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EV - perl interface to libev, a high performance full-featured event loop


  use EV;
  my $w = EV::timer 2, 0, sub {
     warn "is called after 2s";
  my $w = EV::timer 2, 2, sub {
     warn "is called roughly every 2s (repeat = 2)";
  undef $w; # destroy event watcher again
  my $w = EV::periodic 0, 60, 0, sub {
     warn "is called every minute, on the minute, exactly";
  # IO
  my $w = EV::io *STDIN, EV::READ, sub {
     my ($w, $revents) = @_; # all callbacks receive the watcher and event mask
     warn "stdin is readable, you entered: ", <STDIN>;
  my $w = EV::signal 'QUIT', sub {
     warn "sigquit received\n";

  my $w = EV::child 666, sub {
     my ($w, $revents) = @_;
     my $status = $w->rstatus;

  my $w = EV::stat "/etc/passwd", 10, sub {
     my ($w, $revents) = @_;
     warn $w->path, " has changed somehow.\n";
  EV::loop;           # loop until EV::unloop is called or all watchers stop
  EV::loop EV::LOOP_ONESHOT;  # block until at least one event could be handled
  EV::loop EV::LOOP_NONBLOCK; # try to handle same events, but do not block


This module provides an interface to libev ( While the documentation below is comprehensive, one might also consult the documentation of libev itself ( for more subtle details on watcher semantics or some discussion on the available backends, or how to force a specific backend with LIBEV_FLAGS.



Must contain a reference to a function that is called when a callback throws an exception (with $@ containing thr error). The default prints an informative message and continues.

If this callback throws an exception it will be silently ignored.

$time = EV::time

Returns the current time in (fractional) seconds since the epoch.

$time = EV::now

Returns the time the last event loop iteration has been started. This is the time that (relative) timers are based on, and refering to it is usually faster then calling EV::time.

$method = EV::method

Returns an integer describing the backend used by libev (EV::METHOD_SELECT or EV::METHOD_EPOLL).

EV::loop [$flags]

Begin checking for events and calling callbacks. It returns when a callback calls EV::unloop.

The $flags argument can be one of the following:

   0                  as above
   EV::LOOP_ONESHOT   block at most once (wait, but do not loop)
   EV::LOOP_NONBLOCK  do not block at all (fetch/handle events but do not wait)
EV::unloop [$how]

When called with no arguments or an argument of EV::UNLOOP_ONE, makes the innermost call to EV::loop return.

When called with an argument of EV::UNLOOP_ALL, all calls to EV::loop will return as fast as possible.

$count = EV::loop_count

Return the number of times the event loop has polled for new events. Sometiems useful as a generation counter.

EV::once $fh_or_undef, $events, $timeout, $cb->($revents)

This function rolls together an I/O and a timer watcher for a single one-shot event without the need for managing a watcher object.

If $fh_or_undef is a filehandle or file descriptor, then $events must be a bitset containing either EV::READ, EV::WRITE or EV::READ | EV::WRITE, indicating the type of I/O event you want to wait for. If you do not want to wait for some I/O event, specify undef for $fh_or_undef and 0 for $events).

If timeout is undef or negative, then there will be no timeout. Otherwise a EV::timer with this value will be started.

When an error occurs or either the timeout or I/O watcher triggers, then the callback will be called with the received event set (in general you can expect it to be a combination of EV:ERROR, EV::READ, EV::WRITE and EV::TIMEOUT).

EV::once doesn't return anything: the watchers stay active till either of them triggers, then they will be stopped and freed, and the callback invoked.


A watcher is an object that gets created 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:

  my $watcher = EV::io *STDIN, EV::READ, sub {
     my ($watcher, $revents) = @_;
     warn "yeah, STDIN should not be readable without blocking!\n"

All watchers can be active (waiting for events) or inactive (paused). Only active watchers will have their callbacks invoked. All callbacks will be called with at least two arguments: the watcher and a bitmask of received events.

Each watcher type has its associated bit in revents, so you can use the same callback for multiple watchers. The event mask is named after the type, i..e. EV::child sets EV::CHILD, EV::prepare sets EV::PREPARE, EV::periodic sets EV::PERIODIC and so on, with the exception of IO events (which can set both EV::READ and EV::WRITE bits), and EV::timer (which uses EV::TIMEOUT).

In the rare case where one wants to create a watcher but not start it at the same time, each constructor has a variant with a trailing _ns in its name, e.g. EV::io has a non-starting variant EV::io_ns and so on.

Please note that a watcher will automatically be stopped when the watcher object is destroyed, so you need to keep the watcher objects returned by the constructors.

Also, all methods changing some aspect of a watcher (->set, ->priority, ->fh and so on) automatically stop and start it again if it is active, which means pending events get lost.


This section lists methods common to all watchers.


Starts a watcher if it isn't active already. Does nothing to an already active watcher. By default, all watchers start out in the active state (see the description of the _ns variants if you need stopped watchers).


Stop a watcher if it is active. Also clear any pending events (events that have been received but that didn't yet result in a callback invocation), regardless of wether the watcher was active or not.

$bool = $w->is_active

Returns true if the watcher is active, false otherwise.

$current_data = $w->data
$old_data = $w->data ($new_data)

Queries a freely usable data scalar on the watcher and optionally changes it. This is a way to associate custom data with a watcher:

   my $w = EV::timer 60, 0, sub {
      warn $_[0]->data;
   $w->data ("print me!");
$current_cb = $w->cb
$old_cb = $w->cb ($new_cb)

Queries the callback on the watcher and optionally changes it. You can do this at any time without the watcher restarting.

$current_priority = $w->priority
$old_priority = $w->priority ($new_priority)

Queries the priority on the watcher and optionally changes it. Pending watchers with higher priority will be invoked first. The valid range of priorities lies between EV::MAXPRI (default 2) and EV::MINPRI (default -2). If the priority is outside this range it will automatically be normalised to the nearest valid priority.

The default priority of any newly-created watcher is 0.

Note that the priority semantics have not yet been fleshed out and are subject to almost certain change.

$w->trigger ($revents)

Call the callback *now* with the given event mask.

$previous_state = $w->keepalive ($bool)

Normally, EV::loop will return when there are no active watchers (which is a "deadlock" because no progress can be made anymore). This is convinient because it allows you to start your watchers (and your jobs), call EV::loop once and when it returns you know that all your jobs are finished (or they forgot to register some watchers for their task :).

Sometimes, however, this gets in your way, for example when you the module that calls EV::loop (usually the main program) is not the same module as a long-living watcher (for example a DNS client module written by somebody else even). Then you might want any outstanding requests to be handled, but you would not want to keep EV::loop from returning just because you happen to have this long-running UDP port watcher.

In this case you can clear the keepalive status, which means that even though your watcher is active, it won't keep EV::loop from returning.

The initial value for keepalive is true (enabled), and you cna change it any time.

Example: Register an IO watcher for some UDP socket but do not keep the event loop from running just because of that watcher.

   my $udp_socket = ...
   my $udp_watcher = EV::io $udp_socket, EV::READ, sub { ... };
   $udp_watcher->keepalive (0);


Each of the following subsections describes a single watcher type.

IO WATCHERS - is this file descriptor readable or writable?

$w = EV::io $fileno_or_fh, $eventmask, $callback
$w = EV::io_ns $fileno_or_fh, $eventmask, $callback

As long as the returned watcher object is alive, call the $callback when at least one of events specified in $eventmask occurs.

The $eventmask can be one or more of these constants ORed together:

  EV::READ     wait until read() wouldn't block anymore
  EV::WRITE    wait until write() wouldn't block anymore

The io_ns variant doesn't start (activate) the newly created watcher.

$w->set ($fileno_or_fh, $eventmask)

Reconfigures the watcher, see the constructor above for details. Can be called at any time.

$current_fh = $w->fh
$old_fh = $w->fh ($new_fh)

Returns the previously set filehandle and optionally set a new one.

$current_eventmask = $w->events
$old_eventmask = $w->events ($new_eventmask)

Returns the previously set event mask and optionally set a new one.

TIMER WATCHERS - relative and optionally repeating timeouts

$w = EV::timer $after, $repeat, $callback
$w = EV::timer_ns $after, $repeat, $callback

Calls the callback after $after seconds (which may be fractional). If $repeat is non-zero, the timer will be restarted (with the $repeat value as $after) after the callback returns.

This means that the callback would be called roughly after $after seconds, and then every $repeat seconds. The timer does his best not to drift, but it will not invoke the timer more often then once per event loop iteration, and might drift in other cases. If that isn't acceptable, look at EV::periodic, which can provide long-term stable timers.

The timer is based on a monotonic clock, that is, if somebody is sitting in front of the machine while the timer is running and changes the system clock, the timer will nevertheless run (roughly) the same time.

The timer_ns variant doesn't start (activate) the newly created watcher.

$w->set ($after, $repeat)

Reconfigures the watcher, see the constructor above for details. Can be called at any time.


Similar to the start method, but has special semantics for repeating timers:

If the timer is active and non-repeating, it will be stopped.

If the timer is active and repeating, reset the timeout to occur $repeat seconds after now.

If the timer is inactive and repeating, start it using the repeat value.

Otherwise do nothing.

This behaviour is useful when you have a timeout for some IO operation. You create a timer object with the same value for $after and $repeat, and then, in the read/write watcher, run the again method on the timeout.

PERIODIC WATCHERS - to cron or not to cron?

$w = EV::periodic $at, $interval, $reschedule_cb, $callback
$w = EV::periodic_ns $at, $interval, $reschedule_cb, $callback

Similar to EV::timer, but is not based on relative timeouts but on absolute times. Apart from creating "simple" timers that trigger "at" the specified time, it can also be used for non-drifting absolute timers and more complex, cron-like, setups that are not adversely affected by time jumps (i.e. when the system clock is changed by explicit date -s or other means such as ntpd). It is also the most complex watcher type in EV.

It has three distinct "modes":

  • absolute timer ($interval = $reschedule_cb = 0)

    This time simply fires at the wallclock time $at and doesn't repeat. It will not adjust when a time jump occurs, that is, if it is to be run at January 1st 2011 then it will run when the system time reaches or surpasses this time.

  • non-repeating interval timer ($interval > 0, $reschedule_cb = 0)

    In this mode the watcher will always be scheduled to time out at the next $at + N * $interval time (for some integer N) and then repeat, regardless of any time jumps.

    This can be used to create timers that do not drift with respect to system time:

       my $hourly = EV::periodic 0, 3600, 0, sub { print "once/hour\n" };

    That doesn't mean there will always be 3600 seconds in between triggers, but only that the the clalback will be called when the system time shows a full hour (UTC).

    Another way to think about it (for the mathematically inclined) is that EV::periodic will try to run the callback in this mode at the next possible time where $time = $at (mod $interval), regardless of any time jumps.

  • manual reschedule mode ($reschedule_cb = coderef)

    In this mode $interval and $at are both being ignored. Instead, each time the periodic watcher gets scheduled, the reschedule callback ($reschedule_cb) will be called with the watcher as first, and the current time as second argument.

    This callback MUST NOT stop or destroy this or any other periodic watcher, ever. If you need to stop it, return 1e30 and stop it afterwards.

    It must return the next time to trigger, based on the passed time value (that is, the lowest time value larger than to the second argument). It will usually be called just before the callback will be triggered, but might be called at other times, too.

    This can be used to create very complex timers, such as a timer that triggers on each midnight, local time (actually 24 hours after the last midnight, to keep the example simple. If you know a way to do it correctly in about the same space (without requiring elaborate modules), drop me a note :):

       my $daily = EV::periodic 0, 0, sub {
          my ($w, $now) = @_;
          use Time::Local ();
          my (undef, undef, undef, $d, $m, $y) = localtime $now;
          86400 + Time::Local::timelocal 0, 0, 0, $d, $m, $y
       }, sub {
          print "it's midnight or likely shortly after, now\n";

The periodic_ns variant doesn't start (activate) the newly created watcher.

$w->set ($at, $interval, $reschedule_cb)

Reconfigures the watcher, see the constructor above for details. Can be called at any time.


Simply stops and starts the watcher again.

SIGNAL WATCHERS - signal me when a signal gets signalled!

$w = EV::signal $signal, $callback
$w = EV::signal_ns $signal, $callback

Call the callback when $signal is received (the signal can be specified by number or by name, just as with kill or %SIG).

EV will grab the signal for the process (the kernel only allows one component to receive a signal at a time) when you start a signal watcher, and removes it again when you stop it. Perl does the same when you add/remove callbacks to %SIG, so watch out.

You can have as many signal watchers per signal as you want.

The signal_ns variant doesn't start (activate) the newly created watcher.

$w->set ($signal)

Reconfigures the watcher, see the constructor above for details. Can be called at any time.

$current_signum = $w->signal
$old_signum = $w->signal ($new_signal)

Returns the previously set signal (always as a number not name) and optionally set a new one.

CHILD WATCHERS - watch out for process status changes

$w = EV::child $pid, $callback
$w = EV::child_ns $pid, $callback

Call the callback when a status change for pid $pid (or any pid if $pid is 0) has been received. More precisely: when the process receives a SIGCHLD, EV will fetch the outstanding exit/wait status for all changed/zombie children and call the callback.

It is valid (and fully supported) to install a child watcher after a child has exited but before the event loop has started its next iteration (for example, first you fork, then the new child process might exit, and only then do you install a child watcher in the parent for the new pid).

You can access both exit (or tracing) status and pid by using the rstatus and rpid methods on the watcher object.

You can have as many pid watchers per pid as you want, they will all be called.

The child_ns variant doesn't start (activate) the newly created watcher.

$w->set ($pid)

Reconfigures the watcher, see the constructor above for details. Can be called at any time.

$current_pid = $w->pid
$old_pid = $w->pid ($new_pid)

Returns the previously set process id and optionally set a new one.

$exit_status = $w->rstatus

Return the exit/wait status (as returned by waitpid, see the waitpid entry in perlfunc).

$pid = $w->rpid

Return the pid of the awaited child (useful when you have installed a watcher for all pids).

STAT WATCHERS - did the file attributes just change?

$w = EV::stat $path, $interval, $callback
$w = EV::stat_ns $path, $interval, $callback

Call the callback when a file status change has been detected on $path. The $path does not need to exist, changing from "path exists" to "path does not exist" is a status change like any other.

The $interval is a recommended polling interval for systems where OS-supported change notifications don't exist or are not supported. If you use 0 then an unspecified default is used (which is highly recommended!), which is to be expected to be around five seconds usually.

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

The stat_ns variant doesn't start (activate) the newly created watcher.

... = $w->stat

This call is very similar to the perl stat built-in: It stats (using lstat) the path specified in the watcher and sets perls stat cache (as well as EV's idea of the current stat values) to the values found.

In scalar context, a boolean is return indicating success or failure of the stat. In list context, the same 13-value list as with stat is returned (except that the blksize and blocks fields are not reliable).

In the case of an error, errno is set to ENOENT (regardless of the actual error value) and the nlink value is forced to zero (if the stat was successful then nlink is guaranteed to be non-zero).

See also the next two entries for more info.

... = $w->attr

Just like $w->stat, but without the initial stat'ing: this returns the values most recently detected by EV. See the next entry for more info.

... = $w->prev

Just like $w->stat, but without the initial stat'ing: this returns the previous set of values, before the change.

That is, when the watcher callback is invoked, $w->prev will be set to the values found before a change was detected, while $w->attr returns the values found leading to the change detection. The difference (if any) between prev and attr is what triggered the callback.

If you did something to the filesystem object and do not want to trigger yet another change, you can call stat to update EV's idea of what the current attributes are.

$w->set ($path, $interval)

Reconfigures the watcher, see the constructor above for details. Can be called at any time.

$current_path = $w->path
$old_path = $w->path ($new_path)

Returns the previously set path and optionally set a new one.

$current_interval = $w->interval
$old_interval = $w->interval ($new_interval)

Returns the previously set interval and optionally set a new one. Can be used to query the actual interval used.

IDLE WATCHERS - when you've got nothing better to do...

$w = EV::idle $callback
$w = EV::idle_ns $callback

Call the callback when there are no pending io, timer/periodic, signal or child events, i.e. when the process is idle.

The process will not block as long as any idle watchers are active, and they will be called repeatedly until stopped.

The idle_ns variant doesn't start (activate) the newly created watcher.

PREPARE WATCHERS - customise your event loop!

$w = EV::prepare $callback
$w = EV::prepare_ns $callback

Call the callback just before the process would block. You can still create/modify any watchers at this point.

See the EV::check watcher, below, for explanations and an example.

The prepare_ns variant doesn't start (activate) the newly created watcher.

CHECK WATCHERS - customise your event loop even more!

$w = EV::check $callback
$w = EV::check_ns $callback

Call the callback just after the process wakes up again (after it has gathered events), but before any other callbacks have been invoked.

This is used to integrate other event-based software into the EV mainloop: You register a prepare callback and in there, you create io and timer watchers as required by the other software. Here is a real-world example of integrating Net::SNMP (with some details left out):

   our @snmp_watcher;

   our $snmp_prepare = EV::prepare sub {
      # do nothing unless active
         or return;

      # make the dispatcher handle any outstanding stuff
      ... not shown

      # create an IO watcher for each and every socket
      @snmp_watcher = (
         (map { EV::io $_, EV::READ, sub { } }
             keys %{ $dispatcher->{_descriptors} }),

         EV::timer +($event->[Net::SNMP::Dispatcher::_ACTIVE]
                     ? $event->[Net::SNMP::Dispatcher::_TIME] - EV::now : 0),
                    0, sub { },

The callbacks are irrelevant (and are not even being called), the only purpose of those watchers is to wake up the process as soon as one of those events occurs (socket readable, or timer timed out). The corresponding EV::check watcher will then clean up:

   our $snmp_check = EV::check sub {
      # destroy all watchers
      @snmp_watcher = ();

      # make the dispatcher handle any new stuff
      ... not shown

The callbacks of the created watchers will not be called as the watchers are destroyed before this cna happen (remember EV::check gets called first).

The check_ns variant doesn't start (activate) the newly created watcher.

FORK WATCHERS - the audacity to resume the event loop after a fork

Fork watchers are called when a fork () was detected. The invocation is done before the event loop blocks next and before check watchers are being called, and only in the child after the fork.

$w = EV::fork $callback
$w = EV::fork_ns $callback

Call the callback before the event loop is resumed in the child process after a fork.

The fork_ns variant doesn't start (activate) the newly created watcher.


Threads are not supported by this module in any way. Perl pseudo-threads is evil stuff and must die. As soon as Perl gains real threads I will work on thread support for it.


Most of the "improved" event delivering mechanisms of modern operating systems have quite a few problems with fork(2) (to put it bluntly: it is not supported and usually destructive). Libev makes it possible to work around this by having a function that recreates the kernel state after fork in the child.

On non-win32 platforms, this module requires the pthread_atfork functionality to do this automatically for you. This function is quite buggy on most BSDs, though, so YMMV. The overhead for this is quite negligible, because everything the function currently does is set a flag that is checked only when the event loop gets used the next time, so when you do fork but not use EV, the overhead is minimal.

On win32, there is no notion of fork so all this doesn't apply, of course.


EV::ADNS (asynchronous dns), Glib::EV (makes Glib/Gtk2 use EV as event loop), Coro::EV (efficient coroutines with EV).


 Marc Lehmann <>