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GETSOCKOPT(2)               BSD Programmer's Manual              GETSOCKOPT(2)

     getsockopt, setsockopt - get and set options on sockets

     #include <&lt;sys/types.h>&gt;
     #include <&lt;sys/socket.h>&gt;

     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'' 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 con-
     trolling 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 pro-
     tocol 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 op-
     tion, 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 not-
     ed, 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 de-
     livery 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 de-
     cides 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 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 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 connections, 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 operations process all of the data supplied by the call, de-
     livering data to the protocol for transmission and blocking as necessary
     for flow control.  Nonblocking output operations will process as much da-
     ta as permitted subject to flow control without blocking, but will pro-
     cess 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 mi-
     croseconds 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 de-
     livered 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.

     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.

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

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

     The getsockopt system call appeared in 4.2BSD.

4.3-Reno Berkeley Distribution    May 2, 1995                                3