NAME

       socket — create an endpoint for communication

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <sys/types.h>
       #include <sys/socket.h>

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

DESCRIPTION

       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, formerly called PF_UNIX,
             PF_UNIX         Host-internal protocols, deprecated, use PF_LOCAL,
             PF_INET         Internet version 4 protocols,
             PF_PUP          PUP protocols, like BSP,
             PF_APPLETALK    AppleTalk protocols,
             PF_ROUTE        Internal Routing protocol,
             PF_LINK         Link layer interface,
             PF_IPX          Novell Internet Packet eXchange protocol,
             PF_RTIP         Help Identify RTIP packets,
             PF_PIP          Help Identify PIP packets,
             PF_ISDN         Integrated Services Digital Network,
             PF_KEY          Internal key-management function,
             PF_INET6        Internet version 6 protocols,
             PF_NATM         Native ATM access,
             PF_ATM          ATM,
             PF_NETGRAPH     Netgraph sockets

       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 is
       protocol specific, and presently unimplemented.  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 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 protocols(5).

       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.

RETURN VALUES

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

ERRORS

       The socket() system 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.

       [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.

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

SEE ALSO

       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), getprotoent(3),  netgraph(4),
       protocols(5)

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

       “BSD Interprocess Communication Tutorial”, PS1, 8.

HISTORY

       The socket() system call appeared in 4.2BSD.

Debian                                           January 5, 2009                                       SOCKET(2)