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Revision: 1.111
Committed: Sun Jun 23 02:02:24 2019 UTC (4 years, 10 months ago) by root
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# Content
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133 .\" ========================================================================
134 .\"
135 .IX Title "LIBEV 3"
136 .TH LIBEV 3 "2019-06-22" "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 Libev also has a few internal error-checking \f(CW\*(C`assert\*(C'\fRions, and also has
297 extensive consistency checking code. These do not trigger under normal
298 circumstances, as they indicate either a bug in libev or worse.
299 .SH "GLOBAL FUNCTIONS"
300 .IX Header "GLOBAL FUNCTIONS"
301 These functions can be called anytime, even before initialising the
302 library in any way.
303 .IP "ev_tstamp ev_time ()" 4
304 .IX Item "ev_tstamp ev_time ()"
305 Returns the current time as libev would use it. Please note that the
306 \&\f(CW\*(C`ev_now\*(C'\fR function is usually faster and also often returns the timestamp
307 you actually want to know. Also interesting is the combination of
308 \&\f(CW\*(C`ev_now_update\*(C'\fR and \f(CW\*(C`ev_now\*(C'\fR.
309 .IP "ev_sleep (ev_tstamp interval)" 4
310 .IX Item "ev_sleep (ev_tstamp interval)"
311 Sleep for the given interval: The current thread will be blocked
312 until either it is interrupted or the given time interval has
313 passed (approximately \- it might return a bit earlier even if not
314 interrupted). Returns immediately if \f(CW\*(C`interval <= 0\*(C'\fR.
315 .Sp
316 Basically this is a sub-second-resolution \f(CW\*(C`sleep ()\*(C'\fR.
317 .Sp
318 The range of the \f(CW\*(C`interval\*(C'\fR is limited \- libev only guarantees to work
319 with sleep times of up to one day (\f(CW\*(C`interval <= 86400\*(C'\fR).
320 .IP "int ev_version_major ()" 4
321 .IX Item "int ev_version_major ()"
322 .PD 0
323 .IP "int ev_version_minor ()" 4
324 .IX Item "int ev_version_minor ()"
325 .PD
326 You can find out the major and minor \s-1ABI\s0 version numbers of the library
327 you linked against by calling the functions \f(CW\*(C`ev_version_major\*(C'\fR and
328 \&\f(CW\*(C`ev_version_minor\*(C'\fR. If you want, you can compare against the global
329 symbols \f(CW\*(C`EV_VERSION_MAJOR\*(C'\fR and \f(CW\*(C`EV_VERSION_MINOR\*(C'\fR, which specify the
330 version of the library your program was compiled against.
331 .Sp
332 These version numbers refer to the \s-1ABI\s0 version of the library, not the
333 release version.
334 .Sp
335 Usually, it's a good idea to terminate if the major versions mismatch,
336 as this indicates an incompatible change. Minor versions are usually
337 compatible to older versions, so a larger minor version alone is usually
338 not a problem.
339 .Sp
340 Example: Make sure we haven't accidentally been linked against the wrong
341 version (note, however, that this will not detect other \s-1ABI\s0 mismatches,
342 such as \s-1LFS\s0 or reentrancy).
343 .Sp
344 .Vb 3
345 \& assert (("libev version mismatch",
346 \& ev_version_major () == EV_VERSION_MAJOR
347 \& && ev_version_minor () >= EV_VERSION_MINOR));
348 .Ve
349 .IP "unsigned int ev_supported_backends ()" 4
350 .IX Item "unsigned int ev_supported_backends ()"
351 Return the set of all backends (i.e. their corresponding \f(CW\*(C`EV_BACKEND_*\*(C'\fR
352 value) compiled into this binary of libev (independent of their
353 availability on the system you are running on). See \f(CW\*(C`ev_default_loop\*(C'\fR for
354 a description of the set values.
355 .Sp
356 Example: make sure we have the epoll method, because yeah this is cool and
357 a must have and can we have a torrent of it please!!!11
358 .Sp
359 .Vb 2
360 \& assert (("sorry, no epoll, no sex",
361 \& ev_supported_backends () & EVBACKEND_EPOLL));
362 .Ve
363 .IP "unsigned int ev_recommended_backends ()" 4
364 .IX Item "unsigned int ev_recommended_backends ()"
365 Return the set of all backends compiled into this binary of libev and
366 also recommended for this platform, meaning it will work for most file
367 descriptor types. This set is often smaller than the one returned by
368 \&\f(CW\*(C`ev_supported_backends\*(C'\fR, as for example kqueue is broken on most BSDs
369 and will not be auto-detected unless you explicitly request it (assuming
370 you know what you are doing). This is the set of backends that libev will
371 probe for if you specify no backends explicitly.
372 .IP "unsigned int ev_embeddable_backends ()" 4
373 .IX Item "unsigned int ev_embeddable_backends ()"
374 Returns the set of backends that are embeddable in other event loops. This
375 value is platform-specific but can include backends not available on the
376 current system. To find which embeddable backends might be supported on
377 the current system, you would need to look at \f(CW\*(C`ev_embeddable_backends ()
378 & ev_supported_backends ()\*(C'\fR, likewise for recommended ones.
379 .Sp
380 See the description of \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
381 .IP "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())" 4
382 .IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())"
383 Sets the allocation function to use (the prototype is similar \- the
384 semantics are identical to the \f(CW\*(C`realloc\*(C'\fR C89/SuS/POSIX function). It is
385 used to allocate and free memory (no surprises here). If it returns zero
386 when memory needs to be allocated (\f(CW\*(C`size != 0\*(C'\fR), the library might abort
387 or take some potentially destructive action.
388 .Sp
389 Since some systems (at least OpenBSD and Darwin) fail to implement
390 correct \f(CW\*(C`realloc\*(C'\fR semantics, libev will use a wrapper around the system
391 \&\f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions by default.
392 .Sp
393 You could override this function in high-availability programs to, say,
394 free some memory if it cannot allocate memory, to use a special allocator,
395 or even to sleep a while and retry until some memory is available.
396 .Sp
397 Example: The following is the \f(CW\*(C`realloc\*(C'\fR function that libev itself uses
398 which should work with \f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions of all kinds and
399 is probably a good basis for your own implementation.
400 .Sp
401 .Vb 5
402 \& static void *
403 \& ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
404 \& {
405 \& if (size)
406 \& return realloc (ptr, size);
407 \&
408 \& free (ptr);
409 \& return 0;
410 \& }
411 .Ve
412 .Sp
413 Example: Replace the libev allocator with one that waits a bit and then
414 retries.
415 .Sp
416 .Vb 8
417 \& static void *
418 \& persistent_realloc (void *ptr, size_t size)
419 \& {
420 \& if (!size)
421 \& {
422 \& free (ptr);
423 \& return 0;
424 \& }
425 \&
426 \& for (;;)
427 \& {
428 \& void *newptr = realloc (ptr, size);
429 \&
430 \& if (newptr)
431 \& return newptr;
432 \&
433 \& sleep (60);
434 \& }
435 \& }
436 \&
437 \& ...
438 \& ev_set_allocator (persistent_realloc);
439 .Ve
440 .IP "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())" 4
441 .IX Item "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())"
442 Set the callback function to call on a retryable system call error (such
443 as failed select, poll, epoll_wait). The message is a printable string
444 indicating the system call or subsystem causing the problem. If this
445 callback is set, then libev will expect it to remedy the situation, no
446 matter what, when it returns. That is, libev will generally retry the
447 requested operation, or, if the condition doesn't go away, do bad stuff
448 (such as abort).
449 .Sp
450 Example: This is basically the same thing that libev does internally, too.
451 .Sp
452 .Vb 6
453 \& static void
454 \& fatal_error (const char *msg)
455 \& {
456 \& perror (msg);
457 \& abort ();
458 \& }
459 \&
460 \& ...
461 \& ev_set_syserr_cb (fatal_error);
462 .Ve
463 .IP "ev_feed_signal (int signum)" 4
464 .IX Item "ev_feed_signal (int signum)"
465 This function can be used to \*(L"simulate\*(R" a signal receive. It is completely
466 safe to call this function at any time, from any context, including signal
467 handlers or random threads.
468 .Sp
469 Its main use is to customise signal handling in your process, especially
470 in the presence of threads. For example, you could block signals
471 by default in all threads (and specifying \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when
472 creating any loops), and in one thread, use \f(CW\*(C`sigwait\*(C'\fR or any other
473 mechanism to wait for signals, then \*(L"deliver\*(R" them to libev by calling
474 \&\f(CW\*(C`ev_feed_signal\*(C'\fR.
475 .SH "FUNCTIONS CONTROLLING EVENT LOOPS"
476 .IX Header "FUNCTIONS CONTROLLING EVENT LOOPS"
477 An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR (the \f(CW\*(C`struct\*(C'\fR is
478 \&\fInot\fR optional in this case unless libev 3 compatibility is disabled, as
479 libev 3 had an \f(CW\*(C`ev_loop\*(C'\fR function colliding with the struct name).
480 .PP
481 The library knows two types of such loops, the \fIdefault\fR loop, which
482 supports child process events, and dynamically created event loops which
483 do not.
484 .IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4
485 .IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"
486 This returns the \*(L"default\*(R" event loop object, which is what you should
487 normally use when you just need \*(L"the event loop\*(R". Event loop objects and
488 the \f(CW\*(C`flags\*(C'\fR parameter are described in more detail in the entry for
489 \&\f(CW\*(C`ev_loop_new\*(C'\fR.
490 .Sp
491 If the default loop is already initialised then this function simply
492 returns it (and ignores the flags. If that is troubling you, check
493 \&\f(CW\*(C`ev_backend ()\*(C'\fR afterwards). Otherwise it will create it with the given
494 flags, which should almost always be \f(CW0\fR, unless the caller is also the
495 one calling \f(CW\*(C`ev_run\*(C'\fR or otherwise qualifies as \*(L"the main program\*(R".
496 .Sp
497 If you don't know what event loop to use, use the one returned from this
498 function (or via the \f(CW\*(C`EV_DEFAULT\*(C'\fR macro).
499 .Sp
500 Note that this function is \fInot\fR thread-safe, so if you want to use it
501 from multiple threads, you have to employ some kind of mutex (note also
502 that this case is unlikely, as loops cannot be shared easily between
503 threads anyway).
504 .Sp
505 The default loop is the only loop that can handle \f(CW\*(C`ev_child\*(C'\fR watchers,
506 and to do this, it always registers a handler for \f(CW\*(C`SIGCHLD\*(C'\fR. If this is
507 a problem for your application you can either create a dynamic loop with
508 \&\f(CW\*(C`ev_loop_new\*(C'\fR which doesn't do that, or you can simply overwrite the
509 \&\f(CW\*(C`SIGCHLD\*(C'\fR signal handler \fIafter\fR calling \f(CW\*(C`ev_default_init\*(C'\fR.
510 .Sp
511 Example: This is the most typical usage.
512 .Sp
513 .Vb 2
514 \& if (!ev_default_loop (0))
515 \& fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
516 .Ve
517 .Sp
518 Example: Restrict libev to the select and poll backends, and do not allow
519 environment settings to be taken into account:
520 .Sp
521 .Vb 1
522 \& ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
523 .Ve
524 .IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4
525 .IX Item "struct ev_loop *ev_loop_new (unsigned int flags)"
526 This will create and initialise a new event loop object. If the loop
527 could not be initialised, returns false.
528 .Sp
529 This function is thread-safe, and one common way to use libev with
530 threads is indeed to create one loop per thread, and using the default
531 loop in the \*(L"main\*(R" or \*(L"initial\*(R" thread.
532 .Sp
533 The flags argument can be used to specify special behaviour or specific
534 backends to use, and is usually specified as \f(CW0\fR (or \f(CW\*(C`EVFLAG_AUTO\*(C'\fR).
535 .Sp
536 The following flags are supported:
537 .RS 4
538 .ie n .IP """EVFLAG_AUTO""" 4
539 .el .IP "\f(CWEVFLAG_AUTO\fR" 4
540 .IX Item "EVFLAG_AUTO"
541 The default flags value. Use this if you have no clue (it's the right
542 thing, believe me).
543 .ie n .IP """EVFLAG_NOENV""" 4
544 .el .IP "\f(CWEVFLAG_NOENV\fR" 4
545 .IX Item "EVFLAG_NOENV"
546 If this flag bit is or'ed into the flag value (or the program runs setuid
547 or setgid) then libev will \fInot\fR look at the environment variable
548 \&\f(CW\*(C`LIBEV_FLAGS\*(C'\fR. Otherwise (the default), this environment variable will
549 override the flags completely if it is found in the environment. This is
550 useful to try out specific backends to test their performance, to work
551 around bugs, or to make libev threadsafe (accessing environment variables
552 cannot be done in a threadsafe way, but usually it works if no other
553 thread modifies them).
554 .ie n .IP """EVFLAG_FORKCHECK""" 4
555 .el .IP "\f(CWEVFLAG_FORKCHECK\fR" 4
556 .IX Item "EVFLAG_FORKCHECK"
557 Instead of calling \f(CW\*(C`ev_loop_fork\*(C'\fR manually after a fork, you can also
558 make libev check for a fork in each iteration by enabling this flag.
559 .Sp
560 This works by calling \f(CW\*(C`getpid ()\*(C'\fR on every iteration of the loop,
561 and thus this might slow down your event loop if you do a lot of loop
562 iterations and little real work, but is usually not noticeable (on my
563 GNU/Linux system for example, \f(CW\*(C`getpid\*(C'\fR is actually a simple 5\-insn
564 sequence without a system call and thus \fIvery\fR fast, but my GNU/Linux
565 system also has \f(CW\*(C`pthread_atfork\*(C'\fR which is even faster). (Update: glibc
566 versions 2.25 apparently removed the \f(CW\*(C`getpid\*(C'\fR optimisation again).
567 .Sp
568 The big advantage of this flag is that you can forget about fork (and
569 forget about forgetting to tell libev about forking, although you still
570 have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR) when you use this flag.
571 .Sp
572 This flag setting cannot be overridden or specified in the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR
573 environment variable.
574 .ie n .IP """EVFLAG_NOINOTIFY""" 4
575 .el .IP "\f(CWEVFLAG_NOINOTIFY\fR" 4
576 .IX Item "EVFLAG_NOINOTIFY"
577 When this flag is specified, then libev will not attempt to use the
578 \&\fIinotify\fR \s-1API\s0 for its \f(CW\*(C`ev_stat\*(C'\fR watchers. Apart from debugging and
579 testing, this flag can be useful to conserve inotify file descriptors, as
580 otherwise each loop using \f(CW\*(C`ev_stat\*(C'\fR watchers consumes one inotify handle.
581 .ie n .IP """EVFLAG_SIGNALFD""" 4
582 .el .IP "\f(CWEVFLAG_SIGNALFD\fR" 4
583 .IX Item "EVFLAG_SIGNALFD"
584 When this flag is specified, then libev will attempt to use the
585 \&\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
586 delivers signals synchronously, which makes it both faster and might make
587 it possible to get the queued signal data. It can also simplify signal
588 handling with threads, as long as you properly block signals in your
589 threads that are not interested in handling them.
590 .Sp
591 Signalfd will not be used by default as this changes your signal mask, and
592 there are a lot of shoddy libraries and programs (glib's threadpool for
593 example) that can't properly initialise their signal masks.
594 .ie n .IP """EVFLAG_NOSIGMASK""" 4
595 .el .IP "\f(CWEVFLAG_NOSIGMASK\fR" 4
596 .IX Item "EVFLAG_NOSIGMASK"
597 When this flag is specified, then libev will avoid to modify the signal
598 mask. Specifically, this means you have to make sure signals are unblocked
599 when you want to receive them.
600 .Sp
601 This behaviour is useful when you want to do your own signal handling, or
602 want to handle signals only in specific threads and want to avoid libev
603 unblocking the signals.
604 .Sp
605 It's also required by \s-1POSIX\s0 in a threaded program, as libev calls
606 \&\f(CW\*(C`sigprocmask\*(C'\fR, whose behaviour is officially unspecified.
607 .Sp
608 This flag's behaviour will become the default in future versions of libev.
609 .ie n .IP """EVBACKEND_SELECT"" (value 1, portable select backend)" 4
610 .el .IP "\f(CWEVBACKEND_SELECT\fR (value 1, portable select backend)" 4
611 .IX Item "EVBACKEND_SELECT (value 1, portable select backend)"
612 This is your standard \fBselect\fR\|(2) backend. Not \fIcompletely\fR standard, as
613 libev tries to roll its own fd_set with no limits on the number of fds,
614 but if that fails, expect a fairly low limit on the number of fds when
615 using this backend. It doesn't scale too well (O(highest_fd)), but its
616 usually the fastest backend for a low number of (low-numbered :) fds.
617 .Sp
618 To get good performance out of this backend you need a high amount of
619 parallelism (most of the file descriptors should be busy). If you are
620 writing a server, you should \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many
621 connections as possible during one iteration. You might also want to have
622 a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of
623 readiness notifications you get per iteration.
624 .Sp
625 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
626 \&\f(CW\*(C`writefds\*(C'\fR set (and to work around Microsoft Windows bugs, also onto the
627 \&\f(CW\*(C`exceptfds\*(C'\fR set on that platform).
628 .ie n .IP """EVBACKEND_POLL"" (value 2, poll backend, available everywhere except on windows)" 4
629 .el .IP "\f(CWEVBACKEND_POLL\fR (value 2, poll backend, available everywhere except on windows)" 4
630 .IX Item "EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)"
631 And this is your standard \fBpoll\fR\|(2) backend. It's more complicated
632 than select, but handles sparse fds better and has no artificial
633 limit on the number of fds you can use (except it will slow down
634 considerably with a lot of inactive fds). It scales similarly to select,
635 i.e. O(total_fds). See the entry for \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for
636 performance tips.
637 .Sp
638 This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLLHUP\*(C'\fR, and
639 \&\f(CW\*(C`EV_WRITE\*(C'\fR to \f(CW\*(C`POLLOUT | POLLERR | POLLHUP\*(C'\fR.
640 .ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4
641 .el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4
642 .IX Item "EVBACKEND_EPOLL (value 4, Linux)"
643 Use the linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9
644 kernels).
645 .Sp
646 For few fds, this backend is a bit little slower than poll and select, but
647 it scales phenomenally better. While poll and select usually scale like
648 O(total_fds) where total_fds is the total number of fds (or the highest
649 fd), epoll scales either O(1) or O(active_fds).
650 .Sp
651 The epoll mechanism deserves honorable mention as the most misdesigned
652 of the more advanced event mechanisms: mere annoyances include silently
653 dropping file descriptors, requiring a system call per change per file
654 descriptor (and unnecessary guessing of parameters), problems with dup,
655 returning before the timeout value, resulting in additional iterations
656 (and only giving 5ms accuracy while select on the same platform gives
657 0.1ms) and so on. The biggest issue is fork races, however \- if a program
658 forks then \fIboth\fR parent and child process have to recreate the epoll
659 set, which can take considerable time (one syscall per file descriptor)
660 and is of course hard to detect.
661 .Sp
662 Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work,
663 but of course \fIdoesn't\fR, and epoll just loves to report events for
664 totally \fIdifferent\fR file descriptors (even already closed ones, so
665 one cannot even remove them from the set) than registered in the set
666 (especially on \s-1SMP\s0 systems). Libev tries to counter these spurious
667 notifications by employing an additional generation counter and comparing
668 that against the events to filter out spurious ones, recreating the set
669 when required. Epoll also erroneously rounds down timeouts, but gives you
670 no way to know when and by how much, so sometimes you have to busy-wait
671 because epoll returns immediately despite a nonzero timeout. And last
672 not least, it also refuses to work with some file descriptors which work
673 perfectly fine with \f(CW\*(C`select\*(C'\fR (files, many character devices...).
674 .Sp
675 Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
676 cobbled together in a hurry, no thought to design or interaction with
677 others. Oh, the pain, will it ever stop...
678 .Sp
679 While stopping, setting and starting an I/O watcher in the same iteration
680 will result in some caching, there is still a system call per such
681 incident (because the same \fIfile descriptor\fR could point to a different
682 \&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed
683 file descriptors might not work very well if you register events for both
684 file descriptors.
685 .Sp
686 Best performance from this backend is achieved by not unregistering all
687 watchers for a file descriptor until it has been closed, if possible,
688 i.e. keep at least one watcher active per fd at all times. Stopping and
689 starting a watcher (without re-setting it) also usually doesn't cause
690 extra overhead. A fork can both result in spurious notifications as well
691 as in libev having to destroy and recreate the epoll object, which can
692 take considerable time and thus should be avoided.
693 .Sp
694 All this means that, in practice, \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR can be as fast or
695 faster than epoll for maybe up to a hundred file descriptors, depending on
696 the usage. So sad.
697 .Sp
698 While nominally embeddable in other event loops, this feature is broken in
699 a lot of kernel revisions, but probably(!) works in current versions.
700 .Sp
701 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
702 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
703 .ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4
704 .el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4
705 .IX Item "EVBACKEND_LINUXAIO (value 64, Linux)"
706 Use the linux-specific linux aio (\fInot\fR \f(CWaio(7)\fR) event interface
707 available in post\-4.18 kernels.
708 .Sp
709 If this backend works for you (as of this writing, it was very
710 experimental and only supports a subset of file types), it is the best
711 event interface available on linux and might be well worth it enabling it
712 \&\- if it isn't available in your kernel this will be detected and another
713 backend will be chosen.
714 .Sp
715 This backend can batch oneshot requests and uses a user-space ring buffer
716 to receive events. It also doesn't suffer from most of the design problems
717 of epoll (such as not being able to remove event sources from the epoll
718 set), and generally sounds too good to be true. Because, this being the
719 linux kernel, of course it suffers from a whole new set of limitations.
720 .Sp
721 For one, it is not easily embeddable (but probably could be done using
722 an event fd at some extra overhead). It also is subject to various
723 arbitrary limits that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR
724 and \fI/proc/sys/fs/aio\-nr\fR), which could lead to it being skipped during
725 initialisation.
726 .Sp
727 Most problematic in practise, however, is that, like kqueue, it requires
728 special support from drivers, and, not surprisingly, not all drivers
729 implement it. For example, in linux 4.19, tcp sockets, pipes, event fds,
730 files, \fI/dev/null\fR and a few others are supported, but ttys are not, so
731 this is not (yet?) a generic event polling interface but is probably still
732 be very useful in a web server or similar program.
733 .Sp
734 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
735 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
736 .ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
737 .el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4
738 .IX Item "EVBACKEND_KQUEUE (value 8, most BSD clones)"
739 Kqueue deserves special mention, as at the time of this writing, it
740 was broken on all BSDs except NetBSD (usually it doesn't work reliably
741 with anything but sockets and pipes, except on Darwin, where of course
742 it's completely useless). Unlike epoll, however, whose brokenness
743 is by design, these kqueue bugs can (and eventually will) be fixed
744 without \s-1API\s0 changes to existing programs. For this reason it's not being
745 \&\*(L"auto-detected\*(R" unless you explicitly specify it in the flags (i.e. using
746 \&\f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a known-to-be-good (\-enough)
747 system like NetBSD.
748 .Sp
749 You still can embed kqueue into a normal poll or select backend and use it
750 only for sockets (after having made sure that sockets work with kqueue on
751 the target platform). See \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
752 .Sp
753 It scales in the same way as the epoll backend, but the interface to the
754 kernel is more efficient (which says nothing about its actual speed, of
755 course). While stopping, setting and starting an I/O watcher does never
756 cause an extra system call as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to
757 two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you
758 might have to leak fd's on fork, but it's more sane than epoll) and it
759 drops fds silently in similarly hard-to-detect cases.
760 .Sp
761 This backend usually performs well under most conditions.
762 .Sp
763 While nominally embeddable in other event loops, this doesn't work
764 everywhere, so you might need to test for this. And since it is broken
765 almost everywhere, you should only use it when you have a lot of sockets
766 (for which it usually works), by embedding it into another event loop
767 (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
768 also broken on \s-1OS X\s0)) and, did I mention it, using it only for sockets.
769 .Sp
770 This backend maps \f(CW\*(C`EV_READ\*(C'\fR into an \f(CW\*(C`EVFILT_READ\*(C'\fR kevent with
771 \&\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
772 \&\f(CW\*(C`NOTE_EOF\*(C'\fR.
773 .ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4
774 .el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4
775 .IX Item "EVBACKEND_DEVPOLL (value 16, Solaris 8)"
776 This is not implemented yet (and might never be, unless you send me an
777 implementation). According to reports, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets
778 and is not embeddable, which would limit the usefulness of this backend
779 immensely.
780 .ie n .IP """EVBACKEND_PORT"" (value 32, Solaris 10)" 4
781 .el .IP "\f(CWEVBACKEND_PORT\fR (value 32, Solaris 10)" 4
782 .IX Item "EVBACKEND_PORT (value 32, Solaris 10)"
783 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
784 it's really slow, but it still scales very well (O(active_fds)).
785 .Sp
786 While this backend scales well, it requires one system call per active
787 file descriptor per loop iteration. For small and medium numbers of file
788 descriptors a \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend
789 might perform better.
790 .Sp
791 On the positive side, this backend actually performed fully to
792 specification in all tests and is fully embeddable, which is a rare feat
793 among the OS-specific backends (I vastly prefer correctness over speed
794 hacks).
795 .Sp
796 On the negative side, the interface is \fIbizarre\fR \- so bizarre that
797 even sun itself gets it wrong in their code examples: The event polling
798 function sometimes returns events to the caller even though an error
799 occurred, but with no indication whether it has done so or not (yes, it's
800 even documented that way) \- deadly for edge-triggered interfaces where you
801 absolutely have to know whether an event occurred or not because you have
802 to re-arm the watcher.
803 .Sp
804 Fortunately libev seems to be able to work around these idiocies.
805 .Sp
806 This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
807 \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
808 .ie n .IP """EVBACKEND_ALL""" 4
809 .el .IP "\f(CWEVBACKEND_ALL\fR" 4
810 .IX Item "EVBACKEND_ALL"
811 Try all backends (even potentially broken ones that wouldn't be tried
812 with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as
813 \&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR.
814 .Sp
815 It is definitely not recommended to use this flag, use whatever
816 \&\f(CW\*(C`ev_recommended_backends ()\*(C'\fR returns, or simply do not specify a backend
817 at all.
818 .ie n .IP """EVBACKEND_MASK""" 4
819 .el .IP "\f(CWEVBACKEND_MASK\fR" 4
820 .IX Item "EVBACKEND_MASK"
821 Not a backend at all, but a mask to select all backend bits from a
822 \&\f(CW\*(C`flags\*(C'\fR value, in case you want to mask out any backends from a flags
823 value (e.g. when modifying the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR environment variable).
824 .RE
825 .RS 4
826 .Sp
827 If one or more of the backend flags are or'ed into the flags value,
828 then only these backends will be tried (in the reverse order as listed
829 here). If none are specified, all backends in \f(CW\*(C`ev_recommended_backends
830 ()\*(C'\fR will be tried.
831 .Sp
832 Example: Try to create a event loop that uses epoll and nothing else.
833 .Sp
834 .Vb 3
835 \& struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
836 \& if (!epoller)
837 \& fatal ("no epoll found here, maybe it hides under your chair");
838 .Ve
839 .Sp
840 Example: Use whatever libev has to offer, but make sure that kqueue is
841 used if available.
842 .Sp
843 .Vb 1
844 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
845 .Ve
846 .Sp
847 Example: Similarly, on linux, you mgiht want to take advantage of the
848 linux aio backend if possible, but fall back to something else if that
849 isn't available.
850 .Sp
851 .Vb 1
852 \& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
853 .Ve
854 .RE
855 .IP "ev_loop_destroy (loop)" 4
856 .IX Item "ev_loop_destroy (loop)"
857 Destroys an event loop object (frees all memory and kernel state
858 etc.). None of the active event watchers will be stopped in the normal
859 sense, so e.g. \f(CW\*(C`ev_is_active\*(C'\fR might still return true. It is your
860 responsibility to either stop all watchers cleanly yourself \fIbefore\fR
861 calling this function, or cope with the fact afterwards (which is usually
862 the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them
863 for example).
864 .Sp
865 Note that certain global state, such as signal state (and installed signal
866 handlers), will not be freed by this function, and related watchers (such
867 as signal and child watchers) would need to be stopped manually.
868 .Sp
869 This function is normally used on loop objects allocated by
870 \&\f(CW\*(C`ev_loop_new\*(C'\fR, but it can also be used on the default loop returned by
871 \&\f(CW\*(C`ev_default_loop\*(C'\fR, in which case it is not thread-safe.
872 .Sp
873 Note that it is not advisable to call this function on the default loop
874 except in the rare occasion where you really need to free its resources.
875 If you need dynamically allocated loops it is better to use \f(CW\*(C`ev_loop_new\*(C'\fR
876 and \f(CW\*(C`ev_loop_destroy\*(C'\fR.
877 .IP "ev_loop_fork (loop)" 4
878 .IX Item "ev_loop_fork (loop)"
879 This function sets a flag that causes subsequent \f(CW\*(C`ev_run\*(C'\fR iterations
880 to reinitialise the kernel state for backends that have one. Despite
881 the name, you can call it anytime you are allowed to start or stop
882 watchers (except inside an \f(CW\*(C`ev_prepare\*(C'\fR callback), but it makes most
883 sense after forking, in the child process. You \fImust\fR call it (or use
884 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR) in the child before resuming or calling \f(CW\*(C`ev_run\*(C'\fR.
885 .Sp
886 In addition, if you want to reuse a loop (via this function or
887 \&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR), you \fIalso\fR have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR.
888 .Sp
889 Again, you \fIhave\fR to call it on \fIany\fR loop that you want to re-use after
890 a fork, \fIeven if you do not plan to use the loop in the parent\fR. This is
891 because some kernel interfaces *cough* \fIkqueue\fR *cough* do funny things
892 during fork.
893 .Sp
894 On the other hand, you only need to call this function in the child
895 process if and only if you want to use the event loop in the child. If
896 you just fork+exec or create a new loop in the child, you don't have to
897 call it at all (in fact, \f(CW\*(C`epoll\*(C'\fR is so badly broken that it makes a
898 difference, but libev will usually detect this case on its own and do a
899 costly reset of the backend).
900 .Sp
901 The function itself is quite fast and it's usually not a problem to call
902 it just in case after a fork.
903 .Sp
904 Example: Automate calling \f(CW\*(C`ev_loop_fork\*(C'\fR on the default loop when
905 using pthreads.
906 .Sp
907 .Vb 5
908 \& static void
909 \& post_fork_child (void)
910 \& {
911 \& ev_loop_fork (EV_DEFAULT);
912 \& }
913 \&
914 \& ...
915 \& pthread_atfork (0, 0, post_fork_child);
916 .Ve
917 .IP "int ev_is_default_loop (loop)" 4
918 .IX Item "int ev_is_default_loop (loop)"
919 Returns true when the given loop is, in fact, the default loop, and false
920 otherwise.
921 .IP "unsigned int ev_iteration (loop)" 4
922 .IX Item "unsigned int ev_iteration (loop)"
923 Returns the current iteration count for the event loop, which is identical
924 to the number of times libev did poll for new events. It starts at \f(CW0\fR
925 and happily wraps around with enough iterations.
926 .Sp
927 This value can sometimes be useful as a generation counter of sorts (it
928 \&\*(L"ticks\*(R" the number of loop iterations), as it roughly corresponds with
929 \&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls \- and is incremented between the
930 prepare and check phases.
931 .IP "unsigned int ev_depth (loop)" 4
932 .IX Item "unsigned int ev_depth (loop)"
933 Returns the number of times \f(CW\*(C`ev_run\*(C'\fR was entered minus the number of
934 times \f(CW\*(C`ev_run\*(C'\fR was exited normally, in other words, the recursion depth.
935 .Sp
936 Outside \f(CW\*(C`ev_run\*(C'\fR, this number is zero. In a callback, this number is
937 \&\f(CW1\fR, unless \f(CW\*(C`ev_run\*(C'\fR was invoked recursively (or from another thread),
938 in which case it is higher.
939 .Sp
940 Leaving \f(CW\*(C`ev_run\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread,
941 throwing an exception etc.), doesn't count as \*(L"exit\*(R" \- consider this
942 as a hint to avoid such ungentleman-like behaviour unless it's really
943 convenient, in which case it is fully supported.
944 .IP "unsigned int ev_backend (loop)" 4
945 .IX Item "unsigned int ev_backend (loop)"
946 Returns one of the \f(CW\*(C`EVBACKEND_*\*(C'\fR flags indicating the event backend in
947 use.
948 .IP "ev_tstamp ev_now (loop)" 4
949 .IX Item "ev_tstamp ev_now (loop)"
950 Returns the current \*(L"event loop time\*(R", which is the time the event loop
951 received events and started processing them. This timestamp does not
952 change as long as callbacks are being processed, and this is also the base
953 time used for relative timers. You can treat it as the timestamp of the
954 event occurring (or more correctly, libev finding out about it).
955 .IP "ev_now_update (loop)" 4
956 .IX Item "ev_now_update (loop)"
957 Establishes the current time by querying the kernel, updating the time
958 returned by \f(CW\*(C`ev_now ()\*(C'\fR in the progress. This is a costly operation and
959 is usually done automatically within \f(CW\*(C`ev_run ()\*(C'\fR.
960 .Sp
961 This function is rarely useful, but when some event callback runs for a
962 very long time without entering the event loop, updating libev's idea of
963 the current time is a good idea.
964 .Sp
965 See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.
966 .IP "ev_suspend (loop)" 4
967 .IX Item "ev_suspend (loop)"
968 .PD 0
969 .IP "ev_resume (loop)" 4
970 .IX Item "ev_resume (loop)"
971 .PD
972 These two functions suspend and resume an event loop, for use when the
973 loop is not used for a while and timeouts should not be processed.
974 .Sp
975 A typical use case would be an interactive program such as a game: When
976 the user presses \f(CW\*(C`^Z\*(C'\fR to suspend the game and resumes it an hour later it
977 would be best to handle timeouts as if no time had actually passed while
978 the program was suspended. This can be achieved by calling \f(CW\*(C`ev_suspend\*(C'\fR
979 in your \f(CW\*(C`SIGTSTP\*(C'\fR handler, sending yourself a \f(CW\*(C`SIGSTOP\*(C'\fR and calling
980 \&\f(CW\*(C`ev_resume\*(C'\fR directly afterwards to resume timer processing.
981 .Sp
982 Effectively, all \f(CW\*(C`ev_timer\*(C'\fR watchers will be delayed by the time spend
983 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
984 will be rescheduled (that is, they will lose any events that would have
985 occurred while suspended).
986 .Sp
987 After calling \f(CW\*(C`ev_suspend\*(C'\fR you \fBmust not\fR call \fIany\fR function on the
988 given loop other than \f(CW\*(C`ev_resume\*(C'\fR, and you \fBmust not\fR call \f(CW\*(C`ev_resume\*(C'\fR
989 without a previous call to \f(CW\*(C`ev_suspend\*(C'\fR.
990 .Sp
991 Calling \f(CW\*(C`ev_suspend\*(C'\fR/\f(CW\*(C`ev_resume\*(C'\fR has the side effect of updating the
992 event loop time (see \f(CW\*(C`ev_now_update\*(C'\fR).
993 .IP "bool ev_run (loop, int flags)" 4
994 .IX Item "bool ev_run (loop, int flags)"
995 Finally, this is it, the event handler. This function usually is called
996 after you have initialised all your watchers and you want to start
997 handling events. It will ask the operating system for any new events, call
998 the watcher callbacks, and then repeat the whole process indefinitely: This
999 is why event loops are called \fIloops\fR.
1000 .Sp
1001 If the flags argument is specified as \f(CW0\fR, it will keep handling events
1002 until either no event watchers are active anymore or \f(CW\*(C`ev_break\*(C'\fR was
1003 called.
1004 .Sp
1005 The return value is false if there are no more active watchers (which
1006 usually means \*(L"all jobs done\*(R" or \*(L"deadlock\*(R"), and true in all other cases
1007 (which usually means " you should call \f(CW\*(C`ev_run\*(C'\fR again").
1008 .Sp
1009 Please note that an explicit \f(CW\*(C`ev_break\*(C'\fR is usually better than
1010 relying on all watchers to be stopped when deciding when a program has
1011 finished (especially in interactive programs), but having a program
1012 that automatically loops as long as it has to and no longer by virtue
1013 of relying on its watchers stopping correctly, that is truly a thing of
1014 beauty.
1015 .Sp
1016 This function is \fImostly\fR exception-safe \- you can break out of a
1017 \&\f(CW\*(C`ev_run\*(C'\fR call by calling \f(CW\*(C`longjmp\*(C'\fR in a callback, throwing a \*(C+
1018 exception and so on. This does not decrement the \f(CW\*(C`ev_depth\*(C'\fR value, nor
1019 will it clear any outstanding \f(CW\*(C`EVBREAK_ONE\*(C'\fR breaks.
1020 .Sp
1021 A flags value of \f(CW\*(C`EVRUN_NOWAIT\*(C'\fR will look for new events, will handle
1022 those events and any already outstanding ones, but will not wait and
1023 block your process in case there are no events and will return after one
1024 iteration of the loop. This is sometimes useful to poll and handle new
1025 events while doing lengthy calculations, to keep the program responsive.
1026 .Sp
1027 A flags value of \f(CW\*(C`EVRUN_ONCE\*(C'\fR will look for new events (waiting if
1028 necessary) and will handle those and any already outstanding ones. It
1029 will block your process until at least one new event arrives (which could
1030 be an event internal to libev itself, so there is no guarantee that a
1031 user-registered callback will be called), and will return after one
1032 iteration of the loop.
1033 .Sp
1034 This is useful if you are waiting for some external event in conjunction
1035 with something not expressible using other libev watchers (i.e. "roll your
1036 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
1037 usually a better approach for this kind of thing.
1038 .Sp
1039 Here are the gory details of what \f(CW\*(C`ev_run\*(C'\fR does (this is for your
1040 understanding, not a guarantee that things will work exactly like this in
1041 future versions):
1042 .Sp
1043 .Vb 10
1044 \& \- Increment loop depth.
1045 \& \- Reset the ev_break status.
1046 \& \- Before the first iteration, call any pending watchers.
1047 \& LOOP:
1048 \& \- If EVFLAG_FORKCHECK was used, check for a fork.
1049 \& \- If a fork was detected (by any means), queue and call all fork watchers.
1050 \& \- Queue and call all prepare watchers.
1051 \& \- If ev_break was called, goto FINISH.
1052 \& \- If we have been forked, detach and recreate the kernel state
1053 \& as to not disturb the other process.
1054 \& \- Update the kernel state with all outstanding changes.
1055 \& \- Update the "event loop time" (ev_now ()).
1056 \& \- Calculate for how long to sleep or block, if at all
1057 \& (active idle watchers, EVRUN_NOWAIT or not having
1058 \& any active watchers at all will result in not sleeping).
1059 \& \- Sleep if the I/O and timer collect interval say so.
1060 \& \- Increment loop iteration counter.
1061 \& \- Block the process, waiting for any events.
1062 \& \- Queue all outstanding I/O (fd) events.
1063 \& \- Update the "event loop time" (ev_now ()), and do time jump adjustments.
1064 \& \- Queue all expired timers.
1065 \& \- Queue all expired periodics.
1066 \& \- Queue all idle watchers with priority higher than that of pending events.
1067 \& \- Queue all check watchers.
1068 \& \- Call all queued watchers in reverse order (i.e. check watchers first).
1069 \& Signals and child watchers are implemented as I/O watchers, and will
1070 \& be handled here by queueing them when their watcher gets executed.
1071 \& \- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
1072 \& were used, or there are no active watchers, goto FINISH, otherwise
1073 \& continue with step LOOP.
1074 \& FINISH:
1075 \& \- Reset the ev_break status iff it was EVBREAK_ONE.
1076 \& \- Decrement the loop depth.
1077 \& \- Return.
1078 .Ve
1079 .Sp
1080 Example: Queue some jobs and then loop until no events are outstanding
1081 anymore.
1082 .Sp
1083 .Vb 4
1084 \& ... queue jobs here, make sure they register event watchers as long
1085 \& ... as they still have work to do (even an idle watcher will do..)
1086 \& ev_run (my_loop, 0);
1087 \& ... jobs done or somebody called break. yeah!
1088 .Ve
1089 .IP "ev_break (loop, how)" 4
1090 .IX Item "ev_break (loop, how)"
1091 Can be used to make a call to \f(CW\*(C`ev_run\*(C'\fR return early (but only after it
1092 has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either
1093 \&\f(CW\*(C`EVBREAK_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_run\*(C'\fR call return, or
1094 \&\f(CW\*(C`EVBREAK_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_run\*(C'\fR calls return.
1095 .Sp
1096 This \*(L"break state\*(R" will be cleared on the next call to \f(CW\*(C`ev_run\*(C'\fR.
1097 .Sp
1098 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
1099 which case it will have no effect.
1100 .IP "ev_ref (loop)" 4
1101 .IX Item "ev_ref (loop)"
1102 .PD 0
1103 .IP "ev_unref (loop)" 4
1104 .IX Item "ev_unref (loop)"
1105 .PD
1106 Ref/unref can be used to add or remove a reference count on the event
1107 loop: Every watcher keeps one reference, and as long as the reference
1108 count is nonzero, \f(CW\*(C`ev_run\*(C'\fR will not return on its own.
1109 .Sp
1110 This is useful when you have a watcher that you never intend to
1111 unregister, but that nevertheless should not keep \f(CW\*(C`ev_run\*(C'\fR from
1112 returning. In such a case, call \f(CW\*(C`ev_unref\*(C'\fR after starting, and \f(CW\*(C`ev_ref\*(C'\fR
1113 before stopping it.
1114 .Sp
1115 As an example, libev itself uses this for its internal signal pipe: It
1116 is not visible to the libev user and should not keep \f(CW\*(C`ev_run\*(C'\fR from
1117 exiting if no event watchers registered by it are active. It is also an
1118 excellent way to do this for generic recurring timers or from within
1119 third-party libraries. Just remember to \fIunref after start\fR and \fIref
1120 before stop\fR (but only if the watcher wasn't active before, or was active
1121 before, respectively. Note also that libev might stop watchers itself
1122 (e.g. non-repeating timers) in which case you have to \f(CW\*(C`ev_ref\*(C'\fR
1123 in the callback).
1124 .Sp
1125 Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_run\*(C'\fR
1126 running when nothing else is active.
1127 .Sp
1128 .Vb 4
1129 \& ev_signal exitsig;
1130 \& ev_signal_init (&exitsig, sig_cb, SIGINT);
1131 \& ev_signal_start (loop, &exitsig);
1132 \& ev_unref (loop);
1133 .Ve
1134 .Sp
1135 Example: For some weird reason, unregister the above signal handler again.
1136 .Sp
1137 .Vb 2
1138 \& ev_ref (loop);
1139 \& ev_signal_stop (loop, &exitsig);
1140 .Ve
1141 .IP "ev_set_io_collect_interval (loop, ev_tstamp interval)" 4
1142 .IX Item "ev_set_io_collect_interval (loop, ev_tstamp interval)"
1143 .PD 0
1144 .IP "ev_set_timeout_collect_interval (loop, ev_tstamp interval)" 4
1145 .IX Item "ev_set_timeout_collect_interval (loop, ev_tstamp interval)"
1146 .PD
1147 These advanced functions influence the time that libev will spend waiting
1148 for events. Both time intervals are by default \f(CW0\fR, meaning that libev
1149 will try to invoke timer/periodic callbacks and I/O callbacks with minimum
1150 latency.
1151 .Sp
1152 Setting these to a higher value (the \f(CW\*(C`interval\*(C'\fR \fImust\fR be >= \f(CW0\fR)
1153 allows libev to delay invocation of I/O and timer/periodic callbacks
1154 to increase efficiency of loop iterations (or to increase power-saving
1155 opportunities).
1156 .Sp
1157 The idea is that sometimes your program runs just fast enough to handle
1158 one (or very few) event(s) per loop iteration. While this makes the
1159 program responsive, it also wastes a lot of \s-1CPU\s0 time to poll for new
1160 events, especially with backends like \f(CW\*(C`select ()\*(C'\fR which have a high
1161 overhead for the actual polling but can deliver many events at once.
1162 .Sp
1163 By setting a higher \fIio collect interval\fR you allow libev to spend more
1164 time collecting I/O events, so you can handle more events per iteration,
1165 at the cost of increasing latency. Timeouts (both \f(CW\*(C`ev_periodic\*(C'\fR and
1166 \&\f(CW\*(C`ev_timer\*(C'\fR) will not be affected. Setting this to a non-null value will
1167 introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. The
1168 sleep time ensures that libev will not poll for I/O events more often then
1169 once per this interval, on average (as long as the host time resolution is
1170 good enough).
1171 .Sp
1172 Likewise, by setting a higher \fItimeout collect interval\fR you allow libev
1173 to spend more time collecting timeouts, at the expense of increased
1174 latency/jitter/inexactness (the watcher callback will be called
1175 later). \f(CW\*(C`ev_io\*(C'\fR watchers will not be affected. Setting this to a non-null
1176 value will not introduce any overhead in libev.
1177 .Sp
1178 Many (busy) programs can usually benefit by setting the I/O collect
1179 interval to a value near \f(CW0.1\fR or so, which is often enough for
1180 interactive servers (of course not for games), likewise for timeouts. It
1181 usually doesn't make much sense to set it to a lower value than \f(CW0.01\fR,
1182 as this approaches the timing granularity of most systems. Note that if
1183 you do transactions with the outside world and you can't increase the
1184 parallelity, then this setting will limit your transaction rate (if you
1185 need to poll once per transaction and the I/O collect interval is 0.01,
1186 then you can't do more than 100 transactions per second).
1187 .Sp
1188 Setting the \fItimeout collect interval\fR can improve the opportunity for
1189 saving power, as the program will \*(L"bundle\*(R" timer callback invocations that
1190 are \*(L"near\*(R" in time together, by delaying some, thus reducing the number of
1191 times the process sleeps and wakes up again. Another useful technique to
1192 reduce iterations/wake\-ups is to use \f(CW\*(C`ev_periodic\*(C'\fR watchers and make sure
1193 they fire on, say, one-second boundaries only.
1194 .Sp
1195 Example: we only need 0.1s timeout granularity, and we wish not to poll
1196 more often than 100 times per second:
1197 .Sp
1198 .Vb 2
1199 \& ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
1200 \& ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
1201 .Ve
1202 .IP "ev_invoke_pending (loop)" 4
1203 .IX Item "ev_invoke_pending (loop)"
1204 This call will simply invoke all pending watchers while resetting their
1205 pending state. Normally, \f(CW\*(C`ev_run\*(C'\fR does this automatically when required,
1206 but when overriding the invoke callback this call comes handy. This
1207 function can be invoked from a watcher \- this can be useful for example
1208 when you want to do some lengthy calculation and want to pass further
1209 event handling to another thread (you still have to make sure only one
1210 thread executes within \f(CW\*(C`ev_invoke_pending\*(C'\fR or \f(CW\*(C`ev_run\*(C'\fR of course).
1211 .IP "int ev_pending_count (loop)" 4
1212 .IX Item "int ev_pending_count (loop)"
1213 Returns the number of pending watchers \- zero indicates that no watchers
1214 are pending.
1215 .IP "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(\s-1EV_P\s0))" 4
1216 .IX Item "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))"
1217 This overrides the invoke pending functionality of the loop: Instead of
1218 invoking all pending watchers when there are any, \f(CW\*(C`ev_run\*(C'\fR will call
1219 this callback instead. This is useful, for example, when you want to
1220 invoke the actual watchers inside another context (another thread etc.).
1221 .Sp
1222 If you want to reset the callback, use \f(CW\*(C`ev_invoke_pending\*(C'\fR as new
1223 callback.
1224 .IP "ev_set_loop_release_cb (loop, void (*release)(\s-1EV_P\s0) throw (), void (*acquire)(\s-1EV_P\s0) throw ())" 4
1225 .IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())"
1226 Sometimes you want to share the same loop between multiple threads. This
1227 can be done relatively simply by putting mutex_lock/unlock calls around
1228 each call to a libev function.
1229 .Sp
1230 However, \f(CW\*(C`ev_run\*(C'\fR can run an indefinite time, so it is not feasible
1231 to wait for it to return. One way around this is to wake up the event
1232 loop via \f(CW\*(C`ev_break\*(C'\fR and \f(CW\*(C`ev_async_send\*(C'\fR, another way is to set these
1233 \&\fIrelease\fR and \fIacquire\fR callbacks on the loop.
1234 .Sp
1235 When set, then \f(CW\*(C`release\*(C'\fR will be called just before the thread is
1236 suspended waiting for new events, and \f(CW\*(C`acquire\*(C'\fR is called just
1237 afterwards.
1238 .Sp
1239 Ideally, \f(CW\*(C`release\*(C'\fR will just call your mutex_unlock function, and
1240 \&\f(CW\*(C`acquire\*(C'\fR will just call the mutex_lock function again.
1241 .Sp
1242 While event loop modifications are allowed between invocations of
1243 \&\f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR (that's their only purpose after all), no
1244 modifications done will affect the event loop, i.e. adding watchers will
1245 have no effect on the set of file descriptors being watched, or the time
1246 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
1247 to take note of any changes you made.
1248 .Sp
1249 In theory, threads executing \f(CW\*(C`ev_run\*(C'\fR will be async-cancel safe between
1250 invocations of \f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR.
1251 .Sp
1252 See also the locking example in the \f(CW\*(C`THREADS\*(C'\fR section later in this
1253 document.
1254 .IP "ev_set_userdata (loop, void *data)" 4
1255 .IX Item "ev_set_userdata (loop, void *data)"
1256 .PD 0
1257 .IP "void *ev_userdata (loop)" 4
1258 .IX Item "void *ev_userdata (loop)"
1259 .PD
1260 Set and retrieve a single \f(CW\*(C`void *\*(C'\fR associated with a loop. When
1261 \&\f(CW\*(C`ev_set_userdata\*(C'\fR has never been called, then \f(CW\*(C`ev_userdata\*(C'\fR returns
1262 \&\f(CW0\fR.
1263 .Sp
1264 These two functions can be used to associate arbitrary data with a loop,
1265 and are intended solely for the \f(CW\*(C`invoke_pending_cb\*(C'\fR, \f(CW\*(C`release\*(C'\fR and
1266 \&\f(CW\*(C`acquire\*(C'\fR callbacks described above, but of course can be (ab\-)used for
1267 any other purpose as well.
1268 .IP "ev_verify (loop)" 4
1269 .IX Item "ev_verify (loop)"
1270 This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR support has been
1271 compiled in, which is the default for non-minimal builds. It tries to go
1272 through all internal structures and checks them for validity. If anything
1273 is found to be inconsistent, it will print an error message to standard
1274 error and call \f(CW\*(C`abort ()\*(C'\fR.
1275 .Sp
1276 This can be used to catch bugs inside libev itself: under normal
1277 circumstances, this function will never abort as of course libev keeps its
1278 data structures consistent.
1279 .SH "ANATOMY OF A WATCHER"
1280 .IX Header "ANATOMY OF A WATCHER"
1281 In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the
1282 watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer
1283 watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.
1284 .PP
1285 A watcher is an opaque structure that you allocate and register to record
1286 your interest in some event. To make a concrete example, imagine you want
1287 to wait for \s-1STDIN\s0 to become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher
1288 for that:
1289 .PP
1290 .Vb 5
1291 \& static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
1292 \& {
1293 \& ev_io_stop (w);
1294 \& ev_break (loop, EVBREAK_ALL);
1295 \& }
1296 \&
1297 \& struct ev_loop *loop = ev_default_loop (0);
1298 \&
1299 \& ev_io stdin_watcher;
1300 \&
1301 \& ev_init (&stdin_watcher, my_cb);
1302 \& ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
1303 \& ev_io_start (loop, &stdin_watcher);
1304 \&
1305 \& ev_run (loop, 0);
1306 .Ve
1307 .PP
1308 As you can see, you are responsible for allocating the memory for your
1309 watcher structures (and it is \fIusually\fR a bad idea to do this on the
1310 stack).
1311 .PP
1312 Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR
1313 or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).
1314 .PP
1315 Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init (watcher
1316 *, callback)\*(C'\fR, which expects a callback to be provided. This callback is
1317 invoked each time the event occurs (or, in the case of I/O watchers, each
1318 time the event loop detects that the file descriptor given is readable
1319 and/or writable).
1320 .PP
1321 Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR
1322 macro to configure it, with arguments specific to the watcher type. There
1323 is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.
1324 .PP
1325 To make the watcher actually watch out for events, you have to start it
1326 with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher
1327 *)\*(C'\fR), and you can stop watching for events at any time by calling the
1328 corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.
1329 .PP
1330 As long as your watcher is active (has been started but not stopped) you
1331 must not touch the values stored in it. Most specifically you must never
1332 reinitialise it or call its \f(CW\*(C`ev_TYPE_set\*(C'\fR macro.
1333 .PP
1334 Each and every callback receives the event loop pointer as first, the
1335 registered watcher structure as second, and a bitset of received events as
1336 third argument.
1337 .PP
1338 The received events usually include a single bit per event type received
1339 (you can receive multiple events at the same time). The possible bit masks
1340 are:
1341 .ie n .IP """EV_READ""" 4
1342 .el .IP "\f(CWEV_READ\fR" 4
1343 .IX Item "EV_READ"
1344 .PD 0
1345 .ie n .IP """EV_WRITE""" 4
1346 .el .IP "\f(CWEV_WRITE\fR" 4
1347 .IX Item "EV_WRITE"
1348 .PD
1349 The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or
1350 writable.
1351 .ie n .IP """EV_TIMER""" 4
1352 .el .IP "\f(CWEV_TIMER\fR" 4
1353 .IX Item "EV_TIMER"
1354 The \f(CW\*(C`ev_timer\*(C'\fR watcher has timed out.
1355 .ie n .IP """EV_PERIODIC""" 4
1356 .el .IP "\f(CWEV_PERIODIC\fR" 4
1357 .IX Item "EV_PERIODIC"
1358 The \f(CW\*(C`ev_periodic\*(C'\fR watcher has timed out.
1359 .ie n .IP """EV_SIGNAL""" 4
1360 .el .IP "\f(CWEV_SIGNAL\fR" 4
1361 .IX Item "EV_SIGNAL"
1362 The signal specified in the \f(CW\*(C`ev_signal\*(C'\fR watcher has been received by a thread.
1363 .ie n .IP """EV_CHILD""" 4
1364 .el .IP "\f(CWEV_CHILD\fR" 4
1365 .IX Item "EV_CHILD"
1366 The pid specified in the \f(CW\*(C`ev_child\*(C'\fR watcher has received a status change.
1367 .ie n .IP """EV_STAT""" 4
1368 .el .IP "\f(CWEV_STAT\fR" 4
1369 .IX Item "EV_STAT"
1370 The path specified in the \f(CW\*(C`ev_stat\*(C'\fR watcher changed its attributes somehow.
1371 .ie n .IP """EV_IDLE""" 4
1372 .el .IP "\f(CWEV_IDLE\fR" 4
1373 .IX Item "EV_IDLE"
1374 The \f(CW\*(C`ev_idle\*(C'\fR watcher has determined that you have nothing better to do.
1375 .ie n .IP """EV_PREPARE""" 4
1376 .el .IP "\f(CWEV_PREPARE\fR" 4
1377 .IX Item "EV_PREPARE"
1378 .PD 0
1379 .ie n .IP """EV_CHECK""" 4
1380 .el .IP "\f(CWEV_CHECK\fR" 4
1381 .IX Item "EV_CHECK"
1382 .PD
1383 All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_run\*(C'\fR starts to
1384 gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are queued (not invoked)
1385 just after \f(CW\*(C`ev_run\*(C'\fR has gathered them, but before it queues any callbacks
1386 for any received events. That means \f(CW\*(C`ev_prepare\*(C'\fR watchers are the last
1387 watchers invoked before the event loop sleeps or polls for new events, and
1388 \&\f(CW\*(C`ev_check\*(C'\fR watchers will be invoked before any other watchers of the same
1389 or lower priority within an event loop iteration.
1390 .Sp
1391 Callbacks of both watcher types can start and stop as many watchers as
1392 they want, and all of them will be taken into account (for example, a
1393 \&\f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep \f(CW\*(C`ev_run\*(C'\fR from
1394 blocking).
1395 .ie n .IP """EV_EMBED""" 4
1396 .el .IP "\f(CWEV_EMBED\fR" 4
1397 .IX Item "EV_EMBED"
1398 The embedded event loop specified in the \f(CW\*(C`ev_embed\*(C'\fR watcher needs attention.
1399 .ie n .IP """EV_FORK""" 4
1400 .el .IP "\f(CWEV_FORK\fR" 4
1401 .IX Item "EV_FORK"
1402 The event loop has been resumed in the child process after fork (see
1403 \&\f(CW\*(C`ev_fork\*(C'\fR).
1404 .ie n .IP """EV_CLEANUP""" 4
1405 .el .IP "\f(CWEV_CLEANUP\fR" 4
1406 .IX Item "EV_CLEANUP"
1407 The event loop is about to be destroyed (see \f(CW\*(C`ev_cleanup\*(C'\fR).
1408 .ie n .IP """EV_ASYNC""" 4
1409 .el .IP "\f(CWEV_ASYNC\fR" 4
1410 .IX Item "EV_ASYNC"
1411 The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).
1412 .ie n .IP """EV_CUSTOM""" 4
1413 .el .IP "\f(CWEV_CUSTOM\fR" 4
1414 .IX Item "EV_CUSTOM"
1415 Not ever sent (or otherwise used) by libev itself, but can be freely used
1416 by libev users to signal watchers (e.g. via \f(CW\*(C`ev_feed_event\*(C'\fR).
1417 .ie n .IP """EV_ERROR""" 4
1418 .el .IP "\f(CWEV_ERROR\fR" 4
1419 .IX Item "EV_ERROR"
1420 An unspecified error has occurred, the watcher has been stopped. This might
1421 happen because the watcher could not be properly started because libev
1422 ran out of memory, a file descriptor was found to be closed or any other
1423 problem. Libev considers these application bugs.
1424 .Sp
1425 You best act on it by reporting the problem and somehow coping with the
1426 watcher being stopped. Note that well-written programs should not receive
1427 an error ever, so when your watcher receives it, this usually indicates a
1428 bug in your program.
1429 .Sp
1430 Libev will usually signal a few \*(L"dummy\*(R" events together with an error, for
1431 example it might indicate that a fd is readable or writable, and if your
1432 callbacks is well-written it can just attempt the operation and cope with
1433 the error from \fBread()\fR or \fBwrite()\fR. This will not work in multi-threaded
1434 programs, though, as the fd could already be closed and reused for another
1435 thing, so beware.
1436 .SS "\s-1GENERIC WATCHER FUNCTIONS\s0"
1437 .IX Subsection "GENERIC WATCHER FUNCTIONS"
1438 .ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4
1439 .el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4
1440 .IX Item "ev_init (ev_TYPE *watcher, callback)"
1441 This macro initialises the generic portion of a watcher. The contents
1442 of the watcher object can be arbitrary (so \f(CW\*(C`malloc\*(C'\fR will do). Only
1443 the generic parts of the watcher are initialised, you \fIneed\fR to call
1444 the type-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR macro afterwards to initialise the
1445 type-specific parts. For each type there is also a \f(CW\*(C`ev_TYPE_init\*(C'\fR macro
1446 which rolls both calls into one.
1447 .Sp
1448 You can reinitialise a watcher at any time as long as it has been stopped
1449 (or never started) and there are no pending events outstanding.
1450 .Sp
1451 The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
1452 int revents)\*(C'\fR.
1453 .Sp
1454 Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
1455 .Sp
1456 .Vb 3
1457 \& ev_io w;
1458 \& ev_init (&w, my_cb);
1459 \& ev_io_set (&w, STDIN_FILENO, EV_READ);
1460 .Ve
1461 .ie n .IP """ev_TYPE_set"" (ev_TYPE *watcher, [args])" 4
1462 .el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *watcher, [args])" 4
1463 .IX Item "ev_TYPE_set (ev_TYPE *watcher, [args])"
1464 This macro initialises the type-specific parts of a watcher. You need to
1465 call \f(CW\*(C`ev_init\*(C'\fR at least once before you call this macro, but you can
1466 call \f(CW\*(C`ev_TYPE_set\*(C'\fR any number of times. You must not, however, call this
1467 macro on a watcher that is active (it can be pending, however, which is a
1468 difference to the \f(CW\*(C`ev_init\*(C'\fR macro).
1469 .Sp
1470 Although some watcher types do not have type-specific arguments
1471 (e.g. \f(CW\*(C`ev_prepare\*(C'\fR) you still need to call its \f(CW\*(C`set\*(C'\fR macro.
1472 .Sp
1473 See \f(CW\*(C`ev_init\*(C'\fR, above, for an example.
1474 .ie n .IP """ev_TYPE_init"" (ev_TYPE *watcher, callback, [args])" 4
1475 .el .IP "\f(CWev_TYPE_init\fR (ev_TYPE *watcher, callback, [args])" 4
1476 .IX Item "ev_TYPE_init (ev_TYPE *watcher, callback, [args])"
1477 This convenience macro rolls both \f(CW\*(C`ev_init\*(C'\fR and \f(CW\*(C`ev_TYPE_set\*(C'\fR macro
1478 calls into a single call. This is the most convenient method to initialise
1479 a watcher. The same limitations apply, of course.
1480 .Sp
1481 Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.
1482 .Sp
1483 .Vb 1
1484 \& ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
1485 .Ve
1486 .ie n .IP """ev_TYPE_start"" (loop, ev_TYPE *watcher)" 4
1487 .el .IP "\f(CWev_TYPE_start\fR (loop, ev_TYPE *watcher)" 4
1488 .IX Item "ev_TYPE_start (loop, ev_TYPE *watcher)"
1489 Starts (activates) the given watcher. Only active watchers will receive
1490 events. If the watcher is already active nothing will happen.
1491 .Sp
1492 Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this
1493 whole section.
1494 .Sp
1495 .Vb 1
1496 \& ev_io_start (EV_DEFAULT_UC, &w);
1497 .Ve
1498 .ie n .IP """ev_TYPE_stop"" (loop, ev_TYPE *watcher)" 4
1499 .el .IP "\f(CWev_TYPE_stop\fR (loop, ev_TYPE *watcher)" 4
1500 .IX Item "ev_TYPE_stop (loop, ev_TYPE *watcher)"
1501 Stops the given watcher if active, and clears the pending status (whether
1502 the watcher was active or not).
1503 .Sp
1504 It is possible that stopped watchers are pending \- for example,
1505 non-repeating timers are being stopped when they become pending \- but
1506 calling \f(CW\*(C`ev_TYPE_stop\*(C'\fR ensures that the watcher is neither active nor
1507 pending. If you want to free or reuse the memory used by the watcher it is
1508 therefore a good idea to always call its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function.
1509 .IP "bool ev_is_active (ev_TYPE *watcher)" 4
1510 .IX Item "bool ev_is_active (ev_TYPE *watcher)"
1511 Returns a true value iff the watcher is active (i.e. it has been started
1512 and not yet been stopped). As long as a watcher is active you must not modify
1513 it.
1514 .IP "bool ev_is_pending (ev_TYPE *watcher)" 4
1515 .IX Item "bool ev_is_pending (ev_TYPE *watcher)"
1516 Returns a true value iff the watcher is pending, (i.e. it has outstanding
1517 events but its callback has not yet been invoked). As long as a watcher
1518 is pending (but not active) you must not call an init function on it (but
1519 \&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must
1520 make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR
1521 it).
1522 .IP "callback ev_cb (ev_TYPE *watcher)" 4
1523 .IX Item "callback ev_cb (ev_TYPE *watcher)"
1524 Returns the callback currently set on the watcher.
1525 .IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
1526 .IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
1527 Change the callback. You can change the callback at virtually any time
1528 (modulo threads).
1529 .IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
1530 .IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
1531 .PD 0
1532 .IP "int ev_priority (ev_TYPE *watcher)" 4
1533 .IX Item "int ev_priority (ev_TYPE *watcher)"
1534 .PD
1535 Set and query the priority of the watcher. The priority is a small
1536 integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR
1537 (default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked
1538 before watchers with lower priority, but priority will not keep watchers
1539 from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).
1540 .Sp
1541 If you need to suppress invocation when higher priority events are pending
1542 you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.
1543 .Sp
1544 You \fImust not\fR change the priority of a watcher as long as it is active or
1545 pending.
1546 .Sp
1547 Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
1548 fine, as long as you do not mind that the priority value you query might
1549 or might not have been clamped to the valid range.
1550 .Sp
1551 The default priority used by watchers when no priority has been set is
1552 always \f(CW0\fR, which is supposed to not be too high and not be too low :).
1553 .Sp
1554 See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
1555 priorities.
1556 .IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
1557 .IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
1558 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
1559 \&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback
1560 can deal with that fact, as both are simply passed through to the
1561 callback.
1562 .IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4
1563 .IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"
1564 If the watcher is pending, this function clears its pending status and
1565 returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the
1566 watcher isn't pending it does nothing and returns \f(CW0\fR.
1567 .Sp
1568 Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its
1569 callback to be invoked, which can be accomplished with this function.
1570 .IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
1571 .IX Item "ev_feed_event (loop, ev_TYPE *watcher, int revents)"
1572 Feeds the given event set into the event loop, as if the specified event
1573 had happened for the specified watcher (which must be a pointer to an
1574 initialised but not necessarily started event watcher). Obviously you must
1575 not free the watcher as long as it has pending events.
1576 .Sp
1577 Stopping the watcher, letting libev invoke it, or calling
1578 \&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
1579 not started in the first place.
1580 .Sp
1581 See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
1582 functions that do not need a watcher.
1583 .PP
1584 See also the \*(L"\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\*(R"\s0 and \*(L"\s-1BUILDING YOUR
1585 OWN COMPOSITE WATCHERS\*(R"\s0 idioms.
1586 .SS "\s-1WATCHER STATES\s0"
1587 .IX Subsection "WATCHER STATES"
1588 There are various watcher states mentioned throughout this manual \-
1589 active, pending and so on. In this section these states and the rules to
1590 transition between them will be described in more detail \- and while these
1591 rules might look complicated, they usually do \*(L"the right thing\*(R".
1592 .IP "initialised" 4
1593 .IX Item "initialised"
1594 Before a watcher can be registered with the event loop it has to be
1595 initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
1596 \&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR function.
1597 .Sp
1598 In this state it is simply some block of memory that is suitable for
1599 use in an event loop. It can be moved around, freed, reused etc. at
1600 will \- as long as you either keep the memory contents intact, or call
1601 \&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
1602 .IP "started/running/active" 4
1603 .IX Item "started/running/active"
1604 Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR it becomes
1605 property of the event loop, and is actively waiting for events. While in
1606 this state it cannot be accessed (except in a few documented ways), moved,
1607 freed or anything else \- the only legal thing is to keep a pointer to it,
1608 and call libev functions on it that are documented to work on active watchers.
1609 .IP "pending" 4
1610 .IX Item "pending"
1611 If a watcher is active and libev determines that an event it is interested
1612 in has occurred (such as a timer expiring), it will become pending. It will
1613 stay in this pending state until either it is stopped or its callback is
1614 about to be invoked, so it is not normally pending inside the watcher
1615 callback.
1616 .Sp
1617 The watcher might or might not be active while it is pending (for example,
1618 an expired non-repeating timer can be pending but no longer active). If it
1619 is stopped, it can be freely accessed (e.g. by calling \f(CW\*(C`ev_TYPE_set\*(C'\fR),
1620 but it is still property of the event loop at this time, so cannot be
1621 moved, freed or reused. And if it is active the rules described in the
1622 previous item still apply.
1623 .Sp
1624 It is also possible to feed an event on a watcher that is not active (e.g.
1625 via \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
1626 active.
1627 .IP "stopped" 4
1628 .IX Item "stopped"
1629 A watcher can be stopped implicitly by libev (in which case it might still
1630 be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function. The
1631 latter will clear any pending state the watcher might be in, regardless
1632 of whether it was active or not, so stopping a watcher explicitly before
1633 freeing it is often a good idea.
1634 .Sp
1635 While stopped (and not pending) the watcher is essentially in the
1636 initialised state, that is, it can be reused, moved, modified in any way
1637 you wish (but when you trash the memory block, you need to \f(CW\*(C`ev_TYPE_init\*(C'\fR
1638 it again).
1639 .SS "\s-1WATCHER PRIORITY MODELS\s0"
1640 .IX Subsection "WATCHER PRIORITY MODELS"
1641 Many event loops support \fIwatcher priorities\fR, which are usually small
1642 integers that influence the ordering of event callback invocation
1643 between watchers in some way, all else being equal.
1644 .PP
1645 In libev, Watcher priorities can be set using \f(CW\*(C`ev_set_priority\*(C'\fR. See its
1646 description for the more technical details such as the actual priority
1647 range.
1648 .PP
1649 There are two common ways how these these priorities are being interpreted
1650 by event loops:
1651 .PP
1652 In the more common lock-out model, higher priorities \*(L"lock out\*(R" invocation
1653 of lower priority watchers, which means as long as higher priority
1654 watchers receive events, lower priority watchers are not being invoked.
1655 .PP
1656 The less common only-for-ordering model uses priorities solely to order
1657 callback invocation within a single event loop iteration: Higher priority
1658 watchers are invoked before lower priority ones, but they all get invoked
1659 before polling for new events.
1660 .PP
1661 Libev uses the second (only-for-ordering) model for all its watchers
1662 except for idle watchers (which use the lock-out model).
1663 .PP
1664 The rationale behind this is that implementing the lock-out model for
1665 watchers is not well supported by most kernel interfaces, and most event
1666 libraries will just poll for the same events again and again as long as
1667 their callbacks have not been executed, which is very inefficient in the
1668 common case of one high-priority watcher locking out a mass of lower
1669 priority ones.
1670 .PP
1671 Static (ordering) priorities are most useful when you have two or more
1672 watchers handling the same resource: a typical usage example is having an
1673 \&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR to handle
1674 timeouts. Under load, data might be received while the program handles
1675 other jobs, but since timers normally get invoked first, the timeout
1676 handler will be executed before checking for data. In that case, giving
1677 the timer a lower priority than the I/O watcher ensures that I/O will be
1678 handled first even under adverse conditions (which is usually, but not
1679 always, what you want).
1680 .PP
1681 Since idle watchers use the \*(L"lock-out\*(R" model, meaning that idle watchers
1682 will only be executed when no same or higher priority watchers have
1683 received events, they can be used to implement the \*(L"lock-out\*(R" model when
1684 required.
1685 .PP
1686 For example, to emulate how many other event libraries handle priorities,
1687 you can associate an \f(CW\*(C`ev_idle\*(C'\fR watcher to each such watcher, and in
1688 the normal watcher callback, you just start the idle watcher. The real
1689 processing is done in the idle watcher callback. This causes libev to
1690 continuously poll and process kernel event data for the watcher, but when
1691 the lock-out case is known to be rare (which in turn is rare :), this is
1692 workable.
1693 .PP
1694 Usually, however, the lock-out model implemented that way will perform
1695 miserably under the type of load it was designed to handle. In that case,
1696 it might be preferable to stop the real watcher before starting the
1697 idle watcher, so the kernel will not have to process the event in case
1698 the actual processing will be delayed for considerable time.
1699 .PP
1700 Here is an example of an I/O watcher that should run at a strictly lower
1701 priority than the default, and which should only process data when no
1702 other events are pending:
1703 .PP
1704 .Vb 2
1705 \& ev_idle idle; // actual processing watcher
1706 \& ev_io io; // actual event watcher
1707 \&
1708 \& static void
1709 \& io_cb (EV_P_ ev_io *w, int revents)
1710 \& {
1711 \& // stop the I/O watcher, we received the event, but
1712 \& // are not yet ready to handle it.
1713 \& ev_io_stop (EV_A_ w);
1714 \&
1715 \& // start the idle watcher to handle the actual event.
1716 \& // it will not be executed as long as other watchers
1717 \& // with the default priority are receiving events.
1718 \& ev_idle_start (EV_A_ &idle);
1719 \& }
1720 \&
1721 \& static void
1722 \& idle_cb (EV_P_ ev_idle *w, int revents)
1723 \& {
1724 \& // actual processing
1725 \& read (STDIN_FILENO, ...);
1726 \&
1727 \& // have to start the I/O watcher again, as
1728 \& // we have handled the event
1729 \& ev_io_start (EV_P_ &io);
1730 \& }
1731 \&
1732 \& // initialisation
1733 \& ev_idle_init (&idle, idle_cb);
1734 \& ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1735 \& ev_io_start (EV_DEFAULT_ &io);
1736 .Ve
1737 .PP
1738 In the \*(L"real\*(R" world, it might also be beneficial to start a timer, so that
1739 low-priority connections can not be locked out forever under load. This
1740 enables your program to keep a lower latency for important connections
1741 during short periods of high load, while not completely locking out less
1742 important ones.
1743 .SH "WATCHER TYPES"
1744 .IX Header "WATCHER TYPES"
1745 This section describes each watcher in detail, but will not repeat
1746 information given in the last section. Any initialisation/set macros,
1747 functions and members specific to the watcher type are explained.
1748 .PP
1749 Members are additionally marked with either \fI[read\-only]\fR, meaning that,
1750 while the watcher is active, you can look at the member and expect some
1751 sensible content, but you must not modify it (you can modify it while the
1752 watcher is stopped to your hearts content), or \fI[read\-write]\fR, which
1753 means you can expect it to have some sensible content while the watcher
1754 is active, but you can also modify it. Modifying it may not do something
1755 sensible or take immediate effect (or do anything at all), but libev will
1756 not crash or malfunction in any way.
1757 .ie n .SS """ev_io"" \- is this file descriptor readable or writable?"
1758 .el .SS "\f(CWev_io\fP \- is this file descriptor readable or writable?"
1759 .IX Subsection "ev_io - is this file descriptor readable or writable?"
1760 I/O watchers check whether a file descriptor is readable or writable
1761 in each iteration of the event loop, or, more precisely, when reading
1762 would not block the process and writing would at least be able to write
1763 some data. This behaviour is called level-triggering because you keep
1764 receiving events as long as the condition persists. Remember you can stop
1765 the watcher if you don't want to act on the event and neither want to
1766 receive future events.
1767 .PP
1768 In general you can register as many read and/or write event watchers per
1769 fd as you want (as long as you don't confuse yourself). Setting all file
1770 descriptors to non-blocking mode is also usually a good idea (but not
1771 required if you know what you are doing).
1772 .PP
1773 Another thing you have to watch out for is that it is quite easy to
1774 receive \*(L"spurious\*(R" readiness notifications, that is, your callback might
1775 be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block
1776 because there is no data. It is very easy to get into this situation even
1777 with a relatively standard program structure. Thus it is best to always
1778 use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning \f(CW\*(C`EAGAIN\*(C'\fR is far
1779 preferable to a program hanging until some data arrives.
1780 .PP
1781 If you cannot run the fd in non-blocking mode (for example you should
1782 not play around with an Xlib connection), then you have to separately
1783 re-test whether a file descriptor is really ready with a known-to-be good
1784 interface such as poll (fortunately in the case of Xlib, it already does
1785 this on its own, so its quite safe to use). Some people additionally
1786 use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
1787 indefinitely.
1788 .PP
1789 But really, best use non-blocking mode.
1790 .PP
1791 \fIThe special problem of disappearing file descriptors\fR
1792 .IX Subsection "The special problem of disappearing file descriptors"
1793 .PP
1794 Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
1795 a file descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other
1796 means, such as \f(CW\*(C`dup2\*(C'\fR). The reason is that you register interest in some
1797 file descriptor, but when it goes away, the operating system will silently
1798 drop this interest. If another file descriptor with the same number then
1799 is registered with libev, there is no efficient way to see that this is,
1800 in fact, a different file descriptor.
1801 .PP
1802 To avoid having to explicitly tell libev about such cases, libev follows
1803 the following policy: Each time \f(CW\*(C`ev_io_set\*(C'\fR is being called, libev
1804 will assume that this is potentially a new file descriptor, otherwise
1805 it is assumed that the file descriptor stays the same. That means that
1806 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
1807 descriptor even if the file descriptor number itself did not change.
1808 .PP
1809 This is how one would do it normally anyway, the important point is that
1810 the libev application should not optimise around libev but should leave
1811 optimisations to libev.
1812 .PP
1813 \fIThe special problem of dup'ed file descriptors\fR
1814 .IX Subsection "The special problem of dup'ed file descriptors"
1815 .PP
1816 Some backends (e.g. epoll), cannot register events for file descriptors,
1817 but only events for the underlying file descriptions. That means when you
1818 have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register
1819 events for them, only one file descriptor might actually receive events.
1820 .PP
1821 There is no workaround possible except not registering events
1822 for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
1823 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1824 .PP
1825 \fIThe special problem of files\fR
1826 .IX Subsection "The special problem of files"
1827 .PP
1828 Many people try to use \f(CW\*(C`select\*(C'\fR (or libev) on file descriptors
1829 representing files, and expect it to become ready when their program
1830 doesn't block on disk accesses (which can take a long time on their own).
1831 .PP
1832 However, this cannot ever work in the \*(L"expected\*(R" way \- you get a readiness
1833 notification as soon as the kernel knows whether and how much data is
1834 there, and in the case of open files, that's always the case, so you
1835 always get a readiness notification instantly, and your read (or possibly
1836 write) will still block on the disk I/O.
1837 .PP
1838 Another way to view it is that in the case of sockets, pipes, character
1839 devices and so on, there is another party (the sender) that delivers data
1840 on its own, but in the case of files, there is no such thing: the disk
1841 will not send data on its own, simply because it doesn't know what you
1842 wish to read \- you would first have to request some data.
1843 .PP
1844 Since files are typically not-so-well supported by advanced notification
1845 mechanism, libev tries hard to emulate \s-1POSIX\s0 behaviour with respect
1846 to files, even though you should not use it. The reason for this is
1847 convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
1848 usually a tty, often a pipe, but also sometimes files or special devices
1849 (for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
1850 \&\fI/dev/urandom\fR), and even though the file might better be served with
1851 asynchronous I/O instead of with non-blocking I/O, it is still useful when
1852 it \*(L"just works\*(R" instead of freezing.
1853 .PP
1854 So avoid file descriptors pointing to files when you know it (e.g. use
1855 libeio), but use them when it is convenient, e.g. for \s-1STDIN/STDOUT,\s0 or
1856 when you rarely read from a file instead of from a socket, and want to
1857 reuse the same code path.
1858 .PP
1859 \fIThe special problem of fork\fR
1860 .IX Subsection "The special problem of fork"
1861 .PP
1862 Some backends (epoll, kqueue, probably linuxaio) do not support \f(CW\*(C`fork ()\*(C'\fR
1863 at all or exhibit useless behaviour. Libev fully supports fork, but needs
1864 to be told about it in the child if you want to continue to use it in the
1865 child.
1866 .PP
1867 To support fork in your child processes, you have to call \f(CW\*(C`ev_loop_fork
1868 ()\*(C'\fR after a fork in the child, enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to
1869 \&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
1870 .PP
1871 \fIThe special problem of \s-1SIGPIPE\s0\fR
1872 .IX Subsection "The special problem of SIGPIPE"
1873 .PP
1874 While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:
1875 when writing to a pipe whose other end has been closed, your program gets
1876 sent a \s-1SIGPIPE,\s0 which, by default, aborts your program. For most programs
1877 this is sensible behaviour, for daemons, this is usually undesirable.
1878 .PP
1879 So when you encounter spurious, unexplained daemon exits, make sure you
1880 ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon
1881 somewhere, as that would have given you a big clue).
1882 .PP
1883 \fIThe special problem of \f(BIaccept()\fIing when you can't\fR
1884 .IX Subsection "The special problem of accept()ing when you can't"
1885 .PP
1886 Many implementations of the \s-1POSIX\s0 \f(CW\*(C`accept\*(C'\fR function (for example,
1887 found in post\-2004 Linux) have the peculiar behaviour of not removing a
1888 connection from the pending queue in all error cases.
1889 .PP
1890 For example, larger servers often run out of file descriptors (because
1891 of resource limits), causing \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR but not
1892 rejecting the connection, leading to libev signalling readiness on
1893 the next iteration again (the connection still exists after all), and
1894 typically causing the program to loop at 100% \s-1CPU\s0 usage.
1895 .PP
1896 Unfortunately, the set of errors that cause this issue differs between
1897 operating systems, there is usually little the app can do to remedy the
1898 situation, and no known thread-safe method of removing the connection to
1899 cope with overload is known (to me).
1900 .PP
1901 One of the easiest ways to handle this situation is to just ignore it
1902 \&\- when the program encounters an overload, it will just loop until the
1903 situation is over. While this is a form of busy waiting, no \s-1OS\s0 offers an
1904 event-based way to handle this situation, so it's the best one can do.
1905 .PP
1906 A better way to handle the situation is to log any errors other than
1907 \&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, making sure not to flood the log with such
1908 messages, and continue as usual, which at least gives the user an idea of
1909 what could be wrong (\*(L"raise the ulimit!\*(R"). For extra points one could stop
1910 the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
1911 usage.
1912 .PP
1913 If your program is single-threaded, then you could also keep a dummy file
1914 descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
1915 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,
1916 close that fd, and create a new dummy fd. This will gracefully refuse
1917 clients under typical overload conditions.
1918 .PP
1919 The last way to handle it is to simply log the error and \f(CW\*(C`exit\*(C'\fR, as
1920 is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
1921 opportunity for a DoS attack.
1922 .PP
1923 \fIWatcher-Specific Functions\fR
1924 .IX Subsection "Watcher-Specific Functions"
1925 .IP "ev_io_init (ev_io *, callback, int fd, int events)" 4
1926 .IX Item "ev_io_init (ev_io *, callback, int fd, int events)"
1927 .PD 0
1928 .IP "ev_io_set (ev_io *, int fd, int events)" 4
1929 .IX Item "ev_io_set (ev_io *, int fd, int events)"
1930 .PD
1931 Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to
1932 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
1933 \&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR, to express the desire to receive the given events.
1934 .IP "int fd [read\-only]" 4
1935 .IX Item "int fd [read-only]"
1936 The file descriptor being watched.
1937 .IP "int events [read\-only]" 4
1938 .IX Item "int events [read-only]"
1939 The events being watched.
1940 .PP
1941 \fIExamples\fR
1942 .IX Subsection "Examples"
1943 .PP
1944 Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well
1945 readable, but only once. Since it is likely line-buffered, you could
1946 attempt to read a whole line in the callback.
1947 .PP
1948 .Vb 6
1949 \& static void
1950 \& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
1951 \& {
1952 \& ev_io_stop (loop, w);
1953 \& .. read from stdin here (or from w\->fd) and handle any I/O errors
1954 \& }
1955 \&
1956 \& ...
1957 \& struct ev_loop *loop = ev_default_init (0);
1958 \& ev_io stdin_readable;
1959 \& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
1960 \& ev_io_start (loop, &stdin_readable);
1961 \& ev_run (loop, 0);
1962 .Ve
1963 .ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
1964 .el .SS "\f(CWev_timer\fP \- relative and optionally repeating timeouts"
1965 .IX Subsection "ev_timer - relative and optionally repeating timeouts"
1966 Timer watchers are simple relative timers that generate an event after a
1967 given time, and optionally repeating in regular intervals after that.
1968 .PP
1969 The timers are based on real time, that is, if you register an event that
1970 times out after an hour and you reset your system clock to January last
1971 year, it will still time out after (roughly) one hour. \*(L"Roughly\*(R" because
1972 detecting time jumps is hard, and some inaccuracies are unavoidable (the
1973 monotonic clock option helps a lot here).
1974 .PP
1975 The callback is guaranteed to be invoked only \fIafter\fR its timeout has
1976 passed (not \fIat\fR, so on systems with very low-resolution clocks this
1977 might introduce a small delay, see \*(L"the special problem of being too
1978 early\*(R", below). If multiple timers become ready during the same loop
1979 iteration then the ones with earlier time-out values are invoked before
1980 ones of the same priority with later time-out values (but this is no
1981 longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
1982 .PP
1983 \fIBe smart about timeouts\fR
1984 .IX Subsection "Be smart about timeouts"
1985 .PP
1986 Many real-world problems involve some kind of timeout, usually for error
1987 recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
1988 you want to raise some error after a while.
1989 .PP
1990 What follows are some ways to handle this problem, from obvious and
1991 inefficient to smart and efficient.
1992 .PP
1993 In the following, a 60 second activity timeout is assumed \- a timeout that
1994 gets reset to 60 seconds each time there is activity (e.g. each time some
1995 data or other life sign was received).
1996 .IP "1. Use a timer and stop, reinitialise and start it on activity." 4
1997 .IX Item "1. Use a timer and stop, reinitialise and start it on activity."
1998 This is the most obvious, but not the most simple way: In the beginning,
1999 start the watcher:
2000 .Sp
2001 .Vb 2
2002 \& ev_timer_init (timer, callback, 60., 0.);
2003 \& ev_timer_start (loop, timer);
2004 .Ve
2005 .Sp
2006 Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
2007 and start it again:
2008 .Sp
2009 .Vb 3
2010 \& ev_timer_stop (loop, timer);
2011 \& ev_timer_set (timer, 60., 0.);
2012 \& ev_timer_start (loop, timer);
2013 .Ve
2014 .Sp
2015 This is relatively simple to implement, but means that each time there is
2016 some activity, libev will first have to remove the timer from its internal
2017 data structure and then add it again. Libev tries to be fast, but it's
2018 still not a constant-time operation.
2019 .ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
2020 .el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
2021 .IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
2022 This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
2023 \&\f(CW\*(C`ev_timer_start\*(C'\fR.
2024 .Sp
2025 To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
2026 of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
2027 successfully read or write some data. If you go into an idle state where
2028 you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
2029 the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
2030 .Sp
2031 That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
2032 \&\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
2033 member and \f(CW\*(C`ev_timer_again\*(C'\fR.
2034 .Sp
2035 At start:
2036 .Sp
2037 .Vb 3
2038 \& ev_init (timer, callback);
2039 \& timer\->repeat = 60.;
2040 \& ev_timer_again (loop, timer);
2041 .Ve
2042 .Sp
2043 Each time there is some activity:
2044 .Sp
2045 .Vb 1
2046 \& ev_timer_again (loop, timer);
2047 .Ve
2048 .Sp
2049 It is even possible to change the time-out on the fly, regardless of
2050 whether the watcher is active or not:
2051 .Sp
2052 .Vb 2
2053 \& timer\->repeat = 30.;
2054 \& ev_timer_again (loop, timer);
2055 .Ve
2056 .Sp
2057 This is slightly more efficient then stopping/starting the timer each time
2058 you want to modify its timeout value, as libev does not have to completely
2059 remove and re-insert the timer from/into its internal data structure.
2060 .Sp
2061 It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
2062 .IP "3. Let the timer time out, but then re-arm it as required." 4
2063 .IX Item "3. Let the timer time out, but then re-arm it as required."
2064 This method is more tricky, but usually most efficient: Most timeouts are
2065 relatively long compared to the intervals between other activity \- in
2066 our example, within 60 seconds, there are usually many I/O events with
2067 associated activity resets.
2068 .Sp
2069 In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
2070 but remember the time of last activity, and check for a real timeout only
2071 within the callback:
2072 .Sp
2073 .Vb 3
2074 \& ev_tstamp timeout = 60.;
2075 \& ev_tstamp last_activity; // time of last activity
2076 \& ev_timer timer;
2077 \&
2078 \& static void
2079 \& callback (EV_P_ ev_timer *w, int revents)
2080 \& {
2081 \& // calculate when the timeout would happen
2082 \& ev_tstamp after = last_activity \- ev_now (EV_A) + timeout;
2083 \&
2084 \& // if negative, it means we the timeout already occurred
2085 \& if (after < 0.)
2086 \& {
2087 \& // timeout occurred, take action
2088 \& }
2089 \& else
2090 \& {
2091 \& // callback was invoked, but there was some recent
2092 \& // activity. simply restart the timer to time out
2093 \& // after "after" seconds, which is the earliest time
2094 \& // the timeout can occur.
2095 \& ev_timer_set (w, after, 0.);
2096 \& ev_timer_start (EV_A_ w);
2097 \& }
2098 \& }
2099 .Ve
2100 .Sp
2101 To summarise the callback: first calculate in how many seconds the
2102 timeout will occur (by calculating the absolute time when it would occur,
2103 \&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
2104 (EV_A)\*(C'\fR from that).
2105 .Sp
2106 If this value is negative, then we are already past the timeout, i.e. we
2107 timed out, and need to do whatever is needed in this case.
2108 .Sp
2109 Otherwise, we now the earliest time at which the timeout would trigger,
2110 and simply start the timer with this timeout value.
2111 .Sp
2112 In other words, each time the callback is invoked it will check whether
2113 the timeout occurred. If not, it will simply reschedule itself to check
2114 again at the earliest time it could time out. Rinse. Repeat.
2115 .Sp
2116 This scheme causes more callback invocations (about one every 60 seconds
2117 minus half the average time between activity), but virtually no calls to
2118 libev to change the timeout.
2119 .Sp
2120 To start the machinery, simply initialise the watcher and set
2121 \&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
2122 now), then call the callback, which will \*(L"do the right thing\*(R" and start
2123 the timer:
2124 .Sp
2125 .Vb 3
2126 \& last_activity = ev_now (EV_A);
2127 \& ev_init (&timer, callback);
2128 \& callback (EV_A_ &timer, 0);
2129 .Ve
2130 .Sp
2131 When there is some activity, simply store the current time in
2132 \&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
2133 .Sp
2134 .Vb 2
2135 \& if (activity detected)
2136 \& last_activity = ev_now (EV_A);
2137 .Ve
2138 .Sp
2139 When your timeout value changes, then the timeout can be changed by simply
2140 providing a new value, stopping the timer and calling the callback, which
2141 will again do the right thing (for example, time out immediately :).
2142 .Sp
2143 .Vb 3
2144 \& timeout = new_value;
2145 \& ev_timer_stop (EV_A_ &timer);
2146 \& callback (EV_A_ &timer, 0);
2147 .Ve
2148 .Sp
2149 This technique is slightly more complex, but in most cases where the
2150 time-out is unlikely to be triggered, much more efficient.
2151 .IP "4. Wee, just use a double-linked list for your timeouts." 4
2152 .IX Item "4. Wee, just use a double-linked list for your timeouts."
2153 If there is not one request, but many thousands (millions...), all
2154 employing some kind of timeout with the same timeout value, then one can
2155 do even better:
2156 .Sp
2157 When starting the timeout, calculate the timeout value and put the timeout
2158 at the \fIend\fR of the list.
2159 .Sp
2160 Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
2161 the list is expected to fire (for example, using the technique #3).
2162 .Sp
2163 When there is some activity, remove the timer from the list, recalculate
2164 the timeout, append it to the end of the list again, and make sure to
2165 update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
2166 .Sp
2167 This way, one can manage an unlimited number of timeouts in O(1) time for
2168 starting, stopping and updating the timers, at the expense of a major
2169 complication, and having to use a constant timeout. The constant timeout
2170 ensures that the list stays sorted.
2171 .PP
2172 So which method the best?
2173 .PP
2174 Method #2 is a simple no-brain-required solution that is adequate in most
2175 situations. Method #3 requires a bit more thinking, but handles many cases
2176 better, and isn't very complicated either. In most case, choosing either
2177 one is fine, with #3 being better in typical situations.
2178 .PP
2179 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
2180 rather complicated, but extremely efficient, something that really pays
2181 off after the first million or so of active timers, i.e. it's usually
2182 overkill :)
2183 .PP
2184 \fIThe special problem of being too early\fR
2185 .IX Subsection "The special problem of being too early"
2186 .PP
2187 If you ask a timer to call your callback after three seconds, then
2188 you expect it to be invoked after three seconds \- but of course, this
2189 cannot be guaranteed to infinite precision. Less obviously, it cannot be
2190 guaranteed to any precision by libev \- imagine somebody suspending the
2191 process with a \s-1STOP\s0 signal for a few hours for example.
2192 .PP
2193 So, libev tries to invoke your callback as soon as possible \fIafter\fR the
2194 delay has occurred, but cannot guarantee this.
2195 .PP
2196 A less obvious failure mode is calling your callback too early: many event
2197 loops compare timestamps with a \*(L"elapsed delay >= requested delay\*(R", but
2198 this can cause your callback to be invoked much earlier than you would
2199 expect.
2200 .PP
2201 To see why, imagine a system with a clock that only offers full second
2202 resolution (think windows if you can't come up with a broken enough \s-1OS\s0
2203 yourself). If you schedule a one-second timer at the time 500.9, then the
2204 event loop will schedule your timeout to elapse at a system time of 500
2205 (500.9 truncated to the resolution) + 1, or 501.
2206 .PP
2207 If an event library looks at the timeout 0.1s later, it will see \*(L"501 >=
2208 501\*(R" and invoke the callback 0.1s after it was started, even though a
2209 one-second delay was requested \- this is being \*(L"too early\*(R", despite best
2210 intentions.
2211 .PP
2212 This is the reason why libev will never invoke the callback if the elapsed
2213 delay equals the requested delay, but only when the elapsed delay is
2214 larger than the requested delay. In the example above, libev would only invoke
2215 the callback at system time 502, or 1.1s after the timer was started.
2216 .PP
2217 So, while libev cannot guarantee that your callback will be invoked
2218 exactly when requested, it \fIcan\fR and \fIdoes\fR guarantee that the requested
2219 delay has actually elapsed, or in other words, it always errs on the \*(L"too
2220 late\*(R" side of things.
2221 .PP
2222 \fIThe special problem of time updates\fR
2223 .IX Subsection "The special problem of time updates"
2224 .PP
2225 Establishing the current time is a costly operation (it usually takes
2226 at least one system call): \s-1EV\s0 therefore updates its idea of the current
2227 time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
2228 growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
2229 lots of events in one iteration.
2230 .PP
2231 The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
2232 time. This is usually the right thing as this timestamp refers to the time
2233 of the event triggering whatever timeout you are modifying/starting. If
2234 you suspect event processing to be delayed and you \fIneed\fR to base the
2235 timeout on the current time, use something like the following to adjust
2236 for it:
2237 .PP
2238 .Vb 1
2239 \& ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
2240 .Ve
2241 .PP
2242 If the event loop is suspended for a long time, you can also force an
2243 update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
2244 ()\*(C'\fR, although that will push the event time of all outstanding events
2245 further into the future.
2246 .PP
2247 \fIThe special problem of unsynchronised clocks\fR
2248 .IX Subsection "The special problem of unsynchronised clocks"
2249 .PP
2250 Modern systems have a variety of clocks \- libev itself uses the normal
2251 \&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
2252 jumps).
2253 .PP
2254 Neither of these clocks is synchronised with each other or any other clock
2255 on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
2256 than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
2257 a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
2258 than a directly following call to \f(CW\*(C`time\*(C'\fR.
2259 .PP
2260 The moral of this is to only compare libev-related timestamps with
2261 \&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, at least if you want better precision than
2262 a second or so.
2263 .PP
2264 One more problem arises due to this lack of synchronisation: if libev uses
2265 the system monotonic clock and you compare timestamps from \f(CW\*(C`ev_time\*(C'\fR
2266 or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
2267 invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
2268 .PP
2269 This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
2270 libev makes sure your callback is not invoked before the delay happened,
2271 \&\fImeasured according to the real time\fR, not the system clock.
2272 .PP
2273 If your timeouts are based on a physical timescale (e.g. \*(L"time out this
2274 connection after 100 seconds\*(R") then this shouldn't bother you as it is
2275 exactly the right behaviour.
2276 .PP
2277 If you want to compare wall clock/system timestamps to your timers, then
2278 you need to use \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
2279 time, where your comparisons will always generate correct results.
2280 .PP
2281 \fIThe special problems of suspended animation\fR
2282 .IX Subsection "The special problems of suspended animation"
2283 .PP
2284 When you leave the server world it is quite customary to hit machines that
2285 can suspend/hibernate \- what happens to the clocks during such a suspend?
2286 .PP
2287 Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
2288 all processes, while the clocks (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) continue
2289 to run until the system is suspended, but they will not advance while the
2290 system is suspended. That means, on resume, it will be as if the program
2291 was frozen for a few seconds, but the suspend time will not be counted
2292 towards \f(CW\*(C`ev_timer\*(C'\fR when a monotonic clock source is used. The real time
2293 clock advanced as expected, but if it is used as sole clocksource, then a
2294 long suspend would be detected as a time jump by libev, and timers would
2295 be adjusted accordingly.
2296 .PP
2297 I would not be surprised to see different behaviour in different between
2298 operating systems, \s-1OS\s0 versions or even different hardware.
2299 .PP
2300 The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) will see a
2301 time jump in the monotonic clocks and the realtime clock. If the program
2302 is suspended for a very long time, and monotonic clock sources are in use,
2303 then you can expect \f(CW\*(C`ev_timer\*(C'\fRs to expire as the full suspension time
2304 will be counted towards the timers. When no monotonic clock source is in
2305 use, then libev will again assume a timejump and adjust accordingly.
2306 .PP
2307 It might be beneficial for this latter case to call \f(CW\*(C`ev_suspend\*(C'\fR
2308 and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
2309 deterministic behaviour in this case (you can do nothing against
2310 \&\f(CW\*(C`SIGSTOP\*(C'\fR).
2311 .PP
2312 \fIWatcher-Specific Functions and Data Members\fR
2313 .IX Subsection "Watcher-Specific Functions and Data Members"
2314 .IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
2315 .IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
2316 .PD 0
2317 .IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
2318 .IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
2319 .PD
2320 Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
2321 negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, then it will
2322 automatically be stopped once the timeout is reached. If it is positive,
2323 then the timer will automatically be configured to trigger again \f(CW\*(C`repeat\*(C'\fR
2324 seconds later, again, and again, until stopped manually.
2325 .Sp
2326 The timer itself will do a best-effort at avoiding drift, that is, if
2327 you configure a timer to trigger every 10 seconds, then it will normally
2328 trigger at exactly 10 second intervals. If, however, your program cannot
2329 keep up with the timer (because it takes longer than those 10 seconds to
2330 do stuff) the timer will not fire more than once per event loop iteration.
2331 .IP "ev_timer_again (loop, ev_timer *)" 4
2332 .IX Item "ev_timer_again (loop, ev_timer *)"
2333 This will act as if the timer timed out, and restarts it again if it is
2334 repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
2335 timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
2336 .Sp
2337 The exact semantics are as in the following rules, all of which will be
2338 applied to the watcher:
2339 .RS 4
2340 .IP "If the timer is pending, the pending status is always cleared." 4
2341 .IX Item "If the timer is pending, the pending status is always cleared."
2342 .PD 0
2343 .IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
2344 .IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
2345 .ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
2346 .el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
2347 .IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
2348 .RE
2349 .RS 4
2350 .PD
2351 .Sp
2352 This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
2353 usage example.
2354 .RE
2355 .IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
2356 .IX Item "ev_tstamp ev_timer_remaining (loop, ev_timer *)"
2357 Returns the remaining time until a timer fires. If the timer is active,
2358 then this time is relative to the current event loop time, otherwise it's
2359 the timeout value currently configured.
2360 .Sp
2361 That is, after an \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
2362 \&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
2363 will return \f(CW4\fR. When the timer expires and is restarted, it will return
2364 roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
2365 too), and so on.
2366 .IP "ev_tstamp repeat [read\-write]" 4
2367 .IX Item "ev_tstamp repeat [read-write]"
2368 The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
2369 or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
2370 which is also when any modifications are taken into account.
2371 .PP
2372 \fIExamples\fR
2373 .IX Subsection "Examples"
2374 .PP
2375 Example: Create a timer that fires after 60 seconds.
2376 .PP
2377 .Vb 5
2378 \& static void
2379 \& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
2380 \& {
2381 \& .. one minute over, w is actually stopped right here
2382 \& }
2383 \&
2384 \& ev_timer mytimer;
2385 \& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
2386 \& ev_timer_start (loop, &mytimer);
2387 .Ve
2388 .PP
2389 Example: Create a timeout timer that times out after 10 seconds of
2390 inactivity.
2391 .PP
2392 .Vb 5
2393 \& static void
2394 \& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
2395 \& {
2396 \& .. ten seconds without any activity
2397 \& }
2398 \&
2399 \& ev_timer mytimer;
2400 \& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
2401 \& ev_timer_again (&mytimer); /* start timer */
2402 \& ev_run (loop, 0);
2403 \&
2404 \& // and in some piece of code that gets executed on any "activity":
2405 \& // reset the timeout to start ticking again at 10 seconds
2406 \& ev_timer_again (&mytimer);
2407 .Ve
2408 .ie n .SS """ev_periodic"" \- to cron or not to cron?"
2409 .el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
2410 .IX Subsection "ev_periodic - to cron or not to cron?"
2411 Periodic watchers are also timers of a kind, but they are very versatile
2412 (and unfortunately a bit complex).
2413 .PP
2414 Unlike \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
2415 relative time, the physical time that passes) but on wall clock time
2416 (absolute time, the thing you can read on your calendar or clock). The
2417 difference is that wall clock time can run faster or slower than real
2418 time, and time jumps are not uncommon (e.g. when you adjust your
2419 wrist-watch).
2420 .PP
2421 You can tell a periodic watcher to trigger after some specific point
2422 in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
2423 seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, that is, an absolute time
2424 not a delay) and then reset your system clock to January of the previous
2425 year, then it will take a year or more to trigger the event (unlike an
2426 \&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
2427 it, as it uses a relative timeout).
2428 .PP
2429 \&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
2430 timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
2431 other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
2432 watchers, as those cannot react to time jumps.
2433 .PP
2434 As with timers, the callback is guaranteed to be invoked only when the
2435 point in time where it is supposed to trigger has passed. If multiple
2436 timers become ready during the same loop iteration then the ones with
2437 earlier time-out values are invoked before ones with later time-out values
2438 (but this is no longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
2439 .PP
2440 \fIWatcher-Specific Functions and Data Members\fR
2441 .IX Subsection "Watcher-Specific Functions and Data Members"
2442 .IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2443 .IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2444 .PD 0
2445 .IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
2446 .IX Item "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
2447 .PD
2448 Lots of arguments, let's sort it out... There are basically three modes of
2449 operation, and we will explain them from simplest to most complex:
2450 .RS 4
2451 .IP "\(bu" 4
2452 absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
2453 .Sp
2454 In this configuration the watcher triggers an event after the wall clock
2455 time \f(CW\*(C`offset\*(C'\fR has passed. It will not repeat and will not adjust when a
2456 time jump occurs, that is, if it is to be run at January 1st 2011 then it
2457 will be stopped and invoked when the system clock reaches or surpasses
2458 this point in time.
2459 .IP "\(bu" 4
2460 repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
2461 .Sp
2462 In this mode the watcher will always be scheduled to time out at the next
2463 \&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
2464 negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
2465 argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
2466 .Sp
2467 This can be used to create timers that do not drift with respect to the
2468 system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
2469 hour, on the hour (with respect to \s-1UTC\s0):
2470 .Sp
2471 .Vb 1
2472 \& ev_periodic_set (&periodic, 0., 3600., 0);
2473 .Ve
2474 .Sp
2475 This doesn't mean there will always be 3600 seconds in between triggers,
2476 but only that the callback will be called when the system time shows a
2477 full hour (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
2478 by 3600.
2479 .Sp
2480 Another way to think about it (for the mathematically inclined) is that
2481 \&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
2482 time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
2483 .Sp
2484 The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
2485 interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
2486 microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR and should have
2487 at most a similar magnitude as the current time (say, within a factor of
2488 ten). Typical values for offset are, in fact, \f(CW0\fR or something between
2489 \&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
2490 .Sp
2491 Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
2492 speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
2493 will of course deteriorate. Libev itself tries to be exact to be about one
2494 millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
2495 .IP "\(bu" 4
2496 manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
2497 .Sp
2498 In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR are both being
2499 ignored. Instead, each time the periodic watcher gets scheduled, the
2500 reschedule callback will be called with the watcher as first, and the
2501 current time as second argument.
2502 .Sp
2503 \&\s-1NOTE:\s0 \fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
2504 or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
2505 allowed by documentation here\fR.
2506 .Sp
2507 If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
2508 it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
2509 only event loop modification you are allowed to do).
2510 .Sp
2511 The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
2512 *w, ev_tstamp now)\*(C'\fR, e.g.:
2513 .Sp
2514 .Vb 5
2515 \& static ev_tstamp
2516 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2517 \& {
2518 \& return now + 60.;
2519 \& }
2520 .Ve
2521 .Sp
2522 It must return the next time to trigger, based on the passed time value
2523 (that is, the lowest time value larger than to the second argument). It
2524 will usually be called just before the callback will be triggered, but
2525 might be called at other times, too.
2526 .Sp
2527 \&\s-1NOTE:\s0 \fIThis callback must always return a time that is higher than or
2528 equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
2529 .Sp
2530 This can be used to create very complex timers, such as a timer that
2531 triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
2532 the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
2533 this. Here is a (completely untested, no error checking) example on how to
2534 do this:
2535 .Sp
2536 .Vb 1
2537 \& #include <time.h>
2538 \&
2539 \& static ev_tstamp
2540 \& my_rescheduler (ev_periodic *w, ev_tstamp now)
2541 \& {
2542 \& time_t tnow = (time_t)now;
2543 \& struct tm tm;
2544 \& localtime_r (&tnow, &tm);
2545 \&
2546 \& tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
2547 \& ++tm.tm_mday; // midnight next day
2548 \&
2549 \& return mktime (&tm);
2550 \& }
2551 .Ve
2552 .Sp
2553 Note: this code might run into trouble on days that have more then two
2554 midnights (beginning and end).
2555 .RE
2556 .RS 4
2557 .RE
2558 .IP "ev_periodic_again (loop, ev_periodic *)" 4
2559 .IX Item "ev_periodic_again (loop, ev_periodic *)"
2560 Simply stops and restarts the periodic watcher again. This is only useful
2561 when you changed some parameters or the reschedule callback would return
2562 a different time than the last time it was called (e.g. in a crond like
2563 program when the crontabs have changed).
2564 .IP "ev_tstamp ev_periodic_at (ev_periodic *)" 4
2565 .IX Item "ev_tstamp ev_periodic_at (ev_periodic *)"
2566 When active, returns the absolute time that the watcher is supposed
2567 to trigger next. This is not the same as the \f(CW\*(C`offset\*(C'\fR argument to
2568 \&\f(CW\*(C`ev_periodic_set\*(C'\fR, but indeed works even in interval and manual
2569 rescheduling modes.
2570 .IP "ev_tstamp offset [read\-write]" 4
2571 .IX Item "ev_tstamp offset [read-write]"
2572 When repeating, this contains the offset value, otherwise this is the
2573 absolute point in time (the \f(CW\*(C`offset\*(C'\fR value passed to \f(CW\*(C`ev_periodic_set\*(C'\fR,
2574 although libev might modify this value for better numerical stability).
2575 .Sp
2576 Can be modified any time, but changes only take effect when the periodic
2577 timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2578 .IP "ev_tstamp interval [read\-write]" 4
2579 .IX Item "ev_tstamp interval [read-write]"
2580 The current interval value. Can be modified any time, but changes only
2581 take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
2582 called.
2583 .IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
2584 .IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
2585 The current reschedule callback, or \f(CW0\fR, if this functionality is
2586 switched off. Can be changed any time, but changes only take effect when
2587 the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
2588 .PP
2589 \fIExamples\fR
2590 .IX Subsection "Examples"
2591 .PP
2592 Example: Call a callback every hour, or, more precisely, whenever the
2593 system time is divisible by 3600. The callback invocation times have
2594 potentially a lot of jitter, but good long-term stability.
2595 .PP
2596 .Vb 5
2597 \& static void
2598 \& clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
2599 \& {
2600 \& ... its now a full hour (UTC, or TAI or whatever your clock follows)
2601 \& }
2602 \&
2603 \& ev_periodic hourly_tick;
2604 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
2605 \& ev_periodic_start (loop, &hourly_tick);
2606 .Ve
2607 .PP
2608 Example: The same as above, but use a reschedule callback to do it:
2609 .PP
2610 .Vb 1
2611 \& #include <math.h>
2612 \&
2613 \& static ev_tstamp
2614 \& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
2615 \& {
2616 \& return now + (3600. \- fmod (now, 3600.));
2617 \& }
2618 \&
2619 \& ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
2620 .Ve
2621 .PP
2622 Example: Call a callback every hour, starting now:
2623 .PP
2624 .Vb 4
2625 \& ev_periodic hourly_tick;
2626 \& ev_periodic_init (&hourly_tick, clock_cb,
2627 \& fmod (ev_now (loop), 3600.), 3600., 0);
2628 \& ev_periodic_start (loop, &hourly_tick);
2629 .Ve
2630 .ie n .SS """ev_signal"" \- signal me when a signal gets signalled!"
2631 .el .SS "\f(CWev_signal\fP \- signal me when a signal gets signalled!"
2632 .IX Subsection "ev_signal - signal me when a signal gets signalled!"
2633 Signal watchers will trigger an event when the process receives a specific
2634 signal one or more times. Even though signals are very asynchronous, libev
2635 will try its best to deliver signals synchronously, i.e. as part of the
2636 normal event processing, like any other event.
2637 .PP
2638 If you want signals to be delivered truly asynchronously, just use
2639 \&\f(CW\*(C`sigaction\*(C'\fR as you would do without libev and forget about sharing
2640 the signal. You can even use \f(CW\*(C`ev_async\*(C'\fR from a signal handler to
2641 synchronously wake up an event loop.
2642 .PP
2643 You can configure as many watchers as you like for the same signal, but
2644 only within the same loop, i.e. you can watch for \f(CW\*(C`SIGINT\*(C'\fR in your
2645 default loop and for \f(CW\*(C`SIGIO\*(C'\fR in another loop, but you cannot watch for
2646 \&\f(CW\*(C`SIGINT\*(C'\fR in both the default loop and another loop at the same time. At
2647 the moment, \f(CW\*(C`SIGCHLD\*(C'\fR is permanently tied to the default loop.
2648 .PP
2649 Only after the first watcher for a signal is started will libev actually
2650 register something with the kernel. It thus coexists with your own signal
2651 handlers as long as you don't register any with libev for the same signal.
2652 .PP
2653 If possible and supported, libev will install its handlers with
2654 \&\f(CW\*(C`SA_RESTART\*(C'\fR (or equivalent) behaviour enabled, so system calls should
2655 not be unduly interrupted. If you have a problem with system calls getting
2656 interrupted by signals you can block all signals in an \f(CW\*(C`ev_check\*(C'\fR watcher
2657 and unblock them in an \f(CW\*(C`ev_prepare\*(C'\fR watcher.
2658 .PP
2659 \fIThe special problem of inheritance over fork/execve/pthread_create\fR
2660 .IX Subsection "The special problem of inheritance over fork/execve/pthread_create"
2661 .PP
2662 Both the signal mask (\f(CW\*(C`sigprocmask\*(C'\fR) and the signal disposition
2663 (\f(CW\*(C`sigaction\*(C'\fR) are unspecified after starting a signal watcher (and after
2664 stopping it again), that is, libev might or might not block the signal,
2665 and might or might not set or restore the installed signal handler (but
2666 see \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR).
2667 .PP
2668 While this does not matter for the signal disposition (libev never
2669 sets signals to \f(CW\*(C`SIG_IGN\*(C'\fR, so handlers will be reset to \f(CW\*(C`SIG_DFL\*(C'\fR on
2670 \&\f(CW\*(C`execve\*(C'\fR), this matters for the signal mask: many programs do not expect
2671 certain signals to be blocked.
2672 .PP
2673 This means that before calling \f(CW\*(C`exec\*(C'\fR (from the child) you should reset
2674 the signal mask to whatever \*(L"default\*(R" you expect (all clear is a good
2675 choice usually).
2676 .PP
2677 The simplest way to ensure that the signal mask is reset in the child is
2678 to install a fork handler with \f(CW\*(C`pthread_atfork\*(C'\fR that resets it. That will
2679 catch fork calls done by libraries (such as the libc) as well.
2680 .PP
2681 In current versions of libev, the signal will not be blocked indefinitely
2682 unless you use the \f(CW\*(C`signalfd\*(C'\fR \s-1API\s0 (\f(CW\*(C`EV_SIGNALFD\*(C'\fR). While this reduces
2683 the window of opportunity for problems, it will not go away, as libev
2684 \&\fIhas\fR to modify the signal mask, at least temporarily.
2685 .PP
2686 So I can't stress this enough: \fIIf you do not reset your signal mask when
2687 you expect it to be empty, you have a race condition in your code\fR. This
2688 is not a libev-specific thing, this is true for most event libraries.
2689 .PP
2690 \fIThe special problem of threads signal handling\fR
2691 .IX Subsection "The special problem of threads signal handling"
2692 .PP
2693 \&\s-1POSIX\s0 threads has problematic signal handling semantics, specifically,
2694 a lot of functionality (sigfd, sigwait etc.) only really works if all
2695 threads in a process block signals, which is hard to achieve.
2696 .PP
2697 When you want to use sigwait (or mix libev signal handling with your own
2698 for the same signals), you can tackle this problem by globally blocking
2699 all signals before creating any threads (or creating them with a fully set
2700 sigprocmask) and also specifying the \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when creating
2701 loops. Then designate one thread as \*(L"signal receiver thread\*(R" which handles
2702 these signals. You can pass on any signals that libev might be interested
2703 in by calling \f(CW\*(C`ev_feed_signal\*(C'\fR.
2704 .PP
2705 \fIWatcher-Specific Functions and Data Members\fR
2706 .IX Subsection "Watcher-Specific Functions and Data Members"
2707 .IP "ev_signal_init (ev_signal *, callback, int signum)" 4
2708 .IX Item "ev_signal_init (ev_signal *, callback, int signum)"
2709 .PD 0
2710 .IP "ev_signal_set (ev_signal *, int signum)" 4
2711 .IX Item "ev_signal_set (ev_signal *, int signum)"
2712 .PD
2713 Configures the watcher to trigger on the given signal number (usually one
2714 of the \f(CW\*(C`SIGxxx\*(C'\fR constants).
2715 .IP "int signum [read\-only]" 4
2716 .IX Item "int signum [read-only]"
2717 The signal the watcher watches out for.
2718 .PP
2719 \fIExamples\fR
2720 .IX Subsection "Examples"
2721 .PP
2722 Example: Try to exit cleanly on \s-1SIGINT.\s0
2723 .PP
2724 .Vb 5
2725 \& static void
2726 \& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
2727 \& {
2728 \& ev_break (loop, EVBREAK_ALL);
2729 \& }
2730 \&
2731 \& ev_signal signal_watcher;
2732 \& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
2733 \& ev_signal_start (loop, &signal_watcher);
2734 .Ve
2735 .ie n .SS """ev_child"" \- watch out for process status changes"
2736 .el .SS "\f(CWev_child\fP \- watch out for process status changes"
2737 .IX Subsection "ev_child - watch out for process status changes"
2738 Child watchers trigger when your process receives a \s-1SIGCHLD\s0 in response to
2739 some child status changes (most typically when a child of yours dies or
2740 exits). It is permissible to install a child watcher \fIafter\fR the child
2741 has been forked (which implies it might have already exited), as long
2742 as the event loop isn't entered (or is continued from a watcher), i.e.,
2743 forking and then immediately registering a watcher for the child is fine,
2744 but forking and registering a watcher a few event loop iterations later or
2745 in the next callback invocation is not.
2746 .PP
2747 Only the default event loop is capable of handling signals, and therefore
2748 you can only register child watchers in the default event loop.
2749 .PP
2750 Due to some design glitches inside libev, child watchers will always be
2751 handled at maximum priority (their priority is set to \f(CW\*(C`EV_MAXPRI\*(C'\fR by
2752 libev)
2753 .PP
2754 \fIProcess Interaction\fR
2755 .IX Subsection "Process Interaction"
2756 .PP
2757 Libev grabs \f(CW\*(C`SIGCHLD\*(C'\fR as soon as the default event loop is
2758 initialised. This is necessary to guarantee proper behaviour even if the
2759 first child watcher is started after the child exits. The occurrence
2760 of \f(CW\*(C`SIGCHLD\*(C'\fR is recorded asynchronously, but child reaping is done
2761 synchronously as part of the event loop processing. Libev always reaps all
2762 children, even ones not watched.
2763 .PP
2764 \fIOverriding the Built-In Processing\fR
2765 .IX Subsection "Overriding the Built-In Processing"
2766 .PP
2767 Libev offers no special support for overriding the built-in child
2768 processing, but if your application collides with libev's default child
2769 handler, you can override it easily by installing your own handler for
2770 \&\f(CW\*(C`SIGCHLD\*(C'\fR after initialising the default loop, and making sure the
2771 default loop never gets destroyed. You are encouraged, however, to use an
2772 event-based approach to child reaping and thus use libev's support for
2773 that, so other libev users can use \f(CW\*(C`ev_child\*(C'\fR watchers freely.
2774 .PP
2775 \fIStopping the Child Watcher\fR
2776 .IX Subsection "Stopping the Child Watcher"
2777 .PP
2778 Currently, the child watcher never gets stopped, even when the
2779 child terminates, so normally one needs to stop the watcher in the
2780 callback. Future versions of libev might stop the watcher automatically
2781 when a child exit is detected (calling \f(CW\*(C`ev_child_stop\*(C'\fR twice is not a
2782 problem).
2783 .PP
2784 \fIWatcher-Specific Functions and Data Members\fR
2785 .IX Subsection "Watcher-Specific Functions and Data Members"
2786 .IP "ev_child_init (ev_child *, callback, int pid, int trace)" 4
2787 .IX Item "ev_child_init (ev_child *, callback, int pid, int trace)"
2788 .PD 0
2789 .IP "ev_child_set (ev_child *, int pid, int trace)" 4
2790 .IX Item "ev_child_set (ev_child *, int pid, int trace)"
2791 .PD
2792 Configures the watcher to wait for status changes of process \f(CW\*(C`pid\*(C'\fR (or
2793 \&\fIany\fR process if \f(CW\*(C`pid\*(C'\fR is specified as \f(CW0\fR). The callback can look
2794 at the \f(CW\*(C`rstatus\*(C'\fR member of the \f(CW\*(C`ev_child\*(C'\fR watcher structure to see
2795 the status word (use the macros from \f(CW\*(C`sys/wait.h\*(C'\fR and see your systems
2796 \&\f(CW\*(C`waitpid\*(C'\fR documentation). The \f(CW\*(C`rpid\*(C'\fR member contains the pid of the
2797 process causing the status change. \f(CW\*(C`trace\*(C'\fR must be either \f(CW0\fR (only
2798 activate the watcher when the process terminates) or \f(CW1\fR (additionally
2799 activate the watcher when the process is stopped or continued).
2800 .IP "int pid [read\-only]" 4
2801 .IX Item "int pid [read-only]"
2802 The process id this watcher watches out for, or \f(CW0\fR, meaning any process id.
2803 .IP "int rpid [read\-write]" 4
2804 .IX Item "int rpid [read-write]"
2805 The process id that detected a status change.
2806 .IP "int rstatus [read\-write]" 4
2807 .IX Item "int rstatus [read-write]"
2808 The process exit/trace status caused by \f(CW\*(C`rpid\*(C'\fR (see your systems
2809 \&\f(CW\*(C`waitpid\*(C'\fR and \f(CW\*(C`sys/wait.h\*(C'\fR documentation for details).
2810 .PP
2811 \fIExamples\fR
2812 .IX Subsection "Examples"
2813 .PP
2814 Example: \f(CW\*(C`fork()\*(C'\fR a new process and install a child handler to wait for
2815 its completion.
2816 .PP
2817 .Vb 1
2818 \& ev_child cw;
2819 \&
2820 \& static void
2821 \& child_cb (EV_P_ ev_child *w, int revents)
2822 \& {
2823 \& ev_child_stop (EV_A_ w);
2824 \& printf ("process %d exited with status %x\en", w\->rpid, w\->rstatus);
2825 \& }
2826 \&
2827 \& pid_t pid = fork ();
2828 \&
2829 \& if (pid < 0)
2830 \& // error
2831 \& else if (pid == 0)
2832 \& {
2833 \& // the forked child executes here
2834 \& exit (1);
2835 \& }
2836 \& else
2837 \& {
2838 \& ev_child_init (&cw, child_cb, pid, 0);
2839 \& ev_child_start (EV_DEFAULT_ &cw);
2840 \& }
2841 .Ve
2842 .ie n .SS """ev_stat"" \- did the file attributes just change?"
2843 .el .SS "\f(CWev_stat\fP \- did the file attributes just change?"
2844 .IX Subsection "ev_stat - did the file attributes just change?"
2845 This watches a file system path for attribute changes. That is, it calls
2846 \&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
2847 and sees if it changed compared to the last time, invoking the callback
2848 if it did. Starting the watcher \f(CW\*(C`stat\*(C'\fR's the file, so only changes that
2849 happen after the watcher has been started will be reported.
2850 .PP
2851 The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
2852 not exist\*(R" is a status change like any other. The condition \*(L"path does not
2853 exist\*(R" (or more correctly \*(L"path cannot be stat'ed\*(R") is signified by the
2854 \&\f(CW\*(C`st_nlink\*(C'\fR field being zero (which is otherwise always forced to be at
2855 least one) and all the other fields of the stat buffer having unspecified
2856 contents.
2857 .PP
2858 The path \fImust not\fR end in a slash or contain special components such as
2859 \&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
2860 your working directory changes, then the behaviour is undefined.
2861 .PP
2862 Since there is no portable change notification interface available, the
2863 portable implementation simply calls \f(CWstat(2)\fR regularly on the path
2864 to see if it changed somehow. You can specify a recommended polling
2865 interval for this case. If you specify a polling interval of \f(CW0\fR (highly
2866 recommended!) then a \fIsuitable, unspecified default\fR value will be used
2867 (which you can expect to be around five seconds, although this might
2868 change dynamically). Libev will also impose a minimum interval which is
2869 currently around \f(CW0.1\fR, but that's usually overkill.
2870 .PP
2871 This watcher type is not meant for massive numbers of stat watchers,
2872 as even with OS-supported change notifications, this can be
2873 resource-intensive.
2874 .PP
2875 At the time of this writing, the only OS-specific interface implemented
2876 is the Linux inotify interface (implementing kqueue support is left as an
2877 exercise for the reader. Note, however, that the author sees no way of
2878 implementing \f(CW\*(C`ev_stat\*(C'\fR semantics with kqueue, except as a hint).
2879 .PP
2880 \fI\s-1ABI\s0 Issues (Largefile Support)\fR
2881 .IX Subsection "ABI Issues (Largefile Support)"
2882 .PP
2883 Libev by default (unless the user overrides this) uses the default
2884 compilation environment, which means that on systems with large file
2885 support disabled by default, you get the 32 bit version of the stat
2886 structure. When using the library from programs that change the \s-1ABI\s0 to
2887 use 64 bit file offsets the programs will fail. In that case you have to
2888 compile libev with the same flags to get binary compatibility. This is
2889 obviously the case with any flags that change the \s-1ABI,\s0 but the problem is
2890 most noticeably displayed with ev_stat and large file support.
2891 .PP
2892 The solution for this is to lobby your distribution maker to make large
2893 file interfaces available by default (as e.g. FreeBSD does) and not
2894 optional. Libev cannot simply switch on large file support because it has
2895 to exchange stat structures with application programs compiled using the
2896 default compilation environment.
2897 .PP
2898 \fIInotify and Kqueue\fR
2899 .IX Subsection "Inotify and Kqueue"
2900 .PP
2901 When \f(CW\*(C`inotify (7)\*(C'\fR support has been compiled into libev and present at
2902 runtime, it will be used to speed up change detection where possible. The
2903 inotify descriptor will be created lazily when the first \f(CW\*(C`ev_stat\*(C'\fR
2904 watcher is being started.
2905 .PP
2906 Inotify presence does not change the semantics of \f(CW\*(C`ev_stat\*(C'\fR watchers
2907 except that changes might be detected earlier, and in some cases, to avoid
2908 making regular \f(CW\*(C`stat\*(C'\fR calls. Even in the presence of inotify support
2909 there are many cases where libev has to resort to regular \f(CW\*(C`stat\*(C'\fR polling,
2910 but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
2911 many bugs), the path exists (i.e. stat succeeds), and the path resides on
2912 a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
2913 xfs are fully working) libev usually gets away without polling.
2914 .PP
2915 There is no support for kqueue, as apparently it cannot be used to
2916 implement this functionality, due to the requirement of having a file
2917 descriptor open on the object at all times, and detecting renames, unlinks
2918 etc. is difficult.
2919 .PP
2920 \fI\f(CI\*(C`stat ()\*(C'\fI is a synchronous operation\fR
2921 .IX Subsection "stat () is a synchronous operation"
2922 .PP
2923 Libev doesn't normally do any kind of I/O itself, and so is not blocking
2924 the process. The exception are \f(CW\*(C`ev_stat\*(C'\fR watchers \- those call \f(CW\*(C`stat
2925 ()\*(C'\fR, which is a synchronous operation.
2926 .PP
2927 For local paths, this usually doesn't matter: unless the system is very
2928 busy or the intervals between stat's are large, a stat call will be fast,
2929 as the path data is usually in memory already (except when starting the
2930 watcher).
2931 .PP
2932 For networked file systems, calling \f(CW\*(C`stat ()\*(C'\fR can block an indefinite
2933 time due to network issues, and even under good conditions, a stat call
2934 often takes multiple milliseconds.
2935 .PP
2936 Therefore, it is best to avoid using \f(CW\*(C`ev_stat\*(C'\fR watchers on networked
2937 paths, although this is fully supported by libev.
2938 .PP
2939 \fIThe special problem of stat time resolution\fR
2940 .IX Subsection "The special problem of stat time resolution"
2941 .PP
2942 The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
2943 and even on systems where the resolution is higher, most file systems
2944 still only support whole seconds.
2945 .PP
2946 That means that, if the time is the only thing that changes, you can
2947 easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
2948 calls your callback, which does something. When there is another update
2949 within the same second, \f(CW\*(C`ev_stat\*(C'\fR will be unable to detect unless the
2950 stat data does change in other ways (e.g. file size).
2951 .PP
2952 The solution to this is to delay acting on a change for slightly more
2953 than a second (or till slightly after the next full second boundary), using
2954 a roughly one-second-delay \f(CW\*(C`ev_timer\*(C'\fR (e.g. \f(CW\*(C`ev_timer_set (w, 0., 1.02);
2955 ev_timer_again (loop, w)\*(C'\fR).
2956 .PP
2957 The \f(CW.02\fR offset is added to work around small timing inconsistencies
2958 of some operating systems (where the second counter of the current time
2959 might be be delayed. One such system is the Linux kernel, where a call to
2960 \&\f(CW\*(C`gettimeofday\*(C'\fR might return a timestamp with a full second later than
2961 a subsequent \f(CW\*(C`time\*(C'\fR call \- if the equivalent of \f(CW\*(C`time ()\*(C'\fR is used to
2962 update file times then there will be a small window where the kernel uses
2963 the previous second to update file times but libev might already execute
2964 the timer callback).
2965 .PP
2966 \fIWatcher-Specific Functions and Data Members\fR
2967 .IX Subsection "Watcher-Specific Functions and Data Members"
2968 .IP "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)" 4
2969 .IX Item "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)"
2970 .PD 0
2971 .IP "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)" 4
2972 .IX Item "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)"
2973 .PD
2974 Configures the watcher to wait for status changes of the given
2975 \&\f(CW\*(C`path\*(C'\fR. The \f(CW\*(C`interval\*(C'\fR is a hint on how quickly a change is expected to
2976 be detected and should normally be specified as \f(CW0\fR to let libev choose
2977 a suitable value. The memory pointed to by \f(CW\*(C`path\*(C'\fR must point to the same
2978 path for as long as the watcher is active.
2979 .Sp
2980 The callback will receive an \f(CW\*(C`EV_STAT\*(C'\fR event when a change was detected,
2981 relative to the attributes at the time the watcher was started (or the
2982 last change was detected).
2983 .IP "ev_stat_stat (loop, ev_stat *)" 4
2984 .IX Item "ev_stat_stat (loop, ev_stat *)"
2985 Updates the stat buffer immediately with new values. If you change the
2986 watched path in your callback, you could call this function to avoid
2987 detecting this change (while introducing a race condition if you are not
2988 the only one changing the path). Can also be useful simply to find out the
2989 new values.
2990 .IP "ev_statdata attr [read\-only]" 4
2991 .IX Item "ev_statdata attr [read-only]"
2992 The most-recently detected attributes of the file. Although the type is
2993 \&\f(CW\*(C`ev_statdata\*(C'\fR, this is usually the (or one of the) \f(CW\*(C`struct stat\*(C'\fR types
2994 suitable for your system, but you can only rely on the POSIX-standardised
2995 members to be present. If the \f(CW\*(C`st_nlink\*(C'\fR member is \f(CW0\fR, then there was
2996 some error while \f(CW\*(C`stat\*(C'\fRing the file.
2997 .IP "ev_statdata prev [read\-only]" 4
2998 .IX Item "ev_statdata prev [read-only]"
2999 The previous attributes of the file. The callback gets invoked whenever
3000 \&\f(CW\*(C`prev\*(C'\fR != \f(CW\*(C`attr\*(C'\fR, or, more precisely, one or more of these members
3001 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,
3002 \&\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.
3003 .IP "ev_tstamp interval [read\-only]" 4
3004 .IX Item "ev_tstamp interval [read-only]"
3005 The specified interval.
3006 .IP "const char *path [read\-only]" 4
3007 .IX Item "const char *path [read-only]"
3008 The file system path that is being watched.
3009 .PP
3010 \fIExamples\fR
3011 .IX Subsection "Examples"
3012 .PP
3013 Example: Watch \f(CW\*(C`/etc/passwd\*(C'\fR for attribute changes.
3014 .PP
3015 .Vb 10
3016 \& static void
3017 \& passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
3018 \& {
3019 \& /* /etc/passwd changed in some way */
3020 \& if (w\->attr.st_nlink)
3021 \& {
3022 \& printf ("passwd current size %ld\en", (long)w\->attr.st_size);
3023 \& printf ("passwd current atime %ld\en", (long)w\->attr.st_mtime);
3024 \& printf ("passwd current mtime %ld\en", (long)w\->attr.st_mtime);
3025 \& }
3026 \& else
3027 \& /* you shalt not abuse printf for puts */
3028 \& puts ("wow, /etc/passwd is not there, expect problems. "
3029 \& "if this is windows, they already arrived\en");
3030 \& }
3031 \&
3032 \& ...
3033 \& ev_stat passwd;
3034 \&
3035 \& ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
3036 \& ev_stat_start (loop, &passwd);
3037 .Ve
3038 .PP
3039 Example: Like above, but additionally use a one-second delay so we do not
3040 miss updates (however, frequent updates will delay processing, too, so
3041 one might do the work both on \f(CW\*(C`ev_stat\*(C'\fR callback invocation \fIand\fR on
3042 \&\f(CW\*(C`ev_timer\*(C'\fR callback invocation).
3043 .PP
3044 .Vb 2
3045 \& static ev_stat passwd;
3046 \& static ev_timer timer;
3047 \&
3048 \& static void
3049 \& timer_cb (EV_P_ ev_timer *w, int revents)
3050 \& {
3051 \& ev_timer_stop (EV_A_ w);
3052 \&
3053 \& /* now it\*(Aqs one second after the most recent passwd change */
3054 \& }
3055 \&
3056 \& static void
3057 \& stat_cb (EV_P_ ev_stat *w, int revents)
3058 \& {
3059 \& /* reset the one\-second timer */
3060 \& ev_timer_again (EV_A_ &timer);
3061 \& }
3062 \&
3063 \& ...
3064 \& ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
3065 \& ev_stat_start (loop, &passwd);
3066 \& ev_timer_init (&timer, timer_cb, 0., 1.02);
3067 .Ve
3068 .ie n .SS """ev_idle"" \- when you've got nothing better to do..."
3069 .el .SS "\f(CWev_idle\fP \- when you've got nothing better to do..."
3070 .IX Subsection "ev_idle - when you've got nothing better to do..."
3071 Idle watchers trigger events when no other events of the same or higher
3072 priority are pending (prepare, check and other idle watchers do not count
3073 as receiving \*(L"events\*(R").
3074 .PP
3075 That is, as long as your process is busy handling sockets or timeouts
3076 (or even signals, imagine) of the same or higher priority it will not be
3077 triggered. But when your process is idle (or only lower-priority watchers
3078 are pending), the idle watchers are being called once per event loop
3079 iteration \- until stopped, that is, or your process receives more events
3080 and becomes busy again with higher priority stuff.
3081 .PP
3082 The most noteworthy effect is that as long as any idle watchers are
3083 active, the process will not block when waiting for new events.
3084 .PP
3085 Apart from keeping your process non-blocking (which is a useful
3086 effect on its own sometimes), idle watchers are a good place to do
3087 \&\*(L"pseudo-background processing\*(R", or delay processing stuff to after the
3088 event loop has handled all outstanding events.
3089 .PP
3090 \fIAbusing an \f(CI\*(C`ev_idle\*(C'\fI watcher for its side-effect\fR
3091 .IX Subsection "Abusing an ev_idle watcher for its side-effect"
3092 .PP
3093 As long as there is at least one active idle watcher, libev will never
3094 sleep unnecessarily. Or in other words, it will loop as fast as possible.
3095 For this to work, the idle watcher doesn't need to be invoked at all \- the
3096 lowest priority will do.
3097 .PP
3098 This mode of operation can be useful together with an \f(CW\*(C`ev_check\*(C'\fR watcher,
3099 to do something on each event loop iteration \- for example to balance load
3100 between different connections.
3101 .PP
3102 See \*(L"Abusing an ev_check watcher for its side-effect\*(R" for a longer
3103 example.
3104 .PP
3105 \fIWatcher-Specific Functions and Data Members\fR
3106 .IX Subsection "Watcher-Specific Functions and Data Members"
3107 .IP "ev_idle_init (ev_idle *, callback)" 4
3108 .IX Item "ev_idle_init (ev_idle *, callback)"
3109 Initialises and configures the idle watcher \- it has no parameters of any
3110 kind. There is a \f(CW\*(C`ev_idle_set\*(C'\fR macro, but using it is utterly pointless,
3111 believe me.
3112 .PP
3113 \fIExamples\fR
3114 .IX Subsection "Examples"
3115 .PP
3116 Example: Dynamically allocate an \f(CW\*(C`ev_idle\*(C'\fR watcher, start it, and in the
3117 callback, free it. Also, use no error checking, as usual.
3118 .PP
3119 .Vb 5
3120 \& static void
3121 \& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
3122 \& {
3123 \& // stop the watcher
3124 \& ev_idle_stop (loop, w);
3125 \&
3126 \& // now we can free it
3127 \& free (w);
3128 \&
3129 \& // now do something you wanted to do when the program has
3130 \& // no longer anything immediate to do.
3131 \& }
3132 \&
3133 \& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
3134 \& ev_idle_init (idle_watcher, idle_cb);
3135 \& ev_idle_start (loop, idle_watcher);
3136 .Ve
3137 .ie n .SS """ev_prepare"" and ""ev_check"" \- customise your event loop!"
3138 .el .SS "\f(CWev_prepare\fP and \f(CWev_check\fP \- customise your event loop!"
3139 .IX Subsection "ev_prepare and ev_check - customise your event loop!"
3140 Prepare and check watchers are often (but not always) used in pairs:
3141 prepare watchers get invoked before the process blocks and check watchers
3142 afterwards.
3143 .PP
3144 You \fImust not\fR call \f(CW\*(C`ev_run\*(C'\fR (or similar functions that enter the
3145 current event loop) or \f(CW\*(C`ev_loop_fork\*(C'\fR from either \f(CW\*(C`ev_prepare\*(C'\fR or
3146 \&\f(CW\*(C`ev_check\*(C'\fR watchers. Other loops than the current one are fine,
3147 however. The rationale behind this is that you do not need to check
3148 for recursion in those watchers, i.e. the sequence will always be
3149 \&\f(CW\*(C`ev_prepare\*(C'\fR, blocking, \f(CW\*(C`ev_check\*(C'\fR so if you have one watcher of each
3150 kind they will always be called in pairs bracketing the blocking call.
3151 .PP
3152 Their main purpose is to integrate other event mechanisms into libev and
3153 their use is somewhat advanced. They could be used, for example, to track
3154 variable changes, implement your own watchers, integrate net-snmp or a
3155 coroutine library and lots more. They are also occasionally useful if
3156 you cache some data and want to flush it before blocking (for example,
3157 in X programs you might want to do an \f(CW\*(C`XFlush ()\*(C'\fR in an \f(CW\*(C`ev_prepare\*(C'\fR
3158 watcher).
3159 .PP
3160 This is done by examining in each prepare call which file descriptors
3161 need to be watched by the other library, registering \f(CW\*(C`ev_io\*(C'\fR watchers
3162 for them and starting an \f(CW\*(C`ev_timer\*(C'\fR watcher for any timeouts (many
3163 libraries provide exactly this functionality). Then, in the check watcher,
3164 you check for any events that occurred (by checking the pending status
3165 of all watchers and stopping them) and call back into the library. The
3166 I/O and timer callbacks will never actually be called (but must be valid
3167 nevertheless, because you never know, you know?).
3168 .PP
3169 As another example, the Perl Coro module uses these hooks to integrate
3170 coroutines into libev programs, by yielding to other active coroutines
3171 during each prepare and only letting the process block if no coroutines
3172 are ready to run (it's actually more complicated: it only runs coroutines
3173 with priority higher than or equal to the event loop and one coroutine
3174 of lower priority, but only once, using idle watchers to keep the event
3175 loop from blocking if lower-priority coroutines are active, thus mapping
3176 low-priority coroutines to idle/background tasks).
3177 .PP
3178 When used for this purpose, it is recommended to give \f(CW\*(C`ev_check\*(C'\fR watchers
3179 highest (\f(CW\*(C`EV_MAXPRI\*(C'\fR) priority, to ensure that they are being run before
3180 any other watchers after the poll (this doesn't matter for \f(CW\*(C`ev_prepare\*(C'\fR
3181 watchers).
3182 .PP
3183 Also, \f(CW\*(C`ev_check\*(C'\fR watchers (and \f(CW\*(C`ev_prepare\*(C'\fR watchers, too) should not
3184 activate (\*(L"feed\*(R") events into libev. While libev fully supports this, they
3185 might get executed before other \f(CW\*(C`ev_check\*(C'\fR watchers did their job. As
3186 \&\f(CW\*(C`ev_check\*(C'\fR watchers are often used to embed other (non-libev) event
3187 loops those other event loops might be in an unusable state until their
3188 \&\f(CW\*(C`ev_check\*(C'\fR watcher ran (always remind yourself to coexist peacefully with
3189 others).
3190 .PP
3191 \fIAbusing an \f(CI\*(C`ev_check\*(C'\fI watcher for its side-effect\fR
3192 .IX Subsection "Abusing an ev_check watcher for its side-effect"
3193 .PP
3194 \&\f(CW\*(C`ev_check\*(C'\fR (and less often also \f(CW\*(C`ev_prepare\*(C'\fR) watchers can also be
3195 useful because they are called once per event loop iteration. For
3196 example, if you want to handle a large number of connections fairly, you
3197 normally only do a bit of work for each active connection, and if there
3198 is more work to do, you wait for the next event loop iteration, so other
3199 connections have a chance of making progress.
3200 .PP
3201 Using an \f(CW\*(C`ev_check\*(C'\fR watcher is almost enough: it will be called on the
3202 next event loop iteration. However, that isn't as soon as possible \-
3203 without external events, your \f(CW\*(C`ev_check\*(C'\fR watcher will not be invoked.
3204 .PP
3205 This is where \f(CW\*(C`ev_idle\*(C'\fR watchers come in handy \- all you need is a
3206 single global idle watcher that is active as long as you have one active
3207 \&\f(CW\*(C`ev_check\*(C'\fR watcher. The \f(CW\*(C`ev_idle\*(C'\fR watcher makes sure the event loop
3208 will not sleep, and the \f(CW\*(C`ev_check\*(C'\fR watcher makes sure a callback gets
3209 invoked. Neither watcher alone can do that.
3210 .PP
3211 \fIWatcher-Specific Functions and Data Members\fR
3212 .IX Subsection "Watcher-Specific Functions and Data Members"
3213 .IP "ev_prepare_init (ev_prepare *, callback)" 4
3214 .IX Item "ev_prepare_init (ev_prepare *, callback)"
3215 .PD 0
3216 .IP "ev_check_init (ev_check *, callback)" 4
3217 .IX Item "ev_check_init (ev_check *, callback)"
3218 .PD
3219 Initialises and configures the prepare or check watcher \- they have no
3220 parameters of any kind. There are \f(CW\*(C`ev_prepare_set\*(C'\fR and \f(CW\*(C`ev_check_set\*(C'\fR
3221 macros, but using them is utterly, utterly, utterly and completely
3222 pointless.
3223 .PP
3224 \fIExamples\fR
3225 .IX Subsection "Examples"
3226 .PP
3227 There are a number of principal ways to embed other event loops or modules
3228 into libev. Here are some ideas on how to include libadns into libev
3229 (there is a Perl module named \f(CW\*(C`EV::ADNS\*(C'\fR that does this, which you could
3230 use as a working example. Another Perl module named \f(CW\*(C`EV::Glib\*(C'\fR embeds a
3231 Glib main context into libev, and finally, \f(CW\*(C`Glib::EV\*(C'\fR embeds \s-1EV\s0 into the
3232 Glib event loop).
3233 .PP
3234 Method 1: Add \s-1IO\s0 watchers and a timeout watcher in a prepare handler,
3235 and in a check watcher, destroy them and call into libadns. What follows
3236 is pseudo-code only of course. This requires you to either use a low
3237 priority for the check watcher or use \f(CW\*(C`ev_clear_pending\*(C'\fR explicitly, as
3238 the callbacks for the IO/timeout watchers might not have been called yet.
3239 .PP
3240 .Vb 2
3241 \& static ev_io iow [nfd];
3242 \& static ev_timer tw;
3243 \&
3244 \& static void
3245 \& io_cb (struct ev_loop *loop, ev_io *w, int revents)
3246 \& {
3247 \& }
3248 \&
3249 \& // create io watchers for each fd and a timer before blocking
3250 \& static void
3251 \& adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
3252 \& {
3253 \& int timeout = 3600000;
3254 \& struct pollfd fds [nfd];
3255 \& // actual code will need to loop here and realloc etc.
3256 \& adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
3257 \&
3258 \& /* the callback is illegal, but won\*(Aqt be called as we stop during check */
3259 \& ev_timer_init (&tw, 0, timeout * 1e\-3, 0.);
3260 \& ev_timer_start (loop, &tw);
3261 \&
3262 \& // create one ev_io per pollfd
3263 \& for (int i = 0; i < nfd; ++i)
3264 \& {
3265 \& ev_io_init (iow + i, io_cb, fds [i].fd,
3266 \& ((fds [i].events & POLLIN ? EV_READ : 0)
3267 \& | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
3268 \&
3269 \& fds [i].revents = 0;
3270 \& ev_io_start (loop, iow + i);
3271 \& }
3272 \& }
3273 \&
3274 \& // stop all watchers after blocking
3275 \& static void
3276 \& adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
3277 \& {
3278 \& ev_timer_stop (loop, &tw);
3279 \&
3280 \& for (int i = 0; i < nfd; ++i)
3281 \& {
3282 \& // set the relevant poll flags
3283 \& // could also call adns_processreadable etc. here
3284 \& struct pollfd *fd = fds + i;
3285 \& int revents = ev_clear_pending (iow + i);
3286 \& if (revents & EV_READ ) fd\->revents |= fd\->events & POLLIN;
3287 \& if (revents & EV_WRITE) fd\->revents |= fd\->events & POLLOUT;
3288 \&
3289 \& // now stop the watcher
3290 \& ev_io_stop (loop, iow + i);
3291 \& }
3292 \&
3293 \& adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
3294 \& }
3295 .Ve
3296 .PP
3297 Method 2: This would be just like method 1, but you run \f(CW\*(C`adns_afterpoll\*(C'\fR
3298 in the prepare watcher and would dispose of the check watcher.
3299 .PP
3300 Method 3: If the module to be embedded supports explicit event
3301 notification (libadns does), you can also make use of the actual watcher
3302 callbacks, and only destroy/create the watchers in the prepare watcher.
3303 .PP
3304 .Vb 5
3305 \& static void
3306 \& timer_cb (EV_P_ ev_timer *w, int revents)
3307 \& {
3308 \& adns_state ads = (adns_state)w\->data;
3309 \& update_now (EV_A);
3310 \&
3311 \& adns_processtimeouts (ads, &tv_now);
3312 \& }
3313 \&
3314 \& static void
3315 \& io_cb (EV_P_ ev_io *w, int revents)
3316 \& {
3317 \& adns_state ads = (adns_state)w\->data;
3318 \& update_now (EV_A);
3319 \&
3320 \& if (revents & EV_READ ) adns_processreadable (ads, w\->fd, &tv_now);
3321 \& if (revents & EV_WRITE) adns_processwriteable (ads, w\->fd, &tv_now);
3322 \& }
3323 \&
3324 \& // do not ever call adns_afterpoll
3325 .Ve
3326 .PP
3327 Method 4: Do not use a prepare or check watcher because the module you
3328 want to embed is not flexible enough to support it. Instead, you can
3329 override their poll function. The drawback with this solution is that the
3330 main loop is now no longer controllable by \s-1EV.\s0 The \f(CW\*(C`Glib::EV\*(C'\fR module uses
3331 this approach, effectively embedding \s-1EV\s0 as a client into the horrible
3332 libglib event loop.
3333 .PP
3334 .Vb 4
3335 \& static gint
3336 \& event_poll_func (GPollFD *fds, guint nfds, gint timeout)
3337 \& {
3338 \& int got_events = 0;
3339 \&
3340 \& for (n = 0; n < nfds; ++n)
3341 \& // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
3342 \&
3343 \& if (timeout >= 0)
3344 \& // create/start timer
3345 \&
3346 \& // poll
3347 \& ev_run (EV_A_ 0);
3348 \&
3349 \& // stop timer again
3350 \& if (timeout >= 0)
3351 \& ev_timer_stop (EV_A_ &to);
3352 \&
3353 \& // stop io watchers again \- their callbacks should have set
3354 \& for (n = 0; n < nfds; ++n)
3355 \& ev_io_stop (EV_A_ iow [n]);
3356 \&
3357 \& return got_events;
3358 \& }
3359 .Ve
3360 .ie n .SS """ev_embed"" \- when one backend isn't enough..."
3361 .el .SS "\f(CWev_embed\fP \- when one backend isn't enough..."
3362 .IX Subsection "ev_embed - when one backend isn't enough..."
3363 This is a rather advanced watcher type that lets you embed one event loop
3364 into another (currently only \f(CW\*(C`ev_io\*(C'\fR events are supported in the embedded
3365 loop, other types of watchers might be handled in a delayed or incorrect
3366 fashion and must not be used).
3367 .PP
3368 There are primarily two reasons you would want that: work around bugs and
3369 prioritise I/O.
3370 .PP
3371 As an example for a bug workaround, the kqueue backend might only support
3372 sockets on some platform, so it is unusable as generic backend, but you
3373 still want to make use of it because you have many sockets and it scales
3374 so nicely. In this case, you would create a kqueue-based loop and embed
3375 it into your default loop (which might use e.g. poll). Overall operation
3376 will be a bit slower because first libev has to call \f(CW\*(C`poll\*(C'\fR and then
3377 \&\f(CW\*(C`kevent\*(C'\fR, but at least you can use both mechanisms for what they are
3378 best: \f(CW\*(C`kqueue\*(C'\fR for scalable sockets and \f(CW\*(C`poll\*(C'\fR if you want it to work :)
3379 .PP
3380 As for prioritising I/O: under rare circumstances you have the case where
3381 some fds have to be watched and handled very quickly (with low latency),
3382 and even priorities and idle watchers might have too much overhead. In
3383 this case you would put all the high priority stuff in one loop and all
3384 the rest in a second one, and embed the second one in the first.
3385 .PP
3386 As long as the watcher is active, the callback will be invoked every
3387 time there might be events pending in the embedded loop. The callback
3388 must then call \f(CW\*(C`ev_embed_sweep (mainloop, watcher)\*(C'\fR to make a single
3389 sweep and invoke their callbacks (the callback doesn't need to invoke the
3390 \&\f(CW\*(C`ev_embed_sweep\*(C'\fR function directly, it could also start an idle watcher
3391 to give the embedded loop strictly lower priority for example).
3392 .PP
3393 You can also set the callback to \f(CW0\fR, in which case the embed watcher
3394 will automatically execute the embedded loop sweep whenever necessary.
3395 .PP
3396 Fork detection will be handled transparently while the \f(CW\*(C`ev_embed\*(C'\fR watcher
3397 is active, i.e., the embedded loop will automatically be forked when the
3398 embedding loop forks. In other cases, the user is responsible for calling
3399 \&\f(CW\*(C`ev_loop_fork\*(C'\fR on the embedded loop.
3400 .PP
3401 Unfortunately, not all backends are embeddable: only the ones returned by
3402 \&\f(CW\*(C`ev_embeddable_backends\*(C'\fR are, which, unfortunately, does not include any
3403 portable one.
3404 .PP
3405 So when you want to use this feature you will always have to be prepared
3406 that you cannot get an embeddable loop. The recommended way to get around
3407 this is to have a separate variables for your embeddable loop, try to
3408 create it, and if that fails, use the normal loop for everything.
3409 .PP
3410 \fI\f(CI\*(C`ev_embed\*(C'\fI and fork\fR
3411 .IX Subsection "ev_embed and fork"
3412 .PP
3413 While the \f(CW\*(C`ev_embed\*(C'\fR watcher is running, forks in the embedding loop will
3414 automatically be applied to the embedded loop as well, so no special
3415 fork handling is required in that case. When the watcher is not running,
3416 however, it is still the task of the libev user to call \f(CW\*(C`ev_loop_fork ()\*(C'\fR
3417 as applicable.
3418 .PP
3419 \fIWatcher-Specific Functions and Data Members\fR
3420 .IX Subsection "Watcher-Specific Functions and Data Members"
3421 .IP "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)" 4
3422 .IX Item "ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)"
3423 .PD 0
3424 .IP "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)" 4
3425 .IX Item "ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)"
3426 .PD
3427 Configures the watcher to embed the given loop, which must be
3428 embeddable. If the callback is \f(CW0\fR, then \f(CW\*(C`ev_embed_sweep\*(C'\fR will be
3429 invoked automatically, otherwise it is the responsibility of the callback
3430 to invoke it (it will continue to be called until the sweep has been done,
3431 if you do not want that, you need to temporarily stop the embed watcher).
3432 .IP "ev_embed_sweep (loop, ev_embed *)" 4
3433 .IX Item "ev_embed_sweep (loop, ev_embed *)"
3434 Make a single, non-blocking sweep over the embedded loop. This works
3435 similarly to \f(CW\*(C`ev_run (embedded_loop, EVRUN_NOWAIT)\*(C'\fR, but in the most
3436 appropriate way for embedded loops.
3437 .IP "struct ev_loop *other [read\-only]" 4
3438 .IX Item "struct ev_loop *other [read-only]"
3439 The embedded event loop.
3440 .PP
3441 \fIExamples\fR
3442 .IX Subsection "Examples"
3443 .PP
3444 Example: Try to get an embeddable event loop and embed it into the default
3445 event loop. If that is not possible, use the default loop. The default
3446 loop is stored in \f(CW\*(C`loop_hi\*(C'\fR, while the embeddable loop is stored in
3447 \&\f(CW\*(C`loop_lo\*(C'\fR (which is \f(CW\*(C`loop_hi\*(C'\fR in the case no embeddable loop can be
3448 used).
3449 .PP
3450 .Vb 3
3451 \& struct ev_loop *loop_hi = ev_default_init (0);
3452 \& struct ev_loop *loop_lo = 0;
3453 \& ev_embed embed;
3454 \&
3455 \& // see if there is a chance of getting one that works
3456 \& // (remember that a flags value of 0 means autodetection)
3457 \& loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
3458 \& ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
3459 \& : 0;
3460 \&
3461 \& // if we got one, then embed it, otherwise default to loop_hi
3462 \& if (loop_lo)
3463 \& {
3464 \& ev_embed_init (&embed, 0, loop_lo);
3465 \& ev_embed_start (loop_hi, &embed);
3466 \& }
3467 \& else
3468 \& loop_lo = loop_hi;
3469 .Ve
3470 .PP
3471 Example: Check if kqueue is available but not recommended and create
3472 a kqueue backend for use with sockets (which usually work with any
3473 kqueue implementation). Store the kqueue/socket\-only event loop in
3474 \&\f(CW\*(C`loop_socket\*(C'\fR. (One might optionally use \f(CW\*(C`EVFLAG_NOENV\*(C'\fR, too).
3475 .PP
3476 .Vb 3
3477 \& struct ev_loop *loop = ev_default_init (0);
3478 \& struct ev_loop *loop_socket = 0;
3479 \& ev_embed embed;
3480 \&
3481 \& if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
3482 \& if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
3483 \& {
3484 \& ev_embed_init (&embed, 0, loop_socket);
3485 \& ev_embed_start (loop, &embed);
3486 \& }
3487 \&
3488 \& if (!loop_socket)
3489 \& loop_socket = loop;
3490 \&
3491 \& // now use loop_socket for all sockets, and loop for everything else
3492 .Ve
3493 .ie n .SS """ev_fork"" \- the audacity to resume the event loop after a fork"
3494 .el .SS "\f(CWev_fork\fP \- the audacity to resume the event loop after a fork"
3495 .IX Subsection "ev_fork - the audacity to resume the event loop after a fork"
3496 Fork watchers are called when a \f(CW\*(C`fork ()\*(C'\fR was detected (usually because
3497 whoever is a good citizen cared to tell libev about it by calling
3498 \&\f(CW\*(C`ev_loop_fork\*(C'\fR). The invocation is done before the event loop blocks next
3499 and before \f(CW\*(C`ev_check\*(C'\fR watchers are being called, and only in the child
3500 after the fork. If whoever good citizen calling \f(CW\*(C`ev_default_fork\*(C'\fR cheats
3501 and calls it in the wrong process, the fork handlers will be invoked, too,
3502 of course.
3503 .PP
3504 \fIThe special problem of life after fork \- how is it possible?\fR
3505 .IX Subsection "The special problem of life after fork - how is it possible?"
3506 .PP
3507 Most uses of \f(CW\*(C`fork ()\*(C'\fR consist of forking, then some simple calls to set
3508 up/change the process environment, followed by a call to \f(CW\*(C`exec()\*(C'\fR. This
3509 sequence should be handled by libev without any problems.
3510 .PP
3511 This changes when the application actually wants to do event handling
3512 in the child, or both parent in child, in effect \*(L"continuing\*(R" after the
3513 fork.
3514 .PP
3515 The default mode of operation (for libev, with application help to detect
3516 forks) is to duplicate all the state in the child, as would be expected
3517 when \fIeither\fR the parent \fIor\fR the child process continues.
3518 .PP
3519 When both processes want to continue using libev, then this is usually the
3520 wrong result. In that case, usually one process (typically the parent) is
3521 supposed to continue with all watchers in place as before, while the other
3522 process typically wants to start fresh, i.e. without any active watchers.
3523 .PP
3524 The cleanest and most efficient way to achieve that with libev is to
3525 simply create a new event loop, which of course will be \*(L"empty\*(R", and
3526 use that for new watchers. This has the advantage of not touching more
3527 memory than necessary, and thus avoiding the copy-on-write, and the
3528 disadvantage of having to use multiple event loops (which do not support
3529 signal watchers).
3530 .PP
3531 When this is not possible, or you want to use the default loop for
3532 other reasons, then in the process that wants to start \*(L"fresh\*(R", call
3533 \&\f(CW\*(C`ev_loop_destroy (EV_DEFAULT)\*(C'\fR followed by \f(CW\*(C`ev_default_loop (...)\*(C'\fR.
3534 Destroying the default loop will \*(L"orphan\*(R" (not stop) all registered
3535 watchers, so you have to be careful not to execute code that modifies
3536 those watchers. Note also that in that case, you have to re-register any
3537 signal watchers.
3538 .PP
3539 \fIWatcher-Specific Functions and Data Members\fR
3540 .IX Subsection "Watcher-Specific Functions and Data Members"
3541 .IP "ev_fork_init (ev_fork *, callback)" 4
3542 .IX Item "ev_fork_init (ev_fork *, callback)"
3543 Initialises and configures the fork watcher \- it has no parameters of any
3544 kind. There is a \f(CW\*(C`ev_fork_set\*(C'\fR macro, but using it is utterly pointless,
3545 really.
3546 .ie n .SS """ev_cleanup"" \- even the best things end"
3547 .el .SS "\f(CWev_cleanup\fP \- even the best things end"
3548 .IX Subsection "ev_cleanup - even the best things end"
3549 Cleanup watchers are called just before the event loop is being destroyed
3550 by a call to \f(CW\*(C`ev_loop_destroy\*(C'\fR.
3551 .PP
3552 While there is no guarantee that the event loop gets destroyed, cleanup
3553 watchers provide a convenient method to install cleanup hooks for your
3554 program, worker threads and so on \- you just to make sure to destroy the
3555 loop when you want them to be invoked.
3556 .PP
3557 Cleanup watchers are invoked in the same way as any other watcher. Unlike
3558 all other watchers, they do not keep a reference to the event loop (which
3559 makes a lot of sense if you think about it). Like all other watchers, you
3560 can call libev functions in the callback, except \f(CW\*(C`ev_cleanup_start\*(C'\fR.
3561 .PP
3562 \fIWatcher-Specific Functions and Data Members\fR
3563 .IX Subsection "Watcher-Specific Functions and Data Members"
3564 .IP "ev_cleanup_init (ev_cleanup *, callback)" 4
3565 .IX Item "ev_cleanup_init (ev_cleanup *, callback)"
3566 Initialises and configures the cleanup watcher \- it has no parameters of
3567 any kind. There is a \f(CW\*(C`ev_cleanup_set\*(C'\fR macro, but using it is utterly
3568 pointless, I assure you.
3569 .PP
3570 Example: Register an atexit handler to destroy the default loop, so any
3571 cleanup functions are called.
3572 .PP
3573 .Vb 5
3574 \& static void
3575 \& program_exits (void)
3576 \& {
3577 \& ev_loop_destroy (EV_DEFAULT_UC);
3578 \& }
3579 \&
3580 \& ...
3581 \& atexit (program_exits);
3582 .Ve
3583 .ie n .SS """ev_async"" \- how to wake up an event loop"
3584 .el .SS "\f(CWev_async\fP \- how to wake up an event loop"
3585 .IX Subsection "ev_async - how to wake up an event loop"
3586 In general, you cannot use an \f(CW\*(C`ev_loop\*(C'\fR from multiple threads or other
3587 asynchronous sources such as signal handlers (as opposed to multiple event
3588 loops \- those are of course safe to use in different threads).
3589 .PP
3590 Sometimes, however, you need to wake up an event loop you do not control,
3591 for example because it belongs to another thread. This is what \f(CW\*(C`ev_async\*(C'\fR
3592 watchers do: as long as the \f(CW\*(C`ev_async\*(C'\fR watcher is active, you can signal
3593 it by calling \f(CW\*(C`ev_async_send\*(C'\fR, which is thread\- and signal safe.
3594 .PP
3595 This functionality is very similar to \f(CW\*(C`ev_signal\*(C'\fR watchers, as signals,
3596 too, are asynchronous in nature, and signals, too, will be compressed
3597 (i.e. the number of callback invocations may be less than the number of
3598 \&\f(CW\*(C`ev_async_send\*(C'\fR calls). In fact, you could use signal watchers as a kind
3599 of \*(L"global async watchers\*(R" by using a watcher on an otherwise unused
3600 signal, and \f(CW\*(C`ev_feed_signal\*(C'\fR to signal this watcher from another thread,
3601 even without knowing which loop owns the signal.
3602 .PP
3603 \fIQueueing\fR
3604 .IX Subsection "Queueing"
3605 .PP
3606 \&\f(CW\*(C`ev_async\*(C'\fR does not support queueing of data in any way. The reason
3607 is that the author does not know of a simple (or any) algorithm for a
3608 multiple-writer-single-reader queue that works in all cases and doesn't
3609 need elaborate support such as pthreads or unportable memory access
3610 semantics.
3611 .PP
3612 That means that if you want to queue data, you have to provide your own
3613 queue. But at least I can tell you how to implement locking around your
3614 queue:
3615 .IP "queueing from a signal handler context" 4
3616 .IX Item "queueing from a signal handler context"
3617 To implement race-free queueing, you simply add to the queue in the signal
3618 handler but you block the signal handler in the watcher callback. Here is
3619 an example that does that for some fictitious \s-1SIGUSR1\s0 handler:
3620 .Sp
3621 .Vb 1
3622 \& static ev_async mysig;
3623 \&
3624 \& static void
3625 \& sigusr1_handler (void)
3626 \& {
3627 \& sometype data;
3628 \&
3629 \& // no locking etc.
3630 \& queue_put (data);
3631 \& ev_async_send (EV_DEFAULT_ &mysig);
3632 \& }
3633 \&
3634 \& static void
3635 \& mysig_cb (EV_P_ ev_async *w, int revents)
3636 \& {
3637 \& sometype data;
3638 \& sigset_t block, prev;
3639 \&
3640 \& sigemptyset (&block);
3641 \& sigaddset (&block, SIGUSR1);
3642 \& sigprocmask (SIG_BLOCK, &block, &prev);
3643 \&
3644 \& while (queue_get (&data))
3645 \& process (data);
3646 \&
3647 \& if (sigismember (&prev, SIGUSR1)
3648 \& sigprocmask (SIG_UNBLOCK, &block, 0);
3649 \& }
3650 .Ve
3651 .Sp
3652 (Note: pthreads in theory requires you to use \f(CW\*(C`pthread_setmask\*(C'\fR
3653 instead of \f(CW\*(C`sigprocmask\*(C'\fR when you use threads, but libev doesn't do it
3654 either...).
3655 .IP "queueing from a thread context" 4
3656 .IX Item "queueing from a thread context"
3657 The strategy for threads is different, as you cannot (easily) block
3658 threads but you can easily preempt them, so to queue safely you need to
3659 employ a traditional mutex lock, such as in this pthread example:
3660 .Sp
3661 .Vb 2
3662 \& static ev_async mysig;
3663 \& static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
3664 \&
3665 \& static void
3666 \& otherthread (void)
3667 \& {
3668 \& // only need to lock the actual queueing operation
3669 \& pthread_mutex_lock (&mymutex);
3670 \& queue_put (data);
3671 \& pthread_mutex_unlock (&mymutex);
3672 \&
3673 \& ev_async_send (EV_DEFAULT_ &mysig);
3674 \& }
3675 \&
3676 \& static void
3677 \& mysig_cb (EV_P_ ev_async *w, int revents)
3678 \& {
3679 \& pthread_mutex_lock (&mymutex);
3680 \&
3681 \& while (queue_get (&data))
3682 \& process (data);
3683 \&
3684 \& pthread_mutex_unlock (&mymutex);
3685 \& }
3686 .Ve
3687 .PP
3688 \fIWatcher-Specific Functions and Data Members\fR
3689 .IX Subsection "Watcher-Specific Functions and Data Members"
3690 .IP "ev_async_init (ev_async *, callback)" 4
3691 .IX Item "ev_async_init (ev_async *, callback)"
3692 Initialises and configures the async watcher \- it has no parameters of any
3693 kind. There is a \f(CW\*(C`ev_async_set\*(C'\fR macro, but using it is utterly pointless,
3694 trust me.
3695 .IP "ev_async_send (loop, ev_async *)" 4
3696 .IX Item "ev_async_send (loop, ev_async *)"
3697 Sends/signals/activates the given \f(CW\*(C`ev_async\*(C'\fR watcher, that is, feeds
3698 an \f(CW\*(C`EV_ASYNC\*(C'\fR event on the watcher into the event loop, and instantly
3699 returns.
3700 .Sp
3701 Unlike \f(CW\*(C`ev_feed_event\*(C'\fR, this call is safe to do from other threads,
3702 signal or similar contexts (see the discussion of \f(CW\*(C`EV_ATOMIC_T\*(C'\fR in the
3703 embedding section below on what exactly this means).
3704 .Sp
3705 Note that, as with other watchers in libev, multiple events might get
3706 compressed into a single callback invocation (another way to look at
3707 this is that \f(CW\*(C`ev_async\*(C'\fR watchers are level-triggered: they are set on
3708 \&\f(CW\*(C`ev_async_send\*(C'\fR, reset when the event loop detects that).
3709 .Sp
3710 This call incurs the overhead of at most one extra system call per event
3711 loop iteration, if the event loop is blocked, and no syscall at all if
3712 the event loop (or your program) is processing events. That means that
3713 repeated calls are basically free (there is no need to avoid calls for
3714 performance reasons) and that the overhead becomes smaller (typically
3715 zero) under load.
3716 .IP "bool = ev_async_pending (ev_async *)" 4
3717 .IX Item "bool = ev_async_pending (ev_async *)"
3718 Returns a non-zero value when \f(CW\*(C`ev_async_send\*(C'\fR has been called on the
3719 watcher but the event has not yet been processed (or even noted) by the
3720 event loop.
3721 .Sp
3722 \&\f(CW\*(C`ev_async_send\*(C'\fR sets a flag in the watcher and wakes up the loop. When
3723 the loop iterates next and checks for the watcher to have become active,
3724 it will reset the flag again. \f(CW\*(C`ev_async_pending\*(C'\fR can be used to very
3725 quickly check whether invoking the loop might be a good idea.
3726 .Sp
3727 Not that this does \fInot\fR check whether the watcher itself is pending,
3728 only whether it has been requested to make this watcher pending: there
3729 is a time window between the event loop checking and resetting the async
3730 notification, and the callback being invoked.
3731 .SH "OTHER FUNCTIONS"
3732 .IX Header "OTHER FUNCTIONS"
3733 There are some other functions of possible interest. Described. Here. Now.
3734 .IP "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)" 4
3735 .IX Item "ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)"
3736 This function combines a simple timer and an I/O watcher, calls your
3737 callback on whichever event happens first and automatically stops both
3738 watchers. This is useful if you want to wait for a single event on an fd
3739 or timeout without having to allocate/configure/start/stop/free one or
3740 more watchers yourself.
3741 .Sp
3742 If \f(CW\*(C`fd\*(C'\fR is less than 0, then no I/O watcher will be started and the
3743 \&\f(CW\*(C`events\*(C'\fR argument is being ignored. Otherwise, an \f(CW\*(C`ev_io\*(C'\fR watcher for
3744 the given \f(CW\*(C`fd\*(C'\fR and \f(CW\*(C`events\*(C'\fR set will be created and started.
3745 .Sp
3746 If \f(CW\*(C`timeout\*(C'\fR is less than 0, then no timeout watcher will be
3747 started. Otherwise an \f(CW\*(C`ev_timer\*(C'\fR watcher with after = \f(CW\*(C`timeout\*(C'\fR (and
3748 repeat = 0) will be started. \f(CW0\fR is a valid timeout.
3749 .Sp
3750 The callback has the type \f(CW\*(C`void (*cb)(int revents, void *arg)\*(C'\fR and is
3751 passed an \f(CW\*(C`revents\*(C'\fR set like normal event callbacks (a combination of
3752 \&\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
3753 value passed to \f(CW\*(C`ev_once\*(C'\fR. Note that it is possible to receive \fIboth\fR
3754 a timeout and an io event at the same time \- you probably should give io
3755 events precedence.
3756 .Sp
3757 Example: wait up to ten seconds for data to appear on \s-1STDIN_FILENO.\s0
3758 .Sp
3759 .Vb 7
3760 \& static void stdin_ready (int revents, void *arg)
3761 \& {
3762 \& if (revents & EV_READ)
3763 \& /* stdin might have data for us, joy! */;
3764 \& else if (revents & EV_TIMER)
3765 \& /* doh, nothing entered */;
3766 \& }
3767 \&
3768 \& ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
3769 .Ve
3770 .IP "ev_feed_fd_event (loop, int fd, int revents)" 4
3771 .IX Item "ev_feed_fd_event (loop, int fd, int revents)"
3772 Feed an event on the given fd, as if a file descriptor backend detected
3773 the given events.
3774 .IP "ev_feed_signal_event (loop, int signum)" 4
3775 .IX Item "ev_feed_signal_event (loop, int signum)"
3776 Feed an event as if the given signal occurred. See also \f(CW\*(C`ev_feed_signal\*(C'\fR,
3777 which is async-safe.
3778 .SH "COMMON OR USEFUL IDIOMS (OR BOTH)"
3779 .IX Header "COMMON OR USEFUL IDIOMS (OR BOTH)"
3780 This section explains some common idioms that are not immediately
3781 obvious. Note that examples are sprinkled over the whole manual, and this
3782 section only contains stuff that wouldn't fit anywhere else.
3783 .SS "\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\s0"
3784 .IX Subsection "ASSOCIATING CUSTOM DATA WITH A WATCHER"
3785 Each watcher has, by default, a \f(CW\*(C`void *data\*(C'\fR member that you can read
3786 or modify at any time: libev will completely ignore it. This can be used
3787 to associate arbitrary data with your watcher. If you need more data and
3788 don't want to allocate memory separately and store a pointer to it in that
3789 data member, you can also \*(L"subclass\*(R" the watcher type and provide your own
3790 data:
3791 .PP
3792 .Vb 7
3793 \& struct my_io
3794 \& {
3795 \& ev_io io;
3796 \& int otherfd;
3797 \& void *somedata;
3798 \& struct whatever *mostinteresting;
3799 \& };
3800 \&
3801 \& ...
3802 \& struct my_io w;
3803 \& ev_io_init (&w.io, my_cb, fd, EV_READ);
3804 .Ve
3805 .PP
3806 And since your callback will be called with a pointer to the watcher, you
3807 can cast it back to your own type:
3808 .PP
3809 .Vb 5
3810 \& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
3811 \& {
3812 \& struct my_io *w = (struct my_io *)w_;
3813 \& ...
3814 \& }
3815 .Ve
3816 .PP
3817 More interesting and less C\-conformant ways of casting your callback
3818 function type instead have been omitted.
3819 .SS "\s-1BUILDING YOUR OWN COMPOSITE WATCHERS\s0"
3820 .IX Subsection "BUILDING YOUR OWN COMPOSITE WATCHERS"
3821 Another common scenario is to use some data structure with multiple
3822 embedded watchers, in effect creating your own watcher that combines
3823 multiple libev event sources into one \*(L"super-watcher\*(R":
3824 .PP
3825 .Vb 6
3826 \& struct my_biggy
3827 \& {
3828 \& int some_data;
3829 \& ev_timer t1;
3830 \& ev_timer t2;
3831 \& }
3832 .Ve
3833 .PP
3834 In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more
3835 complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct in
3836 the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies or \*(C+ coders), or you need
3837 to use some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for
3838 real programmers):
3839 .PP
3840 .Vb 1
3841 \& #include <stddef.h>
3842 \&
3843 \& static void
3844 \& t1_cb (EV_P_ ev_timer *w, int revents)
3845 \& {
3846 \& struct my_biggy big = (struct my_biggy *)
3847 \& (((char *)w) \- offsetof (struct my_biggy, t1));
3848 \& }
3849 \&
3850 \& static void
3851 \& t2_cb (EV_P_ ev_timer *w, int revents)
3852 \& {
3853 \& struct my_biggy big = (struct my_biggy *)
3854 \& (((char *)w) \- offsetof (struct my_biggy, t2));
3855 \& }
3856 .Ve
3857 .SS "\s-1AVOIDING FINISHING BEFORE RETURNING\s0"
3858 .IX Subsection "AVOIDING FINISHING BEFORE RETURNING"
3859 Often you have structures like this in event-based programs:
3860 .PP
3861 .Vb 4
3862 \& callback ()
3863 \& {
3864 \& free (request);
3865 \& }
3866 \&
3867 \& request = start_new_request (..., callback);
3868 .Ve
3869 .PP
3870 The intent is to start some \*(L"lengthy\*(R" operation. The \f(CW\*(C`request\*(C'\fR could be
3871 used to cancel the operation, or do other things with it.
3872 .PP
3873 It's not uncommon to have code paths in \f(CW\*(C`start_new_request\*(C'\fR that
3874 immediately invoke the callback, for example, to report errors. Or you add
3875 some caching layer that finds that it can skip the lengthy aspects of the
3876 operation and simply invoke the callback with the result.
3877 .PP
3878 The problem here is that this will happen \fIbefore\fR \f(CW\*(C`start_new_request\*(C'\fR
3879 has returned, so \f(CW\*(C`request\*(C'\fR is not set.
3880 .PP
3881 Even if you pass the request by some safer means to the callback, you
3882 might want to do something to the request after starting it, such as
3883 canceling it, which probably isn't working so well when the callback has
3884 already been invoked.
3885 .PP
3886 A common way around all these issues is to make sure that
3887 \&\f(CW\*(C`start_new_request\*(C'\fR \fIalways\fR returns before the callback is invoked. If
3888 \&\f(CW\*(C`start_new_request\*(C'\fR immediately knows the result, it can artificially
3889 delay invoking the callback by using a \f(CW\*(C`prepare\*(C'\fR or \f(CW\*(C`idle\*(C'\fR watcher for
3890 example, or more sneakily, by reusing an existing (stopped) watcher and
3891 pushing it into the pending queue:
3892 .PP
3893 .Vb 2
3894 \& ev_set_cb (watcher, callback);
3895 \& ev_feed_event (EV_A_ watcher, 0);
3896 .Ve
3897 .PP
3898 This way, \f(CW\*(C`start_new_request\*(C'\fR can safely return before the callback is
3899 invoked, while not delaying callback invocation too much.
3900 .SS "\s-1MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS\s0"
3901 .IX Subsection "MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS"
3902 Often (especially in \s-1GUI\s0 toolkits) there are places where you have
3903 \&\fImodal\fR interaction, which is most easily implemented by recursively
3904 invoking \f(CW\*(C`ev_run\*(C'\fR.
3905 .PP
3906 This brings the problem of exiting \- a callback might want to finish the
3907 main \f(CW\*(C`ev_run\*(C'\fR call, but not the nested one (e.g. user clicked \*(L"Quit\*(R", but
3908 a modal \*(L"Are you sure?\*(R" dialog is still waiting), or just the nested one
3909 and not the main one (e.g. user clocked \*(L"Ok\*(R" in a modal dialog), or some
3910 other combination: In these cases, a simple \f(CW\*(C`ev_break\*(C'\fR will not work.
3911 .PP
3912 The solution is to maintain \*(L"break this loop\*(R" variable for each \f(CW\*(C`ev_run\*(C'\fR
3913 invocation, and use a loop around \f(CW\*(C`ev_run\*(C'\fR until the condition is
3914 triggered, using \f(CW\*(C`EVRUN_ONCE\*(C'\fR:
3915 .PP
3916 .Vb 2
3917 \& // main loop
3918 \& int exit_main_loop = 0;
3919 \&
3920 \& while (!exit_main_loop)
3921 \& ev_run (EV_DEFAULT_ EVRUN_ONCE);
3922 \&
3923 \& // in a modal watcher
3924 \& int exit_nested_loop = 0;
3925 \&
3926 \& while (!exit_nested_loop)
3927 \& ev_run (EV_A_ EVRUN_ONCE);
3928 .Ve
3929 .PP
3930 To exit from any of these loops, just set the corresponding exit variable:
3931 .PP
3932 .Vb 2
3933 \& // exit modal loop
3934 \& exit_nested_loop = 1;
3935 \&
3936 \& // exit main program, after modal loop is finished
3937 \& exit_main_loop = 1;
3938 \&
3939 \& // exit both
3940 \& exit_main_loop = exit_nested_loop = 1;
3941 .Ve
3942 .SS "\s-1THREAD LOCKING EXAMPLE\s0"
3943 .IX Subsection "THREAD LOCKING EXAMPLE"
3944 Here is a fictitious example of how to run an event loop in a different
3945 thread from where callbacks are being invoked and watchers are
3946 created/added/removed.
3947 .PP
3948 For a real-world example, see the \f(CW\*(C`EV::Loop::Async\*(C'\fR perl module,
3949 which uses exactly this technique (which is suited for many high-level
3950 languages).
3951 .PP
3952 The example uses a pthread mutex to protect the loop data, a condition
3953 variable to wait for callback invocations, an async watcher to notify the
3954 event loop thread and an unspecified mechanism to wake up the main thread.
3955 .PP
3956 First, you need to associate some data with the event loop:
3957 .PP
3958 .Vb 6
3959 \& typedef struct {
3960 \& mutex_t lock; /* global loop lock */
3961 \& ev_async async_w;
3962 \& thread_t tid;
3963 \& cond_t invoke_cv;
3964 \& } userdata;
3965 \&
3966 \& void prepare_loop (EV_P)
3967 \& {
3968 \& // for simplicity, we use a static userdata struct.
3969 \& static userdata u;
3970 \&
3971 \& ev_async_init (&u\->async_w, async_cb);
3972 \& ev_async_start (EV_A_ &u\->async_w);
3973 \&
3974 \& pthread_mutex_init (&u\->lock, 0);
3975 \& pthread_cond_init (&u\->invoke_cv, 0);
3976 \&
3977 \& // now associate this with the loop
3978 \& ev_set_userdata (EV_A_ u);
3979 \& ev_set_invoke_pending_cb (EV_A_ l_invoke);
3980 \& ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
3981 \&
3982 \& // then create the thread running ev_run
3983 \& pthread_create (&u\->tid, 0, l_run, EV_A);
3984 \& }
3985 .Ve
3986 .PP
3987 The callback for the \f(CW\*(C`ev_async\*(C'\fR watcher does nothing: the watcher is used
3988 solely to wake up the event loop so it takes notice of any new watchers
3989 that might have been added:
3990 .PP
3991 .Vb 5
3992 \& static void
3993 \& async_cb (EV_P_ ev_async *w, int revents)
3994 \& {
3995 \& // just used for the side effects
3996 \& }
3997 .Ve
3998 .PP
3999 The \f(CW\*(C`l_release\*(C'\fR and \f(CW\*(C`l_acquire\*(C'\fR callbacks simply unlock/lock the mutex
4000 protecting the loop data, respectively.
4001 .PP
4002 .Vb 6
4003 \& static void
4004 \& l_release (EV_P)
4005 \& {
4006 \& userdata *u = ev_userdata (EV_A);
4007 \& pthread_mutex_unlock (&u\->lock);
4008 \& }
4009 \&
4010 \& static void
4011 \& l_acquire (EV_P)
4012 \& {
4013 \& userdata *u = ev_userdata (EV_A);
4014 \& pthread_mutex_lock (&u\->lock);
4015 \& }
4016 .Ve
4017 .PP
4018 The event loop thread first acquires the mutex, and then jumps straight
4019 into \f(CW\*(C`ev_run\*(C'\fR:
4020 .PP
4021 .Vb 4
4022 \& void *
4023 \& l_run (void *thr_arg)
4024 \& {
4025 \& struct ev_loop *loop = (struct ev_loop *)thr_arg;
4026 \&
4027 \& l_acquire (EV_A);
4028 \& pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4029 \& ev_run (EV_A_ 0);
4030 \& l_release (EV_A);
4031 \&
4032 \& return 0;
4033 \& }
4034 .Ve
4035 .PP
4036 Instead of invoking all pending watchers, the \f(CW\*(C`l_invoke\*(C'\fR callback will
4037 signal the main thread via some unspecified mechanism (signals? pipe
4038 writes? \f(CW\*(C`Async::Interrupt\*(C'\fR?) and then waits until all pending watchers
4039 have been called (in a while loop because a) spurious wakeups are possible
4040 and b) skipping inter-thread-communication when there are no pending
4041 watchers is very beneficial):
4042 .PP
4043 .Vb 4
4044 \& static void
4045 \& l_invoke (EV_P)
4046 \& {
4047 \& userdata *u = ev_userdata (EV_A);
4048 \&
4049 \& while (ev_pending_count (EV_A))
4050 \& {
4051 \& wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4052 \& pthread_cond_wait (&u\->invoke_cv, &u\->lock);
4053 \& }
4054 \& }
4055 .Ve
4056 .PP
4057 Now, whenever the main thread gets told to invoke pending watchers, it
4058 will grab the lock, call \f(CW\*(C`ev_invoke_pending\*(C'\fR and then signal the loop
4059 thread to continue:
4060 .PP
4061 .Vb 4
4062 \& static void
4063 \& real_invoke_pending (EV_P)
4064 \& {
4065 \& userdata *u = ev_userdata (EV_A);
4066 \&
4067 \& pthread_mutex_lock (&u\->lock);
4068 \& ev_invoke_pending (EV_A);
4069 \& pthread_cond_signal (&u\->invoke_cv);
4070 \& pthread_mutex_unlock (&u\->lock);
4071 \& }
4072 .Ve
4073 .PP
4074 Whenever you want to start/stop a watcher or do other modifications to an
4075 event loop, you will now have to lock:
4076 .PP
4077 .Vb 2
4078 \& ev_timer timeout_watcher;
4079 \& userdata *u = ev_userdata (EV_A);
4080 \&
4081 \& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4082 \&
4083 \& pthread_mutex_lock (&u\->lock);
4084 \& ev_timer_start (EV_A_ &timeout_watcher);
4085 \& ev_async_send (EV_A_ &u\->async_w);
4086 \& pthread_mutex_unlock (&u\->lock);
4087 .Ve
4088 .PP
4089 Note that sending the \f(CW\*(C`ev_async\*(C'\fR watcher is required because otherwise
4090 an event loop currently blocking in the kernel will have no knowledge
4091 about the newly added timer. By waking up the loop it will pick up any new
4092 watchers in the next event loop iteration.
4093 .SS "\s-1THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS\s0"
4094 .IX Subsection "THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS"
4095 While the overhead of a callback that e.g. schedules a thread is small, it
4096 is still an overhead. If you embed libev, and your main usage is with some
4097 kind of threads or coroutines, you might want to customise libev so that
4098 doesn't need callbacks anymore.
4099 .PP
4100 Imagine you have coroutines that you can switch to using a function
4101 \&\f(CW\*(C`switch_to (coro)\*(C'\fR, that libev runs in a coroutine called \f(CW\*(C`libev_coro\*(C'\fR
4102 and that due to some magic, the currently active coroutine is stored in a
4103 global called \f(CW\*(C`current_coro\*(C'\fR. Then you can build your own \*(L"wait for libev
4104 event\*(R" primitive by changing \f(CW\*(C`EV_CB_DECLARE\*(C'\fR and \f(CW\*(C`EV_CB_INVOKE\*(C'\fR (note
4105 the differing \f(CW\*(C`;\*(C'\fR conventions):
4106 .PP
4107 .Vb 2
4108 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4109 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4110 .Ve
4111 .PP
4112 That means instead of having a C callback function, you store the
4113 coroutine to switch to in each watcher, and instead of having libev call
4114 your callback, you instead have it switch to that coroutine.
4115 .PP
4116 A coroutine might now wait for an event with a function called
4117 \&\f(CW\*(C`wait_for_event\*(C'\fR. (the watcher needs to be started, as always, but it doesn't
4118 matter when, or whether the watcher is active or not when this function is
4119 called):
4120 .PP
4121 .Vb 6
4122 \& void
4123 \& wait_for_event (ev_watcher *w)
4124 \& {
4125 \& ev_set_cb (w, current_coro);
4126 \& switch_to (libev_coro);
4127 \& }
4128 .Ve
4129 .PP
4130 That basically suspends the coroutine inside \f(CW\*(C`wait_for_event\*(C'\fR and
4131 continues the libev coroutine, which, when appropriate, switches back to
4132 this or any other coroutine.
4133 .PP
4134 You can do similar tricks if you have, say, threads with an event queue \-
4135 instead of storing a coroutine, you store the queue object and instead of
4136 switching to a coroutine, you push the watcher onto the queue and notify
4137 any waiters.
4138 .PP
4139 To embed libev, see \*(L"\s-1EMBEDDING\*(R"\s0, but in short, it's easiest to create two
4140 files, \fImy_ev.h\fR and \fImy_ev.c\fR that include the respective libev files:
4141 .PP
4142 .Vb 4
4143 \& // my_ev.h
4144 \& #define EV_CB_DECLARE(type) struct my_coro *cb;
4145 \& #define EV_CB_INVOKE(watcher) switch_to ((watcher)\->cb)
4146 \& #include "../libev/ev.h"
4147 \&
4148 \& // my_ev.c
4149 \& #define EV_H "my_ev.h"
4150 \& #include "../libev/ev.c"
4151 .Ve
4152 .PP
4153 And then use \fImy_ev.h\fR when you would normally use \fIev.h\fR, and compile
4154 \&\fImy_ev.c\fR into your project. When properly specifying include paths, you
4155 can even use \fIev.h\fR as header file name directly.
4156 .SH "LIBEVENT EMULATION"
4157 .IX Header "LIBEVENT EMULATION"
4158 Libev offers a compatibility emulation layer for libevent. It cannot
4159 emulate the internals of libevent, so here are some usage hints:
4160 .IP "\(bu" 4
4161 Only the libevent\-1.4.1\-beta \s-1API\s0 is being emulated.
4162 .Sp
4163 This was the newest libevent version available when libev was implemented,
4164 and is still mostly unchanged in 2010.
4165 .IP "\(bu" 4
4166 Use it by including <event.h>, as usual.
4167 .IP "\(bu" 4
4168 The following members are fully supported: ev_base, ev_callback,
4169 ev_arg, ev_fd, ev_res, ev_events.
4170 .IP "\(bu" 4
4171 Avoid using ev_flags and the EVLIST_*\-macros, while it is
4172 maintained by libev, it does not work exactly the same way as in libevent (consider
4173 it a private \s-1API\s0).
4174 .IP "\(bu" 4
4175 Priorities are not currently supported. Initialising priorities
4176 will fail and all watchers will have the same priority, even though there
4177 is an ev_pri field.
4178 .IP "\(bu" 4
4179 In libevent, the last base created gets the signals, in libev, the
4180 base that registered the signal gets the signals.
4181 .IP "\(bu" 4
4182 Other members are not supported.
4183 .IP "\(bu" 4
4184 The libev emulation is \fInot\fR \s-1ABI\s0 compatible to libevent, you need
4185 to use the libev header file and library.
4186 .SH "\*(C+ SUPPORT"
4187 .IX Header " SUPPORT"
4188 .SS "C \s-1API\s0"
4189 .IX Subsection "C API"
4190 The normal C \s-1API\s0 should work fine when used from \*(C+: both ev.h and the
4191 libev sources can be compiled as \*(C+. Therefore, code that uses the C \s-1API\s0
4192 will work fine.
4193 .PP
4194 Proper exception specifications might have to be added to callbacks passed
4195 to libev: exceptions may be thrown only from watcher callbacks, all other
4196 callbacks (allocator, syserr, loop acquire/release and periodic reschedule
4197 callbacks) must not throw exceptions, and might need a \f(CW\*(C`noexcept\*(C'\fR
4198 specification. If you have code that needs to be compiled as both C and
4199 \&\*(C+ you can use the \f(CW\*(C`EV_NOEXCEPT\*(C'\fR macro for this:
4200 .PP
4201 .Vb 6
4202 \& static void
4203 \& fatal_error (const char *msg) EV_NOEXCEPT
4204 \& {
4205 \& perror (msg);
4206 \& abort ();
4207 \& }
4208 \&
4209 \& ...
4210 \& ev_set_syserr_cb (fatal_error);
4211 .Ve
4212 .PP
4213 The only \s-1API\s0 functions that can currently throw exceptions are \f(CW\*(C`ev_run\*(C'\fR,
4214 \&\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
4215 because it runs cleanup watchers).
4216 .PP
4217 Throwing exceptions in watcher callbacks is only supported if libev itself
4218 is compiled with a \*(C+ compiler or your C and \*(C+ environments allow
4219 throwing exceptions through C libraries (most do).
4220 .SS "\*(C+ \s-1API\s0"
4221 .IX Subsection " API"
4222 Libev comes with some simplistic wrapper classes for \*(C+ that mainly allow
4223 you to use some convenience methods to start/stop watchers and also change
4224 the callback model to a model using method callbacks on objects.
4225 .PP
4226 To use it,
4227 .PP
4228 .Vb 1
4229 \& #include <ev++.h>
4230 .Ve
4231 .PP
4232 This automatically includes \fIev.h\fR and puts all of its definitions (many
4233 of them macros) into the global namespace. All \*(C+ specific things are
4234 put into the \f(CW\*(C`ev\*(C'\fR namespace. It should support all the same embedding
4235 options as \fIev.h\fR, most notably \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR.
4236 .PP
4237 Care has been taken to keep the overhead low. The only data member the \*(C+
4238 classes add (compared to plain C\-style watchers) is the event loop pointer
4239 that the watcher is associated with (or no additional members at all if
4240 you disable \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR when embedding libev).
4241 .PP
4242 Currently, functions, static and non-static member functions and classes
4243 with \f(CW\*(C`operator ()\*(C'\fR can be used as callbacks. Other types should be easy
4244 to add as long as they only need one additional pointer for context. If
4245 you need support for other types of functors please contact the author
4246 (preferably after implementing it).
4247 .PP
4248 For all this to work, your \*(C+ compiler either has to use the same calling
4249 conventions as your C compiler (for static member functions), or you have
4250 to embed libev and compile libev itself as \*(C+.
4251 .PP
4252 Here is a list of things available in the \f(CW\*(C`ev\*(C'\fR namespace:
4253 .ie n .IP """ev::READ"", ""ev::WRITE"" etc." 4
4254 .el .IP "\f(CWev::READ\fR, \f(CWev::WRITE\fR etc." 4
4255 .IX Item "ev::READ, ev::WRITE etc."
4256 These are just enum values with the same values as the \f(CW\*(C`EV_READ\*(C'\fR etc.
4257 macros from \fIev.h\fR.
4258 .ie n .IP """ev::tstamp"", ""ev::now""" 4
4259 .el .IP "\f(CWev::tstamp\fR, \f(CWev::now\fR" 4
4260 .IX Item "ev::tstamp, ev::now"
4261 Aliases to the same types/functions as with the \f(CW\*(C`ev_\*(C'\fR prefix.
4262 .ie n .IP """ev::io"", ""ev::timer"", ""ev::periodic"", ""ev::idle"", ""ev::sig"" etc." 4
4263 .el .IP "\f(CWev::io\fR, \f(CWev::timer\fR, \f(CWev::periodic\fR, \f(CWev::idle\fR, \f(CWev::sig\fR etc." 4
4264 .IX Item "ev::io, ev::timer, ev::periodic, ev::idle, ev::sig etc."
4265 For each \f(CW\*(C`ev_TYPE\*(C'\fR watcher in \fIev.h\fR there is a corresponding class of
4266 the same name in the \f(CW\*(C`ev\*(C'\fR namespace, with the exception of \f(CW\*(C`ev_signal\*(C'\fR
4267 which is called \f(CW\*(C`ev::sig\*(C'\fR to avoid clashes with the \f(CW\*(C`signal\*(C'\fR macro
4268 defined by many implementations.
4269 .Sp
4270 All of those classes have these methods:
4271 .RS 4
4272 .IP "ev::TYPE::TYPE ()" 4
4273 .IX Item "ev::TYPE::TYPE ()"
4274 .PD 0
4275 .IP "ev::TYPE::TYPE (loop)" 4
4276 .IX Item "ev::TYPE::TYPE (loop)"
4277 .IP "ev::TYPE::~TYPE" 4
4278 .IX Item "ev::TYPE::~TYPE"
4279 .PD
4280 The constructor (optionally) takes an event loop to associate the watcher
4281 with. If it is omitted, it will use \f(CW\*(C`EV_DEFAULT\*(C'\fR.
4282 .Sp
4283 The constructor calls \f(CW\*(C`ev_init\*(C'\fR for you, which means you have to call the
4284 \&\f(CW\*(C`set\*(C'\fR method before starting it.
4285 .Sp
4286 It will not set a callback, however: You have to call the templated \f(CW\*(C`set\*(C'\fR
4287 method to set a callback before you can start the watcher.
4288 .Sp
4289 (The reason why you have to use a method is a limitation in \*(C+ which does
4290 not allow explicit template arguments for constructors).
4291 .Sp
4292 The destructor automatically stops the watcher if it is active.
4293 .IP "w\->set<class, &class::method> (object *)" 4
4294 .IX Item "w->set<class, &class::method> (object *)"
4295 This method sets the callback method to call. The method has to have a
4296 signature of \f(CW\*(C`void (*)(ev_TYPE &, int)\*(C'\fR, it receives the watcher as
4297 first argument and the \f(CW\*(C`revents\*(C'\fR as second. The object must be given as
4298 parameter and is stored in the \f(CW\*(C`data\*(C'\fR member of the watcher.
4299 .Sp
4300 This method synthesizes efficient thunking code to call your method from
4301 the C callback that libev requires. If your compiler can inline your
4302 callback (i.e. it is visible to it at the place of the \f(CW\*(C`set\*(C'\fR call and
4303 your compiler is good :), then the method will be fully inlined into the
4304 thunking function, making it as fast as a direct C callback.
4305 .Sp
4306 Example: simple class declaration and watcher initialisation
4307 .Sp
4308 .Vb 4
4309 \& struct myclass
4310 \& {
4311 \& void io_cb (ev::io &w, int revents) { }
4312 \& }
4313 \&
4314 \& myclass obj;
4315 \& ev::io iow;
4316 \& iow.set <myclass, &myclass::io_cb> (&obj);
4317 .Ve
4318 .IP "w\->set (object *)" 4
4319 .IX Item "w->set (object *)"
4320 This is a variation of a method callback \- leaving out the method to call
4321 will default the method to \f(CW\*(C`operator ()\*(C'\fR, which makes it possible to use
4322 functor objects without having to manually specify the \f(CW\*(C`operator ()\*(C'\fR all
4323 the time. Incidentally, you can then also leave out the template argument
4324 list.
4325 .Sp
4326 The \f(CW\*(C`operator ()\*(C'\fR method prototype must be \f(CW\*(C`void operator ()(watcher &w,
4327 int revents)\*(C'\fR.
4328 .Sp
4329 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4330 .Sp
4331 Example: use a functor object as callback.
4332 .Sp
4333 .Vb 7
4334 \& struct myfunctor
4335 \& {
4336 \& void operator() (ev::io &w, int revents)
4337 \& {
4338 \& ...
4339 \& }
4340 \& }
4341 \&
4342 \& myfunctor f;
4343 \&
4344 \& ev::io w;
4345 \& w.set (&f);
4346 .Ve
4347 .IP "w\->set<function> (void *data = 0)" 4
4348 .IX Item "w->set<function> (void *data = 0)"
4349 Also sets a callback, but uses a static method or plain function as
4350 callback. The optional \f(CW\*(C`data\*(C'\fR argument will be stored in the watcher's
4351 \&\f(CW\*(C`data\*(C'\fR member and is free for you to use.
4352 .Sp
4353 The prototype of the \f(CW\*(C`function\*(C'\fR must be \f(CW\*(C`void (*)(ev::TYPE &w, int)\*(C'\fR.
4354 .Sp
4355 See the method\-\f(CW\*(C`set\*(C'\fR above for more details.
4356 .Sp
4357 Example: Use a plain function as callback.
4358 .Sp
4359 .Vb 2
4360 \& static void io_cb (ev::io &w, int revents) { }
4361 \& iow.set <io_cb> ();
4362 .Ve
4363 .IP "w\->set (loop)" 4
4364 .IX Item "w->set (loop)"
4365 Associates a different \f(CW\*(C`struct ev_loop\*(C'\fR with this watcher. You can only
4366 do this when the watcher is inactive (and not pending either).
4367 .IP "w\->set ([arguments])" 4
4368 .IX Item "w->set ([arguments])"
4369 Basically the same as \f(CW\*(C`ev_TYPE_set\*(C'\fR (except for \f(CW\*(C`ev::embed\*(C'\fR watchers>),
4370 with the same arguments. Either this method or a suitable start method
4371 must be called at least once. Unlike the C counterpart, an active watcher
4372 gets automatically stopped and restarted when reconfiguring it with this
4373 method.
4374 .Sp
4375 For \f(CW\*(C`ev::embed\*(C'\fR watchers this method is called \f(CW\*(C`set_embed\*(C'\fR, to avoid
4376 clashing with the \f(CW\*(C`set (loop)\*(C'\fR method.
4377 .IP "w\->start ()" 4
4378 .IX Item "w->start ()"
4379 Starts the watcher. Note that there is no \f(CW\*(C`loop\*(C'\fR argument, as the
4380 constructor already stores the event loop.
4381 .IP "w\->start ([arguments])" 4
4382 .IX Item "w->start ([arguments])"
4383 Instead of calling \f(CW\*(C`set\*(C'\fR and \f(CW\*(C`start\*(C'\fR methods separately, it is often
4384 convenient to wrap them in one call. Uses the same type of arguments as
4385 the configure \f(CW\*(C`set\*(C'\fR method of the watcher.
4386 .IP "w\->stop ()" 4
4387 .IX Item "w->stop ()"
4388 Stops the watcher if it is active. Again, no \f(CW\*(C`loop\*(C'\fR argument.
4389 .ie n .IP "w\->again () (""ev::timer"", ""ev::periodic"" only)" 4
4390 .el .IP "w\->again () (\f(CWev::timer\fR, \f(CWev::periodic\fR only)" 4
4391 .IX Item "w->again () (ev::timer, ev::periodic only)"
4392 For \f(CW\*(C`ev::timer\*(C'\fR and \f(CW\*(C`ev::periodic\*(C'\fR, this invokes the corresponding
4393 \&\f(CW\*(C`ev_TYPE_again\*(C'\fR function.
4394 .ie n .IP "w\->sweep () (""ev::embed"" only)" 4
4395 .el .IP "w\->sweep () (\f(CWev::embed\fR only)" 4
4396 .IX Item "w->sweep () (ev::embed only)"
4397 Invokes \f(CW\*(C`ev_embed_sweep\*(C'\fR.
4398 .ie n .IP "w\->update () (""ev::stat"" only)" 4
4399 .el .IP "w\->update () (\f(CWev::stat\fR only)" 4
4400 .IX Item "w->update () (ev::stat only)"
4401 Invokes \f(CW\*(C`ev_stat_stat\*(C'\fR.
4402 .RE
4403 .RS 4
4404 .RE
4405 .PP
4406 Example: Define a class with two I/O and idle watchers, start the I/O
4407 watchers in the constructor.
4408 .PP
4409 .Vb 5
4410 \& class myclass
4411 \& {
4412 \& ev::io io ; void io_cb (ev::io &w, int revents);
4413 \& ev::io io2 ; void io2_cb (ev::io &w, int revents);
4414 \& ev::idle idle; void idle_cb (ev::idle &w, int revents);
4415 \&
4416 \& myclass (int fd)
4417 \& {
4418 \& io .set <myclass, &myclass::io_cb > (this);
4419 \& io2 .set <myclass, &myclass::io2_cb > (this);
4420 \& idle.set <myclass, &myclass::idle_cb> (this);
4421 \&
4422 \& io.set (fd, ev::WRITE); // configure the watcher
4423 \& io.start (); // start it whenever convenient
4424 \&
4425 \& io2.start (fd, ev::READ); // set + start in one call
4426 \& }
4427 \& };
4428 .Ve
4429 .SH "OTHER LANGUAGE BINDINGS"
4430 .IX Header "OTHER LANGUAGE BINDINGS"
4431 Libev does not offer other language bindings itself, but bindings for a
4432 number of languages exist in the form of third-party packages. If you know
4433 any interesting language binding in addition to the ones listed here, drop
4434 me a note.
4435 .IP "Perl" 4
4436 .IX Item "Perl"
4437 The \s-1EV\s0 module implements the full libev \s-1API\s0 and is actually used to test
4438 libev. \s-1EV\s0 is developed together with libev. Apart from the \s-1EV\s0 core module,
4439 there are additional modules that implement libev-compatible interfaces
4440 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),
4441 \&\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
4442 and \f(CW\*(C`EV::Glib\*(C'\fR).
4443 .Sp
4444 It can be found and installed via \s-1CPAN,\s0 its homepage is at
4445 <http://software.schmorp.de/pkg/EV>.
4446 .IP "Python" 4
4447 .IX Item "Python"
4448 Python bindings can be found at <http://code.google.com/p/pyev/>. It
4449 seems to be quite complete and well-documented.
4450 .IP "Ruby" 4
4451 .IX Item "Ruby"
4452 Tony Arcieri has written a ruby extension that offers access to a subset
4453 of the libev \s-1API\s0 and adds file handle abstractions, asynchronous \s-1DNS\s0 and
4454 more on top of it. It can be found via gem servers. Its homepage is at
4455 <http://rev.rubyforge.org/>.
4456 .Sp
4457 Roger Pack reports that using the link order \f(CW\*(C`\-lws2_32 \-lmsvcrt\-ruby\-190\*(C'\fR
4458 makes rev work even on mingw.
4459 .IP "Haskell" 4
4460 .IX Item "Haskell"
4461 A haskell binding to libev is available at
4462 <http://hackage.haskell.org/cgi\-bin/hackage\-scripts/package/hlibev>.
4463 .IP "D" 4
4464 .IX Item "D"
4465 Leandro Lucarella has written a D language binding (\fIev.d\fR) for libev, to
4466 be found at <http://www.llucax.com.ar/proj/ev.d/index.html>.
4467 .IP "Ocaml" 4
4468 .IX Item "Ocaml"
4469 Erkki Seppala has written Ocaml bindings for libev, to be found at
4470 <http://modeemi.cs.tut.fi/~flux/software/ocaml\-ev/>.
4471 .IP "Lua" 4
4472 .IX Item "Lua"
4473 Brian Maher has written a partial interface to libev for lua (at the
4474 time of this writing, only \f(CW\*(C`ev_io\*(C'\fR and \f(CW\*(C`ev_timer\*(C'\fR), to be found at
4475 <http://github.com/brimworks/lua\-ev>.
4476 .IP "Javascript" 4
4477 .IX Item "Javascript"
4478 Node.js (<http://nodejs.org>) uses libev as the underlying event library.
4479 .IP "Others" 4
4480 .IX Item "Others"
4481 There are others, and I stopped counting.
4482 .SH "MACRO MAGIC"
4483 .IX Header "MACRO MAGIC"
4484 Libev can be compiled with a variety of options, the most fundamental
4485 of which is \f(CW\*(C`EV_MULTIPLICITY\*(C'\fR. This option determines whether (most)
4486 functions and callbacks have an initial \f(CW\*(C`struct ev_loop *\*(C'\fR argument.
4487 .PP
4488 To make it easier to write programs that cope with either variant, the
4489 following macros are defined:
4490 .ie n .IP """EV_A"", ""EV_A_""" 4
4491 .el .IP "\f(CWEV_A\fR, \f(CWEV_A_\fR" 4
4492 .IX Item "EV_A, EV_A_"
4493 This provides the loop \fIargument\fR for functions, if one is required (\*(L"ev
4494 loop argument\*(R"). The \f(CW\*(C`EV_A\*(C'\fR form is used when this is the sole argument,
4495 \&\f(CW\*(C`EV_A_\*(C'\fR is used when other arguments are following. Example:
4496 .Sp
4497 .Vb 3
4498 \& ev_unref (EV_A);
4499 \& ev_timer_add (EV_A_ watcher);
4500 \& ev_run (EV_A_ 0);
4501 .Ve
4502 .Sp
4503 It assumes the variable \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR is in scope,
4504 which is often provided by the following macro.
4505 .ie n .IP """EV_P"", ""EV_P_""" 4
4506 .el .IP "\f(CWEV_P\fR, \f(CWEV_P_\fR" 4
4507 .IX Item "EV_P, EV_P_"
4508 This provides the loop \fIparameter\fR for functions, if one is required (\*(L"ev
4509 loop parameter\*(R"). The \f(CW\*(C`EV_P\*(C'\fR form is used when this is the sole parameter,
4510 \&\f(CW\*(C`EV_P_\*(C'\fR is used when other parameters are following. Example:
4511 .Sp
4512 .Vb 2
4513 \& // this is how ev_unref is being declared
4514 \& static void ev_unref (EV_P);
4515 \&
4516 \& // this is how you can declare your typical callback
4517 \& static void cb (EV_P_ ev_timer *w, int revents)
4518 .Ve
4519 .Sp
4520 It declares a parameter \f(CW\*(C`loop\*(C'\fR of type \f(CW\*(C`struct ev_loop *\*(C'\fR, quite
4521 suitable for use with \f(CW\*(C`EV_A\*(C'\fR.
4522 .ie n .IP """EV_DEFAULT"", ""EV_DEFAULT_""" 4
4523 .el .IP "\f(CWEV_DEFAULT\fR, \f(CWEV_DEFAULT_\fR" 4
4524 .IX Item "EV_DEFAULT, EV_DEFAULT_"
4525 Similar to the other two macros, this gives you the value of the default
4526 loop, if multiple loops are supported (\*(L"ev loop default\*(R"). The default loop
4527 will be initialised if it isn't already initialised.
4528 .Sp
4529 For non-multiplicity builds, these macros do nothing, so you always have
4530 to initialise the loop somewhere.
4531 .ie n .IP """EV_DEFAULT_UC"", ""EV_DEFAULT_UC_""" 4
4532 .el .IP "\f(CWEV_DEFAULT_UC\fR, \f(CWEV_DEFAULT_UC_\fR" 4
4533 .IX Item "EV_DEFAULT_UC, EV_DEFAULT_UC_"
4534 Usage identical to \f(CW\*(C`EV_DEFAULT\*(C'\fR and \f(CW\*(C`EV_DEFAULT_\*(C'\fR, but requires that the
4535 default loop has been initialised (\f(CW\*(C`UC\*(C'\fR == unchecked). Their behaviour
4536 is undefined when the default loop has not been initialised by a previous
4537 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.
4538 .Sp
4539 It is often prudent to use \f(CW\*(C`EV_DEFAULT\*(C'\fR when initialising the first
4540 watcher in a function but use \f(CW\*(C`EV_DEFAULT_UC\*(C'\fR afterwards.
4541 .PP
4542 Example: Declare and initialise a check watcher, utilising the above
4543 macros so it will work regardless of whether multiple loops are supported
4544 or not.
4545 .PP
4546 .Vb 5
4547 \& static void
4548 \& check_cb (EV_P_ ev_timer *w, int revents)
4549 \& {
4550 \& ev_check_stop (EV_A_ w);
4551 \& }
4552 \&
4553 \& ev_check check;
4554 \& ev_check_init (&check, check_cb);
4555 \& ev_check_start (EV_DEFAULT_ &check);
4556 \& ev_run (EV_DEFAULT_ 0);
4557 .Ve
4558 .SH "EMBEDDING"
4559 .IX Header "EMBEDDING"
4560 Libev can (and often is) directly embedded into host
4561 applications. Examples of applications that embed it include the Deliantra
4562 Game Server, the \s-1EV\s0 perl module, the \s-1GNU\s0 Virtual Private Ethernet (gvpe)
4563 and rxvt-unicode.
4564 .PP
4565 The goal is to enable you to just copy the necessary files into your
4566 source directory without having to change even a single line in them, so
4567 you can easily upgrade by simply copying (or having a checked-out copy of
4568 libev somewhere in your source tree).
4569 .SS "\s-1FILESETS\s0"
4570 .IX Subsection "FILESETS"
4571 Depending on what features you need you need to include one or more sets of files
4572 in your application.
4573 .PP
4574 \fI\s-1CORE EVENT LOOP\s0\fR
4575 .IX Subsection "CORE EVENT LOOP"
4576 .PP
4577 To include only the libev core (all the \f(CW\*(C`ev_*\*(C'\fR functions), with manual
4578 configuration (no autoconf):
4579 .PP
4580 .Vb 2
4581 \& #define EV_STANDALONE 1
4582 \& #include "ev.c"
4583 .Ve
4584 .PP
4585 This will automatically include \fIev.h\fR, too, and should be done in a
4586 single C source file only to provide the function implementations. To use
4587 it, do the same for \fIev.h\fR in all files wishing to use this \s-1API\s0 (best
4588 done by writing a wrapper around \fIev.h\fR that you can include instead and
4589 where you can put other configuration options):
4590 .PP
4591 .Vb 2
4592 \& #define EV_STANDALONE 1
4593 \& #include "ev.h"
4594 .Ve
4595 .PP
4596 Both header files and implementation files can be compiled with a \*(C+
4597 compiler (at least, that's a stated goal, and breakage will be treated
4598 as a bug).
4599 .PP
4600 You need the following files in your source tree, or in a directory
4601 in your include path (e.g. in libev/ when using \-Ilibev):
4602 .PP
4603 .Vb 4
4604 \& ev.h
4605 \& ev.c
4606 \& ev_vars.h
4607 \& ev_wrap.h
4608 \&
4609 \& ev_win32.c required on win32 platforms only
4610 \&
4611 \& ev_select.c only when select backend is enabled
4612 \& ev_poll.c only when poll backend is enabled
4613 \& ev_epoll.c only when the epoll backend is enabled
4614 \& ev_linuxaio.c only when the linux aio backend is enabled
4615 \& ev_kqueue.c only when the kqueue backend is enabled
4616 \& ev_port.c only when the solaris port backend is enabled
4617 .Ve
4618 .PP
4619 \&\fIev.c\fR includes the backend files directly when enabled, so you only need
4620 to compile this single file.
4621 .PP
4622 \fI\s-1LIBEVENT COMPATIBILITY API\s0\fR
4623 .IX Subsection "LIBEVENT COMPATIBILITY API"
4624 .PP
4625 To include the libevent compatibility \s-1API,\s0 also include:
4626 .PP
4627 .Vb 1
4628 \& #include "event.c"
4629 .Ve
4630 .PP
4631 in the file including \fIev.c\fR, and:
4632 .PP
4633 .Vb 1
4634 \& #include "event.h"
4635 .Ve
4636 .PP
4637 in the files that want to use the libevent \s-1API.\s0 This also includes \fIev.h\fR.
4638 .PP
4639 You need the following additional files for this:
4640 .PP
4641 .Vb 2
4642 \& event.h
4643 \& event.c
4644 .Ve
4645 .PP
4646 \fI\s-1AUTOCONF SUPPORT\s0\fR
4647 .IX Subsection "AUTOCONF SUPPORT"
4648 .PP
4649 Instead of using \f(CW\*(C`EV_STANDALONE=1\*(C'\fR and providing your configuration in
4650 whatever way you want, you can also \f(CW\*(C`m4_include([libev.m4])\*(C'\fR in your
4651 \&\fIconfigure.ac\fR and leave \f(CW\*(C`EV_STANDALONE\*(C'\fR undefined. \fIev.c\fR will then
4652 include \fIconfig.h\fR and configure itself accordingly.
4653 .PP
4654 For this of course you need the m4 file:
4655 .PP
4656 .Vb 1
4657 \& libev.m4
4658 .Ve
4659 .SS "\s-1PREPROCESSOR SYMBOLS/MACROS\s0"
4660 .IX Subsection "PREPROCESSOR SYMBOLS/MACROS"
4661 Libev can be configured via a variety of preprocessor symbols you have to
4662 define before including (or compiling) any of its files. The default in
4663 the absence of autoconf is documented for every option.
4664 .PP
4665 Symbols marked with \*(L"(h)\*(R" do not change the \s-1ABI,\s0 and can have different
4666 values when compiling libev vs. including \fIev.h\fR, so it is permissible
4667 to redefine them before including \fIev.h\fR without breaking compatibility
4668 to a compiled library. All other symbols change the \s-1ABI,\s0 which means all
4669 users of libev and the libev code itself must be compiled with compatible
4670 settings.
4671 .IP "\s-1EV_COMPAT3\s0 (h)" 4
4672 .IX Item "EV_COMPAT3 (h)"
4673 Backwards compatibility is a major concern for libev. This is why this
4674 release of libev comes with wrappers for the functions and symbols that
4675 have been renamed between libev version 3 and 4.
4676 .Sp
4677 You can disable these wrappers (to test compatibility with future
4678 versions) by defining \f(CW\*(C`EV_COMPAT3\*(C'\fR to \f(CW0\fR when compiling your
4679 sources. This has the additional advantage that you can drop the \f(CW\*(C`struct\*(C'\fR
4680 from \f(CW\*(C`struct ev_loop\*(C'\fR declarations, as libev will provide an \f(CW\*(C`ev_loop\*(C'\fR
4681 typedef in that case.
4682 .Sp
4683 In some future version, the default for \f(CW\*(C`EV_COMPAT3\*(C'\fR will become \f(CW0\fR,
4684 and in some even more future version the compatibility code will be
4685 removed completely.
4686 .IP "\s-1EV_STANDALONE\s0 (h)" 4
4687 .IX Item "EV_STANDALONE (h)"
4688 Must always be \f(CW1\fR if you do not use autoconf configuration, which
4689 keeps libev from including \fIconfig.h\fR, and it also defines dummy
4690 implementations for some libevent functions (such as logging, which is not
4691 supported). It will also not define any of the structs usually found in
4692 \&\fIevent.h\fR that are not directly supported by the libev core alone.
4693 .Sp
4694 In standalone mode, libev will still try to automatically deduce the
4695 configuration, but has to be more conservative.
4696 .IP "\s-1EV_USE_FLOOR\s0" 4
4697 .IX Item "EV_USE_FLOOR"
4698 If defined to be \f(CW1\fR, libev will use the \f(CW\*(C`floor ()\*(C'\fR function for its
4699 periodic reschedule calculations, otherwise libev will fall back on a
4700 portable (slower) implementation. If you enable this, you usually have to
4701 link against libm or something equivalent. Enabling this when the \f(CW\*(C`floor\*(C'\fR
4702 function is not available will fail, so the safe default is to not enable
4703 this.
4704 .IP "\s-1EV_USE_MONOTONIC\s0" 4
4705 .IX Item "EV_USE_MONOTONIC"
4706 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4707 monotonic clock option at both compile time and runtime. Otherwise no
4708 use of the monotonic clock option will be attempted. If you enable this,
4709 you usually have to link against librt or something similar. Enabling it
4710 when the functionality isn't available is safe, though, although you have
4711 to make sure you link against any libraries where the \f(CW\*(C`clock_gettime\*(C'\fR
4712 function is hiding in (often \fI\-lrt\fR). See also \f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4713 .IP "\s-1EV_USE_REALTIME\s0" 4
4714 .IX Item "EV_USE_REALTIME"
4715 If defined to be \f(CW1\fR, libev will try to detect the availability of the
4716 real-time clock option at compile time (and assume its availability
4717 at runtime if successful). Otherwise no use of the real-time clock
4718 option will be attempted. This effectively replaces \f(CW\*(C`gettimeofday\*(C'\fR
4719 by \f(CW\*(C`clock_get (CLOCK_REALTIME, ...)\*(C'\fR and will not normally affect
4720 correctness. See the note about libraries in the description of
4721 \&\f(CW\*(C`EV_USE_MONOTONIC\*(C'\fR, though. Defaults to the opposite value of
4722 \&\f(CW\*(C`EV_USE_CLOCK_SYSCALL\*(C'\fR.
4723 .IP "\s-1EV_USE_CLOCK_SYSCALL\s0" 4
4724 .IX Item "EV_USE_CLOCK_SYSCALL"
4725 If defined to be \f(CW1\fR, libev will try to use a direct syscall instead
4726 of calling the system-provided \f(CW\*(C`clock_gettime\*(C'\fR function. This option
4727 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
4728 unconditionally pulls in \f(CW\*(C`libpthread\*(C'\fR, slowing down single-threaded
4729 programs needlessly. Using a direct syscall is slightly slower (in
4730 theory), because no optimised vdso implementation can be used, but avoids
4731 the pthread dependency. Defaults to \f(CW1\fR on GNU/Linux with glibc 2.x or
4732 higher, as it simplifies linking (no need for \f(CW\*(C`\-lrt\*(C'\fR).
4733 .IP "\s-1EV_USE_NANOSLEEP\s0" 4
4734 .IX Item "EV_USE_NANOSLEEP"
4735 If defined to be \f(CW1\fR, libev will assume that \f(CW\*(C`nanosleep ()\*(C'\fR is available
4736 and will use it for delays. Otherwise it will use \f(CW\*(C`select ()\*(C'\fR.
4737 .IP "\s-1EV_USE_EVENTFD\s0" 4
4738 .IX Item "EV_USE_EVENTFD"
4739 If defined to be \f(CW1\fR, then libev will assume that \f(CW\*(C`eventfd ()\*(C'\fR is
4740 available and will probe for kernel support at runtime. This will improve
4741 \&\f(CW\*(C`ev_signal\*(C'\fR and \f(CW\*(C`ev_async\*(C'\fR performance and reduce resource consumption.
4742 If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
4743 2.7 or newer, otherwise disabled.
4744 .IP "\s-1EV_USE_SELECT\s0" 4
4745 .IX Item "EV_USE_SELECT"
4746 If undefined or defined to be \f(CW1\fR, libev will compile in support for the
4747 \&\f(CW\*(C`select\*(C'\fR(2) backend. No attempt at auto-detection will be done: if no
4748 other method takes over, select will be it. Otherwise the select backend
4749 will not be compiled in.
4750 .IP "\s-1EV_SELECT_USE_FD_SET\s0" 4
4751 .IX Item "EV_SELECT_USE_FD_SET"
4752 If defined to \f(CW1\fR, then the select backend will use the system \f(CW\*(C`fd_set\*(C'\fR
4753 structure. This is useful if libev doesn't compile due to a missing
4754 \&\f(CW\*(C`NFDBITS\*(C'\fR or \f(CW\*(C`fd_mask\*(C'\fR definition or it mis-guesses the bitset layout
4755 on exotic systems. This usually limits the range of file descriptors to
4756 some low limit such as 1024 or might have other limitations (winsocket
4757 only allows 64 sockets). The \f(CW\*(C`FD_SETSIZE\*(C'\fR macro, set before compilation,
4758 configures the maximum size of the \f(CW\*(C`fd_set\*(C'\fR.
4759 .IP "\s-1EV_SELECT_IS_WINSOCKET\s0" 4
4760 .IX Item "EV_SELECT_IS_WINSOCKET"
4761 When defined to \f(CW1\fR, the select backend will assume that
4762 select/socket/connect etc. don't understand file descriptors but
4763 wants osf handles on win32 (this is the case when the select to
4764 be used is the winsock select). This means that it will call
4765 \&\f(CW\*(C`_get_osfhandle\*(C'\fR on the fd to convert it to an \s-1OS\s0 handle. Otherwise,
4766 it is assumed that all these functions actually work on fds, even
4767 on win32. Should not be defined on non\-win32 platforms.
4768 .IP "\s-1EV_FD_TO_WIN32_HANDLE\s0(fd)" 4
4769 .IX Item "EV_FD_TO_WIN32_HANDLE(fd)"
4770 If \f(CW\*(C`EV_SELECT_IS_WINSOCKET\*(C'\fR is enabled, then libev needs a way to map
4771 file descriptors to socket handles. When not defining this symbol (the
4772 default), then libev will call \f(CW\*(C`_get_osfhandle\*(C'\fR, which is usually
4773 correct. In some cases, programs use their own file descriptor management,
4774 in which case they can provide this function to map fds to socket handles.
4775 .IP "\s-1EV_WIN32_HANDLE_TO_FD\s0(handle)" 4
4776 .IX Item "EV_WIN32_HANDLE_TO_FD(handle)"
4777 If \f(CW\*(C`EV_SELECT_IS_WINSOCKET\*(C'\fR then libev maps handles to file descriptors
4778 using the standard \f(CW\*(C`_open_osfhandle\*(C'\fR function. For programs implementing
4779 their own fd to handle mapping, overwriting this function makes it easier
4780 to do so. This can be done by defining this macro to an appropriate value.
4781 .IP "\s-1EV_WIN32_CLOSE_FD\s0(fd)" 4
4782 .IX Item "EV_WIN32_CLOSE_FD(fd)"
4783 If programs implement their own fd to handle mapping on win32, then this
4784 macro can be used to override the \f(CW\*(C`close\*(C'\fR function, useful to unregister
4785 file descriptors again. Note that the replacement function has to close
4786 the underlying \s-1OS\s0 handle.
4787 .IP "\s-1EV_USE_WSASOCKET\s0" 4
4788 .IX Item "EV_USE_WSASOCKET"
4789 If defined to be \f(CW1\fR, libev will use \f(CW\*(C`WSASocket\*(C'\fR to create its internal
4790 communication socket, which works better in some environments. Otherwise,
4791 the normal \f(CW\*(C`socket\*(C'\fR function will be used, which works better in other
4792 environments.
4793 .IP "\s-1EV_USE_POLL\s0" 4
4794 .IX Item "EV_USE_POLL"
4795 If defined to be \f(CW1\fR, libev will compile in support for the \f(CW\*(C`poll\*(C'\fR(2)
4796 backend. Otherwise it will be enabled on non\-win32 platforms. It
4797 takes precedence over select.
4798 .IP "\s-1EV_USE_EPOLL\s0" 4
4799 .IX Item "EV_USE_EPOLL"
4800 If defined to be \f(CW1\fR, libev will compile in support for the Linux
4801 \&\f(CW\*(C`epoll\*(C'\fR(7) backend. Its availability will be detected at runtime,
4802 otherwise another method will be used as fallback. This is the preferred
4803 backend for GNU/Linux systems. If undefined, it will be enabled if the
4804 headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
4805 .IP "\s-1EV_USE_LINUXAIO\s0" 4
4806 .IX Item "EV_USE_LINUXAIO"
4807 If defined to be \f(CW1\fR, libev will compile in support for the Linux
4808 aio backend. Due to it's currenbt limitations it has to be requested
4809 explicitly. If undefined, it will be enabled on linux, otherwise
4810 disabled.
4811 .IP "\s-1EV_USE_KQUEUE\s0" 4
4812 .IX Item "EV_USE_KQUEUE"
4813 If defined to be \f(CW1\fR, libev will compile in support for the \s-1BSD\s0 style
4814 \&\f(CW\*(C`kqueue\*(C'\fR(2) backend. Its actual availability will be detected at runtime,
4815 otherwise another method will be used as fallback. This is the preferred
4816 backend for \s-1BSD\s0 and BSD-like systems, although on most BSDs kqueue only
4817 supports some types of fds correctly (the only platform we found that
4818 supports ptys for example was NetBSD), so kqueue might be compiled in, but
4819 not be used unless explicitly requested. The best way to use it is to find
4820 out whether kqueue supports your type of fd properly and use an embedded
4821 kqueue loop.
4822 .IP "\s-1EV_USE_PORT\s0" 4
4823 .IX Item "EV_USE_PORT"
4824 If defined to be \f(CW1\fR, libev will compile in support for the Solaris
4825 10 port style backend. Its availability will be detected at runtime,
4826 otherwise another method will be used as fallback. This is the preferred
4827 backend for Solaris 10 systems.
4828 .IP "\s-1EV_USE_DEVPOLL\s0" 4
4829 .IX Item "EV_USE_DEVPOLL"
4830 Reserved for future expansion, works like the \s-1USE\s0 symbols above.
4831 .IP "\s-1EV_USE_INOTIFY\s0" 4
4832 .IX Item "EV_USE_INOTIFY"
4833 If defined to be \f(CW1\fR, libev will compile in support for the Linux inotify
4834 interface to speed up \f(CW\*(C`ev_stat\*(C'\fR watchers. Its actual availability will
4835 be detected at runtime. If undefined, it will be enabled if the headers
4836 indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
4837 .IP "\s-1EV_NO_SMP\s0" 4
4838 .IX Item "EV_NO_SMP"
4839 If defined to be \f(CW1\fR, libev will assume that memory is always coherent
4840 between threads, that is, threads can be used, but threads never run on
4841 different cpus (or different cpu cores). This reduces dependencies
4842 and makes libev faster.
4843 .IP "\s-1EV_NO_THREADS\s0" 4
4844 .IX Item "EV_NO_THREADS"
4845 If defined to be \f(CW1\fR, libev will assume that it will never be called from
4846 different threads (that includes signal handlers), which is a stronger
4847 assumption than \f(CW\*(C`EV_NO_SMP\*(C'\fR, above. This reduces dependencies and makes
4848 libev faster.
4849 .IP "\s-1EV_ATOMIC_T\s0" 4
4850 .IX Item "EV_ATOMIC_T"
4851 Libev requires an integer type (suitable for storing \f(CW0\fR or \f(CW1\fR) whose
4852 access is atomic with respect to other threads or signal contexts. No
4853 such type is easily found in the C language, so you can provide your own
4854 type that you know is safe for your purposes. It is used both for signal
4855 handler \*(L"locking\*(R" as well as for signal and thread safety in \f(CW\*(C`ev_async\*(C'\fR
4856 watchers.
4857 .Sp
4858 In the absence of this define, libev will use \f(CW\*(C`sig_atomic_t volatile\*(C'\fR
4859 (from \fIsignal.h\fR), which is usually good enough on most platforms.
4860 .IP "\s-1EV_H\s0 (h)" 4
4861 .IX Item "EV_H (h)"
4862 The name of the \fIev.h\fR header file used to include it. The default if
4863 undefined is \f(CW"ev.h"\fR in \fIevent.h\fR, \fIev.c\fR and \fIev++.h\fR. This can be
4864 used to virtually rename the \fIev.h\fR header file in case of conflicts.
4865 .IP "\s-1EV_CONFIG_H\s0 (h)" 4
4866 .IX Item "EV_CONFIG_H (h)"
4867 If \f(CW\*(C`EV_STANDALONE\*(C'\fR isn't \f(CW1\fR, this variable can be used to override
4868 \&\fIev.c\fR's idea of where to find the \fIconfig.h\fR file, similarly to
4869 \&\f(CW\*(C`EV_H\*(C'\fR, above.
4870 .IP "\s-1EV_EVENT_H\s0 (h)" 4
4871 .IX Item "EV_EVENT_H (h)"
4872 Similarly to \f(CW\*(C`EV_H\*(C'\fR, this macro can be used to override \fIevent.c\fR's idea
4873 of how the \fIevent.h\fR header can be found, the default is \f(CW"event.h"\fR.
4874 .IP "\s-1EV_PROTOTYPES\s0 (h)" 4
4875 .IX Item "EV_PROTOTYPES (h)"
4876 If defined to be \f(CW0\fR, then \fIev.h\fR will not define any function
4877 prototypes, but still define all the structs and other symbols. This is
4878 occasionally useful if you want to provide your own wrapper functions
4879 around libev functions.
4880 .IP "\s-1EV_MULTIPLICITY\s0" 4
4881 .IX Item "EV_MULTIPLICITY"
4882 If undefined or defined to \f(CW1\fR, then all event-loop-specific functions
4883 will have the \f(CW\*(C`struct ev_loop *\*(C'\fR as first argument, and you can create
4884 additional independent event loops. Otherwise there will be no support
4885 for multiple event loops and there is no first event loop pointer
4886 argument. Instead, all functions act on the single default loop.
4887 .Sp
4888 Note that \f(CW\*(C`EV_DEFAULT\*(C'\fR and \f(CW\*(C`EV_DEFAULT_\*(C'\fR will no longer provide a
4889 default loop when multiplicity is switched off \- you always have to
4890 initialise the loop manually in this case.
4891 .IP "\s-1EV_MINPRI\s0" 4
4892 .IX Item "EV_MINPRI"
4893 .PD 0
4894 .IP "\s-1EV_MAXPRI\s0" 4
4895 .IX Item "EV_MAXPRI"
4896 .PD
4897 The range of allowed priorities. \f(CW\*(C`EV_MINPRI\*(C'\fR must be smaller or equal to
4898 \&\f(CW\*(C`EV_MAXPRI\*(C'\fR, but otherwise there are no non-obvious limitations. You can
4899 provide for more priorities by overriding those symbols (usually defined
4900 to be \f(CW\*(C`\-2\*(C'\fR and \f(CW2\fR, respectively).
4901 .Sp
4902 When doing priority-based operations, libev usually has to linearly search
4903 all the priorities, so having many of them (hundreds) uses a lot of space
4904 and time, so using the defaults of five priorities (\-2 .. +2) is usually
4905 fine.
4906 .Sp
4907 If your embedding application does not need any priorities, defining these
4908 both to \f(CW0\fR will save some memory and \s-1CPU.\s0
4909 .IP "\s-1EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE, EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE, EV_ASYNC_ENABLE, EV_CHILD_ENABLE.\s0" 4
4910 .IX Item "EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE, EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE, EV_ASYNC_ENABLE, EV_CHILD_ENABLE."
4911 If undefined or defined to be \f(CW1\fR (and the platform supports it), then
4912 the respective watcher type is supported. If defined to be \f(CW0\fR, then it
4913 is not. Disabling watcher types mainly saves code size.
4914 .IP "\s-1EV_FEATURES\s0" 4
4915 .IX Item "EV_FEATURES"
4916 If you need to shave off some kilobytes of code at the expense of some
4917 speed (but with the full \s-1API\s0), you can define this symbol to request
4918 certain subsets of functionality. The default is to enable all features
4919 that can be enabled on the platform.
4920 .Sp
4921 A typical way to use this symbol is to define it to \f(CW0\fR (or to a bitset
4922 with some broad features you want) and then selectively re-enable
4923 additional parts you want, for example if you want everything minimal,
4924 but multiple event loop support, async and child watchers and the poll
4925 backend, use this:
4926 .Sp
4927 .Vb 5
4928 \& #define EV_FEATURES 0
4929 \& #define EV_MULTIPLICITY 1
4930 \& #define EV_USE_POLL 1
4931 \& #define EV_CHILD_ENABLE 1
4932 \& #define EV_ASYNC_ENABLE 1
4933 .Ve
4934 .Sp
4935 The actual value is a bitset, it can be a combination of the following
4936 values (by default, all of these are enabled):
4937 .RS 4
4938 .ie n .IP "1 \- faster/larger code" 4
4939 .el .IP "\f(CW1\fR \- faster/larger code" 4
4940 .IX Item "1 - faster/larger code"
4941 Use larger code to speed up some operations.
4942 .Sp
4943 Currently this is used to override some inlining decisions (enlarging the
4944 code size by roughly 30% on amd64).
4945 .Sp
4946 When optimising for size, use of compiler flags such as \f(CW\*(C`\-Os\*(C'\fR with
4947 gcc is recommended, as well as \f(CW\*(C`\-DNDEBUG\*(C'\fR, as libev contains a number of
4948 assertions.
4949 .Sp
4950 The default is off when \f(CW\*(C`_\|_OPTIMIZE_SIZE_\|_\*(C'\fR is defined by your compiler
4951 (e.g. gcc with \f(CW\*(C`\-Os\*(C'\fR).
4952 .ie n .IP "2 \- faster/larger data structures" 4
4953 .el .IP "\f(CW2\fR \- faster/larger data structures" 4
4954 .IX Item "2 - faster/larger data structures"
4955 Replaces the small 2\-heap for timer management by a faster 4\-heap, larger
4956 hash table sizes and so on. This will usually further increase code size
4957 and can additionally have an effect on the size of data structures at
4958 runtime.
4959 .Sp
4960 The default is off when \f(CW\*(C`_\|_OPTIMIZE_SIZE_\|_\*(C'\fR is defined by your compiler
4961 (e.g. gcc with \f(CW\*(C`\-Os\*(C'\fR).
4962 .ie n .IP "4 \- full \s-1API\s0 configuration" 4
4963 .el .IP "\f(CW4\fR \- full \s-1API\s0 configuration" 4
4964 .IX Item "4 - full API configuration"
4965 This enables priorities (sets \f(CW\*(C`EV_MAXPRI\*(C'\fR=2 and \f(CW\*(C`EV_MINPRI\*(C'\fR=\-2), and
4966 enables multiplicity (\f(CW\*(C`EV_MULTIPLICITY\*(C'\fR=1).
4967 .ie n .IP "8 \- full \s-1API\s0" 4
4968 .el .IP "\f(CW8\fR \- full \s-1API\s0" 4
4969 .IX Item "8 - full API"
4970 This enables a lot of the \*(L"lesser used\*(R" \s-1API\s0 functions. See \f(CW\*(C`ev.h\*(C'\fR for
4971 details on which parts of the \s-1API\s0 are still available without this
4972 feature, and do not complain if this subset changes over time.
4973 .ie n .IP "16 \- enable all optional watcher types" 4
4974 .el .IP "\f(CW16\fR \- enable all optional watcher types" 4
4975 .IX Item "16 - enable all optional watcher types"
4976 Enables all optional watcher types. If you want to selectively enable
4977 only some watcher types other than I/O and timers (e.g. prepare,
4978 embed, async, child...) you can enable them manually by defining
4979 \&\f(CW\*(C`EV_watchertype_ENABLE\*(C'\fR to \f(CW1\fR instead.
4980 .ie n .IP "32 \- enable all backends" 4
4981 .el .IP "\f(CW32\fR \- enable all backends" 4
4982 .IX Item "32 - enable all backends"
4983 This enables all backends \- without this feature, you need to enable at
4984 least one backend manually (\f(CW\*(C`EV_USE_SELECT\*(C'\fR is a good choice).
4985 .ie n .IP "64 \- enable OS-specific ""helper"" APIs" 4
4986 .el .IP "\f(CW64\fR \- enable OS-specific ``helper'' APIs" 4
4987 .IX Item "64 - enable OS-specific helper APIs"
4988 Enable inotify, eventfd, signalfd and similar OS-specific helper APIs by
4989 default.
4990 .RE
4991 .RS 4
4992 .Sp
4993 Compiling with \f(CW\*(C`gcc \-Os \-DEV_STANDALONE \-DEV_USE_EPOLL=1 \-DEV_FEATURES=0\*(C'\fR
4994 reduces the compiled size of libev from 24.7Kb code/2.8Kb data to 6.5Kb
4995 code/0.3Kb data on my GNU/Linux amd64 system, while still giving you I/O
4996 watchers, timers and monotonic clock support.
4997 .Sp
4998 With an intelligent-enough linker (gcc+binutils are intelligent enough
4999 when you use \f(CW\*(C`\-Wl,\-\-gc\-sections \-ffunction\-sections\*(C'\fR) functions unused by
5000 your program might be left out as well \- a binary starting a timer and an
5001 I/O watcher then might come out at only 5Kb.
5002 .RE
5003 .IP "\s-1EV_API_STATIC\s0" 4
5004 .IX Item "EV_API_STATIC"
5005 If this symbol is defined (by default it is not), then all identifiers
5006 will have static linkage. This means that libev will not export any
5007 identifiers, and you cannot link against libev anymore. This can be useful
5008 when you embed libev, only want to use libev functions in a single file,
5009 and do not want its identifiers to be visible.
5010 .Sp
5011 To use this, define \f(CW\*(C`EV_API_STATIC\*(C'\fR and include \fIev.c\fR in the file that
5012 wants to use libev.
5013 .Sp
5014 This option only works when libev is compiled with a C compiler, as \*(C+
5015 doesn't support the required declaration syntax.
5016 .IP "\s-1EV_AVOID_STDIO\s0" 4
5017 .IX Item "EV_AVOID_STDIO"
5018 If this is set to \f(CW1\fR at compiletime, then libev will avoid using stdio
5019 functions (printf, scanf, perror etc.). This will increase the code size
5020 somewhat, but if your program doesn't otherwise depend on stdio and your
5021 libc allows it, this avoids linking in the stdio library which is quite
5022 big.
5023 .Sp
5024 Note that error messages might become less precise when this option is
5025 enabled.
5026 .IP "\s-1EV_NSIG\s0" 4
5027 .IX Item "EV_NSIG"
5028 The highest supported signal number, +1 (or, the number of
5029 signals): Normally, libev tries to deduce the maximum number of signals
5030 automatically, but sometimes this fails, in which case it can be
5031 specified. Also, using a lower number than detected (\f(CW32\fR should be
5032 good for about any system in existence) can save some memory, as libev
5033 statically allocates some 12\-24 bytes per signal number.
5034 .IP "\s-1EV_PID_HASHSIZE\s0" 4
5035 .IX Item "EV_PID_HASHSIZE"
5036 \&\f(CW\*(C`ev_child\*(C'\fR watchers use a small hash table to distribute workload by
5037 pid. The default size is \f(CW16\fR (or \f(CW1\fR with \f(CW\*(C`EV_FEATURES\*(C'\fR disabled),
5038 usually more than enough. If you need to manage thousands of children you
5039 might want to increase this value (\fImust\fR be a power of two).
5040 .IP "\s-1EV_INOTIFY_HASHSIZE\s0" 4
5041 .IX Item "EV_INOTIFY_HASHSIZE"
5042 \&\f(CW\*(C`ev_stat\*(C'\fR watchers use a small hash table to distribute workload by
5043 inotify watch id. The default size is \f(CW16\fR (or \f(CW1\fR with \f(CW\*(C`EV_FEATURES\*(C'\fR
5044 disabled), usually more than enough. If you need to manage thousands of
5045 \&\f(CW\*(C`ev_stat\*(C'\fR watchers you might want to increase this value (\fImust\fR be a
5046 power of two).
5047 .IP "\s-1EV_USE_4HEAP\s0" 4
5048 .IX Item "EV_USE_4HEAP"
5049 Heaps are not very cache-efficient. To improve the cache-efficiency of the
5050 timer and periodics heaps, libev uses a 4\-heap when this symbol is defined
5051 to \f(CW1\fR. The 4\-heap uses more complicated (longer) code but has noticeably
5052 faster performance with many (thousands) of watchers.
5053 .Sp
5054 The default is \f(CW1\fR, unless \f(CW\*(C`EV_FEATURES\*(C'\fR overrides it, in which case it
5055 will be \f(CW0\fR.
5056 .IP "\s-1EV_HEAP_CACHE_AT\s0" 4
5057 .IX Item "EV_HEAP_CACHE_AT"
5058 Heaps are not very cache-efficient. To improve the cache-efficiency of the
5059 timer and periodics heaps, libev can cache the timestamp (\fIat\fR) within
5060 the heap structure (selected by defining \f(CW\*(C`EV_HEAP_CACHE_AT\*(C'\fR to \f(CW1\fR),
5061 which uses 8\-12 bytes more per watcher and a few hundred bytes more code,
5062 but avoids random read accesses on heap changes. This improves performance
5063 noticeably with many (hundreds) of watchers.
5064 .Sp
5065 The default is \f(CW1\fR, unless \f(CW\*(C`EV_FEATURES\*(C'\fR overrides it, in which case it
5066 will be \f(CW0\fR.
5067 .IP "\s-1EV_VERIFY\s0" 4
5068 .IX Item "EV_VERIFY"
5069 Controls how much internal verification (see \f(CW\*(C`ev_verify ()\*(C'\fR) will
5070 be done: If set to \f(CW0\fR, no internal verification code will be compiled
5071 in. If set to \f(CW1\fR, then verification code will be compiled in, but not
5072 called. If set to \f(CW2\fR, then the internal verification code will be
5073 called once per loop, which can slow down libev. If set to \f(CW3\fR, then the
5074 verification code will be called very frequently, which will slow down
5075 libev considerably.
5076 .Sp
5077 The default is \f(CW1\fR, unless \f(CW\*(C`EV_FEATURES\*(C'\fR overrides it, in which case it
5078 will be \f(CW0\fR.
5079 .IP "\s-1EV_COMMON\s0" 4
5080 .IX Item "EV_COMMON"
5081 By default, all watchers have a \f(CW\*(C`void *data\*(C'\fR member. By redefining
5082 this macro to something else you can include more and other types of
5083 members. You have to define it each time you include one of the files,
5084 though, and it must be identical each time.
5085 .Sp
5086 For example, the perl \s-1EV\s0 module uses something like this:
5087 .Sp
5088 .Vb 3
5089 \& #define EV_COMMON \e
5090 \& SV *self; /* contains this struct */ \e
5091 \& SV *cb_sv, *fh /* note no trailing ";" */
5092 .Ve
5093 .IP "\s-1EV_CB_DECLARE\s0 (type)" 4
5094 .IX Item "EV_CB_DECLARE (type)"
5095 .PD 0
5096 .IP "\s-1EV_CB_INVOKE\s0 (watcher, revents)" 4
5097 .IX Item "EV_CB_INVOKE (watcher, revents)"
5098 .IP "ev_set_cb (ev, cb)" 4
5099 .IX Item "ev_set_cb (ev, cb)"
5100 .PD
5101 Can be used to change the callback member declaration in each watcher,
5102 and the way callbacks are invoked and set. Must expand to a struct member
5103 definition and a statement, respectively. See the \fIev.h\fR header file for
5104 their default definitions. One possible use for overriding these is to
5105 avoid the \f(CW\*(C`struct ev_loop *\*(C'\fR as first argument in all cases, or to use
5106 method calls instead of plain function calls in \*(C+.
5107 .SS "\s-1EXPORTED API SYMBOLS\s0"
5108 .IX Subsection "EXPORTED API SYMBOLS"
5109 If you need to re-export the \s-1API\s0 (e.g. via a \s-1DLL\s0) and you need a list of
5110 exported symbols, you can use the provided \fISymbol.*\fR files which list
5111 all public symbols, one per line:
5112 .PP
5113 .Vb 2
5114 \& Symbols.ev for libev proper
5115 \& Symbols.event for the libevent emulation
5116 .Ve
5117 .PP
5118 This can also be used to rename all public symbols to avoid clashes with
5119 multiple versions of libev linked together (which is obviously bad in
5120 itself, but sometimes it is inconvenient to avoid this).
5121 .PP
5122 A sed command like this will create wrapper \f(CW\*(C`#define\*(C'\fR's that you need to
5123 include before including \fIev.h\fR:
5124 .PP
5125 .Vb 1
5126 \& <Symbols.ev sed \-e "s/.*/#define & myprefix_&/" >wrap.h
5127 .Ve
5128 .PP
5129 This would create a file \fIwrap.h\fR which essentially looks like this:
5130 .PP
5131 .Vb 4
5132 \& #define ev_backend myprefix_ev_backend
5133 \& #define ev_check_start myprefix_ev_check_start
5134 \& #define ev_check_stop myprefix_ev_check_stop
5135 \& ...
5136 .Ve
5137 .SS "\s-1EXAMPLES\s0"
5138 .IX Subsection "EXAMPLES"
5139 For a real-world example of a program the includes libev
5140 verbatim, you can have a look at the \s-1EV\s0 perl module
5141 (<http://software.schmorp.de/pkg/EV.html>). It has the libev files in
5142 the \fIlibev/\fR subdirectory and includes them in the \fI\s-1EV/EVAPI\s0.h\fR (public
5143 interface) and \fI\s-1EV\s0.xs\fR (implementation) files. Only the \fI\s-1EV\s0.xs\fR file
5144 will be compiled. It is pretty complex because it provides its own header
5145 file.
5146 .PP
5147 The usage in rxvt-unicode is simpler. It has a \fIev_cpp.h\fR header file
5148 that everybody includes and which overrides some configure choices:
5149 .PP
5150 .Vb 8
5151 \& #define EV_FEATURES 8
5152 \& #define EV_USE_SELECT 1
5153 \& #define EV_PREPARE_ENABLE 1
5154 \& #define EV_IDLE_ENABLE 1
5155 \& #define EV_SIGNAL_ENABLE 1
5156 \& #define EV_CHILD_ENABLE 1
5157 \& #define EV_USE_STDEXCEPT 0
5158 \& #define EV_CONFIG_H <config.h>
5159 \&
5160 \& #include "ev++.h"
5161 .Ve
5162 .PP
5163 And a \fIev_cpp.C\fR implementation file that contains libev proper and is compiled:
5164 .PP
5165 .Vb 2
5166 \& #include "ev_cpp.h"
5167 \& #include "ev.c"
5168 .Ve
5169 .SH "INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT"
5170 .IX Header "INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT"
5171 .SS "\s-1THREADS AND COROUTINES\s0"
5172 .IX Subsection "THREADS AND COROUTINES"
5173 \fI\s-1THREADS\s0\fR
5174 .IX Subsection "THREADS"
5175 .PP
5176 All libev functions are reentrant and thread-safe unless explicitly
5177 documented otherwise, but libev implements no locking itself. This means
5178 that you can use as many loops as you want in parallel, as long as there
5179 are no concurrent calls into any libev function with the same loop
5180 parameter (\f(CW\*(C`ev_default_*\*(C'\fR calls have an implicit default loop parameter,
5181 of course): libev guarantees that different event loops share no data
5182 structures that need any locking.
5183 .PP
5184 Or to put it differently: calls with different loop parameters can be done
5185 concurrently from multiple threads, calls with the same loop parameter
5186 must be done serially (but can be done from different threads, as long as
5187 only one thread ever is inside a call at any point in time, e.g. by using
5188 a mutex per loop).
5189 .PP
5190 Specifically to support threads (and signal handlers), libev implements
5191 so-called \f(CW\*(C`ev_async\*(C'\fR watchers, which allow some limited form of
5192 concurrency on the same event loop, namely waking it up \*(L"from the
5193 outside\*(R".
5194 .PP
5195 If you want to know which design (one loop, locking, or multiple loops
5196 without or something else still) is best for your problem, then I cannot
5197 help you, but here is some generic advice:
5198 .IP "\(bu" 4
5199 most applications have a main thread: use the default libev loop
5200 in that thread, or create a separate thread running only the default loop.
5201 .Sp
5202 This helps integrating other libraries or software modules that use libev
5203 themselves and don't care/know about threading.
5204 .IP "\(bu" 4
5205 one loop per thread is usually a good model.
5206 .Sp
5207 Doing this is almost never wrong, sometimes a better-performance model
5208 exists, but it is always a good start.
5209 .IP "\(bu" 4
5210 other models exist, such as the leader/follower pattern, where one
5211 loop is handed through multiple threads in a kind of round-robin fashion.
5212 .Sp
5213 Choosing a model is hard \- look around, learn, know that usually you can do
5214 better than you currently do :\-)
5215 .IP "\(bu" 4
5216 often you need to talk to some other thread which blocks in the
5217 event loop.
5218 .Sp
5219 \&\f(CW\*(C`ev_async\*(C'\fR watchers can be used to wake them up from other threads safely
5220 (or from signal contexts...).
5221 .Sp
5222 An example use would be to communicate signals or other events that only
5223 work in the default loop by registering the signal watcher with the
5224 default loop and triggering an \f(CW\*(C`ev_async\*(C'\fR watcher from the default loop
5225 watcher callback into the event loop interested in the signal.
5226 .PP
5227 See also \*(L"\s-1THREAD LOCKING EXAMPLE\*(R"\s0.
5228 .PP
5229 \fI\s-1COROUTINES\s0\fR
5230 .IX Subsection "COROUTINES"
5231 .PP
5232 Libev is very accommodating to coroutines (\*(L"cooperative threads\*(R"):
5233 libev fully supports nesting calls to its functions from different
5234 coroutines (e.g. you can call \f(CW\*(C`ev_run\*(C'\fR on the same loop from two
5235 different coroutines, and switch freely between both coroutines running
5236 the loop, as long as you don't confuse yourself). The only exception is
5237 that you must not do this from \f(CW\*(C`ev_periodic\*(C'\fR reschedule callbacks.
5238 .PP
5239 Care has been taken to ensure that libev does not keep local state inside
5240 \&\f(CW\*(C`ev_run\*(C'\fR, and other calls do not usually allow for coroutine switches as
5241 they do not call any callbacks.
5242 .SS "\s-1COMPILER WARNINGS\s0"
5243 .IX Subsection "COMPILER WARNINGS"
5244 Depending on your compiler and compiler settings, you might get no or a
5245 lot of warnings when compiling libev code. Some people are apparently
5246 scared by this.
5247 .PP
5248 However, these are unavoidable for many reasons. For one, each compiler
5249 has different warnings, and each user has different tastes regarding
5250 warning options. \*(L"Warn-free\*(R" code therefore cannot be a goal except when
5251 targeting a specific compiler and compiler-version.
5252 .PP
5253 Another reason is that some compiler warnings require elaborate
5254 workarounds, or other changes to the code that make it less clear and less
5255 maintainable.
5256 .PP
5257 And of course, some compiler warnings are just plain stupid, or simply
5258 wrong (because they don't actually warn about the condition their message
5259 seems to warn about). For example, certain older gcc versions had some
5260 warnings that resulted in an extreme number of false positives. These have
5261 been fixed, but some people still insist on making code warn-free with
5262 such buggy versions.
5263 .PP
5264 While libev is written to generate as few warnings as possible,
5265 \&\*(L"warn-free\*(R" code is not a goal, and it is recommended not to build libev
5266 with any compiler warnings enabled unless you are prepared to cope with
5267 them (e.g. by ignoring them). Remember that warnings are just that:
5268 warnings, not errors, or proof of bugs.
5269 .SS "\s-1VALGRIND\s0"
5270 .IX Subsection "VALGRIND"
5271 Valgrind has a special section here because it is a popular tool that is
5272 highly useful. Unfortunately, valgrind reports are very hard to interpret.
5273 .PP
5274 If you think you found a bug (memory leak, uninitialised data access etc.)
5275 in libev, then check twice: If valgrind reports something like:
5276 .PP
5277 .Vb 3
5278 \& ==2274== definitely lost: 0 bytes in 0 blocks.
5279 \& ==2274== possibly lost: 0 bytes in 0 blocks.
5280 \& ==2274== still reachable: 256 bytes in 1 blocks.
5281 .Ve
5282 .PP
5283 Then there is no memory leak, just as memory accounted to global variables
5284 is not a memleak \- the memory is still being referenced, and didn't leak.
5285 .PP
5286 Similarly, under some circumstances, valgrind might report kernel bugs
5287 as if it were a bug in libev (e.g. in realloc or in the poll backend,
5288 although an acceptable workaround has been found here), or it might be
5289 confused.
5290 .PP
5291 Keep in mind that valgrind is a very good tool, but only a tool. Don't
5292 make it into some kind of religion.
5293 .PP
5294 If you are unsure about something, feel free to contact the mailing list
5295 with the full valgrind report and an explanation on why you think this
5296 is a bug in libev (best check the archives, too :). However, don't be
5297 annoyed when you get a brisk \*(L"this is no bug\*(R" answer and take the chance
5298 of learning how to interpret valgrind properly.
5299 .PP
5300 If you need, for some reason, empty reports from valgrind for your project
5301 I suggest using suppression lists.
5302 .SH "PORTABILITY NOTES"
5303 .IX Header "PORTABILITY NOTES"
5304 .SS "\s-1GNU/LINUX 32 BIT LIMITATIONS\s0"
5305 .IX Subsection "GNU/LINUX 32 BIT LIMITATIONS"
5306 GNU/Linux is the only common platform that supports 64 bit file/large file
5307 interfaces but \fIdisables\fR them by default.
5308 .PP
5309 That means that libev compiled in the default environment doesn't support
5310 files larger than 2GiB or so, which mainly affects \f(CW\*(C`ev_stat\*(C'\fR watchers.
5311 .PP
5312 Unfortunately, many programs try to work around this GNU/Linux issue
5313 by enabling the large file \s-1API,\s0 which makes them incompatible with the
5314 standard libev compiled for their system.
5315 .PP
5316 Likewise, libev cannot enable the large file \s-1API\s0 itself as this would
5317 suddenly make it incompatible to the default compile time environment,
5318 i.e. all programs not using special compile switches.
5319 .SS "\s-1OS/X AND DARWIN BUGS\s0"
5320 .IX Subsection "OS/X AND DARWIN BUGS"
5321 The whole thing is a bug if you ask me \- basically any system interface
5322 you touch is broken, whether it is locales, poll, kqueue or even the
5323 OpenGL drivers.
5324 .PP
5325 \fI\f(CI\*(C`kqueue\*(C'\fI is buggy\fR
5326 .IX Subsection "kqueue is buggy"
5327 .PP
5328 The kqueue syscall is broken in all known versions \- most versions support
5329 only sockets, many support pipes.
5330 .PP
5331 Libev tries to work around this by not using \f(CW\*(C`kqueue\*(C'\fR by default on this
5332 rotten platform, but of course you can still ask for it when creating a
5333 loop \- embedding a socket-only kqueue loop into a select-based one is
5334 probably going to work well.
5335 .PP
5336 \fI\f(CI\*(C`poll\*(C'\fI is buggy\fR
5337 .IX Subsection "poll is buggy"
5338 .PP
5339 Instead of fixing \f(CW\*(C`kqueue\*(C'\fR, Apple replaced their (working) \f(CW\*(C`poll\*(C'\fR
5340 implementation by something calling \f(CW\*(C`kqueue\*(C'\fR internally around the 10.5.6
5341 release, so now \f(CW\*(C`kqueue\*(C'\fR \fIand\fR \f(CW\*(C`poll\*(C'\fR are broken.
5342 .PP
5343 Libev tries to work around this by not using \f(CW\*(C`poll\*(C'\fR by default on
5344 this rotten platform, but of course you can still ask for it when creating
5345 a loop.
5346 .PP
5347 \fI\f(CI\*(C`select\*(C'\fI is buggy\fR
5348 .IX Subsection "select is buggy"
5349 .PP
5350 All that's left is \f(CW\*(C`select\*(C'\fR, and of course Apple found a way to fuck this
5351 one up as well: On \s-1OS/X,\s0 \f(CW\*(C`select\*(C'\fR actively limits the number of file
5352 descriptors you can pass in to 1024 \- your program suddenly crashes when
5353 you use more.
5354 .PP
5355 There is an undocumented \*(L"workaround\*(R" for this \- defining
5356 \&\f(CW\*(C`_DARWIN_UNLIMITED_SELECT\*(C'\fR, which libev tries to use, so select \fIshould\fR
5357 work on \s-1OS/X.\s0
5358 .SS "\s-1SOLARIS PROBLEMS AND WORKAROUNDS\s0"
5359 .IX Subsection "SOLARIS PROBLEMS AND WORKAROUNDS"
5360 \fI\f(CI\*(C`errno\*(C'\fI reentrancy\fR
5361 .IX Subsection "errno reentrancy"
5362 .PP
5363 The default compile environment on Solaris is unfortunately so
5364 thread-unsafe that you can't even use components/libraries compiled
5365 without \f(CW\*(C`\-D_REENTRANT\*(C'\fR in a threaded program, which, of course, isn't
5366 defined by default. A valid, if stupid, implementation choice.
5367 .PP
5368 If you want to use libev in threaded environments you have to make sure
5369 it's compiled with \f(CW\*(C`_REENTRANT\*(C'\fR defined.
5370 .PP
5371 \fIEvent port backend\fR
5372 .IX Subsection "Event port backend"
5373 .PP
5374 The scalable event interface for Solaris is called \*(L"event
5375 ports\*(R". Unfortunately, this mechanism is very buggy in all major
5376 releases. If you run into high \s-1CPU\s0 usage, your program freezes or you get
5377 a large number of spurious wakeups, make sure you have all the relevant
5378 and latest kernel patches applied. No, I don't know which ones, but there
5379 are multiple ones to apply, and afterwards, event ports actually work
5380 great.
5381 .PP
5382 If you can't get it to work, you can try running the program by setting
5383 the environment variable \f(CW\*(C`LIBEV_FLAGS=3\*(C'\fR to only allow \f(CW\*(C`poll\*(C'\fR and
5384 \&\f(CW\*(C`select\*(C'\fR backends.
5385 .SS "\s-1AIX POLL BUG\s0"
5386 .IX Subsection "AIX POLL BUG"
5387 \&\s-1AIX\s0 unfortunately has a broken \f(CW\*(C`poll.h\*(C'\fR header. Libev works around
5388 this by trying to avoid the poll backend altogether (i.e. it's not even
5389 compiled in), which normally isn't a big problem as \f(CW\*(C`select\*(C'\fR works fine
5390 with large bitsets on \s-1AIX,\s0 and \s-1AIX\s0 is dead anyway.
5391 .SS "\s-1WIN32 PLATFORM LIMITATIONS AND WORKAROUNDS\s0"
5392 .IX Subsection "WIN32 PLATFORM LIMITATIONS AND WORKAROUNDS"
5393 \fIGeneral issues\fR
5394 .IX Subsection "General issues"
5395 .PP
5396 Win32 doesn't support any of the standards (e.g. \s-1POSIX\s0) that libev
5397 requires, and its I/O model is fundamentally incompatible with the \s-1POSIX\s0
5398 model. Libev still offers limited functionality on this platform in
5399 the form of the \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR backend, and only supports socket
5400 descriptors. This only applies when using Win32 natively, not when using
5401 e.g. cygwin. Actually, it only applies to the microsofts own compilers,
5402 as every compiler comes with a slightly differently broken/incompatible
5403 environment.
5404 .PP
5405 Lifting these limitations would basically require the full
5406 re-implementation of the I/O system. If you are into this kind of thing,
5407 then note that glib does exactly that for you in a very portable way (note
5408 also that glib is the slowest event library known to man).
5409 .PP
5410 There is no supported compilation method available on windows except
5411 embedding it into other applications.
5412 .PP
5413 Sensible signal handling is officially unsupported by Microsoft \- libev
5414 tries its best, but under most conditions, signals will simply not work.
5415 .PP
5416 Not a libev limitation but worth mentioning: windows apparently doesn't
5417 accept large writes: instead of resulting in a partial write, windows will
5418 either accept everything or return \f(CW\*(C`ENOBUFS\*(C'\fR if the buffer is too large,
5419 so make sure you only write small amounts into your sockets (less than a
5420 megabyte seems safe, but this apparently depends on the amount of memory
5421 available).
5422 .PP
5423 Due to the many, low, and arbitrary limits on the win32 platform and
5424 the abysmal performance of winsockets, using a large number of sockets
5425 is not recommended (and not reasonable). If your program needs to use
5426 more than a hundred or so sockets, then likely it needs to use a totally
5427 different implementation for windows, as libev offers the \s-1POSIX\s0 readiness
5428 notification model, which cannot be implemented efficiently on windows
5429 (due to Microsoft monopoly games).
5430 .PP
5431 A typical way to use libev under windows is to embed it (see the embedding
5432 section for details) and use the following \fIevwrap.h\fR header file instead
5433 of \fIev.h\fR:
5434 .PP
5435 .Vb 2
5436 \& #define EV_STANDALONE /* keeps ev from requiring config.h */
5437 \& #define EV_SELECT_IS_WINSOCKET 1 /* configure libev for windows select */
5438 \&
5439 \& #include "ev.h"
5440 .Ve
5441 .PP
5442 And compile the following \fIevwrap.c\fR file into your project (make sure
5443 you do \fInot\fR compile the \fIev.c\fR or any other embedded source files!):
5444 .PP
5445 .Vb 2
5446 \& #include "evwrap.h"
5447 \& #include "ev.c"
5448 .Ve
5449 .PP
5450 \fIThe winsocket \f(CI\*(C`select\*(C'\fI function\fR
5451 .IX Subsection "The winsocket select function"
5452 .PP
5453 The winsocket \f(CW\*(C`select\*(C'\fR function doesn't follow \s-1POSIX\s0 in that it
5454 requires socket \fIhandles\fR and not socket \fIfile descriptors\fR (it is
5455 also extremely buggy). This makes select very inefficient, and also
5456 requires a mapping from file descriptors to socket handles (the Microsoft
5457 C runtime provides the function \f(CW\*(C`_open_osfhandle\*(C'\fR for this). See the
5458 discussion of the \f(CW\*(C`EV_SELECT_USE_FD_SET\*(C'\fR, \f(CW\*(C`EV_SELECT_IS_WINSOCKET\*(C'\fR and
5459 \&\f(CW\*(C`EV_FD_TO_WIN32_HANDLE\*(C'\fR preprocessor symbols for more info.
5460 .PP
5461 The configuration for a \*(L"naked\*(R" win32 using the Microsoft runtime
5462 libraries and raw winsocket select is:
5463 .PP
5464 .Vb 2
5465 \& #define EV_USE_SELECT 1
5466 \& #define EV_SELECT_IS_WINSOCKET 1 /* forces EV_SELECT_USE_FD_SET, too */
5467 .Ve
5468 .PP
5469 Note that winsockets handling of fd sets is O(n), so you can easily get a
5470 complexity in the O(nX) range when using win32.
5471 .PP
5472 \fILimited number of file descriptors\fR
5473 .IX Subsection "Limited number of file descriptors"
5474 .PP
5475 Windows has numerous arbitrary (and low) limits on things.
5476 .PP
5477 Early versions of winsocket's select only supported waiting for a maximum
5478 of \f(CW64\fR handles (probably owning to the fact that all windows kernels
5479 can only wait for \f(CW64\fR things at the same time internally; Microsoft
5480 recommends spawning a chain of threads and wait for 63 handles and the
5481 previous thread in each. Sounds great!).
5482 .PP
5483 Newer versions support more handles, but you need to define \f(CW\*(C`FD_SETSIZE\*(C'\fR
5484 to some high number (e.g. \f(CW2048\fR) before compiling the winsocket select
5485 call (which might be in libev or elsewhere, for example, perl and many
5486 other interpreters do their own select emulation on windows).
5487 .PP
5488 Another limit is the number of file descriptors in the Microsoft runtime
5489 libraries, which by default is \f(CW64\fR (there must be a hidden \fI64\fR
5490 fetish or something like this inside Microsoft). You can increase this
5491 by calling \f(CW\*(C`_setmaxstdio\*(C'\fR, which can increase this limit to \f(CW2048\fR
5492 (another arbitrary limit), but is broken in many versions of the Microsoft
5493 runtime libraries. This might get you to about \f(CW512\fR or \f(CW2048\fR sockets
5494 (depending on windows version and/or the phase of the moon). To get more,
5495 you need to wrap all I/O functions and provide your own fd management, but
5496 the cost of calling select (O(nX)) will likely make this unworkable.
5497 .SS "\s-1PORTABILITY REQUIREMENTS\s0"
5498 .IX Subsection "PORTABILITY REQUIREMENTS"
5499 In addition to a working ISO-C implementation and of course the
5500 backend-specific APIs, libev relies on a few additional extensions:
5501 .ie n .IP """void (*)(ev_watcher_type *, int revents)"" must have compatible calling conventions regardless of ""ev_watcher_type *""." 4
5502 .el .IP "\f(CWvoid (*)(ev_watcher_type *, int revents)\fR must have compatible calling conventions regardless of \f(CWev_watcher_type *\fR." 4
5503 .IX Item "void (*)(ev_watcher_type *, int revents) must have compatible calling conventions regardless of ev_watcher_type *."
5504 Libev assumes not only that all watcher pointers have the same internal
5505 structure (guaranteed by \s-1POSIX\s0 but not by \s-1ISO C\s0 for example), but it also
5506 assumes that the same (machine) code can be used to call any watcher
5507 callback: The watcher callbacks have different type signatures, but libev
5508 calls them using an \f(CW\*(C`ev_watcher *\*(C'\fR internally.
5509 .IP "null pointers and integer zero are represented by 0 bytes" 4
5510 .IX Item "null pointers and integer zero are represented by 0 bytes"
5511 Libev uses \f(CW\*(C`memset\*(C'\fR to initialise structs and arrays to \f(CW0\fR bytes, and
5512 relies on this setting pointers and integers to null.
5513 .IP "pointer accesses must be thread-atomic" 4
5514 .IX Item "pointer accesses must be thread-atomic"
5515 Accessing a pointer value must be atomic, it must both be readable and
5516 writable in one piece \- this is the case on all current architectures.
5517 .ie n .IP """sig_atomic_t volatile"" must be thread-atomic as well" 4
5518 .el .IP "\f(CWsig_atomic_t volatile\fR must be thread-atomic as well" 4
5519 .IX Item "sig_atomic_t volatile must be thread-atomic as well"
5520 The type \f(CW\*(C`sig_atomic_t volatile\*(C'\fR (or whatever is defined as
5521 \&\f(CW\*(C`EV_ATOMIC_T\*(C'\fR) must be atomic with respect to accesses from different
5522 threads. This is not part of the specification for \f(CW\*(C`sig_atomic_t\*(C'\fR, but is
5523 believed to be sufficiently portable.
5524 .ie n .IP """sigprocmask"" must work in a threaded environment" 4
5525 .el .IP "\f(CWsigprocmask\fR must work in a threaded environment" 4
5526 .IX Item "sigprocmask must work in a threaded environment"
5527 Libev uses \f(CW\*(C`sigprocmask\*(C'\fR to temporarily block signals. This is not
5528 allowed in a threaded program (\f(CW\*(C`pthread_sigmask\*(C'\fR has to be used). Typical
5529 pthread implementations will either allow \f(CW\*(C`sigprocmask\*(C'\fR in the \*(L"main
5530 thread\*(R" or will block signals process-wide, both behaviours would
5531 be compatible with libev. Interaction between \f(CW\*(C`sigprocmask\*(C'\fR and
5532 \&\f(CW\*(C`pthread_sigmask\*(C'\fR could complicate things, however.
5533 .Sp
5534 The most portable way to handle signals is to block signals in all threads
5535 except the initial one, and run the signal handling loop in the initial
5536 thread as well.
5537 .ie n .IP """long"" must be large enough for common memory allocation sizes" 4
5538 .el .IP "\f(CWlong\fR must be large enough for common memory allocation sizes" 4
5539 .IX Item "long must be large enough for common memory allocation sizes"
5540 To improve portability and simplify its \s-1API,\s0 libev uses \f(CW\*(C`long\*(C'\fR internally
5541 instead of \f(CW\*(C`size_t\*(C'\fR when allocating its data structures. On non-POSIX
5542 systems (Microsoft...) this might be unexpectedly low, but is still at
5543 least 31 bits everywhere, which is enough for hundreds of millions of
5544 watchers.
5545 .ie n .IP """double"" must hold a time value in seconds with enough accuracy" 4
5546 .el .IP "\f(CWdouble\fR must hold a time value in seconds with enough accuracy" 4
5547 .IX Item "double must hold a time value in seconds with enough accuracy"
5548 The type \f(CW\*(C`double\*(C'\fR is used to represent timestamps. It is required to
5549 have at least 51 bits of mantissa (and 9 bits of exponent), which is
5550 good enough for at least into the year 4000 with millisecond accuracy
5551 (the design goal for libev). This requirement is overfulfilled by
5552 implementations using \s-1IEEE 754,\s0 which is basically all existing ones.
5553 .Sp
5554 With \s-1IEEE 754\s0 doubles, you get microsecond accuracy until at least the
5555 year 2255 (and millisecond accuracy till the year 287396 \- by then, libev
5556 is either obsolete or somebody patched it to use \f(CW\*(C`long double\*(C'\fR or
5557 something like that, just kidding).
5558 .PP
5559 If you know of other additional requirements drop me a note.
5560 .SH "ALGORITHMIC COMPLEXITIES"
5561 .IX Header "ALGORITHMIC COMPLEXITIES"
5562 In this section the complexities of (many of) the algorithms used inside
5563 libev will be documented. For complexity discussions about backends see
5564 the documentation for \f(CW\*(C`ev_default_init\*(C'\fR.
5565 .PP
5566 All of the following are about amortised time: If an array needs to be
5567 extended, libev needs to realloc and move the whole array, but this
5568 happens asymptotically rarer with higher number of elements, so O(1) might
5569 mean that libev does a lengthy realloc operation in rare cases, but on
5570 average it is much faster and asymptotically approaches constant time.
5571 .IP "Starting and stopping timer/periodic watchers: O(log skipped_other_timers)" 4
5572 .IX Item "Starting and stopping timer/periodic watchers: O(log skipped_other_timers)"
5573 This means that, when you have a watcher that triggers in one hour and
5574 there are 100 watchers that would trigger before that, then inserting will
5575 have to skip roughly seven (\f(CW\*(C`ld 100\*(C'\fR) of these watchers.
5576 .IP "Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)" 4
5577 .IX Item "Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)"
5578 That means that changing a timer costs less than removing/adding them,
5579 as only the relative motion in the event queue has to be paid for.
5580 .IP "Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)" 4
5581 .IX Item "Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)"
5582 These just add the watcher into an array or at the head of a list.
5583 .IP "Stopping check/prepare/idle/fork/async watchers: O(1)" 4
5584 .IX Item "Stopping check/prepare/idle/fork/async watchers: O(1)"
5585 .PD 0
5586 .IP "Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % \s-1EV_PID_HASHSIZE\s0))" 4
5587 .IX Item "Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))"
5588 .PD
5589 These watchers are stored in lists, so they need to be walked to find the
5590 correct watcher to remove. The lists are usually short (you don't usually
5591 have many watchers waiting for the same fd or signal: one is typical, two
5592 is rare).
5593 .IP "Finding the next timer in each loop iteration: O(1)" 4
5594 .IX Item "Finding the next timer in each loop iteration: O(1)"
5595 By virtue of using a binary or 4\-heap, the next timer is always found at a
5596 fixed position in the storage array.
5597 .IP "Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)" 4
5598 .IX Item "Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)"
5599 A change means an I/O watcher gets started or stopped, which requires
5600 libev to recalculate its status (and possibly tell the kernel, depending
5601 on backend and whether \f(CW\*(C`ev_io_set\*(C'\fR was used).
5602 .IP "Activating one watcher (putting it into the pending state): O(1)" 4
5603 .IX Item "Activating one watcher (putting it into the pending state): O(1)"
5604 .PD 0
5605 .IP "Priority handling: O(number_of_priorities)" 4
5606 .IX Item "Priority handling: O(number_of_priorities)"
5607 .PD
5608 Priorities are implemented by allocating some space for each
5609 priority. When doing priority-based operations, libev usually has to
5610 linearly search all the priorities, but starting/stopping and activating
5611 watchers becomes O(1) with respect to priority handling.
5612 .IP "Sending an ev_async: O(1)" 4
5613 .IX Item "Sending an ev_async: O(1)"
5614 .PD 0
5615 .IP "Processing ev_async_send: O(number_of_async_watchers)" 4
5616 .IX Item "Processing ev_async_send: O(number_of_async_watchers)"
5617 .IP "Processing signals: O(max_signal_number)" 4
5618 .IX Item "Processing signals: O(max_signal_number)"
5619 .PD
5620 Sending involves a system call \fIiff\fR there were no other \f(CW\*(C`ev_async_send\*(C'\fR
5621 calls in the current loop iteration and the loop is currently
5622 blocked. Checking for async and signal events involves iterating over all
5623 running async watchers or all signal numbers.
5624 .SH "PORTING FROM LIBEV 3.X TO 4.X"
5625 .IX Header "PORTING FROM LIBEV 3.X TO 4.X"
5626 The major version 4 introduced some incompatible changes to the \s-1API.\s0
5627 .PP
5628 At the moment, the \f(CW\*(C`ev.h\*(C'\fR header file provides compatibility definitions
5629 for all changes, so most programs should still compile. The compatibility
5630 layer might be removed in later versions of libev, so better update to the
5631 new \s-1API\s0 early than late.
5632 .ie n .IP """EV_COMPAT3"" backwards compatibility mechanism" 4
5633 .el .IP "\f(CWEV_COMPAT3\fR backwards compatibility mechanism" 4
5634 .IX Item "EV_COMPAT3 backwards compatibility mechanism"
5635 The backward compatibility mechanism can be controlled by
5636 \&\f(CW\*(C`EV_COMPAT3\*(C'\fR. See \*(L"\s-1PREPROCESSOR SYMBOLS/MACROS\*(R"\s0 in the \*(L"\s-1EMBEDDING\*(R"\s0
5637 section.
5638 .ie n .IP """ev_default_destroy"" and ""ev_default_fork"" have been removed" 4
5639 .el .IP "\f(CWev_default_destroy\fR and \f(CWev_default_fork\fR have been removed" 4
5640 .IX Item "ev_default_destroy and ev_default_fork have been removed"
5641 These calls can be replaced easily by their \f(CW\*(C`ev_loop_xxx\*(C'\fR counterparts:
5642 .Sp
5643 .Vb 2
5644 \& ev_loop_destroy (EV_DEFAULT_UC);
5645 \& ev_loop_fork (EV_DEFAULT);
5646 .Ve
5647 .IP "function/symbol renames" 4
5648 .IX Item "function/symbol renames"
5649 A number of functions and symbols have been renamed:
5650 .Sp
5651 .Vb 3
5652 \& ev_loop => ev_run
5653 \& EVLOOP_NONBLOCK => EVRUN_NOWAIT
5654 \& EVLOOP_ONESHOT => EVRUN_ONCE
5655 \&
5656 \& ev_unloop => ev_break
5657 \& EVUNLOOP_CANCEL => EVBREAK_CANCEL
5658 \& EVUNLOOP_ONE => EVBREAK_ONE
5659 \& EVUNLOOP_ALL => EVBREAK_ALL
5660 \&
5661 \& EV_TIMEOUT => EV_TIMER
5662 \&
5663 \& ev_loop_count => ev_iteration
5664 \& ev_loop_depth => ev_depth
5665 \& ev_loop_verify => ev_verify
5666 .Ve
5667 .Sp
5668 Most functions working on \f(CW\*(C`struct ev_loop\*(C'\fR objects don't have an
5669 \&\f(CW\*(C`ev_loop_\*(C'\fR prefix, so it was removed; \f(CW\*(C`ev_loop\*(C'\fR, \f(CW\*(C`ev_unloop\*(C'\fR and
5670 associated constants have been renamed to not collide with the \f(CW\*(C`struct
5671 ev_loop\*(C'\fR anymore and \f(CW\*(C`EV_TIMER\*(C'\fR now follows the same naming scheme
5672 as all other watcher types. Note that \f(CW\*(C`ev_loop_fork\*(C'\fR is still called
5673 \&\f(CW\*(C`ev_loop_fork\*(C'\fR because it would otherwise clash with the \f(CW\*(C`ev_fork\*(C'\fR
5674 typedef.
5675 .ie n .IP """EV_MINIMAL"" mechanism replaced by ""EV_FEATURES""" 4
5676 .el .IP "\f(CWEV_MINIMAL\fR mechanism replaced by \f(CWEV_FEATURES\fR" 4
5677 .IX Item "EV_MINIMAL mechanism replaced by EV_FEATURES"
5678 The preprocessor symbol \f(CW\*(C`EV_MINIMAL\*(C'\fR has been replaced by a different
5679 mechanism, \f(CW\*(C`EV_FEATURES\*(C'\fR. Programs using \f(CW\*(C`EV_MINIMAL\*(C'\fR usually compile
5680 and work, but the library code will of course be larger.
5681 .SH "GLOSSARY"
5682 .IX Header "GLOSSARY"
5683 .IP "active" 4
5684 .IX Item "active"
5685 A watcher is active as long as it has been started and not yet stopped.
5686 See \*(L"\s-1WATCHER STATES\*(R"\s0 for details.
5687 .IP "application" 4
5688 .IX Item "application"
5689 In this document, an application is whatever is using libev.
5690 .IP "backend" 4
5691 .IX Item "backend"
5692 The part of the code dealing with the operating system interfaces.
5693 .IP "callback" 4
5694 .IX Item "callback"
5695 The address of a function that is called when some event has been
5696 detected. Callbacks are being passed the event loop, the watcher that
5697 received the event, and the actual event bitset.
5698 .IP "callback/watcher invocation" 4
5699 .IX Item "callback/watcher invocation"
5700 The act of calling the callback associated with a watcher.
5701 .IP "event" 4
5702 .IX Item "event"
5703 A change of state of some external event, such as data now being available
5704 for reading on a file descriptor, time having passed or simply not having
5705 any other events happening anymore.
5706 .Sp
5707 In libev, events are represented as single bits (such as \f(CW\*(C`EV_READ\*(C'\fR or
5708 \&\f(CW\*(C`EV_TIMER\*(C'\fR).
5709 .IP "event library" 4
5710 .IX Item "event library"
5711 A software package implementing an event model and loop.
5712 .IP "event loop" 4
5713 .IX Item "event loop"
5714 An entity that handles and processes external events and converts them
5715 into callback invocations.
5716 .IP "event model" 4
5717 .IX Item "event model"
5718 The model used to describe how an event loop handles and processes
5719 watchers and events.
5720 .IP "pending" 4
5721 .IX Item "pending"
5722 A watcher is pending as soon as the corresponding event has been
5723 detected. See \*(L"\s-1WATCHER STATES\*(R"\s0 for details.
5724 .IP "real time" 4
5725 .IX Item "real time"
5726 The physical time that is observed. It is apparently strictly monotonic :)
5727 .IP "wall-clock time" 4
5728 .IX Item "wall-clock time"
5729 The time and date as shown on clocks. Unlike real time, it can actually
5730 be wrong and jump forwards and backwards, e.g. when you adjust your
5731 clock.
5732 .IP "watcher" 4
5733 .IX Item "watcher"
5734 A data structure that describes interest in certain events. Watchers need
5735 to be started (attached to an event loop) before they can receive events.
5736 .SH "AUTHOR"
5737 .IX Header "AUTHOR"
5738 Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael
5739 Magnusson and Emanuele Giaquinta, and minor corrections by many others.