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/IEC 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.1-2008.

       SOCK_NONBLOCK and SOCK_CLOEXEC are Linux-specific.

HISTORY

       POSIX.1-2001, 4.4BSD.

       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).

       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).