'\" t

.\" Copyright (c) 1983, 1991 The Regents of the University of California.

.\" All rights reserved.

.\"

.\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)

.\" Redistribution and use in source and binary forms, with or without

.\" modification, are permitted provided that the following conditions

.\" are met:

.\" 1. Redistributions of source code must retain the above copyright

.\"    notice, this list of conditions and the following disclaimer.

.\" 2. Redistributions in binary form must reproduce the above copyright

.\"    notice, this list of conditions and the following disclaimer in the

.\"    documentation and/or other materials provided with the distribution.

.\" 3. All advertising materials mentioning features or use of this software

.\"    must display the following acknowledgement:

.\"	This product includes software developed by the University of

.\"	California, Berkeley and its contributors.

.\" 4. Neither the name of the University nor the names of its contributors

.\"    may be used to endorse or promote products derived from this software

.\"    without specific prior written permission.

.\"

.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND

.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE

.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

.\" SUCH DAMAGE.

.\" %%%LICENSE_END

.\"

.\"     $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 2017-09-15 "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>

.PP

.BI "int socket(int " domain ", int " type ", int " protocol );

.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.

.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}

T{

.B AF_ALG

T}:T{

Interface to kernel crypto API

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.

.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.

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 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).

.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

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.

.PP

Other errors may be generated by the underlying protocol modules.

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, 4.4BSD.

.PP

The

.B SOCK_NONBLOCK

and

.B SOCK_CLOEXEC

flags are Linux-specific.

.PP

.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 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.

.PP

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.

.PP

The

.B AF_ALG

protocol type was added in Linux 2.6.38.

More information on this interface is provided with the kernel HTML

documentation at

.IR https://www.kernel.org/doc/htmldocs/crypto\-API/User.html .

.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 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 ip (7),

.BR socket (7),

.BR tcp (7),

.BR udp (7),

.BR unix (7)

.PP

\(lqAn Introductory 4.3BSD Interprocess Communication Tutorial\(rq

and

\(lqBSD Interprocess Communication Tutorial\(rq,

reprinted in

.I UNIX Programmer's Supplementary Documents Volume 1.

.SH COLOPHON

This page is part of release 4.15 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

\%https://www.kernel.org/doc/man\-pages/.