ctypes — A foreign function library for Python¶
Source code: Lib/ctypes
ctypes is a foreign function library for Python. It provides C compatible
data types, and allows calling functions in DLLs or shared libraries. It can be
used to wrap these libraries in pure Python.
ctypes tutorial¶
Note: The code samples in this tutorial use doctest to make sure that
they actually work. Since some code samples behave differently under Linux,
Windows, or macOS, they contain doctest directives in comments.
Note: Some code samples reference the ctypes c_int type. On platforms
where sizeof(long) == sizeof(int) it is an alias to c_long.
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 cdll, and on Windows windll and oledll
objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects. cdll
loads libraries which export functions using the standard cdecl calling
convention, while windll libraries call functions using the stdcall
calling convention. oledll also uses the stdcall calling convention, and
assumes the functions return a Windows HRESULT error code. The error
code is used to automatically raise an OSError exception when the
function call fails.
3.3 sürümünde değişti: Windows errors used to raise WindowsError, which is now an alias
of OSError.
Here are some examples for Windows. Note that msvcrt is the MS standard C
library containing most standard C functions, and uses the cdecl calling
convention:
>>> from ctypes import *
>>> print(windll.kernel32)
<WinDLL 'kernel32', handle ... at ...>
>>> print(cdll.msvcrt)
<CDLL 'msvcrt', handle ... at ...>
>>> libc = cdll.msvcrt
>>>
Windows appends the usual .dll file suffix automatically.
Not
Accessing the standard C library through cdll.msvcrt will use an
outdated version of the library that may be incompatible with the one
being used by Python. Where possible, use native Python functionality,
or else import and use the msvcrt module.
On Linux, it is required to specify the filename including the extension to
load a library, so attribute access can not be used to load libraries. Either the
LoadLibrary() method of the dll loaders should be used,
or you should load the library by creating an instance of CDLL by calling
the constructor:
>>> cdll.LoadLibrary("libc.so.6")
<CDLL 'libc.so.6', handle ... at ...>
>>> libc = CDLL("libc.so.6")
>>> libc
<CDLL 'libc.so.6', handle ... at ...>
>>>
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:
/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
windll does not try to select one of them by magic, you must access the
version you need by specifying GetModuleHandleA or GetModuleHandleW
explicitly, and then call it with bytes or string objects respectively.
Sometimes, dlls export functions with names which aren’t valid Python
identifiers, like "??2@YAPAXI@Z". In this case you have to use
getattr() to retrieve the function:
>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")
<_FuncPtr object at 0x...>
>>>
On Windows, some dlls export functions not by name but by ordinal. These functions can be accessed by indexing the dll object with the ordinal number:
>>> cdll.kernel32[1]
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0]
Traceback (most recent call last):
File "<stdin>", line 1, in <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())
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)))
0x1d000000
>>>
ValueError 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
>>>
There are, however, enough ways to crash Python with ctypes, so you
should be careful anyway. The faulthandler module can be helpful in
debugging crashes (e.g. from segmentation faults produced by erroneous C library
calls).
None, integers, bytes objects and (unicode) strings are the only native
Python objects that can directly be used as parameters in these function calls.
None is passed as a C NULL pointer, bytes objects and strings are passed
as pointer to the memory block that contains their data (char* or
wchar_t*). Python integers are passed as the 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 |
bool (1) |
|
char |
1-character bytes object |
|
|
1-character string |
|
char |
int |
|
unsigned char |
int |
|
short |
int |
|
unsigned short |
int |
|
int |
int |
|
unsigned int |
int |
|
long |
int |
|
unsigned long |
int |
|
__int64 or long long |
int |
|
unsigned __int64 or unsigned long long |
int |
|
|
int |
|
|
int |
|
|
int |
|
float |
float |
|
double |
float |
|
long double |
float |
|
char* (NUL terminated) |
bytes object or |
|
wchar_t* (NUL terminated) |
string or |
|
void* |
int or |
The constructor accepts any object with a truth value.
Additionally, if IEC 60559 compatible complex arithmetic (Annex G) is supported, the following complex types are available:
ctypes type |
C type |
Python type |
|---|---|---|
float complex |
complex |
|
double complex |
complex |
|
long double complex |
complex |
All these types can be created by calling them with an optional initializer of the correct type and value:
>>> c_int()
c_long(0)
>>> c_wchar_p("Hello, World")
c_wchar_p(140018365411392)
>>> c_ushort(-3)
c_ushort(65533)
>>>
Since these types are mutable, their value can also be changed afterwards:
>>> i = c_int(42)
>>> print(i)
c_long(42)
>>> print(i.value)
42
>>> i.value = -99
>>> print(i.value)
-99
>>>
Assigning a new value to instances of the pointer types c_char_p,
c_wchar_p, and c_void_p changes the memory location they
point to, not the contents of the memory block (of course not, because Python
bytes objects are immutable):
>>> s = "Hello, World"
>>> c_s = c_wchar_p(s)
>>> print(c_s)
c_wchar_p(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
create_string_buffer() function which creates these in various ways. The
current memory block contents can be accessed (or changed) with the raw
property; if you want to access it as NUL terminated string, use the value
property:
>>> from ctypes import *
>>> p = create_string_buffer(3) # create a 3 byte buffer, initialized to NUL bytes
>>> print(sizeof(p), repr(p.raw))
3 b'\x00\x00\x00'
>>> p = create_string_buffer(b"Hello") # create a buffer containing a NUL terminated string
>>> print(sizeof(p), repr(p.raw))
6 b'Hello\x00'
>>> print(repr(p.value))
b'Hello'
>>> p = create_string_buffer(b"Hello", 10) # create a 10 byte buffer
>>> print(sizeof(p), repr(p.raw))
10 b'Hello\x00\x00\x00\x00\x00'
>>> p.value = b"Hi"
>>> print(sizeof(p), repr(p.raw))
10 b'Hi\x00lo\x00\x00\x00\x00\x00'
>>>
The create_string_buffer() function replaces the 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
create_unicode_buffer() function.
Calling functions, continued¶
Note that printf prints to the real standard output channel, not to
sys.stdout, so these examples will only work at the console prompt, not
from within IDLE or PythonWin:
>>> printf = libc.printf
>>> printf(b"Hello, %s\n", b"World!")
Hello, World!
14
>>> printf(b"Hello, %S\n", "World!")
Hello, World!
14
>>> printf(b"%d bottles of beer\n", 42)
42 bottles of beer
19
>>> printf(b"%f bottles of beer\n", 42.5)
Traceback (most recent call last):
File "<stdin>", line 1, in <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 3.140000
31
>>>
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 argtypes
attribute for the regular, non-variadic, function arguments:
libc.printf.argtypes = [ctypes.c_char_p]
Because specifying the attribute does not inhibit portability it is advised to always
specify argtypes 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:
>>> class Bottles:
... def __init__(self, number):
... self._as_parameter_ = number
...
>>> bottles = Bottles(42)
>>> printf(b"%d bottles of beer\n", bottles)
42 bottles of beer
19
>>>
If you don’t want to store the instance’s data in the _as_parameter_
instance variable, you could define a property which makes the
attribute available on request.
Specifying the required argument types (function prototypes)¶
It is possible to specify the required argument types of functions exported from
DLLs by setting the argtypes attribute.
argtypes must be a sequence of C data types (the printf() function is
probably not a good example here, because it takes a variable number and
different types of parameters depending on the format string, on the other hand
this is quite handy to experiment with this feature):
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
String 'Hi', Int 10, Double 2.200000
37
>>>
Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:
>>> printf(b"%d %d %d", 1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <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 from_param() class method for them to be able to use them
in the argtypes sequence. The from_param() class method receives
the Python object passed to the function call, it should do a typecheck or
whatever is needed to make sure this object is acceptable, and then return the
object itself, its _as_parameter_ attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, bytes, a ctypes instance, or an object with an
_as_parameter_ attribute.
Return types¶
By default functions are assumed to return the C int type. Other
return types can be specified by setting the restype attribute of the
function object.
The C prototype of time() is time_t time(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_t
The argument types can be specified using 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))
1150640792
Here is a more advanced example, it uses the strchr() function, which expects
a string pointer and a char, and returns a pointer to a string:
>>> strchr = libc.strchr
>>> strchr(b"abcdef", ord("d"))
8059983
>>> strchr.restype = c_char_p # c_char_p is a pointer to a string
>>> strchr(b"abcdef", ord("d"))
b'def'
>>> print(strchr(b"abcdef", ord("x")))
None
>>>
If you want to avoid the ord("x") calls above, you can set the
argtypes attribute, and the second argument will be converted from a
single character Python bytes object into a C char:
>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> strchr(b"abcdef", b"d")
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 restype attribute, if the foreign function returns an integer. The
callable will be called with the integer the C function returns, and the
result of this call will be used as the result of your function call. This is
useful to check for error return values and automatically raise an exception:
>>> GetModuleHandle = windll.kernel32.GetModuleHandleA
>>> def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
>>>
>>> GetModuleHandle.restype = ValidHandle
>>> GetModuleHandle(None)
486539264
>>> GetModuleHandle("something silly")
Traceback (most recent call last):
File "<stdin>", line 1, in <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
GetLastError() to retrieve it.
Please note that a much more powerful error checking mechanism is available
through the errcheck attribute;
see the reference manual for details.
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 byref() function which is used to pass parameters
by reference. The same effect can be achieved with the pointer() function,
although pointer() does a lot more work since it constructs a real pointer
object, so it is faster to use byref() if you don’t need the pointer
object in Python itself:
>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer(b'\000' * 32)
>>> print(i.value, f.value, repr(s.value))
0 0.0 b''
>>> libc.sscanf(b"1 3.14 Hello", b"%d %f %s",
... byref(i), byref(f), s)
3
>>> print(i.value, f.value, repr(s.value))
1 3.1400001049 b'Hello'
>>>
Structures and unions¶
Structures and unions must derive from the Structure and Union
base classes which are defined in the ctypes module. Each subclass must
define a _fields_ attribute. _fields_ must be a list of
2-tuples, containing a field name and a field type.
The field type must be a ctypes type like c_int, or any other
derived ctypes type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = [("x", c_int),
... ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print(point.x, point.y)
10 20
>>> point = POINT(y=5)
>>> print(point.x, point.y)
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <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 descriptors can be retrieved from the class, they are useful for debugging because they can provide useful information:
>>> print(POINT.x)
<Field type=c_long, ofs=0, size=4>
>>> print(POINT.y)
<Field type=c_long, ofs=4, size=4>
>>>
Uyarı
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
_layout_ class attribute in the subclass definition; see
the attribute documentation for details.
It is possible to specify the maximum alignment for the fields by setting
the _pack_ class attribute to a positive integer.
This matches what #pragma pack(n) does in MSVC.
It is also possible to set a minimum alignment for how the subclass itself is packed in the
same way #pragma align(n) works in MSVC.
This can be achieved by specifying a :_align_ class attribute
in the subclass definition.
ctypes uses the native byte order for Structures and Unions. To build
structures with non-native byte order, you can use one of the
BigEndianStructure, LittleEndianStructure,
BigEndianUnion, and LittleEndianUnion base classes. These
classes cannot contain pointer fields.
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 _fields_ tuples:
>>> class Int(Structure):
... _fields_ = [("first_16", c_int, 16),
... ("second_16", c_int, 16)]
...
>>> print(Int.first_16)
<Field type=c_long, ofs=0:0, bits=16>
>>> print(Int.second_16)
<Field type=c_long, ofs=0:16, bits=16>
>>>
Arrays¶
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
>>>
Pointers¶
Pointer instances are created by calling the pointer() function on a
ctypes type:
>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>
Pointer instances have a contents attribute which
returns the object to which the pointer points, the i object above:
>>> pi.contents
c_long(42)
>>>
Note that ctypes does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute:
>>> pi.contents is i
False
>>> pi.contents is pi.contents
False
>>>
Assigning another c_int instance to the pointer’s contents attribute
would cause the pointer to point to the memory location where this is stored:
>>> i = c_int(99)
>>> pi.contents = i
>>> pi.contents
c_long(99)
>>>
Pointer instances can also be indexed with integers:
>>> pi[0]
99
>>>
Assigning to an integer index changes the pointed to value:
>>> print(i)
c_long(99)
>>> pi[0] = 22
>>> print(i)
c_long(22)
>>>
It is also possible to use indexes different from 0, but you must know what you’re doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.
Behind the scenes, the pointer() function does more than simply create
pointer instances, it has to create pointer types first. This is done with the
POINTER() function, which accepts any ctypes type, and returns a
new type:
>>> PI = POINTER(c_int)
>>> PI
<class 'ctypes.LP_c_long'>
>>> PI(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <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] = 1234
Traceback (most recent call last):
....
ValueError: NULL pointer access
>>>
Type conversions¶
Usually, ctypes does strict type checking. This means, if you have
POINTER(c_int) in the argtypes list of a function or as the type of
a member field in a structure definition, only instances of exactly the same
type are accepted. There are some exceptions to this rule, where ctypes accepts
other objects. For example, you can pass compatible array instances instead of
pointer types. So, for POINTER(c_int), ctypes accepts an array of c_int:
>>> class Bar(Structure):
... _fields_ = [("count", c_int), ("values", POINTER(c_int))]
...
>>> bar = Bar()
>>> bar.values = (c_int * 3)(1, 2, 3)
>>> bar.count = 3
>>> for i in range(bar.count):
... print(bar.values[i])
...
1
2
3
>>>
In addition, if a function argument is explicitly declared to be a pointer type
(such as POINTER(c_int)) in argtypes, an object of the pointed
type (c_int in this case) can be passed to the function. ctypes will apply
the required byref() conversion in this case automatically.
To set a POINTER type field to NULL, you can assign None:
>>> bar.values = None
>>>
Sometimes you have instances of incompatible types. In C, you can cast one type
into another type. ctypes provides a cast() function which can be
used in the same way. The Bar structure defined above accepts
POINTER(c_int) pointers or c_int arrays for its values field,
but not instances of other types:
>>> bar.values = (c_byte * 4)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
>>>
For these cases, the cast() function is handy.
The cast() function can be used to cast a ctypes instance into a pointer
to a different ctypes data type. cast() takes two parameters, a ctypes
object that is or can be converted to a pointer of some kind, and a ctypes
pointer type. It returns an instance of the second argument, which references
the same memory block as the first argument:
>>> a = (c_byte * 4)()
>>> cast(a, POINTER(c_int))
<ctypes.LP_c_long object at ...>
>>>
So, cast() can be used to assign to the values field of Bar the
structure:
>>> bar = Bar()
>>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
>>> print(bar.values[0])
0
>>>
Incomplete Types¶
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 ctypes, we can define the cell class and set the
_fields_ attribute later, after the class statement:
>>> from ctypes import *
>>> class cell(Structure):
... pass
...
>>> cell._fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
>>>
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
>>>
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 CFUNCTYPE() factory function creates types for callback functions
using the cdecl calling convention. On Windows, the WINFUNCTYPE()
factory function creates types for callback functions using the stdcall
calling convention.
Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.
I will present an example here which uses the standard C library’s
qsort() function, that is used to sort items with the help of a callback
function. qsort() will be used to sort an array of integers:
>>> IntArray5 = c_int * 5
>>> ia = IntArray5(5, 1, 7, 33, 99)
>>> qsort = libc.qsort
>>> qsort.restype = None
>>>
qsort() must be called with a pointer to the data to sort, the number of
items in the data array, the size of one item, and a pointer to the comparison
function, the callback. The callback will then be called with two pointers to
items, and it must return a negative integer if the first item is smaller than
the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an
integer. First we create the type for the callback function:
>>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
>>>
To get started, here is a simple callback that shows the values it gets passed:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return 0
...
>>> cmp_func = CMPFUNC(py_cmp_func)
>>>
The result:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 5 7
py_cmp_func 1 7
>>>
Now we can actually compare the two items and return a useful result:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return a[0] - b[0]
...
>>>
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func))
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
>>>
As we can easily check, our array is sorted now:
>>> for i in ia: print(i, end=" ")
...
1 5 7 33 99
>>>
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
>>>
Not
Make sure you keep references to CFUNCTYPE() objects as long as they
are used from C code. ctypes doesn’t, and if you don’t, they may be
garbage collected, crashing your program when a callback is made.
Also, note that if the callback function is called in a thread created
outside of Python’s control (e.g. by the foreign code that calls the
callback), ctypes creates a new dummy Python thread on every invocation. This
behavior is correct for most purposes, but it means that values stored with
threading.local will not survive across different callbacks, even when
those calls are made from the same C thread.
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 Py_Version, Python
runtime version number encoded in a single constant integer.
ctypes can access values like this with the in_dll() class methods of
the type. pythonapi is a predefined symbol giving access to the Python C
api:
>>> 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
PyImport_FrozenModules pointer exported by Python.
Quoting the docs for that value:
This pointer is initialized to point to an array of
_frozenrecords, terminated by one whose members are allNULLor zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.
So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with ctypes:
>>> from ctypes import *
>>>
>>> class struct_frozen(Structure):
... _fields_ = [("name", c_char_p),
... ("code", POINTER(c_ubyte)),
... ("size", c_int),
... ("get_code", POINTER(c_ubyte)), # Function pointer
... ]
...
>>>
We have defined the _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.
Surprises¶
There are some edges in ctypes where you might expect something other
than what actually happens.
Consider the following example:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class RECT(Structure):
... _fields_ = ("a", POINT), ("b", POINT)
...
>>> p1 = POINT(1, 2)
>>> p2 = POINT(3, 4)
>>> rc = RECT(p1, p2)
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
>>> # now swap the two points
>>> rc.a, rc.b = rc.b, rc.a
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
>>>
Hm. We certainly expected the last statement to print 3 4 1 2. What
happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a line above:
>>> temp0, temp1 = rc.b, rc.a
>>> rc.a = temp0
>>> rc.b = temp1
>>>
Note that temp0 and temp1 are objects still using the internal buffer of
the rc object above. So executing rc.a = temp0 copies the buffer
contents of temp0 into rc ‘s buffer. This, in turn, changes the
contents of temp1. So, the last assignment rc.b = temp1, doesn’t have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn’t copy the sub-object, instead it retrieves a wrapper object accessing the root-object’s underlying buffer.
Another example that may behave 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
>>>
Not
Objects instantiated from 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 descriptors accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the contents of the object is stored. Accessing the contents again
constructs a new Python object each time!
Variable-sized data types¶
ctypes provides some support for variable-sized arrays and structures.
The resize() function can be used to resize the memory buffer of an
existing ctypes object. The function takes the object as first argument, and
the requested size in bytes as the second argument. The memory block cannot be
made smaller than the natural memory block specified by the objects type, a
ValueError is raised if this is tried:
>>> short_array = (c_short * 4)()
>>> print(sizeof(short_array))
8
>>> resize(short_array, 4)
Traceback (most recent call last):
...
ValueError: minimum size is 8
>>> resize(short_array, 32)
>>> sizeof(short_array)
32
>>> sizeof(type(short_array))
8
>>>
This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:
>>> short_array[:]
[0, 0, 0, 0]
>>> short_array[7]
Traceback (most recent call last):
...
IndexError: invalid index
>>>
Another way to use variable-sized data types with ctypes is to use the
dynamic nature of Python, and (re-)define the data type after the required size
is already known, on a case by case basis.
ctypes reference¶
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 _CFuncPtr class:
>>> import ctypes
>>> lib = ctypes.CDLL(None)
>>> issubclass(lib._FuncPtr, ctypes._CFuncPtr)
True
>>> lib._FuncPtr is ctypes._CFuncPtr
False
- class ctypes._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
Nonefor 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
restypeand assign a callable to theerrcheckattribute.
- argtypes¶
Assign a tuple of ctypes types to specify the argument types that the function accepts. Functions using the
stdcallcalling convention can only be called with the same number of arguments as the length of this tuple; functions using the C calling convention accept additional, unspecified arguments as well.When a foreign function is called, each actual argument is passed to the
from_param()class method of the items in theargtypestuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, ac_char_pitem in theargtypestuple will convert a string passed as argument into a bytes object using ctypes conversion rules.New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a
from_param()method which returns a value usable as argument (integer, string, ctypes instance). This allows 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
restypeattribute.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 may raise an auditing event
ctypes.call_function with arguments function pointer 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 Callback functions 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
errnovariable is exchanged with the realerrnovalue 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
stdcallcalling convention. The function will release the GIL during the call. use_errno and use_last_error have the same meaning as above.Availability: Windows
- 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
OSErroris raised if the COM method call fails. If iid is specified, aCOMErroris 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
argtypestuple.
The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.
paramflags must be a tuple of the same length as argtypes.
Each item in this tuple contains further information about a parameter, it must be a tuple containing one, two, or three items.
The first item is an integer containing a combination of direction flags for the parameter:
- 1
Specifies an input parameter to the function.
- 2
Output parameter. The foreign function fills in a value.
- 4
Input parameter which defaults to the integer zero.
The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.
The optional third item is the default value for this parameter.
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 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 ctypes:
>>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
>>> from ctypes.wintypes import BOOL, HWND, RECT
>>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
>>> paramflags = (1, "hwnd"), (2, "lprect")
>>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)
>>>
Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.
Output parameters can be combined with the errcheck protocol to do
further output processing and error checking. The win32 GetWindowRect api
function returns a BOOL to signal success or failure, so this function could
do the error checking, and raises an exception when the api call failed:
>>> def errcheck(result, func, args):
... if not result:
... raise WinError()
... return args
...
>>> GetWindowRect.errcheck = errcheck
>>>
If the errcheck function returns the argument tuple it receives
unchanged, ctypes continues the normal processing it does on the output
parameters. If you want to return a tuple of window coordinates instead of a
RECT instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place:
>>> def errcheck(result, func, args):
... if not result:
... raise WinError()
... rc = args[1]
... return rc.left, rc.top, rc.bottom, rc.right
...
>>> GetWindowRect.errcheck = errcheck
>>>
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 auditing event
ctypes.addressofwith argumentobj.
- 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
HRESULTvalue.If src is not
NULL, itsAddRefmethod 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 itsReleasemethod when necessary.Availability: Windows
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_or_size, size=None)¶
This function creates a mutable character buffer. The returned object is a ctypes array of
c_char.init_or_size must be an integer which specifies the size of the array, or a bytes object which will be used to initialize the array items.
If a bytes object is specified as first argument, the buffer is made one item larger than its length so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the bytes should not be used.
Raises an auditing event
ctypes.create_string_bufferwith argumentsinit,size.
- ctypes.create_unicode_buffer(init_or_size, size=None)¶
This function creates a mutable unicode character buffer. The returned object is a ctypes array of
c_wchar.init_or_size must be an integer which specifies the size of the array, or a string which will be used to initialize the array items.
If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the string should not be used.
Raises an auditing event
ctypes.create_unicode_bufferwith argumentsinit,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.
Availability: Windows
- 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
_ctypesextension dll exports.Availability: Windows
- ctypes.util.find_library(name)¶
Try to find a library and return a pathname. name is the library name without any prefix like
lib, suffix like.so,.dylibor version number (this is the form used for the posix linker option-l). If no library can be found, returnsNone.The exact functionality is system dependent.
- 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,
Noneis returned.If you need to free memory, for example, allocated by an extension module with a call to the
free(void *), it is important that you use the function in the same library that allocated the memory.Availability: Windows
- 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
GetLastError().Availability: Windows
- 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.Availability: Windows
- ctypes.get_errno()¶
Returns the current value of the ctypes-private copy of the system
errnovariable in the calling thread.Raises an auditing event
ctypes.get_errnowith no arguments.
- ctypes.get_last_error()¶
Returns the current value of the ctypes-private copy of the system
LastErrorvariable in the calling thread.Availability: Windows
Raises an auditing event
ctypes.get_last_errorwith 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 and return a new ctypes pointer type. Pointer types are cached and reused internally, so calling this function repeatedly is cheap. type must be a ctypes type.
- 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
errnovariable in the calling thread to value and return the previous value.Raises an auditing event
ctypes.set_errnowith argumenterrno.
- ctypes.set_last_error(value)¶
Sets the current value of the ctypes-private copy of the system
LastErrorvariable in the calling thread to value and return the previous value.Availability: Windows
Raises an auditing event
ctypes.set_last_errorwith argumenterror.
- ctypes.sizeof(obj_or_type)¶
Returns the size in bytes of a ctypes type or instance memory buffer. Does the same as the C
sizeofoperator.
- 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 auditing event
ctypes.string_atwith argumentsptr,size.
- ctypes.WinError(code=None, descr=None)¶
Creates an instance of
OSError. If code is not specified,GetLastError()is called to determine the error code. If descr is not specified,FormatError()is called to get a textual description of the error.Availability: Windows
3.3 sürümünde değişti: An instance of
WindowsErrorused to be created, which is now an alias ofOSError.
- 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 auditing event
ctypes.wstring_atwith argumentsptr,size.
Data types¶
- class ctypes._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
addressof()helper function. Another instance variable is exposed as_objects; this contains other Python objects that need to be kept alive in case the memory block contains pointers.Common methods of ctypes data types, these are all class methods (to be exact, they are methods of the metaclass):
- 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
ValueErroris raised.Raises an auditing event
ctypes.cdata/bufferwith argumentspointer,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
ValueErroris raised.Raises an auditing event
ctypes.cdata/bufferwith argumentspointer,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 auditing event
ctypes.cdatawith argumentaddress.
- 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
argtypestuple; 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 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
_b_base_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
Noneor 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¶
- class ctypes._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.
_SimpleCDatais a subclass of_CData, so it inherits their methods and attributes. ctypes data types that are not and do not contain pointers can now be pickled.Instances have a single attribute:
- 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
valueattribute is retrieved from a ctypes instance, usually a new object is returned each time.ctypesdoes not implement original object return, always a new object is constructed. The same is true for all other ctypes object instances.
Fundamental data types, when returned as foreign function call results, or, for
example, by retrieving structure field members or array items, are transparently
converted to native Python types. In other words, if a foreign function has a
restype of c_char_p, you will always receive a Python bytes
object, not a c_char_p instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions restype is a subclass of c_void_p, you will
receive an instance of this subclass from the function call. Of course, you can
get the value of the pointer by accessing the value attribute.
These are the fundamental ctypes data types:
- class ctypes.c_byte¶
Represents the C signed char datatype, and interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.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.
- class ctypes.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.
- class ctypes.c_double¶
Represents the C double datatype. The constructor accepts an optional float initializer.
- class ctypes.c_longdouble¶
Represents the C long double datatype. The constructor accepts an optional float initializer. On platforms where
sizeof(long double) == sizeof(double)it is an alias toc_double.
- class ctypes.c_float¶
Represents the C float datatype. The constructor accepts an optional float initializer.
- class ctypes.c_double_complex¶
Represents the C double complex datatype, if available. The constructor accepts an optional
complexinitializer.Added in version 3.14.
- class ctypes.c_float_complex¶
Represents the C float complex datatype, if available. The constructor accepts an optional
complexinitializer.Added in version 3.14.
- class ctypes.c_longdouble_complex¶
Represents the C long double complex datatype, if available. The constructor accepts an optional
complexinitializer.Added in version 3.14.
- class ctypes.c_int¶
Represents the C signed int datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where
sizeof(int) == sizeof(long)it is an alias toc_long.
- class ctypes.c_int64¶
Represents the C 64-bit signed int datatype. Usually an alias for
c_longlong.
- class ctypes.c_long¶
Represents the C signed long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_longlong¶
Represents the C signed long long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_short¶
Represents the C signed short datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_size_t¶
Represents the C
size_tdatatype.
- class ctypes.c_ssize_t¶
Represents the C
ssize_tdatatype.Added in version 3.2.
- class ctypes.c_time_t¶
Represents the C
time_tdatatype.Added in version 3.12.
- class ctypes.c_ubyte¶
Represents the C unsigned char datatype, it interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_uint¶
Represents the C unsigned int datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where
sizeof(int) == sizeof(long)it is an alias forc_ulong.
- class ctypes.c_uint16¶
Represents the C 16-bit unsigned int datatype. Usually an alias for
c_ushort.
- class ctypes.c_uint64¶
Represents the C 64-bit unsigned int datatype. Usually an alias for
c_ulonglong.
- class ctypes.c_ulong¶
Represents the C unsigned long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_ulonglong¶
Represents the C unsigned long long datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_ushort¶
Represents the C unsigned short datatype. The constructor accepts an optional integer initializer; no overflow checking is done.
- class ctypes.c_void_p¶
Represents the C void* type. The value is represented as integer. The constructor accepts an optional integer initializer.
- class ctypes.c_wchar¶
Represents the C
wchar_tdatatype, 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 ctypes.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.
- class ctypes.c_bool¶
Represent the C bool datatype (more accurately, _Bool from C99). Its value can be
TrueorFalse, and the constructor accepts any object that has a truth value.
- class ctypes.HRESULT¶
Represents a
HRESULTvalue, which contains success or error information for a function or method call.Availability: Windows
- class ctypes.py_object¶
Represents the C PyObject* datatype. Calling this without an argument creates a
NULLPyObject* pointer.
The ctypes.wintypes module provides quite some other Windows specific
data types, for example HWND, WPARAM, or DWORD.
Some useful structures like MSG or RECT are also defined.
Structured data types¶
- class ctypes.Union(*args, **kw)¶
Abstract base class for unions in native byte order.
Unions share common attributes and behavior with structures; see
Structuredocumentation for details.
- class ctypes.BigEndianUnion(*args, **kw)¶
Abstract base class for unions in big endian byte order.
Added in version 3.11.
- class ctypes.LittleEndianUnion(*args, **kw)¶
Abstract base class for unions in little endian byte order.
Added in version 3.11.
- class ctypes.BigEndianStructure(*args, **kw)¶
Abstract base class for structures in big endian byte order.
- class ctypes.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 ctypes.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
_fields_class variable.ctypeswill create descriptors 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
c_int, a third optional item can be given. It must be a small positive integer defining the bit width of the field.Field names must be unique within one structure or union. This is not checked, only one field can be accessed when names are repeated.
It is possible to define the
_fields_class variable after the class statement that defines the Structure subclass, this allows creating data types that directly or indirectly reference themselves:class List(Structure): pass List._fields_ = [("pnext", POINTER(List)), ... ]
The
_fields_class variable can only be set once. Later assignments will raise anAttributeError.Additionally, the
_fields_class variable must be defined before the structure or union type is first used: an instance or subclass is created,sizeof()is called on it, and so on. Later assignments to_fields_will raise anAttributeError. 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
_fields_defined in the sub-subclass, if any.
- _pack_¶
An optional small integer that allows overriding the alignment of structure fields in the instance.
_pack_must already be defined when_fields_is assigned, otherwise it will have no effect. Setting this attribute to 0 is the same as not setting it at all.
- _align_¶
An optional small integer that allows overriding the alignment of the structure when being packed or unpacked to/from memory. Setting this attribute to 0 is the same as not setting it at all.
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,_pack_must be unset or zero.
If not set explicitly,
ctypeswill 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
_pack_is specified:"ms"Otherwise:
"gcc-sysv"
_layout_must already be defined when_fields_is assigned, otherwise it will have no effect.
- _anonymous_¶
An optional sequence that lists the names of unnamed (anonymous) fields.
_anonymous_must be already defined when_fields_is assigned, otherwise it will have no effect.The fields listed in this variable must be structure or union type fields.
ctypeswill 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
TYPEDESCstructure describes a COM data type, thevtfield specifies which one of the union fields is valid. Since theufield is defined as anonymous field, it is now possible to access the members directly off the TYPEDESC instance.td.lptdescandtd.u.lptdescare 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
_fields_variable, the fields specified in this are appended to the fields of the base class.Structure and union constructors accept both positional and keyword arguments. Positional arguments are used to initialize member fields in the same order as they are appear in
_fields_. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize_fields_with the same name, or create new attributes for names not present in_fields_.
Arrays and pointers¶
- class ctypes.Array(*args)¶
Abstract base class for arrays.
The recommended way to create concrete array types is by multiplying any
ctypesdata type with a non-negative integer. Alternatively, you can subclass this type and define_length_and_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 anArray.- _length_¶
A positive integer specifying the number of elements in the array. Out-of-range subscripts result in an
IndexError. Will be returned bylen().
- _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 actypesdata type and length an integer.This function is soft deprecated in favor of multiplication. There are no plans to remove it.
- class ctypes._Pointer¶
Private, abstract base class for pointers.
Concrete pointer types are created by calling
POINTER()with the type that will be pointed to; this is done automatically bypointer().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
len()will raiseTypeError. 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¶
- exception ctypes.ArgumentError¶
This exception is raised when a foreign function call cannot convert one of the passed arguments.
- exception ctypes.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
ProgIDfor the class or application that raised the error. helpfile is the path of the help file. helpcontext is the help context identifier. progid is theProgIDof the interface that defined the error.
Availability: Windows
Added in version 3.14.