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SOCKET(2)                     System Calls Manual                    SOCKET(2)

       socket - create an endpoint for communication

       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/socket.h>&gt;

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

       socket()  creates  an endpoint for communication and returns a descrip-

       The domain parameter specifies a  communications  domain  within  which
       communication  will  take place; this selects the protocol family which
       should be used.  The protocol family  generally  is  the  same  as  the
       address  family  for  the addresses supplied in later operations on the
       socket.  These families are defined in the include file <&lt;sys/socket.h>&gt;.
       The currently understood formats are

              PF_UNIX             (UNIX system internal protocols),

              PF_INET             (ARPA Internet protocols), and

              PF_IMPLINK          (IMP "host at IMP" link layer).

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


       A SOCK_STREAM type provides  sequenced,  reliable,  two-way  connection
       based  byte streams.  An out-of-band data transmission mechanism may be
       supported.  A SOCK_DGRAM  socket  supports  datagrams  (connectionless,
       unreliable  messages  of  a  fixed (typically small) maximum length). A
       SOCK_SEQPACKET socket may provide a sequenced, reliable,  two-way  con-
       nection-based  data  transmission  path  for datagrams of fixed maximum
       length; a consumer may be required to read an entire packet  with  each
       read  system  call.   This facility is protocol specific, and presently
       not implemented for any  protocol  family.   SOCK_RAW  sockets  provide
       access  to  internal  network interfaces.  The types SOCK_RAW, which is
       available only to the super-user, and SOCK_RDM, for which no  implemen-
       tation currently exists, are not described here.

       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.  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 particular
       to the "communication domain" in which communication is to take  place;
       see protocols(5).

       Sockets  of  type  SOCK_STREAM are full-duplex byte streams, similar to
       pipes.  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 cre-
       ated with a connect(2) call.  Once connected, data may  be  transferred
       using  read(2V)  and write(2V) calls or some variant of the send(2) and
       recv(2) calls.  When a session has been completed a close(2V),  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 used to  implement  a  SOCK_STREAM  insure
       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
       broken and calls will indicate  an  error  with  -1  returns  and  with
       ETIMEDOUT  as the specific code in the global variable errno.  The pro-
       tocols optionally keep sockets "warm" by forcing transmissions  roughly
       every  minute in the absence of other activity.  An error is then indi-
       cated if no response can be elicited on an  otherwise  idle  connection
       for  a  extended  period (for instance 5 minutes).  A SIGPIPE signal is
       raised if a process sends on a broken stream; this  causes  naive  pro-
       cesses, 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(2V) calls will  return  only
       the  amount of data requested, and any remaining in the arriving packet
       will be discarded.

       SOCK_DGRAM and SOCK_RAW sockets allow sending of  datagrams  to  corre-
       spondents  named  in  send(2)  calls.  Datagrams are generally received
       with recv(2), which returns the next datagram with its return address.

       An fcntl(2V) call can be used to specify a process group to  receive  a
       SIGURG  signal  when  the out-of-band data arrives.  It may also enable
       non-blocking I/O and asynchronous notification of I/O events with SIGIO

       The  operation of sockets is controlled by socket level options.  These
       options are defined in the file socket.h.  getsockopt(2)  and  setsock-
       opt() are used to get and set options, respectively.

       socket()  returns a non-negative descriptor on success.  On failure, it
       returns -1 and sets errno to indicate the error.

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

       EMFILE              The per-process descriptor table is full.

       ENFILE              The system file table is full.

       ENOBUFS             Insufficient buffer space is available.  The socket
                           cannot be created until  sufficient  resources  are

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

       EPROTOTYPE          The protocol is the wrong type for the socket.

       accept(2), bind(2), close(2V), connect(2),  fcntl(2V),  getsockname(2),
       getsockopt(2),   ioctl(2),  listen(2),  read(2V),  recv(2),  select(2),
       send(2), shutdown(2), socketpair(2), write(2V), protocols(5)

                                21 January 1990                      SOCKET(2)