'\" t
-.\" Don't change the first line, it tells man that we need tbl.
.\" 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
.\" 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
-.\" SO_TIMESTAMPNS
-.\" SO_MARK
-.\" SO_TIMESTAMPING
-.\" SO_PROTOCOL (2.6.32)
-.\" SO_DOMAIN (2.6.32)
+.\" SO_PEERNAME (2.4?)
+.\" get only
+.\" Seems to do something similar to getpeernam(), 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_RXQ_OVFL (2.6.33)
+.\" commit 3b885787ea4112eaa80945999ea0901bf742707f
+.\" Author: Neil Horman <nhorman@tuxdriver.com>
+.\" SO_WIFI_STATUS (3.3)
+.\" commit 6e3e939f3b1bf8534b32ad09ff199d88800835a0
+.\" Author: Johannes Berg <johannes.berg@intel.com>
+.\" Also: SCM_WIFI_STATUS
+.\" SO_PEEK_OFF (3.4)
+.\" commit ef64a54f6e558155b4f149bb10666b9e914b6c54
+.\" Author: Pavel Emelyanov <xemul@parallels.com>
+.\" SO_NOFCS (3.4)
+.\" commit 3bdc0eba0b8b47797f4a76e377dd8360f317450f
+.\" Author: Ben Greear <greearb@candelatech.com>
.\"
-.TH SOCKET 7 2008-12-03 Linux "Linux Programmer's Manual"
+.TH SOCKET 7 2013-03-15 Linux "Linux Programmer's Manual"
.SH NAME
socket \- Linux socket interface
.SH SYNOPSIS
See
.BR socket (2)
for more information on families and types.
-.SS Socket Layer Functions
+.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 accept (2)
is used to get a new socket with a new incoming connection.
.BR socketpair (2)
-returns two connected anonymous sockets (only implemented for a few
+returns two connected anonymous sockets (implemented only for a few
local families like
.BR AF_UNIX )
.PP
.\" or
.\" .BR close (2).
.TE
-
.PP
An alternative to
.BR poll (2)
See the
.I Signals
discussion below.
-.SS Socket Options
+.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
These socket options can be set by using
.BR setsockopt (2)
and read with
with the socket level set to
.B SOL_SOCKET
for all sockets:
+.\" 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
.BR listen (2).
The value 0 indicates that this is not a listening socket,
the value 1 indicates that this is a listening socket.
-Can only be read
-with
-.BR getsockopt (2).
+This socket option is read-only.
.TP
.B SO_BINDTODEVICE
Bind this socket to a particular device like \(lqeth0\(rq,
If a socket is bound to an interface,
only packets received from that particular interface are processed by the
socket.
-Note that this only works for some socket types, particularly
+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 (8)
+.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
-receive packets sent to a broadcast address and they are allowed to send
+When enabled, datagram sockets are allowed to send
packets to a broadcast address.
This option has no effect on stream-oriented sockets.
.TP
.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.
-Only valid as a
-.BR getsockopt (2).
+This socket option is read-only.
Expects an integer.
.TP
.B SO_DONTROUTE
-Don't send via a gateway, only send to directly connected hosts.
+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 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 only passed when the
+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.
.TP
.B SO_PEERCRED
Return the credentials of the foreign process connected to this socket.
-This is only possible for connected
+This is possible only for connected
.B AF_UNIX
stream sockets and
.B AF_UNIX
.BR connect (2)
or
.BR socketpair (2).
-Argument is a
+The argument is a
.I ucred
-structure.
-Only valid as a
-.BR getsockopt (2).
+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
.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)
.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" behaviour
+.\" 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
and the timeout has been reached then \-1 is returned with
.I errno
set to
-.B EAGAIN
+.BR EAGAIN
or
-.B EWOULDBLOCK
+.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.
.BR select (2),
.BR poll (2),
.BR epoll_wait (2),
-etc.
+and so on.
.TP
.B SO_REUSEADDR
Indicates that the rules used in validating addresses supplied in a
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).
for details on control messages.
.TP
.B SO_TYPE
-Gets the socket type as an integer (like
+Gets the socket type as an integer (e.g.,
.BR SOCK_STREAM ).
-Can only be read
-with
-.BR getsockopt (2).
+This socket option is read-only.
.SS Signals
When writing onto a connection-oriented socket that has been shut down
(by the local or the remote end)
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 Ancillary messages
.SS /proc interfaces
The core socket networking parameters can be accessed
via files in the directory
for a description of
.IR "struct timeval" .
.\"
-This ioctl should only be used if the socket option
+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
library.
.\" .SH AUTHORS
.\" This man page was written by Andi Kleen.
-.SH "SEE ALSO"
+.SH SEE ALSO
.BR getsockopt (2),
+.BR connect (2),
.BR setsockopt (2),
.BR socket (2),
.BR capabilities (7),
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