Provided by: erlang-manpages_20.2.2+dfsg-1ubuntu2_all 
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.
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:socket_address()} |
{fd, Fd :: integer() >= 0} |
{ifaddr, inet:socket_address()} |
inet:address_family() |
{port, inet:port_number()} |
{tcp_module, module()} |
option()
listen_option() =
{ip, inet:socket_address()} |
{fd, Fd :: integer() >= 0} |
{ifaddr, inet:socket_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
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 listening socket. Socket must be a socket returned
from listen/2. Timeout specifies a time-out value in milliseconds. Defaults to infinity.
Returns:
* {ok, Socket} if a connection is established
* {error, closed} if ListenSocket is closed
* {error, timeout} if no connection is established within the specified time
* {error, system_limit} if all available ports in the Erlang emulator are in use
* 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 (unless {active, false} is specified in the option list for the listening
socket, in which case packets are retrieved by calling recv/2):
{tcp, Socket, Data}
Note:
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.
close(Socket) -> ok
Types:
Socket = socket()
Closes a TCP socket.
Note that in most implementations of TCP, doing a close 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.
* Use gen_tcp:shutdown(Sock, write) to signal that no more data is to be sent and wait for the
read side of the socket to be closed.
* Use the socket option {packet, N} (or something similar) to make it possible for the receiver
to close the connection when it knowns it has received all the data.
connect(Address, Port, Options) -> {ok, Socket} | {error, Reason}
connect(Address, Port, Options, Timeout) ->
{ok, Socket} | {error, Reason}
Types:
Address = inet:socket_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. Argument Address can be
a hostname or an IP address.
The following options are available:
{ip, Address}:
If the host has many network interfaces, this option specifies which one to use.
{ifaddr, Address}:
Same as {ip, Address}. If the host has many 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, Address} and/or {port, port_number()} is combined with this option,
the fd is bound to the specified interface and port before connecting. If these options are
not specified, it is assumed that the fd is already bound appropriately.
inet:
Sets up the socket for IPv4.
inet6:
Sets up the socket for IPv6.
local:
Sets up a Unix Domain Socket. See inet:local_address()
{port, Port}:
Specifies which local port number to use.
{tcp_module, module()}:
Overrides 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 (unless {active, false} is
specified in the option list for the socket, in which case packets are retrieved by calling
recv/2):
{tcp_error, Socket, Reason}
The optional Timeout parameter specifies a time-out in milliseconds. Defaults to infinity.
Note:
The default values for options specified to connect can be affected by the Kernel configuration
parameter inet_default_connect_options. For details, see inet(3erl).
controlling_process(Socket, Pid) -> ok | {error, Reason}
Types:
Socket = socket()
Pid = pid()
Reason = closed | not_owner | badarg | inet:posix()
Assigns a new controlling process Pid to Socket. The controlling process is the process that
receives messages from the socket. If called by any other process than the current controlling
process, {error, not_owner} is returned. If the process identified by Pid is not an existing local
pid, {error, badarg} is returned. {error, badarg} may also be returned in some cases when Socket
is closed during the execution of this function.
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 transfere is
complete may cause this.
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 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 following options are available:
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 defines the maximum length that the queue of pending
connections can grow to. Defaults to 5.
{ip, Address}:
If the host has many network interfaces, this option specifies which one to listen on.
{port, Port}:
Specifies 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, Address}:
Same as {ip, Address}. If the host has many network interfaces, this option specifies which
one to use.
inet6:
Sets up the socket for IPv6.
inet:
Sets up the socket for IPv4.
{tcp_module, module()}:
Overrides 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 should be used in calls to accept/1,2 to accept incoming
connection requests.
Note:
The default values for options specified to listen can be affected by the Kernel configuration
parameter inet_default_listen_options. For details, see inet(3erl).
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 in ERTS.
Receives a packet from a socket in passive mode. A closed socket is indicated by return value
{error, closed}.
Argument Length is only meaningful when the socket is in raw mode and denotes the number of bytes
to read. If Length is 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 is
closed from the other side.
The optional Timeout parameter specifies a time-out in milliseconds. Defaults to infinity.
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 a time-out option, use socket option send_timeout if time-outs are
desired. See section Examples.
shutdown(Socket, How) -> ok | {error, Reason}
Types:
Socket = socket()
How = read | write | read_write
Reason = inet:posix()
Closes 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, the socket is shut down
immediately and any error encountered is returned in Reason.
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 {error, closed} is returned on the next recv/2 or send/2.
Option {exit_on_close, false} is useful if the peer has done a shutdown on the write side.
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.