socket - create an endpoint for communication
int socket(int domain, int type, int protocol);
Socket creates an endpoint for communication and returns a descriptor.
The domain parameter 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
Name Purpose Man page
PF_UNIX,PF_LOCAL Local communication unix(7)
PF_INET IPv4 Internet protocols ip(7)
PF_INET6 IPv6 Internet protocols
PF_IPX IPX - Novell protocols
PF_NETLINK Kernel user interface device netlink(7)
PF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
PF_AX25 Amateur radio AX.25 protocol
PF_ATMPVC Access to raw ATM PVCs
PF_APPLETALK Appletalk ddp(7)
PF_PACKET Low level packet interface packet(7)
The socket has the indicated type, which specifies the communication
semantics. Currently defined types are:
SOCK_STREAM [Toc] [Back]
Provides sequenced, reliable, two-way, connection-based byte
streams. An out-of-band data transmission mechanism may be supported.
SOCK_DGRAM [Toc] [Back]
Supports datagrams (connectionless, unreliable messages of a
fixed maximum length).
SOCK_SEQPACKET [Toc] [Back]
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 read system
SOCK_RAW [Toc] [Back]
Provides raw network protocol access.
SOCK_RDM [Toc] [Back]
Provides a reliable datagram layer that does not guarantee
SOCK_PACKET [Toc] [Back]
Obsolete and should not be used in new programs; see packet(7).
Some socket types may not be implemented by all protocol families; for
example, SOCK_SEQPACKET is not implemented for AF_INET.
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. 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, similar to
pipes. 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
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 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 send(2) calls. Datagrams are generally received
with recvfrom(2), which returns the next datagram with its return
SOCK_PACKET is an obsolete socket type to receive raw packets directly
from the device driver. Use packet(7) instead.
An fcntl(2) call with the the F_SETOWN argument can be used to specify
a process group to receive a SIGURG signal when the out-of-band data
arrives or SIGPIPE signal when a SOCK_STREAM connection breaks unexpectedly.
It 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 SIOSETOWN
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 in ip(7).
The operation of sockets is controlled by socket level options. These
options are defined in <sys/socket.h>. Setsockopt(2) and getsockopt(2)
are used to set and get options, respectively.
-1 is returned if an error occurs; otherwise the return value is a
descriptor referencing the socket.
The protocol type or the specified protocol is not supported
within this domain.
ENFILE Not enough kernel memory to allocate a new socket structure.
EMFILE Process file table overflow.
EACCES Permission to create a socket of the specified type and/or protocol
ENOBUFS or ENOMEM
Insufficient memory is available. The socket cannot be created
until sufficient resources are freed.
EINVAL Unknown protocol, or protocol family not available.
Other errors may be generated by the underlying protocol modules.
4.4BSD (the socket function call appeared in 4.2BSD). Generally portable
to/from non-BSD systems supporting clones of the BSD socket layer
(including System V variants).
The manifest constants used under BSD 4.* for protocol families are
PF_UNIX, PF_INET, etc., while AF_UNIX etc. 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.
SOCK_UUCP is not implemented yet.
accept(2), bind(2), connect(2), getprotoent(3), getsockname(2), get-
sockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
shutdown(2), socketpair(2), write(2)
"An Introductory 4.3 BSD Interprocess Communication Tutorial" is
reprinted in UNIX Programmer's Supplementary Documents Volume 1.
"BSD Interprocess Communication Tutorial" is reprinted in UNIX Program-
mer's Supplementary Documents Volume 1.
Linux Man Page 1999-04-24 SOCKET(2)
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