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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_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
     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-

     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.

     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.

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.

SEE ALSO

     ioctl(2), socket(2), getprotoent(3), sysctl(3), protocols(5), sysctl(8),
     accept_filter(9)

BUGS

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

HISTORY

     The getsockopt() system call appeared in 4.2BSD.

FreeBSD 5.4			  May 2, 1995			   FreeBSD 5.4

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