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SOCKET

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 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.

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 include:

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)
AF_ALG	Interface to kernel crypto API

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 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.

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. 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 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 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 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.

RETURN VALUE

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 protocol is denied.
EAFNOSUPPORT: The implementation does not support the specified address family.
EINVAL: Unknown protocol, or protocol family not available.
EINVAL: Invalid flags in type.
EMFILE: The per-process limit on the number of open file descriptors has been reached.
ENFILE: The system-wide 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.

CONFORMING TO

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

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 variants).

NOTES

POSIX.1 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, and so on, while AF_UNIX, 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.

The AF_ALG protocol type was added in Linux 2.6.38. More information on this interface is provided with the kernel HTML documentation at https://www.kernel.org/doc/htmldocs/crypto-API/User.html.

EXAMPLE

An example of the use of socket() is shown in getaddrinfo(3).

COLOPHON

This page is part of release 4.13 of the Linux 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/.