gen_tcp(3erl) Erlang Module Definition gen_tcp(3erl)
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 con-
nection:
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 con-
nection, 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} |
{tclass, integer() >= 0} |
{ttl, integer() >= 0} |
{recvtos, boolean()} |
{recvtclass, boolean()} |
{recvttl, boolean()} |
{ipv6_v6only, boolean()}
pktoptions_value() = {pktoptions, inet:ancillary_data()}
If the platform implements the IPv4 option IP_PKTOPTIONS, or the
IPv6 option IPV6_PKTOPTIONS or IPV6_2292PKTOPTIONS for the
socket this value is returned from inet:getopts/2 when called
with the option name pktoptions.
Note:
This option appears to be VERY Linux specific, and its existence
in future Linux kernel versions is also worrying since the op-
tion is part of RFC 2292 which is since long (2003) obsoleted by
RFC 3542 that explicitly removes this possibility to get packet
information from a stream socket. For comparision: it has ex-
isted in FreeBSD but is now removed, at least since FreeBSD 10.
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 |
pktoptions | 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() =
{ip, inet:socket_address()} |
{fd, Fd :: integer() >= 0} |
{ifaddr, inet:socket_address()} |
inet:address_family() |
{port, inet:port_number()} |
{tcp_module, module()} |
{netns, file:filename_all()} |
{bind_to_device, binary()} |
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()} |
{netns, file:filename_all()} |
{bind_to_device, binary()} |
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 speci-
fies 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 listen-
ing 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, multi-
ple simultaneous accept calls can be issued from different pro-
cesses, 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 be-
fore 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 = timeout | inet:posix()
Connects to a server on TCP port Port on the host with IP ad-
dress Address. Argument Address can be a hostname or an IP ad-
dress.
The following options are available:
{ip, Address}:
If the host has many network interfaces, this option speci-
fies which one to use.
{ifaddr, Address}:
Same as {ip, Address}. If the host has many network inter-
faces, 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 speci-
fied, it is assumed that the fd is already bound appropri-
ately.
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 de-
tails) 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 de-
livered (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 millisec-
onds. Defaults to infinity.
Note:
Keep in mind that if the underlying OS connect() call returns a
timeout, gen_tcp:connect will also return a timeout (i.e. {er-
ror, etimedout}), even if a larger Timeout was specified.
Note:
The default values for options specified to connect can be af-
fected by the Kernel configuration parameter inet_default_con-
nect_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 identi-
fied by Pid is not an existing local pid, {error, badarg} is re-
turned. {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 cor-
rectly and messages may remain in the caller's mailbox. For in-
stance changing the sockets active mode before the transfer 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 num-
ber, 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 speci-
fies 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 inter-
faces, 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 ac-
cept/1,2 to accept incoming connection requests.
Note:
The default values for options specified to listen can be af-
fected by the Kernel configuration parameter inet_default_lis-
ten_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 | timeout | 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 millisec-
onds. 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 mul-
tiple 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 for-
warded 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 de-
tected:
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 proto-
col is strictly one way, option send_timeout comes in handy.
Ericsson AB kernel 7.0 gen_tcp(3erl)