SOCKET(2)                        System Calls                        SOCKET(2)


       socket - create an endpoint for communication


       #include <sys/types.h>
       #include <sys/socket.h>

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


       Socket  creates an endpoint for communication and returns a descriptor.

       The domain parameter specifies a  communications  domain  within  which
       communication  will  take place; this selects the protocol family which
       should be used.   These  families  are  defined  in  the  include  file
       <sys/socket.h> The currently understood formats are

              PF_LOCAL  (Host-internal protocols, formerly called PF_UNIX),
              PF_INET        (ARPA Internet protocols),
              PF_ISO         (ISO protocols),
              PF_CCITT  (ITU-T protocols, like X.25), and
              PF_NS          (Xerox Network Systems protocols).

       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
       implemented only for PF_NS.  SOCK_RAW sockets provide access to  inter-
       nal  network  protocols  and  interfaces.  The types SOCK_RAW, which is
       available only to the super-user, and SOCK_RDM, which is  planned,  but
       not yet implemented, 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(2)  and  write(2)  calls or some variant of the send(2) and
       recv(2) calls.  (Some protocol families, such as the  Internet  family,
       support the notion of an which permits data to be sent piggybacked onto
       a connect operation by using the sendto(2) call.)  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

       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 by forcing transmissions roughly every
       minute in the absence of other activity.  An error is then indicated if
       no  response  can  be  elicited  on  an otherwise idle connection for a
       extended period (e.g. 5 minutes).  A SIGPIPE signal is raised if a pro-
       cess  sends  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 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  recvfrom(2),  which  returns  the  next  datagram with its return

       An fcntl(2) 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 via SIGIO.

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


       A  -1  is  returned if an error occurs, otherwise the return value is a
       descriptor referencing the socket.


       The socket call fails if:

                     The protocol type or the specified protocol is  not  sup-
                     ported within this domain.

              EMFILE The per-process descriptor table is full.

              ENFILE The system file table is full.

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

                     Insufficient buffer space is available.  The socket  can-
                     not be created until sufficient resources are freed.


       accept(2),  bind(2),  connect(2),  getprotoent(3), getsockname(2), get-
       sockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2),  send(2),
       shutdown(2), socketpair(2), write(2)

              An  Introductory 4.3 BSD Interprocess Communication Tutorial PS1
              volume 7

              BSD Interprocess Communication Tutorial PS1 volume 8


       The socket function call appeared in 4.2BSD.

GNO                            15 February 1995                      SOCKET(2)

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