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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
     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
     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
     #include <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 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

     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_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_ERROR           get and clear error on the socket (get only)
           SO_SETFIB          set the associated FIB (routing table) for the
                              socket (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 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(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
     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 allocated 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,
     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 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
     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
     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 microseconds used to limit waits for input operations to
     complete.  In the current implementation, 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.

     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
     returned from the sysctl net.fibs.

     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
     configuration 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 datagram 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;

     Finally, SO_ACCEPTCONN, SO_TYPE 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_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.

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), sysctl(3),
     protocols(5), sysctl(8), accept_filter(9), bintime(9)

HISTORY

     The getsockopt() system call appeared in 4.2BSD.

BUGS

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