--- libev/ev.pod 2007/11/12 08:20:02 1.8 +++ libev/ev.pod 2007/12/22 06:16:36 1.99 @@ -6,10 +6,56 @@ #include +=head1 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 occuring), and it will manage +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 @@ -23,65 +69,150 @@ =head1 FEATURES -Libev supports select, poll, the linux-specific epoll and the bsd-specific -kqueue mechanisms for file descriptor events, relative timers, absolute -timers with customised rescheduling, signal events, process status change -events (related to SIGCHLD), and event watchers dealing with the event -loop mechanism itself (idle, prepare and check watchers). It also is quite -fast (see this L comparing -it to libevent for example). +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 bvalue 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 ev_io watcher for that: +become readable, you would create an C watcher for that: static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents) { @@ -297,63 +657,71 @@ 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 set method. - -You cna check whether an event is active by calling the C macro. To see whether an event is outstanding (but the -callback for it has not been called yet) you cna use the C macro. +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 rceeived events usually include a single bit per event type received +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 EV_READ +=item C -=item EV_WRITE +=item C -The file descriptor in the ev_io watcher has become readable and/or +The file descriptor in the C watcher has become readable and/or writable. -=item EV_TIMEOUT +=item C + +The C watcher has timed out. -The ev_timer watcher has timed out. +=item C -=item EV_PERIODIC +The C watcher has timed out. -The ev_periodic watcher has timed out. +=item C -=item EV_SIGNAL +The signal specified in the C watcher has been received by a thread. -The signal specified in the ev_signal watcher has been received by a thread. +=item C -=item EV_CHILD +The pid specified in the C watcher has received a status change. -The pid specified in the ev_child watcher has received a status change. +=item C -=item EV_IDLE +The path specified in the C watcher changed its attributes somehow. -The ev_idle watcher has determined that you have nothing better to do. +=item C -=item EV_PREPARE +The C watcher has determined that you have nothing better to do. -=item EV_CHECK +=item C -All ev_prepare watchers are invoked just I C starts -to gather new events, and all ev_check watchers are invoked just after +=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 ev_prepare watcher might start an idle watcher to keep +(for example, a C watcher might start an idle watcher to keep C from blocking). -=item EV_ERROR +=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 An unspecified error has occured, the watcher has been stopped. This might happen because the watcher could not be properly started because libev @@ -369,10 +737,130 @@ =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 cna be used +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 @@ -395,24 +883,66 @@ ... } -More interesting and less C-conformant ways of catsing your callback type -have been omitted.... +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. +information given in the last section. Any initialisation/set macros, +functions and members specific to the watcher type are explained. -=head2 struct ev_io - is my file descriptor readable or writable +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 (This behaviour is called -level-triggering because you keep receiving events as long as the -condition persists. Remember you cna stop the watcher if you don't want to -act on the event and neither want to receive future events). +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 oer +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). @@ -420,11 +950,73 @@ You have to be careful with dup'ed file descriptors, though. Some backends (the linux epoll backend is a notable example) cannot handle dup'ed file descriptors correctly if you register interest in two or more fds pointing -to the same file/socket etc. description. +to the same underlying file/socket/etc. description (that is, they share +the same underlying "file open"). 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 EVMETHOD_SELECT and -EVMETHOD_POLL). +(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 menas when you +have C'ed file descriptors and register events for them, only one +file descriptor might actually receive events. + +There is no workaorund 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 @@ -432,23 +1024,64 @@ =item ev_io_set (ev_io *, int fd, int events) -Configures an ev_io watcher. The fd is the file descriptor to rceeive -events for and events is either C, C or C to receive the given 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 -=head2 struct ev_timer - relative and optionally recurring timeouts +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 youreset your system clock to last years +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 soem inaccuracies are unavoidable (the +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) @@ -463,7 +1096,7 @@ 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 (ecause it takes longer than those 10 seconds to do stuff) the +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) @@ -471,38 +1104,99 @@ 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 started but nonrepeating, stop it. +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 repeat -value), or reset the running timer to the repeat value. +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 ev_timer with after=repeat=60 and calling ev_timer_again 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 stop -the timer, and again will automatically restart it if need be. +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 -=head2 ev_periodic - to cron or not to cron it +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 ev_timer's, they are not based on real time (or relative time) +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 ev_timer, which would trigger -roughly 10 seconds later and of course not if you reset your system time -again). +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 eahc midnight, local time. +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 @@ -513,21 +1207,20 @@ 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 (interval = reschedule_cb = 0) +=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 (interval > 0, reschedule_cb = 0) +=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) and then repeat, regardless -of any time jumps. +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: @@ -536,25 +1229,30 @@ 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 correct, when the system time is evenly divisible +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 -ev_periodic will try to run the callback in this mode at the next possible +C will try to run the callback in this mode at the next possible time where C