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Revision: 1.121
Committed: Wed Mar 18 12:21:48 2020 UTC (4 years, 4 months ago) by root
Branch: MAIN
CVS Tags: rel-4_33
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# Content
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132 .rm #[ #] #H #V #F C
133 .\" ========================================================================
134 .\"
135 .IX Title "LIBEV 3"
136 .TH LIBEV 3 "2020-03-12" "libev-4.31" "libev - high performance full featured event loop"
137 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
138 .\" way too many mistakes in technical documents.
139 .if n .ad l
140 .nh
141 .SH "NAME"
142 libev \- a high performance full\-featured event loop written in C
144 .IX Header "SYNOPSIS"
145 .Vb 1
146 \& #include <ev.h>
147 .Ve
148 .SS "\s-1EXAMPLE PROGRAM\s0"
149 .IX Subsection "EXAMPLE PROGRAM"
150 .Vb 2
151 \& // a single header file is required
152 \& #include <ev.h>
153 \&
154 \& #include <stdio.h> // for puts
155 \&
156 \& // every watcher type has its own typedef\*(Aqd struct
157 \& // with the name ev_TYPE
158 \& ev_io stdin_watcher;
159 \& ev_timer timeout_watcher;
160 \&
161 \& // all watcher callbacks have a similar signature
162 \& // this callback is called when data is readable on stdin
163 \& static void
164 \& stdin_cb (EV_P_ ev_io *w, int revents)
165 \& {
166 \& puts ("stdin ready");
167 \& // for one\-shot events, one must manually stop the watcher
168 \& // with its corresponding stop function.
169 \& ev_io_stop (EV_A_ w);
170 \&
171 \& // this causes all nested ev_run\*(Aqs to stop iterating
172 \& ev_break (EV_A_ EVBREAK_ALL);
173 \& }
174 \&
175 \& // another callback, this time for a time\-out
176 \& static void
177 \& timeout_cb (EV_P_ ev_timer *w, int revents)
178 \& {
179 \& puts ("timeout");
180 \& // this causes the innermost ev_run to stop iterating
181 \& ev_break (EV_A_ EVBREAK_ONE);
182 \& }
183 \&
184 \& int
185 \& main (void)
186 \& {
187 \& // use the default event loop unless you have special needs
188 \& struct ev_loop *loop = EV_DEFAULT;
189 \&
190 \& // initialise an io watcher, then start it
191 \& // this one will watch for stdin to become readable
192 \& ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
193 \& ev_io_start (loop, &stdin_watcher);
194 \&
195 \& // initialise a timer watcher, then start it
196 \& // simple non\-repeating 5.5 second timeout
197 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
198 \& ev_timer_start (loop, &timeout_watcher);
199 \&
200 \& // now wait for events to arrive
201 \& ev_run (loop, 0);
202 \&
203 \& // break was called, so exit
204 \& return 0;
205 \& }
206 .Ve
209 This document documents the libev software package.
210 .PP
211 The newest version of this document is also available as an html-formatted
212 web page you might find easier to navigate when reading it for the first
213 time: <>.
214 .PP
215 While this document tries to be as complete as possible in documenting
216 libev, its usage and the rationale behind its design, it is not a tutorial
217 on event-based programming, nor will it introduce event-based programming
218 with libev.
219 .PP
220 Familiarity with event based programming techniques in general is assumed
221 throughout this document.
224 This manual tries to be very detailed, but unfortunately, this also makes
225 it very long. If you just want to know the basics of libev, I suggest
226 reading \*(L"\s-1ANATOMY OF A WATCHER\*(R"\s0, then the \*(L"\s-1EXAMPLE PROGRAM\*(R"\s0 above and
227 look up the missing functions in \*(L"\s-1GLOBAL FUNCTIONS\*(R"\s0 and the \f(CW\*(C`ev_io\*(C'\fR and
228 \&\f(CW\*(C`ev_timer\*(C'\fR sections in \*(L"\s-1WATCHER TYPES\*(R"\s0.
230 .IX Header "ABOUT LIBEV"
231 Libev is an event loop: you register interest in certain events (such as a
232 file descriptor being readable or a timeout occurring), and it will manage
233 these event sources and provide your program with events.
234 .PP
235 To do this, it must take more or less complete control over your process
236 (or thread) by executing the \fIevent loop\fR handler, and will then
237 communicate events via a callback mechanism.
238 .PP
239 You register interest in certain events by registering so-called \fIevent
240 watchers\fR, which are relatively small C structures you initialise with the
241 details of the event, and then hand it over to libev by \fIstarting\fR the
242 watcher.
243 .SS "\s-1FEATURES\s0"
244 .IX Subsection "FEATURES"
245 Libev supports \f(CW\*(C`select\*(C'\fR, \f(CW\*(C`poll\*(C'\fR, the Linux-specific aio and \f(CW\*(C`epoll\*(C'\fR
246 interfaces, the BSD-specific \f(CW\*(C`kqueue\*(C'\fR and the Solaris-specific event port
247 mechanisms for file descriptor events (\f(CW\*(C`ev_io\*(C'\fR), the Linux \f(CW\*(C`inotify\*(C'\fR
248 interface (for \f(CW\*(C`ev_stat\*(C'\fR), Linux eventfd/signalfd (for faster and cleaner
249 inter-thread wakeup (\f(CW\*(C`ev_async\*(C'\fR)/signal handling (\f(CW\*(C`ev_signal\*(C'\fR)) relative
250 timers (\f(CW\*(C`ev_timer\*(C'\fR), absolute timers with customised rescheduling
251 (\f(CW\*(C`ev_periodic\*(C'\fR), synchronous signals (\f(CW\*(C`ev_signal\*(C'\fR), process status
252 change events (\f(CW\*(C`ev_child\*(C'\fR), and event watchers dealing with the event
253 loop mechanism itself (\f(CW\*(C`ev_idle\*(C'\fR, \f(CW\*(C`ev_embed\*(C'\fR, \f(CW\*(C`ev_prepare\*(C'\fR and
254 \&\f(CW\*(C`ev_check\*(C'\fR watchers) as well as file watchers (\f(CW\*(C`ev_stat\*(C'\fR) and even
255 limited support for fork events (\f(CW\*(C`ev_fork\*(C'\fR).
256 .PP
257 It also is quite fast (see this
258 benchmark <> comparing it to libevent
259 for example).
260 .SS "\s-1CONVENTIONS\s0"
261 .IX Subsection "CONVENTIONS"
262 Libev is very configurable. In this manual the default (and most common)
263 configuration will be described, which supports multiple event loops. For
264 more info about various configuration options please have a look at
265 \&\fB\s-1EMBED\s0\fR section in this manual. If libev was configured without support
266 for multiple event loops, then all functions taking an initial argument of
267 name \f(CW\*(C`loop\*(C'\fR (which is always of type \f(CW\*(C`struct ev_loop *\*(C'\fR) will not have
268 this argument.
271 Libev represents time as a single floating point number, representing
272 the (fractional) number of seconds since the (\s-1POSIX\s0) epoch (in practice
273 somewhere near the beginning of 1970, details are complicated, don't
274 ask). This type is called \f(CW\*(C`ev_tstamp\*(C'\fR, which is what you should use
275 too. It usually aliases to the \f(CW\*(C`double\*(C'\fR type in C. When you need to do
276 any calculations on it, you should treat it as some floating point value.
277 .PP
278 Unlike the name component \f(CW\*(C`stamp\*(C'\fR might indicate, it is also used for
279 time differences (e.g. delays) throughout libev.
282 Libev knows three classes of errors: operating system errors, usage errors
283 and internal errors (bugs).
284 .PP
285 When libev catches an operating system error it cannot handle (for example
286 a system call indicating a condition libev cannot fix), it calls the callback
287 set via \f(CW\*(C`ev_set_syserr_cb\*(C'\fR, which is supposed to fix the problem or
288 abort. The default is to print a diagnostic message and to call \f(CW\*(C`abort
289 ()\*(C'\fR.
290 .PP
291 When libev detects a usage error such as a negative timer interval, then
292 it will print a diagnostic message and abort (via the \f(CW\*(C`assert\*(C'\fR mechanism,
293 so \f(CW\*(C`NDEBUG\*(C'\fR will disable this checking): these are programming errors in
294 the libev caller and need to be fixed there.
295 .PP
296 Via the \f(CW\*(C`EV_FREQUENT\*(C'\fR macro you can compile in and/or enable extensive
297 consistency checking code inside libev that can be used to check for
298 internal inconsistencies, suually caused by application bugs.
299 .PP
300 Libev also has a few internal error-checking \f(CW\*(C`assert\*(C'\fRions. These do not
301 trigger under normal circumstances, as they indicate either a bug in libev
302 or worse.
305 These functions can be called anytime, even before initialising the
306 library in any way.
307 .IP "ev_tstamp ev_time ()" 4
308 .IX Item "ev_tstamp ev_time ()"
309 Returns the current time as libev would use it. Please note that the
310 \&\f(CW\*(C`ev_now\*(C'\fR function is usually faster and also often returns the timestamp
311 you actually want to know. Also interesting is the combination of
312 \&\f(CW\*(C`ev_now_update\*(C'\fR and \f(CW\*(C`ev_now\*(C'\fR.
313 .IP "ev_sleep (ev_tstamp interval)" 4
314 .IX Item "ev_sleep (ev_tstamp interval)"
315 Sleep for the given interval: The current thread will be blocked
316 until either it is interrupted or the given time interval has
317 passed (approximately \- it might return a bit earlier even if not
318 interrupted). Returns immediately if \f(CW\*(C`interval <= 0\*(C'\fR.
319 .Sp
320 Basically this is a sub-second-resolution \f(CW\*(C`sleep ()\*(C'\fR.
321 .Sp
322 The range of the \f(CW\*(C`interval\*(C'\fR is limited \- libev only guarantees to work
323 with sleep times of up to one day (\f(CW\*(C`interval <= 86400\*(C'\fR).
324 .IP "int ev_version_major ()" 4
325 .IX Item "int ev_version_major ()"
326 .PD 0
327 .IP "int ev_version_minor ()" 4
328 .IX Item "int ev_version_minor ()"
329 .PD
330 You can find out the major and minor \s-1ABI\s0 version numbers of the library
331 you linked against by calling the functions \f(CW\*(C`ev_version_major\*(C'\fR and
332 \&\f(CW\*(C`ev_version_minor\*(C'\fR. If you want, you can compare against the global
333 symbols \f(CW\*(C`EV_VERSION_MAJOR\*(C'\fR and \f(CW\*(C`EV_VERSION_MINOR\*(C'\fR, which specify the
334 version of the library your program was compiled against.
335 .Sp
336 These version numbers refer to the \s-1ABI\s0 version of the library, not the
337 release version.
338 .Sp
339 Usually, it's a good idea to terminate if the major versions mismatch,
340 as this indicates an incompatible change. Minor versions are usually
341 compatible to older versions, so a larger minor version alone is usually
342 not a problem.
343 .Sp
344 Example: Make sure we haven't accidentally been linked against the wrong
345 version (note, however, that this will not detect other \s-1ABI\s0 mismatches,
346 such as \s-1LFS\s0 or reentrancy).
347 .Sp
348 .Vb 3
349 \& assert (("libev version mismatch",
350 \& ev_version_major () == EV_VERSION_MAJOR
351 \& && ev_version_minor () >= EV_VERSION_MINOR));
352 .Ve
353 .IP "unsigned int ev_supported_backends ()" 4
354 .IX Item "unsigned int ev_supported_backends ()"
355 Return the set of all backends (i.e. their corresponding \f(CW\*(C`EV_BACKEND_*\*(C'\fR
356 value) compiled into this binary of libev (independent of their
357 availability on the system you are running on). See \f(CW\*(C`ev_default_loop\*(C'\fR for
358 a description of the set values.
359 .Sp
360 Example: make sure we have the epoll method, because yeah this is cool and
361 a must have and can we have a torrent of it please!!!11
362 .Sp
363 .Vb 2
364 \& assert (("sorry, no epoll, no sex",
365 \& ev_supported_backends () & EVBACKEND_EPOLL));
366 .Ve
367 .IP "unsigned int ev_recommended_backends ()" 4
368 .IX Item "unsigned int ev_recommended_backends ()"
369 Return the set of all backends compiled into this binary of libev and
370 also recommended for this platform, meaning it will work for most file
371 descriptor types. This set is often smaller than the one returned by
372 \&\f(CW\*(C`ev_supported_backends\*(C'\fR, as for example kqueue is broken on most BSDs
373 and will not be auto-detected unless you explicitly request it (assuming
374 you know what you are doing). This is the set of backends that libev will
375 probe for if you specify no backends explicitly.
376 .IP "unsigned int ev_embeddable_backends ()" 4
377 .IX Item "unsigned int ev_embeddable_backends ()"
378 Returns the set of backends that are embeddable in other event loops. This
379 value is platform-specific but can include backends not available on the
380 current system. To find which embeddable backends might be supported on
381 the current system, you would need to look at \f(CW\*(C`ev_embeddable_backends ()
382 & ev_supported_backends ()\*(C'\fR, likewise for recommended ones.
383 .Sp
384 See the description of \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
385 .IP "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())" 4
386 .IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())"
387 Sets the allocation function to use (the prototype is similar \- the
388 semantics are identical to the \f(CW\*(C`realloc\*(C'\fR C89/SuS/POSIX function). It is
389 used to allocate and free memory (no surprises here). If it returns zero
390 when memory needs to be allocated (\f(CW\*(C`size != 0\*(C'\fR), the library might abort
391 or take some potentially destructive action.
392 .Sp
393 Since some systems (at least OpenBSD and Darwin) fail to implement
394 correct \f(CW\*(C`realloc\*(C'\fR semantics, libev will use a wrapper around the system
395 \&\f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions by default.
396 .Sp
397 You could override this function in high-availability programs to, say,
398 free some memory if it cannot allocate memory, to use a special allocator,
399 or even to sleep a while and retry until some memory is available.
400 .Sp
401 Example: The following is the \f(CW\*(C`realloc\*(C'\fR function that libev itself uses
402 which should work with \f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions of all kinds and
403 is probably a good basis for your own implementation.
404 .Sp
405 .Vb 5
406 \& static void *
407 \& ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
408 \& {
409 \& if (size)
410 \& return realloc (ptr, size);
411 \&
412 \& free (ptr);
413 \& return 0;
414 \& }
415 .Ve
416 .Sp
417 Example: Replace the libev allocator with one that waits a bit and then
418 retries.
419 .Sp
420 .Vb 8
421 \& static void *
422 \& persistent_realloc (void *ptr, size_t size)
423 \& {
424 \& if (!size)
425 \& {
426 \& free (ptr);
427 \& return 0;
428 \& }
429 \&
430 \& for (;;)
431 \& {
432 \& void *newptr = realloc (ptr, size);
433 \&
434 \& if (newptr)
435 \& return newptr;
436 \&
437 \& sleep (60);
438 \& }
439 \& }
440 \&
441 \& ...
442 \& ev_set_allocator (persistent_realloc);
443 .Ve
444 .IP "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())" 4
445 .IX Item "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())"
446 Set the callback function to call on a retryable system call error (such
447 as failed select, poll, epoll_wait). The message is a printable string
448 indicating the system call or subsystem causing the problem. If this
449 callback is set, then libev will expect it to remedy the situation, no
450 matter what, when it returns. That is, libev will generally retry the
451 requested operation, or, if the condition doesn't go away, do bad stuff
452 (such as abort).
453 .Sp
454 Example: This is basically the same thing that libev does internally, too.
455 .Sp
456 .Vb 6
457 \& static void
458 \& fatal_error (const char *msg)
459 \& {
460 \& perror (msg);
461 \& abort ();
462 \& }
463 \&
464 \& ...
465 \& ev_set_syserr_cb (fatal_error);
466 .Ve
467 .IP "ev_feed_signal (int signum)" 4
468 .IX Item "ev_feed_signal (int signum)"
469 This function can be used to \*(L"simulate\*(R" a signal receive. It is completely
470 safe to call this function at any time, from any context, including signal
471 handlers or random threads.
472 .Sp
473 Its main use is to customise signal handling in your process, especially
474 in the presence of threads. For example, you could block signals
475 by default in all threads (and specifying \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when
476 creating any loops), and in one thread, use \f(CW\*(C`sigwait\*(C'\fR or any other
477 mechanism to wait for signals, then \*(L"deliver\*(R" them to libev by calling
478 \&\f(CW\*(C`ev_feed_signal\*(C'\fR.
481 An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR (the \f(CW\*(C`struct\*(C'\fR is
482 \&\fInot\fR optional in this case unless libev 3 compatibility is disabled, as
483 libev 3 had an \f(CW\*(C`ev_loop\*(C'\fR function colliding with the struct name).
484 .PP
485 The library knows two types of such loops, the \fIdefault\fR loop, which
486 supports child process events, and dynamically created event loops which
487 do not.
488 .IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4
489 .IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"
490 This returns the \*(L"default\*(R" event loop object, which is what you should
491 normally use when you just need \*(L"the event loop\*(R". Event loop objects and
492 the \f(CW\*(C`flags\*(C'\fR parameter are described in more detail in the entry for
493 \&\f(CW\*(C`ev_loop_new\*(C'\fR.
494 .Sp
495 If the default loop is already initialised then this function simply
496 returns it (and ignores the flags. If that is troubling you, check
497 \&\f(CW\*(C`ev_backend ()\*(C'\fR afterwards). Otherwise it will create it with the given
498 flags, which should almost always be \f(CW0\fR, unless the caller is also the
499 one calling \f(CW\*(C`ev_run\*(C'\fR or otherwise qualifies as \*(L"the main program\*(R".
500 .Sp
501 If you don't know what event loop to use, use the one returned from this
502 function (or via the \f(CW\*(C`EV_DEFAULT\*(C'\fR macro).
503 .Sp
504 Note that this function is \fInot\fR thread-safe, so if you want to use it
505 from multiple threads, you have to employ some kind of mutex (note also
506 that this case is unlikely, as loops cannot be shared easily between
507 threads anyway).
508 .Sp
509 The default loop is the only loop that can handle \f(CW\*(C`ev_child\*(C'\fR watchers,
510 and to do this, it always registers a handler for \f(CW\*(C`SIGCHLD\*(C'\fR. If this is
511 a problem for your application you can either create a dynamic loop with
512 \&\f(CW\*(C`ev_loop_new\*(C'\fR which doesn't do that, or you can simply overwrite the
513 \&\f(CW\*(C`SIGCHLD\*(C'\fR signal handler \fIafter\fR calling \f(CW\*(C`ev_default_init\*(C'\fR.
514 .Sp
515 Example: This is the most typical usage.
516 .Sp
517 .Vb 2
518 \& if (!ev_default_loop (0))
519 \& fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
520 .Ve
521 .Sp
522 Example: Restrict libev to the select and poll backends, and do not allow
523 environment settings to be taken into account:
524 .Sp
525 .Vb 1
527 .Ve
528 .IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4
529 .IX Item "struct ev_loop *ev_loop_new (unsigned int flags)"
530 This will create and initialise a new event loop object. If the loop
531 could not be initialised, returns false.
532 .Sp
533 This function is thread-safe, and one common way to use libev with
534 threads is indeed to create one loop per thread, and using the default
535 loop in the \*(L"main\*(R" or \*(L"initial\*(R" thread.
536 .Sp
537 The flags argument can be used to specify special behaviour or specific
538 backends to use, and is usually specified as \f(CW0\fR (or \f(CW\*(C`EVFLAG_AUTO\*(C'\fR).
539 .Sp
540 The following flags are supported:
541 .RS 4
542 .ie n .IP """EVFLAG_AUTO""" 4
543 .el .IP "\f(CWEVFLAG_AUTO\fR" 4
544 .IX Item "EVFLAG_AUTO"
545 The default flags value. Use this if you have no clue (it's the right
546 thing, believe me).
547 .ie n .IP """EVFLAG_NOENV""" 4
548 .el .IP "\f(CWEVFLAG_NOENV\fR" 4
550 If this flag bit is or'ed into the flag value (or the program runs setuid
551 or setgid) then libev will \fInot\fR look at the environment variable
552 \&\f(CW\*(C`LIBEV_FLAGS\*(C'\fR. Otherwise (the default), this environment variable will
553 override the flags completely if it is found in the environment. This is
554 useful to try out specific backends to test their performance, to work
555 around bugs, or to make libev threadsafe (accessing environment variables
556 cannot be done in a threadsafe way, but usually it works if no other
557 thread modifies them).
558 .ie n .IP """EVFLAG_FORKCHECK""" 4
559 .el .IP "\f(CWEVFLAG_FORKCHECK\fR" 4
561 Instead of calling \f(CW\*(C`ev_loop_fork\*(C'\fR manually after a fork, you can also
562 make libev check for a fork in each iteration by enabling this flag.
563 .Sp
564 This works by calling \f(CW\*(C`getpid ()\*(C'\fR on every iteration of the loop,
565 and thus this might slow down your event loop if you do a lot of loop
566 iterations and little real work, but is usually not noticeable (on my
567 GNU/Linux system for example, \f(CW\*(C`getpid\*(C'\fR is actually a simple 5\-insn
568 sequence without a system call and thus \fIvery\fR fast, but my GNU/Linux
569 system also has \f(CW\*(C`pthread_atfork\*(C'\fR which is even faster). (Update: glibc
570 versions 2.25 apparently removed the \f(CW\*(C`getpid\*(C'\fR optimisation again).
571 .Sp
572 The big advantage of this flag is that you can forget about fork (and
573 forget about forgetting to tell libev about forking, although you still
574 have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR) when you use this flag.
575 .Sp
576 This flag setting cannot be overridden or specified in the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR
577 environment variable.
578 .ie n .IP """EVFLAG_NOINOTIFY""" 4
579 .el .IP "\f(CWEVFLAG_NOINOTIFY\fR" 4
581 When this flag is specified, then libev will not attempt to use the
582 \&\fIinotify\fR \s-1API\s0 for its \f(CW\*(C`ev_stat\*(C'\fR watchers. Apart from debugging and
583 testing, this flag can be useful to conserve inotify file descriptors, as
584 otherwise each loop using \f(CW\*(C`ev_stat\*(C'\fR watchers consumes one inotify handle.
585 .ie n .IP """EVFLAG_SIGNALFD""" 4
586 .el .IP "\f(CWEVFLAG_SIGNALFD\fR" 4
588 When this flag is specified, then libev will attempt to use the
589 \&\fIsignalfd\fR \s-1API\s0 for its \f(CW\*(C`ev_signal\*(C'\fR (and \f(CW\*(C`ev_child\*(C'\fR) watchers. This \s-1API\s0
590 delivers signals synchronously, which makes it both faster and might make
591 it possible to get the queued signal data. It can also simplify signal
592 handling with threads, as long as you properly block signals in your
593 threads that are not interested in handling them.
594 .Sp
595 Signalfd will not be used by default as this changes your signal mask, and
596 there are a lot of shoddy libraries and programs (glib's threadpool for
597 example) that can't properly initialise their signal masks.
598 .ie n .IP """EVFLAG_NOSIGMASK""" 4
599 .el .IP "\f(CWEVFLAG_NOSIGMASK\fR" 4
601 When this flag is specified, then libev will avoid to modify the signal
602 mask. Specifically, this means you have to make sure signals are unblocked
603 when you want to receive them.
604 .Sp
605 This behaviour is useful when you want to do your own signal handling, or
606 want to handle signals only in specific threads and want to avoid libev
607 unblocking the signals.
608 .Sp
609 It's also required by \s-1POSIX\s0 in a threaded program, as libev calls
610 \&\f(CW\*(C`sigprocmask\*(C'\fR, whose behaviour is officially unspecified.
611 .ie n .IP """EVFLAG_NOTIMERFD""" 4
612 .el .IP "\f(CWEVFLAG_NOTIMERFD\fR" 4
614 When this flag is specified, the libev will avoid using a \f(CW\*(C`timerfd\*(C'\fR to
615 detect time jumps. It will still be able to detect time jumps, but takes
616 longer and has a lower accuracy in doing so, but saves a file descriptor
617 per loop.
618 .Sp
619 The current implementation only tries to use a \f(CW\*(C`timerfd\*(C'\fR when the first
620 \&\f(CW\*(C`ev_periodic\*(C'\fR watcher is started and falls back on other methods if it
621 cannot be created, but this behaviour might change in the future.
622 .ie n .IP """EVBACKEND_SELECT"" (value 1, portable select backend)" 4
623 .el .IP "\f(CWEVBACKEND_SELECT\fR (value 1, portable select backend)" 4
624 .IX Item "EVBACKEND_SELECT (value 1, portable select backend)"
625 This is your standard \fBselect\fR\|(2) backend. Not \fIcompletely\fR standard, as
626 libev tries to roll its own fd_set with no limits on the number of fds,
627 but if that fails, expect a fairly low limit on the number of fds when
628 using this backend. It doesn't scale too well (O(highest_fd)), but its
629 usually the fastest backend for a low number of (low-numbered :) fds.
630 .Sp
631 To get good performance out of this backend you need a high amount of
632 parallelism (most of the file descriptors should be busy). If you are
633 writing a server, you should \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many
634 connections as possible during one iteration. You might also want to have
635 a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of
636 readiness notifications you get per iteration.
637 .Sp
638 This backend maps \f(CW\*(C`EV_READ\*(C'\fR to the \f(CW\*(C`readfds\*(C'\fR set and \f(CW\*(C`EV_WRITE\*(C'\fR to the
639 \&\f(CW\*(C`writefds\*(C'\fR set (and to work around Microsoft Windows bugs, also onto the
640 \&\f(CW\*(C`exceptfds\*(C'\fR set on that platform).
641 .ie n .IP """EVBACKEND_POLL"" (value 2, poll backend, available everywhere except on windows)" 4
642 .el .IP "\f(CWEVBACKEND_POLL\fR (value 2, poll backend, available everywhere except on windows)" 4
643 .IX Item "EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)"
644 And this is your standard \fBpoll\fR\|(2) backend. It's more complicated
645 than select, but handles sparse fds better and has no artificial
646 limit on the number of fds you can use (except it will slow down
647 considerably with a lot of inactive fds). It scales similarly to select,
648 i.e. O(total_fds). See the entry for \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for
649 performance tips.
650 .Sp
651 This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLLHUP\*(C'\fR, and
652 \&\f(CW\*(C`EV_WRITE\*(C'\fR to \f(CW\*(C`POLLOUT | POLLERR | POLLHUP\*(C'\fR.
653 .ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4
654 .el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4
655 .IX Item "EVBACKEND_EPOLL (value 4, Linux)"
656 Use the Linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9
657 kernels).
658 .Sp
659 For few fds, this backend is a bit little slower than poll and select, but
660 it scales phenomenally better. While poll and select usually scale like
661 O(total_fds) where total_fds is the total number of fds (or the highest
662 fd), epoll scales either O(1) or O(active_fds).
663 .Sp
664 The epoll mechanism deserves honorable mention as the most misdesigned
665 of the more advanced event mechanisms: mere annoyances include silently
666 dropping file descriptors, requiring a system call per change per file
667 descriptor (and unnecessary guessing of parameters), problems with dup,
668 returning before the timeout value, resulting in additional iterations
669 (and only giving 5ms accuracy while select on the same platform gives
670 0.1ms) and so on. The biggest issue is fork races, however \- if a program
671 forks then \fIboth\fR parent and child process have to recreate the epoll
672 set, which can take considerable time (one syscall per file descriptor)
673 and is of course hard to detect.
674 .Sp
675 Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work,
676 but of course \fIdoesn't\fR, and epoll just loves to report events for
677 totally \fIdifferent\fR file descriptors (even already closed ones, so
678 one cannot even remove them from the set) than registered in the set
679 (especially on \s-1SMP\s0 systems). Libev tries to counter these spurious
680 notifications by employing an additional generation counter and comparing
681 that against the events to filter out spurious ones, recreating the set
682 when required. Epoll also erroneously rounds down timeouts, but gives you
683 no way to know when and by how much, so sometimes you have to busy-wait
684 because epoll returns immediately despite a nonzero timeout. And last
685 not least, it also refuses to work with some file descriptors which work
686 perfectly fine with \f(CW\*(C`select\*(C'\fR (files, many character devices...).
687 .Sp
688 Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
689 cobbled together in a hurry, no thought to design or interaction with
690 others. Oh, the pain, will it ever stop...
691 .Sp
692 While stopping, setting and starting an I/O watcher in the same iteration
693 will result in some caching, there is still a system call per such
694 incident (because the same \fIfile descriptor\fR could point to a different
695 \&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed
696 file descriptors might not work very well if you register events for both
697 file descriptors.
698 .Sp
699 Best performance from this backend is achieved by not unregistering all
700 watchers for a file descriptor until it has been closed, if possible,
701 i.e. keep at least one watcher active per fd at all times. Stopping and
702 starting a watcher (without re-setting it) also usually doesn't cause
703 extra overhead. A fork can both result in spurious notifications as well
704 as in libev having to destroy and recreate the epoll object, which can
705 take considerable time and thus should be avoided.
706 .Sp
707 All this means that, in practice, \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR can be as fast or
708 faster than epoll for maybe up to a hundred file descriptors, depending on
709 the usage. So sad.
710 .Sp
711 While nominally embeddable in other event loops, this feature is broken in
712 a lot of kernel revisions, but probably(!) works in current versions.
713 .Sp
714 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
715 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
716 .ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4
717 .el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4
718 .IX Item "EVBACKEND_LINUXAIO (value 64, Linux)"
719 Use the Linux-specific Linux \s-1AIO\s0 (\fInot\fR \f(CWaio(7)\fR but \f(CWio_submit(2)\fR) event interface available in post\-4.18 kernels (but libev
720 only tries to use it in 4.19+).
721 .Sp
722 This is another Linux train wreck of an event interface.
723 .Sp
724 If this backend works for you (as of this writing, it was very
725 experimental), it is the best event interface available on Linux and might
726 be well worth enabling it \- if it isn't available in your kernel this will
727 be detected and this backend will be skipped.
728 .Sp
729 This backend can batch oneshot requests and supports a user-space ring
730 buffer to receive events. It also doesn't suffer from most of the design
731 problems of epoll (such as not being able to remove event sources from
732 the epoll set), and generally sounds too good to be true. Because, this
733 being the Linux kernel, of course it suffers from a whole new set of
734 limitations, forcing you to fall back to epoll, inheriting all its design
735 issues.
736 .Sp
737 For one, it is not easily embeddable (but probably could be done using
738 an event fd at some extra overhead). It also is subject to a system wide
739 limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR. If no \s-1AIO\s0
740 requests are left, this backend will be skipped during initialisation, and
741 will switch to epoll when the loop is active.
742 .Sp
743 Most problematic in practice, however, is that not all file descriptors
744 work with it. For example, in Linux 5.1, \s-1TCP\s0 sockets, pipes, event fds,
745 files, \fI/dev/null\fR and many others are supported, but ttys do not work
746 properly (a known bug that the kernel developers don't care about, see
747 <>), so this is not
748 (yet?) a generic event polling interface.
749 .Sp
750 Overall, it seems the Linux developers just don't want it to have a
751 generic event handling mechanism other than \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`poll\*(C'\fR.
752 .Sp
753 To work around all these problem, the current version of libev uses its
754 epoll backend as a fallback for file descriptor types that do not work. Or
755 falls back completely to epoll if the kernel acts up.
756 .Sp
757 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
758 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
759 .ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
760 .el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4
761 .IX Item "EVBACKEND_KQUEUE (value 8, most BSD clones)"
762 Kqueue deserves special mention, as at the time this backend was
763 implemented, it was broken on all BSDs except NetBSD (usually it doesn't
764 work reliably with anything but sockets and pipes, except on Darwin,
765 where of course it's completely useless). Unlike epoll, however, whose
766 brokenness is by design, these kqueue bugs can be (and mostly have been)
767 fixed without \s-1API\s0 changes to existing programs. For this reason it's not
768 being \*(L"auto-detected\*(R" on all platforms unless you explicitly specify it
769 in the flags (i.e. using \f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a
770 known-to-be-good (\-enough) system like NetBSD.
771 .Sp
772 You still can embed kqueue into a normal poll or select backend and use it
773 only for sockets (after having made sure that sockets work with kqueue on
774 the target platform). See \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
775 .Sp
776 It scales in the same way as the epoll backend, but the interface to the
777 kernel is more efficient (which says nothing about its actual speed, of
778 course). While stopping, setting and starting an I/O watcher does never
779 cause an extra system call as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to
780 two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you
781 might have to leak fds on fork, but it's more sane than epoll) and it
782 drops fds silently in similarly hard-to-detect cases.
783 .Sp
784 This backend usually performs well under most conditions.
785 .Sp
786 While nominally embeddable in other event loops, this doesn't work
787 everywhere, so you might need to test for this. And since it is broken
788 almost everywhere, you should only use it when you have a lot of sockets
789 (for which it usually works), by embedding it into another event loop
790 (e.g. \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR (but \f(CW\*(C`poll\*(C'\fR is of course
791 also broken on \s-1OS X\s0)) and, did I mention it, using it only for sockets.
792 .Sp
793 This backend maps \f(CW\*(C`EV_READ\*(C'\fR into an \f(CW\*(C`EVFILT_READ\*(C'\fR kevent with
794 \&\f(CW\*(C`NOTE_EOF\*(C'\fR, and \f(CW\*(C`EV_WRITE\*(C'\fR into an \f(CW\*(C`EVFILT_WRITE\*(C'\fR kevent with
795 \&\f(CW\*(C`NOTE_EOF\*(C'\fR.
796 .ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4
797 .el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4
798 .IX Item "EVBACKEND_DEVPOLL (value 16, Solaris 8)"
799 This is not implemented yet (and might never be, unless you send me an
800 implementation). According to reports, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets
801 and is not embeddable, which would limit the usefulness of this backend
802 immensely.
803 .ie n .IP """EVBACKEND_PORT"" (value 32, Solaris 10)" 4
804 .el .IP "\f(CWEVBACKEND_PORT\fR (value 32, Solaris 10)" 4
805 .IX Item "EVBACKEND_PORT (value 32, Solaris 10)"
806 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
807 it's really slow, but it still scales very well (O(active_fds)).
808 .Sp
809 While this backend scales well, it requires one system call per active
810 file descriptor per loop iteration. For small and medium numbers of file
811 descriptors a \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend
812 might perform better.
813 .Sp
814 On the positive side, this backend actually performed fully to
815 specification in all tests and is fully embeddable, which is a rare feat
816 among the OS-specific backends (I vastly prefer correctness over speed
817 hacks).
818 .Sp
819 On the negative side, the interface is \fIbizarre\fR \- so bizarre that
820 even sun itself gets it wrong in their code examples: The event polling
821 function sometimes returns events to the caller even though an error
822 occurred, but with no indication whether it has done so or not (yes, it's
823 even documented that way) \- deadly for edge-triggered interfaces where you
824 absolutely have to know whether an event occurred or not because you have
825 to re-arm the watcher.
826 .Sp
827 Fortunately libev seems to be able to work around these idiocies.
828 .Sp
829 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
830 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
831 .ie n .IP """EVBACKEND_ALL""" 4
832 .el .IP "\f(CWEVBACKEND_ALL\fR" 4
834 Try all backends (even potentially broken ones that wouldn't be tried
835 with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as
837 .Sp
838 It is definitely not recommended to use this flag, use whatever
839 \&\f(CW\*(C`ev_recommended_backends ()\*(C'\fR returns, or simply do not specify a backend
840 at all.
841 .ie n .IP """EVBACKEND_MASK""" 4
842 .el .IP "\f(CWEVBACKEND_MASK\fR" 4
844 Not a backend at all, but a mask to select all backend bits from a
845 \&\f(CW\*(C`flags\*(C'\fR value, in case you want to mask out any backends from a flags
846 value (e.g. when modifying the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR environment variable).
847 .RE
848 .RS 4
849 .Sp
850 If one or more of the backend flags are or'ed into the flags value,
851 then only these backends will be tried (in the reverse order as listed
852 here). If none are specified, all backends in \f(CW\*(C`ev_recommended_backends
853 ()\*(C'\fR will be tried.
854 .Sp
855 Example: Try to create a event loop that uses epoll and nothing else.
856 .Sp
857 .Vb 3
858 \& struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
859 \& if (!epoller)
860 \& fatal ("no epoll found here, maybe it hides under your chair");
861 .Ve
862 .Sp
863 Example: Use whatever libev has to offer, but make sure that kqueue is
864 used if available.
865 .Sp
866 .Vb 1
867 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
868 .Ve
869 .Sp
870 Example: Similarly, on linux, you mgiht want to take advantage of the
871 linux aio backend if possible, but fall back to something else if that
872 isn't available.
873 .Sp
874 .Vb 1
875 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
876 .Ve
877 .RE
878 .IP "ev_loop_destroy (loop)" 4
879 .IX Item "ev_loop_destroy (loop)"
880 Destroys an event loop object (frees all memory and kernel state
881 etc.). None of the active event watchers will be stopped in the normal
882 sense, so e.g. \f(CW\*(C`ev_is_active\*(C'\fR might still return true. It is your
883 responsibility to either stop all watchers cleanly yourself \fIbefore\fR
884 calling this function, or cope with the fact afterwards (which is usually
885 the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them
886 for example).
887 .Sp
888 Note that certain global state, such as signal state (and installed signal
889 handlers), will not be freed by this function, and related watchers (such
890 as signal and child watchers) would need to be stopped manually.
891 .Sp
892 This function is normally used on loop objects allocated by
893 \&\f(CW\*(C`ev_loop_new\*(C'\fR, but it can also be used on the default loop returned by
894 \&\f(CW\*(C`ev_default_loop\*(C'\fR, in which case it is not thread-safe.
895 .Sp
896 Note that it is not advisable to call this function on the default loop
897 except in the rare occasion where you really need to free its resources.
898 If you need dynamically allocated loops it is better to use \f(CW\*(C`ev_loop_new\*(C'\fR
899 and \f(CW\*(C`ev_loop_destroy\*(C'\fR.
900 .IP "ev_loop_fork (loop)" 4
901 .IX Item "ev_loop_fork (loop)"
902 This function sets a flag that causes subsequent \f(CW\*(C`ev_run\*(C'\fR iterations
903 to reinitialise the kernel state for backends that have one. Despite
904 the name, you can call it anytime you are allowed to start or stop
905 watchers (except inside an \f(CW\*(C`ev_prepare\*(C'\fR callback), but it makes most
906 sense after forking, in the child process. You \fImust\fR call it (or use
907 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR) in the child before resuming or calling \f(CW\*(C`ev_run\*(C'\fR.
908 .Sp
909 In addition, if you want to reuse a loop (via this function or
910 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR), you \fIalso\fR have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR.
911 .Sp
912 Again, you \fIhave\fR to call it on \fIany\fR loop that you want to re-use after
913 a fork, \fIeven if you do not plan to use the loop in the parent\fR. This is
914 because some kernel interfaces *cough* \fIkqueue\fR *cough* do funny things
915 during fork.
916 .Sp
917 On the other hand, you only need to call this function in the child
918 process if and only if you want to use the event loop in the child. If
919 you just fork+exec or create a new loop in the child, you don't have to
920 call it at all (in fact, \f(CW\*(C`epoll\*(C'\fR is so badly broken that it makes a
921 difference, but libev will usually detect this case on its own and do a
922 costly reset of the backend).
923 .Sp
924 The function itself is quite fast and it's usually not a problem to call
925 it just in case after a fork.
926 .Sp
927 Example: Automate calling \f(CW\*(C`ev_loop_fork\*(C'\fR on the default loop when
928 using pthreads.
929 .Sp
930 .Vb 5
931 \& static void
932 \& post_fork_child (void)
933 \& {
934 \& ev_loop_fork (EV_DEFAULT);
935 \& }
936 \&
937 \& ...
938 \& pthread_atfork (0, 0, post_fork_child);
939 .Ve
940 .IP "int ev_is_default_loop (loop)" 4
941 .IX Item "int ev_is_default_loop (loop)"
942 Returns true when the given loop is, in fact, the default loop, and false
943 otherwise.
944 .IP "unsigned int ev_iteration (loop)" 4
945 .IX Item "unsigned int ev_iteration (loop)"
946 Returns the current iteration count for the event loop, which is identical
947 to the number of times libev did poll for new events. It starts at \f(CW0\fR
948 and happily wraps around with enough iterations.
949 .Sp
950 This value can sometimes be useful as a generation counter of sorts (it
951 \&\*(L"ticks\*(R" the number of loop iterations), as it roughly corresponds with
952 \&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls \- and is incremented between the
953 prepare and check phases.
954 .IP "unsigned int ev_depth (loop)" 4
955 .IX Item "unsigned int ev_depth (loop)"
956 Returns the number of times \f(CW\*(C`ev_run\*(C'\fR was entered minus the number of
957 times \f(CW\*(C`ev_run\*(C'\fR was exited normally, in other words, the recursion depth.
958 .Sp
959 Outside \f(CW\*(C`ev_run\*(C'\fR, this number is zero. In a callback, this number is
960 \&\f(CW1\fR, unless \f(CW\*(C`ev_run\*(C'\fR was invoked recursively (or from another thread),
961 in which case it is higher.
962 .Sp
963 Leaving \f(CW\*(C`ev_run\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread,
964 throwing an exception etc.), doesn't count as \*(L"exit\*(R" \- consider this
965 as a hint to avoid such ungentleman-like behaviour unless it's really
966 convenient, in which case it is fully supported.
967 .IP "unsigned int ev_backend (loop)" 4
968 .IX Item "unsigned int ev_backend (loop)"
969 Returns one of the \f(CW\*(C`EVBACKEND_*\*(C'\fR flags indicating the event backend in
970 use.
971 .IP "ev_tstamp ev_now (loop)" 4
972 .IX Item "ev_tstamp ev_now (loop)"
973 Returns the current \*(L"event loop time\*(R", which is the time the event loop
974 received events and started processing them. This timestamp does not
975 change as long as callbacks are being processed, and this is also the base
976 time used for relative timers. You can treat it as the timestamp of the
977 event occurring (or more correctly, libev finding out about it).
978 .IP "ev_now_update (loop)" 4
979 .IX Item "ev_now_update (loop)"
980 Establishes the current time by querying the kernel, updating the time
981 returned by \f(CW\*(C`ev_now ()\*(C'\fR in the progress. This is a costly operation and
982 is usually done automatically within \f(CW\*(C`ev_run ()\*(C'\fR.
983 .Sp
984 This function is rarely useful, but when some event callback runs for a
985 very long time without entering the event loop, updating libev's idea of
986 the current time is a good idea.
987 .Sp
988 See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.
989 .IP "ev_suspend (loop)" 4
990 .IX Item "ev_suspend (loop)"
991 .PD 0
992 .IP "ev_resume (loop)" 4
993 .IX Item "ev_resume (loop)"
994 .PD
995 These two functions suspend and resume an event loop, for use when the
996 loop is not used for a while and timeouts should not be processed.
997 .Sp
998 A typical use case would be an interactive program such as a game: When
999 the user presses \f(CW\*(C`^Z\*(C'\fR to suspend the game and resumes it an hour later it
1000 would be best to handle timeouts as if no time had actually passed while
1001 the program was suspended. This can be achieved by calling \f(CW\*(C`ev_suspend\*(C'\fR
1002 in your \f(CW\*(C`SIGTSTP\*(C'\fR handler, sending yourself a \f(CW\*(C`SIGSTOP\*(C'\fR and calling
1003 \&\f(CW\*(C`ev_resume\*(C'\fR directly afterwards to resume timer processing.
1004 .Sp
1005 Effectively, all \f(CW\*(C`ev_timer\*(C'\fR watchers will be delayed by the time spend
1006 between \f(CW\*(C`ev_suspend\*(C'\fR and \f(CW\*(C`ev_resume\*(C'\fR, and all \f(CW\*(C`ev_periodic\*(C'\fR watchers
1007 will be rescheduled (that is, they will lose any events that would have
1008 occurred while suspended).
1009 .Sp
1010 After calling \f(CW\*(C`ev_suspend\*(C'\fR you \fBmust not\fR call \fIany\fR function on the
1011 given loop other than \f(CW\*(C`ev_resume\*(C'\fR, and you \fBmust not\fR call \f(CW\*(C`ev_resume\*(C'\fR
1012 without a previous call to \f(CW\*(C`ev_suspend\*(C'\fR.
1013 .Sp
1014 Calling \f(CW\*(C`ev_suspend\*(C'\fR/\f(CW\*(C`ev_resume\*(C'\fR has the side effect of updating the
1015 event loop time (see \f(CW\*(C`ev_now_update\*(C'\fR).
1016 .IP "bool ev_run (loop, int flags)" 4
1017 .IX Item "bool ev_run (loop, int flags)"
1018 Finally, this is it, the event handler. This function usually is called
1019 after you have initialised all your watchers and you want to start
1020 handling events. It will ask the operating system for any new events, call
1021 the watcher callbacks, and then repeat the whole process indefinitely: This
1022 is why event loops are called \fIloops\fR.
1023 .Sp
1024 If the flags argument is specified as \f(CW0\fR, it will keep handling events
1025 until either no event watchers are active anymore or \f(CW\*(C`ev_break\*(C'\fR was
1026 called.
1027 .Sp
1028 The return value is false if there are no more active watchers (which
1029 usually means \*(L"all jobs done\*(R" or \*(L"deadlock\*(R"), and true in all other cases
1030 (which usually means " you should call \f(CW\*(C`ev_run\*(C'\fR again").
1031 .Sp
1032 Please note that an explicit \f(CW\*(C`ev_break\*(C'\fR is usually better than
1033 relying on all watchers to be stopped when deciding when a program has
1034 finished (especially in interactive programs), but having a program
1035 that automatically loops as long as it has to and no longer by virtue
1036 of relying on its watchers stopping correctly, that is truly a thing of
1037 beauty.
1038 .Sp
1039 This function is \fImostly\fR exception-safe \- you can break out of a
1040 \&\f(CW\*(C`ev_run\*(C'\fR call by calling \f(CW\*(C`longjmp\*(C'\fR in a callback, throwing a \*(C+
1041 exception and so on. This does not decrement the \f(CW\*(C`ev_depth\*(C'\fR value, nor
1042 will it clear any outstanding \f(CW\*(C`EVBREAK_ONE\*(C'\fR breaks.
1043 .Sp
1044 A flags value of \f(CW\*(C`EVRUN_NOWAIT\*(C'\fR will look for new events, will handle
1045 those events and any already outstanding ones, but will not wait and
1046 block your process in case there are no events and will return after one
1047 iteration of the loop. This is sometimes useful to poll and handle new
1048 events while doing lengthy calculations, to keep the program responsive.
1049 .Sp
1050 A flags value of \f(CW\*(C`EVRUN_ONCE\*(C'\fR will look for new events (waiting if
1051 necessary) and will handle those and any already outstanding ones. It
1052 will block your process until at least one new event arrives (which could
1053 be an event internal to libev itself, so there is no guarantee that a
1054 user-registered callback will be called), and will return after one
1055 iteration of the loop.
1056 .Sp
1057 This is useful if you are waiting for some external event in conjunction
1058 with something not expressible using other libev watchers (i.e. "roll your
1059 own \f(CW\*(C`ev_run\*(C'\fR"). However, a pair of \f(CW\*(C`ev_prepare\*(C'\fR/\f(CW\*(C`ev_check\*(C'\fR watchers is
1060 usually a better approach for this kind of thing.
1061 .Sp
1062 Here are the gory details of what \f(CW\*(C`ev_run\*(C'\fR does (this is for your
1063 understanding, not a guarantee that things will work exactly like this in
1064 future versions):
1065 .Sp
1066 .Vb 10
1067 \& \- Increment loop depth.
1068 \& \- Reset the ev_break status.
1069 \& \- Before the first iteration, call any pending watchers.
1070 \& LOOP:
1071 \& \- If EVFLAG_FORKCHECK was used, check for a fork.
1072 \& \- If a fork was detected (by any means), queue and call all fork watchers.
1073 \& \- Queue and call all prepare watchers.
1074 \& \- If ev_break was called, goto FINISH.
1075 \& \- If we have been forked, detach and recreate the kernel state
1076 \& as to not disturb the other process.
1077 \& \- Update the kernel state with all outstanding changes.
1078 \& \- Update the "event loop time" (ev_now ()).
1079 \& \- Calculate for how long to sleep or block, if at all
1080 \& (active idle watchers, EVRUN_NOWAIT or not having
1081 \& any active watchers at all will result in not sleeping).
1082 \& \- Sleep if the I/O and timer collect interval say so.
1083 \& \- Increment loop iteration counter.
1084 \& \- Block the process, waiting for any events.
1085 \& \- Queue all outstanding I/O (fd) events.
1086 \& \- Update the "event loop time" (ev_now ()), and do time jump adjustments.
1087 \& \- Queue all expired timers.
1088 \& \- Queue all expired periodics.
1089 \& \- Queue all idle watchers with priority higher than that of pending events.
1090 \& \- Queue all check watchers.
1091 \& \- Call all queued watchers in reverse order (i.e. check watchers first).
1092 \& Signals and child watchers are implemented as I/O watchers, and will
1093 \& be handled here by queueing them when their watcher gets executed.
1094 \& \- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
1095 \& were used, or there are no active watchers, goto FINISH, otherwise
1096 \& continue with step LOOP.
1097 \& FINISH:
1098 \& \- Reset the ev_break status iff it was EVBREAK_ONE.
1099 \& \- Decrement the loop depth.
1100 \& \- Return.
1101 .Ve
1102 .Sp
1103 Example: Queue some jobs and then loop until no events are outstanding
1104 anymore.
1105 .Sp
1106 .Vb 4
1107 \& ... queue jobs here, make sure they register event watchers as long
1108 \& ... as they still have work to do (even an idle watcher will do..)
1109 \& ev_run (my_loop, 0);
1110 \& ... jobs done or somebody called break. yeah!
1111 .Ve
1112 .IP "ev_break (loop, how)" 4
1113 .IX Item "ev_break (loop, how)"
1114 Can be used to make a call to \f(CW\*(C`ev_run\*(C'\fR return early (but only after it
1115 has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either
1116 \&\f(CW\*(C`EVBREAK_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_run\*(C'\fR call return, or
1117 \&\f(CW\*(C`EVBREAK_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_run\*(C'\fR calls return.
1118 .Sp
1119 This \*(L"break state\*(R" will be cleared on the next call to \f(CW\*(C`ev_run\*(C'\fR.
1120 .Sp
1121 It is safe to call \f(CW\*(C`ev_break\*(C'\fR from outside any \f(CW\*(C`ev_run\*(C'\fR calls, too, in
1122 which case it will have no effect.
1123 .IP "ev_ref (loop)" 4
1124 .IX Item "ev_ref (loop)"
1125 .PD 0
1126 .IP "ev_unref (loop)" 4
1127 .IX Item "ev_unref (loop)"
1128 .PD
1129 Ref/unref can be used to add or remove a reference count on the event
1130 loop: Every watcher keeps one reference, and as long as the reference
1131 count is nonzero, \f(CW\*(C`ev_run\*(C'\fR will not return on its own.
1132 .Sp
1133 This is useful when you have a watcher that you never intend to
1134 unregister, but that nevertheless should not keep \f(CW\*(C`ev_run\*(C'\fR from
1135 returning. In such a case, call \f(CW\*(C`ev_unref\*(C'\fR after starting, and \f(CW\*(C`ev_ref\*(C'\fR
1136 before stopping it.
1137 .Sp
1138 As an example, libev itself uses this for its internal signal pipe: It
1139 is not visible to the libev user and should not keep \f(CW\*(C`ev_run\*(C'\fR from
1140 exiting if no event watchers registered by it are active. It is also an
1141 excellent way to do this for generic recurring timers or from within
1142 third-party libraries. Just remember to \fIunref after start\fR and \fIref
1143 before stop\fR (but only if the watcher wasn't active before, or was active
1144 before, respectively. Note also that libev might stop watchers itself
1145 (e.g. non-repeating timers) in which case you have to \f(CW\*(C`ev_ref\*(C'\fR
1146 in the callback).
1147 .Sp
1148 Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_run\*(C'\fR
1149 running when nothing else is active.
1150 .Sp
1151 .Vb 4
1152 \& ev_signal exitsig;
1153 \& ev_signal_init (&exitsig, sig_cb, SIGINT);
1154 \& ev_signal_start (loop, &exitsig);
1155 \& ev_unref (loop);
1156 .Ve
1157 .Sp
1158 Example: For some weird reason, unregister the above signal handler again.
1159 .Sp
1160 .Vb 2
1161 \& ev_ref (loop);
1162 \& ev_signal_stop (loop, &exitsig);
1163 .Ve
1164 .IP "ev_set_io_collect_interval (loop, ev_tstamp interval)" 4
1165 .IX Item "ev_set_io_collect_interval (loop, ev_tstamp interval)"
1166 .PD 0
1167 .IP "ev_set_timeout_collect_interval (loop, ev_tstamp interval)" 4
1168 .IX Item "ev_set_timeout_collect_interval (loop, ev_tstamp interval)"
1169 .PD
1170 These advanced functions influence the time that libev will spend waiting
1171 for events. Both time intervals are by default \f(CW0\fR, meaning that libev
1172 will try to invoke timer/periodic callbacks and I/O callbacks with minimum
1173 latency.
1174 .Sp
1175 Setting these to a higher value (the \f(CW\*(C`interval\*(C'\fR \fImust\fR be >= \f(CW0\fR)
1176 allows libev to delay invocation of I/O and timer/periodic callbacks
1177 to increase efficiency of loop iterations (or to increase power-saving
1178 opportunities).
1179 .Sp
1180 The idea is that sometimes your program runs just fast enough to handle
1181 one (or very few) event(s) per loop iteration. While this makes the
1182 program responsive, it also wastes a lot of \s-1CPU\s0 time to poll for new
1183 events, especially with backends like \f(CW\*(C`select ()\*(C'\fR which have a high
1184 overhead for the actual polling but can deliver many events at once.
1185 .Sp
1186 By setting a higher \fIio collect interval\fR you allow libev to spend more
1187 time collecting I/O events, so you can handle more events per iteration,
1188 at the cost of increasing latency. Timeouts (both \f(CW\*(C`ev_periodic\*(C'\fR and
1189 \&\f(CW\*(C`ev_timer\*(C'\fR) will not be affected. Setting this to a non-null value will
1190 introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. The
1191 sleep time ensures that libev will not poll for I/O events more often then
1192 once per this interval, on average (as long as the host time resolution is
1193 good enough).
1194 .Sp
1195 Likewise, by setting a higher \fItimeout collect interval\fR you allow libev
1196 to spend more time collecting timeouts, at the expense of increased
1197 latency/jitter/inexactness (the watcher callback will be called
1198 later). \f(CW\*(C`ev_io\*(C'\fR watchers will not be affected. Setting this to a non-null
1199 value will not introduce any overhead in libev.
1200 .Sp
1201 Many (busy) programs can usually benefit by setting the I/O collect
1202 interval to a value near \f(CW0.1\fR or so, which is often enough for
1203 interactive servers (of course not for games), likewise for timeouts. It
1204 usually doesn't make much sense to set it to a lower value than \f(CW0.01\fR,
1205 as this approaches the timing granularity of most systems. Note that if
1206 you do transactions with the outside world and you can't increase the
1207 parallelity, then this setting will limit your transaction rate (if you
1208 need to poll once per transaction and the I/O collect interval is 0.01,
1209 then you can't do more than 100 transactions per second).
1210 .Sp
1211 Setting the \fItimeout collect interval\fR can improve the opportunity for
1212 saving power, as the program will \*(L"bundle\*(R" timer callback invocations that
1213 are \*(L"near\*(R" in time together, by delaying some, thus reducing the number of
1214 times the process sleeps and wakes up again. Another useful technique to
1215 reduce iterations/wake\-ups is to use \f(CW\*(C`ev_periodic\*(C'\fR watchers and make sure
1216 they fire on, say, one-second boundaries only.
1217 .Sp
1218 Example: we only need 0.1s timeout granularity, and we wish not to poll
1219 more often than 100 times per second:
1220 .Sp
1221 .Vb 2
1222 \& ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
1223 \& ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
1224 .Ve
1225 .IP "ev_invoke_pending (loop)" 4
1226 .IX Item "ev_invoke_pending (loop)"
1227 This call will simply invoke all pending watchers while resetting their
1228 pending state. Normally, \f(CW\*(C`ev_run\*(C'\fR does this automatically when required,
1229 but when overriding the invoke callback this call comes handy. This
1230 function can be invoked from a watcher \- this can be useful for example
1231 when you want to do some lengthy calculation and want to pass further
1232 event handling to another thread (you still have to make sure only one
1233 thread executes within \f(CW\*(C`ev_invoke_pending\*(C'\fR or \f(CW\*(C`ev_run\*(C'\fR of course).
1234 .IP "int ev_pending_count (loop)" 4
1235 .IX Item "int ev_pending_count (loop)"
1236 Returns the number of pending watchers \- zero indicates that no watchers
1237 are pending.
1238 .IP "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(\s-1EV_P\s0))" 4
1239 .IX Item "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))"
1240 This overrides the invoke pending functionality of the loop: Instead of
1241 invoking all pending watchers when there are any, \f(CW\*(C`ev_run\*(C'\fR will call
1242 this callback instead. This is useful, for example, when you want to
1243 invoke the actual watchers inside another context (another thread etc.).
1244 .Sp
1245 If you want to reset the callback, use \f(CW\*(C`ev_invoke_pending\*(C'\fR as new
1246 callback.
1247 .IP "ev_set_loop_release_cb (loop, void (*release)(\s-1EV_P\s0) throw (), void (*acquire)(\s-1EV_P\s0) throw ())" 4
1248 .IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())"
1249 Sometimes you want to share the same loop between multiple threads. This
1250 can be done relatively simply by putting mutex_lock/unlock calls around
1251 each call to a libev function.
1252 .Sp
1253 However, \f(CW\*(C`ev_run\*(C'\fR can run an indefinite time, so it is not feasible
1254 to wait for it to return. One way around this is to wake up the event
1255 loop via \f(CW\*(C`ev_break\*(C'\fR and \f(CW\*(C`ev_async_send\*(C'\fR, another way is to set these
1256 \&\fIrelease\fR and \fIacquire\fR callbacks on the loop.
1257 .Sp
1258 When set, then \f(CW\*(C`release\*(C'\fR will be called just before the thread is
1259 suspended waiting for new events, and \f(CW\*(C`acquire\*(C'\fR is called just
1260 afterwards.
1261 .Sp
1262 Ideally, \f(CW\*(C`release\*(C'\fR will just call your mutex_unlock function, and
1263 \&\f(CW\*(C`acquire\*(C'\fR will just call the mutex_lock function again.
1264 .Sp
1265 While event loop modifications are allowed between invocations of
1266 \&\f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR (that's their only purpose after all), no
1267 modifications done will affect the event loop, i.e. adding watchers will
1268 have no effect on the set of file descriptors being watched, or the time
1269 waited. Use an \f(CW\*(C`ev_async\*(C'\fR watcher to wake up \f(CW\*(C`ev_run\*(C'\fR when you want it
1270 to take note of any changes you made.
1271 .Sp
1272 In theory, threads executing \f(CW\*(C`ev_run\*(C'\fR will be async-cancel safe between
1273 invocations of \f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR.
1274 .Sp
1275 See also the locking example in the \f(CW\*(C`THREADS\*(C'\fR section later in this
1276 document.
1277 .IP "ev_set_userdata (loop, void *data)" 4
1278 .IX Item "ev_set_userdata (loop, void *data)"
1279 .PD 0
1280 .IP "void *ev_userdata (loop)" 4
1281 .IX Item "void *ev_userdata (loop)"
1282 .PD
1283 Set and retrieve a single \f(CW\*(C`void *\*(C'\fR associated with a loop. When
1284 \&\f(CW\*(C`ev_set_userdata\*(C'\fR has never been called, then \f(CW\*(C`ev_userdata\*(C'\fR returns
1285 \&\f(CW0\fR.
1286 .Sp
1287 These two functions can be used to associate arbitrary data with a loop,
1288 and are intended solely for the \f(CW\*(C`invoke_pending_cb\*(C'\fR, \f(CW\*(C`release\*(C'\fR and
1289 \&\f(CW\*(C`acquire\*(C'\fR callbacks described above, but of course can be (ab\-)used for
1290 any other purpose as well.
1291 .IP "ev_verify (loop)" 4
1292 .IX Item "ev_verify (loop)"
1293 This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR support has been
1294 compiled in, which is the default for non-minimal builds. It tries to go
1295 through all internal structures and checks them for validity. If anything
1296 is found to be inconsistent, it will print an error message to standard
1297 error and call \f(CW\*(C`abort ()\*(C'\fR.
1298 .Sp
1299 This can be used to catch bugs inside libev itself: under normal
1300 circumstances, this function will never abort as of course libev keeps its
1301 data structures consistent.
1304 In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the
1305 watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer
1306 watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.
1307 .PP
1308 A watcher is an opaque structure that you allocate and register to record
1309 your interest in some event. To make a concrete example, imagine you want
1310 to wait for \s-1STDIN\s0 to become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher
1311 for that:
1312 .PP
1313 .Vb 5
1314 \& static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
1315 \& {
1316 \& ev_io_stop (w);
1317 \& ev_break (loop, EVBREAK_ALL);
1318 \& }
1319 \&
1320 \& struct ev_loop *loop = ev_default_loop (0);
1321 \&
1322 \& ev_io stdin_watcher;
1323 \&
1324 \& ev_init (&stdin_watcher, my_cb);
1325 \& ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
1326 \& ev_io_start (loop, &stdin_watcher);
1327 \&
1328 \& ev_run (loop, 0);
1329 .Ve
1330 .PP
1331 As you can see, you are responsible for allocating the memory for your
1332 watcher structures (and it is \fIusually\fR a bad idea to do this on the
1333 stack).
1334 .PP
1335 Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR
1336 or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).
1337 .PP
1338 Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init (watcher
1339 *, callback)\*(C'\fR, which expects a callback to be provided. This callback is
1340 invoked each time the event occurs (or, in the case of I/O watchers, each
1341 time the event loop detects that the file descriptor given is readable
1342 and/or writable).
1343 .PP
1344 Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR
1345 macro to configure it, with arguments specific to the watcher type. There
1346 is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.
1347 .PP
1348 To make the watcher actually watch out for events, you have to start it
1349 with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher
1350 *)\*(C'\fR), and you can stop watching for events at any time by calling the
1351 corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.
1352 .PP
1353 As long as your watcher is active (has been started but not stopped) you
1354 must not touch the values stored in it except when explicitly documented
1355 otherwise. Most specifically you must never reinitialise it or call its
1356 \&\f(CW\*(C`ev_TYPE_set\*(C'\fR macro.
1357 .PP
1358 Each and every callback receives the event loop pointer as first, the
1359 registered watcher structure as second, and a bitset of received events as
1360 third argument.
1361 .PP
1362 The received events usually include a single bit per event type received
1363 (you can receive multiple events at the same time). The possible bit masks
1364 are:
1365 .ie n .IP """EV_READ""" 4
1366 .el .IP "\f(CWEV_READ\fR" 4
1367 .IX Item "EV_READ"
1368 .PD 0
1369 .ie n .IP """EV_WRITE""" 4
1370 .el .IP "\f(CWEV_WRITE\fR" 4
1371 .IX Item "EV_WRITE"
1372 .PD
1373 The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or
1374 writable.
1375 .ie n .IP """EV_TIMER""" 4
1376 .el .IP "\f(CWEV_TIMER\fR" 4
1377 .IX Item "EV_TIMER"
1378 The \f(CW\*(C`ev_timer\*(C'\fR watcher has timed out.
1379 .ie n .IP """EV_PERIODIC""" 4
1380 .el .IP "\f(CWEV_PERIODIC\fR" 4
1381 .IX Item "EV_PERIODIC"
1382 The \f(CW\*(C`ev_periodic\*(C'\fR watcher has timed out.
1383 .ie n .IP """EV_SIGNAL""" 4
1384 .el .IP "\f(CWEV_SIGNAL\fR" 4
1385 .IX Item "EV_SIGNAL"
1386 The signal specified in the \f(CW\*(C`ev_signal\*(C'\fR watcher has been received by a thread.
1387 .ie n .IP """EV_CHILD""" 4
1388 .el .IP "\f(CWEV_CHILD\fR" 4
1389 .IX Item "EV_CHILD"
1390 The pid specified in the \f(CW\*(C`ev_child\*(C'\fR watcher has received a status change.
1391 .ie n .IP """EV_STAT""" 4
1392 .el .IP "\f(CWEV_STAT\fR" 4
1393 .IX Item "EV_STAT"
1394 The path specified in the \f(CW\*(C`ev_stat\*(C'\fR watcher changed its attributes somehow.
1395 .ie n .IP """EV_IDLE""" 4
1396 .el .IP "\f(CWEV_IDLE\fR" 4
1397 .IX Item "EV_IDLE"
1398 The \f(CW\*(C`ev_idle\*(C'\fR watcher has determined that you have nothing better to do.
1399 .ie n .IP """EV_PREPARE""" 4
1400 .el .IP "\f(CWEV_PREPARE\fR" 4
1401 .IX Item "EV_PREPARE"
1402 .PD 0
1403 .ie n .IP """EV_CHECK""" 4
1404 .el .IP "\f(CWEV_CHECK\fR" 4
1405 .IX Item "EV_CHECK"
1406 .PD
1407 All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_run\*(C'\fR starts to
1408 gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are queued (not invoked)
1409 just after \f(CW\*(C`ev_run\*(C'\fR has gathered them, but before it queues any callbacks
1410 for any received events. That means \f(CW\*(C`ev_prepare\*(C'\fR watchers are the last
1411 watchers invoked before the event loop sleeps or polls for new events, and
1412 \&\f(CW\*(C`ev_check\*(C'\fR watchers will be invoked before any other watchers of the same
1413 or lower priority within an event loop iteration.
1414 .Sp
1415 Callbacks of both watcher types can start and stop as many watchers as
1416 they want, and all of them will be taken into account (for example, a
1417 \&\f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep \f(CW\*(C`ev_run\*(C'\fR from
1418 blocking).
1419 .ie n .IP """EV_EMBED""" 4
1420 .el .IP "\f(CWEV_EMBED\fR" 4
1421 .IX Item "EV_EMBED"
1422 The embedded event loop specified in the \f(CW\*(C`ev_embed\*(C'\fR watcher needs attention.
1423 .ie n .IP """EV_FORK""" 4
1424 .el .IP "\f(CWEV_FORK\fR" 4
1425 .IX Item "EV_FORK"
1426 The event loop has been resumed in the child process after fork (see
1427 \&\f(CW\*(C`ev_fork\*(C'\fR).
1428 .ie n .IP """EV_CLEANUP""" 4
1429 .el .IP "\f(CWEV_CLEANUP\fR" 4
1430 .IX Item "EV_CLEANUP"
1431 The event loop is about to be destroyed (see \f(CW\*(C`ev_cleanup\*(C'\fR).
1432 .ie n .IP """EV_ASYNC""" 4
1433 .el .IP "\f(CWEV_ASYNC\fR" 4
1434 .IX Item "EV_ASYNC"
1435 The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).
1436 .ie n .IP """EV_CUSTOM""" 4
1437 .el .IP "\f(CWEV_CUSTOM\fR" 4
1438 .IX Item "EV_CUSTOM"
1439 Not ever sent (or otherwise used) by libev itself, but can be freely used
1440 by libev users to signal watchers (e.g. via \f(CW\*(C`ev_feed_event\*(C'\fR).
1441 .ie n .IP """EV_ERROR""" 4
1442 .el .IP "\f(CWEV_ERROR\fR" 4
1443 .IX Item "EV_ERROR"
1444 An unspecified error has occurred, the watcher has been stopped. This might
1445 happen because the watcher could not be properly started because libev
1446 ran out of memory, a file descriptor was found to be closed or any other
1447 problem. Libev considers these application bugs.
1448 .Sp
1449 You best act on it by reporting the problem and somehow coping with the
1450 watcher being stopped. Note that well-written programs should not receive
1451 an error ever, so when your watcher receives it, this usually indicates a
1452 bug in your program.
1453 .Sp
1454 Libev will usually signal a few \*(L"dummy\*(R" events together with an error, for
1455 example it might indicate that a fd is readable or writable, and if your
1456 callbacks is well-written it can just attempt the operation and cope with
1457 the error from \fBread()\fR or \fBwrite()\fR. This will not work in multi-threaded
1458 programs, though, as the fd could already be closed and reused for another
1459 thing, so beware.
1462 .ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4
1463 .el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4
1464 .IX Item "ev_init (ev_TYPE *watcher, callback)"
1465 This macro initialises the generic portion of a watcher. The contents
1466 of the watcher object can be arbitrary (so \f(CW\*(C`malloc\*(C'\fR will do). Only
1467 the generic parts of the watcher are initialised, you \fIneed\fR to call
1468 the type-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR macro afterwards to initialise the
1469 type-specific parts. For each type there is also a \f(CW\*(C`ev_TYPE_init\*(C'\fR macro
1470 which rolls both calls into one.
1471 .Sp
1472 You can reinitialise a watcher at any time as long as it has been stopped
1473 (or never started) and there are no pending events outstanding.
1474 .Sp
1475 The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
1476 int revents)\*(C'\fR.
1477 .Sp
1478 Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
1479 .Sp
1480 .Vb 3
1481 \& ev_io w;
1482 \& ev_init (&w, my_cb);
1483 \& ev_io_set (&w, STDIN_FILENO, EV_READ);
1484 .Ve
1485 .ie n .IP """ev_TYPE_set"" (ev_TYPE *watcher, [args])" 4
1486 .el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *watcher, [args])" 4
1487 .IX Item "ev_TYPE_set (ev_TYPE *watcher, [args])"
1488 This macro initialises the type-specific parts of a watcher. You need to
1489 call \f(CW\*(C`ev_init\*(C'\fR at least once before you call this macro, but you can
1490 call \f(CW\*(C`ev_TYPE_set\*(C'\fR any number of times. You must not, however, call this
1491 macro on a watcher that is active (it can be pending, however, which is a
1492 difference to the \f(CW\*(C`ev_init\*(C'\fR macro).
1493 .Sp
1494 Although some watcher types do not have type-specific arguments
1495 (e.g. \f(CW\*(C`ev_prepare\*(C'\fR) you still need to call its \f(CW\*(C`set\*(C'\fR macro.
1496 .Sp
1497 See \f(CW\*(C`ev_init\*(C'\fR, above, for an example.
1498 .ie n .IP """ev_TYPE_init"" (ev_TYPE *watcher, callback, [args])" 4
1499 .el .IP "\f(CWev_TYPE_init\fR (ev_TYPE *watcher, callback, [args])" 4
1500 .IX Item "ev_TYPE_init (ev_TYPE *watcher, callback, [args])"
1501 This convenience macro rolls both \f(CW\*(C`ev_init\*(C'\fR and \f(CW\*(C`ev_TYPE_set\*(C'\fR macro
1502 calls into a single call. This is the most convenient method to initialise
1503 a watcher. The same limitations apply, of course.
1504 .Sp
1505 Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.
1506 .Sp
1507 .Vb 1
1508 \& ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
1509 .Ve
1510 .ie n .IP """ev_TYPE_start"" (loop, ev_TYPE *watcher)" 4
1511 .el .IP "\f(CWev_TYPE_start\fR (loop, ev_TYPE *watcher)" 4
1512 .IX Item "ev_TYPE_start (loop, ev_TYPE *watcher)"
1513 Starts (activates) the given watcher. Only active watchers will receive
1514 events. If the watcher is already active nothing will happen.
1515 .Sp
1516 Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this
1517 whole section.
1518 .Sp
1519 .Vb 1
1520 \& ev_io_start (EV_DEFAULT_UC, &w);
1521 .Ve
1522 .ie n .IP """ev_TYPE_stop"" (loop, ev_TYPE *watcher)" 4
1523 .el .IP "\f(CWev_TYPE_stop\fR (loop, ev_TYPE *watcher)" 4
1524 .IX Item "ev_TYPE_stop (loop, ev_TYPE *watcher)"
1525 Stops the given watcher if active, and clears the pending status (whether
1526 the watcher was active or not).
1527 .Sp
1528 It is possible that stopped watchers are pending \- for example,
1529 non-repeating timers are being stopped when they become pending \- but
1530 calling \f(CW\*(C`ev_TYPE_stop\*(C'\fR ensures that the watcher is neither active nor
1531 pending. If you want to free or reuse the memory used by the watcher it is
1532 therefore a good idea to always call its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function.
1533 .IP "bool ev_is_active (ev_TYPE *watcher)" 4
1534 .IX Item "bool ev_is_active (ev_TYPE *watcher)"
1535 Returns a true value iff the watcher is active (i.e. it has been started
1536 and not yet been stopped). As long as a watcher is active you must not modify
1537 it.
1538 .IP "bool ev_is_pending (ev_TYPE *watcher)" 4
1539 .IX Item "bool ev_is_pending (ev_TYPE *watcher)"
1540 Returns a true value iff the watcher is pending, (i.e. it has outstanding
1541 events but its callback has not yet been invoked). As long as a watcher
1542 is pending (but not active) you must not call an init function on it (but
1543 \&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must
1544 make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR
1545 it).
1546 .IP "callback ev_cb (ev_TYPE *watcher)" 4
1547 .IX Item "callback ev_cb (ev_TYPE *watcher)"
1548 Returns the callback currently set on the watcher.
1549 .IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
1550 .IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
1551 Change the callback. You can change the callback at virtually any time
1552 (modulo threads).
1553 .IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
1554 .IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
1555 .PD 0
1556 .IP "int ev_priority (ev_TYPE *watcher)" 4
1557 .IX Item "int ev_priority (ev_TYPE *watcher)"
1558 .PD
1559 Set and query the priority of the watcher. The priority is a small
1560 integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR
1561 (default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked
1562 before watchers with lower priority, but priority will not keep watchers
1563 from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).
1564 .Sp
1565 If you need to suppress invocation when higher priority events are pending
1566 you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.
1567 .Sp
1568 You \fImust not\fR change the priority of a watcher as long as it is active or
1569 pending.
1570 .Sp
1571 Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
1572 fine, as long as you do not mind that the priority value you query might
1573 or might not have been clamped to the valid range.
1574 .Sp
1575 The default priority used by watchers when no priority has been set is
1576 always \f(CW0\fR, which is supposed to not be too high and not be too low :).
1577 .Sp
1578 See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
1579 priorities.
1580 .IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
1581 .IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
1582 Invoke the \f(CW\*(C`watcher\*(C'\fR with the given \f(CW\*(C`loop\*(C'\fR and \f(CW\*(C`revents\*(C'\fR. Neither
1583 \&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback
1584 can deal with that fact, as both are simply passed through to the
1585 callback.
1586 .IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4
1587 .IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"
1588 If the watcher is pending, this function clears its pending status and
1589 returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the
1590 watcher isn't pending it does nothing and returns \f(CW0\fR.
1591 .Sp
1592 Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its
1593 callback to be invoked, which can be accomplished with this function.
1594 .IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
1595 .IX Item "ev_feed_event (loop, ev_TYPE *watcher, int revents)"
1596 Feeds the given event set into the event loop, as if the specified event
1597 had happened for the specified watcher (which must be a pointer to an
1598 initialised but not necessarily started event watcher). Obviously you must
1599 not free the watcher as long as it has pending events.
1600 .Sp
1601 Stopping the watcher, letting libev invoke it, or calling
1602 \&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
1603 not started in the first place.
1604 .Sp
1605 See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
1606 functions that do not need a watcher.
1607 .PP
1608 See also the \*(L"\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\*(R"\s0 and \*(L"\s-1BUILDING YOUR
1609 OWN COMPOSITE WATCHERS\*(R"\s0 idioms.
1610 .SS "\s-1WATCHER STATES\s0"
1611 .IX Subsection "WATCHER STATES"
1612 There are various watcher states mentioned throughout this manual \-
1613 active, pending and so on. In this section these states and the rules to
1614 transition between them will be described in more detail \- and while these
1615 rules might look complicated, they usually do \*(L"the right thing\*(R".
1616 .IP "initialised" 4
1617 .IX Item "initialised"
1618 Before a watcher can be registered with the event loop it has to be
1619 initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
1620 \&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR function.
1621 .Sp
1622 In this state it is simply some block of memory that is suitable for
1623 use in an event loop. It can be moved around, freed, reused etc. at
1624 will \- as long as you either keep the memory contents intact, or call
1625 \&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
1626 .IP "started/running/active" 4
1627 .IX Item "started/running/active"
1628 Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR it becomes
1629 property of the event loop, and is actively waiting for events. While in
1630 this state it cannot be accessed (except in a few documented ways), moved,
1631 freed or anything else \- the only legal thing is to keep a pointer to it,
1632 and call libev functions on it that are documented to work on active watchers.
1633 .IP "pending" 4
1634 .IX Item "pending"
1635 If a watcher is active and libev determines that an event it is interested
1636 in has occurred (such as a timer expiring), it will become pending. It will
1637 stay in this pending state until either it is stopped or its callback is
1638 about to be invoked, so it is not normally pending inside the watcher
1639 callback.
1640 .Sp
1641 The watcher might or might not be active while it is pending (for example,
1642 an expired non-repeating timer can be pending but no longer active). If it
1643 is stopped, it can be freely accessed (e.g. by calling \f(CW\*(C`ev_TYPE_set\*(C'\fR),
1644 but it is still property of the event loop at this time, so cannot be
1645 moved, freed or reused. And if it is active the rules described in the
1646 previous item still apply.
1647 .Sp
1648 It is also possible to feed an event on a watcher that is not active (e.g.
1649 via \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
1650 active.
1651 .IP "stopped" 4
1652 .IX Item "stopped"
1653 A watcher can be stopped implicitly by libev (in which case it might still
1654 be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function. The
1655 latter will clear any pending state the watcher might be in, regardless
1656 of whether it was active or not, so stopping a watcher explicitly before
1657 freeing it is often a good idea.
1658 .Sp
1659 While stopped (and not pending) the watcher is essentially in the
1660 initialised state, that is, it can be reused, moved, modified in any way
1661 you wish (but when you trash the memory block, you need to \f(CW\*(C`ev_TYPE_init\*(C'\fR
1662 it again).
1665 Many event loops support \fIwatcher priorities\fR, which are usually small
1666 integers that influence the ordering of event callback invocation
1667 between watchers in some way, all else being equal.
1668 .PP
1669 In libev, watcher priorities can be set using \f(CW\*(C`ev_set_priority\*(C'\fR. See its
1670 description for the more technical details such as the actual priority
1671 range.
1672 .PP
1673 There are two common ways how these these priorities are being interpreted
1674 by event loops:
1675 .PP
1676 In the more common lock-out model, higher priorities \*(L"lock out\*(R" invocation
1677 of lower priority watchers, which means as long as higher priority
1678 watchers receive events, lower priority watchers are not being invoked.
1679 .PP
1680 The less common only-for-ordering model uses priorities solely to order
1681 callback invocation within a single event loop iteration: Higher priority
1682 watchers are invoked before lower priority ones, but they all get invoked
1683 before polling for new events.
1684 .PP
1685 Libev uses the second (only-for-ordering) model for all its watchers
1686 except for idle watchers (which use the lock-out model).
1687 .PP
1688 The rationale behind this is that implementing the lock-out model for
1689 watchers is not well supported by most kernel interfaces, and most event
1690 libraries will just poll for the same events again and again as long as
1691 their callbacks have not been executed, which is very inefficient in the
1692 common case of one high-priority watcher locking out a mass of lower
1693 priority ones.
1694 .PP
1695 Static (ordering) priorities are most useful when you have two or more
1696 watchers handling the same resource: a typical usage example is having an
1697 \&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR to handle
1698 timeouts. Under load, data might be received while the program handles
1699 other jobs, but since timers normally get invoked first, the timeout
1700 handler will be executed before checking for data. In that case, giving
1701 the timer a lower priority than the I/O watcher ensures that I/O will be
1702 handled first even under adverse conditions (which is usually, but not
1703 always, what you want).
1704 .PP
1705 Since idle watchers use the \*(L"lock-out\*(R" model, meaning that idle watchers
1706 will only be executed when no same or higher priority watchers have
1707 received events, they can be used to implement the \*(L"lock-out\*(R" model when
1708 required.
1709 .PP
1710 For example, to emulate how many other event libraries handle priorities,
1711 you can associate an \f(CW\*(C`ev_idle\*(C'\fR watcher to each such watcher, and in
1712 the normal watcher callback, you just start the idle watcher. The real
1713 processing is done in the idle watcher callback. This causes libev to
1714 continuously poll and process kernel event data for the watcher, but when
1715 the lock-out case is known to be rare (which in turn is rare :), this is
1716 workable.
1717 .PP
1718 Usually, however, the lock-out model implemented that way will perform
1719 miserably under the type of load it was designed to handle. In that case,
1720 it might be preferable to stop the real watcher before starting the
1721 idle watcher, so the kernel will not have to process the event in case
1722 the actual processing will be delayed for considerable time.
1723 .PP
1724 Here is an example of an I/O watcher that should run at a strictly lower
1725 priority than the default, and which should only process data when no
1726 other events are pending:
1727 .PP
1728 .Vb 2
1729 \& ev_idle idle; // actual processing watcher
1730 \& ev_io io; // actual event watcher
1731 \&
1732 \& static void
1733 \& io_cb (EV_P_ ev_io *w, int revents)
1734 \& {
1735 \& // stop the I/O watcher, we received the event, but
1736 \& // are not yet ready to handle it.
1737 \& ev_io_stop (EV_A_ w);
1738 \&
1739 \& // start the idle watcher to handle the actual event.
1740 \& // it will not be executed as long as other watchers
1741 \& // with the default priority are receiving events.
1742 \& ev_idle_start (EV_A_ &idle);
1743 \& }
1744 \&
1745 \& static void
1746 \& idle_cb (EV_P_ ev_idle *w, int revents)
1747 \& {
1748 \& // actual processing
1749 \& read (STDIN_FILENO, ...);
1750 \&
1751 \& // have to start the I/O watcher again, as
1752 \& // we have handled the event
1753 \& ev_io_start (EV_P_ &io);
1754 \& }
1755 \&
1756 \& // initialisation
1757 \& ev_idle_init (&idle, idle_cb);
1758 \& ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1759 \& ev_io_start (EV_DEFAULT_ &io);
1760 .Ve
1761 .PP
1762 In the \*(L"real\*(R" world, it might also be beneficial to start a timer, so that
1763 low-priority connections can not be locked out forever under load. This
1764 enables your program to keep a lower latency for important connections
1765 during short periods of high load, while not completely locking out less
1766 important ones.
1768 .IX Header "WATCHER TYPES"
1769 This section describes each watcher in detail, but will not repeat
1770 information given in the last section. Any initialisation/set macros,
1771 functions and members specific to the watcher type are explained.
1772 .PP
1773 Most members are additionally marked with either \fI[read\-only]\fR, meaning
1774 that, while the watcher is active, you can look at the member and expect
1775 some sensible content, but you must not modify it (you can modify it while
1776 the watcher is stopped to your hearts content), or \fI[read\-write]\fR, which
1777 means you can expect it to have some sensible content while the watcher is
1778 active, but you can also modify it (within the same thread as the event
1779 loop, i.e. without creating data races). Modifying it may not do something
1780 sensible or take immediate effect (or do anything at all), but libev will
1781 not crash or malfunction in any way.
1782 .PP
1783 In any case, the documentation for each member will explain what the
1784 effects are, and if there are any additional access restrictions.
1785 .ie n .SS """ev_io"" \- is this file descriptor readable or writable?"
1786 .el .SS "\f(CWev_io\fP \- is this file descriptor readable or writable?"
1787 .IX Subsection "ev_io - is this file descriptor readable or writable?"
1788 I/O watchers check whether a file descriptor is readable or writable
1789 in each iteration of the event loop, or, more precisely, when reading
1790 would not block the process and writing would at least be able to write
1791 some data. This behaviour is called level-triggering because you keep
1792 receiving events as long as the condition persists. Remember you can stop
1793 the watcher if you don't want to act on the event and neither want to
1794 receive future events.
1795 .PP
1796 In general you can register as many read and/or write event watchers per
1797 fd as you want (as long as you don't confuse yourself). Setting all file
1798 descriptors to non-blocking mode is also usually a good idea (but not
1799 required if you know what you are doing).
1800 .PP
1801 Another thing you have to watch out for is that it is quite easy to
1802 receive \*(L"spurious\*(R" readiness notifications, that is, your callback might
1803 be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block
1804 because there is no data. It is very easy to get into this situation even
1805 with a relatively standard program structure. Thus it is best to always
1806 use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning \f(CW\*(C`EAGAIN\*(C'\fR is far
1807 preferable to a program hanging until some data arrives.
1808 .PP
1809 If you cannot run the fd in non-blocking mode (for example you should
1810 not play around with an Xlib connection), then you have to separately
1811 re-test whether a file descriptor is really ready with a known-to-be good
1812 interface such as poll (fortunately in the case of Xlib, it already does
1813 this on its own, so its quite safe to use). Some people additionally
1814 use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
1815 indefinitely.
1816 .PP
1817 But really, best use non-blocking mode.
1818 .PP
1819 \fIThe special problem of disappearing file descriptors\fR
1820 .IX Subsection "The special problem of disappearing file descriptors"
1821 .PP
1822 Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
1823 a file descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other
1824 means, such as \f(CW\*(C`dup2\*(C'\fR). The reason is that you register interest in some
1825 file descriptor, but when it goes away, the operating system will silently
1826 drop this interest. If another file descriptor with the same number then
1827 is registered with libev, there is no efficient way to see that this is,
1828 in fact, a different file descriptor.
1829 .PP
1830 To avoid having to explicitly tell libev about such cases, libev follows
1831 the following policy: Each time \f(CW\*(C`ev_io_set\*(C'\fR is being called, libev
1832 will assume that this is potentially a new file descriptor, otherwise
1833 it is assumed that the file descriptor stays the same. That means that
1834 you \fIhave\fR to call \f(CW\*(C`ev_io_set\*(C'\fR (or \f(CW\*(C`ev_io_init\*(C'\fR) when you change the
1835 descriptor even if the file descriptor number itself did not change.
1836 .PP
1837 This is how one would do it normally anyway, the important point is that
1838 the libev application should not optimise around libev but should leave
1839 optimisations to libev.
1840 .PP
1841 \fIThe special problem of dup'ed file descriptors\fR
1842 .IX Subsection "The special problem of dup'ed file descriptors"
1843 .PP
1844 Some backends (e.g. epoll), cannot register events for file descriptors,
1845 but only events for the underlying file descriptions. That means when you
1846 have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register
1847 events for them, only one file descriptor might actually receive events.
1848 .PP
1849 There is no workaround possible except not registering events
1850 for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
1851 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1852 .PP
1853 \fIThe special problem of files\fR
1854 .IX Subsection "The special problem of files"
1855 .PP
1856 Many people try to use \f(CW\*(C`select\*(C'\fR (or libev) on file descriptors
1857 representing files, and expect it to become ready when their program
1858 doesn't block on disk accesses (which can take a long time on their own).
1859 .PP
1860 However, this cannot ever work in the \*(L"expected\*(R" way \- you get a readiness
1861 notification as soon as the kernel knows whether and how much data is
1862 there, and in the case of open files, that's always the case, so you
1863 always get a readiness notification instantly, and your read (or possibly
1864 write) will still block on the disk I/O.
1865 .PP
1866 Another way to view it is that in the case of sockets, pipes, character
1867 devices and so on, there is another party (the sender) that delivers data
1868 on its own, but in the case of files, there is no such thing: the disk
1869 will not send data on its own, simply because it doesn't know what you
1870 wish to read \- you would first have to request some data.
1871 .PP
1872 Since files are typically not-so-well supported by advanced notification
1873 mechanism, libev tries hard to emulate \s-1POSIX\s0 behaviour with respect
1874 to files, even though you should not use it. The reason for this is
1875 convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
1876 usually a tty, often a pipe, but also sometimes files or special devices
1877 (for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
1878 \&\fI/dev/urandom\fR), and even though the file might better be served with
1879 asynchronous I/O instead of with non-blocking I/O, it is still useful when
1880 it \*(L"just works\*(R" instead of freezing.
1881 .PP
1882 So avoid file descriptors pointing to files when you know it (e.g. use
1883 libeio), but use them when it is convenient, e.g. for \s-1STDIN/STDOUT,\s0 or
1884 when you rarely read from a file instead of from a socket, and want to
1885 reuse the same code path.
1886 .PP
1887 \fIThe special problem of fork\fR
1888 .IX Subsection "The special problem of fork"
1889 .PP
1890 Some backends (epoll, kqueue, linuxaio, iouring) do not support \f(CW\*(C`fork ()\*(C'\fR
1891 at all or exhibit useless behaviour. Libev fully supports fork, but needs
1892 to be told about it in the child if you want to continue to use it in the
1893 child.
1894 .PP
1895 To support fork in your child processes, you have to call \f(CW\*(C`ev_loop_fork
1896 ()\*(C'\fR after a fork in the child, enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to
1897 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1898 .PP
1899 \fIThe special problem of \s-1SIGPIPE\s0\fR
1900 .IX Subsection "The special problem of SIGPIPE"
1901 .PP
1902 While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:
1903 when writing to a pipe whose other end has been closed, your program gets
1904 sent a \s-1SIGPIPE,\s0 which, by default, aborts your program. For most programs
1905 this is sensible behaviour, for daemons, this is usually undesirable.
1906 .PP
1907 So when you encounter spurious, unexplained daemon exits, make sure you
1908 ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon
1909 somewhere, as that would have given you a big clue).
1910 .PP
1911 \fIThe special problem of \f(BIaccept()\fIing when you can't\fR
1912 .IX Subsection "The special problem of accept()ing when you can't"
1913 .PP
1914 Many implementations of the \s-1POSIX\s0 \f(CW\*(C`accept\*(C'\fR function (for example,
1915 found in post\-2004 Linux) have the peculiar behaviour of not removing a
1916 connection from the pending queue in all error cases.
1917 .PP
1918 For example, larger servers often run out of file descriptors (because
1919 of resource limits), causing \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR but not
1920 rejecting the connection, leading to libev signalling readiness on
1921 the next iteration again (the connection still exists after all), and
1922 typically causing the program to loop at 100% \s-1CPU\s0 usage.
1923 .PP
1924 Unfortunately, the set of errors that cause this issue differs between
1925 operating systems, there is usually little the app can do to remedy the
1926 situation, and no known thread-safe method of removing the connection to
1927 cope with overload is known (to me).
1928 .PP
1929 One of the easiest ways to handle this situation is to just ignore it
1930 \&\- when the program encounters an overload, it will just loop until the
1931 situation is over. While this is a form of busy waiting, no \s-1OS\s0 offers an
1932 event-based way to handle this situation, so it's the best one can do.
1933 .PP
1934 A better way to handle the situation is to log any errors other than
1935 \&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, making sure not to flood the log with such
1936 messages, and continue as usual, which at least gives the user an idea of
1937 what could be wrong (\*(L"raise the ulimit!\*(R"). For extra points one could stop
1938 the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
1939 usage.
1940 .PP
1941 If your program is single-threaded, then you could also keep a dummy file
1942 descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
1943 when you run into \f(CW\*(C`ENFILE\*(C'\fR or \f(CW\*(C`EMFILE\*(C'\fR, close it, run \f(CW\*(C`accept\*(C'\fR,
1944 close that fd, and create a new dummy fd. This will gracefully refuse
1945 clients under typical overload conditions.
1946 .PP
1947 The last way to handle it is to simply log the error and \f(CW\*(C`exit\*(C'\fR, as
1948 is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
1949 opportunity for a DoS attack.
1950 .PP
1951 \fIWatcher-Specific Functions\fR
1952 .IX Subsection "Watcher-Specific Functions"
1953 .IP "ev_io_init (ev_io *, callback, int fd, int events)" 4
1954 .IX Item "ev_io_init (ev_io *, callback, int fd, int events)"
1955 .PD 0
1956 .IP "ev_io_set (ev_io *, int fd, int events)" 4
1957 .IX Item "ev_io_set (ev_io *, int fd, int events)"
1958 .PD
1959 Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to
1960 receive events for and \f(CW\*(C`events\*(C'\fR is either \f(CW\*(C`EV_READ\*(C'\fR, \f(CW\*(C`EV_WRITE\*(C'\fR, both
1961 \&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR or \f(CW0\fR, to express the desire to receive the given
1962 events.
1963 .Sp
1964 Note that setting the \f(CW\*(C`events\*(C'\fR to \f(CW0\fR and starting the watcher is
1965 supported, but not specially optimized \- if your program sometimes happens
1966 to generate this combination this is fine, but if it is easy to avoid
1967 starting an io watcher watching for no events you should do so.
1968 .IP "ev_io_modify (ev_io *, int events)" 4
1969 .IX Item "ev_io_modify (ev_io *, int events)"
1970 Similar to \f(CW\*(C`ev_io_set\*(C'\fR, but only changes the requested events. Using this
1971 might be faster with some backends, as libev can assume that the \f(CW\*(C`fd\*(C'\fR
1972 still refers to the same underlying file description, something it cannot
1973 do when using \f(CW\*(C`ev_io_set\*(C'\fR.
1974 .IP "int fd [no\-modify]" 4
1975 .IX Item "int fd [no-modify]"
1976 The file descriptor being watched. While it can be read at any time, you
1977 must not modify this member even when the watcher is stopped \- always use
1978 \&\f(CW\*(C`ev_io_set\*(C'\fR for that.
1979 .IP "int events [no\-modify]" 4
1980 .IX Item "int events [no-modify]"
1981 The set of events the fd is being watched for, among other flags. Remember
1982 that this is a bit set \- to test for \f(CW\*(C`EV_READ\*(C'\fR, use \f(CW\*(C`w\->events &
1983 EV_READ\*(C'\fR, and similarly for \f(CW\*(C`EV_WRITE\*(C'\fR.
1984 .Sp
1985 As with \f(CW\*(C`fd\*(C'\fR, you must not modify this member even when the watcher is
1986 stopped, always use \f(CW\*(C`ev_io_set\*(C'\fR or \f(CW\*(C`ev_io_modify\*(C'\fR for that.
1987 .PP
1988 \fIExamples\fR
1989 .IX Subsection "Examples"
1990 .PP
1991 Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well
1992 readable, but only once. Since it is likely line-buffered, you could
1993 attempt to read a whole line in the callback.
1994 .PP
1995 .Vb 6
1996 \& static void
1997 \& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1998 \& {
1999 \& ev_io_stop (loop, w);
2000 \& .. read from stdin here (or from w\->fd) and handle any I/O errors
2001 \& }
2002 \&
2003 \& ...
2004 \& struct ev_loop *loop = ev_default_init (0);
2005 \& ev_io stdin_readable;
2006 \& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
2007 \& ev_io_start (loop, &stdin_readable);
2008 \& ev_run (loop, 0);
2009 .Ve
2010 .ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
2011 .el .SS "\f(CWev_timer\fP \- relative and optionally repeating timeouts"
2012 .IX Subsection "ev_timer - relative and optionally repeating timeouts"
2013 Timer watchers are simple relative timers that generate an event after a
2014 given time, and optionally repeating in regular intervals after that.
2015 .PP
2016 The timers are based on real time, that is, if you register an event that
2017 times out after an hour and you reset your system clock to January last
2018 year, it will still time out after (roughly) one hour. \*(L"Roughly\*(R" because
2019 detecting time jumps is hard, and some inaccuracies are unavoidable (the
2020 monotonic clock option helps a lot here).
2021 .PP
2022 The callback is guaranteed to be invoked only \fIafter\fR its timeout has
2023 passed (not \fIat\fR, so on systems with very low-resolution clocks this
2024 might introduce a small delay, see \*(L"the special problem of being too
2025 early\*(R", below). If multiple timers become ready during the same loop
2026 iteration then the ones with earlier time-out values are invoked before
2027 ones of the same priority with later time-out values (but this is no
2028 longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2029 .PP
2030 \fIBe smart about timeouts\fR
2031 .IX Subsection "Be smart about timeouts"
2032 .PP
2033 Many real-world problems involve some kind of timeout, usually for error
2034 recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
2035 you want to raise some error after a while.
2036 .PP
2037 What follows are some ways to handle this problem, from obvious and
2038 inefficient to smart and efficient.
2039 .PP
2040 In the following, a 60 second activity timeout is assumed \- a timeout that
2041 gets reset to 60 seconds each time there is activity (e.g. each time some
2042 data or other life sign was received).
2043 .IP "1. Use a timer and stop, reinitialise and start it on activity." 4
2044 .IX Item "1. Use a timer and stop, reinitialise and start it on activity."
2045 This is the most obvious, but not the most simple way: In the beginning,
2046 start the watcher:
2047 .Sp
2048 .Vb 2
2049 \& ev_timer_init (timer, callback, 60., 0.);
2050 \& ev_timer_start (loop, timer);
2051 .Ve
2052 .Sp
2053 Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
2054 and start it again:
2055 .Sp
2056 .Vb 3
2057 \& ev_timer_stop (loop, timer);
2058 \& ev_timer_set (timer, 60., 0.);
2059 \& ev_timer_start (loop, timer);
2060 .Ve
2061 .Sp
2062 This is relatively simple to implement, but means that each time there is
2063 some activity, libev will first have to remove the timer from its internal
2064 data structure and then add it again. Libev tries to be fast, but it's
2065 still not a constant-time operation.
2066 .ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
2067 .el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
2068 .IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
2069 This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
2070 \&\f(CW\*(C`ev_timer_start\*(C'\fR.
2071 .Sp
2072 To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
2073 of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
2074 successfully read or write some data. If you go into an idle state where
2075 you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
2076 the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
2077 .Sp
2078 That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
2079 \&\f(CW\*(C`after\*(C'\fR argument to \f(CW\*(C`ev_timer_set\*(C'\fR, and only ever use the \f(CW\*(C`repeat\*(C'\fR
2080 member and \f(CW\*(C`ev_timer_again\*(C'\fR.
2081 .Sp
2082 At start:
2083 .Sp
2084 .Vb 3
2085 \& ev_init (timer, callback);
2086 \& timer\->repeat = 60.;
2087 \& ev_timer_again (loop, timer);
2088 .Ve
2089 .Sp
2090 Each time there is some activity:
2091 .Sp
2092 .Vb 1
2093 \& ev_timer_again (loop, timer);
2094 .Ve
2095 .Sp
2096 It is even possible to change the time-out on the fly, regardless of
2097 whether the watcher is active or not:
2098 .Sp
2099 .Vb 2
2100 \& timer\->repeat = 30.;
2101 \& ev_timer_again (loop, timer);
2102 .Ve
2103 .Sp
2104 This is slightly more efficient then stopping/starting the timer each time
2105 you want to modify its timeout value, as libev does not have to completely
2106 remove and re-insert the timer from/into its internal data structure.
2107 .Sp
2108 It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
2109 .IP "3. Let the timer time out, but then re-arm it as required." 4
2110 .IX Item "3. Let the timer time out, but then re-arm it as required."
2111 This method is more tricky, but usually most efficient: Most timeouts are
2112 relatively long compared to the intervals between other activity \- in
2113 our example, within 60 seconds, there are usually many I/O events with
2114 associated activity resets.
2115 .Sp
2116 In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
2117 but remember the time of last activity, and check for a real timeout only
2118 within the callback:
2119 .Sp
2120 .Vb 3
2121 \& ev_tstamp timeout = 60.;
2122 \& ev_tstamp last_activity; // time of last activity
2123 \& ev_timer timer;
2124 \&
2125 \& static void
2126 \& callback (EV_P_ ev_timer *w, int revents)
2127 \& {
2128 \& // calculate when the timeout would happen
2129 \& ev_tstamp after = last_activity \- ev_now (EV_A) + timeout;
2130 \&
2131 \& // if negative, it means we the timeout already occurred
2132 \& if (after < 0.)
2133 \& {
2134 \& // timeout occurred, take action
2135 \& }
2136 \& else
2137 \& {
2138 \& // callback was invoked, but there was some recent
2139 \& // activity. simply restart the timer to time out
2140 \& // after "after" seconds, which is the earliest time
2141 \& // the timeout can occur.
2142 \& ev_timer_set (w, after, 0.);
2143 \& ev_timer_start (EV_A_ w);
2144 \& }
2145 \& }
2146 .Ve
2147 .Sp
2148 To summarise the callback: first calculate in how many seconds the
2149 timeout will occur (by calculating the absolute time when it would occur,
2150 \&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
2151 (EV_A)\*(C'\fR from that).
2152 .Sp
2153 If this value is negative, then we are already past the timeout, i.e. we
2154 timed out, and need to do whatever is needed in this case.
2155 .Sp
2156 Otherwise, we now the earliest time at which the timeout would trigger,
2157 and simply start the timer with this timeout value.
2158 .Sp
2159 In other words, each time the callback is invoked it will check whether
2160 the timeout occurred. If not, it will simply reschedule itself to check
2161 again at the earliest time it could time out. Rinse. Repeat.
2162 .Sp
2163 This scheme causes more callback invocations (about one every 60 seconds
2164 minus half the average time between activity), but virtually no calls to
2165 libev to change the timeout.
2166 .Sp
2167 To start the machinery, simply initialise the watcher and set
2168 \&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
2169 now), then call the callback, which will \*(L"do the right thing\*(R" and start
2170 the timer:
2171 .Sp
2172 .Vb 3
2173 \& last_activity = ev_now (EV_A);
2174 \& ev_init (&timer, callback);
2175 \& callback (EV_A_ &timer, 0);
2176 .Ve
2177 .Sp
2178 When there is some activity, simply store the current time in
2179 \&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
2180 .Sp
2181 .Vb 2
2182 \& if (activity detected)
2183 \& last_activity = ev_now (EV_A);
2184 .Ve
2185 .Sp
2186 When your timeout value changes, then the timeout can be changed by simply
2187 providing a new value, stopping the timer and calling the callback, which
2188 will again do the right thing (for example, time out immediately :).
2189 .Sp
2190 .Vb 3
2191 \& timeout = new_value;
2192 \& ev_timer_stop (EV_A_ &timer);
2193 \& callback (EV_A_ &timer, 0);
2194 .Ve
2195 .Sp
2196 This technique is slightly more complex, but in most cases where the
2197 time-out is unlikely to be triggered, much more efficient.
2198 .IP "4. Wee, just use a double-linked list for your timeouts." 4
2199 .IX Item "4. Wee, just use a double-linked list for your timeouts."
2200 If there is not one request, but many thousands (millions...), all
2201 employing some kind of timeout with the same timeout value, then one can
2202 do even better:
2203 .Sp
2204 When starting the timeout, calculate the timeout value and put the timeout
2205 at the \fIend\fR of the list.
2206 .Sp
2207 Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
2208 the list is expected to fire (for example, using the technique #3).
2209 .Sp
2210 When there is some activity, remove the timer from the list, recalculate
2211 the timeout, append it to the end of the list again, and make sure to
2212 update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
2213 .Sp
2214 This way, one can manage an unlimited number of timeouts in O(1) time for
2215 starting, stopping and updating the timers, at the expense of a major
2216 complication, and having to use a constant timeout. The constant timeout
2217 ensures that the list stays sorted.
2218 .PP
2219 So which method the best?
2220 .PP
2221 Method #2 is a simple no-brain-required solution that is adequate in most
2222 situations. Method #3 requires a bit more thinking, but handles many cases
2223 better, and isn't very complicated either. In most case, choosing either
2224 one is fine, with #3 being better in typical situations.
2225 .PP
2226 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
2227 rather complicated, but extremely efficient, something that really pays
2228 off after the first million or so of active timers, i.e. it's usually
2229 overkill :)
2230 .PP
2231 \fIThe special problem of being too early\fR
2232 .IX Subsection "The special problem of being too early"
2233 .PP
2234 If you ask a timer to call your callback after three seconds, then
2235 you expect it to be invoked after three seconds \- but of course, this
2236 cannot be guaranteed to infinite precision. Less obviously, it cannot be
2237 guaranteed to any precision by libev \- imagine somebody suspending the
2238 process with a \s-1STOP\s0 signal for a few hours for example.
2239 .PP
2240 So, libev tries to invoke your callback as soon as possible \fIafter\fR the
2241 delay has occurred, but cannot guarantee this.
2242 .PP
2243 A less obvious failure mode is calling your callback too early: many event
2244 loops compare timestamps with a \*(L"elapsed delay >= requested delay\*(R", but
2245 this can cause your callback to be invoked much earlier than you would
2246 expect.
2247 .PP
2248 To see why, imagine a system with a clock that only offers full second
2249 resolution (think windows if you can't come up with a broken enough \s-1OS\s0
2250 yourself). If you schedule a one-second timer at the time 500.9, then the
2251 event loop will schedule your timeout to elapse at a system time of 500
2252 (500.9 truncated to the resolution) + 1, or 501.
2253 .PP
2254 If an event library looks at the timeout 0.1s later, it will see \*(L"501 >=
2255 501\*(R" and invoke the callback 0.1s after it was started, even though a
2256 one-second delay was requested \- this is being \*(L"too early\*(R", despite best
2257 intentions.
2258 .PP
2259 This is the reason why libev will never invoke the callback if the elapsed
2260 delay equals the requested delay, but only when the elapsed delay is
2261 larger than the requested delay. In the example above, libev would only invoke
2262 the callback at system time 502, or 1.1s after the timer was started.
2263 .PP
2264 So, while libev cannot guarantee that your callback will be invoked
2265 exactly when requested, it \fIcan\fR and \fIdoes\fR guarantee that the requested
2266 delay has actually elapsed, or in other words, it always errs on the \*(L"too
2267 late\*(R" side of things.
2268 .PP
2269 \fIThe special problem of time updates\fR
2270 .IX Subsection "The special problem of time updates"
2271 .PP
2272 Establishing the current time is a costly operation (it usually takes
2273 at least one system call): \s-1EV\s0 therefore updates its idea of the current
2274 time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
2275 growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
2276 lots of events in one iteration.
2277 .PP
2278 The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
2279 time. This is usually the right thing as this timestamp refers to the time
2280 of the event triggering whatever timeout you are modifying/starting. If
2281 you suspect event processing to be delayed and you \fIneed\fR to base the
2282 timeout on the current time, use something like the following to adjust
2283 for it:
2284 .PP
2285 .Vb 1
2286 \& ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
2287 .Ve
2288 .PP
2289 If the event loop is suspended for a long time, you can also force an
2290 update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
2291 ()\*(C'\fR, although that will push the event time of all outstanding events
2292 further into the future.
2293 .PP
2294 \fIThe special problem of unsynchronised clocks\fR
2295 .IX Subsection "The special problem of unsynchronised clocks"
2296 .PP
2297 Modern systems have a variety of clocks \- libev itself uses the normal
2298 \&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
2299 jumps).
2300 .PP
2301 Neither of these clocks is synchronised with each other or any other clock
2302 on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
2303 than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
2304 a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
2305 than a directly following call to \f(CW\*(C`time\*(C'\fR.
2306 .PP
2307 The moral of this is to only compare libev-related timestamps with
2308 \&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, at least if you want better precision than
2309 a second or so.
2310 .PP
2311 One more problem arises due to this lack of synchronisation: if libev uses
2312 the system monotonic clock and you compare timestamps from \f(CW\*(C`ev_time\*(C'\fR
2313 or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
2314 invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
2315 .PP
2316 This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
2317 libev makes sure your callback is not invoked before the delay happened,
2318 \&\fImeasured according to the real time\fR, not the system clock.
2319 .PP
2320 If your timeouts are based on a physical timescale (e.g. \*(L"time out this
2321 connection after 100 seconds\*(R") then this shouldn't bother you as it is
2322 exactly the right behaviour.
2323 .PP
2324 If you want to compare wall clock/system timestamps to your timers, then
2325 you need to use \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
2326 time, where your comparisons will always generate correct results.
2327 .PP
2328 \fIThe special problems of suspended animation\fR
2329 .IX Subsection "The special problems of suspended animation"
2330 .PP
2331 When you leave the server world it is quite customary to hit machines that
2332 can suspend/hibernate \- what happens to the clocks during such a suspend?
2333 .PP
2334 Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
2335 all processes, while the clocks (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) continue
2336 to run until the system is suspended, but they will not advance while the
2337 system is suspended. That means, on resume, it will be as if the program
2338 was frozen for a few seconds, but the suspend time will not be counted
2339 towards \f(CW\*(C`ev_timer\*(C'\fR when a monotonic clock source is used. The real time
2340 clock advanced as expected, but if it is used as sole clocksource, then a
2341 long suspend would be detected as a time jump by libev, and timers would
2342 be adjusted accordingly.
2343 .PP
2344 I would not be surprised to see different behaviour in different between
2345 operating systems, \s-1OS\s0 versions or even different hardware.
2346 .PP
2347 The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) will see a
2348 time jump in the monotonic clocks and the realtime clock. If the program
2349 is suspended for a very long time, and monotonic clock sources are in use,
2350 then you can expect \f(CW\*(C`ev_timer\*(C'\fRs to expire as the full suspension time
2351 will be counted towards the timers. When no monotonic clock source is in
2352 use, then libev will again assume a timejump and adjust accordingly.
2353 .PP
2354 It might be beneficial for this latter case to call \f(CW\*(C`ev_suspend\*(C'\fR
2355 and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
2356 deterministic behaviour in this case (you can do nothing against
2357 \&\f(CW\*(C`SIGSTOP\*(C'\fR).
2358 .PP
2359 \fIWatcher-Specific Functions and Data Members\fR
2360 .IX Subsection "Watcher-Specific Functions and Data Members"
2361 .IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
2362 .IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
2363 .PD 0
2364 .IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
2365 .IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
2366 .PD
2367 Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
2368 negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, then it will
2369 automatically be stopped once the timeout is reached. If it is positive,
2370 then the timer will automatically be configured to trigger again \f(CW\*(C`repeat\*(C'\fR
2371 seconds later, again, and again, until stopped manually.
2372 .Sp
2373 The timer itself will do a best-effort at avoiding drift, that is, if
2374 you configure a timer to trigger every 10 seconds, then it will normally
2375 trigger at exactly 10 second intervals. If, however, your program cannot
2376 keep up with the timer (because it takes longer than those 10 seconds to
2377 do stuff) the timer will not fire more than once per event loop iteration.
2378 .IP "ev_timer_again (loop, ev_timer *)" 4
2379 .IX Item "ev_timer_again (loop, ev_timer *)"
2380 This will act as if the timer timed out, and restarts it again if it is
2381 repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
2382 timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
2383 .Sp
2384 The exact semantics are as in the following rules, all of which will be
2385 applied to the watcher:
2386 .RS 4
2387 .IP "If the timer is pending, the pending status is always cleared." 4
2388 .IX Item "If the timer is pending, the pending status is always cleared."
2389 .PD 0
2390 .IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
2391 .IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
2392 .ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
2393 .el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
2394 .IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
2395 .RE
2396 .RS 4
2397 .PD
2398 .Sp
2399 This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
2400 usage example.
2401 .RE
2402 .IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
2403 .IX Item "ev_tstamp ev_timer_remaining (loop, ev_timer *)"
2404 Returns the remaining time until a timer fires. If the timer is active,
2405 then this time is relative to the current event loop time, otherwise it's
2406 the timeout value currently configured.
2407 .Sp
2408 That is, after an \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
2409 \&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
2410 will return \f(CW4\fR. When the timer expires and is restarted, it will return
2411 roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
2412 too), and so on.
2413 .IP "ev_tstamp repeat [read\-write]" 4
2414 .IX Item "ev_tstamp repeat [read-write]"
2415 The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
2416 or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
2417 which is also when any modifications are taken into account.
2418 .PP
2419 \fIExamples\fR
2420 .IX Subsection "Examples"
2421 .PP
2422 Example: Create a timer that fires after 60 seconds.
2423 .PP
2424 .Vb 5
2425 \& static void
2426 \& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2427 \& {
2428 \& .. one minute over, w is actually stopped right here
2429 \& }
2430 \&
2431 \& ev_timer mytimer;
2432 \& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2433 \& ev_timer_start (loop, &mytimer);
2434 .Ve
2435 .PP
2436 Example: Create a timeout timer that times out after 10 seconds of
2437 inactivity.
2438 .PP
2439 .Vb 5
2440 \& static void
2441 \& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2442 \& {
2443 \& .. ten seconds without any activity
2444 \& }
2445 \&
2446 \& ev_timer mytimer;
2447 \& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2448 \& ev_timer_again (&mytimer); /* start timer */
2449 \& ev_run (loop, 0);
2450 \&
2451 \& // and in some piece of code that gets executed on any "activity":
2452 \& // reset the timeout to start ticking again at 10 seconds
2453 \& ev_timer_again (&mytimer);
2454 .Ve
2455 .ie n .SS """ev_periodic"" \- to cron or not to cron?"
2456 .el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
2457 .IX Subsection "ev_periodic - to cron or not to cron?"
2458 Periodic watchers are also timers of a kind, but they are very versatile
2459 (and unfortunately a bit complex).
2460 .PP
2461 Unlike \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
2462 relative time, the physical time that passes) but on wall clock time
2463 (absolute time, the thing you can read on your calendar or clock). The
2464 difference is that wall clock time can run faster or slower than real
2465 time, and time jumps are not uncommon (e.g. when you adjust your
2466 wrist-watch).
2467 .PP
2468 You can tell a periodic watcher to trigger after some specific point
2469 in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
2470 seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, that is, an absolute time
2471 not a delay) and then reset your system clock to January of the previous
2472 year, then it will take a year or more to trigger the event (unlike an
2473 \&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
2474 it, as it uses a relative timeout).
2475 .PP
2476 \&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
2477 timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
2478 other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
2479 watchers, as those cannot react to time jumps.
2480 .PP
2481 As with timers, the callback is guaranteed to be invoked only when the
2482 point in time where it is supposed to trigger has passed. If multiple
2483 timers become ready during the same loop iteration then the ones with
2484 earlier time-out values are invoked before ones with later time-out values
2485 (but this is no longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2486 .PP
2487 \fIWatcher-Specific Functions and Data Members\fR
2488 .IX Subsection "Watcher-Specific Functions and Data Members"
2489 .IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2490 .IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2491 .PD 0
2492 .IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2493 .IX Item "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2494 .PD
2495 Lots of arguments, let's sort it out... There are basically three modes of
2496 operation, and we will explain them from simplest to most complex:
2497 .RS 4
2498 .IP "\(bu" 4
2499 absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
2500 .Sp
2501 In this configuration the watcher triggers an event after the wall clock
2502 time \f(CW\*(C`offset\*(C'\fR has passed. It will not repeat and will not adjust when a
2503 time jump occurs, that is, if it is to be run at January 1st 2011 then it
2504 will be stopped and invoked when the system clock reaches or surpasses
2505 this point in time.
2506 .IP "\(bu" 4
2507 repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
2508 .Sp
2509 In this mode the watcher will always be scheduled to time out at the next
2510 \&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
2511 negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
2512 argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
2513 .Sp
2514 This can be used to create timers that do not drift with respect to the
2515 system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
2516 hour, on the hour (with respect to \s-1UTC\s0):
2517 .Sp
2518 .Vb 1
2519 \& ev_periodic_set (&periodic, 0., 3600., 0);
2520 .Ve
2521 .Sp
2522 This doesn't mean there will always be 3600 seconds in between triggers,
2523 but only that the callback will be called when the system time shows a
2524 full hour (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
2525 by 3600.
2526 .Sp
2527 Another way to think about it (for the mathematically inclined) is that
2528 \&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
2529 time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
2530 .Sp
2531 The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
2532 interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
2533 microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR and should have
2534 at most a similar magnitude as the current time (say, within a factor of
2535 ten). Typical values for offset are, in fact, \f(CW0\fR or something between
2536 \&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
2537 .Sp
2538 Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
2539 speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
2540 will of course deteriorate. Libev itself tries to be exact to be about one
2541 millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
2542 .IP "\(bu" 4
2543 manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
2544 .Sp
2545 In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR are both being
2546 ignored. Instead, each time the periodic watcher gets scheduled, the
2547 reschedule callback will be called with the watcher as first, and the
2548 current time as second argument.
2549 .Sp
2550 \&\s-1NOTE:\s0 \fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
2551 or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
2552 allowed by documentation here\fR.
2553 .Sp
2554 If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
2555 it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
2556 only event loop modification you are allowed to do).
2557 .Sp
2558 The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
2559 *w, ev_tstamp now)\*(C'\fR, e.g.:
2560 .Sp
2561 .Vb 5
2562 \& static ev_tstamp
2563 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2564 \& {
2565 \& return now + 60.;
2566 \& }
2567 .Ve
2568 .Sp
2569 It must return the next time to trigger, based on the passed time value
2570 (that is, the lowest time value larger than to the second argument). It
2571 will usually be called just before the callback will be triggered, but
2572 might be called at other times, too.
2573 .Sp
2574 \&\s-1NOTE:\s0 \fIThis callback must always return a time that is higher than or
2575 equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
2576 .Sp
2577 This can be used to create very complex timers, such as a timer that
2578 triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
2579 the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
2580 this. Here is a (completely untested, no error checking) example on how to
2581 do this:
2582 .Sp
2583 .Vb 1
2584 \& #include <time.h>
2585 \&
2586 \& static ev_tstamp
2587 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2588 \& {
2589 \& time_t tnow = (time_t)now;
2590 \& struct tm tm;
2591 \& localtime_r (&tnow, &tm);
2592 \&
2593 \& tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2594 \& ++tm.tm_mday; // midnight next day
2595 \&
2596 \& return mktime (&tm);
2597 \& }
2598 .Ve
2599 .Sp
2600 Note: this code might run into trouble on days that have more then two
2601 midnights (beginning and end).
2602 .RE
2603 .RS 4
2604 .RE
2605 .IP "ev_periodic_again (loop, ev_periodic *)" 4
2606 .IX Item "ev_periodic_again (loop, ev_periodic *)"
2607 Simply stops and restarts the periodic watcher again. This is only useful
2608 when you changed some parameters or the reschedule callback would return
2609 a different time than the last time it was called (e.g. in a crond like
2610 program when the crontabs have changed).
2611 .IP "ev_tstamp ev_periodic_at (ev_periodic *)" 4
2612 .IX Item "ev_tstamp ev_periodic_at (ev_periodic *)"
2613 When active, returns the absolute time that the watcher is supposed
2614 to trigger next. This is not the same as the \f(CW\*(C`offset\*(C'\fR argument to
2615 \&\f(CW\*(C`ev_periodic_set\*(C'\fR, but indeed works even in interval and manual
2616 rescheduling modes.
2617 .IP "ev_tstamp offset [read\-write]" 4
2618 .IX Item "ev_tstamp offset [read-write]"
2619 When repeating, this contains the offset value, otherwise this is the
2620 absolute point in time (the \f(CW\*(C`offset\*(C'\fR value passed to \f(CW\*(C`ev_periodic_set\*(C'\fR,
2621 although libev might modify this value for better numerical stability).
2622 .Sp
2623 Can be modified any time, but changes only take effect when the periodic
2624 timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2625 .IP "ev_tstamp interval [read\-write]" 4
2626 .IX Item "ev_tstamp interval [read-write]"
2627 The current interval value. Can be modified any time, but changes only
2628 take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
2629 called.
2630 .IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
2631 .IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
2632 The current reschedule callback, or \f(CW0\fR, if this functionality is
2633 switched off. Can be changed any time, but changes only take effect when
2634 the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2635 .PP
2636 \fIExamples\fR
2637 .IX Subsection "Examples"
2638 .PP
2639 Example: Call a callback every hour, or, more precisely, whenever the
2640 system time is divisible by 3600. The callback invocation times have
2641 potentially a lot of jitter, but good long-term stability.
2642 .PP
2643 .Vb 5
2644 \& static void
2645 \& clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2646 \& {
2647 \& ... its now a full hour (UTC, or TAI or whatever your clock follows)
2648 \& }
2649 \&
2650 \& ev_periodic hourly_tick;
2651 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2652 \& ev_periodic_start (loop, &hourly_tick);
2653 .Ve
2654 .PP
2655 Example: The same as above, but use a reschedule callback to do it:
2656 .PP
2657 .Vb 1
2658 \& #include <math.h>
2659 \&
2660 \& static ev_tstamp
2661 \& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2662 \& {
2663 \& return now + (3600. \- fmod (now, 3600.));
2664 \& }
2665 \&
2666 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2667 .Ve
2668 .PP
2669 Example: Call a callback every hour, starting now:
2670 .PP
2671 .Vb 4
2672 \& ev_periodic hourly_tick;
2673 \& ev_periodic_init (&hourly_tick, clock_cb,
2674 \& fmod (ev_now (loop), 3600.), 3600., 0);
2675 \& ev_periodic_start (loop, &hourly_tick);
2676 .Ve
2677 .ie n .SS """ev_signal"" \- signal me when a signal gets signalled!"
2678 .el .SS "\f(CWev_signal\fP \- signal me when a signal gets signalled!"
2679 .IX Subsection "ev_signal - signal me when a signal gets signalled!"
2680 Signal watchers will trigger an event when the process receives a specific
2681 signal one or more times. Even though signals are very asynchronous, libev
2682 will try its best to deliver signals synchronously, i.e. as part of the
2683 normal event processing, like any other event.
2684 .PP
2685 If you want signals to be delivered truly asynchronously, just use
2686 \&\f(CW\*(C`sigaction\*(C'\fR as you would do without libev and forget about sharing
2687 the signal. You can even use \f(CW\*(C`ev_async\*(C'\fR from a signal handler to
2688 synchronously wake up an event loop.
2689 .PP
2690 You can configure as many watchers as you like for the same signal, but
2691 only within the same loop, i.e. you can watch for \f(CW\*(C`SIGINT\*(C'\fR in your
2692 default loop and for \f(CW\*(C`SIGIO\*(C'\fR in another loop, but you cannot watch for
2693 \&\f(CW\*(C`SIGINT\*(C'\fR in both the default loop and another loop at the same time. At
2694 the moment, \f(CW\*(C`SIGCHLD\*(C'\fR is permanently tied to the default loop.
2695 .PP
2696 Only after the first watcher for a signal is started will libev actually
2697 register something with the kernel. It thus coexists with your own signal
2698 handlers as long as you don't register any with libev for the same signal.
2699 .PP
2700 If possible and supported, libev will install its handlers with
2701 \&\f(CW\*(C`SA_RESTART\*(C'\fR (or equivalent) behaviour enabled, so system calls should
2702 not be unduly interrupted. If you have a problem with system calls getting
2703 interrupted by signals you can block all signals in an \f(CW\*(C`ev_check\*(C'\fR watcher
2704 and unblock them in an \f(CW\*(C`ev_prepare\*(C'\fR watcher.
2705 .PP
2706 \fIThe special problem of inheritance over fork/execve/pthread_create\fR
2707 .IX Subsection "The special problem of inheritance over fork/execve/pthread_create"
2708 .PP
2709 Both the signal mask (\f(CW\*(C`sigprocmask\*(C'\fR) and the signal disposition
2710 (\f(CW\*(C`sigaction\*(C'\fR) are unspecified after starting a signal watcher (and after
2711 stopping it again), that is, libev might or might not block the signal,
2712 and might or might not set or restore the installed signal handler (but
2713 see \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR).
2714 .PP
2715 While this does not matter for the signal disposition (libev never
2716 sets signals to \f(CW\*(C`SIG_IGN\*(C'\fR, so handlers will be reset to \f(CW\*(C`SIG_DFL\*(C'\fR on
2717 \&\f(CW\*(C`execve\*(C'\fR), this matters for the signal mask: many programs do not expect
2718 certain signals to be blocked.
2719 .PP
2720 This means that before calling \f(CW\*(C`exec\*(C'\fR (from the child) you should reset
2721 the signal mask to whatever \*(L"default\*(R" you expect (all clear is a good
2722 choice usually).
2723 .PP
2724 The simplest way to ensure that the signal mask is reset in the child is
2725 to install a fork handler with \f(CW\*(C`pthread_atfork\*(C'\fR that resets it. That will
2726 catch fork calls done by libraries (such as the libc) as well.
2727 .PP
2728 In current versions of libev, the signal will not be blocked indefinitely
2729 unless you use the \f(CW\*(C`signalfd\*(C'\fR \s-1API\s0 (\f(CW\*(C`EV_SIGNALFD\*(C'\fR). While this reduces
2730 the window of opportunity for problems, it will not go away, as libev
2731 \&\fIhas\fR to modify the signal mask, at least temporarily.
2732 .PP
2733 So I can't stress this enough: \fIIf you do not reset your signal mask when
2734 you expect it to be empty, you have a race condition in your code\fR. This
2735 is not a libev-specific thing, this is true for most event libraries.
2736 .PP
2737 \fIThe special problem of threads signal handling\fR
2738 .IX Subsection "The special problem of threads signal handling"
2739 .PP
2740 \&\s-1POSIX\s0 threads has problematic signal handling semantics, specifically,
2741 a lot of functionality (sigfd, sigwait etc.) only really works if all
2742 threads in a process block signals, which is hard to achieve.
2743 .PP
2744 When you want to use sigwait (or mix libev signal handling with your own
2745 for the same signals), you can tackle this problem by globally blocking
2746 all signals before creating any threads (or creating them with a fully set
2747 sigprocmask) and also specifying the \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when creating
2748 loops. Then designate one thread as \*(L"signal receiver thread\*(R" which handles
2749 these signals. You can pass on any signals that libev might be interested
2750 in by calling \f(CW\*(C`ev_feed_signal\*(C'\fR.
2751 .PP
2752 \fIWatcher-Specific Functions and Data Members\fR
2753 .IX Subsection "Watcher-Specific Functions and Data Members"
2754 .IP "ev_signal_init (ev_signal *, callback, int signum)" 4
2755 .IX Item "ev_signal_init (ev_signal *, callback, int signum)"
2756 .PD 0
2757 .IP "ev_signal_set (ev_signal *, int signum)" 4
2758 .IX Item "ev_signal_set (ev_signal *, int signum)"
2759 .PD
2760 Configures the watcher to trigger on the given signal number (usually one
2761 of the \f(CW\*(C`SIGxxx\*(C'\fR constants).
2762 .IP "int signum [read\-only]" 4
2763 .IX Item "int signum [read-only]"
2764 The signal the watcher watches out for.
2765 .PP
2766 \fIExamples\fR
2767 .IX Subsection "Examples"
2768 .PP
2769 Example: Try to exit cleanly on \s-1SIGINT.\s0
2770 .PP
2771 .Vb 5
2772 \& static void
2773 \& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2774 \& {
2775 \& ev_break (loop, EVBREAK_ALL);
2776 \& }
2777 \&
2778 \& ev_signal signal_watcher;
2779 \& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2780 \& ev_signal_start (loop, &signal_watcher);
2781 .Ve
2782 .ie n .SS """ev_child"" \- watch out for process status changes"
2783 .el .SS "\f(CWev_child\fP \- watch out for process status changes"
2784 .IX Subsection "ev_child - watch out for process status changes"
2785 Child watchers trigger when your process receives a \s-1SIGCHLD\s0 in response to
2786 some child status changes (most typically when a child of yours dies or
2787 exits). It is permissible to install a child watcher \fIafter\fR the child
2788 has been forked (which implies it might have already exited), as long
2789 as the event loop isn't entered (or is continued from a watcher), i.e.,
2790 forking and then immediately registering a watcher for the child is fine,
2791 but forking and registering a watcher a few event loop iterations later or
2792 in the next callback invocation is not.
2793 .PP
2794 Only the default event loop is capable of handling signals, and therefore
2795 you can only register child watchers in the default event loop.
2796 .PP
2797 Due to some design glitches inside libev, child watchers will always be
2798 handled at maximum priority (their priority is set to \f(CW\*(C`EV_MAXPRI\*(C'\fR by
2799 libev)
2800 .PP
2801 \fIProcess Interaction\fR
2802 .IX Subsection "Process Interaction"
2803 .PP
2804 Libev grabs \f(CW\*(C`SIGCHLD\*(C'\fR as soon as the default event loop is
2805 initialised. This is necessary to guarantee proper behaviour even if the
2806 first child watcher is started after the child exits. The occurrence
2807 of \f(CW\*(C`SIGCHLD\*(C'\fR is recorded asynchronously, but child reaping is done
2808 synchronously as part of the event loop processing. Libev always reaps all
2809 children, even ones not watched.
2810 .PP
2811 \fIOverriding the Built-In Processing\fR
2812 .IX Subsection "Overriding the Built-In Processing"
2813 .PP
2814 Libev offers no special support for overriding the built-in child
2815 processing, but if your application collides with libev's default child
2816 handler, you can override it easily by installing your own handler for
2817 \&\f(CW\*(C`SIGCHLD\*(C'\fR after initialising the default loop, and making sure the
2818 default loop never gets destroyed. You are encouraged, however, to use an
2819 event-based approach to child reaping and thus use libev's support for
2820 that, so other libev users can use \f(CW\*(C`ev_child\*(C'\fR watchers freely.
2821 .PP
2822 \fIStopping the Child Watcher\fR
2823 .IX Subsection "Stopping the Child Watcher"
2824 .PP
2825 Currently, the child watcher never gets stopped, even when the
2826 child terminates, so normally one needs to stop the watcher in the
2827 callback. Future versions of libev might stop the watcher automatically
2828 when a child exit is detected (calling \f(CW\*(C`ev_child_stop\*(C'\fR twice is not a
2829 problem).
2830 .PP
2831 \fIWatcher-Specific Functions and Data Members\fR
2832 .IX Subsection "Watcher-Specific Functions and Data Members"
2833 .IP "ev_child_init (ev_child *, callback, int pid, int trace)" 4
2834 .IX Item "ev_child_init (ev_child *, callback, int pid, int trace)"
2835 .PD 0
2836 .IP "ev_child_set (ev_child *, int pid, int trace)" 4
2837 .IX Item "ev_child_set (ev_child *, int pid, int trace)"
2838 .PD
2839 Configures the watcher to wait for status changes of process \f(CW\*(C`pid\*(C'\fR (or
2840 \&\fIany\fR process if \f(CW\*(C`pid\*(C'\fR is specified as \f(CW0\fR). The callback can look
2841 at the \f(CW\*(C`rstatus\*(C'\fR member of the \f(CW\*(C`ev_child\*(C'\fR watcher structure to see
2842 the status word (use the macros from \f(CW\*(C`sys/wait.h\*(C'\fR and see your systems
2843 \&\f(CW\*(C`waitpid\*(C'\fR documentation). The \f(CW\*(C`rpid\*(C'\fR member contains the pid of the
2844 process causing the status change. \f(CW\*(C`trace\*(C'\fR must be either \f(CW0\fR (only
2845 activate the watcher when the process terminates) or \f(CW1\fR (additionally
2846 activate the watcher when the process is stopped or continued).
2847 .IP "int pid [read\-only]" 4
2848 .IX Item "int pid [read-only]"
2849 The process id this watcher watches out for, or \f(CW0\fR, meaning any process id.
2850 .IP "int rpid [read\-write]" 4
2851 .IX Item "int rpid [read-write]"
2852 The process id that detected a status change.
2853 .IP "int rstatus [read\-write]" 4
2854 .IX Item "int rstatus [read-write]"
2855 The process exit/trace status caused by \f(CW\*(C`rpid\*(C'\fR (see your systems
2856 \&\f(CW\*(C`waitpid\*(C'\fR and \f(CW\*(C`sys/wait.h\*(C'\fR documentation for details).
2857 .PP
2858 \fIExamples\fR
2859 .IX Subsection "Examples"
2860 .PP
2861 Example: \f(CW\*(C`fork()\*(C'\fR a new process and install a child handler to wait for
2862 its completion.
2863 .PP
2864 .Vb 1
2865 \& ev_child cw;
2866 \&
2867 \& static void
2868 \& child_cb (EV_P_ ev_child *w, int revents)
2869 \& {
2870 \& ev_child_stop (EV_A_ w);
2871 \& printf ("process %d exited with status %x\en", w\->rpid, w\->rstatus);
2872 \& }
2873 \&
2874 \& pid_t pid = fork ();
2875 \&
2876 \& if (pid < 0)
2877 \& // error
2878 \& else if (pid == 0)
2879 \& {
2880 \& // the forked child executes here
2881 \& exit (1);
2882 \& }
2883 \& else
2884 \& {
2885 \& ev_child_init (&cw, child_cb, pid, 0);
2886 \& ev_child_start (EV_DEFAULT_ &cw);
2887 \& }
2888 .Ve
2889 .ie n .SS """ev_stat"" \- did the file attributes just change?"
2890 .el .SS "\f(CWev_stat\fP \- did the file attributes just change?"
2891 .IX Subsection "ev_stat - did the file attributes just change?"
2892 This watches a file system path for attribute changes. That is, it calls
2893 \&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
2894 and sees if it changed compared to the last time, invoking the callback
2895 if it did. Starting the watcher \f(CW\*(C`stat\*(C'\fR's the file, so only changes that
2896 happen after the watcher has been started will be reported.
2897 .PP
2898 The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
2899 not exist\*(R" is a status change like any other. The condition \*(L"path does not
2900 exist\*(R" (or more correctly \*(L"path cannot be stat'ed\*(R") is signified by the
2901 \&\f(CW\*(C`st_nlink\*(C'\fR field being zero (which is otherwise always forced to be at
2902 least one) and all the other fields of the stat buffer having unspecified
2903 contents.
2904 .PP
2905 The path \fImust not\fR end in a slash or contain special components such as
2906 \&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
2907 your working directory changes, then the behaviour is undefined.
2908 .PP
2909 Since there is no portable change notification interface available, the
2910 portable implementation simply calls \f(CWstat(2)\fR regularly on the path
2911 to see if it changed somehow. You can specify a recommended polling
2912 interval for this case. If you specify a polling interval of \f(CW0\fR (highly
2913 recommended!) then a \fIsuitable, unspecified default\fR value will be used
2914 (which you can expect to be around five seconds, although this might
2915 change dynamically). Libev will also impose a minimum interval which is
2916 currently around \f(CW0.1\fR, but that's usually overkill.
2917 .PP
2918 This watcher type is not meant for massive numbers of stat watchers,
2919 as even with OS-supported change notifications, this can be
2920 resource-intensive.
2921 .PP
2922 At the time of this writing, the only OS-specific interface implemented
2923 is the Linux inotify interface (implementing kqueue support is left as an
2924 exercise for the reader. Note, however, that the author sees no way of
2925 implementing \f(CW\*(C`ev_stat\*(C'\fR semantics with kqueue, except as a hint).
2926 .PP
2927 \fI\s-1ABI\s0 Issues (Largefile Support)\fR
2928 .IX Subsection "ABI Issues (Largefile Support)"
2929 .PP
2930 Libev by default (unless the user overrides this) uses the default
2931 compilation environment, which means that on systems with large file
2932 support disabled by default, you get the 32 bit version of the stat
2933 structure. When using the library from programs that change the \s-1ABI\s0 to
2934 use 64 bit file offsets the programs will fail. In that case you have to
2935 compile libev with the same flags to get binary compatibility. This is
2936 obviously the case with any flags that change the \s-1ABI,\s0 but the problem is
2937 most noticeably displayed with ev_stat and large file support.
2938 .PP
2939 The solution for this is to lobby your distribution maker to make large
2940 file interfaces available by default (as e.g. FreeBSD does) and not
2941 optional. Libev cannot simply switch on large file support because it has
2942 to exchange stat structures with application programs compiled using the
2943 default compilation environment.
2944 .PP
2945 \fIInotify and Kqueue\fR
2946 .IX Subsection "Inotify and Kqueue"
2947 .PP
2948 When \f(CW\*(C`inotify (7)\*(C'\fR support has been compiled into libev and present at
2949 runtime, it will be used to speed up change detection where possible. The
2950 inotify descriptor will be created lazily when the first \f(CW\*(C`ev_stat\*(C'\fR
2951 watcher is being started.
2952 .PP
2953 Inotify presence does not change the semantics of \f(CW\*(C`ev_stat\*(C'\fR watchers
2954 except that changes might be detected earlier, and in some cases, to avoid
2955 making regular \f(CW\*(C`stat\*(C'\fR calls. Even in the presence of inotify support
2956 there are many cases where libev has to resort to regular \f(CW\*(C`stat\*(C'\fR polling,
2957 but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
2958 many bugs), the path exists (i.e. stat succeeds), and the path resides on
2959 a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
2960 xfs are fully working) libev usually gets away without polling.
2961 .PP
2962 There is no support for kqueue, as apparently it cannot be used to
2963 implement this functionality, due to the requirement of having a file
2964 descriptor open on the object at all times, and detecting renames, unlinks
2965 etc. is difficult.
2966 .PP
2967 \fI\f(CI\*(C`stat ()\*(C'\fI is a synchronous operation\fR
2968 .IX Subsection "stat () is a synchronous operation"
2969 .PP
2970 Libev doesn't normally do any kind of I/O itself, and so is not blocking
2971 the process. The exception are \f(CW\*(C`ev_stat\*(C'\fR watchers \- those call \f(CW\*(C`stat
2972 ()\*(C'\fR, which is a synchronous operation.
2973 .PP
2974 For local paths, this usually doesn't matter: unless the system is very
2975 busy or the intervals between stat's are large, a stat call will be fast,
2976 as the path data is usually in memory already (except when starting the
2977 watcher).
2978 .PP
2979 For networked file systems, calling \f(CW\*(C`stat ()\*(C'\fR can block an indefinite
2980 time due to network issues, and even under good conditions, a stat call
2981 often takes multiple milliseconds.
2982 .PP
2983 Therefore, it is best to avoid using \f(CW\*(C`ev_stat\*(C'\fR watchers on networked
2984 paths, although this is fully supported by libev.
2985 .PP
2986 \fIThe special problem of stat time resolution\fR
2987 .IX Subsection "The special problem of stat time resolution"
2988 .PP
2989 The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
2990 and even on systems where the resolution is higher, most file systems
2991 still only support whole seconds.
2992 .PP
2993 That means that, if the time is the only thing that changes, you can
2994 easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
2995 calls your callback, which does something. When there is another update
2996 within the same second, \f(CW\*(C`ev_stat\*(C'\fR will be unable to detect unless the
2997 stat data does change in other ways (e.g. file size).
2998 .PP
2999 The solution to this is to delay acting on a change for slightly more
3000 than a second (or till slightly after the next full second boundary), using
3001 a roughly one-second-delay \f(CW\*(C`ev_timer\*(C'\fR (e.g. \f(CW\*(C`ev_timer_set (w, 0., 1.02);
3002 ev_timer_again (loop, w)\*(C'\fR).
3003 .PP
3004 The \f(CW.02\fR offset is added to work around small timing inconsistencies
3005 of some operating systems (where the second counter of the current time
3006 might be be delayed. One such system is the Linux kernel, where a call to
3007 \&\f(CW\*(C`gettimeofday\*(C'\fR might return a timestamp with a full second later than
3008 a subsequent \f(CW\*(C`time\*(C'\fR call \- if the equivalent of \f(CW\*(C`time ()\*(C'\fR is used to
3009 update file times then there will be a small window where the kernel uses
3010 the previous second to update file times but libev might already execute
3011 the timer callback).
3012 .PP
3013 \fIWatcher-Specific Functions and Data Members\fR
3014 .IX Subsection "Watcher-Specific Functions and Data Members"
3015 .IP "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)" 4
3016 .IX Item "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)"
3017 .PD 0
3018 .IP "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)" 4
3019 .IX Item "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)"
3020 .PD
3021 Configures the watcher to wait for status changes of the given
3022 \&\f(CW\*(C`path\*(C'\fR. The \f(CW\*(C`interval\*(C'\fR is a hint on how quickly a change is expected to
3023 be detected and should normally be specified as \f(CW0\fR to let libev choose
3024 a suitable value. The memory pointed to by \f(CW\*(C`path\*(C'\fR must point to the same
3025 path for as long as the watcher is active.
3026 .Sp
3027 The callback will receive an \f(CW\*(C`EV_STAT\*(C'\fR event when a change was detected,
3028 relative to the attributes at the time the watcher was started (or the
3029 last change was detected).
3030 .IP "ev_stat_stat (loop, ev_stat *)" 4
3031 .IX Item "ev_stat_stat (loop, ev_stat *)"
3032 Updates the stat buffer immediately with new values. If you change the
3033 watched path in your callback, you could call this function to avoid
3034 detecting this change (while introducing a race condition if you are not
3035 the only one changing the path). Can also be useful simply to find out the
3036 new values.
3037 .IP "ev_statdata attr [read\-only]" 4
3038 .IX Item "ev_statdata attr [read-only]"
3039 The most-recently detected attributes of the file. Although the type is
3040 \&\f(CW\*(C`ev_statdata\*(C'\fR, this is usually the (or one of the) \f(CW\*(C`struct stat\*(C'\fR types
3041 suitable for your system, but you can only rely on the POSIX-standardised
3042 members to be present. If the \f(CW\*(C`st_nlink\*(C'\fR member is \f(CW0\fR, then there was
3043 some error while \f(CW\*(C`stat\*(C'\fRing the file.
3044 .IP "ev_statdata prev [read\-only]" 4
3045 .IX Item "ev_statdata prev [read-only]"
3046 The previous attributes of the file. The callback gets invoked whenever
3047 \&\f(CW\*(C`prev\*(C'\fR != \f(CW\*(C`attr\*(C'\fR, or, more precisely, one or more of these members
3048 differ: \f(CW\*(C`st_dev\*(C'\fR, \f(CW\*(C`st_ino\*(C'\fR, \f(CW\*(C`st_mode\*(C'\fR, \f(CW\*(C`st_nlink\*(C'\fR, \f(CW\*(C`st_uid\*(C'\fR,
3049 \&\f(CW\*(C`st_gid\*(C'\fR, \f(CW\*(C`st_rdev\*(C'\fR, \f(CW\*(C`st_size\*(C'\fR, \f(CW\*(C`st_atime\*(C'\fR, \f(CW\*(C`st_mtime\*(C'\fR, \f(CW\*(C`st_ctime\*(C'\fR.
3050 .IP "ev_tstamp interval [read\-only]" 4
3051 .IX Item "ev_tstamp interval [read-only]"
3052 The specified interval.
3053 .IP "const char *path [read\-only]" 4
3054 .IX Item "const char *path [read-only]"
3055 The file system path that is being watched.
3056 .PP
3057 \fIExamples\fR
3058 .IX Subsection "Examples"
3059 .PP
3060 Example: Watch \f(CW\*(C`/etc/passwd\*(C'\fR for attribute changes.
3061 .PP
3062 .Vb 10
3063 \& static void
3064 \& passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
3065 \& {
3066 \& /* /etc/passwd changed in some way */
3067 \& if (w\->attr.st_nlink)
3068 \& {
3069 \& printf ("passwd current size %ld\en", (long)w\->attr.st_size);
3070 \& printf ("passwd current atime %ld\en", (long)w\->attr.st_mtime);
3071 \& printf ("passwd current mtime %ld\en", (long)w\->attr.st_mtime);
3072 \& }
3073 \& else
3074 \& /* you shalt not abuse printf for puts */
3075 \& puts ("wow, /etc/passwd is not there, expect problems. "
3076 \& "if this is windows, they already arrived\en");
3077 \& }
3078 \&
3079 \& ...
3080 \& ev_stat passwd;
3081 \&
3082 \& ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
3083 \& ev_stat_start (loop, &passwd);
3084 .Ve
3085 .PP
3086 Example: Like above, but additionally use a one-second delay so we do not
3087 miss updates (however, frequent updates will delay processing, too, so
3088 one might do the work both on \f(CW\*(C`ev_stat\*(C'\fR callback invocation \fIand\fR on
3089 \&\f(CW\*(C`ev_timer\*(C'\fR callback invocation).
3090 .PP
3091 .Vb 2
3092 \& static ev_stat passwd;
3093 \& static ev_timer timer;
3094 \&
3095 \& static void
3096 \& timer_cb (EV_P_ ev_timer *w, int revents)
3097 \& {
3098 \& ev_timer_stop (EV_A_ w);
3099 \&
3100 \& /* now it\*(Aqs one second after the most recent passwd change */
3101 \& }
3102 \&
3103 \& static void
3104 \& stat_cb (EV_P_ ev_stat *w, int revents)
3105 \& {
3106 \& /* reset the one\-second timer */
3107 \& ev_timer_again (EV_A_ &timer);
3108 \& }
3109 \&
3110 \& ...
3111 \& ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
3112 \& ev_stat_start (loop, &passwd);
3113 \& ev_timer_init (&timer, timer_cb, 0., 1.02);
3114 .Ve
3115 .ie n .SS """ev_idle"" \- when you've got nothing better to do..."
3116 .el .SS "\f(CWev_idle\fP \- when you've got nothing better to do..."
3117 .IX Subsection "ev_idle - when you've got nothing better to do..."
3118 Idle watchers trigger events when no other events of the same or higher
3119 priority are pending (prepare, check and other idle watchers do not count
3120 as receiving \*(L"events\*(R").
3121 .PP
3122 That is, as long as your process is busy handling sockets or timeouts
3123 (or even signals, imagine) of the same or higher priority it will not be
3124 triggered. But when your process is idle (or only lower-priority watchers
3125 are pending), the idle watchers are being called once per event loop
3126 iteration \- until stopped, that is, or your process receives more events
3127 and becomes busy again with higher priority stuff.
3128 .PP
3129 The most noteworthy effect is that as long as any idle watchers are
3130 active, the process will not block when waiting for new events.
3131 .PP
3132 Apart from keeping your process non-blocking (which is a useful
3133 effect on its own sometimes), idle watchers are a good place to do
3134 \&\*(L"pseudo-background processing\*(R", or delay processing stuff to after the
3135 event loop has handled all outstanding events.
3136 .PP
3137 \fIAbusing an \f(CI\*(C`ev_idle\*(C'\fI watcher for its side-effect\fR
3138 .IX Subsection "Abusing an ev_idle watcher for its side-effect"
3139 .PP
3140 As long as there is at least one active idle watcher, libev will never
3141 sleep unnecessarily. Or in other words, it will loop as fast as possible.
3142 For this to work, the idle watcher doesn't need to be invoked at all \- the
3143 lowest priority will do.
3144 .PP
3145 This mode of operation can be useful together with an \f(CW\*(C`ev_check\*(C'\fR watcher,
3146 to do something on each event loop iteration \- for example to balance load
3147 between different connections.
3148 .PP
3149 See \*(L"Abusing an ev_check watcher for its side-effect\*(R" for a longer
3150 example.
3151 .PP
3152 \fIWatcher-Specific Functions and Data Members\fR
3153 .IX Subsection "Watcher-Specific Functions and Data Members"
3154 .IP "ev_idle_init (ev_idle *, callback)" 4
3155 .IX Item "ev_idle_init (ev_idle *, callback)"
3156 Initialises and configures the idle watcher \- it has no parameters of any
3157 kind. There is a \f(CW\*(C`ev_idle_set\*(C'\fR macro, but using it is utterly pointless,
3158 believe me.
3159 .PP
3160 \fIExamples\fR
3161 .IX Subsection "Examples"
3162 .PP
3163 Example: Dynamically allocate an \f(CW\*(C`ev_idle\*(C'\fR watcher, start it, and in the
3164 callback, free it. Also, use no error checking, as usual.
3165 .PP
3166 .Vb 5
3167 \& static void
3168 \& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
3169 \& {
3170 \& // stop the watcher
3171 \& ev_idle_stop (loop, w);
3172 \&
3173 \& // now we can free it
3174 \& free (w);
3175 \&
3176 \& // now do something you wanted to do when the program has
3177 \& // no longer anything immediate to do.
3178 \& }
3179 \&
3180 \& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
3181 \& ev_idle_init (idle_watcher, idle_cb);
3182 \& ev_idle_start (loop, idle_watcher);
3183 .Ve
3184 .ie n .SS """ev_prepare"" and ""ev_check"" \- customise your event loop!"
3185 .el .SS "\f(CWev_prepare\fP and \f(CWev_check\fP \- customise your event loop!"
3186 .IX Subsection "ev_prepare and ev_check - customise your event loop!"
3187 Prepare and check watchers are often (but not always) used in pairs:
3188 prepare watchers get invoked before the process blocks and check watchers
3189 afterwards.
3190 .PP
3191 You \fImust not\fR call \f(CW\*(C`ev_run\*(C'\fR (or similar functions that enter the
3192 current event loop) or \f(CW\*(C`ev_loop_fork\*(C'\fR from either \f(CW\*(C`ev_prepare\*(C'\fR or
3193 \&\f(CW\*(C`ev_check\*(C'\fR watchers. Other loops than the current one are fine,
3194 however. The rationale behind this is that you do not need to check
3195 for recursion in those watchers, i.e. the sequence will always be
3196 \&\f(CW\*(C`ev_prepare\*(C'\fR, blocking, \f(CW\*(C`ev_check\*(C'\fR so if you have one watcher of each
3197 kind they will always be called in pairs bracketing the blocking call.
3198 .PP
3199 Their main purpose is to integrate other event mechanisms into libev and
3200 their use is somewhat advanced. They could be used, for example, to track
3201 variable changes, implement your own watchers, integrate net-snmp or a
3202 coroutine library and lots more. They are also occasionally useful if
3203 you cache some data and want to flush it before blocking (for example,
3204 in X programs you might want to do an \f(CW\*(C`XFlush ()\*(C'\fR in an \f(CW\*(C`ev_prepare\*(C'\fR
3205 watcher).
3206 .PP
3207 This is done by examining in each prepare call which file descriptors
3208 need to be watched by the other library, registering \f(CW\*(C`ev_io\*(C'\fR watchers
3209 for them and starting an \f(CW\*(C`ev_timer\*(C'\fR watcher for any timeouts (many
3210 libraries provide exactly this functionality). Then, in the check watcher,
3211 you check for any events that occurred (by checking the pending status
3212 of all watchers and stopping them) and call back into the library. The
3213 I/O and timer callbacks will never actually be called (but must be valid
3214 nevertheless, because you never know, you know?).
3215 .PP
3216 As another example, the Perl Coro module uses these hooks to integrate
3217 coroutines into libev programs, by yielding to other active coroutines
3218 during each prepare and only letting the process block if no coroutines
3219 are ready to run (it's actually more complicated: it only runs coroutines
3220 with priority higher than or equal to the event loop and one coroutine
3221 of lower priority, but only once, using idle watchers to keep the event
3222 loop from blocking if lower-priority coroutines are active, thus mapping
3223 low-priority coroutines to idle/background tasks).
3224 .PP
3225 When used for this purpose, it is recommended to give \f(CW\*(C`ev_check\*(C'\fR watchers
3226 highest (\f(CW\*(C`EV_MAXPRI\*(C'\fR) priority, to ensure that they are being run before
3227 any other watchers after the poll (this doesn't matter for \f(CW\*(C`ev_prepare\*(C'\fR
3228 watchers).
3229 .PP
3230 Also, \f(CW\*(C`ev_check\*(C'\fR watchers (and \f(CW\*(C`ev_prepare\*(C'\fR watchers, too) should not
3231 activate (\*(L"feed\*(R") events into libev. While libev fully supports this, they
3232 might get executed before other \f(CW\*(C`ev_check\*(C'\fR watchers did their job. As
3233 \&\f(CW\*(C`ev_check\*(C'\fR watchers are often used to embed other (non-libev) event
3234 loops those other event loops might be in an unusable state until their
3235 \&\f(CW\*(C`ev_check\*(C'\fR watcher ran (always remind yourself to coexist peacefully with
3236 others).
3237 .PP
3238 \fIAbusing an \f(CI\*(C`ev_check\*(C'\fI watcher for its side-effect\fR
3239 .IX Subsection "Abusing an ev_check watcher for its side-effect"
3240 .PP
3241 \&\f(CW\*(C`ev_check\*(C'\fR (and less often also \f(CW\*(C`ev_prepare\*(C'\fR) watchers can also be
3242 useful because they are called once per event loop iteration. For
3243 example, if you want to handle a large number of connections fairly, you
3244 normally only do a bit of work for each active connection, and if there
3245 is more work to do, you wait for the next event loop iteration, so other
3246 connections have a chance of making progress.
3247 .PP
3248 Using an \f(CW\*(C`ev_check\*(C'\fR watcher is almost enough: it will be called on the
3249 next event loop iteration. However, that isn't as soon as possible \-
3250 without external events, your \f(CW\*(C`ev_check\*(C'\fR watcher will not be invoked.
3251 .PP
3252 This is where \f(CW\*(C`ev_idle\*(C'\fR watchers come in handy \- all you need is a
3253 single global idle watcher that is active as long as you have one active
3254 \&\f(CW\*(C`ev_check\*(C'\fR watcher. The \f(CW\*(C`ev_idle\*(C'\fR watcher makes sure the event loop
3255 will not sleep, and the \f(CW\*(C`ev_check\*(C'\fR watcher makes sure a callback gets
3256 invoked. Neither watcher alone can do that.
3257 .PP
3258 \fIWatcher-Specific Functions and Data Members\fR
3259 .IX Subsection "Watcher-Specific Functions and Data Members"
3260 .IP "ev_prepare_init (ev_prepare *, callback)" 4
3261 .IX Item "ev_prepare_init (ev_prepare *, callback)"
3262 .PD 0
3263 .IP "ev_check_init (ev_check *, callback)" 4
3264 .IX Item "ev_check_init (ev_check *, callback)"
3265 .PD
3266 Initialises and configures the prepare or check watcher \- they have no
3267 parameters of any kind. There are \f(CW\*(C`ev_prepare_set\*(C'\fR and \f(CW\*(C`ev_check_set\*(C'\fR
3268 macros, but using them is utterly, utterly, utterly and completely
3269 pointless.
3270 .PP
3271 \fIExamples\fR
3272 .IX Subsection "Examples"
3273 .PP
3274 There are a number of principal ways to embed other event loops or modules
3275 into libev. Here are some ideas on how to include libadns into libev
3276 (there is a Perl module named \f(CW\*(C`EV::ADNS\*(C'\fR that does this, which you could
3277 use as a working example. Another Perl module named \f(CW\*(C`EV::Glib\*(C'\fR embeds a
3278 Glib main context into libev, and finally, \f(CW\*(C`Glib::EV\*(C'\fR embeds \s-1EV\s0 into the
3279 Glib event loop).
3280 .PP
3281 Method 1: Add \s-1IO\s0 watchers and a timeout watcher in a prepare handler,
3282 and in a check watcher, destroy them and call into libadns. What follows
3283 is pseudo-code only of course. This requires you to either use a low
3284 priority for the check watcher or use \f(CW\*(C`ev_clear_pending\*(C'\fR explicitly, as
3285 the callbacks for the IO/timeout watchers might not have been called yet.
3286 .PP
3287 .Vb 2
3288 \& static ev_io iow [nfd];
3289 \& static ev_timer tw;
3290 \&
3291 \& static void
3292 \& io_cb (struct ev_loop *loop, ev_io *w, int revents)
3293 \& {
3294 \& }
3295 \&
3296 \& // create io watchers for each fd and a timer before blocking
3297 \& static void
3298 \& adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3299 \& {
3300 \& int timeout = 3600000;
3301 \& struct pollfd fds [nfd];
3302 \& // actual code will need to loop here and realloc etc.
3303 \& adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3304 \&
3305 \& /* the callback is illegal, but won\*(Aqt be called as we stop during check */
3306 \& ev_timer_init (&tw, 0, timeout * 1e\-3, 0.);
3307 \& ev_timer_start (loop, &tw);
3308 \&
3309 \& // create one ev_io per pollfd
3310 \& for (int i = 0; i < nfd; ++i)
3311 \& {
3312 \& ev_io_init (iow + i, io_cb, fds [i].fd,
3313 \& ((fds [i].events & POLLIN ? EV_READ : 0)
3314 \& | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3315 \&
3316 \& fds [i].revents = 0;
3317 \& ev_io_start (loop, iow + i);
3318 \& }
3319 \& }
3320 \&
3321 \& // stop all watchers after blocking
3322 \& static void
3323 \& adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3324 \& {
3325 \& ev_timer_stop (loop, &tw);
3326 \&
3327 \& for (int i = 0; i < nfd; ++i)
3328 \& {
3329 \& // set the relevant poll flags
3330 \& // could also call adns_processreadable etc. here
3331 \& struct pollfd *fd = fds + i;
3332 \& int revents = ev_clear_pending (iow + i);
3333 \& if (revents & EV_READ ) fd\->revents |= fd\->events & POLLIN;
3334 \& if (revents & EV_WRITE) fd\->revents |= fd\->events & POLLOUT;
3335 \&
3336 \& // now stop the watcher
3337 \& ev_io_stop (loop, iow + i);
3338 \& }
3339 \&
3340 \& adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3341 \& }
3342 .Ve
3343 .PP
3344 Method 2: This would be just like method 1, but you run \f(CW\*(C`adns_afterpoll\*(C'\fR
3345 in the prepare watcher and would dispose of the check watcher.
3346 .PP
3347 Method 3: If the module to be embedded supports explicit event
3348 notification (libadns does), you can also make use of the actual watcher
3349 callbacks, and only destroy/create the watchers in the prepare watcher.
3350 .PP
3351 .Vb 5
3352 \& static void
3353 \& timer_cb (EV_P_ ev_timer *w, int revents)
3354 \& {
3355 \& adns_state ads = (adns_state)w\->data;
3356 \& update_now (EV_A);
3357 \&
3358 \& adns_processtimeouts (ads, &tv_now);
3359 \& }
3360 \&
3361 \& static void
3362 \& io_cb (EV_P_ ev_io *w, int revents)
3363 \& {
3364 \& adns_state ads = (adns_state)w\->data;
3365 \& update_now (EV_A);
3366 \&
3367 \& if (revents & EV_READ ) adns_processreadable (ads, w\->fd, &tv_now);
3368 \& if (revents & EV_WRITE) adns_processwriteable (ads, w\->fd, &tv_now);
3369 \& }
3370 \&
3371 \& // do not ever call adns_afterpoll
3372 .Ve
3373 .PP
3374 Method 4: Do not use a prepare or check watcher because the module you
3375 want to embed is not flexible enough to support it. Instead, you can
3376 override their poll function. The drawback with this solution is that the
3377 main loop is now no longer controllable by \s-1EV.\s0 The \f(CW\*(C`Glib::EV\*(C'\fR module uses
3378 this approach, effectively embedding \s-1EV\s0 as a client into the horrible
3379 libglib event loop.
3380 .PP
3381 .Vb 4
3382 \& static gint
3383 \& event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3384 \& {
3385 \& int got_events = 0;
3386 \&
3387 \& for (n = 0; n < nfds; ++n)
3388 \& // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3389 \&
3390 \& if (timeout >= 0)
3391 \& // create/start timer
3392 \&
3393 \& // poll
3394 \& ev_run (EV_A_ 0);
3395 \&
3396 \& // stop timer again
3397 \& if (timeout >= 0)
3398 \& ev_timer_stop (EV_A_ &to);
3399 \&
3400 \& // stop io watchers again \- their callbacks should have set
3401 \& for (n = 0; n < nfds; ++n)
3402 \& ev_io_stop (EV_A_ iow [n]);
3403 \&
3404 \& return got_events;
3405 \& }
3406 .Ve
3407 .ie n .SS """ev_embed"" \- when one backend isn't enough..."
3408 .el .SS "\f(CWev_embed\fP \- when one backend isn't enough..."
3409 .IX Subsection "ev_embed - when one backend isn't enough..."
3410 This is a rather advanced watcher type that lets you embed one event loop
3411 into another (currently only \f(CW\*(C`ev_io\*(C'\fR events are supported in the embedded
3412 loop, other types of watchers might be handled in a delayed or incorrect
3413 fashion and must not be used).
3414 .PP
3415 There are primarily two reasons you would want that: work around bugs and
3416 prioritise I/O.
3417 .PP
3418 As an example for a bug workaround, the kqueue backend might only support
3419 sockets on some platform, so it is unusable as generic backend, but you
3420 still want to make use of it because you have many sockets and it scales
3421 so nicely. In this case, you would create a kqueue-based loop and embed
3422 it into your default loop (which might use e.g. poll). Overall operation
3423 will be a bit slower because first libev has to call \f(CW\*(C`poll\*(C'\fR and then
3424 \&\f(CW\*(C`kevent\*(C'\fR, but at least you can use both mechanisms for what they are
3425 best: \f(CW\*(C`kqueue\*(C'\fR for scalable sockets and \f(CW\*(C`poll\*(C'\fR if you want it to work :)
3426 .PP
3427 As for prioritising I/O: under rare circumstances you have the case where
3428 some fds have to be watched and handled very quickly (with low latency),
3429 and even priorities and idle watchers might have too much overhead. In
3430 this case you would put all the high priority stuff in one loop and all
3431 the rest in a second one, and embed the second one in the first.
3432 .PP
3433 As long as the watcher is active, the callback will be invoked every
3434 time there might be events pending in the embedded loop. The callback
3435 must then call \f(CW\*(C`ev_embed_sweep (mainloop, watcher)\*(C'\fR to make a single
3436 sweep and invoke their callbacks (the callback doesn't need to invoke the
3437 \&\f(CW\*(C`ev_embed_sweep\*(C'\fR function directly, it could also start an idle watcher
3438 to give the embedded loop strictly lower priority for example).
3439 .PP
3440 You can also set the callback to \f(CW0\fR, in which case the embed watcher
3441 will automatically execute the embedded loop sweep whenever necessary.
3442 .PP
3443 Fork detection will be handled transparently while the \f(CW\*(C`ev_embed\*(C'\fR watcher
3444 is active, i.e., the embedded loop will automatically be forked when the
3445 embedding loop forks. In other cases, the user is responsible for calling
3446 \&\f(CW\*(C`ev_loop_fork\*(C'\fR on the embedded loop.
3447 .PP
3448 Unfortunately, not all backends are embeddable: only the ones returned by
3449 \&\f(CW\*(C`ev_embeddable_backends\*(C'\fR are, which, unfortunately, does not include any
3450 portable one.
3451 .PP
3452 So when you want to use this feature you will always have to be prepared
3453 that you cannot get an embeddable loop. The recommended way to get around
3454 this is to have a separate variables for your embeddable loop, try to
3455 create it, and if that fails, use the normal loop for everything.
3456 .PP
3457 \fI\f(CI\*(C`ev_embed\*(C'\fI and fork\fR
3458 .IX Subsection "ev_embed and fork"
3459 .PP
3460 While the \f(CW\*(C`ev_embed\*(C'\fR watcher is running, forks in the embedding loop will
3461 automatically be applied to the embedded loop as well, so no special
3462 fork handling is required in that case. When the watcher is not running,
3463 however, it is still the task of the libev user to call \f(CW\*(C`ev_loop_fork ()\*(C'\fR
3464 as applicable.
3465 .PP
3466 \fIWatcher-Specific Functions and Data Members\fR
3467 .IX Subsection "Watcher-Specific Functions and Data Members"
3468 .IP "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)" 4
3469 .IX Item "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)"
3470 .PD 0
3471 .IP "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)" 4
3472 .IX Item "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)"
3473 .PD
3474 Configures the watcher to embed the given loop, which must be
3475 embeddable. If the callback is \f(CW0\fR, then \f(CW\*(C`ev_embed_sweep\*(C'\fR will be
3476 invoked automatically, otherwise it is the responsibility of the callback
3477 to invoke it (it will continue to be called until the sweep has been done,
3478 if you do not want that, you need to temporarily stop the embed watcher).
3479 .IP "ev_embed_sweep (loop, ev_embed *)" 4
3480 .IX Item "ev_embed_sweep (loop, ev_embed *)"
3481 Make a single, non-blocking sweep over the embedded loop. This works
3482 similarly to \f(CW\*(C`ev_run (embedded_loop, EVRUN_NOWAIT)\*(C'\fR, but in the most
3483 appropriate way for embedded loops.
3484 .IP "struct ev_loop *other [read\-only]" 4
3485 .IX Item "struct ev_loop *other [read-only]"
3486 The embedded event loop.
3487 .PP
3488 \fIExamples\fR
3489 .IX Subsection "Examples"
3490 .PP
3491 Example: Try to get an embeddable event loop and embed it into the default
3492 event loop. If that is not possible, use the default loop. The default
3493 loop is stored in \f(CW\*(C`loop_hi\*(C'\fR, while the embeddable loop is stored in
3494 \&\f(CW\*(C`loop_lo\*(C'\fR (which is \f(CW\*(C`loop_hi\*(C'\fR in the case no embeddable loop can be
3495 used).
3496 .PP
3497 .Vb 3
3498 \& struct ev_loop *loop_hi = ev_default_init (0);
3499 \& struct ev_loop *loop_lo = 0;
3500 \& ev_embed embed;
3501 \&
3502 \& // see if there is a chance of getting one that works
3503 \& // (remember that a flags value of 0 means autodetection)
3504 \& loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3505 \& ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3506 \& : 0;
3507 \&
3508 \& // if we got one, then embed it, otherwise default to loop_hi
3509 \& if (loop_lo)
3510 \& {
3511 \& ev_embed_init (&embed, 0, loop_lo);
3512 \& ev_embed_start (loop_hi, &embed);
3513 \& }
3514 \& else
3515 \& loop_lo = loop_hi;
3516 .Ve
3517 .PP
3518 Example: Check if kqueue is available but not recommended and create
3519 a kqueue backend for use with sockets (which usually work with any
3520 kqueue implementation). Store the kqueue/socket\-only event loop in
3521 \&\f(CW\*(C`loop_socket\*(C'\fR. (One might optionally use \f(CW\*(C`EVFLAG_NOENV\*(C'\fR, too).
3522 .PP
3523 .Vb 3
3524 \& struct ev_loop *loop = ev_default_init (0);
3525 \& struct ev_loop *loop_socket = 0;
3526 \& ev_embed embed;
3527 \&
3528 \& if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3529 \& if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3530 \& {
3531 \& ev_embed_init (&embed, 0, loop_socket);
3532 \& ev_embed_start (loop, &embed);
3533 \& }
3534 \&
3535 \& if (!loop_socket)
3536 \& loop_socket = loop;
3537 \&
3538 \& // now use loop_socket for all sockets, and loop for everything else
3539 .Ve
3540 .ie n .SS """ev_fork"" \- the audacity to resume the event loop after a fork"
3541 .el .SS "\f(CWev_fork\fP \- the audacity to resume the event loop after a fork"
3542 .IX Subsection "ev_fork - the audacity to resume the event loop after a fork"
3543 Fork watchers are called when a \f(CW\*(C`fork ()\*(C'\fR was detected (usually because
3544 whoever is a good citizen cared to tell libev about it by calling
3545 \&\f(CW\*(C`ev_loop_fork\*(C'\fR). The invocation is done before the event loop blocks next
3546 and before \f(CW\*(C`ev_check\*(C'\fR watchers are being called, and only in the child
3547 after the fork. If whoever good citizen calling \f(CW\*(C`ev_default_fork\*(C'\fR cheats
3548 and calls it in the wrong process, the fork handlers will be invoked, too,
3549 of course.
3550 .PP
3551 \fIThe special problem of life after fork \- how is it possible?\fR
3552 .IX Subsection "The special problem of life after fork - how is it possible?"
3553 .PP
3554 Most uses of \f(CW\*(C`fork ()\*(C'\fR consist of forking, then some simple calls to set
3555 up/change the process environment, followed by a call to \f(CW\*(C`exec()\*(C'\fR. This
3556 sequence should be handled by libev without any problems.
3557 .PP
3558 This changes when the application actually wants to do event handling
3559 in the child, or both parent in child, in effect \*(L"continuing\*(R" after the
3560 fork.
3561 .PP
3562 The default mode of operation (for libev, with application help to detect
3563 forks) is to duplicate all the state in the child, as would be expected
3564 when \fIeither\fR the parent \fIor\fR the child process continues.
3565 .PP
3566 When both processes want to continue using libev, then this is usually the
3567 wrong result. In that case, usually one process (typically the parent) is
3568 supposed to continue with all watchers in place as before, while the other
3569 process typically wants to start fresh, i.e. without any active watchers.
3570 .PP
3571 The cleanest and most efficient way to achieve that with libev is to
3572 simply create a new event loop, which of course will be \*(L"empty\*(R", and
3573 use that for new watchers. This has the advantage of not touching more
3574 memory than necessary, and thus avoiding the copy-on-write, and the
3575 disadvantage of having to use multiple event loops (which do not support
3576 signal watchers).
3577 .PP
3578 When this is not possible, or you want to use the default loop for
3579 other reasons, then in the process that wants to start \*(L"fresh\*(R", call
3580 \&\f(CW\*(C`ev_loop_destroy (EV_DEFAULT)\*(C'\fR followed by \f(CW\*(C`ev_default_loop (...)\*(C'\fR.
3581 Destroying the default loop will \*(L"orphan\*(R" (not stop) all registered
3582 watchers, so you have to be careful not to execute code that modifies
3583 those watchers. Note also that in that case, you have to re-register any
3584 signal watchers.
3585 .PP
3586 \fIWatcher-Specific Functions and Data Members\fR
3587 .IX Subsection "Watcher-Specific Functions and Data Members"
3588 .IP "ev_fork_init (ev_fork *, callback)" 4
3589 .IX Item "ev_fork_init (ev_fork *, callback)"
3590 Initialises and configures the fork watcher \- it has no parameters of any
3591 kind. There is a \f(CW\*(C`ev_fork_set\*(C'\fR macro, but using it is utterly pointless,
3592 really.
3593 .ie n .SS """ev_cleanup"" \- even the best things end"
3594 .el .SS "\f(CWev_cleanup\fP \- even the best things end"
3595 .IX Subsection "ev_cleanup - even the best things end"
3596 Cleanup watchers are called just before the event loop is being destroyed
3597 by a call to \f(CW\*(C`ev_loop_destroy\*(C'\fR.
3598 .PP
3599 While there is no guarantee that the event loop gets destroyed, cleanup
3600 watchers provide a convenient method to install cleanup hooks for your
3601 program, worker threads and so on \- you just to make sure to destroy the
3602 loop when you want them to be invoked.
3603 .PP
3604 Cleanup watchers are invoked in the same way as any other watcher. Unlike
3605 all other watchers, they do not keep a reference to the event loop (which
3606 makes a lot of sense if you think about it). Like all other watchers, you
3607 can call libev functions in the callback, except \f(CW\*(C`ev_cleanup_start\*(C'\fR.
3608 .PP
3609 \fIWatcher-Specific Functions and Data Members\fR
3610 .IX Subsection "Watcher-Specific Functions and Data Members"
3611 .IP "ev_cleanup_init (ev_cleanup *, callback)" 4
3612 .IX Item "ev_cleanup_init (ev_cleanup *, callback)"
3613 Initialises and configures the cleanup watcher \- it has no parameters of
3614 any kind. There is a \f(CW\*(C`ev_cleanup_set\*(C'\fR macro, but using it is utterly
3615 pointless, I assure you.
3616 .PP
3617 Example: Register an atexit handler to destroy the default loop, so any
3618 cleanup functions are called.
3619 .PP
3620 .Vb 5
3621 \& static void
3622 \& program_exits (void)
3623 \& {
3624 \& ev_loop_destroy (EV_DEFAULT_UC);
3625 \& }
3626 \&
3627 \& ...
3628 \& atexit (program_exits);
3629 .Ve
3630 .ie n .SS """ev_async"" \- how to wake up an event loop"
3631 .el .SS "\f(CWev_async\fP \- how to wake up an event loop"
3632 .IX Subsection "ev_async - how to wake up an event loop"
3633 In general, you cannot use an \f(CW\*(C`ev_loop\*(C'\fR from multiple threads or other
3634 asynchronous sources such as signal handlers (as opposed to multiple event
3635 loops \- those are of course safe to use in different threads).
3636 .PP
3637 Sometimes, however, you need to wake up an event loop you do not control,
3638 for example because it belongs to another thread. This is what \f(CW\*(C`ev_async\*(C'\fR
3639 watchers do: as long as the \f(CW\*(C`ev_async\*(C'\fR watcher is active, you can signal
3640 it by calling \f(CW\*(C`ev_async_send\*(C'\fR, which is thread\- and signal safe.
3641 .PP
3642 This functionality is very similar to \f(CW\*(C`ev_signal\*(C'\fR watchers, as signals,
3643 too, are asynchronous in nature, and signals, too, will be compressed
3644 (i.e. the number of callback invocations may be less than the number of
3645 \&\f(CW\*(C`ev_async_send\*(C'\fR calls). In fact, you could use signal watchers as a kind
3646 of \*(L"global async watchers\*(R" by using a watcher on an otherwise unused
3647 signal, and \f(CW\*(C`ev_feed_signal\*(C'\fR to signal this watcher from another thread,
3648 even without knowing which loop owns the signal.
3649 .PP
3650 \fIQueueing\fR
3651 .IX Subsection "Queueing"
3652 .PP
3653 \&\f(CW\*(C`ev_async\*(C'\fR does not support queueing of data in any way. The reason
3654 is that the author does not know of a simple (or any) algorithm for a
3655 multiple-writer-single-reader queue that works in all cases and doesn't
3656 need elaborate support such as pthreads or unportable memory access
3657 semantics.
3658 .PP
3659 That means that if you want to queue data, you have to provide your own
3660 queue. But at least I can tell you how to implement locking around your
3661 queue:
3662 .IP "queueing from a signal handler context" 4
3663 .IX Item "queueing from a signal handler context"
3664 To implement race-free queueing, you simply add to the queue in the signal
3665 handler but you block the signal handler in the watcher callback. Here is
3666 an example that does that for some fictitious \s-1SIGUSR1\s0 handler:
3667 .Sp
3668 .Vb 1
3669 \& static ev_async mysig;
3670 \&
3671 \& static void
3672 \& sigusr1_handler (void)
3673 \& {
3674 \& sometype data;
3675 \&
3676 \& // no locking etc.
3677 \& queue_put (data);
3678 \& ev_async_send (EV_DEFAULT_ &mysig);
3679 \& }
3680 \&
3681 \& static void
3682 \& mysig_cb (EV_P_ ev_async *w, int revents)
3683 \& {
3684 \& sometype data;
3685 \& sigset_t block, prev;
3686 \&
3687 \& sigemptyset (&block);
3688 \& sigaddset (&block, SIGUSR1);
3689 \& sigprocmask (SIG_BLOCK, &block, &prev);
3690 \&
3691 \& while (queue_get (&data))
3692 \& process (data);
3693 \&
3694 \& if (sigismember (&prev, SIGUSR1)
3695 \& sigprocmask (SIG_UNBLOCK, &block, 0);
3696 \& }
3697 .Ve
3698 .Sp
3699 (Note: pthreads in theory requires you to use \f(CW\*(C`pthread_setmask\*(C'\fR
3700 instead of \f(CW\*(C`sigprocmask\*(C'\fR when you use threads, but libev doesn't do it
3701 either...).
3702 .IP "queueing from a thread context" 4
3703 .IX Item "queueing from a thread context"
3704 The strategy for threads is different, as you cannot (easily) block
3705 threads but you can easily preempt them, so to queue safely you need to
3706 employ a traditional mutex lock, such as in this pthread example:
3707 .Sp
3708 .Vb 2
3709 \& static ev_async mysig;
3710 \& static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3711 \&
3712 \& static void
3713 \& otherthread (void)
3714 \& {
3715 \& // only need to lock the actual queueing operation
3716 \& pthread_mutex_lock (&mymutex);
3717 \& queue_put (data);
3718 \& pthread_mutex_unlock (&mymutex);
3719 \&
3720 \& ev_async_send (EV_DEFAULT_ &mysig);
3721 \& }
3722 \&
3723 \& static void
3724 \& mysig_cb (EV_P_ ev_async *w, int revents)
3725 \& {
3726 \& pthread_mutex_lock (&mymutex);
3727 \&
3728 \& while (queue_get (&data))
3729 \& process (data);
3730 \&
3731 \& pthread_mutex_unlock (&mymutex);
3732 \& }
3733 .Ve
3734 .PP
3735 \fIWatcher-Specific Functions and Data Members\fR
3736 .IX Subsection "Watcher-Specific Functions and Data Members"
3737 .IP "ev_async_init (ev_async *, callback)" 4
3738 .IX Item "ev_async_init (ev_async *, callback)"
3739 Initialises and configures the async watcher \- it has no parameters of any
3740 kind. There is a \f(CW\*(C`ev_async_set\*(C'\fR macro, but using it is utterly pointless,
3741 trust me.
3742 .IP "ev_async_send (loop, ev_async *)" 4
3743 .IX Item "ev_async_send (loop, ev_async *)"
3744 Sends/signals/activates the given \f(CW\*(C`ev_async\*(C'\fR watcher, that is, feeds
3745 an \f(CW\*(C`EV_ASYNC\*(C'\fR event on the watcher into the event loop, and instantly
3746 returns.
3747 .Sp
3748 Unlike \f(CW\*(C`ev_feed_event\*(C'\fR, this call is safe to do from other threads,
3749 signal or similar contexts (see the discussion of \f(CW\*(C`EV_ATOMIC_T\*(C'\fR in the
3750 embedding section below on what exactly this means).
3751 .Sp
3752 Note that, as with other watchers in libev, multiple events might get
3753 compressed into a single callback invocation (another way to look at
3754 this is that \f(CW\*(C`ev_async\*(C'\fR watchers are level-triggered: they are set on
3755 \&\f(CW\*(C`ev_async_send\*(C'\fR, reset when the event loop detects that).
3756 .Sp
3757 This call incurs the overhead of at most one extra system call per event
3758 loop iteration, if the event loop is blocked, and no syscall at all if
3759 the event loop (or your program) is processing events. That means that
3760 repeated calls are basically free (there is no need to avoid calls for
3761 performance reasons) and that the overhead becomes smaller (typically
3762 zero) under load.
3763 .IP "bool = ev_async_pending (ev_async *)" 4
3764 .IX Item "bool = ev_async_pending (ev_async *)"
3765 Returns a non-zero value when \f(CW\*(C`ev_async_send\*(C'\fR has been called on the
3766 watcher but the event has not yet been processed (or even noted) by the
3767 event loop.
3768 .Sp
3769 \&\f(CW\*(C`ev_async_send\*(C'\fR sets a flag in the watcher and wakes up the loop. When
3770 the loop iterates next and checks for the watcher to have become active,
3771 it will reset the flag again. \f(CW\*(C`ev_async_pending\*(C'\fR can be used to very
3772 quickly check whether invoking the loop might be a good idea.
3773 .Sp
3774 Not that this does \fInot\fR check whether the watcher itself is pending,
3775 only whether it has been requested to make this watcher pending: there
3776 is a time window between the event loop checking and resetting the async
3777 notification, and the callback being invoked.
3780 There are some other functions of possible interest. Described. Here. Now.
3781 .IP "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)" 4
3782 .IX Item "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)"
3783 This function combines a simple timer and an I/O watcher, calls your
3784 callback on whichever event happens first and automatically stops both
3785 watchers. This is useful if you want to wait for a single event on an fd
3786 or timeout without having to allocate/configure/start/stop/free one or
3787 more watchers yourself.
3788 .Sp
3789 If \f(CW\*(C`fd\*(C'\fR is less than 0, then no I/O watcher will be started and the
3790 \&\f(CW\*(C`events\*(C'\fR argument is being ignored. Otherwise, an \f(CW\*(C`ev_io\*(C'\fR watcher for
3791 the given \f(CW\*(C`fd\*(C'\fR and \f(CW\*(C`events\*(C'\fR set will be created and started.
3792 .Sp
3793 If \f(CW\*(C`timeout\*(C'\fR is less than 0, then no timeout watcher will be
3794 started. Otherwise an \f(CW\*(C`ev_timer\*(C'\fR watcher with after = \f(CW\*(C`timeout\*(C'\fR (and
3795 repeat = 0) will be started. \f(CW0\fR is a valid timeout.
3796 .Sp
3797 The callback has the type \f(CW\*(C`void (*cb)(int revents, void *arg)\*(C'\fR and is
3798 passed an \f(CW\*(C`revents\*(C'\fR set like normal event callbacks (a combination of
3799 \&\f(CW\*(C`EV_ERROR\*(C'\fR, \f(CW\*(C`EV_READ\*(C'\fR, \f(CW\*(C`EV_WRITE\*(C'\fR or \f(CW\*(C`EV_TIMER\*(C'\fR) and the \f(CW\*(C`arg\*(C'\fR
3800 value passed to \f(CW\*(C`ev_once\*(C'\fR. Note that it is possible to receive \fIboth\fR
3801 a timeout and an io event at the same time \- you probably should give io
3802 events precedence.
3803 .Sp
3804 Example: wait up to ten seconds for data to appear on \s-1STDIN_FILENO.\s0
3805 .Sp
3806 .Vb 7
3807 \& static void stdin_ready (int revents, void *arg)
3808 \& {
3809 \& if (revents & EV_READ)
3810 \& /* stdin might have data for us, joy! */;
3811 \& else if (revents & EV_TIMER)
3812 \& /* doh, nothing entered */;
3813 \& }
3814 \&
3815 \& ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3816 .Ve
3817 .IP "ev_feed_fd_event (loop, int fd, int revents)" 4
3818 .IX Item "ev_feed_fd_event (loop, int fd, int revents)"
3819 Feed an event on the given fd, as if a file descriptor backend detected
3820 the given events.
3821 .IP "ev_feed_signal_event (loop, int signum)" 4
3822 .IX Item "ev_feed_signal_event (loop, int signum)"
3823 Feed an event as if the given signal occurred. See also \f(CW\*(C`ev_feed_signal\*(C'\fR,
3824 which is async-safe.
3827 This section explains some common idioms that are not immediately
3828 obvious. Note that examples are sprinkled over the whole manual, and this
3829 section only contains stuff that wouldn't fit anywhere else.
3832 Each watcher has, by default, a \f(CW\*(C`void *data\*(C'\fR member that you can read
3833 or modify at any time: libev will completely ignore it. This can be used
3834 to associate arbitrary data with your watcher. If you need more data and
3835 don't want to allocate memory separately and store a pointer to it in that
3836 data member, you can also \*(L"subclass\*(R" the watcher type and provide your own
3837 data:
3838 .PP
3839 .Vb 7
3840 \& struct my_io
3841 \& {
3842 \& ev_io io;
3843 \& int otherfd;
3844 \& void *somedata;
3845 \& struct whatever *mostinteresting;
3846 \& };
3847 \&
3848 \& ...
3849 \& struct my_io w;
3850 \& ev_io_init (&, my_cb, fd, EV_READ);
3851 .Ve
3852 .PP
3853 And since your callback will be called with a pointer to the watcher, you
3854 can cast it back to your own type:
3855 .PP
3856 .Vb 5
3857 \& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3858 \& {
3859 \& struct my_io *w = (struct my_io *)w_;
3860 \& ...
3861 \& }
3862 .Ve
3863 .PP
3864 More interesting and less C\-conformant ways of casting your callback
3865 function type instead have been omitted.
3868 Another common scenario is to use some data structure with multiple
3869 embedded watchers, in effect creating your own watcher that combines
3870 multiple libev event sources into one \*(L"super-watcher\*(R":
3871 .PP
3872 .Vb 6
3873 \& struct my_biggy
3874 \& {
3875 \& int some_data;
3876 \& ev_timer t1;
3877 \& ev_timer t2;
3878 \& }
3879 .Ve
3880 .PP
3881 In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more
3882 complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct in
3883 the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies or \*(C+ coders), or you need
3884 to use some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for
3885 real programmers):
3886 .PP
3887 .Vb 1
3888 \& #include <stddef.h>
3889 \&
3890 \& static void
3891 \& t1_cb (EV_P_ ev_timer *w, int revents)
3892 \& {
3893 \& struct my_biggy big = (struct my_biggy *)
3894 \& (((char *)w) \- offsetof (struct my_biggy, t1));
3895 \& }
3896 \&
3897 \& static void
3898 \& t2_cb (EV_P_ ev_timer *w, int revents)
3899 \& {
3900 \& struct my_biggy big = (struct my_biggy *)
3901 \& (((char *)w) \- offsetof (struct my_biggy, t2));
3902 \& }
3903 .Ve
3906 Often you have structures like this in event-based programs:
3907 .PP
3908 .Vb 4
3909 \& callback ()
3910 \& {
3911 \& free (request);
3912 \& }
3913 \&
3914 \& request = start_new_request (..., callback);
3915 .Ve
3916 .PP
3917 The intent is to start some \*(L"lengthy\*(R" operation. The \f(CW\*(C`request\*(C'\fR could be
3918 used to cancel the operation, or do other things with it.
3919 .PP
3920 It's not uncommon to have code paths in \f(CW\*(C`start_new_request\*(C'\fR that
3921 immediately invoke the callback, for example, to report errors. Or you add
3922 some caching layer that finds that it can skip the lengthy aspects of the
3923 operation and simply invoke the callback with the result.
3924 .PP
3925 The problem here is that this will happen \fIbefore\fR \f(CW\*(C`start_new_request\*(C'\fR
3926 has returned, so \f(CW\*(C`request\*(C'\fR is not set.
3927 .PP
3928 Even if you pass the request by some safer means to the callback, you
3929 might want to do something to the request after starting it, such as
3930 canceling it, which probably isn't working so well when the callback has
3931 already been invoked.
3932 .PP
3933 A common way around all these issues is to make sure that
3934 \&\f(CW\*(C`start_new_request\*(C'\fR \fIalways\fR returns before the callback is invoked. If
3935 \&\f(CW\*(C`start_new_request\*(C'\fR immediately knows the result, it can artificially
3936 delay invoking the callback by using a \f(CW\*(C`prepare\*(C'\fR or \f(CW\*(C`idle\*(C'\fR watcher for
3937 example, or more sneakily, by reusing an existing (stopped) watcher and
3938 pushing it into the pending queue:
3939 .PP
3940 .Vb 2
3941 \& ev_set_cb (watcher, callback);
3942 \& ev_feed_event (EV_A_ watcher, 0);
3943 .Ve
3944 .PP
3945 This way, \f(CW\*(C`start_new_request\*(C'\fR can safely return before the callback is
3946 invoked, while not delaying callback invocation too much.
3949 Often (especially in \s-1GUI\s0 toolkits) there are places where you have
3950 \&\fImodal\fR interaction, which is most easily implemented by recursively
3951 invoking \f(CW\*(C`ev_run\*(C'\fR.
3952 .PP
3953 This brings the problem of exiting \- a callback might want to finish the
3954 main \f(CW\*(C`ev_run\*(C'\fR call, but not the nested one (e.g. user clicked \*(L"Quit\*(R", but
3955 a modal \*(L"Are you sure?\*(R" dialog is still waiting), or just the nested one
3956 and not the main one (e.g. user clocked \*(L"Ok\*(R" in a modal dialog), or some
3957 other combination: In these cases, a simple \f(CW\*(C`ev_break\*(C'\fR will not work.
3958 .PP
3959 The solution is to maintain \*(L"break this loop\*(R" variable for each \f(CW\*(C`ev_run\*(C'\fR
3960 invocation, and use a loop around \f(CW\*(C`ev_run\*(C'\fR until the condition is
3961 triggered, using \f(CW\*(C`EVRUN_ONCE\*(C'\fR:
3962 .PP
3963 .Vb 2
3964 \& // main loop
3965 \& int exit_main_loop = 0;
3966 \&
3967 \& while (!exit_main_loop)
3968 \& ev_run (EV_DEFAULT_ EVRUN_ONCE);
3969 \&
3970 \& // in a modal watcher
3971 \& int exit_nested_loop = 0;
3972 \&
3973 \& while (!exit_nested_loop)
3974 \& ev_run (EV_A_ EVRUN_ONCE);
3975 .Ve
3976 .PP
3977 To exit from any of these loops, just set the corresponding exit variable:
3978 .PP
3979 .Vb 2
3980 \& // exit modal loop
3981 \& exit_nested_loop = 1;
3982 \&
3983 \& // exit main program, after modal loop is finished
3984 \& exit_main_loop = 1;
3985 \&
3986 \& // exit both
3987 \& exit_main_loop = exit_nested_loop = 1;
3988 .Ve
3991 Here is a fictitious example of how to run an event loop in a different
3992 thread from where callbacks are being invoked and watchers are
3993 created/added/removed.
3994 .PP
3995 For a real-world example, see the \f(CW\*(C`EV::Loop::Async\*(C'\fR perl module,
3996 which uses exactly this technique (which is suited for many high-level
3997 languages).
3998 .PP
3999 The example uses a pthread mutex to protect the loop data, a condition
4000 variable to wait for callback invocations, an async watcher to notify the
4001 event loop thread and an unspecified mechanism to wake up the main thread.
4002 .PP
4003 First, you need to associate some data with the event loop:
4004 .PP
4005 .Vb 6
4006 \& typedef struct {
4007 \& mutex_t lock; /* global loop lock */
4008 \& ev_async async_w;
4009 \& thread_t tid;
4010 \& cond_t invoke_cv;
4011 \& } userdata;
4012 \&
4013 \& void prepare_loop (EV_P)
4014 \& {
4015 \& // for simplicity, we use a static userdata struct.
4016 \& static userdata u;
4017 \&
4018 \& ev_async_init (&u\->async_w, async_cb);
4019 \& ev_async_start (EV_A_ &u\->async_w);
4020 \&
4021 \& pthread_mutex_init (&u\->lock, 0);
4022 \& pthread_cond_init (&u\->invoke_cv, 0);
4023 \&
4024 \& // now associate this with the loop
4025 \& ev_set_userdata (EV_A_ u);
4026 \& ev_set_invoke_pending_cb (EV_A_ l_invoke);
4027 \& ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
4028 \&
4029 \& // then create the thread running ev_run
4030 \& pthread_create (&u\->tid, 0, l_run, EV_A);
4031 \& }
4032 .Ve
4033 .PP
4034 The callback for the \f(CW\*(C`ev_async\*(C'\fR watcher does nothing: the watcher is used
4035 solely to wake up the event loop so it takes notice of any new watchers
4036 that might have been added:
4037 .PP
4038 .Vb 5
4039 \& static void
4040 \& async_cb (EV_P_ ev_async *w, int revents)
4041 \& {
4042 \& // just used for the side effects
4043 \& }
4044 .Ve
4045 .PP
4046 The \f(CW\*(C`l_release\*(C'\fR and \f(CW\*(C`l_acquire\*(C'\fR callbacks simply unlock/lock the mutex
4047 protecting the loop data, respectively.
4048 .PP
4049 .Vb 6
4050 \& static void
4051 \& l_release (EV_P)
4052 \& {
4053 \& userdata *u = ev_userdata (EV_A);
4054 \& pthread_mutex_unlock (&u\->lock);
4055 \& }
4056 \&
4057 \& static void
4058 \& l_acquire (EV_P)
4059 \& {
4060 \& userdata *u = ev_userdata (EV_A);
4061 \& pthread_mutex_lock (&u\->lock);
4062 \& }
4063 .Ve
4064 .PP
4065 The event loop thread first acquires the mutex, and then jumps straight
4066 into \f(CW\*(C`ev_run\*(C'\fR:
4067 .PP
4068 .Vb 4
4069 \& void *
4070 \& l_run (void *thr_arg)
4071 \& {
4072 \& struct ev_loop *loop = (struct ev_loop *)thr_arg;
4073 \&
4074 \& l_acquire (EV_A);
4075 \& pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4076 \& ev_run (EV_A_ 0);
4077 \& l_release (EV_A);
4078 \&
4079 \& return 0;
4080 \& }
4081 .Ve
4082 .PP
4083 Instead of invoking all pending watchers, the \f(CW\*(C`l_invoke\*(C'\fR callback will
4084 signal the main thread via some unspecified mechanism (signals? pipe
4085 writes? \f(CW\*(C`Async::Interrupt\*(C'\fR?) and then waits until all pending watchers
4086 have been called (in a while loop because a) spurious wakeups are possible
4087 and b) skipping inter-thread-communication when there are no pending
4088 watchers is very beneficial):
4089 .PP
4090 .Vb 4
4091 \& static void
4092 \& l_invoke (EV_P)
4093 \& {
4094 \& userdata *u = ev_userdata (EV_A);
4095 \&
4096 \& while (ev_pending_count (EV_A))
4097 \& {
4098 \& wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4099 \& pthread_cond_wait (&u\->invoke_cv, &u\->lock);
4100 \& }
4101 \& }
4102 .Ve
4103 .PP
4104 Now, whenever the main thread gets told to invoke pending watchers, it
4105 will grab the lock, call \f(CW\*(C`ev_invoke_pending\*(C'\fR and then signal the loop
4106 thread to continue:
4107 .PP
4108 .Vb 4
4109 \& static void
4110 \& real_invoke_pending (EV_P)
4111 \& {
4112 \& userdata *u = ev_userdata (EV_A);
4113 \&
4114 \& pthread_mutex_lock (&u\->lock);
4115 \& ev_invoke_pending (EV_A);
4116 \& pthread_cond_signal (&u\->invoke_cv);
4117 \& pthread_mutex_unlock (&u\->lock);
4118 \& }
4119 .Ve
4120 .PP
4121 Whenever you want to start/stop a watcher or do other modifications to an
4122 event loop, you will now have to lock:
4123 .PP
4124 .Vb 2
4125 \& ev_timer timeout_watcher;
4126 \& userdata *u = ev_userdata (EV_A);
4127 \&
4128 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4129 \&
4130 \& pthread_mutex_lock (&u\->lock);
4131 \& ev_timer_start (EV_A_ &timeout_watcher);
4132 \& ev_async_send (EV_A_ &u\->async_w);
4133 \& pthread_mutex_unlock (&u\->lock);
4134 .Ve
4135 .PP
4136 Note that sending the \f(CW\*(C`ev_async\*(C'\fR watcher is required because otherwise
4137 an event loop currently blocking in the kernel will have no knowledge
4138 about the newly added timer. By waking up the loop it will pick up any new
4139 watchers in the next event loop iteration.
4142 While the overhead of a callback that e.g. schedules a thread is small, it
4143 is still an overhead. If you embed libev, and your main usage is with some
4144 kind of threads or coroutines, you might want to customise libev so that
4145 doesn't need callbacks anymore.
4146 .PP
4147 Imagine you have coroutines that you can switch to using a function
4148 \&\f(CW\*(C`switch_to (coro)\*(C'\fR, that libev runs in a coroutine called \f(CW\*(C`libev_coro\*(C'\fR
4149 and that due to some magic, the currently active coroutine is stored in a
4150 global called \f(CW\*(C`current_coro\*(C'\fR. Then you can build your own \*(L"wait for libev
4151 event\*(R" primitive by changing \f(CW\*(C`EV_CB_DECLARE\*(C'\fR and \f(CW\*(C`EV_CB_INVOKE\*(C'\fR (note
4152 the differing \f(CW\*(C`;\*(C'\fR conventions):
4153 .PP
4154 .Vb 2
4155 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4156 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4157 .Ve
4158 .PP
4159 That means instead of having a C callback function, you store the
4160 coroutine to switch to in each watcher, and instead of having libev call
4161 your callback, you instead have it switch to that coroutine.
4162 .PP
4163 A coroutine might now wait for an event with a function called
4164 \&\f(CW\*(C`wait_for_event\*(C'\fR. (the watcher needs to be started, as always, but it doesn't
4165 matter when, or whether the watcher is active or not when this function is
4166 called):
4167 .PP
4168 .Vb 6
4169 \& void
4170 \& wait_for_event (ev_watcher *w)
4171 \& {
4172 \& ev_set_cb (w, current_coro);
4173 \& switch_to (libev_coro);
4174 \& }
4175 .Ve
4176 .PP
4177 That basically suspends the coroutine inside \f(CW\*(C`wait_for_event\*(C'\fR and
4178 continues the libev coroutine, which, when appropriate, switches back to
4179 this or any other coroutine.
4180 .PP
4181 You can do similar tricks if you have, say, threads with an event queue \-
4182 instead of storing a coroutine, you store the queue object and instead of
4183 switching to a coroutine, you push the watcher onto the queue and notify
4184 any waiters.
4185 .PP
4186 To embed libev, see \*(L"\s-1EMBEDDING\*(R"\s0, but in short, it's easiest to create two
4187 files, \fImy_ev.h\fR and \fImy_ev.c\fR that include the respective libev files:
4188 .PP
4189 .Vb 4
4190 \& // my_ev.h
4191 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4192 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4193 \& #include "../libev/ev.h"
4194 \&
4195 \& // my_ev.c
4196 \& #define EV_H "my_ev.h"
4197 \& #include "../libev/ev.c"
4198 .Ve
4199 .PP
4200 And then use \fImy_ev.h\fR when you would normally use \fIev.h\fR, and compile
4201 \&\fImy_ev.c\fR into your project. When properly specifying include paths, you
4202 can even use \fIev.h\fR as header file name directly.
4205 Libev offers a compatibility emulation layer for libevent. It cannot
4206 emulate the internals of libevent, so here are some usage hints:
4207 .IP "\(bu" 4
4208 Only the libevent\-1.4.1\-beta \s-1API\s0 is being emulated.
4209 .Sp
4210 This was the newest libevent version available when libev was implemented,
4211 and is still mostly unchanged in 2010.
4212 .IP "\(bu" 4
4213 Use it by including <event.h>, as usual.
4214 .IP "\(bu" 4
4215 The following members are fully supported: ev_base, ev_callback,
4216 ev_arg, ev_fd, ev_res, ev_events.
4217 .IP "\(bu" 4
4218 Avoid using ev_flags and the EVLIST_*\-macros, while it is
4219 maintained by libev, it does not work exactly the same way as in libevent (consider
4220 it a private \s-1API\s0).
4221 .IP "\(bu" 4
4222 Priorities are not currently supported. Initialising priorities
4223 will fail and all watchers will have the same priority, even though there
4224 is an ev_pri field.
4225 .IP "\(bu" 4
4226 In libevent, the last base created gets the signals, in libev, the
4227 base that registered the signal gets the signals.
4228 .IP "\(bu" 4
4229 Other members are not supported.
4230 .IP "\(bu" 4
4231 The libev emulation is \fInot\fR \s-1ABI\s0 compatible to libevent, you need
4232 to use the libev header file and library.
4233 .SH "\*(C+ SUPPORT"
4234 .IX Header " SUPPORT"
4235 .SS "C \s-1API\s0"
4236 .IX Subsection "C API"
4237 The normal C \s-1API\s0 should work fine when used from \*(C+: both ev.h and the
4238 libev sources can be compiled as \*(C+. Therefore, code that uses the C \s-1API\s0
4239 will work fine.
4240 .PP
4241 Proper exception specifications might have to be added to callbacks passed
4242 to libev: exceptions may be thrown only from watcher callbacks, all other
4243 callbacks (allocator, syserr, loop acquire/release and periodic reschedule
4244 callbacks) must not throw exceptions, and might need a \f(CW\*(C`noexcept\*(C'\fR
4245 specification. If you have code that needs to be compiled as both C and
4246 \&\*(C+ you can use the \f(CW\*(C`EV_NOEXCEPT\*(C'\fR macro for this:
4247 .PP
4248 .Vb 6
4249 \& static void
4250 \& fatal_error (const char *msg) EV_NOEXCEPT
4251 \& {
4252 \& perror (msg);
4253 \& abort ();
4254 \& }
4255 \&
4256 \& ...
4257 \& ev_set_syserr_cb (fatal_error);
4258 .Ve
4259 .PP
4260 The only \s-1API\s0 functions that can currently throw exceptions are \f(CW\*(C`ev_run\*(C'\fR,
4261 \&\f(CW\*(C`ev_invoke\*(C'\fR, \f(CW\*(C`ev_invoke_pending\*(C'\fR and \f(CW\*(C`ev_loop_destroy\*(C'\fR (the latter
4262 because it runs cleanup watchers).
4263 .PP
4264 Throwing exceptions in watcher callbacks is only supported if libev itself
4265 is compiled with a \*(C+ compiler or your C and \*(C+ environments allow
4266 throwing exceptions through C libraries (most do).
4267 .SS "\*(C+ \s-1API\s0"
4268 .IX Subsection " API"
4269 Libev comes with some simplistic wrapper classes for \*(C+ that mainly allow
4270 you to use some convenience methods to start/stop watchers and also change
4271 the callback model to a model using method callbacks on objects.
4272 .PP
4273 To use it,
4274 .PP
4275 .Vb 1
4276 \& #include <ev++.h>
4277 .Ve
4278 .PP
4279 This automatically includes \fIev.h\fR and puts all of its definitions (many
4280 of them macros) into the global namespace. All \*(C+ specific things are
4281 put into the \f(CW\*(C`ev\*(C'\fR namespace. It should support all the same embedding
4282 options as \fIev.h\fR, most notably \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR.
4283 .PP
4284 Care has been taken to keep the overhead low. The only data member the \*(C+
4285 classes add (compared to plain C\-style watchers) is the event loop pointer
4286 that the watcher is associated with (or no additional members at all if
4287 you disable \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR when embedding libev).
4288 .PP
4289 Currently, functions, static and non-static member functions and classes
4290 with \f(CW\*(C`operator ()\*(C'\fR can be used as callbacks. Other types should be easy
4291 to add as long as they only need one additional pointer for context. If
4292 you need support for other types of functors please contact the author
4293 (preferably after implementing it).
4294 .PP
4295 For all this to work, your \*(C+ compiler either has to use the same calling
4296 conventions as your C compiler (for static member functions), or you have
4297 to embed libev and compile libev itself as \*(C+.
4298 .PP
4299 Here is a list of things available in the \f(CW\*(C`ev\*(C'\fR namespace:
4300 .ie n .IP """ev::READ"", ""ev::WRITE"" etc." 4
4301 .el .IP "\f(CWev::READ\fR, \f(CWev::WRITE\fR etc." 4
4302 .IX Item "ev::READ, ev::WRITE etc."
4303 These are just enum values with the same values as the \f(CW\*(C`EV_READ\*(C'\fR etc.
4304 macros from \fIev.h\fR.
4305 .ie n .IP """ev::tstamp"", ""ev::now""" 4
4306 .el .IP "\f(CWev::tstamp\fR, \f(CWev::now\fR" 4
4307 .IX Item "ev::tstamp, ev::now"
4308 Aliases to the same types/functions as with the \f(CW\*(C`ev_\*(C'\fR prefix.
4309 .ie n .IP """ev::io"", ""ev::timer"", ""ev::periodic"", ""ev::idle"", ""ev::sig"" etc." 4
4310 .el .IP "\f(CWev::io\fR, \f(CWev::timer\fR, \f(CWev::periodic\fR, \f(CWev::idle\fR, \f(CWev::sig\fR etc." 4
4311 .IX Item "ev::io, ev::timer, ev::periodic, ev::idle, ev::sig etc."
4312 For each \f(CW\*(C`ev_TYPE\*(C'\fR watcher in \fIev.h\fR there is a corresponding class of
4313 the same name in the \f(CW\*(C`ev\*(C'\fR namespace, with the exception of \f(CW\*(C`ev_signal\*(C'\fR
4314 which is called \f(CW\*(C`ev::sig\*(C'\fR to avoid clashes with the \f(CW\*(C`signal\*(C'\fR macro
4315 defined by many implementations.
4316 .Sp
4317 All of those classes have these methods:
4318 .RS 4
4319 .IP "ev::TYPE::TYPE ()" 4
4320 .IX Item "ev::TYPE::TYPE ()"
4321 .PD 0
4322 .IP "ev::TYPE::TYPE (loop)" 4
4323 .IX Item "ev::TYPE::TYPE (loop)"
4324 .IP "ev::TYPE::~TYPE" 4
4325 .IX Item "ev::TYPE::~TYPE"
4326 .PD
4327 The constructor (optionally) takes an event loop to associate the watcher
4328 with. If it is omitted, it will use \f(CW\*(C`EV_DEFAULT\*(C'\fR.
4329 .Sp
4330 The constructor calls \f(CW\*(C`ev_init\*(C'\fR for you, which means you have to call the
4331 \&\f(CW\*(C`set\*(C'\fR method before starting it.
4332 .Sp
4333 It will not set a callback, however: You have to call the templated \f(CW\*(C`set\*(C'\fR
4334 method to set a callback before you can start the watcher.
4335 .Sp
4336 (The reason why you have to use a method is a limitation in \*(C+ which does
4337 not allow explicit template arguments for constructors).
4338 .Sp
4339 The destructor automatically stops the watcher if it is active.
4340 .IP "w\->set<class, &class::method> (object *)" 4
4341 .IX Item "w->set<class, &class::method> (object *)"
4342 This method sets the callback method to call. The method has to have a
4343 signature of \f(CW\*(C`void (*)(ev_TYPE &, int)\*(C'\fR, it receives the watcher as
4344 first argument and the \f(CW\*(C`revents\*(C'\fR as second. The object must be given as
4345 parameter and is stored in the \f(CW\*(C`data\*(C'\fR member of the watcher.
4346 .Sp
4347 This method synthesizes efficient thunking code to call your method from
4348 the C callback that libev requires. If your compiler can inline your
4349 callback (i.e. it is visible to it at the place of the \f(CW\*(C`set\*(C'\fR call and
4350 your compiler is good :), then the method will be fully inlined into the
4351 thunking function, making it as fast as a direct C callback.
4352 .Sp
4353 Example: simple class declaration and watcher initialisation
4354 .Sp
4355 .Vb 4
4356 \& struct myclass
4357 \& {
4358 \& void io_cb (ev::io &w, int revents) { }
4359 \& }
4360 \&
4361 \& myclass obj;
4362 \& ev::io iow;
4363 \& iow.set <myclass, &myclass::io_cb> (&obj);
4364 .Ve
4365 .IP "w\->set (object *)" 4
4366 .IX Item "w->set (object *)"
4367 This is a variation of a method callback \- leaving out the method to call
4368 will default the method to \f(CW\*(C`operator ()\*(C'\fR, which makes it possible to use
4369 functor objects without having to manually specify the \f(CW\*(C`operator ()\*(C'\fR all
4370 the time. Incidentally, you can then also leave out the template argument
4371 list.
4372 .Sp
4373 The \f(CW\*(C`operator ()\*(C'\fR method prototype must be \f(CW\*(C`void operator ()(watcher &w,
4374 int revents)\*(C'\fR.
4375 .Sp
4376 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4377 .Sp
4378 Example: use a functor object as callback.
4379 .Sp
4380 .Vb 7
4381 \& struct myfunctor
4382 \& {
4383 \& void operator() (ev::io &w, int revents)
4384 \& {
4385 \& ...
4386 \& }
4387 \& }
4388 \&
4389 \& myfunctor f;
4390 \&
4391 \& ev::io w;
4392 \& w.set (&f);
4393 .Ve
4394 .IP "w\->set<function> (void *data = 0)" 4
4395 .IX Item "w->set<function> (void *data = 0)"
4396 Also sets a callback, but uses a static method or plain function as
4397 callback. The optional \f(CW\*(C`data\*(C'\fR argument will be stored in the watcher's
4398 \&\f(CW\*(C`data\*(C'\fR member and is free for you to use.
4399 .Sp
4400 The prototype of the \f(CW\*(C`function\*(C'\fR must be \f(CW\*(C`void (*)(ev::TYPE &w, int)\*(C'\fR.
4401 .Sp
4402 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4403 .Sp
4404 Example: Use a plain function as callback.
4405 .Sp
4406 .Vb 2
4407 \& static void io_cb (ev::io &w, int revents) { }
4408 \& iow.set <io_cb> ();
4409 .Ve
4410 .IP "w\->set (loop)" 4
4411 .IX Item "w->set (loop)"
4412 Associates a different \f(CW\*(C`struct ev_loop\*(C'\fR with this watcher. You can only
4413 do this when the watcher is inactive (and not pending either).
4414 .IP "w\->set ([arguments])" 4
4415 .IX Item "w->set ([arguments])"
4416 Basically the same as \f(CW\*(C`ev_TYPE_set\*(C'\fR (except for \f(CW\*(C`ev::embed\*(C'\fR watchers>),
4417 with the same arguments. Either this method or a suitable start method
4418 must be called at least once. Unlike the C counterpart, an active watcher
4419 gets automatically stopped and restarted when reconfiguring it with this
4420 method.
4421 .Sp
4422 For \f(CW\*(C`ev::embed\*(C'\fR watchers this method is called \f(CW\*(C`set_embed\*(C'\fR, to avoid
4423 clashing with the \f(CW\*(C`set (loop)\*(C'\fR method.
4424 .Sp
4425 For \f(CW\*(C`ev::io\*(C'\fR watchers there is an additional \f(CW\*(C`set\*(C'\fR method that acepts a
4426 new event mask only, and internally calls \f(CW\*(C`ev_io_modfify\*(C'\fR.
4427 .IP "w\->start ()" 4
4428 .IX Item "w->start ()"
4429 Starts the watcher. Note that there is no \f(CW\*(C`loop\*(C'\fR argument, as the
4430 constructor already stores the event loop.
4431 .IP "w\->start ([arguments])" 4
4432 .IX Item "w->start ([arguments])"
4433 Instead of calling \f(CW\*(C`set\*(C'\fR and \f(CW\*(C`start\*(C'\fR methods separately, it is often
4434 convenient to wrap them in one call. Uses the same type of arguments as
4435 the configure \f(CW\*(C`set\*(C'\fR method of the watcher.
4436 .IP "w\->stop ()" 4
4437 .IX Item "w->stop ()"
4438 Stops the watcher if it is active. Again, no \f(CW\*(C`loop\*(C'\fR argument.
4439 .ie n .IP "w\->again () (""ev::timer"", ""ev::periodic"" only)" 4
4440 .el .IP "w\->again () (\f(CWev::timer\fR, \f(CWev::periodic\fR only)" 4
4441 .IX Item "w->again () (ev::timer, ev::periodic only)"
4442 For \f(CW\*(C`ev::timer\*(C'\fR and \f(CW\*(C`ev::periodic\*(C'\fR, this invokes the corresponding
4443 \&\f(CW\*(C`ev_TYPE_again\*(C'\fR function.
4444 .ie n .IP "w\->sweep () (""ev::embed"" only)" 4
4445 .el .IP "w\->sweep () (\f(CWev::embed\fR only)" 4
4446 .IX Item "w->sweep () (ev::embed only)"
4447 Invokes \f(CW\*(C`ev_embed_sweep\*(C'\fR.
4448 .ie n .IP "w\->update () (""ev::stat"" only)" 4
4449 .el .IP "w\->update () (\f(CWev::stat\fR only)" 4
4450 .IX Item "w->update () (ev::stat only)"
4451 Invokes \f(CW\*(C`ev_stat_stat\*(C'\fR.
4452 .RE
4453 .RS 4
4454 .RE
4455 .PP
4456 Example: Define a class with two I/O and idle watchers, start the I/O
4457 watchers in the constructor.
4458 .PP
4459 .Vb 5
4460 \& class myclass
4461 \& {
4462 \& ev::io io ; void io_cb (ev::io &w, int revents);
4463 \& ev::io io2 ; void io2_cb (ev::io &w, int revents);
4464 \& ev::idle idle; void idle_cb (ev::idle &w, int revents);
4465 \&
4466 \& myclass (int fd)
4467 \& {
4468 \& io .set <myclass, &myclass::io_cb > (this);
4469 \& io2 .set <myclass, &myclass::io2_cb > (this);
4470 \& idle.set <myclass, &myclass::idle_cb> (this);
4471 \&
4472 \& io.set (fd, ev::WRITE); // configure the watcher
4473 \& io.start (); // start it whenever convenient
4474 \&
4475 \& io2.start (fd, ev::READ); // set + start in one call
4476 \& }
4477 \& };
4478 .Ve
4481 Libev does not offer other language bindings itself, but bindings for a
4482 number of languages exist in the form of third-party packages. If you know
4483 any interesting language binding in addition to the ones listed here, drop
4484 me a note.
4485 .IP "Perl" 4
4486 .IX Item "Perl"
4487 The \s-1EV\s0 module implements the full libev \s-1API\s0 and is actually used to test
4488 libev. \s-1EV\s0 is developed together with libev. Apart from the \s-1EV\s0 core module,
4489 there are additional modules that implement libev-compatible interfaces
4490 to \f(CW\*(C`libadns\*(C'\fR (\f(CW\*(C`EV::ADNS\*(C'\fR, but \f(CW\*(C`AnyEvent::DNS\*(C'\fR is preferred nowadays),
4491 \&\f(CW\*(C`Net::SNMP\*(C'\fR (\f(CW\*(C`Net::SNMP::EV\*(C'\fR) and the \f(CW\*(C`libglib\*(C'\fR event core (\f(CW\*(C`Glib::EV\*(C'\fR
4492 and \f(CW\*(C`EV::Glib\*(C'\fR).
4493 .Sp
4494 It can be found and installed via \s-1CPAN,\s0 its homepage is at
4495 <>.
4496 .IP "Python" 4
4497 .IX Item "Python"
4498 Python bindings can be found at <>. It
4499 seems to be quite complete and well-documented.
4500 .IP "Ruby" 4
4501 .IX Item "Ruby"
4502 Tony Arcieri has written a ruby extension that offers access to a subset
4503 of the libev \s-1API\s0 and adds file handle abstractions, asynchronous \s-1DNS\s0 and
4504 more on top of it. It can be found via gem servers. Its homepage is at
4505 <>.
4506 .Sp
4507 Roger Pack reports that using the link order \f(CW\*(C`\-lws2_32 \-lmsvcrt\-ruby\-190\*(C'\fR
4508 makes rev work even on mingw.
4509 .IP "Haskell" 4
4510 .IX Item "Haskell"
4511 A haskell binding to libev is available at
4512 <\-bin/hackage\-scripts/package/hlibev>.
4513 .IP "D" 4
4514 .IX Item "D"
4515 Leandro Lucarella has written a D language binding (\fIev.d\fR) for libev, to
4516 be found at <>.
4517 .IP "Ocaml" 4
4518 .IX Item "Ocaml"
4519 Erkki Seppala has written Ocaml bindings for libev, to be found at
4520 <\-ev/>.
4521 .IP "Lua" 4
4522 .IX Item "Lua"
4523 Brian Maher has written a partial interface to libev for lua (at the
4524 time of this writing, only \f(CW\*(C`ev_io\*(C'\fR and \f(CW\*(C`ev_timer\*(C'\fR), to be found at
4525 <\-ev>.
4526 .IP "Javascript" 4
4527 .IX Item "Javascript"
4528 Node.js (<>) uses libev as the underlying event library.
4529 .IP "Others" 4
4530 .IX Item "Others"
4531 There are others, and I stopped counting.
4533 .IX Header "MACRO MAGIC"
4534 Libev can be compiled with a variety of options, the most fundamental
4535 of which is \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR. This option determines whether (most)
4536 functions and callbacks have an initial \f(CW\*(C`struct ev_loop *\*(C'\fR argument.
4537 .PP
4538 To make it easier to write programs that cope with either variant, the
4539 following macros are defined:
4540 .ie n .IP """EV_A"", ""EV_A_""" 4
4541 .el .IP "\f(CWEV_A\fR, \f(CWEV_A_\fR" 4
4542 .IX Item "EV_A, EV_A_"
4543 This provides the loop \fIargument\fR for functions, if one is required (\*(L"ev
4544 loop argument\*(R"). The \f(CW\*(C`EV_A\*(C'\fR form is used when this is the sole argument,
4545 \&\f(CW\*(C`EV_A_\*(C'\fR is used when other arguments are following. Example:
4546 .Sp
4547 .Vb 3
4548 \& ev_unref (EV_A);
4549 \& ev_timer_add (EV_A_ watcher);
4550 \& ev_run (EV_A_ 0);
4551 .Ve
4552 .Sp
4553 It assumes the variable \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR is in scope,
4554 which is often provided by the following macro.
4555 .ie n .IP """EV_P"", ""EV_P_""" 4
4556 .el .IP "\f(CWEV_P\fR, \f(CWEV_P_\fR" 4
4557 .IX Item "EV_P, EV_P_"
4558 This provides the loop \fIparameter\fR for functions, if one is required (\*(L"ev
4559 loop parameter\*(R"). The \f(CW\*(C`EV_P\*(C'\fR form is used when this is the sole parameter,
4560 \&\f(CW\*(C`EV_P_\*(C'\fR is used when other parameters are following. Example:
4561 .Sp
4562 .Vb 2
4563 \& // this is how ev_unref is being declared
4564 \& static void ev_unref (EV_P);
4565 \&
4566 \& // this is how you can declare your typical callback
4567 \& static void cb (EV_P_ ev_timer *w, int revents)
4568 .Ve
4569 .Sp
4570 It declares a parameter \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR, quite
4571 suitable for use with \f(CW\*(C`EV_A\*(C'\fR.
4572 .ie n .IP """EV_DEFAULT"", ""EV_DEFAULT_""" 4
4573 .el .IP "\f(CWEV_DEFAULT\fR, \f(CWEV_DEFAULT_\fR" 4
4575 Similar to the other two macros, this gives you the value of the default
4576 loop, if multiple loops are supported (\*(L"ev loop default\*(R"). The default loop
4577 will be initialised if it isn't already initialised.
4578 .Sp
4579 For non-multiplicity builds, these macros do nothing, so you always have
4580 to initialise the loop somewhere.
4581 .ie n .IP """EV_DEFAULT_UC"", ""EV_DEFAULT_UC_""" 4
4582 .el .IP "\f(CWEV_DEFAULT_UC\fR, \f(CWEV_DEFAULT_UC_\fR" 4
4584 Usage identical to \f(CW\*(C`EV_DEFAULT\*(C'\fR and \f(CW\*(C`EV_DEFAULT_\*(C'\fR, but requires that the
4585 default loop has been initialised (\f(CW\*(C`UC\*(C'\fR == unchecked). Their behaviour
4586 is undefined when the default loop has not been initialised by a previous
4587 execution of \f(CW\*(C`EV_DEFAULT\*(C'\fR, \f(CW\*(C`EV_DEFAULT_\*(C'\fR or \f(CW\*(C`ev_default_init (...)\*(C'\fR.
4588 .Sp
4589 It is often prudent to use \f(CW\*(C`EV_DEFAULT\*(C'\fR when initialising the first
4590 watcher in a function but use \f(CW\*(C`EV_DEFAULT_UC\*(C'\fR afterwards.
4591 .PP
4592 Example: Declare and initialise a check watcher, utilising the above
4593 macros so it will work regardless of whether multiple loops are supported
4594 or not.
4595 .PP
4596 .Vb 5
4597 \& static void
4598 \& check_cb (EV_P_ ev_timer *w, int revents)
4599 \& {
4600 \& ev_check_stop (EV_A_ w);
4601 \& }
4602 \&
4603 \& ev_check check;
4604 \& ev_check_init (&check, check_cb);
4605 \& ev_check_start (EV_DEFAULT_ &check);
4606 \& ev_run (EV_DEFAULT_ 0);
4607 .Ve
4609 .IX Header "EMBEDDING"
4610 Libev can (and often is) directly embedded into host
4611 applications. Examples of applications that embed it include the Deliantra
4612 Game Server, the \s-1EV\s0 perl module, the \s-1GNU\s0 Virtual Private Ethernet (gvpe)
4613 and rxvt-unicode.
4614 .PP
4615 The goal is to enable you to just copy the necessary files into your
4616 source directory without having to change even a single line in them, so
4617 you can easily upgrade by simply copying (or having a checked-out copy of
4618 libev somewhere in your source tree).
4619 .SS "\s-1FILESETS\s0"
4620 .IX Subsection "FILESETS"
4621 Depending on what features you need you need to include one or more sets of files
4622 in your application.
4623 .PP
4624 \fI\s-1CORE EVENT LOOP\s0\fR
4625 .IX Subsection "CORE EVENT LOOP"
4626 .PP
4627 To include only the libev core (all the \f(CW\*(C`ev_*\*(C'\fR functions), with manual
4628 configuration (no autoconf):
4629 .PP
4630 .Vb 2
4631 \& #define EV_STANDALONE 1
4632 \& #include "ev.c"
4633 .Ve
4634 .PP
4635 This will automatically include \fIev.h\fR, too, and should be done in a
4636 single C source file only to provide the function implementations. To use
4637 it, do the same for \fIev.h\fR in all files wishing to use this \s-1API\s0 (best
4638 done by writing a wrapper around \fIev.h\fR that you can include instead and
4639 where you can put other configuration options):
4640 .PP
4641 .Vb 2
4642 \& #define EV_STANDALONE 1
4643 \& #include "ev.h"
4644 .Ve
4645 .PP
4646 Both header files and implementation files can be compiled with a \*(C+
4647 compiler (at least, that's a stated goal, and breakage will be treated
4648 as a bug).
4649 .PP
4650 You need the following files in your source tree, or in a directory
4651 in your include path (e.g. in libev/ when using \-Ilibev):
4652 .PP
4653 .Vb 4
4654 \& ev.h
4655 \& ev.c
4656 \& ev_vars.h
4657 \& ev_wrap.h
4658 \&
4659 \& ev_win32.c required on win32 platforms only
4660 \&
4661 \& ev_select.c only when select backend is enabled
4662 \& ev_poll.c only when poll backend is enabled
4663 \& ev_epoll.c only when the epoll backend is enabled
4664 \& ev_linuxaio.c only when the linux aio backend is enabled
4665 \& ev_iouring.c only when the linux io_uring backend is enabled
4666 \& ev_kqueue.c only when the kqueue backend is enabled
4667 \& ev_port.c only when the solaris port backend is enabled
4668 .Ve
4669 .PP
4670 \&\fIev.c\fR includes the backend files directly when enabled, so you only need
4671 to compile this single file.
4672 .PP
4675 .PP
4676 To include the libevent compatibility \s-1API,\s0 also include:
4677 .PP
4678 .Vb 1
4679 \& #include "event.c"
4680 .Ve
4681 .PP
4682 in the file including \fIev.c\fR, and:
4683 .PP
4684 .Vb 1
4685 \& #include "event.h"
4686 .Ve
4687 .PP
4688 in the files that want to use the libevent \s-1API.\s0 This also includes \fIev.h\fR.
4689 .PP
4690 You need the following additional files for this:
4691 .PP
4692 .Vb 2
4693 \& event.h
4694 \& event.c
4695 .Ve
4696 .PP
4697 \fI\s-1AUTOCONF SUPPORT\s0\fR
4698 .IX Subsection "AUTOCONF SUPPORT"
4699 .PP
4700 Instead of using \f(CW\*(C`EV_STANDALONE=1\*(C'\fR and providing your configuration in
4701 whatever way you want, you can also \f(CW\*(C`m4_include([libev.m4])\*(C'\fR in your
4702 \&\\fR and leave \f(CW\*(C`EV_STANDALONE\*(C'\fR undefined. \fIev.c\fR will then
4703 include \fIconfig.h\fR and configure itself accordingly.
4704 .PP
4705 For this of course you need the m4 file:
4706 .PP
4707 .Vb 1
4708 \& libev.m4
4709 .Ve
4712 Libev can be configured via a variety of preprocessor symbols you have to
4713 define before including (or compiling) any of its files. The default in
4714 the absence of autoconf is documented for every option.
4715 .PP
4716 Symbols marked with \*(L"(h)\*(R" do not change the \s-1ABI,\s0 and can have different
4717 values when compiling libev vs. including \fIev.h\fR, so it is permissible
4718 to redefine them before including \fIev.h\fR without breaking compatibility
4719 to a compiled library. All other symbols change the \s-1ABI,\s0 which means all
4720 users of libev and the libev code itself must be compiled with compatible
4721 settings.
4722 .IP "\s-1EV_COMPAT3\s0 (h)" 4
4723 .IX Item "EV_COMPAT3 (h)"
4724 Backwards compatibility is a major concern for libev. This is why this
4725 release of libev comes with wrappers for the functions and symbols that
4726 have been renamed between libev version 3 and 4.
4727 .Sp
4728 You can disable these wrappers (to test compatibility with future
4729 versions) by defining \f(CW\*(C`EV_COMPAT3\*(C'\fR to \f(CW0\fR when compiling your
4730 sources. This has the additional advantage that you can drop the \f(CW\*(C`struct\*(C'\fR
4731 from \f(CW\*(C`struct ev_loop\*(C'\fR declarations, as libev will provide an \f(CW\*(C`ev_loop\*(C'\fR
4732 typedef in that case.
4733 .Sp
4734 In some future version, the default for \f(CW\*(C`EV_COMPAT3\*(C'\fR will become \f(CW0\fR,
4735 and in some even more future version the compatibility code will be
4736 removed completely.
4737 .IP "\s-1EV_STANDALONE\s0 (h)" 4
4738 .IX Item "EV_STANDALONE (h)"
4739 Must always be \f(CW1\fR if you do not use autoconf configuration, which
4740 keeps libev from including \fIconfig.h\fR, and it also defines dummy
4741 implementations for some libevent functions (such as logging, which is not
4742 supported). It will also not define any of the structs usually found in
4743 \&\fIevent.h\fR that are not directly supported by the libev core alone.
4744 .Sp
4745 In standalone mode, libev will still try to automatically deduce the
4746 configuration, but has to be more conservative.
4747 .IP "\s-1EV_USE_FLOOR\s0" 4
4748 .IX Item "EV_USE_FLOOR"
4749 If defined to be \f(CW1\fR, libev will use the \f(CW\*(C`floor ()\*(C'\fR function for its
4750 periodic reschedule calculations, otherwise libev will fall back on a
4751 portable (slower) implementation. If you enable this, you usually have to
4752 link against libm or something equivalent. Enabling this when the \f(CW\*(C`floor\*(C'\fR
4753 function is not available will fail, so the safe default is to not enable
4754 this.
4755 .IP "\s-1EV_USE_MONOTONIC\s0" 4
4757 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4758 monotonic clock option at both compile time and runtime. Otherwise no
4759 use of the monotonic clock option will be attempted. If you enable this,
4760 you usually have to link against librt or something similar. Enabling it
4761 when the functionality isn't available is safe, though, although you have
4762 to make sure you link against any libraries where the \f(CW\*(C`clock_gettime\*(C'\fR
4763 function is hiding in (often \fI\-lrt\fR). See also \f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4764 .IP "\s-1EV_USE_REALTIME\s0" 4
4766 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4767 real-time clock option at compile time (and assume its availability
4768 at runtime if successful). Otherwise no use of the real-time clock
4769 option will be attempted. This effectively replaces \f(CW\*(C`gettimeofday\*(C'\fR
4770 by \f(CW\*(C`clock_get (CLOCK_REALTIME, ...)\*(C'\fR and will not normally affect
4771 correctness. See the note about libraries in the description of
4772 \&\f(CW\*(C`EV_USE_MONOTONIC\*(C'\fR, though. Defaults to the opposite value of
4773 \&\f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4774 .IP "\s-1EV_USE_CLOCK_SYSCALL\s0" 4
4776 If defined to be \f(CW1\fR, libev will try to use a direct syscall instead
4777 of calling the system-provided \f(CW\*(C`clock_gettime\*(C'\fR function. This option
4778 exists because on GNU/Linux, \f(CW\*(C`clock_gettime\*(C'\fR is in \f(CW\*(C`librt\*(C'\fR, but \f(CW\*(C`librt\*(C'\fR
4779 unconditionally pulls in \f(CW\*(C`libpthread\*(C'\fR, slowing down single-threaded
4780 programs needlessly. Using a direct syscall is slightly slower (in
4781 theory), because no optimised vdso implementat