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  <header>
    <copyright>
      <year>1997</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>gen_udp</title>
    <prepared>tony@erix.ericsson.se</prepared>
    <docno></docno>
    <date>1997-12-03</date>
    <rev>A</rev>
  </header>
  <module since="">gen_udp</module>
  <modulesummary>Interface to UDP sockets.</modulesummary>
  <description>
    <p>This module provides functions for communicating
      with sockets using the UDP protocol.</p>

    <note>
      <p>
        Functions that create sockets can take an optional option;
        <c>{inet_backend,&nbsp;Backend}</c> that, if specified,
        has to be the first option.  This selects
        the implementation backend towards the platform's socket API.
      </p>
      <p>
        This is a <em>temporary</em> option
        that will be ignored in a future release.
      </p>
      <p>
        The default is <c>Backend&nbsp;=&nbsp;inet</c>
        that selects the traditional <c>inet_drv.c</c> driver.
        The other choice is <c>Backend&nbsp;=&nbsp;socket</c>
        that selects the new <seeerl marker="socket"><c>socket</c></seeerl>
        module and its NIF implementation.
      </p>
      <p>
        The system default can be changed when the node is started
        with the application <c>kernel</c>'s configuration variable
        <c>inet_backend</c>.
      </p>
      <p>For <c>gen_udp</c> with <c>inet_backend = socket</c> 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
      <c>inet</c> (default) and <c>socket</c> are different: </p>
      <list type="bulleted">
	<item>
	  <p>
	    The option
	    <seeerl marker="inet#option-read_packets">read_packets</seeerl>
	    is currently <em>ignored</em>.
	  </p>
	</item>
      </list>
    </note>

  </description>

  <datatypes>
    <datatype>
      <name name="option"/>
    </datatype>
    <datatype>
      <name name="option_name"/>
    </datatype>
    <datatype>
      <name name="open_option"/>
    </datatype>
    <datatype>
      <name>socket()</name>
      <desc>
        <p>As returned by
          <seemfa marker="#open/1"><c>open/1,2</c></seemfa>.</p>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="close" arity="1" since=""/>
      <fsummary>Close a UDP socket.</fsummary>
      <desc>
        <p>Closes a UDP socket.</p>
      </desc>
    </func>

    <func>
      <name name="controlling_process" arity="2" since=""/>
      <fsummary>Change controlling process of a socket.</fsummary>
      <desc>
        <p>Assigns a new controlling process <c><anno>Pid</anno></c> to
          <c><anno>Socket</anno></c>. The controlling process is the process
          that receives messages from the socket. If called by any other
          process than the current controlling process,
          <c>{error, not_owner}</c> is returned. If the process identified
          by <c><anno>Pid</anno></c> is not an existing local pid,
          <c>{error, badarg}</c> is returned. <c>{error, badarg}</c> may also
          be returned in some cases when <c><anno>Socket</anno></c> is closed
          during the execution of this function.</p>
      </desc>
    </func>

    <func>
      <name name="connect" arity="2" since="OTP 24.3" anchor="connect-sockaddr"/>
      <fsummary>Connect a UDP port.</fsummary>
      <desc> 
	<p>Connecting a UDP socket only means storing the specified
	(destination) socket address, as specified by
	<c><anno>SockAddr</anno></c>, so that the
	system knows where to send data. </p>

	<p>This means that it is not necessary to specify the destination
	address when sending a datagram. That is, we can use
	<seeerl marker="#send-2"><c>send/2</c></seeerl>. </p>

	<p>It also means that the socket will only received data
	from this address. </p>

	<!--
	<p>Depending on the system implementation of connect and send,
	this could mean that sending becomes much faster. </p>
	-->
      </desc>
    </func>

    <func>
      <name name="connect" arity="3" since="OTP 24.3" anchor="connect-addr-port"/>
      <fsummary>Connect a UDP port.</fsummary>
      <desc>
	<p>Connecting a UDP socket only means storing the specified
	(destination) socket address, as specified by
	<c><anno>Address</anno></c> and <c><anno>Port</anno></c>,
	so that the system knows where to send data. </p>

	<p>This means that it is not necessary to specify the destination
	address when sending a datagram. That is, we can use
	<seeerl marker="#send-2"><c>send/2</c></seeerl>. </p>

	<p>It also means that the socket will only received data
	from this address. </p>

	<!--
	<p>Depending on the system implementation of connect and send,
	this could mean that sending becomes much faster. </p>
	-->
      </desc>
    </func>

    <func>
      <name name="open" arity="1" since=""/>
      <name name="open" arity="2" since=""/>
      <fsummary>Associate a UDP port number with the process calling it.</fsummary>
      <desc>
        <p>Associates a UDP port number (<c><anno>Port</anno></c>) with the
          calling process.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>list</c></tag>
	  <item>
	    <p>Received <c>Packet</c> is delivered as a list.</p>
	  </item>
          <tag><c>binary</c></tag>
	  <item>
	    <p>Received <c>Packet</c> is delivered as a binary.</p>
	  </item>
          <tag><c>{ip, Address}</c></tag>
	  <item>
	    <p>If the host has many network interfaces, this option
	    specifies which one to use.</p>
	  </item>
          <tag><c>{ifaddr, Address}</c></tag>
	  <item>
	    <p>Same as <c>{ip, Address}</c>. If the host has many
            network interfaces, this option specifies which one to
	    use.</p>
	    <p>However, if this instead is an
	    <seetype marker="socket#sockaddr_in">
	    <c>socket:sockaddr_in()</c></seetype>
	    or
	    <seetype marker="socket#sockaddr_in6">
	    <c>socket:sockaddr_in6()</c></seetype>
	    this takes precedence over any value previously set with the
	    <c>ip</c> options.
	    If the <c>ip</c> option comes <em>after</em> the <c>ifaddr</c> option,
	    it may be used to <em>update</em> its corresponding field of the
	    <c>ifaddr</c> option (the <c>addr</c> field). </p>
	  </item>
          <tag><c>{fd, integer() >= 0}</c></tag>
	  <item>
	    <p>If a socket has somehow been opened without using
            <c>gen_udp</c>, use this option to pass the file descriptor
            for it. If <c><anno>Port</anno></c> is not set to <c>0</c> and/or
            <c>{ip, ip_address()}</c> is combined with this option, the
            <c>fd</c> is bound to the specified interface and port after it is
            being opened. If these options are not specified, it is assumed that
	    the <c>fd</c> is already bound appropriately.</p>
	  </item>
          <tag><c>inet6</c></tag>
	  <item>
	    <p>Sets up the socket for IPv6.</p>
	  </item>
          <tag><c>inet</c></tag>
	  <item>
	    <p>Sets up the socket for IPv4.</p>
	  </item>
	  <tag><c>local</c></tag>
	  <item>
	    <p>Sets up a Unix Domain Socket. See
	    <seetype marker="inet#local_address">
	      <c>inet:local_address()</c>
	    </seetype>
	    </p>
          </item>
          <tag><c>{udp_module, module()}</c></tag>
	  <item>
	    <p>Overrides which callback module is used. Defaults to
	    <c>inet_udp</c> for IPv4 and <c>inet6_udp</c> for IPv6.</p>
	  </item>
          <tag><c>{multicast_if, Address}</c></tag>
	  <item>
	    <p>Sets the local device for a multicast socket.</p>
	  </item>
          <tag><c>{multicast_loop, true | false}</c></tag>
	  <item>
	    <p>When <c>true</c>, sent multicast packets are looped back to
	    the local sockets.</p>
	  </item>
          <tag><c>{multicast_ttl, Integer}</c></tag>
	  <item>
	    <p>Option <c>multicast_ttl</c> changes the time-to-live (TTL)
            for outgoing multicast datagrams to control the scope of the
            multicasts.</p>
            <p>Datagrams with a TTL of 1 are not forwarded beyond the local
	    network. Defaults to <c>1</c>.</p>
	  </item>
          <tag><c>{add_membership, {MultiAddress, InterfaceAddress}}</c></tag>
	  <item>
	    <p>Joins a multicast group.</p>
	  </item>
          <tag><c>{drop_membership, {MultiAddress, InterfaceAddress}}</c></tag>
	  <item>
	    <p>Leaves a multicast group.</p>
	  </item>
          <tag><c>Opt</c></tag>
	  <item>
	    <p>See
            <seemfa marker="inet#setopts/2"><c>inet:setopts/2</c></seemfa>.
	    </p>
	  </item>
        </taglist>
        <p>The returned socket <c><anno>Socket</anno></c> is used to send
          packets from this port with
          <seemfa marker="#send/4"><c>send/4</c></seemfa>.
          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:</p>
        <code type="none">
{udp, Socket, IP, InPortNo, Packet} % Without ancillary data
{udp, Socket, IP, InPortNo, AncData, Packet} % With ancillary data
	</code>
	<p>
	  The message contains an <c>AncData</c> field
	  if any of the socket
	  <seetype marker="#option">options</seetype>
	  <seeerl marker="inet#option-recvtos"><c>recvtos</c></seeerl>,
	  <seeerl marker="inet#option-recvtclass"><c>recvtclass</c></seeerl>
	  or
	  <seeerl marker="inet#option-recvttl"><c>recvttl</c></seeerl>
	  are active, otherwise it does not.
	</p>
	<p>
	</p>
        <p>If the socket is not in an active mode, data can be
          retrieved through the
          <seemfa marker="#recv/2"><c>recv/2,3</c></seemfa> calls.
          Notice that arriving UDP packets that are longer than
          the receive buffer option specifies can be truncated
          without warning.</p>
        <p>When a socket in <c>{active, N}</c> mode (see
          <seemfa marker="inet#setopts/2"><c>inet:setopts/2</c></seemfa>
          for details), transitions to passive (<c>{active, false}</c>) mode,
          the controlling process is notified by a message of the following
          form:</p>
        <code type="none">
{udp_passive, Socket}</code>
        <p><c>IP</c> and <c>InPortNo</c> define the address from which
          <c>Packet</c> comes. <c>Packet</c> is a list of bytes if
          option <c>list</c> is specified. <c>Packet</c> is a
          binary if option <c>binary</c> is specified.</p>
        <p>Default value for the receive buffer option is
          <c>{recbuf, 8192}</c>.</p>
        <p>If <c><anno>Port</anno> == 0</c>, the underlying OS assigns a free
          UDP port, use
          <seemfa marker="inet#port/1"><c>inet:port/1</c></seemfa>
          to retrieve it.</p>
      </desc>
    </func>

    <func>
      <name name="recv" arity="2" since=""/>
      <name name="recv" arity="3" since=""/>
      <fsummary>Receive a packet from a passive socket.</fsummary>
      <desc>
        <p>
	  Receives a packet from a socket in passive mode. Optional parameter
          <c><anno>Timeout</anno></c> specifies a time-out in milliseconds.
          Defaults to <c>infinity</c>.
	</p>
	<p>
	  If any of the socket
	  <seetype marker="#option">options</seetype>
	  <seeerl marker="inet#option-recvtos"><c>recvtos</c></seeerl>,
	  <seeerl marker="inet#option-recvtclass"><c>recvtclass</c></seeerl>
	  or
	  <seeerl marker="inet#option-recvttl"><c>recvttl</c></seeerl>
	  are active, the <c><anno>RecvData</anno></c> tuple contains an
	  <c><anno>AncData</anno></c> field,
	  otherwise it does not.
	</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="2" since="OTP 24.3" anchor="send-2"/>
      <fsummary>Send a packet on a connected socket.</fsummary>
      <desc>
        <p>
	  Sends a packet on a connected socket
	  (see
	  <seeerl marker="#connect-sockaddr"><c>connect/2</c></seeerl>
	  and
	  <seeerl marker="#connect-addr-port"><c>connect/3</c></seeerl>). </p>
      </desc>
    </func>

    <func>
      <name name="send" arity="3" clause_i="1" anchor="send-3" since="OTP 22.1"/>
      <fsummary>Send a packet.</fsummary>
      <desc>
        <p>
	  Sends a packet to the specified <c><anno>Destination</anno></c>.
	</p>
	<p>
	  This function is equivalent to
	  <seeerl marker="#send-4-AncData"><c>send(<anno>Socket</anno>, <anno>Destination</anno>, [], <anno>Packet</anno>)</c></seeerl>.
	</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="4" clause_i="1" anchor="send-4-1" since=""/>
      <fsummary>Send a packet.</fsummary>
      <desc>
        <p>
	  Sends a packet to the specified <c><anno>Host</anno></c>
	  and <c><anno>Port</anno></c>.
	</p>
	<p>
	  This clause is equivalent to
	  <seemfa marker="#send/5"><c>send(<anno>Socket</anno>, <anno>Host</anno>, <anno>Port</anno>, [], <anno>Packet</anno>)</c></seemfa>.
	</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="4" clause_i="2" anchor="send-4-AncData" since="OTP 22.1"/>
      <fsummary>Send a packet.</fsummary>
      <desc>
        <p>
	  Sends a packet to the specified <c><anno>Destination</anno></c>
	  with ancillary data <c><anno>AncData</anno></c>.
	</p>
	<note>
	  <p>
	    The ancillary data <c><anno>AncData</anno></c>
	    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 <c>{error, einval}</c>.
	    Using more than one of an ancillary data item type
	    may also not be supported.
	    <c><anno>AncData</anno> =:= []</c> is always supported.
	  </p>
	</note>
      </desc>
    </func>

    <func>
      <name name="send" arity="4" clause_i="3" anchor="send-4-3" since="OTP 22.1"/>
      <fsummary>Send a packet.</fsummary>
      <desc>
        <p>
	  Sends a packet to the specified <c><anno>Destination</anno></c>.
	  Since <c><anno>Destination</anno></c> is complete,
	  <c><anno>PortZero</anno></c> is redundant and has to be <c>0</c>.
	</p>
	<p>
	  This is a legacy clause mostly for
	  <c><anno>Destination</anno> = {local, Binary}</c>
	  where <c><anno>PortZero</anno></c> is superfluous.
	  It is equivalent to
	  <seeerl marker="#send-4-AncData"><c>send(<anno>Socket</anno>, <anno>Destination</anno>, [], <anno>Packet</anno>)</c></seeerl>, the clause right above here.
	</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="5" since="OTP 22.1"/>
      <fsummary>Send a packet.</fsummary>
      <desc>
        <p>
	  Sends a packet to the specified <c><anno>Host</anno></c>
	  and <c><anno>Port</anno></c>,
	  with ancillary data <c><anno>AncData</anno></c>.
	</p>
	<p>
	  Argument <c><anno>Host</anno></c> can be
	  a hostname or a socket address,
	  and <c><anno>Port</anno></c> can be a port number
	  or a service name atom.
	  These are resolved into a <c>Destination</c> and after that
	  this function is equivalent to
	  <seeerl marker="#send-4-AncData"><c>send(<anno>Socket</anno>, Destination, <anno>AncData</anno>, <anno>Packet</anno>)</c></seeerl>, read there about ancillary data.
	</p>
      </desc>
    </func>
  </funcs>
</erlref>
