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

       socket - create an endpoint for communication

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

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

       socket()	 creates  an endpoint for communication and returns a descrip‐

       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

       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
		       entire 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

       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
       exist, in which case a particular protocol must be  specified  in  this
       manner.	 The  protocol number to use is specific to the “communication
       domain” 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

       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
       incoming 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
       address 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
       arrives	or  SIGPIPE  signal when a SOCK_STREAM connection breaks unex‐
       pectedly.  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
       error; 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.

       On success, a file descriptor for  the  new  socket  is	returned.   On
       error, -1 is returned, and errno is set appropriately.

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

	      The implementation does not support the specified	 address  fam‐

       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

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

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

       Other errors may be generated by the underlying protocol modules.

       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

       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.

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

       accept(2),  bind(2),  connect(2),  fcntl(2),  getpeername(2),  getsock‐
       name(2),	 getsockopt(2),	  ioctl(2),   listen(2),   read(2),   recv(2),
       select(2),   send(2),  shutdown(2),  socketpair(2),  write(2),  getpro‐
       toent(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.

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2013-12-31			     SOCKET(2)

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