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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
       descriptor.

       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
       include:

       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
                       mechanism 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
                       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.

       Since  Linux  2.6.27,  the  type  argument  serves a second purpose: in
       addition 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
       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
       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 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
       correspondents  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
       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
              protocol is denied.

       EAFNOSUPPORT
              The  implementation  does  not  support  the  specified  address
              family.

       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,  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 subsequent
       standards use AF_* everywhere.

       The header file <sys/types.h> is only required for  libc4  or  earlier.
       Some packages, like util-linux, claim portability to all Linux versions
       and libraries.  They certainly need this header file.

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),
       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   4.3BSD  Interprocess  Communication  Tutorial"  is
       reprinted in UNIX Programmer's Supplementary Documents Volume 1.

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

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.