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 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 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 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 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.

       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),
       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.

COLOPHON

       This  page  is  part  of  release  4.15  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)