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

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

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

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.

See also:

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

   Module ``ssl``
      A TLS/SSL wrapper for socket objects.


Socket families
===============

Depending on the system and the build options, various socket families
are supported by this module.

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 integer.

* For ``AF_INET6`` address family, a four-tuple ``(host, port,
  flowinfo, scopeid)`` is used, where *flowinfo* and *scopeid*
  represent the ``sin6_flowinfo`` and ``sin6_scope_id`` members 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.

* ``AF_NETLINK`` sockets are represented as pairs ``(pid, groups)``.

* Linux-only support for TIPC is 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.

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

* Certain other address families (``AF_BLUETOOTH``, ``AF_PACKET``)
  support specific representations.

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``.  This behavior is
not compatible with IPv6, 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.

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 ``socket.error`` or one
of its subclasses.

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


Module contents
===============

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

exception exception socket.error

   A subclass of ``IOError``, this exception is raised for socket-
   related errors.  It is recommended that you inspect its ``errno``
   attribute to discriminate between different kinds of errors.

   See also:

      The ``errno`` module contains symbolic names for the error codes
      defined by the underlying operating system.

exception exception socket.herror

   A subclass of ``socket.error``, this exception is raised for
   address-related errors, i.e. for functions that use *h_errno* in
   the POSIX C API, including ``gethostbyname_ex()`` and
   ``gethostbyaddr()``. The accompanying value is a pair ``(h_errno,
   string)`` representing an error returned by a library call.
   *h_errno* is a numeric value, while *string* represents the
   description of *h_errno*, as returned by the ``hstrerror()`` C
   function.

exception exception socket.gaierror

   A subclass of ``socket.error``, this exception is raised for
   address-related errors by ``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 numeric *error* value will match one of the
   ``EAI_*`` constants defined in this module.

exception exception socket.timeout

   A subclass of ``socket.error``, this exception is raised when a
   timeout occurs on a socket which has had timeouts enabled via a
   prior call to ``settimeout()`` (or implicitly through
   ``setdefaulttimeout()``).  The accompanying value is a string whose
   value is currently always "timed out".

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.  More
   constants may be available depending on the system.

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()``.  More constants may be available
   depending on the system. (Only ``SOCK_STREAM`` and ``SOCK_DGRAM``
   appear to be generally useful.)

socket.SOCK_CLOEXEC
socket.SOCK_NONBLOCK

   These two constants, if defined, can be combined with the socket
   types and allow you to set some flags atomically (thus avoiding
   possible race conditions and the need for separate calls).

   See also:

      Secure File Descriptor Handling for a more thorough explanation.

   Availability: Linux >= 2.6.27.

   New in version 3.2.

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.

TIPC_*

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

socket.has_ipv6

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

socket.create_connection(address[, timeout[, source_address]])

   Connect to a TCP service listening on the Internet *address* (a
   2-tuple ``(host, port)``), and return the socket object.  This is a
   higher-level function than ``socket.connect()``: if *host* is a
   non-numeric hostname, it will try to resolve it for both
   ``AF_INET`` and ``AF_INET6``, and then try to connect to all
   possible addresses in turn until a connection succeeds.  This makes
   it easy to write clients that are compatible to both IPv4 and IPv6.

   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.

   If supplied, *source_address* must be a 2-tuple ``(host, port)``
   for the socket to bind to as its source address before connecting.
   If host or port are '' or 0 respectively the OS default behavior
   will be used.

   Changed in version 3.2: *source_address* was added.

   Changed in version 3.2: support for the ``with`` statement was
   added.

socket.getaddrinfo(host, port, family=0, type=0, proto=0, flags=0)

   Translate the *host*/*port* argument into a sequence of 5-tuples
   that contain all the necessary arguments for creating a socket
   connected to that service. *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``.  By passing ``None`` as the value of *host* and *port*,
   you can pass ``NULL`` to the underlying C API.

   The *family*, *type* and *proto* arguments can be optionally
   specified in order to narrow the list of addresses returned.
   Passing zero as a value for each of these arguments selects the
   full range of results. The *flags* argument can be one or several
   of the ``AI_*`` constants, and will influence how results are
   computed and returned. For example, ``AI_NUMERICHOST`` will disable
   domain name resolution and will raise an error if *host* is a
   domain name.

   The function returns a list of 5-tuples with the following
   structure:

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

   In these tuples, *family*, *type*, *proto* are all integers and are
   meant to be passed to the ``socket()`` function.  *canonname* will
   be a string representing the canonical name of the *host* if
   ``AI_CANONNAME`` is part of the *flags* argument; else *canonname*
   will be empty.  *sockaddr* is a tuple describing a socket address,
   whose format depends on the returned *family* (a ``(address,
   port)`` 2-tuple for ``AF_INET``, a ``(address, port, flow info,
   scope id)`` 4-tuple for ``AF_INET6``), and is meant to be passed to
   the ``socket.connect()`` method.

   The following example fetches address information for a
   hypothetical TCP connection to ``www.python.org`` on port 80
   (results may differ on your system if IPv6 isn't enabled):

      >>> socket.getaddrinfo("www.python.org", 80, proto=socket.SOL_TCP)
      [(2, 1, 6, '', ('82.94.164.162', 80)),
       (10, 1, 6, '', ('2001:888:2000:d::a2', 80, 0, 0))]

   Changed in version 3.2: parameters can now be passed as single
   keyword arguments.

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.

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.

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.

   Changed in version 3.2: The returned socket objects now support the
   whole socket API, rather than a subset.

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.

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 bytes
   object 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.

   ``inet_aton()`` also accepts strings with less than three dots; see
   the Unix manual page *inet(3)* for details.

   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 ``inet_pton()`` should
   be used instead for IPv4/v6 dual stack support.

socket.inet_ntoa(packed_ip)

   Convert a 32-bit packed IPv4 address (a bytes object 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 byte sequence passed to this function is not exactly 4 bytes
   in length, ``socket.error`` will be raised. ``inet_ntoa()`` does
   not support IPv6, and ``inet_ntop()`` 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).

socket.inet_ntop(address_family, packed_ip)

   Convert a packed IP address (a bytes object 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).

socket.getdefaulttimeout()

   Return the default timeout in seconds (float) 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``.

socket.setdefaulttimeout(timeout)

   Set the default timeout in seconds (float) for new socket objects.
   When the socket module is first imported, the default is ``None``.
   See ``settimeout()`` for possible values and their respective
   meanings.

socket.SocketType

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


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.)

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.

   Note: ``close()`` releases the resource associated with a connection
     but does not necessarily close the connection immediately.  If
     you want to close the connection in a timely fashion, call
     ``shutdown()`` before ``close()``.

socket.connect(address)

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

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.

socket.detach()

   Put the socket object into closed state without actually closing
   the underlying file descriptor.  The file descriptor is returned,
   and can be reused for other purposes.

   New in version 3.2.

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 bytes object.  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 byte strings).

socket.gettimeout()

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

socket.ioctl(control, option)

   Platform :
      Windows

   The ``ioctl()`` method is a limited interface to the WSAIoctl
   system interface.  Please refer to the Win32 documentation for more
   information.

   On other platforms, the generic ``fcntl.fcntl()`` and
   ``fcntl.ioctl()`` functions may be used; they accept a socket
   object as their first argument.

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 0; the maximum value is system-dependent (usually 5), the
   minimum value is forced to 0.

socket.makefile(mode='r', buffering=None, *, encoding=None, errors=None, newline=None)

   Return a *file object* associated with the socket.  The exact
   returned type depends on the arguments given to ``makefile()``.
   These arguments are interpreted the same way as by the built-in
   ``open()`` function.

   Closing the file object won't close the socket unless there are no
   remaining references to the socket.  The socket must be in blocking
   mode; it can have a timeout, but the file object's internal buffer
   may end up in a inconsistent state if a timeout occurs.

   Note: On Windows, the file-like object created by ``makefile()`` cannot
     be used where a file object with a file descriptor is expected,
     such as the stream arguments of ``subprocess.Popen()``.

socket.recv(bufsize[, flags])

   Receive data from the socket.  The return value is a bytes object
   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 ``(bytes,
   address)`` where *bytes* is a bytes object 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 bytestring.  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.)

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 bytestring.  If *nbytes* is not
   specified (or 0), receive up to the size available in the given
   buffer.  Returns the number of bytes received.  See the Unix manual
   page *recv(2)* for the meaning of the optional argument *flags*; it
   defaults to zero.

socket.send(bytes[, 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. For further information on this
   topic, consult the *Socket Programming HOWTO*.

socket.sendall(bytes[, 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 *bytes* 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(bytes, address)
socket.sendto(bytes, 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
   false, the socket is set to non-blocking, else to blocking mode.

   This method is a shorthand for certain ``settimeout()`` calls:

   * ``sock.setblocking(True)`` is equivalent to
     ``sock.settimeout(None)``

   * ``sock.setblocking(False)`` is equivalent to
     ``sock.settimeout(0.0)``

socket.settimeout(value)

   Set a timeout on blocking socket operations.  The *value* argument
   can be a nonnegative floating point number expressing seconds, or
   ``None``. If a non-zero value is given, subsequent socket
   operations will raise a ``timeout`` exception if the timeout period
   *value* has elapsed before the operation has completed.  If zero is
   given, the socket is put in non-blocking mode. If ``None`` is
   given, the socket is put in blocking mode.

   For further information, please consult the *notes on socket
   timeouts*.

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 bytes object representing a buffer.  In the latter case it is up
   to the caller to ensure that the bytestring contains the proper
   bits (see the optional built-in module ``struct`` for a way to
   encode C structures as bytestrings).

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.
   Depending on the platform, shutting down one half of the connection
   can also close the opposite half (e.g. on Mac OS X,
   ``shutdown(SHUT_WR)`` does not allow further reads on the other end
   of the connection).

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.

socket.type

   The socket type.

socket.proto

   The socket protocol.


Notes on socket timeouts
========================

A socket object can be in one of three modes: blocking, non-blocking,
or timeout.  Sockets are by default always created in blocking mode,
but this can be changed by calling ``setdefaulttimeout()``.

* In *blocking mode*, operations block until complete or the system
  returns an error (such as connection timed out).

* In *non-blocking mode*, operations fail (with an error that is
  unfortunately system-dependent) if they cannot be completed
  immediately: functions from the ``select`` can be used to know when
  and whether a socket is available for reading or writing.

* In *timeout mode*, operations fail if they cannot be completed
  within the timeout specified for the socket (they raise a
  ``timeout`` exception) or if the system returns an error.

Note: At the operating system level, sockets in *timeout mode* are
  internally set in non-blocking mode.  Also, the blocking and timeout
  modes are shared between file descriptors and socket objects that
  refer to the same network endpoint. This implementation detail can
  have visible consequences if e.g. you decide to use the ``fileno()``
  of a socket.


Timeouts and the ``connect`` method
-----------------------------------

The ``connect()`` operation is also subject to the timeout setting,
and in general it is recommended to call ``settimeout()`` before
calling ``connect()`` or pass a timeout parameter to
``create_connection()``.  However, the system network stack may also
return a connection timeout error of its own regardless of any Python
socket timeout setting.


Timeouts and the ``accept`` method
----------------------------------

If ``getdefaulttimeout()`` is not ``None``, sockets returned by the
``accept()`` method inherit that timeout.  Otherwise, the behaviour
depends on settings of the listening socket:

* if the listening socket is in *blocking mode* or in *timeout mode*,
  the socket returned by ``accept()`` is in *blocking mode*;

* if the listening socket is in *non-blocking mode*, whether the
  socket returned by ``accept()`` is in blocking or non-blocking mode
  is operating system-dependent.  If you want to ensure cross-platform
  behaviour, it is recommended you manually override this setting.


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 ``sendall()``/``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 True:
       data = conn.recv(1024)
       if not data: break
       conn.sendall(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.sendall(b'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 as msg:
           s = None
           continue
       try:
           s.bind(sa)
           s.listen(1)
       except socket.error as 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 True:
       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 as msg:
           s = None
           continue
       try:
           s.connect(sa)
       except socket.error as msg:
           s.close()
           s = None
           continue
       break
   if s is None:
       print('could not open socket')
       sys.exit(1)
   s.sendall(b'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)

Running an example several times with too small delay between
executions, could lead to this error:

   socket.error: [Errno 98] Address already in use

This is because the previous execution has left the socket in a
``TIME_WAIT`` state, and can't be immediately reused.

There is a ``socket`` flag to set, in order to prevent this,
``socket.SO_REUSEADDR``:

   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
   s.bind((HOST, PORT))

the ``SO_REUSEADDR`` flag tells the kernel to reuse a local socket in
``TIME_WAIT`` state, without waiting for its natural timeout to
expire.

See also:

   For an introduction to socket programming (in C), see the following
   papers:

   * *An Introductory 4.3BSD Interprocess Communication Tutorial*, by
     Stuart Sechrest

   * *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.
