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Version 4.2

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Interface to UDP sockets


The gen_udp module provides functions for communicating with sockets using the UDP protocol.


option() =
    {active, true | false | once | -32768..32767} |
    {add_membership, {inet:ip_address(), inet:ip_address()}} |
    {broadcast, boolean()} |
    {buffer, integer() >= 0} |
    {deliver, port | term} |
    {dontroute, boolean()} |
    {drop_membership, {inet:ip_address(), inet:ip_address()}} |
    {header, integer() >= 0} |
    {high_msgq_watermark, integer() >= 1} |
    {low_msgq_watermark, integer() >= 1} |
    {mode, list | binary} |
    list |
    binary |
    {multicast_if, inet:ip_address()} |
    {multicast_loop, boolean()} |
    {multicast_ttl, integer() >= 0} |
    {priority, integer() >= 0} |
     Protocol :: integer() >= 0,
     OptionNum :: integer() >= 0,
     ValueBin :: binary()} |
    {read_packets, integer() >= 0} |
    {recbuf, integer() >= 0} |
    {reuseaddr, boolean()} |
    {sndbuf, integer() >= 0} |
    {tos, integer() >= 0} |
    {ipv6_v6only, boolean()}

option_name() =
    active |
    broadcast |
    buffer |
    deliver |
    dontroute |
    header |
    high_msgq_watermark |
    low_msgq_watermark |
    mode |
    multicast_if |
    multicast_loop |
    multicast_ttl |
    priority |
     Protocol :: integer() >= 0,
     OptionNum :: integer() >= 0,
     ValueSpec ::
         (ValueSize :: integer() >= 0) | (ValueBin :: binary())} |
    read_packets |
    recbuf |
    reuseaddr |
    sndbuf |
    tos |


As returned by open/1,2.


open(Port) -> {ok, Socket} | {error, Reason}
open(Port, Opts) -> {ok, Socket} | {error, Reason}


Opts = [Option]
Option =
    {ip, inet:ip_address()} |
    {fd, integer() >= 0} |
    {ifaddr, inet:ip_address()} |
    inet:address_family() |
    {port, inet:port_number()} |
Socket = socket()
Reason = inet:posix()

Associates a UDP port number (Port) with the calling process.

The available options are:


Received Packet is delivered as a list.


Received Packet is delivered as a binary.

{ip, ip_address()}

If the host has several network interfaces, this option specifies which one to use.

{ifaddr, ip_address()}

Same as {ip, ip_address()}. If the host has several network interfaces, this option specifies which one to use.

{fd, integer() >= 0}

If a socket has somehow been opened without using gen_udp, use this option to pass the file descriptor for it. If Port is not set to 0 and/or {ip, ip_address()} is combined with this option the fd will be bound to the given interface and port after being opened. If these options are not given it is assumed that the fd is already bound appropriately.


Set up the socket for IPv6.


Set up the socket for IPv4.

{udp_module, module()}

Override which callback module is used. Defaults to inet_udp for IPv4 and inet6_udp for IPv6.

{multicast_if, Address}

Set the local device for a multicast socket.

{multicast_loop, true | false}

When true sent multicast packets will be looped back to the local sockets.

{multicast_ttl, Integer}

The multicast_ttl option changes the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts.

Datagrams with a TTL of 1 are not forwarded beyond the local network.
Default: 1

{add_membership, {MultiAddress, InterfaceAddress}}

Join a multicast group.

{drop_membership, {MultiAddress, InterfaceAddress}}

Leave multicast group.


See inet:setopts/2.

The returned socket Socket is used to send packets from this port with send/4. When UDP packets arrive at the opened port, if the socket is in an active mode the packets are delivered as messages to the controlling process:

{udp, Socket, IP, InPortNo, Packet}

If the socket is not in an active mode, data can be retrieved via the recv/2,3 calls. Note that arriving UDP packets that are longer than the receive buffer option specifies, might be truncated without warning.

When a socket in {active, N} mode (see inet:setopts/2 for details) transitions to passive ({active, false}) mode, the controlling process is notified by a message of the following form:

{udp_passive, Socket}

IP and InPortNo define the address from which Packet came. Packet is a list of bytes if the option list was specified. Packet is a binary if the option binary was specified.

Default value for the receive buffer option is {recbuf, 8192}.

If Port == 0, the underlying OS assigns a free UDP port, use inet:port/1 to retrieve it.

send(Socket, Address, Port, Packet) -> ok | {error, Reason}


Socket = socket()
Packet = iodata()
Reason = not_owner | inet:posix()

Sends a packet to the specified address and port. The Address argument can be either a hostname, or an IP address.

recv(Socket, Length) ->
        {ok, {Address, Port, Packet}} | {error, Reason}

recv(Socket, Length, Timeout) ->
        {ok, {Address, Port, Packet}} | {error, Reason}


Socket = socket()
Length = integer() >= 0
Timeout = timeout()
Packet = string() | binary()
Reason = not_owner | inet:posix()

This function receives a packet from a socket in passive mode.

The optional Timeout parameter specifies a timeout in milliseconds. The default value is infinity.

controlling_process(Socket, Pid) -> ok | {error, Reason}


Socket = socket()
Pid = pid()
Reason = closed | not_owner | inet:posix()

Assigns a new controlling process Pid to Socket. The controlling process is the process which receives messages from the socket. If called by any other process than the current controlling process, {error, not_owner} is returned.

close(Socket) -> ok


Socket = socket()

Closes a UDP socket.