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 下甩出的异常类型 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正确的调用协议。
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 类型 |
---|---|---|
|
bool (1) |
|
|
单字符字节对象 |
|
|
单字符字符串 |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
int |
|
|
float |
|
|
float |
|
|
float |
|
|
字节串对象或 |
|
|
字符串或 |
|
|
int 或 |
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函数的函数签名),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(或者传递引用)¶
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¶
指针可以通过 ctypes
中的 pointer()
函数进行创建:
>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>
指针实例拥有 contents
属性,它存储了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,但是你必须确保这么做的后果,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
>>>
类型强制转换¶
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完整的类型。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。
这些工厂函数都是用返回值类型作为第一个参数,回掉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,它使用一个回掉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 CFUNCTYPE()
对象的引用与它们在 C 代码中的使用期一样长。 ctypes
不会确保这一点,而如果你不这样做,它们可能会被垃圾回收,导致你的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 中导出的值¶
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进行了一层包装。
另一个和预期可能有偏差的例子是这样:
>>> 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,women访问新增元素会产生以下错误:
>>> 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¶
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 realerrno
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 whereWINFUNCTYPE()
is the same asCFUNCTYPE()
. 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 argumentobj
.
-
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 argumentsinit
,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 argumentsinit
,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, returnsNone
.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 argumenterrno
.
-
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 argumenterror
.
-
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 argumentsaddress
,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 argumentsaddress
,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 argumentspointer
,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 argumentspointer
,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 argumentaddress
.
-
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 wheresizeof(long double) == sizeof(double)
it is an alias toc_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 wheresizeof(int) == sizeof(long)
it is an alias toc_long
.
-
class
ctypes.
c_int64
¶ Represents the C 64-bit
signed int
datatype. Usually an alias forc_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 wheresizeof(int) == sizeof(long)
it is an alias forc_ulong
.
-
class
ctypes.
c_uint16
¶ Represents the C 16-bit
unsigned int
datatype. Usually an alias forc_ushort
.
-
class
ctypes.
c_uint64
¶ Represents the C 64-bit
unsigned int
datatype. Usually an alias forc_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 beTrue
orFalse
, 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 aNULL
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 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 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, thevt
field specifies which one of the union fields is valid. Since theu
field is defined as anonymous field, it is now possible to access the members directly off the TYPEDESC instance.td.lptdesc
andtd.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 anArray
.-
_length_
¶ A positive integer specifying the number of elements in the array. Out-of-range subscripts result in an
IndexError
. Will be returned bylen()
.
-
_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 bypointer()
.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 raiseTypeError
. 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.
-