socket(3N)

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
     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.  There must
     be an entry in the netconfig(4) file for at least each protocol family
     and type required.  If protocol has been specified, but no exact match
     for the tuplet family, type, protocol is found, then the first entry
     containing the specified family and type with zero for protocol will be
     used.  The currently understood formats are:

     PF_UNIX             UNIX system internal protocols

     PF_INET             ARPA Internet protocols

     The socket has the indicated type, which specifies the communication
     semantics.  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
     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 not
     implemented for any protocol family.  SOCK_RAW sockets provide access to
     internal network interfaces.  The types SOCK_RAW, which is available only
     to a privileged user, and SOCK_RDM, for which no implementation currently
     exists, are not described here.

     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, multiple 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.  If a protocol is
     specified by the caller, then it will be packaged into a socket level
     option request and sent to the underlying protocol layers.

     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(3N) call.  Once connected, data may be transferred using
     read(2) and write(2) calls or some variant of the send(3N) and recv(3N)
     calls.  When a session has been completed, a close(2) may be performed.
     Out-of-band data may also be transmitted as described on the send(3N)
     manual page and received as described on the recv(3N) manual page.

     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
     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
     instance 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 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 datagrams to be sent to
     correspondents named in sendto calls.  Datagrams are generally received
     with recvfrom, 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 with SIGIO
     signals.

     The operation of sockets is controlled by socket level options.  These
     options are defined in the file /usr/include/sys/socket.h.
     setsockopt(3N) and getsockopt(3N) are used to set and get options,
     respectively.

RETURN VALUE
     A -1 is returned if an error occurs.  Otherwise the return value is a
     descriptor referencing the socket.

ERRORS
     The socket() call fails if:

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

     EMFILE              The per-process descriptor table is full.

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

     ENOMEM              Insufficient user memory is available.

     ENOSR               There were insufficient STREAMS resources available
                         to complete the operation.

SEE ALSO
     close(2), fcntl(2), ioctl(2), read(2), write(2), accept(3N), bind(3N),
     connect(3N), getsockname(3N), getsockopt(3N), listen(3N), recv(3N),
     send(3N), shutdown(3N), socketpair(3N)