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