GETSOCKOPT(2) System Calls GETSOCKOPT(2)
getsockopt, setsockopt - get and set options on sockets
int getsockopt (int s, int level, int optname, void *optval, int
int setsockopt (int s, int level, int optname, const void *optval, int
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 pro-
tocol 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 sup-
plied 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; con-
sult the appropriate entries in section 4 of the manual.
Most socket-level options utilize 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
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_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 sup-
plied 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 per-
mits 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 con-
nection is considered broken and processes using the socket are noti-
fied 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_LINGERisset, 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,
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 ver-
sions 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
SO_SNDLOWAT is an option to set the minimum count for output opera-
tions. 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) 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
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 par-
tial count or with the error EWOULDBLOCK 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 opera-
tions 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 opera-
tion has been blocked for this much time without receiving additional
data, it returns with a short count or with the error EWOULDBLOCK if no
data were received.
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 use-
ful 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.
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, this error
may also be returned if optlen is not in a valid part of
the process address space.
ioctl(2), socket(2), getprotoent(3) protocols(5)
Several of the socket options should be handled at lower levels of the
The setsockopt system call appeared in 4.2BSD.
GNO 16 January 1997 GETSOCKOPT(2)
Man(1) output converted with