man7/socket.7

'\" 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.
.\" 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.
.\"
.\" 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 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 2012-12-15 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
like
.BR AF_INET ", " AF_IPX ", " AF_PACKET
and
.I socket types
like
.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 (only implemented 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
These socket options 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:
.\" 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 only works for some socket types, particularly
.B AF_INET
sockets.
It is not supported for packet sockets (use normal
.BR bind (2)
there).
.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, only send 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 only passed 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
.B SO_PEERCRED
Return the credentials of the foreign process connected to this socket.
This is only possible 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).
Argument is a
.I ucred
structure.
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" 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
.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
.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.
.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 only be used 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 only allow port reuse 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 getsockopt (2),
.BR connect (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)