Python C/C++ 拓展使用接口库(build-in) ctypes 使用手册

Python C/C++ 拓展使用接口库(build-in) ctypes 使用手册

ctypes 是一个Python 标准库中的一个库.为了实现调用 DLL,或者共享库等C数据类型而设计.它可以把这些C库包装后在纯Python环境下调用.

注意:代码中 c_int 类型其实只是 c_long 的别名,在32位系统中他们被定义为相同的数据类型.

1.1 加载动态链接库

ctypes 可以导出 cdll,在windows上则为 windll和oledll

究竟什么是 cdll,windll和oledll? 他们是DLL生成时的调用约定(不同语言生成的dll也会有细微差别). 这样来说用cdll方法导出DLL中的方法是使用cdecl方式的,windll方法则是stdcall方式的,oledll下面再具体解释.也可以查阅官方说法

CDLL:代码方式 cdecl 。

WINDLL:代码方式win32 stdcall 。

oledll使用win32调用代码方式 且返回值是windows里返回的hresult值,双字节的值说明函数执行结果,其最高bit位为0则执行成功,1则为执行失败。详细见http://www.blogjava.net/JAVA-HE/archive/2010/01/04/308134.html。

cdecl和stdcall异同,参数入栈顺序均是从右向左,不同的是栈的清除工作,cdecl是由调用者负责清除,stdcall由被调用者清除。

在python3.3中改变:WIndows的错误类型 WindowsError,现在只是OSError的别名.

下面是一个Windows中的例子.其中 msvcrt 是MS(微软)标准C库,它包含了大多数的标准C库函数,并使用 cdecl 代码方式来调用:

>>> from ctypes import *                   # 导入 ctypes模块
>>> print(windll.kernel32)                 # 使用windll约定方式导出 kernel32.dll 中的功能和信息
<WinDLL 'kernel32', handle ... at ...>
>>> print(cdll.msvcrt)                     # 使用cdll约定方式导出 msvcrt.dll 中的功能和信息
<CDLL 'msvcrt', handle ... at ...>
>>> libc = cdll.msvcrt
>>>

在windows中,通常 .dll 后缀会自动加上

而在Linux系统中,必须具体指定文件名(包含后缀)才能加载,所以基于"属性"的调用就不可能了,例如 windll.kernel32.dll 中最后一个"."到底是 kernal32 的属性还是后缀名的一部分?不得而知. 所以我们必须使用 LoadLibrary() 方法来加载 dll,或者通过实例化CDLL来加载dll.

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

在 Linux系统中(Ubuntu等) 动态链接库的编译与windows不同,后缀也不同,通常为 .so 文件,放在 usr/lib 文件夹下,而windows的dll大多放在Windows\System32文件夹下.其实原理差不多.我们这里统一称为dll代表 Dynamic Link Library,而非单指windows下的动态链接库文件.

1.2 从dll中获取函数

函数是从dll对象中的属性来获取得到的

接着上面的代码

>>> libc.printf
<_FuncPtr object at 0x...>                   # 可以看到 libc.printf 函数的信息
>>> print(windll.kernel32.GetModuleHandleA)
<_FuncPtr object at 0x...>                   # 与上述相同
>>> print(windll.kernel32.MyOwnFunction)     # 返回错误信息没有该函数属性
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "ctypes.py", line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
>>>

注意win32系统dll像 kernel32 和 user32 通常既会返回ANSI也会返回UNICODE的函数版本. UNICODE版本通常会有"W"作为名字后缀,ANSI则是A.

比如 win32 中的 GetModuleHandle函数会根据 module的名字返回一个 module handle, 库内部根据宏定义选择以下两个版本之一作为函数原型:

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

windll 不会神奇的选择其中的一项,你必须显示的指定调用,然后使用指定的类型参数(宽字符)

有些时候 dll 导出的函数不是Python中的有效值,比如 "??2@YAPAXI@Z". 在这种情况下你必须使用函数 getattr() 来获取函数:

>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")   # 因为 cdll.msvcrt.??2@YAPAXI@Z 不合法,变量名(属性)不可以这样定义
<_FuncPtr object at 0x...>
>>>

在windows中,一些dll的导出函数并不是按名字来的,而是下标数字. 这些函数可以用下标序号来获取,比如:

>>> cdll.kernel32[1]               # 通过下标获取函数信息
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0]               # 可见并不是按顺序排列的...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "ctypes.py", line 310, in __getitem__
func = _StdcallFuncPtr(name, self)
AttributeError: function ordinal 0 not found
>>>

1.3 调用函数

你可以像调用Python一样调用这些函数.在这个例子中我们使用 time() 函数,该函数返回自Unix时间戳(1970年1月1日00:00:00 UTC)到现在的累计总秒数.(会不会int32值不够用?int32可以代表68年间的总秒数uint32则136年,uint64则是584942417354年)

下面的例子中函数都以 NULL 指针来调用( None在python中代表 NULL)

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

ctypes试图阻止你用错误的参数和代码风格,但这种徒劳只在Windows下有效.

>>> windll.kernel32.GetModuleHandleA()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
>>> windll.kernel32.GetModuleHandleA(0, 0)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
>>>

下面一个例子会报错,原因是错误的使用 cdecl 风格来调用 stdcall 风格的函数,反过来也是错的

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

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

为了正确地使用调用函数风格你必须到C头文件中去查看,或者查阅有关文档.

在Windows中,ctypes使用win32结构的异常处理机制来防止程序 crash掉,当你传入无效参数的时候.

>>> windll.kernel32.GetModuleHandleA(32)  # 试图获得一个并不存在的模块
Traceback (most recent call last):
File "<stdin>", line 1, in ?
OSError: exception: access violation reading 0x00000020  # 捕获到的异常
>>>

但令 ctypes crash掉有诸多方法(甚至没有异常捕获到),所以你必须很小心. faulthandler 模块(python内置)可以帮助你debug crash的具体原因.

None,integers,bytes,(unicode)strings 是仅有的可以被直接作为函数调用参数的Python原生结构.其中 None 对应C语言中 Null, bytes和 strings 作为内存块的指针 (char ,wchar_t ). Python中的 integers 对应C中的 int 类型,他们的值可被直接转换成C类型.

在我们使用其他类型的参数来调用C函数前,先来看一下 ctypes 中的数据类型

1.4 基本数据类型

ctypes 定义了一些基础C兼容的类型

ctypes type	C type	Python type
c_bool	_Bool	bool(1)
c_char	char	1-character bytes object
c_wchar	wchar_t	1-charactor string
c_byte	char	int
c_ubyte	unsigned char	int
c_short	short	int
c_ushort	unsigned short	int
c_int	int	int
c_uint	unsigned int	int
c_long	long	int
c_ulong	unsigned long	int
c_longlong	__int64 or long long	int
c_ulonglong	unsigned __int64 or unsigned long long	int
c_size_t	size_t	int
c_ssize_t	ssize_t or Py_ssize_t	int
c_float	float	float
c_double	double	float
c_longdouble	long double	float
c_char_p	char * (NUL terminated)	bytes object or None
c_wchar_p	wchar_t * (NUL terminated)	string or None
c_void_p	void *	int or None

构造函数接受任意对象(只要是值为真)

所有这些类型都可以用相应的类型和值来调用构造函数.

>>> c_int()     # 在文章前已经提及 ctypes 中 c_int只是 c_long的别名而已
c_long(0)
>>> c_wchar_p("Hello, World")
c_wchar_p('Hello, World')
>>> c_ushort(-3)
c_ushort(65533)
>>>

因为这些类型都是可变的(mutable),他们的值同样可以在定义之后被修改

>>> i = c_int(42)
>>> print(i)
c_long(42)
>>> print(i.value)
42
>>> i.value = -99  # 注意别忘了Python的特性,ctypes所有类型都是一个对象包装
>>> print(i.value) # 如果你使用i=-99,则 i会直接被Python原生int替换...
-99
>>>

给指针类型赋新值等于改变他们指向内存的位置,而不是修改他们所指内存中的值,指针类型有c_char_p, c_wchar_p和 c_void_p.(这非常好理解,因为Python中的 bytes 对象是不可修改的常量):

>>> s = "Hello, World"
>>> c_s = c_wchar_p(s)
>>> print(c_s)
c_wchar_p('Hello, World')
>>> c_s.value = "Hi, there"
>>> print(c_s)
c_wchar_p('Hi, there')
>>> print(s)                 # first object is unchanged
Hello, World
>>>

你应该小心,不要把这些指针传给试图改变内存的函数. 如果你确实需要改变内存数据而非替换指针地址, ctypes提供了create_string_buffer()函数.

内存块可以使用 raw 属性来访问和修改; 如果你希望访问一个以 NUL 为结尾 string, 则使用 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"                       # 这里注意 p.value = b'HI'并不是把value替换成常量b'HI'的指针,而是直接修改了buffer
>>> print(sizeof(p), repr(p.raw))
10 b'Hi\x00lo\x00\x00\x00\x00\x00'        # 从这里看得到，确实是 buffer 被修改了,上一次的值Hello 中的lo还在内存之中.
>>>

create_string_buffer() 函数代替了以前的 c_buffer() 函数(现在依旧可用,作为别名). 为了创建可修改的unicode wchar_t类型内存块, 请使用 create_unicode_buffer() 函数.

1.5 再谈调用函数

注意 printf 打印变量至标准输出通道, 而不是 sys.stdout, 所以这些例子只有在控制台有输出,而不会输出在 IDLE 或者 PythonWin之中.

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

就像之前提到的那样, 只有四种类型 integers:42, (unicode)strings:"World!", bytes objects:b"World!", NULL:None. 其他所有相应类型类型都需要用 ctypes进行相应的包装才能被使用:

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

1.6 使用自定义数据类型调用函数

你可以自定义 ctypes 的参数变换来使用自定义数据类型作为函数参数. ctypes 查看 as_parameter 这个属性,并使用它作为函数参数. 当然,它必须是Python支持的四种类型之一:

>>> 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
>>>

如果你不想事先储存实例数据在 as_paramter 之中,那你也可以动态地给任意一个对象增加这个属性值.

1.7 指定参数类型(函数原型定义)

通过设置 argtypes 属性,我们可以指定函数的参数类型.

argtypes 必须是C数据类型的一个数列(printf 可能并不是个很好的例子,但是可以用来实验这个特性):

>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]  # 指定4个参数,按顺序
>>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
String 'Hi', Int 10, Double 2.200000
37
>>>

指定参数类型防止使用者不小心传入错误的参数类型(就像C函数的原型定义那样),并试图转换无效参数至有效的数据类型:

>>> printf(b"%d %d %d", 1, 2, 3)               # 与 argtypes 中定义的参数数列不匹配并报错
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: wrong type
>>> printf(b"%s %d %f\n", b"X", 2, 3)          # 可以看到最后一个参数被 c_double(3)转化成了有效值
X 2 3.000000
13
>>>

如果你定义了一个自己的类,并试图将它作为参数传入函数时,你必须实现它的 from_param() 类方法,为了能在 argtypes 数列中使用他们. from_param() 类方法会获得函数调用时对应参数位置传入的Python对象并且由您自己判断并实现您觉得必要的一些类型检查工作,最后返回该传入的对象或者该对象的 as_parameter 属性,又或者是你想返回的任何东西(返回内容完全看你的心情). 当然返回结果也必须是四种原生数据类型中的一种,或者依旧是一个带有 _as_parameter_属性的对象.(注:所有这些只有在你想使用 argtypes 来做函数的参数类型限定时才是必须的)

1.8 返回值类型

ctypes默认函数的返回值应该是 C int类型的. 其他类型的返回值则要使用函数对象的 restpye 属性来设置.

下面是一个更高级的例子,它使用 strchr 函数(接受一个 string 指针和一个 char,查找字符串中首次出现字符char的位置并返回指针)

>>> strchr = libc.strchr
>>> strchr(b"abcdef", ord("d"))
8059983            # 这里ctypes并不知道返回的是什么,所以默认直接就把指针地址打印了出来(int型)
>>> strchr.restype = c_char_p   # c_char_p is a pointer to a string
>>> strchr(b"abcdef", ord("d"))
b'def'             # 这里设置过了,ctypes知道返回的是c_char_p类型,所以打印该指针指向的字符串数据
>>> print(strchr(b"abcdef", ord("x")))
None
>>>

如果你想避免使用 ord()函数(用来返回char字符的数字编码), 你可以设置 argtypes ,那么第二个参数就会从Python的单字节对象转换成 C char类型数据:

>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> strchr(b"abcdef", b"d")
'def'
>>> strchr(b"abcdef", b"def")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: one character string expected
>>> print(strchr(b"abcdef", b"x"))
None
>>> strchr(b"abcdef", b"d")
'def'
>>>

--------------------余下部分还未翻译--------------------

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.restype = ValidHandle

GetModuleHandle(None)

486539264

GetModuleHandle("something silly")

Traceback (most recent call last):

File "", line 1, in ?

File "", line 3, in ValidHandle

WindowsError: [Errno 126] The specified module could not be found.

WinError is a function which will call Windows FormatMessage() api to get the string representation of an error code, and returns an exception. WinError takes an optional error code parameter, if no one is used, it calls GetLastError() to retrieve it.

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

15.17.1.9. 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:



15.17.1.10. 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:



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:



Nested structures can also be initialized in the constructor in several ways:



Field descriptors can be retrieved from the class, they are useful for debugging because they can provide useful information:



Warning:

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.

15.17.1.11. Structure/union alignment and byte order

By default, Structure and Union fields are aligned in the same way the C compiler does it. It is possible to override this behavior be specifying a pack class attribute in the subclass definition. This must be set to a positive integer and specifies the maximum alignment for the fields. This is what #pragma pack(n) also does in MSVC.

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

15.17.1.12. 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:



15.17.1.13. Arrays

Arrays are sequences, containing a fixed number of instances of the same type.

The recommended way to create array types is by multiplying a data type with a positive integer:

TenPointsArrayType = POINT * 10

Here is an example of an somewhat artificial data type, a structure containing 4 POINTs among other stuff:



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:



15.17.1.14. Pointers

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



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



Note that ctypes does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:



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:



Pointer instances can also be indexed with integers:



Assigning to an integer index changes the pointed to value:



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:



Calling the pointer type without an argument creates a NULL pointer. NULL pointers have a False boolean value:



ctypes checks for NULL when dereferencing pointers (but dereferencing invalid non-NULL pointers would crash Python):





15.17.1.15. 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:



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:



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:



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:



So, cast() can be used to assign to the values field of Bar the structure:



15.17.1.16. 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:



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:



Lets try it. We create two instances of cell, and let them point to each other, and finally follow the pointer chain a few times:



15.17.1.17. 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 normal cdecl calling convention, and, on Windows, the WINFUNCTYPE factory function creates types for callback functions using the stdcall calling convention.

Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.

I will present an example here which uses the standard C library’s qsort() function, this is used to sort items with the help of a callback function. qsort() will be used to sort an array of integers:



qsort() must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer else.

So our callback function receives pointers to integers, and must return an integer. First we create the type for the callback function:



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



Create the C callable callback:



And we’re ready to go:



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



Here is what we get on Windows:



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



Ah, we’re nearly done! The last step is to actually compare the two items and return a useful result:



Final run on Windows:



and on Linux:



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

As we can easily check, our array is sorted now:



Note:

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.

15.17.1.18. 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:



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 Python docs: This pointer is initialized to point to an array of “struct _frozen” records, terminated by one whose members are all NULL or zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.

So manipulating this pointer could even prove useful. To restrict the example size, we show only how this table can be read with ctypes:



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:



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:



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.

15.17.1.19. Surprises

There are some edge cases in ctypes where you might expect something other than what actually happens.

Consider the following example:

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:



Note that temp0 and temp1 are objects still using the internal buffer of the rc object above. So executing rc.a = temp0 copies the buffer contents of temp0 into rc ‘s buffer. This, in turn, changes the contents of temp1. So, the last assignment rc.b = temp1, doesn’t have the expected effect.

Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn’t copy the sub-object, instead it retrieves a wrapper object accessing the root-object’s underlying buffer.

Another example that may behave different from what one would expect is this:



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!

15.17.1.20. 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:



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:



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.

15.17.2. ctypes reference

15.17.2.1. Finding shared libraries

When programming in a compiled language, shared libraries are accessed when compiling/linking a program, and when the program is run.

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

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

ctypes.util.find_library(name)

Try to find a library and return a pathname. name is the library name without any prefix like lib, suffix like .so, .dylib or version number (this is the form used for the posix linker option -l). If no library can be found, returns None.

The exact functionality is system dependent.

On Linux, find_library() tries to run external programs (/sbin/ldconfig, gcc, and objdump) to find the library file. It returns the filename of the library file. Here are some examples:



On OS X, find_library() tries several predefined naming schemes and paths to locate the library, and returns a full pathname if successful:



On Windows, find_library() searches along the system search path, and returns the full pathname, but since there is no predefined naming scheme a call like find_library("c") will fail and return None.

If wrapping a shared library with ctypes, it may be better to determine the shared library name at development time, and hardcode that into the wrapper module instead of using find_library() to locate the library at runtime.

15.17.2.2. Loading shared libraries

There are several ways to load shared libraries into the Python process. One way is to instantiate one of the following classes:

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

Instances of this class represent loaded shared libraries. Functions in these libraries use the standard C calling convention, and are assumed to return int.

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

Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the stdcall calling convention, and are assumed to return the windows specific HRESULT code. HRESULT values contain information specifying whether the function call failed or succeeded, together with additional error code. If the return value signals a failure, an WindowsError is automatically raised.

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

Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the stdcall calling convention, and are assumed to return int by default.

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

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

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

Instances of this class behave like CDLL instances, except that the Python GIL is not released during the function call, and after the function execution the Python error flag is checked. If the error flag is set, a Python exception is raised.

Thus, this is only useful to call Python C api functions directly.

All these classes can be instantiated by calling them with at least one argument, the pathname of the shared library. If you have an existing handle to an already loaded shared library, it can be passed as the handle named parameter, otherwise the underlying platforms dlopen or LoadLibrary function is used to load the library into the process, and to get a handle to it.

The mode parameter can be used to specify how the library is loaded. For details, consult the dlopen(3) manpage, on Windows, mode is ignored.

The use_errno parameter, when set to True, enables a ctypes mechanism that allows accessing the system errno error number in a safe way. ctypes maintains a thread-local copy of the systems errno variable; if you call foreign functions created with use_errno=True then the errno value before the function call is swapped with the ctypes private copy, the same happens immediately after the function call.

The function ctypes.get_errno() returns the value of the ctypes private copy, and the function ctypes.set_errno() changes the ctypes private copy to a new value and returns the former value.

The use_last_error parameter, when set to True, enables the same mechanism for the Windows error code which is managed by the GetLastError() and SetLastError() Windows API functions; ctypes.get_last_error() and ctypes.set_last_error() are used to request and change the ctypes private copy of the windows error code.

New in version 2.6: The use_last_error and use_errno optional parameters were added.

ctypes.RTLD_GLOBAL

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

ctypes.RTLD_LOCAL

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

ctypes.DEFAULT_MODE

The default mode which is used to load shared libraries. On OSX 10.3, this is RTLD_GLOBAL, otherwise it is the same as RTLD_LOCAL.

Instances of these classes have no public methods. Functions exported by the shared library can be accessed as attributes or by index. Please note that accessing the function through an attribute caches the result and therefore accessing it repeatedly returns the same object each time. On the other hand, accessing it through an index returns a new object each time:



The following public attributes are available, their name starts with an underscore to not clash with exported function names:

PyDLL._handle

The system handle used to access the library.

PyDLL._name

The name of the library passed in the constructor.

Shared libraries can also be loaded by using one of the prefabricated objects, which are instances of the LibraryLoader class, either by calling the LoadLibrary() method, or by retrieving the library as attribute of the loader instance.

class ctypes.LibraryLoader(dlltype)

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

getattr() has special behavior: It allows loading a shared library by accessing it as attribute of a library loader instance. The result is cached, so repeated attribute accesses return the same library each time.

LoadLibrary(name)

Load a shared library into the process and return it. This method always returns a new instance of the library.

These prefabricated library loaders are available:

ctypes.cdll

Creates CDLL instances.

ctypes.windll

Windows only: Creates WinDLL instances.

ctypes.oledll

Windows only: Creates OleDLL instances.

ctypes.pydll

Creates PyDLL instances.

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

ctypes.pythonapi

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

15.17.2.3. 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 for void, a function not returning anything.

It is possible to assign a callable Python object that is not a ctypes type, in this case the function is assumed to return a C int, and the callable will be called with this integer, allowing further processing or error checking. Using this is deprecated, for more flexible post processing or error checking use a ctypes data type as restype and assign a callable to the errcheck attribute.

argtypes

Assign a tuple of ctypes types to specify the argument types that the function accepts. Functions using the stdcall calling convention can only be called with the same number of arguments as the length of this tuple; functions using the C calling convention accept additional, unspecified arguments as well.

When a foreign function is called, each actual argument is passed to the from_param() class method of the items in the argtypes tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, a c_char_p item in the argtypes tuple will convert a unicode string passed as argument into an byte string using ctypes conversion rules.

New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a from_param() method which returns a value usable as argument (integer, string, ctypes instance). This allows 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.

15.17.2.4. Function prototypes

Foreign functions can also be created by instantiating function prototypes. Function prototypes are similar to function prototypes in C; they describe a function (return type, argument types, calling convention) without defining an implementation. The factory functions must be called with the desired result type and the argument types of the function.

ctypes.CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)

The returned function prototype creates functions that use the standard C calling convention. The function will release the GIL during the call. If use_errno is set to True, the ctypes private copy of the system errno variable is exchanged with the real errno value before and after the call; use_last_error does the same for the Windows error code.

Changed in version 2.6: The optional use_errno and use_last_error parameters were added.

ctypes.WINFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)

Windows only: The returned function prototype creates functions that use the stdcall calling convention, except on Windows CE where WINFUNCTYPE() is the same as CFUNCTYPE(). The function will release the GIL during the call. use_errno and use_last_error have the same meaning as above.

ctypes.PYFUNCTYPE(restype, *argtypes)

The returned function prototype creates functions that use the Python calling convention. The function will not release the GIL during the call.

Function prototypes created by these factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call:

prototype(address)

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

prototype(callable)

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

prototype(func_spec[, paramflags])

Returns a foreign function exported by a shared library. func_spec must be a 2-tuple (name_or_ordinal, library). The first item is the name of the exported function as string, or the ordinal of the exported function as small integer. The second item is the shared library instance.

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

Returns a foreign function that will call a COM method. vtbl_index is the index into the virtual function table, a small non-negative integer. name is name of the COM method. iid is an optional pointer to the interface identifier which is used in extended error reporting.

COM methods use a special calling convention: They require a pointer to the COM interface as first argument, in addition to those parameters that are specified in the argtypes tuple.

The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.

paramflags must be a tuple of the same length as argtypes.

Each item in this tuple contains further information about a parameter, it must be a tuple containing one, two, or three items.

The first item is an integer containing a combination of direction flags for the parameter:

1

Specifies an input parameter to the function.

2

Output parameter. The foreign function fills in a value.

4

Input parameter which defaults to the integer zero.

The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.

The optional third item is the default value for this parameter.

This example demonstrates how to wrap the Windows MessageBoxA function so that it supports default parameters and named arguments. The C declaration from the windows header file is this:

WINUSERAPI int WINAPI

MessageBoxA(

HWND hWnd,

LPCSTR lpText,

LPCSTR lpCaption,

UINT uType);

Here is the wrapping with ctypes:



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



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:



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:



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:



15.17.2.5. 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.

New in version 2.6: The offset optional argument was added.

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])

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

init_or_size must be an integer which specifies the size of the array, or a string which will be used to initialize the array items.

If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the string should not be used.

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

ctypes.create_unicode_buffer(init_or_size[, size])

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

init_or_size must be an integer which specifies the size of the array, or a unicode string which will be used to initialize the array items.

If a unicode string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the string should not be used.

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

ctypes.DllCanUnloadNow()

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

ctypes.DllGetClassObject()

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

ctypes.util.find_library(name)

Try to find a library and return a pathname. name is the library name without any prefix like lib, suffix like .so, .dylib or version number (this is the form used for the posix linker option -l). If no library can be found, returns None.

The exact functionality is system dependent.

Changed in version 2.6: Windows only: find_library("m") or find_library("c") return the result of a call to find_msvcrt().

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.

New in version 2.6.

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.

New in version 2.6.

ctypes.get_last_error()

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

New in version 2.6.

ctypes.memmove(dst, src, count)

Same as the standard C memmove library function: copies count bytes from src to dst. dst and src must be integers or ctypes instances that can be converted to pointers.

ctypes.memset(dst, c, count)

Same as the standard C memset library function: fills the memory block at address dst with count bytes of value c. dst must be an integer specifying an address, or a ctypes instance.

ctypes.POINTER(type)

This factory function creates and returns a new ctypes pointer type. Pointer types are cached an reused internally, so calling this function repeatedly is cheap. type must be a ctypes type.

ctypes.pointer(obj)

This function creates a new pointer instance, pointing to obj. The returned object is of the type POINTER(type(obj)).

Note: If you just want to pass a pointer to an object to a foreign function call, you should use byref(obj) which is much faster.

ctypes.resize(obj, size)

This function resizes the internal memory buffer of obj, which must be an instance of a ctypes type. It is not possible to make the buffer smaller than the native size of the objects type, as given by sizeof(type(obj)), but it is possible to enlarge the buffer.

ctypes.set_conversion_mode(encoding, errors)

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

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

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.

New in version 2.6.

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.

New in version 2.6.

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])

This function returns the string starting at memory address address. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated.

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

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

ctypes.wstring_at(address[, size])

This function returns the wide character string starting at memory address address as unicode string. If size is specified, it is used as the number of characters of the string, otherwise the string is assumed to be zero-terminated.

15.17.2.6. 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.

New in version 2.6.

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.

New in version 2.6.

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.

15.17.2.7. Fundamental data types

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.

Changed in version 2.6: 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 string, for character pointer types it is a Python string or unicode string.

When the value attribute is retrieved from a ctypes instance, usually a new object is returned each time. ctypes does not implement original object return, always a new object is constructed. The same is true for all other ctypes object instances.

Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python string, not a c_char_p instance.

Subclasses of fundamental data types do not inherit this 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 string.

class ctypes.c_double

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

class ctypes.c_longdouble

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

New in version 2.6.

class ctypes.c_float

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

class ctypes.c_int

Represents the C signed int datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where sizeof(int) == sizeof(long) it is an alias to c_long.

class ctypes.c_int8

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

class ctypes.c_int16

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

class ctypes.c_int32

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

class ctypes.c_int64

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

class ctypes.c_long

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

class ctypes.c_longlong

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

class ctypes.c_short

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

class ctypes.c_size_t

Represents the C size_t datatype.

class ctypes.c_ssize_t

Represents the C ssize_t datatype.

New in version 2.7.

class ctypes.c_ubyte

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

class ctypes.c_uint

Represents the C unsigned int datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where sizeof(int) == sizeof(long) it is an alias for c_ulong.

class ctypes.c_uint8

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

class ctypes.c_uint16

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

class ctypes.c_uint32

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

class ctypes.c_uint64

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

class ctypes.c_ulong

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

class ctypes.c_ulonglong

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

class ctypes.c_ushort

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

class ctypes.c_void_p

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

class ctypes.c_wchar

Represents the C wchar_t datatype, and interprets the value as a single character unicode string. The constructor accepts an optional string initializer, the length of the string must be exactly one character.

class ctypes.c_wchar_p

Represents the C wchar_t * datatype, which must be a pointer to a zero-terminated wide character string. The constructor accepts an integer address, or a string.

class ctypes.c_bool

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

New in version 2.6.

class ctypes.HRESULT

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

class ctypes.py_object

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

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

15.17.2.8. 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 defined sub-subclasses of structure types, they inherit the fields of the base class plus the fields defined in the sub-subclass, if any.

pack

An optional small integer that allows 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 allow accessing the nested fields directly, without the need to create the structure or union field.

Here is an example type (Windows):

class _U(Union):

fields = [("lptdesc", POINTER(TYPEDESC)),

("lpadesc", POINTER(ARRAYDESC)),

("hreftype", HREFTYPE)]

class TYPEDESC(Structure):

anonymous = ("u",)

fields = [("u", _U),

("vt", VARTYPE)]

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

td = TYPEDESC()

td.vt = VT_PTR

td.lptdesc = POINTER(some_type)

td.u.lptdesc = POINTER(some_type)

It is possible to defined sub-subclasses of structures, they inherit the fields of the base class. If the subclass definition has a separate fields variable, the fields specified in this are appended to the fields of the base class.

Structure and union constructors accept both positional and keyword arguments. Positional arguments are used to initialize member fields in the same order as they are appear in fields. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize fields with the same name, or create new attributes for names not present in fields.

15.17.2.9. Arrays and pointers

class ctypes.Array(*args)

Abstract base class for arrays.

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

length

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

type

Specifies the type of each element in the array.

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

class ctypes._Pointer

Private, abstract base class for pointers.

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

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

type

Specifies the type pointed to.

contents

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