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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, Mac OS X, BeOS, 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
 17For an introduction to socket programming (in C), see the following papers: An
 18Introductory 4.3BSD Interprocess Communication Tutorial, by Stuart Sechrest and
 19An Advanced 4.3BSD Interprocess Communication Tutorial, by Samuel J.  Leffler et
 20al, both in the UNIX Programmer's Manual, Supplementary Documents 1 (sections
 21PS1:7 and PS1:8).  The platform-specific reference material for the various
 22socket-related system calls are also a valuable source of information on the
 23details of socket semantics.  For Unix, refer to the manual pages; for Windows,
 24see the WinSock (or Winsock 2) specification. For IPv6-ready APIs, readers may
 25want to refer to :rfc:`3493` titled Basic Socket Interface Extensions for IPv6.
 26
 27.. index:: object: socket
 28
 29The Python interface is a straightforward transliteration of the Unix system
 30call and library interface for sockets to Python's object-oriented style: the
 31:func:`socket` function returns a :dfn:`socket object` whose methods implement
 32the various socket system calls.  Parameter types are somewhat higher-level than
 33in the C interface: as with :meth:`read` and :meth:`write` operations on Python
 34files, buffer allocation on receive operations is automatic, and buffer length
 35is implicit on send operations.
 36
 37Socket addresses are represented as follows: A single string is used for the
 38:const:`AF_UNIX` address family. A pair ``(host, port)`` is used for the
 39:const:`AF_INET` address family, where *host* is a string representing either a
 40hostname in Internet domain notation like ``'daring.cwi.nl'`` or an IPv4 address
 41like ``'100.50.200.5'``, and *port* is an integral port number. For
 42:const:`AF_INET6` address family, a four-tuple ``(host, port, flowinfo,
 43scopeid)`` is used, where *flowinfo* and *scopeid* represents ``sin6_flowinfo``
 44and ``sin6_scope_id`` member in :const:`struct sockaddr_in6` in C. For
 45:mod:`socket` module methods, *flowinfo* and *scopeid* can be omitted just for
 46backward compatibility. Note, however, omission of *scopeid* can cause problems
 47in manipulating scoped IPv6 addresses. Other address families are currently not
 48supported. The address format required by a particular socket object is
 49automatically selected based on the address family specified when the socket
 50object was created.
 51
 52For IPv4 addresses, two special forms are accepted instead of a host address:
 53the empty string represents :const:`INADDR_ANY`, and the string
 54``'<broadcast>'`` represents :const:`INADDR_BROADCAST`. The behavior is not
 55available for IPv6 for backward compatibility, therefore, you may want to avoid
 56these if you intend to support IPv6 with your Python programs.
 57
 58If you use a hostname in the *host* portion of IPv4/v6 socket address, the
 59program may show a nondeterministic behavior, as Python uses the first address
 60returned from the DNS resolution.  The socket address will be resolved
 61differently into an actual IPv4/v6 address, depending on the results from DNS
 62resolution and/or the host configuration.  For deterministic behavior use a
 63numeric address in *host* portion.
 64
 65.. versionadded:: 2.5
 66   AF_NETLINK sockets are represented as  pairs ``pid, groups``.
 67
 68.. versionadded:: 2.6
 69   Linux-only support for TIPC is also available using the :const:`AF_TIPC`
 70   address family. TIPC is an open, non-IP based networked protocol designed
 71   for use in clustered computer environments.  Addresses are represented by a
 72   tuple, and the fields depend on the address type. The general tuple form is
 73   ``(addr_type, v1, v2, v3 [, scope])``, where:
 74
 75   - *addr_type* is one of TIPC_ADDR_NAMESEQ, TIPC_ADDR_NAME, or
 76     TIPC_ADDR_ID.
 77   - *scope* is one of TIPC_ZONE_SCOPE, TIPC_CLUSTER_SCOPE, and
 78     TIPC_NODE_SCOPE.
 79   - If *addr_type* is TIPC_ADDR_NAME, then *v1* is the server type, *v2* is
 80     the port identifier, and *v3* should be 0.
 81
 82     If *addr_type* is TIPC_ADDR_NAMESEQ, then *v1* is the server type, *v2*
 83     is the lower port number, and *v3* is the upper port number.
 84
 85     If *addr_type* is TIPC_ADDR_ID, then *v1* is the node, *v2* is the
 86     reference, and *v3* should be set to 0.
 87
 88
 89All errors raise exceptions.  The normal exceptions for invalid argument types
 90and out-of-memory conditions can be raised; errors related to socket or address
 91semantics raise the error :exc:`socket.error`.
 92
 93Non-blocking mode is supported through :meth:`~socket.setblocking`.  A
 94generalization of this based on timeouts is supported through
 95:meth:`~socket.settimeout`.
 96
 97The module :mod:`socket` exports the following constants and functions:
 98
 99
100.. exception:: error
101
102   .. index:: module: errno
103
104   This exception is raised for socket-related errors. The accompanying value is
105   either a string telling what went wrong or a pair ``(errno, string)``
106   representing an error returned by a system call, similar to the value
107   accompanying :exc:`os.error`. See the module :mod:`errno`, which contains names
108   for the error codes defined by the underlying operating system.
109
110   .. versionchanged:: 2.6
111      :exc:`socket.error` is now a child class of :exc:`IOError`.
112
113
114.. exception:: herror
115
116   This exception is raised for address-related errors, i.e. for functions that use
117   *h_errno* in the C API, including :func:`gethostbyname_ex` and
118   :func:`gethostbyaddr`.
119
120   The accompanying value is a pair ``(h_errno, string)`` representing an error
121   returned by a library call. *string* represents the description of *h_errno*, as
122   returned by the :cfunc:`hstrerror` C function.
123
124
125.. exception:: gaierror
126
127   This exception is raised for address-related errors, for :func:`getaddrinfo` and
128   :func:`getnameinfo`. The accompanying value is a pair ``(error, string)``
129   representing an error returned by a library call. *string* represents the
130   description of *error*, as returned by the :cfunc:`gai_strerror` C function. The
131   *error* value will match one of the :const:`EAI_\*` constants defined in this
132   module.
133
134
135.. exception:: timeout
136
137   This exception is raised when a timeout occurs on a socket which has had
138   timeouts enabled via a prior call to :meth:`settimeout`.  The accompanying value
139   is a string whose value is currently always "timed out".
140
141   .. versionadded:: 2.3
142
143
144.. data:: AF_UNIX
145          AF_INET
146          AF_INET6
147
148   These constants represent the address (and protocol) families, used for the
149   first argument to :func:`socket`.  If the :const:`AF_UNIX` constant is not
150   defined then this protocol is unsupported.
151
152
153.. data:: SOCK_STREAM
154          SOCK_DGRAM
155          SOCK_RAW
156          SOCK_RDM
157          SOCK_SEQPACKET
158
159   These constants represent the socket types, used for the second argument to
160   :func:`socket`. (Only :const:`SOCK_STREAM` and :const:`SOCK_DGRAM` appear to be
161   generally useful.)
162
163
164.. data:: SO_*
165          SOMAXCONN
166          MSG_*
167          SOL_*
168          IPPROTO_*
169          IPPORT_*
170          INADDR_*
171          IP_*
172          IPV6_*
173          EAI_*
174          AI_*
175          NI_*
176          TCP_*
177
178   Many constants of these forms, documented in the Unix documentation on sockets
179   and/or the IP protocol, are also defined in the socket module. They are
180   generally used in arguments to the :meth:`setsockopt` and :meth:`getsockopt`
181   methods of socket objects.  In most cases, only those symbols that are defined
182   in the Unix header files are defined; for a few symbols, default values are
183   provided.
184
185.. data:: SIO_*
186          RCVALL_*
187
188   Constants for Windows' WSAIoctl(). The constants are used as arguments to the
189   :meth:`ioctl` method of socket objects.
190
191   .. versionadded:: 2.6
192
193.. data:: TIPC_*
194
195   TIPC related constants, matching the ones exported by the C socket API. See
196   the TIPC documentation for more information.
197
198   .. versionadded:: 2.6
199
200.. data:: has_ipv6
201
202   This constant contains a boolean value which indicates if IPv6 is supported on
203   this platform.
204
205   .. versionadded:: 2.3
206
207
208.. function:: create_connection(address[, timeout[, source_address]])
209
210   Convenience function.  Connect to *address* (a 2-tuple ``(host, port)``),
211   and return the socket object.  Passing the optional *timeout* parameter will
212   set the timeout on the socket instance before attempting to connect.  If no
213   *timeout* is supplied, the global default timeout setting returned by
214   :func:`getdefaulttimeout` is used.
215
216   If supplied, *source_address* must be a 2-tuple ``(host, port)`` for the
217   socket to bind to as its source address before connecting.  If host or port
218   are '' or 0 respectively the OS default behavior will be used.
219
220   .. versionadded:: 2.6
221
222   .. versionchanged:: 2.7
223      *source_address* was added.
224
225
226.. function:: getaddrinfo(host, port, family=0, socktype=0, proto=0, flags=0)
227
228   Translate the *host*/*port* argument into a sequence of 5-tuples that contain
229   all the necessary arguments for creating a socket connected to that service.
230   *host* is a domain name, a string representation of an IPv4/v6 address
231   or ``None``. *port* is a string service name such as ``'http'``, a numeric
232   port number or ``None``.  By passing ``None`` as the value of *host*
233   and *port*, you can pass ``NULL`` to the underlying C API.
234
235   The *family*, *socktype* and *proto* arguments can be optionally specified
236   in order to narrow the list of addresses returned.  Passing zero as a
237   value for each of these arguments selects the full range of results.
238   The *flags* argument can be one or several of the ``AI_*`` constants,
239   and will influence how results are computed and returned.
240   For example, :const:`AI_NUMERICHOST` will disable domain name resolution
241   and will raise an error if *host* is a domain name.
242
243   The function returns a list of 5-tuples with the following structure:
244
245   ``(family, socktype, proto, canonname, sockaddr)``
246
247   In these tuples, *family*, *socktype*, *proto* are all integers and are
248   meant to be passed to the :func:`socket` function.  *canonname* will be
249   a string representing the canonical name of the *host* if
250   :const:`AI_CANONNAME` is part of the *flags* argument; else *canonname*
251   will be empty.  *sockaddr* is a tuple describing a socket address, whose
252   format depends on the returned *family* (a ``(address, port)`` 2-tuple for
253   :const:`AF_INET`, a ``(address, port, flow info, scope id)`` 4-tuple for
254   :const:`AF_INET6`), and is meant to be passed to the :meth:`socket.connect`
255   method.
256
257   The following example fetches address information for a hypothetical TCP
258   connection to ``www.python.org`` on port 80 (results may differ on your
259   system if IPv6 isn't enabled)::
260
261      >>> socket.getaddrinfo("www.python.org", 80, 0, 0, socket.SOL_TCP)
262      [(2, 1, 6, '', ('82.94.164.162', 80)),
263       (10, 1, 6, '', ('2001:888:2000:d::a2', 80, 0, 0))]
264
265   .. versionadded:: 2.2
266
267
268.. function:: getfqdn([name])
269
270   Return a fully qualified domain name for *name*. If *name* is omitted or empty,
271   it is interpreted as the local host.  To find the fully qualified name, the
272   hostname returned by :func:`gethostbyaddr` is checked, followed by aliases for the
273   host, if available.  The first name which includes a period is selected.  In
274   case no fully qualified domain name is available, the hostname as returned by
275   :func:`gethostname` is returned.
276
277   .. versionadded:: 2.0
278
279
280.. function:: gethostbyname(hostname)
281
282   Translate a host name to IPv4 address format.  The IPv4 address is returned as a
283   string, such as  ``'100.50.200.5'``.  If the host name is an IPv4 address itself
284   it is returned unchanged.  See :func:`gethostbyname_ex` for a more complete
285   interface. :func:`gethostbyname` does not support IPv6 name resolution, and
286   :func:`getaddrinfo` should be used instead for IPv4/v6 dual stack support.
287
288
289.. function:: gethostbyname_ex(hostname)
290
291   Translate a host name to IPv4 address format, extended interface. Return a
292   triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the primary
293   host name responding to the given *ip_address*, *aliaslist* is a (possibly
294   empty) list of alternative host names for the same address, and *ipaddrlist* is
295   a list of IPv4 addresses for the same interface on the same host (often but not
296   always a single address). :func:`gethostbyname_ex` does not support IPv6 name
297   resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual
298   stack support.
299
300
301.. function:: gethostname()
302
303   Return a string containing the hostname of the machine where  the Python
304   interpreter is currently executing.
305
306   If you want to know the current machine's IP address, you may want to use
307   ``gethostbyname(gethostname())``. This operation assumes that there is a
308   valid address-to-host mapping for the host, and the assumption does not
309   always hold.
310
311   Note: :func:`gethostname` doesn't always return the fully qualified domain
312   name; use ``getfqdn()`` (see above).
313
314
315.. function:: gethostbyaddr(ip_address)
316
317   Return a triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the
318   primary host name responding to the given *ip_address*, *aliaslist* is a
319   (possibly empty) list of alternative host names for the same address, and
320   *ipaddrlist* is a list of IPv4/v6 addresses for the same interface on the same
321   host (most likely containing only a single address). To find the fully qualified
322   domain name, use the function :func:`getfqdn`. :func:`gethostbyaddr` supports
323   both IPv4 and IPv6.
324
325
326.. function:: getnameinfo(sockaddr, flags)
327
328   Translate a socket address *sockaddr* into a 2-tuple ``(host, port)``. Depending
329   on the settings of *flags*, the result can contain a fully-qualified domain name
330   or numeric address representation in *host*.  Similarly, *port* can contain a
331   string port name or a numeric port number.
332
333   .. versionadded:: 2.2
334
335
336.. function:: getprotobyname(protocolname)
337
338   Translate an Internet protocol name (for example, ``'icmp'``) to a constant
339   suitable for passing as the (optional) third argument to the :func:`socket`
340   function.  This is usually only needed for sockets opened in "raw" mode
341   (:const:`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen
342   automatically if the protocol is omitted or zero.
343
344
345.. function:: getservbyname(servicename[, protocolname])
346
347   Translate an Internet service name and protocol name to a port number for that
348   service.  The optional protocol name, if given, should be ``'tcp'`` or
349   ``'udp'``, otherwise any protocol will match.
350
351
352.. function:: getservbyport(port[, protocolname])
353
354   Translate an Internet port number and protocol name to a service name for that
355   service.  The optional protocol name, if given, should be ``'tcp'`` or
356   ``'udp'``, otherwise any protocol will match.
357
358
359.. function:: socket([family[, type[, proto]]])
360
361   Create a new socket using the given address family, socket type and protocol
362   number.  The address family should be :const:`AF_INET` (the default),
363   :const:`AF_INET6` or :const:`AF_UNIX`.  The socket type should be
364   :const:`SOCK_STREAM` (the default), :const:`SOCK_DGRAM` or perhaps one of the
365   other ``SOCK_`` constants.  The protocol number is usually zero and may be
366   omitted in that case.
367
368
369.. function:: socketpair([family[, type[, proto]]])
370
371   Build a pair of connected socket objects using the given address family, socket
372   type, and protocol number.  Address family, socket type, and protocol number are
373   as for the :func:`socket` function above. The default family is :const:`AF_UNIX`
374   if defined on the platform; otherwise, the default is :const:`AF_INET`.
375   Availability: Unix.
376
377   .. versionadded:: 2.4
378
379
380.. function:: fromfd(fd, family, type[, proto])
381
382   Duplicate the file descriptor *fd* (an integer as returned by a file object's
383   :meth:`fileno` method) and build a socket object from the result.  Address
384   family, socket type and protocol number are as for the :func:`socket` function
385   above. The file descriptor should refer to a socket, but this is not checked ---
386   subsequent operations on the object may fail if the file descriptor is invalid.
387   This function is rarely needed, but can be used to get or set socket options on
388   a socket passed to a program as standard input or output (such as a server
389   started by the Unix inet daemon).  The socket is assumed to be in blocking mode.
390   Availability: Unix.
391
392
393.. function:: ntohl(x)
394
395   Convert 32-bit positive integers from network to host byte order.  On machines
396   where the host byte order is the same as network byte order, this is a no-op;
397   otherwise, it performs a 4-byte swap operation.
398
399
400.. function:: ntohs(x)
401
402   Convert 16-bit positive integers from network to host byte order.  On machines
403   where the host byte order is the same as network byte order, this is a no-op;
404   otherwise, it performs a 2-byte swap operation.
405
406
407.. function:: htonl(x)
408
409   Convert 32-bit positive integers from host to network byte order.  On machines
410   where the host byte order is the same as network byte order, this is a no-op;
411   otherwise, it performs a 4-byte swap operation.
412
413
414.. function:: htons(x)
415
416   Convert 16-bit positive integers from host to network byte order.  On machines
417   where the host byte order is the same as network byte order, this is a no-op;
418   otherwise, it performs a 2-byte swap operation.
419
420
421.. function:: inet_aton(ip_string)
422
423   Convert an IPv4 address from dotted-quad string format (for example,
424   '123.45.67.89') to 32-bit packed binary format, as a string four characters in
425   length.  This is useful when conversing with a program that uses the standard C
426   library and needs objects of type :ctype:`struct in_addr`, which is the C type
427   for the 32-bit packed binary this function returns.
428
429   :func:`inet_aton` also accepts strings with less than three dots; see the
430   Unix manual page :manpage:`inet(3)` for details.
431
432   If the IPv4 address string passed to this function is invalid,
433   :exc:`socket.error` will be raised. Note that exactly what is valid depends on
434   the underlying C implementation of :cfunc:`inet_aton`.
435
436   :func:`inet_aton` does not support IPv6, and :func:`inet_pton` should be used
437   instead for IPv4/v6 dual stack support.
438
439
440.. function:: inet_ntoa(packed_ip)
441
442   Convert a 32-bit packed IPv4 address (a string four characters in length) to its
443   standard dotted-quad string representation (for example, '123.45.67.89').  This
444   is useful when conversing with a program that uses the standard C library and
445   needs objects of type :ctype:`struct in_addr`, which is the C type for the
446   32-bit packed binary data this function takes as an argument.
447
448   If the string passed to this function is not exactly 4 bytes in length,
449   :exc:`socket.error` will be raised. :func:`inet_ntoa` does not support IPv6, and
450   :func:`inet_ntop` should be used instead for IPv4/v6 dual stack support.
451
452
453.. function:: inet_pton(address_family, ip_string)
454
455   Convert an IP address from its family-specific string format to a packed, binary
456   format. :func:`inet_pton` is useful when a library or network protocol calls for
457   an object of type :ctype:`struct in_addr` (similar to :func:`inet_aton`) or
458   :ctype:`struct in6_addr`.
459
460   Supported values for *address_family* are currently :const:`AF_INET` and
461   :const:`AF_INET6`. If the IP address string *ip_string* is invalid,
462   :exc:`socket.error` will be raised. Note that exactly what is valid depends on
463   both the value of *address_family* and the underlying implementation of
464   :cfunc:`inet_pton`.
465
466   Availability: Unix (maybe not all platforms).
467
468   .. versionadded:: 2.3
469
470
471.. function:: inet_ntop(address_family, packed_ip)
472
473   Convert a packed IP address (a string of some number of characters) to its
474   standard, family-specific string representation (for example, ``'7.10.0.5'`` or
475   ``'5aef:2b::8'``) :func:`inet_ntop` is useful when a library or network protocol
476   returns an object of type :ctype:`struct in_addr` (similar to :func:`inet_ntoa`)
477   or :ctype:`struct in6_addr`.
478
479   Supported values for *address_family* are currently :const:`AF_INET` and
480   :const:`AF_INET6`. If the string *packed_ip* is not the correct length for the
481   specified address family, :exc:`ValueError` will be raised.  A
482   :exc:`socket.error` is raised for errors from the call to :func:`inet_ntop`.
483
484   Availability: Unix (maybe not all platforms).
485
486   .. versionadded:: 2.3
487
488
489.. function:: getdefaulttimeout()
490
491   Return the default timeout in floating seconds for new socket objects. A value
492   of ``None`` indicates that new socket objects have no timeout. When the socket
493   module is first imported, the default is ``None``.
494
495   .. versionadded:: 2.3
496
497
498.. function:: setdefaulttimeout(timeout)
499
500   Set the default timeout in floating seconds for new socket objects. A value of
501   ``None`` indicates that new socket objects have no timeout. When the socket
502   module is first imported, the default is ``None``.
503
504   .. versionadded:: 2.3
505
506
507.. data:: SocketType
508
509   This is a Python type object that represents the socket object type. It is the
510   same as ``type(socket(...))``.
511
512
513.. seealso::
514
515   Module :mod:`SocketServer`
516      Classes that simplify writing network servers.
517
518   Module :mod:`ssl`
519      A TLS/SSL wrapper for socket objects.
520
521
522.. _socket-objects:
523
524Socket Objects
525--------------
526
527Socket objects have the following methods.  Except for :meth:`makefile` these
528correspond to Unix system calls applicable to sockets.
529
530
531.. method:: socket.accept()
532
533   Accept a connection. The socket must be bound to an address and listening for
534   connections. The return value is a pair ``(conn, address)`` where *conn* is a
535   *new* socket object usable to send and receive data on the connection, and
536   *address* is the address bound to the socket on the other end of the connection.
537
538
539.. method:: socket.bind(address)
540
541   Bind the socket to *address*.  The socket must not already be bound. (The format
542   of *address* depends on the address family --- see above.)
543
544   .. note::
545
546      This method has historically accepted a pair of parameters for :const:`AF_INET`
547      addresses instead of only a tuple.  This was never intentional and is no longer
548      available in Python 2.0 and later.
549
550
551.. method:: socket.close()
552
553   Close the socket.  All future operations on the socket object will fail. The
554   remote end will receive no more data (after queued data is flushed). Sockets are
555   automatically closed when they are garbage-collected.
556
557   .. note::
558      :meth:`close()` releases the resource associated with a connection but
559      does not necessarily close the connection immediately.  If you want
560      to close the connection in a timely fashion, call :meth:`shutdown()`
561      before :meth:`close()`.
562
563
564.. method:: socket.connect(address)
565
566   Connect to a remote socket at *address*. (The format of *address* depends on the
567   address family --- see above.)
568
569   .. note::
570
571      This method has historically accepted a pair of parameters for :const:`AF_INET`
572      addresses instead of only a tuple.  This was never intentional and is no longer
573      available in Python 2.0 and later.
574
575
576.. method:: socket.connect_ex(address)
577
578   Like ``connect(address)``, but return an error indicator instead of raising an
579   exception for errors returned by the C-level :cfunc:`connect` call (other
580   problems, such as "host not found," can still raise exceptions).  The error
581   indicator is ``0`` if the operation succeeded, otherwise the value of the
582   :cdata:`errno` variable.  This is useful to support, for example, asynchronous
583   connects.
584
585   .. note::
586
587      This method has historically accepted a pair of parameters for :const:`AF_INET`
588      addresses instead of only a tuple. This was never intentional and is no longer
589      available in Python 2.0 and later.
590
591
592.. method:: socket.fileno()
593
594   Return the socket's file descriptor (a small integer).  This is useful with
595   :func:`select.select`.
596
597   Under Windows the small integer returned by this method cannot be used where a
598   file descriptor can be used (such as :func:`os.fdopen`).  Unix does not have
599   this limitation.
600
601
602.. method:: socket.getpeername()
603
604   Return the remote address to which the socket is connected.  This is useful to
605   find out the port number of a remote IPv4/v6 socket, for instance. (The format
606   of the address returned depends on the address family --- see above.)  On some
607   systems this function is not supported.
608
609
610.. method:: socket.getsockname()
611
612   Return the socket's own address.  This is useful to find out the port number of
613   an IPv4/v6 socket, for instance. (The format of the address returned depends on
614   the address family --- see above.)
615
616
617.. method:: socket.getsockopt(level, optname[, buflen])
618
619   Return the value of the given socket option (see the Unix man page
620   :manpage:`getsockopt(2)`).  The needed symbolic constants (:const:`SO_\*` etc.)
621   are defined in this module.  If *buflen* is absent, an integer option is assumed
622   and its integer value is returned by the function.  If *buflen* is present, it
623   specifies the maximum length of the buffer used to receive the option in, and
624   this buffer is returned as a string.  It is up to the caller to decode the
625   contents of the buffer (see the optional built-in module :mod:`struct` for a way
626   to decode C structures encoded as strings).
627
628
629.. method:: socket.ioctl(control, option)
630
631   :platform: Windows
632
633   The :meth:`ioctl` method is a limited interface to the WSAIoctl system
634   interface.  Please refer to the `Win32 documentation
635   <http://msdn.microsoft.com/en-us/library/ms741621%28VS.85%29.aspx>`_ for more
636   information.
637
638   On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
639   functions may be used; they accept a socket object as their first argument.
640
641   .. versionadded:: 2.6
642
643
644.. method:: socket.listen(backlog)
645
646   Listen for connections made to the socket.  The *backlog* argument specifies the
647   maximum number of queued connections and should be at least 1; the maximum value
648   is system-dependent (usually 5).
649
650
651.. method:: socket.makefile([mode[, bufsize]])
652
653   .. index:: single: I/O control; buffering
654
655   Return a :dfn:`file object` associated with the socket.  (File objects are
656   described in :ref:`bltin-file-objects`.) The file object
657   references a :cfunc:`dup`\ ped version of the socket file descriptor, so the
658   file object and socket object may be closed or garbage-collected independently.
659   The socket must be in blocking mode (it can not have a timeout). The optional
660   *mode* and *bufsize* arguments are interpreted the same way as by the built-in
661   :func:`file` function.
662
663   .. note::
664
665      On Windows, the file-like object created by :meth:`makefile` cannot be
666      used where a file object with a file descriptor is expected, such as the
667      stream arguments of :meth:`subprocess.Popen`.
668
669
670.. method:: socket.recv(bufsize[, flags])
671
672   Receive data from the socket.  The return value is a string representing the
673   data received.  The maximum amount of data to be received at once is specified
674   by *bufsize*.  See the Unix manual page :manpage:`recv(2)` for the meaning of
675   the optional argument *flags*; it defaults to zero.
676
677   .. note::
678
679      For best match with hardware and network realities, the value of  *bufsize*
680      should be a relatively small power of 2, for example, 4096.
681
682
683.. method:: socket.recvfrom(bufsize[, flags])
684
685   Receive data from the socket.  The return value is a pair ``(string, address)``
686   where *string* is a string representing the data received and *address* is the
687   address of the socket sending the data.  See the Unix manual page
688   :manpage:`recv(2)` for the meaning of the optional argument *flags*; it defaults
689   to zero. (The format of *address* depends on the address family --- see above.)
690
691
692.. method:: socket.recvfrom_into(buffer[, nbytes[, flags]])
693
694   Receive data from the socket, writing it into *buffer* instead of  creating a
695   new string.  The return value is a pair ``(nbytes, address)`` where *nbytes* is
696   the number of bytes received and *address* is the address of the socket sending
697   the data.  See the Unix manual page :manpage:`recv(2)` for the meaning of the
698   optional argument *flags*; it defaults to zero.  (The format of *address*
699   depends on the address family --- see above.)
700
701   .. versionadded:: 2.5
702
703
704.. method:: socket.recv_into(buffer[, nbytes[, flags]])
705
706   Receive up to *nbytes* bytes from the socket, storing the data into a buffer
707   rather than creating a new string.  If *nbytes* is not specified (or 0),
708   receive up to the size available in the given buffer.  Returns the number of
709   bytes received.  See the Unix manual page :manpage:`recv(2)` for the meaning
710   of the optional argument *flags*; it defaults to zero.
711
712   .. versionadded:: 2.5
713
714
715.. method:: socket.send(string[, flags])
716
717   Send data to the socket.  The socket must be connected to a remote socket.  The
718   optional *flags* argument has the same meaning as for :meth:`recv` above.
719   Returns the number of bytes sent. Applications are responsible for checking that
720   all data has been sent; if only some of the data was transmitted, the
721   application needs to attempt delivery of the remaining data.
722
723
724.. method:: socket.sendall(string[, flags])
725
726   Send data to the socket.  The socket must be connected to a remote socket.  The
727   optional *flags* argument has the same meaning as for :meth:`recv` above.
728   Unlike :meth:`send`, this method continues to send data from *string* until
729   either all data has been sent or an error occurs.  ``None`` is returned on
730   success.  On error, an exception is raised, and there is no way to determine how
731   much data, if any, was successfully sent.
732
733
734.. method:: socket.sendto(string[, flags], address)
735
736   Send data to the socket.  The socket should not be connected to a remote socket,
737   since the destination socket is specified by *address*.  The optional *flags*
738   argument has the same meaning as for :meth:`recv` above.  Return the number of
739   bytes sent. (The format of *address* depends on the address family --- see
740   above.)
741
742
743.. method:: socket.setblocking(flag)
744
745   Set blocking or non-blocking mode of the socket: if *flag* is 0, the socket is
746   set to non-blocking, else to blocking mode.  Initially all sockets are in
747   blocking mode.  In non-blocking mode, if a :meth:`recv` call doesn't find any
748   data, or if a :meth:`send` call can't immediately dispose of the data, a
749   :exc:`error` exception is raised; in blocking mode, the calls block until they
750   can proceed. ``s.setblocking(0)`` is equivalent to ``s.settimeout(0.0)``;
751   ``s.setblocking(1)`` is equivalent to ``s.settimeout(None)``.
752
753
754.. method:: socket.settimeout(value)
755
756   Set a timeout on blocking socket operations.  The *value* argument can be a
757   nonnegative float expressing seconds, or ``None``. If a float is given,
758   subsequent socket operations will raise a :exc:`timeout` exception if the
759   timeout period *value* has elapsed before the operation has completed.  Setting
760   a timeout of ``None`` disables timeouts on socket operations.
761   ``s.settimeout(0.0)`` is equivalent to ``s.setblocking(0)``;
762   ``s.settimeout(None)`` is equivalent to ``s.setblocking(1)``.
763
764   .. versionadded:: 2.3
765
766
767.. method:: socket.gettimeout()
768
769   Return the timeout in floating seconds associated with socket operations, or
770   ``None`` if no timeout is set.  This reflects the last call to
771   :meth:`setblocking` or :meth:`settimeout`.
772
773   .. versionadded:: 2.3
774
775Some notes on socket blocking and timeouts: A socket object can be in one of
776three modes: blocking, non-blocking, or timeout.  Sockets are always created in
777blocking mode.  In blocking mode, operations block until complete or
778the system returns an error (such as connection timed out).  In
779non-blocking mode, operations fail (with an error that is unfortunately
780system-dependent) if they cannot be completed immediately.  In timeout mode,
781operations fail if they cannot be completed within the timeout specified for the
782socket or if the system returns an error.  The :meth:`~socket.setblocking`
783method is simply a shorthand for certain :meth:`~socket.settimeout` calls.
784
785Timeout mode internally sets the socket in non-blocking mode.  The blocking and
786timeout modes are shared between file descriptors and socket objects that refer
787to the same network endpoint.  A consequence of this is that file objects
788returned by the :meth:`~socket.makefile` method must only be used when the
789socket is in blocking mode; in timeout or non-blocking mode file operations
790that cannot be completed immediately will fail.
791
792Note that the :meth:`~socket.connect` operation is subject to the timeout
793setting, and in general it is recommended to call :meth:`~socket.settimeout`
794before calling :meth:`~socket.connect` or pass a timeout parameter to
795:meth:`create_connection`.  The system network stack may return a connection
796timeout error of its own regardless of any Python socket timeout setting.
797
798
799.. method:: socket.setsockopt(level, optname, value)
800
801   .. index:: module: struct
802
803   Set the value of the given socket option (see the Unix manual page
804   :manpage:`setsockopt(2)`).  The needed symbolic constants are defined in the
805   :mod:`socket` module (:const:`SO_\*` etc.).  The value can be an integer or a
806   string representing a buffer.  In the latter case it is up to the caller to
807   ensure that the string contains the proper bits (see the optional built-in
808   module :mod:`struct` for a way to encode C structures as strings).
809
810
811.. method:: socket.shutdown(how)
812
813   Shut down one or both halves of the connection.  If *how* is :const:`SHUT_RD`,
814   further receives are disallowed.  If *how* is :const:`SHUT_WR`, further sends
815   are disallowed.  If *how* is :const:`SHUT_RDWR`, further sends and receives are
816   disallowed.  Depending on the platform, shutting down one half of the connection
817   can also close the opposite half (e.g. on Mac OS X, ``shutdown(SHUT_WR)`` does
818   not allow further reads on the other end of the connection).
819
820Note that there are no methods :meth:`read` or :meth:`write`; use
821:meth:`~socket.recv` and :meth:`~socket.send` without *flags* argument instead.
822
823Socket objects also have these (read-only) attributes that correspond to the
824values given to the :class:`socket` constructor.
825
826
827.. attribute:: socket.family
828
829   The socket family.
830
831   .. versionadded:: 2.5
832
833
834.. attribute:: socket.type
835
836   The socket type.
837
838   .. versionadded:: 2.5
839
840
841.. attribute:: socket.proto
842
843   The socket protocol.
844
845   .. versionadded:: 2.5
846
847
848.. _socket-example:
849
850Example
851-------
852
853Here are four minimal example programs using the TCP/IP protocol: a server that
854echoes all data that it receives back (servicing only one client), and a client
855using it.  Note that a server must perform the sequence :func:`socket`,
856:meth:`~socket.bind`, :meth:`~socket.listen`, :meth:`~socket.accept` (possibly
857repeating the :meth:`~socket.accept` to service more than one client), while a
858client only needs the sequence :func:`socket`, :meth:`~socket.connect`.  Also
859note that the server does not :meth:`~socket.send`/:meth:`~socket.recv` on the
860socket it is listening on but on the new socket returned by
861:meth:`~socket.accept`.
862
863The first two examples support IPv4 only. ::
864
865   # Echo server program
866   import socket
867
868   HOST = ''                 # Symbolic name meaning all available interfaces
869   PORT = 50007              # Arbitrary non-privileged port
870   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
871   s.bind((HOST, PORT))
872   s.listen(1)
873   conn, addr = s.accept()
874   print 'Connected by', addr
875   while 1:
876       data = conn.recv(1024)
877       if not data: break
878       conn.send(data)
879   conn.close()
880
881::
882
883   # Echo client program
884   import socket
885
886   HOST = 'daring.cwi.nl'    # The remote host
887   PORT = 50007              # The same port as used by the server
888   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
889   s.connect((HOST, PORT))
890   s.send('Hello, world')
891   data = s.recv(1024)
892   s.close()
893   print 'Received', repr(data)
894
895The next two examples are identical to the above two, but support both IPv4 and
896IPv6. The server side will listen to the first address family available (it
897should listen to both instead). On most of IPv6-ready systems, IPv6 will take
898precedence and the server may not accept IPv4 traffic. The client side will try
899to connect to the all addresses returned as a result of the name resolution, and
900sends traffic to the first one connected successfully. ::
901
902   # Echo server program
903   import socket
904   import sys
905
906   HOST = None               # Symbolic name meaning all available interfaces
907   PORT = 50007              # Arbitrary non-privileged port
908   s = None
909   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
910                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
911       af, socktype, proto, canonname, sa = res
912       try:
913           s = socket.socket(af, socktype, proto)
914       except socket.error, msg:
915           s = None
916           continue
917       try:
918           s.bind(sa)
919           s.listen(1)
920       except socket.error, msg:
921           s.close()
922           s = None
923           continue
924       break
925   if s is None:
926       print 'could not open socket'
927       sys.exit(1)
928   conn, addr = s.accept()
929   print 'Connected by', addr
930   while 1:
931       data = conn.recv(1024)
932       if not data: break
933       conn.send(data)
934   conn.close()
935
936::
937
938   # Echo client program
939   import socket
940   import sys
941
942   HOST = 'daring.cwi.nl'    # The remote host
943   PORT = 50007              # The same port as used by the server
944   s = None
945   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
946       af, socktype, proto, canonname, sa = res
947       try:
948           s = socket.socket(af, socktype, proto)
949       except socket.error, msg:
950           s = None
951           continue
952       try:
953           s.connect(sa)
954       except socket.error, msg:
955           s.close()
956           s = None
957           continue
958       break
959   if s is None:
960       print 'could not open socket'
961       sys.exit(1)
962   s.send('Hello, world')
963   data = s.recv(1024)
964   s.close()
965   print 'Received', repr(data)
966
967
968The last example shows how to write a very simple network sniffer with raw
969sockets on Windows. The example requires administrator privileges to modify
970the interface::
971
972   import socket
973
974   # the public network interface
975   HOST = socket.gethostbyname(socket.gethostname())
976
977   # create a raw socket and bind it to the public interface
978   s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
979   s.bind((HOST, 0))
980
981   # Include IP headers
982   s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)
983
984   # receive all packages
985   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)
986
987   # receive a package
988   print s.recvfrom(65565)
989
990   # disabled promiscuous mode
991   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)