socket(2)



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

NAME
       socket - create an endpoint for communication

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

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

DESCRIPTION
       socket()  creates  an  endpoint  for  communication  and returns a file
       descriptor that refers to that endpoint.  The file descriptor  returned
       by  a  successful  call will be the lowest-numbered file descriptor not
       currently open for the process.

       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
       include:

       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 mecha-
                       nism 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  guar-
                       antee 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.

       Since Linux 2.6.27, the type argument serves a second purpose: in addi-
       tion  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 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 an 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.

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.

       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
              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
       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
       variants).

NOTES
       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  sub-
       sequent 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
       https://www.kernel.org/doc/htmldocs/crypto-API/User.html.

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

SEE ALSO
       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),  getpro-
       toent(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.

COLOPHON
       This page is part of release 4.13 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                         SOCKET(2)

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