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GETSOCKOPT(2)               BSD System Calls Manual              GETSOCKOPT(2)

NAME
     getsockopt, setsockopt -- get and set options on sockets

SYNOPSIS
     #include <sys/socket.h>

     int
     getsockopt(int socket, int level, int option_name, void *restrict option_value,
         socklen_t *restrict option_len);

     int
     setsockopt(int socket, int level, int option_name, const void *option_value, socklen_t option_len);

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 manipu-late manipulate
     late 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 option_value and option_len 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(), option_len is a value-result parameter, initially containing the size of the buffer
     pointed to by option_value, and modified on return to indicate the actual size of the value returned.
     If no option value is to be supplied or returned, option_value may be NULL.

     option_name 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 option_value.  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_NOSIGPIPE    do not generate SIGPIPE, instead return EPIPE
           SO_NREAD        number of bytes to be read (get only)
           SO_NWRITE       number of bytes written not yet sent by the protocol (get only)
           SO_LINGER_SEC   linger on close if data present with timeout in seconds

     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 des-tination destination
     tination address.

     SO_LINGER controls the action taken when unsent messages are queued on socket and a close(2) is per-formed. performed.
     formed.  If the socket promises reliable delivery 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 decides it is unable
     to deliver the information (a timeout period, termed the linger interval, is specified 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.

     SO_LINGER_SEC is the same option as SO_LINGER except the linger time is in seconds for SO_LINGER_SEC.

     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 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, 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) oper-ation operation
     ation 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 2048.

     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 lowwater
     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 com-plete. complete.
     plete.  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 com-plete. complete.
     plete.  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.  The struct timeval parameter must represent a positive
     time interval; otherwise, setsockopt() returns with the error EDOM.

     SO_NOSIGPIPE is an option that prevents SIGPIPE from being raised when a write fails on a socket to
     which there is no reader; instead, the write to the socket returns with the error EPIPE when there is
     no reader.

     Finally, SO_TYPE, SO_ERROR, SO_NREAD, and SO_NWRITE 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.

     SO_NREAD returns the amount of data in the input buffer that is available to be received.  For datagram
     oriented sockets, SO_NREAD returns the size of the first packet -- this differs from the ioctl() com-mand command
     mand FIONREAD that returns the total amount of data available.

     SO_NWRITE returns the amount of data in the output buffer not yet sent by the protocol.

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 getsockopt() and setsockopt() system calls will succeed unless:

     [EBADF]            The argument socket is not a valid file descriptor.

     [EFAULT]           The address pointed to by option_value is not in a valid part of the process address
                        space.  For getsockopt(), this error may also be returned if option_len is not in a
                        valid part of the process address space.

     [EINVAL]           The option is invalid at the level indicated.

     [ENOBUFS]          Insufficient system resources available for the call to complete.

     [ENOMEM]           Insufficient memory available for the system call to complete.

     [ENOPROTOOPT]      The option is unknown at the level indicated.

     [ENOTSOCK]         The argument socket is not a socket (e.g., a plain file).

     The setsockopt() system call will succeed unless:

     [EDOM]             The argument option_value is out of bounds.

     [EISCONN]          socket is already connected and a specified option cannot be set while this is the
                        case.

     [EINVAL]           The socket has been shut down.

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

     The include file <sys/types.h> is necessary.

SEE ALSO
     socket(2), bind(2), ioctl(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.

4.3-Reno Berkeley Distribution  April 19, 1994  4.3-Reno Berkeley Distribution