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GETSOCKOPT(2)		  FreeBSD System Calls Manual		 GETSOCKOPT(2)

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

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/socket.h>

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

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

DESCRIPTION
     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 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 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 NULL.

     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.	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_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)	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	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
     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 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 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 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)
     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	opera-
     tions.  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 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 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 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.

     SO_ACCEPTFILTER places an accept_filter(9)	on the socket, which will fil-
     ter incoming connections on a listening stream socket before being	pre-
     sented 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() sys-
     tem 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 nor-
     mally 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 = sizeof(struct timeval);
	  cmsg_level = SOL_SOCKET;
	  cmsg_type = SCM_TIMESTAMP;

     and for SO_BINTIME:

	  cmsg_len = sizeof(struct bintime);
	  cmsg_level = SOL_SOCKET;
	  cmsg_type = SCM_BINTIME;

     Finally, SO_ACCEPTCONN, SO_TYPE 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_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.

RETURN VALUES
     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.

ERRORS
     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.

SEE ALSO
     ioctl(2), listen(2), recvmsg(2), socket(2), getprotoent(3), sysctl(3),
     protocols(5), sysctl(8), accept_filter(9),	bintime(9)

HISTORY
     The getsockopt() system call appeared in 4.2BSD.

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

FreeBSD	9.3			 March 8, 2007			   FreeBSD 9.3

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | BUGS

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