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 communications domain within which
communication will take place; this selects the protocol family which
should be used. The protocol family generally is the same as the address
family for the addresses supplied in later operations on the socket.
These families are defined in the include file sys/socket.h. There must
be an entry in the netconfig(4) file for at least each protocol family
and type required. If protocol has been specified, but no exact match
for the tuplet family, type, protocol is found, then the first entry
containing the specified family and type with zero for protocol will be
used. The currently understood formats are:
PF_UNIX UNIX system internal protocols
PF_INET ARPA Internet protocols
The socket has the indicated type, which specifies the communication
semantics. Currently defined types are:
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 not
implemented for any protocol family. SOCK_RAW sockets provide access to
internal network interfaces. The types SOCK_RAW, which is available only
to a privileged user, and SOCK_RDM, for which no implementation currently
exists, are not described here.
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, multiple 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. If a protocol is
specified by the caller, then it will be packaged into a socket level
option request and sent to the underlying protocol layers.
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(3N) call. Once connected, data may be transferred using
read(2) and write(2) calls or some variant of the send(3N) and recv(3N)
calls. When a session has been completed, a close(2) may be performed.
Out-of-band data may also be transmitted as described on the send(3N)
manual page and received as described on the recv(3N) manual page.
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 a extended period (for
instance 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 exit.
SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM
sockets. The only difference is that read calls will return only the
amount of data requested, and any remaining in the arriving packet will
SOCK_DGRAM and SOCK_RAW sockets allow datagrams to be sent to
correspondents named in sendto calls. Datagrams are generally received
with recvfrom, which returns the next datagram with its return address.
An fcntl(2) 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 with SIGIO
The operation of sockets is controlled by socket level options. These
options are defined in the file /usr/include/sys/socket.h.
A -1 is returned if an error occurs. Otherwise the return value is a
descriptor referencing the socket.
The socket() 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.
EACCESS Permission to create a socket of the specified type
and/or protocol is denied.
ENOMEM Insufficient user memory is available.
ENOSR There were insufficient STREAMS resources available
to complete the operation.
close(2), fcntl(2), ioctl(2), read(2), write(2), accept(3N), bind(3N),
connect(3N), getsockname(3N), getsockopt(3N), listen(3N), recv(3N),
send(3N), shutdown(3N), socketpair(3N)
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