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

     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

          AF_UNIX  (UNIX internal protocols)

          AF_INET  (ARPA Internet protocols)

          AF_ISO   (ISO protocols)

          AF_NS    (Xerox Network Systems protocols)

                   (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 mes-
     sages of a  fixed  (typically  small)  maximum  length).   A
     SOCK_SEQPACKET  socket  may  provide  a sequenced, reliable,
     two-way  connection-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 AF_NS.  SOCK_RAW sockets provide access
     to  internal  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 sup-
     port a particular socket type within a given  protocol  fam-
     ily.  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 con-
     nection 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 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  protocols
     optionally   keep  sockets  warm  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 process 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  correspondents  named  in  send(2) calls.  Datagrams are
     generally received with recvfrom(2), which returns the  next
     datagram with its return address.

     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 notifi-
     cation of I/O events via SIGIO.
     The operation of  sockets  is  controlled  by  socket  level
     options.    These   options   are   defined   in   the  file
     sys/socket.h.  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 protocol type or the specified protocol  is  not
             supported within this domain.

     EMFILE  The per-process descriptor table is full.

     ENFILE  The system file table is full.

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

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


     4.4BSD (the socket function call appeared in  4.2BSD).  Gen-
     erally portable to/from non-BSD systems supporting clones of
     the BSD socket layer (including System V variants).


     accept(2),   bind(2),   connect(2),   getsockname(2),   get-
     sockopt(2),  ioctl(2),  read(2), recv(2), select(2), socket-
     pair(2), write(2)

     "An  Introductory   4.3   BSD   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