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

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>
    <title>gen_tcp</title>
    <prepared>tony@erix.ericsson.se</prepared>
    <docno></docno>
    <date>1997-10-24</date>
    <rev>A</rev>
  </header>
  <module since="">gen_tcp</module>
  <modulesummary>Interface to TCP/IP sockets.</modulesummary>
  <description>
    <p>This module provides functions for communicating
      with sockets using the TCP/IP protocol.</p>
    <p>The following code fragment is a simple example of
      a client connecting to a server at port 5678, transferring a
      binary, and closing the connection:</p>
    <code type="none">
client() ->
    SomeHostInNet = "localhost", % to make it runnable on one machine
    {ok, Sock} = gen_tcp:connect(SomeHostInNet, 5678, 
                                 [binary, {packet, 0}]),
    ok = gen_tcp:send(Sock, "Some Data"),
    ok = gen_tcp:close(Sock).</code>
    <p>At the other end, a server is listening on port 5678, accepts
      the connection, and receives the binary:</p>
    <code type="none">
server() ->
    {ok, LSock} = gen_tcp:listen(5678, [binary, {packet, 0}, 
                                        {active, false}]),
    {ok, Sock} = gen_tcp:accept(LSock),
    {ok, Bin} = do_recv(Sock, []),
    ok = gen_tcp:close(Sock),
    ok = gen_tcp:close(LSock),
    Bin.

do_recv(Sock, Bs) ->
    case gen_tcp:recv(Sock, 0) of
        {ok, B} ->
            do_recv(Sock, [Bs, B]);
        {error, closed} ->
            {ok, list_to_binary(Bs)}
    end.</code>
    <p>For more examples, see section
    <seeerl marker="#examples">Examples</seeerl>.</p>

    <note>
      <p>For <c>gen_tcp</c> with <c>inet_backend = socket</c> we have tried
      to be as "compatible" as possible but it has not always been possible.
      Here is a list of cases when the behaviour of inet-backend
      <c>inet</c> (default) and <c>socket</c> are different: </p>
      <list type="bulleted">
        <item>
	  <p><seeerl marker="#non_blocking_send">Non-blocking send</seeerl></p>
	  <p>If a user calling
	  <seemfa marker="#send/2"><c>gen_tcp:send/2</c></seemfa>     
	  with <c>inet_backend = inet</c>,
	  tries to send more data than there is room for in the OS buffers,
	  the "rest data" is buffered by the inet driver (and later sent in the
	  background).
	  The effect for the user is that the call is non-blocking.</p>
	  <p>This is <em>not</em> the effect when
	  <c>inet_backend = socket</c>, since there is no buffering.
	  Instead the user hangs either until all data has been sent or the
	  <c>send_timeout</c> timeout has been reached. </p>
	</item>

	<item><p>Remote close detected by background send. </p>
	<p>An background send will detect a 'remote close' and
	(the inet driver will) mark the socket as 'closed'.
	No other action is taken.
	If the socket has <c>active</c> set to <c>false</c> (passive) at
	this point and no one is reading, this will not be noticed.
	But as soon as the socket is "activated" (<c>active</c> set to
	not <c>false</c>,
	<seemfa marker="#send/2">send/2</seemfa>
	is called or
	<seemfa marker="#recv/2">recv/2,3</seemfa>
	is called),
	an error message will be sent to the
	caller or (socket) owner: <c>{tcp_error, Socket, econnreset}</c>.
	Any data in the OS receive buffers will be lost! </p>
	<p>This behaviour is <em>not</em> replicated by the socket
	implementation.
	A send operation will detect a remote close and immediately
	return this to the caller, but do nothing else.
	A reader will therefor be able to extract any data from the OS buffers.
	If the socket is set to <c>active</c> to not <c>false</c>, the data
	will be received as expected (<c>{tcp, ...}</c> and then a closed
	message (<c>{tcp_closed, ...}</c> will be received (not an error). </p>
	</item>

        <item>
	  <p>The option
	  <seeerl marker="inet#option-show_econnreset">show_econnreset</seeerl>
	  basically do <em>not</em>
	  work as described when used with <c>inet_backend = socket</c>.
	  The "issue" is that a remote close (as described above)
	  <em>do</em> allow a reader to extract what is in the read buffers
	  before a close is "delivered".</p>
	</item>

        <item>
	  <p>The option
	  <seeerl marker="inet#option-nodelay">nodelay</seeerl>
	  is a TCP specific option that is <em>not</em> compatible with
	  <c>domain = local</c>. </p>
	  <p>When using <c>inet_backend = socket</c>, trying to create a socket
	  (via listen or connect) with <c>domain = local</c>
	  (for example with option {ifaddr, {local,"/tmp/test"}})
	  <em>will fail</em> with <c>{error, enotsup}</c>. </p>
	  <p>This does not actually work for <c>inet_backend = inet</c> either,
	  but in that case the error is simply <em>ignored</em>,
	  which is a <em>bad</em> idea. We have choosen to <em>not</em>
	  ignore this error for <c>inet_backend = socket</c>. </p>
	</item>

	<item>
	  <p><seeerl marker="#async_shutdown_write">Async shutdown write</seeerl></p>
	  <p>Calling
	  <seemfa marker="#shutdown/2">gen_tcp:shutdown(Socket, write | read_write)</seemfa>
	  on a socket created with <c>inet_backend = socket</c>
	  will take <em>immediate</em> effect,
	  unlike for a socket created with <c>inet_backend = inet</c>.</p>
	  <p>See <seeerl marker="#async_shutdown_write">async shutdown write</seeerl>
	  for more info. </p>
	</item>
      </list>
    </note>
  </description>

  <datatypes>
    <datatype>
      <name name="option"/>
    </datatype>
    <datatype>
      <name name="pktoptions_value"/>
      <desc>
	<p>
	  If the platform implements the IPv4 option
	  <c>IP_PKTOPTIONS</c>, or the IPv6 option
	  <c>IPV6_PKTOPTIONS</c> or <c>IPV6_2292PKTOPTIONS</c> for the socket
	  this value is returned from
	  <seemfa marker="inet#getopts/2"><c>inet:getopts/2</c></seemfa>
	  when called with the option name
	  <seetype marker="#option_name"><c>pktoptions</c></seetype>.
	</p>
	<note>
	  <p>
	    This option appears to be VERY Linux specific,
	    and its existence in future Linux kernel versions
	    is also worrying since the option is part of RFC 2292
	    which is since long (2003) obsoleted by RFC 3542
	    that <em>explicitly</em> removes this possibility to get
	    packet information from a stream socket.
	    For comparision: it has existed in FreeBSD but is now removed,
	    at least since FreeBSD 10.
	  </p>
	</note>
      </desc>
    </datatype>
    <datatype>
      <name name="option_name"/>
    </datatype>
    <datatype>
      <name name="connect_option"/>
    </datatype>
    <datatype>
      <name name="listen_option"/>
    </datatype>
    <datatype>
      <name>socket()</name>
      <desc><p>As returned by
        <seemfa marker="#accept/1"><c>accept/1,2</c></seemfa> and
        <seemfa marker="#connect/3"><c>connect/3,4</c></seemfa>.</p>
        <marker id="connect"></marker>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="accept" arity="1" since=""/>
      <name name="accept" arity="2" since=""/>
      <fsummary>Accept an incoming connection request on a listening socket.</fsummary>
      <type_desc variable="ListenSocket">Returned by
        <seemfa marker="#listen/2"><c>listen/2</c></seemfa>.
      </type_desc>
      <desc>
        <p>Accepts an incoming connection request on a listening socket.
          <c><anno>Socket</anno></c> must be a socket returned from
          <seemfa marker="#listen/2"><c>listen/2</c></seemfa>.
          <c><anno>Timeout</anno></c> specifies a time-out value in
          milliseconds. Defaults to <c>infinity</c>.</p>
        <p>Returns:</p>
        <list type="bulleted">
          <item><p><c>{ok, <anno>Socket</anno>}</c> if a connection is
            established</p></item>
          <item><p><c>{error, closed}</c> if <c><anno>ListenSocket</anno></c>
            is closed</p></item>
          <item><p><c>{error, timeout}</c> if no connection is established
            within the specified time</p></item>
          <item><p><c>{error, system_limit}</c> if all available ports in the
            Erlang emulator are in use</p></item>
          <item><p>A POSIX error value if something else goes wrong, see
            <seeerl marker="inet"><c>inet(3)</c></seeerl> for possible
            error values</p></item>
        </list>
        <p>Packets can be sent to the returned socket <c><anno>Socket</anno></c>
          using
          <seemfa marker="#send/2"><c>send/2</c></seemfa>.
          Packets sent from the peer are delivered as messages (unless
          <c>{active, false}</c> is specified in the option list for the
          listening socket, in which case packets are retrieved by calling
          <seemfa marker="#recv/2"><c>recv/2</c></seemfa>):</p>
        <code type="none">
{tcp, Socket, Data}</code>
        <note>
          <p>The <c>accept</c> call does
            <em>not</em> have to be issued from the socket owner
            process. Using version 5.5.3 and higher of the emulator,
            multiple simultaneous accept calls can be issued from
            different processes, which allows for a pool of acceptor
            processes handling incoming connections.</p>
        </note>
      </desc>
    </func>

    <func>
      <name name="close" arity="1" since=""/>
      <fsummary>Close a TCP socket.</fsummary>
      <desc>
        <p>Closes a TCP socket.</p>
        <p>Note that in most implementations of TCP, doing a <c>close</c> does
         not guarantee that any data sent is delivered to the recipient before
         the close is detected at the remote side. If you want to guarantee
         delivery of the data to the recipient there are two common ways to
         achieve this.</p>
         <list type="ordered">
           <item><p>Use <seemfa marker="#shutdown/2">
             <c>gen_tcp:shutdown(Sock, write)</c></seemfa> to signal that
             no more data is to be sent and wait for the read side of the
             socket to be closed.</p>
           </item>
           <item><p>Use the socket option <seeerl marker="inet#packet">
             <c>{packet, N}</c></seeerl> (or something similar) to make
             it possible for the receiver to close the connection when it
             knowns it has received all the data.</p>
           </item>
         </list>
      </desc>
    </func>

    <func>
      <name name="connect" arity="3" since=""/>
      <name name="connect" arity="4" since=""/>
      <fsummary>Connect to a TCP port.</fsummary>
      <desc>
        <p>Connects to a server on TCP port <c><anno>Port</anno></c> on the host
          with IP address <c><anno>Address</anno></c>. Argument
          <c><anno>Address</anno></c> can be a hostname or an IP address.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>{ip, Address}</c></tag>
          <item><p>If the host has many network interfaces, this option
            specifies which one to use.</p></item>
          <tag><c>{ifaddr, Address}</c></tag>
          <item><p>Same as <c>{ip, Address}</c>. If the host has many
            network interfaces, this option specifies which one to use.</p>
          </item>
          <tag><c>{fd, integer() >= 0}</c></tag>
          <item><p>If a socket has somehow been connected without using
            <c>gen_tcp</c>, use this option to pass the file descriptor
            for it. If <c>{ip, Address}</c> and/or
            <c>{port, port_number()}</c> is combined with this option,  the
            <c>fd</c> is bound to the specified interface and port before
            connecting. If these options are not specified, it is assumed that
            the <c>fd</c> is already bound appropriately.</p></item>
          <tag><c>inet</c></tag>
          <item><p>Sets up the socket for IPv4.</p></item>
          <tag><c>inet6</c></tag>
          <item><p>Sets up the socket for IPv6.</p></item>
	  <tag><c>local</c></tag>
	  <item>
            <p>
	      Sets up a Unix Domain Socket. See
	      <seetype marker="inet#local_address">
		<c>inet:local_address()</c>
	      </seetype>
	    </p>
          </item>
          <tag><c>{port, Port}</c></tag>
          <item><p>Specifies which local port number to use.</p></item>
          <tag><c>{tcp_module, module()}</c></tag>
          <item><p>Overrides which callback module is used. Defaults to
            <c>inet_tcp</c> for IPv4 and <c>inet6_tcp</c> for IPv6.</p></item>
          <tag><c>Opt</c></tag>
          <item><p>See
            <seemfa marker="inet#setopts/2"><c>inet:setopts/2</c></seemfa>.</p>
          </item>
        </taglist>
        <p>Packets can be sent to the returned socket <c><anno>Socket</anno></c>
          using <seemfa marker="#send/2"><c>send/2</c></seemfa>.
          Packets sent from the peer are delivered as messages:</p>
        <code type="none">
{tcp, Socket, Data}</code>
        <p>If the socket is in <c>{active, N}</c> mode (see
          <seemfa marker="inet#setopts/2"><c>inet:setopts/2</c></seemfa>
          for details) and its message counter drops to <c>0</c>, the following
          message is delivered to indicate that the
          socket has transitioned to passive (<c>{active, false}</c>) mode:</p>
        <code type="none">
{tcp_passive, Socket}</code>
        <p>If the socket is closed, the following message is delivered:</p>
        <code type="none">
{tcp_closed, Socket}</code>
        <p>If an error occurs on the socket, the following message is delivered
          (unless <c>{active, false}</c> is specified in the option list for
          the socket, in which case packets are retrieved by calling
          <seemfa marker="#recv/2"><c>recv/2</c></seemfa>):</p>
        <code type="none">
{tcp_error, Socket, Reason}</code>
        <p>The optional <c><anno>Timeout</anno></c> parameter specifies a
          time-out in milliseconds. Defaults to <c>infinity</c>.</p>
        <note>
          <p>Keep in mind that if the underlying OS <c>connect()</c> call returns
            a timeout, <c>gen_tcp:connect</c> will also return a timeout
            (i.e. <c>{error, etimedout}</c>), even if a larger <c>Timeout</c> was
            specified.</p>
        </note>
        <note>
          <p>The default values for options specified to <c>connect</c> can
            be affected by the Kernel configuration parameter
            <c>inet_default_connect_options</c>. For details, see
            <seeerl marker="inet"><c>inet(3)</c></seeerl>.</p>
        </note>
      </desc>
    </func>

    <func>
      <name name="controlling_process" arity="2" since=""/>
      <fsummary>Change controlling process of a socket.</fsummary>
      <desc>
        <p>Assigns a new controlling process <c><anno>Pid</anno></c> to
          <c><anno>Socket</anno></c>. The controlling process is the process
          that receives messages from the socket. If called by any other
          process than the current controlling process,
          <c>{error, not_owner}</c> is returned. If the process identified
          by <c><anno>Pid</anno></c> is not an existing local pid,
          <c>{error, badarg}</c> is returned. <c>{error, badarg}</c> may also
          be returned in some cases when <c><anno>Socket</anno></c> is closed
          during the execution of this function.</p>
          <p>If the socket is set in active mode, this function
          will transfer any messages in the mailbox of the caller
          to the new controlling process.
          If any other process is interacting with the socket while
          the transfer is happening, the transfer may not work correctly
          and messages may remain in the caller's mailbox. For instance
          changing the sockets active mode before the transfer is complete
          may cause this.</p>
      </desc>
    </func>

    <func>
      <name name="listen" arity="2" since=""/>
      <fsummary>Set up a socket to listen on a port.</fsummary>
      <desc>
        <p>Sets up a socket to listen on port <c><anno>Port</anno></c> on
          the local host.</p>
        <p>If <c><anno>Port</anno> == 0</c>, the underlying OS assigns an
          available port number, use
          <seemfa marker="inet#port/1"><c>inet:port/1</c></seemfa>
          to retrieve it.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>list</c></tag>
          <item><p>Received <c>Packet</c> is delivered as a list.</p></item>
          <tag><c>binary</c></tag>
          <item><p>Received <c>Packet</c> is delivered as a binary.</p></item>
          <tag><c>{backlog, B}</c></tag>
          <item><p><c>B</c> is an integer &gt;= <c>0</c>. The backlog value
            defines the maximum length that the queue of pending connections
            can grow to. Defaults to <c>5</c>.</p></item>
          <tag><c>{ip, Address}</c></tag>
          <item><p>If the host has many network interfaces, this option
            specifies which one to listen on.</p></item>
          <tag><c>{port, Port}</c></tag>
          <item><p>Specifies which local port number to use.</p></item>
          <tag><c>{fd, Fd}</c></tag>
          <item><p>If a socket has somehow been connected without using
            <c>gen_tcp</c>, use this option to pass the file
            descriptor for it.</p></item>
          <tag><c>{ifaddr, Address}</c></tag>
          <item><p>Same as <c>{ip, Address}</c>. If the host has many
            network interfaces, this option specifies which one to use.</p>
          </item>
          <tag><c>inet6</c></tag>
          <item><p>Sets up the socket for IPv6.</p></item>
          <tag><c>inet</c></tag>
          <item><p>Sets up the socket for IPv4.</p></item>
          <tag><c>{tcp_module, module()}</c></tag>
          <item><p>Overrides which callback module is used. Defaults to
            <c>inet_tcp</c> for IPv4 and <c>inet6_tcp</c> for IPv6.</p></item>
          <tag><c>Opt</c></tag>
          <item><p>See
            <seemfa marker="inet#setopts/2"><c>inet:setopts/2</c></seemfa>.
            </p></item>
        </taglist>
        <p>The returned socket <c><anno>ListenSocket</anno></c> should be used
          in calls to <seemfa marker="#accept/1"><c>accept/1,2</c></seemfa> to
          accept incoming connection requests.</p>
        <note>
          <p>The default values for options specified to <c>listen</c> can
            be affected by the Kernel configuration parameter
            <c>inet_default_listen_options</c>. For details, see
            <seeerl marker="inet"><c>inet(3)</c></seeerl>.</p>
        </note>
      </desc>
    </func>

    <func>
      <name name="recv" arity="2" since=""/>
      <name name="recv" arity="3" since=""/>
      <fsummary>Receive a packet from a passive socket.</fsummary>
      <type_desc variable="HttpPacket">See the description of
        <c>HttpPacket</c> in
        <seemfa marker="erts:erlang#decode_packet/3"><c>erlang:decode_packet/3</c></seemfa>
      in ERTS.
      </type_desc>
      <desc>
        <p>Receives a packet from a socket in passive
          mode. A closed socket is indicated by return value
          <c>{error, closed}</c>.</p>
        <p>Argument <c><anno>Length</anno></c> is only meaningful when
          the socket is in <c>raw</c> mode and denotes the number of
          bytes to read. If <c><anno>Length</anno></c> is <c>0</c>, all
          available bytes are returned.
          If <c><anno>Length</anno></c> &gt; <c>0</c>, exactly
          <c><anno>Length</anno></c> bytes are returned, or an error;
          possibly discarding less than <c><anno>Length</anno></c> bytes of
          data when the socket is closed from the other side.</p>
        <p>The optional <c><anno>Timeout</anno></c> parameter specifies a
          time-out in milliseconds. Defaults to <c>infinity</c>.</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="2" since=""/>
      <fsummary>Send a packet.</fsummary>
      <desc>
        <p>Sends a packet on a socket.</p>
        <p>There is no <c>send</c> call with a time-out option, use socket
        option <c>send_timeout</c> if time-outs are desired. See section
        <seeerl marker="#examples">Examples</seeerl>.</p>
	<p>The return value <c>{error, {timeout, RestData}}</c> can
	only be returned when <c>inet_backend = socket</c>. </p>
        <marker id="non_blocking_send"></marker>
        <note>
          <p>Non-blocking send.</p>
          <p>If the user tries to send more data than there is room for
          in the OS send buffers, the 'rest data' is put into (inet driver)
          internal buffers and later sent in the background.
          The function immediately returns ok (<em>not</em> informing the
          caller that not all of the data was actually sent).
          Any issue while sending the 'rest data' is maybe returned later. </p>
          <p>When using <c>inet_backend = socket</c>, the
          behaviour is different. There is <em>no</em>
          buffering done (like the inet-driver does), instead
          the caller will "hang" until all of the data has been sent or
          send timeout (as specified by the <c>send_timeout</c> option)
          expires (the function can hang even when using 'inet' backend
          if the internal buffers are full). </p>
	  <p>If this happens when using <c>packet =/= raw</c>, we have a partial
	  package written. A new package therefor <em>must not</em> be written
	  at this point, as there is no way for the peer to distinguish this
	  from the data portion of the current package. Instead, set package
	  to raw, send the rest data (as raw data) and then set package to
	  the wanted package type again. </p>
        </note>
      </desc>
    </func>

    <func>
      <name name="shutdown" arity="2" since=""/>
      <fsummary>Asynchronously close a socket.</fsummary>
      <desc>
        <p>Closes a socket in one or two directions.</p>
        <p><c><anno>How</anno> == write</c> means closing the socket for
          writing, reading from it is still possible.</p>
        <p>If <c><anno>How</anno> == read</c> or there is no outgoing
          data buffered in the <c><anno>Socket</anno></c> port,
          the socket is shut down immediately and any error encountered
          is returned in <c><anno>Reason</anno></c>.</p>
        <p>If there is data buffered in the socket port, the attempt
          to shutdown the socket is postponed until that data is written to the
          kernel socket send buffer. If any errors are encountered, the socket
          is closed and <c>{error, closed}</c> is returned on the next
          <seemfa marker="#recv/2"><c>recv/2</c></seemfa> or
          <seemfa marker="#send/2"><c>send/2</c></seemfa>.</p>
        <p>Option <c>{exit_on_close, false}</c> is useful if the peer has done
          a shutdown on the write side.</p>
        <marker id="async_shutdown_write"></marker>
        <note>
          <p>Async shutdown write (write or read_write).</p>
          <p>If the shutdown attempt is made while the inet-driver
	  is sending buffered data in the background,
	  the shutdown is postponed until all buffered data has been sent.
	  The function immediately returns <c>ok</c> and the caller is
          <em>not</em> informed (that the shutdown has <em>not yet</em>
          been performed). </p>
          <p>When using <c>inet_backend = socket</c>, the
          behaviour is different. A shutdown with
          <c>How == write | read_write</c>, the operation will
          take <em>immediate</em> effect (unlike the inet-driver,
	  which basically saves the operation for later). </p>
        </note>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Examples</title>
    <marker id="examples"></marker>
    <p>The following example illustrates use of option
    <c>{active,once}</c> and multiple accepts by implementing a server
    as a number of worker processes doing accept on a single listening
    socket. Function <c>start/2</c> takes the number of worker
    processes and the port number on which to listen for incoming
    connections. If <c>LPort</c> is specified as <c>0</c>, an
    ephemeral port number is used, which is why the start function
    returns the actual port number allocated:</p>
    <code type="none">
start(Num,LPort) ->
    case gen_tcp:listen(LPort,[{active, false},{packet,2}]) of
        {ok, ListenSock} ->
            start_servers(Num,ListenSock),
            {ok, Port} = inet:port(ListenSock),
            Port;
        {error,Reason} ->
            {error,Reason}
    end.

start_servers(0,_) ->
    ok;
start_servers(Num,LS) ->
    spawn(?MODULE,server,[LS]),
    start_servers(Num-1,LS).

server(LS) ->
    case gen_tcp:accept(LS) of
        {ok,S} ->
            loop(S),
            server(LS);
        Other ->
            io:format("accept returned ~w - goodbye!~n",[Other]),
            ok
    end.

loop(S) ->
    inet:setopts(S,[{active,once}]),
    receive
        {tcp,S,Data} ->
            Answer = process(Data), % Not implemented in this example
            gen_tcp:send(S,Answer),
            loop(S);
        {tcp_closed,S} ->
            io:format("Socket ~w closed [~w]~n",[S,self()]),
            ok
    end.</code>
    <p>Example of a simple client:</p>
    <code type="none">
client(PortNo,Message) ->
    {ok,Sock} = gen_tcp:connect("localhost",PortNo,[{active,false},
                                                    {packet,2}]),
    gen_tcp:send(Sock,Message),
    A = gen_tcp:recv(Sock,0),
    gen_tcp:close(Sock),
    A.</code>
    <p>The <c>send</c> call does not accept a time-out
      option because time-outs on send is handled through socket
      option <c>send_timeout</c>. The behavior of a send operation with
      no receiver is mainly defined by the underlying TCP
      stack and the network infrastructure. To write
      code that handles a hanging receiver that can eventually cause
      the sender to hang on a <c>send</c> do like the following.</p>
    <p>Consider a process that receives data from a client process
      to be forwarded to a server on the network. The process is
      connected to the server through TCP/IP and does not get any acknowledge
      for each message it sends, but has to rely on the send time-out
      option to detect that the other end is unresponsive. Option
      <c>send_timeout</c> can be used when connecting:</p>
    <code type="none">
...
{ok,Sock} = gen_tcp:connect(HostAddress, Port,
                            [{active,false},
                             {send_timeout, 5000},
                             {packet,2}]),
                loop(Sock), % See below
...</code>
    <p>In the loop where requests are handled, send time-outs can now be
      detected:</p>
    <code type="none">
loop(Sock) ->
    receive
        {Client, send_data, Binary} ->
            case gen_tcp:send(Sock,[Binary]) of
                {error, timeout} ->
                    io:format("Send timeout, closing!~n",
                              []),
                    handle_send_timeout(), % Not implemented here
                    Client ! {self(),{error_sending, timeout}},
                    %% Usually, it's a good idea to give up in case of a 
                    %% send timeout, as you never know how much actually 
                    %% reached the server, maybe only a packet header?!
                    gen_tcp:close(Sock);
                {error, OtherSendError} ->
                    io:format("Some other error on socket (~p), closing",
                              [OtherSendError]),
                    Client ! {self(),{error_sending, OtherSendError}},
                    gen_tcp:close(Sock);
                ok ->
                    Client ! {self(), data_sent},
                    loop(Sock)
            end
    end.</code>
    <p>Usually it suffices to detect time-outs on receive, as most
      protocols include some sort of acknowledgment from the server,
      but if the protocol is strictly one way, option <c>send_timeout</c>
      comes in handy.</p>
  </section>
</erlref>

