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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1997</year><year>2020</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>inet</title>
    <prepared>bjorn@erix.ericsson.se</prepared>
    <docno></docno>
    <date>1998-02-04</date>
    <rev>A</rev>
  </header>
  <module since="">inet</module>
  <modulesummary>Access to TCP/IP protocols.</modulesummary>
  <description>
    <p>This module provides access to TCP/IP protocols.</p>
    <p>See also
      <seeguide marker="erts:inet_cfg">ERTS User's Guide:
      Inet Configuration</seeguide> for more information about how to
      configure an Erlang runtime system for IP communication.</p>
    <p>The following two Kernel configuration parameters affect the
      behavior of all sockets opened on an Erlang node:</p>
    <list type="bulleted">
      <item><p><c>inet_default_connect_options</c> can contain a list of
        default options used for all sockets returned when doing
        <c>connect</c>.</p></item>
      <item><p><c>inet_default_listen_options</c> can contain a list of
        default options used when issuing a <c>listen</c> call.</p></item>
    </list>
    <p>When <c>accept</c> is issued, the values of the listening socket options
      are inherited. No such application variable is therefore needed for
      <c>accept</c>.</p>
    <p>Using the Kernel configuration parameters above, one
      can set default options for all TCP sockets on a node, but use this
      with care. Options such as <c>{delay_send,true}</c> can be
      specified in this way. The following is an example of starting an Erlang
      node with all sockets using delayed send:</p>
    <pre>
$ <input>erl -sname test -kernel \</input>
<input>inet_default_connect_options '[{delay_send,true}]' \</input>
<input>inet_default_listen_options '[{delay_send,true}]'</input></pre>
    <p>Notice that default option <c>{active, true}</c>
      cannot be changed, for internal reasons.</p>
    <p>Addresses as inputs to functions can be either a string or a
      tuple. For example, the IP address 150.236.20.73 can be passed to
      <c>gethostbyaddr/1</c>, either as string <c>"150.236.20.73"</c>
      or as tuple <c>{150, 236, 20, 73}</c>.</p>
    <p><em>IPv4 address examples:</em></p>
    <code type="none">
Address          ip_address()
-------          ------------
127.0.0.1        {127,0,0,1}
192.168.42.2     {192,168,42,2}</code>
    <p><em>IPv6 address examples:</em></p>
    <code type="none">
Address          ip_address()
-------          ------------
::1             {0,0,0,0,0,0,0,1}
::192.168.42.2  {0,0,0,0,0,0,(192 bsl 8) bor 168,(42 bsl 8) bor 2}
::FFFF:192.168.42.2
                {0,0,0,0,0,16#FFFF,(192 bsl 8) bor 168,(42 bsl 8) bor 2}
3ffe:b80:1f8d:2:204:acff:fe17:bf38
                {16#3ffe,16#b80,16#1f8d,16#2,16#204,16#acff,16#fe17,16#bf38}
fe80::204:acff:fe17:bf38
                {16#fe80,0,0,0,16#204,16#acff,16#fe17,16#bf38}</code>
    <p>Function
      <seemfa marker="#parse_address/1"><c>parse_address/1</c></seemfa>
        can be useful:</p>
    <pre>
1> <input>inet:parse_address("192.168.42.2").</input>
{ok,{192,168,42,2}}
2> <input>inet:parse_address("::FFFF:192.168.42.2").</input>
{ok,{0,0,0,0,0,65535,49320,10754}}</pre>
  </description>

  <datatypes>
    <datatype>
      <name name="hostent"/>
      <desc>
        <p>The record is defined in the Kernel include file
          <c>"inet.hrl"</c>.</p>
        <p>Add the following directive to the module:</p>
        <code>
-include_lib("kernel/include/inet.hrl").</code>
      </desc>
    </datatype>
    <datatype>
      <name name="hostname"/>
    </datatype>
    <datatype>
      <name name="ip_address"/>
    </datatype>
    <datatype>
      <name name="ip4_address"/>
    </datatype>
    <datatype>
      <name name="ip6_address"/>
    </datatype>
    <datatype>
      <name name="port_number"/>
    </datatype>
    <datatype>
      <name name="family_address" since="OTP 22.1"/>
      <desc>
	<p>
	  A general address format on the form <c>{Family, Destination}</c>
	  where <c>Family</c> is an atom such as <c>local</c>
	  and the format of <c>Destination</c> depends on <c>Family</c>,
	  and is a complete address
	  (for example an IP address including port number).
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="inet_address" since="OTP 22.1"/>
      <desc>
	<warning>
	  <p>
	    This address format is for now experimental
	    and for completeness to make all address families have a
	    <c>{Family, Destination}</c> representation.
	  </p>
	</warning>
      </desc>
    </datatype>
    <datatype>
      <name name="inet6_address" since="OTP 22.1"/>
      <desc>
	<warning>
	  <p>
	    This address format is for now experimental
	    and for completeness to make all address families have a
	    <c>{Family, Destination}</c> representation.
	  </p>
	</warning>
      </desc>
    </datatype>
    <datatype>
      <name name="local_address"/>
      <desc>
	<p>
	  This address family only works on Unix-like systems.
	</p>
	<p>
	  <c><anno>File</anno></c> is normally a file pathname
	  in a local filesystem.  It is limited in length by the
	  operating system, traditionally to 108 bytes.
	</p>
	<p>
	  A <c>binary()</c> is passed as is to the operating system,
	  but a <c>string()</c> is encoded according to the
	  <seemfa marker="file#native_name_encoding/0">
	    system filename encoding mode.
	  </seemfa>
	</p>
	<p>
	  Other addresses are possible, for example Linux implements
	  "Abstract Addresses".  See the documentation for
	  Unix Domain Sockets on your system,
	  normally <c>unix</c> in manual section 7.
	</p>
	<p>
	  In most API functions where you can use
	  this address family the port number must be <c>0</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="socket_address"/>
    </datatype>
    <datatype>
      <name name="socket_getopt"/>
    </datatype>
    <datatype>
      <name name="socket_setopt"/>
    </datatype>
    <datatype>
      <name name="returned_non_ip_address"/>
      <desc>
	<p>
	  Addresses besides
	  <seetype marker="#ip_address">
	    <c>ip_address()</c>
	  </seetype>
	  ones that are returned from socket API functions.
	  See in particular
	  <seetype marker="#local_address">
	    <c>local_address()</c>.
	  </seetype>
	  The <c>unspec</c> family corresponds to AF_UNSPEC and can
	  occur if the other side has no socket address.
	  The <c>undefined</c> family can only occur in the unlikely
	  event of an address family that the VM does not recognize.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="ancillary_data"/>
      <desc>
	<p>
	  Ancillary data received with the data packet,
	  read with the socket option
	  <seetype marker="gen_tcp#pktoptions_value">
	    <c>pktoptions</c>
	  </seetype>
	  from a TCP socket,
	  or to set in a call to
	  <seeerl marker="gen_udp#send-4-AncData"><c>gen_udp:send/4</c></seeerl>
	  or
	  <seemfa marker="gen_udp#send/5"><c>gen_udp:send/5</c></seemfa>.
	</p>
	<p>
	  The value(s) correspond to the currently active socket
	  <seetype marker="#socket_setopt">options</seetype>
	  <seeerl marker="inet#option-recvtos"><c>recvtos</c></seeerl>,
	  <seeerl marker="inet#option-recvtclass"><c>recvtclass</c></seeerl>
	  and
	  <seeerl marker="inet#option-recvttl"><c>recvttl</c></seeerl>,
	  or for a single send operation the option(s) to override
	  the currently active socket option(s).
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="getifaddrs_ifopts"/>
      <desc>
	<p>
	  Interface address description list returned from
	  <seemfa marker="#getifaddrs/0"><c>getifaddrs/0,1</c></seemfa>
	  for a named interface, translated from the returned
	  data of the POSIX API function <c>getaddrinfo()</c>.
	</p>
        <p>
	  <c><anno>Hwaddr</anno></c> is hardware dependent,
	  for example, on Ethernet interfaces it is
	the 6-byte Ethernet address (MAC address (EUI-48 address)).
	</p>
        <p>
	  The tuples <c>{addr,<anno>Addr</anno>}</c>,
	  <c>{netmask,<anno>Netmask</anno>}</c>, and possibly
	  <c>{broadaddr,<anno>Broadaddr</anno>}</c> or
	  <c>{dstaddr,<anno>Dstaddr</anno>}</c>
	  are repeated in the list
	  if the interface has got multiple addresses.
	  An interface may have multiple <c>{flag,_}</c> tuples
	  for example if it has different flags
	  for different address families.
	  Multiple <c>{hwaddr,<anno>Hwaddr</anno>}</c> tuples
	  is hard to say anything definite about, though.
	  The tuple <c>{flag,<anno>Flags</anno>}</c> is mandatory,
	  all others are optional.
	</p>
        <p>
	  Do not rely too much on the order
	  of <c><anno>Flags</anno></c> atoms
          or the <c><anno>Ifopt</anno></c> tuples.
	  There are however some rules:
	</p>
        <list type="bulleted">
	  <item><p>
	    A <c>{flag,_}</c> tuple applies to all other tuples that follow.
	  </p></item>
          <item><p>
	    Immediately after <c>{addr,_}</c> follows <c>{netmask,_}</c>.
	  </p></item>
          <item><p>
	    Immediately thereafter may <c>{broadaddr,_}</c> follow
	    if <c>broadcast</c> is member of <c><anno>Flags</anno></c>,
	    or <c>{dstaddr,_}</c> if <c>pointtopoint</c>
	    is member of <c><anno>Flags</anno></c>.
	    Both <c>{dstaddr,_}</c> and <c>{broadaddr,_}</c>
	    does not occur for the same <c>{addr,_}</c>.
	  </p></item>
          <item><p>
	    Any <c>{netmask,_}</c>, <c>{broadaddr,_}</c>, or
            <c>{dstaddr,_}</c> tuples that follow an
	    <c>{addr,<anno>Addr</anno>}</c>
            tuple concerns the address <c><anno>Addr</anno></c>.
	  </p></item>
        </list>
        <p>
	  The tuple <c>{hwaddr,_}</c> is not returned on Solaris, as the
          hardware address historically belongs to the link layer
	  and it is not returned by the Solaris API function
	  <c>getaddrinfo()</c>.
	</p>
	<warning>
	  <p>
	    On Windows, the data is fetched from different
	    OS API functions, so the <c><anno>Netmask</anno></c>
	    and <c><anno>Broadaddr</anno></c> values may be calculated,
	    just as some <c><anno>Flags</anno></c> values.
	  </p>
	</warning>
      </desc>
    </datatype>
    <datatype>
      <name name="posix"/>
      <desc>
        <p>An atom that is named from the POSIX error codes used in Unix,
          and in the runtime libraries of most C compilers. See section
          <seeerl marker="#error_codes">POSIX Error Codes</seeerl>.</p>
      </desc>
    </datatype>
    <datatype>
      <name>socket()</name>
      <desc>
        <p>See
          <seetype marker="gen_tcp#socket"><c>gen_tcp:type-socket</c></seetype>
          and
          <seetype marker="gen_udp#socket"><c>gen_udp:type-socket</c></seetype>.
        </p>
      </desc>
    </datatype>
    <datatype>
      <name name="address_family"/>
    </datatype>
    <datatype>
      <name name="socket_protocol"/>
    </datatype>
    <datatype>
      <name name="stat_option"/>
    </datatype>
  </datatypes>

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

    <func>
      <name name="format_error" arity="1" since=""/>
      <fsummary>Return a descriptive string for an error reason.</fsummary>
      <desc>
        <p>Returns a diagnostic error string. For possible POSIX values and
          corresponding strings, see section
          <seeerl marker="#error_codes">POSIX Error Codes</seeerl>.</p>
      </desc>
    </func>

    <func>
      <name name="get_rc" arity="0" since=""/>
      <fsummary>Return a list of IP configuration parameters.</fsummary>
      <desc>
        <p>
	  Returns the state of the <c>Inet</c> configuration database in
          form of a list of recorded configuration parameters. For more
          information, see <seeguide marker="erts:inet_cfg">ERTS User's Guide:
          Inet Configuration</seeguide>.
	</p>
        <p>
	  Only actual parameters with other than default values
	  are returned, for example not directives that specify
	  other sources for configuration parameters nor
	  directives that clear parameters.
	</p>
      </desc>
    </func>

    <func>
      <name name="getaddr" arity="2" since=""/>
      <fsummary>Return the IP address for a host.</fsummary>
      <desc>
        <p>Returns the IP address for <c><anno>Host</anno></c> as a tuple of
          integers. <c><anno>Host</anno></c> can be an IP address, a single
          hostname, or a fully qualified hostname.</p>
      </desc>
    </func>

    <func>
      <name name="getaddrs" arity="2" since=""/>
      <fsummary>Return the IP addresses for a host.</fsummary>
      <desc>
        <p>Returns a list of all IP addresses for <c><anno>Host</anno></c>.
          <c><anno>Host</anno></c> can be an IP address, a single hostname, or
          a fully qualified hostname.</p>
      </desc>
    </func>

    <func>
      <name name="gethostbyaddr" arity="1" since=""/>
      <fsummary>Return a hostent record for the host with the specified
        address.</fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record for the host with the specified
          address.</p></desc>
    </func>

    <func>
      <name name="gethostbyname" arity="1" since=""/>
      <fsummary>Return a hostent record for the host with the specified name.
      </fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record for the host with the specified
          hostname.</p>
        <p>If resolver option <c>inet6</c> is <c>true</c>,
          an IPv6 address is looked up.</p>
      </desc>
    </func>

    <func>
      <name name="gethostbyname" arity="2" since=""/>
      <fsummary>Return a hostent record for the host with the specified name.
      </fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record for the host with the specified
          name, restricted to the specified address family.</p>
      </desc>
    </func>

    <func>
      <name name="gethostname" arity="0" since=""/>
      <fsummary>Return the local hostname.</fsummary>
      <desc>
        <p>Returns the local hostname. Never fails.</p>
      </desc>
    </func>

    <func>
      <name name="getifaddrs" arity="0" since="OTP R14B01"/>
      <fsummary>Return a list of interfaces and their addresses.</fsummary>
      <desc>
        <p>
	  Returns a list of 2-tuples containing interface names and
	  the interfaces' addresses. <c><anno>Ifname</anno></c>
	  is a Unicode string and
	  <c><anno>Ifopts</anno></c> is a list of
	  interface address description tuples.
	</p>
        <p>
	  The interface address description tuples
	  are documented under the type of the
          <seetype marker="#getifaddrs_ifopts">
	    <c><anno>Ifopts</anno></c>
	  </seetype>
	  value.
	</p>
      </desc>
    </func>

    <func>
      <name since="OTP 21.2">getifaddrs(Opts) ->
        {ok, [{Ifname, Ifopts}]} | {error, Posix}
      </name>
      <fsummary>Return a list of interfaces and their addresses.</fsummary>
      <type>
	<v>
	  Opts = [{netns, Namespace}]
	</v>
	<v>
	  Namespace =
	  <seetype marker="file#filename_all">
	    file:filename_all()
	  </seetype>
	</v>
	<v>Ifname = string()</v>
	<v>
	  Ifopts =
	  <seetype marker="#getifaddrs_ifopts">
	    getifaddrs_ifopts()
	  </seetype>
	</v>
	<v>Posix = <seetype marker="#posix">posix()</seetype></v>
      </type>
      <desc>
	<p>
	  The same as
	  <seemfa marker="#getifaddrs/0"><c>getifaddrs/0</c></seemfa>
	  but the <c>Option</c>
	  <c>{netns, Namespace}</c> sets a network namespace
	  for the OS call, on platforms that supports that feature.
	</p>
	<p>
	  See the socket option
	  <seeerl marker="#option-netns">
	    <c>{netns, Namespace}</c>
	  </seeerl>
	  under
	  <seemfa marker="#setopts/2"><c>setopts/2</c></seemfa>.
	</p>
      </desc>
    </func>

    <func>
      <name name="getopts" arity="2" since=""/>
      <fsummary>Get one or more options for a socket.</fsummary>
      <desc>
        <p>Gets one or more options for a socket. For a list of available
          options, see
          <seemfa marker="#setopts/2"><c>setopts/2</c></seemfa>.
	  See also the description for the type
          <seetype marker="gen_tcp#pktoptions_value">
	    <c>gen_tcp:pktoptions_value()</c>
	  </seetype>.</p>
        <p>The number of elements in the returned
          <c><anno>OptionValues</anno></c>
          list does not necessarily correspond to the number of options
          asked for. If the operating system fails to support an option,
          it is left out in the returned list. An error tuple is returned
          only when getting options for the socket is impossible (that is,
          the socket is closed or the buffer size in a raw request
          is too large). This behavior is kept for backward
          compatibility reasons.</p>
        <p>A raw option request
          <c>RawOptReq = {raw, Protocol, OptionNum, ValueSpec}</c>
          can be used to get information about
          socket options not (explicitly) supported by the emulator. The
          use of raw socket options makes the code non-portable, but
          allows the Erlang programmer to take advantage of unusual features
          present on a particular platform.</p>
        <p><c>RawOptReq</c> consists of tag <c>raw</c> followed
          by the protocol level, the option number, and either a binary
          or the size, in bytes, of the
          buffer in which the option value is to be stored. A binary is to be
          used when the underlying <c>getsockopt</c> requires <em>input</em>
          in the argument field. In this case, the binary size
          is to correspond to the required buffer
          size of the return value. The supplied values in a <c>RawOptReq</c>
          correspond to the second, third, and fourth/fifth parameters to the
          <c>getsockopt</c> call in the C socket API. The value stored
          in the buffer is returned as a binary <c>ValueBin</c>,
          where all values are coded in the native endianess.</p>
        <p>Asking for and inspecting raw socket options require low-level
          information about the current operating system and TCP stack.</p>
        <p><em>Example:</em></p>
        <p>Consider a Linux machine where option
          <c>TCP_INFO</c> can be used to collect TCP statistics
          for a socket. Assume you are interested in field
          <c>tcpi_sacked</c> of <c>struct tcp_info</c>
          filled in when asking for <c>TCP_INFO</c>. To be able to access
          this information, you need to know the following:</p>
        <list type="bulleted">
          <item>The numeric value of protocol level <c>IPPROTO_TCP</c></item>
          <item>The numeric value of option <c>TCP_INFO</c></item>
          <item>The size of <c>struct tcp_info</c></item>
          <item>The size and offset of the specific field</item>
        </list>
        <p>By inspecting the headers or writing a small C program, it is found
          that <c>IPPROTO_TCP</c> is 6, <c>TCP_INFO</c> is 11, the structure
          size is 92 (bytes), the offset of <c>tcpi_sacked</c> is 28 bytes,
          and the value is a 32-bit integer. The following code can be used
          to retrieve the value:</p>
        <code type="none"><![CDATA[
get_tcpi_sacked(Sock) ->
    {ok,[{raw,_,_,Info}]} = inet:getopts(Sock,[{raw,6,11,92}]),
    <<_:28/binary,TcpiSacked:32/native,_/binary>> = Info,
    TcpiSacked.]]></code>
        <p>Preferably, you would check the machine type, the operating system,
          and the Kernel version before executing anything similar to
          this code.</p>
      </desc>
    </func>

    <func>
      <name name="getstat" arity="1" since=""/>
      <name name="getstat" arity="2" since=""/>
      <fsummary>Get one or more statistic options for a socket.</fsummary>
      <type name="stat_option"/>
      <desc>
        <p>Gets one or more statistic options for a socket.</p>
        <p><c>getstat(<anno>Socket</anno>)</c> is equivalent to
          <c>getstat(<anno>Socket</anno>, [recv_avg, recv_cnt, recv_dvi,
          recv_max, recv_oct, send_avg, send_cnt, send_pend, send_max,
          send_oct])</c>.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>recv_avg</c></tag>
          <item>
            <p>Average size of packets, in bytes, received by the socket.</p>
          </item>
          <tag><c>recv_cnt</c></tag>
          <item>
            <p>Number of packets received by the socket.</p>
          </item>
          <tag><c>recv_dvi</c></tag>
          <item>
            <p>Average packet size deviation, in bytes, received by the socket.</p>
          </item>
          <tag><c>recv_max</c></tag>
          <item>
            <p>Size of the largest packet, in bytes, received by the socket.</p>
          </item>
          <tag><c>recv_oct</c></tag>
          <item>
            <p>Number of bytes received by the socket.</p>
          </item>
          <tag><c>send_avg</c></tag>
          <item>
            <p>Average size of packets, in bytes, sent from the socket.</p>
          </item>
          <tag><c>send_cnt</c></tag>
          <item>
            <p>Number of packets sent from the socket.</p>
          </item>
          <tag><c>send_pend</c></tag>
          <item>
            <p>Number of bytes waiting to be sent by the socket.</p>
          </item>
          <tag><c>send_max</c></tag>
          <item>
            <p>Size of the largest packet, in bytes, sent from the socket.</p>
          </item>
          <tag><c>send_oct</c></tag>
          <item>
            <p>Number of bytes sent from the socket.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="i" arity="0"  since="OTP 21.0"/>
      <name name="i" arity="1"  since="OTP 21.0"/>
      <name name="i" arity="2"  since="OTP 21.0"/>
      <fsummary>Displays information and statistics about sockets on the terminal</fsummary>
      <desc>
        <p>
          Lists all TCP, UDP and SCTP sockets, including those that the Erlang runtime system uses as well as
          those created by the application.
        </p>
        <p>
          The following options are available:
        </p>

        <taglist>
          <tag><c>port</c></tag>
          <item>
            <p>The internal index of the port.</p>
          </item>
          <tag><c>module</c></tag>
          <item>
            <p>The callback module of the socket.</p>
          </item>
          <tag><c>recv</c></tag>
          <item>
            <p>Number of bytes received by the socket.</p>
          </item>
          <tag><c>sent</c></tag>
          <item>
            <p>Number of bytes sent from the socket.</p>
          </item>
          <tag><c>owner</c></tag>
          <item>
            <p>The socket owner process.</p>
          </item>
          <tag><c>local_address</c></tag>
          <item>
            <p>The local address of the socket.</p>
          </item>
          <tag><c>foreign_address</c></tag>
          <item>
            <p>The address and port of the other end of the connection.</p>
          </item>
          <tag><c>state</c></tag>
          <item>
            <p>The connection state.</p>
          </item>
          <tag><c>type</c></tag>
          <item>
            <p>STREAM or DGRAM or SEQPACKET.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="info" arity="1"  since="@OTP-17203@"/>
      <fsummary>Retreive miscellaneous information about a socket</fsummary>
      <desc>
        <p>
          Produces a term containg miscellaneous information about a socket.
        </p>
      </desc>
    </func>

    <func>
      <name name="ntoa" arity="1"  since="OTP R16B02"/>
      <fsummary>Convert IPv6/IPV4 address to ASCII.</fsummary>
      <desc>
	<p>Parses an
          <seetype marker="#ip_address"><c>ip_address()</c></seetype>
          and returns an IPv4 or IPv6 address string.</p>
      </desc>
    </func>

    <func>
      <name name="parse_address" arity="1"  since="OTP R16B"/>
      <fsummary>Parse an IPv4 or IPv6 address.</fsummary>
      <desc>
	<p>Parses an IPv4 or IPv6 address string and returns an
          <seetype marker="#ip4_address"><c>ip4_address()</c></seetype> or
          <seetype marker="#ip6_address"><c>ip6_address()</c></seetype>.
          Accepts a shortened IPv4 address string.</p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv4_address" arity="1"  since="OTP R16B"/>
      <fsummary>Parse an IPv4 address.</fsummary>
      <desc>
	<p>Parses an IPv4 address string and returns an
          <seetype marker="#ip4_address"><c>ip4_address()</c></seetype>.
	  Accepts a shortened IPv4 address string.</p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv4strict_address" arity="1"  since="OTP R16B"/>
      <fsummary>Parse an IPv4 address strict.</fsummary>
      <desc>
	<p>Parses an IPv4 address string containing four fields, that is,
          <em>not</em> shortened, and returns an
          <seetype marker="#ip4_address"><c>ip4_address()</c></seetype>.
        </p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv6_address" arity="1"  since="OTP R16B"/>
      <fsummary>Parse an IPv6 address.</fsummary>
      <desc>
	<p>Parses an IPv6 address string and returns an
          <seetype marker="#ip6_address"><c>ip6_address()</c></seetype>.
          If an IPv4 address string is specified, an IPv4-mapped IPv6 address
          is returned.</p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv6strict_address" arity="1"  since="OTP R16B"/>
      <fsummary>Parse an IPv6 address strict.</fsummary>
      <desc>
	<p>Parses an IPv6 address string and returns an
          <seetype marker="#ip6_address"><c>ip6_address()</c></seetype>.
          Does <em>not</em> accept IPv4 addresses.</p>
      </desc>
    </func>

    <func>
      <name name="ipv4_mapped_ipv6_address" arity="1"  since="OTP 21.0"/>
      <fsummary>Convert to and from IPv4-mapped IPv6 address.</fsummary>
      <desc>
	<p>
	  Convert an IPv4 address to an IPv4-mapped IPv6 address
	  or the reverse.  When converting from an IPv6 address
	  all but the 2 low words are ignored so this function also
	  works on some other types of addresses than IPv4-mapped.
	</p>
      </desc>
    </func>

    <func>
      <name name="parse_strict_address" arity="1"  since="OTP R16B"/>
      <fsummary>Parse an IPv4 or IPv6 address strict.</fsummary>
      <desc>
	<p>Parses an IPv4 or IPv6 address string and returns an
          <seetype marker="#ip4_address"><c>ip4_address()</c></seetype> or
          <seetype marker="#ip6_address"><c>ip6_address()</c></seetype>.
          Does <em>not</em> accept a shortened IPv4 address string.</p>
      </desc>
    </func>

    <func>
      <name name="peername" arity="1" since=""/>
      <fsummary>Return the address and port for the other end of a connection.
      </fsummary>
      <desc>
        <p>Returns the address and port for the other end of a connection.</p>
	<p>Notice that for SCTP sockets, this function returns only
	  one of the peer addresses of the socket. Function
	  <seemfa marker="#peernames/1"><c>peernames/1,2</c></seemfa>
	  returns all.</p>
      </desc>
    </func>

    <func>
      <name name="peernames" arity="1" since="OTP R16B03"/>
      <fsummary>Return all address/port numbers for the other end of a
        connection.</fsummary>
      <desc>
	<p>Equivalent to
	  <seemfa marker="#peernames/2"><c>peernames(<anno>Socket</anno>, 0)</c></seemfa>.
        </p>
        <p>Notice that the behavior of this function for an SCTP
	  one-to-many style socket is not defined by the
	  <url href="http://tools.ietf.org/html/draft-ietf-tsvwg-sctpsocket-13">SCTP Sockets API Extensions</url>.</p>
      </desc>
    </func>

    <func>
      <name name="peernames" arity="2" since="OTP R16B03"/>
      <fsummary>Return all address/port numbers for the other end of a
        connection.</fsummary>
      <desc>
	<p>Returns a list of all address/port number pairs for the other end
	  of an association <c><anno>Assoc</anno></c> of a socket.</p>
	<p>This function can return multiple addresses for multihomed
	  sockets, such as SCTP sockets. For other sockets it
	  returns a one-element list.</p>
	<p>Notice that parameter <c><anno>Assoc</anno></c> is by the
	  <url href="http://tools.ietf.org/html/draft-ietf-tsvwg-sctpsocket-13">SCTP Sockets API Extensions</url>
	  defined to be ignored for
	  one-to-one style sockets. What the special value <c>0</c>
	  means, hence its behavior for one-to-many style sockets,
	  is unfortunately undefined.</p>
      </desc>
    </func>

    <func>
      <name name="port" arity="1" since=""/>
      <fsummary>Return the local port number for a socket.</fsummary>
      <desc>
        <p>Returns the local port number for a socket.</p>
      </desc>
    </func>

    <func>
      <name name="setopts" arity="2" since=""/>
      <fsummary>Set one or more options for a socket.</fsummary>
      <desc>
        <p>Sets one or more options for a socket.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>{active, true | false | once | N}</c></tag>
          <item>
            <p>If the value is <c>true</c>, which is the default,
              everything received from the socket is sent as
              messages to the receiving process.</p>
            <p>If the value is <c>false</c> (passive mode), the process must
              explicitly receive incoming data by calling
              <seemfa marker="gen_tcp#recv/2"><c>gen_tcp:recv/2,3</c></seemfa>,
              <seemfa marker="gen_udp#recv/2"><c>gen_udp:recv/2,3</c></seemfa>,
              or <seemfa marker="gen_sctp#recv/1"><c>gen_sctp:recv/1,2</c></seemfa>
              (depending on the type of socket).</p>
            <p>If the value is <c>once</c> (<c>{active, once}</c>),
              <em>one</em> data message from the socket is sent
              to the process. To receive one more message,
              <c>setopts/2</c> must be called again with option
              <c>{active, once}</c>.</p>
            <p>If the value is an integer <c>N</c> in the range -32768 to 32767
              (inclusive), the value is added to the socket's count of data
              messages sent to the controlling process. A socket's default
              message count is <c>0</c>. If a negative value is specified, and
              its magnitude is equal to or greater than the socket's current
              message count, the socket's message count is set to <c>0</c>.
              Once the socket's message count reaches <c>0</c>, either because
              of sending
              received data messages to the process or by being explicitly set,
              the process is then notified by a special message, specific to
              the type of socket, that the socket has entered passive
              mode. Once the socket enters passive mode, to receive more
              messages <c>setopts/2</c> must be called again to set the
              socket back into an active mode.</p>
            <p>When using <c>{active, once}</c> or <c>{active, N}</c>, the
              socket changes behavior automatically when data is received.
              This can be confusing in combination with connection-oriented
              sockets (that is, <c>gen_tcp</c>), as a socket
              with <c>{active, false}</c> behavior reports closing
              differently than a socket with <c>{active, true}</c>
              behavior. To simplify programming, a socket where
              the peer closed, and this is detected while in
              <c>{active, false}</c> mode, still generates message
              <c>{tcp_closed,Socket}</c> when set to <c>{active, once}</c>,
              <c>{active, true}</c>, or <c>{active, N}</c> mode.
              It is therefore safe to assume that message
              <c>{tcp_closed,Socket}</c>, possibly followed by socket port
              termination (depending on option <c>exit_on_close</c>)
              eventually appears when a socket changes
              back and forth between <c>{active, true}</c> and
              <c>{active, false}</c> mode. However,
              <em>when</em> peer closing is detected it is all up to the
              underlying TCP/IP stack and protocol.</p>
            <p>Notice that <c>{active, true}</c> mode provides no flow
              control; a fast sender can easily overflow the
              receiver with incoming messages. The same is true for
              <c>{active, N}</c> mode, while the message count is greater
              than zero.</p>
            <p>Use active mode only if
              your high-level protocol provides its own flow control
              (for example, acknowledging received messages) or the
              amount of data exchanged is small. <c>{active, false}</c>
              mode, use of the <c>{active, once}</c> mode, or <c>{active, N}</c>
              mode with values of <c>N</c> appropriate for the application
              provides flow control. The other side cannot send
              faster than the receiver can read.</p>
          </item>
          <tag><c>{broadcast, Boolean}</c> (UDP sockets)</tag>
          <item>
            <p>Enables/disables permission to send broadcasts.</p>
            <marker id="option-buffer"></marker>
          </item>
          <tag><c>{buffer, Size}</c></tag>
          <item>
            <p>The size of the user-level buffer used by
              the driver. Not to be confused with options <c>sndbuf</c>
              and <c>recbuf</c>, which correspond to the
              Kernel socket buffers. For TCP it is recommended
              to have <c>val(buffer) &gt;= val(recbuf)</c> to
              avoid performance issues because of unnecessary copying.
              For UDP the same recommendation applies, but the max should
              not be larger than the MTU of the network path.
              <c>val(buffer)</c> is automatically set to the above
              maximum when <c>recbuf</c> is set.
              However, as the size set for <c>recbuf</c>
              usually become larger, you are encouraged to use
              <seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>
              to analyze the behavior of your operating system.</p>
            <p>Note that this is also the maximum amount of data that can be
	       received from a single recv call. If you are using higher than
	       normal MTU consider setting buffer higher.</p>
          </item>
          <tag><c>{delay_send, Boolean}</c></tag>
          <item>
            <p>Normally, when an Erlang process sends to a socket,
              the driver tries to send the data immediately. If that
              fails, the driver uses any means available to queue
              up the message to be sent whenever the operating system
              says it can handle it. Setting <c>{delay_send, true}</c>
              makes <em>all</em> messages queue up. The messages sent
              to the network are then larger but fewer.
              The option affects the scheduling of send
              requests versus Erlang processes instead of changing any
              real property of the socket. The option is
              implementation-specific. Defaults to <c>false</c>.</p>
          </item>
          <tag><c>{deliver, port | term}</c></tag>
          <item>
            <p>When <c>{active, true}</c>, data is delivered on the form
              <c>port</c> : <c>{S, {data, [H1,..Hsz | Data]}}</c> or
              <c>term</c> : <c>{tcp, S, [H1..Hsz | Data]}</c>.</p>
          </item>
          <tag><c>{dontroute, Boolean}</c></tag>
          <item>
            <p>Enables/disables routing bypass for outgoing messages.</p>
          </item>
          <tag><c>{exit_on_close, Boolean}</c></tag>
          <item>
            <p>This option is set to <c>true</c> by default.</p>
            <p>The only reason to set it to <c>false</c> is if you want
              to continue sending data to the socket after a close is
              detected, for example, if the peer uses
              <seemfa marker="gen_tcp#shutdown/2"><c>gen_tcp:shutdown/2</c></seemfa>
              to shut down the write side.</p>
          </item>
          <tag><c>{header, Size}</c></tag>
          <item>
            <p>This option is only meaningful if option <c>binary</c>
              was specified when the socket was created. If option
              <c>header</c> is specified, the first
              <c>Size</c> number bytes of data received from the socket
              are elements of a list, and the remaining data is
              a binary specified as the tail of the same list. For example,
              if <c>Size == 2</c>, the data received matches
              <c>[Byte1,Byte2|Binary]</c>.</p>
          </item>
          <tag><c>{high_msgq_watermark, Size}</c></tag>
          <item>
	    <p>The socket message queue is set to a busy
	      state when the amount of data on the message
	      queue reaches this limit. Notice that this limit only
	      concerns data that has not yet reached the ERTS internal
	      socket implementation. Defaults to 8 kB.</p>
	    <p>Senders of data to the socket are suspended if
	      either the socket message queue is busy or the socket
	      itself is busy.</p>
	    <p>For more information, see options <c>low_msgq_watermark</c>,
	      <c>high_watermark</c>, and <c>low_watermark</c>.</p>
	    <p>Notice that distribution sockets disable the use of
	      <c>high_msgq_watermark</c> and <c>low_msgq_watermark</c>.
	      Instead use the
	      <seeerl marker="erts:erlang#system_info_dist_buf_busy_limit">distribution buffer busy limit</seeerl>,
              which is a similar feature.</p>
          </item>
          <tag><c>{high_watermark, Size}</c> (TCP/IP sockets)</tag>
          <item>
	    <p>The socket is set to a busy state when the amount
	      of data queued internally by the ERTS socket implementation
	      reaches this limit. Defaults to 8 kB.</p>
	    <p>Senders of data to the socket are suspended if
	      either the socket message queue is busy or the socket
	      itself is busy.</p>
	    <p>For more information, see options <c>low_watermark</c>,
	      <c>high_msgq_watermark</c>, and <c>low_msqg_watermark</c>.</p>
          </item>
	  <tag><c>{ipv6_v6only, Boolean}</c></tag>
	  <item>
	    <p>Restricts the socket to use only IPv6, prohibiting any
	      IPv4 connections. This is only applicable for
	      IPv6 sockets (option <c>inet6</c>).</p>
            <p>On most platforms this option must be set on the socket
	      before associating it to an address. It is therefore only
	      reasonable to specify it when creating the socket and not
	      to use it when calling function
	      (<seemfa marker="#setopts/2"><c>setopts/2</c></seemfa>)
	      containing this description.</p>
	    <p>The behavior of a socket with this option set to
	      <c>true</c> is the only portable one. The original
	      idea when IPv6 was new of using IPv6 for all traffic
	      is now not recommended by FreeBSD (you can use
	      <c>{ipv6_v6only,false}</c> to override the recommended
	      system default value),
	      forbidden by OpenBSD (the supported GENERIC kernel),
	      and impossible on Windows (which has separate
	      IPv4 and IPv6 protocol stacks). Most Linux distros
	      still have a system default value of <c>false</c>.
	      This policy shift among operating systems to
	      separate IPv6 from IPv4 traffic has evolved, as
	      it gradually proved hard and complicated to get
	      a dual stack implementation correct and secure.</p>
	    <p>On some platforms, the only allowed value for this option
	      is <c>true</c>, for example, OpenBSD and Windows. Trying to set
	      this option to <c>false</c>, when creating the socket, fails
	      in this case.</p>
	    <p>Setting this option on platforms where it does not exist
	      is ignored. Getting this option with
	      <seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>
	      returns no value, that is, the returned list does not contain an
	      <c>{ipv6_v6only,_}</c> tuple. On Windows, the option
	      does not exist, but it is emulated as a
	      read-only option with value <c>true</c>.</p>
	    <p>Therefore, setting this option to <c>true</c>
	      when creating a socket never fails, except possibly on a
              platform where you
	      have customized the kernel to only allow <c>false</c>,
	      which can be doable (but awkward) on, for example, OpenBSD.</p>
	    <p>If you read back the option value using
	      <seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>
	      and get no value, the option does not exist in the host
              operating system. The behavior of both an IPv6 and an IPv4
	      socket listening on the same port, and for an IPv6 socket
	      getting IPv4 traffic is then no longer predictable.</p>
	  </item>
          <tag><c>{keepalive, Boolean}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Enables/disables periodic transmission on a connected
              socket when no other data is exchanged. If
              the other end does not respond, the connection is
              considered broken and an error message is sent to
              the controlling process. Defaults to <c>false</c>.</p>
            <marker id="option-linger"></marker>
          </item>
          <tag><c>{linger, {true|false, Seconds}}</c></tag>
	  <item>
            <p>Determines the time-out, in seconds, for flushing unsent data
              in the <c>close/1</c> socket call. </p>
	    <p>The first component is if linger is enabled, the second component
	    is the flushing time-out, in seconds. There are 3 alternatives:</p>
	    <taglist>
	      <tag><c>{false, _}</c></tag>
	      <item>
		<p>close/1 or shutdown/2 returns immediately,
		not waiting for data to be flushed, with closing
		happening in the background.</p>
	      </item>
	      <tag><c>{true, 0}</c></tag>
	      <item>
		<p>Aborts the connection when it is closed.
		Discards any data still remaining in the send buffers
		and sends RST to the peer.</p>
		<p>This avoids TCP's TIME_WAIT state, but leaves open
		the possibility that another "incarnation" of this connection
		being created.</p>
	      </item>
	      <tag><c>{true, Time} when Time > 0</c></tag>
	      <item>
		<p>close/1 or shutdown/2 will not return until
		all queued messages for the socket have been successfully
		sent or the linger timeout (Time) has been reached.</p>
	      </item>
	    </taglist>
          </item>
          <tag><c>{low_msgq_watermark, Size}</c></tag>
          <item>
	    <p>If the socket message queue is in a busy state, the
	      socket message queue is set in a not busy state when
	      the amount of data queued in the message queue falls
	      below this limit. Notice that this limit only concerns data
	      that has not yet reached the ERTS internal socket
	      implementation. Defaults to 4 kB.</p>
	    <p>Senders that are suspended because of either a
	      busy message queue or a busy socket are resumed
	      when the socket message queue and the socket
	      are not busy.</p>
	    <p>For more information, see options <c>high_msgq_watermark</c>,
	      <c>high_watermark</c>, and <c>low_watermark</c>.</p>
	    <p>Notice that distribution sockets disable the use of
	      <c>high_msgq_watermark</c> and <c>low_msgq_watermark</c>.
	      Instead they use the
	      <seeerl marker="erts:erlang#system_info_dist_buf_busy_limit">distribution
	      buffer busy limit</seeerl>, which is a similar feature.</p>
          </item>
          <tag><c>{low_watermark, Size}</c> (TCP/IP sockets)</tag>
          <item>
	    <p>If the socket is in a busy state, the socket is
	      set in a not busy state when the amount of data
	      queued internally by the ERTS socket implementation
	      falls below this limit. Defaults to 4 kB.</p>
	    <p>Senders that are suspended because of a
	      busy message queue or a busy socket are resumed
	      when the socket message queue and the socket are not busy.</p>
	    <p>For more information, see options <c>high_watermark</c>,
	      <c>high_msgq_watermark</c>, and <c>low_msgq_watermark</c>.</p>
          </item>
          <tag><c>{mode, Mode :: binary | list}</c></tag>
          <item>
            <p>
	      Received <c>Packet</c> is delivered as defined by <c>Mode</c>.
            </p>
          </item>
	  <tag>
	    <marker id="option-netns"/>
	    <c>{netns, Namespace :: file:filename_all()}</c>
	  </tag>
	  <item>
	    <p>
	      Sets a network namespace for the socket. Parameter
	      <c>Namespace</c> is a filename defining the namespace, for
	      example, <c>"/var/run/netns/example"</c>, typically created by
	      command <c>ip netns add example</c>. This option must be used in
	      a function call that creates a socket, that is,
	      <seemfa marker="gen_tcp#connect/3"><c>gen_tcp:connect/3,4</c></seemfa>,
	      <seemfa marker="gen_tcp#listen/2"><c>gen_tcp:listen/2</c></seemfa>,
	      <seemfa marker="gen_udp#open/1"><c>gen_udp:open/1,2</c></seemfa>
	      or
	      <seemfa marker="gen_sctp#open/0"><c>gen_sctp:open/0,1,2</c></seemfa>,
	      and also
	      <seemfa marker="#getifaddrs/1"><c>getifaddrs/1</c></seemfa>.
	    </p>
	    <p>This option uses the Linux-specific syscall
	      <c>setns()</c>, such as in Linux kernel 3.0 or later,
	      and therefore only exists when the runtime system
	      is compiled for such an operating system.</p>
	    <p>The virtual machine also needs elevated privileges, either
	      running as superuser or (for Linux) having capability
	      <c>CAP_SYS_ADMIN</c> according to the documentation for
	      <c>setns(2)</c>.
	      However, during testing also <c>CAP_SYS_PTRACE</c>
	      and <c>CAP_DAC_READ_SEARCH</c> have proven to be necessary.</p>
            <p><em>Example:</em></p>
	    <code>
setcap cap_sys_admin,cap_sys_ptrace,cap_dac_read_search+epi beam.smp</code>
            <p>Notice that the filesystem containing the virtual machine
	      executable (<c>beam.smp</c> in the example) must be local,
	      mounted without flag <c>nosetuid</c>,
	      support extended attributes, and
	      the kernel must support file capabilities.
	      All this runs out of the box on at least Ubuntu 12.04 LTS,
	      except that SCTP sockets appear to not support
	      network namespaces.</p>
	    <p><c>Namespace</c> is a filename and is encoded
	      and decoded as discussed in module
	      <seeerl marker="file">file</seeerl>, with the
              following exceptions:</p>
	    <list type="bulleted">
	      <item><p>Emulator flag <c>+fnu</c> is ignored.</p></item>
	      <item><p><seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>
	      for this option returns a binary for the filename if the stored
	      filename cannot be decoded. This is only to occur if you set the
	      option using a binary that cannot be decoded with the emulator's
	      filename encoding:
	      <seemfa marker="file#native_name_encoding/0"><c>file:native_name_encoding/0</c></seemfa>.</p></item>
	    </list>
          </item>
	  <tag><c>{bind_to_device, Ifname :: binary()}</c></tag>
	  <item>
	    <p>Binds a socket to a specific network interface. This option
	      must be used in a function call that creates a socket, that is,
	      <seemfa marker="gen_tcp#connect/3"><c>gen_tcp:connect/3,4</c></seemfa>,
	      <seemfa marker="gen_tcp#listen/2"><c>gen_tcp:listen/2</c></seemfa>,
	      <seemfa marker="gen_udp#open/1"><c>gen_udp:open/1,2</c></seemfa>, or
	      <seemfa marker="gen_sctp#open/0"><c>gen_sctp:open/0,1,2</c></seemfa>.</p>
	    <p>Unlike <seemfa marker="#getifaddrs/0"><c>getifaddrs/0</c></seemfa>, Ifname
	      is encoded a binary. In the unlikely case that a system is using
	      non-7-bit-ASCII characters in network device names, special care
	      has to be taken when encoding this argument.</p>
	    <p>This option uses the Linux-specific socket option
	      <c>SO_BINDTODEVICE</c>, such as in Linux kernel 2.0.30 or later,
	      and therefore only exists when the runtime system
	      is compiled for such an operating system.</p>
	    <p>Before Linux 3.8, this socket option could be set, but could not retrieved
	      with <seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>. Since Linux 3.8,
	      it is readable.</p>
	    <p>The virtual machine also needs elevated privileges, either
	      running as superuser or (for Linux) having capability
	    <c>CAP_NET_RAW</c>.</p>
	    <p>The primary use case for this option is to bind sockets into
	      <url href="http://www.kernel.org/doc/Documentation/networking/vrf.txt">Linux VRF instances</url>.
	    </p>
	  </item>
          <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>
            <marker id="option-nodelay"></marker>
          </item>
          <tag><c>{nodelay, Boolean}</c>(TCP/IP sockets)</tag>
          <item>
            <p>If <c>Boolean == true</c>, option <c>TCP_NODELAY</c>
            is turned on for the socket, which means that also small
            amounts of data are sent immediately.</p>
	    <p>This option is <em>not</em> supported for
	    <c>domain = local</c>, but if <c>inet_backend =/= socket</c>
	    this error will be <em>ignored</em>. </p>
          </item>
          <tag><c>{nopush, Boolean}</c>(TCP/IP sockets)</tag>
          <item>
            <p>This translates to <c>TCP_NOPUSH</c> on BSD and
              to <c>TCP_CORK</c> on Linux.</p>
            <p>If <c>Boolean == true</c>, the corresponding option
              is turned on for the socket, which means that small
              amounts of data are accumulated until a full MSS-worth
              of data is available or this option is turned off.</p>
            <p>Note that while <c>TCP_NOPUSH</c> socket option is available on OSX, its semantics
              is very different (e.g., unsetting it does not cause immediate send
              of accumulated data). Hence, <c>nopush</c> option is intentionally ignored on OSX.</p>
          </item>
          <tag><c>{packet, PacketType}</c>(TCP/IP sockets)</tag>
          <item>
            <p><marker id="packet"/>Defines the type of packets to use for a socket.
	      Possible values:</p>
            <taglist>
              <tag><c>raw | 0</c></tag>
              <item>
                <p>No packaging is done.</p>
              </item>
              <tag><c>1 | 2 | 4</c></tag>
              <item>
                <p>Packets consist of a header specifying the number of
                  bytes in the packet, followed by that number of bytes.
                  The header length can be one, two, or four bytes, and
                  containing an unsigned integer in big-endian byte order.
                  Each send operation generates the header, and the header
                  is stripped off on each receive operation.</p>
                <p>The 4-byte header is limited to 2Gb.</p>
              </item>
              <tag><c>asn1 | cdr | sunrm | fcgi | tpkt | line</c></tag>
              <item>
                <p>These packet types only have effect on receiving.
                  When sending a packet, it is the responsibility of
                  the application to supply a correct header. On
                  receiving, however, one message is sent to
                  the controlling process for each complete packet
                  received, and, similarly, each call to
                  <c>gen_tcp:recv/2,3</c> returns one complete packet.
                  The header is <em>not</em> stripped off.</p>
                <p>The meanings of the packet types are as follows:</p>
		<list type="bulleted">
		  <item><c>asn1</c> - ASN.1 BER</item>
		  <item><c>sunrm</c> - Sun's RPC encoding</item>
		  <item><c>cdr</c> - CORBA (GIOP 1.1)</item>
		  <item><c>fcgi</c> - Fast CGI</item>
		  <item><c>tpkt</c> - TPKT format [RFC1006]</item>
		  <item><c>line</c> - Line mode, a packet is a line-terminated
		    with newline, lines longer than the receive buffer are
		    truncated</item>
		</list>
	      </item>
              <tag><c>http | http_bin</c></tag>
              <item>
                <p>The Hypertext Transfer Protocol. The packets
                 are returned with the format according to <c>HttpPacket</c>
                 described in
                 <seemfa marker="erts:erlang#decode_packet/3">
                 <c>erlang:decode_packet/3</c></seemfa> in ERTS.
                 A socket in passive
                 mode returns <c>{ok, HttpPacket}</c> from <c>gen_tcp:recv</c>
                 while an active socket sends messages like
		 <c>{http, Socket, HttpPacket}</c>.</p>
              </item>
              <tag><c>httph | httph_bin</c></tag>
              <item>
                <p>These two types are often not needed, as the socket
		automatically switches from <c>http</c>/<c>http_bin</c> to
		<c>httph</c>/<c>httph_bin</c> internally after the first line
		is read. However, there can be occasions when they are
		useful, such as parsing trailers from chunked encoding.</p>
              </item>
            </taglist>
          </item>
          <tag><c>{packet_size, Integer}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Sets the maximum allowed length of the packet body. If
              the packet header indicates that the length of the packet
              is longer than the maximum allowed length, the packet is
              considered invalid. The same occurs if the packet header
              is too large for the socket receive buffer.</p>
	    <p>For line-oriented protocols (<c>line</c>, <c>http*</c>),
	      option <c>packet_size</c> also guarantees that lines up to the
	      indicated length are accepted and not considered invalid
	      because of internal buffer limitations.</p>
          </item>
          <tag><c>{line_delimiter, Char}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Sets the line delimiting character for line-oriented protocols
              (<c>line</c>). Defaults to <c>$\n</c>.</p>
          </item>
          <tag><c>{raw, Protocol, OptionNum, ValueBin}</c></tag>
          <item>
            <p>See below.</p>
          </item>
          <tag><c>{read_packets, Integer}</c>(UDP sockets)</tag>
          <item>
            <p>Sets the maximum number of UDP packets to read without
              intervention from the socket when data is available.
              When this many packets have been read and delivered
              to the destination process, new packets are not read
              until a new notification of available data has arrived.
              Defaults to <c>5</c>. If this parameter is set too
              high, the system can become unresponsive because of
              UDP packet flooding.</p>
          </item>
          <tag><c>{recbuf, Size}</c></tag>
          <item>
            <p>The minimum size of the receive buffer to use for
              the socket. You are encouraged to use
              <seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>
              to retrieve the size set by your operating system.</p>
	    <marker id="option-recvtclass"></marker>
          </item>
          <tag><c>{recvtclass, Boolean}</c></tag>
          <item>
            <p>
	      If set to <c>true</c> activates returning the received
	      <c>TCLASS</c> value on platforms that implements
	      the protocol <c>IPPROTO_IPV6</c>
	      option <c>IPV6_RECVTCLASS</c> or <c>IPV6_2292RECVTCLASS</c>
	      for the socket.
	      The value is returned as a <c>{tclass,TCLASS}</c> tuple
	      regardless of if the platform returns an <c>IPV6_TCLASS</c>
	      or an <c>IPV6_RECVTCLASS</c> CMSG value.
	    </p>
	    <p>
	      For packet oriented sockets that supports receiving
	      ancillary data with the payload data
	      (<c>gen_udp</c> and <c>gen_sctp</c>),
	      the <c>TCLASS</c> value is returned
	      in an extended return tuple contained in an
	      <seetype marker="inet#ancillary_data">
		ancillary data
	      </seetype>
	      list.
	      For stream oriented sockets (<c>gen_tcp</c>)
	      the only way to get the <c>TCLASS</c>
	      value is if the platform supports the
	      <seetype marker="gen_tcp#pktoptions_value">
		<c>pktoptions</c>
	      </seetype>
	      option.
	    </p>
	    <marker id="option-recvtos"></marker>
          </item>
          <tag><c>{recvtos, Boolean}</c></tag>
          <item>
            <p>
	      If set to <c>true</c> activates returning the received
	      <c>TOS</c> value on platforms that implements
	      the protocol <c>IPPROTO_IP</c> option <c>IP_RECVTOS</c>
	      for the socket.
	      The value is returned as a <c>{tos,TOS}</c> tuple
	      regardless of if the platform returns an <c>IP_TOS</c>
	      or an <c>IP_RECVTOS</c> CMSG value.
	    </p>
	    <p>
	      For packet oriented sockets that supports receiving
	      ancillary data with the payload data
	      (<c>gen_udp</c> and <c>gen_sctp</c>),
	      the <c>TOS</c> value is returned
	      in an extended return tuple contained in an
	      <seetype marker="inet#ancillary_data">
		ancillary data
	      </seetype>
	      list.
	      For stream oriented sockets (<c>gen_tcp</c>)
	      the only way to get the <c>TOS</c>
	      value is if the platform supports the
	      <seetype marker="gen_tcp#pktoptions_value">
		<c>pktoptions</c>
	      </seetype>
	      option.
	    </p>
	    <marker id="option-recvttl"></marker>
          </item>
          <tag><c>{recvttl, Boolean}</c></tag>
          <item>
            <p>
	      If set to <c>true</c> activates returning the received
	      <c>TTL</c> value on platforms that implements
	      the protocol <c>IPPROTO_IP</c> option <c>IP_RECVTTL</c>
	      for the socket.
	      The value is returned as a <c>{ttl,TTL}</c> tuple
	      regardless of if the platform returns an <c>IP_TTL</c>
	      or an <c>IP_RECVTTL</c> CMSG value.
	    </p>
	    <p>
	      For packet oriented sockets that supports receiving
	      ancillary data with the payload data
	      (<c>gen_udp</c> and <c>gen_sctp</c>),
	      the <c>TTL</c> value is returned
	      in an extended return tuple contained in an
	      <seetype marker="inet#ancillary_data">
		ancillary data
	      </seetype>
	      list.
	      For stream oriented sockets (<c>gen_tcp</c>)
	      the only way to get the <c>TTL</c>
	      value is if the platform supports the
	      <seetype marker="gen_tcp#pktoptions_value">
		<c>pktoptions</c>
	      </seetype>
	      option.
	    </p>
          </item>
          <tag><c>{reuseaddr, Boolean}</c></tag>
          <item>
            <p>Allows or disallows local reuse of port numbers. By
              default, reuse is disallowed.</p>
          </item>
          <tag><c>{send_timeout, Integer}</c></tag>
          <item>
            <p>Only allowed for connection-oriented sockets.</p>
            <p>Specifies a longest time to wait for a send operation to
              be accepted by the underlying TCP stack. When the limit is
              exceeded, the send operation returns
              <c>{error,timeout}</c>. How much of a packet that
              got sent is unknown; the socket is therefore to be closed
              whenever a time-out has occurred (see <c>send_timeout_close</c>
              below). Defaults to <c>infinity</c>.</p>
          </item>
          <tag><c>{send_timeout_close, Boolean}</c></tag>
          <item>
            <p>Only allowed for connection-oriented sockets.</p>
            <p>Used together with <c>send_timeout</c> to specify whether
              the socket is to be automatically closed when the send operation
              returns <c>{error,timeout}</c>. The recommended setting is
              <c>true</c>, which automatically closes the socket.
              Defaults to <c>false</c> because of backward compatibility.</p>
            <marker id="option-show_econnreset"></marker>
          </item>
          <tag><c>{show_econnreset, Boolean}</c> (TCP/IP sockets) </tag>
          <item>
            <p>When this option is set to <c>false</c>, which is
              default, an RST received from the TCP peer is treated
              as a normal close (as though an FIN was sent). A caller to
              <seemfa marker="gen_tcp#recv/2"><c>gen_tcp:recv/2</c></seemfa>
              gets <c>{error, closed}</c>. In active
              mode, the controlling process receives a
              <c>{tcp_closed, Socket}</c> message, indicating that the
              peer has closed the connection.</p>
            <p>Setting this option to <c>true</c> allows you to
              distinguish between a connection that was closed normally,
              and one that was aborted (intentionally or unintentionally)
              by the TCP peer. A call to
              <seemfa marker="gen_tcp#recv/2"><c>gen_tcp:recv/2</c></seemfa>
              returns <c>{error, econnreset}</c>. In
              active mode, the controlling process receives a
              <c>{tcp_error, Socket, econnreset}</c> message
              before the usual <c>{tcp_closed, Socket}</c>, as is
              the case for any other socket error. Calls to
              <seemfa marker="gen_tcp#send/2"><c>gen_tcp:send/2</c></seemfa>
              also returns <c>{error, econnreset}</c> when it
              is detected that a TCP peer has sent an RST.</p>
            <p>A connected socket returned from
              <seemfa marker="gen_tcp#accept/1"><c>gen_tcp:accept/1</c></seemfa>
              inherits the <c>show_econnreset</c> setting from the
              listening socket.</p>
            <marker id="option-sndbuf"></marker>
          </item>
          <tag><c>{sndbuf, Size}</c></tag>
          <item>
            <p>The minimum size of the send buffer to use for the socket.
              You are encouraged to use
              <seemfa marker="#getopts/2"><c>getopts/2</c></seemfa>,
              to retrieve the size set by your operating system.</p>
          </item>
          <tag><c>{priority, Integer}</c></tag>
          <item>
            <p>Sets the <c>SO_PRIORITY</c> socket level option on platforms
              where this is implemented. The behavior and allowed range varies
              between different systems.
              The option is ignored on platforms where it
              is not implemented. Use with caution.</p>
          </item>
          <tag><c>{tos, Integer}</c></tag>
          <item>
            <p>Sets <c>IP_TOS IP</c> level options on platforms where this is
              implemented. The behavior and allowed range varies between
              different systems.
              The option is ignored on platforms where it is not
              implemented. Use with caution.</p>
          </item>
          <tag><c>{tclass, Integer}</c></tag>
          <item>
            <p>
	      Sets <c>IPV6_TCLASS IP</c> level options on platforms
	      where this is implemented. The behavior and allowed range
	      varies between different systems.
              The option is ignored on platforms where it is not
              implemented. Use with caution.</p>
          </item>
        </taglist>
        <p>In addition to these options, <em>raw</em>
          option specifications can be used. The raw options are
          specified as a tuple of arity four, beginning with tag
          <c>raw</c>, followed by the protocol level, the option number,
          and the option value specified as a binary. This
          corresponds to the second, third, and fourth arguments to the
          <c>setsockopt</c> call in the C socket API. The option value
          must be coded in the native endianess of the platform and,
          if a structure is required, must follow the structure
          alignment conventions on the specific platform.</p>
        <p>Using raw socket options requires detailed knowledge about
          the current operating system and TCP stack.</p>
        <p><em>Example:</em></p>
        <p>This example concerns the use of raw options. Consider a Linux
          system where you want to set option <c>TCP_LINGER2</c> on
          protocol level <c>IPPROTO_TCP</c> in the stack. You know
          that on this particular system it defaults to 60 (seconds),
          but you want to lower it to 30 for a particular
          socket. Option <c>TCP_LINGER2</c> is not explicitly
          supported by <c>inet</c>, but you know that the protocol level
          translates to number 6, the option number to number 8,
          and the value is to be specified as a 32-bit integer. You can use
          this code line to set the option for the socket named
          <c>Sock</c>:</p>
        <code type="none"><![CDATA[
inet:setopts(Sock,[{raw,6,8,<<30:32/native>>}]),]]></code>
        <p>As many options are silently discarded by the stack if they
          are specified out of range; it can be a good idea to check that
          a raw option is accepted. The following code places the value
          in variable <c>TcpLinger2:</c></p>
        <code type="none"><![CDATA[
{ok,[{raw,6,8,<<TcpLinger2:32/native>>}]}=inet:getopts(Sock,[{raw,6,8,4}]),]]></code>
        <p>Code such as these examples is inherently non-portable,
          even different versions of the same OS on the same platform
          can respond differently to this kind of option
          manipulation. Use with care.</p>
        <p>Notice that the default options for TCP/IP sockets can be
          changed with the Kernel configuration parameters mentioned in
          the beginning of this manual page.</p>
      </desc>
    </func>

    <func>
      <name name="sockname" arity="1" since=""/>
      <fsummary>Return the local address and port number for a socket.
      </fsummary>
      <desc>
        <p>Returns the local address and port number for a socket.</p>
	<p>Notice that for SCTP sockets this function returns only
	  one of the socket addresses. Function
	  <seemfa marker="#socknames/1"><c>socknames/1,2</c></seemfa>
	  returns all.</p>
      </desc>
    </func>

    <func>
      <name name="socknames" arity="1" since="OTP R16B03"/>
      <fsummary>Return all local address/port numbers for a socket.</fsummary>
      <desc>
	<p>Equivalent to
	  <seemfa marker="#socknames/2"><c>socknames(<anno>Socket</anno>, 0)</c></seemfa>.
        </p>
      </desc>
    </func>

    <func>
      <name name="socknames" arity="2" since="OTP R16B03"/>
      <fsummary>Return all local address/port numbers for a socket.</fsummary>
      <desc>
	<p>Returns a list of all local address/port number pairs for a socket
	  for the specified association <c><anno>Assoc</anno></c>.</p>
	<p>This function can return multiple addresses for multihomed
	  sockets, such as SCTP sockets. For other sockets it
	  returns a one-element list.</p>
	<p>Notice that parameter <c><anno>Assoc</anno></c> is by the
	  <url href="http://tools.ietf.org/html/draft-ietf-tsvwg-sctpsocket-13">SCTP Sockets API Extensions</url>
	  defined to be ignored for one-to-one style sockets.
	  For one-to-many style sockets, the special value <c>0</c>
	  is defined to mean that the returned addresses must be
	  without any particular association.
	  How different SCTP implementations interpret this varies somewhat.
        </p>
      </desc>
    </func>
  </funcs>

  <section>
    <marker id="error_codes"></marker>
    <title>POSIX Error Codes</title>
    <list type="bulleted">
      <item><c>e2big</c> - Too long argument list</item>
      <item><c>eacces</c> - Permission denied</item>
      <item><c>eaddrinuse</c> - Address already in use</item>
      <item><c>eaddrnotavail</c> - Cannot assign requested address</item>
      <item><c>eadv</c> - Advertise error</item>
      <item><c>eafnosupport</c> - Address family not supported by
        protocol family</item>
      <item><c>eagain</c> - Resource temporarily unavailable</item>
      <item><c>ealign</c> - EALIGN</item>
      <item><c>ealready</c> - Operation already in progress</item>
      <item><c>ebade</c> - Bad exchange descriptor</item>
      <item><c>ebadf</c> - Bad file number</item>
      <item><c>ebadfd</c> - File descriptor in bad state</item>
      <item><c>ebadmsg</c> - Not a data message</item>
      <item><c>ebadr</c> - Bad request descriptor</item>
      <item><c>ebadrpc</c> - Bad RPC structure</item>
      <item><c>ebadrqc</c> - Bad request code</item>
      <item><c>ebadslt</c> - Invalid slot</item>
      <item><c>ebfont</c> - Bad font file format</item>
      <item><c>ebusy</c> - File busy</item>
      <item><c>echild</c> - No children</item>
      <item><c>echrng</c> - Channel number out of range</item>
      <item><c>ecomm</c> - Communication error on send</item>
      <item><c>econnaborted</c> - Software caused connection abort</item>
      <item><c>econnrefused</c> - Connection refused</item>
      <item><c>econnreset</c> - Connection reset by peer</item>
      <item><c>edeadlk</c> - Resource deadlock avoided</item>
      <item><c>edeadlock</c> - Resource deadlock avoided</item>
      <item><c>edestaddrreq</c> - Destination address required</item>
      <item><c>edirty</c> - Mounting a dirty fs without force</item>
      <item><c>edom</c> - Math argument out of range</item>
      <item><c>edotdot</c> - Cross mount point</item>
      <item><c>edquot</c> - Disk quota exceeded</item>
      <item><c>eduppkg</c> - Duplicate package name</item>
      <item><c>eexist</c> - File already exists</item>
      <item><c>efault</c> - Bad address in system call argument</item>
      <item><c>efbig</c> - File too large</item>
      <item><c>ehostdown</c> - Host is down</item>
      <item><c>ehostunreach</c> - Host is unreachable</item>
      <item><c>eidrm</c> - Identifier removed</item>
      <item><c>einit</c> - Initialization error</item>
      <item><c>einprogress</c> - Operation now in progress</item>
      <item><c>eintr</c> - Interrupted system call</item>
      <item><c>einval</c> - Invalid argument</item>
      <item><c>eio</c> - I/O error</item>
      <item><c>eisconn</c> - Socket is already connected</item>
      <item><c>eisdir</c> - Illegal operation on a directory</item>
      <item><c>eisnam</c> - Is a named file</item>
      <item><c>el2hlt</c> - Level 2 halted</item>
      <item><c>el2nsync</c> - Level 2 not synchronized</item>
      <item><c>el3hlt</c> - Level 3 halted</item>
      <item><c>el3rst</c> - Level 3 reset</item>
      <item><c>elbin</c> - ELBIN</item>
      <item><c>elibacc</c> - Cannot access a needed shared library</item>
      <item><c>elibbad</c> - Accessing a corrupted shared library</item>
      <item><c>elibexec</c> - Cannot exec a shared library directly</item>
      <item><c>elibmax</c> - Attempting to link in more shared
        libraries than system limit</item>
      <item><c>elibscn</c> - <c>.lib</c> section in <c>a.out</c>
        corrupted</item>
      <item><c>elnrng</c> - Link number out of range</item>
      <item><c>eloop</c> - Too many levels of symbolic links</item>
      <item><c>emfile</c> - Too many open files</item>
      <item><c>emlink</c> - Too many links</item>
      <item><c>emsgsize</c> - Message too long</item>
      <item><c>emultihop</c> - Multihop attempted</item>
      <item><c>enametoolong</c> - Filename too long</item>
      <item><c>enavail</c> - Unavailable</item>
      <item><c>enet</c> - ENET</item>
      <item><c>enetdown</c> - Network is down</item>
      <item><c>enetreset</c> - Network dropped connection on reset</item>
      <item><c>enetunreach</c> - Network is unreachable</item>
      <item><c>enfile</c> - File table overflow</item>
      <item><c>enoano</c> - Anode table overflow</item>
      <item><c>enobufs</c> - No buffer space available</item>
      <item><c>enocsi</c> - No CSI structure available</item>
      <item><c>enodata</c> - No data available</item>
      <item><c>enodev</c> - No such device</item>
      <item><c>enoent</c> - No such file or directory</item>
      <item><c>enoexec</c> - Exec format error</item>
      <item><c>enolck</c> - No locks available</item>
      <item><c>enolink</c> - Link has been severed</item>
      <item><c>enomem</c> - Not enough memory</item>
      <item><c>enomsg</c> - No message of desired type</item>
      <item><c>enonet</c> - Machine is not on the network</item>
      <item><c>enopkg</c> - Package not installed</item>
      <item><c>enoprotoopt</c> - Bad protocol option</item>
      <item><c>enospc</c> - No space left on device</item>
      <item><c>enosr</c> - Out of stream resources or not a stream device</item>
      <item><c>enosym</c> - Unresolved symbol name</item>
      <item><c>enosys</c> - Function not implemented</item>
      <item><c>enotblk</c> - Block device required</item>
      <item><c>enotconn</c> - Socket is not connected</item>
      <item><c>enotdir</c> - Not a directory</item>
      <item><c>enotempty</c> - Directory not empty</item>
      <item><c>enotnam</c> - Not a named file</item>
      <item><c>enotsock</c> - Socket operation on non-socket</item>
      <item><c>enotsup</c> - Operation not supported</item>
      <item><c>enotty</c> - Inappropriate device for <c>ioctl</c></item>
      <item><c>enotuniq</c> - Name not unique on network</item>
      <item><c>enxio</c> - No such device or address</item>
      <item><c>eopnotsupp</c> - Operation not supported on socket</item>
      <item><c>eperm</c> - Not owner</item>
      <item><c>epfnosupport</c> - Protocol family not supported</item>
      <item><c>epipe</c> - Broken pipe</item>
      <item><c>eproclim</c> - Too many processes</item>
      <item><c>eprocunavail</c> - Bad procedure for program</item>
      <item><c>eprogmismatch</c> - Wrong program version</item>
      <item><c>eprogunavail</c> - RPC program unavailable</item>
      <item><c>eproto</c> - Protocol error</item>
      <item><c>eprotonosupport</c> - Protocol not supported</item>
      <item><c>eprototype</c> - Wrong protocol type for socket</item>
      <item><c>erange</c> - Math result unrepresentable</item>
      <item><c>erefused</c> - EREFUSED</item>
      <item><c>eremchg</c> - Remote address changed</item>
      <item><c>eremdev</c> - Remote device</item>
      <item><c>eremote</c> - Pathname hit remote filesystem</item>
      <item><c>eremoteio</c> - Remote I/O error</item>
      <item><c>eremoterelease</c> - EREMOTERELEASE</item>
      <item><c>erofs</c> - Read-only filesystem</item>
      <item><c>erpcmismatch</c> - Wrong RPC version</item>
      <item><c>erremote</c> - Object is remote</item>
      <item><c>eshutdown</c> - Cannot send after socket shutdown</item>
      <item><c>esocktnosupport</c> - Socket type not supported</item>
      <item><c>espipe</c> - Invalid seek</item>
      <item><c>esrch</c> - No such process</item>
      <item><c>esrmnt</c> - Srmount error</item>
      <item><c>estale</c> - Stale remote file handle</item>
      <item><c>esuccess</c> - Error 0</item>
      <item><c>etime</c> - Timer expired</item>
      <item><c>etimedout</c> - Connection timed out</item>
      <item><c>etoomanyrefs</c> - Too many references</item>
      <item><c>etxtbsy</c> - Text file or pseudo-device busy</item>
      <item><c>euclean</c> - Structure needs cleaning</item>
      <item><c>eunatch</c> - Protocol driver not attached</item>
      <item><c>eusers</c> - Too many users</item>
      <item><c>eversion</c> - Version mismatch</item>
      <item><c>ewouldblock</c> - Operation would block</item>
      <item><c>exdev</c> - Cross-domain link</item>
      <item><c>exfull</c> - Message tables full</item>
      <item><c>nxdomain</c> - Hostname or domain name cannot be found</item>
    </list>
  </section>
</erlref>
