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GETSOCKOPT(2)                 System Calls Manual                GETSOCKOPT(2)

     getsockopt, setsockopt, getsockopt2 - get and set options on sockets

     Standard C Library (libc, -lc)

     #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);

     getsockopt2(int s, int level, int optname, void * restrict optval,
         socklen_t * restrict optlen);

     getsockopt(), setsockopt() and getsockopt2() manipulate the options
     associated with a socket.

     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.  Options may exist at
     multiple protocol levels.  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() and getsockopt2() they identify a buffer
     in which the value for the requested option(s) are to be returned.  For
     getsockopt2() they are also used to provide an extra argument to select
     which value to return.  For getsockopt() and getsockopt2(), 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 the size of the requested option value to be
     stored in optval is greater than the size indicated in optlen then only
     optlen bytes will be stored in optval and the result will be silently
     truncated.  If no option value is to be supplied 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; consult
     the appropriate entries in section 4 of the manual, including: faith(4),
     icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4),
     tcp(4), and unix(4).

     Most socket-level options use 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() or getsockopt2() 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
           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_TIMESTAMP       enables reception of a timestamp with datagrams
           SO_ACCEPTFILTER    set accept filter on listening socket
           SO_RERROR          enables receive size error reporting
           SO_NOSIGPIPE       controls generation of SIGPIPE for the socket
           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
     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 option
     permits 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
     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 address.  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, but the
     default can be changed by setting the SO_RERROR flag using sysctl(8) and

     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,
     measured 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
     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 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
     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)
     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
     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 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 EAGAIN 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 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 EAGAIN 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
     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.  The cmsghdr fields have the following values:

     cmsg_len = sizeof(struct timeval)
     cmsg_level = SOL_SOCKET
     cmsg_type = SCM_TIMESTAMP

     SO_ACCEPTFILTER places an accept_filter(9) on the socket, which will
     filter incoming connections on a listening socket before being presented
     for accept(2).  The setsockopt() system call will fail if the socket
     already has a filter set, and listen(2) must be called on the socket
     before trying to install a filter.  The optval argument should point to a
     struct accept_filter_arg that will select and configure the
     accept_filter(9), defined as follows:

     struct  accept_filter_arg {
             char    af_name[16];
             char    af_arg[256-16];

     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.

     Finally, SO_TYPE and SO_ERROR are options used only with getsockopt() or
     getsockopt2().  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 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.

     [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]           The socket s was not suitable for installing an

     [ENOPROTOOPT]      The option is unknown at the level indicated.

     [ENOTSOCK]         The argument s is a file, not a socket.

     ioctl(2), poll(2), select(2), socket(2), getprotoent(3), faith(4),
     icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4),
     tcp(4), unix(4), protocols(5), accept_filter(9)

     The getsockopt() system call appeared in 4.2BSD.

     The getsockopt2() system call appeared in NetBSD 9.0.

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

NetBSD 10.99                    August 7, 2021                    NetBSD 10.99