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Revision: 1.126
Committed: Sat Jun 3 08:53:03 2023 UTC (12 months, 2 weeks ago) by root
Branch: MAIN
CVS Tags: HEAD
Changes since 1.125: +37 -21 lines
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File Contents

# Content
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133 .\" ========================================================================
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135 .IX Title "LIBEV 3"
136 .TH LIBEV 3 "2023-05-15" "libev-4.33" "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
143 .SH "SYNOPSIS"
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
207 .SH "ABOUT THIS DOCUMENT"
208 .IX Header "ABOUT THIS DOCUMENT"
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: <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
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.
222 .SH "WHAT TO READ WHEN IN A HURRY"
223 .IX Header "WHAT TO READ WHEN IN A HURRY"
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.
229 .SH "ABOUT LIBEV"
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 <http://libev.schmorp.de/bench.html> 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.
269 .SS "\s-1TIME REPRESENTATION\s0"
270 .IX Subsection "TIME REPRESENTATION"
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.
280 .SH "ERROR HANDLING"
281 .IX Header "ERROR HANDLING"
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.
303 .SH "GLOBAL FUNCTIONS"
304 .IX Header "GLOBAL FUNCTIONS"
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.
479 .SH "FUNCTIONS CONTROLLING EVENT LOOPS"
480 .IX Header "FUNCTIONS CONTROLLING EVENT LOOPS"
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
526 \& ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
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
549 .IX Item "EVFLAG_NOENV"
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
560 .IX Item "EVFLAG_FORKCHECK"
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
580 .IX Item "EVFLAG_NOINOTIFY"
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
587 .IX Item "EVFLAG_SIGNALFD"
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
600 .IX Item "EVFLAG_NOSIGMASK"
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
613 .IX Item "EVFLAG_NOTIMERFD"
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 <https://lore.kernel.org/patchwork/patch/1047453/>), 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
807 Solaris, 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
833 .IX Item "EVBACKEND_ALL"
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
836 \&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR.
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
843 .IX Item "EVBACKEND_MASK"
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, async and child watchers are implemented as I/O watchers, and
1093 \& will 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.
1302 .SH "ANATOMY OF A WATCHER"
1303 .IX Header "ANATOMY OF A WATCHER"
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.
1460 .SS "\s-1GENERIC WATCHER FUNCTIONS\s0"
1461 .IX Subsection "GENERIC WATCHER FUNCTIONS"
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 unless documented otherwise.
1538 .Sp
1539 Obviously, it is safe to call this on an active watcher, or actually any
1540 watcher that is initialised.
1541 .IP "bool ev_is_pending (ev_TYPE *watcher)" 4
1542 .IX Item "bool ev_is_pending (ev_TYPE *watcher)"
1543 Returns a true value iff the watcher is pending, (i.e. it has outstanding
1544 events but its callback has not yet been invoked). As long as a watcher
1545 is pending (but not active) you must not call an init function on it (but
1546 \&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must
1547 make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR
1548 it).
1549 .Sp
1550 It is safe to call this on any watcher in any state as long as it is
1551 initialised.
1552 .IP "callback ev_cb (ev_TYPE *watcher)" 4
1553 .IX Item "callback ev_cb (ev_TYPE *watcher)"
1554 Returns the callback currently set on the watcher.
1555 .IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
1556 .IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
1557 Change the callback. You can change the callback at virtually any time
1558 (modulo threads).
1559 .IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
1560 .IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
1561 .PD 0
1562 .IP "int ev_priority (ev_TYPE *watcher)" 4
1563 .IX Item "int ev_priority (ev_TYPE *watcher)"
1564 .PD
1565 Set and query the priority of the watcher. The priority is a small
1566 integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR
1567 (default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked
1568 before watchers with lower priority, but priority will not keep watchers
1569 from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).
1570 .Sp
1571 If you need to suppress invocation when higher priority events are pending
1572 you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.
1573 .Sp
1574 You \fImust not\fR change the priority of a watcher as long as it is active
1575 or pending. Reading the priority with \f(CW\*(C`ev_priority\*(C'\fR is fine in any state.
1576 .Sp
1577 Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
1578 fine, as long as you do not mind that the priority value you query might
1579 or might not have been clamped to the valid range.
1580 .Sp
1581 The default priority used by watchers when no priority has been set is
1582 always \f(CW0\fR, which is supposed to not be too high and not be too low :).
1583 .Sp
1584 See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
1585 priorities.
1586 .IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
1587 .IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
1588 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
1589 \&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback
1590 can deal with that fact, as both are simply passed through to the
1591 callback.
1592 .IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4
1593 .IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"
1594 If the watcher is pending, this function clears its pending status and
1595 returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the
1596 watcher isn't pending it does nothing and returns \f(CW0\fR.
1597 .Sp
1598 Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its
1599 callback to be invoked, which can be accomplished with this function.
1600 .IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
1601 .IX Item "ev_feed_event (loop, ev_TYPE *watcher, int revents)"
1602 Feeds the given event set into the event loop, as if the specified event
1603 had happened for the specified watcher (which must be a pointer to an
1604 initialised but not necessarily started event watcher, though it can be
1605 active). Obviously you must not free the watcher as long as it has pending
1606 events.
1607 .Sp
1608 Stopping the watcher, letting libev invoke it, or calling
1609 \&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
1610 not started in the first place.
1611 .Sp
1612 See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
1613 functions that do not need a watcher.
1614 .PP
1615 See also the \*(L"\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\*(R"\s0 and \*(L"\s-1BUILDING YOUR
1616 OWN COMPOSITE WATCHERS\*(R"\s0 idioms.
1617 .SS "\s-1WATCHER STATES\s0"
1618 .IX Subsection "WATCHER STATES"
1619 There are various watcher states mentioned throughout this manual \-
1620 active, pending and so on. In this section these states and the rules to
1621 transition between them will be described in more detail \- and while these
1622 rules might look complicated, they usually do \*(L"the right thing\*(R".
1623 .IP "initialised" 4
1624 .IX Item "initialised"
1625 Before a watcher can be registered with the event loop it has to be
1626 initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
1627 \&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR function.
1628 .Sp
1629 In this state it is simply some block of memory that is suitable for
1630 use in an event loop. It can be moved around, freed, reused etc. at
1631 will \- as long as you either keep the memory contents intact, or call
1632 \&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
1633 .IP "started/running/active" 4
1634 .IX Item "started/running/active"
1635 Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR it becomes
1636 property of the event loop, and is actively waiting for events. While in
1637 this state it cannot be accessed (except in a few documented ways, such as
1638 stoping it), moved, freed or anything else \- the only legal thing is to
1639 keep a pointer to it, and call libev functions on it that are documented
1640 to work on active watchers.
1641 .Sp
1642 As a rule of thumb, before accessing a member or calling any function on
1643 a watcher, it should be stopped (or freshly initialised). If that is not
1644 convenient, you can check the documentation for that function or member to
1645 see if it is safe to use on an active watcher.
1646 .IP "pending" 4
1647 .IX Item "pending"
1648 If a watcher is active and libev determines that an event it is interested
1649 in has occurred (such as a timer expiring), it will become pending. It
1650 will stay in this pending state until either it is explicitly stopped or
1651 its callback is about to be invoked, so it is not normally pending inside
1652 the watcher callback.
1653 .Sp
1654 Generally, the watcher might or might not be active while it is pending
1655 (for example, an expired non-repeating timer can be pending but no longer
1656 active). If it is pending but not active, it can be freely accessed (e.g.
1657 by calling \f(CW\*(C`ev_TYPE_set\*(C'\fR), but it is still property of the event loop at
1658 this time, so cannot be moved, freed or reused. And if it is active the
1659 rules described in the previous item still apply.
1660 .Sp
1661 Explicitly stopping a watcher will also clear the pending state
1662 unconditionally, so it is safe to stop a watcher and then free it.
1663 .Sp
1664 It is also possible to feed an event on a watcher that is not active (e.g.
1665 via \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
1666 active.
1667 .IP "stopped" 4
1668 .IX Item "stopped"
1669 A watcher can be stopped implicitly by libev (in which case it might still
1670 be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function. The
1671 latter will clear any pending state the watcher might be in, regardless
1672 of whether it was active or not, so stopping a watcher explicitly before
1673 freeing it is often a good idea.
1674 .Sp
1675 While stopped (and not pending) the watcher is essentially in the
1676 initialised state, that is, it can be reused, moved, modified in any way
1677 you wish (but when you trash the memory block, you need to \f(CW\*(C`ev_TYPE_init\*(C'\fR
1678 it again).
1679 .SS "\s-1WATCHER PRIORITY MODELS\s0"
1680 .IX Subsection "WATCHER PRIORITY MODELS"
1681 Many event loops support \fIwatcher priorities\fR, which are usually small
1682 integers that influence the ordering of event callback invocation
1683 between watchers in some way, all else being equal.
1684 .PP
1685 In libev, watcher priorities can be set using \f(CW\*(C`ev_set_priority\*(C'\fR. See its
1686 description for the more technical details such as the actual priority
1687 range.
1688 .PP
1689 There are two common ways how these these priorities are being interpreted
1690 by event loops:
1691 .PP
1692 In the more common lock-out model, higher priorities \*(L"lock out\*(R" invocation
1693 of lower priority watchers, which means as long as higher priority
1694 watchers receive events, lower priority watchers are not being invoked.
1695 .PP
1696 The less common only-for-ordering model uses priorities solely to order
1697 callback invocation within a single event loop iteration: Higher priority
1698 watchers are invoked before lower priority ones, but they all get invoked
1699 before polling for new events.
1700 .PP
1701 Libev uses the second (only-for-ordering) model for all its watchers
1702 except for idle watchers (which use the lock-out model).
1703 .PP
1704 The rationale behind this is that implementing the lock-out model for
1705 watchers is not well supported by most kernel interfaces, and most event
1706 libraries will just poll for the same events again and again as long as
1707 their callbacks have not been executed, which is very inefficient in the
1708 common case of one high-priority watcher locking out a mass of lower
1709 priority ones.
1710 .PP
1711 Static (ordering) priorities are most useful when you have two or more
1712 watchers handling the same resource: a typical usage example is having an
1713 \&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR to handle
1714 timeouts. Under load, data might be received while the program handles
1715 other jobs, but since timers normally get invoked first, the timeout
1716 handler will be executed before checking for data. In that case, giving
1717 the timer a lower priority than the I/O watcher ensures that I/O will be
1718 handled first even under adverse conditions (which is usually, but not
1719 always, what you want).
1720 .PP
1721 Since idle watchers use the \*(L"lock-out\*(R" model, meaning that idle watchers
1722 will only be executed when no same or higher priority watchers have
1723 received events, they can be used to implement the \*(L"lock-out\*(R" model when
1724 required.
1725 .PP
1726 For example, to emulate how many other event libraries handle priorities,
1727 you can associate an \f(CW\*(C`ev_idle\*(C'\fR watcher to each such watcher, and in
1728 the normal watcher callback, you just start the idle watcher. The real
1729 processing is done in the idle watcher callback. This causes libev to
1730 continuously poll and process kernel event data for the watcher, but when
1731 the lock-out case is known to be rare (which in turn is rare :), this is
1732 workable.
1733 .PP
1734 Usually, however, the lock-out model implemented that way will perform
1735 miserably under the type of load it was designed to handle. In that case,
1736 it might be preferable to stop the real watcher before starting the
1737 idle watcher, so the kernel will not have to process the event in case
1738 the actual processing will be delayed for considerable time.
1739 .PP
1740 Here is an example of an I/O watcher that should run at a strictly lower
1741 priority than the default, and which should only process data when no
1742 other events are pending:
1743 .PP
1744 .Vb 2
1745 \& ev_idle idle; // actual processing watcher
1746 \& ev_io io; // actual event watcher
1747 \&
1748 \& static void
1749 \& io_cb (EV_P_ ev_io *w, int revents)
1750 \& {
1751 \& // stop the I/O watcher, we received the event, but
1752 \& // are not yet ready to handle it.
1753 \& ev_io_stop (EV_A_ w);
1754 \&
1755 \& // start the idle watcher to handle the actual event.
1756 \& // it will not be executed as long as other watchers
1757 \& // with the default priority are receiving events.
1758 \& ev_idle_start (EV_A_ &idle);
1759 \& }
1760 \&
1761 \& static void
1762 \& idle_cb (EV_P_ ev_idle *w, int revents)
1763 \& {
1764 \& // actual processing
1765 \& read (STDIN_FILENO, ...);
1766 \&
1767 \& // have to start the I/O watcher again, as
1768 \& // we have handled the event
1769 \& ev_io_start (EV_P_ &io);
1770 \& }
1771 \&
1772 \& // initialisation
1773 \& ev_idle_init (&idle, idle_cb);
1774 \& ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1775 \& ev_io_start (EV_DEFAULT_ &io);
1776 .Ve
1777 .PP
1778 In the \*(L"real\*(R" world, it might also be beneficial to start a timer, so that
1779 low-priority connections can not be locked out forever under load. This
1780 enables your program to keep a lower latency for important connections
1781 during short periods of high load, while not completely locking out less
1782 important ones.
1783 .SH "WATCHER TYPES"
1784 .IX Header "WATCHER TYPES"
1785 This section describes each watcher in detail, but will not repeat
1786 information given in the last section. Any initialisation/set macros,
1787 functions and members specific to the watcher type are explained.
1788 .PP
1789 Most members are additionally marked with either \fI[read\-only]\fR, meaning
1790 that, while the watcher is active, you can look at the member and expect
1791 some sensible content, but you must not modify it (you can modify it while
1792 the watcher is stopped to your hearts content), or \fI[read\-write]\fR, which
1793 means you can expect it to have some sensible content while the watcher is
1794 active, but you can also modify it (within the same thread as the event
1795 loop, i.e. without creating data races). Modifying it may not do something
1796 sensible or take immediate effect (or do anything at all), but libev will
1797 not crash or malfunction in any way.
1798 .PP
1799 In any case, the documentation for each member will explain what the
1800 effects are, and if there are any additional access restrictions.
1801 .ie n .SS """ev_io"" \- is this file descriptor readable or writable?"
1802 .el .SS "\f(CWev_io\fP \- is this file descriptor readable or writable?"
1803 .IX Subsection "ev_io - is this file descriptor readable or writable?"
1804 I/O watchers check whether a file descriptor is readable or writable
1805 in each iteration of the event loop, or, more precisely, when reading
1806 would not block the process and writing would at least be able to write
1807 some data. This behaviour is called level-triggering because you keep
1808 receiving events as long as the condition persists. Remember you can stop
1809 the watcher if you don't want to act on the event and neither want to
1810 receive future events.
1811 .PP
1812 In general you can register as many read and/or write event watchers per
1813 fd as you want (as long as you don't confuse yourself). Setting all file
1814 descriptors to non-blocking mode is also usually a good idea (but not
1815 required if you know what you are doing).
1816 .PP
1817 Another thing you have to watch out for is that it is quite easy to
1818 receive \*(L"spurious\*(R" readiness notifications, that is, your callback might
1819 be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block
1820 because there is no data. It is very easy to get into this situation even
1821 with a relatively standard program structure. Thus it is best to always
1822 use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning \f(CW\*(C`EAGAIN\*(C'\fR is far
1823 preferable to a program hanging until some data arrives.
1824 .PP
1825 If you cannot run the fd in non-blocking mode (for example you should
1826 not play around with an Xlib connection), then you have to separately
1827 re-test whether a file descriptor is really ready with a known-to-be good
1828 interface such as poll (fortunately in the case of Xlib, it already does
1829 this on its own, so its quite safe to use). Some people additionally
1830 use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
1831 indefinitely.
1832 .PP
1833 But really, best use non-blocking mode.
1834 .PP
1835 \fIThe special problem of disappearing file descriptors\fR
1836 .IX Subsection "The special problem of disappearing file descriptors"
1837 .PP
1838 Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
1839 a file descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other
1840 means, such as \f(CW\*(C`dup2\*(C'\fR). The reason is that you register interest in some
1841 file descriptor, but when it goes away, the operating system will silently
1842 drop this interest. If another file descriptor with the same number then
1843 is registered with libev, there is no efficient way to see that this is,
1844 in fact, a different file descriptor.
1845 .PP
1846 To avoid having to explicitly tell libev about such cases, libev follows
1847 the following policy: Each time \f(CW\*(C`ev_io_set\*(C'\fR is being called, libev
1848 will assume that this is potentially a new file descriptor, otherwise
1849 it is assumed that the file descriptor stays the same. That means that
1850 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
1851 descriptor even if the file descriptor number itself did not change.
1852 .PP
1853 This is how one would do it normally anyway, the important point is that
1854 the libev application should not optimise around libev but should leave
1855 optimisations to libev.
1856 .PP
1857 \fIThe special problem of dup'ed file descriptors\fR
1858 .IX Subsection "The special problem of dup'ed file descriptors"
1859 .PP
1860 Some backends (e.g. epoll), cannot register events for file descriptors,
1861 but only events for the underlying file descriptions. That means when you
1862 have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register
1863 events for them, only one file descriptor might actually receive events.
1864 .PP
1865 There is no workaround possible except not registering events
1866 for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
1867 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1868 .PP
1869 \fIThe special problem of files\fR
1870 .IX Subsection "The special problem of files"
1871 .PP
1872 Many people try to use \f(CW\*(C`select\*(C'\fR (or libev) on file descriptors
1873 representing files, and expect it to become ready when their program
1874 doesn't block on disk accesses (which can take a long time on their own).
1875 .PP
1876 However, this cannot ever work in the \*(L"expected\*(R" way \- you get a readiness
1877 notification as soon as the kernel knows whether and how much data is
1878 there, and in the case of open files, that's always the case, so you
1879 always get a readiness notification instantly, and your read (or possibly
1880 write) will still block on the disk I/O.
1881 .PP
1882 Another way to view it is that in the case of sockets, pipes, character
1883 devices and so on, there is another party (the sender) that delivers data
1884 on its own, but in the case of files, there is no such thing: the disk
1885 will not send data on its own, simply because it doesn't know what you
1886 wish to read \- you would first have to request some data.
1887 .PP
1888 Since files are typically not-so-well supported by advanced notification
1889 mechanism, libev tries hard to emulate \s-1POSIX\s0 behaviour with respect
1890 to files, even though you should not use it. The reason for this is
1891 convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
1892 usually a tty, often a pipe, but also sometimes files or special devices
1893 (for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
1894 \&\fI/dev/urandom\fR), and even though the file might better be served with
1895 asynchronous I/O instead of with non-blocking I/O, it is still useful when
1896 it \*(L"just works\*(R" instead of freezing.
1897 .PP
1898 So avoid file descriptors pointing to files when you know it (e.g. use
1899 libeio), but use them when it is convenient, e.g. for \s-1STDIN/STDOUT,\s0 or
1900 when you rarely read from a file instead of from a socket, and want to
1901 reuse the same code path.
1902 .PP
1903 \fIThe special problem of fork\fR
1904 .IX Subsection "The special problem of fork"
1905 .PP
1906 Some backends (epoll, kqueue, linuxaio, iouring) do not support \f(CW\*(C`fork ()\*(C'\fR
1907 at all or exhibit useless behaviour. Libev fully supports fork, but needs
1908 to be told about it in the child if you want to continue to use it in the
1909 child.
1910 .PP
1911 To support fork in your child processes, you have to call \f(CW\*(C`ev_loop_fork
1912 ()\*(C'\fR after a fork in the child, enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to
1913 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1914 .PP
1915 \fIThe special problem of \s-1SIGPIPE\s0\fR
1916 .IX Subsection "The special problem of SIGPIPE"
1917 .PP
1918 While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:
1919 when writing to a pipe whose other end has been closed, your program gets
1920 sent a \s-1SIGPIPE,\s0 which, by default, aborts your program. For most programs
1921 this is sensible behaviour, for daemons, this is usually undesirable.
1922 .PP
1923 So when you encounter spurious, unexplained daemon exits, make sure you
1924 ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon
1925 somewhere, as that would have given you a big clue).
1926 .PP
1927 \fIThe special problem of \f(BIaccept()\fIing when you can't\fR
1928 .IX Subsection "The special problem of accept()ing when you can't"
1929 .PP
1930 Many implementations of the \s-1POSIX\s0 \f(CW\*(C`accept\*(C'\fR function (for example,
1931 found in post\-2004 Linux) have the peculiar behaviour of not removing a
1932 connection from the pending queue in all error cases.
1933 .PP
1934 For example, larger servers often run out of file descriptors (because
1935 of resource limits), causing \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR but not
1936 rejecting the connection, leading to libev signalling readiness on
1937 the next iteration again (the connection still exists after all), and
1938 typically causing the program to loop at 100% \s-1CPU\s0 usage.
1939 .PP
1940 Unfortunately, the set of errors that cause this issue differs between
1941 operating systems, there is usually little the app can do to remedy the
1942 situation, and no known thread-safe method of removing the connection to
1943 cope with overload is known (to me).
1944 .PP
1945 One of the easiest ways to handle this situation is to just ignore it
1946 \&\- when the program encounters an overload, it will just loop until the
1947 situation is over. While this is a form of busy waiting, no \s-1OS\s0 offers an
1948 event-based way to handle this situation, so it's the best one can do.
1949 .PP
1950 A better way to handle the situation is to log any errors other than
1951 \&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, making sure not to flood the log with such
1952 messages, and continue as usual, which at least gives the user an idea of
1953 what could be wrong (\*(L"raise the ulimit!\*(R"). For extra points one could stop
1954 the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
1955 usage.
1956 .PP
1957 If your program is single-threaded, then you could also keep a dummy file
1958 descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
1959 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,
1960 close that fd, and create a new dummy fd. This will gracefully refuse
1961 clients under typical overload conditions.
1962 .PP
1963 The last way to handle it is to simply log the error and \f(CW\*(C`exit\*(C'\fR, as
1964 is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
1965 opportunity for a DoS attack.
1966 .PP
1967 \fIWatcher-Specific Functions\fR
1968 .IX Subsection "Watcher-Specific Functions"
1969 .IP "ev_io_init (ev_io *, callback, int fd, int events)" 4
1970 .IX Item "ev_io_init (ev_io *, callback, int fd, int events)"
1971 .PD 0
1972 .IP "ev_io_set (ev_io *, int fd, int events)" 4
1973 .IX Item "ev_io_set (ev_io *, int fd, int events)"
1974 .PD
1975 Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to
1976 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
1977 \&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR or \f(CW0\fR, to express the desire to receive the given
1978 events.
1979 .Sp
1980 Note that setting the \f(CW\*(C`events\*(C'\fR to \f(CW0\fR and starting the watcher is
1981 supported, but not specially optimized \- if your program sometimes happens
1982 to generate this combination this is fine, but if it is easy to avoid
1983 starting an io watcher watching for no events you should do so.
1984 .IP "ev_io_modify (ev_io *, int events)" 4
1985 .IX Item "ev_io_modify (ev_io *, int events)"
1986 Similar to \f(CW\*(C`ev_io_set\*(C'\fR, but only changes the requested events. Using this
1987 might be faster with some backends, as libev can assume that the \f(CW\*(C`fd\*(C'\fR
1988 still refers to the same underlying file description, something it cannot
1989 do when using \f(CW\*(C`ev_io_set\*(C'\fR.
1990 .IP "int fd [no\-modify]" 4
1991 .IX Item "int fd [no-modify]"
1992 The file descriptor being watched. While it can be read at any time, you
1993 must not modify this member even when the watcher is stopped \- always use
1994 \&\f(CW\*(C`ev_io_set\*(C'\fR for that.
1995 .IP "int events [no\-modify]" 4
1996 .IX Item "int events [no-modify]"
1997 The set of events the fd is being watched for, among other flags. Remember
1998 that this is a bit set \- to test for \f(CW\*(C`EV_READ\*(C'\fR, use \f(CW\*(C`w\->events &
1999 EV_READ\*(C'\fR, and similarly for \f(CW\*(C`EV_WRITE\*(C'\fR.
2000 .Sp
2001 As with \f(CW\*(C`fd\*(C'\fR, you must not modify this member even when the watcher is
2002 stopped, always use \f(CW\*(C`ev_io_set\*(C'\fR or \f(CW\*(C`ev_io_modify\*(C'\fR for that.
2003 .PP
2004 \fIExamples\fR
2005 .IX Subsection "Examples"
2006 .PP
2007 Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well
2008 readable, but only once. Since it is likely line-buffered, you could
2009 attempt to read a whole line in the callback.
2010 .PP
2011 .Vb 6
2012 \& static void
2013 \& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
2014 \& {
2015 \& ev_io_stop (loop, w);
2016 \& .. read from stdin here (or from w\->fd) and handle any I/O errors
2017 \& }
2018 \&
2019 \& ...
2020 \& struct ev_loop *loop = ev_default_init (0);
2021 \& ev_io stdin_readable;
2022 \& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
2023 \& ev_io_start (loop, &stdin_readable);
2024 \& ev_run (loop, 0);
2025 .Ve
2026 .ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
2027 .el .SS "\f(CWev_timer\fP \- relative and optionally repeating timeouts"
2028 .IX Subsection "ev_timer - relative and optionally repeating timeouts"
2029 Timer watchers are simple relative timers that generate an event after a
2030 given time, and optionally repeating in regular intervals after that.
2031 .PP
2032 The timers are based on real time, that is, if you register an event that
2033 times out after an hour and you reset your system clock to January last
2034 year, it will still time out after (roughly) one hour. \*(L"Roughly\*(R" because
2035 detecting time jumps is hard, and some inaccuracies are unavoidable (the
2036 monotonic clock option helps a lot here).
2037 .PP
2038 The callback is guaranteed to be invoked only \fIafter\fR its timeout has
2039 passed (not \fIat\fR, so on systems with very low-resolution clocks this
2040 might introduce a small delay, see \*(L"the special problem of being too
2041 early\*(R", below). If multiple timers become ready during the same loop
2042 iteration then the ones with earlier time-out values are invoked before
2043 ones of the same priority with later time-out values (but this is no
2044 longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2045 .PP
2046 \fIBe smart about timeouts\fR
2047 .IX Subsection "Be smart about timeouts"
2048 .PP
2049 Many real-world problems involve some kind of timeout, usually for error
2050 recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
2051 you want to raise some error after a while.
2052 .PP
2053 What follows are some ways to handle this problem, from obvious and
2054 inefficient to smart and efficient.
2055 .PP
2056 In the following, a 60 second activity timeout is assumed \- a timeout that
2057 gets reset to 60 seconds each time there is activity (e.g. each time some
2058 data or other life sign was received).
2059 .IP "1. Use a timer and stop, reinitialise and start it on activity." 4
2060 .IX Item "1. Use a timer and stop, reinitialise and start it on activity."
2061 This is the most obvious, but not the most simple way: In the beginning,
2062 start the watcher:
2063 .Sp
2064 .Vb 2
2065 \& ev_timer_init (timer, callback, 60., 0.);
2066 \& ev_timer_start (loop, timer);
2067 .Ve
2068 .Sp
2069 Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
2070 and start it again:
2071 .Sp
2072 .Vb 3
2073 \& ev_timer_stop (loop, timer);
2074 \& ev_timer_set (timer, 60., 0.);
2075 \& ev_timer_start (loop, timer);
2076 .Ve
2077 .Sp
2078 This is relatively simple to implement, but means that each time there is
2079 some activity, libev will first have to remove the timer from its internal
2080 data structure and then add it again. Libev tries to be fast, but it's
2081 still not a constant-time operation.
2082 .ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
2083 .el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
2084 .IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
2085 This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
2086 \&\f(CW\*(C`ev_timer_start\*(C'\fR.
2087 .Sp
2088 To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
2089 of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
2090 successfully read or write some data. If you go into an idle state where
2091 you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
2092 the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
2093 .Sp
2094 That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
2095 \&\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
2096 member and \f(CW\*(C`ev_timer_again\*(C'\fR.
2097 .Sp
2098 At start:
2099 .Sp
2100 .Vb 3
2101 \& ev_init (timer, callback);
2102 \& timer\->repeat = 60.;
2103 \& ev_timer_again (loop, timer);
2104 .Ve
2105 .Sp
2106 Each time there is some activity:
2107 .Sp
2108 .Vb 1
2109 \& ev_timer_again (loop, timer);
2110 .Ve
2111 .Sp
2112 It is even possible to change the time-out on the fly, regardless of
2113 whether the watcher is active or not:
2114 .Sp
2115 .Vb 2
2116 \& timer\->repeat = 30.;
2117 \& ev_timer_again (loop, timer);
2118 .Ve
2119 .Sp
2120 This is slightly more efficient then stopping/starting the timer each time
2121 you want to modify its timeout value, as libev does not have to completely
2122 remove and re-insert the timer from/into its internal data structure.
2123 .Sp
2124 It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
2125 .IP "3. Let the timer time out, but then re-arm it as required." 4
2126 .IX Item "3. Let the timer time out, but then re-arm it as required."
2127 This method is more tricky, but usually most efficient: Most timeouts are
2128 relatively long compared to the intervals between other activity \- in
2129 our example, within 60 seconds, there are usually many I/O events with
2130 associated activity resets.
2131 .Sp
2132 In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
2133 but remember the time of last activity, and check for a real timeout only
2134 within the callback:
2135 .Sp
2136 .Vb 3
2137 \& ev_tstamp timeout = 60.;
2138 \& ev_tstamp last_activity; // time of last activity
2139 \& ev_timer timer;
2140 \&
2141 \& static void
2142 \& callback (EV_P_ ev_timer *w, int revents)
2143 \& {
2144 \& // calculate when the timeout would happen
2145 \& ev_tstamp after = last_activity \- ev_now (EV_A) + timeout;
2146 \&
2147 \& // if negative, it means we the timeout already occurred
2148 \& if (after < 0.)
2149 \& {
2150 \& // timeout occurred, take action
2151 \& }
2152 \& else
2153 \& {
2154 \& // callback was invoked, but there was some recent
2155 \& // activity. simply restart the timer to time out
2156 \& // after "after" seconds, which is the earliest time
2157 \& // the timeout can occur.
2158 \& ev_timer_set (w, after, 0.);
2159 \& ev_timer_start (EV_A_ w);
2160 \& }
2161 \& }
2162 .Ve
2163 .Sp
2164 To summarise the callback: first calculate in how many seconds the
2165 timeout will occur (by calculating the absolute time when it would occur,
2166 \&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
2167 (EV_A)\*(C'\fR from that).
2168 .Sp
2169 If this value is negative, then we are already past the timeout, i.e. we
2170 timed out, and need to do whatever is needed in this case.
2171 .Sp
2172 Otherwise, we now the earliest time at which the timeout would trigger,
2173 and simply start the timer with this timeout value.
2174 .Sp
2175 In other words, each time the callback is invoked it will check whether
2176 the timeout occurred. If not, it will simply reschedule itself to check
2177 again at the earliest time it could time out. Rinse. Repeat.
2178 .Sp
2179 This scheme causes more callback invocations (about one every 60 seconds
2180 minus half the average time between activity), but virtually no calls to
2181 libev to change the timeout.
2182 .Sp
2183 To start the machinery, simply initialise the watcher and set
2184 \&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
2185 now), then call the callback, which will \*(L"do the right thing\*(R" and start
2186 the timer:
2187 .Sp
2188 .Vb 3
2189 \& last_activity = ev_now (EV_A);
2190 \& ev_init (&timer, callback);
2191 \& callback (EV_A_ &timer, 0);
2192 .Ve
2193 .Sp
2194 When there is some activity, simply store the current time in
2195 \&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
2196 .Sp
2197 .Vb 2
2198 \& if (activity detected)
2199 \& last_activity = ev_now (EV_A);
2200 .Ve
2201 .Sp
2202 When your timeout value changes, then the timeout can be changed by simply
2203 providing a new value, stopping the timer and calling the callback, which
2204 will again do the right thing (for example, time out immediately :).
2205 .Sp
2206 .Vb 3
2207 \& timeout = new_value;
2208 \& ev_timer_stop (EV_A_ &timer);
2209 \& callback (EV_A_ &timer, 0);
2210 .Ve
2211 .Sp
2212 This technique is slightly more complex, but in most cases where the
2213 time-out is unlikely to be triggered, much more efficient.
2214 .IP "4. Wee, just use a double-linked list for your timeouts." 4
2215 .IX Item "4. Wee, just use a double-linked list for your timeouts."
2216 If there is not one request, but many thousands (millions...), all
2217 employing some kind of timeout with the same timeout value, then one can
2218 do even better:
2219 .Sp
2220 When starting the timeout, calculate the timeout value and put the timeout
2221 at the \fIend\fR of the list.
2222 .Sp
2223 Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
2224 the list is expected to fire (for example, using the technique #3).
2225 .Sp
2226 When there is some activity, remove the timer from the list, recalculate
2227 the timeout, append it to the end of the list again, and make sure to
2228 update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
2229 .Sp
2230 This way, one can manage an unlimited number of timeouts in O(1) time for
2231 starting, stopping and updating the timers, at the expense of a major
2232 complication, and having to use a constant timeout. The constant timeout
2233 ensures that the list stays sorted.
2234 .PP
2235 So which method the best?
2236 .PP
2237 Method #2 is a simple no-brain-required solution that is adequate in most
2238 situations. Method #3 requires a bit more thinking, but handles many cases
2239 better, and isn't very complicated either. In most case, choosing either
2240 one is fine, with #3 being better in typical situations.
2241 .PP
2242 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
2243 rather complicated, but extremely efficient, something that really pays
2244 off after the first million or so of active timers, i.e. it's usually
2245 overkill :)
2246 .PP
2247 \fIThe special problem of being too early\fR
2248 .IX Subsection "The special problem of being too early"
2249 .PP
2250 If you ask a timer to call your callback after three seconds, then
2251 you expect it to be invoked after three seconds \- but of course, this
2252 cannot be guaranteed to infinite precision. Less obviously, it cannot be
2253 guaranteed to any precision by libev \- imagine somebody suspending the
2254 process with a \s-1STOP\s0 signal for a few hours for example.
2255 .PP
2256 So, libev tries to invoke your callback as soon as possible \fIafter\fR the
2257 delay has occurred, but cannot guarantee this.
2258 .PP
2259 A less obvious failure mode is calling your callback too early: many event
2260 loops compare timestamps with a \*(L"elapsed delay >= requested delay\*(R", but
2261 this can cause your callback to be invoked much earlier than you would
2262 expect.
2263 .PP
2264 To see why, imagine a system with a clock that only offers full second
2265 resolution (think windows if you can't come up with a broken enough \s-1OS\s0
2266 yourself). If you schedule a one-second timer at the time 500.9, then the
2267 event loop will schedule your timeout to elapse at a system time of 500
2268 (500.9 truncated to the resolution) + 1, or 501.
2269 .PP
2270 If an event library looks at the timeout 0.1s later, it will see \*(L"501 >=
2271 501\*(R" and invoke the callback 0.1s after it was started, even though a
2272 one-second delay was requested \- this is being \*(L"too early\*(R", despite best
2273 intentions.
2274 .PP
2275 This is the reason why libev will never invoke the callback if the elapsed
2276 delay equals the requested delay, but only when the elapsed delay is
2277 larger than the requested delay. In the example above, libev would only invoke
2278 the callback at system time 502, or 1.1s after the timer was started.
2279 .PP
2280 So, while libev cannot guarantee that your callback will be invoked
2281 exactly when requested, it \fIcan\fR and \fIdoes\fR guarantee that the requested
2282 delay has actually elapsed, or in other words, it always errs on the \*(L"too
2283 late\*(R" side of things.
2284 .PP
2285 \fIThe special problem of time updates\fR
2286 .IX Subsection "The special problem of time updates"
2287 .PP
2288 Establishing the current time is a costly operation (it usually takes
2289 at least one system call): \s-1EV\s0 therefore updates its idea of the current
2290 time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
2291 growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
2292 lots of events in one iteration.
2293 .PP
2294 The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
2295 time. This is usually the right thing as this timestamp refers to the time
2296 of the event triggering whatever timeout you are modifying/starting. If
2297 you suspect event processing to be delayed and you \fIneed\fR to base the
2298 timeout on the current time, use something like the following to adjust
2299 for it:
2300 .PP
2301 .Vb 1
2302 \& ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
2303 .Ve
2304 .PP
2305 If the event loop is suspended for a long time, you can also force an
2306 update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
2307 ()\*(C'\fR, although that will push the event time of all outstanding events
2308 further into the future.
2309 .PP
2310 \fIThe special problem of unsynchronised clocks\fR
2311 .IX Subsection "The special problem of unsynchronised clocks"
2312 .PP
2313 Modern systems have a variety of clocks \- libev itself uses the normal
2314 \&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
2315 jumps).
2316 .PP
2317 Neither of these clocks is synchronised with each other or any other clock
2318 on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
2319 than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
2320 a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
2321 than a directly following call to \f(CW\*(C`time\*(C'\fR.
2322 .PP
2323 The moral of this is to only compare libev-related timestamps with
2324 \&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, at least if you want better precision than
2325 a second or so.
2326 .PP
2327 One more problem arises due to this lack of synchronisation: if libev uses
2328 the system monotonic clock and you compare timestamps from \f(CW\*(C`ev_time\*(C'\fR
2329 or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
2330 invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
2331 .PP
2332 This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
2333 libev makes sure your callback is not invoked before the delay happened,
2334 \&\fImeasured according to the real time\fR, not the system clock.
2335 .PP
2336 If your timeouts are based on a physical timescale (e.g. \*(L"time out this
2337 connection after 100 seconds\*(R") then this shouldn't bother you as it is
2338 exactly the right behaviour.
2339 .PP
2340 If you want to compare wall clock/system timestamps to your timers, then
2341 you need to use \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
2342 time, where your comparisons will always generate correct results.
2343 .PP
2344 \fIThe special problems of suspended animation\fR
2345 .IX Subsection "The special problems of suspended animation"
2346 .PP
2347 When you leave the server world it is quite customary to hit machines that
2348 can suspend/hibernate \- what happens to the clocks during such a suspend?
2349 .PP
2350 Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
2351 all processes, while the clocks (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) continue
2352 to run until the system is suspended, but they will not advance while the
2353 system is suspended. That means, on resume, it will be as if the program
2354 was frozen for a few seconds, but the suspend time will not be counted
2355 towards \f(CW\*(C`ev_timer\*(C'\fR when a monotonic clock source is used. The real time
2356 clock advanced as expected, but if it is used as sole clocksource, then a
2357 long suspend would be detected as a time jump by libev, and timers would
2358 be adjusted accordingly.
2359 .PP
2360 I would not be surprised to see different behaviour in different between
2361 operating systems, \s-1OS\s0 versions or even different hardware.
2362 .PP
2363 The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) will see a
2364 time jump in the monotonic clocks and the realtime clock. If the program
2365 is suspended for a very long time, and monotonic clock sources are in use,
2366 then you can expect \f(CW\*(C`ev_timer\*(C'\fRs to expire as the full suspension time
2367 will be counted towards the timers. When no monotonic clock source is in
2368 use, then libev will again assume a timejump and adjust accordingly.
2369 .PP
2370 It might be beneficial for this latter case to call \f(CW\*(C`ev_suspend\*(C'\fR
2371 and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
2372 deterministic behaviour in this case (you can do nothing against
2373 \&\f(CW\*(C`SIGSTOP\*(C'\fR).
2374 .PP
2375 \fIWatcher-Specific Functions and Data Members\fR
2376 .IX Subsection "Watcher-Specific Functions and Data Members"
2377 .IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
2378 .IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
2379 .PD 0
2380 .IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
2381 .IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
2382 .PD
2383 Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
2384 negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, then it will
2385 automatically be stopped once the timeout is reached. If it is positive,
2386 then the timer will automatically be configured to trigger again \f(CW\*(C`repeat\*(C'\fR
2387 seconds later, again, and again, until stopped manually.
2388 .Sp
2389 The timer itself will do a best-effort at avoiding drift, that is, if
2390 you configure a timer to trigger every 10 seconds, then it will normally
2391 trigger at exactly 10 second intervals. If, however, your program cannot
2392 keep up with the timer (because it takes longer than those 10 seconds to
2393 do stuff) the timer will not fire more than once per event loop iteration.
2394 .IP "ev_timer_again (loop, ev_timer *)" 4
2395 .IX Item "ev_timer_again (loop, ev_timer *)"
2396 This will act as if the timer timed out, and restarts it again if it is
2397 repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
2398 timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
2399 .Sp
2400 The exact semantics are as in the following rules, all of which will be
2401 applied to the watcher:
2402 .RS 4
2403 .IP "If the timer is pending, the pending status is always cleared." 4
2404 .IX Item "If the timer is pending, the pending status is always cleared."
2405 .PD 0
2406 .IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
2407 .IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
2408 .ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
2409 .el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
2410 .IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
2411 .RE
2412 .RS 4
2413 .PD
2414 .Sp
2415 This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
2416 usage example.
2417 .RE
2418 .IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
2419 .IX Item "ev_tstamp ev_timer_remaining (loop, ev_timer *)"
2420 Returns the remaining time until a timer fires. If the timer is active,
2421 then this time is relative to the current event loop time, otherwise it's
2422 the timeout value currently configured.
2423 .Sp
2424 That is, after an \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
2425 \&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
2426 will return \f(CW4\fR. When the timer expires and is restarted, it will return
2427 roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
2428 too), and so on.
2429 .IP "ev_tstamp repeat [read\-write]" 4
2430 .IX Item "ev_tstamp repeat [read-write]"
2431 The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
2432 or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
2433 which is also when any modifications are taken into account.
2434 .PP
2435 \fIExamples\fR
2436 .IX Subsection "Examples"
2437 .PP
2438 Example: Create a timer that fires after 60 seconds.
2439 .PP
2440 .Vb 5
2441 \& static void
2442 \& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2443 \& {
2444 \& .. one minute over, w is actually stopped right here
2445 \& }
2446 \&
2447 \& ev_timer mytimer;
2448 \& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2449 \& ev_timer_start (loop, &mytimer);
2450 .Ve
2451 .PP
2452 Example: Create a timeout timer that times out after 10 seconds of
2453 inactivity.
2454 .PP
2455 .Vb 5
2456 \& static void
2457 \& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2458 \& {
2459 \& .. ten seconds without any activity
2460 \& }
2461 \&
2462 \& ev_timer mytimer;
2463 \& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2464 \& ev_timer_again (&mytimer); /* start timer */
2465 \& ev_run (loop, 0);
2466 \&
2467 \& // and in some piece of code that gets executed on any "activity":
2468 \& // reset the timeout to start ticking again at 10 seconds
2469 \& ev_timer_again (&mytimer);
2470 .Ve
2471 .ie n .SS """ev_periodic"" \- to cron or not to cron?"
2472 .el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
2473 .IX Subsection "ev_periodic - to cron or not to cron?"
2474 Periodic watchers are also timers of a kind, but they are very versatile
2475 (and unfortunately a bit complex).
2476 .PP
2477 Unlike \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
2478 relative time, the physical time that passes) but on wall clock time
2479 (absolute time, the thing you can read on your calendar or clock). The
2480 difference is that wall clock time can run faster or slower than real
2481 time, and time jumps are not uncommon (e.g. when you adjust your
2482 wrist-watch).
2483 .PP
2484 You can tell a periodic watcher to trigger after some specific point
2485 in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
2486 seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, that is, an absolute time
2487 not a delay) and then reset your system clock to January of the previous
2488 year, then it will take a year or more to trigger the event (unlike an
2489 \&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
2490 it, as it uses a relative timeout).
2491 .PP
2492 \&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
2493 timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
2494 other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
2495 watchers, as those cannot react to time jumps.
2496 .PP
2497 As with timers, the callback is guaranteed to be invoked only when the
2498 point in time where it is supposed to trigger has passed. If multiple
2499 timers become ready during the same loop iteration then the ones with
2500 earlier time-out values are invoked before ones with later time-out values
2501 (but this is no longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2502 .PP
2503 \fIWatcher-Specific Functions and Data Members\fR
2504 .IX Subsection "Watcher-Specific Functions and Data Members"
2505 .IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2506 .IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2507 .PD 0
2508 .IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2509 .IX Item "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2510 .PD
2511 Lots of arguments, let's sort it out... There are basically three modes of
2512 operation, and we will explain them from simplest to most complex:
2513 .RS 4
2514 .IP "\(bu" 4
2515 absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
2516 .Sp
2517 In this configuration the watcher triggers an event after the wall clock
2518 time \f(CW\*(C`offset\*(C'\fR has passed. It will not repeat and will not adjust when a
2519 time jump occurs, that is, if it is to be run at January 1st 2011 then it
2520 will be stopped and invoked when the system clock reaches or surpasses
2521 this point in time.
2522 .IP "\(bu" 4
2523 repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
2524 .Sp
2525 In this mode the watcher will always be scheduled to time out at the next
2526 \&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
2527 negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
2528 argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
2529 .Sp
2530 This can be used to create timers that do not drift with respect to the
2531 system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
2532 hour, on the hour (with respect to \s-1UTC\s0):
2533 .Sp
2534 .Vb 1
2535 \& ev_periodic_set (&periodic, 0., 3600., 0);
2536 .Ve
2537 .Sp
2538 This doesn't mean there will always be 3600 seconds in between triggers,
2539 but only that the callback will be called when the system time shows a
2540 full hour (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
2541 by 3600.
2542 .Sp
2543 Another way to think about it (for the mathematically inclined) is that
2544 \&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
2545 time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
2546 .Sp
2547 The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
2548 interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
2549 microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR and should have
2550 at most a similar magnitude as the current time (say, within a factor of
2551 ten). Typical values for offset are, in fact, \f(CW0\fR or something between
2552 \&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
2553 .Sp
2554 Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
2555 speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
2556 will of course deteriorate. Libev itself tries to be exact to be about one
2557 millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
2558 .IP "\(bu" 4
2559 manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
2560 .Sp
2561 In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR are both being
2562 ignored. Instead, each time the periodic watcher gets scheduled, the
2563 reschedule callback will be called with the watcher as first, and the
2564 current time as second argument.
2565 .Sp
2566 \&\s-1NOTE:\s0 \fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
2567 or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
2568 allowed by documentation here\fR.
2569 .Sp
2570 If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
2571 it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
2572 only event loop modification you are allowed to do).
2573 .Sp
2574 The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
2575 *w, ev_tstamp now)\*(C'\fR, e.g.:
2576 .Sp
2577 .Vb 5
2578 \& static ev_tstamp
2579 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2580 \& {
2581 \& return now + 60.;
2582 \& }
2583 .Ve
2584 .Sp
2585 It must return the next time to trigger, based on the passed time value
2586 (that is, the lowest time value larger than to the second argument). It
2587 will usually be called just before the callback will be triggered, but
2588 might be called at other times, too.
2589 .Sp
2590 \&\s-1NOTE:\s0 \fIThis callback must always return a time that is higher than or
2591 equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
2592 .Sp
2593 This can be used to create very complex timers, such as a timer that
2594 triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
2595 the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
2596 this. Here is a (completely untested, no error checking) example on how to
2597 do this:
2598 .Sp
2599 .Vb 1
2600 \& #include <time.h>
2601 \&
2602 \& static ev_tstamp
2603 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2604 \& {
2605 \& time_t tnow = (time_t)now;
2606 \& struct tm tm;
2607 \& localtime_r (&tnow, &tm);
2608 \&
2609 \& tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2610 \& ++tm.tm_mday; // midnight next day
2611 \&
2612 \& return mktime (&tm);
2613 \& }
2614 .Ve
2615 .Sp
2616 Note: this code might run into trouble on days that have more then two
2617 midnights (beginning and end).
2618 .RE
2619 .RS 4
2620 .RE
2621 .IP "ev_periodic_again (loop, ev_periodic *)" 4
2622 .IX Item "ev_periodic_again (loop, ev_periodic *)"
2623 Simply stops and restarts the periodic watcher again. This is only useful
2624 when you changed some parameters or the reschedule callback would return
2625 a different time than the last time it was called (e.g. in a crond like
2626 program when the crontabs have changed).
2627 .IP "ev_tstamp ev_periodic_at (ev_periodic *)" 4
2628 .IX Item "ev_tstamp ev_periodic_at (ev_periodic *)"
2629 When active, returns the absolute time that the watcher is supposed
2630 to trigger next. This is not the same as the \f(CW\*(C`offset\*(C'\fR argument to
2631 \&\f(CW\*(C`ev_periodic_set\*(C'\fR, but indeed works even in interval and manual
2632 rescheduling modes.
2633 .IP "ev_tstamp offset [read\-write]" 4
2634 .IX Item "ev_tstamp offset [read-write]"
2635 When repeating, this contains the offset value, otherwise this is the
2636 absolute point in time (the \f(CW\*(C`offset\*(C'\fR value passed to \f(CW\*(C`ev_periodic_set\*(C'\fR,
2637 although libev might modify this value for better numerical stability).
2638 .Sp
2639 Can be modified any time, but changes only take effect when the periodic
2640 timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2641 .IP "ev_tstamp interval [read\-write]" 4
2642 .IX Item "ev_tstamp interval [read-write]"
2643 The current interval value. Can be modified any time, but changes only
2644 take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
2645 called.
2646 .IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
2647 .IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
2648 The current reschedule callback, or \f(CW0\fR, if this functionality is
2649 switched off. Can be changed any time, but changes only take effect when
2650 the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2651 .PP
2652 \fIExamples\fR
2653 .IX Subsection "Examples"
2654 .PP
2655 Example: Call a callback every hour, or, more precisely, whenever the
2656 system time is divisible by 3600. The callback invocation times have
2657 potentially a lot of jitter, but good long-term stability.
2658 .PP
2659 .Vb 5
2660 \& static void
2661 \& clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2662 \& {
2663 \& ... its now a full hour (UTC, or TAI or whatever your clock follows)
2664 \& }
2665 \&
2666 \& ev_periodic hourly_tick;
2667 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2668 \& ev_periodic_start (loop, &hourly_tick);
2669 .Ve
2670 .PP
2671 Example: The same as above, but use a reschedule callback to do it:
2672 .PP
2673 .Vb 1
2674 \& #include <math.h>
2675 \&
2676 \& static ev_tstamp
2677 \& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2678 \& {
2679 \& return now + (3600. \- fmod (now, 3600.));
2680 \& }
2681 \&
2682 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2683 .Ve
2684 .PP
2685 Example: Call a callback every hour, starting now:
2686 .PP
2687 .Vb 4
2688 \& ev_periodic hourly_tick;
2689 \& ev_periodic_init (&hourly_tick, clock_cb,
2690 \& fmod (ev_now (loop), 3600.), 3600., 0);
2691 \& ev_periodic_start (loop, &hourly_tick);
2692 .Ve
2693 .ie n .SS """ev_signal"" \- signal me when a signal gets signalled!"
2694 .el .SS "\f(CWev_signal\fP \- signal me when a signal gets signalled!"
2695 .IX Subsection "ev_signal - signal me when a signal gets signalled!"
2696 Signal watchers will trigger an event when the process receives a specific
2697 signal one or more times. Even though signals are very asynchronous, libev
2698 will try its best to deliver signals synchronously, i.e. as part of the
2699 normal event processing, like any other event.
2700 .PP
2701 If you want signals to be delivered truly asynchronously, just use
2702 \&\f(CW\*(C`sigaction\*(C'\fR as you would do without libev and forget about sharing
2703 the signal. You can even use \f(CW\*(C`ev_async\*(C'\fR from a signal handler to
2704 synchronously wake up an event loop.
2705 .PP
2706 You can configure as many watchers as you like for the same signal, but
2707 only within the same loop, i.e. you can watch for \f(CW\*(C`SIGINT\*(C'\fR in your
2708 default loop and for \f(CW\*(C`SIGIO\*(C'\fR in another loop, but you cannot watch for
2709 \&\f(CW\*(C`SIGINT\*(C'\fR in both the default loop and another loop at the same time. At
2710 the moment, \f(CW\*(C`SIGCHLD\*(C'\fR is permanently tied to the default loop.
2711 .PP
2712 Only after the first watcher for a signal is started will libev actually
2713 register something with the kernel. It thus coexists with your own signal
2714 handlers as long as you don't register any with libev for the same signal.
2715 .PP
2716 If possible and supported, libev will install its handlers with
2717 \&\f(CW\*(C`SA_RESTART\*(C'\fR (or equivalent) behaviour enabled, so system calls should
2718 not be unduly interrupted. If you have a problem with system calls getting
2719 interrupted by signals you can block all signals in an \f(CW\*(C`ev_check\*(C'\fR watcher
2720 and unblock them in an \f(CW\*(C`ev_prepare\*(C'\fR watcher.
2721 .PP
2722 \fIThe special problem of inheritance over fork/execve/pthread_create\fR
2723 .IX Subsection "The special problem of inheritance over fork/execve/pthread_create"
2724 .PP
2725 Both the signal mask (\f(CW\*(C`sigprocmask\*(C'\fR) and the signal disposition
2726 (\f(CW\*(C`sigaction\*(C'\fR) are unspecified after starting a signal watcher (and after
2727 stopping it again), that is, libev might or might not block the signal,
2728 and might or might not set or restore the installed signal handler (but
2729 see \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR).
2730 .PP
2731 While this does not matter for the signal disposition (libev never
2732 sets signals to \f(CW\*(C`SIG_IGN\*(C'\fR, so handlers will be reset to \f(CW\*(C`SIG_DFL\*(C'\fR on
2733 \&\f(CW\*(C`execve\*(C'\fR), this matters for the signal mask: many programs do not expect
2734 certain signals to be blocked.
2735 .PP
2736 This means that before calling \f(CW\*(C`exec\*(C'\fR (from the child) you should reset
2737 the signal mask to whatever \*(L"default\*(R" you expect (all clear is a good
2738 choice usually).
2739 .PP
2740 The simplest way to ensure that the signal mask is reset in the child is
2741 to install a fork handler with \f(CW\*(C`pthread_atfork\*(C'\fR that resets it. That will
2742 catch fork calls done by libraries (such as the libc) as well.
2743 .PP
2744 In current versions of libev, the signal will not be blocked indefinitely
2745 unless you use the \f(CW\*(C`signalfd\*(C'\fR \s-1API\s0 (\f(CW\*(C`EV_SIGNALFD\*(C'\fR). While this reduces
2746 the window of opportunity for problems, it will not go away, as libev
2747 \&\fIhas\fR to modify the signal mask, at least temporarily.
2748 .PP
2749 So I can't stress this enough: \fIIf you do not reset your signal mask when
2750 you expect it to be empty, you have a race condition in your code\fR. This
2751 is not a libev-specific thing, this is true for most event libraries.
2752 .PP
2753 \fIThe special problem of threads signal handling\fR
2754 .IX Subsection "The special problem of threads signal handling"
2755 .PP
2756 \&\s-1POSIX\s0 threads has problematic signal handling semantics, specifically,
2757 a lot of functionality (sigfd, sigwait etc.) only really works if all
2758 threads in a process block signals, which is hard to achieve.
2759 .PP
2760 When you want to use sigwait (or mix libev signal handling with your own
2761 for the same signals), you can tackle this problem by globally blocking
2762 all signals before creating any threads (or creating them with a fully set
2763 sigprocmask) and also specifying the \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when creating
2764 loops. Then designate one thread as \*(L"signal receiver thread\*(R" which handles
2765 these signals. You can pass on any signals that libev might be interested
2766 in by calling \f(CW\*(C`ev_feed_signal\*(C'\fR.
2767 .PP
2768 \fIWatcher-Specific Functions and Data Members\fR
2769 .IX Subsection "Watcher-Specific Functions and Data Members"
2770 .IP "ev_signal_init (ev_signal *, callback, int signum)" 4
2771 .IX Item "ev_signal_init (ev_signal *, callback, int signum)"
2772 .PD 0
2773 .IP "ev_signal_set (ev_signal *, int signum)" 4
2774 .IX Item "ev_signal_set (ev_signal *, int signum)"
2775 .PD
2776 Configures the watcher to trigger on the given signal number (usually one
2777 of the \f(CW\*(C`SIGxxx\*(C'\fR constants).
2778 .IP "int signum [read\-only]" 4
2779 .IX Item "int signum [read-only]"
2780 The signal the watcher watches out for.
2781 .PP
2782 \fIExamples\fR
2783 .IX Subsection "Examples"
2784 .PP
2785 Example: Try to exit cleanly on \s-1SIGINT.\s0
2786 .PP
2787 .Vb 5
2788 \& static void
2789 \& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2790 \& {
2791 \& ev_break (loop, EVBREAK_ALL);
2792 \& }
2793 \&
2794 \& ev_signal signal_watcher;
2795 \& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2796 \& ev_signal_start (loop, &signal_watcher);
2797 .Ve
2798 .ie n .SS """ev_child"" \- watch out for process status changes"
2799 .el .SS "\f(CWev_child\fP \- watch out for process status changes"
2800 .IX Subsection "ev_child - watch out for process status changes"
2801 Child watchers trigger when your process receives a \s-1SIGCHLD\s0 in response to
2802 some child status changes (most typically when a child of yours dies or
2803 exits). It is permissible to install a child watcher \fIafter\fR the child
2804 has been forked (which implies it might have already exited), as long
2805 as the event loop isn't entered (or is continued from a watcher), i.e.,
2806 forking and then immediately registering a watcher for the child is fine,
2807 but forking and registering a watcher a few event loop iterations later or
2808 in the next callback invocation is not.
2809 .PP
2810 Only the default event loop is capable of handling signals, and therefore
2811 you can only register child watchers in the default event loop.
2812 .PP
2813 Due to some design glitches inside libev, child watchers will always be
2814 handled at maximum priority (their priority is set to \f(CW\*(C`EV_MAXPRI\*(C'\fR by
2815 libev)
2816 .PP
2817 \fIProcess Interaction\fR
2818 .IX Subsection "Process Interaction"
2819 .PP
2820 Libev grabs \f(CW\*(C`SIGCHLD\*(C'\fR as soon as the default event loop is
2821 initialised. This is necessary to guarantee proper behaviour even if the
2822 first child watcher is started after the child exits. The occurrence
2823 of \f(CW\*(C`SIGCHLD\*(C'\fR is recorded asynchronously, but child reaping is done
2824 synchronously as part of the event loop processing. Libev always reaps all
2825 children, even ones not watched.
2826 .PP
2827 \fIOverriding the Built-In Processing\fR
2828 .IX Subsection "Overriding the Built-In Processing"
2829 .PP
2830 Libev offers no special support for overriding the built-in child
2831 processing, but if your application collides with libev's default child
2832 handler, you can override it easily by installing your own handler for
2833 \&\f(CW\*(C`SIGCHLD\*(C'\fR after initialising the default loop, and making sure the
2834 default loop never gets destroyed. You are encouraged, however, to use an
2835 event-based approach to child reaping and thus use libev's support for
2836 that, so other libev users can use \f(CW\*(C`ev_child\*(C'\fR watchers freely.
2837 .PP
2838 \fIStopping the Child Watcher\fR
2839 .IX Subsection "Stopping the Child Watcher"
2840 .PP
2841 Currently, the child watcher never gets stopped, even when the
2842 child terminates, so normally one needs to stop the watcher in the
2843 callback. Future versions of libev might stop the watcher automatically
2844 when a child exit is detected (calling \f(CW\*(C`ev_child_stop\*(C'\fR twice is not a
2845 problem).
2846 .PP
2847 \fIWatcher-Specific Functions and Data Members\fR
2848 .IX Subsection "Watcher-Specific Functions and Data Members"
2849 .IP "ev_child_init (ev_child *, callback, int pid, int trace)" 4
2850 .IX Item "ev_child_init (ev_child *, callback, int pid, int trace)"
2851 .PD 0
2852 .IP "ev_child_set (ev_child *, int pid, int trace)" 4
2853 .IX Item "ev_child_set (ev_child *, int pid, int trace)"
2854 .PD
2855 Configures the watcher to wait for status changes of process \f(CW\*(C`pid\*(C'\fR (or
2856 \&\fIany\fR process if \f(CW\*(C`pid\*(C'\fR is specified as \f(CW0\fR). The callback can look
2857 at the \f(CW\*(C`rstatus\*(C'\fR member of the \f(CW\*(C`ev_child\*(C'\fR watcher structure to see
2858 the status word (use the macros from \f(CW\*(C`sys/wait.h\*(C'\fR and see your systems
2859 \&\f(CW\*(C`waitpid\*(C'\fR documentation). The \f(CW\*(C`rpid\*(C'\fR member contains the pid of the
2860 process causing the status change. \f(CW\*(C`trace\*(C'\fR must be either \f(CW0\fR (only
2861 activate the watcher when the process terminates) or \f(CW1\fR (additionally
2862 activate the watcher when the process is stopped or continued).
2863 .IP "int pid [read\-only]" 4
2864 .IX Item "int pid [read-only]"
2865 The process id this watcher watches out for, or \f(CW0\fR, meaning any process id.
2866 .IP "int rpid [read\-write]" 4
2867 .IX Item "int rpid [read-write]"
2868 The process id that detected a status change.
2869 .IP "int rstatus [read\-write]" 4
2870 .IX Item "int rstatus [read-write]"
2871 The process exit/trace status caused by \f(CW\*(C`rpid\*(C'\fR (see your systems
2872 \&\f(CW\*(C`waitpid\*(C'\fR and \f(CW\*(C`sys/wait.h\*(C'\fR documentation for details).
2873 .PP
2874 \fIExamples\fR
2875 .IX Subsection "Examples"
2876 .PP
2877 Example: \f(CW\*(C`fork()\*(C'\fR a new process and install a child handler to wait for
2878 its completion.
2879 .PP
2880 .Vb 1
2881 \& ev_child cw;
2882 \&
2883 \& static void
2884 \& child_cb (EV_P_ ev_child *w, int revents)
2885 \& {
2886 \& ev_child_stop (EV_A_ w);
2887 \& printf ("process %d exited with status %x\en", w\->rpid, w\->rstatus);
2888 \& }
2889 \&
2890 \& pid_t pid = fork ();
2891 \&
2892 \& if (pid < 0)
2893 \& // error
2894 \& else if (pid == 0)
2895 \& {
2896 \& // the forked child executes here
2897 \& exit (1);
2898 \& }
2899 \& else
2900 \& {
2901 \& ev_child_init (&cw, child_cb, pid, 0);
2902 \& ev_child_start (EV_DEFAULT_ &cw);
2903 \& }
2904 .Ve
2905 .ie n .SS """ev_stat"" \- did the file attributes just change?"
2906 .el .SS "\f(CWev_stat\fP \- did the file attributes just change?"
2907 .IX Subsection "ev_stat - did the file attributes just change?"
2908 This watches a file system path for attribute changes. That is, it calls
2909 \&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
2910 and sees if it changed compared to the last time, invoking the callback
2911 if it did. Starting the watcher \f(CW\*(C`stat\*(C'\fR's the file, so only changes that
2912 happen after the watcher has been started will be reported.
2913 .PP
2914 The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
2915 not exist\*(R" is a status change like any other. The condition \*(L"path does not
2916 exist\*(R" (or more correctly \*(L"path cannot be stat'ed\*(R") is signified by the
2917 \&\f(CW\*(C`st_nlink\*(C'\fR field being zero (which is otherwise always forced to be at
2918 least one) and all the other fields of the stat buffer having unspecified
2919 contents.
2920 .PP
2921 The path \fImust not\fR end in a slash or contain special components such as
2922 \&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
2923 your working directory changes, then the behaviour is undefined.
2924 .PP
2925 Since there is no portable change notification interface available, the
2926 portable implementation simply calls \f(CWstat(2)\fR regularly on the path
2927 to see if it changed somehow. You can specify a recommended polling
2928 interval for this case. If you specify a polling interval of \f(CW0\fR (highly
2929 recommended!) then a \fIsuitable, unspecified default\fR value will be used
2930 (which you can expect to be around five seconds, although this might
2931 change dynamically). Libev will also impose a minimum interval which is
2932 currently around \f(CW0.1\fR, but that's usually overkill.
2933 .PP
2934 This watcher type is not meant for massive numbers of stat watchers,
2935 as even with OS-supported change notifications, this can be
2936 resource-intensive.
2937 .PP
2938 At the time of this writing, the only OS-specific interface implemented
2939 is the Linux inotify interface (implementing kqueue support is left as an
2940 exercise for the reader. Note, however, that the author sees no way of
2941 implementing \f(CW\*(C`ev_stat\*(C'\fR semantics with kqueue, except as a hint).
2942 .PP
2943 \fI\s-1ABI\s0 Issues (Largefile Support)\fR
2944 .IX Subsection "ABI Issues (Largefile Support)"
2945 .PP
2946 Libev by default (unless the user overrides this) uses the default
2947 compilation environment, which means that on systems with large file
2948 support disabled by default, you get the 32 bit version of the stat
2949 structure. When using the library from programs that change the \s-1ABI\s0 to
2950 use 64 bit file offsets the programs will fail. In that case you have to
2951 compile libev with the same flags to get binary compatibility. This is
2952 obviously the case with any flags that change the \s-1ABI,\s0 but the problem is
2953 most noticeably displayed with ev_stat and large file support.
2954 .PP
2955 The solution for this is to lobby your distribution maker to make large
2956 file interfaces available by default (as e.g. FreeBSD does) and not
2957 optional. Libev cannot simply switch on large file support because it has
2958 to exchange stat structures with application programs compiled using the
2959 default compilation environment.
2960 .PP
2961 \fIInotify and Kqueue\fR
2962 .IX Subsection "Inotify and Kqueue"
2963 .PP
2964 When \f(CW\*(C`inotify (7)\*(C'\fR support has been compiled into libev and present at
2965 runtime, it will be used to speed up change detection where possible. The
2966 inotify descriptor will be created lazily when the first \f(CW\*(C`ev_stat\*(C'\fR
2967 watcher is being started.
2968 .PP
2969 Inotify presence does not change the semantics of \f(CW\*(C`ev_stat\*(C'\fR watchers
2970 except that changes might be detected earlier, and in some cases, to avoid
2971 making regular \f(CW\*(C`stat\*(C'\fR calls. Even in the presence of inotify support
2972 there are many cases where libev has to resort to regular \f(CW\*(C`stat\*(C'\fR polling,
2973 but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
2974 many bugs), the path exists (i.e. stat succeeds), and the path resides on
2975 a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
2976 xfs are fully working) libev usually gets away without polling.
2977 .PP
2978 There is no support for kqueue, as apparently it cannot be used to
2979 implement this functionality, due to the requirement of having a file
2980 descriptor open on the object at all times, and detecting renames, unlinks
2981 etc. is difficult.
2982 .PP
2983 \fI\f(CI\*(C`stat ()\*(C'\fI is a synchronous operation\fR
2984 .IX Subsection "stat () is a synchronous operation"
2985 .PP
2986 Libev doesn't normally do any kind of I/O itself, and so is not blocking
2987 the process. The exception are \f(CW\*(C`ev_stat\*(C'\fR watchers \- those call \f(CW\*(C`stat
2988 ()\*(C'\fR, which is a synchronous operation.
2989 .PP
2990 For local paths, this usually doesn't matter: unless the system is very
2991 busy or the intervals between stat's are large, a stat call will be fast,
2992 as the path data is usually in memory already (except when starting the
2993 watcher).
2994 .PP
2995 For networked file systems, calling \f(CW\*(C`stat ()\*(C'\fR can block an indefinite
2996 time due to network issues, and even under good conditions, a stat call
2997 often takes multiple milliseconds.
2998 .PP
2999 Therefore, it is best to avoid using \f(CW\*(C`ev_stat\*(C'\fR watchers on networked
3000 paths, although this is fully supported by libev.
3001 .PP
3002 \fIThe special problem of stat time resolution\fR
3003 .IX Subsection "The special problem of stat time resolution"
3004 .PP
3005 The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
3006 and even on systems where the resolution is higher, most file systems
3007 still only support whole seconds.
3008 .PP
3009 That means that, if the time is the only thing that changes, you can
3010 easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
3011 calls your callback, which does something. When there is another update
3012 within the same second, \f(CW\*(C`ev_stat\*(C'\fR will be unable to detect unless the
3013 stat data does change in other ways (e.g. file size).
3014 .PP
3015 The solution to this is to delay acting on a change for slightly more
3016 than a second (or till slightly after the next full second boundary), using
3017 a roughly one-second-delay \f(CW\*(C`ev_timer\*(C'\fR (e.g. \f(CW\*(C`ev_timer_set (w, 0., 1.02);
3018 ev_timer_again (loop, w)\*(C'\fR).
3019 .PP
3020 The \f(CW.02\fR offset is added to work around small timing inconsistencies
3021 of some operating systems (where the second counter of the current time
3022 might be be delayed. One such system is the Linux kernel, where a call to
3023 \&\f(CW\*(C`gettimeofday\*(C'\fR might return a timestamp with a full second later than
3024 a subsequent \f(CW\*(C`time\*(C'\fR call \- if the equivalent of \f(CW\*(C`time ()\*(C'\fR is used to
3025 update file times then there will be a small window where the kernel uses
3026 the previous second to update file times but libev might already execute
3027 the timer callback).
3028 .PP
3029 \fIWatcher-Specific Functions and Data Members\fR
3030 .IX Subsection "Watcher-Specific Functions and Data Members"
3031 .IP "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)" 4
3032 .IX Item "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)"
3033 .PD 0
3034 .IP "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)" 4
3035 .IX Item "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)"
3036 .PD
3037 Configures the watcher to wait for status changes of the given
3038 \&\f(CW\*(C`path\*(C'\fR. The \f(CW\*(C`interval\*(C'\fR is a hint on how quickly a change is expected to
3039 be detected and should normally be specified as \f(CW0\fR to let libev choose
3040 a suitable value. The memory pointed to by \f(CW\*(C`path\*(C'\fR must point to the same
3041 path for as long as the watcher is active.
3042 .Sp
3043 The callback will receive an \f(CW\*(C`EV_STAT\*(C'\fR event when a change was detected,
3044 relative to the attributes at the time the watcher was started (or the
3045 last change was detected).
3046 .IP "ev_stat_stat (loop, ev_stat *)" 4
3047 .IX Item "ev_stat_stat (loop, ev_stat *)"
3048 Updates the stat buffer immediately with new values. If you change the
3049 watched path in your callback, you could call this function to avoid
3050 detecting this change (while introducing a race condition if you are not
3051 the only one changing the path). Can also be useful simply to find out the
3052 new values.
3053 .IP "ev_statdata attr [read\-only]" 4
3054 .IX Item "ev_statdata attr [read-only]"
3055 The most-recently detected attributes of the file. Although the type is
3056 \&\f(CW\*(C`ev_statdata\*(C'\fR, this is usually the (or one of the) \f(CW\*(C`struct stat\*(C'\fR types
3057 suitable for your system, but you can only rely on the POSIX-standardised
3058 members to be present. If the \f(CW\*(C`st_nlink\*(C'\fR member is \f(CW0\fR, then there was
3059 some error while \f(CW\*(C`stat\*(C'\fRing the file.
3060 .IP "ev_statdata prev [read\-only]" 4
3061 .IX Item "ev_statdata prev [read-only]"
3062 The previous attributes of the file. The callback gets invoked whenever
3063 \&\f(CW\*(C`prev\*(C'\fR != \f(CW\*(C`attr\*(C'\fR, or, more precisely, one or more of these members
3064 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,
3065 \&\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.
3066 .IP "ev_tstamp interval [read\-only]" 4
3067 .IX Item "ev_tstamp interval [read-only]"
3068 The specified interval.
3069 .IP "const char *path [read\-only]" 4
3070 .IX Item "const char *path [read-only]"
3071 The file system path that is being watched.
3072 .PP
3073 \fIExamples\fR
3074 .IX Subsection "Examples"
3075 .PP
3076 Example: Watch \f(CW\*(C`/etc/passwd\*(C'\fR for attribute changes.
3077 .PP
3078 .Vb 10
3079 \& static void
3080 \& passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
3081 \& {
3082 \& /* /etc/passwd changed in some way */
3083 \& if (w\->attr.st_nlink)
3084 \& {
3085 \& printf ("passwd current size %ld\en", (long)w\->attr.st_size);
3086 \& printf ("passwd current atime %ld\en", (long)w\->attr.st_mtime);
3087 \& printf ("passwd current mtime %ld\en", (long)w\->attr.st_mtime);
3088 \& }
3089 \& else
3090 \& /* you shalt not abuse printf for puts */
3091 \& puts ("wow, /etc/passwd is not there, expect problems. "
3092 \& "if this is windows, they already arrived\en");
3093 \& }
3094 \&
3095 \& ...
3096 \& ev_stat passwd;
3097 \&
3098 \& ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
3099 \& ev_stat_start (loop, &passwd);
3100 .Ve
3101 .PP
3102 Example: Like above, but additionally use a one-second delay so we do not
3103 miss updates (however, frequent updates will delay processing, too, so
3104 one might do the work both on \f(CW\*(C`ev_stat\*(C'\fR callback invocation \fIand\fR on
3105 \&\f(CW\*(C`ev_timer\*(C'\fR callback invocation).
3106 .PP
3107 .Vb 2
3108 \& static ev_stat passwd;
3109 \& static ev_timer timer;
3110 \&
3111 \& static void
3112 \& timer_cb (EV_P_ ev_timer *w, int revents)
3113 \& {
3114 \& ev_timer_stop (EV_A_ w);
3115 \&
3116 \& /* now it\*(Aqs one second after the most recent passwd change */
3117 \& }
3118 \&
3119 \& static void
3120 \& stat_cb (EV_P_ ev_stat *w, int revents)
3121 \& {
3122 \& /* reset the one\-second timer */
3123 \& ev_timer_again (EV_A_ &timer);
3124 \& }
3125 \&
3126 \& ...
3127 \& ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
3128 \& ev_stat_start (loop, &passwd);
3129 \& ev_timer_init (&timer, timer_cb, 0., 1.02);
3130 .Ve
3131 .ie n .SS """ev_idle"" \- when you've got nothing better to do..."
3132 .el .SS "\f(CWev_idle\fP \- when you've got nothing better to do..."
3133 .IX Subsection "ev_idle - when you've got nothing better to do..."
3134 Idle watchers trigger events when no other events of the same or higher
3135 priority are pending (prepare, check and other idle watchers do not count
3136 as receiving \*(L"events\*(R").
3137 .PP
3138 That is, as long as your process is busy handling sockets or timeouts
3139 (or even signals, imagine) of the same or higher priority it will not be
3140 triggered. But when your process is idle (or only lower-priority watchers
3141 are pending), the idle watchers are being called once per event loop
3142 iteration \- until stopped, that is, or your process receives more events
3143 and becomes busy again with higher priority stuff.
3144 .PP
3145 The most noteworthy effect is that as long as any idle watchers are
3146 active, the process will not block when waiting for new events.
3147 .PP
3148 Apart from keeping your process non-blocking (which is a useful
3149 effect on its own sometimes), idle watchers are a good place to do
3150 \&\*(L"pseudo-background processing\*(R", or delay processing stuff to after the
3151 event loop has handled all outstanding events.
3152 .PP
3153 \fIAbusing an \f(CI\*(C`ev_idle\*(C'\fI watcher for its side-effect\fR
3154 .IX Subsection "Abusing an ev_idle watcher for its side-effect"
3155 .PP
3156 As long as there is at least one active idle watcher, libev will never
3157 sleep unnecessarily. Or in other words, it will loop as fast as possible.
3158 For this to work, the idle watcher doesn't need to be invoked at all \- the
3159 lowest priority will do.
3160 .PP
3161 This mode of operation can be useful together with an \f(CW\*(C`ev_check\*(C'\fR watcher,
3162 to do something on each event loop iteration \- for example to balance load
3163 between different connections.
3164 .PP
3165 See \*(L"Abusing an ev_check watcher for its side-effect\*(R" for a longer
3166 example.
3167 .PP
3168 \fIWatcher-Specific Functions and Data Members\fR
3169 .IX Subsection "Watcher-Specific Functions and Data Members"
3170 .IP "ev_idle_init (ev_idle *, callback)" 4
3171 .IX Item "ev_idle_init (ev_idle *, callback)"
3172 Initialises and configures the idle watcher \- it has no parameters of any
3173 kind. There is a \f(CW\*(C`ev_idle_set\*(C'\fR macro, but using it is utterly pointless,
3174 believe me.
3175 .PP
3176 \fIExamples\fR
3177 .IX Subsection "Examples"
3178 .PP
3179 Example: Dynamically allocate an \f(CW\*(C`ev_idle\*(C'\fR watcher, start it, and in the
3180 callback, free it. Also, use no error checking, as usual.
3181 .PP
3182 .Vb 5
3183 \& static void
3184 \& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
3185 \& {
3186 \& // stop the watcher
3187 \& ev_idle_stop (loop, w);
3188 \&
3189 \& // now we can free it
3190 \& free (w);
3191 \&
3192 \& // now do something you wanted to do when the program has
3193 \& // no longer anything immediate to do.
3194 \& }
3195 \&
3196 \& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
3197 \& ev_idle_init (idle_watcher, idle_cb);
3198 \& ev_idle_start (loop, idle_watcher);
3199 .Ve
3200 .ie n .SS """ev_prepare"" and ""ev_check"" \- customise your event loop!"
3201 .el .SS "\f(CWev_prepare\fP and \f(CWev_check\fP \- customise your event loop!"
3202 .IX Subsection "ev_prepare and ev_check - customise your event loop!"
3203 Prepare and check watchers are often (but not always) used in pairs:
3204 prepare watchers get invoked before the process blocks and check watchers
3205 afterwards.
3206 .PP
3207 You \fImust not\fR call \f(CW\*(C`ev_run\*(C'\fR (or similar functions that enter the
3208 current event loop) or \f(CW\*(C`ev_loop_fork\*(C'\fR from either \f(CW\*(C`ev_prepare\*(C'\fR or
3209 \&\f(CW\*(C`ev_check\*(C'\fR watchers. Other loops than the current one are fine,
3210 however. The rationale behind this is that you do not need to check
3211 for recursion in those watchers, i.e. the sequence will always be
3212 \&\f(CW\*(C`ev_prepare\*(C'\fR, blocking, \f(CW\*(C`ev_check\*(C'\fR so if you have one watcher of each
3213 kind they will always be called in pairs bracketing the blocking call.
3214 .PP
3215 Their main purpose is to integrate other event mechanisms into libev and
3216 their use is somewhat advanced. They could be used, for example, to track
3217 variable changes, implement your own watchers, integrate net-snmp or a
3218 coroutine library and lots more. They are also occasionally useful if
3219 you cache some data and want to flush it before blocking (for example,
3220 in X programs you might want to do an \f(CW\*(C`XFlush ()\*(C'\fR in an \f(CW\*(C`ev_prepare\*(C'\fR
3221 watcher).
3222 .PP
3223 This is done by examining in each prepare call which file descriptors
3224 need to be watched by the other library, registering \f(CW\*(C`ev_io\*(C'\fR watchers
3225 for them and starting an \f(CW\*(C`ev_timer\*(C'\fR watcher for any timeouts (many
3226 libraries provide exactly this functionality). Then, in the check watcher,
3227 you check for any events that occurred (by checking the pending status
3228 of all watchers and stopping them) and call back into the library. The
3229 I/O and timer callbacks will never actually be called (but must be valid
3230 nevertheless, because you never know, you know?).
3231 .PP
3232 As another example, the Perl Coro module uses these hooks to integrate
3233 coroutines into libev programs, by yielding to other active coroutines
3234 during each prepare and only letting the process block if no coroutines
3235 are ready to run (it's actually more complicated: it only runs coroutines
3236 with priority higher than or equal to the event loop and one coroutine
3237 of lower priority, but only once, using idle watchers to keep the event
3238 loop from blocking if lower-priority coroutines are active, thus mapping
3239 low-priority coroutines to idle/background tasks).
3240 .PP
3241 When used for this purpose, it is recommended to give \f(CW\*(C`ev_check\*(C'\fR watchers
3242 highest (\f(CW\*(C`EV_MAXPRI\*(C'\fR) priority, to ensure that they are being run before
3243 any other watchers after the poll (this doesn't matter for \f(CW\*(C`ev_prepare\*(C'\fR
3244 watchers).
3245 .PP
3246 Also, \f(CW\*(C`ev_check\*(C'\fR watchers (and \f(CW\*(C`ev_prepare\*(C'\fR watchers, too) should not
3247 activate (\*(L"feed\*(R") events into libev. While libev fully supports this, they
3248 might get executed before other \f(CW\*(C`ev_check\*(C'\fR watchers did their job. As
3249 \&\f(CW\*(C`ev_check\*(C'\fR watchers are often used to embed other (non-libev) event
3250 loops those other event loops might be in an unusable state until their
3251 \&\f(CW\*(C`ev_check\*(C'\fR watcher ran (always remind yourself to coexist peacefully with
3252 others).
3253 .PP
3254 \fIAbusing an \f(CI\*(C`ev_check\*(C'\fI watcher for its side-effect\fR
3255 .IX Subsection "Abusing an ev_check watcher for its side-effect"
3256 .PP
3257 \&\f(CW\*(C`ev_check\*(C'\fR (and less often also \f(CW\*(C`ev_prepare\*(C'\fR) watchers can also be
3258 useful because they are called once per event loop iteration. For
3259 example, if you want to handle a large number of connections fairly, you
3260 normally only do a bit of work for each active connection, and if there
3261 is more work to do, you wait for the next event loop iteration, so other
3262 connections have a chance of making progress.
3263 .PP
3264 Using an \f(CW\*(C`ev_check\*(C'\fR watcher is almost enough: it will be called on the
3265 next event loop iteration. However, that isn't as soon as possible \-
3266 without external events, your \f(CW\*(C`ev_check\*(C'\fR watcher will not be invoked.
3267 .PP
3268 This is where \f(CW\*(C`ev_idle\*(C'\fR watchers come in handy \- all you need is a
3269 single global idle watcher that is active as long as you have one active
3270 \&\f(CW\*(C`ev_check\*(C'\fR watcher. The \f(CW\*(C`ev_idle\*(C'\fR watcher makes sure the event loop
3271 will not sleep, and the \f(CW\*(C`ev_check\*(C'\fR watcher makes sure a callback gets
3272 invoked. Neither watcher alone can do that.
3273 .PP
3274 \fIWatcher-Specific Functions and Data Members\fR
3275 .IX Subsection "Watcher-Specific Functions and Data Members"
3276 .IP "ev_prepare_init (ev_prepare *, callback)" 4
3277 .IX Item "ev_prepare_init (ev_prepare *, callback)"
3278 .PD 0
3279 .IP "ev_check_init (ev_check *, callback)" 4
3280 .IX Item "ev_check_init (ev_check *, callback)"
3281 .PD
3282 Initialises and configures the prepare or check watcher \- they have no
3283 parameters of any kind. There are \f(CW\*(C`ev_prepare_set\*(C'\fR and \f(CW\*(C`ev_check_set\*(C'\fR
3284 macros, but using them is utterly, utterly, utterly and completely
3285 pointless.
3286 .PP
3287 \fIExamples\fR
3288 .IX Subsection "Examples"
3289 .PP
3290 There are a number of principal ways to embed other event loops or modules
3291 into libev. Here are some ideas on how to include libadns into libev
3292 (there is a Perl module named \f(CW\*(C`EV::ADNS\*(C'\fR that does this, which you could
3293 use as a working example. Another Perl module named \f(CW\*(C`EV::Glib\*(C'\fR embeds a
3294 Glib main context into libev, and finally, \f(CW\*(C`Glib::EV\*(C'\fR embeds \s-1EV\s0 into the
3295 Glib event loop).
3296 .PP
3297 Method 1: Add \s-1IO\s0 watchers and a timeout watcher in a prepare handler,
3298 and in a check watcher, destroy them and call into libadns. What follows
3299 is pseudo-code only of course. This requires you to either use a low
3300 priority for the check watcher or use \f(CW\*(C`ev_clear_pending\*(C'\fR explicitly, as
3301 the callbacks for the IO/timeout watchers might not have been called yet.
3302 .PP
3303 .Vb 2
3304 \& static ev_io iow [nfd];
3305 \& static ev_timer tw;
3306 \&
3307 \& static void
3308 \& io_cb (struct ev_loop *loop, ev_io *w, int revents)
3309 \& {
3310 \& }
3311 \&
3312 \& // create io watchers for each fd and a timer before blocking
3313 \& static void
3314 \& adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3315 \& {
3316 \& int timeout = 3600000;
3317 \& struct pollfd fds [nfd];
3318 \& // actual code will need to loop here and realloc etc.
3319 \& adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3320 \&
3321 \& /* the callback is illegal, but won\*(Aqt be called as we stop during check */
3322 \& ev_timer_init (&tw, 0, timeout * 1e\-3, 0.);
3323 \& ev_timer_start (loop, &tw);
3324 \&
3325 \& // create one ev_io per pollfd
3326 \& for (int i = 0; i < nfd; ++i)
3327 \& {
3328 \& ev_io_init (iow + i, io_cb, fds [i].fd,
3329 \& ((fds [i].events & POLLIN ? EV_READ : 0)
3330 \& | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3331 \&
3332 \& fds [i].revents = 0;
3333 \& ev_io_start (loop, iow + i);
3334 \& }
3335 \& }
3336 \&
3337 \& // stop all watchers after blocking
3338 \& static void
3339 \& adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3340 \& {
3341 \& ev_timer_stop (loop, &tw);
3342 \&
3343 \& for (int i = 0; i < nfd; ++i)
3344 \& {
3345 \& // set the relevant poll flags
3346 \& // could also call adns_processreadable etc. here
3347 \& struct pollfd *fd = fds + i;
3348 \& int revents = ev_clear_pending (iow + i);
3349 \& if (revents & EV_READ ) fd\->revents |= fd\->events & POLLIN;
3350 \& if (revents & EV_WRITE) fd\->revents |= fd\->events & POLLOUT;
3351 \&
3352 \& // now stop the watcher
3353 \& ev_io_stop (loop, iow + i);
3354 \& }
3355 \&
3356 \& adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3357 \& }
3358 .Ve
3359 .PP
3360 Method 2: This would be just like method 1, but you run \f(CW\*(C`adns_afterpoll\*(C'\fR
3361 in the prepare watcher and would dispose of the check watcher.
3362 .PP
3363 Method 3: If the module to be embedded supports explicit event
3364 notification (libadns does), you can also make use of the actual watcher
3365 callbacks, and only destroy/create the watchers in the prepare watcher.
3366 .PP
3367 .Vb 5
3368 \& static void
3369 \& timer_cb (EV_P_ ev_timer *w, int revents)
3370 \& {
3371 \& adns_state ads = (adns_state)w\->data;
3372 \& update_now (EV_A);
3373 \&
3374 \& adns_processtimeouts (ads, &tv_now);
3375 \& }
3376 \&
3377 \& static void
3378 \& io_cb (EV_P_ ev_io *w, int revents)
3379 \& {
3380 \& adns_state ads = (adns_state)w\->data;
3381 \& update_now (EV_A);
3382 \&
3383 \& if (revents & EV_READ ) adns_processreadable (ads, w\->fd, &tv_now);
3384 \& if (revents & EV_WRITE) adns_processwriteable (ads, w\->fd, &tv_now);
3385 \& }
3386 \&
3387 \& // do not ever call adns_afterpoll
3388 .Ve
3389 .PP
3390 Method 4: Do not use a prepare or check watcher because the module you
3391 want to embed is not flexible enough to support it. Instead, you can
3392 override their poll function. The drawback with this solution is that the
3393 main loop is now no longer controllable by \s-1EV.\s0 The \f(CW\*(C`Glib::EV\*(C'\fR module uses
3394 this approach, effectively embedding \s-1EV\s0 as a client into the horrible
3395 libglib event loop.
3396 .PP
3397 .Vb 4
3398 \& static gint
3399 \& event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3400 \& {
3401 \& int got_events = 0;
3402 \&
3403 \& for (n = 0; n < nfds; ++n)
3404 \& // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3405 \&
3406 \& if (timeout >= 0)
3407 \& // create/start timer
3408 \&
3409 \& // poll
3410 \& ev_run (EV_A_ 0);
3411 \&
3412 \& // stop timer again
3413 \& if (timeout >= 0)
3414 \& ev_timer_stop (EV_A_ &to);
3415 \&
3416 \& // stop io watchers again \- their callbacks should have set
3417 \& for (n = 0; n < nfds; ++n)
3418 \& ev_io_stop (EV_A_ iow [n]);
3419 \&
3420 \& return got_events;
3421 \& }
3422 .Ve
3423 .ie n .SS """ev_embed"" \- when one backend isn't enough..."
3424 .el .SS "\f(CWev_embed\fP \- when one backend isn't enough..."
3425 .IX Subsection "ev_embed - when one backend isn't enough..."
3426 This is a rather advanced watcher type that lets you embed one event loop
3427 into another (currently only \f(CW\*(C`ev_io\*(C'\fR events are supported in the embedded
3428 loop, other types of watchers might be handled in a delayed or incorrect
3429 fashion and must not be used).
3430 .PP
3431 There are primarily two reasons you would want that: work around bugs and
3432 prioritise I/O.
3433 .PP
3434 As an example for a bug workaround, the kqueue backend might only support
3435 sockets on some platform, so it is unusable as generic backend, but you
3436 still want to make use of it because you have many sockets and it scales
3437 so nicely. In this case, you would create a kqueue-based loop and embed
3438 it into your default loop (which might use e.g. poll). Overall operation
3439 will be a bit slower because first libev has to call \f(CW\*(C`poll\*(C'\fR and then
3440 \&\f(CW\*(C`kevent\*(C'\fR, but at least you can use both mechanisms for what they are
3441 best: \f(CW\*(C`kqueue\*(C'\fR for scalable sockets and \f(CW\*(C`poll\*(C'\fR if you want it to work :)
3442 .PP
3443 As for prioritising I/O: under rare circumstances you have the case where
3444 some fds have to be watched and handled very quickly (with low latency),
3445 and even priorities and idle watchers might have too much overhead. In
3446 this case you would put all the high priority stuff in one loop and all
3447 the rest in a second one, and embed the second one in the first.
3448 .PP
3449 As long as the watcher is active, the callback will be invoked every
3450 time there might be events pending in the embedded loop. The callback
3451 must then call \f(CW\*(C`ev_embed_sweep (mainloop, watcher)\*(C'\fR to make a single
3452 sweep and invoke their callbacks (the callback doesn't need to invoke the
3453 \&\f(CW\*(C`ev_embed_sweep\*(C'\fR function directly, it could also start an idle watcher
3454 to give the embedded loop strictly lower priority for example).
3455 .PP
3456 You can also set the callback to \f(CW0\fR, in which case the embed watcher
3457 will automatically execute the embedded loop sweep whenever necessary.
3458 .PP
3459 Fork detection will be handled transparently while the \f(CW\*(C`ev_embed\*(C'\fR watcher
3460 is active, i.e., the embedded loop will automatically be forked when the
3461 embedding loop forks. In other cases, the user is responsible for calling
3462 \&\f(CW\*(C`ev_loop_fork\*(C'\fR on the embedded loop.
3463 .PP
3464 Unfortunately, not all backends are embeddable: only the ones returned by
3465 \&\f(CW\*(C`ev_embeddable_backends\*(C'\fR are, which, unfortunately, does not include any
3466 portable one.
3467 .PP
3468 So when you want to use this feature you will always have to be prepared
3469 that you cannot get an embeddable loop. The recommended way to get around
3470 this is to have a separate variables for your embeddable loop, try to
3471 create it, and if that fails, use the normal loop for everything.
3472 .PP
3473 \fI\f(CI\*(C`ev_embed\*(C'\fI and fork\fR
3474 .IX Subsection "ev_embed and fork"
3475 .PP
3476 While the \f(CW\*(C`ev_embed\*(C'\fR watcher is running, forks in the embedding loop will
3477 automatically be applied to the embedded loop as well, so no special
3478 fork handling is required in that case. When the watcher is not running,
3479 however, it is still the task of the libev user to call \f(CW\*(C`ev_loop_fork ()\*(C'\fR
3480 as applicable.
3481 .PP
3482 \fIWatcher-Specific Functions and Data Members\fR
3483 .IX Subsection "Watcher-Specific Functions and Data Members"
3484 .IP "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)" 4
3485 .IX Item "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)"
3486 .PD 0
3487 .IP "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)" 4
3488 .IX Item "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)"
3489 .PD
3490 Configures the watcher to embed the given loop, which must be
3491 embeddable. If the callback is \f(CW0\fR, then \f(CW\*(C`ev_embed_sweep\*(C'\fR will be
3492 invoked automatically, otherwise it is the responsibility of the callback
3493 to invoke it (it will continue to be called until the sweep has been done,
3494 if you do not want that, you need to temporarily stop the embed watcher).
3495 .IP "ev_embed_sweep (loop, ev_embed *)" 4
3496 .IX Item "ev_embed_sweep (loop, ev_embed *)"
3497 Make a single, non-blocking sweep over the embedded loop. This works
3498 similarly to \f(CW\*(C`ev_run (embedded_loop, EVRUN_NOWAIT)\*(C'\fR, but in the most
3499 appropriate way for embedded loops.
3500 .IP "struct ev_loop *other [read\-only]" 4
3501 .IX Item "struct ev_loop *other [read-only]"
3502 The embedded event loop.
3503 .PP
3504 \fIExamples\fR
3505 .IX Subsection "Examples"
3506 .PP
3507 Example: Try to get an embeddable event loop and embed it into the default
3508 event loop. If that is not possible, use the default loop. The default
3509 loop is stored in \f(CW\*(C`loop_hi\*(C'\fR, while the embeddable loop is stored in
3510 \&\f(CW\*(C`loop_lo\*(C'\fR (which is \f(CW\*(C`loop_hi\*(C'\fR in the case no embeddable loop can be
3511 used).
3512 .PP
3513 .Vb 3
3514 \& struct ev_loop *loop_hi = ev_default_init (0);
3515 \& struct ev_loop *loop_lo = 0;
3516 \& ev_embed embed;
3517 \&
3518 \& // see if there is a chance of getting one that works
3519 \& // (remember that a flags value of 0 means autodetection)
3520 \& loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3521 \& ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3522 \& : 0;
3523 \&
3524 \& // if we got one, then embed it, otherwise default to loop_hi
3525 \& if (loop_lo)
3526 \& {
3527 \& ev_embed_init (&embed, 0, loop_lo);
3528 \& ev_embed_start (loop_hi, &embed);
3529 \& }
3530 \& else
3531 \& loop_lo = loop_hi;
3532 .Ve
3533 .PP
3534 Example: Check if kqueue is available but not recommended and create
3535 a kqueue backend for use with sockets (which usually work with any
3536 kqueue implementation). Store the kqueue/socket\-only event loop in
3537 \&\f(CW\*(C`loop_socket\*(C'\fR. (One might optionally use \f(CW\*(C`EVFLAG_NOENV\*(C'\fR, too).
3538 .PP
3539 .Vb 3
3540 \& struct ev_loop *loop = ev_default_init (0);
3541 \& struct ev_loop *loop_socket = 0;
3542 \& ev_embed embed;
3543 \&
3544 \& if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3545 \& if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3546 \& {
3547 \& ev_embed_init (&embed, 0, loop_socket);
3548 \& ev_embed_start (loop, &embed);
3549 \& }
3550 \&
3551 \& if (!loop_socket)
3552 \& loop_socket = loop;
3553 \&
3554 \& // now use loop_socket for all sockets, and loop for everything else
3555 .Ve
3556 .ie n .SS """ev_fork"" \- the audacity to resume the event loop after a fork"
3557 .el .SS "\f(CWev_fork\fP \- the audacity to resume the event loop after a fork"
3558 .IX Subsection "ev_fork - the audacity to resume the event loop after a fork"
3559 Fork watchers are called when a \f(CW\*(C`fork ()\*(C'\fR was detected (usually because
3560 whoever is a good citizen cared to tell libev about it by calling
3561 \&\f(CW\*(C`ev_loop_fork\*(C'\fR). The invocation is done before the event loop blocks next
3562 and before \f(CW\*(C`ev_check\*(C'\fR watchers are being called, and only in the child
3563 after the fork. If whoever good citizen calling \f(CW\*(C`ev_default_fork\*(C'\fR cheats
3564 and calls it in the wrong process, the fork handlers will be invoked, too,
3565 of course.
3566 .PP
3567 \fIThe special problem of life after fork \- how is it possible?\fR
3568 .IX Subsection "The special problem of life after fork - how is it possible?"
3569 .PP
3570 Most uses of \f(CW\*(C`fork ()\*(C'\fR consist of forking, then some simple calls to set
3571 up/change the process environment, followed by a call to \f(CW\*(C`exec()\*(C'\fR. This
3572 sequence should be handled by libev without any problems.
3573 .PP
3574 This changes when the application actually wants to do event handling
3575 in the child, or both parent in child, in effect \*(L"continuing\*(R" after the
3576 fork.
3577 .PP
3578 The default mode of operation (for libev, with application help to detect
3579 forks) is to duplicate all the state in the child, as would be expected
3580 when \fIeither\fR the parent \fIor\fR the child process continues.
3581 .PP
3582 When both processes want to continue using libev, then this is usually the
3583 wrong result. In that case, usually one process (typically the parent) is
3584 supposed to continue with all watchers in place as before, while the other
3585 process typically wants to start fresh, i.e. without any active watchers.
3586 .PP
3587 The cleanest and most efficient way to achieve that with libev is to
3588 simply create a new event loop, which of course will be \*(L"empty\*(R", and
3589 use that for new watchers. This has the advantage of not touching more
3590 memory than necessary, and thus avoiding the copy-on-write, and the
3591 disadvantage of having to use multiple event loops (which do not support
3592 signal watchers).
3593 .PP
3594 When this is not possible, or you want to use the default loop for
3595 other reasons, then in the process that wants to start \*(L"fresh\*(R", call
3596 \&\f(CW\*(C`ev_loop_destroy (EV_DEFAULT)\*(C'\fR followed by \f(CW\*(C`ev_default_loop (...)\*(C'\fR.
3597 Destroying the default loop will \*(L"orphan\*(R" (not stop) all registered
3598 watchers, so you have to be careful not to execute code that modifies
3599 those watchers. Note also that in that case, you have to re-register any
3600 signal watchers.
3601 .PP
3602 \fIWatcher-Specific Functions and Data Members\fR
3603 .IX Subsection "Watcher-Specific Functions and Data Members"
3604 .IP "ev_fork_init (ev_fork *, callback)" 4
3605 .IX Item "ev_fork_init (ev_fork *, callback)"
3606 Initialises and configures the fork watcher \- it has no parameters of any
3607 kind. There is a \f(CW\*(C`ev_fork_set\*(C'\fR macro, but using it is utterly pointless,
3608 really.
3609 .ie n .SS """ev_cleanup"" \- even the best things end"
3610 .el .SS "\f(CWev_cleanup\fP \- even the best things end"
3611 .IX Subsection "ev_cleanup - even the best things end"
3612 Cleanup watchers are called just before the event loop is being destroyed
3613 by a call to \f(CW\*(C`ev_loop_destroy\*(C'\fR.
3614 .PP
3615 While there is no guarantee that the event loop gets destroyed, cleanup
3616 watchers provide a convenient method to install cleanup hooks for your
3617 program, worker threads and so on \- you just to make sure to destroy the
3618 loop when you want them to be invoked.
3619 .PP
3620 Cleanup watchers are invoked in the same way as any other watcher. Unlike
3621 all other watchers, they do not keep a reference to the event loop (which
3622 makes a lot of sense if you think about it). Like all other watchers, you
3623 can call libev functions in the callback, except \f(CW\*(C`ev_cleanup_start\*(C'\fR.
3624 .PP
3625 \fIWatcher-Specific Functions and Data Members\fR
3626 .IX Subsection "Watcher-Specific Functions and Data Members"
3627 .IP "ev_cleanup_init (ev_cleanup *, callback)" 4
3628 .IX Item "ev_cleanup_init (ev_cleanup *, callback)"
3629 Initialises and configures the cleanup watcher \- it has no parameters of
3630 any kind. There is a \f(CW\*(C`ev_cleanup_set\*(C'\fR macro, but using it is utterly
3631 pointless, I assure you.
3632 .PP
3633 Example: Register an atexit handler to destroy the default loop, so any
3634 cleanup functions are called.
3635 .PP
3636 .Vb 5
3637 \& static void
3638 \& program_exits (void)
3639 \& {
3640 \& ev_loop_destroy (EV_DEFAULT_UC);
3641 \& }
3642 \&
3643 \& ...
3644 \& atexit (program_exits);
3645 .Ve
3646 .ie n .SS """ev_async"" \- how to wake up an event loop"
3647 .el .SS "\f(CWev_async\fP \- how to wake up an event loop"
3648 .IX Subsection "ev_async - how to wake up an event loop"
3649 In general, you cannot use an \f(CW\*(C`ev_loop\*(C'\fR from multiple threads or other
3650 asynchronous sources such as signal handlers (as opposed to multiple event
3651 loops \- those are of course safe to use in different threads).
3652 .PP
3653 Sometimes, however, you need to wake up an event loop you do not control,
3654 for example because it belongs to another thread. This is what \f(CW\*(C`ev_async\*(C'\fR
3655 watchers do: as long as the \f(CW\*(C`ev_async\*(C'\fR watcher is active, you can signal
3656 it by calling \f(CW\*(C`ev_async_send\*(C'\fR, which is thread\- and signal safe.
3657 .PP
3658 This functionality is very similar to \f(CW\*(C`ev_signal\*(C'\fR watchers, as signals,
3659 too, are asynchronous in nature, and signals, too, will be compressed
3660 (i.e. the number of callback invocations may be less than the number of
3661 \&\f(CW\*(C`ev_async_send\*(C'\fR calls). In fact, you could use signal watchers as a kind
3662 of \*(L"global async watchers\*(R" by using a watcher on an otherwise unused
3663 signal, and \f(CW\*(C`ev_feed_signal\*(C'\fR to signal this watcher from another thread,
3664 even without knowing which loop owns the signal.
3665 .PP
3666 \fIQueueing\fR
3667 .IX Subsection "Queueing"
3668 .PP
3669 \&\f(CW\*(C`ev_async\*(C'\fR does not support queueing of data in any way. The reason
3670 is that the author does not know of a simple (or any) algorithm for a
3671 multiple-writer-single-reader queue that works in all cases and doesn't
3672 need elaborate support such as pthreads or unportable memory access
3673 semantics.
3674 .PP
3675 That means that if you want to queue data, you have to provide your own
3676 queue. But at least I can tell you how to implement locking around your
3677 queue:
3678 .IP "queueing from a signal handler context" 4
3679 .IX Item "queueing from a signal handler context"
3680 To implement race-free queueing, you simply add to the queue in the signal
3681 handler but you block the signal handler in the watcher callback. Here is
3682 an example that does that for some fictitious \s-1SIGUSR1\s0 handler:
3683 .Sp
3684 .Vb 1
3685 \& static ev_async mysig;
3686 \&
3687 \& static void
3688 \& sigusr1_handler (void)
3689 \& {
3690 \& sometype data;
3691 \&
3692 \& // no locking etc.
3693 \& queue_put (data);
3694 \& ev_async_send (EV_DEFAULT_ &mysig);
3695 \& }
3696 \&
3697 \& static void
3698 \& mysig_cb (EV_P_ ev_async *w, int revents)
3699 \& {
3700 \& sometype data;
3701 \& sigset_t block, prev;
3702 \&
3703 \& sigemptyset (&block);
3704 \& sigaddset (&block, SIGUSR1);
3705 \& sigprocmask (SIG_BLOCK, &block, &prev);
3706 \&
3707 \& while (queue_get (&data))
3708 \& process (data);
3709 \&
3710 \& if (sigismember (&prev, SIGUSR1)
3711 \& sigprocmask (SIG_UNBLOCK, &block, 0);
3712 \& }
3713 .Ve
3714 .Sp
3715 (Note: pthreads in theory requires you to use \f(CW\*(C`pthread_setmask\*(C'\fR
3716 instead of \f(CW\*(C`sigprocmask\*(C'\fR when you use threads, but libev doesn't do it
3717 either...).
3718 .IP "queueing from a thread context" 4
3719 .IX Item "queueing from a thread context"
3720 The strategy for threads is different, as you cannot (easily) block
3721 threads but you can easily preempt them, so to queue safely you need to
3722 employ a traditional mutex lock, such as in this pthread example:
3723 .Sp
3724 .Vb 2
3725 \& static ev_async mysig;
3726 \& static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3727 \&
3728 \& static void
3729 \& otherthread (void)
3730 \& {
3731 \& // only need to lock the actual queueing operation
3732 \& pthread_mutex_lock (&mymutex);
3733 \& queue_put (data);
3734 \& pthread_mutex_unlock (&mymutex);
3735 \&
3736 \& ev_async_send (EV_DEFAULT_ &mysig);
3737 \& }
3738 \&
3739 \& static void
3740 \& mysig_cb (EV_P_ ev_async *w, int revents)
3741 \& {
3742 \& pthread_mutex_lock (&mymutex);
3743 \&
3744 \& while (queue_get (&data))
3745 \& process (data);
3746 \&
3747 \& pthread_mutex_unlock (&mymutex);
3748 \& }
3749 .Ve
3750 .PP
3751 \fIWatcher-Specific Functions and Data Members\fR
3752 .IX Subsection "Watcher-Specific Functions and Data Members"
3753 .IP "ev_async_init (ev_async *, callback)" 4
3754 .IX Item "ev_async_init (ev_async *, callback)"
3755 Initialises and configures the async watcher \- it has no parameters of any
3756 kind. There is a \f(CW\*(C`ev_async_set\*(C'\fR macro, but using it is utterly pointless,
3757 trust me.
3758 .IP "ev_async_send (loop, ev_async *)" 4
3759 .IX Item "ev_async_send (loop, ev_async *)"
3760 Sends/signals/activates the given \f(CW\*(C`ev_async\*(C'\fR watcher, that is, feeds
3761 an \f(CW\*(C`EV_ASYNC\*(C'\fR event on the watcher into the event loop, and instantly
3762 returns.
3763 .Sp
3764 Unlike \f(CW\*(C`ev_feed_event\*(C'\fR, this call is safe to do from other threads,
3765 signal or similar contexts (see the discussion of \f(CW\*(C`EV_ATOMIC_T\*(C'\fR in the
3766 embedding section below on what exactly this means).
3767 .Sp
3768 Note that, as with other watchers in libev, multiple events might get
3769 compressed into a single callback invocation (another way to look at
3770 this is that \f(CW\*(C`ev_async\*(C'\fR watchers are level-triggered: they are set on
3771 \&\f(CW\*(C`ev_async_send\*(C'\fR, reset when the event loop detects that).
3772 .Sp
3773 This call incurs the overhead of at most one extra system call per event
3774 loop iteration, if the event loop is blocked, and no syscall at all if
3775 the event loop (or your program) is processing events. That means that
3776 repeated calls are basically free (there is no need to avoid calls for
3777 performance reasons) and that the overhead becomes smaller (typically
3778 zero) under load.
3779 .IP "bool = ev_async_pending (ev_async *)" 4
3780 .IX Item "bool = ev_async_pending (ev_async *)"
3781 Returns a non-zero value when \f(CW\*(C`ev_async_send\*(C'\fR has been called on the
3782 watcher but the event has not yet been processed (or even noted) by the
3783 event loop.
3784 .Sp
3785 \&\f(CW\*(C`ev_async_send\*(C'\fR sets a flag in the watcher and wakes up the loop. When
3786 the loop iterates next and checks for the watcher to have become active,
3787 it will reset the flag again. \f(CW\*(C`ev_async_pending\*(C'\fR can be used to very
3788 quickly check whether invoking the loop might be a good idea.
3789 .Sp
3790 Not that this does \fInot\fR check whether the watcher itself is pending,
3791 only whether it has been requested to make this watcher pending: there
3792 is a time window between the event loop checking and resetting the async
3793 notification, and the callback being invoked.
3794 .SH "OTHER FUNCTIONS"
3795 .IX Header "OTHER FUNCTIONS"
3796 There are some other functions of possible interest. Described. Here. Now.
3797 .IP "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)" 4
3798 .IX Item "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)"
3799 This function combines a simple timer and an I/O watcher, calls your
3800 callback on whichever event happens first and automatically stops both
3801 watchers. This is useful if you want to wait for a single event on an fd
3802 or timeout without having to allocate/configure/start/stop/free one or
3803 more watchers yourself.
3804 .Sp
3805 If \f(CW\*(C`fd\*(C'\fR is less than 0, then no I/O watcher will be started and the
3806 \&\f(CW\*(C`events\*(C'\fR argument is being ignored. Otherwise, an \f(CW\*(C`ev_io\*(C'\fR watcher for
3807 the given \f(CW\*(C`fd\*(C'\fR and \f(CW\*(C`events\*(C'\fR set will be created and started.
3808 .Sp
3809 If \f(CW\*(C`timeout\*(C'\fR is less than 0, then no timeout watcher will be
3810 started. Otherwise an \f(CW\*(C`ev_timer\*(C'\fR watcher with after = \f(CW\*(C`timeout\*(C'\fR (and
3811 repeat = 0) will be started. \f(CW0\fR is a valid timeout.
3812 .Sp
3813 The callback has the type \f(CW\*(C`void (*cb)(int revents, void *arg)\*(C'\fR and is
3814 passed an \f(CW\*(C`revents\*(C'\fR set like normal event callbacks (a combination of
3815 \&\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
3816 value passed to \f(CW\*(C`ev_once\*(C'\fR. Note that it is possible to receive \fIboth\fR
3817 a timeout and an io event at the same time \- you probably should give io
3818 events precedence.
3819 .Sp
3820 Example: wait up to ten seconds for data to appear on \s-1STDIN_FILENO.\s0
3821 .Sp
3822 .Vb 7
3823 \& static void stdin_ready (int revents, void *arg)
3824 \& {
3825 \& if (revents & EV_READ)
3826 \& /* stdin might have data for us, joy! */;
3827 \& else if (revents & EV_TIMER)
3828 \& /* doh, nothing entered */;
3829 \& }
3830 \&
3831 \& ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3832 .Ve
3833 .IP "ev_feed_fd_event (loop, int fd, int revents)" 4
3834 .IX Item "ev_feed_fd_event (loop, int fd, int revents)"
3835 Feed an event on the given fd, as if a file descriptor backend detected
3836 the given events.
3837 .IP "ev_feed_signal_event (loop, int signum)" 4
3838 .IX Item "ev_feed_signal_event (loop, int signum)"
3839 Feed an event as if the given signal occurred. See also \f(CW\*(C`ev_feed_signal\*(C'\fR,
3840 which is async-safe.
3841 .SH "COMMON OR USEFUL IDIOMS (OR BOTH)"
3842 .IX Header "COMMON OR USEFUL IDIOMS (OR BOTH)"
3843 This section explains some common idioms that are not immediately
3844 obvious. Note that examples are sprinkled over the whole manual, and this
3845 section only contains stuff that wouldn't fit anywhere else.
3846 .SS "\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\s0"
3847 .IX Subsection "ASSOCIATING CUSTOM DATA WITH A WATCHER"
3848 Each watcher has, by default, a \f(CW\*(C`void *data\*(C'\fR member that you can read
3849 or modify at any time: libev will completely ignore it. This can be used
3850 to associate arbitrary data with your watcher. If you need more data and
3851 don't want to allocate memory separately and store a pointer to it in that
3852 data member, you can also \*(L"subclass\*(R" the watcher type and provide your own
3853 data:
3854 .PP
3855 .Vb 7
3856 \& struct my_io
3857 \& {
3858 \& ev_io io;
3859 \& int otherfd;
3860 \& void *somedata;
3861 \& struct whatever *mostinteresting;
3862 \& };
3863 \&
3864 \& ...
3865 \& struct my_io w;
3866 \& ev_io_init (&w.io, my_cb, fd, EV_READ);
3867 .Ve
3868 .PP
3869 And since your callback will be called with a pointer to the watcher, you
3870 can cast it back to your own type:
3871 .PP
3872 .Vb 5
3873 \& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3874 \& {
3875 \& struct my_io *w = (struct my_io *)w_;
3876 \& ...
3877 \& }
3878 .Ve
3879 .PP
3880 More interesting and less C\-conformant ways of casting your callback
3881 function type instead have been omitted.
3882 .SS "\s-1BUILDING YOUR OWN COMPOSITE WATCHERS\s0"
3883 .IX Subsection "BUILDING YOUR OWN COMPOSITE WATCHERS"
3884 Another common scenario is to use some data structure with multiple
3885 embedded watchers, in effect creating your own watcher that combines
3886 multiple libev event sources into one \*(L"super-watcher\*(R":
3887 .PP
3888 .Vb 6
3889 \& struct my_biggy
3890 \& {
3891 \& int some_data;
3892 \& ev_timer t1;
3893 \& ev_timer t2;
3894 \& }
3895 .Ve
3896 .PP
3897 In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more
3898 complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct in
3899 the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies or \*(C+ coders), or you need
3900 to use some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for
3901 real programmers):
3902 .PP
3903 .Vb 1
3904 \& #include <stddef.h>
3905 \&
3906 \& static void
3907 \& t1_cb (EV_P_ ev_timer *w, int revents)
3908 \& {
3909 \& struct my_biggy big = (struct my_biggy *)
3910 \& (((char *)w) \- offsetof (struct my_biggy, t1));
3911 \& }
3912 \&
3913 \& static void
3914 \& t2_cb (EV_P_ ev_timer *w, int revents)
3915 \& {
3916 \& struct my_biggy big = (struct my_biggy *)
3917 \& (((char *)w) \- offsetof (struct my_biggy, t2));
3918 \& }
3919 .Ve
3920 .SS "\s-1AVOIDING FINISHING BEFORE RETURNING\s0"
3921 .IX Subsection "AVOIDING FINISHING BEFORE RETURNING"
3922 Often you have structures like this in event-based programs:
3923 .PP
3924 .Vb 4
3925 \& callback ()
3926 \& {
3927 \& free (request);
3928 \& }
3929 \&
3930 \& request = start_new_request (..., callback);
3931 .Ve
3932 .PP
3933 The intent is to start some \*(L"lengthy\*(R" operation. The \f(CW\*(C`request\*(C'\fR could be
3934 used to cancel the operation, or do other things with it.
3935 .PP
3936 It's not uncommon to have code paths in \f(CW\*(C`start_new_request\*(C'\fR that
3937 immediately invoke the callback, for example, to report errors. Or you add
3938 some caching layer that finds that it can skip the lengthy aspects of the
3939 operation and simply invoke the callback with the result.
3940 .PP
3941 The problem here is that this will happen \fIbefore\fR \f(CW\*(C`start_new_request\*(C'\fR
3942 has returned, so \f(CW\*(C`request\*(C'\fR is not set.
3943 .PP
3944 Even if you pass the request by some safer means to the callback, you
3945 might want to do something to the request after starting it, such as
3946 canceling it, which probably isn't working so well when the callback has
3947 already been invoked.
3948 .PP
3949 A common way around all these issues is to make sure that
3950 \&\f(CW\*(C`start_new_request\*(C'\fR \fIalways\fR returns before the callback is invoked. If
3951 \&\f(CW\*(C`start_new_request\*(C'\fR immediately knows the result, it can artificially
3952 delay invoking the callback by using a \f(CW\*(C`prepare\*(C'\fR or \f(CW\*(C`idle\*(C'\fR watcher for
3953 example, or more sneakily, by reusing an existing (stopped) watcher and
3954 pushing it into the pending queue:
3955 .PP
3956 .Vb 2
3957 \& ev_set_cb (watcher, callback);
3958 \& ev_feed_event (EV_A_ watcher, 0);
3959 .Ve
3960 .PP
3961 This way, \f(CW\*(C`start_new_request\*(C'\fR can safely return before the callback is
3962 invoked, while not delaying callback invocation too much.
3963 .SS "\s-1MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS\s0"
3964 .IX Subsection "MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS"
3965 Often (especially in \s-1GUI\s0 toolkits) there are places where you have
3966 \&\fImodal\fR interaction, which is most easily implemented by recursively
3967 invoking \f(CW\*(C`ev_run\*(C'\fR.
3968 .PP
3969 This brings the problem of exiting \- a callback might want to finish the
3970 main \f(CW\*(C`ev_run\*(C'\fR call, but not the nested one (e.g. user clicked \*(L"Quit\*(R", but
3971 a modal \*(L"Are you sure?\*(R" dialog is still waiting), or just the nested one
3972 and not the main one (e.g. user clocked \*(L"Ok\*(R" in a modal dialog), or some
3973 other combination: In these cases, a simple \f(CW\*(C`ev_break\*(C'\fR will not work.
3974 .PP
3975 The solution is to maintain \*(L"break this loop\*(R" variable for each \f(CW\*(C`ev_run\*(C'\fR
3976 invocation, and use a loop around \f(CW\*(C`ev_run\*(C'\fR until the condition is
3977 triggered, using \f(CW\*(C`EVRUN_ONCE\*(C'\fR:
3978 .PP
3979 .Vb 2
3980 \& // main loop
3981 \& int exit_main_loop = 0;
3982 \&
3983 \& while (!exit_main_loop)
3984 \& ev_run (EV_DEFAULT_ EVRUN_ONCE);
3985 \&
3986 \& // in a modal watcher
3987 \& int exit_nested_loop = 0;
3988 \&
3989 \& while (!exit_nested_loop)
3990 \& ev_run (EV_A_ EVRUN_ONCE);
3991 .Ve
3992 .PP
3993 To exit from any of these loops, just set the corresponding exit variable:
3994 .PP
3995 .Vb 2
3996 \& // exit modal loop
3997 \& exit_nested_loop = 1;
3998 \&
3999 \& // exit main program, after modal loop is finished
4000 \& exit_main_loop = 1;
4001 \&
4002 \& // exit both
4003 \& exit_main_loop = exit_nested_loop = 1;
4004 .Ve
4005 .SS "\s-1THREAD LOCKING EXAMPLE\s0"
4006 .IX Subsection "THREAD LOCKING EXAMPLE"
4007 Here is a fictitious example of how to run an event loop in a different
4008 thread from where callbacks are being invoked and watchers are
4009 created/added/removed.
4010 .PP
4011 For a real-world example, see the \f(CW\*(C`EV::Loop::Async\*(C'\fR perl module,
4012 which uses exactly this technique (which is suited for many high-level
4013 languages).
4014 .PP
4015 The example uses a pthread mutex to protect the loop data, a condition
4016 variable to wait for callback invocations, an async watcher to notify the
4017 event loop thread and an unspecified mechanism to wake up the main thread.
4018 .PP
4019 First, you need to associate some data with the event loop:
4020 .PP
4021 .Vb 6
4022 \& typedef struct {
4023 \& pthread_mutex_t lock; /* global loop lock */
4024 \& pthread_t tid;
4025 \& pthread_cond_t invoke_cv;
4026 \& ev_async async_w;
4027 \& } userdata;
4028 \&
4029 \& void prepare_loop (EV_P)
4030 \& {
4031 \& // for simplicity, we use a static userdata struct.
4032 \& static userdata u;
4033 \&
4034 \& ev_async_init (&u.async_w, async_cb);
4035 \& ev_async_start (EV_A_ &u.async_w);
4036 \&
4037 \& pthread_mutex_init (&u.lock, 0);
4038 \& pthread_cond_init (&u.invoke_cv, 0);
4039 \&
4040 \& // now associate this with the loop
4041 \& ev_set_userdata (EV_A_ &u);
4042 \& ev_set_invoke_pending_cb (EV_A_ l_invoke);
4043 \& ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
4044 \&
4045 \& // then create the thread running ev_run
4046 \& pthread_create (&u.tid, 0, l_run, EV_A);
4047 \& }
4048 .Ve
4049 .PP
4050 The callback for the \f(CW\*(C`ev_async\*(C'\fR watcher does nothing: the watcher is used
4051 solely to wake up the event loop so it takes notice of any new watchers
4052 that might have been added:
4053 .PP
4054 .Vb 5
4055 \& static void
4056 \& async_cb (EV_P_ ev_async *w, int revents)
4057 \& {
4058 \& // just used for the side effects
4059 \& }
4060 .Ve
4061 .PP
4062 The \f(CW\*(C`l_release\*(C'\fR and \f(CW\*(C`l_acquire\*(C'\fR callbacks simply unlock/lock the mutex
4063 protecting the loop data, respectively.
4064 .PP
4065 .Vb 6
4066 \& static void
4067 \& l_release (EV_P)
4068 \& {
4069 \& userdata *u = ev_userdata (EV_A);
4070 \& pthread_mutex_unlock (&u\->lock);
4071 \& }
4072 \&
4073 \& static void
4074 \& l_acquire (EV_P)
4075 \& {
4076 \& userdata *u = ev_userdata (EV_A);
4077 \& pthread_mutex_lock (&u\->lock);
4078 \& }
4079 .Ve
4080 .PP
4081 The event loop thread first acquires the mutex, and then jumps straight
4082 into \f(CW\*(C`ev_run\*(C'\fR:
4083 .PP
4084 .Vb 4
4085 \& void *
4086 \& l_run (void *thr_arg)
4087 \& {
4088 \& struct ev_loop *loop = (struct ev_loop *)thr_arg;
4089 \&
4090 \& l_acquire (EV_A);
4091 \& pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4092 \& ev_run (EV_A_ 0);
4093 \& l_release (EV_A);
4094 \&
4095 \& return 0;
4096 \& }
4097 .Ve
4098 .PP
4099 Instead of invoking all pending watchers, the \f(CW\*(C`l_invoke\*(C'\fR callback will
4100 signal the main thread via some unspecified mechanism (signals? pipe
4101 writes? \f(CW\*(C`Async::Interrupt\*(C'\fR?) and then waits until all pending watchers
4102 have been called (in a while loop because a) spurious wakeups are possible
4103 and b) skipping inter-thread-communication when there are no pending
4104 watchers is very beneficial):
4105 .PP
4106 .Vb 4
4107 \& static void
4108 \& l_invoke (EV_P)
4109 \& {
4110 \& userdata *u = ev_userdata (EV_A);
4111 \&
4112 \& while (ev_pending_count (EV_A))
4113 \& {
4114 \& wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4115 \& pthread_cond_wait (&u\->invoke_cv, &u\->lock);
4116 \& }
4117 \& }
4118 .Ve
4119 .PP
4120 Now, whenever the main thread gets told to invoke pending watchers, it
4121 will grab the lock, call \f(CW\*(C`ev_invoke_pending\*(C'\fR and then signal the loop
4122 thread to continue:
4123 .PP
4124 .Vb 4
4125 \& static void
4126 \& real_invoke_pending (EV_P)
4127 \& {
4128 \& userdata *u = ev_userdata (EV_A);
4129 \&
4130 \& pthread_mutex_lock (&u\->lock);
4131 \& ev_invoke_pending (EV_A);
4132 \& pthread_cond_signal (&u\->invoke_cv);
4133 \& pthread_mutex_unlock (&u\->lock);
4134 \& }
4135 .Ve
4136 .PP
4137 Whenever you want to start/stop a watcher or do other modifications to an
4138 event loop, you will now have to lock:
4139 .PP
4140 .Vb 2
4141 \& ev_timer timeout_watcher;
4142 \& userdata *u = ev_userdata (EV_A);
4143 \&
4144 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4145 \&
4146 \& pthread_mutex_lock (&u\->lock);
4147 \& ev_timer_start (EV_A_ &timeout_watcher);
4148 \& ev_async_send (EV_A_ &u\->async_w);
4149 \& pthread_mutex_unlock (&u\->lock);
4150 .Ve
4151 .PP
4152 Note that sending the \f(CW\*(C`ev_async\*(C'\fR watcher is required because otherwise
4153 an event loop currently blocking in the kernel will have no knowledge
4154 about the newly added timer. By waking up the loop it will pick up any new
4155 watchers in the next event loop iteration.
4156 .SS "\s-1THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS\s0"
4157 .IX Subsection "THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS"
4158 While the overhead of a callback that e.g. schedules a thread is small, it
4159 is still an overhead. If you embed libev, and your main usage is with some
4160 kind of threads or coroutines, you might want to customise libev so that
4161 doesn't need callbacks anymore.
4162 .PP
4163 Imagine you have coroutines that you can switch to using a function
4164 \&\f(CW\*(C`switch_to (coro)\*(C'\fR, that libev runs in a coroutine called \f(CW\*(C`libev_coro\*(C'\fR
4165 and that due to some magic, the currently active coroutine is stored in a
4166 global called \f(CW\*(C`current_coro\*(C'\fR. Then you can build your own \*(L"wait for libev
4167 event\*(R" primitive by changing \f(CW\*(C`EV_CB_DECLARE\*(C'\fR and \f(CW\*(C`EV_CB_INVOKE\*(C'\fR (note
4168 the differing \f(CW\*(C`;\*(C'\fR conventions):
4169 .PP
4170 .Vb 2
4171 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4172 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4173 .Ve
4174 .PP
4175 That means instead of having a C callback function, you store the
4176 coroutine to switch to in each watcher, and instead of having libev call
4177 your callback, you instead have it switch to that coroutine.
4178 .PP
4179 A coroutine might now wait for an event with a function called
4180 \&\f(CW\*(C`wait_for_event\*(C'\fR. (the watcher needs to be started, as always, but it doesn't
4181 matter when, or whether the watcher is active or not when this function is
4182 called):
4183 .PP
4184 .Vb 6
4185 \& void
4186 \& wait_for_event (ev_watcher *w)
4187 \& {
4188 \& ev_set_cb (w, current_coro);
4189 \& switch_to (libev_coro);
4190 \& }
4191 .Ve
4192 .PP
4193 That basically suspends the coroutine inside \f(CW\*(C`wait_for_event\*(C'\fR and
4194 continues the libev coroutine, which, when appropriate, switches back to
4195 this or any other coroutine.
4196 .PP
4197 You can do similar tricks if you have, say, threads with an event queue \-
4198 instead of storing a coroutine, you store the queue object and instead of
4199 switching to a coroutine, you push the watcher onto the queue and notify
4200 any waiters.
4201 .PP
4202 To embed libev, see \*(L"\s-1EMBEDDING\*(R"\s0, but in short, it's easiest to create two
4203 files, \fImy_ev.h\fR and \fImy_ev.c\fR that include the respective libev files:
4204 .PP
4205 .Vb 4
4206 \& // my_ev.h
4207 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4208 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4209 \& #include "../libev/ev.h"
4210 \&
4211 \& // my_ev.c
4212 \& #define EV_H "my_ev.h"
4213 \& #include "../libev/ev.c"
4214 .Ve
4215 .PP
4216 And then use \fImy_ev.h\fR when you would normally use \fIev.h\fR, and compile
4217 \&\fImy_ev.c\fR into your project. When properly specifying include paths, you
4218 can even use \fIev.h\fR as header file name directly.
4219 .SH "LIBEVENT EMULATION"
4220 .IX Header "LIBEVENT EMULATION"
4221 Libev offers a compatibility emulation layer for libevent. It cannot
4222 emulate the internals of libevent, so here are some usage hints:
4223 .IP "\(bu" 4
4224 Only the libevent\-1.4.1\-beta \s-1API\s0 is being emulated.
4225 .Sp
4226 This was the newest libevent version available when libev was implemented,
4227 and is still mostly unchanged in 2010.
4228 .IP "\(bu" 4
4229 Use it by including <event.h>, as usual.
4230 .IP "\(bu" 4
4231 The following members are fully supported: ev_base, ev_callback,
4232 ev_arg, ev_fd, ev_res, ev_events.
4233 .IP "\(bu" 4
4234 Avoid using ev_flags and the EVLIST_*\-macros, while it is
4235 maintained by libev, it does not work exactly the same way as in libevent (consider
4236 it a private \s-1API\s0).
4237 .IP "\(bu" 4
4238 Priorities are not currently supported. Initialising priorities
4239 will fail and all watchers will have the same priority, even though there
4240 is an ev_pri field.
4241 .IP "\(bu" 4
4242 In libevent, the last base created gets the signals, in libev, the
4243 base that registered the signal gets the signals.
4244 .IP "\(bu" 4
4245 Other members are not supported.
4246 .IP "\(bu" 4
4247 The libev emulation is \fInot\fR \s-1ABI\s0 compatible to libevent, you need
4248 to use the libev header file and library.
4249 .SH "\*(C+ SUPPORT"
4250 .IX Header " SUPPORT"
4251 .SS "C \s-1API\s0"
4252 .IX Subsection "C API"
4253 The normal C \s-1API\s0 should work fine when used from \*(C+: both ev.h and the
4254 libev sources can be compiled as \*(C+. Therefore, code that uses the C \s-1API\s0
4255 will work fine.
4256 .PP
4257 Proper exception specifications might have to be added to callbacks passed
4258 to libev: exceptions may be thrown only from watcher callbacks, all other
4259 callbacks (allocator, syserr, loop acquire/release and periodic reschedule
4260 callbacks) must not throw exceptions, and might need a \f(CW\*(C`noexcept\*(C'\fR
4261 specification. If you have code that needs to be compiled as both C and
4262 \&\*(C+ you can use the \f(CW\*(C`EV_NOEXCEPT\*(C'\fR macro for this:
4263 .PP
4264 .Vb 6
4265 \& static void
4266 \& fatal_error (const char *msg) EV_NOEXCEPT
4267 \& {
4268 \& perror (msg);
4269 \& abort ();
4270 \& }
4271 \&
4272 \& ...
4273 \& ev_set_syserr_cb (fatal_error);
4274 .Ve
4275 .PP
4276 The only \s-1API\s0 functions that can currently throw exceptions are \f(CW\*(C`ev_run\*(C'\fR,
4277 \&\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
4278 because it runs cleanup watchers).
4279 .PP
4280 Throwing exceptions in watcher callbacks is only supported if libev itself
4281 is compiled with a \*(C+ compiler or your C and \*(C+ environments allow
4282 throwing exceptions through C libraries (most do).
4283 .SS "\*(C+ \s-1API\s0"
4284 .IX Subsection " API"
4285 Libev comes with some simplistic wrapper classes for \*(C+ that mainly allow
4286 you to use some convenience methods to start/stop watchers and also change
4287 the callback model to a model using method callbacks on objects.
4288 .PP
4289 To use it,
4290 .PP
4291 .Vb 1
4292 \& #include <ev++.h>
4293 .Ve
4294 .PP
4295 This automatically includes \fIev.h\fR and puts all of its definitions (many
4296 of them macros) into the global namespace. All \*(C+ specific things are
4297 put into the \f(CW\*(C`ev\*(C'\fR namespace. It should support all the same embedding
4298 options as \fIev.h\fR, most notably \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR.
4299 .PP
4300 Care has been taken to keep the overhead low. The only data member the \*(C+
4301 classes add (compared to plain C\-style watchers) is the event loop pointer
4302 that the watcher is associated with (or no additional members at all if
4303 you disable \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR when embedding libev).
4304 .PP
4305 Currently, functions, static and non-static member functions and classes
4306 with \f(CW\*(C`operator ()\*(C'\fR can be used as callbacks. Other types should be easy
4307 to add as long as they only need one additional pointer for context. If
4308 you need support for other types of functors please contact the author
4309 (preferably after implementing it).
4310 .PP
4311 For all this to work, your \*(C+ compiler either has to use the same calling
4312 conventions as your C compiler (for static member functions), or you have
4313 to embed libev and compile libev itself as \*(C+.
4314 .PP
4315 Here is a list of things available in the \f(CW\*(C`ev\*(C'\fR namespace:
4316 .ie n .IP """ev::READ"", ""ev::WRITE"" etc." 4
4317 .el .IP "\f(CWev::READ\fR, \f(CWev::WRITE\fR etc." 4
4318 .IX Item "ev::READ, ev::WRITE etc."
4319 These are just enum values with the same values as the \f(CW\*(C`EV_READ\*(C'\fR etc.
4320 macros from \fIev.h\fR.
4321 .ie n .IP """ev::tstamp"", ""ev::now""" 4
4322 .el .IP "\f(CWev::tstamp\fR, \f(CWev::now\fR" 4
4323 .IX Item "ev::tstamp, ev::now"
4324 Aliases to the same types/functions as with the \f(CW\*(C`ev_\*(C'\fR prefix.
4325 .ie n .IP """ev::io"", ""ev::timer"", ""ev::periodic"", ""ev::idle"", ""ev::sig"" etc." 4
4326 .el .IP "\f(CWev::io\fR, \f(CWev::timer\fR, \f(CWev::periodic\fR, \f(CWev::idle\fR, \f(CWev::sig\fR etc." 4
4327 .IX Item "ev::io, ev::timer, ev::periodic, ev::idle, ev::sig etc."
4328 For each \f(CW\*(C`ev_TYPE\*(C'\fR watcher in \fIev.h\fR there is a corresponding class of
4329 the same name in the \f(CW\*(C`ev\*(C'\fR namespace, with the exception of \f(CW\*(C`ev_signal\*(C'\fR
4330 which is called \f(CW\*(C`ev::sig\*(C'\fR to avoid clashes with the \f(CW\*(C`signal\*(C'\fR macro
4331 defined by many implementations.
4332 .Sp
4333 All of those classes have these methods:
4334 .RS 4
4335 .IP "ev::TYPE::TYPE ()" 4
4336 .IX Item "ev::TYPE::TYPE ()"
4337 .PD 0
4338 .IP "ev::TYPE::TYPE (loop)" 4
4339 .IX Item "ev::TYPE::TYPE (loop)"
4340 .IP "ev::TYPE::~TYPE" 4
4341 .IX Item "ev::TYPE::~TYPE"
4342 .PD
4343 The constructor (optionally) takes an event loop to associate the watcher
4344 with. If it is omitted, it will use \f(CW\*(C`EV_DEFAULT\*(C'\fR.
4345 .Sp
4346 The constructor calls \f(CW\*(C`ev_init\*(C'\fR for you, which means you have to call the
4347 \&\f(CW\*(C`set\*(C'\fR method before starting it.
4348 .Sp
4349 It will not set a callback, however: You have to call the templated \f(CW\*(C`set\*(C'\fR
4350 method to set a callback before you can start the watcher.
4351 .Sp
4352 (The reason why you have to use a method is a limitation in \*(C+ which does
4353 not allow explicit template arguments for constructors).
4354 .Sp
4355 The destructor automatically stops the watcher if it is active.
4356 .IP "w\->set<class, &class::method> (object *)" 4
4357 .IX Item "w->set<class, &class::method> (object *)"
4358 This method sets the callback method to call. The method has to have a
4359 signature of \f(CW\*(C`void (*)(ev_TYPE &, int)\*(C'\fR, it receives the watcher as
4360 first argument and the \f(CW\*(C`revents\*(C'\fR as second. The object must be given as
4361 parameter and is stored in the \f(CW\*(C`data\*(C'\fR member of the watcher.
4362 .Sp
4363 This method synthesizes efficient thunking code to call your method from
4364 the C callback that libev requires. If your compiler can inline your
4365 callback (i.e. it is visible to it at the place of the \f(CW\*(C`set\*(C'\fR call and
4366 your compiler is good :), then the method will be fully inlined into the
4367 thunking function, making it as fast as a direct C callback.
4368 .Sp
4369 Example: simple class declaration and watcher initialisation
4370 .Sp
4371 .Vb 4
4372 \& struct myclass
4373 \& {
4374 \& void io_cb (ev::io &w, int revents) { }
4375 \& }
4376 \&
4377 \& myclass obj;
4378 \& ev::io iow;
4379 \& iow.set <myclass, &myclass::io_cb> (&obj);
4380 .Ve
4381 .IP "w\->set (object *)" 4
4382 .IX Item "w->set (object *)"
4383 This is a variation of a method callback \- leaving out the method to call
4384 will default the method to \f(CW\*(C`operator ()\*(C'\fR, which makes it possible to use
4385 functor objects without having to manually specify the \f(CW\*(C`operator ()\*(C'\fR all
4386 the time. Incidentally, you can then also leave out the template argument
4387 list.
4388 .Sp
4389 The \f(CW\*(C`operator ()\*(C'\fR method prototype must be \f(CW\*(C`void operator ()(watcher &w,
4390 int revents)\*(C'\fR.
4391 .Sp
4392 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4393 .Sp
4394 Example: use a functor object as callback.
4395 .Sp
4396 .Vb 7
4397 \& struct myfunctor
4398 \& {
4399 \& void operator() (ev::io &w, int revents)
4400 \& {
4401 \& ...
4402 \& }
4403 \& }
4404 \&
4405 \& myfunctor f;
4406 \&
4407 \& ev::io w;
4408 \& w.set (&f);
4409 .Ve
4410 .IP "w\->set<function> (void *data = 0)" 4
4411 .IX Item "w->set<function> (void *data = 0)"
4412 Also sets a callback, but uses a static method or plain function as
4413 callback. The optional \f(CW\*(C`data\*(C'\fR argument will be stored in the watcher's
4414 \&\f(CW\*(C`data\*(C'\fR member and is free for you to use.
4415 .Sp
4416 The prototype of the \f(CW\*(C`function\*(C'\fR must be \f(CW\*(C`void (*)(ev::TYPE &w, int)\*(C'\fR.
4417 .Sp
4418 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4419 .Sp
4420 Example: Use a plain function as callback.
4421 .Sp
4422 .Vb 2
4423 \& static void io_cb (ev::io &w, int revents) { }
4424 \& iow.set <io_cb> ();
4425 .Ve
4426 .IP "w\->set (loop)" 4
4427 .IX Item "w->set (loop)"
4428 Associates a different \f(CW\*(C`struct ev_loop\*(C'\fR with this watcher. You can only
4429 do this when the watcher is inactive (and not pending either).
4430 .IP "w\->set ([arguments])" 4
4431 .IX Item "w->set ([arguments])"
4432 Basically the same as \f(CW\*(C`ev_TYPE_set\*(C'\fR (except for \f(CW\*(C`ev::embed\*(C'\fR watchers>),
4433 with the same arguments. Either this method or a suitable start method
4434 must be called at least once. Unlike the C counterpart, an active watcher
4435 gets automatically stopped and restarted when reconfiguring it with this
4436 method.
4437 .Sp
4438 For \f(CW\*(C`ev::embed\*(C'\fR watchers this method is called \f(CW\*(C`set_embed\*(C'\fR, to avoid
4439 clashing with the \f(CW\*(C`set (loop)\*(C'\fR method.
4440 .Sp
4441 For \f(CW\*(C`ev::io\*(C'\fR watchers there is an additional \f(CW\*(C`set\*(C'\fR method that acepts a
4442 new event mask only, and internally calls \f(CW\*(C`ev_io_modify\*(C'\fR.
4443 .IP "w\->start ()" 4
4444 .IX Item "w->start ()"
4445 Starts the watcher. Note that there is no \f(CW\*(C`loop\*(C'\fR argument, as the
4446 constructor already stores the event loop.
4447 .IP "w\->start ([arguments])" 4
4448 .IX Item "w->start ([arguments])"
4449 Instead of calling \f(CW\*(C`set\*(C'\fR and \f(CW\*(C`start\*(C'\fR methods separately, it is often
4450 convenient to wrap them in one call. Uses the same type of arguments as
4451 the configure \f(CW\*(C`set\*(C'\fR method of the watcher.
4452 .IP "w\->stop ()" 4
4453 .IX Item "w->stop ()"
4454 Stops the watcher if it is active. Again, no \f(CW\*(C`loop\*(C'\fR argument.
4455 .ie n .IP "w\->again () (""ev::timer"", ""ev::periodic"" only)" 4
4456 .el .IP "w\->again () (\f(CWev::timer\fR, \f(CWev::periodic\fR only)" 4
4457 .IX Item "w->again () (ev::timer, ev::periodic only)"
4458 For \f(CW\*(C`ev::timer\*(C'\fR and \f(CW\*(C`ev::periodic\*(C'\fR, this invokes the corresponding
4459 \&\f(CW\*(C`ev_TYPE_again\*(C'\fR function.
4460 .ie n .IP "w\->sweep () (""ev::embed"" only)" 4
4461 .el .IP "w\->sweep () (\f(CWev::embed\fR only)" 4
4462 .IX Item "w->sweep () (ev::embed only)"
4463 Invokes \f(CW\*(C`ev_embed_sweep\*(C'\fR.
4464 .ie n .IP "w\->update () (""ev::stat"" only)" 4
4465 .el .IP "w\->update () (\f(CWev::stat\fR only)" 4
4466 .IX Item "w->update () (ev::stat only)"
4467 Invokes \f(CW\*(C`ev_stat_stat\*(C'\fR.
4468 .RE
4469 .RS 4
4470 .RE
4471 .PP
4472 Example: Define a class with two I/O and idle watchers, start the I/O
4473 watchers in the constructor.
4474 .PP
4475 .Vb 5
4476 \& class myclass
4477 \& {
4478 \& ev::io io ; void io_cb (ev::io &w, int revents);
4479 \& ev::io io2 ; void io2_cb (ev::io &w, int revents);
4480 \& ev::idle idle; void idle_cb (ev::idle &w, int revents);
4481 \&
4482 \& myclass (int fd)
4483 \& {
4484 \& io .set <myclass, &myclass::io_cb > (this);
4485 \& io2 .set <myclass, &myclass::io2_cb > (this);
4486 \& idle.set <myclass, &myclass::idle_cb> (this);
4487 \&
4488 \& io.set (fd, ev::WRITE); // configure the watcher
4489 \& io.start (); // start it whenever convenient
4490 \&
4491 \& io2.start (fd, ev::READ); // set + start in one call
4492 \& }
4493 \& };
4494 .Ve
4495 .SH "OTHER LANGUAGE BINDINGS"
4496 .IX Header "OTHER LANGUAGE BINDINGS"
4497 Libev does not offer other language bindings itself, but bindings for a
4498 number of languages exist in the form of third-party packages. If you know
4499 any interesting language binding in addition to the ones listed here, drop
4500 me a note.
4501 .IP "Perl" 4
4502 .IX Item "Perl"
4503 The \s-1EV\s0 module implements the full libev \s-1API\s0 and is actually used to test
4504 libev. \s-1EV\s0 is developed together with libev. Apart from the \s-1EV\s0 core module,
4505 there are additional modules that implement libev-compatible interfaces
4506 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),
4507 \&\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
4508 and \f(CW\*(C`EV::Glib\*(C'\fR).
4509 .Sp
4510 It can be found and installed via \s-1CPAN,\s0 its homepage is at
4511 <http://software.schmorp.de/pkg/EV>.
4512 .IP "Python" 4
4513 .IX Item "Python"
4514 Python bindings can be found at <http://code.google.com/p/pyev/>. It
4515 seems to be quite complete and well-documented.
4516 .IP "Ruby" 4
4517 .IX Item "Ruby"
4518 Tony Arcieri has written a ruby extension that offers access to a subset
4519 of the libev \s-1API\s0 and adds file handle abstractions, asynchronous \s-1DNS\s0 and
4520 more on top of it. It can be found via gem servers. Its homepage is at
4521 <http://rev.rubyforge.org/>.
4522 .Sp
4523 Roger Pack reports that using the link order \f(CW\*(C`\-lws2_32 \-lmsvcrt\-ruby\-190\*(C'\fR
4524 makes rev work even on mingw.
4525 .IP "Haskell" 4
4526 .IX Item "Haskell"
4527 A haskell binding to libev is available at
4528 <http://hackage.haskell.org/cgi\-bin/hackage\-scripts/package/hlibev>.
4529 .IP "D" 4
4530 .IX Item "D"
4531 Leandro Lucarella has written a D language binding (\fIev.d\fR) for libev, to
4532 be found at <http://www.llucax.com.ar/proj/ev.d/index.html>.
4533 .IP "Ocaml" 4
4534 .IX Item "Ocaml"
4535 Erkki Seppala has written Ocaml bindings for libev, to be found at
4536 <http://modeemi.cs.tut.fi/~flux/software/ocaml\-ev/>.
4537 .IP "Lua" 4
4538 .IX Item "Lua"
4539 Brian Maher has written a partial interface to libev for lua (at the
4540 time of this writing, only \f(CW\*(C`ev_io\*(C'\fR and \f(CW\*(C`ev_timer\*(C'\fR), to be found at
4541 <http://github.com/brimworks/lua\-ev>.
4542 .IP "Javascript" 4
4543 .IX Item "Javascript"
4544 Node.js (<http://nodejs.org>) uses libev as the underlying event library.
4545 .IP "Others" 4
4546 .IX Item "Others"
4547 There are others, and I stopped counting.
4548 .SH "MACRO MAGIC"
4549 .IX Header "MACRO MAGIC"
4550 Libev can be compiled with a variety of options, the most fundamental
4551 of which is \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR. This option determines whether (most)
4552 functions and callbacks have an initial \f(CW\*(C`struct ev_loop *\*(C'\fR argument.
4553 .PP
4554 To make it easier to write programs that cope with either variant, the
4555 following macros are defined:
4556 .ie n .IP """EV_A"", ""EV_A_""" 4
4557 .el .IP "\f(CWEV_A\fR, \f(CWEV_A_\fR" 4
4558 .IX Item "EV_A, EV_A_"
4559 This provides the loop \fIargument\fR for functions, if one is required (\*(L"ev
4560 loop argument\*(R"). The \f(CW\*(C`EV_A\*(C'\fR form is used when this is the sole argument,
4561 \&\f(CW\*(C`EV_A_\*(C'\fR is used when other arguments are following. Example:
4562 .Sp
4563 .Vb 3
4564 \& ev_unref (EV_A);
4565 \& ev_timer_add (EV_A_ watcher);
4566 \& ev_run (EV_A_ 0);
4567 .Ve
4568 .Sp
4569 It assumes the variable \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR is in scope,
4570 which is often provided by the following macro.
4571 .ie n .IP """EV_P"", ""EV_P_""" 4
4572 .el .IP "\f(CWEV_P\fR, \f(CWEV_P_\fR" 4
4573 .IX Item "EV_P, EV_P_"
4574 This provides the loop \fIparameter\fR for functions, if one is required (\*(L"ev
4575 loop parameter\*(R"). The \f(CW\*(C`EV_P\*(C'\fR form is used when this is the sole parameter,
4576 \&\f(CW\*(C`EV_P_\*(C'\fR is used when other parameters are following. Example:
4577 .Sp
4578 .Vb 2
4579 \& // this is how ev_unref is being declared
4580 \& static void ev_unref (EV_P);
4581 \&
4582 \& // this is how you can declare your typical callback
4583 \& static void cb (EV_P_ ev_timer *w, int revents)
4584 .Ve
4585 .Sp
4586 It declares a parameter \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR, quite
4587 suitable for use with \f(CW\*(C`EV_A\*(C'\fR.
4588 .ie n .IP """EV_DEFAULT"", ""EV_DEFAULT_""" 4
4589 .el .IP "\f(CWEV_DEFAULT\fR, \f(CWEV_DEFAULT_\fR" 4
4590 .IX Item "EV_DEFAULT, EV_DEFAULT_"
4591 Similar to the other two macros, this gives you the value of the default
4592 loop, if multiple loops are supported (\*(L"ev loop default\*(R"). The default loop
4593 will be initialised if it isn't already initialised.
4594 .Sp
4595 For non-multiplicity builds, these macros do nothing, so you always have
4596 to initialise the loop somewhere.
4597 .ie n .IP """EV_DEFAULT_UC"", ""EV_DEFAULT_UC_""" 4
4598 .el .IP "\f(CWEV_DEFAULT_UC\fR, \f(CWEV_DEFAULT_UC_\fR" 4
4599 .IX Item "EV_DEFAULT_UC, EV_DEFAULT_UC_"
4600 Usage identical to \f(CW\*(C`EV_DEFAULT\*(C'\fR and \f(CW\*(C`EV_DEFAULT_\*(C'\fR, but requires that the
4601 default loop has been initialised (\f(CW\*(C`UC\*(C'\fR == unchecked). Their behaviour
4602 is undefined when the default loop has not been initialised by a previous
4603 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.
4604 .Sp
4605 It is often prudent to use \f(CW\*(C`EV_DEFAULT\*(C'\fR when initialising the first
4606 watcher in a function but use \f(CW\*(C`EV_DEFAULT_UC\*(C'\fR afterwards.
4607 .PP
4608 Example: Declare and initialise a check watcher, utilising the above
4609 macros so it will work regardless of whether multiple loops are supported
4610 or not.
4611 .PP
4612 .Vb 5
4613 \& static void
4614 \& check_cb (EV_P_ ev_timer *w, int revents)
4615 \& {
4616 \& ev_check_stop (EV_A_ w);
4617 \& }
4618 \&
4619 \& ev_check check;
4620 \& ev_check_init (&check, check_cb);
4621 \& ev_check_start (EV_DEFAULT_ &check);
4622 \& ev_run (EV_DEFAULT_ 0);
4623 .Ve
4624 .SH "EMBEDDING"
4625 .IX Header "EMBEDDING"
4626 Libev can (and often is) directly embedded into host
4627 applications. Examples of applications that embed it include the Deliantra
4628 Game Server, the \s-1EV\s0 perl module, the \s-1GNU\s0 Virtual Private Ethernet (gvpe)
4629 and rxvt-unicode.
4630 .PP
4631 The goal is to enable you to just copy the necessary files into your
4632 source directory without having to change even a single line in them, so
4633 you can easily upgrade by simply copying (or having a checked-out copy of
4634 libev somewhere in your source tree).
4635 .SS "\s-1FILESETS\s0"
4636 .IX Subsection "FILESETS"
4637 Depending on what features you need you need to include one or more sets of files
4638 in your application.
4639 .PP
4640 \fI\s-1CORE EVENT LOOP\s0\fR
4641 .IX Subsection "CORE EVENT LOOP"
4642 .PP
4643 To include only the libev core (all the \f(CW\*(C`ev_*\*(C'\fR functions), with manual
4644 configuration (no autoconf):
4645 .PP
4646 .Vb 2
4647 \& #define EV_STANDALONE 1
4648 \& #include "ev.c"
4649 .Ve
4650 .PP
4651 This will automatically include \fIev.h\fR, too, and should be done in a
4652 single C source file only to provide the function implementations. To use
4653 it, do the same for \fIev.h\fR in all files wishing to use this \s-1API\s0 (best
4654 done by writing a wrapper around \fIev.h\fR that you can include instead and
4655 where you can put other configuration options):
4656 .PP
4657 .Vb 2
4658 \& #define EV_STANDALONE 1
4659 \& #include "ev.h"
4660 .Ve
4661 .PP
4662 Both header files and implementation files can be compiled with a \*(C+
4663 compiler (at least, that's a stated goal, and breakage will be treated
4664 as a bug).
4665 .PP
4666 You need the following files in your source tree, or in a directory
4667 in your include path (e.g. in libev/ when using \-Ilibev):
4668 .PP
4669 .Vb 4
4670 \& ev.h
4671 \& ev.c
4672 \& ev_vars.h
4673 \& ev_wrap.h
4674 \&
4675 \& ev_win32.c required on win32 platforms only
4676 \&
4677 \& ev_select.c only when select backend is enabled
4678 \& ev_poll.c only when poll backend is enabled
4679 \& ev_epoll.c only when the epoll backend is enabled
4680 \& ev_linuxaio.c only when the linux aio backend is enabled
4681 \& ev_iouring.c only when the linux io_uring backend is enabled
4682 \& ev_kqueue.c only when the kqueue backend is enabled
4683 \& ev_port.c only when the solaris port backend is enabled
4684 .Ve
4685 .PP
4686 \&\fIev.c\fR includes the backend files directly when enabled, so you only need
4687 to compile this single file.
4688 .PP
4689 \fI\s-1LIBEVENT COMPATIBILITY API\s0\fR
4690 .IX Subsection "LIBEVENT COMPATIBILITY API"
4691 .PP
4692 To include the libevent compatibility \s-1API,\s0 also include:
4693 .PP
4694 .Vb 1
4695 \& #include "event.c"
4696 .Ve
4697 .PP
4698 in the file including \fIev.c\fR, and:
4699 .PP
4700 .Vb 1
4701 \& #include "event.h"
4702 .Ve
4703 .PP
4704 in the files that want to use the libevent \s-1API.\s0 This also includes \fIev.h\fR.
4705 .PP
4706 You need the following additional files for this:
4707 .PP
4708 .Vb 2
4709 \& event.h
4710 \& event.c
4711 .Ve
4712 .PP
4713 \fI\s-1AUTOCONF SUPPORT\s0\fR
4714 .IX Subsection "AUTOCONF SUPPORT"
4715 .PP
4716 Instead of using \f(CW\*(C`EV_STANDALONE=1\*(C'\fR and providing your configuration in
4717 whatever way you want, you can also \f(CW\*(C`m4_include([libev.m4])\*(C'\fR in your
4718 \&\fIconfigure.ac\fR and leave \f(CW\*(C`EV_STANDALONE\*(C'\fR undefined. \fIev.c\fR will then
4719 include \fIconfig.h\fR and configure itself accordingly.
4720 .PP
4721 For this of course you need the m4 file:
4722 .PP
4723 .Vb 1
4724 \& libev.m4
4725 .Ve
4726 .SS "\s-1PREPROCESSOR SYMBOLS/MACROS\s0"
4727 .IX Subsection "PREPROCESSOR SYMBOLS/MACROS"
4728 Libev can be configured via a variety of preprocessor symbols you have to
4729 define before including (or compiling) any of its files. The default in
4730 the absence of autoconf is documented for every option.
4731 .PP
4732 Symbols marked with \*(L"(h)\*(R" do not change the \s-1ABI,\s0 and can have different
4733 values when compiling libev vs. including \fIev.h\fR, so it is permissible
4734 to redefine them before including \fIev.h\fR without breaking compatibility
4735 to a compiled library. All other symbols change the \s-1ABI,\s0 which means all
4736 users of libev and the libev code itself must be compiled with compatible
4737 settings.
4738 .IP "\s-1EV_COMPAT3\s0 (h)" 4
4739 .IX Item "EV_COMPAT3 (h)"
4740 Backwards compatibility is a major concern for libev. This is why this
4741 release of libev comes with wrappers for the functions and symbols that
4742 have been renamed between libev version 3 and 4.
4743 .Sp
4744 You can disable these wrappers (to test compatibility with future
4745 versions) by defining \f(CW\*(C`EV_COMPAT3\*(C'\fR to \f(CW0\fR when compiling your
4746 sources. This has the additional advantage that you can drop the \f(CW\*(C`struct\*(C'\fR
4747 from \f(CW\*(C`struct ev_loop\*(C'\fR declarations, as libev will provide an \f(CW\*(C`ev_loop\*(C'\fR
4748 typedef in that case.
4749 .Sp
4750 In some future version, the default for \f(CW\*(C`EV_COMPAT3\*(C'\fR will become \f(CW0\fR,
4751 and in some even more future version the compatibility code will be
4752 removed completely.
4753 .IP "\s-1EV_STANDALONE\s0 (h)" 4
4754 .IX Item "EV_STANDALONE (h)"
4755 Must always be \f(CW1\fR if you do not use autoconf configuration, which
4756 keeps libev from including \fIconfig.h\fR, and it also defines dummy
4757 implementations for some libevent functions (such as logging, which is not
4758 supported). It will also not define any of the structs usually found in
4759 \&\fIevent.h\fR that are not directly supported by the libev core alone.
4760 .Sp
4761 In standalone mode, libev will still try to automatically deduce the
4762 configuration, but has to be more conservative.
4763 .IP "\s-1EV_USE_FLOOR\s0" 4
4764 .IX Item "EV_USE_FLOOR"
4765 If defined to be \f(CW1\fR, libev will use the \f(CW\*(C`floor ()\*(C'\fR function for its
4766 periodic reschedule calculations, otherwise libev will fall back on a
4767 portable (slower) implementation. If you enable this, you usually have to
4768 link against libm or something equivalent. Enabling this when the \f(CW\*(C`floor\*(C'\fR
4769 function is not available will fail, so the safe default is to not enable
4770 this.
4771 .IP "\s-1EV_USE_MONOTONIC\s0" 4
4772 .IX Item "EV_USE_MONOTONIC"
4773 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4774 monotonic clock option at both compile time and runtime. Otherwise no
4775 use of the monotonic clock option will be attempted. If you enable this,
4776 you usually have to link against librt or something similar. Enabling it
4777 when the functionality isn't available is safe, though, although you have
4778 to make sure you link against any libraries where the \f(CW\*(C`clock_gettime\*(C'\fR
4779 function is hiding in (often \fI\-lrt\fR). See also \f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4780 .IP "\s-1EV_USE_REALTIME\s0" 4
4781 .IX Item "EV_USE_REALTIME"
4782 If defined to be \f(CW1\fR, libev will try to detect the availability of the