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


  #include <ev.h>


  // a single header file is required
  #include <ev.h>
  // 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_ struct 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_ struct 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;


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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), 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, and when you need to do any calculations on it, you should treat it as some floatingpoint 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 syscall 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 assertions, 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 subsecond-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 autodetected 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))

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));

Set the callback function to call on a retryable syscall error (such as failed select, poll, epoll_wait). The message is a printable string indicating the system call or subsystem causing the problem. If this callback is set, then libev will expect it to remedy the sitution, no matter what, when it returns. That is, libev will 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 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 bes hared 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 app 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 EVFLAG_AUTO).

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 ored 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 bugs.


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 syscall 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 overriden or specified in the LIBEV_FLAGS environment variable.

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 parallelity (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.

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.

EVBACKEND_EPOLL (value 4, Linux)

For few fds, this backend is a bit little slower than poll and select, but it scales phenomenally better. While poll and select usually scale like O(total_fds) where n is the total number of fds (or the highest fd), epoll scales either O(1) or O(active_fds). The epoll design has a number of shortcomings, such as silently dropping events in some hard-to-detect cases and requiring a syscall per fd change, no fork support and bad support for dup.

While stopping, setting and starting an I/O watcher in the same iteration will result in some caching, there is still a syscall per such incident (because the fd 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 fds.

Please note that epoll sometimes generates spurious notifications, so you need to use non-blocking I/O or other means to avoid blocking when no data (or space) is available.

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.

While nominally embeddeble in other event loops, this feature is broken in all kernel versions tested so far.

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). For this reason it's not being ``autodetected'' unless you explicitly specify it explicitly 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 syscall as with EVBACKEND_EPOLL, it still adds up to two event changes per incident, support for fork () is very bad 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) and using it only for sockets.

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 available.

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, ignoring the spurious readiness notifications, this backend actually performed to specification in all tests and is fully embeddable, which is a rare feat among the OS-specific backends.


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 these are ored 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.

The most typical usage is like this:

  if (!ev_default_loop (0))
    fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");

Restrict libev to the select and poll backends, and do not allow environment settings to be taken into account:


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 else.

  struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
  if (!epoller)
    fatal ("no epoll found here, maybe it hides under your chair");
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 yoursef 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, 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, and how you do this is entirely your own problem.

int ev_is_default_loop (loop)

Returns true when the given loop actually is the default loop, 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_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_loop (loop, int flags)

Finally, this is it, the event handler. This function usually is called after you initialised all your watchers and you want to start handling events.

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

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 is a thing of beauty.

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

A flags value of EVLOOP_ONESHOT will look for new events (waiting if neccessary) and will handle those and any outstanding ones. It will block your process until at least one new event arrives, 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. 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, queue and call all fork watchers.
   - Queue and call all prepare watchers.
   - If we have been forked, recreate the kernel state.
   - Update the kernel state with all outstanding changes.
   - Update the "event loop time".
   - 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" and do time jump handling.
   - Queue all outstanding timers.
   - Queue all outstanding periodics.
   - If no 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. 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.

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. If you have a watcher you never unregister that should not keep ev_loop from returning, ev_unref() after starting, and ev_ref() before stopping it. For 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).

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

  struct 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 again.

  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 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.

The background 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.

Likewise, by setting a higher timeout collect interval you allow libev to spend more time collecting timeouts, at the expense of increased latency (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 io 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 approsaches the timing granularity of most systems.

ev_loop_verify (loop)

This function only does something when EV_VERIFY support has been compiled in. 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.


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

  static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
    ev_io_stop (w);
    ev_unloop (loop, EVUNLOOP_ALL);
  struct ev_loop *loop = ev_default_loop (0);
  struct ev_io stdin_watcher;
  ev_init (&stdin_watcher, my_cb);
  ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
  ev_io_start (loop, &stdin_watcher);
  ev_loop (loop, 0);

As you can see, you are responsible for allocating the memory for your watcher structures (and it is usually a bad idea to do this on the stack, although this can sometimes be quite valid).

Each watcher structure must be initialised by a call to ev_init (watcher *, callback), which expects a callback to be provided. This callback gets invoked each time the event occurs (or, in the case of io watchers, each time the event loop detects that the file descriptor given is readable and/or writable).

Each watcher type has its own ev_<type>_set (watcher *, ...) macro with arguments specific to this watcher type. There is also a macro to combine initialisation and setting in one call: 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 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 change.


The path specified in the ev_stat watcher changed its attributes somehow.


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 attention.


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


The given async watcher has been asynchronously notified (see ev_async).


An unspecified error has occured, the watcher has been stopped. This might happen because the watcher could not be properly started because libev ran out of memory, a file descriptor was found to be closed or any other problem. You best act on it by reporting the problem and somehow coping with the watcher being stopped.

Libev will usually signal a few ``dummy'' events together with an error, for example it might indicate that a fd is readable or writable, and if your callbacks is well-written it can just attempt the operation and cope with the error from read() or write(). This will not work in multithreaded programs, though, so beware.


In the following description, TYPE stands for the watcher type, e.g. timer for ev_timer watchers and io for ev_io watchers.

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 outstanding.

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

ev_TYPE_set (ev_TYPE *, [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.

ev_TYPE_init (ev_TYPE *watcher, callback, [args])

This convinience macro rolls both ev_init and ev_TYPE_set macro calls into a single call. This is the most convinient method to initialise a watcher. The same limitations apply, of course.

ev_TYPE_start (loop *, ev_TYPE *watcher)

Starts (activates) the given watcher. Only active watchers will receive events. If the watcher is already active nothing will happen.

ev_TYPE_stop (loop *, ev_TYPE *watcher)

Stops the given watcher again (if active) and clears the pending status. It is possible that stopped watchers are pending (for example, non-repeating timers are being stopped when they become pending), but 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, 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).

This means that priorities are only used for ordering callback invocation after new events have been received. This is useful, for example, to reduce latency after idling, or more often, to bind two watchers on the same event and make sure one is called first.

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

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

The default priority used by watchers when no priority has been set is always 0, which is supposed to not be too high and not be too low :).

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 adjusted to be within valid range.

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.

int ev_clear_pending (loop, ev_TYPE *watcher)

If the watcher is pending, this function returns 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.


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

And since your callback will be called with a pointer to the watcher, you can cast it back to your own type:

  static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
    struct my_io *w = (struct my_io *)w_;

More interesting and less C-conformant ways of casting your callback type instead have been omitted.

Another common scenario is having some data structure with multiple watchers:

  struct my_biggy
    int some_data;
    ev_timer t1;
    ev_timer t2;

In this case getting the pointer to my_biggy is a bit more complicated, you need to use offsetof:

  #include <stddef.h>
  static void
  t1_cb (EV_P_ struct ev_timer *w, int revents)
    struct my_biggy big = (struct my_biggy *
      (((char *)w) - offsetof (struct my_biggy, t1));
  static void
  t2_cb (EV_P_ struct ev_timer *w, int revents)
    struct my_biggy big = (struct my_biggy *
      (((char *)w) - offsetof (struct my_biggy, t2));


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 must do this, 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).

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 arrives.

If you cannot run the fd in non-blocking mode (for example you should not play around with an Xlib connection), then you have to seperately re-test whether a file descriptor is really ready with a known-to-be good interface such as poll (fortunately in our Xlib example, Xlib already does this on its own, so its quite safe to use).

The special problem of disappearing file descriptors

Some backends (e.g. kqueue, epoll) need to be told about closing a file descriptor (either by calling close explicitly or by any other means, such as dup). 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 change.

This is how one would do it normally anyway, the important point is that the libev application should not optimise around libev but should leave optimisations to libev.

The special problem of dup'ed file descriptors

Some backends (e.g. epoll), cannot register events for file descriptors, but only events for the underlying file descriptions. That means when you have 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 EVBACKEND_SELECT or EVBACKEND_POLL.

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 EVBACKEND_POLL.

The special problem of SIGPIPE

While not really specific to libev, it is easy to forget about SIGPIPE: when reading from a pipe whose other end has been closed, your program gets send 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 rceeive events for and events is either EV_READ, EV_WRITE or EV_READ | EV_WRITE to receive the given events.

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, struct ev_io *w, int revents)
     ev_io_stop (loop, w);
    .. read from stdin here (or from w->fd) and haqndle any I/O errors
  struct ev_loop *loop = ev_default_init (0);
  struct ev_io stdin_readable;
  ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
  ev_io_start (loop, &stdin_readable);
  ev_loop (loop, 0);

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) and hour. ``Roughly'' because detecting time jumps is hard, and some inaccuracies are unavoidable (the monotonic clock option helps a lot here).

The relative timeouts are calculated relative to the 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 this:

   ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);

The callback is guarenteed to be invoked only after its timeout has passed, but if multiple timers become ready during the same loop iteration then order of execution is undefined.

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 nonrepeating, stop it (as if it timed out).

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

This sounds a bit complicated, but here is a useful and typical example: Imagine you have a tcp connection and you want a so-called idle timeout, that is, you want to be called when there have been, say, 60 seconds of inactivity on the socket. The easiest way to do this is to configure an ev_timer with a repeat value of 60 and then call ev_timer_again each time you successfully read or write some data. If you go into an idle state where you do not expect data to travel on the socket, you can ev_timer_stop the timer, and ev_timer_again will automatically restart it if need be.

That means you can ignore the after value and ev_timer_start altogether and only ever use the repeat value and ev_timer_again:

   ev_timer_init (timer, callback, 0., 5.);
   ev_timer_again (loop, timer);
   timer->again = 17.;
   ev_timer_again (loop, timer);
   timer->again = 10.;
   ev_timer_again (loop, timer);

This is more slightly efficient then stopping/starting the timer each time you want to modify its timeout value.

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, struct ev_timer *w, int revents)
    .. one minute over, w is actually stopped right here
  struct ev_timer mytimer;
  ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
  ev_timer_start (loop, &mytimer);

Example: Create a timeout timer that times out after 10 seconds of inactivity.

  static void
  timeout_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
    .. ten seconds without any activity
  struct ev_timer mytimer;
  ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
  ev_timer_again (&mytimer); /* start timer */
  ev_loop (loop, 0);
  // and in some piece of code that gets executed on any "activity":
  // reset the timeout to start ticking again at 10 seconds
  ev_timer_again (&mytimer);

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's, they are not based on real time (or relative time) but on wallclock time (absolute time). You can tell a periodic watcher to trigger after some specific point in time. For example, if you tell a periodic watcher to trigger in 10 seconds (by specifiying 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 more than year to trigger the event (unlike an ev_timer, which would still trigger roughly 10 seconds later as it uses a relative timeout).

ev_periodics can also be used to implement vastly more complex timers, such as triggering an event on each ``midnight, local time'', or other complicated, rules.

As with timers, the callback is guarenteed to be invoked only when the time (at) has passed, but if multiple periodic timers become ready during the same loop iteration then order of execution is undefined.

Watcher-Specific Functions and Data Members

ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)
ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)

Lots of arguments, lets sort it out... There are basically three modes of operation, and we will explain them from simplest to complex:

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.

ev_tstamp offset [read-write]

When repeating, this contains the offset value, otherwise this is the absolute point in time (the at value passed to ev_periodic_set).

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)(struct 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 called.


Example: Call a callback every hour, or, more precisely, whenever the system clock is divisible by 3600. The callback invocation times have potentially a lot of jittering, but good long-term stability.

  static void
  clock_cb (struct ev_loop *loop, struct ev_io *w, int revents)
    ... its now a full hour (UTC, or TAI or whatever your clock follows)
  struct ev_periodic hourly_tick;
  ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
  ev_periodic_start (loop, &hourly_tick);

Example: The same as above, but use a reschedule callback to do it:

  #include <math.h>
  static ev_tstamp
  my_scheduler_cb (struct ev_periodic *w, ev_tstamp now)
    return fmod (now, 3600.) + 3600.;
  ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);

Example: Call a callback every hour, starting now:

  struct ev_periodic hourly_tick;
  ev_periodic_init (&hourly_tick, clock_cb,
                    fmod (ev_now (loop), 3600.), 3600., 0);
  ev_periodic_start (loop, &hourly_tick);

ev_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.

You can configure as many watchers as you like per signal. Only when the first watcher gets started will libev actually register a signal watcher with the kernel (thus it coexists with your own signal handlers as long as you don't register any with libev). Similarly, when the last signal watcher for a signal is stopped libev will reset the signal handler to SIG_DFL (regardless of what it was set to before).

If possible and supported, libev will install its handlers with SA_RESTART behaviour enabled, so syscalls should not be unduly interrupted. If you have a problem with syscalls getting interrupted by signals you can block all signals in an ev_check watcher and unblock them in an ev_prepare watcher.

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 and SIGTERM.

  static void
  sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents)
    ev_unloop (loop, EVUNLOOP_ALL);
  struct ev_signal signal_watcher;
  ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
  ev_signal_start (loop, &sigint_cb);

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). 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).

Only the default event loop is capable of handling signals, and therefore you can only rgeister child watchers in the default event loop.

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 occurance 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.

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 continued).

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_ struct 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 filesystem path for attribute changes. That is, it calls stat regularly (or when the OS says it changed) and sees if it changed compared to the last time, invoking the callback if it did.

The path does not need to exist: changing from ``path exists'' to ``path does not exist'' is a status change like any other. The condition ``path does not exist'' is signified by the 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 should be absolute and must not end in a slash. If it is relative and your working directory changes, the behaviour is undefined.

Since there is no standard to do this, the portable implementation simply calls 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 thats usually overkill.

This watcher type is not meant for massive numbers of stat watchers, as even with OS-supported change notifications, this can be resource-intensive.

At the time of this writing, only the Linux inotify interface is implemented (implementing kqueue support is left as an exercise for the reader, note, however, that the author sees no way of implementing ev_stat semantics with kqueue). Inotify will be used to give hints only and should not change the semantics of ev_stat watchers, which means that libev sometimes needs to fall back to regular polling again even with inotify, but changes are usually detected immediately, and if the file exists there will be no polling.

ABI Issues (Largefile Support)

Libev by default (unless the user overrides this) uses the default compilation environment, which means that on systems with optionally disabled large file support, 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 noticably with ev_stat and largefile support.


When inotify (7) support has been compiled into libev (generally only available on Linux) 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.

(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).

The special problem of stat time resolution

The stat () syscall only supports full-second resolution portably, and even on systems where the resolution is higher, many filesystems 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 it as the stat data does not change.

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 interval)
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 EV_STAT 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 stating 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, st_ctime.

ev_tstamp interval [read-only]

The specified interval.

const char *path [read-only]

The filesystem 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 you've got nothing better to do...

Idle watchers trigger events when no other events of the same or higher priority are pending (prepare, check and other idle watchers do not count).

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_signal *, 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, struct ev_idle *w, int revents)
    free (w);
    // now do something you wanted to do when the program has
    // no longer anything immediate to do.
  struct ev_idle *idle_watcher = malloc (sizeof (struct ev_idle));
  ev_idle_init (idle_watcher, idle_cb);
  ev_idle_start (loop, idle_cb);

ev_prepare and ev_check - customise your event loop!

Prepare and check watchers are usually (but not always) used in tandem: 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. This 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 just this functionality). Then, in the check watcher you check for any events that occured (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. 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 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 (ev_loop *loop, ev_io *w, int revents)
  // create io watchers for each fd and a timer before blocking
  static void
  adns_prepare_cb (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);
    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 (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 watcher.

Method 3: If the module to be embedded supports explicit event notification (adns does), you can also make use of the actual watcher callbacks, and only destroy/create the watchers in the prepare watcher.

  static void
  timer_cb (EV_P_ ev_timer *w, int revents)
    adns_state ads = (adns_state)w->data;
    update_now (EV_A);
    adns_processtimeouts (ads, &tv_now);
  static void
  io_cb (EV_P_ ev_io *w, int revents)
    adns_state ads = (adns_state)w->data;
    update_now (EV_A);
    if (revents & EV_READ ) adns_processreadable  (ads, w->fd, &tv_now);
    if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
  // do not ever call adns_afterpoll

Method 4: Do not use a prepare or check watcher because the module you want to embed is too inflexible to support it. Instead, youc na 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 does this.

  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 isn't 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 poll and then call kevent, but at least you can use both at what they are best.

As for prioritising I/O: rarely you have the case where some fds have to be watched and handled very quickly (with low latency), and even priorities and idle watchers might have too much overhead. In this case you would put all the high priority stuff in one loop and all the rest in a second one, and embed the second one in the first.

As long as the watcher is active, the callback will be invoked every time there might be events pending in the embedded loop. The callback must then call ev_embed_sweep (mainloop, watcher) to make a single sweep and invoke their callbacks (you 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.

As long as the watcher is started it will automatically handle events. The callback will be invoked whenever some events have been handled. You can set the callback to 0 to avoid having to specify one if you are not interested in that.

Also, there have not currently been made special provisions for forking: when you fork, you not only have to call ev_loop_fork on both loops, but you will also have to stop and restart any ev_embed watchers yourself.

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 everything.

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 thta, 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 apropriate 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 mebeddable loop is stored in loop_lo (which is loop_hi in the acse no embeddable loop can be used).

  struct ev_loop *loop_hi = ev_default_init (0);
  struct ev_loop *loop_lo = 0;
  struct 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;
  struct 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.

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.

That means that if you want to queue data, you have to provide your own queue. But at least I can tell you would implement locking around your queue:

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 fictitiuous 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 example:

   static ev_async mysig;
   static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
   static void
   otherthread (void)
     // only need to lock the actual queueing operation
     pthread_mutex_lock (&mymutex);
     queue_put (data);
     pthread_mutex_unlock (&mymutex);
     ev_async_send (EV_DEFAULT_ &mysig);
   static void
   mysig_cb (EV_P_ ev_async *w, int revents)
     pthread_mutex_lock (&mymutex);
     while (queue_get (&data))
       process (data);
     pthread_mutex_unlock (&mymutex);

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_asynd_set macro, but using it is utterly pointless, believe 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 in other threads, signal or similar contexts (see the dicusssion of EV_ATOMIC_T in the embedding section below on what exactly this means).

This call incurs the overhead of a syscall only once per loop iteration, so while the overhead might be noticable, it doesn't apply to repeated calls to ev_async_send.

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 wether invoking the loop might be a good idea.

Not that this does not check wether the watcher itself is pending, only wether it has been requested to make this watcher pending.


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

ev_once (loop, int fd, int events, ev_tstamp timeout, callback)

This function combines a simple timer and an I/O watcher, calls your callback on whichever event happens first and automatically stop both watchers. This is useful if you want to wait for a single event on an fd or timeout without 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 events is being ignored. Otherwise, an ev_io watcher for the given fd and events set will be craeted 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. While 0 is a valid timeout, it is of dubious value.

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:

  static void stdin_ready (int revents, void *arg)
    if (revents & EV_TIMEOUT)
      /* doh, nothing entered */;
    else if (revents & EV_READ)
      /* stdin might have data for us, joy! */;
  ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
ev_feed_event (ev_loop *, 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).

ev_feed_fd_event (ev_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 (ev_loop *loop, int signum)

Feed an event as if the given signal occured (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:


Libev comes with some simplistic wrapper classes for C++ that mainly allow you to use some convinience methods to start/stop watchers and also change the callback model to a model using method callbacks on objects.

To use it,

  #include <ev++.h>

This automatically includes 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 (struct ev_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 constructors).

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<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 use.

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

See the method-set above for more details.


  static void io_cb (ev::io &w, int revents) { }
  iow.set <io_cb> ();
w->set (struct ev_loop *)

Associates a different struct ev_loop with this watcher. You can only do this when the watcher is inactive (and not pending either).

w->set ([args])

Basically the same as ev_TYPE_set, with the same args. 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 numbe rof 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), 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 found at


Tony Arcieri has written a ruby extension that offers access to a subset of the libev API and adds filehandle abstractions, asynchronous DNS and more on top of it. It can be found via gem servers. Its homepage is at


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


Libev can be compiled with a variety of options, the most fundamantal 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:


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. Example:

  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.


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 rxvt-unicode.

The goal is to enable you to just copy the necessary files into your source directory without having to change even a single line in them, so you can easily upgrade by simply copying (or having a checked-out copy of libev somewhere in your source tree).


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


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, thats 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 ev.h.

You need the following additional files for this:



Instead of using EV_STANDALONE=1 and providing your config 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 absense of autoconf is noted 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.


If defined to be 1, libev will try to detect the availability of the monotonic clock option at both compiletime 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).


If defined to be 1, libev will try to detect the availability of the realtime clock option at compiletime (and assume its availability at runtime if successful). Otherwise no use of the realtime 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.


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 autodetection 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 misguesses 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, might influence the size of the fd_set used.


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 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 absense of this define, libev will use sig_atomic_t volatile (from signal.h), which is usually good enough on most platforms.


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 "event.h".


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 app 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.


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, define this symbol to 1. Currently this is used to override some inlining decisions, saves roughly 30% codesize of amd64. It also selects a much smaller 2-heap for timer management over the default 4-heap.


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 heap, libev uses a 4-heap when this symbol is defined to 1. The 4-heap uses more complicated (longer) code but has noticably faster performance with many (thousands) of watchers.

The default is 1 unless EV_MINIMAL is set in which case it is 0 (disabled).


Heaps are not very cache-efficient. To improve the cache-efficiency of the timer and periodics heap, 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 noticably with with many (hundreds) of watchers.

The default is 1 unless EV_MINIMAL is set in which case it is 0 (disabled).


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_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 inconvinient 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 file.

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_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 compiled:

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



Libev itself is completely threadsafe, but it uses no locking. This means that you can use as many loops as you want in parallel, as long as only one thread ever calls into one libev function with the same loop parameter.

Or put differently: calls with different loop parameters can be done in parallel 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).

If you want to know which design is best for your problem, then I cannot help you but by giving some generic advice:


Libev is much more accomodating to coroutines (``cooperative threads''): libev fully supports nesting calls to it's 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 invested into making sure that libev does not keep local state inside ev_loop, and other calls do not usually allow coroutine switches.


In this section the complexities of (many of) the algorithms used inside libev will be explained. 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 never with higher number of elements, so O(1) might mean it might do a lengthy realloc operation in rare cases, but on average it is much faster and asymptotically approaches constant time.

Starting and stopping timer/periodic watchers: O(log skipped_other_timers)

This means that, when you have a watcher that triggers in one hour and there are 100 watchers that would trigger before that then inserting will have to skip roughly seven (ld 100) of these watchers.

Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)

That means that changing a timer costs less than removing/adding them as only the relative motion in the event queue has to be paid for.

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 then 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).

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: O(number_of_watchers_for_this_fd)

A change means an I/O watcher gets started or stopped, which requires libev to recalculate its status (and possibly tell the kernel, depending on backend and wether 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) w.r.t. 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 syscall 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.

Win32 platform limitations and workarounds

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 man).

There is no supported compilation method available on windows except embedding it into other applications.

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 (microsoft monopoly games).

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. 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(n²) 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. 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 does its 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(n²)) will likely make this unworkable.


In addition to a working ISO-C implementation, libev relies on a few additional extensions:

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 w.r.t. 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, however.

The most portable way to handle signals is to block signals in all threads except the initial one, and run the default loop in the initial thread as well.

long must be large enough for common memory allocation sizes

To improve portability and simplify using libev, 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 (basically all existing ones).

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


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 targetting 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 cindition their message seems to warn about).

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, but valgrind reports are very hard to interpret.

If you think you found a bug (memory leak, uninitialised data access etc.) in libev, then check twice: If valgrind reports something like:

   ==2274==    definitely lost: 0 bytes in 0 blocks.
   ==2274==      possibly lost: 0 bytes in 0 blocks.
   ==2274==    still reachable: 256 bytes in 1 blocks.

then there is no memory leak. Similarly, under some circumstances, valgrind might report kernel bugs as if it were a bug in libev, or it might be confused (it is a very good tool, but only a tool).

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. 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.


Marc Lehmann <>.