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     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

     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

     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_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_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 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_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_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

     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_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 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 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

     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:

          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;

     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.

     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 connections.


     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.


     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 system.