getsockopt, setsockopt - get and set options on sockets
int getsockopt(int s, int level, int optname, void *optval,
int setsockopt(int s, int level, int optname, const void *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 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(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 in bytes of the
buffer pointed to by optval, and modified on return to indicate the
actual size of the value returned. If the size of the option value is
greater than the value of optlen, then the option will be truncated
silently to optlen bytes.
If no option value is to be supplied or returned, 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 ``socket'' level options,
described below. Options at other protocol levels vary in format and
name; consult the appropriate entries in section (7P).
Most socket-level options listed in the table below 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_REUSEPORT toggle local port reuse for multicast programs
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_REUSEPORT indicates that the rules used in validating ports supplied
in a bind(2) call should allow reuse of local ports. It allows multiple
programs to receive UDP multicast/broadcast datagrams on the same port if
they all set SO_REUSEPORT before binding the 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. 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 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 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 status 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. The error status is an errno value as
described in intro(2).
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.
[EINVAL] The option length is too small. Most socket-level
options expect optlen to be sizeof(int).
ioctl(2), socket(2), getprotoent(3N), ip(7P), tcp(7P), udp(7P)
ABI-compliant versions of the above calls can be obtained from
Several of the socket options should be handled at lower levels of the
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