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socket - create an endpoint for communication
Standard C Library (libc, -lc)
socket(int domain, int type, int protocol);
The socket() system call creates an endpoint for communication and
returns a descriptor.
The domain argument specifies a communications domain within which
communication will take place; this selects the protocol family which
should be used. These families are defined in the include file The
currently understood formats are:
PF_LOCAL Host-internal protocols, formerly called PF_UNIX,
PF_UNIX Host-internal protocols, deprecated, use PF_LOCAL,
PF_INET Internet version 4 protocols,
PF_PUP PUP protocols, like BSP,
PF_APPLETALK AppleTalk protocols,
PF_ROUTE Internal Routing protocol,
PF_LINK Link layer interface,
PF_IPX Novell Internet Packet eXchange protocol,
PF_RTIP Help Identify RTIP packets,
PF_PIP Help Identify PIP packets,
PF_ISDN Integrated Services Digital Network,
PF_KEY Internal key-management function,
PF_INET6 Internet version 6 protocols,
PF_NATM Native ATM access,
PF_NETGRAPH Netgraph sockets
The socket has the indicated type, which specifies the semantics of
communication. Currently defined types are:
SOCK_STREAM Stream socket,
SOCK_DGRAM Datagram socket,
SOCK_RAW Raw-protocol interface,
SOCK_RDM Reliably-delivered packet,
SOCK_SEQPACKET Sequenced packet stream
A SOCK_STREAM type provides sequenced, reliable, two-way connection based
byte streams. An out-of-band data transmission mechanism may be
supported. A SOCK_DGRAM socket supports datagrams (connectionless,
unreliable messages of a fixed (typically small) maximum length). A
SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way
connection-based data transmission path for datagrams of fixed maximum
length; a consumer may be required to read an entire packet with each
read system call. This facility is protocol specific, and presently
unimplemented. SOCK_RAW sockets provide access to internal network
protocols and interfaces. The types SOCK_RAW, which is available only to
the super-user, and SOCK_RDM, which is planned, but not yet implemented,
are not described here.
The protocol argument 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 particular to
the “communication domain” in which communication is to take place; see
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
pipes. 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) system call. Once connected, data may be transferred
using read(2) and write(2) calls or some variant of the send(2) and
recv(2) functions. (Some protocol families, such as the Internet family,
support the notion of an “implied connect”, which permits data to be sent
piggybacked onto a connect operation by using the sendto(2) system call.)
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 used to implement a SOCK_STREAM insure 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 broken and
calls will indicate an error with -1 returns and with ETIMEDOUT as the
specific code in the global variable errno. The protocols optionally
keep sockets “warm” by forcing transmissions roughly every minute in the
absence of other activity. An error is then indicated if no response can
be elicited on an otherwise idle connection for an extended period (e.g.
5 minutes). A SIGPIPE signal is raised if a process sends on a broken
stream; this causes naive processes, which do not handle the signal, to
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
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
An fcntl(2) system call can be used to specify a process group to receive
a SIGURG signal when the out-of-band data arrives. It may also enable
non-blocking I/O and asynchronous notification of I/O events via SIGIO.
The operation of sockets is controlled by socket level options. These
options are defined in the file The setsockopt(2) and getsockopt(2)
system calls are used to set and get options, respectively.
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
The socket() system call fails if:
[EPROTONOSUPPORT] The protocol type or the specified protocol is not
supported within this domain.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[EACCES] Permission to create a socket of the specified type
and/or protocol is denied.
[ENOBUFS] Insufficient buffer space is available. The socket
cannot be created until sufficient resources are
[EPERM] User has insufficient privileges to carry out the
accept(2), bind(2), connect(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), netgraph(4),
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"BSD Interprocess Communication Tutorial", PS1, 8.
The socket() system call appeared in 4.2BSD.