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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/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 as-
     sociated 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 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
	   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_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 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,	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, 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)	opera-
     tion 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 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 EWOULDBLOCK if no data were sent.	In the current
     implementation, 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 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.

     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.

     [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), socket(2), getprotoent(3), sysctl(3), protocols(5), sysctl(8),

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

     The getsockopt() system call appeared in 4.2BSD.

BSD				  May 2, 1995				   BSD


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