Ubuntu Manpage: socket - create an endpoint for communication

NAME

       socket - create an endpoint for communication

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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
formats currently understood by the Linux kernel include:

Name Purpose Man page
AF_UNIX Local communication unix(7)
AF_LOCAL Synonym for AF_UNIX
AF_INET IPv4 Internet protocols ip(7)
AF_AX25 Amateur radio AX.25 protocol ax25(4)
AF_IPX IPX - Novell protocols
AF_APPLETALK AppleTalk ddp(7)
AF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
AF_INET6 IPv6 Internet protocols ipv6(7)
AF_DECnet DECet protocol sockets
AF_KEY Key management protocol, originally
developed for usage with IPsec
AF_NETLINK Kernel user interface device netlink(7)
AF_PACKET Low-level packet interface packet(7)
AF_RDS Reliable Datagram Sockets (RDS) protocol rds(7)
rds-rdma(7)
AF_PPPOX Generic PPP transport layer, for setting
up L2 tunnels (L2TP and PPPoE)
AF_LLC Logical link control (IEEE 802.2 LLC)
protocol
AF_IB InfiniBand native addressing
AF_MPLS Multiprotocol Label Switching
AF_CAN Controller Area Network automotive bus
protocol
AF_TIPC TIPC, "cluster domain sockets" protocol
AF_BLUETOOTH Bluetooth low-level socket protocol
AF_ALG Interface to kernel crypto API
AF_VSOCK VSOCK (originally "VMWare VSockets") vsock(7)
protocol for hypervisor-guest
communication
AF_KCM KCM (kernel connection multiplexer)
interface
AF_XDP XDP (express data path) interface

Further details of the above address families, as well as information on several other
address families, can be found in address_families(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 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 open file description (see
open(2)) referred to by the new file descriptor. 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.

RETURN VALUE

       On  success,  a file descriptor for the new socket is returned.  On error, -1 is returned,
       and errno is set to indicate the error.

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.

STANDARDS

       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

       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.

EXAMPLES

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