Navigation
- index
- modules |
- next |
- previous |
- Python »
- 3.5.1rc1 Documentation »
- The Python Standard Library »
- 16. Generic Operating System Services »
ctypes is a foreign function library for Python. It provides C compatible data types, and allows calling functions in DLLs or shared libraries. It can be used to wrap these libraries in pure Python.
Note: The code samples in this tutorial use doctest to make sure that they actually work. Since some code samples behave differently under Linux, Windows, or Mac OS X, they contain doctest directives in comments.
Note: Some code samples reference the ctypes c_int type. This type is an alias for the c_long type on 32-bit systems. So, you should not be confused if c_long is printed if you would expect c_int — they are actually the same type.
ctypes exports the cdll, and on Windows windll and oledll objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects. cdll loads libraries which export functions using the standard cdecl calling convention, while windll libraries call functions using the stdcall calling convention. oledll also uses the stdcall calling convention, and assumes the functions return a Windows HRESULT error code. The error code is used to automatically raise an OSError exception when the function call fails.
Changed in version 3.3: Windows errors used to raise WindowsError, which is now an alias of OSError.
Here are some examples for Windows. Note that msvcrt is the MS standard C library containing most standard C functions, and uses the cdecl calling convention:
>>> from ctypes import *
>>> print(windll.kernel32)
<WinDLL 'kernel32', handle ... at ...>
>>> print(cdll.msvcrt)
<CDLL 'msvcrt', handle ... at ...>
>>> libc = cdll.msvcrt
>>>
Windows appends the usual .dll file suffix automatically.
On Linux, it is required to specify the filename including the extension to load a library, so attribute access can not be used to load libraries. Either the LoadLibrary() method of the dll loaders should be used, or you should load the library by creating an instance of CDLL by calling the constructor:
>>> cdll.LoadLibrary("libc.so.6")
<CDLL 'libc.so.6', handle ... at ...>
>>> libc = CDLL("libc.so.6")
>>> libc
<CDLL 'libc.so.6', handle ... at ...>
>>>
Functions are accessed as attributes of dll objects:
>>> from ctypes import *
>>> libc.printf
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.GetModuleHandleA)
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.MyOwnFunction)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "ctypes.py", line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
>>>
Note that win32 system dlls like kernel32 and user32 often export ANSI as well as UNICODE versions of a function. The UNICODE version is exported with an W appended to the name, while the ANSI version is exported with an A appended to the name. The win32 GetModuleHandle function, which returns a module handle for a given module name, has the following C prototype, and a macro is used to expose one of them as GetModuleHandle depending on whether UNICODE is defined or not:
/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
windll does not try to select one of them by magic, you must access the version you need by specifying GetModuleHandleA or GetModuleHandleW explicitly, and then call it with bytes or string objects respectively.
Sometimes, dlls export functions with names which aren’t valid Python identifiers, like "??2@YAPAXI@Z". In this case you have to use getattr() to retrieve the function:
>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")
<_FuncPtr object at 0x...>
>>>
On Windows, some dlls export functions not by name but by ordinal. These functions can be accessed by indexing the dll object with the ordinal number:
>>> cdll.kernel32[1]
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0]
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
>>>
You can call these functions like any other Python callable. This example uses the time() function, which returns system time in seconds since the Unix epoch, and the GetModuleHandleA() function, which returns a win32 module handle.
This example calls both functions with a NULL pointer (None should be used as the NULL pointer):
>>> print(libc.time(None))
1150640792
>>> print(hex(windll.kernel32.GetModuleHandleA(None)))
0x1d000000
>>>
ctypes tries to protect you from calling functions with the wrong number of arguments or the wrong calling convention. Unfortunately this only works on Windows. It does this by examining the stack after the function returns, so although an error is raised the function has been called:
>>> 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)
>>>
The same exception is raised when you call an stdcall function with the cdecl calling convention, or vice versa:
>>> 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)
>>>
To find out the correct calling convention you have to look into the C header file or the documentation for the function you want to call.
On Windows, ctypes uses win32 structured exception handling to prevent crashes from general protection faults when functions are called with invalid argument values:
>>> windll.kernel32.GetModuleHandleA(32)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
OSError: exception: access violation reading 0x00000020
>>>
There are, however, enough ways to crash Python with ctypes, so you should be careful anyway. The faulthandler module can be helpful in debugging crashes (e.g. from segmentation faults produced by erroneous C library calls).
None, integers, bytes objects and (unicode) strings are the only native Python objects that can directly be used as parameters in these function calls. None is passed as a C NULL pointer, bytes objects and strings are passed as pointer to the memory block that contains their data (char * or wchar_t *). Python integers are passed as the platforms default C int type, their value is masked to fit into the C type.
Before we move on calling functions with other parameter types, we have to learn more about ctypes data types.
ctypes defines a number of primitive C compatible data types:
| ctypes type | C type | Python type |
|---|---|---|
| c_bool | _Bool | bool (1) |
| c_char | char | 1-character bytes object |
| c_wchar | wchar_t | 1-character 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 |
All these types can be created by calling them with an optional initializer of the correct type and value:
>>> c_int()
c_long(0)
>>> c_wchar_p("Hello, World")
c_wchar_p('Hello, World')
>>> c_ushort(-3)
c_ushort(65533)
>>>
Since these types are mutable, their value can also be changed afterwards:
>>> i = c_int(42)
>>> print(i)
c_long(42)
>>> print(i.value)
42
>>> i.value = -99
>>> print(i.value)
-99
>>>
Assigning a new value to instances of the pointer types c_char_p, c_wchar_p, and c_void_p changes the memory location they point to, not the contents of the memory block (of course not, because Python bytes objects are immutable):
>>> 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
>>>
You should be careful, however, not to pass them to functions expecting pointers to mutable memory. If you need mutable memory blocks, ctypes has a create_string_buffer() function which creates these in various ways. The current memory block contents can be accessed (or changed) with the raw property; if you want to access it as NUL terminated string, use the value property:
>>> from ctypes import *
>>> p = create_string_buffer(3) # create a 3 byte buffer, initialized to NUL bytes
>>> print(sizeof(p), repr(p.raw))
3 b'\x00\x00\x00'
>>> p = create_string_buffer(b"Hello") # create a buffer containing a NUL terminated string
>>> print(sizeof(p), repr(p.raw))
6 b'Hello\x00'
>>> print(repr(p.value))
b'Hello'
>>> p = create_string_buffer(b"Hello", 10) # create a 10 byte buffer
>>> print(sizeof(p), repr(p.raw))
10 b'Hello\x00\x00\x00\x00\x00'
>>> p.value = b"Hi"
>>> print(sizeof(p), repr(p.raw))
10 b'Hi\x00lo\x00\x00\x00\x00\x00'
>>>
The create_string_buffer() function replaces the c_buffer() function (which is still available as an alias), as well as the c_string() function from earlier ctypes releases. To create a mutable memory block containing unicode characters of the C type wchar_t use the create_unicode_buffer() function.
Note that printf prints to the real standard output channel, not to sys.stdout, so these examples will only work at the console prompt, not from within IDLE or PythonWin:
>>> 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
>>>
As has been mentioned before, all Python types except integers, strings, and bytes objects have to be wrapped in their corresponding ctypes type, so that they can be converted to the required C data type:
>>> printf(b"An int %d, a double %f\n", 1234, c_double(3.14))
An int 1234, a double 3.140000
31
>>>
You can also customize ctypes argument conversion to allow instances of your own classes be used as function arguments. ctypes looks for an _as_parameter_ attribute and uses this as the function argument. Of course, it must be one of integer, string, or bytes:
>>> class Bottles:
... def __init__(self, number):
... self._as_parameter_ = number
...
>>> bottles = Bottles(42)
>>> printf(b"%d bottles of beer\n", bottles)
42 bottles of beer
19
>>>
If you don’t want to store the instance’s data in the _as_parameter_ instance variable, you could define a property which makes the attribute available on request.
It is possible to specify the required argument types of functions exported from DLLs by setting the argtypes attribute.
argtypes must be a sequence of C data types (the printf function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
String 'Hi', Int 10, Double 2.200000
37
>>>
Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:
>>> printf(b"%d %d %d", 1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: wrong type
>>> printf(b"%s %d %f\n", b"X", 2, 3)
X 2 3.000000
13
>>>
If you have defined your own classes which you pass to function calls, you have to implement a from_param() class method for them to be able to use them in the argtypes sequence. The from_param() class method receives the Python object passed to the function call, it should do a typecheck or whatever is needed to make sure this object is acceptable, and then return the object itself, its _as_parameter_ attribute, or whatever you want to pass as the C function argument in this case. Again, the result should be an integer, string, bytes, a ctypes instance, or an object with an _as_parameter_ attribute.
By default functions are assumed to return the C int type. Other return types can be specified by setting the restype attribute of the function object.
Here is a more advanced example, it uses the strchr function, which expects a string pointer and a char, and returns a pointer to a string:
>>> strchr = libc.strchr
>>> strchr(b"abcdef", ord("d"))
8059983
>>> strchr.restype = c_char_p # c_char_p is a pointer to a string
>>> strchr(b"abcdef", ord("d"))
b'def'
>>> print(strchr(b"abcdef", ord("x")))
None
>>>
If you want to avoid the ord("x") calls above, you can set the argtypes attribute, and the second argument will be converted from a single character Python bytes object into a C char:
>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> strchr(b"abcdef", b"d")
'def'
>>> strchr(b"abcdef", b"def")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: one character string expected
>>> print(strchr(b"abcdef", b"x"))
None
>>> strchr(b"abcdef", b"d")
'def'
>>>
You can also use a callable Python object (a function or a class for example) as the restype attribute, if the foreign function returns an integer. The callable will be called with the integer the C function returns, and the result of this call will be used as the result of your function call. This is useful to check for error return values and automatically raise an exception:
>>> GetModuleHandle = windll.kernel32.GetModuleHandleA
>>> def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
>>>
>>> GetModuleHandle.restype = ValidHandle
>>> GetModuleHandle(None)
486539264
>>> GetModuleHandle("something silly")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in ValidHandle
OSError: [Errno 126] The specified module could not be found.
>>>
WinError is a function which will call Windows FormatMessage() api to get the string representation of an error code, and returns an exception. WinError takes an optional error code parameter, if no one is used, it calls GetLastError() to retrieve it.
Please note that a much more powerful error checking mechanism is available through the errcheck attribute; see the reference manual for details.
Sometimes a C api function expects a pointer to a data type as parameter, probably to write into the corresponding location, or if the data is too large to be passed by value. This is also known as passing parameters by reference.
ctypes exports the byref() function which is used to pass parameters by reference. The same effect can be achieved with the pointer() function, although pointer() does a lot more work since it constructs a real pointer object, so it is faster to use byref() if you don’t need the pointer object in Python itself:
>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer(b'\000' * 32)
>>> print(i.value, f.value, repr(s.value))
0 0.0 b''
>>> libc.sscanf(b"1 3.14 Hello", b"%d %f %s",
... byref(i), byref(f), s)
3
>>> print(i.value, f.value, repr(s.value))
1 3.1400001049 b'Hello'
>>>
Structures and unions must derive from the Structure and Union base classes which are defined in the ctypes module. Each subclass must define a _fields_ attribute. _fields_ must be a list of 2-tuples, containing a field name and a field type.
The field type must be a ctypes type like c_int, or any other derived ctypes type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = [("x", c_int),
... ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print(point.x, point.y)
10 20
>>> point = POINT(y=5)
>>> print(point.x, point.y)
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
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)
<Field type=c_long, ofs=0, size=4>
>>> print(POINT.y)
<Field type=c_long, ofs=4, size=4>
>>>
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.
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.
It is possible to create structures and unions containing bit fields. Bit fields are only possible for integer fields, the bit width is specified as the third item in the _fields_ tuples:
>>> class Int(Structure):
... _fields_ = [("first_16", c_int, 16),
... ("second_16", c_int, 16)]
...
>>> print(Int.first_16)
<Field type=c_long, ofs=0:0, bits=16>
>>> print(Int.second_16)
<Field type=c_long, ofs=0:16, bits=16>
>>>
Arrays 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, end=" ")
...
1 2 3 4 5 6 7 8 9 10
>>>
Pointer instances are created by calling the pointer() function on a ctypes type:
>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>
Pointer instances have a contents attribute which returns the object to which the pointer points, the i object above:
>>> pi.contents
c_long(42)
>>>
Note that ctypes does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:
>>> pi.contents is i
False
>>> pi.contents is pi.contents
False
>>>
Assigning another c_int instance to the pointer’s contents attribute would cause the pointer to point to the memory location where this is stored:
>>> i = c_int(99)
>>> pi.contents = i
>>> pi.contents
c_long(99)
>>>
Pointer instances can also be indexed with integers:
>>> pi[0]
99
>>>
Assigning to an integer index changes the pointed to value:
>>> print(i)
c_long(99)
>>> pi[0] = 22
>>> print(i)
c_long(22)
>>>
It is also possible to use indexes different from 0, but you must know what you’re doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.
Behind the scenes, the pointer() function does more than simply create pointer instances, it has to create pointer types first. This is done with the POINTER() function, which accepts any ctypes type, and returns a new type:
>>> PI = POINTER(c_int)
>>> PI
<class 'ctypes.LP_c_long'>
>>> PI(42)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
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
>>>
Usually, ctypes does strict type checking. This means, if you have POINTER(c_int) in the argtypes list of a function or as the type of a member field in a structure definition, only instances of exactly the same type are accepted. There are some exceptions to this rule, where ctypes accepts other objects. For example, you can pass compatible array instances instead of pointer types. So, for POINTER(c_int), ctypes accepts an array of c_int:
>>> class Bar(Structure):
... _fields_ = [("count", c_int), ("values", POINTER(c_int))]
...
>>> bar = Bar()
>>> bar.values = (c_int * 3)(1, 2, 3)
>>> bar.count = 3
>>> for i in range(bar.count):
... print(bar.values[i])
...
1
2
3
>>>
In addition, if a function argument is explicitly declared to be a pointer type (such as POINTER(c_int)) in argtypes, an object of the pointed type (c_int in this case) can be passed to the function. ctypes will apply the required byref() conversion in this case automatically.
To set a POINTER type field to NULL, you can assign None:
>>> bar.values = None
>>>
Sometimes you have instances of incompatible types. In C, you can cast one type into another type. ctypes provides a cast() function which can be used in the same way. The Bar structure defined above accepts POINTER(c_int) pointers or c_int arrays for its values field, but not instances of other types:
>>> bar.values = (c_byte * 4)()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
>>>
For these cases, the cast() function is handy.
The cast() function can be used to cast a ctypes instance into a pointer to a different ctypes data type. cast() takes two parameters, a ctypes object that is or can be converted to a pointer of some kind, and a ctypes pointer type. It returns an instance of the second argument, which references the same memory block as the first argument:
>>> a = (c_byte * 4)()
>>> cast(a, POINTER(c_int))
<ctypes.LP_c_long object at ...>
>>>
So, cast() can be used to assign to the values field of Bar the structure:
>>> bar = Bar()
>>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
>>> print(bar.values[0])
0
>>>
Incomplete Types are structures, unions or arrays whose members are not yet specified. In C, they are specified by forward declarations, which are defined later:
struct cell; /* forward declaration */
struct cell {
char *name;
struct cell *next;
};
The straightforward translation into ctypes code would be this, but it does not work:
>>> class cell(Structure):
... _fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 2, in cell
NameError: name 'cell' is not defined
>>>
because the new class cell is not available in the class statement itself. In ctypes, we can define the cell class and set the _fields_ attribute later, after the class statement:
>>> from ctypes import *
>>> class cell(Structure):
... pass
...
>>> cell._fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
>>>
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, end=" ")
... p = p.next[0]
...
foo bar foo bar foo bar foo bar
>>>
ctypes allows to create C callable function pointers from Python callables. These are sometimes called callback functions.
First, you must create a class for the callback function. The class knows the calling convention, the return type, and the number and types of arguments this function will receive.
The CFUNCTYPE() factory function creates types for callback functions using the cdecl calling convention. On Windows, the WINFUNCTYPE() factory function creates types for callback functions using the stdcall calling convention.
Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.
I will present an example here which uses the standard C library’s qsort() function, that is used to sort items with the help of a callback function. qsort() will be used to sort an array of integers:
>>> IntArray5 = c_int * 5
>>> ia = IntArray5(5, 1, 7, 33, 99)
>>> qsort = libc.qsort
>>> qsort.restype = None
>>>
qsort() must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an integer. First we create the type for the callback function:
>>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
>>>
To get started, here is a simple callback that shows the values it gets passed:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return 0
...
>>> cmp_func = CMPFUNC(py_cmp_func)
>>>
The result:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 5 7
py_cmp_func 1 7
>>>
Now we can actually compare the two items and return a useful result:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return a[0] - b[0]
...
>>>
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func))
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
>>>
As we can easily check, our array is sorted now:
>>> for i in ia: print(i, end=" ")
...
1 5 7 33 99
>>>
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.
Some shared libraries not only export functions, they also export variables. An example in the Python library itself is the Py_OptimizeFlag, an integer set to 0, 1, or 2, depending on the -O or -OO flag given on startup.
ctypes can access values like this with the in_dll() class methods of the type. pythonapi is a predefined symbol giving access to the Python C api:
>>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag")
>>> print(opt_flag)
c_long(0)
>>>
If the interpreter would have been started with -O, the sample would have printed c_long(1), or c_long(2) if -OO would have been specified.
An extended example which also demonstrates the use of pointers accesses the PyImport_FrozenModules pointer exported by Python.
Quoting the docs for that value:
This pointer is initialized to point to an array of struct _frozen records, terminated by one whose members are 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.
There are some edges in ctypes where you might expect something other than what actually happens.
Consider the following example:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class RECT(Structure):
... _fields_ = ("a", POINT), ("b", POINT)
...
>>> p1 = POINT(1, 2)
>>> p2 = POINT(3, 4)
>>> rc = RECT(p1, p2)
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
>>> # now swap the two points
>>> rc.a, rc.b = rc.b, rc.a
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
>>>
Hm. We certainly expected the last statement to print 3 4 1 2. What happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a line above:
>>> temp0, temp1 = rc.b, rc.a
>>> rc.a = temp0
>>> rc.b = temp1
>>>
Note that temp0 and temp1 are objects still using the internal buffer of the rc object above. So executing rc.a = temp0 copies the buffer contents of temp0 into rc ‘s buffer. This, in turn, changes the contents of temp1. So, the last assignment rc.b = temp1, doesn’t have the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn’t copy the sub-object, instead it retrieves a wrapper object accessing the root-object’s underlying buffer.
Another example that may behave 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!
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.
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:
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.
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 to do 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.
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 to adapt 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 string passed as argument into a bytes object using ctypes conversion rules.
New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a from_param() method which returns a value usable as argument (integer, string, ctypes instance). This allows to define adapters that can adapt custom objects as function parameters.
Assign a Python function or another callable to this attribute. The callable will be called with three or more arguments:
result is what the foreign function returns, as specified by the restype attribute.
func is the foreign function object itself, this allows to reuse 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 to specialize 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.
This exception is raised when a foreign function call cannot convert one of the passed arguments.
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.
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.
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.
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
>>>
Returns the address of the memory buffer as integer. obj must be an instance of a ctypes type.
Returns the alignment requirements of a ctypes type. obj_or_type must be a ctypes type or instance.
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.
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.
This function creates a mutable character buffer. The returned object is a ctypes array of c_char.
init_or_size must be an integer which specifies the size of the array, or a bytes object which will be used to initialize the array items.
If a bytes object is specified as first argument, the buffer is made one item larger than its length so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows to specify the size of the array if the length of the bytes should not be used.
This function creates a mutable unicode character buffer. The returned object is a ctypes array of c_wchar.
init_or_size must be an integer which specifies the size of the array, or a string which will be used to initialize the array items.
If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows to specify the size of the array if the length of the string should not be used.
Windows only: This function is a hook which allows to implement in-process COM servers with ctypes. It is called from the DllCanUnloadNow function that the _ctypes extension dll exports.
Windows only: This function is a hook which allows to implement in-process COM servers with ctypes. It is called from the DllGetClassObject function that the _ctypes extension dll exports.
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.
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.
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.
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.
Returns the current value of the ctypes-private copy of the system errno variable in the calling thread.
Windows only: returns the current value of the ctypes-private copy of the system LastError variable in the calling thread.
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.
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.
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.
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.
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.
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.
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.
Returns the size in bytes of a ctypes type or instance memory buffer. Does the same as the C sizeof operator.
This function returns the C string starting at memory address address as a bytes object. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated.
Windows only: this function is probably the worst-named thing in ctypes. It creates an instance of OSError. If code is not specified, GetLastError is called to determine the error code. If descr is not specified, FormatError() is called to get a textual description of the error.
Changed in version 3.3: An instance of WindowsError used to be created.
This function returns the wide character string starting at memory address address as a string. If size is specified, it is used as the number of characters of the string, otherwise the string is assumed to be zero-terminated.
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):
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.
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.
This method returns a ctypes type instance using the memory specified by address which must be an integer.
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.
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:
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.
This read-only variable is true when the ctypes data instance has allocated the memory block itself, false otherwise.
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.
This non-public class is the base class of all fundamental ctypes data types. It is mentioned here because it contains the common attributes of the fundamental ctypes data types. _SimpleCData is a subclass of _CData, so it inherits their methods and attributes. ctypes data types that are not and do not contain pointers can now be pickled.
Instances have a single attribute:
This attribute contains the actual value of the instance. For integer and pointer types, it is an integer, for character types, it is a single character bytes object or string, for character pointer types it is a Python bytes object or string.
When the value attribute is retrieved from a ctypes instance, usually a new object is returned each time. ctypes does not implement original object return, always a new object is constructed. The same is true for all other ctypes object instances.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python bytes object, not a c_char_p instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a foreign functions restype is a subclass of c_void_p, you will receive an instance of this subclass from the function call. Of course, you can get the value of the pointer by accessing the value attribute.
These are the fundamental ctypes data types:
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.
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.
Represents the C char * datatype when it points to a zero-terminated string. For a general character pointer that may also point to binary data, POINTER(c_char) must be used. The constructor accepts an integer address, or a bytes object.
Represents the C double datatype. The constructor accepts an optional float initializer.
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.
Represents the C float datatype. The constructor accepts an optional float initializer.
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.
Represents the C 64-bit signed int datatype. Usually an alias for c_longlong.
Represents the C signed long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C signed long long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C signed short datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C size_t datatype.
Represents the C ssize_t datatype.
New in version 3.2.
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.
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.
Represents the C 16-bit unsigned int datatype. Usually an alias for c_ushort.
Represents the C 64-bit unsigned int datatype. Usually an alias for c_ulonglong.
Represents the C unsigned long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C unsigned long long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C unsigned short datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C void * type. The value is represented as integer. The constructor accepts an optional integer initializer.
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.
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.
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.
Windows only: Represents a HRESULT value, which contains success or error information for a function or method call.
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.
Abstract base class for unions in native byte order.
Abstract base class for structures in big endian byte order.
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.
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
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 to create 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.
An optional small integer that allows to override the alignment of structure fields in the instance. _pack_ must already be defined when _fields_ is assigned, otherwise it will have no effect.
An optional sequence that lists the names of unnamed (anonymous) fields. _anonymous_ must be already defined when _fields_ is assigned, otherwise it will have no effect.
The fields listed in this variable must be structure or union type fields. ctypes will create descriptors in the structure type that allows to access 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_.