getsockopt, setsockopt - get and set options on sockets
getsockopt(int s, int level, int optname, void *optval,
setsockopt(int s, int level, int optname, const void
getsockopt() and setsockopt() manipulate the options associated with a
socket. Options may exist at multiple protocol levels; they
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
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
setsockopt(). For getsockopt() they identify a buffer in
which the value
for the requested option(s) are to be returned. For
is a value-result parameter, initially containing the size
of the buffer
pointed to by optval, and modified on return to indicate the
of the value returned. If no option value is to be supplied
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
the appropriate entries in section 4 of the manual.
Most socket-level options utilize an int parameter for
setsockopt(), the parameter should be non-zero to enable a
or zero if the option is to be disabled. SO_LINGER
uses a struct
linger parameter, defined in <sys/socket.h>, which specifies
state of the option and the linger interval (see below).
SO_RCVTIMEO use a struct timeval parameter, defined in
The following options are recognized at the socket level.
Except as noted,
each may be examined with getsockopt() and set with
SO_DEBUG enables recording of debugging information
SO_REUSEADDR enables local address reuse
SO_REUSEPORT enables duplicate address and port
SO_KEEPALIVE enables keep connections alive
SO_DONTROUTE enables routing bypass for outgoing
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
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_TYPE get the type of the socket (get only)
SO_ERROR get and clear error on the socket (get
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
multiple instances of a program to each receive UDP/IP
broadcast datagrams destined for the bound port.
the periodic transmission of messages on a connected socket.
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
messages should bypass the standard routing facilities.
messages are directed to the appropriate network interface
the network portion of the destination address.
SO_LINGER controls the action taken when unsent messages are
socket and a close(2) is performed. If the socket promises
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
in seconds, termed the linger interval, is specified
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
or read(2) calls without the MSG_OOB flag. Some protocols
as if this option is set. SO_SNDBUF and SO_RCVBUF are options to adjust
the normal buffer sizes allocated for output and input
The buffer size may be increased for high-volume
may be decreased to limit the possible backlog of incoming
system places an absolute limit on these values.
SO_SNDLOWAT is an option to set the minimum count for output
Most output operations process all of the data supplied by
the call, delivering
data to the protocol for transmission and blocking
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
In general, receive calls will block until any (nonzero) amount
of data is received, then return with the smaller of the
or the amount requested. The default value for SO_RCVLOWAT
is 1. If
SO_RCVLOWAT is set to a larger value, blocking receive calls
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
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 EWOULDBLOCK if no data was sent. In
implementation, this timer is restarted each time additional
data are delivered
to the protocol, implying that the limit applies to
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
It accepts a struct timeval parameter with the number of seconds
and microseconds used to limit waits for input operations to
In the current implementation, this timer is restarted each
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
count or with the error EWOULDBLOCK if no data were received.
Finally, SO_TYPE and SO_ERROR are options used only with
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
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.
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(),
may also be returned if optlen is not in a
valid part of
the process address space.
connect(2), ioctl(2), poll(2), select(2), socket(2), getprotoent(3),
SO_PEERCRED is not supported, see getpeereid(2) instead.
The getsockopt() system call appeared in 4.2BSD.
Several of the socket options should be handled at lower
levels of the
OpenBSD 3.6 February 15, 1999
[ Back ]