getsockopt, setsockopt - get and set options on sockets
getsockopt(s, level, optname, optval, optlen)
int s, level, optname;
setsockopt(s, level, optname, optval, optlen)
int s, level, optname;
Getsockopt and setsockopt manipulate 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 speci-
fied 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 num-
ber of TCP; see getprotoent(3N).
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 supplied as 0.
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 for-
mat and name; consult the appropriate entries in section(4P).
Most socket-level options take an int parameter for optval. For setsockopt, the parameter
should 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).
The following options are recognized at the socket level. Except as noted, each may be
examined with getsockopt and set with setsockopt.
SO_DEBUG toggle recording of debugging information
SO_REUSEADDR toggle local address reuse
SO_KEEPALIVE toggle keep connections alive
SO_DONTROUTE toggle routing bypass for outgoing messages
SO_LINGER linger on close if data present
SO_BROADCAST toggle permission to transmit broadcast messages
SO_OOBINLINE toggle reception of out-of-band data in band
SO_SNDBUF set buffer size for output
SO_RCVBUF set buffer size for input
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_KEEPALIVE enables the periodic transmission of messages on a con-
nected socket. Should the connected party fail to respond to these messages, the connec-
tion is considered broken and processes using the socket are notified via a SIGPIPE sig-
nal. 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 messags 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 attempt until it is able to transmit
the data or until it decides it is unable to deliver the information (a timeout period,
termed the linger interval, is specified in the setsockopt call when SO_LINGER is
requested). If SO_LINGER is disabled and a close 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 or
read calls without the MSG_OOB flag. 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. Finally,
SO_TYPE and SO_ERROR are options used only with setsockopt. 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 asynchro-
A 0 is returned if the call succeeds, -1 if it fails.
The call succeeds unless:
[EBADF] The argument s is not a valid descriptor.
[ENOTSOCK] The argument s is a file, not a socket.
[ENOPROTOOPT] The option is unknown at the level indicated.
[EFAULT] The address pointed to by optval is not in a valid part of the process
address space. For getsockopt, this error may also be returned if
optlen is not in a valid part of the process address space.
ioctl(2), socket(2), getprotoent(3N)
Several of the socket options should be handled at lower levels of the system.
4.2 Berkeley Distribution May 23, 1986 GETSOCKOPT(2)