
``socket`` --- Low-level networking interface
*********************************************

This module provides access to the BSD *socket* interface. It is
available on all modern Unix systems, Windows, Mac OS X, BeOS, OS/2,
and probably additional platforms.

Note: Some behavior may be platform dependent, since calls are made to the
  operating system socket APIs.

For an introduction to socket programming (in C), see the following
papers: An Introductory 4.3BSD Interprocess Communication Tutorial, by
Stuart Sechrest and An Advanced 4.3BSD Interprocess Communication
Tutorial, by Samuel J.  Leffler et al, both in the UNIX Programmer's
Manual, Supplementary Documents 1 (sections PS1:7 and PS1:8).  The
platform-specific reference material for the various socket-related
system calls are also a valuable source of information on the details
of socket semantics.  For Unix, refer to the manual pages; for
Windows, see the WinSock (or Winsock 2) specification. For IPv6-ready
APIs, readers may want to refer to **RFC 3493** titled Basic Socket
Interface Extensions for IPv6.

The Python interface is a straightforward transliteration of the Unix
system call and library interface for sockets to Python's object-
oriented style: the ``socket()`` function returns a *socket object*
whose methods implement the various socket system calls.  Parameter
types are somewhat higher-level than in the C interface: as with
``read()`` and ``write()`` operations on Python files, buffer
allocation on receive operations is automatic, and buffer length is
implicit on send operations.

Socket addresses are represented as follows: A single string is used
for the ``AF_UNIX`` address family. A pair ``(host, port)`` is used
for the ``AF_INET`` address family, where *host* is a string
representing either a hostname in Internet domain notation like
``'daring.cwi.nl'`` or an IPv4 address like ``'100.50.200.5'``, and
*port* is an integral port number. For ``AF_INET6`` address family, a
four-tuple ``(host, port, flowinfo, scopeid)`` is used, where
*flowinfo* and *scopeid* represents ``sin6_flowinfo`` and
``sin6_scope_id`` member in ``struct sockaddr_in6`` in C. For
``socket`` module methods, *flowinfo* and *scopeid* can be omitted
just for backward compatibility. Note, however, omission of *scopeid*
can cause problems in manipulating scoped IPv6 addresses. Other
address families are currently not supported. The address format
required by a particular socket object is automatically selected based
on the address family specified when the socket object was created.

For IPv4 addresses, two special forms are accepted instead of a host
address: the empty string represents ``INADDR_ANY``, and the string
``'<broadcast>'`` represents ``INADDR_BROADCAST``. The behavior is not
available for IPv6 for backward compatibility, therefore, you may want
to avoid these if you intend to support IPv6 with your Python
programs.

If you use a hostname in the *host* portion of IPv4/v6 socket address,
the program may show a nondeterministic behavior, as Python uses the
first address returned from the DNS resolution.  The socket address
will be resolved differently into an actual IPv4/v6 address, depending
on the results from DNS resolution and/or the host configuration.  For
deterministic behavior use a numeric address in *host* portion.

New in version 2.5: AF_NETLINK sockets are represented as  pairs
``pid, groups``.

New in version 2.6: Linux-only support for TIPC is also available
using the ``AF_TIPC`` address family. TIPC is an open, non-IP based
networked protocol designed for use in clustered computer
environments.  Addresses are represented by a tuple, and the fields
depend on the address type. The general tuple form is ``(addr_type,
v1, v2, v3 [, scope])``, where:

   * *addr_type* is one of TIPC_ADDR_NAMESEQ, TIPC_ADDR_NAME, or
     TIPC_ADDR_ID.

   * *scope* is one of TIPC_ZONE_SCOPE, TIPC_CLUSTER_SCOPE, and
     TIPC_NODE_SCOPE.

   * If *addr_type* is TIPC_ADDR_NAME, then *v1* is the server type,
     *v2* is the port identifier, and *v3* should be 0.

     If *addr_type* is TIPC_ADDR_NAMESEQ, then *v1* is the server
     type, *v2* is the lower port number, and *v3* is the upper port
     number.

     If *addr_type* is TIPC_ADDR_ID, then *v1* is the node, *v2* is
     the reference, and *v3* should be set to 0.

All errors raise exceptions.  The normal exceptions for invalid
argument types and out-of-memory conditions can be raised; errors
related to socket or address semantics raise the error
``socket.error``.

Non-blocking mode is supported through ``setblocking()``.  A
generalization of this based on timeouts is supported through
``settimeout()``.

The module ``socket`` exports the following constants and functions:

exception socket.error

   This exception is raised for socket-related errors. The
   accompanying value is either a string telling what went wrong or a
   pair ``(errno, string)`` representing an error returned by a system
   call, similar to the value accompanying ``os.error``. See the
   module ``errno``, which contains names for the error codes defined
   by the underlying operating system.

   Changed in version 2.6: ``socket.error`` is now a child class of
   ``IOError``.

exception socket.herror

   This exception is raised for address-related errors, i.e. for
   functions that use *h_errno* in the C API, including
   ``gethostbyname_ex()`` and ``gethostbyaddr()``.

   The accompanying value is a pair ``(h_errno, string)`` representing
   an error returned by a library call. *string* represents the
   description of *h_errno*, as returned by the ``hstrerror`` C
   function.

exception socket.gaierror

   This exception is raised for address-related errors, for
   ``getaddrinfo()`` and ``getnameinfo()``. The accompanying value is
   a pair ``(error, string)`` representing an error returned by a
   library call. *string* represents the description of *error*, as
   returned by the ``gai_strerror`` C function. The *error* value will
   match one of the ``EAI_*`` constants defined in this module.

exception socket.timeout

   This exception is raised when a timeout occurs on a socket which
   has had timeouts enabled via a prior call to ``settimeout()``.  The
   accompanying value is a string whose value is currently always
   "timed out".

   New in version 2.3.

socket.AF_UNIX
socket.AF_INET
socket.AF_INET6

   These constants represent the address (and protocol) families, used
   for the first argument to ``socket()``.  If the ``AF_UNIX``
   constant is not defined then this protocol is unsupported.

socket.SOCK_STREAM
socket.SOCK_DGRAM
socket.SOCK_RAW
socket.SOCK_RDM
socket.SOCK_SEQPACKET

   These constants represent the socket types, used for the second
   argument to ``socket()``. (Only ``SOCK_STREAM`` and ``SOCK_DGRAM``
   appear to be generally useful.)

SO_*
socket.SOMAXCONN
MSG_*
SOL_*
IPPROTO_*
IPPORT_*
INADDR_*
IP_*
IPV6_*
EAI_*
AI_*
NI_*
TCP_*

   Many constants of these forms, documented in the Unix documentation
   on sockets and/or the IP protocol, are also defined in the socket
   module. They are generally used in arguments to the
   ``setsockopt()`` and ``getsockopt()`` methods of socket objects.
   In most cases, only those symbols that are defined in the Unix
   header files are defined; for a few symbols, default values are
   provided.

SIO_*
RCVALL_*

   Constants for Windows' WSAIoctl(). The constants are used as
   arguments to the ``ioctl()`` method of socket objects.

   New in version 2.6.

TIPC_*

   TIPC related constants, matching the ones exported by the C socket
   API. See the TIPC documentation for more information.

   New in version 2.6.

socket.has_ipv6

   This constant contains a boolean value which indicates if IPv6 is
   supported on this platform.

   New in version 2.3.

socket.create_connection(address[, timeout])

   Convenience function.  Connect to *address* (a 2-tuple ``(host,
   port)``), and return the socket object.  Passing the optional
   *timeout* parameter will set the timeout on the socket instance
   before attempting to connect.  If no *timeout* is supplied, the
   global default timeout setting returned by ``getdefaulttimeout()``
   is used.

   New in version 2.6.

socket.getaddrinfo(host, port[, family[, socktype[, proto[, flags]]]])

   Resolves the *host*/*port* argument, into a sequence of 5-tuples
   that contain all the necessary arguments for creating the
   corresponding socket. *host* is a domain name, a string
   representation of an IPv4/v6 address or ``None``. *port* is a
   string service name such as ``'http'``, a numeric port number or
   ``None``. The rest of the arguments are optional and must be
   numeric if specified. By passing ``None`` as the value of *host*
   and *port*, , you can pass ``NULL`` to the C API.

   The ``getaddrinfo()`` function returns a list of 5-tuples with the
   following structure:

   ``(family, socktype, proto, canonname, sockaddr)``

   *family*, *socktype*, *proto* are all integers and are meant to be
   passed to the ``socket()`` function. *canonname* is a string
   representing the canonical name of the *host*. It can be a numeric
   IPv4/v6 address when ``AI_CANONNAME`` is specified for a numeric
   *host*. *sockaddr* is a tuple describing a socket address, as
   described above. See the source for ``socket`` and other library
   modules for a typical usage of the function.

   New in version 2.2.

socket.getfqdn([name])

   Return a fully qualified domain name for *name*. If *name* is
   omitted or empty, it is interpreted as the local host.  To find the
   fully qualified name, the hostname returned by ``gethostbyaddr()``
   is checked, followed by aliases for the host, if available.  The
   first name which includes a period is selected.  In case no fully
   qualified domain name is available, the hostname as returned by
   ``gethostname()`` is returned.

   New in version 2.0.

socket.gethostbyname(hostname)

   Translate a host name to IPv4 address format.  The IPv4 address is
   returned as a string, such as  ``'100.50.200.5'``.  If the host
   name is an IPv4 address itself it is returned unchanged.  See
   ``gethostbyname_ex()`` for a more complete interface.
   ``gethostbyname()`` does not support IPv6 name resolution, and
   ``getaddrinfo()`` should be used instead for IPv4/v6 dual stack
   support.

socket.gethostbyname_ex(hostname)

   Translate a host name to IPv4 address format, extended interface.
   Return a triple ``(hostname, aliaslist, ipaddrlist)`` where
   *hostname* is the primary host name responding to the given
   *ip_address*, *aliaslist* is a (possibly empty) list of alternative
   host names for the same address, and *ipaddrlist* is a list of IPv4
   addresses for the same interface on the same host (often but not
   always a single address). ``gethostbyname_ex()`` does not support
   IPv6 name resolution, and ``getaddrinfo()`` should be used instead
   for IPv4/v6 dual stack support.

socket.gethostname()

   Return a string containing the hostname of the machine where  the
   Python interpreter is currently executing. If you want to know the
   current machine's IP address, you may want to use
   ``gethostbyname(gethostname())``. This operation assumes that there
   is a valid address-to-host mapping for the host, and the assumption
   does not always hold. Note: ``gethostname()`` doesn't always return
   the fully qualified domain name; use ``getfqdn()`` (see above).

socket.gethostbyaddr(ip_address)

   Return a triple ``(hostname, aliaslist, ipaddrlist)`` where
   *hostname* is the primary host name responding to the given
   *ip_address*, *aliaslist* is a (possibly empty) list of alternative
   host names for the same address, and *ipaddrlist* is a list of
   IPv4/v6 addresses for the same interface on the same host (most
   likely containing only a single address). To find the fully
   qualified domain name, use the function ``getfqdn()``.
   ``gethostbyaddr()`` supports both IPv4 and IPv6.

socket.getnameinfo(sockaddr, flags)

   Translate a socket address *sockaddr* into a 2-tuple ``(host,
   port)``. Depending on the settings of *flags*, the result can
   contain a fully-qualified domain name or numeric address
   representation in *host*.  Similarly, *port* can contain a string
   port name or a numeric port number.

   New in version 2.2.

socket.getprotobyname(protocolname)

   Translate an Internet protocol name (for example, ``'icmp'``) to a
   constant suitable for passing as the (optional) third argument to
   the ``socket()`` function.  This is usually only needed for sockets
   opened in "raw" mode (``SOCK_RAW``); for the normal socket modes,
   the correct protocol is chosen automatically if the protocol is
   omitted or zero.

socket.getservbyname(servicename[, protocolname])

   Translate an Internet service name and protocol name to a port
   number for that service.  The optional protocol name, if given,
   should be ``'tcp'`` or ``'udp'``, otherwise any protocol will
   match.

socket.getservbyport(port[, protocolname])

   Translate an Internet port number and protocol name to a service
   name for that service.  The optional protocol name, if given,
   should be ``'tcp'`` or ``'udp'``, otherwise any protocol will
   match.

socket.socket([family[, type[, proto]]])

   Create a new socket using the given address family, socket type and
   protocol number.  The address family should be ``AF_INET`` (the
   default), ``AF_INET6`` or ``AF_UNIX``.  The socket type should be
   ``SOCK_STREAM`` (the default), ``SOCK_DGRAM`` or perhaps one of the
   other ``SOCK_`` constants.  The protocol number is usually zero and
   may be omitted in that case.

socket.socketpair([family[, type[, proto]]])

   Build a pair of connected socket objects using the given address
   family, socket type, and protocol number.  Address family, socket
   type, and protocol number are as for the ``socket()`` function
   above. The default family is ``AF_UNIX`` if defined on the
   platform; otherwise, the default is ``AF_INET``. Availability:
   Unix.

   New in version 2.4.

socket.fromfd(fd, family, type[, proto])

   Duplicate the file descriptor *fd* (an integer as returned by a
   file object's ``fileno()`` method) and build a socket object from
   the result.  Address family, socket type and protocol number are as
   for the ``socket()`` function above. The file descriptor should
   refer to a socket, but this is not checked --- subsequent
   operations on the object may fail if the file descriptor is
   invalid. This function is rarely needed, but can be used to get or
   set socket options on a socket passed to a program as standard
   input or output (such as a server started by the Unix inet daemon).
   The socket is assumed to be in blocking mode. Availability: Unix.

socket.ntohl(x)

   Convert 32-bit positive integers from network to host byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 4-byte swap
   operation.

socket.ntohs(x)

   Convert 16-bit positive integers from network to host byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 2-byte swap
   operation.

socket.htonl(x)

   Convert 32-bit positive integers from host to network byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 4-byte swap
   operation.

socket.htons(x)

   Convert 16-bit positive integers from host to network byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 2-byte swap
   operation.

socket.inet_aton(ip_string)

   Convert an IPv4 address from dotted-quad string format (for
   example, '123.45.67.89') to 32-bit packed binary format, as a
   string four characters in length.  This is useful when conversing
   with a program that uses the standard C library and needs objects
   of type ``struct in_addr``, which is the C type for the 32-bit
   packed binary this function returns.

   If the IPv4 address string passed to this function is invalid,
   ``socket.error`` will be raised. Note that exactly what is valid
   depends on the underlying C implementation of ``inet_aton``.

   ``inet_aton()`` does not support IPv6, and ``getnameinfo()`` should
   be used instead for IPv4/v6 dual stack support.

socket.inet_ntoa(packed_ip)

   Convert a 32-bit packed IPv4 address (a string four characters in
   length) to its standard dotted-quad string representation (for
   example, '123.45.67.89').  This is useful when conversing with a
   program that uses the standard C library and needs objects of type
   ``struct in_addr``, which is the C type for the 32-bit packed
   binary data this function takes as an argument.

   If the string passed to this function is not exactly 4 bytes in
   length, ``socket.error`` will be raised. ``inet_ntoa()`` does not
   support IPv6, and ``getnameinfo()`` should be used instead for
   IPv4/v6 dual stack support.

socket.inet_pton(address_family, ip_string)

   Convert an IP address from its family-specific string format to a
   packed, binary format. ``inet_pton()`` is useful when a library or
   network protocol calls for an object of type ``struct in_addr``
   (similar to ``inet_aton()``) or ``struct in6_addr``.

   Supported values for *address_family* are currently ``AF_INET`` and
   ``AF_INET6``. If the IP address string *ip_string* is invalid,
   ``socket.error`` will be raised. Note that exactly what is valid
   depends on both the value of *address_family* and the underlying
   implementation of ``inet_pton``.

   Availability: Unix (maybe not all platforms).

   New in version 2.3.

socket.inet_ntop(address_family, packed_ip)

   Convert a packed IP address (a string of some number of characters)
   to its standard, family-specific string representation (for
   example, ``'7.10.0.5'`` or ``'5aef:2b::8'``) ``inet_ntop()`` is
   useful when a library or network protocol returns an object of type
   ``struct in_addr`` (similar to ``inet_ntoa()``) or ``struct
   in6_addr``.

   Supported values for *address_family* are currently ``AF_INET`` and
   ``AF_INET6``. If the string *packed_ip* is not the correct length
   for the specified address family, ``ValueError`` will be raised.  A
   ``socket.error`` is raised for errors from the call to
   ``inet_ntop()``.

   Availability: Unix (maybe not all platforms).

   New in version 2.3.

socket.getdefaulttimeout()

   Return the default timeout in floating seconds for new socket
   objects. A value of ``None`` indicates that new socket objects have
   no timeout. When the socket module is first imported, the default
   is ``None``.

   New in version 2.3.

socket.setdefaulttimeout(timeout)

   Set the default timeout in floating seconds for new socket objects.
   A value of ``None`` indicates that new socket objects have no
   timeout. When the socket module is first imported, the default is
   ``None``.

   New in version 2.3.

socket.SocketType

   This is a Python type object that represents the socket object
   type. It is the same as ``type(socket(...))``.

See also:

   Module ``SocketServer``
      Classes that simplify writing network servers.


Socket Objects
==============

Socket objects have the following methods.  Except for ``makefile()``
these correspond to Unix system calls applicable to sockets.

socket.accept()

   Accept a connection. The socket must be bound to an address and
   listening for connections. The return value is a pair ``(conn,
   address)`` where *conn* is a *new* socket object usable to send and
   receive data on the connection, and *address* is the address bound
   to the socket on the other end of the connection.

socket.bind(address)

   Bind the socket to *address*.  The socket must not already be
   bound. (The format of *address* depends on the address family ---
   see above.)

   Note: This method has historically accepted a pair of parameters for
     ``AF_INET`` addresses instead of only a tuple.  This was never
     intentional and is no longer available in Python 2.0 and later.

socket.close()

   Close the socket.  All future operations on the socket object will
   fail. The remote end will receive no more data (after queued data
   is flushed). Sockets are automatically closed when they are
   garbage-collected.

socket.connect(address)

   Connect to a remote socket at *address*. (The format of *address*
   depends on the address family --- see above.)

   Note: This method has historically accepted a pair of parameters for
     ``AF_INET`` addresses instead of only a tuple.  This was never
     intentional and is no longer available in Python 2.0 and later.

socket.connect_ex(address)

   Like ``connect(address)``, but return an error indicator instead of
   raising an exception for errors returned by the C-level ``connect``
   call (other problems, such as "host not found," can still raise
   exceptions).  The error indicator is ``0`` if the operation
   succeeded, otherwise the value of the ``errno`` variable.  This is
   useful to support, for example, asynchronous connects.

   Note: This method has historically accepted a pair of parameters for
     ``AF_INET`` addresses instead of only a tuple. This was never
     intentional and is no longer available in Python 2.0 and later.

socket.fileno()

   Return the socket's file descriptor (a small integer).  This is
   useful with ``select.select()``.

   Under Windows the small integer returned by this method cannot be
   used where a file descriptor can be used (such as ``os.fdopen()``).
   Unix does not have this limitation.

socket.getpeername()

   Return the remote address to which the socket is connected.  This
   is useful to find out the port number of a remote IPv4/v6 socket,
   for instance. (The format of the address returned depends on the
   address family --- see above.)  On some systems this function is
   not supported.

socket.getsockname()

   Return the socket's own address.  This is useful to find out the
   port number of an IPv4/v6 socket, for instance. (The format of the
   address returned depends on the address family --- see above.)

socket.getsockopt(level, optname[, buflen])

   Return the value of the given socket option (see the Unix man page
   *getsockopt(2)*).  The needed symbolic constants (``SO_*`` etc.)
   are defined in this module.  If *buflen* is absent, an integer
   option is assumed and its integer value is returned by the
   function.  If *buflen* is present, it specifies the maximum length
   of the buffer used to receive the option in, and this buffer is
   returned as a string.  It is up to the caller to decode the
   contents of the buffer (see the optional built-in module ``struct``
   for a way to decode C structures encoded as strings).

socket.ioctl(control, option)

   Platform:
      Windows

   The ``ioctl()`` method is a limited interface to the WSAIoctl
   system interface. Please refer to the MSDN documentation for more
   information.

   New in version 2.6.

socket.listen(backlog)

   Listen for connections made to the socket.  The *backlog* argument
   specifies the maximum number of queued connections and should be at
   least 1; the maximum value is system-dependent (usually 5).

socket.makefile([mode[, bufsize]])

   Return a *file object* associated with the socket.  (File objects
   are described in *File Objects*.) The file object references a
   ``dup``ped version of the socket file descriptor, so the file
   object and socket object may be closed or garbage-collected
   independently. The socket must be in blocking mode (it can not have
   a timeout). The optional *mode* and *bufsize* arguments are
   interpreted the same way as by the built-in ``file()`` function.

socket.recv(bufsize[, flags])

   Receive data from the socket.  The return value is a string
   representing the data received.  The maximum amount of data to be
   received at once is specified by *bufsize*.  See the Unix manual
   page *recv(2)* for the meaning of the optional argument *flags*; it
   defaults to zero.

   Note: For best match with hardware and network realities, the value of
     *bufsize* should be a relatively small power of 2, for example,
     4096.

socket.recvfrom(bufsize[, flags])

   Receive data from the socket.  The return value is a pair
   ``(string, address)`` where *string* is a string representing the
   data received and *address* is the address of the socket sending
   the data.  See the Unix manual page *recv(2)* for the meaning of
   the optional argument *flags*; it defaults to zero. (The format of
   *address* depends on the address family --- see above.)

socket.recvfrom_into(buffer[, nbytes[, flags]])

   Receive data from the socket, writing it into *buffer* instead of
   creating a new string.  The return value is a pair ``(nbytes,
   address)`` where *nbytes* is the number of bytes received and
   *address* is the address of the socket sending the data.  See the
   Unix manual page *recv(2)* for the meaning of the optional argument
   *flags*; it defaults to zero.  (The format of *address* depends on
   the address family --- see above.)

   New in version 2.5.

socket.recv_into(buffer[, nbytes[, flags]])

   Receive up to *nbytes* bytes from the socket, storing the data into
   a buffer rather than creating a new string.     If *nbytes* is not
   specified (or 0), receive up to the size available in the given
   buffer. See the Unix manual page *recv(2)* for the meaning of the
   optional argument *flags*; it defaults to zero.

   New in version 2.5.

socket.send(string[, flags])

   Send data to the socket.  The socket must be connected to a remote
   socket.  The optional *flags* argument has the same meaning as for
   ``recv()`` above. Returns the number of bytes sent. Applications
   are responsible for checking that all data has been sent; if only
   some of the data was transmitted, the application needs to attempt
   delivery of the remaining data.

socket.sendall(string[, flags])

   Send data to the socket.  The socket must be connected to a remote
   socket.  The optional *flags* argument has the same meaning as for
   ``recv()`` above. Unlike ``send()``, this method continues to send
   data from *string* until either all data has been sent or an error
   occurs.  ``None`` is returned on success.  On error, an exception
   is raised, and there is no way to determine how much data, if any,
   was successfully sent.

socket.sendto(string[, flags], address)

   Send data to the socket.  The socket should not be connected to a
   remote socket, since the destination socket is specified by
   *address*.  The optional *flags* argument has the same meaning as
   for ``recv()`` above.  Return the number of bytes sent. (The format
   of *address* depends on the address family --- see above.)

socket.setblocking(flag)

   Set blocking or non-blocking mode of the socket: if *flag* is 0,
   the socket is set to non-blocking, else to blocking mode.
   Initially all sockets are in blocking mode.  In non-blocking mode,
   if a ``recv()`` call doesn't find any data, or if a ``send()`` call
   can't immediately dispose of the data, a ``error`` exception is
   raised; in blocking mode, the calls block until they can proceed.
   ``s.setblocking(0)`` is equivalent to ``s.settimeout(0)``;
   ``s.setblocking(1)`` is equivalent to ``s.settimeout(None)``.

socket.settimeout(value)

   Set a timeout on blocking socket operations.  The *value* argument
   can be a nonnegative float expressing seconds, or ``None``. If a
   float is given, subsequent socket operations will raise an
   ``timeout`` exception if the timeout period *value* has elapsed
   before the operation has completed.  Setting a timeout of ``None``
   disables timeouts on socket operations. ``s.settimeout(0.0)`` is
   equivalent to ``s.setblocking(0)``; ``s.settimeout(None)`` is
   equivalent to ``s.setblocking(1)``.

   New in version 2.3.

socket.gettimeout()

   Return the timeout in floating seconds associated with socket
   operations, or ``None`` if no timeout is set.  This reflects the
   last call to ``setblocking()`` or ``settimeout()``.

   New in version 2.3.

Some notes on socket blocking and timeouts: A socket object can be in
one of three modes: blocking, non-blocking, or timeout.  Sockets are
always created in blocking mode.  In blocking mode, operations block
until complete.  In non-blocking mode, operations fail (with an error
that is unfortunately system-dependent) if they cannot be completed
immediately.  In timeout mode, operations fail if they cannot be
completed within the timeout specified for the socket.  The
``setblocking()`` method is simply a shorthand for certain
``settimeout()`` calls.

Timeout mode internally sets the socket in non-blocking mode.  The
blocking and timeout modes are shared between file descriptors and
socket objects that refer to the same network endpoint.  A consequence
of this is that file objects returned by the ``makefile()`` method
must only be used when the socket is in blocking mode; in timeout or
non-blocking mode file operations that cannot be completed immediately
will fail.

Note that the ``connect()`` operation is subject to the timeout
setting, and in general it is recommended to call ``settimeout()``
before calling ``connect()``.

socket.setsockopt(level, optname, value)

   Set the value of the given socket option (see the Unix manual page
   *setsockopt(2)*).  The needed symbolic constants are defined in the
   ``socket`` module (``SO_*`` etc.).  The value can be an integer or
   a string representing a buffer.  In the latter case it is up to the
   caller to ensure that the string contains the proper bits (see the
   optional built-in module ``struct`` for a way to encode C
   structures as strings).

socket.shutdown(how)

   Shut down one or both halves of the connection.  If *how* is
   ``SHUT_RD``, further receives are disallowed.  If *how* is
   ``SHUT_WR``, further sends are disallowed.  If *how* is
   ``SHUT_RDWR``, further sends and receives are disallowed.

Note that there are no methods ``read()`` or ``write()``; use
``recv()`` and ``send()`` without *flags* argument instead.

Socket objects also have these (read-only) attributes that correspond
to the values given to the ``socket`` constructor.

socket.family

   The socket family.

   New in version 2.5.

socket.type

   The socket type.

   New in version 2.5.

socket.proto

   The socket protocol.

   New in version 2.5.


Example
=======

Here are four minimal example programs using the TCP/IP protocol: a
server that echoes all data that it receives back (servicing only one
client), and a client using it.  Note that a server must perform the
sequence ``socket()``, ``bind()``, ``listen()``, ``accept()``
(possibly repeating the ``accept()`` to service more than one client),
while a client only needs the sequence ``socket()``, ``connect()``.
Also note that the server does not ``send()``/``recv()`` on the
socket it is listening on but on the new socket returned by
``accept()``.

The first two examples support IPv4 only.

   # Echo server program
   import socket

   HOST = ''                 # Symbolic name meaning all available interfaces
   PORT = 50007              # Arbitrary non-privileged port
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.bind((HOST, PORT))
   s.listen(1)
   conn, addr = s.accept()
   print 'Connected by', addr
   while 1:
       data = conn.recv(1024)
       if not data: break
       conn.send(data)
   conn.close()

   # Echo client program
   import socket

   HOST = 'daring.cwi.nl'    # The remote host
   PORT = 50007              # The same port as used by the server
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.connect((HOST, PORT))
   s.send('Hello, world')
   data = s.recv(1024)
   s.close()
   print 'Received', repr(data)

The next two examples are identical to the above two, but support both
IPv4 and IPv6. The server side will listen to the first address family
available (it should listen to both instead). On most of IPv6-ready
systems, IPv6 will take precedence and the server may not accept IPv4
traffic. The client side will try to connect to the all addresses
returned as a result of the name resolution, and sends traffic to the
first one connected successfully.

   # Echo server program
   import socket
   import sys

   HOST = None               # Symbolic name meaning all available interfaces
   PORT = 50007              # Arbitrary non-privileged port
   s = None
   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
       af, socktype, proto, canonname, sa = res
       try:
        s = socket.socket(af, socktype, proto)
       except socket.error, msg:
        s = None
        continue
       try:
        s.bind(sa)
        s.listen(1)
       except socket.error, msg:
        s.close()
        s = None
        continue
       break
   if s is None:
       print 'could not open socket'
       sys.exit(1)
   conn, addr = s.accept()
   print 'Connected by', addr
   while 1:
       data = conn.recv(1024)
       if not data: break
       conn.send(data)
   conn.close()

   # Echo client program
   import socket
   import sys

   HOST = 'daring.cwi.nl'    # The remote host
   PORT = 50007              # The same port as used by the server
   s = None
   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
       af, socktype, proto, canonname, sa = res
       try:
        s = socket.socket(af, socktype, proto)
       except socket.error, msg:
        s = None
        continue
       try:
        s.connect(sa)
       except socket.error, msg:
        s.close()
        s = None
        continue
       break
   if s is None:
       print 'could not open socket'
       sys.exit(1)
   s.send('Hello, world')
   data = s.recv(1024)
   s.close()
   print 'Received', repr(data)

The last example shows how to write a very simple network sniffer with
raw sockets on Windows. The example requires administrator privileges
to modify the interface:

   import socket

   # the public network interface
   HOST = socket.gethostbyname(socket.gethostname())

   # create a raw socket and bind it to the public interface
   s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
   s.bind((HOST, 0))

   # Include IP headers
   s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)

   # receive all packages
   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)

   # receive a package
   print s.recvfrom(65565)

   # disabled promiscuous mode
   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)
