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Author image Marc A. Lehmann


AnyEvent::Handle - non-blocking I/O on file handles via AnyEvent


   use AnyEvent;
   use AnyEvent::Handle;

   my $cv = AnyEvent->condvar;

   my $handle =
      AnyEvent::Handle->new (
         fh => \*STDIN,
         on_eof => sub {

   # send some request line
   $handle->push_write ("getinfo\015\012");

   # read the response line
   $handle->push_read (line => sub {
      my ($handle, $line) = @_;
      warn "read line <$line>\n";



This module is a helper module to make it easier to do event-based I/O on filehandles. For utility functions for doing non-blocking connects and accepts on sockets see AnyEvent::Util.

In the following, when the documentation refers to of "bytes" then this means characters. As sysread and syswrite are used for all I/O, their treatment of characters applies to this module as well.

All callbacks will be invoked with the handle object as their first argument.


new (%args)

The constructor supports these arguments (all as key => value pairs).

fh => $filehandle [MANDATORY]

The filehandle this AnyEvent::Handle object will operate on.

NOTE: The filehandle will be set to non-blocking (using AnyEvent::Util::fh_nonblocking).

on_eof => $cb->($self)

Set the callback to be called on EOF.

While not mandatory, it is highly recommended to set an eof callback, otherwise you might end up with a closed socket while you are still waiting for data.

on_error => $cb->($self)

This is the fatal error callback, that is called when, well, a fatal error occurs, such as not being able to resolve the hostname, failure to connect or a read error.

The object will not be in a usable state when this callback has been called.

On callback entrance, the value of $! contains the operating system error (or ENOSPC, EPIPE or EBADMSG).

While not mandatory, it is highly recommended to set this callback, as you will not be notified of errors otherwise. The default simply calls die.

on_read => $cb->($self)

This sets the default read callback, which is called when data arrives and no read request is in the queue.

To access (and remove data from) the read buffer, use the ->rbuf method or access the $self-{rbuf}> member directly.

When an EOF condition is detected then AnyEvent::Handle will first try to feed all the remaining data to the queued callbacks and on_read before calling the on_eof callback. If no progress can be made, then a fatal error will be raised (with $! set to EPIPE).

on_drain => $cb->()

This sets the callback that is called when the write buffer becomes empty (or when the callback is set and the buffer is empty already).

To append to the write buffer, use the ->push_write method.

rbuf_max => <bytes>

If defined, then a fatal error will be raised (with $! set to ENOSPC) when the read buffer ever (strictly) exceeds this size. This is useful to avoid denial-of-service attacks.

For example, a server accepting connections from untrusted sources should be configured to accept only so-and-so much data that it cannot act on (for example, when expecting a line, an attacker could send an unlimited amount of data without a callback ever being called as long as the line isn't finished).

read_size => <bytes>

The default read block size (the amount of bytes this module will try to read on each [loop iteration). Default: 4096.

low_water_mark => <bytes>

Sets the amount of bytes (default: 0) that make up an "empty" write buffer: If the write reaches this size or gets even samller it is considered empty.

tls => "accept" | "connect" | Net::SSLeay::SSL object

When this parameter is given, it enables TLS (SSL) mode, that means it will start making tls handshake and will transparently encrypt/decrypt data.

TLS mode requires Net::SSLeay to be installed (it will be loaded automatically when you try to create a TLS handle).

For the TLS server side, use accept, and for the TLS client side of a connection, use connect mode.

You can also provide your own TLS connection object, but you have to make sure that you call either Net::SSLeay::set_connect_state or Net::SSLeay::set_accept_state on it before you pass it to AnyEvent::Handle.

See the starttls method if you need to start TLs negotiation later.

tls_ctx => $ssl_ctx

Use the given Net::SSLeay::CTX object to create the new TLS connection (unless a connection object was specified directly). If this parameter is missing, then AnyEvent::Handle will use AnyEvent::Handle::TLS_CTX.

$fh = $handle->fh

This method returns the file handle of the AnyEvent::Handle object.

$handle->on_error ($cb)

Replace the current on_error callback (see the on_error constructor argument).

$handle->on_eof ($cb)

Replace the current on_eof callback (see the on_eof constructor argument).


AnyEvent::Handle manages two queues per handle, one for writing and one for reading.

The write queue is very simple: you can add data to its end, and AnyEvent::Handle will automatically try to get rid of it for you.

When data could be written and the write buffer is shorter then the low water mark, the on_drain callback will be invoked.

$handle->on_drain ($cb)

Sets the on_drain callback or clears it (see the description of on_drain in the constructor).

$handle->push_write ($data)

Queues the given scalar to be written. You can push as much data as you want (only limited by the available memory), as AnyEvent::Handle buffers it independently of the kernel.

$handle->push_write (type => @args)
$handle->unshift_write (type => @args)

Instead of formatting your data yourself, you can also let this module do the job by specifying a type and type-specific arguments.

Predefined types are (if you have ideas for additional types, feel free to drop by and tell us):

netstring => $string

Formats the given value as netstring (http://cr.yp.to/proto/netstrings.txt, this is not a recommendation to use them).

AnyEvent::Handle::register_write_type type => $coderef->($self, @args)

This function (not method) lets you add your own types to push_write. Whenever the given type is used, push_write will invoke the code reference with the handle object and the remaining arguments.

The code reference is supposed to return a single octet string that will be appended to the write buffer.

Note that this is a function, and all types registered this way will be global, so try to use unique names.


AnyEvent::Handle manages two queues per handle, one for writing and one for reading.

The read queue is more complex than the write queue. It can be used in two ways, the "simple" way, using only on_read and the "complex" way, using a queue.

In the simple case, you just install an on_read callback and whenever new data arrives, it will be called. You can then remove some data (if enough is there) from the read buffer ($handle->rbuf) if you want or not.

In the more complex case, you want to queue multiple callbacks. In this case, AnyEvent::Handle will call the first queued callback each time new data arrives and removes it when it has done its job (see push_read, below).

This way you can, for example, push three line-reads, followed by reading a chunk of data, and AnyEvent::Handle will execute them in order.

Example 1: EPP protocol parser. EPP sends 4 byte length info, followed by the specified number of bytes which give an XML datagram.

   # in the default state, expect some header bytes
   $handle->on_read (sub {
      # some data is here, now queue the length-header-read (4 octets)
      shift->unshift_read_chunk (4, sub {
         # header arrived, decode
         my $len = unpack "N", $_[1];

         # now read the payload
         shift->unshift_read_chunk ($len, sub {
            my $xml = $_[1];
            # handle xml

Example 2: Implement a client for a protocol that replies either with "OK" and another line or "ERROR" for one request, and 64 bytes for the second request. Due tot he availability of a full queue, we can just pipeline sending both requests and manipulate the queue as necessary in the callbacks:

   # request one
   $handle->push_write ("request 1\015\012");

   # we expect "ERROR" or "OK" as response, so push a line read
   $handle->push_read_line (sub {
      # if we got an "OK", we have to _prepend_ another line,
      # so it will be read before the second request reads its 64 bytes
      # which are already in the queue when this callback is called
      # we don't do this in case we got an error
      if ($_[1] eq "OK") {
         $_[0]->unshift_read_line (sub {
            my $response = $_[1];

   # request two
   $handle->push_write ("request 2\015\012");

   # simply read 64 bytes, always
   $handle->push_read_chunk (64, sub {
      my $response = $_[1];
$handle->on_read ($cb)

This replaces the currently set on_read callback, or clears it (when the new callback is undef). See the description of on_read in the constructor.


Returns the read buffer (as a modifiable lvalue).

You can access the read buffer directly as the ->{rbuf} member, if you want.

NOTE: The read buffer should only be used or modified if the on_read, push_read or unshift_read methods are used. The other read methods automatically manage the read buffer.

$handle->push_read ($cb)
$handle->unshift_read ($cb)

Append the given callback to the end of the queue (push_read) or prepend it (unshift_read).

The callback is called each time some additional read data arrives.

It must check whether enough data is in the read buffer already.

If not enough data is available, it must return the empty list or a false value, in which case it will be called repeatedly until enough data is available (or an error condition is detected).

If enough data was available, then the callback must remove all data it is interested in (which can be none at all) and return a true value. After returning true, it will be removed from the queue.

$handle->push_read (type => @args, $cb)
$handle->unshift_read (type => @args, $cb)

Instead of providing a callback that parses the data itself you can chose between a number of predefined parsing formats, for chunks of data, lines etc.

Predefined types are (if you have ideas for additional types, feel free to drop by and tell us):

chunk => $octets, $cb->($self, $data)

Invoke the callback only once $octets bytes have been read. Pass the data read to the callback. The callback will never be called with less data.

Example: read 2 bytes.

   $handle->push_read (chunk => 2, sub {
      warn "yay ", unpack "H*", $_[1];
line => [$eol, ]$cb->($self, $line, $eol)

The callback will be called only once a full line (including the end of line marker, $eol) has been read. This line (excluding the end of line marker) will be passed to the callback as second argument ($line), and the end of line marker as the third argument ($eol).

The end of line marker, $eol, can be either a string, in which case it will be interpreted as a fixed record end marker, or it can be a regex object (e.g. created by qr), in which case it is interpreted as a regular expression.

The end of line marker argument $eol is optional, if it is missing (NOT undef), then qr|\015?\012| is used (which is good for most internet protocols).

Partial lines at the end of the stream will never be returned, as they are not marked by the end of line marker.

netstring => $cb->($string)

A netstring (http://cr.yp.to/proto/netstrings.txt, this is not an endorsement).

Throws an error with $! set to EBADMSG on format violations.

AnyEvent::Handle::register_read_type type => $coderef->($self, $cb, @args)

This function (not method) lets you add your own types to push_read.

Whenever the given type is used, push_read will invoke the code reference with the handle object, the callback and the remaining arguments.

The code reference is supposed to return a callback (usually a closure) that works as a plain read callback (see ->push_read ($cb)).

It should invoke the passed callback when it is done reading (remember to pass $self as first argument as all other callbacks do that).

Note that this is a function, and all types registered this way will be global, so try to use unique names.

For examples, see the source of this module (perldoc -m AnyEvent::Handle, search for register_read_type)).


In rare cases you actually do not want to read anything from the socket. In this case you can call stop_read. Neither on_read no any queued callbacks will be executed then. To start reading again, call start_read.

$handle->starttls ($tls[, $tls_ctx])

Instead of starting TLS negotiation immediately when the AnyEvent::Handle object is created, you can also do that at a later time by calling starttls.

The first argument is the same as the tls constructor argument (either "connect", "accept" or an existing Net::SSLeay object).

The second argument is the optional Net::SSLeay::CTX object that is used when AnyEvent::Handle has to create its own TLS connection object.


Destroys the SSL connection, if any. Partial read or write data will be lost.


This function creates and returns the Net::SSLeay::CTX object used by default for TLS mode.

The context is created like this:


   my $CTX = Net::SSLeay::CTX_new;

   Net::SSLeay::CTX_set_options $CTX, Net::SSLeay::OP_ALL


Robin Redeker <elmex at ta-sa.org>, Marc Lehmann <schmorp@schmorp.de>.