socket - create an endpoint for communication
/* See NOTES */
int socket(int domain, int type, int
() creates an endpoint for communication and returns a descriptor.
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
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 supported.
- Supports datagrams (connectionless, unreliable messages of a fixed maximum
- 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 ordering.
- 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, similar to
pipes. They do not preserve record boundaries. A stream socket must be in a
state before any data may be sent or received on it. A
connection to another socket is created with a connect(2)
connected, data may be transferred using read(2)
calls or some variant of the send(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
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
a 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
- The implementation does not support the specified address family.
- Unknown protocol, or protocol family not available.
- Invalid flags in type.
- Process file table overflow.
- The system 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.
- The protocol type or the specified protocol is not supported within this
Other errors may be generated by the underlying protocol modules.
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.1-2001 does not require the inclusion of <sys/types.h>
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.
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.
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