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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.