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

NAME
       socket -	create an endpoint for communication

SYNOPSIS
       #include	<sys/types.h>	       /* See NOTES */
       #include	<sys/socket.h>

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

DESCRIPTION
       socket()	 creates  an endpoint for communication	and returns a descrip-
       tor.

       The domain argument specifies a communication domain; this selects  the
       protocol	 family	 which will be used for	communication.	These families
       are defined in _sys/socket.h_.  The currently  understood  formats  in-
       clude:

       Name		   Purpose			    Man	page
       AF_UNIX,	AF_LOCAL   Local communication		    unix(7)
       AF_INET		   IPv4	Internet protocols	    ip(7)
       AF_INET6		   IPv6	Internet protocols	    ipv6(7)
       AF_IPX		   IPX - Novell	protocols
       AF_NETLINK	   Kernel user interface device	    netlink(7)
       AF_X25		   ITU-T X.25 /	ISO-8208 protocol   x25(7)
       AF_AX25		   Amateur radio AX.25 protocol
       AF_ATMPVC	   Access to raw ATM PVCs
       AF_APPLETALK	   AppleTalk			    ddp(7)
       AF_PACKET	   Low level packet interface	    packet(7)

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

       SOCK_STREAM     Provides	sequenced, reliable, two-way, connection-based
		       byte  streams.  An out-of-band data transmission	mecha-
		       nism may	be supported.

       SOCK_DGRAM      Supports	datagrams (connectionless, unreliable messages
		       of a fixed maximum length).

       SOCK_SEQPACKET  Provides	 a  sequenced,	reliable,  two-way connection-
		       based data transmission path  for  datagrams  of	 fixed
		       maximum	length;	 a consumer is required	to read	an en-
		       tire packet with	each input system call.

       SOCK_RAW	       Provides	raw network protocol access.

       SOCK_RDM	       Provides	a reliable datagram layer that does not	 guar-
		       antee ordering.

       SOCK_PACKET     Obsolete	 and  should  not be used in new programs; see
		       packet(7).

       Some socket types may not be implemented	by all protocol	families.

       Since Linux 2.6.27, the type argument serves a second purpose: in addi-
       tion  to	specifying a socket type, it may include the bitwise OR	of any
       of the following	values,	to modify the behavior of socket():

       SOCK_NONBLOCK   Set the O_NONBLOCK file status flag  on	the  new  open
		       file description.  Using	this flag saves	extra calls to
		       fcntl(2)	to achieve the same result.

       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file
		       descriptor.   See the description of the	O_CLOEXEC flag
		       in open(2) for reasons why this may be useful.

       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, in which case protocol  can
       be specified as 0.  However, it is possible that	many protocols may ex-
       ist, in which case a particular protocol	must be	specified in this man-
       ner.   The protocol number to use is specific to	the "communication do-
       main" in	which communication is to take place; see  protocols(5).   See
       getprotoent(3) on how to	map protocol name strings to protocol numbers.

       Sockets	of  type  SOCK_STREAM are full-duplex byte streams, similar to
       pipes.  They do not preserve record boundaries.	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.  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 which 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	to  be
       dead.   When  SO_KEEPALIVE is enabled on	the socket the protocol	checks
       in a protocol-specific manner if	the other end is still alive.  A  SIG-
       PIPE  signal  is	 raised	 if  a	process	 sends or receives 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 data remaining	in the
       arriving	packet will be discarded.  Also	all message boundaries in  in-
       coming datagrams	are preserved.

       SOCK_DGRAM  and	SOCK_RAW  sockets allow	sending	of datagrams to	corre-
       spondents named in sendto(2) calls.  Datagrams are  generally  received
       with  recvfrom(2),  which  returns the next datagram along with the ad-
       dress of	its sender.

       SOCK_PACKET is an obsolete socket type to receive raw packets  directly
       from the	device driver.	Use packet(7) instead.

       An  fcntl(2)  F_SETOWN  operation  can  be used to specify a process or
       process group to	receive	a SIGURG signal	when the out-of-band data  ar-
       rives  or SIGPIPE signal	when a SOCK_STREAM connection breaks unexpect-
       edly.  This operation may also be used to set the  process  or  process
       group that receives the I/O and asynchronous notification of I/O	events
       via SIGIO.  Using F_SETOWN is equivalent	to an ioctl(2) call  with  the
       FIOSETOWN or SIOCSPGRP argument.

       When  the  network  signals  an	error condition	to the protocol	module
       (e.g., using a ICMP message for IP) the pending error flag is  set  for
       the  socket.   The  next	operation on this socket will return the error
       code of the pending error.  For some protocols it is possible to	enable
       a per-socket error queue	to retrieve detailed information about the er-
       ror; see	IP_RECVERR in ip(7).

       The operation of	sockets	is controlled by socket	level options.	 These
       options are defined in _sys/socket.h_.  The functions setsockopt(2) and
       getsockopt(2) are used to set and get options, respectively.

RETURN VALUE
       On success, a file descriptor for the new socket	is returned.   On  er-
       ror, -1 is returned, and	errno is set appropriately.

ERRORS
       EACCES Permission  to create a socket of	the specified type and/or pro-
	      tocol is denied.

       EAFNOSUPPORT
	      The implementation does not support the specified	 address  fam-
	      ily.

       EINVAL Unknown protocol,	or protocol family not available.

       EINVAL Invalid flags in type.

       EMFILE Process file table overflow.

       ENFILE The  system  limit  on  the  total number	of open	files has been
	      reached.

       ENOBUFS or ENOMEM
	      Insufficient memory is available.	 The socket cannot be  created
	      until sufficient resources are freed.

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

       Other errors may	be generated by	the underlying protocol	modules.

CONFORMING TO
       4.4BSD, POSIX.1-2001.

       The SOCK_NONBLOCK and SOCK_CLOEXEC flags	are Linux-specific.

       socket()	appeared in 4.2BSD.  It	is generally portable to/from  non-BSD
       systems	supporting  clones of the BSD socket layer (including System V
       variants).

NOTES
       POSIX.1-2001 does not require the inclusion of _sys/types.h_, and  this
       header  file  is	not required on	Linux.	However, some historical (BSD)
       implementations required	this header file,  and	portable  applications
       are probably wise to include it.

       The  manifest  constants	 used  under 4.x BSD for protocol families are
       PF_UNIX,	PF_INET, and so	on, while AF_UNIX, AF_INET, and	so on are used
       for address families.  However, already the BSD man page	promises: "The
       protocol	family generally is the	same as	the address family", and  sub-
       sequent standards use AF_* everywhere.

EXAMPLE
       An example of the use of	socket() is shown in getaddrinfo(3).

SEE ALSO
       accept(2),  bind(2),  connect(2),  fcntl(2),  getpeername(2),  getsock-
       name(2),	getsockopt(2),	ioctl(2),  listen(2),  read(2),	 recv(2),  se-
       lect(2),	send(2), shutdown(2), socketpair(2), write(2), getprotoent(3),
       ip(7), socket(7), tcp(7), udp(7), unix(7)

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

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2013-12-31			     SOCKET(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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