IO::Async::Socket - event callbacks and send buffering for a socket filehandle


   use IO::Async::Socket;

   use IO::Async::Loop;
   my $loop = IO::Async::Loop->new;

   my $socket = IO::Async::Socket->new(
      on_recv => sub {
         my ( $self, $dgram, $addr ) = @_;

         print "Received reply: $dgram\n",
      on_recv_error => sub {
         my ( $self, $errno ) = @_;
         die "Cannot recv - $errno\n";
   $loop->add( $socket );

      host     => "",
      service  => "echo",
      socktype => 'dgram',

   $socket->send( "A TEST DATAGRAM" );



This subclass of IO::Async::Handle contains a socket filehandle. It provides a queue of outgoing data. It invokes the on_recv handler when new data is received from the filehandle. Data may be sent to the filehandle by calling the send method.

It is primarily intended for SOCK_DGRAM or SOCK_RAW sockets (such as UDP or packet-capture); for SOCK_STREAM sockets (such as TCP) an instance of IO::Async::Stream is more appropriate.


The following events are invoked, either using subclass methods or CODE references in parameters:

on_recv $data, $addr

Invoke on receipt of a packet, datagram, or stream segment.

The on_recv handler is invoked once for each packet, datagram, or stream segment that is received. It is passed the data itself, and the sender's address.

on_recv_error $errno

Optional. Invoked when the recv method on the receiving handle fails.

on_send_error $errno

Optional. Invoked when the send method on the sending handle fails.

The on_recv_error and on_send_error handlers are passed the value of $! at the time the error occurred. (The $! variable itself, by its nature, may have changed from the original error by the time this handler runs so it should always use the value passed in).

If an error occurs when the corresponding error callback is not supplied, and there is not a subclass method for it, then the close method is called instead.


Optional. Invoked when the sending data buffer becomes empty.


The following named parameters may be passed to new or configure:

read_handle => IO

The IO handle to receive from. Must implement fileno and recv methods.

write_handle => IO

The IO handle to send to. Must implement fileno and send methods.

handle => IO

Shortcut to specifying the same IO handle for both of the above.

on_recv => CODE

on_recv_error => CODE

on_outgoing_empty => CODE

on_send_error => CODE

autoflush => BOOL

Optional. If true, the send method will atempt to send data to the operating system immediately, without waiting for the loop to indicate the filehandle is write-ready.

recv_len => INT

Optional. Sets the buffer size for recv calls. Defaults to 64 KiB.

recv_all => BOOL

Optional. If true, repeatedly call recv when the receiving handle first becomes read-ready. By default this is turned off, meaning at most one fixed-size buffer is received. If there is still more data in the kernel's buffer, the handle will stil be readable, and will be received from again.

This behaviour allows multiple streams and sockets to be multiplexed simultaneously, meaning that a large bulk transfer on one cannot starve other filehandles of processing time. Turning this option on may improve bulk data transfer rate, at the risk of delaying or stalling processing on other filehandles.

send_all => INT

Optional. Analogous to the recv_all option, but for sending. When autoflush is enabled, this option only affects deferred sending if the initial attempt failed.

The condition requiring an on_recv handler is checked at the time the object is added to a Loop; it is allowed to create a IO::Async::Socket object with a read handle but without a on_recv handler, provided that one is later given using configure before the stream is added to its containing Loop, either directly or by being a child of another Notifier already in a Loop, or added to one.



   $socket->send( $data, $flags, $addr )

This method adds a segment of data to be sent, or sends it immediately, according to the autoflush parameter. $flags and $addr are optional.

If the autoflush option is set, this method will try immediately to send the data to the underlying filehandle, optionally using the given flags and destination address. If this completes successfully then it will have been sent by the time this method returns. If it fails to send, then the data is queued as if autoflush were not set, and will be flushed as normal.


Send-first on a UDP Socket

UDP is carried by the SOCK_DGRAM socket type, for which the string 'dgram' is a convenient shortcut:

      host     => $hostname,
      service  => $service,
      socktype => 'dgram',

Receive-first on a UDP Socket

A typical server pattern with UDP involves binding a well-known port number instead of connecting to one, and waiting on incoming packets.

      service  => 12345,
      socktype => 'dgram',


  • IO::Handle - Supply object methods for I/O handles


Paul Evans <>