SOCKET(2)                     System Calls Manual                    SOCKET(2)


     socket - create an endpoint for communication


     #include <sys/socket.h>

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


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

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

           AF_UNIX      UNIX internal protocols
           AF_INET      ARPA Internet protocols
           AF_INET6     IPv6 (Internet Protocol version 6) protocols

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


     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
     implemented only for AF_UNIX.  SOCK_RAW sockets provide access to
     internal network protocols and interfaces, and are available only to the

     Any combination of the following flags may additionally be used in the
     type argument:

           SOCK_CLOEXEC    Set close-on-exec flag on the new descriptor.
           SOCK_NONBLOCK   Set non-blocking I/O mode on the new socket.
           SOCK_DNS        For domains AF_INET or AF_INET6, only allow
                           connect(2), sendto(2), or sendmsg(2) to the DNS
                           port (typically 53).

     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
     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.  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 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

     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.


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


     The socket() call fails if:

     [EAFNOSUPPORT]     The specified address family is not supported on this

     [EPROTONOSUPPORT]  The protocol type or the specified protocol is not
                        supported within this domain.

     [EPROTOTYPE]       The combination of the specified protocol and type is
                        not supported.

     [EMFILE]           The per-process descriptor table is full.

     [ENFILE]           The system file table is full.

     [ENOBUFS]          Insufficient resources were available in the system to
                        perform the operation.

     [EACCES]           Permission to create a socket of the specified type
                        and/or protocol is denied.


     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), inet(4), inet6(4),
     netintro(4), unix(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.


     The socket() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').


     The socket() system call first appeared in 4.1cBSD.

OpenBSD 5.9                    October 20, 2015                    OpenBSD 5.9

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