SOCKET(2)	    UNIX Programmer's Manual		SOCKET(2)


NAME
     socket - create an endpoint for communication

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

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

DESCRIPTION
     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>.
     The currently understood formats are

	  PF_UNIX     (UNIX internal protocols),
	  PF_INET     (ARPA Internet protocols),
	  PF_NS       (Xerox Network Systems protocols), and
	  PF_IMPLINK  (IMP "host at IMP" link layer).

     The socket has the indicated type, which specifies the
     semantics of communication.  Currently defined types are:

	  SOCK_STREAM
	  SOCK_DGRAM
	  SOCK_RAW
	  SOCK_SEQPACKET
	  SOCK_RDM

     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 mes-
     sages 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
     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.


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SOCKET(2)	    UNIX Programmer's Manual		SOCKET(2)


     The protocol specifies a particular protocol to be used with
     the socket.  Normally only a single protocol exists to sup-
     port a particular socket type within a given protocol fam-
     ily.  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 protocols(3N).

     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 con-
     nection 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
     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 signal, to exit.

     SOCK_SEQPACKET sockets employ the same system calls as
     SOCK_STREAM sockets.  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 correspondents 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 notifi-
     cation 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


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SOCKET(2)	    UNIX Programmer's Manual		SOCKET(2)


     set and get options, respectively.

RETURN VALUE
     A -1 is returned if an error occurs, otherwise the return
     value is a descriptor referencing the socket.

ERRORS
     The socket call fails if:

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

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

     [ENFILE]		 The system file table is full.

     [EACCESS]		 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 suf-
			 ficient resources are freed.

SEE ALSO
     accept(2), bind(2), connect(2), getsockname(2), get-
     sockopt(2), ioctl(2), listen(2), read(2), recv(2),
     select(2), send(2), shutdown(2), socketpair(2), write(2)
     ``An Introductory 4.3BSD Interprocess Communication
     Tutorial.'' (reprinted in UNIX Programmer's Supplementary
     Documents Volume 1, PS1:7) ``An Advanced 4.3BSD Interprocess
     Communication Tutorial.'' (reprinted in UNIX Programmer's
     Supplementary Documents Volume 1, PS1:8)


Printed 11/26/99	  May 23, 1986				3


 
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