socket - create an endpoint for communication


   #include <sys/types.h>          /* See NOTES */
   #include <sys/socket.h>

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


   socket()  creates  an  endpoint  for  communication  and returns a file
   descriptor that refers to that endpoint.

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

   Name                Purpose                          Man page
   AF_UNIX, AF_LOCAL   Local communication              unix(7)
   AF_INET             IPv4 Internet protocols          ip(7)
   AF_INET6            IPv6 Internet protocols          ipv6(7)
   AF_IPX              IPX - Novell protocols
   AF_NETLINK          Kernel user interface device     netlink(7)
   AF_X25              ITU-T X.25 / ISO-8208 protocol   x25(7)
   AF_AX25             Amateur radio AX.25 protocol
   AF_ATMPVC           Access to raw ATM PVCs
   AF_APPLETALK        AppleTalk                        ddp(7)
   AF_PACKET           Low level packet interface       packet(7)
   AF_ALG              Interface to kernel crypto API

   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 supported.

   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  consumer  is  required  to read an
                   entire packet with each input system 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

   Some socket types may not be implemented by all protocol families.

   Since Linux 2.6.27, the type  argument  serves  a  second  purpose:  in
   addition  to specifying a socket type, it may include the bitwise OR of
   any of the following values, to modify the behavior of socket():

   SOCK_NONBLOCK   Set the O_NONBLOCK file status flag  on  the  new  open
                   file description.  Using this flag saves extra calls to
                   fcntl(2) to achieve the same result.

   SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file
                   descriptor.   See the description of the O_CLOEXEC flag
                   in open(2) for reasons why this may be useful.

   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.  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
   SIGPIPE 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
   correspondents  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
   unexpectedly.   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 <sys/socket.h>.  The functions setsockopt(2) and
   getsockopt(2) are used to set and get options, respectively.


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


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

          The  implementation  does  not  support  the  specified  address

   EINVAL Unknown protocol, or protocol family not available.

   EINVAL Invalid flags in type.

   EMFILE The per-process limit on the number of open file descriptors has
          been reached.

   ENFILE The system-wide limit on the total number of open files has been

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

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

   Other errors may be generated by the underlying protocol modules.


   POSIX.1-2001, POSIX.1-2008, 4.4BSD.

   The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.

   socket() appeared in 4.2BSD.  It is generally portable to/from  non-BSD
   systems  supporting  clones of the BSD socket layer (including System V


   POSIX.1 does not require  the  inclusion  of  <sys/types.h>,  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, and so on, while AF_UNIX, AF_INET, and so on are used
   for address families.  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  AF_ALG  protocol type was added in Linux 2.6.38.  More information
   on this interface is provided with the  kernel  HTML  documentation  at


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


   accept(2),  bind(2),  close(2),  connect(2),  fcntl(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), ip(7), socket(7), tcp(7), udp(7), unix(7)

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


   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at


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