SOCKET(2)                    BSD 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);

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

           PF_LOCAL        Host-internal protocols (alias for PF_UNIX),
           PF_UNIX         Host-internal protocols,
           PF_INET         Internet version 4 protocols,
           PF_INET6        Internet version 6 protocols,
           PF_ROUTE        Internal routing protocol,
           PF_LINK         Link layer interface,
           PF_KEY          Internal key-management function,
           PF_NATM         Asynchronous transfer mode protocols,
           PF_NETGRAPH     Netgraph sockets,
           PF_IEEE80211    IEEE 802.11 wireless link-layer protocols (WiFi),
           PF_BLUETOOTH    Bluetooth protocols,
           PF_INET_SDP     OFED socket direct protocol (IPv4),
           PF_INET6_SDP    OFED socket direct protocol (IPv6)

     Each protocol family is connected to an address family, which has the same
     name except that the prefix is “AF_” in place of “PF_”.  Other protocol
     families may be also defined, beginning with “PF_”, with corresponding
     address families.

     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 may have
     protocol-specific properties.  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.

     Additionally, the following flags are allowed in the type argument:

           SOCK_CLOEXEC    Set close-on-exec on the new descriptor,
           SOCK_NONBLOCK   Set non-blocking mode on the new socket

     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

     The protocol argument may be set to zero (0) to request the default
     implementation of a socket type for the protocol, if any.

     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 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).  By default, a SIGPIPE signal is raised if a process sends on a
     broken stream, but this behavior may be inhibited via setsockopt(2).

     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 <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() system 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 freed.

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

     [EPROTONOSUPPORT]  The protocol type 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), getpeername(2), getsockname(2),
     getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
     shutdown(2), socketpair(2), write(2), CMSG_DATA(3), getprotoent(3),
     netgraph(4), protocols(5)

     “An Introductory 4.3 BSD Interprocess Communication Tutorial”, PS1, 7.

     “BSD Interprocess Communication Tutorial”, PS1, 8.

     The socket() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).  The
     POSIX standard specifies only the AF_INET, AF_INET6, and AF_UNIX constants
     for address families, and requires the use of AF_* constants for the domain
     argument of socket().  The SOCK_CLOEXEC flag is expected to conform to the
     next revision of the POSIX standard.  The SOCK_RDM type, the PF_*
     constants, and other address families are FreeBSD extensions.

     The socket() system call appeared in 4.2BSD.

BSD                              August 19, 2018                             BSD