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cpython/Doc/library/socket.rst
Gregory P. Smith b6471db8a7 Further documentation updates to be consistent with doc standards and comments 
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:mod:`socket` --- Low-level networking interface
================================================
.. module:: socket
:synopsis: 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.
.. index:: object: socket
The Python interface is a straightforward transliteration of the Unix system
call and library interface for sockets to Python's object-oriented style: the
:func:`socket` function returns a :dfn:`socket object` whose methods implement
the various socket system calls. Parameter types are somewhat higher-level than
in the C interface: as with :meth:`read` and :meth:`write` operations on Python
files, buffer allocation on receive operations is automatic, and buffer length
is implicit on send operations.
.. seealso::
Module :mod:`socketserver`
Classes that simplify writing network servers.
Module :mod:`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 :const:`AF_UNIX` address family.
- A pair ``(host, port)`` is used for the :const:`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 :const:`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 :const:`struct sockaddr_in6` in C. For
:mod:`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.
- :const:`AF_NETLINK` sockets are represented as pairs ``(pid, groups)``.
- Linux-only support for TIPC is available using the :const:`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 (:const:`AF_BLUETOOTH`, :const:`AF_PACKET`)
support specific representations.
.. XXX document them!
For IPv4 addresses, two special forms are accepted instead of a host address:
the empty string represents :const:`INADDR_ANY`, and the string
``'<broadcast>'`` represents :const:`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 :exc:`socket.error` or one of its subclasses.
Non-blocking mode is supported through :meth:`~socket.setblocking`. A
generalization of this based on timeouts is supported through
:meth:`~socket.settimeout`.
Module contents
---------------
The module :mod:`socket` exports the following constants and functions:
.. exception:: error
.. index:: module: errno
A subclass of :exc:`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.
.. seealso::
The :mod:`errno` module contains symbolic names for the error codes
defined by the underlying operating system.
.. exception:: herror
A subclass of :exc:`socket.error`, this exception is raised for
address-related errors, i.e. for functions that use *h_errno* in the POSIX
C API, including :func:`gethostbyname_ex` and :func:`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
:c:func:`hstrerror` C function.
.. exception:: gaierror
A subclass of :exc:`socket.error`, this exception is raised for
address-related errors by :func:`getaddrinfo` and :func:`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 :c:func:`gai_strerror` C function. The
numeric *error* value will match one of the :const:`EAI_\*` constants
defined in this module.
.. exception:: timeout
A subclass of :exc:`socket.error`, this exception is raised when a timeout
occurs on a socket which has had timeouts enabled via a prior call to
:meth:`~socket.settimeout` (or implicitly through
:func:`~socket.setdefaulttimeout`). The accompanying value is a string
whose value is currently always "timed out".
.. data:: AF_UNIX
AF_INET
AF_INET6
These constants represent the address (and protocol) families, used for the
first argument to :func:`socket`. If the :const:`AF_UNIX` constant is not
defined then this protocol is unsupported. More constants may be available
depending on the system.
.. data:: SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_RDM
SOCK_SEQPACKET
These constants represent the socket types, used for the second argument to
:func:`socket`. More constants may be available depending on the system.
(Only :const:`SOCK_STREAM` and :const:`SOCK_DGRAM` appear to be generally
useful.)
.. data:: SOCK_CLOEXEC
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).
.. seealso::
`Secure File Descriptor Handling <http://udrepper.livejournal.com/20407.html>`_
for a more thorough explanation.
Availability: Linux >= 2.6.27.
.. versionadded:: 3.2
.. data:: SO_*
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 :meth:`setsockopt` and :meth:`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.
.. data:: SIO_*
RCVALL_*
Constants for Windows' WSAIoctl(). The constants are used as arguments to the
:meth:`ioctl` method of socket objects.
.. data:: TIPC_*
TIPC related constants, matching the ones exported by the C socket API. See
the TIPC documentation for more information.
.. data:: has_ipv6
This constant contains a boolean value which indicates if IPv6 is supported on
this platform.
.. function:: create_connection(address[, timeout[, source_address]])
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
:func:`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.
.. versionchanged:: 3.2
*source_address* was added.
.. versionchanged:: 3.2
support for the :keyword:`with` statement was added.
.. function:: 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, :const:`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 :func:`socket` function. *canonname* will be
a string representing the canonical name of the *host* if
:const:`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
:const:`AF_INET`, a ``(address, port, flow info, scope id)`` 4-tuple for
:const:`AF_INET6`), and is meant to be passed to the :meth:`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))]
.. versionchanged:: 3.2
parameters can now be passed as single keyword arguments.
.. function:: 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 :func:`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
:func:`gethostname` is returned.
.. function:: 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 :func:`gethostbyname_ex` for a more complete
interface. :func:`gethostbyname` does not support IPv6 name resolution, and
:func:`getaddrinfo` should be used instead for IPv4/v6 dual stack support.
.. function:: 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). :func:`gethostbyname_ex` does not support IPv6 name
resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual
stack support.
.. function:: 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: :func:`gethostname` doesn't always return the fully qualified domain
name; use ``getfqdn()`` (see above).
.. function:: 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 :func:`getfqdn`. :func:`gethostbyaddr` supports
both IPv4 and IPv6.
.. function:: 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.
.. function:: getprotobyname(protocolname)
Translate an Internet protocol name (for example, ``'icmp'``) to a constant
suitable for passing as the (optional) third argument to the :func:`socket`
function. This is usually only needed for sockets opened in "raw" mode
(:const:`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen
automatically if the protocol is omitted or zero.
.. function:: 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.
.. function:: 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.
.. function:: socket([family[, type[, proto]]])
Create a new socket using the given address family, socket type and protocol
number. The address family should be :const:`AF_INET` (the default),
:const:`AF_INET6` or :const:`AF_UNIX`. The socket type should be
:const:`SOCK_STREAM` (the default), :const:`SOCK_DGRAM` or perhaps one of the
other ``SOCK_`` constants. The protocol number is usually zero and may be
omitted in that case.
.. function:: 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 :func:`socket` function above. The default family is :const:`AF_UNIX`
if defined on the platform; otherwise, the default is :const:`AF_INET`.
Availability: Unix.
.. versionchanged:: 3.2
The returned socket objects now support the whole socket API, rather
than a subset.
.. function:: fromfd(fd, family, type[, proto])
Duplicate the file descriptor *fd* (an integer as returned by a file object's
:meth:`fileno` method) and build a socket object from the result. Address
family, socket type and protocol number are as for the :func:`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.
.. function:: 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.
.. function:: 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.
.. function:: 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.
.. function:: 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.
.. function:: 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 :c:type:`struct in_addr`, which is the C type
for the 32-bit packed binary this function returns.
:func:`inet_aton` also accepts strings with less than three dots; see the
Unix manual page :manpage:`inet(3)` for details.
If the IPv4 address string passed to this function is invalid,
:exc:`socket.error` will be raised. Note that exactly what is valid depends on
the underlying C implementation of :c:func:`inet_aton`.
:func:`inet_aton` does not support IPv6, and :func:`inet_pton` should be used
instead for IPv4/v6 dual stack support.
.. function:: 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 :c: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, :exc:`socket.error` will be raised. :func:`inet_ntoa` does not
support IPv6, and :func:`inet_ntop` should be used instead for IPv4/v6 dual
stack support.
.. function:: inet_pton(address_family, ip_string)
Convert an IP address from its family-specific string format to a packed,
binary format. :func:`inet_pton` is useful when a library or network protocol
calls for an object of type :c:type:`struct in_addr` (similar to
:func:`inet_aton`) or :c:type:`struct in6_addr`.
Supported values for *address_family* are currently :const:`AF_INET` and
:const:`AF_INET6`. If the IP address string *ip_string* is invalid,
:exc:`socket.error` will be raised. Note that exactly what is valid depends on
both the value of *address_family* and the underlying implementation of
:c:func:`inet_pton`.
Availability: Unix (maybe not all platforms).
.. function:: 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'``). :func:`inet_ntop` is useful when a library or network protocol
returns an object of type :c:type:`struct in_addr` (similar to :func:`inet_ntoa`)
or :c:type:`struct in6_addr`.
Supported values for *address_family* are currently :const:`AF_INET` and
:const:`AF_INET6`. If the string *packed_ip* is not the correct length for the
specified address family, :exc:`ValueError` will be raised. A
:exc:`socket.error` is raised for errors from the call to :func:`inet_ntop`.
Availability: Unix (maybe not all platforms).
.. function:: 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``.
.. function:: setdefaulttimeout(timeout)
Set the default timeout in floating seconds for new socket objects. When
the socket module is first imported, the default is ``None``. See
:meth:`~socket.settimeout` for possible values and their respective
meanings.
.. function:: sethostname(name)
Set the machine's hostname to *name*. This will raise a
:exc:`socket.error` if you don't have enough rights.
Availability: Unix.
.. versionadded:: 3.3
.. function:: if_nameindex()
Return a list of network interface information
(index int, name string) tuples.
:exc:`socket.error` if the system call fails.
Availability: Unix.
.. versionadded:: 3.3
.. function:: if_nametoindex(if_name)
Return a network interface index number corresponding to an
interface name.
:exc:`socket.error` if no interface with the given name exists.
Availability: Unix.
.. versionadded:: 3.3
.. function:: if_indextoname(if_index)
Return a network interface name corresponding to a
interface index number.
:exc:`socket.error` if no interface with the given index exists.
Availability: Unix.
.. versionadded:: 3.3
.. data:: SocketType
This is a Python type object that represents the socket object type. It is the
same as ``type(socket(...))``.
.. _socket-objects:
Socket Objects
--------------
Socket objects have the following methods. Except for :meth:`makefile` these
correspond to Unix system calls applicable to sockets.
.. method:: 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.
.. method:: 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.)
.. method:: 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::
:meth:`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 :meth:`shutdown()`
before :meth:`close()`.
.. method:: socket.connect(address)
Connect to a remote socket at *address*. (The format of *address* depends on the
address family --- see above.)
.. method:: socket.connect_ex(address)
Like ``connect(address)``, but return an error indicator instead of raising an
exception for errors returned by the C-level :c:func:`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
:c:data:`errno` variable. This is useful to support, for example, asynchronous
connects.
.. method:: 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.
.. versionadded:: 3.2
.. method:: socket.fileno()
Return the socket's file descriptor (a small integer). This is useful with
:func:`select.select`.
Under Windows the small integer returned by this method cannot be used where a
file descriptor can be used (such as :func:`os.fdopen`). Unix does not have
this limitation.
.. method:: 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.
.. method:: 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.)
.. method:: socket.getsockopt(level, optname[, buflen])
Return the value of the given socket option (see the Unix man page
:manpage:`getsockopt(2)`). The needed symbolic constants (:const:`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 :mod:`struct` for a way
to decode C structures encoded as byte strings).
.. method:: 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
:meth:`setblocking` or :meth:`settimeout`.
.. method:: socket.ioctl(control, option)
:platform: Windows
The :meth:`ioctl` method is a limited interface to the WSAIoctl system
interface. Please refer to the `Win32 documentation
<http://msdn.microsoft.com/en-us/library/ms741621%28VS.85%29.aspx>`_ for more
information.
On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
functions may be used; they accept a socket object as their first argument.
.. method:: 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.
.. method:: socket.makefile(mode='r', buffering=None, *, encoding=None, \
errors=None, newline=None)
.. index:: single: I/O control; buffering
Return a :term:`file object` associated with the socket. The exact returned
type depends on the arguments given to :meth:`makefile`. These arguments are
interpreted the same way as by the built-in :func:`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 :meth:`makefile` cannot be
used where a file object with a file descriptor is expected, such as the
stream arguments of :meth:`subprocess.Popen`.
.. method:: 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 :manpage:`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.
.. method:: 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
:manpage:`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.)
.. method:: 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 :manpage:`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.)
.. method:: 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 :manpage:`recv(2)` for the meaning
of the optional argument *flags*; it defaults to zero.
.. method:: 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 :meth:`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.
.. method:: 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 :meth:`recv` above.
Unlike :meth:`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.
.. method:: 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 :meth:`recv` above. Return the number of
bytes sent. (The format of *address* depends on the address family --- see
above.)
.. method:: 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 :meth:`~socket.settimeout` calls:
* ``sock.setblocking(True)`` is equivalent to ``sock.settimeout(None)``
* ``sock.setblocking(False)`` is equivalent to ``sock.settimeout(0.0)``
.. method:: 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
:exc:`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 :ref:`notes on socket timeouts <socket-timeouts>`.
.. method:: socket.setsockopt(level, optname, value)
.. index:: module: struct
Set the value of the given socket option (see the Unix manual page
:manpage:`setsockopt(2)`). The needed symbolic constants are defined in the
:mod:`socket` module (:const:`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 :mod:`struct` for a way to encode C structures as bytestrings).
.. method:: socket.shutdown(how)
Shut down one or both halves of the connection. If *how* is :const:`SHUT_RD`,
further receives are disallowed. If *how* is :const:`SHUT_WR`, further sends
are disallowed. If *how* is :const:`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 :meth:`read` or :meth:`write`; use
:meth:`~socket.recv` and :meth:`~socket.send` without *flags* argument instead.
Socket objects also have these (read-only) attributes that correspond to the
values given to the :class:`socket` constructor.
.. attribute:: socket.family
The socket family.
.. attribute:: socket.type
The socket type.
.. attribute:: socket.proto
The socket protocol.
.. _socket-timeouts:
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 :func:`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
:mod:`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 :exc:`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 :meth:`~socket.fileno()` of a socket.
Timeouts and the ``connect`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The :meth:`~socket.connect` operation is also subject to the timeout
setting, and in general it is recommended to call :meth:`~socket.settimeout`
before calling :meth:`~socket.connect` or pass a timeout parameter to
:meth:`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 :func:`getdefaulttimeout` is not :const:`None`, sockets returned by
the :meth:`~socket.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 :meth:`~socket.accept` is in *blocking mode*;
* if the listening socket is in *non-blocking mode*, whether the socket
returned by :meth:`~socket.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.
.. _socket-example:
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 :func:`socket`,
:meth:`~socket.bind`, :meth:`~socket.listen`, :meth:`~socket.accept` (possibly
repeating the :meth:`~socket.accept` to service more than one client), while a
client only needs the sequence :func:`socket`, :meth:`~socket.connect`. Also
note that the server does not :meth:`~socket.send`/:meth:`~socket.recv` on the
socket it is listening on but on the new socket returned by
:meth:`~socket.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.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(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.send(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)
.. seealso::
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.
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