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



NAME
       socket - create an endpoint for communication

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

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

       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  con-
	      sumer is required to read an entire packet with each read system
	      call.

       SOCK_RAW
	      Provides raw network protocol access.

       SOCK_RDM
	      Provides a reliable  datagram  layer  that  does	not  guarantee
	      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;  for
       example, SOCK_SEQPACKET 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 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
       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
       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 pro-
       cess group to receive a SIGURG signal when the out-of-band data arrives
       or  SIGPIPE  signal  when a SOCK_STREAM connection breaks unexpectedly.
       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.

RETURN VALUE
       On success, a file descriptor for  the  new  socket  is	returned.   On
       error, -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.

       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.  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).

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

BUGS
       SOCK_UUCP is not implemented yet.

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),
       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"  is
       reprinted in UNIX Programmer's Supplementary Documents Volume 1.

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



Linux 2.6.7			  2004-06-17			     SOCKET(2)
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