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-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") protocol for
hypervisor-guest communication |
vsock (7) |
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.1-2001, POSIX.1-2008, 4.4BSD.
The
SOCK_NONBLOCK and
SOCK_CLOEXEC flags are Linux-specific.
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.