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

     socket -- create an endpoint for communication

     #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:

	   AF_UNIX	UNIX internal protocols
	   AF_INET	Internet Protocol version 4 (IPv4) protocol family
	   AF_INET6	Internet Protocol version 6 (IPv6) protocol family

     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 AF_UNIX.  SOCK_RAW sockets provide access to internal network proto-
     cols and interfaces, and are available only to the	superuser.

     Any combination of	the following flags may	additionally be	used in	the
     type argument:

	   SOCK_CLOEXEC	   Set close-on-exec flag on the new descriptor.
	   SOCK_NONBLOCK   Set non-blocking I/O	mode on	the new	socket.
	   SOCK_DNS	   For domains AF_INET or AF_INET6, only allow
			   connect(2), sendto(2), or sendmsg(2)	to the DNS
			   port	(typically 53).

     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
     protocols(5).  A value of 0 for protocol will let the system select an
     appropriate protocol for the requested socket type.

     Sockets of	type SOCK_STREAM are full-duplex byte streams.	A stream
     socket must be in a connected state before	any data may be	sent or	re-
     ceived 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.
     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 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 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) 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.

     If	successful, socket() returns a non-negative integer, the socket	file
     descriptor.  Otherwise, a value of	-1 is returned and errno is set	to in-
     dicate the	error.

     The socket() call fails if:

     [EAFNOSUPPORT]	The specified address family is	not supported on this

     [EPROTONOSUPPORT]	The protocol type or the specified protocol is not
			supported within this domain.

     [EPROTOTYPE]	The combination	of the specified protocol and type is
			not supported.

     [EMFILE]		The per-process	descriptor table is full.

     [ENFILE]		The system file	table is full.

     [ENOBUFS]		Insufficient resources were available in the system to
			perform	the operation.

     [EACCES]		Permission to create a socket of the specified type
			and/or protocol	is denied.

     accept(2),	bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2),
     listen(2),	poll(2), read(2), recv(2), select(2), send(2), setsockopt(2),
     shutdown(2), socketpair(2), write(2), getprotoent(3), inet(4), inet6(4),
     netintro(4), unix(4)

     An	Introductory 4.3 BSD Interprocess Communication	Tutorial, reprinted in
     UNIX Programmer's Supplementary Documents Volume 1.

     BSD Interprocess Communication Tutorial, reprinted	in UNIX	Programmer's
     Supplementary Documents Volume 1.

     The socket() function conforms to IEEE Std	1003.1-2008 ("POSIX.1").  The
     SOCK_CLOEXEC and SOCK_NONBLOCK flags are expected to conform to a future
     revision of that standard.

     The SOCK_DNS flag is an OpenBSD extension.

     The socket() system call first appeared in	4.1cBSD.  Support for the
     SOCK_CLOEXEC and SOCK_NONBLOCK flags appeared in OpenBSD 5.7.  Support
     for the SOCK_DNS flag appeared in OpenBSD 5.9.

FreeBSD	13.0			 May 26, 2019			  FreeBSD 13.0


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