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 <sys/socket.h>.  The currently understood
       formats include:

              UNIX protocols for local communication

              IPv4 Internet protocols; see ip(4)

              IPv6 Internet protocols

       PF_IPX IPX - Novell protocols

              Kernel user interface device

       PF_X25 ITU-T X.25 / ISO-8208 protocol

              Amateur radio AX.25 protocol

              Access to raw ATM PVCs

              Appletalk; see ddp(4)

              Low level packet interface; see packet(4)

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

              Provides  sequenced,  reliable, two-way connection-
              based byte streams.  An out-of-band data  transmis­
              Supports datagrams (connectionless, unreliable mes­
              sages of a fixed maximum length).

              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 read system call.

              Provides raw network protocol access.

              Provides  a  reliable  datagram layer that does not
              gurantee ordering.

              Obsolete and should not be used  in  new  programs;
              see packet(4).

       Some  socket  types may not be implemented by all protocol
       families; for example,  SOCK_SEQPACKET  is  currently  not
       implemented in Linux for AF_INET.

       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 proto­
       col 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 communi­
       cation is to take place; see  protocols(5).   See  getpro­
       toent(3)  on  how to map protocol name strings to protocol

       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 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 When SO_KEEPALIVE
       is enabled on the socket the protocol checks in  a  proto­
       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 arriv­
       ing 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 send(2) calls.   Datagrams  are
       generally  received  with  recvfrom(2),  which returns the
       next datagram with its return address.

       SOCK_PACKET is an obsolete  socket  type  to  receive  raw
       packets  directly  from  the  device driver. Use packet(4)

       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 unexpectedly.  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 SIOSETOWN argument.

       When  the network signals an error condition to the proto­
       col 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(4).

       The  operation  of  sockets  is controlled by socket level
       options.  These options  are  defined  in  <sys/socket.h>.
       Setsockopt(2)  and  getsockopt(2)  are used to set and get
       options, respectively.


       -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.

       ENFILE  Not enough kernel memory to allocate a new  socket

       EMFILE  Process file table overflow.
               type and/or protocol is denied.

               Insufficient memory is available.  The socket can­
               not  be  created  until  sufficient  resources are

       EINVAL  Unknown protocol, or protocol  family  not  avail­

       Other  errors  may be generated by the underlying protocol


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


       accept(2), bind(2), connect(2),  getprotoent(3),  getsock­
       name(2),   getsockopt(2),  ioctl(2),  listen(2),  read(2),
       recv(2), select(2), send(2),  shutdown(2),  socketpair(2),

       "An  Introductory 4.3 BSD Interprocess Communication Tuto­
       rial" is reprinted in UNIX Programmer's Supplementary Doc­
       uments Volume 1.

       "BSD  Interprocess Communication Tutorial" is reprinted in
       UNIX Programmer's Supplementary Documents Volume 1.