GETSOCKOPT(2)                BSD System Calls Manual               GETSOCKOPT(2)

     getsockopt, setsockopt — get and set options on sockets

     Standard C Library (libc, -lc)

     #include <sys/types.h>
     #include <sys/socket.h>

     getsockopt(int s, int level, int optname, void * restrict optval,
         socklen_t * restrict optlen);

     setsockopt(int s, int level, int optname, const void *optval,
         socklen_t optlen);

     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_DEBUG           enables recording of debugging information
           SO_REUSEADDR       enables local address reuse
           SO_REUSEPORT       enables duplicate address and port bindings
           SO_REUSEPORT_LB    enables duplicate address and port bindings with
                              load balancing
           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_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_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
           SO_LISTENINCQLEN    get incomplete queue length of the socket (get
           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_MAX_PACING_RATE  set the maximum transmit rate in bytes per second
                               for the socket

     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 processes
     if they all set SO_REUSEPORT_LB before binding the port.  Incoming TCP and
     UDP connections are distributed among the sharing processes based on a hash
     function of local port number, foreign IP address and port number. A
     maximum of 256 processes can share one socket.

     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 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

     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

     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
     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

     SO_RCVTIMEO is an option to set a timeout value for input operations.  It
     accepts a struct timeval argument 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 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 will return a timestamp corresponding to when the
     datagram was received.  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
     instead of SCM_TIMESTAMP when SO_TIMESTAMP is enabled. These SO_TS_CLOCK
     values are recognized in FreeBSD:

           SO_TS_REALTIME_MICROrealtime (SCM_TIMESTAMP, struct timeval), default
           SO_TS_BINTIME  realtime (SCM_BINTIME, struct bintime)
           SO_TS_REALTIME realtime (SCM_REALTIME, struct timespec)
           SO_TS_MONOTONICmonotonic time (SCM_MONOTONIC, struct timespec)

     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.

     Finally, 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

     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.

     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.

     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]           Installing an accept_filter(9) on a non-listening socket
                        was attempted.

     [ENOMEM]           A memory allocation failed that was required to service
                        the request.

     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)

     The getsockopt() and setsockopt() system calls appeared in 4.2BSD.

     Several of the socket options should be handled at lower levels of the

BSD                              August 21, 2018                             BSD