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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)getsockopt.2 8.4 (Berkeley) 5/2/95 29.\" $FreeBSD: src/lib/libc/sys/getsockopt.2,v 1.12.2.11 2002/01/09 17:44:15 yar Exp $ 30.\" 31.Dd October 30, 2023 32.Dt GETSOCKOPT 2 33.Os 34.Sh NAME 35.Nm getsockopt , 36.Nm setsockopt 37.Nd get and set options on sockets 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In sys/types.h 42.In sys/socket.h 43.Ft int 44.Fn getsockopt "int s" "int level" "int optname" "void *optval" "socklen_t *optlen" 45.Ft int 46.Fn setsockopt "int s" "int level" "int optname" "const void *optval" "socklen_t optlen" 47.Sh DESCRIPTION 48.Fn Getsockopt 49and 50.Fn setsockopt 51manipulate the 52.Em options 53associated with a socket. Options may exist at multiple 54protocol levels; they are always present at the uppermost 55.Dq socket 56level. 57.Pp 58When manipulating socket options the level at which the 59option resides and the name of the option must be specified. 60To manipulate options at the socket level, 61.Fa level 62is specified as 63.Dv SOL_SOCKET . 64To manipulate options at any 65other level the protocol number of the appropriate protocol 66controlling the option is supplied. For example, 67to indicate that an option is to be interpreted by the 68.Tn TCP 69protocol, 70.Fa level 71should be set to the protocol number of 72.Tn TCP ; 73see 74.Xr getprotoent 3 . 75.Pp 76The parameters 77.Fa optval 78and 79.Fa optlen 80are used to access option values for 81.Fn setsockopt . 82For 83.Fn getsockopt 84they identify a buffer in which the value for the 85requested option(s) are to be returned. For 86.Fn getsockopt , 87.Fa optlen 88is a value-result parameter, initially containing the 89size of the buffer pointed to by 90.Fa optval , 91and modified on return to indicate the actual size of 92the value returned. If no option value is 93to be supplied or returned, 94.Fa optval 95may be NULL. 96.Pp 97.Fa Optname 98and any specified options are passed uninterpreted to the appropriate 99protocol module for interpretation. 100The include file 101.In sys/socket.h 102contains definitions for 103socket level options, described below. 104Options at other protocol levels vary in format and 105name; consult the appropriate entries in 106section 1074 of the manual. 108.Pp 109Most socket-level options utilize an 110.Fa int 111parameter for 112.Fa optval . 113For 114.Fn setsockopt , 115the parameter should be non-zero to enable a boolean option, 116or zero if the option is to be disabled. 117.Dv SO_LINGER 118uses a 119.Fa struct linger 120parameter, defined in 121.In sys/socket.h , 122which specifies the desired state of the option and the 123linger interval (see below). 124.Dv SO_SNDTIMEO 125and 126.Dv SO_RCVTIMEO 127use a 128.Fa struct timeval 129parameter, defined in 130.In sys/time.h . 131.Pp 132The following options are recognized at the socket level. 133Except as noted, each may be examined with 134.Fn getsockopt 135and set with 136.Fn setsockopt . 137.Bl -column SO_ACCEPTFILTER -offset indent 138.It Dv SO_DEBUG Ta "enables recording of debugging information" 139.It Dv SO_REUSEADDR Ta "enables local address reuse" 140.It Dv SO_REUSEPORT Ta "enables duplicate address and port bindings" 141.It Dv SO_KEEPALIVE Ta "enables keep connections alive" 142.It Dv SO_DONTROUTE Ta "enables routing bypass for outgoing messages" 143.It Dv SO_LINGER Ta "linger on close if data present" 144.It Dv SO_BROADCAST Ta "enables permission to transmit broadcast messages" 145.It Dv SO_OOBINLINE Ta "enables reception of out-of-band data in band" 146.It Dv SO_SNDBUF Ta "set buffer size for output" 147.It Dv SO_RCVBUF Ta "set buffer size for input" 148.It Dv SO_SNDLOWAT Ta "set minimum count for output" 149.It Dv SO_RCVLOWAT Ta "set minimum count for input" 150.It Dv SO_SNDTIMEO Ta "set timeout value for output" 151.It Dv SO_RCVTIMEO Ta "set timeout value for input" 152.It Dv SO_ACCEPTFILTER Ta "set accept filter on listening socket" 153.It Dv SO_TYPE Ta "get the type of the socket (get only)" 154.It Dv SO_ERROR Ta "get and clear error on the socket (get only)" 155.It Dv SO_RERROR Ta "enables receive error reporting" 156.El 157.Pp 158.Dv SO_DEBUG 159enables debugging in the underlying protocol modules. 160.Pp 161.Dv SO_REUSEADDR 162indicates that the rules used in validating addresses supplied 163in a 164.Xr bind 2 165call should allow reuse of local addresses. 166.Pp 167.Dv SO_REUSEPORT 168allows completely duplicate bindings by multiple processes 169if they all set 170.Dv SO_REUSEPORT 171before binding the port. 172This option permits multiple instances of a program to each 173receive UDP/IP multicast or broadcast datagrams destined for the bound port. 174.Pp 175.Dv SO_KEEPALIVE 176enables the 177periodic transmission of messages on a connected socket. Should the 178connected party fail to respond to these messages, the connection is 179considered broken and processes using the socket are notified via a 180.Dv SIGPIPE 181signal when attempting to send data. 182.Pp 183.Dv SO_DONTROUTE 184indicates that outgoing messages should 185bypass the standard routing facilities. Instead, messages are directed 186to the appropriate network interface according to the network portion 187of the destination address. 188.Pp 189.Dv SO_RERROR 190indicates that receive buffer overflows should be handled as errors. 191Historically receive buffer overflows have been ignored and programs 192could not tell if they missed messages or messages had been truncated 193because of overflows. 194Since programs historically do not expect to get receive overflow errors, 195this behavior is not the default. 196.Pp 197.Dv SO_LINGER 198controls the action taken when unsent messages 199are queued on socket and a 200.Xr close 2 201is performed. 202If the socket promises reliable delivery of data and 203.Dv SO_LINGER 204is set, 205the system will block the process on the 206.Xr close 2 207attempt until it is able to transmit the data or until it decides it 208is unable to deliver the information (a timeout period, termed the 209linger interval, is specified in seconds in the 210.Fn setsockopt 211call when 212.Dv SO_LINGER 213is requested). 214If 215.Dv SO_LINGER 216is disabled and a 217.Xr close 2 218is issued, the system will process the close in a manner that allows 219the process to continue as quickly as possible. 220.Pp 221The option 222.Dv SO_BROADCAST 223requests permission to send broadcast datagrams 224on the socket. 225Broadcast was a privileged operation in earlier versions of the system. 226With protocols that support out-of-band data, the 227.Dv SO_OOBINLINE 228option 229requests that out-of-band data be placed in the normal data input queue 230as received; it will then be accessible with 231.Xr recv 2 232or 233.Xr read 2 234calls without the 235.Dv MSG_OOB 236flag. 237Some protocols always behave as if this option is set. 238.Pp 239.Dv SO_SNDBUF 240and 241.Dv SO_RCVBUF 242are options to adjust the normal 243buffer sizes allocated for output and input buffers, respectively. 244The buffer size may be increased for high-volume connections, 245or may be decreased to limit the possible backlog of incoming data. 246The system places an absolute maximum on these values, which is accessible 247through the 248.Xr sysctl 3 249MIB variable 250.Va kern.ipc.maxsockbuf . 251.Pp 252.Dv SO_SNDLOWAT 253is an option to set the minimum count for output operations. 254Most output operations process all of the data supplied 255by the call, delivering data to the protocol for transmission 256and blocking as necessary for flow control. 257Nonblocking output operations will process as much data as permitted 258subject to flow control without blocking, but will process no data 259if flow control does not allow the smaller of the low water mark value 260or the entire request to be processed. 261A 262.Xr select 2 263operation testing the ability to write to a socket will return true 264only if the low water mark amount could be processed. 265The default value for 266.Dv SO_SNDLOWAT 267is set to a convenient size for network efficiency, often 1024. 268.Pp 269.Dv SO_RCVLOWAT 270is an option to set the minimum count for input operations. 271In general, receive calls will block until any (non-zero) amount of data 272is received, then return with the smaller of the amount available or the amount 273requested. 274The default value for 275.Dv SO_RCVLOWAT 276is 1. 277If 278.Dv SO_RCVLOWAT 279is set to a larger value, blocking receive calls normally 280wait until they have received the smaller of the low water mark value 281or the requested amount. 282Receive calls may still return less than the low water mark if an error 283occurs, a signal is caught, or the type of data next in the receive queue 284is different from that which was returned. 285.Pp 286.Dv SO_SNDTIMEO 287is an option to set a timeout value for output operations. 288It accepts a 289.Fa struct timeval 290parameter with the number of seconds and microseconds 291used to limit waits for output operations to complete. 292If a send operation has blocked for this much time, 293it returns with a partial count 294or with the error 295.Er EWOULDBLOCK 296if no data were sent. 297In the current implementation, this timer is restarted each time additional 298data are delivered to the protocol, 299implying that the limit applies to output portions ranging in size 300from the low water mark to the high water mark for output. 301.Pp 302.Dv SO_RCVTIMEO 303is an option to set a timeout value for input operations. 304It accepts a 305.Fa struct timeval 306parameter with the number of seconds and microseconds 307used to limit waits for input operations to complete. 308In the current implementation, this timer is restarted each time additional 309data are received by the protocol, 310and thus the limit is in effect an inactivity timer. 311If a receive operation has been blocked for this much time without 312receiving additional data, it returns with a short count 313or with the error 314.Er EWOULDBLOCK 315if no data were received. 316.Pp 317.Dv SO_ACCEPTFILTER 318places an 319.Xr accept_filter 9 320on the socket, 321which will filter incoming connections 322on a listening stream socket before being presented for 323.Xr accept 2 . 324Once more, 325.Xr listen 2 326must be called on the socket before 327trying to install the filter on it, 328or else the 329.Fn setsockopt 330call will fail. 331.Bd -literal -offset indent 332struct accept_filter_arg { 333 char af_name[16]; 334 char af_arg[256-16]; 335}; 336.Ed 337.Pp 338.Fa optval 339should point to a 340.Fa struct accept_filter_arg 341that will select and configure the 342.Xr accept_filter 9 . 343.Fa af_name 344should be filled with the name of the accept filter 345that the application wishes to place on the listening socket. 346.Fa af_arg 347is an optional parameter that can be passed to the accept 348filter specified by 349.Fa af_name 350to provide additional configuration options at attach time. 351Passing in an 352.Fa optval 353of NULL will remove the filter. 354.Pp 355Finally, 356.Dv SO_TYPE 357and 358.Dv SO_ERROR 359are options used only with 360.Fn getsockopt . 361.Dv SO_TYPE 362returns the type of the socket, such as 363.Dv SOCK_STREAM ; 364it is useful for servers that inherit sockets on startup. 365.Dv SO_ERROR 366returns any pending error on the socket and clears 367the error status. 368It may be used to check for asynchronous errors on connected 369datagram sockets or for other asynchronous errors. 370.Sh RETURN VALUES 371.Rv -std 372.Sh ERRORS 373The call succeeds unless: 374.Bl -tag -width Er 375.It Bq Er EBADF 376The argument 377.Fa s 378is not a valid descriptor. 379.It Bq Er ENOTSOCK 380The argument 381.Fa s 382is a file, not a socket. 383.It Bq Er ENOPROTOOPT 384The option is unknown at the level indicated. 385.It Bq Er EFAULT 386The address pointed to by 387.Fa optval 388is not in a valid part of the process address space. 389For 390.Fn getsockopt , 391this error may also be returned if 392.Fa optlen 393is not in a valid part of the process address space. 394.It Bq Er EINVAL 395Installing an 396.Xr accept_filter 9 397on a non-listening socket was attempted. 398.El 399.Sh SEE ALSO 400.Xr ioctl 2 , 401.Xr socket 2 , 402.Xr getprotoent 3 , 403.Xr sysctl 3 , 404.Xr protocols 5 , 405.Xr sysctl 8 , 406.Xr accept_filter 9 407.Sh HISTORY 408The 409.Fn getsockopt 410system call appeared in 411.Bx 4.2 . 412.Sh BUGS 413Several of the socket options should be handled at lower levels of the system. 414