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Revision: 1.115
Committed: Wed Jun 26 00:01:46 2019 UTC (5 years ago) by root
Branch: MAIN
CVS Tags: EV-rel-4_27, rel-4_27
Changes since 1.114: +10 -3 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 "2019-06-25" "libev-4.25" "libev - high performance full featured event loop"
137 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
138 .\" way too many mistakes in technical documents.
139 .if n .ad l
140 .nh
141 .SH "NAME"
142 libev \- a high performance full\-featured event loop written in C
143 .SH "SYNOPSIS"
144 .IX Header "SYNOPSIS"
145 .Vb 1
146 \& #include <ev.h>
147 .Ve
148 .SS "\s-1EXAMPLE PROGRAM\s0"
149 .IX Subsection "EXAMPLE PROGRAM"
150 .Vb 2
151 \& // a single header file is required
152 \& #include <ev.h>
153 \&
154 \& #include <stdio.h> // for puts
155 \&
156 \& // every watcher type has its own typedef\*(Aqd struct
157 \& // with the name ev_TYPE
158 \& ev_io stdin_watcher;
159 \& ev_timer timeout_watcher;
160 \&
161 \& // all watcher callbacks have a similar signature
162 \& // this callback is called when data is readable on stdin
163 \& static void
164 \& stdin_cb (EV_P_ ev_io *w, int revents)
165 \& {
166 \& puts ("stdin ready");
167 \& // for one\-shot events, one must manually stop the watcher
168 \& // with its corresponding stop function.
169 \& ev_io_stop (EV_A_ w);
170 \&
171 \& // this causes all nested ev_run\*(Aqs to stop iterating
172 \& ev_break (EV_A_ EVBREAK_ALL);
173 \& }
174 \&
175 \& // another callback, this time for a time\-out
176 \& static void
177 \& timeout_cb (EV_P_ ev_timer *w, int revents)
178 \& {
179 \& puts ("timeout");
180 \& // this causes the innermost ev_run to stop iterating
181 \& ev_break (EV_A_ EVBREAK_ONE);
182 \& }
183 \&
184 \& int
185 \& main (void)
186 \& {
187 \& // use the default event loop unless you have special needs
188 \& struct ev_loop *loop = EV_DEFAULT;
189 \&
190 \& // initialise an io watcher, then start it
191 \& // this one will watch for stdin to become readable
192 \& ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
193 \& ev_io_start (loop, &stdin_watcher);
194 \&
195 \& // initialise a timer watcher, then start it
196 \& // simple non\-repeating 5.5 second timeout
197 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
198 \& ev_timer_start (loop, &timeout_watcher);
199 \&
200 \& // now wait for events to arrive
201 \& ev_run (loop, 0);
202 \&
203 \& // break was called, so exit
204 \& return 0;
205 \& }
206 .Ve
207 .SH "ABOUT THIS DOCUMENT"
208 .IX Header "ABOUT THIS DOCUMENT"
209 This document documents the libev software package.
210 .PP
211 The newest version of this document is also available as an html-formatted
212 web page you might find easier to navigate when reading it for the first
213 time: <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
214 .PP
215 While this document tries to be as complete as possible in documenting
216 libev, its usage and the rationale behind its design, it is not a tutorial
217 on event-based programming, nor will it introduce event-based programming
218 with libev.
219 .PP
220 Familiarity with event based programming techniques in general is assumed
221 throughout this document.
222 .SH "WHAT TO READ WHEN IN A HURRY"
223 .IX Header "WHAT TO READ WHEN IN A HURRY"
224 This manual tries to be very detailed, but unfortunately, this also makes
225 it very long. If you just want to know the basics of libev, I suggest
226 reading \*(L"\s-1ANATOMY OF A WATCHER\*(R"\s0, then the \*(L"\s-1EXAMPLE PROGRAM\*(R"\s0 above and
227 look up the missing functions in \*(L"\s-1GLOBAL FUNCTIONS\*(R"\s0 and the \f(CW\*(C`ev_io\*(C'\fR and
228 \&\f(CW\*(C`ev_timer\*(C'\fR sections in \*(L"\s-1WATCHER TYPES\*(R"\s0.
229 .SH "ABOUT LIBEV"
230 .IX Header "ABOUT LIBEV"
231 Libev is an event loop: you register interest in certain events (such as a
232 file descriptor being readable or a timeout occurring), and it will manage
233 these event sources and provide your program with events.
234 .PP
235 To do this, it must take more or less complete control over your process
236 (or thread) by executing the \fIevent loop\fR handler, and will then
237 communicate events via a callback mechanism.
238 .PP
239 You register interest in certain events by registering so-called \fIevent
240 watchers\fR, which are relatively small C structures you initialise with the
241 details of the event, and then hand it over to libev by \fIstarting\fR the
242 watcher.
243 .SS "\s-1FEATURES\s0"
244 .IX Subsection "FEATURES"
245 Libev supports \f(CW\*(C`select\*(C'\fR, \f(CW\*(C`poll\*(C'\fR, the Linux-specific aio and \f(CW\*(C`epoll\*(C'\fR
246 interfaces, the BSD-specific \f(CW\*(C`kqueue\*(C'\fR and the Solaris-specific event port
247 mechanisms for file descriptor events (\f(CW\*(C`ev_io\*(C'\fR), the Linux \f(CW\*(C`inotify\*(C'\fR
248 interface (for \f(CW\*(C`ev_stat\*(C'\fR), Linux eventfd/signalfd (for faster and cleaner
249 inter-thread wakeup (\f(CW\*(C`ev_async\*(C'\fR)/signal handling (\f(CW\*(C`ev_signal\*(C'\fR)) relative
250 timers (\f(CW\*(C`ev_timer\*(C'\fR), absolute timers with customised rescheduling
251 (\f(CW\*(C`ev_periodic\*(C'\fR), synchronous signals (\f(CW\*(C`ev_signal\*(C'\fR), process status
252 change events (\f(CW\*(C`ev_child\*(C'\fR), and event watchers dealing with the event
253 loop mechanism itself (\f(CW\*(C`ev_idle\*(C'\fR, \f(CW\*(C`ev_embed\*(C'\fR, \f(CW\*(C`ev_prepare\*(C'\fR and
254 \&\f(CW\*(C`ev_check\*(C'\fR watchers) as well as file watchers (\f(CW\*(C`ev_stat\*(C'\fR) and even
255 limited support for fork events (\f(CW\*(C`ev_fork\*(C'\fR).
256 .PP
257 It also is quite fast (see this
258 benchmark <http://libev.schmorp.de/bench.html> comparing it to libevent
259 for example).
260 .SS "\s-1CONVENTIONS\s0"
261 .IX Subsection "CONVENTIONS"
262 Libev is very configurable. In this manual the default (and most common)
263 configuration will be described, which supports multiple event loops. For
264 more info about various configuration options please have a look at
265 \&\fB\s-1EMBED\s0\fR section in this manual. If libev was configured without support
266 for multiple event loops, then all functions taking an initial argument of
267 name \f(CW\*(C`loop\*(C'\fR (which is always of type \f(CW\*(C`struct ev_loop *\*(C'\fR) will not have
268 this argument.
269 .SS "\s-1TIME REPRESENTATION\s0"
270 .IX Subsection "TIME REPRESENTATION"
271 Libev represents time as a single floating point number, representing
272 the (fractional) number of seconds since the (\s-1POSIX\s0) epoch (in practice
273 somewhere near the beginning of 1970, details are complicated, don't
274 ask). This type is called \f(CW\*(C`ev_tstamp\*(C'\fR, which is what you should use
275 too. It usually aliases to the \f(CW\*(C`double\*(C'\fR type in C. When you need to do
276 any calculations on it, you should treat it as some floating point value.
277 .PP
278 Unlike the name component \f(CW\*(C`stamp\*(C'\fR might indicate, it is also used for
279 time differences (e.g. delays) throughout libev.
280 .SH "ERROR HANDLING"
281 .IX Header "ERROR HANDLING"
282 Libev knows three classes of errors: operating system errors, usage errors
283 and internal errors (bugs).
284 .PP
285 When libev catches an operating system error it cannot handle (for example
286 a system call indicating a condition libev cannot fix), it calls the callback
287 set via \f(CW\*(C`ev_set_syserr_cb\*(C'\fR, which is supposed to fix the problem or
288 abort. The default is to print a diagnostic message and to call \f(CW\*(C`abort
289 ()\*(C'\fR.
290 .PP
291 When libev detects a usage error such as a negative timer interval, then
292 it will print a diagnostic message and abort (via the \f(CW\*(C`assert\*(C'\fR mechanism,
293 so \f(CW\*(C`NDEBUG\*(C'\fR will disable this checking): these are programming errors in
294 the libev caller and need to be fixed there.
295 .PP
296 Via the \f(CW\*(C`EV_FREQUENT\*(C'\fR macro you can compile in and/or enable extensive
297 consistency checking code inside libev that can be used to check for
298 internal inconsistencies, suually caused by application bugs.
299 .PP
300 Libev also has a few internal error-checking \f(CW\*(C`assert\*(C'\fRions. These do not
301 trigger under normal circumstances, as they indicate either a bug in libev
302 or worse.
303 .SH "GLOBAL FUNCTIONS"
304 .IX Header "GLOBAL FUNCTIONS"
305 These functions can be called anytime, even before initialising the
306 library in any way.
307 .IP "ev_tstamp ev_time ()" 4
308 .IX Item "ev_tstamp ev_time ()"
309 Returns the current time as libev would use it. Please note that the
310 \&\f(CW\*(C`ev_now\*(C'\fR function is usually faster and also often returns the timestamp
311 you actually want to know. Also interesting is the combination of
312 \&\f(CW\*(C`ev_now_update\*(C'\fR and \f(CW\*(C`ev_now\*(C'\fR.
313 .IP "ev_sleep (ev_tstamp interval)" 4
314 .IX Item "ev_sleep (ev_tstamp interval)"
315 Sleep for the given interval: The current thread will be blocked
316 until either it is interrupted or the given time interval has
317 passed (approximately \- it might return a bit earlier even if not
318 interrupted). Returns immediately if \f(CW\*(C`interval <= 0\*(C'\fR.
319 .Sp
320 Basically this is a sub-second-resolution \f(CW\*(C`sleep ()\*(C'\fR.
321 .Sp
322 The range of the \f(CW\*(C`interval\*(C'\fR is limited \- libev only guarantees to work
323 with sleep times of up to one day (\f(CW\*(C`interval <= 86400\*(C'\fR).
324 .IP "int ev_version_major ()" 4
325 .IX Item "int ev_version_major ()"
326 .PD 0
327 .IP "int ev_version_minor ()" 4
328 .IX Item "int ev_version_minor ()"
329 .PD
330 You can find out the major and minor \s-1ABI\s0 version numbers of the library
331 you linked against by calling the functions \f(CW\*(C`ev_version_major\*(C'\fR and
332 \&\f(CW\*(C`ev_version_minor\*(C'\fR. If you want, you can compare against the global
333 symbols \f(CW\*(C`EV_VERSION_MAJOR\*(C'\fR and \f(CW\*(C`EV_VERSION_MINOR\*(C'\fR, which specify the
334 version of the library your program was compiled against.
335 .Sp
336 These version numbers refer to the \s-1ABI\s0 version of the library, not the
337 release version.
338 .Sp
339 Usually, it's a good idea to terminate if the major versions mismatch,
340 as this indicates an incompatible change. Minor versions are usually
341 compatible to older versions, so a larger minor version alone is usually
342 not a problem.
343 .Sp
344 Example: Make sure we haven't accidentally been linked against the wrong
345 version (note, however, that this will not detect other \s-1ABI\s0 mismatches,
346 such as \s-1LFS\s0 or reentrancy).
347 .Sp
348 .Vb 3
349 \& assert (("libev version mismatch",
350 \& ev_version_major () == EV_VERSION_MAJOR
351 \& && ev_version_minor () >= EV_VERSION_MINOR));
352 .Ve
353 .IP "unsigned int ev_supported_backends ()" 4
354 .IX Item "unsigned int ev_supported_backends ()"
355 Return the set of all backends (i.e. their corresponding \f(CW\*(C`EV_BACKEND_*\*(C'\fR
356 value) compiled into this binary of libev (independent of their
357 availability on the system you are running on). See \f(CW\*(C`ev_default_loop\*(C'\fR for
358 a description of the set values.
359 .Sp
360 Example: make sure we have the epoll method, because yeah this is cool and
361 a must have and can we have a torrent of it please!!!11
362 .Sp
363 .Vb 2
364 \& assert (("sorry, no epoll, no sex",
365 \& ev_supported_backends () & EVBACKEND_EPOLL));
366 .Ve
367 .IP "unsigned int ev_recommended_backends ()" 4
368 .IX Item "unsigned int ev_recommended_backends ()"
369 Return the set of all backends compiled into this binary of libev and
370 also recommended for this platform, meaning it will work for most file
371 descriptor types. This set is often smaller than the one returned by
372 \&\f(CW\*(C`ev_supported_backends\*(C'\fR, as for example kqueue is broken on most BSDs
373 and will not be auto-detected unless you explicitly request it (assuming
374 you know what you are doing). This is the set of backends that libev will
375 probe for if you specify no backends explicitly.
376 .IP "unsigned int ev_embeddable_backends ()" 4
377 .IX Item "unsigned int ev_embeddable_backends ()"
378 Returns the set of backends that are embeddable in other event loops. This
379 value is platform-specific but can include backends not available on the
380 current system. To find which embeddable backends might be supported on
381 the current system, you would need to look at \f(CW\*(C`ev_embeddable_backends ()
382 & ev_supported_backends ()\*(C'\fR, likewise for recommended ones.
383 .Sp
384 See the description of \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
385 .IP "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())" 4
386 .IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())"
387 Sets the allocation function to use (the prototype is similar \- the
388 semantics are identical to the \f(CW\*(C`realloc\*(C'\fR C89/SuS/POSIX function). It is
389 used to allocate and free memory (no surprises here). If it returns zero
390 when memory needs to be allocated (\f(CW\*(C`size != 0\*(C'\fR), the library might abort
391 or take some potentially destructive action.
392 .Sp
393 Since some systems (at least OpenBSD and Darwin) fail to implement
394 correct \f(CW\*(C`realloc\*(C'\fR semantics, libev will use a wrapper around the system
395 \&\f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions by default.
396 .Sp
397 You could override this function in high-availability programs to, say,
398 free some memory if it cannot allocate memory, to use a special allocator,
399 or even to sleep a while and retry until some memory is available.
400 .Sp
401 Example: The following is the \f(CW\*(C`realloc\*(C'\fR function that libev itself uses
402 which should work with \f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions of all kinds and
403 is probably a good basis for your own implementation.
404 .Sp
405 .Vb 5
406 \& static void *
407 \& ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
408 \& {
409 \& if (size)
410 \& return realloc (ptr, size);
411 \&
412 \& free (ptr);
413 \& return 0;
414 \& }
415 .Ve
416 .Sp
417 Example: Replace the libev allocator with one that waits a bit and then
418 retries.
419 .Sp
420 .Vb 8
421 \& static void *
422 \& persistent_realloc (void *ptr, size_t size)
423 \& {
424 \& if (!size)
425 \& {
426 \& free (ptr);
427 \& return 0;
428 \& }
429 \&
430 \& for (;;)
431 \& {
432 \& void *newptr = realloc (ptr, size);
433 \&
434 \& if (newptr)
435 \& return newptr;
436 \&
437 \& sleep (60);
438 \& }
439 \& }
440 \&
441 \& ...
442 \& ev_set_allocator (persistent_realloc);
443 .Ve
444 .IP "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())" 4
445 .IX Item "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())"
446 Set the callback function to call on a retryable system call error (such
447 as failed select, poll, epoll_wait). The message is a printable string
448 indicating the system call or subsystem causing the problem. If this
449 callback is set, then libev will expect it to remedy the situation, no
450 matter what, when it returns. That is, libev will generally retry the
451 requested operation, or, if the condition doesn't go away, do bad stuff
452 (such as abort).
453 .Sp
454 Example: This is basically the same thing that libev does internally, too.
455 .Sp
456 .Vb 6
457 \& static void
458 \& fatal_error (const char *msg)
459 \& {
460 \& perror (msg);
461 \& abort ();
462 \& }
463 \&
464 \& ...
465 \& ev_set_syserr_cb (fatal_error);
466 .Ve
467 .IP "ev_feed_signal (int signum)" 4
468 .IX Item "ev_feed_signal (int signum)"
469 This function can be used to \*(L"simulate\*(R" a signal receive. It is completely
470 safe to call this function at any time, from any context, including signal
471 handlers or random threads.
472 .Sp
473 Its main use is to customise signal handling in your process, especially
474 in the presence of threads. For example, you could block signals
475 by default in all threads (and specifying \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when
476 creating any loops), and in one thread, use \f(CW\*(C`sigwait\*(C'\fR or any other
477 mechanism to wait for signals, then \*(L"deliver\*(R" them to libev by calling
478 \&\f(CW\*(C`ev_feed_signal\*(C'\fR.
479 .SH "FUNCTIONS CONTROLLING EVENT LOOPS"
480 .IX Header "FUNCTIONS CONTROLLING EVENT LOOPS"
481 An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR (the \f(CW\*(C`struct\*(C'\fR is
482 \&\fInot\fR optional in this case unless libev 3 compatibility is disabled, as
483 libev 3 had an \f(CW\*(C`ev_loop\*(C'\fR function colliding with the struct name).
484 .PP
485 The library knows two types of such loops, the \fIdefault\fR loop, which
486 supports child process events, and dynamically created event loops which
487 do not.
488 .IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4
489 .IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"
490 This returns the \*(L"default\*(R" event loop object, which is what you should
491 normally use when you just need \*(L"the event loop\*(R". Event loop objects and
492 the \f(CW\*(C`flags\*(C'\fR parameter are described in more detail in the entry for
493 \&\f(CW\*(C`ev_loop_new\*(C'\fR.
494 .Sp
495 If the default loop is already initialised then this function simply
496 returns it (and ignores the flags. If that is troubling you, check
497 \&\f(CW\*(C`ev_backend ()\*(C'\fR afterwards). Otherwise it will create it with the given
498 flags, which should almost always be \f(CW0\fR, unless the caller is also the
499 one calling \f(CW\*(C`ev_run\*(C'\fR or otherwise qualifies as \*(L"the main program\*(R".
500 .Sp
501 If you don't know what event loop to use, use the one returned from this
502 function (or via the \f(CW\*(C`EV_DEFAULT\*(C'\fR macro).
503 .Sp
504 Note that this function is \fInot\fR thread-safe, so if you want to use it
505 from multiple threads, you have to employ some kind of mutex (note also
506 that this case is unlikely, as loops cannot be shared easily between
507 threads anyway).
508 .Sp
509 The default loop is the only loop that can handle \f(CW\*(C`ev_child\*(C'\fR watchers,
510 and to do this, it always registers a handler for \f(CW\*(C`SIGCHLD\*(C'\fR. If this is
511 a problem for your application you can either create a dynamic loop with
512 \&\f(CW\*(C`ev_loop_new\*(C'\fR which doesn't do that, or you can simply overwrite the
513 \&\f(CW\*(C`SIGCHLD\*(C'\fR signal handler \fIafter\fR calling \f(CW\*(C`ev_default_init\*(C'\fR.
514 .Sp
515 Example: This is the most typical usage.
516 .Sp
517 .Vb 2
518 \& if (!ev_default_loop (0))
519 \& fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
520 .Ve
521 .Sp
522 Example: Restrict libev to the select and poll backends, and do not allow
523 environment settings to be taken into account:
524 .Sp
525 .Vb 1
526 \& ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
527 .Ve
528 .IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4
529 .IX Item "struct ev_loop *ev_loop_new (unsigned int flags)"
530 This will create and initialise a new event loop object. If the loop
531 could not be initialised, returns false.
532 .Sp
533 This function is thread-safe, and one common way to use libev with
534 threads is indeed to create one loop per thread, and using the default
535 loop in the \*(L"main\*(R" or \*(L"initial\*(R" thread.
536 .Sp
537 The flags argument can be used to specify special behaviour or specific
538 backends to use, and is usually specified as \f(CW0\fR (or \f(CW\*(C`EVFLAG_AUTO\*(C'\fR).
539 .Sp
540 The following flags are supported:
541 .RS 4
542 .ie n .IP """EVFLAG_AUTO""" 4
543 .el .IP "\f(CWEVFLAG_AUTO\fR" 4
544 .IX Item "EVFLAG_AUTO"
545 The default flags value. Use this if you have no clue (it's the right
546 thing, believe me).
547 .ie n .IP """EVFLAG_NOENV""" 4
548 .el .IP "\f(CWEVFLAG_NOENV\fR" 4
549 .IX Item "EVFLAG_NOENV"
550 If this flag bit is or'ed into the flag value (or the program runs setuid
551 or setgid) then libev will \fInot\fR look at the environment variable
552 \&\f(CW\*(C`LIBEV_FLAGS\*(C'\fR. Otherwise (the default), this environment variable will
553 override the flags completely if it is found in the environment. This is
554 useful to try out specific backends to test their performance, to work
555 around bugs, or to make libev threadsafe (accessing environment variables
556 cannot be done in a threadsafe way, but usually it works if no other
557 thread modifies them).
558 .ie n .IP """EVFLAG_FORKCHECK""" 4
559 .el .IP "\f(CWEVFLAG_FORKCHECK\fR" 4
560 .IX Item "EVFLAG_FORKCHECK"
561 Instead of calling \f(CW\*(C`ev_loop_fork\*(C'\fR manually after a fork, you can also
562 make libev check for a fork in each iteration by enabling this flag.
563 .Sp
564 This works by calling \f(CW\*(C`getpid ()\*(C'\fR on every iteration of the loop,
565 and thus this might slow down your event loop if you do a lot of loop
566 iterations and little real work, but is usually not noticeable (on my
567 GNU/Linux system for example, \f(CW\*(C`getpid\*(C'\fR is actually a simple 5\-insn
568 sequence without a system call and thus \fIvery\fR fast, but my GNU/Linux
569 system also has \f(CW\*(C`pthread_atfork\*(C'\fR which is even faster). (Update: glibc
570 versions 2.25 apparently removed the \f(CW\*(C`getpid\*(C'\fR optimisation again).
571 .Sp
572 The big advantage of this flag is that you can forget about fork (and
573 forget about forgetting to tell libev about forking, although you still
574 have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR) when you use this flag.
575 .Sp
576 This flag setting cannot be overridden or specified in the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR
577 environment variable.
578 .ie n .IP """EVFLAG_NOINOTIFY""" 4
579 .el .IP "\f(CWEVFLAG_NOINOTIFY\fR" 4
580 .IX Item "EVFLAG_NOINOTIFY"
581 When this flag is specified, then libev will not attempt to use the
582 \&\fIinotify\fR \s-1API\s0 for its \f(CW\*(C`ev_stat\*(C'\fR watchers. Apart from debugging and
583 testing, this flag can be useful to conserve inotify file descriptors, as
584 otherwise each loop using \f(CW\*(C`ev_stat\*(C'\fR watchers consumes one inotify handle.
585 .ie n .IP """EVFLAG_SIGNALFD""" 4
586 .el .IP "\f(CWEVFLAG_SIGNALFD\fR" 4
587 .IX Item "EVFLAG_SIGNALFD"
588 When this flag is specified, then libev will attempt to use the
589 \&\fIsignalfd\fR \s-1API\s0 for its \f(CW\*(C`ev_signal\*(C'\fR (and \f(CW\*(C`ev_child\*(C'\fR) watchers. This \s-1API\s0
590 delivers signals synchronously, which makes it both faster and might make
591 it possible to get the queued signal data. It can also simplify signal
592 handling with threads, as long as you properly block signals in your
593 threads that are not interested in handling them.
594 .Sp
595 Signalfd will not be used by default as this changes your signal mask, and
596 there are a lot of shoddy libraries and programs (glib's threadpool for
597 example) that can't properly initialise their signal masks.
598 .ie n .IP """EVFLAG_NOSIGMASK""" 4
599 .el .IP "\f(CWEVFLAG_NOSIGMASK\fR" 4
600 .IX Item "EVFLAG_NOSIGMASK"
601 When this flag is specified, then libev will avoid to modify the signal
602 mask. Specifically, this means you have to make sure signals are unblocked
603 when you want to receive them.
604 .Sp
605 This behaviour is useful when you want to do your own signal handling, or
606 want to handle signals only in specific threads and want to avoid libev
607 unblocking the signals.
608 .Sp
609 It's also required by \s-1POSIX\s0 in a threaded program, as libev calls
610 \&\f(CW\*(C`sigprocmask\*(C'\fR, whose behaviour is officially unspecified.
611 .Sp
612 This flag's behaviour will become the default in future versions of libev.
613 .ie n .IP """EVBACKEND_SELECT"" (value 1, portable select backend)" 4
614 .el .IP "\f(CWEVBACKEND_SELECT\fR (value 1, portable select backend)" 4
615 .IX Item "EVBACKEND_SELECT (value 1, portable select backend)"
616 This is your standard \fBselect\fR\|(2) backend. Not \fIcompletely\fR standard, as
617 libev tries to roll its own fd_set with no limits on the number of fds,
618 but if that fails, expect a fairly low limit on the number of fds when
619 using this backend. It doesn't scale too well (O(highest_fd)), but its
620 usually the fastest backend for a low number of (low-numbered :) fds.
621 .Sp
622 To get good performance out of this backend you need a high amount of
623 parallelism (most of the file descriptors should be busy). If you are
624 writing a server, you should \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many
625 connections as possible during one iteration. You might also want to have
626 a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of
627 readiness notifications you get per iteration.
628 .Sp
629 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
630 \&\f(CW\*(C`writefds\*(C'\fR set (and to work around Microsoft Windows bugs, also onto the
631 \&\f(CW\*(C`exceptfds\*(C'\fR set on that platform).
632 .ie n .IP """EVBACKEND_POLL"" (value 2, poll backend, available everywhere except on windows)" 4
633 .el .IP "\f(CWEVBACKEND_POLL\fR (value 2, poll backend, available everywhere except on windows)" 4
634 .IX Item "EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)"
635 And this is your standard \fBpoll\fR\|(2) backend. It's more complicated
636 than select, but handles sparse fds better and has no artificial
637 limit on the number of fds you can use (except it will slow down
638 considerably with a lot of inactive fds). It scales similarly to select,
639 i.e. O(total_fds). See the entry for \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for
640 performance tips.
641 .Sp
642 This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLLHUP\*(C'\fR, and
643 \&\f(CW\*(C`EV_WRITE\*(C'\fR to \f(CW\*(C`POLLOUT | POLLERR | POLLHUP\*(C'\fR.
644 .ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4
645 .el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4
646 .IX Item "EVBACKEND_EPOLL (value 4, Linux)"
647 Use the Linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9
648 kernels).
649 .Sp
650 For few fds, this backend is a bit little slower than poll and select, but
651 it scales phenomenally better. While poll and select usually scale like
652 O(total_fds) where total_fds is the total number of fds (or the highest
653 fd), epoll scales either O(1) or O(active_fds).
654 .Sp
655 The epoll mechanism deserves honorable mention as the most misdesigned
656 of the more advanced event mechanisms: mere annoyances include silently
657 dropping file descriptors, requiring a system call per change per file
658 descriptor (and unnecessary guessing of parameters), problems with dup,
659 returning before the timeout value, resulting in additional iterations
660 (and only giving 5ms accuracy while select on the same platform gives
661 0.1ms) and so on. The biggest issue is fork races, however \- if a program
662 forks then \fIboth\fR parent and child process have to recreate the epoll
663 set, which can take considerable time (one syscall per file descriptor)
664 and is of course hard to detect.
665 .Sp
666 Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work,
667 but of course \fIdoesn't\fR, and epoll just loves to report events for
668 totally \fIdifferent\fR file descriptors (even already closed ones, so
669 one cannot even remove them from the set) than registered in the set
670 (especially on \s-1SMP\s0 systems). Libev tries to counter these spurious
671 notifications by employing an additional generation counter and comparing
672 that against the events to filter out spurious ones, recreating the set
673 when required. Epoll also erroneously rounds down timeouts, but gives you
674 no way to know when and by how much, so sometimes you have to busy-wait
675 because epoll returns immediately despite a nonzero timeout. And last
676 not least, it also refuses to work with some file descriptors which work
677 perfectly fine with \f(CW\*(C`select\*(C'\fR (files, many character devices...).
678 .Sp
679 Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
680 cobbled together in a hurry, no thought to design or interaction with
681 others. Oh, the pain, will it ever stop...
682 .Sp
683 While stopping, setting and starting an I/O watcher in the same iteration
684 will result in some caching, there is still a system call per such
685 incident (because the same \fIfile descriptor\fR could point to a different
686 \&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed
687 file descriptors might not work very well if you register events for both
688 file descriptors.
689 .Sp
690 Best performance from this backend is achieved by not unregistering all
691 watchers for a file descriptor until it has been closed, if possible,
692 i.e. keep at least one watcher active per fd at all times. Stopping and
693 starting a watcher (without re-setting it) also usually doesn't cause
694 extra overhead. A fork can both result in spurious notifications as well
695 as in libev having to destroy and recreate the epoll object, which can
696 take considerable time and thus should be avoided.
697 .Sp
698 All this means that, in practice, \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR can be as fast or
699 faster than epoll for maybe up to a hundred file descriptors, depending on
700 the usage. So sad.
701 .Sp
702 While nominally embeddable in other event loops, this feature is broken in
703 a lot of kernel revisions, but probably(!) works in current versions.
704 .Sp
705 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
706 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
707 .ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4
708 .el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4
709 .IX Item "EVBACKEND_LINUXAIO (value 64, Linux)"
710 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
711 only tries to use it in 4.19+).
712 .Sp
713 This is another Linux train wreck of an event interface.
714 .Sp
715 If this backend works for you (as of this writing, it was very
716 experimental), it is the best event interface available on Linux and might
717 be well worth enabling it \- if it isn't available in your kernel this will
718 be detected and this backend will be skipped.
719 .Sp
720 This backend can batch oneshot requests and supports a user-space ring
721 buffer to receive events. It also doesn't suffer from most of the design
722 problems of epoll (such as not being able to remove event sources from
723 the epoll set), and generally sounds too good to be true. Because, this
724 being the Linux kernel, of course it suffers from a whole new set of
725 limitations, forcing you to fall back to epoll, inheriting all its design
726 issues.
727 .Sp
728 For one, it is not easily embeddable (but probably could be done using
729 an event fd at some extra overhead). It also is subject to a system wide
730 limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR. If no \s-1AIO\s0
731 requests are left, this backend will be skipped during initialisation, and
732 will switch to epoll when the loop is active.
733 .Sp
734 Most problematic in practice, however, is that not all file descriptors
735 work with it. For example, in Linux 5.1, \s-1TCP\s0 sockets, pipes, event fds,
736 files, \fI/dev/null\fR and many others are supported, but ttys do not work
737 properly (a known bug that the kernel developers don't care about, see
738 <https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
739 (yet?) a generic event polling interface.
740 .Sp
741 Overall, it seems the Linux developers just don't want it to have a
742 generic event handling mechanism other than \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`poll\*(C'\fR.
743 .Sp
744 To work around all these problem, the current version of libev uses its
745 epoll backend as a fallback for file descriptor types that do not work. Or
746 falls back completely to epoll if the kernel acts up.
747 .Sp
748 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
749 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
750 .ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
751 .el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4
752 .IX Item "EVBACKEND_KQUEUE (value 8, most BSD clones)"
753 Kqueue deserves special mention, as at the time this backend was
754 implemented, it was broken on all BSDs except NetBSD (usually it doesn't
755 work reliably with anything but sockets and pipes, except on Darwin,
756 where of course it's completely useless). Unlike epoll, however, whose
757 brokenness is by design, these kqueue bugs can be (and mostly have been)
758 fixed without \s-1API\s0 changes to existing programs. For this reason it's not
759 being \*(L"auto-detected\*(R" on all platforms unless you explicitly specify it
760 in the flags (i.e. using \f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a
761 known-to-be-good (\-enough) system like NetBSD.
762 .Sp
763 You still can embed kqueue into a normal poll or select backend and use it
764 only for sockets (after having made sure that sockets work with kqueue on
765 the target platform). See \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
766 .Sp
767 It scales in the same way as the epoll backend, but the interface to the
768 kernel is more efficient (which says nothing about its actual speed, of
769 course). While stopping, setting and starting an I/O watcher does never
770 cause an extra system call as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to
771 two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you
772 might have to leak fds on fork, but it's more sane than epoll) and it
773 drops fds silently in similarly hard-to-detect cases.
774 .Sp
775 This backend usually performs well under most conditions.
776 .Sp
777 While nominally embeddable in other event loops, this doesn't work
778 everywhere, so you might need to test for this. And since it is broken
779 almost everywhere, you should only use it when you have a lot of sockets
780 (for which it usually works), by embedding it into another event loop
781 (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
782 also broken on \s-1OS X\s0)) and, did I mention it, using it only for sockets.
783 .Sp
784 This backend maps \f(CW\*(C`EV_READ\*(C'\fR into an \f(CW\*(C`EVFILT_READ\*(C'\fR kevent with
785 \&\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
786 \&\f(CW\*(C`NOTE_EOF\*(C'\fR.
787 .ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4
788 .el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4
789 .IX Item "EVBACKEND_DEVPOLL (value 16, Solaris 8)"
790 This is not implemented yet (and might never be, unless you send me an
791 implementation). According to reports, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets
792 and is not embeddable, which would limit the usefulness of this backend
793 immensely.
794 .ie n .IP """EVBACKEND_PORT"" (value 32, Solaris 10)" 4
795 .el .IP "\f(CWEVBACKEND_PORT\fR (value 32, Solaris 10)" 4
796 .IX Item "EVBACKEND_PORT (value 32, Solaris 10)"
797 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
798 it's really slow, but it still scales very well (O(active_fds)).
799 .Sp
800 While this backend scales well, it requires one system call per active
801 file descriptor per loop iteration. For small and medium numbers of file
802 descriptors a \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend
803 might perform better.
804 .Sp
805 On the positive side, this backend actually performed fully to
806 specification in all tests and is fully embeddable, which is a rare feat
807 among the OS-specific backends (I vastly prefer correctness over speed
808 hacks).
809 .Sp
810 On the negative side, the interface is \fIbizarre\fR \- so bizarre that
811 even sun itself gets it wrong in their code examples: The event polling
812 function sometimes returns events to the caller even though an error
813 occurred, but with no indication whether it has done so or not (yes, it's
814 even documented that way) \- deadly for edge-triggered interfaces where you
815 absolutely have to know whether an event occurred or not because you have
816 to re-arm the watcher.
817 .Sp
818 Fortunately libev seems to be able to work around these idiocies.
819 .Sp
820 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
821 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
822 .ie n .IP """EVBACKEND_ALL""" 4
823 .el .IP "\f(CWEVBACKEND_ALL\fR" 4
824 .IX Item "EVBACKEND_ALL"
825 Try all backends (even potentially broken ones that wouldn't be tried
826 with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as
827 \&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR.
828 .Sp
829 It is definitely not recommended to use this flag, use whatever
830 \&\f(CW\*(C`ev_recommended_backends ()\*(C'\fR returns, or simply do not specify a backend
831 at all.
832 .ie n .IP """EVBACKEND_MASK""" 4
833 .el .IP "\f(CWEVBACKEND_MASK\fR" 4
834 .IX Item "EVBACKEND_MASK"
835 Not a backend at all, but a mask to select all backend bits from a
836 \&\f(CW\*(C`flags\*(C'\fR value, in case you want to mask out any backends from a flags
837 value (e.g. when modifying the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR environment variable).
838 .RE
839 .RS 4
840 .Sp
841 If one or more of the backend flags are or'ed into the flags value,
842 then only these backends will be tried (in the reverse order as listed
843 here). If none are specified, all backends in \f(CW\*(C`ev_recommended_backends
844 ()\*(C'\fR will be tried.
845 .Sp
846 Example: Try to create a event loop that uses epoll and nothing else.
847 .Sp
848 .Vb 3
849 \& struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
850 \& if (!epoller)
851 \& fatal ("no epoll found here, maybe it hides under your chair");
852 .Ve
853 .Sp
854 Example: Use whatever libev has to offer, but make sure that kqueue is
855 used if available.
856 .Sp
857 .Vb 1
858 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
859 .Ve
860 .Sp
861 Example: Similarly, on linux, you mgiht want to take advantage of the
862 linux aio backend if possible, but fall back to something else if that
863 isn't available.
864 .Sp
865 .Vb 1
866 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
867 .Ve
868 .RE
869 .IP "ev_loop_destroy (loop)" 4
870 .IX Item "ev_loop_destroy (loop)"
871 Destroys an event loop object (frees all memory and kernel state
872 etc.). None of the active event watchers will be stopped in the normal
873 sense, so e.g. \f(CW\*(C`ev_is_active\*(C'\fR might still return true. It is your
874 responsibility to either stop all watchers cleanly yourself \fIbefore\fR
875 calling this function, or cope with the fact afterwards (which is usually
876 the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them
877 for example).
878 .Sp
879 Note that certain global state, such as signal state (and installed signal
880 handlers), will not be freed by this function, and related watchers (such
881 as signal and child watchers) would need to be stopped manually.
882 .Sp
883 This function is normally used on loop objects allocated by
884 \&\f(CW\*(C`ev_loop_new\*(C'\fR, but it can also be used on the default loop returned by
885 \&\f(CW\*(C`ev_default_loop\*(C'\fR, in which case it is not thread-safe.
886 .Sp
887 Note that it is not advisable to call this function on the default loop
888 except in the rare occasion where you really need to free its resources.
889 If you need dynamically allocated loops it is better to use \f(CW\*(C`ev_loop_new\*(C'\fR
890 and \f(CW\*(C`ev_loop_destroy\*(C'\fR.
891 .IP "ev_loop_fork (loop)" 4
892 .IX Item "ev_loop_fork (loop)"
893 This function sets a flag that causes subsequent \f(CW\*(C`ev_run\*(C'\fR iterations
894 to reinitialise the kernel state for backends that have one. Despite
895 the name, you can call it anytime you are allowed to start or stop
896 watchers (except inside an \f(CW\*(C`ev_prepare\*(C'\fR callback), but it makes most
897 sense after forking, in the child process. You \fImust\fR call it (or use
898 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR) in the child before resuming or calling \f(CW\*(C`ev_run\*(C'\fR.
899 .Sp
900 In addition, if you want to reuse a loop (via this function or
901 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR), you \fIalso\fR have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR.
902 .Sp
903 Again, you \fIhave\fR to call it on \fIany\fR loop that you want to re-use after
904 a fork, \fIeven if you do not plan to use the loop in the parent\fR. This is
905 because some kernel interfaces *cough* \fIkqueue\fR *cough* do funny things
906 during fork.
907 .Sp
908 On the other hand, you only need to call this function in the child
909 process if and only if you want to use the event loop in the child. If
910 you just fork+exec or create a new loop in the child, you don't have to
911 call it at all (in fact, \f(CW\*(C`epoll\*(C'\fR is so badly broken that it makes a
912 difference, but libev will usually detect this case on its own and do a
913 costly reset of the backend).
914 .Sp
915 The function itself is quite fast and it's usually not a problem to call
916 it just in case after a fork.
917 .Sp
918 Example: Automate calling \f(CW\*(C`ev_loop_fork\*(C'\fR on the default loop when
919 using pthreads.
920 .Sp
921 .Vb 5
922 \& static void
923 \& post_fork_child (void)
924 \& {
925 \& ev_loop_fork (EV_DEFAULT);
926 \& }
927 \&
928 \& ...
929 \& pthread_atfork (0, 0, post_fork_child);
930 .Ve
931 .IP "int ev_is_default_loop (loop)" 4
932 .IX Item "int ev_is_default_loop (loop)"
933 Returns true when the given loop is, in fact, the default loop, and false
934 otherwise.
935 .IP "unsigned int ev_iteration (loop)" 4
936 .IX Item "unsigned int ev_iteration (loop)"
937 Returns the current iteration count for the event loop, which is identical
938 to the number of times libev did poll for new events. It starts at \f(CW0\fR
939 and happily wraps around with enough iterations.
940 .Sp
941 This value can sometimes be useful as a generation counter of sorts (it
942 \&\*(L"ticks\*(R" the number of loop iterations), as it roughly corresponds with
943 \&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls \- and is incremented between the
944 prepare and check phases.
945 .IP "unsigned int ev_depth (loop)" 4
946 .IX Item "unsigned int ev_depth (loop)"
947 Returns the number of times \f(CW\*(C`ev_run\*(C'\fR was entered minus the number of
948 times \f(CW\*(C`ev_run\*(C'\fR was exited normally, in other words, the recursion depth.
949 .Sp
950 Outside \f(CW\*(C`ev_run\*(C'\fR, this number is zero. In a callback, this number is
951 \&\f(CW1\fR, unless \f(CW\*(C`ev_run\*(C'\fR was invoked recursively (or from another thread),
952 in which case it is higher.
953 .Sp
954 Leaving \f(CW\*(C`ev_run\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread,
955 throwing an exception etc.), doesn't count as \*(L"exit\*(R" \- consider this
956 as a hint to avoid such ungentleman-like behaviour unless it's really
957 convenient, in which case it is fully supported.
958 .IP "unsigned int ev_backend (loop)" 4
959 .IX Item "unsigned int ev_backend (loop)"
960 Returns one of the \f(CW\*(C`EVBACKEND_*\*(C'\fR flags indicating the event backend in
961 use.
962 .IP "ev_tstamp ev_now (loop)" 4
963 .IX Item "ev_tstamp ev_now (loop)"
964 Returns the current \*(L"event loop time\*(R", which is the time the event loop
965 received events and started processing them. This timestamp does not
966 change as long as callbacks are being processed, and this is also the base
967 time used for relative timers. You can treat it as the timestamp of the
968 event occurring (or more correctly, libev finding out about it).
969 .IP "ev_now_update (loop)" 4
970 .IX Item "ev_now_update (loop)"
971 Establishes the current time by querying the kernel, updating the time
972 returned by \f(CW\*(C`ev_now ()\*(C'\fR in the progress. This is a costly operation and
973 is usually done automatically within \f(CW\*(C`ev_run ()\*(C'\fR.
974 .Sp
975 This function is rarely useful, but when some event callback runs for a
976 very long time without entering the event loop, updating libev's idea of
977 the current time is a good idea.
978 .Sp
979 See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.
980 .IP "ev_suspend (loop)" 4
981 .IX Item "ev_suspend (loop)"
982 .PD 0
983 .IP "ev_resume (loop)" 4
984 .IX Item "ev_resume (loop)"
985 .PD
986 These two functions suspend and resume an event loop, for use when the
987 loop is not used for a while and timeouts should not be processed.
988 .Sp
989 A typical use case would be an interactive program such as a game: When
990 the user presses \f(CW\*(C`^Z\*(C'\fR to suspend the game and resumes it an hour later it
991 would be best to handle timeouts as if no time had actually passed while
992 the program was suspended. This can be achieved by calling \f(CW\*(C`ev_suspend\*(C'\fR
993 in your \f(CW\*(C`SIGTSTP\*(C'\fR handler, sending yourself a \f(CW\*(C`SIGSTOP\*(C'\fR and calling
994 \&\f(CW\*(C`ev_resume\*(C'\fR directly afterwards to resume timer processing.
995 .Sp
996 Effectively, all \f(CW\*(C`ev_timer\*(C'\fR watchers will be delayed by the time spend
997 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
998 will be rescheduled (that is, they will lose any events that would have
999 occurred while suspended).
1000 .Sp
1001 After calling \f(CW\*(C`ev_suspend\*(C'\fR you \fBmust not\fR call \fIany\fR function on the
1002 given loop other than \f(CW\*(C`ev_resume\*(C'\fR, and you \fBmust not\fR call \f(CW\*(C`ev_resume\*(C'\fR
1003 without a previous call to \f(CW\*(C`ev_suspend\*(C'\fR.
1004 .Sp
1005 Calling \f(CW\*(C`ev_suspend\*(C'\fR/\f(CW\*(C`ev_resume\*(C'\fR has the side effect of updating the
1006 event loop time (see \f(CW\*(C`ev_now_update\*(C'\fR).
1007 .IP "bool ev_run (loop, int flags)" 4
1008 .IX Item "bool ev_run (loop, int flags)"
1009 Finally, this is it, the event handler. This function usually is called
1010 after you have initialised all your watchers and you want to start
1011 handling events. It will ask the operating system for any new events, call
1012 the watcher callbacks, and then repeat the whole process indefinitely: This
1013 is why event loops are called \fIloops\fR.
1014 .Sp
1015 If the flags argument is specified as \f(CW0\fR, it will keep handling events
1016 until either no event watchers are active anymore or \f(CW\*(C`ev_break\*(C'\fR was
1017 called.
1018 .Sp
1019 The return value is false if there are no more active watchers (which
1020 usually means \*(L"all jobs done\*(R" or \*(L"deadlock\*(R"), and true in all other cases
1021 (which usually means " you should call \f(CW\*(C`ev_run\*(C'\fR again").
1022 .Sp
1023 Please note that an explicit \f(CW\*(C`ev_break\*(C'\fR is usually better than
1024 relying on all watchers to be stopped when deciding when a program has
1025 finished (especially in interactive programs), but having a program
1026 that automatically loops as long as it has to and no longer by virtue
1027 of relying on its watchers stopping correctly, that is truly a thing of
1028 beauty.
1029 .Sp
1030 This function is \fImostly\fR exception-safe \- you can break out of a
1031 \&\f(CW\*(C`ev_run\*(C'\fR call by calling \f(CW\*(C`longjmp\*(C'\fR in a callback, throwing a \*(C+
1032 exception and so on. This does not decrement the \f(CW\*(C`ev_depth\*(C'\fR value, nor
1033 will it clear any outstanding \f(CW\*(C`EVBREAK_ONE\*(C'\fR breaks.
1034 .Sp
1035 A flags value of \f(CW\*(C`EVRUN_NOWAIT\*(C'\fR will look for new events, will handle
1036 those events and any already outstanding ones, but will not wait and
1037 block your process in case there are no events and will return after one
1038 iteration of the loop. This is sometimes useful to poll and handle new
1039 events while doing lengthy calculations, to keep the program responsive.
1040 .Sp
1041 A flags value of \f(CW\*(C`EVRUN_ONCE\*(C'\fR will look for new events (waiting if
1042 necessary) and will handle those and any already outstanding ones. It
1043 will block your process until at least one new event arrives (which could
1044 be an event internal to libev itself, so there is no guarantee that a
1045 user-registered callback will be called), and will return after one
1046 iteration of the loop.
1047 .Sp
1048 This is useful if you are waiting for some external event in conjunction
1049 with something not expressible using other libev watchers (i.e. "roll your
1050 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
1051 usually a better approach for this kind of thing.
1052 .Sp
1053 Here are the gory details of what \f(CW\*(C`ev_run\*(C'\fR does (this is for your
1054 understanding, not a guarantee that things will work exactly like this in
1055 future versions):
1056 .Sp
1057 .Vb 10
1058 \& \- Increment loop depth.
1059 \& \- Reset the ev_break status.
1060 \& \- Before the first iteration, call any pending watchers.
1061 \& LOOP:
1062 \& \- If EVFLAG_FORKCHECK was used, check for a fork.
1063 \& \- If a fork was detected (by any means), queue and call all fork watchers.
1064 \& \- Queue and call all prepare watchers.
1065 \& \- If ev_break was called, goto FINISH.
1066 \& \- If we have been forked, detach and recreate the kernel state
1067 \& as to not disturb the other process.
1068 \& \- Update the kernel state with all outstanding changes.
1069 \& \- Update the "event loop time" (ev_now ()).
1070 \& \- Calculate for how long to sleep or block, if at all
1071 \& (active idle watchers, EVRUN_NOWAIT or not having
1072 \& any active watchers at all will result in not sleeping).
1073 \& \- Sleep if the I/O and timer collect interval say so.
1074 \& \- Increment loop iteration counter.
1075 \& \- Block the process, waiting for any events.
1076 \& \- Queue all outstanding I/O (fd) events.
1077 \& \- Update the "event loop time" (ev_now ()), and do time jump adjustments.
1078 \& \- Queue all expired timers.
1079 \& \- Queue all expired periodics.
1080 \& \- Queue all idle watchers with priority higher than that of pending events.
1081 \& \- Queue all check watchers.
1082 \& \- Call all queued watchers in reverse order (i.e. check watchers first).
1083 \& Signals and child watchers are implemented as I/O watchers, and will
1084 \& be handled here by queueing them when their watcher gets executed.
1085 \& \- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
1086 \& were used, or there are no active watchers, goto FINISH, otherwise
1087 \& continue with step LOOP.
1088 \& FINISH:
1089 \& \- Reset the ev_break status iff it was EVBREAK_ONE.
1090 \& \- Decrement the loop depth.
1091 \& \- Return.
1092 .Ve
1093 .Sp
1094 Example: Queue some jobs and then loop until no events are outstanding
1095 anymore.
1096 .Sp
1097 .Vb 4
1098 \& ... queue jobs here, make sure they register event watchers as long
1099 \& ... as they still have work to do (even an idle watcher will do..)
1100 \& ev_run (my_loop, 0);
1101 \& ... jobs done or somebody called break. yeah!
1102 .Ve
1103 .IP "ev_break (loop, how)" 4
1104 .IX Item "ev_break (loop, how)"
1105 Can be used to make a call to \f(CW\*(C`ev_run\*(C'\fR return early (but only after it
1106 has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either
1107 \&\f(CW\*(C`EVBREAK_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_run\*(C'\fR call return, or
1108 \&\f(CW\*(C`EVBREAK_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_run\*(C'\fR calls return.
1109 .Sp
1110 This \*(L"break state\*(R" will be cleared on the next call to \f(CW\*(C`ev_run\*(C'\fR.
1111 .Sp
1112 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
1113 which case it will have no effect.
1114 .IP "ev_ref (loop)" 4
1115 .IX Item "ev_ref (loop)"
1116 .PD 0
1117 .IP "ev_unref (loop)" 4
1118 .IX Item "ev_unref (loop)"
1119 .PD
1120 Ref/unref can be used to add or remove a reference count on the event
1121 loop: Every watcher keeps one reference, and as long as the reference
1122 count is nonzero, \f(CW\*(C`ev_run\*(C'\fR will not return on its own.
1123 .Sp
1124 This is useful when you have a watcher that you never intend to
1125 unregister, but that nevertheless should not keep \f(CW\*(C`ev_run\*(C'\fR from
1126 returning. In such a case, call \f(CW\*(C`ev_unref\*(C'\fR after starting, and \f(CW\*(C`ev_ref\*(C'\fR
1127 before stopping it.
1128 .Sp
1129 As an example, libev itself uses this for its internal signal pipe: It
1130 is not visible to the libev user and should not keep \f(CW\*(C`ev_run\*(C'\fR from
1131 exiting if no event watchers registered by it are active. It is also an
1132 excellent way to do this for generic recurring timers or from within
1133 third-party libraries. Just remember to \fIunref after start\fR and \fIref
1134 before stop\fR (but only if the watcher wasn't active before, or was active
1135 before, respectively. Note also that libev might stop watchers itself
1136 (e.g. non-repeating timers) in which case you have to \f(CW\*(C`ev_ref\*(C'\fR
1137 in the callback).
1138 .Sp
1139 Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_run\*(C'\fR
1140 running when nothing else is active.
1141 .Sp
1142 .Vb 4
1143 \& ev_signal exitsig;
1144 \& ev_signal_init (&exitsig, sig_cb, SIGINT);
1145 \& ev_signal_start (loop, &exitsig);
1146 \& ev_unref (loop);
1147 .Ve
1148 .Sp
1149 Example: For some weird reason, unregister the above signal handler again.
1150 .Sp
1151 .Vb 2
1152 \& ev_ref (loop);
1153 \& ev_signal_stop (loop, &exitsig);
1154 .Ve
1155 .IP "ev_set_io_collect_interval (loop, ev_tstamp interval)" 4
1156 .IX Item "ev_set_io_collect_interval (loop, ev_tstamp interval)"
1157 .PD 0
1158 .IP "ev_set_timeout_collect_interval (loop, ev_tstamp interval)" 4
1159 .IX Item "ev_set_timeout_collect_interval (loop, ev_tstamp interval)"
1160 .PD
1161 These advanced functions influence the time that libev will spend waiting
1162 for events. Both time intervals are by default \f(CW0\fR, meaning that libev
1163 will try to invoke timer/periodic callbacks and I/O callbacks with minimum
1164 latency.
1165 .Sp
1166 Setting these to a higher value (the \f(CW\*(C`interval\*(C'\fR \fImust\fR be >= \f(CW0\fR)
1167 allows libev to delay invocation of I/O and timer/periodic callbacks
1168 to increase efficiency of loop iterations (or to increase power-saving
1169 opportunities).
1170 .Sp
1171 The idea is that sometimes your program runs just fast enough to handle
1172 one (or very few) event(s) per loop iteration. While this makes the
1173 program responsive, it also wastes a lot of \s-1CPU\s0 time to poll for new
1174 events, especially with backends like \f(CW\*(C`select ()\*(C'\fR which have a high
1175 overhead for the actual polling but can deliver many events at once.
1176 .Sp
1177 By setting a higher \fIio collect interval\fR you allow libev to spend more
1178 time collecting I/O events, so you can handle more events per iteration,
1179 at the cost of increasing latency. Timeouts (both \f(CW\*(C`ev_periodic\*(C'\fR and
1180 \&\f(CW\*(C`ev_timer\*(C'\fR) will not be affected. Setting this to a non-null value will
1181 introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. The
1182 sleep time ensures that libev will not poll for I/O events more often then
1183 once per this interval, on average (as long as the host time resolution is
1184 good enough).
1185 .Sp
1186 Likewise, by setting a higher \fItimeout collect interval\fR you allow libev
1187 to spend more time collecting timeouts, at the expense of increased
1188 latency/jitter/inexactness (the watcher callback will be called
1189 later). \f(CW\*(C`ev_io\*(C'\fR watchers will not be affected. Setting this to a non-null
1190 value will not introduce any overhead in libev.
1191 .Sp
1192 Many (busy) programs can usually benefit by setting the I/O collect
1193 interval to a value near \f(CW0.1\fR or so, which is often enough for
1194 interactive servers (of course not for games), likewise for timeouts. It
1195 usually doesn't make much sense to set it to a lower value than \f(CW0.01\fR,
1196 as this approaches the timing granularity of most systems. Note that if
1197 you do transactions with the outside world and you can't increase the
1198 parallelity, then this setting will limit your transaction rate (if you
1199 need to poll once per transaction and the I/O collect interval is 0.01,
1200 then you can't do more than 100 transactions per second).
1201 .Sp
1202 Setting the \fItimeout collect interval\fR can improve the opportunity for
1203 saving power, as the program will \*(L"bundle\*(R" timer callback invocations that
1204 are \*(L"near\*(R" in time together, by delaying some, thus reducing the number of
1205 times the process sleeps and wakes up again. Another useful technique to
1206 reduce iterations/wake\-ups is to use \f(CW\*(C`ev_periodic\*(C'\fR watchers and make sure
1207 they fire on, say, one-second boundaries only.
1208 .Sp
1209 Example: we only need 0.1s timeout granularity, and we wish not to poll
1210 more often than 100 times per second:
1211 .Sp
1212 .Vb 2
1213 \& ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
1214 \& ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
1215 .Ve
1216 .IP "ev_invoke_pending (loop)" 4
1217 .IX Item "ev_invoke_pending (loop)"
1218 This call will simply invoke all pending watchers while resetting their
1219 pending state. Normally, \f(CW\*(C`ev_run\*(C'\fR does this automatically when required,
1220 but when overriding the invoke callback this call comes handy. This
1221 function can be invoked from a watcher \- this can be useful for example
1222 when you want to do some lengthy calculation and want to pass further
1223 event handling to another thread (you still have to make sure only one
1224 thread executes within \f(CW\*(C`ev_invoke_pending\*(C'\fR or \f(CW\*(C`ev_run\*(C'\fR of course).
1225 .IP "int ev_pending_count (loop)" 4
1226 .IX Item "int ev_pending_count (loop)"
1227 Returns the number of pending watchers \- zero indicates that no watchers
1228 are pending.
1229 .IP "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(\s-1EV_P\s0))" 4
1230 .IX Item "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))"
1231 This overrides the invoke pending functionality of the loop: Instead of
1232 invoking all pending watchers when there are any, \f(CW\*(C`ev_run\*(C'\fR will call
1233 this callback instead. This is useful, for example, when you want to
1234 invoke the actual watchers inside another context (another thread etc.).
1235 .Sp
1236 If you want to reset the callback, use \f(CW\*(C`ev_invoke_pending\*(C'\fR as new
1237 callback.
1238 .IP "ev_set_loop_release_cb (loop, void (*release)(\s-1EV_P\s0) throw (), void (*acquire)(\s-1EV_P\s0) throw ())" 4
1239 .IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())"
1240 Sometimes you want to share the same loop between multiple threads. This
1241 can be done relatively simply by putting mutex_lock/unlock calls around
1242 each call to a libev function.
1243 .Sp
1244 However, \f(CW\*(C`ev_run\*(C'\fR can run an indefinite time, so it is not feasible
1245 to wait for it to return. One way around this is to wake up the event
1246 loop via \f(CW\*(C`ev_break\*(C'\fR and \f(CW\*(C`ev_async_send\*(C'\fR, another way is to set these
1247 \&\fIrelease\fR and \fIacquire\fR callbacks on the loop.
1248 .Sp
1249 When set, then \f(CW\*(C`release\*(C'\fR will be called just before the thread is
1250 suspended waiting for new events, and \f(CW\*(C`acquire\*(C'\fR is called just
1251 afterwards.
1252 .Sp
1253 Ideally, \f(CW\*(C`release\*(C'\fR will just call your mutex_unlock function, and
1254 \&\f(CW\*(C`acquire\*(C'\fR will just call the mutex_lock function again.
1255 .Sp
1256 While event loop modifications are allowed between invocations of
1257 \&\f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR (that's their only purpose after all), no
1258 modifications done will affect the event loop, i.e. adding watchers will
1259 have no effect on the set of file descriptors being watched, or the time
1260 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
1261 to take note of any changes you made.
1262 .Sp
1263 In theory, threads executing \f(CW\*(C`ev_run\*(C'\fR will be async-cancel safe between
1264 invocations of \f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR.
1265 .Sp
1266 See also the locking example in the \f(CW\*(C`THREADS\*(C'\fR section later in this
1267 document.
1268 .IP "ev_set_userdata (loop, void *data)" 4
1269 .IX Item "ev_set_userdata (loop, void *data)"
1270 .PD 0
1271 .IP "void *ev_userdata (loop)" 4
1272 .IX Item "void *ev_userdata (loop)"
1273 .PD
1274 Set and retrieve a single \f(CW\*(C`void *\*(C'\fR associated with a loop. When
1275 \&\f(CW\*(C`ev_set_userdata\*(C'\fR has never been called, then \f(CW\*(C`ev_userdata\*(C'\fR returns
1276 \&\f(CW0\fR.
1277 .Sp
1278 These two functions can be used to associate arbitrary data with a loop,
1279 and are intended solely for the \f(CW\*(C`invoke_pending_cb\*(C'\fR, \f(CW\*(C`release\*(C'\fR and
1280 \&\f(CW\*(C`acquire\*(C'\fR callbacks described above, but of course can be (ab\-)used for
1281 any other purpose as well.
1282 .IP "ev_verify (loop)" 4
1283 .IX Item "ev_verify (loop)"
1284 This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR support has been
1285 compiled in, which is the default for non-minimal builds. It tries to go
1286 through all internal structures and checks them for validity. If anything
1287 is found to be inconsistent, it will print an error message to standard
1288 error and call \f(CW\*(C`abort ()\*(C'\fR.
1289 .Sp
1290 This can be used to catch bugs inside libev itself: under normal
1291 circumstances, this function will never abort as of course libev keeps its
1292 data structures consistent.
1293 .SH "ANATOMY OF A WATCHER"
1294 .IX Header "ANATOMY OF A WATCHER"
1295 In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the
1296 watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer
1297 watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.
1298 .PP
1299 A watcher is an opaque structure that you allocate and register to record
1300 your interest in some event. To make a concrete example, imagine you want
1301 to wait for \s-1STDIN\s0 to become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher
1302 for that:
1303 .PP
1304 .Vb 5
1305 \& static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
1306 \& {
1307 \& ev_io_stop (w);
1308 \& ev_break (loop, EVBREAK_ALL);
1309 \& }
1310 \&
1311 \& struct ev_loop *loop = ev_default_loop (0);
1312 \&
1313 \& ev_io stdin_watcher;
1314 \&
1315 \& ev_init (&stdin_watcher, my_cb);
1316 \& ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
1317 \& ev_io_start (loop, &stdin_watcher);
1318 \&
1319 \& ev_run (loop, 0);
1320 .Ve
1321 .PP
1322 As you can see, you are responsible for allocating the memory for your
1323 watcher structures (and it is \fIusually\fR a bad idea to do this on the
1324 stack).
1325 .PP
1326 Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR
1327 or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).
1328 .PP
1329 Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init (watcher
1330 *, callback)\*(C'\fR, which expects a callback to be provided. This callback is
1331 invoked each time the event occurs (or, in the case of I/O watchers, each
1332 time the event loop detects that the file descriptor given is readable
1333 and/or writable).
1334 .PP
1335 Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR
1336 macro to configure it, with arguments specific to the watcher type. There
1337 is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.
1338 .PP
1339 To make the watcher actually watch out for events, you have to start it
1340 with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher
1341 *)\*(C'\fR), and you can stop watching for events at any time by calling the
1342 corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.
1343 .PP
1344 As long as your watcher is active (has been started but not stopped) you
1345 must not touch the values stored in it. Most specifically you must never
1346 reinitialise it or call its \f(CW\*(C`ev_TYPE_set\*(C'\fR macro.
1347 .PP
1348 Each and every callback receives the event loop pointer as first, the
1349 registered watcher structure as second, and a bitset of received events as
1350 third argument.
1351 .PP
1352 The received events usually include a single bit per event type received
1353 (you can receive multiple events at the same time). The possible bit masks
1354 are:
1355 .ie n .IP """EV_READ""" 4
1356 .el .IP "\f(CWEV_READ\fR" 4
1357 .IX Item "EV_READ"
1358 .PD 0
1359 .ie n .IP """EV_WRITE""" 4
1360 .el .IP "\f(CWEV_WRITE\fR" 4
1361 .IX Item "EV_WRITE"
1362 .PD
1363 The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or
1364 writable.
1365 .ie n .IP """EV_TIMER""" 4
1366 .el .IP "\f(CWEV_TIMER\fR" 4
1367 .IX Item "EV_TIMER"
1368 The \f(CW\*(C`ev_timer\*(C'\fR watcher has timed out.
1369 .ie n .IP """EV_PERIODIC""" 4
1370 .el .IP "\f(CWEV_PERIODIC\fR" 4
1371 .IX Item "EV_PERIODIC"
1372 The \f(CW\*(C`ev_periodic\*(C'\fR watcher has timed out.
1373 .ie n .IP """EV_SIGNAL""" 4
1374 .el .IP "\f(CWEV_SIGNAL\fR" 4
1375 .IX Item "EV_SIGNAL"
1376 The signal specified in the \f(CW\*(C`ev_signal\*(C'\fR watcher has been received by a thread.
1377 .ie n .IP """EV_CHILD""" 4
1378 .el .IP "\f(CWEV_CHILD\fR" 4
1379 .IX Item "EV_CHILD"
1380 The pid specified in the \f(CW\*(C`ev_child\*(C'\fR watcher has received a status change.
1381 .ie n .IP """EV_STAT""" 4
1382 .el .IP "\f(CWEV_STAT\fR" 4
1383 .IX Item "EV_STAT"
1384 The path specified in the \f(CW\*(C`ev_stat\*(C'\fR watcher changed its attributes somehow.
1385 .ie n .IP """EV_IDLE""" 4
1386 .el .IP "\f(CWEV_IDLE\fR" 4
1387 .IX Item "EV_IDLE"
1388 The \f(CW\*(C`ev_idle\*(C'\fR watcher has determined that you have nothing better to do.
1389 .ie n .IP """EV_PREPARE""" 4
1390 .el .IP "\f(CWEV_PREPARE\fR" 4
1391 .IX Item "EV_PREPARE"
1392 .PD 0
1393 .ie n .IP """EV_CHECK""" 4
1394 .el .IP "\f(CWEV_CHECK\fR" 4
1395 .IX Item "EV_CHECK"
1396 .PD
1397 All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_run\*(C'\fR starts to
1398 gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are queued (not invoked)
1399 just after \f(CW\*(C`ev_run\*(C'\fR has gathered them, but before it queues any callbacks
1400 for any received events. That means \f(CW\*(C`ev_prepare\*(C'\fR watchers are the last
1401 watchers invoked before the event loop sleeps or polls for new events, and
1402 \&\f(CW\*(C`ev_check\*(C'\fR watchers will be invoked before any other watchers of the same
1403 or lower priority within an event loop iteration.
1404 .Sp
1405 Callbacks of both watcher types can start and stop as many watchers as
1406 they want, and all of them will be taken into account (for example, a
1407 \&\f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep \f(CW\*(C`ev_run\*(C'\fR from
1408 blocking).
1409 .ie n .IP """EV_EMBED""" 4
1410 .el .IP "\f(CWEV_EMBED\fR" 4
1411 .IX Item "EV_EMBED"
1412 The embedded event loop specified in the \f(CW\*(C`ev_embed\*(C'\fR watcher needs attention.
1413 .ie n .IP """EV_FORK""" 4
1414 .el .IP "\f(CWEV_FORK\fR" 4
1415 .IX Item "EV_FORK"
1416 The event loop has been resumed in the child process after fork (see
1417 \&\f(CW\*(C`ev_fork\*(C'\fR).
1418 .ie n .IP """EV_CLEANUP""" 4
1419 .el .IP "\f(CWEV_CLEANUP\fR" 4
1420 .IX Item "EV_CLEANUP"
1421 The event loop is about to be destroyed (see \f(CW\*(C`ev_cleanup\*(C'\fR).
1422 .ie n .IP """EV_ASYNC""" 4
1423 .el .IP "\f(CWEV_ASYNC\fR" 4
1424 .IX Item "EV_ASYNC"
1425 The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).
1426 .ie n .IP """EV_CUSTOM""" 4
1427 .el .IP "\f(CWEV_CUSTOM\fR" 4
1428 .IX Item "EV_CUSTOM"
1429 Not ever sent (or otherwise used) by libev itself, but can be freely used
1430 by libev users to signal watchers (e.g. via \f(CW\*(C`ev_feed_event\*(C'\fR).
1431 .ie n .IP """EV_ERROR""" 4
1432 .el .IP "\f(CWEV_ERROR\fR" 4
1433 .IX Item "EV_ERROR"
1434 An unspecified error has occurred, the watcher has been stopped. This might
1435 happen because the watcher could not be properly started because libev
1436 ran out of memory, a file descriptor was found to be closed or any other
1437 problem. Libev considers these application bugs.
1438 .Sp
1439 You best act on it by reporting the problem and somehow coping with the
1440 watcher being stopped. Note that well-written programs should not receive
1441 an error ever, so when your watcher receives it, this usually indicates a
1442 bug in your program.
1443 .Sp
1444 Libev will usually signal a few \*(L"dummy\*(R" events together with an error, for
1445 example it might indicate that a fd is readable or writable, and if your
1446 callbacks is well-written it can just attempt the operation and cope with
1447 the error from \fBread()\fR or \fBwrite()\fR. This will not work in multi-threaded
1448 programs, though, as the fd could already be closed and reused for another
1449 thing, so beware.
1450 .SS "\s-1GENERIC WATCHER FUNCTIONS\s0"
1451 .IX Subsection "GENERIC WATCHER FUNCTIONS"
1452 .ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4
1453 .el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4
1454 .IX Item "ev_init (ev_TYPE *watcher, callback)"
1455 This macro initialises the generic portion of a watcher. The contents
1456 of the watcher object can be arbitrary (so \f(CW\*(C`malloc\*(C'\fR will do). Only
1457 the generic parts of the watcher are initialised, you \fIneed\fR to call
1458 the type-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR macro afterwards to initialise the
1459 type-specific parts. For each type there is also a \f(CW\*(C`ev_TYPE_init\*(C'\fR macro
1460 which rolls both calls into one.
1461 .Sp
1462 You can reinitialise a watcher at any time as long as it has been stopped
1463 (or never started) and there are no pending events outstanding.
1464 .Sp
1465 The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
1466 int revents)\*(C'\fR.
1467 .Sp
1468 Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
1469 .Sp
1470 .Vb 3
1471 \& ev_io w;
1472 \& ev_init (&w, my_cb);
1473 \& ev_io_set (&w, STDIN_FILENO, EV_READ);
1474 .Ve
1475 .ie n .IP """ev_TYPE_set"" (ev_TYPE *watcher, [args])" 4
1476 .el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *watcher, [args])" 4
1477 .IX Item "ev_TYPE_set (ev_TYPE *watcher, [args])"
1478 This macro initialises the type-specific parts of a watcher. You need to
1479 call \f(CW\*(C`ev_init\*(C'\fR at least once before you call this macro, but you can
1480 call \f(CW\*(C`ev_TYPE_set\*(C'\fR any number of times. You must not, however, call this
1481 macro on a watcher that is active (it can be pending, however, which is a
1482 difference to the \f(CW\*(C`ev_init\*(C'\fR macro).
1483 .Sp
1484 Although some watcher types do not have type-specific arguments
1485 (e.g. \f(CW\*(C`ev_prepare\*(C'\fR) you still need to call its \f(CW\*(C`set\*(C'\fR macro.
1486 .Sp
1487 See \f(CW\*(C`ev_init\*(C'\fR, above, for an example.
1488 .ie n .IP """ev_TYPE_init"" (ev_TYPE *watcher, callback, [args])" 4
1489 .el .IP "\f(CWev_TYPE_init\fR (ev_TYPE *watcher, callback, [args])" 4
1490 .IX Item "ev_TYPE_init (ev_TYPE *watcher, callback, [args])"
1491 This convenience macro rolls both \f(CW\*(C`ev_init\*(C'\fR and \f(CW\*(C`ev_TYPE_set\*(C'\fR macro
1492 calls into a single call. This is the most convenient method to initialise
1493 a watcher. The same limitations apply, of course.
1494 .Sp
1495 Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.
1496 .Sp
1497 .Vb 1
1498 \& ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
1499 .Ve
1500 .ie n .IP """ev_TYPE_start"" (loop, ev_TYPE *watcher)" 4
1501 .el .IP "\f(CWev_TYPE_start\fR (loop, ev_TYPE *watcher)" 4
1502 .IX Item "ev_TYPE_start (loop, ev_TYPE *watcher)"
1503 Starts (activates) the given watcher. Only active watchers will receive
1504 events. If the watcher is already active nothing will happen.
1505 .Sp
1506 Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this
1507 whole section.
1508 .Sp
1509 .Vb 1
1510 \& ev_io_start (EV_DEFAULT_UC, &w);
1511 .Ve
1512 .ie n .IP """ev_TYPE_stop"" (loop, ev_TYPE *watcher)" 4
1513 .el .IP "\f(CWev_TYPE_stop\fR (loop, ev_TYPE *watcher)" 4
1514 .IX Item "ev_TYPE_stop (loop, ev_TYPE *watcher)"
1515 Stops the given watcher if active, and clears the pending status (whether
1516 the watcher was active or not).
1517 .Sp
1518 It is possible that stopped watchers are pending \- for example,
1519 non-repeating timers are being stopped when they become pending \- but
1520 calling \f(CW\*(C`ev_TYPE_stop\*(C'\fR ensures that the watcher is neither active nor
1521 pending. If you want to free or reuse the memory used by the watcher it is
1522 therefore a good idea to always call its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function.
1523 .IP "bool ev_is_active (ev_TYPE *watcher)" 4
1524 .IX Item "bool ev_is_active (ev_TYPE *watcher)"
1525 Returns a true value iff the watcher is active (i.e. it has been started
1526 and not yet been stopped). As long as a watcher is active you must not modify
1527 it.
1528 .IP "bool ev_is_pending (ev_TYPE *watcher)" 4
1529 .IX Item "bool ev_is_pending (ev_TYPE *watcher)"
1530 Returns a true value iff the watcher is pending, (i.e. it has outstanding
1531 events but its callback has not yet been invoked). As long as a watcher
1532 is pending (but not active) you must not call an init function on it (but
1533 \&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must
1534 make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR
1535 it).
1536 .IP "callback ev_cb (ev_TYPE *watcher)" 4
1537 .IX Item "callback ev_cb (ev_TYPE *watcher)"
1538 Returns the callback currently set on the watcher.
1539 .IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
1540 .IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
1541 Change the callback. You can change the callback at virtually any time
1542 (modulo threads).
1543 .IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
1544 .IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
1545 .PD 0
1546 .IP "int ev_priority (ev_TYPE *watcher)" 4
1547 .IX Item "int ev_priority (ev_TYPE *watcher)"
1548 .PD
1549 Set and query the priority of the watcher. The priority is a small
1550 integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR
1551 (default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked
1552 before watchers with lower priority, but priority will not keep watchers
1553 from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).
1554 .Sp
1555 If you need to suppress invocation when higher priority events are pending
1556 you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.
1557 .Sp
1558 You \fImust not\fR change the priority of a watcher as long as it is active or
1559 pending.
1560 .Sp
1561 Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
1562 fine, as long as you do not mind that the priority value you query might
1563 or might not have been clamped to the valid range.
1564 .Sp
1565 The default priority used by watchers when no priority has been set is
1566 always \f(CW0\fR, which is supposed to not be too high and not be too low :).
1567 .Sp
1568 See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
1569 priorities.
1570 .IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
1571 .IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
1572 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
1573 \&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback
1574 can deal with that fact, as both are simply passed through to the
1575 callback.
1576 .IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4
1577 .IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"
1578 If the watcher is pending, this function clears its pending status and
1579 returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the
1580 watcher isn't pending it does nothing and returns \f(CW0\fR.
1581 .Sp
1582 Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its
1583 callback to be invoked, which can be accomplished with this function.
1584 .IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
1585 .IX Item "ev_feed_event (loop, ev_TYPE *watcher, int revents)"
1586 Feeds the given event set into the event loop, as if the specified event
1587 had happened for the specified watcher (which must be a pointer to an
1588 initialised but not necessarily started event watcher). Obviously you must
1589 not free the watcher as long as it has pending events.
1590 .Sp
1591 Stopping the watcher, letting libev invoke it, or calling
1592 \&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
1593 not started in the first place.
1594 .Sp
1595 See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
1596 functions that do not need a watcher.
1597 .PP
1598 See also the \*(L"\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\*(R"\s0 and \*(L"\s-1BUILDING YOUR
1599 OWN COMPOSITE WATCHERS\*(R"\s0 idioms.
1600 .SS "\s-1WATCHER STATES\s0"
1601 .IX Subsection "WATCHER STATES"
1602 There are various watcher states mentioned throughout this manual \-
1603 active, pending and so on. In this section these states and the rules to
1604 transition between them will be described in more detail \- and while these
1605 rules might look complicated, they usually do \*(L"the right thing\*(R".
1606 .IP "initialised" 4
1607 .IX Item "initialised"
1608 Before a watcher can be registered with the event loop it has to be
1609 initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
1610 \&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR function.
1611 .Sp
1612 In this state it is simply some block of memory that is suitable for
1613 use in an event loop. It can be moved around, freed, reused etc. at
1614 will \- as long as you either keep the memory contents intact, or call
1615 \&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
1616 .IP "started/running/active" 4
1617 .IX Item "started/running/active"
1618 Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR it becomes
1619 property of the event loop, and is actively waiting for events. While in
1620 this state it cannot be accessed (except in a few documented ways), moved,
1621 freed or anything else \- the only legal thing is to keep a pointer to it,
1622 and call libev functions on it that are documented to work on active watchers.
1623 .IP "pending" 4
1624 .IX Item "pending"
1625 If a watcher is active and libev determines that an event it is interested
1626 in has occurred (such as a timer expiring), it will become pending. It will
1627 stay in this pending state until either it is stopped or its callback is
1628 about to be invoked, so it is not normally pending inside the watcher
1629 callback.
1630 .Sp
1631 The watcher might or might not be active while it is pending (for example,
1632 an expired non-repeating timer can be pending but no longer active). If it
1633 is stopped, it can be freely accessed (e.g. by calling \f(CW\*(C`ev_TYPE_set\*(C'\fR),
1634 but it is still property of the event loop at this time, so cannot be
1635 moved, freed or reused. And if it is active the rules described in the
1636 previous item still apply.
1637 .Sp
1638 It is also possible to feed an event on a watcher that is not active (e.g.
1639 via \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
1640 active.
1641 .IP "stopped" 4
1642 .IX Item "stopped"
1643 A watcher can be stopped implicitly by libev (in which case it might still
1644 be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function. The
1645 latter will clear any pending state the watcher might be in, regardless
1646 of whether it was active or not, so stopping a watcher explicitly before
1647 freeing it is often a good idea.
1648 .Sp
1649 While stopped (and not pending) the watcher is essentially in the
1650 initialised state, that is, it can be reused, moved, modified in any way
1651 you wish (but when you trash the memory block, you need to \f(CW\*(C`ev_TYPE_init\*(C'\fR
1652 it again).
1653 .SS "\s-1WATCHER PRIORITY MODELS\s0"
1654 .IX Subsection "WATCHER PRIORITY MODELS"
1655 Many event loops support \fIwatcher priorities\fR, which are usually small
1656 integers that influence the ordering of event callback invocation
1657 between watchers in some way, all else being equal.
1658 .PP
1659 In libev, Watcher priorities can be set using \f(CW\*(C`ev_set_priority\*(C'\fR. See its
1660 description for the more technical details such as the actual priority
1661 range.
1662 .PP
1663 There are two common ways how these these priorities are being interpreted
1664 by event loops:
1665 .PP
1666 In the more common lock-out model, higher priorities \*(L"lock out\*(R" invocation
1667 of lower priority watchers, which means as long as higher priority
1668 watchers receive events, lower priority watchers are not being invoked.
1669 .PP
1670 The less common only-for-ordering model uses priorities solely to order
1671 callback invocation within a single event loop iteration: Higher priority
1672 watchers are invoked before lower priority ones, but they all get invoked
1673 before polling for new events.
1674 .PP
1675 Libev uses the second (only-for-ordering) model for all its watchers
1676 except for idle watchers (which use the lock-out model).
1677 .PP
1678 The rationale behind this is that implementing the lock-out model for
1679 watchers is not well supported by most kernel interfaces, and most event
1680 libraries will just poll for the same events again and again as long as
1681 their callbacks have not been executed, which is very inefficient in the
1682 common case of one high-priority watcher locking out a mass of lower
1683 priority ones.
1684 .PP
1685 Static (ordering) priorities are most useful when you have two or more
1686 watchers handling the same resource: a typical usage example is having an
1687 \&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR to handle
1688 timeouts. Under load, data might be received while the program handles
1689 other jobs, but since timers normally get invoked first, the timeout
1690 handler will be executed before checking for data. In that case, giving
1691 the timer a lower priority than the I/O watcher ensures that I/O will be
1692 handled first even under adverse conditions (which is usually, but not
1693 always, what you want).
1694 .PP
1695 Since idle watchers use the \*(L"lock-out\*(R" model, meaning that idle watchers
1696 will only be executed when no same or higher priority watchers have
1697 received events, they can be used to implement the \*(L"lock-out\*(R" model when
1698 required.
1699 .PP
1700 For example, to emulate how many other event libraries handle priorities,
1701 you can associate an \f(CW\*(C`ev_idle\*(C'\fR watcher to each such watcher, and in
1702 the normal watcher callback, you just start the idle watcher. The real
1703 processing is done in the idle watcher callback. This causes libev to
1704 continuously poll and process kernel event data for the watcher, but when
1705 the lock-out case is known to be rare (which in turn is rare :), this is
1706 workable.
1707 .PP
1708 Usually, however, the lock-out model implemented that way will perform
1709 miserably under the type of load it was designed to handle. In that case,
1710 it might be preferable to stop the real watcher before starting the
1711 idle watcher, so the kernel will not have to process the event in case
1712 the actual processing will be delayed for considerable time.
1713 .PP
1714 Here is an example of an I/O watcher that should run at a strictly lower
1715 priority than the default, and which should only process data when no
1716 other events are pending:
1717 .PP
1718 .Vb 2
1719 \& ev_idle idle; // actual processing watcher
1720 \& ev_io io; // actual event watcher
1721 \&
1722 \& static void
1723 \& io_cb (EV_P_ ev_io *w, int revents)
1724 \& {
1725 \& // stop the I/O watcher, we received the event, but
1726 \& // are not yet ready to handle it.
1727 \& ev_io_stop (EV_A_ w);
1728 \&
1729 \& // start the idle watcher to handle the actual event.
1730 \& // it will not be executed as long as other watchers
1731 \& // with the default priority are receiving events.
1732 \& ev_idle_start (EV_A_ &idle);
1733 \& }
1734 \&
1735 \& static void
1736 \& idle_cb (EV_P_ ev_idle *w, int revents)
1737 \& {
1738 \& // actual processing
1739 \& read (STDIN_FILENO, ...);
1740 \&
1741 \& // have to start the I/O watcher again, as
1742 \& // we have handled the event
1743 \& ev_io_start (EV_P_ &io);
1744 \& }
1745 \&
1746 \& // initialisation
1747 \& ev_idle_init (&idle, idle_cb);
1748 \& ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1749 \& ev_io_start (EV_DEFAULT_ &io);
1750 .Ve
1751 .PP
1752 In the \*(L"real\*(R" world, it might also be beneficial to start a timer, so that
1753 low-priority connections can not be locked out forever under load. This
1754 enables your program to keep a lower latency for important connections
1755 during short periods of high load, while not completely locking out less
1756 important ones.
1757 .SH "WATCHER TYPES"
1758 .IX Header "WATCHER TYPES"
1759 This section describes each watcher in detail, but will not repeat
1760 information given in the last section. Any initialisation/set macros,
1761 functions and members specific to the watcher type are explained.
1762 .PP
1763 Members are additionally marked with either \fI[read\-only]\fR, meaning that,
1764 while the watcher is active, you can look at the member and expect some
1765 sensible content, but you must not modify it (you can modify it while the
1766 watcher is stopped to your hearts content), or \fI[read\-write]\fR, which
1767 means you can expect it to have some sensible content while the watcher
1768 is active, but you can also modify it. Modifying it may not do something
1769 sensible or take immediate effect (or do anything at all), but libev will
1770 not crash or malfunction in any way.
1771 .ie n .SS """ev_io"" \- is this file descriptor readable or writable?"
1772 .el .SS "\f(CWev_io\fP \- is this file descriptor readable or writable?"
1773 .IX Subsection "ev_io - is this file descriptor readable or writable?"
1774 I/O watchers check whether a file descriptor is readable or writable
1775 in each iteration of the event loop, or, more precisely, when reading
1776 would not block the process and writing would at least be able to write
1777 some data. This behaviour is called level-triggering because you keep
1778 receiving events as long as the condition persists. Remember you can stop
1779 the watcher if you don't want to act on the event and neither want to
1780 receive future events.
1781 .PP
1782 In general you can register as many read and/or write event watchers per
1783 fd as you want (as long as you don't confuse yourself). Setting all file
1784 descriptors to non-blocking mode is also usually a good idea (but not
1785 required if you know what you are doing).
1786 .PP
1787 Another thing you have to watch out for is that it is quite easy to
1788 receive \*(L"spurious\*(R" readiness notifications, that is, your callback might
1789 be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block
1790 because there is no data. It is very easy to get into this situation even
1791 with a relatively standard program structure. Thus it is best to always
1792 use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning \f(CW\*(C`EAGAIN\*(C'\fR is far
1793 preferable to a program hanging until some data arrives.
1794 .PP
1795 If you cannot run the fd in non-blocking mode (for example you should
1796 not play around with an Xlib connection), then you have to separately
1797 re-test whether a file descriptor is really ready with a known-to-be good
1798 interface such as poll (fortunately in the case of Xlib, it already does
1799 this on its own, so its quite safe to use). Some people additionally
1800 use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
1801 indefinitely.
1802 .PP
1803 But really, best use non-blocking mode.
1804 .PP
1805 \fIThe special problem of disappearing file descriptors\fR
1806 .IX Subsection "The special problem of disappearing file descriptors"
1807 .PP
1808 Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
1809 a file descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other
1810 means, such as \f(CW\*(C`dup2\*(C'\fR). The reason is that you register interest in some
1811 file descriptor, but when it goes away, the operating system will silently
1812 drop this interest. If another file descriptor with the same number then
1813 is registered with libev, there is no efficient way to see that this is,
1814 in fact, a different file descriptor.
1815 .PP
1816 To avoid having to explicitly tell libev about such cases, libev follows
1817 the following policy: Each time \f(CW\*(C`ev_io_set\*(C'\fR is being called, libev
1818 will assume that this is potentially a new file descriptor, otherwise
1819 it is assumed that the file descriptor stays the same. That means that
1820 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
1821 descriptor even if the file descriptor number itself did not change.
1822 .PP
1823 This is how one would do it normally anyway, the important point is that
1824 the libev application should not optimise around libev but should leave
1825 optimisations to libev.
1826 .PP
1827 \fIThe special problem of dup'ed file descriptors\fR
1828 .IX Subsection "The special problem of dup'ed file descriptors"
1829 .PP
1830 Some backends (e.g. epoll), cannot register events for file descriptors,
1831 but only events for the underlying file descriptions. That means when you
1832 have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register
1833 events for them, only one file descriptor might actually receive events.
1834 .PP
1835 There is no workaround possible except not registering events
1836 for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
1837 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1838 .PP
1839 \fIThe special problem of files\fR
1840 .IX Subsection "The special problem of files"
1841 .PP
1842 Many people try to use \f(CW\*(C`select\*(C'\fR (or libev) on file descriptors
1843 representing files, and expect it to become ready when their program
1844 doesn't block on disk accesses (which can take a long time on their own).
1845 .PP
1846 However, this cannot ever work in the \*(L"expected\*(R" way \- you get a readiness
1847 notification as soon as the kernel knows whether and how much data is
1848 there, and in the case of open files, that's always the case, so you
1849 always get a readiness notification instantly, and your read (or possibly
1850 write) will still block on the disk I/O.
1851 .PP
1852 Another way to view it is that in the case of sockets, pipes, character
1853 devices and so on, there is another party (the sender) that delivers data
1854 on its own, but in the case of files, there is no such thing: the disk
1855 will not send data on its own, simply because it doesn't know what you
1856 wish to read \- you would first have to request some data.
1857 .PP
1858 Since files are typically not-so-well supported by advanced notification
1859 mechanism, libev tries hard to emulate \s-1POSIX\s0 behaviour with respect
1860 to files, even though you should not use it. The reason for this is
1861 convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
1862 usually a tty, often a pipe, but also sometimes files or special devices
1863 (for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
1864 \&\fI/dev/urandom\fR), and even though the file might better be served with
1865 asynchronous I/O instead of with non-blocking I/O, it is still useful when
1866 it \*(L"just works\*(R" instead of freezing.
1867 .PP
1868 So avoid file descriptors pointing to files when you know it (e.g. use
1869 libeio), but use them when it is convenient, e.g. for \s-1STDIN/STDOUT,\s0 or
1870 when you rarely read from a file instead of from a socket, and want to
1871 reuse the same code path.
1872 .PP
1873 \fIThe special problem of fork\fR
1874 .IX Subsection "The special problem of fork"
1875 .PP
1876 Some backends (epoll, kqueue, probably linuxaio) do not support \f(CW\*(C`fork ()\*(C'\fR
1877 at all or exhibit useless behaviour. Libev fully supports fork, but needs
1878 to be told about it in the child if you want to continue to use it in the
1879 child.
1880 .PP
1881 To support fork in your child processes, you have to call \f(CW\*(C`ev_loop_fork
1882 ()\*(C'\fR after a fork in the child, enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to
1883 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1884 .PP
1885 \fIThe special problem of \s-1SIGPIPE\s0\fR
1886 .IX Subsection "The special problem of SIGPIPE"
1887 .PP
1888 While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:
1889 when writing to a pipe whose other end has been closed, your program gets
1890 sent a \s-1SIGPIPE,\s0 which, by default, aborts your program. For most programs
1891 this is sensible behaviour, for daemons, this is usually undesirable.
1892 .PP
1893 So when you encounter spurious, unexplained daemon exits, make sure you
1894 ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon
1895 somewhere, as that would have given you a big clue).
1896 .PP
1897 \fIThe special problem of \f(BIaccept()\fIing when you can't\fR
1898 .IX Subsection "The special problem of accept()ing when you can't"
1899 .PP
1900 Many implementations of the \s-1POSIX\s0 \f(CW\*(C`accept\*(C'\fR function (for example,
1901 found in post\-2004 Linux) have the peculiar behaviour of not removing a
1902 connection from the pending queue in all error cases.
1903 .PP
1904 For example, larger servers often run out of file descriptors (because
1905 of resource limits), causing \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR but not
1906 rejecting the connection, leading to libev signalling readiness on
1907 the next iteration again (the connection still exists after all), and
1908 typically causing the program to loop at 100% \s-1CPU\s0 usage.
1909 .PP
1910 Unfortunately, the set of errors that cause this issue differs between
1911 operating systems, there is usually little the app can do to remedy the
1912 situation, and no known thread-safe method of removing the connection to
1913 cope with overload is known (to me).
1914 .PP
1915 One of the easiest ways to handle this situation is to just ignore it
1916 \&\- when the program encounters an overload, it will just loop until the
1917 situation is over. While this is a form of busy waiting, no \s-1OS\s0 offers an
1918 event-based way to handle this situation, so it's the best one can do.
1919 .PP
1920 A better way to handle the situation is to log any errors other than
1921 \&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, making sure not to flood the log with such
1922 messages, and continue as usual, which at least gives the user an idea of
1923 what could be wrong (\*(L"raise the ulimit!\*(R"). For extra points one could stop
1924 the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
1925 usage.
1926 .PP
1927 If your program is single-threaded, then you could also keep a dummy file
1928 descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
1929 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,
1930 close that fd, and create a new dummy fd. This will gracefully refuse
1931 clients under typical overload conditions.
1932 .PP
1933 The last way to handle it is to simply log the error and \f(CW\*(C`exit\*(C'\fR, as
1934 is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
1935 opportunity for a DoS attack.
1936 .PP
1937 \fIWatcher-Specific Functions\fR
1938 .IX Subsection "Watcher-Specific Functions"
1939 .IP "ev_io_init (ev_io *, callback, int fd, int events)" 4
1940 .IX Item "ev_io_init (ev_io *, callback, int fd, int events)"
1941 .PD 0
1942 .IP "ev_io_set (ev_io *, int fd, int events)" 4
1943 .IX Item "ev_io_set (ev_io *, int fd, int events)"
1944 .PD
1945 Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to
1946 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
1947 \&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR, to express the desire to receive the given events.
1948 .IP "int fd [read\-only]" 4
1949 .IX Item "int fd [read-only]"
1950 The file descriptor being watched.
1951 .IP "int events [read\-only]" 4
1952 .IX Item "int events [read-only]"
1953 The events being watched.
1954 .PP
1955 \fIExamples\fR
1956 .IX Subsection "Examples"
1957 .PP
1958 Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well
1959 readable, but only once. Since it is likely line-buffered, you could
1960 attempt to read a whole line in the callback.
1961 .PP
1962 .Vb 6
1963 \& static void
1964 \& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1965 \& {
1966 \& ev_io_stop (loop, w);
1967 \& .. read from stdin here (or from w\->fd) and handle any I/O errors
1968 \& }
1969 \&
1970 \& ...
1971 \& struct ev_loop *loop = ev_default_init (0);
1972 \& ev_io stdin_readable;
1973 \& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
1974 \& ev_io_start (loop, &stdin_readable);
1975 \& ev_run (loop, 0);
1976 .Ve
1977 .ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
1978 .el .SS "\f(CWev_timer\fP \- relative and optionally repeating timeouts"
1979 .IX Subsection "ev_timer - relative and optionally repeating timeouts"
1980 Timer watchers are simple relative timers that generate an event after a
1981 given time, and optionally repeating in regular intervals after that.
1982 .PP
1983 The timers are based on real time, that is, if you register an event that
1984 times out after an hour and you reset your system clock to January last
1985 year, it will still time out after (roughly) one hour. \*(L"Roughly\*(R" because
1986 detecting time jumps is hard, and some inaccuracies are unavoidable (the
1987 monotonic clock option helps a lot here).
1988 .PP
1989 The callback is guaranteed to be invoked only \fIafter\fR its timeout has
1990 passed (not \fIat\fR, so on systems with very low-resolution clocks this
1991 might introduce a small delay, see \*(L"the special problem of being too
1992 early\*(R", below). If multiple timers become ready during the same loop
1993 iteration then the ones with earlier time-out values are invoked before
1994 ones of the same priority with later time-out values (but this is no
1995 longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
1996 .PP
1997 \fIBe smart about timeouts\fR
1998 .IX Subsection "Be smart about timeouts"
1999 .PP
2000 Many real-world problems involve some kind of timeout, usually for error
2001 recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
2002 you want to raise some error after a while.
2003 .PP
2004 What follows are some ways to handle this problem, from obvious and
2005 inefficient to smart and efficient.
2006 .PP
2007 In the following, a 60 second activity timeout is assumed \- a timeout that
2008 gets reset to 60 seconds each time there is activity (e.g. each time some
2009 data or other life sign was received).
2010 .IP "1. Use a timer and stop, reinitialise and start it on activity." 4
2011 .IX Item "1. Use a timer and stop, reinitialise and start it on activity."
2012 This is the most obvious, but not the most simple way: In the beginning,
2013 start the watcher:
2014 .Sp
2015 .Vb 2
2016 \& ev_timer_init (timer, callback, 60., 0.);
2017 \& ev_timer_start (loop, timer);
2018 .Ve
2019 .Sp
2020 Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
2021 and start it again:
2022 .Sp
2023 .Vb 3
2024 \& ev_timer_stop (loop, timer);
2025 \& ev_timer_set (timer, 60., 0.);
2026 \& ev_timer_start (loop, timer);
2027 .Ve
2028 .Sp
2029 This is relatively simple to implement, but means that each time there is
2030 some activity, libev will first have to remove the timer from its internal
2031 data structure and then add it again. Libev tries to be fast, but it's
2032 still not a constant-time operation.
2033 .ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
2034 .el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
2035 .IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
2036 This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
2037 \&\f(CW\*(C`ev_timer_start\*(C'\fR.
2038 .Sp
2039 To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
2040 of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
2041 successfully read or write some data. If you go into an idle state where
2042 you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
2043 the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
2044 .Sp
2045 That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
2046 \&\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
2047 member and \f(CW\*(C`ev_timer_again\*(C'\fR.
2048 .Sp
2049 At start:
2050 .Sp
2051 .Vb 3
2052 \& ev_init (timer, callback);
2053 \& timer\->repeat = 60.;
2054 \& ev_timer_again (loop, timer);
2055 .Ve
2056 .Sp
2057 Each time there is some activity:
2058 .Sp
2059 .Vb 1
2060 \& ev_timer_again (loop, timer);
2061 .Ve
2062 .Sp
2063 It is even possible to change the time-out on the fly, regardless of
2064 whether the watcher is active or not:
2065 .Sp
2066 .Vb 2
2067 \& timer\->repeat = 30.;
2068 \& ev_timer_again (loop, timer);
2069 .Ve
2070 .Sp
2071 This is slightly more efficient then stopping/starting the timer each time
2072 you want to modify its timeout value, as libev does not have to completely
2073 remove and re-insert the timer from/into its internal data structure.
2074 .Sp
2075 It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
2076 .IP "3. Let the timer time out, but then re-arm it as required." 4
2077 .IX Item "3. Let the timer time out, but then re-arm it as required."
2078 This method is more tricky, but usually most efficient: Most timeouts are
2079 relatively long compared to the intervals between other activity \- in
2080 our example, within 60 seconds, there are usually many I/O events with
2081 associated activity resets.
2082 .Sp
2083 In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
2084 but remember the time of last activity, and check for a real timeout only
2085 within the callback:
2086 .Sp
2087 .Vb 3
2088 \& ev_tstamp timeout = 60.;
2089 \& ev_tstamp last_activity; // time of last activity
2090 \& ev_timer timer;
2091 \&
2092 \& static void
2093 \& callback (EV_P_ ev_timer *w, int revents)
2094 \& {
2095 \& // calculate when the timeout would happen
2096 \& ev_tstamp after = last_activity \- ev_now (EV_A) + timeout;
2097 \&
2098 \& // if negative, it means we the timeout already occurred
2099 \& if (after < 0.)
2100 \& {
2101 \& // timeout occurred, take action
2102 \& }
2103 \& else
2104 \& {
2105 \& // callback was invoked, but there was some recent
2106 \& // activity. simply restart the timer to time out
2107 \& // after "after" seconds, which is the earliest time
2108 \& // the timeout can occur.
2109 \& ev_timer_set (w, after, 0.);
2110 \& ev_timer_start (EV_A_ w);
2111 \& }
2112 \& }
2113 .Ve
2114 .Sp
2115 To summarise the callback: first calculate in how many seconds the
2116 timeout will occur (by calculating the absolute time when it would occur,
2117 \&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
2118 (EV_A)\*(C'\fR from that).
2119 .Sp
2120 If this value is negative, then we are already past the timeout, i.e. we
2121 timed out, and need to do whatever is needed in this case.
2122 .Sp
2123 Otherwise, we now the earliest time at which the timeout would trigger,
2124 and simply start the timer with this timeout value.
2125 .Sp
2126 In other words, each time the callback is invoked it will check whether
2127 the timeout occurred. If not, it will simply reschedule itself to check
2128 again at the earliest time it could time out. Rinse. Repeat.
2129 .Sp
2130 This scheme causes more callback invocations (about one every 60 seconds
2131 minus half the average time between activity), but virtually no calls to
2132 libev to change the timeout.
2133 .Sp
2134 To start the machinery, simply initialise the watcher and set
2135 \&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
2136 now), then call the callback, which will \*(L"do the right thing\*(R" and start
2137 the timer:
2138 .Sp
2139 .Vb 3
2140 \& last_activity = ev_now (EV_A);
2141 \& ev_init (&timer, callback);
2142 \& callback (EV_A_ &timer, 0);
2143 .Ve
2144 .Sp
2145 When there is some activity, simply store the current time in
2146 \&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
2147 .Sp
2148 .Vb 2
2149 \& if (activity detected)
2150 \& last_activity = ev_now (EV_A);
2151 .Ve
2152 .Sp
2153 When your timeout value changes, then the timeout can be changed by simply
2154 providing a new value, stopping the timer and calling the callback, which
2155 will again do the right thing (for example, time out immediately :).
2156 .Sp
2157 .Vb 3
2158 \& timeout = new_value;
2159 \& ev_timer_stop (EV_A_ &timer);
2160 \& callback (EV_A_ &timer, 0);
2161 .Ve
2162 .Sp
2163 This technique is slightly more complex, but in most cases where the
2164 time-out is unlikely to be triggered, much more efficient.
2165 .IP "4. Wee, just use a double-linked list for your timeouts." 4
2166 .IX Item "4. Wee, just use a double-linked list for your timeouts."
2167 If there is not one request, but many thousands (millions...), all
2168 employing some kind of timeout with the same timeout value, then one can
2169 do even better:
2170 .Sp
2171 When starting the timeout, calculate the timeout value and put the timeout
2172 at the \fIend\fR of the list.
2173 .Sp
2174 Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
2175 the list is expected to fire (for example, using the technique #3).
2176 .Sp
2177 When there is some activity, remove the timer from the list, recalculate
2178 the timeout, append it to the end of the list again, and make sure to
2179 update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
2180 .Sp
2181 This way, one can manage an unlimited number of timeouts in O(1) time for
2182 starting, stopping and updating the timers, at the expense of a major
2183 complication, and having to use a constant timeout. The constant timeout
2184 ensures that the list stays sorted.
2185 .PP
2186 So which method the best?
2187 .PP
2188 Method #2 is a simple no-brain-required solution that is adequate in most
2189 situations. Method #3 requires a bit more thinking, but handles many cases
2190 better, and isn't very complicated either. In most case, choosing either
2191 one is fine, with #3 being better in typical situations.
2192 .PP
2193 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
2194 rather complicated, but extremely efficient, something that really pays
2195 off after the first million or so of active timers, i.e. it's usually
2196 overkill :)
2197 .PP
2198 \fIThe special problem of being too early\fR
2199 .IX Subsection "The special problem of being too early"
2200 .PP
2201 If you ask a timer to call your callback after three seconds, then
2202 you expect it to be invoked after three seconds \- but of course, this
2203 cannot be guaranteed to infinite precision. Less obviously, it cannot be
2204 guaranteed to any precision by libev \- imagine somebody suspending the
2205 process with a \s-1STOP\s0 signal for a few hours for example.
2206 .PP
2207 So, libev tries to invoke your callback as soon as possible \fIafter\fR the
2208 delay has occurred, but cannot guarantee this.
2209 .PP
2210 A less obvious failure mode is calling your callback too early: many event
2211 loops compare timestamps with a \*(L"elapsed delay >= requested delay\*(R", but
2212 this can cause your callback to be invoked much earlier than you would
2213 expect.
2214 .PP
2215 To see why, imagine a system with a clock that only offers full second
2216 resolution (think windows if you can't come up with a broken enough \s-1OS\s0
2217 yourself). If you schedule a one-second timer at the time 500.9, then the
2218 event loop will schedule your timeout to elapse at a system time of 500
2219 (500.9 truncated to the resolution) + 1, or 501.
2220 .PP
2221 If an event library looks at the timeout 0.1s later, it will see \*(L"501 >=
2222 501\*(R" and invoke the callback 0.1s after it was started, even though a
2223 one-second delay was requested \- this is being \*(L"too early\*(R", despite best
2224 intentions.
2225 .PP
2226 This is the reason why libev will never invoke the callback if the elapsed
2227 delay equals the requested delay, but only when the elapsed delay is
2228 larger than the requested delay. In the example above, libev would only invoke
2229 the callback at system time 502, or 1.1s after the timer was started.
2230 .PP
2231 So, while libev cannot guarantee that your callback will be invoked
2232 exactly when requested, it \fIcan\fR and \fIdoes\fR guarantee that the requested
2233 delay has actually elapsed, or in other words, it always errs on the \*(L"too
2234 late\*(R" side of things.
2235 .PP
2236 \fIThe special problem of time updates\fR
2237 .IX Subsection "The special problem of time updates"
2238 .PP
2239 Establishing the current time is a costly operation (it usually takes
2240 at least one system call): \s-1EV\s0 therefore updates its idea of the current
2241 time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
2242 growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
2243 lots of events in one iteration.
2244 .PP
2245 The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
2246 time. This is usually the right thing as this timestamp refers to the time
2247 of the event triggering whatever timeout you are modifying/starting. If
2248 you suspect event processing to be delayed and you \fIneed\fR to base the
2249 timeout on the current time, use something like the following to adjust
2250 for it:
2251 .PP
2252 .Vb 1
2253 \& ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
2254 .Ve
2255 .PP
2256 If the event loop is suspended for a long time, you can also force an
2257 update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
2258 ()\*(C'\fR, although that will push the event time of all outstanding events
2259 further into the future.
2260 .PP
2261 \fIThe special problem of unsynchronised clocks\fR
2262 .IX Subsection "The special problem of unsynchronised clocks"
2263 .PP
2264 Modern systems have a variety of clocks \- libev itself uses the normal
2265 \&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
2266 jumps).
2267 .PP
2268 Neither of these clocks is synchronised with each other or any other clock
2269 on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
2270 than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
2271 a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
2272 than a directly following call to \f(CW\*(C`time\*(C'\fR.
2273 .PP
2274 The moral of this is to only compare libev-related timestamps with
2275 \&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, at least if you want better precision than
2276 a second or so.
2277 .PP
2278 One more problem arises due to this lack of synchronisation: if libev uses
2279 the system monotonic clock and you compare timestamps from \f(CW\*(C`ev_time\*(C'\fR
2280 or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
2281 invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
2282 .PP
2283 This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
2284 libev makes sure your callback is not invoked before the delay happened,
2285 \&\fImeasured according to the real time\fR, not the system clock.
2286 .PP
2287 If your timeouts are based on a physical timescale (e.g. \*(L"time out this
2288 connection after 100 seconds\*(R") then this shouldn't bother you as it is
2289 exactly the right behaviour.
2290 .PP
2291 If you want to compare wall clock/system timestamps to your timers, then
2292 you need to use \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
2293 time, where your comparisons will always generate correct results.
2294 .PP
2295 \fIThe special problems of suspended animation\fR
2296 .IX Subsection "The special problems of suspended animation"
2297 .PP
2298 When you leave the server world it is quite customary to hit machines that
2299 can suspend/hibernate \- what happens to the clocks during such a suspend?
2300 .PP
2301 Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
2302 all processes, while the clocks (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) continue
2303 to run until the system is suspended, but they will not advance while the
2304 system is suspended. That means, on resume, it will be as if the program
2305 was frozen for a few seconds, but the suspend time will not be counted
2306 towards \f(CW\*(C`ev_timer\*(C'\fR when a monotonic clock source is used. The real time
2307 clock advanced as expected, but if it is used as sole clocksource, then a
2308 long suspend would be detected as a time jump by libev, and timers would
2309 be adjusted accordingly.
2310 .PP
2311 I would not be surprised to see different behaviour in different between
2312 operating systems, \s-1OS\s0 versions or even different hardware.
2313 .PP
2314 The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) will see a
2315 time jump in the monotonic clocks and the realtime clock. If the program
2316 is suspended for a very long time, and monotonic clock sources are in use,
2317 then you can expect \f(CW\*(C`ev_timer\*(C'\fRs to expire as the full suspension time
2318 will be counted towards the timers. When no monotonic clock source is in
2319 use, then libev will again assume a timejump and adjust accordingly.
2320 .PP
2321 It might be beneficial for this latter case to call \f(CW\*(C`ev_suspend\*(C'\fR
2322 and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
2323 deterministic behaviour in this case (you can do nothing against
2324 \&\f(CW\*(C`SIGSTOP\*(C'\fR).
2325 .PP
2326 \fIWatcher-Specific Functions and Data Members\fR
2327 .IX Subsection "Watcher-Specific Functions and Data Members"
2328 .IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
2329 .IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
2330 .PD 0
2331 .IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
2332 .IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
2333 .PD
2334 Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
2335 negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, then it will
2336 automatically be stopped once the timeout is reached. If it is positive,
2337 then the timer will automatically be configured to trigger again \f(CW\*(C`repeat\*(C'\fR
2338 seconds later, again, and again, until stopped manually.
2339 .Sp
2340 The timer itself will do a best-effort at avoiding drift, that is, if
2341 you configure a timer to trigger every 10 seconds, then it will normally
2342 trigger at exactly 10 second intervals. If, however, your program cannot
2343 keep up with the timer (because it takes longer than those 10 seconds to
2344 do stuff) the timer will not fire more than once per event loop iteration.
2345 .IP "ev_timer_again (loop, ev_timer *)" 4
2346 .IX Item "ev_timer_again (loop, ev_timer *)"
2347 This will act as if the timer timed out, and restarts it again if it is
2348 repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
2349 timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
2350 .Sp
2351 The exact semantics are as in the following rules, all of which will be
2352 applied to the watcher:
2353 .RS 4
2354 .IP "If the timer is pending, the pending status is always cleared." 4
2355 .IX Item "If the timer is pending, the pending status is always cleared."
2356 .PD 0
2357 .IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
2358 .IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
2359 .ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
2360 .el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
2361 .IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
2362 .RE
2363 .RS 4
2364 .PD
2365 .Sp
2366 This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
2367 usage example.
2368 .RE
2369 .IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
2370 .IX Item "ev_tstamp ev_timer_remaining (loop, ev_timer *)"
2371 Returns the remaining time until a timer fires. If the timer is active,
2372 then this time is relative to the current event loop time, otherwise it's
2373 the timeout value currently configured.
2374 .Sp
2375 That is, after an \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
2376 \&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
2377 will return \f(CW4\fR. When the timer expires and is restarted, it will return
2378 roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
2379 too), and so on.
2380 .IP "ev_tstamp repeat [read\-write]" 4
2381 .IX Item "ev_tstamp repeat [read-write]"
2382 The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
2383 or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
2384 which is also when any modifications are taken into account.
2385 .PP
2386 \fIExamples\fR
2387 .IX Subsection "Examples"
2388 .PP
2389 Example: Create a timer that fires after 60 seconds.
2390 .PP
2391 .Vb 5
2392 \& static void
2393 \& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2394 \& {
2395 \& .. one minute over, w is actually stopped right here
2396 \& }
2397 \&
2398 \& ev_timer mytimer;
2399 \& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2400 \& ev_timer_start (loop, &mytimer);
2401 .Ve
2402 .PP
2403 Example: Create a timeout timer that times out after 10 seconds of
2404 inactivity.
2405 .PP
2406 .Vb 5
2407 \& static void
2408 \& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2409 \& {
2410 \& .. ten seconds without any activity
2411 \& }
2412 \&
2413 \& ev_timer mytimer;
2414 \& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2415 \& ev_timer_again (&mytimer); /* start timer */
2416 \& ev_run (loop, 0);
2417 \&
2418 \& // and in some piece of code that gets executed on any "activity":
2419 \& // reset the timeout to start ticking again at 10 seconds
2420 \& ev_timer_again (&mytimer);
2421 .Ve
2422 .ie n .SS """ev_periodic"" \- to cron or not to cron?"
2423 .el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
2424 .IX Subsection "ev_periodic - to cron or not to cron?"
2425 Periodic watchers are also timers of a kind, but they are very versatile
2426 (and unfortunately a bit complex).
2427 .PP
2428 Unlike \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
2429 relative time, the physical time that passes) but on wall clock time
2430 (absolute time, the thing you can read on your calendar or clock). The
2431 difference is that wall clock time can run faster or slower than real
2432 time, and time jumps are not uncommon (e.g. when you adjust your
2433 wrist-watch).
2434 .PP
2435 You can tell a periodic watcher to trigger after some specific point
2436 in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
2437 seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, that is, an absolute time
2438 not a delay) and then reset your system clock to January of the previous
2439 year, then it will take a year or more to trigger the event (unlike an
2440 \&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
2441 it, as it uses a relative timeout).
2442 .PP
2443 \&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
2444 timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
2445 other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
2446 watchers, as those cannot react to time jumps.
2447 .PP
2448 As with timers, the callback is guaranteed to be invoked only when the
2449 point in time where it is supposed to trigger has passed. If multiple
2450 timers become ready during the same loop iteration then the ones with
2451 earlier time-out values are invoked before ones with later time-out values
2452 (but this is no longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2453 .PP
2454 \fIWatcher-Specific Functions and Data Members\fR
2455 .IX Subsection "Watcher-Specific Functions and Data Members"
2456 .IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2457 .IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2458 .PD 0
2459 .IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2460 .IX Item "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2461 .PD
2462 Lots of arguments, let's sort it out... There are basically three modes of
2463 operation, and we will explain them from simplest to most complex:
2464 .RS 4
2465 .IP "\(bu" 4
2466 absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
2467 .Sp
2468 In this configuration the watcher triggers an event after the wall clock
2469 time \f(CW\*(C`offset\*(C'\fR has passed. It will not repeat and will not adjust when a
2470 time jump occurs, that is, if it is to be run at January 1st 2011 then it
2471 will be stopped and invoked when the system clock reaches or surpasses
2472 this point in time.
2473 .IP "\(bu" 4
2474 repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
2475 .Sp
2476 In this mode the watcher will always be scheduled to time out at the next
2477 \&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
2478 negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
2479 argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
2480 .Sp
2481 This can be used to create timers that do not drift with respect to the
2482 system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
2483 hour, on the hour (with respect to \s-1UTC\s0):
2484 .Sp
2485 .Vb 1
2486 \& ev_periodic_set (&periodic, 0., 3600., 0);
2487 .Ve
2488 .Sp
2489 This doesn't mean there will always be 3600 seconds in between triggers,
2490 but only that the callback will be called when the system time shows a
2491 full hour (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
2492 by 3600.
2493 .Sp
2494 Another way to think about it (for the mathematically inclined) is that
2495 \&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
2496 time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
2497 .Sp
2498 The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
2499 interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
2500 microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR and should have
2501 at most a similar magnitude as the current time (say, within a factor of
2502 ten). Typical values for offset are, in fact, \f(CW0\fR or something between
2503 \&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
2504 .Sp
2505 Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
2506 speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
2507 will of course deteriorate. Libev itself tries to be exact to be about one
2508 millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
2509 .IP "\(bu" 4
2510 manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
2511 .Sp
2512 In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR are both being
2513 ignored. Instead, each time the periodic watcher gets scheduled, the
2514 reschedule callback will be called with the watcher as first, and the
2515 current time as second argument.
2516 .Sp
2517 \&\s-1NOTE:\s0 \fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
2518 or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
2519 allowed by documentation here\fR.
2520 .Sp
2521 If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
2522 it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
2523 only event loop modification you are allowed to do).
2524 .Sp
2525 The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
2526 *w, ev_tstamp now)\*(C'\fR, e.g.:
2527 .Sp
2528 .Vb 5
2529 \& static ev_tstamp
2530 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2531 \& {
2532 \& return now + 60.;
2533 \& }
2534 .Ve
2535 .Sp
2536 It must return the next time to trigger, based on the passed time value
2537 (that is, the lowest time value larger than to the second argument). It
2538 will usually be called just before the callback will be triggered, but
2539 might be called at other times, too.
2540 .Sp
2541 \&\s-1NOTE:\s0 \fIThis callback must always return a time that is higher than or
2542 equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
2543 .Sp
2544 This can be used to create very complex timers, such as a timer that
2545 triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
2546 the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
2547 this. Here is a (completely untested, no error checking) example on how to
2548 do this:
2549 .Sp
2550 .Vb 1
2551 \& #include <time.h>
2552 \&
2553 \& static ev_tstamp
2554 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2555 \& {
2556 \& time_t tnow = (time_t)now;
2557 \& struct tm tm;
2558 \& localtime_r (&tnow, &tm);
2559 \&
2560 \& tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2561 \& ++tm.tm_mday; // midnight next day
2562 \&
2563 \& return mktime (&tm);
2564 \& }
2565 .Ve
2566 .Sp
2567 Note: this code might run into trouble on days that have more then two
2568 midnights (beginning and end).
2569 .RE
2570 .RS 4
2571 .RE
2572 .IP "ev_periodic_again (loop, ev_periodic *)" 4
2573 .IX Item "ev_periodic_again (loop, ev_periodic *)"
2574 Simply stops and restarts the periodic watcher again. This is only useful
2575 when you changed some parameters or the reschedule callback would return
2576 a different time than the last time it was called (e.g. in a crond like
2577 program when the crontabs have changed).
2578 .IP "ev_tstamp ev_periodic_at (ev_periodic *)" 4
2579 .IX Item "ev_tstamp ev_periodic_at (ev_periodic *)"
2580 When active, returns the absolute time that the watcher is supposed
2581 to trigger next. This is not the same as the \f(CW\*(C`offset\*(C'\fR argument to
2582 \&\f(CW\*(C`ev_periodic_set\*(C'\fR, but indeed works even in interval and manual
2583 rescheduling modes.
2584 .IP "ev_tstamp offset [read\-write]" 4
2585 .IX Item "ev_tstamp offset [read-write]"
2586 When repeating, this contains the offset value, otherwise this is the
2587 absolute point in time (the \f(CW\*(C`offset\*(C'\fR value passed to \f(CW\*(C`ev_periodic_set\*(C'\fR,
2588 although libev might modify this value for better numerical stability).
2589 .Sp
2590 Can be modified any time, but changes only take effect when the periodic
2591 timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2592 .IP "ev_tstamp interval [read\-write]" 4
2593 .IX Item "ev_tstamp interval [read-write]"
2594 The current interval value. Can be modified any time, but changes only
2595 take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
2596 called.
2597 .IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
2598 .IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
2599 The current reschedule callback, or \f(CW0\fR, if this functionality is
2600 switched off. Can be changed any time, but changes only take effect when
2601 the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2602 .PP
2603 \fIExamples\fR
2604 .IX Subsection "Examples"
2605 .PP
2606 Example: Call a callback every hour, or, more precisely, whenever the
2607 system time is divisible by 3600. The callback invocation times have
2608 potentially a lot of jitter, but good long-term stability.
2609 .PP
2610 .Vb 5
2611 \& static void
2612 \& clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2613 \& {
2614 \& ... its now a full hour (UTC, or TAI or whatever your clock follows)
2615 \& }
2616 \&
2617 \& ev_periodic hourly_tick;
2618 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2619 \& ev_periodic_start (loop, &hourly_tick);
2620 .Ve
2621 .PP
2622 Example: The same as above, but use a reschedule callback to do it:
2623 .PP
2624 .Vb 1
2625 \& #include <math.h>
2626 \&
2627 \& static ev_tstamp
2628 \& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2629 \& {
2630 \& return now + (3600. \- fmod (now, 3600.));
2631 \& }
2632 \&
2633 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2634 .Ve
2635 .PP
2636 Example: Call a callback every hour, starting now:
2637 .PP
2638 .Vb 4
2639 \& ev_periodic hourly_tick;
2640 \& ev_periodic_init (&hourly_tick, clock_cb,
2641 \& fmod (ev_now (loop), 3600.), 3600., 0);
2642 \& ev_periodic_start (loop, &hourly_tick);
2643 .Ve
2644 .ie n .SS """ev_signal"" \- signal me when a signal gets signalled!"
2645 .el .SS "\f(CWev_signal\fP \- signal me when a signal gets signalled!"
2646 .IX Subsection "ev_signal - signal me when a signal gets signalled!"
2647 Signal watchers will trigger an event when the process receives a specific
2648 signal one or more times. Even though signals are very asynchronous, libev
2649 will try its best to deliver signals synchronously, i.e. as part of the
2650 normal event processing, like any other event.
2651 .PP
2652 If you want signals to be delivered truly asynchronously, just use
2653 \&\f(CW\*(C`sigaction\*(C'\fR as you would do without libev and forget about sharing
2654 the signal. You can even use \f(CW\*(C`ev_async\*(C'\fR from a signal handler to
2655 synchronously wake up an event loop.
2656 .PP
2657 You can configure as many watchers as you like for the same signal, but
2658 only within the same loop, i.e. you can watch for \f(CW\*(C`SIGINT\*(C'\fR in your
2659 default loop and for \f(CW\*(C`SIGIO\*(C'\fR in another loop, but you cannot watch for
2660 \&\f(CW\*(C`SIGINT\*(C'\fR in both the default loop and another loop at the same time. At
2661 the moment, \f(CW\*(C`SIGCHLD\*(C'\fR is permanently tied to the default loop.
2662 .PP
2663 Only after the first watcher for a signal is started will libev actually
2664 register something with the kernel. It thus coexists with your own signal
2665 handlers as long as you don't register any with libev for the same signal.
2666 .PP
2667 If possible and supported, libev will install its handlers with
2668 \&\f(CW\*(C`SA_RESTART\*(C'\fR (or equivalent) behaviour enabled, so system calls should
2669 not be unduly interrupted. If you have a problem with system calls getting
2670 interrupted by signals you can block all signals in an \f(CW\*(C`ev_check\*(C'\fR watcher
2671 and unblock them in an \f(CW\*(C`ev_prepare\*(C'\fR watcher.
2672 .PP
2673 \fIThe special problem of inheritance over fork/execve/pthread_create\fR
2674 .IX Subsection "The special problem of inheritance over fork/execve/pthread_create"
2675 .PP
2676 Both the signal mask (\f(CW\*(C`sigprocmask\*(C'\fR) and the signal disposition
2677 (\f(CW\*(C`sigaction\*(C'\fR) are unspecified after starting a signal watcher (and after
2678 stopping it again), that is, libev might or might not block the signal,
2679 and might or might not set or restore the installed signal handler (but
2680 see \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR).
2681 .PP
2682 While this does not matter for the signal disposition (libev never
2683 sets signals to \f(CW\*(C`SIG_IGN\*(C'\fR, so handlers will be reset to \f(CW\*(C`SIG_DFL\*(C'\fR on
2684 \&\f(CW\*(C`execve\*(C'\fR), this matters for the signal mask: many programs do not expect
2685 certain signals to be blocked.
2686 .PP
2687 This means that before calling \f(CW\*(C`exec\*(C'\fR (from the child) you should reset
2688 the signal mask to whatever \*(L"default\*(R" you expect (all clear is a good
2689 choice usually).
2690 .PP
2691 The simplest way to ensure that the signal mask is reset in the child is
2692 to install a fork handler with \f(CW\*(C`pthread_atfork\*(C'\fR that resets it. That will
2693 catch fork calls done by libraries (such as the libc) as well.
2694 .PP
2695 In current versions of libev, the signal will not be blocked indefinitely
2696 unless you use the \f(CW\*(C`signalfd\*(C'\fR \s-1API\s0 (\f(CW\*(C`EV_SIGNALFD\*(C'\fR). While this reduces
2697 the window of opportunity for problems, it will not go away, as libev
2698 \&\fIhas\fR to modify the signal mask, at least temporarily.
2699 .PP
2700 So I can't stress this enough: \fIIf you do not reset your signal mask when
2701 you expect it to be empty, you have a race condition in your code\fR. This
2702 is not a libev-specific thing, this is true for most event libraries.
2703 .PP
2704 \fIThe special problem of threads signal handling\fR
2705 .IX Subsection "The special problem of threads signal handling"
2706 .PP
2707 \&\s-1POSIX\s0 threads has problematic signal handling semantics, specifically,
2708 a lot of functionality (sigfd, sigwait etc.) only really works if all
2709 threads in a process block signals, which is hard to achieve.
2710 .PP
2711 When you want to use sigwait (or mix libev signal handling with your own
2712 for the same signals), you can tackle this problem by globally blocking
2713 all signals before creating any threads (or creating them with a fully set
2714 sigprocmask) and also specifying the \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when creating
2715 loops. Then designate one thread as \*(L"signal receiver thread\*(R" which handles
2716 these signals. You can pass on any signals that libev might be interested
2717 in by calling \f(CW\*(C`ev_feed_signal\*(C'\fR.
2718 .PP
2719 \fIWatcher-Specific Functions and Data Members\fR
2720 .IX Subsection "Watcher-Specific Functions and Data Members"
2721 .IP "ev_signal_init (ev_signal *, callback, int signum)" 4
2722 .IX Item "ev_signal_init (ev_signal *, callback, int signum)"
2723 .PD 0
2724 .IP "ev_signal_set (ev_signal *, int signum)" 4
2725 .IX Item "ev_signal_set (ev_signal *, int signum)"
2726 .PD
2727 Configures the watcher to trigger on the given signal number (usually one
2728 of the \f(CW\*(C`SIGxxx\*(C'\fR constants).
2729 .IP "int signum [read\-only]" 4
2730 .IX Item "int signum [read-only]"
2731 The signal the watcher watches out for.
2732 .PP
2733 \fIExamples\fR
2734 .IX Subsection "Examples"
2735 .PP
2736 Example: Try to exit cleanly on \s-1SIGINT.\s0
2737 .PP
2738 .Vb 5
2739 \& static void
2740 \& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2741 \& {
2742 \& ev_break (loop, EVBREAK_ALL);
2743 \& }
2744 \&
2745 \& ev_signal signal_watcher;
2746 \& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2747 \& ev_signal_start (loop, &signal_watcher);
2748 .Ve
2749 .ie n .SS """ev_child"" \- watch out for process status changes"
2750 .el .SS "\f(CWev_child\fP \- watch out for process status changes"
2751 .IX Subsection "ev_child - watch out for process status changes"
2752 Child watchers trigger when your process receives a \s-1SIGCHLD\s0 in response to
2753 some child status changes (most typically when a child of yours dies or
2754 exits). It is permissible to install a child watcher \fIafter\fR the child
2755 has been forked (which implies it might have already exited), as long
2756 as the event loop isn't entered (or is continued from a watcher), i.e.,
2757 forking and then immediately registering a watcher for the child is fine,
2758 but forking and registering a watcher a few event loop iterations later or
2759 in the next callback invocation is not.
2760 .PP
2761 Only the default event loop is capable of handling signals, and therefore
2762 you can only register child watchers in the default event loop.
2763 .PP
2764 Due to some design glitches inside libev, child watchers will always be
2765 handled at maximum priority (their priority is set to \f(CW\*(C`EV_MAXPRI\*(C'\fR by
2766 libev)
2767 .PP
2768 \fIProcess Interaction\fR
2769 .IX Subsection "Process Interaction"
2770 .PP
2771 Libev grabs \f(CW\*(C`SIGCHLD\*(C'\fR as soon as the default event loop is
2772 initialised. This is necessary to guarantee proper behaviour even if the
2773 first child watcher is started after the child exits. The occurrence
2774 of \f(CW\*(C`SIGCHLD\*(C'\fR is recorded asynchronously, but child reaping is done
2775 synchronously as part of the event loop processing. Libev always reaps all
2776 children, even ones not watched.
2777 .PP
2778 \fIOverriding the Built-In Processing\fR
2779 .IX Subsection "Overriding the Built-In Processing"
2780 .PP
2781 Libev offers no special support for overriding the built-in child
2782 processing, but if your application collides with libev's default child
2783 handler, you can override it easily by installing your own handler for
2784 \&\f(CW\*(C`SIGCHLD\*(C'\fR after initialising the default loop, and making sure the
2785 default loop never gets destroyed. You are encouraged, however, to use an
2786 event-based approach to child reaping and thus use libev's support for
2787 that, so other libev users can use \f(CW\*(C`ev_child\*(C'\fR watchers freely.
2788 .PP
2789 \fIStopping the Child Watcher\fR
2790 .IX Subsection "Stopping the Child Watcher"
2791 .PP
2792 Currently, the child watcher never gets stopped, even when the
2793 child terminates, so normally one needs to stop the watcher in the
2794 callback. Future versions of libev might stop the watcher automatically
2795 when a child exit is detected (calling \f(CW\*(C`ev_child_stop\*(C'\fR twice is not a
2796 problem).
2797 .PP
2798 \fIWatcher-Specific Functions and Data Members\fR
2799 .IX Subsection "Watcher-Specific Functions and Data Members"
2800 .IP "ev_child_init (ev_child *, callback, int pid, int trace)" 4
2801 .IX Item "ev_child_init (ev_child *, callback, int pid, int trace)"
2802 .PD 0
2803 .IP "ev_child_set (ev_child *, int pid, int trace)" 4
2804 .IX Item "ev_child_set (ev_child *, int pid, int trace)"
2805 .PD
2806 Configures the watcher to wait for status changes of process \f(CW\*(C`pid\*(C'\fR (or
2807 \&\fIany\fR process if \f(CW\*(C`pid\*(C'\fR is specified as \f(CW0\fR). The callback can look
2808 at the \f(CW\*(C`rstatus\*(C'\fR member of the \f(CW\*(C`ev_child\*(C'\fR watcher structure to see
2809 the status word (use the macros from \f(CW\*(C`sys/wait.h\*(C'\fR and see your systems
2810 \&\f(CW\*(C`waitpid\*(C'\fR documentation). The \f(CW\*(C`rpid\*(C'\fR member contains the pid of the
2811 process causing the status change. \f(CW\*(C`trace\*(C'\fR must be either \f(CW0\fR (only
2812 activate the watcher when the process terminates) or \f(CW1\fR (additionally
2813 activate the watcher when the process is stopped or continued).
2814 .IP "int pid [read\-only]" 4
2815 .IX Item "int pid [read-only]"
2816 The process id this watcher watches out for, or \f(CW0\fR, meaning any process id.
2817 .IP "int rpid [read\-write]" 4
2818 .IX Item "int rpid [read-write]"
2819 The process id that detected a status change.
2820 .IP "int rstatus [read\-write]" 4
2821 .IX Item "int rstatus [read-write]"
2822 The process exit/trace status caused by \f(CW\*(C`rpid\*(C'\fR (see your systems
2823 \&\f(CW\*(C`waitpid\*(C'\fR and \f(CW\*(C`sys/wait.h\*(C'\fR documentation for details).
2824 .PP
2825 \fIExamples\fR
2826 .IX Subsection "Examples"
2827 .PP
2828 Example: \f(CW\*(C`fork()\*(C'\fR a new process and install a child handler to wait for
2829 its completion.
2830 .PP
2831 .Vb 1
2832 \& ev_child cw;
2833 \&
2834 \& static void
2835 \& child_cb (EV_P_ ev_child *w, int revents)
2836 \& {
2837 \& ev_child_stop (EV_A_ w);
2838 \& printf ("process %d exited with status %x\en", w\->rpid, w\->rstatus);
2839 \& }
2840 \&
2841 \& pid_t pid = fork ();
2842 \&
2843 \& if (pid < 0)
2844 \& // error
2845 \& else if (pid == 0)
2846 \& {
2847 \& // the forked child executes here
2848 \& exit (1);
2849 \& }
2850 \& else
2851 \& {
2852 \& ev_child_init (&cw, child_cb, pid, 0);
2853 \& ev_child_start (EV_DEFAULT_ &cw);
2854 \& }
2855 .Ve
2856 .ie n .SS """ev_stat"" \- did the file attributes just change?"
2857 .el .SS "\f(CWev_stat\fP \- did the file attributes just change?"
2858 .IX Subsection "ev_stat - did the file attributes just change?"
2859 This watches a file system path for attribute changes. That is, it calls
2860 \&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
2861 and sees if it changed compared to the last time, invoking the callback
2862 if it did. Starting the watcher \f(CW\*(C`stat\*(C'\fR's the file, so only changes that
2863 happen after the watcher has been started will be reported.
2864 .PP
2865 The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
2866 not exist\*(R" is a status change like any other. The condition \*(L"path does not
2867 exist\*(R" (or more correctly \*(L"path cannot be stat'ed\*(R") is signified by the
2868 \&\f(CW\*(C`st_nlink\*(C'\fR field being zero (which is otherwise always forced to be at
2869 least one) and all the other fields of the stat buffer having unspecified
2870 contents.
2871 .PP
2872 The path \fImust not\fR end in a slash or contain special components such as
2873 \&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
2874 your working directory changes, then the behaviour is undefined.
2875 .PP
2876 Since there is no portable change notification interface available, the
2877 portable implementation simply calls \f(CWstat(2)\fR regularly on the path
2878 to see if it changed somehow. You can specify a recommended polling
2879 interval for this case. If you specify a polling interval of \f(CW0\fR (highly
2880 recommended!) then a \fIsuitable, unspecified default\fR value will be used
2881 (which you can expect to be around five seconds, although this might
2882 change dynamically). Libev will also impose a minimum interval which is
2883 currently around \f(CW0.1\fR, but that's usually overkill.
2884 .PP
2885 This watcher type is not meant for massive numbers of stat watchers,
2886 as even with OS-supported change notifications, this can be
2887 resource-intensive.
2888 .PP
2889 At the time of this writing, the only OS-specific interface implemented
2890 is the Linux inotify interface (implementing kqueue support is left as an
2891 exercise for the reader. Note, however, that the author sees no way of
2892 implementing \f(CW\*(C`ev_stat\*(C'\fR semantics with kqueue, except as a hint).
2893 .PP
2894 \fI\s-1ABI\s0 Issues (Largefile Support)\fR
2895 .IX Subsection "ABI Issues (Largefile Support)"
2896 .PP
2897 Libev by default (unless the user overrides this) uses the default
2898 compilation environment, which means that on systems with large file
2899 support disabled by default, you get the 32 bit version of the stat
2900 structure. When using the library from programs that change the \s-1ABI\s0 to
2901 use 64 bit file offsets the programs will fail. In that case you have to
2902 compile libev with the same flags to get binary compatibility. This is
2903 obviously the case with any flags that change the \s-1ABI,\s0 but the problem is
2904 most noticeably displayed with ev_stat and large file support.
2905 .PP
2906 The solution for this is to lobby your distribution maker to make large
2907 file interfaces available by default (as e.g. FreeBSD does) and not
2908 optional. Libev cannot simply switch on large file support because it has
2909 to exchange stat structures with application programs compiled using the
2910 default compilation environment.
2911 .PP
2912 \fIInotify and Kqueue\fR
2913 .IX Subsection "Inotify and Kqueue"
2914 .PP
2915 When \f(CW\*(C`inotify (7)\*(C'\fR support has been compiled into libev and present at
2916 runtime, it will be used to speed up change detection where possible. The
2917 inotify descriptor will be created lazily when the first \f(CW\*(C`ev_stat\*(C'\fR
2918 watcher is being started.
2919 .PP
2920 Inotify presence does not change the semantics of \f(CW\*(C`ev_stat\*(C'\fR watchers
2921 except that changes might be detected earlier, and in some cases, to avoid
2922 making regular \f(CW\*(C`stat\*(C'\fR calls. Even in the presence of inotify support
2923 there are many cases where libev has to resort to regular \f(CW\*(C`stat\*(C'\fR polling,
2924 but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
2925 many bugs), the path exists (i.e. stat succeeds), and the path resides on
2926 a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
2927 xfs are fully working) libev usually gets away without polling.
2928 .PP
2929 There is no support for kqueue, as apparently it cannot be used to
2930 implement this functionality, due to the requirement of having a file
2931 descriptor open on the object at all times, and detecting renames, unlinks
2932 etc. is difficult.
2933 .PP
2934 \fI\f(CI\*(C`stat ()\*(C'\fI is a synchronous operation\fR
2935 .IX Subsection "stat () is a synchronous operation"
2936 .PP
2937 Libev doesn't normally do any kind of I/O itself, and so is not blocking
2938 the process. The exception are \f(CW\*(C`ev_stat\*(C'\fR watchers \- those call \f(CW\*(C`stat
2939 ()\*(C'\fR, which is a synchronous operation.
2940 .PP
2941 For local paths, this usually doesn't matter: unless the system is very
2942 busy or the intervals between stat's are large, a stat call will be fast,
2943 as the path data is usually in memory already (except when starting the
2944 watcher).
2945 .PP
2946 For networked file systems, calling \f(CW\*(C`stat ()\*(C'\fR can block an indefinite
2947 time due to network issues, and even under good conditions, a stat call
2948 often takes multiple milliseconds.
2949 .PP
2950 Therefore, it is best to avoid using \f(CW\*(C`ev_stat\*(C'\fR watchers on networked
2951 paths, although this is fully supported by libev.
2952 .PP
2953 \fIThe special problem of stat time resolution\fR
2954 .IX Subsection "The special problem of stat time resolution"
2955 .PP
2956 The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
2957 and even on systems where the resolution is higher, most file systems
2958 still only support whole seconds.
2959 .PP
2960 That means that, if the time is the only thing that changes, you can
2961 easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
2962 calls your callback, which does something. When there is another update
2963 within the same second, \f(CW\*(C`ev_stat\*(C'\fR will be unable to detect unless the
2964 stat data does change in other ways (e.g. file size).
2965 .PP
2966 The solution to this is to delay acting on a change for slightly more
2967 than a second (or till slightly after the next full second boundary), using
2968 a roughly one-second-delay \f(CW\*(C`ev_timer\*(C'\fR (e.g. \f(CW\*(C`ev_timer_set (w, 0., 1.02);
2969 ev_timer_again (loop, w)\*(C'\fR).
2970 .PP
2971 The \f(CW.02\fR offset is added to work around small timing inconsistencies
2972 of some operating systems (where the second counter of the current time
2973 might be be delayed. One such system is the Linux kernel, where a call to
2974 \&\f(CW\*(C`gettimeofday\*(C'\fR might return a timestamp with a full second later than
2975 a subsequent \f(CW\*(C`time\*(C'\fR call \- if the equivalent of \f(CW\*(C`time ()\*(C'\fR is used to
2976 update file times then there will be a small window where the kernel uses
2977 the previous second to update file times but libev might already execute
2978 the timer callback).
2979 .PP
2980 \fIWatcher-Specific Functions and Data Members\fR
2981 .IX Subsection "Watcher-Specific Functions and Data Members"
2982 .IP "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)" 4
2983 .IX Item "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)"
2984 .PD 0
2985 .IP "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)" 4
2986 .IX Item "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)"
2987 .PD
2988 Configures the watcher to wait for status changes of the given
2989 \&\f(CW\*(C`path\*(C'\fR. The \f(CW\*(C`interval\*(C'\fR is a hint on how quickly a change is expected to
2990 be detected and should normally be specified as \f(CW0\fR to let libev choose
2991 a suitable value. The memory pointed to by \f(CW\*(C`path\*(C'\fR must point to the same
2992 path for as long as the watcher is active.
2993 .Sp
2994 The callback will receive an \f(CW\*(C`EV_STAT\*(C'\fR event when a change was detected,
2995 relative to the attributes at the time the watcher was started (or the
2996 last change was detected).
2997 .IP "ev_stat_stat (loop, ev_stat *)" 4
2998 .IX Item "ev_stat_stat (loop, ev_stat *)"
2999 Updates the stat buffer immediately with new values. If you change the
3000 watched path in your callback, you could call this function to avoid
3001 detecting this change (while introducing a race condition if you are not
3002 the only one changing the path). Can also be useful simply to find out the
3003 new values.
3004 .IP "ev_statdata attr [read\-only]" 4
3005 .IX Item "ev_statdata attr [read-only]"
3006 The most-recently detected attributes of the file. Although the type is
3007 \&\f(CW\*(C`ev_statdata\*(C'\fR, this is usually the (or one of the) \f(CW\*(C`struct stat\*(C'\fR types
3008 suitable for your system, but you can only rely on the POSIX-standardised
3009 members to be present. If the \f(CW\*(C`st_nlink\*(C'\fR member is \f(CW0\fR, then there was
3010 some error while \f(CW\*(C`stat\*(C'\fRing the file.
3011 .IP "ev_statdata prev [read\-only]" 4
3012 .IX Item "ev_statdata prev [read-only]"
3013 The previous attributes of the file. The callback gets invoked whenever
3014 \&\f(CW\*(C`prev\*(C'\fR != \f(CW\*(C`attr\*(C'\fR, or, more precisely, one or more of these members
3015 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,
3016 \&\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.
3017 .IP "ev_tstamp interval [read\-only]" 4
3018 .IX Item "ev_tstamp interval [read-only]"
3019 The specified interval.
3020 .IP "const char *path [read\-only]" 4
3021 .IX Item "const char *path [read-only]"
3022 The file system path that is being watched.
3023 .PP
3024 \fIExamples\fR
3025 .IX Subsection "Examples"
3026 .PP
3027 Example: Watch \f(CW\*(C`/etc/passwd\*(C'\fR for attribute changes.
3028 .PP
3029 .Vb 10
3030 \& static void
3031 \& passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
3032 \& {
3033 \& /* /etc/passwd changed in some way */
3034 \& if (w\->attr.st_nlink)
3035 \& {
3036 \& printf ("passwd current size %ld\en", (long)w\->attr.st_size);
3037 \& printf ("passwd current atime %ld\en", (long)w\->attr.st_mtime);
3038 \& printf ("passwd current mtime %ld\en", (long)w\->attr.st_mtime);
3039 \& }
3040 \& else
3041 \& /* you shalt not abuse printf for puts */
3042 \& puts ("wow, /etc/passwd is not there, expect problems. "
3043 \& "if this is windows, they already arrived\en");
3044 \& }
3045 \&
3046 \& ...
3047 \& ev_stat passwd;
3048 \&
3049 \& ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
3050 \& ev_stat_start (loop, &passwd);
3051 .Ve
3052 .PP
3053 Example: Like above, but additionally use a one-second delay so we do not
3054 miss updates (however, frequent updates will delay processing, too, so
3055 one might do the work both on \f(CW\*(C`ev_stat\*(C'\fR callback invocation \fIand\fR on
3056 \&\f(CW\*(C`ev_timer\*(C'\fR callback invocation).
3057 .PP
3058 .Vb 2
3059 \& static ev_stat passwd;
3060 \& static ev_timer timer;
3061 \&
3062 \& static void
3063 \& timer_cb (EV_P_ ev_timer *w, int revents)
3064 \& {
3065 \& ev_timer_stop (EV_A_ w);
3066 \&
3067 \& /* now it\*(Aqs one second after the most recent passwd change */
3068 \& }
3069 \&
3070 \& static void
3071 \& stat_cb (EV_P_ ev_stat *w, int revents)
3072 \& {
3073 \& /* reset the one\-second timer */
3074 \& ev_timer_again (EV_A_ &timer);
3075 \& }
3076 \&
3077 \& ...
3078 \& ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
3079 \& ev_stat_start (loop, &passwd);
3080 \& ev_timer_init (&timer, timer_cb, 0., 1.02);
3081 .Ve
3082 .ie n .SS """ev_idle"" \- when you've got nothing better to do..."
3083 .el .SS "\f(CWev_idle\fP \- when you've got nothing better to do..."
3084 .IX Subsection "ev_idle - when you've got nothing better to do..."
3085 Idle watchers trigger events when no other events of the same or higher
3086 priority are pending (prepare, check and other idle watchers do not count
3087 as receiving \*(L"events\*(R").
3088 .PP
3089 That is, as long as your process is busy handling sockets or timeouts
3090 (or even signals, imagine) of the same or higher priority it will not be
3091 triggered. But when your process is idle (or only lower-priority watchers
3092 are pending), the idle watchers are being called once per event loop
3093 iteration \- until stopped, that is, or your process receives more events
3094 and becomes busy again with higher priority stuff.
3095 .PP
3096 The most noteworthy effect is that as long as any idle watchers are
3097 active, the process will not block when waiting for new events.
3098 .PP
3099 Apart from keeping your process non-blocking (which is a useful
3100 effect on its own sometimes), idle watchers are a good place to do
3101 \&\*(L"pseudo-background processing\*(R", or delay processing stuff to after the
3102 event loop has handled all outstanding events.
3103 .PP
3104 \fIAbusing an \f(CI\*(C`ev_idle\*(C'\fI watcher for its side-effect\fR
3105 .IX Subsection "Abusing an ev_idle watcher for its side-effect"
3106 .PP
3107 As long as there is at least one active idle watcher, libev will never
3108 sleep unnecessarily. Or in other words, it will loop as fast as possible.
3109 For this to work, the idle watcher doesn't need to be invoked at all \- the
3110 lowest priority will do.
3111 .PP
3112 This mode of operation can be useful together with an \f(CW\*(C`ev_check\*(C'\fR watcher,
3113 to do something on each event loop iteration \- for example to balance load
3114 between different connections.
3115 .PP
3116 See \*(L"Abusing an ev_check watcher for its side-effect\*(R" for a longer
3117 example.
3118 .PP
3119 \fIWatcher-Specific Functions and Data Members\fR
3120 .IX Subsection "Watcher-Specific Functions and Data Members"
3121 .IP "ev_idle_init (ev_idle *, callback)" 4
3122 .IX Item "ev_idle_init (ev_idle *, callback)"
3123 Initialises and configures the idle watcher \- it has no parameters of any
3124 kind. There is a \f(CW\*(C`ev_idle_set\*(C'\fR macro, but using it is utterly pointless,
3125 believe me.
3126 .PP
3127 \fIExamples\fR
3128 .IX Subsection "Examples"
3129 .PP
3130 Example: Dynamically allocate an \f(CW\*(C`ev_idle\*(C'\fR watcher, start it, and in the
3131 callback, free it. Also, use no error checking, as usual.
3132 .PP
3133 .Vb 5
3134 \& static void
3135 \& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
3136 \& {
3137 \& // stop the watcher
3138 \& ev_idle_stop (loop, w);
3139 \&
3140 \& // now we can free it
3141 \& free (w);
3142 \&
3143 \& // now do something you wanted to do when the program has
3144 \& // no longer anything immediate to do.
3145 \& }
3146 \&
3147 \& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
3148 \& ev_idle_init (idle_watcher, idle_cb);
3149 \& ev_idle_start (loop, idle_watcher);
3150 .Ve
3151 .ie n .SS """ev_prepare"" and ""ev_check"" \- customise your event loop!"
3152 .el .SS "\f(CWev_prepare\fP and \f(CWev_check\fP \- customise your event loop!"
3153 .IX Subsection "ev_prepare and ev_check - customise your event loop!"
3154 Prepare and check watchers are often (but not always) used in pairs:
3155 prepare watchers get invoked before the process blocks and check watchers
3156 afterwards.
3157 .PP
3158 You \fImust not\fR call \f(CW\*(C`ev_run\*(C'\fR (or similar functions that enter the
3159 current event loop) or \f(CW\*(C`ev_loop_fork\*(C'\fR from either \f(CW\*(C`ev_prepare\*(C'\fR or
3160 \&\f(CW\*(C`ev_check\*(C'\fR watchers. Other loops than the current one are fine,
3161 however. The rationale behind this is that you do not need to check
3162 for recursion in those watchers, i.e. the sequence will always be
3163 \&\f(CW\*(C`ev_prepare\*(C'\fR, blocking, \f(CW\*(C`ev_check\*(C'\fR so if you have one watcher of each
3164 kind they will always be called in pairs bracketing the blocking call.
3165 .PP
3166 Their main purpose is to integrate other event mechanisms into libev and
3167 their use is somewhat advanced. They could be used, for example, to track
3168 variable changes, implement your own watchers, integrate net-snmp or a
3169 coroutine library and lots more. They are also occasionally useful if
3170 you cache some data and want to flush it before blocking (for example,
3171 in X programs you might want to do an \f(CW\*(C`XFlush ()\*(C'\fR in an \f(CW\*(C`ev_prepare\*(C'\fR
3172 watcher).
3173 .PP
3174 This is done by examining in each prepare call which file descriptors
3175 need to be watched by the other library, registering \f(CW\*(C`ev_io\*(C'\fR watchers
3176 for them and starting an \f(CW\*(C`ev_timer\*(C'\fR watcher for any timeouts (many
3177 libraries provide exactly this functionality). Then, in the check watcher,
3178 you check for any events that occurred (by checking the pending status
3179 of all watchers and stopping them) and call back into the library. The
3180 I/O and timer callbacks will never actually be called (but must be valid
3181 nevertheless, because you never know, you know?).
3182 .PP
3183 As another example, the Perl Coro module uses these hooks to integrate
3184 coroutines into libev programs, by yielding to other active coroutines
3185 during each prepare and only letting the process block if no coroutines
3186 are ready to run (it's actually more complicated: it only runs coroutines
3187 with priority higher than or equal to the event loop and one coroutine
3188 of lower priority, but only once, using idle watchers to keep the event
3189 loop from blocking if lower-priority coroutines are active, thus mapping
3190 low-priority coroutines to idle/background tasks).
3191 .PP
3192 When used for this purpose, it is recommended to give \f(CW\*(C`ev_check\*(C'\fR watchers
3193 highest (\f(CW\*(C`EV_MAXPRI\*(C'\fR) priority, to ensure that they are being run before
3194 any other watchers after the poll (this doesn't matter for \f(CW\*(C`ev_prepare\*(C'\fR
3195 watchers).
3196 .PP
3197 Also, \f(CW\*(C`ev_check\*(C'\fR watchers (and \f(CW\*(C`ev_prepare\*(C'\fR watchers, too) should not
3198 activate (\*(L"feed\*(R") events into libev. While libev fully supports this, they
3199 might get executed before other \f(CW\*(C`ev_check\*(C'\fR watchers did their job. As
3200 \&\f(CW\*(C`ev_check\*(C'\fR watchers are often used to embed other (non-libev) event
3201 loops those other event loops might be in an unusable state until their
3202 \&\f(CW\*(C`ev_check\*(C'\fR watcher ran (always remind yourself to coexist peacefully with
3203 others).
3204 .PP
3205 \fIAbusing an \f(CI\*(C`ev_check\*(C'\fI watcher for its side-effect\fR
3206 .IX Subsection "Abusing an ev_check watcher for its side-effect"
3207 .PP
3208 \&\f(CW\*(C`ev_check\*(C'\fR (and less often also \f(CW\*(C`ev_prepare\*(C'\fR) watchers can also be
3209 useful because they are called once per event loop iteration. For
3210 example, if you want to handle a large number of connections fairly, you
3211 normally only do a bit of work for each active connection, and if there
3212 is more work to do, you wait for the next event loop iteration, so other
3213 connections have a chance of making progress.
3214 .PP
3215 Using an \f(CW\*(C`ev_check\*(C'\fR watcher is almost enough: it will be called on the
3216 next event loop iteration. However, that isn't as soon as possible \-
3217 without external events, your \f(CW\*(C`ev_check\*(C'\fR watcher will not be invoked.
3218 .PP
3219 This is where \f(CW\*(C`ev_idle\*(C'\fR watchers come in handy \- all you need is a
3220 single global idle watcher that is active as long as you have one active
3221 \&\f(CW\*(C`ev_check\*(C'\fR watcher. The \f(CW\*(C`ev_idle\*(C'\fR watcher makes sure the event loop
3222 will not sleep, and the \f(CW\*(C`ev_check\*(C'\fR watcher makes sure a callback gets
3223 invoked. Neither watcher alone can do that.
3224 .PP
3225 \fIWatcher-Specific Functions and Data Members\fR
3226 .IX Subsection "Watcher-Specific Functions and Data Members"
3227 .IP "ev_prepare_init (ev_prepare *, callback)" 4
3228 .IX Item "ev_prepare_init (ev_prepare *, callback)"
3229 .PD 0
3230 .IP "ev_check_init (ev_check *, callback)" 4
3231 .IX Item "ev_check_init (ev_check *, callback)"
3232 .PD
3233 Initialises and configures the prepare or check watcher \- they have no
3234 parameters of any kind. There are \f(CW\*(C`ev_prepare_set\*(C'\fR and \f(CW\*(C`ev_check_set\*(C'\fR
3235 macros, but using them is utterly, utterly, utterly and completely
3236 pointless.
3237 .PP
3238 \fIExamples\fR
3239 .IX Subsection "Examples"
3240 .PP
3241 There are a number of principal ways to embed other event loops or modules
3242 into libev. Here are some ideas on how to include libadns into libev
3243 (there is a Perl module named \f(CW\*(C`EV::ADNS\*(C'\fR that does this, which you could
3244 use as a working example. Another Perl module named \f(CW\*(C`EV::Glib\*(C'\fR embeds a
3245 Glib main context into libev, and finally, \f(CW\*(C`Glib::EV\*(C'\fR embeds \s-1EV\s0 into the
3246 Glib event loop).
3247 .PP
3248 Method 1: Add \s-1IO\s0 watchers and a timeout watcher in a prepare handler,
3249 and in a check watcher, destroy them and call into libadns. What follows
3250 is pseudo-code only of course. This requires you to either use a low
3251 priority for the check watcher or use \f(CW\*(C`ev_clear_pending\*(C'\fR explicitly, as
3252 the callbacks for the IO/timeout watchers might not have been called yet.
3253 .PP
3254 .Vb 2
3255 \& static ev_io iow [nfd];
3256 \& static ev_timer tw;
3257 \&
3258 \& static void
3259 \& io_cb (struct ev_loop *loop, ev_io *w, int revents)
3260 \& {
3261 \& }
3262 \&
3263 \& // create io watchers for each fd and a timer before blocking
3264 \& static void
3265 \& adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3266 \& {
3267 \& int timeout = 3600000;
3268 \& struct pollfd fds [nfd];
3269 \& // actual code will need to loop here and realloc etc.
3270 \& adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3271 \&
3272 \& /* the callback is illegal, but won\*(Aqt be called as we stop during check */
3273 \& ev_timer_init (&tw, 0, timeout * 1e\-3, 0.);
3274 \& ev_timer_start (loop, &tw);
3275 \&
3276 \& // create one ev_io per pollfd
3277 \& for (int i = 0; i < nfd; ++i)
3278 \& {
3279 \& ev_io_init (iow + i, io_cb, fds [i].fd,
3280 \& ((fds [i].events & POLLIN ? EV_READ : 0)
3281 \& | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3282 \&
3283 \& fds [i].revents = 0;
3284 \& ev_io_start (loop, iow + i);
3285 \& }
3286 \& }
3287 \&
3288 \& // stop all watchers after blocking
3289 \& static void
3290 \& adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3291 \& {
3292 \& ev_timer_stop (loop, &tw);
3293 \&
3294 \& for (int i = 0; i < nfd; ++i)
3295 \& {
3296 \& // set the relevant poll flags
3297 \& // could also call adns_processreadable etc. here
3298 \& struct pollfd *fd = fds + i;
3299 \& int revents = ev_clear_pending (iow + i);
3300 \& if (revents & EV_READ ) fd\->revents |= fd\->events & POLLIN;
3301 \& if (revents & EV_WRITE) fd\->revents |= fd\->events & POLLOUT;
3302 \&
3303 \& // now stop the watcher
3304 \& ev_io_stop (loop, iow + i);
3305 \& }
3306 \&
3307 \& adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3308 \& }
3309 .Ve
3310 .PP
3311 Method 2: This would be just like method 1, but you run \f(CW\*(C`adns_afterpoll\*(C'\fR
3312 in the prepare watcher and would dispose of the check watcher.
3313 .PP
3314 Method 3: If the module to be embedded supports explicit event
3315 notification (libadns does), you can also make use of the actual watcher
3316 callbacks, and only destroy/create the watchers in the prepare watcher.
3317 .PP
3318 .Vb 5
3319 \& static void
3320 \& timer_cb (EV_P_ ev_timer *w, int revents)
3321 \& {
3322 \& adns_state ads = (adns_state)w\->data;
3323 \& update_now (EV_A);
3324 \&
3325 \& adns_processtimeouts (ads, &tv_now);
3326 \& }
3327 \&
3328 \& static void
3329 \& io_cb (EV_P_ ev_io *w, int revents)
3330 \& {
3331 \& adns_state ads = (adns_state)w\->data;
3332 \& update_now (EV_A);
3333 \&
3334 \& if (revents & EV_READ ) adns_processreadable (ads, w\->fd, &tv_now);
3335 \& if (revents & EV_WRITE) adns_processwriteable (ads, w\->fd, &tv_now);
3336 \& }
3337 \&
3338 \& // do not ever call adns_afterpoll
3339 .Ve
3340 .PP
3341 Method 4: Do not use a prepare or check watcher because the module you
3342 want to embed is not flexible enough to support it. Instead, you can
3343 override their poll function. The drawback with this solution is that the
3344 main loop is now no longer controllable by \s-1EV.\s0 The \f(CW\*(C`Glib::EV\*(C'\fR module uses
3345 this approach, effectively embedding \s-1EV\s0 as a client into the horrible
3346 libglib event loop.
3347 .PP
3348 .Vb 4
3349 \& static gint
3350 \& event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3351 \& {
3352 \& int got_events = 0;
3353 \&
3354 \& for (n = 0; n < nfds; ++n)
3355 \& // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3356 \&
3357 \& if (timeout >= 0)
3358 \& // create/start timer
3359 \&
3360 \& // poll
3361 \& ev_run (EV_A_ 0);
3362 \&
3363 \& // stop timer again
3364 \& if (timeout >= 0)
3365 \& ev_timer_stop (EV_A_ &to);
3366 \&
3367 \& // stop io watchers again \- their callbacks should have set
3368 \& for (n = 0; n < nfds; ++n)
3369 \& ev_io_stop (EV_A_ iow [n]);
3370 \&
3371 \& return got_events;
3372 \& }
3373 .Ve
3374 .ie n .SS """ev_embed"" \- when one backend isn't enough..."
3375 .el .SS "\f(CWev_embed\fP \- when one backend isn't enough..."
3376 .IX Subsection "ev_embed - when one backend isn't enough..."
3377 This is a rather advanced watcher type that lets you embed one event loop
3378 into another (currently only \f(CW\*(C`ev_io\*(C'\fR events are supported in the embedded
3379 loop, other types of watchers might be handled in a delayed or incorrect
3380 fashion and must not be used).
3381 .PP
3382 There are primarily two reasons you would want that: work around bugs and
3383 prioritise I/O.
3384 .PP
3385 As an example for a bug workaround, the kqueue backend might only support
3386 sockets on some platform, so it is unusable as generic backend, but you
3387 still want to make use of it because you have many sockets and it scales
3388 so nicely. In this case, you would create a kqueue-based loop and embed
3389 it into your default loop (which might use e.g. poll). Overall operation
3390 will be a bit slower because first libev has to call \f(CW\*(C`poll\*(C'\fR and then
3391 \&\f(CW\*(C`kevent\*(C'\fR, but at least you can use both mechanisms for what they are
3392 best: \f(CW\*(C`kqueue\*(C'\fR for scalable sockets and \f(CW\*(C`poll\*(C'\fR if you want it to work :)
3393 .PP
3394 As for prioritising I/O: under rare circumstances you have the case where
3395 some fds have to be watched and handled very quickly (with low latency),
3396 and even priorities and idle watchers might have too much overhead. In
3397 this case you would put all the high priority stuff in one loop and all
3398 the rest in a second one, and embed the second one in the first.
3399 .PP
3400 As long as the watcher is active, the callback will be invoked every
3401 time there might be events pending in the embedded loop. The callback
3402 must then call \f(CW\*(C`ev_embed_sweep (mainloop, watcher)\*(C'\fR to make a single
3403 sweep and invoke their callbacks (the callback doesn't need to invoke the
3404 \&\f(CW\*(C`ev_embed_sweep\*(C'\fR function directly, it could also start an idle watcher
3405 to give the embedded loop strictly lower priority for example).
3406 .PP
3407 You can also set the callback to \f(CW0\fR, in which case the embed watcher
3408 will automatically execute the embedded loop sweep whenever necessary.
3409 .PP
3410 Fork detection will be handled transparently while the \f(CW\*(C`ev_embed\*(C'\fR watcher
3411 is active, i.e., the embedded loop will automatically be forked when the
3412 embedding loop forks. In other cases, the user is responsible for calling
3413 \&\f(CW\*(C`ev_loop_fork\*(C'\fR on the embedded loop.
3414 .PP
3415 Unfortunately, not all backends are embeddable: only the ones returned by
3416 \&\f(CW\*(C`ev_embeddable_backends\*(C'\fR are, which, unfortunately, does not include any
3417 portable one.
3418 .PP
3419 So when you want to use this feature you will always have to be prepared
3420 that you cannot get an embeddable loop. The recommended way to get around
3421 this is to have a separate variables for your embeddable loop, try to
3422 create it, and if that fails, use the normal loop for everything.
3423 .PP
3424 \fI\f(CI\*(C`ev_embed\*(C'\fI and fork\fR
3425 .IX Subsection "ev_embed and fork"
3426 .PP
3427 While the \f(CW\*(C`ev_embed\*(C'\fR watcher is running, forks in the embedding loop will
3428 automatically be applied to the embedded loop as well, so no special
3429 fork handling is required in that case. When the watcher is not running,
3430 however, it is still the task of the libev user to call \f(CW\*(C`ev_loop_fork ()\*(C'\fR
3431 as applicable.
3432 .PP
3433 \fIWatcher-Specific Functions and Data Members\fR
3434 .IX Subsection "Watcher-Specific Functions and Data Members"
3435 .IP "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)" 4
3436 .IX Item "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)"
3437 .PD 0
3438 .IP "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)" 4
3439 .IX Item "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)"
3440 .PD
3441 Configures the watcher to embed the given loop, which must be
3442 embeddable. If the callback is \f(CW0\fR, then \f(CW\*(C`ev_embed_sweep\*(C'\fR will be
3443 invoked automatically, otherwise it is the responsibility of the callback
3444 to invoke it (it will continue to be called until the sweep has been done,
3445 if you do not want that, you need to temporarily stop the embed watcher).
3446 .IP "ev_embed_sweep (loop, ev_embed *)" 4
3447 .IX Item "ev_embed_sweep (loop, ev_embed *)"
3448 Make a single, non-blocking sweep over the embedded loop. This works
3449 similarly to \f(CW\*(C`ev_run (embedded_loop, EVRUN_NOWAIT)\*(C'\fR, but in the most
3450 appropriate way for embedded loops.
3451 .IP "struct ev_loop *other [read\-only]" 4
3452 .IX Item "struct ev_loop *other [read-only]"
3453 The embedded event loop.
3454 .PP
3455 \fIExamples\fR
3456 .IX Subsection "Examples"
3457 .PP
3458 Example: Try to get an embeddable event loop and embed it into the default
3459 event loop. If that is not possible, use the default loop. The default
3460 loop is stored in \f(CW\*(C`loop_hi\*(C'\fR, while the embeddable loop is stored in
3461 \&\f(CW\*(C`loop_lo\*(C'\fR (which is \f(CW\*(C`loop_hi\*(C'\fR in the case no embeddable loop can be
3462 used).
3463 .PP
3464 .Vb 3
3465 \& struct ev_loop *loop_hi = ev_default_init (0);
3466 \& struct ev_loop *loop_lo = 0;
3467 \& ev_embed embed;
3468 \&
3469 \& // see if there is a chance of getting one that works
3470 \& // (remember that a flags value of 0 means autodetection)
3471 \& loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3472 \& ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3473 \& : 0;
3474 \&
3475 \& // if we got one, then embed it, otherwise default to loop_hi
3476 \& if (loop_lo)
3477 \& {
3478 \& ev_embed_init (&embed, 0, loop_lo);
3479 \& ev_embed_start (loop_hi, &embed);
3480 \& }
3481 \& else
3482 \& loop_lo = loop_hi;
3483 .Ve
3484 .PP
3485 Example: Check if kqueue is available but not recommended and create
3486 a kqueue backend for use with sockets (which usually work with any
3487 kqueue implementation). Store the kqueue/socket\-only event loop in
3488 \&\f(CW\*(C`loop_socket\*(C'\fR. (One might optionally use \f(CW\*(C`EVFLAG_NOENV\*(C'\fR, too).
3489 .PP
3490 .Vb 3
3491 \& struct ev_loop *loop = ev_default_init (0);
3492 \& struct ev_loop *loop_socket = 0;
3493 \& ev_embed embed;
3494 \&
3495 \& if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3496 \& if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3497 \& {
3498 \& ev_embed_init (&embed, 0, loop_socket);
3499 \& ev_embed_start (loop, &embed);
3500 \& }
3501 \&
3502 \& if (!loop_socket)
3503 \& loop_socket = loop;
3504 \&
3505 \& // now use loop_socket for all sockets, and loop for everything else
3506 .Ve
3507 .ie n .SS """ev_fork"" \- the audacity to resume the event loop after a fork"
3508 .el .SS "\f(CWev_fork\fP \- the audacity to resume the event loop after a fork"
3509 .IX Subsection "ev_fork - the audacity to resume the event loop after a fork"
3510 Fork watchers are called when a \f(CW\*(C`fork ()\*(C'\fR was detected (usually because
3511 whoever is a good citizen cared to tell libev about it by calling
3512 \&\f(CW\*(C`ev_loop_fork\*(C'\fR). The invocation is done before the event loop blocks next
3513 and before \f(CW\*(C`ev_check\*(C'\fR watchers are being called, and only in the child
3514 after the fork. If whoever good citizen calling \f(CW\*(C`ev_default_fork\*(C'\fR cheats
3515 and calls it in the wrong process, the fork handlers will be invoked, too,
3516 of course.
3517 .PP
3518 \fIThe special problem of life after fork \- how is it possible?\fR
3519 .IX Subsection "The special problem of life after fork - how is it possible?"
3520 .PP
3521 Most uses of \f(CW\*(C`fork ()\*(C'\fR consist of forking, then some simple calls to set
3522 up/change the process environment, followed by a call to \f(CW\*(C`exec()\*(C'\fR. This
3523 sequence should be handled by libev without any problems.
3524 .PP
3525 This changes when the application actually wants to do event handling
3526 in the child, or both parent in child, in effect \*(L"continuing\*(R" after the
3527 fork.
3528 .PP
3529 The default mode of operation (for libev, with application help to detect
3530 forks) is to duplicate all the state in the child, as would be expected
3531 when \fIeither\fR the parent \fIor\fR the child process continues.
3532 .PP
3533 When both processes want to continue using libev, then this is usually the
3534 wrong result. In that case, usually one process (typically the parent) is
3535 supposed to continue with all watchers in place as before, while the other
3536 process typically wants to start fresh, i.e. without any active watchers.
3537 .PP
3538 The cleanest and most efficient way to achieve that with libev is to
3539 simply create a new event loop, which of course will be \*(L"empty\*(R", and
3540 use that for new watchers. This has the advantage of not touching more
3541 memory than necessary, and thus avoiding the copy-on-write, and the
3542 disadvantage of having to use multiple event loops (which do not support
3543 signal watchers).
3544 .PP
3545 When this is not possible, or you want to use the default loop for
3546 other reasons, then in the process that wants to start \*(L"fresh\*(R", call
3547 \&\f(CW\*(C`ev_loop_destroy (EV_DEFAULT)\*(C'\fR followed by \f(CW\*(C`ev_default_loop (...)\*(C'\fR.
3548 Destroying the default loop will \*(L"orphan\*(R" (not stop) all registered
3549 watchers, so you have to be careful not to execute code that modifies
3550 those watchers. Note also that in that case, you have to re-register any
3551 signal watchers.
3552 .PP
3553 \fIWatcher-Specific Functions and Data Members\fR
3554 .IX Subsection "Watcher-Specific Functions and Data Members"
3555 .IP "ev_fork_init (ev_fork *, callback)" 4
3556 .IX Item "ev_fork_init (ev_fork *, callback)"
3557 Initialises and configures the fork watcher \- it has no parameters of any
3558 kind. There is a \f(CW\*(C`ev_fork_set\*(C'\fR macro, but using it is utterly pointless,
3559 really.
3560 .ie n .SS """ev_cleanup"" \- even the best things end"
3561 .el .SS "\f(CWev_cleanup\fP \- even the best things end"
3562 .IX Subsection "ev_cleanup - even the best things end"
3563 Cleanup watchers are called just before the event loop is being destroyed
3564 by a call to \f(CW\*(C`ev_loop_destroy\*(C'\fR.
3565 .PP
3566 While there is no guarantee that the event loop gets destroyed, cleanup
3567 watchers provide a convenient method to install cleanup hooks for your
3568 program, worker threads and so on \- you just to make sure to destroy the
3569 loop when you want them to be invoked.
3570 .PP
3571 Cleanup watchers are invoked in the same way as any other watcher. Unlike
3572 all other watchers, they do not keep a reference to the event loop (which
3573 makes a lot of sense if you think about it). Like all other watchers, you
3574 can call libev functions in the callback, except \f(CW\*(C`ev_cleanup_start\*(C'\fR.
3575 .PP
3576 \fIWatcher-Specific Functions and Data Members\fR
3577 .IX Subsection "Watcher-Specific Functions and Data Members"
3578 .IP "ev_cleanup_init (ev_cleanup *, callback)" 4
3579 .IX Item "ev_cleanup_init (ev_cleanup *, callback)"
3580 Initialises and configures the cleanup watcher \- it has no parameters of
3581 any kind. There is a \f(CW\*(C`ev_cleanup_set\*(C'\fR macro, but using it is utterly
3582 pointless, I assure you.
3583 .PP
3584 Example: Register an atexit handler to destroy the default loop, so any
3585 cleanup functions are called.
3586 .PP
3587 .Vb 5
3588 \& static void
3589 \& program_exits (void)
3590 \& {
3591 \& ev_loop_destroy (EV_DEFAULT_UC);
3592 \& }
3593 \&
3594 \& ...
3595 \& atexit (program_exits);
3596 .Ve
3597 .ie n .SS """ev_async"" \- how to wake up an event loop"
3598 .el .SS "\f(CWev_async\fP \- how to wake up an event loop"
3599 .IX Subsection "ev_async - how to wake up an event loop"
3600 In general, you cannot use an \f(CW\*(C`ev_loop\*(C'\fR from multiple threads or other
3601 asynchronous sources such as signal handlers (as opposed to multiple event
3602 loops \- those are of course safe to use in different threads).
3603 .PP
3604 Sometimes, however, you need to wake up an event loop you do not control,
3605 for example because it belongs to another thread. This is what \f(CW\*(C`ev_async\*(C'\fR
3606 watchers do: as long as the \f(CW\*(C`ev_async\*(C'\fR watcher is active, you can signal
3607 it by calling \f(CW\*(C`ev_async_send\*(C'\fR, which is thread\- and signal safe.
3608 .PP
3609 This functionality is very similar to \f(CW\*(C`ev_signal\*(C'\fR watchers, as signals,
3610 too, are asynchronous in nature, and signals, too, will be compressed
3611 (i.e. the number of callback invocations may be less than the number of
3612 \&\f(CW\*(C`ev_async_send\*(C'\fR calls). In fact, you could use signal watchers as a kind
3613 of \*(L"global async watchers\*(R" by using a watcher on an otherwise unused
3614 signal, and \f(CW\*(C`ev_feed_signal\*(C'\fR to signal this watcher from another thread,
3615 even without knowing which loop owns the signal.
3616 .PP
3617 \fIQueueing\fR
3618 .IX Subsection "Queueing"
3619 .PP
3620 \&\f(CW\*(C`ev_async\*(C'\fR does not support queueing of data in any way. The reason
3621 is that the author does not know of a simple (or any) algorithm for a
3622 multiple-writer-single-reader queue that works in all cases and doesn't
3623 need elaborate support such as pthreads or unportable memory access
3624 semantics.
3625 .PP
3626 That means that if you want to queue data, you have to provide your own
3627 queue. But at least I can tell you how to implement locking around your
3628 queue:
3629 .IP "queueing from a signal handler context" 4
3630 .IX Item "queueing from a signal handler context"
3631 To implement race-free queueing, you simply add to the queue in the signal
3632 handler but you block the signal handler in the watcher callback. Here is
3633 an example that does that for some fictitious \s-1SIGUSR1\s0 handler:
3634 .Sp
3635 .Vb 1
3636 \& static ev_async mysig;
3637 \&
3638 \& static void
3639 \& sigusr1_handler (void)
3640 \& {
3641 \& sometype data;
3642 \&
3643 \& // no locking etc.
3644 \& queue_put (data);
3645 \& ev_async_send (EV_DEFAULT_ &mysig);
3646 \& }
3647 \&
3648 \& static void
3649 \& mysig_cb (EV_P_ ev_async *w, int revents)
3650 \& {
3651 \& sometype data;
3652 \& sigset_t block, prev;
3653 \&
3654 \& sigemptyset (&block);
3655 \& sigaddset (&block, SIGUSR1);
3656 \& sigprocmask (SIG_BLOCK, &block, &prev);
3657 \&
3658 \& while (queue_get (&data))
3659 \& process (data);
3660 \&
3661 \& if (sigismember (&prev, SIGUSR1)
3662 \& sigprocmask (SIG_UNBLOCK, &block, 0);
3663 \& }
3664 .Ve
3665 .Sp
3666 (Note: pthreads in theory requires you to use \f(CW\*(C`pthread_setmask\*(C'\fR
3667 instead of \f(CW\*(C`sigprocmask\*(C'\fR when you use threads, but libev doesn't do it
3668 either...).
3669 .IP "queueing from a thread context" 4
3670 .IX Item "queueing from a thread context"
3671 The strategy for threads is different, as you cannot (easily) block
3672 threads but you can easily preempt them, so to queue safely you need to
3673 employ a traditional mutex lock, such as in this pthread example:
3674 .Sp
3675 .Vb 2
3676 \& static ev_async mysig;
3677 \& static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3678 \&
3679 \& static void
3680 \& otherthread (void)
3681 \& {
3682 \& // only need to lock the actual queueing operation
3683 \& pthread_mutex_lock (&mymutex);
3684 \& queue_put (data);
3685 \& pthread_mutex_unlock (&mymutex);
3686 \&
3687 \& ev_async_send (EV_DEFAULT_ &mysig);
3688 \& }
3689 \&
3690 \& static void
3691 \& mysig_cb (EV_P_ ev_async *w, int revents)
3692 \& {
3693 \& pthread_mutex_lock (&mymutex);
3694 \&
3695 \& while (queue_get (&data))
3696 \& process (data);
3697 \&
3698 \& pthread_mutex_unlock (&mymutex);
3699 \& }
3700 .Ve
3701 .PP
3702 \fIWatcher-Specific Functions and Data Members\fR
3703 .IX Subsection "Watcher-Specific Functions and Data Members"
3704 .IP "ev_async_init (ev_async *, callback)" 4
3705 .IX Item "ev_async_init (ev_async *, callback)"
3706 Initialises and configures the async watcher \- it has no parameters of any
3707 kind. There is a \f(CW\*(C`ev_async_set\*(C'\fR macro, but using it is utterly pointless,
3708 trust me.
3709 .IP "ev_async_send (loop, ev_async *)" 4
3710 .IX Item "ev_async_send (loop, ev_async *)"
3711 Sends/signals/activates the given \f(CW\*(C`ev_async\*(C'\fR watcher, that is, feeds
3712 an \f(CW\*(C`EV_ASYNC\*(C'\fR event on the watcher into the event loop, and instantly
3713 returns.
3714 .Sp
3715 Unlike \f(CW\*(C`ev_feed_event\*(C'\fR, this call is safe to do from other threads,
3716 signal or similar contexts (see the discussion of \f(CW\*(C`EV_ATOMIC_T\*(C'\fR in the
3717 embedding section below on what exactly this means).
3718 .Sp
3719 Note that, as with other watchers in libev, multiple events might get
3720 compressed into a single callback invocation (another way to look at
3721 this is that \f(CW\*(C`ev_async\*(C'\fR watchers are level-triggered: they are set on
3722 \&\f(CW\*(C`ev_async_send\*(C'\fR, reset when the event loop detects that).
3723 .Sp
3724 This call incurs the overhead of at most one extra system call per event
3725 loop iteration, if the event loop is blocked, and no syscall at all if
3726 the event loop (or your program) is processing events. That means that
3727 repeated calls are basically free (there is no need to avoid calls for
3728 performance reasons) and that the overhead becomes smaller (typically
3729 zero) under load.
3730 .IP "bool = ev_async_pending (ev_async *)" 4
3731 .IX Item "bool = ev_async_pending (ev_async *)"
3732 Returns a non-zero value when \f(CW\*(C`ev_async_send\*(C'\fR has been called on the
3733 watcher but the event has not yet been processed (or even noted) by the
3734 event loop.
3735 .Sp
3736 \&\f(CW\*(C`ev_async_send\*(C'\fR sets a flag in the watcher and wakes up the loop. When
3737 the loop iterates next and checks for the watcher to have become active,
3738 it will reset the flag again. \f(CW\*(C`ev_async_pending\*(C'\fR can be used to very
3739 quickly check whether invoking the loop might be a good idea.
3740 .Sp
3741 Not that this does \fInot\fR check whether the watcher itself is pending,
3742 only whether it has been requested to make this watcher pending: there
3743 is a time window between the event loop checking and resetting the async
3744 notification, and the callback being invoked.
3745 .SH "OTHER FUNCTIONS"
3746 .IX Header "OTHER FUNCTIONS"
3747 There are some other functions of possible interest. Described. Here. Now.
3748 .IP "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)" 4
3749 .IX Item "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)"
3750 This function combines a simple timer and an I/O watcher, calls your
3751 callback on whichever event happens first and automatically stops both
3752 watchers. This is useful if you want to wait for a single event on an fd
3753 or timeout without having to allocate/configure/start/stop/free one or
3754 more watchers yourself.
3755 .Sp
3756 If \f(CW\*(C`fd\*(C'\fR is less than 0, then no I/O watcher will be started and the
3757 \&\f(CW\*(C`events\*(C'\fR argument is being ignored. Otherwise, an \f(CW\*(C`ev_io\*(C'\fR watcher for
3758 the given \f(CW\*(C`fd\*(C'\fR and \f(CW\*(C`events\*(C'\fR set will be created and started.
3759 .Sp
3760 If \f(CW\*(C`timeout\*(C'\fR is less than 0, then no timeout watcher will be
3761 started. Otherwise an \f(CW\*(C`ev_timer\*(C'\fR watcher with after = \f(CW\*(C`timeout\*(C'\fR (and
3762 repeat = 0) will be started. \f(CW0\fR is a valid timeout.
3763 .Sp
3764 The callback has the type \f(CW\*(C`void (*cb)(int revents, void *arg)\*(C'\fR and is
3765 passed an \f(CW\*(C`revents\*(C'\fR set like normal event callbacks (a combination of
3766 \&\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
3767 value passed to \f(CW\*(C`ev_once\*(C'\fR. Note that it is possible to receive \fIboth\fR
3768 a timeout and an io event at the same time \- you probably should give io
3769 events precedence.
3770 .Sp
3771 Example: wait up to ten seconds for data to appear on \s-1STDIN_FILENO.\s0
3772 .Sp
3773 .Vb 7
3774 \& static void stdin_ready (int revents, void *arg)
3775 \& {
3776 \& if (revents & EV_READ)
3777 \& /* stdin might have data for us, joy! */;
3778 \& else if (revents & EV_TIMER)
3779 \& /* doh, nothing entered */;
3780 \& }
3781 \&
3782 \& ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3783 .Ve
3784 .IP "ev_feed_fd_event (loop, int fd, int revents)" 4
3785 .IX Item "ev_feed_fd_event (loop, int fd, int revents)"
3786 Feed an event on the given fd, as if a file descriptor backend detected
3787 the given events.
3788 .IP "ev_feed_signal_event (loop, int signum)" 4
3789 .IX Item "ev_feed_signal_event (loop, int signum)"
3790 Feed an event as if the given signal occurred. See also \f(CW\*(C`ev_feed_signal\*(C'\fR,
3791 which is async-safe.
3792 .SH "COMMON OR USEFUL IDIOMS (OR BOTH)"
3793 .IX Header "COMMON OR USEFUL IDIOMS (OR BOTH)"
3794 This section explains some common idioms that are not immediately
3795 obvious. Note that examples are sprinkled over the whole manual, and this
3796 section only contains stuff that wouldn't fit anywhere else.
3797 .SS "\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\s0"
3798 .IX Subsection "ASSOCIATING CUSTOM DATA WITH A WATCHER"
3799 Each watcher has, by default, a \f(CW\*(C`void *data\*(C'\fR member that you can read
3800 or modify at any time: libev will completely ignore it. This can be used
3801 to associate arbitrary data with your watcher. If you need more data and
3802 don't want to allocate memory separately and store a pointer to it in that
3803 data member, you can also \*(L"subclass\*(R" the watcher type and provide your own
3804 data:
3805 .PP
3806 .Vb 7
3807 \& struct my_io
3808 \& {
3809 \& ev_io io;
3810 \& int otherfd;
3811 \& void *somedata;
3812 \& struct whatever *mostinteresting;
3813 \& };
3814 \&
3815 \& ...
3816 \& struct my_io w;
3817 \& ev_io_init (&w.io, my_cb, fd, EV_READ);
3818 .Ve
3819 .PP
3820 And since your callback will be called with a pointer to the watcher, you
3821 can cast it back to your own type:
3822 .PP
3823 .Vb 5
3824 \& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3825 \& {
3826 \& struct my_io *w = (struct my_io *)w_;
3827 \& ...
3828 \& }
3829 .Ve
3830 .PP
3831 More interesting and less C\-conformant ways of casting your callback
3832 function type instead have been omitted.
3833 .SS "\s-1BUILDING YOUR OWN COMPOSITE WATCHERS\s0"
3834 .IX Subsection "BUILDING YOUR OWN COMPOSITE WATCHERS"
3835 Another common scenario is to use some data structure with multiple
3836 embedded watchers, in effect creating your own watcher that combines
3837 multiple libev event sources into one \*(L"super-watcher\*(R":
3838 .PP
3839 .Vb 6
3840 \& struct my_biggy
3841 \& {
3842 \& int some_data;
3843 \& ev_timer t1;
3844 \& ev_timer t2;
3845 \& }
3846 .Ve
3847 .PP
3848 In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more
3849 complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct in
3850 the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies or \*(C+ coders), or you need
3851 to use some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for
3852 real programmers):
3853 .PP
3854 .Vb 1
3855 \& #include <stddef.h>
3856 \&
3857 \& static void
3858 \& t1_cb (EV_P_ ev_timer *w, int revents)
3859 \& {
3860 \& struct my_biggy big = (struct my_biggy *)
3861 \& (((char *)w) \- offsetof (struct my_biggy, t1));
3862 \& }
3863 \&
3864 \& static void
3865 \& t2_cb (EV_P_ ev_timer *w, int revents)
3866 \& {
3867 \& struct my_biggy big = (struct my_biggy *)
3868 \& (((char *)w) \- offsetof (struct my_biggy, t2));
3869 \& }
3870 .Ve
3871 .SS "\s-1AVOIDING FINISHING BEFORE RETURNING\s0"
3872 .IX Subsection "AVOIDING FINISHING BEFORE RETURNING"
3873 Often you have structures like this in event-based programs:
3874 .PP
3875 .Vb 4
3876 \& callback ()
3877 \& {
3878 \& free (request);
3879 \& }
3880 \&
3881 \& request = start_new_request (..., callback);
3882 .Ve
3883 .PP
3884 The intent is to start some \*(L"lengthy\*(R" operation. The \f(CW\*(C`request\*(C'\fR could be
3885 used to cancel the operation, or do other things with it.
3886 .PP
3887 It's not uncommon to have code paths in \f(CW\*(C`start_new_request\*(C'\fR that
3888 immediately invoke the callback, for example, to report errors. Or you add
3889 some caching layer that finds that it can skip the lengthy aspects of the
3890 operation and simply invoke the callback with the result.
3891 .PP
3892 The problem here is that this will happen \fIbefore\fR \f(CW\*(C`start_new_request\*(C'\fR
3893 has returned, so \f(CW\*(C`request\*(C'\fR is not set.
3894 .PP
3895 Even if you pass the request by some safer means to the callback, you
3896 might want to do something to the request after starting it, such as
3897 canceling it, which probably isn't working so well when the callback has
3898 already been invoked.
3899 .PP
3900 A common way around all these issues is to make sure that
3901 \&\f(CW\*(C`start_new_request\*(C'\fR \fIalways\fR returns before the callback is invoked. If
3902 \&\f(CW\*(C`start_new_request\*(C'\fR immediately knows the result, it can artificially
3903 delay invoking the callback by using a \f(CW\*(C`prepare\*(C'\fR or \f(CW\*(C`idle\*(C'\fR watcher for
3904 example, or more sneakily, by reusing an existing (stopped) watcher and
3905 pushing it into the pending queue:
3906 .PP
3907 .Vb 2
3908 \& ev_set_cb (watcher, callback);
3909 \& ev_feed_event (EV_A_ watcher, 0);
3910 .Ve
3911 .PP
3912 This way, \f(CW\*(C`start_new_request\*(C'\fR can safely return before the callback is
3913 invoked, while not delaying callback invocation too much.
3914 .SS "\s-1MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS\s0"
3915 .IX Subsection "MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS"
3916 Often (especially in \s-1GUI\s0 toolkits) there are places where you have
3917 \&\fImodal\fR interaction, which is most easily implemented by recursively
3918 invoking \f(CW\*(C`ev_run\*(C'\fR.
3919 .PP
3920 This brings the problem of exiting \- a callback might want to finish the
3921 main \f(CW\*(C`ev_run\*(C'\fR call, but not the nested one (e.g. user clicked \*(L"Quit\*(R", but
3922 a modal \*(L"Are you sure?\*(R" dialog is still waiting), or just the nested one
3923 and not the main one (e.g. user clocked \*(L"Ok\*(R" in a modal dialog), or some
3924 other combination: In these cases, a simple \f(CW\*(C`ev_break\*(C'\fR will not work.
3925 .PP
3926 The solution is to maintain \*(L"break this loop\*(R" variable for each \f(CW\*(C`ev_run\*(C'\fR
3927 invocation, and use a loop around \f(CW\*(C`ev_run\*(C'\fR until the condition is
3928 triggered, using \f(CW\*(C`EVRUN_ONCE\*(C'\fR:
3929 .PP
3930 .Vb 2
3931 \& // main loop
3932 \& int exit_main_loop = 0;
3933 \&
3934 \& while (!exit_main_loop)
3935 \& ev_run (EV_DEFAULT_ EVRUN_ONCE);
3936 \&
3937 \& // in a modal watcher
3938 \& int exit_nested_loop = 0;
3939 \&
3940 \& while (!exit_nested_loop)
3941 \& ev_run (EV_A_ EVRUN_ONCE);
3942 .Ve
3943 .PP
3944 To exit from any of these loops, just set the corresponding exit variable:
3945 .PP
3946 .Vb 2
3947 \& // exit modal loop
3948 \& exit_nested_loop = 1;
3949 \&
3950 \& // exit main program, after modal loop is finished
3951 \& exit_main_loop = 1;
3952 \&
3953 \& // exit both
3954 \& exit_main_loop = exit_nested_loop = 1;
3955 .Ve
3956 .SS "\s-1THREAD LOCKING EXAMPLE\s0"
3957 .IX Subsection "THREAD LOCKING EXAMPLE"
3958 Here is a fictitious example of how to run an event loop in a different
3959 thread from where callbacks are being invoked and watchers are
3960 created/added/removed.
3961 .PP
3962 For a real-world example, see the \f(CW\*(C`EV::Loop::Async\*(C'\fR perl module,
3963 which uses exactly this technique (which is suited for many high-level
3964 languages).
3965 .PP
3966 The example uses a pthread mutex to protect the loop data, a condition
3967 variable to wait for callback invocations, an async watcher to notify the
3968 event loop thread and an unspecified mechanism to wake up the main thread.
3969 .PP
3970 First, you need to associate some data with the event loop:
3971 .PP
3972 .Vb 6
3973 \& typedef struct {
3974 \& mutex_t lock; /* global loop lock */
3975 \& ev_async async_w;
3976 \& thread_t tid;
3977 \& cond_t invoke_cv;
3978 \& } userdata;
3979 \&
3980 \& void prepare_loop (EV_P)
3981 \& {
3982 \& // for simplicity, we use a static userdata struct.
3983 \& static userdata u;
3984 \&
3985 \& ev_async_init (&u\->async_w, async_cb);
3986 \& ev_async_start (EV_A_ &u\->async_w);
3987 \&
3988 \& pthread_mutex_init (&u\->lock, 0);
3989 \& pthread_cond_init (&u\->invoke_cv, 0);
3990 \&
3991 \& // now associate this with the loop
3992 \& ev_set_userdata (EV_A_ u);
3993 \& ev_set_invoke_pending_cb (EV_A_ l_invoke);
3994 \& ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
3995 \&
3996 \& // then create the thread running ev_run
3997 \& pthread_create (&u\->tid, 0, l_run, EV_A);
3998 \& }
3999 .Ve
4000 .PP
4001 The callback for the \f(CW\*(C`ev_async\*(C'\fR watcher does nothing: the watcher is used
4002 solely to wake up the event loop so it takes notice of any new watchers
4003 that might have been added:
4004 .PP
4005 .Vb 5
4006 \& static void
4007 \& async_cb (EV_P_ ev_async *w, int revents)
4008 \& {
4009 \& // just used for the side effects
4010 \& }
4011 .Ve
4012 .PP
4013 The \f(CW\*(C`l_release\*(C'\fR and \f(CW\*(C`l_acquire\*(C'\fR callbacks simply unlock/lock the mutex
4014 protecting the loop data, respectively.
4015 .PP
4016 .Vb 6
4017 \& static void
4018 \& l_release (EV_P)
4019 \& {
4020 \& userdata *u = ev_userdata (EV_A);
4021 \& pthread_mutex_unlock (&u\->lock);
4022 \& }
4023 \&
4024 \& static void
4025 \& l_acquire (EV_P)
4026 \& {
4027 \& userdata *u = ev_userdata (EV_A);
4028 \& pthread_mutex_lock (&u\->lock);
4029 \& }
4030 .Ve
4031 .PP
4032 The event loop thread first acquires the mutex, and then jumps straight
4033 into \f(CW\*(C`ev_run\*(C'\fR:
4034 .PP
4035 .Vb 4
4036 \& void *
4037 \& l_run (void *thr_arg)
4038 \& {
4039 \& struct ev_loop *loop = (struct ev_loop *)thr_arg;
4040 \&
4041 \& l_acquire (EV_A);
4042 \& pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4043 \& ev_run (EV_A_ 0);
4044 \& l_release (EV_A);
4045 \&
4046 \& return 0;
4047 \& }
4048 .Ve
4049 .PP
4050 Instead of invoking all pending watchers, the \f(CW\*(C`l_invoke\*(C'\fR callback will
4051 signal the main thread via some unspecified mechanism (signals? pipe
4052 writes? \f(CW\*(C`Async::Interrupt\*(C'\fR?) and then waits until all pending watchers
4053 have been called (in a while loop because a) spurious wakeups are possible
4054 and b) skipping inter-thread-communication when there are no pending
4055 watchers is very beneficial):
4056 .PP
4057 .Vb 4
4058 \& static void
4059 \& l_invoke (EV_P)
4060 \& {
4061 \& userdata *u = ev_userdata (EV_A);
4062 \&
4063 \& while (ev_pending_count (EV_A))
4064 \& {
4065 \& wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4066 \& pthread_cond_wait (&u\->invoke_cv, &u\->lock);
4067 \& }
4068 \& }
4069 .Ve
4070 .PP
4071 Now, whenever the main thread gets told to invoke pending watchers, it
4072 will grab the lock, call \f(CW\*(C`ev_invoke_pending\*(C'\fR and then signal the loop
4073 thread to continue:
4074 .PP
4075 .Vb 4
4076 \& static void
4077 \& real_invoke_pending (EV_P)
4078 \& {
4079 \& userdata *u = ev_userdata (EV_A);
4080 \&
4081 \& pthread_mutex_lock (&u\->lock);
4082 \& ev_invoke_pending (EV_A);
4083 \& pthread_cond_signal (&u\->invoke_cv);
4084 \& pthread_mutex_unlock (&u\->lock);
4085 \& }
4086 .Ve
4087 .PP
4088 Whenever you want to start/stop a watcher or do other modifications to an
4089 event loop, you will now have to lock:
4090 .PP
4091 .Vb 2
4092 \& ev_timer timeout_watcher;
4093 \& userdata *u = ev_userdata (EV_A);
4094 \&
4095 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4096 \&
4097 \& pthread_mutex_lock (&u\->lock);
4098 \& ev_timer_start (EV_A_ &timeout_watcher);
4099 \& ev_async_send (EV_A_ &u\->async_w);
4100 \& pthread_mutex_unlock (&u\->lock);
4101 .Ve
4102 .PP
4103 Note that sending the \f(CW\*(C`ev_async\*(C'\fR watcher is required because otherwise
4104 an event loop currently blocking in the kernel will have no knowledge
4105 about the newly added timer. By waking up the loop it will pick up any new
4106 watchers in the next event loop iteration.
4107 .SS "\s-1THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS\s0"
4108 .IX Subsection "THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS"
4109 While the overhead of a callback that e.g. schedules a thread is small, it
4110 is still an overhead. If you embed libev, and your main usage is with some
4111 kind of threads or coroutines, you might want to customise libev so that
4112 doesn't need callbacks anymore.
4113 .PP
4114 Imagine you have coroutines that you can switch to using a function
4115 \&\f(CW\*(C`switch_to (coro)\*(C'\fR, that libev runs in a coroutine called \f(CW\*(C`libev_coro\*(C'\fR
4116 and that due to some magic, the currently active coroutine is stored in a
4117 global called \f(CW\*(C`current_coro\*(C'\fR. Then you can build your own \*(L"wait for libev
4118 event\*(R" primitive by changing \f(CW\*(C`EV_CB_DECLARE\*(C'\fR and \f(CW\*(C`EV_CB_INVOKE\*(C'\fR (note
4119 the differing \f(CW\*(C`;\*(C'\fR conventions):
4120 .PP
4121 .Vb 2
4122 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4123 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4124 .Ve
4125 .PP
4126 That means instead of having a C callback function, you store the
4127 coroutine to switch to in each watcher, and instead of having libev call
4128 your callback, you instead have it switch to that coroutine.
4129 .PP
4130 A coroutine might now wait for an event with a function called
4131 \&\f(CW\*(C`wait_for_event\*(C'\fR. (the watcher needs to be started, as always, but it doesn't
4132 matter when, or whether the watcher is active or not when this function is
4133 called):
4134 .PP
4135 .Vb 6
4136 \& void
4137 \& wait_for_event (ev_watcher *w)
4138 \& {
4139 \& ev_set_cb (w, current_coro);
4140 \& switch_to (libev_coro);
4141 \& }
4142 .Ve
4143 .PP
4144 That basically suspends the coroutine inside \f(CW\*(C`wait_for_event\*(C'\fR and
4145 continues the libev coroutine, which, when appropriate, switches back to
4146 this or any other coroutine.
4147 .PP
4148 You can do similar tricks if you have, say, threads with an event queue \-
4149 instead of storing a coroutine, you store the queue object and instead of
4150 switching to a coroutine, you push the watcher onto the queue and notify
4151 any waiters.
4152 .PP
4153 To embed libev, see \*(L"\s-1EMBEDDING\*(R"\s0, but in short, it's easiest to create two
4154 files, \fImy_ev.h\fR and \fImy_ev.c\fR that include the respective libev files:
4155 .PP
4156 .Vb 4
4157 \& // my_ev.h
4158 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4159 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4160 \& #include "../libev/ev.h"
4161 \&
4162 \& // my_ev.c
4163 \& #define EV_H "my_ev.h"
4164 \& #include "../libev/ev.c"
4165 .Ve
4166 .PP
4167 And then use \fImy_ev.h\fR when you would normally use \fIev.h\fR, and compile
4168 \&\fImy_ev.c\fR into your project. When properly specifying include paths, you
4169 can even use \fIev.h\fR as header file name directly.
4170 .SH "LIBEVENT EMULATION"
4171 .IX Header "LIBEVENT EMULATION"
4172 Libev offers a compatibility emulation layer for libevent. It cannot
4173 emulate the internals of libevent, so here are some usage hints:
4174 .IP "\(bu" 4
4175 Only the libevent\-1.4.1\-beta \s-1API\s0 is being emulated.
4176 .Sp
4177 This was the newest libevent version available when libev was implemented,
4178 and is still mostly unchanged in 2010.
4179 .IP "\(bu" 4
4180 Use it by including <event.h>, as usual.
4181 .IP "\(bu" 4
4182 The following members are fully supported: ev_base, ev_callback,
4183 ev_arg, ev_fd, ev_res, ev_events.
4184 .IP "\(bu" 4
4185 Avoid using ev_flags and the EVLIST_*\-macros, while it is
4186 maintained by libev, it does not work exactly the same way as in libevent (consider
4187 it a private \s-1API\s0).
4188 .IP "\(bu" 4
4189 Priorities are not currently supported. Initialising priorities
4190 will fail and all watchers will have the same priority, even though there
4191 is an ev_pri field.
4192 .IP "\(bu" 4
4193 In libevent, the last base created gets the signals, in libev, the
4194 base that registered the signal gets the signals.
4195 .IP "\(bu" 4
4196 Other members are not supported.
4197 .IP "\(bu" 4
4198 The libev emulation is \fInot\fR \s-1ABI\s0 compatible to libevent, you need
4199 to use the libev header file and library.
4200 .SH "\*(C+ SUPPORT"
4201 .IX Header " SUPPORT"
4202 .SS "C \s-1API\s0"
4203 .IX Subsection "C API"
4204 The normal C \s-1API\s0 should work fine when used from \*(C+: both ev.h and the
4205 libev sources can be compiled as \*(C+. Therefore, code that uses the C \s-1API\s0
4206 will work fine.
4207 .PP
4208 Proper exception specifications might have to be added to callbacks passed
4209 to libev: exceptions may be thrown only from watcher callbacks, all other
4210 callbacks (allocator, syserr, loop acquire/release and periodic reschedule
4211 callbacks) must not throw exceptions, and might need a \f(CW\*(C`noexcept\*(C'\fR
4212 specification. If you have code that needs to be compiled as both C and
4213 \&\*(C+ you can use the \f(CW\*(C`EV_NOEXCEPT\*(C'\fR macro for this:
4214 .PP
4215 .Vb 6
4216 \& static void
4217 \& fatal_error (const char *msg) EV_NOEXCEPT
4218 \& {
4219 \& perror (msg);
4220 \& abort ();
4221 \& }
4222 \&
4223 \& ...
4224 \& ev_set_syserr_cb (fatal_error);
4225 .Ve
4226 .PP
4227 The only \s-1API\s0 functions that can currently throw exceptions are \f(CW\*(C`ev_run\*(C'\fR,
4228 \&\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
4229 because it runs cleanup watchers).
4230 .PP
4231 Throwing exceptions in watcher callbacks is only supported if libev itself
4232 is compiled with a \*(C+ compiler or your C and \*(C+ environments allow
4233 throwing exceptions through C libraries (most do).
4234 .SS "\*(C+ \s-1API\s0"
4235 .IX Subsection " API"
4236 Libev comes with some simplistic wrapper classes for \*(C+ that mainly allow
4237 you to use some convenience methods to start/stop watchers and also change
4238 the callback model to a model using method callbacks on objects.
4239 .PP
4240 To use it,
4241 .PP
4242 .Vb 1
4243 \& #include <ev++.h>
4244 .Ve
4245 .PP
4246 This automatically includes \fIev.h\fR and puts all of its definitions (many
4247 of them macros) into the global namespace. All \*(C+ specific things are
4248 put into the \f(CW\*(C`ev\*(C'\fR namespace. It should support all the same embedding
4249 options as \fIev.h\fR, most notably \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR.
4250 .PP
4251 Care has been taken to keep the overhead low. The only data member the \*(C+
4252 classes add (compared to plain C\-style watchers) is the event loop pointer
4253 that the watcher is associated with (or no additional members at all if
4254 you disable \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR when embedding libev).
4255 .PP
4256 Currently, functions, static and non-static member functions and classes
4257 with \f(CW\*(C`operator ()\*(C'\fR can be used as callbacks. Other types should be easy
4258 to add as long as they only need one additional pointer for context. If
4259 you need support for other types of functors please contact the author
4260 (preferably after implementing it).
4261 .PP
4262 For all this to work, your \*(C+ compiler either has to use the same calling
4263 conventions as your C compiler (for static member functions), or you have
4264 to embed libev and compile libev itself as \*(C+.
4265 .PP
4266 Here is a list of things available in the \f(CW\*(C`ev\*(C'\fR namespace:
4267 .ie n .IP """ev::READ"", ""ev::WRITE"" etc." 4
4268 .el .IP "\f(CWev::READ\fR, \f(CWev::WRITE\fR etc." 4
4269 .IX Item "ev::READ, ev::WRITE etc."
4270 These are just enum values with the same values as the \f(CW\*(C`EV_READ\*(C'\fR etc.
4271 macros from \fIev.h\fR.
4272 .ie n .IP """ev::tstamp"", ""ev::now""" 4
4273 .el .IP "\f(CWev::tstamp\fR, \f(CWev::now\fR" 4
4274 .IX Item "ev::tstamp, ev::now"
4275 Aliases to the same types/functions as with the \f(CW\*(C`ev_\*(C'\fR prefix.
4276 .ie n .IP """ev::io"", ""ev::timer"", ""ev::periodic"", ""ev::idle"", ""ev::sig"" etc." 4
4277 .el .IP "\f(CWev::io\fR, \f(CWev::timer\fR, \f(CWev::periodic\fR, \f(CWev::idle\fR, \f(CWev::sig\fR etc." 4
4278 .IX Item "ev::io, ev::timer, ev::periodic, ev::idle, ev::sig etc."
4279 For each \f(CW\*(C`ev_TYPE\*(C'\fR watcher in \fIev.h\fR there is a corresponding class of
4280 the same name in the \f(CW\*(C`ev\*(C'\fR namespace, with the exception of \f(CW\*(C`ev_signal\*(C'\fR
4281 which is called \f(CW\*(C`ev::sig\*(C'\fR to avoid clashes with the \f(CW\*(C`signal\*(C'\fR macro
4282 defined by many implementations.
4283 .Sp
4284 All of those classes have these methods:
4285 .RS 4
4286 .IP "ev::TYPE::TYPE ()" 4
4287 .IX Item "ev::TYPE::TYPE ()"
4288 .PD 0
4289 .IP "ev::TYPE::TYPE (loop)" 4
4290 .IX Item "ev::TYPE::TYPE (loop)"
4291 .IP "ev::TYPE::~TYPE" 4
4292 .IX Item "ev::TYPE::~TYPE"
4293 .PD
4294 The constructor (optionally) takes an event loop to associate the watcher
4295 with. If it is omitted, it will use \f(CW\*(C`EV_DEFAULT\*(C'\fR.
4296 .Sp
4297 The constructor calls \f(CW\*(C`ev_init\*(C'\fR for you, which means you have to call the
4298 \&\f(CW\*(C`set\*(C'\fR method before starting it.
4299 .Sp
4300 It will not set a callback, however: You have to call the templated \f(CW\*(C`set\*(C'\fR
4301 method to set a callback before you can start the watcher.
4302 .Sp
4303 (The reason why you have to use a method is a limitation in \*(C+ which does
4304 not allow explicit template arguments for constructors).
4305 .Sp
4306 The destructor automatically stops the watcher if it is active.
4307 .IP "w\->set<class, &class::method> (object *)" 4
4308 .IX Item "w->set<class, &class::method> (object *)"
4309 This method sets the callback method to call. The method has to have a
4310 signature of \f(CW\*(C`void (*)(ev_TYPE &, int)\*(C'\fR, it receives the watcher as
4311 first argument and the \f(CW\*(C`revents\*(C'\fR as second. The object must be given as
4312 parameter and is stored in the \f(CW\*(C`data\*(C'\fR member of the watcher.
4313 .Sp
4314 This method synthesizes efficient thunking code to call your method from
4315 the C callback that libev requires. If your compiler can inline your
4316 callback (i.e. it is visible to it at the place of the \f(CW\*(C`set\*(C'\fR call and
4317 your compiler is good :), then the method will be fully inlined into the
4318 thunking function, making it as fast as a direct C callback.
4319 .Sp
4320 Example: simple class declaration and watcher initialisation
4321 .Sp
4322 .Vb 4
4323 \& struct myclass
4324 \& {
4325 \& void io_cb (ev::io &w, int revents) { }
4326 \& }
4327 \&
4328 \& myclass obj;
4329 \& ev::io iow;
4330 \& iow.set <myclass, &myclass::io_cb> (&obj);
4331 .Ve
4332 .IP "w\->set (object *)" 4
4333 .IX Item "w->set (object *)"
4334 This is a variation of a method callback \- leaving out the method to call
4335 will default the method to \f(CW\*(C`operator ()\*(C'\fR, which makes it possible to use
4336 functor objects without having to manually specify the \f(CW\*(C`operator ()\*(C'\fR all
4337 the time. Incidentally, you can then also leave out the template argument
4338 list.
4339 .Sp
4340 The \f(CW\*(C`operator ()\*(C'\fR method prototype must be \f(CW\*(C`void operator ()(watcher &w,
4341 int revents)\*(C'\fR.
4342 .Sp
4343 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4344 .Sp
4345 Example: use a functor object as callback.
4346 .Sp
4347 .Vb 7
4348 \& struct myfunctor
4349 \& {
4350 \& void operator() (ev::io &w, int revents)
4351 \& {
4352 \& ...
4353 \& }
4354 \& }
4355 \&
4356 \& myfunctor f;
4357 \&
4358 \& ev::io w;
4359 \& w.set (&f);
4360 .Ve
4361 .IP "w\->set<function> (void *data = 0)" 4
4362 .IX Item "w->set<function> (void *data = 0)"
4363 Also sets a callback, but uses a static method or plain function as
4364 callback. The optional \f(CW\*(C`data\*(C'\fR argument will be stored in the watcher's
4365 \&\f(CW\*(C`data\*(C'\fR member and is free for you to use.
4366 .Sp
4367 The prototype of the \f(CW\*(C`function\*(C'\fR must be \f(CW\*(C`void (*)(ev::TYPE &w, int)\*(C'\fR.
4368 .Sp
4369 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4370 .Sp
4371 Example: Use a plain function as callback.
4372 .Sp
4373 .Vb 2
4374 \& static void io_cb (ev::io &w, int revents) { }
4375 \& iow.set <io_cb> ();
4376 .Ve
4377 .IP "w\->set (loop)" 4
4378 .IX Item "w->set (loop)"
4379 Associates a different \f(CW\*(C`struct ev_loop\*(C'\fR with this watcher. You can only
4380 do this when the watcher is inactive (and not pending either).
4381 .IP "w\->set ([arguments])" 4
4382 .IX Item "w->set ([arguments])"
4383 Basically the same as \f(CW\*(C`ev_TYPE_set\*(C'\fR (except for \f(CW\*(C`ev::embed\*(C'\fR watchers>),
4384 with the same arguments. Either this method or a suitable start method
4385 must be called at least once. Unlike the C counterpart, an active watcher
4386 gets automatically stopped and restarted when reconfiguring it with this
4387 method.
4388 .Sp
4389 For \f(CW\*(C`ev::embed\*(C'\fR watchers this method is called \f(CW\*(C`set_embed\*(C'\fR, to avoid
4390 clashing with the \f(CW\*(C`set (loop)\*(C'\fR method.
4391 .IP "w\->start ()" 4
4392 .IX Item "w->start ()"
4393 Starts the watcher. Note that there is no \f(CW\*(C`loop\*(C'\fR argument, as the
4394 constructor already stores the event loop.
4395 .IP "w\->start ([arguments])" 4
4396 .IX Item "w->start ([arguments])"
4397 Instead of calling \f(CW\*(C`set\*(C'\fR and \f(CW\*(C`start\*(C'\fR methods separately, it is often
4398 convenient to wrap them in one call. Uses the same type of arguments as
4399 the configure \f(CW\*(C`set\*(C'\fR method of the watcher.
4400 .IP "w\->stop ()" 4
4401 .IX Item "w->stop ()"
4402 Stops the watcher if it is active. Again, no \f(CW\*(C`loop\*(C'\fR argument.
4403 .ie n .IP "w\->again () (""ev::timer"", ""ev::periodic"" only)" 4
4404 .el .IP "w\->again () (\f(CWev::timer\fR, \f(CWev::periodic\fR only)" 4
4405 .IX Item "w->again () (ev::timer, ev::periodic only)"
4406 For \f(CW\*(C`ev::timer\*(C'\fR and \f(CW\*(C`ev::periodic\*(C'\fR, this invokes the corresponding
4407 \&\f(CW\*(C`ev_TYPE_again\*(C'\fR function.
4408 .ie n .IP "w\->sweep () (""ev::embed"" only)" 4
4409 .el .IP "w\->sweep () (\f(CWev::embed\fR only)" 4
4410 .IX Item "w->sweep () (ev::embed only)"
4411 Invokes \f(CW\*(C`ev_embed_sweep\*(C'\fR.
4412 .ie n .IP "w\->update () (""ev::stat"" only)" 4
4413 .el .IP "w\->update () (\f(CWev::stat\fR only)" 4
4414 .IX Item "w->update () (ev::stat only)"
4415 Invokes \f(CW\*(C`ev_stat_stat\*(C'\fR.
4416 .RE
4417 .RS 4
4418 .RE
4419 .PP
4420 Example: Define a class with two I/O and idle watchers, start the I/O
4421 watchers in the constructor.
4422 .PP
4423 .Vb 5
4424 \& class myclass
4425 \& {
4426 \& ev::io io ; void io_cb (ev::io &w, int revents);
4427 \& ev::io io2 ; void io2_cb (ev::io &w, int revents);
4428 \& ev::idle idle; void idle_cb (ev::idle &w, int revents);
4429 \&
4430 \& myclass (int fd)
4431 \& {
4432 \& io .set <myclass, &myclass::io_cb > (this);
4433 \& io2 .set <myclass, &myclass::io2_cb > (this);
4434 \& idle.set <myclass, &myclass::idle_cb> (this);
4435 \&
4436 \& io.set (fd, ev::WRITE); // configure the watcher
4437 \& io.start (); // start it whenever convenient
4438 \&
4439 \& io2.start (fd, ev::READ); // set + start in one call
4440 \& }
4441 \& };
4442 .Ve
4443 .SH "OTHER LANGUAGE BINDINGS"
4444 .IX Header "OTHER LANGUAGE BINDINGS"
4445 Libev does not offer other language bindings itself, but bindings for a
4446 number of languages exist in the form of third-party packages. If you know
4447 any interesting language binding in addition to the ones listed here, drop
4448 me a note.
4449 .IP "Perl" 4
4450 .IX Item "Perl"
4451 The \s-1EV\s0 module implements the full libev \s-1API\s0 and is actually used to test
4452 libev. \s-1EV\s0 is developed together with libev. Apart from the \s-1EV\s0 core module,
4453 there are additional modules that implement libev-compatible interfaces
4454 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),
4455 \&\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
4456 and \f(CW\*(C`EV::Glib\*(C'\fR).
4457 .Sp
4458 It can be found and installed via \s-1CPAN,\s0 its homepage is at
4459 <http://software.schmorp.de/pkg/EV>.
4460 .IP "Python" 4
4461 .IX Item "Python"
4462 Python bindings can be found at <http://code.google.com/p/pyev/>. It
4463 seems to be quite complete and well-documented.
4464 .IP "Ruby" 4
4465 .IX Item "Ruby"
4466 Tony Arcieri has written a ruby extension that offers access to a subset
4467 of the libev \s-1API\s0 and adds file handle abstractions, asynchronous \s-1DNS\s0 and
4468 more on top of it. It can be found via gem servers. Its homepage is at
4469 <http://rev.rubyforge.org/>.
4470 .Sp
4471 Roger Pack reports that using the link order \f(CW\*(C`\-lws2_32 \-lmsvcrt\-ruby\-190\*(C'\fR
4472 makes rev work even on mingw.
4473 .IP "Haskell" 4
4474 .IX Item "Haskell"
4475 A haskell binding to libev is available at
4476 <http://hackage.haskell.org/cgi\-bin/hackage\-scripts/package/hlibev>.
4477 .IP "D" 4
4478 .IX Item "D"
4479 Leandro Lucarella has written a D language binding (\fIev.d\fR) for libev, to
4480 be found at <http://www.llucax.com.ar/proj/ev.d/index.html>.
4481 .IP "Ocaml" 4
4482 .IX Item "Ocaml"
4483 Erkki Seppala has written Ocaml bindings for libev, to be found at
4484 <http://modeemi.cs.tut.fi/~flux/software/ocaml\-ev/>.
4485 .IP "Lua" 4
4486 .IX Item "Lua"
4487 Brian Maher has written a partial interface to libev for lua (at the
4488 time of this writing, only \f(CW\*(C`ev_io\*(C'\fR and \f(CW\*(C`ev_timer\*(C'\fR), to be found at
4489 <http://github.com/brimworks/lua\-ev>.
4490 .IP "Javascript" 4
4491 .IX Item "Javascript"
4492 Node.js (<http://nodejs.org>) uses libev as the underlying event library.
4493 .IP "Others" 4
4494 .IX Item "Others"
4495 There are others, and I stopped counting.
4496 .SH "MACRO MAGIC"
4497 .IX Header "MACRO MAGIC"
4498 Libev can be compiled with a variety of options, the most fundamental
4499 of which is \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR. This option determines whether (most)
4500 functions and callbacks have an initial \f(CW\*(C`struct ev_loop *\*(C'\fR argument.
4501 .PP
4502 To make it easier to write programs that cope with either variant, the
4503 following macros are defined:
4504 .ie n .IP """EV_A"", ""EV_A_""" 4
4505 .el .IP "\f(CWEV_A\fR, \f(CWEV_A_\fR" 4
4506 .IX Item "EV_A, EV_A_"
4507 This provides the loop \fIargument\fR for functions, if one is required (\*(L"ev
4508 loop argument\*(R"). The \f(CW\*(C`EV_A\*(C'\fR form is used when this is the sole argument,
4509 \&\f(CW\*(C`EV_A_\*(C'\fR is used when other arguments are following. Example:
4510 .Sp
4511 .Vb 3
4512 \& ev_unref (EV_A);
4513 \& ev_timer_add (EV_A_ watcher);
4514 \& ev_run (EV_A_ 0);
4515 .Ve
4516 .Sp
4517 It assumes the variable \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR is in scope,
4518 which is often provided by the following macro.
4519 .ie n .IP """EV_P"", ""EV_P_""" 4
4520 .el .IP "\f(CWEV_P\fR, \f(CWEV_P_\fR" 4
4521 .IX Item "EV_P, EV_P_"
4522 This provides the loop \fIparameter\fR for functions, if one is required (\*(L"ev
4523 loop parameter\*(R"). The \f(CW\*(C`EV_P\*(C'\fR form is used when this is the sole parameter,
4524 \&\f(CW\*(C`EV_P_\*(C'\fR is used when other parameters are following. Example:
4525 .Sp
4526 .Vb 2
4527 \& // this is how ev_unref is being declared
4528 \& static void ev_unref (EV_P);
4529 \&
4530 \& // this is how you can declare your typical callback
4531 \& static void cb (EV_P_ ev_timer *w, int revents)
4532 .Ve
4533 .Sp
4534 It declares a parameter \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR, quite
4535 suitable for use with \f(CW\*(C`EV_A\*(C'\fR.
4536 .ie n .IP """EV_DEFAULT"", ""EV_DEFAULT_""" 4
4537 .el .IP "\f(CWEV_DEFAULT\fR, \f(CWEV_DEFAULT_\fR" 4
4538 .IX Item "EV_DEFAULT, EV_DEFAULT_"
4539 Similar to the other two macros, this gives you the value of the default
4540 loop, if multiple loops are supported (\*(L"ev loop default\*(R"). The default loop
4541 will be initialised if it isn't already initialised.
4542 .Sp
4543 For non-multiplicity builds, these macros do nothing, so you always have
4544 to initialise the loop somewhere.
4545 .ie n .IP """EV_DEFAULT_UC"", ""EV_DEFAULT_UC_""" 4
4546 .el .IP "\f(CWEV_DEFAULT_UC\fR, \f(CWEV_DEFAULT_UC_\fR" 4
4547 .IX Item "EV_DEFAULT_UC, EV_DEFAULT_UC_"
4548 Usage identical to \f(CW\*(C`EV_DEFAULT\*(C'\fR and \f(CW\*(C`EV_DEFAULT_\*(C'\fR, but requires that the
4549 default loop has been initialised (\f(CW\*(C`UC\*(C'\fR == unchecked). Their behaviour
4550 is undefined when the default loop has not been initialised by a previous
4551 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.
4552 .Sp
4553 It is often prudent to use \f(CW\*(C`EV_DEFAULT\*(C'\fR when initialising the first
4554 watcher in a function but use \f(CW\*(C`EV_DEFAULT_UC\*(C'\fR afterwards.
4555 .PP
4556 Example: Declare and initialise a check watcher, utilising the above
4557 macros so it will work regardless of whether multiple loops are supported
4558 or not.
4559 .PP
4560 .Vb 5
4561 \& static void
4562 \& check_cb (EV_P_ ev_timer *w, int revents)
4563 \& {
4564 \& ev_check_stop (EV_A_ w);
4565 \& }
4566 \&
4567 \& ev_check check;
4568 \& ev_check_init (&check, check_cb);
4569 \& ev_check_start (EV_DEFAULT_ &check);
4570 \& ev_run (EV_DEFAULT_ 0);
4571 .Ve
4572 .SH "EMBEDDING"
4573 .IX Header "EMBEDDING"
4574 Libev can (and often is) directly embedded into host
4575 applications. Examples of applications that embed it include the Deliantra
4576 Game Server, the \s-1EV\s0 perl module, the \s-1GNU\s0 Virtual Private Ethernet (gvpe)
4577 and rxvt-unicode.
4578 .PP
4579 The goal is to enable you to just copy the necessary files into your
4580 source directory without having to change even a single line in them, so
4581 you can easily upgrade by simply copying (or having a checked-out copy of
4582 libev somewhere in your source tree).
4583 .SS "\s-1FILESETS\s0"
4584 .IX Subsection "FILESETS"
4585 Depending on what features you need you need to include one or more sets of files
4586 in your application.
4587 .PP
4588 \fI\s-1CORE EVENT LOOP\s0\fR
4589 .IX Subsection "CORE EVENT LOOP"
4590 .PP
4591 To include only the libev core (all the \f(CW\*(C`ev_*\*(C'\fR functions), with manual
4592 configuration (no autoconf):
4593 .PP
4594 .Vb 2
4595 \& #define EV_STANDALONE 1
4596 \& #include "ev.c"
4597 .Ve
4598 .PP
4599 This will automatically include \fIev.h\fR, too, and should be done in a
4600 single C source file only to provide the function implementations. To use
4601 it, do the same for \fIev.h\fR in all files wishing to use this \s-1API\s0 (best
4602 done by writing a wrapper around \fIev.h\fR that you can include instead and
4603 where you can put other configuration options):
4604 .PP
4605 .Vb 2
4606 \& #define EV_STANDALONE 1
4607 \& #include "ev.h"
4608 .Ve
4609 .PP
4610 Both header files and implementation files can be compiled with a \*(C+
4611 compiler (at least, that's a stated goal, and breakage will be treated
4612 as a bug).
4613 .PP
4614 You need the following files in your source tree, or in a directory
4615 in your include path (e.g. in libev/ when using \-Ilibev):
4616 .PP
4617 .Vb 4
4618 \& ev.h
4619 \& ev.c
4620 \& ev_vars.h
4621 \& ev_wrap.h
4622 \&
4623 \& ev_win32.c required on win32 platforms only
4624 \&
4625 \& ev_select.c only when select backend is enabled
4626 \& ev_poll.c only when poll backend is enabled
4627 \& ev_epoll.c only when the epoll backend is enabled
4628 \& ev_linuxaio.c only when the linux aio backend is enabled
4629 \& ev_kqueue.c only when the kqueue backend is enabled
4630 \& ev_port.c only when the solaris port backend is enabled
4631 .Ve
4632 .PP
4633 \&\fIev.c\fR includes the backend files directly when enabled, so you only need
4634 to compile this single file.
4635 .PP
4636 \fI\s-1LIBEVENT COMPATIBILITY API\s0\fR
4637 .IX Subsection "LIBEVENT COMPATIBILITY API"
4638 .PP
4639 To include the libevent compatibility \s-1API,\s0 also include:
4640 .PP
4641 .Vb 1
4642 \& #include "event.c"
4643 .Ve
4644 .PP
4645 in the file including \fIev.c\fR, and:
4646 .PP
4647 .Vb 1
4648 \& #include "event.h"
4649 .Ve
4650 .PP
4651 in the files that want to use the libevent \s-1API.\s0 This also includes \fIev.h\fR.
4652 .PP
4653 You need the following additional files for this:
4654 .PP
4655 .Vb 2
4656 \& event.h
4657 \& event.c
4658 .Ve
4659 .PP
4660 \fI\s-1AUTOCONF SUPPORT\s0\fR
4661 .IX Subsection "AUTOCONF SUPPORT"
4662 .PP
4663 Instead of using \f(CW\*(C`EV_STANDALONE=1\*(C'\fR and providing your configuration in
4664 whatever way you want, you can also \f(CW\*(C`m4_include([libev.m4])\*(C'\fR in your
4665 \&\fIconfigure.ac\fR and leave \f(CW\*(C`EV_STANDALONE\*(C'\fR undefined. \fIev.c\fR will then
4666 include \fIconfig.h\fR and configure itself accordingly.
4667 .PP
4668 For this of course you need the m4 file:
4669 .PP
4670 .Vb 1
4671 \& libev.m4
4672 .Ve
4673 .SS "\s-1PREPROCESSOR SYMBOLS/MACROS\s0"
4674 .IX Subsection "PREPROCESSOR SYMBOLS/MACROS"
4675 Libev can be configured via a variety of preprocessor symbols you have to
4676 define before including (or compiling) any of its files. The default in
4677 the absence of autoconf is documented for every option.
4678 .PP
4679 Symbols marked with \*(L"(h)\*(R" do not change the \s-1ABI,\s0 and can have different
4680 values when compiling libev vs. including \fIev.h\fR, so it is permissible
4681 to redefine them before including \fIev.h\fR without breaking compatibility
4682 to a compiled library. All other symbols change the \s-1ABI,\s0 which means all
4683 users of libev and the libev code itself must be compiled with compatible
4684 settings.
4685 .IP "\s-1EV_COMPAT3\s0 (h)" 4
4686 .IX Item "EV_COMPAT3 (h)"
4687 Backwards compatibility is a major concern for libev. This is why this
4688 release of libev comes with wrappers for the functions and symbols that
4689 have been renamed between libev version 3 and 4.
4690 .Sp
4691 You can disable these wrappers (to test compatibility with future
4692 versions) by defining \f(CW\*(C`EV_COMPAT3\*(C'\fR to \f(CW0\fR when compiling your
4693 sources. This has the additional advantage that you can drop the \f(CW\*(C`struct\*(C'\fR
4694 from \f(CW\*(C`struct ev_loop\*(C'\fR declarations, as libev will provide an \f(CW\*(C`ev_loop\*(C'\fR
4695 typedef in that case.
4696 .Sp
4697 In some future version, the default for \f(CW\*(C`EV_COMPAT3\*(C'\fR will become \f(CW0\fR,
4698 and in some even more future version the compatibility code will be
4699 removed completely.
4700 .IP "\s-1EV_STANDALONE\s0 (h)" 4
4701 .IX Item "EV_STANDALONE (h)"
4702 Must always be \f(CW1\fR if you do not use autoconf configuration, which
4703 keeps libev from including \fIconfig.h\fR, and it also defines dummy
4704 implementations for some libevent functions (such as logging, which is not
4705 supported). It will also not define any of the structs usually found in
4706 \&\fIevent.h\fR that are not directly supported by the libev core alone.
4707 .Sp
4708 In standalone mode, libev will still try to automatically deduce the
4709 configuration, but has to be more conservative.
4710 .IP "\s-1EV_USE_FLOOR\s0" 4
4711 .IX Item "EV_USE_FLOOR"
4712 If defined to be \f(CW1\fR, libev will use the \f(CW\*(C`floor ()\*(C'\fR function for its
4713 periodic reschedule calculations, otherwise libev will fall back on a
4714 portable (slower) implementation. If you enable this, you usually have to
4715 link against libm or something equivalent. Enabling this when the \f(CW\*(C`floor\*(C'\fR
4716 function is not available will fail, so the safe default is to not enable
4717 this.
4718 .IP "\s-1EV_USE_MONOTONIC\s0" 4
4719 .IX Item "EV_USE_MONOTONIC"
4720 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4721 monotonic clock option at both compile time and runtime. Otherwise no
4722 use of the monotonic clock option will be attempted. If you enable this,
4723 you usually have to link against librt or something similar. Enabling it
4724 when the functionality isn't available is safe, though, although you have
4725 to make sure you link against any libraries where the \f(CW\*(C`clock_gettime\*(C'\fR
4726 function is hiding in (often \fI\-lrt\fR). See also \f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4727 .IP "\s-1EV_USE_REALTIME\s0" 4
4728 .IX Item "EV_USE_REALTIME"
4729 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4730 real-time clock option at compile time (and assume its availability
4731 at runtime if successful). Otherwise no use of the real-time clock
4732 option will be attempted. This effectively replaces \f(CW\*(C`gettimeofday\*(C'\fR
4733 by \f(CW\*(C`clock_get (CLOCK_REALTIME, ...)\*(C'\fR and will not normally affect
4734 correctness. See the note about libraries in the description of
4735 \&\f(CW\*(C`EV_USE_MONOTONIC\*(C'\fR, though. Defaults to the opposite value of
4736 \&\f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4737 .IP "\s-1EV_USE_CLOCK_SYSCALL\s0" 4
4738 .IX Item "EV_USE_CLOCK_SYSCALL"
4739 If defined to be \f(CW1\fR, libev will try to use a direct syscall instead
4740 of calling the system-provided \f(CW\*(C`clock_gettime\*(C'\fR function. This option
4741 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
4742 unconditionally pulls in \f(CW\*(C`libpthread\*(C'\fR, slowing down single-threaded
4743 programs needlessly. Using a direct syscall is slightly slower (in
4744 theory), because no optimised vdso implementation can be used, but avoids
4745 the pthread dependency. Defaults to \f(CW1\fR on GNU/Linux with glibc 2.x or
4746 higher, as it simplifies linking (no need for \f(CW\*(C`\-lrt\*(C'\fR).
4747 .IP "\s-1EV_USE_NANOSLEEP\s0" 4
4748 .IX Item "EV_USE_NANOSLEEP"
4749 If defined to be \f(CW1\fR, libev will assume that \f(CW\*(C`nanosleep ()\*(C'\fR is available
4750 and will use it for delays. Otherwise it will use \f(CW\*(C`select ()\*(C'\fR.
4751 .IP "\s-1EV_USE_EVENTFD\s0" 4
4752 .IX Item "EV_USE_EVENTFD"
4753 If defined to be \f(CW1\fR, then libev will assume that \f(CW\*(C`eventfd ()\*(C'\fR is
4754 available and will probe for kernel support at runtime. This will improve
4755 \&\f(CW\*(C`ev_signal\*(C'\fR and \f(CW\*(C`ev_async\*(C'\fR performance and reduce resource consumption.
4756 If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
4757 2.7 or newer, otherwise disabled.
4758 .IP "\s-1EV_USE_SELECT\s0" 4
4759 .IX Item "EV_USE_SELECT"
4760 If undefined or defined to be \f(CW1\fR, libev will compile in support for the
4761 \&\f(CW\*(C`select\*(C'\fR(2) backend. No attempt at auto-detection will be done: if no
4762 other method takes over, select will be it. Otherwise the select backend
4763 will not be compiled in.
4764 .IP "\s-1EV_SELECT_USE_FD_SET\s0" 4
4765 .IX Item "EV_SELECT_USE_FD_SET"
4766 If defined to \f(CW1\fR, then the select backend will use the system \f(CW\*(C`fd_set\*(C'\fR
4767 structure. This is useful if libev doesn't compile due to a missing
4768 \&\f(CW\*(C`NFDBITS\*(C'\fR or \f(CW\*(C`fd_mask\*(C'\fR definition or it mis-guesses the bitset layout
4769 on exotic systems. This usually limits the range of file descriptors to
4770 some low limit such as 1024 or might have other limitations (winsocket
4771 only allows 64 sockets). The \f(CW\*(C`FD_SETSIZE\*(C'\fR macro, set before compilation,
4772 configures the maximum size of the \f(CW\*(C`fd_set\*(C'\fR.
4773 .IP "\s-1EV_SELECT_IS_WINSOCKET\s0" 4
4774 .IX Item "EV_SELECT_IS_WINSOCKET"
4775 When defined to \f(CW1\fR, the select backend will assume that
4776 select/socket/connect etc. don't understand file descriptors but
4777 wants osf handles on win32 (this is the case when the select to
4778 be used is the winsock select). This means that it will call
4779 \&\f(CW\*(C`_get_osfhandle\*(C'\fR on the fd to convert it to an \s-1OS\s0 handle. Otherwise,
4780 it is assumed that all these functions actually work on fds, even