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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 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- 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.. data:: SocketType
 684
 685   This is a Python type object that represents the socket object type. It is the
 686   same as ``type(socket(...))``.
 687
 688
 689.. _socket-objects:
 690
 691Socket Objects
 692--------------
 693
 694Socket objects have the following methods.  Except for :meth:`makefile` these
 695correspond to Unix system calls applicable to sockets.
 696
 697
 698.. method:: socket.accept()
 699
 700   Accept a connection. The socket must be bound to an address and listening for
 701   connections. The return value is a pair ``(conn, address)`` where *conn* is a
 702   *new* socket object usable to send and receive data on the connection, and
 703   *address* is the address bound to the socket on the other end of the connection.
 704
 705
 706.. method:: socket.bind(address)
 707
 708   Bind the socket to *address*.  The socket must not already be bound. (The format
 709   of *address* depends on the address family --- see above.)
 710
 711
 712.. method:: socket.close()
 713
 714   Close the socket.  All future operations on the socket object will fail. The
 715   remote end will receive no more data (after queued data is flushed). Sockets are
 716   automatically closed when they are garbage-collected.
 717
 718   .. note::
 719      :meth:`close()` releases the resource associated with a connection but
 720      does not necessarily close the connection immediately.  If you want
 721      to close the connection in a timely fashion, call :meth:`shutdown()`
 722      before :meth:`close()`.
 723
 724
 725.. method:: socket.connect(address)
 726
 727   Connect to a remote socket at *address*. (The format of *address* depends on the
 728   address family --- see above.)
 729
 730
 731.. method:: socket.connect_ex(address)
 732
 733   Like ``connect(address)``, but return an error indicator instead of raising an
 734   exception for errors returned by the C-level :c:func:`connect` call (other
 735   problems, such as "host not found," can still raise exceptions).  The error
 736   indicator is ``0`` if the operation succeeded, otherwise the value of the
 737   :c:data:`errno` variable.  This is useful to support, for example, asynchronous
 738   connects.
 739
 740
 741.. method:: socket.detach()
 742
 743   Put the socket object into closed state without actually closing the
 744   underlying file descriptor.  The file descriptor is returned, and can
 745   be reused for other purposes.
 746
 747   .. versionadded:: 3.2
 748
 749
 750.. method:: socket.fileno()
 751
 752   Return the socket's file descriptor (a small integer).  This is useful with
 753   :func:`select.select`.
 754
 755   Under Windows the small integer returned by this method cannot be used where a
 756   file descriptor can be used (such as :func:`os.fdopen`).  Unix does not have
 757   this limitation.
 758
 759
 760.. method:: socket.getpeername()
 761
 762   Return the remote address to which the socket is connected.  This is useful to
 763   find out the port number of a remote IPv4/v6 socket, for instance. (The format
 764   of the address returned depends on the address family --- see above.)  On some
 765   systems this function is not supported.
 766
 767
 768.. method:: socket.getsockname()
 769
 770   Return the socket's own address.  This is useful to find out the port number of
 771   an IPv4/v6 socket, for instance. (The format of the address returned depends on
 772   the address family --- see above.)
 773
 774
 775.. method:: socket.getsockopt(level, optname[, buflen])
 776
 777   Return the value of the given socket option (see the Unix man page
 778   :manpage:`getsockopt(2)`).  The needed symbolic constants (:const:`SO_\*` etc.)
 779   are defined in this module.  If *buflen* is absent, an integer option is assumed
 780   and its integer value is returned by the function.  If *buflen* is present, it
 781   specifies the maximum length of the buffer used to receive the option in, and
 782   this buffer is returned as a bytes object.  It is up to the caller to decode the
 783   contents of the buffer (see the optional built-in module :mod:`struct` for a way
 784   to decode C structures encoded as byte strings).
 785
 786
 787.. method:: socket.gettimeout()
 788
 789   Return the timeout in seconds (float) associated with socket operations,
 790   or ``None`` if no timeout is set.  This reflects the last call to
 791   :meth:`setblocking` or :meth:`settimeout`.
 792
 793
 794.. method:: socket.ioctl(control, option)
 795
 796   :platform: Windows
 797
 798   The :meth:`ioctl` method is a limited interface to the WSAIoctl system
 799   interface.  Please refer to the `Win32 documentation
 800   <http://msdn.microsoft.com/en-us/library/ms741621%28VS.85%29.aspx>`_ for more
 801   information.
 802
 803   On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
 804   functions may be used; they accept a socket object as their first argument.
 805
 806.. method:: socket.listen(backlog)
 807
 808   Listen for connections made to the socket.  The *backlog* argument specifies the
 809   maximum number of queued connections and should be at least 0; the maximum value
 810   is system-dependent (usually 5), the minimum value is forced to 0.
 811
 812
 813.. method:: socket.makefile(mode='r', buffering=None, *, encoding=None, \
 814                            errors=None, newline=None)
 815
 816   .. index:: single: I/O control; buffering
 817
 818   Return a :term:`file object` associated with the socket.  The exact returned
 819   type depends on the arguments given to :meth:`makefile`.  These arguments are
 820   interpreted the same way as by the built-in :func:`open` function.
 821
 822   Closing the file object won't close the socket unless there are no remaining
 823   references to the socket.  The socket must be in blocking mode; it can have
 824   a timeout, but the file object's internal buffer may end up in a inconsistent
 825   state if a timeout occurs.
 826
 827   .. note::
 828
 829      On Windows, the file-like object created by :meth:`makefile` cannot be
 830      used where a file object with a file descriptor is expected, such as the
 831      stream arguments of :meth:`subprocess.Popen`.
 832
 833
 834.. method:: socket.recv(bufsize[, flags])
 835
 836   Receive data from the socket.  The return value is a bytes object representing the
 837   data received.  The maximum amount of data to be received at once is specified
 838   by *bufsize*.  See the Unix manual page :manpage:`recv(2)` for the meaning of
 839   the optional argument *flags*; it defaults to zero.
 840
 841   .. note::
 842
 843      For best match with hardware and network realities, the value of  *bufsize*
 844      should be a relatively small power of 2, for example, 4096.
 845
 846
 847.. method:: socket.recvfrom(bufsize[, flags])
 848
 849   Receive data from the socket.  The return value is a pair ``(bytes, address)``
 850   where *bytes* is a bytes object representing the data received and *address* is the
 851   address of the socket sending the data.  See the Unix manual page
 852   :manpage:`recv(2)` for the meaning of the optional argument *flags*; it defaults
 853   to zero. (The format of *address* depends on the address family --- see above.)
 854
 855
 856.. method:: socket.recvmsg(bufsize[, ancbufsize[, flags]])
 857
 858   Receive normal data (up to *bufsize* bytes) and ancillary data from
 859   the socket.  The *ancbufsize* argument sets the size in bytes of
 860   the internal buffer used to receive the ancillary data; it defaults
 861   to 0, meaning that no ancillary data will be received.  Appropriate
 862   buffer sizes for ancillary data can be calculated using
 863   :func:`CMSG_SPACE` or :func:`CMSG_LEN`, and items which do not fit
 864   into the buffer might be truncated or discarded.  The *flags*
 865   argument defaults to 0 and has the same meaning as for
 866   :meth:`recv`.
 867
 868   The return value is a 4-tuple: ``(data, ancdata, msg_flags,
 869   address)``.  The *data* item is a :class:`bytes` object holding the
 870   non-ancillary data received.  The *ancdata* item is a list of zero
 871   or more tuples ``(cmsg_level, cmsg_type, cmsg_data)`` representing
 872   the ancillary data (control messages) received: *cmsg_level* and
 873   *cmsg_type* are integers specifying the protocol level and
 874   protocol-specific type respectively, and *cmsg_data* is a
 875   :class:`bytes` object holding the associated data.  The *msg_flags*
 876   item is the bitwise OR of various flags indicating conditions on
 877   the received message; see your system documentation for details.
 878   If the receiving socket is unconnected, *address* is the address of
 879   the sending socket, if available; otherwise, its value is
 880   unspecified.
 881
 882   On some systems, :meth:`sendmsg` and :meth:`recvmsg` can be used to
 883   pass file descriptors between processes over an :const:`AF_UNIX`
 884   socket.  When this facility is used (it is often restricted to
 885   :const:`SOCK_STREAM` sockets), :meth:`recvmsg` will return, in its
 886   ancillary data, items of the form ``(socket.SOL_SOCKET,
 887   socket.SCM_RIGHTS, fds)``, where *fds* is a :class:`bytes` object
 888   representing the new file descriptors as a binary array of the
 889   native C :c:type:`int` type.  If :meth:`recvmsg` raises an
 890   exception after the system call returns, it will first attempt to
 891   close any file descriptors received via this mechanism.
 892
 893   Some systems do not indicate the truncated length of ancillary data
 894   items which have been only partially received.  If an item appears
 895   to extend beyond the end of the buffer, :meth:`recvmsg` will issue
 896   a :exc:`RuntimeWarning`, and will return the part of it which is
 897   inside the buffer provided it has not been truncated before the
 898   start of its associated data.
 899
 900   On systems which support the :const:`SCM_RIGHTS` mechanism, the
 901   following function will receive up to *maxfds* file descriptors,
 902   returning the message data and a list containing the descriptors
 903   (while ignoring unexpected conditions such as unrelated control
 904   messages being received).  See also :meth:`sendmsg`. ::
 905
 906      import socket, array
 907
 908      def recv_fds(sock, msglen, maxfds):
 909          fds = array.array("i")   # Array of ints
 910          msg, ancdata, flags, addr = sock.recvmsg(msglen, socket.CMSG_LEN(maxfds * fds.itemsize))
 911          for cmsg_level, cmsg_type, cmsg_data in ancdata:
 912              if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
 913                  # Append data, ignoring any truncated integers at the end.
 914                  fds.fromstring(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
 915          return msg, list(fds)
 916
 917   Availability: most Unix platforms, possibly others.
 918
 919   .. versionadded:: 3.3
 920
 921
 922.. method:: socket.recvmsg_into(buffers[, ancbufsize[, flags]])
 923
 924   Receive normal data and ancillary data from the socket, behaving as
 925   :meth:`recvmsg` would, but scatter the non-ancillary data into a
 926   series of buffers instead of returning a new bytes object.  The
 927   *buffers* argument must be an iterable of objects that export
 928   writable buffers (e.g. :class:`bytearray` objects); these will be
 929   filled with successive chunks of the non-ancillary data until it
 930   has all been written or there are no more buffers.  The operating
 931   system may set a limit (:func:`~os.sysconf` value ``SC_IOV_MAX``)
 932   on the number of buffers that can be used.  The *ancbufsize* and
 933   *flags* arguments have the same meaning as for :meth:`recvmsg`.
 934
 935   The return value is a 4-tuple: ``(nbytes, ancdata, msg_flags,
 936   address)``, where *nbytes* is the total number of bytes of
 937   non-ancillary data written into the buffers, and *ancdata*,
 938   *msg_flags* and *address* are the same as for :meth:`recvmsg`.
 939
 940   Example::
 941
 942      >>> import socket
 943      >>> s1, s2 = socket.socketpair()
 944      >>> b1 = bytearray(b'----')
 945      >>> b2 = bytearray(b'0123456789')
 946      >>> b3 = bytearray(b'--------------')
 947      >>> s1.send(b'Mary had a little lamb')
 948      22
 949      >>> s2.recvmsg_into([b1, memoryview(b2)[2:9], b3])
 950      (22, [], 0, None)
 951      >>> [b1, b2, b3]
 952      [bytearray(b'Mary'), bytearray(b'01 had a 9'), bytearray(b'little lamb---')]
 953
 954   Availability: most Unix platforms, possibly others.
 955
 956   .. versionadded:: 3.3
 957
 958
 959.. method:: socket.recvfrom_into(buffer[, nbytes[, flags]])
 960
 961   Receive data from the socket, writing it into *buffer* instead of creating a
 962   new bytestring.  The return value is a pair ``(nbytes, address)`` where *nbytes* is
 963   the number of bytes received and *address* is the address of the socket sending
 964   the data.  See the Unix manual page :manpage:`recv(2)` for the meaning of the
 965   optional argument *flags*; it defaults to zero.  (The format of *address*
 966   depends on the address family --- see above.)
 967
 968
 969.. method:: socket.recv_into(buffer[, nbytes[, flags]])
 970
 971   Receive up to *nbytes* bytes from the socket, storing the data into a buffer
 972   rather than creating a new bytestring.  If *nbytes* is not specified (or 0),
 973   receive up to the size available in the given buffer.  Returns the number of
 974   bytes received.  See the Unix manual page :manpage:`recv(2)` for the meaning
 975   of the optional argument *flags*; it defaults to zero.
 976
 977
 978.. method:: socket.send(bytes[, flags])
 979
 980   Send data to the socket.  The socket must be connected to a remote socket.  The
 981   optional *flags* argument has the same meaning as for :meth:`recv` above.
 982   Returns the number of bytes sent. Applications are responsible for checking that
 983   all data has been sent; if only some of the data was transmitted, the
 984   application needs to attempt delivery of the remaining data. For further
 985   information on this topic, consult the :ref:`socket-howto`.
 986
 987
 988.. method:: socket.sendall(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   Unlike :meth:`send`, this method continues to send data from *bytes* until
 993   either all data has been sent or an error occurs.  ``None`` is returned on
 994   success.  On error, an exception is raised, and there is no way to determine how
 995   much data, if any, was successfully sent.
 996
 997
 998.. method:: socket.sendto(bytes[, flags], address)
 999
1000   Send data to the socket.  The socket should not be connected to a remote socket,
1001   since the destination socket is specified by *address*.  The optional *flags*
1002   argument has the same meaning as for :meth:`recv` above.  Return the number of
1003   bytes sent. (The format of *address* depends on the address family --- see
1004   above.)
1005
1006
1007.. method:: socket.sendmsg(buffers[, ancdata[, flags[, address]]])
1008
1009   Send normal and ancillary data to the socket, gathering the
1010   non-ancillary data from a series of buffers and concatenating it
1011   into a single message.  The *buffers* argument specifies the
1012   non-ancillary data as an iterable of buffer-compatible objects
1013   (e.g. :class:`bytes` objects); the operating system may set a limit
1014   (:func:`~os.sysconf` value ``SC_IOV_MAX``) on the number of buffers
1015   that can be used.  The *ancdata* argument specifies the ancillary
1016   data (control messages) as an iterable of zero or more tuples
1017   ``(cmsg_level, cmsg_type, cmsg_data)``, where *cmsg_level* and
1018   *cmsg_type* are integers specifying the protocol level and
1019   protocol-specific type respectively, and *cmsg_data* is a
1020   buffer-compatible object holding the associated data.  Note that
1021   some systems (in particular, systems without :func:`CMSG_SPACE`)
1022   might support sending only one control message per call.  The
1023   *flags* argument defaults to 0 and has the same meaning as for
1024   :meth:`send`.  If *address* is supplied and not ``None``, it sets a
1025   destination address for the message.  The return value is the
1026   number of bytes of non-ancillary data sent.
1027
1028   The following function sends the list of file descriptors *fds*
1029   over an :const:`AF_UNIX` socket, on systems which support the
1030   :const:`SCM_RIGHTS` mechanism.  See also :meth:`recvmsg`. ::
1031
1032      import socket, array
1033
1034      def send_fds(sock, msg, fds):
1035          return sock.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds))])
1036
1037   Availability: most Unix platforms, possibly others.
1038
1039   .. versionadded:: 3.3
1040
1041
1042.. method:: socket.setblocking(flag)
1043
1044   Set blocking or non-blocking mode of the socket: if *flag* is false, the
1045   socket is set to non-blocking, else to blocking mode.
1046
1047   This method is a shorthand for certain :meth:`~socket.settimeout` calls:
1048
1049   * ``sock.setblocking(True)`` is equivalent to ``sock.settimeout(None)``
1050
1051   * ``sock.setblocking(False)`` is equivalent to ``sock.settimeout(0.0)``
1052
1053
1054.. method:: socket.settimeout(value)
1055
1056   Set a timeout on blocking socket operations.  The *value* argument can be a
1057   nonnegative floating point number expressing seconds, or ``None``.
1058   If a non-zero value is given, subsequent socket operations will raise a
1059   :exc:`timeout` exception if the timeout period *value* has elapsed before
1060   the operation has completed.  If zero is given, the socket is put in
1061   non-blocking mode. If ``None`` is given, the socket is put in blocking mode.
1062
1063   For further information, please consult the :ref:`notes on socket timeouts <socket-timeouts>`.
1064
1065
1066.. method:: socket.setsockopt(level, optname, value)
1067
1068   .. index:: module: struct
1069
1070   Set the value of the given socket option (see the Unix manual page
1071   :manpage:`setsockopt(2)`).  The needed symbolic constants are defined in the
1072   :mod:`socket` module (:const:`SO_\*` etc.).  The value can be an integer or a
1073   bytes object representing a buffer.  In the latter case it is up to the caller to
1074   ensure that the bytestring contains the proper bits (see the optional built-in
1075   module :mod:`struct` for a way to encode C structures as bytestrings).
1076
1077
1078.. method:: socket.shutdown(how)
1079
1080   Shut down one or both halves of the connection.  If *how* is :const:`SHUT_RD`,
1081   further receives are disallowed.  If *how* is :const:`SHUT_WR`, further sends
1082   are disallowed.  If *how* is :const:`SHUT_RDWR`, further sends and receives are
1083   disallowed.
1084
1085Note that there are no methods :meth:`read` or :meth:`write`; use
1086:meth:`~socket.recv` and :meth:`~socket.send` without *flags* argument instead.
1087
1088Socket objects also have these (read-only) attributes that correspond to the
1089values given to the :class:`socket` constructor.
1090
1091
1092.. attribute:: socket.family
1093
1094   The socket family.
1095
1096
1097.. attribute:: socket.type
1098
1099   The socket type.
1100
1101
1102.. attribute:: socket.proto
1103
1104   The socket protocol.
1105
1106
1107
1108.. _socket-timeouts:
1109
1110Notes on socket timeouts
1111------------------------
1112
1113A socket object can be in one of three modes: blocking, non-blocking, or
1114timeout.  Sockets are by default always created in blocking mode, but this
1115can be changed by calling :func:`setdefaulttimeout`.
1116
1117* In *blocking mode*, operations block until complete or the system returns
1118  an error (such as connection timed out).
1119
1120* In *non-blocking mode*, operations fail (with an error that is unfortunately
1121  system-dependent) if they cannot be completed immediately: functions from the
1122  :mod:`select` can be used to know when and whether a socket is available for
1123  reading or writing.
1124
1125* In *timeout mode*, operations fail if they cannot be completed within the
1126  timeout specified for the socket (they raise a :exc:`timeout` exception)
1127  or if the system returns an error.
1128
1129.. note::
1130   At the operating system level, sockets in *timeout mode* are internally set
1131   in non-blocking mode.  Also, the blocking and timeout modes are shared between
1132   file descriptors and socket objects that refer to the same network endpoint.
1133   This implementation detail can have visible consequences if e.g. you decide
1134   to use the :meth:`~socket.fileno()` of a socket.
1135
1136Timeouts and the ``connect`` method
1137^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1138
1139The :meth:`~socket.connect` operation is also subject to the timeout
1140setting, and in general it is recommended to call :meth:`~socket.settimeout`
1141before calling :meth:`~socket.connect` or pass a timeout parameter to
1142:meth:`create_connection`.  However, the system network stack may also
1143return a connection timeout error of its own regardless of any Python socket
1144timeout setting.
1145
1146Timeouts and the ``accept`` method
1147^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1148
1149If :func:`getdefaulttimeout` is not :const:`None`, sockets returned by
1150the :meth:`~socket.accept` method inherit that timeout.  Otherwise, the
1151behaviour depends on settings of the listening socket:
1152
1153* if the listening socket is in *blocking mode* or in *timeout mode*,
1154  the socket returned by :meth:`~socket.accept` is in *blocking mode*;
1155
1156* if the listening socket is in *non-blocking mode*, whether the socket
1157  returned by :meth:`~socket.accept` is in blocking or non-blocking mode
1158  is operating system-dependent.  If you want to ensure cross-platform
1159  behaviour, it is recommended you manually override this setting.
1160
1161
1162.. _socket-example:
1163
1164Example
1165-------
1166
1167Here are four minimal example programs using the TCP/IP protocol: a server that
1168echoes all data that it receives back (servicing only one client), and a client
1169using it.  Note that a server must perform the sequence :func:`socket`,
1170:meth:`~socket.bind`, :meth:`~socket.listen`, :meth:`~socket.accept` (possibly
1171repeating the :meth:`~socket.accept` to service more than one client), while a
1172client only needs the sequence :func:`socket`, :meth:`~socket.connect`.  Also
1173note that the server does not :meth:`~socket.sendall`/:meth:`~socket.recv` on
1174the socket it is listening on but on the new socket returned by
1175:meth:`~socket.accept`.
1176
1177The first two examples support IPv4 only. ::
1178
1179   # Echo server program
1180   import socket
1181
1182   HOST = ''                 # Symbolic name meaning all available interfaces
1183   PORT = 50007              # Arbitrary non-privileged port
1184   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1185   s.bind((HOST, PORT))
1186   s.listen(1)
1187   conn, addr = s.accept()
1188   print('Connected by', addr)
1189   while True:
1190       data = conn.recv(1024)
1191       if not data: break
1192       conn.sendall(data)
1193   conn.close()
1194
1195::
1196
1197   # Echo client program
1198   import socket
1199
1200   HOST = 'daring.cwi.nl'    # The remote host
1201   PORT = 50007              # The same port as used by the server
1202   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1203   s.connect((HOST, PORT))
1204   s.sendall(b'Hello, world')
1205   data = s.recv(1024)
1206   s.close()
1207   print('Received', repr(data))
1208
1209The next two examples are identical to the above two, but support both IPv4 and
1210IPv6. The server side will listen to the first address family available (it
1211should listen to both instead). On most of IPv6-ready systems, IPv6 will take
1212precedence and the server may not accept IPv4 traffic. The client side will try
1213to connect to the all addresses returned as a result of the name resolution, and
1214sends traffic to the first one connected successfully. ::
1215
1216   # Echo server program
1217   import socket
1218   import sys
1219
1220   HOST = None               # Symbolic name meaning all available interfaces
1221   PORT = 50007              # Arbitrary non-privileged port
1222   s = None
1223   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
1224                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
1225       af, socktype, proto, canonname, sa = res
1226       try:
1227           s = socket.socket(af, socktype, proto)
1228       except OSError as msg:
1229           s = None
1230           continue
1231       try:
1232           s.bind(sa)
1233           s.listen(1)
1234       except OSError as msg:
1235           s.close()
1236           s = None
1237           continue
1238       break
1239   if s is None:
1240       print('could not open socket')
1241       sys.exit(1)
1242   conn, addr = s.accept()
1243   print('Connected by', addr)
1244   while True:
1245       data = conn.recv(1024)
1246       if not data:…

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