man7/socket.7

   1 '\" t
   2 .\" Don't change the first line, it tells man that we need tbl.
   3 .\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
   4 .\" and copyright (c) 1999 Matthew Wilcox.
   5 .\" Permission is granted to distribute possibly modified copies
   6 .\" of this page provided the header is included verbatim,
   7 .\" and in case of nontrivial modification author and date
   8 .\" of the modification is added to the header.
   9 .\"
  10 .\" 2002-10-30, Michael Kerrisk, <mtk.manpages@gmail.com>
  11 .\"     Added description of SO_ACCEPTCONN
  12 .\" 2004-05-20, aeb, added SO_RCVTIMEO/SO_SNDTIMEO text.
  13 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  14 .\"     Added notes on capability requirements
  15 .\"     A few small grammar fixes
  16 .\" 2010-06-13 Jan Engelhardt <jengelh@medozas.de>
  17 .\"     Documented SO_DOMAIN and SO_PROTOCOL.
  18 .\" FIXME
  19 .\" The following are not yet documented:
  20 .\"     SO_PEERNAME
  21 .\"     SO_TIMESTAMPNS
  22 .\"     SO_TIMESTAMPING (2.6.30)
  23 .\"     SO_RXQ_OVFL (2.6.33)
  24 .\"
  25 .TH SOCKET 7 2012-07-07 Linux "Linux Programmer's Manual"
  26 .SH NAME
  27 socket \- Linux socket interface
  28 .SH SYNOPSIS
  29 .B #include <sys/socket.h>
  30 .sp
  31 .IB sockfd " = socket(int " socket_family ", int " socket_type ", int " protocol );
  32 .SH DESCRIPTION
  33 This manual page describes the Linux networking socket layer user
  34 interface.
  35 The BSD compatible sockets
  36 are the uniform interface
  37 between the user process and the network protocol stacks in the kernel.
  38 The protocol modules are grouped into
  39 .I protocol families
  40 like
  41 .BR AF_INET ", " AF_IPX ", " AF_PACKET
  42 and
  43 .I socket types
  44 like
  45 .B SOCK_STREAM
  46 or
  47 .BR SOCK_DGRAM .
  48 See
  49 .BR socket (2)
  50 for more information on families and types.
  51 .SS Socket Layer Functions
  52 These functions are used by the user process to send or receive packets
  53 and to do other socket operations.
  54 For more information see their respective manual pages.
  55
  56 .BR socket (2)
  57 creates a socket,
  58 .BR connect (2)
  59 connects a socket to a remote socket address,
  60 the
  61 .BR bind (2)
  62 function binds a socket to a local socket address,
  63 .BR listen (2)
  64 tells the socket that new connections shall be accepted, and
  65 .BR accept (2)
  66 is used to get a new socket with a new incoming connection.
  67 .BR socketpair (2)
  68 returns two connected anonymous sockets (only implemented for a few
  69 local families like
  70 .BR AF_UNIX )
  71 .PP
  72 .BR send (2),
  73 .BR sendto (2),
  74 and
  75 .BR sendmsg (2)
  76 send data over a socket, and
  77 .BR recv (2),
  78 .BR recvfrom (2),
  79 .BR recvmsg (2)
  80 receive data from a socket.
  81 .BR poll (2)
  82 and
  83 .BR select (2)
  84 wait for arriving data or a readiness to send data.
  85 In addition, the standard I/O operations like
  86 .BR write (2),
  87 .BR writev (2),
  88 .BR sendfile (2),
  89 .BR read (2),
  90 and
  91 .BR readv (2)
  92 can be used to read and write data.
  93 .PP
  94 .BR getsockname (2)
  95 returns the local socket address and
  96 .BR getpeername (2)
  97 returns the remote socket address.
  98 .BR getsockopt (2)
  99 and
 100 .BR setsockopt (2)
 101 are used to set or get socket layer or protocol options.
 102 .BR ioctl (2)
 103 can be used to set or read some other options.
 104 .PP
 105 .BR close (2)
 106 is used to close a socket.
 107 .BR shutdown (2)
 108 closes parts of a full-duplex socket connection.
 109 .PP
 110 Seeking, or calling
 111 .BR pread (2)
 112 or
 113 .BR pwrite (2)
 114 with a nonzero position is not supported on sockets.
 115 .PP
 116 It is possible to do nonblocking I/O on sockets by setting the
 117 .B O_NONBLOCK
 118 flag on a socket file descriptor using
 119 .BR fcntl (2).
 120 Then all operations that would block will (usually)
 121 return with
 122 .B EAGAIN
 123 (operation should be retried later);
 124 .BR connect (2)
 125 will return
 126 .B EINPROGRESS
 127 error.
 128 The user can then wait for various events via
 129 .BR poll (2)
 130 or
 131 .BR select (2).
 132 .TS
 133 tab(:) allbox;
 134 c s s
 135 l l l.
 136 I/O events
 137 Event:Poll flag:Occurrence
 138 Read:POLLIN:T{
 139 New data arrived.
 140 T}
 141 Read:POLLIN:T{
 142 A connection setup has been completed
 143 (for connection-oriented sockets)
 144 T}
 145 Read:POLLHUP:T{
 146 A disconnection request has been initiated by the other end.
 147 T}
 148 Read:POLLHUP:T{
 149 A connection is broken (only for connection-oriented protocols).
 150 When the socket is written
 151 .B SIGPIPE
 152 is also sent.
 153 T}
 154 Write:POLLOUT:T{
 155 Socket has enough send buffer space for writing new data.
 156 T}
 157 Read/Write:T{
 158 POLLIN|
 159 .br
 160 POLLOUT
 161 T}:T{
 162 An outgoing
 163 .BR connect (2)
 164 finished.
 165 T}
 166 Read/Write:POLLERR:An asynchronous error occurred.
 167 Read/Write:POLLHUP:The other end has shut down one direction.
 168 Exception:POLLPRI:T{
 169 Urgent data arrived.
 170 .B SIGURG
 171 is sent then.
 172 T}
 173 .\" FIXME . The following is not true currently:
 174 .\" It is no I/O event when the connection
 175 .\" is broken from the local end using
 176 .\" .BR shutdown (2)
 177 .\" or
 178 .\" .BR close (2).
 179 .TE
 180
 181 .PP
 182 An alternative to
 183 .BR poll (2)
 184 and
 185 .BR select (2)
 186 is to let the kernel inform the application about events
 187 via a
 188 .B SIGIO
 189 signal.
 190 For that the
 191 .B O_ASYNC
 192 flag must be set on a socket file descriptor via
 193 .BR fcntl (2)
 194 and a valid signal handler for
 195 .B SIGIO
 196 must be installed via
 197 .BR sigaction (2).
 198 See the
 199 .I Signals
 200 discussion below.
 201 .SS Socket Options
 202 These socket options can be set by using
 203 .BR setsockopt (2)
 204 and read with
 205 .BR getsockopt (2)
 206 with the socket level set to
 207 .B SOL_SOCKET
 208 for all sockets:
 209 .\" FIXME
 210 .\" In the list below, the text used to describe argument types
 211 .\" for each socket option should be more consistent
 212 .\"
 213 .\" SO_ACCEPTCONN is in POSIX.1-2001, and its origin is explained in
 214 .\" W R Stevens, UNPv1
 215 .TP
 216 .B SO_ACCEPTCONN
 217 Returns a value indicating whether or not this socket has been marked
 218 to accept connections with
 219 .BR listen (2).
 220 The value 0 indicates that this is not a listening socket,
 221 the value 1 indicates that this is a listening socket.
 222 This socket option is read-only.
 223 .TP
 224 .B SO_BINDTODEVICE
 225 Bind this socket to a particular device like \(lqeth0\(rq,
 226 as specified in the passed interface name.
 227 If the
 228 name is an empty string or the option length is zero, the socket device
 229 binding is removed.
 230 The passed option is a variable-length null-terminated
 231 interface name string with the maximum size of
 232 .BR IFNAMSIZ .
 233 If a socket is bound to an interface,
 234 only packets received from that particular interface are processed by the
 235 socket.
 236 Note that this only works for some socket types, particularly
 237 .B AF_INET
 238 sockets.
 239 It is not supported for packet sockets (use normal
 240 .BR bind (2)
 241 there).
 242 .TP
 243 .B SO_BROADCAST
 244 Set or get the broadcast flag.
 245 When enabled, datagram sockets are allowed to send
 246 packets to a broadcast address.
 247 This option has no effect on stream-oriented sockets.
 248 .TP
 249 .B SO_BSDCOMPAT
 250 Enable BSD bug-to-bug compatibility.
 251 This is used by the UDP protocol module in Linux 2.0 and 2.2.
 252 If enabled ICMP errors received for a UDP socket will not be passed
 253 to the user program.
 254 In later kernel versions, support for this option has been phased out:
 255 Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning
 256 (printk()) if a program uses this option.
 257 Linux 2.0 also enabled BSD bug-to-bug compatibility
 258 options (random header changing, skipping of the broadcast flag) for raw
 259 sockets with this option, but that was removed in Linux 2.2.
 260 .TP
 261 .B SO_DEBUG
 262 Enable socket debugging.
 263 Only allowed for processes with the
 264 .B CAP_NET_ADMIN
 265 capability or an effective user ID of 0.
 266 .TP
 267 .BR SO_DOMAIN " (since Linux 2.6.32)"
 268 Retrieves the socket domain as an integer, returning a value such as
 269 .BR AF_INET6 .
 270 See
 271 .BR socket (2)
 272 for details.
 273 This socket option is read-only.
 274 .TP
 275 .B SO_ERROR
 276 Get and clear the pending socket error.
 277 This socket option is read-only.
 278 Expects an integer.
 279 .TP
 280 .B SO_DONTROUTE
 281 Don't send via a gateway, only send to directly connected hosts.
 282 The same effect can be achieved by setting the
 283 .B MSG_DONTROUTE
 284 flag on a socket
 285 .BR send (2)
 286 operation.
 287 Expects an integer boolean flag.
 288 .TP
 289 .B SO_KEEPALIVE
 290 Enable sending of keep-alive messages on connection-oriented sockets.
 291 Expects an integer boolean flag.
 292 .TP
 293 .B SO_LINGER
 294 Sets or gets the
 295 .B SO_LINGER
 296 option.
 297 The argument is a
 298 .I linger
 299 structure.
 300 .sp
 301 .in +4n
 302 .nf
 303 struct linger {
 304     int l_onoff;    /* linger active */
 305     int l_linger;   /* how many seconds to linger for */
 306 };
 307 .fi
 308 .in
 309 .IP
 310 When enabled, a
 311 .BR close (2)
 312 or
 313 .BR shutdown (2)
 314 will not return until all queued messages for the socket have been
 315 successfully sent or the linger timeout has been reached.
 316 Otherwise,
 317 the call returns immediately and the closing is done in the background.
 318 When the socket is closed as part of
 319 .BR exit (2),
 320 it always lingers in the background.
 321 .TP
 322 .BR SO_MARK " (since Linux 2.6.25)"
 323 .\" commit 4a19ec5800fc3bb64e2d87c4d9fdd9e636086fe0
 324 .\" and    914a9ab386a288d0f22252fc268ecbc048cdcbd5
 325 Set the mark for each packet sent through this socket
 326 (similar to the netfilter MARK target but socket-based).
 327 Changing the mark can be used for mark-based
 328 routing without netfilter or for packet filtering.
 329 Setting this option requires the
 330 .B CAP_NET_ADMIN
 331 capability.
 332 .TP
 333 .B SO_OOBINLINE
 334 If this option is enabled,
 335 out-of-band data is directly placed into the receive data stream.
 336 Otherwise out-of-band data is only passed when the
 337 .B MSG_OOB
 338 flag is set during receiving.
 339 .\" don't document it because it can do too much harm.
 340 .\".B SO_NO_CHECK
 341 .TP
 342 .B SO_PASSCRED
 343 Enable or disable the receiving of the
 344 .B SCM_CREDENTIALS
 345 control message.
 346 For more information see
 347 .BR unix (7).
 348 .\" FIXME Document SO_PASSSEC, added in 2.6.18; there is some info
 349 .\" in the 2.6.18 ChangeLog
 350 .TP
 351 .B SO_PEERCRED
 352 Return the credentials of the foreign process connected to this socket.
 353 This is only possible for connected
 354 .B AF_UNIX
 355 stream sockets and
 356 .B AF_UNIX
 357 stream and datagram socket pairs created using
 358 .BR socketpair (2);
 359 see
 360 .BR unix (7).
 361 The returned credentials are those that were in effect at the time
 362 of the call to
 363 .BR connect (2)
 364 or
 365 .BR socketpair (2).
 366 Argument is a
 367 .I ucred
 368 structure.
 369 This socket option is read-only.
 370 .TP
 371 .B SO_PRIORITY
 372 Set the protocol-defined priority for all packets to be sent on
 373 this socket.
 374 Linux uses this value to order the networking queues:
 375 packets with a higher priority may be processed first depending
 376 on the selected device queueing discipline.
 377 For
 378 .BR ip (7),
 379 this also sets the IP type-of-service (TOS) field for outgoing packets.
 380 Setting a priority outside the range 0 to 6 requires the
 381 .B CAP_NET_ADMIN
 382 capability.
 383 .TP
 384 .BR SO_PROTOCOL " (since Linux 2.6.32)"
 385 Retrieves the socket protocol as an integer, returning a value such as
 386 .BR IPPROTO_SCTP .
 387 See
 388 .BR socket (2)
 389 for details.
 390 This socket option is read-only.
 391 .TP
 392 .B SO_RCVBUF
 393 Sets or gets the maximum socket receive buffer in bytes.
 394 The kernel doubles this value (to allow space for bookkeeping overhead)
 395 when it is set using
 396 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 397 .BR setsockopt (2),
 398 and this doubled value is returned by
 399 .BR getsockopt (2).
 400 .\" The following thread on LMKL is quite informative:
 401 .\" getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard" behaviour
 402 .\" 17 July 2012
 403 .\" http://thread.gmane.org/gmane.linux.kernel/1328935
 404 The default value is set by the
 405 .I /proc/sys/net/core/rmem_default
 406 file, and the maximum allowed value is set by the
 407 .I /proc/sys/net/core/rmem_max
 408 file.
 409 The minimum (doubled) value for this option is 256.
 410 .TP
 411 .BR SO_RCVBUFFORCE " (since Linux 2.6.14)"
 412 Using this socket option, a privileged
 413 .RB ( CAP_NET_ADMIN )
 414 process can perform the same task as
 415 .BR SO_RCVBUF ,
 416 but the
 417 .I rmem_max
 418 limit can be overridden.
 419 .TP
 420 .BR SO_RCVLOWAT " and " SO_SNDLOWAT
 421 Specify the minimum number of bytes in the buffer until the socket layer
 422 will pass the data to the protocol
 423 .RB ( SO_SNDLOWAT )
 424 or the user on receiving
 425 .RB ( SO_RCVLOWAT ).
 426 These two values are initialized to 1.
 427 .B SO_SNDLOWAT
 428 is not changeable on Linux
 429 .RB ( setsockopt (2)
 430 fails with the error
 431 .BR ENOPROTOOPT ).
 432 .B SO_RCVLOWAT
 433 is changeable
 434 only since Linux 2.4.
 435 The
 436 .BR select (2)
 437 and
 438 .BR poll (2)
 439 system calls currently do not respect the
 440 .B SO_RCVLOWAT
 441 setting on Linux,
 442 and mark a socket readable when even a single byte of data is available.
 443 A subsequent read from the socket will block until
 444 .B SO_RCVLOWAT
 445 bytes are available.
 446 .\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=111049368106984&w=2
 447 .\" Tested on kernel 2.6.14 -- mtk, 30 Nov 05
 448 .TP
 449 .BR SO_RCVTIMEO " and " SO_SNDTIMEO
 450 .\" Not implemented in 2.0.
 451 .\" Implemented in 2.1.11 for getsockopt: always return a zero struct.
 452 .\" Implemented in 2.3.41 for setsockopt, and actually used.
 453 Specify the receiving or sending timeouts until reporting an error.
 454 The argument is a
 455 .IR "struct timeval" .
 456 If an input or output function blocks for this period of time, and
 457 data has been sent or received, the return value of that function
 458 will be the amount of data transferred; if no data has been transferred
 459 and the timeout has been reached then \-1 is returned with
 460 .I errno
 461 set to
 462 .BR EAGAIN
 463 or
 464 .BR EWOULDBLOCK ,
 465 .\" in fact to EAGAIN
 466 or
 467 .B EINPROGRESS
 468 (for
 469 .BR connect (2))
 470 just as if the socket was specified to be nonblocking.
 471 If the timeout is set to zero (the default)
 472 then the operation will never timeout.
 473 Timeouts only have effect for system calls that perform socket I/O (e.g.,
 474 .BR read (2),
 475 .BR recvmsg (2),
 476 .BR send (2),
 477 .BR sendmsg (2));
 478 timeouts have no effect for
 479 .BR select (2),
 480 .BR poll (2),
 481 .BR epoll_wait (2),
 482 and so on.
 483 .TP
 484 .B SO_REUSEADDR
 485 Indicates that the rules used in validating addresses supplied in a
 486 .BR bind (2)
 487 call should allow reuse of local addresses.
 488 For
 489 .B AF_INET
 490 sockets this
 491 means that a socket may bind, except when there
 492 is an active listening socket bound to the address.
 493 When the listening socket is bound to
 494 .B INADDR_ANY
 495 with a specific port then it is not possible
 496 to bind to this port for any local address.
 497 Argument is an integer boolean flag.
 498 .TP
 499 .B SO_SNDBUF
 500 Sets or gets the maximum socket send buffer in bytes.
 501 The kernel doubles this value (to allow space for bookkeeping overhead)
 502 when it is set using
 503 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
 504 .\" See also the comment to SO_RCVBUF (17 Jul 2012 LKML mail)
 505 .BR setsockopt (2),
 506 and this doubled value is returned by
 507 .BR getsockopt (2).
 508 The default value is set by the
 509 .I /proc/sys/net/core/wmem_default
 510 file and the maximum allowed value is set by the
 511 .I /proc/sys/net/core/wmem_max
 512 file.
 513 The minimum (doubled) value for this option is 2048.
 514 .TP
 515 .BR SO_SNDBUFFORCE " (since Linux 2.6.14)"
 516 Using this socket option, a privileged
 517 .RB ( CAP_NET_ADMIN )
 518 process can perform the same task as
 519 .BR SO_SNDBUF ,
 520 but the
 521 .I wmem_max
 522 limit can be overridden.
 523 .TP
 524 .B SO_TIMESTAMP
 525 Enable or disable the receiving of the
 526 .B SO_TIMESTAMP
 527 control message.
 528 The timestamp control message is sent with level
 529 .B SOL_SOCKET
 530 and the
 531 .I cmsg_data
 532 field is a
 533 .I "struct timeval"
 534 indicating the
 535 reception time of the last packet passed to the user in this call.
 536 See
 537 .BR cmsg (3)
 538 for details on control messages.
 539 .TP
 540 .B SO_TYPE
 541 Gets the socket type as an integer (e.g.,
 542 .BR SOCK_STREAM ).
 543 This socket option is read-only.
 544 .SS Signals
 545 When writing onto a connection-oriented socket that has been shut down
 546 (by the local or the remote end)
 547 .B SIGPIPE
 548 is sent to the writing process and
 549 .B EPIPE
 550 is returned.
 551 The signal is not sent when the write call
 552 specified the
 553 .B MSG_NOSIGNAL
 554 flag.
 555 .PP
 556 When requested with the
 557 .B FIOSETOWN
 558 .BR fcntl (2)
 559 or
 560 .B SIOCSPGRP
 561 .BR ioctl (2),
 562 .B SIGIO
 563 is sent when an I/O event occurs.
 564 It is possible to use
 565 .BR poll (2)
 566 or
 567 .BR select (2)
 568 in the signal handler to find out which socket the event occurred on.
 569 An alternative (in Linux 2.2) is to set a real-time signal using the
 570 .B F_SETSIG
 571 .BR fcntl (2);
 572 the handler of the real time signal will be called with
 573 the file descriptor in the
 574 .I si_fd
 575 field of its
 576 .IR siginfo_t .
 577 See
 578 .BR fcntl (2)
 579 for more information.
 580 .PP
 581 Under some circumstances (e.g., multiple processes accessing a
 582 single socket), the condition that caused the
 583 .B SIGIO
 584 may have already disappeared when the process reacts to the signal.
 585 If this happens, the process should wait again because Linux
 586 will resend the signal later.
 587 .\" .SS Ancillary Messages
 588 .SS /proc interfaces
 589 The core socket networking parameters can be accessed
 590 via files in the directory
 591 .IR /proc/sys/net/core/ .
 592 .TP
 593 .I rmem_default
 594 contains the default setting in bytes of the socket receive buffer.
 595 .TP
 596 .I rmem_max
 597 contains the maximum socket receive buffer size in bytes which a user may
 598 set by using the
 599 .B SO_RCVBUF
 600 socket option.
 601 .TP
 602 .I wmem_default
 603 contains the default setting in bytes of the socket send buffer.
 604 .TP
 605 .I wmem_max
 606 contains the maximum socket send buffer size in bytes which a user may
 607 set by using the
 608 .B SO_SNDBUF
 609 socket option.
 610 .TP
 611 .IR message_cost " and " message_burst
 612 configure the token bucket filter used to load limit warning messages
 613 caused by external network events.
 614 .TP
 615 .I netdev_max_backlog
 616 Maximum number of packets in the global input queue.
 617 .TP
 618 .I optmem_max
 619 Maximum length of ancillary data and user control data like the iovecs
 620 per socket.
 621 .\" netdev_fastroute is not documented because it is experimental
 622 .SS Ioctls
 623 These operations can be accessed using
 624 .BR ioctl (2):
 625
 626 .in +4n
 627 .nf
 628 .IB error " = ioctl(" ip_socket ", " ioctl_type ", " &value_result ");"
 629 .fi
 630 .in
 631 .TP
 632 .B SIOCGSTAMP
 633 Return a
 634 .I struct timeval
 635 with the receive timestamp of the last packet passed to the user.
 636 This is useful for accurate round trip time measurements.
 637 See
 638 .BR setitimer (2)
 639 for a description of
 640 .IR "struct timeval" .
 641 .\"
 642 This ioctl should only be used if the socket option
 643 .B SO_TIMESTAMP
 644 is not set on the socket.
 645 Otherwise, it returns the timestamp of the
 646 last packet that was received while
 647 .B SO_TIMESTAMP
 648 was not set, or it fails if no such packet has been received,
 649 (i.e.,
 650 .BR ioctl (2)
 651 returns \-1 with
 652 .I errno
 653 set to
 654 .BR ENOENT ).
 655 .TP
 656 .B SIOCSPGRP
 657 Set the process or process group to send
 658 .B SIGIO
 659 or
 660 .B SIGURG
 661 signals
 662 to when an
 663 asynchronous I/O operation has finished or urgent data is available.
 664 The argument is a pointer to a
 665 .IR pid_t .
 666 If the argument is positive, send the signals to that process.
 667 If the
 668 argument is negative, send the signals to the process group with the ID
 669 of the absolute value of the argument.
 670 The process may only choose itself or its own process group to receive
 671 signals unless it has the
 672 .B CAP_KILL
 673 capability or an effective UID of 0.
 674 .TP
 675 .B FIOASYNC
 676 Change the
 677 .B O_ASYNC
 678 flag to enable or disable asynchronous I/O mode of the socket.
 679 Asynchronous I/O mode means that the
 680 .B SIGIO
 681 signal or the signal set with
 682 .B F_SETSIG
 683 is raised when a new I/O event occurs.
 684 .IP
 685 Argument is an integer boolean flag.
 686 (This operation is synonymous with the use of
 687 .BR fcntl (2)
 688 to set the
 689 .B O_ASYNC
 690 flag.)
 691 .\"
 692 .TP
 693 .B SIOCGPGRP
 694 Get the current process or process group that receives
 695 .B SIGIO
 696 or
 697 .B SIGURG
 698 signals,
 699 or 0
 700 when none is set.
 701 .PP
 702 Valid
 703 .BR fcntl (2)
 704 operations:
 705 .TP
 706 .B FIOGETOWN
 707 The same as the
 708 .B SIOCGPGRP
 709 .BR ioctl (2).
 710 .TP
 711 .B FIOSETOWN
 712 The same as the
 713 .B SIOCSPGRP
 714 .BR ioctl (2).
 715 .SH VERSIONS
 716 .B SO_BINDTODEVICE
 717 was introduced in Linux 2.0.30.
 718 .B SO_PASSCRED
 719 is new in Linux 2.2.
 720 The
 721 .I /proc
 722 interfaces was introduced in Linux 2.2.
 723 .B SO_RCVTIMEO
 724 and
 725 .B SO_SNDTIMEO
 726 are supported since Linux 2.3.41.
 727 Earlier, timeouts were fixed to
 728 a protocol-specific setting, and could not be read or written.
 729 .SH NOTES
 730 Linux assumes that half of the send/receive buffer is used for internal
 731 kernel structures; thus the values in the corresponding
 732 .I /proc
 733 files are twice what can be observed on the wire.
 734
 735 Linux will only allow port reuse with the
 736 .B SO_REUSEADDR
 737 option
 738 when this option was set both in the previous program that performed a
 739 .BR bind (2)
 740 to the port and in the program that wants to reuse the port.
 741 This differs from some implementations (e.g., FreeBSD)
 742 where only the later program needs to set the
 743 .B SO_REUSEADDR
 744 option.
 745 Typically this difference is invisible, since, for example, a server
 746 program is designed to always set this option.
 747 .SH BUGS
 748 The
 749 .B CONFIG_FILTER
 750 socket options
 751 .B SO_ATTACH_FILTER
 752 and
 753 .B SO_DETACH_FILTER
 754 .\" FIXME Document SO_ATTACH_FILTER and SO_DETACH_FILTER
 755 are not documented.
 756 The suggested interface to use them is via the libpcap
 757 library.
 758 .\" .SH AUTHORS
 759 .\" This man page was written by Andi Kleen.
 760 .SH "SEE ALSO"
 761 .BR getsockopt (2),
 762 .BR connect (2),
 763 .BR setsockopt (2),
 764 .BR socket (2),
 765 .BR capabilities (7),
 766 .BR ddp (7),
 767 .BR ip (7),
 768 .BR packet (7),
 769 .BR tcp (7),
 770 .BR udp (7),
 771 .BR unix (7)