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# getsockopt

    GETSOCKOPT(2)                 System Calls Manual                GETSOCKOPT(2)

    NAME
         getsockopt, setsockopt - get and set options on sockets

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

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

         int
         setsockopt(int s, int level, int optname, const void *optval,
                 socklen_t optlen);

    DESCRIPTION
         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 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 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
         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_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
         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 mea-
         sured in seconds, termed the linger interval, is specified in the
         setsockopt() 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 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,
         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) or
         poll(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 nor-
         mally 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 was 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 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.

    RETURN VALUES
         A 0 is returned if the call succeeds, -1 if it fails.

    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.

    SEE ALSO
         connect(2), ioctl(2), poll(2), select(2), poll(2), socket(2),
         getprotoent(3), protocols(5)

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

    HISTORY
         The getsockopt() system call appeared in 4.2BSD.

    BSD                            February 15, 1999                           BSD

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