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/.