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SOCKET(2)                  Linux Programmer's Manual                 SOCKET(2)



NAME
       socket - create an endpoint for communication

SYNOPSIS
       #include <&lt;sys/types.h>&gt;          /* See NOTES */
       #include <&lt;sys/socket.h>&gt;

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

DESCRIPTION
       socket()  creates  an endpoint for communication and returns a descrip-
       tor.

       The domain argument specifies a communication domain; this selects  the
       protocol  family  which will be used for communication.  These families
       are  defined  in  &lt;sys/socket.h&gt;.   The  currently  understood  formats
       include:

       tab(:);  l l l.  Name:Purpose:Man page T{ PF_UNIX, PF_LOCAL T}:T{ Local
       communication T}:T{ unix(7) T} T{ PF_INET T}:IPv4 Internet protocols:T{
       ip(7) T} T{ PF_INET6 T}:IPv6 Internet protocols:T{ ipv6(7) T} T{ PF_IPX
       T}:IPX - Novell protocols: T{ PF_NETLINK T}:T{  Kernel  user  interface
       device  T}:T{  netlink(7)  T} T{ PF_X25 T}:ITU-T X.25 / ISO-8208 proto-
       col:T{ x25(7) T} T{ PF_AX25 T}:T{ Amateur radio AX.25 protocol  T}:  T{
       PF_ATMPVC  T}:Access  to  raw ATM PVCs: T{ PF_APPLETALK T}:Appletalk:T{
       ddp(7) T} T{ PF_PACKET T}:T{ Low level packet interface T}:T{ packet(7)
       T}

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

       SOCK_STREAM
              Provides sequenced,  reliable,  two-way,  connection-based  byte
              streams.  An out-of-band data transmission mechanism may be sup-
              ported.

       SOCK_DGRAM
              Supports datagrams (connectionless,  unreliable  messages  of  a
              fixed maximum length).

       SOCK_SEQPACKET
              Provides  a  sequenced,  reliable, two-way connection-based data
              transmission path for datagrams of fixed maximum length; a  con-
              sumer  is required to read an entire packet with each input sys-
              tem call.

       SOCK_RAW
              Provides raw network protocol access.

       SOCK_RDM
              Provides a reliable  datagram  layer  that  does  not  guarantee
              ordering.

       SOCK_PACKET
              Obsolete and should not be used in new programs; see packet(7).

       Some  socket types may not be implemented by all protocol families; for
       example, SOCK_SEQPACKET is not implemented for AF_INET.

       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, in which case protocol  can
       be  specified  as  0.   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 specific to the "communication
       domain" in which communication is to take place; see protocols(5).  See
       getprotoent(3) on how to map protocol name strings to protocol numbers.

       Sockets  of  type  SOCK_STREAM are full-duplex byte streams, similar to
       pipes.  They do not preserve record boundaries.  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 which 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  to  be
       dead.   When  SO_KEEPALIVE is enabled on the socket the protocol checks
       in a protocol-specific manner if the other end is still alive.  A  SIG-
       PIPE  signal  is  raised  if  a  process  sends or receives 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(2)  calls  will
       return only the amount of data requested, and any data remaining in the
       arriving packet will be discarded.   Also  all  message  boundaries  in
       incoming datagrams are preserved.

       SOCK_DGRAM  and  SOCK_RAW  sockets allow sending of datagrams to corre-
       spondents named in sendto(2) calls.  Datagrams are  generally  received
       with  recvfrom(2),  which  returns  the  next  datagram  along with the
       address of its sender.

       SOCK_PACKET is an obsolete socket type to receive raw packets  directly
       from the device driver.  Use packet(7) instead.

       An  fcntl(2)  F_SETOWN  operation  can  be used to specify a process or
       process group to receive a SIGURG  signal  when  the  out-of-band  data
       arrives  or  SIGPIPE  signal when a SOCK_STREAM connection breaks unex-
       pectedly.  This operation may also  be  used  to  set  the  process  or
       process  group  that  receives the I/O and asynchronous notification of
       I/O events via SIGIO.  Using F_SETOWN is equivalent to an ioctl(2) call
       with the FIOSETOWN or SIOCSPGRP argument.

       When  the  network  signals  an  error condition to the protocol module
       (e.g., using a ICMP message for IP) the pending error flag is  set  for
       the  socket.   The  next operation on this socket will return the error
       code of the pending error.  For some protocols it is possible to enable
       a  per-socket  error  queue  to retrieve detailed information about the
       error; see IP_RECVERR in ip(7).

       The operation of sockets is controlled by socket level options.   These
       options are defined in &lt;sys/socket.h&gt;.  The functions setsockopt(2) and
       getsockopt(2) are used to set and get options, respectively.

RETURN VALUE
       On success, a file descriptor for  the  new  socket  is  returned.   On
       error, -1 is returned, and errno is set appropriately.

ERRORS
       EACCES Permission  to create a socket of the specified type and/or pro-
              tocol is denied.

       EAFNOSUPPORT
              The implementation does not support the specified  address  fam-
              ily.

       EINVAL Unknown protocol, or protocol family not available.

       EMFILE Process file table overflow.

       ENFILE The  system  limit  on  the  total number of open files has been
              reached.

       ENOBUFS or ENOMEM
              Insufficient memory is available.  The socket cannot be  created
              until sufficient resources are freed.

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

       Other errors may be generated by the underlying protocol modules.

CONFORMING TO
       4.4BSD, POSIX.1-2001.  socket() appeared in 4.2BSD.   It  is  generally
       portable  to/from  non-BSD  systems supporting clones of the BSD socket
       layer (including System V variants).

NOTES
       POSIX.1-2001 does not require the inclusion of &lt;sys/types.h&gt;, and  this
       header  file  is not required on Linux.  However, some historical (BSD)
       implementations required this header file,  and  portable  applications
       are probably wise to include it.

       The  manifest  constants  used  under 4.x BSD for protocol families are
       PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for  address  fami-
       lies.  However, already the BSD man page promises: "The protocol family
       generally is the same as the address family", and subsequent  standards
       use AF_* everywhere.

       The  header  file  &lt;sys/types.h&gt; is only required for libc4 or earlier.
       Some packages, like util-linux, claim portability to all Linux versions
       and libraries.  They certainly need this header file.

BUGS
       SOCK_UUCP is not implemented yet.

EXAMPLE
       An example of the use of socket() is shown in getaddrinfo(3).

SEE ALSO
       accept(2),  bind(2),  connect(2),  fcntl(2),  getpeername(2),  getsock-
       name(2),  getsockopt(2),   ioctl(2),   listen(2),   read(2),   recv(2),
       select(2),   send(2),  shutdown(2),  socketpair(2),  write(2),  getpro-
       toent(3), ip(7), socket(7), tcp(7), udp(7), unix(7)

       "An  Introductory  4.3BSD  Interprocess  Communication   Tutorial"   is
       reprinted in UNIX Programmer's Supplementary Documents Volume 1.

       "BSD Interprocess Communication Tutorial" is reprinted in UNIX Program-
       mer's Supplementary Documents Volume 1.

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2004-06-17                         SOCKET(2)