Source code:
———
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.
This is an
. If it is missing from your copy of CPython, look for documentation from your distributor (that is, whoever provided Python to you). If you are the distributor, see
Requirements for optional modules
.
Warning
ctypes provides low-level access to native libraries and the process’s memory, bypassing Python’s safety mechanisms and allowing execution of arbitrary native code. Incorrect use can corrupt data and objects, reveal sensitive information, cause crashes, or otherwise compromise the running process.
ctypes tutorial
Note: Some code samples reference the ctypes
type. On platforms where sizeof(long)==sizeof(int) it is an alias to
. So, you should not be confused if c_long is printed if you would expect c_int — they are actually the same type.
Loading dynamic link libraries
ctypes exports the
, and on Windows
and
objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects. cdll loads libraries which export functions using the standard cdecl calling convention, while windll libraries call functions using the stdcall calling convention.
also uses the stdcall calling convention, and assumes the functions return a Windows HRESULT error code. The error code is used to automatically raise an
exception when the function call fails.
Changed in version 3.3: Windows errors used to raise
, which is now an alias of
.
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:
>>> fromctypesimport*>>> print(windll.kernel32)<WinDLL 'kernel32', handle ... at ...>>>> print(cdll.msvcrt)<CDLL 'msvcrt', handle ... at ...>>>> libc=cdll.msvcrt>>>Windows appends the usual .dll file suffix automatically.
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Accessing the standard C library through cdll.msvcrt will use an outdated version of the library that may be incompatible with the one being used by Python.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
method of the dll loaders should be used, or you should load the library by creating an instance of
by calling the constructor.
For example, on Linux:
>>> cdll.LoadLibrary("libc.so.6")<CDLL 'libc.so.6', handle ... at ...>>>> libc=CDLL("libc.so.6")>>> libc<CDLL 'libc.so.6', handle ... at ...>>>>On macOS:
>>> cdll.LoadLibrary("libc.dylib")<CDLL 'libc.dylib', handle ... at ...>>>> libc=CDLL("libc.dylib")>>> libc<CDLL 'libc.dylib', handle ... at ...>Accessing functions from loaded dlls
Functions are accessed as attributes of dll objects:
>>> libc.printf<_FuncPtr object at 0x...>>>> print(windll.kernel32.GetModuleHandleA)<_FuncPtr object at 0x...>>>> print(windll.kernel32.MyOwnFunction)Traceback (most recent call last): File "<stdin>", line 1, in <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:
/*ANSIversion*/HMODULEGetModuleHandleA(LPCSTRlpModuleName);/*UNICODEversion*/HMODULEGetModuleHandleW(LPCWSTRlpModuleName);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
to retrieve the function:
>>> getattr(cdll.msvcrt,"??2@YAPAXI@Z")<_FuncPtr object at 0x...>>>>On Windows, some dlls export functions not by name but by ordinal. These functions can be accessed by indexing the dll object with the ordinal number:
>>> cdll.kernel32[1]<_FuncPtr object at 0x...>>>> cdll.kernel32[0]Traceback (most recent call last): File "<stdin>", line 1, in <module> File "ctypes.py", line 310, in __getitem__func=_StdcallFuncPtr(name,self)AttributeError: function ordinal 0 not found>>>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())1804289383On 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)))0x1d000000>>>
is raised when you call an stdcall function with the cdecl calling convention, or vice versa:
>>> cdll.kernel32.GetModuleHandleA(None)Traceback (most recent call last): File "<stdin>", line 1, in <module>ValueError: Procedure probably called with not enough arguments (4 bytes missing)>>>>>> windll.msvcrt.printf(b"spam")Traceback (most recent call last): File "<stdin>", line 1, in <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, ctypes uses win32 structured exception handling to prevent crashes from general protection faults when functions are called with invalid argument values:
>>> windll.kernel32.GetModuleHandleA(32)Traceback (most recent call last): File "<stdin>", line 1, in <module>OSError: exception: access violation reading 0x00000020>>>The
module can help debug crashes, such as segmentation faults produced by erroneous C library calls.
None, integers, bytes objects and (unicode) strings are the only native Python objects that can directly be used as parameters in these function calls. None is passed as a C NULL pointer, bytes objects and strings are passed as pointer to the memory block that contains their data (char* or wchar_t*). Python integers are passed as the platform’s default C int type, their value is masked to fit into the C type.
Before we move on calling functions with other parameter types, we have to learn more about ctypes data types.
Fundamental data types
ctypes defines a number of primitive C compatible data types:
ctypes type
C type
Python type
_Bool
'?'
char
1-character
'c'
wchar_t
1-character
'u'
char
'b'
unsignedchar
'B'
short
'h'
unsignedshort
'H'
int
'i' *
int8_t
*
int16_t
*
int32_t
*
int64_t
*
unsignedint
'I' *
uint8_t
*
uint16_t
*
uint32_t
*
uint64_t
*
long
'l'
unsignedlong
'L'
longlong
'q' *
unsignedlonglong
'Q' *
size_t
*
*
time_t
*
float
'f'
double
'd'
longdouble
'g' *
char* (NUL terminated)
or None
'z'
wchar_t* (NUL terminated)
or None
'Z'
void*
or None
'P'
*
'O'
shortint
'v'
Additionally, if IEC 60559 compatible complex arithmetic (Annex G) is supported in both C and libffi, the following complex types are available:
ctypes type
C type
Python type
floatcomplex
'Zf'
doublecomplex
'Zd'
longdoublecomplex
'Zg'
Changed in version 3.15: The
types F, D and G have been replaced with Zf, Zd and Zg.
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
,
(for int),
or
. This means
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
,
, and
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 changedc_wchar_p(139966783348904)>>> print(c_s.value)Hi, there>>> print(s)# first object is unchangedHello, 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
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:
>>> fromctypesimport*>>> 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
function replaces the old c_buffer() function (which is still available as an alias). To create a mutable memory block containing unicode characters of the C type wchar_t, use the
function.
Calling functions, continued
Note that printf prints to the real standard output channel, not to
, 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 beer19>>> 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 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 3.14000031>>>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
attribute for the regular, non-variadic, function arguments:
libc.printf.argtypes=[ctypes.c_char_p]Because specifying the attribute does not inhibit portability it is advised to always specify
for all variadic functions.
Calling functions with your own custom data types
You can also customize ctypes argument conversion to allow instances of your own classes be used as function arguments. ctypes looks for an _as_parameter_ attribute and uses this as the function argument. The attribute must be an integer, string, bytes, a ctypes instance, or an object with an _as_parameter_ attribute:
>>> classBottles:... def__init__(self,number):... self._as_parameter_=number...>>> bottles=Bottles(42)>>> printf(b"%d bottles of beer\n",bottles)42 bottles of beer19>>>If you don’t want to store the instance’s data in the _as_parameter_ instance variable, you could define a
which makes the attribute available on request.
Specifying the required argument types (function prototypes)
It is possible to specify the required argument types of functions exported from DLLs by setting the
attribute.
must be a sequence of C data types (the printf() function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):
>>> printf.argtypes=[c_char_p,c_char_p,c_int,c_double]>>> printf(b"String '%s', Int %d, Double %f\n",b"Hi",10,2.2)String 'Hi', Int 10, Double 2.20000037>>>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 3.00000013>>>If you have defined your own classes which you pass to function calls, you have to implement a
class method for them to be able to use them in the
sequence. The from_param() class method receives the Python object passed to the function call, it should do a typecheck or whatever is needed to make sure this object is acceptable, and then return the object itself, its _as_parameter_ attribute, or whatever you want to pass as the C function argument in this case. Again, the result should be an integer, string, bytes, a ctypes instance, or an object with an _as_parameter_ attribute.
Return types
By default functions are assumed to return the C int type. Other return types can be specified by setting the
attribute of the function object.
The C prototype of time() is time_ttime(time_t*). Because time_t might be of a different type than the default return type int, you should specify the restype attribute:
>>> libc.time.restype=c_time_tThe argument types can be specified using
:
>>> libc.time.argtypes=(POINTER(c_time_t),)To call the function with a NULL pointer as first argument, use None:
>>> print(libc.time(None))1150640792Here is a more advanced example, it uses the strchr() function, which expects a string pointer and a char, and returns a pointer to a string:
>>> strchr=libc.strchr>>> strchr(b"abcdef",ord("d"))8059983>>> strchr.restype=c_char_p# c_char_p is a pointer to a string>>> strchr(b"abcdef",ord("d"))b'def'>>> print(strchr(b"abcdef",ord("x")))None>>>If you want to avoid the
calls above, you can set the
attribute, and the second argument will be converted from a single character Python bytes object into a C char:
>>> strchr.restype=c_char_p>>> strchr.argtypes=[c_char_p,c_char]>>> strchr(b"abcdef",b"d")b'def'>>> strchr(b"abcdef",b"def")Traceback (most recent call last):ctypes.ArgumentError: argument 2: TypeError: one character bytes, bytearray or integer expected>>> print(strchr(b"abcdef",b"x"))None>>> strchr(b"abcdef",b"d")b'def'>>>You can also use a callable Python object (a function or a class for example) as the
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>>> defValidHandle(value):... ifvalue==0:... raiseWinError()... returnvalue...>>>>>> GetModuleHandle.restype=ValidHandle>>> GetModuleHandle(None)486539264>>> GetModuleHandle("something silly")Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in ValidHandleOSError: [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
to retrieve it.
Please note that a much more powerful error checking mechanism is available through the
attribute; see the reference manual for details.
Specifying function pointers using type annotations
@ctypes.util.wrap_dll_function(dll)
A
that generates
and
from a function signature, using the
of the function and its
.
The decorated function should look like this:
@wrap_dll_function(dll_to_wrap)deffunction_ptr_name(arg_name:ctypes_type,...)->ctypes_type:"""Optional docstring. There should be no function body."""The body of the decorated function is ignored, and any parameters that are missing type annotations are skipped. The names of the parameters are ignored and do not have to match the underlying C implementation.
If the decorated function does not have a return type annotation, a
is raised. A ValueError is also raised if it has a keyword-only, *args, or **kwargs parameter, since
describes positional arguments only. If the name of the function does not exist in dll, an
is raised.
For example:
importctypesfromctypes.utilimportwrap_dll_function@wrap_dll_function(ctypes.pythonapi)defPyObject_GetAttrString(op:ctypes.py_object,attr:ctypes.c_char_p)->ctypes.py_object:passPyObject_GetAttrString(42,b"real")Added in version 3.16.0a0 (unreleased).
Passing pointers (or: passing parameters by reference)
Sometimes a C api function expects a pointer to a data type as parameter, probably to write into the corresponding location, or if the data is too large to be passed by value. This is also known as passing parameters by reference.
ctypes exports the
function which is used to pass parameters by reference. The same effect can be achieved with the
function, although pointer() does a lot more work since it constructs a real pointer object, so it is faster to use byref() if you don’t need the pointer object in Python itself:
>>> i=c_int()>>> f=c_float()>>> s=create_string_buffer(b'\000'*32)>>> print(i.value,f.value,repr(s.value))0 0.0 b''>>> libc.sscanf(b"1 3.14 Hello",b"%d%f%s",... byref(i),byref(f),s)3>>> print(i.value,f.value,repr(s.value))1 3.1400001049 b'Hello'>>>Structures and unions
Structures and unions must derive from the
and
base classes which are defined in the ctypes module. Each subclass must define a
attribute. _fields_ must be a list of 2-tuples, containing a field name and a field type.
The field type must be a ctypes type like
, or any other derived ctypes type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:
>>> fromctypesimport*>>> classPOINT(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:
>>> classRECT(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
s can be retrieved from the class, they are useful for debugging because they can provide useful information. See
:
>>> POINT.x<ctypes.CField 'x' type=c_int, ofs=0, size=4>>>> POINT.y<ctypes.CField 'y' type=c_int, ofs=4, size=4>>>>Warning
ctypes does not support passing unions or structures with bit-fields to functions by value. While this may work on 32-bit x86, it’s not guaranteed by the library to work in the general case. Unions and structures with bit-fields should always be passed to functions by pointer.
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
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
and/or
, respectively. See the attribute documentation for details.
ctypes uses the native byte order for Structures and Unions. To build structures with non-native byte order, you can use one of the
,
,
, and
base classes. These classes cannot contain pointer fields.
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
tuples:
>>> classInt(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
attribute for details on the default behavior and how to change it.
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*10Here is an example of a somewhat artificial data type, a structure containing 4 POINTs among other stuff:
>>> fromctypesimport*>>> classPOINT(Structure):... _fields_=("x",c_int),("y",c_int)...>>> classMyStruct(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()forptinarr:print(pt.x,pt.y)The above code print a series of 00 lines, because the array contents is initialized to zeros.
Initializers of the correct type can also be specified:
>>> fromctypesimport*>>> 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...>>>> foriinii:print(i,end=" ")...1 2 3 4 5 6 7 8 9 10>>>Pointers
Pointer instances are created by calling the
function on a ctypes type:
>>> fromctypesimport*>>> i=c_int(42)>>> pi=pointer(i)>>>Pointer instances have a
attribute which returns the object to which the pointer points, the i object above:
>>> pi.contentsc_long(42)>>>Note that ctypes does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:
>>> pi.contentsisiFalse>>> pi.contentsispi.contentsFalse>>>Assigning another
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.contentsc_long(99)>>>Pointer instances can also be indexed with integers:
>>> pi[0]99>>>Assigning to an integer index changes the pointed to value:
>>> print(i)c_long(99)>>> pi[0]=22>>> print(i)c_long(22)>>>It is also possible to use indexes different from 0, but you must know what you’re doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.
Behind the scenes, the
function does more than simply create pointer instances, it has to create pointer types first. This is done with the
function, which accepts any ctypes type, and returns a new type:
>>> PI=POINTER(c_int)>>> PI<class 'ctypes.LP_c_long'>>>> PI(42)Traceback (most recent call last): File "<stdin>", line 1, in <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>>>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]=1234Traceback (most recent call last):....ValueError: NULL pointer access>>>Thread safety without the GIL
From Python 3.13 onward, the
can be disabled on the
. In ctypes, reads and writes to a single object concurrently is safe, but not across multiple objects:
>>> 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
to synchronize access to memory:
>>> importthreading>>> lock=threading.Lock()>>> # Thread 1>>> withlock:... pointer_a.contents=24>>> # Thread 2>>> withlock:... pointer_b.contents=42
Type conversions
Usually, ctypes does strict type checking. This means, if you have POINTER(c_int) in the
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:
>>> classBar(Structure):... _fields_=[("count",c_int),("values",POINTER(c_int))]...>>> bar=Bar()>>> bar.values=(c_int*3)(1,2,3)>>> bar.count=3>>> foriinrange(bar.count):... print(bar.values[i])...123>>>In addition, if a function argument is explicitly declared to be a pointer type (such as POINTER(c_int)) in
, an object of the pointed type (c_int in this case) can be passed to the function. ctypes will apply the required
conversion in this case automatically.
To set a POINTER type field to NULL, you can assign None:
>>> bar.values=None>>>Sometimes you have instances of incompatible types. In C, you can cast one type into another type. ctypes provides a
function which can be used in the same way. The Bar structure defined above accepts POINTER(c_int) pointers or
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
function is handy.
The
function can be used to cast a ctypes instance into a pointer to a different ctypes data type. cast() takes two parameters, a ctypes object that is or can be converted to a pointer of some kind, and a ctypes pointer type. It returns an instance of the second argument, which references the same memory block as the first argument:
>>> a=(c_byte*4)()>>> cast(a,POINTER(c_int))<ctypes.LP_c_long object at ...>>>>So,
can be used to assign to the values field of Bar the structure:
>>> bar=Bar()>>> bar.values=cast((c_byte*4)(),POINTER(c_int))>>> print(bar.values[0])0>>>Incomplete Types
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:
structcell;/*forwarddeclaration*/structcell{char*name;structcell*next;};The straightforward translation into ctypes code would be this, but it does not work:
>>> classcell(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 cellNameError: name 'cell' is not defined>>>because the new classcell is not available in the class statement itself. In ctypes, we can define the cell class and set the
attribute later, after the class statement:
>>> fromctypesimport*>>> classcell(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>>> foriinrange(8):... print(p.name,end=" ")... p=p.next[0]...foo bar foo bar foo bar foo bar>>>Callback functions
ctypes allows creating C callable function pointers from Python callables. These are sometimes called callback functions.
First, you must create a class for the callback function. The class knows the calling convention, the return type, and the number and types of arguments this function will receive.
The
factory function creates types for callback functions using the cdecl calling convention. On Windows, the
factory function creates types for callback functions using the stdcall calling convention.
Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.
I will present an example here which uses the standard C library’s qsort() function, that is used to sort items with the help of a callback function. qsort() will be used to sort an array of integers:
>>> IntArray5=c_int*5>>> ia=IntArray5(5,1,7,33,99)>>> qsort=libc.qsort>>> qsort.restype=None>>>qsort() must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an integer. First we create the type for the callback function:
>>> CMPFUNC=CFUNCTYPE(c_int,POINTER(c_int),POINTER(c_int))>>>To get started, here is a simple callback that shows the values it gets passed:
>>> defpy_cmp_func(a,b):... print("py_cmp_func",a[0],b[0])... return0...>>> cmp_func=CMPFUNC(py_cmp_func)>>>The result:
>>> qsort(ia,len(ia),sizeof(c_int),cmp_func)py_cmp_func 5 1py_cmp_func 33 99py_cmp_func 7 33py_cmp_func 5 7py_cmp_func 1 7>>>Now we can actually compare the two items and return a useful result:
>>> defpy_cmp_func(a,b):... print("py_cmp_func",a[0],b[0])... returna[0]-b[0]...>>>>>> qsort(ia,len(ia),sizeof(c_int),CMPFUNC(py_cmp_func))py_cmp_func 5 1py_cmp_func 33 99py_cmp_func 7 33py_cmp_func 1 7py_cmp_func 5 7>>>As we can easily check, our array is sorted now:
>>> foriinia: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))... defpy_cmp_func(a,b):... print("py_cmp_func",a[0],b[0])... returna[0]-b[0]...>>> qsort(ia,len(ia),sizeof(c_int),py_cmp_func)py_cmp_func 5 1py_cmp_func 33 99py_cmp_func 7 33py_cmp_func 1 7py_cmp_func 5 7>>>Note
Make sure you keep references to
objects as long as they are used from C code. ctypes doesn’t, and if you don’t, they may be garbage collected, crashing your program when a callback is made.
Also, note that if the callback function is called in a thread created outside of Python’s control (e.g. by the foreign code that calls the callback), ctypes creates a new dummy Python thread on every invocation. This behavior is correct for most purposes, but it means that values stored with
will not survive across different callbacks, even when those calls are made from the same C thread.
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
, Python runtime version number encoded in a single constant integer.
ctypes can access values like this with the
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))0x30c00a0An extended example which also demonstrates the use of pointers accesses the
pointer exported by Python.
Quoting the docs for that value:
This pointer is initialized to point to an array of
records, terminated by one whose members are all NULL or zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.
So manipulating this pointer could even prove useful. To restrict the example size, we show only how this table can be read with ctypes:
>>> fromctypesimport*>>>>>> classstruct_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
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:
>>> foritemintable:... ifitem.nameisNone:... break... print(item.name.decode("ascii"),item.size)..._frozen_importlib 31764_frozen_importlib_external 41499zipimport 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.
Surprises
There are some edges in ctypes where you might expect something other than what actually happens.
Consider the following example:
>>> fromctypesimport*>>> classPOINT(Structure):... _fields_=("x",c_int),("y",c_int)...>>> classRECT(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 3412. 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.valueb'abc def ghi'>>> s.valueiss.valueFalse>>>Note
Objects instantiated from
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
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!
Variable-sized data types
ctypes provides some support for variable-sized arrays and structures.
The
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
is raised if this is tried:
>>> short_array=(c_short*4)()>>> print(sizeof(short_array))8>>> resize(short_array,4)Traceback (most recent call last):...ValueError: minimum size is 8>>> resize(short_array,32)>>> sizeof(short_array)32>>> sizeof(type(short_array))8>>>This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:
>>> short_array[:][0, 0, 0, 0]>>> short_array[7]Traceback (most recent call last):...IndexError: invalid index>>>Another way to use variable-sized data types with ctypes is to use the dynamic nature of Python, and (re-)define the data type after the required size is already known, on a case by case basis.
ctypes reference
Loading shared libraries
There are several ways to load shared libraries into the Python process. One way is to instantiate
or one of its subclasses:
classctypes.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 int. The Python
is released before calling any function exported by these libraries, and reacquired afterwards. For different function behavior, use a subclass:
,
, or
.
If you have an existing
to an already loaded shared library, it can be passed as the handle argument to wrap the opened library in a new CDLL object. In this case, name is only used to set the
attribute, but it may be adjusted and/or validated.
If handle is None, the underlying platform’s
or
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
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
for more information.
On non-Windows systems, name can be None. In this case, 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.)
CPython implementation detail
Since CPython is linked to libc, a Nonename is often used to access the C standard library:
>>> printf=ctypes.CDLL(None).printf>>> printf.argtypes=[ctypes.c_char_p]>>> printf(b"hello\n")hello6To access the Python C API, prefer
which works across platforms.
The mode parameter can be used to specify how the library is loaded. For details, consult the
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
error number in a safe way. ctypes maintains a thread-local copy of the system’s errno variable; if you call foreign functions created with use_errno=True then the errno value before the function call is swapped with the ctypes private copy, the same happens immediately after the function call.
The function
returns the value of the ctypes private copy, and the function
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
and SetLastError() Windows API functions;
and
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
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 CDLL instance may fail even if the DLL name exists. When a dependent DLL of the loaded DLL is not found, a
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.
Changed in version 3.8: Added winmode parameter.
Changed in version 3.12: The name parameter can now be a
.
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:
>>> fromctypesimportCDLL>>> libc=CDLL("libc.so.6")# On Linux>>> libc.time==libc.timeTrue>>> libc['time']==libc['time']FalseThe following public attributes are available. Their name starts with an underscore to not clash with exported function names:
_handle
The system handle used to access the library.
_name
The name of the library passed in the constructor.
classctypes.OleDLL
See
, the superclass, for common information.
Functions in this library use the stdcall calling convention, and are assumed to return the windows specific
code. HRESULT values contain information specifying whether the function call failed or succeeded, together with additional error code. If the return value signals a failure, an
is automatically raised.
Changed in version 3.3:
used to be raised, which is now an alias of
.
classctypes.WinDLL
See
, the superclass, for common information.
Functions in these libraries use the stdcall calling convention, and are assumed to return int by default.
classctypes.PyDLL
See
, 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.
ctypes.RTLD_GLOBAL
Flag to use as mode parameter. On platforms where this flag is not available, it is defined as the integer zero.
ctypes.RTLD_LOCAL
Flag to use as mode parameter. On platforms where this is not available, it is the same as RTLD_GLOBAL.
ctypes.DEFAULT_MODE
The default mode which is used to load shared libraries. On OSX 10.3, this is RTLD_GLOBAL, otherwise it is the same as RTLD_LOCAL.
Shared libraries can also be loaded by using one of the prefabricated objects, which are instances of the
class, either by calling the
method, or by retrieving the library as attribute of the loader instance.
classctypes.LibraryLoader(dlltype)
Class which loads shared libraries. dlltype should be one of the
,
,
, or
types.
__getattr__() has special behavior: It allows loading a shared library by accessing it as attribute of a library loader instance. The result is cached, so repeated attribute accesses return the same library each time.
LoadLibrary(name)
Load a shared library into the process and return it. This method always returns a new instance of the library.
These prefabricated library loaders are available:
ctypes.cdll
Creates
instances.
ctypes.windll
Creates
instances.
ctypes.oledll
Creates
instances.
ctypes.pydll
Creates
instances.
ctypes.pythonapi
An instance of
that exposes Python C API functions as attributes. Note that all these functions are assumed to return C int, which is of course not always the truth, so you have to assign the correct restype attribute to use these functions.
Loading a library through any of these objects raises an
ctypes.dlopen with string argument name, the name used to load the library.
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).
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.
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 ctypes.util module provides a function, 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
.
If wrapping a shared library with ctypes, consider determining the shared library name at development time, and hardcoding it into the wrapper module instead of using find_library() to locate the library at runtime. Also consider adding a configuration option or environment variable to let users select a library to use, and then perhaps use find_library() as a default or fallback.
ctypes.util.find_library(name)
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 -l.) The result is in a format suitable for passing to
.
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,
since version 3.15: This function is kept for use in cases where it works, but not expected to be updated for additional platforms and configurations.
On Linux, 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.
Changed in version 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:
>>> fromctypes.utilimportfind_library>>> find_library("m")'libm.so.6'>>> find_library("c")'libc.so.6'>>> find_library("bz2")'libbz2.so.1.0'>>>On macOS and Android, find_library() uses the system’s standard naming schemes and paths to locate the library, and returns a full pathname if successful:
>>> fromctypes.utilimportfind_library>>> find_library("c")'/usr/lib/libc.dylib'>>> find_library("m")'/usr/lib/libm.dylib'>>> find_library("bz2")'/usr/lib/libbz2.dylib'>>> find_library("AGL")'/System/Library/Frameworks/AGL.framework/AGL'>>>On Windows, find_library() searches along the system search path, and returns the full pathname, but since there is no predefined naming scheme a call like find_library("c") will fail and return None.
ctypes.util.find_msvcrt()
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.
since version 3.16: This function now always returns None, as there are no more VC runtime DLLs that are a single file and supported by Microsoft.
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 ctypes.util module provides the
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:
>>> fromctypes.utilimportdllist>>> dllist()['', 'linux-vdso.so.1', '/lib/x86_64-linux-gnu/libm.so.6', '/lib/x86_64-linux-gnu/libc.so.6', ... ]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 _FuncPtr (not exposed in ctypes) which inherits from the private
class:
>>> importctypes>>> lib=ctypes.CDLL(None)>>> issubclass(lib._FuncPtr,ctypes._CFuncPtr)True>>> lib._FuncPtrisctypes._CFuncPtrFalseclassctypes._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.
restype
Assign a ctypes type to specify the result type of the foreign function. Use None for void, a function not returning anything.
It is possible to assign a callable Python object that is not a ctypes type, in this case the function is assumed to return a C int, and the callable will be called with this integer, allowing further processing or error checking. Using this is deprecated, for more flexible post processing or error checking use a ctypes data type as restype and assign a callable to the
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
class method of the items in the
tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, a
item in the argtypes tuple will convert a string passed as argument into a bytes object using ctypes conversion rules.
New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a
method which returns a value usable as argument (integer, string, ctypes instance). This allows defining adapters that can adapt custom objects as function parameters.
errcheck
Assign a Python function or another callable to this attribute. The callable will be called with three or more arguments:
callable(result, func, arguments)result is what the foreign function returns, as specified by the restype attribute.
func is the foreign function object itself, this allows reusing the same callable object to check or post process the results of several functions.
arguments is a tuple containing the parameters originally passed to the function call, this allows specializing the behavior on the arguments used.
The object that this function returns will be returned from the foreign function call, but it can also check the result value and raise an exception if the foreign function call failed.
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.
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 functionpointer and arguments.
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
for examples.
ctypes.CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)
The returned function prototype creates functions that use the standard C calling convention. The function will release the GIL during the call. If use_errno is set to true, the ctypes private copy of the system
variable is exchanged with the real errno value before and after the call; use_last_error does the same for the Windows error code.
ctypes.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.
ctypes.PYFUNCTYPE(restype, *argtypes)
The returned function prototype creates functions that use the Python calling convention. The function will not release the GIL during the call.
Function prototypes created by these factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call:
prototype(address)Returns a foreign function at the specified address which must be an integer.
prototype(callable)Create a C callable function (a callback function) from a Python callable.
prototype(func_spec[, paramflags])Returns a foreign function exported by a shared library. func_spec must be a 2-tuple (name_or_ordinal,library). The first item is the name of the exported function as string, or the ordinal of the exported function as small integer. The second item is the shared library instance.
prototype(vtbl_index, name[, paramflags[, iid]])Returns a foreign function that will call a COM method. vtbl_index is the index into the virtual function table, a small non-negative integer. name is name of the COM method. iid is an optional pointer to the interface identifier which is used in extended error reporting.
If iid is not specified, an
is raised if the COM method call fails. If iid is specified, a
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 argtypes tuple.
The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.
paramflags must be a tuple of the same length as
.
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:
1Specifies an input parameter to the function.
2Output parameter. The foreign function fills in a value.
4Input 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:
WINUSERAPIintWINAPIMessageBoxW(HWNDhWnd,LPCWSTRlpText,LPCWSTRlpCaption,UINTuType);Here is the wrapping with ctypes:
>>> fromctypesimportc_int,WINFUNCTYPE,windll>>> fromctypes.wintypesimportHWND,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:
WINUSERAPIBOOLWINAPIGetWindowRect(HWNDhWnd,LPRECTlpRect);Here is the wrapping with ctypes:
>>> fromctypesimportPOINTER,WINFUNCTYPE,windll,WinError>>> fromctypes.wintypesimportBOOL,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
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:
>>> deferrcheck(result,func,args):... ifnotresult:... raiseWinError()... returnargs...>>> GetWindowRect.errcheck=errcheck>>>If the
function returns the argument tuple it receives unchanged, ctypes continues the normal processing it does on the output parameters. If you want to return a tuple of window coordinates instead of a RECT instance, you can retrieve the fields in the function and return them instead, the normal processing will no longer take place:
>>> deferrcheck(result,func,args):... ifnotresult:... raiseWinError()... rc=args[1]... returnrc.left,rc.top,rc.bottom,rc.right...>>> GetWindowRect.errcheck=errcheck>>>Utility functions
ctypes.addressof(obj)
Returns the address of the memory buffer as integer. obj must be an instance of a ctypes type.
Raises an
ctypes.addressof with argument obj.
ctypes.alignment(obj_or_type)
Returns the alignment requirements of a ctypes type. obj_or_type must be a ctypes type or instance.
ctypes.byref(obj[, offset])
Returns a light-weight pointer to obj, which must be an instance of a ctypes type. offset defaults to zero, and must be an integer that will be added to the internal pointer value.
byref(obj,offset) corresponds to this C code:
(((char*)&obj)+offset)The returned object can only be used as a foreign function call parameter. It behaves similar to pointer(obj), but the construction is a lot faster.
ctypes.CopyComPointer(src, dst)
Copies a COM pointer from src to dst and returns the Windows specific 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.
Added in version 3.14.
ctypes.cast(obj, type)
This function is similar to the cast operator in C. It returns a new instance of type which points to the same memory block as obj. type must be a pointer type, and obj must be an object that can be interpreted as a pointer.
ctypes.create_string_buffer(init, size=None)
ctypes.create_string_buffer(size)This function creates a mutable character buffer. The returned object is a ctypes array of
.
If size is given (and not None), it must be an
. It specifies the size of the returned array.
If the init argument is given, it must be
. 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 longRaises an
ctypes.create_string_buffer with arguments init, size.
ctypes.create_unicode_buffer(init, size=None)
ctypes.create_unicode_buffer(size)This function creates a mutable unicode character buffer. The returned object is a ctypes array of
.
The function takes the same arguments as
except init must be a string and size counts
.
Raises an
ctypes.create_unicode_buffer with arguments init, size.
ctypes.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.
ctypes.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.
ctypes.util.dllist()
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
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.
: Windows, macOS, iOS, glibc, BSD libc, musl
Added in version 3.14.
ctypes.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
.
ctypes.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.
ctypes.get_errno()
Returns the current value of the ctypes-private copy of the system
variable in the calling thread.
Raises an
ctypes.get_errno with no arguments.
ctypes.get_last_error()
Returns the current value of the ctypes-private copy of the system LastError variable in the calling thread.
Raises an
ctypes.get_last_error with no arguments.
ctypes.memmove(dst, src, count)
Same as the standard C memmove library function: copies count bytes from src to dst. dst and src must be integers or ctypes instances that can be converted to pointers.
ctypes.memset(dst, c, count)
Same as the standard C memset library function: fills the memory block at address dst with count bytes of value c. dst must be an integer specifying an address, or a ctypes instance.
ctypes.POINTER(type, /)
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.
CPython implementation 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.
ctypes.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.
ctypes.resize(obj, size)
This function resizes the internal memory buffer of obj, which must be an instance of a ctypes type. It is not possible to make the buffer smaller than the native size of the objects type, as given by sizeof(type(obj)), but it is possible to enlarge the buffer.
ctypes.set_errno(value)
Set the current value of the ctypes-private copy of the system
variable in the calling thread to value and return the previous value.
Raises an
ctypes.set_errno with argument errno.
ctypes.set_last_error(value)
Sets the current value of the ctypes-private copy of the system LastError variable in the calling thread to value and return the previous value.
Raises an
ctypes.set_last_error with argument error.
ctypes.sizeof(obj_or_type)
Returns the size in bytes of a ctypes type or instance memory buffer. Does the same as the C sizeof operator.
ctypes.string_at(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.
Raises an
ctypes.string_at with arguments ptr, size.
ctypes.WinError(code=None, descr=None)
Creates an instance of
. If code is not specified,
is called to determine the error code. If descr is not specified,
is called to get a textual description of the error.
Changed in version 3.3: An instance of
used to be created, which is now an alias of
.
ctypes.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.
Raises an
ctypes.wstring_at with arguments ptr, size.
ctypes.memoryview_at(ptr, size, readonly=False)
Return a
object of length size that references memory starting at void *ptr.
If readonly is true, the returned 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
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
only takes integers, ptr can also be given as a
or a
object.)
Raises an
ctypes.memoryview_at with arguments address, size, readonly.
Added in version 3.14.
Data types
classctypes._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
helper function. Another instance variable is exposed as
; 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
):
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
is raised.
Raises an
ctypes.cdata/buffer with arguments pointer, size, offset.
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
is raised.
Raises an
ctypes.cdata/buffer with arguments pointer, size, offset.
from_address(address)
This method returns a ctypes type instance using the memory specified by address which must be an integer.
This method, and others that indirectly call this method, raises an
ctypes.cdata with argument address.
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
tuple; it must return an object that can be used as a function call parameter.
All ctypes data types have a default implementation of this classmethod that normally returns obj if that is an instance of the type. Some types accept other objects as well.
in_dll(library, name)
This method returns a ctypes type instance exported by a shared library. name is the name of the symbol that exports the data, library is the loaded shared library.
Common class variables of ctypes data types:
__pointer_type__
The pointer type that was created by calling
for corresponding ctypes data type. If a pointer type was not yet created, the attribute is missing.
Added in version 3.14.
Common instance variables of ctypes data types:
_b_base_
Sometimes ctypes data instances do not own the memory block they contain, instead they share part of the memory block of a base object. The
read-only member is the root ctypes object that owns the memory block.
_b_needsfree_
This read-only variable is true when the ctypes data instance has allocated the memory block itself, false otherwise.
_objects
This member is either None or a dictionary containing Python objects that need to be kept alive so that the memory block contents is kept valid. This object is only exposed for debugging; never modify the contents of this dictionary.
Fundamental data types
classctypes._SimpleCData
This non-public class is the base class of all fundamental ctypes data types. It is mentioned here because it contains the common attributes of the fundamental ctypes data types. _SimpleCData is a subclass of
, 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:
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. 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:
_type_
Class attribute that contains an internal type code, as a string. See
for a summary.
Types marked * in the summary may be (or always are) aliases of a different _SimpleCData subclass, and will not necessarily use the listed type code. For example, if the platform’s long, longlong and time_t C types are the same, then
,
and
all refer to a single class, c_long, whose
code is 'l'. The 'L' code will be unused.
See also
The
and
modules, as well as third-party modules like
, 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
of
, you will always receive a Python bytes object, not a c_char_p instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a foreign functions restype is a subclass of
, 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:
classctypes.c_byte
Represents the C signedchar datatype, and interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.
classctypes.c_char
Represents the C char datatype, and interprets the value as a single character. The constructor accepts an optional string initializer, the length of the string must be exactly one character.
classctypes.c_char_p
Represents the C char* datatype when it points to a zero-terminated string. For a general character pointer that may also point to binary data, POINTER(c_char) must be used. The constructor accepts an integer address, or a bytes object.
classctypes.c_double
Represents the C double datatype. The constructor accepts an optional float initializer.
classctypes.c_longdouble
Represents the C longdouble datatype. The constructor accepts an optional float initializer. On platforms where sizeof(longdouble)==sizeof(double) it is an alias to
.
classctypes.c_float
Represents the C float datatype. The constructor accepts an optional float initializer.
classctypes.c_double_complex
Represents the C doublecomplex datatype, if available. The constructor accepts an optional
initializer.
Added in version 3.14.
classctypes.c_float_complex
Represents the C floatcomplex datatype, if available. The constructor accepts an optional
initializer.
Added in version 3.14.
classctypes.c_longdouble_complex
Represents the C longdoublecomplex datatype, if available. The constructor accepts an optional
initializer.
Added in version 3.14.
classctypes.c_int
Represents the C signedint 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
.
classctypes.c_int8
Represents the C 8-bit signedint datatype. It is an alias for
.
classctypes.c_int16
Represents the C 16-bit signedint datatype. Usually an alias for
.
classctypes.c_int32
Represents the C 32-bit signedint datatype. Usually an alias for
.
classctypes.c_int64
Represents the C 64-bit signedint datatype. Usually an alias for
.
classctypes.c_long
Represents the C signedlong datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
classctypes.c_longlong
Represents the C signedlonglong datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where sizeof(longlong)==sizeof(long) it is an alias to
.
classctypes.c_short
Represents the C signedshort datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
classctypes.c_size_t
Represents the C size_t datatype. Usually an alias for another unsigned integer type.
classctypes.c_ssize_t
Represents the
datatype. This is a signed version of size_t; that is, the POSIX ssize_t type. Usually an alias for another integer type.
Added in version 3.2.
classctypes.c_time_t
Represents the C time_t datatype. Usually an alias for another integer type.
Added in version 3.12.
classctypes.c_ubyte
Represents the C unsignedchar datatype, it interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.
classctypes.c_uint
Represents the C unsignedint 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
.
classctypes.c_uint8
Represents the C 8-bit unsignedint datatype. It is an alias for
.
classctypes.c_uint16
Represents the C 16-bit unsignedint datatype. Usually an alias for
.
classctypes.c_uint32
Represents the C 32-bit unsignedint datatype. Usually an alias for
.
classctypes.c_uint64
Represents the C 64-bit unsignedint datatype. Usually an alias for
.
classctypes.c_ulong
Represents the C unsignedlong datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
classctypes.c_ulonglong
Represents the C unsignedlonglong datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where sizeof(longlong)==sizeof(long) it is an alias to
.
classctypes.c_ushort
Represents the C unsignedshort datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
classctypes.c_void_p
Represents the C void* type. The value is represented as integer. The constructor accepts an optional integer initializer.
classctypes.c_wchar
Represents the C wchar_t datatype, and interprets the value as a single character unicode string. The constructor accepts an optional string initializer, the length of the string must be exactly one character.
classctypes.c_wchar_p
Represents the C wchar_t* datatype, which must be a pointer to a zero-terminated wide character string. The constructor accepts an integer address, or a string.
classctypes.c_bool
Represent the C bool datatype (more accurately, _Bool from C99). Its value can be True or False, and the constructor accepts any object that has a truth value.
classctypes.HRESULT
Represents a HRESULT value, which contains success or error information for a function or method call.
classctypes.py_object
Represents the C
* datatype. Calling this without an argument creates a NULLPyObject* pointer.
Changed in version 3.14: py_object is now a
.
The ctypes.wintypes module provides quite some other Windows specific data types, for example HWND, WPARAM, VARIANT_BOOL or DWORD. Some useful structures like MSG or RECT are also defined.
Structured data types
classctypes.Union(*args, **kw)
Abstract base class for unions in native byte order.
Unions share common attributes and behavior with structures; see
documentation for details.
classctypes.BigEndianUnion(*args, **kw)
Abstract base class for unions in big endian byte order.
Added in version 3.11.
classctypes.LittleEndianUnion(*args, **kw)
Abstract base class for unions in little endian byte order.
Added in version 3.11.
classctypes.BigEndianStructure(*args, **kw)
Abstract base class for structures in big endian byte order.
classctypes.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.
classctypes.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
class variable. ctypes will create
s which allow reading and writing the fields by direct attribute accesses. These are the
_fields_
A sequence defining the structure fields. The items must be 2-tuples or 3-tuples. The first item is the name of the field, the second item specifies the type of the field; it can be any ctypes data type.
For integer type fields like
, 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
class variable after the class statement that defines the Structure subclass, this allows creating data types that directly or indirectly reference themselves:
classList(Structure):passList._fields_=[("pnext",POINTER(List)),...]The _fields_ class variable can only be set once. Later assignments will raise an
.
Additionally, the _fields_ class variable must be defined before the structure or union type is first used: an instance or subclass is created,
is called on it, and so on. Later assignments to _fields_ will raise an
. If _fields_ has not been set before such use, the structure or union will have no own fields, as if _fields_ was empty.
Sub-subclasses of structure types inherit the fields of the base class plus the
defined in the sub-subclass, if any.
_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
).
Setting _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 #pragmapack(N) in C, except ctypes may allow larger n than what the compiler accepts.
_pack_ must already be defined when
is assigned, otherwise it will have no effect.
Deprecated since version 3.14, will be removed in version 3.19: For historical reasons, if _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
to 'ms' explicitly.
_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 #pragmaalign(N) on MSVC, except ctypes may allow values that the compiler would reject.
_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.
_align_ must already be defined when
is assigned, otherwise it will have no effect.
Added in version 3.13.
_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,
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
is specified: "ms". (This is deprecated; see _pack_ documentation.)
Otherwise: "gcc-sysv"
_layout_ must already be defined when
is assigned, otherwise it will have no effect.
Added in version 3.14.
_anonymous_
An optional sequence that lists the names of unnamed (anonymous) fields.
must be already defined when
is assigned, otherwise it will have no effect.
The fields listed in this variable must be structure or union type fields. ctypes will create descriptors in the structure type that allows 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)]classTYPEDESC(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_PTRtd.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
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
. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize _fields_ with the same name, or create new attributes for names not present in _fields_.
classctypes.CField(*args, **kw)
Descriptor for fields of a
and
. For example:
>>> classColor(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_offset2>>> Color.intense<ctypes.CField 'intense' type=c_bool, ofs=3, bit_size=1, bit_offset=0>>>> Color.blinking.bit_offset1All attributes are read-only.
CField objects are created via
; do not instantiate the class directly.
Added in version 3.14: Previously, descriptors only had offset and size attributes and a readable string representation; the CField class was not available directly.
name
Name of the field, as a string.
type
Type of the field, as a
.
offset
byte_offset
Offset of the field, in bytes.
For bitfields, this is the offset of the underlying byte-aligned storage unit; see
.
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.
size
For non-bitfields, equivalent to
.
For bitfields, this contains a backwards-compatible bit-packed value that combines
and
. Prefer using the explicit attributes instead.
is_bitfield
True if this is a bitfield.
bit_offset
bit_size
The location of a bitfield within its storage unit, that is, within
bytes of memory starting at
.
To get the field’s value, read the storage unit as an integer,
by bit_offset and take the bit_size least significant bits.
For non-bitfields, bit_offset is zero and bit_size is equal to byte_size*8.
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.util.struct(*, align=None, layout, endian='native', pack=None)
A
that allows generating structure types using an annotation-based syntax, similar to the
module.
For example:
fromctypes.utilimportstructfromctypesimportc_int@structclassPoint:x:c_inty:c_intpoint=Point(1,2)align, layout, and pack supply the value for the
,
, and
attributes, respectively.
endian controls which structure class will be used as the base.
If endian is 'native',
will be used.
If endian is 'big',
will be used.
If endian is 'little',
will be used.
Any other value will raise a
.
For controlling field-specific data, wrap the annotation in
with
as the second argument, like so:
@structclassPyObject:ob_refcnt:c_ssize_tob_type:c_void_p@structclassPyHovercraftObject:ob_base:Annotated[PyObject,CFieldInfo(anonymous=True)]Added in version 3.16.0a0 (unreleased).
classctypes.util.CFieldInfo(anonymous=False, bit_width=None)
Information regarding a structure field defined by the
decorator. This should be used in the second argument of a
wrapping a ctypes type.
anonymous specifies whether the field will be present in the
attribute of the generated class.
If bit_width is non-None, the annotated field will be bit_width number of bits in the generated structure. This is equivalent to passing a third item in
.
Added in version 3.16.0a0 (unreleased).
Arrays and pointers
classctypes.Array(*args)
Abstract base class for arrays.
The recommended way to create concrete array types is by multiplying any ctypes data type with a non-negative integer. Alternatively, you can subclass this type and define
and
class variables. Array elements can be read and written using standard subscript and slice accesses; for slice reads, the resulting object is not itself an Array.
Arrays are
over the type of their elements.
_length_
A positive integer specifying the number of elements in the array. Out-of-range subscripts result in an
. Will be returned by
.
_type_
Specifies the type of each element in the array.
Array subclass constructors accept positional arguments, used to initialize the elements in order.
ctypes.ARRAY(type, length)
Create an array. Equivalent to type*length, where type is a ctypes data type and length an integer.
since version 3.14: In favor of multiplication.
classctypes._Pointer
Private, abstract base class for pointers.
Concrete pointer types are created by calling
with the type that will be pointed to; this is done automatically by
.
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
will raise
. Negative subscripts will read from the memory before the pointer (as in C), and out-of-range subscripts will probably crash with an access violation (if you’re lucky).
_type_
Specifies the type pointed to.
contents
Returns the object to which to pointer points. Assigning to this attribute changes the pointer to point to the assigned object.
Exceptions
exceptionctypes.ArgumentError
This exception is raised when a foreign function call cannot convert one of the passed arguments.
exceptionctypes.COMError(hresult, text, details)
This exception is raised when a COM method call failed.
hresult
The integer value representing the error code.
text
The error message.
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.
Added in version 3.14.