gen_tcp(3erl)						     Erlang Module Definition						     gen_tcp(3erl)

NAME

       gen_tcp - Interface to TCP/IP sockets

DESCRIPTION

       The gen_tcp module provides functions for communicating with sockets using the TCP/IP protocol.

       The  following  code  fragment provides a simple example of a client connecting to a server at port 5678, transferring a binary and closing
       the connection:

       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).

       At the other end a server is listening on port 5678, accepts the connection and receives the binary:

       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),
	   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.

       For more examples, see the examples section.

DATA TYPES

       ip_address()
	 see inet(3erl)

       posix()
	 see inet(3erl)

       socket()
	 as returned by accept/1,2 and connect/3,4

EXPORTS

       connect(Address, Port, Options) -> {ok, Socket} | {error, Reason}
       connect(Address, Port, Options, Timeout) -> {ok, Socket} | {error, Reason}

	      Types  Address = string() | atom() | ip_address()
		     Port = 0..65535
		     Options = [Opt]
		     Opt -- see below
		     Timeout = int() | infinity
		     Socket = socket()
		     Reason = posix()

	      Connects to a server on TCP port Port on the host with IP address Address . The Address argument can be either a hostname, or an	IP
	      address.

	      The available options are:

		list :
		  Received Packet is delivered as a list.

		binary :
		  Received Packet is delivered as a binary.

		{ip, ip_address()} :
		  If the host has several network interfaces, this option specifies which one to use.

		{port, Port} :
		  Specify which local port number to use.

		{fd, int()} :
		  If a socket has somehow been connected without using gen_tcp , use this option to pass the file descriptor for it.

		inet6 :
		  Set up the socket for IPv6.

		inet :
		  Set up the socket for IPv4.

		Opt :
		  See inet:setopts/2 .

	      Packets can be sent to the returned socket Socket using send/2 . Packets sent from the peer are delivered as messages:

	      {tcp, Socket, Data}

	      If the socket is closed, the following message is delivered:

	      {tcp_closed, Socket}

	      If an error occurs on the socket, the following message is delivered:

	      {tcp_error, Socket, Reason}

	      unless {active, false} is specified in the option list for the socket, in which case packets are retrieved by calling recv/2 .

	      The optional Timeout parameter specifies a timeout in milliseconds. The default value is infinity .

   Note:
       The  default  values  for options given to connect can be affected by the Kernel configuration parameter inet_default_connect_options . See
       inet(3erl) for details.

       listen(Port, Options) -> {ok, ListenSocket} | {error, Reason}

	      Types  Port = 0..65535
		     Options = [Opt]
		     Opt -- see below
		     ListenSocket -- see below
		     Reason = posix()

	      Sets up a socket to listen on the port Port on the local host.

	      If Port == 0 , the underlying OS assigns an available port number, use inet:port/1 to retrieve it.

	      The available options are:

		list :
		  Received Packet is delivered as a list.

		binary :
		  Received Packet is delivered as a binary.

		{backlog, B} :
		  B is an integer >= 0. The backlog value defaults to 5. The backlog value defines the maximum length that the	queue  of  pending
		  connections may grow to.

		{ip, ip_address()} :
		  If the host has several network interfaces, this option specifies which one to listen on.

		{fd, Fd} :
		  If a socket has somehow been connected without using gen_tcp , use this option to pass the file descriptor for it.

		inet6 :
		  Set up the socket for IPv6.

		inet :
		  Set up the socket for IPv4.

		Opt :
		  See inet:setopts/2 .

	      The returned socket ListenSocket can only be used in calls to accept/1,2 .

   Note:
       The  default  values  for  options  given to listen can be affected by the Kernel configuration parameter inet_default_listen_options . See
       inet(3erl) for details.

       accept(ListenSocket) -> {ok, Socket} | {error, Reason}
       accept(ListenSocket, Timeout) -> {ok, Socket} | {error, Reason}

	      Types  ListenSocket -- see listen/2
		     Timeout = int() | infinity
		     Socket = socket()
		     Reason = closed | timeout | posix()

	      Accepts an incoming connection request on a listen socket. Socket must be a socket returned from	listen/2  .  Timeout  specifies  a
	      timeout value in ms, defaults to infinity .

	      Returns {ok, Socket} if a connection is established, or {error, closed} if ListenSocket is closed, or {error, timeout} if no connec-
	      tion is established within the specified time. May also return a POSIX error value if something else goes wrong, see inet(3erl)  for
	      possible error values.

	      Packets can be sent to the returned socket Socket using send/2 . Packets sent from the peer are delivered as messages:

	      {tcp, Socket, Data}

	      unless {active, false} was specified in the option list for the listen socket, in which case packets are retrieved by calling recv/2
	      .

   Note:
       It is worth noting that the accept call does not 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.

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

	      Types  Socket = socket()
		     Packet = [char()] | binary()
		     Reason = posix()

	      Sends a packet on a socket.

	      There is no send call with timeout option, you use the send_timeout socket option if timeouts are desired. See the examples section.

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

	      Types  Socket = socket()
		     Length = int()
		     Packet = [char()] | binary() | HttpPacket
		     Timeout = int() | infinity
		     Reason = closed | posix()
		     HttpPacket = see the description of

	      This function receives a packet from a socket in passive mode. A closed socket is indicated by a return value {error, closed} .

	      The Length argument is only meaningful when the socket is in raw mode and denotes the number of bytes to read. If Length	=  0,  all
	      available  bytes	are  returned. If Length > 0, exactly Length bytes are returned, or an error; possibly discarding less than Length
	      bytes of data when the socket gets closed from the other side.

	      The optional Timeout parameter specifies a timeout in milliseconds. The default value is infinity .

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

	      Types  Socket = socket()
		     Pid = pid()
		     Reason = closed | not_owner | posix()

	      Assigns a new controlling process Pid to Socket . The controlling process is the process which receives messages from the socket. If
	      called by any other process than the current controlling process, {error, eperm} is returned.

       close(Socket) -> ok | {error, Reason}

	      Types  Socket = socket()
		     Reason = posix()

	      Closes a TCP socket.

       shutdown(Socket, How) -> ok | {error, Reason}

	      Types  Socket = socket()
		     How = read | write | read_write
		     Reason = posix()

	      Immediately close a socket in one or two directions.

	      How == write means closing the socket for writing, reading from it is still possible.

	      To be able to handle that the peer has done a shutdown on the write side, the {exit_on_close, false} option is useful.

EXAMPLES

       The  following  example	illustrates  usage of the {active,once} option and multiple accepts by implementing a server as a number of worker
       processes doing accept on one single listen socket. The start/2 function takes the number of worker processes as well as a port	number	to
       listen for incoming connections on. If LPort is specified as 0 , an ephemeral portnumber is used, why the start function returns the actual
       portnumber allocated:

       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.

       A simple client could look like this:

       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.

       The fact that the send call does not accept a timeout option, is because timeouts on send is handled through the socket option send_timeout
       .  The  behavior  of a send operation with no receiver is in a very high degree defined by the underlying TCP stack, as well as the network
       infrastructure. If one wants to write code that handles a hanging receiver that might eventually cause the sender to hang on a  send  call,
       one writes code like the following.

       Consider  a  process that receives data from a client process that is to be forwarded to a server on the network. The process has connected
       to the server via TCP/IP and does not get any acknowledge for each message it sends, but has to rely on the send timeout option	to  detect
       that the other end is unresponsive. We could use the send_timeout option when connecting:

	   ...
	   {ok,Sock} = gen_tcp:connect(HostAddress, Port,
				       [{active,false},
					{send_timeout, 5000},
					{packet,2}]),
			   loop(Sock), % See below
	   ...

       In the loop where requests are handled, we can now detect send timeouts:

       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.

       Usually	it  would suffice to detect timeouts on receive, as most protocols include some sort of acknowledgment from the server, but if the
       protocol is strictly one way, the send_timeout option comes in handy!

Ericsson AB							   kernel 2.14.3						     gen_tcp(3erl)