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 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

       Name               Purpose                          Man page
       PF_UNIX,PF_LOCAL   Local communication              unix(7)
       PF_INET            IPv4 Internet protocols          ip(7)
       PF_INET6           IPv6 Internet protocols
       PF_IPX             IPX - Novell protocols
       PF_NETLINK         Kernel user interface device     netlink(7)
       PF_X25             ITU-T X.25 / ISO-8208 protocol   x25(7)
       PF_AX25            Amateur radio AX.25 protocol
       PF_ATMPVC          Access to raw ATM PVCs
       PF_APPLETALK       Appletalk                        ddp(7)
       PF_PACKET          Low level packet interface       packet(7)

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

              Provides sequenced,  reliable,  two-way,  connection-based  byte
              streams.  An out-of-band data transmission mechanism may be sup-

              Supports datagrams (connectionless,  unreliable  messages  of  a
              fixed maximum length).

              Provides  a  sequenced,  reliable, two-way connection-based data
              transmission path for datagrams of fixed maximum length; a  con-
              sumer is required to read an entire packet with each read system

              Provides raw network protocol access.

              Provides a reliable  datagram  layer  that  does  not  guarantee

       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

       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  SIG-
       PIPE  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 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 corre-
       spondents named in send(2) calls.   Datagrams  are  generally  received
       with  recvfrom(2),  which  returns  the  next  datagram with its return

       SOCK_PACKET is an obsolete socket type to receive raw packets  directly
       from the device driver. Use packet(7) instead.

       An  fcntl(2) call with the the F_SETOWN argument can be used to specify
       a process group to receive a SIGURG signal when  the  out-of-band  data
       arrives  or  SIGPIPE  signal when a SOCK_STREAM connection breaks unex-
       pectedly.  It 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.

              The implementation does not support the specified  address  fam-

       ENFILE Not enough kernel memory to allocate a new socket structure.

       EMFILE Process file table overflow.

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

              Insufficient memory is available.  The socket cannot be  created
              until sufficient resources are freed.

       EINVAL Unknown protocol, or protocol family not available.

       Other errors may be generated by the underlying protocol modules.


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


       The  manifest  constants  used  under BSD 4.* for protocol families are
       PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for  address  fami-
       lies.  However, already the BSD man page promises: "The protocol family
       generally is the same as the address family", and subsequent  standards
       use AF_* everywhere.


       SOCK_UUCP is not implemented yet.


       accept(2),  bind(2),  connect(2),  fcntl(2),  getpeername(2),  getsock-
       name(2),  getsockopt(2),   ioctl(2),   listen(2),   read(2),   recv(2),
       select(2),   send(2),  shutdown(2),  socketpair(2),  write(2),  getpro-
       toent(3), ip(7), socket(7), tcp(7), udp(7), unix(7)

       "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 Program-
       mer's Supplementary Documents Volume 1.

Linux Man Page                    1999-04-24                         socket(2)