SOCKET(2)                 OpenBSD Programmer's 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_ISO          (ISO protocols),
           AF_NS           (Xerox Network Systems protocols), 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 support-
     ed.  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 inter-
     faces.  The types SOCK_RAW, which is available only to the super-user,
     and SOCK_RDM, which is planned, but not yet implemented, are not de-
     scribed 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 re-
     ceived as described in recv(2).
     The communications protocols used to implement a SOCK_STREAM insure 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 ab-
     sence 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 signal, 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 op-
     tions are defined in the file <sys/socket.h>. setsockopt(2) and getsock-
     opt(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 de-
     scriptor referencing the socket.

ERRORS
     The socket() call fails if:

     [EPROTONOSUPPORT]
                   The protocol type or the specified protocol is not support-
                   ed 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.

OpenBSD 2.6                      June 4, 1993                                2