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SOCKET(2)		  FreeBSD System Calls Manual		     SOCKET(2)

     socket -- create an endpoint for communication

     #include <sys/types.h>
     #include <sys/socket.h>

     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 com-
     munication	will take place; this selects the protocol family which	should
     be	used.  These families are defined in the include file <sys/socket.h>.
     The currently understood formats are

	   PF_LOCAL	   (Host-internal protocols, formerly called PF_UNIX),
	   PF_INET	   (ARPA Internet protocols),
	   PF_ISO	   (ISO	protocols),
	   PF_CCITT	   (ITU-T protocols, like X.25),
	   PF_NS	   (Xerox Network Systems protocols), and

     The socket	has the	indicated type,	which specifies	the semantics of com-
     munication.  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 sup-
     ported.  A	SOCK_DGRAM socket supports datagrams (connectionless, unreli-
     able messages of a	fixed (typically small)	maximum	length).  A
     SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connec-
     tion-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 implemented only
     for PF_NS.	 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 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 speci-
     fied 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) 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.  (Some protocol families, such as the Internet family, support the
     notion of an ``implied connect,'' which permits data to be	sent piggy-
     backed onto a connect operation by	using the sendto(2) 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 a 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 sig-
     nal, to exit.

     SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM	sock-
     ets.  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.

     SOCK_DGRAM	and SOCK_RAW sockets allow sending of datagrams	to correspon-
     dents named in send(2) calls.  Datagrams are generally received with
     recvfrom(2), 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 via SIGIO.

     The operation of sockets is controlled by socket level options.  These
     options are defined in the	file <sys/socket.h>.  Setsockopt(2) and
     getsockopt(2) 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() 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

     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), protocols(5)

     "An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.

     "BSD Interprocess Communication Tutorial",	PS1, 8.

     The socket() function call	appeared in 4.2BSD.

4.2 Berkeley Distribution      November	24, 1997     4.2 Berkeley Distribution


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