:mod:`!ctypes` --- A foreign function library for Python
========================================================

.. module:: ctypes
   :synopsis: A foreign function library for Python.

.. moduleauthor:: Thomas Heller <theller@python.net>

**Source code:** :source:`Lib/ctypes`

--------------

:mod:`!ctypes` is a foreign function library for Python.  It provides C compatible
data types, and allows calling functions in DLLs or shared libraries.  It can be
used to wrap these libraries in pure Python.

.. include:: ../includes/optional-module.rst

.. warning::

   :mod:`!ctypes` provides low-level access to native libraries and the
   process's memory, bypassing Python's safety mechanisms and allowing
   execution of arbitrary native code.
   Incorrect use can corrupt data and objects, reveal sensitive information,
   cause crashes, or otherwise compromise the running process.


.. _ctypes-ctypes-tutorial:

ctypes tutorial
---------------

Note: Some code samples reference the ctypes :class:`c_int` type.  On platforms
where ``sizeof(long) == sizeof(int)`` it is an alias to :class:`c_long`.
So, you should not be confused if :class:`c_long` is printed if you would expect
:class:`c_int` --- they are actually the same type.

.. _ctypes-loading-dynamic-link-libraries:

Loading dynamic link libraries
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:mod:`!ctypes` exports the :py:data:`~ctypes.cdll`, and on Windows
:py:data:`~ctypes.windll` and :py:data:`~ctypes.oledll`
objects, for loading dynamic link libraries.

You load libraries by accessing them as attributes of these objects.
:py:data:`!cdll` loads libraries which export functions using the
standard ``cdecl`` calling convention, while :py:data:`!windll`
libraries call functions using the ``stdcall``
calling convention.
:py:data:`~oledll` also uses the ``stdcall`` calling convention, and
assumes the functions return a Windows :c:type:`!HRESULT` error code. The error
code is used to automatically raise an :class:`OSError` exception when the
function call fails.

.. versionchanged:: 3.3
   Windows errors used to raise :exc:`WindowsError`, which is now an alias
   of :exc:`OSError`.


Here are some examples for Windows. Note that ``msvcrt`` is the MS standard C
library containing most standard C functions, and uses the ``cdecl`` calling
convention::

   >>> from ctypes import *
   >>> print(windll.kernel32)  # doctest: +WINDOWS
   <WinDLL 'kernel32', handle ... at ...>
   >>> print(cdll.msvcrt)      # doctest: +WINDOWS
   <CDLL 'msvcrt', handle ... at ...>
   >>> libc = cdll.msvcrt      # doctest: +WINDOWS
   >>>

Windows appends the usual ``.dll`` file suffix automatically.

.. note::
    Accessing the standard C library through ``cdll.msvcrt`` will use an
    outdated version of the library that may be incompatible with the one
    being used by Python. Where possible, use native Python functionality,
    or else import and use the ``msvcrt`` module.

Other systems require the filename *including* the extension to
load a library, so attribute access can not be used to load libraries. Either the
:meth:`~LibraryLoader.LoadLibrary` method of the dll loaders should be used,
or you should load the library by creating an instance of :py:class:`CDLL`
by calling the constructor.

For example, on Linux::

   >>> cdll.LoadLibrary("libc.so.6")  # doctest: +LINUX
   <CDLL 'libc.so.6', handle ... at ...>
   >>> libc = CDLL("libc.so.6")       # doctest: +LINUX
   >>> libc                           # doctest: +LINUX
   <CDLL 'libc.so.6', handle ... at ...>
   >>>

On macOS::

   >>> cdll.LoadLibrary("libc.dylib")  # doctest: +MACOS
   <CDLL 'libc.dylib', handle ... at ...>
   >>> libc = CDLL("libc.dylib")       # doctest: +MACOS
   >>> libc                            # doctest: +MACOS
   <CDLL 'libc.dylib', handle ... at ...>



.. _ctypes-accessing-functions-from-loaded-dlls:

Accessing functions from loaded dlls
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Functions are accessed as attributes of dll objects::

   >>> libc.printf
   <_FuncPtr object at 0x...>
   >>> print(windll.kernel32.GetModuleHandleA)  # doctest: +WINDOWS
   <_FuncPtr object at 0x...>
   >>> print(windll.kernel32.MyOwnFunction)     # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "ctypes.py", line 239, in __getattr__
       func = _StdcallFuncPtr(name, self)
   AttributeError: function 'MyOwnFunction' not found
   >>>

Note that win32 system dlls like ``kernel32`` and ``user32`` often export ANSI
as well as UNICODE versions of a function. The UNICODE version is exported with
a ``W`` appended to the name, while the ANSI version is exported with an ``A``
appended to the name. The win32 ``GetModuleHandle`` function, which returns a
*module handle* for a given module name, has the following C prototype, and a
macro is used to expose one of them as ``GetModuleHandle`` depending on whether
UNICODE is defined or not::

   /* ANSI version */
   HMODULE GetModuleHandleA(LPCSTR lpModuleName);
   /* UNICODE version */
   HMODULE GetModuleHandleW(LPCWSTR lpModuleName);

*windll* does not try to select one of them by magic, you must access the
version you need by specifying ``GetModuleHandleA`` or ``GetModuleHandleW``
explicitly, and then call it with bytes or string objects respectively.

Sometimes, dlls export functions with names which aren't valid Python
identifiers, like ``"??2@YAPAXI@Z"``. In this case you have to use
:func:`getattr` to retrieve the function::

   >>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")  # doctest: +WINDOWS
   <_FuncPtr object at 0x...>
   >>>

On Windows, some dlls export functions not by name but by ordinal. These
functions can be accessed by indexing the dll object with the ordinal number::

   >>> cdll.kernel32[1]  # doctest: +WINDOWS
   <_FuncPtr object at 0x...>
   >>> cdll.kernel32[0]  # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "ctypes.py", line 310, in __getitem__
       func = _StdcallFuncPtr(name, self)
   AttributeError: function ordinal 0 not found
   >>>


.. _ctypes-calling-functions:

Calling functions
^^^^^^^^^^^^^^^^^

You can call these functions like any other Python callable. This example uses
the ``rand()`` function, which takes no arguments and returns a pseudo-random integer::

   >>> print(libc.rand())  # doctest: +SKIP
   1804289383

On Windows, you can call the ``GetModuleHandleA()`` function, which returns a win32 module
handle (passing ``None`` as single argument to call it with a ``NULL`` pointer)::

   >>> print(hex(windll.kernel32.GetModuleHandleA(None)))  # doctest: +WINDOWS
   0x1d000000
   >>>

:exc:`ValueError` is raised when you call an ``stdcall`` function with the
``cdecl`` calling convention, or vice versa::

   >>> cdll.kernel32.GetModuleHandleA(None)  # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ValueError: Procedure probably called with not enough arguments (4 bytes missing)
   >>>

   >>> windll.msvcrt.printf(b"spam")  # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ValueError: Procedure probably called with too many arguments (4 bytes in excess)
   >>>

To find out the correct calling convention you have to look into the C header
file or the documentation for the function you want to call.

On Windows, :mod:`!ctypes` uses win32 structured exception handling to prevent
crashes from general protection faults when functions are called with invalid
argument values::

   >>> windll.kernel32.GetModuleHandleA(32)  # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   OSError: exception: access violation reading 0x00000020
   >>>

The :mod:`faulthandler` module can help debug crashes,
such as segmentation faults produced by erroneous C library calls.

``None``, integers, bytes objects and (unicode) strings are the only native
Python objects that can directly be used as parameters in these function calls.
``None`` is passed as a C ``NULL`` pointer, bytes objects and strings are passed
as pointer to the memory block that contains their data (:c:expr:`char *` or
:c:expr:`wchar_t *`).  Python integers are passed as the platform's default C
:c:expr:`int` type, their value is masked to fit into the C type.

Before we move on calling functions with other parameter types, we have to learn
more about :mod:`!ctypes` data types.


.. _ctypes-fundamental-data-types:

Fundamental data types
^^^^^^^^^^^^^^^^^^^^^^

:mod:`!ctypes` defines a number of primitive C compatible data types:

.. list-table::
   :header-rows: 1

   * - ctypes type
     - C type
     - Python type
     - :py:attr:`~_SimpleCData._type_`
   * - :class:`c_bool`
     - :c:expr:`_Bool`
     - :py:class:`bool`
     - ``'?'``
   * - :class:`c_char`
     - :c:expr:`char`
     - 1-character :py:class:`bytes`
     - ``'c'``
   * - :class:`c_wchar`
     - :c:type:`wchar_t`
     - 1-character :py:class:`str`
     - ``'u'``
   * - :class:`c_byte`
     - :c:expr:`char`
     - :py:class:`int`
     - ``'b'``
   * - :class:`c_ubyte`
     - :c:expr:`unsigned char`
     - :py:class:`int`
     - ``'B'``
   * - :class:`c_short`
     - :c:expr:`short`
     - :py:class:`int`
     - ``'h'``
   * - :class:`c_ushort`
     - :c:expr:`unsigned short`
     - :py:class:`int`
     - ``'H'``
   * - :class:`c_int`
     - :c:expr:`int`
     - :py:class:`int`
     - ``'i'`` \*
   * - :class:`c_int8`
     - :c:type:`int8_t`
     - :py:class:`int`
     - \*
   * - :class:`c_int16`
     - :c:type:`int16_t`
     - :py:class:`int`
     - \*
   * - :class:`c_int32`
     - :c:type:`int32_t`
     - :py:class:`int`
     - \*
   * - :class:`c_int64`
     - :c:type:`int64_t`
     - :py:class:`int`
     - \*
   * - :class:`c_uint`
     - :c:expr:`unsigned int`
     - :py:class:`int`
     - ``'I'`` \*
   * - :class:`c_uint8`
     - :c:type:`uint8_t`
     - :py:class:`int`
     - \*
   * - :class:`c_uint16`
     - :c:type:`uint16_t`
     - :py:class:`int`
     - \*
   * - :class:`c_uint32`
     - :c:type:`uint32_t`
     - :py:class:`int`
     - \*
   * - :class:`c_uint64`
     - :c:type:`uint64_t`
     - :py:class:`int`
     - \*
   * - :class:`c_long`
     - :c:expr:`long`
     - :py:class:`int`
     - ``'l'``
   * - :class:`c_ulong`
     - :c:expr:`unsigned long`
     - :py:class:`int`
     - ``'L'``
   * - :class:`c_longlong`
     - :c:expr:`long long`
     - :py:class:`int`
     - ``'q'`` \*
   * - :class:`c_ulonglong`
     - :c:expr:`unsigned long long`
     - :py:class:`int`
     - ``'Q'`` \*
   * - :class:`c_size_t`
     - :c:type:`size_t`
     - :py:class:`int`
     - \*
   * - :class:`c_ssize_t`
     - :c:type:`Py_ssize_t`
     - :py:class:`int`
     - \*
   * - :class:`c_time_t`
     - :c:type:`time_t`
     - :py:class:`int`
     - \*
   * - :class:`c_float`
     - :c:expr:`float`
     - :py:class:`float`
     - ``'f'``
   * - :class:`c_double`
     - :c:expr:`double`
     - :py:class:`float`
     - ``'d'``
   * - :class:`c_longdouble`
     - :c:expr:`long double`
     - :py:class:`float`
     - ``'g'`` \*
   * - :class:`c_char_p`
     - :c:expr:`char *` (NUL terminated)
     - :py:class:`bytes` or ``None``
     - ``'z'``
   * - :class:`c_wchar_p`
     - :c:expr:`wchar_t *` (NUL terminated)
     - :py:class:`str` or ``None``
     - ``'Z'``
   * - :class:`c_void_p`
     - :c:expr:`void *`
     - :py:class:`int` or ``None``
     - ``'P'``
   * - :class:`py_object`
     - :c:expr:`PyObject *`
     - :py:class:`object`
     - ``'O'``
   * - :ref:`VARIANT_BOOL <ctypes-wintypes>`
     - :c:expr:`short int`
     - :py:class:`bool`
     - ``'v'``

Additionally, if IEC 60559 compatible complex arithmetic (Annex G) is supported
in both C and ``libffi``, the following complex types are available:

.. list-table::
   :header-rows: 1

   * - ctypes type
     - C type
     - Python type
     - :py:attr:`~_SimpleCData._type_`
   * - :class:`c_float_complex`
     - :c:expr:`float complex`
     - :py:class:`complex`
     - ``'F'``
   * - :class:`c_double_complex`
     - :c:expr:`double complex`
     - :py:class:`complex`
     - ``'D'``
   * - :class:`c_longdouble_complex`
     - :c:expr:`long double complex`
     - :py:class:`complex`
     - ``'G'``


All these types can be created by calling them with an optional initializer of
the correct type and value::

   >>> c_int()
   c_long(0)
   >>> c_wchar_p("Hello, World")
   c_wchar_p(140018365411392)
   >>> c_ushort(-3)
   c_ushort(65533)
   >>>

The constructors for numeric types will convert input using
:py:meth:`~object.__bool__`,
:py:meth:`~object.__index__` (for ``int``),
:py:meth:`~object.__float__` or :py:meth:`~object.__complex__`.
This means :py:class:`~ctypes.c_bool` accepts any object with a truth value::

   >>> empty_list = []
   >>> c_bool(empty_list)
   c_bool(False)

Since these types are mutable, their value can also be changed afterwards::

   >>> i = c_int(42)
   >>> print(i)
   c_long(42)
   >>> print(i.value)
   42
   >>> i.value = -99
   >>> print(i.value)
   -99
   >>>

Assigning a new value to instances of the pointer types :class:`c_char_p`,
:class:`c_wchar_p`, and :class:`c_void_p` changes the *memory location* they
point to, *not the contents* of the memory block (of course not, because Python
string objects are immutable)::

   >>> s = "Hello, World"
   >>> c_s = c_wchar_p(s)
   >>> print(c_s)
   c_wchar_p(139966785747344)
   >>> print(c_s.value)
   Hello World
   >>> c_s.value = "Hi, there"
   >>> print(c_s)              # the memory location has changed
   c_wchar_p(139966783348904)
   >>> print(c_s.value)
   Hi, there
   >>> print(s)                # first object is unchanged
   Hello, World
   >>>

You should be careful, however, not to pass them to functions expecting pointers
to mutable memory. If you need mutable memory blocks, ctypes has a
:func:`create_string_buffer` function which creates these in various ways.  The
current memory block contents can be accessed (or changed) with the ``raw``
property; if you want to access it as NUL terminated string, use the ``value``
property::

   >>> from ctypes import *
   >>> p = create_string_buffer(3)            # create a 3 byte buffer, initialized to NUL bytes
   >>> print(sizeof(p), repr(p.raw))
   3 b'\x00\x00\x00'
   >>> p = create_string_buffer(b"Hello")     # create a buffer containing a NUL terminated string
   >>> print(sizeof(p), repr(p.raw))
   6 b'Hello\x00'
   >>> print(repr(p.value))
   b'Hello'
   >>> p = create_string_buffer(b"Hello", 10) # create a 10 byte buffer
   >>> print(sizeof(p), repr(p.raw))
   10 b'Hello\x00\x00\x00\x00\x00'
   >>> p.value = b"Hi"
   >>> print(sizeof(p), repr(p.raw))
   10 b'Hi\x00lo\x00\x00\x00\x00\x00'
   >>>

The :func:`create_string_buffer` function replaces the old :func:`!c_buffer`
function (which is still available as an alias).  To create a mutable memory
block containing unicode characters of the C type :c:type:`wchar_t`, use the
:func:`create_unicode_buffer` function.


.. _ctypes-calling-functions-continued:

Calling functions, continued
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Note that printf prints to the real standard output channel, *not* to
:data:`sys.stdout`, so these examples will only work at the console prompt, not
from within *IDLE* or *PythonWin*::

   >>> printf = libc.printf
   >>> printf(b"Hello, %s\n", b"World!")
   Hello, World!
   14
   >>> printf(b"Hello, %S\n", "World!")
   Hello, World!
   14
   >>> printf(b"%d bottles of beer\n", 42)
   42 bottles of beer
   19
   >>> printf(b"%f bottles of beer\n", 42.5)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ctypes.ArgumentError: argument 2: TypeError: Don't know how to convert parameter 2
   >>>

As has been mentioned before, all Python types except integers, strings, and
bytes objects have to be wrapped in their corresponding :mod:`!ctypes` type, so
that they can be converted to the required C data type::

   >>> printf(b"An int %d, a double %f\n", 1234, c_double(3.14))
   An int 1234, a double 3.140000
   31
   >>>

.. _ctypes-calling-variadic-functions:

Calling variadic functions
^^^^^^^^^^^^^^^^^^^^^^^^^^

On a lot of platforms calling variadic functions through ctypes is exactly the same
as calling functions with a fixed number of parameters. On some platforms, and in
particular ARM64 for Apple Platforms, the calling convention for variadic functions
is different than that for regular functions.

On those platforms it is required to specify the :attr:`~_CFuncPtr.argtypes`
attribute for the regular, non-variadic, function arguments:

.. code-block:: python3

   libc.printf.argtypes = [ctypes.c_char_p]

Because specifying the attribute does not inhibit portability it is advised to always
specify :attr:`~_CFuncPtr.argtypes` for all variadic functions.


.. _ctypes-calling-functions-with-own-custom-data-types:

Calling functions with your own custom data types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can also customize :mod:`!ctypes` argument conversion to allow instances of
your own classes be used as function arguments. :mod:`!ctypes` looks for an
:attr:`!_as_parameter_` attribute and uses this as the function argument. The
attribute must be an integer, string, bytes, a :mod:`!ctypes` instance, or an
object with an :attr:`!_as_parameter_` attribute::

   >>> class Bottles:
   ...     def __init__(self, number):
   ...         self._as_parameter_ = number
   ...
   >>> bottles = Bottles(42)
   >>> printf(b"%d bottles of beer\n", bottles)
   42 bottles of beer
   19
   >>>

If you don't want to store the instance's data in the :attr:`!_as_parameter_`
instance variable, you could define a :class:`property` which makes the
attribute available on request.


.. _ctypes-specifying-required-argument-types:

Specifying the required argument types (function prototypes)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is possible to specify the required argument types of functions exported from
DLLs by setting the :attr:`~_CFuncPtr.argtypes` attribute.

:attr:`~_CFuncPtr.argtypes` must be a sequence of C data types (the :func:`!printf` function is
probably not a good example here, because it takes a variable number and
different types of parameters depending on the format string, on the other hand
this is quite handy to experiment with this feature)::

   >>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
   >>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
   String 'Hi', Int 10, Double 2.200000
   37
   >>>

Specifying a format protects against incompatible argument types (just as a
prototype for a C function), and tries to convert the arguments to valid types::

   >>> printf(b"%d %d %d", 1, 2, 3)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ctypes.ArgumentError: argument 2: TypeError: 'int' object cannot be interpreted as ctypes.c_char_p
   >>> printf(b"%s %d %f\n", b"X", 2, 3)
   X 2 3.000000
   13
   >>>

If you have defined your own classes which you pass to function calls, you have
to implement a :meth:`~_CData.from_param` class method for them to be able to use them
in the :attr:`~_CFuncPtr.argtypes` sequence. The :meth:`~_CData.from_param` class method receives
the Python object passed to the function call, it should do a typecheck or
whatever is needed to make sure this object is acceptable, and then return the
object itself, its :attr:`!_as_parameter_` attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, bytes, a :mod:`!ctypes` instance, or an object with an
:attr:`!_as_parameter_` attribute.


.. _ctypes-return-types:

Return types
^^^^^^^^^^^^

.. testsetup::

   from ctypes import CDLL, c_char, c_char_p
   from ctypes.util import find_library
   libc = CDLL(find_library('c'))
   strchr = libc.strchr


By default functions are assumed to return the C :c:expr:`int` type.  Other
return types can be specified by setting the :attr:`~_CFuncPtr.restype` attribute of the
function object.

The C prototype of :c:func:`time` is ``time_t time(time_t *)``. Because :c:type:`time_t`
might be of a different type than the default return type :c:expr:`int`, you should
specify the :attr:`!restype` attribute::

   >>> libc.time.restype = c_time_t

The argument types can be specified using :attr:`~_CFuncPtr.argtypes`::

   >>> libc.time.argtypes = (POINTER(c_time_t),)

To call the function with a ``NULL`` pointer as first argument, use ``None``::

   >>> print(libc.time(None))  # doctest: +SKIP
   1150640792

Here is a more advanced example, it uses the :func:`!strchr` function, which expects
a string pointer and a char, and returns a pointer to a string::

   >>> strchr = libc.strchr
   >>> strchr(b"abcdef", ord("d"))  # doctest: +SKIP
   8059983
   >>> strchr.restype = c_char_p    # c_char_p is a pointer to a string
   >>> strchr(b"abcdef", ord("d"))
   b'def'
   >>> print(strchr(b"abcdef", ord("x")))
   None
   >>>

If you want to avoid the :func:`ord("x") <ord>` calls above, you can set the
:attr:`~_CFuncPtr.argtypes` attribute, and the second argument will be converted from a
single character Python bytes object into a C char:

.. doctest::

   >>> strchr.restype = c_char_p
   >>> strchr.argtypes = [c_char_p, c_char]
   >>> strchr(b"abcdef", b"d")
   b'def'
   >>> strchr(b"abcdef", b"def")
   Traceback (most recent call last):
   ctypes.ArgumentError: argument 2: TypeError: one character bytes, bytearray or integer expected
   >>> print(strchr(b"abcdef", b"x"))
   None
   >>> strchr(b"abcdef", b"d")
   b'def'
   >>>

You can also use a callable Python object (a function or a class for example) as
the :attr:`~_CFuncPtr.restype` attribute, if the foreign function returns an integer.  The
callable will be called with the *integer* the C function returns, and the
result of this call will be used as the result of your function call. This is
useful to check for error return values and automatically raise an exception::

   >>> GetModuleHandle = windll.kernel32.GetModuleHandleA  # doctest: +WINDOWS
   >>> def ValidHandle(value):
   ...     if value == 0:
   ...         raise WinError()
   ...     return value
   ...
   >>>
   >>> GetModuleHandle.restype = ValidHandle  # doctest: +WINDOWS
   >>> GetModuleHandle(None)  # doctest: +WINDOWS
   486539264
   >>> GetModuleHandle("something silly")  # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 3, in ValidHandle
   OSError: [Errno 126] The specified module could not be found.
   >>>

``WinError`` is a function which will call Windows ``FormatMessage()`` api to
get the string representation of an error code, and *returns* an exception.
``WinError`` takes an optional error code parameter, if no one is used, it calls
:func:`GetLastError` to retrieve it.

Please note that a much more powerful error checking mechanism is available
through the :attr:`~_CFuncPtr.errcheck` attribute;
see the reference manual for details.


.. _ctypes-passing-pointers:

Passing pointers (or: passing parameters by reference)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes a C api function expects a *pointer* to a data type as parameter,
probably to write into the corresponding location, or if the data is too large
to be passed by value. This is also known as *passing parameters by reference*.

:mod:`!ctypes` exports the :func:`byref` function which is used to pass parameters
by reference.  The same effect can be achieved with the :func:`pointer` function,
although :func:`pointer` does a lot more work since it constructs a real pointer
object, so it is faster to use :func:`byref` if you don't need the pointer
object in Python itself::

   >>> i = c_int()
   >>> f = c_float()
   >>> s = create_string_buffer(b'\000' * 32)
   >>> print(i.value, f.value, repr(s.value))
   0 0.0 b''
   >>> libc.sscanf(b"1 3.14 Hello", b"%d %f %s",
   ...             byref(i), byref(f), s)
   3
   >>> print(i.value, f.value, repr(s.value))
   1 3.1400001049 b'Hello'
   >>>


.. _ctypes-structures-unions:

Structures and unions
^^^^^^^^^^^^^^^^^^^^^

Structures and unions must derive from the :class:`Structure` and :class:`Union`
base classes which are defined in the :mod:`!ctypes` module. Each subclass must
define a :attr:`~Structure._fields_` attribute.  :attr:`!_fields_` must be a list of
*2-tuples*, containing a *field name* and a *field type*.

The field type must be a :mod:`!ctypes` type like :class:`c_int`, or any other
derived :mod:`!ctypes` type: structure, union, array, pointer.

Here is a simple example of a POINT structure, which contains two integers named
*x* and *y*, and also shows how to initialize a structure in the constructor::

   >>> from ctypes import *
   >>> class POINT(Structure):
   ...     _fields_ = [("x", c_int),
   ...                 ("y", c_int)]
   ...
   >>> point = POINT(10, 20)
   >>> print(point.x, point.y)
   10 20
   >>> point = POINT(y=5)
   >>> print(point.x, point.y)
   0 5
   >>> POINT(1, 2, 3)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: too many initializers
   >>>

You can, however, build much more complicated structures.  A structure can
itself contain other structures by using a structure as a field type.

Here is a RECT structure which contains two POINTs named *upperleft* and
*lowerright*::

   >>> class RECT(Structure):
   ...     _fields_ = [("upperleft", POINT),
   ...                 ("lowerright", POINT)]
   ...
   >>> rc = RECT(point)
   >>> print(rc.upperleft.x, rc.upperleft.y)
   0 5
   >>> print(rc.lowerright.x, rc.lowerright.y)
   0 0
   >>>

Nested structures can also be initialized in the constructor in several ways::

   >>> r = RECT(POINT(1, 2), POINT(3, 4))
   >>> r = RECT((1, 2), (3, 4))

Field :term:`descriptor`\s can be retrieved from the *class*, they are useful
for debugging because they can provide useful information.
See :class:`CField`::

   >>> POINT.x
   <ctypes.CField 'x' type=c_int, ofs=0, size=4>
   >>> POINT.y
   <ctypes.CField 'y' type=c_int, ofs=4, size=4>
   >>>


.. _ctypes-structureunion-alignment-byte-order:

.. warning::

   :mod:`!ctypes` does not support passing unions or structures with bit-fields
   to functions by value.  While this may work on 32-bit x86, it's not
   guaranteed by the library to work in the general case.  Unions and
   structures with bit-fields should always be passed to functions by pointer.

Structure/union layout, alignment and byte order
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By default, Structure and Union fields are laid out in the same way the C
compiler does it.  It is possible to override this behavior entirely by specifying a
:attr:`~Structure._layout_` class attribute in the subclass definition; see
the attribute documentation for details.

It is possible to specify the maximum alignment for the fields and/or for the
structure itself by setting the class attributes :attr:`~Structure._pack_`
and/or :attr:`~Structure._align_`, respectively.
See the attribute documentation for details.

:mod:`!ctypes` uses the native byte order for Structures and Unions.  To build
structures with non-native byte order, you can use one of the
:class:`BigEndianStructure`, :class:`LittleEndianStructure`,
:class:`BigEndianUnion`, and :class:`LittleEndianUnion` base classes.  These
classes cannot contain pointer fields.


.. _ctypes-bit-fields-in-structures-unions:

Bit fields in structures and unions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is possible to create structures and unions containing bit fields. Bit fields
are only possible for integer fields, the bit width is specified as the third
item in the :attr:`~Structure._fields_` tuples::

   >>> class Int(Structure):
   ...     _fields_ = [("first_16", c_int, 16),
   ...                 ("second_16", c_int, 16)]
   ...
   >>> print(Int.first_16)
   <ctypes.CField 'first_16' type=c_int, ofs=0, bit_size=16, bit_offset=0>
   >>> print(Int.second_16)
   <ctypes.CField 'second_16' type=c_int, ofs=0, bit_size=16, bit_offset=16>

It is important to note that bit field allocation and layout in memory are not
defined as a C standard; their implementation is compiler-specific.
By default, Python will attempt to match the behavior of a "native" compiler
for the current platform.
See the :attr:`~Structure._layout_` attribute for details on the default
behavior and how to change it.


.. _ctypes-arrays:

Arrays
^^^^^^

Arrays are sequences, containing a fixed number of instances of the same type.

The recommended way to create array types is by multiplying a data type with a
positive integer::

   TenPointsArrayType = POINT * 10

Here is an example of a somewhat artificial data type, a structure containing 4
POINTs among other stuff::

   >>> from ctypes import *
   >>> class POINT(Structure):
   ...     _fields_ = ("x", c_int), ("y", c_int)
   ...
   >>> class MyStruct(Structure):
   ...     _fields_ = [("a", c_int),
   ...                 ("b", c_float),
   ...                 ("point_array", POINT * 4)]
   >>>
   >>> print(len(MyStruct().point_array))
   4
   >>>

Instances are created in the usual way, by calling the class::

   arr = TenPointsArrayType()
   for pt in arr:
       print(pt.x, pt.y)

The above code print a series of ``0 0`` lines, because the array contents is
initialized to zeros.

Initializers of the correct type can also be specified::

   >>> from ctypes import *
   >>> TenIntegers = c_int * 10
   >>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
   >>> print(ii)
   <c_long_Array_10 object at 0x...>
   >>> for i in ii: print(i, end=" ")
   ...
   1 2 3 4 5 6 7 8 9 10
   >>>


.. _ctypes-pointers:

Pointers
^^^^^^^^

Pointer instances are created by calling the :func:`pointer` function on a
:mod:`!ctypes` type::

   >>> from ctypes import *
   >>> i = c_int(42)
   >>> pi = pointer(i)
   >>>

Pointer instances have a :attr:`~_Pointer.contents` attribute which
returns the object to which the pointer points, the ``i`` object above::

   >>> pi.contents
   c_long(42)
   >>>

Note that :mod:`!ctypes` does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute::

   >>> pi.contents is i
   False
   >>> pi.contents is pi.contents
   False
   >>>

Assigning another :class:`c_int` instance to the pointer's contents attribute
would cause the pointer to point to the memory location where this is stored::

   >>> i = c_int(99)
   >>> pi.contents = i
   >>> pi.contents
   c_long(99)
   >>>

.. XXX Document dereferencing pointers, and that it is preferred over the
   .contents attribute.

Pointer instances can also be indexed with integers::

   >>> pi[0]
   99
   >>>

Assigning to an integer index changes the pointed to value::

   >>> print(i)
   c_long(99)
   >>> pi[0] = 22
   >>> print(i)
   c_long(22)
   >>>

It is also possible to use indexes different from 0, but you must know what
you're doing, just as in C: You can access or change arbitrary memory locations.
Generally you only use this feature if you receive a pointer from a C function,
and you *know* that the pointer actually points to an array instead of a single
item.

Behind the scenes, the :func:`pointer` function does more than simply create
pointer instances, it has to create pointer *types* first. This is done with the
:func:`POINTER` function, which accepts any :mod:`!ctypes` type, and returns a
new type::

   >>> PI = POINTER(c_int)
   >>> PI
   <class 'ctypes.LP_c_long'>
   >>> PI(42)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: expected c_long instead of int
   >>> PI(c_int(42))
   <ctypes.LP_c_long object at 0x...>
   >>>

Calling the pointer type without an argument creates a ``NULL`` pointer.
``NULL`` pointers have a ``False`` boolean value::

   >>> null_ptr = POINTER(c_int)()
   >>> print(bool(null_ptr))
   False
   >>>

:mod:`!ctypes` checks for ``NULL`` when dereferencing pointers (but dereferencing
invalid non-\ ``NULL`` pointers would crash Python)::

   >>> null_ptr[0]
   Traceback (most recent call last):
       ....
   ValueError: NULL pointer access
   >>>

   >>> null_ptr[0] = 1234
   Traceback (most recent call last):
       ....
   ValueError: NULL pointer access
   >>>

.. _ctypes-thread-safety:

Thread safety without the GIL
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

From Python 3.13 onward, the :term:`GIL` can be disabled on the :term:`free-threaded build`.
In ctypes, reads and writes to a single object concurrently is safe, but not across multiple objects:

   .. code-block:: pycon

      >>> number = c_int(42)
      >>> pointer_a = pointer(number)
      >>> pointer_b = pointer(number)

In the above, it's only safe for one object to read and write to the address at once if the GIL is disabled.
So, ``pointer_a`` can be shared and written to across multiple threads, but only if ``pointer_b``
is not also attempting to do the same. If this is an issue, consider using a :class:`threading.Lock`
to synchronize access to memory:

   .. code-block:: pycon

      >>> import threading
      >>> lock = threading.Lock()
      >>> # Thread 1
      >>> with lock:
      ...    pointer_a.contents = 24
      >>> # Thread 2
      >>> with lock:
      ...    pointer_b.contents = 42


.. _ctypes-type-conversions:

Type conversions
^^^^^^^^^^^^^^^^

Usually, ctypes does strict type checking.  This means, if you have
``POINTER(c_int)`` in the :attr:`~_CFuncPtr.argtypes` list of a function or as the type of
a member field in a structure definition, only instances of exactly the same
type are accepted.  There are some exceptions to this rule, where ctypes accepts
other objects.  For example, you can pass compatible array instances instead of
pointer types.  So, for ``POINTER(c_int)``, ctypes accepts an array of c_int::

   >>> class Bar(Structure):
   ...     _fields_ = [("count", c_int), ("values", POINTER(c_int))]
   ...
   >>> bar = Bar()
   >>> bar.values = (c_int * 3)(1, 2, 3)
   >>> bar.count = 3
   >>> for i in range(bar.count):
   ...     print(bar.values[i])
   ...
   1
   2
   3
   >>>

In addition, if a function argument is explicitly declared to be a pointer type
(such as ``POINTER(c_int)``) in :attr:`~_CFuncPtr.argtypes`, an object of the pointed
type (``c_int`` in this case) can be passed to the function.  ctypes will apply
the required :func:`byref` conversion in this case automatically.

To set a POINTER type field to ``NULL``, you can assign ``None``::

   >>> bar.values = None
   >>>

.. XXX list other conversions...

Sometimes you have instances of incompatible types.  In C, you can cast one type
into another type.  :mod:`!ctypes` provides a :func:`cast` function which can be
used in the same way.  The ``Bar`` structure defined above accepts
``POINTER(c_int)`` pointers or :class:`c_int` arrays for its ``values`` field,
but not instances of other types::

   >>> bar.values = (c_byte * 4)()
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
   >>>

For these cases, the :func:`cast` function is handy.

The :func:`cast` function can be used to cast a ctypes instance into a pointer
to a different ctypes data type.  :func:`cast` takes two parameters, a ctypes
object that is or can be converted to a pointer of some kind, and a ctypes
pointer type.  It returns an instance of the second argument, which references
the same memory block as the first argument::

   >>> a = (c_byte * 4)()
   >>> cast(a, POINTER(c_int))
   <ctypes.LP_c_long object at ...>
   >>>

So, :func:`cast` can be used to assign to the ``values`` field of ``Bar`` the
structure::

   >>> bar = Bar()
   >>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
   >>> print(bar.values[0])
   0
   >>>


.. _ctypes-incomplete-types:

Incomplete Types
^^^^^^^^^^^^^^^^

*Incomplete Types* are structures, unions or arrays whose members are not yet
specified. In C, they are specified by forward declarations, which are defined
later::

   struct cell; /* forward declaration */

   struct cell {
       char *name;
       struct cell *next;
   };

The straightforward translation into ctypes code would be this, but it does not
work::

   >>> class cell(Structure):
   ...     _fields_ = [("name", c_char_p),
   ...                 ("next", POINTER(cell))]
   ...
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 2, in cell
   NameError: name 'cell' is not defined
   >>>

because the new ``class cell`` is not available in the class statement itself.
In :mod:`!ctypes`, we can define the ``cell`` class and set the
:attr:`~Structure._fields_` attribute later, after the class statement::

   >>> from ctypes import *
   >>> class cell(Structure):
   ...     pass
   ...
   >>> cell._fields_ = [("name", c_char_p),
   ...                  ("next", POINTER(cell))]
   >>>

Let's try it. We create two instances of ``cell``, and let them point to each
other, and finally follow the pointer chain a few times::

   >>> c1 = cell()
   >>> c1.name = b"foo"
   >>> c2 = cell()
   >>> c2.name = b"bar"
   >>> c1.next = pointer(c2)
   >>> c2.next = pointer(c1)
   >>> p = c1
   >>> for i in range(8):
   ...     print(p.name, end=" ")
   ...     p = p.next[0]
   ...
   foo bar foo bar foo bar foo bar
   >>>


.. _ctypes-callback-functions:

Callback functions
^^^^^^^^^^^^^^^^^^

:mod:`!ctypes` allows creating C callable function pointers from Python callables.
These are sometimes called *callback functions*.

First, you must create a class for the callback function. The class knows the
calling convention, the return type, and the number and types of arguments this
function will receive.

The :func:`CFUNCTYPE` factory function creates types for callback functions
using the ``cdecl`` calling convention. On Windows, the :func:`WINFUNCTYPE`
factory function creates types for callback functions using the ``stdcall``
calling convention.

Both of these factory functions are called with the result type as first
argument, and the callback functions expected argument types as the remaining
arguments.

I will present an example here which uses the standard C library's
:c:func:`!qsort` function, that is used to sort items with the help of a callback
function.  :c:func:`!qsort` will be used to sort an array of integers::

   >>> IntArray5 = c_int * 5
   >>> ia = IntArray5(5, 1, 7, 33, 99)
   >>> qsort = libc.qsort
   >>> qsort.restype = None
   >>>

:func:`!qsort` must be called with a pointer to the data to sort, the number of
items in the data array, the size of one item, and a pointer to the comparison
function, the callback. The callback will then be called with two pointers to
items, and it must return a negative integer if the first item is smaller than
the second, a zero if they are equal, and a positive integer otherwise.

So our callback function receives pointers to integers, and must return an
integer. First we create the ``type`` for the callback function::

   >>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
   >>>

To get started, here is a simple callback that shows the values it gets
passed::

   >>> def py_cmp_func(a, b):
   ...     print("py_cmp_func", a[0], b[0])
   ...     return 0
   ...
   >>> cmp_func = CMPFUNC(py_cmp_func)
   >>>

The result::

   >>> qsort(ia, len(ia), sizeof(c_int), cmp_func)  # doctest: +LINUX
   py_cmp_func 5 1
   py_cmp_func 33 99
   py_cmp_func 7 33
   py_cmp_func 5 7
   py_cmp_func 1 7
   >>>

Now we can actually compare the two items and return a useful result::

   >>> def py_cmp_func(a, b):
   ...     print("py_cmp_func", a[0], b[0])
   ...     return a[0] - b[0]
   ...
   >>>
   >>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +LINUX
   py_cmp_func 5 1
   py_cmp_func 33 99
   py_cmp_func 7 33
   py_cmp_func 1 7
   py_cmp_func 5 7
   >>>

As we can easily check, our array is sorted now::

   >>> for i in ia: print(i, end=" ")
   ...
   1 5 7 33 99
   >>>

The function factories can be used as decorator factories, so we may as well
write::

   >>> @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
   ... def py_cmp_func(a, b):
   ...     print("py_cmp_func", a[0], b[0])
   ...     return a[0] - b[0]
   ...
   >>> qsort(ia, len(ia), sizeof(c_int), py_cmp_func)
   py_cmp_func 5 1
   py_cmp_func 33 99
   py_cmp_func 7 33
   py_cmp_func 1 7
   py_cmp_func 5 7
   >>>

.. note::

   Make sure you keep references to :func:`CFUNCTYPE` objects as long as they
   are used from C code. :mod:`!ctypes` doesn't, and if you don't, they may be
   garbage collected, crashing your program when a callback is made.

   Also, note that if the callback function is called in a thread created
   outside of Python's control (e.g. by the foreign code that calls the
   callback), ctypes creates a new dummy Python thread on every invocation. This
   behavior is correct for most purposes, but it means that values stored with
   :class:`threading.local` will *not* survive across different callbacks, even when
   those calls are made from the same C thread.

.. _ctypes-accessing-values-exported-from-dlls:

Accessing values exported from dlls
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the :c:data:`Py_Version`, Python
runtime version number encoded in a single constant integer.

:mod:`!ctypes` can access values like this with the :meth:`~_CData.in_dll` class methods of
the type.  *pythonapi* is a predefined symbol giving access to the Python C
api::

   >>> version = ctypes.c_int.in_dll(ctypes.pythonapi, "Py_Version")
   >>> print(hex(version.value))
   0x30c00a0

An extended example which also demonstrates the use of pointers accesses the
:c:data:`PyImport_FrozenModules` pointer exported by Python.

Quoting the docs for that value:

   This pointer is initialized to point to an array of :c:struct:`_frozen`
   records, terminated by one whose members are all ``NULL`` or zero.  When a frozen
   module is imported, it is searched in this table.  Third-party code could play
   tricks with this to provide a dynamically created collection of frozen modules.

So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with :mod:`!ctypes`::

   >>> from ctypes import *
   >>>
   >>> class struct_frozen(Structure):
   ...     _fields_ = [("name", c_char_p),
   ...                 ("code", POINTER(c_ubyte)),
   ...                 ("size", c_int),
   ...                 ("get_code", POINTER(c_ubyte)),  # Function pointer
   ...                ]
   ...
   >>>

We have defined the :c:struct:`_frozen` data type, so we can get the pointer
to the table::

   >>> FrozenTable = POINTER(struct_frozen)
   >>> table = FrozenTable.in_dll(pythonapi, "_PyImport_FrozenBootstrap")
   >>>

Since ``table`` is a ``pointer`` to the array of ``struct_frozen`` records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it's better to break out of the loop when we
hit the ``NULL`` entry::

   >>> for item in table:
   ...     if item.name is None:
   ...         break
   ...     print(item.name.decode("ascii"), item.size)
   ...
   _frozen_importlib 31764
   _frozen_importlib_external 41499
   zipimport 12345
   >>>

The fact that standard Python has a frozen module and a frozen package
(indicated by the negative ``size`` member) is not well known, it is only used
for testing. Try it out with ``import __hello__`` for example.


.. _ctypes-surprises:

Surprises
^^^^^^^^^

There are some edges in :mod:`!ctypes` where you might expect something other
than what actually happens.

Consider the following example::

   >>> from ctypes import *
   >>> class POINT(Structure):
   ...     _fields_ = ("x", c_int), ("y", c_int)
   ...
   >>> class RECT(Structure):
   ...     _fields_ = ("a", POINT), ("b", POINT)
   ...
   >>> p1 = POINT(1, 2)
   >>> p2 = POINT(3, 4)
   >>> rc = RECT(p1, p2)
   >>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
   1 2 3 4
   >>> # now swap the two points
   >>> rc.a, rc.b = rc.b, rc.a
   >>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
   3 4 3 4
   >>>

Hm. We certainly expected the last statement to print ``3 4 1 2``. What
happened? Here are the steps of the ``rc.a, rc.b = rc.b, rc.a`` line above::

   >>> temp0, temp1 = rc.b, rc.a
   >>> rc.a = temp0
   >>> rc.b = temp1
   >>>

Note that ``temp0`` and ``temp1`` are objects still using the internal buffer of
the ``rc`` object above. So executing ``rc.a = temp0`` copies the buffer
contents of ``temp0`` into ``rc`` 's buffer.  This, in turn, changes the
contents of ``temp1``. So, the last assignment ``rc.b = temp1``, doesn't have
the expected effect.

Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays
doesn't *copy* the sub-object, instead it retrieves a wrapper object accessing
the root-object's underlying buffer.

Another example that may behave differently from what one would expect is this::

   >>> s = c_char_p()
   >>> s.value = b"abc def ghi"
   >>> s.value
   b'abc def ghi'
   >>> s.value is s.value
   False
   >>>

.. note::

   Objects instantiated from :class:`c_char_p` can only have their value set to bytes
   or integers.

Why is it printing ``False``?  ctypes instances are objects containing a memory
block plus some :term:`descriptor`\s accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the ``contents`` of the object is stored.  Accessing the contents again
constructs a new Python object each time!


.. _ctypes-variable-sized-data-types:

Variable-sized data types
^^^^^^^^^^^^^^^^^^^^^^^^^

:mod:`!ctypes` provides some support for variable-sized arrays and structures.

The :func:`resize` function can be used to resize the memory buffer of an
existing ctypes object.  The function takes the object as first argument, and
the requested size in bytes as the second argument.  The memory block cannot be
made smaller than the natural memory block specified by the objects type, a
:exc:`ValueError` is raised if this is tried::

   >>> short_array = (c_short * 4)()
   >>> print(sizeof(short_array))
   8
   >>> resize(short_array, 4)
   Traceback (most recent call last):
       ...
   ValueError: minimum size is 8
   >>> resize(short_array, 32)
   >>> sizeof(short_array)
   32
   >>> sizeof(type(short_array))
   8
   >>>

This is nice and fine, but how would one access the additional elements
contained in this array?  Since the type still only knows about 4 elements, we
get errors accessing other elements::

   >>> short_array[:]
   [0, 0, 0, 0]
   >>> short_array[7]
   Traceback (most recent call last):
       ...
   IndexError: invalid index
   >>>

Another way to use variable-sized data types with :mod:`!ctypes` is to use the
dynamic nature of Python, and (re-)define the data type after the required size
is already known, on a case by case basis.


.. _ctypes-ctypes-reference:

ctypes reference
----------------


.. _ctypes-finding-shared-libraries:

Finding shared libraries
^^^^^^^^^^^^^^^^^^^^^^^^

When programming in a compiled language, shared libraries are accessed when
compiling/linking a program, and when the program is run.

The purpose of the :func:`~ctypes.util.find_library` function is to locate a library in a way
similar to what the compiler or runtime loader does (on platforms with several
versions of a shared library the most recent should be loaded), while the ctypes
library loaders act like when a program is run, and call the runtime loader
directly.

The :mod:`!ctypes.util` module provides a function which can help to determine
the library to load.


.. data:: find_library(name)
   :module: ctypes.util
   :noindex:

   Try to find a library and return a pathname.  *name* is the library name without
   any prefix like *lib*, suffix like ``.so``, ``.dylib`` or version number (this
   is the form used for the posix linker option :option:`!-l`).  If no library can
   be found, returns ``None``.

The exact functionality is system dependent.

On Linux, :func:`~ctypes.util.find_library` tries to run external programs
(``/sbin/ldconfig``, ``gcc``, ``objdump`` and ``ld``) to find the library file.
It returns the filename of the library file.

Note that if the output of these programs does not correspond to the dynamic
linker used by Python, the result of this function may be misleading.

.. versionchanged:: 3.6
   On Linux, the value of the environment variable ``LD_LIBRARY_PATH`` is used
   when searching for libraries, if a library cannot be found by any other means.

Here are some examples::

   >>> from ctypes.util import find_library
   >>> find_library("m")
   'libm.so.6'
   >>> find_library("c")
   'libc.so.6'
   >>> find_library("bz2")
   'libbz2.so.1.0'
   >>>

On macOS and Android, :func:`~ctypes.util.find_library` uses the system's
standard naming schemes and paths to locate the library, and returns a full
pathname if successful::

   >>> from ctypes.util import find_library
   >>> find_library("c")
   '/usr/lib/libc.dylib'
   >>> find_library("m")
   '/usr/lib/libm.dylib'
   >>> find_library("bz2")
   '/usr/lib/libbz2.dylib'
   >>> find_library("AGL")
   '/System/Library/Frameworks/AGL.framework/AGL'
   >>>

On Windows, :func:`~ctypes.util.find_library` searches along the system search path, and
returns the full pathname, but since there is no predefined naming scheme a call
like ``find_library("c")`` will fail and return ``None``.

If wrapping a shared library with :mod:`!ctypes`, it *may* be better to determine
the shared library name at development time, and hardcode that into the wrapper
module instead of using :func:`~ctypes.util.find_library` to locate the library at runtime.


.. _ctypes-listing-loaded-shared-libraries:

Listing loaded shared libraries
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When writing code that relies on code loaded from shared libraries, it can be
useful to know which shared libraries have already been loaded into the current
process.

The :mod:`!ctypes.util` module provides the :func:`~ctypes.util.dllist` function,
which calls the different APIs provided by the various platforms to help determine
which shared libraries have already been loaded into the current process.

The exact output of this function will be system dependent. On most platforms,
the first entry of this list represents the current process itself, which may
be an empty string.
For example, on glibc-based Linux, the return may look like::

   >>> from ctypes.util import dllist
   >>> dllist()
   ['', 'linux-vdso.so.1', '/lib/x86_64-linux-gnu/libm.so.6', '/lib/x86_64-linux-gnu/libc.so.6', ... ]

.. _ctypes-loading-shared-libraries:

Loading shared libraries
^^^^^^^^^^^^^^^^^^^^^^^^

There are several ways to load shared libraries into the Python process.  One
way is to instantiate :py:class:`CDLL` or one of its subclasses:


.. class:: CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=None)

   Represents a loaded shared library.

   Functions in this library use the standard C calling convention, and are
   assumed to return :c:expr:`int`.
   The Python :term:`global interpreter lock` is released before calling any
   function exported by these libraries, and reacquired afterwards.
   For different function behavior, use a subclass: :py:class:`~ctypes.OleDLL`,
   :py:class:`~ctypes.WinDLL`, or :py:class:`~ctypes.PyDLL`.

   If you have an existing :py:attr:`handle <ctypes.CDLL._handle>` to an already
   loaded shared library, it can be passed as the *handle* argument to wrap
   the opened library in a new :py:class:`!CDLL` object.
   In this case, *name* is only used to set the :py:attr:`~ctypes.CDLL._name`
   attribute, but it may be adjusted and/or validated.

   If *handle* is ``None``, the underlying platform's :manpage:`dlopen(3)` or
   :c:func:`!LoadLibrary` function is used to load the library into
   the process, and to get a handle to it.

   *name* is the pathname of the shared library to open.
   If *name* does not contain a path separator, the library is found
   in a platform-specific way.

   On non-Windows systems, *name* can  be ``None``. In this case,
   :c:func:`!dlopen` is called with ``NULL``, which opens the main program
   as a "library".
   (Some systems do the same is *name* is empty; ``None``/``NULL`` is more
   portable.)

   .. admonition:: CPython implementation detail

      Since CPython is linked to ``libc``, a ``None`` *name* is often used
      to access the C standard library::

         >>> printf = ctypes.CDLL(None).printf
         >>> printf.argtypes = [ctypes.c_char_p]
         >>> printf(b"hello\n")
         hello
         6

      To access the Python C API, prefer :py:data:`ctypes.pythonapi` which
      works across platforms.

   The *mode* parameter can be used to specify how the library is loaded.  For
   details, consult the :manpage:`dlopen(3)` manpage.  On Windows, *mode* is
   ignored.  On posix systems, RTLD_NOW is always added, and is not
   configurable.

   The *use_errno* parameter, when set to true, enables a ctypes mechanism that
   allows accessing the system :data:`errno` error number in a safe way.
   :mod:`!ctypes` maintains a thread-local copy of the system's :data:`errno`
   variable; if you call foreign functions created with ``use_errno=True`` then the
   :data:`errno` value before the function call is swapped with the ctypes private
   copy, the same happens immediately after the function call.

   The function :func:`ctypes.get_errno` returns the value of the ctypes private
   copy, and the function :func:`ctypes.set_errno` changes the ctypes private copy
   to a new value and returns the former value.

   The *use_last_error* parameter, when set to true, enables the same mechanism for
   the Windows error code which is managed by the :func:`GetLastError` and
   :func:`!SetLastError` Windows API functions; :func:`ctypes.get_last_error` and
   :func:`ctypes.set_last_error` are used to request and change the ctypes private
   copy of the windows error code.

   The *winmode* parameter is used on Windows to specify how the library is loaded
   (since *mode* is ignored). It takes any value that is valid for the Win32 API
   ``LoadLibraryEx`` flags parameter. When omitted, the default is to use the
   flags that result in the most secure DLL load, which avoids issues such as DLL
   hijacking. Passing the full path to the DLL is the safest way to ensure the
   correct library and dependencies are loaded.

   On Windows creating a :class:`CDLL` instance may fail even if the DLL name
   exists. When a dependent DLL of the loaded DLL is not found, a
   :exc:`OSError` error is raised with the message *"[WinError 126] The
   specified module could not be found".* This error message does not contain
   the name of the missing DLL because the Windows API does not return this
   information making this error hard to diagnose. To resolve this error and
   determine which DLL is not found, you need to find the list of dependent
   DLLs and determine which one is not found using Windows debugging and
   tracing tools.

   .. seealso::

      `Microsoft DUMPBIN tool <https://learn.microsoft.com/en-us/cpp/build/reference/dumpbin-reference?view=msvc-170>`_
      -- A tool to find DLL dependents.

   .. versionchanged:: 3.8
      Added *winmode* parameter.

   .. versionchanged:: 3.12

      The *name* parameter can now be a :term:`path-like object`.

   Instances of this class have no public methods.  Functions exported by the
   shared library can be accessed as attributes or by index.  Please note that
   accessing the function through an attribute caches the result and therefore
   accessing it repeatedly returns the same object each time.  On the other hand,
   accessing it through an index returns a new object each time::

      >>> from ctypes import CDLL
      >>> libc = CDLL("libc.so.6")  # On Linux
      >>> libc.time == libc.time
      True
      >>> libc['time'] == libc['time']
      False

   The following public attributes are available. Their name starts with an
   underscore to not clash with exported function names:

   .. attribute:: _handle

      The system handle used to access the library.

   .. attribute:: _name

      The name of the library passed in the constructor.

.. class:: OleDLL

   See :py:class:`~ctypes.CDLL`, the superclass, for common information.

   Functions in this library use the ``stdcall`` calling convention, and are
   assumed to return the windows specific :class:`HRESULT` code.  :class:`HRESULT`
   values contain information specifying whether the function call failed or
   succeeded, together with additional error code.  If the return value signals a
   failure, an :class:`OSError` is automatically raised.

   .. availability:: Windows

   .. versionchanged:: 3.3
      :exc:`WindowsError` used to be raised,
      which is now an alias of :exc:`OSError`.


.. class:: WinDLL

   See :py:class:`~ctypes.CDLL`, the superclass, for common information.

   Functions in these libraries use the ``stdcall`` calling convention, and are
   assumed to return :c:expr:`int` by default.

   .. availability:: Windows

.. class:: PyDLL

   See :py:class:`~ctypes.CDLL`, the superclass, for common information.

   When functions in this library are called, the
   Python GIL is *not* released during the function call, and after the function
   execution the Python error flag is checked. If the error flag is set, a Python
   exception is raised.

   Thus, this is only useful to call Python C API functions directly.


.. data:: RTLD_GLOBAL

   Flag to use as *mode* parameter.  On platforms where this flag is not available,
   it is defined as the integer zero.


.. data:: RTLD_LOCAL

   Flag to use as *mode* parameter.  On platforms where this is not available, it
   is the same as *RTLD_GLOBAL*.


.. data:: DEFAULT_MODE

   The default mode which is used to load shared libraries.  On OSX 10.3, this is
   *RTLD_GLOBAL*, otherwise it is the same as *RTLD_LOCAL*.


Shared libraries can also be loaded by using one of the prefabricated objects,
which are instances of the :class:`LibraryLoader` class, either by calling the
:meth:`~LibraryLoader.LoadLibrary` method, or by retrieving the library as
attribute of the loader instance.

.. class:: LibraryLoader(dlltype)

   Class which loads shared libraries.  *dlltype* should be one of the
   :class:`CDLL`, :class:`PyDLL`, :class:`WinDLL`, or :class:`OleDLL` types.

   :meth:`!__getattr__` has special behavior: It allows loading a shared library by
   accessing it as attribute of a library loader instance.  The result is cached,
   so repeated attribute accesses return the same library each time.

   .. method:: LoadLibrary(name)

      Load a shared library into the process and return it.  This method always
      returns a new instance of the library.


These prefabricated library loaders are available:

.. data:: cdll

   Creates :class:`CDLL` instances.


.. data:: windll

   Creates :class:`WinDLL` instances.

   .. availability:: Windows


.. data:: oledll

   Creates :class:`OleDLL` instances.

   .. availability:: Windows


.. data:: pydll

   Creates :class:`PyDLL` instances.


For accessing the C Python api directly, a ready-to-use Python shared library
object is available:

.. data:: pythonapi

   An instance of :class:`PyDLL` that exposes Python C API functions as
   attributes.  Note that all these functions are assumed to return C
   :c:expr:`int`, which is of course not always the truth, so you have to assign
   the correct :attr:`!restype` attribute to use these functions.

.. audit-event:: ctypes.dlopen name ctypes.LibraryLoader

   Loading a library through any of these objects raises an
   :ref:`auditing event <auditing>` ``ctypes.dlopen`` with string argument
   ``name``, the name used to load the library.

.. audit-event:: ctypes.dlsym library,name ctypes.LibraryLoader

   Accessing a function on a loaded library raises an auditing event
   ``ctypes.dlsym`` with arguments ``library`` (the library object) and ``name``
   (the symbol's name as a string or integer).

.. audit-event:: ctypes.dlsym/handle handle,name ctypes.LibraryLoader

   In cases when only the library handle is available rather than the object,
   accessing a function raises an auditing event ``ctypes.dlsym/handle`` with
   arguments ``handle`` (the raw library handle) and ``name``.

.. _ctypes-foreign-functions:

Foreign functions
^^^^^^^^^^^^^^^^^

As explained in the previous section, foreign functions can be accessed as
attributes of loaded shared libraries.  The function objects created in this way
by default accept any number of arguments, accept any ctypes data instances as
arguments, and return the default result type specified by the library loader.

They are instances of a private local class :class:`!_FuncPtr` (not exposed
in :mod:`!ctypes`) which inherits from the private :class:`_CFuncPtr` class:

.. doctest::

   >>> import ctypes
   >>> lib = ctypes.CDLL(None)
   >>> issubclass(lib._FuncPtr, ctypes._CFuncPtr)
   True
   >>> lib._FuncPtr is ctypes._CFuncPtr
   False

.. class:: _CFuncPtr

   Base class for C callable foreign functions.

   Instances of foreign functions are also C compatible data types; they
   represent C function pointers.

   This behavior can be customized by assigning to special attributes of the
   foreign function object.

   .. attribute:: restype

      Assign a ctypes type to specify the result type of the foreign function.
      Use ``None`` for :c:expr:`void`, a function not returning anything.

      It is possible to assign a callable Python object that is not a ctypes
      type, in this case the function is assumed to return a C :c:expr:`int`, and
      the callable will be called with this integer, allowing further
      processing or error checking.  Using this is deprecated, for more flexible
      post processing or error checking use a ctypes data type as
      :attr:`!restype` and assign a callable to the :attr:`errcheck` attribute.

   .. attribute:: argtypes

      Assign a tuple of ctypes types to specify the argument types that the
      function accepts.  Functions using the ``stdcall`` calling convention can
      only be called with the same number of arguments as the length of this
      tuple; functions using the C calling convention accept additional,
      unspecified arguments as well.

      When a foreign function is called, each actual argument is passed to the
      :meth:`~_CData.from_param` class method of the items in the :attr:`argtypes`
      tuple, this method allows adapting the actual argument to an object that
      the foreign function accepts.  For example, a :class:`c_char_p` item in
      the :attr:`argtypes` tuple will convert a string passed as argument into
      a bytes object using ctypes conversion rules.

      New: It is now possible to put items in argtypes which are not ctypes
      types, but each item must have a :meth:`~_CData.from_param` method which returns a
      value usable as argument (integer, string, ctypes instance).  This allows
      defining adapters that can adapt custom objects as function parameters.

   .. attribute:: errcheck

      Assign a Python function or another callable to this attribute. The
      callable will be called with three or more arguments:

      .. function:: callable(result, func, arguments)
         :noindex:
         :module:

         *result* is what the foreign function returns, as specified by the
         :attr:`!restype` attribute.

         *func* is the foreign function object itself, this allows reusing the
         same callable object to check or post process the results of several
         functions.

         *arguments* is a tuple containing the parameters originally passed to
         the function call, this allows specializing the behavior on the
         arguments used.

      The object that this function returns will be returned from the
      foreign function call, but it can also check the result value
      and raise an exception if the foreign function call failed.


.. audit-event:: ctypes.set_exception code foreign-functions

   On Windows, when a foreign function call raises a system exception (for
   example, due to an access violation), it will be captured and replaced with
   a suitable Python exception. Further, an auditing event
   ``ctypes.set_exception`` with argument ``code`` will be raised, allowing an
   audit hook to replace the exception with its own.

.. audit-event:: ctypes.call_function func_pointer,arguments foreign-functions

   Some ways to invoke foreign function calls as well as some of the
   functions in this module may raise an auditing event
   ``ctypes.call_function`` with arguments ``function pointer`` and ``arguments``.

.. _ctypes-function-prototypes:

Function prototypes
^^^^^^^^^^^^^^^^^^^

Foreign functions can also be created by instantiating function prototypes.
Function prototypes are similar to function prototypes in C; they describe a
function (return type, argument types, calling convention) without defining an
implementation.  The factory functions must be called with the desired result
type and the argument types of the function, and can be used as decorator
factories, and as such, be applied to functions through the ``@wrapper`` syntax.
See :ref:`ctypes-callback-functions` for examples.


.. function:: CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)

   The returned function prototype creates functions that use the standard C
   calling convention.  The function will release the GIL during the call.  If
   *use_errno* is set to true, the ctypes private copy of the system
   :data:`errno` variable is exchanged with the real :data:`errno` value before
   and after the call; *use_last_error* does the same for the Windows error
   code.


.. function:: WINFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)

   The returned function prototype creates functions that use the
   ``stdcall`` calling convention.  The function will
   release the GIL during the call.  *use_errno* and *use_last_error* have the
   same meaning as above.

   .. availability:: Windows


.. function:: PYFUNCTYPE(restype, *argtypes)

   The returned function prototype creates functions that use the Python calling
   convention.  The function will *not* release the GIL during the call.

Function prototypes created by these factory functions can be instantiated in
different ways, depending on the type and number of the parameters in the call:

.. function:: prototype(address)
   :noindex:
   :module:

   Returns a foreign function at the specified address which must be an integer.


.. function:: prototype(callable)
   :noindex:
   :module:

   Create a C callable function (a callback function) from a Python *callable*.


.. function:: prototype(func_spec[, paramflags])
   :noindex:
   :module:

   Returns a foreign function exported by a shared library. *func_spec* must
   be a 2-tuple ``(name_or_ordinal, library)``. The first item is the name of
   the exported function as string, or the ordinal of the exported function
   as small integer.  The second item is the shared library instance.


.. function:: prototype(vtbl_index, name[, paramflags[, iid]])
   :noindex:
   :module:

   Returns a foreign function that will call a COM method. *vtbl_index* is
   the index into the virtual function table, a small non-negative
   integer. *name* is name of the COM method. *iid* is an optional pointer to
   the interface identifier which is used in extended error reporting.

   If *iid* is not specified, an :exc:`OSError` is raised if the COM method
   call fails. If *iid* is specified, a :exc:`~ctypes.COMError` is raised
   instead.

   COM methods use a special calling convention: They require a pointer to
   the COM interface as first argument, in addition to those parameters that
   are specified in the :attr:`!argtypes` tuple.

   .. availability:: Windows


The optional *paramflags* parameter creates foreign function wrappers with much
more functionality than the features described above.

*paramflags* must be a tuple of the same length as :attr:`~_CFuncPtr.argtypes`.

Each item in this tuple contains further information about a parameter, it must
be a tuple containing one, two, or three items.

The first item is an integer containing a combination of direction
flags for the parameter:

   1
      Specifies an input parameter to the function.

   2
      Output parameter.  The foreign function fills in a value.

   4
      Input parameter which defaults to the integer zero.

The optional second item is the parameter name as string.  If this is specified,
the foreign function can be called with named parameters.

The optional third item is the default value for this parameter.


The following example demonstrates how to wrap the Windows ``MessageBoxW`` function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this::

   WINUSERAPI int WINAPI
   MessageBoxW(
       HWND hWnd,
       LPCWSTR lpText,
       LPCWSTR lpCaption,
       UINT uType);

Here is the wrapping with :mod:`!ctypes`::

   >>> from ctypes import c_int, WINFUNCTYPE, windll
   >>> from ctypes.wintypes import HWND, LPCWSTR, UINT
   >>> prototype = WINFUNCTYPE(c_int, HWND, LPCWSTR, LPCWSTR, UINT)
   >>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", "Hello from ctypes"), (1, "flags", 0)
   >>> MessageBox = prototype(("MessageBoxW", windll.user32), paramflags)

The ``MessageBox`` foreign function can now be called in these ways::

   >>> MessageBox()
   >>> MessageBox(text="Spam, spam, spam")
   >>> MessageBox(flags=2, text="foo bar")

A second example demonstrates output parameters.  The win32 ``GetWindowRect``
function retrieves the dimensions of a specified window by copying them into
``RECT`` structure that the caller has to supply.  Here is the C declaration::

   WINUSERAPI BOOL WINAPI
   GetWindowRect(
        HWND hWnd,
        LPRECT lpRect);

Here is the wrapping with :mod:`!ctypes`::

   >>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
   >>> from ctypes.wintypes import BOOL, HWND, RECT
   >>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
   >>> paramflags = (1, "hwnd"), (2, "lprect")
   >>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)
   >>>

Functions with output parameters will automatically return the output parameter
value if there is a single one, or a tuple containing the output parameter
values when there are more than one, so the GetWindowRect function now returns a
RECT instance, when called.

Output parameters can be combined with the :attr:`~_CFuncPtr.errcheck` protocol to do
further output processing and error checking.  The win32 ``GetWindowRect`` api
function returns a ``BOOL`` to signal success or failure, so this function could
do the error checking, and raises an exception when the api call failed::

   >>> def errcheck(result, func, args):
   ...     if not result:
   ...         raise WinError()
   ...     return args
   ...
   >>> GetWindowRect.errcheck = errcheck
   >>>

If the :attr:`~_CFuncPtr.errcheck` function returns the argument tuple it receives
unchanged, :mod:`!ctypes` continues the normal processing it does on the output
parameters.  If you want to return a tuple of window coordinates instead of a
``RECT`` instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place::

   >>> def errcheck(result, func, args):
   ...     if not result:
   ...         raise WinError()
   ...     rc = args[1]
   ...     return rc.left, rc.top, rc.bottom, rc.right
   ...
   >>> GetWindowRect.errcheck = errcheck
   >>>


.. _ctypes-utility-functions:

Utility functions
^^^^^^^^^^^^^^^^^

.. function:: addressof(obj)

   Returns the address of the memory buffer as integer.  *obj* must be an
   instance of a ctypes type.

   .. audit-event:: ctypes.addressof obj ctypes.addressof


.. function:: alignment(obj_or_type)

   Returns the alignment requirements of a ctypes type. *obj_or_type* must be a
   ctypes type or instance.


.. function:: byref(obj[, offset])

   Returns a light-weight pointer to *obj*, which must be an instance of a
   ctypes type.  *offset* defaults to zero, and must be an integer that will be
   added to the internal pointer value.

   ``byref(obj, offset)`` corresponds to this C code::

      (((char *)&obj) + offset)

   The returned object can only be used as a foreign function call parameter.
   It behaves similar to ``pointer(obj)``, but the construction is a lot faster.


.. function:: CopyComPointer(src, dst)

   Copies a COM pointer from *src* to *dst* and returns the Windows specific
   :c:type:`!HRESULT` value.

   If *src* is not ``NULL``, its ``AddRef`` method is called, incrementing the
   reference count.

   In contrast, the reference count of *dst* will not be decremented before
   assigning the new value. Unless *dst* is ``NULL``, the caller is responsible
   for decrementing the reference count by calling its ``Release`` method when
   necessary.

   .. availability:: Windows

   .. versionadded:: 3.14


.. function:: cast(obj, type)

   This function is similar to the cast operator in C. It returns a new instance
   of *type* which points to the same memory block as *obj*.  *type* must be a
   pointer type, and *obj* must be an object that can be interpreted as a
   pointer.


.. function:: create_string_buffer(init, size=None)
              create_string_buffer(size)

   This function creates a mutable character buffer. The returned object is a
   ctypes array of :class:`c_char`.

   If *size* is given (and not ``None``), it must be an :class:`int`.
   It specifies the size of the returned array.

   If the *init* argument is given, it must be :class:`bytes`. It is used
   to initialize the array items. Bytes not initialized this way are
   set to zero (NUL).

   If *size* is not given (or if it is ``None``), the buffer is made one element
   larger than *init*, effectively adding a NUL terminator.

   If both arguments are given, *size* must not be less than ``len(init)``.

   .. warning::

      If *size* is equal to ``len(init)``, a NUL terminator is
      not added. Do not treat such a buffer as a C string.

   For example::

      >>> bytes(create_string_buffer(2))
      b'\x00\x00'
      >>> bytes(create_string_buffer(b'ab'))
      b'ab\x00'
      >>> bytes(create_string_buffer(b'ab', 2))
      b'ab'
      >>> bytes(create_string_buffer(b'ab', 4))
      b'ab\x00\x00'
      >>> bytes(create_string_buffer(b'abcdef', 2))
      Traceback (most recent call last):
         ...
      ValueError: byte string too long

   .. audit-event:: ctypes.create_string_buffer init,size ctypes.create_string_buffer


.. function:: create_unicode_buffer(init, size=None)
              create_unicode_buffer(size)

   This function creates a mutable unicode character buffer. The returned object is
   a ctypes array of :class:`c_wchar`.

   The function takes the same arguments as :func:`~create_string_buffer` except
   *init* must be a string and *size* counts :class:`c_wchar`.

   .. audit-event:: ctypes.create_unicode_buffer init,size ctypes.create_unicode_buffer


.. function:: DllCanUnloadNow()

   This function is a hook which allows implementing in-process
   COM servers with ctypes.  It is called from the DllCanUnloadNow function that
   the _ctypes extension dll exports.

   .. availability:: Windows


.. function:: DllGetClassObject()

   This function is a hook which allows implementing in-process
   COM servers with ctypes.  It is called from the DllGetClassObject function
   that the ``_ctypes`` extension dll exports.

   .. availability:: Windows


.. function:: find_library(name)
   :module: ctypes.util

   Try to find a library and return a pathname.  *name* is the library name
   without any prefix like ``lib``, suffix like ``.so``, ``.dylib`` or version
   number (this is the form used for the posix linker option :option:`!-l`).  If
   no library can be found, returns ``None``.

   The exact functionality is system dependent.

   See :ref:`ctypes-finding-shared-libraries` for complete documentation.


.. function:: find_msvcrt()
   :module: ctypes.util

   Returns the filename of the VC runtime library used by Python,
   and by the extension modules.  If the name of the library cannot be
   determined, ``None`` is returned.

   If you need to free memory, for example, allocated by an extension module
   with a call to the ``free(void *)``, it is important that you use the
   function in the same library that allocated the memory.

   .. availability:: Windows


.. function:: dllist()
   :module: ctypes.util

   Try to provide a list of paths of the shared libraries loaded into the current
   process.  These paths are not normalized or processed in any way.  The function
   can raise :exc:`OSError` if the underlying platform APIs fail.
   The exact functionality is system dependent.

   On most platforms, the first element of the list represents the current
   executable file. It may be an empty string.

   .. availability:: Windows, macOS, iOS, glibc, BSD libc, musl
   .. versionadded:: 3.14

.. function:: FormatError([code])

   Returns a textual description of the error code *code*.  If no error code is
   specified, the last error code is used by calling the Windows API function
   :func:`GetLastError`.

   .. availability:: Windows


.. function:: GetLastError()

   Returns the last error code set by Windows in the calling thread.
   This function calls the Windows ``GetLastError()`` function directly,
   it does not return the ctypes-private copy of the error code.

   .. availability:: Windows


.. function:: get_errno()

   Returns the current value of the ctypes-private copy of the system
   :data:`errno` variable in the calling thread.

   .. audit-event:: ctypes.get_errno "" ctypes.get_errno

.. function:: get_last_error()

   Returns the current value of the ctypes-private copy of the system
   :data:`!LastError` variable in the calling thread.

   .. availability:: Windows

   .. audit-event:: ctypes.get_last_error "" ctypes.get_last_error


.. function:: memmove(dst, src, count)

   Same as the standard C memmove library function: copies *count* bytes from
   *src* to *dst*. *dst* and *src* must be integers or ctypes instances that can
   be converted to pointers.


.. function:: memset(dst, c, count)

   Same as the standard C memset library function: fills the memory block at
   address *dst* with *count* bytes of value *c*. *dst* must be an integer
   specifying an address, or a ctypes instance.


.. function:: POINTER(type, /)

   Create or return a ctypes pointer type. Pointer types are cached and
   reused internally, so calling this function repeatedly is cheap.
   *type* must be a ctypes type.

   .. impl-detail::

      The resulting pointer type is cached in the ``__pointer_type__``
      attribute of *type*.
      It is possible to set this attribute before the first call to
      ``POINTER`` in order to set a custom pointer type.
      However, doing this is discouraged: manually creating a suitable
      pointer type is difficult without relying on implementation
      details that may change in future Python versions.


.. function:: pointer(obj, /)

   Create a new pointer instance, pointing to *obj*.
   The returned object is of the type ``POINTER(type(obj))``.

   Note: If you just want to pass a pointer to an object to a foreign function
   call, you should use ``byref(obj)`` which is much faster.


.. function:: resize(obj, size)

   This function resizes the internal memory buffer of *obj*, which must be an
   instance of a ctypes type.  It is not possible to make the buffer smaller
   than the native size of the objects type, as given by ``sizeof(type(obj))``,
   but it is possible to enlarge the buffer.


.. function:: set_errno(value)

   Set the current value of the ctypes-private copy of the system :data:`errno`
   variable in the calling thread to *value* and return the previous value.

   .. audit-event:: ctypes.set_errno errno ctypes.set_errno


.. function:: set_last_error(value)

   Sets the current value of the ctypes-private copy of the system
   :data:`!LastError` variable in the calling thread to *value* and return the
   previous value.

   .. availability:: Windows

   .. audit-event:: ctypes.set_last_error error ctypes.set_last_error


.. function:: sizeof(obj_or_type)

   Returns the size in bytes of a ctypes type or instance memory buffer.
   Does the same as the C ``sizeof`` operator.


.. function:: string_at(ptr, size=-1)

   Return the byte string at *void \*ptr*.
   If *size* is specified, it is used as size, otherwise the string is assumed
   to be zero-terminated.

   .. audit-event:: ctypes.string_at ptr,size ctypes.string_at


.. function:: WinError(code=None, descr=None)

   Creates an instance of :exc:`OSError`.  If *code* is not specified,
   :func:`GetLastError` is called to determine the error code. If *descr* is not
   specified, :func:`FormatError` is called to get a textual description of the
   error.

   .. availability:: Windows

   .. versionchanged:: 3.3
      An instance of :exc:`WindowsError` used to be created, which is now an
      alias of :exc:`OSError`.


.. function:: wstring_at(ptr, size=-1)

   Return the wide-character string at *void \*ptr*.
   If *size* is specified, it is used as the number of
   characters of the string, otherwise the string is assumed to be
   zero-terminated.

   .. audit-event:: ctypes.wstring_at ptr,size ctypes.wstring_at


.. function:: memoryview_at(ptr, size, readonly=False)

   Return a :class:`memoryview` object of length *size* that references memory
   starting at *void \*ptr*.

   If *readonly* is true, the returned :class:`!memoryview` object can
   not be used to modify the underlying memory.
   (Changes made by other means will still be reflected in the returned
   object.)

   This function is similar to :func:`string_at` with the key
   difference of not making a copy of the specified memory.
   It is a semantically equivalent (but more efficient) alternative to
   ``memoryview((c_byte * size).from_address(ptr))``.
   (While :meth:`~_CData.from_address` only takes integers, *ptr* can also
   be given as a :class:`ctypes.POINTER` or a :func:`~ctypes.byref` object.)

   .. audit-event:: ctypes.memoryview_at address,size,readonly

   .. versionadded:: 3.14


.. _ctypes-data-types:

Data types
^^^^^^^^^^


.. class:: _CData

   This non-public class is the common base class of all ctypes data types.
   Among other things, all ctypes type instances contain a memory block that
   hold C compatible data; the address of the memory block is returned by the
   :func:`addressof` helper function. Another instance variable is exposed as
   :attr:`_objects`; this contains other Python objects that need to be kept
   alive in case the memory block contains pointers.

   Common methods of ctypes data types, these are all class methods (to be
   exact, they are methods of the :term:`metaclass`):

   .. method:: _CData.from_buffer(source[, offset])

      This method returns a ctypes instance that shares the buffer of the
      *source* object.  The *source* object must support the writeable buffer
      interface.  The optional *offset* parameter specifies an offset into the
      source buffer in bytes; the default is zero.  If the source buffer is not
      large enough a :exc:`ValueError` is raised.

      .. audit-event:: ctypes.cdata/buffer pointer,size,offset ctypes._CData.from_buffer

   .. method:: _CData.from_buffer_copy(source[, offset])

      This method creates a ctypes instance, copying the buffer from the
      *source* object buffer which must be readable.  The optional *offset*
      parameter specifies an offset into the source buffer in bytes; the default
      is zero.  If the source buffer is not large enough a :exc:`ValueError` is
      raised.

      .. audit-event:: ctypes.cdata/buffer pointer,size,offset ctypes._CData.from_buffer_copy

   .. method:: from_address(address)

      This method returns a ctypes type instance using the memory specified by
      *address* which must be an integer.

      .. audit-event:: ctypes.cdata address ctypes._CData.from_address

         This method, and others that indirectly call this method, raises an
         :ref:`auditing event <auditing>` ``ctypes.cdata`` with argument
         ``address``.

   .. method:: from_param(obj)

      This method adapts *obj* to a ctypes type.  It is called with the actual
      object used in a foreign function call when the type is present in the
      foreign function's :attr:`~_CFuncPtr.argtypes` tuple;
      it must return an object that can be used as a function call parameter.

      All ctypes data types have a default implementation of this classmethod
      that normally returns *obj* if that is an instance of the type.  Some
      types accept other objects as well.

   .. method:: in_dll(library, name)

      This method returns a ctypes type instance exported by a shared
      library. *name* is the name of the symbol that exports the data, *library*
      is the loaded shared library.

   Common class variables of ctypes data types:

   .. attribute:: __pointer_type__

      The pointer type that was created by calling
      :func:`POINTER` for corresponding ctypes data type. If a pointer type
      was not yet created, the attribute is missing.

      .. versionadded:: 3.14

   Common instance variables of ctypes data types:

   .. attribute:: _b_base_

      Sometimes ctypes data instances do not own the memory block they contain,
      instead they share part of the memory block of a base object.  The
      :attr:`_b_base_` read-only member is the root ctypes object that owns the
      memory block.

   .. attribute:: _b_needsfree_

      This read-only variable is true when the ctypes data instance has
      allocated the memory block itself, false otherwise.

   .. attribute:: _objects

      This member is either ``None`` or a dictionary containing Python objects
      that need to be kept alive so that the memory block contents is kept
      valid.  This object is only exposed for debugging; never modify the
      contents of this dictionary.


.. _ctypes-fundamental-data-types-2:

Fundamental data types
^^^^^^^^^^^^^^^^^^^^^^

.. class:: _SimpleCData

   This non-public class is the base class of all fundamental ctypes data
   types. It is mentioned here because it contains the common attributes of the
   fundamental ctypes data types.  :class:`_SimpleCData` is a subclass of
   :class:`_CData`, so it inherits their methods and attributes. ctypes data
   types that are not and do not contain pointers can now be pickled.

   Instances have a single attribute:

   .. attribute:: value

      This attribute contains the actual value of the instance. For integer and
      pointer types, it is an integer, for character types, it is a single
      character bytes object or string, for character pointer types it is a
      Python bytes object or string.

      When the ``value`` attribute is retrieved from a ctypes instance, usually
      a new object is returned each time.  :mod:`!ctypes` does *not* implement
      original object return, always a new object is constructed.  The same is
      true for all other ctypes object instances.

   Each subclass has a class attribute:

   .. attribute:: _type_

      Class attribute that contains an internal type code, as a
      single-character string.
      See :ref:`ctypes-fundamental-data-types` for a summary.

      Types marked \* in the summary may be (or always are) aliases of a
      different :class:`_SimpleCData` subclass, and will not necessarily
      use the listed type code.
      For example, if the platform's :c:expr:`long`, :c:expr:`long long`
      and :c:expr:`time_t` C types are the same, then :class:`c_long`,
      :class:`c_longlong` and :class:`c_time_t` all refer to a single class,
      :class:`c_long`, whose :attr:`_type_` code is ``'l'``.
      The ``'L'`` code will be unused.

      .. seealso::

         The :mod:`array` and :ref:`struct <format-characters>` modules,
         as well as third-party modules like `numpy <https://numpy.org/doc/stable/reference/arrays.interface.html#object.__array_interface__>`__,
         use similar -- but slightly different -- type codes.


Fundamental data types, when returned as foreign function call results, or, for
example, by retrieving structure field members or array items, are transparently
converted to native Python types.  In other words, if a foreign function has a
:attr:`~_CFuncPtr.restype` of :class:`c_char_p`, you will always receive a Python bytes
object, *not* a :class:`c_char_p` instance.

.. XXX above is false, it actually returns a Unicode string

Subclasses of fundamental data types do *not* inherit this behavior. So, if a
foreign functions :attr:`!restype` is a subclass of :class:`c_void_p`, you will
receive an instance of this subclass from the function call. Of course, you can
get the value of the pointer by accessing the ``value`` attribute.

These are the fundamental ctypes data types:

.. class:: c_byte

   Represents the C :c:expr:`signed char` datatype, and interprets the value as
   small integer.  The constructor accepts an optional integer initializer; no
   overflow checking is done.


.. class:: c_char

   Represents the C :c:expr:`char` datatype, and interprets the value as a single
   character.  The constructor accepts an optional string initializer, the
   length of the string must be exactly one character.


.. class:: c_char_p

   Represents the C :c:expr:`char *` datatype when it points to a zero-terminated
   string.  For a general character pointer that may also point to binary data,
   ``POINTER(c_char)`` must be used.  The constructor accepts an integer
   address, or a bytes object.


.. class:: c_double

   Represents the C :c:expr:`double` datatype.  The constructor accepts an
   optional float initializer.


.. class:: c_longdouble

   Represents the C :c:expr:`long double` datatype.  The constructor accepts an
   optional float initializer.  On platforms where ``sizeof(long double) ==
   sizeof(double)`` it is an alias to :class:`c_double`.

.. class:: c_float

   Represents the C :c:expr:`float` datatype.  The constructor accepts an
   optional float initializer.


.. class:: c_double_complex

   Represents the C :c:expr:`double complex` datatype, if available.  The
   constructor accepts an optional :class:`complex` initializer.

   .. versionadded:: 3.14


.. class:: c_float_complex

   Represents the C :c:expr:`float complex` datatype, if available.  The
   constructor accepts an optional :class:`complex` initializer.

   .. versionadded:: 3.14


.. class:: c_longdouble_complex

   Represents the C :c:expr:`long double complex` datatype, if available.  The
   constructor accepts an optional :class:`complex` initializer.

   .. versionadded:: 3.14


.. class:: c_int

   Represents the C :c:expr:`signed int` datatype.  The constructor accepts an
   optional integer initializer; no overflow checking is done.  On platforms
   where ``sizeof(int) == sizeof(long)`` it is an alias to :class:`c_long`.


.. class:: c_int8

   Represents the C 8-bit :c:expr:`signed int` datatype.  It is an alias for
   :class:`c_byte`.


.. class:: c_int16

   Represents the C 16-bit :c:expr:`signed int` datatype.  Usually an alias for
   :class:`c_short`.


.. class:: c_int32

   Represents the C 32-bit :c:expr:`signed int` datatype.  Usually an alias for
   :class:`c_int`.


.. class:: c_int64

   Represents the C 64-bit :c:expr:`signed int` datatype.  Usually an alias for
   :class:`c_longlong`.


.. class:: c_long

   Represents the C :c:expr:`signed long` datatype.  The constructor accepts an
   optional integer initializer; no overflow checking is done.


.. class:: c_longlong

   Represents the C :c:expr:`signed long long` datatype.  The constructor accepts
   an optional integer initializer; no overflow checking is done.
   On platforms where ``sizeof(long long) == sizeof(long)`` it is an alias
   to :class:`c_long`.


.. class:: c_short

   Represents the C :c:expr:`signed short` datatype.  The constructor accepts an
   optional integer initializer; no overflow checking is done.


.. class:: c_size_t

   Represents the C :c:type:`size_t` datatype.
   Usually an alias for another unsigned integer type.


.. class:: c_ssize_t

   Represents the :c:type:`Py_ssize_t` datatype.
   This is a signed version of :c:type:`size_t`;
   that is, the POSIX :c:type:`ssize_t` type.
   Usually an alias for another integer type.

   .. versionadded:: 3.2


.. class:: c_time_t

   Represents the C :c:type:`time_t` datatype.
   Usually an alias for another integer type.

   .. versionadded:: 3.12


.. class:: c_ubyte

   Represents the C :c:expr:`unsigned char` datatype, it interprets the value as
   small integer.  The constructor accepts an optional integer initializer; no
   overflow checking is done.


.. class:: c_uint

   Represents the C :c:expr:`unsigned int` datatype.  The constructor accepts an
   optional integer initializer; no overflow checking is done.  On platforms
   where ``sizeof(int) == sizeof(long)`` it is an alias for :class:`c_ulong`.


.. class:: c_uint8

   Represents the C 8-bit :c:expr:`unsigned int` datatype.  It is an alias for
   :class:`c_ubyte`.


.. class:: c_uint16

   Represents the C 16-bit :c:expr:`unsigned int` datatype.  Usually an alias for
   :class:`c_ushort`.


.. class:: c_uint32

   Represents the C 32-bit :c:expr:`unsigned int` datatype.  Usually an alias for
   :class:`c_uint`.


.. class:: c_uint64

   Represents the C 64-bit :c:expr:`unsigned int` datatype.  Usually an alias for
   :class:`c_ulonglong`.


.. class:: c_ulong

   Represents the C :c:expr:`unsigned long` datatype.  The constructor accepts an
   optional integer initializer; no overflow checking is done.


.. class:: c_ulonglong

   Represents the C :c:expr:`unsigned long long` datatype.  The constructor
   accepts an optional integer initializer; no overflow checking is done.
   On platforms where ``sizeof(long long) == sizeof(long)`` it is an alias
   to :class:`c_long`.


.. class:: c_ushort

   Represents the C :c:expr:`unsigned short` datatype.  The constructor accepts
   an optional integer initializer; no overflow checking is done.


.. class:: c_void_p

   Represents the C :c:expr:`void *` type.  The value is represented as integer.
   The constructor accepts an optional integer initializer.


.. class:: c_wchar

   Represents the C :c:type:`wchar_t` datatype, and interprets the value as a
   single character unicode string.  The constructor accepts an optional string
   initializer, the length of the string must be exactly one character.


.. class:: c_wchar_p

   Represents the C :c:expr:`wchar_t *` datatype, which must be a pointer to a
   zero-terminated wide character string.  The constructor accepts an integer
   address, or a string.


.. class:: c_bool

   Represent the C :c:expr:`bool` datatype (more accurately, :c:expr:`_Bool` from
   C99).  Its value can be ``True`` or ``False``, and the constructor accepts any object
   that has a truth value.


.. class:: HRESULT

   Represents a :c:type:`!HRESULT` value, which contains success or
   error information for a function or method call.

   .. availability:: Windows


.. class:: py_object

   Represents the C :c:expr:`PyObject *` datatype.  Calling this without an
   argument creates a ``NULL`` :c:expr:`PyObject *` pointer.

   .. versionchanged:: 3.14
      :class:`!py_object` is now a :term:`generic type`.

.. _ctypes-wintypes:

The :mod:`!ctypes.wintypes` module provides quite some other Windows specific
data types, for example :c:type:`!HWND`, :c:type:`!WPARAM`,
:c:type:`!VARIANT_BOOL` or :c:type:`!DWORD`.
Some useful structures like :c:type:`!MSG` or :c:type:`!RECT` are also defined.


.. _ctypes-structured-data-types:

Structured data types
^^^^^^^^^^^^^^^^^^^^^


.. class:: Union(*args, **kw)

   Abstract base class for unions in native byte order.

   Unions share common attributes and behavior with structures;
   see :class:`Structure` documentation for details.

.. class:: BigEndianUnion(*args, **kw)

   Abstract base class for unions in *big endian* byte order.

   .. versionadded:: 3.11

.. class:: LittleEndianUnion(*args, **kw)

   Abstract base class for unions in *little endian* byte order.

   .. versionadded:: 3.11

.. class:: BigEndianStructure(*args, **kw)

   Abstract base class for structures in *big endian* byte order.


.. class:: LittleEndianStructure(*args, **kw)

   Abstract base class for structures in *little endian* byte order.

Structures and unions with non-native byte order cannot contain pointer type
fields, or any other data types containing pointer type fields.


.. class:: Structure(*args, **kw)

   Abstract base class for structures in *native* byte order.

   Concrete structure and union types must be created by subclassing one of these
   types, and at least define a :attr:`_fields_` class variable. :mod:`!ctypes` will
   create :term:`descriptor`\s which allow reading and writing the fields by direct
   attribute accesses.  These are the


   .. attribute:: _fields_

      A sequence defining the structure fields.  The items must be 2-tuples or
      3-tuples.  The first item is the name of the field, the second item
      specifies the type of the field; it can be any ctypes data type.

      For integer type fields like :class:`c_int`, a third optional item can be
      given.  It must be a small positive integer defining the bit width of the
      field.

      Field names must be unique within one structure or union.  This is not
      checked, only one field can be accessed when names are repeated.

      It is possible to define the :attr:`_fields_` class variable *after* the
      class statement that defines the Structure subclass, this allows creating
      data types that directly or indirectly reference themselves::

         class List(Structure):
             pass
         List._fields_ = [("pnext", POINTER(List)),
                          ...
                         ]

      The :attr:`!_fields_` class variable can only be set once.
      Later assignments will raise an :exc:`AttributeError`.

      Additionally, the :attr:`!_fields_` class variable must be defined before
      the structure or union type is first used: an instance or subclass is
      created, :func:`sizeof` is called on it, and so on.
      Later assignments to :attr:`!_fields_` will raise an :exc:`AttributeError`.
      If :attr:`!_fields_` has not been set before such use,
      the structure or union will have no own fields, as if :attr:`!_fields_`
      was empty.

      Sub-subclasses of structure types inherit the fields of the base class
      plus the :attr:`_fields_` defined in the sub-subclass, if any.


   .. attribute:: _pack_

      An optional small integer that allows overriding the alignment of
      structure fields in the instance.

      This is only implemented for the MSVC-compatible memory layout
      (see :attr:`_layout_`).

      Setting :attr:`!_pack_` to 0 is the same as not setting it at all.
      Otherwise, the value must be a positive power of two.
      The effect is equivalent to ``#pragma pack(N)`` in C, except
      :mod:`!ctypes` may allow larger *n* than what the compiler accepts.

      :attr:`!_pack_` must already be defined
      when :attr:`_fields_` is assigned, otherwise it will have no effect.

      .. deprecated-removed:: 3.14 3.19

         For historical reasons, if :attr:`!_pack_` is non-zero,
         the MSVC-compatible layout will be used by default.
         On non-Windows platforms, this default is deprecated and is slated to
         become an error in Python 3.19.
         If it is intended, set :attr:`~Structure._layout_` to ``'ms'``
         explicitly.

   .. attribute:: _align_

      An optional small integer that allows increasing the alignment of
      the structure when being packed or unpacked to/from memory.

      The value must not be negative.
      The effect is equivalent to ``__attribute__((aligned(N)))`` on GCC
      or ``#pragma align(N)`` on MSVC, except :mod:`!ctypes` may allow
      values that the compiler would reject.

      :attr:`!_align_` can only *increase* a structure's alignment
      requirements. Setting it to 0 or 1 has no effect.

      Using values that are not powers of two is discouraged and may lead to
      surprising behavior.

      :attr:`!_align_` must already be defined
      when :attr:`_fields_` is assigned, otherwise it will have no effect.

      .. versionadded:: 3.13

   .. attribute:: _layout_

      An optional string naming the struct/union layout. It can currently
      be set to:

      - ``"ms"``: the layout used by the Microsoft compiler (MSVC).
        On GCC and Clang, this layout can be selected with
        ``__attribute__((ms_struct))``.
      - ``"gcc-sysv"``: the layout used by GCC with the System V or “SysV-like”
        data model, as used on Linux and macOS.
        With this layout, :attr:`~Structure._pack_` must be unset or zero.

      If not set explicitly, ``ctypes`` will use a default that
      matches the platform conventions. This default may change in future
      Python releases (for example, when a new platform gains official support,
      or when a difference between similar platforms is found).
      Currently the default will be:

      - On Windows: ``"ms"``
      - When :attr:`~Structure._pack_` is specified: ``"ms"``.
        (This is deprecated; see :attr:`~Structure._pack_` documentation.)
      - Otherwise: ``"gcc-sysv"``

      :attr:`!_layout_` must already be defined when
      :attr:`~Structure._fields_` is assigned, otherwise it will have no effect.

      .. versionadded:: 3.14

   .. attribute:: _anonymous_

      An optional sequence that lists the names of unnamed (anonymous) fields.
      :attr:`_anonymous_` must be already defined when :attr:`_fields_` is
      assigned, otherwise it will have no effect.

      The fields listed in this variable must be structure or union type fields.
      :mod:`!ctypes` will create descriptors in the structure type that allows
      accessing the nested fields directly, without the need to create the
      structure or union field.

      Here is an example type (Windows)::

         class _U(Union):
             _fields_ = [("lptdesc", POINTER(TYPEDESC)),
                         ("lpadesc", POINTER(ARRAYDESC)),
                         ("hreftype", HREFTYPE)]

         class TYPEDESC(Structure):
             _anonymous_ = ("u",)
             _fields_ = [("u", _U),
                         ("vt", VARTYPE)]


      The ``TYPEDESC`` structure describes a COM data type, the ``vt`` field
      specifies which one of the union fields is valid.  Since the ``u`` field
      is defined as anonymous field, it is now possible to access the members
      directly off the TYPEDESC instance. ``td.lptdesc`` and ``td.u.lptdesc``
      are equivalent, but the former is faster since it does not need to create
      a temporary union instance::

         td = TYPEDESC()
         td.vt = VT_PTR
         td.lptdesc = POINTER(some_type)
         td.u.lptdesc = POINTER(some_type)

   It is possible to define sub-subclasses of structures, they inherit the
   fields of the base class.  If the subclass definition has a separate
   :attr:`_fields_` variable, the fields specified in this are appended to the
   fields of the base class.

   Structure and union constructors accept both positional and keyword
   arguments.  Positional arguments are used to initialize member fields in the
   same order as they are appear in :attr:`_fields_`.  Keyword arguments in the
   constructor are interpreted as attribute assignments, so they will initialize
   :attr:`_fields_` with the same name, or create new attributes for names not
   present in :attr:`_fields_`.


.. class:: CField(*args, **kw)

   Descriptor for fields of a :class:`Structure` and :class:`Union`.
   For example::

      >>> class Color(Structure):
      ...     _fields_ = (
      ...         ('red', c_uint8),
      ...         ('green', c_uint8),
      ...         ('blue', c_uint8),
      ...         ('intense', c_bool, 1),
      ...         ('blinking', c_bool, 1),
      ...    )
      ...
      >>> Color.red
      <ctypes.CField 'red' type=c_ubyte, ofs=0, size=1>
      >>> Color.green.type
      <class 'ctypes.c_ubyte'>
      >>> Color.blue.byte_offset
      2
      >>> Color.intense
      <ctypes.CField 'intense' type=c_bool, ofs=3, bit_size=1, bit_offset=0>
      >>> Color.blinking.bit_offset
      1

   All attributes are read-only.

   :class:`!CField` objects are created via :attr:`~Structure._fields_`;
   do not instantiate the class directly.

   .. versionadded:: 3.14

      Previously, descriptors only had ``offset`` and ``size`` attributes
      and a readable string representation; the :class:`!CField` class was not
      available directly.

   .. attribute:: name

      Name of the field, as a string.

   .. attribute:: type

      Type of the field, as a :ref:`ctypes class <ctypes-data-types>`.

   .. attribute:: offset
                  byte_offset

      Offset of the field, in bytes.

      For bitfields, this is the offset of the underlying byte-aligned
      *storage unit*; see :attr:`~CField.bit_offset`.

   .. attribute:: byte_size

      Size of the field, in bytes.

      For bitfields, this is the size of the underlying *storage unit*.
      Typically, it has the same size as the bitfield's type.

   .. attribute:: size

      For non-bitfields, equivalent to :attr:`~CField.byte_size`.

      For bitfields, this contains a backwards-compatible bit-packed
      value that combines :attr:`~CField.bit_size` and
      :attr:`~CField.bit_offset`.
      Prefer using the explicit attributes instead.

   .. attribute:: is_bitfield

      True if this is a bitfield.

   .. attribute:: bit_offset
                  bit_size

      The location of a bitfield within its *storage unit*, that is, within
      :attr:`~CField.byte_size` bytes of memory starting at
      :attr:`~CField.byte_offset`.

      To get the field's value, read the storage unit as an integer,
      :ref:`shift left <shifting>` by :attr:`!bit_offset` and
      take the :attr:`!bit_size` least significant bits.

      For non-bitfields, :attr:`!bit_offset` is zero
      and :attr:`!bit_size` is equal to ``byte_size * 8``.

   .. attribute:: is_anonymous

      True if this field is anonymous, that is, it contains nested sub-fields
      that should be merged into a containing structure or union.


.. _ctypes-arrays-pointers:

Arrays and pointers
^^^^^^^^^^^^^^^^^^^

.. class:: Array(*args)

   Abstract base class for arrays.

   The recommended way to create concrete array types is by multiplying any
   :mod:`!ctypes` data type with a non-negative integer.  Alternatively, you can subclass
   this type and define :attr:`_length_` and :attr:`_type_` class variables.
   Array elements can be read and written using standard
   subscript and slice accesses; for slice reads, the resulting object is
   *not* itself an :class:`Array`.

   Arrays are :ref:`generic <generics>` over the type of their elements.


   .. attribute:: _length_

        A positive integer specifying the number of elements in the array.
        Out-of-range subscripts result in an :exc:`IndexError`. Will be
        returned by :func:`len`.


   .. attribute:: _type_

        Specifies the type of each element in the array.


   Array subclass constructors accept positional arguments, used to
   initialize the elements in order.

.. function:: ARRAY(type, length)

   Create an array.
   Equivalent to ``type * length``, where *type* is a
   :mod:`!ctypes` data type and *length* an integer.

   This function is :term:`soft deprecated` in favor of multiplication.
   There are no plans to remove it.


.. class:: _Pointer

   Private, abstract base class for pointers.

   Concrete pointer types are created by calling :func:`POINTER` with the
   type that will be pointed to; this is done automatically by
   :func:`pointer`.

   If a pointer points to an array, its elements can be read and
   written using standard subscript and slice accesses.  Pointer objects
   have no size, so :func:`len` will raise :exc:`TypeError`.  Negative
   subscripts will read from the memory *before* the pointer (as in C), and
   out-of-range subscripts will probably crash with an access violation (if
   you're lucky).


   .. attribute:: _type_

        Specifies the type pointed to.

   .. attribute:: contents

        Returns the object to which to pointer points.  Assigning to this
        attribute changes the pointer to point to the assigned object.


.. _ctypes-exceptions:

Exceptions
^^^^^^^^^^

.. exception:: ArgumentError

   This exception is raised when a foreign function call cannot convert one of the
   passed arguments.


.. exception:: COMError(hresult, text, details)

   This exception is raised when a COM method call failed.

   .. attribute:: hresult

      The integer value representing the error code.

   .. attribute:: text

      The error message.

   .. attribute:: details

      The 5-tuple ``(descr, source, helpfile, helpcontext, progid)``.

      *descr* is the textual description.  *source* is the language-dependent
      ``ProgID`` for the class or application that raised the error.  *helpfile*
      is the path of the help file.  *helpcontext* is the help context
      identifier.  *progid* is the ``ProgID`` of the interface that defined the
      error.

   .. availability:: Windows

   .. versionadded:: 3.14
