'\" t
.\" Copyright (c) 1983, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"     $Id: socket.2,v 1.4 1999/05/13 11:33:42 freitag Exp $
.\"
.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998, 1999 by Andi Kleen <ak@muc.de>
.\" Modified 2002-07-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.TH socket 2 2024-01-28 "Linux man-pages 6.06"
.SH NAME
socket \- create an endpoint for communication
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.P
.BI "int socket(int " domain ", int " type ", int " protocol );
.fi
.SH DESCRIPTION
.BR socket ()
creates an endpoint for communication and returns a file descriptor
that refers to that endpoint.
The file descriptor returned by a successful call will be
the lowest-numbered file descriptor not currently open for the process.
.P
The
.I domain
argument specifies a communication domain; this selects the protocol
family which will be used for communication.
These families are defined in
.IR <sys/socket.h> .
The formats currently understood by the Linux kernel include:
.TS
tab(:);
l1 lw40 l.
Name:Purpose:Man page
T{
.B AF_UNIX
T}:T{
Local communication
T}:T{
.BR unix (7)
T}
T{
.B AF_LOCAL
T}:T{
Synonym for
.B AF_UNIX
T}:T{
T}
T{
.B AF_INET
T}:IPv4 Internet protocols:T{
.BR ip (7)
T}
T{
.B AF_AX25
T}:T{
Amateur radio AX.25 protocol
T}:T{
.\" Part of ax25-tools
.BR ax25 (4)
T}
T{
.B AF_IPX
T}:IPX \- Novell protocols:
T{
.B AF_APPLETALK
T}:AppleTalk:T{
.BR ddp (7)
T}
T{
.B AF_X25
T}:ITU-T X.25 / ISO/IEC\~8208 protocol:T{
.BR x25 (7)
T}
T{
.B AF_INET6
T}:IPv6 Internet protocols:T{
.BR ipv6 (7)
T}
T{
.B AF_DECnet
T}:T{
DECet protocol sockets
T}
T{
.B AF_KEY
T}:T{
Key management protocol, originally developed for usage with IPsec
T}
T{
.B AF_NETLINK
T}:T{
Kernel user interface device
T}:T{
.BR netlink (7)
T}
T{
.B AF_PACKET
T}:T{
Low-level packet interface
T}:T{
.BR packet (7)
T}
T{
.B AF_RDS
T}:T{
.\" commit: 639b321b4d8f4e412bfbb2a4a19bfebc1e68ace4
Reliable Datagram Sockets (RDS) protocol
T}:T{
.\" rds-tools: https://github.com/oracle/rds-tools/blob/master/rds.7
.\" rds-tools: https://github.com/oracle/rds-tools/blob/master/rds-rdma.7
.BR rds (7)
.br
.BR rds\-rdma (7)
T}
T{
.B AF_PPPOX
T}:T{
Generic PPP transport layer, for setting up L2 tunnels
(L2TP and PPPoE)
T}
T{
.B AF_LLC
T}:T{
.\" linux-history commit: 34beb106cde7da233d4df35dd3d6cf4fee937caa
Logical link control (IEEE 802.2 LLC) protocol
T}
T{
.B AF_IB
T}:T{
.\" commits: 8d36eb01da5d371f..ce117ffac2e93334
InfiniBand native addressing
T}
T{
.B AF_MPLS
T}:T{
.\" commits: 0189197f441602acdca3f97750d392a895b778fd
Multiprotocol Label Switching
T}
T{
.B AF_CAN
T}:T{
.\" commits: 8dbde28d9711475a..5423dd67bd0108a1
Controller Area Network automotive bus protocol
T}
T{
.B AF_TIPC
T}:T{
.\" commits: b97bf3fd8f6a16966d4f18983b2c40993ff937d4
TIPC, "cluster domain sockets" protocol
T}
T{
.B AF_BLUETOOTH
T}:T{
.\" commits: 8d36eb01da5d371f..ce117ffac2e93334
Bluetooth low-level socket protocol
T}
T{
.B AF_ALG
T}:T{
.\" commit: 03c8efc1ffeb6b82a22c1af8dd908af349563314
Interface to kernel crypto API
T}
T{
.B AF_VSOCK
T}:T{
.\" commit: d021c344051af91f42c5ba9fdedc176740cbd238
VSOCK (originally "VMWare VSockets") protocol
for hypervisor-guest communication
T}:T{
.BR vsock (7)
T}
T{
.B AF_KCM
T}:T{
.\" commit: 03c8efc1ffeb6b82a22c1af8dd908af349563314
KCM (kernel connection multiplexer) interface
T}
T{
.B AF_XDP
T}:T{
.\" commit: c0c77d8fb787cfe0c3fca689c2a30d1dad4eaba7
XDP (express data path) interface
T}
.TE
.P
Further details of the above address families,
as well as information on several other address families, can be found in
.BR address_families (7).
.P
The socket has the indicated
.IR type ,
which specifies the communication semantics.
Currently defined types
are:
.TP 16
.B SOCK_STREAM
Provides sequenced, reliable, two-way, connection-based byte streams.
An out-of-band data transmission mechanism may be supported.
.TP
.B SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages of a fixed
maximum length).
.TP
.B SOCK_SEQPACKET
Provides a sequenced, reliable, two-way connection-based data
transmission path for datagrams of fixed maximum length; a consumer is
required to read an entire packet with each input system call.
.TP
.B SOCK_RAW
Provides raw network protocol access.
.TP
.B SOCK_RDM
Provides a reliable datagram layer that does not guarantee ordering.
.TP
.B SOCK_PACKET
Obsolete and should not be used in new programs;
see
.BR packet (7).
.P
Some socket types may not be implemented by all protocol families.
.P
Since Linux 2.6.27, the
.I type
argument serves a second purpose:
in addition to specifying a socket type,
it may include the bitwise OR of any of the following values,
to modify the behavior of
.BR socket ():
.TP 16
.B SOCK_NONBLOCK
Set the
.B O_NONBLOCK
file status flag on the open file description (see
.BR open (2))
referred to by the new file descriptor.
Using this flag saves extra calls to
.BR fcntl (2)
to achieve the same result.
.TP
.B SOCK_CLOEXEC
Set the close-on-exec
.RB ( FD_CLOEXEC )
flag on the new file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.P
The
.I protocol
specifies a particular protocol to be used with the socket.
Normally only a single protocol exists to support a particular
socket type within a given protocol family, in which case
.I protocol
can be specified as 0.
However, it is possible that many protocols may exist, in
which case a particular protocol must be specified in this manner.
The protocol number to use is specific to the \*(lqcommunication domain\*(rq
in which communication is to take place; see
.BR protocols (5).
See
.BR getprotoent (3)
on how to map protocol name strings to protocol numbers.
.P
Sockets of type
.B SOCK_STREAM
are full-duplex byte streams.
They do not preserve
record boundaries.
A stream socket must be in
a
.I connected
state before any data may be sent or received on it.
A connection to
another socket is created with a
.BR connect (2)
call.
Once connected, data may be transferred using
.BR read (2)
and
.BR write (2)
calls or some variant of the
.BR send (2)
and
.BR recv (2)
calls.
When a session has been completed a
.BR close (2)
may be performed.
Out-of-band data may also be transmitted as described in
.BR send (2)
and received as described in
.BR recv (2).
.P
The communications protocols which implement a
.B SOCK_STREAM
ensure that data is not lost or duplicated.
If a piece of data for which
the peer protocol has buffer space cannot be successfully transmitted
within a reasonable length of time, then the connection is considered
to be dead.
When
.B SO_KEEPALIVE
is enabled on the socket the protocol checks in a protocol-specific
manner if the other end is still alive.
A
.B SIGPIPE
signal is raised if a process sends or receives
on a broken stream; this causes naive processes,
which do not handle the signal, to exit.
.B SOCK_SEQPACKET
sockets employ the same system calls as
.B SOCK_STREAM
sockets.
The only difference is that
.BR read (2)
calls will return only the amount of data requested,
and any data remaining in the arriving packet will be discarded.
Also all message boundaries in incoming datagrams are preserved.
.P
.B SOCK_DGRAM
and
.B SOCK_RAW
sockets allow sending of datagrams to correspondents named in
.BR sendto (2)
calls.
Datagrams are generally received with
.BR recvfrom (2),
which returns the next datagram along with the address of its sender.
.P
.B SOCK_PACKET
is an obsolete socket type to receive raw packets directly from the
device driver.
Use
.BR packet (7)
instead.
.P
An
.BR fcntl (2)
.B F_SETOWN
operation can be used to specify a process or process group to receive a
.B SIGURG
signal when the out-of-band data arrives or
.B SIGPIPE
signal when a
.B SOCK_STREAM
connection breaks unexpectedly.
This operation may also be used to set the process or process group
that receives the I/O and asynchronous notification of I/O events via
.BR SIGIO .
Using
.B F_SETOWN
is equivalent to an
.BR ioctl (2)
call with the
.B FIOSETOWN
or
.B SIOCSPGRP
argument.
.P
When the network signals an error condition to the protocol module (e.g.,
using an ICMP message for IP) the pending error flag is set for the socket.
The next operation on this socket will return the error code of the pending
error.
For some protocols it is possible to enable a per-socket error queue
to retrieve detailed information about the error; see
.B IP_RECVERR
in
.BR ip (7).
.P
The operation of sockets is controlled by socket level
.IR options .
These options are defined in
.IR <sys/socket.h> .
The functions
.BR setsockopt (2)
and
.BR getsockopt (2)
are used to set and get options.
.SH RETURN VALUE
On success, a file descriptor for the new socket is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
Permission to create a socket of the specified type and/or protocol
is denied.
.TP
.B EAFNOSUPPORT
The implementation does not support the specified address family.
.TP
.B EINVAL
Unknown protocol, or protocol family not available.
.TP
.B EINVAL
.\" Since Linux 2.6.27
Invalid flags in
.IR type .
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.BR ENOBUFS " or " ENOMEM
Insufficient memory is available.
The socket cannot be
created until sufficient resources are freed.
.TP
.B EPROTONOSUPPORT
The protocol type or the specified protocol is not
supported within this domain.
.P
Other errors may be generated by the underlying protocol modules.
.SH STANDARDS
POSIX.1-2008.
.P
.B SOCK_NONBLOCK
and
.B SOCK_CLOEXEC
are Linux-specific.
.SH HISTORY
POSIX.1-2001, 4.4BSD.
.P
.BR socket ()
appeared in 4.2BSD.
It is generally portable to/from
non-BSD systems supporting clones of the BSD socket layer (including
System\ V variants).
.P
The manifest constants used under 4.x BSD for protocol families
are
.BR PF_UNIX ,
.BR PF_INET ,
and so on, while
.BR AF_UNIX ,
.BR AF_INET ,
and so on are used for address
families.
However, already the BSD man page promises: "The protocol
family generally is the same as the address family", and subsequent
standards use AF_* everywhere.
.SH EXAMPLES
An example of the use of
.BR socket ()
is shown in
.BR getaddrinfo (3).
.SH SEE ALSO
.BR accept (2),
.BR bind (2),
.BR close (2),
.BR connect (2),
.BR fcntl (2),
.BR getpeername (2),
.BR getsockname (2),
.BR getsockopt (2),
.BR ioctl (2),
.BR listen (2),
.BR read (2),
.BR recv (2),
.BR select (2),
.BR send (2),
.BR shutdown (2),
.BR socketpair (2),
.BR write (2),
.BR getprotoent (3),
.BR address_families (7),
.BR ip (7),
.BR socket (7),
.BR tcp (7),
.BR udp (7),
.BR unix (7)
.P
\[lq]An Introductory 4.3BSD Interprocess Communication Tutorial\[rq]
and
\[lq]BSD Interprocess Communication Tutorial\[rq],
reprinted in
.I UNIX Programmer's Supplementary Documents Volume 1.