# ctypes reference

`ctypes` is a foreign function library for Python. It provides C compatible data types, and allows to call functions in dlls/shared libraries. It can be used to wrap these libraries in pure Python.

Contents

- Finding shared libraries
- Loading shared libraries
- Foreign functions
- Function prototypes
- Utility functions
- Data types
- Fundamental data types
- Structured data types
- Arrays and pointers

# 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.

`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 dependend.

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 sone 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 successfull:

    >>> 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:

`CDLL(name, mode=DEFAULT_MODE, handle=None)`
  Instances of this class represent loaded shared libraries. Functions in these libraries use the standard C calling convention, and are assumed to return `int`.
`OleDLL(name, mode=DEFAULT_MODE, handle=None)`
  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.
`WinDLL(name, mode=DEFAULT_MODE, handle=None)`

  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 GIL is released before calling any function exported by these libraries, and reaquired afterwards.

`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 shard 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.

`RTLD_GLOBAL`
  Flag to use as `mode` parameter. On platforms where this flag is not available, it is defined as the integer zero.
`RTLD_LOCAL`
  Flag to use as `mode` parameter. On platforms where this is not available, it is the same as `RTLD_GLOBAL`.
`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 behaviour: 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:

`_handle`
  The system handle used to access the library.
`_name`
  The name of the library passed in the contructor.

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.

`LibraryLoader(dlltype)`

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

  `__getattr__` has special behaviour: 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:

`cdll`
  Creates `CDLL` instances.
`windll`
  Windows only: Creates `WinDLL` instances.
`oledll`
  Windows only: Creates `OleDLL` instances.
`pydll`
  Creates `PyDLL` instances.

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

`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:

`_FuncPtr`
  Base class for C callable foreign functions.

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

This behaviour 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 postprocessing 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 unicode string passed as argument into an byte string 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 postprocess the results of several functions.

  `arguments` is a tuple containing the parameters originally passed to the function call, this allows to specialize the behaviour 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.

`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.

`CFUNCTYPE(restype, *argtypes)`
  The returned function prototype creates functions that use the standard C calling convention. The function will release the GIL during the call.
`WINFUNCTYPE(restype, *argtypes)`
  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.
`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 the 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.
`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 nonnegative 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 1, 2, or 3 items.

The first item is an integer containing 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

`addressof(obj)`
  Returns the address of the memory buffer as integer. `obj` must be an instance of a ctypes type.
`alignment(obj_or_type)`
  Returns the alignment requirements of a ctypes type. `obj_or_type` must be a ctypes type or instance.
`byref(obj)`
  Returns a light-weight pointer to `obj`, which must be an instance of a ctypes type. 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.
`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.
`create_string_buffer(init_or_size[, size])`

  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 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 unicode string, it is converted into an 8-bit string according to ctypes conversion rules.

`create_unicode_buffer(init_or_size[, size])`

  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 unicode string which will be used to initialize the array items.

  If a unicode 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 8-bit string, it is converted into an unicode string according to ctypes conversion rules.

`DllCanUnloadNow()`
  Windows only: This function is a hook which allows to implement inprocess COM servers with ctypes. It is called from the DllCanUnloadNow function that the _ctypes extension dll exports.
`DllGetClassObject()`
  Windows only: This function is a hook which allows to implement inprocess COM servers with ctypes. It is called from the DllGetClassObject function that the `_ctypes` extension dll exports.
`FormatError([code])`
  Windows only: Returns a textual description of the error code. If no error code is specified, the last error code is used by calling the Windows api function GetLastError.
`GetLastError()`
  Windows only: Returns the last error code set by Windows in the calling thread.
`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.
`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.
`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.
`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.

`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.
`set_conversion_mode(encoding, errors)`

  This function sets the rules that ctypes objects use when converting between 8-bit strings 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')`.

`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.
`string_at(address[, size])`
  This function returns the string starting at memory address address. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated.
`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 spcified, `FormatError` is called to get a textual description of the error.
`wstring_at(address)`
  This function returns the wide character string starting at memory address `address` as unicode 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

`_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_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 functions `argtypes` tuple; it must return an object that can be used as function call parameter.

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

`in_dll(name, library)`
  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_` readonly member is the root ctypes object that owns the memory block.
`_b_needsfree_`
  This readonly 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

`_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.

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 string, for character pointer types it is a Python string or unicode 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 string, not a `c_char_p` instance.

Subclasses of fundamental data types do not inherit this behaviour. 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:

`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.
`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.
`c_char_p`
  Represents the C char * datatype, which must be a pointer to a zero-terminated string. The constructor accepts an integer address, or a string.
`c_double`
  Represents the C double datatype. The constructor accepts an optional float initializer.
`c_float`
  Represents the C double datatype. The constructor accepts an optional float initializer.
`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`.
`c_int8`
  Represents the C 8-bit `signed int` datatype. Usually an alias for `c_byte`.
`c_int16`
  Represents the C 16-bit signed int datatype. Usually an alias for `c_short`.
`c_int32`
  Represents the C 32-bit signed int datatype. Usually an alias for `c_int`.
`c_int64`
  Represents the C 64-bit `signed int` datatype. Usually an alias for `c_longlong`.
`c_long`
  Represents the C `signed long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
`c_longlong`
  Represents the C `signed long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
`c_short`
  Represents the C `signed short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
`c_size_t`
  Represents the C `size_t` datatype.
`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.
`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`.
`c_uint8`
  Represents the C 8-bit unsigned int datatype. Usually an alias for `c_ubyte`.
`c_uint16`
  Represents the C 16-bit unsigned int datatype. Usually an alias for `c_ushort`.
`c_uint32`
  Represents the C 32-bit unsigned int datatype. Usually an alias for `c_uint`.
`c_uint64`
  Represents the C 64-bit unsigned int datatype. Usually an alias for `c_ulonglong`.
`c_ulong`
  Represents the C `unsigned long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
`c_ulonglong`
  Represents the C `unsigned long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
`c_ushort`
  Represents the C `unsigned short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
`c_void_p`
  Represents the C `void *` type. The value is represented as integer. The constructor accepts an optional integer initializer.
`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.
`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.
`HRESULT`
  Windows only: Represents a `HRESULT` value, which contains success or error information for a function or method call.
`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

`Union(*args, **kw)`
  Abstract base class for unions in native byte order.
`BigEndianStructure(*args, **kw)`
  Abstract base class for structures in big endian byte order.
`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.

`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 descriptors 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):
          _fields_ = [("u", _U),
                      ("vt", VARTYPE)]

          _anonymous_ = ("u",)

  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 pointers and arrays in the tutorial.