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

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

ctypes 是一個Python 標準庫中的一個庫.爲了實現調用 DLL,或者共享庫等C數據類型而設計.它能夠把這些C庫包裝後在純Python環境下調用.

注意:代碼中 c_int 類型其實只是 c_long 的別名,在32位系統中他們被定義爲相同的數據類型.

1.1 加載動態連接庫

ctypes 能夠導出 cdll,在windows上則爲 windll和oledll
究竟什麼是 cdll,windll和oledll? 他們是DLL生成時的調用約定(不一樣語言生成的dll也會有細微差異). 這樣來講用cdll方法導出DLL中的方法是使用cdecl方式的,windll方法則是stdcall方式的,oledll下面再具體解釋.也能夠查閱官方說法

CDLL:代碼方式 cdecl 。

WINDLL:代碼方式win32 stdcall 。

oledll使用win32調用代碼方式 且返回值是windows裏返回的hresult值,雙字節的值說明函數執行結果,其最高bit位爲0則執行成功,1則爲執行失敗。詳細見http://www.blogjava.net/JAVA-HE/archive/2010/01/04/308134.html。

cdecl和stdcall異同,參數入棧順序均是從右向左,不一樣的是棧的清除工做,cdecl是由調用者負責清除,stdcall由被調用者清除。

在python3.3中改變:WIndows的錯誤類型 WindowsError,如今只是OSError的別名.

下面是一個Windows中的例子.其中 msvcrt 是MS(微軟)標準C庫,它包含了大多數的標準C庫函數,並使用 cdecl 代碼方式來調用:

>>> from ctypes import *                   # 導入 ctypes模塊
>>> print(windll.kernel32)                 # 使用windll約定方式導出 kernel32.dll 中的功能和信息
<WinDLL 'kernel32', handle ... at ...>  
>>> print(cdll.msvcrt)                     # 使用cdll約定方式導出 msvcrt.dll 中的功能和信息
<CDLL 'msvcrt', handle ... at ...>
>>> libc = cdll.msvcrt 
>>>

在windows中,一般 .dll 後綴會自動加上
而在Linux系統中,必須具體指定文件名(包含後綴)才能加載,因此基於"屬性"的調用就不可能了,例如 windll.kernel32.dll 中最後一個"."究竟是 kernal32 的屬性仍是後綴名的一部分?不得而知. 因此咱們必須使用 LoadLibrary() 方法來加載 dll,或者經過實例化CDLL來加載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系統中(Ubuntu等) 動態連接庫的編譯與windows不一樣,後綴也不一樣,一般爲 .so 文件,放在 usr/lib 文件夾下,而windows的dll大多放在Windows\System32文件夾下.其實原理差很少.咱們這裏統一稱爲dll表明 Dynamic Link Library,而非單指windows下的動態連接庫文件.

1.2 從dll中獲取函數

函數是從dll對象中的屬性來獲取獲得的
接着上面的代碼

>>> libc.printf
<_FuncPtr object at 0x...>                   # 能夠看到 libc.printf 函數的信息
>>> print(windll.kernel32.GetModuleHandleA) 
<_FuncPtr object at 0x...>                   # 與上述相同
>>> print(windll.kernel32.MyOwnFunction)     # 返回錯誤信息沒有該函數屬性
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系統dll像 kernel32 和 user32 一般既會返回ANSI也會返回UNICODE的函數版本. UNICODE版本一般會有"W"做爲名字後綴,ANSI則是A.
好比 win32 中的 GetModuleHandle函數會根據 module的名字返回一個 module handle, 庫內部根據宏定義選擇如下兩個版本之一做爲函數原型:

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

windll 不會神奇的選擇其中的一項,你必須顯示的指定調用,而後使用指定的類型參數(寬字符)
有些時候 dll 導出的函數不是Python中的有效值,好比 "??2@YAPAXI@Z". 在這種狀況下你必須使用函數 getattr() 來獲取函數:

>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")   # 由於 cdll.msvcrt.??2@YAPAXI@Z 不合法,變量名(屬性)不能夠這樣定義
<_FuncPtr object at 0x...>
>>>

在windows中,一些dll的導出函數並非按名字來的,而是下標數字. 這些函數能夠用下標序號來獲取,好比:

>>> cdll.kernel32[1]               # 經過下標獲取函數信息
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0]               # 可見並非按順序排列的...
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 調用函數

你能夠像調用Python同樣調用這些函數.在這個例子中咱們使用 time() 函數,該函數返回自Unix時間戳(1970年1月1日00:00:00 UTC)到如今的累計總秒數.(會不會int32值不夠用?int32能夠表明68年間的總秒數uint32則136年,uint64則是584942417354年)
下面的例子中函數都以 NULL 指針來調用( None在python中表明 NULL)

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

ctypes試圖阻止你用錯誤的參數和代碼風格,但這種徒勞只在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)
>>>

下面一個例子會報錯,緣由是錯誤的使用 cdecl 風格來調用 stdcall 風格的函數,反過來也是錯的

>>> 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)
>>>

爲了正確地使用調用函數風格你必須到C頭文件中去查看,或者查閱有關文檔.

在Windows中,ctypes使用win32結構的異常處理機制來防止程序 crash掉,當你傳入無效參數的時候.

>>> windll.kernel32.GetModuleHandleA(32)  # 試圖得到一個並不存在的模塊
Traceback (most recent call last):
    File "<stdin>", line 1, in ?
OSError: exception: access violation reading 0x00000020  # 捕獲到的異常
>>>

但令 ctypes crash掉有諸多方法(甚至沒有異常捕獲到),因此你必須很當心. faulthandler 模塊(python內置)能夠幫助你debug crash的具體緣由.

None,integers,bytes,(unicode)strings 是僅有的能夠被直接做爲函數調用參數的Python原生結構.其中 None 對應C語言中 Null, bytes和 strings 做爲內存塊的指針 (char ,wchar_t ). Python中的 integers 對應C中的 int 類型,他們的值可被直接轉換成C類型.

在咱們使用其餘類型的參數來調用C函數前,先來看一下 ctypes 中的數據類型

1.4 基本數據類型

ctypes 定義了一些基礎C兼容的類型

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

構造函數接受任意對象(只要是值爲真)
全部這些類型均可以用相應的類型和值來調用構造函數.

>>> c_int()     # 在文章前已經說起 ctypes 中 c_int只是 c_long的別名而已
c_long(0)
>>> c_wchar_p("Hello, World")
c_wchar_p('Hello, World')
>>> c_ushort(-3)
c_ushort(65533)
>>>

由於這些類型都是可變的(mutable),他們的值一樣能夠在定義以後被修改

>>> i = c_int(42)
>>> print(i)
c_long(42)
>>> print(i.value)
42
>>> i.value = -99  # 注意別忘了Python的特性,ctypes全部類型都是一個對象包裝
>>> print(i.value) # 若是你使用i=-99,則 i會直接被Python原生int替換...
-99
>>>

給指針類型賦新值等於改變他們指向內存的位置,而不是修改他們所指內存中的值,指針類型有c_char_p, c_wchar_p和 c_void_p.(這很是好理解,由於Python中的 bytes 對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
>>>

你應該當心,不要把這些指針傳給試圖改變內存的函數. 若是你確實須要改變內存數據而非替換指針地址, ctypes提供了create_string_buffer()函數.
內存塊可使用 raw 屬性來訪問和修改; 若是你但願訪問一個以 NUL 爲結尾 string, 則使用 value屬性:

>>> 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"                       # 這裏注意 p.value = b'HI'並非把value替換成常量b'HI'的指針,而是直接修改了buffer
>>> print(sizeof(p), repr(p.raw))
10 b'Hi\x00lo\x00\x00\x00\x00\x00'        # 從這裏看獲得，確實是 buffer 被修改了,上一次的值Hello 中的lo還在內存之中.
>>>

create_string_buffer() 函數代替了之前的 c_buffer() 函數(如今依舊可用,做爲別名). 爲了建立可修改的unicode wchar_t類型內存塊, 請使用 create_unicode_buffer() 函數.

1.5 再談調用函數

注意 printf 打印變量至標準輸出通道, 而不是 sys.stdout, 因此這些例子只有在控制檯有輸出,而不會輸出在 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
>>>

就像以前提到的那樣, 只有四種類型 integers:42, (unicode)strings:"World!", bytes objects:b"World!", NULL:None. 其餘全部相應類型類型都須要用 ctypes進行相應的包裝才能被使用:

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

1.6 使用自定義數據類型調用函數

你能夠自定義 ctypes 的參數變換來使用自定義數據類型做爲函數參數. ctypes 查看 as_parameter 這個屬性,並使用它做爲函數參數. 固然,它必須是Python支持的四種類型之一:

>>> 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
>>>

若是你不想事先儲存實例數據在 as_paramter 之中,那你也能夠動態地給任意一個對象增長這個屬性值.

1.7 指定參數類型(函數原型定義)

經過設置 argtypes 屬性,咱們能夠指定函數的參數類型.
argtypes 必須是C數據類型的一個數列(printf 可能並非個很好的例子,可是能夠用來實驗這個特性):

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

指定參數類型防止使用者不當心傳入錯誤的參數類型(就像C函數的原型定義那樣),並試圖轉換無效參數至有效的數據類型:

>>> printf(b"%d %d %d", 1, 2, 3)               # 與 argtypes 中定義的參數數列不匹配並報錯
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)          # 能夠看到最後一個參數被 c_double(3)轉化成了有效值
X 2 3.000000
13
>>>

若是你定義了一個本身的類,並試圖將它做爲參數傳入函數時,你必須實現它的 from_param() 類方法,爲了能在 argtypes 數列中使用他們. from_param() 類方法會得到函數調用時對應參數位置傳入的Python對象而且由您本身判斷並實現您以爲必要的一些類型檢查工做,最後返回該傳入的對象或者該對象的 as_parameter 屬性,又或者是你想返回的任何東西(返回內容徹底看你的心情). 固然返回結果也必須是四種原生數據類型中的一種,或者依舊是一個帶有 _as_parameter_屬性的對象.(注:全部這些只有在你想使用 argtypes 來作函數的參數類型限定時纔是必須的)

1.8 返回值類型

ctypes默認函數的返回值應該是 C int類型的. 其餘類型的返回值則要使用函數對象的 restpye 屬性來設置.
下面是一個更高級的例子,它使用 strchr 函數(接受一個 string 指針和一個 char,查找字符串中首次出現字符char的位置並返回指針)

>>> strchr = libc.strchr
>>> strchr(b"abcdef", ord("d")) 
8059983            # 這裏ctypes並不知道返回的是什麼,因此默認直接就把指針地址打印了出來(int型)
>>> strchr.restype = c_char_p   # c_char_p is a pointer to a string
>>> strchr(b"abcdef", ord("d"))
b'def'             # 這裏設置過了,ctypes知道返回的是c_char_p類型,因此打印該指針指向的字符串數據
>>> print(strchr(b"abcdef", ord("x")))
None
>>>

若是你想避免使用 ord()函數(用來返回char字符的數字編碼), 你能夠設置 argtypes ,那麼第二個參數就會從Python的單字節對象轉換成 C char類型數據:

>>> 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'
>>>

--------------------餘下部分還未翻譯--------------------

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

i = c_int()
f = c_float()
s = create_string_buffer('\000' * 32)
print i.value, f.value, repr(s.value)
0 0.0 ''
libc.sscanf("1 3.14 Hello", "%d %f %s",
... byref(i), byref(f), s)
3
print i.value, f.value, repr(s.value)
1 3.1400001049 'Hello'

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:

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 " ", line 1, in ?
ValueError: 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

print POINT.y

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:

class Int(Structure):
... fields = [("first_16", c_int, 16),
... ("second_16", c_int, 16)]
...
print Int.first_16

print Int.second_16

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:

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,
...
1 2 3 4 5 6 7 8 9 10

15.17.1.14. 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 " ", line 1, in ?
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

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:

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 " ", line 1, in ?
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

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:

class cell(Structure):
... fields = [("name", c_char_p),
... ("next", POINTER(cell))]
...
Traceback (most recent call last):
File " ", line 1, in ?
File " ", 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))]

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:

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,
... p = p.next[0]
...
foo bar foo bar foo bar foo bar

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:

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 else.

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))

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

def py_cmp_func(a, b):
... print "py_cmp_func", a, b
... return 0
...

Create the C callable callback:

cmp_func = CMPFUNC(py_cmp_func)

And we’re ready to go:

qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>

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

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

Here is what we get on Windows:

qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func 7 1
py_cmp_func 33 1
py_cmp_func 99 1
py_cmp_func 5 1
py_cmp_func 7 5
py_cmp_func 33 5
py_cmp_func 99 5
py_cmp_func 7 99
py_cmp_func 33 99
py_cmp_func 7 33

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

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

Ah, we’re nearly done! The last step is to 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]
...

Final run on Windows:

qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func))
py_cmp_func 33 7
py_cmp_func 99 33
py_cmp_func 5 99
py_cmp_func 1 99
py_cmp_func 33 7
py_cmp_func 1 33
py_cmp_func 5 33
py_cmp_func 5 7
py_cmp_func 1 7
py_cmp_func 5 1

and on Linux:

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

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:

for i in ia: print i,
...
1 5 7 33 99

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:

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 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:

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:
... print item.name, item.size
... if item.name is None:
... break
...
hello 104
phello -104
phello.spam 104
None 0

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:

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 different from what one would expect is this:

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

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:

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.

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:

from ctypes.util import find_library
find_library("m")
'libm.so.6'
find_library("c")
'libc.so.6'
find_library("bz2")
'libbz2.so.1.0'

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

from ctypes.util import find_library
find_library("c")
'/usr/lib/libc.dylib'
find_library("m")
'/usr/lib/libm.dylib'
find_library("bz2")
'/usr/lib/libbz2.dylib'
find_library("AGL")
'/System/Library/Frameworks/AGL.framework/AGL'

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

If wrapping a shared library with ctypes, it may be better to determine the shared library name at development 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:

libc.time == libc.time
True
libc['time'] == libc['time']
False

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:

from ctypes import c_int, WINFUNCTYPE, windll
from ctypes.wintypes import HWND, LPCSTR, UINT
prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", None), (1, "flags", 0)
MessageBox = prototype(("MessageBoxA", windll.user32), paramflags)

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

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

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

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

Here is the wrapping with ctypes:

from ctypes import POINTER, WINFUNCTYPE, windll, WinError
from ctypes.wintypes import BOOL, HWND, RECT
prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
paramflags = (1, "hwnd"), (2, "lprect")
GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)

Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.

Output parameters can be combined with the errcheck protocol to do further output processing and error checking. The win32 GetWindowRect api function returns a BOOL to signal success or failure, so this function could do the error checking, and raises an exception when the api call failed:

def errcheck(result, func, args):
... if not result:
... raise WinError()
... return args
...
GetWindowRect.errcheck = errcheck

If the errcheck function returns the argument tuple it receives unchanged, ctypes continues the normal processing it does on the output parameters. If you want to return a tuple of window coordinates instead of a RECT instance, you can retrieve the fields in the function and return them instead, the normal processing will no longer take place:

def errcheck(result, func, args):
... if not result:
... raise WinError()
... rc = args[1]
... return rc.left, rc.top, rc.bottom, rc.right
...
GetWindowRect.errcheck = errcheck

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.