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15 <h3 id="TOP">Index</h3>
17 <ul><li><a href="#NAME">NAME</a></li>
18 <li><a href="#SYNOPSIS">SYNOPSIS</a></li>
19 <li><a href="#DESCRIPTION">DESCRIPTION</a></li>
20 <li><a href="#FEATURES">FEATURES</a></li>
21 <li><a href="#CONVENTIONS">CONVENTIONS</a></li>
22 <li><a href="#TIME_REPRESENTATION">TIME REPRESENTATION</a></li>
23 <li><a href="#GLOBAL_FUNCTIONS">GLOBAL FUNCTIONS</a></li>
24 <li><a href="#FUNCTIONS_CONTROLLING_THE_EVENT_LOOP">FUNCTIONS CONTROLLING THE EVENT LOOP</a></li>
25 <li><a href="#ANATOMY_OF_A_WATCHER">ANATOMY OF A WATCHER</a>
26 <ul><li><a href="#ASSOCIATING_CUSTOM_DATA_WITH_A_WATCH">ASSOCIATING CUSTOM DATA WITH A WATCHER</a></li>
29 <li><a href="#WATCHER_TYPES">WATCHER TYPES</a>
30 <ul><li><a href="#code_ev_io_code_is_this_file_descrip"><code>ev_io</code> - is this file descriptor readable or writable</a></li>
31 <li><a href="#code_ev_timer_code_relative_and_opti"><code>ev_timer</code> - relative and optionally recurring timeouts</a></li>
32 <li><a href="#code_ev_periodic_code_to_cron_or_not"><code>ev_periodic</code> - to cron or not to cron</a></li>
33 <li><a href="#code_ev_signal_code_signal_me_when_a"><code>ev_signal</code> - signal me when a signal gets signalled</a></li>
34 <li><a href="#code_ev_child_code_wait_for_pid_stat"><code>ev_child</code> - wait for pid status changes</a></li>
35 <li><a href="#code_ev_idle_code_when_you_ve_got_no"><code>ev_idle</code> - when you've got nothing better to do</a></li>
36 <li><a href="#code_ev_prepare_code_and_code_ev_che"><code>ev_prepare</code> and <code>ev_check</code> - customise your event loop</a></li>
39 <li><a href="#OTHER_FUNCTIONS">OTHER FUNCTIONS</a></li>
40 <li><a href="#AUTHOR">AUTHOR</a>
45 <h1 id="NAME">NAME</h1><p><a href="#TOP" class="toplink">Top</a></p>
46 <div id="NAME_CONTENT">
47 <p>libev - a high performance full-featured event loop written in C</p>
50 <h1 id="SYNOPSIS">SYNOPSIS</h1><p><a href="#TOP" class="toplink">Top</a></p>
51 <div id="SYNOPSIS_CONTENT">
52 <pre> #include <ev.h>
57 <h1 id="DESCRIPTION">DESCRIPTION</h1><p><a href="#TOP" class="toplink">Top</a></p>
58 <div id="DESCRIPTION_CONTENT">
59 <p>Libev is an event loop: you register interest in certain events (such as a
60 file descriptor being readable or a timeout occuring), and it will manage
61 these event sources and provide your program with events.</p>
62 <p>To do this, it must take more or less complete control over your process
63 (or thread) by executing the <i>event loop</i> handler, and will then
64 communicate events via a callback mechanism.</p>
65 <p>You register interest in certain events by registering so-called <i>event
66 watchers</i>, which are relatively small C structures you initialise with the
67 details of the event, and then hand it over to libev by <i>starting</i> the
71 <h1 id="FEATURES">FEATURES</h1><p><a href="#TOP" class="toplink">Top</a></p>
72 <div id="FEATURES_CONTENT">
73 <p>Libev supports select, poll, the linux-specific epoll and the bsd-specific
74 kqueue mechanisms for file descriptor events, relative timers, absolute
75 timers with customised rescheduling, signal events, process status change
76 events (related to SIGCHLD), and event watchers dealing with the event
77 loop mechanism itself (idle, prepare and check watchers). It also is quite
78 fast (see this <a href="http://libev.schmorp.de/bench.html">benchmark</a> comparing
79 it to libevent for example).</p>
82 <h1 id="CONVENTIONS">CONVENTIONS</h1><p><a href="#TOP" class="toplink">Top</a></p>
83 <div id="CONVENTIONS_CONTENT">
84 <p>Libev is very configurable. In this manual the default configuration
85 will be described, which supports multiple event loops. For more info
86 about various configuration options please have a look at the file
87 <cite>README.embed</cite> in the libev distribution. If libev was configured without
88 support for multiple event loops, then all functions taking an initial
89 argument of name <code>loop</code> (which is always of type <code>struct ev_loop *</code>)
90 will not have this argument.</p>
93 <h1 id="TIME_REPRESENTATION">TIME REPRESENTATION</h1><p><a href="#TOP" class="toplink">Top</a></p>
94 <div id="TIME_REPRESENTATION_CONTENT">
95 <p>Libev represents time as a single floating point number, representing the
96 (fractional) number of seconds since the (POSIX) epoch (somewhere near
97 the beginning of 1970, details are complicated, don't ask). This type is
98 called <code>ev_tstamp</code>, which is what you should use too. It usually aliases
99 to the double type in C.</p>
102 <h1 id="GLOBAL_FUNCTIONS">GLOBAL FUNCTIONS</h1><p><a href="#TOP" class="toplink">Top</a></p>
103 <div id="GLOBAL_FUNCTIONS_CONTENT">
105 <dt>ev_tstamp ev_time ()</dt>
107 <p>Returns the current time as libev would use it.</p>
109 <dt>int ev_version_major ()</dt>
110 <dt>int ev_version_minor ()</dt>
112 <p>You can find out the major and minor version numbers of the library
113 you linked against by calling the functions <code>ev_version_major</code> and
114 <code>ev_version_minor</code>. If you want, you can compare against the global
115 symbols <code>EV_VERSION_MAJOR</code> and <code>EV_VERSION_MINOR</code>, which specify the
116 version of the library your program was compiled against.</p>
117 <p>Usually, it's a good idea to terminate if the major versions mismatch,
118 as this indicates an incompatible change. Minor versions are usually
119 compatible to older versions, so a larger minor version alone is usually
122 <dt>ev_set_allocator (void *(*cb)(void *ptr, long size))</dt>
124 <p>Sets the allocation function to use (the prototype is similar to the
125 realloc C function, the semantics are identical). It is used to allocate
126 and free memory (no surprises here). If it returns zero when memory
127 needs to be allocated, the library might abort or take some potentially
128 destructive action. The default is your system realloc function.</p>
129 <p>You could override this function in high-availability programs to, say,
130 free some memory if it cannot allocate memory, to use a special allocator,
131 or even to sleep a while and retry until some memory is available.</p>
133 <dt>ev_set_syserr_cb (void (*cb)(const char *msg));</dt>
135 <p>Set the callback function to call on a retryable syscall error (such
136 as failed select, poll, epoll_wait). The message is a printable string
137 indicating the system call or subsystem causing the problem. If this
138 callback is set, then libev will expect it to remedy the sitution, no
139 matter what, when it returns. That is, libev will generally retry the
140 requested operation, or, if the condition doesn't go away, do bad stuff
146 <h1 id="FUNCTIONS_CONTROLLING_THE_EVENT_LOOP">FUNCTIONS CONTROLLING THE EVENT LOOP</h1><p><a href="#TOP" class="toplink">Top</a></p>
147 <div id="FUNCTIONS_CONTROLLING_THE_EVENT_LOOP-2">
148 <p>An event loop is described by a <code>struct ev_loop *</code>. The library knows two
149 types of such loops, the <i>default</i> loop, which supports signals and child
150 events, and dynamically created loops which do not.</p>
151 <p>If you use threads, a common model is to run the default event loop
152 in your main thread (or in a separate thread) and for each thread you
153 create, you also create another event loop. Libev itself does no locking
154 whatsoever, so if you mix calls to the same event loop in different
155 threads, make sure you lock (this is usually a bad idea, though, even if
156 done correctly, because it's hideous and inefficient).</p>
158 <dt>struct ev_loop *ev_default_loop (unsigned int flags)</dt>
160 <p>This will initialise the default event loop if it hasn't been initialised
161 yet and return it. If the default loop could not be initialised, returns
162 false. If it already was initialised it simply returns it (and ignores the
164 <p>If you don't know what event loop to use, use the one returned from this
166 <p>The flags argument can be used to specify special behaviour or specific
167 backends to use, and is usually specified as 0 (or EVFLAG_AUTO).</p>
168 <p>It supports the following flags:</p>
171 <dt><code>EVFLAG_AUTO</code></dt>
173 <p>The default flags value. Use this if you have no clue (it's the right
174 thing, believe me).</p>
176 <dt><code>EVFLAG_NOENV</code></dt>
178 <p>If this flag bit is ored into the flag value (or the program runs setuid
179 or setgid) then libev will <i>not</i> look at the environment variable
180 <code>LIBEV_FLAGS</code>. Otherwise (the default), this environment variable will
181 override the flags completely if it is found in the environment. This is
182 useful to try out specific backends to test their performance, or to work
185 <dt><code>EVMETHOD_SELECT</code> (portable select backend)</dt>
186 <dt><code>EVMETHOD_POLL</code> (poll backend, available everywhere except on windows)</dt>
187 <dt><code>EVMETHOD_EPOLL</code> (linux only)</dt>
188 <dt><code>EVMETHOD_KQUEUE</code> (some bsds only)</dt>
189 <dt><code>EVMETHOD_DEVPOLL</code> (solaris 8 only)</dt>
190 <dt><code>EVMETHOD_PORT</code> (solaris 10 only)</dt>
192 <p>If one or more of these are ored into the flags value, then only these
193 backends will be tried (in the reverse order as given here). If one are
194 specified, any backend will do.</p>
199 <dt>struct ev_loop *ev_loop_new (unsigned int flags)</dt>
201 <p>Similar to <code>ev_default_loop</code>, but always creates a new event loop that is
202 always distinct from the default loop. Unlike the default loop, it cannot
203 handle signal and child watchers, and attempts to do so will be greeted by
204 undefined behaviour (or a failed assertion if assertions are enabled).</p>
206 <dt>ev_default_destroy ()</dt>
208 <p>Destroys the default loop again (frees all memory and kernel state
209 etc.). This stops all registered event watchers (by not touching them in
210 any way whatsoever, although you cannot rely on this :).</p>
212 <dt>ev_loop_destroy (loop)</dt>
214 <p>Like <code>ev_default_destroy</code>, but destroys an event loop created by an
215 earlier call to <code>ev_loop_new</code>.</p>
217 <dt>ev_default_fork ()</dt>
219 <p>This function reinitialises the kernel state for backends that have
220 one. Despite the name, you can call it anytime, but it makes most sense
221 after forking, in either the parent or child process (or both, but that
222 again makes little sense).</p>
223 <p>You <i>must</i> call this function after forking if and only if you want to
224 use the event library in both processes. If you just fork+exec, you don't
226 <p>The function itself is quite fast and it's usually not a problem to call
227 it just in case after a fork. To make this easy, the function will fit in
228 quite nicely into a call to <code>pthread_atfork</code>:</p>
229 <pre> pthread_atfork (0, 0, ev_default_fork);
233 <dt>ev_loop_fork (loop)</dt>
235 <p>Like <code>ev_default_fork</code>, but acts on an event loop created by
236 <code>ev_loop_new</code>. Yes, you have to call this on every allocated event loop
237 after fork, and how you do this is entirely your own problem.</p>
239 <dt>unsigned int ev_method (loop)</dt>
241 <p>Returns one of the <code>EVMETHOD_*</code> flags indicating the event backend in
244 <dt>ev_tstamp ev_now (loop)</dt>
246 <p>Returns the current "event loop time", which is the time the event loop
247 got events and started processing them. This timestamp does not change
248 as long as callbacks are being processed, and this is also the base time
249 used for relative timers. You can treat it as the timestamp of the event
250 occuring (or more correctly, the mainloop finding out about it).</p>
252 <dt>ev_loop (loop, int flags)</dt>
254 <p>Finally, this is it, the event handler. This function usually is called
255 after you initialised all your watchers and you want to start handling
257 <p>If the flags argument is specified as 0, it will not return until either
258 no event watchers are active anymore or <code>ev_unloop</code> was called.</p>
259 <p>A flags value of <code>EVLOOP_NONBLOCK</code> will look for new events, will handle
260 those events and any outstanding ones, but will not block your process in
261 case there are no events and will return after one iteration of the loop.</p>
262 <p>A flags value of <code>EVLOOP_ONESHOT</code> will look for new events (waiting if
263 neccessary) and will handle those and any outstanding ones. It will block
264 your process until at least one new event arrives, and will return after
265 one iteration of the loop.</p>
266 <p>This flags value could be used to implement alternative looping
267 constructs, but the <code>prepare</code> and <code>check</code> watchers provide a better and
268 more generic mechanism.</p>
270 <dt>ev_unloop (loop, how)</dt>
272 <p>Can be used to make a call to <code>ev_loop</code> return early (but only after it
273 has processed all outstanding events). The <code>how</code> argument must be either
274 <code>EVUNLOOP_ONCE</code>, which will make the innermost <code>ev_loop</code> call return, or
275 <code>EVUNLOOP_ALL</code>, which will make all nested <code>ev_loop</code> calls return.</p>
277 <dt>ev_ref (loop)</dt>
278 <dt>ev_unref (loop)</dt>
280 <p>Ref/unref can be used to add or remove a reference count on the event
281 loop: Every watcher keeps one reference, and as long as the reference
282 count is nonzero, <code>ev_loop</code> will not return on its own. If you have
283 a watcher you never unregister that should not keep <code>ev_loop</code> from
284 returning, ev_unref() after starting, and ev_ref() before stopping it. For
285 example, libev itself uses this for its internal signal pipe: It is not
286 visible to the libev user and should not keep <code>ev_loop</code> from exiting if
287 no event watchers registered by it are active. It is also an excellent
288 way to do this for generic recurring timers or from within third-party
289 libraries. Just remember to <i>unref after start</i> and <i>ref before stop</i>.</p>
294 <h1 id="ANATOMY_OF_A_WATCHER">ANATOMY OF A WATCHER</h1><p><a href="#TOP" class="toplink">Top</a></p>
295 <div id="ANATOMY_OF_A_WATCHER_CONTENT">
296 <p>A watcher is a structure that you create and register to record your
297 interest in some event. For instance, if you want to wait for STDIN to
298 become readable, you would create an <code>ev_io</code> watcher for that:</p>
299 <pre> static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
302 ev_unloop (loop, EVUNLOOP_ALL);
305 struct ev_loop *loop = ev_default_loop (0);
306 struct ev_io stdin_watcher;
307 ev_init (&stdin_watcher, my_cb);
308 ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
309 ev_io_start (loop, &stdin_watcher);
313 <p>As you can see, you are responsible for allocating the memory for your
314 watcher structures (and it is usually a bad idea to do this on the stack,
315 although this can sometimes be quite valid).</p>
316 <p>Each watcher structure must be initialised by a call to <code>ev_init
317 (watcher *, callback)</code>, which expects a callback to be provided. This
318 callback gets invoked each time the event occurs (or, in the case of io
319 watchers, each time the event loop detects that the file descriptor given
320 is readable and/or writable).</p>
321 <p>Each watcher type has its own <code>ev_<type>_set (watcher *, ...)</code> macro
322 with arguments specific to this watcher type. There is also a macro
323 to combine initialisation and setting in one call: <code>ev_<type>_init
324 (watcher *, callback, ...)</code>.</p>
325 <p>To make the watcher actually watch out for events, you have to start it
326 with a watcher-specific start function (<code>ev_<type>_start (loop, watcher
327 *)</code>), and you can stop watching for events at any time by calling the
328 corresponding stop function (<code>ev_<type>_stop (loop, watcher *)</code>.</p>
329 <p>As long as your watcher is active (has been started but not stopped) you
330 must not touch the values stored in it. Most specifically you must never
331 reinitialise it or call its set method.</p>
332 <p>You can check whether an event is active by calling the <code>ev_is_active
333 (watcher *)</code> macro. To see whether an event is outstanding (but the
334 callback for it has not been called yet) you can use the <code>ev_is_pending
335 (watcher *)</code> macro.</p>
336 <p>Each and every callback receives the event loop pointer as first, the
337 registered watcher structure as second, and a bitset of received events as
339 <p>The received events usually include a single bit per event type received
340 (you can receive multiple events at the same time). The possible bit masks
343 <dt><code>EV_READ</code></dt>
344 <dt><code>EV_WRITE</code></dt>
346 <p>The file descriptor in the <code>ev_io</code> watcher has become readable and/or
349 <dt><code>EV_TIMEOUT</code></dt>
351 <p>The <code>ev_timer</code> watcher has timed out.</p>
353 <dt><code>EV_PERIODIC</code></dt>
355 <p>The <code>ev_periodic</code> watcher has timed out.</p>
357 <dt><code>EV_SIGNAL</code></dt>
359 <p>The signal specified in the <code>ev_signal</code> watcher has been received by a thread.</p>
361 <dt><code>EV_CHILD</code></dt>
363 <p>The pid specified in the <code>ev_child</code> watcher has received a status change.</p>
365 <dt><code>EV_IDLE</code></dt>
367 <p>The <code>ev_idle</code> watcher has determined that you have nothing better to do.</p>
369 <dt><code>EV_PREPARE</code></dt>
370 <dt><code>EV_CHECK</code></dt>
372 <p>All <code>ev_prepare</code> watchers are invoked just <i>before</i> <code>ev_loop</code> starts
373 to gather new events, and all <code>ev_check</code> watchers are invoked just after
374 <code>ev_loop</code> has gathered them, but before it invokes any callbacks for any
375 received events. Callbacks of both watcher types can start and stop as
376 many watchers as they want, and all of them will be taken into account
377 (for example, a <code>ev_prepare</code> watcher might start an idle watcher to keep
378 <code>ev_loop</code> from blocking).</p>
380 <dt><code>EV_ERROR</code></dt>
382 <p>An unspecified error has occured, the watcher has been stopped. This might
383 happen because the watcher could not be properly started because libev
384 ran out of memory, a file descriptor was found to be closed or any other
385 problem. You best act on it by reporting the problem and somehow coping
386 with the watcher being stopped.</p>
387 <p>Libev will usually signal a few "dummy" events together with an error,
388 for example it might indicate that a fd is readable or writable, and if
389 your callbacks is well-written it can just attempt the operation and cope
390 with the error from read() or write(). This will not work in multithreaded
391 programs, though, so beware.</p>
396 <h2 id="ASSOCIATING_CUSTOM_DATA_WITH_A_WATCH">ASSOCIATING CUSTOM DATA WITH A WATCHER</h2>
397 <div id="ASSOCIATING_CUSTOM_DATA_WITH_A_WATCH-2">
398 <p>Each watcher has, by default, a member <code>void *data</code> that you can change
399 and read at any time, libev will completely ignore it. This can be used
400 to associate arbitrary data with your watcher. If you need more data and
401 don't want to allocate memory and store a pointer to it in that data
402 member, you can also "subclass" the watcher type and provide your own
409 struct whatever *mostinteresting;
413 <p>And since your callback will be called with a pointer to the watcher, you
414 can cast it back to your own type:</p>
415 <pre> static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
417 struct my_io *w = (struct my_io *)w_;
422 <p>More interesting and less C-conformant ways of catsing your callback type
423 have been omitted....</p>
430 <h1 id="WATCHER_TYPES">WATCHER TYPES</h1><p><a href="#TOP" class="toplink">Top</a></p>
431 <div id="WATCHER_TYPES_CONTENT">
432 <p>This section describes each watcher in detail, but will not repeat
433 information given in the last section.</p>
436 <h2 id="code_ev_io_code_is_this_file_descrip"><code>ev_io</code> - is this file descriptor readable or writable</h2>
437 <div id="code_ev_io_code_is_this_file_descrip-2">
438 <p>I/O watchers check whether a file descriptor is readable or writable
439 in each iteration of the event loop (This behaviour is called
440 level-triggering because you keep receiving events as long as the
441 condition persists. Remember you can stop the watcher if you don't want to
442 act on the event and neither want to receive future events).</p>
443 <p>In general you can register as many read and/or write event watchers oer
444 fd as you want (as long as you don't confuse yourself). Setting all file
445 descriptors to non-blocking mode is also usually a good idea (but not
446 required if you know what you are doing).</p>
447 <p>You have to be careful with dup'ed file descriptors, though. Some backends
448 (the linux epoll backend is a notable example) cannot handle dup'ed file
449 descriptors correctly if you register interest in two or more fds pointing
450 to the same file/socket etc. description.</p>
451 <p>If you must do this, then force the use of a known-to-be-good backend
452 (at the time of this writing, this includes only EVMETHOD_SELECT and
455 <dt>ev_io_init (ev_io *, callback, int fd, int events)</dt>
456 <dt>ev_io_set (ev_io *, int fd, int events)</dt>
458 <p>Configures an <code>ev_io</code> watcher. The fd is the file descriptor to rceeive
459 events for and events is either <code>EV_READ</code>, <code>EV_WRITE</code> or <code>EV_READ |
460 EV_WRITE</code> to receive the given events.</p>
465 <h2 id="code_ev_timer_code_relative_and_opti"><code>ev_timer</code> - relative and optionally recurring timeouts</h2>
466 <div id="code_ev_timer_code_relative_and_opti-2">
467 <p>Timer watchers are simple relative timers that generate an event after a
468 given time, and optionally repeating in regular intervals after that.</p>
469 <p>The timers are based on real time, that is, if you register an event that
470 times out after an hour and youreset your system clock to last years
471 time, it will still time out after (roughly) and hour. "Roughly" because
472 detecting time jumps is hard, and soem inaccuracies are unavoidable (the
473 monotonic clock option helps a lot here).</p>
474 <p>The relative timeouts are calculated relative to the <code>ev_now ()</code>
475 time. This is usually the right thing as this timestamp refers to the time
476 of the event triggering whatever timeout you are modifying/starting. If
477 you suspect event processing to be delayed and you *need* to base the timeout
478 ion the current time, use something like this to adjust for this:</p>
479 <pre> ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
483 <dt>ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)</dt>
484 <dt>ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)</dt>
486 <p>Configure the timer to trigger after <code>after</code> seconds. If <code>repeat</code> is
487 <code>0.</code>, then it will automatically be stopped. If it is positive, then the
488 timer will automatically be configured to trigger again <code>repeat</code> seconds
489 later, again, and again, until stopped manually.</p>
490 <p>The timer itself will do a best-effort at avoiding drift, that is, if you
491 configure a timer to trigger every 10 seconds, then it will trigger at
492 exactly 10 second intervals. If, however, your program cannot keep up with
493 the timer (ecause it takes longer than those 10 seconds to do stuff) the
494 timer will not fire more than once per event loop iteration.</p>
496 <dt>ev_timer_again (loop)</dt>
498 <p>This will act as if the timer timed out and restart it again if it is
499 repeating. The exact semantics are:</p>
500 <p>If the timer is started but nonrepeating, stop it.</p>
501 <p>If the timer is repeating, either start it if necessary (with the repeat
502 value), or reset the running timer to the repeat value.</p>
503 <p>This sounds a bit complicated, but here is a useful and typical
504 example: Imagine you have a tcp connection and you want a so-called idle
505 timeout, that is, you want to be called when there have been, say, 60
506 seconds of inactivity on the socket. The easiest way to do this is to
507 configure an <code>ev_timer</code> with after=repeat=60 and calling ev_timer_again each
508 time you successfully read or write some data. If you go into an idle
509 state where you do not expect data to travel on the socket, you can stop
510 the timer, and again will automatically restart it if need be.</p>
515 <h2 id="code_ev_periodic_code_to_cron_or_not"><code>ev_periodic</code> - to cron or not to cron</h2>
516 <div id="code_ev_periodic_code_to_cron_or_not-2">
517 <p>Periodic watchers are also timers of a kind, but they are very versatile
518 (and unfortunately a bit complex).</p>
519 <p>Unlike <code>ev_timer</code>'s, they are not based on real time (or relative time)
520 but on wallclock time (absolute time). You can tell a periodic watcher
521 to trigger "at" some specific point in time. For example, if you tell a
522 periodic watcher to trigger in 10 seconds (by specifiying e.g. c<ev_now ()
523 + 10.>) and then reset your system clock to the last year, then it will
524 take a year to trigger the event (unlike an <code>ev_timer</code>, which would trigger
525 roughly 10 seconds later and of course not if you reset your system time
527 <p>They can also be used to implement vastly more complex timers, such as
528 triggering an event on eahc midnight, local time.</p>
530 <dt>ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)</dt>
531 <dt>ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)</dt>
533 <p>Lots of arguments, lets sort it out... There are basically three modes of
534 operation, and we will explain them from simplest to complex:</p>
541 <dt>* absolute timer (interval = reschedule_cb = 0)</dt>
543 <p>In this configuration the watcher triggers an event at the wallclock time
544 <code>at</code> and doesn't repeat. It will not adjust when a time jump occurs,
545 that is, if it is to be run at January 1st 2011 then it will run when the
546 system time reaches or surpasses this time.</p>
548 <dt>* non-repeating interval timer (interval > 0, reschedule_cb = 0)</dt>
550 <p>In this mode the watcher will always be scheduled to time out at the next
551 <code>at + N * interval</code> time (for some integer N) and then repeat, regardless
552 of any time jumps.</p>
553 <p>This can be used to create timers that do not drift with respect to system
555 <pre> ev_periodic_set (&periodic, 0., 3600., 0);
558 <p>This doesn't mean there will always be 3600 seconds in between triggers,
559 but only that the the callback will be called when the system time shows a
560 full hour (UTC), or more correctly, when the system time is evenly divisible
562 <p>Another way to think about it (for the mathematically inclined) is that
563 <code>ev_periodic</code> will try to run the callback in this mode at the next possible
564 time where <code>time = at (mod interval)</code>, regardless of any time jumps.</p>
566 <dt>* manual reschedule mode (reschedule_cb = callback)</dt>
568 <p>In this mode the values for <code>interval</code> and <code>at</code> are both being
569 ignored. Instead, each time the periodic watcher gets scheduled, the
570 reschedule callback will be called with the watcher as first, and the
571 current time as second argument.</p>
572 <p>NOTE: <i>This callback MUST NOT stop or destroy the periodic or any other
573 periodic watcher, ever, or make any event loop modifications</i>. If you need
574 to stop it, return <code>now + 1e30</code> (or so, fudge fudge) and stop it afterwards.</p>
575 <p>Also, <i>this callback must always return a time that is later than the
576 passed <code>now</code> value</i>. Not even <code>now</code> itself will be ok.</p>
577 <p>Its prototype is <code>ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
578 ev_tstamp now)</code>, e.g.:</p>
579 <pre> static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
585 <p>It must return the next time to trigger, based on the passed time value
586 (that is, the lowest time value larger than to the second argument). It
587 will usually be called just before the callback will be triggered, but
588 might be called at other times, too.</p>
589 <p>This can be used to create very complex timers, such as a timer that
590 triggers on each midnight, local time. To do this, you would calculate the
591 next midnight after <code>now</code> and return the timestamp value for this. How you do this
592 is, again, up to you (but it is not trivial).</p>
597 <dt>ev_periodic_again (loop, ev_periodic *)</dt>
599 <p>Simply stops and restarts the periodic watcher again. This is only useful
600 when you changed some parameters or the reschedule callback would return
601 a different time than the last time it was called (e.g. in a crond like
602 program when the crontabs have changed).</p>
607 <h2 id="code_ev_signal_code_signal_me_when_a"><code>ev_signal</code> - signal me when a signal gets signalled</h2>
608 <div id="code_ev_signal_code_signal_me_when_a-2">
609 <p>Signal watchers will trigger an event when the process receives a specific
610 signal one or more times. Even though signals are very asynchronous, libev
611 will try it's best to deliver signals synchronously, i.e. as part of the
612 normal event processing, like any other event.</p>
613 <p>You can configure as many watchers as you like per signal. Only when the
614 first watcher gets started will libev actually register a signal watcher
615 with the kernel (thus it coexists with your own signal handlers as long
616 as you don't register any with libev). Similarly, when the last signal
617 watcher for a signal is stopped libev will reset the signal handler to
618 SIG_DFL (regardless of what it was set to before).</p>
620 <dt>ev_signal_init (ev_signal *, callback, int signum)</dt>
621 <dt>ev_signal_set (ev_signal *, int signum)</dt>
623 <p>Configures the watcher to trigger on the given signal number (usually one
624 of the <code>SIGxxx</code> constants).</p>
629 <h2 id="code_ev_child_code_wait_for_pid_stat"><code>ev_child</code> - wait for pid status changes</h2>
630 <div id="code_ev_child_code_wait_for_pid_stat-2">
631 <p>Child watchers trigger when your process receives a SIGCHLD in response to
632 some child status changes (most typically when a child of yours dies).</p>
634 <dt>ev_child_init (ev_child *, callback, int pid)</dt>
635 <dt>ev_child_set (ev_child *, int pid)</dt>
637 <p>Configures the watcher to wait for status changes of process <code>pid</code> (or
638 <i>any</i> process if <code>pid</code> is specified as <code>0</code>). The callback can look
639 at the <code>rstatus</code> member of the <code>ev_child</code> watcher structure to see
640 the status word (use the macros from <code>sys/wait.h</code> and see your systems
641 <code>waitpid</code> documentation). The <code>rpid</code> member contains the pid of the
642 process causing the status change.</p>
647 <h2 id="code_ev_idle_code_when_you_ve_got_no"><code>ev_idle</code> - when you've got nothing better to do</h2>
648 <div id="code_ev_idle_code_when_you_ve_got_no-2">
649 <p>Idle watchers trigger events when there are no other events are pending
650 (prepare, check and other idle watchers do not count). That is, as long
651 as your process is busy handling sockets or timeouts (or even signals,
652 imagine) it will not be triggered. But when your process is idle all idle
653 watchers are being called again and again, once per event loop iteration -
654 until stopped, that is, or your process receives more events and becomes
656 <p>The most noteworthy effect is that as long as any idle watchers are
657 active, the process will not block when waiting for new events.</p>
658 <p>Apart from keeping your process non-blocking (which is a useful
659 effect on its own sometimes), idle watchers are a good place to do
660 "pseudo-background processing", or delay processing stuff to after the
661 event loop has handled all outstanding events.</p>
663 <dt>ev_idle_init (ev_signal *, callback)</dt>
665 <p>Initialises and configures the idle watcher - it has no parameters of any
666 kind. There is a <code>ev_idle_set</code> macro, but using it is utterly pointless,
672 <h2 id="code_ev_prepare_code_and_code_ev_che"><code>ev_prepare</code> and <code>ev_check</code> - customise your event loop</h2>
673 <div id="code_ev_prepare_code_and_code_ev_che-2">
674 <p>Prepare and check watchers are usually (but not always) used in tandem:
675 Prepare watchers get invoked before the process blocks and check watchers
677 <p>Their main purpose is to integrate other event mechanisms into libev. This
678 could be used, for example, to track variable changes, implement your own
679 watchers, integrate net-snmp or a coroutine library and lots more.</p>
680 <p>This is done by examining in each prepare call which file descriptors need
681 to be watched by the other library, registering <code>ev_io</code> watchers for
682 them and starting an <code>ev_timer</code> watcher for any timeouts (many libraries
683 provide just this functionality). Then, in the check watcher you check for
684 any events that occured (by checking the pending status of all watchers
685 and stopping them) and call back into the library. The I/O and timer
686 callbacks will never actually be called (but must be valid neverthelles,
687 because you never know, you know?).</p>
688 <p>As another example, the Perl Coro module uses these hooks to integrate
689 coroutines into libev programs, by yielding to other active coroutines
690 during each prepare and only letting the process block if no coroutines
691 are ready to run (its actually more complicated, it only runs coroutines
692 with priority higher than the event loop and one lower priority once,
693 using idle watchers to keep the event loop from blocking if lower-priority
694 coroutines exist, thus mapping low-priority coroutines to idle/background
697 <dt>ev_prepare_init (ev_prepare *, callback)</dt>
698 <dt>ev_check_init (ev_check *, callback)</dt>
700 <p>Initialises and configures the prepare or check watcher - they have no
701 parameters of any kind. There are <code>ev_prepare_set</code> and <code>ev_check_set</code>
702 macros, but using them is utterly, utterly and completely pointless.</p>
707 <h1 id="OTHER_FUNCTIONS">OTHER FUNCTIONS</h1><p><a href="#TOP" class="toplink">Top</a></p>
708 <div id="OTHER_FUNCTIONS_CONTENT">
709 <p>There are some other functions of possible interest. Described. Here. Now.</p>
711 <dt>ev_once (loop, int fd, int events, ev_tstamp timeout, callback)</dt>
713 <p>This function combines a simple timer and an I/O watcher, calls your
714 callback on whichever event happens first and automatically stop both
715 watchers. This is useful if you want to wait for a single event on an fd
716 or timeout without havign to allocate/configure/start/stop/free one or
717 more watchers yourself.</p>
718 <p>If <code>fd</code> is less than 0, then no I/O watcher will be started and events
719 is being ignored. Otherwise, an <code>ev_io</code> watcher for the given <code>fd</code> and
720 <code>events</code> set will be craeted and started.</p>
721 <p>If <code>timeout</code> is less than 0, then no timeout watcher will be
722 started. Otherwise an <code>ev_timer</code> watcher with after = <code>timeout</code> (and
723 repeat = 0) will be started. While <code>0</code> is a valid timeout, it is of
725 <p>The callback has the type <code>void (*cb)(int revents, void *arg)</code> and gets
726 passed an events set like normal event callbacks (with a combination of
727 <code>EV_ERROR</code>, <code>EV_READ</code>, <code>EV_WRITE</code> or <code>EV_TIMEOUT</code>) and the <code>arg</code>
728 value passed to <code>ev_once</code>:</p>
729 <pre> static void stdin_ready (int revents, void *arg)
731 if (revents & EV_TIMEOUT)
732 /* doh, nothing entered */;
733 else if (revents & EV_READ)
734 /* stdin might have data for us, joy! */;
737 ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
741 <dt>ev_feed_event (loop, watcher, int events)</dt>
743 <p>Feeds the given event set into the event loop, as if the specified event
744 had happened for the specified watcher (which must be a pointer to an
745 initialised but not necessarily started event watcher).</p>
747 <dt>ev_feed_fd_event (loop, int fd, int revents)</dt>
749 <p>Feed an event on the given fd, as if a file descriptor backend detected
750 the given events it.</p>
752 <dt>ev_feed_signal_event (loop, int signum)</dt>
754 <p>Feed an event as if the given signal occured (loop must be the default loop!).</p>
759 <h1 id="AUTHOR">AUTHOR</h1><p><a href="#TOP" class="toplink">Top</a></p>
760 <div id="AUTHOR_CONTENT">
761 <p>Marc Lehmann <libev@schmorp.de>.</p>
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