Doc/library/ctypes.rst
!ctypes --- A foreign function library for Python.. module:: ctypes :synopsis: A foreign function library for Python.
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.
.. include:: ../includes/optional-module.rst
.. _ctypes-ctypes-tutorial:
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.
.. _ctypes-loading-dynamic-link-libraries:
Loading dynamic link libraries ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
:mod:!ctypes exports the :py:data:~ctypes.cdll, and on Windows
:py:data:~ctypes.windll and :py:data:~ctypes.oledll
objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects.
:py:data:!cdll loads libraries which export functions using the
standard cdecl calling convention, while :py:data:!windll
libraries call functions using the stdcall
calling convention.
:py:data:~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outdated version of the library that may be incompatible with the one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.
Other systems require 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 :py:class:CDLL
by calling the constructor.
For example, on Linux::
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 ...>
On macOS::
cdll.LoadLibrary("libc.dylib") # doctest: +MACOS <CDLL 'libc.dylib', handle ... at ...> libc = CDLL("libc.dylib") # doctest: +MACOS libc # doctest: +MACOS <CDLL 'libc.dylib', handle ... at ...>
.. _ctypes-accessing-functions-from-loaded-dlls:
Accessing functions from loaded dlls ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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
.. _ctypes-calling-functions:
Calling functions ^^^^^^^^^^^^^^^^^
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.
.. _ctypes-fundamental-data-types:
Fundamental data types ^^^^^^^^^^^^^^^^^^^^^^
:mod:!ctypes defines a number of primitive C compatible data types:
.. list-table:: :header-rows: 1
~_SimpleCData._type_c_bool_Boolbool'?'c_charcharbytes'c'c_wcharwchar_tstr'u'c_bytecharint'b'c_ubyteunsigned charint'B'c_shortshortint'h'c_ushortunsigned shortint'H'c_intintint'i' *c_int8int8_tintc_int16int16_tintc_int32int32_tintc_int64int64_tintc_uintunsigned intint'I' *c_uint8uint8_tintc_uint16uint16_tintc_uint32uint32_tintc_uint64uint64_tintc_longlongint'l'c_ulongunsigned longint'L'c_longlonglong longint'q' *c_ulonglongunsigned long longint'Q' *c_size_tsize_tintc_ssize_tPy_ssize_tintc_time_ttime_tintc_floatfloatfloat'f'c_doubledoublefloat'd'c_longdoublelong doublefloat'g' *c_char_pchar * (NUL terminated)bytes or None'z'c_wchar_pwchar_t * (NUL terminated)str or None'Z'c_void_pvoid *int or None'P'py_objectPyObject *object'O'VARIANT_BOOL <ctypes-wintypes>short intbool'v'Additionally, if IEC 60559 compatible complex arithmetic (Annex G) is supported
in both C and libffi, the following complex types are available:
.. list-table:: :header-rows: 1
~_SimpleCData._type_c_float_complexfloat complexcomplex'F'c_double_complexdouble complexcomplex'D'c_longdouble_complexlong double complexcomplex'G'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)
The constructors for numeric types will convert input using
:py:meth:~object.__bool__,
:py:meth:~object.__index__ (for int),
:py:meth:~object.__float__ or :py:meth:~object.__complex__.
This means :py:class:~ctypes.c_bool accepts any object with a truth value::
empty_list = [] c_bool(empty_list) c_bool(False)
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
string 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.
.. _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 <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
.. _ctypes-calling-variadic-functions:
Calling variadic functions ^^^^^^^^^^^^^^^^^^^^^^^^^^
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:
.. code-block:: python3
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.
.. _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. 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.
.. _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:~_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.
.. _ctypes-return-types:
Return types ^^^^^^^^^^^^
.. 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:
.. doctest::
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.
.. _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:~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.
See :class:CField::
POINT.x <ctypes.CField 'x' type=c_int, ofs=0, size=4> POINT.y <ctypes.CField 'y' type=c_int, 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 layout, alignment and byte order ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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 and/or for the
structure itself by setting the class attributes :attr:~Structure._pack_
and/or :attr:~Structure._align_, respectively.
See the attribute documentation for details.
: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:~Structure._fields_ tuples::
class Int(Structure): ... fields = [("first_16", c_int, 16), ... ("second_16", c_int, 16)] ... print(Int.first_16) <ctypes.CField 'first_16' type=c_int, ofs=0, bit_size=16, bit_offset=0> print(Int.second_16) <ctypes.CField 'second_16' type=c_int, ofs=0, bit_size=16, bit_offset=16>
It is important to note that bit field allocation and layout in memory are not
defined as a C standard; their implementation is compiler-specific.
By default, Python will attempt to match the behavior of a "native" compiler
for the current platform.
See the :attr:~Structure._layout_ attribute for details on the default
behavior and how to change it.
.. _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 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
.. _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:~_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)
.. 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 <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
.. _ctypes-thread-safety:
Thread safety without the GIL ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
From Python 3.13 onward, the :term:GIL can be disabled on the :term:free-threaded build.
In ctypes, reads and writes to a single object concurrently is safe, but not across multiple objects:
.. code-block:: pycon
>>> number = c_int(42)
>>> pointer_a = pointer(number)
>>> pointer_b = pointer(number)
In the above, it's only safe for one object to read and write to the address at once if the GIL is disabled.
So, pointer_a can be shared and written to across multiple threads, but only if pointer_b
is not also attempting to do the same. If this is an issue, consider using a :class:threading.Lock
to synchronize access to memory:
.. code-block:: pycon
>>> import threading
>>> lock = threading.Lock()
>>> # Thread 1
>>> with lock:
... pointer_a.contents = 24
>>> # Thread 2
>>> with lock:
... pointer_b.contents = 42
.. _ctypes-type-conversions:
Type conversions ^^^^^^^^^^^^^^^^
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
.. 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 <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
.. _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 <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
.. _ctypes-callback-functions:
Callback functions ^^^^^^^^^^^^^^^^^^
: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.
.. _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_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.
.. _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 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!
.. _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-loading-shared-libraries:
Loading shared libraries ^^^^^^^^^^^^^^^^^^^^^^^^
There are several ways to load shared libraries into the Python process. One
way is to instantiate :py:class:CDLL or one of its subclasses:
.. class:: CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=None)
Represents a loaded shared library.
Functions in this library use the standard C calling convention, and are
assumed to return :c:expr:int.
The Python :term:global interpreter lock is released before calling any
function exported by these libraries, and reacquired afterwards.
For different function behavior, use a subclass: :py:class:~ctypes.OleDLL,
:py:class:~ctypes.WinDLL, or :py:class:~ctypes.PyDLL.
If you have an existing :py:attr:handle <ctypes.CDLL._handle> to an already
loaded shared library, it can be passed as the handle argument to wrap
the opened library in a new :py:class:!CDLL object.
In this case, name is only used to set the :py:attr:~ctypes.CDLL._name
attribute, but it may be adjusted and/or validated.
If handle is None, the underlying platform's :manpage:dlopen(3) or
LoadLibraryExW_ function is used to load the library into
the process, and to get a handle to it.
name is the pathname of the shared library to open. If name does not contain a path separator, the library is found in a platform-specific way.
On Windows, the .DLL suffix may be missing. (For details, see
LoadLibraryExW_ documentation.)
Other platform-specific prefixes and suffixes (for example, lib,
.so, .dylib, or version numbers) must be present in name;
they are not added automatically.
See :ref:ctypes-finding-shared-libraries for more information.
On non-Windows systems, name can be None. In this case,
:c:func:!dlopen is called with NULL, which opens the main program
as a "library".
(Some systems do the same is name is empty; None/NULL is more
portable.)
.. admonition:: CPython implementation detail
Since CPython is linked to ``libc``, a ``None`` *name* is often used
to access the C standard library::
>>> printf = ctypes.CDLL(None).printf
>>> printf.argtypes = [ctypes.c_char_p]
>>> printf(b"hello\n")
hello
6
To access the Python C API, prefer :py:data:`ctypes.pythonapi` which
works across platforms.
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
LoadLibraryExW_ 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.
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.
.. seealso::
`Microsoft DUMPBIN tool <https://learn.microsoft.com/en-us/cpp/build/reference/dumpbin-reference?view=msvc-170>`_
-- A tool to find DLL dependents.
.. versionchanged:: 3.8 Added winmode parameter.
.. versionchanged:: 3.12
The *name* parameter can now be a :term:`path-like object`.
Instances of this class 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:: _handle
The system handle used to access the library.
.. attribute:: _name
The name of the library passed in the constructor.
.. _LoadLibraryExW: https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-loadlibraryexw
.. class:: OleDLL
See :py:class:~ctypes.CDLL, the superclass, for common information.
Functions in this library 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.
.. availability:: Windows
.. versionchanged:: 3.3
:exc:WindowsError used to be raised,
which is now an alias of :exc:OSError.
.. class:: WinDLL
See :py:class:~ctypes.CDLL, the superclass, for common information.
Functions in these libraries use the stdcall calling convention, and are
assumed to return :c:expr:int by default.
.. availability:: Windows
.. class:: PyDLL
See :py:class:~ctypes.CDLL, the superclass, for common information.
When functions in this library are called, 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.
.. data:: RTLD_GLOBAL
Flag to use as mode parameter. On platforms where this flag is not available, it is defined as the integer zero.
.. data:: RTLD_LOCAL
Flag to use as mode parameter. On platforms where this is not available, it is the same as RTLD_GLOBAL.
.. data:: DEFAULT_MODE
The default mode which is used to load shared libraries. On OSX 10.3, this is RTLD_GLOBAL, otherwise it is the same as RTLD_LOCAL.
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:: 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 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
Creates :class:`CDLL` instances.
.. data:: windll
Creates :class:`WinDLL` instances.
.. availability:: Windows
.. data:: oledll
Creates :class:`OleDLL` instances.
.. availability:: Windows
.. data:: pydll
Creates :class:`PyDLL` instances.
.. data:: pythonapi
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.
.. _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 programmer specifies a short name; the C compiler, linker, and runtime dynamic library loader then interact in system-specific ways to find the filename of the library to load.
While the mapping from short names to filenames is not consistently exposed
by platforms, the :mod:!ctypes.util module provides a function,
:func:!find_library, that attempts to match it.
However, as backwards compatibility concerns make it difficult to adjust
its behavior for new platforms and configurations, the function
is :term:soft deprecated.
If wrapping a shared library with :mod:!ctypes, consider determining the
shared library name at development time, and hardcoding it into the wrapper
module instead of using :func:!find_library to locate the library
at runtime.
Also consider addding a configuration option or environment variable to let
users select a library to use, and then perhaps use :func:!find_library
as a default or fallback.
.. function:: find_library(name) :module: ctypes.util
Try to find a library and return a pathname.
name is the "short" 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.)
The result is in a format suitable for passing to :py:class:~ctypes.CDLL.
If no library can be found, return None.
The exact functionality is system dependent, and is not guaranteed to match the behavior of the compiler, linker, and loader used for (or by) Python. It is recommended to only use this function as a default or fallback,
.. deprecated:: 3.15
This function is :term:`soft deprecated`.
It is kept for use in cases where it works, but not expected to be
updated for additional platforms and configurations.
On Linux, :func:!find_library tries to run external
programs (/sbin/ldconfig, gcc, objdump and ld) to find the
library file.
If the output of these programs does not correspond to the dynamic
linker used by Python, the result of this function may be misleading.
.. 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:!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:!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.
.. function:: find_msvcrt() :module: ctypes.util
Returns 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.
Notably, this will happen for recent versions of the VC runtime library,
which are not directly loadable.
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.
.. availability:: Windows
.. _ctypes-listing-loaded-shared-libraries:
Listing loaded shared libraries ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
When writing code that relies on code loaded from shared libraries, it can be useful to know which shared libraries have already been loaded into the current process.
The :mod:!ctypes.util module provides the :func:~ctypes.util.dllist function,
which calls the different APIs provided by the various platforms to help determine
which shared libraries have already been loaded into the current process.
The exact output of this function will be system dependent. On most platforms, the first entry of this list represents the current process itself, which may be an empty string. For example, on glibc-based Linux, the return may look like::
from ctypes.util import dllist dllist() ['', 'linux-vdso.so.1', '/lib/x86_64-linux-gnu/libm.so.6', '/lib/x86_64-linux-gnu/libc.so.6', ... ]
.. _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 local class :class:!_FuncPtr (not exposed
in :mod:!ctypes) which inherits from the private :class:_CFuncPtr class:
.. doctest::
import ctypes lib = ctypes.CDLL(None) issubclass(lib._FuncPtr, ctypes._CFuncPtr) True lib._FuncPtr is ctypes._CFuncPtr False
.. class:: _CFuncPtr
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: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.
.. 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 as well as some of the
functions in this module may raise an auditing event
ctypes.call_function with arguments function pointer and 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, 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)
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.
.. availability:: Windows
.. 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.
If iid is not specified, an :exc:OSError is raised if the COM method
call fails. If iid is specified, a :exc:~ctypes.COMError is raised
instead.
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.
.. availability:: Windows
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
.. _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.
.. 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:: CopyComPointer(src, dst)
Copies a COM pointer from src to dst and returns the Windows specific
:c:type:!HRESULT value.
If src is not NULL, its AddRef method is called, incrementing the
reference count.
In contrast, the reference count of dst will not be decremented before
assigning the new value. Unless dst is NULL, the caller is responsible
for decrementing the reference count by calling its Release method when
necessary.
.. availability:: Windows
.. versionadded:: 3.14
.. 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, size=None) create_string_buffer(size)
This function creates a mutable character buffer. The returned object is a
ctypes array of :class:c_char.
If size is given (and not None), it must be an :class:int.
It specifies the size of the returned array.
If the init argument is given, it must be :class:bytes. It is used
to initialize the array items. Bytes not initialized this way are
set to zero (NUL).
If size is not given (or if it is None), the buffer is made one element
larger than init, effectively adding a NUL terminator.
If both arguments are given, size must not be less than len(init).
.. warning::
If *size* is equal to ``len(init)``, a NUL terminator is
not added. Do not treat such a buffer as a C string.
For example::
>>> bytes(create_string_buffer(2))
b'\x00\x00'
>>> bytes(create_string_buffer(b'ab'))
b'ab\x00'
>>> bytes(create_string_buffer(b'ab', 2))
b'ab'
>>> bytes(create_string_buffer(b'ab', 4))
b'ab\x00\x00'
>>> bytes(create_string_buffer(b'abcdef', 2))
Traceback (most recent call last):
...
ValueError: byte string too long
.. audit-event:: ctypes.create_string_buffer init,size ctypes.create_string_buffer
.. function:: create_unicode_buffer(init, size=None) create_unicode_buffer(size)
This function creates a mutable unicode character buffer. The returned object is
a ctypes array of :class:c_wchar.
The function takes the same arguments as :func:~create_string_buffer except
init must be a string and size counts :class:c_wchar.
.. audit-event:: ctypes.create_unicode_buffer init,size ctypes.create_unicode_buffer
.. function:: DllCanUnloadNow()
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.
.. availability:: Windows
.. function:: DllGetClassObject()
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.
.. availability:: Windows
.. function:: dllist() :module: ctypes.util
Try to provide a list of paths of the shared libraries loaded into the current
process. These paths are not normalized or processed in any way. The function
can raise :exc:OSError if the underlying platform APIs fail.
The exact functionality is system dependent.
On most platforms, the first element of the list represents the current executable file. It may be an empty string.
.. availability:: Windows, macOS, iOS, glibc, BSD libc, musl .. versionadded:: 3.14
.. function:: FormatError([code])
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
:func:GetLastError.
.. availability:: Windows
.. function:: GetLastError()
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.
.. availability:: Windows
.. 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()
Returns the current value of the ctypes-private copy of the system
:data:!LastError variable in the calling thread.
.. availability:: Windows
.. 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 or return a ctypes pointer type. Pointer types are cached and reused internally, so calling this function repeatedly is cheap. type must be a ctypes type.
.. impl-detail::
The resulting pointer type is cached in the ``__pointer_type__``
attribute of *type*.
It is possible to set this attribute before the first call to
``POINTER`` in order to set a custom pointer type.
However, doing this is discouraged: manually creating a suitable
pointer type is difficult without relying on implementation
details that may change in future Python versions.
.. 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)
Sets 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.
.. availability:: Windows
.. 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)
Creates an instance of :exc:OSError. If code is not specified,
:func: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.
.. availability:: Windows
.. 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
.. function:: memoryview_at(ptr, size, readonly=False)
Return a :class:memoryview object of length size that references memory
starting at void *ptr.
If readonly is true, the returned :class:!memoryview object can
not be used to modify the underlying memory.
(Changes made by other means will still be reflected in the returned
object.)
This function is similar to :func:string_at with the key
difference of not making a copy of the specified memory.
It is a semantically equivalent (but more efficient) alternative to
memoryview((c_byte * size).from_address(ptr)).
(While :meth:~_CData.from_address only takes integers, ptr can also
be given as a :class:ctypes.POINTER or a :func:~ctypes.byref object.)
.. audit-event:: ctypes.memoryview_at address,size,readonly
.. versionadded:: 3.14
.. _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.
.. 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 class variables of ctypes data types:
.. attribute:: pointer_type
The pointer type that was created by calling
:func:`POINTER` for corresponding ctypes data type. If a pointer type
was not yet created, the attribute is missing.
.. versionadded:: 3.14
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.
Each subclass has a class attribute:
.. attribute:: type
Class attribute that contains an internal type code, as a
single-character string.
See :ref:`ctypes-fundamental-data-types` for a summary.
Types marked \* in the summary may be (or always are) aliases of a
different :class:`_SimpleCData` subclass, and will not necessarily
use the listed type code.
For example, if the platform's :c:expr:`long`, :c:expr:`long long`
and :c:expr:`time_t` C types are the same, then :class:`c_long`,
:class:`c_longlong` and :class:`c_time_t` all refer to a single class,
:class:`c_long`, whose :attr:`_type_` code is ``'l'``.
The ``'L'`` code will be unused.
.. seealso::
The :mod:`array` and :ref:`struct <format-characters>` modules,
as well as third-party modules like `numpy <https://numpy.org/doc/stable/reference/arrays.interface.html#object.__array_interface__>`__,
use similar -- but slightly different -- type codes.
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.
.. 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:expr: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: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.
.. class:: c_char_p
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.
.. class:: c_double
Represents the C :c:expr:double datatype. The constructor accepts an
optional float initializer.
.. class:: c_longdouble
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.
.. class:: c_float
Represents the C :c:expr:float datatype. The constructor accepts an
optional float initializer.
.. class:: c_double_complex
Represents the C :c:expr:double complex datatype, if available. The
constructor accepts an optional :class:complex initializer.
.. versionadded:: 3.14
.. class:: c_float_complex
Represents the C :c:expr:float complex datatype, if available. The
constructor accepts an optional :class:complex initializer.
.. versionadded:: 3.14
.. class:: c_longdouble_complex
Represents the C :c:expr:long double complex datatype, if available. The
constructor accepts an optional :class:complex initializer.
.. versionadded:: 3.14
.. class:: c_int
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.
.. class:: c_int8
Represents the C 8-bit :c:expr:signed int datatype. It is an alias for
:class:c_byte.
.. class:: c_int16
Represents the C 16-bit :c:expr:signed int datatype. Usually an alias for
:class:c_short.
.. class:: c_int32
Represents the C 32-bit :c:expr:signed int datatype. Usually an alias for
:class:c_int.
.. class:: c_int64
Represents the C 64-bit :c:expr:signed int datatype. Usually an alias for
:class:c_longlong.
.. class:: c_long
Represents the C :c:expr:signed long datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
.. class:: c_longlong
Represents the C :c:expr:signed long long datatype. The constructor accepts
an optional integer initializer; no overflow checking is done.
On platforms where sizeof(long long) == sizeof(long) it is an alias
to :class:c_long.
.. class:: c_short
Represents the C :c:expr: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.
Usually an alias for another unsigned integer type.
.. class:: c_ssize_t
Represents the :c:type:Py_ssize_t datatype.
This is a signed version of :c:type:size_t;
that is, the POSIX :c:type:ssize_t type.
Usually an alias for another integer type.
.. versionadded:: 3.2
.. class:: c_time_t
Represents the C :c:type:time_t datatype.
Usually an alias for another integer type.
.. versionadded:: 3.12
.. class:: c_ubyte
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.
.. class:: c_uint
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.
.. class:: c_uint8
Represents the C 8-bit :c:expr:unsigned int datatype. It is an alias for
:class:c_ubyte.
.. class:: c_uint16
Represents the C 16-bit :c:expr:unsigned int datatype. Usually an alias for
:class:c_ushort.
.. class:: c_uint32
Represents the C 32-bit :c:expr:unsigned int datatype. Usually an alias for
:class:c_uint.
.. class:: c_uint64
Represents the C 64-bit :c:expr:unsigned int datatype. Usually an alias for
:class:c_ulonglong.
.. class:: c_ulong
Represents the C :c:expr:unsigned long datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
.. class:: c_ulonglong
Represents the C :c:expr:unsigned long long datatype. The constructor
accepts an optional integer initializer; no overflow checking is done.
On platforms where sizeof(long long) == sizeof(long) it is an alias
to :class:c_long.
.. class:: c_ushort
Represents the C :c:expr:unsigned short datatype. The constructor accepts
an optional integer initializer; no overflow checking is done.
.. class:: c_void_p
Represents the C :c:expr: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: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.
.. class:: c_bool
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.
.. class:: HRESULT
Represents a :c:type:!HRESULT value, which contains success or
error information for a function or method call.
.. availability:: Windows
.. class:: py_object
Represents the C :c:expr:PyObject * datatype. Calling this without an
argument creates a NULL :c:expr:PyObject * pointer.
.. versionchanged:: 3.14
:class:!py_object is now a :term:generic type.
.. _ctypes-wintypes:
The :mod:!ctypes.wintypes module provides quite some other Windows specific
data types, for example :c:type:!HWND, :c:type:!WPARAM,
:c:type:!VARIANT_BOOL 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.
Unions share common attributes and behavior with structures;
see :class:Structure documentation for details.
.. class:: BigEndianUnion(*args, **kw)
Abstract base class for unions in big endian byte order.
.. versionadded:: 3.11
.. class:: LittleEndianUnion(*args, **kw)
Abstract base class for unions in little endian byte order.
.. versionadded:: 3.11
.. 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 and unions 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 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.
This is only implemented for the MSVC-compatible memory layout
(see :attr:`_layout_`).
Setting :attr:`!_pack_` to 0 is the same as not setting it at all.
Otherwise, the value must be a positive power of two.
The effect is equivalent to ``#pragma pack(N)`` in C, except
:mod:`!ctypes` may allow larger *n* than what the compiler accepts.
:attr:`!_pack_` must already be defined
when :attr:`_fields_` is assigned, otherwise it will have no effect.
.. deprecated-removed:: 3.14 3.19
For historical reasons, if :attr:`!_pack_` is non-zero,
the MSVC-compatible layout will be used by default.
On non-Windows platforms, this default is deprecated and is slated to
become an error in Python 3.19.
If it is intended, set :attr:`~Structure._layout_` to ``'ms'``
explicitly.
.. attribute:: align
An optional small integer that allows increasing the alignment of
the structure when being packed or unpacked to/from memory.
The value must not be negative.
The effect is equivalent to ``__attribute__((aligned(N)))`` on GCC
or ``#pragma align(N)`` on MSVC, except :mod:`!ctypes` may allow
values that the compiler would reject.
:attr:`!_align_` can only *increase* a structure's alignment
requirements. Setting it to 0 or 1 has no effect.
Using values that are not powers of two is discouraged and may lead to
surprising behavior.
:attr:`!_align_` must already be defined
when :attr:`_fields_` is assigned, otherwise it will have no effect.
.. 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"``.
(This is deprecated; see :attr:`~Structure._pack_` documentation.)
- Otherwise: ``"gcc-sysv"``
:attr:`!_layout_` must already be defined when
:attr:`~Structure._fields_` is assigned, otherwise it will have no effect.
.. versionadded:: 3.14
.. 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_.
.. class:: CField(*args, **kw)
Descriptor for fields of a :class:Structure and :class:Union.
For example::
>>> class Color(Structure):
... _fields_ = (
... ('red', c_uint8),
... ('green', c_uint8),
... ('blue', c_uint8),
... ('intense', c_bool, 1),
... ('blinking', c_bool, 1),
... )
...
>>> Color.red
<ctypes.CField 'red' type=c_ubyte, ofs=0, size=1>
>>> Color.green.type
<class 'ctypes.c_ubyte'>
>>> Color.blue.byte_offset
2
>>> Color.intense
<ctypes.CField 'intense' type=c_bool, ofs=3, bit_size=1, bit_offset=0>
>>> Color.blinking.bit_offset
1
All attributes are read-only.
:class:!CField objects are created via :attr:~Structure._fields_;
do not instantiate the class directly.
.. versionadded:: 3.14
Previously, descriptors only had ``offset`` and ``size`` attributes
and a readable string representation; the :class:`!CField` class was not
available directly.
.. attribute:: name
Name of the field, as a string.
.. attribute:: type
Type of the field, as a :ref:`ctypes class <ctypes-data-types>`.
.. attribute:: offset byte_offset
Offset of the field, in bytes.
For bitfields, this is the offset of the underlying byte-aligned
*storage unit*; see :attr:`~CField.bit_offset`.
.. attribute:: byte_size
Size of the field, in bytes.
For bitfields, this is the size of the underlying *storage unit*.
Typically, it has the same size as the bitfield's type.
.. attribute:: size
For non-bitfields, equivalent to :attr:`~CField.byte_size`.
For bitfields, this contains a backwards-compatible bit-packed
value that combines :attr:`~CField.bit_size` and
:attr:`~CField.bit_offset`.
Prefer using the explicit attributes instead.
.. attribute:: is_bitfield
True if this is a bitfield.
.. attribute:: bit_offset bit_size
The location of a bitfield within its *storage unit*, that is, within
:attr:`~CField.byte_size` bytes of memory starting at
:attr:`~CField.byte_offset`.
To get the field's value, read the storage unit as an integer,
:ref:`shift left <shifting>` by :attr:`!bit_offset` and
take the :attr:`!bit_size` least significant bits.
For non-bitfields, :attr:`!bit_offset` is zero
and :attr:`!bit_size` is equal to ``byte_size * 8``.
.. attribute:: is_anonymous
True if this field is anonymous, that is, it contains nested sub-fields
that should be merged into a containing structure or union.
.. _ctypes-arrays-pointers:
Arrays and pointers ^^^^^^^^^^^^^^^^^^^
.. class:: Array(*args)
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.
.. class:: _Pointer
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.
.. _ctypes-exceptions:
Exceptions ^^^^^^^^^^
.. exception:: ArgumentError
This exception is raised when a foreign function call cannot convert one of the passed arguments.
.. exception:: COMError(hresult, text, details)
This exception is raised when a COM method call failed.
.. attribute:: hresult
The integer value representing the error code.
.. attribute:: text
The error message.
.. attribute:: details
The 5-tuple ``(descr, source, helpfile, helpcontext, progid)``.
*descr* is the textual description. *source* is the language-dependent
``ProgID`` for the class or application that raised the error. *helpfile*
is the path of the help file. *helpcontext* is the help context
identifier. *progid* is the ``ProgID`` of the interface that defined the
error.
.. availability:: Windows
.. versionadded:: 3.14