.c only when the solaris port backend is enabled
		4491
		4492	F<ev.c> includes the backend files directly when enabled, so you only need
		4493	to compile this single file.
		4494
		4495	=head3 LIBEVENT COMPATIBILITY API
		4496
		4497	To include the libevent compatibility API, also include:
		4498
		4499	#include "event.c"
		4500
		4501	in the file including F<ev.c>, and:
		4502
		4503	#include "event.h"
		4504
		4505	in the files that want to use the libevent API. This also includes F<ev.h>.
		4506
		4507	You need the following additional files for this:
		4508
		4509	event.h
		4510	event.c
		4511
		4512	=head3 AUTOCONF SUPPORT
		4513
		4514	Instead of using C<EV_STANDALONE=1> and providing your configuration in
		4515	whatever way you want, you can also C<m4_include([libev.m4])> in your
		4516	F<configure.ac> and leave C<EV_STANDALONE> undefined. F<ev.c> will then
		4517	include F<config.h> and configure itself accordingly.
		4518
		4519	For this of course you need the m4 file:
		4520
		4521	libev.m4
		4522
		4523	=head2 PREPROCESSOR SYMBOLS/MACROS
		4524
		4525	Libev can be configured via a variety of preprocessor symbols you have to
		4526	define before including (or compiling) any of its files. The default in
		4527	the absence of autoconf is documented for every option.
		4528
		4529	Symbols marked with "(h)" do not change the ABI, and can have different
		4530	values when compiling libev vs. including F<ev.h>, so it is permissible
		4531	to redefine them before including F<ev.h> without breaking compatibility
		4532	to a compiled library. All other symbols change the ABI, which means all
		4533	users of libev and the libev code itself must be compiled with compatible
		4534	settings.
		4535
		4536	=over 4
		4537
		4538	=item EV_COMPAT3 (h)
		4539
		4540	Backwards compatibility is a major concern for libev. This is why this
		4541	release of libev comes with wrappers for the functions and symbols that
		4542	have been renamed between libev version 3 and 4.
		4543
		4544	You can disable these wrappers (to test compatibility with future
		4545	versions) by defining C<EV_COMPAT3> to C<0> when compiling your
		4546	sources. This has the additional advantage that you can drop the C<struct>
		4547	from C<struct ev_loop> declarations, as libev will provide an C<ev_loop>
		4548	typedef in that case.
		4549
		4550	In some future version, the default for C<EV_COMPAT3> will become C<0>,
		4551	and in some even more future version the compatibility code will be
		4552	removed completely.
		4553
		4554	=item EV_STANDALONE (h)
		4555
		4556	Must always be C<1> if you do not use autoconf configuration, which
		4557	keeps libev from including F<config.h>, and it also defines dummy
		4558	implementations for some libevent functions (such as logging, which is not
		4559	supported). It will also not define any of the structs usually found in
		4560	F<event.h> that are not directly supported by the libev core alone.
		4561
		4562	In standalone mode, libev will still try to automatically deduce the
		4563	configuration, but has to be more conservative.
		4564
		4565	=item EV_USE_FLOOR
		4566
		4567	If defined to be C<1>, libev will use the C<floor ()> function for its
		4568	periodic reschedule calculations, otherwise libev will fall back on a
		4569	portable (slower) implementation. If you enable this, you usually have to
		4570	link against libm or something equivalent. Enabling this when the C<floor>
		4571	function is not available will fail, so the safe default is to not enable
		4572	this.
		4573
		4574	=item EV_USE_MONOTONIC
		4575
		4576	If defined to be C<1>, libev will try to detect the availability of the
		4577	monotonic clock option at both compile time and runtime. Otherwise no
		4578	use of the monotonic clock option will be attempted. If you enable this,
		4579	you usually have to link against librt or something similar. Enabling it
		4580	when the functionality isn't available is safe, though, although you have
		4581	to make sure you link against any libraries where the C<clock_gettime>
		4582	function is hiding in (often F<-lrt>). See also C<EV_USE_CLOCK_SYSCALL>.
		4583
		4584	=item EV_USE_REALTIME
		4585
		4586	If defined to be C<1>, libev will try to detect the availability of the
		4587	real-time clock option at compile time (and assume its availability
		4588	at runtime if successful). Otherwise no use of the real-time clock
		4589	option will be attempted. This effectively replaces C<gettimeofday>
		4590	by C<clock_get (CLOCK_REALTIME, ...)> and will not normally affect
		4591	correctness. See the note about libraries in the description of
		4592	C<EV_USE_MONOTONIC>, though. Defaults to the opposite value of
		4593	C<EV_USE_CLOCK_SYSCALL>.
		4594
		4595	=item EV_USE_CLOCK_SYSCALL
		4596
		4597	If defined to be C<1>, libev will try to use a direct syscall instead
		4598	of calling the system-provided C<clock_gettime> function. This option
		4599	exists because on GNU/Linux, C<clock_gettime> is in C<librt>, but C<librt>
		4600	unconditionally pulls in C<libpthread>, slowing down single-threaded
		4601	programs needlessly. Using a direct syscall is slightly slower (in
		4602	theory), because no optimised vdso implementation can be used, but avoids
		4603	the pthread dependency. Defaults to C<1> on GNU/Linux with glibc 2.x or
		4604	higher, as it simplifies linking (no need for C<-lrt>).
		4605
		4606	=item EV_USE_NANOSLEEP
		4607
		4608	If defined to be C<1>, libev will assume that C<nanosleep ()> is available
		4609	and will use it for delays. Otherwise it will use C<select ()>.
		4610
		4611	=item EV_USE_EVENTFD
		4612
		4613	If defined to be C<1>, then libev will assume that C<eventfd ()> is
		4614	available and will probe for kernel support at runtime. This will improve
		4615	C<ev_signal> and C<ev_async> performance and reduce resource consumption.
		4616	If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
		4617	2.7 or newer, otherwise disabled.
		4618
		4619	=item EV_USE_SELECT
		4620
		4621	If undefined or defined to be C<1>, libev will compile in support for the
		4622	C<select>(2) backend. No attempt at auto-detection will be done: if no
		4623	other method takes over, select will be it. Otherwise the select backend
		4624	will not be compiled in.
		4625
		4626	=item EV_SELECT_USE_FD_SET
		4627
		4628	If defined to C<1>, then the select backend will use the system C<fd_set>
		4629	structure. This is useful if libev doesn't compile due to a missing
		4630	C<NFDBITS> or C<fd_mask> definition or it mis-guesses the bitset layout
		4631	on exotic systems. This usually limits the range of file descriptors to
		4632	some low limit such as 1024 or might have other limitations (winsocket
		4633	only allows 64 sockets). The C<FD_SETSIZE> macro, set before compilation,
		4634	configures the maximum size of the C<fd_set>.
		4635
		4636	=item EV_SELECT_IS_WINSOCKET
		4637
		4638	When defined to C<1>, the select backend will assume that
		4639	select/socket/connect etc. don't understand file descriptors but
		4640	wants osf handles on win32 (this is the case when the select to
		4641	be used is the winsock select). This means that it will call
		4642	C<_get_osfhandle> on the fd to convert it to an OS handle. Otherwise,
		4643	it is assumed that all these functions actually work on fds, even
		4644	on win32. Should not be defined on non-win32 platforms.
		4645
		4646	=item EV_FD_TO_WIN32_HANDLE(fd)
		4647
		4648	If C<EV_SELECT_IS_WINSOCKET> is enabled, then libev needs a way to map
		4649	file descriptors to socket handles. When not defining this symbol (the
		4650	default), then libev will call C<_get_osfhandle>, which is usually
		4651	correct. In some cases, programs use their own file descriptor management,
		4652	in which case they can provide this function to map fds to socket handles.
		4653
		4654	=item EV_WIN32_HANDLE_TO_FD(handle)
		4655
		4656	If C<EV_SELECT_IS_WINSOCKET> then libev maps handles to file descriptors
		4657	using the standard C<_open_osfhandle> function. For programs implementing
		4658	their own fd to handle mapping, overwriting this function makes it easier
		4659	to do so. This can be done by defining this macro to an appropriate value.
		4660
		4661	=item EV_WIN32_CLOSE_FD(fd)
		4662
		4663	If programs implement their own fd to handle mapping on win32, then this
		4664	macro can be used to override the C<close> function, useful to unregister
		4665	file descriptors again. Note that the replacement function has to close
		4666	the underlying OS handle.
		4667
		4668	=item EV_USE_WSASOCKET
		4669
		4670	If defined to be C<1>, libev will use C<WSASocket> to create its internal
		4671	communication socket, which works better in some environments. Otherwise,
		4672	the normal C<socket> function will be used, which works better in other
		4673	environments.
		4674
		4675	=item EV_USE_POLL
		4676
		4677	If defined to be C<1>, libev will compile in support for the C<poll>(2)
		4678	backend. Otherwise it will be enabled on non-win32 platforms. It
		4679	takes precedence over select.
		4680
		4681	=item EV_USE_EPOLL
		4682
		4683	If defined to be C<1>, libev will compile in support for the Linux
		4684	C<epoll>(7) backend. Its availability will be detected at runtime,
		4685	otherwise another method will be used as fallback. This is the preferred
		4686	backend for GNU/Linux systems. If undefined, it will be enabled if the
		4687	headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
		4688
		4689	=item EV_USE_LINUXAIO
		4690
		4691	If defined to be C<1>, libev will compile in support for the Linux
		4692	aio backend. Due to it's currenbt limitations it has to be requested
		4693	explicitly. If undefined, it will be enabled on linux, otherwise
		4694	disabled.
		4695
		4696	=item EV_USE_KQUEUE
		4697
		4698	If defined to be C<1>, libev will compile in support for the BSD style
		4699	C<kqueue>(2) backend. Its actual availability will be detected at runtime,
		4700	otherwise another method will be used as fallback. This is the preferred
		4701	backend for BSD and BSD-like systems, although on most BSDs kqueue only
		4702	supports some types of fds correctly (the only platform we found that
		4703	supports ptys for example was NetBSD), so kqueue might be compiled in, but
		4704	not be used unless explicitly requested. The best way to use it is to find
		4705	out whether kqueue supports your type of fd properly and use an embedded
		4706	kqueue loop.
		4707
		4708	=item EV_USE_PORT
		4709
		4710	If defined to be C<1>, libev will compile in support for the Solaris
		4711	10 port style backend. Its availability will be detected at runtime,
		4712	otherwise another method will be used as fallback. This is the preferred
		4713	backend for Solaris 10 systems.
		4714
		4715	=item EV_USE_DEVPOLL
		4716
		4717	Reserved for future expansion, works like the USE symbols above.
		4718
		4719	=item EV_USE_INOTIFY
		4720
		4721	If defined to be C<1>, libev will compile in support for the Linux inotify
		4722	interface to speed up C<ev_stat> watchers. Its actual availability will
		4723	be detected at runtime. If undefined, it will be enabled if the headers
		4724	indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
		4725
		4726	=item EV_NO_SMP
		4727
		4728	If defined to be C<1>, libev will assume that memory is always coherent
		4729	between threads, that is, threads can be used, but threads never run on
		4730	different cpus (or different cpu cores). This reduces dependencies
		4731	and makes libev faster.
		4732
		4733	=item EV_NO_THREADS
		4734
		4735	If defined to be C<1>, libev will assume that it will never be called from
		4736	different threads (that includes signal handlers), which is a stronger
		4737	assumption than C<EV_NO_SMP>, above. This reduces dependencies and makes
		4738	libev faster.
		4739
		4740	=item EV_ATOMIC_T
		4741
		4742	Libev requires an integer type (suitable for storing C<0> or C<1>) whose
		4743	access is atomic with respect to other threads or signal contexts. No
		4744	such type is easily found in the C language, so you can provide your own
		4745	type that you know is safe for your purposes. It is used both for signal
		4746	handler "locking" as well as for signal and thread safety in C<ev_async>
		4747	watchers.
		4748
		4749	In the absence of this define, libev will use C<sig_atomic_t volatile>
		4750	(from F<signal.h>), which is usually good enough on most platforms.
		4751
		4752	=item EV_H (h)
		4753
		4754	The name of the F<ev.h> header file used to include it. The default if
		4755	undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be
		4756	used to virtually rename the F<ev.h> header file in case of conflicts.
		4757
		4758	=item EV_CONFIG_H (h)
		4759
		4760	If C<EV_STANDALONE> isn't C<1>, this variable can be used to override
		4761	F<ev.c>'s idea of where to find the F<config.h> file, similarly to
		4762	C<EV_H>, above.
		4763
		4764	=item EV_EVENT_H (h)
		4765
		4766	Similarly to C<EV_H>, this macro can be used to override F<event.c>'s idea
		4767	of how the F<event.h> header can be found, the default is C<"event.h">.
		4768
		4769	=item EV_PROTOTYPES (h)
		4770
		4771	If defined to be C<0>, then F<ev.h> will not define any function
		4772	prototypes, but still define all the structs and other symbols. This is
		4773	occasionally useful if you want to provide your own wrapper functions
		4774	around libev functions.
		4775
		4776	=item EV_MULTIPLICITY
		4777
		4778	If undefined or defined to C<1>, then all event-loop-specific functions
		4779	will have the C<struct ev_loop *> as first argument, and you can create
		4780	additional independent event loops. Otherwise there will be no support
		4781	for multiple event loops and there is no first event loop pointer
		4782	argument. Instead, all functions act on the single default loop.
		4783
		4784	Note that C<EV_DEFAULT> and C<EV_DEFAULT_> will no longer provide a
		4785	default loop when multiplicity is switched off - you always have to
		4786	initialise the loop manually in this case.
		4787
		4788	=item EV_MINPRI
		4789
		4790	=item EV_MAXPRI
		4791
		4792	The range of allowed priorities. C<EV_MINPRI> must be smaller or equal to
		4793	C<EV_MAXPRI>, but otherwise there are no non-obvious limitations. You can
		4794	provide for more priorities by overriding those symbols (usually defined
		4795	to be C<-2> and C<2>, respectively).
		4796
		4797	When doing priority-based operations, libev usually has to linearly search
		4798	all the priorities, so having many of them (hundreds) uses a lot of space
		4799	and time, so using the defaults of five priorities (-2 .. +2) is usually
		4800	fine.
		4801
		4802	If your embedding application does not need any priorities, defining these
		4803	both to C<0> will save some memory and CPU.
		4804
		4805	=item EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE,
		4806	EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE,
		4807	EV_ASYNC_ENABLE, EV_CHILD_ENABLE.
		4808
		4809	If undefined or defined to be C<1> (and the platform supports it), then
		4810	the respective watcher type is supported. If defined to be C<0>, then it
		4811	is not. Disabling watcher types mainly saves code size.
		4812
		4813	=item EV_FEATURES
		4814
		4815	If you need to shave off some kilobytes of code at the expense of some
		4816	speed (but with the full API), you can define this symbol to request
		4817	certain subsets of functionality. The default is to enable all features
		4818	that can be enabled on the platform.
		4819
		4820	A typical way to use this symbol is to define it to C<0> (or to a bitset
		4821	with some broad features you want) and then selectively re-enable
		4822	additional parts you want, for example if you want everything minimal,
		4823	but multiple event loop support, async and child watchers and the poll
		4824	backend, use this:
		4825
		4826	#define EV_FEATURES 0
		4827	#define EV_MULTIPLICITY 1
		4828	#define EV_USE_POLL 1
		4829	#define EV_CHILD_ENABLE 1
		4830	#define EV_ASYNC_ENABLE 1
		4831
		4832	The actual value is a bitset, it can be a combination of the following
		4833	values (by default, all of these are enabled):
		4834
		4835	=over 4
		4836
		4837	=item C<1> - faster/larger code
		4838
		4839	Use larger code to speed up some operations.
		4840
		4841	Currently this is used to override some inlining decisions (enlarging the
		4842	code size by roughly 30% on amd64).
		4843
		4844	When optimising for size, use of compiler flags such as C<-Os> with
		4845	gcc is recommended, as well as C<-DNDEBUG>, as libev contains a number of
		4846	assertions.
		4847
		4848	The default is off when C<__OPTIMIZE_SIZE__> is defined by your compiler
		4849	(e.g. gcc with C<-Os>).
		4850
		4851	=item C<2> - faster/larger data structures
		4852
		4853	Replaces the small 2-heap for timer management by a faster 4-heap, larger
		4854	hash table sizes and so on. This will usually further increase code size
		4855	and can additionally have an effect on the size of data structures at
		4856	runtime.
		4857
		4858	The default is off when C<__OPTIMIZE_SIZE__> is defined by your compiler
		4859	(e.g. gcc with C<-Os>).
		4860
		4861	=item C<4> - full API configuration
		4862
		4863	This enables priorities (sets C<EV_MAXPRI>=2 and C<EV_MINPRI>=-2), and
		4864	enables multiplicity (C<EV_MULTIPLICITY>=1).
		4865
		4866	=item C<8> - full API
		4867
		4868	This enables a lot of the "lesser used" API functions. See C<ev.h> for
		4869	details on which parts of the API are still available without this
		4870	feature, and do not complain if this subset changes over time.
		4871
		4872	=item C<16> - enable all optional watcher types
		4873
		4874	Enables all optional watcher types. If you want to selectively enable
		4875	only some watcher types other than I/O and timers (e.g. prepare,
		4876	embed, async, child...) you can enable them manually by defining
		4877	C<EV_watchertype_ENABLE> to C<1> instead.
		4878
		4879	=item C<32> - enable all backends
		4880
		4881	This enables all backends - without this feature, you need to enable at
		4882	least one backend manually (C<EV_USE_SELECT> is a good choice).
		4883
		4884	=item C<64> - enable OS-specific "helper" APIs
		4885
		4886	Enable inotify, eventfd, signalfd and similar OS-specific helper APIs by
		4887	default.
		4888
		4889	=back
		4890
		4891	Compiling with C<gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1 -DEV_FEATURES=0>
		4892	reduces the compiled size of libev from 24.7Kb code/2.8Kb data to 6.5Kb
		4893	code/0.3Kb data on my GNU/Linux amd64 system, while still giving you I/O
		4894	watchers, timers and monotonic clock support.
		4895
		4896	With an intelligent-enough linker (gcc+binutils are intelligent enough
		4897	when you use C<-Wl,--gc-sections -ffunction-sections>) functions unused by
		4898	your program might be left out as well - a binary starting a timer and an
		4899	I/O watcher then might come out at only 5Kb.
		4900
		4901	=item EV_API_STATIC
		4902
		4903	If this symbol is defined (by default it is not), then all identifiers
		4904	will have static linkage. This means that libev will not export any
		4905	identifiers, and you cannot link against libev anymore. This can be useful
		4906	when you embed libev, only want to use libev functions in a single file,
		4907	and do not want its identifiers to be visible.
		4908
		4909	To use this, define C<EV_API_STATIC> and include F<ev.c> in the file that
		4910	wants to use libev.
		4911
		4912	This option only works when libev is compiled with a C compiler, as C++
		4913	doesn't support the required declaration syntax.
		4914
		4915	=item EV_AVOID_STDIO
		4916
		4917	If this is set to C<1> at compiletime, then libev will avoid using stdio
		4918	functions (printf, scanf, perror etc.). This will increase the code size
		4919	somewhat, but if your program doesn't otherwise depend on stdio and your
		4920	libc allows it, this avoids linking in the stdio library which is quite
		4921	big.
		4922
		4923	Note that error messages might become less precise when this option is
		4924	enabled.
		4925
		4926	=item EV_NSIG
		4927
		4928	The highest supported signal number, +1 (or, the number of
		4929	signals): Normally, libev tries to deduce the maximum number of signals
		4930	automatically, but sometimes this fails, in which case it can be
		4931	specified. Also, using a lower number than detected (C<32> should be
		4932	good for about any system in existence) can save some memory, as libev
		4933	statically allocates some 12-24 bytes per signal number.
		4934
		4935	=item EV_PID_HASHSIZE
		4936
		4937	C<ev_child> watchers use a small hash table to distribute workload by
		4938	pid. The default size is C<16> (or C<1> with C<EV_FEATURES> disabled),
		4939	usually more than enough. If you need to manage thousands of children you
		4940	might want to increase this value (I<must> be a power of two).
		4941
		4942	=item EV_INOTIFY_HASHSIZE
		4943
		4944	C<ev_stat> watchers use a small hash table to distribute workload by
		4945	inotify watch id. The default size is C<16> (or C<1> with C<EV_FEATURES>
		4946	disabled), usually more than enough. If you need to manage thousands of
		4947	C<ev_stat> watchers you might want to increase this value (I<must> be a
		4948	power of two).
		4949
		4950	=item EV_USE_4HEAP
		4951
		4952	Heaps are not very cache-efficient. To improve the cache-efficiency of the
		4953	timer and periodics heaps, libev uses a 4-heap when this symbol is defined
		4954	to C<1>. The 4-heap uses more complicated (longer) code but has noticeably
		4955	faster performance with many (thousands) of watchers.
		4956
		4957	The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
		4958	will be C<0>.
		4959
		4960	=item EV_HEAP_CACHE_AT
		4961
		4962	Heaps are not very cache-efficient. To improve the cache-efficiency of the
		4963	timer and periodics heaps, libev can cache the timestamp (I<at>) within
		4964	the heap structure (selected by defining C<EV_HEAP_CACHE_AT> to C<1>),
		4965	which uses 8-12 bytes more per watcher and a few hundred bytes more code,
		4966	but avoids random read accesses on heap changes. This improves performance
		4967	noticeably with many (hundreds) of watchers.
		4968
		4969	The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
		4970	will be C<0>.
		4971
		4972	=item EV_VERIFY
		4973
		4974	Controls how much internal verification (see C<ev_verify ()>) will
		4975	be done: If set to C<0>, no internal verification code will be compiled
		4976	in. If set to C<1>, then verification code will be compiled in, but not
		4977	called. If set to C<2>, then the internal verification code will be
		4978	called once per loop, which can slow down libev. If set to C<3>, then the
		4979	verification code will be called very frequently, which will slow down
		4980	libev considerably.
		4981
		4982	Verification errors are reported via C's C<assert> mechanism, so if you
		4983	disable that (e.g. by defining C<NDEBUG>) then no errors will be reported.
		4984
		4985	The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
		4986	will be C<0>.
		4987
		4988	=item EV_COMMON
		4989
		4990	By default, all watchers have a C<void *data> member. By redefining
		4991	this macro to something else you can include more and other types of
		4992	members. You have to define it each time you include one of the files,
		4993	though, and it must be identical each time.
		4994
		4995	For example, the perl EV module uses something like this:
		4996
		4997	#define EV_COMMON \
		4998	SV *self; /* contains this struct */ \
		4999	SV *cb_sv, *fh /* note no trailing ";" */
		5000
		5001	=item EV_CB_DECLARE (type)
		5002
		5003	=item EV_CB_INVOKE (watcher, revents)
		5004
		5005	=item ev_set_cb (ev, cb)
		5006
		5007	Can be used to change the callback member declaration in each watcher,
		5008	and the way callbacks are invoked and set. Must expand to a struct member
		5009	definition and a statement, respectively. See the F<ev.h> header file for
		5010	their default definitions. One possible use for overriding these is to
		5011	avoid the C<struct ev_loop *> as first argument in all cases, or to use
		5012	method calls instead of plain function calls in C++.
		5013
		5014	=back
		5015
		5016	=head2 EXPORTED API SYMBOLS
		5017
		5018	If you need to re-export the API (e.g. via a DLL) and you need a list of
		5019	exported symbols, you can use the provided F<Symbol.*> files which list
		5020	all public symbols, one per line:
		5021
		5022	Symbols.ev for libev proper
		5023	Symbols.event for the libevent emulation
		5024
		5025	This can also be used to rename all public symbols to avoid clashes with
		5026	multiple versions of libev linked together (which is obviously bad in
		5027	itself, but sometimes it is inconvenient to avoid this).
		5028
		5029	A sed command like this will create wrapper C<#define>'s that you need to
		5030	include before including F<ev.h>:
		5031
		5032	<Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h
		5033
		5034	This would create a file F<wrap.h> which essentially looks like this:
		5035
		5036	#define ev_backend myprefix_ev_backend
		5037	#define ev_check_start myprefix_ev_check_start
		5038	#define ev_check_stop myprefix_ev_check_stop
		5039	...
		5040
		5041	=head2 EXAMPLES
		5042
		5043	For a real-world example of a program the includes libev
		5044	verbatim, you can have a look at the EV perl module
		5045	(L<http://software.schmorp.de/pkg/EV.html>). It has the libev files in
		5046	the F<libev/> subdirectory and includes them in the F<EV/EVAPI.h> (public
		5047	interface) and F<EV.xs> (implementation) files. Only the F<EV.xs> file
		5048	will be compiled. It is pretty complex because it provides its own header
		5049	file.
		5050
		5051	The usage in rxvt-unicode is simpler. It has a F<ev_cpp.h> header file
		5052	that everybody includes and which overrides some configure choices:
		5053
		5054	#define EV_FEATURES 8
		5055	#define EV_USE_SELECT 1
		5056	#define EV_PREPARE_ENABLE 1
		5057	#define EV_IDLE_ENABLE 1
		5058	#define EV_SIGNAL_ENABLE 1
		5059	#define EV_CHILD_ENABLE 1
		5060	#define EV_USE_STDEXCEPT 0
		5061	#define EV_CONFIG_H <config.h>
		5062
		5063	#include "ev++.h"
		5064
		5065	And a F<ev_cpp.C> implementation file that contains libev proper and is compiled:
		5066
		5067	#include "ev_cpp.h"
		5068	#include "ev.c"
		5069
		5070	=head1 INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT
		5071
		5072	=head2 THREADS AND COROUTINES
		5073
		5074	=head3 THREADS
		5075
		5076	All libev functions are reentrant and thread-safe unless explicitly
		5077	documented otherwise, but libev implements no locking itself. This means
		5078	that you can use as many loops as you want in parallel, as long as there
		5079	are no concurrent calls into any libev function with the same loop
		5080	parameter (C<ev_default_*> calls have an implicit default loop parameter,
		5081	of course): libev guarantees that different event loops share no data
		5082	structures that need any locking.
		5083
		5084	Or to put it differently: calls with different loop parameters can be done
		5085	concurrently from multiple threads, calls with the same loop parameter
		5086	must be done serially (but can be done from different threads, as long as
		5087	only one thread ever is inside a call at any point in time, e.g. by using
		5088	a mutex per loop).
		5089
		5090	Specifically to support threads (and signal handlers), libev implements
		5091	so-called C<ev_async> watchers, which allow some limited form of
		5092	concurrency on the same event loop, namely waking it up "from the
		5093	outside".
		5094
		5095	If you want to know which design (one loop, locking, or multiple loops
		5096	without or something else still) is best for your problem, then I cannot
		5097	help you, but here is some generic advice:
		5098
		5099	=over 4
		5100
		5101	=item * most applications have a main thread: use the default libev loop
		5102	in that thread, or create a separate thread running only the default loop.
		5103
		5104	This helps integrating other libraries or software modules that use libev
		5105	themselves and don't care/know about threading.
		5106
		5107	=item * one loop per thread is usually a good model.
		5108
		5109	Doing this is almost never wrong, sometimes a better-performance model
		5110	exists, but it is always a good start.
		5111
		5112	=item * other models exist, such as the leader/follower pattern, where one
		5113	loop is handed through multiple threads in a kind of round-robin fashion.
		5114
		5115	Choosing a model is hard - look around, learn, know that usually you can do
		5116	better than you currently do :-)
		5117
		5118	=item * often you need to talk to some other thread which blocks in the
		5119	event loop.
		5120
		5121	C<ev_async> watchers can be used to wake them up from other threads safely
		5122	(or from signal contexts...).
		5123
		5124	An example use would be to communicate signals or other events that only
		5125	work in the default loop by registering the signal watcher with the
		5126	default loop and triggering an C<ev_async> watcher from the default loop
		5127	watcher callback into the event loop interested in the signal.
		5128
		5129	=back
		5130
		5131	See also L</THREAD LOCKING EXAMPLE>.
		5132
		5133	=head3 COROUTINES
		5134
		5135	Libev is very accommodating to coroutines ("cooperative threads"):
		5136	libev fully supports nesting calls to its functions from different
		5137	coroutines (e.g. you can call C<ev_run> on the same loop from two
		5138	different coroutines, and switch freely between both coroutines running
		5139	the loop, as long as you don't confuse yourself). The only exception is
		5140	that you must not do this from C<ev_periodic> reschedule callbacks.
		5141
		5142	Care has been taken to ensure that libev does not keep local state inside
		5143	C<ev_run>, and other calls do not usually allow for coroutine switches as
		5144	they do not call any callbacks.
		5145
		5146	=head2 COMPILER WARNINGS
		5147
		5148	Depending on your compiler and compiler settings, you might get no or a
		5149	lot of warnings when compiling libev code. Some people are apparently
		5150	scared by this.
		5151
		5152	However, these are unavoidable for many reasons. For one, each compiler
		5153	has different warnings, and each user has different tastes regarding
		5154	warning options. "Warn-free" code therefore cannot be a goal except when
		5155	targeting a specific compiler and compiler-version.
		5156
		5157	Another reason is that some compiler warnings require elaborate
		5158	workarounds, or other changes to the code that make it less clear and less
		5159	maintainable.
		5160
		5161	And of course, some compiler warnings are just plain stupid, or simply
		5162	wrong (because they don't actually warn about the condition their message
		5163	seems to warn about). For example, certain older gcc versions had some
		5164	warnings that resulted in an extreme number of false positives. These have
		5165	been fixed, but some people still insist on making code warn-free with
		5166	such buggy versions.
		5167
		5168	While libev is written to generate as few warnings as possible,
		5169	"warn-free" code is not a goal, and it is recommended not to build libev
		5170	with any compiler warnings enabled unless you are prepared to cope with
		5171	them (e.g. by ignoring them). Remember that warnings are just that:
		5172	warnings, not errors, or proof of bugs.
		5173
		5174
		5175	=head2 VALGRIND
		5176
		5177	Valgrind has a special section here because it is a popular tool that is
		5178	highly useful. Unfortunately, valgrind reports are very hard to interpret.
		5179
		5180	If you think you found a bug (memory leak, uninitialised data access etc.)
		5181	in libev, then check twice: If valgrind reports something like:
		5182
		5183	==2274== definitely lost: 0 bytes in 0 blocks.
		5184	==2274== possibly lost: 0 bytes in 0 blocks.
		5185	==2274== still reachable: 256 bytes in 1 blocks.
		5186
		5187	Then there is no memory leak, just as memory accounted to global variables
		5188	is not a memleak - the memory is still being referenced, and didn't leak.
		5189
		5190	Similarly, under some circumstances, valgrind might report kernel bugs
		5191	as if it were a bug in libev (e.g. in realloc or in the poll backend,
		5192	although an acceptable workaround has been found here), or it might be
		5193	confused.
		5194
		5195	Keep in mind that valgrind is a very good tool, but only a tool. Don't
		5196	make it into some kind of religion.
		5197
		5198	If you are unsure about something, feel free to contact the mailing list
		5199	with the full valgrind report and an explanation on why you think this
		5200	is a bug in libev (best check the archives, too :). However, don't be
		5201	annoyed when you get a brisk "this is no bug" answer and take the chance
		5202	of learning how to interpret valgrind properly.
		5203
		5204	If you need, for some reason, empty reports from valgrind for your project
		5205	I suggest using suppression lists.
		5206
		5207
		5208	=head1 PORTABILITY NOTES
		5209
		5210	=head2 GNU/LINUX 32 BIT LIMITATIONS
		5211
		5212	GNU/Linux is the only common platform that supports 64 bit file/large file
		5213	interfaces but I<disables> them by default.
		5214
		5215	That means that libev compiled in the default environment doesn't support
		5216	files larger than 2GiB or so, which mainly affects C<ev_stat> watchers.
		5217
		5218	Unfortunately, many programs try to work around this GNU/Linux issue
		5219	by enabling the large file API, which makes them incompatible with the
		5220	standard libev compiled for their system.
		5221
		5222	Likewise, libev cannot enable the large file API itself as this would
		5223	suddenly make it incompatible to the default compile time environment,
		5224	i.e. all programs not using special compile switches.
		5225
		5226	=head2 OS/X AND DARWIN BUGS
		5227
		5228	The whole thing is a bug if you ask me - basically any system interface
		5229	you touch is broken, whether it is locales, poll, kqueue or even the
		5230	OpenGL drivers.
		5231
		5232	=head3 C<kqueue> is buggy
		5233
		5234	The kqueue syscall is broken in all known versions - most versions support
		5235	only sockets, many support pipes.
		5236
		5237	Libev tries to work around this by not using C<kqueue> by default on this
		5238	rotten platform, but of course you can still ask for it when creating a
		5239	loop - embedding a socket-only kqueue loop into a select-based one is
		5240	probably going to work well.
		5241
		5242	=head3 C<poll> is buggy
		5243
		5244	Instead of fixing C<kqueue>, Apple replaced their (working) C<poll>
		5245	implementation by something calling C<kqueue> internally around the 10.5.6
		5246	release, so now C<kqueue> I<and> C<poll> are broken.
		5247
		5248	Libev tries to work around this by not using C<poll> by default on
		5249	this rotten platform, but of course you can still ask for it when creating
		5250	a loop.
		5251
		5252	=head3 C<select> is buggy
		5253
		5254	All that's left is C<select>, and of course Apple found a way to fuck this
		5255	one up as well: On OS/X, C<select> actively limits the number of file
		5256	descriptors you can pass in to 1024 - your program suddenly crashes when
		5257	you use more.
		5258
		5259	There is an undocumented "workaround" for this - defining
		5260	C<_DARWIN_UNLIMITED_SELECT>, which libev tries to use, so select I<should>
		5261	work on OS/X.
		5262
		5263	=head2 SOLARIS PROBLEMS AND WORKAROUNDS
		5264
		5265	=head3 C<errno> reentrancy
		5266
		5267	The default compile environment on Solaris is unfortunately so
		5268	thread-unsafe that you can't even use components/libraries compiled
		5269	without C<-D_REENTRANT> in a threaded program, which, of course, isn't
		5270	defined by default. A valid, if stupid, implementation choice.
		5271
		5272	If you want to use libev in threaded environments you have to make sure
		5273	it's compiled with C<_REENTRANT> defined.
		5274
		5275	=head3 Event port backend
		5276
		5277	The scalable event interface for Solaris is called "event
		5278	ports". Unfortunately, this mechanism is very buggy in all major
		5279	releases. If you run into high CPU usage, your program freezes or you get
		5280	a large number of spurious wakeups, make sure you have all the relevant
		5281	and latest kernel patches applied. No, I don't know which ones, but there
		5282	are multiple ones to apply, and afterwards, event ports actually work
		5283	great.
		5284
		5285	If you can't get it to work, you can try running the program by setting
		5286	the environment variable C<LIBEV_FLAGS=3> to only allow C<poll> and
		5287	C<select> backends.
		5288
		5289	=head2 AIX POLL BUG
		5290
		5291	AIX unfortunately has a broken C<poll.h> header. Libev works around
		5292	this by trying to avoid the poll backend altogether (i.e. it's not even
		5293	compiled in), which normally isn't a big problem as C<select> works fine
		5294	with large bitsets on AIX, and AIX is dead anyway.
		5295
		5296	=head2 WIN32 PLATFORM LIMITATIONS AND WORKAROUNDS
		5297
		5298	=head3 General issues
		5299
		5300	Win32 doesn't support any of the standards (e.g. POSIX) that libev
		5301	requires, and its I/O model is fundamentally incompatible with the POSIX
		5302	model. Libev still offers limited functionality on this platform in
		5303	the form of the C<EVBACKEND_SELECT> backend, and only supports socket
		5304	descriptors. This only applies when using Win32 natively, not when using
		5305	e.g. cygwin. Actually, it only applies to the microsofts own compilers,
		5306	as every compiler comes with a slightly differently broken/incompatible
		5307	environment.
		5308
		5309	Lifting these limitations would basically require the full
		5310	re-implementation of the I/O system. If you are into this kind of thing,
		5311	then note that glib does exactly that for you in a very portable way (note
		5312	also that glib is the slowest event library known to man).
		5313
		5314	There is no supported compilation method available on windows except
		5315	embedding it into other applications.
		5316
		5317	Sensible signal handling is officially unsupported by Microsoft - libev
		5318	tries its best, but under most conditions, signals will simply not work.
		5319
		5320	Not a libev limitation but worth mentioning: windows apparently doesn't
		5321	accept large writes: instead of resulting in a partial write, windows will
		5322	either accept everything or return C<ENOBUFS> if the buffer is too large,
		5323	so make sure you only write small amounts into your sockets (less than a
		5324	megabyte seems safe, but this apparently depends on the amount of memory
		5325	available).
		5326
		5327	Due to the many, low, and arbitrary limits on the win32 platform and
		5328	the abysmal performance of winsockets, using a large number of sockets
		5329	is not recommended (and not reasonable). If your program needs to use
		5330	more than a hundred or so sockets, then likely it needs to use a totally
		5331	different implementation for windows, as libev offers the POSIX readiness
		5332	notification model, which cannot be implemented efficiently on windows
		5333	(due to Microsoft monopoly games).
		5334
		5335	A typical way to use libev under windows is to embed it (see the embedding
		5336	section for details) and use the following F<evwrap.h> header file instead
		5337	of F<ev.h>:
		5338
		5339	#define EV_STANDALONE /* keeps ev from requiring config.h */
		5340	#define EV_SELECT_IS_WINSOCKET 1 /* configure libev for windows select */
		5341
		5342	#include "ev.h"
		5343
		5344	And compile the following F<evwrap.c> file into your project (make sure
		5345	you do I<not> compile the F<ev.c> or any other embedded source files!):
		5346
		5347	#include "evwrap.h"
		5348	#include "ev.c"
		5349
		5350	=head3 The winsocket C<select> function
		5351
		5352	The winsocket C<select> function doesn't follow POSIX in that it
		5353	requires socket I<handles> and not socket I<file descriptors> (it is
		5354	also extremely buggy). This makes select very inefficient, and also
		5355	requires a mapping from file descriptors to socket handles (the Microsoft
		5356	C runtime provides the function C<_open_osfhandle> for this). See the
		5357	discussion of the C<EV_SELECT_USE_FD_SET>, C<EV_SELECT_IS_WINSOCKET> and
		5358	C<EV_FD_TO_WIN32_HANDLE> preprocessor symbols for more info.
		5359
		5360	The configuration for a "naked" win32 using the Microsoft runtime
		5361	libraries and raw winsocket select is:
		5362
		5363	#define EV_USE_SELECT 1
		5364	#define EV_SELECT_IS_WINSOCKET 1 /* forces EV_SELECT_USE_FD_SET, too */
		5365
		5366	Note that winsockets handling of fd sets is O(n), so you can easily get a
		5367	complexity in the O(n²) range when using win32.
		5368
		5369	=head3 Limited number of file descriptors
		5370
		5371	Windows has numerous arbitrary (and low) limits on things.
		5372
		5373	Early versions of winsocket's select only supported waiting for a maximum
		5374	of C<64> handles (probably owning to the fact that all windows kernels
		5375	can only wait for C<64> things at the same time internally; Microsoft
		5376	recommends spawning a chain of threads and wait for 63 handles and the
		5377	previous thread in each. Sounds great!).
		5378
		5379	Newer versions support more handles, but you need to define C<FD_SETSIZE>
		5380	to some high number (e.g. C<2048>) before compiling the winsocket select
		5381	call (which might be in libev or elsewhere, for example, perl and many
		5382	other interpreters do their own select emulation on windows).
		5383
		5384	Another limit is the number of file descriptors in the Microsoft runtime
		5385	libraries, which by default is C<64> (there must be a hidden I<64>
		5386	fetish or something like this inside Microsoft). You can increase this
		5387	by calling C<_setmaxstdio>, which can increase this limit to C<2048>
		5388	(another arbitrary limit), but is broken in many versions of the Microsoft
		5389	runtime libraries. This might get you to about C<512> or C<2048> sockets
		5390	(depending on windows version and/or the phase of the moon). To get more,
		5391	you need to wrap all I/O functions and provide your own fd management, but
		5392	the cost of calling select (O(n²)) will likely make this unworkable.
		5393
		5394	=head2 PORTABILITY REQUIREMENTS
		5395
		5396	In addition to a working ISO-C implementation and of course the
		5397	backend-specific APIs, libev relies on a few additional extensions:
		5398
		5399	=over 4
		5400
		5401	=item C<void (*)(ev_watcher_type *, int revents)> must have compatible
		5402	calling conventions regardless of C<ev_watcher_type *>.
		5403
		5404	Libev assumes not only that all watcher pointers have the same internal
		5405	structure (guaranteed by POSIX but not by ISO C for example), but it also
		5406	assumes that the same (machine) code can be used to call any watcher
		5407	callback: The watcher callbacks have different type signatures, but libev
		5408	calls them using an C<ev_watcher *> internally.
		5409
		5410	=item null pointers and integer zero are represented by 0 bytes
		5411
		5412	Libev uses C<memset> to initialise structs and arrays to C<0> bytes, and
		5413	relies on this setting pointers and integers to null.
		5414
		5415	=item pointer accesses must be thread-atomic
		5416
		5417	Accessing a pointer value must be atomic, it must both be readable and
		5418	writable in one piece - this is the case on all current architectures.
		5419
		5420	=item C<sig_atomic_t volatile> must be thread-atomic as well
		5421
		5422	The type C<sig_atomic_t volatile> (or whatever is defined as
		5423	C<EV_ATOMIC_T>) must be atomic with respect to accesses from different
		5424	threads. This is not part of the specification for C<sig_atomic_t>, but is
		5425	believed to be sufficiently portable.
		5426
		5427	=item C<sigprocmask> must work in a threaded environment
		5428
		5429	Libev uses C<sigprocmask> to temporarily block signals. This is not
		5430	allowed in a threaded program (C<pthread_sigmask> has to be used). Typical
		5431	pthread implementations will either allow C<sigprocmask> in the "main
		5432	thread" or will block signals process-wide, both behaviours would
		5433	be compatible with libev. Interaction between C<sigprocmask> and
		5434	C<pthread_sigmask> could complicate things, however.
		5435
		5436	The most portable way to handle signals is to block signals in all threads
		5437	except the initial one, and run the signal handling loop in the initial
		5438	thread as well.
		5439
		5440	=item C<long> must be large enough for common memory allocation sizes
		5441
		5442	To improve portability and simplify its API, libev uses C<long> internally
		5443	instead of C<size_t> when allocating its data structures. On non-POSIX
		5444	systems (Microsoft...) this might be unexpectedly low, but is still at
		5445	least 31 bits everywhere, which is enough for hundreds of millions of
		5446	watchers.
		5447
		5448	=item C<double> must hold a time value in seconds with enough accuracy
		5449
		5450	The type C<double> is used to represent timestamps. It is required to
		5451	have at least 51 bits of mantissa (and 9 bits of exponent), which is
		5452	good enough for at least into the year 4000 with millisecond accuracy
		5453	(the design goal for libev). This requirement is overfulfilled by
		5454	implementations using IEEE 754, which is basically all existing ones.
		5455
		5456	With IEEE 754 doubles, you get microsecond accuracy until at least the
		5457	year 2255 (and millisecond accuracy till the year 287396 - by then, libev
		5458	is either obsolete or somebody patched it to use C<long double> or
		5459	something like that, just kidding).
		5460
		5461	=back
		5462
		5463	If you know of other additional requirements drop me a note.
		5464
		5465
		5466	=head1 ALGORITHMIC COMPLEXITIES
		5467
		5468	In this section the complexities of (many of) the algorithms used inside
		5469	libev will be documented. For complexity discussions about backends see
		5470	the documentation for C<ev_default_init>.
		5471
		5472	All of the following are about amortised time: If an array needs to be
		5473	extended, libev needs to realloc and move the whole array, but this
		5474	happens asymptotically rarer with higher number of elements, so O(1) might
		5475	mean that libev does a lengthy realloc operation in rare cases, but on
		5476	average it is much faster and asymptotically approaches constant time.
		5477
		5478	=over 4
		5479
		5480	=item Starting and stopping timer/periodic watchers: O(log skipped_other_timers)
		5481
		5482	This means that, when you have a watcher that triggers in one hour and
		5483	there are 100 watchers that would trigger before that, then inserting will
		5484	have to skip roughly seven (C<ld 100>) of these watchers.
		5485
		5486	=item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)
		5487
		5488	That means that changing a timer costs less than removing/adding them,
		5489	as only the relative motion in the event queue has to be paid for.
		5490
		5491	=item Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)
		5492
		5493	These just add the watcher into an array or at the head of a list.
		5494
		5495	=item Stopping check/prepare/idle/fork/async watchers: O(1)
		5496
		5497	=item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
		5498
		5499	These watchers are stored in lists, so they need to be walked to find the
		5500	correct watcher to remove. The lists are usually short (you don't usually
		5501	have many watchers waiting for the same fd or signal: one is typical, two
		5502	is rare).
		5503
		5504	=item Finding the next timer in each loop iteration: O(1)
		5505
		5506	By virtue of using a binary or 4-heap, the next timer is always found at a
		5507	fixed position in the storage array.
		5508
		5509	=item Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)
		5510
		5511	A change means an I/O watcher gets started or stopped, which requires
		5512	libev to recalculate its status (and possibly tell the kernel, depending
		5513	on backend and whether C<ev_io_set> was used).
		5514
		5515	=item Activating one watcher (putting it into the pending state): O(1)
		5516
		5517	=item Priority handling: O(number_of_priorities)
		5518
		5519	Priorities are implemented by allocating some space for each
		5520	priority. When doing priority-based operations, libev usually has to
		5521	linearly search all the priorities, but starting/stopping and activating
		5522	watchers becomes O(1) with respect to priority handling.
		5523
		5524	=item Sending an ev_async: O(1)
		5525
		5526	=item Processing ev_async_send: O(number_of_async_watchers)
		5527
		5528	=item Processing signals: O(max_signal_number)
		5529
		5530	Sending involves a system call I<iff> there were no other C<ev_async_send>
		5531	calls in the current loop iteration and the loop is currently
		5532	blocked. Checking for async and signal events involves iterating over all
		5533	running async watchers or all signal numbers.
		5534
		5535	=back
		5536
		5537
		5538	=head1 PORTING FROM LIBEV 3.X TO 4.X
		5539
		5540	The major version 4 introduced some incompatible changes to the API.
		5541
		5542	At the moment, the C<ev.h> header file provides compatibility definitions
		5543	for all changes, so most programs should still compile. The compatibility
		5544	layer might be removed in later versions of libev, so better update to the
		5545	new API early than late.
		5546
		5547	=over 4
		5548
		5549	=item C<EV_COMPAT3> backwards compatibility mechanism
		5550
		5551	The backward compatibility mechanism can be controlled by
		5552	C<EV_COMPAT3>. See L</"PREPROCESSOR SYMBOLS/MACROS"> in the L</EMBEDDING>
		5553	section.
		5554
		5555	=item C<ev_default_destroy> and C<ev_default_fork> have been removed
		5556
		5557	These calls can be replaced easily by their C<ev_loop_xxx> counterparts:
		5558
		5559	ev_loop_destroy (EV_DEFAULT_UC);
		5560	ev_loop_fork (EV_DEFAULT);
		5561
		5562	=item function/symbol renames
		5563
		5564	A number of functions and symbols have been renamed:
		5565
		5566	ev_loop => ev_run
		5567	EVLOOP_NONBLOCK => EVRUN_NOWAIT
		5568	EVLOOP_ONESHOT => EVRUN_ONCE
		5569
		5570	ev_unloop => ev_break
		5571	EVUNLOOP_CANCEL => EVBREAK_CANCEL
		5572	EVUNLOOP_ONE => EVBREAK_ONE
		5573	EVUNLOOP_ALL => EVBREAK_ALL
		5574
		5575	EV_TIMEOUT => EV_TIMER
		5576
		5577	ev_loop_count => ev_iteration
		5578	ev_loop_depth => ev_depth
		5579	ev_loop_verify => ev_verify
		5580
		5581	Most functions working on C<struct ev_loop> objects don't have an
		5582	C<ev_loop_> prefix, so it was removed; C<ev_loop>, C<ev_unloop> and
		5583	associated constants have been renamed to not collide with the C<struct
		5584	ev_loop> anymore and C<EV_TIMER> now follows the same naming scheme
		5585	as all other watcher types. Note that C<ev_loop_fork> is still called
		5586	C<ev_loop_fork> because it would otherwise clash with the C<ev_fork>
		5587	typedef.
		5588
		5589	=item C<EV_MINIMAL> mechanism replaced by C<EV_FEATURES>
		5590
		5591	The preprocessor symbol C<EV_MINIMAL> has been replaced by a different
		5592	mechanism, C<EV_FEATURES>. Programs using C<EV_MINIMAL> usually compile
		5593	and work, but the library code will of course be larger.
		5594
		5595	=back
		5596
		5597
		5598	=head1 GLOSSARY
		5599
		5600	=over 4
		5601
		5602	=item active
		5603
		5604	A watcher is active as long as it has been started and not yet stopped.
		5605	See L</WATCHER STATES> for details.
		5606
		5607	=item application
		5608
		5609	In this document, an application is whatever is using libev.
		5610
		5611	=item backend
		5612
		5613	The part of the code dealing with the operating system interfaces.
		5614
		5615	=item callback
		5616
		5617	The address of a function that is called when some event has been
		5618	detected. Callbacks are being passed the event loop, the watcher that
		5619	received the event, and the actual event bitset.
		5620
		5621	=item callback/watcher invocation
		5622
		5623	The act of calling the callback associated with a watcher.
		5624
		5625	=item event
		5626
		5627	A change of state of some external event, such as data now being available
		5628	for reading on a file descriptor, time having passed or simply not having
		5629	any other events happening anymore.
		5630
		5631	In libev, events are represented as single bits (such as C<EV_READ> or
		5632	C<EV_TIMER>).
		5633
		5634	=item event library
		5635
		5636	A software package implementing an event model and loop.
		5637
		5638	=item event loop
		5639
		5640	An entity that handles and processes external events and converts them
		5641	into callback invocations.
		5642
		5643	=item event model
		5644
		5645	The model used to describe how an event loop handles and processes
		5646	watchers and events.
		5647
		5648	=item pending
		5649
		5650	A watcher is pending as soon as the corresponding event has been
		5651	detected. See L</WATCHER STATES> for details.
		5652
		5653	=item real time
		5654
		5655	The physical time that is observed. It is apparently strictly monotonic :)
		5656
		5657	=item wall-clock time
		5658
		5659	The time and date as shown on clocks. Unlike real time, it can actually
		5660	be wrong and jump forwards and backwards, e.g. when you adjust your
		5661	clock.
		5662
		5663	=item watcher
		5664
		5665	A data structure that describes interest in certain events. Watchers need
		5666	to be started (attached to an event loop) before they can receive events.
		5667
		5668	=back
		5669
742	=head1 AUTHOR	5670	=head1 AUTHOR
743		5671
744	Marc Lehmann <libev@schmorp.de>.	5672	Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael
		5673	Magnusson and Emanuele Giaquinta, and minor corrections by many others.
745		5674

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