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SOCKET(2)		    BSD	System Calls Manual		     SOCKET(2)

     socket -- create an endpoint for communication

     Standard C	Library	(libc, -lc)

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

     socket(int	domain,	int type, int protocol);

     The socket() system call creates an endpoint for communication and	re-
     turns a descriptor.

     The domain	argument specifies a communications domain within which	commu-
     nication 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_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_ATM	   ATM,
	   PF_NETGRAPH	   Netgraph sockets

     The socket	has the	indicated type,	which specifies	the semantics of com-
     munication.  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 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 unimplemented.
     SOCK_RAW sockets provide access to	internal network protocols and inter-
     faces.  The types SOCK_RAW, which is available only to the	super-user,
     and SOCK_RDM, which is planned, but not yet implemented, are not de-
     scribed 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	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) 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 <sys/socket.h>.  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 de-
     scriptor 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

     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.

BSD			       November	24, 1997			   BSD


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