AnyEventpm (3) manual page

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AnyEvent - provide framework for multiple event loops

Event, Coro, Glib, Tk - various supported event loops


use AnyEvent;

my $w = AnyEvent->io (fh => ..., poll => “[rw]+", cb => sub { my ($poll_got) = @_;

* only one io watcher per $fh and $poll type is allowed (i.e. on a socket you can have one r + one w or one rw watcher, not any more (limitation by Tk).

* the $poll_got passed to the handler needs to be checked by looking for single characters (e.g. with a regex), as it can contain more event types than were requested (e.g. a ‘w’ watcher might generate ‘rw’ events, limitation by Glib).

* AnyEvent will keep filehandles alive, so as long as the watcher exists, the filehandle exists.

my $w = AnyEvent->timer (after => $seconds, cb => sub { ...

* io and time watchers get canceled whenever $w is destroyed, so keep a copy

* timers can only be used once and must be recreated for repeated operation (limitation by Glib and Tk).

my $w = AnyEvent->condvar; # kind of main loop replacement $w->wait; # enters main loop till $condvar gets ->broadcast $w->broadcast; # wake up current and all future wait’s

* condvars are used to give blocking behaviour when neccessary. Create a condvar for any “request” or “event” your module might create, “->broadcast” it when the event happens and provide a function that calls “->wait” for it. See the examples below.


AnyEvent provides an identical interface to multiple event loops. This allows module authors to utilizy an event loop without forcing module users to use the same event loop (as only a single event loop can coexist peacefully at any one time).

The interface itself is vaguely similar but not identical to the Event module.

On the first call of any method, the module tries to detect the currently loaded event loop by probing wether any of the following modules is loaded: Coro::Event, Event, Glib, Tk. The first one found is used. If none is found, the module tries to load these modules in the order given. The first one that could be successfully loaded will be used. If still none could be found, it will issue an error.

Supplying Your Own Event Model Interface

If you need to support another event library which isn’t directly supported by AnyEvent, you can supply your own interface to it by pushing, before the first watch gets created, the package name of the event module and the package name of the interface to use onto @AnyEvent::REGISTRY. You can do that before and even without loading AnyEvent.


push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];

This tells AnyEvent to (literally) use the “urxvt::anyevent::” module when it finds the “urxvt” module is loaded. When AnyEvent is loaded and requested to find a suitable event model, it will first check for the urxvt module.

The above isn’t fictitious, the rxvt-unicode (a.k.a. urxvt) uses the above line exactly. An interface isn’t included in AnyEvent because it doesn’t make sense outside the embedded interpreter inside rxvt-uni_code, and it is updated and maintained as part of the rxvt-unicode distribution.

Environment Variables

The following environment variables are used by this module:

“PERL_ANYEVENT_VERBOSE” when set to 2 or higher, reports which event model gets used.


The following program uses an io watcher to read data from stdin, a timer to display a message once per second, and a condvar to exit the program when the user enters quit:

use AnyEvent;

my $cv = AnyEvent->condvar;

my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => ‘r’, cb => sub {

warn “io event <$_[0]>\n";
# will always output <r> chomp (my $input = <STDIN>); # read a line
warn “read: $input\n";
# output what has been read $cv->broadcast if $input =~ /^q/i; # quit program if /^q/i });

my $time_watcher; # can only be used once

sub new_timer {
$timer = AnyEvent->timer (after => 1, cb => sub { warn “timeout\n"; # print ‘timeout’ about every second &new_timer; # and restart the time });

new_timer; # create first timer

$cv->wait; # wait until user enters /^q/i

Real-world Example

Consider the Net::FCP module. It features (among others) the following API calls, which are to freenet what HTTP GET requests are to http:

my $data = $fcp->client_get ($url); # blocks

my $transaction = $fcp->txn_client_get ($url); # does not block $transaction->cb ( sub { ... } ); # set optional result callback my $data = $transaction->result; # possibly blocks

The “client_get” method works like “LWP::Simple::get": it requests the given URL and waits till the data has arrived. It is defined to be:

sub client_get { $_[0]->txn_client_get ($_[1])->result }

And in fact is automatically generated. This is the blocking API of Net::FCP, and it works as simple as in any other, similar, module.

More complicated is “txn_client_get": It only creates a transaction (completion, result, ...) object and initiates the transaction.

my $txn = bless { }, Net::FCP::Txn::;

It also creates a condition variable that is used to signal the completion of the request:

$txn->{finished} = AnyAvent->condvar;

It then creates a socket in non-blocking mode.

socket $txn->{fh}, ...;
fcntl $txn->{fh}, F_SETFL, O_NONBLOCK; connect $txn->{fh}, ...
and Carp::croak “unable to connect: $!\n";

Then it creates a write-watcher which gets called whenever an error occurs or the connection succeeds:

$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => ‘w’, cb => sub { $txn->fh_ready_w });

And returns this transaction object. The “fh_ready_w” callback gets called as soon as the event loop detects that the socket is ready for writing.

The “fh_ready_w” method makes the socket blocking again, writes the request data and replaces the watcher by a read watcher (waiting for reply data). The actual code is more complicated, but that doesn’t matter for this example:

fcntl $txn->{fh}, F_SETFL, 0;
syswrite $txn->{fh}, $txn->{request}
or die “connection or write error"; $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => ‘r’, cb => sub { $txn->fh_ready_r });

Again, “fh_ready_r” waits till all data has arrived, and then stores the result and signals any possible waiters that the request ahs finished:

sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};

if (end-of-file or data complete) { $txn->{result} = $txn->{buf};
$txb->{cb}->($txn) of $txn->{cb}; # also call callback }

The “result” method, finally, just waits for the finished signal (if the request was already finished, it doesn’t wait, of course, and returns the data:

return $txn->{result};

The actual code goes further and collects all errors ("die"s, exceptions) that occured during request processing. The “result” method detects wether an exception as thrown (it is stored inside the $txn object) and just throws the exception, which means connection errors and other problems get reported tot he code that tries to use the result, not in a random callback.

All of this enables the following usage styles:

1. Blocking:

my $data = $fcp->client_get ($url);

2. Blocking, but parallelizing:

my @datas = map $_->result,
map $fcp->txn_client_get ($_), @urls;

Both blocking examples work without the module user having to know anything about events.

3a. Event-based in a main program, using any support Event module:

use Event;

$fcp->txn_client_get ($url)->cb (sub { my $txn = shift;
my $data = $txn->result;


3b. The module user could use AnyEvent, too:

use AnyEvent;

my $quit = AnyEvent->condvar;

$fcp->txn_client_get ($url)->cb (sub { ...


See Also

Event modules: Coro::Event, Coro, Event, Glib::Event, Glib.

Implementations: AnyEvent::Impl::Coro, AnyEvent::Impl::Event, AnyEvent::Impl::Glib, AnyEvent::Impl::Tk.

Nontrivial usage example: Net::FCP.

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