--- libev/ev.pod 2007/12/21 05:10:01 1.95 +++ libev/ev.pod 2009/03/04 12:51:37 1.226 @@ -4,55 +4,71 @@ =head1 SYNOPSIS - #include + #include -=head1 EXAMPLE PROGRAM +=head2 EXAMPLE PROGRAM - #include + // a single header file is required + #include - ev_io stdin_watcher; - ev_timer timeout_watcher; + #include // for puts - /* 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); + // every watcher type has its own typedef'd struct + // with the name ev_TYPE + ev_io stdin_watcher; + ev_timer timeout_watcher; - /* loop till timeout or data ready */ - ev_loop (loop, 0); + // all watcher callbacks have a similar signature + // this callback is called when data is readable on stdin + static void + stdin_cb (EV_P_ ev_io *w, int revents) + { + puts ("stdin ready"); + // for one-shot events, one must manually stop the watcher + // with its corresponding stop function. + ev_io_stop (EV_A_ w); + + // this causes all nested ev_loop's to stop iterating + ev_unloop (EV_A_ EVUNLOOP_ALL); + } + + // another callback, this time for a time-out + static void + timeout_cb (EV_P_ ev_timer *w, int revents) + { + puts ("timeout"); + // this causes the innermost ev_loop to stop iterating + ev_unloop (EV_A_ EVUNLOOP_ONE); + } - return 0; - } + int + main (void) + { + // use the default event loop unless you have special needs + struct ev_loop *loop = ev_default_loop (0); + + // initialise an io watcher, then start it + // this one will watch for stdin to become readable + ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ); + ev_io_start (loop, &stdin_watcher); + + // initialise a timer watcher, then start it + // simple non-repeating 5.5 second timeout + ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.); + ev_timer_start (loop, &timeout_watcher); + + // now wait for events to arrive + ev_loop (loop, 0); + + // unloop was called, so exit + return 0; + } =head1 DESCRIPTION -The newest version of this document is also available as a html-formatted +The newest version of this document is also available as an html-formatted web page you might find easier to navigate when reading it for the first -time: L. +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 @@ -67,7 +83,7 @@ details of the event, and then hand it over to libev by I the watcher. -=head1 FEATURES +=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/jitter/inexactness (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 I/O 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 approaches the timing granularity of most systems. + +Setting the I can improve the opportunity for +saving power, as the program will "bundle" timer callback invocations that +are "near" in time together, by delaying some, thus reducing the number of +times the process sleeps and wakes up again. Another useful technique to +reduce iterations/wake-ups is to use C watchers and make sure +they fire on, say, one-second boundaries only. + +=item ev_loop_verify (loop) + +This function only does something when C support has been +compiled in, which is the default for non-minimal builds. It tries to go +through all internal structures and checks them for validity. If anything +is found to be inconsistent, it will print an error message to standard +error and call C. + +This can be used to catch bugs inside libev itself: under normal +circumstances, this function will never abort as of course libev keeps its +data structures consistent. =back =head1 ANATOMY OF A WATCHER +In the following description, uppercase C in names stands for the +watcher type, e.g. C can mean C for timer +watchers and C for I/O watchers. + 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); + static void my_cb (struct ev_loop *loop, ev_io *w, int revents) + { + ev_io_stop (w); + ev_unloop (loop, EVUNLOOP_ALL); + } + + struct ev_loop *loop = ev_default_loop (0); + + 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). +watcher structures (and it is I a bad idea to do this on the +stack). + +Each watcher has an associated watcher structure (called C +or simply C, as typedefs are provided for all watcher structs). 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 +callback gets invoked each time the event occurs (or, in the case of I/O 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, ...) >>. +Each watcher type further has its own C<< ev_TYPE_set (watcher *, ...) >> +macro to configure it, with arguments specific to the watcher type. There +is also a macro to combine initialisation and setting in one call: C<< +ev_TYPE_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 +with a watcher-specific start function (C<< ev_TYPE_start (loop, watcher *) >>), and you can stop watching for events at any time by calling the -corresponding stop function (C<< ev__stop (loop, watcher *) >>. +corresponding stop function (C<< ev_TYPE_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. +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 @@ -679,27 +924,33 @@ 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 +An unspecified error has occurred, 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. +problem. Libev considers these application bugs. -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. +You best act on it by reporting the problem and somehow coping with the +watcher being stopped. Note that well-written programs should not receive +an error ever, so when your watcher receives it, this usually indicates a +bug in your program. + +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 multi-threaded +programs, though, as the fd could already be closed and reused for another +thing, 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) @@ -714,9 +965,15 @@ 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. +Example: Initialise an C watcher in two steps. + + ev_io w; + ev_init (&w, my_cb); + ev_io_set (&w, STDIN_FILENO, EV_READ); + =item C (ev_TYPE *, [args]) This macro initialises the type-specific parts of a watcher. You need to @@ -728,25 +985,38 @@ Although some watcher types do not have type-specific arguments (e.g. C) you still need to call its C macro. +See C, above, for an example. + =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 +This convenience macro rolls both C and C macro +calls into a single call. This is the most convenient method to initialise a watcher. The same limitations apply, of course. +Example: Initialise and set an C watcher in one step. + + ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ); + =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. +Example: Start the C watcher that is being abused as example in this +whole section. + + ev_io_start (EV_DEFAULT_UC, &w); + =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. +Stops the given watcher if active, and clears the pending status (whether +the watcher was active or not). + +It is possible that stopped watchers are pending - for example, +non-repeating timers are being stopped when they become pending - but +calling 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) @@ -798,80 +1068,91 @@ 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. +or might not have been clamped to the 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. +can deal with that fact, as both are simply passed through to the +callback. =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 +If the watcher is pending, this function 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>. +Sometimes it can be useful to "poll" a watcher instead of waiting for its +callback to be invoked, which can be accomplished with this function. + =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 +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; - } + struct my_io + { + ev_io io; + int otherfd; + void *somedata; + struct whatever *mostinteresting; + }; + + ... + struct my_io w; + ev_io_init (&w.io, my_cb, fd, EV_READ); 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_; - ... - } + static void my_cb (struct ev_loop *loop, 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: +Another common scenario is to use some data structure with multiple +embedded watchers: + + struct my_biggy + { + int some_data; + ev_timer t1; + ev_timer t2; + } - 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)); - } +In this case getting the pointer to C is a bit more +complicated: Either you store the address of your C struct +in the C member of the watcher (for woozies), or you need to use +some pointer arithmetic using C inside your watchers (for real +programmers): + + #include + + static void + t1_cb (EV_P_ 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_ ev_timer *w, int revents) + { + struct my_biggy big = (struct my_biggy * + (((char *)w) - offsetof (struct my_biggy, t2)); + } =head1 WATCHER TYPES @@ -905,36 +1186,34 @@ descriptors to non-blocking mode is also usually a good idea (but not required if you know what you are doing). -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 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 C and -C). +If you cannot use non-blocking mode, 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 +receive "spurious" readiness 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 +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). +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 separately +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). Some people additionally +use C and an interval timer, just to be sure you won't block +indefinitely. + +But really, best use non-blocking mode. =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 (either due to calling C explicitly or 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 @@ -954,12 +1233,12 @@ =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. +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 workaorund possible except not registering events -for potentially C'ed file descriptors or to resort to +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 @@ -973,6 +1252,17 @@ enable C, or resort to C or C. +=head3 The special problem of SIGPIPE + +While not really specific to libev, it is easy to forget about C: +when writing to a pipe whose other end has been closed, your program gets +sent a SIGPIPE, which, by default, aborts your program. For most programs +this is sensible behaviour, for daemons, this is usually undesirable. + +So when you encounter spurious, unexplained daemon exits, make sure you +ignore SIGPIPE (and maybe make sure you log the exit status of your daemon +somewhere, as that would have given you a big clue). + =head3 Watcher-Specific Functions @@ -983,8 +1273,8 @@ =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. +receive events for and C is either C, C or +C, to express the desire to receive the given events. =item int fd [read-only] @@ -996,23 +1286,25 @@ =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); + static void + stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents) + { + ev_io_stop (loop, w); + .. read from stdin here (or from w->fd) and handle any I/O errors + } + + ... + struct ev_loop *loop = ev_default_init (0); + 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 @@ -1021,22 +1313,209 @@ 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 +times out after an hour and you reset your system clock to January last +year, it will still time out after (roughly) one hour. "Roughly" because detecting time jumps is hard, and some inaccuracies are unavoidable (the monotonic clock option helps a lot here). +The callback is guaranteed to be invoked only I its timeout has +passed, but if multiple timers become ready during the same loop iteration +then order of execution is undefined. + +=head3 Be smart about timeouts + +Many real-world problems involve some kind of timeout, usually for error +recovery. A typical example is an HTTP request - if the other side hangs, +you want to raise some error after a while. + +What follows are some ways to handle this problem, from obvious and +inefficient to smart and efficient. + +In the following, a 60 second activity timeout is assumed - a timeout that +gets reset to 60 seconds each time there is activity (e.g. each time some +data or other life sign was received). + +=over 4 + +=item 1. Use a timer and stop, reinitialise and start it on activity. + +This is the most obvious, but not the most simple way: In the beginning, +start the watcher: + + ev_timer_init (timer, callback, 60., 0.); + ev_timer_start (loop, timer); + +Then, each time there is some activity, C it, initialise it +and start it again: + + ev_timer_stop (loop, timer); + ev_timer_set (timer, 60., 0.); + ev_timer_start (loop, timer); + +This is relatively simple to implement, but means that each time there is +some activity, libev will first have to remove the timer from its internal +data structure and then add it again. Libev tries to be fast, but it's +still not a constant-time operation. + +=item 2. Use a timer and re-start it with C inactivity. + +This is the easiest way, and involves using C instead of +C. + +To implement this, configure an C with a C value +of C<60> and then call C at start and 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 both the C function and the +C argument to C, and only ever use the C +member and C. + +At start: + + ev_timer_init (timer, callback); + timer->repeat = 60.; + ev_timer_again (loop, timer); + +Each time there is some activity: + + ev_timer_again (loop, timer); + +It is even possible to change the time-out on the fly, regardless of +whether the watcher is active or not: + + timer->repeat = 30.; + ev_timer_again (loop, timer); + +This is slightly more efficient then stopping/starting the timer each time +you want to modify its timeout value, as libev does not have to completely +remove and re-insert the timer from/into its internal data structure. + +It is, however, even simpler than the "obvious" way to do it. + +=item 3. Let the timer time out, but then re-arm it as required. + +This method is more tricky, but usually most efficient: Most timeouts are +relatively long compared to the intervals between other activity - in +our example, within 60 seconds, there are usually many I/O events with +associated activity resets. + +In this case, it would be more efficient to leave the C alone, +but remember the time of last activity, and check for a real timeout only +within the callback: + + ev_tstamp last_activity; // time of last activity + + static void + callback (EV_P_ ev_timer *w, int revents) + { + ev_tstamp now = ev_now (EV_A); + ev_tstamp timeout = last_activity + 60.; + + // if last_activity + 60. is older than now, we did time out + if (timeout < now) + { + // timeout occured, take action + } + else + { + // callback was invoked, but there was some activity, re-arm + // the watcher to fire in last_activity + 60, which is + // guaranteed to be in the future, so "again" is positive: + w->repeat = timeout - now; + ev_timer_again (EV_A_ w); + } + } + +To summarise the callback: first calculate the real timeout (defined +as "60 seconds after the last activity"), then check if that time has +been reached, which means something I, in fact, time out. Otherwise +the callback was invoked too early (C is in the future), so +re-schedule the timer to fire at that future time, to see if maybe we have +a timeout then. + +Note how C is used, taking advantage of the +C optimisation when the timer is already running. + +This scheme causes more callback invocations (about one every 60 seconds +minus half the average time between activity), but virtually no calls to +libev to change the timeout. + +To start the timer, simply initialise the watcher and set C +to the current time (meaning we just have some activity :), then call the +callback, which will "do the right thing" and start the timer: + + ev_timer_init (timer, callback); + last_activity = ev_now (loop); + callback (loop, timer, EV_TIMEOUT); + +And when there is some activity, simply store the current time in +C, no libev calls at all: + + last_actiivty = ev_now (loop); + +This technique is slightly more complex, but in most cases where the +time-out is unlikely to be triggered, much more efficient. + +Changing the timeout is trivial as well (if it isn't hard-coded in the +callback :) - just change the timeout and invoke the callback, which will +fix things for you. + +=item 4. Wee, just use a double-linked list for your timeouts. + +If there is not one request, but many thousands (millions...), all +employing some kind of timeout with the same timeout value, then one can +do even better: + +When starting the timeout, calculate the timeout value and put the timeout +at the I of the list. + +Then use an C to fire when the timeout at the I of +the list is expected to fire (for example, using the technique #3). + +When there is some activity, remove the timer from the list, recalculate +the timeout, append it to the end of the list again, and make sure to +update the C if it was taken from the beginning of the list. + +This way, one can manage an unlimited number of timeouts in O(1) time for +starting, stopping and updating the timers, at the expense of a major +complication, and having to use a constant timeout. The constant timeout +ensures that the list stays sorted. + +=back + +So which method the best? + +Method #2 is a simple no-brain-required solution that is adequate in most +situations. Method #3 requires a bit more thinking, but handles many cases +better, and isn't very complicated either. In most case, choosing either +one is fine, with #3 being better in typical situations. + +Method #1 is almost always a bad idea, and buys you nothing. Method #4 is +rather complicated, but extremely efficient, something that really pays +off after the first million or so of active timers, i.e. it's usually +overkill :) + +=head3 The special problem of time updates + +Establishing the current time is a costly operation (it usually takes at +least two system calls): EV therefore updates its idea of the current +time only before and after C collects new events, which causes a +growing difference between C and C when handling +lots of events in one iteration. + 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: +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. +If the event loop is suspended for a long time, you can also force an +update of the time returned by C by calling C. =head3 Watcher-Specific Functions and Data Members @@ -1046,91 +1525,72 @@ =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. +Configure the timer to trigger after C seconds. If C +is C<0.>, then it will automatically be stopped once the timeout is +reached. 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 normally +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) +=item ev_timer_again (loop, ev_timer *) 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 started but non-repeating, 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. +This sounds a bit complicated, see "Be smart about timeouts", above, for a +usage example. =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), +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); + static void + one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents) + { + .. one minute over, w is actually stopped right here + } + + 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); + static void + timeout_cb (struct ev_loop *loop, ev_timer *w, int revents) + { + .. ten seconds without any activity + } + + 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? @@ -1139,20 +1599,21 @@ (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. +but on wall clock time (absolute time). You can tell a periodic watcher +to trigger after some specific point in time. For example, if you tell a +periodic watcher to trigger in 10 seconds (by specifying e.g. C, that is, an absolute time not a delay) and then reset your system +clock to January of the previous year, then it will take more than year +to trigger the event (unlike an C, which would still trigger +roughly 10 seconds later as it uses a relative timeout). + +Cs 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 guaranteed to be invoked only when the +time (C) has 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 @@ -1160,33 +1621,34 @@ =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) +=item ev_periodic_set (ev_periodic *, ev_tstamp at, ev_tstamp interval, 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: +operation, and we will explain them from simplest to most 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. +In this configuration the watcher triggers an event after the wall clock +time C has passed. It will not repeat and will not adjust when a time +jump occurs, that is, if it is to be run at January 1st 2011 then it will +only run when the system clock reaches or surpasses this time. -=item * non-repeating interval timer (at = offset, interval > 0, reschedule_cb = 0) +=item * 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: +This can be used to create timers that do not drift with respect to the +system clock, for example, here is a C that triggers each +hour, on the hour: 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 +but only that 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. @@ -1196,7 +1658,12 @@ For numerical stability it is preferable that the C value is near C (the current time), but there is no range requirement for -this value. +this value, and in fact is often specified as zero. + +Note also that there is an upper limit to how often a timer can fire (CPU +speed for example), so if C is very small then timing stability +will of course deteriorate. Libev itself tries to be exact to be about one +millisecond (if the OS supports it and the machine is fast enough). =item * manual reschedule mode (at and interval ignored, reschedule_cb = callback) @@ -1206,14 +1673,17 @@ current time as second argument. NOTE: I. If you need to stop it, -return C (or so, fudge fudge) and stop it afterwards (e.g. by -starting an C watcher, which is legal). +ever, or make ANY other event loop modifications whatsoever>. + +If you need to stop it, return C (or so, fudge fudge) and stop +it afterwards (e.g. by starting an C watcher, which is the +only event loop modification you are allowed to do). -Its prototype is C, e.g.: +The callback prototype is C, e.g.: - static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now) + static ev_tstamp + my_rescheduler (ev_periodic *w, ev_tstamp now) { return now + 60.; } @@ -1223,11 +1693,11 @@ will usually be called just before the callback will be triggered, but might be called at other times, too. -NOTE: I<< This callback must always return a time that is later than the -passed C value >>. Not even C itself will do, it I be larger. +NOTE: I<< This callback must always return a time that is higher than or +equal to the passed C value >>. This can be used to create very complex timers, such as a timer that -triggers on each midnight, local time. To do this, you would calculate the +triggers on "next midnight, local time". To do this, you would calculate the next midnight after C and return the timestamp value for this. How you do this is, again, up to you (but it is not trivial, which is the main reason I omitted it as an example). @@ -1241,6 +1711,11 @@ a different time than the last time it was called (e.g. in a crond like program when the crontabs have changed). +=item ev_tstamp ev_periodic_at (ev_periodic *) + +When active, returns the absolute time that the watcher is supposed to +trigger next. + =item ev_tstamp offset [read-write] When repeating, this contains the offset value, otherwise this is the @@ -1255,51 +1730,48 @@ take effect when the periodic timer fires or C is being called. -=item ev_tstamp (*reschedule_cb)(struct ev_periodic *w, ev_tstamp now) [read-write] +=item ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write] The current reschedule callback, or C<0>, if this functionality is switched off. Can be changed any time, but changes only take effect when the periodic timer fires or C is being called. -=item ev_tstamp at [read-only] - -When active, contains the absolute time that the watcher is supposed to -trigger next. - =back +=head3 Examples + Example: Call a callback every hour, or, more precisely, whenever the -system clock is divisible by 3600. The callback invocation times have -potentially a lot of jittering, but good long-term stability. +system time is divisible by 3600. The callback invocation times have +potentially a lot of jitter, but good long-term stability. + + static void + clock_cb (struct ev_loop *loop, ev_io *w, int revents) + { + ... its now a full hour (UTC, or TAI or whatever your clock follows) + } - static void - clock_cb (struct ev_loop *loop, struct ev_io *w, int revents) - { - ... its now a full hour (UTC, or TAI or whatever your clock follows) - } - - struct ev_periodic hourly_tick; - ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0); - ev_periodic_start (loop, &hourly_tick); + ev_periodic hourly_tick; + ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0); + ev_periodic_start (loop, &hourly_tick); Example: The same as above, but use a reschedule callback to do it: - #include + #include - static ev_tstamp - my_scheduler_cb (struct ev_periodic *w, ev_tstamp now) - { - return fmod (now, 3600.) + 3600.; - } + static ev_tstamp + my_scheduler_cb (ev_periodic *w, ev_tstamp now) + { + return now + (3600. - fmod (now, 3600.)); + } - ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb); + ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb); Example: Call a callback every hour, starting now: - struct ev_periodic hourly_tick; - ev_periodic_init (&hourly_tick, clock_cb, - fmod (ev_now (loop), 3600.), 3600., 0); - ev_periodic_start (loop, &hourly_tick); + ev_periodic hourly_tick; + ev_periodic_init (&hourly_tick, clock_cb, + fmod (ev_now (loop), 3600.), 3600., 0); + ev_periodic_start (loop, &hourly_tick); =head2 C - signal me when a signal gets signalled! @@ -1309,12 +1781,22 @@ will try it's best to deliver signals synchronously, i.e. as part of the normal event processing, like any other event. +If you want signals asynchronously, just use C as you would +do without libev and forget about sharing the signal. You can even use +C from a signal handler to synchronously wake up an event loop. + You can configure as many watchers as you like per signal. Only when the -first watcher gets started will libev actually register a signal watcher -with the kernel (thus it coexists with your own signal handlers as long -as you don't register any with libev). Similarly, when the last signal -watcher for a signal is stopped libev will reset the signal handler to -SIG_DFL (regardless of what it was set to before). +first watcher gets started will libev actually register a signal handler +with the kernel (thus it coexists with your own signal handlers as long as +you don't register any with libev for the same signal). Similarly, when +the last signal watcher for a signal is stopped, libev will reset the +signal handler to SIG_DFL (regardless of what it was set to before). + +If possible and supported, libev will install its handlers with +C behaviour enabled, so system calls should not be unduly +interrupted. If you have a problem with system calls getting interrupted by +signals you can block all signals in an C watcher and unblock +them in an C watcher. =head3 Watcher-Specific Functions and Data Members @@ -1333,26 +1815,77 @@ =back +=head3 Examples + +Example: Try to exit cleanly on SIGINT. + + static void + sigint_cb (struct ev_loop *loop, ev_signal *w, int revents) + { + ev_unloop (loop, EVUNLOOP_ALL); + } + + ev_signal signal_watcher; + ev_signal_init (&signal_watcher, sigint_cb, SIGINT); + ev_signal_start (loop, &signal_watcher); + =head2 C - watch out for process status changes Child watchers trigger when your process receives a SIGCHLD in response to -some child status changes (most typically when a child of yours dies). +some child status changes (most typically when a child of yours dies or +exits). It is permissible to install a child watcher I the child +has been forked (which implies it might have already exited), as long +as the event loop isn't entered (or is continued from a watcher), i.e., +forking and then immediately registering a watcher for the child is fine, +but forking and registering a watcher a few event loop iterations later is +not. + +Only the default event loop is capable of handling signals, and therefore +you can only register child watchers in the default event loop. + +=head3 Process Interaction + +Libev grabs C as soon as the default event loop is +initialised. This is necessary to guarantee proper behaviour even if +the first child watcher is started after the child exits. The occurrence +of C is recorded asynchronously, but child reaping is done +synchronously as part of the event loop processing. Libev always reaps all +children, even ones not watched. + +=head3 Overriding the Built-In Processing + +Libev offers no special support for overriding the built-in child +processing, but if your application collides with libev's default child +handler, you can override it easily by installing your own handler for +C after initialising the default loop, and making sure the +default loop never gets destroyed. You are encouraged, however, to use an +event-based approach to child reaping and thus use libev's support for +that, so other libev users can use C watchers freely. + +=head3 Stopping the Child Watcher + +Currently, the child watcher never gets stopped, even when the +child terminates, so normally one needs to stop the watcher in the +callback. Future versions of libev might stop the watcher automatically +when a child exit is detected. =head3 Watcher-Specific Functions and Data Members =over 4 -=item ev_child_init (ev_child *, callback, int pid) +=item ev_child_init (ev_child *, callback, int pid, int trace) -=item ev_child_set (ev_child *, int pid) +=item ev_child_set (ev_child *, int pid, int trace) Configures the watcher to wait for status changes of process C (or I process if C is specified as C<0>). The callback can look at the C member of the C watcher structure to see the status word (use the macros from C and see your systems C documentation). The C member contains the pid of the -process causing the status change. +process causing the status change. C must be either C<0> (only +activate the watcher when the process terminates) or C<1> (additionally +activate the watcher when the process is stopped or continued). =item int pid [read-only] @@ -1369,54 +1902,153 @@ =back -Example: Try to exit cleanly on SIGINT and SIGTERM. +=head3 Examples - static void - sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents) - { - ev_unloop (loop, EVUNLOOP_ALL); - } - - struct ev_signal signal_watcher; - ev_signal_init (&signal_watcher, sigint_cb, SIGINT); - ev_signal_start (loop, &sigint_cb); +Example: C a new process and install a child handler to wait for +its completion. + + ev_child cw; + + static void + child_cb (EV_P_ ev_child *w, int revents) + { + ev_child_stop (EV_A_ w); + printf ("process %d exited with status %x\n", w->rpid, w->rstatus); + } + + pid_t pid = fork (); + + if (pid < 0) + // error + else if (pid == 0) + { + // the forked child executes here + exit (1); + } + else + { + ev_child_init (&cw, child_cb, pid, 0); + ev_child_start (EV_DEFAULT_ &cw); + } =head2 C - did the file attributes just change? -This watches a filesystem path for attribute changes. That is, it calls -C regularly (or when the OS says it changed) and sees if it changed -compared to the last time, invoking the callback if it did. +This watches a file system path for attribute changes. That is, it calls +C on that path in regular intervals (or when the OS says it changed) +and sees if it changed compared to the last time, invoking the callback if +it did. The path does not need to exist: changing from "path exists" to "path does -not exist" is a status change like any other. The condition "path does -not exist" is signified by the C field being zero (which is -otherwise always forced to be at least one) and all the other fields of -the stat buffer having unspecified contents. - -The path I be absolute and I end in a slash. If it is -relative and your working directory changes, the behaviour is undefined. - -Since there is no standard to do this, the portable implementation simply -calls C regularly on the path to see if it changed somehow. You -can specify a recommended polling interval for this case. If you specify -a polling interval of C<0> (highly recommended!) then a I value will be used (which you can expect to be around -five seconds, although this might change dynamically). Libev will also -impose a minimum interval which is currently around C<0.1>, but thats -usually overkill. +not exist" is a status change like any other. The condition "path does not +exist" (or more correctly "path cannot be stat'ed") is signified by the +C field being zero (which is otherwise always forced to be at +least one) and all the other fields of the stat buffer having unspecified +contents. + +The path I end in a slash or contain special components such as +C<.> or C<..>. The path I be absolute: If it is relative and +your working directory changes, then the behaviour is undefined. + +Since there is no portable change notification interface available, the +portable implementation simply calls C regularly on the path +to see if it changed somehow. You can specify a recommended polling +interval for this case. If you specify a polling interval of C<0> (highly +recommended!) then a I value will be used +(which you can expect to be around five seconds, although this might +change dynamically). Libev will also impose a minimum interval which is +currently around C<0.1>, but that's usually overkill. This watcher type is not meant for massive numbers of stat watchers, as even with OS-supported change notifications, this can be resource-intensive. -At the time of this writing, only the Linux inotify interface is -implemented (implementing kqueue support is left as an exercise for the -reader). Inotify will be used to give hints only and should not change the -semantics of C watchers, which means that libev sometimes needs -to fall back to regular polling again even with inotify, but changes are -usually detected immediately, and if the file exists there will be no -polling. +At the time of this writing, the only OS-specific interface implemented +is the Linux inotify interface (implementing kqueue support is left as an +exercise for the reader. Note, however, that the author sees no way of +implementing C semantics with kqueue, except as a hint). + +=head3 ABI Issues (Largefile Support) + +Libev by default (unless the user overrides this) uses the default +compilation environment, which means that on systems with large file +support disabled by default, you get the 32 bit version of the stat +structure. When using the library from programs that change the ABI to +use 64 bit file offsets the programs will fail. In that case you have to +compile libev with the same flags to get binary compatibility. This is +obviously the case with any flags that change the ABI, but the problem is +most noticeably displayed with ev_stat and large file support. + +The solution for this is to lobby your distribution maker to make large +file interfaces available by default (as e.g. FreeBSD does) and not +optional. Libev cannot simply switch on large file support because it has +to exchange stat structures with application programs compiled using the +default compilation environment. + +=head3 Inotify and Kqueue + +When C support has been compiled into libev and present at +runtime, it will be used to speed up change detection where possible. The +inotify descriptor will be created lazily when the first C +watcher is being started. + +Inotify presence does not change the semantics of C watchers +except that changes might be detected earlier, and in some cases, to avoid +making regular C calls. Even in the presence of inotify support +there are many cases where libev has to resort to regular C polling, +but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too +many bugs), the path exists (i.e. stat succeeds), and the path resides on +a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and +xfs are fully working) libev usually gets away without polling. + +There is no support for kqueue, as apparently it cannot be used to +implement this functionality, due to the requirement of having a file +descriptor open on the object at all times, and detecting renames, unlinks +etc. is difficult. + +=head3 C is a synchronous operation + +Libev doesn't normally do any kind of I/O itself, and so is not blocking +the process. The exception are C watchers - those call C, which is a synchronous operation. + +For local paths, this usually doesn't matter: unless the system is very +busy or the intervals between stat's are large, a stat call will be fast, +as the path data is usually in memory already (except when starting the +watcher). + +For networked file systems, calling C can block an indefinite +time due to network issues, and even under good conditions, a stat call +often takes multiple milliseconds. + +Therefore, it is best to avoid using C watchers on networked +paths, although this is fully supported by libev. + +=head3 The special problem of stat time resolution + +The C system call only supports full-second resolution portably, +and even on systems where the resolution is higher, most file systems +still only support whole seconds. + +That means that, if the time is the only thing that changes, you can +easily miss updates: on the first update, C detects a change and +calls your callback, which does something. When there is another update +within the same second, C will be unable to detect unless the +stat data does change in other ways (e.g. file size). + +The solution to this is to delay acting on a change for slightly more +than a second (or till slightly after the next full second boundary), using +a roughly one-second-delay C (e.g. C). + +The C<.02> offset is added to work around small timing inconsistencies +of some operating systems (where the second counter of the current time +might be be delayed. One such system is the Linux kernel, where a call to +C might return a timestamp with a full second later than +a subsequent C