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+NAME="NET-COMMON-TCPIP-MANPAGES-GETSOCKOPT">getsockopt</H1
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+>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
+ </PRE
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