Doc/library/ctypes.rst

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

.. module:: ctypes
   :synopsis: A foreign function library for Python.
.. moduleauthor:: Thomas Heller <theller@python.net>


: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.


.. _ctypes-ctypes-tutorial:

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

Note: The code samples in this tutorial use :mod:`doctest` to make sure that
they actually work.  Since some code samples behave differently under Linux,
Windows, or Mac OS X, they contain doctest directives in comments.

Note: Some code samples reference the ctypes :class:`c_int` type. This type is
an alias for the :class:`c_long` type on 32-bit systems.  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 *cdll*, and on Windows *windll* and *oledll*
objects, for loading dynamic link libraries.

You load libraries by accessing them as attributes of these objects. *cdll*
loads libraries which export functions using the standard ``cdecl`` calling
convention, while *windll* libraries call functions using the ``stdcall``
calling convention. *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 a :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.

On Linux, it is required to specify the filename *including* the extension to
load a library, so attribute access can not be used to load libraries. Either the
:meth:`LoadLibrary` method of the dll loaders should be used, or you should load
the library by creating an instance of CDLL by calling the constructor::

   >>> 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 ...>
   >>>

.. XXX Add section for Mac OS X.


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

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

Functions are accessed as attributes of dll objects::

   >>> from ctypes import *
   >>> 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 ?
     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
an ``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 ?
     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 ``time()`` function, which returns system time in seconds since the Unix
epoch, and the ``GetModuleHandleA()`` function, which returns a win32 module
handle.

This example calls both functions with a NULL pointer (``None`` should be used
as the NULL pointer)::

   >>> print(libc.time(None)) # doctest: +SKIP
   1150640792
   >>> print(hex(windll.kernel32.GetModuleHandleA(None))) # doctest: +WINDOWS
   0x1d000000
   >>>

:mod:`ctypes` tries to protect you from calling functions with the wrong number
of arguments or the wrong calling convention.  Unfortunately this only works on
Windows.  It does this by examining the stack after the function returns, so
although an error is raised the function *has* been called::

   >>> windll.kernel32.GetModuleHandleA() # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ValueError: Procedure probably called with not enough arguments (4 bytes missing)
   >>> windll.kernel32.GetModuleHandleA(0, 0) # doctest: +WINDOWS
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ValueError: Procedure probably called with too many arguments (4 bytes in excess)
   >>>

The same exception 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 ?
   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 ?
   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 ?
   OSError: exception: access violation reading 0x00000020
   >>>

There are, however, enough ways to crash Python with :mod:`ctypes`, so you
should be careful anyway.  The :mod:`faulthandler` module can be helpful in
debugging crashes (e.g. from 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:type:`char *` or
:c:type:`wchar_t *`).  Python integers are passed as the platforms default C
:c:type:`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 :

+----------------------+------------------------------------------+----------------------------+
| ctypes type          | C type                                   | Python type                |
+======================+==========================================+============================+
| :class:`c_bool`      | :c:type:`_Bool`                          | bool (1)                   |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_char`      | :c:type:`char`                           | 1-character bytes object   |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_wchar`     | :c:type:`wchar_t`                        | 1-character string         |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_byte`      | :c:type:`char`                           | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_ubyte`     | :c:type:`unsigned char`                  | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_short`     | :c:type:`short`                          | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_ushort`    | :c:type:`unsigned short`                 | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_int`       | :c:type:`int`                            | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_uint`      | :c:type:`unsigned int`                   | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_long`      | :c:type:`long`                           | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_ulong`     | :c:type:`unsigned long`                  | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_longlong`  | :c:type:`__int64` or :c:type:`long long` | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_ulonglong` | :c:type:`unsigned __int64` or            | int                        |
|                      | :c:type:`unsigned long long`             |                            |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_size_t`    | :c:type:`size_t`                         | int                        |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_ssize_t`   | :c:type:`ssize_t` or                     | int                        |
|                      | :c:type:`Py_ssize_t`                     |                            |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_float`     | :c:type:`float`                          | float                      |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_double`    | :c:type:`double`                         | float                      |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_longdouble`| :c:type:`long double`                    | float                      |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_char_p`    | :c:type:`char *` (NUL terminated)        | bytes object or ``None``   |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_wchar_p`   | :c:type:`wchar_t *` (NUL terminated)     | string or ``None``         |
+----------------------+------------------------------------------+----------------------------+
| :class:`c_void_p`    | :c:type:`void *`                         | int or ``None``            |
+----------------------+------------------------------------------+----------------------------+

(1)
   The constructor accepts any object with a truth value.

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('Hello, World')
   >>> c_ushort(-3)
   c_ushort(65533)
   >>>

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
bytes objects are immutable)::

   >>> s = "Hello, World"
   >>> c_s = c_wchar_p(s)
   >>> print(c_s)
   c_wchar_p('Hello, World')
   >>> c_s.value = "Hi, there"
   >>> print(c_s)
   c_wchar_p('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 :func:`c_buffer` function
(which is still available as an alias), as well as the :func:`c_string` function
from earlier ctypes releases.  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 ?
   ArgumentError: argument 2: exceptions.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-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.  Of
course, it must be one of integer, string, or bytes::

   >>> 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:`argtypes` attribute.

:attr:`argtypes` must be a sequence of C data types (the ``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 ?
   ArgumentError: argument 2: exceptions.TypeError: wrong type
   >>> 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:`from_param` class method for them to be able to use them
in the :attr:`argtypes` sequence. The :meth:`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
^^^^^^^^^^^^

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

Here is a more advanced example, it uses the ``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 ``ord("x")`` calls above, you can set the
:attr:`argtypes` attribute, and the second argument will be converted from a
single character Python bytes object into a C char::

   >>> strchr.restype = c_char_p
   >>> strchr.argtypes = [c_char_p, c_char]
   >>> strchr(b"abcdef", b"d")
   'def'
   >>> strchr(b"abcdef", b"def")
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ArgumentError: argument 2: exceptions.TypeError: one character string expected
   >>> print(strchr(b"abcdef", b"x"))
   None
   >>> strchr(b"abcdef", b"d")
   'def'
   >>>

You can also use a callable Python object (a function or a class for example) as
the :attr:`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 ?
     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:`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:`_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 ?
   ValueError: 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::

   >>> print(POINT.x)
   <Field type=c_long, ofs=0, size=4>
   >>> print(POINT.y)
   <Field type=c_long, 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 alignment and byte order
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By default, Structure and Union fields are aligned in the same way the C
compiler does it. It is possible to override this behavior be specifying a
:attr:`_pack_` class attribute in the subclass definition. This must be set to a
positive integer and specifies the maximum alignment for the fields. This is
what ``#pragma pack(n)`` also does in MSVC.

: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:`_fields_` tuples::

   >>> class Int(Structure):
   ...     _fields_ = [("first_16", c_int, 16),
   ...                 ("second_16", c_int, 16)]
   ...
   >>> print(Int.first_16)
   <Field type=c_long, ofs=0:0, bits=16>
   >>> print(Int.second_16)
   <Field type=c_long, ofs=0:16, bits=16>
   >>>


.. _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 an 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:`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 ?
   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-type-conversions:

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

Usually, ctypes does strict type checking.  This means, if you have
``POINTER(c_int)`` in the :attr:`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:`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 ?
   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 ?
     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:`_fields_`
attribute later, after the class statement::

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

Lets 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 = "foo"
   >>> c2 = cell()
   >>> c2.name = "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 to create 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
   >>>

**Important note for callback functions:**

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.


.. _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_OptimizeFlag`, an integer
set to 0, 1, or 2, depending on the :option:`-O` or :option:`-OO` flag given on
startup.

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

   >>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag")
   >>> print(opt_flag)
   c_long(0)
   >>>

If the interpreter would have been started with :option:`-O`, the sample would
have printed ``c_long(1)``, or ``c_long(2)`` if :option:`-OO` would have been
specified.

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:type:`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)]
   ...
   >>>

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

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

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:
   ...    print(item.name, item.size)
   ...    if item.name is None:
   ...        break
   ...
   __hello__ 104
   __phello__ -104
   __phello__.spam 104
   None 0
   >>>

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 different from what one would expect is this::

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

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:`find_library` function is to locate a library in a way
similar to what the compiler 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:`find_library` tries to run external programs
(``/sbin/ldconfig``, ``gcc``, and ``objdump``) to find the library file.  It
returns the filename of the library file.  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 OS X, :func:`find_library` tries several predefined 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:`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:`find_library` to locate the library at runtime.


.. _ctypes-loading-shared-libraries:

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

There are several ways to loaded shared libraries into the Python process.  One
way is to instantiate one of the following classes:


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

   Instances of this class represent loaded shared libraries. Functions in these
   libraries use the standard C calling convention, and are assumed to return
   :c:type:`int`.


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

   Windows only: Instances of this class represent loaded shared libraries,
   functions in these libraries 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.

   .. versionchanged:: 3.3
      :exc:`WindowsError` used to be raised.


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

   Windows only: Instances of this class represent loaded shared libraries,
   functions in these libraries use the ``stdcall`` calling convention, and are
   assumed to return :c:type:`int` by default.

   On Windows CE only the standard calling convention is used, for convenience the
   :class:`WinDLL` and :class:`OleDLL` use the standard calling convention on this
   platform.

The Python :term:`global interpreter lock` is released before calling any
function exported by these libraries, and reacquired afterwards.


.. class:: PyDLL(name, mode=DEFAULT_MODE, handle=None)

   Instances of this class behave like :class:`CDLL` instances, except that 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.

All these classes can be instantiated by calling them with at least one
argument, the pathname of the shared library.  If you have an existing handle to
an already loaded shared library, it can be passed as the ``handle`` named
parameter, otherwise the underlying platforms ``dlopen`` or ``LoadLibrary``
function is used to load the library into the process, and to get a handle to
it.

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.

The *use_errno* parameter, when set to True, enables a ctypes mechanism that
allows to access the system :data:`errno` error number in a safe way.
:mod:`ctypes` maintains a thread-local copy of the systems :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.

.. data:: RTLD_GLOBAL
   :noindex:

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


.. data:: RTLD_LOCAL
   :noindex:

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


.. data:: DEFAULT_MODE
   :noindex:

   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*.

Instances of these classes have no public methods, however :meth:`__getattr__`
and :meth:`__getitem__` have special behavior: functions exported by the shared
library can be accessed as attributes of by index.  Please note that both
:meth:`__getattr__` and :meth:`__getitem__` cache their result, so calling them
repeatedly returns the same object each time.

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


.. attribute:: PyDLL._handle

   The system handle used to access the library.


.. attribute:: PyDLL._name

   The name of the library passed in the constructor.

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:`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 to load 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
   :noindex:

   Creates :class:`CDLL` instances.


.. data:: windll
   :noindex:

   Windows only: Creates :class:`WinDLL` instances.


.. data:: oledll
   :noindex:

   Windows only: Creates :class:`OleDLL` instances.


.. data:: pydll
   :noindex:

   Creates :class:`PyDLL` instances.


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

.. data:: pythonapi
   :noindex:

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


.. _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 class:


.. class:: _FuncPtr

   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:type:`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:type:`int`, and
      the callable will be called with this integer, allowing to do 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:`from_param` class method of the items in the :attr:`argtypes`
      tuple, this method allows to adapt 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:`from_param` method which returns a
      value usable as argument (integer, string, ctypes instance).  This allows
      to define 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 to reuse 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 to specialize 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.


.. exception:: ArgumentError

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


.. 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)

   Windows only: The returned function prototype creates functions that use the
   ``stdcall`` calling convention, except on Windows CE where
   :func:`WINFUNCTYPE` is the same as :func:`CFUNCTYPE`.  The function will
   release the GIL during the call.  *use_errno* and *use_last_error* have the
   same meaning as above.


.. 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.

      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.

   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:`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.

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

   WINUSERAPI int WINAPI
   MessageBoxA(
       HWND hWnd ,
       LPCSTR lpText,
       LPCSTR lpCaption,
       UINT uType);

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

   >>> from ctypes import c_int, WINFUNCTYPE, windll
   >>> from ctypes.wintypes import HWND, LPCSTR, UINT
   >>> prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
   >>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", None), (1, "flags", 0)
   >>> MessageBox = prototype(("MessageBoxA", 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:`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:`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.


.. 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:: 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_or_size, size=None)

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

   *init_or_size* must be an integer which specifies the size of the array, or a
   bytes object which will be used to initialize the array items.

   If a bytes object is specified as first argument, the buffer is made one item
   larger than its length so that the last element in the array is a NUL
   termination character. An integer can be passed as second argument which allows
   to specify the size of the array if the length of the bytes should not be used.



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

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

   *init_or_size* must be an integer which specifies the size of the array, or a
   string which will be used to initialize the array items.

   If a string is specified as first argument, the buffer is made one item
   larger than the length of the string so that the last element in the array is a
   NUL termination character. An integer can be passed as second argument which
   allows to specify the size of the array if the length of the string should not
   be used.



.. function:: DllCanUnloadNow()

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


.. function:: DllGetClassObject()

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


.. 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.


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

   Windows only: return the filename of the VC runtype 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.


.. function:: FormatError([code])

   Windows only: 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 GetLastError.


.. function:: GetLastError()

   Windows only: 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.

.. function:: get_errno()

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

.. function:: get_last_error()

   Windows only: returns the current value of the ctypes-private copy of the system
   :data:`LastError` variable in the calling thread.

.. 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)

   This factory function creates and returns a new ctypes pointer type. Pointer
   types are cached an reused internally, so calling this function repeatedly is
   cheap. *type* must be a ctypes type.


.. function:: pointer(obj)

   This function creates 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.



.. function:: set_last_error(value)

   Windows only: set 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.



.. 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(address, size=-1)

   This function returns the C string starting at memory address *address* as a bytes
   object. If size is specified, it is used as size, otherwise the string is assumed
   to be zero-terminated.


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

   Windows only: this function is probably the worst-named thing in ctypes. It
   creates an instance of OSError.  If *code* is not specified,
   ``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.

   .. versionchanged:: 3.3
      An instance of :exc:`WindowsError` used to be created.


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

   This function returns the wide character string starting at memory address
   *address* as a string.  If *size* is specified, it is used as the number of
   characters of the string, otherwise the string is assumed to be
   zero-terminated.


.. _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.


   .. 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.

   .. method:: from_address(address)

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

   .. 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:`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 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.


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:`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:type:`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:type:`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:type:`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:type:`double` datatype.  The constructor accepts an
   optional float initializer.


.. class:: c_longdouble

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


.. class:: c_int

   Represents the C :c:type:`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:type:`signed int` datatype.  Usually an alias for
   :class:`c_byte`.


.. class:: c_int16

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


.. class:: c_int32

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


.. class:: c_int64

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


.. class:: c_long

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


.. class:: c_longlong

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


.. class:: c_short

   Represents the C :c:type:`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.


.. class:: c_ssize_t

   Represents the C :c:type:`ssize_t` datatype.

   .. versionadded:: 3.2


.. class:: c_ubyte

   Represents the C :c:type:`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:type:`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:type:`unsigned int` datatype.  Usually an alias for
   :class:`c_ubyte`.


.. class:: c_uint16

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


.. class:: c_uint32

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


.. class:: c_uint64

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


.. class:: c_ulong

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


.. class:: c_ulonglong

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


.. class:: c_ushort

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


.. class:: c_void_p

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


.. class:: HRESULT

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


.. class:: py_object

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

The :mod:`ctypes.wintypes` module provides quite some other Windows specific
data types, for example :c:type:`HWND`, :c:type:`WPARAM`, 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.


.. 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 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 to create
      data types that directly or indirectly reference themselves::

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

      The :attr:`_fields_` class variable must, however, be defined before the
      type is first used (an instance is created, :func:`sizeof` is called on it,
      and so on).  Later assignments to the :attr:`_fields_` class variable will
      raise an AttributeError.

      Structure and union subclass constructors accept both positional and named
      arguments.  Positional arguments are used to initialize the fields in the
      same order as they appear in the :attr:`_fields_` definition, named
      arguments are used to initialize the fields with the corresponding name.

      It is possible to defined sub-subclasses of structure types, they 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 to override the alignment of
      structure fields in the instance.  :attr:`_pack_` must already be defined
      when :attr:`_fields_` is assigned, otherwise it will have no effect.


   .. 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 to
      access 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 defined 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_`.


.. _ctypes-arrays-pointers:

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

Not yet written - please see the sections :ref:`ctypes-pointers` and section
:ref:`ctypes-arrays` in the tutorial.