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Author image Marc A. Lehmann


AnyEvent - provide framework for multiple event loops

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


   use AnyEvent;

   my $w = AnyEvent->io (fh => $fh, poll => "r|w", cb => sub {

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

   my $w = AnyEvent->condvar; # stores wether a condition was flagged
   $w->wait; # enters "main loop" till $condvar gets ->broadcast
   $w->broadcast; # wake up current and all future wait's


AnyEvent provides an identical interface to multiple event loops. This allows module authors to utilise 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, AnyEvent will fall back to a pure-perl event loop, which is also not very efficient.

Because AnyEvent first checks for modules that are already loaded, loading an Event model explicitly before first using AnyEvent will likely make that model the default. For example:

   use Tk;
   use AnyEvent;

   # .. AnyEvent will likely default to Tk

The pure-perl implementation of AnyEvent is called AnyEvent::Impl::Perl. Like other event modules you can load it explicitly.


AnyEvent has the central concept of a watcher, which is an object that stores relevant data for each kind of event you are waiting for, such as the callback to call, the filehandle to watch, etc.

These watchers are normal Perl objects with normal Perl lifetime. After creating a watcher it will immediately "watch" for events and invoke the callback. To disable the watcher you have to destroy it (e.g. by setting the variable that stores it to undef or otherwise deleting all references to it).

All watchers are created by calling a method on the AnyEvent class.


You can create I/O watcher by calling the AnyEvent->io method with the following mandatory arguments:

fh the Perl filehandle (not filedescriptor) to watch for events. poll must be a string that is either r or w, that creates a watcher waiting for "r"eadable or "w"ritable events. cb teh callback to invoke everytime the filehandle becomes ready.

Only one io watcher per fh and poll combination is allowed (i.e. on a socket you can have one r + one w, not any more (limitation comes from Tk - if you are sure you are not using Tk this limitation is gone).

Filehandles will be kept alive, so as long as the watcher exists, the filehandle exists, too.


   # wait for readability of STDIN, then read a line and disable the watcher
   my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
      chomp (my $input = <STDIN>);
      warn "read: $input\n";
      undef $w;


You can create a time watcher by calling the AnyEvent->timer method with the following mandatory arguments:

after after how many seconds (fractions are supported) should the timer activate. cb the callback to invoke.

The timer callback will be invoked at most once: if you want a repeating timer you have to create a new watcher (this is a limitation by both Tk and Glib).


   # fire an event after 7.7 seconds
   my $w = AnyEvent->timer (after => 7.7, cb => sub {
      warn "timeout\n";

   # to cancel the timer:
   undef $w


Condition watchers can be created by calling the AnyEvent->condvar method without any arguments.

A condition watcher watches for a condition - precisely that the ->broadcast method has been called.

The watcher has only two methods:


Wait (blocking if necessary) until the ->broadcast method has been called on c<$cv>, while servicing other watchers normally.

Not all event models support a blocking wait - some die in that case, so if you are using this from a module, never require a blocking wait, but let the caller decide wether the call will block or not (for example, by coupling condition variables with some kind of request results and supporting callbacks so the caller knows that getting the result will not block, while still suppporting blockign waits if the caller so desires).

You can only wait once on a condition - additional calls will return immediately.


Flag the condition as ready - a running ->wait and all further calls to wait will return after this method has been called. If nobody is waiting the broadcast will be remembered..


   # wait till the result is ready
   my $result_ready = AnyEvent->condvar;

   # do something such as adding a timer
   # or socket watcher the calls $result_ready->broadcast
   # when the "result" is ready.



You can listen for signals using a signal watcher, signal is the signal name without any SIG prefix. Multiple signals events can be clumped together into one callback invocation, and callback invocation might or might not be asynchronous.

These watchers might use %SIG, so programs overwriting those signals directly will likely not work correctly.

Example: exit on SIGINT

   my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });


You can also listen for the status of a child process specified by the pid argument. The watcher will only trigger once. This works by installing a signal handler for SIGCHLD.

Example: wait for pid 1333

  my $w = AnyEvent->child (pid => 1333, cb => sub { warn "exit status $?" });



Contains undef until the first watcher is being created. Then it contains the event model that is being used, which is the name of the Perl class implementing the model. This class is usually one of the AnyEvent::Impl:xxx modules, but can be any other class in the case AnyEvent has been extended at runtime (e.g. in rxvt-unicode).

The known classes so far are:

   AnyEvent::Impl::Coro      based on Coro::Event, best choise.
   AnyEvent::Impl::Event     based on Event, also best choice :)
   AnyEvent::Impl::Glib      based on Glib, second-best choice.
   AnyEvent::Impl::Tk        based on Tk, very bad choice.
   AnyEvent::Impl::Perl      pure-perl implementation, inefficient.

Returns $AnyEvent::MODEL, forcing autodetection of the event model if necessary. You should only call this function right before you would have created an AnyEvent watcher anyway, that is, very late at runtime.


As a module author, you should "use AnyEvent" and call AnyEvent methods freely, but you should not load a specific event module or rely on it.

Be careful when you create watchers in the module body - Anyevent will decide which event module to use as soon as the first method is called, so by calling AnyEvent in your module body you force the user of your module to load the event module first.


There will always be a single main program - the only place that should dictate which event model to use.

If it doesn't care, it can just "use AnyEvent" and use it itself, or not do anything special and let AnyEvent decide which implementation to chose.

If the main program relies on a specific event model (for example, in Gtk2 programs you have to rely on either Glib or Glib::Event), you should load it before loading AnyEvent or any module that uses it, generally, as early as possible. The reason is that modules might create watchers when they are loaded, and AnyEvent will decide on the event model to use as soon as it creates watchers, and it might chose the wrong one unless you load the correct one yourself.

You can chose to use a rather inefficient pure-perl implementation by loading the AnyEvent::Impl::Perl module, but letting AnyEvent chose is generally better.


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 watcher 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:: package/class when it finds the urxvt package/module is loaded. When AnyEvent is loaded and asked to find a suitable event model, it will first check for the presence of urxvt.

The class should provide implementations for all watcher types (see AnyEvent::Impl::Event (source code), AnyEvent::Impl::Glib (Source code) and so on for actual examples, use perldoc -m AnyEvent::Impl::Glib to see the sources).

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

rxvt-unicode also cheats a bit by not providing blocking access to condition variables: code blocking while waiting for a condition will die. This still works with most modules/usages, and blocking calls must not be in an interactive application, so it makes sense.


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


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 $txn->{fh}, ...
      and !$!{EWOULDBLOCK}
      and !$!{EINPROGRESS}
      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 (dies, 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 $fcp->txn_client_get ($_),

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 {



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.