getsockopt() and
setsockopt() manipulate the
options associated with a socket. Options may exist at multiple protocol levels; they are always present at the uppermost “socket” level.
When manipulating socket options the level at which the option resides and the name of the option must be specified. To manipulate options at the socket level,
level is specified as
SOL_SOCKET. To manipulate options at any other level the protocol number of the appropriate protocol controlling the option is supplied. For example, to indicate that an option is to be interpreted by the TCP protocol,
level should be set to the protocol number of TCP; see
getprotoent(3).
The parameters
optval and
optlen are used to access option values for
setsockopt(). For
getsockopt() they identify a buffer in which the value for the requested option(s) are to be returned. For
getsockopt(),
optlen is a value-result parameter, initially containing the size of the buffer pointed to by
optval, and modified on return to indicate the actual size of the value returned. If no option value is to be supplied or returned,
optval may be
NULL.
optname and any specified options are passed uninterpreted to the appropriate protocol module for interpretation. The include file <
sys/socket.h> contains definitions for socket level options, described below. Options at other protocol levels vary in format and name; consult the appropriate entries in section 4 of the manual, including:
clnp(4),
faith(4),
icmp6(4),
ip(4),
ip6(4),
ipsec(4),
multicast(4),
pim(4),
route(4),
tcp(4),
tp(4), and
unix(4).
Most socket-level options use an
int parameter for
optval. For
setsockopt(), the parameter should be non-zero to enable a boolean option, or zero if the option is to be disabled.
SO_LINGER uses a
struct linger parameter, defined in <
sys/socket.h>, which specifies the desired state of the option and the linger interval (see below).
SO_SNDTIMEO and
SO_RCVTIMEO use a
struct timeval parameter, defined in <
sys/time.h>.
The following options are recognized at the socket level. Except as noted, each may be examined with
getsockopt() and set with
setsockopt().
SO_DEBUG
enables recording of debugging information
SO_REUSEADDR
enables local address reuse
SO_REUSEPORT
enables duplicate address and port bindings
SO_KEEPALIVE
enables keep connections alive
SO_DONTROUTE
enables routing bypass for outgoing messages
SO_LINGER
linger on close if data present
SO_BROADCAST
enables permission to transmit broadcast messages
SO_OOBINLINE
enables reception of out-of-band data in band
SO_SNDBUF
set buffer size for output
SO_RCVBUF
set buffer size for input
SO_SNDLOWAT
set minimum count for output
SO_RCVLOWAT
set minimum count for input
SO_SNDTIMEO
set timeout value for output
SO_RCVTIMEO
set timeout value for input
SO_TIMESTAMP
enables reception of a timestamp with datagrams
SO_ACCEPTFILTER
set accept filter on listening socket
SO_TYPE
get the type of the socket (get only)
SO_ERROR
get and clear error on the socket (get only)
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses supplied in a
bind(2) call should allow reuse of local addresses.
SO_REUSEPORT allows completely duplicate bindings by multiple processes if they all set
SO_REUSEPORT before binding the port. This option permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port.
SO_KEEPALIVE enables the periodic transmission of messages on a connected socket. Should the connected party fail to respond to these messages, the connection is considered broken and processes using the socket are notified via a
SIGPIPE signal when attempting to send data.
SO_DONTROUTE indicates that outgoing messages should bypass the standard routing facilities. Instead, messages are directed to the appropriate network interface according to the network portion of the destination address.
SO_LINGER controls the action taken when unsent messages are queued on socket and a
close(2) is performed. If the socket promises reliable delivery of data and
SO_LINGER is set, the system will block the process on the
close(2) attempt until it is able to transmit the data or until it decides it is unable to deliver the information (a timeout period, measured in seconds, termed the linger interval, is specified in the
setsockopt() call when
SO_LINGER is requested). If
SO_LINGER is disabled and a
close(2) is issued, the system will process the close in a manner that allows the process to continue as quickly as possible.
The option
SO_BROADCAST requests permission to send broadcast datagrams on the socket. Broadcast was a privileged operation in earlier versions of the system. With protocols that support out-of-band data, the
SO_OOBINLINE option requests that out-of-band data be placed in the normal data input queue as received; it will then be accessible with
recv(2) or
read(2) calls without the
MSG_OOB flag. Some protocols always behave as if this option is set.
SO_SNDBUF and
SO_RCVBUF are options to adjust the normal buffer sizes allocated for output and input buffers, respectively. The buffer size may be increased for high-volume connections, or may be decreased to limit the possible backlog of incoming data. The system places an absolute limit on these values.
SO_SNDLOWAT is an option to set the minimum count for output operations. Most output operations process all of the data supplied by the call, delivering data to the protocol for transmission and blocking as necessary for flow control. Nonblocking output operations will process as much data as permitted subject to flow control without blocking, but will process no data if flow control does not allow the smaller of the low water mark value or the entire request to be processed. A
select(2) or
poll(2) operation testing the ability to write to a socket will return true only if the low water mark amount could be processed. The default value for
SO_SNDLOWAT is set to a convenient size for network efficiency, often 1024.
SO_RCVLOWAT is an option to set the minimum count for input operations. In general, receive calls will block until any (non-zero) amount of data is received, then return with the smaller of the amount available or the amount requested. 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. Receive calls may still 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 than that returned.
SO_SNDTIMEO is an option to set a timeout value for output operations. It accepts a
struct timeval parameter with the number of seconds and microseconds used to limit waits for output operations to complete. If a send operation has blocked for this much time, it returns with a partial count or with the error
EAGAIN if no data were sent. In the current implementation, this timer is restarted each time additional data are delivered to the protocol, implying that the limit applies to output portions ranging in size from the low water mark to the high water mark for output.
SO_RCVTIMEO is an option to set a timeout value for input operations. It accepts a
struct timeval parameter with the number of seconds and microseconds used to limit waits for input operations to complete. In the current implementation, this timer is restarted each time additional data are received by the protocol, and thus the limit is in effect an inactivity timer. If a receive operation has been blocked for this much time without receiving additional data, it returns with a short count or with the error
EAGAIN if no data were received.
If the
SO_TIMESTAMP option is enabled on a
SOCK_DGRAM socket, the
recvmsg(2) call will return a timestamp corresponding to when the datagram was received. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by a struct timeval. The cmsghdr fields have the following values:
cmsg_len = sizeof(struct timeval)
cmsg_level = SOL_SOCKET
cmsg_type = SCM_TIMESTAMP
SO_ACCEPTFILTER places an
accept_filter(9) on the socket, which will filter incoming connections on a listening socket before being presented for
accept(2). The
setsockopt() system call will fail if the socket already has a filter set, and
listen(2) must be called on the socket before trying to install a filter. The
optval argument should point to a
struct accept_filter_arg that will select and configure the
accept_filter(9), defined as follows:
struct accept_filter_arg {
char af_name[16];
char af_arg[256-16];
};
The
af_name argument should be filled with the name of the accept filter that the application wishes to place on the listening socket. The optional argument
af_arg can be passed to the accept filter specified by
af_name to provide additional configuration options at attach time. Passing in an
optval of
NULL will remove the filter.
Finally,
SO_TYPE and
SO_ERROR are options used only with
getsockopt().
SO_TYPE returns the type of the socket, such as
SOCK_STREAM; it is useful for servers that inherit sockets on startup.
SO_ERROR returns any pending error on the socket and clears the error status. It may be used to check for asynchronous errors on connected datagram sockets or for other asynchronous errors.