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 de‐
       scriptor 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 maxi‐
                       mum 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 pos‐
       sible 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 get‐
       protoent(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 con‐
       nect(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 gener‐
       ally 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_SE‐
       TOWN 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 his‐
       torical (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 4.16 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)