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OpenDarwin 7.2.1 - man page for getsockopt (opendarwin section 2)

GETSOCKOPT(2)			     BSD System Calls Manual			    GETSOCKOPT(2)

     getsockopt, setsockopt -- get and set options on sockets

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

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

     setsockopt(int s, int level, int optname, const void *optval, int 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''

     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

     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

     Optname and any specified options are passed uninterpreted to the appropriate protocol mod-
     ule 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 param-
     eter 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_NOSIGPIPE    do not generate SIGPIPE, instead return EPIPE

     SO_DEBUG enables debugging in the underlying protocol modules.  SO_REUSEADDR indicates that
     the rules used in validating addresses supplied 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 permits 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 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 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 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 the setsockopt() call when SO_LINGER is requested).  If
     SO_LINGER is disabled and a close 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 normal data input queue as received; it will then be accessible with recv or read
     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, respectively.  The buffer size may be increased for high-volume connec-
     tions, 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 opera-
     tions process all of the data supplied by the call, delivering 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 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 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 imple-
     mentation, 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 parameter with the number of seconds and
     microseconds used to limit waits for input operations to complete.  In the current implemen-
     tation, this timer is restarted each time additional 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. The struct timeval parameter must represent
     a positive time interval less than SHRT_MAX * 10 milliseconds (5 minutes and 28 seconds)
     otherwise setsockopt() returns with the error EDOM.

     SO_NOSIGPIPE is an option that prevents SIGPIPE from being raised when a write fails on a
     socket to which there is no reader; instead the write to the socket returns with the error
     EPIPE when there is no reader.

     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 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.

     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.

     [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.

     [EDOM]		The argument value is out of bounds.

     ioctl(2), socket(2), getprotoent(3) protocols(5)

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

     The getsockopt() system call appeared in 4.2BSD.

4.3-Reno Berkeley Distribution		  April 19, 1994	   4.3-Reno Berkeley Distribution

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