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/policy-and-header-refactor/Doc/library/socket.rst

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  1:mod:`socket` --- Low-level networking interface
  2================================================
  3
  4.. module:: socket
  5   :synopsis: Low-level networking interface.
  6
  7
  8This module provides access to the BSD *socket* interface. It is available on
  9all modern Unix systems, Windows, MacOS, OS/2, and probably additional
 10platforms.
 11
 12.. note::
 13
 14   Some behavior may be platform dependent, since calls are made to the operating
 15   system socket APIs.
 16
 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
 65AF_NETLINK sockets are represented as  pairs ``pid, groups``.
 66
 67
 68Linux-only support for TIPC is also available using the :const:`AF_TIPC`
 69address family. TIPC is an open, non-IP based networked protocol designed
 70for use in clustered computer environments.  Addresses are represented by a
 71tuple, and the fields depend on the address type. The general tuple form is
 72``(addr_type, v1, v2, v3 [, scope])``, where:
 73
 74   - *addr_type* is one of TIPC_ADDR_NAMESEQ, TIPC_ADDR_NAME, or
 75     TIPC_ADDR_ID.
 76   - *scope* is one of TIPC_ZONE_SCOPE, TIPC_CLUSTER_SCOPE, and
 77     TIPC_NODE_SCOPE.
 78   - If *addr_type* is TIPC_ADDR_NAME, then *v1* is the server type, *v2* is
 79     the port identifier, and *v3* should be 0.
 80
 81     If *addr_type* is TIPC_ADDR_NAMESEQ, then *v1* is the server type, *v2*
 82     is the lower port number, and *v3* is the upper port number.
 83
 84     If *addr_type* is TIPC_ADDR_ID, then *v1* is the node, *v2* is the
 85     reference, and *v3* should be set to 0.
 86
 87
 88All errors raise exceptions.  The normal exceptions for invalid argument types
 89and out-of-memory conditions can be raised; errors related to socket or address
 90semantics raise the error :exc:`socket.error`.
 91
 92Non-blocking mode is supported through :meth:`setblocking`.  A generalization of
 93this based on timeouts is supported through :meth:`settimeout`.
 94
 95The module :mod:`socket` exports the following constants and functions:
 96
 97
 98.. exception:: error
 99
100   .. index:: module: errno
101
102   This exception is raised for socket-related errors. The accompanying value is
103   either a string telling what went wrong or a pair ``(errno, string)``
104   representing an error returned by a system call, similar to the value
105   accompanying :exc:`os.error`. See the module :mod:`errno`, which contains names
106   for the error codes defined by the underlying operating system.
107
108
109.. exception:: herror
110
111   This exception is raised for address-related errors, i.e. for functions that use
112   *h_errno* in the C API, including :func:`gethostbyname_ex` and
113   :func:`gethostbyaddr`.
114
115   The accompanying value is a pair ``(h_errno, string)`` representing an error
116   returned by a library call. *string* represents the description of *h_errno*, as
117   returned by the :cfunc:`hstrerror` C function.
118
119
120.. exception:: gaierror
121
122   This exception is raised for address-related errors, for :func:`getaddrinfo` and
123   :func:`getnameinfo`. The accompanying value is a pair ``(error, string)``
124   representing an error returned by a library call. *string* represents the
125   description of *error*, as returned by the :cfunc:`gai_strerror` C function. The
126   *error* value will match one of the :const:`EAI_\*` constants defined in this
127   module.
128
129
130.. exception:: timeout
131
132   This exception is raised when a timeout occurs on a socket which has had
133   timeouts enabled via a prior call to :meth:`settimeout`.  The accompanying value
134   is a string whose value is currently always "timed out".
135
136
137.. data:: AF_UNIX
138          AF_INET
139          AF_INET6
140
141   These constants represent the address (and protocol) families, used for the
142   first argument to :func:`socket`.  If the :const:`AF_UNIX` constant is not
143   defined then this protocol is unsupported.
144
145
146.. data:: SOCK_STREAM
147          SOCK_DGRAM
148          SOCK_RAW
149          SOCK_RDM
150          SOCK_SEQPACKET
151
152   These constants represent the socket types, used for the second argument to
153   :func:`socket`. (Only :const:`SOCK_STREAM` and :const:`SOCK_DGRAM` appear to be
154   generally useful.)
155
156
157.. data:: SO_*
158          SOMAXCONN
159          MSG_*
160          SOL_*
161          IPPROTO_*
162          IPPORT_*
163          INADDR_*
164          IP_*
165          IPV6_*
166          EAI_*
167          AI_*
168          NI_*
169          TCP_*
170
171   Many constants of these forms, documented in the Unix documentation on sockets
172   and/or the IP protocol, are also defined in the socket module. They are
173   generally used in arguments to the :meth:`setsockopt` and :meth:`getsockopt`
174   methods of socket objects.  In most cases, only those symbols that are defined
175   in the Unix header files are defined; for a few symbols, default values are
176   provided.
177
178.. data:: SIO_*
179          RCVALL_*
180
181   Constants for Windows' WSAIoctl(). The constants are used as arguments to the
182   :meth:`ioctl` method of socket objects.
183
184
185.. data:: TIPC_*
186
187   TIPC related constants, matching the ones exported by the C socket API. See
188   the TIPC documentation for more information.
189
190
191.. data:: has_ipv6
192
193   This constant contains a boolean value which indicates if IPv6 is supported on
194   this platform.
195
196
197.. function:: create_connection(address[, timeout[, source_address]])
198
199   Convenience function.  Connect to *address* (a 2-tuple ``(host, port)``),
200   and return the socket object.  Passing the optional *timeout* parameter will
201   set the timeout on the socket instance before attempting to connect.  If no
202   *timeout* is supplied, the global default timeout setting returned by
203   :func:`getdefaulttimeout` is used.
204
205   If supplied, *source_address* must be a 2-tuple ``(host, port)`` for the
206   socket to bind to as its source address before connecting.  If host or port
207   are '' or 0 respectively the OS default behavior will be used.
208
209   .. versionchanged:: 3.2
210      *source_address* was added.
211
212
213.. function:: getaddrinfo(host, port[, family[, socktype[, proto[, flags]]]])
214
215   Resolves the *host*/*port* argument, into a sequence of 5-tuples that contain
216   all the necessary arguments for creating the corresponding socket. *host* is a domain
217   name, a string representation of an IPv4/v6 address or ``None``. *port* is a string
218   service name such as ``'http'``, a numeric port number or ``None``.
219   The rest of the arguments are optional and must be numeric if specified.
220   By passing ``None`` as the value of *host* and *port*, , you can pass ``NULL`` to the C API.
221
222   The :func:`getaddrinfo` function returns a list of 5-tuples with the following
223   structure:
224
225   ``(family, socktype, proto, canonname, sockaddr)``
226
227   *family*, *socktype*, *proto* are all integers and are meant to be passed to the
228   :func:`socket` function. *canonname* is a string representing the canonical name
229   of the *host*. It can be a numeric IPv4/v6 address when :const:`AI_CANONNAME` is
230   specified for a numeric *host*. *sockaddr* is a tuple describing a socket
231   address, as described above. See the source for :mod:`socket` and other
232   library modules for a typical usage of the function.
233
234
235.. function:: getfqdn([name])
236
237   Return a fully qualified domain name for *name*. If *name* is omitted or empty,
238   it is interpreted as the local host.  To find the fully qualified name, the
239   hostname returned by :func:`gethostbyaddr` is checked, followed by aliases for the
240   host, if available.  The first name which includes a period is selected.  In
241   case no fully qualified domain name is available, the hostname as returned by
242   :func:`gethostname` is returned.
243
244
245.. function:: gethostbyname(hostname)
246
247   Translate a host name to IPv4 address format.  The IPv4 address is returned as a
248   string, such as  ``'100.50.200.5'``.  If the host name is an IPv4 address itself
249   it is returned unchanged.  See :func:`gethostbyname_ex` for a more complete
250   interface. :func:`gethostbyname` does not support IPv6 name resolution, and
251   :func:`getaddrinfo` should be used instead for IPv4/v6 dual stack support.
252
253
254.. function:: gethostbyname_ex(hostname)
255
256   Translate a host name to IPv4 address format, extended interface. Return a
257   triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the primary
258   host name responding to the given *ip_address*, *aliaslist* is a (possibly
259   empty) list of alternative host names for the same address, and *ipaddrlist* is
260   a list of IPv4 addresses for the same interface on the same host (often but not
261   always a single address). :func:`gethostbyname_ex` does not support IPv6 name
262   resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual
263   stack support.
264
265
266.. function:: gethostname()
267
268   Return a string containing the hostname of the machine where  the Python
269   interpreter is currently executing.
270
271   If you want to know the current machine's IP address, you may want to use
272   ``gethostbyname(gethostname())``. This operation assumes that there is a
273   valid address-to-host mapping for the host, and the assumption does not
274   always hold.
275
276   Note: :func:`gethostname` doesn't always return the fully qualified domain
277   name; use ``getfqdn()`` (see above).
278
279
280.. function:: gethostbyaddr(ip_address)
281
282   Return a triple ``(hostname, aliaslist, ipaddrlist)`` where *hostname* is the
283   primary host name responding to the given *ip_address*, *aliaslist* is a
284   (possibly empty) list of alternative host names for the same address, and
285   *ipaddrlist* is a list of IPv4/v6 addresses for the same interface on the same
286   host (most likely containing only a single address). To find the fully qualified
287   domain name, use the function :func:`getfqdn`. :func:`gethostbyaddr` supports
288   both IPv4 and IPv6.
289
290
291.. function:: getnameinfo(sockaddr, flags)
292
293   Translate a socket address *sockaddr* into a 2-tuple ``(host, port)``. Depending
294   on the settings of *flags*, the result can contain a fully-qualified domain name
295   or numeric address representation in *host*.  Similarly, *port* can contain a
296   string port name or a numeric port number.
297
298
299.. function:: getprotobyname(protocolname)
300
301   Translate an Internet protocol name (for example, ``'icmp'``) to a constant
302   suitable for passing as the (optional) third argument to the :func:`socket`
303   function.  This is usually only needed for sockets opened in "raw" mode
304   (:const:`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen
305   automatically if the protocol is omitted or zero.
306
307
308.. function:: getservbyname(servicename[, protocolname])
309
310   Translate an Internet service name and protocol name to a port number for that
311   service.  The optional protocol name, if given, should be ``'tcp'`` or
312   ``'udp'``, otherwise any protocol will match.
313
314
315.. function:: getservbyport(port[, protocolname])
316
317   Translate an Internet port number and protocol name to a service name for that
318   service.  The optional protocol name, if given, should be ``'tcp'`` or
319   ``'udp'``, otherwise any protocol will match.
320
321
322.. function:: socket([family[, type[, proto]]])
323
324   Create a new socket using the given address family, socket type and protocol
325   number.  The address family should be :const:`AF_INET` (the default),
326   :const:`AF_INET6` or :const:`AF_UNIX`.  The socket type should be
327   :const:`SOCK_STREAM` (the default), :const:`SOCK_DGRAM` or perhaps one of the
328   other ``SOCK_`` constants.  The protocol number is usually zero and may be
329   omitted in that case.
330
331
332.. function:: socketpair([family[, type[, proto]]])
333
334   Build a pair of connected socket objects using the given address family, socket
335   type, and protocol number.  Address family, socket type, and protocol number are
336   as for the :func:`socket` function above. The default family is :const:`AF_UNIX`
337   if defined on the platform; otherwise, the default is :const:`AF_INET`.
338   Availability: Unix.
339
340
341.. function:: fromfd(fd, family, type[, proto])
342
343   Duplicate the file descriptor *fd* (an integer as returned by a file object's
344   :meth:`fileno` method) and build a socket object from the result.  Address
345   family, socket type and protocol number are as for the :func:`socket` function
346   above. The file descriptor should refer to a socket, but this is not checked ---
347   subsequent operations on the object may fail if the file descriptor is invalid.
348   This function is rarely needed, but can be used to get or set socket options on
349   a socket passed to a program as standard input or output (such as a server
350   started by the Unix inet daemon).  The socket is assumed to be in blocking mode.
351   Availability: Unix.
352
353
354.. function:: ntohl(x)
355
356   Convert 32-bit positive integers from network to host byte order.  On machines
357   where the host byte order is the same as network byte order, this is a no-op;
358   otherwise, it performs a 4-byte swap operation.
359
360
361.. function:: ntohs(x)
362
363   Convert 16-bit positive integers from network to host byte order.  On machines
364   where the host byte order is the same as network byte order, this is a no-op;
365   otherwise, it performs a 2-byte swap operation.
366
367
368.. function:: htonl(x)
369
370   Convert 32-bit positive integers from host to network byte order.  On machines
371   where the host byte order is the same as network byte order, this is a no-op;
372   otherwise, it performs a 4-byte swap operation.
373
374
375.. function:: htons(x)
376
377   Convert 16-bit positive integers from host to network byte order.  On machines
378   where the host byte order is the same as network byte order, this is a no-op;
379   otherwise, it performs a 2-byte swap operation.
380
381
382.. function:: inet_aton(ip_string)
383
384   Convert an IPv4 address from dotted-quad string format (for example,
385   '123.45.67.89') to 32-bit packed binary format, as a bytes object four characters in
386   length.  This is useful when conversing with a program that uses the standard C
387   library and needs objects of type :ctype:`struct in_addr`, which is the C type
388   for the 32-bit packed binary this function returns.
389
390   :func:`inet_aton` also accepts strings with less than three dots; see the
391   Unix manual page :manpage:`inet(3)` for details.
392
393   If the IPv4 address string passed to this function is invalid,
394   :exc:`socket.error` will be raised. Note that exactly what is valid depends on
395   the underlying C implementation of :cfunc:`inet_aton`.
396
397   :func:`inet_aton` does not support IPv6, and :func:`inet_pton` should be used
398   instead for IPv4/v6 dual stack support.
399
400
401.. function:: inet_ntoa(packed_ip)
402
403   Convert a 32-bit packed IPv4 address (a bytes object four characters in
404   length) to its standard dotted-quad string representation (for example,
405   '123.45.67.89').  This is useful when conversing with a program that uses the
406   standard C library and needs objects of type :ctype:`struct in_addr`, which
407   is the C type for the 32-bit packed binary data this function takes as an
408   argument.
409
410   If the byte sequence passed to this function is not exactly 4 bytes in
411   length, :exc:`socket.error` will be raised. :func:`inet_ntoa` does not
412   support IPv6, and :func:`inet_ntop` should be used instead for IPv4/v6 dual
413   stack support.
414
415
416.. function:: inet_pton(address_family, ip_string)
417
418   Convert an IP address from its family-specific string format to a packed,
419   binary format. :func:`inet_pton` is useful when a library or network protocol
420   calls for an object of type :ctype:`struct in_addr` (similar to
421   :func:`inet_aton`) or :ctype:`struct in6_addr`.
422
423   Supported values for *address_family* are currently :const:`AF_INET` and
424   :const:`AF_INET6`. If the IP address string *ip_string* is invalid,
425   :exc:`socket.error` will be raised. Note that exactly what is valid depends on
426   both the value of *address_family* and the underlying implementation of
427   :cfunc:`inet_pton`.
428
429   Availability: Unix (maybe not all platforms).
430
431
432.. function:: inet_ntop(address_family, packed_ip)
433
434   Convert a packed IP address (a bytes object of some number of characters) to its
435   standard, family-specific string representation (for example, ``'7.10.0.5'`` or
436   ``'5aef:2b::8'``). :func:`inet_ntop` is useful when a library or network protocol
437   returns an object of type :ctype:`struct in_addr` (similar to :func:`inet_ntoa`)
438   or :ctype:`struct in6_addr`.
439
440   Supported values for *address_family* are currently :const:`AF_INET` and
441   :const:`AF_INET6`. If the string *packed_ip* is not the correct length for the
442   specified address family, :exc:`ValueError` will be raised.  A
443   :exc:`socket.error` is raised for errors from the call to :func:`inet_ntop`.
444
445   Availability: Unix (maybe not all platforms).
446
447
448.. function:: getdefaulttimeout()
449
450   Return the default timeout in floating seconds for new socket objects. A value
451   of ``None`` indicates that new socket objects have no timeout. When the socket
452   module is first imported, the default is ``None``.
453
454
455.. function:: setdefaulttimeout(timeout)
456
457   Set the default timeout in floating seconds for new socket objects. A value of
458   ``None`` indicates that new socket objects have no timeout. When the socket
459   module is first imported, the default is ``None``.
460
461
462.. data:: SocketType
463
464   This is a Python type object that represents the socket object type. It is the
465   same as ``type(socket(...))``.
466
467
468.. seealso::
469
470   Module :mod:`socketserver`
471      Classes that simplify writing network servers.
472
473
474.. _socket-objects:
475
476Socket Objects
477--------------
478
479Socket objects have the following methods.  Except for :meth:`makefile` these
480correspond to Unix system calls applicable to sockets.
481
482
483.. method:: socket.accept()
484
485   Accept a connection. The socket must be bound to an address and listening for
486   connections. The return value is a pair ``(conn, address)`` where *conn* is a
487   *new* socket object usable to send and receive data on the connection, and
488   *address* is the address bound to the socket on the other end of the connection.
489
490
491.. method:: socket.bind(address)
492
493   Bind the socket to *address*.  The socket must not already be bound. (The format
494   of *address* depends on the address family --- see above.)
495
496
497.. method:: socket.close()
498
499   Close the socket.  All future operations on the socket object will fail. The
500   remote end will receive no more data (after queued data is flushed). Sockets are
501   automatically closed when they are garbage-collected.
502
503
504.. method:: socket.connect(address)
505
506   Connect to a remote socket at *address*. (The format of *address* depends on the
507   address family --- see above.)
508
509
510.. method:: socket.connect_ex(address)
511
512   Like ``connect(address)``, but return an error indicator instead of raising an
513   exception for errors returned by the C-level :cfunc:`connect` call (other
514   problems, such as "host not found," can still raise exceptions).  The error
515   indicator is ``0`` if the operation succeeded, otherwise the value of the
516   :cdata:`errno` variable.  This is useful to support, for example, asynchronous
517   connects.
518
519
520.. method:: socket.fileno()
521
522   Return the socket's file descriptor (a small integer).  This is useful with
523   :func:`select.select`.
524
525   Under Windows the small integer returned by this method cannot be used where a
526   file descriptor can be used (such as :func:`os.fdopen`).  Unix does not have
527   this limitation.
528
529
530.. method:: socket.getpeername()
531
532   Return the remote address to which the socket is connected.  This is useful to
533   find out the port number of a remote IPv4/v6 socket, for instance. (The format
534   of the address returned depends on the address family --- see above.)  On some
535   systems this function is not supported.
536
537
538.. method:: socket.getsockname()
539
540   Return the socket's own address.  This is useful to find out the port number of
541   an IPv4/v6 socket, for instance. (The format of the address returned depends on
542   the address family --- see above.)
543
544
545.. method:: socket.getsockopt(level, optname[, buflen])
546
547   Return the value of the given socket option (see the Unix man page
548   :manpage:`getsockopt(2)`).  The needed symbolic constants (:const:`SO_\*` etc.)
549   are defined in this module.  If *buflen* is absent, an integer option is assumed
550   and its integer value is returned by the function.  If *buflen* is present, it
551   specifies the maximum length of the buffer used to receive the option in, and
552   this buffer is returned as a bytes object.  It is up to the caller to decode the
553   contents of the buffer (see the optional built-in module :mod:`struct` for a way
554   to decode C structures encoded as byte strings).
555
556
557.. method:: socket.ioctl(control, option)
558
559   :platform: Windows
560
561   The :meth:`ioctl` method is a limited interface to the WSAIoctl system
562   interface. Please refer to the MSDN documentation for more information.
563
564   On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
565   functions may be used; they accept a socket object as their first argument.
566
567.. method:: socket.listen(backlog)
568
569   Listen for connections made to the socket.  The *backlog* argument specifies the
570   maximum number of queued connections and should be at least 1; the maximum value
571   is system-dependent (usually 5).
572
573
574.. method:: socket.makefile(mode='r', buffering=None, *, encoding=None, newline=None)
575
576   .. index:: single: I/O control; buffering
577
578   Return a :dfn:`file object` associated with the socket.  The exact
579   returned type depends on the arguments given to :meth:`makefile`.  These
580   arguments are interpreted the same way as by the built-in :func:`open`
581   function.
582
583   The returned file object references a :cfunc:`dup`\ ped version of the
584   socket file descriptor, so the file object and socket object may be
585   closed or garbage-collected independently.  The socket must be in
586   blocking mode (it can not have a timeout).
587
588
589.. method:: socket.recv(bufsize[, flags])
590
591   Receive data from the socket.  The return value is a bytes object representing the
592   data received.  The maximum amount of data to be received at once is specified
593   by *bufsize*.  See the Unix manual page :manpage:`recv(2)` for the meaning of
594   the optional argument *flags*; it defaults to zero.
595
596   .. note::
597
598      For best match with hardware and network realities, the value of  *bufsize*
599      should be a relatively small power of 2, for example, 4096.
600
601
602.. method:: socket.recvfrom(bufsize[, flags])
603
604   Receive data from the socket.  The return value is a pair ``(bytes, address)``
605   where *bytes* is a bytes object representing the data received and *address* is the
606   address of the socket sending the data.  See the Unix manual page
607   :manpage:`recv(2)` for the meaning of the optional argument *flags*; it defaults
608   to zero. (The format of *address* depends on the address family --- see above.)
609
610
611.. method:: socket.recvfrom_into(buffer[, nbytes[, flags]])
612
613   Receive data from the socket, writing it into *buffer* instead of creating a
614   new bytestring.  The return value is a pair ``(nbytes, address)`` where *nbytes* is
615   the number of bytes received and *address* is the address of the socket sending
616   the data.  See the Unix manual page :manpage:`recv(2)` for the meaning of the
617   optional argument *flags*; it defaults to zero.  (The format of *address*
618   depends on the address family --- see above.)
619
620
621.. method:: socket.recv_into(buffer[, nbytes[, flags]])
622
623   Receive up to *nbytes* bytes from the socket, storing the data into a buffer
624   rather than creating a new bytestring.  If *nbytes* is not specified (or 0),
625   receive up to the size available in the given buffer.  Returns the number of
626   bytes received.  See the Unix manual page :manpage:`recv(2)` for the meaning
627   of the optional argument *flags*; it defaults to zero.
628
629
630.. method:: socket.send(bytes[, flags])
631
632   Send data to the socket.  The socket must be connected to a remote socket.  The
633   optional *flags* argument has the same meaning as for :meth:`recv` above.
634   Returns the number of bytes sent. Applications are responsible for checking that
635   all data has been sent; if only some of the data was transmitted, the
636   application needs to attempt delivery of the remaining data.
637
638
639.. method:: socket.sendall(bytes[, flags])
640
641   Send data to the socket.  The socket must be connected to a remote socket.  The
642   optional *flags* argument has the same meaning as for :meth:`recv` above.
643   Unlike :meth:`send`, this method continues to send data from *bytes* until
644   either all data has been sent or an error occurs.  ``None`` is returned on
645   success.  On error, an exception is raised, and there is no way to determine how
646   much data, if any, was successfully sent.
647
648
649.. method:: socket.sendto(bytes[, flags], address)
650
651   Send data to the socket.  The socket should not be connected to a remote socket,
652   since the destination socket is specified by *address*.  The optional *flags*
653   argument has the same meaning as for :meth:`recv` above.  Return the number of
654   bytes sent. (The format of *address* depends on the address family --- see
655   above.)
656
657
658.. method:: socket.setblocking(flag)
659
660   Set blocking or non-blocking mode of the socket: if *flag* is 0, the socket is
661   set to non-blocking, else to blocking mode.  Initially all sockets are in
662   blocking mode.  In non-blocking mode, if a :meth:`recv` call doesn't find any
663   data, or if a :meth:`send` call can't immediately dispose of the data, a
664   :exc:`error` exception is raised; in blocking mode, the calls block until they
665   can proceed. ``s.setblocking(0)`` is equivalent to ``s.settimeout(0)``;
666   ``s.setblocking(1)`` is equivalent to ``s.settimeout(None)``.
667
668
669.. method:: socket.settimeout(value)
670
671   Set a timeout on blocking socket operations.  The *value* argument can be a
672   nonnegative float expressing seconds, or ``None``. If a float is given,
673   subsequent socket operations will raise an :exc:`timeout` exception if the
674   timeout period *value* has elapsed before the operation has completed.  Setting
675   a timeout of ``None`` disables timeouts on socket operations.
676   ``s.settimeout(0.0)`` is equivalent to ``s.setblocking(0)``;
677   ``s.settimeout(None)`` is equivalent to ``s.setblocking(1)``.
678
679
680.. method:: socket.gettimeout()
681
682   Return the timeout in floating seconds associated with socket operations, or
683   ``None`` if no timeout is set.  This reflects the last call to
684   :meth:`setblocking` or :meth:`settimeout`.
685
686
687Some notes on socket blocking and timeouts: A socket object can be in one of
688three modes: blocking, non-blocking, or timeout.  Sockets are always created in
689blocking mode.  In blocking mode, operations block until complete or
690the system returns an error (such as connection timed out).  In
691non-blocking mode, operations fail (with an error that is unfortunately
692system-dependent) if they cannot be completed immediately.  In timeout mode,
693operations fail if they cannot be completed within the timeout specified for the
694socket or if the system returns an error.  The :meth:`setblocking` method is simply
695a shorthand for certain :meth:`settimeout` calls.
696
697Timeout mode internally sets the socket in non-blocking mode.  The blocking and
698timeout modes are shared between file descriptors and socket objects that refer
699to the same network endpoint.  A consequence of this is that file objects
700returned by the :meth:`makefile` method must only be used when the socket is in
701blocking mode; in timeout or non-blocking mode file operations that cannot be
702completed immediately will fail.
703
704Note that the :meth:`connect` operation is subject to the timeout setting, and
705in general it is recommended to call :meth:`settimeout` before calling
706:meth:`connect` or pass a timeout parameter to :meth:`create_connection`.
707The system network stack may return a connection timeout error
708of its own regardless of any Python socket timeout setting.
709
710
711.. method:: socket.setsockopt(level, optname, value)
712
713   .. index:: module: struct
714
715   Set the value of the given socket option (see the Unix manual page
716   :manpage:`setsockopt(2)`).  The needed symbolic constants are defined in the
717   :mod:`socket` module (:const:`SO_\*` etc.).  The value can be an integer or a
718   bytes object representing a buffer.  In the latter case it is up to the caller to
719   ensure that the bytestring contains the proper bits (see the optional built-in
720   module :mod:`struct` for a way to encode C structures as bytestrings).
721
722
723.. method:: socket.shutdown(how)
724
725   Shut down one or both halves of the connection.  If *how* is :const:`SHUT_RD`,
726   further receives are disallowed.  If *how* is :const:`SHUT_WR`, further sends
727   are disallowed.  If *how* is :const:`SHUT_RDWR`, further sends and receives are
728   disallowed.
729
730Note that there are no methods :meth:`read` or :meth:`write`; use :meth:`recv`
731and :meth:`send` without *flags* argument instead.
732
733Socket objects also have these (read-only) attributes that correspond to the
734values given to the :class:`socket` constructor.
735
736
737.. attribute:: socket.family
738
739   The socket family.
740
741
742.. attribute:: socket.type
743
744   The socket type.
745
746
747.. attribute:: socket.proto
748
749   The socket protocol.
750
751
752.. _socket-example:
753
754Example
755-------
756
757Here are four minimal example programs using the TCP/IP protocol: a server that
758echoes all data that it receives back (servicing only one client), and a client
759using it.  Note that a server must perform the sequence :func:`socket`,
760:meth:`bind`, :meth:`listen`, :meth:`accept` (possibly repeating the
761:meth:`accept` to service more than one client), while a client only needs the
762sequence :func:`socket`, :meth:`connect`.  Also note that the server does not
763:meth:`send`/:meth:`recv` on the  socket it is listening on but on the new
764socket returned by :meth:`accept`.
765
766The first two examples support IPv4 only. ::
767
768   # Echo server program
769   import socket
770
771   HOST = ''                 # Symbolic name meaning all available interfaces
772   PORT = 50007              # Arbitrary non-privileged port
773   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
774   s.bind((HOST, PORT))
775   s.listen(1)
776   conn, addr = s.accept()
777   print('Connected by', addr)
778   while True:
779       data = conn.recv(1024)
780       if not data: break
781       conn.send(data)
782   conn.close()
783
784::
785
786   # Echo client program
787   import socket
788
789   HOST = 'daring.cwi.nl'    # The remote host
790   PORT = 50007              # The same port as used by the server
791   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
792   s.connect((HOST, PORT))
793   s.send(b'Hello, world')
794   data = s.recv(1024)
795   s.close()
796   print('Received', repr(data))
797
798The next two examples are identical to the above two, but support both IPv4 and
799IPv6. The server side will listen to the first address family available (it
800should listen to both instead). On most of IPv6-ready systems, IPv6 will take
801precedence and the server may not accept IPv4 traffic. The client side will try
802to connect to the all addresses returned as a result of the name resolution, and
803sends traffic to the first one connected successfully. ::
804
805   # Echo server program
806   import socket
807   import sys
808
809   HOST = None               # Symbolic name meaning all available interfaces
810   PORT = 50007              # Arbitrary non-privileged port
811   s = None
812   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
813                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
814       af, socktype, proto, canonname, sa = res
815       try:
816           s = socket.socket(af, socktype, proto)
817       except socket.error as msg:
818           s = None
819           continue
820       try:
821           s.bind(sa)
822           s.listen(1)
823       except socket.error as msg:
824           s.close()
825           s = None
826           continue
827       break
828   if s is None:
829       print('could not open socket')
830       sys.exit(1)
831   conn, addr = s.accept()
832   print('Connected by', addr)
833   while True:
834       data = conn.recv(1024)
835       if not data: break
836       conn.send(data)
837   conn.close()
838
839::
840
841   # Echo client program
842   import socket
843   import sys
844
845   HOST = 'daring.cwi.nl'    # The remote host
846   PORT = 50007              # The same port as used by the server
847   s = None
848   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
849       af, socktype, proto, canonname, sa = res
850       try:
851           s = socket.socket(af, socktype, proto)
852       except socket.error as msg:
853           s = None
854           continue
855       try:
856           s.connect(sa)
857       except socket.error as msg:
858           s.close()
859           s = None
860           continue
861       break
862   if s is None:
863       print('could not open socket')
864       sys.exit(1)
865   s.send(b'Hello, world')
866   data = s.recv(1024)
867   s.close()
868   print('Received', repr(data))
869
870
871The last example shows how to write a very simple network sniffer with raw
872sockets on Windows. The example requires administrator privileges to modify
873the interface::
874
875   import socket
876
877   # the public network interface
878   HOST = socket.gethostbyname(socket.gethostname())
879
880   # create a raw socket and bind it to the public interface
881   s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
882   s.bind((HOST, 0))
883
884   # Include IP headers
885   s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)
886
887   # receive all packages
888   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)
889
890   # receive a package
891   print(s.recvfrom(65565))
892
893   # disabled promiscuous mode
894   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)