PageRenderTime 42ms CodeModel.GetById 2ms app.highlight 30ms RepoModel.GetById 1ms app.codeStats 0ms

/Doc/library/socket.rst

https://bitbucket.org/carljm/cpython
ReStructuredText | 1340 lines | 940 code | 400 blank | 0 comment | 0 complexity | 3ae255c32bc5b69883e55d2234ca7025 MD5 | raw file
   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
  43Socket addresses are represented as follows:
  44
  45- A single string is used for the :const:`AF_UNIX` address family.
  46
  47- A pair ``(host, port)`` is used for the :const:`AF_INET` address family,
  48  where *host* is a string representing either a hostname in Internet domain
  49  notation like ``'daring.cwi.nl'`` or an IPv4 address like ``'100.50.200.5'``,
  50  and *port* is an integral port number.
  51
  52- For :const:`AF_INET6` address family, a four-tuple ``(host, port, flowinfo,
  53  scopeid)`` is used, where *flowinfo* and *scopeid* represent the ``sin6_flowinfo``
  54  and ``sin6_scope_id`` members in :const:`struct sockaddr_in6` in C.  For
  55  :mod:`socket` module methods, *flowinfo* and *scopeid* can be omitted just for
  56  backward compatibility.  Note, however, omission of *scopeid* can cause problems
  57  in manipulating scoped IPv6 addresses.
  58
  59- :const:`AF_NETLINK` sockets are represented as pairs ``(pid, groups)``.
  60
  61- Linux-only support for TIPC is available using the :const:`AF_TIPC`
  62  address family.  TIPC is an open, non-IP based networked protocol designed
  63  for use in clustered computer environments.  Addresses are represented by a
  64  tuple, and the fields depend on the address type. The general tuple form is
  65  ``(addr_type, v1, v2, v3 [, scope])``, where:
  66
  67  - *addr_type* is one of TIPC_ADDR_NAMESEQ, TIPC_ADDR_NAME, or
  68    TIPC_ADDR_ID.
  69  - *scope* is one of TIPC_ZONE_SCOPE, TIPC_CLUSTER_SCOPE, and
  70    TIPC_NODE_SCOPE.
  71  - If *addr_type* is TIPC_ADDR_NAME, then *v1* is the server type, *v2* is
  72    the port identifier, and *v3* should be 0.
  73
  74    If *addr_type* is TIPC_ADDR_NAMESEQ, then *v1* is the server type, *v2*
  75    is the lower port number, and *v3* is the upper port number.
  76
  77    If *addr_type* is TIPC_ADDR_ID, then *v1* is the node, *v2* is the
  78    reference, and *v3* should be set to 0.
  79
  80    If *addr_type* is TIPC_ADDR_ID, then *v1* is the node, *v2* is the
  81    reference, and *v3* should be set to 0.
  82
  83- A tuple ``(interface, )`` is used for the :const:`AF_CAN` address family,
  84  where *interface* is a string representing a network interface name like
  85  ``'can0'``. The network interface name ``''`` can be used to receive packets
  86  from all network interfaces of this family.
  87
  88- Certain other address families (:const:`AF_BLUETOOTH`, :const:`AF_PACKET`)
  89  support specific representations.
  90
  91  .. XXX document them!
  92
  93For IPv4 addresses, two special forms are accepted instead of a host address:
  94the empty string represents :const:`INADDR_ANY`, and the string
  95``'<broadcast>'`` represents :const:`INADDR_BROADCAST`.  This behavior is not
  96compatible with IPv6, therefore, you may want to avoid these if you intend
  97to support IPv6 with your Python programs.
  98
  99If you use a hostname in the *host* portion of IPv4/v6 socket address, the
 100program may show a nondeterministic behavior, as Python uses the first address
 101returned from the DNS resolution.  The socket address will be resolved
 102differently into an actual IPv4/v6 address, depending on the results from DNS
 103resolution and/or the host configuration.  For deterministic behavior use a
 104numeric address in *host* portion.
 105
 106All errors raise exceptions.  The normal exceptions for invalid argument types
 107and out-of-memory conditions can be raised; starting from Python 3.3, errors
 108related to socket or address semantics raise :exc:`OSError` or one of its
 109subclasses (they used to raise :exc:`socket.error`).
 110
 111Non-blocking mode is supported through :meth:`~socket.setblocking`.  A
 112generalization of this based on timeouts is supported through
 113:meth:`~socket.settimeout`.
 114
 115
 116Module contents
 117---------------
 118
 119The module :mod:`socket` exports the following constants and functions:
 120
 121
 122.. exception:: error
 123
 124   A deprecated alias of :exc:`OSError`.
 125
 126   .. versionchanged:: 3.3
 127      Following :pep:`3151`, this class was made an alias of :exc:`OSError`.
 128
 129
 130.. exception:: herror
 131
 132   A subclass of :exc:`OSError`, this exception is raised for
 133   address-related errors, i.e. for functions that use *h_errno* in the POSIX
 134   C API, including :func:`gethostbyname_ex` and :func:`gethostbyaddr`.
 135   The accompanying value is a pair ``(h_errno, string)`` representing an
 136   error returned by a library call.  *h_errno* is a numeric value, while
 137   *string* represents the description of *h_errno*, as returned by the
 138   :c:func:`hstrerror` C function.
 139
 140   .. versionchanged:: 3.3
 141      This class was made a subclass of :exc:`OSError`.
 142
 143.. exception:: gaierror
 144
 145   A subclass of :exc:`OSError`, this exception is raised for
 146   address-related errors by :func:`getaddrinfo` and :func:`getnameinfo`.
 147   The accompanying value is a pair ``(error, string)`` representing an error
 148   returned by a library call.  *string* represents the description of
 149   *error*, as returned by the :c:func:`gai_strerror` C function.  The
 150   numeric *error* value will match one of the :const:`EAI_\*` constants
 151   defined in this module.
 152
 153   .. versionchanged:: 3.3
 154      This class was made a subclass of :exc:`OSError`.
 155
 156.. exception:: timeout
 157
 158   A subclass of :exc:`OSError`, this exception is raised when a timeout
 159   occurs on a socket which has had timeouts enabled via a prior call to
 160   :meth:`~socket.settimeout` (or implicitly through
 161   :func:`~socket.setdefaulttimeout`).  The accompanying value is a string
 162   whose value is currently always "timed out".
 163
 164   .. versionchanged:: 3.3
 165      This class was made a subclass of :exc:`OSError`.
 166
 167.. data:: AF_UNIX
 168          AF_INET
 169          AF_INET6
 170
 171   These constants represent the address (and protocol) families, used for the
 172   first argument to :func:`socket`.  If the :const:`AF_UNIX` constant is not
 173   defined then this protocol is unsupported.  More constants may be available
 174   depending on the system.
 175
 176
 177.. data:: SOCK_STREAM
 178          SOCK_DGRAM
 179          SOCK_RAW
 180          SOCK_RDM
 181          SOCK_SEQPACKET
 182
 183   These constants represent the socket types, used for the second argument to
 184   :func:`socket`.  More constants may be available depending on the system.
 185   (Only :const:`SOCK_STREAM` and :const:`SOCK_DGRAM` appear to be generally
 186   useful.)
 187
 188.. data:: SOCK_CLOEXEC
 189          SOCK_NONBLOCK
 190
 191   These two constants, if defined, can be combined with the socket types and
 192   allow you to set some flags atomically (thus avoiding possible race
 193   conditions and the need for separate calls).
 194
 195   .. seealso::
 196
 197      `Secure File Descriptor Handling <http://udrepper.livejournal.com/20407.html>`_
 198      for a more thorough explanation.
 199
 200   Availability: Linux >= 2.6.27.
 201
 202   .. versionadded:: 3.2
 203
 204.. data:: SO_*
 205          SOMAXCONN
 206          MSG_*
 207          SOL_*
 208          SCM_*
 209          IPPROTO_*
 210          IPPORT_*
 211          INADDR_*
 212          IP_*
 213          IPV6_*
 214          EAI_*
 215          AI_*
 216          NI_*
 217          TCP_*
 218
 219   Many constants of these forms, documented in the Unix documentation on sockets
 220   and/or the IP protocol, are also defined in the socket module. They are
 221   generally used in arguments to the :meth:`setsockopt` and :meth:`getsockopt`
 222   methods of socket objects.  In most cases, only those symbols that are defined
 223   in the Unix header files are defined; for a few symbols, default values are
 224   provided.
 225
 226.. data:: AF_CAN
 227          PF_CAN
 228          SOL_CAN_*
 229          CAN_*
 230
 231   Many constants of these forms, documented in the Linux documentation, are
 232   also defined in the socket module.
 233
 234   Availability: Linux >= 2.6.25.
 235
 236   .. versionadded:: 3.3
 237
 238
 239.. data:: SIO_*
 240          RCVALL_*
 241
 242   Constants for Windows' WSAIoctl(). The constants are used as arguments to the
 243   :meth:`ioctl` method of socket objects.
 244
 245
 246.. data:: TIPC_*
 247
 248   TIPC related constants, matching the ones exported by the C socket API. See
 249   the TIPC documentation for more information.
 250
 251
 252.. data:: has_ipv6
 253
 254   This constant contains a boolean value which indicates if IPv6 is supported on
 255   this platform.
 256
 257
 258.. function:: create_connection(address[, timeout[, source_address]])
 259
 260   Convenience function.  Connect to *address* (a 2-tuple ``(host, port)``),
 261   and return the socket object.  Passing the optional *timeout* parameter will
 262   set the timeout on the socket instance before attempting to connect.  If no
 263   *timeout* is supplied, the global default timeout setting returned by
 264   :func:`getdefaulttimeout` is used.
 265
 266   If supplied, *source_address* must be a 2-tuple ``(host, port)`` for the
 267   socket to bind to as its source address before connecting.  If host or port
 268   are '' or 0 respectively the OS default behavior will be used.
 269
 270   .. versionchanged:: 3.2
 271      *source_address* was added.
 272
 273   .. versionchanged:: 3.2
 274      support for the :keyword:`with` statement was added.
 275
 276
 277.. function:: getaddrinfo(host, port, family=0, type=0, proto=0, flags=0)
 278
 279   Translate the *host*/*port* argument into a sequence of 5-tuples that contain
 280   all the necessary arguments for creating a socket connected to that service.
 281   *host* is a domain name, a string representation of an IPv4/v6 address
 282   or ``None``. *port* is a string service name such as ``'http'``, a numeric
 283   port number or ``None``.  By passing ``None`` as the value of *host*
 284   and *port*, you can pass ``NULL`` to the underlying C API.
 285
 286   The *family*, *type* and *proto* arguments can be optionally specified
 287   in order to narrow the list of addresses returned.  Passing zero as a
 288   value for each of these arguments selects the full range of results.
 289   The *flags* argument can be one or several of the ``AI_*`` constants,
 290   and will influence how results are computed and returned.
 291   For example, :const:`AI_NUMERICHOST` will disable domain name resolution
 292   and will raise an error if *host* is a domain name.
 293
 294   The function returns a list of 5-tuples with the following structure:
 295
 296   ``(family, type, proto, canonname, sockaddr)``
 297
 298   In these tuples, *family*, *type*, *proto* are all integers and are
 299   meant to be passed to the :func:`socket` function.  *canonname* will be
 300   a string representing the canonical name of the *host* if
 301   :const:`AI_CANONNAME` is part of the *flags* argument; else *canonname*
 302   will be empty.  *sockaddr* is a tuple describing a socket address, whose
 303   format depends on the returned *family* (a ``(address, port)`` 2-tuple for
 304   :const:`AF_INET`, a ``(address, port, flow info, scope id)`` 4-tuple for
 305   :const:`AF_INET6`), and is meant to be passed to the :meth:`socket.connect`
 306   method.
 307
 308   The following example fetches address information for a hypothetical TCP
 309   connection to ``www.python.org`` on port 80 (results may differ on your
 310   system if IPv6 isn't enabled)::
 311
 312      >>> socket.getaddrinfo("www.python.org", 80, proto=socket.SOL_TCP)
 313      [(2, 1, 6, '', ('82.94.164.162', 80)),
 314       (10, 1, 6, '', ('2001:888:2000:d::a2', 80, 0, 0))]
 315
 316   .. versionchanged:: 3.2
 317      parameters can now be passed as single keyword arguments.
 318
 319.. function:: getfqdn([name])
 320
 321   Return a fully qualified domain name for *name*. If *name* is omitted or empty,
 322   it is interpreted as the local host.  To find the fully qualified name, the
 323   hostname returned by :func:`gethostbyaddr` is checked, followed by aliases for the
 324   host, if available.  The first name which includes a period is selected.  In
 325   case no fully qualified domain name is available, the hostname as returned by
 326   :func:`gethostname` is returned.
 327
 328
 329.. function:: gethostbyname(hostname)
 330
 331   Translate a host name to IPv4 address format.  The IPv4 address is returned as a
 332   string, such as  ``'100.50.200.5'``.  If the host name is an IPv4 address itself
 333   it is returned unchanged.  See :func:`gethostbyname_ex` for a more complete
 334   interface. :func:`gethostbyname` does not support IPv6 name resolution, and
 335   :func:`getaddrinfo` should be used instead for IPv4/v6 dual stack support.
 336
 337
 338.. function:: gethostbyname_ex(hostname)
 339
 340   Translate a host name to IPv4 address format, extended interface. Return a
 341   triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the primary
 342   host name responding to the given *ip_address*, *aliaslist* is a (possibly
 343   empty) list of alternative host names for the same address, and *ipaddrlist* is
 344   a list of IPv4 addresses for the same interface on the same host (often but not
 345   always a single address). :func:`gethostbyname_ex` does not support IPv6 name
 346   resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual
 347   stack support.
 348
 349
 350.. function:: gethostname()
 351
 352   Return a string containing the hostname of the machine where  the Python
 353   interpreter is currently executing.
 354
 355   If you want to know the current machine's IP address, you may want to use
 356   ``gethostbyname(gethostname())``. This operation assumes that there is a
 357   valid address-to-host mapping for the host, and the assumption does not
 358   always hold.
 359
 360   Note: :func:`gethostname` doesn't always return the fully qualified domain
 361   name; use ``getfqdn()`` (see above).
 362
 363
 364.. function:: gethostbyaddr(ip_address)
 365
 366   Return a triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the
 367   primary host name responding to the given *ip_address*, *aliaslist* is a
 368   (possibly empty) list of alternative host names for the same address, and
 369   *ipaddrlist* is a list of IPv4/v6 addresses for the same interface on the same
 370   host (most likely containing only a single address). To find the fully qualified
 371   domain name, use the function :func:`getfqdn`. :func:`gethostbyaddr` supports
 372   both IPv4 and IPv6.
 373
 374
 375.. function:: getnameinfo(sockaddr, flags)
 376
 377   Translate a socket address *sockaddr* into a 2-tuple ``(host, port)``. Depending
 378   on the settings of *flags*, the result can contain a fully-qualified domain name
 379   or numeric address representation in *host*.  Similarly, *port* can contain a
 380   string port name or a numeric port number.
 381
 382
 383.. function:: getprotobyname(protocolname)
 384
 385   Translate an Internet protocol name (for example, ``'icmp'``) to a constant
 386   suitable for passing as the (optional) third argument to the :func:`socket`
 387   function.  This is usually only needed for sockets opened in "raw" mode
 388   (:const:`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen
 389   automatically if the protocol is omitted or zero.
 390
 391
 392.. function:: getservbyname(servicename[, protocolname])
 393
 394   Translate an Internet service name and protocol name to a port number for that
 395   service.  The optional protocol name, if given, should be ``'tcp'`` or
 396   ``'udp'``, otherwise any protocol will match.
 397
 398
 399.. function:: getservbyport(port[, protocolname])
 400
 401   Translate an Internet port number and protocol name to a service name for that
 402   service.  The optional protocol name, if given, should be ``'tcp'`` or
 403   ``'udp'``, otherwise any protocol will match.
 404
 405
 406.. function:: socket([family[, type[, proto]]])
 407
 408   Create a new socket using the given address family, socket type and protocol
 409   number.  The address family should be :const:`AF_INET` (the default),
 410   :const:`AF_INET6`, :const:`AF_UNIX` or :const:`AF_CAN`. The socket type
 411   should be :const:`SOCK_STREAM` (the default), :const:`SOCK_DGRAM`,
 412   :const:`SOCK_RAW` or perhaps one of the other ``SOCK_`` constants. The
 413   protocol number is usually zero and may be omitted in that case or
 414   :const:`CAN_RAW` in case the address family is :const:`AF_CAN`.
 415
 416   .. versionchanged:: 3.3
 417      The AF_CAN family was added.
 418
 419
 420.. function:: socketpair([family[, type[, proto]]])
 421
 422   Build a pair of connected socket objects using the given address family, socket
 423   type, and protocol number.  Address family, socket type, and protocol number are
 424   as for the :func:`socket` function above. The default family is :const:`AF_UNIX`
 425   if defined on the platform; otherwise, the default is :const:`AF_INET`.
 426   Availability: Unix.
 427
 428   .. versionchanged:: 3.2
 429      The returned socket objects now support the whole socket API, rather
 430      than a subset.
 431
 432
 433.. function:: fromfd(fd, family, type[, proto])
 434
 435   Duplicate the file descriptor *fd* (an integer as returned by a file object's
 436   :meth:`fileno` method) and build a socket object from the result.  Address
 437   family, socket type and protocol number are as for the :func:`socket` function
 438   above. The file descriptor should refer to a socket, but this is not checked ---
 439   subsequent operations on the object may fail if the file descriptor is invalid.
 440   This function is rarely needed, but can be used to get or set socket options on
 441   a socket passed to a program as standard input or output (such as a server
 442   started by the Unix inet daemon).  The socket is assumed to be in blocking mode.
 443
 444
 445.. function:: ntohl(x)
 446
 447   Convert 32-bit positive integers from network to host byte order.  On machines
 448   where the host byte order is the same as network byte order, this is a no-op;
 449   otherwise, it performs a 4-byte swap operation.
 450
 451
 452.. function:: ntohs(x)
 453
 454   Convert 16-bit positive integers from network to host byte order.  On machines
 455   where the host byte order is the same as network byte order, this is a no-op;
 456   otherwise, it performs a 2-byte swap operation.
 457
 458
 459.. function:: htonl(x)
 460
 461   Convert 32-bit positive integers from host to network byte order.  On machines
 462   where the host byte order is the same as network byte order, this is a no-op;
 463   otherwise, it performs a 4-byte swap operation.
 464
 465
 466.. function:: htons(x)
 467
 468   Convert 16-bit positive integers from host to network byte order.  On machines
 469   where the host byte order is the same as network byte order, this is a no-op;
 470   otherwise, it performs a 2-byte swap operation.
 471
 472
 473.. function:: inet_aton(ip_string)
 474
 475   Convert an IPv4 address from dotted-quad string format (for example,
 476   '123.45.67.89') to 32-bit packed binary format, as a bytes object four characters in
 477   length.  This is useful when conversing with a program that uses the standard C
 478   library and needs objects of type :c:type:`struct in_addr`, which is the C type
 479   for the 32-bit packed binary this function returns.
 480
 481   :func:`inet_aton` also accepts strings with less than three dots; see the
 482   Unix manual page :manpage:`inet(3)` for details.
 483
 484   If the IPv4 address string passed to this function is invalid,
 485   :exc:`OSError` will be raised. Note that exactly what is valid depends on
 486   the underlying C implementation of :c:func:`inet_aton`.
 487
 488   :func:`inet_aton` does not support IPv6, and :func:`inet_pton` should be used
 489   instead for IPv4/v6 dual stack support.
 490
 491
 492.. function:: inet_ntoa(packed_ip)
 493
 494   Convert a 32-bit packed IPv4 address (a bytes object four characters in
 495   length) to its standard dotted-quad string representation (for example,
 496   '123.45.67.89').  This is useful when conversing with a program that uses the
 497   standard C library and needs objects of type :c:type:`struct in_addr`, which
 498   is the C type for the 32-bit packed binary data this function takes as an
 499   argument.
 500
 501   If the byte sequence passed to this function is not exactly 4 bytes in
 502   length, :exc:`OSError` will be raised. :func:`inet_ntoa` does not
 503   support IPv6, and :func:`inet_ntop` should be used instead for IPv4/v6 dual
 504   stack support.
 505
 506
 507.. function:: inet_pton(address_family, ip_string)
 508
 509   Convert an IP address from its family-specific string format to a packed,
 510   binary format. :func:`inet_pton` is useful when a library or network protocol
 511   calls for an object of type :c:type:`struct in_addr` (similar to
 512   :func:`inet_aton`) or :c:type:`struct in6_addr`.
 513
 514   Supported values for *address_family* are currently :const:`AF_INET` and
 515   :const:`AF_INET6`. If the IP address string *ip_string* is invalid,
 516   :exc:`OSError` will be raised. Note that exactly what is valid depends on
 517   both the value of *address_family* and the underlying implementation of
 518   :c:func:`inet_pton`.
 519
 520   Availability: Unix (maybe not all platforms).
 521
 522
 523.. function:: inet_ntop(address_family, packed_ip)
 524
 525   Convert a packed IP address (a bytes object of some number of characters) to its
 526   standard, family-specific string representation (for example, ``'7.10.0.5'`` or
 527   ``'5aef:2b::8'``). :func:`inet_ntop` is useful when a library or network protocol
 528   returns an object of type :c:type:`struct in_addr` (similar to :func:`inet_ntoa`)
 529   or :c:type:`struct in6_addr`.
 530
 531   Supported values for *address_family* are currently :const:`AF_INET` and
 532   :const:`AF_INET6`. If the string *packed_ip* is not the correct length for the
 533   specified address family, :exc:`ValueError` will be raised.  A
 534   :exc:`OSError` is raised for errors from the call to :func:`inet_ntop`.
 535
 536   Availability: Unix (maybe not all platforms).
 537
 538
 539..
 540   XXX: Are sendmsg(), recvmsg() and CMSG_*() available on any
 541   non-Unix platforms?  The old (obsolete?) 4.2BSD form of the
 542   interface, in which struct msghdr has no msg_control or
 543   msg_controllen members, is not currently supported.
 544
 545.. function:: CMSG_LEN(length)
 546
 547   Return the total length, without trailing padding, of an ancillary
 548   data item with associated data of the given *length*.  This value
 549   can often be used as the buffer size for :meth:`~socket.recvmsg` to
 550   receive a single item of ancillary data, but :rfc:`3542` requires
 551   portable applications to use :func:`CMSG_SPACE` and thus include
 552   space for padding, even when the item will be the last in the
 553   buffer.  Raises :exc:`OverflowError` if *length* is outside the
 554   permissible range of values.
 555
 556   Availability: most Unix platforms, possibly others.
 557
 558   .. versionadded:: 3.3
 559
 560
 561.. function:: CMSG_SPACE(length)
 562
 563   Return the buffer size needed for :meth:`~socket.recvmsg` to
 564   receive an ancillary data item with associated data of the given
 565   *length*, along with any trailing padding.  The buffer space needed
 566   to receive multiple items is the sum of the :func:`CMSG_SPACE`
 567   values for their associated data lengths.  Raises
 568   :exc:`OverflowError` if *length* is outside the permissible range
 569   of values.
 570
 571   Note that some systems might support ancillary data without
 572   providing this function.  Also note that setting the buffer size
 573   using the results of this function may not precisely limit the
 574   amount of ancillary data that can be received, since additional
 575   data may be able to fit into the padding area.
 576
 577   Availability: most Unix platforms, possibly others.
 578
 579   .. versionadded:: 3.3
 580
 581
 582.. function:: getdefaulttimeout()
 583
 584   Return the default timeout in seconds (float) for new socket objects. A value
 585   of ``None`` indicates that new socket objects have no timeout. When the socket
 586   module is first imported, the default is ``None``.
 587
 588
 589.. function:: setdefaulttimeout(timeout)
 590
 591   Set the default timeout in seconds (float) for new socket objects.  When
 592   the socket module is first imported, the default is ``None``.  See
 593   :meth:`~socket.settimeout` for possible values and their respective
 594   meanings.
 595
 596
 597.. function:: sethostname(name)
 598
 599   Set the machine's hostname to *name*.  This will raise a
 600   :exc:`OSError` if you don't have enough rights.
 601
 602   Availability: Unix.
 603
 604   .. versionadded:: 3.3
 605
 606
 607.. function:: if_nameindex()
 608
 609   Return a list of network interface information
 610   (index int, name string) tuples.
 611   :exc:`OSError` if the system call fails.
 612
 613   Availability: Unix.
 614
 615   .. versionadded:: 3.3
 616
 617
 618.. function:: if_nametoindex(if_name)
 619
 620   Return a network interface index number corresponding to an
 621   interface name.
 622   :exc:`OSError` if no interface with the given name exists.
 623
 624   Availability: Unix.
 625
 626   .. versionadded:: 3.3
 627
 628
 629.. function:: if_indextoname(if_index)
 630
 631   Return a network interface name corresponding to a
 632   interface index number.
 633   :exc:`OSError` if no interface with the given index exists.
 634
 635   Availability: Unix.
 636
 637   .. versionadded:: 3.3
 638
 639
 640.. data:: SocketType
 641
 642   This is a Python type object that represents the socket object type. It is the
 643   same as ``type(socket(...))``.
 644
 645
 646.. _socket-objects:
 647
 648Socket Objects
 649--------------
 650
 651Socket objects have the following methods.  Except for :meth:`makefile` these
 652correspond to Unix system calls applicable to sockets.
 653
 654
 655.. method:: socket.accept()
 656
 657   Accept a connection. The socket must be bound to an address and listening for
 658   connections. The return value is a pair ``(conn, address)`` where *conn* is a
 659   *new* socket object usable to send and receive data on the connection, and
 660   *address* is the address bound to the socket on the other end of the connection.
 661
 662
 663.. method:: socket.bind(address)
 664
 665   Bind the socket to *address*.  The socket must not already be bound. (The format
 666   of *address* depends on the address family --- see above.)
 667
 668
 669.. method:: socket.close()
 670
 671   Close the socket.  All future operations on the socket object will fail. The
 672   remote end will receive no more data (after queued data is flushed). Sockets are
 673   automatically closed when they are garbage-collected.
 674
 675   .. note::
 676      :meth:`close()` releases the resource associated with a connection but
 677      does not necessarily close the connection immediately.  If you want
 678      to close the connection in a timely fashion, call :meth:`shutdown()`
 679      before :meth:`close()`.
 680
 681
 682.. method:: socket.connect(address)
 683
 684   Connect to a remote socket at *address*. (The format of *address* depends on the
 685   address family --- see above.)
 686
 687
 688.. method:: socket.connect_ex(address)
 689
 690   Like ``connect(address)``, but return an error indicator instead of raising an
 691   exception for errors returned by the C-level :c:func:`connect` call (other
 692   problems, such as "host not found," can still raise exceptions).  The error
 693   indicator is ``0`` if the operation succeeded, otherwise the value of the
 694   :c:data:`errno` variable.  This is useful to support, for example, asynchronous
 695   connects.
 696
 697
 698.. method:: socket.detach()
 699
 700   Put the socket object into closed state without actually closing the
 701   underlying file descriptor.  The file descriptor is returned, and can
 702   be reused for other purposes.
 703
 704   .. versionadded:: 3.2
 705
 706
 707.. method:: socket.fileno()
 708
 709   Return the socket's file descriptor (a small integer).  This is useful with
 710   :func:`select.select`.
 711
 712   Under Windows the small integer returned by this method cannot be used where a
 713   file descriptor can be used (such as :func:`os.fdopen`).  Unix does not have
 714   this limitation.
 715
 716
 717.. method:: socket.getpeername()
 718
 719   Return the remote address to which the socket is connected.  This is useful to
 720   find out the port number of a remote IPv4/v6 socket, for instance. (The format
 721   of the address returned depends on the address family --- see above.)  On some
 722   systems this function is not supported.
 723
 724
 725.. method:: socket.getsockname()
 726
 727   Return the socket's own address.  This is useful to find out the port number of
 728   an IPv4/v6 socket, for instance. (The format of the address returned depends on
 729   the address family --- see above.)
 730
 731
 732.. method:: socket.getsockopt(level, optname[, buflen])
 733
 734   Return the value of the given socket option (see the Unix man page
 735   :manpage:`getsockopt(2)`).  The needed symbolic constants (:const:`SO_\*` etc.)
 736   are defined in this module.  If *buflen* is absent, an integer option is assumed
 737   and its integer value is returned by the function.  If *buflen* is present, it
 738   specifies the maximum length of the buffer used to receive the option in, and
 739   this buffer is returned as a bytes object.  It is up to the caller to decode the
 740   contents of the buffer (see the optional built-in module :mod:`struct` for a way
 741   to decode C structures encoded as byte strings).
 742
 743
 744.. method:: socket.gettimeout()
 745
 746   Return the timeout in seconds (float) associated with socket operations,
 747   or ``None`` if no timeout is set.  This reflects the last call to
 748   :meth:`setblocking` or :meth:`settimeout`.
 749
 750
 751.. method:: socket.ioctl(control, option)
 752
 753   :platform: Windows
 754
 755   The :meth:`ioctl` method is a limited interface to the WSAIoctl system
 756   interface.  Please refer to the `Win32 documentation
 757   <http://msdn.microsoft.com/en-us/library/ms741621%28VS.85%29.aspx>`_ for more
 758   information.
 759
 760   On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
 761   functions may be used; they accept a socket object as their first argument.
 762
 763.. method:: socket.listen(backlog)
 764
 765   Listen for connections made to the socket.  The *backlog* argument specifies the
 766   maximum number of queued connections and should be at least 0; the maximum value
 767   is system-dependent (usually 5), the minimum value is forced to 0.
 768
 769
 770.. method:: socket.makefile(mode='r', buffering=None, *, encoding=None, \
 771                            errors=None, newline=None)
 772
 773   .. index:: single: I/O control; buffering
 774
 775   Return a :term:`file object` associated with the socket.  The exact returned
 776   type depends on the arguments given to :meth:`makefile`.  These arguments are
 777   interpreted the same way as by the built-in :func:`open` function.
 778
 779   Closing the file object won't close the socket unless there are no remaining
 780   references to the socket.  The socket must be in blocking mode; it can have
 781   a timeout, but the file object's internal buffer may end up in a inconsistent
 782   state if a timeout occurs.
 783
 784   .. note::
 785
 786      On Windows, the file-like object created by :meth:`makefile` cannot be
 787      used where a file object with a file descriptor is expected, such as the
 788      stream arguments of :meth:`subprocess.Popen`.
 789
 790
 791.. method:: socket.recv(bufsize[, flags])
 792
 793   Receive data from the socket.  The return value is a bytes object representing the
 794   data received.  The maximum amount of data to be received at once is specified
 795   by *bufsize*.  See the Unix manual page :manpage:`recv(2)` for the meaning of
 796   the optional argument *flags*; it defaults to zero.
 797
 798   .. note::
 799
 800      For best match with hardware and network realities, the value of  *bufsize*
 801      should be a relatively small power of 2, for example, 4096.
 802
 803
 804.. method:: socket.recvfrom(bufsize[, flags])
 805
 806   Receive data from the socket.  The return value is a pair ``(bytes, address)``
 807   where *bytes* is a bytes object representing the data received and *address* is the
 808   address of the socket sending the data.  See the Unix manual page
 809   :manpage:`recv(2)` for the meaning of the optional argument *flags*; it defaults
 810   to zero. (The format of *address* depends on the address family --- see above.)
 811
 812
 813.. method:: socket.recvmsg(bufsize[, ancbufsize[, flags]])
 814
 815   Receive normal data (up to *bufsize* bytes) and ancillary data from
 816   the socket.  The *ancbufsize* argument sets the size in bytes of
 817   the internal buffer used to receive the ancillary data; it defaults
 818   to 0, meaning that no ancillary data will be received.  Appropriate
 819   buffer sizes for ancillary data can be calculated using
 820   :func:`CMSG_SPACE` or :func:`CMSG_LEN`, and items which do not fit
 821   into the buffer might be truncated or discarded.  The *flags*
 822   argument defaults to 0 and has the same meaning as for
 823   :meth:`recv`.
 824
 825   The return value is a 4-tuple: ``(data, ancdata, msg_flags,
 826   address)``.  The *data* item is a :class:`bytes` object holding the
 827   non-ancillary data received.  The *ancdata* item is a list of zero
 828   or more tuples ``(cmsg_level, cmsg_type, cmsg_data)`` representing
 829   the ancillary data (control messages) received: *cmsg_level* and
 830   *cmsg_type* are integers specifying the protocol level and
 831   protocol-specific type respectively, and *cmsg_data* is a
 832   :class:`bytes` object holding the associated data.  The *msg_flags*
 833   item is the bitwise OR of various flags indicating conditions on
 834   the received message; see your system documentation for details.
 835   If the receiving socket is unconnected, *address* is the address of
 836   the sending socket, if available; otherwise, its value is
 837   unspecified.
 838
 839   On some systems, :meth:`sendmsg` and :meth:`recvmsg` can be used to
 840   pass file descriptors between processes over an :const:`AF_UNIX`
 841   socket.  When this facility is used (it is often restricted to
 842   :const:`SOCK_STREAM` sockets), :meth:`recvmsg` will return, in its
 843   ancillary data, items of the form ``(socket.SOL_SOCKET,
 844   socket.SCM_RIGHTS, fds)``, where *fds* is a :class:`bytes` object
 845   representing the new file descriptors as a binary array of the
 846   native C :c:type:`int` type.  If :meth:`recvmsg` raises an
 847   exception after the system call returns, it will first attempt to
 848   close any file descriptors received via this mechanism.
 849
 850   Some systems do not indicate the truncated length of ancillary data
 851   items which have been only partially received.  If an item appears
 852   to extend beyond the end of the buffer, :meth:`recvmsg` will issue
 853   a :exc:`RuntimeWarning`, and will return the part of it which is
 854   inside the buffer provided it has not been truncated before the
 855   start of its associated data.
 856
 857   On systems which support the :const:`SCM_RIGHTS` mechanism, the
 858   following function will receive up to *maxfds* file descriptors,
 859   returning the message data and a list containing the descriptors
 860   (while ignoring unexpected conditions such as unrelated control
 861   messages being received).  See also :meth:`sendmsg`. ::
 862
 863      import socket, array
 864
 865      def recv_fds(sock, msglen, maxfds):
 866          fds = array.array("i")   # Array of ints
 867          msg, ancdata, flags, addr = sock.recvmsg(msglen, socket.CMSG_LEN(maxfds * fds.itemsize))
 868          for cmsg_level, cmsg_type, cmsg_data in ancdata:
 869              if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
 870                  # Append data, ignoring any truncated integers at the end.
 871                  fds.fromstring(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
 872          return msg, list(fds)
 873
 874   Availability: most Unix platforms, possibly others.
 875
 876   .. versionadded:: 3.3
 877
 878
 879.. method:: socket.recvmsg_into(buffers[, ancbufsize[, flags]])
 880
 881   Receive normal data and ancillary data from the socket, behaving as
 882   :meth:`recvmsg` would, but scatter the non-ancillary data into a
 883   series of buffers instead of returning a new bytes object.  The
 884   *buffers* argument must be an iterable of objects that export
 885   writable buffers (e.g. :class:`bytearray` objects); these will be
 886   filled with successive chunks of the non-ancillary data until it
 887   has all been written or there are no more buffers.  The operating
 888   system may set a limit (:func:`~os.sysconf` value ``SC_IOV_MAX``)
 889   on the number of buffers that can be used.  The *ancbufsize* and
 890   *flags* arguments have the same meaning as for :meth:`recvmsg`.
 891
 892   The return value is a 4-tuple: ``(nbytes, ancdata, msg_flags,
 893   address)``, where *nbytes* is the total number of bytes of
 894   non-ancillary data written into the buffers, and *ancdata*,
 895   *msg_flags* and *address* are the same as for :meth:`recvmsg`.
 896
 897   Example::
 898
 899      >>> import socket
 900      >>> s1, s2 = socket.socketpair()
 901      >>> b1 = bytearray(b'----')
 902      >>> b2 = bytearray(b'0123456789')
 903      >>> b3 = bytearray(b'--------------')
 904      >>> s1.send(b'Mary had a little lamb')
 905      22
 906      >>> s2.recvmsg_into([b1, memoryview(b2)[2:9], b3])
 907      (22, [], 0, None)
 908      >>> [b1, b2, b3]
 909      [bytearray(b'Mary'), bytearray(b'01 had a 9'), bytearray(b'little lamb---')]
 910
 911   Availability: most Unix platforms, possibly others.
 912
 913   .. versionadded:: 3.3
 914
 915
 916.. method:: socket.recvfrom_into(buffer[, nbytes[, flags]])
 917
 918   Receive data from the socket, writing it into *buffer* instead of creating a
 919   new bytestring.  The return value is a pair ``(nbytes, address)`` where *nbytes* is
 920   the number of bytes received and *address* is the address of the socket sending
 921   the data.  See the Unix manual page :manpage:`recv(2)` for the meaning of the
 922   optional argument *flags*; it defaults to zero.  (The format of *address*
 923   depends on the address family --- see above.)
 924
 925
 926.. method:: socket.recv_into(buffer[, nbytes[, flags]])
 927
 928   Receive up to *nbytes* bytes from the socket, storing the data into a buffer
 929   rather than creating a new bytestring.  If *nbytes* is not specified (or 0),
 930   receive up to the size available in the given buffer.  Returns the number of
 931   bytes received.  See the Unix manual page :manpage:`recv(2)` for the meaning
 932   of the optional argument *flags*; it defaults to zero.
 933
 934
 935.. method:: socket.send(bytes[, flags])
 936
 937   Send data to the socket.  The socket must be connected to a remote socket.  The
 938   optional *flags* argument has the same meaning as for :meth:`recv` above.
 939   Returns the number of bytes sent. Applications are responsible for checking that
 940   all data has been sent; if only some of the data was transmitted, the
 941   application needs to attempt delivery of the remaining data.
 942
 943
 944.. method:: socket.sendall(bytes[, flags])
 945
 946   Send data to the socket.  The socket must be connected to a remote socket.  The
 947   optional *flags* argument has the same meaning as for :meth:`recv` above.
 948   Unlike :meth:`send`, this method continues to send data from *bytes* until
 949   either all data has been sent or an error occurs.  ``None`` is returned on
 950   success.  On error, an exception is raised, and there is no way to determine how
 951   much data, if any, was successfully sent.
 952
 953
 954.. method:: socket.sendto(bytes[, flags], address)
 955
 956   Send data to the socket.  The socket should not be connected to a remote socket,
 957   since the destination socket is specified by *address*.  The optional *flags*
 958   argument has the same meaning as for :meth:`recv` above.  Return the number of
 959   bytes sent. (The format of *address* depends on the address family --- see
 960   above.)
 961
 962
 963.. method:: socket.sendmsg(buffers[, ancdata[, flags[, address]]])
 964
 965   Send normal and ancillary data to the socket, gathering the
 966   non-ancillary data from a series of buffers and concatenating it
 967   into a single message.  The *buffers* argument specifies the
 968   non-ancillary data as an iterable of buffer-compatible objects
 969   (e.g. :class:`bytes` objects); the operating system may set a limit
 970   (:func:`~os.sysconf` value ``SC_IOV_MAX``) on the number of buffers
 971   that can be used.  The *ancdata* argument specifies the ancillary
 972   data (control messages) as an iterable of zero or more tuples
 973   ``(cmsg_level, cmsg_type, cmsg_data)``, where *cmsg_level* and
 974   *cmsg_type* are integers specifying the protocol level and
 975   protocol-specific type respectively, and *cmsg_data* is a
 976   buffer-compatible object holding the associated data.  Note that
 977   some systems (in particular, systems without :func:`CMSG_SPACE`)
 978   might support sending only one control message per call.  The
 979   *flags* argument defaults to 0 and has the same meaning as for
 980   :meth:`send`.  If *address* is supplied and not ``None``, it sets a
 981   destination address for the message.  The return value is the
 982   number of bytes of non-ancillary data sent.
 983
 984   The following function sends the list of file descriptors *fds*
 985   over an :const:`AF_UNIX` socket, on systems which support the
 986   :const:`SCM_RIGHTS` mechanism.  See also :meth:`recvmsg`. ::
 987
 988      import socket, array
 989
 990      def send_fds(sock, msg, fds):
 991          return sock.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds))])
 992
 993   Availability: most Unix platforms, possibly others.
 994
 995   .. versionadded:: 3.3
 996
 997
 998.. method:: socket.setblocking(flag)
 999
1000   Set blocking or non-blocking mode of the socket: if *flag* is false, the
1001   socket is set to non-blocking, else to blocking mode.
1002
1003   This method is a shorthand for certain :meth:`~socket.settimeout` calls:
1004
1005   * ``sock.setblocking(True)`` is equivalent to ``sock.settimeout(None)``
1006
1007   * ``sock.setblocking(False)`` is equivalent to ``sock.settimeout(0.0)``
1008
1009
1010.. method:: socket.settimeout(value)
1011
1012   Set a timeout on blocking socket operations.  The *value* argument can be a
1013   nonnegative floating point number expressing seconds, or ``None``.
1014   If a non-zero value is given, subsequent socket operations will raise a
1015   :exc:`timeout` exception if the timeout period *value* has elapsed before
1016   the operation has completed.  If zero is given, the socket is put in
1017   non-blocking mode. If ``None`` is given, the socket is put in blocking mode.
1018
1019   For further information, please consult the :ref:`notes on socket timeouts <socket-timeouts>`.
1020
1021
1022.. method:: socket.setsockopt(level, optname, value)
1023
1024   .. index:: module: struct
1025
1026   Set the value of the given socket option (see the Unix manual page
1027   :manpage:`setsockopt(2)`).  The needed symbolic constants are defined in the
1028   :mod:`socket` module (:const:`SO_\*` etc.).  The value can be an integer or a
1029   bytes object representing a buffer.  In the latter case it is up to the caller to
1030   ensure that the bytestring contains the proper bits (see the optional built-in
1031   module :mod:`struct` for a way to encode C structures as bytestrings).
1032
1033
1034.. method:: socket.shutdown(how)
1035
1036   Shut down one or both halves of the connection.  If *how* is :const:`SHUT_RD`,
1037   further receives are disallowed.  If *how* is :const:`SHUT_WR`, further sends
1038   are disallowed.  If *how* is :const:`SHUT_RDWR`, further sends and receives are
1039   disallowed.  Depending on the platform, shutting down one half of the connection
1040   can also close the opposite half (e.g. on Mac OS X, ``shutdown(SHUT_WR)`` does
1041   not allow further reads on the other end of the connection).
1042
1043Note that there are no methods :meth:`read` or :meth:`write`; use
1044:meth:`~socket.recv` and :meth:`~socket.send` without *flags* argument instead.
1045
1046Socket objects also have these (read-only) attributes that correspond to the
1047values given to the :class:`socket` constructor.
1048
1049
1050.. attribute:: socket.family
1051
1052   The socket family.
1053
1054
1055.. attribute:: socket.type
1056
1057   The socket type.
1058
1059
1060.. attribute:: socket.proto
1061
1062   The socket protocol.
1063
1064
1065
1066.. _socket-timeouts:
1067
1068Notes on socket timeouts
1069------------------------
1070
1071A socket object can be in one of three modes: blocking, non-blocking, or
1072timeout.  Sockets are by default always created in blocking mode, but this
1073can be changed by calling :func:`setdefaulttimeout`.
1074
1075* In *blocking mode*, operations block until complete or the system returns
1076  an error (such as connection timed out).
1077
1078* In *non-blocking mode*, operations fail (with an error that is unfortunately
1079  system-dependent) if they cannot be completed immediately: functions from the
1080  :mod:`select` can be used to know when and whether a socket is available for
1081  reading or writing.
1082
1083* In *timeout mode*, operations fail if they cannot be completed within the
1084  timeout specified for the socket (they raise a :exc:`timeout` exception)
1085  or if the system returns an error.
1086
1087.. note::
1088   At the operating system level, sockets in *timeout mode* are internally set
1089   in non-blocking mode.  Also, the blocking and timeout modes are shared between
1090   file descriptors and socket objects that refer to the same network endpoint.
1091   This implementation detail can have visible consequences if e.g. you decide
1092   to use the :meth:`~socket.fileno()` of a socket.
1093
1094Timeouts and the ``connect`` method
1095^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1096
1097The :meth:`~socket.connect` operation is also subject to the timeout
1098setting, and in general it is recommended to call :meth:`~socket.settimeout`
1099before calling :meth:`~socket.connect` or pass a timeout parameter to
1100:meth:`create_connection`.  However, the system network stack may also
1101return a connection timeout error of its own regardless of any Python socket
1102timeout setting.
1103
1104Timeouts and the ``accept`` method
1105^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1106
1107If :func:`getdefaulttimeout` is not :const:`None`, sockets returned by
1108the :meth:`~socket.accept` method inherit that timeout.  Otherwise, the
1109behaviour depends on settings of the listening socket:
1110
1111* if the listening socket is in *blocking mode* or in *timeout mode*,
1112  the socket returned by :meth:`~socket.accept` is in *blocking mode*;
1113
1114* if the listening socket is in *non-blocking mode*, whether the socket
1115  returned by :meth:`~socket.accept` is in blocking or non-blocking mode
1116  is operating system-dependent.  If you want to ensure cross-platform
1117  behaviour, it is recommended you manually override this setting.
1118
1119
1120.. _socket-example:
1121
1122Example
1123-------
1124
1125Here are four minimal example programs using the TCP/IP protocol: a server that
1126echoes all data that it receives back (servicing only one client), and a client
1127using it.  Note that a server must perform the sequence :func:`socket`,
1128:meth:`~socket.bind`, :meth:`~socket.listen`, :meth:`~socket.accept` (possibly
1129repeating the :meth:`~socket.accept` to service more than one client), while a
1130client only needs the sequence :func:`socket`, :meth:`~socket.connect`.  Also
1131note that the server does not :meth:`~socket.send`/:meth:`~socket.recv` on the
1132socket it is listening on but on the new socket returned by
1133:meth:`~socket.accept`.
1134
1135The first two examples support IPv4 only. ::
1136
1137   # Echo server program
1138   import socket
1139
1140   HOST = ''                 # Symbolic name meaning all available interfaces
1141   PORT = 50007              # Arbitrary non-privileged port
1142   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1143   s.bind((HOST, PORT))
1144   s.listen(1)
1145   conn, addr = s.accept()
1146   print('Connected by', addr)
1147   while True:
1148       data = conn.recv(1024)
1149       if not data: break
1150       conn.send(data)
1151   conn.close()
1152
1153::
1154
1155   # Echo client program
1156   import socket
1157
1158   HOST = 'daring.cwi.nl'    # The remote host
1159   PORT = 50007              # The same port as used by the server
1160   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1161   s.connect((HOST, PORT))
1162   s.send(b'Hello, world')
1163   data = s.recv(1024)
1164   s.close()
1165   print('Received', repr(data))
1166
1167The next two examples are identical to the above two, but support both IPv4 and
1168IPv6. The server side will listen to the first address family available (it
1169should listen to both instead). On most of IPv6-ready systems, IPv6 will take
1170precedence and the server may not accept IPv4 traffic. The client side will try
1171to connect to the all addresses returned as a result of the name resolution, and
1172sends traffic to the first one connected successfully. ::
1173
1174   # Echo server program
1175   import socket
1176   import sys
1177
1178   HOST = None               # Symbolic name meaning all available interfaces
1179   PORT = 50007              # Arbitrary non-privileged port
1180   s = None
1181   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
1182                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
1183       af, socktype, proto, canonname, sa = res
1184       try:
1185           s = socket.socket(af, socktype, proto)
1186       except OSError as msg:
1187           s = None
1188           continue
1189       try:
1190           s.bind(sa)
1191           s.listen(1)
1192       except OSError as msg:
1193           s.close()
1194           s = None
1195           continue
1196       break
1197   if s is None:
1198       print('could not open socket')
1199       sys.exit(1)
1200   conn, addr = s.accept()
1201   print('Connected by', addr)
1202   while True:
1203       data = conn.recv(1024)
1204       if not data: break
1205       conn.send(data)
1206   conn.close()
1207
1208::
1209
1210   # Echo client program
1211   import socket
1212   import sys
1213
1214   HOST = 'daring.cwi.nl'    # The remote host
1215   PORT = 50007              # The same port as used by the server
1216   s = None
1217   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
1218       af, socktype, proto, canonname, sa = res
1219       try:
1220           s = socket.socket(af, socktype, proto)
1221       except OSError as msg:
1222           s = None
1223           continue
1224       try:
1225           s.connect(sa)
1226       except OSError as msg:
1227           s.close()
1228           s = None
1229           continue
1230       break
1231   if s is None:
1232       print('could not open socket')
1233       sys.exit(1)
1234   s.send(b'Hello, world')
1235   data = s.recv(1024)
1236   s.close()
1237   print('Received', repr(data))
1238
1239
1240The next example shows how to write a very simple network sniffer with raw
1241sockets on Windows. The example requires administrator privileges to modify
1242the interface::
1243
1244   import socket
1245
1246   # the public network interface
1247   HOST = socket.gethostbyname(socket.gethostname())
1248
1249   # create a raw socket and bind it to the public interface
1250   s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
1251   s.bind((HOST, 0))
1252
1253   # Include IP headers
1254   s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)
1255
1256   # receive all packages
1257   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)
1258
1259   # receive a package
1260   print(s.recvfrom(65565))
1261
1262   # disabled promiscuous mode
1263   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)
1264
1265The last example shows how to use the socket interface to communicate to a CAN
1266network. This example might require special priviledge::
1267
1268   import socket
1269   import struct
1270
1271
1272   # CAN frame packing/unpacking (see `struct can_frame` in <linux/can.h>)
1273
1274   can_frame_fmt = "=IB3x8s"
1275   can_frame_size = struct.calcsize(can_frame_fmt)
1276
1277   def build_can_frame(can_id, data):
1278       can_dlc = len(data)
1279       data = data.ljust(8, b'\x00')
1280       return struct.pack(can_frame_fmt, can_id, can_dlc, data)
1281
1282   def dissect_can_frame(frame):
1283       can_id, can_dlc, data = struct.unpack(can_frame_fmt, frame)
1284       return (can_id, can_dlc, data[:can_dlc])
1285
1286
1287   # create a raw socket and bind it to the `vcan0` interface
1288   s = socket.socket(socket.AF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
1289   s.bind(('vcan0',))
1290
1291   while True:
1292       cf, addr = s.recvfrom(can_frame_size)
1293
1294       print('Received: can_id=%x, can_dlc=%x, data=%s' % dissect_can_frame(cf))
1295
1296       try:
1297           s.send(cf)
1298       except OSError:
1299           print('Error sending CAN frame')
1300
1301       try:
1302           s.send(build_can_frame(0x01, b'\x01\x02\x03'))
1303       except OSError:
1304           print('Error sending CAN frame')
1305
1306Running an example several times with too small delay between executions, could
1307lead to this error::
1308
1309   OSError: [Errno 98] Address already in use
1310
1311This is because the previous execution has left the socket in a ``TIME_WAIT``
1312state, and can't be immediately reused.
1313
1314There is a :mod:`socket` flag to set, in order to prevent this,
1315:data:`socket.SO_REUSEADDR`::
1316
1317   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
1318   s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
1319   s.bind((HOST, PORT))
1320
1321the :data:`SO_REUSEADDR` flag tells the kernel to reuse a local socket in
1322``TIME_WAIT`` state, without waiting for its natural timeout to expire.
1323
1324
1325.. seealso::
1326
1327   For an introduction to socket programming (in C), see the following papers:
1328
1329   - *An Introductory 4.3BSD Interprocess Communication Tutorial*, by Stuart Sechrest
1330
1331   - *An Advanced 4.3BSD Interprocess Communication Tutorial*, by Samuel J.  Leffler et
1332     al,
1333
1334   both in the UNIX Programmer's Manual, Supplementary Documents 1 (sections
1335   PS1:7 and PS1:8).  The platform-specific reference material for the various
1336   socket-related system calls are also a valuable source of information on the
1337   details of socket semantics.  For Unix, refer to the manual pages; for Windows,
1338   see the WinSock (or Winsock 2) specification.  For IPv6-ready APIs, readers may
1339   want to refer to :rfc:`3493` titled Basic Socket Interface Extensions for IPv6.
1340