SETSOCKOPT(P) POSIX Programmer's Manual SETSOCKOPT(P)
setsockopt - set the socket options
int setsockopt(int socket, int level, int option_name,
const void *option_value, socklen_t option_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
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 Con-
trol Protocol), set level to IPPROTO_TCP as defined in the <netinet/in.h> header.
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 debug-
ging in the underlying protocol modules. This option takes an int value. This is a
Permits sending of broadcast messages, if this is supported by the protocol. This
option takes an int value. This is a Boolean option.
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
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 option.
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 desti-
nation 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, block-
ing 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
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 error.
The setsockopt() function shall fail if:
EBADF The socket argument is not a valid file descriptor.
EDOM The send and receive timeout values are too big to fit into the timeout fields in
the socket structure.
EINVAL 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:
ENOMEM There was insufficient memory available for the operation to complete.
Insufficient resources are available in the system to complete the call.
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 setsockopt().
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, <netinet/in.h>, <sys/socket.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SETSOCKOPT(P)