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       libev - a high performance full-featured event loop written in C


          #include <ev.h>

          // 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_loop's to stop iterating
            ev_unloop (EV_A_ EVUNLOOP_ALL);

          // another callback, this time for a time-out
          static void
          timeout_cb (EV_P_ ev_timer *w, int revents)
            puts ("timeout");
            // this causes the innermost ev_loop to stop iterating
            ev_unloop (EV_A_ EVUNLOOP_ONE);

          main (void)
            // use the default event loop unless you have special needs
            struct ev_loop *loop = ev_default_loop (0);

            // initialise an io watcher, then start it
            // this one will watch for stdin to become readable
            ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
            ev_io_start (loop, &stdin_watcher);

            // initialise a timer watcher, then start it
            // simple non-repeating 5.5 second timeout
            ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
            ev_timer_start (loop, &timeout_watcher);

            // now wait for events to arrive
            ev_loop (loop, 0);

            // unloop was called, so exit
            return 0;


       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:

       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.

       Familarity with event based programming techniques in general is
       assumed throughout this document.


       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 event loop handler, and will then
       communicate events via a callback mechanism.

       You register interest in certain events by registering so-called event
       watchers, which are relatively small C structures you initialise with
       the details of the event, and then hand it over to libev by starting
       the watcher.

       Libev supports "select", "poll", the Linux-specific "epoll", the BSD-
       specific "kqueue" and the Solaris-specific event port mechanisms for
       file descriptor events ("ev_io"), the Linux "inotify" interface (for
       "ev_stat"), Linux eventfd/signalfd (for faster and cleaner inter-thread
       wakeup ("ev_async")/signal handling ("ev_signal")) relative timers
       ("ev_timer"), absolute timers with customised rescheduling
       ("ev_periodic"), synchronous signals ("ev_signal"), process status
       change events ("ev_child"), and event watchers dealing with the event
       loop mechanism itself ("ev_idle", "ev_embed", "ev_prepare" and
       "ev_check" watchers) as well as file watchers ("ev_stat") and even
       limited support for fork events ("ev_fork").

       It also is quite fast (see this <benchmark> comparing it to libevent
       for example).

       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 EMBED section in this manual. If libev was configured without
       support for multiple event loops, then all functions taking an initial
       argument of name "loop" (which is always of type "struct ev_loop *")
       will not have this argument.

       Libev represents time as a single floating point number, representing
       the (fractional) number of seconds since the (POSIX) epoch (somewhere
       near the beginning of 1970, details are complicated, don’t ask). This
       type is called "ev_tstamp", which is what you should use too. It
       usually aliases to the "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 "stamp" might indicate, it is also used for
       time differences throughout libev.


       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 "ev_set_syserr_cb", which is supposed to fix
       the problem or abort. The default is to print a diagnostic message and
       to call "abort ()".

       When libev detects a usage error such as a negative timer interval,
       then it will print a diagnostic message and abort (via the "assert"
       mechanism, so "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 "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.


       These functions can be called anytime, even before initialising the
       library in any way.

       ev_tstamp ev_time ()
           Returns the current time as libev would use it. Please note that
           the "ev_now" function is usually faster and also often returns the
           timestamp you actually want to know.

       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 "sleep ()".

       int ev_version_major ()
       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
           "ev_version_major" and "ev_version_minor". If you want, you can
           compare against the global symbols "EV_VERSION_MAJOR" and
           "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.

              assert (("libev version mismatch",
                       ev_version_major () == EV_VERSION_MAJOR
                       && ev_version_minor () >= EV_VERSION_MINOR));

       unsigned int ev_supported_backends ()
           Return the set of all backends (i.e. their corresponding
           "EV_BACKEND_*" value) compiled into this binary of libev
           (independent of their availability on the system you are running
           on). See "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));

       unsigned int ev_recommended_backends ()
           Return the set of all backends compiled into this binary of libev
           and also recommended for this platform. This set is often smaller
           than the one returned by "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.

       unsigned int ev_embeddable_backends ()
           Returns the set of backends that are embeddable in other event
           loops. This is the theoretical, all-platform, value. To find which
           backends might be supported on the current system, you would need
           to look at "ev_embeddable_backends () & ev_supported_backends ()",
           likewise for recommended ones.

           See the description of "ev_embed" watchers for more info.

       ev_set_allocator (void *(*cb)(void *ptr, long size)) [NOT REENTRANT]
           Sets the allocation function to use (the prototype is similar - the
           semantics are identical to the "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 ("size != 0"), the
           library might abort or take some potentially destructive action.

           Since some systems (at least OpenBSD and Darwin) fail to implement
           correct "realloc" semantics, libev will use a wrapper around the
           system "realloc" and "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 "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);

       ev_set_syserr_cb (void (*cb)(const char *msg)); [NOT REENTRANT]
           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);


       An event loop is described by a "struct ev_loop *" (the "struct" is not
       optional in this case, as there is also an "ev_loop" function).

       The library knows two types of such loops, the default loop, which
       supports signals and child events, and dynamically created loops which
       do not.

       struct ev_loop *ev_default_loop (unsigned int flags)
           This will initialise the default event loop if it hasn’t been
           initialised yet and return it. If the default loop could not be
           initialised, returns false. If it already was initialised it simply
           returns it (and ignores the flags. If that is troubling you, check
           "ev_backend ()" afterwards).

           If you don’t know what event loop to use, use the one returned from
           this function.

           Note that this function is not thread-safe, so if you want to use
           it from multiple threads, you have to lock (note also that this is
           unlikely, as loops cannot be shared easily between threads anyway).

           The default loop is the only loop that can handle "ev_signal" and
           "ev_child" watchers, and to do this, it always registers a handler
           for "SIGCHLD". If this is a problem for your application you can
           either create a dynamic loop with "ev_loop_new" that doesn’t do
           that, or you can simply overwrite the "SIGCHLD" signal handler
           after calling "ev_default_init".

           The flags argument can be used to specify special behaviour or
           specific backends to use, and is usually specified as 0 (or

           The following flags are supported:

               The default flags value. Use this if you have no clue (it’s the
               right thing, believe me).

               If this flag bit is or’ed into the flag value (or the program
               runs setuid or setgid) then libev will not look at the
               environment variable "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

               Instead of calling "ev_default_fork" or "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 "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, "getpid" is
               actually a simple 5-insn sequence without a system call and
               thus very fast, but my GNU/Linux system also has
               "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
               "LIBEV_FLAGS" environment variable.

               When this flag is specified, then libev will not attempt to use
               the inotify API for it’s "ev_stat" watchers. Apart from
               debugging and testing, this flag can be useful to conserve
               inotify file descriptors, as otherwise each loop using
               "ev_stat" watchers consumes one inotify handle.

               When this flag is specified, then libev will attempt to use the
               signalfd API for it’s "ev_signal" (and "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.

           "EVBACKEND_SELECT"  (value 1, portable select backend)
               This is your standard select(2) backend. Not 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 "accept ()" in a
               loop to accept as many connections as possible during one
               iteration. You might also want to have a look at
               "ev_set_io_collect_interval ()" to increase the amount of
               readiness notifications you get per iteration.

               This backend maps "EV_READ" to the "readfds" set and "EV_WRITE"
               to the "writefds" set (and to work around Microsoft Windows
               bugs, also onto the "exceptfds" set on that platform).

           "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 "EVBACKEND_SELECT", above, for performance tips.

               This backend maps "EV_READ" to "POLLIN | POLLERR | POLLHUP",
               and "EV_WRITE" to "POLLOUT | POLLERR | POLLHUP".

           "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

               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 and so
               on. The biggest issue is fork races, however - if a program
               forks then 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 should
               work, but of course doesnt, and epoll just loves to report
               events for totally 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

               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 file descriptor could
               point to a different file description now), so its best to
               avoid that. Also, "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, "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 "EV_READ" and "EV_WRITE" in the same way as

           "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 "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
               "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 "EVBACKEND_EPOLL", it still adds up to two event
               changes per incident. Support for "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. "EVBACKEND_SELECT"
               or "EVBACKEND_POLL" (but "poll" is of course also broken on OS
               X)) and, did I mention it, using it only for sockets.

               This backend maps "EV_READ" into an "EVFILT_READ" kevent with
               "NOTE_EOF", and "EV_WRITE" into an "EVFILT_WRITE" kevent with

           "EVBACKEND_DEVPOLL" (value 16, Solaris 8)
               This is not implemented yet (and might never be, unless you
               send me an implementation). According to reports, "/dev/poll"
               only supports sockets and is not embeddable, which would limit
               the usefulness of this backend immensely.

           "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)).

               Please note that Solaris event ports can deliver a lot of
               spurious notifications, so you need to use non-blocking I/O or
               other means to avoid blocking when no data (or space) is

               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" "EVBACKEND_SELECT" or
               "EVBACKEND_POLL" backend might perform better.

               On the positive side, with the exception of the spurious
               readiness notifications, 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).

               This backend maps "EV_READ" and "EV_WRITE" in the same way as

               Try all backends (even potentially broken ones that wouldn’t be
               tried with "EVFLAG_AUTO"). Since this is a mask, you can do
               stuff such as "EVBACKEND_ALL & ~EVBACKEND_KQUEUE".

               It is definitely not recommended to use this flag.

           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
           "ev_recommended_backends ()" will be tried.

           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);

           Example: Use whatever libev has to offer, but make sure that kqueue
           is used if available (warning, breaks stuff, best use only with
           your own private event loop and only if you know the OS supports
           your types of fds):

              ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);

       struct ev_loop *ev_loop_new (unsigned int flags)
           Similar to "ev_default_loop", but always creates a new event loop
           that is always distinct from the default loop. Unlike the default
           loop, it cannot handle signal and child watchers, and attempts to
           do so will be greeted by undefined behaviour (or a failed assertion
           if assertions are enabled).

           Note that this function is thread-safe, and the recommended 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.

           Example: Try to create a event loop that uses epoll and nothing

              struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
              if (!epoller)
                fatal ("no epoll found here, maybe it hides under your chair");

       ev_default_destroy ()
           Destroys the default loop again (frees all memory and kernel state
           etc.). None of the active event watchers will be stopped in the
           normal sense, so e.g. "ev_is_active" might still return true. It is
           your responsibility to either stop all watchers cleanly yourself
           before calling this function, or cope with the fact afterwards
           (which is usually the easiest thing, you can just ignore the
           watchers and/or "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.

           In general it is not advisable to call this function except in the
           rare occasion where you really need to free e.g. the signal
           handling pipe fds. If you need dynamically allocated loops it is
           better to use "ev_loop_new" and "ev_loop_destroy".

       ev_loop_destroy (loop)
           Like "ev_default_destroy", but destroys an event loop created by an
           earlier call to "ev_loop_new".

       ev_default_fork ()
           This function sets a flag that causes subsequent "ev_loop"
           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 (or both child and
           parent, but that again makes little sense). You must call it in the
           child before using any of the libev functions, and it will only
           take effect at the next "ev_loop" iteration.

           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 library in the
           child. If you just fork+exec, you don’t have to call it at all.

           The function itself is quite fast and it’s usually not a problem to
           call it just in case after a fork. To make this easy, the function
           will fit in quite nicely into a call to "pthread_atfork":

               pthread_atfork (0, 0, ev_default_fork);

       ev_loop_fork (loop)
           Like "ev_default_fork", but acts on an event loop created by
           "ev_loop_new". Yes, you have to call this on every allocated event
           loop after fork that you want to re-use in the child, and how you
           do this is entirely your own problem.

       int ev_is_default_loop (loop)
           Returns true when the given loop is, in fact, the default loop, and
           false otherwise.

       unsigned int ev_loop_count (loop)
           Returns the count of loop iterations for the loop, which is
           identical to the number of times libev did poll for new events. It
           starts at 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 "ev_prepare" and "ev_check" calls.

       unsigned int ev_loop_depth (loop)
           Returns the number of times "ev_loop" was entered minus the number
           of times "ev_loop" was exited, in other words, the recursion depth.

           Outside "ev_loop", this number is zero. In a callback, this number
           is 1, unless "ev_loop" was invoked recursively (or from another
           thread), in which case it is higher.

           Leaving "ev_loop" abnormally (setjmp/longjmp, cancelling the thread
           etc.), doesn’t count as exit.

       unsigned int ev_backend (loop)
           Returns one of the "EVBACKEND_*" flags indicating the event backend
           in use.

       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).

       ev_now_update (loop)
           Establishes the current time by querying the kernel, updating the
           time returned by "ev_now ()" in the progress. This is a costly
           operation and is usually done automatically within "ev_loop ()".

           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 "The special problem of time updates" in the "ev_timer"

       ev_suspend (loop)
       ev_resume (loop)
           These two functions suspend and resume a 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 "^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 "ev_suspend" in your "SIGTSTP" handler, sending
           yourself a "SIGSTOP" and calling "ev_resume" directly afterwards to
           resume timer processing.

           Effectively, all "ev_timer" watchers will be delayed by the time
           spend between "ev_suspend" and "ev_resume", and all "ev_periodic"
           watchers will be rescheduled (that is, they will lose any events
           that would have occured while suspended).

           After calling "ev_suspend" you must not call any function on the
           given loop other than "ev_resume", and you must not call
           "ev_resume" without a previous call to "ev_suspend".

           Calling "ev_suspend"/"ev_resume" has the side effect of updating
           the event loop time (see "ev_now_update").

       ev_loop (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.

           If the flags argument is specified as 0, it will not return until
           either no event watchers are active anymore or "ev_unloop" was

           Please note that an explicit "ev_unloop" 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.

           A flags value of "EVLOOP_NONBLOCK" will look for new events, will
           handle those events and any already outstanding ones, but will not
           block your process in case there are no events and will return
           after one iteration of the loop.

           A flags value of "EVLOOP_ONESHOT" 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 "ev_loop""). However, a pair of
           "ev_prepare"/"ev_check" watchers is usually a better approach for
           this kind of thing.

           Here are the gory details of what "ev_loop" does:

              - Before the first iteration, call any pending watchers.
              * 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 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, EVLOOP_NONBLOCK or not having
                any active watchers at all will result in not sleeping).
              - Sleep if the I/O and timer collect interval say so.
              - 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.
              - Unless any events are pending now, queue all idle watchers.
              - 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_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
                were used, or there are no active watchers, return, otherwise
                continue with step *.

           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_loop (my_loop, 0);
              ... jobs done or somebody called unloop. yeah!

       ev_unloop (loop, how)
           Can be used to make a call to "ev_loop" return early (but only
           after it has processed all outstanding events). The "how" argument
           must be either "EVUNLOOP_ONE", which will make the innermost
           "ev_loop" call return, or "EVUNLOOP_ALL", which will make all
           nested "ev_loop" calls return.

           This "unloop state" will be cleared when entering "ev_loop" again.

           It is safe to call "ev_unloop" from otuside any "ev_loop" calls.

       ev_ref (loop)
       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, "ev_loop" 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 "ev_loop" from
           returning. In such a case, call "ev_unref" after starting, and
           "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 "ev_loop"
           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 unref after
           start and 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 "ev_ref" in the callback).

           Example: Create a signal watcher, but keep it from keeping
           "ev_loop" running when nothing else is active.

              ev_signal exitsig;
              ev_signal_init (&exitsig, sig_cb, SIGINT);
              ev_signal_start (loop, &exitsig);
              evf_unref (loop);

           Example: For some weird reason, unregister the above signal handler

              ev_ref (loop);
              ev_signal_stop (loop, &exitsig);

       ev_set_io_collect_interval (loop, ev_tstamp interval)
       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 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 "interval" must be >= 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 "select ()"
           which have a high overhead for the actual polling but can deliver
           many events at once.

           By setting a higher 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
           "ev_periodic" and "ev_timer") will be not affected. Setting this to
           a non-null value will introduce an additional "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

           Likewise, by setting a higher 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). "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 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 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 transations per second).

           Setting the 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 "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);

       ev_invoke_pending (loop)
           This call will simply invoke all pending watchers while resetting
           their pending state. Normally, "ev_loop" does this automatically
           when required, but when overriding the invoke callback this call
           comes handy.

       int ev_pending_count (loop)
           Returns the number of pending watchers - zero indicates that no
           watchers are pending.

       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,
           "ev_loop" 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 "ev_invoke_pending" as new

       ev_set_loop_release_cb (loop, void (*release)(EV_P), void
           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, "ev_loop" 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 loop via "ev_unloop" and "av_async_send", another way is to
           set these release and acquire callbacks on the loop.

           When set, then "release" will be called just before the thread is
           suspended waiting for new events, and "acquire" is called just

           Ideally, "release" will just call your mutex_unlock function, and
           "acquire" will just call the mutex_lock function again.

           While event loop modifications are allowed between invocations of
           "release" and "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 "ev_async" watcher to wake up "ev_loop"
           when you want it to take note of any changes you made.

           In theory, threads executing "ev_loop" will be async-cancel safe
           between invocations of "release" and "acquire".

           See also the locking example in the "THREADS" section later in this

       ev_set_userdata (loop, void *data)
       ev_userdata (loop)
           Set and retrieve a single "void *" associated with a loop. When
           "ev_set_userdata" has never been called, then "ev_userdata" returns

           These two functions can be used to associate arbitrary data with a
           loop, and are intended solely for the "invoke_pending_cb",
           "release" and "acquire" callbacks described above, but of course
           can be (ab-)used for any other purpose as well.

       ev_loop_verify (loop)
           This function only does something when "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 "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.


       In the following description, uppercase "TYPE" in names stands for the
       watcher type, e.g. "ev_TYPE_start" can mean "ev_timer_start" for timer
       watchers and "ev_io_start" for I/O watchers.

       A watcher is a structure that you create and register to record your
       interest in some event. For instance, if you want to wait for STDIN to
       become readable, you would create an "ev_io" watcher for that:

          static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
            ev_io_stop (w);
            ev_unloop (loop, EVUNLOOP_ALL);

          struct ev_loop *loop = ev_default_loop (0);

          ev_io stdin_watcher;

          ev_init (&stdin_watcher, my_cb);
          ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
          ev_io_start (loop, &stdin_watcher);

          ev_loop (loop, 0);

       As you can see, you are responsible for allocating the memory for your
       watcher structures (and it is usually a bad idea to do this on the

       Each watcher has an associated watcher structure (called "struct
       ev_TYPE" or simply "ev_TYPE", as typedefs are provided for all watcher

       Each watcher structure must be initialised by a call to "ev_init
       (watcher *, callback)", which expects a callback to be provided. This
       callback gets invoked each time the event occurs (or, in the case of
       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 "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: "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 ("ev_TYPE_start (loop, watcher
       *)"), and you can stop watching for events at any time by calling the
       corresponding stop function ("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 "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:

           The file descriptor in the "ev_io" watcher has become readable
           and/or writable.

           The "ev_timer" watcher has timed out.

           The "ev_periodic" watcher has timed out.

           The signal specified in the "ev_signal" watcher has been received
           by a thread.

           The pid specified in the "ev_child" watcher has received a status

           The path specified in the "ev_stat" watcher changed its attributes

           The "ev_idle" watcher has determined that you have nothing better
           to do.

           All "ev_prepare" watchers are invoked just before "ev_loop" starts
           to gather new events, and all "ev_check" watchers are invoked just
           after "ev_loop" has gathered them, but before it invokes any
           callbacks for any received events. Callbacks of both watcher types
           can start and stop as many watchers as they want, and all of them
           will be taken into account (for example, a "ev_prepare" watcher
           might start an idle watcher to keep "ev_loop" from blocking).

           The embedded event loop specified in the "ev_embed" watcher needs

           The event loop has been resumed in the child process after fork
           (see "ev_fork").

           The given async watcher has been asynchronously notified (see

           Not ever sent (or otherwise used) by libev itself, but can be
           freely used by libev users to signal watchers (e.g. via

           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

           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.

       "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 "malloc" will
           do). Only the generic parts of the watcher are initialised, you
           need to call the type-specific "ev_TYPE_set" macro afterwards to
           initialise the type-specific parts. For each type there is also a
           "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

           The callback is always of type "void (*)(struct ev_loop *loop,
           ev_TYPE *watcher, int revents)".

           Example: Initialise an "ev_io" watcher in two steps.

              ev_io w;
              ev_init (&w, my_cb);
              ev_io_set (&w, STDIN_FILENO, EV_READ);

       "ev_TYPE_set" (ev_TYPE *watcher, [args])
           This macro initialises the type-specific parts of a watcher. You
           need to call "ev_init" at least once before you call this macro,
           but you can call "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 "ev_init" macro).

           Although some watcher types do not have type-specific arguments
           (e.g. "ev_prepare") you still need to call its "set" macro.

           See "ev_init", above, for an example.

       "ev_TYPE_init" (ev_TYPE *watcher, callback, [args])
           This convenience macro rolls both "ev_init" and "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 "ev_io" watcher in one step.

              ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);

       "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

           Example: Start the "ev_io" watcher that is being abused as example
           in this whole section.

              ev_io_start (EV_DEFAULT_UC, &w);

       "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 "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
           "ev_TYPE_stop" function.

       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.

       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 "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 "free ()" it).

       callback ev_cb (ev_TYPE *watcher)
           Returns the callback currently set on the watcher.

       ev_cb_set (ev_TYPE *watcher, callback)
           Change the callback. You can change the callback at virtually any
           time (modulo threads).

       ev_set_priority (ev_TYPE *watcher, int priority)
       int ev_priority (ev_TYPE *watcher)
           Set and query the priority of the watcher. The priority is a small
           integer between "EV_MAXPRI" (default: 2) and "EV_MINPRI" (default:
           "-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 "ev_idle" watchers).

           If you need to suppress invocation when higher priority events are
           pending you need to look at "ev_idle" watchers, which provide this

           You must not change the priority of a watcher as long as it is
           active or pending.

           Setting a priority outside the range of "EV_MINPRI" to "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 0, which is supposed to not be too high and not be too
           low :).

           See "WATCHER PRIORITY MODELS", below, for a more thorough treatment
           of priorities.

       ev_invoke (loop, ev_TYPE *watcher, int revents)
           Invoke the "watcher" with the given "loop" and "revents". Neither
           "loop" nor "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.

       int ev_clear_pending (loop, ev_TYPE *watcher)
           If the watcher is pending, this function clears its pending status
           and returns its "revents" bitset (as if its callback was invoked).
           If the watcher isn’t pending it does nothing and returns 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

       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
           "ev_clear_pending" will clear the pending event, even if the
           watcher was not started in the first place.

           See also "ev_feed_fd_event" and "ev_feed_signal_event" for related
           functions that do not need a watcher.

       Each watcher has, by default, a member "void *data" that you can change
       and read at any time: libev will completely ignore it. This can be used
       to associate arbitrary data with your watcher. If you need more data
       and don’t want to allocate memory and store a pointer to it in that
       data member, you can also "subclass" the watcher type and provide your
       own data:

          struct my_io
            ev_io io;
            int otherfd;
            void *somedata;
            struct whatever *mostinteresting;

          struct my_io w;
          ev_io_init (&, 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
       type instead have been omitted.

       Another common scenario is to use some data structure with multiple
       embedded watchers:

          struct my_biggy
            int some_data;
            ev_timer t1;
            ev_timer t2;

       In this case getting the pointer to "my_biggy" is a bit more
       complicated: Either you store the address of your "my_biggy" struct in
       the "data" member of the watcher (for woozies), or you need to use some
       pointer arithmetic using "offsetof" inside your watchers (for real

          #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));

       Many event loops support 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 "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

       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 "ev_io" watcher to receive data, and an associated "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 "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 continously 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 ahndle 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.


       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 [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 [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.

   "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).

       If you cannot use non-blocking mode, then force the use of a known-to-
       be-good backend (at the time of this writing, this includes only
       "EVBACKEND_SELECT" and "EVBACKEND_POLL"). The same applies to file
       descriptors for which non-blocking operation makes no sense (such as
       files) - libev doesn’t guarentee any specific behaviour in that case.

       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 "EV_READ" but a subsequent "read"(2) will actually block
       because there is no data. Not only are some backends known to create a
       lot of those (for example Solaris ports), it is very easy to get into
       this situation even with a relatively standard program structure. Thus
       it is best to always use non-blocking I/O: An extra "read"(2) returning
       "EAGAIN" is far preferable to a program hanging until some data

       If you cannot run the fd in non-blocking mode (for example you should
       not play around with an Xlib connection), then you have to separately
       re-test whether a file descriptor is really ready with a known-to-be
       good interface such as poll (fortunately in our Xlib example, Xlib
       already does this on its own, so its quite safe to use). Some people
       additionally use "SIGALRM" and an interval timer, just to be sure you
       won’t block indefinitely.

       But really, best use non-blocking mode.

       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 "close" explicitly or any other
       means, such as "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 "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 have to call "ev_io_set" (or "ev_io_init") when you
       change the descriptor even if the file descriptor number itself did not

       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.

       The special problem of duped 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 "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 "dup ()"’ed file descriptors, or to resort to

       The special problem of fork

       Some backends (epoll, kqueue) do not support "fork ()" at all or
       exhibit useless behaviour. Libev fully supports fork, but needs to be
       told about it in the child.

       To support fork in your programs, you either have to call
       "ev_default_fork ()" or "ev_loop_fork ()" after a fork in the child,
       enable "EVFLAG_FORKCHECK", or resort to "EVBACKEND_SELECT" or

       The special problem of SIGPIPE

       While not really specific to libev, it is easy to forget about
       "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).

       Watcher-Specific Functions

       ev_io_init (ev_io *, callback, int fd, int events)
       ev_io_set (ev_io *, int fd, int events)
           Configures an "ev_io" watcher. The "fd" is the file descriptor to
           receive events for and "events" is either "EV_READ", "EV_WRITE" or
           "EV_READ | EV_WRITE", to express the desire to receive the given

       int fd [read-only]
           The file descriptor being watched.

       int events [read-only]
           The events being watched.


       Example: Call "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_loop (loop, 0);

   "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 after its timeout has
       passed (not 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 "ev_loop"

       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).

       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, "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.

       2. Use a timer and re-start it with "ev_timer_again" inactivity.
           This is the easiest way, and involves using "ev_timer_again"
           instead of "ev_timer_start".

           To implement this, configure an "ev_timer" with a "repeat" value of
           60 and then call "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
           "ev_timer_stop" the timer, and "ev_timer_again" will automatically
           restart it if need be.

           That means you can ignore both the "ev_timer_start" function and
           the "after" argument to "ev_timer_set", and only ever use the
           "repeat" member and "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.

       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 "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 occured, take action
                    // 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 did, in fact,
           time out. Otherwise the callback was invoked too early ("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 "ev_timer_again" is used, taking advantage of the
           "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
           "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_TIMEOUT);

           And when there is some activity, simply store the current time in
           "last_activity", no libev calls at all:

              last_actiivty = ev_now (loop);

           This technique is slightly more complex, but in most cases where
           the time-out is unlikely to be triggered, much more efficient.

           Changing the timeout is trivial as well (if it isn’t hard-coded in
           the callback :) - just change the timeout and invoke the callback,
           which will fix things for you.

       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 end of the list.

           Then use an "ev_timer" to fire when the timeout at the 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 "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.

       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

       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 :)

       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 "ev_loop" collects new events, which
       causes a growing difference between "ev_now ()" and "ev_time ()" when
       handling lots of events in one iteration.

       The relative timeouts are calculated relative to the "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 need to base the
       timeout on the current time, use something like this to adjust for

          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 "ev_now ()" by calling "ev_now_update

       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

       Some quick tests made with a Linux 2.6.28 indicate that a suspend
       freezes all processes, while the clocks ("times", "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 "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 "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 "ev_suspend" and
       "ev_resume" in code that handles "SIGTSTP", to at least get
       deterministic behaviour in this case (you can do nothing against

       Watcher-Specific Functions and Data Members

       ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
       ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
           Configure the timer to trigger after "after" seconds. If "repeat"
           is 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 "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.

       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

           If the timer is repeating, either start it if necessary (with the
           "repeat" value), or reset the running timer to the "repeat" value.

           This sounds a bit complicated, see "Be smart about timeouts",
           above, for a usage example.

       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 "ev_timer_set (w, 5, 7)", "ev_timer_remaining"
           returns 5. When the timer is started and one second passes,
           "ev_timer_remain" will return 4. When the timer expires and is
           restarted, it will return roughly 7 (likely slightly less as
           callback invocation takes some time, too), and so on.

       ev_tstamp repeat [read-write]
           The current "repeat" value. Will be used each time the watcher
           times out or "ev_timer_again" is called, and determines the next
           timeout (if any), which is also when any modifications are taken
           into account.


       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

          static void
          timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
            .. ten seconds without any activity

          ev_timer mytimer;
          ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
          ev_timer_again (&mytimer); /* start timer */
          ev_loop (loop, 0);

          // and in some piece of code that gets executed on any "activity":
          // reset the timeout to start ticking again at 10 seconds
          ev_timer_again (&mytimer);

   "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 "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-

       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. "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 "ev_timer", which would still trigger roughly 10 seconds
       after starting it, as it uses a relative timeout).

       "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 "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 "ev_loop"

       Watcher-Specific Functions and Data Members

       ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp
       interval, reschedule_cb)
       ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval,
           Lots of arguments, let’s sort it out... There are basically three
           modes of operation, and we will explain them from simplest to most

           ·   absolute timer (offset = absolute time, interval = 0,
               reschedule_cb = 0)

               In this configuration the watcher triggers an event after the
               wall clock time "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.

           ·   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 "offset + N * interval" time (for some integer N,
               which can also be negative) and then repeat, regardless of any
               time jumps. The "offset" argument is merely an offset into the
               "interval" periods.

               This can be used to create timers that do not drift with
               respect to the system clock, for example, here is an
               "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 "ev_periodic" will try to run the callback in this mode
               at the next possible time where "time = offset (mod interval)",
               regardless of any time jumps.

               For numerical stability it is preferable that the "offset"
               value is near "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 "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).

           ·   manual reschedule mode (offset ignored, interval ignored,
               reschedule_cb = callback)

               In this mode the values for "interval" and "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: 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 "now + 1e30" (or so, fudge
               fudge) and stop it afterwards (e.g. by starting an "ev_prepare"
               watcher, which is the only event loop modification you are
               allowed to do).

               The callback prototype is "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,

               NOTE: This callback must always return a time that is higher
               than or equal to the passed "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 "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).

       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).

       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 "offset" argument to
           "ev_periodic_set", but indeed works even in interval and manual
           rescheduling modes.

       ev_tstamp offset [read-write]
           When repeating, this contains the offset value, otherwise this is
           the absolute point in time (the "offset" value passed to
           "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 "ev_periodic_again" is being called.

       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
           "ev_periodic_again" is being called.

       ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]
           The current reschedule callback, or 0, if this functionality is
           switched off. Can be changed any time, but changes only take effect
           when the periodic timer fires or "ev_periodic_again" is being


       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_io *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);

   "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 it’s 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
       "sigaction" as you would do without libev and forget about sharing the
       signal. You can even use "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 "SIGINT" in your
       default loop and for "SIGIO" in another loop, but you cannot watch for
       "SIGINT" in both the default loop and another loop at the same time. At
       the moment, "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

       If possible and supported, libev will install its handlers with
       "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
       "ev_check" watcher and unblock them in an "ev_prepare" watcher.

       The special problem of inheritance over fork/execve/pthread_create

       Both the signal mask ("sigprocmask") and the signal disposition
       ("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

       While this does not matter for the signal disposition (libev never sets
       signals to "SIG_IGN", so handlers will be reset to "SIG_DFL" on
       "execve"), this matters for the signal mask: many programs do not
       expect certain signals to be blocked.

       This means that before calling "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 "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 "signalfd" API ("EV_SIGNALFD"). While
       this reduces the window of opportunity for problems, it will not go
       away, as libev has to modify the signal mask, at least temporarily.

       So I can’t stress this enough: 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

       Watcher-Specific Functions and Data Members

       ev_signal_init (ev_signal *, callback, int signum)
       ev_signal_set (ev_signal *, int signum)
           Configures the watcher to trigger on the given signal number
           (usually one of the "SIGxxx" constants).

       int signum [read-only]
           The signal the watcher watches out for.


       Example: Try to exit cleanly on SIGINT.

          static void
          sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
            ev_unloop (loop, EVUNLOOP_ALL);

          ev_signal signal_watcher;
          ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
          ev_signal_start (loop, &signal_watcher);

   "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 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

       Due to some design glitches inside libev, child watchers will always be
       handled at maximum priority (their priority is set to "EV_MAXPRI" by

       Process Interaction

       Libev grabs "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 "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.

       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
       "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 "ev_child" watchers freely.

       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 "ev_child_stop" twice is not a

       Watcher-Specific Functions and Data Members

       ev_child_init (ev_child *, callback, int pid, int trace)
       ev_child_set (ev_child *, int pid, int trace)
           Configures the watcher to wait for status changes of process "pid"
           (or any process if "pid" is specified as 0). The callback can look
           at the "rstatus" member of the "ev_child" watcher structure to see
           the status word (use the macros from "sys/wait.h" and see your
           systems "waitpid" documentation). The "rpid" member contains the
           pid of the process causing the status change. "trace" must be
           either 0 (only activate the watcher when the process terminates) or
           1 (additionally activate the watcher when the process is stopped or

       int pid [read-only]
           The process id this watcher watches out for, or 0, meaning any
           process id.

       int rpid [read-write]
           The process id that detected a status change.

       int rstatus [read-write]
           The process exit/trace status caused by "rpid" (see your systems
           "waitpid" and "sys/wait.h" documentation for details).


       Example: "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);
              ev_child_init (&cw, child_cb, pid, 0);
              ev_child_start (EV_DEFAULT_ &cw);

   "ev_stat" - did the file attributes just change?
       This watches a file system path for attribute changes. That is, it
       calls "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 "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 must not end in a slash or contain special components such as
       "." or "..". The path 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 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 0 (highly
       recommended!) then a 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 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-

       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 "ev_stat" semantics with kqueue, except as a hint).

       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

       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.

       Inotify and Kqueue

       When "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 "ev_stat"
       watcher is being started.

       Inotify presence does not change the semantics of "ev_stat" watchers
       except that changes might be detected earlier, and in some cases, to
       avoid making regular "stat" calls. Even in the presence of inotify
       support there are many cases where libev has to resort to regular
       "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.

       "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 "ev_stat" watchers - those call
       "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 "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 "ev_stat" watchers on networked
       paths, although this is fully supported by libev.

       The special problem of stat time resolution

       The "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, "ev_stat" detects a change
       and calls your callback, which does something. When there is another
       update within the same second, "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 "ev_timer" (e.g. "ev_timer_set (w, 0.,
       1.02); ev_timer_again (loop, w)").

       The .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 "gettimeofday" might return a timestamp with a full second later
       than a subsequent "time" call - if the equivalent of "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).

       Watcher-Specific Functions and Data Members

       ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp
       ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)
           Configures the watcher to wait for status changes of the given
           "path". The "interval" is a hint on how quickly a change is
           expected to be detected and should normally be specified as 0 to
           let libev choose a suitable value. The memory pointed to by "path"
           must point to the same path for as long as the watcher is active.

           The callback will receive an "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).

       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.

       ev_statdata attr [read-only]
           The most-recently detected attributes of the file. Although the
           type is "ev_statdata", this is usually the (or one of the) "struct
           stat" types suitable for your system, but you can only rely on the
           POSIX-standardised members to be present. If the "st_nlink" member
           is 0, then there was some error while "stat"ing the file.

       ev_statdata prev [read-only]
           The previous attributes of the file. The callback gets invoked
           whenever "prev" != "attr", or, more precisely, one or more of these
           members differ: "st_dev", "st_ino", "st_mode", "st_nlink",
           "st_uid", "st_gid", "st_rdev", "st_size", "st_atime", "st_mtime",

       ev_tstamp interval [read-only]
           The specified interval.

       const char *path [read-only]
           The file system path that is being watched.


       Example: Watch "/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);
              /* 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 "ev_stat" callback invocation and on
       "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);

   "ev_idle" - when youve 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.

       Watcher-Specific Functions and Data Members

       ev_idle_init (ev_idle *, callback)
           Initialises and configures the idle watcher - it has no parameters
           of any kind. There is a "ev_idle_set" macro, but using it is
           utterly pointless, believe me.


       Example: Dynamically allocate an "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);

   "ev_prepare" and "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 must not call "ev_loop" or similar functions that enter the current
       event loop from either "ev_prepare" or "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 "ev_prepare", blocking, "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 "XFlush ()" in an
       "ev_prepare" watcher).

       This is done by examining in each prepare call which file descriptors
       need to be watched by the other library, registering "ev_io" watchers
       for them and starting an "ev_timer" watcher for any timeouts (many
       libraries provide 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 "ev_check" watchers highest ("EV_MAXPRI")
       priority, to ensure that they are being run before any other watchers
       after the poll (this doesn’t matter for "ev_prepare" watchers).

       Also, "ev_check" watchers (and "ev_prepare" watchers, too) should not
       activate ("feed") events into libev. While libev fully supports this,
       they might get executed before other "ev_check" watchers did their job.
       As "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
       "ev_check" watcher ran (always remind yourself to coexist peacefully
       with others).

       Watcher-Specific Functions and Data Members

       ev_prepare_init (ev_prepare *, callback)
       ev_check_init (ev_check *, callback)
           Initialises and configures the prepare or check watcher - they have
           no parameters of any kind. There are "ev_prepare_set" and
           "ev_check_set" macros, but using them is utterly, utterly, utterly
           and completely pointless.


       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 "EV::ADNS" that does this, which
       you could use as a working example. Another Perl module named
       "EV::Glib" embeds a Glib main context into libev, and finally,
       "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 "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
       "adns_afterpoll" in the prepare watcher and would dispose of the check

       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

          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 "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_loop (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;

   "ev_embed" - when one backend isnt enough...
       This is a rather advanced watcher type that lets you embed one event
       loop into another (currently only "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
       "poll" and then "kevent", but at least you can use both mechanisms for
       what they are best: "kqueue" for scalable sockets and "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

       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 "ev_embed_sweep (mainloop, watcher)" to make a single
       sweep and invoke their callbacks (the callback doesn’t need to invoke
       the "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 0, in which case the embed watcher
       will automatically execute the embedded loop sweep whenever necessary.

       Fork detection will be handled transparently while the "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 "ev_loop_fork" on the embedded loop.

       Unfortunately, not all backends are embeddable: only the ones returned
       by "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

       "ev_embed" and fork

       While the "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
       "ev_loop_fork ()" as applicable.

       Watcher-Specific Functions and Data Members

       ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
       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 0, then "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).

       ev_embed_sweep (loop, ev_embed *)
           Make a single, non-blocking sweep over the embedded loop. This
           works similarly to "ev_loop (embedded_loop, EVLOOP_NONBLOCK)", but
           in the most appropriate way for embedded loops.

       struct ev_loop *other [read-only]
           The embedded event loop.


       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 "loop_hi", while the embeddable loop is
       stored in "loop_lo" (which is "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);
            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
       "loop_socket". (One might optionally use "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

   "ev_fork" - the audacity to resume the event loop after a fork
       Fork watchers are called when a "fork ()" was detected (usually because
       whoever is a good citizen cared to tell libev about it by calling
       "ev_default_fork" or "ev_loop_fork"). The invocation is done before the
       event loop blocks next and before "ev_check" watchers are being called,
       and only in the child after the fork. If whoever good citizen calling
       "ev_default_fork" cheats and calls it in the wrong process, the fork
       handlers will be invoked, too, of course.

       The special problem of life after fork - how is it possible?

       Most uses of "fork()" consist of forking, then some simple calls to ste
       up/change the process environment, followed by a call to "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

       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 either the parent 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
       "ev_default_destroy ()" followed by "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.

       Watcher-Specific Functions and Data Members

       ev_fork_init (ev_signal *, callback)
           Initialises and configures the fork watcher - it has no parameters
           of any kind. There is a "ev_fork_set" macro, but using it is
           utterly pointless, believe me.

   "ev_async" - how to wake up another event loop
       In general, you cannot use an "ev_loop" 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 another event loop you do not
       control, for example because it belongs to another thread. This is what
       "ev_async" watchers do: as long as the "ev_async" watcher is active,
       you can signal it by calling "ev_async_send", which is thread- and
       signal safe.

       This functionality is very similar to "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
       "ev_async_sent" calls).

       Unlike "ev_signal" watchers, "ev_async" works with any event loop, not
       just the default loop.


       "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

       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

       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 "pthread_setmask"
           instead of "sigprocmask" when you use threads, but libev doesn’t do
           it either...).

       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

              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);

       Watcher-Specific Functions and Data Members

       ev_async_init (ev_async *, callback)
           Initialises and configures the async watcher - it has no parameters
           of any kind. There is a "ev_async_set" macro, but using it is
           utterly pointless, trust me.

       ev_async_send (loop, ev_async *)
           Sends/signals/activates the given "ev_async" watcher, that is,
           feeds an "EV_ASYNC" event on the watcher into the event loop.
           Unlike "ev_feed_event", this call is safe to do from other threads,
           signal or similar contexts (see the discussion of "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 "ev_async" watchers are level-triggered, set
           on "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 "ev_async_send" for the same
           event loop.

       bool = ev_async_pending (ev_async *)
           Returns a non-zero value when "ev_async_send" has been called on
           the watcher but the event has not yet been processed (or even
           noted) by the event loop.

           "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. "ev_async_pending" can
           be used to very quickly check whether invoking the loop might be a
           good idea.

           Not that this does 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.


       There are some other functions of possible interest. Described. Here.

       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 "fd" is less than 0, then no I/O watcher will be started and the
           "events" argument is being ignored. Otherwise, an "ev_io" watcher
           for the given "fd" and "events" set will be created and started.

           If "timeout" is less than 0, then no timeout watcher will be
           started. Otherwise an "ev_timer" watcher with after = "timeout"
           (and repeat = 0) will be started. 0 is a valid timeout.

           The callback has the type "void (*cb)(int revents, void *arg)" and
           gets passed an "revents" set like normal event callbacks (a
           combination of "EV_ERROR", "EV_READ", "EV_WRITE" or "EV_TIMEOUT")
           and the "arg" value passed to "ev_once". Note that it is possible
           to receive 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_TIMEOUT)
                  /* doh, nothing entered */;

              ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);

       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.

       ev_feed_signal_event (loop, int signum)
           Feed an event as if the given signal occurred ("loop" must be the
           default loop!).


       Libev offers a compatibility emulation layer for libevent. It cannot
       emulate the internals of libevent, so here are some usage hints:

       ·   Use it by including <event.h>, as usual.

       ·   The following members are fully supported: ev_base, ev_callback,
           ev_arg, ev_fd, ev_res, ev_events.

       ·   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).

       ·   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.

       ·   In libevent, the last base created gets the signals, in libev, the
           first base created (== the default loop) gets the signals.

       ·   Other members are not supported.

       ·   The libev emulation is not ABI compatible to libevent, you need to
           use the libev header file and library.


       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

       To use it,

          #include <ev++.h>

       This automatically includes ev.h and puts all of its definitions (many
       of them macros) into the global namespace. All C++ specific things are
       put into the "ev" namespace. It should support all the same embedding
       options as ev.h, most notably "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 "EV_MULTIPLICITY" when embedding libev).

       Currently, functions, and static and non-static member functions 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 "ev" namespace:

       "ev::READ", "ev::WRITE" etc.
           These are just enum values with the same values as the "EV_READ"
           etc.  macros from ev.h.

       "ev::tstamp", "ev::now"
           Aliases to the same types/functions as with the "ev_" prefix.

       "ev::io", "ev::timer", "ev::periodic", "ev::idle", "ev::sig" etc.
           For each "ev_TYPE" watcher in ev.h there is a corresponding class
           of the same name in the "ev" namespace, with the exception of
           "ev_signal" which is called "ev::sig" to avoid clashes with the
           "signal" macro defines by many implementations.

           All of those classes have these methods:

           ev::TYPE::TYPE ()
           ev::TYPE::TYPE (loop)
               The constructor (optionally) takes an event loop to associate
               the watcher with. If it is omitted, it will use "EV_DEFAULT".

               The constructor calls "ev_init" for you, which means you have
               to call the "set" method before starting it.

               It will not set a callback, however: You have to call the
               templated "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

               The destructor automatically stops the watcher if it is active.

           w->set<class, &class::method> (object *)
               This method sets the callback method to call. The method has to
               have a signature of "void (*)(ev_TYPE &, int)", it receives the
               watcher as first argument and the "revents" as second. The
               object must be given as parameter and is stored in the "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 "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);

           w->set (object *)
               This is an experimental feature that might go away in a future

               This is a variation of a method callback - leaving out the
               method to call will default the method to "operator ()", which
               makes it possible to use functor objects without having to
               manually specify the "operator ()" all the time. Incidentally,
               you can then also leave out the template argument list.

               The "operator ()" method prototype must be "void operator
               ()(watcher &w, int revents)".

               See the method-"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);

           w->set<function> (void *data = 0)
               Also sets a callback, but uses a static method or plain
               function as callback. The optional "data" argument will be
               stored in the watcher’s "data" member and is free for you to

               The prototype of the "function" must be "void (*)(ev::TYPE &w,

               See the method-"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> ();

           w->set (loop)
               Associates a different "struct ev_loop" with this watcher. You
               can only do this when the watcher is inactive (and not pending

           w->set ([arguments])
               Basically the same as "ev_TYPE_set", with the same arguments.
               Must be called at least once. Unlike the C counterpart, an
               active watcher gets automatically stopped and restarted when
               reconfiguring it with this method.

           w->start ()
               Starts the watcher. Note that there is no "loop" argument, as
               the constructor already stores the event loop.

           w->stop ()
               Stops the watcher if it is active. Again, no "loop" argument.

           w->again () ("ev::timer", "ev::periodic" only)
               For "ev::timer" and "ev::periodic", this invokes the
               corresponding "ev_TYPE_again" function.

           w->sweep () ("ev::embed" only)
               Invokes "ev_embed_sweep".

           w->update () ("ev::stat" only)
               Invokes "ev_stat_stat".

       Example: Define a class with an IO and idle watcher, start one of them
       in the constructor.

          class myclass
            ev::io   io  ; void io_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);
              idle.set <myclass, &myclass::idle_cb> (this);

              io.start (fd, ev::READ);


       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.

           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 "libadns" ("EV::ADNS", but "AnyEvent::DNS"
           is preferred nowadays), "Net::SNMP" ("Net::SNMP::EV") and the
           "libglib" event core ("Glib::EV" and "EV::Glib").

           It can be found and installed via CPAN, its homepage is at

           Python bindings can be found at <>.
           It seems to be quite complete and well-documented.

           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 <>.

           Roger Pack reports that using the link order "-lws2_32
           -lmsvcrt-ruby-190" makes rev work even on mingw.

           A haskell binding to libev is available at

       D   Leandro Lucarella has written a D language binding (ev.d) for
           libev, to be found at <>.

           Erkki Seppala has written Ocaml bindings for libev, to be found at

       Lua Brian Maher has written a partial interface to libev for lua (only
           "ev_io" and "ev_timer"), to be found at


       Libev can be compiled with a variety of options, the most fundamental
       of which is "EV_MULTIPLICITY". This option determines whether (most)
       functions and callbacks have an initial "struct ev_loop *" argument.

       To make it easier to write programs that cope with either variant, the
       following macros are defined:

       "EV_A", "EV_A_"
           This provides the loop argument for functions, if one is required
           ("ev loop argument"). The "EV_A" form is used when this is the sole
           argument, "EV_A_" is used when other arguments are following.

              ev_unref (EV_A);
              ev_timer_add (EV_A_ watcher);
              ev_loop (EV_A_ 0);

           It assumes the variable "loop" of type "struct ev_loop *" is in
           scope, which is often provided by the following macro.

       "EV_P", "EV_P_"
           This provides the loop parameter for functions, if one is required
           ("ev loop parameter"). The "EV_P" form is used when this is the
           sole parameter, "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 "loop" of type "struct ev_loop *", quite
           suitable for use with "EV_A".

           Similar to the other two macros, this gives you the value of the
           default loop, if multiple loops are supported ("ev loop default").

           Usage identical to "EV_DEFAULT" and "EV_DEFAULT_", but requires
           that the default loop has been initialised ("UC" == unchecked).
           Their behaviour is undefined when the default loop has not been
           initialised by a previous execution of "EV_DEFAULT", "EV_DEFAULT_"
           or "ev_default_init (...)".

           It is often prudent to use "EV_DEFAULT" when initialising the first
           watcher in a function but use "EV_DEFAULT_UC" afterwards.

       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_loop (EV_DEFAULT_ 0);


       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

       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).

       Depending on what features you need you need to include one or more
       sets of files in your application.


       To include only the libev core (all the "ev_*" functions), with manual
       configuration (no autoconf):

          #define EV_STANDALONE 1
          #include "ev.c"

       This will automatically include 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 ev.h in all files wishing to use this API (best
       done by writing a wrapper around 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_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)

       ev.c includes the backend files directly when enabled, so you only need
       to compile this single file.


       To include the libevent compatibility API, also include:

          #include "event.c"

       in the file including ev.c, and:

          #include "event.h"

       in the files that want to use the libevent API. This also includes

       You need the following additional files for this:



       Instead of using "EV_STANDALONE=1" and providing your configuration in
       whatever way you want, you can also "m4_include([libev.m4])" in your and leave "EV_STANDALONE" undefined. ev.c will then
       include config.h and configure itself accordingly.

       For this of course you need the m4 file:


       Libev can be configured via a variety of preprocessor symbols you have
       to define before including any of its files. The default in the absence
       of autoconf is documented for every option.

           Must always be 1 if you do not use autoconf configuration, which
           keeps libev from including 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 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.

           If defined to be 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 "clock_gettime" function is hiding
           in (often -lrt). See also "EV_USE_CLOCK_SYSCALL".

           If defined to be 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
           "gettimeofday" by "clock_get (CLOCK_REALTIME, ...)" and will not
           normally affect correctness. See the note about libraries in the
           description of "EV_USE_MONOTONIC", though. Defaults to the opposite
           value of "EV_USE_CLOCK_SYSCALL".

           If defined to be 1, libev will try to use a direct syscall instead
           of calling the system-provided "clock_gettime" function. This
           option exists because on GNU/Linux, "clock_gettime" is in "librt",
           but "librt" unconditionally pulls in "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 1 on GNU/Linux with glibc 2.x or higher, as it
           simplifies linking (no need for "-lrt").

           If defined to be 1, libev will assume that "nanosleep ()" is
           available and will use it for delays. Otherwise it will use "select

           If defined to be 1, then libev will assume that "eventfd ()" is
           available and will probe for kernel support at runtime. This will
           improve "ev_signal" and "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.

           If undefined or defined to be 1, libev will compile in support for
           the "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.

           If defined to 1, then the select backend will use the system
           "fd_set" structure. This is useful if libev doesn’t compile due to
           a missing "NFDBITS" or "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 "FD_SETSIZE"
           macro, set before compilation, configures the maximum size of the

           When defined to 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
           "_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.

           If "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 "_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.

           If "EV_SELECT_IS_WINSOCKET" then libev maps handles to file
           descriptors using the standard "_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.

           If programs implement their own fd to handle mapping on win32, then
           this macro can be used to override the "close" function, useful to
           unregister file descriptors again. Note that the replacement
           function has to close the underlying OS handle.

           If defined to be 1, libev will compile in support for the "poll"(2)
           backend. Otherwise it will be enabled on non-win32 platforms. It
           takes precedence over select.

           If defined to be 1, libev will compile in support for the Linux
           "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.

           If defined to be 1, libev will compile in support for the BSD style
           "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.

           If defined to be 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.

           Reserved for future expansion, works like the USE symbols above.

           If defined to be 1, libev will compile in support for the Linux
           inotify interface to speed up "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.

           Libev requires an integer type (suitable for storing 0 or 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 "ev_async" watchers.

           In the absence of this define, libev will use "sig_atomic_t
           volatile" (from signal.h), which is usually good enough on most

           The name of the ev.h header file used to include it. The default if
           undefined is "ev.h" in event.h, ev.c and ev++.h. This can be used
           to virtually rename the ev.h header file in case of conflicts.

           If "EV_STANDALONE" isn’t 1, this variable can be used to override
           ev.c’s idea of where to find the config.h file, similarly to
           "EV_H", above.

           Similarly to "EV_H", this macro can be used to override event.c’s
           idea of how the event.h header can be found, the default is

           If defined to be 0, then 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.

           If undefined or defined to 1, then all event-loop-specific
           functions will have the "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.

           The range of allowed priorities. "EV_MINPRI" must be smaller or
           equal to "EV_MAXPRI", but otherwise there are no non-obvious
           limitations. You can provide for more priorities by overriding
           those symbols (usually defined to be "-2" and 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 0 will save some memory and CPU.

           If undefined or defined to be 1, then periodic timers are
           supported. If defined to be 0, then they are not. Disabling them
           saves a few kB of code.

           If undefined or defined to be 1, then idle watchers are supported.
           If defined to be 0, then they are not. Disabling them saves a few
           kB of code.

           If undefined or defined to be 1, then embed watchers are supported.
           If defined to be 0, then they are not. Embed watchers rely on most
           other watcher types, which therefore must not be disabled.

           If undefined or defined to be 1, then stat watchers are supported.
           If defined to be 0, then they are not.

           If undefined or defined to be 1, then fork watchers are supported.
           If defined to be 0, then they are not.

           If undefined or defined to be 1, then async watchers are supported.
           If defined to be 0, then they are not.

           If you need to shave off some kilobytes of code at the expense of
           some speed (but with the full API), define this symbol to 1.
           Currently this is used to override some inlining decisions, saves
           roughly 30% code size on amd64. It also selects a much smaller
           2-heap for timer management over the default 4-heap.

           You can save even more by disabling watcher types you do not need
           and setting "EV_MAXPRI" == "EV_MINPRI". Also, disabling "assert"
           ("-DNDEBUG") will usually reduce code size a lot.

           Defining "EV_MINIMAL" to 2 will additionally reduce the core API to
           provide a bare-bones event library. See "ev.h" for details on what
           parts of the API are still available, and do not complain if this
           subset changes over time.

           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 (32
           should be good for about any system in existance) can save some
           memory, as libev statically allocates some 12-24 bytes per signal

           "ev_child" watchers use a small hash table to distribute workload
           by pid. The default size is 16 (or 1 with "EV_MINIMAL"), usually
           more than enough. If you need to manage thousands of children you
           might want to increase this value (must be a power of two).

           "ev_stat" watchers use a small hash table to distribute workload by
           inotify watch id. The default size is 16 (or 1 with "EV_MINIMAL"),
           usually more than enough. If you need to manage thousands of
           "ev_stat" watchers you might want to increase this value (must be a
           power of two).

           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 1. The 4-heap uses more complicated (longer)
           code but has noticeably faster performance with many (thousands) of

           The default is 1 unless "EV_MINIMAL" is set in which case it is 0

           Heaps are not very cache-efficient. To improve the cache-efficiency
           of the timer and periodics heaps, libev can cache the timestamp
           (at) within the heap structure (selected by defining
           "EV_HEAP_CACHE_AT" to 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 1 unless "EV_MINIMAL" is set in which case it is 0

           Controls how much internal verification (see "ev_loop_verify ()")
           will be done: If set to 0, no internal verification code will be
           compiled in. If set to 1, then verification code will be compiled
           in, but not called. If set to 2, then the internal verification
           code will be called once per loop, which can slow down libev. If
           set to 3, then the verification code will be called very
           frequently, which will slow down libev considerably.

           The default is 1, unless "EV_MINIMAL" is set, in which case it will
           be 0.

           By default, all watchers have a "void *data" member. By redefining
           this macro to a 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 ";" */

       EV_CB_DECLARE (type)
       EV_CB_INVOKE (watcher, revents)
       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
           ev.h header file for their default definitions. One possible use
           for overriding these is to avoid the "struct ev_loop *" as first
           argument in all cases, or to use method calls instead of plain
           function calls in C++.

       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 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 "#define"’s that you need
       to include before including ev.h:

          <Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h

       This would create a file 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

       For a real-world example of a program the includes libev verbatim, you
       can have a look at the EV perl module
       (<>). It has the libev files in
       the libev/ subdirectory and includes them in the EV/EVAPI.h (public
       interface) and EV.xs (implementation) files. Only the EV.xs file will
       be compiled. It is pretty complex because it provides its own header

       The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file
       that everybody includes and which overrides some configure choices:

          #define EV_MINIMAL 1
          #define EV_USE_POLL 0
          #define EV_MULTIPLICITY 0
          #define EV_PERIODIC_ENABLE 0
          #define EV_STAT_ENABLE 0
          #define EV_FORK_ENABLE 0
          #define EV_CONFIG_H <config.h>
          #define EV_MINPRI 0
          #define EV_MAXPRI 0

          #include "ev++.h"

       And a ev_cpp.C implementation file that contains libev proper and is

          #include "ev_cpp.h"
          #include "ev.c"



       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 ("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 "ev_async" watchers, which allow some limited form of
       concurrency on the same event loop, namely waking it up "from the

       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:

       ·   most applications have a main thread: use the default libev loop in
           that thread, or create a separate thread running only the default

           This helps integrating other libraries or software modules that use
           libev themselves and don’t care/know about threading.

       ·   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.

       ·   other models exist, such as the leader/follower pattern, where one
           loop is handed through multiple threads in a kind of round-robin

           Choosing a model is hard - look around, learn, know that usually
           you can do better than you currently do :-)

       ·   often you need to talk to some other thread which blocks in the
           event loop.

           "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 "ev_async" watcher from the
           default loop watcher callback into the event loop interested in the


       Here is a fictitious example of how to run an event loop in a different
       thread than where callbacks are being invoked and watchers are

       For a real-world example, see the "EV::Loop::Async" perl module, which
       uses exactly this technique (which is suited for many high-level

       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

       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 "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 "l_release" and "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 "ev_loop":

          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_loop (EV_A_ 0);
            l_release (EV_A);

            return 0;

       Instead of invoking all pending watchers, the "l_invoke" callback will
       signal the main thread via some unspecified mechanism (signals? pipe
       writes? "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 "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 "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.


       Libev is very accommodating to coroutines ("cooperative threads"):
       libev fully supports nesting calls to its functions from different
       coroutines (e.g. you can call "ev_loop" 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 "ev_periodic" reschedule callbacks.

       Care has been taken to ensure that libev does not keep local state
       inside "ev_loop", and other calls do not usually allow for coroutine
       switches as they do not call any callbacks.

       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 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.

       Valgrind has a special section here because it is a popular tool that
       is highly useful. Unfortunately, valgrind reports are very hard to

       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

       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.


       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 "EVBACKEND_SELECT" backend, and only supports socket
       descriptors. This only applies when using Win32 natively, not when
       using e.g. cygwin.

       Lifting these limitations would basically require the full re-
       implementation of the I/O system. If you are into these kinds of
       things, 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

       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

       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 "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 evwrap.h header
       file instead of 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 evwrap.c file into your project (make sure
       you do not compile the ev.c or any other embedded source files!):

          #include "evwrap.h"
          #include "ev.c"

       The winsocket select function
           The winsocket "select" function doesn’t follow POSIX in that it
           requires socket handles and not socket 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 "_open_osfhandle" for
           this). See the discussion of the "EV_SELECT_USE_FD_SET",
           "EV_SELECT_IS_WINSOCKET" and "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(nAX) range when using win32.

       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 64 handles (probably owning to the fact that all windows
           kernels can only wait for 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
           "FD_SETSIZE" to some high number (e.g. 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 64 (there must be a hidden
           64 fetish or something like this inside Microsoft). You can
           increase this by calling "_setmaxstdio", which can increase this
           limit to 2048 (another arbitrary limit), but is broken in many
           versions of the Microsoft runtime libraries. This might get you to
           about 512 or 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(nAX)) will likely make this unworkable.

       In addition to a working ISO-C implementation and of course the
       backend-specific APIs, libev relies on a few additional extensions:

       "void (*)(ev_watcher_type *, int revents)" must have compatible calling
       conventions regardless of "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
           "ev_watcher *" internally.

       "sig_atomic_t volatile" must be thread-atomic as well
           The type "sig_atomic_t volatile" (or whatever is defined as
           "EV_ATOMIC_T") must be atomic with respect to accesses from
           different threads. This is not part of the specification for
           "sig_atomic_t", but is believed to be sufficiently portable.

       "sigprocmask" must work in a threaded environment
           Libev uses "sigprocmask" to temporarily block signals. This is not
           allowed in a threaded program ("pthread_sigmask" has to be used).
           Typical pthread implementations will either allow "sigprocmask" in
           the "main thread" or will block signals process-wide, both
           behaviours would be compatible with libev. Interaction between
           "sigprocmask" and "pthread_sigmask" could complicate things,

           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.

       "long" must be large enough for common memory allocation sizes
           To improve portability and simplify its API, libev uses "long"
           internally instead of "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.

       "double" must hold a time value in seconds with enough accuracy
           The type "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. This
           requirement is fulfilled by implementations implementing IEEE 754,
           which is basically all existing ones. With IEEE 754 doubles, you
           get microsecond accuracy until at least 2200.

       If you know of other additional requirements drop me a note.


       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 "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

       Starting and stopping timer/periodic watchers: O(log
           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 ("ld 100") of these

       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

       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.

       Stopping check/prepare/idle/fork/async watchers: O(1)
       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).

       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.

       Each change on a file descriptor per loop iteration:
           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 "ev_io_set" was used).

       Activating one watcher (putting it into the pending state): O(1)
       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.

       Sending an ev_async: O(1)
       Processing ev_async_send: O(number_of_async_watchers)
       Processing signals: O(max_signal_number)
           Sending involves a system call iff there were no other
           "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.


           A watcher is active as long as it has been started (has been
           attached to an event loop) but not yet stopped (disassociated from
           the event loop).

           In this document, an application is whatever is using libev.

           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.

       callback invocation
           The act of calling the callback associated with a watcher.

           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 "EV_READ"
           or "EV_TIMEOUT").

       event library
           A software package implementing an event model and loop.

       event loop
           An entity that handles and processes external events and converts
           them into callback invocations.

       event model
           The model used to describe how an event loop handles and processes
           watchers and events.

           A watcher is pending as soon as the corresponding event has been
           detected, and stops being pending as soon as the watcher will be
           invoked or its pending status is explicitly cleared by the

           A watcher can be pending, but not active. Stopping a watcher also
           clears its pending status.

       real time
           The physical time that is observed. It is apparently strictly
           monotonic :)

       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.

           A data structure that describes interest in certain events.
           Watchers need to be started (attached to an event loop) before they
           can receive events.

       watcher invocation
           The act of calling the callback associated with a watcher.


       Marc Lehmann <>, with repeated corrections by Mikael