int setsockopt(int socket, int level, int
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 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> 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 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 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 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 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 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 error.
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 setsockopt().
Options may exist at multiple protocol levels. The SO_ options are always present at the uppermost socket level.
COPYRIGHTPortions 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 .