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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
       formats currently understood by the Linux kernel include:

       Name         Purpose                                    Man page
       AF_UNIX      Local communication                        unix(7)
       AF_LOCAL     Synonym for AF_UNIX
       AF_INET      IPv4 Internet protocols                    ip(7)
       AF_AX25      Amateur radio AX.25 protocol               ax25(4)
       AF_IPX       IPX - Novell protocols
       AF_APPLETALK AppleTalk                                  ddp(7)
       AF_X25       ITU-T X.25 / ISO-8208 protocol             x25(7)
       AF_INET6     IPv6 Internet protocols                    ipv6(7)
       AF_DECnet    DECet protocol sockets
       AF_KEY       Key   management   protocol,  originally
                    developed for usage with IPsec
       AF_NETLINK   Kernel user interface device               netlink(7)
       AF_PACKET    Low-level packet interface                 packet(7)
       AF_RDS       Reliable Datagram Sockets (RDS) protocol   rds(7)
                                                               rds-rdma(7)
       AF_PPPOX     Generic PPP transport layer, for setting
                    up up L2 tunnels (L2TP and PPPoE)
       AF_LLC       Logical  link  control  (IEEE 802.2 LLC)
                    protocol
       AF_IB        InfiniBand native addressing
       AF_MPLS      Multiprotocol Label Switching
       AF_CAN       Controller Area Network  automotive  bus
                    protocol
       AF_TIPC      TIPC, "cluster domain sockets" protocol
       AF_BLUETOOTH Bluetooth low-level socket protocol
       AF_ALG       Interface to kernel crypto API
       AF_VSOCK     VSOCK   (originally  "VMWare  VSockets")   vsock(7)
                    protocol      for       hypervisor-guest
                    communication
       AF_KCM       KCM   (kernel   connection  multiplexor)
                    interface
       AF_XDP       XDP (express data path) interface

       Further details of the above address families, as well as  information  on  several  other
       address families, can be found in address_families(7).

       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 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 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  open  file  description  (see
                       open(2))  referred  to  by the new file descriptor.  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 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.

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 protocol is denied.

       EAFNOSUPPORT
              The implementation does not support the specified address family.

       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 subsequent standards use AF_* everywhere.

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),  getprotoent(3),  address_families(7), 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 5.02 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/.