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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 the Address of a Socket) 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.