-spec accept(ListenSocket) -> {ok, Socket} | {error, Reason}
                when
                    ListenSocket :: socket(),
                    Socket :: socket(),
                    Reason :: closed | system_limit | inet:posix().

Equivalent to accept(ListenSocket, infinity).

-spec accept(ListenSocket, Timeout) -> {ok, Socket} | {error, Reason}
                when
                    ListenSocket :: socket(),
                    Timeout :: timeout(),
                    Socket :: socket(),
                    Reason :: closed | timeout | system_limit | inet:posix().

Accept an incoming connection request on a listen 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 Timeout
• {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 about possible values

To send packets (outbound) on the returned Socket, use send/2. Packets sent from the peer (inbound) are delivered as messages to the socket owner; the process that created the socket. Unless {active, false} is specified in the option list when creating the listening socket.

See connect/4 about active mode socket messages and passive mode.

Note

The accept call doesn't 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.

-spec close(Socket) -> ok when Socket :: socket().

Close a TCP socket.

Note that in most implementations of TCP, doing a close does not guarantee that the data sent is delivered to the recipient. It is guaranteed that the recepient will see all sent data before getting the close, but the sender gets no indication of that.

If the sender needs to know that the recepient has received all data there are two common ways to achieve this:

1. Use gen_tcp:shutdown(Sock, write) to signal that no more data is to be sent and wait for the other side to acknowledge seeing its read side being closed, by closing its write side, which shows as a socket close on this side.
2. Implement an acknowledgement in the protocol on top of TCP that both connection ends adhere to, indicating that all data has been seen. The socket option {packet, N} may be useful.

-spec connect(SockAddr, Opts) -> {ok, Socket} | {error, Reason}
                 when
                     SockAddr :: socket:sockaddr_in() | socket:sockaddr_in6(),
                     Opts :: [inet:inet_backend() | connect_option()],
                     Socket :: socket(),
                     Reason :: inet:posix().

Equivalent to connect(SockAddr, Opts, infinity).

-spec connect(Address, Port, Opts) -> {ok, Socket} | {error, Reason}
                 when
                     Address :: inet:socket_address() | inet:hostname(),
                     Port :: inet:port_number(),
                     Opts :: [inet:inet_backend() | connect_option()],
                     Socket :: socket(),
                     Reason :: inet:posix();
             (SockAddr, Opts, Timeout) -> {ok, Socket} | {error, Reason}
                 when
                     SockAddr :: socket:sockaddr_in() | socket:sockaddr_in6(),
                     Opts :: [inet:inet_backend() | connect_option()],
                     Timeout :: timeout(),
                     Socket :: socket(),
                     Reason :: timeout | inet:posix().

Create a socket connected to the specified address.

With arguments Address and Port

Equivalent to connect(Address, Port, Opts, infinity).

With argument SockAddr (since OTP 24.3)

Connects to a remote listen socket specified by SockAddr where socket:sockaddr_in6/0 for example allows specifying the scope_id for link local IPv6 addresses.

IPv4 addresses on the same map/0 format is also allowed.

Equivalent to connect/4, besides the format of the destination address.

-spec connect(Address, Port, Opts, Timeout) -> {ok, Socket} | {error, Reason}
                 when
                     Address :: inet:socket_address() | inet:hostname(),
                     Port :: inet:port_number(),
                     Opts :: [inet:inet_backend() | connect_option()],
                     Timeout :: timeout(),
                     Socket :: socket(),
                     Reason :: timeout | inet:posix().

Create a socket connected to the specified address.

Creates a socket and connects it to a server on TCP port Port on the host with IP address Address, that may also be a hostname.

Opts (connect options)

• {ip, Address} - If the local host has many IP addresses, this option specifies which one to use.

• {ifaddr, Address} - Same as {ip, Address}.

  However, if Address instead is a socket:sockaddr_in/0 or socket:sockaddr_in6/0 this takes precedence over any value previously set with the ip and port options. If these options (ip or/and port) however comes after this option, they may be used to update the corresponding fields of this option (for ip, the addr field, and for port, the port field).

• {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/0

• {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.

• option/0 - See inet:setopts/2.

Socket Data

Packets can be sent to the peer (outbound) with send(Socket, Packet). Packets sent from the peer (inbound) are delivered as messages to the socket owner; the process that created the socket, unless {active, false} is specified in the Options list.

Active mode socket messages

• {tcp, Socket, Data} - Inbound data from the socket.

• {tcp_passive, Socket} - The socket was in {active, N} mode (see inet:setopts/2 for details) and its message counter reached 0, indicating that the socket has transitioned to passive ({active, false}) mode.

• {tcp_closed, Socket} - The socket was closed.

• {tcp_error, Socket, Reason} A socket error occurred.

Passive mode

If {active, false} is specified in the option list for the socket, packets and errors are retrieved by calling recv/2,3 (send/2 may also return errors).

Timeout

The optional Timeout parameter specifies a connect time-out in milliseconds. 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 (for example 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.

-spec controlling_process(Socket, Pid) -> ok | {error, Reason}
                             when
                                 Socket :: socket(),
                                 Pid :: pid(),
                                 Reason :: closed | not_owner | badarg | inet:posix().

Change the controlling process (owner) of a socket.

Assigns a new controlling process Pid to Socket. The controlling process is the process that the socket sends messages to. If this function is called from 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/0, {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 in active mode, this function will transfer any messages from the socket in the mailbox of the caller to the new controlling process.

If any other process is interacting with the socket during the transfer, it may not work correctly and messages may remain in the caller's mailbox. For instance, changing the sockets active mode during the transfer could cause this.

-spec listen(Port, Options) -> {ok, ListenSocket} | {error, Reason}
                when
                    Port :: inet:port_number(),
                    Options :: [inet:inet_backend() | listen_option()],
                    ListenSocket :: socket(),
                    Reason :: system_limit | inet:posix().

Create a listen socket.

Creates a socket and sets it to listen on port Port on the local host.

If Port == 0, the underlying OS assigns an available (ephemeral) port number, use inet:port/1 to retrieve it.

The following options are available:

• list - Received Packets are delivered as lists of bytes, [byte/0].

• binary - Received Packets are delivered as binary/0s.

• {backlog, B} - B :: non_neg_integer/0. The backlog value defines the maximum length that the queue of pending connections can grow to. Defaults to 5.

• inet6 - Sets up the socket for IPv6.

• inet - Sets up the socket for IPv4.

• {fd, Fd} - If a socket has somehow been created without using gen_tcp, use this option to pass the file descriptor for it.

• {ip, Address} - If the host has many IP addresses, this option specifies which one to listen on.

• {port, Port} - Specifies which local port number to use.

• {ifaddr, Address} - Same as {ip, Address}.

  However, if this instead is an socket:sockaddr_in/0 or socket:sockaddr_in6/0 this takes precedence over any value previously set with the ip and port options. If these options (ip or/and port) however comes after this option, they may be used to update their corresponding fields of this option (for ip, the addr field, and for port, the port field).

• {tcp_module, module()} - Overrides which callback module is used. Defaults to inet_tcp for IPv4 and inet6_tcp for IPv6.

• option/0 - See inet:setopts/2.

The returned socket ListenSocket should be used when calling accept/1,2 to accept an incoming connection request.

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.

-spec recv(Socket, Length) -> {ok, Packet} | {error, Reason}
              when
                  Socket :: socket(),
                  Length :: non_neg_integer(),
                  Packet :: string() | binary() | HttpPacket,
                  Reason :: closed | inet:posix(),
                  HttpPacket :: term().

Equivalent to recv(Socket, Length, infinity).

-spec recv(Socket, Length, Timeout) -> {ok, Packet} | {error, Reason}
              when
                  Socket :: socket(),
                  Length :: non_neg_integer(),
                  Timeout :: timeout(),
                  Packet :: string() | binary() | HttpPacket,
                  Reason :: closed | timeout | inet:posix(),
                  HttpPacket :: term().

Receive a packet, from a socket in passive mode.

A closed socket is indicated by the return value {error, closed}. If the socket is not in passive mode, the return value is {error, einval}.

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; except if the socket is closed from the other side, then the last read before the one returning {error, closed} may return less than Length bytes of data.

The optional Timeout parameter specifies a time-out in milliseconds. Defaults to infinity.

Any process can receive data from a passive socket, even if that process is not the controlling process of the socket. However, only one process can call this function on a socket at any given time. Using simultaneous calls to recv is not recommended as the behavior depends on the socket implementation, and could return errors such as {error, ealready}.

-spec send(Socket, Packet) -> ok | {error, Reason}
              when
                  Socket :: socket(),
                  Packet :: iodata(),
                  Reason :: closed | {timeout, RestData} | inet:posix(),
                  RestData :: binary() | erlang:iovec().

Send a packet on a socket.

There is no send/2 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 stored in (inet driver) internal buffers and later sent in the background. The function immediately returns ok (not informing the caller that some date isn'nt sent yet). Any issue while sending the 'rest data' may be returned later.

When using inet_backend = socket, the behaviour is different. There is no buffering, instead the caller will "hang" until all of the data has been sent or the send timeout (as specified by the send_timeout option) expires (the function can "hang" even when using the inet backend if the internal buffers are full).

If this happens when using packet =/= raw, a partial package has been written. A new package therefore mustn't be written at this point, as there is no way for the peer to distinguish this from data in the current package. Instead, set package to raw, send the rest data (as raw data) and then set package to the correct package type again.

-spec shutdown(Socket, How) -> ok | {error, Reason}
                  when Socket :: socket(), How :: read | write | read_write, Reason :: inet:posix().

Close the socket in one or both 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 shutdown is performed immediately and any error encountered is returned in Reason.

If there is data buffered in the socket port, shutdown isn't performed on the socket until that buffered data has been written to the OS protocol stack. If any errors are encountered, the socket is closed and {error, closed} is returned by the next recv/2 or send/2 call.

Option {exit_on_close, false} is useful if the peer performs a shurdown of its write side. Then the socket stays open for writing after receive has indicated that the socket was closed.

Note

Async shutdown write (How :: write | 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. This function immediately returns ok, and the caller isn't informed (that the shutdown has been postponed).

When using inet_backend = socket, the behaviour is different. A shutdown with How :: write | read_write will always be performed immediately.