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

       socket - create an endpoint for communication

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

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

       Socket creates an endpoint for communication and returns a descriptor.

       The  domain  parameter  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 include:

       Name		  Purpose			   Man page
       PF_UNIX,PF_LOCAL   Local communication		   unix(7)
       PF_INET		  IPv4 Internet protocols	   ip(7)
       PF_INET6 	  IPv6 Internet protocols
       PF_IPX		  IPX - Novell protocols
       PF_NETLINK	  Kernel user interface device	   netlink(7)
       PF_X25		  ITU-T X.25 / ISO-8208 protocol   x25(7)
       PF_AX25		  Amateur radio AX.25 protocol
       PF_ATMPVC	  Access to raw ATM PVCs
       PF_APPLETALK	  Appletalk			   ddp(7)
       PF_PACKET	  Low level packet interface	   packet(7)

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

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

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

	      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 read system call.

	      Provides raw network protocol access.

	      Provides a reliable datagram layer that does not guarantee ordering.

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

       Some  socket types may not be implemented by all protocol families; for example, SOCK_SEQ-
       PACKET is not implemented for AF_INET.

       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 a case protocol can be specified as 0.	However, it is possible that many  proto-
       cols 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  con-
       nect(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  con-
       sidered	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 SIGPIPE 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 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 correspondents named in
       send(2) calls.  Datagrams are generally received with recvfrom(2), which returns the  next
       datagram with its return address.

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

       An fcntl(2) call with the the F_SETOWN argument can be used to specify a process group  to
       receive	a  SIGURG  signal  when  the  out-of-band  data  arrives or SIGPIPE signal when a
       SOCK_STREAM connection breaks unexpectedly.  It 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.

       -1 is returned if an error occurs; otherwise the return value is a descriptor  referencing
       the socket.

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

	      The implementation does not support the specified address family.

       ENFILE Not enough kernel memory to allocate a new socket structure.

       EMFILE Process file table overflow.

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

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

       EINVAL Unknown protocol, or protocol family not available.

       Other errors may be generated by the underlying protocol modules.

       4.4BSD (the socket function call appeared in 4.2BSD). Generally portable  to/from  non-BSD
       systems supporting clones of the BSD socket layer (including System V variants).

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

       SOCK_UUCP is not implemented yet.

       accept(2), bind(2), connect(2), fcntl(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), ip(7), socket(7), tcp(7), udp(7), unix(7)

       "An Introductory 4.3 BSD Interprocess Communication Tutorial" is reprinted  in  UNIX  Pro-
       grammer's Supplementary Documents Volume 1.

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

Linux Man Page				    1999-04-24					SOCKET(2)
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