focal (2) socket.2.gz

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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 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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.