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GETSOCKOPT(2)		       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 asso-
     ciated  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 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 getprotoent(3).

     The optval and optlen arguments are used to access option values  for  set-
     sockopt().   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 be-
     low.  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 setsock-
     opt(), 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.   For  protocol-
     specific options, see protocol manual pages, e.g.	ip(4) or tcp(4).  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_PROTOCOL	      get the protocol number for the socket (get only)
	   SO_PROTOTYPE       SunOS alias for the Linux SO_PROTOCOL (get only)
	   SO_ERROR	      get and clear error on the socket (get only)
	   SO_SETFIB	      set  the	associated  FIB  (routing table) for the
			      socket (set only)

     The following options are recognized in FreeBSD:

	   SO_LABEL	       get MAC label of the socket (get only)
	   SO_PEERLABEL        get socket's peer's MAC label (get only)
	   SO_LISTENQLIMIT     get backlog limit of the socket (get only)
	   SO_LISTENQLEN       get complete queue  length  of  the  socket  (get
			       only)
	   SO_LISTENINCQLEN    get  incomplete	queue  length of the socket (get
			       only)
	   SO_USER_COOKIE      set the 'so_user_cookie'  value	for  the  socket
			       (uint32_t, set 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) 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  permits
     multiple  instances of a program to each receive UDP/IP multicast or broad-
     cast 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 net-
     work 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 deliv-
     ery 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  re-
     quests  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 allo-
     cated  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 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, deliv-
     ering 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) 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 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  microsec-
     onds  used to limit waits for output operations to complete.  If a send op-
     eration 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 implemen-
     tation, 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 argument with the number of seconds and  microsec-
     onds  used to limit waits for input operations to complete.  In the current
     implementation, this timer is restarted each time additional data	are  re-
     ceived  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_SETFIB can be used to over-ride the default FIB (routing table) for  the
     given  socket.   The  value  must be from 0 to one less than the number re-
     turned from the sysctl net.fibs.

     SO_USER_COOKIE can be used to set the uint32_t so_user_cookie field in  the
     socket.   The value is an uint32_t, and can be used in the kernel code that
     manipulates traffic related to the socket.  The default value for the field
     is 0.  As an example, the value can be used as the skipto	target	or  pipe
     number in ipfw/dummynet.

     SO_ACCEPTFILTER places an accept_filter(9) on the socket, which will filter
     incoming  connections  on	a listening stream socket before being presented
     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() system 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  configura-
     tion  options at attach time.  Passing in an optval of NULL will remove the
     filter.

     The SO_NOSIGPIPE option controls generation of the SIGPIPE signal	normally
     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 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 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;

     SO_ACCEPTCONN,  SO_TYPE,  SO_PROTOCOL  (and  its  alias  SO_PROTOTYPE)  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_PROTOCOL returns the protocol number for
     the socket, for AF_INET and AF_INET6 address  families.   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.

     Finally,  SO_LABEL  returns  the MAC label of the socket.	SO_PEERLABEL re-
     turns the MAC label of the socket's peer.	Note that your	kernel	must  be
     compiled	with   MAC   support.	 See   mac(3)	for   more  information.
     SO_LISTENQLIMIT returns the maximal number of queued connections, as set by
     listen(2).  SO_LISTENQLEN returns the number of unaccepted complete connec-
     tions.  SO_LISTENINCQLEN returns the number of unaccepted	incomplete  con-
     nections.

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),	 mac(3),
     sysctl(3),  ip(4),  ip6(4),  sctp(4),  tcp(4), protocols(5), sysctl(8), ac-
     cept_filter(9), bintime(9)

HISTORY
     The getsockopt() and setsockopt() system calls appeared in 4.2BSD.

BUGS
     Several of the socket options should be handled at lower levels of the sys-
     tem.

FreeBSD 9.1			February 26, 2012		   GETSOCKOPT(2)
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