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# socket

    SOCKET(2)                     System Calls Manual                    SOCKET(2)

    NAME
         socket - create an endpoint for communication

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

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

    DESCRIPTION
         socket() creates an endpoint for communication and returns a descriptor.

         The domain parameter specifies a communications domain within which com-
         munication 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_INET6        (ARPA IPv6 protocols),
               AF_ISO          (ISO protocols),
               AF_NS           (Xerox Network Systems protocols),
               AF_IPX          (Internetwork Packet Exchange), and
               AF_IMPLINK      (IMP host at IMP link layer).

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

               SOCK_STREAM
               SOCK_DGRAM
               SOCK_RAW
               SOCK_SEQPACKET
               SOCK_RDM

         A SOCK_STREAM type provides sequenced, reliable, two-way connection based
         byte streams.  An out-of-band data transmission mechanism may be sup-
         ported.  A SOCK_DGRAM socket supports datagrams (connectionless, unreli-
         able messages of a fixed (typically small) maximum length).  A
         SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connec-
         tion-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 internal network protocols
         and interfaces.  The types SOCK_RAW, which is available only to the supe-
         ruser, 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 speci-
         fied in this manner.  The protocol number to use is particular to the
         communication domain in which communication is to take place; see
         protocols(5).  A value of 0 for protocol will let the system select an
         appropriate protocol for the requested socket type.

         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 used to 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 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 sig-
         nal, to exit.

         SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sock-
         ets.  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 correspon-
         dents 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 notification 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.

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

    ERRORS
         The socket() 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.

    SEE ALSO
         accept(2), bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2),
         listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2),
         shutdown(2), socketpair(2), write(2), getprotoent(3), netintro(4)

         An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in
         UNIX Programmer's Supplementary Documents Volume 1.

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

    HISTORY
         The socket() function call appeared in 4.2BSD.

    BSD                              June 4, 1993                              BSD

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