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socket - create an endpoint for communication
#include <sys/types.h> /* See NOTES */
int socket(int domain, int type, int protocol);
socket() creates an endpoint for communication and returns a
The domain argument 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
Name Purpose Man page
AF_UNIX, AF_LOCAL Local communication unix(7)
AF_INET IPv4 Internet protocols ip(7)
AF_INET6 IPv6 Internet protocols ipv6(7)
AF_IPX IPX - Novell protocols
AF_NETLINK Kernel user interface device netlink(7)
AF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
AF_AX25 Amateur radio AX.25 protocol
AF_ATMPVC Access to raw ATM PVCs
AF_APPLETALK Appletalk ddp(7)
AF_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 supported.
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 consumer is required to read an
entire packet with each input system call.
SOCK_RAW Provides raw network protocol access.
SOCK_RDM Provides a reliable datagram layer that does not
SOCK_PACKET Obsolete and should not be used in new programs; see
Some socket types may not be implemented by all protocol families; for
example, SOCK_SEQPACKET is not implemented for AF_INET.
Since Linux 2.6.27, the 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 socket():
SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the new open
file description. Using this flag saves extra calls to
fcntl(2) to achieve the same result.
SOCK_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag
in open(2) for reasons why this may be useful.
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
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
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. 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
correspondents 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
process 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.
EACCES Permission to create a socket of the specified type and/or
protocol is denied.
The implementation does not support the specified address
EINVAL Unknown protocol, or protocol family not available.
EINVAL Invalid flags in type.
EMFILE Process file table overflow.
ENFILE The system limit on the total number of open files has been
ENOBUFS or ENOMEM
Insufficient memory is available. The socket cannot be created
until sufficient resources are freed.
The protocol type or the specified protocol is not supported
within this domain.
Other errors may be generated by the underlying protocol modules.
The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.
socket() appeared in 4.2BSD. It is generally portable to/from non-BSD
systems supporting clones of the BSD socket layer (including System V
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
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.
An example of the use of socket() is shown in getaddrinfo(3).
accept(2), bind(2), connect(2), fcntl(2), getpeername(2),
getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2),
select(2), send(2), shutdown(2), socketpair(2), write(2),
getprotoent(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
Programmer's Supplementary Documents Volume 1.
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