Provided by: erlang-manpages_18.3-dfsg-1ubuntu3.1_all 
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
option() =
{active, true | false | once | -32768..32767} |
{buffer, integer() >= 0} |
{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} |
{ipv6_v6only, boolean()}
option_name() =
active |
buffer |
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 |
ipv6_v6only
connect_option() =
{ip, inet:ip_address()} |
{fd, Fd :: integer() >= 0} |
{ifaddr, inet:ip_address()} |
inet:address_family() |
{port, inet:port_number()} |
{tcp_module, module()} |
option()
listen_option() =
{ip, inet:ip_address()} |
{fd, Fd :: integer() >= 0} |
{ifaddr, inet:ip_address()} |
inet:address_family() |
{port, inet:port_number()} |
{backlog, B :: integer() >= 0} |
{tcp_module, module()} |
option()
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 = inet:ip_address() | inet:hostname()
Port = inet:port_number()
Options = [connect_option()]
Timeout = timeout()
Socket = socket()
Reason = inet: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:
{ip, ip_address()}:
If the host has several network interfaces, this option specifies which one to
use.
{ifaddr, ip_address()}:
Same as {ip, ip_address()}. If the host has several network interfaces, this
option specifies which one to use.
{fd, integer() >= 0}:
If a socket has somehow been connected without using gen_tcp, use this option
to pass the file descriptor for it. If {ip, ip_address()} and/or {port,
port_number()} is combined with this option the fd will be bound to the given
interface and port before connecting. If these options are not given it is
assumed that the fd is already bound appropriately.
inet:
Set up the socket for IPv4.
inet6:
Set up the socket for IPv6.
{port, Port}:
Specify which local port number to use.
{tcp_module, module()}:
Override which callback module is used. Defaults to inet_tcp for IPv4 and
inet6_tcp for IPv6.
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 in {active, N} mode (see inet:setopts/2 for details) and its
message counter drops to 0, the following message is delivered to indicate that the
socket has transitioned to passive ({active, false}) mode:
{tcp_passive, Socket}
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 = inet:port_number()
Options = [listen_option()]
ListenSocket = socket()
Reason = system_limit | inet: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.
{port, Port}:
Specify which local port number to use.
{fd, Fd}:
If a socket has somehow been connected without using gen_tcp, use this option
to pass the file descriptor for it.
{ifaddr, ip_address()}:
Same as {ip, ip_address()}. If the host has several network interfaces, this
option specifies which one to use.
inet6:
Set up the socket for IPv6.
inet:
Set up the socket for IPv4.
{tcp_module, module()}:
Override which callback module is used. Defaults to inet_tcp for IPv4 and
inet6_tcp for IPv6.
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 = socket()
Returned by listen/2.
Timeout = timeout()
Socket = socket()
Reason = closed | timeout | system_limit | inet: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 connection is established within
the specified time, or {error, system_limit} if all available ports in the Erlang
emulator are in use. 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 = iodata()
Reason = closed | inet: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 = integer() >= 0
Timeout = timeout()
Packet = string() | binary() | HttpPacket
Reason = closed | inet:posix()
HttpPacket = term()
See the description of HttpPacket in erlang:decode_packet/3.
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 | inet: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, not_owner} is returned.
close(Socket) -> ok
Types:
Socket = socket()
Closes a TCP socket.
shutdown(Socket, How) -> ok | {error, Reason}
Types:
Socket = socket()
How = read | write | read_write
Reason = inet:posix()
Close a socket in one or two directions.
How == write means closing the socket for writing, reading from it is still
possible.
If How == read, or there is no outgoing data buffered in the Socket port, then the
socket is shutdown immediately and any error encountered is returned in Reason.
If there is data buffered in the socket port, then the attempt to shutdown the
socket is postponed until that data is written to the kernel socket send buffer.
Any errors encountered will result in the socket being closed and {error, closed}
being returned on the next recv/2 or send/2.
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!