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#### 16.12.2 Socket-Level Options

— Constant: int SOL_SOCKET

> Use this constant as the level argument to `getsockopt` or `setsockopt` to manipulate the socket-level options described in this section.

Here is a table of socket-level option names; all are defined in the header file `sys/socket.h`.

`SO_DEBUG`

  This option toggles recording of debugging information in the underlying protocol modules. The value has type `int`; a nonzero value means “yes”.

`SO_REUSEADDR`
  This option controls whether `bind` (see Setting Address) should permit reuse of local addresses for this socket. If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet. The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same port number.

  The value has type `int`; a nonzero value means “yes”.

`SO_KEEPALIVE`
  This option controls whether the underlying protocol should periodically transmit messages on a connected socket. If the peer fails to respond to these messages, the connection is considered broken. The value has type `int`; a nonzero value means “yes”.
`SO_DONTROUTE`
  This option controls whether outgoing messages bypass the normal message routing facilities. If set, messages are sent directly to the network interface instead. The value has type `int`; a nonzero value means “yes”.
`SO_LINGER`
  This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see Closing a Socket. The value has type `struct linger`.
  — Data Type: struct linger

  > This structure type has the following members:

  > `int l_onoff`
  >   This field is interpreted as a boolean. If nonzero, `close` blocks until the data are transmitted or the timeout period has expired.
  > `int l_linger`
  >   This specifies the timeout period, in seconds.

`SO_BROADCAST`
  This option controls whether datagrams may be broadcast from the socket. The value has type `int`; a nonzero value means “yes”.
`SO_OOBINLINE`
  If this option is set, out-of-band data received on the socket is placed in the normal input queue. This permits it to be read using `read` or `recv` without specifying the `MSG_OOB` flag. See Out-of-Band Data. The value has type `int`; a nonzero value means “yes”.
`SO_SNDBUF`
  This option gets or sets the size of the output buffer. The value is a `size_t`, which is the size in bytes.
`SO_RCVBUF`
  This option gets or sets the size of the input buffer. The value is a `size_t`, which is the size in bytes.
`SO_STYLE`
`SO_TYPE`
  This option can be used with `getsockopt` only. It is used to get the socket's communication style. `SO_TYPE` is the historical name, and `SO_STYLE` is the preferred name in GNU. The value has type `int` and its value designates a communication style; see Communication Styles.
`SO_ERROR`

  This option can be used with `getsockopt` only. It is used to reset the error status of the socket. The value is an `int`, which represents the previous error status.