mirror of https://github.com/mkerrisk/man-pages
517 lines
13 KiB
Groff
517 lines
13 KiB
Groff
.\" Copyright (c) 1983, 1991 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\" %%%LICENSE_END
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.\"
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.\" $Id: socket.2,v 1.4 1999/05/13 11:33:42 freitag Exp $
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.\"
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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
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.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 1998, 1999 by Andi Kleen <ak@muc.de>
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.\" Modified 2002-07-17 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.TH SOCKET 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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socket \- create an endpoint for communication
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.SH SYNOPSIS
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.nf
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.B #include <sys/socket.h>
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.PP
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.BI "int socket(int " domain ", int " type ", int " protocol );
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.fi
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.SH DESCRIPTION
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.BR socket ()
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creates an endpoint for communication and returns a file descriptor
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that refers to that endpoint.
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The file descriptor returned by a successful call will be
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the lowest-numbered file descriptor not currently open for the process.
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.PP
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The
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.I domain
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argument specifies a communication domain; this selects the protocol
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family which will be used for communication.
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These families are defined in
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.IR <sys/socket.h> .
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The formats currently understood by the Linux kernel include:
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.TS
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tab(:);
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l1 lw40 l.
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Name:Purpose:Man page
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T{
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.BR AF_UNIX
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T}:T{
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Local communication
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T}:T{
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.BR unix (7)
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T}
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T{
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.B AF_LOCAL
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T}:T{
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Synonym for
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.B AF_UNIX
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T}:T{
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T}
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T{
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.B AF_INET
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T}:IPv4 Internet protocols:T{
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.BR ip (7)
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T}
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T{
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.B AF_AX25
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T}:T{
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Amateur radio AX.25 protocol
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T}:T{
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.\" Part of ax25-tools
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.BR ax25 (4)
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T}
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T{
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.B AF_IPX
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T}:IPX \- Novell protocols:
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T{
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.B AF_APPLETALK
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T}:AppleTalk:T{
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.BR ddp (7)
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T}
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T{
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.B AF_X25
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T}:ITU-T X.25 / ISO-8208 protocol:T{
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.BR x25 (7)
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T}
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T{
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.B AF_INET6
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T}:IPv6 Internet protocols:T{
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.BR ipv6 (7)
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T}
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T{
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.B AF_DECnet
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T}:T{
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DECet protocol sockets
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T}
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T{
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.B AF_KEY
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T}:T{
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Key management protocol, originally developed for usage with IPsec
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T}
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T{
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.B AF_NETLINK
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T}:T{
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Kernel user interface device
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T}:T{
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.BR netlink (7)
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T}
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T{
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.B AF_PACKET
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T}:T{
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Low-level packet interface
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T}:T{
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.BR packet (7)
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T}
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T{
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.B AF_RDS
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T}:T{
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.\" commit: 639b321b4d8f4e412bfbb2a4a19bfebc1e68ace4
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Reliable Datagram Sockets (RDS) protocol
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T}:T{
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.\" rds-tools: https://github.com/oracle/rds-tools/blob/master/rds.7
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.\" rds-tools: https://github.com/oracle/rds-tools/blob/master/rds-rdma.7
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.BR rds (7)
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.br
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.BR rds\-rdma (7)
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T}
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T{
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.B AF_PPPOX
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T}:T{
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Generic PPP transport layer, for setting up L2 tunnels
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(L2TP and PPPoE)
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T}
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T{
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.B AF_LLC
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T}:T{
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.\" linux-history commit: 34beb106cde7da233d4df35dd3d6cf4fee937caa
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Logical link control (IEEE 802.2 LLC) protocol
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T}
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T{
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.B AF_IB
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T}:T{
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.\" commits: 8d36eb01da5d371f..ce117ffac2e93334
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InfiniBand native addressing
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T}
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T{
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.B AF_MPLS
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T}:T{
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.\" commits: 0189197f441602acdca3f97750d392a895b778fd
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Multiprotocol Label Switching
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T}
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T{
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.B AF_CAN
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T}:T{
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.\" commits: 8dbde28d9711475a..5423dd67bd0108a1
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Controller Area Network automotive bus protocol
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T}
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T{
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.B AF_TIPC
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T}:T{
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.\" commits: b97bf3fd8f6a16966d4f18983b2c40993ff937d4
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TIPC, "cluster domain sockets" protocol
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T}
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T{
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.B AF_BLUETOOTH
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T}:T{
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.\" commits: 8d36eb01da5d371f..ce117ffac2e93334
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Bluetooth low-level socket protocol
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T}
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T{
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.B AF_ALG
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T}:T{
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.\" commit: 03c8efc1ffeb6b82a22c1af8dd908af349563314
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Interface to kernel crypto API
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T}
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T{
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.B AF_VSOCK
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T}:T{
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.\" commit: d021c344051af91f42c5ba9fdedc176740cbd238
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VSOCK (originally "VMWare VSockets") protocol
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for hypervisor-guest communication
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T}:T{
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.BR vsock (7)
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T}
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T{
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.B AF_KCM
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T}:T{
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.\" commit: 03c8efc1ffeb6b82a22c1af8dd908af349563314
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KCM (kernel connection multiplexer) interface
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T}
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T{
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.B AF_XDP
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T}:T{
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.\" commit: c0c77d8fb787cfe0c3fca689c2a30d1dad4eaba7
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XDP (express data path) interface
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T}
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.TE
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.PP
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Further details of the above address families,
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as well as information on several other address families, can be found in
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.BR address_families (7).
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.PP
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The socket has the indicated
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.IR type ,
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which specifies the communication semantics.
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Currently defined types
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are:
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.TP 16
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.B SOCK_STREAM
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Provides sequenced, reliable, two-way, connection-based byte streams.
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An out-of-band data transmission mechanism may be supported.
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.TP
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.B SOCK_DGRAM
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Supports datagrams (connectionless, unreliable messages of a fixed
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maximum length).
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.TP
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.B SOCK_SEQPACKET
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Provides a sequenced, reliable, two-way connection-based data
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transmission path for datagrams of fixed maximum length; a consumer is
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required to read an entire packet with each input system call.
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.TP
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.B SOCK_RAW
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Provides raw network protocol access.
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.TP
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.B SOCK_RDM
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Provides a reliable datagram layer that does not guarantee ordering.
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.TP
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.B SOCK_PACKET
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Obsolete and should not be used in new programs;
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see
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.BR packet (7).
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.PP
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Some socket types may not be implemented by all protocol families.
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.PP
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Since Linux 2.6.27, the
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.I type
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argument serves a second purpose:
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in addition to specifying a socket type,
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it may include the bitwise OR of any of the following values,
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to modify the behavior of
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.BR socket ():
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.TP 16
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.B SOCK_NONBLOCK
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Set the
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.BR O_NONBLOCK
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file status flag on the open file description (see
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.BR open (2))
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referred to by the new file descriptor.
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Using this flag saves extra calls to
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.BR fcntl (2)
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to achieve the same result.
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.TP
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.B SOCK_CLOEXEC
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Set the close-on-exec
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.RB ( FD_CLOEXEC )
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flag on the new file descriptor.
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See the description of the
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.B O_CLOEXEC
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flag in
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.BR open (2)
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for reasons why this may be useful.
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.PP
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The
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.I protocol
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specifies a particular protocol to be used with the socket.
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Normally only a single protocol exists to support a particular
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socket type within a given protocol family, in which case
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.I protocol
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can be specified as 0.
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However, it is possible that many protocols may exist, in
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which case a particular protocol must be specified in this manner.
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The protocol number to use is specific to the \*(lqcommunication domain\*(rq
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in which communication is to take place; see
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.BR protocols (5).
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See
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.BR getprotoent (3)
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on how to map protocol name strings to protocol numbers.
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.PP
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Sockets of type
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.B SOCK_STREAM
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are full-duplex byte streams.
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They do not preserve
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record boundaries.
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A stream socket must be in
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a
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.I connected
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state before any data may be sent or received on it.
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A connection to
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another socket is created with a
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.BR connect (2)
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call.
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Once connected, data may be transferred using
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.BR read (2)
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and
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.BR write (2)
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calls or some variant of the
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.BR send (2)
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and
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.BR recv (2)
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calls.
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When a session has been completed a
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.BR close (2)
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may be performed.
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Out-of-band data may also be transmitted as described in
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.BR send (2)
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and received as described in
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.BR recv (2).
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.PP
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The communications protocols which implement a
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.B SOCK_STREAM
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ensure that data is not lost or duplicated.
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If a piece of data for which
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the peer protocol has buffer space cannot be successfully transmitted
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within a reasonable length of time, then the connection is considered
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to be dead.
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When
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.B SO_KEEPALIVE
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is enabled on the socket the protocol checks in a protocol-specific
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manner if the other end is still alive.
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A
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.B SIGPIPE
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signal is raised if a process sends or receives
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on a broken stream; this causes naive processes,
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which do not handle the signal, to exit.
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.B SOCK_SEQPACKET
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sockets employ the same system calls as
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.B SOCK_STREAM
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sockets.
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The only difference is that
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.BR read (2)
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calls will return only the amount of data requested,
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and any data remaining in the arriving packet will be discarded.
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Also all message boundaries in incoming datagrams are preserved.
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.PP
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.B SOCK_DGRAM
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and
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.B SOCK_RAW
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sockets allow sending of datagrams to correspondents named in
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.BR sendto (2)
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calls.
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Datagrams are generally received with
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.BR recvfrom (2),
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which returns the next datagram along with the address of its sender.
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.PP
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.B SOCK_PACKET
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is an obsolete socket type to receive raw packets directly from the
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device driver.
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Use
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.BR packet (7)
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instead.
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.PP
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An
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.BR fcntl (2)
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.B F_SETOWN
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operation can be used to specify a process or process group to receive a
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.B SIGURG
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signal when the out-of-band data arrives or
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.B SIGPIPE
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signal when a
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.B SOCK_STREAM
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connection breaks unexpectedly.
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This operation may also be used to set the process or process group
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that receives the I/O and asynchronous notification of I/O events via
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.BR SIGIO .
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Using
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.B F_SETOWN
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is equivalent to an
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.BR ioctl (2)
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call with the
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.B FIOSETOWN
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or
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.B SIOCSPGRP
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argument.
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.PP
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When the network signals an error condition to the protocol module (e.g.,
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using an ICMP message for IP) the pending error flag is set for the socket.
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The next operation on this socket will return the error code of the pending
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error.
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For some protocols it is possible to enable a per-socket error queue
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to retrieve detailed information about the error; see
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.B IP_RECVERR
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in
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.BR ip (7).
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.PP
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The operation of sockets is controlled by socket level
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.IR options .
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These options are defined in
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.IR <sys/socket.h> .
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The functions
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.BR setsockopt (2)
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and
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.BR getsockopt (2)
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are used to set and get options.
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.SH RETURN VALUE
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On success, a file descriptor for the new socket is returned.
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EACCES
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Permission to create a socket of the specified type and/or protocol
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is denied.
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.TP
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.B EAFNOSUPPORT
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The implementation does not support the specified address family.
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.TP
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.B EINVAL
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Unknown protocol, or protocol family not available.
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.TP
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.B EINVAL
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.\" Since Linux 2.6.27
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Invalid flags in
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.IR type .
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.TP
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.B EMFILE
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The per-process limit on the number of open file descriptors has been reached.
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.TP
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.B ENFILE
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The system-wide limit on the total number of open files has been reached.
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.TP
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.BR ENOBUFS " or " ENOMEM
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Insufficient memory is available.
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The socket cannot be
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created until sufficient resources are freed.
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.TP
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.B EPROTONOSUPPORT
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The protocol type or the specified protocol is not
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supported within this domain.
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.PP
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Other errors may be generated by the underlying protocol modules.
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008, 4.4BSD.
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.PP
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The
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.B SOCK_NONBLOCK
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and
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.B SOCK_CLOEXEC
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flags are Linux-specific.
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.PP
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.BR socket ()
|
|
appeared in 4.2BSD.
|
|
It is generally portable to/from
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non-BSD systems supporting clones of the BSD socket layer (including
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System\ V variants).
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.SH NOTES
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|
The manifest constants used under 4.x BSD for protocol families
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are
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.BR PF_UNIX ,
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.BR PF_INET ,
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and so on, while
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.BR AF_UNIX ,
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|
.BR AF_INET ,
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|
and so on are used for address
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families.
|
|
However, already the BSD man page promises: "The protocol
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|
family generally is the same as the address family", and subsequent
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|
standards use AF_* everywhere.
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.SH EXAMPLES
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|
An example of the use of
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.BR socket ()
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is shown in
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.BR getaddrinfo (3).
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|
.SH SEE ALSO
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|
.BR accept (2),
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.BR bind (2),
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.BR close (2),
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.BR connect (2),
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.BR fcntl (2),
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.BR getpeername (2),
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|
.BR getsockname (2),
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|
.BR getsockopt (2),
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|
.BR ioctl (2),
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.BR listen (2),
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|
.BR read (2),
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.BR recv (2),
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|
.BR select (2),
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|
.BR send (2),
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|
.BR shutdown (2),
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|
.BR socketpair (2),
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.BR write (2),
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.BR getprotoent (3),
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|
.BR address_families (7),
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.BR ip (7),
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.BR socket (7),
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.BR tcp (7),
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.BR udp (7),
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.BR unix (7)
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|
.PP
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|
\(lqAn Introductory 4.3BSD Interprocess Communication Tutorial\(rq
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|
and
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|
\(lqBSD Interprocess Communication Tutorial\(rq,
|
|
reprinted in
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.I UNIX Programmer's Supplementary Documents Volume 1.
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