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Revision: 1.32
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# Content
1 =head1 NAME
2
3 libev - a high performance full-featured event loop written in C
4
5 =head1 SYNOPSIS
6
7 #include <ev.h>
8
9 =head1 DESCRIPTION
10
11 Libev is an event loop: you register interest in certain events (such as a
12 file descriptor being readable or a timeout occuring), and it will manage
13 these event sources and provide your program with events.
14
15 To do this, it must take more or less complete control over your process
16 (or thread) by executing the I<event loop> handler, and will then
17 communicate events via a callback mechanism.
18
19 You register interest in certain events by registering so-called I<event
20 watchers>, which are relatively small C structures you initialise with the
21 details of the event, and then hand it over to libev by I<starting> the
22 watcher.
23
24 =head1 FEATURES
25
26 Libev supports select, poll, the linux-specific epoll and the bsd-specific
27 kqueue mechanisms for file descriptor events, relative timers, absolute
28 timers with customised rescheduling, signal events, process status change
29 events (related to SIGCHLD), and event watchers dealing with the event
30 loop mechanism itself (idle, prepare and check watchers). It also is quite
31 fast (see this L<benchmark|http://libev.schmorp.de/bench.html> comparing
32 it to libevent for example).
33
34 =head1 CONVENTIONS
35
36 Libev is very configurable. In this manual the default configuration
37 will be described, which supports multiple event loops. For more info
38 about various configuration options please have a look at the file
39 F<README.embed> in the libev distribution. If libev was configured without
40 support for multiple event loops, then all functions taking an initial
41 argument of name C<loop> (which is always of type C<struct ev_loop *>)
42 will not have this argument.
43
44 =head1 TIME REPRESENTATION
45
46 Libev represents time as a single floating point number, representing the
47 (fractional) number of seconds since the (POSIX) epoch (somewhere near
48 the beginning of 1970, details are complicated, don't ask). This type is
49 called C<ev_tstamp>, which is what you should use too. It usually aliases
50 to the double type in C.
51
52 =head1 GLOBAL FUNCTIONS
53
54 These functions can be called anytime, even before initialising the
55 library in any way.
56
57 =over 4
58
59 =item ev_tstamp ev_time ()
60
61 Returns the current time as libev would use it. Please note that the
62 C<ev_now> function is usually faster and also often returns the timestamp
63 you actually want to know.
64
65 =item int ev_version_major ()
66
67 =item int ev_version_minor ()
68
69 You can find out the major and minor version numbers of the library
70 you linked against by calling the functions C<ev_version_major> and
71 C<ev_version_minor>. If you want, you can compare against the global
72 symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
73 version of the library your program was compiled against.
74
75 Usually, it's a good idea to terminate if the major versions mismatch,
76 as this indicates an incompatible change. Minor versions are usually
77 compatible to older versions, so a larger minor version alone is usually
78 not a problem.
79
80 =item unsigned int ev_supported_backends ()
81
82 Return the set of all backends (i.e. their corresponding C<EV_BACKEND_*>
83 value) compiled into this binary of libev (independent of their
84 availability on the system you are running on). See C<ev_default_loop> for
85 a description of the set values.
86
87 =item unsigned int ev_recommended_backends ()
88
89 Return the set of all backends compiled into this binary of libev and also
90 recommended for this platform. This set is often smaller than the one
91 returned by C<ev_supported_backends>, as for example kqueue is broken on
92 most BSDs and will not be autodetected unless you explicitly request it
93 (assuming you know what you are doing). This is the set of backends that
94 C<EVFLAG_AUTO> will probe for.
95
96 =item ev_set_allocator (void *(*cb)(void *ptr, long size))
97
98 Sets the allocation function to use (the prototype is similar to the
99 realloc C function, the semantics are identical). It is used to allocate
100 and free memory (no surprises here). If it returns zero when memory
101 needs to be allocated, the library might abort or take some potentially
102 destructive action. The default is your system realloc function.
103
104 You could override this function in high-availability programs to, say,
105 free some memory if it cannot allocate memory, to use a special allocator,
106 or even to sleep a while and retry until some memory is available.
107
108 =item ev_set_syserr_cb (void (*cb)(const char *msg));
109
110 Set the callback function to call on a retryable syscall error (such
111 as failed select, poll, epoll_wait). The message is a printable string
112 indicating the system call or subsystem causing the problem. If this
113 callback is set, then libev will expect it to remedy the sitution, no
114 matter what, when it returns. That is, libev will generally retry the
115 requested operation, or, if the condition doesn't go away, do bad stuff
116 (such as abort).
117
118 =back
119
120 =head1 FUNCTIONS CONTROLLING THE EVENT LOOP
121
122 An event loop is described by a C<struct ev_loop *>. The library knows two
123 types of such loops, the I<default> loop, which supports signals and child
124 events, and dynamically created loops which do not.
125
126 If you use threads, a common model is to run the default event loop
127 in your main thread (or in a separate thread) and for each thread you
128 create, you also create another event loop. Libev itself does no locking
129 whatsoever, so if you mix calls to the same event loop in different
130 threads, make sure you lock (this is usually a bad idea, though, even if
131 done correctly, because it's hideous and inefficient).
132
133 =over 4
134
135 =item struct ev_loop *ev_default_loop (unsigned int flags)
136
137 This will initialise the default event loop if it hasn't been initialised
138 yet and return it. If the default loop could not be initialised, returns
139 false. If it already was initialised it simply returns it (and ignores the
140 flags. If that is troubling you, check C<ev_backend ()> afterwards).
141
142 If you don't know what event loop to use, use the one returned from this
143 function.
144
145 The flags argument can be used to specify special behaviour or specific
146 backends to use, and is usually specified as C<0> (or EVFLAG_AUTO).
147
148 It supports the following flags:
149
150 =over 4
151
152 =item C<EVFLAG_AUTO>
153
154 The default flags value. Use this if you have no clue (it's the right
155 thing, believe me).
156
157 =item C<EVFLAG_NOENV>
158
159 If this flag bit is ored into the flag value (or the program runs setuid
160 or setgid) then libev will I<not> look at the environment variable
161 C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will
162 override the flags completely if it is found in the environment. This is
163 useful to try out specific backends to test their performance, or to work
164 around bugs.
165
166 =item C<EVBACKEND_SELECT> (value 1, portable select backend)
167
168 This is your standard select(2) backend. Not I<completely> standard, as
169 libev tries to roll its own fd_set with no limits on the number of fds,
170 but if that fails, expect a fairly low limit on the number of fds when
171 using this backend. It doesn't scale too well (O(highest_fd)), but its usually
172 the fastest backend for a low number of fds.
173
174 =item C<EVBACKEND_POLL> (value 2, poll backend, available everywhere except on windows)
175
176 And this is your standard poll(2) backend. It's more complicated than
177 select, but handles sparse fds better and has no artificial limit on the
178 number of fds you can use (except it will slow down considerably with a
179 lot of inactive fds). It scales similarly to select, i.e. O(total_fds).
180
181 =item C<EVBACKEND_EPOLL> (value 4, Linux)
182
183 For few fds, this backend is a bit little slower than poll and select,
184 but it scales phenomenally better. While poll and select usually scale like
185 O(total_fds) where n is the total number of fds (or the highest fd), epoll scales
186 either O(1) or O(active_fds).
187
188 While stopping and starting an I/O watcher in the same iteration will
189 result in some caching, there is still a syscall per such incident
190 (because the fd could point to a different file description now), so its
191 best to avoid that. Also, dup()ed file descriptors might not work very
192 well if you register events for both fds.
193
194 Please note that epoll sometimes generates spurious notifications, so you
195 need to use non-blocking I/O or other means to avoid blocking when no data
196 (or space) is available.
197
198 =item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)
199
200 Kqueue deserves special mention, as at the time of this writing, it
201 was broken on all BSDs except NetBSD (usually it doesn't work with
202 anything but sockets and pipes, except on Darwin, where of course its
203 completely useless). For this reason its not being "autodetected" unless
204 you explicitly specify the flags (i.e. you don't use EVFLAG_AUTO).
205
206 It scales in the same way as the epoll backend, but the interface to the
207 kernel is more efficient (which says nothing about its actual speed, of
208 course). While starting and stopping an I/O watcher does not cause an
209 extra syscall as with epoll, it still adds up to four event changes per
210 incident, so its best to avoid that.
211
212 =item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8)
213
214 This is not implemented yet (and might never be).
215
216 =item C<EVBACKEND_PORT> (value 32, Solaris 10)
217
218 This uses the Solaris 10 port mechanism. As with everything on Solaris,
219 it's really slow, but it still scales very well (O(active_fds)).
220
221 Please note that solaris ports can result in a lot of spurious
222 notifications, so you need to use non-blocking I/O or other means to avoid
223 blocking when no data (or space) is available.
224
225 =item C<EVBACKEND_ALL>
226
227 Try all backends (even potentially broken ones that wouldn't be tried
228 with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
229 C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.
230
231 =back
232
233 If one or more of these are ored into the flags value, then only these
234 backends will be tried (in the reverse order as given here). If none are
235 specified, most compiled-in backend will be tried, usually in reverse
236 order of their flag values :)
237
238 =item struct ev_loop *ev_loop_new (unsigned int flags)
239
240 Similar to C<ev_default_loop>, but always creates a new event loop that is
241 always distinct from the default loop. Unlike the default loop, it cannot
242 handle signal and child watchers, and attempts to do so will be greeted by
243 undefined behaviour (or a failed assertion if assertions are enabled).
244
245 =item ev_default_destroy ()
246
247 Destroys the default loop again (frees all memory and kernel state
248 etc.). This stops all registered event watchers (by not touching them in
249 any way whatsoever, although you cannot rely on this :).
250
251 =item ev_loop_destroy (loop)
252
253 Like C<ev_default_destroy>, but destroys an event loop created by an
254 earlier call to C<ev_loop_new>.
255
256 =item ev_default_fork ()
257
258 This function reinitialises the kernel state for backends that have
259 one. Despite the name, you can call it anytime, but it makes most sense
260 after forking, in either the parent or child process (or both, but that
261 again makes little sense).
262
263 You I<must> call this function in the child process after forking if and
264 only if you want to use the event library in both processes. If you just
265 fork+exec, you don't have to call it.
266
267 The function itself is quite fast and it's usually not a problem to call
268 it just in case after a fork. To make this easy, the function will fit in
269 quite nicely into a call to C<pthread_atfork>:
270
271 pthread_atfork (0, 0, ev_default_fork);
272
273 At the moment, C<EVBACKEND_SELECT> and C<EVBACKEND_POLL> are safe to use
274 without calling this function, so if you force one of those backends you
275 do not need to care.
276
277 =item ev_loop_fork (loop)
278
279 Like C<ev_default_fork>, but acts on an event loop created by
280 C<ev_loop_new>. Yes, you have to call this on every allocated event loop
281 after fork, and how you do this is entirely your own problem.
282
283 =item unsigned int ev_backend (loop)
284
285 Returns one of the C<EVBACKEND_*> flags indicating the event backend in
286 use.
287
288 =item ev_tstamp ev_now (loop)
289
290 Returns the current "event loop time", which is the time the event loop
291 got events and started processing them. This timestamp does not change
292 as long as callbacks are being processed, and this is also the base time
293 used for relative timers. You can treat it as the timestamp of the event
294 occuring (or more correctly, the mainloop finding out about it).
295
296 =item ev_loop (loop, int flags)
297
298 Finally, this is it, the event handler. This function usually is called
299 after you initialised all your watchers and you want to start handling
300 events.
301
302 If the flags argument is specified as 0, it will not return until either
303 no event watchers are active anymore or C<ev_unloop> was called.
304
305 A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle
306 those events and any outstanding ones, but will not block your process in
307 case there are no events and will return after one iteration of the loop.
308
309 A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
310 neccessary) and will handle those and any outstanding ones. It will block
311 your process until at least one new event arrives, and will return after
312 one iteration of the loop.
313
314 This flags value could be used to implement alternative looping
315 constructs, but the C<prepare> and C<check> watchers provide a better and
316 more generic mechanism.
317
318 Here are the gory details of what ev_loop does:
319
320 1. If there are no active watchers (reference count is zero), return.
321 2. Queue and immediately call all prepare watchers.
322 3. If we have been forked, recreate the kernel state.
323 4. Update the kernel state with all outstanding changes.
324 5. Update the "event loop time".
325 6. Calculate for how long to block.
326 7. Block the process, waiting for events.
327 8. Update the "event loop time" and do time jump handling.
328 9. Queue all outstanding timers.
329 10. Queue all outstanding periodics.
330 11. If no events are pending now, queue all idle watchers.
331 12. Queue all check watchers.
332 13. Call all queued watchers in reverse order (i.e. check watchers first).
333 14. If ev_unloop has been called or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
334 was used, return, otherwise continue with step #1.
335
336 =item ev_unloop (loop, how)
337
338 Can be used to make a call to C<ev_loop> return early (but only after it
339 has processed all outstanding events). The C<how> argument must be either
340 C<EVUNLOOP_ONE>, which will make the innermost C<ev_loop> call return, or
341 C<EVUNLOOP_ALL>, which will make all nested C<ev_loop> calls return.
342
343 =item ev_ref (loop)
344
345 =item ev_unref (loop)
346
347 Ref/unref can be used to add or remove a reference count on the event
348 loop: Every watcher keeps one reference, and as long as the reference
349 count is nonzero, C<ev_loop> will not return on its own. If you have
350 a watcher you never unregister that should not keep C<ev_loop> from
351 returning, ev_unref() after starting, and ev_ref() before stopping it. For
352 example, libev itself uses this for its internal signal pipe: It is not
353 visible to the libev user and should not keep C<ev_loop> from exiting if
354 no event watchers registered by it are active. It is also an excellent
355 way to do this for generic recurring timers or from within third-party
356 libraries. Just remember to I<unref after start> and I<ref before stop>.
357
358 =back
359
360 =head1 ANATOMY OF A WATCHER
361
362 A watcher is a structure that you create and register to record your
363 interest in some event. For instance, if you want to wait for STDIN to
364 become readable, you would create an C<ev_io> watcher for that:
365
366 static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
367 {
368 ev_io_stop (w);
369 ev_unloop (loop, EVUNLOOP_ALL);
370 }
371
372 struct ev_loop *loop = ev_default_loop (0);
373 struct ev_io stdin_watcher;
374 ev_init (&stdin_watcher, my_cb);
375 ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
376 ev_io_start (loop, &stdin_watcher);
377 ev_loop (loop, 0);
378
379 As you can see, you are responsible for allocating the memory for your
380 watcher structures (and it is usually a bad idea to do this on the stack,
381 although this can sometimes be quite valid).
382
383 Each watcher structure must be initialised by a call to C<ev_init
384 (watcher *, callback)>, which expects a callback to be provided. This
385 callback gets invoked each time the event occurs (or, in the case of io
386 watchers, each time the event loop detects that the file descriptor given
387 is readable and/or writable).
388
389 Each watcher type has its own C<< ev_<type>_set (watcher *, ...) >> macro
390 with arguments specific to this watcher type. There is also a macro
391 to combine initialisation and setting in one call: C<< ev_<type>_init
392 (watcher *, callback, ...) >>.
393
394 To make the watcher actually watch out for events, you have to start it
395 with a watcher-specific start function (C<< ev_<type>_start (loop, watcher
396 *) >>), and you can stop watching for events at any time by calling the
397 corresponding stop function (C<< ev_<type>_stop (loop, watcher *) >>.
398
399 As long as your watcher is active (has been started but not stopped) you
400 must not touch the values stored in it. Most specifically you must never
401 reinitialise it or call its set macro.
402
403 You can check whether an event is active by calling the C<ev_is_active
404 (watcher *)> macro. To see whether an event is outstanding (but the
405 callback for it has not been called yet) you can use the C<ev_is_pending
406 (watcher *)> macro.
407
408 Each and every callback receives the event loop pointer as first, the
409 registered watcher structure as second, and a bitset of received events as
410 third argument.
411
412 The received events usually include a single bit per event type received
413 (you can receive multiple events at the same time). The possible bit masks
414 are:
415
416 =over 4
417
418 =item C<EV_READ>
419
420 =item C<EV_WRITE>
421
422 The file descriptor in the C<ev_io> watcher has become readable and/or
423 writable.
424
425 =item C<EV_TIMEOUT>
426
427 The C<ev_timer> watcher has timed out.
428
429 =item C<EV_PERIODIC>
430
431 The C<ev_periodic> watcher has timed out.
432
433 =item C<EV_SIGNAL>
434
435 The signal specified in the C<ev_signal> watcher has been received by a thread.
436
437 =item C<EV_CHILD>
438
439 The pid specified in the C<ev_child> watcher has received a status change.
440
441 =item C<EV_IDLE>
442
443 The C<ev_idle> watcher has determined that you have nothing better to do.
444
445 =item C<EV_PREPARE>
446
447 =item C<EV_CHECK>
448
449 All C<ev_prepare> watchers are invoked just I<before> C<ev_loop> starts
450 to gather new events, and all C<ev_check> watchers are invoked just after
451 C<ev_loop> has gathered them, but before it invokes any callbacks for any
452 received events. Callbacks of both watcher types can start and stop as
453 many watchers as they want, and all of them will be taken into account
454 (for example, a C<ev_prepare> watcher might start an idle watcher to keep
455 C<ev_loop> from blocking).
456
457 =item C<EV_ERROR>
458
459 An unspecified error has occured, the watcher has been stopped. This might
460 happen because the watcher could not be properly started because libev
461 ran out of memory, a file descriptor was found to be closed or any other
462 problem. You best act on it by reporting the problem and somehow coping
463 with the watcher being stopped.
464
465 Libev will usually signal a few "dummy" events together with an error,
466 for example it might indicate that a fd is readable or writable, and if
467 your callbacks is well-written it can just attempt the operation and cope
468 with the error from read() or write(). This will not work in multithreaded
469 programs, though, so beware.
470
471 =back
472
473 =head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
474
475 Each watcher has, by default, a member C<void *data> that you can change
476 and read at any time, libev will completely ignore it. This can be used
477 to associate arbitrary data with your watcher. If you need more data and
478 don't want to allocate memory and store a pointer to it in that data
479 member, you can also "subclass" the watcher type and provide your own
480 data:
481
482 struct my_io
483 {
484 struct ev_io io;
485 int otherfd;
486 void *somedata;
487 struct whatever *mostinteresting;
488 }
489
490 And since your callback will be called with a pointer to the watcher, you
491 can cast it back to your own type:
492
493 static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
494 {
495 struct my_io *w = (struct my_io *)w_;
496 ...
497 }
498
499 More interesting and less C-conformant ways of catsing your callback type
500 have been omitted....
501
502
503 =head1 WATCHER TYPES
504
505 This section describes each watcher in detail, but will not repeat
506 information given in the last section.
507
508 =head2 C<ev_io> - is this file descriptor readable or writable
509
510 I/O watchers check whether a file descriptor is readable or writable
511 in each iteration of the event loop (This behaviour is called
512 level-triggering because you keep receiving events as long as the
513 condition persists. Remember you can stop the watcher if you don't want to
514 act on the event and neither want to receive future events).
515
516 In general you can register as many read and/or write event watchers per
517 fd as you want (as long as you don't confuse yourself). Setting all file
518 descriptors to non-blocking mode is also usually a good idea (but not
519 required if you know what you are doing).
520
521 You have to be careful with dup'ed file descriptors, though. Some backends
522 (the linux epoll backend is a notable example) cannot handle dup'ed file
523 descriptors correctly if you register interest in two or more fds pointing
524 to the same underlying file/socket etc. description (that is, they share
525 the same underlying "file open").
526
527 If you must do this, then force the use of a known-to-be-good backend
528 (at the time of this writing, this includes only C<EVBACKEND_SELECT> and
529 C<EVBACKEND_POLL>).
530
531 =over 4
532
533 =item ev_io_init (ev_io *, callback, int fd, int events)
534
535 =item ev_io_set (ev_io *, int fd, int events)
536
537 Configures an C<ev_io> watcher. The fd is the file descriptor to rceeive
538 events for and events is either C<EV_READ>, C<EV_WRITE> or C<EV_READ |
539 EV_WRITE> to receive the given events.
540
541 Please note that most of the more scalable backend mechanisms (for example
542 epoll and solaris ports) can result in spurious readyness notifications
543 for file descriptors, so you practically need to use non-blocking I/O (and
544 treat callback invocation as hint only), or retest separately with a safe
545 interface before doing I/O (XLib can do this), or force the use of either
546 C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>, which don't suffer from this
547 problem. Also note that it is quite easy to have your callback invoked
548 when the readyness condition is no longer valid even when employing
549 typical ways of handling events, so its a good idea to use non-blocking
550 I/O unconditionally.
551
552 =back
553
554 =head2 C<ev_timer> - relative and optionally recurring timeouts
555
556 Timer watchers are simple relative timers that generate an event after a
557 given time, and optionally repeating in regular intervals after that.
558
559 The timers are based on real time, that is, if you register an event that
560 times out after an hour and you reset your system clock to last years
561 time, it will still time out after (roughly) and hour. "Roughly" because
562 detecting time jumps is hard, and some inaccuracies are unavoidable (the
563 monotonic clock option helps a lot here).
564
565 The relative timeouts are calculated relative to the C<ev_now ()>
566 time. This is usually the right thing as this timestamp refers to the time
567 of the event triggering whatever timeout you are modifying/starting. If
568 you suspect event processing to be delayed and you I<need> to base the timeout
569 on the current time, use something like this to adjust for this:
570
571 ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
572
573 The callback is guarenteed to be invoked only when its timeout has passed,
574 but if multiple timers become ready during the same loop iteration then
575 order of execution is undefined.
576
577 =over 4
578
579 =item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
580
581 =item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
582
583 Configure the timer to trigger after C<after> seconds. If C<repeat> is
584 C<0.>, then it will automatically be stopped. If it is positive, then the
585 timer will automatically be configured to trigger again C<repeat> seconds
586 later, again, and again, until stopped manually.
587
588 The timer itself will do a best-effort at avoiding drift, that is, if you
589 configure a timer to trigger every 10 seconds, then it will trigger at
590 exactly 10 second intervals. If, however, your program cannot keep up with
591 the timer (because it takes longer than those 10 seconds to do stuff) the
592 timer will not fire more than once per event loop iteration.
593
594 =item ev_timer_again (loop)
595
596 This will act as if the timer timed out and restart it again if it is
597 repeating. The exact semantics are:
598
599 If the timer is started but nonrepeating, stop it.
600
601 If the timer is repeating, either start it if necessary (with the repeat
602 value), or reset the running timer to the repeat value.
603
604 This sounds a bit complicated, but here is a useful and typical
605 example: Imagine you have a tcp connection and you want a so-called idle
606 timeout, that is, you want to be called when there have been, say, 60
607 seconds of inactivity on the socket. The easiest way to do this is to
608 configure an C<ev_timer> with after=repeat=60 and calling ev_timer_again each
609 time you successfully read or write some data. If you go into an idle
610 state where you do not expect data to travel on the socket, you can stop
611 the timer, and again will automatically restart it if need be.
612
613 =back
614
615 =head2 C<ev_periodic> - to cron or not to cron
616
617 Periodic watchers are also timers of a kind, but they are very versatile
618 (and unfortunately a bit complex).
619
620 Unlike C<ev_timer>'s, they are not based on real time (or relative time)
621 but on wallclock time (absolute time). You can tell a periodic watcher
622 to trigger "at" some specific point in time. For example, if you tell a
623 periodic watcher to trigger in 10 seconds (by specifiying e.g. c<ev_now ()
624 + 10.>) and then reset your system clock to the last year, then it will
625 take a year to trigger the event (unlike an C<ev_timer>, which would trigger
626 roughly 10 seconds later and of course not if you reset your system time
627 again).
628
629 They can also be used to implement vastly more complex timers, such as
630 triggering an event on eahc midnight, local time.
631
632 As with timers, the callback is guarenteed to be invoked only when the
633 time (C<at>) has been passed, but if multiple periodic timers become ready
634 during the same loop iteration then order of execution is undefined.
635
636 =over 4
637
638 =item ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)
639
640 =item ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)
641
642 Lots of arguments, lets sort it out... There are basically three modes of
643 operation, and we will explain them from simplest to complex:
644
645 =over 4
646
647 =item * absolute timer (interval = reschedule_cb = 0)
648
649 In this configuration the watcher triggers an event at the wallclock time
650 C<at> and doesn't repeat. It will not adjust when a time jump occurs,
651 that is, if it is to be run at January 1st 2011 then it will run when the
652 system time reaches or surpasses this time.
653
654 =item * non-repeating interval timer (interval > 0, reschedule_cb = 0)
655
656 In this mode the watcher will always be scheduled to time out at the next
657 C<at + N * interval> time (for some integer N) and then repeat, regardless
658 of any time jumps.
659
660 This can be used to create timers that do not drift with respect to system
661 time:
662
663 ev_periodic_set (&periodic, 0., 3600., 0);
664
665 This doesn't mean there will always be 3600 seconds in between triggers,
666 but only that the the callback will be called when the system time shows a
667 full hour (UTC), or more correctly, when the system time is evenly divisible
668 by 3600.
669
670 Another way to think about it (for the mathematically inclined) is that
671 C<ev_periodic> will try to run the callback in this mode at the next possible
672 time where C<time = at (mod interval)>, regardless of any time jumps.
673
674 =item * manual reschedule mode (reschedule_cb = callback)
675
676 In this mode the values for C<interval> and C<at> are both being
677 ignored. Instead, each time the periodic watcher gets scheduled, the
678 reschedule callback will be called with the watcher as first, and the
679 current time as second argument.
680
681 NOTE: I<This callback MUST NOT stop or destroy any periodic watcher,
682 ever, or make any event loop modifications>. If you need to stop it,
683 return C<now + 1e30> (or so, fudge fudge) and stop it afterwards (e.g. by
684 starting a prepare watcher).
685
686 Its prototype is C<ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
687 ev_tstamp now)>, e.g.:
688
689 static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
690 {
691 return now + 60.;
692 }
693
694 It must return the next time to trigger, based on the passed time value
695 (that is, the lowest time value larger than to the second argument). It
696 will usually be called just before the callback will be triggered, but
697 might be called at other times, too.
698
699 NOTE: I<< This callback must always return a time that is later than the
700 passed C<now> value >>. Not even C<now> itself will do, it I<must> be larger.
701
702 This can be used to create very complex timers, such as a timer that
703 triggers on each midnight, local time. To do this, you would calculate the
704 next midnight after C<now> and return the timestamp value for this. How
705 you do this is, again, up to you (but it is not trivial, which is the main
706 reason I omitted it as an example).
707
708 =back
709
710 =item ev_periodic_again (loop, ev_periodic *)
711
712 Simply stops and restarts the periodic watcher again. This is only useful
713 when you changed some parameters or the reschedule callback would return
714 a different time than the last time it was called (e.g. in a crond like
715 program when the crontabs have changed).
716
717 =back
718
719 =head2 C<ev_signal> - signal me when a signal gets signalled
720
721 Signal watchers will trigger an event when the process receives a specific
722 signal one or more times. Even though signals are very asynchronous, libev
723 will try it's best to deliver signals synchronously, i.e. as part of the
724 normal event processing, like any other event.
725
726 You can configure as many watchers as you like per signal. Only when the
727 first watcher gets started will libev actually register a signal watcher
728 with the kernel (thus it coexists with your own signal handlers as long
729 as you don't register any with libev). Similarly, when the last signal
730 watcher for a signal is stopped libev will reset the signal handler to
731 SIG_DFL (regardless of what it was set to before).
732
733 =over 4
734
735 =item ev_signal_init (ev_signal *, callback, int signum)
736
737 =item ev_signal_set (ev_signal *, int signum)
738
739 Configures the watcher to trigger on the given signal number (usually one
740 of the C<SIGxxx> constants).
741
742 =back
743
744 =head2 C<ev_child> - wait for pid status changes
745
746 Child watchers trigger when your process receives a SIGCHLD in response to
747 some child status changes (most typically when a child of yours dies).
748
749 =over 4
750
751 =item ev_child_init (ev_child *, callback, int pid)
752
753 =item ev_child_set (ev_child *, int pid)
754
755 Configures the watcher to wait for status changes of process C<pid> (or
756 I<any> process if C<pid> is specified as C<0>). The callback can look
757 at the C<rstatus> member of the C<ev_child> watcher structure to see
758 the status word (use the macros from C<sys/wait.h> and see your systems
759 C<waitpid> documentation). The C<rpid> member contains the pid of the
760 process causing the status change.
761
762 =back
763
764 =head2 C<ev_idle> - when you've got nothing better to do
765
766 Idle watchers trigger events when there are no other events are pending
767 (prepare, check and other idle watchers do not count). That is, as long
768 as your process is busy handling sockets or timeouts (or even signals,
769 imagine) it will not be triggered. But when your process is idle all idle
770 watchers are being called again and again, once per event loop iteration -
771 until stopped, that is, or your process receives more events and becomes
772 busy.
773
774 The most noteworthy effect is that as long as any idle watchers are
775 active, the process will not block when waiting for new events.
776
777 Apart from keeping your process non-blocking (which is a useful
778 effect on its own sometimes), idle watchers are a good place to do
779 "pseudo-background processing", or delay processing stuff to after the
780 event loop has handled all outstanding events.
781
782 =over 4
783
784 =item ev_idle_init (ev_signal *, callback)
785
786 Initialises and configures the idle watcher - it has no parameters of any
787 kind. There is a C<ev_idle_set> macro, but using it is utterly pointless,
788 believe me.
789
790 =back
791
792 =head2 C<ev_prepare> and C<ev_check> - customise your event loop
793
794 Prepare and check watchers are usually (but not always) used in tandem:
795 prepare watchers get invoked before the process blocks and check watchers
796 afterwards.
797
798 Their main purpose is to integrate other event mechanisms into libev. This
799 could be used, for example, to track variable changes, implement your own
800 watchers, integrate net-snmp or a coroutine library and lots more.
801
802 This is done by examining in each prepare call which file descriptors need
803 to be watched by the other library, registering C<ev_io> watchers for
804 them and starting an C<ev_timer> watcher for any timeouts (many libraries
805 provide just this functionality). Then, in the check watcher you check for
806 any events that occured (by checking the pending status of all watchers
807 and stopping them) and call back into the library. The I/O and timer
808 callbacks will never actually be called (but must be valid nevertheless,
809 because you never know, you know?).
810
811 As another example, the Perl Coro module uses these hooks to integrate
812 coroutines into libev programs, by yielding to other active coroutines
813 during each prepare and only letting the process block if no coroutines
814 are ready to run (it's actually more complicated: it only runs coroutines
815 with priority higher than or equal to the event loop and one coroutine
816 of lower priority, but only once, using idle watchers to keep the event
817 loop from blocking if lower-priority coroutines are active, thus mapping
818 low-priority coroutines to idle/background tasks).
819
820 =over 4
821
822 =item ev_prepare_init (ev_prepare *, callback)
823
824 =item ev_check_init (ev_check *, callback)
825
826 Initialises and configures the prepare or check watcher - they have no
827 parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set>
828 macros, but using them is utterly, utterly and completely pointless.
829
830 =back
831
832 =head1 OTHER FUNCTIONS
833
834 There are some other functions of possible interest. Described. Here. Now.
835
836 =over 4
837
838 =item ev_once (loop, int fd, int events, ev_tstamp timeout, callback)
839
840 This function combines a simple timer and an I/O watcher, calls your
841 callback on whichever event happens first and automatically stop both
842 watchers. This is useful if you want to wait for a single event on an fd
843 or timeout without having to allocate/configure/start/stop/free one or
844 more watchers yourself.
845
846 If C<fd> is less than 0, then no I/O watcher will be started and events
847 is being ignored. Otherwise, an C<ev_io> watcher for the given C<fd> and
848 C<events> set will be craeted and started.
849
850 If C<timeout> is less than 0, then no timeout watcher will be
851 started. Otherwise an C<ev_timer> watcher with after = C<timeout> (and
852 repeat = 0) will be started. While C<0> is a valid timeout, it is of
853 dubious value.
854
855 The callback has the type C<void (*cb)(int revents, void *arg)> and gets
856 passed an C<revents> set like normal event callbacks (a combination of
857 C<EV_ERROR>, C<EV_READ>, C<EV_WRITE> or C<EV_TIMEOUT>) and the C<arg>
858 value passed to C<ev_once>:
859
860 static void stdin_ready (int revents, void *arg)
861 {
862 if (revents & EV_TIMEOUT)
863 /* doh, nothing entered */;
864 else if (revents & EV_READ)
865 /* stdin might have data for us, joy! */;
866 }
867
868 ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
869
870 =item ev_feed_event (loop, watcher, int events)
871
872 Feeds the given event set into the event loop, as if the specified event
873 had happened for the specified watcher (which must be a pointer to an
874 initialised but not necessarily started event watcher).
875
876 =item ev_feed_fd_event (loop, int fd, int revents)
877
878 Feed an event on the given fd, as if a file descriptor backend detected
879 the given events it.
880
881 =item ev_feed_signal_event (loop, int signum)
882
883 Feed an event as if the given signal occured (loop must be the default loop!).
884
885 =back
886
887 =head1 LIBEVENT EMULATION
888
889 Libev offers a compatibility emulation layer for libevent. It cannot
890 emulate the internals of libevent, so here are some usage hints:
891
892 =over 4
893
894 =item * Use it by including <event.h>, as usual.
895
896 =item * The following members are fully supported: ev_base, ev_callback,
897 ev_arg, ev_fd, ev_res, ev_events.
898
899 =item * Avoid using ev_flags and the EVLIST_*-macros, while it is
900 maintained by libev, it does not work exactly the same way as in libevent (consider
901 it a private API).
902
903 =item * Priorities are not currently supported. Initialising priorities
904 will fail and all watchers will have the same priority, even though there
905 is an ev_pri field.
906
907 =item * Other members are not supported.
908
909 =item * The libev emulation is I<not> ABI compatible to libevent, you need
910 to use the libev header file and library.
911
912 =back
913
914 =head1 C++ SUPPORT
915
916 TBD.
917
918 =head1 AUTHOR
919
920 Marc Lehmann <libev@schmorp.de>.
921