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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 Programmers Supplementary Documents Volume 1.

       “BSD   Interprocess   Communication  Tutorial”  is  reprinted  in  UNIX
       Programmers Supplementary Documents Volume 1.

COLOPHON

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