1libev::ev(3)          User Contributed Perl Documentation         libev::ev(3)
2
3
4

NAME

6       libev - a high performance full-featured event loop written in C
7

SYNOPSIS

9          #include <ev.h>
10
11   EXAMPLE PROGRAM
12          // a single header file is required
13          #include <ev.h>
14
15          #include <stdio.h> // for puts
16
17          // every watcher type has its own typedef'd struct
18          // with the name ev_TYPE
19          ev_io stdin_watcher;
20          ev_timer timeout_watcher;
21
22          // all watcher callbacks have a similar signature
23          // this callback is called when data is readable on stdin
24          static void
25          stdin_cb (EV_P_ ev_io *w, int revents)
26          {
27            puts ("stdin ready");
28            // for one-shot events, one must manually stop the watcher
29            // with its corresponding stop function.
30            ev_io_stop (EV_A_ w);
31
32            // this causes all nested ev_run's to stop iterating
33            ev_break (EV_A_ EVBREAK_ALL);
34          }
35
36          // another callback, this time for a time-out
37          static void
38          timeout_cb (EV_P_ ev_timer *w, int revents)
39          {
40            puts ("timeout");
41            // this causes the innermost ev_run to stop iterating
42            ev_break (EV_A_ EVBREAK_ONE);
43          }
44
45          int
46          main (void)
47          {
48            // use the default event loop unless you have special needs
49            struct ev_loop *loop = EV_DEFAULT;
50
51            // initialise an io watcher, then start it
52            // this one will watch for stdin to become readable
53            ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
54            ev_io_start (loop, &stdin_watcher);
55
56            // initialise a timer watcher, then start it
57            // simple non-repeating 5.5 second timeout
58            ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
59            ev_timer_start (loop, &timeout_watcher);
60
61            // now wait for events to arrive
62            ev_run (loop, 0);
63
64            // break was called, so exit
65            return 0;
66          }
67

ABOUT THIS DOCUMENT

69       This document documents the libev software package.
70
71       The newest version of this document is also available as an html-
72       formatted web page you might find easier to navigate when reading it
73       for the first time:
74       <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
75
76       While this document tries to be as complete as possible in documenting
77       libev, its usage and the rationale behind its design, it is not a
78       tutorial on event-based programming, nor will it introduce event-based
79       programming with libev.
80
81       Familiarity with event based programming techniques in general is
82       assumed throughout this document.
83

WHAT TO READ WHEN IN A HURRY

85       This manual tries to be very detailed, but unfortunately, this also
86       makes it very long. If you just want to know the basics of libev, I
87       suggest reading "ANATOMY OF A WATCHER", then the "EXAMPLE PROGRAM"
88       above and look up the missing functions in "GLOBAL FUNCTIONS" and the
89       "ev_io" and "ev_timer" sections in "WATCHER TYPES".
90

ABOUT LIBEV

92       Libev is an event loop: you register interest in certain events (such
93       as a file descriptor being readable or a timeout occurring), and it
94       will manage these event sources and provide your program with events.
95
96       To do this, it must take more or less complete control over your
97       process (or thread) by executing the event loop handler, and will then
98       communicate events via a callback mechanism.
99
100       You register interest in certain events by registering so-called event
101       watchers, which are relatively small C structures you initialise with
102       the details of the event, and then hand it over to libev by starting
103       the watcher.
104
105   FEATURES
106       Libev supports "select", "poll", the Linux-specific aio and "epoll"
107       interfaces, the BSD-specific "kqueue" and the Solaris-specific event
108       port mechanisms for file descriptor events ("ev_io"), the Linux
109       "inotify" interface (for "ev_stat"), Linux eventfd/signalfd (for faster
110       and cleaner inter-thread wakeup ("ev_async")/signal handling
111       ("ev_signal")) relative timers ("ev_timer"), absolute timers with
112       customised rescheduling ("ev_periodic"), synchronous signals
113       ("ev_signal"), process status change events ("ev_child"), and event
114       watchers dealing with the event loop mechanism itself ("ev_idle",
115       "ev_embed", "ev_prepare" and "ev_check" watchers) as well as file
116       watchers ("ev_stat") and even limited support for fork events
117       ("ev_fork").
118
119       It also is quite fast (see this benchmark
120       <http://libev.schmorp.de/bench.html> comparing it to libevent for
121       example).
122
123   CONVENTIONS
124       Libev is very configurable. In this manual the default (and most
125       common) configuration will be described, which supports multiple event
126       loops. For more info about various configuration options please have a
127       look at EMBED section in this manual. If libev was configured without
128       support for multiple event loops, then all functions taking an initial
129       argument of name "loop" (which is always of type "struct ev_loop *")
130       will not have this argument.
131
132   TIME REPRESENTATION
133       Libev represents time as a single floating point number, representing
134       the (fractional) number of seconds since the (POSIX) epoch (in practice
135       somewhere near the beginning of 1970, details are complicated, don't
136       ask). This type is called "ev_tstamp", which is what you should use
137       too. It usually aliases to the "double" type in C. When you need to do
138       any calculations on it, you should treat it as some floating point
139       value.
140
141       Unlike the name component "stamp" might indicate, it is also used for
142       time differences (e.g. delays) throughout libev.
143

ERROR HANDLING

145       Libev knows three classes of errors: operating system errors, usage
146       errors and internal errors (bugs).
147
148       When libev catches an operating system error it cannot handle (for
149       example a system call indicating a condition libev cannot fix), it
150       calls the callback set via "ev_set_syserr_cb", which is supposed to fix
151       the problem or abort. The default is to print a diagnostic message and
152       to call "abort ()".
153
154       When libev detects a usage error such as a negative timer interval,
155       then it will print a diagnostic message and abort (via the "assert"
156       mechanism, so "NDEBUG" will disable this checking): these are
157       programming errors in the libev caller and need to be fixed there.
158
159       Via the "EV_FREQUENT" macro you can compile in and/or enable extensive
160       consistency checking code inside libev that can be used to check for
161       internal inconsistencies, suually caused by application bugs.
162
163       Libev also has a few internal error-checking "assert"ions. These do not
164       trigger under normal circumstances, as they indicate either a bug in
165       libev or worse.
166

GLOBAL FUNCTIONS

168       These functions can be called anytime, even before initialising the
169       library in any way.
170
171       ev_tstamp ev_time ()
172           Returns the current time as libev would use it. Please note that
173           the "ev_now" function is usually faster and also often returns the
174           timestamp you actually want to know. Also interesting is the
175           combination of "ev_now_update" and "ev_now".
176
177       ev_sleep (ev_tstamp interval)
178           Sleep for the given interval: The current thread will be blocked
179           until either it is interrupted or the given time interval has
180           passed (approximately - it might return a bit earlier even if not
181           interrupted). Returns immediately if "interval <= 0".
182
183           Basically this is a sub-second-resolution "sleep ()".
184
185           The range of the "interval" is limited - libev only guarantees to
186           work with sleep times of up to one day ("interval <= 86400").
187
188       int ev_version_major ()
189       int ev_version_minor ()
190           You can find out the major and minor ABI version numbers of the
191           library you linked against by calling the functions
192           "ev_version_major" and "ev_version_minor". If you want, you can
193           compare against the global symbols "EV_VERSION_MAJOR" and
194           "EV_VERSION_MINOR", which specify the version of the library your
195           program was compiled against.
196
197           These version numbers refer to the ABI version of the library, not
198           the release version.
199
200           Usually, it's a good idea to terminate if the major versions
201           mismatch, as this indicates an incompatible change. Minor versions
202           are usually compatible to older versions, so a larger minor version
203           alone is usually not a problem.
204
205           Example: Make sure we haven't accidentally been linked against the
206           wrong version (note, however, that this will not detect other ABI
207           mismatches, such as LFS or reentrancy).
208
209              assert (("libev version mismatch",
210                       ev_version_major () == EV_VERSION_MAJOR
211                       && ev_version_minor () >= EV_VERSION_MINOR));
212
213       unsigned int ev_supported_backends ()
214           Return the set of all backends (i.e. their corresponding
215           "EV_BACKEND_*" value) compiled into this binary of libev
216           (independent of their availability on the system you are running
217           on). See "ev_default_loop" for a description of the set values.
218
219           Example: make sure we have the epoll method, because yeah this is
220           cool and a must have and can we have a torrent of it please!!!11
221
222              assert (("sorry, no epoll, no sex",
223                       ev_supported_backends () & EVBACKEND_EPOLL));
224
225       unsigned int ev_recommended_backends ()
226           Return the set of all backends compiled into this binary of libev
227           and also recommended for this platform, meaning it will work for
228           most file descriptor types. This set is often smaller than the one
229           returned by "ev_supported_backends", as for example kqueue is
230           broken on most BSDs and will not be auto-detected unless you
231           explicitly request it (assuming you know what you are doing). This
232           is the set of backends that libev will probe for if you specify no
233           backends explicitly.
234
235       unsigned int ev_embeddable_backends ()
236           Returns the set of backends that are embeddable in other event
237           loops. This value is platform-specific but can include backends not
238           available on the current system. To find which embeddable backends
239           might be supported on the current system, you would need to look at
240           "ev_embeddable_backends () & ev_supported_backends ()", likewise
241           for recommended ones.
242
243           See the description of "ev_embed" watchers for more info.
244
245       ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())
246           Sets the allocation function to use (the prototype is similar - the
247           semantics are identical to the "realloc" C89/SuS/POSIX function).
248           It is used to allocate and free memory (no surprises here). If it
249           returns zero when memory needs to be allocated ("size != 0"), the
250           library might abort or take some potentially destructive action.
251
252           Since some systems (at least OpenBSD and Darwin) fail to implement
253           correct "realloc" semantics, libev will use a wrapper around the
254           system "realloc" and "free" functions by default.
255
256           You could override this function in high-availability programs to,
257           say, free some memory if it cannot allocate memory, to use a
258           special allocator, or even to sleep a while and retry until some
259           memory is available.
260
261           Example: The following is the "realloc" function that libev itself
262           uses which should work with "realloc" and "free" functions of all
263           kinds and is probably a good basis for your own implementation.
264
265              static void *
266              ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
267              {
268                if (size)
269                  return realloc (ptr, size);
270
271                free (ptr);
272                return 0;
273              }
274
275           Example: Replace the libev allocator with one that waits a bit and
276           then retries.
277
278              static void *
279              persistent_realloc (void *ptr, size_t size)
280              {
281                if (!size)
282                  {
283                    free (ptr);
284                    return 0;
285                  }
286
287                for (;;)
288                  {
289                    void *newptr = realloc (ptr, size);
290
291                    if (newptr)
292                      return newptr;
293
294                    sleep (60);
295                  }
296              }
297
298              ...
299              ev_set_allocator (persistent_realloc);
300
301       ev_set_syserr_cb (void (*cb)(const char *msg) throw ())
302           Set the callback function to call on a retryable system call error
303           (such as failed select, poll, epoll_wait). The message is a
304           printable string indicating the system call or subsystem causing
305           the problem. If this callback is set, then libev will expect it to
306           remedy the situation, no matter what, when it returns. That is,
307           libev will generally retry the requested operation, or, if the
308           condition doesn't go away, do bad stuff (such as abort).
309
310           Example: This is basically the same thing that libev does
311           internally, too.
312
313              static void
314              fatal_error (const char *msg)
315              {
316                perror (msg);
317                abort ();
318              }
319
320              ...
321              ev_set_syserr_cb (fatal_error);
322
323       ev_feed_signal (int signum)
324           This function can be used to "simulate" a signal receive. It is
325           completely safe to call this function at any time, from any
326           context, including signal handlers or random threads.
327
328           Its main use is to customise signal handling in your process,
329           especially in the presence of threads. For example, you could block
330           signals by default in all threads (and specifying
331           "EVFLAG_NOSIGMASK" when creating any loops), and in one thread, use
332           "sigwait" or any other mechanism to wait for signals, then
333           "deliver" them to libev by calling "ev_feed_signal".
334

FUNCTIONS CONTROLLING EVENT LOOPS

336       An event loop is described by a "struct ev_loop *" (the "struct" is not
337       optional in this case unless libev 3 compatibility is disabled, as
338       libev 3 had an "ev_loop" function colliding with the struct name).
339
340       The library knows two types of such loops, the default loop, which
341       supports child process events, and dynamically created event loops
342       which do not.
343
344       struct ev_loop *ev_default_loop (unsigned int flags)
345           This returns the "default" event loop object, which is what you
346           should normally use when you just need "the event loop". Event loop
347           objects and the "flags" parameter are described in more detail in
348           the entry for "ev_loop_new".
349
350           If the default loop is already initialised then this function
351           simply returns it (and ignores the flags. If that is troubling you,
352           check "ev_backend ()" afterwards). Otherwise it will create it with
353           the given flags, which should almost always be 0, unless the caller
354           is also the one calling "ev_run" or otherwise qualifies as "the
355           main program".
356
357           If you don't know what event loop to use, use the one returned from
358           this function (or via the "EV_DEFAULT" macro).
359
360           Note that this function is not thread-safe, so if you want to use
361           it from multiple threads, you have to employ some kind of mutex
362           (note also that this case is unlikely, as loops cannot be shared
363           easily between threads anyway).
364
365           The default loop is the only loop that can handle "ev_child"
366           watchers, and to do this, it always registers a handler for
367           "SIGCHLD". If this is a problem for your application you can either
368           create a dynamic loop with "ev_loop_new" which doesn't do that, or
369           you can simply overwrite the "SIGCHLD" signal handler after calling
370           "ev_default_init".
371
372           Example: This is the most typical usage.
373
374              if (!ev_default_loop (0))
375                fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
376
377           Example: Restrict libev to the select and poll backends, and do not
378           allow environment settings to be taken into account:
379
380              ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
381
382       struct ev_loop *ev_loop_new (unsigned int flags)
383           This will create and initialise a new event loop object. If the
384           loop could not be initialised, returns false.
385
386           This function is thread-safe, and one common way to use libev with
387           threads is indeed to create one loop per thread, and using the
388           default loop in the "main" or "initial" thread.
389
390           The flags argument can be used to specify special behaviour or
391           specific backends to use, and is usually specified as 0 (or
392           "EVFLAG_AUTO").
393
394           The following flags are supported:
395
396           "EVFLAG_AUTO"
397               The default flags value. Use this if you have no clue (it's the
398               right thing, believe me).
399
400           "EVFLAG_NOENV"
401               If this flag bit is or'ed into the flag value (or the program
402               runs setuid or setgid) then libev will not look at the
403               environment variable "LIBEV_FLAGS". Otherwise (the default),
404               this environment variable will override the flags completely if
405               it is found in the environment. This is useful to try out
406               specific backends to test their performance, to work around
407               bugs, or to make libev threadsafe (accessing environment
408               variables cannot be done in a threadsafe way, but usually it
409               works if no other thread modifies them).
410
411           "EVFLAG_FORKCHECK"
412               Instead of calling "ev_loop_fork" manually after a fork, you
413               can also make libev check for a fork in each iteration by
414               enabling this flag.
415
416               This works by calling "getpid ()" on every iteration of the
417               loop, and thus this might slow down your event loop if you do a
418               lot of loop iterations and little real work, but is usually not
419               noticeable (on my GNU/Linux system for example, "getpid" is
420               actually a simple 5-insn sequence without a system call and
421               thus very fast, but my GNU/Linux system also has
422               "pthread_atfork" which is even faster). (Update: glibc versions
423               2.25 apparently removed the "getpid" optimisation again).
424
425               The big advantage of this flag is that you can forget about
426               fork (and forget about forgetting to tell libev about forking,
427               although you still have to ignore "SIGPIPE") when you use this
428               flag.
429
430               This flag setting cannot be overridden or specified in the
431               "LIBEV_FLAGS" environment variable.
432
433           "EVFLAG_NOINOTIFY"
434               When this flag is specified, then libev will not attempt to use
435               the inotify API for its "ev_stat" watchers. Apart from
436               debugging and testing, this flag can be useful to conserve
437               inotify file descriptors, as otherwise each loop using
438               "ev_stat" watchers consumes one inotify handle.
439
440           "EVFLAG_SIGNALFD"
441               When this flag is specified, then libev will attempt to use the
442               signalfd API for its "ev_signal" (and "ev_child") watchers.
443               This API delivers signals synchronously, which makes it both
444               faster and might make it possible to get the queued signal
445               data. It can also simplify signal handling with threads, as
446               long as you properly block signals in your threads that are not
447               interested in handling them.
448
449               Signalfd will not be used by default as this changes your
450               signal mask, and there are a lot of shoddy libraries and
451               programs (glib's threadpool for example) that can't properly
452               initialise their signal masks.
453
454           "EVFLAG_NOSIGMASK"
455               When this flag is specified, then libev will avoid to modify
456               the signal mask. Specifically, this means you have to make sure
457               signals are unblocked when you want to receive them.
458
459               This behaviour is useful when you want to do your own signal
460               handling, or want to handle signals only in specific threads
461               and want to avoid libev unblocking the signals.
462
463               It's also required by POSIX in a threaded program, as libev
464               calls "sigprocmask", whose behaviour is officially unspecified.
465
466           "EVFLAG_NOTIMERFD"
467               When this flag is specified, the libev will avoid using a
468               "timerfd" to detect time jumps. It will still be able to detect
469               time jumps, but takes longer and has a lower accuracy in doing
470               so, but saves a file descriptor per loop.
471
472               The current implementation only tries to use a "timerfd" when
473               the first "ev_periodic" watcher is started and falls back on
474               other methods if it cannot be created, but this behaviour might
475               change in the future.
476
477           "EVBACKEND_SELECT"  (value 1, portable select backend)
478               This is your standard select(2) backend. Not completely
479               standard, as libev tries to roll its own fd_set with no limits
480               on the number of fds, but if that fails, expect a fairly low
481               limit on the number of fds when using this backend. It doesn't
482               scale too well (O(highest_fd)), but its usually the fastest
483               backend for a low number of (low-numbered :) fds.
484
485               To get good performance out of this backend you need a high
486               amount of parallelism (most of the file descriptors should be
487               busy). If you are writing a server, you should "accept ()" in a
488               loop to accept as many connections as possible during one
489               iteration. You might also want to have a look at
490               "ev_set_io_collect_interval ()" to increase the amount of
491               readiness notifications you get per iteration.
492
493               This backend maps "EV_READ" to the "readfds" set and "EV_WRITE"
494               to the "writefds" set (and to work around Microsoft Windows
495               bugs, also onto the "exceptfds" set on that platform).
496
497           "EVBACKEND_POLL"    (value 2, poll backend, available everywhere
498           except on windows)
499               And this is your standard poll(2) backend. It's more
500               complicated than select, but handles sparse fds better and has
501               no artificial limit on the number of fds you can use (except it
502               will slow down considerably with a lot of inactive fds). It
503               scales similarly to select, i.e. O(total_fds). See the entry
504               for "EVBACKEND_SELECT", above, for performance tips.
505
506               This backend maps "EV_READ" to "POLLIN | POLLERR | POLLHUP",
507               and "EV_WRITE" to "POLLOUT | POLLERR | POLLHUP".
508
509           "EVBACKEND_EPOLL"   (value 4, Linux)
510               Use the Linux-specific epoll(7) interface (for both pre- and
511               post-2.6.9 kernels).
512
513               For few fds, this backend is a bit little slower than poll and
514               select, but it scales phenomenally better. While poll and
515               select usually scale like O(total_fds) where total_fds is the
516               total number of fds (or the highest fd), epoll scales either
517               O(1) or O(active_fds).
518
519               The epoll mechanism deserves honorable mention as the most
520               misdesigned of the more advanced event mechanisms: mere
521               annoyances include silently dropping file descriptors,
522               requiring a system call per change per file descriptor (and
523               unnecessary guessing of parameters), problems with dup,
524               returning before the timeout value, resulting in additional
525               iterations (and only giving 5ms accuracy while select on the
526               same platform gives 0.1ms) and so on. The biggest issue is fork
527               races, however - if a program forks then both parent and child
528               process have to recreate the epoll set, which can take
529               considerable time (one syscall per file descriptor) and is of
530               course hard to detect.
531
532               Epoll is also notoriously buggy - embedding epoll fds should
533               work, but of course doesn't, and epoll just loves to report
534               events for totally different file descriptors (even already
535               closed ones, so one cannot even remove them from the set) than
536               registered in the set (especially on SMP systems). Libev tries
537               to counter these spurious notifications by employing an
538               additional generation counter and comparing that against the
539               events to filter out spurious ones, recreating the set when
540               required. Epoll also erroneously rounds down timeouts, but
541               gives you no way to know when and by how much, so sometimes you
542               have to busy-wait because epoll returns immediately despite a
543               nonzero timeout. And last not least, it also refuses to work
544               with some file descriptors which work perfectly fine with
545               "select" (files, many character devices...).
546
547               Epoll is truly the train wreck among event poll mechanisms, a
548               frankenpoll, cobbled together in a hurry, no thought to design
549               or interaction with others. Oh, the pain, will it ever stop...
550
551               While stopping, setting and starting an I/O watcher in the same
552               iteration will result in some caching, there is still a system
553               call per such incident (because the same file descriptor could
554               point to a different file description now), so its best to
555               avoid that. Also, "dup ()"'ed file descriptors might not work
556               very well if you register events for both file descriptors.
557
558               Best performance from this backend is achieved by not
559               unregistering all watchers for a file descriptor until it has
560               been closed, if possible, i.e. keep at least one watcher active
561               per fd at all times. Stopping and starting a watcher (without
562               re-setting it) also usually doesn't cause extra overhead. A
563               fork can both result in spurious notifications as well as in
564               libev having to destroy and recreate the epoll object, which
565               can take considerable time and thus should be avoided.
566
567               All this means that, in practice, "EVBACKEND_SELECT" can be as
568               fast or faster than epoll for maybe up to a hundred file
569               descriptors, depending on the usage. So sad.
570
571               While nominally embeddable in other event loops, this feature
572               is broken in a lot of kernel revisions, but probably(!) works
573               in current versions.
574
575               This backend maps "EV_READ" and "EV_WRITE" in the same way as
576               "EVBACKEND_POLL".
577
578           "EVBACKEND_LINUXAIO"   (value 64, Linux)
579               Use the Linux-specific Linux AIO (not aio(7) but io_submit(2))
580               event interface available in post-4.18 kernels (but libev only
581               tries to use it in 4.19+).
582
583               This is another Linux train wreck of an event interface.
584
585               If this backend works for you (as of this writing, it was very
586               experimental), it is the best event interface available on
587               Linux and might be well worth enabling it - if it isn't
588               available in your kernel this will be detected and this backend
589               will be skipped.
590
591               This backend can batch oneshot requests and supports a user-
592               space ring buffer to receive events. It also doesn't suffer
593               from most of the design problems of epoll (such as not being
594               able to remove event sources from the epoll set), and generally
595               sounds too good to be true. Because, this being the Linux
596               kernel, of course it suffers from a whole new set of
597               limitations, forcing you to fall back to epoll, inheriting all
598               its design issues.
599
600               For one, it is not easily embeddable (but probably could be
601               done using an event fd at some extra overhead). It also is
602               subject to a system wide limit that can be configured in
603               /proc/sys/fs/aio-max-nr. If no AIO requests are left, this
604               backend will be skipped during initialisation, and will switch
605               to epoll when the loop is active.
606
607               Most problematic in practice, however, is that not all file
608               descriptors work with it. For example, in Linux 5.1, TCP
609               sockets, pipes, event fds, files, /dev/null and many others are
610               supported, but ttys do not work properly (a known bug that the
611               kernel developers don't care about, see
612               <https://lore.kernel.org/patchwork/patch/1047453/>), so this is
613               not (yet?) a generic event polling interface.
614
615               Overall, it seems the Linux developers just don't want it to
616               have a generic event handling mechanism other than "select" or
617               "poll".
618
619               To work around all these problem, the current version of libev
620               uses its epoll backend as a fallback for file descriptor types
621               that do not work. Or falls back completely to epoll if the
622               kernel acts up.
623
624               This backend maps "EV_READ" and "EV_WRITE" in the same way as
625               "EVBACKEND_POLL".
626
627           "EVBACKEND_KQUEUE"  (value 8, most BSD clones)
628               Kqueue deserves special mention, as at the time this backend
629               was implemented, it was broken on all BSDs except NetBSD
630               (usually it doesn't work reliably with anything but sockets and
631               pipes, except on Darwin, where of course it's completely
632               useless). Unlike epoll, however, whose brokenness is by design,
633               these kqueue bugs can be (and mostly have been) fixed without
634               API changes to existing programs. For this reason it's not
635               being "auto-detected" on all platforms unless you explicitly
636               specify it in the flags (i.e. using "EVBACKEND_KQUEUE") or
637               libev was compiled on a known-to-be-good (-enough) system like
638               NetBSD.
639
640               You still can embed kqueue into a normal poll or select backend
641               and use it only for sockets (after having made sure that
642               sockets work with kqueue on the target platform). See
643               "ev_embed" watchers for more info.
644
645               It scales in the same way as the epoll backend, but the
646               interface to the kernel is more efficient (which says nothing
647               about its actual speed, of course). While stopping, setting and
648               starting an I/O watcher does never cause an extra system call
649               as with "EVBACKEND_EPOLL", it still adds up to two event
650               changes per incident. Support for "fork ()" is very bad (you
651               might have to leak fds on fork, but it's more sane than epoll)
652               and it drops fds silently in similarly hard-to-detect cases.
653
654               This backend usually performs well under most conditions.
655
656               While nominally embeddable in other event loops, this doesn't
657               work everywhere, so you might need to test for this. And since
658               it is broken almost everywhere, you should only use it when you
659               have a lot of sockets (for which it usually works), by
660               embedding it into another event loop (e.g. "EVBACKEND_SELECT"
661               or "EVBACKEND_POLL" (but "poll" is of course also broken on OS
662               X)) and, did I mention it, using it only for sockets.
663
664               This backend maps "EV_READ" into an "EVFILT_READ" kevent with
665               "NOTE_EOF", and "EV_WRITE" into an "EVFILT_WRITE" kevent with
666               "NOTE_EOF".
667
668           "EVBACKEND_DEVPOLL" (value 16, Solaris 8)
669               This is not implemented yet (and might never be, unless you
670               send me an implementation). According to reports, "/dev/poll"
671               only supports sockets and is not embeddable, which would limit
672               the usefulness of this backend immensely.
673
674           "EVBACKEND_PORT"    (value 32, Solaris 10)
675               This uses the Solaris 10 event port mechanism. As with
676               everything on Solaris, it's really slow, but it still scales
677               very well (O(active_fds)).
678
679               While this backend scales well, it requires one system call per
680               active file descriptor per loop iteration. For small and medium
681               numbers of file descriptors a "slow" "EVBACKEND_SELECT" or
682               "EVBACKEND_POLL" backend might perform better.
683
684               On the positive side, this backend actually performed fully to
685               specification in all tests and is fully embeddable, which is a
686               rare feat among the OS-specific backends (I vastly prefer
687               correctness over speed hacks).
688
689               On the negative side, the interface is bizarre - so bizarre
690               that even sun itself gets it wrong in their code examples: The
691               event polling function sometimes returns events to the caller
692               even though an error occurred, but with no indication whether
693               it has done so or not (yes, it's even documented that way) -
694               deadly for edge-triggered interfaces where you absolutely have
695               to know whether an event occurred or not because you have to
696               re-arm the watcher.
697
698               Fortunately libev seems to be able to work around these
699               idiocies.
700
701               This backend maps "EV_READ" and "EV_WRITE" in the same way as
702               "EVBACKEND_POLL".
703
704           "EVBACKEND_ALL"
705               Try all backends (even potentially broken ones that wouldn't be
706               tried with "EVFLAG_AUTO"). Since this is a mask, you can do
707               stuff such as "EVBACKEND_ALL & ~EVBACKEND_KQUEUE".
708
709               It is definitely not recommended to use this flag, use whatever
710               "ev_recommended_backends ()" returns, or simply do not specify
711               a backend at all.
712
713           "EVBACKEND_MASK"
714               Not a backend at all, but a mask to select all backend bits
715               from a "flags" value, in case you want to mask out any backends
716               from a flags value (e.g. when modifying the "LIBEV_FLAGS"
717               environment variable).
718
719           If one or more of the backend flags are or'ed into the flags value,
720           then only these backends will be tried (in the reverse order as
721           listed here). If none are specified, all backends in
722           "ev_recommended_backends ()" will be tried.
723
724           Example: Try to create a event loop that uses epoll and nothing
725           else.
726
727              struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
728              if (!epoller)
729                fatal ("no epoll found here, maybe it hides under your chair");
730
731           Example: Use whatever libev has to offer, but make sure that kqueue
732           is used if available.
733
734              struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
735
736           Example: Similarly, on linux, you mgiht want to take advantage of
737           the linux aio backend if possible, but fall back to something else
738           if that isn't available.
739
740              struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
741
742       ev_loop_destroy (loop)
743           Destroys an event loop object (frees all memory and kernel state
744           etc.). None of the active event watchers will be stopped in the
745           normal sense, so e.g. "ev_is_active" might still return true. It is
746           your responsibility to either stop all watchers cleanly yourself
747           before calling this function, or cope with the fact afterwards
748           (which is usually the easiest thing, you can just ignore the
749           watchers and/or "free ()" them for example).
750
751           Note that certain global state, such as signal state (and installed
752           signal handlers), will not be freed by this function, and related
753           watchers (such as signal and child watchers) would need to be
754           stopped manually.
755
756           This function is normally used on loop objects allocated by
757           "ev_loop_new", but it can also be used on the default loop returned
758           by "ev_default_loop", in which case it is not thread-safe.
759
760           Note that it is not advisable to call this function on the default
761           loop except in the rare occasion where you really need to free its
762           resources.  If you need dynamically allocated loops it is better to
763           use "ev_loop_new" and "ev_loop_destroy".
764
765       ev_loop_fork (loop)
766           This function sets a flag that causes subsequent "ev_run"
767           iterations to reinitialise the kernel state for backends that have
768           one. Despite the name, you can call it anytime you are allowed to
769           start or stop watchers (except inside an "ev_prepare" callback),
770           but it makes most sense after forking, in the child process. You
771           must call it (or use "EVFLAG_FORKCHECK") in the child before
772           resuming or calling "ev_run".
773
774           In addition, if you want to reuse a loop (via this function or
775           "EVFLAG_FORKCHECK"), you also have to ignore "SIGPIPE".
776
777           Again, you have to call it on any loop that you want to re-use
778           after a fork, even if you do not plan to use the loop in the
779           parent. This is because some kernel interfaces *cough* kqueue
780           *cough* do funny things during fork.
781
782           On the other hand, you only need to call this function in the child
783           process if and only if you want to use the event loop in the child.
784           If you just fork+exec or create a new loop in the child, you don't
785           have to call it at all (in fact, "epoll" is so badly broken that it
786           makes a difference, but libev will usually detect this case on its
787           own and do a costly reset of the backend).
788
789           The function itself is quite fast and it's usually not a problem to
790           call it just in case after a fork.
791
792           Example: Automate calling "ev_loop_fork" on the default loop when
793           using pthreads.
794
795              static void
796              post_fork_child (void)
797              {
798                ev_loop_fork (EV_DEFAULT);
799              }
800
801              ...
802              pthread_atfork (0, 0, post_fork_child);
803
804       int ev_is_default_loop (loop)
805           Returns true when the given loop is, in fact, the default loop, and
806           false otherwise.
807
808       unsigned int ev_iteration (loop)
809           Returns the current iteration count for the event loop, which is
810           identical to the number of times libev did poll for new events. It
811           starts at 0 and happily wraps around with enough iterations.
812
813           This value can sometimes be useful as a generation counter of sorts
814           (it "ticks" the number of loop iterations), as it roughly
815           corresponds with "ev_prepare" and "ev_check" calls - and is
816           incremented between the prepare and check phases.
817
818       unsigned int ev_depth (loop)
819           Returns the number of times "ev_run" was entered minus the number
820           of times "ev_run" was exited normally, in other words, the
821           recursion depth.
822
823           Outside "ev_run", this number is zero. In a callback, this number
824           is 1, unless "ev_run" was invoked recursively (or from another
825           thread), in which case it is higher.
826
827           Leaving "ev_run" abnormally (setjmp/longjmp, cancelling the thread,
828           throwing an exception etc.), doesn't count as "exit" - consider
829           this as a hint to avoid such ungentleman-like behaviour unless it's
830           really convenient, in which case it is fully supported.
831
832       unsigned int ev_backend (loop)
833           Returns one of the "EVBACKEND_*" flags indicating the event backend
834           in use.
835
836       ev_tstamp ev_now (loop)
837           Returns the current "event loop time", which is the time the event
838           loop received events and started processing them. This timestamp
839           does not change as long as callbacks are being processed, and this
840           is also the base time used for relative timers. You can treat it as
841           the timestamp of the event occurring (or more correctly, libev
842           finding out about it).
843
844       ev_now_update (loop)
845           Establishes the current time by querying the kernel, updating the
846           time returned by "ev_now ()" in the progress. This is a costly
847           operation and is usually done automatically within "ev_run ()".
848
849           This function is rarely useful, but when some event callback runs
850           for a very long time without entering the event loop, updating
851           libev's idea of the current time is a good idea.
852
853           See also "The special problem of time updates" in the "ev_timer"
854           section.
855
856       ev_suspend (loop)
857       ev_resume (loop)
858           These two functions suspend and resume an event loop, for use when
859           the loop is not used for a while and timeouts should not be
860           processed.
861
862           A typical use case would be an interactive program such as a game:
863           When the user presses "^Z" to suspend the game and resumes it an
864           hour later it would be best to handle timeouts as if no time had
865           actually passed while the program was suspended. This can be
866           achieved by calling "ev_suspend" in your "SIGTSTP" handler, sending
867           yourself a "SIGSTOP" and calling "ev_resume" directly afterwards to
868           resume timer processing.
869
870           Effectively, all "ev_timer" watchers will be delayed by the time
871           spend between "ev_suspend" and "ev_resume", and all "ev_periodic"
872           watchers will be rescheduled (that is, they will lose any events
873           that would have occurred while suspended).
874
875           After calling "ev_suspend" you must not call any function on the
876           given loop other than "ev_resume", and you must not call
877           "ev_resume" without a previous call to "ev_suspend".
878
879           Calling "ev_suspend"/"ev_resume" has the side effect of updating
880           the event loop time (see "ev_now_update").
881
882       bool ev_run (loop, int flags)
883           Finally, this is it, the event handler. This function usually is
884           called after you have initialised all your watchers and you want to
885           start handling events. It will ask the operating system for any new
886           events, call the watcher callbacks, and then repeat the whole
887           process indefinitely: This is why event loops are called loops.
888
889           If the flags argument is specified as 0, it will keep handling
890           events until either no event watchers are active anymore or
891           "ev_break" was called.
892
893           The return value is false if there are no more active watchers
894           (which usually means "all jobs done" or "deadlock"), and true in
895           all other cases (which usually means " you should call "ev_run"
896           again").
897
898           Please note that an explicit "ev_break" is usually better than
899           relying on all watchers to be stopped when deciding when a program
900           has finished (especially in interactive programs), but having a
901           program that automatically loops as long as it has to and no longer
902           by virtue of relying on its watchers stopping correctly, that is
903           truly a thing of beauty.
904
905           This function is mostly exception-safe - you can break out of a
906           "ev_run" call by calling "longjmp" in a callback, throwing a C++
907           exception and so on. This does not decrement the "ev_depth" value,
908           nor will it clear any outstanding "EVBREAK_ONE" breaks.
909
910           A flags value of "EVRUN_NOWAIT" will look for new events, will
911           handle those events and any already outstanding ones, but will not
912           wait and block your process in case there are no events and will
913           return after one iteration of the loop. This is sometimes useful to
914           poll and handle new events while doing lengthy calculations, to
915           keep the program responsive.
916
917           A flags value of "EVRUN_ONCE" will look for new events (waiting if
918           necessary) and will handle those and any already outstanding ones.
919           It will block your process until at least one new event arrives
920           (which could be an event internal to libev itself, so there is no
921           guarantee that a user-registered callback will be called), and will
922           return after one iteration of the loop.
923
924           This is useful if you are waiting for some external event in
925           conjunction with something not expressible using other libev
926           watchers (i.e. "roll your own "ev_run""). However, a pair of
927           "ev_prepare"/"ev_check" watchers is usually a better approach for
928           this kind of thing.
929
930           Here are the gory details of what "ev_run" does (this is for your
931           understanding, not a guarantee that things will work exactly like
932           this in future versions):
933
934              - Increment loop depth.
935              - Reset the ev_break status.
936              - Before the first iteration, call any pending watchers.
937              LOOP:
938              - If EVFLAG_FORKCHECK was used, check for a fork.
939              - If a fork was detected (by any means), queue and call all fork watchers.
940              - Queue and call all prepare watchers.
941              - If ev_break was called, goto FINISH.
942              - If we have been forked, detach and recreate the kernel state
943                as to not disturb the other process.
944              - Update the kernel state with all outstanding changes.
945              - Update the "event loop time" (ev_now ()).
946              - Calculate for how long to sleep or block, if at all
947                (active idle watchers, EVRUN_NOWAIT or not having
948                any active watchers at all will result in not sleeping).
949              - Sleep if the I/O and timer collect interval say so.
950              - Increment loop iteration counter.
951              - Block the process, waiting for any events.
952              - Queue all outstanding I/O (fd) events.
953              - Update the "event loop time" (ev_now ()), and do time jump adjustments.
954              - Queue all expired timers.
955              - Queue all expired periodics.
956              - Queue all idle watchers with priority higher than that of pending events.
957              - Queue all check watchers.
958              - Call all queued watchers in reverse order (i.e. check watchers first).
959                Signals and child watchers are implemented as I/O watchers, and will
960                be handled here by queueing them when their watcher gets executed.
961              - If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
962                were used, or there are no active watchers, goto FINISH, otherwise
963                continue with step LOOP.
964              FINISH:
965              - Reset the ev_break status iff it was EVBREAK_ONE.
966              - Decrement the loop depth.
967              - Return.
968
969           Example: Queue some jobs and then loop until no events are
970           outstanding anymore.
971
972              ... queue jobs here, make sure they register event watchers as long
973              ... as they still have work to do (even an idle watcher will do..)
974              ev_run (my_loop, 0);
975              ... jobs done or somebody called break. yeah!
976
977       ev_break (loop, how)
978           Can be used to make a call to "ev_run" return early (but only after
979           it has processed all outstanding events). The "how" argument must
980           be either "EVBREAK_ONE", which will make the innermost "ev_run"
981           call return, or "EVBREAK_ALL", which will make all nested "ev_run"
982           calls return.
983
984           This "break state" will be cleared on the next call to "ev_run".
985
986           It is safe to call "ev_break" from outside any "ev_run" calls, too,
987           in which case it will have no effect.
988
989       ev_ref (loop)
990       ev_unref (loop)
991           Ref/unref can be used to add or remove a reference count on the
992           event loop: Every watcher keeps one reference, and as long as the
993           reference count is nonzero, "ev_run" will not return on its own.
994
995           This is useful when you have a watcher that you never intend to
996           unregister, but that nevertheless should not keep "ev_run" from
997           returning. In such a case, call "ev_unref" after starting, and
998           "ev_ref" before stopping it.
999
1000           As an example, libev itself uses this for its internal signal pipe:
1001           It is not visible to the libev user and should not keep "ev_run"
1002           from exiting if no event watchers registered by it are active. It
1003           is also an excellent way to do this for generic recurring timers or
1004           from within third-party libraries. Just remember to unref after
1005           start and ref before stop (but only if the watcher wasn't active
1006           before, or was active before, respectively. Note also that libev
1007           might stop watchers itself (e.g. non-repeating timers) in which
1008           case you have to "ev_ref" in the callback).
1009
1010           Example: Create a signal watcher, but keep it from keeping "ev_run"
1011           running when nothing else is active.
1012
1013              ev_signal exitsig;
1014              ev_signal_init (&exitsig, sig_cb, SIGINT);
1015              ev_signal_start (loop, &exitsig);
1016              ev_unref (loop);
1017
1018           Example: For some weird reason, unregister the above signal handler
1019           again.
1020
1021              ev_ref (loop);
1022              ev_signal_stop (loop, &exitsig);
1023
1024       ev_set_io_collect_interval (loop, ev_tstamp interval)
1025       ev_set_timeout_collect_interval (loop, ev_tstamp interval)
1026           These advanced functions influence the time that libev will spend
1027           waiting for events. Both time intervals are by default 0, meaning
1028           that libev will try to invoke timer/periodic callbacks and I/O
1029           callbacks with minimum latency.
1030
1031           Setting these to a higher value (the "interval" must be >= 0)
1032           allows libev to delay invocation of I/O and timer/periodic
1033           callbacks to increase efficiency of loop iterations (or to increase
1034           power-saving opportunities).
1035
1036           The idea is that sometimes your program runs just fast enough to
1037           handle one (or very few) event(s) per loop iteration. While this
1038           makes the program responsive, it also wastes a lot of CPU time to
1039           poll for new events, especially with backends like "select ()"
1040           which have a high overhead for the actual polling but can deliver
1041           many events at once.
1042
1043           By setting a higher io collect interval you allow libev to spend
1044           more time collecting I/O events, so you can handle more events per
1045           iteration, at the cost of increasing latency. Timeouts (both
1046           "ev_periodic" and "ev_timer") will not be affected. Setting this to
1047           a non-null value will introduce an additional "ev_sleep ()" call
1048           into most loop iterations. The sleep time ensures that libev will
1049           not poll for I/O events more often then once per this interval, on
1050           average (as long as the host time resolution is good enough).
1051
1052           Likewise, by setting a higher timeout collect interval you allow
1053           libev to spend more time collecting timeouts, at the expense of
1054           increased latency/jitter/inexactness (the watcher callback will be
1055           called later). "ev_io" watchers will not be affected. Setting this
1056           to a non-null value will not introduce any overhead in libev.
1057
1058           Many (busy) programs can usually benefit by setting the I/O collect
1059           interval to a value near 0.1 or so, which is often enough for
1060           interactive servers (of course not for games), likewise for
1061           timeouts. It usually doesn't make much sense to set it to a lower
1062           value than 0.01, as this approaches the timing granularity of most
1063           systems. Note that if you do transactions with the outside world
1064           and you can't increase the parallelity, then this setting will
1065           limit your transaction rate (if you need to poll once per
1066           transaction and the I/O collect interval is 0.01, then you can't do
1067           more than 100 transactions per second).
1068
1069           Setting the timeout collect interval can improve the opportunity
1070           for saving power, as the program will "bundle" timer callback
1071           invocations that are "near" in time together, by delaying some,
1072           thus reducing the number of times the process sleeps and wakes up
1073           again. Another useful technique to reduce iterations/wake-ups is to
1074           use "ev_periodic" watchers and make sure they fire on, say, one-
1075           second boundaries only.
1076
1077           Example: we only need 0.1s timeout granularity, and we wish not to
1078           poll more often than 100 times per second:
1079
1080              ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
1081              ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
1082
1083       ev_invoke_pending (loop)
1084           This call will simply invoke all pending watchers while resetting
1085           their pending state. Normally, "ev_run" does this automatically
1086           when required, but when overriding the invoke callback this call
1087           comes handy. This function can be invoked from a watcher - this can
1088           be useful for example when you want to do some lengthy calculation
1089           and want to pass further event handling to another thread (you
1090           still have to make sure only one thread executes within
1091           "ev_invoke_pending" or "ev_run" of course).
1092
1093       int ev_pending_count (loop)
1094           Returns the number of pending watchers - zero indicates that no
1095           watchers are pending.
1096
1097       ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))
1098           This overrides the invoke pending functionality of the loop:
1099           Instead of invoking all pending watchers when there are any,
1100           "ev_run" will call this callback instead. This is useful, for
1101           example, when you want to invoke the actual watchers inside another
1102           context (another thread etc.).
1103
1104           If you want to reset the callback, use "ev_invoke_pending" as new
1105           callback.
1106
1107       ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void
1108       (*acquire)(EV_P) throw ())
1109           Sometimes you want to share the same loop between multiple threads.
1110           This can be done relatively simply by putting mutex_lock/unlock
1111           calls around each call to a libev function.
1112
1113           However, "ev_run" can run an indefinite time, so it is not feasible
1114           to wait for it to return. One way around this is to wake up the
1115           event loop via "ev_break" and "ev_async_send", another way is to
1116           set these release and acquire callbacks on the loop.
1117
1118           When set, then "release" will be called just before the thread is
1119           suspended waiting for new events, and "acquire" is called just
1120           afterwards.
1121
1122           Ideally, "release" will just call your mutex_unlock function, and
1123           "acquire" will just call the mutex_lock function again.
1124
1125           While event loop modifications are allowed between invocations of
1126           "release" and "acquire" (that's their only purpose after all), no
1127           modifications done will affect the event loop, i.e. adding watchers
1128           will have no effect on the set of file descriptors being watched,
1129           or the time waited. Use an "ev_async" watcher to wake up "ev_run"
1130           when you want it to take note of any changes you made.
1131
1132           In theory, threads executing "ev_run" will be async-cancel safe
1133           between invocations of "release" and "acquire".
1134
1135           See also the locking example in the "THREADS" section later in this
1136           document.
1137
1138       ev_set_userdata (loop, void *data)
1139       void *ev_userdata (loop)
1140           Set and retrieve a single "void *" associated with a loop. When
1141           "ev_set_userdata" has never been called, then "ev_userdata" returns
1142           0.
1143
1144           These two functions can be used to associate arbitrary data with a
1145           loop, and are intended solely for the "invoke_pending_cb",
1146           "release" and "acquire" callbacks described above, but of course
1147           can be (ab-)used for any other purpose as well.
1148
1149       ev_verify (loop)
1150           This function only does something when "EV_VERIFY" support has been
1151           compiled in, which is the default for non-minimal builds. It tries
1152           to go through all internal structures and checks them for validity.
1153           If anything is found to be inconsistent, it will print an error
1154           message to standard error and call "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
1158           keeps its data structures consistent.
1159

ANATOMY OF A WATCHER

1161       In the following description, uppercase "TYPE" in names stands for the
1162       watcher type, e.g. "ev_TYPE_start" can mean "ev_timer_start" for timer
1163       watchers and "ev_io_start" for I/O watchers.
1164
1165       A watcher is an opaque structure that you allocate and register to
1166       record your interest in some event. To make a concrete example, imagine
1167       you want to wait for STDIN to become readable, you would create an
1168       "ev_io" watcher for that:
1169
1170          static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
1171          {
1172            ev_io_stop (w);
1173            ev_break (loop, EVBREAK_ALL);
1174          }
1175
1176          struct ev_loop *loop = ev_default_loop (0);
1177
1178          ev_io stdin_watcher;
1179
1180          ev_init (&stdin_watcher, my_cb);
1181          ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
1182          ev_io_start (loop, &stdin_watcher);
1183
1184          ev_run (loop, 0);
1185
1186       As you can see, you are responsible for allocating the memory for your
1187       watcher structures (and it is usually a bad idea to do this on the
1188       stack).
1189
1190       Each watcher has an associated watcher structure (called "struct
1191       ev_TYPE" or simply "ev_TYPE", as typedefs are provided for all watcher
1192       structs).
1193
1194       Each watcher structure must be initialised by a call to "ev_init
1195       (watcher *, callback)", which expects a callback to be provided. This
1196       callback is invoked each time the event occurs (or, in the case of I/O
1197       watchers, each time the event loop detects that the file descriptor
1198       given is readable and/or writable).
1199
1200       Each watcher type further has its own "ev_TYPE_set (watcher *, ...)"
1201       macro to configure it, with arguments specific to the watcher type.
1202       There is also a macro to combine initialisation and setting in one
1203       call: "ev_TYPE_init (watcher *, callback, ...)".
1204
1205       To make the watcher actually watch out for events, you have to start it
1206       with a watcher-specific start function ("ev_TYPE_start (loop, watcher
1207       *)"), and you can stop watching for events at any time by calling the
1208       corresponding stop function ("ev_TYPE_stop (loop, watcher *)".
1209
1210       As long as your watcher is active (has been started but not stopped)
1211       you must not touch the values stored in it. Most specifically you must
1212       never reinitialise it or call its "ev_TYPE_set" macro.
1213
1214       Each and every callback receives the event loop pointer as first, the
1215       registered watcher structure as second, and a bitset of received events
1216       as third argument.
1217
1218       The received events usually include a single bit per event type
1219       received (you can receive multiple events at the same time). The
1220       possible bit masks are:
1221
1222       "EV_READ"
1223       "EV_WRITE"
1224           The file descriptor in the "ev_io" watcher has become readable
1225           and/or writable.
1226
1227       "EV_TIMER"
1228           The "ev_timer" watcher has timed out.
1229
1230       "EV_PERIODIC"
1231           The "ev_periodic" watcher has timed out.
1232
1233       "EV_SIGNAL"
1234           The signal specified in the "ev_signal" watcher has been received
1235           by a thread.
1236
1237       "EV_CHILD"
1238           The pid specified in the "ev_child" watcher has received a status
1239           change.
1240
1241       "EV_STAT"
1242           The path specified in the "ev_stat" watcher changed its attributes
1243           somehow.
1244
1245       "EV_IDLE"
1246           The "ev_idle" watcher has determined that you have nothing better
1247           to do.
1248
1249       "EV_PREPARE"
1250       "EV_CHECK"
1251           All "ev_prepare" watchers are invoked just before "ev_run" starts
1252           to gather new events, and all "ev_check" watchers are queued (not
1253           invoked) just after "ev_run" has gathered them, but before it
1254           queues any callbacks for any received events. That means
1255           "ev_prepare" watchers are the last watchers invoked before the
1256           event loop sleeps or polls for new events, and "ev_check" watchers
1257           will be invoked before any other watchers of the same or lower
1258           priority within an event loop iteration.
1259
1260           Callbacks of both watcher types can start and stop as many watchers
1261           as they want, and all of them will be taken into account (for
1262           example, a "ev_prepare" watcher might start an idle watcher to keep
1263           "ev_run" from blocking).
1264
1265       "EV_EMBED"
1266           The embedded event loop specified in the "ev_embed" watcher needs
1267           attention.
1268
1269       "EV_FORK"
1270           The event loop has been resumed in the child process after fork
1271           (see "ev_fork").
1272
1273       "EV_CLEANUP"
1274           The event loop is about to be destroyed (see "ev_cleanup").
1275
1276       "EV_ASYNC"
1277           The given async watcher has been asynchronously notified (see
1278           "ev_async").
1279
1280       "EV_CUSTOM"
1281           Not ever sent (or otherwise used) by libev itself, but can be
1282           freely used by libev users to signal watchers (e.g. via
1283           "ev_feed_event").
1284
1285       "EV_ERROR"
1286           An unspecified error has occurred, the watcher has been stopped.
1287           This might happen because the watcher could not be properly started
1288           because libev ran out of memory, a file descriptor was found to be
1289           closed or any other problem. Libev considers these application
1290           bugs.
1291
1292           You best act on it by reporting the problem and somehow coping with
1293           the watcher being stopped. Note that well-written programs should
1294           not receive an error ever, so when your watcher receives it, this
1295           usually indicates a bug in your program.
1296
1297           Libev will usually signal a few "dummy" events together with an
1298           error, for example it might indicate that a fd is readable or
1299           writable, and if your callbacks is well-written it can just attempt
1300           the operation and cope with the error from read() or write(). This
1301           will not work in multi-threaded programs, though, as the fd could
1302           already be closed and reused for another thing, so beware.
1303
1304   GENERIC WATCHER FUNCTIONS
1305       "ev_init" (ev_TYPE *watcher, callback)
1306           This macro initialises the generic portion of a watcher. The
1307           contents of the watcher object can be arbitrary (so "malloc" will
1308           do). Only the generic parts of the watcher are initialised, you
1309           need to call the type-specific "ev_TYPE_set" macro afterwards to
1310           initialise the type-specific parts. For each type there is also a
1311           "ev_TYPE_init" macro which rolls both calls into one.
1312
1313           You can reinitialise a watcher at any time as long as it has been
1314           stopped (or never started) and there are no pending events
1315           outstanding.
1316
1317           The callback is always of type "void (*)(struct ev_loop *loop,
1318           ev_TYPE *watcher, int revents)".
1319
1320           Example: Initialise an "ev_io" watcher in two steps.
1321
1322              ev_io w;
1323              ev_init (&w, my_cb);
1324              ev_io_set (&w, STDIN_FILENO, EV_READ);
1325
1326       "ev_TYPE_set" (ev_TYPE *watcher, [args])
1327           This macro initialises the type-specific parts of a watcher. You
1328           need to call "ev_init" at least once before you call this macro,
1329           but you can call "ev_TYPE_set" any number of times. You must not,
1330           however, call this macro on a watcher that is active (it can be
1331           pending, however, which is a difference to the "ev_init" macro).
1332
1333           Although some watcher types do not have type-specific arguments
1334           (e.g. "ev_prepare") you still need to call its "set" macro.
1335
1336           See "ev_init", above, for an example.
1337
1338       "ev_TYPE_init" (ev_TYPE *watcher, callback, [args])
1339           This convenience macro rolls both "ev_init" and "ev_TYPE_set" macro
1340           calls into a single call. This is the most convenient method to
1341           initialise a watcher. The same limitations apply, of course.
1342
1343           Example: Initialise and set an "ev_io" watcher in one step.
1344
1345              ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
1346
1347       "ev_TYPE_start" (loop, ev_TYPE *watcher)
1348           Starts (activates) the given watcher. Only active watchers will
1349           receive events. If the watcher is already active nothing will
1350           happen.
1351
1352           Example: Start the "ev_io" watcher that is being abused as example
1353           in this whole section.
1354
1355              ev_io_start (EV_DEFAULT_UC, &w);
1356
1357       "ev_TYPE_stop" (loop, ev_TYPE *watcher)
1358           Stops the given watcher if active, and clears the pending status
1359           (whether the watcher was active or not).
1360
1361           It is possible that stopped watchers are pending - for example,
1362           non-repeating timers are being stopped when they become pending -
1363           but calling "ev_TYPE_stop" ensures that the watcher is neither
1364           active nor pending. If you want to free or reuse the memory used by
1365           the watcher it is therefore a good idea to always call its
1366           "ev_TYPE_stop" function.
1367
1368       bool ev_is_active (ev_TYPE *watcher)
1369           Returns a true value iff the watcher is active (i.e. it has been
1370           started and not yet been stopped). As long as a watcher is active
1371           you must not modify it.
1372
1373       bool ev_is_pending (ev_TYPE *watcher)
1374           Returns a true value iff the watcher is pending, (i.e. it has
1375           outstanding events but its callback has not yet been invoked). As
1376           long as a watcher is pending (but not active) you must not call an
1377           init function on it (but "ev_TYPE_set" is safe), you must not
1378           change its priority, and you must make sure the watcher is
1379           available to libev (e.g. you cannot "free ()" it).
1380
1381       callback ev_cb (ev_TYPE *watcher)
1382           Returns the callback currently set on the watcher.
1383
1384       ev_set_cb (ev_TYPE *watcher, callback)
1385           Change the callback. You can change the callback at virtually any
1386           time (modulo threads).
1387
1388       ev_set_priority (ev_TYPE *watcher, int priority)
1389       int ev_priority (ev_TYPE *watcher)
1390           Set and query the priority of the watcher. The priority is a small
1391           integer between "EV_MAXPRI" (default: 2) and "EV_MINPRI" (default:
1392           "-2"). Pending watchers with higher priority will be invoked before
1393           watchers with lower priority, but priority will not keep watchers
1394           from being executed (except for "ev_idle" watchers).
1395
1396           If you need to suppress invocation when higher priority events are
1397           pending you need to look at "ev_idle" watchers, which provide this
1398           functionality.
1399
1400           You must not change the priority of a watcher as long as it is
1401           active or pending.
1402
1403           Setting a priority outside the range of "EV_MINPRI" to "EV_MAXPRI"
1404           is fine, as long as you do not mind that the priority value you
1405           query might or might not have been clamped to the valid range.
1406
1407           The default priority used by watchers when no priority has been set
1408           is always 0, which is supposed to not be too high and not be too
1409           low :).
1410
1411           See "WATCHER PRIORITY MODELS", below, for a more thorough treatment
1412           of priorities.
1413
1414       ev_invoke (loop, ev_TYPE *watcher, int revents)
1415           Invoke the "watcher" with the given "loop" and "revents". Neither
1416           "loop" nor "revents" need to be valid as long as the watcher
1417           callback can deal with that fact, as both are simply passed through
1418           to the callback.
1419
1420       int ev_clear_pending (loop, ev_TYPE *watcher)
1421           If the watcher is pending, this function clears its pending status
1422           and returns its "revents" bitset (as if its callback was invoked).
1423           If the watcher isn't pending it does nothing and returns 0.
1424
1425           Sometimes it can be useful to "poll" a watcher instead of waiting
1426           for its callback to be invoked, which can be accomplished with this
1427           function.
1428
1429       ev_feed_event (loop, ev_TYPE *watcher, int revents)
1430           Feeds the given event set into the event loop, as if the specified
1431           event had happened for the specified watcher (which must be a
1432           pointer to an initialised but not necessarily started event
1433           watcher). Obviously you must not free the watcher as long as it has
1434           pending events.
1435
1436           Stopping the watcher, letting libev invoke it, or calling
1437           "ev_clear_pending" will clear the pending event, even if the
1438           watcher was not started in the first place.
1439
1440           See also "ev_feed_fd_event" and "ev_feed_signal_event" for related
1441           functions that do not need a watcher.
1442
1443       See also the "ASSOCIATING CUSTOM DATA WITH A WATCHER" and "BUILDING
1444       YOUR OWN COMPOSITE WATCHERS" idioms.
1445
1446   WATCHER STATES
1447       There are various watcher states mentioned throughout this manual -
1448       active, pending and so on. In this section these states and the rules
1449       to transition between them will be described in more detail - and while
1450       these rules might look complicated, they usually do "the right thing".
1451
1452       initialised
1453           Before a watcher can be registered with the event loop it has to be
1454           initialised. This can be done with a call to "ev_TYPE_init", or
1455           calls to "ev_init" followed by the watcher-specific "ev_TYPE_set"
1456           function.
1457
1458           In this state it is simply some block of memory that is suitable
1459           for use in an event loop. It can be moved around, freed, reused
1460           etc. at will - as long as you either keep the memory contents
1461           intact, or call "ev_TYPE_init" again.
1462
1463       started/running/active
1464           Once a watcher has been started with a call to "ev_TYPE_start" it
1465           becomes property of the event loop, and is actively waiting for
1466           events. While in this state it cannot be accessed (except in a few
1467           documented ways), moved, freed or anything else - the only legal
1468           thing is to keep a pointer to it, and call libev functions on it
1469           that are documented to work on active watchers.
1470
1471       pending
1472           If a watcher is active and libev determines that an event it is
1473           interested in has occurred (such as a timer expiring), it will
1474           become pending. It will stay in this pending state until either it
1475           is stopped or its callback is about to be invoked, so it is not
1476           normally pending inside the watcher callback.
1477
1478           The watcher might or might not be active while it is pending (for
1479           example, an expired non-repeating timer can be pending but no
1480           longer active). If it is stopped, it can be freely accessed (e.g.
1481           by calling "ev_TYPE_set"), but it is still property of the event
1482           loop at this time, so cannot be moved, freed or reused. And if it
1483           is active the rules described in the previous item still apply.
1484
1485           It is also possible to feed an event on a watcher that is not
1486           active (e.g.  via "ev_feed_event"), in which case it becomes
1487           pending without being active.
1488
1489       stopped
1490           A watcher can be stopped implicitly by libev (in which case it
1491           might still be pending), or explicitly by calling its
1492           "ev_TYPE_stop" function. The latter will clear any pending state
1493           the watcher might be in, regardless of whether it was active or
1494           not, so stopping a watcher explicitly before freeing it is often a
1495           good idea.
1496
1497           While stopped (and not pending) the watcher is essentially in the
1498           initialised state, that is, it can be reused, moved, modified in
1499           any way you wish (but when you trash the memory block, you need to
1500           "ev_TYPE_init" it again).
1501
1502   WATCHER PRIORITY MODELS
1503       Many event loops support watcher priorities, which are usually small
1504       integers that influence the ordering of event callback invocation
1505       between watchers in some way, all else being equal.
1506
1507       In libev, watcher priorities can be set using "ev_set_priority". See
1508       its description for the more technical details such as the actual
1509       priority range.
1510
1511       There are two common ways how these these priorities are being
1512       interpreted by event loops:
1513
1514       In the more common lock-out model, higher priorities "lock out"
1515       invocation of lower priority watchers, which means as long as higher
1516       priority watchers receive events, lower priority watchers are not being
1517       invoked.
1518
1519       The less common only-for-ordering model uses priorities solely to order
1520       callback invocation within a single event loop iteration: Higher
1521       priority watchers are invoked before lower priority ones, but they all
1522       get invoked before polling for new events.
1523
1524       Libev uses the second (only-for-ordering) model for all its watchers
1525       except for idle watchers (which use the lock-out model).
1526
1527       The rationale behind this is that implementing the lock-out model for
1528       watchers is not well supported by most kernel interfaces, and most
1529       event libraries will just poll for the same events again and again as
1530       long as their callbacks have not been executed, which is very
1531       inefficient in the common case of one high-priority watcher locking out
1532       a mass of lower priority ones.
1533
1534       Static (ordering) priorities are most useful when you have two or more
1535       watchers handling the same resource: a typical usage example is having
1536       an "ev_io" watcher to receive data, and an associated "ev_timer" to
1537       handle timeouts. Under load, data might be received while the program
1538       handles other jobs, but since timers normally get invoked first, the
1539       timeout handler will be executed before checking for data. In that
1540       case, giving the timer a lower priority than the I/O watcher ensures
1541       that I/O will be handled first even under adverse conditions (which is
1542       usually, but not always, what you want).
1543
1544       Since idle watchers use the "lock-out" model, meaning that idle
1545       watchers will only be executed when no same or higher priority watchers
1546       have received events, they can be used to implement the "lock-out"
1547       model when required.
1548
1549       For example, to emulate how many other event libraries handle
1550       priorities, you can associate an "ev_idle" watcher to each such
1551       watcher, and in the normal watcher callback, you just start the idle
1552       watcher. The real processing is done in the idle watcher callback. This
1553       causes libev to continuously poll and process kernel event data for the
1554       watcher, but when the lock-out case is known to be rare (which in turn
1555       is rare :), this is workable.
1556
1557       Usually, however, the lock-out model implemented that way will perform
1558       miserably under the type of load it was designed to handle. In that
1559       case, it might be preferable to stop the real watcher before starting
1560       the idle watcher, so the kernel will not have to process the event in
1561       case the actual processing will be delayed for considerable time.
1562
1563       Here is an example of an I/O watcher that should run at a strictly
1564       lower priority than the default, and which should only process data
1565       when no other events are pending:
1566
1567          ev_idle idle; // actual processing watcher
1568          ev_io io;     // actual event watcher
1569
1570          static void
1571          io_cb (EV_P_ ev_io *w, int revents)
1572          {
1573            // stop the I/O watcher, we received the event, but
1574            // are not yet ready to handle it.
1575            ev_io_stop (EV_A_ w);
1576
1577            // start the idle watcher to handle the actual event.
1578            // it will not be executed as long as other watchers
1579            // with the default priority are receiving events.
1580            ev_idle_start (EV_A_ &idle);
1581          }
1582
1583          static void
1584          idle_cb (EV_P_ ev_idle *w, int revents)
1585          {
1586            // actual processing
1587            read (STDIN_FILENO, ...);
1588
1589            // have to start the I/O watcher again, as
1590            // we have handled the event
1591            ev_io_start (EV_P_ &io);
1592          }
1593
1594          // initialisation
1595          ev_idle_init (&idle, idle_cb);
1596          ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1597          ev_io_start (EV_DEFAULT_ &io);
1598
1599       In the "real" world, it might also be beneficial to start a timer, so
1600       that low-priority connections can not be locked out forever under load.
1601       This enables your program to keep a lower latency for important
1602       connections during short periods of high load, while not completely
1603       locking out less important ones.
1604

WATCHER TYPES

1606       This section describes each watcher in detail, but will not repeat
1607       information given in the last section. Any initialisation/set macros,
1608       functions and members specific to the watcher type are explained.
1609
1610       Members are additionally marked with either [read-only], meaning that,
1611       while the watcher is active, you can look at the member and expect some
1612       sensible content, but you must not modify it (you can modify it while
1613       the watcher is stopped to your hearts content), or [read-write], which
1614       means you can expect it to have some sensible content while the watcher
1615       is active, but you can also modify it. Modifying it may not do
1616       something sensible or take immediate effect (or do anything at all),
1617       but libev will not crash or malfunction in any way.
1618
1619   "ev_io" - is this file descriptor readable or writable?
1620       I/O watchers check whether a file descriptor is readable or writable in
1621       each iteration of the event loop, or, more precisely, when reading
1622       would not block the process and writing would at least be able to write
1623       some data. This behaviour is called level-triggering because you keep
1624       receiving events as long as the condition persists. Remember you can
1625       stop the watcher if you don't want to act on the event and neither want
1626       to receive future events.
1627
1628       In general you can register as many read and/or write event watchers
1629       per fd as you want (as long as you don't confuse yourself). Setting all
1630       file descriptors to non-blocking mode is also usually a good idea (but
1631       not required if you know what you are doing).
1632
1633       Another thing you have to watch out for is that it is quite easy to
1634       receive "spurious" readiness notifications, that is, your callback
1635       might be called with "EV_READ" but a subsequent "read"(2) will actually
1636       block because there is no data. It is very easy to get into this
1637       situation even with a relatively standard program structure. Thus it is
1638       best to always use non-blocking I/O: An extra "read"(2) returning
1639       "EAGAIN" is far preferable to a program hanging until some data
1640       arrives.
1641
1642       If you cannot run the fd in non-blocking mode (for example you should
1643       not play around with an Xlib connection), then you have to separately
1644       re-test whether a file descriptor is really ready with a known-to-be
1645       good interface such as poll (fortunately in the case of Xlib, it
1646       already does this on its own, so its quite safe to use). Some people
1647       additionally use "SIGALRM" and an interval timer, just to be sure you
1648       won't block indefinitely.
1649
1650       But really, best use non-blocking mode.
1651
1652       The special problem of disappearing file descriptors
1653
1654       Some backends (e.g. kqueue, epoll, linuxaio) need to be told about
1655       closing a file descriptor (either due to calling "close" explicitly or
1656       any other means, such as "dup2"). The reason is that you register
1657       interest in some file descriptor, but when it goes away, the operating
1658       system will silently drop this interest. If another file descriptor
1659       with the same number then is registered with libev, there is no
1660       efficient way to see that this is, in fact, a different file
1661       descriptor.
1662
1663       To avoid having to explicitly tell libev about such cases, libev
1664       follows the following policy:  Each time "ev_io_set" is being called,
1665       libev will assume that this is potentially a new file descriptor,
1666       otherwise it is assumed that the file descriptor stays the same. That
1667       means that you have to call "ev_io_set" (or "ev_io_init") when you
1668       change the descriptor even if the file descriptor number itself did not
1669       change.
1670
1671       This is how one would do it normally anyway, the important point is
1672       that the libev application should not optimise around libev but should
1673       leave optimisations to libev.
1674
1675       The special problem of dup'ed file descriptors
1676
1677       Some backends (e.g. epoll), cannot register events for file
1678       descriptors, but only events for the underlying file descriptions. That
1679       means when you have "dup ()"'ed file descriptors or weirder
1680       constellations, and register events for them, only one file descriptor
1681       might actually receive events.
1682
1683       There is no workaround possible except not registering events for
1684       potentially "dup ()"'ed file descriptors, or to resort to
1685       "EVBACKEND_SELECT" or "EVBACKEND_POLL".
1686
1687       The special problem of files
1688
1689       Many people try to use "select" (or libev) on file descriptors
1690       representing files, and expect it to become ready when their program
1691       doesn't block on disk accesses (which can take a long time on their
1692       own).
1693
1694       However, this cannot ever work in the "expected" way - you get a
1695       readiness notification as soon as the kernel knows whether and how much
1696       data is there, and in the case of open files, that's always the case,
1697       so you always get a readiness notification instantly, and your read (or
1698       possibly write) will still block on the disk I/O.
1699
1700       Another way to view it is that in the case of sockets, pipes, character
1701       devices and so on, there is another party (the sender) that delivers
1702       data on its own, but in the case of files, there is no such thing: the
1703       disk will not send data on its own, simply because it doesn't know what
1704       you wish to read - you would first have to request some data.
1705
1706       Since files are typically not-so-well supported by advanced
1707       notification mechanism, libev tries hard to emulate POSIX behaviour
1708       with respect to files, even though you should not use it. The reason
1709       for this is convenience: sometimes you want to watch STDIN or STDOUT,
1710       which is usually a tty, often a pipe, but also sometimes files or
1711       special devices (for example, "epoll" on Linux works with /dev/random
1712       but not with /dev/urandom), and even though the file might better be
1713       served with asynchronous I/O instead of with non-blocking I/O, it is
1714       still useful when it "just works" instead of freezing.
1715
1716       So avoid file descriptors pointing to files when you know it (e.g. use
1717       libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or
1718       when you rarely read from a file instead of from a socket, and want to
1719       reuse the same code path.
1720
1721       The special problem of fork
1722
1723       Some backends (epoll, kqueue, linuxaio, iouring) do not support "fork
1724       ()" at all or exhibit useless behaviour. Libev fully supports fork, but
1725       needs to be told about it in the child if you want to continue to use
1726       it in the child.
1727
1728       To support fork in your child processes, you have to call "ev_loop_fork
1729       ()" after a fork in the child, enable "EVFLAG_FORKCHECK", or resort to
1730       "EVBACKEND_SELECT" or "EVBACKEND_POLL".
1731
1732       The special problem of SIGPIPE
1733
1734       While not really specific to libev, it is easy to forget about
1735       "SIGPIPE": when writing to a pipe whose other end has been closed, your
1736       program gets sent a SIGPIPE, which, by default, aborts your program.
1737       For most programs this is sensible behaviour, for daemons, this is
1738       usually undesirable.
1739
1740       So when you encounter spurious, unexplained daemon exits, make sure you
1741       ignore SIGPIPE (and maybe make sure you log the exit status of your
1742       daemon somewhere, as that would have given you a big clue).
1743
1744       The special problem of accept()ing when you can't
1745
1746       Many implementations of the POSIX "accept" function (for example, found
1747       in post-2004 Linux) have the peculiar behaviour of not removing a
1748       connection from the pending queue in all error cases.
1749
1750       For example, larger servers often run out of file descriptors (because
1751       of resource limits), causing "accept" to fail with "ENFILE" but not
1752       rejecting the connection, leading to libev signalling readiness on the
1753       next iteration again (the connection still exists after all), and
1754       typically causing the program to loop at 100% CPU usage.
1755
1756       Unfortunately, the set of errors that cause this issue differs between
1757       operating systems, there is usually little the app can do to remedy the
1758       situation, and no known thread-safe method of removing the connection
1759       to cope with overload is known (to me).
1760
1761       One of the easiest ways to handle this situation is to just ignore it -
1762       when the program encounters an overload, it will just loop until the
1763       situation is over. While this is a form of busy waiting, no OS offers
1764       an event-based way to handle this situation, so it's the best one can
1765       do.
1766
1767       A better way to handle the situation is to log any errors other than
1768       "EAGAIN" and "EWOULDBLOCK", making sure not to flood the log with such
1769       messages, and continue as usual, which at least gives the user an idea
1770       of what could be wrong ("raise the ulimit!"). For extra points one
1771       could stop the "ev_io" watcher on the listening fd "for a while", which
1772       reduces CPU usage.
1773
1774       If your program is single-threaded, then you could also keep a dummy
1775       file descriptor for overload situations (e.g. by opening /dev/null),
1776       and when you run into "ENFILE" or "EMFILE", close it, run "accept",
1777       close that fd, and create a new dummy fd. This will gracefully refuse
1778       clients under typical overload conditions.
1779
1780       The last way to handle it is to simply log the error and "exit", as is
1781       often done with "malloc" failures, but this results in an easy
1782       opportunity for a DoS attack.
1783
1784       Watcher-Specific Functions
1785
1786       ev_io_init (ev_io *, callback, int fd, int events)
1787       ev_io_set (ev_io *, int fd, int events)
1788           Configures an "ev_io" watcher. The "fd" is the file descriptor to
1789           receive events for and "events" is either "EV_READ", "EV_WRITE" or
1790           "EV_READ | EV_WRITE", to express the desire to receive the given
1791           events.
1792
1793       int fd [read-only]
1794           The file descriptor being watched.
1795
1796       int events [read-only]
1797           The events being watched.
1798
1799       Examples
1800
1801       Example: Call "stdin_readable_cb" when STDIN_FILENO has become, well
1802       readable, but only once. Since it is likely line-buffered, you could
1803       attempt to read a whole line in the callback.
1804
1805          static void
1806          stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1807          {
1808             ev_io_stop (loop, w);
1809            .. read from stdin here (or from w->fd) and handle any I/O errors
1810          }
1811
1812          ...
1813          struct ev_loop *loop = ev_default_init (0);
1814          ev_io stdin_readable;
1815          ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
1816          ev_io_start (loop, &stdin_readable);
1817          ev_run (loop, 0);
1818
1819   "ev_timer" - relative and optionally repeating timeouts
1820       Timer watchers are simple relative timers that generate an event after
1821       a given time, and optionally repeating in regular intervals after that.
1822
1823       The timers are based on real time, that is, if you register an event
1824       that times out after an hour and you reset your system clock to January
1825       last year, it will still time out after (roughly) one hour. "Roughly"
1826       because detecting time jumps is hard, and some inaccuracies are
1827       unavoidable (the monotonic clock option helps a lot here).
1828
1829       The callback is guaranteed to be invoked only after its timeout has
1830       passed (not at, so on systems with very low-resolution clocks this
1831       might introduce a small delay, see "the special problem of being too
1832       early", below). If multiple timers become ready during the same loop
1833       iteration then the ones with earlier time-out values are invoked before
1834       ones of the same priority with later time-out values (but this is no
1835       longer true when a callback calls "ev_run" recursively).
1836
1837       Be smart about timeouts
1838
1839       Many real-world problems involve some kind of timeout, usually for
1840       error recovery. A typical example is an HTTP request - if the other
1841       side hangs, you want to raise some error after a while.
1842
1843       What follows are some ways to handle this problem, from obvious and
1844       inefficient to smart and efficient.
1845
1846       In the following, a 60 second activity timeout is assumed - a timeout
1847       that gets reset to 60 seconds each time there is activity (e.g. each
1848       time some data or other life sign was received).
1849
1850       1. Use a timer and stop, reinitialise and start it on activity.
1851           This is the most obvious, but not the most simple way: In the
1852           beginning, start the watcher:
1853
1854              ev_timer_init (timer, callback, 60., 0.);
1855              ev_timer_start (loop, timer);
1856
1857           Then, each time there is some activity, "ev_timer_stop" it,
1858           initialise it and start it again:
1859
1860              ev_timer_stop (loop, timer);
1861              ev_timer_set (timer, 60., 0.);
1862              ev_timer_start (loop, timer);
1863
1864           This is relatively simple to implement, but means that each time
1865           there is some activity, libev will first have to remove the timer
1866           from its internal data structure and then add it again. Libev tries
1867           to be fast, but it's still not a constant-time operation.
1868
1869       2. Use a timer and re-start it with "ev_timer_again" inactivity.
1870           This is the easiest way, and involves using "ev_timer_again"
1871           instead of "ev_timer_start".
1872
1873           To implement this, configure an "ev_timer" with a "repeat" value of
1874           60 and then call "ev_timer_again" at start and each time you
1875           successfully read or write some data. If you go into an idle state
1876           where you do not expect data to travel on the socket, you can
1877           "ev_timer_stop" the timer, and "ev_timer_again" will automatically
1878           restart it if need be.
1879
1880           That means you can ignore both the "ev_timer_start" function and
1881           the "after" argument to "ev_timer_set", and only ever use the
1882           "repeat" member and "ev_timer_again".
1883
1884           At start:
1885
1886              ev_init (timer, callback);
1887              timer->repeat = 60.;
1888              ev_timer_again (loop, timer);
1889
1890           Each time there is some activity:
1891
1892              ev_timer_again (loop, timer);
1893
1894           It is even possible to change the time-out on the fly, regardless
1895           of whether the watcher is active or not:
1896
1897              timer->repeat = 30.;
1898              ev_timer_again (loop, timer);
1899
1900           This is slightly more efficient then stopping/starting the timer
1901           each time you want to modify its timeout value, as libev does not
1902           have to completely remove and re-insert the timer from/into its
1903           internal data structure.
1904
1905           It is, however, even simpler than the "obvious" way to do it.
1906
1907       3. Let the timer time out, but then re-arm it as required.
1908           This method is more tricky, but usually most efficient: Most
1909           timeouts are relatively long compared to the intervals between
1910           other activity - in our example, within 60 seconds, there are
1911           usually many I/O events with associated activity resets.
1912
1913           In this case, it would be more efficient to leave the "ev_timer"
1914           alone, but remember the time of last activity, and check for a real
1915           timeout only within the callback:
1916
1917              ev_tstamp timeout = 60.;
1918              ev_tstamp last_activity; // time of last activity
1919              ev_timer timer;
1920
1921              static void
1922              callback (EV_P_ ev_timer *w, int revents)
1923              {
1924                // calculate when the timeout would happen
1925                ev_tstamp after = last_activity - ev_now (EV_A) + timeout;
1926
1927                // if negative, it means we the timeout already occurred
1928                if (after < 0.)
1929                  {
1930                    // timeout occurred, take action
1931                  }
1932                else
1933                  {
1934                    // callback was invoked, but there was some recent
1935                    // activity. simply restart the timer to time out
1936                    // after "after" seconds, which is the earliest time
1937                    // the timeout can occur.
1938                    ev_timer_set (w, after, 0.);
1939                    ev_timer_start (EV_A_ w);
1940                  }
1941              }
1942
1943           To summarise the callback: first calculate in how many seconds the
1944           timeout will occur (by calculating the absolute time when it would
1945           occur, "last_activity + timeout", and subtracting the current time,
1946           "ev_now (EV_A)" from that).
1947
1948           If this value is negative, then we are already past the timeout,
1949           i.e. we timed out, and need to do whatever is needed in this case.
1950
1951           Otherwise, we now the earliest time at which the timeout would
1952           trigger, and simply start the timer with this timeout value.
1953
1954           In other words, each time the callback is invoked it will check
1955           whether the timeout occurred. If not, it will simply reschedule
1956           itself to check again at the earliest time it could time out.
1957           Rinse. Repeat.
1958
1959           This scheme causes more callback invocations (about one every 60
1960           seconds minus half the average time between activity), but
1961           virtually no calls to libev to change the timeout.
1962
1963           To start the machinery, simply initialise the watcher and set
1964           "last_activity" to the current time (meaning there was some
1965           activity just now), then call the callback, which will "do the
1966           right thing" and start the timer:
1967
1968              last_activity = ev_now (EV_A);
1969              ev_init (&timer, callback);
1970              callback (EV_A_ &timer, 0);
1971
1972           When there is some activity, simply store the current time in
1973           "last_activity", no libev calls at all:
1974
1975              if (activity detected)
1976                last_activity = ev_now (EV_A);
1977
1978           When your timeout value changes, then the timeout can be changed by
1979           simply providing a new value, stopping the timer and calling the
1980           callback, which will again do the right thing (for example, time
1981           out immediately :).
1982
1983              timeout = new_value;
1984              ev_timer_stop (EV_A_ &timer);
1985              callback (EV_A_ &timer, 0);
1986
1987           This technique is slightly more complex, but in most cases where
1988           the time-out is unlikely to be triggered, much more efficient.
1989
1990       4. Wee, just use a double-linked list for your timeouts.
1991           If there is not one request, but many thousands (millions...), all
1992           employing some kind of timeout with the same timeout value, then
1993           one can do even better:
1994
1995           When starting the timeout, calculate the timeout value and put the
1996           timeout at the end of the list.
1997
1998           Then use an "ev_timer" to fire when the timeout at the beginning of
1999           the list is expected to fire (for example, using the technique #3).
2000
2001           When there is some activity, remove the timer from the list,
2002           recalculate the timeout, append it to the end of the list again,
2003           and make sure to update the "ev_timer" if it was taken from the
2004           beginning of the list.
2005
2006           This way, one can manage an unlimited number of timeouts in O(1)
2007           time for starting, stopping and updating the timers, at the expense
2008           of a major complication, and having to use a constant timeout. The
2009           constant timeout ensures that the list stays sorted.
2010
2011       So which method the best?
2012
2013       Method #2 is a simple no-brain-required solution that is adequate in
2014       most situations. Method #3 requires a bit more thinking, but handles
2015       many cases better, and isn't very complicated either. In most case,
2016       choosing either one is fine, with #3 being better in typical
2017       situations.
2018
2019       Method #1 is almost always a bad idea, and buys you nothing. Method #4
2020       is rather complicated, but extremely efficient, something that really
2021       pays off after the first million or so of active timers, i.e. it's
2022       usually overkill :)
2023
2024       The special problem of being too early
2025
2026       If you ask a timer to call your callback after three seconds, then you
2027       expect it to be invoked after three seconds - but of course, this
2028       cannot be guaranteed to infinite precision. Less obviously, it cannot
2029       be guaranteed to any precision by libev - imagine somebody suspending
2030       the process with a STOP signal for a few hours for example.
2031
2032       So, libev tries to invoke your callback as soon as possible after the
2033       delay has occurred, but cannot guarantee this.
2034
2035       A less obvious failure mode is calling your callback too early: many
2036       event loops compare timestamps with a "elapsed delay >= requested
2037       delay", but this can cause your callback to be invoked much earlier
2038       than you would expect.
2039
2040       To see why, imagine a system with a clock that only offers full second
2041       resolution (think windows if you can't come up with a broken enough OS
2042       yourself). If you schedule a one-second timer at the time 500.9, then
2043       the event loop will schedule your timeout to elapse at a system time of
2044       500 (500.9 truncated to the resolution) + 1, or 501.
2045
2046       If an event library looks at the timeout 0.1s later, it will see "501
2047       >= 501" and invoke the callback 0.1s after it was started, even though
2048       a one-second delay was requested - this is being "too early", despite
2049       best intentions.
2050
2051       This is the reason why libev will never invoke the callback if the
2052       elapsed delay equals the requested delay, but only when the elapsed
2053       delay is larger than the requested delay. In the example above, libev
2054       would only invoke the callback at system time 502, or 1.1s after the
2055       timer was started.
2056
2057       So, while libev cannot guarantee that your callback will be invoked
2058       exactly when requested, it can and does guarantee that the requested
2059       delay has actually elapsed, or in other words, it always errs on the
2060       "too late" side of things.
2061
2062       The special problem of time updates
2063
2064       Establishing the current time is a costly operation (it usually takes
2065       at least one system call): EV therefore updates its idea of the current
2066       time only before and after "ev_run" collects new events, which causes a
2067       growing difference between "ev_now ()" and "ev_time ()" when handling
2068       lots of events in one iteration.
2069
2070       The relative timeouts are calculated relative to the "ev_now ()" time.
2071       This is usually the right thing as this timestamp refers to the time of
2072       the event triggering whatever timeout you are modifying/starting. If
2073       you suspect event processing to be delayed and you need to base the
2074       timeout on the current time, use something like the following to adjust
2075       for it:
2076
2077          ev_timer_set (&timer, after + (ev_time () - ev_now ()), 0.);
2078
2079       If the event loop is suspended for a long time, you can also force an
2080       update of the time returned by "ev_now ()" by calling "ev_now_update
2081       ()", although that will push the event time of all outstanding events
2082       further into the future.
2083
2084       The special problem of unsynchronised clocks
2085
2086       Modern systems have a variety of clocks - libev itself uses the normal
2087       "wall clock" clock and, if available, the monotonic clock (to avoid
2088       time jumps).
2089
2090       Neither of these clocks is synchronised with each other or any other
2091       clock on the system, so "ev_time ()" might return a considerably
2092       different time than "gettimeofday ()" or "time ()". On a GNU/Linux
2093       system, for example, a call to "gettimeofday" might return a second
2094       count that is one higher than a directly following call to "time".
2095
2096       The moral of this is to only compare libev-related timestamps with
2097       "ev_time ()" and "ev_now ()", at least if you want better precision
2098       than a second or so.
2099
2100       One more problem arises due to this lack of synchronisation: if libev
2101       uses the system monotonic clock and you compare timestamps from
2102       "ev_time" or "ev_now" from when you started your timer and when your
2103       callback is invoked, you will find that sometimes the callback is a bit
2104       "early".
2105
2106       This is because "ev_timer"s work in real time, not wall clock time, so
2107       libev makes sure your callback is not invoked before the delay
2108       happened, measured according to the real time, not the system clock.
2109
2110       If your timeouts are based on a physical timescale (e.g. "time out this
2111       connection after 100 seconds") then this shouldn't bother you as it is
2112       exactly the right behaviour.
2113
2114       If you want to compare wall clock/system timestamps to your timers,
2115       then you need to use "ev_periodic"s, as these are based on the wall
2116       clock time, where your comparisons will always generate correct
2117       results.
2118
2119       The special problems of suspended animation
2120
2121       When you leave the server world it is quite customary to hit machines
2122       that can suspend/hibernate - what happens to the clocks during such a
2123       suspend?
2124
2125       Some quick tests made with a Linux 2.6.28 indicate that a suspend
2126       freezes all processes, while the clocks ("times", "CLOCK_MONOTONIC")
2127       continue to run until the system is suspended, but they will not
2128       advance while the system is suspended. That means, on resume, it will
2129       be as if the program was frozen for a few seconds, but the suspend time
2130       will not be counted towards "ev_timer" when a monotonic clock source is
2131       used. The real time clock advanced as expected, but if it is used as
2132       sole clocksource, then a long suspend would be detected as a time jump
2133       by libev, and timers would be adjusted accordingly.
2134
2135       I would not be surprised to see different behaviour in different
2136       between operating systems, OS versions or even different hardware.
2137
2138       The other form of suspend (job control, or sending a SIGSTOP) will see
2139       a time jump in the monotonic clocks and the realtime clock. If the
2140       program is suspended for a very long time, and monotonic clock sources
2141       are in use, then you can expect "ev_timer"s to expire as the full
2142       suspension time will be counted towards the timers. When no monotonic
2143       clock source is in use, then libev will again assume a timejump and
2144       adjust accordingly.
2145
2146       It might be beneficial for this latter case to call "ev_suspend" and
2147       "ev_resume" in code that handles "SIGTSTP", to at least get
2148       deterministic behaviour in this case (you can do nothing against
2149       "SIGSTOP").
2150
2151       Watcher-Specific Functions and Data Members
2152
2153       ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
2154       ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
2155           Configure the timer to trigger after "after" seconds (fractional
2156           and negative values are supported). If "repeat" is 0., then it will
2157           automatically be stopped once the timeout is reached. If it is
2158           positive, then the timer will automatically be configured to
2159           trigger again "repeat" seconds later, again, and again, until
2160           stopped manually.
2161
2162           The timer itself will do a best-effort at avoiding drift, that is,
2163           if you configure a timer to trigger every 10 seconds, then it will
2164           normally trigger at exactly 10 second intervals. If, however, your
2165           program cannot keep up with the timer (because it takes longer than
2166           those 10 seconds to do stuff) the timer will not fire more than
2167           once per event loop iteration.
2168
2169       ev_timer_again (loop, ev_timer *)
2170           This will act as if the timer timed out, and restarts it again if
2171           it is repeating. It basically works like calling "ev_timer_stop",
2172           updating the timeout to the "repeat" value and calling
2173           "ev_timer_start".
2174
2175           The exact semantics are as in the following rules, all of which
2176           will be applied to the watcher:
2177
2178           If the timer is pending, the pending status is always cleared.
2179           If the timer is started but non-repeating, stop it (as if it timed
2180           out, without invoking it).
2181           If the timer is repeating, make the "repeat" value the new timeout
2182           and start the timer, if necessary.
2183
2184           This sounds a bit complicated, see "Be smart about timeouts",
2185           above, for a usage example.
2186
2187       ev_tstamp ev_timer_remaining (loop, ev_timer *)
2188           Returns the remaining time until a timer fires. If the timer is
2189           active, then this time is relative to the current event loop time,
2190           otherwise it's the timeout value currently configured.
2191
2192           That is, after an "ev_timer_set (w, 5, 7)", "ev_timer_remaining"
2193           returns 5. When the timer is started and one second passes,
2194           "ev_timer_remaining" will return 4. When the timer expires and is
2195           restarted, it will return roughly 7 (likely slightly less as
2196           callback invocation takes some time, too), and so on.
2197
2198       ev_tstamp repeat [read-write]
2199           The current "repeat" value. Will be used each time the watcher
2200           times out or "ev_timer_again" is called, and determines the next
2201           timeout (if any), which is also when any modifications are taken
2202           into account.
2203
2204       Examples
2205
2206       Example: Create a timer that fires after 60 seconds.
2207
2208          static void
2209          one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2210          {
2211            .. one minute over, w is actually stopped right here
2212          }
2213
2214          ev_timer mytimer;
2215          ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2216          ev_timer_start (loop, &mytimer);
2217
2218       Example: Create a timeout timer that times out after 10 seconds of
2219       inactivity.
2220
2221          static void
2222          timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2223          {
2224            .. ten seconds without any activity
2225          }
2226
2227          ev_timer mytimer;
2228          ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2229          ev_timer_again (&mytimer); /* start timer */
2230          ev_run (loop, 0);
2231
2232          // and in some piece of code that gets executed on any "activity":
2233          // reset the timeout to start ticking again at 10 seconds
2234          ev_timer_again (&mytimer);
2235
2236   "ev_periodic" - to cron or not to cron?
2237       Periodic watchers are also timers of a kind, but they are very
2238       versatile (and unfortunately a bit complex).
2239
2240       Unlike "ev_timer", periodic watchers are not based on real time (or
2241       relative time, the physical time that passes) but on wall clock time
2242       (absolute time, the thing you can read on your calendar or clock). The
2243       difference is that wall clock time can run faster or slower than real
2244       time, and time jumps are not uncommon (e.g. when you adjust your wrist-
2245       watch).
2246
2247       You can tell a periodic watcher to trigger after some specific point in
2248       time: for example, if you tell a periodic watcher to trigger "in 10
2249       seconds" (by specifying e.g. "ev_now () + 10.", that is, an absolute
2250       time not a delay) and then reset your system clock to January of the
2251       previous year, then it will take a year or more to trigger the event
2252       (unlike an "ev_timer", which would still trigger roughly 10 seconds
2253       after starting it, as it uses a relative timeout).
2254
2255       "ev_periodic" watchers can also be used to implement vastly more
2256       complex timers, such as triggering an event on each "midnight, local
2257       time", or other complicated rules. This cannot easily be done with
2258       "ev_timer" watchers, as those cannot react to time jumps.
2259
2260       As with timers, the callback is guaranteed to be invoked only when the
2261       point in time where it is supposed to trigger has passed. If multiple
2262       timers become ready during the same loop iteration then the ones with
2263       earlier time-out values are invoked before ones with later time-out
2264       values (but this is no longer true when a callback calls "ev_run"
2265       recursively).
2266
2267       Watcher-Specific Functions and Data Members
2268
2269       ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp
2270       interval, reschedule_cb)
2271       ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval,
2272       reschedule_cb)
2273           Lots of arguments, let's sort it out... There are basically three
2274           modes of operation, and we will explain them from simplest to most
2275           complex:
2276
2277           ·   absolute timer (offset = absolute time, interval = 0,
2278               reschedule_cb = 0)
2279
2280               In this configuration the watcher triggers an event after the
2281               wall clock time "offset" has passed. It will not repeat and
2282               will not adjust when a time jump occurs, that is, if it is to
2283               be run at January 1st 2011 then it will be stopped and invoked
2284               when the system clock reaches or surpasses this point in time.
2285
2286           ·   repeating interval timer (offset = offset within interval,
2287               interval > 0, reschedule_cb = 0)
2288
2289               In this mode the watcher will always be scheduled to time out
2290               at the next "offset + N * interval" time (for some integer N,
2291               which can also be negative) and then repeat, regardless of any
2292               time jumps. The "offset" argument is merely an offset into the
2293               "interval" periods.
2294
2295               This can be used to create timers that do not drift with
2296               respect to the system clock, for example, here is an
2297               "ev_periodic" that triggers each hour, on the hour (with
2298               respect to UTC):
2299
2300                  ev_periodic_set (&periodic, 0., 3600., 0);
2301
2302               This doesn't mean there will always be 3600 seconds in between
2303               triggers, but only that the callback will be called when the
2304               system time shows a full hour (UTC), or more correctly, when
2305               the system time is evenly divisible by 3600.
2306
2307               Another way to think about it (for the mathematically inclined)
2308               is that "ev_periodic" will try to run the callback in this mode
2309               at the next possible time where "time = offset (mod interval)",
2310               regardless of any time jumps.
2311
2312               The "interval" MUST be positive, and for numerical stability,
2313               the interval value should be higher than "1/8192" (which is
2314               around 100 microseconds) and "offset" should be higher than 0
2315               and should have at most a similar magnitude as the current time
2316               (say, within a factor of ten). Typical values for offset are,
2317               in fact, 0 or something between 0 and "interval", which is also
2318               the recommended range.
2319
2320               Note also that there is an upper limit to how often a timer can
2321               fire (CPU speed for example), so if "interval" is very small
2322               then timing stability will of course deteriorate. Libev itself
2323               tries to be exact to be about one millisecond (if the OS
2324               supports it and the machine is fast enough).
2325
2326           ·   manual reschedule mode (offset ignored, interval ignored,
2327               reschedule_cb = callback)
2328
2329               In this mode the values for "interval" and "offset" are both
2330               being ignored. Instead, each time the periodic watcher gets
2331               scheduled, the reschedule callback will be called with the
2332               watcher as first, and the current time as second argument.
2333
2334               NOTE: This callback MUST NOT stop or destroy any periodic
2335               watcher, ever, or make ANY other event loop modifications
2336               whatsoever, unless explicitly allowed by documentation here.
2337
2338               If you need to stop it, return "now + 1e30" (or so, fudge
2339               fudge) and stop it afterwards (e.g. by starting an "ev_prepare"
2340               watcher, which is the only event loop modification you are
2341               allowed to do).
2342
2343               The callback prototype is "ev_tstamp
2344               (*reschedule_cb)(ev_periodic *w, ev_tstamp now)", e.g.:
2345
2346                  static ev_tstamp
2347                  my_rescheduler (ev_periodic *w, ev_tstamp now)
2348                  {
2349                    return now + 60.;
2350                  }
2351
2352               It must return the next time to trigger, based on the passed
2353               time value (that is, the lowest time value larger than to the
2354               second argument). It will usually be called just before the
2355               callback will be triggered, but might be called at other times,
2356               too.
2357
2358               NOTE: This callback must always return a time that is higher
2359               than or equal to the passed "now" value.
2360
2361               This can be used to create very complex timers, such as a timer
2362               that triggers on "next midnight, local time". To do this, you
2363               would calculate the next midnight after "now" and return the
2364               timestamp value for this. Here is a (completely untested, no
2365               error checking) example on how to do this:
2366
2367                  #include <time.h>
2368
2369                  static ev_tstamp
2370                  my_rescheduler (ev_periodic *w, ev_tstamp now)
2371                  {
2372                    time_t tnow = (time_t)now;
2373                    struct tm tm;
2374                    localtime_r (&tnow, &tm);
2375
2376                    tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2377                    ++tm.tm_mday; // midnight next day
2378
2379                    return mktime (&tm);
2380                  }
2381
2382               Note: this code might run into trouble on days that have more
2383               then two midnights (beginning and end).
2384
2385       ev_periodic_again (loop, ev_periodic *)
2386           Simply stops and restarts the periodic watcher again. This is only
2387           useful when you changed some parameters or the reschedule callback
2388           would return a different time than the last time it was called
2389           (e.g. in a crond like program when the crontabs have changed).
2390
2391       ev_tstamp ev_periodic_at (ev_periodic *)
2392           When active, returns the absolute time that the watcher is supposed
2393           to trigger next. This is not the same as the "offset" argument to
2394           "ev_periodic_set", but indeed works even in interval and manual
2395           rescheduling modes.
2396
2397       ev_tstamp offset [read-write]
2398           When repeating, this contains the offset value, otherwise this is
2399           the absolute point in time (the "offset" value passed to
2400           "ev_periodic_set", although libev might modify this value for
2401           better numerical stability).
2402
2403           Can be modified any time, but changes only take effect when the
2404           periodic timer fires or "ev_periodic_again" is being called.
2405
2406       ev_tstamp interval [read-write]
2407           The current interval value. Can be modified any time, but changes
2408           only take effect when the periodic timer fires or
2409           "ev_periodic_again" is being called.
2410
2411       ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]
2412           The current reschedule callback, or 0, if this functionality is
2413           switched off. Can be changed any time, but changes only take effect
2414           when the periodic timer fires or "ev_periodic_again" is being
2415           called.
2416
2417       Examples
2418
2419       Example: Call a callback every hour, or, more precisely, whenever the
2420       system time is divisible by 3600. The callback invocation times have
2421       potentially a lot of jitter, but good long-term stability.
2422
2423          static void
2424          clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2425          {
2426            ... its now a full hour (UTC, or TAI or whatever your clock follows)
2427          }
2428
2429          ev_periodic hourly_tick;
2430          ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2431          ev_periodic_start (loop, &hourly_tick);
2432
2433       Example: The same as above, but use a reschedule callback to do it:
2434
2435          #include <math.h>
2436
2437          static ev_tstamp
2438          my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2439          {
2440            return now + (3600. - fmod (now, 3600.));
2441          }
2442
2443          ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2444
2445       Example: Call a callback every hour, starting now:
2446
2447          ev_periodic hourly_tick;
2448          ev_periodic_init (&hourly_tick, clock_cb,
2449                            fmod (ev_now (loop), 3600.), 3600., 0);
2450          ev_periodic_start (loop, &hourly_tick);
2451
2452   "ev_signal" - signal me when a signal gets signalled!
2453       Signal watchers will trigger an event when the process receives a
2454       specific signal one or more times. Even though signals are very
2455       asynchronous, libev will try its best to deliver signals synchronously,
2456       i.e. as part of the normal event processing, like any other event.
2457
2458       If you want signals to be delivered truly asynchronously, just use
2459       "sigaction" as you would do without libev and forget about sharing the
2460       signal. You can even use "ev_async" from a signal handler to
2461       synchronously wake up an event loop.
2462
2463       You can configure as many watchers as you like for the same signal, but
2464       only within the same loop, i.e. you can watch for "SIGINT" in your
2465       default loop and for "SIGIO" in another loop, but you cannot watch for
2466       "SIGINT" in both the default loop and another loop at the same time. At
2467       the moment, "SIGCHLD" is permanently tied to the default loop.
2468
2469       Only after the first watcher for a signal is started will libev
2470       actually register something with the kernel. It thus coexists with your
2471       own signal handlers as long as you don't register any with libev for
2472       the same signal.
2473
2474       If possible and supported, libev will install its handlers with
2475       "SA_RESTART" (or equivalent) behaviour enabled, so system calls should
2476       not be unduly interrupted. If you have a problem with system calls
2477       getting interrupted by signals you can block all signals in an
2478       "ev_check" watcher and unblock them in an "ev_prepare" watcher.
2479
2480       The special problem of inheritance over fork/execve/pthread_create
2481
2482       Both the signal mask ("sigprocmask") and the signal disposition
2483       ("sigaction") are unspecified after starting a signal watcher (and
2484       after stopping it again), that is, libev might or might not block the
2485       signal, and might or might not set or restore the installed signal
2486       handler (but see "EVFLAG_NOSIGMASK").
2487
2488       While this does not matter for the signal disposition (libev never sets
2489       signals to "SIG_IGN", so handlers will be reset to "SIG_DFL" on
2490       "execve"), this matters for the signal mask: many programs do not
2491       expect certain signals to be blocked.
2492
2493       This means that before calling "exec" (from the child) you should reset
2494       the signal mask to whatever "default" you expect (all clear is a good
2495       choice usually).
2496
2497       The simplest way to ensure that the signal mask is reset in the child
2498       is to install a fork handler with "pthread_atfork" that resets it. That
2499       will catch fork calls done by libraries (such as the libc) as well.
2500
2501       In current versions of libev, the signal will not be blocked
2502       indefinitely unless you use the "signalfd" API ("EV_SIGNALFD"). While
2503       this reduces the window of opportunity for problems, it will not go
2504       away, as libev has to modify the signal mask, at least temporarily.
2505
2506       So I can't stress this enough: If you do not reset your signal mask
2507       when you expect it to be empty, you have a race condition in your code.
2508       This is not a libev-specific thing, this is true for most event
2509       libraries.
2510
2511       The special problem of threads signal handling
2512
2513       POSIX threads has problematic signal handling semantics, specifically,
2514       a lot of functionality (sigfd, sigwait etc.) only really works if all
2515       threads in a process block signals, which is hard to achieve.
2516
2517       When you want to use sigwait (or mix libev signal handling with your
2518       own for the same signals), you can tackle this problem by globally
2519       blocking all signals before creating any threads (or creating them with
2520       a fully set sigprocmask) and also specifying the "EVFLAG_NOSIGMASK"
2521       when creating loops. Then designate one thread as "signal receiver
2522       thread" which handles these signals. You can pass on any signals that
2523       libev might be interested in by calling "ev_feed_signal".
2524
2525       Watcher-Specific Functions and Data Members
2526
2527       ev_signal_init (ev_signal *, callback, int signum)
2528       ev_signal_set (ev_signal *, int signum)
2529           Configures the watcher to trigger on the given signal number
2530           (usually one of the "SIGxxx" constants).
2531
2532       int signum [read-only]
2533           The signal the watcher watches out for.
2534
2535       Examples
2536
2537       Example: Try to exit cleanly on SIGINT.
2538
2539          static void
2540          sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2541          {
2542            ev_break (loop, EVBREAK_ALL);
2543          }
2544
2545          ev_signal signal_watcher;
2546          ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2547          ev_signal_start (loop, &signal_watcher);
2548
2549   "ev_child" - watch out for process status changes
2550       Child watchers trigger when your process receives a SIGCHLD in response
2551       to some child status changes (most typically when a child of yours dies
2552       or exits). It is permissible to install a child watcher after the child
2553       has been forked (which implies it might have already exited), as long
2554       as the event loop isn't entered (or is continued from a watcher), i.e.,
2555       forking and then immediately registering a watcher for the child is
2556       fine, but forking and registering a watcher a few event loop iterations
2557       later or in the next callback invocation is not.
2558
2559       Only the default event loop is capable of handling signals, and
2560       therefore you can only register child watchers in the default event
2561       loop.
2562
2563       Due to some design glitches inside libev, child watchers will always be
2564       handled at maximum priority (their priority is set to "EV_MAXPRI" by
2565       libev)
2566
2567       Process Interaction
2568
2569       Libev grabs "SIGCHLD" as soon as the default event loop is initialised.
2570       This is necessary to guarantee proper behaviour even if the first child
2571       watcher is started after the child exits. The occurrence of "SIGCHLD"
2572       is recorded asynchronously, but child reaping is done synchronously as
2573       part of the event loop processing. Libev always reaps all children,
2574       even ones not watched.
2575
2576       Overriding the Built-In Processing
2577
2578       Libev offers no special support for overriding the built-in child
2579       processing, but if your application collides with libev's default child
2580       handler, you can override it easily by installing your own handler for
2581       "SIGCHLD" after initialising the default loop, and making sure the
2582       default loop never gets destroyed. You are encouraged, however, to use
2583       an event-based approach to child reaping and thus use libev's support
2584       for that, so other libev users can use "ev_child" watchers freely.
2585
2586       Stopping the Child Watcher
2587
2588       Currently, the child watcher never gets stopped, even when the child
2589       terminates, so normally one needs to stop the watcher in the callback.
2590       Future versions of libev might stop the watcher automatically when a
2591       child exit is detected (calling "ev_child_stop" twice is not a
2592       problem).
2593
2594       Watcher-Specific Functions and Data Members
2595
2596       ev_child_init (ev_child *, callback, int pid, int trace)
2597       ev_child_set (ev_child *, int pid, int trace)
2598           Configures the watcher to wait for status changes of process "pid"
2599           (or any process if "pid" is specified as 0). The callback can look
2600           at the "rstatus" member of the "ev_child" watcher structure to see
2601           the status word (use the macros from "sys/wait.h" and see your
2602           systems "waitpid" documentation). The "rpid" member contains the
2603           pid of the process causing the status change. "trace" must be
2604           either 0 (only activate the watcher when the process terminates) or
2605           1 (additionally activate the watcher when the process is stopped or
2606           continued).
2607
2608       int pid [read-only]
2609           The process id this watcher watches out for, or 0, meaning any
2610           process id.
2611
2612       int rpid [read-write]
2613           The process id that detected a status change.
2614
2615       int rstatus [read-write]
2616           The process exit/trace status caused by "rpid" (see your systems
2617           "waitpid" and "sys/wait.h" documentation for details).
2618
2619       Examples
2620
2621       Example: "fork()" a new process and install a child handler to wait for
2622       its completion.
2623
2624          ev_child cw;
2625
2626          static void
2627          child_cb (EV_P_ ev_child *w, int revents)
2628          {
2629            ev_child_stop (EV_A_ w);
2630            printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
2631          }
2632
2633          pid_t pid = fork ();
2634
2635          if (pid < 0)
2636            // error
2637          else if (pid == 0)
2638            {
2639              // the forked child executes here
2640              exit (1);
2641            }
2642          else
2643            {
2644              ev_child_init (&cw, child_cb, pid, 0);
2645              ev_child_start (EV_DEFAULT_ &cw);
2646            }
2647
2648   "ev_stat" - did the file attributes just change?
2649       This watches a file system path for attribute changes. That is, it
2650       calls "stat" on that path in regular intervals (or when the OS says it
2651       changed) and sees if it changed compared to the last time, invoking the
2652       callback if it did. Starting the watcher "stat"'s the file, so only
2653       changes that happen after the watcher has been started will be
2654       reported.
2655
2656       The path does not need to exist: changing from "path exists" to "path
2657       does not exist" is a status change like any other. The condition "path
2658       does not exist" (or more correctly "path cannot be stat'ed") is
2659       signified by the "st_nlink" field being zero (which is otherwise always
2660       forced to be at least one) and all the other fields of the stat buffer
2661       having unspecified contents.
2662
2663       The path must not end in a slash or contain special components such as
2664       "." or "..". The path should be absolute: If it is relative and your
2665       working directory changes, then the behaviour is undefined.
2666
2667       Since there is no portable change notification interface available, the
2668       portable implementation simply calls stat(2) regularly on the path to
2669       see if it changed somehow. You can specify a recommended polling
2670       interval for this case. If you specify a polling interval of 0 (highly
2671       recommended!) then a suitable, unspecified default value will be used
2672       (which you can expect to be around five seconds, although this might
2673       change dynamically). Libev will also impose a minimum interval which is
2674       currently around 0.1, but that's usually overkill.
2675
2676       This watcher type is not meant for massive numbers of stat watchers, as
2677       even with OS-supported change notifications, this can be resource-
2678       intensive.
2679
2680       At the time of this writing, the only OS-specific interface implemented
2681       is the Linux inotify interface (implementing kqueue support is left as
2682       an exercise for the reader. Note, however, that the author sees no way
2683       of implementing "ev_stat" semantics with kqueue, except as a hint).
2684
2685       ABI Issues (Largefile Support)
2686
2687       Libev by default (unless the user overrides this) uses the default
2688       compilation environment, which means that on systems with large file
2689       support disabled by default, you get the 32 bit version of the stat
2690       structure. When using the library from programs that change the ABI to
2691       use 64 bit file offsets the programs will fail. In that case you have
2692       to compile libev with the same flags to get binary compatibility. This
2693       is obviously the case with any flags that change the ABI, but the
2694       problem is most noticeably displayed with ev_stat and large file
2695       support.
2696
2697       The solution for this is to lobby your distribution maker to make large
2698       file interfaces available by default (as e.g. FreeBSD does) and not
2699       optional. Libev cannot simply switch on large file support because it
2700       has to exchange stat structures with application programs compiled
2701       using the default compilation environment.
2702
2703       Inotify and Kqueue
2704
2705       When "inotify (7)" support has been compiled into libev and present at
2706       runtime, it will be used to speed up change detection where possible.
2707       The inotify descriptor will be created lazily when the first "ev_stat"
2708       watcher is being started.
2709
2710       Inotify presence does not change the semantics of "ev_stat" watchers
2711       except that changes might be detected earlier, and in some cases, to
2712       avoid making regular "stat" calls. Even in the presence of inotify
2713       support there are many cases where libev has to resort to regular
2714       "stat" polling, but as long as kernel 2.6.25 or newer is used (2.6.24
2715       and older have too many bugs), the path exists (i.e. stat succeeds),
2716       and the path resides on a local filesystem (libev currently assumes
2717       only ext2/3, jfs, reiserfs and xfs are fully working) libev usually
2718       gets away without polling.
2719
2720       There is no support for kqueue, as apparently it cannot be used to
2721       implement this functionality, due to the requirement of having a file
2722       descriptor open on the object at all times, and detecting renames,
2723       unlinks etc. is difficult.
2724
2725       "stat ()" is a synchronous operation
2726
2727       Libev doesn't normally do any kind of I/O itself, and so is not
2728       blocking the process. The exception are "ev_stat" watchers - those call
2729       "stat ()", which is a synchronous operation.
2730
2731       For local paths, this usually doesn't matter: unless the system is very
2732       busy or the intervals between stat's are large, a stat call will be
2733       fast, as the path data is usually in memory already (except when
2734       starting the watcher).
2735
2736       For networked file systems, calling "stat ()" can block an indefinite
2737       time due to network issues, and even under good conditions, a stat call
2738       often takes multiple milliseconds.
2739
2740       Therefore, it is best to avoid using "ev_stat" watchers on networked
2741       paths, although this is fully supported by libev.
2742
2743       The special problem of stat time resolution
2744
2745       The "stat ()" system call only supports full-second resolution
2746       portably, and even on systems where the resolution is higher, most file
2747       systems still only support whole seconds.
2748
2749       That means that, if the time is the only thing that changes, you can
2750       easily miss updates: on the first update, "ev_stat" detects a change
2751       and calls your callback, which does something. When there is another
2752       update within the same second, "ev_stat" will be unable to detect
2753       unless the stat data does change in other ways (e.g. file size).
2754
2755       The solution to this is to delay acting on a change for slightly more
2756       than a second (or till slightly after the next full second boundary),
2757       using a roughly one-second-delay "ev_timer" (e.g. "ev_timer_set (w, 0.,
2758       1.02); ev_timer_again (loop, w)").
2759
2760       The .02 offset is added to work around small timing inconsistencies of
2761       some operating systems (where the second counter of the current time
2762       might be be delayed. One such system is the Linux kernel, where a call
2763       to "gettimeofday" might return a timestamp with a full second later
2764       than a subsequent "time" call - if the equivalent of "time ()" is used
2765       to update file times then there will be a small window where the kernel
2766       uses the previous second to update file times but libev might already
2767       execute the timer callback).
2768
2769       Watcher-Specific Functions and Data Members
2770
2771       ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp
2772       interval)
2773       ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)
2774           Configures the watcher to wait for status changes of the given
2775           "path". The "interval" is a hint on how quickly a change is
2776           expected to be detected and should normally be specified as 0 to
2777           let libev choose a suitable value. The memory pointed to by "path"
2778           must point to the same path for as long as the watcher is active.
2779
2780           The callback will receive an "EV_STAT" event when a change was
2781           detected, relative to the attributes at the time the watcher was
2782           started (or the last change was detected).
2783
2784       ev_stat_stat (loop, ev_stat *)
2785           Updates the stat buffer immediately with new values. If you change
2786           the watched path in your callback, you could call this function to
2787           avoid detecting this change (while introducing a race condition if
2788           you are not the only one changing the path). Can also be useful
2789           simply to find out the new values.
2790
2791       ev_statdata attr [read-only]
2792           The most-recently detected attributes of the file. Although the
2793           type is "ev_statdata", this is usually the (or one of the) "struct
2794           stat" types suitable for your system, but you can only rely on the
2795           POSIX-standardised members to be present. If the "st_nlink" member
2796           is 0, then there was some error while "stat"ing the file.
2797
2798       ev_statdata prev [read-only]
2799           The previous attributes of the file. The callback gets invoked
2800           whenever "prev" != "attr", or, more precisely, one or more of these
2801           members differ: "st_dev", "st_ino", "st_mode", "st_nlink",
2802           "st_uid", "st_gid", "st_rdev", "st_size", "st_atime", "st_mtime",
2803           "st_ctime".
2804
2805       ev_tstamp interval [read-only]
2806           The specified interval.
2807
2808       const char *path [read-only]
2809           The file system path that is being watched.
2810
2811       Examples
2812
2813       Example: Watch "/etc/passwd" for attribute changes.
2814
2815          static void
2816          passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
2817          {
2818            /* /etc/passwd changed in some way */
2819            if (w->attr.st_nlink)
2820              {
2821                printf ("passwd current size  %ld\n", (long)w->attr.st_size);
2822                printf ("passwd current atime %ld\n", (long)w->attr.st_mtime);
2823                printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime);
2824              }
2825            else
2826              /* you shalt not abuse printf for puts */
2827              puts ("wow, /etc/passwd is not there, expect problems. "
2828                    "if this is windows, they already arrived\n");
2829          }
2830
2831          ...
2832          ev_stat passwd;
2833
2834          ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
2835          ev_stat_start (loop, &passwd);
2836
2837       Example: Like above, but additionally use a one-second delay so we do
2838       not miss updates (however, frequent updates will delay processing, too,
2839       so one might do the work both on "ev_stat" callback invocation and on
2840       "ev_timer" callback invocation).
2841
2842          static ev_stat passwd;
2843          static ev_timer timer;
2844
2845          static void
2846          timer_cb (EV_P_ ev_timer *w, int revents)
2847          {
2848            ev_timer_stop (EV_A_ w);
2849
2850            /* now it's one second after the most recent passwd change */
2851          }
2852
2853          static void
2854          stat_cb (EV_P_ ev_stat *w, int revents)
2855          {
2856            /* reset the one-second timer */
2857            ev_timer_again (EV_A_ &timer);
2858          }
2859
2860          ...
2861          ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
2862          ev_stat_start (loop, &passwd);
2863          ev_timer_init (&timer, timer_cb, 0., 1.02);
2864
2865   "ev_idle" - when you've got nothing better to do...
2866       Idle watchers trigger events when no other events of the same or higher
2867       priority are pending (prepare, check and other idle watchers do not
2868       count as receiving "events").
2869
2870       That is, as long as your process is busy handling sockets or timeouts
2871       (or even signals, imagine) of the same or higher priority it will not
2872       be triggered. But when your process is idle (or only lower-priority
2873       watchers are pending), the idle watchers are being called once per
2874       event loop iteration - until stopped, that is, or your process receives
2875       more events and becomes busy again with higher priority stuff.
2876
2877       The most noteworthy effect is that as long as any idle watchers are
2878       active, the process will not block when waiting for new events.
2879
2880       Apart from keeping your process non-blocking (which is a useful effect
2881       on its own sometimes), idle watchers are a good place to do "pseudo-
2882       background processing", or delay processing stuff to after the event
2883       loop has handled all outstanding events.
2884
2885       Abusing an "ev_idle" watcher for its side-effect
2886
2887       As long as there is at least one active idle watcher, libev will never
2888       sleep unnecessarily. Or in other words, it will loop as fast as
2889       possible.  For this to work, the idle watcher doesn't need to be
2890       invoked at all - the lowest priority will do.
2891
2892       This mode of operation can be useful together with an "ev_check"
2893       watcher, to do something on each event loop iteration - for example to
2894       balance load between different connections.
2895
2896       See "Abusing an ev_check watcher for its side-effect" for a longer
2897       example.
2898
2899       Watcher-Specific Functions and Data Members
2900
2901       ev_idle_init (ev_idle *, callback)
2902           Initialises and configures the idle watcher - it has no parameters
2903           of any kind. There is a "ev_idle_set" macro, but using it is
2904           utterly pointless, believe me.
2905
2906       Examples
2907
2908       Example: Dynamically allocate an "ev_idle" watcher, start it, and in
2909       the callback, free it. Also, use no error checking, as usual.
2910
2911          static void
2912          idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
2913          {
2914            // stop the watcher
2915            ev_idle_stop (loop, w);
2916
2917            // now we can free it
2918            free (w);
2919
2920            // now do something you wanted to do when the program has
2921            // no longer anything immediate to do.
2922          }
2923
2924          ev_idle *idle_watcher = malloc (sizeof (ev_idle));
2925          ev_idle_init (idle_watcher, idle_cb);
2926          ev_idle_start (loop, idle_watcher);
2927
2928   "ev_prepare" and "ev_check" - customise your event loop!
2929       Prepare and check watchers are often (but not always) used in pairs:
2930       prepare watchers get invoked before the process blocks and check
2931       watchers afterwards.
2932
2933       You must not call "ev_run" (or similar functions that enter the current
2934       event loop) or "ev_loop_fork" from either "ev_prepare" or "ev_check"
2935       watchers. Other loops than the current one are fine, however. The
2936       rationale behind this is that you do not need to check for recursion in
2937       those watchers, i.e. the sequence will always be "ev_prepare",
2938       blocking, "ev_check" so if you have one watcher of each kind they will
2939       always be called in pairs bracketing the blocking call.
2940
2941       Their main purpose is to integrate other event mechanisms into libev
2942       and their use is somewhat advanced. They could be used, for example, to
2943       track variable changes, implement your own watchers, integrate net-snmp
2944       or a coroutine library and lots more. They are also occasionally useful
2945       if you cache some data and want to flush it before blocking (for
2946       example, in X programs you might want to do an "XFlush ()" in an
2947       "ev_prepare" watcher).
2948
2949       This is done by examining in each prepare call which file descriptors
2950       need to be watched by the other library, registering "ev_io" watchers
2951       for them and starting an "ev_timer" watcher for any timeouts (many
2952       libraries provide exactly this functionality). Then, in the check
2953       watcher, you check for any events that occurred (by checking the
2954       pending status of all watchers and stopping them) and call back into
2955       the library. The I/O and timer callbacks will never actually be called
2956       (but must be valid nevertheless, because you never know, you know?).
2957
2958       As another example, the Perl Coro module uses these hooks to integrate
2959       coroutines into libev programs, by yielding to other active coroutines
2960       during each prepare and only letting the process block if no coroutines
2961       are ready to run (it's actually more complicated: it only runs
2962       coroutines with priority higher than or equal to the event loop and one
2963       coroutine of lower priority, but only once, using idle watchers to keep
2964       the event loop from blocking if lower-priority coroutines are active,
2965       thus mapping low-priority coroutines to idle/background tasks).
2966
2967       When used for this purpose, it is recommended to give "ev_check"
2968       watchers highest ("EV_MAXPRI") priority, to ensure that they are being
2969       run before any other watchers after the poll (this doesn't matter for
2970       "ev_prepare" watchers).
2971
2972       Also, "ev_check" watchers (and "ev_prepare" watchers, too) should not
2973       activate ("feed") events into libev. While libev fully supports this,
2974       they might get executed before other "ev_check" watchers did their job.
2975       As "ev_check" watchers are often used to embed other (non-libev) event
2976       loops those other event loops might be in an unusable state until their
2977       "ev_check" watcher ran (always remind yourself to coexist peacefully
2978       with others).
2979
2980       Abusing an "ev_check" watcher for its side-effect
2981
2982       "ev_check" (and less often also "ev_prepare") watchers can also be
2983       useful because they are called once per event loop iteration. For
2984       example, if you want to handle a large number of connections fairly,
2985       you normally only do a bit of work for each active connection, and if
2986       there is more work to do, you wait for the next event loop iteration,
2987       so other connections have a chance of making progress.
2988
2989       Using an "ev_check" watcher is almost enough: it will be called on the
2990       next event loop iteration. However, that isn't as soon as possible -
2991       without external events, your "ev_check" watcher will not be invoked.
2992
2993       This is where "ev_idle" watchers come in handy - all you need is a
2994       single global idle watcher that is active as long as you have one
2995       active "ev_check" watcher. The "ev_idle" watcher makes sure the event
2996       loop will not sleep, and the "ev_check" watcher makes sure a callback
2997       gets invoked. Neither watcher alone can do that.
2998
2999       Watcher-Specific Functions and Data Members
3000
3001       ev_prepare_init (ev_prepare *, callback)
3002       ev_check_init (ev_check *, callback)
3003           Initialises and configures the prepare or check watcher - they have
3004           no parameters of any kind. There are "ev_prepare_set" and
3005           "ev_check_set" macros, but using them is utterly, utterly, utterly
3006           and completely pointless.
3007
3008       Examples
3009
3010       There are a number of principal ways to embed other event loops or
3011       modules into libev. Here are some ideas on how to include libadns into
3012       libev (there is a Perl module named "EV::ADNS" that does this, which
3013       you could use as a working example. Another Perl module named
3014       "EV::Glib" embeds a Glib main context into libev, and finally,
3015       "Glib::EV" embeds EV into the Glib event loop).
3016
3017       Method 1: Add IO watchers and a timeout watcher in a prepare handler,
3018       and in a check watcher, destroy them and call into libadns. What
3019       follows is pseudo-code only of course. This requires you to either use
3020       a low priority for the check watcher or use "ev_clear_pending"
3021       explicitly, as the callbacks for the IO/timeout watchers might not have
3022       been called yet.
3023
3024          static ev_io iow [nfd];
3025          static ev_timer tw;
3026
3027          static void
3028          io_cb (struct ev_loop *loop, ev_io *w, int revents)
3029          {
3030          }
3031
3032          // create io watchers for each fd and a timer before blocking
3033          static void
3034          adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3035          {
3036            int timeout = 3600000;
3037            struct pollfd fds [nfd];
3038            // actual code will need to loop here and realloc etc.
3039            adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3040
3041            /* the callback is illegal, but won't be called as we stop during check */
3042            ev_timer_init (&tw, 0, timeout * 1e-3, 0.);
3043            ev_timer_start (loop, &tw);
3044
3045            // create one ev_io per pollfd
3046            for (int i = 0; i < nfd; ++i)
3047              {
3048                ev_io_init (iow + i, io_cb, fds [i].fd,
3049                  ((fds [i].events & POLLIN ? EV_READ : 0)
3050                   | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3051
3052                fds [i].revents = 0;
3053                ev_io_start (loop, iow + i);
3054              }
3055          }
3056
3057          // stop all watchers after blocking
3058          static void
3059          adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3060          {
3061            ev_timer_stop (loop, &tw);
3062
3063            for (int i = 0; i < nfd; ++i)
3064              {
3065                // set the relevant poll flags
3066                // could also call adns_processreadable etc. here
3067                struct pollfd *fd = fds + i;
3068                int revents = ev_clear_pending (iow + i);
3069                if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
3070                if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
3071
3072                // now stop the watcher
3073                ev_io_stop (loop, iow + i);
3074              }
3075
3076            adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3077          }
3078
3079       Method 2: This would be just like method 1, but you run
3080       "adns_afterpoll" in the prepare watcher and would dispose of the check
3081       watcher.
3082
3083       Method 3: If the module to be embedded supports explicit event
3084       notification (libadns does), you can also make use of the actual
3085       watcher callbacks, and only destroy/create the watchers in the prepare
3086       watcher.
3087
3088          static void
3089          timer_cb (EV_P_ ev_timer *w, int revents)
3090          {
3091            adns_state ads = (adns_state)w->data;
3092            update_now (EV_A);
3093
3094            adns_processtimeouts (ads, &tv_now);
3095          }
3096
3097          static void
3098          io_cb (EV_P_ ev_io *w, int revents)
3099          {
3100            adns_state ads = (adns_state)w->data;
3101            update_now (EV_A);
3102
3103            if (revents & EV_READ ) adns_processreadable  (ads, w->fd, &tv_now);
3104            if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
3105          }
3106
3107          // do not ever call adns_afterpoll
3108
3109       Method 4: Do not use a prepare or check watcher because the module you
3110       want to embed is not flexible enough to support it. Instead, you can
3111       override their poll function. The drawback with this solution is that
3112       the main loop is now no longer controllable by EV. The "Glib::EV"
3113       module uses this approach, effectively embedding EV as a client into
3114       the horrible libglib event loop.
3115
3116          static gint
3117          event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3118          {
3119            int got_events = 0;
3120
3121            for (n = 0; n < nfds; ++n)
3122              // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3123
3124            if (timeout >= 0)
3125              // create/start timer
3126
3127            // poll
3128            ev_run (EV_A_ 0);
3129
3130            // stop timer again
3131            if (timeout >= 0)
3132              ev_timer_stop (EV_A_ &to);
3133
3134            // stop io watchers again - their callbacks should have set
3135            for (n = 0; n < nfds; ++n)
3136              ev_io_stop (EV_A_ iow [n]);
3137
3138            return got_events;
3139          }
3140
3141   "ev_embed" - when one backend isn't enough...
3142       This is a rather advanced watcher type that lets you embed one event
3143       loop into another (currently only "ev_io" events are supported in the
3144       embedded loop, other types of watchers might be handled in a delayed or
3145       incorrect fashion and must not be used).
3146
3147       There are primarily two reasons you would want that: work around bugs
3148       and prioritise I/O.
3149
3150       As an example for a bug workaround, the kqueue backend might only
3151       support sockets on some platform, so it is unusable as generic backend,
3152       but you still want to make use of it because you have many sockets and
3153       it scales so nicely. In this case, you would create a kqueue-based loop
3154       and embed it into your default loop (which might use e.g. poll).
3155       Overall operation will be a bit slower because first libev has to call
3156       "poll" and then "kevent", but at least you can use both mechanisms for
3157       what they are best: "kqueue" for scalable sockets and "poll" if you
3158       want it to work :)
3159
3160       As for prioritising I/O: under rare circumstances you have the case
3161       where some fds have to be watched and handled very quickly (with low
3162       latency), and even priorities and idle watchers might have too much
3163       overhead. In this case you would put all the high priority stuff in one
3164       loop and all the rest in a second one, and embed the second one in the
3165       first.
3166
3167       As long as the watcher is active, the callback will be invoked every
3168       time there might be events pending in the embedded loop. The callback
3169       must then call "ev_embed_sweep (mainloop, watcher)" to make a single
3170       sweep and invoke their callbacks (the callback doesn't need to invoke
3171       the "ev_embed_sweep" function directly, it could also start an idle
3172       watcher to give the embedded loop strictly lower priority for example).
3173
3174       You can also set the callback to 0, in which case the embed watcher
3175       will automatically execute the embedded loop sweep whenever necessary.
3176
3177       Fork detection will be handled transparently while the "ev_embed"
3178       watcher is active, i.e., the embedded loop will automatically be forked
3179       when the embedding loop forks. In other cases, the user is responsible
3180       for calling "ev_loop_fork" on the embedded loop.
3181
3182       Unfortunately, not all backends are embeddable: only the ones returned
3183       by "ev_embeddable_backends" are, which, unfortunately, does not include
3184       any portable one.
3185
3186       So when you want to use this feature you will always have to be
3187       prepared that you cannot get an embeddable loop. The recommended way to
3188       get around this is to have a separate variables for your embeddable
3189       loop, try to create it, and if that fails, use the normal loop for
3190       everything.
3191
3192       "ev_embed" and fork
3193
3194       While the "ev_embed" watcher is running, forks in the embedding loop
3195       will automatically be applied to the embedded loop as well, so no
3196       special fork handling is required in that case. When the watcher is not
3197       running, however, it is still the task of the libev user to call
3198       "ev_loop_fork ()" as applicable.
3199
3200       Watcher-Specific Functions and Data Members
3201
3202       ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
3203       ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)
3204           Configures the watcher to embed the given loop, which must be
3205           embeddable. If the callback is 0, then "ev_embed_sweep" will be
3206           invoked automatically, otherwise it is the responsibility of the
3207           callback to invoke it (it will continue to be called until the
3208           sweep has been done, if you do not want that, you need to
3209           temporarily stop the embed watcher).
3210
3211       ev_embed_sweep (loop, ev_embed *)
3212           Make a single, non-blocking sweep over the embedded loop. This
3213           works similarly to "ev_run (embedded_loop, EVRUN_NOWAIT)", but in
3214           the most appropriate way for embedded loops.
3215
3216       struct ev_loop *other [read-only]
3217           The embedded event loop.
3218
3219       Examples
3220
3221       Example: Try to get an embeddable event loop and embed it into the
3222       default event loop. If that is not possible, use the default loop. The
3223       default loop is stored in "loop_hi", while the embeddable loop is
3224       stored in "loop_lo" (which is "loop_hi" in the case no embeddable loop
3225       can be used).
3226
3227          struct ev_loop *loop_hi = ev_default_init (0);
3228          struct ev_loop *loop_lo = 0;
3229          ev_embed embed;
3230
3231          // see if there is a chance of getting one that works
3232          // (remember that a flags value of 0 means autodetection)
3233          loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3234            ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3235            : 0;
3236
3237          // if we got one, then embed it, otherwise default to loop_hi
3238          if (loop_lo)
3239            {
3240              ev_embed_init (&embed, 0, loop_lo);
3241              ev_embed_start (loop_hi, &embed);
3242            }
3243          else
3244            loop_lo = loop_hi;
3245
3246       Example: Check if kqueue is available but not recommended and create a
3247       kqueue backend for use with sockets (which usually work with any kqueue
3248       implementation). Store the kqueue/socket-only event loop in
3249       "loop_socket". (One might optionally use "EVFLAG_NOENV", too).
3250
3251          struct ev_loop *loop = ev_default_init (0);
3252          struct ev_loop *loop_socket = 0;
3253          ev_embed embed;
3254
3255          if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3256            if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3257              {
3258                ev_embed_init (&embed, 0, loop_socket);
3259                ev_embed_start (loop, &embed);
3260              }
3261
3262          if (!loop_socket)
3263            loop_socket = loop;
3264
3265          // now use loop_socket for all sockets, and loop for everything else
3266
3267   "ev_fork" - the audacity to resume the event loop after a fork
3268       Fork watchers are called when a "fork ()" was detected (usually because
3269       whoever is a good citizen cared to tell libev about it by calling
3270       "ev_loop_fork"). The invocation is done before the event loop blocks
3271       next and before "ev_check" watchers are being called, and only in the
3272       child after the fork. If whoever good citizen calling "ev_default_fork"
3273       cheats and calls it in the wrong process, the fork handlers will be
3274       invoked, too, of course.
3275
3276       The special problem of life after fork - how is it possible?
3277
3278       Most uses of "fork ()" consist of forking, then some simple calls to
3279       set up/change the process environment, followed by a call to "exec()".
3280       This sequence should be handled by libev without any problems.
3281
3282       This changes when the application actually wants to do event handling
3283       in the child, or both parent in child, in effect "continuing" after the
3284       fork.
3285
3286       The default mode of operation (for libev, with application help to
3287       detect forks) is to duplicate all the state in the child, as would be
3288       expected when either the parent or the child process continues.
3289
3290       When both processes want to continue using libev, then this is usually
3291       the wrong result. In that case, usually one process (typically the
3292       parent) is supposed to continue with all watchers in place as before,
3293       while the other process typically wants to start fresh, i.e. without
3294       any active watchers.
3295
3296       The cleanest and most efficient way to achieve that with libev is to
3297       simply create a new event loop, which of course will be "empty", and
3298       use that for new watchers. This has the advantage of not touching more
3299       memory than necessary, and thus avoiding the copy-on-write, and the
3300       disadvantage of having to use multiple event loops (which do not
3301       support signal watchers).
3302
3303       When this is not possible, or you want to use the default loop for
3304       other reasons, then in the process that wants to start "fresh", call
3305       "ev_loop_destroy (EV_DEFAULT)" followed by "ev_default_loop (...)".
3306       Destroying the default loop will "orphan" (not stop) all registered
3307       watchers, so you have to be careful not to execute code that modifies
3308       those watchers. Note also that in that case, you have to re-register
3309       any signal watchers.
3310
3311       Watcher-Specific Functions and Data Members
3312
3313       ev_fork_init (ev_fork *, callback)
3314           Initialises and configures the fork watcher - it has no parameters
3315           of any kind. There is a "ev_fork_set" macro, but using it is
3316           utterly pointless, really.
3317
3318   "ev_cleanup" - even the best things end
3319       Cleanup watchers are called just before the event loop is being
3320       destroyed by a call to "ev_loop_destroy".
3321
3322       While there is no guarantee that the event loop gets destroyed, cleanup
3323       watchers provide a convenient method to install cleanup hooks for your
3324       program, worker threads and so on - you just to make sure to destroy
3325       the loop when you want them to be invoked.
3326
3327       Cleanup watchers are invoked in the same way as any other watcher.
3328       Unlike all other watchers, they do not keep a reference to the event
3329       loop (which makes a lot of sense if you think about it). Like all other
3330       watchers, you can call libev functions in the callback, except
3331       "ev_cleanup_start".
3332
3333       Watcher-Specific Functions and Data Members
3334
3335       ev_cleanup_init (ev_cleanup *, callback)
3336           Initialises and configures the cleanup watcher - it has no
3337           parameters of any kind. There is a "ev_cleanup_set" macro, but
3338           using it is utterly pointless, I assure you.
3339
3340       Example: Register an atexit handler to destroy the default loop, so any
3341       cleanup functions are called.
3342
3343          static void
3344          program_exits (void)
3345          {
3346            ev_loop_destroy (EV_DEFAULT_UC);
3347          }
3348
3349          ...
3350          atexit (program_exits);
3351
3352   "ev_async" - how to wake up an event loop
3353       In general, you cannot use an "ev_loop" from multiple threads or other
3354       asynchronous sources such as signal handlers (as opposed to multiple
3355       event loops - those are of course safe to use in different threads).
3356
3357       Sometimes, however, you need to wake up an event loop you do not
3358       control, for example because it belongs to another thread. This is what
3359       "ev_async" watchers do: as long as the "ev_async" watcher is active,
3360       you can signal it by calling "ev_async_send", which is thread- and
3361       signal safe.
3362
3363       This functionality is very similar to "ev_signal" watchers, as signals,
3364       too, are asynchronous in nature, and signals, too, will be compressed
3365       (i.e. the number of callback invocations may be less than the number of
3366       "ev_async_send" calls). In fact, you could use signal watchers as a
3367       kind of "global async watchers" by using a watcher on an otherwise
3368       unused signal, and "ev_feed_signal" to signal this watcher from another
3369       thread, even without knowing which loop owns the signal.
3370
3371       Queueing
3372
3373       "ev_async" does not support queueing of data in any way. The reason is
3374       that the author does not know of a simple (or any) algorithm for a
3375       multiple-writer-single-reader queue that works in all cases and doesn't
3376       need elaborate support such as pthreads or unportable memory access
3377       semantics.
3378
3379       That means that if you want to queue data, you have to provide your own
3380       queue. But at least I can tell you how to implement locking around your
3381       queue:
3382
3383       queueing from a signal handler context
3384           To implement race-free queueing, you simply add to the queue in the
3385           signal handler but you block the signal handler in the watcher
3386           callback. Here is an example that does that for some fictitious
3387           SIGUSR1 handler:
3388
3389              static ev_async mysig;
3390
3391              static void
3392              sigusr1_handler (void)
3393              {
3394                sometype data;
3395
3396                // no locking etc.
3397                queue_put (data);
3398                ev_async_send (EV_DEFAULT_ &mysig);
3399              }
3400
3401              static void
3402              mysig_cb (EV_P_ ev_async *w, int revents)
3403              {
3404                sometype data;
3405                sigset_t block, prev;
3406
3407                sigemptyset (&block);
3408                sigaddset (&block, SIGUSR1);
3409                sigprocmask (SIG_BLOCK, &block, &prev);
3410
3411                while (queue_get (&data))
3412                  process (data);
3413
3414                if (sigismember (&prev, SIGUSR1)
3415                  sigprocmask (SIG_UNBLOCK, &block, 0);
3416              }
3417
3418           (Note: pthreads in theory requires you to use "pthread_setmask"
3419           instead of "sigprocmask" when you use threads, but libev doesn't do
3420           it either...).
3421
3422       queueing from a thread context
3423           The strategy for threads is different, as you cannot (easily) block
3424           threads but you can easily preempt them, so to queue safely you
3425           need to employ a traditional mutex lock, such as in this pthread
3426           example:
3427
3428              static ev_async mysig;
3429              static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3430
3431              static void
3432              otherthread (void)
3433              {
3434                // only need to lock the actual queueing operation
3435                pthread_mutex_lock (&mymutex);
3436                queue_put (data);
3437                pthread_mutex_unlock (&mymutex);
3438
3439                ev_async_send (EV_DEFAULT_ &mysig);
3440              }
3441
3442              static void
3443              mysig_cb (EV_P_ ev_async *w, int revents)
3444              {
3445                pthread_mutex_lock (&mymutex);
3446
3447                while (queue_get (&data))
3448                  process (data);
3449
3450                pthread_mutex_unlock (&mymutex);
3451              }
3452
3453       Watcher-Specific Functions and Data Members
3454
3455       ev_async_init (ev_async *, callback)
3456           Initialises and configures the async watcher - it has no parameters
3457           of any kind. There is a "ev_async_set" macro, but using it is
3458           utterly pointless, trust me.
3459
3460       ev_async_send (loop, ev_async *)
3461           Sends/signals/activates the given "ev_async" watcher, that is,
3462           feeds an "EV_ASYNC" event on the watcher into the event loop, and
3463           instantly returns.
3464
3465           Unlike "ev_feed_event", this call is safe to do from other threads,
3466           signal or similar contexts (see the discussion of "EV_ATOMIC_T" in
3467           the embedding section below on what exactly this means).
3468
3469           Note that, as with other watchers in libev, multiple events might
3470           get compressed into a single callback invocation (another way to
3471           look at this is that "ev_async" watchers are level-triggered: they
3472           are set on "ev_async_send", reset when the event loop detects
3473           that).
3474
3475           This call incurs the overhead of at most one extra system call per
3476           event loop iteration, if the event loop is blocked, and no syscall
3477           at all if the event loop (or your program) is processing events.
3478           That means that repeated calls are basically free (there is no need
3479           to avoid calls for performance reasons) and that the overhead
3480           becomes smaller (typically zero) under load.
3481
3482       bool = ev_async_pending (ev_async *)
3483           Returns a non-zero value when "ev_async_send" has been called on
3484           the watcher but the event has not yet been processed (or even
3485           noted) by the event loop.
3486
3487           "ev_async_send" sets a flag in the watcher and wakes up the loop.
3488           When the loop iterates next and checks for the watcher to have
3489           become active, it will reset the flag again. "ev_async_pending" can
3490           be used to very quickly check whether invoking the loop might be a
3491           good idea.
3492
3493           Not that this does not check whether the watcher itself is pending,
3494           only whether it has been requested to make this watcher pending:
3495           there is a time window between the event loop checking and
3496           resetting the async notification, and the callback being invoked.
3497

OTHER FUNCTIONS

3499       There are some other functions of possible interest. Described. Here.
3500       Now.
3501
3502       ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)
3503           This function combines a simple timer and an I/O watcher, calls
3504           your callback on whichever event happens first and automatically
3505           stops both watchers. This is useful if you want to wait for a
3506           single event on an fd or timeout without having to
3507           allocate/configure/start/stop/free one or more watchers yourself.
3508
3509           If "fd" is less than 0, then no I/O watcher will be started and the
3510           "events" argument is being ignored. Otherwise, an "ev_io" watcher
3511           for the given "fd" and "events" set will be created and started.
3512
3513           If "timeout" is less than 0, then no timeout watcher will be
3514           started. Otherwise an "ev_timer" watcher with after = "timeout"
3515           (and repeat = 0) will be started. 0 is a valid timeout.
3516
3517           The callback has the type "void (*cb)(int revents, void *arg)" and
3518           is passed an "revents" set like normal event callbacks (a
3519           combination of "EV_ERROR", "EV_READ", "EV_WRITE" or "EV_TIMER") and
3520           the "arg" value passed to "ev_once". Note that it is possible to
3521           receive both a timeout and an io event at the same time - you
3522           probably should give io events precedence.
3523
3524           Example: wait up to ten seconds for data to appear on STDIN_FILENO.
3525
3526              static void stdin_ready (int revents, void *arg)
3527              {
3528                if (revents & EV_READ)
3529                  /* stdin might have data for us, joy! */;
3530                else if (revents & EV_TIMER)
3531                  /* doh, nothing entered */;
3532              }
3533
3534              ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3535
3536       ev_feed_fd_event (loop, int fd, int revents)
3537           Feed an event on the given fd, as if a file descriptor backend
3538           detected the given events.
3539
3540       ev_feed_signal_event (loop, int signum)
3541           Feed an event as if the given signal occurred. See also
3542           "ev_feed_signal", which is async-safe.
3543

COMMON OR USEFUL IDIOMS (OR BOTH)

3545       This section explains some common idioms that are not immediately
3546       obvious. Note that examples are sprinkled over the whole manual, and
3547       this section only contains stuff that wouldn't fit anywhere else.
3548
3549   ASSOCIATING CUSTOM DATA WITH A WATCHER
3550       Each watcher has, by default, a "void *data" member that you can read
3551       or modify at any time: libev will completely ignore it. This can be
3552       used to associate arbitrary data with your watcher. If you need more
3553       data and don't want to allocate memory separately and store a pointer
3554       to it in that data member, you can also "subclass" the watcher type and
3555       provide your own data:
3556
3557          struct my_io
3558          {
3559            ev_io io;
3560            int otherfd;
3561            void *somedata;
3562            struct whatever *mostinteresting;
3563          };
3564
3565          ...
3566          struct my_io w;
3567          ev_io_init (&w.io, my_cb, fd, EV_READ);
3568
3569       And since your callback will be called with a pointer to the watcher,
3570       you can cast it back to your own type:
3571
3572          static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3573          {
3574            struct my_io *w = (struct my_io *)w_;
3575            ...
3576          }
3577
3578       More interesting and less C-conformant ways of casting your callback
3579       function type instead have been omitted.
3580
3581   BUILDING YOUR OWN COMPOSITE WATCHERS
3582       Another common scenario is to use some data structure with multiple
3583       embedded watchers, in effect creating your own watcher that combines
3584       multiple libev event sources into one "super-watcher":
3585
3586          struct my_biggy
3587          {
3588            int some_data;
3589            ev_timer t1;
3590            ev_timer t2;
3591          }
3592
3593       In this case getting the pointer to "my_biggy" is a bit more
3594       complicated: Either you store the address of your "my_biggy" struct in
3595       the "data" member of the watcher (for woozies or C++ coders), or you
3596       need to use some pointer arithmetic using "offsetof" inside your
3597       watchers (for real programmers):
3598
3599          #include <stddef.h>
3600
3601          static void
3602          t1_cb (EV_P_ ev_timer *w, int revents)
3603          {
3604            struct my_biggy big = (struct my_biggy *)
3605              (((char *)w) - offsetof (struct my_biggy, t1));
3606          }
3607
3608          static void
3609          t2_cb (EV_P_ ev_timer *w, int revents)
3610          {
3611            struct my_biggy big = (struct my_biggy *)
3612              (((char *)w) - offsetof (struct my_biggy, t2));
3613          }
3614
3615   AVOIDING FINISHING BEFORE RETURNING
3616       Often you have structures like this in event-based programs:
3617
3618         callback ()
3619         {
3620           free (request);
3621         }
3622
3623         request = start_new_request (..., callback);
3624
3625       The intent is to start some "lengthy" operation. The "request" could be
3626       used to cancel the operation, or do other things with it.
3627
3628       It's not uncommon to have code paths in "start_new_request" that
3629       immediately invoke the callback, for example, to report errors. Or you
3630       add some caching layer that finds that it can skip the lengthy aspects
3631       of the operation and simply invoke the callback with the result.
3632
3633       The problem here is that this will happen before "start_new_request"
3634       has returned, so "request" is not set.
3635
3636       Even if you pass the request by some safer means to the callback, you
3637       might want to do something to the request after starting it, such as
3638       canceling it, which probably isn't working so well when the callback
3639       has already been invoked.
3640
3641       A common way around all these issues is to make sure that
3642       "start_new_request" always returns before the callback is invoked. If
3643       "start_new_request" immediately knows the result, it can artificially
3644       delay invoking the callback by using a "prepare" or "idle" watcher for
3645       example, or more sneakily, by reusing an existing (stopped) watcher and
3646       pushing it into the pending queue:
3647
3648          ev_set_cb (watcher, callback);
3649          ev_feed_event (EV_A_ watcher, 0);
3650
3651       This way, "start_new_request" can safely return before the callback is
3652       invoked, while not delaying callback invocation too much.
3653
3654   MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS
3655       Often (especially in GUI toolkits) there are places where you have
3656       modal interaction, which is most easily implemented by recursively
3657       invoking "ev_run".
3658
3659       This brings the problem of exiting - a callback might want to finish
3660       the main "ev_run" call, but not the nested one (e.g. user clicked
3661       "Quit", but a modal "Are you sure?" dialog is still waiting), or just
3662       the nested one and not the main one (e.g. user clocked "Ok" in a modal
3663       dialog), or some other combination: In these cases, a simple "ev_break"
3664       will not work.
3665
3666       The solution is to maintain "break this loop" variable for each
3667       "ev_run" invocation, and use a loop around "ev_run" until the condition
3668       is triggered, using "EVRUN_ONCE":
3669
3670          // main loop
3671          int exit_main_loop = 0;
3672
3673          while (!exit_main_loop)
3674            ev_run (EV_DEFAULT_ EVRUN_ONCE);
3675
3676          // in a modal watcher
3677          int exit_nested_loop = 0;
3678
3679          while (!exit_nested_loop)
3680            ev_run (EV_A_ EVRUN_ONCE);
3681
3682       To exit from any of these loops, just set the corresponding exit
3683       variable:
3684
3685          // exit modal loop
3686          exit_nested_loop = 1;
3687
3688          // exit main program, after modal loop is finished
3689          exit_main_loop = 1;
3690
3691          // exit both
3692          exit_main_loop = exit_nested_loop = 1;
3693
3694   THREAD LOCKING EXAMPLE
3695       Here is a fictitious example of how to run an event loop in a different
3696       thread from where callbacks are being invoked and watchers are
3697       created/added/removed.
3698
3699       For a real-world example, see the "EV::Loop::Async" perl module, which
3700       uses exactly this technique (which is suited for many high-level
3701       languages).
3702
3703       The example uses a pthread mutex to protect the loop data, a condition
3704       variable to wait for callback invocations, an async watcher to notify
3705       the event loop thread and an unspecified mechanism to wake up the main
3706       thread.
3707
3708       First, you need to associate some data with the event loop:
3709
3710          typedef struct {
3711            mutex_t lock; /* global loop lock */
3712            ev_async async_w;
3713            thread_t tid;
3714            cond_t invoke_cv;
3715          } userdata;
3716
3717          void prepare_loop (EV_P)
3718          {
3719             // for simplicity, we use a static userdata struct.
3720             static userdata u;
3721
3722             ev_async_init (&u->async_w, async_cb);
3723             ev_async_start (EV_A_ &u->async_w);
3724
3725             pthread_mutex_init (&u->lock, 0);
3726             pthread_cond_init (&u->invoke_cv, 0);
3727
3728             // now associate this with the loop
3729             ev_set_userdata (EV_A_ u);
3730             ev_set_invoke_pending_cb (EV_A_ l_invoke);
3731             ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
3732
3733             // then create the thread running ev_run
3734             pthread_create (&u->tid, 0, l_run, EV_A);
3735          }
3736
3737       The callback for the "ev_async" watcher does nothing: the watcher is
3738       used solely to wake up the event loop so it takes notice of any new
3739       watchers that might have been added:
3740
3741          static void
3742          async_cb (EV_P_ ev_async *w, int revents)
3743          {
3744             // just used for the side effects
3745          }
3746
3747       The "l_release" and "l_acquire" callbacks simply unlock/lock the mutex
3748       protecting the loop data, respectively.
3749
3750          static void
3751          l_release (EV_P)
3752          {
3753            userdata *u = ev_userdata (EV_A);
3754            pthread_mutex_unlock (&u->lock);
3755          }
3756
3757          static void
3758          l_acquire (EV_P)
3759          {
3760            userdata *u = ev_userdata (EV_A);
3761            pthread_mutex_lock (&u->lock);
3762          }
3763
3764       The event loop thread first acquires the mutex, and then jumps straight
3765       into "ev_run":
3766
3767          void *
3768          l_run (void *thr_arg)
3769          {
3770            struct ev_loop *loop = (struct ev_loop *)thr_arg;
3771
3772            l_acquire (EV_A);
3773            pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
3774            ev_run (EV_A_ 0);
3775            l_release (EV_A);
3776
3777            return 0;
3778          }
3779
3780       Instead of invoking all pending watchers, the "l_invoke" callback will
3781       signal the main thread via some unspecified mechanism (signals? pipe
3782       writes? "Async::Interrupt"?) and then waits until all pending watchers
3783       have been called (in a while loop because a) spurious wakeups are
3784       possible and b) skipping inter-thread-communication when there are no
3785       pending watchers is very beneficial):
3786
3787          static void
3788          l_invoke (EV_P)
3789          {
3790            userdata *u = ev_userdata (EV_A);
3791
3792            while (ev_pending_count (EV_A))
3793              {
3794                wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
3795                pthread_cond_wait (&u->invoke_cv, &u->lock);
3796              }
3797          }
3798
3799       Now, whenever the main thread gets told to invoke pending watchers, it
3800       will grab the lock, call "ev_invoke_pending" and then signal the loop
3801       thread to continue:
3802
3803          static void
3804          real_invoke_pending (EV_P)
3805          {
3806            userdata *u = ev_userdata (EV_A);
3807
3808            pthread_mutex_lock (&u->lock);
3809            ev_invoke_pending (EV_A);
3810            pthread_cond_signal (&u->invoke_cv);
3811            pthread_mutex_unlock (&u->lock);
3812          }
3813
3814       Whenever you want to start/stop a watcher or do other modifications to
3815       an event loop, you will now have to lock:
3816
3817          ev_timer timeout_watcher;
3818          userdata *u = ev_userdata (EV_A);
3819
3820          ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
3821
3822          pthread_mutex_lock (&u->lock);
3823          ev_timer_start (EV_A_ &timeout_watcher);
3824          ev_async_send (EV_A_ &u->async_w);
3825          pthread_mutex_unlock (&u->lock);
3826
3827       Note that sending the "ev_async" watcher is required because otherwise
3828       an event loop currently blocking in the kernel will have no knowledge
3829       about the newly added timer. By waking up the loop it will pick up any
3830       new watchers in the next event loop iteration.
3831
3832   THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS
3833       While the overhead of a callback that e.g. schedules a thread is small,
3834       it is still an overhead. If you embed libev, and your main usage is
3835       with some kind of threads or coroutines, you might want to customise
3836       libev so that doesn't need callbacks anymore.
3837
3838       Imagine you have coroutines that you can switch to using a function
3839       "switch_to (coro)", that libev runs in a coroutine called "libev_coro"
3840       and that due to some magic, the currently active coroutine is stored in
3841       a global called "current_coro". Then you can build your own "wait for
3842       libev event" primitive by changing "EV_CB_DECLARE" and "EV_CB_INVOKE"
3843       (note the differing ";" conventions):
3844
3845          #define EV_CB_DECLARE(type)   struct my_coro *cb;
3846          #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
3847
3848       That means instead of having a C callback function, you store the
3849       coroutine to switch to in each watcher, and instead of having libev
3850       call your callback, you instead have it switch to that coroutine.
3851
3852       A coroutine might now wait for an event with a function called
3853       "wait_for_event". (the watcher needs to be started, as always, but it
3854       doesn't matter when, or whether the watcher is active or not when this
3855       function is called):
3856
3857          void
3858          wait_for_event (ev_watcher *w)
3859          {
3860            ev_set_cb (w, current_coro);
3861            switch_to (libev_coro);
3862          }
3863
3864       That basically suspends the coroutine inside "wait_for_event" and
3865       continues the libev coroutine, which, when appropriate, switches back
3866       to this or any other coroutine.
3867
3868       You can do similar tricks if you have, say, threads with an event queue
3869       - instead of storing a coroutine, you store the queue object and
3870       instead of switching to a coroutine, you push the watcher onto the
3871       queue and notify any waiters.
3872
3873       To embed libev, see "EMBEDDING", but in short, it's easiest to create
3874       two files, my_ev.h and my_ev.c that include the respective libev files:
3875
3876          // my_ev.h
3877          #define EV_CB_DECLARE(type)   struct my_coro *cb;
3878          #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
3879          #include "../libev/ev.h"
3880
3881          // my_ev.c
3882          #define EV_H "my_ev.h"
3883          #include "../libev/ev.c"
3884
3885       And then use my_ev.h when you would normally use ev.h, and compile
3886       my_ev.c into your project. When properly specifying include paths, you
3887       can even use ev.h as header file name directly.
3888

LIBEVENT EMULATION

3890       Libev offers a compatibility emulation layer for libevent. It cannot
3891       emulate the internals of libevent, so here are some usage hints:
3892
3893       ·   Only the libevent-1.4.1-beta API is being emulated.
3894
3895           This was the newest libevent version available when libev was
3896           implemented, and is still mostly unchanged in 2010.
3897
3898       ·   Use it by including <event.h>, as usual.
3899
3900       ·   The following members are fully supported: ev_base, ev_callback,
3901           ev_arg, ev_fd, ev_res, ev_events.
3902
3903       ·   Avoid using ev_flags and the EVLIST_*-macros, while it is
3904           maintained by libev, it does not work exactly the same way as in
3905           libevent (consider it a private API).
3906
3907       ·   Priorities are not currently supported. Initialising priorities
3908           will fail and all watchers will have the same priority, even though
3909           there is an ev_pri field.
3910
3911       ·   In libevent, the last base created gets the signals, in libev, the
3912           base that registered the signal gets the signals.
3913
3914       ·   Other members are not supported.
3915
3916       ·   The libev emulation is not ABI compatible to libevent, you need to
3917           use the libev header file and library.
3918

C++ SUPPORT

3920   C API
3921       The normal C API should work fine when used from C++: both ev.h and the
3922       libev sources can be compiled as C++. Therefore, code that uses the C
3923       API will work fine.
3924
3925       Proper exception specifications might have to be added to callbacks
3926       passed to libev: exceptions may be thrown only from watcher callbacks,
3927       all other callbacks (allocator, syserr, loop acquire/release and
3928       periodic reschedule callbacks) must not throw exceptions, and might
3929       need a "noexcept" specification. If you have code that needs to be
3930       compiled as both C and C++ you can use the "EV_NOEXCEPT" macro for
3931       this:
3932
3933          static void
3934          fatal_error (const char *msg) EV_NOEXCEPT
3935          {
3936            perror (msg);
3937            abort ();
3938          }
3939
3940          ...
3941          ev_set_syserr_cb (fatal_error);
3942
3943       The only API functions that can currently throw exceptions are
3944       "ev_run", "ev_invoke", "ev_invoke_pending" and "ev_loop_destroy" (the
3945       latter because it runs cleanup watchers).
3946
3947       Throwing exceptions in watcher callbacks is only supported if libev
3948       itself is compiled with a C++ compiler or your C and C++ environments
3949       allow throwing exceptions through C libraries (most do).
3950
3951   C++ API
3952       Libev comes with some simplistic wrapper classes for C++ that mainly
3953       allow you to use some convenience methods to start/stop watchers and
3954       also change the callback model to a model using method callbacks on
3955       objects.
3956
3957       To use it,
3958
3959          #include <ev++.h>
3960
3961       This automatically includes ev.h and puts all of its definitions (many
3962       of them macros) into the global namespace. All C++ specific things are
3963       put into the "ev" namespace. It should support all the same embedding
3964       options as ev.h, most notably "EV_MULTIPLICITY".
3965
3966       Care has been taken to keep the overhead low. The only data member the
3967       C++ classes add (compared to plain C-style watchers) is the event loop
3968       pointer that the watcher is associated with (or no additional members
3969       at all if you disable "EV_MULTIPLICITY" when embedding libev).
3970
3971       Currently, functions, static and non-static member functions and
3972       classes with "operator ()" can be used as callbacks. Other types should
3973       be easy to add as long as they only need one additional pointer for
3974       context. If you need support for other types of functors please contact
3975       the author (preferably after implementing it).
3976
3977       For all this to work, your C++ compiler either has to use the same
3978       calling conventions as your C compiler (for static member functions),
3979       or you have to embed libev and compile libev itself as C++.
3980
3981       Here is a list of things available in the "ev" namespace:
3982
3983       "ev::READ", "ev::WRITE" etc.
3984           These are just enum values with the same values as the "EV_READ"
3985           etc.  macros from ev.h.
3986
3987       "ev::tstamp", "ev::now"
3988           Aliases to the same types/functions as with the "ev_" prefix.
3989
3990       "ev::io", "ev::timer", "ev::periodic", "ev::idle", "ev::sig" etc.
3991           For each "ev_TYPE" watcher in ev.h there is a corresponding class
3992           of the same name in the "ev" namespace, with the exception of
3993           "ev_signal" which is called "ev::sig" to avoid clashes with the
3994           "signal" macro defined by many implementations.
3995
3996           All of those classes have these methods:
3997
3998           ev::TYPE::TYPE ()
3999           ev::TYPE::TYPE (loop)
4000           ev::TYPE::~TYPE
4001               The constructor (optionally) takes an event loop to associate
4002               the watcher with. If it is omitted, it will use "EV_DEFAULT".
4003
4004               The constructor calls "ev_init" for you, which means you have
4005               to call the "set" method before starting it.
4006
4007               It will not set a callback, however: You have to call the
4008               templated "set" method to set a callback before you can start
4009               the watcher.
4010
4011               (The reason why you have to use a method is a limitation in C++
4012               which does not allow explicit template arguments for
4013               constructors).
4014
4015               The destructor automatically stops the watcher if it is active.
4016
4017           w->set<class, &class::method> (object *)
4018               This method sets the callback method to call. The method has to
4019               have a signature of "void (*)(ev_TYPE &, int)", it receives the
4020               watcher as first argument and the "revents" as second. The
4021               object must be given as parameter and is stored in the "data"
4022               member of the watcher.
4023
4024               This method synthesizes efficient thunking code to call your
4025               method from the C callback that libev requires. If your
4026               compiler can inline your callback (i.e. it is visible to it at
4027               the place of the "set" call and your compiler is good :), then
4028               the method will be fully inlined into the thunking function,
4029               making it as fast as a direct C callback.
4030
4031               Example: simple class declaration and watcher initialisation
4032
4033                  struct myclass
4034                  {
4035                    void io_cb (ev::io &w, int revents) { }
4036                  }
4037
4038                  myclass obj;
4039                  ev::io iow;
4040                  iow.set <myclass, &myclass::io_cb> (&obj);
4041
4042           w->set (object *)
4043               This is a variation of a method callback - leaving out the
4044               method to call will default the method to "operator ()", which
4045               makes it possible to use functor objects without having to
4046               manually specify the "operator ()" all the time. Incidentally,
4047               you can then also leave out the template argument list.
4048
4049               The "operator ()" method prototype must be "void operator
4050               ()(watcher &w, int revents)".
4051
4052               See the method-"set" above for more details.
4053
4054               Example: use a functor object as callback.
4055
4056                  struct myfunctor
4057                  {
4058                    void operator() (ev::io &w, int revents)
4059                    {
4060                      ...
4061                    }
4062                  }
4063
4064                  myfunctor f;
4065
4066                  ev::io w;
4067                  w.set (&f);
4068
4069           w->set<function> (void *data = 0)
4070               Also sets a callback, but uses a static method or plain
4071               function as callback. The optional "data" argument will be
4072               stored in the watcher's "data" member and is free for you to
4073               use.
4074
4075               The prototype of the "function" must be "void (*)(ev::TYPE &w,
4076               int)".
4077
4078               See the method-"set" above for more details.
4079
4080               Example: Use a plain function as callback.
4081
4082                  static void io_cb (ev::io &w, int revents) { }
4083                  iow.set <io_cb> ();
4084
4085           w->set (loop)
4086               Associates a different "struct ev_loop" with this watcher. You
4087               can only do this when the watcher is inactive (and not pending
4088               either).
4089
4090           w->set ([arguments])
4091               Basically the same as "ev_TYPE_set" (except for "ev::embed"
4092               watchers>), with the same arguments. Either this method or a
4093               suitable start method must be called at least once. Unlike the
4094               C counterpart, an active watcher gets automatically stopped and
4095               restarted when reconfiguring it with this method.
4096
4097               For "ev::embed" watchers this method is called "set_embed", to
4098               avoid clashing with the "set (loop)" method.
4099
4100           w->start ()
4101               Starts the watcher. Note that there is no "loop" argument, as
4102               the constructor already stores the event loop.
4103
4104           w->start ([arguments])
4105               Instead of calling "set" and "start" methods separately, it is
4106               often convenient to wrap them in one call. Uses the same type
4107               of arguments as the configure "set" method of the watcher.
4108
4109           w->stop ()
4110               Stops the watcher if it is active. Again, no "loop" argument.
4111
4112           w->again () ("ev::timer", "ev::periodic" only)
4113               For "ev::timer" and "ev::periodic", this invokes the
4114               corresponding "ev_TYPE_again" function.
4115
4116           w->sweep () ("ev::embed" only)
4117               Invokes "ev_embed_sweep".
4118
4119           w->update () ("ev::stat" only)
4120               Invokes "ev_stat_stat".
4121
4122       Example: Define a class with two I/O and idle watchers, start the I/O
4123       watchers in the constructor.
4124
4125          class myclass
4126          {
4127            ev::io   io  ; void io_cb   (ev::io   &w, int revents);
4128            ev::io   io2 ; void io2_cb  (ev::io   &w, int revents);
4129            ev::idle idle; void idle_cb (ev::idle &w, int revents);
4130
4131            myclass (int fd)
4132            {
4133              io  .set <myclass, &myclass::io_cb  > (this);
4134              io2 .set <myclass, &myclass::io2_cb > (this);
4135              idle.set <myclass, &myclass::idle_cb> (this);
4136
4137              io.set (fd, ev::WRITE); // configure the watcher
4138              io.start ();            // start it whenever convenient
4139
4140              io2.start (fd, ev::READ); // set + start in one call
4141            }
4142          };
4143

OTHER LANGUAGE BINDINGS

4145       Libev does not offer other language bindings itself, but bindings for a
4146       number of languages exist in the form of third-party packages. If you
4147       know any interesting language binding in addition to the ones listed
4148       here, drop me a note.
4149
4150       Perl
4151           The EV module implements the full libev API and is actually used to
4152           test libev. EV is developed together with libev. Apart from the EV
4153           core module, there are additional modules that implement libev-
4154           compatible interfaces to "libadns" ("EV::ADNS", but "AnyEvent::DNS"
4155           is preferred nowadays), "Net::SNMP" ("Net::SNMP::EV") and the
4156           "libglib" event core ("Glib::EV" and "EV::Glib").
4157
4158           It can be found and installed via CPAN, its homepage is at
4159           <http://software.schmorp.de/pkg/EV>.
4160
4161       Python
4162           Python bindings can be found at <http://code.google.com/p/pyev/>.
4163           It seems to be quite complete and well-documented.
4164
4165       Ruby
4166           Tony Arcieri has written a ruby extension that offers access to a
4167           subset of the libev API and adds file handle abstractions,
4168           asynchronous DNS and more on top of it. It can be found via gem
4169           servers. Its homepage is at <http://rev.rubyforge.org/>.
4170
4171           Roger Pack reports that using the link order "-lws2_32
4172           -lmsvcrt-ruby-190" makes rev work even on mingw.
4173
4174       Haskell
4175           A haskell binding to libev is available at
4176           <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>.
4177
4178       D   Leandro Lucarella has written a D language binding (ev.d) for
4179           libev, to be found at
4180           <http://www.llucax.com.ar/proj/ev.d/index.html>.
4181
4182       Ocaml
4183           Erkki Seppala has written Ocaml bindings for libev, to be found at
4184           <http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.
4185
4186       Lua Brian Maher has written a partial interface to libev for lua (at
4187           the time of this writing, only "ev_io" and "ev_timer"), to be found
4188           at <http://github.com/brimworks/lua-ev>.
4189
4190       Javascript
4191           Node.js (<http://nodejs.org>) uses libev as the underlying event
4192           library.
4193
4194       Others
4195           There are others, and I stopped counting.
4196

MACRO MAGIC

4198       Libev can be compiled with a variety of options, the most fundamental
4199       of which is "EV_MULTIPLICITY". This option determines whether (most)
4200       functions and callbacks have an initial "struct ev_loop *" argument.
4201
4202       To make it easier to write programs that cope with either variant, the
4203       following macros are defined:
4204
4205       "EV_A", "EV_A_"
4206           This provides the loop argument for functions, if one is required
4207           ("ev loop argument"). The "EV_A" form is used when this is the sole
4208           argument, "EV_A_" is used when other arguments are following.
4209           Example:
4210
4211              ev_unref (EV_A);
4212              ev_timer_add (EV_A_ watcher);
4213              ev_run (EV_A_ 0);
4214
4215           It assumes the variable "loop" of type "struct ev_loop *" is in
4216           scope, which is often provided by the following macro.
4217
4218       "EV_P", "EV_P_"
4219           This provides the loop parameter for functions, if one is required
4220           ("ev loop parameter"). The "EV_P" form is used when this is the
4221           sole parameter, "EV_P_" is used when other parameters are
4222           following. Example:
4223
4224              // this is how ev_unref is being declared
4225              static void ev_unref (EV_P);
4226
4227              // this is how you can declare your typical callback
4228              static void cb (EV_P_ ev_timer *w, int revents)
4229
4230           It declares a parameter "loop" of type "struct ev_loop *", quite
4231           suitable for use with "EV_A".
4232
4233       "EV_DEFAULT", "EV_DEFAULT_"
4234           Similar to the other two macros, this gives you the value of the
4235           default loop, if multiple loops are supported ("ev loop default").
4236           The default loop will be initialised if it isn't already
4237           initialised.
4238
4239           For non-multiplicity builds, these macros do nothing, so you always
4240           have to initialise the loop somewhere.
4241
4242       "EV_DEFAULT_UC", "EV_DEFAULT_UC_"
4243           Usage identical to "EV_DEFAULT" and "EV_DEFAULT_", but requires
4244           that the default loop has been initialised ("UC" == unchecked).
4245           Their behaviour is undefined when the default loop has not been
4246           initialised by a previous execution of "EV_DEFAULT", "EV_DEFAULT_"
4247           or "ev_default_init (...)".
4248
4249           It is often prudent to use "EV_DEFAULT" when initialising the first
4250           watcher in a function but use "EV_DEFAULT_UC" afterwards.
4251
4252       Example: Declare and initialise a check watcher, utilising the above
4253       macros so it will work regardless of whether multiple loops are
4254       supported or not.
4255
4256          static void
4257          check_cb (EV_P_ ev_timer *w, int revents)
4258          {
4259            ev_check_stop (EV_A_ w);
4260          }
4261
4262          ev_check check;
4263          ev_check_init (&check, check_cb);
4264          ev_check_start (EV_DEFAULT_ &check);
4265          ev_run (EV_DEFAULT_ 0);
4266

EMBEDDING

4268       Libev can (and often is) directly embedded into host applications.
4269       Examples of applications that embed it include the Deliantra Game
4270       Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe) and
4271       rxvt-unicode.
4272
4273       The goal is to enable you to just copy the necessary files into your
4274       source directory without having to change even a single line in them,
4275       so you can easily upgrade by simply copying (or having a checked-out
4276       copy of libev somewhere in your source tree).
4277
4278   FILESETS
4279       Depending on what features you need you need to include one or more
4280       sets of files in your application.
4281
4282       CORE EVENT LOOP
4283
4284       To include only the libev core (all the "ev_*" functions), with manual
4285       configuration (no autoconf):
4286
4287          #define EV_STANDALONE 1
4288          #include "ev.c"
4289
4290       This will automatically include ev.h, too, and should be done in a
4291       single C source file only to provide the function implementations. To
4292       use it, do the same for ev.h in all files wishing to use this API (best
4293       done by writing a wrapper around ev.h that you can include instead and
4294       where you can put other configuration options):
4295
4296          #define EV_STANDALONE 1
4297          #include "ev.h"
4298
4299       Both header files and implementation files can be compiled with a C++
4300       compiler (at least, that's a stated goal, and breakage will be treated
4301       as a bug).
4302
4303       You need the following files in your source tree, or in a directory in
4304       your include path (e.g. in libev/ when using -Ilibev):
4305
4306          ev.h
4307          ev.c
4308          ev_vars.h
4309          ev_wrap.h
4310
4311          ev_win32.c      required on win32 platforms only
4312
4313          ev_select.c     only when select backend is enabled
4314          ev_poll.c       only when poll backend is enabled
4315          ev_epoll.c      only when the epoll backend is enabled
4316          ev_linuxaio.c   only when the linux aio backend is enabled
4317          ev_iouring.c    only when the linux io_uring backend is enabled
4318          ev_kqueue.c     only when the kqueue backend is enabled
4319          ev_port.c       only when the solaris port backend is enabled
4320
4321       ev.c includes the backend files directly when enabled, so you only need
4322       to compile this single file.
4323
4324       LIBEVENT COMPATIBILITY API
4325
4326       To include the libevent compatibility API, also include:
4327
4328          #include "event.c"
4329
4330       in the file including ev.c, and:
4331
4332          #include "event.h"
4333
4334       in the files that want to use the libevent API. This also includes
4335       ev.h.
4336
4337       You need the following additional files for this:
4338
4339          event.h
4340          event.c
4341
4342       AUTOCONF SUPPORT
4343
4344       Instead of using "EV_STANDALONE=1" and providing your configuration in
4345       whatever way you want, you can also "m4_include([libev.m4])" in your
4346       configure.ac and leave "EV_STANDALONE" undefined. ev.c will then
4347       include config.h and configure itself accordingly.
4348
4349       For this of course you need the m4 file:
4350
4351          libev.m4
4352
4353   PREPROCESSOR SYMBOLS/MACROS
4354       Libev can be configured via a variety of preprocessor symbols you have
4355       to define before including (or compiling) any of its files. The default
4356       in the absence of autoconf is documented for every option.
4357
4358       Symbols marked with "(h)" do not change the ABI, and can have different
4359       values when compiling libev vs. including ev.h, so it is permissible to
4360       redefine them before including ev.h without breaking compatibility to a
4361       compiled library. All other symbols change the ABI, which means all
4362       users of libev and the libev code itself must be compiled with
4363       compatible settings.
4364
4365       EV_COMPAT3 (h)
4366           Backwards compatibility is a major concern for libev. This is why
4367           this release of libev comes with wrappers for the functions and
4368           symbols that have been renamed between libev version 3 and 4.
4369
4370           You can disable these wrappers (to test compatibility with future
4371           versions) by defining "EV_COMPAT3" to 0 when compiling your
4372           sources. This has the additional advantage that you can drop the
4373           "struct" from "struct ev_loop" declarations, as libev will provide
4374           an "ev_loop" typedef in that case.
4375
4376           In some future version, the default for "EV_COMPAT3" will become 0,
4377           and in some even more future version the compatibility code will be
4378           removed completely.
4379
4380       EV_STANDALONE (h)
4381           Must always be 1 if you do not use autoconf configuration, which
4382           keeps libev from including config.h, and it also defines dummy
4383           implementations for some libevent functions (such as logging, which
4384           is not supported). It will also not define any of the structs
4385           usually found in event.h that are not directly supported by the
4386           libev core alone.
4387
4388           In standalone mode, libev will still try to automatically deduce
4389           the configuration, but has to be more conservative.
4390
4391       EV_USE_FLOOR
4392           If defined to be 1, libev will use the "floor ()" function for its
4393           periodic reschedule calculations, otherwise libev will fall back on
4394           a portable (slower) implementation. If you enable this, you usually
4395           have to link against libm or something equivalent. Enabling this
4396           when the "floor" function is not available will fail, so the safe
4397           default is to not enable this.
4398
4399       EV_USE_MONOTONIC
4400           If defined to be 1, libev will try to detect the availability of
4401           the monotonic clock option at both compile time and runtime.
4402           Otherwise no use of the monotonic clock option will be attempted.
4403           If you enable this, you usually have to link against librt or
4404           something similar. Enabling it when the functionality isn't
4405           available is safe, though, although you have to make sure you link
4406           against any libraries where the "clock_gettime" function is hiding
4407           in (often -lrt). See also "EV_USE_CLOCK_SYSCALL".
4408
4409       EV_USE_REALTIME
4410           If defined to be 1, libev will try to detect the availability of
4411           the real-time clock option at compile time (and assume its
4412           availability at runtime if successful). Otherwise no use of the
4413           real-time clock option will be attempted. This effectively replaces
4414           "gettimeofday" by "clock_get (CLOCK_REALTIME, ...)" and will not
4415           normally affect correctness. See the note about libraries in the
4416           description of "EV_USE_MONOTONIC", though. Defaults to the opposite
4417           value of "EV_USE_CLOCK_SYSCALL".
4418
4419       EV_USE_CLOCK_SYSCALL
4420           If defined to be 1, libev will try to use a direct syscall instead
4421           of calling the system-provided "clock_gettime" function. This
4422           option exists because on GNU/Linux, "clock_gettime" is in "librt",
4423           but "librt" unconditionally pulls in "libpthread", slowing down
4424           single-threaded programs needlessly. Using a direct syscall is
4425           slightly slower (in theory), because no optimised vdso
4426           implementation can be used, but avoids the pthread dependency.
4427           Defaults to 1 on GNU/Linux with glibc 2.x or higher, as it
4428           simplifies linking (no need for "-lrt").
4429
4430       EV_USE_NANOSLEEP
4431           If defined to be 1, libev will assume that "nanosleep ()" is
4432           available and will use it for delays. Otherwise it will use "select
4433           ()".
4434
4435       EV_USE_EVENTFD
4436           If defined to be 1, then libev will assume that "eventfd ()" is
4437           available and will probe for kernel support at runtime. This will
4438           improve "ev_signal" and "ev_async" performance and reduce resource
4439           consumption.  If undefined, it will be enabled if the headers
4440           indicate GNU/Linux + Glibc 2.7 or newer, otherwise disabled.
4441
4442       EV_USE_SIGNALFD
4443           If defined to be 1, then libev will assume that "signalfd ()" is
4444           available and will probe for kernel support at runtime. This
4445           enables the use of EVFLAG_SIGNALFD for faster and simpler signal
4446           handling. If undefined, it will be enabled if the headers indicate
4447           GNU/Linux + Glibc 2.7 or newer, otherwise disabled.
4448
4449       EV_USE_TIMERFD
4450           If defined to be 1, then libev will assume that "timerfd ()" is
4451           available and will probe for kernel support at runtime. This allows
4452           libev to detect time jumps accurately. If undefined, it will be
4453           enabled if the headers indicate GNU/Linux + Glibc 2.8 or newer and
4454           define "TFD_TIMER_CANCEL_ON_SET", otherwise disabled.
4455
4456       EV_USE_EVENTFD
4457           If defined to be 1, then libev will assume that "eventfd ()" is
4458           available and will probe for kernel support at runtime. This will
4459           improve "ev_signal" and "ev_async" performance and reduce resource
4460           consumption.  If undefined, it will be enabled if the headers
4461           indicate GNU/Linux + Glibc 2.7 or newer, otherwise disabled.
4462
4463       EV_USE_SELECT
4464           If undefined or defined to be 1, libev will compile in support for
4465           the "select"(2) backend. No attempt at auto-detection will be done:
4466           if no other method takes over, select will be it. Otherwise the
4467           select backend will not be compiled in.
4468
4469       EV_SELECT_USE_FD_SET
4470           If defined to 1, then the select backend will use the system
4471           "fd_set" structure. This is useful if libev doesn't compile due to
4472           a missing "NFDBITS" or "fd_mask" definition or it mis-guesses the
4473           bitset layout on exotic systems. This usually limits the range of
4474           file descriptors to some low limit such as 1024 or might have other
4475           limitations (winsocket only allows 64 sockets). The "FD_SETSIZE"
4476           macro, set before compilation, configures the maximum size of the
4477           "fd_set".
4478
4479       EV_SELECT_IS_WINSOCKET
4480           When defined to 1, the select backend will assume that
4481           select/socket/connect etc. don't understand file descriptors but
4482           wants osf handles on win32 (this is the case when the select to be
4483           used is the winsock select). This means that it will call
4484           "_get_osfhandle" on the fd to convert it to an OS handle.
4485           Otherwise, it is assumed that all these functions actually work on
4486           fds, even on win32. Should not be defined on non-win32 platforms.
4487
4488       EV_FD_TO_WIN32_HANDLE(fd)
4489           If "EV_SELECT_IS_WINSOCKET" is enabled, then libev needs a way to
4490           map file descriptors to socket handles. When not defining this
4491           symbol (the default), then libev will call "_get_osfhandle", which
4492           is usually correct. In some cases, programs use their own file
4493           descriptor management, in which case they can provide this function
4494           to map fds to socket handles.
4495
4496       EV_WIN32_HANDLE_TO_FD(handle)
4497           If "EV_SELECT_IS_WINSOCKET" then libev maps handles to file
4498           descriptors using the standard "_open_osfhandle" function. For
4499           programs implementing their own fd to handle mapping, overwriting
4500           this function makes it easier to do so. This can be done by
4501           defining this macro to an appropriate value.
4502
4503       EV_WIN32_CLOSE_FD(fd)
4504           If programs implement their own fd to handle mapping on win32, then
4505           this macro can be used to override the "close" function, useful to
4506           unregister file descriptors again. Note that the replacement
4507           function has to close the underlying OS handle.
4508
4509       EV_USE_WSASOCKET
4510           If defined to be 1, libev will use "WSASocket" to create its
4511           internal communication socket, which works better in some
4512           environments. Otherwise, the normal "socket" function will be used,
4513           which works better in other environments.
4514
4515       EV_USE_POLL
4516           If defined to be 1, libev will compile in support for the "poll"(2)
4517           backend. Otherwise it will be enabled on non-win32 platforms. It
4518           takes precedence over select.
4519
4520       EV_USE_EPOLL
4521           If defined to be 1, libev will compile in support for the Linux
4522           "epoll"(7) backend. Its availability will be detected at runtime,
4523           otherwise another method will be used as fallback. This is the
4524           preferred backend for GNU/Linux systems. If undefined, it will be
4525           enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer,
4526           otherwise disabled.
4527
4528       EV_USE_LINUXAIO
4529           If defined to be 1, libev will compile in support for the Linux aio
4530           backend ("EV_USE_EPOLL" must also be enabled). If undefined, it
4531           will be enabled on linux, otherwise disabled.
4532
4533       EV_USE_IOURING
4534           If defined to be 1, libev will compile in support for the Linux
4535           io_uring backend ("EV_USE_EPOLL" must also be enabled). Due to it's
4536           current limitations it has to be requested explicitly. If
4537           undefined, it will be enabled on linux, otherwise disabled.
4538
4539       EV_USE_KQUEUE
4540           If defined to be 1, libev will compile in support for the BSD style
4541           "kqueue"(2) backend. Its actual availability will be detected at
4542           runtime, otherwise another method will be used as fallback. This is
4543           the preferred backend for BSD and BSD-like systems, although on
4544           most BSDs kqueue only supports some types of fds correctly (the
4545           only platform we found that supports ptys for example was NetBSD),
4546           so kqueue might be compiled in, but not be used unless explicitly
4547           requested. The best way to use it is to find out whether kqueue
4548           supports your type of fd properly and use an embedded kqueue loop.
4549
4550       EV_USE_PORT
4551           If defined to be 1, libev will compile in support for the Solaris
4552           10 port style backend. Its availability will be detected at
4553           runtime, otherwise another method will be used as fallback. This is
4554           the preferred backend for Solaris 10 systems.
4555
4556       EV_USE_DEVPOLL
4557           Reserved for future expansion, works like the USE symbols above.
4558
4559       EV_USE_INOTIFY
4560           If defined to be 1, libev will compile in support for the Linux
4561           inotify interface to speed up "ev_stat" watchers. Its actual
4562           availability will be detected at runtime. If undefined, it will be
4563           enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer,
4564           otherwise disabled.
4565
4566       EV_NO_SMP
4567           If defined to be 1, libev will assume that memory is always
4568           coherent between threads, that is, threads can be used, but threads
4569           never run on different cpus (or different cpu cores). This reduces
4570           dependencies and makes libev faster.
4571
4572       EV_NO_THREADS
4573           If defined to be 1, libev will assume that it will never be called
4574           from different threads (that includes signal handlers), which is a
4575           stronger assumption than "EV_NO_SMP", above. This reduces
4576           dependencies and makes libev faster.
4577
4578       EV_ATOMIC_T
4579           Libev requires an integer type (suitable for storing 0 or 1) whose
4580           access is atomic with respect to other threads or signal contexts.
4581           No such type is easily found in the C language, so you can provide
4582           your own type that you know is safe for your purposes. It is used
4583           both for signal handler "locking" as well as for signal and thread
4584           safety in "ev_async" watchers.
4585
4586           In the absence of this define, libev will use "sig_atomic_t
4587           volatile" (from signal.h), which is usually good enough on most
4588           platforms.
4589
4590       EV_H (h)
4591           The name of the ev.h header file used to include it. The default if
4592           undefined is "ev.h" in event.h, ev.c and ev++.h. This can be used
4593           to virtually rename the ev.h header file in case of conflicts.
4594
4595       EV_CONFIG_H (h)
4596           If "EV_STANDALONE" isn't 1, this variable can be used to override
4597           ev.c's idea of where to find the config.h file, similarly to
4598           "EV_H", above.
4599
4600       EV_EVENT_H (h)
4601           Similarly to "EV_H", this macro can be used to override event.c's
4602           idea of how the event.h header can be found, the default is
4603           "event.h".
4604
4605       EV_PROTOTYPES (h)
4606           If defined to be 0, then ev.h will not define any function
4607           prototypes, but still define all the structs and other symbols.
4608           This is occasionally useful if you want to provide your own wrapper
4609           functions around libev functions.
4610
4611       EV_MULTIPLICITY
4612           If undefined or defined to 1, then all event-loop-specific
4613           functions will have the "struct ev_loop *" as first argument, and
4614           you can create additional independent event loops. Otherwise there
4615           will be no support for multiple event loops and there is no first
4616           event loop pointer argument. Instead, all functions act on the
4617           single default loop.
4618
4619           Note that "EV_DEFAULT" and "EV_DEFAULT_" will no longer provide a
4620           default loop when multiplicity is switched off - you always have to
4621           initialise the loop manually in this case.
4622
4623       EV_MINPRI
4624       EV_MAXPRI
4625           The range of allowed priorities. "EV_MINPRI" must be smaller or
4626           equal to "EV_MAXPRI", but otherwise there are no non-obvious
4627           limitations. You can provide for more priorities by overriding
4628           those symbols (usually defined to be "-2" and 2, respectively).
4629
4630           When doing priority-based operations, libev usually has to linearly
4631           search all the priorities, so having many of them (hundreds) uses a
4632           lot of space and time, so using the defaults of five priorities (-2
4633           .. +2) is usually fine.
4634
4635           If your embedding application does not need any priorities,
4636           defining these both to 0 will save some memory and CPU.
4637
4638       EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE,
4639       EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE,
4640       EV_ASYNC_ENABLE, EV_CHILD_ENABLE.
4641           If undefined or defined to be 1 (and the platform supports it),
4642           then the respective watcher type is supported. If defined to be 0,
4643           then it is not. Disabling watcher types mainly saves code size.
4644
4645       EV_FEATURES
4646           If you need to shave off some kilobytes of code at the expense of
4647           some speed (but with the full API), you can define this symbol to
4648           request certain subsets of functionality. The default is to enable
4649           all features that can be enabled on the platform.
4650
4651           A typical way to use this symbol is to define it to 0 (or to a
4652           bitset with some broad features you want) and then selectively re-
4653           enable additional parts you want, for example if you want
4654           everything minimal, but multiple event loop support, async and
4655           child watchers and the poll backend, use this:
4656
4657              #define EV_FEATURES 0
4658              #define EV_MULTIPLICITY 1
4659              #define EV_USE_POLL 1
4660              #define EV_CHILD_ENABLE 1
4661              #define EV_ASYNC_ENABLE 1
4662
4663           The actual value is a bitset, it can be a combination of the
4664           following values (by default, all of these are enabled):
4665
4666           1 - faster/larger code
4667               Use larger code to speed up some operations.
4668
4669               Currently this is used to override some inlining decisions
4670               (enlarging the code size by roughly 30% on amd64).
4671
4672               When optimising for size, use of compiler flags such as "-Os"
4673               with gcc is recommended, as well as "-DNDEBUG", as libev
4674               contains a number of assertions.
4675
4676               The default is off when "__OPTIMIZE_SIZE__" is defined by your
4677               compiler (e.g. gcc with "-Os").
4678
4679           2 - faster/larger data structures
4680               Replaces the small 2-heap for timer management by a faster
4681               4-heap, larger hash table sizes and so on. This will usually
4682               further increase code size and can additionally have an effect
4683               on the size of data structures at runtime.
4684
4685               The default is off when "__OPTIMIZE_SIZE__" is defined by your
4686               compiler (e.g. gcc with "-Os").
4687
4688           4 - full API configuration
4689               This enables priorities (sets "EV_MAXPRI"=2 and
4690               "EV_MINPRI"=-2), and enables multiplicity
4691               ("EV_MULTIPLICITY"=1).
4692
4693           8 - full API
4694               This enables a lot of the "lesser used" API functions. See
4695               "ev.h" for details on which parts of the API are still
4696               available without this feature, and do not complain if this
4697               subset changes over time.
4698
4699           16 - enable all optional watcher types
4700               Enables all optional watcher types.  If you want to selectively
4701               enable only some watcher types other than I/O and timers (e.g.
4702               prepare, embed, async, child...) you can enable them manually
4703               by defining "EV_watchertype_ENABLE" to 1 instead.
4704
4705           32 - enable all backends
4706               This enables all backends - without this feature, you need to
4707               enable at least one backend manually ("EV_USE_SELECT" is a good
4708               choice).
4709
4710           64 - enable OS-specific "helper" APIs
4711               Enable inotify, eventfd, signalfd and similar OS-specific
4712               helper APIs by default.
4713
4714           Compiling with "gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1
4715           -DEV_FEATURES=0" reduces the compiled size of libev from 24.7Kb
4716           code/2.8Kb data to 6.5Kb code/0.3Kb data on my GNU/Linux amd64
4717           system, while still giving you I/O watchers, timers and monotonic
4718           clock support.
4719
4720           With an intelligent-enough linker (gcc+binutils are intelligent
4721           enough when you use "-Wl,--gc-sections -ffunction-sections")
4722           functions unused by your program might be left out as well - a
4723           binary starting a timer and an I/O watcher then might come out at
4724           only 5Kb.
4725
4726       EV_API_STATIC
4727           If this symbol is defined (by default it is not), then all
4728           identifiers will have static linkage. This means that libev will
4729           not export any identifiers, and you cannot link against libev
4730           anymore. This can be useful when you embed libev, only want to use
4731           libev functions in a single file, and do not want its identifiers
4732           to be visible.
4733
4734           To use this, define "EV_API_STATIC" and include ev.c in the file
4735           that wants to use libev.
4736
4737           This option only works when libev is compiled with a C compiler, as
4738           C++ doesn't support the required declaration syntax.
4739
4740       EV_AVOID_STDIO
4741           If this is set to 1 at compiletime, then libev will avoid using
4742           stdio functions (printf, scanf, perror etc.). This will increase
4743           the code size somewhat, but if your program doesn't otherwise
4744           depend on stdio and your libc allows it, this avoids linking in the
4745           stdio library which is quite big.
4746
4747           Note that error messages might become less precise when this option
4748           is enabled.
4749
4750       EV_NSIG
4751           The highest supported signal number, +1 (or, the number of
4752           signals): Normally, libev tries to deduce the maximum number of
4753           signals automatically, but sometimes this fails, in which case it
4754           can be specified. Also, using a lower number than detected (32
4755           should be good for about any system in existence) can save some
4756           memory, as libev statically allocates some 12-24 bytes per signal
4757           number.
4758
4759       EV_PID_HASHSIZE
4760           "ev_child" watchers use a small hash table to distribute workload
4761           by pid. The default size is 16 (or 1 with "EV_FEATURES" disabled),
4762           usually more than enough. If you need to manage thousands of
4763           children you might want to increase this value (must be a power of
4764           two).
4765
4766       EV_INOTIFY_HASHSIZE
4767           "ev_stat" watchers use a small hash table to distribute workload by
4768           inotify watch id. The default size is 16 (or 1 with "EV_FEATURES"
4769           disabled), usually more than enough. If you need to manage
4770           thousands of "ev_stat" watchers you might want to increase this
4771           value (must be a power of two).
4772
4773       EV_USE_4HEAP
4774           Heaps are not very cache-efficient. To improve the cache-efficiency
4775           of the timer and periodics heaps, libev uses a 4-heap when this
4776           symbol is defined to 1. The 4-heap uses more complicated (longer)
4777           code but has noticeably faster performance with many (thousands) of
4778           watchers.
4779
4780           The default is 1, unless "EV_FEATURES" overrides it, in which case
4781           it will be 0.
4782
4783       EV_HEAP_CACHE_AT
4784           Heaps are not very cache-efficient. To improve the cache-efficiency
4785           of the timer and periodics heaps, libev can cache the timestamp
4786           (at) within the heap structure (selected by defining
4787           "EV_HEAP_CACHE_AT" to 1), which uses 8-12 bytes more per watcher
4788           and a few hundred bytes more code, but avoids random read accesses
4789           on heap changes. This improves performance noticeably with many
4790           (hundreds) of watchers.
4791
4792           The default is 1, unless "EV_FEATURES" overrides it, in which case
4793           it will be 0.
4794
4795       EV_VERIFY
4796           Controls how much internal verification (see "ev_verify ()") will
4797           be done: If set to 0, no internal verification code will be
4798           compiled in. If set to 1, then verification code will be compiled
4799           in, but not called. If set to 2, then the internal verification
4800           code will be called once per loop, which can slow down libev. If
4801           set to 3, then the verification code will be called very
4802           frequently, which will slow down libev considerably.
4803
4804           Verification errors are reported via C's "assert" mechanism, so if
4805           you disable that (e.g. by defining "NDEBUG") then no errors will be
4806           reported.
4807
4808           The default is 1, unless "EV_FEATURES" overrides it, in which case
4809           it will be 0.
4810
4811       EV_COMMON
4812           By default, all watchers have a "void *data" member. By redefining
4813           this macro to something else you can include more and other types
4814           of members. You have to define it each time you include one of the
4815           files, though, and it must be identical each time.
4816
4817           For example, the perl EV module uses something like this:
4818
4819              #define EV_COMMON                       \
4820                SV *self; /* contains this struct */  \
4821                SV *cb_sv, *fh /* note no trailing ";" */
4822
4823       EV_CB_DECLARE (type)
4824       EV_CB_INVOKE (watcher, revents)
4825       ev_set_cb (ev, cb)
4826           Can be used to change the callback member declaration in each
4827           watcher, and the way callbacks are invoked and set. Must expand to
4828           a struct member definition and a statement, respectively. See the
4829           ev.h header file for their default definitions. One possible use
4830           for overriding these is to avoid the "struct ev_loop *" as first
4831           argument in all cases, or to use method calls instead of plain
4832           function calls in C++.
4833
4834   EXPORTED API SYMBOLS
4835       If you need to re-export the API (e.g. via a DLL) and you need a list
4836       of exported symbols, you can use the provided Symbol.* files which list
4837       all public symbols, one per line:
4838
4839          Symbols.ev      for libev proper
4840          Symbols.event   for the libevent emulation
4841
4842       This can also be used to rename all public symbols to avoid clashes
4843       with multiple versions of libev linked together (which is obviously bad
4844       in itself, but sometimes it is inconvenient to avoid this).
4845
4846       A sed command like this will create wrapper "#define"'s that you need
4847       to include before including ev.h:
4848
4849          <Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h
4850
4851       This would create a file wrap.h which essentially looks like this:
4852
4853          #define ev_backend     myprefix_ev_backend
4854          #define ev_check_start myprefix_ev_check_start
4855          #define ev_check_stop  myprefix_ev_check_stop
4856          ...
4857
4858   EXAMPLES
4859       For a real-world example of a program the includes libev verbatim, you
4860       can have a look at the EV perl module
4861       (<http://software.schmorp.de/pkg/EV.html>). It has the libev files in
4862       the libev/ subdirectory and includes them in the EV/EVAPI.h (public
4863       interface) and EV.xs (implementation) files. Only the EV.xs file will
4864       be compiled. It is pretty complex because it provides its own header
4865       file.
4866
4867       The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file
4868       that everybody includes and which overrides some configure choices:
4869
4870          #define EV_FEATURES 8
4871          #define EV_USE_SELECT 1
4872          #define EV_PREPARE_ENABLE 1
4873          #define EV_IDLE_ENABLE 1
4874          #define EV_SIGNAL_ENABLE 1
4875          #define EV_CHILD_ENABLE 1
4876          #define EV_USE_STDEXCEPT 0
4877          #define EV_CONFIG_H <config.h>
4878
4879          #include "ev++.h"
4880
4881       And a ev_cpp.C implementation file that contains libev proper and is
4882       compiled:
4883
4884          #include "ev_cpp.h"
4885          #include "ev.c"
4886

INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT

4888   THREADS AND COROUTINES
4889       THREADS
4890
4891       All libev functions are reentrant and thread-safe unless explicitly
4892       documented otherwise, but libev implements no locking itself. This
4893       means that you can use as many loops as you want in parallel, as long
4894       as there are no concurrent calls into any libev function with the same
4895       loop parameter ("ev_default_*" calls have an implicit default loop
4896       parameter, of course): libev guarantees that different event loops
4897       share no data structures that need any locking.
4898
4899       Or to put it differently: calls with different loop parameters can be
4900       done concurrently from multiple threads, calls with the same loop
4901       parameter must be done serially (but can be done from different
4902       threads, as long as only one thread ever is inside a call at any point
4903       in time, e.g. by using a mutex per loop).
4904
4905       Specifically to support threads (and signal handlers), libev implements
4906       so-called "ev_async" watchers, which allow some limited form of
4907       concurrency on the same event loop, namely waking it up "from the
4908       outside".
4909
4910       If you want to know which design (one loop, locking, or multiple loops
4911       without or something else still) is best for your problem, then I
4912       cannot help you, but here is some generic advice:
4913
4914       ·   most applications have a main thread: use the default libev loop in
4915           that thread, or create a separate thread running only the default
4916           loop.
4917
4918           This helps integrating other libraries or software modules that use
4919           libev themselves and don't care/know about threading.
4920
4921       ·   one loop per thread is usually a good model.
4922
4923           Doing this is almost never wrong, sometimes a better-performance
4924           model exists, but it is always a good start.
4925
4926       ·   other models exist, such as the leader/follower pattern, where one
4927           loop is handed through multiple threads in a kind of round-robin
4928           fashion.
4929
4930           Choosing a model is hard - look around, learn, know that usually
4931           you can do better than you currently do :-)
4932
4933       ·   often you need to talk to some other thread which blocks in the
4934           event loop.
4935
4936           "ev_async" watchers can be used to wake them up from other threads
4937           safely (or from signal contexts...).
4938
4939           An example use would be to communicate signals or other events that
4940           only work in the default loop by registering the signal watcher
4941           with the default loop and triggering an "ev_async" watcher from the
4942           default loop watcher callback into the event loop interested in the
4943           signal.
4944
4945       See also "THREAD LOCKING EXAMPLE".
4946
4947       COROUTINES
4948
4949       Libev is very accommodating to coroutines ("cooperative threads"):
4950       libev fully supports nesting calls to its functions from different
4951       coroutines (e.g. you can call "ev_run" on the same loop from two
4952       different coroutines, and switch freely between both coroutines running
4953       the loop, as long as you don't confuse yourself). The only exception is
4954       that you must not do this from "ev_periodic" reschedule callbacks.
4955
4956       Care has been taken to ensure that libev does not keep local state
4957       inside "ev_run", and other calls do not usually allow for coroutine
4958       switches as they do not call any callbacks.
4959
4960   COMPILER WARNINGS
4961       Depending on your compiler and compiler settings, you might get no or a
4962       lot of warnings when compiling libev code. Some people are apparently
4963       scared by this.
4964
4965       However, these are unavoidable for many reasons. For one, each compiler
4966       has different warnings, and each user has different tastes regarding
4967       warning options. "Warn-free" code therefore cannot be a goal except
4968       when targeting a specific compiler and compiler-version.
4969
4970       Another reason is that some compiler warnings require elaborate
4971       workarounds, or other changes to the code that make it less clear and
4972       less maintainable.
4973
4974       And of course, some compiler warnings are just plain stupid, or simply
4975       wrong (because they don't actually warn about the condition their
4976       message seems to warn about). For example, certain older gcc versions
4977       had some warnings that resulted in an extreme number of false
4978       positives. These have been fixed, but some people still insist on
4979       making code warn-free with such buggy versions.
4980
4981       While libev is written to generate as few warnings as possible, "warn-
4982       free" code is not a goal, and it is recommended not to build libev with
4983       any compiler warnings enabled unless you are prepared to cope with them
4984       (e.g. by ignoring them). Remember that warnings are just that:
4985       warnings, not errors, or proof of bugs.
4986
4987   VALGRIND
4988       Valgrind has a special section here because it is a popular tool that
4989       is highly useful. Unfortunately, valgrind reports are very hard to
4990       interpret.
4991
4992       If you think you found a bug (memory leak, uninitialised data access
4993       etc.)  in libev, then check twice: If valgrind reports something like:
4994
4995          ==2274==    definitely lost: 0 bytes in 0 blocks.
4996          ==2274==      possibly lost: 0 bytes in 0 blocks.
4997          ==2274==    still reachable: 256 bytes in 1 blocks.
4998
4999       Then there is no memory leak, just as memory accounted to global
5000       variables is not a memleak - the memory is still being referenced, and
5001       didn't leak.
5002
5003       Similarly, under some circumstances, valgrind might report kernel bugs
5004       as if it were a bug in libev (e.g. in realloc or in the poll backend,
5005       although an acceptable workaround has been found here), or it might be
5006       confused.
5007
5008       Keep in mind that valgrind is a very good tool, but only a tool. Don't
5009       make it into some kind of religion.
5010
5011       If you are unsure about something, feel free to contact the mailing
5012       list with the full valgrind report and an explanation on why you think
5013       this is a bug in libev (best check the archives, too :). However, don't
5014       be annoyed when you get a brisk "this is no bug" answer and take the
5015       chance of learning how to interpret valgrind properly.
5016
5017       If you need, for some reason, empty reports from valgrind for your
5018       project I suggest using suppression lists.
5019

PORTABILITY NOTES

5021   GNU/LINUX 32 BIT LIMITATIONS
5022       GNU/Linux is the only common platform that supports 64 bit file/large
5023       file interfaces but disables them by default.
5024
5025       That means that libev compiled in the default environment doesn't
5026       support files larger than 2GiB or so, which mainly affects "ev_stat"
5027       watchers.
5028
5029       Unfortunately, many programs try to work around this GNU/Linux issue by
5030       enabling the large file API, which makes them incompatible with the
5031       standard libev compiled for their system.
5032
5033       Likewise, libev cannot enable the large file API itself as this would
5034       suddenly make it incompatible to the default compile time environment,
5035       i.e. all programs not using special compile switches.
5036
5037   OS/X AND DARWIN BUGS
5038       The whole thing is a bug if you ask me - basically any system interface
5039       you touch is broken, whether it is locales, poll, kqueue or even the
5040       OpenGL drivers.
5041
5042       "kqueue" is buggy
5043
5044       The kqueue syscall is broken in all known versions - most versions
5045       support only sockets, many support pipes.
5046
5047       Libev tries to work around this by not using "kqueue" by default on
5048       this rotten platform, but of course you can still ask for it when
5049       creating a loop - embedding a socket-only kqueue loop into a select-
5050       based one is probably going to work well.
5051
5052       "poll" is buggy
5053
5054       Instead of fixing "kqueue", Apple replaced their (working) "poll"
5055       implementation by something calling "kqueue" internally around the
5056       10.5.6 release, so now "kqueue" and "poll" are broken.
5057
5058       Libev tries to work around this by not using "poll" by default on this
5059       rotten platform, but of course you can still ask for it when creating a
5060       loop.
5061
5062       "select" is buggy
5063
5064       All that's left is "select", and of course Apple found a way to fuck
5065       this one up as well: On OS/X, "select" actively limits the number of
5066       file descriptors you can pass in to 1024 - your program suddenly
5067       crashes when you use more.
5068
5069       There is an undocumented "workaround" for this - defining
5070       "_DARWIN_UNLIMITED_SELECT", which libev tries to use, so select should
5071       work on OS/X.
5072
5073   SOLARIS PROBLEMS AND WORKAROUNDS
5074       "errno" reentrancy
5075
5076       The default compile environment on Solaris is unfortunately so thread-
5077       unsafe that you can't even use components/libraries compiled without
5078       "-D_REENTRANT" in a threaded program, which, of course, isn't defined
5079       by default. A valid, if stupid, implementation choice.
5080
5081       If you want to use libev in threaded environments you have to make sure
5082       it's compiled with "_REENTRANT" defined.
5083
5084       Event port backend
5085
5086       The scalable event interface for Solaris is called "event ports".
5087       Unfortunately, this mechanism is very buggy in all major releases. If
5088       you run into high CPU usage, your program freezes or you get a large
5089       number of spurious wakeups, make sure you have all the relevant and
5090       latest kernel patches applied. No, I don't know which ones, but there
5091       are multiple ones to apply, and afterwards, event ports actually work
5092       great.
5093
5094       If you can't get it to work, you can try running the program by setting
5095       the environment variable "LIBEV_FLAGS=3" to only allow "poll" and
5096       "select" backends.
5097
5098   AIX POLL BUG
5099       AIX unfortunately has a broken "poll.h" header. Libev works around this
5100       by trying to avoid the poll backend altogether (i.e. it's not even
5101       compiled in), which normally isn't a big problem as "select" works fine
5102       with large bitsets on AIX, and AIX is dead anyway.
5103
5104   WIN32 PLATFORM LIMITATIONS AND WORKAROUNDS
5105       General issues
5106
5107       Win32 doesn't support any of the standards (e.g. POSIX) that libev
5108       requires, and its I/O model is fundamentally incompatible with the
5109       POSIX model. Libev still offers limited functionality on this platform
5110       in the form of the "EVBACKEND_SELECT" backend, and only supports socket
5111       descriptors. This only applies when using Win32 natively, not when
5112       using e.g. cygwin. Actually, it only applies to the microsofts own
5113       compilers, as every compiler comes with a slightly differently
5114       broken/incompatible environment.
5115
5116       Lifting these limitations would basically require the full re-
5117       implementation of the I/O system. If you are into this kind of thing,
5118       then note that glib does exactly that for you in a very portable way
5119       (note also that glib is the slowest event library known to man).
5120
5121       There is no supported compilation method available on windows except
5122       embedding it into other applications.
5123
5124       Sensible signal handling is officially unsupported by Microsoft - libev
5125       tries its best, but under most conditions, signals will simply not
5126       work.
5127
5128       Not a libev limitation but worth mentioning: windows apparently doesn't
5129       accept large writes: instead of resulting in a partial write, windows
5130       will either accept everything or return "ENOBUFS" if the buffer is too
5131       large, so make sure you only write small amounts into your sockets
5132       (less than a megabyte seems safe, but this apparently depends on the
5133       amount of memory available).
5134
5135       Due to the many, low, and arbitrary limits on the win32 platform and
5136       the abysmal performance of winsockets, using a large number of sockets
5137       is not recommended (and not reasonable). If your program needs to use
5138       more than a hundred or so sockets, then likely it needs to use a
5139       totally different implementation for windows, as libev offers the POSIX
5140       readiness notification model, which cannot be implemented efficiently
5141       on windows (due to Microsoft monopoly games).
5142
5143       A typical way to use libev under windows is to embed it (see the
5144       embedding section for details) and use the following evwrap.h header
5145       file instead of ev.h:
5146
5147          #define EV_STANDALONE              /* keeps ev from requiring config.h */
5148          #define EV_SELECT_IS_WINSOCKET 1   /* configure libev for windows select */
5149
5150          #include "ev.h"
5151
5152       And compile the following evwrap.c file into your project (make sure
5153       you do not compile the ev.c or any other embedded source files!):
5154
5155          #include "evwrap.h"
5156          #include "ev.c"
5157
5158       The winsocket "select" function
5159
5160       The winsocket "select" function doesn't follow POSIX in that it
5161       requires socket handles and not socket file descriptors (it is also
5162       extremely buggy). This makes select very inefficient, and also requires
5163       a mapping from file descriptors to socket handles (the Microsoft C
5164       runtime provides the function "_open_osfhandle" for this). See the
5165       discussion of the "EV_SELECT_USE_FD_SET", "EV_SELECT_IS_WINSOCKET" and
5166       "EV_FD_TO_WIN32_HANDLE" preprocessor symbols for more info.
5167
5168       The configuration for a "naked" win32 using the Microsoft runtime
5169       libraries and raw winsocket select is:
5170
5171          #define EV_USE_SELECT 1
5172          #define EV_SELECT_IS_WINSOCKET 1   /* forces EV_SELECT_USE_FD_SET, too */
5173
5174       Note that winsockets handling of fd sets is O(n), so you can easily get
5175       a complexity in the O(n²) range when using win32.
5176
5177       Limited number of file descriptors
5178
5179       Windows has numerous arbitrary (and low) limits on things.
5180
5181       Early versions of winsocket's select only supported waiting for a
5182       maximum of 64 handles (probably owning to the fact that all windows
5183       kernels can only wait for 64 things at the same time internally;
5184       Microsoft recommends spawning a chain of threads and wait for 63
5185       handles and the previous thread in each. Sounds great!).
5186
5187       Newer versions support more handles, but you need to define
5188       "FD_SETSIZE" to some high number (e.g. 2048) before compiling the
5189       winsocket select call (which might be in libev or elsewhere, for
5190       example, perl and many other interpreters do their own select emulation
5191       on windows).
5192
5193       Another limit is the number of file descriptors in the Microsoft
5194       runtime libraries, which by default is 64 (there must be a hidden 64
5195       fetish or something like this inside Microsoft). You can increase this
5196       by calling "_setmaxstdio", which can increase this limit to 2048
5197       (another arbitrary limit), but is broken in many versions of the
5198       Microsoft runtime libraries. This might get you to about 512 or 2048
5199       sockets (depending on windows version and/or the phase of the moon). To
5200       get more, you need to wrap all I/O functions and provide your own fd
5201       management, but the cost of calling select (O(n²)) will likely make
5202       this unworkable.
5203
5204   PORTABILITY REQUIREMENTS
5205       In addition to a working ISO-C implementation and of course the
5206       backend-specific APIs, libev relies on a few additional extensions:
5207
5208       "void (*)(ev_watcher_type *, int revents)" must have compatible calling
5209       conventions regardless of "ev_watcher_type *".
5210           Libev assumes not only that all watcher pointers have the same
5211           internal structure (guaranteed by POSIX but not by ISO C for
5212           example), but it also assumes that the same (machine) code can be
5213           used to call any watcher callback: The watcher callbacks have
5214           different type signatures, but libev calls them using an
5215           "ev_watcher *" internally.
5216
5217       null pointers and integer zero are represented by 0 bytes
5218           Libev uses "memset" to initialise structs and arrays to 0 bytes,
5219           and relies on this setting pointers and integers to null.
5220
5221       pointer accesses must be thread-atomic
5222           Accessing a pointer value must be atomic, it must both be readable
5223           and writable in one piece - this is the case on all current
5224           architectures.
5225
5226       "sig_atomic_t volatile" must be thread-atomic as well
5227           The type "sig_atomic_t volatile" (or whatever is defined as
5228           "EV_ATOMIC_T") must be atomic with respect to accesses from
5229           different threads. This is not part of the specification for
5230           "sig_atomic_t", but is believed to be sufficiently portable.
5231
5232       "sigprocmask" must work in a threaded environment
5233           Libev uses "sigprocmask" to temporarily block signals. This is not
5234           allowed in a threaded program ("pthread_sigmask" has to be used).
5235           Typical pthread implementations will either allow "sigprocmask" in
5236           the "main thread" or will block signals process-wide, both
5237           behaviours would be compatible with libev. Interaction between
5238           "sigprocmask" and "pthread_sigmask" could complicate things,
5239           however.
5240
5241           The most portable way to handle signals is to block signals in all
5242           threads except the initial one, and run the signal handling loop in
5243           the initial thread as well.
5244
5245       "long" must be large enough for common memory allocation sizes
5246           To improve portability and simplify its API, libev uses "long"
5247           internally instead of "size_t" when allocating its data structures.
5248           On non-POSIX systems (Microsoft...) this might be unexpectedly low,
5249           but is still at least 31 bits everywhere, which is enough for
5250           hundreds of millions of watchers.
5251
5252       "double" must hold a time value in seconds with enough accuracy
5253           The type "double" is used to represent timestamps. It is required
5254           to have at least 51 bits of mantissa (and 9 bits of exponent),
5255           which is good enough for at least into the year 4000 with
5256           millisecond accuracy (the design goal for libev). This requirement
5257           is overfulfilled by implementations using IEEE 754, which is
5258           basically all existing ones.
5259
5260           With IEEE 754 doubles, you get microsecond accuracy until at least
5261           the year 2255 (and millisecond accuracy till the year 287396 - by
5262           then, libev is either obsolete or somebody patched it to use "long
5263           double" or something like that, just kidding).
5264
5265       If you know of other additional requirements drop me a note.
5266

ALGORITHMIC COMPLEXITIES

5268       In this section the complexities of (many of) the algorithms used
5269       inside libev will be documented. For complexity discussions about
5270       backends see the documentation for "ev_default_init".
5271
5272       All of the following are about amortised time: If an array needs to be
5273       extended, libev needs to realloc and move the whole array, but this
5274       happens asymptotically rarer with higher number of elements, so O(1)
5275       might mean that libev does a lengthy realloc operation in rare cases,
5276       but on average it is much faster and asymptotically approaches constant
5277       time.
5278
5279       Starting and stopping timer/periodic watchers: O(log
5280       skipped_other_timers)
5281           This means that, when you have a watcher that triggers in one hour
5282           and there are 100 watchers that would trigger before that, then
5283           inserting will have to skip roughly seven ("ld 100") of these
5284           watchers.
5285
5286       Changing timer/periodic watchers (by autorepeat or calling again):
5287       O(log skipped_other_timers)
5288           That means that changing a timer costs less than removing/adding
5289           them, as only the relative motion in the event queue has to be paid
5290           for.
5291
5292       Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)
5293           These just add the watcher into an array or at the head of a list.
5294
5295       Stopping check/prepare/idle/fork/async watchers: O(1)
5296       Stopping an io/signal/child watcher:
5297       O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
5298           These watchers are stored in lists, so they need to be walked to
5299           find the correct watcher to remove. The lists are usually short
5300           (you don't usually have many watchers waiting for the same fd or
5301           signal: one is typical, two is rare).
5302
5303       Finding the next timer in each loop iteration: O(1)
5304           By virtue of using a binary or 4-heap, the next timer is always
5305           found at a fixed position in the storage array.
5306
5307       Each change on a file descriptor per loop iteration:
5308       O(number_of_watchers_for_this_fd)
5309           A change means an I/O watcher gets started or stopped, which
5310           requires libev to recalculate its status (and possibly tell the
5311           kernel, depending on backend and whether "ev_io_set" was used).
5312
5313       Activating one watcher (putting it into the pending state): O(1)
5314       Priority handling: O(number_of_priorities)
5315           Priorities are implemented by allocating some space for each
5316           priority. When doing priority-based operations, libev usually has
5317           to linearly search all the priorities, but starting/stopping and
5318           activating watchers becomes O(1) with respect to priority handling.
5319
5320       Sending an ev_async: O(1)
5321       Processing ev_async_send: O(number_of_async_watchers)
5322       Processing signals: O(max_signal_number)
5323           Sending involves a system call iff there were no other
5324           "ev_async_send" calls in the current loop iteration and the loop is
5325           currently blocked. Checking for async and signal events involves
5326           iterating over all running async watchers or all signal numbers.
5327

PORTING FROM LIBEV 3.X TO 4.X

5329       The major version 4 introduced some incompatible changes to the API.
5330
5331       At the moment, the "ev.h" header file provides compatibility
5332       definitions for all changes, so most programs should still compile. The
5333       compatibility layer might be removed in later versions of libev, so
5334       better update to the new API early than late.
5335
5336       "EV_COMPAT3" backwards compatibility mechanism
5337           The backward compatibility mechanism can be controlled by
5338           "EV_COMPAT3". See "PREPROCESSOR SYMBOLS/MACROS" in the "EMBEDDING"
5339           section.
5340
5341       "ev_default_destroy" and "ev_default_fork" have been removed
5342           These calls can be replaced easily by their "ev_loop_xxx"
5343           counterparts:
5344
5345              ev_loop_destroy (EV_DEFAULT_UC);
5346              ev_loop_fork (EV_DEFAULT);
5347
5348       function/symbol renames
5349           A number of functions and symbols have been renamed:
5350
5351             ev_loop         => ev_run
5352             EVLOOP_NONBLOCK => EVRUN_NOWAIT
5353             EVLOOP_ONESHOT  => EVRUN_ONCE
5354
5355             ev_unloop       => ev_break
5356             EVUNLOOP_CANCEL => EVBREAK_CANCEL
5357             EVUNLOOP_ONE    => EVBREAK_ONE
5358             EVUNLOOP_ALL    => EVBREAK_ALL
5359
5360             EV_TIMEOUT      => EV_TIMER
5361
5362             ev_loop_count   => ev_iteration
5363             ev_loop_depth   => ev_depth
5364             ev_loop_verify  => ev_verify
5365
5366           Most functions working on "struct ev_loop" objects don't have an
5367           "ev_loop_" prefix, so it was removed; "ev_loop", "ev_unloop" and
5368           associated constants have been renamed to not collide with the
5369           "struct ev_loop" anymore and "EV_TIMER" now follows the same naming
5370           scheme as all other watcher types. Note that "ev_loop_fork" is
5371           still called "ev_loop_fork" because it would otherwise clash with
5372           the "ev_fork" typedef.
5373
5374       "EV_MINIMAL" mechanism replaced by "EV_FEATURES"
5375           The preprocessor symbol "EV_MINIMAL" has been replaced by a
5376           different mechanism, "EV_FEATURES". Programs using "EV_MINIMAL"
5377           usually compile and work, but the library code will of course be
5378           larger.
5379

GLOSSARY

5381       active
5382           A watcher is active as long as it has been started and not yet
5383           stopped.  See "WATCHER STATES" for details.
5384
5385       application
5386           In this document, an application is whatever is using libev.
5387
5388       backend
5389           The part of the code dealing with the operating system interfaces.
5390
5391       callback
5392           The address of a function that is called when some event has been
5393           detected. Callbacks are being passed the event loop, the watcher
5394           that received the event, and the actual event bitset.
5395
5396       callback/watcher invocation
5397           The act of calling the callback associated with a watcher.
5398
5399       event
5400           A change of state of some external event, such as data now being
5401           available for reading on a file descriptor, time having passed or
5402           simply not having any other events happening anymore.
5403
5404           In libev, events are represented as single bits (such as "EV_READ"
5405           or "EV_TIMER").
5406
5407       event library
5408           A software package implementing an event model and loop.
5409
5410       event loop
5411           An entity that handles and processes external events and converts
5412           them into callback invocations.
5413
5414       event model
5415           The model used to describe how an event loop handles and processes
5416           watchers and events.
5417
5418       pending
5419           A watcher is pending as soon as the corresponding event has been
5420           detected. See "WATCHER STATES" for details.
5421
5422       real time
5423           The physical time that is observed. It is apparently strictly
5424           monotonic :)
5425
5426       wall-clock time
5427           The time and date as shown on clocks. Unlike real time, it can
5428           actually be wrong and jump forwards and backwards, e.g. when you
5429           adjust your clock.
5430
5431       watcher
5432           A data structure that describes interest in certain events.
5433           Watchers need to be started (attached to an event loop) before they
5434           can receive events.
5435

AUTHOR

5437       Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael
5438       Magnusson and Emanuele Giaquinta, and minor corrections by many others.
5439
5440
5441
5442perl v5.30.1                      2020-01-29                      libev::ev(3)
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