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

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