GETSOCKOPT(2) BSD System Calls Manual GETSOCKOPT(2)
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 *optval,
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 proto-
col 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 supplied or returned,
optval may be NULL.
optname and any specified options are passed uninterpreted to the appro-
priate 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 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 <sys/time.h>.
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; not supported
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_BINDANY enables binding to any address
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_TIMESTAMP enables reception of a timestamp with datagrams
SO_PEERCRED get the credentials from other side of connection
SO_RTABLE set the routing table used for route lookups
SO_SPLICE splice two sockets together or get data length
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 connection is con-
sidered broken and processes using the socket are notified via a SIGPIPE
signal when attempting to send data.
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(2) attempt until it is able to transmit the data or until it
decides it is unable to deliver the information (a timeout period mea-
sured in seconds, 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 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 nor-
mal 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_BINDANY allows the socket to be bound to addresses which are not local
to the machine, so it can be used to make a transparent proxy. Note that
this option is limited to the super-user. In order to receive packets
for these addresses, SO_BINDANY needs to be combined with matching outgo-
ing pf(4) rules with the divert-reply parameter. For example, with the
following rule the socket receives packets for 192.168.0.10 even if it is
not a local address:
pass out inet from 192.168.0.10 divert-reply
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 abso-
lute limit on these values.
SO_SNDLOWAT is an option to set the minimum count for output operations.
Most output operations process all of the data supplied by the call,
delivering data to the protocol for transmission and blocking as neces-
sary 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 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 nor-
mally 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 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 partial
count or with the error EWOULDBLOCK if no data was 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 por-
tions 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 opera-
tions. It accepts a struct timeval parameter with the number of seconds
and microseconds used to limit waits for input operations to complete.
In the current implementation, this timer is restarted each time addi-
tional 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 a short
count or with the error EWOULDBLOCK if no data were received.
If the SO_TIMESTAMP option is enabled on a SOCK_DGRAM socket, the
recvmsg(2) call will return a timestamp corresponding to when the data-
gram was received. The msg_control field in the msghdr structure points
to a buffer that contains a cmsghdr structure followed by a struct
timeval. The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(sizeof(struct timeval))
cmsg_level = SOL_SOCKET
cmsg_type = SCM_TIMESTAMP
SO_PEERCRED fetches the struct sockpeercred credentials from the other
side of the connection (currently only possible on AF_UNIX sockets).
These credentials are from the time that bind(2) or connect(2) were
The SO_RTABLE option gets or sets the routing table which will be used by
the socket for address lookups. If a protocol family of the socket
doesn't support switching routing tables, the ENOPROTOOPT error is
returned. Only the superuser is allowed to change the routing table if
it is already set to a non-zero value. A socket's chosen routing table
is initialized from the process's configuration, previously selected
SO_SPLICE can splice together two TCP or UDP sockets for zero-copy data
transfers. Both sockets must be of the same type. In the first form,
setsockopt() is called with the source socket s and the drain socket's
int file descriptor as optval. In the second form, optval is a struct
splice with the drain socket in sp_fd, a positive maximum number of bytes
or 0 in sp_max and an idle timeout sp_idle in the form of a struct
timeval. If -1 is given as drain socket, the source socket s gets
unspliced. Otherwise the spliced data transfer continues within the ker-
nel until the optional maximum is reached, one of the connections termi-
nates, idle timeout expires or an error occurs. A successful select(2),
poll(2), or kqueue(2) operation testing the ability to read from the
source socket indicates that the splicing has terminated. The error sta-
tus can be examined with SO_ERROR at the source socket. The ETIMEDOUT
error is set if there was no data transferred between two sockets during
the sp_idle period of time. The EFBIG error is set after exactly sp_max
bytes have been transferred. Note that if a maximum is given, it is only
guaranteed that no more bytes are transferred. A short splice can hap-
pen, but then a second call to splice will transfer the remaining data
immediately. The SO_SPLICE option with getsockopt() and an off_t value
as optval can be used to retrieve the number of bytes transferred so far
from the source socket s. A successful new splice resets this number.
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 pend-
ing 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
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
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.
[EOPNOTSUPP] The option is unsupported.
[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.
connect(2), getrtable(2), ioctl(2), poll(2), select(2), socket(2),
getprotoent(3), divert(4), pf.conf(5), protocols(5), sosplice(9)
The getsockopt() and setsockopt() functions conform to IEEE Std
The getsockopt() system call appeared in 4.2BSD.
Several of the socket options should be handled at lower levels of the
BSD March 27, 2017 BSD