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       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 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

       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.


       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 protocol is denied.

              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.

              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 variants).


       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 subsequent 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), 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”  and  “BSD  Interprocess
       Communication Tutorial”, reprinted in UNIX Programmer's Supplementary Documents Volume 1.


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