| :mod:`ctypes` --- A foreign function library for Python |
| ======================================================= |
| |
| .. module:: ctypes |
| :synopsis: A foreign function library for Python. |
| .. moduleauthor:: Thomas Heller <theller@python.net> |
| |
| |
| :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. |
| |
| |
| .. _ctypes-ctypes-tutorial: |
| |
| ctypes tutorial |
| --------------- |
| |
| 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 Mac OS X, they contain doctest directives in comments. |
| |
| Note: Some code samples reference the ctypes :class:`c_int` type. This type is |
| an alias for the :class:`c_long` type on 32-bit systems. 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. |
| |
| |
| .. _ctypes-loading-dynamic-link-libraries: |
| |
| Loading dynamic link libraries |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| :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 a :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. |
| |
| 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:`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 ...> |
| >>> |
| |
| .. XXX Add section for Mac OS X. |
| |
| |
| .. _ctypes-accessing-functions-from-loaded-dlls: |
| |
| Accessing functions from loaded dlls |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Functions are accessed as attributes of dll objects:: |
| |
| >>> from ctypes import * |
| >>> 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 ? |
| 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 |
| :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 ? |
| File "ctypes.py", line 310, in __getitem__ |
| func = _StdcallFuncPtr(name, self) |
| AttributeError: function ordinal 0 not found |
| >>> |
| |
| |
| .. _ctypes-calling-functions: |
| |
| Calling functions |
| ^^^^^^^^^^^^^^^^^ |
| |
| 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)) # doctest: +SKIP |
| 1150640792 |
| >>> print(hex(windll.kernel32.GetModuleHandleA(None))) # doctest: +WINDOWS |
| 0x1d000000 |
| >>> |
| |
| :mod:`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() # doctest: +WINDOWS |
| 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) # doctest: +WINDOWS |
| 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) # doctest: +WINDOWS |
| 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") # doctest: +WINDOWS |
| 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, :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 ? |
| OSError: exception: access violation reading 0x00000020 |
| >>> |
| |
| There are, however, enough ways to crash Python with :mod:`ctypes`, so you |
| should be careful anyway. |
| |
| ``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:type:`char *` or |
| :c:type:`wchar_t *`). Python integers are passed as the platforms default C |
| :c:type:`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. |
| |
| |
| .. _ctypes-fundamental-data-types: |
| |
| Fundamental data types |
| ^^^^^^^^^^^^^^^^^^^^^^ |
| |
| :mod:`ctypes` defines a number of primitive C compatible data types : |
| |
| +----------------------+------------------------------------------+----------------------------+ |
| | ctypes type | C type | Python type | |
| +======================+==========================================+============================+ |
| | :class:`c_bool` | :c:type:`_Bool` | bool (1) | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_char` | :c:type:`char` | 1-character bytes object | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_wchar` | :c:type:`wchar_t` | 1-character string | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_byte` | :c:type:`char` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_ubyte` | :c:type:`unsigned char` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_short` | :c:type:`short` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_ushort` | :c:type:`unsigned short` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_int` | :c:type:`int` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_uint` | :c:type:`unsigned int` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_long` | :c:type:`long` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_ulong` | :c:type:`unsigned long` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_longlong` | :c:type:`__int64` or :c:type:`long long` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_ulonglong` | :c:type:`unsigned __int64` or | int | |
| | | :c:type:`unsigned long long` | | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_size_t` | :c:type:`size_t` | int | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_ssize_t` | :c:type:`ssize_t` or | int | |
| | | :c:type:`Py_ssize_t` | | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_float` | :c:type:`float` | float | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_double` | :c:type:`double` | float | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_longdouble`| :c:type:`long double` | float | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_char_p` | :c:type:`char *` (NUL terminated) | bytes object or ``None`` | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_wchar_p` | :c:type:`wchar_t *` (NUL terminated) | string or ``None`` | |
| +----------------------+------------------------------------------+----------------------------+ |
| | :class:`c_void_p` | :c:type:`void *` | int or ``None`` | |
| +----------------------+------------------------------------------+----------------------------+ |
| |
| (1) |
| The constructor accepts any object with a truth value. |
| |
| 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 :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('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 |
| :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 :func:`c_buffer` function |
| (which is still available as an alias), as well as the :func:`c_string` function |
| from earlier ctypes releases. To create a mutable memory block containing |
| unicode characters of the C type :c:type:`wchar_t` use the |
| :func:`create_unicode_buffer` function. |
| |
| |
| .. _ctypes-calling-functions-continued: |
| |
| Calling functions, continued |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| 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 ? |
| 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 :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 |
| >>> |
| |
| |
| .. _ctypes-calling-functions-with-own-custom-data-types: |
| |
| Calling functions with your own custom data types |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| 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. 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 :attr:`_as_parameter_` |
| instance variable, you could define a :class:`property` which makes the |
| attribute available on request. |
| |
| |
| .. _ctypes-specifying-required-argument-types: |
| |
| Specifying the required argument types (function prototypes) |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| It is possible to specify the required argument types of functions exported from |
| DLLs by setting the :attr:`argtypes` attribute. |
| |
| :attr:`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 :meth:`from_param` class method for them to be able to use them |
| in the :attr:`argtypes` sequence. The :meth:`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. |
| |
| |
| .. _ctypes-return-types: |
| |
| Return types |
| ^^^^^^^^^^^^ |
| |
| By default functions are assumed to return the C :c:type:`int` type. Other |
| return types can be specified by setting the :attr:`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")) # 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 ``ord("x")`` calls above, you can set the |
| :attr:`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 :attr:`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 ? |
| 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:`errcheck` attribute; see the reference manual for details. |
| |
| |
| .. _ctypes-passing-pointers: |
| |
| Passing pointers (or: passing parameters by reference) |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Sometimes a C api function expects a *pointer* to a data type as parameter, |
| probably to write into the corresponding location, or if the data is too large |
| to be passed by value. This is also known as *passing parameters by reference*. |
| |
| :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' |
| >>> |
| |
| |
| .. _ctypes-structures-unions: |
| |
| Structures and unions |
| ^^^^^^^^^^^^^^^^^^^^^ |
| |
| 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:`_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 ? |
| 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 :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> |
| >>> |
| |
| |
| .. _ctypes-structureunion-alignment-byte-order: |
| |
| .. 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. |
| |
| Structure/union alignment and byte order |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| By default, Structure and Union fields are aligned in the same way the C |
| compiler does it. It is possible to override this behavior be specifying a |
| :attr:`_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. |
| |
| :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. |
| |
| |
| .. _ctypes-bit-fields-in-structures-unions: |
| |
| Bit fields in structures and unions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| It is possible to create structures and unions containing bit fields. Bit fields |
| are only possible for integer fields, the bit width is specified as the third |
| item in the :attr:`_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> |
| >>> |
| |
| |
| .. _ctypes-arrays: |
| |
| Arrays |
| ^^^^^^ |
| |
| Arrays are sequences, containing a fixed number of instances of the same type. |
| |
| The recommended way to create array types is by multiplying a data type with a |
| positive integer:: |
| |
| TenPointsArrayType = POINT * 10 |
| |
| Here is an example of an somewhat artificial data type, a structure containing 4 |
| POINTs among other stuff:: |
| |
| >>> from ctypes import * |
| >>> class POINT(Structure): |
| ... _fields_ = ("x", c_int), ("y", c_int) |
| ... |
| >>> class MyStruct(Structure): |
| ... _fields_ = [("a", c_int), |
| ... ("b", c_float), |
| ... ("point_array", POINT * 4)] |
| >>> |
| >>> print(len(MyStruct().point_array)) |
| 4 |
| >>> |
| |
| Instances are created in the usual way, by calling the class:: |
| |
| arr = TenPointsArrayType() |
| for pt in arr: |
| print(pt.x, pt.y) |
| |
| The above code print a series of ``0 0`` lines, because the array contents is |
| initialized to zeros. |
| |
| Initializers of the correct type can also be specified:: |
| |
| >>> from ctypes import * |
| >>> TenIntegers = c_int * 10 |
| >>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) |
| >>> print(ii) |
| <c_long_Array_10 object at 0x...> |
| >>> for i in ii: print(i, end=" ") |
| ... |
| 1 2 3 4 5 6 7 8 9 10 |
| >>> |
| |
| |
| .. _ctypes-pointers: |
| |
| Pointers |
| ^^^^^^^^ |
| |
| 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:`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) |
| >>> |
| |
| .. XXX Document dereferencing pointers, and that it is preferred over the |
| .contents attribute. |
| |
| 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 ? |
| 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 |
| >>> |
| |
| |
| .. _ctypes-type-conversions: |
| |
| Type conversions |
| ^^^^^^^^^^^^^^^^ |
| |
| Usually, ctypes does strict type checking. This means, if you have |
| ``POINTER(c_int)`` in the :attr:`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:`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 |
| >>> |
| |
| .. XXX list other conversions... |
| |
| 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 ? |
| 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 |
| >>> |
| |
| |
| .. _ctypes-incomplete-types: |
| |
| Incomplete Types |
| ^^^^^^^^^^^^^^^^ |
| |
| *Incomplete Types* are structures, unions or arrays whose members are not yet |
| specified. In C, they are specified by forward declarations, which are defined |
| later:: |
| |
| struct cell; /* forward declaration */ |
| |
| struct cell { |
| char *name; |
| struct cell *next; |
| }; |
| |
| The straightforward translation into ctypes code would be this, but it does not |
| work:: |
| |
| >>> class cell(Structure): |
| ... _fields_ = [("name", c_char_p), |
| ... ("next", POINTER(cell))] |
| ... |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| 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:`_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-callback-functions: |
| |
| Callback functions |
| ^^^^^^^^^^^^^^^^^^ |
| |
| :mod:`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 :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 |
| >>> |
| |
| **Important note for callback functions:** |
| |
| 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. |
| |
| |
| .. _ctypes-accessing-values-exported-from-dlls: |
| |
| Accessing values exported from dlls |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Some shared libraries not only export functions, they also export variables. An |
| example in the Python library itself is the :c:data:`Py_OptimizeFlag`, an integer |
| set to 0, 1, or 2, depending on the :option:`-O` or :option:`-OO` flag given on |
| startup. |
| |
| :mod:`ctypes` can access values like this with the :meth:`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 :option:`-O`, the sample would |
| have printed ``c_long(1)``, or ``c_long(2)`` if :option:`-OO` would have been |
| specified. |
| |
| 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:type:`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)] |
| ... |
| >>> |
| |
| We have defined the :c:type:`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. |
| |
| |
| .. _ctypes-surprises: |
| |
| Surprises |
| ^^^^^^^^^ |
| |
| 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 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 :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! |
| |
| |
| .. _ctypes-variable-sized-data-types: |
| |
| Variable-sized data types |
| ^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| :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. |
| |
| |
| .. _ctypes-ctypes-reference: |
| |
| ctypes reference |
| ---------------- |
| |
| |
| .. _ctypes-finding-shared-libraries: |
| |
| Finding shared libraries |
| ^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| When programming in a compiled language, shared libraries are accessed when |
| compiling/linking a program, and when the program is run. |
| |
| The purpose of the :func:`find_library` function is to locate a library in a way |
| similar to what the compiler does (on platforms with several versions of a |
| shared library the most recent should be loaded), while the ctypes library |
| loaders act like when a program is run, and call the runtime loader directly. |
| |
| The :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:`find_library` tries to run external programs |
| (``/sbin/ldconfig``, ``gcc``, and ``objdump``) to find the library file. It |
| returns the filename of the library file. Here are some examples:: |
| |
| >>> from ctypes.util import find_library |
| >>> find_library("m") |
| 'libm.so.6' |
| >>> find_library("c") |
| 'libc.so.6' |
| >>> find_library("bz2") |
| 'libbz2.so.1.0' |
| >>> |
| |
| On OS X, :func:`find_library` tries several predefined naming schemes and paths |
| to locate the library, and returns a full pathname if successful:: |
| |
| >>> from ctypes.util import find_library |
| >>> find_library("c") |
| '/usr/lib/libc.dylib' |
| >>> find_library("m") |
| '/usr/lib/libm.dylib' |
| >>> find_library("bz2") |
| '/usr/lib/libbz2.dylib' |
| >>> find_library("AGL") |
| '/System/Library/Frameworks/AGL.framework/AGL' |
| >>> |
| |
| On Windows, :func:`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:`find_library` to locate the library at runtime. |
| |
| |
| .. _ctypes-loading-shared-libraries: |
| |
| Loading shared libraries |
| ^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| There are several ways to loaded shared libraries into the Python process. One |
| way is to instantiate one of the following classes: |
| |
| |
| .. class:: CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False) |
| |
| Instances of this class represent loaded shared libraries. Functions in these |
| libraries use the standard C calling convention, and are assumed to return |
| :c:type:`int`. |
| |
| |
| .. class:: OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False) |
| |
| Windows only: Instances of this class represent loaded shared libraries, |
| functions in these libraries use the ``stdcall`` calling convention, and are |
| assumed to return the windows specific :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. |
| |
| |
| .. class:: WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False) |
| |
| Windows only: Instances of this class represent loaded shared libraries, |
| functions in these libraries use the ``stdcall`` calling convention, and are |
| assumed to return :c:type:`int` by default. |
| |
| On Windows CE only the standard calling convention is used, for convenience the |
| :class:`WinDLL` and :class:`OleDLL` use the standard calling convention on this |
| platform. |
| |
| The Python :term:`global interpreter lock` is released before calling any |
| function exported by these libraries, and reacquired afterwards. |
| |
| |
| .. class:: PyDLL(name, mode=DEFAULT_MODE, handle=None) |
| |
| 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. |
| |
| All these classes can be instantiated by calling them with at least one |
| argument, the pathname of the shared library. If you have an existing handle to |
| an already loaded shared library, it can be passed as the ``handle`` named |
| parameter, otherwise the underlying platforms ``dlopen`` or ``LoadLibrary`` |
| function is used to load the library into the process, and to get a handle to |
| it. |
| |
| The *mode* parameter can be used to specify how the library is loaded. For |
| details, consult the :manpage:`dlopen(3)` manpage, on Windows, *mode* is |
| ignored. |
| |
| The *use_errno* parameter, when set to True, enables a ctypes mechanism that |
| allows to access the system :data:`errno` error number in a safe way. |
| :mod:`ctypes` maintains a thread-local copy of the systems :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. |
| |
| .. 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, however :meth:`__getattr__` |
| and :meth:`__getitem__` have special behavior: functions exported by the shared |
| library can be accessed as attributes of by index. Please note that both |
| :meth:`__getattr__` and :meth:`__getitem__` cache their result, so calling them |
| repeatedly returns the same object each time. |
| |
| 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:`LoadLibrary` method, or by retrieving the library as attribute of the |
| loader instance. |
| |
| |
| .. class:: LibraryLoader(dlltype) |
| |
| 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 to load 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:type:`int`, which is of course not always the truth, so you have to assign |
| the correct :attr:`restype` attribute to use these functions. |
| |
| |
| .. _ctypes-foreign-functions: |
| |
| Foreign functions |
| ^^^^^^^^^^^^^^^^^ |
| |
| As explained in the previous section, foreign functions can be accessed as |
| attributes of loaded shared libraries. The function objects created in this way |
| by default accept any number of arguments, accept any ctypes data instances as |
| arguments, and return the default result type specified by the library loader. |
| They are instances of a private class: |
| |
| |
| .. class:: _FuncPtr |
| |
| Base class for C callable foreign functions. |
| |
| Instances of foreign functions are also C compatible data types; they |
| represent C function pointers. |
| |
| This behavior can be customized by assigning to special attributes of the |
| foreign function object. |
| |
| .. attribute:: restype |
| |
| Assign a ctypes type to specify the result type of the foreign function. |
| Use ``None`` for :c:type:`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:type:`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 |
| :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:`from_param` class method of the items in the :attr:`argtypes` |
| tuple, this method allows to adapt 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:`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. |
| |
| .. 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 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. |
| |
| |
| .. exception:: ArgumentError |
| |
| This exception is raised when a foreign function call cannot convert one of the |
| passed arguments. |
| |
| |
| .. _ctypes-function-prototypes: |
| |
| Function prototypes |
| ^^^^^^^^^^^^^^^^^^^ |
| |
| Foreign functions can also be created by instantiating function prototypes. |
| Function prototypes are similar to function prototypes in C; they describe a |
| function (return type, argument types, calling convention) without defining an |
| implementation. The factory functions must be called with the desired result |
| type and the argument types of the function. |
| |
| |
| .. 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, except on Windows CE where |
| :func:`WINFUNCTYPE` is the same as :func:`CFUNCTYPE`. 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:`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 :mod:`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 :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:`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:`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 |
| >>> |
| |
| |
| .. _ctypes-utility-functions: |
| |
| Utility functions |
| ^^^^^^^^^^^^^^^^^ |
| |
| .. function:: addressof(obj) |
| |
| Returns the address of the memory buffer as integer. *obj* must be an |
| instance of a ctypes type. |
| |
| |
| .. 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 |
| to specify the size of the array if the length of the bytes should not be used. |
| |
| |
| |
| .. 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 to specify the size of the array if the length of the string should not |
| be used. |
| |
| |
| |
| .. function:: DllCanUnloadNow() |
| |
| 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. |
| |
| |
| .. function:: DllGetClassObject() |
| |
| 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. |
| |
| |
| .. 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 runtype 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. |
| |
| .. 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. |
| |
| .. 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) |
| |
| 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. |
| |
| |
| .. function:: pointer(obj) |
| |
| This function creates a new pointer instance, pointing to *obj*. The returned |
| object is of the type ``POINTER(type(obj))``. |
| |
| Note: If you just want to pass a pointer to an object to a foreign function |
| call, you should use ``byref(obj)`` which is much faster. |
| |
| |
| .. 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. |
| |
| |
| |
| .. 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. |
| |
| |
| |
| .. 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()`` function. |
| |
| |
| .. function:: string_at(address, size=-1) |
| |
| This function returns the C string starting at memory address *address* as a bytes |
| object. If size is specified, it is used as size, otherwise the string is assumed |
| to be zero-terminated. |
| |
| |
| .. function:: WinError(code=None, descr=None) |
| |
| Windows only: this function is probably the worst-named thing in ctypes. It |
| creates an instance of OSError. If *code* is not specified, |
| ``GetLastError`` is called to determine the error code. If *descr* is not |
| specified, :func:`FormatError` is called to get a textual description of the |
| error. |
| |
| .. versionchanged:: 3.3 |
| An instance of :exc:`WindowsError` used to be created. |
| |
| |
| .. function:: wstring_at(address, size=-1) |
| |
| This function returns the wide character string starting at memory address |
| *address* as a string. If *size* is specified, it is used as the number of |
| characters of the string, otherwise the string is assumed to be |
| zero-terminated. |
| |
| |
| .. _ctypes-data-types: |
| |
| Data types |
| ^^^^^^^^^^ |
| |
| |
| .. class:: _CData |
| |
| This non-public class is the common base class of all ctypes data types. |
| Among other things, all ctypes type instances contain a memory block that |
| hold C compatible data; the address of the memory block is returned by the |
| :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. |
| |
| |
| .. 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. |
| |
| .. method:: from_address(address) |
| |
| This method returns a ctypes type instance using the memory specified by |
| *address* which must be an integer. |
| |
| .. 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:`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. |
| |
| |
| .. _ctypes-fundamental-data-types-2: |
| |
| Fundamental data types |
| ^^^^^^^^^^^^^^^^^^^^^^ |
| |
| .. class:: _SimpleCData |
| |
| This non-public class is the base class of all fundamental ctypes data |
| types. It is mentioned here because it contains the common attributes of the |
| fundamental ctypes data types. :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:`restype` of :class:`c_char_p`, you will always receive a Python bytes |
| object, *not* a :class:`c_char_p` instance. |
| |
| .. XXX above is false, it actually returns a Unicode string |
| |
| 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: |
| |
| .. class:: c_byte |
| |
| Represents the C :c:type:`signed char` datatype, and interprets the value as |
| small integer. The constructor accepts an optional integer initializer; no |
| overflow checking is done. |
| |
| |
| .. class:: c_char |
| |
| Represents the C :c:type:`char` datatype, and interprets the value as a single |
| character. The constructor accepts an optional string initializer, the |
| length of the string must be exactly one character. |
| |
| |
| .. class:: c_char_p |
| |
| Represents the C :c:type:`char *` datatype when it points to a zero-terminated |
| string. For a general character pointer that may also point to binary data, |
| ``POINTER(c_char)`` must be used. The constructor accepts an integer |
| address, or a bytes object. |
| |
| |
| .. class:: c_double |
| |
| Represents the C :c:type:`double` datatype. The constructor accepts an |
| optional float initializer. |
| |
| |
| .. class:: c_longdouble |
| |
| Represents the C :c:type:`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`. |
| |
| .. class:: c_float |
| |
| Represents the C :c:type:`float` datatype. The constructor accepts an |
| optional float initializer. |
| |
| |
| .. class:: c_int |
| |
| Represents the C :c:type:`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`. |
| |
| |
| .. class:: c_int8 |
| |
| Represents the C 8-bit :c:type:`signed int` datatype. Usually an alias for |
| :class:`c_byte`. |
| |
| |
| .. class:: c_int16 |
| |
| Represents the C 16-bit :c:type:`signed int` datatype. Usually an alias for |
| :class:`c_short`. |
| |
| |
| .. class:: c_int32 |
| |
| Represents the C 32-bit :c:type:`signed int` datatype. Usually an alias for |
| :class:`c_int`. |
| |
| |
| .. class:: c_int64 |
| |
| Represents the C 64-bit :c:type:`signed int` datatype. Usually an alias for |
| :class:`c_longlong`. |
| |
| |
| .. class:: c_long |
| |
| Represents the C :c:type:`signed long` datatype. The constructor accepts an |
| optional integer initializer; no overflow checking is done. |
| |
| |
| .. class:: c_longlong |
| |
| Represents the C :c:type:`signed long long` datatype. The constructor accepts |
| an optional integer initializer; no overflow checking is done. |
| |
| |
| .. class:: c_short |
| |
| Represents the C :c:type:`signed short` datatype. The constructor accepts an |
| optional integer initializer; no overflow checking is done. |
| |
| |
| .. class:: c_size_t |
| |
| Represents the C :c:type:`size_t` datatype. |
| |
| |
| .. class:: c_ssize_t |
| |
| Represents the C :c:type:`ssize_t` datatype. |
| |
| .. versionadded:: 3.2 |
| |
| |
| .. class:: c_ubyte |
| |
| Represents the C :c:type:`unsigned char` datatype, it interprets the value as |
| small integer. The constructor accepts an optional integer initializer; no |
| overflow checking is done. |
| |
| |
| .. class:: c_uint |
| |
| Represents the C :c:type:`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`. |
| |
| |
| .. class:: c_uint8 |
| |
| Represents the C 8-bit :c:type:`unsigned int` datatype. Usually an alias for |
| :class:`c_ubyte`. |
| |
| |
| .. class:: c_uint16 |
| |
| Represents the C 16-bit :c:type:`unsigned int` datatype. Usually an alias for |
| :class:`c_ushort`. |
| |
| |
| .. class:: c_uint32 |
| |
| Represents the C 32-bit :c:type:`unsigned int` datatype. Usually an alias for |
| :class:`c_uint`. |
| |
| |
| .. class:: c_uint64 |
| |
| Represents the C 64-bit :c:type:`unsigned int` datatype. Usually an alias for |
| :class:`c_ulonglong`. |
| |
| |
| .. class:: c_ulong |
| |
| Represents the C :c:type:`unsigned long` datatype. The constructor accepts an |
| optional integer initializer; no overflow checking is done. |
| |
| |
| .. class:: c_ulonglong |
| |
| Represents the C :c:type:`unsigned long long` datatype. The constructor |
| accepts an optional integer initializer; no overflow checking is done. |
| |
| |
| .. class:: c_ushort |
| |
| Represents the C :c:type:`unsigned short` datatype. The constructor accepts |
| an optional integer initializer; no overflow checking is done. |
| |
| |
| .. class:: c_void_p |
| |
| Represents the C :c:type:`void *` type. The value is represented as integer. |
| The constructor accepts an optional integer initializer. |
| |
| |
| .. class:: c_wchar |
| |
| 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. |
| |
| |
| .. class:: c_wchar_p |
| |
| Represents the C :c:type:`wchar_t *` datatype, which must be a pointer to a |
| zero-terminated wide character string. The constructor accepts an integer |
| address, or a string. |
| |
| |
| .. class:: c_bool |
| |
| Represent the C :c:type:`bool` datatype (more accurately, :c:type:`_Bool` from |
| C99). Its value can be True or False, and the constructor accepts any object |
| that has a truth value. |
| |
| |
| .. class:: HRESULT |
| |
| Windows only: Represents a :c:type:`HRESULT` value, which contains success or |
| error information for a function or method call. |
| |
| |
| .. class:: py_object |
| |
| Represents the C :c:type:`PyObject *` datatype. Calling this without an |
| argument creates a ``NULL`` :c:type:`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. |
| |
| |
| .. _ctypes-structured-data-types: |
| |
| Structured data types |
| ^^^^^^^^^^^^^^^^^^^^^ |
| |
| |
| .. class:: Union(*args, **kw) |
| |
| Abstract base class for unions in native byte order. |
| |
| |
| .. class:: BigEndianStructure(*args, **kw) |
| |
| Abstract base class for structures in *big endian* byte order. |
| |
| |
| .. class:: LittleEndianStructure(*args, **kw) |
| |
| Abstract base class for structures in *little endian* byte order. |
| |
| Structures with non-native byte order cannot contain pointer type fields, or any |
| other data types containing pointer type fields. |
| |
| |
| .. class:: Structure(*args, **kw) |
| |
| Abstract base class for structures in *native* byte order. |
| |
| Concrete structure and union types must be created by subclassing one of these |
| types, and at least define a :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 to create |
| data types that directly or indirectly reference themselves:: |
| |
| class List(Structure): |
| pass |
| List._fields_ = [("pnext", POINTER(List)), |
| ... |
| ] |
| |
| The :attr:`_fields_` class variable must, however, be defined before the |
| type is first used (an instance is created, :func:`sizeof` is called on it, |
| and so on). Later assignments to the :attr:`_fields_` class variable will |
| raise an AttributeError. |
| |
| Structure and union subclass constructors accept both positional and named |
| arguments. Positional arguments are used to initialize the fields in the |
| same order as they appear in the :attr:`_fields_` definition, named |
| arguments are used to initialize the fields with the corresponding name. |
| |
| It is possible to defined sub-subclasses of structure types, they 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 to override 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. |
| |
| |
| .. 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 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 |
| :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_`. |
| |
| |
| .. _ctypes-arrays-pointers: |
| |
| Arrays and pointers |
| ^^^^^^^^^^^^^^^^^^^ |
| |
| Not yet written - please see the sections :ref:`ctypes-pointers` and section |
| :ref:`ctypes-arrays` in the tutorial. |
| |