socket - create an endpoint for communication
/* See NOTES */
int socket(int domain, int type, int
() 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.
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
|AF_UNIX ", " AF_LOCAL
||IPv4 Internet protocols
||IPv6 Internet protocols
||IPX - Novell protocols
||Kernel user interface device
||ITU-T X.25 / ISO-8208 protocol
||Amateur radio AX.25 protocol
||Access to raw ATM PVCs
||Low level packet interface
||Interface to kernel crypto API
The socket has the indicated type
, which specifies the communication
semantics. Currently defined types are:
- Provides sequenced, reliable, two-way, connection-based
byte streams. An out-of-band data transmission mechanism may be
- Supports datagrams (connectionless, unreliable messages of
a fixed maximum length).
- 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.
- Provides raw network protocol access.
- Provides a reliable datagram layer that does not guarantee
- Obsolete and should not be used in new programs; see
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
- 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.
- 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.
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)
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)
calls or some variant of
calls. When a session has been completed
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
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
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.
sockets employ the same system calls as
sockets. The only difference is that read(2)
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.
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.
is an obsolete socket type to receive raw packets directly
from the device driver. Use packet(7)
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
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
. Using F_SETOWN
is equivalent to an ioctl(2)
with the FIOSETOWN
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
The operation of sockets is controlled by socket level options
options are defined in <sys/socket.h>
. The functions
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 appropriately.
- Permission to create a socket of the specified type and/or
protocol is denied.
- The implementation does not support the specified address
- Unknown protocol, or protocol family not available.
- Invalid flags in type.
- The per-process limit on the number of open file
descriptors has been reached.
- The system-wide limit on the total number of open files has
- ENOBUFS or ENOMEM
- Insufficient memory is available. The socket cannot be
created until sufficient resources are freed.
- The protocol type or the specified protocol is not
supported within this domain.
Other errors may be generated by the underlying protocol modules.
POSIX.1-2001, POSIX.1-2008, 4.4BSD.
flags are Linux-specific.
() appeared in 4.2BSD. It is generally portable to/from non-BSD
systems supporting clones of the BSD socket layer (including System V
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
, and so on, while AF_UNIX
, 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.
protocol type was added in Linux 2.6.38. More information on
this interface is provided with the kernel HTML documentation at
An example of the use of socket
() is shown in getaddrinfo(3)
“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 release 4.13 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at