NAME¶

gen_tcp - Interface to TCP/IP sockets.

DESCRIPTION¶

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

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:

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),


    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.

For more examples, see section Examples.

This is a temporary option that will be ignored in a future release.

The default is Backend = inet that selects the traditional inet_drv.c driver. The other choice is Backend = socket that selects the new socket module and its NIF implementation.

The system default can be changed when the node is started with the application kernel's configuration variable inet_backend.

For gen_tcp with inet_backend = socket we have tried to be as "compatible" as possible which has sometimes been impossible. Here is a list of cases when the behaviour of inet-backend inet (default) and socket are different:

DATA TYPES¶

option() = 


    {active, true | false | once | -32768..32767} |


    {buffer, integer() >= 0} |


    {debug, boolean()} |


    {delay_send, boolean()} |


    {deliver, port | term} |


    {dontroute, boolean()} |


    {exit_on_close, boolean()} |


    {header, integer() >= 0} |


    {high_msgq_watermark, integer() >= 1} |


    {high_watermark, integer() >= 0} |


    {keepalive, boolean()} |


    {linger, {boolean(), integer() >= 0}} |


    {low_msgq_watermark, integer() >= 1} |


    {low_watermark, integer() >= 0} |


    {mode, list | binary} |


    list | binary |


    {nodelay, boolean()} |


    {packet,


     0 | 1 | 2 | 4 | raw | sunrm | asn1 | cdr | fcgi | line |


     tpkt | http | httph | http_bin | httph_bin} |


    {packet_size, integer() >= 0} |


    {priority, integer() >= 0} |


    {raw,


     Protocol :: integer() >= 0,


     OptionNum :: integer() >= 0,


     ValueBin :: binary()} |


    {recbuf, integer() >= 0} |


    {reuseaddr, boolean()} |


    {send_timeout, integer() >= 0 | infinity} |


    {send_timeout_close, boolean()} |


    {show_econnreset, boolean()} |


    {sndbuf, integer() >= 0} |


    {tos, integer() >= 0} |


    {tclass, integer() >= 0} |


    {ttl, integer() >= 0} |


    {recvtos, boolean()} |


    {recvtclass, boolean()} |


    {recvttl, boolean()} |


    {ipv6_v6only, boolean()}

pktoptions_value() = {pktoptions, inet:ancillary_data()}

option_name() = 


    active | buffer | debug | delay_send | deliver | dontroute |


    exit_on_close | header | high_msgq_watermark |


    high_watermark | keepalive | linger | low_msgq_watermark |


    low_watermark | mode | nodelay | packet | packet_size |


    priority |


    {raw,


     Protocol :: integer() >= 0,


     OptionNum :: integer() >= 0,


     ValueSpec ::


         (ValueSize :: integer() >= 0) | (ValueBin :: binary())} |


    recbuf | reuseaddr | send_timeout | send_timeout_close |


    show_econnreset | sndbuf | tos | tclass | ttl | recvtos |


    recvtclass | recvttl | pktoptions | ipv6_v6only

connect_option() = 


    {fd, Fd :: integer() >= 0} |


    inet:address_family() |


    {ifaddr,


     socket:sockaddr_in() |


     socket:sockaddr_in6() |


     inet:socket_address()} |


    {ip, inet:socket_address()} |


    {port, inet:port_number()} |


    {tcp_module, module()} |


    {netns, file:filename_all()} |


    {bind_to_device, binary()} |


    option()

listen_option() = 


    {fd, Fd :: integer() >= 0} |


    inet:address_family() |


    {ifaddr,


     socket:sockaddr_in() |


     socket:sockaddr_in6() |


     inet:socket_address()} |


    {ip, inet:socket_address()} |


    {port, inet:port_number()} |


    {backlog, B :: integer() >= 0} |


    {tcp_module, module()} |


    {netns, file:filename_all()} |


    {bind_to_device, binary()} |


    option()

socket()

EXPORTS¶

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

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

{tcp, Socket, Data}

close(Socket) -> ok

connect(SockAddr, Opts) -> {ok, Socket} | {error, Reason}

connect(SockAddr, Opts, Timeout) -> {ok, Socket} | {error, Reason}

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

connect(Address, Port, Opts, Timeout) ->



           {ok, Socket} | {error, Reason}

{tcp, Socket, Data}

{tcp_passive, Socket}

{tcp_closed, Socket}

{tcp_error, Socket, Reason}

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

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

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

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

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

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

EXAMPLES¶

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

Example of a simple client:

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 send call does not accept a time-out option because time-outs on send is handled through socket option send_timeout. 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 send do like the following.

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 send_timeout can be used 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, send time-outs can now be detected:

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 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 send_timeout comes in handy.