creates an endpoint for communication and returns a descriptor.
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
.In sys/socket.h .
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
which specifies the semantics of communication.
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
type provides sequenced, reliable,
two-way connection based byte streams.
An out-of-band data transmission mechanism may be supported.
datagrams (connectionless, unreliable messages of
a fixed (typically small) maximum length).
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.
sockets provide access to internal network protocols and interfaces.
which is available only to the super-user, and
which is planned,
but not yet implemented, are not described here.
Additionally, the following flags are allowed in the
SOCK_CLOEXEC Set close-on-exec on the new descriptor,
SOCK_NONBLOCK Set non-blocking mode on the new socket
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
in which communication
is to take place; see
argument may be set to zero (0) to request the default
implementation of a socket type for the protocol, if any.
Sockets of type
are full-duplex byte streams, similar
A stream socket must be in a
state before any data may be sent or received
A connection to another socket is created with a
Once connected, data may be transferred using
calls or some variant of the
(Some protocol families, such as the Internet family,
support the notion of an
which permits data to be sent piggybacked onto a connect operation by
When a session has been completed a
may be performed.
Out-of-band data may also be transmitted as described in
and received as described in
The communications protocols used to implement a
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 broken and calls
will indicate an error with
-1 returns and with
as the specific code
in the global variable
The protocols optionally keep sockets
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).
By default, a
signal is raised if a process sends
on a broken stream, but this behavior may be inhibited via
sockets employ the same system calls
The only difference
calls will return only the amount of data requested,
and any remaining in the arriving packet will be discarded.
sockets allow sending of datagrams to correspondents
Datagrams are generally received with
which returns the next datagram with its return address.
system call can be used to specify a process group to receive
signal when the out-of-band data arrives.
It may also enable non-blocking I/O
and asynchronous notification of I/O events
The operation of sockets is controlled by socket level
These options are defined in the file
.In sys/socket.h .
system calls are used to set and get options, respectively.