
``ctypes`` --- A foreign function library for Python
****************************************************

``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 tutorial
===============

Note: The code samples in this tutorial use ``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 ``c_int`` type. This type
is an alias for the ``c_long`` type on 32-bit systems.  So, you should
not be confused if ``c_long`` is printed if you would expect ``c_int``
--- they are actually the same type.


Loading dynamic link libraries
------------------------------

``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 ``HRESULT`` error code. The error code is used to
automatically raise a ``WindowsError`` exception when the function
call fails.

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 ``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 ...>
   >>>


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
``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
   >>>


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
   >>>

``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, ``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 ?
   WindowsError: exception: access violation reading 0x00000020
   >>>

There are, however, enough ways to crash Python with ``ctypes``, so
you should be careful anyway.

``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 (``char *`` or ``wchar_t *``).  Python integers
are passed as the platforms default C ``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 ``ctypes`` data types.


Fundamental data types
----------------------

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

+------------------------+------------------------------------------+------------------------------+
| ctypes type            | C type                                   | Python type                  |
+========================+==========================================+==============================+
| ``c_bool``             | ``_Bool``                                | bool (1)                     |
+------------------------+------------------------------------------+------------------------------+
| ``c_char``             | ``char``                                 | 1-character bytes object     |
+------------------------+------------------------------------------+------------------------------+
| ``c_wchar``            | ``wchar_t``                              | 1-character string           |
+------------------------+------------------------------------------+------------------------------+
| ``c_byte``             | ``char``                                 | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_ubyte``            | ``unsigned char``                        | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_short``            | ``short``                                | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_ushort``           | ``unsigned short``                       | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_int``              | ``int``                                  | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_uint``             | ``unsigned int``                         | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_long``             | ``long``                                 | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_ulong``            | ``unsigned long``                        | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_longlong``         | ``__int64`` or ``long long``             | int                          |
+------------------------+------------------------------------------+------------------------------+
| ``c_ulonglong``        | ``unsigned __int64`` or ``unsigned long  | int                          |
|                        | long``                                   |                              |
+------------------------+------------------------------------------+------------------------------+
| ``c_float``            | ``float``                                | float                        |
+------------------------+------------------------------------------+------------------------------+
| ``c_double``           | ``double``                               | float                        |
+------------------------+------------------------------------------+------------------------------+
| ``c_longdouble``       | ``long double``                          | float                        |
+------------------------+------------------------------------------+------------------------------+
| ``c_char_p``           | ``char *`` (NUL terminated)              | bytes object or ``None``     |
+------------------------+------------------------------------------+------------------------------+
| ``c_wchar_p``          | ``wchar_t *`` (NUL terminated)           | string or ``None``           |
+------------------------+------------------------------------------+------------------------------+
| ``c_void_p``           | ``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 ``c_char_p``,
``c_wchar_p``, and ``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 ``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 ``create_string_buffer()`` function replaces the ``c_buffer()``
function (which is still available as an alias), as well as the
``c_string()`` function from earlier ctypes releases.  To create a
mutable memory block containing unicode characters of the C type
``wchar_t`` use the ``create_unicode_buffer()`` function.


Calling functions, continued
----------------------------

Note that printf prints to the real standard output channel, *not* to
``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
``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
   >>>


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

You can also customize ``ctypes`` argument conversion to allow
instances of your own classes be used as function arguments.
``ctypes`` looks for an ``_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
``_as_parameter_`` instance variable, you could define a ``property``
which makes the attribute available on request.


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

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

``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 ``from_param()`` class method for them to be
able to use them in the ``argtypes`` sequence. The ``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
``_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 ``ctypes`` instance, or an object with an
``_as_parameter_`` attribute.


Return types
------------

By default functions are assumed to return the C ``int`` type.  Other
return types can be specified by setting the ``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
``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 ``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
   WindowsError: [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 ``GetLastError()`` to retrieve it.

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


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*.

``ctypes`` exports the ``byref()`` function which is used to pass
parameters by reference.  The same effect can be achieved with the
``pointer()`` function, although ``pointer()`` does a lot more work
since it constructs a real pointer object, so it is faster to use
``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'
   >>>


Structures and unions
---------------------

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

The field type must be a ``ctypes`` type like ``c_int``, or any other
derived ``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. Structures
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 *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>
   >>>


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 ``_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.

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


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 ``_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>
   >>>


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
   >>>


Pointers
--------

Pointer instances are created by calling the ``pointer()`` function on
a ``ctypes`` type:

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

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

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

Note that ``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 ``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)
   >>>

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 ``pointer()`` function does more than simply
create pointer instances, it has to create pointer *types* first. This
is done with the ``POINTER()`` function, which accepts any ``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
   >>>

``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
   >>>


Type conversions
----------------

Usually, ctypes does strict type checking.  This means, if you have
``POINTER(c_int)`` in the ``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
   >>>

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

   >>> bar.values = None
   >>>

Sometimes you have instances of incompatible types.  In C, you can
cast one type into another type.  ``ctypes`` provides a ``cast()``
function which can be used in the same way.  The ``Bar`` structure
defined above accepts ``POINTER(c_int)`` pointers or ``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 ``cast()`` function is handy.

The ``cast()`` function can be used to cast a ctypes instance into a
pointer to a different ctypes data type.  ``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, ``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
   >>>


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

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 ``ctypes``, we can define the ``cell`` class and set the
``_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
   >>>


Callback functions
------------------

``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 CFUNCTYPE factory function creates types for callback functions
using the normal cdecl calling convention, and, on Windows, the
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
``qsort()`` function, this is used to sort items with the help of a
callback function.  ``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
   >>>

``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 else.

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))
   >>>

For the first implementation of the callback function, we simply print
the arguments we get, and return 0 (incremental development ;-):

   >>> def py_cmp_func(a, b):
   ...     print("py_cmp_func", a, b)
   ...     return 0
   ...
   >>>

Create the C callable callback:

   >>> cmp_func = CMPFUNC(py_cmp_func)
   >>>

And we're ready to go:

   >>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
   >>>

We know how to access the contents of a pointer, so lets redefine our
callback:

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

Here is what we get on Windows:

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

It is funny to see that on linux the sort function seems to work much
more efficiently, it is doing less comparisons:

   >>> 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
   >>>

Ah, we're nearly done! The last step is to 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]
   ...
   >>>

Final run on Windows:

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

and on Linux:

   >>> 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
   >>>

It is quite interesting to see that the Windows ``qsort()`` function
needs more comparisons than the linux version!

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 CFUNCTYPE objects as long as they are
used from C code. ``ctypes`` doesn't, and if you don't, they may be
garbage collected, crashing your program when a callback is made.


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
``Py_OptimizeFlag``, an integer set to 0, 1, or 2, depending on the
*-O* or *-OO* flag given on startup.

``ctypes`` can access values like this with the ``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 *-O*, the sample would
have printed ``c_long(1)``, or ``c_long(2)`` if *-OO* would have been
specified.

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

Quoting the docs for that value:

   This pointer is initialized to point to an array of ``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 ``ctypes``:

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

We have defined the ``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.


Surprises
---------

There are some edges in ``ctypes`` where you may be expect something
else 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 *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!


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

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

The ``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 ``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 ``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 reference
================


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

ctypes.util.find_library(name)

   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 *-l*).  If no library can be found, returns ``None``.

The exact functionality is system dependent.

On Linux, ``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, ``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, ``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 ``ctypes``, it *may* be better to
determine the shared library name at development type, and hardcode
that into the wrapper module instead of using ``find_library()`` to
locate the library at runtime.


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 class ctypes.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 ``int``.

class class ctypes.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
   ``HRESULT`` code.  ``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 ``WindowsError`` is automatically raised.

class class ctypes.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 ``int`` by default.

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

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

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

   Instances of this class behave like ``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 *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 ``errno`` error number in a
safe way. ``ctypes`` maintains a thread-local copy of the systems
``errno`` variable; if you call foreign functions created with
``use_errno=True`` then the ``errno`` value before the function call
is swapped with the ctypes private copy, the same happens immediately
after the function call.

The function ``ctypes.get_errno()`` returns the value of the ctypes
private copy, and the function ``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
``GetLastError()`` and ``SetLastError()`` Windows API functions;
``ctypes.get_last_error()`` and ``ctypes.set_last_error()`` are used
to request and change the ctypes private copy of the windows error
code.

ctypes.RTLD_GLOBAL

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

ctypes.RTLD_LOCAL

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

ctypes.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*.

Instances of these classes have no public methods, however
``__getattr__()`` and ``__getitem__()`` have special behavior:
functions exported by the shared library can be accessed as attributes
of by index.  Please note that both ``__getattr__()`` and
``__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:

PyDLL._handle

   The system handle used to access the library.

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 ``LibraryLoader`` class, either by
calling the ``LoadLibrary()`` method, or by retrieving the library as
attribute of the loader instance.

class class ctypes.LibraryLoader(dlltype)

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

   ``__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.

   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:

ctypes.cdll

   Creates ``CDLL`` instances.

ctypes.windll

   Windows only: Creates ``WinDLL`` instances.

ctypes.oledll

   Windows only: Creates ``OleDLL`` instances.

ctypes.pydll

   Creates ``PyDLL`` instances.

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

ctypes.pythonapi

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

   restype

      Assign a ctypes type to specify the result type of the foreign
      function. Use ``None`` for ``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
      ``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 ``restype`` and assign a
      callable to the ``errcheck`` 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 ``from_param()`` class method of the items in the
      ``argtypes`` tuple, this method allows to adapt the actual
      argument to an object that the foreign function accepts.  For
      example, a ``c_char_p`` item in the ``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 ``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.

   errcheck

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

      callable(result, func, arguments)

         *result* is what the foreign function returns, as specified
         by the ``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 exception ctypes.ArgumentError

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


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.

ctypes.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 ``errno`` variable is exchanged with the real
   ``errno`` value before and after the call; *use_last_error* does
   the same for the Windows error code.

ctypes.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 ``WINFUNCTYPE()`` is the same as ``CFUNCTYPE()``.  The
   function will release the GIL during the call.  *use_errno* and
   *use_last_error* have the same meaning as above.

ctypes.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:

   prototype(address)

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

   prototype(callable)

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

   prototype(func_spec[, paramflags])

      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.

   prototype(vtbl_index, name[, paramflags[, iid]])

      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 ``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 ``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 ``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 ``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 ``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 ``errcheck`` function returns the argument tuple it receives
unchanged, ``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
   >>>


Utility functions
-----------------

ctypes.addressof(obj)

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

ctypes.alignment(obj_or_type)

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

ctypes.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.

ctypes.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.

ctypes.create_string_buffer(init_or_size, size=None)

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

   If the first parameter is a string, it is converted into a bytes
   object according to ctypes conversion rules.

ctypes.create_unicode_buffer(init_or_size, size=None)

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

   If the first parameter is a bytes object, it is converted into an
   unicode string according to ctypes conversion rules.

ctypes.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.

ctypes.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.

ctypes.util.find_library(name)

   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 *-l*).  If no library can be found, returns ``None``.

   The exact functionality is system dependent.

ctypes.util.find_msvcrt()

   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.

ctypes.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.

ctypes.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.

ctypes.get_errno()

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

ctypes.get_last_error()

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

ctypes.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.

ctypes.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.

ctypes.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.

ctypes.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.

ctypes.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.

ctypes.set_conversion_mode(encoding, errors)

   This function sets the rules that ctypes objects use when
   converting between bytes objects and (unicode) strings. *encoding*
   must be a string specifying an encoding, like ``'utf-8'`` or
   ``'mbcs'``, *errors* must be a string specifying the error handling
   on encoding/decoding errors. Examples of possible values are
   ``'strict'``, ``'replace'``, or ``'ignore'``.

   ``set_conversion_mode()`` returns a 2-tuple containing the previous
   conversion rules. On windows, the initial conversion rules are
   ``('mbcs', 'ignore')``, on other systems ``('ascii', 'strict')``.

   You can set the *encoding* to ``'undefined'`` to completely disable
   automatic conversions.

ctypes.set_errno(value)

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

ctypes.set_last_error(value)

   Windows only: set the current value of the ctypes-private copy of
   the system ``LastError`` variable in the calling thread to *value*
   and return the previous value.

ctypes.sizeof(obj_or_type)

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

ctypes.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.

ctypes.WinError(code=None, descr=None)

   Windows only: this function is probably the worst-named thing in
   ctypes. It creates an instance of WindowsError.  If *code* is not
   specified, ``GetLastError`` is called to determine the error code.
   If *descr* is not specified, ``FormatError()`` is called to get a
   textual description of the error.

ctypes.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.


Data types
----------

class class ctypes._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 ``addressof()`` helper function. Another
   instance variable is exposed as ``_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 *metaclass*):

   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
      ``ValueError`` is raised.

   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 ``ValueError`` is raised.

   from_address(address)

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

   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 ``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.

   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:

   _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 ``_b_base_`` read-only member is the root ctypes
      object that owns the memory block.

   _b_needsfree_

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

   _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.


Fundamental data types
----------------------

class class ctypes._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.  ``_SimpleCData``
   is a subclass of ``_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:

   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.
      ``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 ``restype`` of ``c_char_p``,
you will always receive a Python bytes object, *not* a ``c_char_p``
instance.

Subclasses of fundamental data types do *not* inherit this behavior.
So, if a foreign functions ``restype`` is a subclass of ``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 class ctypes.c_byte

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

class class ctypes.c_char

   Represents the C ``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 class ctypes.c_char_p

   Represents the C ``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 class ctypes.c_double

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

class class ctypes.c_longdouble

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

class class ctypes.c_float

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

class class ctypes.c_int

   Represents the C ``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
   ``c_long``.

class class ctypes.c_int8

   Represents the C 8-bit ``signed int`` datatype.  Usually an alias
   for ``c_byte``.

class class ctypes.c_int16

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

class class ctypes.c_int32

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

class class ctypes.c_int64

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

class class ctypes.c_long

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

class class ctypes.c_longlong

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

class class ctypes.c_short

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

class class ctypes.c_size_t

   Represents the C ``size_t`` datatype.

class class ctypes.c_ubyte

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

class class ctypes.c_uint

   Represents the C ``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 ``c_ulong``.

class class ctypes.c_uint8

   Represents the C 8-bit ``unsigned int`` datatype.  Usually an alias
   for ``c_ubyte``.

class class ctypes.c_uint16

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

class class ctypes.c_uint32

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

class class ctypes.c_uint64

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

class class ctypes.c_ulong

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

class class ctypes.c_ulonglong

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

class class ctypes.c_ushort

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

class class ctypes.c_void_p

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

class class ctypes.c_wchar

   Represents the C ``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 class ctypes.c_wchar_p

   Represents the C ``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 class ctypes.c_bool

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

class class ctypes.HRESULT

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

class class ctypes.py_object

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

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


Structured data types
---------------------

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

   Abstract base class for unions in native byte order.

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

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

class class ctypes.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 class ctypes.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 ``_fields_`` class
   variable. ``ctypes`` will create *descriptor*s which allow reading
   and writing the fields by direct attribute accesses.  These are the

   _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 ``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 ``_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 ``_fields_`` class variable must, however, be defined before
      the type is first used (an instance is created, ``sizeof()`` is
      called on it, and so on).  Later assignments to the ``_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
      ``_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 ``_fields_``
      defined in the sub-subclass, if any.

   _pack_

      An optional small integer that allows to override the alignment
      of structure fields in the instance.  ``_pack_`` must already be
      defined when ``_fields_`` is assigned, otherwise it will have no
      effect.

   _anonymous_

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

      The fields listed in this variable must be structure or union
      type fields. ``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 ``_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 ``_fields_``.
   Keyword arguments in the constructor are interpreted as attribute
   assignments, so they will initialize ``_fields_`` with the same
   name, or create new attributes for names not present in
   ``_fields_``.


Arrays and pointers
-------------------

Not yet written - please see the sections *Pointers* and section
*Arrays* in the tutorial.
