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NAME

     socket - create an endpoint for communication

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

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

DESCRIPTION

     The socket() system call creates an endpoint for communication and
     returns a descriptor.

     The domain argument 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 The
     currently understood formats are:

           PF_LOCAL        Host-internal protocols, formerly called PF_UNIX,
           PF_UNIX         Host-internal protocols, deprecated, use PF_LOCAL,
           PF_INET         Internet version 4 protocols,
           PF_PUP          PUP protocols, like BSP,
           PF_APPLETALK    AppleTalk protocols,
           PF_ROUTE        Internal Routing protocol,
           PF_LINK         Link layer interface,
           PF_IPX          Novell Internet Packet eXchange protocol,
           PF_RTIP         Help Identify RTIP packets,
           PF_PIP          Help Identify PIP packets,
           PF_ISDN         Integrated Services Digital Network,
           PF_KEY          Internal key-management function,
           PF_INET6        Internet version 6 protocols,
           PF_NATM         Native ATM access,
           PF_ATM          ATM,
           PF_NETGRAPH     Netgraph sockets

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

           SOCK_STREAM     Stream socket,
           SOCK_DGRAM      Datagram socket,
           SOCK_RAW        Raw-protocol interface,
           SOCK_RDM        Reliably-delivered packet,
           SOCK_SEQPACKET  Sequenced packet stream

     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
     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
     unimplemented.  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 argument 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 created
     with a connect(2) system call.  Once connected, data may be transferred
     using read(2) and write(2) calls or some variant of the send(2) and
     recv(2) functions.  (Some protocol families, such as the Internet family,
     support the notion of an “implied connect”, which permits data to be sent
     piggybacked onto a connect operation by using the sendto(2) system 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 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 an 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) system 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 The setsockopt(2) and getsockopt(2)
     system calls are used to set and get options, respectively.

RETURN VALUES

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

ERRORS

     The socket() system call fails if:

     [EPROTONOSUPPORT]  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
                        freed.

     [EPERM]            User has insufficient privileges to carry out the
                        requested operation.

SEE ALSO

     accept(2), bind(2), connect(2), getpeername(2), getsockname(2),
     getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
     shutdown(2), socketpair(2), write(2), getprotoent(3), netgraph(4),
     protocols(5)

     "An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.

     "BSD Interprocess Communication Tutorial", PS1, 8.

HISTORY

     The socket() system call appeared in 4.2BSD.