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.
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.
19 .\" The following are not yet documented:
22 .\" SO_MARK (see https://bugzilla.kernel.org/show_bug.cgi?id=16461)
23 .\" SO_TIMESTAMPING (2.6.30)
24 .\" SO_RXQ_OVFL (2.6.33)
26 .TH SOCKET 7 2010-06-13 Linux "Linux Programmer's Manual"
28 socket \- Linux socket interface
30 .B #include <sys/socket.h>
32 .IB sockfd " = socket(int " socket_family ", int " socket_type ", int " protocol );
34 This manual page describes the Linux networking socket layer user
36 The BSD compatible sockets
37 are the uniform interface
38 between the user process and the network protocol stacks in the kernel.
39 The protocol modules are grouped into
42 .BR AF_INET ", " AF_IPX ", " AF_PACKET
51 for more information on families and types.
52 .SS Socket Layer Functions
53 These functions are used by the user process to send or receive packets
54 and to do other socket operations.
55 For more information see their respective manual pages.
60 connects a socket to a remote socket address,
63 function binds a socket to a local socket address,
65 tells the socket that new connections shall be accepted, and
67 is used to get a new socket with a new incoming connection.
69 returns two connected anonymous sockets (only implemented for a few
77 send data over a socket, and
81 receive data from a socket.
85 wait for arriving data or a readiness to send data.
86 In addition, the standard I/O operations like
93 can be used to read and write data.
96 returns the local socket address and
98 returns the remote socket address.
102 are used to set or get socket layer or protocol options.
104 can be used to set or read some other options.
107 is used to close a socket.
109 closes parts of a full-duplex socket connection.
115 with a nonzero position is not supported on sockets.
117 It is possible to do nonblocking I/O on sockets by setting the
119 flag on a socket file descriptor using
121 Then all operations that would block will (usually)
124 (operation should be retried later);
129 The user can then wait for various events via
138 Event:Poll flag:Occurrence
143 A connection setup has been completed
144 (for connection-oriented sockets)
147 A disconnection request has been initiated by the other end.
150 A connection is broken (only for connection-oriented protocols).
151 When the socket is written
156 Socket has enough send buffer space for writing new data.
167 Read/Write:POLLERR:An asynchronous error occurred.
168 Read/Write:POLLHUP:The other end has shut down one direction.
174 .\" FIXME . The following is not true currently:
175 .\" It is no I/O event when the connection
176 .\" is broken from the local end using
187 is to let the kernel inform the application about events
193 flag must be set on a socket file descriptor via
195 and a valid signal handler for
197 must be installed via
203 These socket options can be set by using
207 with the socket level set to
211 .\" In the list below, the text used to describe argument types
212 .\" for each socket option should be more consistent
214 .\" SO_ACCEPTCONN is in POSIX.1-2001, and its origin is explained in
215 .\" W R Stevens, UNPv1
218 Returns a value indicating whether or not this socket has been marked
219 to accept connections with
221 The value 0 indicates that this is not a listening socket,
222 the value 1 indicates that this is a listening socket.
223 This socket option is read-only.
226 Bind this socket to a particular device like \(lqeth0\(rq,
227 as specified in the passed interface name.
229 name is an empty string or the option length is zero, the socket device
231 The passed option is a variable-length null-terminated
232 interface name string with the maximum size of
234 If a socket is bound to an interface,
235 only packets received from that particular interface are processed by the
237 Note that this only works for some socket types, particularly
240 It is not supported for packet sockets (use normal
245 Set or get the broadcast flag.
246 When enabled, datagram sockets
247 receive packets sent to a broadcast address and they are allowed to send
248 packets to a broadcast address.
249 This option has no effect on stream-oriented sockets.
252 Enable BSD bug-to-bug compatibility.
253 This is used by the UDP protocol module in Linux 2.0 and 2.2.
254 If enabled ICMP errors received for a UDP socket will not be passed
256 In later kernel versions, support for this option has been phased out:
257 Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning
258 (printk()) if a program uses this option.
259 Linux 2.0 also enabled BSD bug-to-bug compatibility
260 options (random header changing, skipping of the broadcast flag) for raw
261 sockets with this option, but that was removed in Linux 2.2.
264 Enable socket debugging.
265 Only allowed for processes with the
267 capability or an effective user ID of 0.
269 .BR SO_DOMAIN " (since Linux 2.6.32)"
270 Retrieves the socket domain as an integer, returning a value such as
275 This socket option is read-only.
278 Get and clear the pending socket error.
279 This socket option is read-only.
283 Don't send via a gateway, only send to directly connected hosts.
284 The same effect can be achieved by setting the
289 Expects an integer boolean flag.
292 Enable sending of keep-alive messages on connection-oriented sockets.
293 Expects an integer boolean flag.
306 int l_onoff; /* linger active */
307 int l_linger; /* how many seconds to linger for */
316 will not return until all queued messages for the socket have been
317 successfully sent or the linger timeout has been reached.
319 the call returns immediately and the closing is done in the background.
320 When the socket is closed as part of
322 it always lingers in the background.
325 If this option is enabled,
326 out-of-band data is directly placed into the receive data stream.
327 Otherwise out-of-band data is only passed when the
329 flag is set during receiving.
330 .\" don't document it because it can do too much harm.
334 Enable or disable the receiving of the
337 For more information see
339 .\" FIXME Document SO_PASSSEC, added in 2.6.18; there is some info
340 .\" in the 2.6.18 ChangeLog
343 Return the credentials of the foreign process connected to this socket.
344 This is only possible for connected
348 stream and datagram socket pairs created using
352 The returned credentials are those that were in effect at the time
360 This socket option is read-only.
363 Set the protocol-defined priority for all packets to be sent on
365 Linux uses this value to order the networking queues:
366 packets with a higher priority may be processed first depending
367 on the selected device queueing discipline.
370 this also sets the IP type-of-service (TOS) field for outgoing packets.
371 Setting a priority outside the range 0 to 6 requires the
375 .BR SO_PROTOCOL " (since Linux 2.6.32)"
376 Retrieves the socket protocol as an integer, returning a value such as
381 This socket option is read-only.
384 Sets or gets the maximum socket receive buffer in bytes.
385 The kernel doubles this value (to allow space for bookkeeping overhead)
387 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
389 and this doubled value is returned by
391 The default value is set by the
392 .I /proc/sys/net/core/rmem_default
393 file, and the maximum allowed value is set by the
394 .I /proc/sys/net/core/rmem_max
396 The minimum (doubled) value for this option is 256.
398 .BR SO_RCVBUFFORCE " (since Linux 2.6.14)"
399 Using this socket option, a privileged
400 .RB ( CAP_NET_ADMIN )
401 process can perform the same task as
405 limit can be overridden.
407 .BR SO_RCVLOWAT " and " SO_SNDLOWAT
408 Specify the minimum number of bytes in the buffer until the socket layer
409 will pass the data to the protocol
411 or the user on receiving
413 These two values are initialized to 1.
415 is not changeable on Linux
421 only since Linux 2.4.
426 system calls currently do not respect the
429 and mark a socket readable when even a single byte of data is available.
430 A subsequent read from the socket will block until
433 .\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=111049368106984&w=2
434 .\" Tested on kernel 2.6.14 -- mtk, 30 Nov 05
436 .BR SO_RCVTIMEO " and " SO_SNDTIMEO
437 .\" Not implemented in 2.0.
438 .\" Implemented in 2.1.11 for getsockopt: always return a zero struct.
439 .\" Implemented in 2.3.41 for setsockopt, and actually used.
440 Specify the receiving or sending timeouts until reporting an error.
442 .IR "struct timeval" .
443 If an input or output function blocks for this period of time, and
444 data has been sent or received, the return value of that function
445 will be the amount of data transferred; if no data has been transferred
446 and the timeout has been reached then \-1 is returned with
452 .\" in fact to EAGAIN
453 just as if the socket was specified to be nonblocking.
454 If the timeout is set to zero (the default)
455 then the operation will never timeout.
456 Timeouts only have effect for system calls that perform socket I/O (e.g.,
461 timeouts have no effect for
468 Indicates that the rules used in validating addresses supplied in a
470 call should allow reuse of local addresses.
474 means that a socket may bind, except when there
475 is an active listening socket bound to the address.
476 When the listening socket is bound to
478 with a specific port then it is not possible
479 to bind to this port for any local address.
480 Argument is an integer boolean flag.
483 Sets or gets the maximum socket send buffer in bytes.
484 The kernel doubles this value (to allow space for bookkeeping overhead)
486 .\" Most (all?) other implementations do not do this -- MTK, Dec 05
488 and this doubled value is returned by
490 The default value is set by the
491 .I /proc/sys/net/core/wmem_default
492 file and the maximum allowed value is set by the
493 .I /proc/sys/net/core/wmem_max
495 The minimum (doubled) value for this option is 2048.
497 .BR SO_SNDBUFFORCE " (since Linux 2.6.14)"
498 Using this socket option, a privileged
499 .RB ( CAP_NET_ADMIN )
500 process can perform the same task as
504 limit can be overridden.
507 Enable or disable the receiving of the
510 The timestamp control message is sent with level
517 reception time of the last packet passed to the user in this call.
520 for details on control messages.
523 Gets the socket type as an integer (e.g.,
525 This socket option is read-only.
527 When writing onto a connection-oriented socket that has been shut down
528 (by the local or the remote end)
530 is sent to the writing process and
533 The signal is not sent when the write call
538 When requested with the
545 is sent when an I/O event occurs.
546 It is possible to use
550 in the signal handler to find out which socket the event occurred on.
551 An alternative (in Linux 2.2) is to set a real-time signal using the
554 the handler of the real time signal will be called with
555 the file descriptor in the
561 for more information.
563 Under some circumstances (e.g., multiple processes accessing a
564 single socket), the condition that caused the
566 may have already disappeared when the process reacts to the signal.
567 If this happens, the process should wait again because Linux
568 will resend the signal later.
569 .\" .SS Ancillary Messages
571 The core socket networking parameters can be accessed
572 via files in the directory
573 .IR /proc/sys/net/core/ .
576 contains the default setting in bytes of the socket receive buffer.
579 contains the maximum socket receive buffer size in bytes which a user may
585 contains the default setting in bytes of the socket send buffer.
588 contains the maximum socket send buffer size in bytes which a user may
593 .IR message_cost " and " message_burst
594 configure the token bucket filter used to load limit warning messages
595 caused by external network events.
597 .I netdev_max_backlog
598 Maximum number of packets in the global input queue.
601 Maximum length of ancillary data and user control data like the iovecs
603 .\" netdev_fastroute is not documented because it is experimental
605 These operations can be accessed using
610 .IB error " = ioctl(" ip_socket ", " ioctl_type ", " &value_result ");"
617 with the receive timestamp of the last packet passed to the user.
618 This is useful for accurate round trip time measurements.
622 .IR "struct timeval" .
624 This ioctl should only be used if the socket option
626 is not set on the socket.
627 Otherwise, it returns the timestamp of the
628 last packet that was received while
630 was not set, or it fails if no such packet has been received,
639 Set the process or process group to send
645 asynchronous I/O operation has finished or urgent data is available.
646 The argument is a pointer to a
648 If the argument is positive, send the signals to that process.
650 argument is negative, send the signals to the process group with the ID
651 of the absolute value of the argument.
652 The process may only choose itself or its own process group to receive
653 signals unless it has the
655 capability or an effective UID of 0.
660 flag to enable or disable asynchronous I/O mode of the socket.
661 Asynchronous I/O mode means that the
663 signal or the signal set with
665 is raised when a new I/O event occurs.
667 Argument is an integer boolean flag.
668 (This operation is synonymous with the use of
676 Get the current process or process group that receives
699 was introduced in Linux 2.0.30.
704 interfaces was introduced in Linux 2.2.
708 are supported since Linux 2.3.41.
709 Earlier, timeouts were fixed to
710 a protocol-specific setting, and could not be read or written.
712 Linux assumes that half of the send/receive buffer is used for internal
713 kernel structures; thus the values in the corresponding
715 files are twice what can be observed on the wire.
717 Linux will only allow port reuse with the
720 when this option was set both in the previous program that performed a
722 to the port and in the program that wants to reuse the port.
723 This differs from some implementations (e.g., FreeBSD)
724 where only the later program needs to set the
727 Typically this difference is invisible, since, for example, a server
728 program is designed to always set this option.
736 .\" FIXME Document SO_ATTACH_FILTER and SO_DETACH_FILTER
738 The suggested interface to use them is via the libpcap
741 .\" This man page was written by Andi Kleen.
746 .BR capabilities (7),