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

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

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

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

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

DESCRIPTION
     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 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_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
     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() 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 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 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 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.

     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.

IMPLEMENTATION NOTES
     In	the non-threaded library getsockopt() is implemented as	the getsockopt
     syscall.

     In	the threaded library, the getsockopt syscall is	assembled to
     _thread_sys_getsockopt() and getsockopt() is implemented as a function
     which locks s for read and	write, then calls _thread_sys_getsockopt().
     Before returning, getsockopt() unlocks s.

     In	the non-threaded library setsockopt() is implemented as	the setsockopt
     syscall.

     In	the threaded library, the setsockopt syscall is	assembled to
     _thread_sys_setsockopt() and setsockopt() is implemented as a function
     which locks s for read and	write, then calls _thread_sys_setsockopt().
     Before returning, setsockopt() unlocks s.

RETURN VALUES
     A 0 is returned if	the call succeeds, -1 if it fails.

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.

SEE ALSO
     ioctl(2), socket(2), getprotoent(3), sysctl(3), protocols(5), sysctl(8)

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

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

4.3-Reno Berkeley Distribution	  May 2, 1995	4.3-Reno Berkeley Distribution

NAME | SYNOPSIS | DESCRIPTION | IMPLEMENTATION NOTES | RETURN VALUES | ERRORS | SEE ALSO | BUGS | HISTORY

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