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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 sup-
                       ported.

       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 order-
                       ing.

       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 implemented for AF_INET.

       Since Linux 2.6.27, the type argument serves a second purpose: in addition to spec-
       ifying 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 descrip-
                       tion.  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 "communi-
       cation 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, similar to pipes.  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 a
       ICMP message for IP) the pending error flag is set for the socket.  The next opera-
       tion on this socket will return the error code of the pending error.  For some pro-
       tocols 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 suffi-
              cient 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 sup-
       porting 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 probably 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), getsock-
       opt(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 Supple-
       mentary Documents Volume 1.

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



Linux                             2009-01-19                         SOCKET(2)

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