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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
     returns 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
     described 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

     The protocol argument may be set to zero (0) to request the default
     implementation of a socket type for the protocol, if any.

     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 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).  By default, a SIGPIPE signal is raised if a process sends on
     a broken stream, but this behavior may be inhibited via setsockopt(2).

     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
     descriptor 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

     [EPERM]		User has insufficient privileges to carry out the
			requested operation.

     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				January 5, 2009				   BSD

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