| EV(3pm) | User Contributed Perl Documentation | EV(3pm) |
EV - perl interface to libev, a high performance full-featured event loop
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
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.
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.
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.
This module does not export any symbols.
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.
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.
If this callback throws an exception it will be silently ignored.
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.
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 occurred 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").
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)
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.
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.
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.
This section lists methods common to all watchers.
my $w = EV::timer 60, 0, sub {
warn $_[0]->data;
};
$w->data ("print me!");
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.
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);
Each of the following subsections describes a single watcher type.
I/O WATCHERS - is this file descriptor readable or writable?
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.
TIMER WATCHERS - relative and optionally repeating timeouts
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.
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.
PERIODIC WATCHERS - to cron or not to cron?
It has three distinct "modes":
This time simply fires at the wallclock time $at and doesn't repeat. It will not adjust when a time jump occurs, that is, if it is to be run at January 1st 2011 then it will run when the system time reaches or surpasses this time.
In this mode the watcher will always be scheduled to time out at the next "$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.
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.
SIGNAL WATCHERS - signal me when a signal gets signalled!
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.
CHILD WATCHERS - watch out for process status changes
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.
STAT WATCHERS - did the file attributes just change?
The $interval is a recommended polling interval for systems where OS-supported change notifications don't exist or are not supported. If you use 0 then an unspecified default is used (which is highly recommended!), which is to be expected to be around five seconds usually.
This watcher type is not meant for massive numbers of stat watchers, as even with OS-supported change notifications, this can be resource-intensive.
The "stat_ns" variant doesn't start (activate) the newly created watcher.
In scalar context, a boolean is return indicating success or failure of the stat. In list context, the same 13-value list as with stat is returned (except that the blksize and blocks fields are not reliable).
In the case of an error, errno is set to "ENOENT" (regardless of the actual error value) and the "nlink" value is forced to zero (if the stat was successful then nlink is guaranteed to be non-zero).
See also the next two entries for more info.
That is, when the watcher callback is invoked, "$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.
IDLE WATCHERS - when you've got nothing better to do...
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!
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!
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.
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.
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 whether 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;
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.
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.
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 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).
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::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.
Marc Lehmann <schmorp@schmorp.de> http://home.schmorp.de/
| 2020-11-09 | perl v5.32.0 |