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


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


9          #include <ev.h>
12          // a single header file is required
13          #include <ev.h>
15          #include <stdio.h> // for puts
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;
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);
32            // this causes all nested ev_run's to stop iterating
33            ev_break (EV_A_ EVBREAK_ALL);
34          }
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          }
45          int
46          main (void)
47          {
48            // use the default event loop unless you have special needs
49            struct ev_loop *loop = EV_DEFAULT;
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);
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);
61            // now wait for events to arrive
62            ev_run (loop, 0);
64            // break was called, so exit
65            return 0;
66          }


69       This document documents the libev software package.
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>.
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.
81       Familiarity with event based programming techniques in general is
82       assumed throughout this document.


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".


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.
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.
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.
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").
119       It also is quite fast (see this benchmark
120       <http://libev.schmorp.de/bench.html> comparing it to libevent for
121       example).
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.
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.
141       Unlike the name component "stamp" might indicate, it is also used for
142       time differences (e.g. delays) throughout libev.


145       Libev knows three classes of errors: operating system errors, usage
146       errors and internal errors (bugs).
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 ()".
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.
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.
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.


168       These functions can be called anytime, even before initialising the
169       library in any way.
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".
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".
183           Basically this is a sub-second-resolution "sleep ()".
185           The range of the "interval" is limited - libev only guarantees to
186           work with sleep times of up to one day ("interval <= 86400").
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.
197           These version numbers refer to the ABI version of the library, not
198           the release version.
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.
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).
209              assert (("libev version mismatch",
210                       ev_version_major () == EV_VERSION_MAJOR
211                       && ev_version_minor () >= EV_VERSION_MINOR));
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.
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
222              assert (("sorry, no epoll, no sex",
223                       ev_supported_backends () & EVBACKEND_EPOLL));
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.
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.
243           See the description of "ev_embed" watchers for more info.
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.
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.
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.
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.
265              static void *
266              ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
267              {
268                if (size)
269                  return realloc (ptr, size);
271                free (ptr);
272                return 0;
273              }
275           Example: Replace the libev allocator with one that waits a bit and
276           then retries.
278              static void *
279              persistent_realloc (void *ptr, size_t size)
280              {
281                if (!size)
282                  {
283                    free (ptr);
284                    return 0;
285                  }
287                for (;;)
288                  {
289                    void *newptr = realloc (ptr, size);
291                    if (newptr)
292                      return newptr;
294                    sleep (60);
295                  }
296              }
298              ...
299              ev_set_allocator (persistent_realloc);
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).
310           Example: This is basically the same thing that libev does
311           internally, too.
313              static void
314              fatal_error (const char *msg)
315              {
316                perror (msg);
317                abort ();
318              }
320              ...
321              ev_set_syserr_cb (fatal_error);
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.
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".


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).
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.
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".
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".
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).
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).
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".
372           Example: This is the most typical usage.
374              if (!ev_default_loop (0))
375                fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
377           Example: Restrict libev to the select and poll backends, and do not
378           allow environment settings to be taken into account:
380              ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
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.
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.
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").
394           The following flags are supported:
396           "EVFLAG_AUTO"
397               The default flags value. Use this if you have no clue (it's the
398               right thing, believe me).
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).
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.
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).
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.
430               This flag setting cannot be overridden or specified in the
431               "LIBEV_FLAGS" environment variable.
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.
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.
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.
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.
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.
463               It's also required by POSIX in a threaded program, as libev
464               calls "sigprocmask", whose behaviour is officially unspecified.
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.
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.
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.
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.
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).
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.
506               This backend maps "EV_READ" to "POLLIN | POLLERR | POLLHUP",
507               and "EV_WRITE" to "POLLOUT | POLLERR | POLLHUP".
509           "EVBACKEND_EPOLL"   (value 4, Linux)
510               Use the Linux-specific epoll(7) interface (for both pre- and
511               post-2.6.9 kernels).
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).
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.
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...).
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...
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.
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.
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.
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.
575               This backend maps "EV_READ" and "EV_WRITE" in the same way as
576               "EVBACKEND_POLL".
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+).
583               This is another Linux train wreck of an event interface.
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.
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.
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.
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.
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".
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.
624               This backend maps "EV_READ" and "EV_WRITE" in the same way as
625               "EVBACKEND_POLL".
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.
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.
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.
654               This backend usually performs well under most conditions.
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.
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".
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.
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)).
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.
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).
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.
698               Fortunately libev seems to be able to work around these
699               idiocies.
701               This backend maps "EV_READ" and "EV_WRITE" in the same way as
702               "EVBACKEND_POLL".
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".
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.
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).
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.
724           Example: Try to create a event loop that uses epoll and nothing
725           else.
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");
731           Example: Use whatever libev has to offer, but make sure that kqueue
732           is used if available.
734              struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
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.
740              struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
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).
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.
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.
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".
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".
774           In addition, if you want to reuse a loop (via this function or
775           "EVFLAG_FORKCHECK"), you also have to ignore "SIGPIPE".
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.
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).
789           The function itself is quite fast and it's usually not a problem to
790           call it just in case after a fork.
792           Example: Automate calling "ev_loop_fork" on the default loop when
793           using pthreads.
795              static void
796              post_fork_child (void)
797              {
798                ev_loop_fork (EV_DEFAULT);
799              }
801              ...
802              pthread_atfork (0, 0, post_fork_child);
804       int ev_is_default_loop (loop)
805           Returns true when the given loop is, in fact, the default loop, and
806           false otherwise.
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.
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.
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.
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.
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.
832       unsigned int ev_backend (loop)
833           Returns one of the "EVBACKEND_*" flags indicating the event backend
834           in use.
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).
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 ()".
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.
853           See also "The special problem of time updates" in the "ev_timer"
854           section.
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.
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.
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).
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".
879           Calling "ev_suspend"/"ev_resume" has the side effect of updating
880           the event loop time (see "ev_now_update").
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.
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.
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").
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.
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.
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.
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.
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.
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):
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.
969           Example: Queue some jobs and then loop until no events are
970           outstanding anymore.
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!
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.
984           This "break state" will be cleared on the next call to "ev_run".
986           It is safe to call "ev_break" from outside any "ev_run" calls, too,
987           in which case it will have no effect.
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.
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.
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).
1010           Example: Create a signal watcher, but keep it from keeping "ev_run"
1011           running when nothing else is active.
1013              ev_signal exitsig;
1014              ev_signal_init (&exitsig, sig_cb, SIGINT);
1015              ev_signal_start (loop, &exitsig);
1016              ev_unref (loop);
1018           Example: For some weird reason, unregister the above signal handler
1019           again.
1021              ev_ref (loop);
1022              ev_signal_stop (loop, &exitsig);
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.
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).
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.
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).
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.
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).
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.
1077           Example: we only need 0.1s timeout granularity, and we wish not to
1078           poll more often than 100 times per second:
1080              ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
1081              ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
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).
1093       int ev_pending_count (loop)
1094           Returns the number of pending watchers - zero indicates that no
1095           watchers are pending.
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.).
1104           If you want to reset the callback, use "ev_invoke_pending" as new
1105           callback.
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.
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.
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.
1122           Ideally, "release" will just call your mutex_unlock function, and
1123           "acquire" will just call the mutex_lock function again.
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.
1132           In theory, threads executing "ev_run" will be async-cancel safe
1133           between invocations of "release" and "acquire".
1135           See also the locking example in the "THREADS" section later in this
1136           document.
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.
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.
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 ()".
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.


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


1607       This section describes each watcher in detail, but will not repeat
1608       information given in the last section. Any initialisation/set macros,
1609       functions and members specific to the watcher type are explained.
1611       Most members are additionally marked with either [read-only], meaning
1612       that, while the watcher is active, you can look at the member and
1613       expect some sensible content, but you must not modify it (you can
1614       modify it while the watcher is stopped to your hearts content), or
1615       [read-write], which means you can expect it to have some sensible
1616       content while the watcher is active, but you can also modify it (within
1617       the same thread as the event loop, i.e. without creating data races).
1618       Modifying it may not do something sensible or take immediate effect (or
1619       do anything at all), but libev will not crash or malfunction in any
1620       way.
1622       In any case, the documentation for each member will explain what the
1623       effects are, and if there are any additional access restrictions.
1625   "ev_io" - is this file descriptor readable or writable?
1626       I/O watchers check whether a file descriptor is readable or writable in
1627       each iteration of the event loop, or, more precisely, when reading
1628       would not block the process and writing would at least be able to write
1629       some data. This behaviour is called level-triggering because you keep
1630       receiving events as long as the condition persists. Remember you can
1631       stop the watcher if you don't want to act on the event and neither want
1632       to receive future events.
1634       In general you can register as many read and/or write event watchers
1635       per fd as you want (as long as you don't confuse yourself). Setting all
1636       file descriptors to non-blocking mode is also usually a good idea (but
1637       not required if you know what you are doing).
1639       Another thing you have to watch out for is that it is quite easy to
1640       receive "spurious" readiness notifications, that is, your callback
1641       might be called with "EV_READ" but a subsequent "read"(2) will actually
1642       block because there is no data. It is very easy to get into this
1643       situation even with a relatively standard program structure. Thus it is
1644       best to always use non-blocking I/O: An extra "read"(2) returning
1645       "EAGAIN" is far preferable to a program hanging until some data
1646       arrives.
1648       If you cannot run the fd in non-blocking mode (for example you should
1649       not play around with an Xlib connection), then you have to separately
1650       re-test whether a file descriptor is really ready with a known-to-be
1651       good interface such as poll (fortunately in the case of Xlib, it
1652       already does this on its own, so its quite safe to use). Some people
1653       additionally use "SIGALRM" and an interval timer, just to be sure you
1654       won't block indefinitely.
1656       But really, best use non-blocking mode.
1658       The special problem of disappearing file descriptors
1660       Some backends (e.g. kqueue, epoll, linuxaio) need to be told about
1661       closing a file descriptor (either due to calling "close" explicitly or
1662       any other means, such as "dup2"). The reason is that you register
1663       interest in some file descriptor, but when it goes away, the operating
1664       system will silently drop this interest. If another file descriptor
1665       with the same number then is registered with libev, there is no
1666       efficient way to see that this is, in fact, a different file
1667       descriptor.
1669       To avoid having to explicitly tell libev about such cases, libev
1670       follows the following policy:  Each time "ev_io_set" is being called,
1671       libev will assume that this is potentially a new file descriptor,
1672       otherwise it is assumed that the file descriptor stays the same. That
1673       means that you have to call "ev_io_set" (or "ev_io_init") when you
1674       change the descriptor even if the file descriptor number itself did not
1675       change.
1677       This is how one would do it normally anyway, the important point is
1678       that the libev application should not optimise around libev but should
1679       leave optimisations to libev.
1681       The special problem of dup'ed file descriptors
1683       Some backends (e.g. epoll), cannot register events for file
1684       descriptors, but only events for the underlying file descriptions. That
1685       means when you have "dup ()"'ed file descriptors or weirder
1686       constellations, and register events for them, only one file descriptor
1687       might actually receive events.
1689       There is no workaround possible except not registering events for
1690       potentially "dup ()"'ed file descriptors, or to resort to
1693       The special problem of files
1695       Many people try to use "select" (or libev) on file descriptors
1696       representing files, and expect it to become ready when their program
1697       doesn't block on disk accesses (which can take a long time on their
1698       own).
1700       However, this cannot ever work in the "expected" way - you get a
1701       readiness notification as soon as the kernel knows whether and how much
1702       data is there, and in the case of open files, that's always the case,
1703       so you always get a readiness notification instantly, and your read (or
1704       possibly write) will still block on the disk I/O.
1706       Another way to view it is that in the case of sockets, pipes, character
1707       devices and so on, there is another party (the sender) that delivers
1708       data on its own, but in the case of files, there is no such thing: the
1709       disk will not send data on its own, simply because it doesn't know what
1710       you wish to read - you would first have to request some data.
1712       Since files are typically not-so-well supported by advanced
1713       notification mechanism, libev tries hard to emulate POSIX behaviour
1714       with respect to files, even though you should not use it. The reason
1715       for this is convenience: sometimes you want to watch STDIN or STDOUT,
1716       which is usually a tty, often a pipe, but also sometimes files or
1717       special devices (for example, "epoll" on Linux works with /dev/random
1718       but not with /dev/urandom), and even though the file might better be
1719       served with asynchronous I/O instead of with non-blocking I/O, it is
1720       still useful when it "just works" instead of freezing.
1722       So avoid file descriptors pointing to files when you know it (e.g. use
1723       libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or
1724       when you rarely read from a file instead of from a socket, and want to
1725       reuse the same code path.
1727       The special problem of fork
1729       Some backends (epoll, kqueue, linuxaio, iouring) do not support "fork
1730       ()" at all or exhibit useless behaviour. Libev fully supports fork, but
1731       needs to be told about it in the child if you want to continue to use
1732       it in the child.
1734       To support fork in your child processes, you have to call "ev_loop_fork
1735       ()" after a fork in the child, enable "EVFLAG_FORKCHECK", or resort to
1738       The special problem of SIGPIPE
1740       While not really specific to libev, it is easy to forget about
1741       "SIGPIPE": when writing to a pipe whose other end has been closed, your
1742       program gets sent a SIGPIPE, which, by default, aborts your program.
1743       For most programs this is sensible behaviour, for daemons, this is
1744       usually undesirable.
1746       So when you encounter spurious, unexplained daemon exits, make sure you
1747       ignore SIGPIPE (and maybe make sure you log the exit status of your
1748       daemon somewhere, as that would have given you a big clue).
1750       The special problem of accept()ing when you can't
1752       Many implementations of the POSIX "accept" function (for example, found
1753       in post-2004 Linux) have the peculiar behaviour of not removing a
1754       connection from the pending queue in all error cases.
1756       For example, larger servers often run out of file descriptors (because
1757       of resource limits), causing "accept" to fail with "ENFILE" but not
1758       rejecting the connection, leading to libev signalling readiness on the
1759       next iteration again (the connection still exists after all), and
1760       typically causing the program to loop at 100% CPU usage.
1762       Unfortunately, the set of errors that cause this issue differs between
1763       operating systems, there is usually little the app can do to remedy the
1764       situation, and no known thread-safe method of removing the connection
1765       to cope with overload is known (to me).
1767       One of the easiest ways to handle this situation is to just ignore it -
1768       when the program encounters an overload, it will just loop until the
1769       situation is over. While this is a form of busy waiting, no OS offers
1770       an event-based way to handle this situation, so it's the best one can
1771       do.
1773       A better way to handle the situation is to log any errors other than
1774       "EAGAIN" and "EWOULDBLOCK", making sure not to flood the log with such
1775       messages, and continue as usual, which at least gives the user an idea
1776       of what could be wrong ("raise the ulimit!"). For extra points one
1777       could stop the "ev_io" watcher on the listening fd "for a while", which
1778       reduces CPU usage.
1780       If your program is single-threaded, then you could also keep a dummy
1781       file descriptor for overload situations (e.g. by opening /dev/null),
1782       and when you run into "ENFILE" or "EMFILE", close it, run "accept",
1783       close that fd, and create a new dummy fd. This will gracefully refuse
1784       clients under typical overload conditions.
1786       The last way to handle it is to simply log the error and "exit", as is
1787       often done with "malloc" failures, but this results in an easy
1788       opportunity for a DoS attack.
1790       Watcher-Specific Functions
1792       ev_io_init (ev_io *, callback, int fd, int events)
1793       ev_io_set (ev_io *, int fd, int events)
1794           Configures an "ev_io" watcher. The "fd" is the file descriptor to
1795           receive events for and "events" is either "EV_READ", "EV_WRITE",
1796           both "EV_READ | EV_WRITE" or 0, to express the desire to receive
1797           the given events.
1799           Note that setting the "events" to 0 and starting the watcher is
1800           supported, but not specially optimized - if your program sometimes
1801           happens to generate this combination this is fine, but if it is
1802           easy to avoid starting an io watcher watching for no events you
1803           should do so.
1805       ev_io_modify (ev_io *, int events)
1806           Similar to "ev_io_set", but only changes the requested events.
1807           Using this might be faster with some backends, as libev can assume
1808           that the "fd" still refers to the same underlying file description,
1809           something it cannot do when using "ev_io_set".
1811       int fd [no-modify]
1812           The file descriptor being watched. While it can be read at any
1813           time, you must not modify this member even when the watcher is
1814           stopped - always use "ev_io_set" for that.
1816       int events [no-modify]
1817           The set of events the fd is being watched for, among other flags.
1818           Remember that this is a bit set - to test for "EV_READ", use
1819           "w->events & EV_READ", and similarly for "EV_WRITE".
1821           As with "fd", you must not modify this member even when the watcher
1822           is stopped, always use "ev_io_set" or "ev_io_modify" for that.
1824       Examples
1826       Example: Call "stdin_readable_cb" when STDIN_FILENO has become, well
1827       readable, but only once. Since it is likely line-buffered, you could
1828       attempt to read a whole line in the callback.
1830          static void
1831          stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1832          {
1833             ev_io_stop (loop, w);
1834            .. read from stdin here (or from w->fd) and handle any I/O errors
1835          }
1837          ...
1838          struct ev_loop *loop = ev_default_init (0);
1839          ev_io stdin_readable;
1840          ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
1841          ev_io_start (loop, &stdin_readable);
1842          ev_run (loop, 0);
1844   "ev_timer" - relative and optionally repeating timeouts
1845       Timer watchers are simple relative timers that generate an event after
1846       a given time, and optionally repeating in regular intervals after that.
1848       The timers are based on real time, that is, if you register an event
1849       that times out after an hour and you reset your system clock to January
1850       last year, it will still time out after (roughly) one hour. "Roughly"
1851       because detecting time jumps is hard, and some inaccuracies are
1852       unavoidable (the monotonic clock option helps a lot here).
1854       The callback is guaranteed to be invoked only after its timeout has
1855       passed (not at, so on systems with very low-resolution clocks this
1856       might introduce a small delay, see "the special problem of being too
1857       early", below). If multiple timers become ready during the same loop
1858       iteration then the ones with earlier time-out values are invoked before
1859       ones of the same priority with later time-out values (but this is no
1860       longer true when a callback calls "ev_run" recursively).
1862       Be smart about timeouts
1864       Many real-world problems involve some kind of timeout, usually for
1865       error recovery. A typical example is an HTTP request - if the other
1866       side hangs, you want to raise some error after a while.
1868       What follows are some ways to handle this problem, from obvious and
1869       inefficient to smart and efficient.
1871       In the following, a 60 second activity timeout is assumed - a timeout
1872       that gets reset to 60 seconds each time there is activity (e.g. each
1873       time some data or other life sign was received).
1875       1. Use a timer and stop, reinitialise and start it on activity.
1876           This is the most obvious, but not the most simple way: In the
1877           beginning, start the watcher:
1879              ev_timer_init (timer, callback, 60., 0.);
1880              ev_timer_start (loop, timer);
1882           Then, each time there is some activity, "ev_timer_stop" it,
1883           initialise it and start it again:
1885              ev_timer_stop (loop, timer);
1886              ev_timer_set (timer, 60., 0.);
1887              ev_timer_start (loop, timer);
1889           This is relatively simple to implement, but means that each time
1890           there is some activity, libev will first have to remove the timer
1891           from its internal data structure and then add it again. Libev tries
1892           to be fast, but it's still not a constant-time operation.
1894       2. Use a timer and re-start it with "ev_timer_again" inactivity.
1895           This is the easiest way, and involves using "ev_timer_again"
1896           instead of "ev_timer_start".
1898           To implement this, configure an "ev_timer" with a "repeat" value of
1899           60 and then call "ev_timer_again" at start and each time you
1900           successfully read or write some data. If you go into an idle state
1901           where you do not expect data to travel on the socket, you can
1902           "ev_timer_stop" the timer, and "ev_timer_again" will automatically
1903           restart it if need be.
1905           That means you can ignore both the "ev_timer_start" function and
1906           the "after" argument to "ev_timer_set", and only ever use the
1907           "repeat" member and "ev_timer_again".
1909           At start:
1911              ev_init (timer, callback);
1912              timer->repeat = 60.;
1913              ev_timer_again (loop, timer);
1915           Each time there is some activity:
1917              ev_timer_again (loop, timer);
1919           It is even possible to change the time-out on the fly, regardless
1920           of whether the watcher is active or not:
1922              timer->repeat = 30.;
1923              ev_timer_again (loop, timer);
1925           This is slightly more efficient then stopping/starting the timer
1926           each time you want to modify its timeout value, as libev does not
1927           have to completely remove and re-insert the timer from/into its
1928           internal data structure.
1930           It is, however, even simpler than the "obvious" way to do it.
1932       3. Let the timer time out, but then re-arm it as required.
1933           This method is more tricky, but usually most efficient: Most
1934           timeouts are relatively long compared to the intervals between
1935           other activity - in our example, within 60 seconds, there are
1936           usually many I/O events with associated activity resets.
1938           In this case, it would be more efficient to leave the "ev_timer"
1939           alone, but remember the time of last activity, and check for a real
1940           timeout only within the callback:
1942              ev_tstamp timeout = 60.;
1943              ev_tstamp last_activity; // time of last activity
1944              ev_timer timer;
1946              static void
1947              callback (EV_P_ ev_timer *w, int revents)
1948              {
1949                // calculate when the timeout would happen
1950                ev_tstamp after = last_activity - ev_now (EV_A) + timeout;
1952                // if negative, it means we the timeout already occurred
1953                if (after < 0.)
1954                  {
1955                    // timeout occurred, take action
1956                  }
1957                else
1958                  {
1959                    // callback was invoked, but there was some recent
1960                    // activity. simply restart the timer to time out
1961                    // after "after" seconds, which is the earliest time
1962                    // the timeout can occur.
1963                    ev_timer_set (w, after, 0.);
1964                    ev_timer_start (EV_A_ w);
1965                  }
1966              }
1968           To summarise the callback: first calculate in how many seconds the
1969           timeout will occur (by calculating the absolute time when it would
1970           occur, "last_activity + timeout", and subtracting the current time,
1971           "ev_now (EV_A)" from that).
1973           If this value is negative, then we are already past the timeout,
1974           i.e. we timed out, and need to do whatever is needed in this case.
1976           Otherwise, we now the earliest time at which the timeout would
1977           trigger, and simply start the timer with this timeout value.
1979           In other words, each time the callback is invoked it will check
1980           whether the timeout occurred. If not, it will simply reschedule
1981           itself to check again at the earliest time it could time out.
1982           Rinse. Repeat.
1984           This scheme causes more callback invocations (about one every 60
1985           seconds minus half the average time between activity), but
1986           virtually no calls to libev to change the timeout.
1988           To start the machinery, simply initialise the watcher and set
1989           "last_activity" to the current time (meaning there was some
1990           activity just now), then call the callback, which will "do the
1991           right thing" and start the timer:
1993              last_activity = ev_now (EV_A);
1994              ev_init (&timer, callback);
1995              callback (EV_A_ &timer, 0);
1997           When there is some activity, simply store the current time in
1998           "last_activity", no libev calls at all:
2000              if (activity detected)
2001                last_activity = ev_now (EV_A);
2003           When your timeout value changes, then the timeout can be changed by
2004           simply providing a new value, stopping the timer and calling the
2005           callback, which will again do the right thing (for example, time
2006           out immediately :).
2008              timeout = new_value;
2009              ev_timer_stop (EV_A_ &timer);
2010              callback (EV_A_ &timer, 0);
2012           This technique is slightly more complex, but in most cases where
2013           the time-out is unlikely to be triggered, much more efficient.
2015       4. Wee, just use a double-linked list for your timeouts.
2016           If there is not one request, but many thousands (millions...), all
2017           employing some kind of timeout with the same timeout value, then
2018           one can do even better:
2020           When starting the timeout, calculate the timeout value and put the
2021           timeout at the end of the list.
2023           Then use an "ev_timer" to fire when the timeout at the beginning of
2024           the list is expected to fire (for example, using the technique #3).
2026           When there is some activity, remove the timer from the list,
2027           recalculate the timeout, append it to the end of the list again,
2028           and make sure to update the "ev_timer" if it was taken from the
2029           beginning of the list.
2031           This way, one can manage an unlimited number of timeouts in O(1)
2032           time for starting, stopping and updating the timers, at the expense
2033           of a major complication, and having to use a constant timeout. The
2034           constant timeout ensures that the list stays sorted.
2036       So which method the best?
2038       Method #2 is a simple no-brain-required solution that is adequate in
2039       most situations. Method #3 requires a bit more thinking, but handles
2040       many cases better, and isn't very complicated either. In most case,
2041       choosing either one is fine, with #3 being better in typical
2042       situations.
2044       Method #1 is almost always a bad idea, and buys you nothing. Method #4
2045       is rather complicated, but extremely efficient, something that really
2046       pays off after the first million or so of active timers, i.e. it's
2047       usually overkill :)
2049       The special problem of being too early
2051       If you ask a timer to call your callback after three seconds, then you
2052       expect it to be invoked after three seconds - but of course, this
2053       cannot be guaranteed to infinite precision. Less obviously, it cannot
2054       be guaranteed to any precision by libev - imagine somebody suspending
2055       the process with a STOP signal for a few hours for example.
2057       So, libev tries to invoke your callback as soon as possible after the
2058       delay has occurred, but cannot guarantee this.
2060       A less obvious failure mode is calling your callback too early: many
2061       event loops compare timestamps with a "elapsed delay >= requested
2062       delay", but this can cause your callback to be invoked much earlier
2063       than you would expect.
2065       To see why, imagine a system with a clock that only offers full second
2066       resolution (think windows if you can't come up with a broken enough OS
2067       yourself). If you schedule a one-second timer at the time 500.9, then
2068       the event loop will schedule your timeout to elapse at a system time of
2069       500 (500.9 truncated to the resolution) + 1, or 501.
2071       If an event library looks at the timeout 0.1s later, it will see "501
2072       >= 501" and invoke the callback 0.1s after it was started, even though
2073       a one-second delay was requested - this is being "too early", despite
2074       best intentions.
2076       This is the reason why libev will never invoke the callback if the
2077       elapsed delay equals the requested delay, but only when the elapsed
2078       delay is larger than the requested delay. In the example above, libev
2079       would only invoke the callback at system time 502, or 1.1s after the
2080       timer was started.
2082       So, while libev cannot guarantee that your callback will be invoked
2083       exactly when requested, it can and does guarantee that the requested
2084       delay has actually elapsed, or in other words, it always errs on the
2085       "too late" side of things.
2087       The special problem of time updates
2089       Establishing the current time is a costly operation (it usually takes
2090       at least one system call): EV therefore updates its idea of the current
2091       time only before and after "ev_run" collects new events, which causes a
2092       growing difference between "ev_now ()" and "ev_time ()" when handling
2093       lots of events in one iteration.
2095       The relative timeouts are calculated relative to the "ev_now ()" time.
2096       This is usually the right thing as this timestamp refers to the time of
2097       the event triggering whatever timeout you are modifying/starting. If
2098       you suspect event processing to be delayed and you need to base the
2099       timeout on the current time, use something like the following to adjust
2100       for it:
2102          ev_timer_set (&timer, after + (ev_time () - ev_now ()), 0.);
2104       If the event loop is suspended for a long time, you can also force an
2105       update of the time returned by "ev_now ()" by calling "ev_now_update
2106       ()", although that will push the event time of all outstanding events
2107       further into the future.
2109       The special problem of unsynchronised clocks
2111       Modern systems have a variety of clocks - libev itself uses the normal
2112       "wall clock" clock and, if available, the monotonic clock (to avoid
2113       time jumps).
2115       Neither of these clocks is synchronised with each other or any other
2116       clock on the system, so "ev_time ()" might return a considerably
2117       different time than "gettimeofday ()" or "time ()". On a GNU/Linux
2118       system, for example, a call to "gettimeofday" might return a second
2119       count that is one higher than a directly following call to "time".
2121       The moral of this is to only compare libev-related timestamps with
2122       "ev_time ()" and "ev_now ()", at least if you want better precision
2123       than a second or so.
2125       One more problem arises due to this lack of synchronisation: if libev
2126       uses the system monotonic clock and you compare timestamps from
2127       "ev_time" or "ev_now" from when you started your timer and when your
2128       callback is invoked, you will find that sometimes the callback is a bit
2129       "early".
2131       This is because "ev_timer"s work in real time, not wall clock time, so
2132       libev makes sure your callback is not invoked before the delay
2133       happened, measured according to the real time, not the system clock.
2135       If your timeouts are based on a physical timescale (e.g. "time out this
2136       connection after 100 seconds") then this shouldn't bother you as it is
2137       exactly the right behaviour.
2139       If you want to compare wall clock/system timestamps to your timers,
2140       then you need to use "ev_periodic"s, as these are based on the wall
2141       clock time, where your comparisons will always generate correct
2142       results.
2144       The special problems of suspended animation
2146       When you leave the server world it is quite customary to hit machines
2147       that can suspend/hibernate - what happens to the clocks during such a
2148       suspend?
2150       Some quick tests made with a Linux 2.6.28 indicate that a suspend
2151       freezes all processes, while the clocks ("times", "CLOCK_MONOTONIC")
2152       continue to run until the system is suspended, but they will not
2153       advance while the system is suspended. That means, on resume, it will
2154       be as if the program was frozen for a few seconds, but the suspend time
2155       will not be counted towards "ev_timer" when a monotonic clock source is
2156       used. The real time clock advanced as expected, but if it is used as
2157       sole clocksource, then a long suspend would be detected as a time jump
2158       by libev, and timers would be adjusted accordingly.
2160       I would not be surprised to see different behaviour in different
2161       between operating systems, OS versions or even different hardware.
2163       The other form of suspend (job control, or sending a SIGSTOP) will see
2164       a time jump in the monotonic clocks and the realtime clock. If the
2165       program is suspended for a very long time, and monotonic clock sources
2166       are in use, then you can expect "ev_timer"s to expire as the full
2167       suspension time will be counted towards the timers. When no monotonic
2168       clock source is in use, then libev will again assume a timejump and
2169       adjust accordingly.
2171       It might be beneficial for this latter case to call "ev_suspend" and
2172       "ev_resume" in code that handles "SIGTSTP", to at least get
2173       deterministic behaviour in this case (you can do nothing against
2174       "SIGSTOP").
2176       Watcher-Specific Functions and Data Members
2178       ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
2179       ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
2180           Configure the timer to trigger after "after" seconds (fractional
2181           and negative values are supported). If "repeat" is 0., then it will
2182           automatically be stopped once the timeout is reached. If it is
2183           positive, then the timer will automatically be configured to
2184           trigger again "repeat" seconds later, again, and again, until
2185           stopped manually.
2187           The timer itself will do a best-effort at avoiding drift, that is,
2188           if you configure a timer to trigger every 10 seconds, then it will
2189           normally trigger at exactly 10 second intervals. If, however, your
2190           program cannot keep up with the timer (because it takes longer than
2191           those 10 seconds to do stuff) the timer will not fire more than
2192           once per event loop iteration.
2194       ev_timer_again (loop, ev_timer *)
2195           This will act as if the timer timed out, and restarts it again if
2196           it is repeating. It basically works like calling "ev_timer_stop",
2197           updating the timeout to the "repeat" value and calling
2198           "ev_timer_start".
2200           The exact semantics are as in the following rules, all of which
2201           will be applied to the watcher:
2203           If the timer is pending, the pending status is always cleared.
2204           If the timer is started but non-repeating, stop it (as if it timed
2205           out, without invoking it).
2206           If the timer is repeating, make the "repeat" value the new timeout
2207           and start the timer, if necessary.
2209           This sounds a bit complicated, see "Be smart about timeouts",
2210           above, for a usage example.
2212       ev_tstamp ev_timer_remaining (loop, ev_timer *)
2213           Returns the remaining time until a timer fires. If the timer is
2214           active, then this time is relative to the current event loop time,
2215           otherwise it's the timeout value currently configured.
2217           That is, after an "ev_timer_set (w, 5, 7)", "ev_timer_remaining"
2218           returns 5. When the timer is started and one second passes,
2219           "ev_timer_remaining" will return 4. When the timer expires and is
2220           restarted, it will return roughly 7 (likely slightly less as
2221           callback invocation takes some time, too), and so on.
2223       ev_tstamp repeat [read-write]
2224           The current "repeat" value. Will be used each time the watcher
2225           times out or "ev_timer_again" is called, and determines the next
2226           timeout (if any), which is also when any modifications are taken
2227           into account.
2229       Examples
2231       Example: Create a timer that fires after 60 seconds.
2233          static void
2234          one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2235          {
2236            .. one minute over, w is actually stopped right here
2237          }
2239          ev_timer mytimer;
2240          ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2241          ev_timer_start (loop, &mytimer);
2243       Example: Create a timeout timer that times out after 10 seconds of
2244       inactivity.
2246          static void
2247          timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2248          {
2249            .. ten seconds without any activity
2250          }
2252          ev_timer mytimer;
2253          ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2254          ev_timer_again (&mytimer); /* start timer */
2255          ev_run (loop, 0);
2257          // and in some piece of code that gets executed on any "activity":
2258          // reset the timeout to start ticking again at 10 seconds
2259          ev_timer_again (&mytimer);
2261   "ev_periodic" - to cron or not to cron?
2262       Periodic watchers are also timers of a kind, but they are very
2263       versatile (and unfortunately a bit complex).
2265       Unlike "ev_timer", periodic watchers are not based on real time (or
2266       relative time, the physical time that passes) but on wall clock time
2267       (absolute time, the thing you can read on your calendar or clock). The
2268       difference is that wall clock time can run faster or slower than real
2269       time, and time jumps are not uncommon (e.g. when you adjust your wrist-
2270       watch).
2272       You can tell a periodic watcher to trigger after some specific point in
2273       time: for example, if you tell a periodic watcher to trigger "in 10
2274       seconds" (by specifying e.g. "ev_now () + 10.", that is, an absolute
2275       time not a delay) and then reset your system clock to January of the
2276       previous year, then it will take a year or more to trigger the event
2277       (unlike an "ev_timer", which would still trigger roughly 10 seconds
2278       after starting it, as it uses a relative timeout).
2280       "ev_periodic" watchers can also be used to implement vastly more
2281       complex timers, such as triggering an event on each "midnight, local
2282       time", or other complicated rules. This cannot easily be done with
2283       "ev_timer" watchers, as those cannot react to time jumps.
2285       As with timers, the callback is guaranteed to be invoked only when the
2286       point in time where it is supposed to trigger has passed. If multiple
2287       timers become ready during the same loop iteration then the ones with
2288       earlier time-out values are invoked before ones with later time-out
2289       values (but this is no longer true when a callback calls "ev_run"
2290       recursively).
2292       Watcher-Specific Functions and Data Members
2294       ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp
2295       interval, reschedule_cb)
2296       ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval,
2297       reschedule_cb)
2298           Lots of arguments, let's sort it out... There are basically three
2299           modes of operation, and we will explain them from simplest to most
2300           complex:
2302           •   absolute timer (offset = absolute time, interval = 0,
2303               reschedule_cb = 0)
2305               In this configuration the watcher triggers an event after the
2306               wall clock time "offset" has passed. It will not repeat and
2307               will not adjust when a time jump occurs, that is, if it is to
2308               be run at January 1st 2011 then it will be stopped and invoked
2309               when the system clock reaches or surpasses this point in time.
2311           •   repeating interval timer (offset = offset within interval,
2312               interval > 0, reschedule_cb = 0)
2314               In this mode the watcher will always be scheduled to time out
2315               at the next "offset + N * interval" time (for some integer N,
2316               which can also be negative) and then repeat, regardless of any
2317               time jumps. The "offset" argument is merely an offset into the
2318               "interval" periods.
2320               This can be used to create timers that do not drift with
2321               respect to the system clock, for example, here is an
2322               "ev_periodic" that triggers each hour, on the hour (with
2323               respect to UTC):
2325                  ev_periodic_set (&periodic, 0., 3600., 0);
2327               This doesn't mean there will always be 3600 seconds in between
2328               triggers, but only that the callback will be called when the
2329               system time shows a full hour (UTC), or more correctly, when
2330               the system time is evenly divisible by 3600.
2332               Another way to think about it (for the mathematically inclined)
2333               is that "ev_periodic" will try to run the callback in this mode
2334               at the next possible time where "time = offset (mod interval)",
2335               regardless of any time jumps.
2337               The "interval" MUST be positive, and for numerical stability,
2338               the interval value should be higher than "1/8192" (which is
2339               around 100 microseconds) and "offset" should be higher than 0
2340               and should have at most a similar magnitude as the current time
2341               (say, within a factor of ten). Typical values for offset are,
2342               in fact, 0 or something between 0 and "interval", which is also
2343               the recommended range.
2345               Note also that there is an upper limit to how often a timer can
2346               fire (CPU speed for example), so if "interval" is very small
2347               then timing stability will of course deteriorate. Libev itself
2348               tries to be exact to be about one millisecond (if the OS
2349               supports it and the machine is fast enough).
2351           •   manual reschedule mode (offset ignored, interval ignored,
2352               reschedule_cb = callback)
2354               In this mode the values for "interval" and "offset" are both
2355               being ignored. Instead, each time the periodic watcher gets
2356               scheduled, the reschedule callback will be called with the
2357               watcher as first, and the current time as second argument.
2359               NOTE: This callback MUST NOT stop or destroy any periodic
2360               watcher, ever, or make ANY other event loop modifications
2361               whatsoever, unless explicitly allowed by documentation here.
2363               If you need to stop it, return "now + 1e30" (or so, fudge
2364               fudge) and stop it afterwards (e.g. by starting an "ev_prepare"
2365               watcher, which is the only event loop modification you are
2366               allowed to do).
2368               The callback prototype is "ev_tstamp
2369               (*reschedule_cb)(ev_periodic *w, ev_tstamp now)", e.g.:
2371                  static ev_tstamp
2372                  my_rescheduler (ev_periodic *w, ev_tstamp now)
2373                  {
2374                    return now + 60.;
2375                  }
2377               It must return the next time to trigger, based on the passed
2378               time value (that is, the lowest time value larger than to the
2379               second argument). It will usually be called just before the
2380               callback will be triggered, but might be called at other times,
2381               too.
2383               NOTE: This callback must always return a time that is higher
2384               than or equal to the passed "now" value.
2386               This can be used to create very complex timers, such as a timer
2387               that triggers on "next midnight, local time". To do this, you
2388               would calculate the next midnight after "now" and return the
2389               timestamp value for this. Here is a (completely untested, no
2390               error checking) example on how to do this:
2392                  #include <time.h>
2394                  static ev_tstamp
2395                  my_rescheduler (ev_periodic *w, ev_tstamp now)
2396                  {
2397                    time_t tnow = (time_t)now;
2398                    struct tm tm;
2399                    localtime_r (&tnow, &tm);
2401                    tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2402                    ++tm.tm_mday; // midnight next day
2404                    return mktime (&tm);
2405                  }
2407               Note: this code might run into trouble on days that have more
2408               then two midnights (beginning and end).
2410       ev_periodic_again (loop, ev_periodic *)
2411           Simply stops and restarts the periodic watcher again. This is only
2412           useful when you changed some parameters or the reschedule callback
2413           would return a different time than the last time it was called
2414           (e.g. in a crond like program when the crontabs have changed).
2416       ev_tstamp ev_periodic_at (ev_periodic *)
2417           When active, returns the absolute time that the watcher is supposed
2418           to trigger next. This is not the same as the "offset" argument to
2419           "ev_periodic_set", but indeed works even in interval and manual
2420           rescheduling modes.
2422       ev_tstamp offset [read-write]
2423           When repeating, this contains the offset value, otherwise this is
2424           the absolute point in time (the "offset" value passed to
2425           "ev_periodic_set", although libev might modify this value for
2426           better numerical stability).
2428           Can be modified any time, but changes only take effect when the
2429           periodic timer fires or "ev_periodic_again" is being called.
2431       ev_tstamp interval [read-write]
2432           The current interval value. Can be modified any time, but changes
2433           only take effect when the periodic timer fires or
2434           "ev_periodic_again" is being called.
2436       ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]
2437           The current reschedule callback, or 0, if this functionality is
2438           switched off. Can be changed any time, but changes only take effect
2439           when the periodic timer fires or "ev_periodic_again" is being
2440           called.
2442       Examples
2444       Example: Call a callback every hour, or, more precisely, whenever the
2445       system time is divisible by 3600. The callback invocation times have
2446       potentially a lot of jitter, but good long-term stability.
2448          static void
2449          clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2450          {
2451            ... its now a full hour (UTC, or TAI or whatever your clock follows)
2452          }
2454          ev_periodic hourly_tick;
2455          ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2456          ev_periodic_start (loop, &hourly_tick);
2458       Example: The same as above, but use a reschedule callback to do it:
2460          #include <math.h>
2462          static ev_tstamp
2463          my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2464          {
2465            return now + (3600. - fmod (now, 3600.));
2466          }
2468          ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2470       Example: Call a callback every hour, starting now:
2472          ev_periodic hourly_tick;
2473          ev_periodic_init (&hourly_tick, clock_cb,
2474                            fmod (ev_now (loop), 3600.), 3600., 0);
2475          ev_periodic_start (loop, &hourly_tick);
2477   "ev_signal" - signal me when a signal gets signalled!
2478       Signal watchers will trigger an event when the process receives a
2479       specific signal one or more times. Even though signals are very
2480       asynchronous, libev will try its best to deliver signals synchronously,
2481       i.e. as part of the normal event processing, like any other event.
2483       If you want signals to be delivered truly asynchronously, just use
2484       "sigaction" as you would do without libev and forget about sharing the
2485       signal. You can even use "ev_async" from a signal handler to
2486       synchronously wake up an event loop.
2488       You can configure as many watchers as you like for the same signal, but
2489       only within the same loop, i.e. you can watch for "SIGINT" in your
2490       default loop and for "SIGIO" in another loop, but you cannot watch for
2491       "SIGINT" in both the default loop and another loop at the same time. At
2492       the moment, "SIGCHLD" is permanently tied to the default loop.
2494       Only after the first watcher for a signal is started will libev
2495       actually register something with the kernel. It thus coexists with your
2496       own signal handlers as long as you don't register any with libev for
2497       the same signal.
2499       If possible and supported, libev will install its handlers with
2500       "SA_RESTART" (or equivalent) behaviour enabled, so system calls should
2501       not be unduly interrupted. If you have a problem with system calls
2502       getting interrupted by signals you can block all signals in an
2503       "ev_check" watcher and unblock them in an "ev_prepare" watcher.
2505       The special problem of inheritance over fork/execve/pthread_create
2507       Both the signal mask ("sigprocmask") and the signal disposition
2508       ("sigaction") are unspecified after starting a signal watcher (and
2509       after stopping it again), that is, libev might or might not block the
2510       signal, and might or might not set or restore the installed signal
2511       handler (but see "EVFLAG_NOSIGMASK").
2513       While this does not matter for the signal disposition (libev never sets
2514       signals to "SIG_IGN", so handlers will be reset to "SIG_DFL" on
2515       "execve"), this matters for the signal mask: many programs do not
2516       expect certain signals to be blocked.
2518       This means that before calling "exec" (from the child) you should reset
2519       the signal mask to whatever "default" you expect (all clear is a good
2520       choice usually).
2522       The simplest way to ensure that the signal mask is reset in the child
2523       is to install a fork handler with "pthread_atfork" that resets it. That
2524       will catch fork calls done by libraries (such as the libc) as well.
2526       In current versions of libev, the signal will not be blocked
2527       indefinitely unless you use the "signalfd" API ("EV_SIGNALFD"). While
2528       this reduces the window of opportunity for problems, it will not go
2529       away, as libev has to modify the signal mask, at least temporarily.
2531       So I can't stress this enough: If you do not reset your signal mask
2532       when you expect it to be empty, you have a race condition in your code.
2533       This is not a libev-specific thing, this is true for most event
2534       libraries.
2536       The special problem of threads signal handling
2538       POSIX threads has problematic signal handling semantics, specifically,
2539       a lot of functionality (sigfd, sigwait etc.) only really works if all
2540       threads in a process block signals, which is hard to achieve.
2542       When you want to use sigwait (or mix libev signal handling with your
2543       own for the same signals), you can tackle this problem by globally
2544       blocking all signals before creating any threads (or creating them with
2545       a fully set sigprocmask) and also specifying the "EVFLAG_NOSIGMASK"
2546       when creating loops. Then designate one thread as "signal receiver
2547       thread" which handles these signals. You can pass on any signals that
2548       libev might be interested in by calling "ev_feed_signal".
2550       Watcher-Specific Functions and Data Members
2552       ev_signal_init (ev_signal *, callback, int signum)
2553       ev_signal_set (ev_signal *, int signum)
2554           Configures the watcher to trigger on the given signal number
2555           (usually one of the "SIGxxx" constants).
2557       int signum [read-only]
2558           The signal the watcher watches out for.
2560       Examples
2562       Example: Try to exit cleanly on SIGINT.
2564          static void
2565          sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2566          {
2567            ev_break (loop, EVBREAK_ALL);
2568          }
2570          ev_signal signal_watcher;
2571          ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2572          ev_signal_start (loop, &signal_watcher);
2574   "ev_child" - watch out for process status changes
2575       Child watchers trigger when your process receives a SIGCHLD in response
2576       to some child status changes (most typically when a child of yours dies
2577       or exits). It is permissible to install a child watcher after the child
2578       has been forked (which implies it might have already exited), as long
2579       as the event loop isn't entered (or is continued from a watcher), i.e.,
2580       forking and then immediately registering a watcher for the child is
2581       fine, but forking and registering a watcher a few event loop iterations
2582       later or in the next callback invocation is not.
2584       Only the default event loop is capable of handling signals, and
2585       therefore you can only register child watchers in the default event
2586       loop.
2588       Due to some design glitches inside libev, child watchers will always be
2589       handled at maximum priority (their priority is set to "EV_MAXPRI" by
2590       libev)
2592       Process Interaction
2594       Libev grabs "SIGCHLD" as soon as the default event loop is initialised.
2595       This is necessary to guarantee proper behaviour even if the first child
2596       watcher is started after the child exits. The occurrence of "SIGCHLD"
2597       is recorded asynchronously, but child reaping is done synchronously as
2598       part of the event loop processing. Libev always reaps all children,
2599       even ones not watched.
2601       Overriding the Built-In Processing
2603       Libev offers no special support for overriding the built-in child
2604       processing, but if your application collides with libev's default child
2605       handler, you can override it easily by installing your own handler for
2606       "SIGCHLD" after initialising the default loop, and making sure the
2607       default loop never gets destroyed. You are encouraged, however, to use
2608       an event-based approach to child reaping and thus use libev's support
2609       for that, so other libev users can use "ev_child" watchers freely.
2611       Stopping the Child Watcher
2613       Currently, the child watcher never gets stopped, even when the child
2614       terminates, so normally one needs to stop the watcher in the callback.
2615       Future versions of libev might stop the watcher automatically when a
2616       child exit is detected (calling "ev_child_stop" twice is not a
2617       problem).
2619       Watcher-Specific Functions and Data Members
2621       ev_child_init (ev_child *, callback, int pid, int trace)
2622       ev_child_set (ev_child *, int pid, int trace)
2623           Configures the watcher to wait for status changes of process "pid"
2624           (or any process if "pid" is specified as 0). The callback can look
2625           at the "rstatus" member of the "ev_child" watcher structure to see
2626           the status word (use the macros from "sys/wait.h" and see your
2627           systems "waitpid" documentation). The "rpid" member contains the
2628           pid of the process causing the status change. "trace" must be
2629           either 0 (only activate the watcher when the process terminates) or
2630           1 (additionally activate the watcher when the process is stopped or
2631           continued).
2633       int pid [read-only]
2634           The process id this watcher watches out for, or 0, meaning any
2635           process id.
2637       int rpid [read-write]
2638           The process id that detected a status change.
2640       int rstatus [read-write]
2641           The process exit/trace status caused by "rpid" (see your systems
2642           "waitpid" and "sys/wait.h" documentation for details).
2644       Examples
2646       Example: "fork()" a new process and install a child handler to wait for
2647       its completion.
2649          ev_child cw;
2651          static void
2652          child_cb (EV_P_ ev_child *w, int revents)
2653          {
2654            ev_child_stop (EV_A_ w);
2655            printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
2656          }
2658          pid_t pid = fork ();
2660          if (pid < 0)
2661            // error
2662          else if (pid == 0)
2663            {
2664              // the forked child executes here
2665              exit (1);
2666            }
2667          else
2668            {
2669              ev_child_init (&cw, child_cb, pid, 0);
2670              ev_child_start (EV_DEFAULT_ &cw);
2671            }
2673   "ev_stat" - did the file attributes just change?
2674       This watches a file system path for attribute changes. That is, it
2675       calls "stat" on that path in regular intervals (or when the OS says it
2676       changed) and sees if it changed compared to the last time, invoking the
2677       callback if it did. Starting the watcher "stat"'s the file, so only
2678       changes that happen after the watcher has been started will be
2679       reported.
2681       The path does not need to exist: changing from "path exists" to "path
2682       does not exist" is a status change like any other. The condition "path
2683       does not exist" (or more correctly "path cannot be stat'ed") is
2684       signified by the "st_nlink" field being zero (which is otherwise always
2685       forced to be at least one) and all the other fields of the stat buffer
2686       having unspecified contents.
2688       The path must not end in a slash or contain special components such as
2689       "." or "..". The path should be absolute: If it is relative and your
2690       working directory changes, then the behaviour is undefined.
2692       Since there is no portable change notification interface available, the
2693       portable implementation simply calls stat(2) regularly on the path to
2694       see if it changed somehow. You can specify a recommended polling
2695       interval for this case. If you specify a polling interval of 0 (highly
2696       recommended!) then a suitable, unspecified default value will be used
2697       (which you can expect to be around five seconds, although this might
2698       change dynamically). Libev will also impose a minimum interval which is
2699       currently around 0.1, but that's usually overkill.
2701       This watcher type is not meant for massive numbers of stat watchers, as
2702       even with OS-supported change notifications, this can be resource-
2703       intensive.
2705       At the time of this writing, the only OS-specific interface implemented
2706       is the Linux inotify interface (implementing kqueue support is left as
2707       an exercise for the reader. Note, however, that the author sees no way
2708       of implementing "ev_stat" semantics with kqueue, except as a hint).
2710       ABI Issues (Largefile Support)
2712       Libev by default (unless the user overrides this) uses the default
2713       compilation environment, which means that on systems with large file
2714       support disabled by default, you get the 32 bit version of the stat
2715       structure. When using the library from programs that change the ABI to
2716       use 64 bit file offsets the programs will fail. In that case you have
2717       to compile libev with the same flags to get binary compatibility. This
2718       is obviously the case with any flags that change the ABI, but the
2719       problem is most noticeably displayed with ev_stat and large file
2720       support.
2722       The solution for this is to lobby your distribution maker to make large
2723       file interfaces available by default (as e.g. FreeBSD does) and not
2724       optional. Libev cannot simply switch on large file support because it
2725       has to exchange stat structures with application programs compiled
2726       using the default compilation environment.
2728       Inotify and Kqueue
2730       When "inotify (7)" support has been compiled into libev and present at
2731       runtime, it will be used to speed up change detection where possible.
2732       The inotify descriptor will be created lazily when the first "ev_stat"
2733       watcher is being started.
2735       Inotify presence does not change the semantics of "ev_stat" watchers
2736       except that changes might be detected earlier, and in some cases, to
2737       avoid making regular "stat" calls. Even in the presence of inotify
2738       support there are many cases where libev has to resort to regular
2739       "stat" polling, but as long as kernel 2.6.25 or newer is used (2.6.24
2740       and older have too many bugs), the path exists (i.e. stat succeeds),
2741       and the path resides on a local filesystem (libev currently assumes
2742       only ext2/3, jfs, reiserfs and xfs are fully working) libev usually
2743       gets away without polling.
2745       There is no support for kqueue, as apparently it cannot be used to
2746       implement this functionality, due to the requirement of having a file
2747       descriptor open on the object at all times, and detecting renames,
2748       unlinks etc. is difficult.
2750       "stat ()" is a synchronous operation
2752       Libev doesn't normally do any kind of I/O itself, and so is not
2753       blocking the process. The exception are "ev_stat" watchers - those call
2754       "stat ()", which is a synchronous operation.
2756       For local paths, this usually doesn't matter: unless the system is very
2757       busy or the intervals between stat's are large, a stat call will be
2758       fast, as the path data is usually in memory already (except when
2759       starting the watcher).
2761       For networked file systems, calling "stat ()" can block an indefinite
2762       time due to network issues, and even under good conditions, a stat call
2763       often takes multiple milliseconds.
2765       Therefore, it is best to avoid using "ev_stat" watchers on networked
2766       paths, although this is fully supported by libev.
2768       The special problem of stat time resolution
2770       The "stat ()" system call only supports full-second resolution
2771       portably, and even on systems where the resolution is higher, most file
2772       systems still only support whole seconds.
2774       That means that, if the time is the only thing that changes, you can
2775       easily miss updates: on the first update, "ev_stat" detects a change
2776       and calls your callback, which does something. When there is another
2777       update within the same second, "ev_stat" will be unable to detect
2778       unless the stat data does change in other ways (e.g. file size).
2780       The solution to this is to delay acting on a change for slightly more
2781       than a second (or till slightly after the next full second boundary),
2782       using a roughly one-second-delay "ev_timer" (e.g. "ev_timer_set (w, 0.,
2783       1.02); ev_timer_again (loop, w)").
2785       The .02 offset is added to work around small timing inconsistencies of
2786       some operating systems (where the second counter of the current time
2787       might be be delayed. One such system is the Linux kernel, where a call
2788       to "gettimeofday" might return a timestamp with a full second later
2789       than a subsequent "time" call - if the equivalent of "time ()" is used
2790       to update file times then there will be a small window where the kernel
2791       uses the previous second to update file times but libev might already
2792       execute the timer callback).
2794       Watcher-Specific Functions and Data Members
2796       ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp
2797       interval)
2798       ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)
2799           Configures the watcher to wait for status changes of the given
2800           "path". The "interval" is a hint on how quickly a change is
2801           expected to be detected and should normally be specified as 0 to
2802           let libev choose a suitable value. The memory pointed to by "path"
2803           must point to the same path for as long as the watcher is active.
2805           The callback will receive an "EV_STAT" event when a change was
2806           detected, relative to the attributes at the time the watcher was
2807           started (or the last change was detected).
2809       ev_stat_stat (loop, ev_stat *)
2810           Updates the stat buffer immediately with new values. If you change
2811           the watched path in your callback, you could call this function to
2812           avoid detecting this change (while introducing a race condition if
2813           you are not the only one changing the path). Can also be useful
2814           simply to find out the new values.
2816       ev_statdata attr [read-only]
2817           The most-recently detected attributes of the file. Although the
2818           type is "ev_statdata", this is usually the (or one of the) "struct
2819           stat" types suitable for your system, but you can only rely on the
2820           POSIX-standardised members to be present. If the "st_nlink" member
2821           is 0, then there was some error while "stat"ing the file.
2823       ev_statdata prev [read-only]
2824           The previous attributes of the file. The callback gets invoked
2825           whenever "prev" != "attr", or, more precisely, one or more of these
2826           members differ: "st_dev", "st_ino", "st_mode", "st_nlink",
2827           "st_uid", "st_gid", "st_rdev", "st_size", "st_atime", "st_mtime",
2828           "st_ctime".
2830       ev_tstamp interval [read-only]
2831           The specified interval.
2833       const char *path [read-only]
2834           The file system path that is being watched.
2836       Examples
2838       Example: Watch "/etc/passwd" for attribute changes.
2840          static void
2841          passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
2842          {
2843            /* /etc/passwd changed in some way */
2844            if (w->attr.st_nlink)
2845              {
2846                printf ("passwd current size  %ld\n", (long)w->attr.st_size);
2847                printf ("passwd current atime %ld\n", (long)w->attr.st_mtime);
2848                printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime);
2849              }
2850            else
2851              /* you shalt not abuse printf for puts */
2852              puts ("wow, /etc/passwd is not there, expect problems. "
2853                    "if this is windows, they already arrived\n");
2854          }
2856          ...
2857          ev_stat passwd;
2859          ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
2860          ev_stat_start (loop, &passwd);
2862       Example: Like above, but additionally use a one-second delay so we do
2863       not miss updates (however, frequent updates will delay processing, too,
2864       so one might do the work both on "ev_stat" callback invocation and on
2865       "ev_timer" callback invocation).
2867          static ev_stat passwd;
2868          static ev_timer timer;
2870          static void
2871          timer_cb (EV_P_ ev_timer *w, int revents)
2872          {
2873            ev_timer_stop (EV_A_ w);
2875            /* now it's one second after the most recent passwd change */
2876          }
2878          static void
2879          stat_cb (EV_P_ ev_stat *w, int revents)
2880          {
2881            /* reset the one-second timer */
2882            ev_timer_again (EV_A_ &timer);
2883          }
2885          ...
2886          ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
2887          ev_stat_start (loop, &passwd);
2888          ev_timer_init (&timer, timer_cb, 0., 1.02);
2890   "ev_idle" - when you've got nothing better to do...
2891       Idle watchers trigger events when no other events of the same or higher
2892       priority are pending (prepare, check and other idle watchers do not
2893       count as receiving "events").
2895       That is, as long as your process is busy handling sockets or timeouts
2896       (or even signals, imagine) of the same or higher priority it will not
2897       be triggered. But when your process is idle (or only lower-priority
2898       watchers are pending), the idle watchers are being called once per
2899       event loop iteration - until stopped, that is, or your process receives
2900       more events and becomes busy again with higher priority stuff.
2902       The most noteworthy effect is that as long as any idle watchers are
2903       active, the process will not block when waiting for new events.
2905       Apart from keeping your process non-blocking (which is a useful effect
2906       on its own sometimes), idle watchers are a good place to do "pseudo-
2907       background processing", or delay processing stuff to after the event
2908       loop has handled all outstanding events.
2910       Abusing an "ev_idle" watcher for its side-effect
2912       As long as there is at least one active idle watcher, libev will never
2913       sleep unnecessarily. Or in other words, it will loop as fast as
2914       possible.  For this to work, the idle watcher doesn't need to be
2915       invoked at all - the lowest priority will do.
2917       This mode of operation can be useful together with an "ev_check"
2918       watcher, to do something on each event loop iteration - for example to
2919       balance load between different connections.
2921       See "Abusing an ev_check watcher for its side-effect" for a longer
2922       example.
2924       Watcher-Specific Functions and Data Members
2926       ev_idle_init (ev_idle *, callback)
2927           Initialises and configures the idle watcher - it has no parameters
2928           of any kind. There is a "ev_idle_set" macro, but using it is
2929           utterly pointless, believe me.
2931       Examples
2933       Example: Dynamically allocate an "ev_idle" watcher, start it, and in
2934       the callback, free it. Also, use no error checking, as usual.
2936          static void
2937          idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
2938          {
2939            // stop the watcher
2940            ev_idle_stop (loop, w);
2942            // now we can free it
2943            free (w);
2945            // now do something you wanted to do when the program has
2946            // no longer anything immediate to do.
2947          }
2949          ev_idle *idle_watcher = malloc (sizeof (ev_idle));
2950          ev_idle_init (idle_watcher, idle_cb);
2951          ev_idle_start (loop, idle_watcher);
2953   "ev_prepare" and "ev_check" - customise your event loop!
2954       Prepare and check watchers are often (but not always) used in pairs:
2955       prepare watchers get invoked before the process blocks and check
2956       watchers afterwards.
2958       You must not call "ev_run" (or similar functions that enter the current
2959       event loop) or "ev_loop_fork" from either "ev_prepare" or "ev_check"
2960       watchers. Other loops than the current one are fine, however. The
2961       rationale behind this is that you do not need to check for recursion in
2962       those watchers, i.e. the sequence will always be "ev_prepare",
2963       blocking, "ev_check" so if you have one watcher of each kind they will
2964       always be called in pairs bracketing the blocking call.
2966       Their main purpose is to integrate other event mechanisms into libev
2967       and their use is somewhat advanced. They could be used, for example, to
2968       track variable changes, implement your own watchers, integrate net-snmp
2969       or a coroutine library and lots more. They are also occasionally useful
2970       if you cache some data and want to flush it before blocking (for
2971       example, in X programs you might want to do an "XFlush ()" in an
2972       "ev_prepare" watcher).
2974       This is done by examining in each prepare call which file descriptors
2975       need to be watched by the other library, registering "ev_io" watchers
2976       for them and starting an "ev_timer" watcher for any timeouts (many
2977       libraries provide exactly this functionality). Then, in the check
2978       watcher, you check for any events that occurred (by checking the
2979       pending status of all watchers and stopping them) and call back into
2980       the library. The I/O and timer callbacks will never actually be called
2981       (but must be valid nevertheless, because you never know, you know?).
2983       As another example, the Perl Coro module uses these hooks to integrate
2984       coroutines into libev programs, by yielding to other active coroutines
2985       during each prepare and only letting the process block if no coroutines
2986       are ready to run (it's actually more complicated: it only runs
2987       coroutines with priority higher than or equal to the event loop and one
2988       coroutine of lower priority, but only once, using idle watchers to keep
2989       the event loop from blocking if lower-priority coroutines are active,
2990       thus mapping low-priority coroutines to idle/background tasks).
2992       When used for this purpose, it is recommended to give "ev_check"
2993       watchers highest ("EV_MAXPRI") priority, to ensure that they are being
2994       run before any other watchers after the poll (this doesn't matter for
2995       "ev_prepare" watchers).
2997       Also, "ev_check" watchers (and "ev_prepare" watchers, too) should not
2998       activate ("feed") events into libev. While libev fully supports this,
2999       they might get executed before other "ev_check" watchers did their job.
3000       As "ev_check" watchers are often used to embed other (non-libev) event
3001       loops those other event loops might be in an unusable state until their
3002       "ev_check" watcher ran (always remind yourself to coexist peacefully
3003       with others).
3005       Abusing an "ev_check" watcher for its side-effect
3007       "ev_check" (and less often also "ev_prepare") watchers can also be
3008       useful because they are called once per event loop iteration. For
3009       example, if you want to handle a large number of connections fairly,
3010       you normally only do a bit of work for each active connection, and if
3011       there is more work to do, you wait for the next event loop iteration,
3012       so other connections have a chance of making progress.
3014       Using an "ev_check" watcher is almost enough: it will be called on the
3015       next event loop iteration. However, that isn't as soon as possible -
3016       without external events, your "ev_check" watcher will not be invoked.
3018       This is where "ev_idle" watchers come in handy - all you need is a
3019       single global idle watcher that is active as long as you have one
3020       active "ev_check" watcher. The "ev_idle" watcher makes sure the event
3021       loop will not sleep, and the "ev_check" watcher makes sure a callback
3022       gets invoked. Neither watcher alone can do that.
3024       Watcher-Specific Functions and Data Members
3026       ev_prepare_init (ev_prepare *, callback)
3027       ev_check_init (ev_check *, callback)
3028           Initialises and configures the prepare or check watcher - they have
3029           no parameters of any kind. There are "ev_prepare_set" and
3030           "ev_check_set" macros, but using them is utterly, utterly, utterly
3031           and completely pointless.
3033       Examples
3035       There are a number of principal ways to embed other event loops or
3036       modules into libev. Here are some ideas on how to include libadns into
3037       libev (there is a Perl module named "EV::ADNS" that does this, which
3038       you could use as a working example. Another Perl module named
3039       "EV::Glib" embeds a Glib main context into libev, and finally,
3040       "Glib::EV" embeds EV into the Glib event loop).
3042       Method 1: Add IO watchers and a timeout watcher in a prepare handler,
3043       and in a check watcher, destroy them and call into libadns. What
3044       follows is pseudo-code only of course. This requires you to either use
3045       a low priority for the check watcher or use "ev_clear_pending"
3046       explicitly, as the callbacks for the IO/timeout watchers might not have
3047       been called yet.
3049          static ev_io iow [nfd];
3050          static ev_timer tw;
3052          static void
3053          io_cb (struct ev_loop *loop, ev_io *w, int revents)
3054          {
3055          }
3057          // create io watchers for each fd and a timer before blocking
3058          static void
3059          adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3060          {
3061            int timeout = 3600000;
3062            struct pollfd fds [nfd];
3063            // actual code will need to loop here and realloc etc.
3064            adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3066            /* the callback is illegal, but won't be called as we stop during check */
3067            ev_timer_init (&tw, 0, timeout * 1e-3, 0.);
3068            ev_timer_start (loop, &tw);
3070            // create one ev_io per pollfd
3071            for (int i = 0; i < nfd; ++i)
3072              {
3073                ev_io_init (iow + i, io_cb, fds [i].fd,
3074                  ((fds [i].events & POLLIN ? EV_READ : 0)
3075                   | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3077                fds [i].revents = 0;
3078                ev_io_start (loop, iow + i);
3079              }
3080          }
3082          // stop all watchers after blocking
3083          static void
3084          adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3085          {
3086            ev_timer_stop (loop, &tw);
3088            for (int i = 0; i < nfd; ++i)
3089              {
3090                // set the relevant poll flags
3091                // could also call adns_processreadable etc. here
3092                struct pollfd *fd = fds + i;
3093                int revents = ev_clear_pending (iow + i);
3094                if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
3095                if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
3097                // now stop the watcher
3098                ev_io_stop (loop, iow + i);
3099              }
3101            adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3102          }
3104       Method 2: This would be just like method 1, but you run
3105       "adns_afterpoll" in the prepare watcher and would dispose of the check
3106       watcher.
3108       Method 3: If the module to be embedded supports explicit event
3109       notification (libadns does), you can also make use of the actual
3110       watcher callbacks, and only destroy/create the watchers in the prepare
3111       watcher.
3113          static void
3114          timer_cb (EV_P_ ev_timer *w, int revents)
3115          {
3116            adns_state ads = (adns_state)w->data;
3117            update_now (EV_A);
3119            adns_processtimeouts (ads, &tv_now);
3120          }
3122          static void
3123          io_cb (EV_P_ ev_io *w, int revents)
3124          {
3125            adns_state ads = (adns_state)w->data;
3126            update_now (EV_A);
3128            if (revents & EV_READ ) adns_processreadable  (ads, w->fd, &tv_now);
3129            if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
3130          }
3132          // do not ever call adns_afterpoll
3134       Method 4: Do not use a prepare or check watcher because the module you
3135       want to embed is not flexible enough to support it. Instead, you can
3136       override their poll function. The drawback with this solution is that
3137       the main loop is now no longer controllable by EV. The "Glib::EV"
3138       module uses this approach, effectively embedding EV as a client into
3139       the horrible libglib event loop.
3141          static gint
3142          event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3143          {
3144            int got_events = 0;
3146            for (n = 0; n < nfds; ++n)
3147              // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3149            if (timeout >= 0)
3150              // create/start timer
3152            // poll
3153            ev_run (EV_A_ 0);
3155            // stop timer again
3156            if (timeout >= 0)
3157              ev_timer_stop (EV_A_ &to);
3159            // stop io watchers again - their callbacks should have set
3160            for (n = 0; n < nfds; ++n)
3161              ev_io_stop (EV_A_ iow [n]);
3163            return got_events;
3164          }
3166   "ev_embed" - when one backend isn't enough...
3167       This is a rather advanced watcher type that lets you embed one event
3168       loop into another (currently only "ev_io" events are supported in the
3169       embedded loop, other types of watchers might be handled in a delayed or
3170       incorrect fashion and must not be used).
3172       There are primarily two reasons you would want that: work around bugs
3173       and prioritise I/O.
3175       As an example for a bug workaround, the kqueue backend might only
3176       support sockets on some platform, so it is unusable as generic backend,
3177       but you still want to make use of it because you have many sockets and
3178       it scales so nicely. In this case, you would create a kqueue-based loop
3179       and embed it into your default loop (which might use e.g. poll).
3180       Overall operation will be a bit slower because first libev has to call
3181       "poll" and then "kevent", but at least you can use both mechanisms for
3182       what they are best: "kqueue" for scalable sockets and "poll" if you
3183       want it to work :)
3185       As for prioritising I/O: under rare circumstances you have the case
3186       where some fds have to be watched and handled very quickly (with low
3187       latency), and even priorities and idle watchers might have too much
3188       overhead. In this case you would put all the high priority stuff in one
3189       loop and all the rest in a second one, and embed the second one in the
3190       first.
3192       As long as the watcher is active, the callback will be invoked every
3193       time there might be events pending in the embedded loop. The callback
3194       must then call "ev_embed_sweep (mainloop, watcher)" to make a single
3195       sweep and invoke their callbacks (the callback doesn't need to invoke
3196       the "ev_embed_sweep" function directly, it could also start an idle
3197       watcher to give the embedded loop strictly lower priority for example).
3199       You can also set the callback to 0, in which case the embed watcher
3200       will automatically execute the embedded loop sweep whenever necessary.
3202       Fork detection will be handled transparently while the "ev_embed"
3203       watcher is active, i.e., the embedded loop will automatically be forked
3204       when the embedding loop forks. In other cases, the user is responsible
3205       for calling "ev_loop_fork" on the embedded loop.
3207       Unfortunately, not all backends are embeddable: only the ones returned
3208       by "ev_embeddable_backends" are, which, unfortunately, does not include
3209       any portable one.
3211       So when you want to use this feature you will always have to be
3212       prepared that you cannot get an embeddable loop. The recommended way to
3213       get around this is to have a separate variables for your embeddable
3214       loop, try to create it, and if that fails, use the normal loop for
3215       everything.
3217       "ev_embed" and fork
3219       While the "ev_embed" watcher is running, forks in the embedding loop
3220       will automatically be applied to the embedded loop as well, so no
3221       special fork handling is required in that case. When the watcher is not
3222       running, however, it is still the task of the libev user to call
3223       "ev_loop_fork ()" as applicable.
3225       Watcher-Specific Functions and Data Members
3227       ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
3228       ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)
3229           Configures the watcher to embed the given loop, which must be
3230           embeddable. If the callback is 0, then "ev_embed_sweep" will be
3231           invoked automatically, otherwise it is the responsibility of the
3232           callback to invoke it (it will continue to be called until the
3233           sweep has been done, if you do not want that, you need to
3234           temporarily stop the embed watcher).
3236       ev_embed_sweep (loop, ev_embed *)
3237           Make a single, non-blocking sweep over the embedded loop. This
3238           works similarly to "ev_run (embedded_loop, EVRUN_NOWAIT)", but in
3239           the most appropriate way for embedded loops.
3241       struct ev_loop *other [read-only]
3242           The embedded event loop.
3244       Examples
3246       Example: Try to get an embeddable event loop and embed it into the
3247       default event loop. If that is not possible, use the default loop. The
3248       default loop is stored in "loop_hi", while the embeddable loop is
3249       stored in "loop_lo" (which is "loop_hi" in the case no embeddable loop
3250       can be used).
3252          struct ev_loop *loop_hi = ev_default_init (0);
3253          struct ev_loop *loop_lo = 0;
3254          ev_embed embed;
3256          // see if there is a chance of getting one that works
3257          // (remember that a flags value of 0 means autodetection)
3258          loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3259            ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3260            : 0;
3262          // if we got one, then embed it, otherwise default to loop_hi
3263          if (loop_lo)
3264            {
3265              ev_embed_init (&embed, 0, loop_lo);
3266              ev_embed_start (loop_hi, &embed);
3267            }
3268          else
3269            loop_lo = loop_hi;
3271       Example: Check if kqueue is available but not recommended and create a
3272       kqueue backend for use with sockets (which usually work with any kqueue
3273       implementation). Store the kqueue/socket-only event loop in
3274       "loop_socket". (One might optionally use "EVFLAG_NOENV", too).
3276          struct ev_loop *loop = ev_default_init (0);
3277          struct ev_loop *loop_socket = 0;
3278          ev_embed embed;
3280          if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3281            if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3282              {
3283                ev_embed_init (&embed, 0, loop_socket);
3284                ev_embed_start (loop, &embed);
3285              }
3287          if (!loop_socket)
3288            loop_socket = loop;
3290          // now use loop_socket for all sockets, and loop for everything else
3292   "ev_fork" - the audacity to resume the event loop after a fork
3293       Fork watchers are called when a "fork ()" was detected (usually because
3294       whoever is a good citizen cared to tell libev about it by calling
3295       "ev_loop_fork"). The invocation is done before the event loop blocks
3296       next and before "ev_check" watchers are being called, and only in the
3297       child after the fork. If whoever good citizen calling "ev_default_fork"
3298       cheats and calls it in the wrong process, the fork handlers will be
3299       invoked, too, of course.
3301       The special problem of life after fork - how is it possible?
3303       Most uses of "fork ()" consist of forking, then some simple calls to
3304       set up/change the process environment, followed by a call to "exec()".
3305       This sequence should be handled by libev without any problems.
3307       This changes when the application actually wants to do event handling
3308       in the child, or both parent in child, in effect "continuing" after the
3309       fork.
3311       The default mode of operation (for libev, with application help to
3312       detect forks) is to duplicate all the state in the child, as would be
3313       expected when either the parent or the child process continues.
3315       When both processes want to continue using libev, then this is usually
3316       the wrong result. In that case, usually one process (typically the
3317       parent) is supposed to continue with all watchers in place as before,
3318       while the other process typically wants to start fresh, i.e. without
3319       any active watchers.
3321       The cleanest and most efficient way to achieve that with libev is to
3322       simply create a new event loop, which of course will be "empty", and
3323       use that for new watchers. This has the advantage of not touching more
3324       memory than necessary, and thus avoiding the copy-on-write, and the
3325       disadvantage of having to use multiple event loops (which do not
3326       support signal watchers).
3328       When this is not possible, or you want to use the default loop for
3329       other reasons, then in the process that wants to start "fresh", call
3330       "ev_loop_destroy (EV_DEFAULT)" followed by "ev_default_loop (...)".
3331       Destroying the default loop will "orphan" (not stop) all registered
3332       watchers, so you have to be careful not to execute code that modifies
3333       those watchers. Note also that in that case, you have to re-register
3334       any signal watchers.
3336       Watcher-Specific Functions and Data Members
3338       ev_fork_init (ev_fork *, callback)
3339           Initialises and configures the fork watcher - it has no parameters
3340           of any kind. There is a "ev_fork_set" macro, but using it is
3341           utterly pointless, really.
3343   "ev_cleanup" - even the best things end
3344       Cleanup watchers are called just before the event loop is being
3345       destroyed by a call to "ev_loop_destroy".
3347       While there is no guarantee that the event loop gets destroyed, cleanup
3348       watchers provide a convenient method to install cleanup hooks for your
3349       program, worker threads and so on - you just to make sure to destroy
3350       the loop when you want them to be invoked.
3352       Cleanup watchers are invoked in the same way as any other watcher.
3353       Unlike all other watchers, they do not keep a reference to the event
3354       loop (which makes a lot of sense if you think about it). Like all other
3355       watchers, you can call libev functions in the callback, except
3356       "ev_cleanup_start".
3358       Watcher-Specific Functions and Data Members
3360       ev_cleanup_init (ev_cleanup *, callback)
3361           Initialises and configures the cleanup watcher - it has no
3362           parameters of any kind. There is a "ev_cleanup_set" macro, but
3363           using it is utterly pointless, I assure you.
3365       Example: Register an atexit handler to destroy the default loop, so any
3366       cleanup functions are called.
3368          static void
3369          program_exits (void)
3370          {
3371            ev_loop_destroy (EV_DEFAULT_UC);
3372          }
3374          ...
3375          atexit (program_exits);
3377   "ev_async" - how to wake up an event loop
3378       In general, you cannot use an "ev_loop" from multiple threads or other
3379       asynchronous sources such as signal handlers (as opposed to multiple
3380       event loops - those are of course safe to use in different threads).
3382       Sometimes, however, you need to wake up an event loop you do not
3383       control, for example because it belongs to another thread. This is what
3384       "ev_async" watchers do: as long as the "ev_async" watcher is active,
3385       you can signal it by calling "ev_async_send", which is thread- and
3386       signal safe.
3388       This functionality is very similar to "ev_signal" watchers, as signals,
3389       too, are asynchronous in nature, and signals, too, will be compressed
3390       (i.e. the number of callback invocations may be less than the number of
3391       "ev_async_send" calls). In fact, you could use signal watchers as a
3392       kind of "global async watchers" by using a watcher on an otherwise
3393       unused signal, and "ev_feed_signal" to signal this watcher from another
3394       thread, even without knowing which loop owns the signal.
3396       Queueing
3398       "ev_async" does not support queueing of data in any way. The reason is
3399       that the author does not know of a simple (or any) algorithm for a
3400       multiple-writer-single-reader queue that works in all cases and doesn't
3401       need elaborate support such as pthreads or unportable memory access
3402       semantics.
3404       That means that if you want to queue data, you have to provide your own
3405       queue. But at least I can tell you how to implement locking around your
3406       queue:
3408       queueing from a signal handler context
3409           To implement race-free queueing, you simply add to the queue in the
3410           signal handler but you block the signal handler in the watcher
3411           callback. Here is an example that does that for some fictitious
3412           SIGUSR1 handler:
3414              static ev_async mysig;
3416              static void
3417              sigusr1_handler (void)
3418              {
3419                sometype data;
3421                // no locking etc.
3422                queue_put (data);
3423                ev_async_send (EV_DEFAULT_ &mysig);
3424              }
3426              static void
3427              mysig_cb (EV_P_ ev_async *w, int revents)
3428              {
3429                sometype data;
3430                sigset_t block, prev;
3432                sigemptyset (&block);
3433                sigaddset (&block, SIGUSR1);
3434                sigprocmask (SIG_BLOCK, &block, &prev);
3436                while (queue_get (&data))
3437                  process (data);
3439                if (sigismember (&prev, SIGUSR1)
3440                  sigprocmask (SIG_UNBLOCK, &block, 0);
3441              }
3443           (Note: pthreads in theory requires you to use "pthread_setmask"
3444           instead of "sigprocmask" when you use threads, but libev doesn't do
3445           it either...).
3447       queueing from a thread context
3448           The strategy for threads is different, as you cannot (easily) block
3449           threads but you can easily preempt them, so to queue safely you
3450           need to employ a traditional mutex lock, such as in this pthread
3451           example:
3453              static ev_async mysig;
3454              static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3456              static void
3457              otherthread (void)
3458              {
3459                // only need to lock the actual queueing operation
3460                pthread_mutex_lock (&mymutex);
3461                queue_put (data);
3462                pthread_mutex_unlock (&mymutex);
3464                ev_async_send (EV_DEFAULT_ &mysig);
3465              }
3467              static void
3468              mysig_cb (EV_P_ ev_async *w, int revents)
3469              {
3470                pthread_mutex_lock (&mymutex);
3472                while (queue_get (&data))
3473                  process (data);
3475                pthread_mutex_unlock (&mymutex);
3476              }
3478       Watcher-Specific Functions and Data Members
3480       ev_async_init (ev_async *, callback)
3481           Initialises and configures the async watcher - it has no parameters
3482           of any kind. There is a "ev_async_set" macro, but using it is
3483           utterly pointless, trust me.
3485       ev_async_send (loop, ev_async *)
3486           Sends/signals/activates the given "ev_async" watcher, that is,
3487           feeds an "EV_ASYNC" event on the watcher into the event loop, and
3488           instantly returns.
3490           Unlike "ev_feed_event", this call is safe to do from other threads,
3491           signal or similar contexts (see the discussion of "EV_ATOMIC_T" in
3492           the embedding section below on what exactly this means).
3494           Note that, as with other watchers in libev, multiple events might
3495           get compressed into a single callback invocation (another way to
3496           look at this is that "ev_async" watchers are level-triggered: they
3497           are set on "ev_async_send", reset when the event loop detects
3498           that).
3500           This call incurs the overhead of at most one extra system call per
3501           event loop iteration, if the event loop is blocked, and no syscall
3502           at all if the event loop (or your program) is processing events.
3503           That means that repeated calls are basically free (there is no need
3504           to avoid calls for performance reasons) and that the overhead
3505           becomes smaller (typically zero) under load.
3507       bool = ev_async_pending (ev_async *)
3508           Returns a non-zero value when "ev_async_send" has been called on
3509           the watcher but the event has not yet been processed (or even
3510           noted) by the event loop.
3512           "ev_async_send" sets a flag in the watcher and wakes up the loop.
3513           When the loop iterates next and checks for the watcher to have
3514           become active, it will reset the flag again. "ev_async_pending" can
3515           be used to very quickly check whether invoking the loop might be a
3516           good idea.
3518           Not that this does not check whether the watcher itself is pending,
3519           only whether it has been requested to make this watcher pending:
3520           there is a time window between the event loop checking and
3521           resetting the async notification, and the callback being invoked.


3524       There are some other functions of possible interest. Described. Here.
3525       Now.
3527       ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)
3528           This function combines a simple timer and an I/O watcher, calls
3529           your callback on whichever event happens first and automatically
3530           stops both watchers. This is useful if you want to wait for a
3531           single event on an fd or timeout without having to
3532           allocate/configure/start/stop/free one or more watchers yourself.
3534           If "fd" is less than 0, then no I/O watcher will be started and the
3535           "events" argument is being ignored. Otherwise, an "ev_io" watcher
3536           for the given "fd" and "events" set will be created and started.
3538           If "timeout" is less than 0, then no timeout watcher will be
3539           started. Otherwise an "ev_timer" watcher with after = "timeout"
3540           (and repeat = 0) will be started. 0 is a valid timeout.
3542           The callback has the type "void (*cb)(int revents, void *arg)" and
3543           is passed an "revents" set like normal event callbacks (a
3544           combination of "EV_ERROR", "EV_READ", "EV_WRITE" or "EV_TIMER") and
3545           the "arg" value passed to "ev_once". Note that it is possible to
3546           receive both a timeout and an io event at the same time - you
3547           probably should give io events precedence.
3549           Example: wait up to ten seconds for data to appear on STDIN_FILENO.
3551              static void stdin_ready (int revents, void *arg)
3552              {
3553                if (revents & EV_READ)
3554                  /* stdin might have data for us, joy! */;
3555                else if (revents & EV_TIMER)
3556                  /* doh, nothing entered */;
3557              }
3559              ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3561       ev_feed_fd_event (loop, int fd, int revents)
3562           Feed an event on the given fd, as if a file descriptor backend
3563           detected the given events.
3565       ev_feed_signal_event (loop, int signum)
3566           Feed an event as if the given signal occurred. See also
3567           "ev_feed_signal", which is async-safe.


3570       This section explains some common idioms that are not immediately
3571       obvious. Note that examples are sprinkled over the whole manual, and
3572       this section only contains stuff that wouldn't fit anywhere else.
3575       Each watcher has, by default, a "void *data" member that you can read
3576       or modify at any time: libev will completely ignore it. This can be
3577       used to associate arbitrary data with your watcher. If you need more
3578       data and don't want to allocate memory separately and store a pointer
3579       to it in that data member, you can also "subclass" the watcher type and
3580       provide your own data:
3582          struct my_io
3583          {
3584            ev_io io;
3585            int otherfd;
3586            void *somedata;
3587            struct whatever *mostinteresting;
3588          };
3590          ...
3591          struct my_io w;
3592          ev_io_init (&w.io, my_cb, fd, EV_READ);
3594       And since your callback will be called with a pointer to the watcher,
3595       you can cast it back to your own type:
3597          static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3598          {
3599            struct my_io *w = (struct my_io *)w_;
3600            ...
3601          }
3603       More interesting and less C-conformant ways of casting your callback
3604       function type instead have been omitted.
3607       Another common scenario is to use some data structure with multiple
3608       embedded watchers, in effect creating your own watcher that combines
3609       multiple libev event sources into one "super-watcher":
3611          struct my_biggy
3612          {
3613            int some_data;
3614            ev_timer t1;
3615            ev_timer t2;
3616          }
3618       In this case getting the pointer to "my_biggy" is a bit more
3619       complicated: Either you store the address of your "my_biggy" struct in
3620       the "data" member of the watcher (for woozies or C++ coders), or you
3621       need to use some pointer arithmetic using "offsetof" inside your
3622       watchers (for real programmers):
3624          #include <stddef.h>
3626          static void
3627          t1_cb (EV_P_ ev_timer *w, int revents)
3628          {
3629            struct my_biggy big = (struct my_biggy *)
3630              (((char *)w) - offsetof (struct my_biggy, t1));
3631          }
3633          static void
3634          t2_cb (EV_P_ ev_timer *w, int revents)
3635          {
3636            struct my_biggy big = (struct my_biggy *)
3637              (((char *)w) - offsetof (struct my_biggy, t2));
3638          }
3641       Often you have structures like this in event-based programs:
3643         callback ()
3644         {
3645           free (request);
3646         }
3648         request = start_new_request (..., callback);
3650       The intent is to start some "lengthy" operation. The "request" could be
3651       used to cancel the operation, or do other things with it.
3653       It's not uncommon to have code paths in "start_new_request" that
3654       immediately invoke the callback, for example, to report errors. Or you
3655       add some caching layer that finds that it can skip the lengthy aspects
3656       of the operation and simply invoke the callback with the result.
3658       The problem here is that this will happen before "start_new_request"
3659       has returned, so "request" is not set.
3661       Even if you pass the request by some safer means to the callback, you
3662       might want to do something to the request after starting it, such as
3663       canceling it, which probably isn't working so well when the callback
3664       has already been invoked.
3666       A common way around all these issues is to make sure that
3667       "start_new_request" always returns before the callback is invoked. If
3668       "start_new_request" immediately knows the result, it can artificially
3669       delay invoking the callback by using a "prepare" or "idle" watcher for
3670       example, or more sneakily, by reusing an existing (stopped) watcher and
3671       pushing it into the pending queue:
3673          ev_set_cb (watcher, callback);
3674          ev_feed_event (EV_A_ watcher, 0);
3676       This way, "start_new_request" can safely return before the callback is
3677       invoked, while not delaying callback invocation too much.
3680       Often (especially in GUI toolkits) there are places where you have
3681       modal interaction, which is most easily implemented by recursively
3682       invoking "ev_run".
3684       This brings the problem of exiting - a callback might want to finish
3685       the main "ev_run" call, but not the nested one (e.g. user clicked
3686       "Quit", but a modal "Are you sure?" dialog is still waiting), or just
3687       the nested one and not the main one (e.g. user clocked "Ok" in a modal
3688       dialog), or some other combination: In these cases, a simple "ev_break"
3689       will not work.
3691       The solution is to maintain "break this loop" variable for each
3692       "ev_run" invocation, and use a loop around "ev_run" until the condition
3693       is triggered, using "EVRUN_ONCE":
3695          // main loop
3696          int exit_main_loop = 0;
3698          while (!exit_main_loop)
3699            ev_run (EV_DEFAULT_ EVRUN_ONCE);
3701          // in a modal watcher
3702          int exit_nested_loop = 0;
3704          while (!exit_nested_loop)
3705            ev_run (EV_A_ EVRUN_ONCE);
3707       To exit from any of these loops, just set the corresponding exit
3708       variable:
3710          // exit modal loop
3711          exit_nested_loop = 1;
3713          // exit main program, after modal loop is finished
3714          exit_main_loop = 1;
3716          // exit both
3717          exit_main_loop = exit_nested_loop = 1;
3720       Here is a fictitious example of how to run an event loop in a different
3721       thread from where callbacks are being invoked and watchers are
3722       created/added/removed.
3724       For a real-world example, see the "EV::Loop::Async" perl module, which
3725       uses exactly this technique (which is suited for many high-level
3726       languages).
3728       The example uses a pthread mutex to protect the loop data, a condition
3729       variable to wait for callback invocations, an async watcher to notify
3730       the event loop thread and an unspecified mechanism to wake up the main
3731       thread.
3733       First, you need to associate some data with the event loop:
3735          typedef struct {
3736            mutex_t lock; /* global loop lock */
3737            ev_async async_w;
3738            thread_t tid;
3739            cond_t invoke_cv;
3740          } userdata;
3742          void prepare_loop (EV_P)
3743          {
3744             // for simplicity, we use a static userdata struct.
3745             static userdata u;
3747             ev_async_init (&u->async_w, async_cb);
3748             ev_async_start (EV_A_ &u->async_w);
3750             pthread_mutex_init (&u->lock, 0);
3751             pthread_cond_init (&u->invoke_cv, 0);
3753             // now associate this with the loop
3754             ev_set_userdata (EV_A_ u);
3755             ev_set_invoke_pending_cb (EV_A_ l_invoke);
3756             ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
3758             // then create the thread running ev_run
3759             pthread_create (&u->tid, 0, l_run, EV_A);
3760          }
3762       The callback for the "ev_async" watcher does nothing: the watcher is
3763       used solely to wake up the event loop so it takes notice of any new
3764       watchers that might have been added:
3766          static void
3767          async_cb (EV_P_ ev_async *w, int revents)
3768          {
3769             // just used for the side effects
3770          }
3772       The "l_release" and "l_acquire" callbacks simply unlock/lock the mutex
3773       protecting the loop data, respectively.
3775          static void
3776          l_release (EV_P)
3777          {
3778            userdata *u = ev_userdata (EV_A);
3779            pthread_mutex_unlock (&u->lock);
3780          }
3782          static void
3783          l_acquire (EV_P)
3784          {
3785            userdata *u = ev_userdata (EV_A);
3786            pthread_mutex_lock (&u->lock);
3787          }
3789       The event loop thread first acquires the mutex, and then jumps straight
3790       into "ev_run":
3792          void *
3793          l_run (void *thr_arg)
3794          {
3795            struct ev_loop *loop = (struct ev_loop *)thr_arg;
3797            l_acquire (EV_A);
3798            pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
3799            ev_run (EV_A_ 0);
3800            l_release (EV_A);
3802            return 0;
3803          }
3805       Instead of invoking all pending watchers, the "l_invoke" callback will
3806       signal the main thread via some unspecified mechanism (signals? pipe
3807       writes? "Async::Interrupt"?) and then waits until all pending watchers
3808       have been called (in a while loop because a) spurious wakeups are
3809       possible and b) skipping inter-thread-communication when there are no
3810       pending watchers is very beneficial):
3812          static void
3813          l_invoke (EV_P)
3814          {
3815            userdata *u = ev_userdata (EV_A);
3817            while (ev_pending_count (EV_A))
3818              {
3819                wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
3820                pthread_cond_wait (&u->invoke_cv, &u->lock);
3821              }
3822          }
3824       Now, whenever the main thread gets told to invoke pending watchers, it
3825       will grab the lock, call "ev_invoke_pending" and then signal the loop
3826       thread to continue:
3828          static void
3829          real_invoke_pending (EV_P)
3830          {
3831            userdata *u = ev_userdata (EV_A);
3833            pthread_mutex_lock (&u->lock);
3834            ev_invoke_pending (EV_A);
3835            pthread_cond_signal (&u->invoke_cv);
3836            pthread_mutex_unlock (&u->lock);
3837          }
3839       Whenever you want to start/stop a watcher or do other modifications to
3840       an event loop, you will now have to lock:
3842          ev_timer timeout_watcher;
3843          userdata *u = ev_userdata (EV_A);
3845          ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
3847          pthread_mutex_lock (&u->lock);
3848          ev_timer_start (EV_A_ &timeout_watcher);
3849          ev_async_send (EV_A_ &u->async_w);
3850          pthread_mutex_unlock (&u->lock);
3852       Note that sending the "ev_async" watcher is required because otherwise
3853       an event loop currently blocking in the kernel will have no knowledge
3854       about the newly added timer. By waking up the loop it will pick up any
3855       new watchers in the next event loop iteration.
3858       While the overhead of a callback that e.g. schedules a thread is small,
3859       it is still an overhead. If you embed libev, and your main usage is
3860       with some kind of threads or coroutines, you might want to customise
3861       libev so that doesn't need callbacks anymore.
3863       Imagine you have coroutines that you can switch to using a function
3864       "switch_to (coro)", that libev runs in a coroutine called "libev_coro"
3865       and that due to some magic, the currently active coroutine is stored in
3866       a global called "current_coro". Then you can build your own "wait for
3867       libev event" primitive by changing "EV_CB_DECLARE" and "EV_CB_INVOKE"
3868       (note the differing ";" conventions):
3870          #define EV_CB_DECLARE(type)   struct my_coro *cb;
3871          #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
3873       That means instead of having a C callback function, you store the
3874       coroutine to switch to in each watcher, and instead of having libev
3875       call your callback, you instead have it switch to that coroutine.
3877       A coroutine might now wait for an event with a function called
3878       "wait_for_event". (the watcher needs to be started, as always, but it
3879       doesn't matter when, or whether the watcher is active or not when this
3880       function is called):
3882          void
3883          wait_for_event (ev_watcher *w)
3884          {
3885            ev_set_cb (w, current_coro);
3886            switch_to (libev_coro);
3887          }
3889       That basically suspends the coroutine inside "wait_for_event" and
3890       continues the libev coroutine, which, when appropriate, switches back
3891       to this or any other coroutine.
3893       You can do similar tricks if you have, say, threads with an event queue
3894       - instead of storing a coroutine, you store the queue object and
3895       instead of switching to a coroutine, you push the watcher onto the
3896       queue and notify any waiters.
3898       To embed libev, see "EMBEDDING", but in short, it's easiest to create
3899       two files, my_ev.h and my_ev.c that include the respective libev files:
3901          // my_ev.h
3902          #define EV_CB_DECLARE(type)   struct my_coro *cb;
3903          #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
3904          #include "../libev/ev.h"
3906          // my_ev.c
3907          #define EV_H "my_ev.h"
3908          #include "../libev/ev.c"
3910       And then use my_ev.h when you would normally use ev.h, and compile
3911       my_ev.c into your project. When properly specifying include paths, you
3912       can even use ev.h as header file name directly.


3915       Libev offers a compatibility emulation layer for libevent. It cannot
3916       emulate the internals of libevent, so here are some usage hints:
3918       •   Only the libevent-1.4.1-beta API is being emulated.
3920           This was the newest libevent version available when libev was
3921           implemented, and is still mostly unchanged in 2010.
3923       •   Use it by including <event.h>, as usual.
3925       •   The following members are fully supported: ev_base, ev_callback,
3926           ev_arg, ev_fd, ev_res, ev_events.
3928       •   Avoid using ev_flags and the EVLIST_*-macros, while it is
3929           maintained by libev, it does not work exactly the same way as in
3930           libevent (consider it a private API).
3932       •   Priorities are not currently supported. Initialising priorities
3933           will fail and all watchers will have the same priority, even though
3934           there is an ev_pri field.
3936       •   In libevent, the last base created gets the signals, in libev, the
3937           base that registered the signal gets the signals.
3939       •   Other members are not supported.
3941       •   The libev emulation is not ABI compatible to libevent, you need to
3942           use the libev header file and library.


3945   C API
3946       The normal C API should work fine when used from C++: both ev.h and the
3947       libev sources can be compiled as C++. Therefore, code that uses the C
3948       API will work fine.
3950       Proper exception specifications might have to be added to callbacks
3951       passed to libev: exceptions may be thrown only from watcher callbacks,
3952       all other callbacks (allocator, syserr, loop acquire/release and
3953       periodic reschedule callbacks) must not throw exceptions, and might
3954       need a "noexcept" specification. If you have code that needs to be
3955       compiled as both C and C++ you can use the "EV_NOEXCEPT" macro for
3956       this:
3958          static void
3959          fatal_error (const char *msg) EV_NOEXCEPT
3960          {
3961            perror (msg);
3962            abort ();
3963          }
3965          ...
3966          ev_set_syserr_cb (fatal_error);
3968       The only API functions that can currently throw exceptions are
3969       "ev_run", "ev_invoke", "ev_invoke_pending" and "ev_loop_destroy" (the
3970       latter because it runs cleanup watchers).
3972       Throwing exceptions in watcher callbacks is only supported if libev
3973       itself is compiled with a C++ compiler or your C and C++ environments
3974       allow throwing exceptions through C libraries (most do).
3976   C++ API
3977       Libev comes with some simplistic wrapper classes for C++ that mainly
3978       allow you to use some convenience methods to start/stop watchers and
3979       also change the callback model to a model using method callbacks on
3980       objects.
3982       To use it,
3984          #include <ev++.h>
3986       This automatically includes ev.h and puts all of its definitions (many
3987       of them macros) into the global namespace. All C++ specific things are
3988       put into the "ev" namespace. It should support all the same embedding
3989       options as ev.h, most notably "EV_MULTIPLICITY".
3991       Care has been taken to keep the overhead low. The only data member the
3992       C++ classes add (compared to plain C-style watchers) is the event loop
3993       pointer that the watcher is associated with (or no additional members
3994       at all if you disable "EV_MULTIPLICITY" when embedding libev).
3996       Currently, functions, static and non-static member functions and
3997       classes with "operator ()" can be used as callbacks. Other types should
3998       be easy to add as long as they only need one additional pointer for
3999       context. If you need support for other types of functors please contact
4000       the author (preferably after implementing it).
4002       For all this to work, your C++ compiler either has to use the same
4003       calling conventions as your C compiler (for static member functions),
4004       or you have to embed libev and compile libev itself as C++.
4006       Here is a list of things available in the "ev" namespace:
4008       "ev::READ", "ev::WRITE" etc.
4009           These are just enum values with the same values as the "EV_READ"
4010           etc.  macros from ev.h.
4012       "ev::tstamp", "ev::now"
4013           Aliases to the same types/functions as with the "ev_" prefix.
4015       "ev::io", "ev::timer", "ev::periodic", "ev::idle", "ev::sig" etc.
4016           For each "ev_TYPE" watcher in ev.h there is a corresponding class
4017           of the same name in the "ev" namespace, with the exception of
4018           "ev_signal" which is called "ev::sig" to avoid clashes with the
4019           "signal" macro defined by many implementations.
4021           All of those classes have these methods:
4023           ev::TYPE::TYPE ()
4024           ev::TYPE::TYPE (loop)
4025           ev::TYPE::~TYPE
4026               The constructor (optionally) takes an event loop to associate
4027               the watcher with. If it is omitted, it will use "EV_DEFAULT".
4029               The constructor calls "ev_init" for you, which means you have
4030               to call the "set" method before starting it.
4032               It will not set a callback, however: You have to call the
4033               templated "set" method to set a callback before you can start
4034               the watcher.
4036               (The reason why you have to use a method is a limitation in C++
4037               which does not allow explicit template arguments for
4038               constructors).
4040               The destructor automatically stops the watcher if it is active.
4042           w->set<class, &class::method> (object *)
4043               This method sets the callback method to call. The method has to
4044               have a signature of "void (*)(ev_TYPE &, int)", it receives the
4045               watcher as first argument and the "revents" as second. The
4046               object must be given as parameter and is stored in the "data"
4047               member of the watcher.
4049               This method synthesizes efficient thunking code to call your
4050               method from the C callback that libev requires. If your
4051               compiler can inline your callback (i.e. it is visible to it at
4052               the place of the "set" call and your compiler is good :), then
4053               the method will be fully inlined into the thunking function,
4054               making it as fast as a direct C callback.
4056               Example: simple class declaration and watcher initialisation
4058                  struct myclass
4059                  {
4060                    void io_cb (ev::io &w, int revents) { }
4061                  }
4063                  myclass obj;
4064                  ev::io iow;
4065                  iow.set <myclass, &myclass::io_cb> (&obj);
4067           w->set (object *)
4068               This is a variation of a method callback - leaving out the
4069               method to call will default the method to "operator ()", which
4070               makes it possible to use functor objects without having to
4071               manually specify the "operator ()" all the time. Incidentally,
4072               you can then also leave out the template argument list.
4074               The "operator ()" method prototype must be "void operator
4075               ()(watcher &w, int revents)".
4077               See the method-"set" above for more details.
4079               Example: use a functor object as callback.
4081                  struct myfunctor
4082                  {
4083                    void operator() (ev::io &w, int revents)
4084                    {
4085                      ...
4086                    }
4087                  }
4089                  myfunctor f;
4091                  ev::io w;
4092                  w.set (&f);
4094           w->set<function> (void *data = 0)
4095               Also sets a callback, but uses a static method or plain
4096               function as callback. The optional "data" argument will be
4097               stored in the watcher's "data" member and is free for you to
4098               use.
4100               The prototype of the "function" must be "void (*)(ev::TYPE &w,
4101               int)".
4103               See the method-"set" above for more details.
4105               Example: Use a plain function as callback.
4107                  static void io_cb (ev::io &w, int revents) { }
4108                  iow.set <io_cb> ();
4110           w->set (loop)
4111               Associates a different "struct ev_loop" with this watcher. You
4112               can only do this when the watcher is inactive (and not pending
4113               either).
4115           w->set ([arguments])
4116               Basically the same as "ev_TYPE_set" (except for "ev::embed"
4117               watchers>), with the same arguments. Either this method or a
4118               suitable start method must be called at least once. Unlike the
4119               C counterpart, an active watcher gets automatically stopped and
4120               restarted when reconfiguring it with this method.
4122               For "ev::embed" watchers this method is called "set_embed", to
4123               avoid clashing with the "set (loop)" method.
4125               For "ev::io" watchers there is an additional "set" method that
4126               acepts a new event mask only, and internally calls
4127               "ev_io_modfify".
4129           w->start ()
4130               Starts the watcher. Note that there is no "loop" argument, as
4131               the constructor already stores the event loop.
4133           w->start ([arguments])
4134               Instead of calling "set" and "start" methods separately, it is
4135               often convenient to wrap them in one call. Uses the same type
4136               of arguments as the configure "set" method of the watcher.
4138           w->stop ()
4139               Stops the watcher if it is active. Again, no "loop" argument.
4141           w->again () ("ev::timer", "ev::periodic" only)
4142               For "ev::timer" and "ev::periodic", this invokes the
4143               corresponding "ev_TYPE_again" function.
4145           w->sweep () ("ev::embed" only)
4146               Invokes "ev_embed_sweep".
4148           w->update () ("ev::stat" only)
4149               Invokes "ev_stat_stat".
4151       Example: Define a class with two I/O and idle watchers, start the I/O
4152       watchers in the constructor.
4154          class myclass
4155          {
4156            ev::io   io  ; void io_cb   (ev::io   &w, int revents);
4157            ev::io   io2 ; void io2_cb  (ev::io   &w, int revents);
4158            ev::idle idle; void idle_cb (ev::idle &w, int revents);
4160            myclass (int fd)
4161            {
4162              io  .set <myclass, &myclass::io_cb  > (this);
4163              io2 .set <myclass, &myclass::io2_cb > (this);
4164              idle.set <myclass, &myclass::idle_cb> (this);
4166              io.set (fd, ev::WRITE); // configure the watcher
4167              io.start ();            // start it whenever convenient
4169              io2.start (fd, ev::READ); // set + start in one call
4170            }
4171          };


4174       Libev does not offer other language bindings itself, but bindings for a
4175       number of languages exist in the form of third-party packages. If you
4176       know any interesting language binding in addition to the ones listed
4177       here, drop me a note.
4179       Perl
4180           The EV module implements the full libev API and is actually used to
4181           test libev. EV is developed together with libev. Apart from the EV
4182           core module, there are additional modules that implement libev-
4183           compatible interfaces to "libadns" ("EV::ADNS", but "AnyEvent::DNS"
4184           is preferred nowadays), "Net::SNMP" ("Net::SNMP::EV") and the
4185           "libglib" event core ("Glib::EV" and "EV::Glib").
4187           It can be found and installed via CPAN, its homepage is at
4188           <http://software.schmorp.de/pkg/EV>.
4190       Python
4191           Python bindings can be found at <http://code.google.com/p/pyev/>.
4192           It seems to be quite complete and well-documented.
4194       Ruby
4195           Tony Arcieri has written a ruby extension that offers access to a
4196           subset of the libev API and adds file handle abstractions,
4197           asynchronous DNS and more on top of it. It can be found via gem
4198           servers. Its homepage is at <http://rev.rubyforge.org/>.
4200           Roger Pack reports that using the link order "-lws2_32
4201           -lmsvcrt-ruby-190" makes rev work even on mingw.
4203       Haskell
4204           A haskell binding to libev is available at
4205           <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>.
4207       D   Leandro Lucarella has written a D language binding (ev.d) for
4208           libev, to be found at
4209           <http://www.llucax.com.ar/proj/ev.d/index.html>.
4211       Ocaml
4212           Erkki Seppala has written Ocaml bindings for libev, to be found at
4213           <http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.
4215       Lua Brian Maher has written a partial interface to libev for lua (at
4216           the time of this writing, only "ev_io" and "ev_timer"), to be found
4217           at <http://github.com/brimworks/lua-ev>.
4219       Javascript
4220           Node.js (<http://nodejs.org>) uses libev as the underlying event
4221           library.
4223       Others
4224           There are others, and I stopped counting.


4227       Libev can be compiled with a variety of options, the most fundamental
4228       of which is "EV_MULTIPLICITY". This option determines whether (most)
4229       functions and callbacks have an initial "struct ev_loop *" argument.
4231       To make it easier to write programs that cope with either variant, the
4232       following macros are defined:
4234       "EV_A", "EV_A_"
4235           This provides the loop argument for functions, if one is required
4236           ("ev loop argument"). The "EV_A" form is used when this is the sole
4237           argument, "EV_A_" is used when other arguments are following.
4238           Example:
4240              ev_unref (EV_A);
4241              ev_timer_add (EV_A_ watcher);
4242              ev_run (EV_A_ 0);
4244           It assumes the variable "loop" of type "struct ev_loop *" is in
4245           scope, which is often provided by the following macro.
4247       "EV_P", "EV_P_"
4248           This provides the loop parameter for functions, if one is required
4249           ("ev loop parameter"). The "EV_P" form is used when this is the
4250           sole parameter, "EV_P_" is used when other parameters are
4251           following. Example:
4253              // this is how ev_unref is being declared
4254              static void ev_unref (EV_P);
4256              // this is how you can declare your typical callback
4257              static void cb (EV_P_ ev_timer *w, int revents)
4259           It declares a parameter "loop" of type "struct ev_loop *", quite
4260           suitable for use with "EV_A".
4262       "EV_DEFAULT", "EV_DEFAULT_"
4263           Similar to the other two macros, this gives you the value of the
4264           default loop, if multiple loops are supported ("ev loop default").
4265           The default loop will be initialised if it isn't already
4266           initialised.
4268           For non-multiplicity builds, these macros do nothing, so you always
4269           have to initialise the loop somewhere.
4271       "EV_DEFAULT_UC", "EV_DEFAULT_UC_"
4272           Usage identical to "EV_DEFAULT" and "EV_DEFAULT_", but requires
4273           that the default loop has been initialised ("UC" == unchecked).
4274           Their behaviour is undefined when the default loop has not been
4275           initialised by a previous execution of "EV_DEFAULT", "EV_DEFAULT_"
4276           or "ev_default_init (...)".
4278           It is often prudent to use "EV_DEFAULT" when initialising the first
4279           watcher in a function but use "EV_DEFAULT_UC" afterwards.
4281       Example: Declare and initialise a check watcher, utilising the above
4282       macros so it will work regardless of whether multiple loops are
4283       supported or not.
4285          static void
4286          check_cb (EV_P_ ev_timer *w, int revents)
4287          {
4288            ev_check_stop (EV_A_ w);
4289          }
4291          ev_check check;
4292          ev_check_init (&check, check_cb);
4293          ev_check_start (EV_DEFAULT_ &check);
4294          ev_run (EV_DEFAULT_ 0);


4297       Libev can (and often is) directly embedded into host applications.
4298       Examples of applications that embed it include the Deliantra Game
4299       Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe) and
4300       rxvt-unicode.
4302       The goal is to enable you to just copy the necessary files into your
4303       source directory without having to change even a single line in them,
4304       so you can easily upgrade by simply copying (or having a checked-out
4305       copy of libev somewhere in your source tree).
4308       Depending on what features you need you need to include one or more
4309       sets of files in your application.
4311       CORE EVENT LOOP
4313       To include only the libev core (all the "ev_*" functions), with manual
4314       configuration (no autoconf):
4316          #define EV_STANDALONE 1
4317          #include "ev.c"
4319       This will automatically include ev.h, too, and should be done in a
4320       single C source file only to provide the function implementations. To
4321       use it, do the same for ev.h in all files wishing to use this API (best
4322       done by writing a wrapper around ev.h that you can include instead and
4323       where you can put other configuration options):
4325          #define EV_STANDALONE 1
4326          #include "ev.h"
4328       Both header files and implementation files can be compiled with a C++
4329       compiler (at least, that's a stated goal, and breakage will be treated
4330       as a bug).
4332       You need the following files in your source tree, or in a directory in
4333       your include path (e.g. in libev/ when using -Ilibev):
4335          ev.h
4336          ev.c
4337          ev_vars.h
4338          ev_wrap.h
4340          ev_win32.c      required on win32 platforms only
4342          ev_select.c     only when select backend is enabled
4343          ev_poll.c       only when poll backend is enabled
4344          ev_epoll.c      only when the epoll backend is enabled
4345          ev_linuxaio.c   only when the linux aio backend is enabled
4346          ev_iouring.c    only when the linux io_uring backend is enabled
4347          ev_kqueue.c     only when the kqueue backend is enabled
4348          ev_port.c       only when the solaris port backend is enabled
4350       ev.c includes the backend files directly when enabled, so you only need
4351       to compile this single file.
4355       To include the libevent compatibility API, also include:
4357          #include "event.c"
4359       in the file including ev.c, and:
4361          #include "event.h"
4363       in the files that want to use the libevent API. This also includes
4364       ev.h.
4366       You need the following additional files for this:
4368          event.h
4369          event.c
4373       Instead of using "EV_STANDALONE=1" and providing your configuration in
4374       whatever way you want, you can also "m4_include([libev.m4])" in your
4375       configure.ac and leave "EV_STANDALONE" undefined. ev.c will then
4376       include config.h and configure itself accordingly.
4378       For this of course you need the m4 file:
4380          libev.m4
4383       Libev can be configured via a variety of preprocessor symbols you have
4384       to define before including (or compiling) any of its files. The default
4385       in the absence of autoconf is documented for every option.
4387       Symbols marked with "(h)" do not change the ABI, and can have different
4388       values when compiling libev vs. including ev.h, so it is permissible to
4389       redefine them before including ev.h without breaking compatibility to a
4390       compiled library. All other symbols change the ABI, which means all
4391       users of libev and the libev code itself must be compiled with
4392       compatible settings.
4394       EV_COMPAT3 (h)
4395           Backwards compatibility is a major concern for libev. This is why
4396           this release of libev comes with wrappers for the functions and
4397           symbols that have been renamed between libev version 3 and 4.
4399           You can disable these wrappers (to test compatibility with future
4400           versions) by defining "EV_COMPAT3" to 0 when compiling your
4401           sources. This has the additional advantage that you can drop the
4402           "struct" from "struct ev_loop" declarations, as libev will provide
4403           an "ev_loop" typedef in that case.
4405           In some future version, the default for "EV_COMPAT3" will become 0,
4406           and in some even more future version the compatibility code will be
4407           removed completely.
4409       EV_STANDALONE (h)
4410           Must always be 1 if you do not use autoconf configuration, which
4411           keeps libev from including config.h, and it also defines dummy
4412           implementations for some libevent functions (such as logging, which
4413           is not supported). It will also not define any of the structs
4414           usually found in event.h that are not directly supported by the
4415           libev core alone.
4417           In standalone mode, libev will still try to automatically deduce
4418           the configuration, but has to be more conservative.
4420       EV_USE_FLOOR
4421           If defined to be 1, libev will use the "floor ()" function for its
4422           periodic reschedule calculations, otherwise libev will fall back on
4423           a portable (slower) implementation. If you enable this, you usually
4424           have to link against libm or something equivalent. Enabling this
4425           when the "floor" function is not available will fail, so the safe
4426           default is to not enable this.
4429           If defined to be 1, libev will try to detect the availability of
4430           the monotonic clock option at both compile time and runtime.
4431           Otherwise no use of the monotonic clock option will be attempted.
4432           If you enable this, you usually have to link against librt or
4433           something similar. Enabling it when the functionality isn't
4434           available is safe, though, although you have to make sure you link
4435           against any libraries where the "clock_gettime" function is hiding
4436           in (often -lrt). See also "EV_USE_CLOCK_SYSCALL".
4438       EV_USE_REALTIME
4439           If defined to be 1, libev will try to detect the availability of
4440           the real-time clock option at compile time (and assume its
4441           availability at runtime if successful). Otherwise no use of the
4442           real-time clock option will be attempted. This effectively replaces
4443           "gettimeofday" by "clock_get (CLOCK_REALTIME, ...)" and will not
4444           normally affect correctness. See the note about libraries in the
4445           description of "EV_USE_MONOTONIC", though. Defaults to the opposite
4446           value of "EV_USE_CLOCK_SYSCALL".
4449           If defined to be 1, libev will try to use a direct syscall instead
4450           of calling the system-provided "clock_gettime" function. This
4451           option exists because on GNU/Linux, "clock_gettime" is in "librt",
4452           but "librt" unconditionally pulls in "libpthread", slowing down
4453           single-threaded programs needlessly. Using a direct syscall is
4454           slightly slower (in theory), because no optimised vdso
4455           implementation can be used, but avoids the pthread dependency.
4456           Defaults to 1 on GNU/Linux with glibc 2.x or higher, as it
4457           simplifies linking (no need for "-lrt").
4460           If defined to be 1, libev will assume that "nanosleep ()" is
4461           available and will use it for delays. Otherwise it will use "select
4462           ()".
4464       EV_USE_EVENTFD
4465           If defined to be 1, then libev will assume that "eventfd ()" is
4466           available and will probe for kernel support at runtime. This will
4467           improve "ev_signal" and "ev_async" performance and reduce resource
4468           consumption.  If undefined, it will be enabled if the headers
4469           indicate GNU/Linux + Glibc 2.7 or newer, otherwise disabled.
4471       EV_USE_SIGNALFD
4472           If defined to be 1, then libev will assume that "signalfd ()" is
4473           available and will probe for kernel support at runtime. This
4474           enables the use of EVFLAG_SIGNALFD for faster and simpler signal
4475           handling. If undefined, it will be enabled if the headers indicate
4476           GNU/Linux + Glibc 2.7 or newer, otherwise disabled.
4478       EV_USE_TIMERFD
4479           If defined to be 1, then libev will assume that "timerfd ()" is
4480           available and will probe for kernel support at runtime. This allows
4481           libev to detect time jumps accurately. If undefined, it will be
4482           enabled if the headers indicate GNU/Linux + Glibc 2.8 or newer and
4483           define "TFD_TIMER_CANCEL_ON_SET", otherwise disabled.
4485       EV_USE_EVENTFD
4486           If defined to be 1, then libev will assume that "eventfd ()" is
4487           available and will probe for kernel support at runtime. This will
4488           improve "ev_signal" and "ev_async" performance and reduce resource
4489           consumption.  If undefined, it will be enabled if the headers
4490           indicate GNU/Linux + Glibc 2.7 or newer, otherwise disabled.
4492       EV_USE_SELECT
4493           If undefined or defined to be 1, libev will compile in support for
4494           the "select"(2) backend. No attempt at auto-detection will be done:
4495           if no other method takes over, select will be it. Otherwise the
4496           select backend will not be compiled in.
4499           If defined to 1, then the select backend will use the system
4500           "fd_set" structure. This is useful if libev doesn't compile due to
4501           a missing "NFDBITS" or "fd_mask" definition or it mis-guesses the
4502           bitset layout on exotic systems. This usually limits the range of
4503           file descriptors to some low limit such as 1024 or might have other
4504           limitations (winsocket only allows 64 sockets). The "FD_SETSIZE"
4505           macro, set before compilation, configures the maximum size of the
4506           "fd_set".
4509           When defined to 1, the select backend will assume that
4510           select/socket/connect etc. don't understand file descriptors but
4511           wants osf handles on win32 (this is the case when the select to be
4512           used is the winsock select). This means that it will call
4513           "_get_osfhandle" on the fd to convert it to an OS handle.
4514           Otherwise, it is assumed that all these functions actually work on
4515           fds, even on win32. Should not be defined on non-win32 platforms.
4517       EV_FD_TO_WIN32_HANDLE(fd)
4518           If "EV_SELECT_IS_WINSOCKET" is enabled, then libev needs a way to
4519           map file descriptors to socket handles. When not defining this
4520           symbol (the default), then libev will call "_get_osfhandle", which
4521           is usually correct. In some cases, programs use their own file
4522           descriptor management, in which case they can provide this function
4523           to map fds to socket handles.
4525       EV_WIN32_HANDLE_TO_FD(handle)
4526           If "EV_SELECT_IS_WINSOCKET" then libev maps handles to file
4527           descriptors using the standard "_open_osfhandle" function. For
4528           programs implementing their own fd to handle mapping, overwriting
4529           this function makes it easier to do so. This can be done by
4530           defining this macro to an appropriate value.
4532       EV_WIN32_CLOSE_FD(fd)
4533           If programs implement their own fd to handle mapping on win32, then
4534           this macro can be used to override the "close" function, useful to
4535           unregister file descriptors again. Note that the replacement
4536           function has to close the underlying OS handle.
4539           If defined to be 1, libev will use "WSASocket" to create its
4540           internal communication socket, which works better in some
4541           environments. Otherwise, the normal "socket" function will be used,
4542           which works better in other environments.
4544       EV_USE_POLL
4545           If defined to be 1, libev will compile in support for the "poll"(2)
4546           backend. Otherwise it will be enabled on non-win32 platforms. It
4547           takes precedence over select.
4549       EV_USE_EPOLL
4550           If defined to be 1, libev will compile in support for the Linux
4551           "epoll"(7) backend. Its availability will be detected at runtime,
4552           otherwise another method will be used as fallback. This is the
4553           preferred backend for GNU/Linux systems. If undefined, it will be
4554           enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer,
4555           otherwise disabled.
4557       EV_USE_LINUXAIO
4558           If defined to be 1, libev will compile in support for the Linux aio
4559           backend ("EV_USE_EPOLL" must also be enabled). If undefined, it
4560           will be enabled on linux, otherwise disabled.
4562       EV_USE_IOURING
4563           If defined to be 1, libev will compile in support for the Linux
4564           io_uring backend ("EV_USE_EPOLL" must also be enabled). Due to it's
4565           current limitations it has to be requested explicitly. If
4566           undefined, it will be enabled on linux, otherwise disabled.
4568       EV_USE_KQUEUE
4569           If defined to be 1, libev will compile in support for the BSD style
4570           "kqueue"(2) backend. Its actual availability will be detected at
4571           runtime, otherwise another method will be used as fallback. This is
4572           the preferred backend for BSD and BSD-like systems, although on
4573           most BSDs kqueue only supports some types of fds correctly (the
4574           only platform we found that supports ptys for example was NetBSD),
4575           so kqueue might be compiled in, but not be used unless explicitly
4576           requested. The best way to use it is to find out whether kqueue
4577           supports your type of fd properly and use an embedded kqueue loop.
4579       EV_USE_PORT
4580           If defined to be 1, libev will compile in support for the Solaris
4581           10 port style backend. Its availability will be detected at
4582           runtime, otherwise another method will be used as fallback. This is
4583           the preferred backend for Solaris 10 systems.
4585       EV_USE_DEVPOLL
4586           Reserved for future expansion, works like the USE symbols above.
4588       EV_USE_INOTIFY
4589           If defined to be 1, libev will compile in support for the Linux
4590           inotify interface to speed up "ev_stat" watchers. Its actual
4591           availability will be detected at runtime. If undefined, it will be
4592           enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer,
4593           otherwise disabled.
4595       EV_NO_SMP
4596           If defined to be 1, libev will assume that memory is always
4597           coherent between threads, that is, threads can be used, but threads
4598           never run on different cpus (or different cpu cores). This reduces
4599           dependencies and makes libev faster.
4601       EV_NO_THREADS
4602           If defined to be 1, libev will assume that it will never be called
4603           from different threads (that includes signal handlers), which is a
4604           stronger assumption than "EV_NO_SMP", above. This reduces
4605           dependencies and makes libev faster.
4607       EV_ATOMIC_T
4608           Libev requires an integer type (suitable for storing 0 or 1) whose
4609           access is atomic with respect to other threads or signal contexts.
4610           No such type is easily found in the C language, so you can provide
4611           your own type that you know is safe for your purposes. It is used
4612           both for signal handler "locking" as well as for signal and thread
4613           safety in "ev_async" watchers.
4615           In the absence of this define, libev will use "sig_atomic_t
4616           volatile" (from signal.h), which is usually good enough on most
4617           platforms.
4619       EV_H (h)
4620           The name of the ev.h header file used to include it. The default if
4621           undefined is "ev.h" in event.h, ev.c and ev++.h. This can be used
4622           to virtually rename the ev.h header file in case of conflicts.
4624       EV_CONFIG_H (h)
4625           If "EV_STANDALONE" isn't 1, this variable can be used to override
4626           ev.c's idea of where to find the config.h file, similarly to
4627           "EV_H", above.
4629       EV_EVENT_H (h)
4630           Similarly to "EV_H", this macro can be used to override event.c's
4631           idea of how the event.h header can be found, the default is
4632           "event.h".
4634       EV_PROTOTYPES (h)
4635           If defined to be 0, then ev.h will not define any function
4636           prototypes, but still define all the structs and other symbols.
4637           This is occasionally useful if you want to provide your own wrapper
4638           functions around libev functions.
4641           If undefined or defined to 1, then all event-loop-specific
4642           functions will have the "struct ev_loop *" as first argument, and
4643           you can create additional independent event loops. Otherwise there
4644           will be no support for multiple event loops and there is no first
4645           event loop pointer argument. Instead, all functions act on the
4646           single default loop.
4648           Note that "EV_DEFAULT" and "EV_DEFAULT_" will no longer provide a
4649           default loop when multiplicity is switched off - you always have to
4650           initialise the loop manually in this case.
4652       EV_MINPRI
4653       EV_MAXPRI
4654           The range of allowed priorities. "EV_MINPRI" must be smaller or
4655           equal to "EV_MAXPRI", but otherwise there are no non-obvious
4656           limitations. You can provide for more priorities by overriding
4657           those symbols (usually defined to be "-2" and 2, respectively).
4659           When doing priority-based operations, libev usually has to linearly
4660           search all the priorities, so having many of them (hundreds) uses a
4661           lot of space and time, so using the defaults of five priorities (-2
4662           .. +2) is usually fine.
4664           If your embedding application does not need any priorities,
4665           defining these both to 0 will save some memory and CPU.
4670           If undefined or defined to be 1 (and the platform supports it),
4671           then the respective watcher type is supported. If defined to be 0,
4672           then it is not. Disabling watcher types mainly saves code size.
4674       EV_FEATURES
4675           If you need to shave off some kilobytes of code at the expense of
4676           some speed (but with the full API), you can define this symbol to
4677           request certain subsets of functionality. The default is to enable
4678           all features that can be enabled on the platform.
4680           A typical way to use this symbol is to define it to 0 (or to a
4681           bitset with some broad features you want) and then selectively re-
4682           enable additional parts you want, for example if you want
4683           everything minimal, but multiple event loop support, async and
4684           child watchers and the poll backend, use this:
4686              #define EV_FEATURES 0
4687              #define EV_MULTIPLICITY 1
4688              #define EV_USE_POLL 1
4689              #define EV_CHILD_ENABLE 1
4690              #define EV_ASYNC_ENABLE 1
4692           The actual value is a bitset, it can be a combination of the
4693           following values (by default, all of these are enabled):
4695           1 - faster/larger code
4696               Use larger code to speed up some operations.
4698               Currently this is used to override some inlining decisions
4699               (enlarging the code size by roughly 30% on amd64).
4701               When optimising for size, use of compiler flags such as "-Os"
4702               with gcc is recommended, as well as "-DNDEBUG", as libev
4703               contains a number of assertions.
4705               The default is off when "__OPTIMIZE_SIZE__" is defined by your
4706               compiler (e.g. gcc with "-Os").
4708           2 - faster/larger data structures
4709               Replaces the small 2-heap for timer management by a faster
4710               4-heap, larger hash table sizes and so on. This will usually
4711               further increase code size and can additionally have an effect
4712               on the size of data structures at runtime.
4714               The default is off when "__OPTIMIZE_SIZE__" is defined by your
4715               compiler (e.g. gcc with "-Os").
4717           4 - full API configuration
4718               This enables priorities (sets "EV_MAXPRI"=2 and
4719               "EV_MINPRI"=-2), and enables multiplicity
4720               ("EV_MULTIPLICITY"=1).
4722           8 - full API
4723               This enables a lot of the "lesser used" API functions. See
4724               "ev.h" for details on which parts of the API are still
4725               available without this feature, and do not complain if this
4726               subset changes over time.
4728           16 - enable all optional watcher types
4729               Enables all optional watcher types.  If you want to selectively
4730               enable only some watcher types other than I/O and timers (e.g.
4731               prepare, embed, async, child...) you can enable them manually
4732               by defining "EV_watchertype_ENABLE" to 1 instead.
4734           32 - enable all backends
4735               This enables all backends - without this feature, you need to
4736               enable at least one backend manually ("EV_USE_SELECT" is a good
4737               choice).
4739           64 - enable OS-specific "helper" APIs
4740               Enable inotify, eventfd, signalfd and similar OS-specific
4741               helper APIs by default.
4743           Compiling with "gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1
4744           -DEV_FEATURES=0" reduces the compiled size of libev from 24.7Kb
4745           code/2.8Kb data to 6.5Kb code/0.3Kb data on my GNU/Linux amd64
4746           system, while still giving you I/O watchers, timers and monotonic
4747           clock support.
4749           With an intelligent-enough linker (gcc+binutils are intelligent
4750           enough when you use "-Wl,--gc-sections -ffunction-sections")
4751           functions unused by your program might be left out as well - a
4752           binary starting a timer and an I/O watcher then might come out at
4753           only 5Kb.
4755       EV_API_STATIC
4756           If this symbol is defined (by default it is not), then all
4757           identifiers will have static linkage. This means that libev will
4758           not export any identifiers, and you cannot link against libev
4759           anymore. This can be useful when you embed libev, only want to use
4760           libev functions in a single file, and do not want its identifiers
4761           to be visible.
4763           To use this, define "EV_API_STATIC" and include ev.c in the file
4764           that wants to use libev.
4766           This option only works when libev is compiled with a C compiler, as
4767           C++ doesn't support the required declaration syntax.
4769       EV_AVOID_STDIO
4770           If this is set to 1 at compiletime, then libev will avoid using
4771           stdio functions (printf, scanf, perror etc.). This will increase
4772           the code size somewhat, but if your program doesn't otherwise
4773           depend on stdio and your libc allows it, this avoids linking in the
4774           stdio library which is quite big.
4776           Note that error messages might become less precise when this option
4777           is enabled.
4779       EV_NSIG
4780           The highest supported signal number, +1 (or, the number of
4781           signals): Normally, libev tries to deduce the maximum number of
4782           signals automatically, but sometimes this fails, in which case it
4783           can be specified. Also, using a lower number than detected (32
4784           should be good for about any system in existence) can save some
4785           memory, as libev statically allocates some 12-24 bytes per signal
4786           number.
4788       EV_PID_HASHSIZE
4789           "ev_child" watchers use a small hash table to distribute workload
4790           by pid. The default size is 16 (or 1 with "EV_FEATURES" disabled),
4791           usually more than enough. If you need to manage thousands of
4792           children you might want to increase this value (must be a power of
4793           two).
4796           "ev_stat" watchers use a small hash table to distribute workload by
4797           inotify watch id. The default size is 16 (or 1 with "EV_FEATURES"
4798           disabled), usually more than enough. If you need to manage
4799           thousands of "ev_stat" watchers you might want to increase this
4800           value (must be a power of two).
4802       EV_USE_4HEAP
4803           Heaps are not very cache-efficient. To improve the cache-efficiency
4804           of the timer and periodics heaps, libev uses a 4-heap when this
4805           symbol is defined to 1. The 4-heap uses more complicated (longer)
4806           code but has noticeably faster performance with many (thousands) of
4807           watchers.
4809           The default is 1, unless "EV_FEATURES" overrides it, in which case
4810           it will be 0.
4812       EV_HEAP_CACHE_AT
4813           Heaps are not very cache-efficient. To improve the cache-efficiency
4814           of the timer and periodics heaps, libev can cache the timestamp
4815           (at) within the heap structure (selected by defining
4816           "EV_HEAP_CACHE_AT" to 1), which uses 8-12 bytes more per watcher
4817           and a few hundred bytes more code, but avoids random read accesses
4818           on heap changes. This improves performance noticeably with many
4819           (hundreds) of watchers.
4821           The default is 1, unless "EV_FEATURES" overrides it, in which case
4822           it will be 0.
4824       EV_VERIFY
4825           Controls how much internal verification (see "ev_verify ()") will
4826           be done: If set to 0, no internal verification code will be
4827           compiled in. If set to 1, then verification code will be compiled
4828           in, but not called. If set to 2, then the internal verification
4829           code will be called once per loop, which can slow down libev. If
4830           set to 3, then the verification code will be called very
4831           frequently, which will slow down libev considerably.
4833           Verification errors are reported via C's "assert" mechanism, so if
4834           you disable that (e.g. by defining "NDEBUG") then no errors will be
4835           reported.
4837           The default is 1, unless "EV_FEATURES" overrides it, in which case
4838           it will be 0.
4840       EV_COMMON
4841           By default, all watchers have a "void *data" member. By redefining
4842           this macro to something else you can include more and other types
4843           of members. You have to define it each time you include one of the
4844           files, though, and it must be identical each time.
4846           For example, the perl EV module uses something like this:
4848              #define EV_COMMON                       \
4849                SV *self; /* contains this struct */  \
4850                SV *cb_sv, *fh /* note no trailing ";" */
4852       EV_CB_DECLARE (type)
4853       EV_CB_INVOKE (watcher, revents)
4854       ev_set_cb (ev, cb)
4855           Can be used to change the callback member declaration in each
4856           watcher, and the way callbacks are invoked and set. Must expand to
4857           a struct member definition and a statement, respectively. See the
4858           ev.h header file for their default definitions. One possible use
4859           for overriding these is to avoid the "struct ev_loop *" as first
4860           argument in all cases, or to use method calls instead of plain
4861           function calls in C++.
4864       If you need to re-export the API (e.g. via a DLL) and you need a list
4865       of exported symbols, you can use the provided Symbol.* files which list
4866       all public symbols, one per line:
4868          Symbols.ev      for libev proper
4869          Symbols.event   for the libevent emulation
4871       This can also be used to rename all public symbols to avoid clashes
4872       with multiple versions of libev linked together (which is obviously bad
4873       in itself, but sometimes it is inconvenient to avoid this).
4875       A sed command like this will create wrapper "#define"'s that you need
4876       to include before including ev.h:
4878          <Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h
4880       This would create a file wrap.h which essentially looks like this:
4882          #define ev_backend     myprefix_ev_backend
4883          #define ev_check_start myprefix_ev_check_start
4884          #define ev_check_stop  myprefix_ev_check_stop
4885          ...
4888       For a real-world example of a program the includes libev verbatim, you
4889       can have a look at the EV perl module
4890       (<http://software.schmorp.de/pkg/EV.html>). It has the libev files in
4891       the libev/ subdirectory and includes them in the EV/EVAPI.h (public
4892       interface) and EV.xs (implementation) files. Only the EV.xs file will
4893       be compiled. It is pretty complex because it provides its own header
4894       file.
4896       The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file
4897       that everybody includes and which overrides some configure choices:
4899          #define EV_FEATURES 8
4900          #define EV_USE_SELECT 1
4901          #define EV_PREPARE_ENABLE 1
4902          #define EV_IDLE_ENABLE 1
4903          #define EV_SIGNAL_ENABLE 1
4904          #define EV_CHILD_ENABLE 1
4905          #define EV_USE_STDEXCEPT 0
4906          #define EV_CONFIG_H <config.h>
4908          #include "ev++.h"
4910       And a ev_cpp.C implementation file that contains libev proper and is
4911       compiled:
4913          #include "ev_cpp.h"
4914          #include "ev.c"


4918       THREADS
4920       All libev functions are reentrant and thread-safe unless explicitly
4921       documented otherwise, but libev implements no locking itself. This
4922       means that you can use as many loops as you want in parallel, as long
4923       as there are no concurrent calls into any libev function with the same
4924       loop parameter ("ev_default_*" calls have an implicit default loop
4925       parameter, of course): libev guarantees that different event loops
4926       share no data structures that need any locking.
4928       Or to put it differently: calls with different loop parameters can be
4929       done concurrently from multiple threads, calls with the same loop
4930       parameter must be done serially (but can be done from different
4931       threads, as long as only one thread ever is inside a call at any point
4932       in time, e.g. by using a mutex per loop).
4934       Specifically to support threads (and signal handlers), libev implements
4935       so-called "ev_async" watchers, which allow some limited form of
4936       concurrency on the same event loop, namely waking it up "from the
4937       outside".
4939       If you want to know which design (one loop, locking, or multiple loops
4940       without or something else still) is best for your problem, then I
4941       cannot help you, but here is some generic advice:
4943       •   most applications have a main thread: use the default libev loop in
4944           that thread, or create a separate thread running only the default
4945           loop.
4947           This helps integrating other libraries or software modules that use
4948           libev themselves and don't care/know about threading.
4950       •   one loop per thread is usually a good model.
4952           Doing this is almost never wrong, sometimes a better-performance
4953           model exists, but it is always a good start.
4955       •   other models exist, such as the leader/follower pattern, where one
4956           loop is handed through multiple threads in a kind of round-robin
4957           fashion.
4959           Choosing a model is hard - look around, learn, know that usually
4960           you can do better than you currently do :-)
4962       •   often you need to talk to some other thread which blocks in the
4963           event loop.
4965           "ev_async" watchers can be used to wake them up from other threads
4966           safely (or from signal contexts...).
4968           An example use would be to communicate signals or other events that
4969           only work in the default loop by registering the signal watcher
4970           with the default loop and triggering an "ev_async" watcher from the
4971           default loop watcher callback into the event loop interested in the
4972           signal.
4974       See also "THREAD LOCKING EXAMPLE".
4976       COROUTINES
4978       Libev is very accommodating to coroutines ("cooperative threads"):
4979       libev fully supports nesting calls to its functions from different
4980       coroutines (e.g. you can call "ev_run" on the same loop from two
4981       different coroutines, and switch freely between both coroutines running
4982       the loop, as long as you don't confuse yourself). The only exception is
4983       that you must not do this from "ev_periodic" reschedule callbacks.
4985       Care has been taken to ensure that libev does not keep local state
4986       inside "ev_run", and other calls do not usually allow for coroutine
4987       switches as they do not call any callbacks.
4990       Depending on your compiler and compiler settings, you might get no or a
4991       lot of warnings when compiling libev code. Some people are apparently
4992       scared by this.
4994       However, these are unavoidable for many reasons. For one, each compiler
4995       has different warnings, and each user has different tastes regarding
4996       warning options. "Warn-free" code therefore cannot be a goal except
4997       when targeting a specific compiler and compiler-version.
4999       Another reason is that some compiler warnings require elaborate
5000       workarounds, or other changes to the code that make it less clear and
5001       less maintainable.
5003       And of course, some compiler warnings are just plain stupid, or simply
5004       wrong (because they don't actually warn about the condition their
5005       message seems to warn about). For example, certain older gcc versions
5006       had some warnings that resulted in an extreme number of false
5007       positives. These have been fixed, but some people still insist on
5008       making code warn-free with such buggy versions.
5010       While libev is written to generate as few warnings as possible, "warn-
5011       free" code is not a goal, and it is recommended not to build libev with
5012       any compiler warnings enabled unless you are prepared to cope with them
5013       (e.g. by ignoring them). Remember that warnings are just that:
5014       warnings, not errors, or proof of bugs.
5017       Valgrind has a special section here because it is a popular tool that
5018       is highly useful. Unfortunately, valgrind reports are very hard to
5019       interpret.
5021       If you think you found a bug (memory leak, uninitialised data access
5022       etc.)  in libev, then check twice: If valgrind reports something like:
5024          ==2274==    definitely lost: 0 bytes in 0 blocks.
5025          ==2274==      possibly lost: 0 bytes in 0 blocks.
5026          ==2274==    still reachable: 256 bytes in 1 blocks.
5028       Then there is no memory leak, just as memory accounted to global
5029       variables is not a memleak - the memory is still being referenced, and
5030       didn't leak.
5032       Similarly, under some circumstances, valgrind might report kernel bugs
5033       as if it were a bug in libev (e.g. in realloc or in the poll backend,
5034       although an acceptable workaround has been found here), or it might be
5035       confused.
5037       Keep in mind that valgrind is a very good tool, but only a tool. Don't
5038       make it into some kind of religion.
5040       If you are unsure about something, feel free to contact the mailing
5041       list with the full valgrind report and an explanation on why you think
5042       this is a bug in libev (best check the archives, too :). However, don't
5043       be annoyed when you get a brisk "this is no bug" answer and take the
5044       chance of learning how to interpret valgrind properly.
5046       If you need, for some reason, empty reports from valgrind for your
5047       project I suggest using suppression lists.


5051       GNU/Linux is the only common platform that supports 64 bit file/large
5052       file interfaces but disables them by default.
5054       That means that libev compiled in the default environment doesn't
5055       support files larger than 2GiB or so, which mainly affects "ev_stat"
5056       watchers.
5058       Unfortunately, many programs try to work around this GNU/Linux issue by
5059       enabling the large file API, which makes them incompatible with the
5060       standard libev compiled for their system.
5062       Likewise, libev cannot enable the large file API itself as this would
5063       suddenly make it incompatible to the default compile time environment,
5064       i.e. all programs not using special compile switches.
5067       The whole thing is a bug if you ask me - basically any system interface
5068       you touch is broken, whether it is locales, poll, kqueue or even the
5069       OpenGL drivers.
5071       "kqueue" is buggy
5073       The kqueue syscall is broken in all known versions - most versions
5074       support only sockets, many support pipes.
5076       Libev tries to work around this by not using "kqueue" by default on
5077       this rotten platform, but of course you can still ask for it when
5078       creating a loop - embedding a socket-only kqueue loop into a select-
5079       based one is probably going to work well.
5081       "poll" is buggy
5083       Instead of fixing "kqueue", Apple replaced their (working) "poll"
5084       implementation by something calling "kqueue" internally around the
5085       10.5.6 release, so now "kqueue" and "poll" are broken.
5087       Libev tries to work around this by not using "poll" by default on this
5088       rotten platform, but of course you can still ask for it when creating a
5089       loop.
5091       "select" is buggy
5093       All that's left is "select", and of course Apple found a way to fuck
5094       this one up as well: On OS/X, "select" actively limits the number of
5095       file descriptors you can pass in to 1024 - your program suddenly
5096       crashes when you use more.
5098       There is an undocumented "workaround" for this - defining
5099       "_DARWIN_UNLIMITED_SELECT", which libev tries to use, so select should
5100       work on OS/X.
5103       "errno" reentrancy
5105       The default compile environment on Solaris is unfortunately so thread-
5106       unsafe that you can't even use components/libraries compiled without
5107       "-D_REENTRANT" in a threaded program, which, of course, isn't defined
5108       by default. A valid, if stupid, implementation choice.
5110       If you want to use libev in threaded environments you have to make sure
5111       it's compiled with "_REENTRANT" defined.
5113       Event port backend
5115       The scalable event interface for Solaris is called "event ports".
5116       Unfortunately, this mechanism is very buggy in all major releases. If
5117       you run into high CPU usage, your program freezes or you get a large
5118       number of spurious wakeups, make sure you have all the relevant and
5119       latest kernel patches applied. No, I don't know which ones, but there
5120       are multiple ones to apply, and afterwards, event ports actually work
5121       great.
5123       If you can't get it to work, you can try running the program by setting
5124       the environment variable "LIBEV_FLAGS=3" to only allow "poll" and
5125       "select" backends.
5128       AIX unfortunately has a broken "poll.h" header. Libev works around this
5129       by trying to avoid the poll backend altogether (i.e. it's not even
5130       compiled in), which normally isn't a big problem as "select" works fine
5131       with large bitsets on AIX, and AIX is dead anyway.
5134       General issues
5136       Win32 doesn't support any of the standards (e.g. POSIX) that libev
5137       requires, and its I/O model is fundamentally incompatible with the
5138       POSIX model. Libev still offers limited functionality on this platform
5139       in the form of the "EVBACKEND_SELECT" backend, and only supports socket
5140       descriptors. This only applies when using Win32 natively, not when
5141       using e.g. cygwin. Actually, it only applies to the microsofts own
5142       compilers, as every compiler comes with a slightly differently
5143       broken/incompatible environment.
5145       Lifting these limitations would basically require the full re-
5146       implementation of the I/O system. If you are into this kind of thing,
5147       then note that glib does exactly that for you in a very portable way
5148       (note also that glib is the slowest event library known to man).
5150       There is no supported compilation method available on windows except
5151       embedding it into other applications.
5153       Sensible signal handling is officially unsupported by Microsoft - libev
5154       tries its best, but under most conditions, signals will simply not
5155       work.
5157       Not a libev limitation but worth mentioning: windows apparently doesn't
5158       accept large writes: instead of resulting in a partial write, windows
5159       will either accept everything or return "ENOBUFS" if the buffer is too
5160       large, so make sure you only write small amounts into your sockets
5161       (less than a megabyte seems safe, but this apparently depends on the
5162       amount of memory available).
5164       Due to the many, low, and arbitrary limits on the win32 platform and
5165       the abysmal performance of winsockets, using a large number of sockets
5166       is not recommended (and not reasonable). If your program needs to use
5167       more than a hundred or so sockets, then likely it needs to use a
5168       totally different implementation for windows, as libev offers the POSIX
5169       readiness notification model, which cannot be implemented efficiently
5170       on windows (due to Microsoft monopoly games).
5172       A typical way to use libev under windows is to embed it (see the
5173       embedding section for details) and use the following evwrap.h header
5174       file instead of ev.h:
5176          #define EV_STANDALONE              /* keeps ev from requiring config.h */
5177          #define EV_SELECT_IS_WINSOCKET 1   /* configure libev for windows select */
5179          #include "ev.h"
5181       And compile the following evwrap.c file into your project (make sure
5182       you do not compile the ev.c or any other embedded source files!):
5184          #include "evwrap.h"
5185          #include "ev.c"
5187       The winsocket "select" function
5189       The winsocket "select" function doesn't follow POSIX in that it
5190       requires socket handles and not socket file descriptors (it is also
5191       extremely buggy). This makes select very inefficient, and also requires
5192       a mapping from file descriptors to socket handles (the Microsoft C
5193       runtime provides the function "_open_osfhandle" for this). See the
5194       discussion of the "EV_SELECT_USE_FD_SET", "EV_SELECT_IS_WINSOCKET" and
5195       "EV_FD_TO_WIN32_HANDLE" preprocessor symbols for more info.
5197       The configuration for a "naked" win32 using the Microsoft runtime
5198       libraries and raw winsocket select is:
5200          #define EV_USE_SELECT 1
5201          #define EV_SELECT_IS_WINSOCKET 1   /* forces EV_SELECT_USE_FD_SET, too */
5203       Note that winsockets handling of fd sets is O(n), so you can easily get
5204       a complexity in the O(n²) range when using win32.
5206       Limited number of file descriptors
5208       Windows has numerous arbitrary (and low) limits on things.
5210       Early versions of winsocket's select only supported waiting for a
5211       maximum of 64 handles (probably owning to the fact that all windows
5212       kernels can only wait for 64 things at the same time internally;
5213       Microsoft recommends spawning a chain of threads and wait for 63
5214       handles and the previous thread in each. Sounds great!).
5216       Newer versions support more handles, but you need to define
5217       "FD_SETSIZE" to some high number (e.g. 2048) before compiling the
5218       winsocket select call (which might be in libev or elsewhere, for
5219       example, perl and many other interpreters do their own select emulation
5220       on windows).
5222       Another limit is the number of file descriptors in the Microsoft
5223       runtime libraries, which by default is 64 (there must be a hidden 64
5224       fetish or something like this inside Microsoft). You can increase this
5225       by calling "_setmaxstdio", which can increase this limit to 2048
5226       (another arbitrary limit), but is broken in many versions of the
5227       Microsoft runtime libraries. This might get you to about 512 or 2048
5228       sockets (depending on windows version and/or the phase of the moon). To
5229       get more, you need to wrap all I/O functions and provide your own fd
5230       management, but the cost of calling select (O(n²)) will likely make
5231       this unworkable.
5234       In addition to a working ISO-C implementation and of course the
5235       backend-specific APIs, libev relies on a few additional extensions:
5237       "void (*)(ev_watcher_type *, int revents)" must have compatible calling
5238       conventions regardless of "ev_watcher_type *".
5239           Libev assumes not only that all watcher pointers have the same
5240           internal structure (guaranteed by POSIX but not by ISO C for
5241           example), but it also assumes that the same (machine) code can be
5242           used to call any watcher callback: The watcher callbacks have
5243           different type signatures, but libev calls them using an
5244           "ev_watcher *" internally.
5246       null pointers and integer zero are represented by 0 bytes
5247           Libev uses "memset" to initialise structs and arrays to 0 bytes,
5248           and relies on this setting pointers and integers to null.
5250       pointer accesses must be thread-atomic
5251           Accessing a pointer value must be atomic, it must both be readable
5252           and writable in one piece - this is the case on all current
5253           architectures.
5255       "sig_atomic_t volatile" must be thread-atomic as well
5256           The type "sig_atomic_t volatile" (or whatever is defined as
5257           "EV_ATOMIC_T") must be atomic with respect to accesses from
5258           different threads. This is not part of the specification for
5259           "sig_atomic_t", but is believed to be sufficiently portable.
5261       "sigprocmask" must work in a threaded environment
5262           Libev uses "sigprocmask" to temporarily block signals. This is not
5263           allowed in a threaded program ("pthread_sigmask" has to be used).
5264           Typical pthread implementations will either allow "sigprocmask" in
5265           the "main thread" or will block signals process-wide, both
5266           behaviours would be compatible with libev. Interaction between
5267           "sigprocmask" and "pthread_sigmask" could complicate things,
5268           however.
5270           The most portable way to handle signals is to block signals in all
5271           threads except the initial one, and run the signal handling loop in
5272           the initial thread as well.
5274       "long" must be large enough for common memory allocation sizes
5275           To improve portability and simplify its API, libev uses "long"
5276           internally instead of "size_t" when allocating its data structures.
5277           On non-POSIX systems (Microsoft...) this might be unexpectedly low,
5278           but is still at least 31 bits everywhere, which is enough for
5279           hundreds of millions of watchers.
5281       "double" must hold a time value in seconds with enough accuracy
5282           The type "double" is used to represent timestamps. It is required
5283           to have at least 51 bits of mantissa (and 9 bits of exponent),
5284           which is good enough for at least into the year 4000 with
5285           millisecond accuracy (the design goal for libev). This requirement
5286           is overfulfilled by implementations using IEEE 754, which is
5287           basically all existing ones.
5289           With IEEE 754 doubles, you get microsecond accuracy until at least
5290           the year 2255 (and millisecond accuracy till the year 287396 - by
5291           then, libev is either obsolete or somebody patched it to use "long
5292           double" or something like that, just kidding).
5294       If you know of other additional requirements drop me a note.


5297       In this section the complexities of (many of) the algorithms used
5298       inside libev will be documented. For complexity discussions about
5299       backends see the documentation for "ev_default_init".
5301       All of the following are about amortised time: If an array needs to be
5302       extended, libev needs to realloc and move the whole array, but this
5303       happens asymptotically rarer with higher number of elements, so O(1)
5304       might mean that libev does a lengthy realloc operation in rare cases,
5305       but on average it is much faster and asymptotically approaches constant
5306       time.
5308       Starting and stopping timer/periodic watchers: O(log
5309       skipped_other_timers)
5310           This means that, when you have a watcher that triggers in one hour
5311           and there are 100 watchers that would trigger before that, then
5312           inserting will have to skip roughly seven ("ld 100") of these
5313           watchers.
5315       Changing timer/periodic watchers (by autorepeat or calling again):
5316       O(log skipped_other_timers)
5317           That means that changing a timer costs less than removing/adding
5318           them, as only the relative motion in the event queue has to be paid
5319           for.
5321       Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)
5322           These just add the watcher into an array or at the head of a list.
5324       Stopping check/prepare/idle/fork/async watchers: O(1)
5325       Stopping an io/signal/child watcher:
5326       O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
5327           These watchers are stored in lists, so they need to be walked to
5328           find the correct watcher to remove. The lists are usually short
5329           (you don't usually have many watchers waiting for the same fd or
5330           signal: one is typical, two is rare).
5332       Finding the next timer in each loop iteration: O(1)
5333           By virtue of using a binary or 4-heap, the next timer is always
5334           found at a fixed position in the storage array.
5336       Each change on a file descriptor per loop iteration:
5337       O(number_of_watchers_for_this_fd)
5338           A change means an I/O watcher gets started or stopped, which
5339           requires libev to recalculate its status (and possibly tell the
5340           kernel, depending on backend and whether "ev_io_set" was used).
5342       Activating one watcher (putting it into the pending state): O(1)
5343       Priority handling: O(number_of_priorities)
5344           Priorities are implemented by allocating some space for each
5345           priority. When doing priority-based operations, libev usually has
5346           to linearly search all the priorities, but starting/stopping and
5347           activating watchers becomes O(1) with respect to priority handling.
5349       Sending an ev_async: O(1)
5350       Processing ev_async_send: O(number_of_async_watchers)
5351       Processing signals: O(max_signal_number)
5352           Sending involves a system call iff there were no other
5353           "ev_async_send" calls in the current loop iteration and the loop is
5354           currently blocked. Checking for async and signal events involves
5355           iterating over all running async watchers or all signal numbers.


5358       The major version 4 introduced some incompatible changes to the API.
5360       At the moment, the "ev.h" header file provides compatibility
5361       definitions for all changes, so most programs should still compile. The
5362       compatibility layer might be removed in later versions of libev, so
5363       better update to the new API early than late.
5365       "EV_COMPAT3" backwards compatibility mechanism
5366           The backward compatibility mechanism can be controlled by
5368           section.
5370       "ev_default_destroy" and "ev_default_fork" have been removed
5371           These calls can be replaced easily by their "ev_loop_xxx"
5372           counterparts:
5374              ev_loop_destroy (EV_DEFAULT_UC);
5375              ev_loop_fork (EV_DEFAULT);
5377       function/symbol renames
5378           A number of functions and symbols have been renamed:
5380             ev_loop         => ev_run
5382             EVLOOP_ONESHOT  => EVRUN_ONCE
5384             ev_unloop       => ev_break
5386             EVUNLOOP_ONE    => EVBREAK_ONE
5387             EVUNLOOP_ALL    => EVBREAK_ALL
5389             EV_TIMEOUT      => EV_TIMER
5391             ev_loop_count   => ev_iteration
5392             ev_loop_depth   => ev_depth
5393             ev_loop_verify  => ev_verify
5395           Most functions working on "struct ev_loop" objects don't have an
5396           "ev_loop_" prefix, so it was removed; "ev_loop", "ev_unloop" and
5397           associated constants have been renamed to not collide with the
5398           "struct ev_loop" anymore and "EV_TIMER" now follows the same naming
5399           scheme as all other watcher types. Note that "ev_loop_fork" is
5400           still called "ev_loop_fork" because it would otherwise clash with
5401           the "ev_fork" typedef.
5403       "EV_MINIMAL" mechanism replaced by "EV_FEATURES"
5404           The preprocessor symbol "EV_MINIMAL" has been replaced by a
5405           different mechanism, "EV_FEATURES". Programs using "EV_MINIMAL"
5406           usually compile and work, but the library code will of course be
5407           larger.


5410       active
5411           A watcher is active as long as it has been started and not yet
5412           stopped.  See "WATCHER STATES" for details.
5414       application
5415           In this document, an application is whatever is using libev.
5417       backend
5418           The part of the code dealing with the operating system interfaces.
5420       callback
5421           The address of a function that is called when some event has been
5422           detected. Callbacks are being passed the event loop, the watcher
5423           that received the event, and the actual event bitset.
5425       callback/watcher invocation
5426           The act of calling the callback associated with a watcher.
5428       event
5429           A change of state of some external event, such as data now being
5430           available for reading on a file descriptor, time having passed or
5431           simply not having any other events happening anymore.
5433           In libev, events are represented as single bits (such as "EV_READ"
5434           or "EV_TIMER").
5436       event library
5437           A software package implementing an event model and loop.
5439       event loop
5440           An entity that handles and processes external events and converts
5441           them into callback invocations.
5443       event model
5444           The model used to describe how an event loop handles and processes
5445           watchers and events.
5447       pending
5448           A watcher is pending as soon as the corresponding event has been
5449           detected. See "WATCHER STATES" for details.
5451       real time
5452           The physical time that is observed. It is apparently strictly
5453           monotonic :)
5455       wall-clock time
5456           The time and date as shown on clocks. Unlike real time, it can
5457           actually be wrong and jump forwards and backwards, e.g. when you
5458           adjust your clock.
5460       watcher
5461           A data structure that describes interest in certain events.
5462           Watchers need to be started (attached to an event loop) before they
5463           can receive events.


5466       Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael
5467       Magnusson and Emanuele Giaquinta, and minor corrections by many others.
5471perl v5.34.0                      2022-01-21                      libev::ev(3)