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@@ -0,0 +1,725 @@ +=head1 NAME + +libev - a high performance full-featured event loop written in C + +=head1 SYNOPSIS + + #include <ev.h> + +=head1 DESCRIPTION + +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 +these event sources and provide your program events. + +To do this, it must take more or less complete control over your process +(or thread) by executing the I<event loop> handler, and will then +communicate events via a callback mechanism. + +You register interest in certain events by registering so-called I<event +watchers>, which are relatively small C structures you initialise with the +details of the event, and then hand it over to libev by I<starting> the +watcher. + +=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). + +=head1 CONVENTIONS + +Libev is very configurable. In this manual the default configuration +will be described, which supports multiple event loops. For more info +about various configuraiton options please have a look at the file +F<README.embed> in the libev distribution. If libev was configured without +support for multiple event loops, then all functions taking an initial +argument of name C<loop> (which is always of type C<struct ev_loop *>) +will not have this argument. + +=head1 TIME AND OTHER GLOBAL FUNCTIONS + +Libev represents time as a single floating point number. This type is +called C<ev_tstamp>, which is what you should use too. It usually aliases +to the double type in C. + +=over 4 + +=item ev_tstamp ev_time () + +Returns the current time as libev would use it. + +=item int ev_version_major () + +=item int ev_version_minor () + +You can find out the major and minor version numbers of the library +you linked against by calling the functions C<ev_version_major> and +C<ev_version_minor>. If you want, you can compare against the global +symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the +version of the library your program was compiled against. + +Usually, its a good idea to terminate if the major versions mismatch, +as this indicates an incompatible change. Minor versions are usually +compatible to older versions, so a larger minor version alone is usually +not a problem. + +=item ev_set_allocator (void *(*cb)(void *ptr, long size)) + +Sets the allocation function to use (the prototype is similar to the +realloc function). It is used to allocate and free memory (no surprises +here). If it returns zero when memory needs to be allocated, the library +might abort or take some potentially destructive action. The default is +your system realloc function. + +You could override this function in high-availability programs to, say, +free some memory if it cannot allocate memory, to use a special allocator, +or even to sleep a while and retry until some memory is available. + +=item ev_set_syserr_cb (void (*cb)(const char *msg)); + +Set the callback function to call on a retryable syscall error (such +as failed select, poll, epoll_wait). The message is a printable string +indicating the system call or subsystem causing the problem. If this +callback is set, then libev will expect it to remedy the sitution, no +matter what, when it returns. That is, libev will geenrally retry the +requested operation, or, if the condition doesn't go away, do bad stuff +(such as abort). + +=back + +=head1 FUNCTIONS CONTROLLING THE EVENT LOOP + +An event loop is described by a C<struct ev_loop *>. The library knows two +types of such loops, the I<default> loop, which supports signals and child +events, and dynamically created loops which do not. + +If you use threads, a common model is to run the default event loop +in your main thread (or in a separate thrad) and for each thread you +create, you also create another event loop. Libev itself does no lockign +whatsoever, so if you mix calls to different event loops, make sure you +lock (this is usually a bad idea, though, even if done right). + +=over 4 + +=item struct ev_loop *ev_default_loop (unsigned int flags) + +This will initialise the default event loop if it hasn't been initialised +yet and return it. If the default loop could not be initialised, returns +false. If it already was initialised it simply returns it (and ignores the +flags). + +If you don't know what event loop to use, use the one returned from this +function. + +The flags argument can be used to specify special behaviour or specific +backends to use, and is usually specified as 0 (or EVFLAG_AUTO) + +It supports the following flags: + +=over 4 + +=item EVFLAG_AUTO + +The default flags value. Use this if you have no clue (its the right +thing, believe me). + +=item EVFLAG_NOENV + +If this flag bit is ored into the flag value then libev will I<not> look +at the environment variable C<LIBEV_FLAGS>. Otherwise (the default), this +environment variable will override the flags completely. This is useful +to try out specific backends to tets their performance, or to work around +bugs. + +=item EVMETHOD_SELECT portable select backend + +=item EVMETHOD_POLL poll backend (everywhere except windows) + +=item EVMETHOD_EPOLL linux only + +=item EVMETHOD_KQUEUE some bsds only + +=item EVMETHOD_DEVPOLL solaris 8 only + +=item EVMETHOD_PORT solaris 10 only + +If one or more of these are ored into the flags value, then only these +backends will be tried (in the reverse order as given here). If one are +specified, any backend will do. + +=back + +=item struct ev_loop *ev_loop_new (unsigned int flags) + +Similar to C<ev_default_loop>, but always creates a new event loop that is +always distinct from the default loop. Unlike the default loop, it cannot +handle signal and child watchers, and attempts to do so will be greeted by +undefined behaviour (or a failed assertion if assertions are enabled). + +=item ev_default_destroy () + +Destroys the default loop again (frees all memory and kernel state +etc.). This stops all registered event watchers (by not touching them in +any way whatsoever, although you cnanot rely on this :). + +=item ev_loop_destroy (loop) + +Like C<ev_default_destroy>, but destroys an event loop created by an +earlier call to C<ev_loop_new>. + +=item ev_default_fork () + +This function reinitialises the kernel state for backends that have +one. Despite the name, you can call it anytime, but it makes most sense +after forking, in either the parent or child process (or both, but that +again makes little sense). + +You I<must> call this function after forking if and only if you want to +use the event library in both processes. If you just fork+exec, you don't +have to call it. + +The function itself is quite fast and its usually not a problem to call +it just in case after a fork. To make this easy, the function will fit in +quite nicely into a call to C<pthread_atfork>: + + pthread_atfork (0, 0, ev_default_fork); + +=item ev_loop_fork (loop) + +Like C<ev_default_fork>, but acts on an event loop created by +C<ev_loop_new>. Yes, you have to call this on every allocated event loop +after fork, and how you do this is entirely your own problem. + +=item unsigned int ev_method (loop) + +Returns one of the C<EVMETHOD_*> flags indicating the event backend in +use. + +=item ev_tstamp = ev_now (loop) + +Returns the current "event loop time", which is the time the event loop +got events and started processing them. This timestamp does not change +as long as callbacks are being processed, and this is also the base time +used for relative timers. You can treat it as the timestamp of the event +occuring (or more correctly, the mainloop finding out about it). + +=item ev_loop (loop, int flags) + +Finally, this is it, the event handler. This function usually is called +after you initialised all your watchers and you want to start handling +events. + +If the flags argument is specified as 0, it will not return until either +no event watchers are active anymore or C<ev_unloop> was called. + +A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle +those events and any outstanding ones, but will not block your process in +case there are no events. + +A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if +neccessary) and will handle those and any outstanding ones. It will block +your process until at least one new event arrives. + +This flags value could be used to implement alternative looping +constructs, but the C<prepare> and C<check> watchers provide a better and +more generic mechanism. + +=item ev_unloop (loop, how) + +Can be used to make a call to C<ev_loop> return early. The C<how> argument +must be either C<EVUNLOOP_ONCE>, which will make the innermost C<ev_loop> +call return, or C<EVUNLOOP_ALL>, which will make all nested C<ev_loop> +calls return. + +=item ev_ref (loop) + +=item ev_unref (loop) + +Ref/unref can be used to add or remove a refcount on the event loop: Every +watcher keeps one reference. If you have a long-runing watcher you never +unregister that should not keep ev_loop from running, ev_unref() after +starting, and ev_ref() before stopping it. Libev itself uses this for +example for its internal signal pipe: It is not visible to you as a user +and should not keep C<ev_loop> from exiting if the work is done. It is +also an excellent way to do this for generic recurring timers or from +within third-party libraries. Just remember to unref after start and ref +before stop. + +=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: + + 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<ev_init +(watcher *, callback)>, 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_<type>_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_<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_<type>_start (loop, watcher +*) >>), and you can stop watching for events at any time by calling the +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 set method. + +You cna check wether an event is active by calling the C<ev_is_active +(watcher *)> macro. To see wether an event is outstanding (but the +callback for it has not been called yet) you cna use the C<ev_is_pending +(watcher *)> 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 +(you can receive multiple events at the same time). The possible bit masks +are: + +=over 4 + +=item EV_READ + +=item EV_WRITE + +The file descriptor in the ev_io watcher has become readable and/or +writable. + +=item EV_TIMEOUT + +The ev_timer watcher has timed out. + +=item EV_PERIODIC + +The ev_periodic watcher has timed out. + +=item EV_SIGNAL + +The signal specified in the ev_signal watcher has been received by a thread. + +=item EV_CHILD + +The pid specified in the ev_child watcher has received a status change. + +=item EV_IDLE + +The ev_idle watcher has determined that you have nothing better to do. + +=item EV_PREPARE + +=item EV_CHECK + +All ev_prepare watchers are invoked just I<before> C<ev_loop> starts +to gather new events, and all ev_check watchers are invoked just after +C<ev_loop> 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 +C<ev_loop> from blocking). + +=item EV_ERROR + +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 ASSOCIATING CUSTOM DATA WITH A WATCHER + +Each watcher has, by default, a member C<void *data> that you can change +and read at any time, libev will completely ignore it. This cna 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 catsing your callback type +have been omitted.... + + +=head1 WATCHER TYPES + +This section describes each watcher in detail, but will not repeat +information given in the last section. + +=head2 struct ev_io - is my file descriptor readable or writable + +I/O watchers check wether 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). + +=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 ev_io watcher. The fd is the file descriptor to rceeive +events for and events is either C<EV_READ>, C<EV_WRITE> or C<EV_READ | +EV_WRITE> to receive the given events. + +=back + +=head2 struct ev_timer - relative and optionally recurring 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 +time, it will still time out after (roughly) and hour. "Roughly" because +detecting time jumps is hard, and soem inaccuracies are unavoidable (the +monotonic clock option helps a lot here). + +=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<after> seconds. If C<repeat> is +C<0.>, then it will automatically be stopped. If it is positive, then the +timer will automatically be configured to trigger again C<repeat> 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 (ecause 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 started but nonrepeating, stop it. + +If the timer is repeating, either start it if necessary (with the repeat +value), or reset the running timer to the repeat 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. + +=back + +=head2 ev_periodic + +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) +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<ev_now () ++ 10.>) 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). + +They can also be used to implement vastly more complex timers, such as +triggering an event on eahc midnight, local time. + +=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 (interval = reschedule_cb = 0) + +In this configuration the watcher triggers an event at the wallclock time +C<at> 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) + +In this mode the watcher will always be scheduled to time out at the next +C<at + N * interval> time (for some integer N) 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 correct, 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 +time where C<time = at (mod interval)>, regardless of any time jumps. + +=item * manual reschedule mode (reschedule_cb = callback) + +In this mode the values for C<interval> and C<at> are both being +ignored. Instead, each time the periodic watcher gets scheduled, the +reschedule callback will be called with the watcher as first, and the +current time as second argument. + +NOTE: I<This callback MUST NOT stop or destroy the periodic or any other +periodic watcher, ever, or make any event loop modificstions>. If you need +to stop it, return 1e30 (or so, fudge fudge) and stop it afterwards. + +Its prototype is c<ev_tstamp (*reschedule_cb)(struct ev_periodic *w, +ev_tstamp now)>, e.g.: + + static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now) + { + return now + 60.; + } + +It must return the next time to trigger, based on the passed time value +(that is, the lowest time value larger than to the second argument). It +will usually be called just before the callback will be triggered, but +might be called at other times, too. + +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 +next midnight after C<now> and return the timestamp value for this. How you do this +is, again, up to you (but it is not trivial). + +=back + +=item ev_periodic_again (loop, ev_periodic *) + +Simply stops and restarts the periodic watcher again. This is only useful +when you changed some parameters or the reschedule callback would return +a different time than the last time it was called (e.g. in a crond like +program when the crontabs have changed). + +=back + +=head2 ev_signal - signal me when a signal gets signalled + +Signal watchers will trigger an event when the process receives a specific +signal one or more times. Even though signals are very asynchronous, libev +will try its best to deliver signals synchronously, i.e. as part of the +normal event processing, like any other event. + +You cna 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). + +=over 4 + +=item ev_signal_init (ev_signal *, callback, int signum) + +=item ev_signal_set (ev_signal *, int signum) + +Configures the watcher to trigger on the given signal number (usually one +of the C<SIGxxx> constants). + +=back + +=head2 ev_child - wait for pid 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). + +=over 4 + +=item ev_child_init (ev_child *, callback, int pid) + +=item ev_child_set (ev_child *, int pid) + +Configures the watcher to wait for status changes of process C<pid> (or +I<any> process if C<pid> is specified as C<0>). The callback can look +at the C<rstatus> member of the C<ev_child> watcher structure to see +the status word (use the macros from C<sys/wait.h>). The C<rpid> member +contains the pid of the process causing the status change. + +=back + +=head2 ev_idle - when you've got nothing better to do + +Idle watchers trigger events when there are no other I/O or timer (or +periodic) events pending. That is, as long as your process is busy +handling sockets or timeouts it will not be called. But when your process +is idle all idle watchers are being called again and again - until +stopped, that is, or your process receives more events. + +The most noteworthy effect is that as long as any idle watchers are +active, the process will not block when waiting for new events. + +Apart from keeping your process non-blocking (which is a useful +effect on its own sometimes), idle watchers are a good place to do +"pseudo-background processing", or delay processing stuff to after the +event loop has handled all outstanding events. + +=over 4 + +=item ev_idle_init (ev_signal *, callback) + +Initialises and configures the idle watcher - it has no parameters of any +kind. There is a C<ev_idle_set> macro, but using it is utterly pointless, +believe me. + +=back + +=head2 prepare and check - your hooks into the event loop + +Prepare and check watchers usually (but not always) are used in +tandom. Prepare watchers get invoked before the process blocks and check +watchers afterwards. + +Their main purpose is to integrate other event mechanisms into libev. This +could be used, for example, to track variable changes, implement your own +watchers, integrate net-snmp or a coroutine library and lots more. + +This is done by examining in each prepare call which file descriptors need +to be watched by the other library, registering ev_io watchers for them +and starting an ev_timer watcher for any timeouts (many libraries provide +just this functionality). Then, in the check watcher you check for any +events that occured (by making your callbacks set soem flags for example) +and call back into the library. + +As another example, the perl Coro module uses these hooks to integrate +coroutines into libev programs, by yielding to other active coroutines +during each prepare and only letting the process block if no coroutines +are ready to run. + +=over 4 + +=item ev_prepare_init (ev_prepare *, callback) + +=item ev_check_init (ev_check *, callback) + +Initialises and configures the prepare or check watcher - they have no +parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set> +macros, but using them is utterly, utterly pointless. + +=back + +=head1 OTHER FUNCTIONS + +There are some other fucntions of possible interest. Described. Here. Now. + +=over 4 + +=item ev_once (loop, int fd, int events, ev_tstamp timeout, callback) + +This function combines a simple timer and an I/O watcher, calls your +callback on whichever event happens first and automatically stop both +watchers. This is useful if you want to wait for a single event on an fd +or timeout without havign to allocate/configure/start/stop/free one or +more watchers yourself. + +If C<fd> is less than 0, then no I/O watcher will be started and events is +ignored. Otherwise, an ev_io watcher for the given C<fd> and C<events> set +will be craeted and started. + +If C<timeout> is less than 0, then no timeout watcher will be +started. Otherwise an ev_timer watcher with after = C<timeout> (and repeat += 0) will be started. + +The callback has the type C<void (*cb)(int revents, void *arg)> and +gets passed an events set (normally a combination of EV_ERROR, EV_READ, +EV_WRITE or EV_TIMEOUT) and the C<arg> value passed to C<ev_once>: + + static void stdin_ready (int revents, void *arg) + { + if (revents & EV_TIMEOUT) + /* doh, nothing entered */ + else if (revents & EV_READ) + /* stdin might have data for us, joy! */ + } + + ev_once (STDIN_FILENO, EV_READm 10., stdin_ready, 0); + +=item ev_feed_event (loop, watcher, int events) + +Feeds the given event set into the event loop, as if the specified event +has happened for the specified watcher (which must be a pointer to an +initialised but not necessarily active event watcher). + +=item ev_feed_fd_event (loop, int fd, int revents) + +Feed an event on the given fd, as if a file descriptor backend detected it. + +=item ev_feed_signal_event (loop, int signum) + +Feed an event as if the given signal occured (loop must be the default loop!). + +=back + +=head1 AUTHOR + +Marc Lehmann <libev@schmorp.de>. + |