The setsockopt() function shall set the option specified by the
option_name argument, at the protocol level specified by the
level argument, to the value pointed to by the option_value
argument for the socket associated with the file descriptor specified by the
The level argument specifies the protocol level at which the option
resides. To set options at the socket level, specify the level argument
as SOL_SOCKET. To set options at other levels, supply the appropriate
level identifier for the protocol controlling the option. For example,
to indicate that an option is interpreted by the TCP (Transport Control
Protocol), set level to IPPROTO_TCP as defined in the
The option_name argument specifies a single option to set. The
option_name argument and any specified options are passed uninterpreted
to the appropriate protocol module for interpretations. The
<sys/socket.h> header defines the socket-level options. The
options are as follows:
Turns on recording of debugging information. This option
enables or disables debugging in the underlying protocol modules. This
option takes an int value. This is a Boolean option.
Permits sending of broadcast messages, if this is supported
by the protocol. This option takes an int value. This is a Boolean
Specifies that the rules used in validating addresses
supplied to bind() should allow reuse of local addresses, if this
is supported by the protocol. This option takes an int value. This
is a Boolean option.
Keeps connections active by enabling the periodic
transmission of messages, if this is supported by the protocol. This
option takes an int value.
If the connected socket fails to respond to these messages, the connection is
broken and threads writing to that socket are notified with a SIGPIPE signal.
This is a Boolean option.
Lingers on a close() if data is present. This option
controls the action taken when unsent messages queue on a socket and
close() is performed. If SO_LINGER is set, the system shall block
the process during close() until it can transmit the data or until
the time expires. If SO_LINGER is not specified, and close() is
issued, the system handles the call in a way that allows the process to
continue as quickly as possible. This option takes a linger
structure, as defined in the <sys/socket.h> header, to
specify the state of the option and linger interval.
Leaves received out-of-band data (data marked urgent)
inline. This option takes an int value. This is a Boolean
Sets send buffer size. This option takes an int
Sets receive buffer size. This option takes an int
Requests that outgoing messages bypass the standard routing
facilities. The destination shall be on a directly-connected network, and
messages are directed to the appropriate network interface according to
the destination address. The effect, if any, of this option depends on
what protocol is in use. This option takes an int value. This is a
Sets the minimum number of bytes to process for socket
input operations. The default value for SO_RCVLOWAT is 1. If SO_RCVLOWAT
is set to a larger value, blocking receive calls normally wait until they
have received the smaller of the low water mark value or the requested
amount. (They may return less than the low water mark if an error occurs,
a signal is caught, or the type of data next in the receive queue is
different from that returned; for example, out-of-band data.) This option
takes an int value. Note that not all implementations allow this
option to be set.
Sets the timeout value that specifies the maximum amount of
time an input function waits until it completes. It accepts a
timeval structure with the number of seconds and microseconds
specifying the limit on how long to wait for an input operation to
complete. If a receive operation has blocked for this much time without
receiving additional data, it shall return with a partial count or
errno set to [EAGAIN] or [EWOULDBLOCK] if no data is received. The
default for this option is zero, which indicates that a receive operation
shall not time out. This option takes a timeval structure. Note
that not all implementations allow this option to be set.
Sets the minimum number of bytes to process for socket
output operations. Non-blocking output operations shall process no data if
flow control does not allow the smaller of the send low water mark value
or the entire request to be processed. This option takes an int
value. Note that not all implementations allow this option to be set.
Sets the timeout value specifying the amount of time that
an output function blocks because flow control prevents data from being
sent. If a send operation has blocked for this time, it shall return with
a partial count or with errno set to [EAGAIN] or [EWOULDBLOCK] if
no data is sent. The default for this option is zero, which indicates that
a send operation shall not time out. This option stores a timeval
structure. Note that not all implementations allow this option to be set.
For Boolean options, 0 indicates that the option is disabled and 1 indicates
that the option is enabled.
Options at other protocol levels vary in format and name.
The setsockopt() function provides an application program with the means
to control socket behavior. An application program can use setsockopt()
to allocate buffer space, control timeouts, or permit socket data broadcasts.
The <sys/socket.h> header defines the socket-level options
available to setsockopt().
Options may exist at multiple protocol levels. The SO_ options are always
present at the uppermost socket level.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html