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GETSOCKOPT(2) FreeBSD System Calls Manual GETSOCKOPT(2)
NAME
getsockopt, setsockopt - get and set options on sockets
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int
getsockopt(int s, int level, int optname, void * restrict optval,
socklen_t * restrict optlen);
int
setsockopt(int s, int level, int optname, const void *optval,
socklen_t optlen);
DESCRIPTION
The getsockopt() and setsockopt() system calls 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
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 optval and optlen arguments 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 argument, 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.
The optname argument 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 4 of the manual.
Most socket-level options utilize an int argument for optval. For
setsockopt(), the argument should be non-zero to enable a boolean option,
or zero if the option is to be disabled. SO_LINGER uses a struct linger
argument, 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 argument, defined in <sys/time.h>.
The following options are recognized at the socket level. For protocol-
specific options, see protocol manual pages, e.g. ip(4) or tcp(4).
Except as noted, each may be examined with getsockopt() and set with
setsockopt().
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_ACCEPTFILTER set accept filter on listening socket
SO_NOSIGPIPE controls generation of SIGPIPE for the socket
SO_TIMESTAMP enables reception of a timestamp with datagrams
SO_BINTIME enables reception of a timestamp with datagrams
SO_ACCEPTCONN get listening status of the socket (get only)
SO_DOMAIN get the domain of the socket (get only)
SO_TYPE get the type of the socket (get only)
SO_PROTOCOL get the protocol number for the socket (get
only)
SO_PROTOTYPE SunOS alias for the Linux SO_PROTOCOL (get only)
SO_ERROR get and clear error on the socket (get only)
SO_RERROR enables receive error reporting
SO_SETFIB set the associated FIB (routing table) for the
socket (set only)
The following options are recognized in FreeBSD:
SO_LABEL get MAC label of the socket (get only)
SO_PEERLABEL get socket's peer's MAC label (get only)
SO_LISTENQLIMIT get backlog limit of the socket (get only)
SO_LISTENQLEN get complete queue length of the socket (get
only)
SO_LISTENINCQLEN get incomplete queue length of the socket (get
only)
SO_USER_COOKIE set the 'so_user_cookie' value for the socket
(uint32_t, set only)
SO_TS_CLOCK set specific format of timestamp returned by
SO_TIMESTAMP
SO_MAX_PACING_RATE set the maximum transmit rate in bytes per
second for the socket
SO_NO_OFFLOAD disables protocol offloads
SO_NO_DDP disables direct data placement offload
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses
supplied in a bind(2) system 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
permits multiple instances of a program to each receive UDP/IP multicast
or broadcast datagrams destined for the bound port.
SO_REUSEPORT_LB allows completely duplicate bindings by multiple sockets
if they all set SO_REUSEPORT_LB before binding the port. Incoming TCP
and UDP connections are distributed among the participating listening
sockets based on a hash function of local port number, and foreign IP
address and port number. A maximum of 256 sockets can be bound to the
same load-balancing group.
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(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 seconds in the setsockopt() system
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 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 maximum on these values, which is accessible through the
sysctl(3) MIB variable "kern.ipc.maxsockbuf".
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
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 from that which was returned.
SO_SNDTIMEO is an option to set a timeout value for output operations.
It accepts a struct timeval argument 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 were sent. In the current
implementation, this timer is restarted each time additional data are
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 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.
SO_SETFIB can be used to over-ride the default FIB (routing table) for
the given socket. The value must be from 0 to one less than the number
returned from the sysctl net.fibs.
SO_USER_COOKIE can be used to set the uint32_t so_user_cookie field in
the socket. The value is an uint32_t, and can be used in the kernel code
that manipulates traffic related to the socket. The default value for
the field is 0. As an example, the value can be used as the skipto
target or pipe number in ipfw/dummynet.
SO_ACCEPTFILTER places an accept_filter(9) on the socket, which will
filter incoming connections on a listening stream socket before being
presented for accept(2). Once more, listen(2) must be called on the
socket before trying to install the filter on it, or else the
setsockopt() system call will fail.
struct accept_filter_arg {
char af_name[16];
char af_arg[256-16];
};
The optval argument should point to a struct accept_filter_arg that will
select and configure the accept_filter(9). The af_name argument should
be filled with the name of the accept filter that the application wishes
to place on the listening socket. The optional argument af_arg can be
passed to the accept filter specified by af_name to provide additional
configuration options at attach time. Passing in an optval of NULL will
remove the filter.
The SO_NOSIGPIPE option controls generation of the SIGPIPE signal
normally sent when writing to a connected socket where the other end has
been closed returns with the error EPIPE.
If the SO_TIMESTAMP or SO_BINTIME option is enabled on a SOCK_DGRAM
socket, the recvmsg(2) call may return a timestamp corresponding to when
the datagram was received. However, it may not, for example due to a
resource shortage. The msg_control field in the msghdr structure points
to a buffer that contains a cmsghdr structure followed by a struct
timeval for SO_TIMESTAMP and struct bintime for SO_BINTIME. The cmsghdr
fields have the following values for TIMESTAMP by default:
cmsg_len = CMSG_LEN(sizeof(struct timeval));
cmsg_level = SOL_SOCKET;
cmsg_type = SCM_TIMESTAMP;
and for SO_BINTIME:
cmsg_len = CMSG_LEN(sizeof(struct bintime));
cmsg_level = SOL_SOCKET;
cmsg_type = SCM_BINTIME;
Additional timestamp types are available by following SO_TIMESTAMP with
SO_TS_CLOCK, which requests a specific timestamp format to be returned
SO_TS_MONOTONIC
monotonic time (SCM_MONOTONIC, struct timespec)
SO_ACCEPTCONN, SO_TYPE, SO_PROTOCOL (and its alias SO_PROTOTYPE) and
SO_ERROR are options used only with getsockopt(). SO_ACCEPTCONN returns
whether the socket is currently accepting connections, that is, whether
or not the listen(2) system call was invoked on the socket. SO_TYPE
returns the type of the socket, such as SOCK_STREAM; it is useful for
servers that inherit sockets on startup. SO_PROTOCOL returns the
protocol number for the socket, for AF_INET and AF_INET6 address
families. 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. SO_RERROR
indicates that receive buffer overflows should be handled as errors.
Historically receive buffer overflows have been ignored and programs
could not tell if they missed messages or messages had been truncated
because of overflows. Since programs historically do not expect to get
receive overflow errors, this behavior is not the default.
SO_LABEL returns the MAC label of the socket. SO_PEERLABEL returns the
MAC label of the socket's peer. Note that your kernel must be compiled
with MAC support. See mac(3) for more information.
SO_LISTENQLIMIT returns the maximal number of queued connections, as set
by listen(2). SO_LISTENQLEN returns the number of unaccepted complete
connections. SO_LISTENINCQLEN returns the number of unaccepted
incomplete connections.
SO_MAX_PACING_RATE instruct the socket and underlying network adapter
layers to limit the transfer rate to the given unsigned 32-bit value in
bytes per second.
SO_NO_OFFLOAD disables support for protocol offloads. At present, this
prevents TCP sockets from using TCP offload engines. SO_NO_DDP disables
support for a specific TCP offload known as direct data placement (DDP).
DDP is an offload supported by Chelsio network adapters that permits
reassembled TCP data streams to be received via zero-copy in user-
supplied buffers using aio_read(2).
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The getsockopt() and setsockopt() system calls succeed 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] Installing an accept_filter(9) on a non-listening
[ENOBUFS] Insufficient resources were available in the system to
perform the operation.
SEE ALSO
ioctl(2), listen(2), recvmsg(2), socket(2), getprotoent(3), mac(3),
sysctl(3), ip(4), ip6(4), sctp(4), tcp(4), protocols(5), sysctl(8),
accept_filter(9), bintime(9)
HISTORY
The getsockopt() and setsockopt() system calls appeared in 4.2BSD.
BUGS
Several of the socket options should be handled at lower levels of the
system.
FreeBSD 14.0-RELEASE-p11 February 8, 2021 FreeBSD 14.0-RELEASE-p11