[ViewVC] Annotation of: cvs/EV/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 $w = EV::child 666, sub {
         my ($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::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

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://cvs.schmorp.de/libev/ev.html>) 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.

BASIC INTERFACE
    $EV::DIED
        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.

  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 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 I/O
    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.

  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.

    $w->trigger ($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 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);

  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
        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.

    $w->again
        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.

    $w->again
        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 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
        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
              $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 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.

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 shortly gets into control for a short time, and
    also ensures slower overall operation.

THREADS
    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.

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

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

Revision:	1.16
Committed:	Fri Dec 7 18:11:23 2007 UTC (16 years, 5 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_71
Changes since 1.15:	+41 -9 lines
Log Message:	* empty log message *