+++ /dev/null
-'\" t
-.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
-.\" and copyright (c) 1999 Matthew Wilcox.
-.\"
-.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
-.\" Permission is granted to distribute possibly modified copies
-.\" of this page provided the header is included verbatim,
-.\" and in case of nontrivial modification author and date
-.\" of the modification is added to the header.
-.\" %%%LICENSE_END
-.\"
-.\" 2002-10-30, Michael Kerrisk, <mtk.manpages@gmail.com>
-.\" Added description of SO_ACCEPTCONN
-.\" 2004-05-20, aeb, added SO_RCVTIMEO/SO_SNDTIMEO text.
-.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
-.\" Added notes on capability requirements
-.\" A few small grammar fixes
-.\" 2010-06-13 Jan Engelhardt <jengelh@medozas.de>
-.\" Documented SO_DOMAIN and SO_PROTOCOL.
-.\" FIXME
-.\" The following are not yet documented:
-.\" SO_PEERNAME (2.4?)
-.\" get only
-.\" Seems to do something similar to getpeername(), but then
-.\" why is it necessary / how does it differ?
-.\" SO_TIMESTAMPNS (2.6.22)
-.\" Documentation/networking/timestamping.txt
-.\" commit 92f37fd2ee805aa77925c1e64fd56088b46094fc
-.\" Author: Eric Dumazet <dada1@cosmosbay.com>
-.\" SO_TIMESTAMPING (2.6.30)
-.\" Documentation/networking/timestamping.txt
-.\" commit cb9eff097831007afb30d64373f29d99825d0068
-.\" Author: Patrick Ohly <patrick.ohly@intel.com>
-.\" SO_WIFI_STATUS (3.3)
-.\" commit 6e3e939f3b1bf8534b32ad09ff199d88800835a0
-.\" Author: Johannes Berg <johannes.berg@intel.com>
-.\" Also: SCM_WIFI_STATUS
-.\" SO_NOFCS (3.4)
-.\" commit 3bdc0eba0b8b47797f4a76e377dd8360f317450f
-.\" Author: Ben Greear <greearb@candelatech.com>
-.\" SO_GET_FILTER (3.8)
-.\" commit a8fc92778080c845eaadc369a0ecf5699a03bef0
-.\" Author: Pavel Emelyanov <xemul@parallels.com>
-.\" SO_REUSEPORT (3.9)
-.\" commit c617f398edd4db2b8567a28e899a88f8f574798d
-.\" https://lwn.net/Articles/542629/
-.\" SO_LOCK_FILTER (3.9)
-.\" commit d59577b6ffd313d0ab3be39cb1ab47e29bdc9182
-.\" Author: Vincent Bernat <bernat@luffy.cx>
-.\" SO_SELECT_ERR_QUEUE (3.10)
-.\" commit 7d4c04fc170087119727119074e72445f2bb192b
-.\" Author: Keller, Jacob E <jacob.e.keller@intel.com>
-.\" SO_MAX_PACING_RATE (3.13)
-.\" commit 62748f32d501f5d3712a7c372bbb92abc7c62bc7
-.\" SO_BPF_EXTENSIONS (3.14)
-.\" commit ea02f9411d9faa3553ed09ce0ec9f00ceae9885e
-.\"
-.TH SOCKET 7 2014-07-08 Linux "Linux Programmer's Manual"
-.SH NAME
-socket \- Linux socket interface
-.SH SYNOPSIS
-.B #include <sys/socket.h>
-.sp
-.IB sockfd " = socket(int " socket_family ", int " socket_type ", int " protocol );
-.SH DESCRIPTION
-This manual page describes the Linux networking socket layer user
-interface.
-The BSD compatible sockets
-are the uniform interface
-between the user process and the network protocol stacks in the kernel.
-The protocol modules are grouped into
-.I protocol families
-such as
-.BR AF_INET ", " AF_IPX ", and " AF_PACKET ,
-and
-.I socket types
-such as
-.B SOCK_STREAM
-or
-.BR SOCK_DGRAM .
-See
-.BR socket (2)
-for more information on families and types.
-.SS Socket-layer functions
-These functions are used by the user process to send or receive packets
-and to do other socket operations.
-For more information see their respective manual pages.
-
-.BR socket (2)
-creates a socket,
-.BR connect (2)
-connects a socket to a remote socket address,
-the
-.BR bind (2)
-function binds a socket to a local socket address,
-.BR listen (2)
-tells the socket that new connections shall be accepted, and
-.BR accept (2)
-is used to get a new socket with a new incoming connection.
-.BR socketpair (2)
-returns two connected anonymous sockets (implemented only for a few
-local families like
-.BR AF_UNIX )
-.PP
-.BR send (2),
-.BR sendto (2),
-and
-.BR sendmsg (2)
-send data over a socket, and
-.BR recv (2),
-.BR recvfrom (2),
-.BR recvmsg (2)
-receive data from a socket.
-.BR poll (2)
-and
-.BR select (2)
-wait for arriving data or a readiness to send data.
-In addition, the standard I/O operations like
-.BR write (2),
-.BR writev (2),
-.BR sendfile (2),
-.BR read (2),
-and
-.BR readv (2)
-can be used to read and write data.
-.PP
-.BR getsockname (2)
-returns the local socket address and
-.BR getpeername (2)
-returns the remote socket address.
-.BR getsockopt (2)
-and
-.BR setsockopt (2)
-are used to set or get socket layer or protocol options.
-.BR ioctl (2)
-can be used to set or read some other options.
-.PP
-.BR close (2)
-is used to close a socket.
-.BR shutdown (2)
-closes parts of a full-duplex socket connection.
-.PP
-Seeking, or calling
-.BR pread (2)
-or
-.BR pwrite (2)
-with a nonzero position is not supported on sockets.
-.PP
-It is possible to do nonblocking I/O on sockets by setting the
-.B O_NONBLOCK
-flag on a socket file descriptor using
-.BR fcntl (2).
-Then all operations that would block will (usually)
-return with
-.B EAGAIN
-(operation should be retried later);
-.BR connect (2)
-will return
-.B EINPROGRESS
-error.
-The user can then wait for various events via
-.BR poll (2)
-or
-.BR select (2).
-.TS
-tab(:) allbox;
-c s s
-l l l.
-I/O events
-Event:Poll flag:Occurrence
-Read:POLLIN:T{
-New data arrived.
-T}
-Read:POLLIN:T{
-A connection setup has been completed
-(for connection-oriented sockets)
-T}
-Read:POLLHUP:T{
-A disconnection request has been initiated by the other end.
-T}
-Read:POLLHUP:T{
-A connection is broken (only for connection-oriented protocols).
-When the socket is written
-.B SIGPIPE
-is also sent.
-T}
-Write:POLLOUT:T{
-Socket has enough send buffer space for writing new data.
-T}
-Read/Write:T{
-POLLIN|
-.br
-POLLOUT
-T}:T{
-An outgoing
-.BR connect (2)
-finished.
-T}
-Read/Write:POLLERR:An asynchronous error occurred.
-Read/Write:POLLHUP:The other end has shut down one direction.
-Exception:POLLPRI:T{
-Urgent data arrived.
-.B SIGURG
-is sent then.
-T}
-.\" FIXME . The following is not true currently:
-.\" It is no I/O event when the connection
-.\" is broken from the local end using
-.\" .BR shutdown (2)
-.\" or
-.\" .BR close (2).
-.TE
-.PP
-An alternative to
-.BR poll (2)
-and
-.BR select (2)
-is to let the kernel inform the application about events
-via a
-.B SIGIO
-signal.
-For that the
-.B O_ASYNC
-flag must be set on a socket file descriptor via
-.BR fcntl (2)
-and a valid signal handler for
-.B SIGIO
-must be installed via
-.BR sigaction (2).
-See the
-.I Signals
-discussion below.
-.SS Socket address structures
-Each socket domain has its own format for socket addresses,
-with a domain-specific address structure.
-Each of these structures begins with an
-integer "family" field (typed as
-.IR sa_family_t )
-that indicates the type of the address structure.
-This allows
-the various system calls (e.g.,
-.BR connect (2),
-.BR bind (2),
-.BR accept (2),
-.BR getsockname (2),
-.BR getpeername (2)),
-which are generic to all socket domains,
-to determine the domain of a particular socket address.
-
-To allow any type of socket address to be passed to
-interfaces in the sockets API,
-the type
-.IR "struct sockaddr"
-is defined.
-The purpose of this type is purely to allow casting of
-domain-specific socket address types to a "generic" type,
-so as to avoid compiler warnings about type mismatches in
-calls to the sockets API.
-
-In addition, the sockets API provides the data type
-.IR "struct sockaddr_storage".
-This type
-is suitable to accommodate all supported domain-specific socket
-address structures; it is large enough and is aligned properly.
-(In particular, it is large enough to hold
-IPv6 socket addresses.)
-The structure includes the following field, which can be used to identify
-the type of socket address actually stored in the structure:
-
-.in +4n
-.nf
- sa_family_t ss_family;
-.fi
-.in
-
-The
-.I sockaddr_storage
-structure is useful in programs that must handle socket addresses
-in a generic way
-(e.g., programs that must deal with both IPv4 and IPv6 socket addresses).
-.SS Socket options
-The socket options listed below can be set by using
-.BR setsockopt (2)
-and read with
-.BR getsockopt (2)
-with the socket level set to
-.B SOL_SOCKET
-for all sockets.
-Unless otherwise noted,
-.I optval
-is a pointer to an
-.IR int .
-.\" FIXME .
-.\" In the list below, the text used to describe argument types
-.\" for each socket option should be more consistent
-.\"
-.\" SO_ACCEPTCONN is in POSIX.1-2001, and its origin is explained in
-.\" W R Stevens, UNPv1
-.TP
-.B SO_ACCEPTCONN
-Returns a value indicating whether or not this socket has been marked
-to accept connections with
-.BR listen (2).
-The value 0 indicates that this is not a listening socket,
-the value 1 indicates that this is a listening socket.
-This socket option is read-only.
-.TP
-.B SO_BINDTODEVICE
-Bind this socket to a particular device like \(lqeth0\(rq,
-as specified in the passed interface name.
-If the
-name is an empty string or the option length is zero, the socket device
-binding is removed.
-The passed option is a variable-length null-terminated
-interface name string with the maximum size of
-.BR IFNAMSIZ .
-If a socket is bound to an interface,
-only packets received from that particular interface are processed by the
-socket.
-Note that this works only for some socket types, particularly
-.B AF_INET
-sockets.
-It is not supported for packet sockets (use normal
-.BR bind (2)
-there).
-
-Before Linux 3.8,
-this socket option could be set, but could not retrieved with
-.BR getsockopt (2).
-Since Linux 3.8, it is readable.
-The
-.I optlen
-argument should contain the buffer size available
-to receive the device name and is recommended to be
-.BR IFNAMSZ
-bytes.
-The real device name length is reported back in the
-.I optlen
-argument.
-.TP
-.B SO_BROADCAST
-Set or get the broadcast flag.
-When enabled, datagram sockets are allowed to send
-packets to a broadcast address.
-This option has no effect on stream-oriented sockets.
-.TP
-.B SO_BSDCOMPAT
-Enable BSD bug-to-bug compatibility.
-This is used by the UDP protocol module in Linux 2.0 and 2.2.
-If enabled, ICMP errors received for a UDP socket will not be passed
-to the user program.
-In later kernel versions, support for this option has been phased out:
-Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning
-(printk()) if a program uses this option.
-Linux 2.0 also enabled BSD bug-to-bug compatibility
-options (random header changing, skipping of the broadcast flag) for raw
-sockets with this option, but that was removed in Linux 2.2.
-.TP
-.B SO_DEBUG
-Enable socket debugging.
-Only allowed for processes with the
-.B CAP_NET_ADMIN
-capability or an effective user ID of 0.
-.TP
-.BR SO_DOMAIN " (since Linux 2.6.32)"
-Retrieves the socket domain as an integer, returning a value such as
-.BR AF_INET6 .
-See
-.BR socket (2)
-for details.
-This socket option is read-only.
-.TP
-.B SO_ERROR
-Get and clear the pending socket error.
-This socket option is read-only.
-Expects an integer.
-.TP
-.B SO_DONTROUTE
-Don't send via a gateway, send only to directly connected hosts.
-The same effect can be achieved by setting the
-.B MSG_DONTROUTE
-flag on a socket
-.BR send (2)
-operation.
-Expects an integer boolean flag.
-.TP
-.B SO_KEEPALIVE
-Enable sending of keep-alive messages on connection-oriented sockets.
-Expects an integer boolean flag.
-.TP
-.B SO_LINGER
-Sets or gets the
-.B SO_LINGER
-option.
-The argument is a
-.I linger
-structure.
-.sp
-.in +4n
-.nf
-struct linger {
- int l_onoff; /* linger active */
- int l_linger; /* how many seconds to linger for */
-};
-.fi
-.in
-.IP
-When enabled, a
-.BR close (2)
-or
-.BR shutdown (2)
-will not return until all queued messages for the socket have been
-successfully sent or the linger timeout has been reached.
-Otherwise,
-the call returns immediately and the closing is done in the background.
-When the socket is closed as part of
-.BR exit (2),
-it always lingers in the background.
-.TP
-.BR SO_MARK " (since Linux 2.6.25)"
-.\" commit 4a19ec5800fc3bb64e2d87c4d9fdd9e636086fe0
-.\" and 914a9ab386a288d0f22252fc268ecbc048cdcbd5
-Set the mark for each packet sent through this socket
-(similar to the netfilter MARK target but socket-based).
-Changing the mark can be used for mark-based
-routing without netfilter or for packet filtering.
-Setting this option requires the
-.B CAP_NET_ADMIN
-capability.
-.TP
-.B SO_OOBINLINE
-If this option is enabled,
-out-of-band data is directly placed into the receive data stream.
-Otherwise, out-of-band data is passed only when the
-.B MSG_OOB
-flag is set during receiving.
-.\" don't document it because it can do too much harm.
-.\".B SO_NO_CHECK
-.TP
-.B SO_PASSCRED
-Enable or disable the receiving of the
-.B SCM_CREDENTIALS
-control message.
-For more information see
-.BR unix (7).
-.\" FIXME Document SO_PASSSEC, added in 2.6.18; there is some info
-.\" in the 2.6.18 ChangeLog
-.TP
-.BR SO_PEEK_OFF " (since Linux 3.4)"
-.\" commit ef64a54f6e558155b4f149bb10666b9e914b6c54
-This option, which is currently supported only for
-.BR unix (7)
-sockets, sets the value of the "peek offset" for the
-.BR recv (2)
-system call when used with
-.BR MSG_PEEK
-flag.
-
-When this option is set to a negative value
-(it is set to \-1 for all new sockets),
-traditional behavior is provided:
-.BR recv (2)
-with the
-.BR MSG_PEEK
-flag will peek data from the front of the queue.
-
-When the option is set to a value greater than or equal to zero,
-then the next peek at data queued in the socket will occur at
-the byte offset specified by the option value.
-At the same time, the "peek offset" will be
-incremented by the number of bytes that were peeked from the queue,
-so that a subsequent peek will return the next data in the queue.
-
-If data is removed from the front of the queue via a call to
-.BR recv (2)
-(or similar) without the
-.BR MSG_PEEK
-flag, the "peek offset" will be decreased by the number of bytes removed.
-In other words, receiving data without the
-.B MSG_PEEK
-flag will cause the "peek offset" to be adjusted to maintain
-the correct relative position in the queued data,
-so that a subsequent peek will retrieve the data that would have been
-retrieved had the data not been removed.
-
-For datagram sockets, if the "peek offset" points to the middle of a packet,
-the data returned will be marked with the
-.BR MSG_TRUNC
-flag.
-
-The following example serves to illustrate the use of
-.BR SO_PEEK_OFF .
-Suppose a stream socket has the following queued input data:
-
- aabbccddeeff
-
-.IP
-The following sequence of
-.BR recv (2)
-calls would have the effect noted in the comments:
-
-.in +4n
-.nf
-int ov = 4; // Set peek offset to 4
-setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
-
-recv(fd, buf, 2, MSG_PEEK); // Peeks "cc"; offset set to 6
-recv(fd, buf, 2, MSG_PEEK); // Peeks "dd"; offset set to 8
-recv(fd, buf, 2, 0); // Reads "aa"; offset set to 6
-recv(fd, buf, 2, MSG_PEEK); // Peeks "ee"; offset set to 8
-.fi
-.in
-.TP
-.B SO_PEERCRED
-Return the credentials of the foreign process connected to this socket.
-This is possible only for connected
-.B AF_UNIX
-stream sockets and
-.B AF_UNIX
-stream and datagram socket pairs created using
-.BR socketpair (2);
-see
-.BR unix (7).
-The returned credentials are those that were in effect at the time
-of the call to
-.BR connect (2)
-or
-.BR socketpair (2).
-The argument is a
-.I ucred
-structure; define the
-.B _GNU_SOURCE
-feature test macro to obtain the definition of that structure from
-.IR <sys/socket.h> .
-This socket option is read-only.
-.TP
-.B SO_PRIORITY
-Set the protocol-defined priority for all packets to be sent on
-this socket.
-Linux uses this value to order the networking queues:
-packets with a higher priority may be processed first depending
-on the selected device queueing discipline.
-For
-.BR ip (7),
-this also sets the IP type-of-service (TOS) field for outgoing packets.
-Setting a priority outside the range 0 to 6 requires the
-.B CAP_NET_ADMIN
-capability.
-.TP
-.BR SO_PROTOCOL " (since Linux 2.6.32)"
-Retrieves the socket protocol as an integer, returning a value such as
-.BR IPPROTO_SCTP .
-See
-.BR socket (2)
-for details.
-This socket option is read-only.
-.TP
-.B SO_RCVBUF
-Sets or gets the maximum socket receive buffer in bytes.
-The kernel doubles this value (to allow space for bookkeeping overhead)
-when it is set using
-.\" Most (all?) other implementations do not do this -- MTK, Dec 05
-.BR setsockopt (2),
-and this doubled value is returned by
-.BR getsockopt (2).
-.\" The following thread on LMKL is quite informative:
-.\" getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard" behavior
-.\" 17 July 2012
-.\" http://thread.gmane.org/gmane.linux.kernel/1328935
-The default value is set by the
-.I /proc/sys/net/core/rmem_default
-file, and the maximum allowed value is set by the
-.I /proc/sys/net/core/rmem_max
-file.
-The minimum (doubled) value for this option is 256.
-.TP
-.BR SO_RCVBUFFORCE " (since Linux 2.6.14)"
-Using this socket option, a privileged
-.RB ( CAP_NET_ADMIN )
-process can perform the same task as
-.BR SO_RCVBUF ,
-but the
-.I rmem_max
-limit can be overridden.
-.TP
-.BR SO_RCVLOWAT " and " SO_SNDLOWAT
-Specify the minimum number of bytes in the buffer until the socket layer
-will pass the data to the protocol
-.RB ( SO_SNDLOWAT )
-or the user on receiving
-.RB ( SO_RCVLOWAT ).
-These two values are initialized to 1.
-.B SO_SNDLOWAT
-is not changeable on Linux
-.RB ( setsockopt (2)
-fails with the error
-.BR ENOPROTOOPT ).
-.B SO_RCVLOWAT
-is changeable
-only since Linux 2.4.
-The
-.BR select (2)
-and
-.BR poll (2)
-system calls currently do not respect the
-.B SO_RCVLOWAT
-setting on Linux,
-and mark a socket readable when even a single byte of data is available.
-A subsequent read from the socket will block until
-.B SO_RCVLOWAT
-bytes are available.
-.\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=111049368106984&w=2
-.\" Tested on kernel 2.6.14 -- mtk, 30 Nov 05
-.TP
-.BR SO_RCVTIMEO " and " SO_SNDTIMEO
-.\" Not implemented in 2.0.
-.\" Implemented in 2.1.11 for getsockopt: always return a zero struct.
-.\" Implemented in 2.3.41 for setsockopt, and actually used.
-Specify the receiving or sending timeouts until reporting an error.
-The argument is a
-.IR "struct timeval" .
-If an input or output function blocks for this period of time, and
-data has been sent or received, the return value of that function
-will be the amount of data transferred; if no data has been transferred
-and the timeout has been reached, then \-1 is returned with
-.I errno
-set to
-.BR EAGAIN
-or
-.BR EWOULDBLOCK ,
-.\" in fact to EAGAIN
-or
-.B EINPROGRESS
-(for
-.BR connect (2))
-just as if the socket was specified to be nonblocking.
-If the timeout is set to zero (the default),
-then the operation will never timeout.
-Timeouts only have effect for system calls that perform socket I/O (e.g.,
-.BR read (2),
-.BR recvmsg (2),
-.BR send (2),
-.BR sendmsg (2));
-timeouts have no effect for
-.BR select (2),
-.BR poll (2),
-.BR epoll_wait (2),
-and so on.
-.TP
-.B SO_REUSEADDR
-Indicates that the rules used in validating addresses supplied in a
-.BR bind (2)
-call should allow reuse of local addresses.
-For
-.B AF_INET
-sockets this
-means that a socket may bind, except when there
-is an active listening socket bound to the address.
-When the listening socket is bound to
-.B INADDR_ANY
-with a specific port then it is not possible
-to bind to this port for any local address.
-Argument is an integer boolean flag.
-.TP
-.BR SO_RXQ_OVFL " (since Linux 2.6.33)"
-.\" commit 3b885787ea4112eaa80945999ea0901bf742707f
-Indicates that an unsigned 32-bit value ancillary message (cmsg)
-should be attached to received skbs indicating
-the number of packets dropped by the socket between
-the last received packet and this received packet.
-.TP
-.B SO_SNDBUF
-Sets or gets the maximum socket send buffer in bytes.
-The kernel doubles this value (to allow space for bookkeeping overhead)
-when it is set using
-.\" Most (all?) other implementations do not do this -- MTK, Dec 05
-.\" See also the comment to SO_RCVBUF (17 Jul 2012 LKML mail)
-.BR setsockopt (2),
-and this doubled value is returned by
-.BR getsockopt (2).
-The default value is set by the
-.I /proc/sys/net/core/wmem_default
-file and the maximum allowed value is set by the
-.I /proc/sys/net/core/wmem_max
-file.
-The minimum (doubled) value for this option is 2048.
-.TP
-.BR SO_SNDBUFFORCE " (since Linux 2.6.14)"
-Using this socket option, a privileged
-.RB ( CAP_NET_ADMIN )
-process can perform the same task as
-.BR SO_SNDBUF ,
-but the
-.I wmem_max
-limit can be overridden.
-.TP
-.B SO_TIMESTAMP
-Enable or disable the receiving of the
-.B SO_TIMESTAMP
-control message.
-The timestamp control message is sent with level
-.B SOL_SOCKET
-and the
-.I cmsg_data
-field is a
-.I "struct timeval"
-indicating the
-reception time of the last packet passed to the user in this call.
-See
-.BR cmsg (3)
-for details on control messages.
-.TP
-.B SO_TYPE
-Gets the socket type as an integer (e.g.,
-.BR SOCK_STREAM ).
-This socket option is read-only.
-.TP
-.BR SO_BUSY_POLL " (since Linux 3.11)"
-Sets the approximate time in microseconds to busy poll on a blocking receive
-when there is no data.
-Increasing this value requires
-.BR CAP_NET_ADMIN .
-The default for this option is controlled by the
-.I /proc/sys/net/core/busy_read
-file.
-
-The value in the
-.I /proc/sys/net/core/busy_poll
-file determines how long
-.BR select (2)
-and
-.BR poll (2)
-will busy poll when they operate on sockets with
-.BR SO_BUSY_POLL
-set and no events to report are found.
-
-In both cases,
-busy polling will only be done when the socket last received data
-from a network device that supports this option.
-
-While busy polling may improve latency of some applications,
-care must be taken when using it since this will increase
-both CPU utilization and power usage.
-.SS Signals
-When writing onto a connection-oriented socket that has been shut down
-(by the local or the remote end)
-.B SIGPIPE
-is sent to the writing process and
-.B EPIPE
-is returned.
-The signal is not sent when the write call
-specified the
-.B MSG_NOSIGNAL
-flag.
-.PP
-When requested with the
-.B FIOSETOWN
-.BR fcntl (2)
-or
-.B SIOCSPGRP
-.BR ioctl (2),
-.B SIGIO
-is sent when an I/O event occurs.
-It is possible to use
-.BR poll (2)
-or
-.BR select (2)
-in the signal handler to find out which socket the event occurred on.
-An alternative (in Linux 2.2) is to set a real-time signal using the
-.B F_SETSIG
-.BR fcntl (2);
-the handler of the real time signal will be called with
-the file descriptor in the
-.I si_fd
-field of its
-.IR siginfo_t .
-See
-.BR fcntl (2)
-for more information.
-.PP
-Under some circumstances (e.g., multiple processes accessing a
-single socket), the condition that caused the
-.B SIGIO
-may have already disappeared when the process reacts to the signal.
-If this happens, the process should wait again because Linux
-will resend the signal later.
-.\" .SS Ancillary messages
-.SS /proc interfaces
-The core socket networking parameters can be accessed
-via files in the directory
-.IR /proc/sys/net/core/ .
-.TP
-.I rmem_default
-contains the default setting in bytes of the socket receive buffer.
-.TP
-.I rmem_max
-contains the maximum socket receive buffer size in bytes which a user may
-set by using the
-.B SO_RCVBUF
-socket option.
-.TP
-.I wmem_default
-contains the default setting in bytes of the socket send buffer.
-.TP
-.I wmem_max
-contains the maximum socket send buffer size in bytes which a user may
-set by using the
-.B SO_SNDBUF
-socket option.
-.TP
-.IR message_cost " and " message_burst
-configure the token bucket filter used to load limit warning messages
-caused by external network events.
-.TP
-.I netdev_max_backlog
-Maximum number of packets in the global input queue.
-.TP
-.I optmem_max
-Maximum length of ancillary data and user control data like the iovecs
-per socket.
-.\" netdev_fastroute is not documented because it is experimental
-.SS Ioctls
-These operations can be accessed using
-.BR ioctl (2):
-
-.in +4n
-.nf
-.IB error " = ioctl(" ip_socket ", " ioctl_type ", " &value_result ");"
-.fi
-.in
-.TP
-.B SIOCGSTAMP
-Return a
-.I struct timeval
-with the receive timestamp of the last packet passed to the user.
-This is useful for accurate round trip time measurements.
-See
-.BR setitimer (2)
-for a description of
-.IR "struct timeval" .
-.\"
-This ioctl should be used only if the socket option
-.B SO_TIMESTAMP
-is not set on the socket.
-Otherwise, it returns the timestamp of the
-last packet that was received while
-.B SO_TIMESTAMP
-was not set, or it fails if no such packet has been received,
-(i.e.,
-.BR ioctl (2)
-returns \-1 with
-.I errno
-set to
-.BR ENOENT ).
-.TP
-.B SIOCSPGRP
-Set the process or process group to send
-.B SIGIO
-or
-.B SIGURG
-signals
-to when an
-asynchronous I/O operation has finished or urgent data is available.
-The argument is a pointer to a
-.IR pid_t .
-If the argument is positive, send the signals to that process.
-If the
-argument is negative, send the signals to the process group with the ID
-of the absolute value of the argument.
-The process may only choose itself or its own process group to receive
-signals unless it has the
-.B CAP_KILL
-capability or an effective UID of 0.
-.TP
-.B FIOASYNC
-Change the
-.B O_ASYNC
-flag to enable or disable asynchronous I/O mode of the socket.
-Asynchronous I/O mode means that the
-.B SIGIO
-signal or the signal set with
-.B F_SETSIG
-is raised when a new I/O event occurs.
-.IP
-Argument is an integer boolean flag.
-(This operation is synonymous with the use of
-.BR fcntl (2)
-to set the
-.B O_ASYNC
-flag.)
-.\"
-.TP
-.B SIOCGPGRP
-Get the current process or process group that receives
-.B SIGIO
-or
-.B SIGURG
-signals,
-or 0
-when none is set.
-.PP
-Valid
-.BR fcntl (2)
-operations:
-.TP
-.B FIOGETOWN
-The same as the
-.B SIOCGPGRP
-.BR ioctl (2).
-.TP
-.B FIOSETOWN
-The same as the
-.B SIOCSPGRP
-.BR ioctl (2).
-.SH VERSIONS
-.B SO_BINDTODEVICE
-was introduced in Linux 2.0.30.
-.B SO_PASSCRED
-is new in Linux 2.2.
-The
-.I /proc
-interfaces was introduced in Linux 2.2.
-.B SO_RCVTIMEO
-and
-.B SO_SNDTIMEO
-are supported since Linux 2.3.41.
-Earlier, timeouts were fixed to
-a protocol-specific setting, and could not be read or written.
-.SH NOTES
-Linux assumes that half of the send/receive buffer is used for internal
-kernel structures; thus the values in the corresponding
-.I /proc
-files are twice what can be observed on the wire.
-
-Linux will allow port reuse only with the
-.B SO_REUSEADDR
-option
-when this option was set both in the previous program that performed a
-.BR bind (2)
-to the port and in the program that wants to reuse the port.
-This differs from some implementations (e.g., FreeBSD)
-where only the later program needs to set the
-.B SO_REUSEADDR
-option.
-Typically this difference is invisible, since, for example, a server
-program is designed to always set this option.
-.SH BUGS
-The
-.B CONFIG_FILTER
-socket options
-.B SO_ATTACH_FILTER
-and
-.B SO_DETACH_FILTER
-.\" FIXME Document SO_ATTACH_FILTER and SO_DETACH_FILTER
-are not documented.
-The suggested interface to use them is via the libpcap
-library.
-.\" .SH AUTHORS
-.\" This man page was written by Andi Kleen.
-.SH SEE ALSO
-.BR connect (2),
-.BR getsockopt (2),
-.BR setsockopt (2),
-.BR socket (2),
-.BR capabilities (7),
-.BR ddp (7),
-.BR ip (7),
-.BR packet (7),
-.BR tcp (7),
-.BR udp (7),
-.BR unix (7)
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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