getsockopt, setsockopt - get and set options on sockets
int getsockopt(int s, int level, int optname, char *optval,
int setsockopt(int s, int level, int optname, char *optval,
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 specified as SOL_SOCKET. To manipulate
options at any other level, level is the protocol number of the protocol
that controls the option. For example, to indicate that an option is to
be interpreted by the TCP protocol, level is set to the TCP protocol
number [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(s)
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,
a 0 optval may be supplied.
optname and any specified options are passed uninterpreted to the
appropriate protocol module for interpretation. The include file
sys/socket.h contains definitions for the socket-level options described
below. Options at other protocol levels vary in format and name.
Most socket-level options take an int for optval. For setsockopt, the
optval 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 that specifies the desired state of the option and the linger
interval (see below). struct linger is defined in
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 is 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 call should allow reuse of local addresses.
SO_KEEPALIVE enables the periodic transmission of messages on a connected
socket. If the connected party fails to respond to these messages, the
connection is considered broken and processes using the socket are
notified using a SIGPIPE signal. 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 a
socket and a close 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. 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 that adjust the normal buffer sizes allocated for output and
input buffers, respectively. The buffer size may be increased for highvolume
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 getsockopt.
SO_TYPE returns the type of the socket (for example, 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.
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.
ENOMEM There was insufficient user memory available for the
operation to complete.
ENOSR There were insufficient STREAMS resources available
for the operation to complete.
close(2), ioctl(2), read(2), socket(3N), getprotoent(3N)
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