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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" 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	op-
     tions at any other	level the protocol number of the appropriate protocol
     controlling the option is supplied.  For example, to indicate that	an op-
     tion 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, including: clnp(4),
     faith(4), icmp6(4), ip(4),	ip6(4),	ipsec(4), multicast(4),	pim(4),
     route(4), tcp(4), tp(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	op-
     tion, 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
	   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	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_DONTROUTE	indicates that outgo-
     ing 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 de-
     livery 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_SNDBUF and SO_RCVBUF	are options to adjust
     the normal	buffer sizes allocated for output and input buffers, respec-
     tively.  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, de-
     livering 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 wa-
     ter 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 mi-
     croseconds	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 im-
     plementation, this	timer is restarted each	time additional	data are de-
     livered 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 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 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 =	sizeof(struct timeval)
     cmsg_level	= SOL_SOCKET
     cmsg_type = SCM_TIMESTAMP

     SO_ACCEPTFILTER places an accept_filter(9)	on the socket, which will fil-
     ter 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 try-
     ing 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	op-
     tional 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	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.

     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), 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), accept_filter(9)

     The getsockopt() system call appeared in 4.2BSD.

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

BSD			       January 23, 2012				   BSD


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