"ctypes" --- Python 的外部函数库
********************************

======================================================================

"ctypes" 是 Python 的外部函数库。它提供了与 C 兼容的数据类型，并允许调
用 DLL 或共享库中的函数。可使用该模块以纯 Python 形式对这些库进行封装
。


ctypes 教程
===========

注意：在本教程中的示例代码使用 "doctest" 进行过测试，保证其正确运行。
由于有些代码在 Linux，Windows 或 Mac OS X 下的表现不同，这些代码会在
doctest 中包含相关的指令注解。

注意：部分示例代码引用了 ctypes "c_int" 类型。在 "sizeof(long) ==
sizeof(int)" 的平台上此类型是 "c_long" 的一个别名。所以，在程序输出
"c_long" 而不是你期望的 "c_int" 时不必感到迷惑 --- 它们实际上是同一种
类型。


载入动态连接库
--------------

"ctypes" 导出了 *cdll* 对象，在 Windows 系统中还导出了 *windll* 和
*oledll* 对象用于载入动态连接库。

通过操作这些对象的属性，你可以载入外部的动态链接库。*cdll* 载入按标准
的 "cdecl" 调用协议导出的函数，而 *windll* 导入的库按 "stdcall" 调用协
议调用其中的函数。 *oledll* 也按 "stdcall" 调用协议调用其中的函数，并
假定该函数返回的是 Windows "HRESULT" 错误代码，并当函数调用失败时，自
动根据该代码甩出一个 "OSError" 异常。

在 3.3 版更改: 原来在 Windows 下抛出的异常类型 "WindowsError" 现在是
"OSError" 的一个别名。

这是一些 Windows 下的例子。注意："msvcrt" 是微软 C 标准库，包含了大部
分 C 标准函数，这些函数都是以 cdecl 调用协议进行调用的。

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

Windows 会自动添加通常的 ".dll" 文件扩展名。

注解:

  通过 "cdll.msvcrt" 调用的标准 C 函数，可能会导致调用一个过时的，与当
  前 Python 所不兼容的函数。因此，请尽量使用标准的 Python 函数，而不要
  使用 "msvcrt" 模块。

在 Linux 下，必须使用 *包含* 文件扩展名的文件名来导入共享库。因此不能
简单使用对象属性的方式来导入库。因此，你可以使用方法 "LoadLibrary()"，
或构造 CDLL 对象来导入库。

   >>> cdll.LoadLibrary("libc.so.6")  
   <CDLL 'libc.so.6', handle ... at ...>
   >>> libc = CDLL("libc.so.6")       
   >>> libc                           
   <CDLL 'libc.so.6', handle ... at ...>
   >>>


操作导入的动态链接库中的函数
----------------------------

通过操作dll对象的属性来操作这些函数。

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

注意：Win32 系统的动态库，比如 "kernel32" 和 "user32"，通常会同时导出
同一个函数的 ANSI 版本和 UNICODE 版本。UNICODE 版本通常会在名字最后以
"W" 结尾，而 ANSI 版本的则以 "A" 结尾。 win32的 "GetModuleHandle" 函数
会根据一个模块名返回一个 *模块句柄*，该函数暨同时包含这样的两个版本的
原型函数，并通过宏 UNICODE 是否定义，来决定宏 "GetModuleHandle" 导出的
是哪个具体函数。

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

*windll* 不会通过这样的魔法手段来帮你决定选择哪一种函数，你必须显式的
调用 "GetModuleHandleA" 或 "GetModuleHandleW"，并分别使用字节对象或字
符串对象作参数。

有时候，dlls的导出的函数名不符合 Python 的标识符规范，比如
""??2@YAPAXI@Z""。此时，你必须使用 "getattr()" 方法来获得该函数。

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

Windows 下，有些 dll 导出的函数没有函数名，而是通过其顺序号调用。对此
类函数，你也可以通过 dll 对象的数值索引来操作这些函数。

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


调用函数
--------

你可以貌似是调用其它 Python 函数那样直接调用这些函数。在这个例子中，我
们调用了 "time()" 函数，该函数返回一个系统时间戳（从 Unix 时间起点到现
在的秒数），而``GetModuleHandleA()`` 函数返回一个 win32 模块句柄。

此函数中调用的两个函数都使用了空指针（用 "None" 作为空指针）:

   >>> print(libc.time(None))  
   1150640792
   >>> print(hex(windll.kernel32.GetModuleHandleA(None)))  
   0x1d000000
   >>>

如果你用 "cdecl" 调用方式调用 "stdcall" 约定的函数，则会甩出一个异常
"ValueError"。反之亦然。

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

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

你必须阅读这些库的头文件或说明文档来确定它们的调用协议。

在 Windows 中，"ctypes" 使用 win32 结构化异常处理来防止由于在调用函数
时使用非法参数导致的程序崩溃。

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

然而，总有许多办法，通过调用 "ctypes" 使得 Python 程序崩溃。因此，你必
须小心使用。 "faulthandler" 模块可以用于帮助诊断程序崩溃的原因。（比如
由于错误的C库函数调用导致的段错误）。

"None"，整型，字节对象和（UNICODE）字符串是仅有的可以直接作为函数参数
使用的四种Python本地数据类型。*None`* 作为C的空指针 ("NULL")，字节和字
符串类型作为一个指向其保存数据的内存块指针 （"char *" 或 "wchar_t *")
。Python 的整型则作为平台默认的C的 "int" 类型，他们的数值被截断以适应C
类型的整型长度。

在我们开始调用函数前，我们必须先了解作为函数参数的 "ctypes" 数据类型。


基础数据类型
------------

"ctypes" 定义了一些和C兼容的基本数据类型：

+------------------------+--------------------------------------------+------------------------------+
| ctypes 类型            | C 类型                                     | Python 数据类型              |
|========================|============================================|==============================|
| "c_bool"               | "_Bool"                                    | bool (1)                     |
+------------------------+--------------------------------------------+------------------------------+
| "c_char"               | "char"                                     | 单字符字节串对象             |
+------------------------+--------------------------------------------+------------------------------+
| "c_wchar"              | "wchar_t"                                  | 单字符字符串                 |
+------------------------+--------------------------------------------+------------------------------+
| "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" 或 "long long"                   | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ulonglong"          | "unsigned __int64" 或 "unsigned long long" | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_size_t"             | "size_t"                                   | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ssize_t"            | "ssize_t" 或 "Py_ssize_t"                  | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_float"              | "float"                                    | float                        |
+------------------------+--------------------------------------------+------------------------------+
| "c_double"             | "double"                                   | float                        |
+------------------------+--------------------------------------------+------------------------------+
| "c_longdouble"         | "long double"                              | float                        |
+------------------------+--------------------------------------------+------------------------------+
| "c_char_p"             | "char *" (NUL terminated)                  | 字节串对象或 "None"          |
+------------------------+--------------------------------------------+------------------------------+
| "c_wchar_p"            | "wchar_t *" (NUL terminated)               | 字符串或 "None"              |
+------------------------+--------------------------------------------+------------------------------+
| "c_void_p"             | "void *"                                   | int 或 "None"                |
+------------------------+--------------------------------------------+------------------------------+

1. 构造函数接受任何具有真值的对象。

所有这些类型都可以通过使用正确类型和值的可选初始值调用它们来创建:

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

由于这些类型是可变的，它们的值也可以在以后更改:

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

当给指针类型的对象 "c_char_p", "c_wchar_p" 和 "c_void_p" 等赋值时，将
改变它们所指向的 *内存地址*，而 *不是* 它们所指向的内存区域的 *内容* (
这是理所当然的，因为 Python 的 bytes 对象是不可变的):

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

但你要注意不能将它们传递给会改变指针所指内存的函数。如果你需要可改变的
内存块，ctypes 提供了 "create_string_buffer()" 函数，它提供多种方式创
建这种内存块。当前的内存块内容可以通过 "raw" 属性存取，如果你希望将它
作为NUL结束的字符串，请使用 "value" 属性:

   >>> 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'
   >>>

"create_string_buffer()" 函数替代以前的ctypes版本中的 "c_buffer()" 函
数 (仍然可当作别名使用)和 "c_string()" 函数。"create_unicode_buffer()"
函数创建包含 unicode 字符的可变内存块，与之对应的C语言类型是 "wchar_t"
。


调用函数，继续
--------------

注意 printf 将打印到真正标准输出设备，而*不是* "sys.stdout"，因此这些
实例只能在控制台提示符下工作，而不能在 *IDLE* 或 *PythonWin* 中运行。

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

正如前面所提到过的，除了整数、字符串以及字节串之外，所有的 Python 类型
都必须使用它们对应的 "ctypes" 类型包装，才能够被正确地转换为所需的C语
言类型。

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


使用自定义的数据类型调用函数
----------------------------

你也可以通过自定义  "ctypes" 参数转换方式来允许自定义类型作为参数。
"ctypes" 会寻找 "_as_parameter_" 属性并使用它作为函数参数。当然，它必
须是数字、字符串或者二进制字符串:

   >>> 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
   >>>

如果你不想把实例的数据存储到 "_as_parameter_" 属性。可以通过定义
"property" 函数计算出这个属性。


指定必选参数的类型(函数原型)
----------------------------

可以通过设置 "argtypes" 属性的方法指定从 DLL 中导出函数的必选参数类型
。

"argtypes" 必须是一个 C 数据类型的序列 (这里的 "printf" 可能不是个好例
子，因为它是变长参数，而且每个参数的类型依赖于格式化字符串，不过尝试这
个功能也很方便):

   >>> 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
   >>>

指定数据类型可以防止不合理的参数传递（就像 C 函数的原型），并且会自动
尝试将参数转换为需要的类型:

   >>> printf(b"%d %d %d", 1, 2, 3)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ArgumentError: argument 2: exceptions.TypeError: wrong type
   >>> printf(b"%s %d %f\n", b"X", 2, 3)
   X 2 3.000000
   13
   >>>

如果你想通过自定义类型传递参数给函数，必须实现 "from_param()" 类方法，
才能够将此自定义类型用于 "argtypes" 序列。"from_param()" 类方法接受一
个 Python 对象作为函数输入，它应该进行类型检查或者其他必要的操作以保证
接收到的对象是合法的，然后返回这个对象，或者它的 "_as_parameter_" 属性
，或者其他你想要传递给 C 函数的参数。这里也一样，返回的结果必须是整型
、字符串、二进制字符串、 "ctypes" 类型，或者一个具有 "_as_parameter_"
属性的对象。


返回类型
--------

默认情况下都会假定函数返回 C "int" 类型。 其他返回类型可以通过设置函数
对象的 "restype" 属性来指定。

这是个更高级的例子，它调用了 "strchr" 函数，这个函数接收一个字符串指针
以及一个字符作为参数，返回另一个字符串指针。

   >>> strchr = libc.strchr
   >>> strchr(b"abcdef", ord("d"))  
   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
   >>>

如果希望避免上述的 "ord("x")" 调用，可以设置  "argtypes" 属性，第二个
参数就会将单字符的 Python 二进制字符对象转换为 C 字符:

   >>> 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 <module>
   ArgumentError: argument 2: exceptions.TypeError: one character string expected
   >>> print(strchr(b"abcdef", b"x"))
   None
   >>> strchr(b"abcdef", b"d")
   'def'
   >>>

如果外部函数返回了一个整数，你也可以使用要给可调用的 Python 对象（比如
函数或者类）作为 "restype" 属性的值。将会以 C 函数返回的 *整数* 对象作
为参数调用这个可调用对象，执行后的结果作为最终函数返回值。这在错误返回
值校验和自动抛出异常等方面比较有用。

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

"WinError" 函数可以调用 Windows 的  "FormatMessage()" API 获取错误码的
字符串说明，然后 *返回* 一个异常。 "WinError" 接收一个可选的错误码作为
参数，如果没有的话，它将调用  "GetLastError()" 获取错误码。

请注意，使用 "errcheck"  属性可以实现更强大的错误检查手段；详情请见参
考手册。


传递指针（或以引用方式传递形参）
--------------------------------

有时候 C 函数接口可能由于要往某个地址写入值，或者数据太大不适合作为值
传递，从而希望接收一个 *指针* 作为数据参数类型。这和 *传递参数引用* 类
似。

"ctypes" 暴露了 "byref()" 函数用于通过引用传递参数，使用 "pointer()"
函数也能达到同样的效果，只不过 "pointer()" 需要更多步骤，因为它要先构
造一个真实指针对象。所以在 Python 代码本身不需要使用这个指针对象的情况
下，使用 "byref()" 效率更高。

   >>> 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'
   >>>


结构体和联合
------------

结构体和联合必须继承自 "ctypes" 模块中的 "Structure" 和 "Union" 。子类
必须定义 "_fields_" 属性。 "_fields_" 是一个二元组列表，二元组中包含
*field name* 和 *field type* 。

type 字段必须是一个 "ctypes" 类型，比如  "c_int"，或者其他 "ctypes" 类
型: 结构体、联合、数组、指针。

这是一个简单的 POINT 结构体，它包含名称为 *x* 和 *y* 的两个变量，还展
示了如何通过构造函数初始化结构体。

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

当然，你可以构造更复杂的结构体。一个结构体可以通过设置 type 字段包含其
他结构体或者自身。

这是以一个 RECT 结构体，他包含了两个 POINT ，分别叫 *upperleft* 和
*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
   >>>

嵌套结构体可以通过几种方式构造初始化:

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

可以通过 *类* 获取字段 *descriptor* ，它能提供很多有用的调试信息。

   >>> print(POINT.x)
   <Field type=c_long, ofs=0, size=4>
   >>> print(POINT.y)
   <Field type=c_long, ofs=4, size=4>
   >>>

警告:

  "ctypes" 不支持带位域的结构体、联合以值的方式传给函数。这可能在 32
  位 x86 平台上可以正常工作，但是对于一般情况，这种行为是未定义的。带
  位域的结构体、联合应该总是通过指针传递给函数。


结构体/联合字段对齐及字节顺序
-----------------------------

默认情况下，结构体和联合的字段与 C 的字节对齐是一样的。也可以在定义子
类的时候指定类的 "_pack_" 属性来覆盖这种行为。 它必须设置为一个正整数
，表示字段的最大对齐字节。这和 MSVC 中的 "#pragma pack(n)" 功能一样。

"ctypes" 中的结构体和联合使用的是本地字节序。要使用非本地字节序，可以
使用 "BigEndianStructure", "LittleEndianStructure", "BigEndianUnion",
and "LittleEndianUnion" 作为基类。这些类不能包含指针字段。


结构体和联合中的位域
--------------------

结构体和联合中是可以包含位域字段的。位域只能用于整型字段，位长度通过
"_fields_" 中的第三个参数指定:

   >>> 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>
   >>>


数组
----

数组是一个序列，包含指定个数元素，且必须类型相同。

创建数组类型的推荐方式是使用一个类型乘以一个正数:

   TenPointsArrayType = POINT * 10

下面是一个构造的数据案例，结构体中包含了4个 POINT 和一些其他东西。

   >>> 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
   >>>

和平常一样，通过调用它创建实例:

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

以上代码会打印几行 "0 0" ，因为数组内容被初始化为 0.

也能通过指定正确类型的数据来初始化:

   >>> 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
   >>>


指针
----

可以将 "ctypes" 类型数据传入 "pointer()" 函数创建指针:

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

指针实例拥有 "contents" 属性，它返回指针指向的真实对象，如上面的 "i"
对象:

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

注意 "ctypes" 并没有 OOR （返回原始对象）, 每次访问这个属性时都会构造
返回一个新的相同对象:

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

将这个指针的 contents 属性赋值为另一个 "c_int" 实例将会导致该指针指向
该实例的内存地址:

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

指针对象也可以通过整数下标进行访问:

   >>> pi[0]
   99
   >>>

通过整数下标赋值可以改变指针所指向的真实内容:

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

使用 0 以外的索引也是合法的，但是你必须确保知道自己为什么这么做，就像
C 语言中: 你可以访问或者修改任意内存内容。 通常只会在函数接收指针是才
会使用这种特性，而且你 *知道* 这个指针指向的是一个数组而不是单个值。

内部细节,  "pointer()" 函数不只是创建了一个指针实例，它首先创建了一个
指针 *类型* 。这是通过调用 "POINTER()" 函数实现的，它接收  "ctypes" 类
型为参数，返回一个新的类型:

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

无参调用指针类型可以创建一个 "NULL" 指针。 "NULL" 指针的布尔值是
"False"

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

解引用指针的时候， "ctypes" 会帮你检测是否指针为 "NULL" (但是解引用无
效的 非 "NULL" 指针仍会导致 Python 崩溃):

   >>> 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
   >>>


类型转换
--------

通常情况下, ctypes 具有严格的类型检查。这代表着, 如果在函数 "argtypes"
中或者结构体定义成员中有 "POINTER(c_int)" 类型，只有相同类型的实例才会
被接受。 也有一些例外。比如，你可以传递兼容的数组实例给指针类型。所以
，对于  "POINTER(c_int)" ，ctypes 也可以接受 c_int 类型的数组:

   >>> 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
   >>>

另外，如果一个函数 "argtypes" 列表中的参数显式的定义为指针类型(如
"POINTER(c_int)" )，指针所指向的 类型 (这个例子中是 "c_int" )也可以传
递给函数。ctypes 会自动调用对应的 "byref()" 转换。

可以给指针内容赋值为 None 将其设置为 "Null"

   >>> bar.values = None
   >>>

有时候你拥有一个不兼容的类型。 在 C 中，你可以将一个类型强制转换为另一
个。 "ctypes" 中的 a "cast()" 函数提供了相同的功能。 上面的结构体
"Bar" 的 "value" 字段接收  "POINTER(c_int)" 指针或者 "c_int" 数组，但
是不能接受其他类型的实例:

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

这种情况下, 需要手动使用 "cast()" 函数。

"cast()" 函数可以将一个指针实例强制转换为另一种 ctypes 类型。 "cast()"
接收两个参数，一个 ctypes 指针对象或者可以被转换为指针的其他类型对象，
和一个 ctypes 指针类型。 返回第二个类型的一个实例，该返回实例和第一个
参数指向同一片内存空间:

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

所以  "cast()" 可以用来给结构体 "Bar" 的 "values" 字段赋值:

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


不完整类型
----------

*不完整类型* 即还没有定义成员的结构体、联合或者数组。在 C 中，它们通常
用于前置声明，然后在后面定义:

   struct cell; /* forward declaration */

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

直接翻译成 ctypes 的代码如下，但是这行不通:

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

因为新的 "cell 类" 在 class 语句结束之前还没有完成定义。在  "ctypes"
中，我们可以先定义 "cell" 类，在 class 语句结束之后再设置 "_fields_"
属性:

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

让我们试试。我们定义两个 "cell" 实例，让它们互相指向对方，然后通过指针
链式访问几次:

   >>> 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
   >>>


回调函数
--------

"ctypes" 允许创建一个指向 Python 可调用对象的 C 函数。它们有时候被称为
*回调函数* 。

首先，你必须为回调函数创建一个类，这个类知道调用约定，包括返回值类型以
及函数接收的参数类型及个数。

"CFUNCTYPE()" 工厂函数使用 "cdecl" 调用约定创建回调函数类型。在
Windows 上， "WINFUNCTYPE()" 工厂函数使用 "stdcall" 调用约定为回调函数
创建类型。

这些工厂函数的第一个参数是返回值类型，回调函数的参数类型作为剩余参数。

这里展示一个使用 C 标准库函数  "qsort()" 的例子，它使用一个回调函数对
数据进行排序。 "qsort()" 将用来给整数数组排序:

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

"qsort()" 必须接收的参数，一个指向待排序数据的指针，元素个数，每个元素
的大小，以及一个指向排序函数的指针，即回调函数。然后回调函数接收两个元
素的指针，如果第一个元素小于第二个，则返回一个负整数，如果相等则返回0
，否则返回一个正整数。

所以，我们的回调函数要接收两个整数指针，返回一个整数。首先我们创建回调
函数的 "类型"

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

首先，这是一个简单的回调，它会显示传入的值:

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

结果:

   >>> qsort(ia, len(ia), sizeof(c_int), cmp_func)  
   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
   >>>

现在我们可以比较两个元素并返回有用的结果了:

   >>> def py_cmp_func(a, b):
   ...     print("py_cmp_func", a[0], b[0])
   ...     return a[0] - b[0]
   ...
   >>>
   >>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) 
   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
   >>>

我们可以轻易地验证，现在数组是有序的了:

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

这些工厂函数可以当作装饰器工厂，所以可以这样写:

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

注解:

  请确保你维持的 "CFUNCTYPE()" 对象的引用周期与它们在 C 代码中的使用期
  一样长。 "ctypes" 不会确保这一点，如果不这样做，它们可能会被垃圾回收
  ，导致程序在执行回调函数时发生崩溃。注意，如果回调函数在Python之外的
  另外一个线程使用(比如，外部代码调用这个回调函数)， ctypes 会在每一次
  调用上创建一个虚拟 Python 线程。这个行为在大多数情况下是合理的，但也
  意味着如果有数据使用 "threading.local" 方式存储，将无法访问，就算它
  们是在同一个 C 线程中调用的 。


访问 dll 的导出变量
-------------------

一些动态链接库不仅仅导出函数，也会导出变量。一个例子就是 Python 库本身
的 "Py_OptimizeFlag" ，根据启动选项 "-O" 、 "-OO" 的不同，它是值可能为
0、1、2 的整型。

"ctypes" 可以通过  "in_dll()" 类方法访问这类变量 。 *pythonapi* 是用于
访问 Python C 接口的预定义符号:

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

如果解释器使用 "-O" 选项启动，这个例子会打印 "c_long(1)" ， 如果使用
"-OO" 启动，则会打印 "c_long(2)" 。

一个扩展例子,  同时也展示了使用指针访问 Python 导出的
"PyImport_FrozenModules" 指针对象。

对文档中这个值的解释说明

   该指针被初始化为指向 "struct _frozen" 数组，以 "NULL" 或者 0 作为结
   束标记。当一个冻结模块被导入，首先要在这个表中搜索。第三方库可以以
   此来提供动态创建的冻结模块集合。

这足以证明修改这个指针是很有用的。为了让实例大小不至于太长，这里只展示
如何使用 "ctypes" 读取这个表:

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

我们定义了  "struct _frozen"  数据类型，接着就可以获取这张表的指针了:

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

由于 "table" 是指向 "struct_frozen" 数组的 "指针" ，我们可以遍历它，只
不过需要自己判断循环是否结束，因为指针本身并不包含长度。它早晚会因为访
问到野指针或者什么的把自己搞崩溃，所以我们最好在遇到  "NULL" 后就让它
退出循环:

   >>> for item in table:
   ...     if item.name is None:
   ...         break
   ...     print(item.name.decode("ascii"), item.size)
   ...
   _frozen_importlib 31764
   _frozen_importlib_external 41499
   __hello__ 161
   __phello__ -161
   __phello__.spam 161
   >>>

Python 的冻结模块和冻结包(由负 "size" 成员表示)并不是广为人知的事情，
它们仅仅用于实验。例如，可以使用 "import __hello__" 尝试一下这个功能。


意外
----

"ctypes" 也有自己的边界，有时候会发生一些意想不到的事情。

比如下面的例子:

   >>> 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
   >>>

嗯。我们预想应该打印 "3 4 1 2" 。但是为什么呢? 这是 "rc.a, rc.b =
rc.b, rc.a" 这行代码展开后的步骤:

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

注意 "temp0" 和 "temp1" 对象始终引用了对象 "rc" 的内容。然后执行 "rc.a
= temp0" 会把 "temp0" 的内容拷贝到 "rc" 的空间。这也改变了  "temp1" 的
内容。最终导致赋值语句 "rc.b = temp1" 没有产生预想的效果。

记住，访问被包含在结构体、联合、数组中的对象并不会将其 *复制* 出来，而
是得到了一个代理对象，它是对根对象的内部内容的一层包装。

下面是另一个可能和预期有偏差的例子:

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

注解:

  使用  "c_char_p"  实例化的对象只能将其值设置为 bytes 或者整数。

为什么这里打印了 "False" ？ ctypes 实例是一些内存块加上一些用于访问这
些内存块的 *descriptor* 组成。将 Python 对象存储在内存块并不会存储对象
本身，而是存储了对象的 "内容" 。每次访问对象的内容都会构造一个新的
Python 对象。


变长数据类型
------------

"ctypes" 对变长数组和结构体提供了一些支持 。

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
   >>>

这非常好，但是要怎么访问数组中额外的元素呢？因为数组类型已经定义包含4
个元素，导致我们访问新增元素时会产生以下错误:

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

使用  "ctypes" 访问变长数据类型的一个可行方法是利用 Python 的动态特性
，根据具体情况，在知道这个数据的大小后，(重新)指定这个数据的类型。


ctypes 参考手册
===============


寻找动态链接库
--------------

在编译型语言中，动态链接库会在编译、链接或者程序运行时访问。

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

"ctypes.util" 模块提供了一个函数，可以帮助确定需要加载的库。

ctypes.util.find_library(name)

   尝试寻找一个库然后返回其路径名， *name* 是库名称, 且去除了 *lib* 等
   前缀和 ".so" 、 ".dylib" 、版本号等后缀(这是 posix 连接器 "-l" 选项
   使用的格式)。如果没有找到对应的库，则返回 "None" 。

确切的功能取决于系统。

在 Linux 上， "find_library()" 会尝试运行外部程序("/sbin/ldconfig",
"gcc", "objdump" 以及 "ld") 来寻找库文件。返回库文件的文件名。

在 3.6 版更改: 在Linux 上，如果其他方式找不到的话，会使用环境变量
"LD_LIBRARY_PATH" 搜索动态链接库。

这是一些例子:

   >>> 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'
   >>>

在 OS X 上， "find_library()" 会尝试几种预定义的命名方案和路径来查找库
，如果成功，则返回完整的路径名:

   >>> 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'
   >>>

在 Windows 上，  "find_library()" 在系统路径中搜索，然后返回全路径，但
是如果没有预定义的命名方案， "find_library("c")" 调用会返回 "None"

使用  "ctypes" 包装动态链接库，更好的方式 *可能* 是在开发的时候就确定
名称，然后硬编码到包装模块中去，而不是在运行时使用 "find_library()" 寻
找库。


加载动态链接库
--------------

有很多方式可以将动态链接库加载到 Python 进程。其中之一是实例化以下类的
其中一个:

class ctypes.CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)

   此类的实例即已加载的动态链接库。库中的函数使用标准 C 调用约定，并假
   定返回 "int" 。

class ctypes.OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)

   仅 Windows : 此类的实例即加载好的动态链接库，其中的函数使用
   "stdcall" 调用约定，并且假定返回 windows 指定的 "HRESULT" 返回码。
   "HRESULT" 的值包含的信息说明函数调用成功还是失败，以及额外错误码。
   如果返回值表示失败，会自动抛出 "OSError" 异常。

   在 3.3 版更改: 以前是引发 "WindowsError"。

class ctypes.WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)

   仅 Windows: 此类的实例即加载好的动态链接库，其中的函数使用
   "stdcall" 调用约定，并假定默认返回 "int" 。

   在 Windows CE 上，只能使用 stdcall 调用约定，为了方便， "WinDLL" 和
   "OleDLL" 在这个平台上都使用标准调用约定。

调用动态库导出的函数之前，Python会释放 *global interpreter lock* ，并
在调用后重新获取。

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

   这个类实例的行为与 "CDLL" 类似，只不过 *不会* 在调用函数的时候释放
   GIL 锁，且调用结束后会检查 Python 错误码。 如果错误码被设置，会抛出
   一个 Python 异常。

   所以，它只在直接调用 Python C 接口函数的时候有用。

通过使用至少一个参数（共享库的路径名）调用它们，可以实例化所有这些类。
也可以传入一个已加载的动态链接库作为 "handler" 参数，其他情况会调用系
统底层的  "dlopen" 或 "LoadLibrary" 函数将库加载到进程，并获取其句柄。

*mode* 可以指定库加载方式。详情请参见  *dlopen(3)* 手册页。 在 Windows
上， 会忽略 *mode* ，在 posix 系统上， 总是会加上 RTLD_NOW ，且无法配
置。

*use_errno* 参数如果设置为 true，可以启用ctypes的机制，通过一种安全的
方法获取系统的  "errno" 错误码。 "ctypes" 维护了一个线程局部变量，它是
系统 "errno" 的一份拷贝；如果调用了使用 "use_errno=True" 创建的外部函
数， "errno" 的值会与 ctypes 自己拷贝的那一份进行交换，函数执行完后立
即再交换一次。

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.

*use_last_error*  参数如果设置为 true，可以在 Windows 上启用相同的策略
，它是通过 Windows API 函数 "GetLastError()"  和 "SetLastError()" 管理
的。 "ctypes.get_last_error()" 和 "ctypes.set_last_error()" 可用于获取
和设置 ctypes 自己维护的 windows 错误码拷贝。

*winmode* 参数用于在 Windows 平台上指定库的加载方式( 因为 *mode* 会被
忽略)。他接受任何与 Win32 API 的 "LoadLibraryEx" 的标志兼容的值作为参
数。省略时，默认设置使用最安全的DLL加载的标志，以避免DLL劫持等问题。传
入 DLL 的全路径是保证正确加载库及其依赖最安全的方法。

在 3.8 版更改: 增加了  *winmode* 参数。

ctypes.RTLD_GLOBAL

   用于 *mode* 参数的标识值。在此标识不可用的系统上，它被定义为整数0。

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

   加载动态链接库的默认模式。在 OSX 10.3 上，它是 *RTLD_GLOBAL* ，其余
   系统上是 *RTLD_LOCAL* 。

这些类的实例没有共用方法。动态链接库的导出函数可以通过属性或者索引的方
式访问。注意，通过属性的方式访问会缓存这个函数，因而每次访问它时返回的
都是同一个对象。另一方面，通过索引访问，每次都会返回一个新的对象:

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

还有下面这些属性可用，他们的名称以下划线开头，以避免和导出函数重名:

PyDLL._handle

   用于访问库的系统句柄。

PyDLL._name

   传入构造函数的库名称。

共享库也可以通用使用一个预制对象来加载，这种对象是 "LibraryLoader" 类
的实例，具体做法或是通过调用 "LoadLibrary()" 方法，或是通过将库作为加
载器实例的属性来提取。

class ctypes.LibraryLoader(dlltype)

   加载共享库的类。 *dlltype* 应当为 "CDLL", "PyDLL", "WinDLL" 或
   "OleDLL" 类型之一。

   "__getattr__()" 具有特殊的行为：它允许通过将一个共享库作为库加载器
   实例的属性进行访问来加载它。 加载结果将被缓存，因此重复的属性访问每
   次都会返回相同的库。

   LoadLibrary(name)

      加载一个共享库到进程中并将其返回。 此方法总是返回一个新的库实例
      。

可用的预制库加载器有如下这些:

ctypes.cdll

   创建 "CDLL" 实例。

ctypes.windll

   仅限 Windows：创建 "WinDLL" 实例.

ctypes.oledll

   仅限 Windows：创建 "OleDLL" 实例。

ctypes.pydll

   创建 "PyDLL" 实例。

要直接访问 C Python api，可以使用一个现成的 Python 共享库对象:

ctypes.pythonapi

   一个 "PyDLL" 的实例，它将 Python C API 函数作为属性公开。 请注意所
   有这些函数都应返回 C "int"，当然这也不是绝对的，因此你必须分配正确
   的 "restype" 属性以使用这些函数。

引发一个 审计事件 "ctypes.dlopen"，附带参数 "name"。

引发一个审计事件 "ctypes.dlsym"，附带参数 "library", "name"。

引发一个审计事件 "ctypes.dlsym/handle"，附带参数 "handle", "name"。


外部函数
--------

正如之前小节的说明，外部函数可作为被加载共享库的属性来访问。 用此方式
创建的函数对象默认接受任意数量的参数，接受任意 ctypes 数据实例作为参数
，并且返回库加载器所指定的默认结果类型。 它们是一个私有类的实例:

class ctypes._FuncPtr

   C 可调用外部函数的基类。

   外部函数的实例也是兼容 C 的数据类型；它们代表 C 函数指针。

   此行为可通过对外部函数对象的特殊属性赋值来自定义。

   restype

      赋值为一个 ctypes 类型来指定外部函数的结果类型。 使用 "None" 表
      示 "void"，即不返回任何结果的函数。

      赋值为一个不为 ctypes 类型的可调用 Python 对象也是可以的，在此情
      况下函数应返回 C "int"，该可调用对象将附带此整数被调用，以允许进
      一步的处理或错误检测。 这种用法已被弃用，为了更灵活的后续处理或
      错误检测请使用一个 ctypes 数据类型作为 "restype" 并将 "errcheck"
      属性赋值为一个可调用对象。

   argtypes

      赋值为一个 ctypes 类型的元组来指定函数所接受的参数类型。 使用
      "stdcall" 调用规范的函数只能附带与此元组长度相同数量的参数进行调
      用；使用 C 调用规范的函数还可接受额外的未指明参数。

      当外部函数被调用时，每个实际参数都会被传给 "argtypes" 元组中条目
      的 "from_param()" 类方法，此方法允许将实际参数适配为此外部函数所
      接受的对象。 例如，"argtypes" 元组中的 "c_char_p" 条目将使用
      ctypes 约定规则把作为参数传入的字符串转换为字节串对象。

      新增：现在可以将不是 ctypes 类型的条目放入 argtypes，但每个条目
      都必须具有 "from_param()" 方法用于返回可作为参数的值（整数、字符
      串、ctypes 实例）。 这样就允许定义可将自定义对象适配为函数形参的
      适配器。

   errcheck

      将一个 Python 函数或其他可调用对象赋值给此属性。 该可调用对象将
      附带三个及以上的参数被调用。

      callable(result, func, arguments)

         *result* 是外部函数返回的结果，由 "restype" 属性指明。

         *func* 是外部函数对象本身，这样就允许重新使用相同的可调用对象
         来对多个函数进行检查或后续处理。

         *arguments* 是一个包含最初传递给函数调用的形参的元组，这样就
         允许对所用参数的行为进行特别处理。

      此函数所返回的对象将会由外部函数调用返回，但它还可以在外部函数调
      用失败时检查结果并引发异常。

exception ctypes.ArgumentError

   此异常会在外部函数无法对某个传入参数执行转换时被引发。

引发一个审计事件 "ctypes.seh_exception" 并附带参数 "code"。

引发一个审计事件 "ctypes.call_function"，附带参数 "func_pointer",
"arguments"。


函数原型
--------

外部函数也可通过实例化函数原型来创建。 函数原型类似于 C 中的函数原型；
它们在不定义具体实现的情况下描述了一个函数（返回类型、参数类型、调用约
定）。 工厂函数必须使用函数所需要的结果类型和参数类型来调用，并可被用
作装饰器工厂函数，在此情况下可以通过 "@wrapper" 语法应用于函数。 请参
阅 回调函数 了解有关示例。

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 "MessageBoxW"
function so that it supports default parameters and named arguments.
The C declaration from the windows header file is this:

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

Here is the wrapping with "ctypes":

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

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

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

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

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

Here is the wrapping with "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.

   Raises an auditing event "ctypes.addressof" with argument "obj".

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 specifying the size of the array if
   the length of the bytes should not be used.

   Raises an auditing event "ctypes.create_string_buffer" with
   arguments "init", "size".

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 specifying the size of the
   array if the length of the string should not be used.

   Raises an auditing event "ctypes.create_unicode_buffer" with
   arguments "init", "size".

ctypes.DllCanUnloadNow()

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

ctypes.DllGetClassObject()

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

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".

   确切的功能取决于系统。

ctypes.util.find_msvcrt()

   Windows only: return the filename of the VC runtime library used by
   Python, and by the extension modules.  If the name of the library
   cannot be determined, "None" is returned.

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

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.

   Raises an auditing event "ctypes.get_errno" with no arguments.

ctypes.get_last_error()

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

   Raises an auditing event "ctypes.get_last_error" with no arguments.

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 and 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_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.

   Raises an auditing event "ctypes.set_errno" with argument "errno".

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.

   Raises an auditing event "ctypes.set_last_error" with argument
   "error".

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" operator.

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.

   Raises an auditing event "ctypes.string_at" with arguments
   "address", "size".

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

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

   在 3.3 版更改: An instance of "WindowsError" used to be created.

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.

   Raises an auditing event "ctypes.wstring_at" with arguments
   "address", "size".


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

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.

      Raises an auditing event "ctypes.cdata/buffer" with arguments
      "pointer", "size", "offset".

   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.

      Raises an auditing event "ctypes.cdata/buffer" with arguments
      "pointer", "size", "offset".

   from_address(address)

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

      This method, and others that indirectly call this method, raises
      an auditing event "ctypes.cdata" with argument "address".

   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.


基础数据类型
------------

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 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 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 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 ctypes.c_double

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

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 ctypes.c_float

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

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 ctypes.c_int8

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

class ctypes.c_int16

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

class ctypes.c_int32

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

class ctypes.c_int64

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

class ctypes.c_long

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

class ctypes.c_longlong

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

class ctypes.c_short

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

class ctypes.c_size_t

   Represents the C "size_t" datatype.

class ctypes.c_ssize_t

   Represents the C "ssize_t" datatype.

   3.2 新版功能.

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 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 ctypes.c_uint8

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

class ctypes.c_uint16

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

class ctypes.c_uint32

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

class ctypes.c_uint64

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

class ctypes.c_ulong

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

class ctypes.c_ulonglong

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

class ctypes.c_ushort

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

class ctypes.c_void_p

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

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 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 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 ctypes.HRESULT

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

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 ctypes.Union(*args, **kw)

   Abstract base class for unions in native byte order.

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

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

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 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 creating 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.

      It is possible to define 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 overriding 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 accessing the nested fields directly, without
      the need to create the structure or union field.

      Here is an example type (Windows):

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

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

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

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

   It is possible to define sub-subclasses of structures, they inherit
   the fields of the base class.  If the subclass definition has a
   separate "_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
-------------------

class ctypes.Array(*args)

   Abstract base class for arrays.

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

   _length_

      A positive integer specifying the number of elements in the
      array. Out-of-range subscripts result in an "IndexError". Will
      be returned by "len()".

   _type_

      Specifies the type of each element in the array.

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

class ctypes._Pointer

   Private, abstract base class for pointers.

   Concrete pointer types are created by calling "POINTER()" with the
   type that will be pointed to; this is done automatically by
   "pointer()".

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

   _type_

      Specifies the type pointed to.

   contents

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