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