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
       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 4.16 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)