# gen_tcp

### MODULE

gen_tcp

### MODULE SUMMARY

Interface to TCP/IP sockets

### DESCRIPTION

The `gen_tcp` module provides functions for communicating with sockets using the TCP/IP protocol.

The available options are described in the setopts/2 function in the `inet` manual page.

The possible `{error, Reason}` results are described in the inet manual page.

The following code fragment provides 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),
        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.

### EXPORTS

`accept(ListenSocket) -> {ok, Socket} | {error, Reason}`
`accept(ListenSocket, Timeout) -> {ok, Socket} | {error, Reason}`

Types:

`ListenSocket = socket()`
`Socket = socket()`
`Timeout = integer()`
`Reason = atom()`

Accepts an incoming connection request on a listen socket. `Socket` must be a socket returned from `listen/1`. If no `Timeout` argument is specified, or it is `infinity`, the `accept` function will not return until a connection has been established or the `ListenSocket` has been closed. If accept returns because the `ListenSocket` has been closed `{error, closed}` is returned. If `Timeout` is specified and no connection is accepted within the given time, `accept` will return `{error, timeout}`.

Packets can be sent to the returned socket using the `send/2` function. Packets sent from the peer will be delivered as messages

            {tcp, Socket, Data}

unless `{active, false}` was specified in the option list for the listen socket, in which case packets should be retrieved by calling `recv/2`.

`close(Socket) -> ok | {error, Reason}`

Types:

`Socket = socket()`
`Reason = atom()`

Closes an TCP socket.

`connect(Address, Port, Options) -> {ok, Socket} | {error, Reason}`
`connect(Address, Port, Options, Timeout) -> {ok, Socket} | {error, Reason}`

Types:

`Address = string() | atom() | ip_address()`
`Port = Timeout = integer()`
`Options = list()`
`Socket = socket()`
`Reason = atom()`

Connects to a server on TCP port `Port` on the host with IP address `Address`. The `Address` argument can be either a hostname, or an IP address.

The available options are:

`list`
  Received `Packet` is delivered as a list.
`binary`
  Received `Packet` is delivered as a binary.
`{ip,IPAddress}`
  If the host has several network interfaces, this option specifies which one to use.
`{port,Port}`
  Specify which local port number to use.
`{fd,Fd}`
  If a socket has somehow been connected without using `gen_tcp`, use this option to pass in the file descriptor for it and create a Socket for it.
`inet6`
  Set up the socket for IPv6
`inet`
  Set up the socket for IPv4
common `inet` options
  The common `inet` options available are described in the setopts/2 function in the `inet` manual page.

Packets can be sent to the returned socket using the `send/2` function. Packets sent from the peer will be delivered as messages

            {tcp, Socket, Data}

If the socket was closed the following message is delivered:

            {tcp_closed, Socket}

If an error occurred on the socket the following message is delivered:

            {tcp_error, Socket, Reason}

unless the socket is in passive mode, in which case packets are retrieved by calling `recv/2`.

The optional `Timeout` parameter specifies a timeout in milliseconds. The default value is `infinity`.

| The default values for options given to `connect` can be affected by the `inet_default_connect_options` kernel application variable. See the description in the `inet` reference page for details.

`controlling_process(Socket, NewOwner) -> ok | {error, eperm}`

Types:

`Socket = socket()`
`NewOwner = pid()`

Assigns a new controlling process to `Socket`. The controlling process is the process which will receive messages from the socket. If called by any other process than the current owner `{error, eperm}` will be returned.

`listen(Port, Options) -> {ok, Socket} | {error, Reason}`

Types:

`Port = integer()`
`Options = list()`
`Socket = socket()`
`Reason = atom()`

Sets up socket to listen on the port `Port` on the local host.

If the port number is zero, the `listen` function picks an available port number (use `inet:port/1` to retrieve it); otherwise, the specified port number is used.

The available options are described in the setopts/2 function in the `inet` manual page.

Additionally, the following options can be given.

`{backlog, B}`
  B is an integer >= 0. The backlog value defaults to 5. The backlog value defines the maximum length the queue of pending connections may grow to.
`{ip,IPAddress}`
  If the host has several network interfaces, this option specifies which one to listen on.
`{fd,Fd}`
  If a listen socket has somehow been opened without using `gen_tcp`, use this option to pass in the file descriptor for it and create a Socket for it.
`inet6`
  Set up the socket for IPv6
`inet`
  Set up the socket for IPv4

The returned socket can only be used in calls to `accept`.

| The default values for options given to `listen` can be affected by the `inet_default_listen_options` kernel application variable. See the description in the `inet` reference page for details.

`recv(Socket, Length) -> {ok, Packet} | {error, Reason}`
`recv(Socket, Length, Timeout)`

Types:

`Socket = socket()`
`Length = integer()`
`Packet = list() | binary()`
`Timeout = integer()`
`Reason = atom()`

This function receives a packet from a socket in passive mode. A closed socket is indicated by a return value of `{error, closed}`.

The `Length` argument is only meaningful when the socket is in `raw` mode and denotes number of bytes to read. If `Length` = 0 all available bytes are returned. If `Length` is > 0, exactly `Length` bytes will be returned, or an error; possibly discarding less than `Length` bytes of data when the socket gets closed from the other side.

The optional `Timeout` parameter specifies a timeout in milliseconds. The default value is `infinity`.

`send(Socket, Packet) -> ok | {error, Reason}`

Types:

`Socket = socket()`
`Packet = list() | binary()`
`Reason = atom()`

Sends a packet on a socket.

`shutdown(Socket, How) -> ok | {error, Reason}`

Types:

`Socket = socket()`
`Reason = atom()`
`How = read | write | read_write`

Immediately close a socket in one or two directions.

Closing a socket in the `write` allows you to still read from the socket.

To be able to handle that the peer has done a shutdown on the write side, you'll need to set the `exit_on_close` option for the socket to `false` as described in the setopts/2 function in `inet` manual page.

### AUTHORS

tony@erix.ericsson.se - support@erlang.ericsson.se

kernel 2.10.9
Copyright © 1991-2005 Ericsson AB