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Revision: 1.117
Committed: Fri Dec 20 20:51:46 2019 UTC (4 years, 6 months ago) by root
Branch: MAIN
CVS Tags: EV-rel-4_31
Changes since 1.116: +30 -2 lines
Log Message:
*** empty log message ***

File Contents

# Content
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133 .\" ========================================================================
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135 .IX Title "LIBEV 3"
136 .TH LIBEV 3 "2019-12-20" "libev-4.27" "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_NOTIMERFD""" 4
599 .el .IP "\f(CWEVFLAG_NOTIMERFD\fR" 4
600 .IX Item "EVFLAG_NOTIMERFD"
601 When this flag is specified, the libev will avoid using a \f(CW\*(C`timerfd\*(C'\fR to
602 detect time jumps. It will still be able to detect time jumps, but takes
603 longer and has a lower accuracy in doing so, but saves a file descriptor
604 per loop.
605 .ie n .IP """EVFLAG_NOSIGMASK""" 4
606 .el .IP "\f(CWEVFLAG_NOSIGMASK\fR" 4
607 .IX Item "EVFLAG_NOSIGMASK"
608 When this flag is specified, then libev will avoid to modify the signal
609 mask. Specifically, this means you have to make sure signals are unblocked
610 when you want to receive them.
611 .Sp
612 This behaviour is useful when you want to do your own signal handling, or
613 want to handle signals only in specific threads and want to avoid libev
614 unblocking the signals.
615 .Sp
616 It's also required by \s-1POSIX\s0 in a threaded program, as libev calls
617 \&\f(CW\*(C`sigprocmask\*(C'\fR, whose behaviour is officially unspecified.
618 .Sp
619 This flag's behaviour will become the default in future versions of libev.
620 .ie n .IP """EVBACKEND_SELECT"" (value 1, portable select backend)" 4
621 .el .IP "\f(CWEVBACKEND_SELECT\fR (value 1, portable select backend)" 4
622 .IX Item "EVBACKEND_SELECT (value 1, portable select backend)"
623 This is your standard \fBselect\fR\|(2) backend. Not \fIcompletely\fR standard, as
624 libev tries to roll its own fd_set with no limits on the number of fds,
625 but if that fails, expect a fairly low limit on the number of fds when
626 using this backend. It doesn't scale too well (O(highest_fd)), but its
627 usually the fastest backend for a low number of (low-numbered :) fds.
628 .Sp
629 To get good performance out of this backend you need a high amount of
630 parallelism (most of the file descriptors should be busy). If you are
631 writing a server, you should \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many
632 connections as possible during one iteration. You might also want to have
633 a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of
634 readiness notifications you get per iteration.
635 .Sp
636 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
637 \&\f(CW\*(C`writefds\*(C'\fR set (and to work around Microsoft Windows bugs, also onto the
638 \&\f(CW\*(C`exceptfds\*(C'\fR set on that platform).
639 .ie n .IP """EVBACKEND_POLL"" (value 2, poll backend, available everywhere except on windows)" 4
640 .el .IP "\f(CWEVBACKEND_POLL\fR (value 2, poll backend, available everywhere except on windows)" 4
641 .IX Item "EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)"
642 And this is your standard \fBpoll\fR\|(2) backend. It's more complicated
643 than select, but handles sparse fds better and has no artificial
644 limit on the number of fds you can use (except it will slow down
645 considerably with a lot of inactive fds). It scales similarly to select,
646 i.e. O(total_fds). See the entry for \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for
647 performance tips.
648 .Sp
649 This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLLHUP\*(C'\fR, and
650 \&\f(CW\*(C`EV_WRITE\*(C'\fR to \f(CW\*(C`POLLOUT | POLLERR | POLLHUP\*(C'\fR.
651 .ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4
652 .el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4
653 .IX Item "EVBACKEND_EPOLL (value 4, Linux)"
654 Use the Linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9
655 kernels).
656 .Sp
657 For few fds, this backend is a bit little slower than poll and select, but
658 it scales phenomenally better. While poll and select usually scale like
659 O(total_fds) where total_fds is the total number of fds (or the highest
660 fd), epoll scales either O(1) or O(active_fds).
661 .Sp
662 The epoll mechanism deserves honorable mention as the most misdesigned
663 of the more advanced event mechanisms: mere annoyances include silently
664 dropping file descriptors, requiring a system call per change per file
665 descriptor (and unnecessary guessing of parameters), problems with dup,
666 returning before the timeout value, resulting in additional iterations
667 (and only giving 5ms accuracy while select on the same platform gives
668 0.1ms) and so on. The biggest issue is fork races, however \- if a program
669 forks then \fIboth\fR parent and child process have to recreate the epoll
670 set, which can take considerable time (one syscall per file descriptor)
671 and is of course hard to detect.
672 .Sp
673 Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work,
674 but of course \fIdoesn't\fR, and epoll just loves to report events for
675 totally \fIdifferent\fR file descriptors (even already closed ones, so
676 one cannot even remove them from the set) than registered in the set
677 (especially on \s-1SMP\s0 systems). Libev tries to counter these spurious
678 notifications by employing an additional generation counter and comparing
679 that against the events to filter out spurious ones, recreating the set
680 when required. Epoll also erroneously rounds down timeouts, but gives you
681 no way to know when and by how much, so sometimes you have to busy-wait
682 because epoll returns immediately despite a nonzero timeout. And last
683 not least, it also refuses to work with some file descriptors which work
684 perfectly fine with \f(CW\*(C`select\*(C'\fR (files, many character devices...).
685 .Sp
686 Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
687 cobbled together in a hurry, no thought to design or interaction with
688 others. Oh, the pain, will it ever stop...
689 .Sp
690 While stopping, setting and starting an I/O watcher in the same iteration
691 will result in some caching, there is still a system call per such
692 incident (because the same \fIfile descriptor\fR could point to a different
693 \&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed
694 file descriptors might not work very well if you register events for both
695 file descriptors.
696 .Sp
697 Best performance from this backend is achieved by not unregistering all
698 watchers for a file descriptor until it has been closed, if possible,
699 i.e. keep at least one watcher active per fd at all times. Stopping and
700 starting a watcher (without re-setting it) also usually doesn't cause
701 extra overhead. A fork can both result in spurious notifications as well
702 as in libev having to destroy and recreate the epoll object, which can
703 take considerable time and thus should be avoided.
704 .Sp
705 All this means that, in practice, \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR can be as fast or
706 faster than epoll for maybe up to a hundred file descriptors, depending on
707 the usage. So sad.
708 .Sp
709 While nominally embeddable in other event loops, this feature is broken in
710 a lot of kernel revisions, but probably(!) works in current versions.
711 .Sp
712 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
713 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
714 .ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4
715 .el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4
716 .IX Item "EVBACKEND_LINUXAIO (value 64, Linux)"
717 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
718 only tries to use it in 4.19+).
719 .Sp
720 This is another Linux train wreck of an event interface.
721 .Sp
722 If this backend works for you (as of this writing, it was very
723 experimental), it is the best event interface available on Linux and might
724 be well worth enabling it \- if it isn't available in your kernel this will
725 be detected and this backend will be skipped.
726 .Sp
727 This backend can batch oneshot requests and supports a user-space ring
728 buffer to receive events. It also doesn't suffer from most of the design
729 problems of epoll (such as not being able to remove event sources from
730 the epoll set), and generally sounds too good to be true. Because, this
731 being the Linux kernel, of course it suffers from a whole new set of
732 limitations, forcing you to fall back to epoll, inheriting all its design
733 issues.
734 .Sp
735 For one, it is not easily embeddable (but probably could be done using
736 an event fd at some extra overhead). It also is subject to a system wide
737 limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR. If no \s-1AIO\s0
738 requests are left, this backend will be skipped during initialisation, and
739 will switch to epoll when the loop is active.
740 .Sp
741 Most problematic in practice, however, is that not all file descriptors
742 work with it. For example, in Linux 5.1, \s-1TCP\s0 sockets, pipes, event fds,
743 files, \fI/dev/null\fR and many others are supported, but ttys do not work
744 properly (a known bug that the kernel developers don't care about, see
745 <https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
746 (yet?) a generic event polling interface.
747 .Sp
748 Overall, it seems the Linux developers just don't want it to have a
749 generic event handling mechanism other than \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`poll\*(C'\fR.
750 .Sp
751 To work around all these problem, the current version of libev uses its
752 epoll backend as a fallback for file descriptor types that do not work. Or
753 falls back completely to epoll if the kernel acts up.
754 .Sp
755 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
756 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
757 .ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
758 .el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4
759 .IX Item "EVBACKEND_KQUEUE (value 8, most BSD clones)"
760 Kqueue deserves special mention, as at the time this backend was
761 implemented, it was broken on all BSDs except NetBSD (usually it doesn't
762 work reliably with anything but sockets and pipes, except on Darwin,
763 where of course it's completely useless). Unlike epoll, however, whose
764 brokenness is by design, these kqueue bugs can be (and mostly have been)
765 fixed without \s-1API\s0 changes to existing programs. For this reason it's not
766 being \*(L"auto-detected\*(R" on all platforms unless you explicitly specify it
767 in the flags (i.e. using \f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a
768 known-to-be-good (\-enough) system like NetBSD.
769 .Sp
770 You still can embed kqueue into a normal poll or select backend and use it
771 only for sockets (after having made sure that sockets work with kqueue on
772 the target platform). See \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
773 .Sp
774 It scales in the same way as the epoll backend, but the interface to the
775 kernel is more efficient (which says nothing about its actual speed, of
776 course). While stopping, setting and starting an I/O watcher does never
777 cause an extra system call as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to
778 two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you
779 might have to leak fds on fork, but it's more sane than epoll) and it
780 drops fds silently in similarly hard-to-detect cases.
781 .Sp
782 This backend usually performs well under most conditions.
783 .Sp
784 While nominally embeddable in other event loops, this doesn't work
785 everywhere, so you might need to test for this. And since it is broken
786 almost everywhere, you should only use it when you have a lot of sockets
787 (for which it usually works), by embedding it into another event loop
788 (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
789 also broken on \s-1OS X\s0)) and, did I mention it, using it only for sockets.
790 .Sp
791 This backend maps \f(CW\*(C`EV_READ\*(C'\fR into an \f(CW\*(C`EVFILT_READ\*(C'\fR kevent with
792 \&\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
793 \&\f(CW\*(C`NOTE_EOF\*(C'\fR.
794 .ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4
795 .el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4
796 .IX Item "EVBACKEND_DEVPOLL (value 16, Solaris 8)"
797 This is not implemented yet (and might never be, unless you send me an
798 implementation). According to reports, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets
799 and is not embeddable, which would limit the usefulness of this backend
800 immensely.
801 .ie n .IP """EVBACKEND_PORT"" (value 32, Solaris 10)" 4
802 .el .IP "\f(CWEVBACKEND_PORT\fR (value 32, Solaris 10)" 4
803 .IX Item "EVBACKEND_PORT (value 32, Solaris 10)"
804 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
805 it's really slow, but it still scales very well (O(active_fds)).
806 .Sp
807 While this backend scales well, it requires one system call per active
808 file descriptor per loop iteration. For small and medium numbers of file
809 descriptors a \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend
810 might perform better.
811 .Sp
812 On the positive side, this backend actually performed fully to
813 specification in all tests and is fully embeddable, which is a rare feat
814 among the OS-specific backends (I vastly prefer correctness over speed
815 hacks).
816 .Sp
817 On the negative side, the interface is \fIbizarre\fR \- so bizarre that
818 even sun itself gets it wrong in their code examples: The event polling
819 function sometimes returns events to the caller even though an error
820 occurred, but with no indication whether it has done so or not (yes, it's
821 even documented that way) \- deadly for edge-triggered interfaces where you
822 absolutely have to know whether an event occurred or not because you have
823 to re-arm the watcher.
824 .Sp
825 Fortunately libev seems to be able to work around these idiocies.
826 .Sp
827 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
828 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
829 .ie n .IP """EVBACKEND_ALL""" 4
830 .el .IP "\f(CWEVBACKEND_ALL\fR" 4
831 .IX Item "EVBACKEND_ALL"
832 Try all backends (even potentially broken ones that wouldn't be tried
833 with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as
834 \&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR.
835 .Sp
836 It is definitely not recommended to use this flag, use whatever
837 \&\f(CW\*(C`ev_recommended_backends ()\*(C'\fR returns, or simply do not specify a backend
838 at all.
839 .ie n .IP """EVBACKEND_MASK""" 4
840 .el .IP "\f(CWEVBACKEND_MASK\fR" 4
841 .IX Item "EVBACKEND_MASK"
842 Not a backend at all, but a mask to select all backend bits from a
843 \&\f(CW\*(C`flags\*(C'\fR value, in case you want to mask out any backends from a flags
844 value (e.g. when modifying the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR environment variable).
845 .RE
846 .RS 4
847 .Sp
848 If one or more of the backend flags are or'ed into the flags value,
849 then only these backends will be tried (in the reverse order as listed
850 here). If none are specified, all backends in \f(CW\*(C`ev_recommended_backends
851 ()\*(C'\fR will be tried.
852 .Sp
853 Example: Try to create a event loop that uses epoll and nothing else.
854 .Sp
855 .Vb 3
856 \& struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
857 \& if (!epoller)
858 \& fatal ("no epoll found here, maybe it hides under your chair");
859 .Ve
860 .Sp
861 Example: Use whatever libev has to offer, but make sure that kqueue is
862 used if available.
863 .Sp
864 .Vb 1
865 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
866 .Ve
867 .Sp
868 Example: Similarly, on linux, you mgiht want to take advantage of the
869 linux aio backend if possible, but fall back to something else if that
870 isn't available.
871 .Sp
872 .Vb 1
873 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
874 .Ve
875 .RE
876 .IP "ev_loop_destroy (loop)" 4
877 .IX Item "ev_loop_destroy (loop)"
878 Destroys an event loop object (frees all memory and kernel state
879 etc.). None of the active event watchers will be stopped in the normal
880 sense, so e.g. \f(CW\*(C`ev_is_active\*(C'\fR might still return true. It is your
881 responsibility to either stop all watchers cleanly yourself \fIbefore\fR
882 calling this function, or cope with the fact afterwards (which is usually
883 the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them
884 for example).
885 .Sp
886 Note that certain global state, such as signal state (and installed signal
887 handlers), will not be freed by this function, and related watchers (such
888 as signal and child watchers) would need to be stopped manually.
889 .Sp
890 This function is normally used on loop objects allocated by
891 \&\f(CW\*(C`ev_loop_new\*(C'\fR, but it can also be used on the default loop returned by
892 \&\f(CW\*(C`ev_default_loop\*(C'\fR, in which case it is not thread-safe.
893 .Sp
894 Note that it is not advisable to call this function on the default loop
895 except in the rare occasion where you really need to free its resources.
896 If you need dynamically allocated loops it is better to use \f(CW\*(C`ev_loop_new\*(C'\fR
897 and \f(CW\*(C`ev_loop_destroy\*(C'\fR.
898 .IP "ev_loop_fork (loop)" 4
899 .IX Item "ev_loop_fork (loop)"
900 This function sets a flag that causes subsequent \f(CW\*(C`ev_run\*(C'\fR iterations
901 to reinitialise the kernel state for backends that have one. Despite
902 the name, you can call it anytime you are allowed to start or stop
903 watchers (except inside an \f(CW\*(C`ev_prepare\*(C'\fR callback), but it makes most
904 sense after forking, in the child process. You \fImust\fR call it (or use
905 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR) in the child before resuming or calling \f(CW\*(C`ev_run\*(C'\fR.
906 .Sp
907 In addition, if you want to reuse a loop (via this function or
908 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR), you \fIalso\fR have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR.
909 .Sp
910 Again, you \fIhave\fR to call it on \fIany\fR loop that you want to re-use after
911 a fork, \fIeven if you do not plan to use the loop in the parent\fR. This is
912 because some kernel interfaces *cough* \fIkqueue\fR *cough* do funny things
913 during fork.
914 .Sp
915 On the other hand, you only need to call this function in the child
916 process if and only if you want to use the event loop in the child. If
917 you just fork+exec or create a new loop in the child, you don't have to
918 call it at all (in fact, \f(CW\*(C`epoll\*(C'\fR is so badly broken that it makes a
919 difference, but libev will usually detect this case on its own and do a
920 costly reset of the backend).
921 .Sp
922 The function itself is quite fast and it's usually not a problem to call
923 it just in case after a fork.
924 .Sp
925 Example: Automate calling \f(CW\*(C`ev_loop_fork\*(C'\fR on the default loop when
926 using pthreads.
927 .Sp
928 .Vb 5
929 \& static void
930 \& post_fork_child (void)
931 \& {
932 \& ev_loop_fork (EV_DEFAULT);
933 \& }
934 \&
935 \& ...
936 \& pthread_atfork (0, 0, post_fork_child);
937 .Ve
938 .IP "int ev_is_default_loop (loop)" 4
939 .IX Item "int ev_is_default_loop (loop)"
940 Returns true when the given loop is, in fact, the default loop, and false
941 otherwise.
942 .IP "unsigned int ev_iteration (loop)" 4
943 .IX Item "unsigned int ev_iteration (loop)"
944 Returns the current iteration count for the event loop, which is identical
945 to the number of times libev did poll for new events. It starts at \f(CW0\fR
946 and happily wraps around with enough iterations.
947 .Sp
948 This value can sometimes be useful as a generation counter of sorts (it
949 \&\*(L"ticks\*(R" the number of loop iterations), as it roughly corresponds with
950 \&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls \- and is incremented between the
951 prepare and check phases.
952 .IP "unsigned int ev_depth (loop)" 4
953 .IX Item "unsigned int ev_depth (loop)"
954 Returns the number of times \f(CW\*(C`ev_run\*(C'\fR was entered minus the number of
955 times \f(CW\*(C`ev_run\*(C'\fR was exited normally, in other words, the recursion depth.
956 .Sp
957 Outside \f(CW\*(C`ev_run\*(C'\fR, this number is zero. In a callback, this number is
958 \&\f(CW1\fR, unless \f(CW\*(C`ev_run\*(C'\fR was invoked recursively (or from another thread),
959 in which case it is higher.
960 .Sp
961 Leaving \f(CW\*(C`ev_run\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread,
962 throwing an exception etc.), doesn't count as \*(L"exit\*(R" \- consider this
963 as a hint to avoid such ungentleman-like behaviour unless it's really
964 convenient, in which case it is fully supported.
965 .IP "unsigned int ev_backend (loop)" 4
966 .IX Item "unsigned int ev_backend (loop)"
967 Returns one of the \f(CW\*(C`EVBACKEND_*\*(C'\fR flags indicating the event backend in
968 use.
969 .IP "ev_tstamp ev_now (loop)" 4
970 .IX Item "ev_tstamp ev_now (loop)"
971 Returns the current \*(L"event loop time\*(R", which is the time the event loop
972 received events and started processing them. This timestamp does not
973 change as long as callbacks are being processed, and this is also the base
974 time used for relative timers. You can treat it as the timestamp of the
975 event occurring (or more correctly, libev finding out about it).
976 .IP "ev_now_update (loop)" 4
977 .IX Item "ev_now_update (loop)"
978 Establishes the current time by querying the kernel, updating the time
979 returned by \f(CW\*(C`ev_now ()\*(C'\fR in the progress. This is a costly operation and
980 is usually done automatically within \f(CW\*(C`ev_run ()\*(C'\fR.
981 .Sp
982 This function is rarely useful, but when some event callback runs for a
983 very long time without entering the event loop, updating libev's idea of
984 the current time is a good idea.
985 .Sp
986 See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.
987 .IP "ev_suspend (loop)" 4
988 .IX Item "ev_suspend (loop)"
989 .PD 0
990 .IP "ev_resume (loop)" 4
991 .IX Item "ev_resume (loop)"
992 .PD
993 These two functions suspend and resume an event loop, for use when the
994 loop is not used for a while and timeouts should not be processed.
995 .Sp
996 A typical use case would be an interactive program such as a game: When
997 the user presses \f(CW\*(C`^Z\*(C'\fR to suspend the game and resumes it an hour later it
998 would be best to handle timeouts as if no time had actually passed while
999 the program was suspended. This can be achieved by calling \f(CW\*(C`ev_suspend\*(C'\fR
1000 in your \f(CW\*(C`SIGTSTP\*(C'\fR handler, sending yourself a \f(CW\*(C`SIGSTOP\*(C'\fR and calling
1001 \&\f(CW\*(C`ev_resume\*(C'\fR directly afterwards to resume timer processing.
1002 .Sp
1003 Effectively, all \f(CW\*(C`ev_timer\*(C'\fR watchers will be delayed by the time spend
1004 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
1005 will be rescheduled (that is, they will lose any events that would have
1006 occurred while suspended).
1007 .Sp
1008 After calling \f(CW\*(C`ev_suspend\*(C'\fR you \fBmust not\fR call \fIany\fR function on the
1009 given loop other than \f(CW\*(C`ev_resume\*(C'\fR, and you \fBmust not\fR call \f(CW\*(C`ev_resume\*(C'\fR
1010 without a previous call to \f(CW\*(C`ev_suspend\*(C'\fR.
1011 .Sp
1012 Calling \f(CW\*(C`ev_suspend\*(C'\fR/\f(CW\*(C`ev_resume\*(C'\fR has the side effect of updating the
1013 event loop time (see \f(CW\*(C`ev_now_update\*(C'\fR).
1014 .IP "bool ev_run (loop, int flags)" 4
1015 .IX Item "bool ev_run (loop, int flags)"
1016 Finally, this is it, the event handler. This function usually is called
1017 after you have initialised all your watchers and you want to start
1018 handling events. It will ask the operating system for any new events, call
1019 the watcher callbacks, and then repeat the whole process indefinitely: This
1020 is why event loops are called \fIloops\fR.
1021 .Sp
1022 If the flags argument is specified as \f(CW0\fR, it will keep handling events
1023 until either no event watchers are active anymore or \f(CW\*(C`ev_break\*(C'\fR was
1024 called.
1025 .Sp
1026 The return value is false if there are no more active watchers (which
1027 usually means \*(L"all jobs done\*(R" or \*(L"deadlock\*(R"), and true in all other cases
1028 (which usually means " you should call \f(CW\*(C`ev_run\*(C'\fR again").
1029 .Sp
1030 Please note that an explicit \f(CW\*(C`ev_break\*(C'\fR is usually better than
1031 relying on all watchers to be stopped when deciding when a program has
1032 finished (especially in interactive programs), but having a program
1033 that automatically loops as long as it has to and no longer by virtue
1034 of relying on its watchers stopping correctly, that is truly a thing of
1035 beauty.
1036 .Sp
1037 This function is \fImostly\fR exception-safe \- you can break out of a
1038 \&\f(CW\*(C`ev_run\*(C'\fR call by calling \f(CW\*(C`longjmp\*(C'\fR in a callback, throwing a \*(C+
1039 exception and so on. This does not decrement the \f(CW\*(C`ev_depth\*(C'\fR value, nor
1040 will it clear any outstanding \f(CW\*(C`EVBREAK_ONE\*(C'\fR breaks.
1041 .Sp
1042 A flags value of \f(CW\*(C`EVRUN_NOWAIT\*(C'\fR will look for new events, will handle
1043 those events and any already outstanding ones, but will not wait and
1044 block your process in case there are no events and will return after one
1045 iteration of the loop. This is sometimes useful to poll and handle new
1046 events while doing lengthy calculations, to keep the program responsive.
1047 .Sp
1048 A flags value of \f(CW\*(C`EVRUN_ONCE\*(C'\fR will look for new events (waiting if
1049 necessary) and will handle those and any already outstanding ones. It
1050 will block your process until at least one new event arrives (which could
1051 be an event internal to libev itself, so there is no guarantee that a
1052 user-registered callback will be called), and will return after one
1053 iteration of the loop.
1054 .Sp
1055 This is useful if you are waiting for some external event in conjunction
1056 with something not expressible using other libev watchers (i.e. "roll your
1057 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
1058 usually a better approach for this kind of thing.
1059 .Sp
1060 Here are the gory details of what \f(CW\*(C`ev_run\*(C'\fR does (this is for your
1061 understanding, not a guarantee that things will work exactly like this in
1062 future versions):
1063 .Sp
1064 .Vb 10
1065 \& \- Increment loop depth.
1066 \& \- Reset the ev_break status.
1067 \& \- Before the first iteration, call any pending watchers.
1068 \& LOOP:
1069 \& \- If EVFLAG_FORKCHECK was used, check for a fork.
1070 \& \- If a fork was detected (by any means), queue and call all fork watchers.
1071 \& \- Queue and call all prepare watchers.
1072 \& \- If ev_break was called, goto FINISH.
1073 \& \- If we have been forked, detach and recreate the kernel state
1074 \& as to not disturb the other process.
1075 \& \- Update the kernel state with all outstanding changes.
1076 \& \- Update the "event loop time" (ev_now ()).
1077 \& \- Calculate for how long to sleep or block, if at all
1078 \& (active idle watchers, EVRUN_NOWAIT or not having
1079 \& any active watchers at all will result in not sleeping).
1080 \& \- Sleep if the I/O and timer collect interval say so.
1081 \& \- Increment loop iteration counter.
1082 \& \- Block the process, waiting for any events.
1083 \& \- Queue all outstanding I/O (fd) events.
1084 \& \- Update the "event loop time" (ev_now ()), and do time jump adjustments.
1085 \& \- Queue all expired timers.
1086 \& \- Queue all expired periodics.
1087 \& \- Queue all idle watchers with priority higher than that of pending events.
1088 \& \- Queue all check watchers.
1089 \& \- Call all queued watchers in reverse order (i.e. check watchers first).
1090 \& Signals and child watchers are implemented as I/O watchers, and will
1091 \& be handled here by queueing them when their watcher gets executed.
1092 \& \- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
1093 \& were used, or there are no active watchers, goto FINISH, otherwise
1094 \& continue with step LOOP.
1095 \& FINISH:
1096 \& \- Reset the ev_break status iff it was EVBREAK_ONE.
1097 \& \- Decrement the loop depth.
1098 \& \- Return.
1099 .Ve
1100 .Sp
1101 Example: Queue some jobs and then loop until no events are outstanding
1102 anymore.
1103 .Sp
1104 .Vb 4
1105 \& ... queue jobs here, make sure they register event watchers as long
1106 \& ... as they still have work to do (even an idle watcher will do..)
1107 \& ev_run (my_loop, 0);
1108 \& ... jobs done or somebody called break. yeah!
1109 .Ve
1110 .IP "ev_break (loop, how)" 4
1111 .IX Item "ev_break (loop, how)"
1112 Can be used to make a call to \f(CW\*(C`ev_run\*(C'\fR return early (but only after it
1113 has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either
1114 \&\f(CW\*(C`EVBREAK_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_run\*(C'\fR call return, or
1115 \&\f(CW\*(C`EVBREAK_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_run\*(C'\fR calls return.
1116 .Sp
1117 This \*(L"break state\*(R" will be cleared on the next call to \f(CW\*(C`ev_run\*(C'\fR.
1118 .Sp
1119 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
1120 which case it will have no effect.
1121 .IP "ev_ref (loop)" 4
1122 .IX Item "ev_ref (loop)"
1123 .PD 0
1124 .IP "ev_unref (loop)" 4
1125 .IX Item "ev_unref (loop)"
1126 .PD
1127 Ref/unref can be used to add or remove a reference count on the event
1128 loop: Every watcher keeps one reference, and as long as the reference
1129 count is nonzero, \f(CW\*(C`ev_run\*(C'\fR will not return on its own.
1130 .Sp
1131 This is useful when you have a watcher that you never intend to
1132 unregister, but that nevertheless should not keep \f(CW\*(C`ev_run\*(C'\fR from
1133 returning. In such a case, call \f(CW\*(C`ev_unref\*(C'\fR after starting, and \f(CW\*(C`ev_ref\*(C'\fR
1134 before stopping it.
1135 .Sp
1136 As an example, libev itself uses this for its internal signal pipe: It
1137 is not visible to the libev user and should not keep \f(CW\*(C`ev_run\*(C'\fR from
1138 exiting if no event watchers registered by it are active. It is also an
1139 excellent way to do this for generic recurring timers or from within
1140 third-party libraries. Just remember to \fIunref after start\fR and \fIref
1141 before stop\fR (but only if the watcher wasn't active before, or was active
1142 before, respectively. Note also that libev might stop watchers itself
1143 (e.g. non-repeating timers) in which case you have to \f(CW\*(C`ev_ref\*(C'\fR
1144 in the callback).
1145 .Sp
1146 Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_run\*(C'\fR
1147 running when nothing else is active.
1148 .Sp
1149 .Vb 4
1150 \& ev_signal exitsig;
1151 \& ev_signal_init (&exitsig, sig_cb, SIGINT);
1152 \& ev_signal_start (loop, &exitsig);
1153 \& ev_unref (loop);
1154 .Ve
1155 .Sp
1156 Example: For some weird reason, unregister the above signal handler again.
1157 .Sp
1158 .Vb 2
1159 \& ev_ref (loop);
1160 \& ev_signal_stop (loop, &exitsig);
1161 .Ve
1162 .IP "ev_set_io_collect_interval (loop, ev_tstamp interval)" 4
1163 .IX Item "ev_set_io_collect_interval (loop, ev_tstamp interval)"
1164 .PD 0
1165 .IP "ev_set_timeout_collect_interval (loop, ev_tstamp interval)" 4
1166 .IX Item "ev_set_timeout_collect_interval (loop, ev_tstamp interval)"
1167 .PD
1168 These advanced functions influence the time that libev will spend waiting
1169 for events. Both time intervals are by default \f(CW0\fR, meaning that libev
1170 will try to invoke timer/periodic callbacks and I/O callbacks with minimum
1171 latency.
1172 .Sp
1173 Setting these to a higher value (the \f(CW\*(C`interval\*(C'\fR \fImust\fR be >= \f(CW0\fR)
1174 allows libev to delay invocation of I/O and timer/periodic callbacks
1175 to increase efficiency of loop iterations (or to increase power-saving
1176 opportunities).
1177 .Sp
1178 The idea is that sometimes your program runs just fast enough to handle
1179 one (or very few) event(s) per loop iteration. While this makes the
1180 program responsive, it also wastes a lot of \s-1CPU\s0 time to poll for new
1181 events, especially with backends like \f(CW\*(C`select ()\*(C'\fR which have a high
1182 overhead for the actual polling but can deliver many events at once.
1183 .Sp
1184 By setting a higher \fIio collect interval\fR you allow libev to spend more
1185 time collecting I/O events, so you can handle more events per iteration,
1186 at the cost of increasing latency. Timeouts (both \f(CW\*(C`ev_periodic\*(C'\fR and
1187 \&\f(CW\*(C`ev_timer\*(C'\fR) will not be affected. Setting this to a non-null value will
1188 introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. The
1189 sleep time ensures that libev will not poll for I/O events more often then
1190 once per this interval, on average (as long as the host time resolution is
1191 good enough).
1192 .Sp
1193 Likewise, by setting a higher \fItimeout collect interval\fR you allow libev
1194 to spend more time collecting timeouts, at the expense of increased
1195 latency/jitter/inexactness (the watcher callback will be called
1196 later). \f(CW\*(C`ev_io\*(C'\fR watchers will not be affected. Setting this to a non-null
1197 value will not introduce any overhead in libev.
1198 .Sp
1199 Many (busy) programs can usually benefit by setting the I/O collect
1200 interval to a value near \f(CW0.1\fR or so, which is often enough for
1201 interactive servers (of course not for games), likewise for timeouts. It
1202 usually doesn't make much sense to set it to a lower value than \f(CW0.01\fR,
1203 as this approaches the timing granularity of most systems. Note that if
1204 you do transactions with the outside world and you can't increase the
1205 parallelity, then this setting will limit your transaction rate (if you
1206 need to poll once per transaction and the I/O collect interval is 0.01,
1207 then you can't do more than 100 transactions per second).
1208 .Sp
1209 Setting the \fItimeout collect interval\fR can improve the opportunity for
1210 saving power, as the program will \*(L"bundle\*(R" timer callback invocations that
1211 are \*(L"near\*(R" in time together, by delaying some, thus reducing the number of
1212 times the process sleeps and wakes up again. Another useful technique to
1213 reduce iterations/wake\-ups is to use \f(CW\*(C`ev_periodic\*(C'\fR watchers and make sure
1214 they fire on, say, one-second boundaries only.
1215 .Sp
1216 Example: we only need 0.1s timeout granularity, and we wish not to poll
1217 more often than 100 times per second:
1218 .Sp
1219 .Vb 2
1220 \& ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
1221 \& ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
1222 .Ve
1223 .IP "ev_invoke_pending (loop)" 4
1224 .IX Item "ev_invoke_pending (loop)"
1225 This call will simply invoke all pending watchers while resetting their
1226 pending state. Normally, \f(CW\*(C`ev_run\*(C'\fR does this automatically when required,
1227 but when overriding the invoke callback this call comes handy. This
1228 function can be invoked from a watcher \- this can be useful for example
1229 when you want to do some lengthy calculation and want to pass further
1230 event handling to another thread (you still have to make sure only one
1231 thread executes within \f(CW\*(C`ev_invoke_pending\*(C'\fR or \f(CW\*(C`ev_run\*(C'\fR of course).
1232 .IP "int ev_pending_count (loop)" 4
1233 .IX Item "int ev_pending_count (loop)"
1234 Returns the number of pending watchers \- zero indicates that no watchers
1235 are pending.
1236 .IP "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(\s-1EV_P\s0))" 4
1237 .IX Item "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))"
1238 This overrides the invoke pending functionality of the loop: Instead of
1239 invoking all pending watchers when there are any, \f(CW\*(C`ev_run\*(C'\fR will call
1240 this callback instead. This is useful, for example, when you want to
1241 invoke the actual watchers inside another context (another thread etc.).
1242 .Sp
1243 If you want to reset the callback, use \f(CW\*(C`ev_invoke_pending\*(C'\fR as new
1244 callback.
1245 .IP "ev_set_loop_release_cb (loop, void (*release)(\s-1EV_P\s0) throw (), void (*acquire)(\s-1EV_P\s0) throw ())" 4
1246 .IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())"
1247 Sometimes you want to share the same loop between multiple threads. This
1248 can be done relatively simply by putting mutex_lock/unlock calls around
1249 each call to a libev function.
1250 .Sp
1251 However, \f(CW\*(C`ev_run\*(C'\fR can run an indefinite time, so it is not feasible
1252 to wait for it to return. One way around this is to wake up the event
1253 loop via \f(CW\*(C`ev_break\*(C'\fR and \f(CW\*(C`ev_async_send\*(C'\fR, another way is to set these
1254 \&\fIrelease\fR and \fIacquire\fR callbacks on the loop.
1255 .Sp
1256 When set, then \f(CW\*(C`release\*(C'\fR will be called just before the thread is
1257 suspended waiting for new events, and \f(CW\*(C`acquire\*(C'\fR is called just
1258 afterwards.
1259 .Sp
1260 Ideally, \f(CW\*(C`release\*(C'\fR will just call your mutex_unlock function, and
1261 \&\f(CW\*(C`acquire\*(C'\fR will just call the mutex_lock function again.
1262 .Sp
1263 While event loop modifications are allowed between invocations of
1264 \&\f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR (that's their only purpose after all), no
1265 modifications done will affect the event loop, i.e. adding watchers will
1266 have no effect on the set of file descriptors being watched, or the time
1267 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
1268 to take note of any changes you made.
1269 .Sp
1270 In theory, threads executing \f(CW\*(C`ev_run\*(C'\fR will be async-cancel safe between
1271 invocations of \f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR.
1272 .Sp
1273 See also the locking example in the \f(CW\*(C`THREADS\*(C'\fR section later in this
1274 document.
1275 .IP "ev_set_userdata (loop, void *data)" 4
1276 .IX Item "ev_set_userdata (loop, void *data)"
1277 .PD 0
1278 .IP "void *ev_userdata (loop)" 4
1279 .IX Item "void *ev_userdata (loop)"
1280 .PD
1281 Set and retrieve a single \f(CW\*(C`void *\*(C'\fR associated with a loop. When
1282 \&\f(CW\*(C`ev_set_userdata\*(C'\fR has never been called, then \f(CW\*(C`ev_userdata\*(C'\fR returns
1283 \&\f(CW0\fR.
1284 .Sp
1285 These two functions can be used to associate arbitrary data with a loop,
1286 and are intended solely for the \f(CW\*(C`invoke_pending_cb\*(C'\fR, \f(CW\*(C`release\*(C'\fR and
1287 \&\f(CW\*(C`acquire\*(C'\fR callbacks described above, but of course can be (ab\-)used for
1288 any other purpose as well.
1289 .IP "ev_verify (loop)" 4
1290 .IX Item "ev_verify (loop)"
1291 This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR support has been
1292 compiled in, which is the default for non-minimal builds. It tries to go
1293 through all internal structures and checks them for validity. If anything
1294 is found to be inconsistent, it will print an error message to standard
1295 error and call \f(CW\*(C`abort ()\*(C'\fR.
1296 .Sp
1297 This can be used to catch bugs inside libev itself: under normal
1298 circumstances, this function will never abort as of course libev keeps its
1299 data structures consistent.
1300 .SH "ANATOMY OF A WATCHER"
1301 .IX Header "ANATOMY OF A WATCHER"
1302 In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the
1303 watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer
1304 watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.
1305 .PP
1306 A watcher is an opaque structure that you allocate and register to record
1307 your interest in some event. To make a concrete example, imagine you want
1308 to wait for \s-1STDIN\s0 to become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher
1309 for that:
1310 .PP
1311 .Vb 5
1312 \& static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
1313 \& {
1314 \& ev_io_stop (w);
1315 \& ev_break (loop, EVBREAK_ALL);
1316 \& }
1317 \&
1318 \& struct ev_loop *loop = ev_default_loop (0);
1319 \&
1320 \& ev_io stdin_watcher;
1321 \&
1322 \& ev_init (&stdin_watcher, my_cb);
1323 \& ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
1324 \& ev_io_start (loop, &stdin_watcher);
1325 \&
1326 \& ev_run (loop, 0);
1327 .Ve
1328 .PP
1329 As you can see, you are responsible for allocating the memory for your
1330 watcher structures (and it is \fIusually\fR a bad idea to do this on the
1331 stack).
1332 .PP
1333 Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR
1334 or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).
1335 .PP
1336 Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init (watcher
1337 *, callback)\*(C'\fR, which expects a callback to be provided. This callback is
1338 invoked each time the event occurs (or, in the case of I/O watchers, each
1339 time the event loop detects that the file descriptor given is readable
1340 and/or writable).
1341 .PP
1342 Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR
1343 macro to configure it, with arguments specific to the watcher type. There
1344 is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.
1345 .PP
1346 To make the watcher actually watch out for events, you have to start it
1347 with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher
1348 *)\*(C'\fR), and you can stop watching for events at any time by calling the
1349 corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.
1350 .PP
1351 As long as your watcher is active (has been started but not stopped) you
1352 must not touch the values stored in it. Most specifically you must never
1353 reinitialise it or call its \f(CW\*(C`ev_TYPE_set\*(C'\fR macro.
1354 .PP
1355 Each and every callback receives the event loop pointer as first, the
1356 registered watcher structure as second, and a bitset of received events as
1357 third argument.
1358 .PP
1359 The received events usually include a single bit per event type received
1360 (you can receive multiple events at the same time). The possible bit masks
1361 are:
1362 .ie n .IP """EV_READ""" 4
1363 .el .IP "\f(CWEV_READ\fR" 4
1364 .IX Item "EV_READ"
1365 .PD 0
1366 .ie n .IP """EV_WRITE""" 4
1367 .el .IP "\f(CWEV_WRITE\fR" 4
1368 .IX Item "EV_WRITE"
1369 .PD
1370 The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or
1371 writable.
1372 .ie n .IP """EV_TIMER""" 4
1373 .el .IP "\f(CWEV_TIMER\fR" 4
1374 .IX Item "EV_TIMER"
1375 The \f(CW\*(C`ev_timer\*(C'\fR watcher has timed out.
1376 .ie n .IP """EV_PERIODIC""" 4
1377 .el .IP "\f(CWEV_PERIODIC\fR" 4
1378 .IX Item "EV_PERIODIC"
1379 The \f(CW\*(C`ev_periodic\*(C'\fR watcher has timed out.
1380 .ie n .IP """EV_SIGNAL""" 4
1381 .el .IP "\f(CWEV_SIGNAL\fR" 4
1382 .IX Item "EV_SIGNAL"
1383 The signal specified in the \f(CW\*(C`ev_signal\*(C'\fR watcher has been received by a thread.
1384 .ie n .IP """EV_CHILD""" 4
1385 .el .IP "\f(CWEV_CHILD\fR" 4
1386 .IX Item "EV_CHILD"
1387 The pid specified in the \f(CW\*(C`ev_child\*(C'\fR watcher has received a status change.
1388 .ie n .IP """EV_STAT""" 4
1389 .el .IP "\f(CWEV_STAT\fR" 4
1390 .IX Item "EV_STAT"
1391 The path specified in the \f(CW\*(C`ev_stat\*(C'\fR watcher changed its attributes somehow.
1392 .ie n .IP """EV_IDLE""" 4
1393 .el .IP "\f(CWEV_IDLE\fR" 4
1394 .IX Item "EV_IDLE"
1395 The \f(CW\*(C`ev_idle\*(C'\fR watcher has determined that you have nothing better to do.
1396 .ie n .IP """EV_PREPARE""" 4
1397 .el .IP "\f(CWEV_PREPARE\fR" 4
1398 .IX Item "EV_PREPARE"
1399 .PD 0
1400 .ie n .IP """EV_CHECK""" 4
1401 .el .IP "\f(CWEV_CHECK\fR" 4
1402 .IX Item "EV_CHECK"
1403 .PD
1404 All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_run\*(C'\fR starts to
1405 gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are queued (not invoked)
1406 just after \f(CW\*(C`ev_run\*(C'\fR has gathered them, but before it queues any callbacks
1407 for any received events. That means \f(CW\*(C`ev_prepare\*(C'\fR watchers are the last
1408 watchers invoked before the event loop sleeps or polls for new events, and
1409 \&\f(CW\*(C`ev_check\*(C'\fR watchers will be invoked before any other watchers of the same
1410 or lower priority within an event loop iteration.
1411 .Sp
1412 Callbacks of both watcher types can start and stop as many watchers as
1413 they want, and all of them will be taken into account (for example, a
1414 \&\f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep \f(CW\*(C`ev_run\*(C'\fR from
1415 blocking).
1416 .ie n .IP """EV_EMBED""" 4
1417 .el .IP "\f(CWEV_EMBED\fR" 4
1418 .IX Item "EV_EMBED"
1419 The embedded event loop specified in the \f(CW\*(C`ev_embed\*(C'\fR watcher needs attention.
1420 .ie n .IP """EV_FORK""" 4
1421 .el .IP "\f(CWEV_FORK\fR" 4
1422 .IX Item "EV_FORK"
1423 The event loop has been resumed in the child process after fork (see
1424 \&\f(CW\*(C`ev_fork\*(C'\fR).
1425 .ie n .IP """EV_CLEANUP""" 4
1426 .el .IP "\f(CWEV_CLEANUP\fR" 4
1427 .IX Item "EV_CLEANUP"
1428 The event loop is about to be destroyed (see \f(CW\*(C`ev_cleanup\*(C'\fR).
1429 .ie n .IP """EV_ASYNC""" 4
1430 .el .IP "\f(CWEV_ASYNC\fR" 4
1431 .IX Item "EV_ASYNC"
1432 The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).
1433 .ie n .IP """EV_CUSTOM""" 4
1434 .el .IP "\f(CWEV_CUSTOM\fR" 4
1435 .IX Item "EV_CUSTOM"
1436 Not ever sent (or otherwise used) by libev itself, but can be freely used
1437 by libev users to signal watchers (e.g. via \f(CW\*(C`ev_feed_event\*(C'\fR).
1438 .ie n .IP """EV_ERROR""" 4
1439 .el .IP "\f(CWEV_ERROR\fR" 4
1440 .IX Item "EV_ERROR"
1441 An unspecified error has occurred, the watcher has been stopped. This might
1442 happen because the watcher could not be properly started because libev
1443 ran out of memory, a file descriptor was found to be closed or any other
1444 problem. Libev considers these application bugs.
1445 .Sp
1446 You best act on it by reporting the problem and somehow coping with the
1447 watcher being stopped. Note that well-written programs should not receive
1448 an error ever, so when your watcher receives it, this usually indicates a
1449 bug in your program.
1450 .Sp
1451 Libev will usually signal a few \*(L"dummy\*(R" events together with an error, for
1452 example it might indicate that a fd is readable or writable, and if your
1453 callbacks is well-written it can just attempt the operation and cope with
1454 the error from \fBread()\fR or \fBwrite()\fR. This will not work in multi-threaded
1455 programs, though, as the fd could already be closed and reused for another
1456 thing, so beware.
1457 .SS "\s-1GENERIC WATCHER FUNCTIONS\s0"
1458 .IX Subsection "GENERIC WATCHER FUNCTIONS"
1459 .ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4
1460 .el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4
1461 .IX Item "ev_init (ev_TYPE *watcher, callback)"
1462 This macro initialises the generic portion of a watcher. The contents
1463 of the watcher object can be arbitrary (so \f(CW\*(C`malloc\*(C'\fR will do). Only
1464 the generic parts of the watcher are initialised, you \fIneed\fR to call
1465 the type-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR macro afterwards to initialise the
1466 type-specific parts. For each type there is also a \f(CW\*(C`ev_TYPE_init\*(C'\fR macro
1467 which rolls both calls into one.
1468 .Sp
1469 You can reinitialise a watcher at any time as long as it has been stopped
1470 (or never started) and there are no pending events outstanding.
1471 .Sp
1472 The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
1473 int revents)\*(C'\fR.
1474 .Sp
1475 Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
1476 .Sp
1477 .Vb 3
1478 \& ev_io w;
1479 \& ev_init (&w, my_cb);
1480 \& ev_io_set (&w, STDIN_FILENO, EV_READ);
1481 .Ve
1482 .ie n .IP """ev_TYPE_set"" (ev_TYPE *watcher, [args])" 4
1483 .el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *watcher, [args])" 4
1484 .IX Item "ev_TYPE_set (ev_TYPE *watcher, [args])"
1485 This macro initialises the type-specific parts of a watcher. You need to
1486 call \f(CW\*(C`ev_init\*(C'\fR at least once before you call this macro, but you can
1487 call \f(CW\*(C`ev_TYPE_set\*(C'\fR any number of times. You must not, however, call this
1488 macro on a watcher that is active (it can be pending, however, which is a
1489 difference to the \f(CW\*(C`ev_init\*(C'\fR macro).
1490 .Sp
1491 Although some watcher types do not have type-specific arguments
1492 (e.g. \f(CW\*(C`ev_prepare\*(C'\fR) you still need to call its \f(CW\*(C`set\*(C'\fR macro.
1493 .Sp
1494 See \f(CW\*(C`ev_init\*(C'\fR, above, for an example.
1495 .ie n .IP """ev_TYPE_init"" (ev_TYPE *watcher, callback, [args])" 4
1496 .el .IP "\f(CWev_TYPE_init\fR (ev_TYPE *watcher, callback, [args])" 4
1497 .IX Item "ev_TYPE_init (ev_TYPE *watcher, callback, [args])"
1498 This convenience macro rolls both \f(CW\*(C`ev_init\*(C'\fR and \f(CW\*(C`ev_TYPE_set\*(C'\fR macro
1499 calls into a single call. This is the most convenient method to initialise
1500 a watcher. The same limitations apply, of course.
1501 .Sp
1502 Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.
1503 .Sp
1504 .Vb 1
1505 \& ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
1506 .Ve
1507 .ie n .IP """ev_TYPE_start"" (loop, ev_TYPE *watcher)" 4
1508 .el .IP "\f(CWev_TYPE_start\fR (loop, ev_TYPE *watcher)" 4
1509 .IX Item "ev_TYPE_start (loop, ev_TYPE *watcher)"
1510 Starts (activates) the given watcher. Only active watchers will receive
1511 events. If the watcher is already active nothing will happen.
1512 .Sp
1513 Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this
1514 whole section.
1515 .Sp
1516 .Vb 1
1517 \& ev_io_start (EV_DEFAULT_UC, &w);
1518 .Ve
1519 .ie n .IP """ev_TYPE_stop"" (loop, ev_TYPE *watcher)" 4
1520 .el .IP "\f(CWev_TYPE_stop\fR (loop, ev_TYPE *watcher)" 4
1521 .IX Item "ev_TYPE_stop (loop, ev_TYPE *watcher)"
1522 Stops the given watcher if active, and clears the pending status (whether
1523 the watcher was active or not).
1524 .Sp
1525 It is possible that stopped watchers are pending \- for example,
1526 non-repeating timers are being stopped when they become pending \- but
1527 calling \f(CW\*(C`ev_TYPE_stop\*(C'\fR ensures that the watcher is neither active nor
1528 pending. If you want to free or reuse the memory used by the watcher it is
1529 therefore a good idea to always call its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function.
1530 .IP "bool ev_is_active (ev_TYPE *watcher)" 4
1531 .IX Item "bool ev_is_active (ev_TYPE *watcher)"
1532 Returns a true value iff the watcher is active (i.e. it has been started
1533 and not yet been stopped). As long as a watcher is active you must not modify
1534 it.
1535 .IP "bool ev_is_pending (ev_TYPE *watcher)" 4
1536 .IX Item "bool ev_is_pending (ev_TYPE *watcher)"
1537 Returns a true value iff the watcher is pending, (i.e. it has outstanding
1538 events but its callback has not yet been invoked). As long as a watcher
1539 is pending (but not active) you must not call an init function on it (but
1540 \&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must
1541 make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR
1542 it).
1543 .IP "callback ev_cb (ev_TYPE *watcher)" 4
1544 .IX Item "callback ev_cb (ev_TYPE *watcher)"
1545 Returns the callback currently set on the watcher.
1546 .IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
1547 .IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
1548 Change the callback. You can change the callback at virtually any time
1549 (modulo threads).
1550 .IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
1551 .IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
1552 .PD 0
1553 .IP "int ev_priority (ev_TYPE *watcher)" 4
1554 .IX Item "int ev_priority (ev_TYPE *watcher)"
1555 .PD
1556 Set and query the priority of the watcher. The priority is a small
1557 integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR
1558 (default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked
1559 before watchers with lower priority, but priority will not keep watchers
1560 from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).
1561 .Sp
1562 If you need to suppress invocation when higher priority events are pending
1563 you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.
1564 .Sp
1565 You \fImust not\fR change the priority of a watcher as long as it is active or
1566 pending.
1567 .Sp
1568 Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
1569 fine, as long as you do not mind that the priority value you query might
1570 or might not have been clamped to the valid range.
1571 .Sp
1572 The default priority used by watchers when no priority has been set is
1573 always \f(CW0\fR, which is supposed to not be too high and not be too low :).
1574 .Sp
1575 See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
1576 priorities.
1577 .IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
1578 .IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
1579 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
1580 \&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback
1581 can deal with that fact, as both are simply passed through to the
1582 callback.
1583 .IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4
1584 .IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"
1585 If the watcher is pending, this function clears its pending status and
1586 returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the
1587 watcher isn't pending it does nothing and returns \f(CW0\fR.
1588 .Sp
1589 Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its
1590 callback to be invoked, which can be accomplished with this function.
1591 .IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
1592 .IX Item "ev_feed_event (loop, ev_TYPE *watcher, int revents)"
1593 Feeds the given event set into the event loop, as if the specified event
1594 had happened for the specified watcher (which must be a pointer to an
1595 initialised but not necessarily started event watcher). Obviously you must
1596 not free the watcher as long as it has pending events.
1597 .Sp
1598 Stopping the watcher, letting libev invoke it, or calling
1599 \&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
1600 not started in the first place.
1601 .Sp
1602 See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
1603 functions that do not need a watcher.
1604 .PP
1605 See also the \*(L"\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\*(R"\s0 and \*(L"\s-1BUILDING YOUR
1606 OWN COMPOSITE WATCHERS\*(R"\s0 idioms.
1607 .SS "\s-1WATCHER STATES\s0"
1608 .IX Subsection "WATCHER STATES"
1609 There are various watcher states mentioned throughout this manual \-
1610 active, pending and so on. In this section these states and the rules to
1611 transition between them will be described in more detail \- and while these
1612 rules might look complicated, they usually do \*(L"the right thing\*(R".
1613 .IP "initialised" 4
1614 .IX Item "initialised"
1615 Before a watcher can be registered with the event loop it has to be
1616 initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
1617 \&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR function.
1618 .Sp
1619 In this state it is simply some block of memory that is suitable for
1620 use in an event loop. It can be moved around, freed, reused etc. at
1621 will \- as long as you either keep the memory contents intact, or call
1622 \&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
1623 .IP "started/running/active" 4
1624 .IX Item "started/running/active"
1625 Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR it becomes
1626 property of the event loop, and is actively waiting for events. While in
1627 this state it cannot be accessed (except in a few documented ways), moved,
1628 freed or anything else \- the only legal thing is to keep a pointer to it,
1629 and call libev functions on it that are documented to work on active watchers.
1630 .IP "pending" 4
1631 .IX Item "pending"
1632 If a watcher is active and libev determines that an event it is interested
1633 in has occurred (such as a timer expiring), it will become pending. It will
1634 stay in this pending state until either it is stopped or its callback is
1635 about to be invoked, so it is not normally pending inside the watcher
1636 callback.
1637 .Sp
1638 The watcher might or might not be active while it is pending (for example,
1639 an expired non-repeating timer can be pending but no longer active). If it
1640 is stopped, it can be freely accessed (e.g. by calling \f(CW\*(C`ev_TYPE_set\*(C'\fR),
1641 but it is still property of the event loop at this time, so cannot be
1642 moved, freed or reused. And if it is active the rules described in the
1643 previous item still apply.
1644 .Sp
1645 It is also possible to feed an event on a watcher that is not active (e.g.
1646 via \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
1647 active.
1648 .IP "stopped" 4
1649 .IX Item "stopped"
1650 A watcher can be stopped implicitly by libev (in which case it might still
1651 be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function. The
1652 latter will clear any pending state the watcher might be in, regardless
1653 of whether it was active or not, so stopping a watcher explicitly before
1654 freeing it is often a good idea.
1655 .Sp
1656 While stopped (and not pending) the watcher is essentially in the
1657 initialised state, that is, it can be reused, moved, modified in any way
1658 you wish (but when you trash the memory block, you need to \f(CW\*(C`ev_TYPE_init\*(C'\fR
1659 it again).
1660 .SS "\s-1WATCHER PRIORITY MODELS\s0"
1661 .IX Subsection "WATCHER PRIORITY MODELS"
1662 Many event loops support \fIwatcher priorities\fR, which are usually small
1663 integers that influence the ordering of event callback invocation
1664 between watchers in some way, all else being equal.
1665 .PP
1666 In libev, watcher priorities can be set using \f(CW\*(C`ev_set_priority\*(C'\fR. See its
1667 description for the more technical details such as the actual priority
1668 range.
1669 .PP
1670 There are two common ways how these these priorities are being interpreted
1671 by event loops:
1672 .PP
1673 In the more common lock-out model, higher priorities \*(L"lock out\*(R" invocation
1674 of lower priority watchers, which means as long as higher priority
1675 watchers receive events, lower priority watchers are not being invoked.
1676 .PP
1677 The less common only-for-ordering model uses priorities solely to order
1678 callback invocation within a single event loop iteration: Higher priority
1679 watchers are invoked before lower priority ones, but they all get invoked
1680 before polling for new events.
1681 .PP
1682 Libev uses the second (only-for-ordering) model for all its watchers
1683 except for idle watchers (which use the lock-out model).
1684 .PP
1685 The rationale behind this is that implementing the lock-out model for
1686 watchers is not well supported by most kernel interfaces, and most event
1687 libraries will just poll for the same events again and again as long as
1688 their callbacks have not been executed, which is very inefficient in the
1689 common case of one high-priority watcher locking out a mass of lower
1690 priority ones.
1691 .PP
1692 Static (ordering) priorities are most useful when you have two or more
1693 watchers handling the same resource: a typical usage example is having an
1694 \&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR to handle
1695 timeouts. Under load, data might be received while the program handles
1696 other jobs, but since timers normally get invoked first, the timeout
1697 handler will be executed before checking for data. In that case, giving
1698 the timer a lower priority than the I/O watcher ensures that I/O will be
1699 handled first even under adverse conditions (which is usually, but not
1700 always, what you want).
1701 .PP
1702 Since idle watchers use the \*(L"lock-out\*(R" model, meaning that idle watchers
1703 will only be executed when no same or higher priority watchers have
1704 received events, they can be used to implement the \*(L"lock-out\*(R" model when
1705 required.
1706 .PP
1707 For example, to emulate how many other event libraries handle priorities,
1708 you can associate an \f(CW\*(C`ev_idle\*(C'\fR watcher to each such watcher, and in
1709 the normal watcher callback, you just start the idle watcher. The real
1710 processing is done in the idle watcher callback. This causes libev to
1711 continuously poll and process kernel event data for the watcher, but when
1712 the lock-out case is known to be rare (which in turn is rare :), this is
1713 workable.
1714 .PP
1715 Usually, however, the lock-out model implemented that way will perform
1716 miserably under the type of load it was designed to handle. In that case,
1717 it might be preferable to stop the real watcher before starting the
1718 idle watcher, so the kernel will not have to process the event in case
1719 the actual processing will be delayed for considerable time.
1720 .PP
1721 Here is an example of an I/O watcher that should run at a strictly lower
1722 priority than the default, and which should only process data when no
1723 other events are pending:
1724 .PP
1725 .Vb 2
1726 \& ev_idle idle; // actual processing watcher
1727 \& ev_io io; // actual event watcher
1728 \&
1729 \& static void
1730 \& io_cb (EV_P_ ev_io *w, int revents)
1731 \& {
1732 \& // stop the I/O watcher, we received the event, but
1733 \& // are not yet ready to handle it.
1734 \& ev_io_stop (EV_A_ w);
1735 \&
1736 \& // start the idle watcher to handle the actual event.
1737 \& // it will not be executed as long as other watchers
1738 \& // with the default priority are receiving events.
1739 \& ev_idle_start (EV_A_ &idle);
1740 \& }
1741 \&
1742 \& static void
1743 \& idle_cb (EV_P_ ev_idle *w, int revents)
1744 \& {
1745 \& // actual processing
1746 \& read (STDIN_FILENO, ...);
1747 \&
1748 \& // have to start the I/O watcher again, as
1749 \& // we have handled the event
1750 \& ev_io_start (EV_P_ &io);
1751 \& }
1752 \&
1753 \& // initialisation
1754 \& ev_idle_init (&idle, idle_cb);
1755 \& ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1756 \& ev_io_start (EV_DEFAULT_ &io);
1757 .Ve
1758 .PP
1759 In the \*(L"real\*(R" world, it might also be beneficial to start a timer, so that
1760 low-priority connections can not be locked out forever under load. This
1761 enables your program to keep a lower latency for important connections
1762 during short periods of high load, while not completely locking out less
1763 important ones.
1764 .SH "WATCHER TYPES"
1765 .IX Header "WATCHER TYPES"
1766 This section describes each watcher in detail, but will not repeat
1767 information given in the last section. Any initialisation/set macros,
1768 functions and members specific to the watcher type are explained.
1769 .PP
1770 Members are additionally marked with either \fI[read\-only]\fR, meaning that,
1771 while the watcher is active, you can look at the member and expect some
1772 sensible content, but you must not modify it (you can modify it while the
1773 watcher is stopped to your hearts content), or \fI[read\-write]\fR, which
1774 means you can expect it to have some sensible content while the watcher
1775 is active, but you can also modify it. Modifying it may not do something
1776 sensible or take immediate effect (or do anything at all), but libev will
1777 not crash or malfunction in any way.
1778 .ie n .SS """ev_io"" \- is this file descriptor readable or writable?"
1779 .el .SS "\f(CWev_io\fP \- is this file descriptor readable or writable?"
1780 .IX Subsection "ev_io - is this file descriptor readable or writable?"
1781 I/O watchers check whether a file descriptor is readable or writable
1782 in each iteration of the event loop, or, more precisely, when reading
1783 would not block the process and writing would at least be able to write
1784 some data. This behaviour is called level-triggering because you keep
1785 receiving events as long as the condition persists. Remember you can stop
1786 the watcher if you don't want to act on the event and neither want to
1787 receive future events.
1788 .PP
1789 In general you can register as many read and/or write event watchers per
1790 fd as you want (as long as you don't confuse yourself). Setting all file
1791 descriptors to non-blocking mode is also usually a good idea (but not
1792 required if you know what you are doing).
1793 .PP
1794 Another thing you have to watch out for is that it is quite easy to
1795 receive \*(L"spurious\*(R" readiness notifications, that is, your callback might
1796 be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block
1797 because there is no data. It is very easy to get into this situation even
1798 with a relatively standard program structure. Thus it is best to always
1799 use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning \f(CW\*(C`EAGAIN\*(C'\fR is far
1800 preferable to a program hanging until some data arrives.
1801 .PP
1802 If you cannot run the fd in non-blocking mode (for example you should
1803 not play around with an Xlib connection), then you have to separately
1804 re-test whether a file descriptor is really ready with a known-to-be good
1805 interface such as poll (fortunately in the case of Xlib, it already does
1806 this on its own, so its quite safe to use). Some people additionally
1807 use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
1808 indefinitely.
1809 .PP
1810 But really, best use non-blocking mode.
1811 .PP
1812 \fIThe special problem of disappearing file descriptors\fR
1813 .IX Subsection "The special problem of disappearing file descriptors"
1814 .PP
1815 Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
1816 a file descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other
1817 means, such as \f(CW\*(C`dup2\*(C'\fR). The reason is that you register interest in some
1818 file descriptor, but when it goes away, the operating system will silently
1819 drop this interest. If another file descriptor with the same number then
1820 is registered with libev, there is no efficient way to see that this is,
1821 in fact, a different file descriptor.
1822 .PP
1823 To avoid having to explicitly tell libev about such cases, libev follows
1824 the following policy: Each time \f(CW\*(C`ev_io_set\*(C'\fR is being called, libev
1825 will assume that this is potentially a new file descriptor, otherwise
1826 it is assumed that the file descriptor stays the same. That means that
1827 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
1828 descriptor even if the file descriptor number itself did not change.
1829 .PP
1830 This is how one would do it normally anyway, the important point is that
1831 the libev application should not optimise around libev but should leave
1832 optimisations to libev.
1833 .PP
1834 \fIThe special problem of dup'ed file descriptors\fR
1835 .IX Subsection "The special problem of dup'ed file descriptors"
1836 .PP
1837 Some backends (e.g. epoll), cannot register events for file descriptors,
1838 but only events for the underlying file descriptions. That means when you
1839 have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register
1840 events for them, only one file descriptor might actually receive events.
1841 .PP
1842 There is no workaround possible except not registering events
1843 for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
1844 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1845 .PP
1846 \fIThe special problem of files\fR
1847 .IX Subsection "The special problem of files"
1848 .PP
1849 Many people try to use \f(CW\*(C`select\*(C'\fR (or libev) on file descriptors
1850 representing files, and expect it to become ready when their program
1851 doesn't block on disk accesses (which can take a long time on their own).
1852 .PP
1853 However, this cannot ever work in the \*(L"expected\*(R" way \- you get a readiness
1854 notification as soon as the kernel knows whether and how much data is
1855 there, and in the case of open files, that's always the case, so you
1856 always get a readiness notification instantly, and your read (or possibly
1857 write) will still block on the disk I/O.
1858 .PP
1859 Another way to view it is that in the case of sockets, pipes, character
1860 devices and so on, there is another party (the sender) that delivers data
1861 on its own, but in the case of files, there is no such thing: the disk
1862 will not send data on its own, simply because it doesn't know what you
1863 wish to read \- you would first have to request some data.
1864 .PP
1865 Since files are typically not-so-well supported by advanced notification
1866 mechanism, libev tries hard to emulate \s-1POSIX\s0 behaviour with respect
1867 to files, even though you should not use it. The reason for this is
1868 convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
1869 usually a tty, often a pipe, but also sometimes files or special devices
1870 (for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
1871 \&\fI/dev/urandom\fR), and even though the file might better be served with
1872 asynchronous I/O instead of with non-blocking I/O, it is still useful when
1873 it \*(L"just works\*(R" instead of freezing.
1874 .PP
1875 So avoid file descriptors pointing to files when you know it (e.g. use
1876 libeio), but use them when it is convenient, e.g. for \s-1STDIN/STDOUT,\s0 or
1877 when you rarely read from a file instead of from a socket, and want to
1878 reuse the same code path.
1879 .PP
1880 \fIThe special problem of fork\fR
1881 .IX Subsection "The special problem of fork"
1882 .PP
1883 Some backends (epoll, kqueue, linuxaio, iouring) do not support \f(CW\*(C`fork ()\*(C'\fR
1884 at all or exhibit useless behaviour. Libev fully supports fork, but needs
1885 to be told about it in the child if you want to continue to use it in the
1886 child.
1887 .PP
1888 To support fork in your child processes, you have to call \f(CW\*(C`ev_loop_fork
1889 ()\*(C'\fR after a fork in the child, enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to
1890 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1891 .PP
1892 \fIThe special problem of \s-1SIGPIPE\s0\fR
1893 .IX Subsection "The special problem of SIGPIPE"
1894 .PP
1895 While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:
1896 when writing to a pipe whose other end has been closed, your program gets
1897 sent a \s-1SIGPIPE,\s0 which, by default, aborts your program. For most programs
1898 this is sensible behaviour, for daemons, this is usually undesirable.
1899 .PP
1900 So when you encounter spurious, unexplained daemon exits, make sure you
1901 ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon
1902 somewhere, as that would have given you a big clue).
1903 .PP
1904 \fIThe special problem of \f(BIaccept()\fIing when you can't\fR
1905 .IX Subsection "The special problem of accept()ing when you can't"
1906 .PP
1907 Many implementations of the \s-1POSIX\s0 \f(CW\*(C`accept\*(C'\fR function (for example,
1908 found in post\-2004 Linux) have the peculiar behaviour of not removing a
1909 connection from the pending queue in all error cases.
1910 .PP
1911 For example, larger servers often run out of file descriptors (because
1912 of resource limits), causing \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR but not
1913 rejecting the connection, leading to libev signalling readiness on
1914 the next iteration again (the connection still exists after all), and
1915 typically causing the program to loop at 100% \s-1CPU\s0 usage.
1916 .PP
1917 Unfortunately, the set of errors that cause this issue differs between
1918 operating systems, there is usually little the app can do to remedy the
1919 situation, and no known thread-safe method of removing the connection to
1920 cope with overload is known (to me).
1921 .PP
1922 One of the easiest ways to handle this situation is to just ignore it
1923 \&\- when the program encounters an overload, it will just loop until the
1924 situation is over. While this is a form of busy waiting, no \s-1OS\s0 offers an
1925 event-based way to handle this situation, so it's the best one can do.
1926 .PP
1927 A better way to handle the situation is to log any errors other than
1928 \&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, making sure not to flood the log with such
1929 messages, and continue as usual, which at least gives the user an idea of
1930 what could be wrong (\*(L"raise the ulimit!\*(R"). For extra points one could stop
1931 the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
1932 usage.
1933 .PP
1934 If your program is single-threaded, then you could also keep a dummy file
1935 descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
1936 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,
1937 close that fd, and create a new dummy fd. This will gracefully refuse
1938 clients under typical overload conditions.
1939 .PP
1940 The last way to handle it is to simply log the error and \f(CW\*(C`exit\*(C'\fR, as
1941 is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
1942 opportunity for a DoS attack.
1943 .PP
1944 \fIWatcher-Specific Functions\fR
1945 .IX Subsection "Watcher-Specific Functions"
1946 .IP "ev_io_init (ev_io *, callback, int fd, int events)" 4
1947 .IX Item "ev_io_init (ev_io *, callback, int fd, int events)"
1948 .PD 0
1949 .IP "ev_io_set (ev_io *, int fd, int events)" 4
1950 .IX Item "ev_io_set (ev_io *, int fd, int events)"
1951 .PD
1952 Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to
1953 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 or
1954 \&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR, to express the desire to receive the given events.
1955 .IP "int fd [read\-only]" 4
1956 .IX Item "int fd [read-only]"
1957 The file descriptor being watched.
1958 .IP "int events [read\-only]" 4
1959 .IX Item "int events [read-only]"
1960 The events being watched.
1961 .PP
1962 \fIExamples\fR
1963 .IX Subsection "Examples"
1964 .PP
1965 Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well
1966 readable, but only once. Since it is likely line-buffered, you could
1967 attempt to read a whole line in the callback.
1968 .PP
1969 .Vb 6
1970 \& static void
1971 \& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1972 \& {
1973 \& ev_io_stop (loop, w);
1974 \& .. read from stdin here (or from w\->fd) and handle any I/O errors
1975 \& }
1976 \&
1977 \& ...
1978 \& struct ev_loop *loop = ev_default_init (0);
1979 \& ev_io stdin_readable;
1980 \& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
1981 \& ev_io_start (loop, &stdin_readable);
1982 \& ev_run (loop, 0);
1983 .Ve
1984 .ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
1985 .el .SS "\f(CWev_timer\fP \- relative and optionally repeating timeouts"
1986 .IX Subsection "ev_timer - relative and optionally repeating timeouts"
1987 Timer watchers are simple relative timers that generate an event after a
1988 given time, and optionally repeating in regular intervals after that.
1989 .PP
1990 The timers are based on real time, that is, if you register an event that
1991 times out after an hour and you reset your system clock to January last
1992 year, it will still time out after (roughly) one hour. \*(L"Roughly\*(R" because
1993 detecting time jumps is hard, and some inaccuracies are unavoidable (the
1994 monotonic clock option helps a lot here).
1995 .PP
1996 The callback is guaranteed to be invoked only \fIafter\fR its timeout has
1997 passed (not \fIat\fR, so on systems with very low-resolution clocks this
1998 might introduce a small delay, see \*(L"the special problem of being too
1999 early\*(R", below). If multiple timers become ready during the same loop
2000 iteration then the ones with earlier time-out values are invoked before
2001 ones of the same priority with later time-out values (but this is no
2002 longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2003 .PP
2004 \fIBe smart about timeouts\fR
2005 .IX Subsection "Be smart about timeouts"
2006 .PP
2007 Many real-world problems involve some kind of timeout, usually for error
2008 recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
2009 you want to raise some error after a while.
2010 .PP
2011 What follows are some ways to handle this problem, from obvious and
2012 inefficient to smart and efficient.
2013 .PP
2014 In the following, a 60 second activity timeout is assumed \- a timeout that
2015 gets reset to 60 seconds each time there is activity (e.g. each time some
2016 data or other life sign was received).
2017 .IP "1. Use a timer and stop, reinitialise and start it on activity." 4
2018 .IX Item "1. Use a timer and stop, reinitialise and start it on activity."
2019 This is the most obvious, but not the most simple way: In the beginning,
2020 start the watcher:
2021 .Sp
2022 .Vb 2
2023 \& ev_timer_init (timer, callback, 60., 0.);
2024 \& ev_timer_start (loop, timer);
2025 .Ve
2026 .Sp
2027 Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
2028 and start it again:
2029 .Sp
2030 .Vb 3
2031 \& ev_timer_stop (loop, timer);
2032 \& ev_timer_set (timer, 60., 0.);
2033 \& ev_timer_start (loop, timer);
2034 .Ve
2035 .Sp
2036 This is relatively simple to implement, but means that each time there is
2037 some activity, libev will first have to remove the timer from its internal
2038 data structure and then add it again. Libev tries to be fast, but it's
2039 still not a constant-time operation.
2040 .ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
2041 .el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
2042 .IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
2043 This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
2044 \&\f(CW\*(C`ev_timer_start\*(C'\fR.
2045 .Sp
2046 To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
2047 of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
2048 successfully read or write some data. If you go into an idle state where
2049 you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
2050 the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
2051 .Sp
2052 That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
2053 \&\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
2054 member and \f(CW\*(C`ev_timer_again\*(C'\fR.
2055 .Sp
2056 At start:
2057 .Sp
2058 .Vb 3
2059 \& ev_init (timer, callback);
2060 \& timer\->repeat = 60.;
2061 \& ev_timer_again (loop, timer);
2062 .Ve
2063 .Sp
2064 Each time there is some activity:
2065 .Sp
2066 .Vb 1
2067 \& ev_timer_again (loop, timer);
2068 .Ve
2069 .Sp
2070 It is even possible to change the time-out on the fly, regardless of
2071 whether the watcher is active or not:
2072 .Sp
2073 .Vb 2
2074 \& timer\->repeat = 30.;
2075 \& ev_timer_again (loop, timer);
2076 .Ve
2077 .Sp
2078 This is slightly more efficient then stopping/starting the timer each time
2079 you want to modify its timeout value, as libev does not have to completely
2080 remove and re-insert the timer from/into its internal data structure.
2081 .Sp
2082 It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
2083 .IP "3. Let the timer time out, but then re-arm it as required." 4
2084 .IX Item "3. Let the timer time out, but then re-arm it as required."
2085 This method is more tricky, but usually most efficient: Most timeouts are
2086 relatively long compared to the intervals between other activity \- in
2087 our example, within 60 seconds, there are usually many I/O events with
2088 associated activity resets.
2089 .Sp
2090 In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
2091 but remember the time of last activity, and check for a real timeout only
2092 within the callback:
2093 .Sp
2094 .Vb 3
2095 \& ev_tstamp timeout = 60.;
2096 \& ev_tstamp last_activity; // time of last activity
2097 \& ev_timer timer;
2098 \&
2099 \& static void
2100 \& callback (EV_P_ ev_timer *w, int revents)
2101 \& {
2102 \& // calculate when the timeout would happen
2103 \& ev_tstamp after = last_activity \- ev_now (EV_A) + timeout;
2104 \&
2105 \& // if negative, it means we the timeout already occurred
2106 \& if (after < 0.)
2107 \& {
2108 \& // timeout occurred, take action
2109 \& }
2110 \& else
2111 \& {
2112 \& // callback was invoked, but there was some recent
2113 \& // activity. simply restart the timer to time out
2114 \& // after "after" seconds, which is the earliest time
2115 \& // the timeout can occur.
2116 \& ev_timer_set (w, after, 0.);
2117 \& ev_timer_start (EV_A_ w);
2118 \& }
2119 \& }
2120 .Ve
2121 .Sp
2122 To summarise the callback: first calculate in how many seconds the
2123 timeout will occur (by calculating the absolute time when it would occur,
2124 \&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
2125 (EV_A)\*(C'\fR from that).
2126 .Sp
2127 If this value is negative, then we are already past the timeout, i.e. we
2128 timed out, and need to do whatever is needed in this case.
2129 .Sp
2130 Otherwise, we now the earliest time at which the timeout would trigger,
2131 and simply start the timer with this timeout value.
2132 .Sp
2133 In other words, each time the callback is invoked it will check whether
2134 the timeout occurred. If not, it will simply reschedule itself to check
2135 again at the earliest time it could time out. Rinse. Repeat.
2136 .Sp
2137 This scheme causes more callback invocations (about one every 60 seconds
2138 minus half the average time between activity), but virtually no calls to
2139 libev to change the timeout.
2140 .Sp
2141 To start the machinery, simply initialise the watcher and set
2142 \&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
2143 now), then call the callback, which will \*(L"do the right thing\*(R" and start
2144 the timer:
2145 .Sp
2146 .Vb 3
2147 \& last_activity = ev_now (EV_A);
2148 \& ev_init (&timer, callback);
2149 \& callback (EV_A_ &timer, 0);
2150 .Ve
2151 .Sp
2152 When there is some activity, simply store the current time in
2153 \&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
2154 .Sp
2155 .Vb 2
2156 \& if (activity detected)
2157 \& last_activity = ev_now (EV_A);
2158 .Ve
2159 .Sp
2160 When your timeout value changes, then the timeout can be changed by simply
2161 providing a new value, stopping the timer and calling the callback, which
2162 will again do the right thing (for example, time out immediately :).
2163 .Sp
2164 .Vb 3
2165 \& timeout = new_value;
2166 \& ev_timer_stop (EV_A_ &timer);
2167 \& callback (EV_A_ &timer, 0);
2168 .Ve
2169 .Sp
2170 This technique is slightly more complex, but in most cases where the
2171 time-out is unlikely to be triggered, much more efficient.
2172 .IP "4. Wee, just use a double-linked list for your timeouts." 4
2173 .IX Item "4. Wee, just use a double-linked list for your timeouts."
2174 If there is not one request, but many thousands (millions...), all
2175 employing some kind of timeout with the same timeout value, then one can
2176 do even better:
2177 .Sp
2178 When starting the timeout, calculate the timeout value and put the timeout
2179 at the \fIend\fR of the list.
2180 .Sp
2181 Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
2182 the list is expected to fire (for example, using the technique #3).
2183 .Sp
2184 When there is some activity, remove the timer from the list, recalculate
2185 the timeout, append it to the end of the list again, and make sure to
2186 update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
2187 .Sp
2188 This way, one can manage an unlimited number of timeouts in O(1) time for
2189 starting, stopping and updating the timers, at the expense of a major
2190 complication, and having to use a constant timeout. The constant timeout
2191 ensures that the list stays sorted.
2192 .PP
2193 So which method the best?
2194 .PP
2195 Method #2 is a simple no-brain-required solution that is adequate in most
2196 situations. Method #3 requires a bit more thinking, but handles many cases
2197 better, and isn't very complicated either. In most case, choosing either
2198 one is fine, with #3 being better in typical situations.
2199 .PP
2200 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
2201 rather complicated, but extremely efficient, something that really pays
2202 off after the first million or so of active timers, i.e. it's usually
2203 overkill :)
2204 .PP
2205 \fIThe special problem of being too early\fR
2206 .IX Subsection "The special problem of being too early"
2207 .PP
2208 If you ask a timer to call your callback after three seconds, then
2209 you expect it to be invoked after three seconds \- but of course, this
2210 cannot be guaranteed to infinite precision. Less obviously, it cannot be
2211 guaranteed to any precision by libev \- imagine somebody suspending the
2212 process with a \s-1STOP\s0 signal for a few hours for example.
2213 .PP
2214 So, libev tries to invoke your callback as soon as possible \fIafter\fR the
2215 delay has occurred, but cannot guarantee this.
2216 .PP
2217 A less obvious failure mode is calling your callback too early: many event
2218 loops compare timestamps with a \*(L"elapsed delay >= requested delay\*(R", but
2219 this can cause your callback to be invoked much earlier than you would
2220 expect.
2221 .PP
2222 To see why, imagine a system with a clock that only offers full second
2223 resolution (think windows if you can't come up with a broken enough \s-1OS\s0
2224 yourself). If you schedule a one-second timer at the time 500.9, then the
2225 event loop will schedule your timeout to elapse at a system time of 500
2226 (500.9 truncated to the resolution) + 1, or 501.
2227 .PP
2228 If an event library looks at the timeout 0.1s later, it will see \*(L"501 >=
2229 501\*(R" and invoke the callback 0.1s after it was started, even though a
2230 one-second delay was requested \- this is being \*(L"too early\*(R", despite best
2231 intentions.
2232 .PP
2233 This is the reason why libev will never invoke the callback if the elapsed
2234 delay equals the requested delay, but only when the elapsed delay is
2235 larger than the requested delay. In the example above, libev would only invoke
2236 the callback at system time 502, or 1.1s after the timer was started.
2237 .PP
2238 So, while libev cannot guarantee that your callback will be invoked
2239 exactly when requested, it \fIcan\fR and \fIdoes\fR guarantee that the requested
2240 delay has actually elapsed, or in other words, it always errs on the \*(L"too
2241 late\*(R" side of things.
2242 .PP
2243 \fIThe special problem of time updates\fR
2244 .IX Subsection "The special problem of time updates"
2245 .PP
2246 Establishing the current time is a costly operation (it usually takes
2247 at least one system call): \s-1EV\s0 therefore updates its idea of the current
2248 time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
2249 growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
2250 lots of events in one iteration.
2251 .PP
2252 The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
2253 time. This is usually the right thing as this timestamp refers to the time
2254 of the event triggering whatever timeout you are modifying/starting. If
2255 you suspect event processing to be delayed and you \fIneed\fR to base the
2256 timeout on the current time, use something like the following to adjust
2257 for it:
2258 .PP
2259 .Vb 1
2260 \& ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
2261 .Ve
2262 .PP
2263 If the event loop is suspended for a long time, you can also force an
2264 update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
2265 ()\*(C'\fR, although that will push the event time of all outstanding events
2266 further into the future.
2267 .PP
2268 \fIThe special problem of unsynchronised clocks\fR
2269 .IX Subsection "The special problem of unsynchronised clocks"
2270 .PP
2271 Modern systems have a variety of clocks \- libev itself uses the normal
2272 \&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
2273 jumps).
2274 .PP
2275 Neither of these clocks is synchronised with each other or any other clock
2276 on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
2277 than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
2278 a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
2279 than a directly following call to \f(CW\*(C`time\*(C'\fR.
2280 .PP
2281 The moral of this is to only compare libev-related timestamps with
2282 \&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, at least if you want better precision than
2283 a second or so.
2284 .PP
2285 One more problem arises due to this lack of synchronisation: if libev uses
2286 the system monotonic clock and you compare timestamps from \f(CW\*(C`ev_time\*(C'\fR
2287 or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
2288 invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
2289 .PP
2290 This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
2291 libev makes sure your callback is not invoked before the delay happened,
2292 \&\fImeasured according to the real time\fR, not the system clock.
2293 .PP
2294 If your timeouts are based on a physical timescale (e.g. \*(L"time out this
2295 connection after 100 seconds\*(R") then this shouldn't bother you as it is
2296 exactly the right behaviour.
2297 .PP
2298 If you want to compare wall clock/system timestamps to your timers, then
2299 you need to use \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
2300 time, where your comparisons will always generate correct results.
2301 .PP
2302 \fIThe special problems of suspended animation\fR
2303 .IX Subsection "The special problems of suspended animation"
2304 .PP
2305 When you leave the server world it is quite customary to hit machines that
2306 can suspend/hibernate \- what happens to the clocks during such a suspend?
2307 .PP
2308 Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
2309 all processes, while the clocks (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) continue
2310 to run until the system is suspended, but they will not advance while the
2311 system is suspended. That means, on resume, it will be as if the program
2312 was frozen for a few seconds, but the suspend time will not be counted
2313 towards \f(CW\*(C`ev_timer\*(C'\fR when a monotonic clock source is used. The real time
2314 clock advanced as expected, but if it is used as sole clocksource, then a
2315 long suspend would be detected as a time jump by libev, and timers would
2316 be adjusted accordingly.
2317 .PP
2318 I would not be surprised to see different behaviour in different between
2319 operating systems, \s-1OS\s0 versions or even different hardware.
2320 .PP
2321 The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) will see a
2322 time jump in the monotonic clocks and the realtime clock. If the program
2323 is suspended for a very long time, and monotonic clock sources are in use,
2324 then you can expect \f(CW\*(C`ev_timer\*(C'\fRs to expire as the full suspension time
2325 will be counted towards the timers. When no monotonic clock source is in
2326 use, then libev will again assume a timejump and adjust accordingly.
2327 .PP
2328 It might be beneficial for this latter case to call \f(CW\*(C`ev_suspend\*(C'\fR
2329 and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
2330 deterministic behaviour in this case (you can do nothing against
2331 \&\f(CW\*(C`SIGSTOP\*(C'\fR).
2332 .PP
2333 \fIWatcher-Specific Functions and Data Members\fR
2334 .IX Subsection "Watcher-Specific Functions and Data Members"
2335 .IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
2336 .IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
2337 .PD 0
2338 .IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
2339 .IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
2340 .PD
2341 Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
2342 negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, then it will
2343 automatically be stopped once the timeout is reached. If it is positive,
2344 then the timer will automatically be configured to trigger again \f(CW\*(C`repeat\*(C'\fR
2345 seconds later, again, and again, until stopped manually.
2346 .Sp
2347 The timer itself will do a best-effort at avoiding drift, that is, if
2348 you configure a timer to trigger every 10 seconds, then it will normally
2349 trigger at exactly 10 second intervals. If, however, your program cannot
2350 keep up with the timer (because it takes longer than those 10 seconds to
2351 do stuff) the timer will not fire more than once per event loop iteration.
2352 .IP "ev_timer_again (loop, ev_timer *)" 4
2353 .IX Item "ev_timer_again (loop, ev_timer *)"
2354 This will act as if the timer timed out, and restarts it again if it is
2355 repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
2356 timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
2357 .Sp
2358 The exact semantics are as in the following rules, all of which will be
2359 applied to the watcher:
2360 .RS 4
2361 .IP "If the timer is pending, the pending status is always cleared." 4
2362 .IX Item "If the timer is pending, the pending status is always cleared."
2363 .PD 0
2364 .IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
2365 .IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
2366 .ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
2367 .el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
2368 .IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
2369 .RE
2370 .RS 4
2371 .PD
2372 .Sp
2373 This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
2374 usage example.
2375 .RE
2376 .IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
2377 .IX Item "ev_tstamp ev_timer_remaining (loop, ev_timer *)"
2378 Returns the remaining time until a timer fires. If the timer is active,
2379 then this time is relative to the current event loop time, otherwise it's
2380 the timeout value currently configured.
2381 .Sp
2382 That is, after an \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
2383 \&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
2384 will return \f(CW4\fR. When the timer expires and is restarted, it will return
2385 roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
2386 too), and so on.
2387 .IP "ev_tstamp repeat [read\-write]" 4
2388 .IX Item "ev_tstamp repeat [read-write]"
2389 The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
2390 or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
2391 which is also when any modifications are taken into account.
2392 .PP
2393 \fIExamples\fR
2394 .IX Subsection "Examples"
2395 .PP
2396 Example: Create a timer that fires after 60 seconds.
2397 .PP
2398 .Vb 5
2399 \& static void
2400 \& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2401 \& {
2402 \& .. one minute over, w is actually stopped right here
2403 \& }
2404 \&
2405 \& ev_timer mytimer;
2406 \& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2407 \& ev_timer_start (loop, &mytimer);
2408 .Ve
2409 .PP
2410 Example: Create a timeout timer that times out after 10 seconds of
2411 inactivity.
2412 .PP
2413 .Vb 5
2414 \& static void
2415 \& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2416 \& {
2417 \& .. ten seconds without any activity
2418 \& }
2419 \&
2420 \& ev_timer mytimer;
2421 \& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2422 \& ev_timer_again (&mytimer); /* start timer */
2423 \& ev_run (loop, 0);
2424 \&
2425 \& // and in some piece of code that gets executed on any "activity":
2426 \& // reset the timeout to start ticking again at 10 seconds
2427 \& ev_timer_again (&mytimer);
2428 .Ve
2429 .ie n .SS """ev_periodic"" \- to cron or not to cron?"
2430 .el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
2431 .IX Subsection "ev_periodic - to cron or not to cron?"
2432 Periodic watchers are also timers of a kind, but they are very versatile
2433 (and unfortunately a bit complex).
2434 .PP
2435 Unlike \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
2436 relative time, the physical time that passes) but on wall clock time
2437 (absolute time, the thing you can read on your calendar or clock). The
2438 difference is that wall clock time can run faster or slower than real
2439 time, and time jumps are not uncommon (e.g. when you adjust your
2440 wrist-watch).
2441 .PP
2442 You can tell a periodic watcher to trigger after some specific point
2443 in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
2444 seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, that is, an absolute time
2445 not a delay) and then reset your system clock to January of the previous
2446 year, then it will take a year or more to trigger the event (unlike an
2447 \&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
2448 it, as it uses a relative timeout).
2449 .PP
2450 \&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
2451 timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
2452 other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
2453 watchers, as those cannot react to time jumps.
2454 .PP
2455 As with timers, the callback is guaranteed to be invoked only when the
2456 point in time where it is supposed to trigger has passed. If multiple
2457 timers become ready during the same loop iteration then the ones with
2458 earlier time-out values are invoked before ones with later time-out values
2459 (but this is no longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2460 .PP
2461 \fIWatcher-Specific Functions and Data Members\fR
2462 .IX Subsection "Watcher-Specific Functions and Data Members"
2463 .IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2464 .IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2465 .PD 0
2466 .IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2467 .IX Item "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2468 .PD
2469 Lots of arguments, let's sort it out... There are basically three modes of
2470 operation, and we will explain them from simplest to most complex:
2471 .RS 4
2472 .IP "\(bu" 4
2473 absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
2474 .Sp
2475 In this configuration the watcher triggers an event after the wall clock
2476 time \f(CW\*(C`offset\*(C'\fR has passed. It will not repeat and will not adjust when a
2477 time jump occurs, that is, if it is to be run at January 1st 2011 then it
2478 will be stopped and invoked when the system clock reaches or surpasses
2479 this point in time.
2480 .IP "\(bu" 4
2481 repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
2482 .Sp
2483 In this mode the watcher will always be scheduled to time out at the next
2484 \&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
2485 negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
2486 argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
2487 .Sp
2488 This can be used to create timers that do not drift with respect to the
2489 system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
2490 hour, on the hour (with respect to \s-1UTC\s0):
2491 .Sp
2492 .Vb 1
2493 \& ev_periodic_set (&periodic, 0., 3600., 0);
2494 .Ve
2495 .Sp
2496 This doesn't mean there will always be 3600 seconds in between triggers,
2497 but only that the callback will be called when the system time shows a
2498 full hour (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
2499 by 3600.
2500 .Sp
2501 Another way to think about it (for the mathematically inclined) is that
2502 \&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
2503 time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
2504 .Sp
2505 The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
2506 interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
2507 microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR and should have
2508 at most a similar magnitude as the current time (say, within a factor of
2509 ten). Typical values for offset are, in fact, \f(CW0\fR or something between
2510 \&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
2511 .Sp
2512 Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
2513 speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
2514 will of course deteriorate. Libev itself tries to be exact to be about one
2515 millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
2516 .IP "\(bu" 4
2517 manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
2518 .Sp
2519 In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR are both being
2520 ignored. Instead, each time the periodic watcher gets scheduled, the
2521 reschedule callback will be called with the watcher as first, and the
2522 current time as second argument.
2523 .Sp
2524 \&\s-1NOTE:\s0 \fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
2525 or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
2526 allowed by documentation here\fR.
2527 .Sp
2528 If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
2529 it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
2530 only event loop modification you are allowed to do).
2531 .Sp
2532 The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
2533 *w, ev_tstamp now)\*(C'\fR, e.g.:
2534 .Sp
2535 .Vb 5
2536 \& static ev_tstamp
2537 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2538 \& {
2539 \& return now + 60.;
2540 \& }
2541 .Ve
2542 .Sp
2543 It must return the next time to trigger, based on the passed time value
2544 (that is, the lowest time value larger than to the second argument). It
2545 will usually be called just before the callback will be triggered, but
2546 might be called at other times, too.
2547 .Sp
2548 \&\s-1NOTE:\s0 \fIThis callback must always return a time that is higher than or
2549 equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
2550 .Sp
2551 This can be used to create very complex timers, such as a timer that
2552 triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
2553 the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
2554 this. Here is a (completely untested, no error checking) example on how to
2555 do this:
2556 .Sp
2557 .Vb 1
2558 \& #include <time.h>
2559 \&
2560 \& static ev_tstamp
2561 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2562 \& {
2563 \& time_t tnow = (time_t)now;
2564 \& struct tm tm;
2565 \& localtime_r (&tnow, &tm);
2566 \&
2567 \& tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2568 \& ++tm.tm_mday; // midnight next day
2569 \&
2570 \& return mktime (&tm);
2571 \& }
2572 .Ve
2573 .Sp
2574 Note: this code might run into trouble on days that have more then two
2575 midnights (beginning and end).
2576 .RE
2577 .RS 4
2578 .RE
2579 .IP "ev_periodic_again (loop, ev_periodic *)" 4
2580 .IX Item "ev_periodic_again (loop, ev_periodic *)"
2581 Simply stops and restarts the periodic watcher again. This is only useful
2582 when you changed some parameters or the reschedule callback would return
2583 a different time than the last time it was called (e.g. in a crond like
2584 program when the crontabs have changed).
2585 .IP "ev_tstamp ev_periodic_at (ev_periodic *)" 4
2586 .IX Item "ev_tstamp ev_periodic_at (ev_periodic *)"
2587 When active, returns the absolute time that the watcher is supposed
2588 to trigger next. This is not the same as the \f(CW\*(C`offset\*(C'\fR argument to
2589 \&\f(CW\*(C`ev_periodic_set\*(C'\fR, but indeed works even in interval and manual
2590 rescheduling modes.
2591 .IP "ev_tstamp offset [read\-write]" 4
2592 .IX Item "ev_tstamp offset [read-write]"
2593 When repeating, this contains the offset value, otherwise this is the
2594 absolute point in time (the \f(CW\*(C`offset\*(C'\fR value passed to \f(CW\*(C`ev_periodic_set\*(C'\fR,
2595 although libev might modify this value for better numerical stability).
2596 .Sp
2597 Can be modified any time, but changes only take effect when the periodic
2598 timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2599 .IP "ev_tstamp interval [read\-write]" 4
2600 .IX Item "ev_tstamp interval [read-write]"
2601 The current interval value. Can be modified any time, but changes only
2602 take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
2603 called.
2604 .IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
2605 .IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
2606 The current reschedule callback, or \f(CW0\fR, if this functionality is
2607 switched off. Can be changed any time, but changes only take effect when
2608 the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2609 .PP
2610 \fIExamples\fR
2611 .IX Subsection "Examples"
2612 .PP
2613 Example: Call a callback every hour, or, more precisely, whenever the
2614 system time is divisible by 3600. The callback invocation times have
2615 potentially a lot of jitter, but good long-term stability.
2616 .PP
2617 .Vb 5
2618 \& static void
2619 \& clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2620 \& {
2621 \& ... its now a full hour (UTC, or TAI or whatever your clock follows)
2622 \& }
2623 \&
2624 \& ev_periodic hourly_tick;
2625 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2626 \& ev_periodic_start (loop, &hourly_tick);
2627 .Ve
2628 .PP
2629 Example: The same as above, but use a reschedule callback to do it:
2630 .PP
2631 .Vb 1
2632 \& #include <math.h>
2633 \&
2634 \& static ev_tstamp
2635 \& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2636 \& {
2637 \& return now + (3600. \- fmod (now, 3600.));
2638 \& }
2639 \&
2640 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2641 .Ve
2642 .PP
2643 Example: Call a callback every hour, starting now:
2644 .PP
2645 .Vb 4
2646 \& ev_periodic hourly_tick;
2647 \& ev_periodic_init (&hourly_tick, clock_cb,
2648 \& fmod (ev_now (loop), 3600.), 3600., 0);
2649 \& ev_periodic_start (loop, &hourly_tick);
2650 .Ve
2651 .ie n .SS """ev_signal"" \- signal me when a signal gets signalled!"
2652 .el .SS "\f(CWev_signal\fP \- signal me when a signal gets signalled!"
2653 .IX Subsection "ev_signal - signal me when a signal gets signalled!"
2654 Signal watchers will trigger an event when the process receives a specific
2655 signal one or more times. Even though signals are very asynchronous, libev
2656 will try its best to deliver signals synchronously, i.e. as part of the
2657 normal event processing, like any other event.
2658 .PP
2659 If you want signals to be delivered truly asynchronously, just use
2660 \&\f(CW\*(C`sigaction\*(C'\fR as you would do without libev and forget about sharing
2661 the signal. You can even use \f(CW\*(C`ev_async\*(C'\fR from a signal handler to
2662 synchronously wake up an event loop.
2663 .PP
2664 You can configure as many watchers as you like for the same signal, but
2665 only within the same loop, i.e. you can watch for \f(CW\*(C`SIGINT\*(C'\fR in your
2666 default loop and for \f(CW\*(C`SIGIO\*(C'\fR in another loop, but you cannot watch for
2667 \&\f(CW\*(C`SIGINT\*(C'\fR in both the default loop and another loop at the same time. At
2668 the moment, \f(CW\*(C`SIGCHLD\*(C'\fR is permanently tied to the default loop.
2669 .PP
2670 Only after the first watcher for a signal is started will libev actually
2671 register something with the kernel. It thus coexists with your own signal
2672 handlers as long as you don't register any with libev for the same signal.
2673 .PP
2674 If possible and supported, libev will install its handlers with
2675 \&\f(CW\*(C`SA_RESTART\*(C'\fR (or equivalent) behaviour enabled, so system calls should
2676 not be unduly interrupted. If you have a problem with system calls getting
2677 interrupted by signals you can block all signals in an \f(CW\*(C`ev_check\*(C'\fR watcher
2678 and unblock them in an \f(CW\*(C`ev_prepare\*(C'\fR watcher.
2679 .PP
2680 \fIThe special problem of inheritance over fork/execve/pthread_create\fR
2681 .IX Subsection "The special problem of inheritance over fork/execve/pthread_create"
2682 .PP
2683 Both the signal mask (\f(CW\*(C`sigprocmask\*(C'\fR) and the signal disposition
2684 (\f(CW\*(C`sigaction\*(C'\fR) are unspecified after starting a signal watcher (and after
2685 stopping it again), that is, libev might or might not block the signal,
2686 and might or might not set or restore the installed signal handler (but
2687 see \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR).
2688 .PP
2689 While this does not matter for the signal disposition (libev never
2690 sets signals to \f(CW\*(C`SIG_IGN\*(C'\fR, so handlers will be reset to \f(CW\*(C`SIG_DFL\*(C'\fR on
2691 \&\f(CW\*(C`execve\*(C'\fR), this matters for the signal mask: many programs do not expect
2692 certain signals to be blocked.
2693 .PP
2694 This means that before calling \f(CW\*(C`exec\*(C'\fR (from the child) you should reset
2695 the signal mask to whatever \*(L"default\*(R" you expect (all clear is a good
2696 choice usually).
2697 .PP
2698 The simplest way to ensure that the signal mask is reset in the child is
2699 to install a fork handler with \f(CW\*(C`pthread_atfork\*(C'\fR that resets it. That will
2700 catch fork calls done by libraries (such as the libc) as well.
2701 .PP
2702 In current versions of libev, the signal will not be blocked indefinitely
2703 unless you use the \f(CW\*(C`signalfd\*(C'\fR \s-1API\s0 (\f(CW\*(C`EV_SIGNALFD\*(C'\fR). While this reduces
2704 the window of opportunity for problems, it will not go away, as libev
2705 \&\fIhas\fR to modify the signal mask, at least temporarily.
2706 .PP
2707 So I can't stress this enough: \fIIf you do not reset your signal mask when
2708 you expect it to be empty, you have a race condition in your code\fR. This
2709 is not a libev-specific thing, this is true for most event libraries.
2710 .PP
2711 \fIThe special problem of threads signal handling\fR
2712 .IX Subsection "The special problem of threads signal handling"
2713 .PP
2714 \&\s-1POSIX\s0 threads has problematic signal handling semantics, specifically,
2715 a lot of functionality (sigfd, sigwait etc.) only really works if all
2716 threads in a process block signals, which is hard to achieve.
2717 .PP
2718 When you want to use sigwait (or mix libev signal handling with your own
2719 for the same signals), you can tackle this problem by globally blocking
2720 all signals before creating any threads (or creating them with a fully set
2721 sigprocmask) and also specifying the \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when creating
2722 loops. Then designate one thread as \*(L"signal receiver thread\*(R" which handles
2723 these signals. You can pass on any signals that libev might be interested
2724 in by calling \f(CW\*(C`ev_feed_signal\*(C'\fR.
2725 .PP
2726 \fIWatcher-Specific Functions and Data Members\fR
2727 .IX Subsection "Watcher-Specific Functions and Data Members"
2728 .IP "ev_signal_init (ev_signal *, callback, int signum)" 4
2729 .IX Item "ev_signal_init (ev_signal *, callback, int signum)"
2730 .PD 0
2731 .IP "ev_signal_set (ev_signal *, int signum)" 4
2732 .IX Item "ev_signal_set (ev_signal *, int signum)"
2733 .PD
2734 Configures the watcher to trigger on the given signal number (usually one
2735 of the \f(CW\*(C`SIGxxx\*(C'\fR constants).
2736 .IP "int signum [read\-only]" 4
2737 .IX Item "int signum [read-only]"
2738 The signal the watcher watches out for.
2739 .PP
2740 \fIExamples\fR
2741 .IX Subsection "Examples"
2742 .PP
2743 Example: Try to exit cleanly on \s-1SIGINT.\s0
2744 .PP
2745 .Vb 5
2746 \& static void
2747 \& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2748 \& {
2749 \& ev_break (loop, EVBREAK_ALL);
2750 \& }
2751 \&
2752 \& ev_signal signal_watcher;
2753 \& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2754 \& ev_signal_start (loop, &signal_watcher);
2755 .Ve
2756 .ie n .SS """ev_child"" \- watch out for process status changes"
2757 .el .SS "\f(CWev_child\fP \- watch out for process status changes"
2758 .IX Subsection "ev_child - watch out for process status changes"
2759 Child watchers trigger when your process receives a \s-1SIGCHLD\s0 in response to
2760 some child status changes (most typically when a child of yours dies or
2761 exits). It is permissible to install a child watcher \fIafter\fR the child
2762 has been forked (which implies it might have already exited), as long
2763 as the event loop isn't entered (or is continued from a watcher), i.e.,
2764 forking and then immediately registering a watcher for the child is fine,
2765 but forking and registering a watcher a few event loop iterations later or
2766 in the next callback invocation is not.
2767 .PP
2768 Only the default event loop is capable of handling signals, and therefore
2769 you can only register child watchers in the default event loop.
2770 .PP
2771 Due to some design glitches inside libev, child watchers will always be
2772 handled at maximum priority (their priority is set to \f(CW\*(C`EV_MAXPRI\*(C'\fR by
2773 libev)
2774 .PP
2775 \fIProcess Interaction\fR
2776 .IX Subsection "Process Interaction"
2777 .PP
2778 Libev grabs \f(CW\*(C`SIGCHLD\*(C'\fR as soon as the default event loop is
2779 initialised. This is necessary to guarantee proper behaviour even if the
2780 first child watcher is started after the child exits. The occurrence
2781 of \f(CW\*(C`SIGCHLD\*(C'\fR is recorded asynchronously, but child reaping is done
2782 synchronously as part of the event loop processing. Libev always reaps all
2783 children, even ones not watched.
2784 .PP
2785 \fIOverriding the Built-In Processing\fR
2786 .IX Subsection "Overriding the Built-In Processing"
2787 .PP
2788 Libev offers no special support for overriding the built-in child
2789 processing, but if your application collides with libev's default child
2790 handler, you can override it easily by installing your own handler for
2791 \&\f(CW\*(C`SIGCHLD\*(C'\fR after initialising the default loop, and making sure the
2792 default loop never gets destroyed. You are encouraged, however, to use an
2793 event-based approach to child reaping and thus use libev's support for
2794 that, so other libev users can use \f(CW\*(C`ev_child\*(C'\fR watchers freely.
2795 .PP
2796 \fIStopping the Child Watcher\fR
2797 .IX Subsection "Stopping the Child Watcher"
2798 .PP
2799 Currently, the child watcher never gets stopped, even when the
2800 child terminates, so normally one needs to stop the watcher in the
2801 callback. Future versions of libev might stop the watcher automatically
2802 when a child exit is detected (calling \f(CW\*(C`ev_child_stop\*(C'\fR twice is not a
2803 problem).
2804 .PP
2805 \fIWatcher-Specific Functions and Data Members\fR
2806 .IX Subsection "Watcher-Specific Functions and Data Members"
2807 .IP "ev_child_init (ev_child *, callback, int pid, int trace)" 4
2808 .IX Item "ev_child_init (ev_child *, callback, int pid, int trace)"
2809 .PD 0
2810 .IP "ev_child_set (ev_child *, int pid, int trace)" 4
2811 .IX Item "ev_child_set (ev_child *, int pid, int trace)"
2812 .PD
2813 Configures the watcher to wait for status changes of process \f(CW\*(C`pid\*(C'\fR (or
2814 \&\fIany\fR process if \f(CW\*(C`pid\*(C'\fR is specified as \f(CW0\fR). The callback can look
2815 at the \f(CW\*(C`rstatus\*(C'\fR member of the \f(CW\*(C`ev_child\*(C'\fR watcher structure to see
2816 the status word (use the macros from \f(CW\*(C`sys/wait.h\*(C'\fR and see your systems
2817 \&\f(CW\*(C`waitpid\*(C'\fR documentation). The \f(CW\*(C`rpid\*(C'\fR member contains the pid of the
2818 process causing the status change. \f(CW\*(C`trace\*(C'\fR must be either \f(CW0\fR (only
2819 activate the watcher when the process terminates) or \f(CW1\fR (additionally
2820 activate the watcher when the process is stopped or continued).
2821 .IP "int pid [read\-only]" 4
2822 .IX Item "int pid [read-only]"
2823 The process id this watcher watches out for, or \f(CW0\fR, meaning any process id.
2824 .IP "int rpid [read\-write]" 4
2825 .IX Item "int rpid [read-write]"
2826 The process id that detected a status change.
2827 .IP "int rstatus [read\-write]" 4
2828 .IX Item "int rstatus [read-write]"
2829 The process exit/trace status caused by \f(CW\*(C`rpid\*(C'\fR (see your systems
2830 \&\f(CW\*(C`waitpid\*(C'\fR and \f(CW\*(C`sys/wait.h\*(C'\fR documentation for details).
2831 .PP
2832 \fIExamples\fR
2833 .IX Subsection "Examples"
2834 .PP
2835 Example: \f(CW\*(C`fork()\*(C'\fR a new process and install a child handler to wait for
2836 its completion.
2837 .PP
2838 .Vb 1
2839 \& ev_child cw;
2840 \&
2841 \& static void
2842 \& child_cb (EV_P_ ev_child *w, int revents)
2843 \& {
2844 \& ev_child_stop (EV_A_ w);
2845 \& printf ("process %d exited with status %x\en", w\->rpid, w\->rstatus);
2846 \& }
2847 \&
2848 \& pid_t pid = fork ();
2849 \&
2850 \& if (pid < 0)
2851 \& // error
2852 \& else if (pid == 0)
2853 \& {
2854 \& // the forked child executes here
2855 \& exit (1);
2856 \& }
2857 \& else
2858 \& {
2859 \& ev_child_init (&cw, child_cb, pid, 0);
2860 \& ev_child_start (EV_DEFAULT_ &cw);
2861 \& }
2862 .Ve
2863 .ie n .SS """ev_stat"" \- did the file attributes just change?"
2864 .el .SS "\f(CWev_stat\fP \- did the file attributes just change?"
2865 .IX Subsection "ev_stat - did the file attributes just change?"
2866 This watches a file system path for attribute changes. That is, it calls
2867 \&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
2868 and sees if it changed compared to the last time, invoking the callback
2869 if it did. Starting the watcher \f(CW\*(C`stat\*(C'\fR's the file, so only changes that
2870 happen after the watcher has been started will be reported.
2871 .PP
2872 The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
2873 not exist\*(R" is a status change like any other. The condition \*(L"path does not
2874 exist\*(R" (or more correctly \*(L"path cannot be stat'ed\*(R") is signified by the
2875 \&\f(CW\*(C`st_nlink\*(C'\fR field being zero (which is otherwise always forced to be at
2876 least one) and all the other fields of the stat buffer having unspecified
2877 contents.
2878 .PP
2879 The path \fImust not\fR end in a slash or contain special components such as
2880 \&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
2881 your working directory changes, then the behaviour is undefined.
2882 .PP
2883 Since there is no portable change notification interface available, the
2884 portable implementation simply calls \f(CWstat(2)\fR regularly on the path
2885 to see if it changed somehow. You can specify a recommended polling
2886 interval for this case. If you specify a polling interval of \f(CW0\fR (highly
2887 recommended!) then a \fIsuitable, unspecified default\fR value will be used
2888 (which you can expect to be around five seconds, although this might
2889 change dynamically). Libev will also impose a minimum interval which is
2890 currently around \f(CW0.1\fR, but that's usually overkill.
2891 .PP
2892 This watcher type is not meant for massive numbers of stat watchers,
2893 as even with OS-supported change notifications, this can be
2894 resource-intensive.
2895 .PP
2896 At the time of this writing, the only OS-specific interface implemented
2897 is the Linux inotify interface (implementing kqueue support is left as an
2898 exercise for the reader. Note, however, that the author sees no way of
2899 implementing \f(CW\*(C`ev_stat\*(C'\fR semantics with kqueue, except as a hint).
2900 .PP
2901 \fI\s-1ABI\s0 Issues (Largefile Support)\fR
2902 .IX Subsection "ABI Issues (Largefile Support)"
2903 .PP
2904 Libev by default (unless the user overrides this) uses the default
2905 compilation environment, which means that on systems with large file
2906 support disabled by default, you get the 32 bit version of the stat
2907 structure. When using the library from programs that change the \s-1ABI\s0 to
2908 use 64 bit file offsets the programs will fail. In that case you have to
2909 compile libev with the same flags to get binary compatibility. This is
2910 obviously the case with any flags that change the \s-1ABI,\s0 but the problem is
2911 most noticeably displayed with ev_stat and large file support.
2912 .PP
2913 The solution for this is to lobby your distribution maker to make large
2914 file interfaces available by default (as e.g. FreeBSD does) and not
2915 optional. Libev cannot simply switch on large file support because it has
2916 to exchange stat structures with application programs compiled using the
2917 default compilation environment.
2918 .PP
2919 \fIInotify and Kqueue\fR
2920 .IX Subsection "Inotify and Kqueue"
2921 .PP
2922 When \f(CW\*(C`inotify (7)\*(C'\fR support has been compiled into libev and present at
2923 runtime, it will be used to speed up change detection where possible. The
2924 inotify descriptor will be created lazily when the first \f(CW\*(C`ev_stat\*(C'\fR
2925 watcher is being started.
2926 .PP
2927 Inotify presence does not change the semantics of \f(CW\*(C`ev_stat\*(C'\fR watchers
2928 except that changes might be detected earlier, and in some cases, to avoid
2929 making regular \f(CW\*(C`stat\*(C'\fR calls. Even in the presence of inotify support
2930 there are many cases where libev has to resort to regular \f(CW\*(C`stat\*(C'\fR polling,
2931 but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
2932 many bugs), the path exists (i.e. stat succeeds), and the path resides on
2933 a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
2934 xfs are fully working) libev usually gets away without polling.
2935 .PP
2936 There is no support for kqueue, as apparently it cannot be used to
2937 implement this functionality, due to the requirement of having a file
2938 descriptor open on the object at all times, and detecting renames, unlinks
2939 etc. is difficult.
2940 .PP
2941 \fI\f(CI\*(C`stat ()\*(C'\fI is a synchronous operation\fR
2942 .IX Subsection "stat () is a synchronous operation"
2943 .PP
2944 Libev doesn't normally do any kind of I/O itself, and so is not blocking
2945 the process. The exception are \f(CW\*(C`ev_stat\*(C'\fR watchers \- those call \f(CW\*(C`stat
2946 ()\*(C'\fR, which is a synchronous operation.
2947 .PP
2948 For local paths, this usually doesn't matter: unless the system is very
2949 busy or the intervals between stat's are large, a stat call will be fast,
2950 as the path data is usually in memory already (except when starting the
2951 watcher).
2952 .PP
2953 For networked file systems, calling \f(CW\*(C`stat ()\*(C'\fR can block an indefinite
2954 time due to network issues, and even under good conditions, a stat call
2955 often takes multiple milliseconds.
2956 .PP
2957 Therefore, it is best to avoid using \f(CW\*(C`ev_stat\*(C'\fR watchers on networked
2958 paths, although this is fully supported by libev.
2959 .PP
2960 \fIThe special problem of stat time resolution\fR
2961 .IX Subsection "The special problem of stat time resolution"
2962 .PP
2963 The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
2964 and even on systems where the resolution is higher, most file systems
2965 still only support whole seconds.
2966 .PP
2967 That means that, if the time is the only thing that changes, you can
2968 easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
2969 calls your callback, which does something. When there is another update
2970 within the same second, \f(CW\*(C`ev_stat\*(C'\fR will be unable to detect unless the
2971 stat data does change in other ways (e.g. file size).
2972 .PP
2973 The solution to this is to delay acting on a change for slightly more
2974 than a second (or till slightly after the next full second boundary), using
2975 a roughly one-second-delay \f(CW\*(C`ev_timer\*(C'\fR (e.g. \f(CW\*(C`ev_timer_set (w, 0., 1.02);
2976 ev_timer_again (loop, w)\*(C'\fR).
2977 .PP
2978 The \f(CW.02\fR offset is added to work around small timing inconsistencies
2979 of some operating systems (where the second counter of the current time
2980 might be be delayed. One such system is the Linux kernel, where a call to
2981 \&\f(CW\*(C`gettimeofday\*(C'\fR might return a timestamp with a full second later than
2982 a subsequent \f(CW\*(C`time\*(C'\fR call \- if the equivalent of \f(CW\*(C`time ()\*(C'\fR is used to
2983 update file times then there will be a small window where the kernel uses
2984 the previous second to update file times but libev might already execute
2985 the timer callback).
2986 .PP
2987 \fIWatcher-Specific Functions and Data Members\fR
2988 .IX Subsection "Watcher-Specific Functions and Data Members"
2989 .IP "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)" 4
2990 .IX Item "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)"
2991 .PD 0
2992 .IP "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)" 4
2993 .IX Item "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)"
2994 .PD
2995 Configures the watcher to wait for status changes of the given
2996 \&\f(CW\*(C`path\*(C'\fR. The \f(CW\*(C`interval\*(C'\fR is a hint on how quickly a change is expected to
2997 be detected and should normally be specified as \f(CW0\fR to let libev choose
2998 a suitable value. The memory pointed to by \f(CW\*(C`path\*(C'\fR must point to the same
2999 path for as long as the watcher is active.
3000 .Sp
3001 The callback will receive an \f(CW\*(C`EV_STAT\*(C'\fR event when a change was detected,
3002 relative to the attributes at the time the watcher was started (or the
3003 last change was detected).
3004 .IP "ev_stat_stat (loop, ev_stat *)" 4
3005 .IX Item "ev_stat_stat (loop, ev_stat *)"
3006 Updates the stat buffer immediately with new values. If you change the
3007 watched path in your callback, you could call this function to avoid
3008 detecting this change (while introducing a race condition if you are not
3009 the only one changing the path). Can also be useful simply to find out the
3010 new values.
3011 .IP "ev_statdata attr [read\-only]" 4
3012 .IX Item "ev_statdata attr [read-only]"
3013 The most-recently detected attributes of the file. Although the type is
3014 \&\f(CW\*(C`ev_statdata\*(C'\fR, this is usually the (or one of the) \f(CW\*(C`struct stat\*(C'\fR types
3015 suitable for your system, but you can only rely on the POSIX-standardised
3016 members to be present. If the \f(CW\*(C`st_nlink\*(C'\fR member is \f(CW0\fR, then there was
3017 some error while \f(CW\*(C`stat\*(C'\fRing the file.
3018 .IP "ev_statdata prev [read\-only]" 4
3019 .IX Item "ev_statdata prev [read-only]"
3020 The previous attributes of the file. The callback gets invoked whenever
3021 \&\f(CW\*(C`prev\*(C'\fR != \f(CW\*(C`attr\*(C'\fR, or, more precisely, one or more of these members
3022 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,
3023 \&\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.
3024 .IP "ev_tstamp interval [read\-only]" 4
3025 .IX Item "ev_tstamp interval [read-only]"
3026 The specified interval.
3027 .IP "const char *path [read\-only]" 4
3028 .IX Item "const char *path [read-only]"
3029 The file system path that is being watched.
3030 .PP
3031 \fIExamples\fR
3032 .IX Subsection "Examples"
3033 .PP
3034 Example: Watch \f(CW\*(C`/etc/passwd\*(C'\fR for attribute changes.
3035 .PP
3036 .Vb 10
3037 \& static void
3038 \& passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
3039 \& {
3040 \& /* /etc/passwd changed in some way */
3041 \& if (w\->attr.st_nlink)
3042 \& {
3043 \& printf ("passwd current size %ld\en", (long)w\->attr.st_size);
3044 \& printf ("passwd current atime %ld\en", (long)w\->attr.st_mtime);
3045 \& printf ("passwd current mtime %ld\en", (long)w\->attr.st_mtime);
3046 \& }
3047 \& else
3048 \& /* you shalt not abuse printf for puts */
3049 \& puts ("wow, /etc/passwd is not there, expect problems. "
3050 \& "if this is windows, they already arrived\en");
3051 \& }
3052 \&
3053 \& ...
3054 \& ev_stat passwd;
3055 \&
3056 \& ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
3057 \& ev_stat_start (loop, &passwd);
3058 .Ve
3059 .PP
3060 Example: Like above, but additionally use a one-second delay so we do not
3061 miss updates (however, frequent updates will delay processing, too, so
3062 one might do the work both on \f(CW\*(C`ev_stat\*(C'\fR callback invocation \fIand\fR on
3063 \&\f(CW\*(C`ev_timer\*(C'\fR callback invocation).
3064 .PP
3065 .Vb 2
3066 \& static ev_stat passwd;
3067 \& static ev_timer timer;
3068 \&
3069 \& static void
3070 \& timer_cb (EV_P_ ev_timer *w, int revents)
3071 \& {
3072 \& ev_timer_stop (EV_A_ w);
3073 \&
3074 \& /* now it\*(Aqs one second after the most recent passwd change */
3075 \& }
3076 \&
3077 \& static void
3078 \& stat_cb (EV_P_ ev_stat *w, int revents)
3079 \& {
3080 \& /* reset the one\-second timer */
3081 \& ev_timer_again (EV_A_ &timer);
3082 \& }
3083 \&
3084 \& ...
3085 \& ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
3086 \& ev_stat_start (loop, &passwd);
3087 \& ev_timer_init (&timer, timer_cb, 0., 1.02);
3088 .Ve
3089 .ie n .SS """ev_idle"" \- when you've got nothing better to do..."
3090 .el .SS "\f(CWev_idle\fP \- when you've got nothing better to do..."
3091 .IX Subsection "ev_idle - when you've got nothing better to do..."
3092 Idle watchers trigger events when no other events of the same or higher
3093 priority are pending (prepare, check and other idle watchers do not count
3094 as receiving \*(L"events\*(R").
3095 .PP
3096 That is, as long as your process is busy handling sockets or timeouts
3097 (or even signals, imagine) of the same or higher priority it will not be
3098 triggered. But when your process is idle (or only lower-priority watchers
3099 are pending), the idle watchers are being called once per event loop
3100 iteration \- until stopped, that is, or your process receives more events
3101 and becomes busy again with higher priority stuff.
3102 .PP
3103 The most noteworthy effect is that as long as any idle watchers are
3104 active, the process will not block when waiting for new events.
3105 .PP
3106 Apart from keeping your process non-blocking (which is a useful
3107 effect on its own sometimes), idle watchers are a good place to do
3108 \&\*(L"pseudo-background processing\*(R", or delay processing stuff to after the
3109 event loop has handled all outstanding events.
3110 .PP
3111 \fIAbusing an \f(CI\*(C`ev_idle\*(C'\fI watcher for its side-effect\fR
3112 .IX Subsection "Abusing an ev_idle watcher for its side-effect"
3113 .PP
3114 As long as there is at least one active idle watcher, libev will never
3115 sleep unnecessarily. Or in other words, it will loop as fast as possible.
3116 For this to work, the idle watcher doesn't need to be invoked at all \- the
3117 lowest priority will do.
3118 .PP
3119 This mode of operation can be useful together with an \f(CW\*(C`ev_check\*(C'\fR watcher,
3120 to do something on each event loop iteration \- for example to balance load
3121 between different connections.
3122 .PP
3123 See \*(L"Abusing an ev_check watcher for its side-effect\*(R" for a longer
3124 example.
3125 .PP
3126 \fIWatcher-Specific Functions and Data Members\fR
3127 .IX Subsection "Watcher-Specific Functions and Data Members"
3128 .IP "ev_idle_init (ev_idle *, callback)" 4
3129 .IX Item "ev_idle_init (ev_idle *, callback)"
3130 Initialises and configures the idle watcher \- it has no parameters of any
3131 kind. There is a \f(CW\*(C`ev_idle_set\*(C'\fR macro, but using it is utterly pointless,
3132 believe me.
3133 .PP
3134 \fIExamples\fR
3135 .IX Subsection "Examples"
3136 .PP
3137 Example: Dynamically allocate an \f(CW\*(C`ev_idle\*(C'\fR watcher, start it, and in the
3138 callback, free it. Also, use no error checking, as usual.
3139 .PP
3140 .Vb 5
3141 \& static void
3142 \& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
3143 \& {
3144 \& // stop the watcher
3145 \& ev_idle_stop (loop, w);
3146 \&
3147 \& // now we can free it
3148 \& free (w);
3149 \&
3150 \& // now do something you wanted to do when the program has
3151 \& // no longer anything immediate to do.
3152 \& }
3153 \&
3154 \& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
3155 \& ev_idle_init (idle_watcher, idle_cb);
3156 \& ev_idle_start (loop, idle_watcher);
3157 .Ve
3158 .ie n .SS """ev_prepare"" and ""ev_check"" \- customise your event loop!"
3159 .el .SS "\f(CWev_prepare\fP and \f(CWev_check\fP \- customise your event loop!"
3160 .IX Subsection "ev_prepare and ev_check - customise your event loop!"
3161 Prepare and check watchers are often (but not always) used in pairs:
3162 prepare watchers get invoked before the process blocks and check watchers
3163 afterwards.
3164 .PP
3165 You \fImust not\fR call \f(CW\*(C`ev_run\*(C'\fR (or similar functions that enter the
3166 current event loop) or \f(CW\*(C`ev_loop_fork\*(C'\fR from either \f(CW\*(C`ev_prepare\*(C'\fR or
3167 \&\f(CW\*(C`ev_check\*(C'\fR watchers. Other loops than the current one are fine,
3168 however. The rationale behind this is that you do not need to check
3169 for recursion in those watchers, i.e. the sequence will always be
3170 \&\f(CW\*(C`ev_prepare\*(C'\fR, blocking, \f(CW\*(C`ev_check\*(C'\fR so if you have one watcher of each
3171 kind they will always be called in pairs bracketing the blocking call.
3172 .PP
3173 Their main purpose is to integrate other event mechanisms into libev and
3174 their use is somewhat advanced. They could be used, for example, to track
3175 variable changes, implement your own watchers, integrate net-snmp or a
3176 coroutine library and lots more. They are also occasionally useful if
3177 you cache some data and want to flush it before blocking (for example,
3178 in X programs you might want to do an \f(CW\*(C`XFlush ()\*(C'\fR in an \f(CW\*(C`ev_prepare\*(C'\fR
3179 watcher).
3180 .PP
3181 This is done by examining in each prepare call which file descriptors
3182 need to be watched by the other library, registering \f(CW\*(C`ev_io\*(C'\fR watchers
3183 for them and starting an \f(CW\*(C`ev_timer\*(C'\fR watcher for any timeouts (many
3184 libraries provide exactly this functionality). Then, in the check watcher,
3185 you check for any events that occurred (by checking the pending status
3186 of all watchers and stopping them) and call back into the library. The
3187 I/O and timer callbacks will never actually be called (but must be valid
3188 nevertheless, because you never know, you know?).
3189 .PP
3190 As another example, the Perl Coro module uses these hooks to integrate
3191 coroutines into libev programs, by yielding to other active coroutines
3192 during each prepare and only letting the process block if no coroutines
3193 are ready to run (it's actually more complicated: it only runs coroutines
3194 with priority higher than or equal to the event loop and one coroutine
3195 of lower priority, but only once, using idle watchers to keep the event
3196 loop from blocking if lower-priority coroutines are active, thus mapping
3197 low-priority coroutines to idle/background tasks).
3198 .PP
3199 When used for this purpose, it is recommended to give \f(CW\*(C`ev_check\*(C'\fR watchers
3200 highest (\f(CW\*(C`EV_MAXPRI\*(C'\fR) priority, to ensure that they are being run before
3201 any other watchers after the poll (this doesn't matter for \f(CW\*(C`ev_prepare\*(C'\fR
3202 watchers).
3203 .PP
3204 Also, \f(CW\*(C`ev_check\*(C'\fR watchers (and \f(CW\*(C`ev_prepare\*(C'\fR watchers, too) should not
3205 activate (\*(L"feed\*(R") events into libev. While libev fully supports this, they
3206 might get executed before other \f(CW\*(C`ev_check\*(C'\fR watchers did their job. As
3207 \&\f(CW\*(C`ev_check\*(C'\fR watchers are often used to embed other (non-libev) event
3208 loops those other event loops might be in an unusable state until their
3209 \&\f(CW\*(C`ev_check\*(C'\fR watcher ran (always remind yourself to coexist peacefully with
3210 others).
3211 .PP
3212 \fIAbusing an \f(CI\*(C`ev_check\*(C'\fI watcher for its side-effect\fR
3213 .IX Subsection "Abusing an ev_check watcher for its side-effect"
3214 .PP
3215 \&\f(CW\*(C`ev_check\*(C'\fR (and less often also \f(CW\*(C`ev_prepare\*(C'\fR) watchers can also be
3216 useful because they are called once per event loop iteration. For
3217 example, if you want to handle a large number of connections fairly, you
3218 normally only do a bit of work for each active connection, and if there
3219 is more work to do, you wait for the next event loop iteration, so other
3220 connections have a chance of making progress.
3221 .PP
3222 Using an \f(CW\*(C`ev_check\*(C'\fR watcher is almost enough: it will be called on the
3223 next event loop iteration. However, that isn't as soon as possible \-
3224 without external events, your \f(CW\*(C`ev_check\*(C'\fR watcher will not be invoked.
3225 .PP
3226 This is where \f(CW\*(C`ev_idle\*(C'\fR watchers come in handy \- all you need is a
3227 single global idle watcher that is active as long as you have one active
3228 \&\f(CW\*(C`ev_check\*(C'\fR watcher. The \f(CW\*(C`ev_idle\*(C'\fR watcher makes sure the event loop
3229 will not sleep, and the \f(CW\*(C`ev_check\*(C'\fR watcher makes sure a callback gets
3230 invoked. Neither watcher alone can do that.
3231 .PP
3232 \fIWatcher-Specific Functions and Data Members\fR
3233 .IX Subsection "Watcher-Specific Functions and Data Members"
3234 .IP "ev_prepare_init (ev_prepare *, callback)" 4
3235 .IX Item "ev_prepare_init (ev_prepare *, callback)"
3236 .PD 0
3237 .IP "ev_check_init (ev_check *, callback)" 4
3238 .IX Item "ev_check_init (ev_check *, callback)"
3239 .PD
3240 Initialises and configures the prepare or check watcher \- they have no
3241 parameters of any kind. There are \f(CW\*(C`ev_prepare_set\*(C'\fR and \f(CW\*(C`ev_check_set\*(C'\fR
3242 macros, but using them is utterly, utterly, utterly and completely
3243 pointless.
3244 .PP
3245 \fIExamples\fR
3246 .IX Subsection "Examples"
3247 .PP
3248 There are a number of principal ways to embed other event loops or modules
3249 into libev. Here are some ideas on how to include libadns into libev
3250 (there is a Perl module named \f(CW\*(C`EV::ADNS\*(C'\fR that does this, which you could
3251 use as a working example. Another Perl module named \f(CW\*(C`EV::Glib\*(C'\fR embeds a
3252 Glib main context into libev, and finally, \f(CW\*(C`Glib::EV\*(C'\fR embeds \s-1EV\s0 into the
3253 Glib event loop).
3254 .PP
3255 Method 1: Add \s-1IO\s0 watchers and a timeout watcher in a prepare handler,
3256 and in a check watcher, destroy them and call into libadns. What follows
3257 is pseudo-code only of course. This requires you to either use a low
3258 priority for the check watcher or use \f(CW\*(C`ev_clear_pending\*(C'\fR explicitly, as
3259 the callbacks for the IO/timeout watchers might not have been called yet.
3260 .PP
3261 .Vb 2
3262 \& static ev_io iow [nfd];
3263 \& static ev_timer tw;
3264 \&
3265 \& static void
3266 \& io_cb (struct ev_loop *loop, ev_io *w, int revents)
3267 \& {
3268 \& }
3269 \&
3270 \& // create io watchers for each fd and a timer before blocking
3271 \& static void
3272 \& adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3273 \& {
3274 \& int timeout = 3600000;
3275 \& struct pollfd fds [nfd];
3276 \& // actual code will need to loop here and realloc etc.
3277 \& adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3278 \&
3279 \& /* the callback is illegal, but won\*(Aqt be called as we stop during check */
3280 \& ev_timer_init (&tw, 0, timeout * 1e\-3, 0.);
3281 \& ev_timer_start (loop, &tw);
3282 \&
3283 \& // create one ev_io per pollfd
3284 \& for (int i = 0; i < nfd; ++i)
3285 \& {
3286 \& ev_io_init (iow + i, io_cb, fds [i].fd,
3287 \& ((fds [i].events & POLLIN ? EV_READ : 0)
3288 \& | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3289 \&
3290 \& fds [i].revents = 0;
3291 \& ev_io_start (loop, iow + i);
3292 \& }
3293 \& }
3294 \&
3295 \& // stop all watchers after blocking
3296 \& static void
3297 \& adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3298 \& {
3299 \& ev_timer_stop (loop, &tw);
3300 \&
3301 \& for (int i = 0; i < nfd; ++i)
3302 \& {
3303 \& // set the relevant poll flags
3304 \& // could also call adns_processreadable etc. here
3305 \& struct pollfd *fd = fds + i;
3306 \& int revents = ev_clear_pending (iow + i);
3307 \& if (revents & EV_READ ) fd\->revents |= fd\->events & POLLIN;
3308 \& if (revents & EV_WRITE) fd\->revents |= fd\->events & POLLOUT;
3309 \&
3310 \& // now stop the watcher
3311 \& ev_io_stop (loop, iow + i);
3312 \& }
3313 \&
3314 \& adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3315 \& }
3316 .Ve
3317 .PP
3318 Method 2: This would be just like method 1, but you run \f(CW\*(C`adns_afterpoll\*(C'\fR
3319 in the prepare watcher and would dispose of the check watcher.
3320 .PP
3321 Method 3: If the module to be embedded supports explicit event
3322 notification (libadns does), you can also make use of the actual watcher
3323 callbacks, and only destroy/create the watchers in the prepare watcher.
3324 .PP
3325 .Vb 5
3326 \& static void
3327 \& timer_cb (EV_P_ ev_timer *w, int revents)
3328 \& {
3329 \& adns_state ads = (adns_state)w\->data;
3330 \& update_now (EV_A);
3331 \&
3332 \& adns_processtimeouts (ads, &tv_now);
3333 \& }
3334 \&
3335 \& static void
3336 \& io_cb (EV_P_ ev_io *w, int revents)
3337 \& {
3338 \& adns_state ads = (adns_state)w\->data;
3339 \& update_now (EV_A);
3340 \&
3341 \& if (revents & EV_READ ) adns_processreadable (ads, w\->fd, &tv_now);
3342 \& if (revents & EV_WRITE) adns_processwriteable (ads, w\->fd, &tv_now);
3343 \& }
3344 \&
3345 \& // do not ever call adns_afterpoll
3346 .Ve
3347 .PP
3348 Method 4: Do not use a prepare or check watcher because the module you
3349 want to embed is not flexible enough to support it. Instead, you can
3350 override their poll function. The drawback with this solution is that the
3351 main loop is now no longer controllable by \s-1EV.\s0 The \f(CW\*(C`Glib::EV\*(C'\fR module uses
3352 this approach, effectively embedding \s-1EV\s0 as a client into the horrible
3353 libglib event loop.
3354 .PP
3355 .Vb 4
3356 \& static gint
3357 \& event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3358 \& {
3359 \& int got_events = 0;
3360 \&
3361 \& for (n = 0; n < nfds; ++n)
3362 \& // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3363 \&
3364 \& if (timeout >= 0)
3365 \& // create/start timer
3366 \&
3367 \& // poll
3368 \& ev_run (EV_A_ 0);
3369 \&
3370 \& // stop timer again
3371 \& if (timeout >= 0)
3372 \& ev_timer_stop (EV_A_ &to);
3373 \&
3374 \& // stop io watchers again \- their callbacks should have set
3375 \& for (n = 0; n < nfds; ++n)
3376 \& ev_io_stop (EV_A_ iow [n]);
3377 \&
3378 \& return got_events;
3379 \& }
3380 .Ve
3381 .ie n .SS """ev_embed"" \- when one backend isn't enough..."
3382 .el .SS "\f(CWev_embed\fP \- when one backend isn't enough..."
3383 .IX Subsection "ev_embed - when one backend isn't enough..."
3384 This is a rather advanced watcher type that lets you embed one event loop
3385 into another (currently only \f(CW\*(C`ev_io\*(C'\fR events are supported in the embedded
3386 loop, other types of watchers might be handled in a delayed or incorrect
3387 fashion and must not be used).
3388 .PP
3389 There are primarily two reasons you would want that: work around bugs and
3390 prioritise I/O.
3391 .PP
3392 As an example for a bug workaround, the kqueue backend might only support
3393 sockets on some platform, so it is unusable as generic backend, but you
3394 still want to make use of it because you have many sockets and it scales
3395 so nicely. In this case, you would create a kqueue-based loop and embed
3396 it into your default loop (which might use e.g. poll). Overall operation
3397 will be a bit slower because first libev has to call \f(CW\*(C`poll\*(C'\fR and then
3398 \&\f(CW\*(C`kevent\*(C'\fR, but at least you can use both mechanisms for what they are
3399 best: \f(CW\*(C`kqueue\*(C'\fR for scalable sockets and \f(CW\*(C`poll\*(C'\fR if you want it to work :)
3400 .PP
3401 As for prioritising I/O: under rare circumstances you have the case where
3402 some fds have to be watched and handled very quickly (with low latency),
3403 and even priorities and idle watchers might have too much overhead. In
3404 this case you would put all the high priority stuff in one loop and all
3405 the rest in a second one, and embed the second one in the first.
3406 .PP
3407 As long as the watcher is active, the callback will be invoked every
3408 time there might be events pending in the embedded loop. The callback
3409 must then call \f(CW\*(C`ev_embed_sweep (mainloop, watcher)\*(C'\fR to make a single
3410 sweep and invoke their callbacks (the callback doesn't need to invoke the
3411 \&\f(CW\*(C`ev_embed_sweep\*(C'\fR function directly, it could also start an idle watcher
3412 to give the embedded loop strictly lower priority for example).
3413 .PP
3414 You can also set the callback to \f(CW0\fR, in which case the embed watcher
3415 will automatically execute the embedded loop sweep whenever necessary.
3416 .PP
3417 Fork detection will be handled transparently while the \f(CW\*(C`ev_embed\*(C'\fR watcher
3418 is active, i.e., the embedded loop will automatically be forked when the
3419 embedding loop forks. In other cases, the user is responsible for calling
3420 \&\f(CW\*(C`ev_loop_fork\*(C'\fR on the embedded loop.
3421 .PP
3422 Unfortunately, not all backends are embeddable: only the ones returned by
3423 \&\f(CW\*(C`ev_embeddable_backends\*(C'\fR are, which, unfortunately, does not include any
3424 portable one.
3425 .PP
3426 So when you want to use this feature you will always have to be prepared
3427 that you cannot get an embeddable loop. The recommended way to get around
3428 this is to have a separate variables for your embeddable loop, try to
3429 create it, and if that fails, use the normal loop for everything.
3430 .PP
3431 \fI\f(CI\*(C`ev_embed\*(C'\fI and fork\fR
3432 .IX Subsection "ev_embed and fork"
3433 .PP
3434 While the \f(CW\*(C`ev_embed\*(C'\fR watcher is running, forks in the embedding loop will
3435 automatically be applied to the embedded loop as well, so no special
3436 fork handling is required in that case. When the watcher is not running,
3437 however, it is still the task of the libev user to call \f(CW\*(C`ev_loop_fork ()\*(C'\fR
3438 as applicable.
3439 .PP
3440 \fIWatcher-Specific Functions and Data Members\fR
3441 .IX Subsection "Watcher-Specific Functions and Data Members"
3442 .IP "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)" 4
3443 .IX Item "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)"
3444 .PD 0
3445 .IP "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)" 4
3446 .IX Item "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)"
3447 .PD
3448 Configures the watcher to embed the given loop, which must be
3449 embeddable. If the callback is \f(CW0\fR, then \f(CW\*(C`ev_embed_sweep\*(C'\fR will be
3450 invoked automatically, otherwise it is the responsibility of the callback
3451 to invoke it (it will continue to be called until the sweep has been done,
3452 if you do not want that, you need to temporarily stop the embed watcher).
3453 .IP "ev_embed_sweep (loop, ev_embed *)" 4
3454 .IX Item "ev_embed_sweep (loop, ev_embed *)"
3455 Make a single, non-blocking sweep over the embedded loop. This works
3456 similarly to \f(CW\*(C`ev_run (embedded_loop, EVRUN_NOWAIT)\*(C'\fR, but in the most
3457 appropriate way for embedded loops.
3458 .IP "struct ev_loop *other [read\-only]" 4
3459 .IX Item "struct ev_loop *other [read-only]"
3460 The embedded event loop.
3461 .PP
3462 \fIExamples\fR
3463 .IX Subsection "Examples"
3464 .PP
3465 Example: Try to get an embeddable event loop and embed it into the default
3466 event loop. If that is not possible, use the default loop. The default
3467 loop is stored in \f(CW\*(C`loop_hi\*(C'\fR, while the embeddable loop is stored in
3468 \&\f(CW\*(C`loop_lo\*(C'\fR (which is \f(CW\*(C`loop_hi\*(C'\fR in the case no embeddable loop can be
3469 used).
3470 .PP
3471 .Vb 3
3472 \& struct ev_loop *loop_hi = ev_default_init (0);
3473 \& struct ev_loop *loop_lo = 0;
3474 \& ev_embed embed;
3475 \&
3476 \& // see if there is a chance of getting one that works
3477 \& // (remember that a flags value of 0 means autodetection)
3478 \& loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3479 \& ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3480 \& : 0;
3481 \&
3482 \& // if we got one, then embed it, otherwise default to loop_hi
3483 \& if (loop_lo)
3484 \& {
3485 \& ev_embed_init (&embed, 0, loop_lo);
3486 \& ev_embed_start (loop_hi, &embed);
3487 \& }
3488 \& else
3489 \& loop_lo = loop_hi;
3490 .Ve
3491 .PP
3492 Example: Check if kqueue is available but not recommended and create
3493 a kqueue backend for use with sockets (which usually work with any
3494 kqueue implementation). Store the kqueue/socket\-only event loop in
3495 \&\f(CW\*(C`loop_socket\*(C'\fR. (One might optionally use \f(CW\*(C`EVFLAG_NOENV\*(C'\fR, too).
3496 .PP
3497 .Vb 3
3498 \& struct ev_loop *loop = ev_default_init (0);
3499 \& struct ev_loop *loop_socket = 0;
3500 \& ev_embed embed;
3501 \&
3502 \& if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3503 \& if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3504 \& {
3505 \& ev_embed_init (&embed, 0, loop_socket);
3506 \& ev_embed_start (loop, &embed);
3507 \& }
3508 \&
3509 \& if (!loop_socket)
3510 \& loop_socket = loop;
3511 \&
3512 \& // now use loop_socket for all sockets, and loop for everything else
3513 .Ve
3514 .ie n .SS """ev_fork"" \- the audacity to resume the event loop after a fork"
3515 .el .SS "\f(CWev_fork\fP \- the audacity to resume the event loop after a fork"
3516 .IX Subsection "ev_fork - the audacity to resume the event loop after a fork"
3517 Fork watchers are called when a \f(CW\*(C`fork ()\*(C'\fR was detected (usually because
3518 whoever is a good citizen cared to tell libev about it by calling
3519 \&\f(CW\*(C`ev_loop_fork\*(C'\fR). The invocation is done before the event loop blocks next
3520 and before \f(CW\*(C`ev_check\*(C'\fR watchers are being called, and only in the child
3521 after the fork. If whoever good citizen calling \f(CW\*(C`ev_default_fork\*(C'\fR cheats
3522 and calls it in the wrong process, the fork handlers will be invoked, too,
3523 of course.
3524 .PP
3525 \fIThe special problem of life after fork \- how is it possible?\fR
3526 .IX Subsection "The special problem of life after fork - how is it possible?"
3527 .PP
3528 Most uses of \f(CW\*(C`fork ()\*(C'\fR consist of forking, then some simple calls to set
3529 up/change the process environment, followed by a call to \f(CW\*(C`exec()\*(C'\fR. This
3530 sequence should be handled by libev without any problems.
3531 .PP
3532 This changes when the application actually wants to do event handling
3533 in the child, or both parent in child, in effect \*(L"continuing\*(R" after the
3534 fork.
3535 .PP
3536 The default mode of operation (for libev, with application help to detect
3537 forks) is to duplicate all the state in the child, as would be expected
3538 when \fIeither\fR the parent \fIor\fR the child process continues.
3539 .PP
3540 When both processes want to continue using libev, then this is usually the
3541 wrong result. In that case, usually one process (typically the parent) is
3542 supposed to continue with all watchers in place as before, while the other
3543 process typically wants to start fresh, i.e. without any active watchers.
3544 .PP
3545 The cleanest and most efficient way to achieve that with libev is to
3546 simply create a new event loop, which of course will be \*(L"empty\*(R", and
3547 use that for new watchers. This has the advantage of not touching more
3548 memory than necessary, and thus avoiding the copy-on-write, and the
3549 disadvantage of having to use multiple event loops (which do not support
3550 signal watchers).
3551 .PP
3552 When this is not possible, or you want to use the default loop for
3553 other reasons, then in the process that wants to start \*(L"fresh\*(R", call
3554 \&\f(CW\*(C`ev_loop_destroy (EV_DEFAULT)\*(C'\fR followed by \f(CW\*(C`ev_default_loop (...)\*(C'\fR.
3555 Destroying the default loop will \*(L"orphan\*(R" (not stop) all registered
3556 watchers, so you have to be careful not to execute code that modifies
3557 those watchers. Note also that in that case, you have to re-register any
3558 signal watchers.
3559 .PP
3560 \fIWatcher-Specific Functions and Data Members\fR
3561 .IX Subsection "Watcher-Specific Functions and Data Members"
3562 .IP "ev_fork_init (ev_fork *, callback)" 4
3563 .IX Item "ev_fork_init (ev_fork *, callback)"
3564 Initialises and configures the fork watcher \- it has no parameters of any
3565 kind. There is a \f(CW\*(C`ev_fork_set\*(C'\fR macro, but using it is utterly pointless,
3566 really.
3567 .ie n .SS """ev_cleanup"" \- even the best things end"
3568 .el .SS "\f(CWev_cleanup\fP \- even the best things end"
3569 .IX Subsection "ev_cleanup - even the best things end"
3570 Cleanup watchers are called just before the event loop is being destroyed
3571 by a call to \f(CW\*(C`ev_loop_destroy\*(C'\fR.
3572 .PP
3573 While there is no guarantee that the event loop gets destroyed, cleanup
3574 watchers provide a convenient method to install cleanup hooks for your
3575 program, worker threads and so on \- you just to make sure to destroy the
3576 loop when you want them to be invoked.
3577 .PP
3578 Cleanup watchers are invoked in the same way as any other watcher. Unlike
3579 all other watchers, they do not keep a reference to the event loop (which
3580 makes a lot of sense if you think about it). Like all other watchers, you
3581 can call libev functions in the callback, except \f(CW\*(C`ev_cleanup_start\*(C'\fR.
3582 .PP
3583 \fIWatcher-Specific Functions and Data Members\fR
3584 .IX Subsection "Watcher-Specific Functions and Data Members"
3585 .IP "ev_cleanup_init (ev_cleanup *, callback)" 4
3586 .IX Item "ev_cleanup_init (ev_cleanup *, callback)"
3587 Initialises and configures the cleanup watcher \- it has no parameters of
3588 any kind. There is a \f(CW\*(C`ev_cleanup_set\*(C'\fR macro, but using it is utterly
3589 pointless, I assure you.
3590 .PP
3591 Example: Register an atexit handler to destroy the default loop, so any
3592 cleanup functions are called.
3593 .PP
3594 .Vb 5
3595 \& static void
3596 \& program_exits (void)
3597 \& {
3598 \& ev_loop_destroy (EV_DEFAULT_UC);
3599 \& }
3600 \&
3601 \& ...
3602 \& atexit (program_exits);
3603 .Ve
3604 .ie n .SS """ev_async"" \- how to wake up an event loop"
3605 .el .SS "\f(CWev_async\fP \- how to wake up an event loop"
3606 .IX Subsection "ev_async - how to wake up an event loop"
3607 In general, you cannot use an \f(CW\*(C`ev_loop\*(C'\fR from multiple threads or other
3608 asynchronous sources such as signal handlers (as opposed to multiple event
3609 loops \- those are of course safe to use in different threads).
3610 .PP
3611 Sometimes, however, you need to wake up an event loop you do not control,
3612 for example because it belongs to another thread. This is what \f(CW\*(C`ev_async\*(C'\fR
3613 watchers do: as long as the \f(CW\*(C`ev_async\*(C'\fR watcher is active, you can signal
3614 it by calling \f(CW\*(C`ev_async_send\*(C'\fR, which is thread\- and signal safe.
3615 .PP
3616 This functionality is very similar to \f(CW\*(C`ev_signal\*(C'\fR watchers, as signals,
3617 too, are asynchronous in nature, and signals, too, will be compressed
3618 (i.e. the number of callback invocations may be less than the number of
3619 \&\f(CW\*(C`ev_async_send\*(C'\fR calls). In fact, you could use signal watchers as a kind
3620 of \*(L"global async watchers\*(R" by using a watcher on an otherwise unused
3621 signal, and \f(CW\*(C`ev_feed_signal\*(C'\fR to signal this watcher from another thread,
3622 even without knowing which loop owns the signal.
3623 .PP
3624 \fIQueueing\fR
3625 .IX Subsection "Queueing"
3626 .PP
3627 \&\f(CW\*(C`ev_async\*(C'\fR does not support queueing of data in any way. The reason
3628 is that the author does not know of a simple (or any) algorithm for a
3629 multiple-writer-single-reader queue that works in all cases and doesn't
3630 need elaborate support such as pthreads or unportable memory access
3631 semantics.
3632 .PP
3633 That means that if you want to queue data, you have to provide your own
3634 queue. But at least I can tell you how to implement locking around your
3635 queue:
3636 .IP "queueing from a signal handler context" 4
3637 .IX Item "queueing from a signal handler context"
3638 To implement race-free queueing, you simply add to the queue in the signal
3639 handler but you block the signal handler in the watcher callback. Here is
3640 an example that does that for some fictitious \s-1SIGUSR1\s0 handler:
3641 .Sp
3642 .Vb 1
3643 \& static ev_async mysig;
3644 \&
3645 \& static void
3646 \& sigusr1_handler (void)
3647 \& {
3648 \& sometype data;
3649 \&
3650 \& // no locking etc.
3651 \& queue_put (data);
3652 \& ev_async_send (EV_DEFAULT_ &mysig);
3653 \& }
3654 \&
3655 \& static void
3656 \& mysig_cb (EV_P_ ev_async *w, int revents)
3657 \& {
3658 \& sometype data;
3659 \& sigset_t block, prev;
3660 \&
3661 \& sigemptyset (&block);
3662 \& sigaddset (&block, SIGUSR1);
3663 \& sigprocmask (SIG_BLOCK, &block, &prev);
3664 \&
3665 \& while (queue_get (&data))
3666 \& process (data);
3667 \&
3668 \& if (sigismember (&prev, SIGUSR1)
3669 \& sigprocmask (SIG_UNBLOCK, &block, 0);
3670 \& }
3671 .Ve
3672 .Sp
3673 (Note: pthreads in theory requires you to use \f(CW\*(C`pthread_setmask\*(C'\fR
3674 instead of \f(CW\*(C`sigprocmask\*(C'\fR when you use threads, but libev doesn't do it
3675 either...).
3676 .IP "queueing from a thread context" 4
3677 .IX Item "queueing from a thread context"
3678 The strategy for threads is different, as you cannot (easily) block
3679 threads but you can easily preempt them, so to queue safely you need to
3680 employ a traditional mutex lock, such as in this pthread example:
3681 .Sp
3682 .Vb 2
3683 \& static ev_async mysig;
3684 \& static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3685 \&
3686 \& static void
3687 \& otherthread (void)
3688 \& {
3689 \& // only need to lock the actual queueing operation
3690 \& pthread_mutex_lock (&mymutex);
3691 \& queue_put (data);
3692 \& pthread_mutex_unlock (&mymutex);
3693 \&
3694 \& ev_async_send (EV_DEFAULT_ &mysig);
3695 \& }
3696 \&
3697 \& static void
3698 \& mysig_cb (EV_P_ ev_async *w, int revents)
3699 \& {
3700 \& pthread_mutex_lock (&mymutex);
3701 \&
3702 \& while (queue_get (&data))
3703 \& process (data);
3704 \&
3705 \& pthread_mutex_unlock (&mymutex);
3706 \& }
3707 .Ve
3708 .PP
3709 \fIWatcher-Specific Functions and Data Members\fR
3710 .IX Subsection "Watcher-Specific Functions and Data Members"
3711 .IP "ev_async_init (ev_async *, callback)" 4
3712 .IX Item "ev_async_init (ev_async *, callback)"
3713 Initialises and configures the async watcher \- it has no parameters of any
3714 kind. There is a \f(CW\*(C`ev_async_set\*(C'\fR macro, but using it is utterly pointless,
3715 trust me.
3716 .IP "ev_async_send (loop, ev_async *)" 4
3717 .IX Item "ev_async_send (loop, ev_async *)"
3718 Sends/signals/activates the given \f(CW\*(C`ev_async\*(C'\fR watcher, that is, feeds
3719 an \f(CW\*(C`EV_ASYNC\*(C'\fR event on the watcher into the event loop, and instantly
3720 returns.
3721 .Sp
3722 Unlike \f(CW\*(C`ev_feed_event\*(C'\fR, this call is safe to do from other threads,
3723 signal or similar contexts (see the discussion of \f(CW\*(C`EV_ATOMIC_T\*(C'\fR in the
3724 embedding section below on what exactly this means).
3725 .Sp
3726 Note that, as with other watchers in libev, multiple events might get
3727 compressed into a single callback invocation (another way to look at
3728 this is that \f(CW\*(C`ev_async\*(C'\fR watchers are level-triggered: they are set on
3729 \&\f(CW\*(C`ev_async_send\*(C'\fR, reset when the event loop detects that).
3730 .Sp
3731 This call incurs the overhead of at most one extra system call per event
3732 loop iteration, if the event loop is blocked, and no syscall at all if
3733 the event loop (or your program) is processing events. That means that
3734 repeated calls are basically free (there is no need to avoid calls for
3735 performance reasons) and that the overhead becomes smaller (typically
3736 zero) under load.
3737 .IP "bool = ev_async_pending (ev_async *)" 4
3738 .IX Item "bool = ev_async_pending (ev_async *)"
3739 Returns a non-zero value when \f(CW\*(C`ev_async_send\*(C'\fR has been called on the
3740 watcher but the event has not yet been processed (or even noted) by the
3741 event loop.
3742 .Sp
3743 \&\f(CW\*(C`ev_async_send\*(C'\fR sets a flag in the watcher and wakes up the loop. When
3744 the loop iterates next and checks for the watcher to have become active,
3745 it will reset the flag again. \f(CW\*(C`ev_async_pending\*(C'\fR can be used to very
3746 quickly check whether invoking the loop might be a good idea.
3747 .Sp
3748 Not that this does \fInot\fR check whether the watcher itself is pending,
3749 only whether it has been requested to make this watcher pending: there
3750 is a time window between the event loop checking and resetting the async
3751 notification, and the callback being invoked.
3752 .SH "OTHER FUNCTIONS"
3753 .IX Header "OTHER FUNCTIONS"
3754 There are some other functions of possible interest. Described. Here. Now.
3755 .IP "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)" 4
3756 .IX Item "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)"
3757 This function combines a simple timer and an I/O watcher, calls your
3758 callback on whichever event happens first and automatically stops both
3759 watchers. This is useful if you want to wait for a single event on an fd
3760 or timeout without having to allocate/configure/start/stop/free one or
3761 more watchers yourself.
3762 .Sp
3763 If \f(CW\*(C`fd\*(C'\fR is less than 0, then no I/O watcher will be started and the
3764 \&\f(CW\*(C`events\*(C'\fR argument is being ignored. Otherwise, an \f(CW\*(C`ev_io\*(C'\fR watcher for
3765 the given \f(CW\*(C`fd\*(C'\fR and \f(CW\*(C`events\*(C'\fR set will be created and started.
3766 .Sp
3767 If \f(CW\*(C`timeout\*(C'\fR is less than 0, then no timeout watcher will be
3768 started. Otherwise an \f(CW\*(C`ev_timer\*(C'\fR watcher with after = \f(CW\*(C`timeout\*(C'\fR (and
3769 repeat = 0) will be started. \f(CW0\fR is a valid timeout.
3770 .Sp
3771 The callback has the type \f(CW\*(C`void (*cb)(int revents, void *arg)\*(C'\fR and is
3772 passed an \f(CW\*(C`revents\*(C'\fR set like normal event callbacks (a combination of
3773 \&\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
3774 value passed to \f(CW\*(C`ev_once\*(C'\fR. Note that it is possible to receive \fIboth\fR
3775 a timeout and an io event at the same time \- you probably should give io
3776 events precedence.
3777 .Sp
3778 Example: wait up to ten seconds for data to appear on \s-1STDIN_FILENO.\s0
3779 .Sp
3780 .Vb 7
3781 \& static void stdin_ready (int revents, void *arg)
3782 \& {
3783 \& if (revents & EV_READ)
3784 \& /* stdin might have data for us, joy! */;
3785 \& else if (revents & EV_TIMER)
3786 \& /* doh, nothing entered */;
3787 \& }
3788 \&
3789 \& ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3790 .Ve
3791 .IP "ev_feed_fd_event (loop, int fd, int revents)" 4
3792 .IX Item "ev_feed_fd_event (loop, int fd, int revents)"
3793 Feed an event on the given fd, as if a file descriptor backend detected
3794 the given events.
3795 .IP "ev_feed_signal_event (loop, int signum)" 4
3796 .IX Item "ev_feed_signal_event (loop, int signum)"
3797 Feed an event as if the given signal occurred. See also \f(CW\*(C`ev_feed_signal\*(C'\fR,
3798 which is async-safe.
3799 .SH "COMMON OR USEFUL IDIOMS (OR BOTH)"
3800 .IX Header "COMMON OR USEFUL IDIOMS (OR BOTH)"
3801 This section explains some common idioms that are not immediately
3802 obvious. Note that examples are sprinkled over the whole manual, and this
3803 section only contains stuff that wouldn't fit anywhere else.
3804 .SS "\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\s0"
3805 .IX Subsection "ASSOCIATING CUSTOM DATA WITH A WATCHER"
3806 Each watcher has, by default, a \f(CW\*(C`void *data\*(C'\fR member that you can read
3807 or modify at any time: libev will completely ignore it. This can be used
3808 to associate arbitrary data with your watcher. If you need more data and
3809 don't want to allocate memory separately and store a pointer to it in that
3810 data member, you can also \*(L"subclass\*(R" the watcher type and provide your own
3811 data:
3812 .PP
3813 .Vb 7
3814 \& struct my_io
3815 \& {
3816 \& ev_io io;
3817 \& int otherfd;
3818 \& void *somedata;
3819 \& struct whatever *mostinteresting;
3820 \& };
3821 \&
3822 \& ...
3823 \& struct my_io w;
3824 \& ev_io_init (&w.io, my_cb, fd, EV_READ);
3825 .Ve
3826 .PP
3827 And since your callback will be called with a pointer to the watcher, you
3828 can cast it back to your own type:
3829 .PP
3830 .Vb 5
3831 \& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3832 \& {
3833 \& struct my_io *w = (struct my_io *)w_;
3834 \& ...
3835 \& }
3836 .Ve
3837 .PP
3838 More interesting and less C\-conformant ways of casting your callback
3839 function type instead have been omitted.
3840 .SS "\s-1BUILDING YOUR OWN COMPOSITE WATCHERS\s0"
3841 .IX Subsection "BUILDING YOUR OWN COMPOSITE WATCHERS"
3842 Another common scenario is to use some data structure with multiple
3843 embedded watchers, in effect creating your own watcher that combines
3844 multiple libev event sources into one \*(L"super-watcher\*(R":
3845 .PP
3846 .Vb 6
3847 \& struct my_biggy
3848 \& {
3849 \& int some_data;
3850 \& ev_timer t1;
3851 \& ev_timer t2;
3852 \& }
3853 .Ve
3854 .PP
3855 In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more
3856 complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct in
3857 the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies or \*(C+ coders), or you need
3858 to use some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for
3859 real programmers):
3860 .PP
3861 .Vb 1
3862 \& #include <stddef.h>
3863 \&
3864 \& static void
3865 \& t1_cb (EV_P_ ev_timer *w, int revents)
3866 \& {
3867 \& struct my_biggy big = (struct my_biggy *)
3868 \& (((char *)w) \- offsetof (struct my_biggy, t1));
3869 \& }
3870 \&
3871 \& static void
3872 \& t2_cb (EV_P_ ev_timer *w, int revents)
3873 \& {
3874 \& struct my_biggy big = (struct my_biggy *)
3875 \& (((char *)w) \- offsetof (struct my_biggy, t2));
3876 \& }
3877 .Ve
3878 .SS "\s-1AVOIDING FINISHING BEFORE RETURNING\s0"
3879 .IX Subsection "AVOIDING FINISHING BEFORE RETURNING"
3880 Often you have structures like this in event-based programs:
3881 .PP
3882 .Vb 4
3883 \& callback ()
3884 \& {
3885 \& free (request);
3886 \& }
3887 \&
3888 \& request = start_new_request (..., callback);
3889 .Ve
3890 .PP
3891 The intent is to start some \*(L"lengthy\*(R" operation. The \f(CW\*(C`request\*(C'\fR could be
3892 used to cancel the operation, or do other things with it.
3893 .PP
3894 It's not uncommon to have code paths in \f(CW\*(C`start_new_request\*(C'\fR that
3895 immediately invoke the callback, for example, to report errors. Or you add
3896 some caching layer that finds that it can skip the lengthy aspects of the
3897 operation and simply invoke the callback with the result.
3898 .PP
3899 The problem here is that this will happen \fIbefore\fR \f(CW\*(C`start_new_request\*(C'\fR
3900 has returned, so \f(CW\*(C`request\*(C'\fR is not set.
3901 .PP
3902 Even if you pass the request by some safer means to the callback, you
3903 might want to do something to the request after starting it, such as
3904 canceling it, which probably isn't working so well when the callback has
3905 already been invoked.
3906 .PP
3907 A common way around all these issues is to make sure that
3908 \&\f(CW\*(C`start_new_request\*(C'\fR \fIalways\fR returns before the callback is invoked. If
3909 \&\f(CW\*(C`start_new_request\*(C'\fR immediately knows the result, it can artificially
3910 delay invoking the callback by using a \f(CW\*(C`prepare\*(C'\fR or \f(CW\*(C`idle\*(C'\fR watcher for
3911 example, or more sneakily, by reusing an existing (stopped) watcher and
3912 pushing it into the pending queue:
3913 .PP
3914 .Vb 2
3915 \& ev_set_cb (watcher, callback);
3916 \& ev_feed_event (EV_A_ watcher, 0);
3917 .Ve
3918 .PP
3919 This way, \f(CW\*(C`start_new_request\*(C'\fR can safely return before the callback is
3920 invoked, while not delaying callback invocation too much.
3921 .SS "\s-1MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS\s0"
3922 .IX Subsection "MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS"
3923 Often (especially in \s-1GUI\s0 toolkits) there are places where you have
3924 \&\fImodal\fR interaction, which is most easily implemented by recursively
3925 invoking \f(CW\*(C`ev_run\*(C'\fR.
3926 .PP
3927 This brings the problem of exiting \- a callback might want to finish the
3928 main \f(CW\*(C`ev_run\*(C'\fR call, but not the nested one (e.g. user clicked \*(L"Quit\*(R", but
3929 a modal \*(L"Are you sure?\*(R" dialog is still waiting), or just the nested one
3930 and not the main one (e.g. user clocked \*(L"Ok\*(R" in a modal dialog), or some
3931 other combination: In these cases, a simple \f(CW\*(C`ev_break\*(C'\fR will not work.
3932 .PP
3933 The solution is to maintain \*(L"break this loop\*(R" variable for each \f(CW\*(C`ev_run\*(C'\fR
3934 invocation, and use a loop around \f(CW\*(C`ev_run\*(C'\fR until the condition is
3935 triggered, using \f(CW\*(C`EVRUN_ONCE\*(C'\fR:
3936 .PP
3937 .Vb 2
3938 \& // main loop
3939 \& int exit_main_loop = 0;
3940 \&
3941 \& while (!exit_main_loop)
3942 \& ev_run (EV_DEFAULT_ EVRUN_ONCE);
3943 \&
3944 \& // in a modal watcher
3945 \& int exit_nested_loop = 0;
3946 \&
3947 \& while (!exit_nested_loop)
3948 \& ev_run (EV_A_ EVRUN_ONCE);
3949 .Ve
3950 .PP
3951 To exit from any of these loops, just set the corresponding exit variable:
3952 .PP
3953 .Vb 2
3954 \& // exit modal loop
3955 \& exit_nested_loop = 1;
3956 \&
3957 \& // exit main program, after modal loop is finished
3958 \& exit_main_loop = 1;
3959 \&
3960 \& // exit both
3961 \& exit_main_loop = exit_nested_loop = 1;
3962 .Ve
3963 .SS "\s-1THREAD LOCKING EXAMPLE\s0"
3964 .IX Subsection "THREAD LOCKING EXAMPLE"
3965 Here is a fictitious example of how to run an event loop in a different
3966 thread from where callbacks are being invoked and watchers are
3967 created/added/removed.
3968 .PP
3969 For a real-world example, see the \f(CW\*(C`EV::Loop::Async\*(C'\fR perl module,
3970 which uses exactly this technique (which is suited for many high-level
3971 languages).
3972 .PP
3973 The example uses a pthread mutex to protect the loop data, a condition
3974 variable to wait for callback invocations, an async watcher to notify the
3975 event loop thread and an unspecified mechanism to wake up the main thread.
3976 .PP
3977 First, you need to associate some data with the event loop:
3978 .PP
3979 .Vb 6
3980 \& typedef struct {
3981 \& mutex_t lock; /* global loop lock */
3982 \& ev_async async_w;
3983 \& thread_t tid;
3984 \& cond_t invoke_cv;
3985 \& } userdata;
3986 \&
3987 \& void prepare_loop (EV_P)
3988 \& {
3989 \& // for simplicity, we use a static userdata struct.
3990 \& static userdata u;
3991 \&
3992 \& ev_async_init (&u\->async_w, async_cb);
3993 \& ev_async_start (EV_A_ &u\->async_w);
3994 \&
3995 \& pthread_mutex_init (&u\->lock, 0);
3996 \& pthread_cond_init (&u\->invoke_cv, 0);
3997 \&
3998 \& // now associate this with the loop
3999 \& ev_set_userdata (EV_A_ u);
4000 \& ev_set_invoke_pending_cb (EV_A_ l_invoke);
4001 \& ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
4002 \&
4003 \& // then create the thread running ev_run
4004 \& pthread_create (&u\->tid, 0, l_run, EV_A);
4005 \& }
4006 .Ve
4007 .PP
4008 The callback for the \f(CW\*(C`ev_async\*(C'\fR watcher does nothing: the watcher is used
4009 solely to wake up the event loop so it takes notice of any new watchers
4010 that might have been added:
4011 .PP
4012 .Vb 5
4013 \& static void
4014 \& async_cb (EV_P_ ev_async *w, int revents)
4015 \& {
4016 \& // just used for the side effects
4017 \& }
4018 .Ve
4019 .PP
4020 The \f(CW\*(C`l_release\*(C'\fR and \f(CW\*(C`l_acquire\*(C'\fR callbacks simply unlock/lock the mutex
4021 protecting the loop data, respectively.
4022 .PP
4023 .Vb 6
4024 \& static void
4025 \& l_release (EV_P)
4026 \& {
4027 \& userdata *u = ev_userdata (EV_A);
4028 \& pthread_mutex_unlock (&u\->lock);
4029 \& }
4030 \&
4031 \& static void
4032 \& l_acquire (EV_P)
4033 \& {
4034 \& userdata *u = ev_userdata (EV_A);
4035 \& pthread_mutex_lock (&u\->lock);
4036 \& }
4037 .Ve
4038 .PP
4039 The event loop thread first acquires the mutex, and then jumps straight
4040 into \f(CW\*(C`ev_run\*(C'\fR:
4041 .PP
4042 .Vb 4
4043 \& void *
4044 \& l_run (void *thr_arg)
4045 \& {
4046 \& struct ev_loop *loop = (struct ev_loop *)thr_arg;
4047 \&
4048 \& l_acquire (EV_A);
4049 \& pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4050 \& ev_run (EV_A_ 0);
4051 \& l_release (EV_A);
4052 \&
4053 \& return 0;
4054 \& }
4055 .Ve
4056 .PP
4057 Instead of invoking all pending watchers, the \f(CW\*(C`l_invoke\*(C'\fR callback will
4058 signal the main thread via some unspecified mechanism (signals? pipe
4059 writes? \f(CW\*(C`Async::Interrupt\*(C'\fR?) and then waits until all pending watchers
4060 have been called (in a while loop because a) spurious wakeups are possible
4061 and b) skipping inter-thread-communication when there are no pending
4062 watchers is very beneficial):
4063 .PP
4064 .Vb 4
4065 \& static void
4066 \& l_invoke (EV_P)
4067 \& {
4068 \& userdata *u = ev_userdata (EV_A);
4069 \&
4070 \& while (ev_pending_count (EV_A))
4071 \& {
4072 \& wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4073 \& pthread_cond_wait (&u\->invoke_cv, &u\->lock);
4074 \& }
4075 \& }
4076 .Ve
4077 .PP
4078 Now, whenever the main thread gets told to invoke pending watchers, it
4079 will grab the lock, call \f(CW\*(C`ev_invoke_pending\*(C'\fR and then signal the loop
4080 thread to continue:
4081 .PP
4082 .Vb 4
4083 \& static void
4084 \& real_invoke_pending (EV_P)
4085 \& {
4086 \& userdata *u = ev_userdata (EV_A);
4087 \&
4088 \& pthread_mutex_lock (&u\->lock);
4089 \& ev_invoke_pending (EV_A);
4090 \& pthread_cond_signal (&u\->invoke_cv);
4091 \& pthread_mutex_unlock (&u\->lock);
4092 \& }
4093 .Ve
4094 .PP
4095 Whenever you want to start/stop a watcher or do other modifications to an
4096 event loop, you will now have to lock:
4097 .PP
4098 .Vb 2
4099 \& ev_timer timeout_watcher;
4100 \& userdata *u = ev_userdata (EV_A);
4101 \&
4102 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4103 \&
4104 \& pthread_mutex_lock (&u\->lock);
4105 \& ev_timer_start (EV_A_ &timeout_watcher);
4106 \& ev_async_send (EV_A_ &u\->async_w);
4107 \& pthread_mutex_unlock (&u\->lock);
4108 .Ve
4109 .PP
4110 Note that sending the \f(CW\*(C`ev_async\*(C'\fR watcher is required because otherwise
4111 an event loop currently blocking in the kernel will have no knowledge
4112 about the newly added timer. By waking up the loop it will pick up any new
4113 watchers in the next event loop iteration.
4114 .SS "\s-1THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS\s0"
4115 .IX Subsection "THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS"
4116 While the overhead of a callback that e.g. schedules a thread is small, it
4117 is still an overhead. If you embed libev, and your main usage is with some
4118 kind of threads or coroutines, you might want to customise libev so that
4119 doesn't need callbacks anymore.
4120 .PP
4121 Imagine you have coroutines that you can switch to using a function
4122 \&\f(CW\*(C`switch_to (coro)\*(C'\fR, that libev runs in a coroutine called \f(CW\*(C`libev_coro\*(C'\fR
4123 and that due to some magic, the currently active coroutine is stored in a
4124 global called \f(CW\*(C`current_coro\*(C'\fR. Then you can build your own \*(L"wait for libev
4125 event\*(R" primitive by changing \f(CW\*(C`EV_CB_DECLARE\*(C'\fR and \f(CW\*(C`EV_CB_INVOKE\*(C'\fR (note
4126 the differing \f(CW\*(C`;\*(C'\fR conventions):
4127 .PP
4128 .Vb 2
4129 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4130 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4131 .Ve
4132 .PP
4133 That means instead of having a C callback function, you store the
4134 coroutine to switch to in each watcher, and instead of having libev call
4135 your callback, you instead have it switch to that coroutine.
4136 .PP
4137 A coroutine might now wait for an event with a function called
4138 \&\f(CW\*(C`wait_for_event\*(C'\fR. (the watcher needs to be started, as always, but it doesn't
4139 matter when, or whether the watcher is active or not when this function is
4140 called):
4141 .PP
4142 .Vb 6
4143 \& void
4144 \& wait_for_event (ev_watcher *w)
4145 \& {
4146 \& ev_set_cb (w, current_coro);
4147 \& switch_to (libev_coro);
4148 \& }
4149 .Ve
4150 .PP
4151 That basically suspends the coroutine inside \f(CW\*(C`wait_for_event\*(C'\fR and
4152 continues the libev coroutine, which, when appropriate, switches back to
4153 this or any other coroutine.
4154 .PP
4155 You can do similar tricks if you have, say, threads with an event queue \-
4156 instead of storing a coroutine, you store the queue object and instead of
4157 switching to a coroutine, you push the watcher onto the queue and notify
4158 any waiters.
4159 .PP
4160 To embed libev, see \*(L"\s-1EMBEDDING\*(R"\s0, but in short, it's easiest to create two
4161 files, \fImy_ev.h\fR and \fImy_ev.c\fR that include the respective libev files:
4162 .PP
4163 .Vb 4
4164 \& // my_ev.h
4165 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4166 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4167 \& #include "../libev/ev.h"
4168 \&
4169 \& // my_ev.c
4170 \& #define EV_H "my_ev.h"
4171 \& #include "../libev/ev.c"
4172 .Ve
4173 .PP
4174 And then use \fImy_ev.h\fR when you would normally use \fIev.h\fR, and compile
4175 \&\fImy_ev.c\fR into your project. When properly specifying include paths, you
4176 can even use \fIev.h\fR as header file name directly.
4177 .SH "LIBEVENT EMULATION"
4178 .IX Header "LIBEVENT EMULATION"
4179 Libev offers a compatibility emulation layer for libevent. It cannot
4180 emulate the internals of libevent, so here are some usage hints:
4181 .IP "\(bu" 4
4182 Only the libevent\-1.4.1\-beta \s-1API\s0 is being emulated.
4183 .Sp
4184 This was the newest libevent version available when libev was implemented,
4185 and is still mostly unchanged in 2010.
4186 .IP "\(bu" 4
4187 Use it by including <event.h>, as usual.
4188 .IP "\(bu" 4
4189 The following members are fully supported: ev_base, ev_callback,
4190 ev_arg, ev_fd, ev_res, ev_events.
4191 .IP "\(bu" 4
4192 Avoid using ev_flags and the EVLIST_*\-macros, while it is
4193 maintained by libev, it does not work exactly the same way as in libevent (consider
4194 it a private \s-1API\s0).
4195 .IP "\(bu" 4
4196 Priorities are not currently supported. Initialising priorities
4197 will fail and all watchers will have the same priority, even though there
4198 is an ev_pri field.
4199 .IP "\(bu" 4
4200 In libevent, the last base created gets the signals, in libev, the
4201 base that registered the signal gets the signals.
4202 .IP "\(bu" 4
4203 Other members are not supported.
4204 .IP "\(bu" 4
4205 The libev emulation is \fInot\fR \s-1ABI\s0 compatible to libevent, you need
4206 to use the libev header file and library.
4207 .SH "\*(C+ SUPPORT"
4208 .IX Header " SUPPORT"
4209 .SS "C \s-1API\s0"
4210 .IX Subsection "C API"
4211 The normal C \s-1API\s0 should work fine when used from \*(C+: both ev.h and the
4212 libev sources can be compiled as \*(C+. Therefore, code that uses the C \s-1API\s0
4213 will work fine.
4214 .PP
4215 Proper exception specifications might have to be added to callbacks passed
4216 to libev: exceptions may be thrown only from watcher callbacks, all other
4217 callbacks (allocator, syserr, loop acquire/release and periodic reschedule
4218 callbacks) must not throw exceptions, and might need a \f(CW\*(C`noexcept\*(C'\fR
4219 specification. If you have code that needs to be compiled as both C and
4220 \&\*(C+ you can use the \f(CW\*(C`EV_NOEXCEPT\*(C'\fR macro for this:
4221 .PP
4222 .Vb 6
4223 \& static void
4224 \& fatal_error (const char *msg) EV_NOEXCEPT
4225 \& {
4226 \& perror (msg);
4227 \& abort ();
4228 \& }
4229 \&
4230 \& ...
4231 \& ev_set_syserr_cb (fatal_error);
4232 .Ve
4233 .PP
4234 The only \s-1API\s0 functions that can currently throw exceptions are \f(CW\*(C`ev_run\*(C'\fR,
4235 \&\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
4236 because it runs cleanup watchers).
4237 .PP
4238 Throwing exceptions in watcher callbacks is only supported if libev itself
4239 is compiled with a \*(C+ compiler or your C and \*(C+ environments allow
4240 throwing exceptions through C libraries (most do).
4241 .SS "\*(C+ \s-1API\s0"
4242 .IX Subsection " API"
4243 Libev comes with some simplistic wrapper classes for \*(C+ that mainly allow
4244 you to use some convenience methods to start/stop watchers and also change
4245 the callback model to a model using method callbacks on objects.
4246 .PP
4247 To use it,
4248 .PP
4249 .Vb 1
4250 \& #include <ev++.h>
4251 .Ve
4252 .PP
4253 This automatically includes \fIev.h\fR and puts all of its definitions (many
4254 of them macros) into the global namespace. All \*(C+ specific things are
4255 put into the \f(CW\*(C`ev\*(C'\fR namespace. It should support all the same embedding
4256 options as \fIev.h\fR, most notably \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR.
4257 .PP
4258 Care has been taken to keep the overhead low. The only data member the \*(C+
4259 classes add (compared to plain C\-style watchers) is the event loop pointer
4260 that the watcher is associated with (or no additional members at all if
4261 you disable \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR when embedding libev).
4262 .PP
4263 Currently, functions, static and non-static member functions and classes
4264 with \f(CW\*(C`operator ()\*(C'\fR can be used as callbacks. Other types should be easy
4265 to add as long as they only need one additional pointer for context. If
4266 you need support for other types of functors please contact the author
4267 (preferably after implementing it).
4268 .PP
4269 For all this to work, your \*(C+ compiler either has to use the same calling
4270 conventions as your C compiler (for static member functions), or you have
4271 to embed libev and compile libev itself as \*(C+.
4272 .PP
4273 Here is a list of things available in the \f(CW\*(C`ev\*(C'\fR namespace:
4274 .ie n .IP """ev::READ"", ""ev::WRITE"" etc." 4
4275 .el .IP "\f(CWev::READ\fR, \f(CWev::WRITE\fR etc." 4
4276 .IX Item "ev::READ, ev::WRITE etc."
4277 These are just enum values with the same values as the \f(CW\*(C`EV_READ\*(C'\fR etc.
4278 macros from \fIev.h\fR.
4279 .ie n .IP """ev::tstamp"", ""ev::now""" 4
4280 .el .IP "\f(CWev::tstamp\fR, \f(CWev::now\fR" 4
4281 .IX Item "ev::tstamp, ev::now"
4282 Aliases to the same types/functions as with the \f(CW\*(C`ev_\*(C'\fR prefix.
4283 .ie n .IP """ev::io"", ""ev::timer"", ""ev::periodic"", ""ev::idle"", ""ev::sig"" etc." 4
4284 .el .IP "\f(CWev::io\fR, \f(CWev::timer\fR, \f(CWev::periodic\fR, \f(CWev::idle\fR, \f(CWev::sig\fR etc." 4
4285 .IX Item "ev::io, ev::timer, ev::periodic, ev::idle, ev::sig etc."
4286 For each \f(CW\*(C`ev_TYPE\*(C'\fR watcher in \fIev.h\fR there is a corresponding class of
4287 the same name in the \f(CW\*(C`ev\*(C'\fR namespace, with the exception of \f(CW\*(C`ev_signal\*(C'\fR
4288 which is called \f(CW\*(C`ev::sig\*(C'\fR to avoid clashes with the \f(CW\*(C`signal\*(C'\fR macro
4289 defined by many implementations.
4290 .Sp
4291 All of those classes have these methods:
4292 .RS 4
4293 .IP "ev::TYPE::TYPE ()" 4
4294 .IX Item "ev::TYPE::TYPE ()"
4295 .PD 0
4296 .IP "ev::TYPE::TYPE (loop)" 4
4297 .IX Item "ev::TYPE::TYPE (loop)"
4298 .IP "ev::TYPE::~TYPE" 4
4299 .IX Item "ev::TYPE::~TYPE"
4300 .PD
4301 The constructor (optionally) takes an event loop to associate the watcher
4302 with. If it is omitted, it will use \f(CW\*(C`EV_DEFAULT\*(C'\fR.
4303 .Sp
4304 The constructor calls \f(CW\*(C`ev_init\*(C'\fR for you, which means you have to call the
4305 \&\f(CW\*(C`set\*(C'\fR method before starting it.
4306 .Sp
4307 It will not set a callback, however: You have to call the templated \f(CW\*(C`set\*(C'\fR
4308 method to set a callback before you can start the watcher.
4309 .Sp
4310 (The reason why you have to use a method is a limitation in \*(C+ which does
4311 not allow explicit template arguments for constructors).
4312 .Sp
4313 The destructor automatically stops the watcher if it is active.
4314 .IP "w\->set<class, &class::method> (object *)" 4
4315 .IX Item "w->set<class, &class::method> (object *)"
4316 This method sets the callback method to call. The method has to have a
4317 signature of \f(CW\*(C`void (*)(ev_TYPE &, int)\*(C'\fR, it receives the watcher as
4318 first argument and the \f(CW\*(C`revents\*(C'\fR as second. The object must be given as
4319 parameter and is stored in the \f(CW\*(C`data\*(C'\fR member of the watcher.
4320 .Sp
4321 This method synthesizes efficient thunking code to call your method from
4322 the C callback that libev requires. If your compiler can inline your
4323 callback (i.e. it is visible to it at the place of the \f(CW\*(C`set\*(C'\fR call and
4324 your compiler is good :), then the method will be fully inlined into the
4325 thunking function, making it as fast as a direct C callback.
4326 .Sp
4327 Example: simple class declaration and watcher initialisation
4328 .Sp
4329 .Vb 4
4330 \& struct myclass
4331 \& {
4332 \& void io_cb (ev::io &w, int revents) { }
4333 \& }
4334 \&
4335 \& myclass obj;
4336 \& ev::io iow;
4337 \& iow.set <myclass, &myclass::io_cb> (&obj);
4338 .Ve
4339 .IP "w\->set (object *)" 4
4340 .IX Item "w->set (object *)"
4341 This is a variation of a method callback \- leaving out the method to call
4342 will default the method to \f(CW\*(C`operator ()\*(C'\fR, which makes it possible to use
4343 functor objects without having to manually specify the \f(CW\*(C`operator ()\*(C'\fR all
4344 the time. Incidentally, you can then also leave out the template argument
4345 list.
4346 .Sp
4347 The \f(CW\*(C`operator ()\*(C'\fR method prototype must be \f(CW\*(C`void operator ()(watcher &w,
4348 int revents)\*(C'\fR.
4349 .Sp
4350 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4351 .Sp
4352 Example: use a functor object as callback.
4353 .Sp
4354 .Vb 7
4355 \& struct myfunctor
4356 \& {
4357 \& void operator() (ev::io &w, int revents)
4358 \& {
4359 \& ...
4360 \& }
4361 \& }
4362 \&
4363 \& myfunctor f;
4364 \&
4365 \& ev::io w;
4366 \& w.set (&f);
4367 .Ve
4368 .IP "w\->set<function> (void *data = 0)" 4
4369 .IX Item "w->set<function> (void *data = 0)"
4370 Also sets a callback, but uses a static method or plain function as
4371 callback. The optional \f(CW\*(C`data\*(C'\fR argument will be stored in the watcher's
4372 \&\f(CW\*(C`data\*(C'\fR member and is free for you to use.
4373 .Sp
4374 The prototype of the \f(CW\*(C`function\*(C'\fR must be \f(CW\*(C`void (*)(ev::TYPE &w, int)\*(C'\fR.
4375 .Sp
4376 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4377 .Sp
4378 Example: Use a plain function as callback.
4379 .Sp
4380 .Vb 2
4381 \& static void io_cb (ev::io &w, int revents) { }
4382 \& iow.set <io_cb> ();
4383 .Ve
4384 .IP "w\->set (loop)" 4
4385 .IX Item "w->set (loop)"
4386 Associates a different \f(CW\*(C`struct ev_loop\*(C'\fR with this watcher. You can only
4387 do this when the watcher is inactive (and not pending either).
4388 .IP "w\->set ([arguments])" 4
4389 .IX Item "w->set ([arguments])"
4390 Basically the same as \f(CW\*(C`ev_TYPE_set\*(C'\fR (except for \f(CW\*(C`ev::embed\*(C'\fR watchers>),
4391 with the same arguments. Either this method or a suitable start method
4392 must be called at least once. Unlike the C counterpart, an active watcher
4393 gets automatically stopped and restarted when reconfiguring it with this
4394 method.
4395 .Sp
4396 For \f(CW\*(C`ev::embed\*(C'\fR watchers this method is called \f(CW\*(C`set_embed\*(C'\fR, to avoid
4397 clashing with the \f(CW\*(C`set (loop)\*(C'\fR method.
4398 .IP "w\->start ()" 4
4399 .IX Item "w->start ()"
4400 Starts the watcher. Note that there is no \f(CW\*(C`loop\*(C'\fR argument, as the
4401 constructor already stores the event loop.
4402 .IP "w\->start ([arguments])" 4
4403 .IX Item "w->start ([arguments])"
4404 Instead of calling \f(CW\*(C`set\*(C'\fR and \f(CW\*(C`start\*(C'\fR methods separately, it is often
4405 convenient to wrap them in one call. Uses the same type of arguments as
4406 the configure \f(CW\*(C`set\*(C'\fR method of the watcher.
4407 .IP "w\->stop ()" 4
4408 .IX Item "w->stop ()"
4409 Stops the watcher if it is active. Again, no \f(CW\*(C`loop\*(C'\fR argument.
4410 .ie n .IP "w\->again () (""ev::timer"", ""ev::periodic"" only)" 4
4411 .el .IP "w\->again () (\f(CWev::timer\fR, \f(CWev::periodic\fR only)" 4
4412 .IX Item "w->again () (ev::timer, ev::periodic only)"
4413 For \f(CW\*(C`ev::timer\*(C'\fR and \f(CW\*(C`ev::periodic\*(C'\fR, this invokes the corresponding
4414 \&\f(CW\*(C`ev_TYPE_again\*(C'\fR function.
4415 .ie n .IP "w\->sweep () (""ev::embed"" only)" 4
4416 .el .IP "w\->sweep () (\f(CWev::embed\fR only)" 4
4417 .IX Item "w->sweep () (ev::embed only)"
4418 Invokes \f(CW\*(C`ev_embed_sweep\*(C'\fR.
4419 .ie n .IP "w\->update () (""ev::stat"" only)" 4
4420 .el .IP "w\->update () (\f(CWev::stat\fR only)" 4
4421 .IX Item "w->update () (ev::stat only)"
4422 Invokes \f(CW\*(C`ev_stat_stat\*(C'\fR.
4423 .RE
4424 .RS 4
4425 .RE
4426 .PP
4427 Example: Define a class with two I/O and idle watchers, start the I/O
4428 watchers in the constructor.
4429 .PP
4430 .Vb 5
4431 \& class myclass
4432 \& {
4433 \& ev::io io ; void io_cb (ev::io &w, int revents);
4434 \& ev::io io2 ; void io2_cb (ev::io &w, int revents);
4435 \& ev::idle idle; void idle_cb (ev::idle &w, int revents);
4436 \&
4437 \& myclass (int fd)
4438 \& {
4439 \& io .set <myclass, &myclass::io_cb > (this);
4440 \& io2 .set <myclass, &myclass::io2_cb > (this);
4441 \& idle.set <myclass, &myclass::idle_cb> (this);
4442 \&
4443 \& io.set (fd, ev::WRITE); // configure the watcher
4444 \& io.start (); // start it whenever convenient
4445 \&
4446 \& io2.start (fd, ev::READ); // set + start in one call
4447 \& }
4448 \& };
4449 .Ve
4450 .SH "OTHER LANGUAGE BINDINGS"
4451 .IX Header "OTHER LANGUAGE BINDINGS"
4452 Libev does not offer other language bindings itself, but bindings for a
4453 number of languages exist in the form of third-party packages. If you know
4454 any interesting language binding in addition to the ones listed here, drop
4455 me a note.
4456 .IP "Perl" 4
4457 .IX Item "Perl"
4458 The \s-1EV\s0 module implements the full libev \s-1API\s0 and is actually used to test
4459 libev. \s-1EV\s0 is developed together with libev. Apart from the \s-1EV\s0 core module,
4460 there are additional modules that implement libev-compatible interfaces
4461 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),
4462 \&\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
4463 and \f(CW\*(C`EV::Glib\*(C'\fR).
4464 .Sp
4465 It can be found and installed via \s-1CPAN,\s0 its homepage is at
4466 <http://software.schmorp.de/pkg/EV>.
4467 .IP "Python" 4
4468 .IX Item "Python"
4469 Python bindings can be found at <http://code.google.com/p/pyev/>. It
4470 seems to be quite complete and well-documented.
4471 .IP "Ruby" 4
4472 .IX Item "Ruby"
4473 Tony Arcieri has written a ruby extension that offers access to a subset
4474 of the libev \s-1API\s0 and adds file handle abstractions, asynchronous \s-1DNS\s0 and
4475 more on top of it. It can be found via gem servers. Its homepage is at
4476 <http://rev.rubyforge.org/>.
4477 .Sp
4478 Roger Pack reports that using the link order \f(CW\*(C`\-lws2_32 \-lmsvcrt\-ruby\-190\*(C'\fR
4479 makes rev work even on mingw.
4480 .IP "Haskell" 4
4481 .IX Item "Haskell"
4482 A haskell binding to libev is available at
4483 <http://hackage.haskell.org/cgi\-bin/hackage\-scripts/package/hlibev>.
4484 .IP "D" 4
4485 .IX Item "D"
4486 Leandro Lucarella has written a D language binding (\fIev.d\fR) for libev, to
4487 be found at <http://www.llucax.com.ar/proj/ev.d/index.html>.
4488 .IP "Ocaml" 4
4489 .IX Item "Ocaml"
4490 Erkki Seppala has written Ocaml bindings for libev, to be found at
4491 <http://modeemi.cs.tut.fi/~flux/software/ocaml\-ev/>.
4492 .IP "Lua" 4
4493 .IX Item "Lua"
4494 Brian Maher has written a partial interface to libev for lua (at the
4495 time of this writing, only \f(CW\*(C`ev_io\*(C'\fR and \f(CW\*(C`ev_timer\*(C'\fR), to be found at
4496 <http://github.com/brimworks/lua\-ev>.
4497 .IP "Javascript" 4
4498 .IX Item "Javascript"
4499 Node.js (<http://nodejs.org>) uses libev as the underlying event library.
4500 .IP "Others" 4
4501 .IX Item "Others"
4502 There are others, and I stopped counting.
4503 .SH "MACRO MAGIC"
4504 .IX Header "MACRO MAGIC"
4505 Libev can be compiled with a variety of options, the most fundamental
4506 of which is \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR. This option determines whether (most)
4507 functions and callbacks have an initial \f(CW\*(C`struct ev_loop *\*(C'\fR argument.
4508 .PP
4509 To make it easier to write programs that cope with either variant, the
4510 following macros are defined:
4511 .ie n .IP """EV_A"", ""EV_A_""" 4
4512 .el .IP "\f(CWEV_A\fR, \f(CWEV_A_\fR" 4
4513 .IX Item "EV_A, EV_A_"
4514 This provides the loop \fIargument\fR for functions, if one is required (\*(L"ev
4515 loop argument\*(R"). The \f(CW\*(C`EV_A\*(C'\fR form is used when this is the sole argument,
4516 \&\f(CW\*(C`EV_A_\*(C'\fR is used when other arguments are following. Example:
4517 .Sp
4518 .Vb 3
4519 \& ev_unref (EV_A);
4520 \& ev_timer_add (EV_A_ watcher);
4521 \& ev_run (EV_A_ 0);
4522 .Ve
4523 .Sp
4524 It assumes the variable \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR is in scope,
4525 which is often provided by the following macro.
4526 .ie n .IP """EV_P"", ""EV_P_""" 4
4527 .el .IP "\f(CWEV_P\fR, \f(CWEV_P_\fR" 4
4528 .IX Item "EV_P, EV_P_"
4529 This provides the loop \fIparameter\fR for functions, if one is required (\*(L"ev
4530 loop parameter\*(R"). The \f(CW\*(C`EV_P\*(C'\fR form is used when this is the sole parameter,
4531 \&\f(CW\*(C`EV_P_\*(C'\fR is used when other parameters are following. Example:
4532 .Sp
4533 .Vb 2
4534 \& // this is how ev_unref is being declared
4535 \& static void ev_unref (EV_P);
4536 \&
4537 \& // this is how you can declare your typical callback
4538 \& static void cb (EV_P_ ev_timer *w, int revents)
4539 .Ve
4540 .Sp
4541 It declares a parameter \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR, quite
4542 suitable for use with \f(CW\*(C`EV_A\*(C'\fR.
4543 .ie n .IP """EV_DEFAULT"", ""EV_DEFAULT_""" 4
4544 .el .IP "\f(CWEV_DEFAULT\fR, \f(CWEV_DEFAULT_\fR" 4
4545 .IX Item "EV_DEFAULT, EV_DEFAULT_"
4546 Similar to the other two macros, this gives you the value of the default
4547 loop, if multiple loops are supported (\*(L"ev loop default\*(R"). The default loop
4548 will be initialised if it isn't already initialised.
4549 .Sp
4550 For non-multiplicity builds, these macros do nothing, so you always have
4551 to initialise the loop somewhere.
4552 .ie n .IP """EV_DEFAULT_UC"", ""EV_DEFAULT_UC_""" 4
4553 .el .IP "\f(CWEV_DEFAULT_UC\fR, \f(CWEV_DEFAULT_UC_\fR" 4
4554 .IX Item "EV_DEFAULT_UC, EV_DEFAULT_UC_"
4555 Usage identical to \f(CW\*(C`EV_DEFAULT\*(C'\fR and \f(CW\*(C`EV_DEFAULT_\*(C'\fR, but requires that the
4556 default loop has been initialised (\f(CW\*(C`UC\*(C'\fR == unchecked). Their behaviour
4557 is undefined when the default loop has not been initialised by a previous
4558 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.
4559 .Sp
4560 It is often prudent to use \f(CW\*(C`EV_DEFAULT\*(C'\fR when initialising the first
4561 watcher in a function but use \f(CW\*(C`EV_DEFAULT_UC\*(C'\fR afterwards.
4562 .PP
4563 Example: Declare and initialise a check watcher, utilising the above
4564 macros so it will work regardless of whether multiple loops are supported
4565 or not.
4566 .PP
4567 .Vb 5
4568 \& static void
4569 \& check_cb (EV_P_ ev_timer *w, int revents)
4570 \& {
4571 \& ev_check_stop (EV_A_ w);
4572 \& }
4573 \&
4574 \& ev_check check;
4575 \& ev_check_init (&check, check_cb);
4576 \& ev_check_start (EV_DEFAULT_ &check);
4577 \& ev_run (EV_DEFAULT_ 0);
4578 .Ve
4579 .SH "EMBEDDING"
4580 .IX Header "EMBEDDING"
4581 Libev can (and often is) directly embedded into host
4582 applications. Examples of applications that embed it include the Deliantra
4583 Game Server, the \s-1EV\s0 perl module, the \s-1GNU\s0 Virtual Private Ethernet (gvpe)
4584 and rxvt-unicode.
4585 .PP
4586 The goal is to enable you to just copy the necessary files into your
4587 source directory without having to change even a single line in them, so
4588 you can easily upgrade by simply copying (or having a checked-out copy of
4589 libev somewhere in your source tree).
4590 .SS "\s-1FILESETS\s0"
4591 .IX Subsection "FILESETS"
4592 Depending on what features you need you need to include one or more sets of files
4593 in your application.
4594 .PP
4595 \fI\s-1CORE EVENT LOOP\s0\fR
4596 .IX Subsection "CORE EVENT LOOP"
4597 .PP
4598 To include only the libev core (all the \f(CW\*(C`ev_*\*(C'\fR functions), with manual
4599 configuration (no autoconf):
4600 .PP
4601 .Vb 2
4602 \& #define EV_STANDALONE 1
4603 \& #include "ev.c"
4604 .Ve
4605 .PP
4606 This will automatically include \fIev.h\fR, too, and should be done in a
4607 single C source file only to provide the function implementations. To use
4608 it, do the same for \fIev.h\fR in all files wishing to use this \s-1API\s0 (best
4609 done by writing a wrapper around \fIev.h\fR that you can include instead and
4610 where you can put other configuration options):
4611 .PP
4612 .Vb 2
4613 \& #define EV_STANDALONE 1
4614 \& #include "ev.h"
4615 .Ve
4616 .PP
4617 Both header files and implementation files can be compiled with a \*(C+
4618 compiler (at least, that's a stated goal, and breakage will be treated
4619 as a bug).
4620 .PP
4621 You need the following files in your source tree, or in a directory
4622 in your include path (e.g. in libev/ when using \-Ilibev):
4623 .PP
4624 .Vb 4
4625 \& ev.h
4626 \& ev.c
4627 \& ev_vars.h
4628 \& ev_wrap.h
4629 \&
4630 \& ev_win32.c required on win32 platforms only
4631 \&
4632 \& ev_select.c only when select backend is enabled
4633 \& ev_poll.c only when poll backend is enabled
4634 \& ev_epoll.c only when the epoll backend is enabled
4635 \& ev_linuxaio.c only when the linux aio backend is enabled
4636 \& ev_iouring.c only when the linux io_uring backend is enabled
4637 \& ev_kqueue.c only when the kqueue backend is enabled
4638 \& ev_port.c only when the solaris port backend is enabled
4639 .Ve
4640 .PP
4641 \&\fIev.c\fR includes the backend files directly when enabled, so you only need
4642 to compile this single file.
4643 .PP
4644 \fI\s-1LIBEVENT COMPATIBILITY API\s0\fR
4645 .IX Subsection "LIBEVENT COMPATIBILITY API"
4646 .PP
4647 To include the libevent compatibility \s-1API,\s0 also include:
4648 .PP
4649 .Vb 1
4650 \& #include "event.c"
4651 .Ve
4652 .PP
4653 in the file including \fIev.c\fR, and:
4654 .PP
4655 .Vb 1
4656 \& #include "event.h"
4657 .Ve
4658 .PP
4659 in the files that want to use the libevent \s-1API.\s0 This also includes \fIev.h\fR.
4660 .PP
4661 You need the following additional files for this:
4662 .PP
4663 .Vb 2
4664 \& event.h
4665 \& event.c
4666 .Ve
4667 .PP
4668 \fI\s-1AUTOCONF SUPPORT\s0\fR
4669 .IX Subsection "AUTOCONF SUPPORT"
4670 .PP
4671 Instead of using \f(CW\*(C`EV_STANDALONE=1\*(C'\fR and providing your configuration in
4672 whatever way you want, you can also \f(CW\*(C`m4_include([libev.m4])\*(C'\fR in your
4673 \&\fIconfigure.ac\fR and leave \f(CW\*(C`EV_STANDALONE\*(C'\fR undefined. \fIev.c\fR will then
4674 include \fIconfig.h\fR and configure itself accordingly.
4675 .PP
4676 For this of course you need the m4 file:
4677 .PP
4678 .Vb 1
4679 \& libev.m4
4680 .Ve
4681 .SS "\s-1PREPROCESSOR SYMBOLS/MACROS\s0"
4682 .IX Subsection "PREPROCESSOR SYMBOLS/MACROS"
4683 Libev can be configured via a variety of preprocessor symbols you have to
4684 define before including (or compiling) any of its files. The default in
4685 the absence of autoconf is documented for every option.
4686 .PP
4687 Symbols marked with \*(L"(h)\*(R" do not change the \s-1ABI,\s0 and can have different
4688 values when compiling libev vs. including \fIev.h\fR, so it is permissible
4689 to redefine them before including \fIev.h\fR without breaking compatibility
4690 to a compiled library. All other symbols change the \s-1ABI,\s0 which means all
4691 users of libev and the libev code itself must be compiled with compatible
4692 settings.
4693 .IP "\s-1EV_COMPAT3\s0 (h)" 4
4694 .IX Item "EV_COMPAT3 (h)"
4695 Backwards compatibility is a major concern for libev. This is why this
4696 release of libev comes with wrappers for the functions and symbols that
4697 have been renamed between libev version 3 and 4.
4698 .Sp
4699 You can disable these wrappers (to test compatibility with future
4700 versions) by defining \f(CW\*(C`EV_COMPAT3\*(C'\fR to \f(CW0\fR when compiling your
4701 sources. This has the additional advantage that you can drop the \f(CW\*(C`struct\*(C'\fR
4702 from \f(CW\*(C`struct ev_loop\*(C'\fR declarations, as libev will provide an \f(CW\*(C`ev_loop\*(C'\fR
4703 typedef in that case.
4704 .Sp
4705 In some future version, the default for \f(CW\*(C`EV_COMPAT3\*(C'\fR will become \f(CW0\fR,
4706 and in some even more future version the compatibility code will be
4707 removed completely.
4708 .IP "\s-1EV_STANDALONE\s0 (h)" 4
4709 .IX Item "EV_STANDALONE (h)"
4710 Must always be \f(CW1\fR if you do not use autoconf configuration, which
4711 keeps libev from including \fIconfig.h\fR, and it also defines dummy
4712 implementations for some libevent functions (such as logging, which is not
4713 supported). It will also not define any of the structs usually found in
4714 \&\fIevent.h\fR that are not directly supported by the libev core alone.
4715 .Sp
4716 In standalone mode, libev will still try to automatically deduce the
4717 configuration, but has to be more conservative.
4718 .IP "\s-1EV_USE_FLOOR\s0" 4
4719 .IX Item "EV_USE_FLOOR"
4720 If defined to be \f(CW1\fR, libev will use the \f(CW\*(C`floor ()\*(C'\fR function for its
4721 periodic reschedule calculations, otherwise libev will fall back on a
4722 portable (slower) implementation. If you enable this, you usually have to
4723 link against libm or something equivalent. Enabling this when the \f(CW\*(C`floor\*(C'\fR
4724 function is not available will fail, so the safe default is to not enable
4725 this.
4726 .IP "\s-1EV_USE_MONOTONIC\s0" 4
4727 .IX Item "EV_USE_MONOTONIC"
4728 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4729 monotonic clock option at both compile time and runtime. Otherwise no
4730 use of the monotonic clock option will be attempted. If you enable this,
4731 you usually have to link against librt or something similar. Enabling it
4732 when the functionality isn't available is safe, though, although you have
4733 to make sure you link against any libraries where the \f(CW\*(C`clock_gettime\*(C'\fR
4734 function is hiding in (often \fI\-lrt\fR). See also \f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4735 .IP "\s-1EV_USE_REALTIME\s0" 4
4736 .IX Item "EV_USE_REALTIME"
4737 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4738 real-time clock option at compile time (and assume its availability
4739 at runtime if successful). Otherwise no use of the real-time clock
4740 option will be attempted. This effectively replaces \f(CW\*(C`gettimeofday\*(C'\fR
4741 by \f(CW\*(C`clock_get (CLOCK_REALTIME, ...)\*(C'\fR and will not normally affect
4742 correctness. See the note about libraries in the description of
4743 \&\f(CW\*(C`EV_USE_MONOTONIC\*(C'\fR, though. Defaults to the opposite value of
4744 \&\f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4745 .IP "\s-1EV_USE_CLOCK_SYSCALL\s0" 4
4746 .IX Item "EV_USE_CLOCK_SYSCALL"
4747 If defined to be \f(CW1\fR, libev will try to use a direct syscall instead
4748 of calling the system-provided \f(CW\*(C`clock_gettime\*(C'\fR function. This option
4749 exists because on GNU/Linux, \f(CW\*(C`clock_gettime\*(C'\fR is in \f(CW\*(C`librt\*(C'\fR, but \f(CW\*(C`librt\*(C'\fR
4750 unconditionally pulls in \f(CW\*(C`libpthread\*(C'\fR, slowing down single-threaded
4751 programs needlessly. Using a direct syscall is slightly slower (in
4752 theory), because no optimised vdso implementation can be used, but avoids
4753 the pthread dependency. Defaults to \f(CW1\fR on GNU/Linux with glibc 2.x or
4754 higher, as it simplifies linking (no need for \f(CW\*(C`\-lrt\*(C'\fR).
4755 .IP "\s-1EV_USE_NANOSLEEP\s0" 4
4756 .IX Item "EV_USE_NANOSLEEP"
4757 If defined to be \f(CW1\fR, libev will assume that \f(CW\*(C`nanosleep ()\*(C'\fR is available
4758 and will use it for delays. Otherwise it will use \f(CW\*(C`select ()\*(C'\fR.
4759 .IP "\s-1EV_USE_EVENTFD\s0" 4
4760 .IX Item "EV_USE_EVENTFD"
4761 If defined to be \f(CW1\fR, then libev will assume that \f(CW\*(C`eventfd ()\*(C'\fR is
4762 available and will probe for kernel support at runtime. This will improve
4763 \&\f(CW\*(C`ev_signal\*(C'\fR and \f(CW\*(C`ev_async\*(C'\fR performance and reduce resource consumption.
4764 If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
4765 2.7 or newer, otherwise disabled.
4766 .IP "\s-1EV_USE_SIGNALFD\s0" 4
4767 .IX Item "EV_USE_SIGNALFD"
4768 If defined to be \f(CW1\fR, then libev will assume that \f(CW\*(C`signalfd ()\*(C'\fR is
4769 available and will probe for kernel support at runtime. This enables
4770 the use of \s-1EVFLAG_SIGNALFD\s0 for faster and simpler signal handling. If
4771 undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
4772 2.7 or newer, otherwise disabled.
4773 .IP "\s-1EV_USE_TIMERFD\s0" 4
4774 .IX Item "EV_USE_TIMERFD"
4775 If defined to be \f(CW1\fR, then libev will assume that \f(CW\*(C`timerfd ()\*(C'\fR is
4776 available and will probe for kernel support at runtime. This allows
4777 libev to detect time jumps accurately. If undefined, it will be enabled
4778 if the headers indicate GNU/Linux + Glibc 2.8 or newer and define
4779 \&\f(CW\*(C`TFD_TIMER_CANCEL_ON_SET\*(C'\fR, otherwise disabled.
4780 .IP "\s-1EV_USE_EVENTFD\s0" 4
4781 .IX Item "EV_USE_EVENTFD"