# gen_udp

###
Module

gen_udp

###
Module Summary

Interface to UDP sockets.

###
Description

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

Note

Functions that create sockets can take an optional option; {inet_backend, Backend} that, if specified, has to be the first option. This selects the implementation backend towards the platform's socket API.

This is a temporary option that will be ignored in a future release.

The default is Backend = inet that selects the traditional inet_drv.c driver. The other choice is Backend = socket that selects the new socket module and its NIF implementation.

The system default can be changed when the node is started with the application kernel's configuration variable inet_backend.

For gen_udp with inet_backend = socket we have tried to be as "compatible" as possible which has sometimes been impossible. Here is a list of cases when the behaviour of inet-backend inet (default) and socket are different:

- The option read_packets is currently ignored.

- Windows require sockets (domain = inet | inet6) to be bound.

  Currently all sockets created on Windows with inet_backend = socket will be bound. If the user does not provide an address, gen_udp will try to 'figure out' an address itself.

###
Data Types

####
option() =
{active, true | false | once | -32768..32767} |
{add_membership, membership()} |
{broadcast, boolean()} |
{buffer, integer() >= 0} |
{debug, boolean()} |
{deliver, port | term} |
{dontroute, boolean()} |
{drop_membership, membership()} |
{exclusiveaddruse, boolean()} |
{header, integer() >= 0} |
{high_msgq_watermark, integer() >= 1} |
{low_msgq_watermark, integer() >= 1} |
{mode, list | binary} |
list | binary |
{multicast_if, multicast_if()} |
{multicast_loop, boolean()} |
{multicast_ttl, integer() >= 0} |
{priority, integer() >= 0} |
{raw,
Protocol :: integer() >= 0,
OptionNum :: integer() >= 0,
ValueBin :: binary()} |
{read_packets, integer() >= 0} |
{recbuf, integer() >= 0} |
{reuseaddr, boolean()} |
{reuseport, boolean()} |
{reuseport_lb, boolean()} |
{sndbuf, integer() >= 0} |
{tos, integer() >= 0} |
{tclass, integer() >= 0} |
{ttl, integer() >= 0} |
{recvtos, boolean()} |
{recvtclass, boolean()} |
{recvttl, boolean()} |
{ipv6_v6only, boolean()}

####
option_name() =
active | broadcast | buffer | debug | deliver | dontroute |
exclusiveaddruse | header | high_msgq_watermark |
low_msgq_watermark | mode | multicast_if | multicast_loop |
multicast_ttl | priority |
{raw,
Protocol :: integer() >= 0,
OptionNum :: integer() >= 0,
ValueSpec ::
(ValueSize :: integer() >= 0) | (ValueBin :: binary())} |
read_packets | recbuf | reuseaddr | reuseport | reuseport_lb |
sndbuf | tos | tclass | ttl | recvtos | recvtclass | recvttl |
pktoptions | ipv6_v6only

####
open_option() =
{ip, inet:socket_address()} |
{fd, integer() >= 0} |
{ifaddr,
socket:sockaddr_in() |
socket:sockaddr_in6() |
inet:socket_address()} |
inet:address_family() |
{port, inet:port_number()} |
{netns, file:filename_all()} |
{bind_to_device, binary()} |
option()

####
socket()

As returned by open/1,2.

####
multicast_if() = ip_multicast_if() | ip6_multicast_if()

####
ip_multicast_if() = inet:ip4_address()

####
ip6_multicast_if() = integer()

For IPv6 this is an interface index (an integer).

####
membership() = ip_membership() | ip6_membership()

####
ip_membership() =
{MultiAddress :: inet:ip4_address(),
Interface :: inet:ip4_address()} |
{MultiAddress :: inet:ip4_address(),
Address :: inet:ip4_address(),
IfIndex :: integer()}

The tuple with size 3 is *not* supported on all platforms. 'ifindex' defaults to zero (0) on platforms that supports the 3-tuple variant.

####
ip6_membership() =
{MultiAddress :: inet:ip6_address(), IfIndex :: integer()}

###
Exports

####
close(Socket) -> ok

### Types

Socket = socket()

Closes a UDP socket.

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

### Types

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

Assigns a new controlling process Pid to Socket. The controlling process is the process that receives messages from the socket. If called by any other process than the current controlling process, {error, not_owner} is returned. If the process identified by Pid is not an existing local pid, {error, badarg} is returned. {error, badarg} may also be returned in some cases when Socket is closed during the execution of this function.

####
connect(Socket, SockAddr) -> ok | {error, Reason}
OTP 24.3

### Types

Socket = socket()
SockAddr = socket:sockaddr_in() | socket:sockaddr_in6()
Reason = inet:posix()

Connecting a UDP socket only means storing the specified (destination) socket address, as specified by SockAddr, so that the system knows where to send data.

This means that it is not necessary to specify the destination address when sending a datagram. That is, we can use send/2.

It also means that the socket will only received data from this address.

####
connect(Socket, Address, Port) -> ok | {error, Reason}
OTP 24.3

### Types

Socket = socket()
Address = inet:socket_address() | inet:hostname()
Port = inet:port_number()
Reason = inet:posix()

Connecting a UDP socket only means storing the specified (destination) socket address, as specified by Address and Port, so that the system knows where to send data.

This means that it is not necessary to specify the destination address when sending a datagram. That is, we can use send/2.

It also means that the socket will only received data from this address.

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

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

### Types

Port = inet:port_number()
Opts = [inet:inet_backend() | open_option()]
Socket = socket()
Reason = system_limit | inet:posix()

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

The following options are available:

list

  Received Packet is delivered as a list.

binary

  Received Packet is delivered as a binary.

{ip, Address}

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

{ifaddr, Address}

  Same as {ip, Address}. If the host has many network interfaces, this option specifies which one to use.

  However, if this instead is an socket:sockaddr_in() or socket:sockaddr_in6() this takes precedence over any value previously set with the ip options. If the ip option comes after the ifaddr option, it may be used to update its corresponding field of the ifaddr option (the addr field).

{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 is bound to the specified interface and port after it is being opened. If these options are not specified, it is assumed that the fd is already bound appropriately.

inet6

  Sets up the socket for IPv6.

inet

  Sets up the socket for IPv4.

local

  Sets up a Unix Domain Socket. See inet:local_address()

{udp_module, module()}

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

{multicast_if, Address}

  Sets the local device for a multicast socket.

{multicast_loop, true | false}

  When true, sent multicast packets are looped back to the local sockets.

{multicast_ttl, Integer}

  Option multicast_ttl changes the time-to-live (TTL) for outgoing multicast datagrams to control the scope of the multicasts.

  Datagrams with a TTL of 1 are not forwarded beyond the local network. Defaults to 1.

{add_membership, {MultiAddress, InterfaceAddress}}

  Joins a multicast group.

{drop_membership, {MultiAddress, InterfaceAddress}}

  Leaves a multicast group.

Opt

  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} % Without ancillary data
    {udp, Socket, IP, InPortNo, AncData, Packet} % With ancillary data

The message contains an AncData field if any of the socket options recvtos, recvtclass or recvttl are active, otherwise it does not.

If the socket is not in an active mode, data can be retrieved through the recv/2,3 calls. Notice that arriving UDP packets that are longer than the receive buffer option specifies can 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 comes. Packet is a list of bytes if option list is specified. Packet is a binary if option binary is 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.

####
recv(Socket, Length) -> {ok, RecvData} | {error, Reason}

####
recv(Socket, Length, Timeout) -> {ok, RecvData} | {error, Reason}

### Types

Socket = socket()
Length = integer() >= 0
Timeout = timeout()
RecvData =
{Address, Port, Packet} | {Address, Port, AncData, Packet}
Address = inet:ip_address() | inet:returned_non_ip_address()
Port = inet:port_number()
AncData = inet:ancillary_data()
Packet = string() | binary()
Reason = not_owner | timeout | inet:posix()

Receives a packet from a socket in passive mode. Optional parameter Timeout specifies a time-out in milliseconds. Defaults to infinity.

If any of the socket options recvtos, recvtclass or recvttl are active, the RecvData tuple contains an AncData field, otherwise it does not.

####
send(Socket, Packet) -> ok | {error, Reason}
OTP 24.3

### Types

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

Sends a packet on a connected socket (see connect/2 and connect/3).

####
send(Socket, Destination, Packet) -> ok | {error, Reason}
OTP 22.1

### Types

Socket = socket()
Destination =
{inet:ip_address(), inet:port_number()} |
inet:family_address() |
socket:sockaddr_in() |
socket:sockaddr_in6()
Packet = iodata()
Reason = not_owner | inet:posix()

Sends a packet to the specified Destination.

This function is equivalent to send(Socket, Destination, [], Packet).

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

### Types

Socket = socket()
Host = inet:hostname() | inet:ip_address()
Port = inet:port_number() | atom()
Packet = iodata()
Reason = not_owner | inet:posix()

Sends a packet to the specified Host and Port.

This clause is equivalent to send(Socket, Host, Port, [], Packet).

####
send(Socket, Destination, AncData, Packet) -> ok | {error, Reason}
OTP 22.1

### Types

Socket = socket()
Destination =
{inet:ip_address(), inet:port_number()} |
inet:family_address() |
socket:sockaddr_in() |
socket:sockaddr_in6()
AncData = inet:ancillary_data()
Packet = iodata()
Reason = not_owner | inet:posix()

Sends a packet to the specified Destination with ancillary data AncData.

Note

The ancillary data AncData contains options that for this single message override the default options for the socket, an operation that may not be supported on all platforms, and if so return {error, einval}. Using more than one of an ancillary data item type may also not be supported. AncData =:= [] is always supported.

####
send(Socket, Destination, PortZero, Packet) ->
ok | {error, Reason}
OTP 22.1

### Types

Socket = socket()
Destination =
{inet:ip_address(), inet:port_number()} |
inet:family_address()
PortZero = inet:port_number()
Packet = iodata()
Reason = not_owner | inet:posix()

Sends a packet to the specified Destination. Since Destination is complete, PortZero is redundant and has to be 0.

This is a legacy clause mostly for Destination = {local, Binary} where PortZero is superfluous. It is equivalent to send(Socket, Destination, [], Packet), the clause right above here.

####
send(Socket, Host, Port, AncData, Packet) -> ok | {error, Reason}
OTP 22.1

### Types

Socket = socket()
Host =
inet:hostname() | inet:ip_address() | inet:local_address()
Port = inet:port_number() | atom()
AncData = inet:ancillary_data()
Packet = iodata()
Reason = not_owner | inet:posix()

Sends a packet to the specified Host and Port, with ancillary data AncData.

Argument Host can be a hostname or a socket address, and Port can be a port number or a service name atom. These are resolved into a Destination and after that this function is equivalent to send(Socket, Destination, AncData, Packet), read there about ancillary data.