gen_tcp - Interface to TCP/IP sockets.
Please see following description for synopsis
gen_tcp(3) Erlang Module Definition gen_tcp(3)
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.
Note:
Functions that create sockets can take an optional option; {inet_back-
end, Backend} that, if specified, has to be the first option. This
selects the implementation backend towards the platform's socket API.
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 "compati-
ble" 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:
* Non-blocking send
If a user calling gen_tcp:send/2 with inet_backend = inet, tries to
send more data than there is room for in the OS buffers, the "rest
data" is buffered by the inet driver (and later sent in the back-
ground). The effect for the user is that the call is non-blocking.
This is not the effect when inet_backend = socket, since there is
no buffering. Instead the user hangs either until all data has been
sent or the send_timeout timeout has been reached.
* Remote close detected by background send.
An background send will detect a 'remote close' and (the inet
driver will) mark the socket as 'closed'. No other action is taken.
If the socket has active set to false (passive) at this point and
no one is reading, this will not be noticed. But as soon as the
socket is "activated" (active set to not false, send/2 is called or
recv/2,3 is called), an error message will be sent to the caller or
(socket) owner: {tcp_error, Socket, econnreset}. Any data in the OS
receive buffers will be lost!
This behaviour is not replicated by the socket implementation. A
send operation will detect a remote close and immediately return
this to the caller, but do nothing else. A reader will therefor be
able to extract any data from the OS buffers. If the socket is set
to active to not false, the data will be received as expected
({tcp, ...} and then a closed message ({tcp_closed, ...} will be
received (not an error).
* The option show_econnreset basically do not work as described when
used with inet_backend = socket. The "issue" is that a remote close
(as described above) do allow a reader to extract what is in the
read buffers before a close is "delivered".
* The option nodelay is a TCP specific option that is not compatible
with domain = local.
When using inet_backend = socket, trying to create a socket (via
listen or connect) with domain = local (for example with option
{ifaddr, {local,"/tmp/test"}}) will fail with {error, enotsup}.
This does not actually work for inet_backend = inet either, but in
that case the error is simply ignored, which is a bad idea. We have
choosen to not ignore this error for inet_backend = socket.
* Async shutdown write
Calling gen_tcp:shutdown(Socket, write | read_write) on a socket
created with inet_backend = socket will take immediate effect,
unlike for a socket created with inet_backend = inet.
See async shutdown write for more info.
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
option 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
existed 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 |
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(3) 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
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 = [inet:inet_backend() | connect_option()]
Timeout = timeout()
Socket = socket()
Reason = timeout | 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 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
details) and its message counter drops to 0, the following mes-
sage 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 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.
{error, etimedout}), even if a larger Timeout was specified.
Note:
The default values for options specified to connect can be
affected by the Kernel configuration parameter inet_default_con-
nect_options. For details, see inet(3).
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
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 cor-
rectly and messages may remain in the caller's mailbox. For
instance 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 = [inet:inet_backend() | 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
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_lis-
ten_options. For details, see inet(3).
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 | {timeout, RestData} | inet:posix()
RestData = binary()
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.
The return value {error, {timeout, RestData}} can only be
returned when inet_backend = socket.
Note:
Non-blocking send.
If the user tries to send more data than there is room for in
the OS send buffers, the 'rest data' is put into (inet driver)
internal buffers and later sent in the background. The function
immediately returns ok (not informing the caller that not all of
the data was actually sent). Any issue while sending the 'rest
data' is maybe returned later.
When using inet_backend = socket, the behaviour is different.
There is no buffering done (like the inet-driver does), instead
the caller will "hang" until all of the data has been sent or
send timeout (as specified by the send_timeout option) expires
(the function can hang even when using 'inet' backend if the
internal buffers are full).
If this happens when using packet =/= raw, we have a partial
package written. A new package therefor must not be written at
this point, as there is no way for the peer to distinguish this
from the data portion of the current package. Instead, set pack-
age to raw, send the rest data (as raw data) and then set pack-
age to the wanted package type again.
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.
Note:
Async shutdown write (write or read_write).
If the shutdown attempt is made while the inet-driver is sending
buffered data in the background, the shutdown is postponed until
all buffered data has been sent. The function immediately
returns ok and the caller is not informed (that the shutdown has
not yet been performed).
When using inet_backend = socket, the behaviour is different. A
shutdown with How == write | read_write, the operation will take
immediate effect (unlike the inet-driver, which basically saves
the operation for later).
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
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 proto-
col is strictly one way, option send_timeout comes in handy.
Ericsson AB kernel 8.2 gen_tcp(3)