path: root/lib/kernel/doc/src/inet.xml

<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1997</year><year>2012</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>inet</title>
    <prepared>[email protected]</prepared>
    <docno></docno>
    <date>1998-02-04</date>
    <rev>A</rev>
  </header>
  <module>inet</module>
  <modulesummary>Access to TCP/IP Protocols</modulesummary>
  <description>
    <p>Provides access to TCP/IP protocols.</p>
    <p>See also <em>ERTS User's Guide, Inet configuration</em> for more
      information on how to configure an Erlang runtime system for IP
      communication.</p>
    <p>Two Kernel configuration parameters affect the behaviour of all
      sockets opened on an Erlang node:
      <c>inet_default_connect_options</c> can contain a list of default
      options used for all sockets returned when doing <c>connect</c>,
      and <c>inet_default_listen_options</c> can contain a list of
      default options used when issuing a <c>listen</c> call. When
      <c>accept</c> is issued, the values of the listensocket options
      are inherited, why no such application variable is needed for
      <c>accept</c>.</p>
    <p>Using the Kernel configuration parameters mentioned above, one
      can set default options for all TCP sockets on a node. This should
      be used with care, but options like <c>{delay_send,true}</c>
      might be specified in this way. An example of starting an Erlang
      node with all sockets using delayed send could look like this:</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>Note that the default option <c>{active, true}</c> currently
      cannot be changed, for internal reasons.</p>
    <p>Addresses as inputs to functions can be either a string or a
      tuple. For instance, the IP address 150.236.20.73 can be passed to
      <c>gethostbyaddr/1</c> either as the string "150.236.20.73"
      or as the tuple <c>{150, 236, 20, 73}</c>.</p>
    <p>IPv4 address examples:</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>IPv6 address examples:</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
                {16#FFFF,0,0,0,0,0,(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#fe80,0,0,0,0,16#204,16#acff,16#fe17,16#bf38}</code>
    <p>A function that may be useful is <c>inet_parse:address/1</c>:</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,{65535,0,0,0,0,0,49320,10754}}</pre>
  </description>

  <datatypes>
    <datatype>
      <name name="hostent"/>
      <desc>
        <p>The record is defined in the Kernel include file "inet.hrl".
          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="posix"/>
      <desc><p>An atom which is named from the Posix error codes
         used in Unix, and in the runtime libraries of most
          C compilers. See
          <seealso marker="#error_codes">POSIX Error Codes</seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name><marker id="type-socket">socket()</marker></name>
      <desc><p>See <seealso marker="gen_tcp#type-socket">gen_tcp(3)</seealso>
         and <seealso marker="gen_udp#type-socket">gen_udp(3)</seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="address_family"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a socket of any type</fsummary>
      <desc>
        <p>Closes a socket of any type.</p>
      </desc>
    </func>
    <func>
      <name name="get_rc" arity="0"/>
      <fsummary>Return a list of IP configuration parameters</fsummary>
      <desc>
        <p>Returns the state of the Inet configuration database in 
          form of a list of recorded configuration parameters. (See the
          ERTS User's Guide, Inet configuration, for more information).
          Only parameters with other than default values are returned.</p>
      </desc>
    </func>
    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Return a descriptive string for an error reason</fsummary>
      <desc>
        <p>Returns a diagnostic error string. See the section below
          for possible Posix values and the corresponding
          strings.</p>
      </desc>
    </func>
    <func>
      <name name="getaddr" arity="2"/>
      <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"/>
      <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"/>
      <fsummary>Return a hostent record for the host with the given address</fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record given an address.</p>
      </desc>
    </func>
    <func>
      <name name="gethostbyname" arity="1"/>
      <fsummary>Return a hostent record for the host with the given name</fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record given a hostname.</p>
      </desc>
    </func>
    <func>
      <name name="gethostbyname" arity="2"/>
      <fsummary>Return a hostent record for the host with the given name</fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record given a hostname, restricted
          to the given address family.</p>
      </desc>
    </func>
    <func>
      <name name="gethostname" arity="0"/>
      <fsummary>Return the local hostname</fsummary>
      <desc>
        <p>Returns the local hostname. Will never fail.</p>
      </desc>
    </func>

    <func>
      <name name="getifaddrs" arity="0"/>
      <fsummary>Return a list of interfaces and their addresses</fsummary>
    <desc>
      <p>
	Returns a list of 2-tuples containing interface names and the
	interface's addresses. <c><anno>Ifname</anno></c> is a Unicode string.
	<c><anno>Hwaddr</anno></c> is hardware dependent, e.g on Ethernet interfaces
	it is the 6-byte Ethernet address (MAC address (EUI-48 address)).
      </p>
      <p>
	The <c>{addr,<anno>Addr</anno>}</c>, <c>{netmask,_}</c> and <c>{broadaddr,_}</c>
	tuples are repeated in the result list iff the interface has multiple
	addresses. If you come across an interface that has
	multiple <c>{flag,_}</c> or <c>{hwaddr,_}</c> tuples you have
	a really strange interface or possibly a bug in this function.
	The <c>{flag,_}</c> tuple is mandatory, all other optional.
      </p>
      <p>
	Do not rely too much on the order of <c><anno>Flag</anno></c> atoms or
	<c><anno>Ifopt</anno></c> tuples. There are some rules, though:
	<list>
	  <item>
	    Immediately after <c>{addr,_}</c> follows <c>{netmask,_}</c>
	  </item>
	  <item>
	    Immediately thereafter follows <c>{broadaddr,_}</c> if
	    the <c>broadcast</c> flag is <em>not</em> set and the
	    <c>pointtopoint</c> flag <em>is</em> set.
	  </item>
	  <item>
	    Any <c>{netmask,_}</c>, <c>{broadaddr,_}</c> or
	    <c>{dstaddr,_}</c> tuples that follow an <c>{addr,_}</c>
	    tuple concerns that address.
	  </item>
	</list>
      </p>
      <p>
	The <c>{hwaddr,_}</c> tuple is not returned on Solaris since the
	hardware address historically belongs to the link layer and only
	the superuser can read such addresses.
      </p>
      <p>
	On Windows, the data is fetched from quite 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>Flag</anno></c> values.
	You have been warned. Report flagrant bugs.
      </p>
    </desc>
    </func>

    <func>
      <name name="getopts" arity="2"/>
      <fsummary>Get one or more options for a socket</fsummary>
      <type name="socket_getopt"/>
      <type name="socket_setopt"/>
      <desc>
        <p>Gets one or more options for a socket. 
          See <seealso marker="#setopts/2">setopts/2</seealso> 
          for a list of available options.</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 simply left out in the returned list. An error tuple is only
          returned when getting options for the socket is impossible
          (i.e. 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 the current platform.</p>
        <p>The <c>RawOptReq</c> consists of the 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
          should be used when the underlying <c>getsockopt</c> requires 
          <em>input</em>
          in the argument field, in which case the size of the binary
          should 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>As an example, consider a Linux machine where the
          <c>TCP_INFO</c> option could be used to collect TCP statistics
          for a socket. Lets say we're interested in the
          <c>tcpi_sacked</c> field of the <c>struct tcp_info</c>
          filled in when asking for <c>TCP_INFO</c>. To
          be able to access this information, we need to know both the
          numeric value of the protocol level <c>IPPROTO_TCP</c>, the
          numeric value of the option <c>TCP_INFO</c>, the size of the
          <c>struct tcp_info</c> and the size and offset of
          the specific field. By inspecting the headers or writing a small C
          program, we found <c>IPPROTO_TCP</c> to be 6,
          <c>TCP_INFO</c> to be 11, the structure size to be 92 (bytes),
          the offset of <c>tcpi_sacked</c> to be 28 bytes and the actual
          value to be a 32 bit integer. We could use the following
          code 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 OS
          and the kernel version prior to executing anything similar to the
          code above.</p>
      </desc>
    </func>

    <func>
      <name name="getstat" arity="1"/>
      <name name="getstat" arity="2"/>
      <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>,&nbsp;[recv_avg,&nbsp;recv_cnt,&nbsp;recv_dvi,&nbsp;recv_max,&nbsp;recv_oct,&nbsp;send_avg,&nbsp;send_cnt,&nbsp;send_dvi,&nbsp;send_max,&nbsp;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 to the socket.</p>
	  </item>
	  <tag><c>recv_cnt</c></tag>
	  <item>
            <p>Number of packets received to the socket.</p>
	  </item>
	  <tag><c>recv_dvi</c></tag>
	  <item>
            <p>Average packet size deviation in bytes received to the socket.</p>
	  </item>
	  <tag><c>recv_max</c></tag>
	  <item>
            <p>The size of the largest packet in bytes received to the socket.</p>
	  </item>
	  <tag><c>recv_oct</c></tag>
	  <item>
            <p>Number of bytes received to 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_dvi</c></tag>
	  <item>
            <p>Average packet size deviation in bytes received sent from the socket.</p>
	  </item>
	  <tag><c>send_max</c></tag>
	  <item>
            <p>The 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="parse_ipv4_address" arity="1" />
      <fsummary>Parse an IPv4 address</fsummary>
      <desc>
	<p>Parses an IPv4 address string and returns an <a href="#type-ip4_address">ip4_address()</a>.
	 Accepts a shortened IPv4 shortened address string.</p>
      </desc>
    </func>
    <func>
      <name name="parse_ipv4strict_address" arity="1" />
      <fsummary>Parse an IPv4 address strict.</fsummary>
      <desc>
	<p>Parses an IPv4 address string containing four fields, i.e <b>not</b> shortened, and returns an <a href="#type-ip4_adress">ip4_address()</a>.</p>
      </desc>
    </func>
    <func>
      <name name="parse_ipv6_address" arity="1" />
      <fsummary>Parse an IPv6 address</fsummary>
      <desc>
	<p>Parses an IPv6 address string and returns an <a href="#type-ip6_address">ip6_address()</a>.
	 If an IPv4 address string is passed, an IPv4-mapped IPv6 address is returned.</p>
      </desc>
    </func>
    <func>
      <name name="parse_ipv6strict_address" arity="1" />
      <fsummary>Parse an IPv6 address strict.</fsummary>
      <desc>
	<p>Parses an IPv6 address string and returns an <a href="#type-ip6_address">ip6_address()</a>. 
	Does <b>not</b> accept IPv4 adresses.</p>
      </desc>
    </func>
    <func>
      <name name="parse_address" arity="1" />
      <fsummary>Parse an IPv4 or IPv6 address.</fsummary>
      <desc>
	<p>Parses an IPv4 or IPv6 address string and returns an <a href="#type-ip4_address">ip4_address()</a> or <a href="#type-ip6_address">ip6_address()</a>. Accepts a shortened IPv4 address string.</p>
      </desc>
    </func>
    <func>
      <name name="parse_strict_address" arity="1" />
      <fsummary>Parse an IPv4 or IPv6 address strict.</fsummary>
      <desc>
	<p>Parses an IPv4 or IPv6 address string and returns an <a href="#type-ip4_address">ip4_address()</a> or <a href="#type-ip6_adress">ip6_address()</a>. Does <b>not</b> accept a shortened IPv4 address string.</p>
      </desc>
    </func>
    <func>
      <name name="peername" arity="1"/>
      <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>
      </desc>
    </func>
    <func>
      <name name="port" arity="1"/>
      <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="sockname" arity="1"/>
      <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>
      </desc>
    </func>
    <func>
      <name name="setopts" arity="2"/>
      <fsummary>Set one or more options for a socket</fsummary>
      <type name="socket_setopt"/>
      <desc>
        <p>Sets one or more options for a socket. The following options
          are available:</p>
        <taglist>
          <tag><c>{active, true | false | once}</c></tag>
          <item>
            <p>If the value is <c>true</c>, which is the default,
              everything received from the socket will be sent as
              messages to the receiving process. If the value is
              <c>false</c> (passive mode), the process must explicitly
              receive incoming data by calling <c>gen_tcp:recv/2,3</c>
              or <c>gen_udp:recv/2,3</c> (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 will be sent
              to the process.  To receive one more message,
              <c>setopts/2</c> must be called again with the
              <c>{active, once}</c> option.</p>
            <p>When using <c>{active, once}</c>, the socket changes
              behaviour automatically when data is received. This can
              sometimes be confusing in combination with connection
              oriented sockets (i.e. <c>gen_tcp</c>) as a socket with
              <c>{active, false}</c> behaviour reports closing
              differently than a socket with <c>{active, true}</c>
              behaviour. To make programming easier, a socket where
              the peer closed and this was detected while in
              <c>{active, false}</c> mode, will still generate the
              message
              <c>{tcp_closed,Socket}</c> when set to <c>{active, once}</c> or <c>{active, true}</c> mode. It is therefore
              safe to assume that the message
              <c>{tcp_closed,Socket}</c>, possibly followed by socket
              port termination (depending on the <c>exit_on_close</c>
              option) will eventually appear 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 is all up to the
              underlying TCP/IP stack and protocol.</p>
            <p>Note that <c>{active,true}</c> mode provides no flow
              control; a fast sender could easily overflow the
              receiver with incoming messages. Use active mode only if
              your high-level protocol provides its own flow control
              (for instance, acknowledging received messages) or the
              amount of data exchanged is small. <c>{active,false}</c>
              mode or use of the <c>{active, once}</c> mode provides
              flow control; the other side will not be able send
              faster than the receiver can read.</p>
          </item>

          <tag><c>{broadcast, Boolean}</c>(UDP sockets)</tag>
          <item>
            <p>Enable/disable permission to send broadcasts.</p>
			<marker id="option-buffer"></marker>
          </item>

		  <tag><c>{buffer, Size}</c></tag>
		  <item>
			  <p>Determines the size of the user-level software buffer used by
				  the driver. Not to be confused with <c>sndbuf</c>
				  and <c>recbuf</c> options which correspond to
				  the kernel socket buffers. It is recommended
				  to have <c>val(buffer) &gt;= max(val(sndbuf),val(recbuf))</c>.
				  In fact, the <c>val(buffer)</c> is automatically set to
				  the above maximum when <c>sndbuf</c> or <c>recbuf</c> values are set.</p>
		  </item>

          <tag><c>{delay_send, Boolean}</c></tag>
          <item>
            <p>Normally, when an Erlang process sends to a socket,
              the driver will try to immediately send the data. If that
              fails, the driver will use 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>
              will make <em>all</em> messages queue up. This makes
              the messages actually sent onto the network be larger but
              fewer. The option actually affects the scheduling of send
              requests versus Erlang processes instead of changing any
              real property of the socket. Needless to say it is an
              implementation specific option. Default is <c>false</c>.</p>
          </item>

		  <tag><c>{deliver, port | term}</c></tag>
		  <item> <p> When <c>{active, true}</c> delivers data on the forms
				  <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>Enable/disable routing bypass for outgoing messages.</p>
          </item>

          <tag><c>{exit_on_close, Boolean}</c></tag>
          <item>
            <p>By default this option is set to <c>true</c>.</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 has
              been detected, for instance if the peer has used
              <seealso marker="gen_tcp#shutdown/2">gen_tcp:shutdown/2</seealso>
              to shutdown the write side.</p>
          </item>

          <tag><c>{header, Size}</c></tag>
          <item>
            <p>This option is only meaningful if the <c>binary</c>
              option was specified when the socket was created. If
              the <c>header</c> option is specified, the first
              <c>Size</c> number bytes of data received from the socket
              will be elements of a list, and the rest of the data will
              be a binary given as the tail of the same list. If for
              example <c>Size == 2</c>, the data received will match
              <c>[Byte1,Byte2|Binary]</c>.</p>
          </item>

		  <tag><c>{high_watermark, Size}</c></tag>
		  <item> <p>
				  Sender is forced busy if sent and enqueued data
				  reaches the highwater mark.
				  <br /> Default: 8192 kB. 
			  </p>
		  </item>

	  <tag><c>{ipv6_v6only, Boolean}</c></tag>
	  <item>
	    <p>
	      Restricts the socket to only use IPv6, prohibiting any
	      IPv4 connections. This is only applicable for
	      IPv6 sockets (option <c>inet6</c>).
	    </p>
	    <p>
	      On most platforms this option has to be set on the socket
	      before associating it to an address. Therefore it is only
	      reasonable to give it when creating the socket and not
	      to use it when calling the function
	      (<seealso marker="#setopts/2">setopts/2</seealso>)
	      containing this description.
	    </p>
	    <p>
	      The behaviour of a socket with this socket option set to
	      <c>true</c> is becoming 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 (that 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 towards
	      separating IPv6 from IPv4 traffic has evolved since
	      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>, e.g. OpenBSD and Windows. Trying to set
	      this option to <c>false</c> when creating the socket
	      will in this case fail.
	    </p>
	    <p>
	      Setting this option on platforms where it does not exist
	      is ignored and getting this option with
	      <seealso marker="#getopts/2">getopts/2</seealso>
	      returns no value i.e the returned list will not contain an
	      <c>{ipv6_v6only,_}</c> tuple. On Windows the option acually
	      does not exist, but it is emulated as being a
	      read-only option with the value <c>true</c>.
	    </p>
	    <p>
	      So it boils down to that setting this option to <c>true</c>
	      when creating a socket will never fail except possibly
	      (at the time of this writing) on a platform where you
	      have customized the kernel to only allow <c>false</c>,
	      which might be doable (but weird) on e.g. OpenBSD.
	    </p>
	    <p>
	      If you read back the option value using
	      <seealso marker="#getopts/2">getopts/2</seealso>
	      and get no value the option does not exist in the host OS
	      and all bets are off regarding the behaviour of both
	      an IPv6 and an IPv4 socket listening on the same port
	      as well as for an IPv6 socket getting IPv4 traffic.
	    </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 being exchanged. If
              the other end does not respond, the connection is
              considered broken and an error message will be sent to
              the controlling process. Default disabled.</p>
			<marker id="option-linger"></marker>
          </item>

		  <tag><c>{linger, {true|false, Seconds}}</c></tag>
		  <item>
			  <p>Determines the timeout in seconds for flushing unsent data in the
				  <c>close/1</c> socket call. If the 1st component of the value
				  tuple is <c>false</c>, the 2nd one is ignored, which means that
				  <c>close/1</c> returns immediately not waiting
				  for data to be flushed. Otherwise, the 2nd component is
				  the flushing time-out in seconds.</p>
		  </item>

		  <tag><c>{low_watermark, Size}</c></tag>
		  <item> <p>
				  If the port has reached its <c>high_watermark</c> it will
				  force busy onto senders. When the port data queue reaches the
				  <c>low_watermark</c> callers are no longer forced busy.
				  <br /> Default: 4096 kB. 
			  </p>
		  </item>

		  <tag><c>{mode, Mode :: binary | list}</c></tag>
          <item>
			  <p>Received <c>Packet</c> is delivered as defined by Mode.</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>
		  </item>

          <tag><c>{nodelay, Boolean}</c>(TCP/IP sockets)</tag>
          <item>
            <p>If <c>Boolean == true</c>, the <c>TCP_NODELAY</c> option
              is turned on for the socket, which means that even small
              amounts of data will be sent immediately.</p>
          </item>
          <tag><c>{packet, PacketType}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Defines the type of packets to use for a socket.
              The following values are valid:</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 length of header can be one, two, or four bytes;
                  containing an unsigned integer in big-endian byte order.
                  Each send operation will generate the header, and the header
                  will be stripped off on each receive operation.</p>
                <p>In current implementation 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, there will be one message 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:
                                    <br></br>
<c>asn1</c> - ASN.1 BER,
                                    <br></br>
<c>sunrm</c> - Sun's RPC encoding,
                                    <br></br>
<c>cdr</c> - CORBA (GIOP 1.1),
                                    <br></br>
<c>fcgi</c> - Fast CGI,
                                    <br></br>
<c>tpkt</c> - TPKT format [RFC1006],
                                    <br></br>
<c>line</c> - Line mode, a packet is a line
                  terminated with newline, lines longer than
                  the receive buffer are truncated.</p>
              </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 <seealso marker="erts:erlang#decode_packet/3">
                 erlang:decode_packet/3</seealso>. A socket in passive
                 mode will return <c>{ok, HttpPacket}</c> from <c>gen_tcp:recv</c>
                 while an active socket will send 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 will
		automatically switch from <c>http</c>/<c>http_bin</c> to
		<c>httph</c>/<c>httph_bin</c> internally after the first line
		has been read. There might be occasions however 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 max allowed length of the packet body. If
              the packet header indicates that the length of the packet
              is longer than the max allowed length, the packet is
              considered invalid. The same happens if the packet header
              is too big 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 due
	    to internal buffer limitations.</p>
          </item>

		  <tag><c>{priority, Priority}</c></tag>
          <item> <p>Set the protocol-defined priority for all packets to be sent
			  on this socket.</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 max 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.
              The default is 5, and if this parameter is set too
              high the system can become unresponsive due to
              UDP packet flooding.</p>
          </item>
          <tag><c>{recbuf, Size}</c></tag>
          <item>
            <p>Gives the size of the receive buffer to use for
              the socket.</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 will return
              <c>{error,timeout}</c>. How much of a packet that actually
              got sent is unknown, why the socket should be closed
              whenever a timeout has occurred (see <c>send_timeout_close</c>).
              Default is <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 will be automatically closed when the send operation
              returns <c>{error,timeout}</c>. The recommended setting is
              <c>true</c> which will automatically close the socket.
              Default is <c>false</c> due to backward compatibility.</p>
			<marker id="option-sndbuf"></marker>
          </item>

          <tag><c>{sndbuf, Size}</c></tag>
          <item>
            <p>Gives the size of the send buffer to use for the socket.</p>
          </item>
          <tag><c>{priority, Integer}</c></tag>
          <item>
            <p>Sets the SO_PRIORITY socket level option on platforms where 
              this is implemented. The behaviour and allowed range varies on 
              different systems. The option is ignored on platforms where the 
              option is not implemented. Use with caution.</p>
          </item>
          <tag><c>{tos, Integer}</c></tag>
          <item>
            <p>Sets IP_TOS IP level options on platforms where this is 
              implemented. The behaviour and allowed range varies on different 
              systems. The option is ignored on platforms where the option is 
              not implemented. Use with caution.</p>
          </item>
        </taglist>

        <p>In addition to the options mentioned above, <em>raw</em>
          option specifications can be used. The raw options are
          specified as a tuple of arity four, beginning with the tag
          <c>raw</c>, followed by the protocol level, the option number
          and the actual option value specified as a binary. This
          corresponds to the second, third and fourth argument to the
          <c>setsockopt</c> call in the C socket API. The option value
          needs to be coded in the native endianess of the platform and,
          if a structure is required, needs to follow the struct
          alignment conventions on the specific platform.</p>
        <p>Using raw socket options require detailed knowledge about
          the current operating system and TCP stack.</p>
        <p>As an example of the usage of raw options, consider a Linux
          system where you want to set the <c>TCP_LINGER2</c> option on
          the <c>IPPROTO_TCP</c> protocol level in the stack. You know
          that on this particular system it defaults to 60 (seconds),
          but you would like to lower it to 30 for a particular
          socket. The <c>TCP_LINGER2</c> option is not explicitly
          supported by inet, but you know that the protocol level
          translates to the number 6, the option number to the number 8
          and the value is to be given as a 32 bit integer. You can use
          this line of code 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 given out of range, it could be a good idea to check that
          a raw option really got accepted. This code places the value
          in the variable TcpLinger2:</p>
        <code type="none"><![CDATA[
        {ok,[{raw,6,8,<<TcpLinger2:32/native>>}]}=inet:getopts(Sock,[{raw,6,8,4}]),]]></code>
        <p>Code such as the examples above is inherently non portable,
          even different versions of the same  OS on the same platform
          may respond differently to this kind of option
          manipulation. Use with care.</p>
        <p>Note that the default options for TCP/IP sockets can be
          changed with the Kernel configuration parameters mentioned in
          the beginning of this document.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <marker id="error_codes"></marker>
    <title>POSIX Error Codes</title>
    <list type="bulleted">
      <item><c>e2big</c> - argument list too long</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> - RPC structure is bad</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 w/o 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> - .lib section in a.out 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> - file name too long</item>
      <item><c>enavail</c> - not available</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 be 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 ioctl</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> - program version wrong</item>
      <item><c>eprogunavail</c> - RPC program not available</item>
      <item><c>eproto</c> - protocol error</item>
      <item><c>eprotonosupport</c> - protocol not supported</item>
      <item><c>eprototype</c> - protocol wrong 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 file system</item>
      <item><c>eremoteio</c> - remote i/o error</item>
      <item><c>eremoterelease</c> - EREMOTERELEASE</item>
      <item><c>erofs</c> - read-only file system</item>
      <item><c>erpcmismatch</c> - RPC version is wrong</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> - the hostname or domain name could not be
       found</item>
    </list>
  </section>
</erlref>