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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="#LIBEVENT_EMULATION">LIBEVENT EMULATION</a></li>
41 <li><a href="#C_SUPPORT">C++ SUPPORT</a></li>
42 <li><a href="#AUTHOR">AUTHOR</a>
47 <h1 id="NAME">NAME</h1><p><a href="#TOP" class="toplink">Top</a></p>
48 <div id="NAME_CONTENT">
49 <p>libev - a high performance full-featured event loop written in C</p>
52 <h1 id="SYNOPSIS">SYNOPSIS</h1><p><a href="#TOP" class="toplink">Top</a></p>
53 <div id="SYNOPSIS_CONTENT">
54 <pre> #include <ev.h>
59 <h1 id="DESCRIPTION">DESCRIPTION</h1><p><a href="#TOP" class="toplink">Top</a></p>
60 <div id="DESCRIPTION_CONTENT">
61 <p>Libev is an event loop: you register interest in certain events (such as a
62 file descriptor being readable or a timeout occuring), and it will manage
63 these event sources and provide your program with events.</p>
64 <p>To do this, it must take more or less complete control over your process
65 (or thread) by executing the <i>event loop</i> handler, and will then
66 communicate events via a callback mechanism.</p>
67 <p>You register interest in certain events by registering so-called <i>event
68 watchers</i>, which are relatively small C structures you initialise with the
69 details of the event, and then hand it over to libev by <i>starting</i> the
73 <h1 id="FEATURES">FEATURES</h1><p><a href="#TOP" class="toplink">Top</a></p>
74 <div id="FEATURES_CONTENT">
75 <p>Libev supports select, poll, the linux-specific epoll and the bsd-specific
76 kqueue mechanisms for file descriptor events, relative timers, absolute
77 timers with customised rescheduling, signal events, process status change
78 events (related to SIGCHLD), and event watchers dealing with the event
79 loop mechanism itself (idle, prepare and check watchers). It also is quite
80 fast (see this <a href="http://libev.schmorp.de/bench.html">benchmark</a> comparing
81 it to libevent for example).</p>
84 <h1 id="CONVENTIONS">CONVENTIONS</h1><p><a href="#TOP" class="toplink">Top</a></p>
85 <div id="CONVENTIONS_CONTENT">
86 <p>Libev is very configurable. In this manual the default configuration
87 will be described, which supports multiple event loops. For more info
88 about various configuration options please have a look at the file
89 <cite>README.embed</cite> in the libev distribution. If libev was configured without
90 support for multiple event loops, then all functions taking an initial
91 argument of name <code>loop</code> (which is always of type <code>struct ev_loop *</code>)
92 will not have this argument.</p>
95 <h1 id="TIME_REPRESENTATION">TIME REPRESENTATION</h1><p><a href="#TOP" class="toplink">Top</a></p>
96 <div id="TIME_REPRESENTATION_CONTENT">
97 <p>Libev represents time as a single floating point number, representing the
98 (fractional) number of seconds since the (POSIX) epoch (somewhere near
99 the beginning of 1970, details are complicated, don't ask). This type is
100 called <code>ev_tstamp</code>, which is what you should use too. It usually aliases
101 to the double type in C.</p>
104 <h1 id="GLOBAL_FUNCTIONS">GLOBAL FUNCTIONS</h1><p><a href="#TOP" class="toplink">Top</a></p>
105 <div id="GLOBAL_FUNCTIONS_CONTENT">
106 <p>These functions can be called anytime, even before initialising the
107 library in any way.</p>
109 <dt>ev_tstamp ev_time ()</dt>
111 <p>Returns the current time as libev would use it.</p>
113 <dt>int ev_version_major ()</dt>
114 <dt>int ev_version_minor ()</dt>
116 <p>You can find out the major and minor version numbers of the library
117 you linked against by calling the functions <code>ev_version_major</code> and
118 <code>ev_version_minor</code>. If you want, you can compare against the global
119 symbols <code>EV_VERSION_MAJOR</code> and <code>EV_VERSION_MINOR</code>, which specify the
120 version of the library your program was compiled against.</p>
121 <p>Usually, it's a good idea to terminate if the major versions mismatch,
122 as this indicates an incompatible change. Minor versions are usually
123 compatible to older versions, so a larger minor version alone is usually
126 <dt>ev_set_allocator (void *(*cb)(void *ptr, long size))</dt>
128 <p>Sets the allocation function to use (the prototype is similar to the
129 realloc C function, the semantics are identical). It is used to allocate
130 and free memory (no surprises here). If it returns zero when memory
131 needs to be allocated, the library might abort or take some potentially
132 destructive action. The default is your system realloc function.</p>
133 <p>You could override this function in high-availability programs to, say,
134 free some memory if it cannot allocate memory, to use a special allocator,
135 or even to sleep a while and retry until some memory is available.</p>
137 <dt>ev_set_syserr_cb (void (*cb)(const char *msg));</dt>
139 <p>Set the callback function to call on a retryable syscall error (such
140 as failed select, poll, epoll_wait). The message is a printable string
141 indicating the system call or subsystem causing the problem. If this
142 callback is set, then libev will expect it to remedy the sitution, no
143 matter what, when it returns. That is, libev will generally retry the
144 requested operation, or, if the condition doesn't go away, do bad stuff
150 <h1 id="FUNCTIONS_CONTROLLING_THE_EVENT_LOOP">FUNCTIONS CONTROLLING THE EVENT LOOP</h1><p><a href="#TOP" class="toplink">Top</a></p>
151 <div id="FUNCTIONS_CONTROLLING_THE_EVENT_LOOP-2">
152 <p>An event loop is described by a <code>struct ev_loop *</code>. The library knows two
153 types of such loops, the <i>default</i> loop, which supports signals and child
154 events, and dynamically created loops which do not.</p>
155 <p>If you use threads, a common model is to run the default event loop
156 in your main thread (or in a separate thread) and for each thread you
157 create, you also create another event loop. Libev itself does no locking
158 whatsoever, so if you mix calls to the same event loop in different
159 threads, make sure you lock (this is usually a bad idea, though, even if
160 done correctly, because it's hideous and inefficient).</p>
162 <dt>struct ev_loop *ev_default_loop (unsigned int flags)</dt>
164 <p>This will initialise the default event loop if it hasn't been initialised
165 yet and return it. If the default loop could not be initialised, returns
166 false. If it already was initialised it simply returns it (and ignores the
168 <p>If you don't know what event loop to use, use the one returned from this
170 <p>The flags argument can be used to specify special behaviour or specific
171 backends to use, and is usually specified as 0 (or EVFLAG_AUTO).</p>
172 <p>It supports the following flags:</p>
175 <dt><code>EVFLAG_AUTO</code></dt>
177 <p>The default flags value. Use this if you have no clue (it's the right
178 thing, believe me).</p>
180 <dt><code>EVFLAG_NOENV</code></dt>
182 <p>If this flag bit is ored into the flag value (or the program runs setuid
183 or setgid) then libev will <i>not</i> look at the environment variable
184 <code>LIBEV_FLAGS</code>. Otherwise (the default), this environment variable will
185 override the flags completely if it is found in the environment. This is
186 useful to try out specific backends to test their performance, or to work
189 <dt><code>EVMETHOD_SELECT</code> (portable select backend)</dt>
190 <dt><code>EVMETHOD_POLL</code> (poll backend, available everywhere except on windows)</dt>
191 <dt><code>EVMETHOD_EPOLL</code> (linux only)</dt>
192 <dt><code>EVMETHOD_KQUEUE</code> (some bsds only)</dt>
193 <dt><code>EVMETHOD_DEVPOLL</code> (solaris 8 only)</dt>
194 <dt><code>EVMETHOD_PORT</code> (solaris 10 only)</dt>
196 <p>If one or more of these are ored into the flags value, then only these
197 backends will be tried (in the reverse order as given here). If one are
198 specified, any backend will do.</p>
203 <dt>struct ev_loop *ev_loop_new (unsigned int flags)</dt>
205 <p>Similar to <code>ev_default_loop</code>, but always creates a new event loop that is
206 always distinct from the default loop. Unlike the default loop, it cannot
207 handle signal and child watchers, and attempts to do so will be greeted by
208 undefined behaviour (or a failed assertion if assertions are enabled).</p>
210 <dt>ev_default_destroy ()</dt>
212 <p>Destroys the default loop again (frees all memory and kernel state
213 etc.). This stops all registered event watchers (by not touching them in
214 any way whatsoever, although you cannot rely on this :).</p>
216 <dt>ev_loop_destroy (loop)</dt>
218 <p>Like <code>ev_default_destroy</code>, but destroys an event loop created by an
219 earlier call to <code>ev_loop_new</code>.</p>
221 <dt>ev_default_fork ()</dt>
223 <p>This function reinitialises the kernel state for backends that have
224 one. Despite the name, you can call it anytime, but it makes most sense
225 after forking, in either the parent or child process (or both, but that
226 again makes little sense).</p>
227 <p>You <i>must</i> call this function after forking if and only if you want to
228 use the event library in both processes. If you just fork+exec, you don't
230 <p>The function itself is quite fast and it's usually not a problem to call
231 it just in case after a fork. To make this easy, the function will fit in
232 quite nicely into a call to <code>pthread_atfork</code>:</p>
233 <pre> pthread_atfork (0, 0, ev_default_fork);
237 <dt>ev_loop_fork (loop)</dt>
239 <p>Like <code>ev_default_fork</code>, but acts on an event loop created by
240 <code>ev_loop_new</code>. Yes, you have to call this on every allocated event loop
241 after fork, and how you do this is entirely your own problem.</p>
243 <dt>unsigned int ev_method (loop)</dt>
245 <p>Returns one of the <code>EVMETHOD_*</code> flags indicating the event backend in
248 <dt>ev_tstamp ev_now (loop)</dt>
250 <p>Returns the current "event loop time", which is the time the event loop
251 got events and started processing them. This timestamp does not change
252 as long as callbacks are being processed, and this is also the base time
253 used for relative timers. You can treat it as the timestamp of the event
254 occuring (or more correctly, the mainloop finding out about it).</p>
256 <dt>ev_loop (loop, int flags)</dt>
258 <p>Finally, this is it, the event handler. This function usually is called
259 after you initialised all your watchers and you want to start handling
261 <p>If the flags argument is specified as 0, it will not return until either
262 no event watchers are active anymore or <code>ev_unloop</code> was called.</p>
263 <p>A flags value of <code>EVLOOP_NONBLOCK</code> will look for new events, will handle
264 those events and any outstanding ones, but will not block your process in
265 case there are no events and will return after one iteration of the loop.</p>
266 <p>A flags value of <code>EVLOOP_ONESHOT</code> will look for new events (waiting if
267 neccessary) and will handle those and any outstanding ones. It will block
268 your process until at least one new event arrives, and will return after
269 one iteration of the loop.</p>
270 <p>This flags value could be used to implement alternative looping
271 constructs, but the <code>prepare</code> and <code>check</code> watchers provide a better and
272 more generic mechanism.</p>
274 <dt>ev_unloop (loop, how)</dt>
276 <p>Can be used to make a call to <code>ev_loop</code> return early (but only after it
277 has processed all outstanding events). The <code>how</code> argument must be either
278 <code>EVUNLOOP_ONE</code>, which will make the innermost <code>ev_loop</code> call return, or
279 <code>EVUNLOOP_ALL</code>, which will make all nested <code>ev_loop</code> calls return.</p>
281 <dt>ev_ref (loop)</dt>
282 <dt>ev_unref (loop)</dt>
284 <p>Ref/unref can be used to add or remove a reference count on the event
285 loop: Every watcher keeps one reference, and as long as the reference
286 count is nonzero, <code>ev_loop</code> will not return on its own. If you have
287 a watcher you never unregister that should not keep <code>ev_loop</code> from
288 returning, ev_unref() after starting, and ev_ref() before stopping it. For
289 example, libev itself uses this for its internal signal pipe: It is not
290 visible to the libev user and should not keep <code>ev_loop</code> from exiting if
291 no event watchers registered by it are active. It is also an excellent
292 way to do this for generic recurring timers or from within third-party
293 libraries. Just remember to <i>unref after start</i> and <i>ref before stop</i>.</p>
298 <h1 id="ANATOMY_OF_A_WATCHER">ANATOMY OF A WATCHER</h1><p><a href="#TOP" class="toplink">Top</a></p>
299 <div id="ANATOMY_OF_A_WATCHER_CONTENT">
300 <p>A watcher is a structure that you create and register to record your
301 interest in some event. For instance, if you want to wait for STDIN to
302 become readable, you would create an <code>ev_io</code> watcher for that:</p>
303 <pre> static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
306 ev_unloop (loop, EVUNLOOP_ALL);
309 struct ev_loop *loop = ev_default_loop (0);
310 struct ev_io stdin_watcher;
311 ev_init (&stdin_watcher, my_cb);
312 ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
313 ev_io_start (loop, &stdin_watcher);
317 <p>As you can see, you are responsible for allocating the memory for your
318 watcher structures (and it is usually a bad idea to do this on the stack,
319 although this can sometimes be quite valid).</p>
320 <p>Each watcher structure must be initialised by a call to <code>ev_init
321 (watcher *, callback)</code>, which expects a callback to be provided. This
322 callback gets invoked each time the event occurs (or, in the case of io
323 watchers, each time the event loop detects that the file descriptor given
324 is readable and/or writable).</p>
325 <p>Each watcher type has its own <code>ev_<type>_set (watcher *, ...)</code> macro
326 with arguments specific to this watcher type. There is also a macro
327 to combine initialisation and setting in one call: <code>ev_<type>_init
328 (watcher *, callback, ...)</code>.</p>
329 <p>To make the watcher actually watch out for events, you have to start it
330 with a watcher-specific start function (<code>ev_<type>_start (loop, watcher
331 *)</code>), and you can stop watching for events at any time by calling the
332 corresponding stop function (<code>ev_<type>_stop (loop, watcher *)</code>.</p>
333 <p>As long as your watcher is active (has been started but not stopped) you
334 must not touch the values stored in it. Most specifically you must never
335 reinitialise it or call its set method.</p>
336 <p>You can check whether an event is active by calling the <code>ev_is_active
337 (watcher *)</code> macro. To see whether an event is outstanding (but the
338 callback for it has not been called yet) you can use the <code>ev_is_pending
339 (watcher *)</code> macro.</p>
340 <p>Each and every callback receives the event loop pointer as first, the
341 registered watcher structure as second, and a bitset of received events as
343 <p>The received events usually include a single bit per event type received
344 (you can receive multiple events at the same time). The possible bit masks
347 <dt><code>EV_READ</code></dt>
348 <dt><code>EV_WRITE</code></dt>
350 <p>The file descriptor in the <code>ev_io</code> watcher has become readable and/or
353 <dt><code>EV_TIMEOUT</code></dt>
355 <p>The <code>ev_timer</code> watcher has timed out.</p>
357 <dt><code>EV_PERIODIC</code></dt>
359 <p>The <code>ev_periodic</code> watcher has timed out.</p>
361 <dt><code>EV_SIGNAL</code></dt>
363 <p>The signal specified in the <code>ev_signal</code> watcher has been received by a thread.</p>
365 <dt><code>EV_CHILD</code></dt>
367 <p>The pid specified in the <code>ev_child</code> watcher has received a status change.</p>
369 <dt><code>EV_IDLE</code></dt>
371 <p>The <code>ev_idle</code> watcher has determined that you have nothing better to do.</p>
373 <dt><code>EV_PREPARE</code></dt>
374 <dt><code>EV_CHECK</code></dt>
376 <p>All <code>ev_prepare</code> watchers are invoked just <i>before</i> <code>ev_loop</code> starts
377 to gather new events, and all <code>ev_check</code> watchers are invoked just after
378 <code>ev_loop</code> has gathered them, but before it invokes any callbacks for any
379 received events. Callbacks of both watcher types can start and stop as
380 many watchers as they want, and all of them will be taken into account
381 (for example, a <code>ev_prepare</code> watcher might start an idle watcher to keep
382 <code>ev_loop</code> from blocking).</p>
384 <dt><code>EV_ERROR</code></dt>
386 <p>An unspecified error has occured, the watcher has been stopped. This might
387 happen because the watcher could not be properly started because libev
388 ran out of memory, a file descriptor was found to be closed or any other
389 problem. You best act on it by reporting the problem and somehow coping
390 with the watcher being stopped.</p>
391 <p>Libev will usually signal a few "dummy" events together with an error,
392 for example it might indicate that a fd is readable or writable, and if
393 your callbacks is well-written it can just attempt the operation and cope
394 with the error from read() or write(). This will not work in multithreaded
395 programs, though, so beware.</p>
400 <h2 id="ASSOCIATING_CUSTOM_DATA_WITH_A_WATCH">ASSOCIATING CUSTOM DATA WITH A WATCHER</h2>
401 <div id="ASSOCIATING_CUSTOM_DATA_WITH_A_WATCH-2">
402 <p>Each watcher has, by default, a member <code>void *data</code> that you can change
403 and read at any time, libev will completely ignore it. This can be used
404 to associate arbitrary data with your watcher. If you need more data and
405 don't want to allocate memory and store a pointer to it in that data
406 member, you can also "subclass" the watcher type and provide your own
413 struct whatever *mostinteresting;
417 <p>And since your callback will be called with a pointer to the watcher, you
418 can cast it back to your own type:</p>
419 <pre> static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
421 struct my_io *w = (struct my_io *)w_;
426 <p>More interesting and less C-conformant ways of catsing your callback type
427 have been omitted....</p>
434 <h1 id="WATCHER_TYPES">WATCHER TYPES</h1><p><a href="#TOP" class="toplink">Top</a></p>
435 <div id="WATCHER_TYPES_CONTENT">
436 <p>This section describes each watcher in detail, but will not repeat
437 information given in the last section.</p>
440 <h2 id="code_ev_io_code_is_this_file_descrip"><code>ev_io</code> - is this file descriptor readable or writable</h2>
441 <div id="code_ev_io_code_is_this_file_descrip-2">
442 <p>I/O watchers check whether a file descriptor is readable or writable
443 in each iteration of the event loop (This behaviour is called
444 level-triggering because you keep receiving events as long as the
445 condition persists. Remember you can stop the watcher if you don't want to
446 act on the event and neither want to receive future events).</p>
447 <p>In general you can register as many read and/or write event watchers per
448 fd as you want (as long as you don't confuse yourself). Setting all file
449 descriptors to non-blocking mode is also usually a good idea (but not
450 required if you know what you are doing).</p>
451 <p>You have to be careful with dup'ed file descriptors, though. Some backends
452 (the linux epoll backend is a notable example) cannot handle dup'ed file
453 descriptors correctly if you register interest in two or more fds pointing
454 to the same underlying file/socket etc. description (that is, they share
455 the same underlying "file open").</p>
456 <p>If you must do this, then force the use of a known-to-be-good backend
457 (at the time of this writing, this includes only EVMETHOD_SELECT and
460 <dt>ev_io_init (ev_io *, callback, int fd, int events)</dt>
461 <dt>ev_io_set (ev_io *, int fd, int events)</dt>
463 <p>Configures an <code>ev_io</code> watcher. The fd is the file descriptor to rceeive
464 events for and events is either <code>EV_READ</code>, <code>EV_WRITE</code> or <code>EV_READ |
465 EV_WRITE</code> to receive the given events.</p>
470 <h2 id="code_ev_timer_code_relative_and_opti"><code>ev_timer</code> - relative and optionally recurring timeouts</h2>
471 <div id="code_ev_timer_code_relative_and_opti-2">
472 <p>Timer watchers are simple relative timers that generate an event after a
473 given time, and optionally repeating in regular intervals after that.</p>
474 <p>The timers are based on real time, that is, if you register an event that
475 times out after an hour and you reset your system clock to last years
476 time, it will still time out after (roughly) and hour. "Roughly" because
477 detecting time jumps is hard, and soem inaccuracies are unavoidable (the
478 monotonic clock option helps a lot here).</p>
479 <p>The relative timeouts are calculated relative to the <code>ev_now ()</code>
480 time. This is usually the right thing as this timestamp refers to the time
481 of the event triggering whatever timeout you are modifying/starting. If
482 you suspect event processing to be delayed and you *need* to base the timeout
483 on the current time, use something like this to adjust for this:</p>
484 <pre> ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
488 <dt>ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)</dt>
489 <dt>ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)</dt>
491 <p>Configure the timer to trigger after <code>after</code> seconds. If <code>repeat</code> is
492 <code>0.</code>, then it will automatically be stopped. If it is positive, then the
493 timer will automatically be configured to trigger again <code>repeat</code> seconds
494 later, again, and again, until stopped manually.</p>
495 <p>The timer itself will do a best-effort at avoiding drift, that is, if you
496 configure a timer to trigger every 10 seconds, then it will trigger at
497 exactly 10 second intervals. If, however, your program cannot keep up with
498 the timer (because it takes longer than those 10 seconds to do stuff) the
499 timer will not fire more than once per event loop iteration.</p>
501 <dt>ev_timer_again (loop)</dt>
503 <p>This will act as if the timer timed out and restart it again if it is
504 repeating. The exact semantics are:</p>
505 <p>If the timer is started but nonrepeating, stop it.</p>
506 <p>If the timer is repeating, either start it if necessary (with the repeat
507 value), or reset the running timer to the repeat value.</p>
508 <p>This sounds a bit complicated, but here is a useful and typical
509 example: Imagine you have a tcp connection and you want a so-called idle
510 timeout, that is, you want to be called when there have been, say, 60
511 seconds of inactivity on the socket. The easiest way to do this is to
512 configure an <code>ev_timer</code> with after=repeat=60 and calling ev_timer_again each
513 time you successfully read or write some data. If you go into an idle
514 state where you do not expect data to travel on the socket, you can stop
515 the timer, and again will automatically restart it if need be.</p>
520 <h2 id="code_ev_periodic_code_to_cron_or_not"><code>ev_periodic</code> - to cron or not to cron</h2>
521 <div id="code_ev_periodic_code_to_cron_or_not-2">
522 <p>Periodic watchers are also timers of a kind, but they are very versatile
523 (and unfortunately a bit complex).</p>
524 <p>Unlike <code>ev_timer</code>'s, they are not based on real time (or relative time)
525 but on wallclock time (absolute time). You can tell a periodic watcher
526 to trigger "at" some specific point in time. For example, if you tell a
527 periodic watcher to trigger in 10 seconds (by specifiying e.g. c<ev_now ()
528 + 10.>) and then reset your system clock to the last year, then it will
529 take a year to trigger the event (unlike an <code>ev_timer</code>, which would trigger
530 roughly 10 seconds later and of course not if you reset your system time
532 <p>They can also be used to implement vastly more complex timers, such as
533 triggering an event on eahc midnight, local time.</p>
535 <dt>ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)</dt>
536 <dt>ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)</dt>
538 <p>Lots of arguments, lets sort it out... There are basically three modes of
539 operation, and we will explain them from simplest to complex:</p>
546 <dt>* absolute timer (interval = reschedule_cb = 0)</dt>
548 <p>In this configuration the watcher triggers an event at the wallclock time
549 <code>at</code> and doesn't repeat. It will not adjust when a time jump occurs,
550 that is, if it is to be run at January 1st 2011 then it will run when the
551 system time reaches or surpasses this time.</p>
553 <dt>* non-repeating interval timer (interval > 0, reschedule_cb = 0)</dt>
555 <p>In this mode the watcher will always be scheduled to time out at the next
556 <code>at + N * interval</code> time (for some integer N) and then repeat, regardless
557 of any time jumps.</p>
558 <p>This can be used to create timers that do not drift with respect to system
560 <pre> ev_periodic_set (&periodic, 0., 3600., 0);
563 <p>This doesn't mean there will always be 3600 seconds in between triggers,
564 but only that the the callback will be called when the system time shows a
565 full hour (UTC), or more correctly, when the system time is evenly divisible
567 <p>Another way to think about it (for the mathematically inclined) is that
568 <code>ev_periodic</code> will try to run the callback in this mode at the next possible
569 time where <code>time = at (mod interval)</code>, regardless of any time jumps.</p>
571 <dt>* manual reschedule mode (reschedule_cb = callback)</dt>
573 <p>In this mode the values for <code>interval</code> and <code>at</code> are both being
574 ignored. Instead, each time the periodic watcher gets scheduled, the
575 reschedule callback will be called with the watcher as first, and the
576 current time as second argument.</p>
577 <p>NOTE: <i>This callback MUST NOT stop or destroy any periodic watcher,
578 ever, or make any event loop modifications</i>. If you need to stop it,
579 return <code>now + 1e30</code> (or so, fudge fudge) and stop it afterwards (e.g. by
580 starting a prepare watcher).</p>
581 <p>Its prototype is <code>ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
582 ev_tstamp now)</code>, e.g.:</p>
583 <pre> static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
589 <p>It must return the next time to trigger, based on the passed time value
590 (that is, the lowest time value larger than to the second argument). It
591 will usually be called just before the callback will be triggered, but
592 might be called at other times, too.</p>
593 <p>NOTE: <i>This callback must always return a time that is later than the
594 passed <code>now</code> value</i>. Not even <code>now</code> itself will do, it <i>must</i> be larger.</p>
595 <p>This can be used to create very complex timers, such as a timer that
596 triggers on each midnight, local time. To do this, you would calculate the
597 next midnight after <code>now</code> and return the timestamp value for this. How
598 you do this is, again, up to you (but it is not trivial, which is the main
599 reason I omitted it as an example).</p>
604 <dt>ev_periodic_again (loop, ev_periodic *)</dt>
606 <p>Simply stops and restarts the periodic watcher again. This is only useful
607 when you changed some parameters or the reschedule callback would return
608 a different time than the last time it was called (e.g. in a crond like
609 program when the crontabs have changed).</p>
614 <h2 id="code_ev_signal_code_signal_me_when_a"><code>ev_signal</code> - signal me when a signal gets signalled</h2>
615 <div id="code_ev_signal_code_signal_me_when_a-2">
616 <p>Signal watchers will trigger an event when the process receives a specific
617 signal one or more times. Even though signals are very asynchronous, libev
618 will try it's best to deliver signals synchronously, i.e. as part of the
619 normal event processing, like any other event.</p>
620 <p>You can configure as many watchers as you like per signal. Only when the
621 first watcher gets started will libev actually register a signal watcher
622 with the kernel (thus it coexists with your own signal handlers as long
623 as you don't register any with libev). Similarly, when the last signal
624 watcher for a signal is stopped libev will reset the signal handler to
625 SIG_DFL (regardless of what it was set to before).</p>
627 <dt>ev_signal_init (ev_signal *, callback, int signum)</dt>
628 <dt>ev_signal_set (ev_signal *, int signum)</dt>
630 <p>Configures the watcher to trigger on the given signal number (usually one
631 of the <code>SIGxxx</code> constants).</p>
636 <h2 id="code_ev_child_code_wait_for_pid_stat"><code>ev_child</code> - wait for pid status changes</h2>
637 <div id="code_ev_child_code_wait_for_pid_stat-2">
638 <p>Child watchers trigger when your process receives a SIGCHLD in response to
639 some child status changes (most typically when a child of yours dies).</p>
641 <dt>ev_child_init (ev_child *, callback, int pid)</dt>
642 <dt>ev_child_set (ev_child *, int pid)</dt>
644 <p>Configures the watcher to wait for status changes of process <code>pid</code> (or
645 <i>any</i> process if <code>pid</code> is specified as <code>0</code>). The callback can look
646 at the <code>rstatus</code> member of the <code>ev_child</code> watcher structure to see
647 the status word (use the macros from <code>sys/wait.h</code> and see your systems
648 <code>waitpid</code> documentation). The <code>rpid</code> member contains the pid of the
649 process causing the status change.</p>
654 <h2 id="code_ev_idle_code_when_you_ve_got_no"><code>ev_idle</code> - when you've got nothing better to do</h2>
655 <div id="code_ev_idle_code_when_you_ve_got_no-2">
656 <p>Idle watchers trigger events when there are no other events are pending
657 (prepare, check and other idle watchers do not count). That is, as long
658 as your process is busy handling sockets or timeouts (or even signals,
659 imagine) it will not be triggered. But when your process is idle all idle
660 watchers are being called again and again, once per event loop iteration -
661 until stopped, that is, or your process receives more events and becomes
663 <p>The most noteworthy effect is that as long as any idle watchers are
664 active, the process will not block when waiting for new events.</p>
665 <p>Apart from keeping your process non-blocking (which is a useful
666 effect on its own sometimes), idle watchers are a good place to do
667 "pseudo-background processing", or delay processing stuff to after the
668 event loop has handled all outstanding events.</p>
670 <dt>ev_idle_init (ev_signal *, callback)</dt>
672 <p>Initialises and configures the idle watcher - it has no parameters of any
673 kind. There is a <code>ev_idle_set</code> macro, but using it is utterly pointless,
679 <h2 id="code_ev_prepare_code_and_code_ev_che"><code>ev_prepare</code> and <code>ev_check</code> - customise your event loop</h2>
680 <div id="code_ev_prepare_code_and_code_ev_che-2">
681 <p>Prepare and check watchers are usually (but not always) used in tandem:
682 prepare watchers get invoked before the process blocks and check watchers
684 <p>Their main purpose is to integrate other event mechanisms into libev. This
685 could be used, for example, to track variable changes, implement your own
686 watchers, integrate net-snmp or a coroutine library and lots more.</p>
687 <p>This is done by examining in each prepare call which file descriptors need
688 to be watched by the other library, registering <code>ev_io</code> watchers for
689 them and starting an <code>ev_timer</code> watcher for any timeouts (many libraries
690 provide just this functionality). Then, in the check watcher you check for
691 any events that occured (by checking the pending status of all watchers
692 and stopping them) and call back into the library. The I/O and timer
693 callbacks will never actually be called (but must be valid nevertheless,
694 because you never know, you know?).</p>
695 <p>As another example, the Perl Coro module uses these hooks to integrate
696 coroutines into libev programs, by yielding to other active coroutines
697 during each prepare and only letting the process block if no coroutines
698 are ready to run (it's actually more complicated: it only runs coroutines
699 with priority higher than or equal to the event loop and one coroutine
700 of lower priority, but only once, using idle watchers to keep the event
701 loop from blocking if lower-priority coroutines are active, thus mapping
702 low-priority coroutines to idle/background tasks).</p>
704 <dt>ev_prepare_init (ev_prepare *, callback)</dt>
705 <dt>ev_check_init (ev_check *, callback)</dt>
707 <p>Initialises and configures the prepare or check watcher - they have no
708 parameters of any kind. There are <code>ev_prepare_set</code> and <code>ev_check_set</code>
709 macros, but using them is utterly, utterly and completely pointless.</p>
714 <h1 id="OTHER_FUNCTIONS">OTHER FUNCTIONS</h1><p><a href="#TOP" class="toplink">Top</a></p>
715 <div id="OTHER_FUNCTIONS_CONTENT">
716 <p>There are some other functions of possible interest. Described. Here. Now.</p>
718 <dt>ev_once (loop, int fd, int events, ev_tstamp timeout, callback)</dt>
720 <p>This function combines a simple timer and an I/O watcher, calls your
721 callback on whichever event happens first and automatically stop both
722 watchers. This is useful if you want to wait for a single event on an fd
723 or timeout without having to allocate/configure/start/stop/free one or
724 more watchers yourself.</p>
725 <p>If <code>fd</code> is less than 0, then no I/O watcher will be started and events
726 is being ignored. Otherwise, an <code>ev_io</code> watcher for the given <code>fd</code> and
727 <code>events</code> set will be craeted and started.</p>
728 <p>If <code>timeout</code> is less than 0, then no timeout watcher will be
729 started. Otherwise an <code>ev_timer</code> watcher with after = <code>timeout</code> (and
730 repeat = 0) will be started. While <code>0</code> is a valid timeout, it is of
732 <p>The callback has the type <code>void (*cb)(int revents, void *arg)</code> and gets
733 passed an <code>revents</code> set like normal event callbacks (a combination of
734 <code>EV_ERROR</code>, <code>EV_READ</code>, <code>EV_WRITE</code> or <code>EV_TIMEOUT</code>) and the <code>arg</code>
735 value passed to <code>ev_once</code>:</p>
736 <pre> static void stdin_ready (int revents, void *arg)
738 if (revents & EV_TIMEOUT)
739 /* doh, nothing entered */;
740 else if (revents & EV_READ)
741 /* stdin might have data for us, joy! */;
744 ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
748 <dt>ev_feed_event (loop, watcher, int events)</dt>
750 <p>Feeds the given event set into the event loop, as if the specified event
751 had happened for the specified watcher (which must be a pointer to an
752 initialised but not necessarily started event watcher).</p>
754 <dt>ev_feed_fd_event (loop, int fd, int revents)</dt>
756 <p>Feed an event on the given fd, as if a file descriptor backend detected
757 the given events it.</p>
759 <dt>ev_feed_signal_event (loop, int signum)</dt>
761 <p>Feed an event as if the given signal occured (loop must be the default loop!).</p>
766 <h1 id="LIBEVENT_EMULATION">LIBEVENT EMULATION</h1><p><a href="#TOP" class="toplink">Top</a></p>
767 <div id="LIBEVENT_EMULATION_CONTENT">
768 <p>Libev offers a compatibility emulation layer for libevent. It cannot
769 emulate the internals of libevent, so here are some usage hints:</p>
771 <dt>* Use it by including <event.h>, as usual.</dt>
772 <dt>* The following members are fully supported: ev_base, ev_callback,
773 ev_arg, ev_fd, ev_res, ev_events.</dt>
774 <dt>* Avoid using ev_flags and the EVLIST_*-macros, while it is
775 maintained by libev, it does not work exactly the same way as in libevent (consider
776 it a private API).</dt>
777 <dt>* Priorities are not currently supported. Initialising priorities
778 will fail and all watchers will have the same priority, even though there
779 is an ev_pri field.</dt>
780 <dt>* Other members are not supported.</dt>
781 <dt>* The libev emulation is <i>not</i> ABI compatible to libevent, you need
782 to use the libev header file and library.</dt>
786 <h1 id="C_SUPPORT">C++ SUPPORT</h1><p><a href="#TOP" class="toplink">Top</a></p>
787 <div id="C_SUPPORT_CONTENT">
791 <h1 id="AUTHOR">AUTHOR</h1><p><a href="#TOP" class="toplink">Top</a></p>
792 <div id="AUTHOR_CONTENT">
793 <p>Marc Lehmann <libev@schmorp.de>.</p>
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