:mod:`!ctypes` --- A foreign function library for Python
.. module:: ctypes :synopsis: A foreign function library for Python.
.. moduleauthor:: Thomas Heller <theller@python.net>
Source code: :source:`Lib/ctypes`
:mod:`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 :mod:`doctest` to make sure that they actually work. Since some code samples behave differently under Linux, Windows, or macOS, they contain doctest directives in comments.
Note: Some code samples reference the ctypes :class:`c_int` type. On platforms
where sizeof(long) == sizeof(int) it is an alias to :class:`c_long`.
So, you should not be confused if :class:`c_long` is printed if you would expect
:class:`c_int` --- they are actually the same type.
:mod:`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 :c:type:`!HRESULT` error code. The error
code is used to automatically raise an :class:`OSError` exception when the
function call fails.
.. versionchanged:: 3.3 Windows errors used to raise :exc:`WindowsError`, which is now an alias of :exc:`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) # doctest: +WINDOWS <WinDLL 'kernel32', handle ... at ...> >>> print(cdll.msvcrt) # doctest: +WINDOWS <CDLL 'msvcrt', handle ... at ...> >>> libc = cdll.msvcrt # doctest: +WINDOWS >>>
Windows appends the usual .dll file suffix automatically.
Note
Accessing the standard C library through cdll.msvcrt will use an
outdated version of the library that may be incompatible with the one
being used by Python. Where possible, use native Python functionality,
or else import and use the msvcrt module.
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 :meth:`~LibraryLoader.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") # doctest: +LINUX
<CDLL 'libc.so.6', handle ... at ...>
>>> libc = CDLL("libc.so.6") # doctest: +LINUX
>>> libc # doctest: +LINUX
<CDLL 'libc.so.6', handle ... at ...>
>>>
Functions are accessed as attributes of dll objects:
>>> libc.printf
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.GetModuleHandleA) # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.MyOwnFunction) # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "ctypes.py", line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
>>>
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
a 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
:func:`getattr` to retrieve the function:
>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z") # doctest: +WINDOWS <_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] # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0] # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "ctypes.py", line 310, in __getitem__
func = _StdcallFuncPtr(name, self)
AttributeError: function ordinal 0 not found
>>>
You can call these functions like any other Python callable. This example uses
the rand() function, which takes no arguments and returns a pseudo-random integer:
>>> print(libc.rand()) # doctest: +SKIP 1804289383
On Windows, you can call the GetModuleHandleA() function, which returns a win32 module
handle (passing None as single argument to call it with a NULL pointer):
>>> print(hex(windll.kernel32.GetModuleHandleA(None))) # doctest: +WINDOWS 0x1d000000 >>>
:exc:`ValueError` is raised when you call an stdcall function with the
cdecl calling convention, or vice versa:
>>> cdll.kernel32.GetModuleHandleA(None) # doctest: +WINDOWS Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: Procedure probably called with not enough arguments (4 bytes missing) >>> >>> windll.msvcrt.printf(b"spam") # doctest: +WINDOWS Traceback (most recent call last): File "<stdin>", line 1, in <module> 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, :mod:`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) # doctest: +WINDOWS Traceback (most recent call last): File "<stdin>", line 1, in <module> OSError: exception: access violation reading 0x00000020 >>>
There are, however, enough ways to crash Python with :mod:`ctypes`, so you should be careful anyway. The :mod:`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 (:c:expr:`char *` or
:c:expr:`wchar_t *`). Python integers are passed as the platform's default C
:c:expr:`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 :mod:`ctypes` data types.
:mod:`ctypes` defines a number of primitive C compatible data types:
- The constructor accepts any object with a truth value.
Additionally, if IEC 60559 compatible complex arithmetic (Annex G) is supported, the following complex types are available:
| ctypes type | C type | Python type |
|---|---|---|
| :class:`c_float_complex` | :c:expr:`float complex` | complex |
| :class:`c_double_complex` | :c:expr:`double complex` | complex |
| :class:`c_longdouble_complex` | :c:expr:`long double complex` | complex |
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(140018365411392)
>>> 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 :class:`c_char_p`, :class:`c_wchar_p`, and :class:`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(139966785747344) >>> print(c_s.value) Hello World >>> c_s.value = "Hi, there" >>> print(c_s) # the memory location has changed c_wchar_p(139966783348904) >>> print(c_s.value) Hi, there >>> print(s) # first object is unchanged Hello, World >>>
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
:func:`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 :func:`create_string_buffer` function replaces the old :func:`!c_buffer` function (which is still available as an alias). To create a mutable memory block containing unicode characters of the C type :c:type:`wchar_t`, use the :func:`create_unicode_buffer` function.
Note that printf prints to the real standard output channel, not to :data:`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 <module> ctypes.ArgumentError: argument 2: 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 :mod:`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 >>>
On a lot of platforms calling variadic functions through ctypes is exactly the same as calling functions with a fixed number of parameters. On some platforms, and in particular ARM64 for Apple Platforms, the calling convention for variadic functions is different than that for regular functions.
On those platforms it is required to specify the :attr:`~_CFuncPtr.argtypes` attribute for the regular, non-variadic, function arguments:
libc.printf.argtypes = [ctypes.c_char_p]Because specifying the attribute does not inhibit portability it is advised to always specify :attr:`~_CFuncPtr.argtypes` for all variadic functions.
You can also customize :mod:`ctypes` argument conversion to allow instances of your own classes be used as function arguments. :mod:`ctypes` looks for an :attr:`!_as_parameter_` attribute and uses this as the function argument. The attribute must be an integer, string, bytes, a :mod:`ctypes` instance, or an object with an :attr:`!_as_parameter_` attribute:
>>> 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 :attr:`!_as_parameter_` instance variable, you could define a :class:`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 :attr:`~_CFuncPtr.argtypes` attribute.
:attr:`~_CFuncPtr.argtypes` must be a sequence of C data types (the :func:`!printf` function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double] >>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2) String 'Hi', Int 10, Double 2.200000 37 >>>
Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:
>>> printf(b"%d %d %d", 1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> ctypes.ArgumentError: argument 2: TypeError: 'int' object cannot be interpreted as ctypes.c_char_p >>> 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 :meth:`~_CData.from_param` class method for them to be able to use them in the :attr:`~_CFuncPtr.argtypes` sequence. The :meth:`~_CData.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 :attr:`!_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 :mod:`ctypes` instance, or an object with an :attr:`!_as_parameter_` attribute.
.. testsetup::
from ctypes import CDLL, c_char, c_char_p
from ctypes.util import find_library
libc = CDLL(find_library('c'))
strchr = libc.strchr
By default functions are assumed to return the C :c:expr:`int` type. Other return types can be specified by setting the :attr:`~_CFuncPtr.restype` attribute of the function object.
The C prototype of :c:func:`time` is time_t time(time_t *). Because :c:type:`time_t`
might be of a different type than the default return type :c:expr:`int`, you should
specify the :attr:`!restype` attribute:
>>> libc.time.restype = c_time_t
The argument types can be specified using :attr:`~_CFuncPtr.argtypes`:
>>> libc.time.argtypes = (POINTER(c_time_t),)
To call the function with a NULL pointer as first argument, use None:
>>> print(libc.time(None)) # doctest: +SKIP 1150640792
Here is a more advanced example, it uses the :func:`!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")) # doctest: +SKIP
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 :func:`ord("x") <ord>` calls above, you can set the :attr:`~_CFuncPtr.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")
b'def'
>>> strchr(b"abcdef", b"def")
Traceback (most recent call last):
ctypes.ArgumentError: argument 2: TypeError: one character bytes, bytearray or integer expected
>>> print(strchr(b"abcdef", b"x"))
None
>>> strchr(b"abcdef", b"d")
b'def'
>>>You can also use a callable Python object (a function or a class for example) as the :attr:`~_CFuncPtr.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 # doctest: +WINDOWS
>>> def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
>>>
>>> GetModuleHandle.restype = ValidHandle # doctest: +WINDOWS
>>> GetModuleHandle(None) # doctest: +WINDOWS
486539264
>>> GetModuleHandle("something silly") # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in ValidHandle
OSError: [Errno 126] The specified module could not be found.
>>>
WinError is a function which will call Windows FormatMessage() api to
get the string representation of an error code, and returns an exception.
WinError takes an optional error code parameter, if no one is used, it calls
:func:`GetLastError` to retrieve it.
Please note that a much more powerful error checking mechanism is available through the :attr:`~_CFuncPtr.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.
:mod:`ctypes` exports the :func:`byref` function which is used to pass parameters by reference. The same effect can be achieved with the :func:`pointer` function, although :func:`pointer` does a lot more work since it constructs a real pointer object, so it is faster to use :func:`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 :class:`Structure` and :class:`Union` base classes which are defined in the :mod:`ctypes` module. Each subclass must define a :attr:`~Structure._fields_` attribute. :attr:`!_fields_` must be a list of 2-tuples, containing a field name and a field type.
The field type must be a :mod:`ctypes` type like :class:`c_int`, or any other derived :mod:`ctypes` type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = [("x", c_int),
... ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print(point.x, point.y)
10 20
>>> point = POINT(y=5)
>>> print(point.x, point.y)
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: too many initializers
>>>
You can, however, build much more complicated structures. A structure can itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft and lowerright:
>>> class RECT(Structure):
... _fields_ = [("upperleft", POINT),
... ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print(rc.upperleft.x, rc.upperleft.y)
0 5
>>> print(rc.lowerright.x, rc.lowerright.y)
0 0
>>>
Nested structures can also be initialized in the constructor in several ways:
>>> r = RECT(POINT(1, 2), POINT(3, 4)) >>> r = RECT((1, 2), (3, 4))
Field :term:`descriptor`s 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
:mod:`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 laid out in the same way the C compiler does it. It is possible to override this behavior entirely by specifying a :attr:`~Structure._layout_` class attribute in the subclass definition; see the attribute documentation for details.
It is possible to specify the maximum alignment for the fields by setting
the :attr:`~Structure._pack_` class attribute to a positive integer.
This matches what #pragma pack(n) does in MSVC.
It is also possible to set a minimum alignment for how the subclass itself is packed in the
same way #pragma align(n) works in MSVC.
This can be achieved by specifying a ::attr:`~Structure._align_` class attribute
in the subclass definition.
:mod:`ctypes` uses the native byte order for Structures and Unions. To build structures with non-native byte order, you can use one of the :class:`BigEndianStructure`, :class:`LittleEndianStructure`, :class:`BigEndianUnion`, and :class:`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 :attr:`~Structure._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 a somewhat artificial data type, a structure containing 4 POINTs among other stuff:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class MyStruct(Structure):
... _fields_ = [("a", c_int),
... ("b", c_float),
... ("point_array", POINT * 4)]
>>>
>>> print(len(MyStruct().point_array))
4
>>>
Instances are created in the usual way, by calling the class:
arr = TenPointsArrayType()
for pt in arr:
print(pt.x, pt.y)
The above code print a series of 0 0 lines, because the array contents is
initialized to zeros.
Initializers of the correct type can also be specified:
>>> from ctypes import * >>> TenIntegers = c_int * 10 >>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) >>> print(ii) <c_long_Array_10 object at 0x...> >>> for i in ii: print(i, end=" ") ... 1 2 3 4 5 6 7 8 9 10 >>>
Pointer instances are created by calling the :func:`pointer` function on a :mod:`ctypes` type:
>>> from ctypes import * >>> i = c_int(42) >>> pi = pointer(i) >>>
Pointer instances have a :attr:`~_Pointer.contents` attribute which
returns the object to which the pointer points, the i object above:
>>> pi.contents c_long(42) >>>
Note that :mod:`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 :class:`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 :func:`pointer` function does more than simply create pointer instances, it has to create pointer types first. This is done with the :func:`POINTER` function, which accepts any :mod:`ctypes` type, and returns a new type:
>>> PI = POINTER(c_int) >>> PI <class 'ctypes.LP_c_long'> >>> PI(42) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: expected c_long instead of int >>> PI(c_int(42)) <ctypes.LP_c_long object at 0x...> >>>
Calling the pointer type without an argument creates a NULL pointer.
NULL pointers have a False boolean value:
>>> null_ptr = POINTER(c_int)() >>> print(bool(null_ptr)) False >>>
:mod:`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 :attr:`~_CFuncPtr.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 :attr:`~_CFuncPtr.argtypes`, an object of the pointed
type (c_int in this case) can be passed to the function. ctypes will apply
the required :func:`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. :mod:`ctypes` provides a :func:`cast` function which can be
used in the same way. The Bar structure defined above accepts
POINTER(c_int) pointers or :class:`c_int` arrays for its values field,
but not instances of other types:
>>> bar.values = (c_byte * 4)() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance >>>
For these cases, the :func:`cast` function is handy.
The :func:`cast` function can be used to cast a ctypes instance into a pointer to a different ctypes data type. :func:`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, :func:`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 <module>
File "<stdin>", line 2, in cell
NameError: name 'cell' is not defined
>>>
because the new class cell is not available in the class statement itself.
In :mod:`ctypes`, we can define the cell class and set the
:attr:`~Structure._fields_` attribute later, after the class statement:
>>> from ctypes import *
>>> class cell(Structure):
... pass
...
>>> cell._fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
>>>
Let's try it. We create two instances of cell, and let them point to each
other, and finally follow the pointer chain a few times:
>>> c1 = cell() >>> c1.name = b"foo" >>> c2 = cell() >>> c2.name = b"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 >>>
:mod:`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 :func:`CFUNCTYPE` factory function creates types for callback functions
using the cdecl calling convention. On Windows, the :func:`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 :c:func:`!qsort` function, that is used to sort items with the help of a callback function. :c:func:`!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 >>>
:func:`!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) # doctest: +LINUX 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)) # doctest: +LINUX
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
>>>
As we can easily check, our array is sorted now:
>>> for i in ia: print(i, end=" ") ... 1 5 7 33 99 >>>
The function factories can be used as decorator factories, so we may as well write:
>>> @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
... def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return a[0] - b[0]
...
>>> qsort(ia, len(ia), sizeof(c_int), py_cmp_func)
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
>>>
Note
Make sure you keep references to :func:`CFUNCTYPE` objects as long as they are used from C code. :mod:`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 :class:`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 :c:data:`Py_Version`, Python runtime version number encoded in a single constant integer.
:mod:`ctypes` can access values like this with the :meth:`~_CData.in_dll` class methods of the type. pythonapi is a predefined symbol giving access to the Python C api:
>>> version = ctypes.c_int.in_dll(ctypes.pythonapi, "Py_Version") >>> print(hex(version.value)) 0x30c00a0
An extended example which also demonstrates the use of pointers accesses the :c:data:`PyImport_FrozenModules` pointer exported by Python.
Quoting the docs for that value:
This pointer is initialized to point to an array of :c: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 :mod:`ctypes`:
>>> from ctypes import *
>>>
>>> class struct_frozen(Structure):
... _fields_ = [("name", c_char_p),
... ("code", POINTER(c_ubyte)),
... ("size", c_int),
... ("get_code", POINTER(c_ubyte)), # Function pointer
... ]
...
>>>
We have defined the :c:struct:`_frozen` data type, so we can get the pointer to the table:
>>> FrozenTable = POINTER(struct_frozen) >>> table = FrozenTable.in_dll(pythonapi, "_PyImport_FrozenBootstrap") >>>
Since table is a pointer to the array of struct_frozen records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it's better to break out of the loop when we
hit the NULL entry:
>>> for item in table:
... if item.name is None:
... break
... print(item.name.decode("ascii"), item.size)
...
_frozen_importlib 31764
_frozen_importlib_external 41499
zipimport 12345
>>>
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 :mod:`ctypes` where you might expect something other than what actually happens.
Consider the following example:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class RECT(Structure):
... _fields_ = ("a", POINT), ("b", POINT)
...
>>> p1 = POINT(1, 2)
>>> p2 = POINT(3, 4)
>>> rc = RECT(p1, p2)
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
>>> # now swap the two points
>>> rc.a, rc.b = rc.b, rc.a
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
>>>
Hm. We certainly expected the last statement to print 3 4 1 2. What
happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a line above:
>>> temp0, temp1 = rc.b, rc.a >>> rc.a = temp0 >>> rc.b = temp1 >>>
Note that temp0 and temp1 are objects still using the internal buffer of
the rc object above. So executing rc.a = temp0 copies the buffer
contents of temp0 into rc 's buffer. This, in turn, changes the
contents of temp1. So, the last assignment rc.b = temp1, doesn't have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn't copy the sub-object, instead it retrieves a wrapper object accessing the root-object's underlying buffer.
Another example that may behave differently from what one would expect is this:
>>> s = c_char_p() >>> s.value = b"abc def ghi" >>> s.value b'abc def ghi' >>> s.value is s.value False >>>
Note
Objects instantiated from :class:`c_char_p` can only have their value set to bytes or integers.
Why is it printing False? ctypes instances are objects containing a memory
block plus some :term:`descriptor`s 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!
:mod:`ctypes` provides some support for variable-sized arrays and structures.
The :func:`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 :exc:`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 :mod:`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.
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 :func:`~ctypes.util.find_library` function is to locate a library in a way similar to what the compiler or runtime loader 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 :mod:`!ctypes.util` module provides a function which can help to determine the library to load.
.. data:: find_library(name) :module: ctypes.util :noindex: 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 :option:`!-l`). If no library can be found, returns ``None``.
The exact functionality is system dependent.
On Linux, :func:`~ctypes.util.find_library` tries to run external programs
(/sbin/ldconfig, gcc, objdump and ld) to find the library file.
It returns the filename of the library file.
.. versionchanged:: 3.6 On Linux, the value of the environment variable ``LD_LIBRARY_PATH`` is used when searching for libraries, if a library cannot be found by any other means.
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 macOS and Android, :func:`~ctypes.util.find_library` uses the system's standard 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, :func:`~ctypes.util.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 :mod:`ctypes`, it may be better to determine the shared library name at development time, and hardcode that into the wrapper module instead of using :func:`~ctypes.util.find_library` to locate the library at runtime.
There are several ways to load shared libraries into the Python process. One way is to instantiate one of the following classes:
Instances of this class represent loaded shared libraries. Functions in these libraries use the standard C calling convention, and are assumed to return :c:expr:`int`.
On Windows creating a :class:`CDLL` instance may fail even if the DLL name exists. When a dependent DLL of the loaded DLL is not found, a :exc:`OSError` error is raised with the message "[WinError 126] The specified module could not be found". This error message does not contain the name of the missing DLL because the Windows API does not return this information making this error hard to diagnose. To resolve this error and determine which DLL is not found, you need to find the list of dependent DLLs and determine which one is not found using Windows debugging and tracing tools.
.. versionchanged:: 3.12 The *name* parameter can now be a :term:`path-like object`.
.. seealso::
`Microsoft DUMPBIN tool <https://docs.microsoft.com/cpp/build/reference/dependents>`_
-- A tool to find DLL dependents.
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 :class:`HRESULT` code. :class:`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 :class:`OSError` is automatically raised.
.. versionchanged:: 3.3 :exc:`WindowsError` used to be raised, which is now an alias of :exc:`OSError`.
.. versionchanged:: 3.12 The *name* parameter can now be a :term:`path-like object`.
Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the stdcall calling convention, and are
assumed to return :c:expr:`int` by default.
.. versionchanged:: 3.12 The *name* parameter can now be a :term:`path-like object`.
The Python :term:`global interpreter lock` is released before calling any function exported by these libraries, and reacquired afterwards.
Instances of this class behave like :class:`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.
.. versionchanged:: 3.12 The *name* parameter can now be a :term:`path-like object`.
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 platform's :c:func:`!dlopen` or
:c:func:`!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 :manpage:`dlopen(3)` manpage. On Windows, mode is ignored. On posix systems, RTLD_NOW is always added, and is not configurable.
The use_errno parameter, when set to true, enables a ctypes mechanism that
allows accessing the system :data:`errno` error number in a safe way.
:mod:`ctypes` maintains a thread-local copy of the system's :data:`errno`
variable; if you call foreign functions created with use_errno=True then the
:data:`errno` value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.
The function :func:`ctypes.get_errno` returns the value of the ctypes private copy, and the function :func:`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 :func:`GetLastError` and :func:`!SetLastError` Windows API functions; :func:`ctypes.get_last_error` and :func:`ctypes.set_last_error` are used to request and change the ctypes private copy of the windows error code.
The winmode parameter is used on Windows to specify how the library is loaded
(since mode is ignored). It takes any value that is valid for the Win32 API
LoadLibraryEx flags parameter. When omitted, the default is to use the
flags that result in the most secure DLL load, which avoids issues such as DLL
hijacking. Passing the full path to the DLL is the safest way to ensure the
correct library and dependencies are loaded.
.. versionchanged:: 3.8 Added *winmode* parameter.
.. data:: RTLD_GLOBAL :noindex: Flag to use as *mode* parameter. On platforms where this flag is not available, it is defined as the integer zero.
.. data:: RTLD_LOCAL :noindex: Flag to use as *mode* parameter. On platforms where this is not available, it is the same as *RTLD_GLOBAL*.
.. data:: DEFAULT_MODE :noindex: 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:
>>> from ctypes import CDLL
>>> libc = CDLL("libc.so.6") # On Linux
>>> 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:
.. attribute:: PyDLL._handle The system handle used to access the library.
.. attribute:: 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 :class:`LibraryLoader` class, either by calling the :meth:`~LibraryLoader.LoadLibrary` method, or by retrieving the library as attribute of the loader instance.
Class which loads shared libraries. dlltype should be one of the :class:`CDLL`, :class:`PyDLL`, :class:`WinDLL`, or :class:`OleDLL` types.
:meth:`!__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.
.. method:: 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:
.. data:: cdll :noindex: Creates :class:`CDLL` instances.
.. data:: windll :noindex: Windows only: Creates :class:`WinDLL` instances.
.. data:: oledll :noindex: Windows only: Creates :class:`OleDLL` instances.
.. data:: pydll :noindex: Creates :class:`PyDLL` instances.
For accessing the C Python api directly, a ready-to-use Python shared library object is available:
.. data:: pythonapi :noindex: An instance of :class:`PyDLL` that exposes Python C API functions as attributes. Note that all these functions are assumed to return C :c:expr:`int`, which is of course not always the truth, so you have to assign the correct :attr:`!restype` attribute to use these functions.
.. audit-event:: ctypes.dlopen name ctypes.LibraryLoader Loading a library through any of these objects raises an :ref:`auditing event <auditing>` ``ctypes.dlopen`` with string argument ``name``, the name used to load the library.
.. audit-event:: ctypes.dlsym library,name ctypes.LibraryLoader Accessing a function on a loaded library raises an auditing event ``ctypes.dlsym`` with arguments ``library`` (the library object) and ``name`` (the symbol's name as a string or integer).
.. audit-event:: ctypes.dlsym/handle handle,name ctypes.LibraryLoader In cases when only the library handle is available rather than the object, accessing a function raises an auditing event ``ctypes.dlsym/handle`` with arguments ``handle`` (the raw library handle) and ``name``.
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 local class :class:`!_FuncPtr` (not exposed in :mod:`!ctypes`) which inherits from the private :class:`_CFuncPtr` class:
>>> import ctypes
>>> lib = ctypes.CDLL(None)
>>> issubclass(lib._FuncPtr, ctypes._CFuncPtr)
True
>>> lib._FuncPtr is ctypes._CFuncPtr
FalseBase 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.
.. attribute:: restype Assign a ctypes type to specify the result type of the foreign function. Use ``None`` for :c:expr:`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 :c:expr:`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 :attr:`!restype` and assign a callable to the :attr:`errcheck` attribute.
.. 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 :meth:`~_CData.from_param` class method of the items in the :attr:`argtypes` tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, a :class:`c_char_p` item in the :attr:`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 :meth:`~_CData.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.
.. attribute:: errcheck
Assign a Python function or another callable to this attribute. The
callable will be called with three or more arguments:
.. function:: callable(result, func, arguments)
:noindex:
:module:
*result* is what the foreign function returns, as specified by the
:attr:`!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:: ArgumentError This exception is raised when a foreign function call cannot convert one of the passed arguments.
.. audit-event:: ctypes.set_exception code foreign-functions On Windows, when a foreign function call raises a system exception (for example, due to an access violation), it will be captured and replaced with a suitable Python exception. Further, an auditing event ``ctypes.set_exception`` with argument ``code`` will be raised, allowing an audit hook to replace the exception with its own.
.. audit-event:: ctypes.call_function func_pointer,arguments foreign-functions Some ways to invoke foreign function calls may raise an auditing event ``ctypes.call_function`` with arguments ``function pointer`` and ``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, and can be used as decorator
factories, and as such, be applied to functions through the @wrapper syntax.
See :ref:`ctypes-callback-functions` for examples.
.. function:: 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 :data:`errno` variable is exchanged with the real :data:`errno` value before and after the call; *use_last_error* does the same for the Windows error code.
.. function:: WINFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False) Windows only: The returned function prototype creates functions that use the ``stdcall`` calling convention. The function will release the GIL during the call. *use_errno* and *use_last_error* have the same meaning as above.
.. function:: 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:
.. function:: prototype(address) :noindex: :module: Returns a foreign function at the specified address which must be an integer.
.. function:: prototype(callable) :noindex: :module: Create a C callable function (a callback function) from a Python *callable*.
.. function:: prototype(func_spec[, paramflags]) :noindex: :module: 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.
.. function:: prototype(vtbl_index, name[, paramflags[, iid]]) :noindex: :module: 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 :attr:`!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 :attr:`~_CFuncPtr.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.
The following example demonstrates how to wrap the Windows MessageBoxW function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this:
WINUSERAPI int WINAPI
MessageBoxW(
HWND hWnd,
LPCWSTR lpText,
LPCWSTR lpCaption,
UINT uType);
Here is the wrapping with :mod:`ctypes`:
>>> from ctypes import c_int, WINFUNCTYPE, windll
>>> from ctypes.wintypes import HWND, LPCWSTR, UINT
>>> prototype = WINFUNCTYPE(c_int, HWND, LPCWSTR, LPCWSTR, UINT)
>>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", "Hello from ctypes"), (1, "flags", 0)
>>> MessageBox = prototype(("MessageBoxW", windll.user32), paramflags)
The MessageBox foreign function can now be called in these ways:
>>> MessageBox() >>> MessageBox(text="Spam, spam, spam") >>> MessageBox(flags=2, text="foo bar")
A second example demonstrates output parameters. The win32 GetWindowRect
function retrieves the dimensions of a specified window by copying them into
RECT structure that the caller has to supply. Here is the C declaration:
WINUSERAPI BOOL WINAPI
GetWindowRect(
HWND hWnd,
LPRECT lpRect);
Here is the wrapping with :mod:`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 :attr:`~_CFuncPtr.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 :attr:`~_CFuncPtr.errcheck` function returns the argument tuple it receives
unchanged, :mod:`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 >>>
.. function:: addressof(obj) Returns the address of the memory buffer as integer. *obj* must be an instance of a ctypes type. .. audit-event:: ctypes.addressof obj ctypes.addressof
.. function:: alignment(obj_or_type) Returns the alignment requirements of a ctypes type. *obj_or_type* must be a ctypes type or instance.
.. function:: 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.
.. function:: 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.
.. function:: create_string_buffer(init_or_size, size=None) This function creates a mutable character buffer. The returned object is a ctypes array of :class:`c_char`. *init_or_size* must be an integer which specifies the size of the array, or a bytes object which will be used to initialize the array items. If a bytes object is specified as first argument, the buffer is made one item larger than its length so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the bytes should not be used. .. audit-event:: ctypes.create_string_buffer init,size ctypes.create_string_buffer
.. function:: create_unicode_buffer(init_or_size, size=None) This function creates a mutable unicode character buffer. The returned object is a ctypes array of :class:`c_wchar`. *init_or_size* must be an integer which specifies the size of the array, or a string which will be used to initialize the array items. If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the string should not be used. .. audit-event:: ctypes.create_unicode_buffer init,size ctypes.create_unicode_buffer
.. function:: 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.
.. function:: 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.
.. function:: find_library(name) :module: ctypes.util 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 :option:`!-l`). If no library can be found, returns ``None``. The exact functionality is system dependent.
.. function:: find_msvcrt() :module: ctypes.util 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.
.. function:: 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.
.. 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.
.. function:: get_errno() Returns the current value of the ctypes-private copy of the system :data:`errno` variable in the calling thread. .. audit-event:: ctypes.get_errno "" ctypes.get_errno
.. function:: get_last_error() Windows only: returns the current value of the ctypes-private copy of the system :data:`!LastError` variable in the calling thread. .. audit-event:: ctypes.get_last_error "" ctypes.get_last_error
.. function:: 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.
.. function:: 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.
.. function:: POINTER(type, /) Create and return a new ctypes pointer type. Pointer types are cached and reused internally, so calling this function repeatedly is cheap. *type* must be a ctypes type.
.. function:: pointer(obj, /) Create 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.
.. function:: 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.
.. function:: set_errno(value) Set the current value of the ctypes-private copy of the system :data:`errno` variable in the calling thread to *value* and return the previous value. .. audit-event:: ctypes.set_errno errno ctypes.set_errno
.. function:: set_last_error(value) Windows only: set the current value of the ctypes-private copy of the system :data:`!LastError` variable in the calling thread to *value* and return the previous value. .. audit-event:: ctypes.set_last_error error ctypes.set_last_error
.. function:: 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.
.. function:: string_at(ptr, size=-1) Return the byte string at *void \*ptr*. If *size* is specified, it is used as size, otherwise the string is assumed to be zero-terminated. .. audit-event:: ctypes.string_at ptr,size ctypes.string_at
.. function:: WinError(code=None, descr=None)
Windows only: this function is probably the worst-named thing in ctypes. It
creates an instance of :exc:`OSError`. If *code* is not specified,
``GetLastError`` is called to determine the error code. If *descr* is not
specified, :func:`FormatError` is called to get a textual description of the
error.
.. versionchanged:: 3.3
An instance of :exc:`WindowsError` used to be created, which is now an
alias of :exc:`OSError`.
.. function:: wstring_at(ptr, size=-1) Return the wide-character string at *void \*ptr*. If *size* is specified, it is used as the number of characters of the string, otherwise the string is assumed to be zero-terminated. .. audit-event:: ctypes.wstring_at ptr,size ctypes.wstring_at
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 :func:`addressof` helper function. Another instance variable is exposed as :attr:`_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 :term:`metaclass`):
.. method:: _CData.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 :exc:`ValueError` is raised. .. audit-event:: ctypes.cdata/buffer pointer,size,offset ctypes._CData.from_buffer
.. method:: _CData.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 :exc:`ValueError` is raised. .. audit-event:: ctypes.cdata/buffer pointer,size,offset ctypes._CData.from_buffer_copy
.. method:: from_address(address)
This method returns a ctypes type instance using the memory specified by
*address* which must be an integer.
.. audit-event:: ctypes.cdata address ctypes._CData.from_address
This method, and others that indirectly call this method, raises an
:ref:`auditing event <auditing>` ``ctypes.cdata`` with argument
``address``.
.. method:: 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 :attr:`~_CFuncPtr.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.
.. method:: 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:
.. attribute:: _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 :attr:`_b_base_` read-only member is the root ctypes object that owns the memory block.
.. attribute:: _b_needsfree_ This read-only variable is true when the ctypes data instance has allocated the memory block itself, false otherwise.
.. attribute:: _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.
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. :class:`_SimpleCData` is a subclass of :class:`_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:
.. attribute:: value This attribute contains the actual value of the instance. For integer and pointer types, it is an integer, for character types, it is a single character bytes object or string, for character pointer types it is a Python bytes object or string. When the ``value`` attribute is retrieved from a ctypes instance, usually a new object is returned each time. :mod:`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 :attr:`~_CFuncPtr.restype` of :class:`c_char_p`, you will always receive a Python bytes object, not a :class:`c_char_p` instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions :attr:`!restype` is a subclass of :class:`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 :c:expr:`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 :c:expr:`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 :c:expr:`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 :c:expr:`double` datatype. The constructor accepts an optional float initializer.
Represents the C :c:expr:`long double` datatype. The constructor accepts an
optional float initializer. On platforms where sizeof(long double) ==
sizeof(double) it is an alias to :class:`c_double`.
Represents the C :c:expr:`float` datatype. The constructor accepts an optional float initializer.
Represents the C :c:expr:`double complex` datatype, if available. The constructor accepts an optional :class:`complex` initializer.
.. versionadded:: 3.14
Represents the C :c:expr:`float complex` datatype, if available. The constructor accepts an optional :class:`complex` initializer.
.. versionadded:: 3.14
Represents the C :c:expr:`long double complex` datatype, if available. The constructor accepts an optional :class:`complex` initializer.
.. versionadded:: 3.14
Represents the C :c:expr:`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 :class:`c_long`.
Represents the C 8-bit :c:expr:`signed int` datatype. Usually an alias for :class:`c_byte`.
Represents the C 16-bit :c:expr:`signed int` datatype. Usually an alias for :class:`c_short`.
Represents the C 32-bit :c:expr:`signed int` datatype. Usually an alias for :class:`c_int`.
Represents the C 64-bit :c:expr:`signed int` datatype. Usually an alias for :class:`c_longlong`.
Represents the C :c:expr:`signed long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C :c:expr:`signed long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C :c:expr:`signed short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C :c:type:`size_t` datatype.
Represents the C :c:type:`ssize_t` datatype.
.. versionadded:: 3.2
Represents the C :c:type:`time_t` datatype.
.. versionadded:: 3.12
Represents the C :c:expr:`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 :c:expr:`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 :class:`c_ulong`.
Represents the C 8-bit :c:expr:`unsigned int` datatype. Usually an alias for :class:`c_ubyte`.
Represents the C 16-bit :c:expr:`unsigned int` datatype. Usually an alias for :class:`c_ushort`.
Represents the C 32-bit :c:expr:`unsigned int` datatype. Usually an alias for :class:`c_uint`.
Represents the C 64-bit :c:expr:`unsigned int` datatype. Usually an alias for :class:`c_ulonglong`.
Represents the C :c:expr:`unsigned long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C :c:expr:`unsigned long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C :c:expr:`unsigned short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C :c:expr:`void *` type. The value is represented as integer. The constructor accepts an optional integer initializer.
Represents the C :c:type:`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 :c:expr:`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 :c:expr:`bool` datatype (more accurately, :c:expr:`_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 :c:type:`!HRESULT` value, which contains success or error information for a function or method call.
Represents the C :c:expr:`PyObject *` datatype. Calling this without an
argument creates a NULL :c:expr:`PyObject *` pointer.
The :mod:`!ctypes.wintypes` module provides quite some other Windows specific data types, for example :c:type:`!HWND`, :c:type:`!WPARAM`, or :c:type:`!DWORD`. Some useful structures like :c:type:`!MSG` or :c:type:`!RECT` are also defined.
Abstract base class for unions in native byte order.
Unions share common attributes and behavior with structures; see :class:`Structure` documentation for details.
Abstract base class for unions in big endian byte order.
.. versionadded:: 3.11
Abstract base class for unions in little endian byte order.
.. versionadded:: 3.11
Abstract base class for structures in big endian byte order.
Abstract base class for structures in little endian byte order.
Structures and unions 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 :attr:`_fields_` class variable. :mod:`ctypes` will create :term:`descriptor`s which allow reading and writing the fields by direct attribute accesses. These are the
.. attribute:: _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 :class:`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 :attr:`_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 :attr:`!_fields_` class variable can only be set once.
Later assignments will raise an :exc:`AttributeError`.
Additionally, the :attr:`!_fields_` class variable must be defined before
the structure or union type is first used: an instance or subclass is
created, :func:`sizeof` is called on it, and so on.
Later assignments to :attr:`!_fields_` will raise an :exc:`AttributeError`.
If :attr:`!_fields_` has not been set before such use,
the structure or union will have no own fields, as if :attr:`!_fields_`
was empty.
Sub-subclasses of structure types inherit the fields of the base class
plus the :attr:`_fields_` defined in the sub-subclass, if any.
.. attribute:: _pack_ An optional small integer that allows overriding the alignment of structure fields in the instance. :attr:`_pack_` must already be defined when :attr:`_fields_` is assigned, otherwise it will have no effect. Setting this attribute to 0 is the same as not setting it at all.
.. attribute:: _align_ An optional small integer that allows overriding the alignment of the structure when being packed or unpacked to/from memory. Setting this attribute to 0 is the same as not setting it at all. .. versionadded:: 3.13
.. attribute:: _layout_
An optional string naming the struct/union layout. It can currently
be set to:
- ``"ms"``: the layout used by the Microsoft compiler (MSVC).
On GCC and Clang, this layout can be selected with
``__attribute__((ms_struct))``.
- ``"gcc-sysv"``: the layout used by GCC with the System V or “SysV-like”
data model, as used on Linux and macOS.
With this layout, :attr:`~Structure._pack_` must be unset or zero.
If not set explicitly, ``ctypes`` will use a default that
matches the platform conventions. This default may change in future
Python releases (for example, when a new platform gains official support,
or when a difference between similar platforms is found).
Currently the default will be:
- On Windows: ``"ms"``
- When :attr:`~Structure._pack_` is specified: ``"ms"``
- Otherwise: ``"gcc-sysv"``
:attr:`!_layout_` must already be defined when
:attr:`~Structure._fields_` is assigned, otherwise it will have no effect.
.. attribute:: _anonymous_
An optional sequence that lists the names of unnamed (anonymous) fields.
:attr:`_anonymous_` must be already defined when :attr:`_fields_` is
assigned, otherwise it will have no effect.
The fields listed in this variable must be structure or union type fields.
:mod:`ctypes` will create descriptors in the structure type that allows
accessing the nested fields directly, without the need to create the
structure or union field.
Here is an example type (Windows)::
class _U(Union):
_fields_ = [("lptdesc", POINTER(TYPEDESC)),
("lpadesc", POINTER(ARRAYDESC)),
("hreftype", HREFTYPE)]
class TYPEDESC(Structure):
_anonymous_ = ("u",)
_fields_ = [("u", _U),
("vt", VARTYPE)]
The ``TYPEDESC`` structure describes a COM data type, 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 define sub-subclasses of structures, they inherit the fields of the base class. If the subclass definition has a separate :attr:`_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 :attr:`_fields_`. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize :attr:`_fields_` with the same name, or create new attributes for names not present in :attr:`_fields_`.
Abstract base class for arrays.
The recommended way to create concrete array types is by multiplying any :mod:`ctypes` data type with a non-negative integer. Alternatively, you can subclass this type and define :attr:`_length_` and :attr:`_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 :class:`Array`.
.. attribute:: _length_
A positive integer specifying the number of elements in the array.
Out-of-range subscripts result in an :exc:`IndexError`. Will be
returned by :func:`len`.
.. attribute:: _type_
Specifies the type of each element in the array.
Array subclass constructors accept positional arguments, used to initialize the elements in order.
.. function:: ARRAY(type, length) Create an array. Equivalent to ``type * length``, where *type* is a :mod:`ctypes` data type and *length* an integer. This function is :term:`soft deprecated` in favor of multiplication. There are no plans to remove it.
Private, abstract base class for pointers.
Concrete pointer types are created by calling :func:`POINTER` with the type that will be pointed to; this is done automatically by :func:`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 :func:`len` will raise :exc:`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).
.. attribute:: _type_
Specifies the type pointed to.
.. attribute:: contents
Returns the object to which to pointer points. Assigning to this
attribute changes the pointer to point to the assigned object.