path: root/README

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

SYNOPSIS
       use EV;
   
       # TIMERS
   
       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>;
       };
   
       # SIGNALS
   
       my $w = EV::signal 'QUIT', sub {
          warn "sigquit received\n";
       };
   
       # CHILD/PID STATUS CHANGES
  
       my $w = EV::child 666, 0, sub {
          my ($w, $revents) = @_;
          my $status = $w->rstatus;
       };
  
       # STAT CHANGES
       my $w = EV::stat "/etc/passwd", 10, sub {
          my ($w, $revents) = @_;
          warn $w->path, " has changed somehow.\n";
       };
   
       # MAINLOOP
       EV::run;                # loop until EV::break is called or all watchers stop
       EV::run EV::RUN_ONCE;   # block until at least one event could be handled
       EV::run EV::RUN_NOWAIT; # try to handle same events, but do not block

BEFORE YOU START USING THIS MODULE
    If you only need timer, I/O, signal, child and idle watchers and not the
    advanced functionality of this module, consider using AnyEvent instead,
    specifically the simplified API described in AE.

    When used with EV as backend, the AE API is as fast as the native EV
    API, but your programs/modules will still run with many other event
    loops.

DESCRIPTION
    This module provides an interface to libev
    (<http://software.schmorp.de/pkg/libev.html>). While the documentation
    below is comprehensive, one might also consult the documentation of
    libev itself (<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod> or
    perldoc EV::libev) for more subtle details on watcher semantics or some
    discussion on the available backends, or how to force a specific backend
    with "LIBEV_FLAGS", or just about in any case because it has much more
    detailed information.

    This module is very fast and scalable. It is actually so fast that you
    can use it through the AnyEvent module, stay portable to other event
    loops (if you don't rely on any watcher types not available through it)
    and still be faster than with any other event loop currently supported
    in Perl.

  PORTING FROM EV 3.X to 4.X
    EV version 4 introduces a number of incompatible changes summarised
    here. According to the depreciation strategy used by libev, there is a
    compatibility layer in place so programs should continue to run
    unchanged (the XS interface lacks this layer, so programs using that one
    need to be updated).

    This compatibility layer will be switched off in some future release.

    All changes relevant to Perl are renames of symbols, functions and
    methods:

      EV::loop          => EV::run
      EV::LOOP_NONBLOCK => EV::RUN_NOWAIT
      EV::LOOP_ONESHOT  => EV::RUN_ONCE

      EV::unloop        => EV::break
      EV::UNLOOP_CANCEL => EV::BREAK_CANCEL
      EV::UNLOOP_ONE    => EV::BREAK_ONE
      EV::UNLOOP_ALL    => EV::BREAK_ALL

      EV::TIMEOUT       => EV::TIMER

      EV::loop_count    => EV::iteration
      EV::loop_depth    => EV::depth
      EV::loop_verify   => EV::verify

    The loop object methods corresponding to the functions above have been
    similarly renamed.

  MODULE EXPORTS
    This module does not export any symbols.

EVENT LOOPS
    EV supports multiple event loops: There is a single "default event loop"
    that can handle everything including signals and child watchers, and any
    number of "dynamic event loops" that can use different backends (with
    various limitations), but no child and signal watchers.

    You do not have to do anything to create the default event loop: When
    the module is loaded a suitable backend is selected on the premise of
    selecting a working backend (which for example rules out kqueue on most
    BSDs). Modules should, unless they have "special needs" always use the
    default loop as this is fastest (perl-wise), best supported by other
    modules (e.g. AnyEvent or Coro) and most portable event loop.

    For specific programs you can create additional event loops dynamically.

    If you want to take advantage of kqueue (which often works properly for
    sockets only) even though the default loop doesn't enable it, you can
    *embed* a kqueue loop into the default loop: running the default loop
    will then also service the kqueue loop to some extent. See the example
    in the section about embed watchers for an example on how to achieve
    that.

    $loop = new EV::Loop [$flags]
        Create a new event loop as per the specified flags. Please refer to
        the "ev_loop_new ()" function description in the libev documentation
        (<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#GLOBAL_FUNCTI
        ONS>, or locally-installed as EV::libev manpage) for more info.

        The loop will automatically be destroyed when it is no longer
        referenced by any watcher and the loop object goes out of scope.

        If you are not embedding the loop, then Using "EV::FLAG_FORKCHECK"
        is recommended, as only the default event loop is protected by this
        module. If you *are* embedding this loop in the default loop, this
        is not necessary, as "EV::embed" automatically does the right thing
        on fork.

    $loop->loop_fork
        Must be called after a fork in the child, before entering or
        continuing the event loop. An alternative is to use
        "EV::FLAG_FORKCHECK" which calls this function automatically, at
        some performance loss (refer to the libev documentation).

    $loop->verify
        Calls "ev_verify" to make internal consistency checks (for debugging
        libev) and abort the program if any data structures were found to be
        corrupted.

    $loop = EV::default_loop [$flags]
        Return the default loop (which is a singleton object). Since this
        module already creates the default loop with default flags,
        specifying flags here will not have any effect unless you destroy
        the default loop first, which isn't supported. So in short: don't do
        it, and if you break it, you get to keep the pieces.

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

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

    $flags = EV::supported_backends
    $flags = EV::recommended_backends
    $flags = EV::embeddable_backends
        Returns the set (see "EV::BACKEND_*" flags) of backends supported by
        this instance of EV, the set of recommended backends (supposed to be
        good) for this platform and the set of embeddable backends (see
        EMBED WATCHERS).

    EV::sleep $seconds
        Block the process for the given number of (fractional) seconds.

    $time = EV::time
        Returns the current time in (fractional) seconds since the epoch.

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

    EV::now_update
    $loop->now_update
        Establishes the current time by querying the kernel, updating the
        time returned by "EV::now" in the progress. This is a costly
        operation and is usually done automatically within "EV::run".

        This function is rarely useful, but when some event callback runs
        for a very long time without entering the event loop, updating
        libev's idea of the current time is a good idea.

    EV::suspend
    $loop->suspend
    EV::resume
    $loop->resume
        These two functions suspend and resume a loop, for use when the loop
        is not used for a while and timeouts should not be processed.

        A typical use case would be an interactive program such as a game:
        When the user presses "^Z" to suspend the game and resumes it an
        hour later it would be best to handle timeouts as if no time had
        actually passed while the program was suspended. This can be
        achieved by calling "suspend" in your "SIGTSTP" handler, sending
        yourself a "SIGSTOP" and calling "resume" directly afterwards to
        resume timer processing.

        Effectively, all "timer" watchers will be delayed by the time spend
        between "suspend" and "resume", and all "periodic" watchers will be
        rescheduled (that is, they will lose any events that would have
        occured while suspended).

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

        Calling "suspend"/"resume" has the side effect of updating the event
        loop time (see "now_update").

    $backend = EV::backend
    $backend = $loop->backend
        Returns an integer describing the backend used by libev
        (EV::BACKEND_SELECT or EV::BACKEND_EPOLL).

    $active = EV::run [$flags]
    $active = $loop->run ([$flags])
        Begin checking for events and calling callbacks. It returns when a
        callback calls EV::break or the flags are nonzero (in which case the
        return value is true) or when there are no active watchers which
        reference the loop (keepalive is true), in which case the return
        value will be false. The return value can generally be interpreted
        as "if true, there is more work left to do".

        The $flags argument can be one of the following:

           0               as above
           EV::RUN_ONCE    block at most once (wait, but do not loop)
           EV::RUN_NOWAIT  do not block at all (fetch/handle events but do not wait)

    EV::break [$how]
    $loop->break ([$how])
        When called with no arguments or an argument of EV::BREAK_ONE, makes
        the innermost call to EV::run return.

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

        When called with an argument of EV::BREAK_CANCEL, any pending break
        will be cancelled.

    $count = EV::iteration
    $count = $loop->iteration
        Return the number of times the event loop has polled for new events.
        Sometimes useful as a generation counter.

    EV::once $fh_or_undef, $events, $timeout, $cb->($revents)
    $loop->once ($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 an "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::TIMER").

        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.

    EV::feed_fd_event $fd, $revents
    $loop->feed_fd_event ($fd, $revents)
        Feed an event on a file descriptor into EV. EV will react to this
        call as if the readyness notifications specified by $revents (a
        combination of "EV::READ" and "EV::WRITE") happened on the file
        descriptor $fd.

    EV::feed_signal_event $signal
        Feed a signal event into the default loop. EV will react to this
        call as if the signal specified by $signal had occured.

    EV::feed_signal $signal
        Feed a signal event into EV - unlike "EV::feed_signal_event", this
        works regardless of which loop has registered the signal, and is
        mainly useful for custom signal implementations.

    EV::set_io_collect_interval $time
    $loop->set_io_collect_interval ($time)
    EV::set_timeout_collect_interval $time
    $loop->set_timeout_collect_interval ($time)
        These advanced functions set the minimum block interval when polling
        for I/O events and the minimum wait interval for timer events. See
        the libev documentation at
        <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#FUNCTIONS_CONT
        ROLLING_THE_EVENT_LOOP> (locally installed as EV::libev) for a more
        detailed discussion.

    $count = EV::pending_count
    $count = $loop->pending_count
        Returns the number of currently pending watchers.

    EV::invoke_pending
    $loop->invoke_pending
        Invoke all currently pending watchers.

WATCHER OBJECTS
    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 now 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 I/O
    events (which can set both EV::READ and EV::WRITE bits).

    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.

  COMMON WATCHER METHODS
    This section lists methods common to all watchers.

    $w->start
        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).

    $w->stop
        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 whether 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.

    $w->invoke ($revents)
        Call the callback *now* with the given event mask.

    $w->feed_event ($revents)
        Feed some events on this watcher into EV. EV will react to this call
        as if the watcher had received the given $revents mask.

    $revents = $w->clear_pending
        If the watcher is pending, this function clears its pending status
        and returns its $revents bitset (as if its callback was invoked). If
        the watcher isn't pending it does nothing and returns 0.

    $previous_state = $w->keepalive ($bool)
        Normally, "EV::run" will return when there are no active watchers
        (which is a "deadlock" because no progress can be made anymore).
        This is convenient because it allows you to start your watchers (and
        your jobs), call "EV::run" 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 the
        module that calls "EV::run" (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::run" 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::run" from
        returning.

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

        Example: Register an I/O 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);

    $loop = $w->loop
        Return the loop that this watcher is attached to.

WATCHER TYPES
    Each of the following subsections describes a single watcher type.

   I/O 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
    $w = $loop->io ($fileno_or_fh, $eventmask, $callback)
    $w = $loop->io_ns ($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
    $w = $loop->timer ($after, $repeat, $callback)
    $w = $loop->timer_ns ($after, $repeat, $callback)
        Calls the callback after $after seconds (which may be fractional or
        negative). 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 = 0)
        Reconfigures the watcher, see the constructor above for details. Can
        be called at any time.

    $w->again
    $w->again ($repeat)
        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.

        If called with a $repeat argument, then it uses this a timer repeat
        value.

    $after = $w->remaining
        Calculates and returns the remaining time till the timer will fire.

    $repeat = $w->repeat
    $old_repeat = $w->repeat ($new_repeat)
        Returns the current value of the repeat attribute and optionally
        sets a new one. Setting the new one will not restart the watcher -
        if the watcher is active, the new repeat value is used whenever it
        expires next.

   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
    $w = $loop->periodic ($at, $interval, $reschedule_cb, $callback)
    $w = $loop->periodic_ns ($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.

        *   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 the lowest integer N)
            and then repeat, regardless of any time jumps. Note that, since
            "N" can be negative, the first trigger can happen before $at.

            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 callback 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, and MUST NOT call any event loop
            functions or methods*. If you need to stop it, return 1e30 and
            stop it afterwards. You may create and start an "EV::prepare"
            watcher for this task.

            It must return the next time to trigger, based on the passed
            time value (that is, the lowest time value larger than or equal
            to 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 one day
            after the last midnight, to keep the example simple):

               my $daily = EV::periodic 0, 0, sub {
                  my ($w, $now) = @_;

                  use Time::Local ();
                  my (undef, undef, undef, $d, $m, $y) = localtime $now;
                  Time::Local::timelocal_nocheck 0, 0, 0, $d + 1, $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.

    $w->again
        Simply stops and starts the watcher again.

    $time = $w->at
        Return the time that the watcher is expected to trigger next.

    $offset = $w->offset
    $old_offset = $w->offset ($new_offset)
        Returns the current value of the offset attribute and optionally
        sets a new one. Setting the new one will not restart the watcher -
        if the watcher is active, the new offset value is used whenever it
        expires next.

    $interval = $w->interval
    $old_interval = $w->interval ($new_interval)
        See above, for the interval attribute.

    $reschedule_cb = $w->reschedule_cb
    $old_reschedule_cb = $w->reschedule_cb ($new_reschedule_cb)
        See above, for the reschedule callback.

   SIGNAL WATCHERS - signal me when a signal gets signalled!
    $w = EV::signal $signal, $callback
    $w = EV::signal_ns $signal, $callback
    $w = $loop->signal ($signal, $callback)
    $w = $loop->signal_ns ($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).

        Only one event loop can grab a given signal - attempting to grab the
        same signal from two EV loops will crash the program immediately or
        cause data corruption.

        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, $trace, $callback
    $w = EV::child_ns $pid, $trace, $callback
    $w = $loop->child ($pid, $trace, $callback)
    $w = $loop->child_ns ($pid, $trace, $callback)
        Call the callback when a status change for pid $pid (or any pid if
        $pid is 0) has been received (a status change happens when the
        process terminates or is killed, or, when trace is true,
        additionally when it is stopped or continued). 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, $trace)
        Reconfigures the watcher, see the constructor above for details. Can
        be called at any time.

    $current_pid = $w->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
    $w = $loop->stat ($path, $interval, $callback)
    $w = $loop->stat_ns ($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
    $w = $loop->idle ($callback)
    $w = $loop->idle_ns ($callback)
        Call the callback when there are no other pending watchers of the
        same or higher priority (excluding check, prepare and other idle
        watchers of the same or lower priority, of course). They are called
        idle watchers because when the watcher is the highest priority
        pending event in the process, the process is considered to be idle
        at that priority.

        If you want a watcher that is only ever called when *no* other
        events are outstanding you have to set the priority to "EV::MINPRI".

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

        For example, if you have idle watchers at priority 0 and 1, and an
        I/O watcher at priority 0, then the idle watcher at priority 1 and
        the I/O watcher will always run when ready. Only when the idle
        watcher at priority 1 is stopped and the I/O watcher at priority 0
        is not pending with the 0-priority idle watcher be invoked.

        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
    $w = $loop->prepare ($callback)
    $w = $loop->prepare_ns ($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
    $w = $loop->check ($callback)
    $w = $loop->check_ns ($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 can be 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
              $dispatcher->{_event_queue_h}
                 or return;

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

              # create an I/O 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 can happen (remember EV::check
        gets called first).

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

    EV::CHECK constant issues
        Like all other watcher types, there is a bitmask constant for use in
        $revents and other places. The "EV::CHECK" is special as it has the
        same name as the "CHECK" sub called by Perl. This doesn't cause big
        issues on newer perls (beginning with 5.8.9), but it means thatthe
        constant must be *inlined*, i.e. runtime calls will not work. That
        means that as long as you always "use EV" and then "EV::CHECK" you
        are on the safe side.

   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
    $w = $loop->fork ($callback)
    $w = $loop->fork_ns ($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.

   EMBED WATCHERS - when one backend isn't enough...
    This is a rather advanced watcher type that lets you embed one event
    loop into another (currently only 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).

    See the libev documentation at
    <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#code_ev_embed_code
    _when_one_backend_> (locally installed as EV::libev) for more details.

    In short, this watcher is most useful on BSD systems without working
    kqueue to still be able to handle a large number of sockets:

       my $socket_loop;
  
       # check wether we use SELECT or POLL _and_ KQUEUE is supported
       if (
         (EV::backend & (EV::BACKEND_POLL | EV::BACKEND_SELECT))
         && (EV::supported_backends & EV::embeddable_backends & EV::BACKEND_KQUEUE)
       ) {
         # use kqueue for sockets
         $socket_loop = new EV::Loop EV::BACKEND_KQUEUE | EV::FLAG_NOENV;
       }
  
       # use the default loop otherwise
       $socket_loop ||= EV::default_loop;

    $w = EV::embed $otherloop[, $callback]
    $w = EV::embed_ns $otherloop[, $callback]
    $w = $loop->embed ($otherloop[, $callback])
    $w = $loop->embed_ns ($otherloop[, $callback])
        Call the callback when the embedded event loop ($otherloop) has any
        I/O activity. The $callback is optional: if it is missing, then the
        embedded event loop will be managed automatically (which is
        recommended), otherwise you have to invoke "sweep" yourself.

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

   ASYNC WATCHERS - how to wake up another event loop
    Async watchers are provided by EV, but have little use in perl directly,
    as perl neither supports threads running in parallel nor direct access
    to signal handlers or other contexts where they could be of value.

    It is, however, possible to use them from the XS level.

    Please see the libev documentation for further details.

    $w = EV::async $callback
    $w = EV::async_ns $callback
    $w = $loop->async ($callback)
    $w = $loop->async_ns ($callback)
    $w->send
    $bool = $w->async_pending

   CLEANUP WATCHERS - how to clean up when the event loop goes away
    Cleanup watchers are not supported on the Perl level, they can only be
    used via XS currently.

PERL SIGNALS
    While Perl signal handling (%SIG) is not affected by EV, the behaviour
    with EV is as the same as any other C library: Perl-signals will only be
    handled when Perl runs, which means your signal handler might be invoked
    only the next time an event callback is invoked.

    The solution is to use EV signal watchers (see "EV::signal"), which will
    ensure proper operations with regards to other event watchers.

    If you cannot do this for whatever reason, you can also force a watcher
    to be called on every event loop iteration by installing a "EV::check"
    watcher:

       my $async_check = EV::check sub { };

    This ensures that perl gets into control for a short time to handle any
    pending signals, and also ensures (slightly) slower overall operation.

ITHREADS
    Ithreads are not supported by this module in any way. Perl
    pseudo-threads is evil stuff and must die. Real threads as provided by
    Coro are fully supported (and enhanced support is available via
    Coro::EV).

FORK
    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.

SEE ALSO
    EV::MakeMaker - MakeMaker interface to XS API, EV::ADNS (asynchronous
    DNS), Glib::EV (makes Glib/Gtk2 use EV as event loop), EV::Glib (embed
    Glib into EV), Coro::EV (efficient thread integration), Net::SNMP::EV
    (asynchronous SNMP), AnyEvent for event-loop agnostic and portable event
    driven programming.

AUTHOR
       Marc Lehmann <schmorp@schmorp.de>
       http://home.schmorp.de/