SOCKET(2)                            BSD                             SOCKET(2)



NAME
     socket - create an endpoint for communication

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

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

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

     The domain parameter specifies a communications domain within that
     communication will take place; this selects the protocol family which
     should be used.  The protocol family generally is the same as the address
     family for the addresses supplied in later operations on the socket.
     These families are defined in the include file <sys/socket.h>.  The
     currently understood formats are

          PF_UNIX     (UNIX internal protocols)
          PF_INET     (ARPA Internet protocols)
          PF_IMPLINK  (IMP "host at IMP" link layer)

     The socket has the indicated type, which specifies the semantics of
     communication.  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 can be
     supported.  A SOCK_DGRAM socket supports datagrams (connectionless,
     unreliable messages of a fixed (typically small) maximum length).  A
     SOCK_SEQPACKET socket can 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 PF_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 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
     getprotoent(3N).

     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 can
     be sent or received on it.  A connection to another socket is created
     with a connect(2) call.  Once connected, data can 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) can be performed.
     Out-of-band data can 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 (for
     example, 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 can 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.

ERRORS
     The socket call fails if any of the following occur:

     [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.

     [EACCESS]           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), read(2), recv(2), select(2), send(2), shutdown(2),
     socketpair(2), write(2)
     "An Introductory 4.3BSD Interprocess Communication Tutorial" (reprinted
     in UNIX Programmer's Supplementary Documents Volume 1, PS1:7), "An
     Advanced 4.3BSD Interprocess Communication Tutorial" (reprinted in UNIX
     Programmer's Supplementary Documents Volume 1, PS1:8)

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