socket(2) System Calls Manual socket(2)
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 de-
scriptor that refers to that endpoint. The file descriptor returned by
a successful call will be the lowest-numbered file descriptor not cur-
rently 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 de-
veloped 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 communica-
tion
AF_KCM KCM (kernel connection multiplexer) in-
terface
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 mecha-
nism 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 en-
tire packet with each input system call.
SOCK_RAW Provides raw network protocol access.
SOCK_RDM Provides a reliable datagram layer that does not guar-
antee 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 addi-
tion 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 fc-
ntl(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 ex-
ist, in which case a particular protocol must be specified in this man-
ner. The protocol number to use is specific to the “communication do-
main” 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 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 in-
coming 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 ad-
dress 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 ar-
rives or SIGPIPE signal when a SOCK_STREAM connection breaks unexpect-
edly. 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 er-
ror; 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 er-
ror, -1 is returned, and errno is set to indicate the error.
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.
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 sub-
sequent standards use AF_* everywhere.
EXAMPLES
An example of the use of socket() is shown in getaddrinfo(3).
SEE ALSO
accept(2), bind(2), close(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), getpro-
toent(3), address_families(7), ip(7), socket(7), tcp(7), udp(7),
unix(7)
“An Introductory 4.3BSD Interprocess Communication Tutorial” and “BSD
Interprocess Communication Tutorial”, reprinted in UNIX Programmer's
Supplementary Documents Volume 1.
Linux man-pages 6.7 2024-01-28 socket(2)
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