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NAME
socket - create an endpoint for communication
SYNOPSIS
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
DESCRIPTION
socket() creates an endpoint for communication and returns a descrip‐
tor.
The domain parameter specifies a communication domain; this selects the
protocol family which will be used for communication. These families
are defined in <sys/socket.h>. The currently understood formats
include:
Name Purpose Man page
PF_UNIX, PF_LOCAL Local communication unix(7)
PF_INET IPv4 Internet protocols ip(7)
PF_INET6 IPv6 Internet protocols ipv6(7)
PF_IPX IPX - Novell protocols
PF_NETLINK Kernel user interface device netlink(7)
PF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
PF_AX25 Amateur radio AX.25 protocol
PF_ATMPVC Access to raw ATM PVCs
PF_APPLETALK Appletalk ddp(7)
PF_PACKET Low level packet interface packet(7)
The socket has the indicated type, which specifies the communication
semantics. Currently defined types are:
SOCK_STREAM
Provides sequenced, reliable, two-way, connection-based byte
streams. An out-of-band data transmission mechanism may be sup‐
ported.
SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages of a
fixed maximum length).
SOCK_SEQPACKET
Provides a sequenced, reliable, two-way connection-based data
transmission path for datagrams of fixed maximum length; a con‐
sumer is required to read an entire packet with each read system
call.
SOCK_RAW
Provides raw network protocol access.
SOCK_RDM
Provides a reliable datagram layer that does not guarantee
ordering.
SOCK_PACKET
Obsolete and should not be used in new programs; see packet(7).
Some socket types may not be implemented by all protocol families; for
example, SOCK_SEQPACKET is not implemented for AF_INET.
The 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 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 “communication
domain” in which communication is to take place; see protocols(5). See
getprotoent(3) on how to map protocol name strings to protocol numbers.
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
pipes. They do not preserve record boundaries. A stream socket must
be in a connected state before any data may be sent or received on it.
A connection to another socket is created with a connect(2) call. Once
connected, data may be transferred using read(2) and write(2) calls or
some variant of the send(2) and recv(2) calls. When a session has been
completed a close(2) may be performed. Out-of-band data may also be
transmitted as described in send(2) and received as described in
recv(2).
The communications protocols which implement a 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 SO_KEEPALIVE is enabled on the socket the protocol checks
in a protocol-specific manner if the other end is still alive. A SIG
PIPE 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. SOCK_SEQPACKET sockets employ the same system calls as
SOCK_STREAM sockets. The only difference is that 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.
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to corre‐
spondents named in sendto(2) calls. Datagrams are generally received
with recvfrom(2), which returns the next datagram along with the
address of its sender.
SOCK_PACKET is an obsolete socket type to receive raw packets directly
from the device driver. Use packet(7) instead.
An fcntl(2) F_SETOWN operation can be used to specify a process or pro‐
cess group to receive a SIGURG signal when the out-of-band data arrives
or SIGPIPE signal when a 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
SIGIO. Using F_SETOWN is equivalent to an ioctl(2) call with the
FIOSETOWN or SIOCSPGRP argument.
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 IP_RECVERR in ip(7).
The operation of sockets is controlled by socket level options. These
options are defined in <sys/socket.h>. The functions setsockopt(2) and
getsockopt(2) are used to set and get options, respectively.
On success, a file descriptor for the new socket is returned. On
error, -1 is returned, and errno is set appropriately.
ERRORS
EACCES Permission to create a socket of the specified type and/or pro‐
tocol is denied.
EAFNOSUPPORT
The implementation does not support the specified address fam‐
ily.
EINVAL Unknown protocol, or protocol family not available.
EMFILE Process file table overflow.
ENFILE The system limit on the total number of open files has been
reached.
ENOBUFS or ENOMEM
Insufficient memory is available. The socket cannot be created
until sufficient resources are freed.
EPROTONOSUPPORT
The protocol type or the specified protocol is not supported
within this domain.
Other errors may be generated by the underlying protocol modules.
4.4BSD, POSIX.1-2001. 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).
NOTES
POSIX.1-2001 does not require the inclusion of <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
PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for address fami‐
lies. 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 <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.
BUGS
SOCK_UUCP is not implemented yet.
EXAMPLE
An example of the use of socket() is shown in getaddrinfo(3).
accept(2), bind(2), connect(2), fcntl(2), getpeername(2), getsock
name(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2),
select(2), send(2), shutdown(2), socketpair(2), write(2), getpro
toent(3), ip(7), socket(7), tcp(7), udp(7), unix(7)
“An Introductory 4.3BSD Interprocess Communication Tutorial” is
reprinted in UNIX Programmer’s Supplementary Documents Volume 1.
“BSD Interprocess Communication Tutorial” is reprinted in UNIX Program‐
mer’s Supplementary Documents Volume 1.