int setsockopt(int socket, intlevel, intoption_name,
const void *option_value, socklen_toption_len);
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 socket argument.
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 <netinet/in.h>
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
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
Specifies that the rules used in validating addresses supplied to
allow reuse of local addresses, if this is supported by the protocol.
This option takes an int value. This is a Boolean
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
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 value.
Sets receive buffer size. This option takes an int value.
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 Boolean option.
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.
Upon successful completion, setsockopt() shall return 0. Otherwise,
-1 shall be returned and errno set to indicate
The setsockopt() function shall fail if:
The socket argument is not a valid file descriptor.
The send and receive timeout values are too big to fit into the timeout
fields in the socket structure.
The specified option is invalid at the specified socket level or the
socket has been shut down.
The socket is already connected, and a specified option cannot be
set while the socket is connected.
The option is not supported by the protocol.
The socket argument does not refer to a socket.
The setsockopt() function may fail if:
There was insufficient memory available for the operation to complete.
Insufficient resources are available in the system to complete the
The following sections are informative.
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
Options may exist at multiple protocol levels. The SO_ options are
always present at the uppermost socket level.
Sockets , bind() , endprotoent() , getsockopt()
, socket() , the Base Definitions volume of IEEE Std 1003.1-2001,
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