=head1 NAME libev - a high performance full-featured event loop written in C =head1 SYNOPSIS #include =head2 EXAMPLE PROGRAM #include ev_io stdin_watcher; ev_timer timeout_watcher; /* called when data readable on stdin */ static void stdin_cb (EV_P_ struct ev_io *w, int revents) { /* puts ("stdin ready"); */ ev_io_stop (EV_A_ w); /* just a syntax example */ ev_unloop (EV_A_ EVUNLOOP_ALL); /* leave all loop calls */ } static void timeout_cb (EV_P_ struct ev_timer *w, int revents) { /* puts ("timeout"); */ ev_unloop (EV_A_ EVUNLOOP_ONE); /* leave one loop call */ } int main (void) { struct ev_loop *loop = ev_default_loop (0); /* initialise an io watcher, then start it */ ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ); ev_io_start (loop, &stdin_watcher); /* simple non-repeating 5.5 second timeout */ ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.); ev_timer_start (loop, &timeout_watcher); /* loop till timeout or data ready */ ev_loop (loop, 0); return 0; } =head1 DESCRIPTION The newest version of this document is also available as a html-formatted web page you might find easier to navigate when reading it for the first time: L. Libev is an event loop: you register interest in certain events (such as a file descriptor being readable or a timeout occurring), and it will manage these event sources and provide your program with events. To do this, it must take more or less complete control over your process (or thread) by executing the I handler, and will then communicate events via a callback mechanism. You register interest in certain events by registering so-called I, which are relatively small C structures you initialise with the details of the event, and then hand it over to libev by I the watcher. =head2 FEATURES Libev supports C which have a high overhead for the actual polling but can deliver many events at once. By setting a higher I you allow libev to spend more time collecting I/O events, so you can handle more events per iteration, at the cost of increasing latency. Timeouts (both C and C) will be not affected. Setting this to a non-null value will introduce an additional C call into most loop iterations. Likewise, by setting a higher I you allow libev to spend more time collecting timeouts, at the expense of increased latency (the watcher callback will be called later). C watchers will not be affected. Setting this to a non-null value will not introduce any overhead in libev. Many (busy) programs can usually benefit by setting the io collect interval to a value near C<0.1> or so, which is often enough for interactive servers (of course not for games), likewise for timeouts. It usually doesn't make much sense to set it to a lower value than C<0.01>, as this approsaches the timing granularity of most systems. =back =head1 ANATOMY OF A WATCHER A watcher is a structure that you create and register to record your interest in some event. For instance, if you want to wait for STDIN to become readable, you would create an C watcher for that: static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents) { ev_io_stop (w); ev_unloop (loop, EVUNLOOP_ALL); } struct ev_loop *loop = ev_default_loop (0); struct ev_io stdin_watcher; ev_init (&stdin_watcher, my_cb); ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ); ev_io_start (loop, &stdin_watcher); ev_loop (loop, 0); As you can see, you are responsible for allocating the memory for your watcher structures (and it is usually a bad idea to do this on the stack, although this can sometimes be quite valid). Each watcher structure must be initialised by a call to C, which expects a callback to be provided. This callback gets invoked each time the event occurs (or, in the case of io watchers, each time the event loop detects that the file descriptor given is readable and/or writable). Each watcher type has its own C<< ev__set (watcher *, ...) >> macro with arguments specific to this watcher type. There is also a macro to combine initialisation and setting in one call: C<< ev__init (watcher *, callback, ...) >>. To make the watcher actually watch out for events, you have to start it with a watcher-specific start function (C<< ev__start (loop, watcher *) >>), and you can stop watching for events at any time by calling the corresponding stop function (C<< ev__stop (loop, watcher *) >>. As long as your watcher is active (has been started but not stopped) you must not touch the values stored in it. Most specifically you must never reinitialise it or call its C macro. Each and every callback receives the event loop pointer as first, the registered watcher structure as second, and a bitset of received events as third argument. The received events usually include a single bit per event type received (you can receive multiple events at the same time). The possible bit masks are: =over 4 =item C =item C The file descriptor in the C watcher has become readable and/or writable. =item C The C watcher has timed out. =item C The C watcher has timed out. =item C The signal specified in the C watcher has been received by a thread. =item C The pid specified in the C watcher has received a status change. =item C The path specified in the C watcher changed its attributes somehow. =item C The C watcher has determined that you have nothing better to do. =item C =item C All C watchers are invoked just I C starts to gather new events, and all C watchers are invoked just after C has gathered them, but before it invokes any callbacks for any received events. Callbacks of both watcher types can start and stop as many watchers as they want, and all of them will be taken into account (for example, a C watcher might start an idle watcher to keep C from blocking). =item C The embedded event loop specified in the C watcher needs attention. =item C The event loop has been resumed in the child process after fork (see C). =item C The given async watcher has been asynchronously notified (see C). =item C An unspecified error has occured, the watcher has been stopped. This might happen because the watcher could not be properly started because libev ran out of memory, a file descriptor was found to be closed or any other problem. You best act on it by reporting the problem and somehow coping with the watcher being stopped. Libev will usually signal a few "dummy" events together with an error, for example it might indicate that a fd is readable or writable, and if your callbacks is well-written it can just attempt the operation and cope with the error from read() or write(). This will not work in multithreaded programs, though, so beware. =back =head2 GENERIC WATCHER FUNCTIONS In the following description, C stands for the watcher type, e.g. C for C watchers and C for C watchers. =over 4 =item C (ev_TYPE *watcher, callback) This macro initialises the generic portion of a watcher. The contents of the watcher object can be arbitrary (so C will do). Only the generic parts of the watcher are initialised, you I to call the type-specific C macro afterwards to initialise the type-specific parts. For each type there is also a C macro which rolls both calls into one. You can reinitialise a watcher at any time as long as it has been stopped (or never started) and there are no pending events outstanding. The callback is always of type C. =item C (ev_TYPE *, [args]) This macro initialises the type-specific parts of a watcher. You need to call C at least once before you call this macro, but you can call C any number of times. You must not, however, call this macro on a watcher that is active (it can be pending, however, which is a difference to the C macro). Although some watcher types do not have type-specific arguments (e.g. C) you still need to call its C macro. =item C (ev_TYPE *watcher, callback, [args]) This convinience macro rolls both C and C macro calls into a single call. This is the most convinient method to initialise a watcher. The same limitations apply, of course. =item C (loop *, ev_TYPE *watcher) Starts (activates) the given watcher. Only active watchers will receive events. If the watcher is already active nothing will happen. =item C (loop *, ev_TYPE *watcher) Stops the given watcher again (if active) and clears the pending status. It is possible that stopped watchers are pending (for example, non-repeating timers are being stopped when they become pending), but C ensures that the watcher is neither active nor pending. If you want to free or reuse the memory used by the watcher it is therefore a good idea to always call its C function. =item bool ev_is_active (ev_TYPE *watcher) Returns a true value iff the watcher is active (i.e. it has been started and not yet been stopped). As long as a watcher is active you must not modify it. =item bool ev_is_pending (ev_TYPE *watcher) Returns a true value iff the watcher is pending, (i.e. it has outstanding events but its callback has not yet been invoked). As long as a watcher is pending (but not active) you must not call an init function on it (but C is safe), you must not change its priority, and you must make sure the watcher is available to libev (e.g. you cannot C it). =item callback ev_cb (ev_TYPE *watcher) Returns the callback currently set on the watcher. =item ev_cb_set (ev_TYPE *watcher, callback) Change the callback. You can change the callback at virtually any time (modulo threads). =item ev_set_priority (ev_TYPE *watcher, priority) =item int ev_priority (ev_TYPE *watcher) Set and query the priority of the watcher. The priority is a small integer between C (default: C<2>) and C (default: C<-2>). Pending watchers with higher priority will be invoked before watchers with lower priority, but priority will not keep watchers from being executed (except for C watchers). This means that priorities are I used for ordering callback invocation after new events have been received. This is useful, for example, to reduce latency after idling, or more often, to bind two watchers on the same event and make sure one is called first. If you need to suppress invocation when higher priority events are pending you need to look at C watchers, which provide this functionality. You I change the priority of a watcher as long as it is active or pending. The default priority used by watchers when no priority has been set is always C<0>, which is supposed to not be too high and not be too low :). Setting a priority outside the range of C to C is fine, as long as you do not mind that the priority value you query might or might not have been adjusted to be within valid range. =item ev_invoke (loop, ev_TYPE *watcher, int revents) Invoke the C with the given C and C. Neither C nor C need to be valid as long as the watcher callback can deal with that fact. =item int ev_clear_pending (loop, ev_TYPE *watcher) If the watcher is pending, this function returns clears its pending status and returns its C bitset (as if its callback was invoked). If the watcher isn't pending it does nothing and returns C<0>. =back =head2 ASSOCIATING CUSTOM DATA WITH A WATCHER Each watcher has, by default, a member C that you can change and read at any time, libev will completely ignore it. This can be used to associate arbitrary data with your watcher. If you need more data and don't want to allocate memory and store a pointer to it in that data member, you can also "subclass" the watcher type and provide your own data: struct my_io { struct ev_io io; int otherfd; void *somedata; struct whatever *mostinteresting; } And since your callback will be called with a pointer to the watcher, you can cast it back to your own type: static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents) { struct my_io *w = (struct my_io *)w_; ... } More interesting and less C-conformant ways of casting your callback type instead have been omitted. Another common scenario is having some data structure with multiple watchers: struct my_biggy { int some_data; ev_timer t1; ev_timer t2; } In this case getting the pointer to C is a bit more complicated, you need to use C: #include static void t1_cb (EV_P_ struct ev_timer *w, int revents) { struct my_biggy big = (struct my_biggy * (((char *)w) - offsetof (struct my_biggy, t1)); } static void t2_cb (EV_P_ struct ev_timer *w, int revents) { struct my_biggy big = (struct my_biggy * (((char *)w) - offsetof (struct my_biggy, t2)); } =head1 WATCHER TYPES This section describes each watcher in detail, but will not repeat information given in the last section. Any initialisation/set macros, functions and members specific to the watcher type are explained. Members are additionally marked with either I<[read-only]>, meaning that, while the watcher is active, you can look at the member and expect some sensible content, but you must not modify it (you can modify it while the watcher is stopped to your hearts content), or I<[read-write]>, which means you can expect it to have some sensible content while the watcher is active, but you can also modify it. Modifying it may not do something sensible or take immediate effect (or do anything at all), but libev will not crash or malfunction in any way. =head2 C - is this file descriptor readable or writable? I/O watchers check whether a file descriptor is readable or writable in each iteration of the event loop, or, more precisely, when reading would not block the process and writing would at least be able to write some data. This behaviour is called level-triggering because you keep receiving events as long as the condition persists. Remember you can stop the watcher if you don't want to act on the event and neither want to receive future events. In general you can register as many read and/or write event watchers per fd as you want (as long as you don't confuse yourself). Setting all file descriptors to non-blocking mode is also usually a good idea (but not required if you know what you are doing). If you must do this, then force the use of a known-to-be-good backend (at the time of this writing, this includes only C and C). Another thing you have to watch out for is that it is quite easy to receive "spurious" readyness notifications, that is your callback might be called with C but a subsequent C(2) will actually block because there is no data. Not only are some backends known to create a lot of those (for example solaris ports), it is very easy to get into this situation even with a relatively standard program structure. Thus it is best to always use non-blocking I/O: An extra C(2) returning C is far preferable to a program hanging until some data arrives. If you cannot run the fd in non-blocking mode (for example you should not play around with an Xlib connection), then you have to seperately re-test whether a file descriptor is really ready with a known-to-be good interface such as poll (fortunately in our Xlib example, Xlib already does this on its own, so its quite safe to use). =head3 The special problem of disappearing file descriptors Some backends (e.g. kqueue, epoll) need to be told about closing a file descriptor (either by calling C explicitly or by any other means, such as C). The reason is that you register interest in some file descriptor, but when it goes away, the operating system will silently drop this interest. If another file descriptor with the same number then is registered with libev, there is no efficient way to see that this is, in fact, a different file descriptor. To avoid having to explicitly tell libev about such cases, libev follows the following policy: Each time C is being called, libev will assume that this is potentially a new file descriptor, otherwise it is assumed that the file descriptor stays the same. That means that you I to call C (or C) when you change the descriptor even if the file descriptor number itself did not change. This is how one would do it normally anyway, the important point is that the libev application should not optimise around libev but should leave optimisations to libev. =head3 The special problem of dup'ed file descriptors Some backends (e.g. epoll), cannot register events for file descriptors, but only events for the underlying file descriptions. That means when you have C'ed file descriptors or weirder constellations, and register events for them, only one file descriptor might actually receive events. There is no workaround possible except not registering events for potentially C'ed file descriptors, or to resort to C or C. =head3 The special problem of fork Some backends (epoll, kqueue) do not support C at all or exhibit useless behaviour. Libev fully supports fork, but needs to be told about it in the child. To support fork in your programs, you either have to call C or C after a fork in the child, enable C, or resort to C or C. =head3 Watcher-Specific Functions =over 4 =item ev_io_init (ev_io *, callback, int fd, int events) =item ev_io_set (ev_io *, int fd, int events) Configures an C watcher. The C is the file descriptor to rceeive events for and events is either C, C or C to receive the given events. =item int fd [read-only] The file descriptor being watched. =item int events [read-only] The events being watched. =back =head3 Examples Example: Call C when STDIN_FILENO has become, well readable, but only once. Since it is likely line-buffered, you could attempt to read a whole line in the callback. static void stdin_readable_cb (struct ev_loop *loop, struct ev_io *w, int revents) { ev_io_stop (loop, w); .. read from stdin here (or from w->fd) and haqndle any I/O errors } ... struct ev_loop *loop = ev_default_init (0); struct ev_io stdin_readable; ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ); ev_io_start (loop, &stdin_readable); ev_loop (loop, 0); =head2 C - relative and optionally repeating timeouts Timer watchers are simple relative timers that generate an event after a given time, and optionally repeating in regular intervals after that. The timers are based on real time, that is, if you register an event that times out after an hour and you reset your system clock to last years time, it will still time out after (roughly) and hour. "Roughly" because detecting time jumps is hard, and some inaccuracies are unavoidable (the monotonic clock option helps a lot here). The relative timeouts are calculated relative to the C time. This is usually the right thing as this timestamp refers to the time of the event triggering whatever timeout you are modifying/starting. If you suspect event processing to be delayed and you I to base the timeout on the current time, use something like this to adjust for this: ev_timer_set (&timer, after + ev_now () - ev_time (), 0.); The callback is guarenteed to be invoked only when its timeout has passed, but if multiple timers become ready during the same loop iteration then order of execution is undefined. =head3 Watcher-Specific Functions and Data Members =over 4 =item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat) =item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat) Configure the timer to trigger after C seconds. If C is C<0.>, then it will automatically be stopped. If it is positive, then the timer will automatically be configured to trigger again C seconds later, again, and again, until stopped manually. The timer itself will do a best-effort at avoiding drift, that is, if you configure a timer to trigger every 10 seconds, then it will trigger at exactly 10 second intervals. If, however, your program cannot keep up with the timer (because it takes longer than those 10 seconds to do stuff) the timer will not fire more than once per event loop iteration. =item ev_timer_again (loop) This will act as if the timer timed out and restart it again if it is repeating. The exact semantics are: If the timer is pending, its pending status is cleared. If the timer is started but nonrepeating, stop it (as if it timed out). If the timer is repeating, either start it if necessary (with the C value), or reset the running timer to the C value. This sounds a bit complicated, but here is a useful and typical example: Imagine you have a tcp connection and you want a so-called idle timeout, that is, you want to be called when there have been, say, 60 seconds of inactivity on the socket. The easiest way to do this is to configure an C with a C value of C<60> and then call C each time you successfully read or write some data. If you go into an idle state where you do not expect data to travel on the socket, you can C the timer, and C will automatically restart it if need be. That means you can ignore the C value and C altogether and only ever use the C value and C: ev_timer_init (timer, callback, 0., 5.); ev_timer_again (loop, timer); ... timer->again = 17.; ev_timer_again (loop, timer); ... timer->again = 10.; ev_timer_again (loop, timer); This is more slightly efficient then stopping/starting the timer each time you want to modify its timeout value. =item ev_tstamp repeat [read-write] The current C value. Will be used each time the watcher times out or C is called and determines the next timeout (if any), which is also when any modifications are taken into account. =back =head3 Examples Example: Create a timer that fires after 60 seconds. static void one_minute_cb (struct ev_loop *loop, struct ev_timer *w, int revents) { .. one minute over, w is actually stopped right here } struct ev_timer mytimer; ev_timer_init (&mytimer, one_minute_cb, 60., 0.); ev_timer_start (loop, &mytimer); Example: Create a timeout timer that times out after 10 seconds of inactivity. static void timeout_cb (struct ev_loop *loop, struct ev_timer *w, int revents) { .. ten seconds without any activity } struct ev_timer mytimer; ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */ ev_timer_again (&mytimer); /* start timer */ ev_loop (loop, 0); // and in some piece of code that gets executed on any "activity": // reset the timeout to start ticking again at 10 seconds ev_timer_again (&mytimer); =head2 C - to cron or not to cron? Periodic watchers are also timers of a kind, but they are very versatile (and unfortunately a bit complex). Unlike C's, they are not based on real time (or relative time) but on wallclock time (absolute time). You can tell a periodic watcher to trigger "at" some specific point in time. For example, if you tell a periodic watcher to trigger in 10 seconds (by specifiying e.g. C) and then reset your system clock to the last year, then it will take a year to trigger the event (unlike an C, which would trigger roughly 10 seconds later). They can also be used to implement vastly more complex timers, such as triggering an event on each midnight, local time or other, complicated, rules. As with timers, the callback is guarenteed to be invoked only when the time (C) has been passed, but if multiple periodic timers become ready during the same loop iteration then order of execution is undefined. =head3 Watcher-Specific Functions and Data Members =over 4 =item ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb) =item ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb) Lots of arguments, lets sort it out... There are basically three modes of operation, and we will explain them from simplest to complex: =over 4 =item * absolute timer (at = time, interval = reschedule_cb = 0) In this configuration the watcher triggers an event at the wallclock time C and doesn't repeat. It will not adjust when a time jump occurs, that is, if it is to be run at January 1st 2011 then it will run when the system time reaches or surpasses this time. =item * non-repeating interval timer (at = offset, interval > 0, reschedule_cb = 0) In this mode the watcher will always be scheduled to time out at the next C time (for some integer N, which can also be negative) and then repeat, regardless of any time jumps. This can be used to create timers that do not drift with respect to system time: ev_periodic_set (&periodic, 0., 3600., 0); This doesn't mean there will always be 3600 seconds in between triggers, but only that the the callback will be called when the system time shows a full hour (UTC), or more correctly, when the system time is evenly divisible by 3600. Another way to think about it (for the mathematically inclined) is that C will try to run the callback in this mode at the next possible time where C