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NetBSD 6.1.5 - man page for getsockopt (netbsd section 2)

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

     getsockopt, setsockopt -- 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);

     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''

     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 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

     optname and any specified options are passed uninterpreted to the appropriate protocol mod-
     ule 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: clnp(4), faith(4),
     icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4), tcp(4), tp(4), and

     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() 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_TIMESTAMP       enables reception of a timestamp with datagrams
	   SO_ACCEPTFILTER    set accept filter on listening socket
	   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_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 opera-
     tions 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 implementa-
     tion, 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 implemen-
     tation, 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 connec-
     tions 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 applica-
     tion 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().  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 accept_filter(9).

     [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), clnp(4), faith(4), icmp6(4), ip(4),
     ip6(4), ipsec(4), multicast(4), pim(4), route(4), tcp(4), tp(4), unix(4), protocols(5),

     The getsockopt() system call appeared in 4.2BSD.

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

BSD					 January 23, 2012				      BSD

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