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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_USER_COOKIE Ta "set the user cookie (uint32_t value)" 154.It Dv SO_TYPE Ta "get the type of the socket (get only)" 155.It Dv SO_ERROR Ta "get and clear error on the socket (get only)" 156.It Dv SO_RERROR Ta "enables receive error reporting" 157.El 158.Pp 159.Dv SO_DEBUG 160enables debugging in the underlying protocol modules. 161.Pp 162.Dv SO_REUSEADDR 163indicates that the rules used in validating addresses supplied 164in a 165.Xr bind 2 166call should allow reuse of local addresses. 167.Pp 168.Dv SO_REUSEPORT 169allows completely duplicate bindings by multiple processes 170if they all set 171.Dv SO_REUSEPORT 172before binding the port. 173This option permits multiple instances of a program to each 174receive UDP/IP multicast or broadcast datagrams destined for the bound port. 175.Pp 176.Dv SO_KEEPALIVE 177enables the 178periodic transmission of messages on a connected socket. Should the 179connected party fail to respond to these messages, the connection is 180considered broken and processes using the socket are notified via a 181.Dv SIGPIPE 182signal when attempting to send data. 183.Pp 184.Dv SO_DONTROUTE 185indicates that outgoing messages should 186bypass the standard routing facilities. Instead, messages are directed 187to the appropriate network interface according to the network portion 188of the destination address. 189.Pp 190.Dv SO_RERROR 191indicates that receive buffer overflows should be handled as errors. 192Historically receive buffer overflows have been ignored and programs 193could not tell if they missed messages or messages had been truncated 194because of overflows. 195Since programs historically do not expect to get receive overflow errors, 196this behavior is not the default. 197.Pp 198.Dv SO_LINGER 199controls the action taken when unsent messages 200are queued on socket and a 201.Xr close 2 202is performed. 203If the socket promises reliable delivery of data and 204.Dv SO_LINGER 205is set, 206the system will block the process on the 207.Xr close 2 208attempt until it is able to transmit the data or until it decides it 209is unable to deliver the information (a timeout period, termed the 210linger interval, is specified in seconds in the 211.Fn setsockopt 212call when 213.Dv SO_LINGER 214is requested). 215If 216.Dv SO_LINGER 217is disabled and a 218.Xr close 2 219is issued, the system will process the close in a manner that allows 220the process to continue as quickly as possible. 221.Pp 222The option 223.Dv SO_BROADCAST 224requests permission to send broadcast datagrams 225on the socket. 226Broadcast was a privileged operation in earlier versions of the system. 227With protocols that support out-of-band data, the 228.Dv SO_OOBINLINE 229option 230requests that out-of-band data be placed in the normal data input queue 231as received; it will then be accessible with 232.Xr recv 2 233or 234.Xr read 2 235calls without the 236.Dv MSG_OOB 237flag. 238Some protocols always behave as if this option is set. 239.Pp 240.Dv SO_SNDBUF 241and 242.Dv SO_RCVBUF 243are options to adjust the normal 244buffer sizes allocated for output and input buffers, respectively. 245The buffer size may be increased for high-volume connections, 246or may be decreased to limit the possible backlog of incoming data. 247The system places an absolute maximum on these values, which is accessible 248through the 249.Xr sysctl 3 250MIB variable 251.Va kern.ipc.maxsockbuf . 252.Pp 253.Dv SO_SNDLOWAT 254is an option to set the minimum count for output operations. 255Most output operations process all of the data supplied 256by the call, delivering data to the protocol for transmission 257and blocking as necessary for flow control. 258Nonblocking output operations will process as much data as permitted 259subject to flow control without blocking, but will process no data 260if flow control does not allow the smaller of the low water mark value 261or the entire request to be processed. 262A 263.Xr select 2 264operation testing the ability to write to a socket will return true 265only if the low water mark amount could be processed. 266The default value for 267.Dv SO_SNDLOWAT 268is set to a convenient size for network efficiency, often 1024. 269.Pp 270.Dv SO_RCVLOWAT 271is an option to set the minimum count for input operations. 272In general, receive calls will block until any (non-zero) amount of data 273is received, then return with the smaller of the amount available or the amount 274requested. 275The default value for 276.Dv SO_RCVLOWAT 277is 1. 278If 279.Dv SO_RCVLOWAT 280is set to a larger value, blocking receive calls normally 281wait until they have received the smaller of the low water mark value 282or the requested amount. 283Receive calls may still return less than the low water mark if an error 284occurs, a signal is caught, or the type of data next in the receive queue 285is different from that which was returned. 286.Pp 287.Dv SO_SNDTIMEO 288is an option to set a timeout value for output operations. 289It accepts a 290.Fa struct timeval 291parameter with the number of seconds and microseconds 292used to limit waits for output operations to complete. 293If a send operation has blocked for this much time, 294it returns with a partial count 295or with the error 296.Er EWOULDBLOCK 297if no data were sent. 298In the current implementation, this timer is restarted each time additional 299data are delivered to the protocol, 300implying that the limit applies to output portions ranging in size 301from the low water mark to the high water mark for output. 302.Pp 303.Dv SO_RCVTIMEO 304is an option to set a timeout value for input operations. 305It accepts a 306.Fa struct timeval 307parameter with the number of seconds and microseconds 308used to limit waits for input operations to complete. 309In the current implementation, this timer is restarted each time additional 310data are received by the protocol, 311and thus the limit is in effect an inactivity timer. 312If a receive operation has been blocked for this much time without 313receiving additional data, it returns with a short count 314or with the error 315.Er EWOULDBLOCK 316if no data were received. 317.Pp 318.Dv SO_ACCEPTFILTER 319places an 320.Xr accept_filter 9 321on the socket, 322which will filter incoming connections 323on a listening stream socket before being presented for 324.Xr accept 2 . 325Once more, 326.Xr listen 2 327must be called on the socket before 328trying to install the filter on it, 329or else the 330.Fn setsockopt 331call will fail. 332.Bd -literal -offset indent 333struct accept_filter_arg { 334 char af_name[16]; 335 char af_arg[256-16]; 336}; 337.Ed 338.Pp 339.Fa optval 340should point to a 341.Fa struct accept_filter_arg 342that will select and configure the 343.Xr accept_filter 9 . 344.Fa af_name 345should be filled with the name of the accept filter 346that the application wishes to place on the listening socket. 347.Fa af_arg 348is an optional parameter that can be passed to the accept 349filter specified by 350.Fa af_name 351to provide additional configuration options at attach time. 352Passing in an 353.Fa optval 354of NULL will remove the filter. 355.Pp 356.Dv SO_USER_COOKIE 357can be used to attach an arbitrary uint32_t value to the socket. 358The value acts like a user-defined cookie and can be used in the kernel 359code that manipulates traffic related to the socket. 360.Pp 361Finally, 362.Dv SO_TYPE 363and 364.Dv SO_ERROR 365are options used only with 366.Fn getsockopt . 367.Dv SO_TYPE 368returns the type of the socket, such as 369.Dv SOCK_STREAM ; 370it is useful for servers that inherit sockets on startup. 371.Dv SO_ERROR 372returns any pending error on the socket and clears 373the error status. 374It may be used to check for asynchronous errors on connected 375datagram sockets or for other asynchronous errors. 376.Sh RETURN VALUES 377.Rv -std 378.Sh ERRORS 379The call succeeds unless: 380.Bl -tag -width Er 381.It Bq Er EBADF 382The argument 383.Fa s 384is not a valid descriptor. 385.It Bq Er ENOTSOCK 386The argument 387.Fa s 388is a file, not a socket. 389.It Bq Er ENOPROTOOPT 390The option is unknown at the level indicated. 391.It Bq Er EFAULT 392The address pointed to by 393.Fa optval 394is not in a valid part of the process address space. 395For 396.Fn getsockopt , 397this error may also be returned if 398.Fa optlen 399is not in a valid part of the process address space. 400.It Bq Er EINVAL 401Installing an 402.Xr accept_filter 9 403on a non-listening socket was attempted. 404.El 405.Sh SEE ALSO 406.Xr ioctl 2 , 407.Xr socket 2 , 408.Xr getprotoent 3 , 409.Xr sysctl 3 , 410.Xr protocols 5 , 411.Xr sysctl 8 , 412.Xr accept_filter 9 413.Sh HISTORY 414The 415.Fn getsockopt 416system call appeared in 417.Bx 4.2 . 418.Sh BUGS 419Several of the socket options should be handled at lower levels of the system. 420