
"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.

The address format required by a particular socket object is
automatically selected based on the address family specified when the
socket object was created.  Socket addresses are represented as
follows:

* The address of an "AF_UNIX" socket bound to a file system node is
  represented as a string, using the file system encoding and the
  "'surrogateescape'" error handler (see **PEP 383**).  An address in
  Linux's abstract namespace is returned as a "bytes" object with an
  initial null byte; note that sockets in this namespace can
  communicate with normal file system sockets, so programs intended to
  run on Linux may need to deal with both types of address.  A string
  or "bytes" object can be used for either type of address when
  passing it as an argument.

     Changed in version 3.3: Previously, "AF_UNIX" socket paths were
     assumed to use UTF-8 encoding.

* 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.

* A tuple "(interface, )" is used for the "AF_CAN" address family,
  where *interface* is a string representing a network interface name
  like "'can0'". The network interface name "''" can be used to
  receive packets from all network interfaces of this family.

* A string or a tuple "(id, unit)" is used for the
  "SYSPROTO_CONTROL" protocol of the "PF_SYSTEM" family. The string is
  the name of a kernel control using a dynamically-assigned ID. The
  tuple can be used if ID and unit number of the kernel control are
  known or if a registered ID is used.

  New in version 3.3.

* 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; starting
from Python 3.3, errors related to socket or address semantics raise
"OSError" or one of its subclasses (they used to raise
"socket.error").

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 elements.


Exceptions
----------

exception exception socket.error

   A deprecated alias of "OSError".

   Changed in version 3.3: Following **PEP 3151**, this class was made
   an alias of "OSError".

exception exception socket.herror

   A subclass of "OSError", 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.

   Changed in version 3.3: This class was made a subclass of
   "OSError".

exception exception socket.gaierror

   A subclass of "OSError", 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.

   Changed in version 3.3: This class was made a subclass of
   "OSError".

exception exception socket.timeout

   A subclass of "OSError", 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".

   Changed in version 3.3: This class was made a subclass of
   "OSError".


Constants
---------

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_*
SCM_*
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.

socket.AF_CAN
socket.PF_CAN
SOL_CAN_*
CAN_*

   Many constants of these forms, documented in the Linux
   documentation, are also defined in the socket module.

   Availability: Linux >= 2.6.25.

   New in version 3.3.

socket.AF_RDS
socket.PF_RDS
socket.SOL_RDS
RDS_*

   Many constants of these forms, documented in the Linux
   documentation, are also defined in the socket module.

   Availability: Linux >= 2.6.30.

   New in version 3.3.

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.


Functions
---------


Creating sockets
~~~~~~~~~~~~~~~~

The following functions all create *socket objects*.

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", "AF_UNIX", "AF_CAN" or "AF_RDS". The socket
   type should be "SOCK_STREAM" (the default), "SOCK_DGRAM",
   "SOCK_RAW" or perhaps one of the other "SOCK_" constants. The
   protocol number is usually zero and may be omitted in that case or
   "CAN_RAW" in case the address family is "AF_CAN".

   Changed in version 3.3: The AF_CAN family was added. The AF_RDS
   family was added.

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.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.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.fromshare(data)

   Instantiate a socket from data obtained from the "socket.share()"
   method.  The socket is assumed to be in blocking mode.

   Availability: Windows.

   New in version 3.3.

socket.SocketType

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


Other functions
~~~~~~~~~~~~~~~

The "socket" module also offers various network-related services:

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 using 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.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,
   "OSError" 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, "OSError" 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,
   "OSError" 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
   "OSError" is raised for errors from the call to "inet_ntop()".

   Availability: Unix (maybe not all platforms).

socket.CMSG_LEN(length)

   Return the total length, without trailing padding, of an ancillary
   data item with associated data of the given *length*.  This value
   can often be used as the buffer size for "recvmsg()" to receive a
   single item of ancillary data, but **RFC 3542** requires portable
   applications to use "CMSG_SPACE()" and thus include space for
   padding, even when the item will be the last in the buffer.  Raises
   "OverflowError" if *length* is outside the permissible range of
   values.

   Availability: most Unix platforms, possibly others.

   New in version 3.3.

socket.CMSG_SPACE(length)

   Return the buffer size needed for "recvmsg()" to receive an
   ancillary data item with associated data of the given *length*,
   along with any trailing padding.  The buffer space needed to
   receive multiple items is the sum of the "CMSG_SPACE()" values for
   their associated data lengths.  Raises "OverflowError" if *length*
   is outside the permissible range of values.

   Note that some systems might support ancillary data without
   providing this function.  Also note that setting the buffer size
   using the results of this function may not precisely limit the
   amount of ancillary data that can be received, since additional
   data may be able to fit into the padding area.

   Availability: most Unix platforms, possibly others.

   New in version 3.3.

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.sethostname(name)

   Set the machine's hostname to *name*.  This will raise a "OSError"
   if you don't have enough rights.

   Availability: Unix.

   New in version 3.3.

socket.if_nameindex()

   Return a list of network interface information (index int, name
   string) tuples. "OSError" if the system call fails.

   Availability: Unix.

   New in version 3.3.

socket.if_nametoindex(if_name)

   Return a network interface index number corresponding to an
   interface name. "OSError" if no interface with the given name
   exists.

   Availability: Unix.

   New in version 3.3.

socket.if_indextoname(if_index)

   Return a network interface name corresponding to a interface index
   number. "OSError" if no interface with the given index exists.

   Availability: Unix.

   New in version 3.3.


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()

   Mark the socket closed.  The underlying system resource (e.g. a
   file descriptor) is also closed when all file objects from
   "makefile()" are closed.  Once that happens, 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,
   but it is recommended to "close()" them explicitly, or to use a
   "with" statement around them.

   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.

   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.

   Closing the file object returned by "makefile()" won't close the
   original socket unless all other file objects have been closed and
   "socket.close()" has been called on the socket object.

   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.recvmsg(bufsize[, ancbufsize[, flags]])

   Receive normal data (up to *bufsize* bytes) and ancillary data from
   the socket.  The *ancbufsize* argument sets the size in bytes of
   the internal buffer used to receive the ancillary data; it defaults
   to 0, meaning that no ancillary data will be received.  Appropriate
   buffer sizes for ancillary data can be calculated using
   "CMSG_SPACE()" or "CMSG_LEN()", and items which do not fit into the
   buffer might be truncated or discarded.  The *flags* argument
   defaults to 0 and has the same meaning as for "recv()".

   The return value is a 4-tuple: "(data, ancdata, msg_flags,
   address)".  The *data* item is a "bytes" object holding the non-
   ancillary data received.  The *ancdata* item is a list of zero or
   more tuples "(cmsg_level, cmsg_type, cmsg_data)" representing the
   ancillary data (control messages) received: *cmsg_level* and
   *cmsg_type* are integers specifying the protocol level and
   protocol-specific type respectively, and *cmsg_data* is a "bytes"
   object holding the associated data.  The *msg_flags* item is the
   bitwise OR of various flags indicating conditions on the received
   message; see your system documentation for details. If the
   receiving socket is unconnected, *address* is the address of the
   sending socket, if available; otherwise, its value is unspecified.

   On some systems, "sendmsg()" and "recvmsg()" can be used to pass
   file descriptors between processes over an "AF_UNIX" socket.  When
   this facility is used (it is often restricted to "SOCK_STREAM"
   sockets), "recvmsg()" will return, in its ancillary data, items of
   the form "(socket.SOL_SOCKET, socket.SCM_RIGHTS, fds)", where *fds*
   is a "bytes" object representing the new file descriptors as a
   binary array of the native C "int" type.  If "recvmsg()" raises an
   exception after the system call returns, it will first attempt to
   close any file descriptors received via this mechanism.

   Some systems do not indicate the truncated length of ancillary data
   items which have been only partially received.  If an item appears
   to extend beyond the end of the buffer, "recvmsg()" will issue a
   "RuntimeWarning", and will return the part of it which is inside
   the buffer provided it has not been truncated before the start of
   its associated data.

   On systems which support the "SCM_RIGHTS" mechanism, the following
   function will receive up to *maxfds* file descriptors, returning
   the message data and a list containing the descriptors (while
   ignoring unexpected conditions such as unrelated control messages
   being received).  See also "sendmsg()".

      import socket, array

      def recv_fds(sock, msglen, maxfds):
          fds = array.array("i")   # Array of ints
          msg, ancdata, flags, addr = sock.recvmsg(msglen, socket.CMSG_LEN(maxfds * fds.itemsize))
          for cmsg_level, cmsg_type, cmsg_data in ancdata:
              if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
                  # Append data, ignoring any truncated integers at the end.
                  fds.fromstring(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
          return msg, list(fds)

   Availability: most Unix platforms, possibly others.

   New in version 3.3.

socket.recvmsg_into(buffers[, ancbufsize[, flags]])

   Receive normal data and ancillary data from the socket, behaving as
   "recvmsg()" would, but scatter the non-ancillary data into a series
   of buffers instead of returning a new bytes object.  The *buffers*
   argument must be an iterable of objects that export writable
   buffers (e.g. "bytearray" objects); these will be filled with
   successive chunks of the non-ancillary data until it has all been
   written or there are no more buffers.  The operating system may set
   a limit ("sysconf()" value "SC_IOV_MAX") on the number of buffers
   that can be used.  The *ancbufsize* and *flags* arguments have the
   same meaning as for "recvmsg()".

   The return value is a 4-tuple: "(nbytes, ancdata, msg_flags,
   address)", where *nbytes* is the total number of bytes of non-
   ancillary data written into the buffers, and *ancdata*, *msg_flags*
   and *address* are the same as for "recvmsg()".

   Example:

      >>> import socket
      >>> s1, s2 = socket.socketpair()
      >>> b1 = bytearray(b'----')
      >>> b2 = bytearray(b'0123456789')
      >>> b3 = bytearray(b'--------------')
      >>> s1.send(b'Mary had a little lamb')
      22
      >>> s2.recvmsg_into([b1, memoryview(b2)[2:9], b3])
      (22, [], 0, None)
      >>> [b1, b2, b3]
      [bytearray(b'Mary'), bytearray(b'01 had a 9'), bytearray(b'little lamb---')]

   Availability: most Unix platforms, possibly others.

   New in version 3.3.

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.sendmsg(buffers[, ancdata[, flags[, address]]])

   Send normal and ancillary data to the socket, gathering the non-
   ancillary data from a series of buffers and concatenating it into a
   single message.  The *buffers* argument specifies the non-ancillary
   data as an iterable of buffer-compatible objects (e.g. "bytes"
   objects); the operating system may set a limit ("sysconf()" value
   "SC_IOV_MAX") on the number of buffers that can be used.  The
   *ancdata* argument specifies the ancillary data (control messages)
   as an iterable of zero or more tuples "(cmsg_level, cmsg_type,
   cmsg_data)", where *cmsg_level* and *cmsg_type* are integers
   specifying the protocol level and protocol-specific type
   respectively, and *cmsg_data* is a buffer-compatible object holding
   the associated data.  Note that some systems (in particular,
   systems without "CMSG_SPACE()") might support sending only one
   control message per call.  The *flags* argument defaults to 0 and
   has the same meaning as for "send()".  If *address* is supplied and
   not "None", it sets a destination address for the message.  The
   return value is the number of bytes of non-ancillary data sent.

   The following function sends the list of file descriptors *fds*
   over an "AF_UNIX" socket, on systems which support the "SCM_RIGHTS"
   mechanism.  See also "recvmsg()".

      import socket, array

      def send_fds(sock, msg, fds):
          return sock.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds))])

   Availability: most Unix platforms, possibly others.

   New in version 3.3.

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.

socket.share(process_id)

   Duplicate a socket and prepare it for sharing with a target
   process.  The target process must be provided with *process_id*.
   The resulting bytes object can then be passed to the target process
   using some form of interprocess communication and the socket can be
   recreated there using "fromshare()". Once this method has been
   called, it is safe to close the socket since the operating system
   has already duplicated it for the target process.

   Availability: Windows.

   New in version 3.3.

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 OSError as msg:
           s = None
           continue
       try:
           s.bind(sa)
           s.listen(1)
       except OSError 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 OSError as msg:
           s = None
           continue
       try:
           s.connect(sa)
       except OSError 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 next 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)

The last example shows how to use the socket interface to communicate
to a CAN network. This example might require special priviledge:

   import socket
   import struct


   # CAN frame packing/unpacking (see 'struct can_frame' in <linux/can.h>)

   can_frame_fmt = "=IB3x8s"
   can_frame_size = struct.calcsize(can_frame_fmt)

   def build_can_frame(can_id, data):
       can_dlc = len(data)
       data = data.ljust(8, b'\x00')
       return struct.pack(can_frame_fmt, can_id, can_dlc, data)

   def dissect_can_frame(frame):
       can_id, can_dlc, data = struct.unpack(can_frame_fmt, frame)
       return (can_id, can_dlc, data[:can_dlc])


   # create a raw socket and bind it to the 'vcan0' interface
   s = socket.socket(socket.AF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
   s.bind(('vcan0',))

   while True:
       cf, addr = s.recvfrom(can_frame_size)

       print('Received: can_id=%x, can_dlc=%x, data=%s' % dissect_can_frame(cf))

       try:
           s.send(cf)
       except OSError:
           print('Error sending CAN frame')

       try:
           s.send(build_can_frame(0x01, b'\x01\x02\x03'))
       except OSError:
           print('Error sending CAN frame')

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

   OSError: [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.
