socket

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 de-
       scriptor that refers to that endpoint.  The file descriptor returned by
       a  successful call will be the lowest-numbered file descriptor not cur-
       rently 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 de-
                    veloped 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 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 communica-
                    tion
       AF_KCM       KCM  (kernel connection multiplexor) in-
                    terface
       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 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 en-
                       tire 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  open  file
                       description  (see  open(2)) referred to by the new file
                       descriptor.  Using this flag saves extra calls  to  fc-
                       ntl(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 ex-
       ist, in which case a particular protocol must be specified in this man-
       ner.  The protocol number to use is specific to the "communication  do-
       main"  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  in-
       coming 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 ad-
       dress 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  ar-
       rives  or SIGPIPE signal when a SOCK_STREAM connection breaks unexpect-
       edly.  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 er-
       ror; 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  er-
       ror, -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.

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),   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 5.05 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                             2019-03-06                         SOCKET(2)