POE::Wheel::SocketFactory(3) User Contributed Perl Documentation POE::Wheel::SocketFactory(3)

POE::Wheel::SocketFactory - non-blocking socket creation

See "SYNOPSIS" in POE::Component::Server::TCP for a much simpler version of this program.

use warnings;
use strict;
use IO::Socket;
use POE qw(Wheel::SocketFactory Wheel::ReadWrite);
  inline_states => {
    _start => sub {
      # Start the server.
      $_[HEAP]{server} = POE::Wheel::SocketFactory->new(
        BindPort => 12345,
        SuccessEvent => "on_client_accept",
        FailureEvent => "on_server_error",
    on_client_accept => sub {
      # Begin interacting with the client.
      my $client_socket = $_[ARG0];
      my $io_wheel = POE::Wheel::ReadWrite->new(
        Handle => $client_socket,
        InputEvent => "on_client_input",
        ErrorEvent => "on_client_error",
      $_[HEAP]{client}{ $io_wheel->ID() } = $io_wheel;
    on_server_error => sub {
      # Shut down server.
      my ($operation, $errnum, $errstr) = @_[ARG0, ARG1, ARG2];
      warn "Server $operation error $errnum: $errstr\n";
      delete $_[HEAP]{server};
    on_client_input => sub {
      # Handle client input.
      my ($input, $wheel_id) = @_[ARG0, ARG1];
      $input =~ tr[a-zA-Z][n-za-mN-ZA-M]; # ASCII rot13
    on_client_error => sub {
      # Handle client error, including disconnect.
      my $wheel_id = $_[ARG3];
      delete $_[HEAP]{client}{$wheel_id};

POE::Wheel::SocketFactory creates sockets upon demand. It can create connectionless UDP sockets, but it really shines for client/server work where establishing connections normally would block.

new() creates a new POE::Wheel::SocketFactory object. For sockets which listen() for and accept() connections, the wheel will generate new sockets for each accepted client. Socket factories for one-shot sockets, such as UDP peers or clients established by connect() only emit a single socket and can be destroyed afterwards without ill effects.

new() always returns a POE::Wheel::SocketFactory object even if it fails to establish the socket. This allows the object to be queried after it has sent its session a "FailureEvent".

new() accepts a healthy number of named parameters, each governing some aspect of socket creation.

Creating the Socket

Socket creation is done with Perl's built-in socket() function. The new() parameters beginning with "Socket" determine how socket() will be called.


"SocketDomain" instructs the wheel to create a socket within a particular domain. Supported domains are "AF_UNIX", "AF_INET", "AF_INET6", "PF_UNIX", "PF_INET", and "PF_INET6". If omitted, the socket will be created in the "AF_INET" domain.

POE::Wheel::SocketFactory contains a table of supported domains and the instructions needed to create them. Please send patches to support additional domains, as needed.

Note: "AF_INET6" and "PF_INET6" are supplied by the Socket module included in Perl 5.8.0 or later. Perl versions before 5.8.0 should not attempt to use IPv6 until someone contributes a workaround.

IPv6 support requires a Socket module that implements getaddrinfo() and unpack_sockaddr_in6(). There may be other modules that perform these functions, but most if not all of them have been deprecated with the advent of proper core Socket support for IPv6.


"SocketType" supplies the socket() call with a particular socket type, which may be "SOCK_STREAM" or "SOCK_DGRAM". "SOCK_STREAM" is the default if "SocketType" is not supplied.


"SocketProtocol" sets the socket() call's protocol. Protocols may be specified by number or name. "SocketProtocol" is ignored for UNIX domain sockets.

The protocol defaults to "tcp" for INET domain sockets. There is no default for other socket domains.

Setting Socket Options

POE::Wheel::SocketFactory uses ioctl(), fcntl() and setsockopt() to set socket options after the socket is created. All sockets are set non-blocking, and bound sockets may be made reusable.


When set, the "Reuse" parameter allows a bound port to be reused immediately. "Reuse" is considered enabled if it contains "yes", "on", or a true numeric value. All other values disable port reuse, as does omitting "Reuse" entirely.

For security purposes, a port cannot be reused for a minute or more after a server has released it. This gives clients time to realize the port has been abandoned. Otherwise a malicious service may snatch up the port and spoof the legitimate service.

It's also terribly annoying to wait a minute or more between server invocations, especially during development.

Bind the Socket to an Address and Port

A socket may optionally be bound to a specific interface and port. The "INADDR_ANY" address may be used to bind to a specific port across all interfaces.

Sockets are bound using bind(). POE::Wheel::SocketFactory parameters beginning with "Bind" control how bind() is called.


"BindAddress" sets an address to bind the socket's local endpoint to. "INADDR_ANY" will be used if "BindAddress" is not specified.

"BindAddress" may contain either a string or a packed Internet address (for "INET" domain sockets). The string parameter should be a dotted numeric address or a resolvable host name. Note that the host name will be resolved with a blocking call. If this is not desired, use POE::Component::Client::DNS to perform a non-blocking name resolution.

When used to bind a "UNIX" domain socket, "BindAddress" should contain a path describing the socket's filename. This is required for server sockets and datagram client sockets. "BindAddress" has no default value for UNIX sockets.


"BindPort" is only meaningful for "INET" domain sockets. It contains a port on the "BindAddress" interface where the socket will be bound. It defaults to 0 if omitted, which will cause the bind() call to choose an indeterminate unallocated port.

"BindPort" may be a port number or a name that can be looked up in the system's services (or equivalent) database.

Connectionless Sockets

Connectionless sockets may interact with remote endpoints without needing to listen() for connections or connect() to remote addresses.

This class of sockets is complete after the bind() call.

Connecting the Socket to a Remote Endpoint

A socket may either listen for connections to arrive, initiate connections to a remote endpoint, or be connectionless (such as in the case of UDP sockets).

POE::Wheel::SocketFactory will initiate a client connection when new() is capped with parameters that describe a remote endpoint. In all other cases, the socket will either listen for connections or be connectionless depending on the socket type.

The following parameters describe a socket's remote endpoint. They determine how POE::Wheel::SocketFactory will call Perl's built-in connect() function.


"RemoteAddress" specifies the remote address to which a socket should connect. If present, POE::Wheel::SocketFactory will create a client socket that attempts to collect to the "RemoteAddress". Otherwise, if the protocol warrants it, the wheel will create a listening socket and attempt to accept connections.

As with the bind address, "RemoteAddress" may be a string containing a dotted quad or a resolvable host name. It may also be a packed Internet address, or a UNIX socket path. It will be packed, with or without an accompanying "RemotePort", as necessary for the socket domain.


"RemotePort" is the port to which the socket should connect. It is required for "INET" client sockets, since the remote endpoint must contain both an address and a port.

The remote port may be numeric, or it may be a symbolic name found in /etc/services or the equivalent for your operating system.

Listening for Connections

Streaming sockets that have no remote endpoint are considered to be server sockets. POE::Wheel::SocketFactory will listen() for connections to these sockets, accept() the new clients, and send the application events with the new client sockets.

POE::Wheel::SocketFactory constructor parameters beginning with "Listen" control how the listen() function is called.


"ListenQueue" specifies the length of the socket's listen() queue. It defaults to "SOMAXCONN" if omitted. "ListenQueue" values greater than "SOMAXCONN" will be clipped to "SOMAXCONN". Excessively large "ListenQueue" values are not necessarily portable, and may cause errors in some rare cases.

Emitting Events

POE::Wheel::SocketFactory emits a small number of events depending on what happens during socket setup or while listening for new connections.

See "PUBLIC EVENTS" for more details.


"SuccessEvent" names the event that will be emitted whenever POE::Wheel::SocketFactory succeeds in creating a new socket.

For connectionless sockets, "SuccessEvent" happens just after the socket is created.

For client connections, "SuccessEvent" is fired when the connection has successfully been established with the remote endpoint.

Server sockets emit a "SuccessEvent" for every successfully accepted client.


"FailureEvent" names the event POE::Wheel::SocketFactory will emit whenever something goes wrong. It usually represents some kind of built-in function call error. See "PUBLIC EVENTS" for details, as some errors are handled internally by this wheel.

event() allows a session to change the events emitted by a wheel without destroying and re-creating the wheel. It accepts one or more of the events listed in "PUBLIC EVENTS". Undefined event names disable those events.

event() is described in more depth in POE::Wheel.

getsockname() behaves like the built-in function of the same name. It returns the local endpoint information for POE::Wheel::SocketFactory's encapsulated listening socket.

getsockname() allows applications to determine the address and port to which POE::Wheel::SocketFactory has bound its listening socket.

Test applications may use getsockname() to find the server socket after POE::Wheel::SocketFactory has bound to INADDR_ANY port 0.

Since there is no event fired immediately after a successful creation of a listening socket, applications can use getsockname() to verify this.

use Socket 'unpack_sockaddr_in';
my $listener = POE::Wheel::SocketFactory->new(
    BindPort     => 123,
    SuccessEvent => 'got_client',
    FailureEvent => 'listener_failed',
    Reuse        => 'on',
my ($port, $addr) = unpack_sockaddr_in($listener->getsockname);
print "Socket successfully bound\n" if $port;

ID() returns the wheel's unique ID. The ID will also be included in every event the wheel generates. Applications can match events back to the objects that generated them.

Applications may occasionally need to block incoming connections. pause_accept() pauses the event watcher that triggers accept(). New inbound connections will stack up in the socket's listen() queue until the queue overflows or the application calls resume_accept().

Pausing accept() can limit the amount of load a server generates. It's also useful in pre-forking servers when the master process shouldn't accept connections at all.

pause_accept() and resume_accept() is quicker and more reliable than dynamically destroying and re-creating a POE::Wheel::SocketFactory object.

resume_accept() resumes the watcher that triggers accept(). See "pause_accept" for a more detailed discussion.

POE::Wheel::SocketFactory emits two public events.

"SuccessEvent" names an event that will be sent to the creating session whenever a POE::Wheel::SocketFactory has created a new socket. For connectionless sockets, it's when the socket is created. For connecting clients, it's after the connection has been established. And for listening servers, "SuccessEvent" is fired after each new client is accepted.

Common SuccessEvent Parameters

In all cases, $_[ARG0] holds the new socket's filehandle, and $_[ARG3] contains the POE::Wheel::SocketFactory's ID. Other parameters vary depending on the socket's domain and whether it's listening or connecting. See below for the differences.

INET SuccessEvent Parameters

For INET sockets, $_[ARG1] and $_[ARG2] hold the socket's remote address and port, respectively. The address is packed; see "inet_ntop" in Socket if a human-readable address is needed.

sub handle_new_client {
  my $accepted_socket = $_[ARG0];
  my $peer_host = inet_ntop(
    ((length($_[ARG1]) == 4) ? AF_INET : AF_INET6),
    "Wheel $_[ARG3] accepted a connection from ",
    "$peer_host port $peer_port\n"

UNIX Client SuccessEvent Parameters

For UNIX client sockets, $_[ARG1] often (but not always) holds the server address. Some systems cannot retrieve a UNIX socket's remote address. $_[ARG2] is always undef for UNIX client sockets.

UNIX Server SuccessEvent Parameters

According to Perl Cookbook, the remote address returned by accept() on UNIX sockets is undefined, so $_[ARG1] and $_[ARG2] are also undefined in this case.

"FailureEvent" names the event that will be emitted when a socket error occurs. POE::Wheel::SocketFactory handles "EAGAIN" internally, so it doesn't count as an error.

"FailureEvent" events include the standard error event parameters:

$_[ARG0] describes which part of socket creation failed. It often holds a Perl built-in function name.

$_[ARG1] and $_[ARG2] describe how the operation failed. They contain the numeric and stringified versions of $!, respectively. An application cannot merely check the global $! variable since it may change during event dispatch.

Finally, $_[ARG3] contains the ID for the POE::Wheel::SocketFactory instance that generated the event. See "ID" and "ID" in POE::Wheel for uses for wheel IDs.

A sample FailureEvent handler:

sub handle_failure {
  my ($operation, $errnum, $errstr, $wheel_id) = @_[ARG0..ARG3];
  warn "Wheel $wheel_id generated $operation error $errnum: $errstr\n";
  delete $_[HEAP]{wheels}{$wheel_id}; # shut down that wheel

POE::Wheel describes the basic operations of all wheels in more depth. You need to know this.

Socket::GetAddrInfo is required for IPv6 work. POE::Wheel::SocketFactory will load it automatically if it's installed. SocketDomain => AF_INET6 is required to trigger IPv6 behaviors. AF_INET6 is exported by the Socket module on all but the oldest versions of Perl 5. If your Socket doesn't provide AF_INET6, try installing Socket6 instead.

The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.

Many (if not all) of the croak/carp/warn/die statements should fire back "FailureEvent" instead.

SocketFactory is only tested with UNIX streams and INET sockets using the UDP and TCP protocols. Others should work after the module's internal configuration tables are updated. Please send patches.

Please see POE for more information about authors and contributors.

2024-07-13 perl v5.38.2