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

     getsockopt, setsockopt -- get and set options on sockets

     #include <&lt;sys/socket.h>&gt;

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

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

     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 even if it is
     not a local address:

           pass out inet from 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
     using setrtable(2).

     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
     asynchronous errors.

     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
     1003.1-2008 (``POSIX.1'').

     The getsockopt() system call appeared in 4.2BSD.

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

BSD                            December 16, 2014                           BSD