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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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socket(2) System Calls Manual socket(2)
socket - create an endpoint for communication
Standard C library (libc, -lc)
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
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.
On success, a file descriptor for the new socket is returned. On
error, -1 is returned, and errno is set to indicate the error.
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.
POSIX.1-2008.
SOCK_NONBLOCK and SOCK_CLOEXEC are Linux-specific.
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.
An example of the use of socket() is shown in getaddrinfo(3).
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), getprotoent(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.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 socket(2)
Pages that refer to this page: lsfd(1), accept(2), bind(2), bpf(2), connect(2), fcntl(2), getsockname(2), getsockopt(2), io_uring_enter2(2), io_uring_enter(2), listen(2), mknod(2), open(2), recv(2), recvmmsg(2), seccomp_unotify(2), send(2), sendfile(2), sendmmsg(2), shutdown(2), socketcall(2), socketpair(2), syscalls(2), audit_open(3), getaddrinfo(3), getifaddrs(3), getnameinfo(3), if_nameindex(3), if_nametoindex(3), io_uring_prep_socket(3), io_uring_prep_socket_direct(3), io_uring_prep_socket_direct_alloc(3), pcap_set_protocol_linux(3pcap), pmda(3), pmdaconnect(3), sockaddr(3type), systemd.exec(5), address_families(7), ddp(7), ip(7), packet(7), raw(7), sctp(7), signal-safety(7), socket(7), tcp(7), unix(7), vsock(7), x25(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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