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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 descriptor.

       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)

       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; for example, SOCK_SEQPACKET is not imple-
       mented for AF_INET.

       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 speci-
       fied  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 commu-
       nication  is to take place; see protocols(5).  See getprotoent(3) on how to map protocol name strings to proto-
       col numbers.

       Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes.  They do not preserve record bound-
       aries.   A stream socket must be in a connected state before any data may be sent or received on it.  A connec-
       tion 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  reason-
       able  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.   Data-
       grams  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 notifica-
       tion 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 a 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 Process file table overflow.

       ENFILE The system 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
       4.4BSD, POSIX.1-2001.

       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.1-2001  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  proba-
       bly wise to include it.

       The  manifest constants used under 4.x BSD for protocol families are PF_UNIX, PF_INET, etc., while AF_UNIX etc.
       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),  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" is reprinted in UNIX Programmer's Supplementary
       Documents Volume 1.

       "BSD Interprocess Communication Tutorial" is reprinted in UNIX Programmer's Supplementary Documents Volume 1.

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-01-19                         SOCKET(2)