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diff --git a/libev/ev.pod b/libev/ev.pod new file mode 100644 index 0000000..6cd777e --- /dev/null +++ b/libev/ev.pod @@ -0,0 +1,5234 @@ +=head1 NAME + +libev - a high performance full-featured event loop written in C + +=head1 SYNOPSIS + + #include <ev.h> + +=head2 EXAMPLE PROGRAM + + // a single header file is required + #include <ev.h> + + #include <stdio.h> // for puts + + // every watcher type has its own typedef'd struct + // with the name ev_TYPE + ev_io stdin_watcher; + ev_timer timeout_watcher; + + // 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_run's to stop iterating + ev_break (EV_A_ EVBREAK_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_run to stop iterating + ev_break (EV_A_ EVBREAK_ONE); + } + + int + main (void) + { + // use the default event loop unless you have special needs + struct ev_loop *loop = EV_DEFAULT; + + // 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_run (loop, 0); + + // unloop was called, so exit + return 0; + } + +=head1 ABOUT THIS DOCUMENT + +This document documents the libev software package. + +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<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>. + +While this document tries to be as complete as possible in documenting +libev, its usage and the rationale behind its design, it is not a tutorial +on event-based programming, nor will it introduce event-based programming +with libev. + +Familiarity with event based programming techniques in general is assumed +throughout this document. + +=head1 WHAT TO READ WHEN IN A HURRY + +This manual tries to be very detailed, but unfortunately, this also makes +it very long. If you just want to know the basics of libev, I suggest +reading L<ANATOMY OF A WATCHER>, then the L<EXAMPLE PROGRAM> above and +look up the missing functions in L<GLOBAL FUNCTIONS> and the C<ev_io> and +C<ev_timer> sections in L<WATCHER TYPES>. + +=head1 ABOUT LIBEV + +Libev is an event loop: you register interest in certain events (such as a +file descriptor being readable or a timeout occurring), and it will manage +these event sources and provide your program with events. + +To do this, it must take more or less complete control over your process +(or thread) by executing the I<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. + +=head2 FEATURES + +Libev supports C<select>, C<poll>, the Linux-specific C<epoll>, the +BSD-specific C<kqueue> and the Solaris-specific event port mechanisms +for file descriptor events (C<ev_io>), the Linux C<inotify> interface +(for C<ev_stat>), Linux eventfd/signalfd (for faster and cleaner +inter-thread wakeup (C<ev_async>)/signal handling (C<ev_signal>)) relative +timers (C<ev_timer>), absolute timers with customised rescheduling +(C<ev_periodic>), synchronous signals (C<ev_signal>), process status +change events (C<ev_child>), and event watchers dealing with the event +loop mechanism itself (C<ev_idle>, C<ev_embed>, C<ev_prepare> and +C<ev_check> watchers) as well as file watchers (C<ev_stat>) and even +limited support for fork events (C<ev_fork>). + +It also is quite fast (see this +L<benchmark|http://libev.schmorp.de/bench.html> comparing it to libevent +for example). + +=head2 CONVENTIONS + +Libev is very configurable. In this manual the default (and most common) +configuration will be described, which supports multiple event loops. For +more info about various configuration options please have a look at +B<EMBED> section in this manual. 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. + +=head2 TIME REPRESENTATION + +Libev represents time as a single floating point number, representing +the (fractional) number of seconds since the (POSIX) epoch (in practice +somewhere near the beginning of 1970, details are complicated, don't +ask). This type is called C<ev_tstamp>, which is what you should use +too. It usually aliases to the C<double> type in C. When you need to do +any calculations on it, you should treat it as some floating point value. + +Unlike the name component C<stamp> might indicate, it is also used for +time differences (e.g. delays) throughout libev. + +=head1 ERROR HANDLING + +Libev knows three classes of errors: operating system errors, usage errors +and internal errors (bugs). + +When libev catches an operating system error it cannot handle (for example +a system call indicating a condition libev cannot fix), it calls the callback +set via C<ev_set_syserr_cb>, which is supposed to fix the problem or +abort. The default is to print a diagnostic message and to call C<abort +()>. + +When libev detects a usage error such as a negative timer interval, then +it will print a diagnostic message and abort (via the C<assert> mechanism, +so C<NDEBUG> will disable this checking): these are programming errors in +the libev caller and need to be fixed there. + +Libev also has a few internal error-checking C<assert>ions, and also has +extensive consistency checking code. These do not trigger under normal +circumstances, as they indicate either a bug in libev or worse. + + +=head1 GLOBAL FUNCTIONS + +These functions can be called anytime, even before initialising the +library in any way. + +=over 4 + +=item ev_tstamp ev_time () + +Returns the current time as libev would use it. Please note that the +C<ev_now> function is usually faster and also often returns the timestamp +you actually want to know. Also interesting is the combination of +C<ev_update_now> and C<ev_now>. + +=item ev_sleep (ev_tstamp interval) + +Sleep for the given interval: The current thread will be blocked until +either it is interrupted or the given time interval has passed. Basically +this is a sub-second-resolution C<sleep ()>. + +=item int ev_version_major () + +=item int ev_version_minor () + +You can find out the major and minor ABI 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. + +These version numbers refer to the ABI version of the library, not the +release version. + +Usually, it's 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. + +Example: Make sure we haven't accidentally been linked against the wrong +version (note, however, that this will not detect other ABI mismatches, +such as LFS or reentrancy). + + assert (("libev version mismatch", + ev_version_major () == EV_VERSION_MAJOR + && ev_version_minor () >= EV_VERSION_MINOR)); + +=item unsigned int ev_supported_backends () + +Return the set of all backends (i.e. their corresponding C<EV_BACKEND_*> +value) compiled into this binary of libev (independent of their +availability on the system you are running on). See C<ev_default_loop> for +a description of the set values. + +Example: make sure we have the epoll method, because yeah this is cool and +a must have and can we have a torrent of it please!!!11 + + assert (("sorry, no epoll, no sex", + ev_supported_backends () & EVBACKEND_EPOLL)); + +=item unsigned int ev_recommended_backends () + +Return the set of all backends compiled into this binary of libev and +also recommended for this platform, meaning it will work for most file +descriptor types. This set is often smaller than the one returned by +C<ev_supported_backends>, as for example kqueue is broken on most BSDs +and will not be auto-detected unless you explicitly request it (assuming +you know what you are doing). This is the set of backends that libev will +probe for if you specify no backends explicitly. + +=item unsigned int ev_embeddable_backends () + +Returns the set of backends that are embeddable in other event loops. This +value is platform-specific but can include backends not available on the +current system. To find which embeddable backends might be supported on +the current system, you would need to look at C<ev_embeddable_backends () +& ev_supported_backends ()>, likewise for recommended ones. + +See the description of C<ev_embed> watchers for more info. + +=item ev_set_allocator (void *(*cb)(void *ptr, long size)) + +Sets the allocation function to use (the prototype is similar - the +semantics are identical to the C<realloc> C89/SuS/POSIX function). It is +used to allocate and free memory (no surprises here). If it returns zero +when memory needs to be allocated (C<size != 0>), the library might abort +or take some potentially destructive action. + +Since some systems (at least OpenBSD and Darwin) fail to implement +correct C<realloc> semantics, libev will use a wrapper around the system +C<realloc> and C<free> functions by default. + +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. + +Example: Replace the libev allocator with one that waits a bit and then +retries (example requires a standards-compliant C<realloc>). + + static void * + persistent_realloc (void *ptr, size_t size) + { + for (;;) + { + void *newptr = realloc (ptr, size); + + if (newptr) + return newptr; + + sleep (60); + } + } + + ... + ev_set_allocator (persistent_realloc); + +=item ev_set_syserr_cb (void (*cb)(const char *msg)) + +Set the callback function to call on a retryable system call 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 situation, no +matter what, when it returns. That is, libev will generally retry the +requested operation, or, if the condition doesn't go away, do bad stuff +(such as abort). + +Example: This is basically the same thing that libev does internally, too. + + static void + fatal_error (const char *msg) + { + perror (msg); + abort (); + } + + ... + ev_set_syserr_cb (fatal_error); + +=item ev_feed_signal (int signum) + +This function can be used to "simulate" a signal receive. It is completely +safe to call this function at any time, from any context, including signal +handlers or random threads. + +Its main use is to customise signal handling in your process, especially +in the presence of threads. For example, you could block signals +by default in all threads (and specifying C<EVFLAG_NOSIGMASK> when +creating any loops), and in one thread, use C<sigwait> or any other +mechanism to wait for signals, then "deliver" them to libev by calling +C<ev_feed_signal>. + +=back + +=head1 FUNCTIONS CONTROLLING EVENT LOOPS + +An event loop is described by a C<struct ev_loop *> (the C<struct> is +I<not> optional in this case unless libev 3 compatibility is disabled, as +libev 3 had an C<ev_loop> function colliding with the struct name). + +The library knows two types of such loops, the I<default> loop, which +supports child process events, and dynamically created event loops which +do not. + +=over 4 + +=item struct ev_loop *ev_default_loop (unsigned int flags) + +This returns the "default" event loop object, which is what you should +normally use when you just need "the event loop". Event loop objects and +the C<flags> parameter are described in more detail in the entry for +C<ev_loop_new>. + +If the default loop is already initialised then this function simply +returns it (and ignores the flags. If that is troubling you, check +C<ev_backend ()> afterwards). Otherwise it will create it with the given +flags, which should almost always be C<0>, unless the caller is also the +one calling C<ev_run> or otherwise qualifies as "the main program". + +If you don't know what event loop to use, use the one returned from this +function (or via the C<EV_DEFAULT> macro). + +Note that this function is I<not> thread-safe, so if you want to use it +from multiple threads, you have to employ some kind of mutex (note also +that this case is unlikely, as loops cannot be shared easily between +threads anyway). + +The default loop is the only loop that can handle C<ev_child> watchers, +and to do this, it always registers a handler for C<SIGCHLD>. If this is +a problem for your application you can either create a dynamic loop with +C<ev_loop_new> which doesn't do that, or you can simply overwrite the +C<SIGCHLD> signal handler I<after> calling C<ev_default_init>. + +Example: This is the most typical usage. + + if (!ev_default_loop (0)) + fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?"); + +Example: Restrict libev to the select and poll backends, and do not allow +environment settings to be taken into account: + + ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV); + +=item struct ev_loop *ev_loop_new (unsigned int flags) + +This will create and initialise a new event loop object. If the loop +could not be initialised, returns false. + +This function is thread-safe, and one common way to use libev with +threads is indeed to create one loop per thread, and using the default +loop in the "main" or "initial" thread. + +The flags argument can be used to specify special behaviour or specific +backends to use, and is usually specified as C<0> (or C<EVFLAG_AUTO>). + +The following flags are supported: + +=over 4 + +=item C<EVFLAG_AUTO> + +The default flags value. Use this if you have no clue (it's the right +thing, believe me). + +=item C<EVFLAG_NOENV> + +If this flag bit is or'ed into the flag value (or the program runs setuid +or setgid) then libev will I<not> look at the environment variable +C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will +override the flags completely if it is found in the environment. This is +useful to try out specific backends to test their performance, or to work +around bugs. + +=item C<EVFLAG_FORKCHECK> + +Instead of calling C<ev_loop_fork> manually after a fork, you can also +make libev check for a fork in each iteration by enabling this flag. + +This works by calling C<getpid ()> on every iteration of the loop, +and thus this might slow down your event loop if you do a lot of loop +iterations and little real work, but is usually not noticeable (on my +GNU/Linux system for example, C<getpid> is actually a simple 5-insn sequence +without a system call and thus I<very> fast, but my GNU/Linux system also has +C<pthread_atfork> which is even faster). + +The big advantage of this flag is that you can forget about fork (and +forget about forgetting to tell libev about forking) when you use this +flag. + +This flag setting cannot be overridden or specified in the C<LIBEV_FLAGS> +environment variable. + +=item C<EVFLAG_NOINOTIFY> + +When this flag is specified, then libev will not attempt to use the +I<inotify> API for its C<ev_stat> watchers. Apart from debugging and +testing, this flag can be useful to conserve inotify file descriptors, as +otherwise each loop using C<ev_stat> watchers consumes one inotify handle. + +=item C<EVFLAG_SIGNALFD> + +When this flag is specified, then libev will attempt to use the +I<signalfd> API for its C<ev_signal> (and C<ev_child>) watchers. This API +delivers signals synchronously, which makes it both faster and might make +it possible to get the queued signal data. It can also simplify signal +handling with threads, as long as you properly block signals in your +threads that are not interested in handling them. + +Signalfd will not be used by default as this changes your signal mask, and +there are a lot of shoddy libraries and programs (glib's threadpool for +example) that can't properly initialise their signal masks. + +=item C<EVFLAG_NOSIGMASK> + +When this flag is specified, then libev will avoid to modify the signal +mask. Specifically, this means you ahve to make sure signals are unblocked +when you want to receive them. + +This behaviour is useful when you want to do your own signal handling, or +want to handle signals only in specific threads and want to avoid libev +unblocking the signals. + +This flag's behaviour will become the default in future versions of libev. + +=item C<EVBACKEND_SELECT> (value 1, portable select backend) + +This is your standard select(2) backend. Not I<completely> standard, as +libev tries to roll its own fd_set with no limits on the number of fds, +but if that fails, expect a fairly low limit on the number of fds when +using this backend. It doesn't scale too well (O(highest_fd)), but its +usually the fastest backend for a low number of (low-numbered :) fds. + +To get good performance out of this backend you need a high amount of +parallelism (most of the file descriptors should be busy). If you are +writing a server, you should C<accept ()> in a loop to accept as many +connections as possible during one iteration. You might also want to have +a look at C<ev_set_io_collect_interval ()> to increase the amount of +readiness notifications you get per iteration. + +This backend maps C<EV_READ> to the C<readfds> set and C<EV_WRITE> to the +C<writefds> set (and to work around Microsoft Windows bugs, also onto the +C<exceptfds> set on that platform). + +=item C<EVBACKEND_POLL> (value 2, poll backend, available everywhere except on windows) + +And this is your standard poll(2) backend. It's more complicated +than select, but handles sparse fds better and has no artificial +limit on the number of fds you can use (except it will slow down +considerably with a lot of inactive fds). It scales similarly to select, +i.e. O(total_fds). See the entry for C<EVBACKEND_SELECT>, above, for +performance tips. + +This backend maps C<EV_READ> to C<POLLIN | POLLERR | POLLHUP>, and +C<EV_WRITE> to C<POLLOUT | POLLERR | POLLHUP>. + +=item C<EVBACKEND_EPOLL> (value 4, Linux) + +Use the linux-specific epoll(7) interface (for both pre- and post-2.6.9 +kernels). + +For few fds, this backend is a bit little slower than poll and select, +but it scales phenomenally better. While poll and select usually scale +like O(total_fds) where n is the total number of fds (or the highest fd), +epoll scales either O(1) or O(active_fds). + +The epoll mechanism deserves honorable mention as the most misdesigned +of the more advanced event mechanisms: mere annoyances include silently +dropping file descriptors, requiring a system call per change per file +descriptor (and unnecessary guessing of parameters), problems with dup, +returning before the timeout value, resulting in additional iterations +(and only giving 5ms accuracy while select on the same platform gives +0.1ms) and so on. The biggest issue is fork races, however - if a program +forks then I<both> parent and child process have to recreate the epoll +set, which can take considerable time (one syscall per file descriptor) +and is of course hard to detect. + +Epoll is also notoriously buggy - embedding epoll fds I<should> work, but +of course I<doesn't>, and epoll just loves to report events for totally +I<different> file descriptors (even already closed ones, so one cannot +even remove them from the set) than registered in the set (especially +on SMP systems). Libev tries to counter these spurious notifications by +employing an additional generation counter and comparing that against the +events to filter out spurious ones, recreating the set when required. Last +not least, it also refuses to work with some file descriptors which work +perfectly fine with C<select> (files, many character devices...). + +Epoll is truly the train wreck analog among event poll mechanisms, +a frankenpoll, cobbled together in a hurry, no thought to design or +interaction with others. + +While stopping, setting and starting an I/O watcher in the same iteration +will result in some caching, there is still a system call per such +incident (because the same I<file descriptor> could point to a different +I<file description> now), so its best to avoid that. Also, C<dup ()>'ed +file descriptors might not work very well if you register events for both +file descriptors. + +Best performance from this backend is achieved by not unregistering all +watchers for a file descriptor until it has been closed, if possible, +i.e. keep at least one watcher active per fd at all times. Stopping and +starting a watcher (without re-setting it) also usually doesn't cause +extra overhead. A fork can both result in spurious notifications as well +as in libev having to destroy and recreate the epoll object, which can +take considerable time and thus should be avoided. + +All this means that, in practice, C<EVBACKEND_SELECT> can be as fast or +faster than epoll for maybe up to a hundred file descriptors, depending on +the usage. So sad. + +While nominally embeddable in other event loops, this feature is broken in +all kernel versions tested so far. + +This backend maps C<EV_READ> and C<EV_WRITE> in the same way as +C<EVBACKEND_POLL>. + +=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones) + +Kqueue deserves special mention, as at the time of this writing, it +was broken on all BSDs except NetBSD (usually it doesn't work reliably +with anything but sockets and pipes, except on Darwin, where of course +it's completely useless). Unlike epoll, however, whose brokenness +is by design, these kqueue bugs can (and eventually will) be fixed +without API changes to existing programs. For this reason it's not being +"auto-detected" unless you explicitly specify it in the flags (i.e. using +C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough) +system like NetBSD. + +You still can embed kqueue into a normal poll or select backend and use it +only for sockets (after having made sure that sockets work with kqueue on +the target platform). See C<ev_embed> watchers for more info. + +It scales in the same way as the epoll backend, but the interface to the +kernel is more efficient (which says nothing about its actual speed, of +course). While stopping, setting and starting an I/O watcher does never +cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to +two event changes per incident. Support for C<fork ()> is very bad (but +sane, unlike epoll) and it drops fds silently in similarly hard-to-detect +cases + +This backend usually performs well under most conditions. + +While nominally embeddable in other event loops, this doesn't work +everywhere, so you might need to test for this. And since it is broken +almost everywhere, you should only use it when you have a lot of sockets +(for which it usually works), by embedding it into another event loop +(e.g. C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> (but C<poll> is of course +also broken on OS X)) and, did I mention it, using it only for sockets. + +This backend maps C<EV_READ> into an C<EVFILT_READ> kevent with +C<NOTE_EOF>, and C<EV_WRITE> into an C<EVFILT_WRITE> kevent with +C<NOTE_EOF>. + +=item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8) + +This is not implemented yet (and might never be, unless you send me an +implementation). According to reports, C</dev/poll> only supports sockets +and is not embeddable, which would limit the usefulness of this backend +immensely. + +=item C<EVBACKEND_PORT> (value 32, Solaris 10) + +This uses the Solaris 10 event port mechanism. As with everything on Solaris, +it's really slow, but it still scales very well (O(active_fds)). + +While this backend scales well, it requires one system call per active +file descriptor per loop iteration. For small and medium numbers of file +descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend +might perform better. + +On the positive side, this backend actually performed fully to +specification in all tests and is fully embeddable, which is a rare feat +among the OS-specific backends (I vastly prefer correctness over speed +hacks). + +On the negative side, the interface is I<bizarre> - so bizarre that +even sun itself gets it wrong in their code examples: The event polling +function sometimes returning events to the caller even though an error +occurred, but with no indication whether it has done so or not (yes, it's +even documented that way) - deadly for edge-triggered interfaces where +you absolutely have to know whether an event occurred or not because you +have to re-arm the watcher. + +Fortunately libev seems to be able to work around these idiocies. + +This backend maps C<EV_READ> and C<EV_WRITE> in the same way as +C<EVBACKEND_POLL>. + +=item C<EVBACKEND_ALL> + +Try all backends (even potentially broken ones that wouldn't be tried +with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as +C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>. + +It is definitely not recommended to use this flag, use whatever +C<ev_recommended_backends ()> returns, or simply do not specify a backend +at all. + +=item C<EVBACKEND_MASK> + +Not a backend at all, but a mask to select all backend bits from a +C<flags> value, in case you want to mask out any backends from a flags +value (e.g. when modifying the C<LIBEV_FLAGS> environment variable). + +=back + +If one or more of the backend flags are or'ed into the flags value, +then only these backends will be tried (in the reverse order as listed +here). If none are specified, all backends in C<ev_recommended_backends +()> will be tried. + +Example: Try to create a event loop that uses epoll and nothing else. + + struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV); + if (!epoller) + fatal ("no epoll found here, maybe it hides under your chair"); + +Example: Use whatever libev has to offer, but make sure that kqueue is +used if available. + + struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE); + +=item ev_loop_destroy (loop) + +Destroys an event loop object (frees all memory and kernel state +etc.). None of the active event watchers will be stopped in the normal +sense, so e.g. C<ev_is_active> might still return true. It is your +responsibility to either stop all watchers cleanly yourself I<before> +calling this function, or cope with the fact afterwards (which is usually +the easiest thing, you can just ignore the watchers and/or C<free ()> them +for example). + +Note that certain global state, such as signal state (and installed signal +handlers), will not be freed by this function, and related watchers (such +as signal and child watchers) would need to be stopped manually. + +This function is normally used on loop objects allocated by +C<ev_loop_new>, but it can also be used on the default loop returned by +C<ev_default_loop>, in which case it is not thread-safe. + +Note that it is not advisable to call this function on the default loop +except in the rare occasion where you really need to free its resources. +If you need dynamically allocated loops it is better to use C<ev_loop_new> +and C<ev_loop_destroy>. + +=item ev_loop_fork (loop) + +This function sets a flag that causes subsequent C<ev_run> iterations to +reinitialise the kernel state for backends that have one. Despite the +name, you can call it anytime, but it makes most sense after forking, in +the child process. You I<must> call it (or use C<EVFLAG_FORKCHECK>) in the +child before resuming or calling C<ev_run>. + +Again, you I<have> to call it on I<any> loop that you want to re-use after +a fork, I<even if you do not plan to use the loop in the parent>. This is +because some kernel interfaces *cough* I<kqueue> *cough* do funny things +during fork. + +On the other hand, you only need to call this function in the child +process if and only if you want to use the event loop in the child. If +you just fork+exec or create a new loop in the child, you don't have to +call it at all (in fact, C<epoll> is so badly broken that it makes a +difference, but libev will usually detect this case on its own and do a +costly reset of the backend). + +The function itself is quite fast and it's usually not a problem to call +it just in case after a fork. + +Example: Automate calling C<ev_loop_fork> on the default loop when +using pthreads. + + static void + post_fork_child (void) + { + ev_loop_fork (EV_DEFAULT); + } + + ... + pthread_atfork (0, 0, post_fork_child); + +=item int ev_is_default_loop (loop) + +Returns true when the given loop is, in fact, the default loop, and false +otherwise. + +=item unsigned int ev_iteration (loop) + +Returns the current iteration count for the event loop, which is identical +to the number of times libev did poll for new events. It starts at C<0> +and happily wraps around with enough iterations. + +This value can sometimes be useful as a generation counter of sorts (it +"ticks" the number of loop iterations), as it roughly corresponds with +C<ev_prepare> and C<ev_check> calls - and is incremented between the +prepare and check phases. + +=item unsigned int ev_depth (loop) + +Returns the number of times C<ev_run> was entered minus the number of +times C<ev_run> was exited normally, in other words, the recursion depth. + +Outside C<ev_run>, this number is zero. In a callback, this number is +C<1>, unless C<ev_run> was invoked recursively (or from another thread), +in which case it is higher. + +Leaving C<ev_run> abnormally (setjmp/longjmp, cancelling the thread, +throwing an exception etc.), doesn't count as "exit" - consider this +as a hint to avoid such ungentleman-like behaviour unless it's really +convenient, in which case it is fully supported. + +=item unsigned int ev_backend (loop) + +Returns one of the C<EVBACKEND_*> 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 +received 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 occurring (or more correctly, libev finding out about it). + +=item ev_now_update (loop) + +Establishes the current time by querying the kernel, updating the time +returned by C<ev_now ()> in the progress. This is a costly operation and +is usually done automatically within C<ev_run ()>. + +This function is rarely useful, but when some event callback runs for a +very long time without entering the event loop, updating libev's idea of +the current time is a good idea. + +See also L<The special problem of time updates> in the C<ev_timer> section. + +=item ev_suspend (loop) + +=item ev_resume (loop) + +These two functions suspend and resume an event loop, for use when the +loop is not used for a while and timeouts should not be processed. + +A typical use case would be an interactive program such as a game: When +the user presses C<^Z> to suspend the game and resumes it an hour later it +would be best to handle timeouts as if no time had actually passed while +the program was suspended. This can be achieved by calling C<ev_suspend> +in your C<SIGTSTP> handler, sending yourself a C<SIGSTOP> and calling +C<ev_resume> directly afterwards to resume timer processing. + +Effectively, all C<ev_timer> watchers will be delayed by the time spend +between C<ev_suspend> and C<ev_resume>, and all C<ev_periodic> watchers +will be rescheduled (that is, they will lose any events that would have +occurred while suspended). + +After calling C<ev_suspend> you B<must not> call I<any> function on the +given loop other than C<ev_resume>, and you B<must not> call C<ev_resume> +without a previous call to C<ev_suspend>. + +Calling C<ev_suspend>/C<ev_resume> has the side effect of updating the +event loop time (see C<ev_now_update>). + +=item ev_run (loop, int flags) + +Finally, this is it, the event handler. This function usually is called +after you have initialised all your watchers and you want to start +handling events. It will ask the operating system for any new events, call +the watcher callbacks, an then repeat the whole process indefinitely: This +is why event loops are called I<loops>. + +If the flags argument is specified as C<0>, it will keep handling events +until either no event watchers are active anymore or C<ev_break> was +called. + +Please note that an explicit C<ev_break> is usually better than +relying on all watchers to be stopped when deciding when a program has +finished (especially in interactive programs), but having a program +that automatically loops as long as it has to and no longer by virtue +of relying on its watchers stopping correctly, that is truly a thing of +beauty. + +This function is also I<mostly> exception-safe - you can break out of +a C<ev_run> call by calling C<longjmp> in a callback, throwing a C++ +exception and so on. This does not decrement the C<ev_depth> value, nor +will it clear any outstanding C<EVBREAK_ONE> breaks. + +A flags value of C<EVRUN_NOWAIT> will look for new events, will handle +those events and any already outstanding ones, but will not wait and +block your process in case there are no events and will return after one +iteration of the loop. This is sometimes useful to poll and handle new +events while doing lengthy calculations, to keep the program responsive. + +A flags value of C<EVRUN_ONCE> will look for new events (waiting if +necessary) and will handle those and any already outstanding ones. It +will block your process until at least one new event arrives (which could +be an event internal to libev itself, so there is no guarantee that a +user-registered callback will be called), and will return after one +iteration of the loop. + +This is useful if you are waiting for some external event in conjunction +with something not expressible using other libev watchers (i.e. "roll your +own C<ev_run>"). However, a pair of C<ev_prepare>/C<ev_check> watchers is +usually a better approach for this kind of thing. + +Here are the gory details of what C<ev_run> does: + + - Increment loop depth. + - Reset the ev_break status. + - Before the first iteration, call any pending watchers. + LOOP: + - If EVFLAG_FORKCHECK was used, check for a fork. + - If a fork was detected (by any means), queue and call all fork watchers. + - Queue and call all prepare watchers. + - If ev_break was called, goto FINISH. + - If we have been forked, detach and recreate the kernel state + as to not disturb the other process. + - Update the kernel state with all outstanding changes. + - Update the "event loop time" (ev_now ()). + - Calculate for how long to sleep or block, if at all + (active idle watchers, EVRUN_NOWAIT or not having + any active watchers at all will result in not sleeping). + - Sleep if the I/O and timer collect interval say so. + - Increment loop iteration counter. + - Block the process, waiting for any events. + - Queue all outstanding I/O (fd) events. + - Update the "event loop time" (ev_now ()), and do time jump adjustments. + - Queue all expired timers. + - Queue all expired periodics. + - Queue all idle watchers with priority higher than that of pending events. + - Queue all check watchers. + - Call all queued watchers in reverse order (i.e. check watchers first). + Signals and child watchers are implemented as I/O watchers, and will + be handled here by queueing them when their watcher gets executed. + - If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT + were used, or there are no active watchers, goto FINISH, otherwise + continue with step LOOP. + FINISH: + - Reset the ev_break status iff it was EVBREAK_ONE. + - Decrement the loop depth. + - Return. + +Example: Queue some jobs and then loop until no events are outstanding +anymore. + + ... queue jobs here, make sure they register event watchers as long + ... as they still have work to do (even an idle watcher will do..) + ev_run (my_loop, 0); + ... jobs done or somebody called unloop. yeah! + +=item ev_break (loop, how) + +Can be used to make a call to C<ev_run> return early (but only after it +has processed all outstanding events). The C<how> argument must be either +C<EVBREAK_ONE>, which will make the innermost C<ev_run> call return, or +C<EVBREAK_ALL>, which will make all nested C<ev_run> calls return. + +This "break state" will be cleared on the next call to C<ev_run>. + +It is safe to call C<ev_break> from outside any C<ev_run> calls, too, in +which case it will have no effect. + +=item ev_ref (loop) + +=item ev_unref (loop) + +Ref/unref can be used to add or remove a reference count on the event +loop: Every watcher keeps one reference, and as long as the reference +count is nonzero, C<ev_run> will not return on its own. + +This is useful when you have a watcher that you never intend to +unregister, but that nevertheless should not keep C<ev_run> from +returning. In such a case, call C<ev_unref> after starting, and C<ev_ref> +before stopping it. + +As an example, libev itself uses this for its internal signal pipe: It +is not visible to the libev user and should not keep C<ev_run> from +exiting if no event watchers registered by it are active. It is also an +excellent way to do this for generic recurring timers or from within +third-party libraries. Just remember to I<unref after start> and I<ref +before stop> (but only if the watcher wasn't active before, or was active +before, respectively. Note also that libev might stop watchers itself +(e.g. non-repeating timers) in which case you have to C<ev_ref> +in the callback). + +Example: Create a signal watcher, but keep it from keeping C<ev_run> +running when nothing else is active. + + ev_signal exitsig; + ev_signal_init (&exitsig, sig_cb, SIGINT); + ev_signal_start (loop, &exitsig); + ev_unref (loop); + +Example: For some weird reason, unregister the above signal handler again. + + ev_ref (loop); + ev_signal_stop (loop, &exitsig); + +=item ev_set_io_collect_interval (loop, ev_tstamp interval) + +=item ev_set_timeout_collect_interval (loop, ev_tstamp interval) + +These advanced functions influence the time that libev will spend waiting +for events. Both time intervals are by default C<0>, meaning that libev +will try to invoke timer/periodic callbacks and I/O callbacks with minimum +latency. + +Setting these to a higher value (the C<interval> I<must> be >= C<0>) +allows libev to delay invocation of I/O and timer/periodic callbacks +to increase efficiency of loop iterations (or to increase power-saving +opportunities). + +The idea is that sometimes your program runs just fast enough to handle +one (or very few) event(s) per loop iteration. While this makes the +program responsive, it also wastes a lot of CPU time to poll for new +events, especially with backends like C<select ()> which have a high +overhead for the actual polling but can deliver many events at once. + +By setting a higher I<io collect interval> 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<ev_periodic> and +C<ev_timer>) will be not affected. Setting this to a non-null value will +introduce an additional C<ev_sleep ()> call into most loop iterations. The +sleep time ensures that libev will not poll for I/O events more often then +once per this interval, on average. + +Likewise, by setting a higher I<timeout collect interval> 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<ev_io> 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. Note that if +you do transactions with the outside world and you can't increase the +parallelity, then this setting will limit your transaction rate (if you +need to poll once per transaction and the I/O collect interval is 0.01, +then you can't do more than 100 transactions per second). + +Setting the I<timeout collect interval> 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<ev_periodic> watchers and make sure +they fire on, say, one-second boundaries only. + +Example: we only need 0.1s timeout granularity, and we wish not to poll +more often than 100 times per second: + + ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1); + ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01); + +=item ev_invoke_pending (loop) + +This call will simply invoke all pending watchers while resetting their +pending state. Normally, C<ev_run> does this automatically when required, +but when overriding the invoke callback this call comes handy. This +function can be invoked from a watcher - this can be useful for example +when you want to do some lengthy calculation and want to pass further +event handling to another thread (you still have to make sure only one +thread executes within C<ev_invoke_pending> or C<ev_run> of course). + +=item int ev_pending_count (loop) + +Returns the number of pending watchers - zero indicates that no watchers +are pending. + +=item ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P)) + +This overrides the invoke pending functionality of the loop: Instead of +invoking all pending watchers when there are any, C<ev_run> will call +this callback instead. This is useful, for example, when you want to +invoke the actual watchers inside another context (another thread etc.). + +If you want to reset the callback, use C<ev_invoke_pending> as new +callback. + +=item ev_set_loop_release_cb (loop, void (*release)(EV_P), void (*acquire)(EV_P)) + +Sometimes you want to share the same loop between multiple threads. This +can be done relatively simply by putting mutex_lock/unlock calls around +each call to a libev function. + +However, C<ev_run> can run an indefinite time, so it is not feasible +to wait for it to return. One way around this is to wake up the event +loop via C<ev_break> and C<av_async_send>, another way is to set these +I<release> and I<acquire> callbacks on the loop. + +When set, then C<release> will be called just before the thread is +suspended waiting for new events, and C<acquire> is called just +afterwards. + +Ideally, C<release> will just call your mutex_unlock function, and +C<acquire> will just call the mutex_lock function again. + +While event loop modifications are allowed between invocations of +C<release> and C<acquire> (that's their only purpose after all), no +modifications done will affect the event loop, i.e. adding watchers will +have no effect on the set of file descriptors being watched, or the time +waited. Use an C<ev_async> watcher to wake up C<ev_run> when you want it +to take note of any changes you made. + +In theory, threads executing C<ev_run> will be async-cancel safe between +invocations of C<release> and C<acquire>. + +See also the locking example in the C<THREADS> section later in this +document. + +=item ev_set_userdata (loop, void *data) + +=item void *ev_userdata (loop) + +Set and retrieve a single C<void *> associated with a loop. When +C<ev_set_userdata> has never been called, then C<ev_userdata> returns +C<0>. + +These two functions can be used to associate arbitrary data with a loop, +and are intended solely for the C<invoke_pending_cb>, C<release> and +C<acquire> callbacks described above, but of course can be (ab-)used for +any other purpose as well. + +=item ev_verify (loop) + +This function only does something when C<EV_VERIFY> 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<abort ()>. + +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<TYPE> in names stands for the +watcher type, e.g. C<ev_TYPE_start> can mean C<ev_timer_start> for timer +watchers and C<ev_io_start> for I/O watchers. + +A watcher is an opaque structure that you allocate and register to record +your interest in some event. To make a concrete example, imagine you want +to wait for STDIN to become readable, you would create an C<ev_io> watcher +for that: + + static void my_cb (struct ev_loop *loop, ev_io *w, int revents) + { + ev_io_stop (w); + ev_break (loop, EVBREAK_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_run (loop, 0); + +As you can see, you are responsible for allocating the memory for your +watcher structures (and it is I<usually> a bad idea to do this on the +stack). + +Each watcher has an associated watcher structure (called C<struct ev_TYPE> +or simply C<ev_TYPE>, as typedefs are provided for all watcher structs). + +Each watcher structure must be initialised by a call to C<ev_init (watcher +*, callback)>, which expects a callback to be provided. This callback is +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 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_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 C<ev_TYPE_set> macro. + +Each and every callback receives the event loop pointer as first, the +registered watcher structure as second, and a bitset of received events as +third argument. + +The received events usually include a single bit per event type received +(you can receive multiple events at the same time). The possible bit masks +are: + +=over 4 + +=item C<EV_READ> + +=item C<EV_WRITE> + +The file descriptor in the C<ev_io> watcher has become readable and/or +writable. + +=item C<EV_TIMER> + +The C<ev_timer> watcher has timed out. + +=item C<EV_PERIODIC> + +The C<ev_periodic> watcher has timed out. + +=item C<EV_SIGNAL> + +The signal specified in the C<ev_signal> watcher has been received by a thread. + +=item C<EV_CHILD> + +The pid specified in the C<ev_child> watcher has received a status change. + +=item C<EV_STAT> + +The path specified in the C<ev_stat> watcher changed its attributes somehow. + +=item C<EV_IDLE> + +The C<ev_idle> watcher has determined that you have nothing better to do. + +=item C<EV_PREPARE> + +=item C<EV_CHECK> + +All C<ev_prepare> watchers are invoked just I<before> C<ev_run> starts +to gather new events, and all C<ev_check> watchers are invoked just after +C<ev_run> has gathered them, but before it invokes any callbacks for any +received events. Callbacks of both watcher types can start and stop as +many watchers as they want, and all of them will be taken into account +(for example, a C<ev_prepare> watcher might start an idle watcher to keep +C<ev_run> from blocking). + +=item C<EV_EMBED> + +The embedded event loop specified in the C<ev_embed> watcher needs attention. + +=item C<EV_FORK> + +The event loop has been resumed in the child process after fork (see +C<ev_fork>). + +=item C<EV_CLEANUP> + +The event loop is about to be destroyed (see C<ev_cleanup>). + +=item C<EV_ASYNC> + +The given async watcher has been asynchronously notified (see C<ev_async>). + +=item C<EV_CUSTOM> + +Not ever sent (or otherwise used) by libev itself, but can be freely used +by libev users to signal watchers (e.g. via C<ev_feed_event>). + +=item C<EV_ERROR> + +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. Libev considers these application bugs. + +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 + +=over 4 + +=item C<ev_init> (ev_TYPE *watcher, callback) + +This macro initialises the generic portion of a watcher. The contents +of the watcher object can be arbitrary (so C<malloc> will do). Only +the generic parts of the watcher are initialised, you I<need> to call +the type-specific C<ev_TYPE_set> macro afterwards to initialise the +type-specific parts. For each type there is also a C<ev_TYPE_init> 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<void (*)(struct ev_loop *loop, ev_TYPE *watcher, +int revents)>. + +Example: Initialise an C<ev_io> watcher in two steps. + + ev_io w; + ev_init (&w, my_cb); + ev_io_set (&w, STDIN_FILENO, EV_READ); + +=item C<ev_TYPE_set> (ev_TYPE *watcher, [args]) + +This macro initialises the type-specific parts of a watcher. You need to +call C<ev_init> at least once before you call this macro, but you can +call C<ev_TYPE_set> 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<ev_init> macro). + +Although some watcher types do not have type-specific arguments +(e.g. C<ev_prepare>) you still need to call its C<set> macro. + +See C<ev_init>, above, for an example. + +=item C<ev_TYPE_init> (ev_TYPE *watcher, callback, [args]) + +This convenience macro rolls both C<ev_init> and C<ev_TYPE_set> 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<ev_io> watcher in one step. + + ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ); + +=item C<ev_TYPE_start> (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<ev_io> watcher that is being abused as example in this +whole section. + + ev_io_start (EV_DEFAULT_UC, &w); + +=item C<ev_TYPE_stop> (loop, ev_TYPE *watcher) + +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<ev_TYPE_stop> 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<ev_TYPE_stop> 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<ev_TYPE_set> is safe), you must not change its priority, and you must +make sure the watcher is available to libev (e.g. you cannot C<free ()> +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, int priority) + +=item int ev_priority (ev_TYPE *watcher) + +Set and query the priority of the watcher. The priority is a small +integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI> +(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<ev_idle> watchers). + +If you need to suppress invocation when higher priority events are pending +you need to look at C<ev_idle> watchers, which provide this functionality. + +You I<must not> change the priority of a watcher as long as it is active or +pending. + +Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is +fine, as long as you do not mind that the priority value you query might +or might not have been clamped to the valid range. + +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 :). + +See L<WATCHER PRIORITY MODELS>, below, for a more thorough treatment of +priorities. + +=item ev_invoke (loop, ev_TYPE *watcher, int revents) + +Invoke the C<watcher> with the given C<loop> and C<revents>. Neither +C<loop> nor C<revents> need to be valid as long as the watcher callback +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 clears its pending status and +returns its C<revents> 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. + +=item ev_feed_event (loop, ev_TYPE *watcher, int revents) + +Feeds the given event set into the event loop, as if the specified event +had happened for the specified watcher (which must be a pointer to an +initialised but not necessarily started event watcher). Obviously you must +not free the watcher as long as it has pending events. + +Stopping the watcher, letting libev invoke it, or calling +C<ev_clear_pending> will clear the pending event, even if the watcher was +not started in the first place. + +See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related +functions that do not need a watcher. + +=back + +See also the L<ASSOCIATING CUSTOM DATA WITH A WATCHER> and L<BUILDING YOUR +OWN COMPOSITE WATCHERS> idioms. + +=head2 WATCHER STATES + +There are various watcher states mentioned throughout this manual - +active, pending and so on. In this section these states and the rules to +transition between them will be described in more detail - and while these +rules might look complicated, they usually do "the right thing". + +=over 4 + +=item initialiased + +Before a watcher can be registered with the event looop it has to be +initialised. This can be done with a call to C<ev_TYPE_init>, or calls to +C<ev_init> followed by the watcher-specific C<ev_TYPE_set> function. + +In this state it is simply some block of memory that is suitable for use +in an event loop. It can be moved around, freed, reused etc. at will. + +=item started/running/active + +Once a watcher has been started with a call to C<ev_TYPE_start> it becomes +property of the event loop, and is actively waiting for events. While in +this state it cannot be accessed (except in a few documented ways), moved, +freed or anything else - the only legal thing is to keep a pointer to it, +and call libev functions on it that are documented to work on active watchers. + +=item pending + +If a watcher is active and libev determines that an event it is interested +in has occurred (such as a timer expiring), it will become pending. It will +stay in this pending state until either it is stopped or its callback is +about to be invoked, so it is not normally pending inside the watcher +callback. + +The watcher might or might not be active while it is pending (for example, +an expired non-repeating timer can be pending but no longer active). If it +is stopped, it can be freely accessed (e.g. by calling C<ev_TYPE_set>), +but it is still property of the event loop at this time, so cannot be +moved, freed or reused. And if it is active the rules described in the +previous item still apply. + +It is also possible to feed an event on a watcher that is not active (e.g. +via C<ev_feed_event>), in which case it becomes pending without being +active. + +=item stopped + +A watcher can be stopped implicitly by libev (in which case it might still +be pending), or explicitly by calling its C<ev_TYPE_stop> function. The +latter will clear any pending state the watcher might be in, regardless +of whether it was active or not, so stopping a watcher explicitly before +freeing it is often a good idea. + +While stopped (and not pending) the watcher is essentially in the +initialised state, that is it can be reused, moved, modified in any way +you wish. + +=back + +=head2 WATCHER PRIORITY MODELS + +Many event loops support I<watcher priorities>, which are usually small +integers that influence the ordering of event callback invocation +between watchers in some way, all else being equal. + +In libev, Watcher priorities can be set using C<ev_set_priority>. See its +description for the more technical details such as the actual priority +range. + +There are two common ways how these these priorities are being interpreted +by event loops: + +In the more common lock-out model, higher priorities "lock out" invocation +of lower priority watchers, which means as long as higher priority +watchers receive events, lower priority watchers are not being invoked. + +The less common only-for-ordering model uses priorities solely to order +callback invocation within a single event loop iteration: Higher priority +watchers are invoked before lower priority ones, but they all get invoked +before polling for new events. + +Libev uses the second (only-for-ordering) model for all its watchers +except for idle watchers (which use the lock-out model). + +The rationale behind this is that implementing the lock-out model for +watchers is not well supported by most kernel interfaces, and most event +libraries will just poll for the same events again and again as long as +their callbacks have not been executed, which is very inefficient in the +common case of one high-priority watcher locking out a mass of lower +priority ones. + +Static (ordering) priorities are most useful when you have two or more +watchers handling the same resource: a typical usage example is having an +C<ev_io> watcher to receive data, and an associated C<ev_timer> to handle +timeouts. Under load, data might be received while the program handles +other jobs, but since timers normally get invoked first, the timeout +handler will be executed before checking for data. In that case, giving +the timer a lower priority than the I/O watcher ensures that I/O will be +handled first even under adverse conditions (which is usually, but not +always, what you want). + +Since idle watchers use the "lock-out" model, meaning that idle watchers +will only be executed when no same or higher priority watchers have +received events, they can be used to implement the "lock-out" model when +required. + +For example, to emulate how many other event libraries handle priorities, +you can associate an C<ev_idle> watcher to each such watcher, and in +the normal watcher callback, you just start the idle watcher. The real +processing is done in the idle watcher callback. This causes libev to +continuously poll and process kernel event data for the watcher, but when +the lock-out case is known to be rare (which in turn is rare :), this is +workable. + +Usually, however, the lock-out model implemented that way will perform +miserably under the type of load it was designed to handle. In that case, +it might be preferable to stop the real watcher before starting the +idle watcher, so the kernel will not have to process the event in case +the actual processing will be delayed for considerable time. + +Here is an example of an I/O watcher that should run at a strictly lower +priority than the default, and which should only process data when no +other events are pending: + + ev_idle idle; // actual processing watcher + ev_io io; // actual event watcher + + static void + io_cb (EV_P_ ev_io *w, int revents) + { + // stop the I/O watcher, we received the event, but + // are not yet ready to handle it. + ev_io_stop (EV_A_ w); + + // start the idle watcher to handle the actual event. + // it will not be executed as long as other watchers + // with the default priority are receiving events. + ev_idle_start (EV_A_ &idle); + } + + static void + idle_cb (EV_P_ ev_idle *w, int revents) + { + // actual processing + read (STDIN_FILENO, ...); + + // have to start the I/O watcher again, as + // we have handled the event + ev_io_start (EV_P_ &io); + } + + // initialisation + ev_idle_init (&idle, idle_cb); + ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ); + ev_io_start (EV_DEFAULT_ &io); + +In the "real" world, it might also be beneficial to start a timer, so that +low-priority connections can not be locked out forever under load. This +enables your program to keep a lower latency for important connections +during short periods of high load, while not completely locking out less +important ones. + + +=head1 WATCHER TYPES + +This section describes each watcher in detail, but will not repeat +information given in the last section. Any initialisation/set macros, +functions and members specific to the watcher type are explained. + +Members are additionally marked with either I<[read-only]>, meaning that, +while the watcher is active, you can look at the member and expect some +sensible content, but you must not modify it (you can modify it while the +watcher is stopped to your hearts content), or I<[read-write]>, which +means you can expect it to have some sensible content while the watcher +is active, but you can also modify it. Modifying it may not do something +sensible or take immediate effect (or do anything at all), but libev will +not crash or malfunction in any way. + + +=head2 C<ev_io> - is this file descriptor readable or writable? + +I/O watchers check whether a file descriptor is readable or writable +in each iteration of the event loop, or, more precisely, when reading +would not block the process and writing would at least be able to write +some data. This behaviour is called level-triggering because you keep +receiving events as long as the condition persists. Remember you can stop +the watcher if you don't want to act on the event and neither want to +receive future events. + +In general you can register as many read and/or write event watchers per +fd as you want (as long as you don't confuse yourself). Setting all file +descriptors to non-blocking mode is also usually a good idea (but not +required if you know what you are doing). + +Another thing you have to watch out for is that it is quite easy to +receive "spurious" readiness notifications, that is, your callback might +be called with C<EV_READ> but a subsequent C<read>(2) will actually block +because there is no data. 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<read>(2) returning C<EAGAIN> 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 separately +re-test whether a file descriptor is really ready with a known-to-be good +interface such as poll (fortunately in the case of Xlib, it already does +this on its own, so its quite safe to use). Some people additionally +use C<SIGALRM> 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 due to calling C<close> explicitly or any other means, +such as C<dup2>). 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<ev_io_set> 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<have> to call C<ev_io_set> (or C<ev_io_init>) when you change the +descriptor even if the file descriptor number itself did not change. + +This is how one would do it normally anyway, the important point is that +the libev application should not optimise around libev but should leave +optimisations to libev. + +=head3 The special problem of dup'ed file descriptors + +Some backends (e.g. epoll), cannot register events for file descriptors, +but only events for the underlying file descriptions. That means when you +have C<dup ()>'ed file descriptors or weirder constellations, and register +events for them, only one file descriptor might actually receive events. + +There is no workaround possible except not registering events +for potentially C<dup ()>'ed file descriptors, or to resort to +C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>. + +=head3 The special problem of files + +Many people try to use C<select> (or libev) on file descriptors +representing files, and expect it to become ready when their program +doesn't block on disk accesses (which can take a long time on their own). + +However, this cannot ever work in the "expected" way - you get a readiness +notification as soon as the kernel knows whether and how much data is +there, and in the case of open files, that's always the case, so you +always get a readiness notification instantly, and your read (or possibly +write) will still block on the disk I/O. + +Another way to view it is that in the case of sockets, pipes, character +devices and so on, there is another party (the sender) that delivers data +on its own, but in the case of files, there is no such thing: the disk +will not send data on its own, simply because it doesn't know what you +wish to read - you would first have to request some data. + +Since files are typically not-so-well supported by advanced notification +mechanism, libev tries hard to emulate POSIX behaviour with respect +to files, even though you should not use it. The reason for this is +convenience: sometimes you want to watch STDIN or STDOUT, which is +usually a tty, often a pipe, but also sometimes files or special devices +(for example, C<epoll> on Linux works with F</dev/random> but not with +F</dev/urandom>), and even though the file might better be served with +asynchronous I/O instead of with non-blocking I/O, it is still useful when +it "just works" instead of freezing. + +So avoid file descriptors pointing to files when you know it (e.g. use +libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or +when you rarely read from a file instead of from a socket, and want to +reuse the same code path. + +=head3 The special problem of fork + +Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit +useless behaviour. Libev fully supports fork, but needs to be told about +it in the child if you want to continue to use it in the child. + +To support fork in your child processes, you have to call C<ev_loop_fork +()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to +C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>. + +=head3 The special problem of SIGPIPE + +While not really specific to libev, it is easy to forget about C<SIGPIPE>: +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 The special problem of accept()ing when you can't + +Many implementations of the POSIX C<accept> function (for example, +found in post-2004 Linux) have the peculiar behaviour of not removing a +connection from the pending queue in all error cases. + +For example, larger servers often run out of file descriptors (because +of resource limits), causing C<accept> to fail with C<ENFILE> but not +rejecting the connection, leading to libev signalling readiness on +the next iteration again (the connection still exists after all), and +typically causing the program to loop at 100% CPU usage. + +Unfortunately, the set of errors that cause this issue differs between +operating systems, there is usually little the app can do to remedy the +situation, and no known thread-safe method of removing the connection to +cope with overload is known (to me). + +One of the easiest ways to handle this situation is to just ignore it +- when the program encounters an overload, it will just loop until the +situation is over. While this is a form of busy waiting, no OS offers an +event-based way to handle this situation, so it's the best one can do. + +A better way to handle the situation is to log any errors other than +C<EAGAIN> and C<EWOULDBLOCK>, making sure not to flood the log with such +messages, and continue as usual, which at least gives the user an idea of +what could be wrong ("raise the ulimit!"). For extra points one could stop +the C<ev_io> watcher on the listening fd "for a while", which reduces CPU +usage. + +If your program is single-threaded, then you could also keep a dummy file +descriptor for overload situations (e.g. by opening F</dev/null>), and +when you run into C<ENFILE> or C<EMFILE>, close it, run C<accept>, +close that fd, and create a new dummy fd. This will gracefully refuse +clients under typical overload conditions. + +The last way to handle it is to simply log the error and C<exit>, as +is often done with C<malloc> failures, but this results in an easy +opportunity for a DoS attack. + +=head3 Watcher-Specific Functions + +=over 4 + +=item ev_io_init (ev_io *, callback, int fd, int events) + +=item ev_io_set (ev_io *, int fd, int events) + +Configures an C<ev_io> watcher. The C<fd> is the file descriptor to +receive events for and C<events> is either C<EV_READ>, C<EV_WRITE> or +C<EV_READ | EV_WRITE>, to express the desire to receive the given events. + +=item int fd [read-only] + +The file descriptor being watched. + +=item int events [read-only] + +The events being watched. + +=back + +=head3 Examples + +Example: Call C<stdin_readable_cb> 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, 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_run (loop, 0); + + +=head2 C<ev_timer> - relative and optionally repeating timeouts + +Timer watchers are simple relative timers that generate an event after a +given time, and optionally repeating in regular intervals after that. + +The timers are based on real time, that is, if you register an event that +times out after an hour and you reset your system clock to 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<after> its timeout has +passed (not I<at>, so on systems with very low-resolution clocks this +might introduce a small delay). If multiple timers become ready during the +same loop iteration then the ones with earlier time-out values are invoked +before ones of the same priority with later time-out values (but this is +no longer true when a callback calls C<ev_run> recursively). + +=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<ev_timer_stop> 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<ev_timer_again> inactivity. + +This is the easiest way, and involves using C<ev_timer_again> instead of +C<ev_timer_start>. + +To implement this, configure an C<ev_timer> with a C<repeat> value +of C<60> and then call C<ev_timer_again> 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<ev_timer_stop> +the timer, and C<ev_timer_again> will automatically restart it if need be. + +That means you can ignore both the C<ev_timer_start> function and the +C<after> argument to C<ev_timer_set>, and only ever use the C<repeat> +member and C<ev_timer_again>. + +At start: + + ev_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<ev_timer> 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 occurred, 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<did>, in fact, time out. Otherwise +the callback was invoked too early (C<timeout> 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<ev_timer_again> is used, taking advantage of the +C<ev_timer_again> 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<last_activity> +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_init (timer, callback); + last_activity = ev_now (loop); + callback (loop, timer, EV_TIMER); + +And when there is some activity, simply store the current time in +C<last_activity>, no libev calls at all: + + last_activity = 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<end> of the list. + +Then use an C<ev_timer> to fire when the timeout at the I<beginning> 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<ev_timer> 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<ev_run> collects new events, which causes a +growing difference between C<ev_now ()> and C<ev_time ()> when handling +lots of events in one iteration. + +The relative timeouts are calculated relative to the C<ev_now ()> +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<need> 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.); + +If the event loop is suspended for a long time, you can also force an +update of the time returned by C<ev_now ()> by calling C<ev_now_update +()>. + +=head3 The special problems of suspended animation + +When you leave the server world it is quite customary to hit machines that +can suspend/hibernate - what happens to the clocks during such a suspend? + +Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes +all processes, while the clocks (C<times>, C<CLOCK_MONOTONIC>) continue +to run until the system is suspended, but they will not advance while the +system is suspended. That means, on resume, it will be as if the program +was frozen for a few seconds, but the suspend time will not be counted +towards C<ev_timer> when a monotonic clock source is used. The real time +clock advanced as expected, but if it is used as sole clocksource, then a +long suspend would be detected as a time jump by libev, and timers would +be adjusted accordingly. + +I would not be surprised to see different behaviour in different between +operating systems, OS versions or even different hardware. + +The other form of suspend (job control, or sending a SIGSTOP) will see a +time jump in the monotonic clocks and the realtime clock. If the program +is suspended for a very long time, and monotonic clock sources are in use, +then you can expect C<ev_timer>s to expire as the full suspension time +will be counted towards the timers. When no monotonic clock source is in +use, then libev will again assume a timejump and adjust accordingly. + +It might be beneficial for this latter case to call C<ev_suspend> +and C<ev_resume> in code that handles C<SIGTSTP>, to at least get +deterministic behaviour in this case (you can do nothing against +C<SIGSTOP>). + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat) + +=item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat) + +Configure the timer to trigger after C<after> seconds. If C<repeat> +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<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 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, 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 non-repeating, stop it (as if it timed out). + +If the timer is repeating, either start it if necessary (with the +C<repeat> value), or reset the running timer to the C<repeat> value. + +This sounds a bit complicated, see L<Be smart about timeouts>, above, for a +usage example. + +=item ev_tstamp ev_timer_remaining (loop, ev_timer *) + +Returns the remaining time until a timer fires. If the timer is active, +then this time is relative to the current event loop time, otherwise it's +the timeout value currently configured. + +That is, after an C<ev_timer_set (w, 5, 7)>, C<ev_timer_remaining> returns +C<5>. When the timer is started and one second passes, C<ev_timer_remaining> +will return C<4>. When the timer expires and is restarted, it will return +roughly C<7> (likely slightly less as callback invocation takes some time, +too), and so on. + +=item ev_tstamp repeat [read-write] + +The current C<repeat> value. Will be used each time the watcher times out +or C<ev_timer_again> 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, 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, 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_run (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<ev_periodic> - to cron or not to cron? + +Periodic watchers are also timers of a kind, but they are very versatile +(and unfortunately a bit complex). + +Unlike C<ev_timer>, periodic watchers are not based on real time (or +relative time, the physical time that passes) but on wall clock time +(absolute time, the thing you can read on your calender or clock). The +difference is that wall clock time can run faster or slower than real +time, and time jumps are not uncommon (e.g. when you adjust your +wrist-watch). + +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<ev_now () + 10.>, that is, an absolute time +not a delay) and then reset your system clock to January of the previous +year, then it will take a year or more to trigger the event (unlike an +C<ev_timer>, which would still trigger roughly 10 seconds after starting +it, as it uses a relative timeout). + +C<ev_periodic> watchers can also be used to implement vastly more complex +timers, such as triggering an event on each "midnight, local time", or +other complicated rules. This cannot be done with C<ev_timer> watchers, as +those cannot react to time jumps. + +As with timers, the callback is guaranteed to be invoked only when the +point in time where it is supposed to trigger has passed. If multiple +timers become ready during the same loop iteration then the ones with +earlier time-out values are invoked before ones with later time-out values +(but this is no longer true when a callback calls C<ev_run> recursively). + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb) + +=item ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb) + +Lots of arguments, let's sort it out... There are basically three modes of +operation, and we will explain them from simplest to most complex: + +=over 4 + +=item * absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0) + +In this configuration the watcher triggers an event after the wall clock +time C<offset> 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 be stopped and invoked when the system clock reaches or surpasses +this point in time. + +=item * repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0) + +In this mode the watcher will always be scheduled to time out at the next +C<offset + N * interval> time (for some integer N, which can also be +negative) and then repeat, regardless of any time jumps. The C<offset> +argument is merely an offset into the C<interval> periods. + +This can be used to create timers that do not drift with respect to the +system clock, for example, here is an C<ev_periodic> that triggers each +hour, on the hour (with respect to UTC): + + ev_periodic_set (&periodic, 0., 3600., 0); + +This doesn't mean there will always be 3600 seconds in between triggers, +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. + +Another way to think about it (for the mathematically inclined) is that +C<ev_periodic> will try to run the callback in this mode at the next possible +time where C<time = offset (mod interval)>, regardless of any time jumps. + +For numerical stability it is preferable that the C<offset> value is near +C<ev_now ()> (the current time), but there is no range requirement for +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<interval> 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 (offset ignored, interval ignored, reschedule_cb = callback) + +In this mode the values for C<interval> and C<offset> 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 any periodic watcher, ever, +or make ANY other event loop modifications whatsoever, unless explicitly +allowed by documentation here>. + +If you need to stop it, return C<now + 1e30> (or so, fudge fudge) and stop +it afterwards (e.g. by starting an C<ev_prepare> watcher, which is the +only event loop modification you are allowed to do). + +The callback prototype is C<ev_tstamp (*reschedule_cb)(ev_periodic +*w, ev_tstamp now)>, e.g.: + + static ev_tstamp + my_rescheduler (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. + +NOTE: I<< This callback must always return a time that is higher than or +equal to the passed C<now> value >>. + +This can be used to create very complex timers, such as a timer that +triggers on "next 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, which is the main +reason I omitted it as an example). + +=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). + +=item ev_tstamp ev_periodic_at (ev_periodic *) + +When active, returns the absolute time that the watcher is supposed +to trigger next. This is not the same as the C<offset> argument to +C<ev_periodic_set>, but indeed works even in interval and manual +rescheduling modes. + +=item ev_tstamp offset [read-write] + +When repeating, this contains the offset value, otherwise this is the +absolute point in time (the C<offset> value passed to C<ev_periodic_set>, +although libev might modify this value for better numerical stability). + +Can be modified any time, but changes only take effect when the periodic +timer fires or C<ev_periodic_again> is being called. + +=item ev_tstamp interval [read-write] + +The current interval value. Can be modified any time, but changes only +take effect when the periodic timer fires or C<ev_periodic_again> is being +called. + +=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<ev_periodic_again> is being called. + +=back + +=head3 Examples + +Example: Call a callback every hour, or, more precisely, whenever the +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_periodic *w, int revents) + { + ... its now a full hour (UTC, or TAI or whatever your clock follows) + } + + 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 <math.h> + + 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); + +Example: Call a callback every hour, starting now: + + 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<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. + +If you want signals to be delivered truly asynchronously, just use +C<sigaction> as you would do without libev and forget about sharing +the signal. You can even use C<ev_async> from a signal handler to +synchronously wake up an event loop. + +You can configure as many watchers as you like for the same signal, but +only within the same loop, i.e. you can watch for C<SIGINT> in your +default loop and for C<SIGIO> in another loop, but you cannot watch for +C<SIGINT> in both the default loop and another loop at the same time. At +the moment, C<SIGCHLD> is permanently tied to the default loop. + +When the first watcher gets started will libev actually register something +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). + +If possible and supported, libev will install its handlers with +C<SA_RESTART> (or equivalent) 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<ev_check> watcher +and unblock them in an C<ev_prepare> watcher. + +=head3 The special problem of inheritance over fork/execve/pthread_create + +Both the signal mask (C<sigprocmask>) and the signal disposition +(C<sigaction>) are unspecified after starting a signal watcher (and after +stopping it again), that is, libev might or might not block the signal, +and might or might not set or restore the installed signal handler. + +While this does not matter for the signal disposition (libev never +sets signals to C<SIG_IGN>, so handlers will be reset to C<SIG_DFL> on +C<execve>), this matters for the signal mask: many programs do not expect +certain signals to be blocked. + +This means that before calling C<exec> (from the child) you should reset +the signal mask to whatever "default" you expect (all clear is a good +choice usually). + +The simplest way to ensure that the signal mask is reset in the child is +to install a fork handler with C<pthread_atfork> that resets it. That will +catch fork calls done by libraries (such as the libc) as well. + +In current versions of libev, the signal will not be blocked indefinitely +unless you use the C<signalfd> API (C<EV_SIGNALFD>). While this reduces +the window of opportunity for problems, it will not go away, as libev +I<has> to modify the signal mask, at least temporarily. + +So I can't stress this enough: I<If you do not reset your signal mask when +you expect it to be empty, you have a race condition in your code>. This +is not a libev-specific thing, this is true for most event libraries. + +=head3 The special problem of threads signal handling + +POSIX threads has problematic signal handling semantics, specifically, +a lot of functionality (sigfd, sigwait etc.) only really works if all +threads in a process block signals, which is hard to achieve. + +When you want to use sigwait (or mix libev signal handling with your own +for the same signals), you can tackle this problem by globally blocking +all signals before creating any threads (or creating them with a fully set +sigprocmask) and also specifying the C<EVFLAG_NOSIGMASK> when creating +loops. Then designate one thread as "signal receiver thread" which handles +these signals. You can pass on any signals that libev might be interested +in by calling C<ev_feed_signal>. + +=head3 Watcher-Specific Functions and Data Members + +=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). + +=item int signum [read-only] + +The signal the watcher watches out for. + +=back + +=head3 Examples + +Example: Try to exit cleanly on SIGINT. + + static void + sigint_cb (struct ev_loop *loop, ev_signal *w, int revents) + { + ev_break (loop, EVBREAK_ALL); + } + + ev_signal signal_watcher; + ev_signal_init (&signal_watcher, sigint_cb, SIGINT); + ev_signal_start (loop, &signal_watcher); + + +=head2 C<ev_child> - 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 or +exits). It is permissible to install a child watcher I<after> 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 or +in the next callback invocation 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. + +Due to some design glitches inside libev, child watchers will always be +handled at maximum priority (their priority is set to C<EV_MAXPRI> by +libev) + +=head3 Process Interaction + +Libev grabs C<SIGCHLD> 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<SIGCHLD> 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<SIGCHLD> 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<ev_child> 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 (calling C<ev_child_stop> twice is not a +problem). + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_child_init (ev_child *, callback, int pid, int trace) + +=item ev_child_set (ev_child *, int pid, int trace) + +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> and see your systems +C<waitpid> documentation). The C<rpid> member contains the pid of the +process causing the status change. C<trace> 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] + +The process id this watcher watches out for, or C<0>, meaning any process id. + +=item int rpid [read-write] + +The process id that detected a status change. + +=item int rstatus [read-write] + +The process exit/trace status caused by C<rpid> (see your systems +C<waitpid> and C<sys/wait.h> documentation for details). + +=back + +=head3 Examples + +Example: C<fork()> 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<ev_stat> - did the file attributes just change? + +This watches a file system path for attribute changes. That is, it calls +C<stat> 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" (or more correctly "path cannot be stat'ed") is signified by the +C<st_nlink> 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<must not> end in a slash or contain special components such as +C<.> or C<..>. The path I<should> 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<stat(2)> 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<suitable, unspecified default> 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, 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<ev_stat> 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<inotify (7)> 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<ev_stat> +watcher is being started. + +Inotify presence does not change the semantics of C<ev_stat> watchers +except that changes might be detected earlier, and in some cases, to avoid +making regular C<stat> calls. Even in the presence of inotify support +there are many cases where libev has to resort to regular C<stat> 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<stat ()> 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<ev_stat> watchers - those call C<stat +()>, 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<stat ()> 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<ev_stat> watchers on networked +paths, although this is fully supported by libev. + +=head3 The special problem of stat time resolution + +The C<stat ()> 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<ev_stat> detects a change and +calls your callback, which does something. When there is another update +within the same second, C<ev_stat> 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<ev_timer> (e.g. C<ev_timer_set (w, 0., 1.02); +ev_timer_again (loop, w)>). + +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<gettimeofday> might return a timestamp with a full second later than +a subsequent C<time> call - if the equivalent of C<time ()> is used to +update file times then there will be a small window where the kernel uses +the previous second to update file times but libev might already execute +the timer callback). + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval) + +=item ev_stat_set (ev_stat *, const char *path, ev_tstamp interval) + +Configures the watcher to wait for status changes of the given +C<path>. The C<interval> is a hint on how quickly a change is expected to +be detected and should normally be specified as C<0> to let libev choose +a suitable value. The memory pointed to by C<path> must point to the same +path for as long as the watcher is active. + +The callback will receive an C<EV_STAT> event when a change was detected, +relative to the attributes at the time the watcher was started (or the +last change was detected). + +=item ev_stat_stat (loop, ev_stat *) + +Updates the stat buffer immediately with new values. If you change the +watched path in your callback, you could call this function to avoid +detecting this change (while introducing a race condition if you are not +the only one changing the path). Can also be useful simply to find out the +new values. + +=item ev_statdata attr [read-only] + +The most-recently detected attributes of the file. Although the type is +C<ev_statdata>, this is usually the (or one of the) C<struct stat> types +suitable for your system, but you can only rely on the POSIX-standardised +members to be present. If the C<st_nlink> member is C<0>, then there was +some error while C<stat>ing the file. + +=item ev_statdata prev [read-only] + +The previous attributes of the file. The callback gets invoked whenever +C<prev> != C<attr>, or, more precisely, one or more of these members +differ: C<st_dev>, C<st_ino>, C<st_mode>, C<st_nlink>, C<st_uid>, +C<st_gid>, C<st_rdev>, C<st_size>, C<st_atime>, C<st_mtime>, C<st_ctime>. + +=item ev_tstamp interval [read-only] + +The specified interval. + +=item const char *path [read-only] + +The file system path that is being watched. + +=back + +=head3 Examples + +Example: Watch C</etc/passwd> for attribute changes. + + static void + passwd_cb (struct ev_loop *loop, ev_stat *w, int revents) + { + /* /etc/passwd changed in some way */ + if (w->attr.st_nlink) + { + printf ("passwd current size %ld\n", (long)w->attr.st_size); + printf ("passwd current atime %ld\n", (long)w->attr.st_mtime); + printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime); + } + else + /* you shalt not abuse printf for puts */ + puts ("wow, /etc/passwd is not there, expect problems. " + "if this is windows, they already arrived\n"); + } + + ... + ev_stat passwd; + + ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.); + ev_stat_start (loop, &passwd); + +Example: Like above, but additionally use a one-second delay so we do not +miss updates (however, frequent updates will delay processing, too, so +one might do the work both on C<ev_stat> callback invocation I<and> on +C<ev_timer> callback invocation). + + static ev_stat passwd; + static ev_timer timer; + + static void + timer_cb (EV_P_ ev_timer *w, int revents) + { + ev_timer_stop (EV_A_ w); + + /* now it's one second after the most recent passwd change */ + } + + static void + stat_cb (EV_P_ ev_stat *w, int revents) + { + /* reset the one-second timer */ + ev_timer_again (EV_A_ &timer); + } + + ... + ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.); + ev_stat_start (loop, &passwd); + ev_timer_init (&timer, timer_cb, 0., 1.02); + + +=head2 C<ev_idle> - when you've got nothing better to do... + +Idle watchers trigger events when no other events of the same or higher +priority are pending (prepare, check and other idle watchers do not count +as receiving "events"). + +That is, as long as your process is busy handling sockets or timeouts +(or even signals, imagine) of the same or higher priority it will not be +triggered. But when your process is idle (or only lower-priority watchers +are pending), the idle watchers are being called once per event loop +iteration - until stopped, that is, or your process receives more events +and becomes busy again with higher priority stuff. + +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. + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_idle_init (ev_idle *, 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 + +=head3 Examples + +Example: Dynamically allocate an C<ev_idle> watcher, start it, and in the +callback, free it. Also, use no error checking, as usual. + + static void + idle_cb (struct ev_loop *loop, ev_idle *w, int revents) + { + free (w); + // now do something you wanted to do when the program has + // no longer anything immediate to do. + } + + ev_idle *idle_watcher = malloc (sizeof (ev_idle)); + ev_idle_init (idle_watcher, idle_cb); + ev_idle_start (loop, idle_watcher); + + +=head2 C<ev_prepare> and C<ev_check> - customise your event loop! + +Prepare and check watchers are usually (but not always) used in pairs: +prepare watchers get invoked before the process blocks and check watchers +afterwards. + +You I<must not> call C<ev_run> or similar functions that enter +the current event loop from either C<ev_prepare> or C<ev_check> +watchers. Other loops than the current one are fine, however. The +rationale behind this is that you do not need to check for recursion in +those watchers, i.e. the sequence will always be C<ev_prepare>, blocking, +C<ev_check> so if you have one watcher of each kind they will always be +called in pairs bracketing the blocking call. + +Their main purpose is to integrate other event mechanisms into libev and +their use is somewhat advanced. They could be used, for example, to track +variable changes, implement your own watchers, integrate net-snmp or a +coroutine library and lots more. They are also occasionally useful if +you cache some data and want to flush it before blocking (for example, +in X programs you might want to do an C<XFlush ()> in an C<ev_prepare> +watcher). + +This is done by examining in each prepare call which file descriptors +need to be watched by the other library, registering C<ev_io> watchers +for them and starting an C<ev_timer> watcher for any timeouts (many +libraries provide exactly this functionality). Then, in the check watcher, +you check for any events that occurred (by checking the pending status +of all watchers and stopping them) and call back into the library. The +I/O and timer callbacks will never actually be called (but must be valid +nevertheless, because you never know, you know?). + +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 (it's actually more complicated: it only runs coroutines +with priority higher than or equal to the event loop and one coroutine +of lower priority, but only once, using idle watchers to keep the event +loop from blocking if lower-priority coroutines are active, thus mapping +low-priority coroutines to idle/background tasks). + +It is recommended to give C<ev_check> watchers highest (C<EV_MAXPRI>) +priority, to ensure that they are being run before any other watchers +after the poll (this doesn't matter for C<ev_prepare> watchers). + +Also, C<ev_check> watchers (and C<ev_prepare> watchers, too) should not +activate ("feed") events into libev. While libev fully supports this, they +might get executed before other C<ev_check> watchers did their job. As +C<ev_check> watchers are often used to embed other (non-libev) event +loops those other event loops might be in an unusable state until their +C<ev_check> watcher ran (always remind yourself to coexist peacefully with +others). + +=head3 Watcher-Specific Functions and Data Members + +=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, utterly and completely +pointless. + +=back + +=head3 Examples + +There are a number of principal ways to embed other event loops or modules +into libev. Here are some ideas on how to include libadns into libev +(there is a Perl module named C<EV::ADNS> that does this, which you could +use as a working example. Another Perl module named C<EV::Glib> embeds a +Glib main context into libev, and finally, C<Glib::EV> embeds EV into the +Glib event loop). + +Method 1: Add IO watchers and a timeout watcher in a prepare handler, +and in a check watcher, destroy them and call into libadns. What follows +is pseudo-code only of course. This requires you to either use a low +priority for the check watcher or use C<ev_clear_pending> explicitly, as +the callbacks for the IO/timeout watchers might not have been called yet. + + static ev_io iow [nfd]; + static ev_timer tw; + + static void + io_cb (struct ev_loop *loop, ev_io *w, int revents) + { + } + + // create io watchers for each fd and a timer before blocking + static void + adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents) + { + int timeout = 3600000; + struct pollfd fds [nfd]; + // actual code will need to loop here and realloc etc. + adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ())); + + /* the callback is illegal, but won't be called as we stop during check */ + ev_timer_init (&tw, 0, timeout * 1e-3, 0.); + ev_timer_start (loop, &tw); + + // create one ev_io per pollfd + for (int i = 0; i < nfd; ++i) + { + ev_io_init (iow + i, io_cb, fds [i].fd, + ((fds [i].events & POLLIN ? EV_READ : 0) + | (fds [i].events & POLLOUT ? EV_WRITE : 0))); + + fds [i].revents = 0; + ev_io_start (loop, iow + i); + } + } + + // stop all watchers after blocking + static void + adns_check_cb (struct ev_loop *loop, ev_check *w, int revents) + { + ev_timer_stop (loop, &tw); + + for (int i = 0; i < nfd; ++i) + { + // set the relevant poll flags + // could also call adns_processreadable etc. here + struct pollfd *fd = fds + i; + int revents = ev_clear_pending (iow + i); + if (revents & EV_READ ) fd->revents |= fd->events & POLLIN; + if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT; + + // now stop the watcher + ev_io_stop (loop, iow + i); + } + + adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop)); + } + +Method 2: This would be just like method 1, but you run C<adns_afterpoll> +in the prepare watcher and would dispose of the check watcher. + +Method 3: If the module to be embedded supports explicit event +notification (libadns does), you can also make use of the actual watcher +callbacks, and only destroy/create the watchers in the prepare watcher. + + static void + timer_cb (EV_P_ ev_timer *w, int revents) + { + adns_state ads = (adns_state)w->data; + update_now (EV_A); + + adns_processtimeouts (ads, &tv_now); + } + + static void + io_cb (EV_P_ ev_io *w, int revents) + { + adns_state ads = (adns_state)w->data; + update_now (EV_A); + + if (revents & EV_READ ) adns_processreadable (ads, w->fd, &tv_now); + if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now); + } + + // do not ever call adns_afterpoll + +Method 4: Do not use a prepare or check watcher because the module you +want to embed is not flexible enough to support it. Instead, you can +override their poll function. The drawback with this solution is that the +main loop is now no longer controllable by EV. The C<Glib::EV> module uses +this approach, effectively embedding EV as a client into the horrible +libglib event loop. + + static gint + event_poll_func (GPollFD *fds, guint nfds, gint timeout) + { + int got_events = 0; + + for (n = 0; n < nfds; ++n) + // create/start io watcher that sets the relevant bits in fds[n] and increment got_events + + if (timeout >= 0) + // create/start timer + + // poll + ev_run (EV_A_ 0); + + // stop timer again + if (timeout >= 0) + ev_timer_stop (EV_A_ &to); + + // stop io watchers again - their callbacks should have set + for (n = 0; n < nfds; ++n) + ev_io_stop (EV_A_ iow [n]); + + return got_events; + } + + +=head2 C<ev_embed> - when one backend isn't enough... + +This is a rather advanced watcher type that lets you embed one event loop +into another (currently only C<ev_io> events are supported in the embedded +loop, other types of watchers might be handled in a delayed or incorrect +fashion and must not be used). + +There are primarily two reasons you would want that: work around bugs and +prioritise I/O. + +As an example for a bug workaround, the kqueue backend might only support +sockets on some platform, so it is unusable as generic backend, but you +still want to make use of it because you have many sockets and it scales +so nicely. In this case, you would create a kqueue-based loop and embed +it into your default loop (which might use e.g. poll). Overall operation +will be a bit slower because first libev has to call C<poll> and then +C<kevent>, but at least you can use both mechanisms for what they are +best: C<kqueue> for scalable sockets and C<poll> if you want it to work :) + +As for prioritising I/O: under rare circumstances you have the case where +some fds have to be watched and handled very quickly (with low latency), +and even priorities and idle watchers might have too much overhead. In +this case you would put all the high priority stuff in one loop and all +the rest in a second one, and embed the second one in the first. + +As long as the watcher is active, the callback will be invoked every +time there might be events pending in the embedded loop. The callback +must then call C<ev_embed_sweep (mainloop, watcher)> to make a single +sweep and invoke their callbacks (the callback doesn't need to invoke the +C<ev_embed_sweep> function directly, it could also start an idle watcher +to give the embedded loop strictly lower priority for example). + +You can also set the callback to C<0>, in which case the embed watcher +will automatically execute the embedded loop sweep whenever necessary. + +Fork detection will be handled transparently while the C<ev_embed> watcher +is active, i.e., the embedded loop will automatically be forked when the +embedding loop forks. In other cases, the user is responsible for calling +C<ev_loop_fork> on the embedded loop. + +Unfortunately, not all backends are embeddable: only the ones returned by +C<ev_embeddable_backends> are, which, unfortunately, does not include any +portable one. + +So when you want to use this feature you will always have to be prepared +that you cannot get an embeddable loop. The recommended way to get around +this is to have a separate variables for your embeddable loop, try to +create it, and if that fails, use the normal loop for everything. + +=head3 C<ev_embed> and fork + +While the C<ev_embed> watcher is running, forks in the embedding loop will +automatically be applied to the embedded loop as well, so no special +fork handling is required in that case. When the watcher is not running, +however, it is still the task of the libev user to call C<ev_loop_fork ()> +as applicable. + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop) + +=item ev_embed_set (ev_embed *, callback, struct ev_loop *embedded_loop) + +Configures the watcher to embed the given loop, which must be +embeddable. If the callback is C<0>, then C<ev_embed_sweep> will be +invoked automatically, otherwise it is the responsibility of the callback +to invoke it (it will continue to be called until the sweep has been done, +if you do not want that, you need to temporarily stop the embed watcher). + +=item ev_embed_sweep (loop, ev_embed *) + +Make a single, non-blocking sweep over the embedded loop. This works +similarly to C<ev_run (embedded_loop, EVRUN_NOWAIT)>, but in the most +appropriate way for embedded loops. + +=item struct ev_loop *other [read-only] + +The embedded event loop. + +=back + +=head3 Examples + +Example: Try to get an embeddable event loop and embed it into the default +event loop. If that is not possible, use the default loop. The default +loop is stored in C<loop_hi>, while the embeddable loop is stored in +C<loop_lo> (which is C<loop_hi> in the case no embeddable loop can be +used). + + struct ev_loop *loop_hi = ev_default_init (0); + struct ev_loop *loop_lo = 0; + ev_embed embed; + + // see if there is a chance of getting one that works + // (remember that a flags value of 0 means autodetection) + loop_lo = ev_embeddable_backends () & ev_recommended_backends () + ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ()) + : 0; + + // if we got one, then embed it, otherwise default to loop_hi + if (loop_lo) + { + ev_embed_init (&embed, 0, loop_lo); + ev_embed_start (loop_hi, &embed); + } + else + loop_lo = loop_hi; + +Example: Check if kqueue is available but not recommended and create +a kqueue backend for use with sockets (which usually work with any +kqueue implementation). Store the kqueue/socket-only event loop in +C<loop_socket>. (One might optionally use C<EVFLAG_NOENV>, too). + + struct ev_loop *loop = ev_default_init (0); + struct ev_loop *loop_socket = 0; + ev_embed embed; + + if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE) + if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE)) + { + ev_embed_init (&embed, 0, loop_socket); + ev_embed_start (loop, &embed); + } + + if (!loop_socket) + loop_socket = loop; + + // now use loop_socket for all sockets, and loop for everything else + + +=head2 C<ev_fork> - the audacity to resume the event loop after a fork + +Fork watchers are called when a C<fork ()> was detected (usually because +whoever is a good citizen cared to tell libev about it by calling +C<ev_default_fork> or C<ev_loop_fork>). The invocation is done before the +event loop blocks next and before C<ev_check> watchers are being called, +and only in the child after the fork. If whoever good citizen calling +C<ev_default_fork> cheats and calls it in the wrong process, the fork +handlers will be invoked, too, of course. + +=head3 The special problem of life after fork - how is it possible? + +Most uses of C<fork()> consist of forking, then some simple calls to set +up/change the process environment, followed by a call to C<exec()>. This +sequence should be handled by libev without any problems. + +This changes when the application actually wants to do event handling +in the child, or both parent in child, in effect "continuing" after the +fork. + +The default mode of operation (for libev, with application help to detect +forks) is to duplicate all the state in the child, as would be expected +when I<either> the parent I<or> the child process continues. + +When both processes want to continue using libev, then this is usually the +wrong result. In that case, usually one process (typically the parent) is +supposed to continue with all watchers in place as before, while the other +process typically wants to start fresh, i.e. without any active watchers. + +The cleanest and most efficient way to achieve that with libev is to +simply create a new event loop, which of course will be "empty", and +use that for new watchers. This has the advantage of not touching more +memory than necessary, and thus avoiding the copy-on-write, and the +disadvantage of having to use multiple event loops (which do not support +signal watchers). + +When this is not possible, or you want to use the default loop for +other reasons, then in the process that wants to start "fresh", call +C<ev_loop_destroy (EV_DEFAULT)> followed by C<ev_default_loop (...)>. +Destroying the default loop will "orphan" (not stop) all registered +watchers, so you have to be careful not to execute code that modifies +those watchers. Note also that in that case, you have to re-register any +signal watchers. + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_fork_init (ev_fork *, callback) + +Initialises and configures the fork watcher - it has no parameters of any +kind. There is a C<ev_fork_set> macro, but using it is utterly pointless, +really. + +=back + + +=head2 C<ev_cleanup> - even the best things end + +Cleanup watchers are called just before the event loop is being destroyed +by a call to C<ev_loop_destroy>. + +While there is no guarantee that the event loop gets destroyed, cleanup +watchers provide a convenient method to install cleanup hooks for your +program, worker threads and so on - you just to make sure to destroy the +loop when you want them to be invoked. + +Cleanup watchers are invoked in the same way as any other watcher. Unlike +all other watchers, they do not keep a reference to the event loop (which +makes a lot of sense if you think about it). Like all other watchers, you +can call libev functions in the callback, except C<ev_cleanup_start>. + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_cleanup_init (ev_cleanup *, callback) + +Initialises and configures the cleanup watcher - it has no parameters of +any kind. There is a C<ev_cleanup_set> macro, but using it is utterly +pointless, I assure you. + +=back + +Example: Register an atexit handler to destroy the default loop, so any +cleanup functions are called. + + static void + program_exits (void) + { + ev_loop_destroy (EV_DEFAULT_UC); + } + + ... + atexit (program_exits); + + +=head2 C<ev_async> - how to wake up an event loop + +In general, you cannot use an C<ev_run> from multiple threads or other +asynchronous sources such as signal handlers (as opposed to multiple event +loops - those are of course safe to use in different threads). + +Sometimes, however, you need to wake up an event loop you do not control, +for example because it belongs to another thread. This is what C<ev_async> +watchers do: as long as the C<ev_async> watcher is active, you can signal +it by calling C<ev_async_send>, which is thread- and signal safe. + +This functionality is very similar to C<ev_signal> watchers, as signals, +too, are asynchronous in nature, and signals, too, will be compressed +(i.e. the number of callback invocations may be less than the number of +C<ev_async_sent> calls). In fact, you could use signal watchers as a kind +of "global async watchers" by using a watcher on an otherwise unused +signal, and C<ev_feed_signal> to signal this watcher from another thread, +even without knowing which loop owns the signal. + +Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not +just the default loop. + +=head3 Queueing + +C<ev_async> does not support queueing of data in any way. The reason +is that the author does not know of a simple (or any) algorithm for a +multiple-writer-single-reader queue that works in all cases and doesn't +need elaborate support such as pthreads or unportable memory access +semantics. + +That means that if you want to queue data, you have to provide your own +queue. But at least I can tell you how to implement locking around your +queue: + +=over 4 + +=item queueing from a signal handler context + +To implement race-free queueing, you simply add to the queue in the signal +handler but you block the signal handler in the watcher callback. Here is +an example that does that for some fictitious SIGUSR1 handler: + + static ev_async mysig; + + static void + sigusr1_handler (void) + { + sometype data; + + // no locking etc. + queue_put (data); + ev_async_send (EV_DEFAULT_ &mysig); + } + + static void + mysig_cb (EV_P_ ev_async *w, int revents) + { + sometype data; + sigset_t block, prev; + + sigemptyset (&block); + sigaddset (&block, SIGUSR1); + sigprocmask (SIG_BLOCK, &block, &prev); + + while (queue_get (&data)) + process (data); + + if (sigismember (&prev, SIGUSR1) + sigprocmask (SIG_UNBLOCK, &block, 0); + } + +(Note: pthreads in theory requires you to use C<pthread_setmask> +instead of C<sigprocmask> when you use threads, but libev doesn't do it +either...). + +=item queueing from a thread context + +The strategy for threads is different, as you cannot (easily) block +threads but you can easily preempt them, so to queue safely you need to +employ a traditional mutex lock, such as in this pthread example: + + static ev_async mysig; + static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER; + + static void + otherthread (void) + { + // only need to lock the actual queueing operation + pthread_mutex_lock (&mymutex); + queue_put (data); + pthread_mutex_unlock (&mymutex); + + ev_async_send (EV_DEFAULT_ &mysig); + } + + static void + mysig_cb (EV_P_ ev_async *w, int revents) + { + pthread_mutex_lock (&mymutex); + + while (queue_get (&data)) + process (data); + + pthread_mutex_unlock (&mymutex); + } + +=back + + +=head3 Watcher-Specific Functions and Data Members + +=over 4 + +=item ev_async_init (ev_async *, callback) + +Initialises and configures the async watcher - it has no parameters of any +kind. There is a C<ev_async_set> macro, but using it is utterly pointless, +trust me. + +=item ev_async_send (loop, ev_async *) + +Sends/signals/activates the given C<ev_async> watcher, that is, feeds +an C<EV_ASYNC> event on the watcher into the event loop. Unlike +C<ev_feed_event>, this call is safe to do from other threads, signal or +similar contexts (see the discussion of C<EV_ATOMIC_T> in the embedding +section below on what exactly this means). + +Note that, as with other watchers in libev, multiple events might get +compressed into a single callback invocation (another way to look at this +is that C<ev_async> watchers are level-triggered, set on C<ev_async_send>, +reset when the event loop detects that). + +This call incurs the overhead of a system call only once per event loop +iteration, so while the overhead might be noticeable, it doesn't apply to +repeated calls to C<ev_async_send> for the same event loop. + +=item bool = ev_async_pending (ev_async *) + +Returns a non-zero value when C<ev_async_send> has been called on the +watcher but the event has not yet been processed (or even noted) by the +event loop. + +C<ev_async_send> sets a flag in the watcher and wakes up the loop. When +the loop iterates next and checks for the watcher to have become active, +it will reset the flag again. C<ev_async_pending> can be used to very +quickly check whether invoking the loop might be a good idea. + +Not that this does I<not> check whether the watcher itself is pending, +only whether it has been requested to make this watcher pending: there +is a time window between the event loop checking and resetting the async +notification, and the callback being invoked. + +=back + + +=head1 OTHER FUNCTIONS + +There are some other functions 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 stops both +watchers. This is useful if you want to wait for a single event on an fd +or timeout without having 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 the +C<events> argument is being ignored. Otherwise, an C<ev_io> watcher for +the given C<fd> and C<events> set will be created and started. + +If C<timeout> is less than 0, then no timeout watcher will be +started. Otherwise an C<ev_timer> watcher with after = C<timeout> (and +repeat = 0) will be started. C<0> is a valid timeout. + +The callback has the type C<void (*cb)(int revents, void *arg)> and is +passed an C<revents> set like normal event callbacks (a combination of +C<EV_ERROR>, C<EV_READ>, C<EV_WRITE> or C<EV_TIMER>) and the C<arg> +value passed to C<ev_once>. Note that it is possible to receive I<both> +a timeout and an io event at the same time - you probably should give io +events precedence. + +Example: wait up to ten seconds for data to appear on STDIN_FILENO. + + static void stdin_ready (int revents, void *arg) + { + if (revents & EV_READ) + /* stdin might have data for us, joy! */; + else if (revents & EV_TIMER) + /* doh, nothing entered */; + } + + ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0); + +=item ev_feed_fd_event (loop, int fd, int revents) + +Feed an event on the given fd, as if a file descriptor backend detected +the given events it. + +=item ev_feed_signal_event (loop, int signum) + +Feed an event as if the given signal occurred. See also C<ev_feed_signal>, +which is async-safe. + +=back + + +=head1 COMMON OR USEFUL IDIOMS (OR BOTH) + +This section explains some common idioms that are not immediately +obvious. Note that examples are sprinkled over the whole manual, and this +section only contains stuff that wouldn't fit anywhere else. + +=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER + +Each watcher has, by default, a C<void *data> member that you can read +or modify 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 separately 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 + { + 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, ev_io *w_, int revents) + { + struct my_io *w = (struct my_io *)w_; + ... + } + +More interesting and less C-conformant ways of casting your callback +function type instead have been omitted. + +=head2 BUILDING YOUR OWN COMPOSITE WATCHERS + +Another common scenario is to use some data structure with multiple +embedded watchers, in effect creating your own watcher that combines +multiple libev event sources into one "super-watcher": + + struct my_biggy + { + int some_data; + ev_timer t1; + ev_timer t2; + } + +In this case getting the pointer to C<my_biggy> is a bit more +complicated: Either you store the address of your C<my_biggy> struct in +the C<data> member of the watcher (for woozies or C++ coders), or you need +to use some pointer arithmetic using C<offsetof> inside your watchers (for +real programmers): + + #include <stddef.h> + + 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)); + } + +=head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS + +Often (especially in GUI toolkits) there are places where you have +I<modal> interaction, which is most easily implemented by recursively +invoking C<ev_run>. + +This brings the problem of exiting - a callback might want to finish the +main C<ev_run> call, but not the nested one (e.g. user clicked "Quit", but +a modal "Are you sure?" dialog is still waiting), or just the nested one +and not the main one (e.g. user clocked "Ok" in a modal dialog), or some +other combination: In these cases, C<ev_break> will not work alone. + +The solution is to maintain "break this loop" variable for each C<ev_run> +invocation, and use a loop around C<ev_run> until the condition is +triggered, using C<EVRUN_ONCE>: + + // main loop + int exit_main_loop = 0; + + while (!exit_main_loop) + ev_run (EV_DEFAULT_ EVRUN_ONCE); + + // in a model watcher + int exit_nested_loop = 0; + + while (!exit_nested_loop) + ev_run (EV_A_ EVRUN_ONCE); + +To exit from any of these loops, just set the corresponding exit variable: + + // exit modal loop + exit_nested_loop = 1; + + // exit main program, after modal loop is finished + exit_main_loop = 1; + + // exit both + exit_main_loop = exit_nested_loop = 1; + +=head2 THREAD LOCKING EXAMPLE + +Here is a fictitious example of how to run an event loop in a different +thread from where callbacks are being invoked and watchers are +created/added/removed. + +For a real-world example, see the C<EV::Loop::Async> perl module, +which uses exactly this technique (which is suited for many high-level +languages). + +The example uses a pthread mutex to protect the loop data, a condition +variable to wait for callback invocations, an async watcher to notify the +event loop thread and an unspecified mechanism to wake up the main thread. + +First, you need to associate some data with the event loop: + + typedef struct { + mutex_t lock; /* global loop lock */ + ev_async async_w; + thread_t tid; + cond_t invoke_cv; + } userdata; + + void prepare_loop (EV_P) + { + // for simplicity, we use a static userdata struct. + static userdata u; + + ev_async_init (&u->async_w, async_cb); + ev_async_start (EV_A_ &u->async_w); + + pthread_mutex_init (&u->lock, 0); + pthread_cond_init (&u->invoke_cv, 0); + + // now associate this with the loop + ev_set_userdata (EV_A_ u); + ev_set_invoke_pending_cb (EV_A_ l_invoke); + ev_set_loop_release_cb (EV_A_ l_release, l_acquire); + + // then create the thread running ev_loop + pthread_create (&u->tid, 0, l_run, EV_A); + } + +The callback for the C<ev_async> watcher does nothing: the watcher is used +solely to wake up the event loop so it takes notice of any new watchers +that might have been added: + + static void + async_cb (EV_P_ ev_async *w, int revents) + { + // just used for the side effects + } + +The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex +protecting the loop data, respectively. + + static void + l_release (EV_P) + { + userdata *u = ev_userdata (EV_A); + pthread_mutex_unlock (&u->lock); + } + + static void + l_acquire (EV_P) + { + userdata *u = ev_userdata (EV_A); + pthread_mutex_lock (&u->lock); + } + +The event loop thread first acquires the mutex, and then jumps straight +into C<ev_run>: + + void * + l_run (void *thr_arg) + { + struct ev_loop *loop = (struct ev_loop *)thr_arg; + + l_acquire (EV_A); + pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0); + ev_run (EV_A_ 0); + l_release (EV_A); + + return 0; + } + +Instead of invoking all pending watchers, the C<l_invoke> callback will +signal the main thread via some unspecified mechanism (signals? pipe +writes? C<Async::Interrupt>?) and then waits until all pending watchers +have been called (in a while loop because a) spurious wakeups are possible +and b) skipping inter-thread-communication when there are no pending +watchers is very beneficial): + + static void + l_invoke (EV_P) + { + userdata *u = ev_userdata (EV_A); + + while (ev_pending_count (EV_A)) + { + wake_up_other_thread_in_some_magic_or_not_so_magic_way (); + pthread_cond_wait (&u->invoke_cv, &u->lock); + } + } + +Now, whenever the main thread gets told to invoke pending watchers, it +will grab the lock, call C<ev_invoke_pending> and then signal the loop +thread to continue: + + static void + real_invoke_pending (EV_P) + { + userdata *u = ev_userdata (EV_A); + + pthread_mutex_lock (&u->lock); + ev_invoke_pending (EV_A); + pthread_cond_signal (&u->invoke_cv); + pthread_mutex_unlock (&u->lock); + } + +Whenever you want to start/stop a watcher or do other modifications to an +event loop, you will now have to lock: + + ev_timer timeout_watcher; + userdata *u = ev_userdata (EV_A); + + ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.); + + pthread_mutex_lock (&u->lock); + ev_timer_start (EV_A_ &timeout_watcher); + ev_async_send (EV_A_ &u->async_w); + pthread_mutex_unlock (&u->lock); + +Note that sending the C<ev_async> watcher is required because otherwise +an event loop currently blocking in the kernel will have no knowledge +about the newly added timer. By waking up the loop it will pick up any new +watchers in the next event loop iteration. + +=head2 THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS + +While the overhead of a callback that e.g. schedules a thread is small, it +is still an overhead. If you embed libev, and your main usage is with some +kind of threads or coroutines, you might want to customise libev so that +doesn't need callbacks anymore. + +Imagine you have coroutines that you can switch to using a function +C<switch_to (coro)>, that libev runs in a coroutine called C<libev_coro> +and that due to some magic, the currently active coroutine is stored in a +global called C<current_coro>. Then you can build your own "wait for libev +event" primitive by changing C<EV_CB_DECLARE> and C<EV_CB_INVOKE> (note +the differing C<;> conventions): + + #define EV_CB_DECLARE(type) struct my_coro *cb; + #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb) + +That means instead of having a C callback function, you store the +coroutine to switch to in each watcher, and instead of having libev call +your callback, you instead have it switch to that coroutine. + +A coroutine might now wait for an event with a function called +C<wait_for_event>. (the watcher needs to be started, as always, but it doesn't +matter when, or whether the watcher is active or not when this function is +called): + + void + wait_for_event (ev_watcher *w) + { + ev_cb_set (w) = current_coro; + switch_to (libev_coro); + } + +That basically suspends the coroutine inside C<wait_for_event> and +continues the libev coroutine, which, when appropriate, switches back to +this or any other coroutine. I am sure if you sue this your own :) + +You can do similar tricks if you have, say, threads with an event queue - +instead of storing a coroutine, you store the queue object and instead of +switching to a coroutine, you push the watcher onto the queue and notify +any waiters. + +To embed libev, see L<EMBEDDING>, but in short, it's easiest to create two +files, F<my_ev.h> and F<my_ev.c> that include the respective libev files: + + // my_ev.h + #define EV_CB_DECLARE(type) struct my_coro *cb; + #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb); + #include "../libev/ev.h" + + // my_ev.c + #define EV_H "my_ev.h" + #include "../libev/ev.c" + +And then use F<my_ev.h> when you would normally use F<ev.h>, and compile +F<my_ev.c> into your project. When properly specifying include paths, you +can even use F<ev.h> as header file name directly. + + +=head1 LIBEVENT EMULATION + +Libev offers a compatibility emulation layer for libevent. It cannot +emulate the internals of libevent, so here are some usage hints: + +=over 4 + +=item * Only the libevent-1.4.1-beta API is being emulated. + +This was the newest libevent version available when libev was implemented, +and is still mostly unchanged in 2010. + +=item * Use it by including <event.h>, as usual. + +=item * The following members are fully supported: ev_base, ev_callback, +ev_arg, ev_fd, ev_res, ev_events. + +=item * Avoid using ev_flags and the EVLIST_*-macros, while it is +maintained by libev, it does not work exactly the same way as in libevent (consider +it a private API). + +=item * Priorities are not currently supported. Initialising priorities +will fail and all watchers will have the same priority, even though there +is an ev_pri field. + +=item * In libevent, the last base created gets the signals, in libev, the +base that registered the signal gets the signals. + +=item * Other members are not supported. + +=item * The libev emulation is I<not> ABI compatible to libevent, you need +to use the libev header file and library. + +=back + +=head1 C++ SUPPORT + +Libev comes with some simplistic wrapper classes for C++ that mainly allow +you to use some convenience methods to start/stop watchers and also change +the callback model to a model using method callbacks on objects. + +To use it, + + #include <ev++.h> + +This automatically includes F<ev.h> and puts all of its definitions (many +of them macros) into the global namespace. All C++ specific things are +put into the C<ev> namespace. It should support all the same embedding +options as F<ev.h>, most notably C<EV_MULTIPLICITY>. + +Care has been taken to keep the overhead low. The only data member the C++ +classes add (compared to plain C-style watchers) is the event loop pointer +that the watcher is associated with (or no additional members at all if +you disable C<EV_MULTIPLICITY> when embedding libev). + +Currently, functions, static and non-static member functions and classes +with C<operator ()> can be used as callbacks. Other types should be easy +to add as long as they only need one additional pointer for context. If +you need support for other types of functors please contact the author +(preferably after implementing it). + +Here is a list of things available in the C<ev> namespace: + +=over 4 + +=item C<ev::READ>, C<ev::WRITE> etc. + +These are just enum values with the same values as the C<EV_READ> etc. +macros from F<ev.h>. + +=item C<ev::tstamp>, C<ev::now> + +Aliases to the same types/functions as with the C<ev_> prefix. + +=item C<ev::io>, C<ev::timer>, C<ev::periodic>, C<ev::idle>, C<ev::sig> etc. + +For each C<ev_TYPE> watcher in F<ev.h> there is a corresponding class of +the same name in the C<ev> namespace, with the exception of C<ev_signal> +which is called C<ev::sig> to avoid clashes with the C<signal> macro +defines by many implementations. + +All of those classes have these methods: + +=over 4 + +=item ev::TYPE::TYPE () + +=item ev::TYPE::TYPE (loop) + +=item ev::TYPE::~TYPE + +The constructor (optionally) takes an event loop to associate the watcher +with. If it is omitted, it will use C<EV_DEFAULT>. + +The constructor calls C<ev_init> for you, which means you have to call the +C<set> method before starting it. + +It will not set a callback, however: You have to call the templated C<set> +method to set a callback before you can start the watcher. + +(The reason why you have to use a method is a limitation in C++ which does +not allow explicit template arguments for constructors). + +The destructor automatically stops the watcher if it is active. + +=item w->set<class, &class::method> (object *) + +This method sets the callback method to call. The method has to have a +signature of C<void (*)(ev_TYPE &, int)>, it receives the watcher as +first argument and the C<revents> as second. The object must be given as +parameter and is stored in the C<data> member of the watcher. + +This method synthesizes efficient thunking code to call your method from +the C callback that libev requires. If your compiler can inline your +callback (i.e. it is visible to it at the place of the C<set> call and +your compiler is good :), then the method will be fully inlined into the +thunking function, making it as fast as a direct C callback. + +Example: simple class declaration and watcher initialisation + + struct myclass + { + void io_cb (ev::io &w, int revents) { } + } + + myclass obj; + ev::io iow; + iow.set <myclass, &myclass::io_cb> (&obj); + +=item w->set (object *) + +This is a variation of a method callback - leaving out the method to call +will default the method to C<operator ()>, which makes it possible to use +functor objects without having to manually specify the C<operator ()> all +the time. Incidentally, you can then also leave out the template argument +list. + +The C<operator ()> method prototype must be C<void operator ()(watcher &w, +int revents)>. + +See the method-C<set> above for more details. + +Example: use a functor object as callback. + + struct myfunctor + { + void operator() (ev::io &w, int revents) + { + ... + } + } + + myfunctor f; + + ev::io w; + w.set (&f); + +=item w->set<function> (void *data = 0) + +Also sets a callback, but uses a static method or plain function as +callback. The optional C<data> argument will be stored in the watcher's +C<data> member and is free for you to use. + +The prototype of the C<function> must be C<void (*)(ev::TYPE &w, int)>. + +See the method-C<set> above for more details. + +Example: Use a plain function as callback. + + static void io_cb (ev::io &w, int revents) { } + iow.set <io_cb> (); + +=item w->set (loop) + +Associates a different C<struct ev_loop> with this watcher. You can only +do this when the watcher is inactive (and not pending either). + +=item w->set ([arguments]) + +Basically the same as C<ev_TYPE_set>, with the same arguments. Either this +method or a suitable start method must be called at least once. Unlike the +C counterpart, an active watcher gets automatically stopped and restarted +when reconfiguring it with this method. + +=item w->start () + +Starts the watcher. Note that there is no C<loop> argument, as the +constructor already stores the event loop. + +=item w->start ([arguments]) + +Instead of calling C<set> and C<start> methods separately, it is often +convenient to wrap them in one call. Uses the same type of arguments as +the configure C<set> method of the watcher. + +=item w->stop () + +Stops the watcher if it is active. Again, no C<loop> argument. + +=item w->again () (C<ev::timer>, C<ev::periodic> only) + +For C<ev::timer> and C<ev::periodic>, this invokes the corresponding +C<ev_TYPE_again> function. + +=item w->sweep () (C<ev::embed> only) + +Invokes C<ev_embed_sweep>. + +=item w->update () (C<ev::stat> only) + +Invokes C<ev_stat_stat>. + +=back + +=back + +Example: Define a class with two I/O and idle watchers, start the I/O +watchers in the constructor. + + class myclass + { + ev::io io ; void io_cb (ev::io &w, int revents); + ev::io2 io2 ; void io2_cb (ev::io &w, int revents); + ev::idle idle; void idle_cb (ev::idle &w, int revents); + + myclass (int fd) + { + io .set <myclass, &myclass::io_cb > (this); + io2 .set <myclass, &myclass::io2_cb > (this); + idle.set <myclass, &myclass::idle_cb> (this); + + io.set (fd, ev::WRITE); // configure the watcher + io.start (); // start it whenever convenient + + io2.start (fd, ev::READ); // set + start in one call + } + }; + + +=head1 OTHER LANGUAGE BINDINGS + +Libev does not offer other language bindings itself, but bindings for a +number of languages exist in the form of third-party packages. If you know +any interesting language binding in addition to the ones listed here, drop +me a note. + +=over 4 + +=item Perl + +The EV module implements the full libev API and is actually used to test +libev. EV is developed together with libev. Apart from the EV core module, +there are additional modules that implement libev-compatible interfaces +to C<libadns> (C<EV::ADNS>, but C<AnyEvent::DNS> is preferred nowadays), +C<Net::SNMP> (C<Net::SNMP::EV>) and the C<libglib> event core (C<Glib::EV> +and C<EV::Glib>). + +It can be found and installed via CPAN, its homepage is at +L<http://software.schmorp.de/pkg/EV>. + +=item Python + +Python bindings can be found at L<http://code.google.com/p/pyev/>. It +seems to be quite complete and well-documented. + +=item Ruby + +Tony Arcieri has written a ruby extension that offers access to a subset +of the libev API and adds file handle abstractions, asynchronous DNS and +more on top of it. It can be found via gem servers. Its homepage is at +L<http://rev.rubyforge.org/>. + +Roger Pack reports that using the link order C<-lws2_32 -lmsvcrt-ruby-190> +makes rev work even on mingw. + +=item Haskell + +A haskell binding to libev is available at +L<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>. + +=item D + +Leandro Lucarella has written a D language binding (F<ev.d>) for libev, to +be found at L<http://proj.llucax.com.ar/wiki/evd>. + +=item Ocaml + +Erkki Seppala has written Ocaml bindings for libev, to be found at +L<http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>. + +=item Lua + +Brian Maher has written a partial interface to libev for lua (at the +time of this writing, only C<ev_io> and C<ev_timer>), to be found at +L<http://github.com/brimworks/lua-ev>. + +=back + + +=head1 MACRO MAGIC + +Libev can be compiled with a variety of options, the most fundamental +of which is C<EV_MULTIPLICITY>. This option determines whether (most) +functions and callbacks have an initial C<struct ev_loop *> argument. + +To make it easier to write programs that cope with either variant, the +following macros are defined: + +=over 4 + +=item C<EV_A>, C<EV_A_> + +This provides the loop I<argument> for functions, if one is required ("ev +loop argument"). The C<EV_A> form is used when this is the sole argument, +C<EV_A_> is used when other arguments are following. Example: + + ev_unref (EV_A); + ev_timer_add (EV_A_ watcher); + ev_run (EV_A_ 0); + +It assumes the variable C<loop> of type C<struct ev_loop *> is in scope, +which is often provided by the following macro. + +=item C<EV_P>, C<EV_P_> + +This provides the loop I<parameter> for functions, if one is required ("ev +loop parameter"). The C<EV_P> form is used when this is the sole parameter, +C<EV_P_> is used when other parameters are following. Example: + + // this is how ev_unref is being declared + static void ev_unref (EV_P); + + // this is how you can declare your typical callback + static void cb (EV_P_ ev_timer *w, int revents) + +It declares a parameter C<loop> of type C<struct ev_loop *>, quite +suitable for use with C<EV_A>. + +=item C<EV_DEFAULT>, C<EV_DEFAULT_> + +Similar to the other two macros, this gives you the value of the default +loop, if multiple loops are supported ("ev loop default"). + +=item C<EV_DEFAULT_UC>, C<EV_DEFAULT_UC_> + +Usage identical to C<EV_DEFAULT> and C<EV_DEFAULT_>, but requires that the +default loop has been initialised (C<UC> == unchecked). Their behaviour +is undefined when the default loop has not been initialised by a previous +execution of C<EV_DEFAULT>, C<EV_DEFAULT_> or C<ev_default_init (...)>. + +It is often prudent to use C<EV_DEFAULT> when initialising the first +watcher in a function but use C<EV_DEFAULT_UC> afterwards. + +=back + +Example: Declare and initialise a check watcher, utilising the above +macros so it will work regardless of whether multiple loops are supported +or not. + + static void + check_cb (EV_P_ ev_timer *w, int revents) + { + ev_check_stop (EV_A_ w); + } + + ev_check check; + ev_check_init (&check, check_cb); + ev_check_start (EV_DEFAULT_ &check); + ev_run (EV_DEFAULT_ 0); + +=head1 EMBEDDING + +Libev can (and often is) directly embedded into host +applications. Examples of applications that embed it include the Deliantra +Game Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe) +and rxvt-unicode. + +The goal is to enable you to just copy the necessary files into your +source directory without having to change even a single line in them, so +you can easily upgrade by simply copying (or having a checked-out copy of +libev somewhere in your source tree). + +=head2 FILESETS + +Depending on what features you need you need to include one or more sets of files +in your application. + +=head3 CORE EVENT LOOP + +To include only the libev core (all the C<ev_*> functions), with manual +configuration (no autoconf): + + #define EV_STANDALONE 1 + #include "ev.c" + +This will automatically include F<ev.h>, too, and should be done in a +single C source file only to provide the function implementations. To use +it, do the same for F<ev.h> in all files wishing to use this API (best +done by writing a wrapper around F<ev.h> that you can include instead and +where you can put other configuration options): + + #define EV_STANDALONE 1 + #include "ev.h" + +Both header files and implementation files can be compiled with a C++ +compiler (at least, that's a stated goal, and breakage will be treated +as a bug). + +You need the following files in your source tree, or in a directory +in your include path (e.g. in libev/ when using -Ilibev): + + ev.h + ev.c + ev_vars.h + ev_wrap.h + + ev_win32.c required on win32 platforms only + + ev_select.c only when select backend is enabled (which is enabled by default) + ev_poll.c only when poll backend is enabled (disabled by default) + ev_epoll.c only when the epoll backend is enabled (disabled by default) + ev_kqueue.c only when the kqueue backend is enabled (disabled by default) + ev_port.c only when the solaris port backend is enabled (disabled by default) + +F<ev.c> includes the backend files directly when enabled, so you only need +to compile this single file. + +=head3 LIBEVENT COMPATIBILITY API + +To include the libevent compatibility API, also include: + + #include "event.c" + +in the file including F<ev.c>, and: + + #include "event.h" + +in the files that want to use the libevent API. This also includes F<ev.h>. + +You need the following additional files for this: + + event.h + event.c + +=head3 AUTOCONF SUPPORT + +Instead of using C<EV_STANDALONE=1> and providing your configuration in +whatever way you want, you can also C<m4_include([libev.m4])> in your +F<configure.ac> and leave C<EV_STANDALONE> undefined. F<ev.c> will then +include F<config.h> and configure itself accordingly. + +For this of course you need the m4 file: + + libev.m4 + +=head2 PREPROCESSOR SYMBOLS/MACROS + +Libev can be configured via a variety of preprocessor symbols you have to +define before including (or compiling) any of its files. The default in +the absence of autoconf is documented for every option. + +Symbols marked with "(h)" do not change the ABI, and can have different +values when compiling libev vs. including F<ev.h>, so it is permissible +to redefine them before including F<ev.h> without breaking compatibility +to a compiled library. All other symbols change the ABI, which means all +users of libev and the libev code itself must be compiled with compatible +settings. + +=over 4 + +=item EV_COMPAT3 (h) + +Backwards compatibility is a major concern for libev. This is why this +release of libev comes with wrappers for the functions and symbols that +have been renamed between libev version 3 and 4. + +You can disable these wrappers (to test compatibility with future +versions) by defining C<EV_COMPAT3> to C<0> when compiling your +sources. This has the additional advantage that you can drop the C<struct> +from C<struct ev_loop> declarations, as libev will provide an C<ev_loop> +typedef in that case. + +In some future version, the default for C<EV_COMPAT3> will become C<0>, +and in some even more future version the compatibility code will be +removed completely. + +=item EV_STANDALONE (h) + +Must always be C<1> if you do not use autoconf configuration, which +keeps libev from including F<config.h>, and it also defines dummy +implementations for some libevent functions (such as logging, which is not +supported). It will also not define any of the structs usually found in +F<event.h> that are not directly supported by the libev core alone. + +In standalone mode, libev will still try to automatically deduce the +configuration, but has to be more conservative. + +=item EV_USE_MONOTONIC + +If defined to be C<1>, libev will try to detect the availability of the +monotonic clock option at both compile time and runtime. Otherwise no +use of the monotonic clock option will be attempted. If you enable this, +you usually have to link against librt or something similar. Enabling it +when the functionality isn't available is safe, though, although you have +to make sure you link against any libraries where the C<clock_gettime> +function is hiding in (often F<-lrt>). See also C<EV_USE_CLOCK_SYSCALL>. + +=item EV_USE_REALTIME + +If defined to be C<1>, libev will try to detect the availability of the +real-time clock option at compile time (and assume its availability +at runtime if successful). Otherwise no use of the real-time clock +option will be attempted. This effectively replaces C<gettimeofday> +by C<clock_get (CLOCK_REALTIME, ...)> and will not normally affect +correctness. See the note about libraries in the description of +C<EV_USE_MONOTONIC>, though. Defaults to the opposite value of +C<EV_USE_CLOCK_SYSCALL>. + +=item EV_USE_CLOCK_SYSCALL + +If defined to be C<1>, libev will try to use a direct syscall instead +of calling the system-provided C<clock_gettime> function. This option +exists because on GNU/Linux, C<clock_gettime> is in C<librt>, but C<librt> +unconditionally pulls in C<libpthread>, slowing down single-threaded +programs needlessly. Using a direct syscall is slightly slower (in +theory), because no optimised vdso implementation can be used, but avoids +the pthread dependency. Defaults to C<1> on GNU/Linux with glibc 2.x or +higher, as it simplifies linking (no need for C<-lrt>). + +=item EV_USE_NANOSLEEP + +If defined to be C<1>, libev will assume that C<nanosleep ()> is available +and will use it for delays. Otherwise it will use C<select ()>. + +=item EV_USE_EVENTFD + +If defined to be C<1>, then libev will assume that C<eventfd ()> is +available and will probe for kernel support at runtime. This will improve +C<ev_signal> and C<ev_async> performance and reduce resource consumption. +If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc +2.7 or newer, otherwise disabled. + +=item EV_USE_SELECT + +If undefined or defined to be C<1>, libev will compile in support for the +C<select>(2) backend. No attempt at auto-detection will be done: if no +other method takes over, select will be it. Otherwise the select backend +will not be compiled in. + +=item EV_SELECT_USE_FD_SET + +If defined to C<1>, then the select backend will use the system C<fd_set> +structure. This is useful if libev doesn't compile due to a missing +C<NFDBITS> or C<fd_mask> definition or it mis-guesses the bitset layout +on exotic systems. This usually limits the range of file descriptors to +some low limit such as 1024 or might have other limitations (winsocket +only allows 64 sockets). The C<FD_SETSIZE> macro, set before compilation, +configures the maximum size of the C<fd_set>. + +=item EV_SELECT_IS_WINSOCKET + +When defined to C<1>, the select backend will assume that +select/socket/connect etc. don't understand file descriptors but +wants osf handles on win32 (this is the case when the select to +be used is the winsock select). This means that it will call +C<_get_osfhandle> on the fd to convert it to an OS handle. Otherwise, +it is assumed that all these functions actually work on fds, even +on win32. Should not be defined on non-win32 platforms. + +=item EV_FD_TO_WIN32_HANDLE(fd) + +If C<EV_SELECT_IS_WINSOCKET> is enabled, then libev needs a way to map +file descriptors to socket handles. When not defining this symbol (the +default), then libev will call C<_get_osfhandle>, which is usually +correct. In some cases, programs use their own file descriptor management, +in which case they can provide this function to map fds to socket handles. + +=item EV_WIN32_HANDLE_TO_FD(handle) + +If C<EV_SELECT_IS_WINSOCKET> then libev maps handles to file descriptors +using the standard C<_open_osfhandle> function. For programs implementing +their own fd to handle mapping, overwriting this function makes it easier +to do so. This can be done by defining this macro to an appropriate value. + +=item EV_WIN32_CLOSE_FD(fd) + +If programs implement their own fd to handle mapping on win32, then this +macro can be used to override the C<close> function, useful to unregister +file descriptors again. Note that the replacement function has to close +the underlying OS handle. + +=item EV_USE_POLL + +If defined to be C<1>, libev will compile in support for the C<poll>(2) +backend. Otherwise it will be enabled on non-win32 platforms. It +takes precedence over select. + +=item EV_USE_EPOLL + +If defined to be C<1>, libev will compile in support for the Linux +C<epoll>(7) backend. Its availability will be detected at runtime, +otherwise another method will be used as fallback. This is the preferred +backend for GNU/Linux systems. If undefined, it will be enabled if the +headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled. + +=item EV_USE_KQUEUE + +If defined to be C<1>, libev will compile in support for the BSD style +C<kqueue>(2) backend. Its actual availability will be detected at runtime, +otherwise another method will be used as fallback. This is the preferred +backend for BSD and BSD-like systems, although on most BSDs kqueue only +supports some types of fds correctly (the only platform we found that +supports ptys for example was NetBSD), so kqueue might be compiled in, but +not be used unless explicitly requested. The best way to use it is to find +out whether kqueue supports your type of fd properly and use an embedded +kqueue loop. + +=item EV_USE_PORT + +If defined to be C<1>, libev will compile in support for the Solaris +10 port style backend. Its availability will be detected at runtime, +otherwise another method will be used as fallback. This is the preferred +backend for Solaris 10 systems. + +=item EV_USE_DEVPOLL + +Reserved for future expansion, works like the USE symbols above. + +=item EV_USE_INOTIFY + +If defined to be C<1>, libev will compile in support for the Linux inotify +interface to speed up C<ev_stat> watchers. Its actual availability will +be detected at runtime. If undefined, it will be enabled if the headers +indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled. + +=item EV_ATOMIC_T + +Libev requires an integer type (suitable for storing C<0> or C<1>) whose +access is atomic with respect to other threads or signal contexts. No such +type is easily found in the C language, so you can provide your own type +that you know is safe for your purposes. It is used both for signal handler "locking" +as well as for signal and thread safety in C<ev_async> watchers. + +In the absence of this define, libev will use C<sig_atomic_t volatile> +(from F<signal.h>), which is usually good enough on most platforms. + +=item EV_H (h) + +The name of the F<ev.h> header file used to include it. The default if +undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be +used to virtually rename the F<ev.h> header file in case of conflicts. + +=item EV_CONFIG_H (h) + +If C<EV_STANDALONE> isn't C<1>, this variable can be used to override +F<ev.c>'s idea of where to find the F<config.h> file, similarly to +C<EV_H>, above. + +=item EV_EVENT_H (h) + +Similarly to C<EV_H>, this macro can be used to override F<event.c>'s idea +of how the F<event.h> header can be found, the default is C<"event.h">. + +=item EV_PROTOTYPES (h) + +If defined to be C<0>, then F<ev.h> will not define any function +prototypes, but still define all the structs and other symbols. This is +occasionally useful if you want to provide your own wrapper functions +around libev functions. + +=item EV_MULTIPLICITY + +If undefined or defined to C<1>, then all event-loop-specific functions +will have the C<struct ev_loop *> as first argument, and you can create +additional independent event loops. Otherwise there will be no support +for multiple event loops and there is no first event loop pointer +argument. Instead, all functions act on the single default loop. + +=item EV_MINPRI + +=item EV_MAXPRI + +The range of allowed priorities. C<EV_MINPRI> must be smaller or equal to +C<EV_MAXPRI>, but otherwise there are no non-obvious limitations. You can +provide for more priorities by overriding those symbols (usually defined +to be C<-2> and C<2>, respectively). + +When doing priority-based operations, libev usually has to linearly search +all the priorities, so having many of them (hundreds) uses a lot of space +and time, so using the defaults of five priorities (-2 .. +2) is usually +fine. + +If your embedding application does not need any priorities, defining these +both to C<0> will save some memory and CPU. + +=item EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE, +EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE, +EV_ASYNC_ENABLE, EV_CHILD_ENABLE. + +If undefined or defined to be C<1> (and the platform supports it), then +the respective watcher type is supported. If defined to be C<0>, then it +is not. Disabling watcher types mainly saves code size. + +=item EV_FEATURES + +If you need to shave off some kilobytes of code at the expense of some +speed (but with the full API), you can define this symbol to request +certain subsets of functionality. The default is to enable all features +that can be enabled on the platform. + +A typical way to use this symbol is to define it to C<0> (or to a bitset +with some broad features you want) and then selectively re-enable +additional parts you want, for example if you want everything minimal, +but multiple event loop support, async and child watchers and the poll +backend, use this: + + #define EV_FEATURES 0 + #define EV_MULTIPLICITY 1 + #define EV_USE_POLL 1 + #define EV_CHILD_ENABLE 1 + #define EV_ASYNC_ENABLE 1 + +The actual value is a bitset, it can be a combination of the following +values: + +=over 4 + +=item C<1> - faster/larger code + +Use larger code to speed up some operations. + +Currently this is used to override some inlining decisions (enlarging the +code size by roughly 30% on amd64). + +When optimising for size, use of compiler flags such as C<-Os> with +gcc is recommended, as well as C<-DNDEBUG>, as libev contains a number of +assertions. + +=item C<2> - faster/larger data structures + +Replaces the small 2-heap for timer management by a faster 4-heap, larger +hash table sizes and so on. This will usually further increase code size +and can additionally have an effect on the size of data structures at +runtime. + +=item C<4> - full API configuration + +This enables priorities (sets C<EV_MAXPRI>=2 and C<EV_MINPRI>=-2), and +enables multiplicity (C<EV_MULTIPLICITY>=1). + +=item C<8> - full API + +This enables a lot of the "lesser used" API functions. See C<ev.h> for +details on which parts of the API are still available without this +feature, and do not complain if this subset changes over time. + +=item C<16> - enable all optional watcher types + +Enables all optional watcher types. If you want to selectively enable +only some watcher types other than I/O and timers (e.g. prepare, +embed, async, child...) you can enable them manually by defining +C<EV_watchertype_ENABLE> to C<1> instead. + +=item C<32> - enable all backends + +This enables all backends - without this feature, you need to enable at +least one backend manually (C<EV_USE_SELECT> is a good choice). + +=item C<64> - enable OS-specific "helper" APIs + +Enable inotify, eventfd, signalfd and similar OS-specific helper APIs by +default. + +=back + +Compiling with C<gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1 -DEV_FEATURES=0> +reduces the compiled size of libev from 24.7Kb code/2.8Kb data to 6.5Kb +code/0.3Kb data on my GNU/Linux amd64 system, while still giving you I/O +watchers, timers and monotonic clock support. + +With an intelligent-enough linker (gcc+binutils are intelligent enough +when you use C<-Wl,--gc-sections -ffunction-sections>) functions unused by +your program might be left out as well - a binary starting a timer and an +I/O watcher then might come out at only 5Kb. + +=item EV_AVOID_STDIO + +If this is set to C<1> at compiletime, then libev will avoid using stdio +functions (printf, scanf, perror etc.). This will increase the code size +somewhat, but if your program doesn't otherwise depend on stdio and your +libc allows it, this avoids linking in the stdio library which is quite +big. + +Note that error messages might become less precise when this option is +enabled. + +=item EV_NSIG + +The highest supported signal number, +1 (or, the number of +signals): Normally, libev tries to deduce the maximum number of signals +automatically, but sometimes this fails, in which case it can be +specified. Also, using a lower number than detected (C<32> should be +good for about any system in existence) can save some memory, as libev +statically allocates some 12-24 bytes per signal number. + +=item EV_PID_HASHSIZE + +C<ev_child> watchers use a small hash table to distribute workload by +pid. The default size is C<16> (or C<1> with C<EV_FEATURES> disabled), +usually more than enough. If you need to manage thousands of children you +might want to increase this value (I<must> be a power of two). + +=item EV_INOTIFY_HASHSIZE + +C<ev_stat> watchers use a small hash table to distribute workload by +inotify watch id. The default size is C<16> (or C<1> with C<EV_FEATURES> +disabled), usually more than enough. If you need to manage thousands of +C<ev_stat> watchers you might want to increase this value (I<must> be a +power of two). + +=item EV_USE_4HEAP + +Heaps are not very cache-efficient. To improve the cache-efficiency of the +timer and periodics heaps, libev uses a 4-heap when this symbol is defined +to C<1>. The 4-heap uses more complicated (longer) code but has noticeably +faster performance with many (thousands) of watchers. + +The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it +will be C<0>. + +=item EV_HEAP_CACHE_AT + +Heaps are not very cache-efficient. To improve the cache-efficiency of the +timer and periodics heaps, libev can cache the timestamp (I<at>) within +the heap structure (selected by defining C<EV_HEAP_CACHE_AT> to C<1>), +which uses 8-12 bytes more per watcher and a few hundred bytes more code, +but avoids random read accesses on heap changes. This improves performance +noticeably with many (hundreds) of watchers. + +The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it +will be C<0>. + +=item EV_VERIFY + +Controls how much internal verification (see C<ev_verify ()>) will +be done: If set to C<0>, no internal verification code will be compiled +in. If set to C<1>, then verification code will be compiled in, but not +called. If set to C<2>, then the internal verification code will be +called once per loop, which can slow down libev. If set to C<3>, then the +verification code will be called very frequently, which will slow down +libev considerably. + +The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it +will be C<0>. + +=item EV_COMMON + +By default, all watchers have a C<void *data> member. By redefining +this macro to something else you can include more and other types of +members. You have to define it each time you include one of the files, +though, and it must be identical each time. + +For example, the perl EV module uses something like this: + + #define EV_COMMON \ + SV *self; /* contains this struct */ \ + SV *cb_sv, *fh /* note no trailing ";" */ + +=item EV_CB_DECLARE (type) + +=item EV_CB_INVOKE (watcher, revents) + +=item ev_set_cb (ev, cb) + +Can be used to change the callback member declaration in each watcher, +and the way callbacks are invoked and set. Must expand to a struct member +definition and a statement, respectively. See the F<ev.h> header file for +their default definitions. One possible use for overriding these is to +avoid the C<struct ev_loop *> as first argument in all cases, or to use +method calls instead of plain function calls in C++. + +=back + +=head2 EXPORTED API SYMBOLS + +If you need to re-export the API (e.g. via a DLL) and you need a list of +exported symbols, you can use the provided F<Symbol.*> files which list +all public symbols, one per line: + + Symbols.ev for libev proper + Symbols.event for the libevent emulation + +This can also be used to rename all public symbols to avoid clashes with +multiple versions of libev linked together (which is obviously bad in +itself, but sometimes it is inconvenient to avoid this). + +A sed command like this will create wrapper C<#define>'s that you need to +include before including F<ev.h>: + + <Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h + +This would create a file F<wrap.h> which essentially looks like this: + + #define ev_backend myprefix_ev_backend + #define ev_check_start myprefix_ev_check_start + #define ev_check_stop myprefix_ev_check_stop + ... + +=head2 EXAMPLES + +For a real-world example of a program the includes libev +verbatim, you can have a look at the EV perl module +(L<http://software.schmorp.de/pkg/EV.html>). It has the libev files in +the F<libev/> subdirectory and includes them in the F<EV/EVAPI.h> (public +interface) and F<EV.xs> (implementation) files. Only the F<EV.xs> file +will be compiled. It is pretty complex because it provides its own header +file. + +The usage in rxvt-unicode is simpler. It has a F<ev_cpp.h> header file +that everybody includes and which overrides some configure choices: + + #define EV_FEATURES 8 + #define EV_USE_SELECT 1 + #define EV_PREPARE_ENABLE 1 + #define EV_IDLE_ENABLE 1 + #define EV_SIGNAL_ENABLE 1 + #define EV_CHILD_ENABLE 1 + #define EV_USE_STDEXCEPT 0 + #define EV_CONFIG_H <config.h> + + #include "ev++.h" + +And a F<ev_cpp.C> implementation file that contains libev proper and is compiled: + + #include "ev_cpp.h" + #include "ev.c" + +=head1 INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT + +=head2 THREADS AND COROUTINES + +=head3 THREADS + +All libev functions are reentrant and thread-safe unless explicitly +documented otherwise, but libev implements no locking itself. This means +that you can use as many loops as you want in parallel, as long as there +are no concurrent calls into any libev function with the same loop +parameter (C<ev_default_*> calls have an implicit default loop parameter, +of course): libev guarantees that different event loops share no data +structures that need any locking. + +Or to put it differently: calls with different loop parameters can be done +concurrently from multiple threads, calls with the same loop parameter +must be done serially (but can be done from different threads, as long as +only one thread ever is inside a call at any point in time, e.g. by using +a mutex per loop). + +Specifically to support threads (and signal handlers), libev implements +so-called C<ev_async> watchers, which allow some limited form of +concurrency on the same event loop, namely waking it up "from the +outside". + +If you want to know which design (one loop, locking, or multiple loops +without or something else still) is best for your problem, then I cannot +help you, but here is some generic advice: + +=over 4 + +=item * most applications have a main thread: use the default libev loop +in that thread, or create a separate thread running only the default loop. + +This helps integrating other libraries or software modules that use libev +themselves and don't care/know about threading. + +=item * one loop per thread is usually a good model. + +Doing this is almost never wrong, sometimes a better-performance model +exists, but it is always a good start. + +=item * other models exist, such as the leader/follower pattern, where one +loop is handed through multiple threads in a kind of round-robin fashion. + +Choosing a model is hard - look around, learn, know that usually you can do +better than you currently do :-) + +=item * often you need to talk to some other thread which blocks in the +event loop. + +C<ev_async> watchers can be used to wake them up from other threads safely +(or from signal contexts...). + +An example use would be to communicate signals or other events that only +work in the default loop by registering the signal watcher with the +default loop and triggering an C<ev_async> watcher from the default loop +watcher callback into the event loop interested in the signal. + +=back + +See also L<THREAD LOCKING EXAMPLE>. + +=head3 COROUTINES + +Libev is very accommodating to coroutines ("cooperative threads"): +libev fully supports nesting calls to its functions from different +coroutines (e.g. you can call C<ev_run> on the same loop from two +different coroutines, and switch freely between both coroutines running +the loop, as long as you don't confuse yourself). The only exception is +that you must not do this from C<ev_periodic> reschedule callbacks. + +Care has been taken to ensure that libev does not keep local state inside +C<ev_run>, and other calls do not usually allow for coroutine switches as +they do not call any callbacks. + +=head2 COMPILER WARNINGS + +Depending on your compiler and compiler settings, you might get no or a +lot of warnings when compiling libev code. Some people are apparently +scared by this. + +However, these are unavoidable for many reasons. For one, each compiler +has different warnings, and each user has different tastes regarding +warning options. "Warn-free" code therefore cannot be a goal except when +targeting a specific compiler and compiler-version. + +Another reason is that some compiler warnings require elaborate +workarounds, or other changes to the code that make it less clear and less +maintainable. + +And of course, some compiler warnings are just plain stupid, or simply +wrong (because they don't actually warn about the condition their message +seems to warn about). For example, certain older gcc versions had some +warnings that resulted in an extreme number of false positives. These have +been fixed, but some people still insist on making code warn-free with +such buggy versions. + +While libev is written to generate as few warnings as possible, +"warn-free" code is not a goal, and it is recommended not to build libev +with any compiler warnings enabled unless you are prepared to cope with +them (e.g. by ignoring them). Remember that warnings are just that: +warnings, not errors, or proof of bugs. + + +=head2 VALGRIND + +Valgrind has a special section here because it is a popular tool that is +highly useful. Unfortunately, valgrind reports are very hard to interpret. + +If you think you found a bug (memory leak, uninitialised data access etc.) +in libev, then check twice: If valgrind reports something like: + + ==2274== definitely lost: 0 bytes in 0 blocks. + ==2274== possibly lost: 0 bytes in 0 blocks. + ==2274== still reachable: 256 bytes in 1 blocks. + +Then there is no memory leak, just as memory accounted to global variables +is not a memleak - the memory is still being referenced, and didn't leak. + +Similarly, under some circumstances, valgrind might report kernel bugs +as if it were a bug in libev (e.g. in realloc or in the poll backend, +although an acceptable workaround has been found here), or it might be +confused. + +Keep in mind that valgrind is a very good tool, but only a tool. Don't +make it into some kind of religion. + +If you are unsure about something, feel free to contact the mailing list +with the full valgrind report and an explanation on why you think this +is a bug in libev (best check the archives, too :). However, don't be +annoyed when you get a brisk "this is no bug" answer and take the chance +of learning how to interpret valgrind properly. + +If you need, for some reason, empty reports from valgrind for your project +I suggest using suppression lists. + + +=head1 PORTABILITY NOTES + +=head2 GNU/LINUX 32 BIT LIMITATIONS + +GNU/Linux is the only common platform that supports 64 bit file/large file +interfaces but I<disables> them by default. + +That means that libev compiled in the default environment doesn't support +files larger than 2GiB or so, which mainly affects C<ev_stat> watchers. + +Unfortunately, many programs try to work around this GNU/Linux issue +by enabling the large file API, which makes them incompatible with the +standard libev compiled for their system. + +Likewise, libev cannot enable the large file API itself as this would +suddenly make it incompatible to the default compile time environment, +i.e. all programs not using special compile switches. + +=head2 OS/X AND DARWIN BUGS + +The whole thing is a bug if you ask me - basically any system interface +you touch is broken, whether it is locales, poll, kqueue or even the +OpenGL drivers. + +=head3 C<kqueue> is buggy + +The kqueue syscall is broken in all known versions - most versions support +only sockets, many support pipes. + +Libev tries to work around this by not using C<kqueue> by default on this +rotten platform, but of course you can still ask for it when creating a +loop - embedding a socket-only kqueue loop into a select-based one is +probably going to work well. + +=head3 C<poll> is buggy + +Instead of fixing C<kqueue>, Apple replaced their (working) C<poll> +implementation by something calling C<kqueue> internally around the 10.5.6 +release, so now C<kqueue> I<and> C<poll> are broken. + +Libev tries to work around this by not using C<poll> by default on +this rotten platform, but of course you can still ask for it when creating +a loop. + +=head3 C<select> is buggy + +All that's left is C<select>, and of course Apple found a way to fuck this +one up as well: On OS/X, C<select> actively limits the number of file +descriptors you can pass in to 1024 - your program suddenly crashes when +you use more. + +There is an undocumented "workaround" for this - defining +C<_DARWIN_UNLIMITED_SELECT>, which libev tries to use, so select I<should> +work on OS/X. + +=head2 SOLARIS PROBLEMS AND WORKAROUNDS + +=head3 C<errno> reentrancy + +The default compile environment on Solaris is unfortunately so +thread-unsafe that you can't even use components/libraries compiled +without C<-D_REENTRANT> in a threaded program, which, of course, isn't +defined by default. A valid, if stupid, implementation choice. + +If you want to use libev in threaded environments you have to make sure +it's compiled with C<_REENTRANT> defined. + +=head3 Event port backend + +The scalable event interface for Solaris is called "event +ports". Unfortunately, this mechanism is very buggy in all major +releases. If you run into high CPU usage, your program freezes or you get +a large number of spurious wakeups, make sure you have all the relevant +and latest kernel patches applied. No, I don't know which ones, but there +are multiple ones to apply, and afterwards, event ports actually work +great. + +If you can't get it to work, you can try running the program by setting +the environment variable C<LIBEV_FLAGS=3> to only allow C<poll> and +C<select> backends. + +=head2 AIX POLL BUG + +AIX unfortunately has a broken C<poll.h> header. Libev works around +this by trying to avoid the poll backend altogether (i.e. it's not even +compiled in), which normally isn't a big problem as C<select> works fine +with large bitsets on AIX, and AIX is dead anyway. + +=head2 WIN32 PLATFORM LIMITATIONS AND WORKAROUNDS + +=head3 General issues + +Win32 doesn't support any of the standards (e.g. POSIX) that libev +requires, and its I/O model is fundamentally incompatible with the POSIX +model. Libev still offers limited functionality on this platform in +the form of the C<EVBACKEND_SELECT> backend, and only supports socket +descriptors. This only applies when using Win32 natively, not when using +e.g. cygwin. Actually, it only applies to the microsofts own compilers, +as every compielr comes with a slightly differently broken/incompatible +environment. + +Lifting these limitations would basically require the full +re-implementation of the I/O system. If you are into this kind of thing, +then note that glib does exactly that for you in a very portable way (note +also that glib is the slowest event library known to man). + +There is no supported compilation method available on windows except +embedding it into other applications. + +Sensible signal handling is officially unsupported by Microsoft - libev +tries its best, but under most conditions, signals will simply not work. + +Not a libev limitation but worth mentioning: windows apparently doesn't +accept large writes: instead of resulting in a partial write, windows will +either accept everything or return C<ENOBUFS> if the buffer is too large, +so make sure you only write small amounts into your sockets (less than a +megabyte seems safe, but this apparently depends on the amount of memory +available). + +Due to the many, low, and arbitrary limits on the win32 platform and +the abysmal performance of winsockets, using a large number of sockets +is not recommended (and not reasonable). If your program needs to use +more than a hundred or so sockets, then likely it needs to use a totally +different implementation for windows, as libev offers the POSIX readiness +notification model, which cannot be implemented efficiently on windows +(due to Microsoft monopoly games). + +A typical way to use libev under windows is to embed it (see the embedding +section for details) and use the following F<evwrap.h> header file instead +of F<ev.h>: + + #define EV_STANDALONE /* keeps ev from requiring config.h */ + #define EV_SELECT_IS_WINSOCKET 1 /* configure libev for windows select */ + + #include "ev.h" + +And compile the following F<evwrap.c> file into your project (make sure +you do I<not> compile the F<ev.c> or any other embedded source files!): + + #include "evwrap.h" + #include "ev.c" + +=head3 The winsocket C<select> function + +The winsocket C<select> function doesn't follow POSIX in that it +requires socket I<handles> and not socket I<file descriptors> (it is +also extremely buggy). This makes select very inefficient, and also +requires a mapping from file descriptors to socket handles (the Microsoft +C runtime provides the function C<_open_osfhandle> for this). See the +discussion of the C<EV_SELECT_USE_FD_SET>, C<EV_SELECT_IS_WINSOCKET> and +C<EV_FD_TO_WIN32_HANDLE> preprocessor symbols for more info. + +The configuration for a "naked" win32 using the Microsoft runtime +libraries and raw winsocket select is: + + #define EV_USE_SELECT 1 + #define EV_SELECT_IS_WINSOCKET 1 /* forces EV_SELECT_USE_FD_SET, too */ + +Note that winsockets handling of fd sets is O(n), so you can easily get a +complexity in the O(n²) range when using win32. + +=head3 Limited number of file descriptors + +Windows has numerous arbitrary (and low) limits on things. + +Early versions of winsocket's select only supported waiting for a maximum +of C<64> handles (probably owning to the fact that all windows kernels +can only wait for C<64> things at the same time internally; Microsoft +recommends spawning a chain of threads and wait for 63 handles and the +previous thread in each. Sounds great!). + +Newer versions support more handles, but you need to define C<FD_SETSIZE> +to some high number (e.g. C<2048>) before compiling the winsocket select +call (which might be in libev or elsewhere, for example, perl and many +other interpreters do their own select emulation on windows). + +Another limit is the number of file descriptors in the Microsoft runtime +libraries, which by default is C<64> (there must be a hidden I<64> +fetish or something like this inside Microsoft). You can increase this +by calling C<_setmaxstdio>, which can increase this limit to C<2048> +(another arbitrary limit), but is broken in many versions of the Microsoft +runtime libraries. This might get you to about C<512> or C<2048> sockets +(depending on windows version and/or the phase of the moon). To get more, +you need to wrap all I/O functions and provide your own fd management, but +the cost of calling select (O(n²)) will likely make this unworkable. + +=head2 PORTABILITY REQUIREMENTS + +In addition to a working ISO-C implementation and of course the +backend-specific APIs, libev relies on a few additional extensions: + +=over 4 + +=item C<void (*)(ev_watcher_type *, int revents)> must have compatible +calling conventions regardless of C<ev_watcher_type *>. + +Libev assumes not only that all watcher pointers have the same internal +structure (guaranteed by POSIX but not by ISO C for example), but it also +assumes that the same (machine) code can be used to call any watcher +callback: The watcher callbacks have different type signatures, but libev +calls them using an C<ev_watcher *> internally. + +=item pointer accesses must be thread-atomic + +Accessing a pointer value must be atomic, it must both be readable and +writable in one piece - this is the case on all current architectures. + +=item C<sig_atomic_t volatile> must be thread-atomic as well + +The type C<sig_atomic_t volatile> (or whatever is defined as +C<EV_ATOMIC_T>) must be atomic with respect to accesses from different +threads. This is not part of the specification for C<sig_atomic_t>, but is +believed to be sufficiently portable. + +=item C<sigprocmask> must work in a threaded environment + +Libev uses C<sigprocmask> to temporarily block signals. This is not +allowed in a threaded program (C<pthread_sigmask> has to be used). Typical +pthread implementations will either allow C<sigprocmask> in the "main +thread" or will block signals process-wide, both behaviours would +be compatible with libev. Interaction between C<sigprocmask> and +C<pthread_sigmask> could complicate things, however. + +The most portable way to handle signals is to block signals in all threads +except the initial one, and run the default loop in the initial thread as +well. + +=item C<long> must be large enough for common memory allocation sizes + +To improve portability and simplify its API, libev uses C<long> internally +instead of C<size_t> when allocating its data structures. On non-POSIX +systems (Microsoft...) this might be unexpectedly low, but is still at +least 31 bits everywhere, which is enough for hundreds of millions of +watchers. + +=item C<double> must hold a time value in seconds with enough accuracy + +The type C<double> is used to represent timestamps. It is required to +have at least 51 bits of mantissa (and 9 bits of exponent), which is +good enough for at least into the year 4000 with millisecond accuracy +(the design goal for libev). This requirement is overfulfilled by +implementations using IEEE 754, which is basically all existing ones. With +IEEE 754 doubles, you get microsecond accuracy until at least 2200. + +=back + +If you know of other additional requirements drop me a note. + + +=head1 ALGORITHMIC COMPLEXITIES + +In this section the complexities of (many of) the algorithms used inside +libev will be documented. For complexity discussions about backends see +the documentation for C<ev_default_init>. + +All of the following are about amortised time: If an array needs to be +extended, libev needs to realloc and move the whole array, but this +happens asymptotically rarer with higher number of elements, so O(1) might +mean that libev does a lengthy realloc operation in rare cases, but on +average it is much faster and asymptotically approaches constant time. + +=over 4 + +=item Starting and stopping timer/periodic watchers: O(log skipped_other_timers) + +This means that, when you have a watcher that triggers in one hour and +there are 100 watchers that would trigger before that, then inserting will +have to skip roughly seven (C<ld 100>) of these watchers. + +=item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers) + +That means that changing a timer costs less than removing/adding them, +as only the relative motion in the event queue has to be paid for. + +=item Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1) + +These just add the watcher into an array or at the head of a list. + +=item Stopping check/prepare/idle/fork/async watchers: O(1) + +=item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE)) + +These watchers are stored in lists, so they need to be walked to find the +correct watcher to remove. The lists are usually short (you don't usually +have many watchers waiting for the same fd or signal: one is typical, two +is rare). + +=item Finding the next timer in each loop iteration: O(1) + +By virtue of using a binary or 4-heap, the next timer is always found at a +fixed position in the storage array. + +=item Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd) + +A change means an I/O watcher gets started or stopped, which requires +libev to recalculate its status (and possibly tell the kernel, depending +on backend and whether C<ev_io_set> was used). + +=item Activating one watcher (putting it into the pending state): O(1) + +=item Priority handling: O(number_of_priorities) + +Priorities are implemented by allocating some space for each +priority. When doing priority-based operations, libev usually has to +linearly search all the priorities, but starting/stopping and activating +watchers becomes O(1) with respect to priority handling. + +=item Sending an ev_async: O(1) + +=item Processing ev_async_send: O(number_of_async_watchers) + +=item Processing signals: O(max_signal_number) + +Sending involves a system call I<iff> there were no other C<ev_async_send> +calls in the current loop iteration. Checking for async and signal events +involves iterating over all running async watchers or all signal numbers. + +=back + + +=head1 PORTING FROM LIBEV 3.X TO 4.X + +The major version 4 introduced some incompatible changes to the API. + +At the moment, the C<ev.h> header file provides compatibility definitions +for all changes, so most programs should still compile. The compatibility +layer might be removed in later versions of libev, so better update to the +new API early than late. + +=over 4 + +=item C<EV_COMPAT3> backwards compatibility mechanism + +The backward compatibility mechanism can be controlled by +C<EV_COMPAT3>. See L<PREPROCESSOR SYMBOLS/MACROS> in the L<EMBEDDING> +section. + +=item C<ev_default_destroy> and C<ev_default_fork> have been removed + +These calls can be replaced easily by their C<ev_loop_xxx> counterparts: + + ev_loop_destroy (EV_DEFAULT_UC); + ev_loop_fork (EV_DEFAULT); + +=item function/symbol renames + +A number of functions and symbols have been renamed: + + ev_loop => ev_run + EVLOOP_NONBLOCK => EVRUN_NOWAIT + EVLOOP_ONESHOT => EVRUN_ONCE + + ev_unloop => ev_break + EVUNLOOP_CANCEL => EVBREAK_CANCEL + EVUNLOOP_ONE => EVBREAK_ONE + EVUNLOOP_ALL => EVBREAK_ALL + + EV_TIMEOUT => EV_TIMER + + ev_loop_count => ev_iteration + ev_loop_depth => ev_depth + ev_loop_verify => ev_verify + +Most functions working on C<struct ev_loop> objects don't have an +C<ev_loop_> prefix, so it was removed; C<ev_loop>, C<ev_unloop> and +associated constants have been renamed to not collide with the C<struct +ev_loop> anymore and C<EV_TIMER> now follows the same naming scheme +as all other watcher types. Note that C<ev_loop_fork> is still called +C<ev_loop_fork> because it would otherwise clash with the C<ev_fork> +typedef. + +=item C<EV_MINIMAL> mechanism replaced by C<EV_FEATURES> + +The preprocessor symbol C<EV_MINIMAL> has been replaced by a different +mechanism, C<EV_FEATURES>. Programs using C<EV_MINIMAL> usually compile +and work, but the library code will of course be larger. + +=back + + +=head1 GLOSSARY + +=over 4 + +=item active + +A watcher is active as long as it has been started and not yet stopped. +See L<WATCHER STATES> for details. + +=item application + +In this document, an application is whatever is using libev. + +=item backend + +The part of the code dealing with the operating system interfaces. + +=item callback + +The address of a function that is called when some event has been +detected. Callbacks are being passed the event loop, the watcher that +received the event, and the actual event bitset. + +=item callback/watcher invocation + +The act of calling the callback associated with a watcher. + +=item event + +A change of state of some external event, such as data now being available +for reading on a file descriptor, time having passed or simply not having +any other events happening anymore. + +In libev, events are represented as single bits (such as C<EV_READ> or +C<EV_TIMER>). + +=item event library + +A software package implementing an event model and loop. + +=item event loop + +An entity that handles and processes external events and converts them +into callback invocations. + +=item event model + +The model used to describe how an event loop handles and processes +watchers and events. + +=item pending + +A watcher is pending as soon as the corresponding event has been +detected. See L<WATCHER STATES> for details. + +=item real time + +The physical time that is observed. It is apparently strictly monotonic :) + +=item wall-clock time + +The time and date as shown on clocks. Unlike real time, it can actually +be wrong and jump forwards and backwards, e.g. when the you adjust your +clock. + +=item watcher + +A data structure that describes interest in certain events. Watchers need +to be started (attached to an event loop) before they can receive events. + +=back + +=head1 AUTHOR + +Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael +Magnusson and Emanuele Giaquinta. + |