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Revision: 1.217
Committed: Mon Nov 17 03:37:08 2008 UTC (15 years, 8 months ago) by root
Branch: MAIN
CVS Tags: rel-3_49
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# Content
1 =head1 NAME
3 libev - a high performance full-featured event loop written in C
5 =head1 SYNOPSIS
7 #include <ev.h>
11 // a single header file is required
12 #include <ev.h>
14 #include <stdio.h> // for puts
16 // every watcher type has its own typedef'd struct
17 // with the name ev_TYPE
18 ev_io stdin_watcher;
19 ev_timer timeout_watcher;
21 // all watcher callbacks have a similar signature
22 // this callback is called when data is readable on stdin
23 static void
24 stdin_cb (EV_P_ ev_io *w, int revents)
25 {
26 puts ("stdin ready");
27 // for one-shot events, one must manually stop the watcher
28 // with its corresponding stop function.
29 ev_io_stop (EV_A_ w);
31 // this causes all nested ev_loop's to stop iterating
32 ev_unloop (EV_A_ EVUNLOOP_ALL);
33 }
35 // another callback, this time for a time-out
36 static void
37 timeout_cb (EV_P_ ev_timer *w, int revents)
38 {
39 puts ("timeout");
40 // this causes the innermost ev_loop to stop iterating
41 ev_unloop (EV_A_ EVUNLOOP_ONE);
42 }
44 int
45 main (void)
46 {
47 // use the default event loop unless you have special needs
48 ev_loop *loop = ev_default_loop (0);
50 // initialise an io watcher, then start it
51 // this one will watch for stdin to become readable
52 ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
53 ev_io_start (loop, &stdin_watcher);
55 // initialise a timer watcher, then start it
56 // simple non-repeating 5.5 second timeout
57 ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
58 ev_timer_start (loop, &timeout_watcher);
60 // now wait for events to arrive
61 ev_loop (loop, 0);
63 // unloop was called, so exit
64 return 0;
65 }
69 The newest version of this document is also available as an html-formatted
70 web page you might find easier to navigate when reading it for the first
71 time: L<>.
73 Libev is an event loop: you register interest in certain events (such as a
74 file descriptor being readable or a timeout occurring), and it will manage
75 these event sources and provide your program with events.
77 To do this, it must take more or less complete control over your process
78 (or thread) by executing the I<event loop> handler, and will then
79 communicate events via a callback mechanism.
81 You register interest in certain events by registering so-called I<event
82 watchers>, which are relatively small C structures you initialise with the
83 details of the event, and then hand it over to libev by I<starting> the
84 watcher.
86 =head2 FEATURES
88 Libev supports C<select>, C<poll>, the Linux-specific C<epoll>, the
89 BSD-specific C<kqueue> and the Solaris-specific event port mechanisms
90 for file descriptor events (C<ev_io>), the Linux C<inotify> interface
91 (for C<ev_stat>), relative timers (C<ev_timer>), absolute timers
92 with customised rescheduling (C<ev_periodic>), synchronous signals
93 (C<ev_signal>), process status change events (C<ev_child>), and event
94 watchers dealing with the event loop mechanism itself (C<ev_idle>,
95 C<ev_embed>, C<ev_prepare> and C<ev_check> watchers) as well as
96 file watchers (C<ev_stat>) and even limited support for fork events
97 (C<ev_fork>).
99 It also is quite fast (see this
100 L<benchmark|> comparing it to libevent
101 for example).
103 =head2 CONVENTIONS
105 Libev is very configurable. In this manual the default (and most common)
106 configuration will be described, which supports multiple event loops. For
107 more info about various configuration options please have a look at
108 B<EMBED> section in this manual. If libev was configured without support
109 for multiple event loops, then all functions taking an initial argument of
110 name C<loop> (which is always of type C<ev_loop *>) will not have
111 this argument.
115 Libev represents time as a single floating point number, representing the
116 (fractional) number of seconds since the (POSIX) epoch (somewhere near
117 the beginning of 1970, details are complicated, don't ask). This type is
118 called C<ev_tstamp>, which is what you should use too. It usually aliases
119 to the C<double> type in C, and when you need to do any calculations on
120 it, you should treat it as some floating point value. Unlike the name
121 component C<stamp> might indicate, it is also used for time differences
122 throughout libev.
126 Libev knows three classes of errors: operating system errors, usage errors
127 and internal errors (bugs).
129 When libev catches an operating system error it cannot handle (for example
130 a system call indicating a condition libev cannot fix), it calls the callback
131 set via C<ev_set_syserr_cb>, which is supposed to fix the problem or
132 abort. The default is to print a diagnostic message and to call C<abort
133 ()>.
135 When libev detects a usage error such as a negative timer interval, then
136 it will print a diagnostic message and abort (via the C<assert> mechanism,
137 so C<NDEBUG> will disable this checking): these are programming errors in
138 the libev caller and need to be fixed there.
140 Libev also has a few internal error-checking C<assert>ions, and also has
141 extensive consistency checking code. These do not trigger under normal
142 circumstances, as they indicate either a bug in libev or worse.
147 These functions can be called anytime, even before initialising the
148 library in any way.
150 =over 4
152 =item ev_tstamp ev_time ()
154 Returns the current time as libev would use it. Please note that the
155 C<ev_now> function is usually faster and also often returns the timestamp
156 you actually want to know.
158 =item ev_sleep (ev_tstamp interval)
160 Sleep for the given interval: The current thread will be blocked until
161 either it is interrupted or the given time interval has passed. Basically
162 this is a sub-second-resolution C<sleep ()>.
164 =item int ev_version_major ()
166 =item int ev_version_minor ()
168 You can find out the major and minor ABI version numbers of the library
169 you linked against by calling the functions C<ev_version_major> and
170 C<ev_version_minor>. If you want, you can compare against the global
171 symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
172 version of the library your program was compiled against.
174 These version numbers refer to the ABI version of the library, not the
175 release version.
177 Usually, it's a good idea to terminate if the major versions mismatch,
178 as this indicates an incompatible change. Minor versions are usually
179 compatible to older versions, so a larger minor version alone is usually
180 not a problem.
182 Example: Make sure we haven't accidentally been linked against the wrong
183 version.
185 assert (("libev version mismatch",
186 ev_version_major () == EV_VERSION_MAJOR
187 && ev_version_minor () >= EV_VERSION_MINOR));
189 =item unsigned int ev_supported_backends ()
191 Return the set of all backends (i.e. their corresponding C<EV_BACKEND_*>
192 value) compiled into this binary of libev (independent of their
193 availability on the system you are running on). See C<ev_default_loop> for
194 a description of the set values.
196 Example: make sure we have the epoll method, because yeah this is cool and
197 a must have and can we have a torrent of it please!!!11
199 assert (("sorry, no epoll, no sex",
200 ev_supported_backends () & EVBACKEND_EPOLL));
202 =item unsigned int ev_recommended_backends ()
204 Return the set of all backends compiled into this binary of libev and also
205 recommended for this platform. This set is often smaller than the one
206 returned by C<ev_supported_backends>, as for example kqueue is broken on
207 most BSDs and will not be auto-detected unless you explicitly request it
208 (assuming you know what you are doing). This is the set of backends that
209 libev will probe for if you specify no backends explicitly.
211 =item unsigned int ev_embeddable_backends ()
213 Returns the set of backends that are embeddable in other event loops. This
214 is the theoretical, all-platform, value. To find which backends
215 might be supported on the current system, you would need to look at
216 C<ev_embeddable_backends () & ev_supported_backends ()>, likewise for
217 recommended ones.
219 See the description of C<ev_embed> watchers for more info.
221 =item ev_set_allocator (void *(*cb)(void *ptr, long size)) [NOT REENTRANT]
223 Sets the allocation function to use (the prototype is similar - the
224 semantics are identical to the C<realloc> C89/SuS/POSIX function). It is
225 used to allocate and free memory (no surprises here). If it returns zero
226 when memory needs to be allocated (C<size != 0>), the library might abort
227 or take some potentially destructive action.
229 Since some systems (at least OpenBSD and Darwin) fail to implement
230 correct C<realloc> semantics, libev will use a wrapper around the system
231 C<realloc> and C<free> functions by default.
233 You could override this function in high-availability programs to, say,
234 free some memory if it cannot allocate memory, to use a special allocator,
235 or even to sleep a while and retry until some memory is available.
237 Example: Replace the libev allocator with one that waits a bit and then
238 retries (example requires a standards-compliant C<realloc>).
240 static void *
241 persistent_realloc (void *ptr, size_t size)
242 {
243 for (;;)
244 {
245 void *newptr = realloc (ptr, size);
247 if (newptr)
248 return newptr;
250 sleep (60);
251 }
252 }
254 ...
255 ev_set_allocator (persistent_realloc);
257 =item ev_set_syserr_cb (void (*cb)(const char *msg)); [NOT REENTRANT]
259 Set the callback function to call on a retryable system call error (such
260 as failed select, poll, epoll_wait). The message is a printable string
261 indicating the system call or subsystem causing the problem. If this
262 callback is set, then libev will expect it to remedy the situation, no
263 matter what, when it returns. That is, libev will generally retry the
264 requested operation, or, if the condition doesn't go away, do bad stuff
265 (such as abort).
267 Example: This is basically the same thing that libev does internally, too.
269 static void
270 fatal_error (const char *msg)
271 {
272 perror (msg);
273 abort ();
274 }
276 ...
277 ev_set_syserr_cb (fatal_error);
279 =back
283 An event loop is described by a C<struct ev_loop *> (the C<struct>
284 is I<not> optional in this case, as there is also an C<ev_loop>
285 I<function>).
287 The library knows two types of such loops, the I<default> loop, which
288 supports signals and child events, and dynamically created loops which do
289 not.
291 =over 4
293 =item struct ev_loop *ev_default_loop (unsigned int flags)
295 This will initialise the default event loop if it hasn't been initialised
296 yet and return it. If the default loop could not be initialised, returns
297 false. If it already was initialised it simply returns it (and ignores the
298 flags. If that is troubling you, check C<ev_backend ()> afterwards).
300 If you don't know what event loop to use, use the one returned from this
301 function.
303 Note that this function is I<not> thread-safe, so if you want to use it
304 from multiple threads, you have to lock (note also that this is unlikely,
305 as loops cannot be shared easily between threads anyway).
307 The default loop is the only loop that can handle C<ev_signal> and
308 C<ev_child> watchers, and to do this, it always registers a handler
309 for C<SIGCHLD>. If this is a problem for your application you can either
310 create a dynamic loop with C<ev_loop_new> that doesn't do that, or you
311 can simply overwrite the C<SIGCHLD> signal handler I<after> calling
312 C<ev_default_init>.
314 The flags argument can be used to specify special behaviour or specific
315 backends to use, and is usually specified as C<0> (or C<EVFLAG_AUTO>).
317 The following flags are supported:
319 =over 4
321 =item C<EVFLAG_AUTO>
323 The default flags value. Use this if you have no clue (it's the right
324 thing, believe me).
326 =item C<EVFLAG_NOENV>
328 If this flag bit is or'ed into the flag value (or the program runs setuid
329 or setgid) then libev will I<not> look at the environment variable
330 C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will
331 override the flags completely if it is found in the environment. This is
332 useful to try out specific backends to test their performance, or to work
333 around bugs.
337 Instead of calling C<ev_default_fork> or C<ev_loop_fork> manually after
338 a fork, you can also make libev check for a fork in each iteration by
339 enabling this flag.
341 This works by calling C<getpid ()> on every iteration of the loop,
342 and thus this might slow down your event loop if you do a lot of loop
343 iterations and little real work, but is usually not noticeable (on my
344 GNU/Linux system for example, C<getpid> is actually a simple 5-insn sequence
345 without a system call and thus I<very> fast, but my GNU/Linux system also has
346 C<pthread_atfork> which is even faster).
348 The big advantage of this flag is that you can forget about fork (and
349 forget about forgetting to tell libev about forking) when you use this
350 flag.
352 This flag setting cannot be overridden or specified in the C<LIBEV_FLAGS>
353 environment variable.
355 =item C<EVBACKEND_SELECT> (value 1, portable select backend)
357 This is your standard select(2) backend. Not I<completely> standard, as
358 libev tries to roll its own fd_set with no limits on the number of fds,
359 but if that fails, expect a fairly low limit on the number of fds when
360 using this backend. It doesn't scale too well (O(highest_fd)), but its
361 usually the fastest backend for a low number of (low-numbered :) fds.
363 To get good performance out of this backend you need a high amount of
364 parallelism (most of the file descriptors should be busy). If you are
365 writing a server, you should C<accept ()> in a loop to accept as many
366 connections as possible during one iteration. You might also want to have
367 a look at C<ev_set_io_collect_interval ()> to increase the amount of
368 readiness notifications you get per iteration.
370 This backend maps C<EV_READ> to the C<readfds> set and C<EV_WRITE> to the
371 C<writefds> set (and to work around Microsoft Windows bugs, also onto the
372 C<exceptfds> set on that platform).
374 =item C<EVBACKEND_POLL> (value 2, poll backend, available everywhere except on windows)
376 And this is your standard poll(2) backend. It's more complicated
377 than select, but handles sparse fds better and has no artificial
378 limit on the number of fds you can use (except it will slow down
379 considerably with a lot of inactive fds). It scales similarly to select,
380 i.e. O(total_fds). See the entry for C<EVBACKEND_SELECT>, above, for
381 performance tips.
383 This backend maps C<EV_READ> to C<POLLIN | POLLERR | POLLHUP>, and
386 =item C<EVBACKEND_EPOLL> (value 4, Linux)
388 For few fds, this backend is a bit little slower than poll and select,
389 but it scales phenomenally better. While poll and select usually scale
390 like O(total_fds) where n is the total number of fds (or the highest fd),
391 epoll scales either O(1) or O(active_fds).
393 The epoll mechanism deserves honorable mention as the most misdesigned
394 of the more advanced event mechanisms: mere annoyances include silently
395 dropping file descriptors, requiring a system call per change per file
396 descriptor (and unnecessary guessing of parameters), problems with dup and
397 so on. The biggest issue is fork races, however - if a program forks then
398 I<both> parent and child process have to recreate the epoll set, which can
399 take considerable time (one syscall per file descriptor) and is of course
400 hard to detect.
402 Epoll is also notoriously buggy - embedding epoll fds I<should> work, but
403 of course I<doesn't>, and epoll just loves to report events for totally
404 I<different> file descriptors (even already closed ones, so one cannot
405 even remove them from the set) than registered in the set (especially
406 on SMP systems). Libev tries to counter these spurious notifications by
407 employing an additional generation counter and comparing that against the
408 events to filter out spurious ones, recreating the set when required.
410 While stopping, setting and starting an I/O watcher in the same iteration
411 will result in some caching, there is still a system call per such
412 incident (because the same I<file descriptor> could point to a different
413 I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
414 file descriptors might not work very well if you register events for both
415 file descriptors.
417 Best performance from this backend is achieved by not unregistering all
418 watchers for a file descriptor until it has been closed, if possible,
419 i.e. keep at least one watcher active per fd at all times. Stopping and
420 starting a watcher (without re-setting it) also usually doesn't cause
421 extra overhead. A fork can both result in spurious notifications as well
422 as in libev having to destroy and recreate the epoll object, which can
423 take considerable time and thus should be avoided.
425 All this means that, in practice, C<EVBACKEND_SELECT> can be as fast or
426 faster than epoll for maybe up to a hundred file descriptors, depending on
427 the usage. So sad.
429 While nominally embeddable in other event loops, this feature is broken in
430 all kernel versions tested so far.
432 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
435 =item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)
437 Kqueue deserves special mention, as at the time of this writing, it
438 was broken on all BSDs except NetBSD (usually it doesn't work reliably
439 with anything but sockets and pipes, except on Darwin, where of course
440 it's completely useless). Unlike epoll, however, whose brokenness
441 is by design, these kqueue bugs can (and eventually will) be fixed
442 without API changes to existing programs. For this reason it's not being
443 "auto-detected" unless you explicitly specify it in the flags (i.e. using
444 C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
445 system like NetBSD.
447 You still can embed kqueue into a normal poll or select backend and use it
448 only for sockets (after having made sure that sockets work with kqueue on
449 the target platform). See C<ev_embed> watchers for more info.
451 It scales in the same way as the epoll backend, but the interface to the
452 kernel is more efficient (which says nothing about its actual speed, of
453 course). While stopping, setting and starting an I/O watcher does never
454 cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to
455 two event changes per incident. Support for C<fork ()> is very bad (but
456 sane, unlike epoll) and it drops fds silently in similarly hard-to-detect
457 cases
459 This backend usually performs well under most conditions.
461 While nominally embeddable in other event loops, this doesn't work
462 everywhere, so you might need to test for this. And since it is broken
463 almost everywhere, you should only use it when you have a lot of sockets
464 (for which it usually works), by embedding it into another event loop
465 (e.g. C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>) and, did I mention it,
466 using it only for sockets.
468 This backend maps C<EV_READ> into an C<EVFILT_READ> kevent with
469 C<NOTE_EOF>, and C<EV_WRITE> into an C<EVFILT_WRITE> kevent with
470 C<NOTE_EOF>.
472 =item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8)
474 This is not implemented yet (and might never be, unless you send me an
475 implementation). According to reports, C</dev/poll> only supports sockets
476 and is not embeddable, which would limit the usefulness of this backend
477 immensely.
479 =item C<EVBACKEND_PORT> (value 32, Solaris 10)
481 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
482 it's really slow, but it still scales very well (O(active_fds)).
484 Please note that Solaris event ports can deliver a lot of spurious
485 notifications, so you need to use non-blocking I/O or other means to avoid
486 blocking when no data (or space) is available.
488 While this backend scales well, it requires one system call per active
489 file descriptor per loop iteration. For small and medium numbers of file
490 descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
491 might perform better.
493 On the positive side, with the exception of the spurious readiness
494 notifications, this backend actually performed fully to specification
495 in all tests and is fully embeddable, which is a rare feat among the
496 OS-specific backends (I vastly prefer correctness over speed hacks).
498 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
501 =item C<EVBACKEND_ALL>
503 Try all backends (even potentially broken ones that wouldn't be tried
504 with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
507 It is definitely not recommended to use this flag.
509 =back
511 If one or more of these are or'ed into the flags value, then only these
512 backends will be tried (in the reverse order as listed here). If none are
513 specified, all backends in C<ev_recommended_backends ()> will be tried.
515 Example: This is the most typical usage.
517 if (!ev_default_loop (0))
518 fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
520 Example: Restrict libev to the select and poll backends, and do not allow
521 environment settings to be taken into account:
525 Example: Use whatever libev has to offer, but make sure that kqueue is
526 used if available (warning, breaks stuff, best use only with your own
527 private event loop and only if you know the OS supports your types of
528 fds):
530 ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);
532 =item struct ev_loop *ev_loop_new (unsigned int flags)
534 Similar to C<ev_default_loop>, but always creates a new event loop that is
535 always distinct from the default loop. Unlike the default loop, it cannot
536 handle signal and child watchers, and attempts to do so will be greeted by
537 undefined behaviour (or a failed assertion if assertions are enabled).
539 Note that this function I<is> thread-safe, and the recommended way to use
540 libev with threads is indeed to create one loop per thread, and using the
541 default loop in the "main" or "initial" thread.
543 Example: Try to create a event loop that uses epoll and nothing else.
545 struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
546 if (!epoller)
547 fatal ("no epoll found here, maybe it hides under your chair");
549 =item ev_default_destroy ()
551 Destroys the default loop again (frees all memory and kernel state
552 etc.). None of the active event watchers will be stopped in the normal
553 sense, so e.g. C<ev_is_active> might still return true. It is your
554 responsibility to either stop all watchers cleanly yourself I<before>
555 calling this function, or cope with the fact afterwards (which is usually
556 the easiest thing, you can just ignore the watchers and/or C<free ()> them
557 for example).
559 Note that certain global state, such as signal state (and installed signal
560 handlers), will not be freed by this function, and related watchers (such
561 as signal and child watchers) would need to be stopped manually.
563 In general it is not advisable to call this function except in the
564 rare occasion where you really need to free e.g. the signal handling
565 pipe fds. If you need dynamically allocated loops it is better to use
566 C<ev_loop_new> and C<ev_loop_destroy>).
568 =item ev_loop_destroy (loop)
570 Like C<ev_default_destroy>, but destroys an event loop created by an
571 earlier call to C<ev_loop_new>.
573 =item ev_default_fork ()
575 This function sets a flag that causes subsequent C<ev_loop> iterations
576 to reinitialise the kernel state for backends that have one. Despite the
577 name, you can call it anytime, but it makes most sense after forking, in
578 the child process (or both child and parent, but that again makes little
579 sense). You I<must> call it in the child before using any of the libev
580 functions, and it will only take effect at the next C<ev_loop> iteration.
582 On the other hand, you only need to call this function in the child
583 process if and only if you want to use the event library in the child. If
584 you just fork+exec, you don't have to call it at all.
586 The function itself is quite fast and it's usually not a problem to call
587 it just in case after a fork. To make this easy, the function will fit in
588 quite nicely into a call to C<pthread_atfork>:
590 pthread_atfork (0, 0, ev_default_fork);
592 =item ev_loop_fork (loop)
594 Like C<ev_default_fork>, but acts on an event loop created by
595 C<ev_loop_new>. Yes, you have to call this on every allocated event loop
596 after fork that you want to re-use in the child, and how you do this is
597 entirely your own problem.
599 =item int ev_is_default_loop (loop)
601 Returns true when the given loop is, in fact, the default loop, and false
602 otherwise.
604 =item unsigned int ev_loop_count (loop)
606 Returns the count of loop iterations for the loop, which is identical to
607 the number of times libev did poll for new events. It starts at C<0> and
608 happily wraps around with enough iterations.
610 This value can sometimes be useful as a generation counter of sorts (it
611 "ticks" the number of loop iterations), as it roughly corresponds with
612 C<ev_prepare> and C<ev_check> calls.
614 =item unsigned int ev_backend (loop)
616 Returns one of the C<EVBACKEND_*> flags indicating the event backend in
617 use.
619 =item ev_tstamp ev_now (loop)
621 Returns the current "event loop time", which is the time the event loop
622 received events and started processing them. This timestamp does not
623 change as long as callbacks are being processed, and this is also the base
624 time used for relative timers. You can treat it as the timestamp of the
625 event occurring (or more correctly, libev finding out about it).
627 =item ev_now_update (loop)
629 Establishes the current time by querying the kernel, updating the time
630 returned by C<ev_now ()> in the progress. This is a costly operation and
631 is usually done automatically within C<ev_loop ()>.
633 This function is rarely useful, but when some event callback runs for a
634 very long time without entering the event loop, updating libev's idea of
635 the current time is a good idea.
637 See also "The special problem of time updates" in the C<ev_timer> section.
639 =item ev_loop (loop, int flags)
641 Finally, this is it, the event handler. This function usually is called
642 after you initialised all your watchers and you want to start handling
643 events.
645 If the flags argument is specified as C<0>, it will not return until
646 either no event watchers are active anymore or C<ev_unloop> was called.
648 Please note that an explicit C<ev_unloop> is usually better than
649 relying on all watchers to be stopped when deciding when a program has
650 finished (especially in interactive programs), but having a program
651 that automatically loops as long as it has to and no longer by virtue
652 of relying on its watchers stopping correctly, that is truly a thing of
653 beauty.
655 A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle
656 those events and any already outstanding ones, but will not block your
657 process in case there are no events and will return after one iteration of
658 the loop.
660 A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
661 necessary) and will handle those and any already outstanding ones. It
662 will block your process until at least one new event arrives (which could
663 be an event internal to libev itself, so there is no guarantee that a
664 user-registered callback will be called), and will return after one
665 iteration of the loop.
667 This is useful if you are waiting for some external event in conjunction
668 with something not expressible using other libev watchers (i.e. "roll your
669 own C<ev_loop>"). However, a pair of C<ev_prepare>/C<ev_check> watchers is
670 usually a better approach for this kind of thing.
672 Here are the gory details of what C<ev_loop> does:
674 - Before the first iteration, call any pending watchers.
675 * If EVFLAG_FORKCHECK was used, check for a fork.
676 - If a fork was detected (by any means), queue and call all fork watchers.
677 - Queue and call all prepare watchers.
678 - If we have been forked, detach and recreate the kernel state
679 as to not disturb the other process.
680 - Update the kernel state with all outstanding changes.
681 - Update the "event loop time" (ev_now ()).
682 - Calculate for how long to sleep or block, if at all
683 (active idle watchers, EVLOOP_NONBLOCK or not having
684 any active watchers at all will result in not sleeping).
685 - Sleep if the I/O and timer collect interval say so.
686 - Block the process, waiting for any events.
687 - Queue all outstanding I/O (fd) events.
688 - Update the "event loop time" (ev_now ()), and do time jump adjustments.
689 - Queue all expired timers.
690 - Queue all expired periodics.
691 - Unless any events are pending now, queue all idle watchers.
692 - Queue all check watchers.
693 - Call all queued watchers in reverse order (i.e. check watchers first).
694 Signals and child watchers are implemented as I/O watchers, and will
695 be handled here by queueing them when their watcher gets executed.
696 - If ev_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
697 were used, or there are no active watchers, return, otherwise
698 continue with step *.
700 Example: Queue some jobs and then loop until no events are outstanding
701 anymore.
703 ... queue jobs here, make sure they register event watchers as long
704 ... as they still have work to do (even an idle watcher will do..)
705 ev_loop (my_loop, 0);
706 ... jobs done or somebody called unloop. yeah!
708 =item ev_unloop (loop, how)
710 Can be used to make a call to C<ev_loop> return early (but only after it
711 has processed all outstanding events). The C<how> argument must be either
712 C<EVUNLOOP_ONE>, which will make the innermost C<ev_loop> call return, or
713 C<EVUNLOOP_ALL>, which will make all nested C<ev_loop> calls return.
715 This "unloop state" will be cleared when entering C<ev_loop> again.
717 It is safe to call C<ev_unloop> from otuside any C<ev_loop> calls.
719 =item ev_ref (loop)
721 =item ev_unref (loop)
723 Ref/unref can be used to add or remove a reference count on the event
724 loop: Every watcher keeps one reference, and as long as the reference
725 count is nonzero, C<ev_loop> will not return on its own.
727 If you have a watcher you never unregister that should not keep C<ev_loop>
728 from returning, call ev_unref() after starting, and ev_ref() before
729 stopping it.
731 As an example, libev itself uses this for its internal signal pipe: It is
732 not visible to the libev user and should not keep C<ev_loop> from exiting
733 if no event watchers registered by it are active. It is also an excellent
734 way to do this for generic recurring timers or from within third-party
735 libraries. Just remember to I<unref after start> and I<ref before stop>
736 (but only if the watcher wasn't active before, or was active before,
737 respectively).
739 Example: Create a signal watcher, but keep it from keeping C<ev_loop>
740 running when nothing else is active.
742 ev_signal exitsig;
743 ev_signal_init (&exitsig, sig_cb, SIGINT);
744 ev_signal_start (loop, &exitsig);
745 evf_unref (loop);
747 Example: For some weird reason, unregister the above signal handler again.
749 ev_ref (loop);
750 ev_signal_stop (loop, &exitsig);
752 =item ev_set_io_collect_interval (loop, ev_tstamp interval)
754 =item ev_set_timeout_collect_interval (loop, ev_tstamp interval)
756 These advanced functions influence the time that libev will spend waiting
757 for events. Both time intervals are by default C<0>, meaning that libev
758 will try to invoke timer/periodic callbacks and I/O callbacks with minimum
759 latency.
761 Setting these to a higher value (the C<interval> I<must> be >= C<0>)
762 allows libev to delay invocation of I/O and timer/periodic callbacks
763 to increase efficiency of loop iterations (or to increase power-saving
764 opportunities).
766 The idea is that sometimes your program runs just fast enough to handle
767 one (or very few) event(s) per loop iteration. While this makes the
768 program responsive, it also wastes a lot of CPU time to poll for new
769 events, especially with backends like C<select ()> which have a high
770 overhead for the actual polling but can deliver many events at once.
772 By setting a higher I<io collect interval> you allow libev to spend more
773 time collecting I/O events, so you can handle more events per iteration,
774 at the cost of increasing latency. Timeouts (both C<ev_periodic> and
775 C<ev_timer>) will be not affected. Setting this to a non-null value will
776 introduce an additional C<ev_sleep ()> call into most loop iterations.
778 Likewise, by setting a higher I<timeout collect interval> you allow libev
779 to spend more time collecting timeouts, at the expense of increased
780 latency/jitter/inexactness (the watcher callback will be called
781 later). C<ev_io> watchers will not be affected. Setting this to a non-null
782 value will not introduce any overhead in libev.
784 Many (busy) programs can usually benefit by setting the I/O collect
785 interval to a value near C<0.1> or so, which is often enough for
786 interactive servers (of course not for games), likewise for timeouts. It
787 usually doesn't make much sense to set it to a lower value than C<0.01>,
788 as this approaches the timing granularity of most systems.
790 Setting the I<timeout collect interval> can improve the opportunity for
791 saving power, as the program will "bundle" timer callback invocations that
792 are "near" in time together, by delaying some, thus reducing the number of
793 times the process sleeps and wakes up again. Another useful technique to
794 reduce iterations/wake-ups is to use C<ev_periodic> watchers and make sure
795 they fire on, say, one-second boundaries only.
797 =item ev_loop_verify (loop)
799 This function only does something when C<EV_VERIFY> support has been
800 compiled in, which is the default for non-minimal builds. It tries to go
801 through all internal structures and checks them for validity. If anything
802 is found to be inconsistent, it will print an error message to standard
803 error and call C<abort ()>.
805 This can be used to catch bugs inside libev itself: under normal
806 circumstances, this function will never abort as of course libev keeps its
807 data structures consistent.
809 =back
814 In the following description, uppercase C<TYPE> in names stands for the
815 watcher type, e.g. C<ev_TYPE_start> can mean C<ev_timer_start> for timer
816 watchers and C<ev_io_start> for I/O watchers.
818 A watcher is a structure that you create and register to record your
819 interest in some event. For instance, if you want to wait for STDIN to
820 become readable, you would create an C<ev_io> watcher for that:
822 static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
823 {
824 ev_io_stop (w);
825 ev_unloop (loop, EVUNLOOP_ALL);
826 }
828 struct ev_loop *loop = ev_default_loop (0);
830 ev_io stdin_watcher;
832 ev_init (&stdin_watcher, my_cb);
833 ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
834 ev_io_start (loop, &stdin_watcher);
836 ev_loop (loop, 0);
838 As you can see, you are responsible for allocating the memory for your
839 watcher structures (and it is I<usually> a bad idea to do this on the
840 stack).
842 Each watcher has an associated watcher structure (called C<struct ev_TYPE>
843 or simply C<ev_TYPE>, as typedefs are provided for all watcher structs).
845 Each watcher structure must be initialised by a call to C<ev_init
846 (watcher *, callback)>, which expects a callback to be provided. This
847 callback gets invoked each time the event occurs (or, in the case of I/O
848 watchers, each time the event loop detects that the file descriptor given
849 is readable and/or writable).
851 Each watcher type further has its own C<< ev_TYPE_set (watcher *, ...) >>
852 macro to configure it, with arguments specific to the watcher type. There
853 is also a macro to combine initialisation and setting in one call: C<<
854 ev_TYPE_init (watcher *, callback, ...) >>.
856 To make the watcher actually watch out for events, you have to start it
857 with a watcher-specific start function (C<< ev_TYPE_start (loop, watcher
858 *) >>), and you can stop watching for events at any time by calling the
859 corresponding stop function (C<< ev_TYPE_stop (loop, watcher *) >>.
861 As long as your watcher is active (has been started but not stopped) you
862 must not touch the values stored in it. Most specifically you must never
863 reinitialise it or call its C<ev_TYPE_set> macro.
865 Each and every callback receives the event loop pointer as first, the
866 registered watcher structure as second, and a bitset of received events as
867 third argument.
869 The received events usually include a single bit per event type received
870 (you can receive multiple events at the same time). The possible bit masks
871 are:
873 =over 4
875 =item C<EV_READ>
877 =item C<EV_WRITE>
879 The file descriptor in the C<ev_io> watcher has become readable and/or
880 writable.
882 =item C<EV_TIMEOUT>
884 The C<ev_timer> watcher has timed out.
886 =item C<EV_PERIODIC>
888 The C<ev_periodic> watcher has timed out.
890 =item C<EV_SIGNAL>
892 The signal specified in the C<ev_signal> watcher has been received by a thread.
894 =item C<EV_CHILD>
896 The pid specified in the C<ev_child> watcher has received a status change.
898 =item C<EV_STAT>
900 The path specified in the C<ev_stat> watcher changed its attributes somehow.
902 =item C<EV_IDLE>
904 The C<ev_idle> watcher has determined that you have nothing better to do.
906 =item C<EV_PREPARE>
908 =item C<EV_CHECK>
910 All C<ev_prepare> watchers are invoked just I<before> C<ev_loop> starts
911 to gather new events, and all C<ev_check> watchers are invoked just after
912 C<ev_loop> has gathered them, but before it invokes any callbacks for any
913 received events. Callbacks of both watcher types can start and stop as
914 many watchers as they want, and all of them will be taken into account
915 (for example, a C<ev_prepare> watcher might start an idle watcher to keep
916 C<ev_loop> from blocking).
918 =item C<EV_EMBED>
920 The embedded event loop specified in the C<ev_embed> watcher needs attention.
922 =item C<EV_FORK>
924 The event loop has been resumed in the child process after fork (see
925 C<ev_fork>).
927 =item C<EV_ASYNC>
929 The given async watcher has been asynchronously notified (see C<ev_async>).
931 =item C<EV_ERROR>
933 An unspecified error has occurred, the watcher has been stopped. This might
934 happen because the watcher could not be properly started because libev
935 ran out of memory, a file descriptor was found to be closed or any other
936 problem. Libev considers these application bugs.
938 You best act on it by reporting the problem and somehow coping with the
939 watcher being stopped. Note that well-written programs should not receive
940 an error ever, so when your watcher receives it, this usually indicates a
941 bug in your program.
943 Libev will usually signal a few "dummy" events together with an error, for
944 example it might indicate that a fd is readable or writable, and if your
945 callbacks is well-written it can just attempt the operation and cope with
946 the error from read() or write(). This will not work in multi-threaded
947 programs, though, as the fd could already be closed and reused for another
948 thing, so beware.
950 =back
954 =over 4
956 =item C<ev_init> (ev_TYPE *watcher, callback)
958 This macro initialises the generic portion of a watcher. The contents
959 of the watcher object can be arbitrary (so C<malloc> will do). Only
960 the generic parts of the watcher are initialised, you I<need> to call
961 the type-specific C<ev_TYPE_set> macro afterwards to initialise the
962 type-specific parts. For each type there is also a C<ev_TYPE_init> macro
963 which rolls both calls into one.
965 You can reinitialise a watcher at any time as long as it has been stopped
966 (or never started) and there are no pending events outstanding.
968 The callback is always of type C<void (*)(struct ev_loop *loop, ev_TYPE *watcher,
969 int revents)>.
971 Example: Initialise an C<ev_io> watcher in two steps.
973 ev_io w;
974 ev_init (&w, my_cb);
975 ev_io_set (&w, STDIN_FILENO, EV_READ);
977 =item C<ev_TYPE_set> (ev_TYPE *, [args])
979 This macro initialises the type-specific parts of a watcher. You need to
980 call C<ev_init> at least once before you call this macro, but you can
981 call C<ev_TYPE_set> any number of times. You must not, however, call this
982 macro on a watcher that is active (it can be pending, however, which is a
983 difference to the C<ev_init> macro).
985 Although some watcher types do not have type-specific arguments
986 (e.g. C<ev_prepare>) you still need to call its C<set> macro.
988 See C<ev_init>, above, for an example.
990 =item C<ev_TYPE_init> (ev_TYPE *watcher, callback, [args])
992 This convenience macro rolls both C<ev_init> and C<ev_TYPE_set> macro
993 calls into a single call. This is the most convenient method to initialise
994 a watcher. The same limitations apply, of course.
996 Example: Initialise and set an C<ev_io> watcher in one step.
998 ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
1000 =item C<ev_TYPE_start> (loop *, ev_TYPE *watcher)
1002 Starts (activates) the given watcher. Only active watchers will receive
1003 events. If the watcher is already active nothing will happen.
1005 Example: Start the C<ev_io> watcher that is being abused as example in this
1006 whole section.
1008 ev_io_start (EV_DEFAULT_UC, &w);
1010 =item C<ev_TYPE_stop> (loop *, ev_TYPE *watcher)
1012 Stops the given watcher if active, and clears the pending status (whether
1013 the watcher was active or not).
1015 It is possible that stopped watchers are pending - for example,
1016 non-repeating timers are being stopped when they become pending - but
1017 calling C<ev_TYPE_stop> ensures that the watcher is neither active nor
1018 pending. If you want to free or reuse the memory used by the watcher it is
1019 therefore a good idea to always call its C<ev_TYPE_stop> function.
1021 =item bool ev_is_active (ev_TYPE *watcher)
1023 Returns a true value iff the watcher is active (i.e. it has been started
1024 and not yet been stopped). As long as a watcher is active you must not modify
1025 it.
1027 =item bool ev_is_pending (ev_TYPE *watcher)
1029 Returns a true value iff the watcher is pending, (i.e. it has outstanding
1030 events but its callback has not yet been invoked). As long as a watcher
1031 is pending (but not active) you must not call an init function on it (but
1032 C<ev_TYPE_set> is safe), you must not change its priority, and you must
1033 make sure the watcher is available to libev (e.g. you cannot C<free ()>
1034 it).
1036 =item callback ev_cb (ev_TYPE *watcher)
1038 Returns the callback currently set on the watcher.
1040 =item ev_cb_set (ev_TYPE *watcher, callback)
1042 Change the callback. You can change the callback at virtually any time
1043 (modulo threads).
1045 =item ev_set_priority (ev_TYPE *watcher, priority)
1047 =item int ev_priority (ev_TYPE *watcher)
1049 Set and query the priority of the watcher. The priority is a small
1050 integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI>
1051 (default: C<-2>). Pending watchers with higher priority will be invoked
1052 before watchers with lower priority, but priority will not keep watchers
1053 from being executed (except for C<ev_idle> watchers).
1055 This means that priorities are I<only> used for ordering callback
1056 invocation after new events have been received. This is useful, for
1057 example, to reduce latency after idling, or more often, to bind two
1058 watchers on the same event and make sure one is called first.
1060 If you need to suppress invocation when higher priority events are pending
1061 you need to look at C<ev_idle> watchers, which provide this functionality.
1063 You I<must not> change the priority of a watcher as long as it is active or
1064 pending.
1066 The default priority used by watchers when no priority has been set is
1067 always C<0>, which is supposed to not be too high and not be too low :).
1069 Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is
1070 fine, as long as you do not mind that the priority value you query might
1071 or might not have been clamped to the valid range.
1073 =item ev_invoke (loop, ev_TYPE *watcher, int revents)
1075 Invoke the C<watcher> with the given C<loop> and C<revents>. Neither
1076 C<loop> nor C<revents> need to be valid as long as the watcher callback
1077 can deal with that fact, as both are simply passed through to the
1078 callback.
1080 =item int ev_clear_pending (loop, ev_TYPE *watcher)
1082 If the watcher is pending, this function clears its pending status and
1083 returns its C<revents> bitset (as if its callback was invoked). If the
1084 watcher isn't pending it does nothing and returns C<0>.
1086 Sometimes it can be useful to "poll" a watcher instead of waiting for its
1087 callback to be invoked, which can be accomplished with this function.
1089 =back
1094 Each watcher has, by default, a member C<void *data> that you can change
1095 and read at any time: libev will completely ignore it. This can be used
1096 to associate arbitrary data with your watcher. If you need more data and
1097 don't want to allocate memory and store a pointer to it in that data
1098 member, you can also "subclass" the watcher type and provide your own
1099 data:
1101 struct my_io
1102 {
1103 ev_io io;
1104 int otherfd;
1105 void *somedata;
1106 struct whatever *mostinteresting;
1107 };
1109 ...
1110 struct my_io w;
1111 ev_io_init (&, my_cb, fd, EV_READ);
1113 And since your callback will be called with a pointer to the watcher, you
1114 can cast it back to your own type:
1116 static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
1117 {
1118 struct my_io *w = (struct my_io *)w_;
1119 ...
1120 }
1122 More interesting and less C-conformant ways of casting your callback type
1123 instead have been omitted.
1125 Another common scenario is to use some data structure with multiple
1126 embedded watchers:
1128 struct my_biggy
1129 {
1130 int some_data;
1131 ev_timer t1;
1132 ev_timer t2;
1133 }
1135 In this case getting the pointer to C<my_biggy> is a bit more
1136 complicated: Either you store the address of your C<my_biggy> struct
1137 in the C<data> member of the watcher (for woozies), or you need to use
1138 some pointer arithmetic using C<offsetof> inside your watchers (for real
1139 programmers):
1141 #include <stddef.h>
1143 static void
1144 t1_cb (EV_P_ ev_timer *w, int revents)
1145 {
1146 struct my_biggy big = (struct my_biggy *
1147 (((char *)w) - offsetof (struct my_biggy, t1));
1148 }
1150 static void
1151 t2_cb (EV_P_ ev_timer *w, int revents)
1152 {
1153 struct my_biggy big = (struct my_biggy *
1154 (((char *)w) - offsetof (struct my_biggy, t2));
1155 }
1158 =head1 WATCHER TYPES
1160 This section describes each watcher in detail, but will not repeat
1161 information given in the last section. Any initialisation/set macros,
1162 functions and members specific to the watcher type are explained.
1164 Members are additionally marked with either I<[read-only]>, meaning that,
1165 while the watcher is active, you can look at the member and expect some
1166 sensible content, but you must not modify it (you can modify it while the
1167 watcher is stopped to your hearts content), or I<[read-write]>, which
1168 means you can expect it to have some sensible content while the watcher
1169 is active, but you can also modify it. Modifying it may not do something
1170 sensible or take immediate effect (or do anything at all), but libev will
1171 not crash or malfunction in any way.
1174 =head2 C<ev_io> - is this file descriptor readable or writable?
1176 I/O watchers check whether a file descriptor is readable or writable
1177 in each iteration of the event loop, or, more precisely, when reading
1178 would not block the process and writing would at least be able to write
1179 some data. This behaviour is called level-triggering because you keep
1180 receiving events as long as the condition persists. Remember you can stop
1181 the watcher if you don't want to act on the event and neither want to
1182 receive future events.
1184 In general you can register as many read and/or write event watchers per
1185 fd as you want (as long as you don't confuse yourself). Setting all file
1186 descriptors to non-blocking mode is also usually a good idea (but not
1187 required if you know what you are doing).
1189 If you cannot use non-blocking mode, then force the use of a
1190 known-to-be-good backend (at the time of this writing, this includes only
1193 Another thing you have to watch out for is that it is quite easy to
1194 receive "spurious" readiness notifications, that is your callback might
1195 be called with C<EV_READ> but a subsequent C<read>(2) will actually block
1196 because there is no data. Not only are some backends known to create a
1197 lot of those (for example Solaris ports), it is very easy to get into
1198 this situation even with a relatively standard program structure. Thus
1199 it is best to always use non-blocking I/O: An extra C<read>(2) returning
1200 C<EAGAIN> is far preferable to a program hanging until some data arrives.
1202 If you cannot run the fd in non-blocking mode (for example you should
1203 not play around with an Xlib connection), then you have to separately
1204 re-test whether a file descriptor is really ready with a known-to-be good
1205 interface such as poll (fortunately in our Xlib example, Xlib already
1206 does this on its own, so its quite safe to use). Some people additionally
1207 use C<SIGALRM> and an interval timer, just to be sure you won't block
1208 indefinitely.
1210 But really, best use non-blocking mode.
1212 =head3 The special problem of disappearing file descriptors
1214 Some backends (e.g. kqueue, epoll) need to be told about closing a file
1215 descriptor (either due to calling C<close> explicitly or any other means,
1216 such as C<dup2>). The reason is that you register interest in some file
1217 descriptor, but when it goes away, the operating system will silently drop
1218 this interest. If another file descriptor with the same number then is
1219 registered with libev, there is no efficient way to see that this is, in
1220 fact, a different file descriptor.
1222 To avoid having to explicitly tell libev about such cases, libev follows
1223 the following policy: Each time C<ev_io_set> is being called, libev
1224 will assume that this is potentially a new file descriptor, otherwise
1225 it is assumed that the file descriptor stays the same. That means that
1226 you I<have> to call C<ev_io_set> (or C<ev_io_init>) when you change the
1227 descriptor even if the file descriptor number itself did not change.
1229 This is how one would do it normally anyway, the important point is that
1230 the libev application should not optimise around libev but should leave
1231 optimisations to libev.
1233 =head3 The special problem of dup'ed file descriptors
1235 Some backends (e.g. epoll), cannot register events for file descriptors,
1236 but only events for the underlying file descriptions. That means when you
1237 have C<dup ()>'ed file descriptors or weirder constellations, and register
1238 events for them, only one file descriptor might actually receive events.
1240 There is no workaround possible except not registering events
1241 for potentially C<dup ()>'ed file descriptors, or to resort to
1244 =head3 The special problem of fork
1246 Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
1247 useless behaviour. Libev fully supports fork, but needs to be told about
1248 it in the child.
1250 To support fork in your programs, you either have to call
1251 C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,
1252 enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or
1255 =head3 The special problem of SIGPIPE
1257 While not really specific to libev, it is easy to forget about C<SIGPIPE>:
1258 when writing to a pipe whose other end has been closed, your program gets
1259 sent a SIGPIPE, which, by default, aborts your program. For most programs
1260 this is sensible behaviour, for daemons, this is usually undesirable.
1262 So when you encounter spurious, unexplained daemon exits, make sure you
1263 ignore SIGPIPE (and maybe make sure you log the exit status of your daemon
1264 somewhere, as that would have given you a big clue).
1267 =head3 Watcher-Specific Functions
1269 =over 4
1271 =item ev_io_init (ev_io *, callback, int fd, int events)
1273 =item ev_io_set (ev_io *, int fd, int events)
1275 Configures an C<ev_io> watcher. The C<fd> is the file descriptor to
1276 receive events for and C<events> is either C<EV_READ>, C<EV_WRITE> or
1277 C<EV_READ | EV_WRITE>, to express the desire to receive the given events.
1279 =item int fd [read-only]
1281 The file descriptor being watched.
1283 =item int events [read-only]
1285 The events being watched.
1287 =back
1289 =head3 Examples
1291 Example: Call C<stdin_readable_cb> when STDIN_FILENO has become, well
1292 readable, but only once. Since it is likely line-buffered, you could
1293 attempt to read a whole line in the callback.
1295 static void
1296 stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1297 {
1298 ev_io_stop (loop, w);
1299 .. read from stdin here (or from w->fd) and handle any I/O errors
1300 }
1302 ...
1303 struct ev_loop *loop = ev_default_init (0);
1304 ev_io stdin_readable;
1305 ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
1306 ev_io_start (loop, &stdin_readable);
1307 ev_loop (loop, 0);
1310 =head2 C<ev_timer> - relative and optionally repeating timeouts
1312 Timer watchers are simple relative timers that generate an event after a
1313 given time, and optionally repeating in regular intervals after that.
1315 The timers are based on real time, that is, if you register an event that
1316 times out after an hour and you reset your system clock to January last
1317 year, it will still time out after (roughly) one hour. "Roughly" because
1318 detecting time jumps is hard, and some inaccuracies are unavoidable (the
1319 monotonic clock option helps a lot here).
1321 The callback is guaranteed to be invoked only I<after> its timeout has
1322 passed, but if multiple timers become ready during the same loop iteration
1323 then order of execution is undefined.
1325 =head3 Be smart about timeouts
1327 Many real-world problems involve some kind of timeout, usually for error
1328 recovery. A typical example is an HTTP request - if the other side hangs,
1329 you want to raise some error after a while.
1331 What follows are some ways to handle this problem, from obvious and
1332 inefficient to smart and efficient.
1334 In the following, a 60 second activity timeout is assumed - a timeout that
1335 gets reset to 60 seconds each time there is activity (e.g. each time some
1336 data or other life sign was received).
1338 =over 4
1340 =item 1. Use a timer and stop, reinitialise and start it on activity.
1342 This is the most obvious, but not the most simple way: In the beginning,
1343 start the watcher:
1345 ev_timer_init (timer, callback, 60., 0.);
1346 ev_timer_start (loop, timer);
1348 Then, each time there is some activity, C<ev_timer_stop> it, initialise it
1349 and start it again:
1351 ev_timer_stop (loop, timer);
1352 ev_timer_set (timer, 60., 0.);
1353 ev_timer_start (loop, timer);
1355 This is relatively simple to implement, but means that each time there is
1356 some activity, libev will first have to remove the timer from its internal
1357 data structure and then add it again. Libev tries to be fast, but it's
1358 still not a constant-time operation.
1360 =item 2. Use a timer and re-start it with C<ev_timer_again> inactivity.
1362 This is the easiest way, and involves using C<ev_timer_again> instead of
1363 C<ev_timer_start>.
1365 To implement this, configure an C<ev_timer> with a C<repeat> value
1366 of C<60> and then call C<ev_timer_again> at start and each time you
1367 successfully read or write some data. If you go into an idle state where
1368 you do not expect data to travel on the socket, you can C<ev_timer_stop>
1369 the timer, and C<ev_timer_again> will automatically restart it if need be.
1371 That means you can ignore both the C<ev_timer_start> function and the
1372 C<after> argument to C<ev_timer_set>, and only ever use the C<repeat>
1373 member and C<ev_timer_again>.
1375 At start:
1377 ev_timer_init (timer, callback);
1378 timer->repeat = 60.;
1379 ev_timer_again (loop, timer);
1381 Each time there is some activity:
1383 ev_timer_again (loop, timer);
1385 It is even possible to change the time-out on the fly, regardless of
1386 whether the watcher is active or not:
1388 timer->repeat = 30.;
1389 ev_timer_again (loop, timer);
1391 This is slightly more efficient then stopping/starting the timer each time
1392 you want to modify its timeout value, as libev does not have to completely
1393 remove and re-insert the timer from/into its internal data structure.
1395 It is, however, even simpler than the "obvious" way to do it.
1397 =item 3. Let the timer time out, but then re-arm it as required.
1399 This method is more tricky, but usually most efficient: Most timeouts are
1400 relatively long compared to the intervals between other activity - in
1401 our example, within 60 seconds, there are usually many I/O events with
1402 associated activity resets.
1404 In this case, it would be more efficient to leave the C<ev_timer> alone,
1405 but remember the time of last activity, and check for a real timeout only
1406 within the callback:
1408 ev_tstamp last_activity; // time of last activity
1410 static void
1411 callback (EV_P_ ev_timer *w, int revents)
1412 {
1413 ev_tstamp now = ev_now (EV_A);
1414 ev_tstamp timeout = last_activity + 60.;
1416 // if last_activity + 60. is older than now, we did time out
1417 if (timeout < now)
1418 {
1419 // timeout occured, take action
1420 }
1421 else
1422 {
1423 // callback was invoked, but there was some activity, re-arm
1424 // the watcher to fire in last_activity + 60, which is
1425 // guaranteed to be in the future, so "again" is positive:
1426 w->repeat = timeout - now;
1427 ev_timer_again (EV_A_ w);
1428 }
1429 }
1431 To summarise the callback: first calculate the real timeout (defined
1432 as "60 seconds after the last activity"), then check if that time has
1433 been reached, which means something I<did>, in fact, time out. Otherwise
1434 the callback was invoked too early (C<timeout> is in the future), so
1435 re-schedule the timer to fire at that future time, to see if maybe we have
1436 a timeout then.
1438 Note how C<ev_timer_again> is used, taking advantage of the
1439 C<ev_timer_again> optimisation when the timer is already running.
1441 This scheme causes more callback invocations (about one every 60 seconds
1442 minus half the average time between activity), but virtually no calls to
1443 libev to change the timeout.
1445 To start the timer, simply initialise the watcher and set C<last_activity>
1446 to the current time (meaning we just have some activity :), then call the
1447 callback, which will "do the right thing" and start the timer:
1449 ev_timer_init (timer, callback);
1450 last_activity = ev_now (loop);
1451 callback (loop, timer, EV_TIMEOUT);
1453 And when there is some activity, simply store the current time in
1454 C<last_activity>, no libev calls at all:
1456 last_actiivty = ev_now (loop);
1458 This technique is slightly more complex, but in most cases where the
1459 time-out is unlikely to be triggered, much more efficient.
1461 Changing the timeout is trivial as well (if it isn't hard-coded in the
1462 callback :) - just change the timeout and invoke the callback, which will
1463 fix things for you.
1465 =item 4. Wee, just use a double-linked list for your timeouts.
1467 If there is not one request, but many thousands (millions...), all
1468 employing some kind of timeout with the same timeout value, then one can
1469 do even better:
1471 When starting the timeout, calculate the timeout value and put the timeout
1472 at the I<end> of the list.
1474 Then use an C<ev_timer> to fire when the timeout at the I<beginning> of
1475 the list is expected to fire (for example, using the technique #3).
1477 When there is some activity, remove the timer from the list, recalculate
1478 the timeout, append it to the end of the list again, and make sure to
1479 update the C<ev_timer> if it was taken from the beginning of the list.
1481 This way, one can manage an unlimited number of timeouts in O(1) time for
1482 starting, stopping and updating the timers, at the expense of a major
1483 complication, and having to use a constant timeout. The constant timeout
1484 ensures that the list stays sorted.
1486 =back
1488 So which method the best?
1490 Method #2 is a simple no-brain-required solution that is adequate in most
1491 situations. Method #3 requires a bit more thinking, but handles many cases
1492 better, and isn't very complicated either. In most case, choosing either
1493 one is fine, with #3 being better in typical situations.
1495 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
1496 rather complicated, but extremely efficient, something that really pays
1497 off after the first million or so of active timers, i.e. it's usually
1498 overkill :)
1500 =head3 The special problem of time updates
1502 Establishing the current time is a costly operation (it usually takes at
1503 least two system calls): EV therefore updates its idea of the current
1504 time only before and after C<ev_loop> collects new events, which causes a
1505 growing difference between C<ev_now ()> and C<ev_time ()> when handling
1506 lots of events in one iteration.
1508 The relative timeouts are calculated relative to the C<ev_now ()>
1509 time. This is usually the right thing as this timestamp refers to the time
1510 of the event triggering whatever timeout you are modifying/starting. If
1511 you suspect event processing to be delayed and you I<need> to base the
1512 timeout on the current time, use something like this to adjust for this:
1514 ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
1516 If the event loop is suspended for a long time, you can also force an
1517 update of the time returned by C<ev_now ()> by calling C<ev_now_update
1518 ()>.
1520 =head3 Watcher-Specific Functions and Data Members
1522 =over 4
1524 =item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
1526 =item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
1528 Configure the timer to trigger after C<after> seconds. If C<repeat>
1529 is C<0.>, then it will automatically be stopped once the timeout is
1530 reached. If it is positive, then the timer will automatically be
1531 configured to trigger again C<repeat> seconds later, again, and again,
1532 until stopped manually.
1534 The timer itself will do a best-effort at avoiding drift, that is, if
1535 you configure a timer to trigger every 10 seconds, then it will normally
1536 trigger at exactly 10 second intervals. If, however, your program cannot
1537 keep up with the timer (because it takes longer than those 10 seconds to
1538 do stuff) the timer will not fire more than once per event loop iteration.
1540 =item ev_timer_again (loop, ev_timer *)
1542 This will act as if the timer timed out and restart it again if it is
1543 repeating. The exact semantics are:
1545 If the timer is pending, its pending status is cleared.
1547 If the timer is started but non-repeating, stop it (as if it timed out).
1549 If the timer is repeating, either start it if necessary (with the
1550 C<repeat> value), or reset the running timer to the C<repeat> value.
1552 This sounds a bit complicated, see "Be smart about timeouts", above, for a
1553 usage example.
1555 =item ev_tstamp repeat [read-write]
1557 The current C<repeat> value. Will be used each time the watcher times out
1558 or C<ev_timer_again> is called, and determines the next timeout (if any),
1559 which is also when any modifications are taken into account.
1561 =back
1563 =head3 Examples
1565 Example: Create a timer that fires after 60 seconds.
1567 static void
1568 one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
1569 {
1570 .. one minute over, w is actually stopped right here
1571 }
1573 ev_timer mytimer;
1574 ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
1575 ev_timer_start (loop, &mytimer);
1577 Example: Create a timeout timer that times out after 10 seconds of
1578 inactivity.
1580 static void
1581 timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
1582 {
1583 .. ten seconds without any activity
1584 }
1586 ev_timer mytimer;
1587 ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
1588 ev_timer_again (&mytimer); /* start timer */
1589 ev_loop (loop, 0);
1591 // and in some piece of code that gets executed on any "activity":
1592 // reset the timeout to start ticking again at 10 seconds
1593 ev_timer_again (&mytimer);
1596 =head2 C<ev_periodic> - to cron or not to cron?
1598 Periodic watchers are also timers of a kind, but they are very versatile
1599 (and unfortunately a bit complex).
1601 Unlike C<ev_timer>'s, they are not based on real time (or relative time)
1602 but on wall clock time (absolute time). You can tell a periodic watcher
1603 to trigger after some specific point in time. For example, if you tell a
1604 periodic watcher to trigger in 10 seconds (by specifying e.g. C<ev_now ()
1605 + 10.>, that is, an absolute time not a delay) and then reset your system
1606 clock to January of the previous year, then it will take more than year
1607 to trigger the event (unlike an C<ev_timer>, which would still trigger
1608 roughly 10 seconds later as it uses a relative timeout).
1610 C<ev_periodic>s can also be used to implement vastly more complex timers,
1611 such as triggering an event on each "midnight, local time", or other
1612 complicated rules.
1614 As with timers, the callback is guaranteed to be invoked only when the
1615 time (C<at>) has passed, but if multiple periodic timers become ready
1616 during the same loop iteration, then order of execution is undefined.
1618 =head3 Watcher-Specific Functions and Data Members
1620 =over 4
1622 =item ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)
1624 =item ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)
1626 Lots of arguments, lets sort it out... There are basically three modes of
1627 operation, and we will explain them from simplest to most complex:
1629 =over 4
1631 =item * absolute timer (at = time, interval = reschedule_cb = 0)
1633 In this configuration the watcher triggers an event after the wall clock
1634 time C<at> has passed. It will not repeat and will not adjust when a time
1635 jump occurs, that is, if it is to be run at January 1st 2011 then it will
1636 only run when the system clock reaches or surpasses this time.
1638 =item * repeating interval timer (at = offset, interval > 0, reschedule_cb = 0)
1640 In this mode the watcher will always be scheduled to time out at the next
1641 C<at + N * interval> time (for some integer N, which can also be negative)
1642 and then repeat, regardless of any time jumps.
1644 This can be used to create timers that do not drift with respect to the
1645 system clock, for example, here is a C<ev_periodic> that triggers each
1646 hour, on the hour:
1648 ev_periodic_set (&periodic, 0., 3600., 0);
1650 This doesn't mean there will always be 3600 seconds in between triggers,
1651 but only that the callback will be called when the system time shows a
1652 full hour (UTC), or more correctly, when the system time is evenly divisible
1653 by 3600.
1655 Another way to think about it (for the mathematically inclined) is that
1656 C<ev_periodic> will try to run the callback in this mode at the next possible
1657 time where C<time = at (mod interval)>, regardless of any time jumps.
1659 For numerical stability it is preferable that the C<at> value is near
1660 C<ev_now ()> (the current time), but there is no range requirement for
1661 this value, and in fact is often specified as zero.
1663 Note also that there is an upper limit to how often a timer can fire (CPU
1664 speed for example), so if C<interval> is very small then timing stability
1665 will of course deteriorate. Libev itself tries to be exact to be about one
1666 millisecond (if the OS supports it and the machine is fast enough).
1668 =item * manual reschedule mode (at and interval ignored, reschedule_cb = callback)
1670 In this mode the values for C<interval> and C<at> are both being
1671 ignored. Instead, each time the periodic watcher gets scheduled, the
1672 reschedule callback will be called with the watcher as first, and the
1673 current time as second argument.
1675 NOTE: I<This callback MUST NOT stop or destroy any periodic watcher,
1676 ever, or make ANY event loop modifications whatsoever>.
1678 If you need to stop it, return C<now + 1e30> (or so, fudge fudge) and stop
1679 it afterwards (e.g. by starting an C<ev_prepare> watcher, which is the
1680 only event loop modification you are allowed to do).
1682 The callback prototype is C<ev_tstamp (*reschedule_cb)(ev_periodic
1683 *w, ev_tstamp now)>, e.g.:
1685 static ev_tstamp
1686 my_rescheduler (ev_periodic *w, ev_tstamp now)
1687 {
1688 return now + 60.;
1689 }
1691 It must return the next time to trigger, based on the passed time value
1692 (that is, the lowest time value larger than to the second argument). It
1693 will usually be called just before the callback will be triggered, but
1694 might be called at other times, too.
1696 NOTE: I<< This callback must always return a time that is higher than or
1697 equal to the passed C<now> value >>.
1699 This can be used to create very complex timers, such as a timer that
1700 triggers on "next midnight, local time". To do this, you would calculate the
1701 next midnight after C<now> and return the timestamp value for this. How
1702 you do this is, again, up to you (but it is not trivial, which is the main
1703 reason I omitted it as an example).
1705 =back
1707 =item ev_periodic_again (loop, ev_periodic *)
1709 Simply stops and restarts the periodic watcher again. This is only useful
1710 when you changed some parameters or the reschedule callback would return
1711 a different time than the last time it was called (e.g. in a crond like
1712 program when the crontabs have changed).
1714 =item ev_tstamp ev_periodic_at (ev_periodic *)
1716 When active, returns the absolute time that the watcher is supposed to
1717 trigger next.
1719 =item ev_tstamp offset [read-write]
1721 When repeating, this contains the offset value, otherwise this is the
1722 absolute point in time (the C<at> value passed to C<ev_periodic_set>).
1724 Can be modified any time, but changes only take effect when the periodic
1725 timer fires or C<ev_periodic_again> is being called.
1727 =item ev_tstamp interval [read-write]
1729 The current interval value. Can be modified any time, but changes only
1730 take effect when the periodic timer fires or C<ev_periodic_again> is being
1731 called.
1733 =item ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]
1735 The current reschedule callback, or C<0>, if this functionality is
1736 switched off. Can be changed any time, but changes only take effect when
1737 the periodic timer fires or C<ev_periodic_again> is being called.
1739 =back
1741 =head3 Examples
1743 Example: Call a callback every hour, or, more precisely, whenever the
1744 system time is divisible by 3600. The callback invocation times have
1745 potentially a lot of jitter, but good long-term stability.
1747 static void
1748 clock_cb (struct ev_loop *loop, ev_io *w, int revents)
1749 {
1750 ... its now a full hour (UTC, or TAI or whatever your clock follows)
1751 }
1753 ev_periodic hourly_tick;
1754 ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
1755 ev_periodic_start (loop, &hourly_tick);
1757 Example: The same as above, but use a reschedule callback to do it:
1759 #include <math.h>
1761 static ev_tstamp
1762 my_scheduler_cb (ev_periodic *w, ev_tstamp now)
1763 {
1764 return now + (3600. - fmod (now, 3600.));
1765 }
1767 ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
1769 Example: Call a callback every hour, starting now:
1771 ev_periodic hourly_tick;
1772 ev_periodic_init (&hourly_tick, clock_cb,
1773 fmod (ev_now (loop), 3600.), 3600., 0);
1774 ev_periodic_start (loop, &hourly_tick);
1777 =head2 C<ev_signal> - signal me when a signal gets signalled!
1779 Signal watchers will trigger an event when the process receives a specific
1780 signal one or more times. Even though signals are very asynchronous, libev
1781 will try it's best to deliver signals synchronously, i.e. as part of the
1782 normal event processing, like any other event.
1784 If you want signals asynchronously, just use C<sigaction> as you would
1785 do without libev and forget about sharing the signal. You can even use
1786 C<ev_async> from a signal handler to synchronously wake up an event loop.
1788 You can configure as many watchers as you like per signal. Only when the
1789 first watcher gets started will libev actually register a signal handler
1790 with the kernel (thus it coexists with your own signal handlers as long as
1791 you don't register any with libev for the same signal). Similarly, when
1792 the last signal watcher for a signal is stopped, libev will reset the
1793 signal handler to SIG_DFL (regardless of what it was set to before).
1795 If possible and supported, libev will install its handlers with
1796 C<SA_RESTART> behaviour enabled, so system calls should not be unduly
1797 interrupted. If you have a problem with system calls getting interrupted by
1798 signals you can block all signals in an C<ev_check> watcher and unblock
1799 them in an C<ev_prepare> watcher.
1801 =head3 Watcher-Specific Functions and Data Members
1803 =over 4
1805 =item ev_signal_init (ev_signal *, callback, int signum)
1807 =item ev_signal_set (ev_signal *, int signum)
1809 Configures the watcher to trigger on the given signal number (usually one
1810 of the C<SIGxxx> constants).
1812 =item int signum [read-only]
1814 The signal the watcher watches out for.
1816 =back
1818 =head3 Examples
1820 Example: Try to exit cleanly on SIGINT.
1822 static void
1823 sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
1824 {
1825 ev_unloop (loop, EVUNLOOP_ALL);
1826 }
1828 ev_signal signal_watcher;
1829 ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
1830 ev_signal_start (loop, &signal_watcher);
1833 =head2 C<ev_child> - watch out for process status changes
1835 Child watchers trigger when your process receives a SIGCHLD in response to
1836 some child status changes (most typically when a child of yours dies or
1837 exits). It is permissible to install a child watcher I<after> the child
1838 has been forked (which implies it might have already exited), as long
1839 as the event loop isn't entered (or is continued from a watcher), i.e.,
1840 forking and then immediately registering a watcher for the child is fine,
1841 but forking and registering a watcher a few event loop iterations later is
1842 not.
1844 Only the default event loop is capable of handling signals, and therefore
1845 you can only register child watchers in the default event loop.
1847 =head3 Process Interaction
1849 Libev grabs C<SIGCHLD> as soon as the default event loop is
1850 initialised. This is necessary to guarantee proper behaviour even if
1851 the first child watcher is started after the child exits. The occurrence
1852 of C<SIGCHLD> is recorded asynchronously, but child reaping is done
1853 synchronously as part of the event loop processing. Libev always reaps all
1854 children, even ones not watched.
1856 =head3 Overriding the Built-In Processing
1858 Libev offers no special support for overriding the built-in child
1859 processing, but if your application collides with libev's default child
1860 handler, you can override it easily by installing your own handler for
1861 C<SIGCHLD> after initialising the default loop, and making sure the
1862 default loop never gets destroyed. You are encouraged, however, to use an
1863 event-based approach to child reaping and thus use libev's support for
1864 that, so other libev users can use C<ev_child> watchers freely.
1866 =head3 Stopping the Child Watcher
1868 Currently, the child watcher never gets stopped, even when the
1869 child terminates, so normally one needs to stop the watcher in the
1870 callback. Future versions of libev might stop the watcher automatically
1871 when a child exit is detected.
1873 =head3 Watcher-Specific Functions and Data Members
1875 =over 4
1877 =item ev_child_init (ev_child *, callback, int pid, int trace)
1879 =item ev_child_set (ev_child *, int pid, int trace)
1881 Configures the watcher to wait for status changes of process C<pid> (or
1882 I<any> process if C<pid> is specified as C<0>). The callback can look
1883 at the C<rstatus> member of the C<ev_child> watcher structure to see
1884 the status word (use the macros from C<sys/wait.h> and see your systems
1885 C<waitpid> documentation). The C<rpid> member contains the pid of the
1886 process causing the status change. C<trace> must be either C<0> (only
1887 activate the watcher when the process terminates) or C<1> (additionally
1888 activate the watcher when the process is stopped or continued).
1890 =item int pid [read-only]
1892 The process id this watcher watches out for, or C<0>, meaning any process id.
1894 =item int rpid [read-write]
1896 The process id that detected a status change.
1898 =item int rstatus [read-write]
1900 The process exit/trace status caused by C<rpid> (see your systems
1901 C<waitpid> and C<sys/wait.h> documentation for details).
1903 =back
1905 =head3 Examples
1907 Example: C<fork()> a new process and install a child handler to wait for
1908 its completion.
1910 ev_child cw;
1912 static void
1913 child_cb (EV_P_ ev_child *w, int revents)
1914 {
1915 ev_child_stop (EV_A_ w);
1916 printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
1917 }
1919 pid_t pid = fork ();
1921 if (pid < 0)
1922 // error
1923 else if (pid == 0)
1924 {
1925 // the forked child executes here
1926 exit (1);
1927 }
1928 else
1929 {
1930 ev_child_init (&cw, child_cb, pid, 0);
1931 ev_child_start (EV_DEFAULT_ &cw);
1932 }
1935 =head2 C<ev_stat> - did the file attributes just change?
1937 This watches a file system path for attribute changes. That is, it calls
1938 C<stat> on that path in regular intervals (or when the OS says it changed)
1939 and sees if it changed compared to the last time, invoking the callback if
1940 it did.
1942 The path does not need to exist: changing from "path exists" to "path does
1943 not exist" is a status change like any other. The condition "path does not
1944 exist" (or more correctly "path cannot be stat'ed") is signified by the
1945 C<st_nlink> field being zero (which is otherwise always forced to be at
1946 least one) and all the other fields of the stat buffer having unspecified
1947 contents.
1949 The path I<must not> end in a slash or contain special components such as
1950 C<.> or C<..>. The path I<should> be absolute: If it is relative and
1951 your working directory changes, then the behaviour is undefined.
1953 Since there is no portable change notification interface available, the
1954 portable implementation simply calls C<stat(2)> regularly on the path
1955 to see if it changed somehow. You can specify a recommended polling
1956 interval for this case. If you specify a polling interval of C<0> (highly
1957 recommended!) then a I<suitable, unspecified default> value will be used
1958 (which you can expect to be around five seconds, although this might
1959 change dynamically). Libev will also impose a minimum interval which is
1960 currently around C<0.1>, but that's usually overkill.
1962 This watcher type is not meant for massive numbers of stat watchers,
1963 as even with OS-supported change notifications, this can be
1964 resource-intensive.
1966 At the time of this writing, the only OS-specific interface implemented
1967 is the Linux inotify interface (implementing kqueue support is left as an
1968 exercise for the reader. Note, however, that the author sees no way of
1969 implementing C<ev_stat> semantics with kqueue, except as a hint).
1971 =head3 ABI Issues (Largefile Support)
1973 Libev by default (unless the user overrides this) uses the default
1974 compilation environment, which means that on systems with large file
1975 support disabled by default, you get the 32 bit version of the stat
1976 structure. When using the library from programs that change the ABI to
1977 use 64 bit file offsets the programs will fail. In that case you have to
1978 compile libev with the same flags to get binary compatibility. This is
1979 obviously the case with any flags that change the ABI, but the problem is
1980 most noticeably displayed with ev_stat and large file support.
1982 The solution for this is to lobby your distribution maker to make large
1983 file interfaces available by default (as e.g. FreeBSD does) and not
1984 optional. Libev cannot simply switch on large file support because it has
1985 to exchange stat structures with application programs compiled using the
1986 default compilation environment.
1988 =head3 Inotify and Kqueue
1990 When C<inotify (7)> support has been compiled into libev and present at
1991 runtime, it will be used to speed up change detection where possible. The
1992 inotify descriptor will be created lazily when the first C<ev_stat>
1993 watcher is being started.
1995 Inotify presence does not change the semantics of C<ev_stat> watchers
1996 except that changes might be detected earlier, and in some cases, to avoid
1997 making regular C<stat> calls. Even in the presence of inotify support
1998 there are many cases where libev has to resort to regular C<stat> polling,
1999 but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
2000 many bugs), the path exists (i.e. stat succeeds), and the path resides on
2001 a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
2002 xfs are fully working) libev usually gets away without polling.
2004 There is no support for kqueue, as apparently it cannot be used to
2005 implement this functionality, due to the requirement of having a file
2006 descriptor open on the object at all times, and detecting renames, unlinks
2007 etc. is difficult.
2009 =head3 C<stat ()> is a synchronous operation
2011 Libev doesn't normally do any kind of I/O itself, and so is not blocking
2012 the process. The exception are C<ev_stat> watchers - those call C<stat
2013 ()>, which is a synchronous operation.
2015 For local paths, this usually doesn't matter: unless the system is very
2016 busy or the intervals between stat's are large, a stat call will be fast,
2017 as the path data is suually in memory already (except when starting the
2018 watcher).
2020 For networked file systems, calling C<stat ()> can block an indefinite
2021 time due to network issues, and even under good conditions, a stat call
2022 often takes multiple milliseconds.
2024 Therefore, it is best to avoid using C<ev_stat> watchers on networked
2025 paths, although this is fully supported by libev.
2027 =head3 The special problem of stat time resolution
2029 The C<stat ()> system call only supports full-second resolution portably,
2030 and even on systems where the resolution is higher, most file systems
2031 still only support whole seconds.
2033 That means that, if the time is the only thing that changes, you can
2034 easily miss updates: on the first update, C<ev_stat> detects a change and
2035 calls your callback, which does something. When there is another update
2036 within the same second, C<ev_stat> will be unable to detect unless the
2037 stat data does change in other ways (e.g. file size).
2039 The solution to this is to delay acting on a change for slightly more
2040 than a second (or till slightly after the next full second boundary), using
2041 a roughly one-second-delay C<ev_timer> (e.g. C<ev_timer_set (w, 0., 1.02);
2042 ev_timer_again (loop, w)>).
2044 The C<.02> offset is added to work around small timing inconsistencies
2045 of some operating systems (where the second counter of the current time
2046 might be be delayed. One such system is the Linux kernel, where a call to
2047 C<gettimeofday> might return a timestamp with a full second later than
2048 a subsequent C<time> call - if the equivalent of C<time ()> is used to
2049 update file times then there will be a small window where the kernel uses
2050 the previous second to update file times but libev might already execute
2051 the timer callback).
2053 =head3 Watcher-Specific Functions and Data Members
2055 =over 4
2057 =item ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)
2059 =item ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)
2061 Configures the watcher to wait for status changes of the given
2062 C<path>. The C<interval> is a hint on how quickly a change is expected to
2063 be detected and should normally be specified as C<0> to let libev choose
2064 a suitable value. The memory pointed to by C<path> must point to the same
2065 path for as long as the watcher is active.
2067 The callback will receive an C<EV_STAT> event when a change was detected,
2068 relative to the attributes at the time the watcher was started (or the
2069 last change was detected).
2071 =item ev_stat_stat (loop, ev_stat *)
2073 Updates the stat buffer immediately with new values. If you change the
2074 watched path in your callback, you could call this function to avoid
2075 detecting this change (while introducing a race condition if you are not
2076 the only one changing the path). Can also be useful simply to find out the
2077 new values.
2079 =item ev_statdata attr [read-only]
2081 The most-recently detected attributes of the file. Although the type is
2082 C<ev_statdata>, this is usually the (or one of the) C<struct stat> types
2083 suitable for your system, but you can only rely on the POSIX-standardised
2084 members to be present. If the C<st_nlink> member is C<0>, then there was
2085 some error while C<stat>ing the file.
2087 =item ev_statdata prev [read-only]
2089 The previous attributes of the file. The callback gets invoked whenever
2090 C<prev> != C<attr>, or, more precisely, one or more of these members
2091 differ: C<st_dev>, C<st_ino>, C<st_mode>, C<st_nlink>, C<st_uid>,
2092 C<st_gid>, C<st_rdev>, C<st_size>, C<st_atime>, C<st_mtime>, C<st_ctime>.
2094 =item ev_tstamp interval [read-only]
2096 The specified interval.
2098 =item const char *path [read-only]
2100 The file system path that is being watched.
2102 =back
2104 =head3 Examples
2106 Example: Watch C</etc/passwd> for attribute changes.
2108 static void
2109 passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
2110 {
2111 /* /etc/passwd changed in some way */
2112 if (w->attr.st_nlink)
2113 {
2114 printf ("passwd current size %ld\n", (long)w->attr.st_size);
2115 printf ("passwd current atime %ld\n", (long)w->attr.st_mtime);
2116 printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime);
2117 }
2118 else
2119 /* you shalt not abuse printf for puts */
2120 puts ("wow, /etc/passwd is not there, expect problems. "
2121 "if this is windows, they already arrived\n");
2122 }
2124 ...
2125 ev_stat passwd;
2127 ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
2128 ev_stat_start (loop, &passwd);
2130 Example: Like above, but additionally use a one-second delay so we do not
2131 miss updates (however, frequent updates will delay processing, too, so
2132 one might do the work both on C<ev_stat> callback invocation I<and> on
2133 C<ev_timer> callback invocation).
2135 static ev_stat passwd;
2136 static ev_timer timer;
2138 static void
2139 timer_cb (EV_P_ ev_timer *w, int revents)
2140 {
2141 ev_timer_stop (EV_A_ w);
2143 /* now it's one second after the most recent passwd change */
2144 }
2146 static void
2147 stat_cb (EV_P_ ev_stat *w, int revents)
2148 {
2149 /* reset the one-second timer */
2150 ev_timer_again (EV_A_ &timer);
2151 }
2153 ...
2154 ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
2155 ev_stat_start (loop, &passwd);
2156 ev_timer_init (&timer, timer_cb, 0., 1.02);
2159 =head2 C<ev_idle> - when you've got nothing better to do...
2161 Idle watchers trigger events when no other events of the same or higher
2162 priority are pending (prepare, check and other idle watchers do not count
2163 as receiving "events").
2165 That is, as long as your process is busy handling sockets or timeouts
2166 (or even signals, imagine) of the same or higher priority it will not be
2167 triggered. But when your process is idle (or only lower-priority watchers
2168 are pending), the idle watchers are being called once per event loop
2169 iteration - until stopped, that is, or your process receives more events
2170 and becomes busy again with higher priority stuff.
2172 The most noteworthy effect is that as long as any idle watchers are
2173 active, the process will not block when waiting for new events.
2175 Apart from keeping your process non-blocking (which is a useful
2176 effect on its own sometimes), idle watchers are a good place to do
2177 "pseudo-background processing", or delay processing stuff to after the
2178 event loop has handled all outstanding events.
2180 =head3 Watcher-Specific Functions and Data Members
2182 =over 4
2184 =item ev_idle_init (ev_signal *, callback)
2186 Initialises and configures the idle watcher - it has no parameters of any
2187 kind. There is a C<ev_idle_set> macro, but using it is utterly pointless,
2188 believe me.
2190 =back
2192 =head3 Examples
2194 Example: Dynamically allocate an C<ev_idle> watcher, start it, and in the
2195 callback, free it. Also, use no error checking, as usual.
2197 static void
2198 idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
2199 {
2200 free (w);
2201 // now do something you wanted to do when the program has
2202 // no longer anything immediate to do.
2203 }
2205 ev_idle *idle_watcher = malloc (sizeof (ev_idle));
2206 ev_idle_init (idle_watcher, idle_cb);
2207 ev_idle_start (loop, idle_cb);
2210 =head2 C<ev_prepare> and C<ev_check> - customise your event loop!
2212 Prepare and check watchers are usually (but not always) used in pairs:
2213 prepare watchers get invoked before the process blocks and check watchers
2214 afterwards.
2216 You I<must not> call C<ev_loop> or similar functions that enter
2217 the current event loop from either C<ev_prepare> or C<ev_check>
2218 watchers. Other loops than the current one are fine, however. The
2219 rationale behind this is that you do not need to check for recursion in
2220 those watchers, i.e. the sequence will always be C<ev_prepare>, blocking,
2221 C<ev_check> so if you have one watcher of each kind they will always be
2222 called in pairs bracketing the blocking call.
2224 Their main purpose is to integrate other event mechanisms into libev and
2225 their use is somewhat advanced. They could be used, for example, to track
2226 variable changes, implement your own watchers, integrate net-snmp or a
2227 coroutine library and lots more. They are also occasionally useful if
2228 you cache some data and want to flush it before blocking (for example,
2229 in X programs you might want to do an C<XFlush ()> in an C<ev_prepare>
2230 watcher).
2232 This is done by examining in each prepare call which file descriptors
2233 need to be watched by the other library, registering C<ev_io> watchers
2234 for them and starting an C<ev_timer> watcher for any timeouts (many
2235 libraries provide exactly this functionality). Then, in the check watcher,
2236 you check for any events that occurred (by checking the pending status
2237 of all watchers and stopping them) and call back into the library. The
2238 I/O and timer callbacks will never actually be called (but must be valid
2239 nevertheless, because you never know, you know?).
2241 As another example, the Perl Coro module uses these hooks to integrate
2242 coroutines into libev programs, by yielding to other active coroutines
2243 during each prepare and only letting the process block if no coroutines
2244 are ready to run (it's actually more complicated: it only runs coroutines
2245 with priority higher than or equal to the event loop and one coroutine
2246 of lower priority, but only once, using idle watchers to keep the event
2247 loop from blocking if lower-priority coroutines are active, thus mapping
2248 low-priority coroutines to idle/background tasks).
2250 It is recommended to give C<ev_check> watchers highest (C<EV_MAXPRI>)
2251 priority, to ensure that they are being run before any other watchers
2252 after the poll (this doesn't matter for C<ev_prepare> watchers).
2254 Also, C<ev_check> watchers (and C<ev_prepare> watchers, too) should not
2255 activate ("feed") events into libev. While libev fully supports this, they
2256 might get executed before other C<ev_check> watchers did their job. As
2257 C<ev_check> watchers are often used to embed other (non-libev) event
2258 loops those other event loops might be in an unusable state until their
2259 C<ev_check> watcher ran (always remind yourself to coexist peacefully with
2260 others).
2262 =head3 Watcher-Specific Functions and Data Members
2264 =over 4
2266 =item ev_prepare_init (ev_prepare *, callback)
2268 =item ev_check_init (ev_check *, callback)
2270 Initialises and configures the prepare or check watcher - they have no
2271 parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set>
2272 macros, but using them is utterly, utterly, utterly and completely
2273 pointless.
2275 =back
2277 =head3 Examples
2279 There are a number of principal ways to embed other event loops or modules
2280 into libev. Here are some ideas on how to include libadns into libev
2281 (there is a Perl module named C<EV::ADNS> that does this, which you could
2282 use as a working example. Another Perl module named C<EV::Glib> embeds a
2283 Glib main context into libev, and finally, C<Glib::EV> embeds EV into the
2284 Glib event loop).
2286 Method 1: Add IO watchers and a timeout watcher in a prepare handler,
2287 and in a check watcher, destroy them and call into libadns. What follows
2288 is pseudo-code only of course. This requires you to either use a low
2289 priority for the check watcher or use C<ev_clear_pending> explicitly, as
2290 the callbacks for the IO/timeout watchers might not have been called yet.
2292 static ev_io iow [nfd];
2293 static ev_timer tw;
2295 static void
2296 io_cb (struct ev_loop *loop, ev_io *w, int revents)
2297 {
2298 }
2300 // create io watchers for each fd and a timer before blocking
2301 static void
2302 adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
2303 {
2304 int timeout = 3600000;
2305 struct pollfd fds [nfd];
2306 // actual code will need to loop here and realloc etc.
2307 adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
2309 /* the callback is illegal, but won't be called as we stop during check */
2310 ev_timer_init (&tw, 0, timeout * 1e-3);
2311 ev_timer_start (loop, &tw);
2313 // create one ev_io per pollfd
2314 for (int i = 0; i < nfd; ++i)
2315 {
2316 ev_io_init (iow + i, io_cb, fds [i].fd,
2317 ((fds [i].events & POLLIN ? EV_READ : 0)
2318 | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
2320 fds [i].revents = 0;
2321 ev_io_start (loop, iow + i);
2322 }
2323 }
2325 // stop all watchers after blocking
2326 static void
2327 adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
2328 {
2329 ev_timer_stop (loop, &tw);
2331 for (int i = 0; i < nfd; ++i)
2332 {
2333 // set the relevant poll flags
2334 // could also call adns_processreadable etc. here
2335 struct pollfd *fd = fds + i;
2336 int revents = ev_clear_pending (iow + i);
2337 if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
2338 if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
2340 // now stop the watcher
2341 ev_io_stop (loop, iow + i);
2342 }
2344 adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
2345 }
2347 Method 2: This would be just like method 1, but you run C<adns_afterpoll>
2348 in the prepare watcher and would dispose of the check watcher.
2350 Method 3: If the module to be embedded supports explicit event
2351 notification (libadns does), you can also make use of the actual watcher
2352 callbacks, and only destroy/create the watchers in the prepare watcher.
2354 static void
2355 timer_cb (EV_P_ ev_timer *w, int revents)
2356 {
2357 adns_state ads = (adns_state)w->data;
2358 update_now (EV_A);
2360 adns_processtimeouts (ads, &tv_now);
2361 }
2363 static void
2364 io_cb (EV_P_ ev_io *w, int revents)
2365 {
2366 adns_state ads = (adns_state)w->data;
2367 update_now (EV_A);
2369 if (revents & EV_READ ) adns_processreadable (ads, w->fd, &tv_now);
2370 if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
2371 }
2373 // do not ever call adns_afterpoll
2375 Method 4: Do not use a prepare or check watcher because the module you
2376 want to embed is not flexible enough to support it. Instead, you can
2377 override their poll function. The drawback with this solution is that the
2378 main loop is now no longer controllable by EV. The C<Glib::EV> module uses
2379 this approach, effectively embedding EV as a client into the horrible
2380 libglib event loop.
2382 static gint
2383 event_poll_func (GPollFD *fds, guint nfds, gint timeout)
2384 {
2385 int got_events = 0;
2387 for (n = 0; n < nfds; ++n)
2388 // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
2390 if (timeout >= 0)
2391 // create/start timer
2393 // poll
2394 ev_loop (EV_A_ 0);
2396 // stop timer again
2397 if (timeout >= 0)
2398 ev_timer_stop (EV_A_ &to);
2400 // stop io watchers again - their callbacks should have set
2401 for (n = 0; n < nfds; ++n)
2402 ev_io_stop (EV_A_ iow [n]);
2404 return got_events;
2405 }
2408 =head2 C<ev_embed> - when one backend isn't enough...
2410 This is a rather advanced watcher type that lets you embed one event loop
2411 into another (currently only C<ev_io> events are supported in the embedded
2412 loop, other types of watchers might be handled in a delayed or incorrect
2413 fashion and must not be used).
2415 There are primarily two reasons you would want that: work around bugs and
2416 prioritise I/O.
2418 As an example for a bug workaround, the kqueue backend might only support
2419 sockets on some platform, so it is unusable as generic backend, but you
2420 still want to make use of it because you have many sockets and it scales
2421 so nicely. In this case, you would create a kqueue-based loop and embed
2422 it into your default loop (which might use e.g. poll). Overall operation
2423 will be a bit slower because first libev has to call C<poll> and then
2424 C<kevent>, but at least you can use both mechanisms for what they are
2425 best: C<kqueue> for scalable sockets and C<poll> if you want it to work :)
2427 As for prioritising I/O: under rare circumstances you have the case where
2428 some fds have to be watched and handled very quickly (with low latency),
2429 and even priorities and idle watchers might have too much overhead. In
2430 this case you would put all the high priority stuff in one loop and all
2431 the rest in a second one, and embed the second one in the first.
2433 As long as the watcher is active, the callback will be invoked every time
2434 there might be events pending in the embedded loop. The callback must then
2435 call C<ev_embed_sweep (mainloop, watcher)> to make a single sweep and invoke
2436 their callbacks (you could also start an idle watcher to give the embedded
2437 loop strictly lower priority for example). You can also set the callback
2438 to C<0>, in which case the embed watcher will automatically execute the
2439 embedded loop sweep.
2441 As long as the watcher is started it will automatically handle events. The
2442 callback will be invoked whenever some events have been handled. You can
2443 set the callback to C<0> to avoid having to specify one if you are not
2444 interested in that.
2446 Also, there have not currently been made special provisions for forking:
2447 when you fork, you not only have to call C<ev_loop_fork> on both loops,
2448 but you will also have to stop and restart any C<ev_embed> watchers
2449 yourself - but you can use a fork watcher to handle this automatically,
2450 and future versions of libev might do just that.
2452 Unfortunately, not all backends are embeddable: only the ones returned by
2453 C<ev_embeddable_backends> are, which, unfortunately, does not include any
2454 portable one.
2456 So when you want to use this feature you will always have to be prepared
2457 that you cannot get an embeddable loop. The recommended way to get around
2458 this is to have a separate variables for your embeddable loop, try to
2459 create it, and if that fails, use the normal loop for everything.
2461 =head3 C<ev_embed> and fork
2463 While the C<ev_embed> watcher is running, forks in the embedding loop will
2464 automatically be applied to the embedded loop as well, so no special
2465 fork handling is required in that case. When the watcher is not running,
2466 however, it is still the task of the libev user to call C<ev_loop_fork ()>
2467 as applicable.
2469 =head3 Watcher-Specific Functions and Data Members
2471 =over 4
2473 =item ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
2475 =item ev_embed_set (ev_embed *, callback, struct ev_loop *embedded_loop)
2477 Configures the watcher to embed the given loop, which must be
2478 embeddable. If the callback is C<0>, then C<ev_embed_sweep> will be
2479 invoked automatically, otherwise it is the responsibility of the callback
2480 to invoke it (it will continue to be called until the sweep has been done,
2481 if you do not want that, you need to temporarily stop the embed watcher).
2483 =item ev_embed_sweep (loop, ev_embed *)
2485 Make a single, non-blocking sweep over the embedded loop. This works
2486 similarly to C<ev_loop (embedded_loop, EVLOOP_NONBLOCK)>, but in the most
2487 appropriate way for embedded loops.
2489 =item struct ev_loop *other [read-only]
2491 The embedded event loop.
2493 =back
2495 =head3 Examples
2497 Example: Try to get an embeddable event loop and embed it into the default
2498 event loop. If that is not possible, use the default loop. The default
2499 loop is stored in C<loop_hi>, while the embeddable loop is stored in
2500 C<loop_lo> (which is C<loop_hi> in the case no embeddable loop can be
2501 used).
2503 struct ev_loop *loop_hi = ev_default_init (0);
2504 struct ev_loop *loop_lo = 0;
2505 ev_embed embed;
2507 // see if there is a chance of getting one that works
2508 // (remember that a flags value of 0 means autodetection)
2509 loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
2510 ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
2511 : 0;
2513 // if we got one, then embed it, otherwise default to loop_hi
2514 if (loop_lo)
2515 {
2516 ev_embed_init (&embed, 0, loop_lo);
2517 ev_embed_start (loop_hi, &embed);
2518 }
2519 else
2520 loop_lo = loop_hi;
2522 Example: Check if kqueue is available but not recommended and create
2523 a kqueue backend for use with sockets (which usually work with any
2524 kqueue implementation). Store the kqueue/socket-only event loop in
2525 C<loop_socket>. (One might optionally use C<EVFLAG_NOENV>, too).
2527 struct ev_loop *loop = ev_default_init (0);
2528 struct ev_loop *loop_socket = 0;
2529 ev_embed embed;
2531 if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
2532 if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
2533 {
2534 ev_embed_init (&embed, 0, loop_socket);
2535 ev_embed_start (loop, &embed);
2536 }
2538 if (!loop_socket)
2539 loop_socket = loop;
2541 // now use loop_socket for all sockets, and loop for everything else
2544 =head2 C<ev_fork> - the audacity to resume the event loop after a fork
2546 Fork watchers are called when a C<fork ()> was detected (usually because
2547 whoever is a good citizen cared to tell libev about it by calling
2548 C<ev_default_fork> or C<ev_loop_fork>). The invocation is done before the
2549 event loop blocks next and before C<ev_check> watchers are being called,
2550 and only in the child after the fork. If whoever good citizen calling
2551 C<ev_default_fork> cheats and calls it in the wrong process, the fork
2552 handlers will be invoked, too, of course.
2554 =head3 Watcher-Specific Functions and Data Members
2556 =over 4
2558 =item ev_fork_init (ev_signal *, callback)
2560 Initialises and configures the fork watcher - it has no parameters of any
2561 kind. There is a C<ev_fork_set> macro, but using it is utterly pointless,
2562 believe me.
2564 =back
2567 =head2 C<ev_async> - how to wake up another event loop
2569 In general, you cannot use an C<ev_loop> from multiple threads or other
2570 asynchronous sources such as signal handlers (as opposed to multiple event
2571 loops - those are of course safe to use in different threads).
2573 Sometimes, however, you need to wake up another event loop you do not
2574 control, for example because it belongs to another thread. This is what
2575 C<ev_async> watchers do: as long as the C<ev_async> watcher is active, you
2576 can signal it by calling C<ev_async_send>, which is thread- and signal
2577 safe.
2579 This functionality is very similar to C<ev_signal> watchers, as signals,
2580 too, are asynchronous in nature, and signals, too, will be compressed
2581 (i.e. the number of callback invocations may be less than the number of
2582 C<ev_async_sent> calls).
2584 Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not
2585 just the default loop.
2587 =head3 Queueing
2589 C<ev_async> does not support queueing of data in any way. The reason
2590 is that the author does not know of a simple (or any) algorithm for a
2591 multiple-writer-single-reader queue that works in all cases and doesn't
2592 need elaborate support such as pthreads.
2594 That means that if you want to queue data, you have to provide your own
2595 queue. But at least I can tell you how to implement locking around your
2596 queue:
2598 =over 4
2600 =item queueing from a signal handler context
2602 To implement race-free queueing, you simply add to the queue in the signal
2603 handler but you block the signal handler in the watcher callback. Here is
2604 an example that does that for some fictitious SIGUSR1 handler:
2606 static ev_async mysig;
2608 static void
2609 sigusr1_handler (void)
2610 {
2611 sometype data;
2613 // no locking etc.
2614 queue_put (data);
2615 ev_async_send (EV_DEFAULT_ &mysig);
2616 }
2618 static void
2619 mysig_cb (EV_P_ ev_async *w, int revents)
2620 {
2621 sometype data;
2622 sigset_t block, prev;
2624 sigemptyset (&block);
2625 sigaddset (&block, SIGUSR1);
2626 sigprocmask (SIG_BLOCK, &block, &prev);
2628 while (queue_get (&data))
2629 process (data);
2631 if (sigismember (&prev, SIGUSR1)
2632 sigprocmask (SIG_UNBLOCK, &block, 0);
2633 }
2635 (Note: pthreads in theory requires you to use C<pthread_setmask>
2636 instead of C<sigprocmask> when you use threads, but libev doesn't do it
2637 either...).
2639 =item queueing from a thread context
2641 The strategy for threads is different, as you cannot (easily) block
2642 threads but you can easily preempt them, so to queue safely you need to
2643 employ a traditional mutex lock, such as in this pthread example:
2645 static ev_async mysig;
2646 static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
2648 static void
2649 otherthread (void)
2650 {
2651 // only need to lock the actual queueing operation
2652 pthread_mutex_lock (&mymutex);
2653 queue_put (data);
2654 pthread_mutex_unlock (&mymutex);
2656 ev_async_send (EV_DEFAULT_ &mysig);
2657 }
2659 static void
2660 mysig_cb (EV_P_ ev_async *w, int revents)
2661 {
2662 pthread_mutex_lock (&mymutex);
2664 while (queue_get (&data))
2665 process (data);
2667 pthread_mutex_unlock (&mymutex);
2668 }
2670 =back
2673 =head3 Watcher-Specific Functions and Data Members
2675 =over 4
2677 =item ev_async_init (ev_async *, callback)
2679 Initialises and configures the async watcher - it has no parameters of any
2680 kind. There is a C<ev_async_set> macro, but using it is utterly pointless,
2681 trust me.
2683 =item ev_async_send (loop, ev_async *)
2685 Sends/signals/activates the given C<ev_async> watcher, that is, feeds
2686 an C<EV_ASYNC> event on the watcher into the event loop. Unlike
2687 C<ev_feed_event>, this call is safe to do from other threads, signal or
2688 similar contexts (see the discussion of C<EV_ATOMIC_T> in the embedding
2689 section below on what exactly this means).
2691 This call incurs the overhead of a system call only once per loop iteration,
2692 so while the overhead might be noticeable, it doesn't apply to repeated
2693 calls to C<ev_async_send>.
2695 =item bool = ev_async_pending (ev_async *)
2697 Returns a non-zero value when C<ev_async_send> has been called on the
2698 watcher but the event has not yet been processed (or even noted) by the
2699 event loop.
2701 C<ev_async_send> sets a flag in the watcher and wakes up the loop. When
2702 the loop iterates next and checks for the watcher to have become active,
2703 it will reset the flag again. C<ev_async_pending> can be used to very
2704 quickly check whether invoking the loop might be a good idea.
2706 Not that this does I<not> check whether the watcher itself is pending, only
2707 whether it has been requested to make this watcher pending.
2709 =back
2714 There are some other functions of possible interest. Described. Here. Now.
2716 =over 4
2718 =item ev_once (loop, int fd, int events, ev_tstamp timeout, callback)
2720 This function combines a simple timer and an I/O watcher, calls your
2721 callback on whichever event happens first and automatically stops both
2722 watchers. This is useful if you want to wait for a single event on an fd
2723 or timeout without having to allocate/configure/start/stop/free one or
2724 more watchers yourself.
2726 If C<fd> is less than 0, then no I/O watcher will be started and the
2727 C<events> argument is being ignored. Otherwise, an C<ev_io> watcher for
2728 the given C<fd> and C<events> set will be created and started.
2730 If C<timeout> is less than 0, then no timeout watcher will be
2731 started. Otherwise an C<ev_timer> watcher with after = C<timeout> (and
2732 repeat = 0) will be started. C<0> is a valid timeout.
2734 The callback has the type C<void (*cb)(int revents, void *arg)> and gets
2735 passed an C<revents> set like normal event callbacks (a combination of
2736 C<EV_ERROR>, C<EV_READ>, C<EV_WRITE> or C<EV_TIMEOUT>) and the C<arg>
2737 value passed to C<ev_once>. Note that it is possible to receive I<both>
2738 a timeout and an io event at the same time - you probably should give io
2739 events precedence.
2741 Example: wait up to ten seconds for data to appear on STDIN_FILENO.
2743 static void stdin_ready (int revents, void *arg)
2744 {
2745 if (revents & EV_READ)
2746 /* stdin might have data for us, joy! */;
2747 else if (revents & EV_TIMEOUT)
2748 /* doh, nothing entered */;
2749 }
2751 ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
2753 =item ev_feed_event (struct ev_loop *, watcher *, int revents)
2755 Feeds the given event set into the event loop, as if the specified event
2756 had happened for the specified watcher (which must be a pointer to an
2757 initialised but not necessarily started event watcher).
2759 =item ev_feed_fd_event (struct ev_loop *, int fd, int revents)
2761 Feed an event on the given fd, as if a file descriptor backend detected
2762 the given events it.
2764 =item ev_feed_signal_event (struct ev_loop *loop, int signum)
2766 Feed an event as if the given signal occurred (C<loop> must be the default
2767 loop!).
2769 =back
2774 Libev offers a compatibility emulation layer for libevent. It cannot
2775 emulate the internals of libevent, so here are some usage hints:
2777 =over 4
2779 =item * Use it by including <event.h>, as usual.
2781 =item * The following members are fully supported: ev_base, ev_callback,
2782 ev_arg, ev_fd, ev_res, ev_events.
2784 =item * Avoid using ev_flags and the EVLIST_*-macros, while it is
2785 maintained by libev, it does not work exactly the same way as in libevent (consider
2786 it a private API).
2788 =item * Priorities are not currently supported. Initialising priorities
2789 will fail and all watchers will have the same priority, even though there
2790 is an ev_pri field.
2792 =item * In libevent, the last base created gets the signals, in libev, the
2793 first base created (== the default loop) gets the signals.
2795 =item * Other members are not supported.
2797 =item * The libev emulation is I<not> ABI compatible to libevent, you need
2798 to use the libev header file and library.
2800 =back
2802 =head1 C++ SUPPORT
2804 Libev comes with some simplistic wrapper classes for C++ that mainly allow
2805 you to use some convenience methods to start/stop watchers and also change
2806 the callback model to a model using method callbacks on objects.
2808 To use it,
2810 #include <ev++.h>
2812 This automatically includes F<ev.h> and puts all of its definitions (many
2813 of them macros) into the global namespace. All C++ specific things are
2814 put into the C<ev> namespace. It should support all the same embedding
2815 options as F<ev.h>, most notably C<EV_MULTIPLICITY>.
2817 Care has been taken to keep the overhead low. The only data member the C++
2818 classes add (compared to plain C-style watchers) is the event loop pointer
2819 that the watcher is associated with (or no additional members at all if
2820 you disable C<EV_MULTIPLICITY> when embedding libev).
2822 Currently, functions, and static and non-static member functions can be
2823 used as callbacks. Other types should be easy to add as long as they only
2824 need one additional pointer for context. If you need support for other
2825 types of functors please contact the author (preferably after implementing
2826 it).
2828 Here is a list of things available in the C<ev> namespace:
2830 =over 4
2832 =item C<ev::READ>, C<ev::WRITE> etc.
2834 These are just enum values with the same values as the C<EV_READ> etc.
2835 macros from F<ev.h>.
2837 =item C<ev::tstamp>, C<ev::now>
2839 Aliases to the same types/functions as with the C<ev_> prefix.
2841 =item C<ev::io>, C<ev::timer>, C<ev::periodic>, C<ev::idle>, C<ev::sig> etc.
2843 For each C<ev_TYPE> watcher in F<ev.h> there is a corresponding class of
2844 the same name in the C<ev> namespace, with the exception of C<ev_signal>
2845 which is called C<ev::sig> to avoid clashes with the C<signal> macro
2846 defines by many implementations.
2848 All of those classes have these methods:
2850 =over 4
2852 =item ev::TYPE::TYPE ()
2854 =item ev::TYPE::TYPE (struct ev_loop *)
2856 =item ev::TYPE::~TYPE
2858 The constructor (optionally) takes an event loop to associate the watcher
2859 with. If it is omitted, it will use C<EV_DEFAULT>.
2861 The constructor calls C<ev_init> for you, which means you have to call the
2862 C<set> method before starting it.
2864 It will not set a callback, however: You have to call the templated C<set>
2865 method to set a callback before you can start the watcher.
2867 (The reason why you have to use a method is a limitation in C++ which does
2868 not allow explicit template arguments for constructors).
2870 The destructor automatically stops the watcher if it is active.
2872 =item w->set<class, &class::method> (object *)
2874 This method sets the callback method to call. The method has to have a
2875 signature of C<void (*)(ev_TYPE &, int)>, it receives the watcher as
2876 first argument and the C<revents> as second. The object must be given as
2877 parameter and is stored in the C<data> member of the watcher.
2879 This method synthesizes efficient thunking code to call your method from
2880 the C callback that libev requires. If your compiler can inline your
2881 callback (i.e. it is visible to it at the place of the C<set> call and
2882 your compiler is good :), then the method will be fully inlined into the
2883 thunking function, making it as fast as a direct C callback.
2885 Example: simple class declaration and watcher initialisation
2887 struct myclass
2888 {
2889 void io_cb (ev::io &w, int revents) { }
2890 }
2892 myclass obj;
2893 ev::io iow;
2894 iow.set <myclass, &myclass::io_cb> (&obj);
2896 =item w->set<function> (void *data = 0)
2898 Also sets a callback, but uses a static method or plain function as
2899 callback. The optional C<data> argument will be stored in the watcher's
2900 C<data> member and is free for you to use.
2902 The prototype of the C<function> must be C<void (*)(ev::TYPE &w, int)>.
2904 See the method-C<set> above for more details.
2906 Example: Use a plain function as callback.
2908 static void io_cb (ev::io &w, int revents) { }
2909 iow.set <io_cb> ();
2911 =item w->set (struct ev_loop *)
2913 Associates a different C<struct ev_loop> with this watcher. You can only
2914 do this when the watcher is inactive (and not pending either).
2916 =item w->set ([arguments])
2918 Basically the same as C<ev_TYPE_set>, with the same arguments. Must be
2919 called at least once. Unlike the C counterpart, an active watcher gets
2920 automatically stopped and restarted when reconfiguring it with this
2921 method.
2923 =item w->start ()
2925 Starts the watcher. Note that there is no C<loop> argument, as the
2926 constructor already stores the event loop.
2928 =item w->stop ()
2930 Stops the watcher if it is active. Again, no C<loop> argument.
2932 =item w->again () (C<ev::timer>, C<ev::periodic> only)
2934 For C<ev::timer> and C<ev::periodic>, this invokes the corresponding
2935 C<ev_TYPE_again> function.
2937 =item w->sweep () (C<ev::embed> only)
2939 Invokes C<ev_embed_sweep>.
2941 =item w->update () (C<ev::stat> only)
2943 Invokes C<ev_stat_stat>.
2945 =back
2947 =back
2949 Example: Define a class with an IO and idle watcher, start one of them in
2950 the constructor.
2952 class myclass
2953 {
2954 ev::io io ; void io_cb (ev::io &w, int revents);
2955 ev::idle idle; void idle_cb (ev::idle &w, int revents);
2957 myclass (int fd)
2958 {
2959 io .set <myclass, &myclass::io_cb > (this);
2960 idle.set <myclass, &myclass::idle_cb> (this);
2962 io.start (fd, ev::READ);
2963 }
2964 };
2969 Libev does not offer other language bindings itself, but bindings for a
2970 number of languages exist in the form of third-party packages. If you know
2971 any interesting language binding in addition to the ones listed here, drop
2972 me a note.
2974 =over 4
2976 =item Perl
2978 The EV module implements the full libev API and is actually used to test
2979 libev. EV is developed together with libev. Apart from the EV core module,
2980 there are additional modules that implement libev-compatible interfaces
2981 to C<libadns> (C<EV::ADNS>, but C<AnyEvent::DNS> is preferred nowadays),
2982 C<Net::SNMP> (C<Net::SNMP::EV>) and the C<libglib> event core (C<Glib::EV>
2983 and C<EV::Glib>).
2985 It can be found and installed via CPAN, its homepage is at
2986 L<>.
2988 =item Python
2990 Python bindings can be found at L<>. It
2991 seems to be quite complete and well-documented. Note, however, that the
2992 patch they require for libev is outright dangerous as it breaks the ABI
2993 for everybody else, and therefore, should never be applied in an installed
2994 libev (if python requires an incompatible ABI then it needs to embed
2995 libev).
2997 =item Ruby
2999 Tony Arcieri has written a ruby extension that offers access to a subset
3000 of the libev API and adds file handle abstractions, asynchronous DNS and
3001 more on top of it. It can be found via gem servers. Its homepage is at
3002 L<>.
3004 =item D
3006 Leandro Lucarella has written a D language binding (F<ev.d>) for libev, to
3007 be found at L<>.
3009 =item Ocaml
3011 Erkki Seppala has written Ocaml bindings for libev, to be found at
3012 L<>.
3014 =back
3017 =head1 MACRO MAGIC
3019 Libev can be compiled with a variety of options, the most fundamental
3020 of which is C<EV_MULTIPLICITY>. This option determines whether (most)
3021 functions and callbacks have an initial C<struct ev_loop *> argument.
3023 To make it easier to write programs that cope with either variant, the
3024 following macros are defined:
3026 =over 4
3028 =item C<EV_A>, C<EV_A_>
3030 This provides the loop I<argument> for functions, if one is required ("ev
3031 loop argument"). The C<EV_A> form is used when this is the sole argument,
3032 C<EV_A_> is used when other arguments are following. Example:
3034 ev_unref (EV_A);
3035 ev_timer_add (EV_A_ watcher);
3036 ev_loop (EV_A_ 0);
3038 It assumes the variable C<loop> of type C<struct ev_loop *> is in scope,
3039 which is often provided by the following macro.
3041 =item C<EV_P>, C<EV_P_>
3043 This provides the loop I<parameter> for functions, if one is required ("ev
3044 loop parameter"). The C<EV_P> form is used when this is the sole parameter,
3045 C<EV_P_> is used when other parameters are following. Example:
3047 // this is how ev_unref is being declared
3048 static void ev_unref (EV_P);
3050 // this is how you can declare your typical callback
3051 static void cb (EV_P_ ev_timer *w, int revents)
3053 It declares a parameter C<loop> of type C<struct ev_loop *>, quite
3054 suitable for use with C<EV_A>.
3056 =item C<EV_DEFAULT>, C<EV_DEFAULT_>
3058 Similar to the other two macros, this gives you the value of the default
3059 loop, if multiple loops are supported ("ev loop default").
3063 Usage identical to C<EV_DEFAULT> and C<EV_DEFAULT_>, but requires that the
3064 default loop has been initialised (C<UC> == unchecked). Their behaviour
3065 is undefined when the default loop has not been initialised by a previous
3066 execution of C<EV_DEFAULT>, C<EV_DEFAULT_> or C<ev_default_init (...)>.
3068 It is often prudent to use C<EV_DEFAULT> when initialising the first
3069 watcher in a function but use C<EV_DEFAULT_UC> afterwards.
3071 =back
3073 Example: Declare and initialise a check watcher, utilising the above
3074 macros so it will work regardless of whether multiple loops are supported
3075 or not.
3077 static void
3078 check_cb (EV_P_ ev_timer *w, int revents)
3079 {
3080 ev_check_stop (EV_A_ w);
3081 }
3083 ev_check check;
3084 ev_check_init (&check, check_cb);
3085 ev_check_start (EV_DEFAULT_ &check);
3086 ev_loop (EV_DEFAULT_ 0);
3088 =head1 EMBEDDING
3090 Libev can (and often is) directly embedded into host
3091 applications. Examples of applications that embed it include the Deliantra
3092 Game Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe)
3093 and rxvt-unicode.
3095 The goal is to enable you to just copy the necessary files into your
3096 source directory without having to change even a single line in them, so
3097 you can easily upgrade by simply copying (or having a checked-out copy of
3098 libev somewhere in your source tree).
3100 =head2 FILESETS
3102 Depending on what features you need you need to include one or more sets of files
3103 in your application.
3105 =head3 CORE EVENT LOOP
3107 To include only the libev core (all the C<ev_*> functions), with manual
3108 configuration (no autoconf):
3110 #define EV_STANDALONE 1
3111 #include "ev.c"
3113 This will automatically include F<ev.h>, too, and should be done in a
3114 single C source file only to provide the function implementations. To use
3115 it, do the same for F<ev.h> in all files wishing to use this API (best
3116 done by writing a wrapper around F<ev.h> that you can include instead and
3117 where you can put other configuration options):
3119 #define EV_STANDALONE 1
3120 #include "ev.h"
3122 Both header files and implementation files can be compiled with a C++
3123 compiler (at least, that's a stated goal, and breakage will be treated
3124 as a bug).
3126 You need the following files in your source tree, or in a directory
3127 in your include path (e.g. in libev/ when using -Ilibev):
3129 ev.h
3130 ev.c
3131 ev_vars.h
3132 ev_wrap.h
3134 ev_win32.c required on win32 platforms only
3136 ev_select.c only when select backend is enabled (which is enabled by default)
3137 ev_poll.c only when poll backend is enabled (disabled by default)
3138 ev_epoll.c only when the epoll backend is enabled (disabled by default)
3139 ev_kqueue.c only when the kqueue backend is enabled (disabled by default)
3140 ev_port.c only when the solaris port backend is enabled (disabled by default)
3142 F<ev.c> includes the backend files directly when enabled, so you only need
3143 to compile this single file.
3147 To include the libevent compatibility API, also include:
3149 #include "event.c"
3151 in the file including F<ev.c>, and:
3153 #include "event.h"
3155 in the files that want to use the libevent API. This also includes F<ev.h>.
3157 You need the following additional files for this:
3159 event.h
3160 event.c
3164 Instead of using C<EV_STANDALONE=1> and providing your configuration in
3165 whatever way you want, you can also C<m4_include([libev.m4])> in your
3166 F<> and leave C<EV_STANDALONE> undefined. F<ev.c> will then
3167 include F<config.h> and configure itself accordingly.
3169 For this of course you need the m4 file:
3171 libev.m4
3175 Libev can be configured via a variety of preprocessor symbols you have to
3176 define before including any of its files. The default in the absence of
3177 autoconf is documented for every option.
3179 =over 4
3181 =item EV_STANDALONE
3183 Must always be C<1> if you do not use autoconf configuration, which
3184 keeps libev from including F<config.h>, and it also defines dummy
3185 implementations for some libevent functions (such as logging, which is not
3186 supported). It will also not define any of the structs usually found in
3187 F<event.h> that are not directly supported by the libev core alone.
3191 If defined to be C<1>, libev will try to detect the availability of the
3192 monotonic clock option at both compile time and runtime. Otherwise no use
3193 of the monotonic clock option will be attempted. If you enable this, you
3194 usually have to link against librt or something similar. Enabling it when
3195 the functionality isn't available is safe, though, although you have
3196 to make sure you link against any libraries where the C<clock_gettime>
3197 function is hiding in (often F<-lrt>).
3199 =item EV_USE_REALTIME
3201 If defined to be C<1>, libev will try to detect the availability of the
3202 real-time clock option at compile time (and assume its availability at
3203 runtime if successful). Otherwise no use of the real-time clock option will
3204 be attempted. This effectively replaces C<gettimeofday> by C<clock_get
3205 (CLOCK_REALTIME, ...)> and will not normally affect correctness. See the
3206 note about libraries in the description of C<EV_USE_MONOTONIC>, though.
3210 If defined to be C<1>, libev will assume that C<nanosleep ()> is available
3211 and will use it for delays. Otherwise it will use C<select ()>.
3213 =item EV_USE_EVENTFD
3215 If defined to be C<1>, then libev will assume that C<eventfd ()> is
3216 available and will probe for kernel support at runtime. This will improve
3217 C<ev_signal> and C<ev_async> performance and reduce resource consumption.
3218 If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
3219 2.7 or newer, otherwise disabled.
3221 =item EV_USE_SELECT
3223 If undefined or defined to be C<1>, libev will compile in support for the
3224 C<select>(2) backend. No attempt at auto-detection will be done: if no
3225 other method takes over, select will be it. Otherwise the select backend
3226 will not be compiled in.
3230 If defined to C<1>, then the select backend will use the system C<fd_set>
3231 structure. This is useful if libev doesn't compile due to a missing
3232 C<NFDBITS> or C<fd_mask> definition or it mis-guesses the bitset layout on
3233 exotic systems. This usually limits the range of file descriptors to some
3234 low limit such as 1024 or might have other limitations (winsocket only
3235 allows 64 sockets). The C<FD_SETSIZE> macro, set before compilation, might
3236 influence the size of the C<fd_set> used.
3240 When defined to C<1>, the select backend will assume that
3241 select/socket/connect etc. don't understand file descriptors but
3242 wants osf handles on win32 (this is the case when the select to
3243 be used is the winsock select). This means that it will call
3244 C<_get_osfhandle> on the fd to convert it to an OS handle. Otherwise,
3245 it is assumed that all these functions actually work on fds, even
3246 on win32. Should not be defined on non-win32 platforms.
3248 =item EV_FD_TO_WIN32_HANDLE
3250 If C<EV_SELECT_IS_WINSOCKET> is enabled, then libev needs a way to map
3251 file descriptors to socket handles. When not defining this symbol (the
3252 default), then libev will call C<_get_osfhandle>, which is usually
3253 correct. In some cases, programs use their own file descriptor management,
3254 in which case they can provide this function to map fds to socket handles.
3256 =item EV_USE_POLL
3258 If defined to be C<1>, libev will compile in support for the C<poll>(2)
3259 backend. Otherwise it will be enabled on non-win32 platforms. It
3260 takes precedence over select.
3262 =item EV_USE_EPOLL
3264 If defined to be C<1>, libev will compile in support for the Linux
3265 C<epoll>(7) backend. Its availability will be detected at runtime,
3266 otherwise another method will be used as fallback. This is the preferred
3267 backend for GNU/Linux systems. If undefined, it will be enabled if the
3268 headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
3270 =item EV_USE_KQUEUE
3272 If defined to be C<1>, libev will compile in support for the BSD style
3273 C<kqueue>(2) backend. Its actual availability will be detected at runtime,
3274 otherwise another method will be used as fallback. This is the preferred
3275 backend for BSD and BSD-like systems, although on most BSDs kqueue only
3276 supports some types of fds correctly (the only platform we found that
3277 supports ptys for example was NetBSD), so kqueue might be compiled in, but
3278 not be used unless explicitly requested. The best way to use it is to find
3279 out whether kqueue supports your type of fd properly and use an embedded
3280 kqueue loop.
3282 =item EV_USE_PORT
3284 If defined to be C<1>, libev will compile in support for the Solaris
3285 10 port style backend. Its availability will be detected at runtime,
3286 otherwise another method will be used as fallback. This is the preferred
3287 backend for Solaris 10 systems.
3289 =item EV_USE_DEVPOLL
3291 Reserved for future expansion, works like the USE symbols above.
3293 =item EV_USE_INOTIFY
3295 If defined to be C<1>, libev will compile in support for the Linux inotify
3296 interface to speed up C<ev_stat> watchers. Its actual availability will
3297 be detected at runtime. If undefined, it will be enabled if the headers
3298 indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
3300 =item EV_ATOMIC_T
3302 Libev requires an integer type (suitable for storing C<0> or C<1>) whose
3303 access is atomic with respect to other threads or signal contexts. No such
3304 type is easily found in the C language, so you can provide your own type
3305 that you know is safe for your purposes. It is used both for signal handler "locking"
3306 as well as for signal and thread safety in C<ev_async> watchers.
3308 In the absence of this define, libev will use C<sig_atomic_t volatile>
3309 (from F<signal.h>), which is usually good enough on most platforms.
3311 =item EV_H
3313 The name of the F<ev.h> header file used to include it. The default if
3314 undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be
3315 used to virtually rename the F<ev.h> header file in case of conflicts.
3317 =item EV_CONFIG_H
3319 If C<EV_STANDALONE> isn't C<1>, this variable can be used to override
3320 F<ev.c>'s idea of where to find the F<config.h> file, similarly to
3321 C<EV_H>, above.
3323 =item EV_EVENT_H
3325 Similarly to C<EV_H>, this macro can be used to override F<event.c>'s idea
3326 of how the F<event.h> header can be found, the default is C<"event.h">.
3328 =item EV_PROTOTYPES
3330 If defined to be C<0>, then F<ev.h> will not define any function
3331 prototypes, but still define all the structs and other symbols. This is
3332 occasionally useful if you want to provide your own wrapper functions
3333 around libev functions.
3337 If undefined or defined to C<1>, then all event-loop-specific functions
3338 will have the C<struct ev_loop *> as first argument, and you can create
3339 additional independent event loops. Otherwise there will be no support
3340 for multiple event loops and there is no first event loop pointer
3341 argument. Instead, all functions act on the single default loop.
3343 =item EV_MINPRI
3345 =item EV_MAXPRI
3347 The range of allowed priorities. C<EV_MINPRI> must be smaller or equal to
3348 C<EV_MAXPRI>, but otherwise there are no non-obvious limitations. You can
3349 provide for more priorities by overriding those symbols (usually defined
3350 to be C<-2> and C<2>, respectively).
3352 When doing priority-based operations, libev usually has to linearly search
3353 all the priorities, so having many of them (hundreds) uses a lot of space
3354 and time, so using the defaults of five priorities (-2 .. +2) is usually
3355 fine.
3357 If your embedding application does not need any priorities, defining these
3358 both to C<0> will save some memory and CPU.
3362 If undefined or defined to be C<1>, then periodic timers are supported. If
3363 defined to be C<0>, then they are not. Disabling them saves a few kB of
3364 code.
3366 =item EV_IDLE_ENABLE
3368 If undefined or defined to be C<1>, then idle watchers are supported. If
3369 defined to be C<0>, then they are not. Disabling them saves a few kB of
3370 code.
3372 =item EV_EMBED_ENABLE
3374 If undefined or defined to be C<1>, then embed watchers are supported. If
3375 defined to be C<0>, then they are not. Embed watchers rely on most other
3376 watcher types, which therefore must not be disabled.
3378 =item EV_STAT_ENABLE
3380 If undefined or defined to be C<1>, then stat watchers are supported. If
3381 defined to be C<0>, then they are not.
3383 =item EV_FORK_ENABLE
3385 If undefined or defined to be C<1>, then fork watchers are supported. If
3386 defined to be C<0>, then they are not.
3388 =item EV_ASYNC_ENABLE
3390 If undefined or defined to be C<1>, then async watchers are supported. If
3391 defined to be C<0>, then they are not.
3393 =item EV_MINIMAL
3395 If you need to shave off some kilobytes of code at the expense of some
3396 speed, define this symbol to C<1>. Currently this is used to override some
3397 inlining decisions, saves roughly 30% code size on amd64. It also selects a
3398 much smaller 2-heap for timer management over the default 4-heap.
3400 =item EV_PID_HASHSIZE
3402 C<ev_child> watchers use a small hash table to distribute workload by
3403 pid. The default size is C<16> (or C<1> with C<EV_MINIMAL>), usually more
3404 than enough. If you need to manage thousands of children you might want to
3405 increase this value (I<must> be a power of two).
3409 C<ev_stat> watchers use a small hash table to distribute workload by
3410 inotify watch id. The default size is C<16> (or C<1> with C<EV_MINIMAL>),
3411 usually more than enough. If you need to manage thousands of C<ev_stat>
3412 watchers you might want to increase this value (I<must> be a power of
3413 two).
3415 =item EV_USE_4HEAP
3417 Heaps are not very cache-efficient. To improve the cache-efficiency of the
3418 timer and periodics heaps, libev uses a 4-heap when this symbol is defined
3419 to C<1>. The 4-heap uses more complicated (longer) code but has noticeably
3420 faster performance with many (thousands) of watchers.
3422 The default is C<1> unless C<EV_MINIMAL> is set in which case it is C<0>
3423 (disabled).
3425 =item EV_HEAP_CACHE_AT
3427 Heaps are not very cache-efficient. To improve the cache-efficiency of the
3428 timer and periodics heaps, libev can cache the timestamp (I<at>) within
3429 the heap structure (selected by defining C<EV_HEAP_CACHE_AT> to C<1>),
3430 which uses 8-12 bytes more per watcher and a few hundred bytes more code,
3431 but avoids random read accesses on heap changes. This improves performance
3432 noticeably with many (hundreds) of watchers.
3434 The default is C<1> unless C<EV_MINIMAL> is set in which case it is C<0>
3435 (disabled).
3437 =item EV_VERIFY
3439 Controls how much internal verification (see C<ev_loop_verify ()>) will
3440 be done: If set to C<0>, no internal verification code will be compiled
3441 in. If set to C<1>, then verification code will be compiled in, but not
3442 called. If set to C<2>, then the internal verification code will be
3443 called once per loop, which can slow down libev. If set to C<3>, then the
3444 verification code will be called very frequently, which will slow down
3445 libev considerably.
3447 The default is C<1>, unless C<EV_MINIMAL> is set, in which case it will be
3448 C<0>.
3450 =item EV_COMMON
3452 By default, all watchers have a C<void *data> member. By redefining
3453 this macro to a something else you can include more and other types of
3454 members. You have to define it each time you include one of the files,
3455 though, and it must be identical each time.
3457 For example, the perl EV module uses something like this:
3459 #define EV_COMMON \
3460 SV *self; /* contains this struct */ \
3461 SV *cb_sv, *fh /* note no trailing ";" */
3463 =item EV_CB_DECLARE (type)
3465 =item EV_CB_INVOKE (watcher, revents)
3467 =item ev_set_cb (ev, cb)
3469 Can be used to change the callback member declaration in each watcher,
3470 and the way callbacks are invoked and set. Must expand to a struct member
3471 definition and a statement, respectively. See the F<ev.h> header file for
3472 their default definitions. One possible use for overriding these is to
3473 avoid the C<struct ev_loop *> as first argument in all cases, or to use
3474 method calls instead of plain function calls in C++.
3476 =back
3480 If you need to re-export the API (e.g. via a DLL) and you need a list of
3481 exported symbols, you can use the provided F<Symbol.*> files which list
3482 all public symbols, one per line:
3484 Symbols.ev for libev proper
3485 Symbols.event for the libevent emulation
3487 This can also be used to rename all public symbols to avoid clashes with
3488 multiple versions of libev linked together (which is obviously bad in
3489 itself, but sometimes it is inconvenient to avoid this).
3491 A sed command like this will create wrapper C<#define>'s that you need to
3492 include before including F<ev.h>:
3494 <Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h
3496 This would create a file F<wrap.h> which essentially looks like this:
3498 #define ev_backend myprefix_ev_backend
3499 #define ev_check_start myprefix_ev_check_start
3500 #define ev_check_stop myprefix_ev_check_stop
3501 ...
3503 =head2 EXAMPLES
3505 For a real-world example of a program the includes libev
3506 verbatim, you can have a look at the EV perl module
3507 (L<>). It has the libev files in
3508 the F<libev/> subdirectory and includes them in the F<EV/EVAPI.h> (public
3509 interface) and F<EV.xs> (implementation) files. Only the F<EV.xs> file
3510 will be compiled. It is pretty complex because it provides its own header
3511 file.
3513 The usage in rxvt-unicode is simpler. It has a F<ev_cpp.h> header file
3514 that everybody includes and which overrides some configure choices:
3516 #define EV_MINIMAL 1
3517 #define EV_USE_POLL 0
3518 #define EV_MULTIPLICITY 0
3519 #define EV_PERIODIC_ENABLE 0
3520 #define EV_STAT_ENABLE 0
3521 #define EV_FORK_ENABLE 0
3522 #define EV_CONFIG_H <config.h>
3523 #define EV_MINPRI 0
3524 #define EV_MAXPRI 0
3526 #include "ev++.h"
3528 And a F<ev_cpp.C> implementation file that contains libev proper and is compiled:
3530 #include "ev_cpp.h"
3531 #include "ev.c"
3537 =head3 THREADS
3539 All libev functions are reentrant and thread-safe unless explicitly
3540 documented otherwise, but libev implements no locking itself. This means
3541 that you can use as many loops as you want in parallel, as long as there
3542 are no concurrent calls into any libev function with the same loop
3543 parameter (C<ev_default_*> calls have an implicit default loop parameter,
3544 of course): libev guarantees that different event loops share no data
3545 structures that need any locking.
3547 Or to put it differently: calls with different loop parameters can be done
3548 concurrently from multiple threads, calls with the same loop parameter
3549 must be done serially (but can be done from different threads, as long as
3550 only one thread ever is inside a call at any point in time, e.g. by using
3551 a mutex per loop).
3553 Specifically to support threads (and signal handlers), libev implements
3554 so-called C<ev_async> watchers, which allow some limited form of
3555 concurrency on the same event loop, namely waking it up "from the
3556 outside".
3558 If you want to know which design (one loop, locking, or multiple loops
3559 without or something else still) is best for your problem, then I cannot
3560 help you, but here is some generic advice:
3562 =over 4
3564 =item * most applications have a main thread: use the default libev loop
3565 in that thread, or create a separate thread running only the default loop.
3567 This helps integrating other libraries or software modules that use libev
3568 themselves and don't care/know about threading.
3570 =item * one loop per thread is usually a good model.
3572 Doing this is almost never wrong, sometimes a better-performance model
3573 exists, but it is always a good start.
3575 =item * other models exist, such as the leader/follower pattern, where one
3576 loop is handed through multiple threads in a kind of round-robin fashion.
3578 Choosing a model is hard - look around, learn, know that usually you can do
3579 better than you currently do :-)
3581 =item * often you need to talk to some other thread which blocks in the
3582 event loop.
3584 C<ev_async> watchers can be used to wake them up from other threads safely
3585 (or from signal contexts...).
3587 An example use would be to communicate signals or other events that only
3588 work in the default loop by registering the signal watcher with the
3589 default loop and triggering an C<ev_async> watcher from the default loop
3590 watcher callback into the event loop interested in the signal.
3592 =back
3594 =head3 COROUTINES
3596 Libev is very accommodating to coroutines ("cooperative threads"):
3597 libev fully supports nesting calls to its functions from different
3598 coroutines (e.g. you can call C<ev_loop> on the same loop from two
3599 different coroutines, and switch freely between both coroutines running the
3600 loop, as long as you don't confuse yourself). The only exception is that
3601 you must not do this from C<ev_periodic> reschedule callbacks.
3603 Care has been taken to ensure that libev does not keep local state inside
3604 C<ev_loop>, and other calls do not usually allow for coroutine switches as
3605 they do not call any callbacks.
3609 Depending on your compiler and compiler settings, you might get no or a
3610 lot of warnings when compiling libev code. Some people are apparently
3611 scared by this.
3613 However, these are unavoidable for many reasons. For one, each compiler
3614 has different warnings, and each user has different tastes regarding
3615 warning options. "Warn-free" code therefore cannot be a goal except when
3616 targeting a specific compiler and compiler-version.
3618 Another reason is that some compiler warnings require elaborate
3619 workarounds, or other changes to the code that make it less clear and less
3620 maintainable.
3622 And of course, some compiler warnings are just plain stupid, or simply
3623 wrong (because they don't actually warn about the condition their message
3624 seems to warn about). For example, certain older gcc versions had some
3625 warnings that resulted an extreme number of false positives. These have
3626 been fixed, but some people still insist on making code warn-free with
3627 such buggy versions.
3629 While libev is written to generate as few warnings as possible,
3630 "warn-free" code is not a goal, and it is recommended not to build libev
3631 with any compiler warnings enabled unless you are prepared to cope with
3632 them (e.g. by ignoring them). Remember that warnings are just that:
3633 warnings, not errors, or proof of bugs.
3636 =head2 VALGRIND
3638 Valgrind has a special section here because it is a popular tool that is
3639 highly useful. Unfortunately, valgrind reports are very hard to interpret.
3641 If you think you found a bug (memory leak, uninitialised data access etc.)
3642 in libev, then check twice: If valgrind reports something like:
3644 ==2274== definitely lost: 0 bytes in 0 blocks.
3645 ==2274== possibly lost: 0 bytes in 0 blocks.
3646 ==2274== still reachable: 256 bytes in 1 blocks.
3648 Then there is no memory leak, just as memory accounted to global variables
3649 is not a memleak - the memory is still being referenced, and didn't leak.
3651 Similarly, under some circumstances, valgrind might report kernel bugs
3652 as if it were a bug in libev (e.g. in realloc or in the poll backend,
3653 although an acceptable workaround has been found here), or it might be
3654 confused.
3656 Keep in mind that valgrind is a very good tool, but only a tool. Don't
3657 make it into some kind of religion.
3659 If you are unsure about something, feel free to contact the mailing list
3660 with the full valgrind report and an explanation on why you think this
3661 is a bug in libev (best check the archives, too :). However, don't be
3662 annoyed when you get a brisk "this is no bug" answer and take the chance
3663 of learning how to interpret valgrind properly.
3665 If you need, for some reason, empty reports from valgrind for your project
3666 I suggest using suppression lists.
3673 Win32 doesn't support any of the standards (e.g. POSIX) that libev
3674 requires, and its I/O model is fundamentally incompatible with the POSIX
3675 model. Libev still offers limited functionality on this platform in
3676 the form of the C<EVBACKEND_SELECT> backend, and only supports socket
3677 descriptors. This only applies when using Win32 natively, not when using
3678 e.g. cygwin.
3680 Lifting these limitations would basically require the full
3681 re-implementation of the I/O system. If you are into these kinds of
3682 things, then note that glib does exactly that for you in a very portable
3683 way (note also that glib is the slowest event library known to man).
3685 There is no supported compilation method available on windows except
3686 embedding it into other applications.
3688 Not a libev limitation but worth mentioning: windows apparently doesn't
3689 accept large writes: instead of resulting in a partial write, windows will
3690 either accept everything or return C<ENOBUFS> if the buffer is too large,
3691 so make sure you only write small amounts into your sockets (less than a
3692 megabyte seems safe, but this apparently depends on the amount of memory
3693 available).
3695 Due to the many, low, and arbitrary limits on the win32 platform and
3696 the abysmal performance of winsockets, using a large number of sockets
3697 is not recommended (and not reasonable). If your program needs to use
3698 more than a hundred or so sockets, then likely it needs to use a totally
3699 different implementation for windows, as libev offers the POSIX readiness
3700 notification model, which cannot be implemented efficiently on windows
3701 (Microsoft monopoly games).
3703 A typical way to use libev under windows is to embed it (see the embedding
3704 section for details) and use the following F<evwrap.h> header file instead
3705 of F<ev.h>:
3707 #define EV_STANDALONE /* keeps ev from requiring config.h */
3708 #define EV_SELECT_IS_WINSOCKET 1 /* configure libev for windows select */
3710 #include "ev.h"
3712 And compile the following F<evwrap.c> file into your project (make sure
3713 you do I<not> compile the F<ev.c> or any other embedded source files!):
3715 #include "evwrap.h"
3716 #include "ev.c"
3718 =over 4
3720 =item The winsocket select function
3722 The winsocket C<select> function doesn't follow POSIX in that it
3723 requires socket I<handles> and not socket I<file descriptors> (it is
3724 also extremely buggy). This makes select very inefficient, and also
3725 requires a mapping from file descriptors to socket handles (the Microsoft
3726 C runtime provides the function C<_open_osfhandle> for this). See the
3727 discussion of the C<EV_SELECT_USE_FD_SET>, C<EV_SELECT_IS_WINSOCKET> and
3728 C<EV_FD_TO_WIN32_HANDLE> preprocessor symbols for more info.
3730 The configuration for a "naked" win32 using the Microsoft runtime
3731 libraries and raw winsocket select is:
3733 #define EV_USE_SELECT 1
3734 #define EV_SELECT_IS_WINSOCKET 1 /* forces EV_SELECT_USE_FD_SET, too */
3736 Note that winsockets handling of fd sets is O(n), so you can easily get a
3737 complexity in the O(n²) range when using win32.
3739 =item Limited number of file descriptors
3741 Windows has numerous arbitrary (and low) limits on things.
3743 Early versions of winsocket's select only supported waiting for a maximum
3744 of C<64> handles (probably owning to the fact that all windows kernels
3745 can only wait for C<64> things at the same time internally; Microsoft
3746 recommends spawning a chain of threads and wait for 63 handles and the
3747 previous thread in each. Great).
3749 Newer versions support more handles, but you need to define C<FD_SETSIZE>
3750 to some high number (e.g. C<2048>) before compiling the winsocket select
3751 call (which might be in libev or elsewhere, for example, perl does its own
3752 select emulation on windows).
3754 Another limit is the number of file descriptors in the Microsoft runtime
3755 libraries, which by default is C<64> (there must be a hidden I<64> fetish
3756 or something like this inside Microsoft). You can increase this by calling
3757 C<_setmaxstdio>, which can increase this limit to C<2048> (another
3758 arbitrary limit), but is broken in many versions of the Microsoft runtime
3759 libraries.
3761 This might get you to about C<512> or C<2048> sockets (depending on
3762 windows version and/or the phase of the moon). To get more, you need to
3763 wrap all I/O functions and provide your own fd management, but the cost of
3764 calling select (O(n²)) will likely make this unworkable.
3766 =back
3770 In addition to a working ISO-C implementation and of course the
3771 backend-specific APIs, libev relies on a few additional extensions:
3773 =over 4
3775 =item C<void (*)(ev_watcher_type *, int revents)> must have compatible
3776 calling conventions regardless of C<ev_watcher_type *>.
3778 Libev assumes not only that all watcher pointers have the same internal
3779 structure (guaranteed by POSIX but not by ISO C for example), but it also
3780 assumes that the same (machine) code can be used to call any watcher
3781 callback: The watcher callbacks have different type signatures, but libev
3782 calls them using an C<ev_watcher *> internally.
3784 =item C<sig_atomic_t volatile> must be thread-atomic as well
3786 The type C<sig_atomic_t volatile> (or whatever is defined as
3787 C<EV_ATOMIC_T>) must be atomic with respect to accesses from different
3788 threads. This is not part of the specification for C<sig_atomic_t>, but is
3789 believed to be sufficiently portable.
3791 =item C<sigprocmask> must work in a threaded environment
3793 Libev uses C<sigprocmask> to temporarily block signals. This is not
3794 allowed in a threaded program (C<pthread_sigmask> has to be used). Typical
3795 pthread implementations will either allow C<sigprocmask> in the "main
3796 thread" or will block signals process-wide, both behaviours would
3797 be compatible with libev. Interaction between C<sigprocmask> and
3798 C<pthread_sigmask> could complicate things, however.
3800 The most portable way to handle signals is to block signals in all threads
3801 except the initial one, and run the default loop in the initial thread as
3802 well.
3804 =item C<long> must be large enough for common memory allocation sizes
3806 To improve portability and simplify its API, libev uses C<long> internally
3807 instead of C<size_t> when allocating its data structures. On non-POSIX
3808 systems (Microsoft...) this might be unexpectedly low, but is still at
3809 least 31 bits everywhere, which is enough for hundreds of millions of
3810 watchers.
3812 =item C<double> must hold a time value in seconds with enough accuracy
3814 The type C<double> is used to represent timestamps. It is required to
3815 have at least 51 bits of mantissa (and 9 bits of exponent), which is good
3816 enough for at least into the year 4000. This requirement is fulfilled by
3817 implementations implementing IEEE 754 (basically all existing ones).
3819 =back
3821 If you know of other additional requirements drop me a note.
3826 In this section the complexities of (many of) the algorithms used inside
3827 libev will be documented. For complexity discussions about backends see
3828 the documentation for C<ev_default_init>.
3830 All of the following are about amortised time: If an array needs to be
3831 extended, libev needs to realloc and move the whole array, but this
3832 happens asymptotically rarer with higher number of elements, so O(1) might
3833 mean that libev does a lengthy realloc operation in rare cases, but on
3834 average it is much faster and asymptotically approaches constant time.
3836 =over 4
3838 =item Starting and stopping timer/periodic watchers: O(log skipped_other_timers)
3840 This means that, when you have a watcher that triggers in one hour and
3841 there are 100 watchers that would trigger before that, then inserting will
3842 have to skip roughly seven (C<ld 100>) of these watchers.
3844 =item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)
3846 That means that changing a timer costs less than removing/adding them,
3847 as only the relative motion in the event queue has to be paid for.
3849 =item Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)
3851 These just add the watcher into an array or at the head of a list.
3853 =item Stopping check/prepare/idle/fork/async watchers: O(1)
3855 =item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
3857 These watchers are stored in lists, so they need to be walked to find the
3858 correct watcher to remove. The lists are usually short (you don't usually
3859 have many watchers waiting for the same fd or signal: one is typical, two
3860 is rare).
3862 =item Finding the next timer in each loop iteration: O(1)
3864 By virtue of using a binary or 4-heap, the next timer is always found at a
3865 fixed position in the storage array.
3867 =item Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)
3869 A change means an I/O watcher gets started or stopped, which requires
3870 libev to recalculate its status (and possibly tell the kernel, depending
3871 on backend and whether C<ev_io_set> was used).
3873 =item Activating one watcher (putting it into the pending state): O(1)
3875 =item Priority handling: O(number_of_priorities)
3877 Priorities are implemented by allocating some space for each
3878 priority. When doing priority-based operations, libev usually has to
3879 linearly search all the priorities, but starting/stopping and activating
3880 watchers becomes O(1) with respect to priority handling.
3882 =item Sending an ev_async: O(1)
3884 =item Processing ev_async_send: O(number_of_async_watchers)
3886 =item Processing signals: O(max_signal_number)
3888 Sending involves a system call I<iff> there were no other C<ev_async_send>
3889 calls in the current loop iteration. Checking for async and signal events
3890 involves iterating over all running async watchers or all signal numbers.
3892 =back
3895 =head1 AUTHOR
3897 Marc Lehmann <>, with repeated corrections by Mikael Magnusson.