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https://github.com/dex4er/perl-anyevent

Mirror of CVS repository :pserver:[email protected]/schmorpforge AnyEvent
https://github.com/dex4er/perl-anyevent

Last synced: 11 days ago
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Mirror of CVS repository :pserver:[email protected]/schmorpforge AnyEvent

Host: GitHub
URL: https://github.com/dex4er/perl-anyevent
Owner: dex4er
License: other
Created: 2013-11-24T11:54:24.000Z (about 11 years ago)
Default Branch: master
Last Pushed: 2015-05-03T14:36:47.000Z (over 9 years ago)
Last Synced: 2024-10-28T09:02:01.199Z (about 2 months ago)
Language: Perl
Homepage: http://cvs.schmorp.de/AnyEvent/
Size: 1.75 MB
Stars: 0
Watchers: 2
Forks: 0
Open Issues: 0
Metadata Files:
- Readme: README
- Changelog: Changes
- License: COPYING

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README

NAME
AnyEvent - the DBI of event loop programming

EV, Event, Glib, Tk, UV, Perl, Event::Lib, Irssi, rxvt-unicode,
IO::Async, Qt, FLTK and POE are various supported event
loops/environments.

SYNOPSIS
use AnyEvent;

# if you prefer function calls, look at the AE manpage for
# an alternative API.

# file handle or descriptor readable
my $w = AnyEvent->io (fh => $fh, poll => "r", cb => sub { ... });

# one-shot or repeating timers
my $w = AnyEvent->timer (after => $seconds, cb => sub { ... });
my $w = AnyEvent->timer (after => $seconds, interval => $seconds, cb => ...);

print AnyEvent->now; # prints current event loop time
print AnyEvent->time; # think Time::HiRes::time or simply CORE::time.

# POSIX signal
my $w = AnyEvent->signal (signal => "TERM", cb => sub { ... });

# child process exit
my $w = AnyEvent->child (pid => $pid, cb => sub {
my ($pid, $status) = @_;
...
});

# called when event loop idle (if applicable)
my $w = AnyEvent->idle (cb => sub { ... });

my $w = AnyEvent->condvar; # stores whether a condition was flagged
$w->send; # wake up current and all future recv's
$w->recv; # enters "main loop" till $condvar gets ->send
# use a condvar in callback mode:
$w->cb (sub { $_[0]->recv });

INTRODUCTION/TUTORIAL
This manpage is mainly a reference manual. If you are interested in a
tutorial or some gentle introduction, have a look at the AnyEvent::Intro
manpage.

SUPPORT
An FAQ document is available as AnyEvent::FAQ.

There also is a mailinglist for discussing all things AnyEvent, and an
IRC channel, too.

See the AnyEvent project page at the Schmorpforge Ta-Sa Software
Repository, at , for more info.

WHY YOU SHOULD USE THIS MODULE (OR NOT)
Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
nowadays. So what is different about AnyEvent?

Executive Summary: AnyEvent is *compatible*, AnyEvent is *free of
policy* and AnyEvent is *small and efficient*.

First and foremost, *AnyEvent is not an event model* itself, it only
interfaces to whatever event model the main program happens to use, in a
pragmatic way. For event models and certain classes of immortals alike,
the statement "there can only be one" is a bitter reality: In general,
only one event loop can be active at the same time in a process.
AnyEvent cannot change this, but it can hide the differences between
those event loops.

The goal of AnyEvent is to offer module authors the ability to do event
programming (waiting for I/O or timer events) without subscribing to a
religion, a way of living, and most importantly: without forcing your
module users into the same thing by forcing them to use the same event
model you use.

For modules like POE or IO::Async (which is a total misnomer as it is
actually doing all I/O *synchronously*...), using them in your module is
like joining a cult: After you join, you are dependent on them and you
cannot use anything else, as they are simply incompatible to everything
that isn't them. What's worse, all the potential users of your module
are *also* forced to use the same event loop you use.

AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works
fine. AnyEvent + Tk works fine etc. etc. but none of these work together
with the rest: POE + EV? No go. Tk + Event? No go. Again: if your module
uses one of those, every user of your module has to use it, too. But if
your module uses AnyEvent, it works transparently with all event models
it supports (including stuff like IO::Async, as long as those use one of
the supported event loops. It is easy to add new event loops to
AnyEvent, too, so it is future-proof).

In addition to being free of having to use *the one and only true event
model*, AnyEvent also is free of bloat and policy: with POE or similar
modules, you get an enormous amount of code and strict rules you have to
follow. AnyEvent, on the other hand, is lean and to the point, by only
offering the functionality that is necessary, in as thin as a wrapper as
technically possible.

Of course, AnyEvent comes with a big (and fully optional!) toolbox of
useful functionality, such as an asynchronous DNS resolver, 100%
non-blocking connects (even with TLS/SSL, IPv6 and on broken platforms
such as Windows) and lots of real-world knowledge and workarounds for
platform bugs and differences.

Now, if you *do want* lots of policy (this can arguably be somewhat
useful) and you want to force your users to use the one and only event
model, you should *not* use this module.

DESCRIPTION
AnyEvent provides a uniform interface to various event loops. This
allows module authors to use event loop functionality without forcing
module users to use a specific event loop implementation (since more
than one event loop cannot coexist peacefully).

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

During the first call of any watcher-creation method, the module tries
to detect the currently loaded event loop by probing whether one of the
following modules is already loaded: EV, AnyEvent::Loop, Event, Glib,
Tk, Event::Lib, Qt, POE. The first one found is used. If none are
detected, the module tries to load the first four modules in the order
given; but note that if EV is not available, the pure-perl
AnyEvent::Loop should always work, so the other two are not normally
tried.

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 *likely* means that, if any module loads another event model and
starts using it, all bets are off - this case should be very rare
though, as very few modules hardcode event loops without announcing this
very loudly.

The pure-perl implementation of AnyEvent is called "AnyEvent::Loop".
Like other event modules you can load it explicitly and enjoy the high
availability of that event loop :)

WATCHERS
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 file handle 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 when the event occurs (of course, only when the event model is
in control).

Note that callbacks must not permanently change global variables
potentially in use by the event loop (such as $_ or $[) and that
callbacks must not "die". The former is good programming practice in
Perl and the latter stems from the fact that exception handling differs
widely between event loops.

To disable a watcher you have to destroy it (e.g. by setting the
variable you store it in to "undef" or otherwise deleting all references
to it).

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

Many watchers either are used with "recursion" (repeating timers for
example), or need to refer to their watcher object in other ways.

One way to achieve that is this pattern:

my $w; $w = AnyEvent->type (arg => value ..., cb => sub {
# you can use $w here, for example to undef it
undef $w;
});

Note that "my $w; $w =" combination. This is necessary because in Perl,
my variables are only visible after the statement in which they are
declared.

I/O WATCHERS
$w = AnyEvent->io (
fh => ,
poll => <"r" or "w">,
cb => ,
);

You can create an I/O watcher by calling the "AnyEvent->io" method with
the following mandatory key-value pairs as arguments:

"fh" is the Perl *file handle* (or a naked file descriptor) to watch for
events (AnyEvent might or might not keep a reference to this file
handle). Note that only file handles pointing to things for which
non-blocking operation makes sense are allowed. This includes sockets,
most character devices, pipes, fifos and so on, but not for example
files or block devices.

"poll" must be a string that is either "r" or "w", which creates a
watcher waiting for "r"eadable or "w"ritable events, respectively.

"cb" is the callback to invoke each time the file handle becomes ready.

Although the callback might get passed parameters, their value and
presence is undefined and you cannot rely on them. Portable AnyEvent
callbacks cannot use arguments passed to I/O watcher callbacks.

The I/O watcher might use the underlying file descriptor or a copy of
it. You must not close a file handle as long as any watcher is active on
the underlying file descriptor.

Some event loops issue spurious readiness notifications, so you should
always use non-blocking calls when reading/writing from/to your file
handles.

Example: 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 = );
warn "read: $input\n";
undef $w;
});

TIME WATCHERS
$w = AnyEvent->timer (after => , cb => );

$w = AnyEvent->timer (
after => ,
interval => ,
cb => ,
);

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

"after" specifies after how many seconds (fractional values are
supported) the callback should be invoked. "cb" is the callback to
invoke in that case.

Although the callback might get passed parameters, their value and
presence is undefined and you cannot rely on them. Portable AnyEvent
callbacks cannot use arguments passed to time watcher callbacks.

The callback will normally be invoked only once. If you specify another
parameter, "interval", as a strictly positive number (> 0), then the
callback will be invoked regularly at that interval (in fractional
seconds) after the first invocation. If "interval" is specified with a
false value, then it is treated as if it were not specified at all.

The callback will be rescheduled before invoking the callback, but no
attempt is made to avoid timer drift in most backends, so the interval
is only approximate.

Example: 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;

Example 2: fire an event after 0.5 seconds, then roughly every second.

my $w = AnyEvent->timer (after => 0.5, interval => 1, cb => sub {
warn "timeout\n";
});

TIMING ISSUES
There are two ways to handle timers: based on real time (relative, "fire
in 10 seconds") and based on wallclock time (absolute, "fire at 12
o'clock").

While most event loops expect timers to specified in a relative way,
they use absolute time internally. This makes a difference when your
clock "jumps", for example, when ntp decides to set your clock backwards
from the wrong date of 2014-01-01 to 2008-01-01, a watcher that is
supposed to fire "after a second" might actually take six years to
finally fire.

AnyEvent cannot compensate for this. The only event loop that is
conscious of these issues is EV, which offers both relative (ev_timer,
based on true relative time) and absolute (ev_periodic, based on
wallclock time) timers.

AnyEvent always prefers relative timers, if available, matching the
AnyEvent API.

AnyEvent has two additional methods that return the "current time":

AnyEvent->time
This returns the "current wallclock time" as a fractional number of
seconds since the Epoch (the same thing as "time" or
"Time::HiRes::time" return, and the result is guaranteed to be
compatible with those).

It progresses independently of any event loop processing, i.e. each
call will check the system clock, which usually gets updated
frequently.

AnyEvent->now
This also returns the "current wallclock time", but unlike "time",
above, this value might change only once per event loop iteration,
depending on the event loop (most return the same time as "time",
above). This is the time that AnyEvent's timers get scheduled
against.

*In almost all cases (in all cases if you don't care), this is the
function to call when you want to know the current time.*

This function is also often faster then "AnyEvent->time", and thus
the preferred method if you want some timestamp (for example,
AnyEvent::Handle uses this to update its activity timeouts).

The rest of this section is only of relevance if you try to be very
exact with your timing; you can skip it without a bad conscience.

For a practical example of when these times differ, consider
Event::Lib and EV and the following set-up:

The event loop is running and has just invoked one of your callbacks
at time=500 (assume no other callbacks delay processing). In your
callback, you wait a second by executing "sleep 1" (blocking the
process for a second) and then (at time=501) you create a relative
timer that fires after three seconds.

With Event::Lib, "AnyEvent->time" and "AnyEvent->now" will both
return 501, because that is the current time, and the timer will be
scheduled to fire at time=504 (501 + 3).

With EV, "AnyEvent->time" returns 501 (as that is the current time),
but "AnyEvent->now" returns 500, as that is the time the last event
processing phase started. With EV, your timer gets scheduled to run
at time=503 (500 + 3).

In one sense, Event::Lib is more exact, as it uses the current time
regardless of any delays introduced by event processing. However,
most callbacks do not expect large delays in processing, so this
causes a higher drift (and a lot more system calls to get the
current time).

In another sense, EV is more exact, as your timer will be scheduled
at the same time, regardless of how long event processing actually
took.

In either case, if you care (and in most cases, you don't), then you
can get whatever behaviour you want with any event loop, by taking
the difference between "AnyEvent->time" and "AnyEvent->now" into
account.

AnyEvent->now_update
Some event loops (such as EV or AnyEvent::Loop) cache the current
time for each loop iteration (see the discussion of AnyEvent->now,
above).

When a callback runs for a long time (or when the process sleeps),
then this "current" time will differ substantially from the real
time, which might affect timers and time-outs.

When this is the case, you can call this method, which will update
the event loop's idea of "current time".

A typical example would be a script in a web server (e.g.
"mod_perl") - when mod_perl executes the script, then the event loop
will have the wrong idea about the "current time" (being potentially
far in the past, when the script ran the last time). In that case
you should arrange a call to "AnyEvent->now_update" each time the
web server process wakes up again (e.g. at the start of your script,
or in a handler).

Note that updating the time *might* cause some events to be handled.

SIGNAL WATCHERS
$w = AnyEvent->signal (signal => , cb => );

You can watch for signals using a signal watcher, "signal" is the signal
*name* in uppercase and without any "SIG" prefix, "cb" is the Perl
callback to be invoked whenever a signal occurs.

Multiple signal occurrences can be clumped together into one callback
invocation, and callback invocation will be synchronous. Synchronous
means that it might take a while until the signal gets handled by the
process, but it is guaranteed not to interrupt any other callbacks.

The main advantage of using these watchers is that you can share a
signal between multiple watchers, and AnyEvent will ensure that signals
will not interrupt your program at bad times.

This watcher might use %SIG (depending on the event loop used), 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 });

Restart Behaviour
While restart behaviour is up to the event loop implementation, most
will not restart syscalls (that includes Async::Interrupt and AnyEvent's
pure perl implementation).

Safe/Unsafe Signals
Perl signals can be either "safe" (synchronous to opcode handling) or
"unsafe" (asynchronous) - the former might delay signal delivery
indefinitely, the latter might corrupt your memory.

AnyEvent signal handlers are, in addition, synchronous to the event
loop, i.e. they will not interrupt your running perl program but will
only be called as part of the normal event handling (just like timer,
I/O etc. callbacks, too).

Signal Races, Delays and Workarounds
Many event loops (e.g. Glib, Tk, Qt, IO::Async) do not support attaching
callbacks to signals in a generic way, which is a pity, as you cannot do
race-free signal handling in perl, requiring C libraries for this.
AnyEvent will try to do its best, which means in some cases, signals
will be delayed. The maximum time a signal might be delayed is 10
seconds by default, but can be overriden via
$ENV{PERL_ANYEVENT_MAX_SIGNAL_LATENCY} or $AnyEvent::MAX_SIGNAL_LATENCY
- see the "ENVIRONMENT VARIABLES" section for details.

All these problems can be avoided by installing the optional
Async::Interrupt module, which works with most event loops. It will not
work with inherently broken event loops such as Event or Event::Lib (and
not with POE currently). For those, you just have to suffer the delays.

CHILD PROCESS WATCHERS
$w = AnyEvent->child (pid => , cb => );

You can also watch for a child process exit and catch its exit status.

The child process is specified by the "pid" argument (on some backends,
using 0 watches for any child process exit, on others this will croak).
The watcher will be triggered only when the child process has finished
and an exit status is available, not on any trace events
(stopped/continued).

The callback will be called with the pid and exit status (as returned by
waitpid), so unlike other watcher types, you *can* rely on child watcher
callback arguments.

This watcher type works by installing a signal handler for "SIGCHLD",
and since it cannot be shared, nothing else should use SIGCHLD or reap
random child processes (waiting for specific child processes, e.g.
inside "system", is just fine).

There is a slight catch to child watchers, however: you usually start
them *after* the child process was created, and this means the process
could have exited already (and no SIGCHLD will be sent anymore).

Not all event models handle this correctly (neither POE nor IO::Async
do, see their AnyEvent::Impl manpages for details), but even for event
models that *do* handle this correctly, they usually need to be loaded
before the process exits (i.e. before you fork in the first place).
AnyEvent's pure perl event loop handles all cases correctly regardless
of when you start the watcher.

This means you cannot create a child watcher as the very first thing in
an AnyEvent program, you *have* to create at least one watcher before
you "fork" the child (alternatively, you can call "AnyEvent::detect").

As most event loops do not support waiting for child events, they will
be emulated by AnyEvent in most cases, in which case the latency and
race problems mentioned in the description of signal watchers apply.

Example: fork a process and wait for it

my $done = AnyEvent->condvar;

# this forks and immediately calls exit in the child. this
# normally has all sorts of bad consequences for your parent,
# so take this as an example only. always fork and exec,
# or call POSIX::_exit, in real code.
my $pid = fork or exit 5;

my $w = AnyEvent->child (
pid => $pid,
cb => sub {
my ($pid, $status) = @_;
warn "pid $pid exited with status $status";
$done->send;
},
);

# do something else, then wait for process exit
$done->recv;

IDLE WATCHERS
$w = AnyEvent->idle (cb => );

This will repeatedly invoke the callback after the process becomes idle,
until either the watcher is destroyed or new events have been detected.

Idle watchers are useful when there is a need to do something, but it is
not so important (or wise) to do it instantly. The callback will be
invoked only when there is "nothing better to do", which is usually
defined as "all outstanding events have been handled and no new events
have been detected". That means that idle watchers ideally get invoked
when the event loop has just polled for new events but none have been
detected. Instead of blocking to wait for more events, the idle watchers
will be invoked.

Unfortunately, most event loops do not really support idle watchers
(only EV, Event and Glib do it in a usable fashion) - for the rest,
AnyEvent will simply call the callback "from time to time".

Example: read lines from STDIN, but only process them when the program
is otherwise idle:

my @lines; # read data
my $idle_w;
my $io_w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
push @lines, scalar ;

# start an idle watcher, if not already done
$idle_w ||= AnyEvent->idle (cb => sub {
# handle only one line, when there are lines left
if (my $line = shift @lines) {
print "handled when idle: $line";
} else {
# otherwise disable the idle watcher again
undef $idle_w;
}
});
});

CONDITION VARIABLES
$cv = AnyEvent->condvar;

$cv->send ();
my @res = $cv->recv;

If you are familiar with some event loops you will know that all of them
require you to run some blocking "loop", "run" or similar function that
will actively watch for new events and call your callbacks.

AnyEvent is slightly different: it expects somebody else to run the
event loop and will only block when necessary (usually when told by the
user).

The tool to do that is called a "condition variable", so called because
they represent a condition that must become true.

Now is probably a good time to look at the examples further below.

Condition variables can be created by calling the "AnyEvent->condvar"
method, usually without arguments. The only argument pair allowed is
"cb", which specifies a callback to be called when the condition
variable becomes true, with the condition variable as the first argument
(but not the results).

After creation, the condition variable is "false" until it becomes
"true" by calling the "send" method (or calling the condition variable
as if it were a callback, read about the caveats in the description for
the "->send" method).

Since condition variables are the most complex part of the AnyEvent API,
here are some different mental models of what they are - pick the ones
you can connect to:

* Condition variables are like callbacks - you can call them (and pass
them instead of callbacks). Unlike callbacks however, you can also
wait for them to be called.

* Condition variables are signals - one side can emit or send them,
the other side can wait for them, or install a handler that is
called when the signal fires.

* Condition variables are like "Merge Points" - points in your program
where you merge multiple independent results/control flows into one.

* Condition variables represent a transaction - functions that start
some kind of transaction can return them, leaving the caller the
choice between waiting in a blocking fashion, or setting a callback.

* Condition variables represent future values, or promises to deliver
some result, long before the result is available.

Condition variables are very useful to signal that something has
finished, for example, if you write a module that does asynchronous http
requests, then a condition variable would be the ideal candidate to
signal the availability of results. The user can either act when the
callback is called or can synchronously "->recv" for the results.

You can also use them to simulate traditional event loops - for example,
you can block your main program until an event occurs - for example, you
could "->recv" in your main program until the user clicks the Quit
button of your app, which would "->send" the "quit" event.

Note that condition variables recurse into the event loop - if you have
two pieces of code that call "->recv" in a round-robin fashion, you
lose. Therefore, condition variables are good to export to your caller,
but you should avoid making a blocking wait yourself, at least in
callbacks, as this asks for trouble.

Condition variables are represented by hash refs in perl, and the keys
used by AnyEvent itself are all named "_ae_XXX" to make subclassing easy
(it is often useful to build your own transaction class on top of
AnyEvent). To subclass, use "AnyEvent::CondVar" as base class and call
its "new" method in your own "new" method.

There are two "sides" to a condition variable - the "producer side"
which eventually calls "-> send", and the "consumer side", which waits
for the send to occur.

Example: wait for a timer.

# condition: "wait till the timer is fired"
my $timer_fired = AnyEvent->condvar;

# create the timer - we could wait for, say
# a handle becomign ready, or even an
# AnyEvent::HTTP request to finish, but
# in this case, we simply use a timer:
my $w = AnyEvent->timer (
after => 1,
cb => sub { $timer_fired->send },
);

# this "blocks" (while handling events) till the callback
# calls ->send
$timer_fired->recv;

Example: wait for a timer, but take advantage of the fact that condition
variables are also callable directly.

my $done = AnyEvent->condvar;
my $delay = AnyEvent->timer (after => 5, cb => $done);
$done->recv;

Example: Imagine an API that returns a condvar and doesn't support
callbacks. This is how you make a synchronous call, for example from the
main program:

use AnyEvent::CouchDB;

...

my @info = $couchdb->info->recv;

And this is how you would just set a callback to be called whenever the
results are available:

$couchdb->info->cb (sub {
my @info = $_[0]->recv;
});

METHODS FOR PRODUCERS
These methods should only be used by the producing side, i.e. the
code/module that eventually sends the signal. Note that it is also the
producer side which creates the condvar in most cases, but it isn't
uncommon for the consumer to create it as well.

$cv->send (...)
Flag the condition as ready - a running "->recv" and all further
calls to "recv" will (eventually) return after this method has been
called. If nobody is waiting the send will be remembered.

If a callback has been set on the condition variable, it is called
immediately from within send.

Any arguments passed to the "send" call will be returned by all
future "->recv" calls.

Condition variables are overloaded so one can call them directly (as
if they were a code reference). Calling them directly is the same as
calling "send".

$cv->croak ($error)
Similar to send, but causes all calls to "->recv" to invoke
"Carp::croak" with the given error message/object/scalar.

This can be used to signal any errors to the condition variable
user/consumer. Doing it this way instead of calling "croak" directly
delays the error detection, but has the overwhelming advantage that
it diagnoses the error at the place where the result is expected,
and not deep in some event callback with no connection to the actual
code causing the problem.

$cv->begin ([group callback])
$cv->end
These two methods can be used to combine many transactions/events
into one. For example, a function that pings many hosts in parallel
might want to use a condition variable for the whole process.

Every call to "->begin" will increment a counter, and every call to
"->end" will decrement it. If the counter reaches 0 in "->end", the
(last) callback passed to "begin" will be executed, passing the
condvar as first argument. That callback is *supposed* to call
"->send", but that is not required. If no group callback was set,
"send" will be called without any arguments.

You can think of "$cv->send" giving you an OR condition (one call
sends), while "$cv->begin" and "$cv->end" giving you an AND
condition (all "begin" calls must be "end"'ed before the condvar
sends).

Let's start with a simple example: you have two I/O watchers (for
example, STDOUT and STDERR for a program), and you want to wait for
both streams to close before activating a condvar:

my $cv = AnyEvent->condvar;

$cv->begin; # first watcher
my $w1 = AnyEvent->io (fh => $fh1, cb => sub {
defined sysread $fh1, my $buf, 4096
or $cv->end;
});

$cv->begin; # second watcher
my $w2 = AnyEvent->io (fh => $fh2, cb => sub {
defined sysread $fh2, my $buf, 4096
or $cv->end;
});

$cv->recv;

This works because for every event source (EOF on file handle),
there is one call to "begin", so the condvar waits for all calls to
"end" before sending.

The ping example mentioned above is slightly more complicated, as
the there are results to be passed back, and the number of tasks
that are begun can potentially be zero:

my $cv = AnyEvent->condvar;

my %result;
$cv->begin (sub { shift->send (\%result) });

for my $host (@list_of_hosts) {
$cv->begin;
ping_host_then_call_callback $host, sub {
$result{$host} = ...;
$cv->end;
};
}

$cv->end;

...

my $results = $cv->recv;

This code fragment supposedly pings a number of hosts and calls
"send" after results for all then have have been gathered - in any
order. To achieve this, the code issues a call to "begin" when it
starts each ping request and calls "end" when it has received some
result for it. Since "begin" and "end" only maintain a counter, the
order in which results arrive is not relevant.

There is an additional bracketing call to "begin" and "end" outside
the loop, which serves two important purposes: first, it sets the
callback to be called once the counter reaches 0, and second, it
ensures that "send" is called even when "no" hosts are being pinged
(the loop doesn't execute once).

This is the general pattern when you "fan out" into multiple (but
potentially zero) subrequests: use an outer "begin"/"end" pair to
set the callback and ensure "end" is called at least once, and then,
for each subrequest you start, call "begin" and for each subrequest
you finish, call "end".

METHODS FOR CONSUMERS
These methods should only be used by the consuming side, i.e. the code
awaits the condition.

$cv->recv
Wait (blocking if necessary) until the "->send" or "->croak" methods
have been called on $cv, while servicing other watchers normally.

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

If an error condition has been set by calling "->croak", then this
function will call "croak".

In list context, all parameters passed to "send" will be returned,
in scalar context only the first one will be returned.

Note that doing a blocking wait in a callback is not supported by
any event loop, that is, recursive invocation of a blocking "->recv"
is not allowed and the "recv" call will "croak" if such a condition
is detected. This requirement can be dropped by relying on
Coro::AnyEvent , which allows you to do a blocking "->recv" from any
thread that doesn't run the event loop itself. Coro::AnyEvent is
loaded automatically when Coro is used with AnyEvent, so code does
not need to do anything special to take advantage of that: any code
that would normally block your program because it calls "recv", be
executed in an "async" thread instead without blocking other
threads.

Not all event models support a blocking wait - some die in that case
(programs might want to do that to stay interactive), so *if you are
using this from a module, never require a blocking wait*. Instead,
let the caller decide whether 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 supporting blocking waits if
the caller so desires).

You can ensure that "->recv" never blocks by setting a callback and
only calling "->recv" from within that callback (or at a later
time). This will work even when the event loop does not support
blocking waits otherwise.

$bool = $cv->ready
Returns true when the condition is "true", i.e. whether "send" or
"croak" have been called.

$cb = $cv->cb ($cb->($cv))
This is a mutator function that returns the callback set and
optionally replaces it before doing so.

The callback will be called when the condition becomes "true", i.e.
when "send" or "croak" are called, with the only argument being the
condition variable itself. If the condition is already true, the
callback is called immediately when it is set. Calling "recv" inside
the callback or at any later time is guaranteed not to block.

SUPPORTED EVENT LOOPS/BACKENDS
The available backend classes are (every class has its own manpage):

Backends that are autoprobed when no other event loop can be found.
EV is the preferred backend when no other event loop seems to be in
use. If EV is not installed, then AnyEvent will fall back to its own
pure-perl implementation, which is available everywhere as it comes
with AnyEvent itself.

AnyEvent::Impl::EV based on EV (interface to libev, best choice).
AnyEvent::Impl::Perl pure-perl AnyEvent::Loop, fast and portable.

Backends that are transparently being picked up when they are used.
These will be used if they are already loaded when the first watcher
is created, in which case it is assumed that the application is
using them. This means that AnyEvent will automatically pick the
right backend when the main program loads an event module before
anything starts to create watchers. Nothing special needs to be done
by the main program.

AnyEvent::Impl::Event based on Event, very stable, few glitches.
AnyEvent::Impl::Glib based on Glib, slow but very stable.
AnyEvent::Impl::Tk based on Tk, very broken.
AnyEvent::Impl::UV based on UV, innovated square wheels.
AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
AnyEvent::Impl::POE based on POE, very slow, some limitations.
AnyEvent::Impl::Irssi used when running within irssi.
AnyEvent::Impl::IOAsync based on IO::Async.
AnyEvent::Impl::Cocoa based on Cocoa::EventLoop.
AnyEvent::Impl::FLTK based on FLTK (fltk 2 binding).

Backends with special needs.
Qt requires the Qt::Application to be instantiated first, but will
otherwise be picked up automatically. As long as the main program
instantiates the application before any AnyEvent watchers are
created, everything should just work.

AnyEvent::Impl::Qt based on Qt.

Event loops that are indirectly supported via other backends.
Some event loops can be supported via other modules:

There is no direct support for WxWidgets (Wx) or Prima.

WxWidgets has no support for watching file handles. However, you can
use WxWidgets through the POE adaptor, as POE has a Wx backend that
simply polls 20 times per second, which was considered to be too
horrible to even consider for AnyEvent.

Prima is not supported as nobody seems to be using it, but it has a
POE backend, so it can be supported through POE.

AnyEvent knows about both Prima and Wx, however, and will try to
load POE when detecting them, in the hope that POE will pick them
up, in which case everything will be automatic.

GLOBAL VARIABLES AND FUNCTIONS
These are not normally required to use AnyEvent, but can be useful to
write AnyEvent extension modules.

$AnyEvent::MODEL
Contains "undef" until the first watcher is being created, before
the backend has been autodetected.

Afterwards 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* it will be "urxvt::anyevent").

AnyEvent::detect
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, as late as
possible at runtime, and not e.g. during initialisation of your
module.

The effect of calling this function is as if a watcher had been
created (specifically, actions that happen "when the first watcher
is created" happen when calling detetc as well).

If you need to do some initialisation before AnyEvent watchers are
created, use "post_detect".

$guard = AnyEvent::post_detect { BLOCK }
Arranges for the code block to be executed as soon as the event
model is autodetected (or immediately if that has already happened).

The block will be executed *after* the actual backend has been
detected ($AnyEvent::MODEL is set), but *before* any watchers have
been created, so it is possible to e.g. patch @AnyEvent::ISA or do
other initialisations - see the sources of AnyEvent::Strict or
AnyEvent::AIO to see how this is used.

The most common usage is to create some global watchers, without
forcing event module detection too early, for example, AnyEvent::AIO
creates and installs the global IO::AIO watcher in a "post_detect"
block to avoid autodetecting the event module at load time.

If called in scalar or list context, then it creates and returns an
object that automatically removes the callback again when it is
destroyed (or "undef" when the hook was immediately executed). See
AnyEvent::AIO for a case where this is useful.

Example: Create a watcher for the IO::AIO module and store it in
$WATCHER, but do so only do so after the event loop is initialised.

our WATCHER;

my $guard = AnyEvent::post_detect {
$WATCHER = AnyEvent->io (fh => IO::AIO::poll_fileno, poll => 'r', cb => \&IO::AIO::poll_cb);
};

# the ||= is important in case post_detect immediately runs the block,
# as to not clobber the newly-created watcher. assigning both watcher and
# post_detect guard to the same variable has the advantage of users being
# able to just C if the watcher causes them grief.

$WATCHER ||= $guard;

@AnyEvent::post_detect
If there are any code references in this array (you can "push" to it
before or after loading AnyEvent), then they will be called directly
after the event loop has been chosen.

You should check $AnyEvent::MODEL before adding to this array,
though: if it is defined then the event loop has already been
detected, and the array will be ignored.

Best use "AnyEvent::post_detect { BLOCK }" when your application
allows it, as it takes care of these details.

This variable is mainly useful for modules that can do something
useful when AnyEvent is used and thus want to know when it is
initialised, but do not need to even load it by default. This array
provides the means to hook into AnyEvent passively, without loading
it.

Example: To load Coro::AnyEvent whenever Coro and AnyEvent are used
together, you could put this into Coro (this is the actual code used
by Coro to accomplish this):

if (defined $AnyEvent::MODEL) {
# AnyEvent already initialised, so load Coro::AnyEvent
require Coro::AnyEvent;
} else {
# AnyEvent not yet initialised, so make sure to load Coro::AnyEvent
# as soon as it is
push @AnyEvent::post_detect, sub { require Coro::AnyEvent };
}

AnyEvent::postpone { BLOCK }
Arranges for the block to be executed as soon as possible, but not
before the call itself returns. In practise, the block will be
executed just before the event loop polls for new events, or shortly
afterwards.

This function never returns anything (to make the "return postpone {
... }" idiom more useful.

To understand the usefulness of this function, consider a function
that asynchronously does something for you and returns some
transaction object or guard to let you cancel the operation. For
example, "AnyEvent::Socket::tcp_connect":

# start a connection attempt unless one is active
$self->{connect_guard} ||= AnyEvent::Socket::tcp_connect "www.example.net", 80, sub {
delete $self->{connect_guard};
...
};

Imagine that this function could instantly call the callback, for
example, because it detects an obvious error such as a negative port
number. Invoking the callback before the function returns causes
problems however: the callback will be called and will try to delete
the guard object. But since the function hasn't returned yet, there
is nothing to delete. When the function eventually returns it will
assign the guard object to "$self->{connect_guard}", where it will
likely never be deleted, so the program thinks it is still trying to
connect.

This is where "AnyEvent::postpone" should be used. Instead of
calling the callback directly on error:

$cb->(undef), return # signal error to callback, BAD!
if $some_error_condition;

It should use "postpone":

AnyEvent::postpone { $cb->(undef) }, return # signal error to callback, later
if $some_error_condition;

AnyEvent::log $level, $msg[, @args]
Log the given $msg at the given $level.

If AnyEvent::Log is not loaded then this function makes a simple
test to see whether the message will be logged. If the test succeeds
it will load AnyEvent::Log and call "AnyEvent::Log::log" -
consequently, look at the AnyEvent::Log documentation for details.

If the test fails it will simply return. Right now this happens when
a numerical loglevel is used and it is larger than the level
specified via $ENV{PERL_ANYEVENT_VERBOSE}.

If you want to sprinkle loads of logging calls around your code,
consider creating a logger callback with the "AnyEvent::Log::logger"
function, which can reduce typing, codesize and can reduce the
logging overhead enourmously.

WHAT TO DO IN A MODULE
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.

Never call "->recv" on a condition variable unless you *know* that the
"->send" method has been called on it already. This is because it will
stall the whole program, and the whole point of using events is to stay
interactive.

It is fine, however, to call "->recv" when the user of your module
requests it (i.e. if you create a http request object ad have a method
called "results" that returns the results, it may call "->recv" freely,
as the user of your module knows what she is doing. Always).

WHAT TO DO IN THE MAIN PROGRAM
There will always be a single main program - the only place that should
dictate which event model to use.

If the program is not event-based, it need not do anything special, even
when it depends on a module that uses an AnyEvent. If the program itself
uses AnyEvent, but does not care which event loop is used, all it needs
to do is "use AnyEvent". In either case, AnyEvent will choose the best
available loop implementation.

If the main program relies on a specific event model - for example, in
Gtk2 programs you have to rely on the Glib module - you should load the
event module before loading AnyEvent or any module that uses it:
generally speaking, you should load it 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 choose the wrong one unless you load the correct one
yourself.

You can chose to use a pure-perl implementation by loading the
"AnyEvent::Loop" module, which gives you similar behaviour everywhere,
but letting AnyEvent chose the model is generally better.

MAINLOOP EMULATION
Sometimes (often for short test scripts, or even standalone programs who
only want to use AnyEvent), you do not want to run a specific event
loop.

In that case, you can use a condition variable like this:

AnyEvent->condvar->recv;

This has the effect of entering the event loop and looping forever.

Note that usually your program has some exit condition, in which case it
is better to use the "traditional" approach of storing a condition
variable somewhere, waiting for it, and sending it when the program
should exit cleanly.

OTHER MODULES
The following is a non-exhaustive list of additional modules that use
AnyEvent as a client and can therefore be mixed easily with other
AnyEvent modules and other event loops in the same program. Some of the
modules come as part of AnyEvent, the others are available via CPAN (see
for a longer
non-exhaustive list), and the list is heavily biased towards modules of
the AnyEvent author himself :)

AnyEvent::Util (part of the AnyEvent distribution)
Contains various utility functions that replace often-used blocking
functions such as "inet_aton" with event/callback-based versions.

AnyEvent::Socket (part of the AnyEvent distribution)
Provides various utility functions for (internet protocol) sockets,
addresses and name resolution. Also functions to create non-blocking
tcp connections or tcp servers, with IPv6 and SRV record support and
more.

AnyEvent::Handle (part of the AnyEvent distribution)
Provide read and write buffers, manages watchers for reads and
writes, supports raw and formatted I/O, I/O queued and fully
transparent and non-blocking SSL/TLS (via AnyEvent::TLS).

AnyEvent::DNS (part of the AnyEvent distribution)
Provides rich asynchronous DNS resolver capabilities.

AnyEvent::HTTP, AnyEvent::IRC, AnyEvent::XMPP, AnyEvent::GPSD,
AnyEvent::IGS, AnyEvent::FCP
Implement event-based interfaces to the protocols of the same name
(for the curious, IGS is the International Go Server and FCP is the
Freenet Client Protocol).

AnyEvent::AIO (part of the AnyEvent distribution)
Truly asynchronous (as opposed to non-blocking) I/O, should be in
the toolbox of every event programmer. AnyEvent::AIO transparently
fuses IO::AIO and AnyEvent together, giving AnyEvent access to
event-based file I/O, and much more.

AnyEvent::Fork, AnyEvent::Fork::RPC, AnyEvent::Fork::Pool,
AnyEvent::Fork::Remote
These let you safely fork new subprocesses, either locally or
remotely (e.g.v ia ssh), using some RPC protocol or not, without the
limitations normally imposed by fork (AnyEvent works fine for
example). Dynamically-resized worker pools are obviously included as
well.

And they are quite tiny and fast as well - "abusing" AnyEvent::Fork
just to exec external programs can easily beat using "fork" and
"exec" (or even "system") in most programs.

AnyEvent::Filesys::Notify
AnyEvent is good for non-blocking stuff, but it can't detect file or
path changes (e.g. "watch this directory for new files", "watch this
file for changes"). The AnyEvent::Filesys::Notify module promises to
do just that in a portbale fashion, supporting inotify on GNU/Linux
and some weird, without doubt broken, stuff on OS X to monitor
files. It can fall back to blocking scans at regular intervals
transparently on other platforms, so it's about as portable as it
gets.

(I haven't used it myself, but it seems the biggest problem with it
is it quite bad performance).

AnyEvent::DBI
Executes DBI requests asynchronously in a proxy process for you,
notifying you in an event-based way when the operation is finished.

AnyEvent::FastPing
The fastest ping in the west.

Coro
Has special support for AnyEvent via Coro::AnyEvent, which allows
you to simply invert the flow control - don't call us, we will call
you:

async {
Coro::AnyEvent::sleep 5; # creates a 5s timer and waits for it
print "5 seconds later!\n";

Coro::AnyEvent::readable *STDIN; # uses an I/O watcher
my $line = ; # works for ttys

AnyEvent::HTTP::http_get "url", Coro::rouse_cb;
my ($body, $hdr) = Coro::rouse_wait;
};

SIMPLIFIED AE API
Starting with version 5.0, AnyEvent officially supports a second, much
simpler, API that is designed to reduce the calling, typing and memory
overhead by using function call syntax and a fixed number of parameters.

See the AE manpage for details.

ERROR AND EXCEPTION HANDLING
In general, AnyEvent does not do any error handling - it relies on the
caller to do that if required. The AnyEvent::Strict module (see also the
"PERL_ANYEVENT_STRICT" environment variable, below) provides strict
checking of all AnyEvent methods, however, which is highly useful during
development.

As for exception handling (i.e. runtime errors and exceptions thrown
while executing a callback), this is not only highly event-loop
specific, but also not in any way wrapped by this module, as this is the
job of the main program.

The pure perl event loop simply re-throws the exception (usually within
"condvar->recv"), the Event and EV modules call "$Event/EV::DIED->()",
Glib uses "install_exception_handler" and so on.

ENVIRONMENT VARIABLES
AnyEvent supports a number of environment variables that tune the
runtime behaviour. They are usually evaluated when AnyEvent is loaded,
initialised, or a submodule that uses them is loaded. Many of them also
cause AnyEvent to load additional modules - for example,
"PERL_ANYEVENT_DEBUG_WRAP" causes the AnyEvent::Debug module to be
loaded.

All the environment variables documented here start with
"PERL_ANYEVENT_", which is what AnyEvent considers its own namespace.
Other modules are encouraged (but by no means required) to use
"PERL_ANYEVENT_SUBMODULE" if they have registered the
AnyEvent::Submodule namespace on CPAN, for any submodule. For example,
AnyEvent::HTTP could be expected to use "PERL_ANYEVENT_HTTP_PROXY" (it
should not access env variables starting with "AE_", see below).

All variables can also be set via the "AE_" prefix, that is, instead of
setting "PERL_ANYEVENT_VERBOSE" you can also set "AE_VERBOSE". In case
there is a clash btween anyevent and another program that uses
"AE_something" you can set the corresponding "PERL_ANYEVENT_something"
variable to the empty string, as those variables take precedence.

When AnyEvent is first loaded, it copies all "AE_xxx" env variables to
their "PERL_ANYEVENT_xxx" counterpart unless that variable already
exists. If taint mode is on, then AnyEvent will remove *all* environment
variables starting with "PERL_ANYEVENT_" from %ENV (or replace them with
"undef" or the empty string, if the corresaponding "AE_" variable is
set).

The exact algorithm is currently:

1. if taint mode enabled, delete all PERL_ANYEVENT_xyz variables from %ENV
2. copy over AE_xyz to PERL_ANYEVENT_xyz unless the latter alraedy exists
3. if taint mode enabled, set all PERL_ANYEVENT_xyz variables to undef.

This ensures that child processes will not see the "AE_" variables.

The following environment variables are currently known to AnyEvent:

"PERL_ANYEVENT_VERBOSE"
By default, AnyEvent will log messages with loglevel 4 ("error") or
higher (see AnyEvent::Log). You can set this environment variable to
a numerical loglevel to make AnyEvent more (or less) talkative.

If you want to do more than just set the global logging level you
should have a look at "PERL_ANYEVENT_LOG", which allows much more
complex specifications.

When set to 0 ("off"), then no messages whatsoever will be logged
with everything else at defaults.

When set to 5 or higher ("warn"), AnyEvent warns about unexpected
conditions, such as not being able to load the event model specified
by "PERL_ANYEVENT_MODEL", or a guard callback throwing an exception
- this is the minimum recommended level for use during development.

When set to 7 or higher (info), AnyEvent reports which event model
it chooses.

When set to 8 or higher (debug), then AnyEvent will report extra
information on which optional modules it loads and how it implements
certain features.

"PERL_ANYEVENT_LOG"
Accepts rather complex logging specifications. For example, you
could log all "debug" messages of some module to stderr, warnings
and above to stderr, and errors and above to syslog, with:

PERL_ANYEVENT_LOG=Some::Module=debug,+log:filter=warn,+%syslog:%syslog=error,syslog

For the rather extensive details, see AnyEvent::Log.

This variable is evaluated when AnyEvent (or AnyEvent::Log) is
loaded, so will take effect even before AnyEvent has initialised
itself.

Note that specifying this environment variable causes the
AnyEvent::Log module to be loaded, while "PERL_ANYEVENT_VERBOSE"
does not, so only using the latter saves a few hundred kB of memory
unless a module explicitly needs the extra features of
AnyEvent::Log.

"PERL_ANYEVENT_STRICT"
AnyEvent does not do much argument checking by default, as thorough
argument checking is very costly. Setting this variable to a true
value will cause AnyEvent to load "AnyEvent::Strict" and then to
thoroughly check the arguments passed to most method calls. If it
finds any problems, it will croak.

In other words, enables "strict" mode.

Unlike "use strict" (or its modern cousin, "use common::sense", it
is definitely recommended to keep it off in production. Keeping
"PERL_ANYEVENT_STRICT=1" in your environment while developing
programs can be very useful, however.

"PERL_ANYEVENT_DEBUG_SHELL"
If this env variable is nonempty, then its contents will be
interpreted by "AnyEvent::Socket::parse_hostport" and
"AnyEvent::Debug::shell" (after replacing every occurance of $$ by
the process pid). The shell object is saved in
$AnyEvent::Debug::SHELL.

This happens when the first watcher is created.

For example, to bind a debug shell on a unix domain socket in
/tmp/debug.sock, you could use this:

PERL_ANYEVENT_DEBUG_SHELL=/tmp/debug\$\$.sock perlprog
# connect with e.g.: socat readline /tmp/debug123.sock

Or to bind to tcp port 4545 on localhost:

PERL_ANYEVENT_DEBUG_SHELL=127.0.0.1:4545 perlprog
# connect with e.g.: telnet localhost 4545

Note that creating sockets in /tmp or on localhost is very unsafe on
multiuser systems.

"PERL_ANYEVENT_DEBUG_WRAP"
Can be set to 0, 1 or 2 and enables wrapping of all watchers for
debugging purposes. See "AnyEvent::Debug::wrap" for details.

"PERL_ANYEVENT_MODEL"
This can be used to specify the event model to be used by AnyEvent,
before auto detection and -probing kicks in.

It normally is a string consisting entirely of ASCII letters (e.g.
"EV" or "IOAsync"). The string "AnyEvent::Impl::" gets prepended and
the resulting module name is loaded and - if the load was successful
- used as event model backend. If it fails to load then AnyEvent
will proceed with auto detection and -probing.

If the string ends with "::" instead (e.g. "AnyEvent::Impl::EV::")
then nothing gets prepended and the module name is used as-is (hint:
"::" at the end of a string designates a module name and quotes it
appropriately).

For example, to force the pure perl model (AnyEvent::Loop::Perl) you
could start your program like this:

PERL_ANYEVENT_MODEL=Perl perl ...

"PERL_ANYEVENT_IO_MODEL"
The current file I/O model - see AnyEvent::IO for more info.

At the moment, only "Perl" (small, pure-perl, synchronous) and
"IOAIO" (truly asynchronous) are supported. The default is "IOAIO"
if AnyEvent::AIO can be loaded, otherwise it is "Perl".

"PERL_ANYEVENT_PROTOCOLS"
Used by both AnyEvent::DNS and AnyEvent::Socket to determine
preferences for IPv4 or IPv6. The default is unspecified (and might
change, or be the result of auto probing).

Must be set to a comma-separated list of protocols or address
families, current supported: "ipv4" and "ipv6". Only protocols
mentioned will be used, and preference will be given to protocols
mentioned earlier in the list.

This variable can effectively be used for denial-of-service attacks
against local programs (e.g. when setuid), although the impact is
likely small, as the program has to handle connection and other
failures anyways.

Examples: "PERL_ANYEVENT_PROTOCOLS=ipv4,ipv6" - prefer IPv4 over
IPv6, but support both and try to use both.
"PERL_ANYEVENT_PROTOCOLS=ipv4" - only support IPv4, never try to
resolve or contact IPv6 addresses.
"PERL_ANYEVENT_PROTOCOLS=ipv6,ipv4" support either IPv4 or IPv6, but
prefer IPv6 over IPv4.

"PERL_ANYEVENT_HOSTS"
This variable, if specified, overrides the /etc/hosts file used by
AnyEvent::Socket"::resolve_sockaddr", i.e. hosts aliases will be
read from that file instead.

"PERL_ANYEVENT_EDNS0"
Used by AnyEvent::DNS to decide whether to use the EDNS0 extension
for DNS. This extension is generally useful to reduce DNS traffic,
especially when DNSSEC is involved, but some (broken) firewalls drop
such DNS packets, which is why it is off by default.

Setting this variable to 1 will cause AnyEvent::DNS to announce
EDNS0 in its DNS requests.

"PERL_ANYEVENT_MAX_FORKS"
The maximum number of child processes that
"AnyEvent::Util::fork_call" will create in parallel.

"PERL_ANYEVENT_MAX_OUTSTANDING_DNS"
The default value for the "max_outstanding" parameter for the
default DNS resolver - this is the maximum number of parallel DNS
requests that are sent to the DNS server.

"PERL_ANYEVENT_MAX_SIGNAL_LATENCY"
Perl has inherently racy signal handling (you can basically choose
between losing signals and memory corruption) - pure perl event
loops (including "AnyEvent::Loop", when "Async::Interrupt" isn't
available) therefore have to poll regularly to avoid losing signals.

Some event loops are racy, but don't poll regularly, and some event
loops are written in C but are still racy. For those event loops,
AnyEvent installs a timer that regularly wakes up the event loop.

By default, the interval for this timer is 10 seconds, but you can
override this delay with this environment variable (or by setting
the $AnyEvent::MAX_SIGNAL_LATENCY variable before creating signal
watchers).

Lower values increase CPU (and energy) usage, higher values can
introduce long delays when reaping children or waiting for signals.

The AnyEvent::Async module, if available, will be used to avoid this
polling (with most event loops).

"PERL_ANYEVENT_RESOLV_CONF"
The absolute path to a resolv.conf-style file to use instead of
/etc/resolv.conf (or the OS-specific configuration) in the default
resolver, or the empty string to select the default configuration.

"PERL_ANYEVENT_CA_FILE", "PERL_ANYEVENT_CA_PATH".
When neither "ca_file" nor "ca_path" was specified during
AnyEvent::TLS context creation, and either of these environment
variables are nonempty, they will be used to specify CA certificate
locations instead of a system-dependent default.

"PERL_ANYEVENT_AVOID_GUARD" and "PERL_ANYEVENT_AVOID_ASYNC_INTERRUPT"
When these are set to 1, then the respective modules are not loaded.
Mostly good for testing AnyEvent itself.

SUPPLYING YOUR OWN EVENT MODEL INTERFACE
This is an advanced topic that you do not normally need to use AnyEvent
in a module. This section is only of use to event loop authors who want
to provide AnyEvent compatibility.

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, so it is reasonably cheap.

Example:

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 already
loaded.

When AnyEvent is loaded and asked to find a suitable event model, it
will first check for the presence of urxvt by trying to "use" the
"urxvt::anyevent" module.

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

If you don't provide "signal" and "child" watchers than AnyEvent will
provide suitable (hopefully) replacements.

The above example isn't fictitious, the *rxvt-unicode* (a.k.a. urxvt)
terminal emulator 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 done in an interactive application, so it makes sense.

EXAMPLE PROGRAM
The following program uses an I/O watcher to read data from STDIN, a
timer to display a message once per second, and a condition variable to
quit 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
chomp (my $input = ); # read a line
warn "read: $input\n"; # output what has been read
$cv->send if $input =~ /^q/i; # quit program if /^q/i
},
);

my $time_watcher = AnyEvent->timer (after => 1, interval => 1, cb => sub {
warn "timeout\n"; # print 'timeout' at most every second
});

$cv->recv; # 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 $txn->{fh}, ...
and !$!{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 has finished:

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

if (end-of-file or data complete) {
$txn->{result} = $txn->{buf};
$txn->{finished}->send;
$txb->{cb}->($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:

$txn->{finished}->recv;
return $txn->{result};

The actual code goes further and collects all errors ("die"s,
exceptions) that occurred during request processing. The "result" method
detects whether 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 to the 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 running in parallel:

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 supported event module:

use EV;

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

EV::loop;

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

use AnyEvent;

my $quit = AnyEvent->condvar;

$fcp->txn_client_get ($url)->cb (sub {
...
$quit->send;
});

$quit->recv;

BENCHMARKS
To give you an idea of the performance and overheads that AnyEvent adds
over the event loops themselves and to give you an impression of the
speed of various event loops I prepared some benchmarks.

BENCHMARKING ANYEVENT OVERHEAD
Here is a benchmark of various supported event models used natively and
through AnyEvent. The benchmark creates a lot of timers (with a zero
timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
which it is), lets them fire exactly once and destroys them again.

Source code for this benchmark is found as eg/bench in the AnyEvent
distribution. It uses the AE interface, which makes a real difference
for the EV and Perl backends only.

Explanation of the columns
*watcher* is the number of event watchers created/destroyed. Since
different event models feature vastly different performances, each event
loop was given a number of watchers so that overall runtime is
acceptable and similar between tested event loop (and keep them from
crashing): Glib would probably take thousands of years if asked to
process the same number of watchers as EV in this benchmark.

*bytes* is the number of bytes (as measured by the resident set size,
RSS) consumed by each watcher. This method of measuring captures both C
and Perl-based overheads.

*create* is the time, in microseconds (millionths of seconds), that it
takes to create a single watcher. The callback is a closure shared
between all watchers, to avoid adding memory overhead. That means
closure creation and memory usage is not included in the figures.

*invoke* is the time, in microseconds, used to invoke a simple callback.
The callback simply counts down a Perl variable and after it was invoked
"watcher" times, it would "->send" a condvar once to signal the end of
this phase.

*destroy* is the time, in microseconds, that it takes to destroy a
single watcher.

Results
name watchers bytes create invoke destroy comment
EV/EV 100000 223 0.47 0.43 0.27 EV native interface
EV/Any 100000 223 0.48 0.42 0.26 EV + AnyEvent watchers
Coro::EV/Any 100000 223 0.47 0.42 0.26 coroutines + Coro::Signal
Perl/Any 100000 431 2.70 0.74 0.92 pure perl implementation
Event/Event 16000 516 31.16 31.84 0.82 Event native interface
Event/Any 16000 1203 42.61 34.79 1.80 Event + AnyEvent watchers
IOAsync/Any 16000 1911 41.92 27.45 16.81 via IO::Async::Loop::IO_Poll
IOAsync/Any 16000 1726 40.69 26.37 15.25 via IO::Async::Loop::Epoll
Glib/Any 16000 1118 89.00 12.57 51.17 quadratic behaviour
Tk/Any 2000 1346 20.96 10.75 8.00 SEGV with >> 2000 watchers
POE/Any 2000 6951 108.97 795.32 14.24 via POE::Loop::Event
POE/Any 2000 6648 94.79 774.40 575.51 via POE::Loop::Select

Discussion
The benchmark does *not* measure scalability of the event loop very
well. For example, a select-based event loop (such as the pure perl one)
can never compete with an event loop that uses epoll when the number of
file descriptors grows high. In this benchmark, all events become ready
at the same time, so select/poll-based implementations get an unnatural
speed boost.

Also, note that the number of watchers usually has a nonlinear effect on
overall speed, that is, creating twice as many watchers doesn't take
twice the time - usually it takes longer. This puts event loops tested
with a higher number of watchers at a disadvantage.

To put the range of results into perspective, consider that on the
benchmark machine, handling an event takes roughly 1600 CPU cycles with
EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000
CPU cycles with POE.

"EV" is the sole leader regarding speed and memory use, which are both
maximal/minimal, respectively. When using the AE API there is zero
overhead (when going through the AnyEvent API create is about 5-6 times
slower, with other times being equal, so still uses far less memory than
any other event loop and is still faster than Event natively).

The pure perl implementation is hit in a few sweet spots (both the
constant timeout and the use of a single fd hit optimisations in the
perl interpreter and the backend itself). Nevertheless this shows that
it adds very little overhead in itself. Like any select-based backend
its performance becomes really bad with lots of file descriptors (and
few of them active), of course, but this was not subject of this
benchmark.

The "Event" module has a relatively high setup and callback invocation
cost, but overall scores in on the third place.

"IO::Async" performs admirably well, about on par with "Event", even
when using its pure perl backend.

"Glib"'s memory usage is quite a bit higher, but it features a faster
callback invocation and overall ends up in the same class as "Event".
However, Glib scales extremely badly, doubling the number of watchers
increases the processing time by more than a factor of four, making it
completely unusable when using larger numbers of watchers (note that
only a single file descriptor was used in the benchmark, so
inefficiencies of "poll" do not account for this).

The "Tk" adaptor works relatively well. The fact that it crashes with
more than 2000 watchers is a big setback, however, as correctness takes
precedence over speed. Nevertheless, its performance is surprising, as
the file descriptor is dup()ed for each watcher. This shows that the
dup() employed by some adaptors is not a big performance issue (it does
incur a hidden memory cost inside the kernel which is not reflected in
the figures above).

"POE", regardless of underlying event loop (whether using its pure perl
select-based backend or the Event module, the POE-EV backend couldn't be
tested because it wasn't working) shows abysmal performance and memory
usage with AnyEvent: Watchers use almost 30 times as much memory as EV
watchers, and 10 times as much memory as Event (the high memory
requirements are caused by requiring a session for each watcher).
Watcher invocation speed is almost 900 times slower than with AnyEvent's
pure perl implementation.

The design of the POE adaptor class in AnyEvent can not really account
for the performance issues, though, as session creation overhead is
small compared to execution of the state machine, which is coded pretty
optimally within AnyEvent::Impl::POE (and while everybody agrees that
using multiple sessions is not a good approach, especially regarding
memory usage, even the author of POE could not come up with a faster
design).

Summary
* Using EV through AnyEvent is faster than any other event loop (even
when used without AnyEvent), but most event loops have acceptable
performance with or without AnyEvent.

* The overhead AnyEvent adds is usually much smaller than the overhead
of the actual event loop, only with extremely fast event loops such
as EV does AnyEvent add significant overhead.

* You should avoid POE like the plague if you want performance or
reasonable memory usage.

BENCHMARKING THE LARGE SERVER CASE
This benchmark actually benchmarks the event loop itself. It works by
creating a number of "servers": each server consists of a socket pair, a
timeout watcher that gets reset on activity (but never fires), and an
I/O watcher waiting for input on one side of the socket. Each time the
socket watcher reads a byte it will write that byte to a random other
"server".

The effect is that there will be a lot of I/O watchers, only part of
which are active at any one point (so there is a constant number of
active fds for each loop iteration, but which fds these are is random).
The timeout is reset each time something is read because that reflects
how most timeouts work (and puts extra pressure on the event loops).

In this benchmark, we use 10000 socket pairs (20000 sockets), of which
100 (1%) are active. This mirrors the activity of large servers with
many connections, most of which are idle at any one point in time.

Source code for this benchmark is found as eg/bench2 in the AnyEvent
distribution. It uses the AE interface, which makes a real difference
for the EV and Perl backends only.

Explanation of the columns
*sockets* is the number of sockets, and twice the number of "servers"
(as each server has a read and write socket end).

*create* is the time it takes to create a socket pair (which is
nontrivial) and two watchers: an I/O watcher and a timeout watcher.

*request*, the most important value, is the time it takes to handle a
single "request", that is, reading the token from the pipe and
forwarding it to another server. This includes deleting the old timeout
and creating a new one that moves the timeout into the future.

Results
name sockets create request
EV 20000 62.66 7.99
Perl 20000 68.32 32.64
IOAsync 20000 174.06 101.15 epoll
IOAsync 20000 174.67 610.84 poll
Event 20000 202.69 242.91
Glib 20000 557.01 1689.52
POE 20000 341.54 12086.32 uses POE::Loop::Event

Discussion
This benchmark *does* measure scalability and overall performance of the
particular event loop.

EV is again fastest. Since it is using epoll on my system, the setup
time is relatively high, though.

Perl surprisingly comes second. It is much faster than the C-based event
loops Event and Glib.

IO::Async performs very well when using its epoll backend, and still
quite good compared to Glib when using its pure perl backend.

Event suffers from high setup time as well (look at its code and you
will understand why). Callback invocation also has a high overhead
compared to the "$_->() for .."-style loop that the Perl event loop
uses. Event uses select or poll in basically all documented
configurations.

Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
clearly fails to perform with many filehandles or in busy servers.

POE is still completely out of the picture, taking over 1000 times as
long as EV, and over 100 times as long as the Perl implementation, even
though it uses a C-based event loop in this case.

Summary
* The pure perl implementation performs extremely well.

* Avoid Glib or POE in large projects where performance matters.

BENCHMARKING SMALL SERVERS
While event loops should scale (and select-based ones do not...) even to
large servers, most programs we (or I :) actually write have only a few
I/O watchers.

In this benchmark, I use the same benchmark program as in the large
server case, but it uses only eight "servers", of which three are active
at any one time. This should reflect performance for a small server
relatively well.

The columns are identical to the previous table.

Results
name sockets create request
EV 16 20.00 6.54
Perl 16 25.75 12.62
Event 16 81.27 35.86
Glib 16 32.63 15.48
POE 16 261.87 276.28 uses POE::Loop::Event

Discussion
The benchmark tries to test the performance of a typical small server.
While knowing how various event loops perform is interesting, keep in
mind that their overhead in this case is usually not as important, due
to the small absolute number of watchers (that is, you need efficiency
and speed most when you have lots of watchers, not when you only have a
few of them).

EV is again fastest.

Perl again comes second. It is noticeably faster than the C-based event
loops Event and Glib, although the difference is too small to really
matter.

POE also performs much better in this case, but is is still far behind
the others.

Summary
* C-based event loops perform very well with small number of watchers,
as the management overhead dominates.

THE IO::Lambda BENCHMARK
Recently I was told about the benchmark in the IO::Lambda manpage, which
could be misinterpreted to make AnyEvent look bad. In fact, the
benchmark simply compares IO::Lambda with POE, and IO::Lambda looks
better (which shouldn't come as a surprise to anybody). As such, the
benchmark is fine, and mostly shows that the AnyEvent backend from
IO::Lambda isn't very optimal. But how would AnyEvent compare when used
without the extra baggage? To explore this, I wrote the equivalent
benchmark for AnyEvent.

The benchmark itself creates an echo-server, and then, for 500 times,
connects to the echo server, sends a line, waits for the reply, and then
creates the next connection. This is a rather bad benchmark, as it
doesn't test the efficiency of the framework or much non-blocking I/O,
but it is a benchmark nevertheless.

name runtime
Lambda/select 0.330 sec
+ optimized 0.122 sec
Lambda/AnyEvent 0.327 sec
+ optimized 0.138 sec
Raw sockets/select 0.077 sec
POE/select, components 0.662 sec
POE/select, raw sockets 0.226 sec
POE/select, optimized 0.404 sec

AnyEvent/select/nb 0.085 sec
AnyEvent/EV/nb 0.068 sec
+state machine 0.134 sec

The benchmark is also a bit unfair (my fault): the IO::Lambda/POE
benchmarks actually make blocking connects and use 100% blocking I/O,
defeating the purpose of an event-based solution. All of the newly
written AnyEvent benchmarks use 100% non-blocking connects (using
AnyEvent::Socket::tcp_connect and the asynchronous pure perl DNS
resolver), so AnyEvent is at a disadvantage here, as non-blocking
connects generally require a lot more bookkeeping and event handling
than blocking connects (which involve a single syscall only).

The last AnyEvent benchmark additionally uses AnyEvent::Handle, which
offers similar expressive power as POE and IO::Lambda, using
conventional Perl syntax. This means that both the echo server and the
client are 100% non-blocking, further placing it at a disadvantage.

As you can see, the AnyEvent + EV combination even beats the
hand-optimised "raw sockets benchmark", while AnyEvent + its pure perl
backend easily beats IO::Lambda and POE.

And even the 100% non-blocking version written using the high-level (and
slow :) AnyEvent::Handle abstraction beats both POE and IO::Lambda
higher level ("unoptimised") abstractions by a large margin, even though
it does all of DNS, tcp-connect and socket I/O in a non-blocking way.

The two AnyEvent benchmarks programs can be found as eg/ae0.pl and
eg/ae2.pl in the AnyEvent distribution, the remaining benchmarks are
part of the IO::Lambda distribution and were used without any changes.

SIGNALS
AnyEvent currently installs handlers for these signals:

SIGCHLD
A handler for "SIGCHLD" is installed by AnyEvent's child watcher
emulation for event loops that do not support them natively. Also,
some event loops install a similar handler.

Additionally, when AnyEvent is loaded and SIGCHLD is set to IGNORE,
then AnyEvent will reset it to default, to avoid losing child exit
statuses.

SIGPIPE
A no-op handler is installed for "SIGPIPE" when $SIG{PIPE} is
"undef" when AnyEvent gets loaded.

The rationale for this is that AnyEvent users usually do not really
depend on SIGPIPE delivery (which is purely an optimisation for
shell use, or badly-written programs), but "SIGPIPE" can cause
spurious and rare program exits as a lot of people do not expect
"SIGPIPE" when writing to some random socket.

The rationale for installing a no-op handler as opposed to ignoring
it is that this way, the handler will be restored to defaults on
exec.

Feel free to install your own handler, or reset it to defaults.

RECOMMENDED/OPTIONAL MODULES
One of AnyEvent's main goals is to be 100% Pure-Perl(tm): only perl (and
its built-in modules) are required to use it.

That does not mean that AnyEvent won't take advantage of some additional
modules if they are installed.

This section explains which additional modules will be used, and how
they affect AnyEvent's operation.

Async::Interrupt
This slightly arcane module is used to implement fast signal
handling: To my knowledge, there is no way to do completely
race-free and quick signal handling in pure perl. To ensure that
signals still get delivered, AnyEvent will start an interval timer
to wake up perl (and catch the signals) with some delay (default is
10 seconds, look for $AnyEvent::MAX_SIGNAL_LATENCY).

If this module is available, then it will be used to implement
signal catching, which means that signals will not be delayed, and
the event loop will not be interrupted regularly, which is more
efficient (and good for battery life on laptops).

This affects not just the pure-perl event loop, but also other event
loops that have no signal handling on their own (e.g. Glib, Tk, Qt).

Some event loops (POE, Event, Event::Lib) offer signal watchers
natively, and either employ their own workarounds (POE) or use
AnyEvent's workaround (using $AnyEvent::MAX_SIGNAL_LATENCY).
Installing Async::Interrupt does nothing for those backends.

EV This module isn't really "optional", as it is simply one of the
backend event loops that AnyEvent can use. However, it is simply the
best event loop available in terms of features, speed and stability:
It supports the AnyEvent API optimally, implements all the watcher
types in XS, does automatic timer adjustments even when no monotonic
clock is available, can take avdantage of advanced kernel interfaces
such as "epoll" and "kqueue", and is the fastest backend *by far*.
You can even embed Glib/Gtk2 in it (or vice versa, see EV::Glib and
Glib::EV).

If you only use backends that rely on another event loop (e.g.
"Tk"), then this module will do nothing for you.

Guard
The guard module, when used, will be used to implement
"AnyEvent::Util::guard". This speeds up guards considerably (and
uses a lot less memory), but otherwise doesn't affect guard
operation much. It is purely used for performance.

JSON and JSON::XS
One of these modules is required when you want to read or write JSON
data via AnyEvent::Handle. JSON is also written in pure-perl, but
can take advantage of the ultra-high-speed JSON::XS module when it
is installed.

Net::SSLeay
Implementing TLS/SSL in Perl is certainly interesting, but not very
worthwhile: If this module is installed, then AnyEvent::Handle (with
the help of AnyEvent::TLS), gains the ability to do TLS/SSL.

Time::HiRes
This module is part of perl since release 5.008. It will be used
when the chosen event library does not come with a timing source of
its own. The pure-perl event loop (AnyEvent::Loop) will additionally
load it to try to use a monotonic clock for timing stability.

AnyEvent::AIO (and IO::AIO)
The default implementation of AnyEvent::IO is to do I/O
synchronously, stopping programs while they access the disk, which
is fine for a lot of programs.

Installing AnyEvent::AIO (and its IO::AIO dependency) makes it
switch to a true asynchronous implementation, so event processing
can continue even while waiting for disk I/O.

FORK
Most event libraries are not fork-safe. The ones who are usually are
because they rely on inefficient but fork-safe "select" or "poll" calls
- higher performance APIs such as BSD's kqueue or the dreaded Linux
epoll are usually badly thought-out hacks that are incompatible with
fork in one way or another. Only EV is fully fork-aware and ensures that
you continue event-processing in both parent and child (or both, if you
know what you are doing).

This means that, in general, you cannot fork and do event processing in
the child if the event library was initialised before the fork (which
usually happens when the first AnyEvent watcher is created, or the
library is loaded).

If you have to fork, you must either do so *before* creating your first
watcher OR you must not use AnyEvent at all in the child OR you must do
something completely out of the scope of AnyEvent (see below).

The problem of doing event processing in the parent *and* the child is
much more complicated: even for backends that *are* fork-aware or
fork-safe, their behaviour is not usually what you want: fork clones all
watchers, that means all timers, I/O watchers etc. are active in both
parent and child, which is almost never what you want. Using "exec" to
start worker children from some kind of manage prrocess is usually
preferred, because it is much easier and cleaner, at the expense of
having to have another binary.

In addition to logical problems with fork, there are also implementation
problems. For example, on POSIX systems, you cannot fork at all in Perl
code if a thread (I am talking of pthreads here) was ever created in the
process, and this is just the tip of the iceberg. In general, using fork
from Perl is difficult, and attempting to use fork without an exec to
implement some kind of parallel processing is almost certainly doomed.

To safely fork and exec, you should use a module such as Proc::FastSpawn
that let's you safely fork and exec new processes.

If you want to do multiprocessing using processes, you can look at the
AnyEvent::Fork module (and some related modules such as
AnyEvent::Fork::RPC, AnyEvent::Fork::Pool and AnyEvent::Fork::Remote).
This module allows you to safely create subprocesses without any
limitations - you can use X11 toolkits or AnyEvent in the children
created by AnyEvent::Fork safely and without any special precautions.

SECURITY CONSIDERATIONS
AnyEvent can be forced to load any event model via
$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used
to execute arbitrary code or directly gain access, it can easily be used
to make the program hang or malfunction in subtle ways, as AnyEvent
watchers will not be active when the program uses a different event
model than specified in the variable.

You can make AnyEvent completely ignore this variable by deleting it
before the first watcher gets created, e.g. with a "BEGIN" block:

BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }

use AnyEvent;

Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can
be used to probe what backend is used and gain other information (which
is probably even less useful to an attacker than PERL_ANYEVENT_MODEL),
and $ENV{PERL_ANYEVENT_STRICT}.

Note that AnyEvent will remove *all* environment variables starting with
"PERL_ANYEVENT_" from %ENV when it is loaded while taint mode is
enabled.

BUGS
Perl 5.8 has numerous memleaks that sometimes hit this module and are
hard to work around. If you suffer from memleaks, first upgrade to Perl
5.10 and check wether the leaks still show up. (Perl 5.10.0 has other
annoying memleaks, such as leaking on "map" and "grep" but it is usually
not as pronounced).

SEE ALSO
Tutorial/Introduction: AnyEvent::Intro.

FAQ: AnyEvent::FAQ.

Utility functions: AnyEvent::Util (misc. grab-bag), AnyEvent::Log
(simply logging).

Development/Debugging: AnyEvent::Strict (stricter checking),
AnyEvent::Debug (interactive shell, watcher tracing).

Supported event modules: AnyEvent::Loop, EV, EV::Glib, Glib::EV, Event,
Glib::Event, Glib, Tk, Event::Lib, Qt, POE, FLTK, Cocoa::EventLoop, UV.

Implementations: AnyEvent::Impl::EV, AnyEvent::Impl::Event,
AnyEvent::Impl::Glib, AnyEvent::Impl::Tk, AnyEvent::Impl::Perl,
AnyEvent::Impl::EventLib, AnyEvent::Impl::Qt, AnyEvent::Impl::POE,
AnyEvent::Impl::IOAsync, AnyEvent::Impl::Irssi, AnyEvent::Impl::FLTK,
AnyEvent::Impl::Cocoa, AnyEvent::Impl::UV.

Non-blocking handles, pipes, stream sockets, TCP clients and servers:
AnyEvent::Handle, AnyEvent::Socket, AnyEvent::TLS.

Asynchronous File I/O: AnyEvent::IO.

Asynchronous DNS: AnyEvent::DNS.

Thread support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event.

Nontrivial usage examples: AnyEvent::GPSD, AnyEvent::IRC,
AnyEvent::HTTP.

AUTHOR
Marc Lehmann
http://anyevent.schmorp.de