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 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
整型 |
|
|
浮点数 |
|
|
浮点数 |
|
|
浮点数 |
|
|
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¶
You can also customize ctypes
argument conversion to allow instances of
your own classes be used as function arguments. ctypes
looks for an
_as_parameter_
attribute and uses this as the function argument. Of
course, it must be one of integer, string, or bytes:
>>> class Bottles:
... def __init__(self, number):
... self._as_parameter_ = number
...
>>> bottles = Bottles(42)
>>> printf(b"%d bottles of beer\n", bottles)
42 bottles of beer
19
>>>
If you don't want to store the instance's data in the _as_parameter_
instance variable, you could define a property
which makes the
attribute available on request.
Specifying the required argument types (function prototypes)¶
It is possible to specify the required argument types of functions exported from
DLLs by setting the argtypes
attribute.
argtypes
must be a sequence of C data types (the printf
function is
probably not a good example here, because it takes a variable number and
different types of parameters depending on the format string, on the other hand
this is quite handy to experiment with this feature):
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
String 'Hi', Int 10, Double 2.200000
37
>>>
Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:
>>> printf(b"%d %d %d", 1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ArgumentError: argument 2: exceptions.TypeError: wrong type
>>> printf(b"%s %d %f\n", b"X", 2, 3)
X 2 3.000000
13
>>>
If you have defined your own classes which you pass to function calls, you have
to implement a from_param()
class method for them to be able to use them
in the argtypes
sequence. The from_param()
class method receives
the Python object passed to the function call, it should do a typecheck or
whatever is needed to make sure this object is acceptable, and then return the
object itself, its _as_parameter_
attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, bytes, a ctypes
instance, or an object with an
_as_parameter_
attribute.
Return types¶
By default functions are assumed to return the C int
type. Other
return types can be specified by setting the restype
attribute of the
function object.
Here is a more advanced example, it uses the strchr
function, which expects
a string pointer and a char, and returns a pointer to a string:
>>> strchr = libc.strchr
>>> strchr(b"abcdef", ord("d"))
8059983
>>> strchr.restype = c_char_p # c_char_p is a pointer to a string
>>> strchr(b"abcdef", ord("d"))
b'def'
>>> print(strchr(b"abcdef", ord("x")))
None
>>>
If you want to avoid the ord("x")
calls above, you can set the
argtypes
attribute, and the second argument will be converted from a
single character Python bytes object into a C char:
>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> strchr(b"abcdef", b"d")
'def'
>>> strchr(b"abcdef", b"def")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ArgumentError: argument 2: exceptions.TypeError: one character string expected
>>> print(strchr(b"abcdef", b"x"))
None
>>> strchr(b"abcdef", b"d")
'def'
>>>
You can also use a callable Python object (a function or a class for example) as
the restype
attribute, if the foreign function returns an integer. The
callable will be called with the integer the C function returns, and the
result of this call will be used as the result of your function call. This is
useful to check for error return values and automatically raise an exception:
>>> GetModuleHandle = windll.kernel32.GetModuleHandleA
>>> 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
is a function which will call Windows FormatMessage()
api to
get the string representation of an error code, and returns an exception.
WinError
takes an optional error code parameter, if no one is used, it calls
GetLastError()
to retrieve it.
Please note that a much more powerful error checking mechanism is available
through the errcheck
attribute; see the reference manual for details.
Passing pointers (or: passing parameters by reference)¶
Sometimes a C api function expects a pointer to a data type as parameter, probably to write into the corresponding location, or if the data is too large to be passed by value. This is also known as passing parameters by reference.
ctypes
exports the byref()
function which is used to pass parameters
by reference. The same effect can be achieved with the pointer()
function,
although pointer()
does a lot more work since it constructs a real pointer
object, so it is faster to use byref()
if you don't need the pointer
object in Python itself:
>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer(b'\000' * 32)
>>> print(i.value, f.value, repr(s.value))
0 0.0 b''
>>> libc.sscanf(b"1 3.14 Hello", b"%d %f %s",
... byref(i), byref(f), s)
3
>>> print(i.value, f.value, repr(s.value))
1 3.1400001049 b'Hello'
>>>
Structures and unions¶
Structures and unions must derive from the Structure
and Union
base classes which are defined in the ctypes
module. Each subclass must
define a _fields_
attribute. _fields_
must be a list of
2-tuples, containing a field name and a field type.
The field type must be a ctypes
type like c_int
, or any other
derived ctypes
type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = [("x", c_int),
... ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print(point.x, point.y)
10 20
>>> point = POINT(y=5)
>>> print(point.x, point.y)
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: too many initializers
>>>
You can, however, build much more complicated structures. A structure can itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft and lowerright:
>>> class RECT(Structure):
... _fields_ = [("upperleft", POINT),
... ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print(rc.upperleft.x, rc.upperleft.y)
0 5
>>> print(rc.lowerright.x, rc.lowerright.y)
0 0
>>>
Nested structures can also be initialized in the constructor in several ways:
>>> r = RECT(POINT(1, 2), POINT(3, 4))
>>> r = RECT((1, 2), (3, 4))
Field descriptors can be retrieved from the class, they are useful for debugging because they can provide useful information:
>>> print(POINT.x)
<Field type=c_long, ofs=0, size=4>
>>> print(POINT.y)
<Field type=c_long, ofs=4, size=4>
>>>
警告
ctypes
does not support passing unions or structures with bit-fields
to functions by value. While this may work on 32-bit x86, it's not
guaranteed by the library to work in the general case. Unions and
structures with bit-fields should always be passed to functions by pointer.
Structure/union alignment and byte order¶
By default, Structure and Union fields are aligned in the same way the C
compiler does it. It is possible to override this behavior by specifying a
_pack_
class attribute in the subclass definition. This must be set to a
positive integer and specifies the maximum alignment for the fields. This is
what #pragma pack(n)
also does in MSVC.
ctypes
uses the native byte order for Structures and Unions. To build
structures with non-native byte order, you can use one of the
BigEndianStructure
, LittleEndianStructure
,
BigEndianUnion
, and LittleEndianUnion
base classes. These
classes cannot contain pointer fields.
Bit fields in structures and unions¶
It is possible to create structures and unions containing bit fields. Bit fields
are only possible for integer fields, the bit width is specified as the third
item in the _fields_
tuples:
>>> class Int(Structure):
... _fields_ = [("first_16", c_int, 16),
... ("second_16", c_int, 16)]
...
>>> print(Int.first_16)
<Field type=c_long, ofs=0:0, bits=16>
>>> print(Int.second_16)
<Field type=c_long, ofs=0:16, bits=16>
>>>
Arrays¶
Arrays are sequences, containing a fixed number of instances of the same type.
The recommended way to create array types is by multiplying a data type with a positive integer:
TenPointsArrayType = POINT * 10
Here is an example of a somewhat artificial data type, a structure containing 4 POINTs among other stuff:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class MyStruct(Structure):
... _fields_ = [("a", c_int),
... ("b", c_float),
... ("point_array", POINT * 4)]
>>>
>>> print(len(MyStruct().point_array))
4
>>>
Instances are created in the usual way, by calling the class:
arr = TenPointsArrayType()
for pt in arr:
print(pt.x, pt.y)
The above code print a series of 0 0
lines, because the array contents is
initialized to zeros.
Initializers of the correct type can also be specified:
>>> from ctypes import *
>>> TenIntegers = c_int * 10
>>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
>>> print(ii)
<c_long_Array_10 object at 0x...>
>>> for i in ii: print(i, end=" ")
...
1 2 3 4 5 6 7 8 9 10
>>>
Pointers¶
Pointer instances are created by calling the pointer()
function on a
ctypes
type:
>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>
Pointer instances have a contents
attribute which
returns the object to which the pointer points, the i
object above:
>>> pi.contents
c_long(42)
>>>
Note that ctypes
does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute:
>>> pi.contents is i
False
>>> pi.contents is pi.contents
False
>>>
Assigning another c_int
instance to the pointer's contents attribute
would cause the pointer to point to the memory location where this is stored:
>>> i = c_int(99)
>>> pi.contents = i
>>> pi.contents
c_long(99)
>>>
Pointer instances can also be indexed with integers:
>>> pi[0]
99
>>>
Assigning to an integer index changes the pointed to value:
>>> print(i)
c_long(99)
>>> pi[0] = 22
>>> print(i)
c_long(22)
>>>
It is also possible to use indexes different from 0, but you must know what you're doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.
Behind the scenes, the pointer()
function does more than simply create
pointer instances, it has to create pointer types first. This is done with the
POINTER()
function, which accepts any ctypes
type, and returns a
new type:
>>> PI = POINTER(c_int)
>>> PI
<class 'ctypes.LP_c_long'>
>>> PI(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: expected c_long instead of int
>>> PI(c_int(42))
<ctypes.LP_c_long object at 0x...>
>>>
Calling the pointer type without an argument creates a NULL
pointer.
NULL
pointers have a False
boolean value:
>>> null_ptr = POINTER(c_int)()
>>> print(bool(null_ptr))
False
>>>
ctypes
checks for NULL
when dereferencing pointers (but dereferencing
invalid non-NULL
pointers would crash Python):
>>> null_ptr[0]
Traceback (most recent call last):
....
ValueError: NULL pointer access
>>>
>>> null_ptr[0] = 1234
Traceback (most recent call last):
....
ValueError: NULL pointer access
>>>
Type conversions¶
Usually, ctypes does strict type checking. This means, if you have
POINTER(c_int)
in the argtypes
list of a function or as the type of
a member field in a structure definition, only instances of exactly the same
type are accepted. There are some exceptions to this rule, where ctypes accepts
other objects. For example, you can pass compatible array instances instead of
pointer types. So, for POINTER(c_int)
, ctypes accepts an array of c_int:
>>> class Bar(Structure):
... _fields_ = [("count", c_int), ("values", POINTER(c_int))]
...
>>> bar = Bar()
>>> bar.values = (c_int * 3)(1, 2, 3)
>>> bar.count = 3
>>> for i in range(bar.count):
... print(bar.values[i])
...
1
2
3
>>>
In addition, if a function argument is explicitly declared to be a pointer type
(such as POINTER(c_int)
) in argtypes
, an object of the pointed
type (c_int
in this case) can be passed to the function. ctypes will apply
the required byref()
conversion in this case automatically.
To set a POINTER type field to NULL
, you can assign None
:
>>> bar.values = None
>>>
Sometimes you have instances of incompatible types. In C, you can cast one type
into another type. ctypes
provides a cast()
function which can be
used in the same way. The Bar
structure defined above accepts
POINTER(c_int)
pointers or c_int
arrays for its values
field,
but not instances of other types:
>>> bar.values = (c_byte * 4)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
>>>
For these cases, the cast()
function is handy.
The cast()
function can be used to cast a ctypes instance into a pointer
to a different ctypes data type. cast()
takes two parameters, a ctypes
object that is or can be converted to a pointer of some kind, and a ctypes
pointer type. It returns an instance of the second argument, which references
the same memory block as the first argument:
>>> a = (c_byte * 4)()
>>> cast(a, POINTER(c_int))
<ctypes.LP_c_long object at ...>
>>>
So, cast()
can be used to assign to the values
field of Bar
the
structure:
>>> bar = Bar()
>>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
>>> print(bar.values[0])
0
>>>
Incomplete Types¶
Incomplete Types are structures, unions or arrays whose members are not yet specified. In C, they are specified by forward declarations, which are defined later:
struct cell; /* forward declaration */
struct cell {
char *name;
struct cell *next;
};
The straightforward translation into ctypes code would be this, but it does not work:
>>> class cell(Structure):
... _fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in cell
NameError: name 'cell' is not defined
>>>
because the new class cell
is not available in the class statement itself.
In ctypes
, we can define the cell
class and set the _fields_
attribute later, after the class statement:
>>> from ctypes import *
>>> class cell(Structure):
... pass
...
>>> cell._fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
>>>
Let's try it. We create two instances of cell
, and let them point to each
other, and finally follow the pointer chain a few times:
>>> c1 = cell()
>>> c1.name = "foo"
>>> c2 = cell()
>>> c2.name = "bar"
>>> c1.next = pointer(c2)
>>> c2.next = pointer(c1)
>>> p = c1
>>> for i in range(8):
... print(p.name, end=" ")
... p = p.next[0]
...
foo bar foo bar foo bar foo bar
>>>
Callback functions¶
ctypes
allows creating C callable function pointers from Python callables.
These are sometimes called callback functions.
First, you must create a class for the callback function. The class knows the calling convention, the return type, and the number and types of arguments this function will receive.
The CFUNCTYPE()
factory function creates types for callback functions
using the cdecl
calling convention. On Windows, the WINFUNCTYPE()
factory function creates types for callback functions using the stdcall
calling convention.
Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.
I will present an example here which uses the standard C library's
qsort()
function, that is used to sort items with the help of a callback
function. qsort()
will be used to sort an array of integers:
>>> IntArray5 = c_int * 5
>>> ia = IntArray5(5, 1, 7, 33, 99)
>>> qsort = libc.qsort
>>> qsort.restype = None
>>>
qsort()
must be called with a pointer to the data to sort, the number of
items in the data array, the size of one item, and a pointer to the comparison
function, the callback. The callback will then be called with two pointers to
items, and it must return a negative integer if the first item is smaller than
the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an
integer. First we create the type
for the callback function:
>>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
>>>
To get started, here is a simple callback that shows the values it gets passed:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return 0
...
>>> cmp_func = CMPFUNC(py_cmp_func)
>>>
The result:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 5 7
py_cmp_func 1 7
>>>
Now we can actually compare the two items and return a useful result:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return a[0] - b[0]
...
>>>
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func))
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
>>>
As we can easily check, our array is sorted now:
>>> for i in ia: print(i, end=" ")
...
1 5 7 33 99
>>>
The function factories can be used as decorator factories, so we may as well write:
>>> @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
Make sure you keep references to CFUNCTYPE()
objects as long as they
are used from C code. ctypes
doesn't, and if you don't, they may be
garbage collected, crashing your program when a callback is made.
Also, note that if the callback function is called in a thread created
outside of Python's control (e.g. by the foreign code that calls the
callback), ctypes creates a new dummy Python thread on every invocation. This
behavior is correct for most purposes, but it means that values stored with
threading.local
will not survive across different callbacks, even when
those calls are made from the same C thread.
Accessing values exported from dlls¶
Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the Py_OptimizeFlag
, an integer
set to 0, 1, or 2, depending on the -O
or -OO
flag given on
startup.
ctypes
can access values like this with the in_dll()
class methods of
the type. pythonapi is a predefined symbol giving access to the Python C
api:
>>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag")
>>> print(opt_flag)
c_long(0)
>>>
If the interpreter would have been started with -O
, the sample would
have printed c_long(1)
, or c_long(2)
if -OO
would have been
specified.
An extended example which also demonstrates the use of pointers accesses the
PyImport_FrozenModules
pointer exported by Python.
Quoting the docs for that value:
This pointer is initialized to point to an array of
struct _frozen
records, terminated by one whose members are allNULL
or zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.
So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with ctypes
:
>>> from ctypes import *
>>>
>>> class struct_frozen(Structure):
... _fields_ = [("name", c_char_p),
... ("code", POINTER(c_ubyte)),
... ("size", c_int)]
...
>>>
We have defined the struct _frozen
data type, so we can get the pointer
to the table:
>>> FrozenTable = POINTER(struct_frozen)
>>> table = FrozenTable.in_dll(pythonapi, "PyImport_FrozenModules")
>>>
Since table
is a pointer
to the array of struct_frozen
records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it's better to break out of the loop when we
hit the NULL
entry:
>>> for item in table:
... 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
>>>
The fact that standard Python has a frozen module and a frozen package
(indicated by the negative size
member) is not well known, it is only used
for testing. Try it out with import __hello__
for example.
Surprises¶
There are some edges in ctypes
where you might expect something other
than what actually happens.
Consider the following example:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class RECT(Structure):
... _fields_ = ("a", POINT), ("b", POINT)
...
>>> p1 = POINT(1, 2)
>>> p2 = POINT(3, 4)
>>> rc = RECT(p1, p2)
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
>>> # now swap the two points
>>> rc.a, rc.b = rc.b, rc.a
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
>>>
Hm. We certainly expected the last statement to print 3 4 1 2
. What
happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a
line above:
>>> temp0, temp1 = rc.b, rc.a
>>> rc.a = temp0
>>> rc.b = temp1
>>>
Note that temp0
and temp1
are objects still using the internal buffer of
the rc
object above. So executing rc.a = temp0
copies the buffer
contents of temp0
into rc
's buffer. This, in turn, changes the
contents of temp1
. So, the last assignment rc.b = temp1
, doesn't have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn't copy the sub-object, instead it retrieves a wrapper object accessing the root-object's underlying buffer.
Another example that may behave differently from what one would expect is this:
>>> s = c_char_p()
>>> s.value = b"abc def ghi"
>>> s.value
b'abc def ghi'
>>> s.value is s.value
False
>>>
注解
Objects instantiated from c_char_p
can only have their value set to bytes
or integers.
Why is it printing False
? ctypes instances are objects containing a memory
block plus some descriptors accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the contents
of the object is stored. Accessing the contents again
constructs a new Python object each time!
Variable-sized data types¶
ctypes
provides some support for variable-sized arrays and structures.
The resize()
function can be used to resize the memory buffer of an
existing ctypes object. The function takes the object as first argument, and
the requested size in bytes as the second argument. The memory block cannot be
made smaller than the natural memory block specified by the objects type, a
ValueError
is raised if this is tried:
>>> short_array = (c_short * 4)()
>>> print(sizeof(short_array))
8
>>> resize(short_array, 4)
Traceback (most recent call last):
...
ValueError: minimum size is 8
>>> resize(short_array, 32)
>>> sizeof(short_array)
32
>>> sizeof(type(short_array))
8
>>>
This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:
>>> short_array[:]
[0, 0, 0, 0]
>>> short_array[7]
Traceback (most recent call last):
...
IndexError: invalid index
>>>
Another way to use variable-sized data types with ctypes
is to use the
dynamic nature of Python, and (re-)define the data type after the required size
is already known, on a case by case basis.
ctypes reference¶
Foreign functions¶
As explained in the previous section, foreign functions can be accessed as attributes of loaded shared libraries. The function objects created in this way by default accept any number of arguments, accept any ctypes data instances as arguments, and return the default result type specified by the library loader. They are instances of a private class:
-
class
ctypes.
_FuncPtr
¶ Base class for C callable foreign functions.
Instances of foreign functions are also C compatible data types; they represent C function pointers.
This behavior can be customized by assigning to special attributes of the foreign function object.
-
restype
¶ Assign a ctypes type to specify the result type of the foreign function. Use
None
forvoid
, a function not returning anything.It is possible to assign a callable Python object that is not a ctypes type, in this case the function is assumed to return a C
int
, and the callable will be called with this integer, allowing further processing or error checking. Using this is deprecated, for more flexible post processing or error checking use a ctypes data type asrestype
and assign a callable to theerrcheck
attribute.
-
argtypes
¶ Assign a tuple of ctypes types to specify the argument types that the function accepts. Functions using the
stdcall
calling convention can only be called with the same number of arguments as the length of this tuple; functions using the C calling convention accept additional, unspecified arguments as well.When a foreign function is called, each actual argument is passed to the
from_param()
class method of the items in theargtypes
tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, ac_char_p
item in theargtypes
tuple will convert a string passed as argument into a bytes object using ctypes conversion rules.New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a
from_param()
method which returns a value usable as argument (integer, string, ctypes instance). This allows defining adapters that can adapt custom objects as function parameters.
-
errcheck
¶ Assign a Python function or another callable to this attribute. The callable will be called with three or more arguments:
-
callable
(result, func, arguments) result is what the foreign function returns, as specified by the
restype
attribute.func is the foreign function object itself, this allows reusing the same callable object to check or post process the results of several functions.
arguments is a tuple containing the parameters originally passed to the function call, this allows specializing the behavior on the arguments used.
The object that this function returns will be returned from the foreign function call, but it can also check the result value and raise an exception if the foreign function call failed.
-
-
-
exception
ctypes.
ArgumentError
¶ This exception is raised when a foreign function call cannot convert one of the passed arguments.
Function prototypes¶
Foreign functions can also be created by instantiating function prototypes.
Function prototypes are similar to function prototypes in C; they describe a
function (return type, argument types, calling convention) without defining an
implementation. The factory functions must be called with the desired result
type and the argument types of the function, and can be used as decorator
factories, and as such, be applied to functions through the @wrapper
syntax.
See Callback functions for examples.
-
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.
-
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.
-
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.
-
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
.The exact functionality is system dependent.
-
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.
-
ctypes.
get_last_error
()¶ Windows only: returns the current value of the ctypes-private copy of the system
LastError
variable in the calling thread.
-
ctypes.
memmove
(dst, src, count)¶ Same as the standard C memmove library function: copies count bytes from src to dst. dst and src must be integers or ctypes instances that can be converted to pointers.
-
ctypes.
memset
(dst, c, count)¶ Same as the standard C memset library function: fills the memory block at address dst with count bytes of value c. dst must be an integer specifying an address, or a ctypes instance.
-
ctypes.
POINTER
(type)¶ This factory function creates and returns a new ctypes pointer type. Pointer types are cached 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.
-
ctypes.
set_last_error
(value)¶ Windows only: set the current value of the ctypes-private copy of the system
LastError
variable in the calling thread to value and return the previous value.
-
ctypes.
sizeof
(obj_or_type)¶ Returns the size in bytes of a ctypes type or instance memory buffer. Does the same as the C
sizeof
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.
-
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.
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.
-
from_buffer_copy
(source[, offset])¶ This method creates a ctypes instance, copying the buffer from the source object buffer which must be readable. The optional offset parameter specifies an offset into the source buffer in bytes; the default is zero. If the source buffer is not large enough a
ValueError
is raised.
-
from_address
(address)¶ This method returns a ctypes type instance using the memory specified by address which must be an integer.
-
from_param
(obj)¶ This method adapts obj to a ctypes type. It is called with the actual object used in a foreign function call when the type is present in the foreign function's
argtypes
tuple; it must return an object that can be used as a function call parameter.All ctypes data types have a default implementation of this classmethod that normally returns obj if that is an instance of the type. Some types accept other objects as well.
-
in_dll
(library, name)¶ This method returns a ctypes type instance exported by a shared library. name is the name of the symbol that exports the data, library is the loaded shared library.
Common instance variables of ctypes data types:
-
_b_base_
¶ Sometimes ctypes data instances do not own the memory block they contain, instead they share part of the memory block of a base object. The
_b_base_
read-only member is the root ctypes object that owns the memory block.
-
_b_needsfree_
¶ This read-only variable is true when the ctypes data instance has allocated the memory block itself, false otherwise.
-
_objects
¶ This member is either
None
or a dictionary containing Python objects that need to be kept alive so that the memory block contents is kept valid. This object is only exposed for debugging; never modify the contents of this dictionary.
-
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.
-