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SOCKET(2)                     System Calls Manual                    SOCKET(2)

     socket - create an endpoint for communication

     Standard C Library (libc, -lc)

     #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:

           PF_LOCAL        local (previously UNIX) domain protocols
           PF_INET         ARPA Internet protocols
           PF_INET6        IPv6 (Internet Protocol version 6) protocols
           PF_NS           Xerox Network Systems protocols
           PF_APPLETALK    AppleTalk protocols
           PF_BLUETOOTH    Bluetooth protocols
           PF_CAN          CAN bus protocols

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


     The following flags can be or'ed to the type to condition the returned
     file descriptor: The following flags are valid:

           SOCK_CLOEXEC Set the close on exec property.
           SOCK_NONBLOCK Sets non-blocking I/O.
           SOCK_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.

     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 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

     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

     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>.  The setsockopt(2) and
     getsockopt(2) system calls 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:

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

     [EAFNOSUPPORT]     The address family (domain) is not supported or the
                        specified domain is not supported by this protocol

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

     [ENFILE]           The system file table is full.

     [ENOBUFS]          Insufficient buffer space is available.  The socket
                        cannot be created until sufficient resources are

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

     [EPROTOTYPE]       The socket type is not supported by the protocol.

     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)

     Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication
     Tutorial.  (see /usr/share/doc/reference/ref3/sockets)

     Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve
     Miller, and Chris Torek, Advanced 4.4BSD IPC Tutorial.  (see

     The socket() function call appeared in 4.2BSD.

NetBSD 9.99                      June 28, 2022                     NetBSD 9.99