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 communica-
       tion.  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  implemented  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  sup-
       port  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 proto-
       cols(5).  See getprotoent(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 proto-
       col-specific manner if the other end is still alive.  A SIGPIPE signal is raised if a process sends or receives on a bro-
       ken 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 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  func-
       tions 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 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), 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 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-01-19                                                  SOCKET(2)