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.\" 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 2009-01-19 "Linux" "Linux Programmer's Manual"
.SH NAME
socket \- create an endpoint for communication
.SH SYNOPSIS
.BR "#include <sys/types.h>" "          /* See NOTES */"
.br
.B #include <sys/socket.h>
.sp
.BI "int socket(int " domain ", int " type ", int " protocol );
.SH DESCRIPTION
.BR socket ()
creates an endpoint for communication and returns a descriptor.
.PP
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 currently understood formats include:
.TS
tab(:);
l l l.
Name:Purpose:Man page
T{
.BR AF_UNIX ", " AF_LOCAL
T}:T{
Local communication
T}:T{
.BR unix (7)
T}
T{
.B AF_INET
T}:IPv4 Internet protocols:T{
.BR ip (7)
T}
T{
.B AF_INET6
T}:IPv6 Internet protocols:T{
.BR ipv6 (7)
T}
T{
.B AF_IPX
T}:IPX \- Novell protocols:
T{
.B AF_NETLINK
T}:T{
Kernel user interface device
T}:T{
.BR netlink (7)
T}
T{
.B AF_X25
T}:ITU-T X.25 / ISO-8208 protocol:T{
.BR x25 (7)
T}
T{
.B AF_AX25
T}:T{
Amateur radio AX.25 protocol
T}:
T{
.B AF_ATMPVC
T}:Access to raw ATM PVCs:
T{
.B AF_APPLETALK
T}:Appletalk:T{
.BR ddp (7)
T}
T{
.B AF_PACKET
T}:T{
Low level packet interface
T}:T{
.BR packet (7)
T}
.TE
.PP
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).
.PP
Some socket types may not be implemented by all protocol families;
for example,
.B SOCK_SEQPACKET
is not implemented for
.BR AF_INET .
.PP
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
.BR O_NONBLOCK
file status flag on the new open file description.
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.
.PP
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.
.PP
Sockets of type
.B SOCK_STREAM
are full-duplex byte streams, similar to pipes.
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).
.PP
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.
.PP
.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.
.PP
.B SOCK_PACKET
is an obsolete socket type to receive raw packets directly from the
device driver.
Use
.BR packet (7)
instead.
.PP
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.
.PP
When the network signals an error condition to the protocol module (e.g.,
using a 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).
.PP
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, respectively.
.SH "RETURN VALUE"
On success, a file descriptor for the new socket is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.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
Process file table overflow.
.TP
.B ENFILE
The system 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.
.PP
Other errors may be generated by the underlying protocol modules.
.SH "CONFORMING TO"
4.4BSD, POSIX.1-2001.

The
.B SOCK_NONBLOCK
and
.B SOCK_CLOEXEC
flags are Linux-specific.

.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).
.SH NOTES
POSIX.1-2001 does not require the inclusion of
.IR <sys/types.h> ,
and this header file is not required on Linux.
However, some historical (BSD) implementations required this header
file, and portable applications are probably wise to include it.

The manifest constants used under 4.x BSD for protocol families
are
.BR PF_UNIX ,
.BR PF_INET ,
etc., while
.B AF_UNIX
etc. 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.

The header file
.I <sys/types.h>
is only required for libc4 or earlier.  Some packages, like
util-linux, claim portability to all Linux versions and libraries.
They certainly need this header file.
.SH EXAMPLE
An example of the use of
.BR socket ()
is shown in
.BR getaddrinfo (3).
.SH "SEE ALSO"
.BR accept (2),
.BR bind (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 ip (7),
.BR socket (7),
.BR tcp (7),
.BR udp (7),
.BR unix (7)
.PP
\(lqAn Introductory 4.3BSD Interprocess Communication Tutorial\(rq
is reprinted in
.I UNIX Programmer's Supplementary Documents Volume 1.
.PP
\(lqBSD Interprocess Communication Tutorial\(rq
is reprinted in
.I UNIX Programmer's Supplementary Documents Volume 1.
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.