/Doc/library/socket.rst

   1:mod:`socket` --- Low-level networking interface
   2================================================
   3
   4.. module:: socket
   5   :synopsis: Low-level networking interface.
   6
   7
   8This module provides access to the BSD *socket* interface. It is available on
   9all modern Unix systems, Windows, MacOS, OS/2, and probably additional
  10platforms.
  11
  12.. note::
  13
  14   Some behavior may be platform dependent, since calls are made to the operating
  15   system socket APIs.
  16
  17.. index:: object: socket
  18
  19The Python interface is a straightforward transliteration of the Unix system
  20call and library interface for sockets to Python's object-oriented style: the
  21:func:`socket` function returns a :dfn:`socket object` whose methods implement
  22the various socket system calls.  Parameter types are somewhat higher-level than
  23in the C interface: as with :meth:`read` and :meth:`write` operations on Python
  24files, buffer allocation on receive operations is automatic, and buffer length
  25is implicit on send operations.
  26
  27
  28.. seealso::
  29
  30   Module :mod:`socketserver`
  31      Classes that simplify writing network servers.
  32
  33   Module :mod:`ssl`
  34      A TLS/SSL wrapper for socket objects.
  35
  36
  37Socket families
  38---------------
  39
  40Depending on the system and the build options, various socket families
  41are supported by this module.
  42
  43The address format required by a particular socket object is automatically
  44selected based on the address family specified when the socket object was
  45created.  Socket addresses are represented as follows:
  46
  47- The address of an :const:`AF_UNIX` socket bound to a file system node
  48  is represented as a string, using the file system encoding and the
  49  ``'surrogateescape'`` error handler (see :pep:`383`).  An address in
  50  Linux's abstract namespace is returned as a :class:`bytes` object with
  51  an initial null byte; note that sockets in this namespace can
  52  communicate with normal file system sockets, so programs intended to
  53  run on Linux may need to deal with both types of address.  A string or
  54  :class:`bytes` object can be used for either type of address when
  55  passing it as an argument.
  56
  57   .. versionchanged:: 3.3
  58      Previously, :const:`AF_UNIX` socket paths were assumed to use UTF-8
  59      encoding.
  60
  61- A pair ``(host, port)`` is used for the :const:`AF_INET` address family,
  62  where *host* is a string representing either a hostname in Internet domain
  63  notation like ``'daring.cwi.nl'`` or an IPv4 address like ``'100.50.200.5'``,
  64  and *port* is an integral port number.
  65
  66- For :const:`AF_INET6` address family, a four-tuple ``(host, port, flowinfo,
  67  scopeid)`` is used, where *flowinfo* and *scopeid* represent the ``sin6_flowinfo``
  68  and ``sin6_scope_id`` members in :const:`struct sockaddr_in6` in C.  For
  69  :mod:`socket` module methods, *flowinfo* and *scopeid* can be omitted just for
  70  backward compatibility.  Note, however, omission of *scopeid* can cause problems
  71  in manipulating scoped IPv6 addresses.
  72
  73- :const:`AF_NETLINK` sockets are represented as pairs ``(pid, groups)``.
  74
  75- Linux-only support for TIPC is available using the :const:`AF_TIPC`
  76  address family.  TIPC is an open, non-IP based networked protocol designed
  77  for use in clustered computer environments.  Addresses are represented by a
  78  tuple, and the fields depend on the address type. The general tuple form is
  79  ``(addr_type, v1, v2, v3 [, scope])``, where:
  80
  81  - *addr_type* is one of :const:`TIPC_ADDR_NAMESEQ`, :const:`TIPC_ADDR_NAME`,
  82    or :const:`TIPC_ADDR_ID`.
  83  - *scope* is one of :const:`TIPC_ZONE_SCOPE`, :const:`TIPC_CLUSTER_SCOPE`, and
  84    :const:`TIPC_NODE_SCOPE`.
  85  - If *addr_type* is :const:`TIPC_ADDR_NAME`, then *v1* is the server type, *v2* is
  86    the port identifier, and *v3* should be 0.
  87
  88    If *addr_type* is :const:`TIPC_ADDR_NAMESEQ`, then *v1* is the server type, *v2*
  89    is the lower port number, and *v3* is the upper port number.
  90
  91    If *addr_type* is :const:`TIPC_ADDR_ID`, then *v1* is the node, *v2* is the
  92    reference, and *v3* should be set to 0.
  93
  94    If *addr_type* is :const:`TIPC_ADDR_ID`, then *v1* is the node, *v2* is the
  95    reference, and *v3* should be set to 0.
  96
  97- A tuple ``(interface, )`` is used for the :const:`AF_CAN` address family,
  98  where *interface* is a string representing a network interface name like
  99  ``'can0'``. The network interface name ``''`` can be used to receive packets
 100  from all network interfaces of this family.
 101
 102- Certain other address families (:const:`AF_BLUETOOTH`, :const:`AF_PACKET`)
 103  support specific representations.
 104
 105  .. XXX document them!
 106
 107For IPv4 addresses, two special forms are accepted instead of a host address:
 108the empty string represents :const:`INADDR_ANY`, and the string
 109``'<broadcast>'`` represents :const:`INADDR_BROADCAST`.  This behavior is not
 110compatible with IPv6, therefore, you may want to avoid these if you intend
 111to support IPv6 with your Python programs.
 112
 113If you use a hostname in the *host* portion of IPv4/v6 socket address, the
 114program may show a nondeterministic behavior, as Python uses the first address
 115returned from the DNS resolution.  The socket address will be resolved
 116differently into an actual IPv4/v6 address, depending on the results from DNS
 117resolution and/or the host configuration.  For deterministic behavior use a
 118numeric address in *host* portion.
 119
 120All errors raise exceptions.  The normal exceptions for invalid argument types
 121and out-of-memory conditions can be raised; starting from Python 3.3, errors
 122related to socket or address semantics raise :exc:`OSError` or one of its
 123subclasses (they used to raise :exc:`socket.error`).
 124
 125Non-blocking mode is supported through :meth:`~socket.setblocking`.  A
 126generalization of this based on timeouts is supported through
 127:meth:`~socket.settimeout`.
 128
 129
 130Module contents
 131---------------
 132
 133The module :mod:`socket` exports the following constants and functions:
 134
 135
 136.. exception:: error
 137
 138   A deprecated alias of :exc:`OSError`.
 139
 140   .. versionchanged:: 3.3
 141      Following :pep:`3151`, this class was made an alias of :exc:`OSError`.
 142
 143
 144.. exception:: herror
 145
 146   A subclass of :exc:`OSError`, this exception is raised for
 147   address-related errors, i.e. for functions that use *h_errno* in the POSIX
 148   C API, including :func:`gethostbyname_ex` and :func:`gethostbyaddr`.
 149   The accompanying value is a pair ``(h_errno, string)`` representing an
 150   error returned by a library call.  *h_errno* is a numeric value, while
 151   *string* represents the description of *h_errno*, as returned by the
 152   :c:func:`hstrerror` C function.
 153
 154   .. versionchanged:: 3.3
 155      This class was made a subclass of :exc:`OSError`.
 156
 157.. exception:: gaierror
 158
 159   A subclass of :exc:`OSError`, this exception is raised for
 160   address-related errors by :func:`getaddrinfo` and :func:`getnameinfo`.
 161   The accompanying value is a pair ``(error, string)`` representing an error
 162   returned by a library call.  *string* represents the description of
 163   *error*, as returned by the :c:func:`gai_strerror` C function.  The
 164   numeric *error* value will match one of the :const:`EAI_\*` constants
 165   defined in this module.
 166
 167   .. versionchanged:: 3.3
 168      This class was made a subclass of :exc:`OSError`.
 169
 170.. exception:: timeout
 171
 172   A subclass of :exc:`OSError`, this exception is raised when a timeout
 173   occurs on a socket which has had timeouts enabled via a prior call to
 174   :meth:`~socket.settimeout` (or implicitly through
 175   :func:`~socket.setdefaulttimeout`).  The accompanying value is a string
 176   whose value is currently always "timed out".
 177
 178   .. versionchanged:: 3.3
 179      This class was made a subclass of :exc:`OSError`.
 180
 181.. data:: AF_UNIX
 182          AF_INET
 183          AF_INET6
 184
 185   These constants represent the address (and protocol) families, used for the
 186   first argument to :func:`socket`.  If the :const:`AF_UNIX` constant is not
 187   defined then this protocol is unsupported.  More constants may be available
 188   depending on the system.
 189
 190
 191.. data:: SOCK_STREAM
 192          SOCK_DGRAM
 193          SOCK_RAW
 194          SOCK_RDM
 195          SOCK_SEQPACKET
 196
 197   These constants represent the socket types, used for the second argument to
 198   :func:`socket`.  More constants may be available depending on the system.
 199   (Only :const:`SOCK_STREAM` and :const:`SOCK_DGRAM` appear to be generally
 200   useful.)
 201
 202.. data:: SOCK_CLOEXEC
 203          SOCK_NONBLOCK
 204
 205   These two constants, if defined, can be combined with the socket types and
 206   allow you to set some flags atomically (thus avoiding possible race
 207   conditions and the need for separate calls).
 208
 209   .. seealso::
 210
 211      `Secure File Descriptor Handling <http://udrepper.livejournal.com/20407.html>`_
 212      for a more thorough explanation.
 213
 214   Availability: Linux >= 2.6.27.
 215
 216   .. versionadded:: 3.2
 217
 218.. data:: SO_*
 219          SOMAXCONN
 220          MSG_*
 221          SOL_*
 222          SCM_*
 223          IPPROTO_*
 224          IPPORT_*
 225          INADDR_*
 226          IP_*
 227          IPV6_*
 228          EAI_*
 229          AI_*
 230          NI_*
 231          TCP_*
 232
 233   Many constants of these forms, documented in the Unix documentation on sockets
 234   and/or the IP protocol, are also defined in the socket module. They are
 235   generally used in arguments to the :meth:`setsockopt` and :meth:`getsockopt`
 236   methods of socket objects.  In most cases, only those symbols that are defined
 237   in the Unix header files are defined; for a few symbols, default values are
 238   provided.
 239
 240.. data:: AF_CAN
 241          PF_CAN
 242          SOL_CAN_*
 243          CAN_*
 244
 245   Many constants of these forms, documented in the Linux documentation, are
 246   also defined in the socket module.
 247
 248   Availability: Linux >= 2.6.25.
 249
 250   .. versionadded:: 3.3
 251
 252
 253.. data:: AF_RDS
 254          PF_RDS
 255          SOL_RDS
 256          RDS_*
 257
 258   Many constants of these forms, documented in the Linux documentation, are
 259   also defined in the socket module.
 260
 261   Availability: Linux >= 2.6.30.
 262
 263   .. versionadded:: 3.3
 264
 265
 266.. data:: SIO_*
 267          RCVALL_*
 268
 269   Constants for Windows' WSAIoctl(). The constants are used as arguments to the
 270   :meth:`ioctl` method of socket objects.
 271
 272
 273.. data:: TIPC_*
 274
 275   TIPC related constants, matching the ones exported by the C socket API. See
 276   the TIPC documentation for more information.
 277
 278
 279.. data:: has_ipv6
 280
 281   This constant contains a boolean value which indicates if IPv6 is supported on
 282   this platform.
 283
 284
 285.. function:: create_connection(address[, timeout[, source_address]])
 286
 287   Connect to a TCP service listening on the Internet *address* (a 2-tuple
 288   ``(host, port)``), and return the socket object.  This is a higher-level
 289   function than :meth:`socket.connect`: if *host* is a non-numeric hostname,
 290   it will try to resolve it for both :data:`AF_INET` and :data:`AF_INET6`,
 291   and then try to connect to all possible addresses in turn until a
 292   connection succeeds.  This makes it easy to write clients that are
 293   compatible to both IPv4 and IPv6.
 294
 295   Passing the optional *timeout* parameter will set the timeout on the
 296   socket instance before attempting to connect.  If no *timeout* is
 297   supplied, the global default timeout setting returned by
 298   :func:`getdefaulttimeout` is used.
 299
 300   If supplied, *source_address* must be a 2-tuple ``(host, port)`` for the
 301   socket to bind to as its source address before connecting.  If host or port
 302   are '' or 0 respectively the OS default behavior will be used.
 303
 304   .. versionchanged:: 3.2
 305      *source_address* was added.
 306
 307   .. versionchanged:: 3.2
 308      support for the :keyword:`with` statement was added.
 309
 310
 311.. function:: getaddrinfo(host, port, family=0, type=0, proto=0, flags=0)
 312
 313   Translate the *host*/*port* argument into a sequence of 5-tuples that contain
 314   all the necessary arguments for creating a socket connected to that service.
 315   *host* is a domain name, a string representation of an IPv4/v6 address
 316   or ``None``. *port* is a string service name such as ``'http'``, a numeric
 317   port number or ``None``.  By passing ``None`` as the value of *host*
 318   and *port*, you can pass ``NULL`` to the underlying C API.
 319
 320   The *family*, *type* and *proto* arguments can be optionally specified
 321   in order to narrow the list of addresses returned.  Passing zero as a
 322   value for each of these arguments selects the full range of results.
 323   The *flags* argument can be one or several of the ``AI_*`` constants,
 324   and will influence how results are computed and returned.
 325   For example, :const:`AI_NUMERICHOST` will disable domain name resolution
 326   and will raise an error if *host* is a domain name.
 327
 328   The function returns a list of 5-tuples with the following structure:
 329
 330   ``(family, type, proto, canonname, sockaddr)``
 331
 332   In these tuples, *family*, *type*, *proto* are all integers and are
 333   meant to be passed to the :func:`socket` function.  *canonname* will be
 334   a string representing the canonical name of the *host* if
 335   :const:`AI_CANONNAME` is part of the *flags* argument; else *canonname*
 336   will be empty.  *sockaddr* is a tuple describing a socket address, whose
 337   format depends on the returned *family* (a ``(address, port)`` 2-tuple for
 338   :const:`AF_INET`, a ``(address, port, flow info, scope id)`` 4-tuple for
 339   :const:`AF_INET6`), and is meant to be passed to the :meth:`socket.connect`
 340   method.
 341
 342   The following example fetches address information for a hypothetical TCP
 343   connection to ``www.python.org`` on port 80 (results may differ on your
 344   system if IPv6 isn't enabled)::
 345
 346      >>> socket.getaddrinfo("www.python.org", 80, proto=socket.SOL_TCP)
 347      [(2, 1, 6, '', ('82.94.164.162', 80)),
 348       (10, 1, 6, '', ('2001:888:2000:d::a2', 80, 0, 0))]
 349
 350   .. versionchanged:: 3.2
 351      parameters can now be passed as single keyword arguments.
 352
 353.. function:: getfqdn([name])
 354
 355   Return a fully qualified domain name for *name*. If *name* is omitted or empty,
 356   it is interpreted as the local host.  To find the fully qualified name, the
 357   hostname returned by :func:`gethostbyaddr` is checked, followed by aliases for the
 358   host, if available.  The first name which includes a period is selected.  In
 359   case no fully qualified domain name is available, the hostname as returned by
 360   :func:`gethostname` is returned.
 361
 362
 363.. function:: gethostbyname(hostname)
 364
 365   Translate a host name to IPv4 address format.  The IPv4 address is returned as a
 366   string, such as  ``'100.50.200.5'``.  If the host name is an IPv4 address itself
 367   it is returned unchanged.  See :func:`gethostbyname_ex` for a more complete
 368   interface. :func:`gethostbyname` does not support IPv6 name resolution, and
 369   :func:`getaddrinfo` should be used instead for IPv4/v6 dual stack support.
 370
 371
 372.. function:: gethostbyname_ex(hostname)
 373
 374   Translate a host name to IPv4 address format, extended interface. Return a
 375   triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the primary
 376   host name responding to the given *ip_address*, *aliaslist* is a (possibly
 377   empty) list of alternative host names for the same address, and *ipaddrlist* is
 378   a list of IPv4 addresses for the same interface on the same host (often but not
 379   always a single address). :func:`gethostbyname_ex` does not support IPv6 name
 380   resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual
 381   stack support.
 382
 383
 384.. function:: gethostname()
 385
 386   Return a string containing the hostname of the machine where  the Python
 387   interpreter is currently executing.
 388
 389   If you want to know the current machine's IP address, you may want to use
 390   ``gethostbyname(gethostname())``. This operation assumes that there is a
 391   valid address-to-host mapping for the host, and the assumption does not
 392   always hold.
 393
 394   Note: :func:`gethostname` doesn't always return the fully qualified domain
 395   name; use ``getfqdn()`` (see above).
 396
 397
 398.. function:: gethostbyaddr(ip_address)
 399
 400   Return a triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the
 401   primary host name responding to the given *ip_address*, *aliaslist* is a
 402   (possibly empty) list of alternative host names for the same address, and
 403   *ipaddrlist* is a list of IPv4/v6 addresses for the same interface on the same
 404   host (most likely containing only a single address). To find the fully qualified
 405   domain name, use the function :func:`getfqdn`. :func:`gethostbyaddr` supports
 406   both IPv4 and IPv6.
 407
 408
 409.. function:: getnameinfo(sockaddr, flags)
 410
 411   Translate a socket address *sockaddr* into a 2-tuple ``(host, port)``. Depending
 412   on the settings of *flags*, the result can contain a fully-qualified domain name
 413   or numeric address representation in *host*.  Similarly, *port* can contain a
 414   string port name or a numeric port number.
 415
 416
 417.. function:: getprotobyname(protocolname)
 418
 419   Translate an Internet protocol name (for example, ``'icmp'``) to a constant
 420   suitable for passing as the (optional) third argument to the :func:`socket`
 421   function.  This is usually only needed for sockets opened in "raw" mode
 422   (:const:`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen
 423   automatically if the protocol is omitted or zero.
 424
 425
 426.. function:: getservbyname(servicename[, protocolname])
 427
 428   Translate an Internet service name and protocol name to a port number for that
 429   service.  The optional protocol name, if given, should be ``'tcp'`` or
 430   ``'udp'``, otherwise any protocol will match.
 431
 432
 433.. function:: getservbyport(port[, protocolname])
 434
 435   Translate an Internet port number and protocol name to a service name for that
 436   service.  The optional protocol name, if given, should be ``'tcp'`` or
 437   ``'udp'``, otherwise any protocol will match.
 438
 439
 440.. function:: socket([family[, type[, proto]]])
 441
 442   Create a new socket using the given address family, socket type and protocol
 443   number.  The address family should be :const:`AF_INET` (the default),
 444   :const:`AF_INET6`, :const:`AF_UNIX`, :const:`AF_CAN` or :const:`AF_RDS`. The
 445   socket type should be :const:`SOCK_STREAM` (the default),
 446   :const:`SOCK_DGRAM`, :const:`SOCK_RAW` or perhaps one of the other ``SOCK_``
 447   constants. The protocol number is usually zero and may be omitted in that
 448   case or :const:`CAN_RAW` in case the address family is :const:`AF_CAN`.
 449
 450   .. versionchanged:: 3.3
 451      The AF_CAN family was added.
 452      The AF_RDS family was added.
 453
 454
 455.. function:: socketpair([family[, type[, proto]]])
 456
 457   Build a pair of connected socket objects using the given address family, socket
 458   type, and protocol number.  Address family, socket type, and protocol number are
 459   as for the :func:`socket` function above. The default family is :const:`AF_UNIX`
 460   if defined on the platform; otherwise, the default is :const:`AF_INET`.
 461   Availability: Unix.
 462
 463   .. versionchanged:: 3.2
 464      The returned socket objects now support the whole socket API, rather
 465      than a subset.
 466
 467
 468.. function:: fromfd(fd, family, type[, proto])
 469
 470   Duplicate the file descriptor *fd* (an integer as returned by a file object's
 471   :meth:`fileno` method) and build a socket object from the result.  Address
 472   family, socket type and protocol number are as for the :func:`socket` function
 473   above. The file descriptor should refer to a socket, but this is not checked ---
 474   subsequent operations on the object may fail if the file descriptor is invalid.
 475   This function is rarely needed, but can be used to get or set socket options on
 476   a socket passed to a program as standard input or output (such as a server
 477   started by the Unix inet daemon).  The socket is assumed to be in blocking mode.
 478
 479
 480.. function:: ntohl(x)
 481
 482   Convert 32-bit positive integers from network to host byte order.  On machines
 483   where the host byte order is the same as network byte order, this is a no-op;
 484   otherwise, it performs a 4-byte swap operation.
 485
 486
 487.. function:: ntohs(x)
 488
 489   Convert 16-bit positive integers from network to host byte order.  On machines
 490   where the host byte order is the same as network byte order, this is a no-op;
 491   otherwise, it performs a 2-byte swap operation.
 492
 493
 494.. function:: htonl(x)
 495
 496   Convert 32-bit positive integers from host to network byte order.  On machines
 497   where the host byte order is the same as network byte order, this is a no-op;
 498   otherwise, it performs a 4-byte swap operation.
 499
 500
 501.. function:: htons(x)
 502
 503   Convert 16-bit positive integers from host to network byte order.  On machines
 504   where the host byte order is the same as network byte order, this is a no-op;
 505   otherwise, it performs a 2-byte swap operation.
 506
 507
 508.. function:: inet_aton(ip_string)
 509
 510   Convert an IPv4 address from dotted-quad string format (for example,
 511   '123.45.67.89') to 32-bit packed binary format, as a bytes object four characters in
 512   length.  This is useful when conversing with a program that uses the standard C
 513   library and needs objects of type :c:type:`struct in_addr`, which is the C type
 514   for the 32-bit packed binary this function returns.
 515
 516   :func:`inet_aton` also accepts strings with less than three dots; see the
 517   Unix manual page :manpage:`inet(3)` for details.
 518
 519   If the IPv4 address string passed to this function is invalid,
 520   :exc:`OSError` will be raised. Note that exactly what is valid depends on
 521   the underlying C implementation of :c:func:`inet_aton`.
 522
 523   :func:`inet_aton` does not support IPv6, and :func:`inet_pton` should be used
 524   instead for IPv4/v6 dual stack support.
 525
 526
 527.. function:: inet_ntoa(packed_ip)
 528
 529   Convert a 32-bit packed IPv4 address (a bytes object four characters in
 530   length) to its standard dotted-quad string representation (for example,
 531   '123.45.67.89').  This is useful when conversing with a program that uses the
 532   standard C library and needs objects of type :c:type:`struct in_addr`, which
 533   is the C type for the 32-bit packed binary data this function takes as an
 534   argument.
 535
 536   If the byte sequence passed to this function is not exactly 4 bytes in
 537   length, :exc:`OSError` will be raised. :func:`inet_ntoa` does not
 538   support IPv6, and :func:`inet_ntop` should be used instead for IPv4/v6 dual
 539   stack support.
 540
 541
 542.. function:: inet_pton(address_family, ip_string)
 543
 544   Convert an IP address from its family-specific string format to a packed,
 545   binary format. :func:`inet_pton` is useful when a library or network protocol
 546   calls for an object of type :c:type:`struct in_addr` (similar to
 547   :func:`inet_aton`) or :c:type:`struct in6_addr`.
 548
 549   Supported values for *address_family* are currently :const:`AF_INET` and
 550   :const:`AF_INET6`. If the IP address string *ip_string* is invalid,
 551   :exc:`OSError` will be raised. Note that exactly what is valid depends on
 552   both the value of *address_family* and the underlying implementation of
 553   :c:func:`inet_pton`.
 554
 555   Availability: Unix (maybe not all platforms).
 556
 557
 558.. function:: inet_ntop(address_family, packed_ip)
 559
 560   Convert a packed IP address (a bytes object of some number of characters) to its
 561   standard, family-specific string representation (for example, ``'7.10.0.5'`` or
 562   ``'5aef:2b::8'``). :func:`inet_ntop` is useful when a library or network protocol
 563   returns an object of type :c:type:`struct in_addr` (similar to :func:`inet_ntoa`)
 564   or :c:type:`struct in6_addr`.
 565
 566   Supported values for *address_family* are currently :const:`AF_INET` and
 567   :const:`AF_INET6`. If the string *packed_ip* is not the correct length for the
 568   specified address family, :exc:`ValueError` will be raised.  A
 569   :exc:`OSError` is raised for errors from the call to :func:`inet_ntop`.
 570
 571   Availability: Unix (maybe not all platforms).
 572
 573
 574..
 575   XXX: Are sendmsg(), recvmsg() and CMSG_*() available on any
 576   non-Unix platforms?  The old (obsolete?) 4.2BSD form of the
 577   interface, in which struct msghdr has no msg_control or
 578   msg_controllen members, is not currently supported.
 579
 580.. function:: CMSG_LEN(length)
 581
 582   Return the total length, without trailing padding, of an ancillary
 583   data item with associated data of the given *length*.  This value
 584   can often be used as the buffer size for :meth:`~socket.recvmsg` to
 585   receive a single item of ancillary data, but :rfc:`3542` requires
 586   portable applications to use :func:`CMSG_SPACE` and thus include
 587   space for padding, even when the item will be the last in the
 588   buffer.  Raises :exc:`OverflowError` if *length* is outside the
 589   permissible range of values.
 590
 591   Availability: most Unix platforms, possibly others.
 592
 593   .. versionadded:: 3.3
 594
 595
 596.. function:: CMSG_SPACE(length)
 597
 598   Return the buffer size needed for :meth:`~socket.recvmsg` to
 599   receive an ancillary data item with associated data of the given
 600   *length*, along with any trailing padding.  The buffer space needed
 601   to receive multiple items is the sum of the :func:`CMSG_SPACE`
 602   values for their associated data lengths.  Raises
 603   :exc:`OverflowError` if *length* is outside the permissible range
 604   of values.
 605
 606   Note that some systems might support ancillary data without
 607   providing this function.  Also note that setting the buffer size
 608   using the results of this function may not precisely limit the
 609   amount of ancillary data that can be received, since additional
 610   data may be able to fit into the padding area.
 611
 612   Availability: most Unix platforms, possibly others.
 613
 614   .. versionadded:: 3.3
 615
 616
 617.. function:: getdefaulttimeout()
 618
 619   Return the default timeout in seconds (float) for new socket objects. A value
 620   of ``None`` indicates that new socket objects have no timeout. When the socket
 621   module is first imported, the default is ``None``.
 622
 623
 624.. function:: setdefaulttimeout(timeout)
 625
 626   Set the default timeout in seconds (float) for new socket objects.  When
 627   the socket module is first imported, the default is ``None``.  See
 628   :meth:`~socket.settimeout` for possible values and their respective
 629   meanings.
 630
 631
 632.. function:: sethostname(name)
 633
 634   Set the machine's hostname to *name*.  This will raise a
 635   :exc:`OSError` if you don't have enough rights.
 636
 637   Availability: Unix.
 638
 639   .. versionadded:: 3.3
 640
 641
 642.. function:: if_nameindex()
 643
 644   Return a list of network interface information
 645   (index int, name string) tuples.
 646   :exc:`OSError` if the system call fails.
 647
 648   Availability: Unix.
 649
 650   .. versionadded:: 3.3
 651
 652
 653.. function:: if_nametoindex(if_name)
 654
 655   Return a network interface index number corresponding to an
 656   interface name.
 657   :exc:`OSError` if no interface with the given name exists.
 658
 659   Availability: Unix.
 660
 661   .. versionadded:: 3.3
 662
 663
 664.. function:: if_indextoname(if_index)
 665
 666   Return a network interface name corresponding to a
 667   interface index number.
 668   :exc:`OSError` if no interface with the given index exists.
 669
 670   Availability: Unix.
 671
 672   .. versionadded:: 3.3
 673
 674
 675.. data:: SocketType
 676
 677   This is a Python type object that represents the socket object type. It is the
 678   same as ``type(socket(...))``.
 679
 680
 681.. _socket-objects:
 682
 683Socket Objects
 684--------------
 685
 686Socket objects have the following methods.  Except for :meth:`makefile` these
 687correspond to Unix system calls applicable to sockets.
 688
 689
 690.. method:: socket.accept()
 691
 692   Accept a connection. The socket must be bound to an address and listening for
 693   connections. The return value is a pair ``(conn, address)`` where *conn* is a
 694   *new* socket object usable to send and receive data on the connection, and
 695   *address* is the address bound to the socket on the other end of the connection.
 696
 697
 698.. method:: socket.bind(address)
 699
 700   Bind the socket to *address*.  The socket must not already be bound. (The format
 701   of *address* depends on the address family --- see above.)
 702
 703
 704.. method:: socket.close()
 705
 706   Close the socket.  All future operations on the socket object will fail. The
 707   remote end will receive no more data (after queued data is flushed). Sockets are
 708   automatically closed when they are garbage-collected.
 709
 710   .. note::
 711      :meth:`close()` releases the resource associated with a connection but
 712      does not necessarily close the connection immediately.  If you want
 713      to close the connection in a timely fashion, call :meth:`shutdown()`
 714      before :meth:`close()`.
 715
 716
 717.. method:: socket.connect(address)
 718
 719   Connect to a remote socket at *address*. (The format of *address* depends on the
 720   address family --- see above.)
 721
 722
 723.. method:: socket.connect_ex(address)
 724
 725   Like ``connect(address)``, but return an error indicator instead of raising an
 726   exception for errors returned by the C-level :c:func:`connect` call (other
 727   problems, such as "host not found," can still raise exceptions).  The error
 728   indicator is ``0`` if the operation succeeded, otherwise the value of the
 729   :c:data:`errno` variable.  This is useful to support, for example, asynchronous
 730   connects.
 731
 732
 733.. method:: socket.detach()
 734
 735   Put the socket object into closed state without actually closing the
 736   underlying file descriptor.  The file descriptor is returned, and can
 737   be reused for other purposes.
 738
 739   .. versionadded:: 3.2
 740
 741
 742.. method:: socket.fileno()
 743
 744   Return the socket's file descriptor (a small integer).  This is useful with
 745   :func:`select.select`.
 746
 747   Under Windows the small integer returned by this method cannot be used where a
 748   file descriptor can be used (such as :func:`os.fdopen`).  Unix does not have
 749   this limitation.
 750
 751
 752.. method:: socket.getpeername()
 753
 754   Return the remote address to which the socket is connected.  This is useful to
 755   find out the port number of a remote IPv4/v6 socket, for instance. (The format
 756   of the address returned depends on the address family --- see above.)  On some
 757   systems this function is not supported.
 758
 759
 760.. method:: socket.getsockname()
 761
 762   Return the socket's own address.  This is useful to find out the port number of
 763   an IPv4/v6 socket, for instance. (The format of the address returned depends on
 764   the address family --- see above.)
 765
 766
 767.. method:: socket.getsockopt(level, optname[, buflen])
 768
 769   Return the value of the given socket option (see the Unix man page
 770   :manpage:`getsockopt(2)`).  The needed symbolic constants (:const:`SO_\*` etc.)
 771   are defined in this module.  If *buflen* is absent, an integer option is assumed
 772   and its integer value is returned by the function.  If *buflen* is present, it
 773   specifies the maximum length of the buffer used to receive the option in, and
 774   this buffer is returned as a bytes object.  It is up to the caller to decode the
 775   contents of the buffer (see the optional built-in module :mod:`struct` for a way
 776   to decode C structures encoded as byte strings).
 777
 778
 779.. method:: socket.gettimeout()
 780
 781   Return the timeout in seconds (float) associated with socket operations,
 782   or ``None`` if no timeout is set.  This reflects the last call to
 783   :meth:`setblocking` or :meth:`settimeout`.
 784
 785
 786.. method:: socket.ioctl(control, option)
 787
 788   :platform: Windows
 789
 790   The :meth:`ioctl` method is a limited interface to the WSAIoctl system
 791   interface.  Please refer to the `Win32 documentation
 792   <http://msdn.microsoft.com/en-us/library/ms741621%28VS.85%29.aspx>`_ for more
 793   information.
 794
 795   On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
 796   functions may be used; they accept a socket object as their first argument.
 797
 798.. method:: socket.listen(backlog)
 799
 800   Listen for connections made to the socket.  The *backlog* argument specifies the
 801   maximum number of queued connections and should be at least 0; the maximum value
 802   is system-dependent (usually 5), the minimum value is forced to 0.
 803
 804
 805.. method:: socket.makefile(mode='r', buffering=None, *, encoding=None, \
 806                            errors=None, newline=None)
 807
 808   .. index:: single: I/O control; buffering
 809
 810   Return a :term:`file object` associated with the socket.  The exact returned
 811   type depends on the arguments given to :meth:`makefile`.  These arguments are
 812   interpreted the same way as by the built-in :func:`open` function.
 813
 814   Closing the file object won't close the socket unless there are no remaining
 815   references to the socket.  The socket must be in blocking mode; it can have
 816   a timeout, but the file object's internal buffer may end up in a inconsistent
 817   state if a timeout occurs.
 818
 819   .. note::
 820
 821      On Windows, the file-like object created by :meth:`makefile` cannot be
 822      used where a file object with a file descriptor is expected, such as the
 823      stream arguments of :meth:`subprocess.Popen`.
 824
 825
 826.. method:: socket.recv(bufsize[, flags])
 827
 828   Receive data from the socket.  The return value is a bytes object representing the
 829   data received.  The maximum amount of data to be received at once is specified
 830   by *bufsize*.  See the Unix manual page :manpage:`recv(2)` for the meaning of
 831   the optional argument *flags*; it defaults to zero.
 832
 833   .. note::
 834
 835      For best match with hardware and network realities, the value of  *bufsize*
 836      should be a relatively small power of 2, for example, 4096.
 837
 838
 839.. method:: socket.recvfrom(bufsize[, flags])
 840
 841   Receive data from the socket.  The return value is a pair ``(bytes, address)``
 842   where *bytes* is a bytes object representing the data received and *address* is the
 843   address of the socket sending the data.  See the Unix manual page
 844   :manpage:`recv(2)` for the meaning of the optional argument *flags*; it defaults
 845   to zero. (The format of *address* depends on the address family --- see above.)
 846
 847
 848.. method:: socket.recvmsg(bufsize[, ancbufsize[, flags]])
 849
 850   Receive normal data (up to *bufsize* bytes) and ancillary data from
 851   the socket.  The *ancbufsize* argument sets the size in bytes of
 852   the internal buffer used to receive the ancillary data; it defaults
 853   to 0, meaning that no ancillary data will be received.  Appropriate
 854   buffer sizes for ancillary data can be calculated using
 855   :func:`CMSG_SPACE` or :func:`CMSG_LEN`, and items which do not fit
 856   into the buffer might be truncated or discarded.  The *flags*
 857   argument defaults to 0 and has the same meaning as for
 858   :meth:`recv`.
 859
 860   The return value is a 4-tuple: ``(data, ancdata, msg_flags,
 861   address)``.  The *data* item is a :class:`bytes` object holding the
 862   non-ancillary data received.  The *ancdata* item is a list of zero
 863   or more tuples ``(cmsg_level, cmsg_type, cmsg_data)`` representing
 864   the ancillary data (control messages) received: *cmsg_level* and
 865   *cmsg_type* are integers specifying the protocol level and
 866   protocol-specific type respectively, and *cmsg_data* is a
 867   :class:`bytes` object holding the associated data.  The *msg_flags*
 868   item is the bitwise OR of various flags indicating conditions on
 869   the received message; see your system documentation for details.
 870   If the receiving socket is unconnected, *address* is the address of
 871   the sending socket, if available; otherwise, its value is
 872   unspecified.
 873
 874   On some systems, :meth:`sendmsg` and :meth:`recvmsg` can be used to
 875   pass file descriptors between processes over an :const:`AF_UNIX`
 876   socket.  When this facility is used (it is often restricted to
 877   :const:`SOCK_STREAM` sockets), :meth:`recvmsg` will return, in its
 878   ancillary data, items of the form ``(socket.SOL_SOCKET,
 879   socket.SCM_RIGHTS, fds)``, where *fds* is a :class:`bytes` object
 880   representing the new file descriptors as a binary array of the
 881   native C :c:type:`int` type.  If :meth:`recvmsg` raises an
 882   exception after the system call returns, it will first attempt to
 883   close any file descriptors received via this mechanism.
 884
 885   Some systems do not indicate the truncated length of ancillary data
 886   items which have been only partially received.  If an item appears
 887   to extend beyond the end of the buffer, :meth:`recvmsg` will issue
 888   a :exc:`RuntimeWarning`, and will return the part of it which is
 889   inside the buffer provided it has not been truncated before the
 890   start of its associated data.
 891
 892   On systems which support the :const:`SCM_RIGHTS` mechanism, the
 893   following function will receive up to *maxfds* file descriptors,
 894   returning the message data and a list containing the descriptors
 895   (while ignoring unexpected conditions such as unrelated control
 896   messages being received).  See also :meth:`sendmsg`. ::
 897
 898      import socket, array
 899
 900      def recv_fds(sock, msglen, maxfds):
 901          fds = array.array("i")   # Array of ints
 902          msg, ancdata, flags, addr = sock.recvmsg(msglen, socket.CMSG_LEN(maxfds * fds.itemsize))
 903          for cmsg_level, cmsg_type, cmsg_data in ancdata:
 904              if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
 905                  # Append data, ignoring any truncated integers at the end.
 906                  fds.fromstring(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
 907          return msg, list(fds)
 908
 909   Availability: most Unix platforms, possibly others.
 910
 911   .. versionadded:: 3.3
 912
 913
 914.. method:: socket.recvmsg_into(buffers[, ancbufsize[, flags]])
 915
 916   Receive normal data and ancillary data from the socket, behaving as
 917   :meth:`recvmsg` would, but scatter the non-ancillary data into a
 918   series of buffers instead of returning a new bytes object.  The
 919   *buffers* argument must be an iterable of objects that export
 920   writable buffers (e.g. :class:`bytearray` objects); these will be
 921   filled with successive chunks of the non-ancillary data until it
 922   has all been written or there are no more buffers.  The operating
 923   system may set a limit (:func:`~os.sysconf` value ``SC_IOV_MAX``)
 924   on the number of buffers that can be used.  The *ancbufsize* and
 925   *flags* arguments have the same meaning as for :meth:`recvmsg`.
 926
 927   The return value is a 4-tuple: ``(nbytes, ancdata, msg_flags,
 928   address)``, where *nbytes* is the total number of bytes of
 929   non-ancillary data written into the buffers, and *ancdata*,
 930   *msg_flags* and *address* are the same as for :meth:`recvmsg`.
 931
 932   Example::
 933
 934      >>> import socket
 935      >>> s1, s2 = socket.socketpair()
 936      >>> b1 = bytearray(b'----')
 937      >>> b2 = bytearray(b'0123456789')
 938      >>> b3 = bytearray(b'--------------')
 939      >>> s1.send(b'Mary had a little lamb')
 940      22
 941      >>> s2.recvmsg_into([b1, memoryview(b2)[2:9], b3])
 942      (22, [], 0, None)
 943      >>> [b1, b2, b3]
 944      [bytearray(b'Mary'), bytearray(b'01 had a 9'), bytearray(b'little lamb---')]
 945
 946   Availability: most Unix platforms, possibly others.
 947
 948   .. versionadded:: 3.3
 949
 950
 951.. method:: socket.recvfrom_into(buffer[, nbytes[, flags]])
 952
 953   Receive data from the socket, writing it into *buffer* instead of creating a
 954   new bytestring.  The return value is a pair ``(nbytes, address)`` where *nbytes* is
 955   the number of bytes received and *address* is the address of the socket sending
 956   the data.  See the Unix manual page :manpage:`recv(2)` for the meaning of the
 957   optional argument *flags*; it defaults to zero.  (The format of *address*
 958   depends on the address family --- see above.)
 959
 960
 961.. method:: socket.recv_into(buffer[, nbytes[, flags]])
 962
 963   Receive up to *nbytes* bytes from the socket, storing the data into a buffer
 964   rather than creating a new bytestring.  If *nbytes* is not specified (or 0),
 965   receive up to the size available in the given buffer.  Returns the number of
 966   bytes received.  See the Unix manual page :manpage:`recv(2)` for the meaning
 967   of the optional argument *flags*; it defaults to zero.
 968
 969
 970.. method:: socket.send(bytes[, flags])
 971
 972   Send data to the socket.  The socket must be connected to a remote socket.  The
 973   optional *flags* argument has the same meaning as for :meth:`recv` above.
 974   Returns the number of bytes sent. Applications are responsible for checking that
 975   all data has been sent; if only some of the data was transmitted, the
 976   application needs to attempt delivery of the remaining data.
 977
 978
 979.. method:: socket.sendall(bytes[, flags])
 980
 981   Send data to the socket.  The socket must be connected to a remote socket.  The
 982   optional *flags* argument has the same meaning as for :meth:`recv` above.
 983   Unlike :meth:`send`, this method continues to send data from *bytes* until
 984   either all data has been sent or an error occurs.  ``None`` is returned on
 985   success.  On error, an exception is raised, and there is no way to determine how
 986   much data, if any, was successfully sent.
 987
 988
 989.. method:: socket.sendto(bytes[, flags], address)
 990
 991   Send data to the socket.  The socket should not be connected to a remote socket,
 992   since the destination socket is specified by *address*.  The optional *flags*
 993   argument has the same meaning as for :meth:`recv` above.  Return the number of
 994   bytes sent. (The format of *address* depends on the address family --- see
 995   above.)
 996
 997
 998.. method:: socket.sendmsg(buffers[, ancdata[, flags[, address]]])
 999
1000   Send normal and ancillary data to the socket, gathering the
1001   non-ancillary data from a series of buffers and concatenating it
1002   into a single message.  The *buffers* argument specifies the
1003   non-ancillary data as an iterable of buffer-compatible objects
1004   (e.g. :class:`bytes` objects); the operating system may set a limit
1005   (:func:`~os.sysconf` value ``SC_IOV_MAX``) on the number of buffers
1006   that can be used.  The *ancdata* argument specifies the ancillary
1007   data (control messages) as an iterable of zero or more tuples
1008   ``(cmsg_level, cmsg_type, cmsg_data)``, where *cmsg_level* and
1009   *cmsg_type* are integers specifying the protocol level and
1010   protocol-specific type respectively, and *cmsg_data* is a
1011   buffer-compatible object holding the associated data.  Note that
1012   some systems (in particular, systems without :func:`CMSG_SPACE`)
1013   might support sending only one control message per call.  The
1014   *flags* argument defaults to 0 and has the same meaning as for
1015   :meth:`send`.  If *address* is supplied and not ``None``, it sets a
1016   destination address for the message.  The return value is the
1017   number of bytes of non-ancillary data sent.
1018
1019   The following function sends the list of file descriptors *fds*
1020   over an :const:`AF_UNIX` socket, on systems which support the
1021   :const:`SCM_RIGHTS` mechanism.  See also :meth:`recvmsg`. ::
1022
1023      import socket, array
1024
1025      def send_fds(sock, msg, fds):
1026          return sock.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds))])
1027
1028   Availability: most Unix platforms, possibly others.
1029
1030   .. versionadded:: 3.3
1031
1032
1033.. method:: socket.setblocking(flag)
1034
1035   Set blocking or non-blocking mode of the socket: if *flag* is false, the
1036   socket is set to non-blocking, else to blocking mode.
1037
1038   This method is a shorthand for certain :meth:`~socket.settimeout` calls:
1039
1040   * ``sock.setblocking(True)`` is equivalent to ``sock.settimeout(None)``
1041
1042   * ``sock.setblocking(False)`` is equivalent to ``sock.settimeout(0.0)``
1043
1044
1045.. method:: socket.settimeout(value)
1046
1047   Set a timeout on blocking socket operations.  The *value* argument can be a
1048   nonnegative floating point number expressing seconds, or ``None``.
1049   If a non-zero value is given, subsequent socket operations will raise a
1050   :exc:`timeout` exception if the timeout period *value* has elapsed before
1051   the operation has completed.  If zero is given, the socket is put in
1052   non-blocking mode. If ``None`` is given, the socket is put in blocking mode.
1053
1054   For further information, please consult the :ref:`notes on socket timeouts <socket-timeouts>`.
1055
1056
1057.. method:: socket.setsockopt(level, optname, value)
1058
1059   .. index:: module: struct
1060
1061   Set the value of the given socket option (see the Unix manual page
1062   :manpage:`setsockopt(2)`).  The needed symbolic constants are defined in the
1063   :mod:`socket` module (:const:`SO_\*` etc.).  The value can be an integer or a
1064   bytes object representing a buffer.  In the latter case it is up to the caller to
1065   ensure that the bytestring contains the proper bits (see the optional built-in
1066   module :mod:`struct` for a way to encode C structures as bytestrings).
1067
1068
1069.. method:: socket.shutdown(how)
1070
1071   Shut down one or both halves of the connection.  If *how* is :const:`SHUT_RD`,
1072   further receives are disallowed.  If *how* is :const:`SHUT_WR`, further sends
1073   are disallowed.  If *how* is :const:`SHUT_RDWR`, further sends and receives are
1074   disallowed.  Depending on the platform, shutting down one half of the connection
1075   can also close the opposite half (e.g. on Mac OS X, ``shutdown(SHUT_WR)`` does
1076   not allow further reads on the other end of the connection).
1077
1078Note that there are no methods :meth:`read` or :meth:`write`; use
1079:meth:`~socket.recv` and :meth:`~socket.send` without *flags* argument instead.
1080
1081Socket objects also have these (read-only) attributes that correspond to the
1082values given to the :class:`socket` constructor.
1083
1084
1085.. attribute:: socket.family
1086
1087   The socket family.
1088
1089
1090.. attribute:: socket.type
1091
1092   The socket type.
1093
1094
1095.. attribute:: socket.proto
1096
1097   The socket protocol.
1098
1099
1100
1101.. _socket-timeouts:
1102
1103Notes on socket timeouts
1104------------------------
1105
1106A socket object can be in one of three modes: blocking, non-blocking, or
1107timeout.  Sockets are by default always created in blocking mode, but this
1108can be changed by calling :func:`setdefaulttimeout`.
1109
1110* In *blocking mode*, operations block until complete or the system returns
1111  an error (such as connection timed out).
1112
1113* In *non-blocking mode*, operations fail (with an error that is unfortunately
1114  system-dependent) if they cannot be completed immediately: functions from the
1115  :mod:`select` can be used to know when and whether a socket is available for
1116  reading or writing.
1117
1118* In *timeout mode*, operations fail if they cannot be completed within the
1119  timeout specified for the socket (they raise a :exc:`timeout` exception)
1120  or if the system returns an error.
1121
1122.. note::
1123   At the operating system level, sockets in *timeout mode* are internally set
1124   in non-blocking mode.  Also, the blocking and timeout modes are shared between
1125   file descriptors and socket objects that refer to the same network endpoint.
1126   This implementation detail can have visible consequences if e.g. you decide
1127   to use the :meth:`~socket.fileno()` of a socket.
1128
1129Timeouts and the ``connect`` method
1130^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1131
1132The :meth:`~socket.connect` operation is also subject to the timeout
1133setting, and in general it is recommended to call :meth:`~socket.settimeout`
1134before calling :meth:`~socket.connect` or pass a timeout parameter to
1135:meth:`create_connection`.  However, the system network stack may also
1136return a connection timeout error of its own regardless of any Python socket
1137timeout setting.
1138
1139Timeouts and the ``accept`` method
1140^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1141
1142If :func:`getdefaulttimeout` is not :const:`None`, sockets returned by
1143the :meth:`~socket.accept` method inherit that timeout.  Otherwise, the
1144behaviour depends on settings of the listening socket:
1145
1146* if the listening socket is in *blocking mode* or in *timeout mode*,
1147  the socket returned by :meth:`~socket.accept` is in *blocking mode*;
1148
1149* if the listening socket is in *non-blocking mode*, whether the socket
1150  returned by :meth:`~socket.accept` is in blocking or non-blocking mode
1151  is operating system-dependent.  If you want to ensure cross-platform
1152  behaviour, it is recommended you manually override this setting.
1153
1154
1155.. _socket-example:
1156
1157Example
1158-------
1159
1160Here are four minimal example programs using the TCP/IP protocol: a server that
1161echoes all data that it receives back (servicing only one client), and a client
1162using it.  Note that a server must perform the sequence :func:`socket`,
1163:meth:`~socket.bind`, :meth:`~socket.listen`, :meth:`~socket.accept` (possibly
1164repeating the :meth:`~socket.accept` to service more than one client), while a
1165client only needs the sequence :func:`socket`, :meth:`~socket.connect`.  Also
1166note that the server does not :meth:`~socket.send`/:meth:`~socket.recv` on the
1167socket it is listening on but on the new socket returned by
1168:meth:`~socket.accept`.
1169
1170The first two examples support IPv4 only. ::
1171
1172   # Echo server program
1173   import socket
1174
1175   HOST = ''                 # Symbolic name meaning all available interfaces
1176   PORT = 50007              # Arbitrary non-privileged port
1177   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1178   s.bind((HOST, PORT))
1179   s.listen(1)
1180   conn, addr = s.accept()
1181   print('Connected by', addr)
1182   while True:
1183       data = conn.recv(1024)
1184       if not data: break
1185       conn.send(data)
1186   conn.close()
1187
1188::
1189
1190   # Echo client program
1191   import socket
1192
1193   HOST = 'daring.cwi.nl'    # The remote host
1194   PORT = 50007              # The same port as used by the server
1195   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1196   s.connect((HOST, PORT))
1197   s.send(b'Hello, world')
1198   data = s.recv(1024)
1199   s.close()
1200   print('Received', repr(data))
1201
1202The next two examples are identical to the above two, but support both IPv4 and
1203IPv6. The server side will listen to the first address family available (it
1204should listen to both instead). On most of IPv6-ready systems, IPv6 will take
1205precedence and the server may not accept IPv4 traffic. The client side will try
1206to connect to the all addresses returned as a result of the name resolution, and
1207sends traffic to the first one connected successfully. ::
1208
1209   # Echo server program
1210   import socket
1211   import sys
1212
1213   HOST = None               # Symbolic name meaning all available interfaces
1214   PORT = 50007              # Arbitrary non-privileged port
1215   s = None
1216   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
1217                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
1218       af, socktype, proto, canonname, sa = res
1219       try:
1220           s = socket.socket(af, socktype, proto)
1221       except OSError as msg:
1222           s = None
1223           continue
1224       try:
1225           s.bind(sa)
1226           s.listen(1)
1227       except OSError as msg:
1228           s.close()
1229           s = None
1230           continue
1231       break
1232   if s is None:
1233       print('could not open socket')
1234       sys.exit(1)
1235   conn, addr = s.accept()
1236   print('Connected by', addr)
1237   while True:
1238       data = conn.recv(1024)
1239       if not data: break
1240       conn.send(data)
1241   conn.close()
1242
1243::
1244
1245   # Echo client program
1246   import socket
1247   import sys
1248
1249   HOST = 'daring.cwi.nl'    # The remote host
1250   PORT = 50007              # The same port as used by the server
1251   s = None
1252   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
1253       af, socktype, proto, canonname, sa = res
1254       try:
1255           s = socket.socket(af, socktype, proto)
1256       except OSError as msg:
1257           s = None
1258           continue
1259       try:
1260           s.connect(sa)
1261       except OSError as msg:
1262           s.close()
1263           s = None
1264           continue
1265       break
1266   if s is None:
1267       print('could not open socket')
1268       sys.exit(1)
1269   s.send(b'Hello, world')
1270   data = s.recv(1024)
1271   s.close()
1272   print('Received', repr(data))
1273
1274
1275The next example shows how to write a very simple network sniffer with raw
1276sockets on Windows. The example requires administrator privileges to modify
1277the interface::
1278
1279   import socket
1280
1281   # the public network interface
1282   HOST = socket.gethostbyname(socket.gethostname())
1283
1284   # create a raw socket and bind it to the public interface
1285   s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
1286   s.bind((HOST, 0))
1287
1288   # Include IP headers
1289   s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)
1290
1291   # receive all packages
1292   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)
1293
1294   # receive a package
1295   print(s.recvfrom(65565))
1296
1297   # disabled promiscuous mode
1298   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)
1299
1300The last example shows how to use the socket interface to communicate to a CAN
1301network. This example might require special priviledge::
1302
1303   import socket
1304   import struct
1305
1306
1307   # CAN frame packing/unpacking (see `struct can_frame` in <linux/can.h>)
1308
1309   can_frame_fmt = "=IB3x8s"
1310   can_frame_size = struct.calcsize(can_frame_fmt)
1311
1312   def build_can_frame(can_id, data):
1313       can_dlc = len(data)
1314       data = data.ljust(8, b'\x00')
1315       return struct.pack(can_frame_fmt, can_id, can_dlc, data)
1316
1317   def dissect_can_frame(frame):
1318       can_id, can_dlc, data = struct.unpack(can_frame_fmt, frame)
1319       return (can_id, can_dlc, data[:can_dlc])
1320
1321
1322   # create a raw socket and bind it to the `vcan0` interface
1323   s = socket.socket(socket.AF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
1324   s.bind(('vcan0',))
1325
1326   while True:
1327       cf, addr = s.recvfrom(can_frame_size)
1328
1329       print('Received: can_id=%x, can_dlc=%x, data=%s' % dissect_can_frame(cf))
1330
1331       try:
1332           s.send(cf)
1333       except OSError:
1334           print('Error sending CAN frame')
1335
1336       try:
1337           s.send(build_can_frame(0x01, b'\x01\x02\x03'))
1338       except OSError:
1339           print('Error sending CAN frame')
1340
1341Running an example several times with too small delay between executions, could
1342lead to this error::
1343
1344   OSError: [Errno 98] Address already in use
1345
1346This is because the previous execution has left the socket in a ``TIME_WAIT``
1347state, and can't be immediately reused.
1348
1349There is a :mod:`socket` flag to set, in order to prevent this,
1350:data:`socket.SO_REUSEADDR`::
1351
1352   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1353   s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
1354   s.bind((HOST, PORT))
1355
1356the :data:`SO_REUSEADDR` flag tells the kernel to reuse a local socket in
1357``TIME_WAIT`` state, without waiting for its natural timeout to expire.
1358
1359
1360.. seealso::
1361
1362   For an introduction to socket programming (in C), see the following papers:
1363
1364   - *An Introductory 4.3BSD Interprocess Communication Tutorial*, by Stuart Sechrest
1365
1366   - *An Advanced 4.3BSD Interprocess Communication Tutorial*, by Samuel J.  Leffler et
1367     al,
1368
1369   both in the UNIX Programmer's Manual, Supplementary Documents 1 (sections
1370   PS1:7 and PS1:8).  The platform-specific reference material for the various
1371   socket-related system calls are also a valuable source of information on the
1372   details of socket semantics.  For Unix, refer to the manual pages; for Windows,
1373   see the WinSock (or Winsock 2) specification.  For IPv6-ready APIs, readers may
1374   want to refer to :rfc:`3493` titled Basic Socket Interface Extensions for IPv6.
1375