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GETSOCKOPT(2)             FreeBSD 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
     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 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
     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 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
     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(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() 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 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
     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 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 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.

     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.

IMPLEMENTATION NOTES
     In the non-threaded library getsockopt() is implemented as the getsockopt
     syscall.

     In the threaded library, the getsockopt syscall is assembled to
     _thread_sys_getsockopt() and getsockopt() is implemented as a function
     which locks s for read and write, then calls _thread_sys_getsockopt().
     Before returning, getsockopt() unlocks s.

     In the non-threaded library setsockopt() is implemented as the setsockopt
     syscall.

     In the threaded library, the setsockopt syscall is assembled to
     _thread_sys_setsockopt() and setsockopt() is implemented as a function
     which locks s for read and write, then calls _thread_sys_setsockopt().
     Before returning, setsockopt() unlocks s.

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

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 4.3r                          May 2, 1995                         BSD 4.3r

NAME | SYNOPSIS | DESCRIPTION | IMPLEMENTATION NOTES | RETURN VALUES | ERRORS | SEE ALSO | BUGS | HISTORY

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