:mod:`ctypes` --- A foreign function library for Python.
.. module:: ctypes :synopsis: A foreign function library for Python.
.. moduleauthor:: Thomas Heller <theller@python.net>
ctypes is a foreign function library for Python. It provides C compatible
data types, and allows calling functions in DLLs or shared libraries. It can be
used to wrap these libraries in pure Python.
Note: The code samples in this tutorial use doctest to make sure that they
actually work. Since some code samples behave differently under Linux, Windows,
or Mac OS X, they contain doctest directives in comments.
Note: Some code samples reference the ctypes :class:`c_int` type. This type is an alias for the :class:`c_long` type on 32-bit systems. So, you should not be confused if :class:`c_long` is printed if you would expect :class:`c_int` --- they are actually the same type.
ctypes 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 :class:`HRESULT` error code. The error
code is used to automatically raise a :class:`WindowsError` exception when
the function call fails.
Here are some examples for Windows. Note that msvcrt is the MS standard C
library containing most standard C functions, and uses the cdecl calling
convention:
>>> from ctypes import * >>> print(windll.kernel32) # doctest: +WINDOWS <WinDLL 'kernel32', handle ... at ...> >>> print(cdll.msvcrt) # doctest: +WINDOWS <CDLL 'msvcrt', handle ... at ...> >>> libc = cdll.msvcrt # doctest: +WINDOWS >>>
Windows appends the usual .dll file suffix automatically.
On Linux, it is required to specify the filename including the extension to load a library, so attribute access can not be used to load libraries. Either the :meth:`LoadLibrary` method of the dll loaders should be used, or you should load the library by creating an instance of CDLL by calling the constructor:
>>> cdll.LoadLibrary("libc.so.6") # doctest: +LINUX
<CDLL 'libc.so.6', handle ... at ...>
>>> libc = CDLL("libc.so.6") # doctest: +LINUX
>>> libc # doctest: +LINUX
<CDLL 'libc.so.6', handle ... at ...>
>>>
Functions are accessed as attributes of dll objects:
>>> from ctypes import *
>>> libc.printf
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.GetModuleHandleA) # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.MyOwnFunction) # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "ctypes.py", line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
>>>
Note that win32 system dlls like kernel32 and user32 often export ANSI
as well as UNICODE versions of a function. The UNICODE version is exported with
an W appended to the name, while the ANSI version is exported with an A
appended to the name. The win32 GetModuleHandle function, which returns a
module handle for a given module name, has the following C prototype, and a
macro is used to expose one of them as GetModuleHandle depending on whether
UNICODE is defined or not:
/* ANSI version */ HMODULE GetModuleHandleA(LPCSTR lpModuleName); /* UNICODE version */ HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
windll does not try to select one of them by magic, you must access the
version you need by specifying GetModuleHandleA or GetModuleHandleW
explicitly, and then call it with strings or unicode strings
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") # doctest: +WINDOWS <_FuncPtr object at 0x...> >>>
On Windows, some dlls export functions not by name but by ordinal. These functions can be accessed by indexing the dll object with the ordinal number:
>>> cdll.kernel32[1] # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0] # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "ctypes.py", line 310, in __getitem__
func = _StdcallFuncPtr(name, self)
AttributeError: function ordinal 0 not found
>>>
You can call these functions like any other Python callable. This example uses
the time() function, which returns system time in seconds since the Unix
epoch, and the GetModuleHandleA() function, which returns a win32 module
handle.
This example calls both functions with a NULL pointer (None should be used
as the NULL pointer):
>>> print(libc.time(None)) # doctest: +SKIP 1150640792 >>> print(hex(windll.kernel32.GetModuleHandleA(None))) # doctest: +WINDOWS 0x1d000000 >>>
ctypes tries to protect you from calling functions with the wrong number of
arguments or the wrong calling convention. Unfortunately this only works on
Windows. It does this by examining the stack after the function returns, so
although an error is raised the function has been called:
>>> windll.kernel32.GetModuleHandleA() # doctest: +WINDOWS Traceback (most recent call last): File "<stdin>", line 1, in ? ValueError: Procedure probably called with not enough arguments (4 bytes missing) >>> windll.kernel32.GetModuleHandleA(0, 0) # doctest: +WINDOWS Traceback (most recent call last): File "<stdin>", line 1, in ? ValueError: Procedure probably called with too many arguments (4 bytes in excess) >>>
The same exception is raised when you call an stdcall function with the
cdecl calling convention, or vice versa:
>>> cdll.kernel32.GetModuleHandleA(None) # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
>>>
>>> windll.msvcrt.printf("spam") # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
>>>
To find out the correct calling convention you have to look into the C header file or the documentation for the function you want to call.
On Windows, ctypes uses win32 structured exception handling to prevent
crashes from general protection faults when functions are called with invalid
argument values:
>>> windll.kernel32.GetModuleHandleA(32) # doctest: +WINDOWS Traceback (most recent call last): File "<stdin>", line 1, in ? WindowsError: exception: access violation reading 0x00000020 >>>
There are, however, enough ways to crash Python with ctypes, so you should
be careful anyway.
None, integers, byte strings 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, byte strings and unicode strings are
passed as pointer to the memory block that contains their data (char * or
wchar_t *). Python integers are passed as the platforms
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.
ctypes defines a number of primitive C compatible data types :
ctypes type C type Python type :class:`c_char` char1-character string :class:`c_wchar` wchar_t1-character unicode string :class:`c_byte` charint :class:`c_ubyte` unsigned charint :class:`c_short` shortint :class:`c_ushort` unsigned shortint :class:`c_int` intint :class:`c_uint` unsigned intint :class:`c_long` longint :class:`c_ulong` unsigned longint :class:`c_longlong` __int64orlong longint :class:`c_ulonglong` unsigned __int64orunsigned long longint :class:`c_float` floatfloat :class:`c_double` doublefloat :class:`c_longdouble` long doublefloat :class:`c_char_p` char *(NUL terminated)string or None:class:`c_wchar_p` wchar_t *(NUL terminated)unicode or None:class:`c_void_p` void *int or None
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_char_p("Hello, World")
c_char_p('Hello, World')
>>> c_ushort(-3)
c_ushort(65533)
>>>
Since these types are mutable, their value can also be changed afterwards:
>>> i = c_int(42) >>> print(i) c_long(42) >>> print(i.value) 42 >>> i.value = -99 >>> print(i.value) -99 >>>
Assigning a new value to instances of the pointer types :class:`c_char_p`, :class:`c_wchar_p`, and :class:`c_void_p` changes the memory location they point to, not the contents of the memory block (of course not, because Python strings are immutable):
>>> s = "Hello, World"
>>> c_s = c_char_p(s)
>>> print(c_s)
c_char_p('Hello, World')
>>> c_s.value = "Hi, there"
>>> print(c_s)
c_char_p('Hi, there')
>>> print(s) # first string 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 '\x00\x00\x00'
>>> p = create_string_buffer("Hello") # create a buffer containing a NUL terminated string
>>> print(sizeof(p), repr(p.raw))
6 'Hello\x00'
>>> print(repr(p.value))
'Hello'
>>> p = create_string_buffer("Hello", 10) # create a 10 byte buffer
>>> print(sizeof(p), repr(p.raw))
10 'Hello\x00\x00\x00\x00\x00'
>>> p.value = "Hi"
>>> print(sizeof(p), repr(p.raw))
10 'Hi\x00lo\x00\x00\x00\x00\x00'
>>>
The create_string_buffer function replaces the c_buffer function (which
is still available as an alias), as well as the c_string function from
earlier ctypes releases. To create a mutable memory block containing unicode
characters of the C type wchar_t use the create_unicode_buffer function.
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("Hello, %s\n", "World!")
Hello, World!
14
>>> printf("Hello, %S", u"World!")
Hello, World!
13
>>> printf("%d bottles of beer\n", 42)
42 bottles of beer
19
>>> printf("%f bottles of beer\n", 42.5)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: Don't know how to convert parameter 2
>>>
As has been mentioned before, all Python types except integers, strings, and
unicode strings have to be wrapped in their corresponding ctypes type, so
that they can be converted to the required C data type:
>>> printf("An int %d, a double %f\n", 1234, c_double(3.14))
Integer 1234, double 3.1400001049
31
>>>
You can also customize ctypes argument conversion to allow instances of your
own classes be used as function arguments. ctypes looks for an
:attr:`_as_parameter_` attribute and uses this as the function argument. Of
course, it must be one of integer, string, or unicode:
>>> class Bottles(object):
... def __init__(self, number):
... self._as_parameter_ = number
...
>>> bottles = Bottles(42)
>>> printf("%d bottles of beer\n", bottles)
42 bottles of beer
19
>>>
If you don't want to store the instance's data in the :attr:`_as_parameter_`
instance variable, you could define a property which makes the data
available.
It is possible to specify the required argument types of functions exported from DLLs by setting the :attr:`argtypes` attribute.
:attr:`argtypes` must be a sequence of C data types (the printf function is
probably not a good example here, because it takes a variable number and
different types of parameters depending on the format string, on the other hand
this is quite handy to experiment with this feature):
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf("String '%s', Int %d, Double %f\n", "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("%d %d %d", 1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: wrong type
>>> printf("%s %d %f", "X", 2, 3)
X 2 3.00000012
12
>>>
If you have defined your own classes which you pass to function calls, you have
to implement a :meth:`from_param` class method for them to be able to use them
in the :attr:`argtypes` sequence. The :meth:`from_param` class method receives
the Python object passed to the function call, it should do a typecheck or
whatever is needed to make sure this object is acceptable, and then return the
object itself, its :attr:`_as_parameter_` attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, unicode, a ctypes instance, or an object with an
:attr:`_as_parameter_` attribute.
By default functions are assumed to return the C int type. Other return
types can be specified by setting the :attr:`restype` attribute of the function
object.
Here is a more advanced example, it uses the strchr function, which expects
a string pointer and a char, and returns a pointer to a string:
>>> strchr = libc.strchr
>>> strchr("abcdef", ord("d")) # doctest: +SKIP
8059983
>>> strchr.restype = c_char_p # c_char_p is a pointer to a string
>>> strchr("abcdef", ord("d"))
'def'
>>> print(strchr("abcdef", ord("x")))
None
>>>
If you want to avoid the ord("x") calls above, you can set the
:attr:`argtypes` attribute, and the second argument will be converted from a
single character Python string into a C char:
>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> strchr("abcdef", "d")
'def'
>>> strchr("abcdef", "def")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: one character string expected
>>> print(strchr("abcdef", "x"))
None
>>> strchr("abcdef", "d")
'def'
>>>
You can also use a callable Python object (a function or a class for example) as
the :attr:`restype` attribute, if the foreign function returns an integer. The
callable will be called with the integer the C function returns, and the
result of this call will be used as the result of your function call. This is
useful to check for error return values and automatically raise an exception:
>>> GetModuleHandle = windll.kernel32.GetModuleHandleA # doctest: +WINDOWS
>>> def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
>>>
>>> GetModuleHandle.restype = ValidHandle # doctest: +WINDOWS
>>> GetModuleHandle(None) # doctest: +WINDOWS
486539264
>>> GetModuleHandle("something silly") # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in ValidHandle
WindowsError: [Errno 126] The specified module could not be found.
>>>
WinError is a function which will call Windows FormatMessage() api to
get the string representation of an error code, and returns an exception.
WinError takes an optional error code parameter, if no one is used, it calls
:func:`GetLastError` to retrieve it.
Please note that a much more powerful error checking mechanism is available through the :attr:`errcheck` attribute; see the reference manual for details.
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 :func:`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 :func:`byref` if you don't need the pointer
object in Python itself:
>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer('\000' * 32)
>>> print(i.value, f.value, repr(s.value))
0 0.0 ''
>>> libc.sscanf("1 3.14 Hello", "%d %f %s",
... byref(i), byref(f), s)
3
>>> print(i.value, f.value, repr(s.value))
1 3.1400001049 'Hello'
>>>
Structures and unions must derive from the :class:`Structure` and :class:`Union`
base classes which are defined in the ctypes module. Each subclass must
define a :attr:`_fields_` attribute. :attr:`_fields_` must be a list of
2-tuples, containing a field name and a field type.
The field type must be a ctypes type like :class:`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 ?
ValueError: too many initializers
>>>
You can, however, build much more complicated structures. Structures can itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft and
lowerright
>>> class RECT(Structure):
... _fields_ = [("upperleft", POINT),
... ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print(rc.upperleft.x, rc.upperleft.y)
0 5
>>> print(rc.lowerright.x, rc.lowerright.y)
0 0
>>>
Nested structures can also be initialized in the constructor in several ways:
>>> r = RECT(POINT(1, 2), POINT(3, 4)) >>> r = RECT((1, 2), (3, 4))
Field :term:`descriptor`s can be retrieved from the class, they are useful for debugging because they can provide useful information:
>>> print(POINT.x) <Field type=c_long, ofs=0, size=4> >>> print(POINT.y) <Field type=c_long, ofs=4, size=4> >>>
By default, Structure and Union fields are aligned in the same way the C
compiler does it. It is possible to override this behavior be specifying a
:attr:`_pack_` class attribute in the subclass definition. This must be set to a
positive integer and specifies the maximum alignment for the fields. This is
what #pragma pack(n) also does in MSVC.
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.
It is possible to create structures and unions containing bit fields. Bit fields are only possible for integer fields, the bit width is specified as the third item in the :attr:`_fields_` tuples:
>>> class Int(Structure):
... _fields_ = [("first_16", c_int, 16),
... ("second_16", c_int, 16)]
...
>>> print(Int.first_16)
<Field type=c_long, ofs=0:0, bits=16>
>>> print(Int.second_16)
<Field type=c_long, ofs=0:16, bits=16>
>>>
Arrays are sequences, containing a fixed number of instances of the same type.
The recommended way to create array types is by multiplying a data type with a positive integer:
TenPointsArrayType = POINT * 10
Here is an example of an somewhat artificial data type, a structure containing 4 POINTs among other stuff:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class MyStruct(Structure):
... _fields_ = [("a", c_int),
... ("b", c_float),
... ("point_array", POINT * 4)]
>>>
>>> print(len(MyStruct().point_array))
4
>>>
Instances are created in the usual way, by calling the class:
arr = TenPointsArrayType()
for pt in arr:
print(pt.x, pt.y)
The above code print a series of 0 0 lines, because the array contents is
initialized to zeros.
Initializers of the correct type can also be specified:
>>> from ctypes import * >>> TenIntegers = c_int * 10 >>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) >>> print(ii) <c_long_Array_10 object at 0x...> >>> for i in ii: print(i, end=" ") ... 1 2 3 4 5 6 7 8 9 10 >>>
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 :class:`c_int` instance to the pointer's contents attribute would cause the pointer to point to the memory location where this is stored:
>>> i = c_int(99) >>> pi.contents = i >>> pi.contents c_long(99) >>>
Pointer instances can also be indexed with integers:
>>> pi[0] 99 >>>
Assigning to an integer index changes the pointed to value:
>>> print(i) c_long(99) >>> pi[0] = 22 >>> print(i) c_long(22) >>>
It is also possible to use indexes different from 0, but you must know what you're doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.
Behind the scenes, the 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 ? 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
>>>
Usually, ctypes does strict type checking. This means, if you have
POINTER(c_int) in the :attr:`argtypes` list of a function or as the type of
a member field in a structure definition, only instances of exactly the same
type are accepted. There are some exceptions to this rule, where ctypes accepts
other objects. For example, you can pass compatible array instances instead of
pointer types. So, for POINTER(c_int), ctypes accepts an array of c_int:
>>> class Bar(Structure):
... _fields_ = [("count", c_int), ("values", POINTER(c_int))]
...
>>> bar = Bar()
>>> bar.values = (c_int * 3)(1, 2, 3)
>>> bar.count = 3
>>> for i in range(bar.count):
... print(bar.values[i])
...
1
2
3
>>>
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 :class:`c_int` arrays for its values field,
but not instances of other types:
>>> bar.values = (c_byte * 4)() Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance >>>
For these cases, the 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 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 {
char *name;
struct cell *next;
} cell;
The straightforward translation into ctypes code would be this, but it does not work:
>>> class cell(Structure):
... _fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 2, in cell
NameError: name 'cell' is not defined
>>>
because the new class cell is not available in the class statement itself.
In ctypes, we can define the cell class and set the :attr:`_fields_`
attribute later, after the class statement:
>>> from ctypes import *
>>> class cell(Structure):
... pass
...
>>> cell._fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
>>>
Lets try it. We create two instances of cell, and let them point to each
other, and finally follow the pointer chain a few times:
>>> c1 = cell() >>> c1.name = "foo" >>> c2 = cell() >>> c2.name = "bar" >>> c1.next = pointer(c2) >>> c2.next = pointer(c1) >>> p = c1 >>> for i in range(8): ... print(p.name, end=" ") ... p = p.next[0] ... foo bar foo bar foo bar foo bar >>>
ctypes allows to create C callable function pointers from Python callables.
These are sometimes called callback functions.
First, you must create a class for the callback function, the class knows the calling convention, the return type, and the number and types of arguments this function will receive.
The CFUNCTYPE factory function creates types for callback functions using the normal cdecl calling convention, and, 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 :func:`qsort` function, this is used to sort items with the help of a callback function. :func:`qsort` will be used to sort an array of integers:
>>> IntArray5 = c_int * 5 >>> ia = IntArray5(5, 1, 7, 33, 99) >>> qsort = libc.qsort >>> qsort.restype = None >>>
:func:`qsort` must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer else.
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)) >>>
For the first implementation of the callback function, we simply print the arguments we get, and return 0 (incremental development ;-):
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a, b)
... return 0
...
>>>
Create the C callable callback:
>>> cmp_func = CMPFUNC(py_cmp_func) >>>
And we're ready to go:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> >>>
We know how to access the contents of a pointer, so lets redefine our callback:
>>> def py_cmp_func(a, b):
... print("py_cmp_func", a[0], b[0])
... return 0
...
>>> cmp_func = CMPFUNC(py_cmp_func)
>>>
Here is what we get on Windows:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS py_cmp_func 7 1 py_cmp_func 33 1 py_cmp_func 99 1 py_cmp_func 5 1 py_cmp_func 7 5 py_cmp_func 33 5 py_cmp_func 99 5 py_cmp_func 7 99 py_cmp_func 33 99 py_cmp_func 7 33 >>>
It is funny to see that on linux the sort function seems to work much more efficient, it is doing less comparisons:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +LINUX py_cmp_func 5 1 py_cmp_func 33 99 py_cmp_func 7 33 py_cmp_func 5 7 py_cmp_func 1 7 >>>
Ah, we're nearly done! The last step is to 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]
...
>>>
Final run on Windows:
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +WINDOWS py_cmp_func 33 7 py_cmp_func 99 33 py_cmp_func 5 99 py_cmp_func 1 99 py_cmp_func 33 7 py_cmp_func 1 33 py_cmp_func 5 33 py_cmp_func 5 7 py_cmp_func 1 7 py_cmp_func 5 1 >>>
and on Linux:
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +LINUX py_cmp_func 5 1 py_cmp_func 33 99 py_cmp_func 7 33 py_cmp_func 1 7 py_cmp_func 5 7 >>>
It is quite interesting to see that the Windows :func:`qsort` function needs more comparisons than the linux version!
As we can easily check, our array is sorted now:
>>> for i in ia: print(i, end=" ") ... 1 5 7 33 99 >>>
Important note for callback functions:
Make sure you keep references to 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.
Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the Py_OptimizeFlag, an integer set
to 0, 1, or 2, depending on the :option:`-O` or :option:`-OO` flag given on
startup.
ctypes can access values like this with the :meth:`in_dll` class methods of
the type. pythonapi is a predefined symbol giving access to the Python C
api:
>>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag") >>> print(opt_flag) c_long(0) >>>
If the interpreter would have been started with :option:`-O`, the sample would
have printed c_long(1), or c_long(2) if :option:`-OO` would have been
specified.
An extended example which also demonstrates the use of pointers accesses the
PyImport_FrozenModules pointer exported by Python.
Quoting the Python docs: This pointer is initialized to point to an array of "struct _frozen" records, terminated by one whose members are all NULL or zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.
So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with ctypes:
>>> from ctypes import *
>>>
>>> class struct_frozen(Structure):
... _fields_ = [("name", c_char_p),
... ("code", POINTER(c_ubyte)),
... ("size", c_int)]
...
>>>
We have defined the struct _frozen data type, so we can get the pointer to
the table:
>>> FrozenTable = POINTER(struct_frozen) >>> table = FrozenTable.in_dll(pythonapi, "PyImport_FrozenModules") >>>
Since table is a pointer to the array of struct_frozen records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it's better to break out of the loop when we
hit the NULL entry:
>>> for item in table: ... print(item.name, item.size) ... if item.name is None: ... break ... __hello__ 104 __phello__ -104 __phello__.spam 104 None 0 >>>
The fact that standard Python has a frozen module and a frozen package
(indicated by the negative size member) is not well known, it is only used for
testing. Try it out with import __hello__ for example.
There are some edges in ctypes where you may be expect something else than
what actually happens.
Consider the following example:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class RECT(Structure):
... _fields_ = ("a", POINT), ("b", POINT)
...
>>> p1 = POINT(1, 2)
>>> p2 = POINT(3, 4)
>>> rc = RECT(p1, p2)
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
>>> # now swap the two points
>>> rc.a, rc.b = rc.b, rc.a
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
>>>
Hm. We certainly expected the last statement to print 3 4 1 2. What
happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a line above:
>>> temp0, temp1 = rc.b, rc.a >>> rc.a = temp0 >>> rc.b = temp1 >>>
Note that temp0 and temp1 are objects still using the internal buffer of
the rc object above. So executing rc.a = temp0 copies the buffer
contents of temp0 into rc 's buffer. This, in turn, changes the
contents of temp1. So, the last assignment rc.b = temp1, doesn't have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn't copy the sub-object, instead it retrieves a wrapper object accessing the root-object's underlying buffer.
Another example that may behave different from what one would expect is this:
>>> s = c_char_p() >>> s.value = "abc def ghi" >>> s.value 'abc def ghi' >>> s.value is s.value False >>>
Why is it printing False? ctypes instances are objects containing a memory
block plus some :term:`descriptor`s accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the contents of the object is stored. Accessing the contents again
constructs a new Python object each time!
ctypes provides some support for variable-sized arrays and structures (this
was added in version 0.9.9.7).
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.
When programming in a compiled language, shared libraries are accessed when compiling/linking a program, and when the program is run.
The purpose of the find_library function is to locate a library in a way
similar to what the compiler does (on platforms with several versions of a
shared library the most recent should be loaded), while the ctypes library
loaders act like when a program is run, and call the runtime loader directly.
The ctypes.util module provides a function which can help to determine the
library to load.
.. data:: find_library(name) :module: ctypes.util :noindex: Try to find a library and return a pathname. *name* is the library name without any prefix like *lib*, suffix like ``.so``, ``.dylib`` or version number (this is the form used for the posix linker option :option:`-l`). If no library can be found, returns ``None``.
The exact functionality is system dependent.
On Linux, find_library tries to run external programs (/sbin/ldconfig, gcc,
and objdump) to find the library file. It returns the filename of the library
file. Here are some examples:
>>> from ctypes.util import find_library
>>> find_library("m")
'libm.so.6'
>>> find_library("c")
'libc.so.6'
>>> find_library("bz2")
'libbz2.so.1.0'
>>>
On OS X, find_library tries several predefined naming schemes and paths to
locate the library, and returns a full pathname if successful:
>>> from ctypes.util import find_library
>>> find_library("c")
'/usr/lib/libc.dylib'
>>> find_library("m")
'/usr/lib/libm.dylib'
>>> find_library("bz2")
'/usr/lib/libbz2.dylib'
>>> find_library("AGL")
'/System/Library/Frameworks/AGL.framework/AGL'
>>>
On Windows, find_library searches along the system search path, and returns
the full pathname, but since there is no predefined naming scheme a call like
find_library("c") will fail and return None.
If wrapping a shared library with ctypes, it may be better to determine
the shared library name at development type, and hardcode that into the wrapper
module instead of using find_library to locate the library at runtime.
There are several ways to loaded shared libraries into the Python process. One way is to instantiate one of the following classes:
Instances of this class represent loaded shared libraries. Functions in these
libraries use the standard C calling convention, and are assumed to return
int.
Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the stdcall calling convention, and are
assumed to return the windows specific :class:`HRESULT` code. :class:`HRESULT`
values contain information specifying whether the function call failed or
succeeded, together with additional error code. If the return value signals a
failure, an :class:`WindowsError` is automatically raised.
Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the stdcall calling convention, and are
assumed to return int by default.
On Windows CE only the standard calling convention is used, for convenience the :class:`WinDLL` and :class:`OleDLL` use the standard calling convention on this platform.
The Python :term:`global interpreter lock` is released before calling any function exported by these libraries, and reacquired afterwards.
Instances of this class behave like :class:`CDLL` instances, except that the Python GIL is not released during the function call, and after the function execution the Python error flag is checked. If the error flag is set, a Python exception is raised.
Thus, this is only useful to call Python C api functions directly.
All these classes can be instantiated by calling them with at least one
argument, the pathname of the shared library. If you have an existing handle to
an already loaded shared library, it can be passed as the handle named
parameter, otherwise the underlying platforms dlopen or :meth:`LoadLibrary`
function is used to load the library into the process, and to get a handle to
it.
The mode parameter can be used to specify how the library is loaded. For
details, consult the dlopen(3) manpage, on Windows, mode is ignored.
The use_errno parameter, when set to True, enables a ctypes mechanism that
allows to access the system :data:`errno` error number in a safe way.
:mod:`ctypes` maintains a thread-local copy of the systems :data:`errno`
variable; if you call foreign functions created with use_errno=True then the
:data:`errno` value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.
The function :func:`ctypes.get_errno` returns the value of the ctypes private copy, and the function :func:`ctypes.set_errno` changes the ctypes private copy to a new value and returns the former value.
The use_last_error parameter, when set to True, enables the same mechanism for the Windows error code which is managed by the :func:`GetLastError` and :func:`SetLastError` Windows API functions; :func:`ctypes.get_last_error` and :func:`ctypes.set_last_error` are used to request and change the ctypes private copy of the windows error code.
.. data:: RTLD_GLOBAL :noindex: Flag to use as *mode* parameter. On platforms where this flag is not available, it is defined as the integer zero.
.. data:: RTLD_LOCAL :noindex: Flag to use as *mode* parameter. On platforms where this is not available, it is the same as *RTLD_GLOBAL*.
.. data:: DEFAULT_MODE :noindex: The default mode which is used to load shared libraries. On OSX 10.3, this is *RTLD_GLOBAL*, otherwise it is the same as *RTLD_LOCAL*.
Instances of these classes have no public methods, however :meth:`__getattr__` and :meth:`__getitem__` have special behavior: functions exported by the shared library can be accessed as attributes of by index. Please note that both :meth:`__getattr__` and :meth:`__getitem__` cache their result, so calling them repeatedly returns the same object each time.
The following public attributes are available, their name starts with an underscore to not clash with exported function names:
.. attribute:: PyDLL._handle The system handle used to access the library.
.. attribute:: PyDLL._name The name of the library passed in the constructor.
Shared libraries can also be loaded by using one of the prefabricated objects, which are instances of the :class:`LibraryLoader` class, either by calling the :meth:`LoadLibrary` method, or by retrieving the library as attribute of the loader instance.
Class which loads shared libraries. dlltype should be one of the
:class:`CDLL`, :class:`PyDLL`, :class:`WinDLL`, or :class:`OleDLL` types.
:meth:`__getattr__` has special behavior: It allows to load a shared library by accessing it as attribute of a library loader instance. The result is cached, so repeated attribute accesses return the same library each time.
.. method:: LoadLibrary(name) Load a shared library into the process and return it. This method always returns a new instance of the library.
These prefabricated library loaders are available:
.. data:: cdll :noindex: Creates :class:`CDLL` instances.
.. data:: windll :noindex: Windows only: Creates :class:`WinDLL` instances.
.. data:: oledll :noindex: Windows only: Creates :class:`OleDLL` instances.
.. data:: pydll :noindex: Creates :class:`PyDLL` instances.
For accessing the C Python api directly, a ready-to-use Python shared library object is available:
.. data:: pythonapi :noindex: An instance of :class:`PyDLL` that exposes Python C api functions as attributes. Note that all these functions are assumed to return C ``int``, which is of course not always the truth, so you have to assign the correct :attr:`restype` attribute to use these functions.
As explained in the previous section, foreign functions can be accessed as attributes of loaded shared libraries. The function objects created in this way by default accept any number of arguments, accept any ctypes data instances as arguments, and return the default result type specified by the library loader. They are instances of a private class:
Base class for C callable foreign functions.
Instances of foreign functions are also C compatible data types; they represent C function pointers.
This behavior can be customized by assigning to special attributes of the foreign function object.
.. attribute:: restype Assign a ctypes type to specify the result type of the foreign function. Use ``None`` for ``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 to do further processing or error checking. Using this is deprecated, for more flexible post processing or error checking use a ctypes data type as :attr:`restype` and assign a callable to the :attr:`errcheck` attribute.
.. attribute:: argtypes Assign a tuple of ctypes types to specify the argument types that the function accepts. Functions using the ``stdcall`` calling convention can only be called with the same number of arguments as the length of this tuple; functions using the C calling convention accept additional, unspecified arguments as well. When a foreign function is called, each actual argument is passed to the :meth:`from_param` class method of the items in the :attr:`argtypes` tuple, this method allows to adapt the actual argument to an object that the foreign function accepts. For example, a :class:`c_char_p` item in the :attr:`argtypes` tuple will convert a unicode string passed as argument into an byte string using ctypes conversion rules. New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a :meth:`from_param` method which returns a value usable as argument (integer, string, ctypes instance). This allows to define adapters that can adapt custom objects as function parameters.
.. attribute:: errcheck
Assign a Python function or another callable to this attribute. The
callable will be called with three or more arguments:
.. function:: callable(result, func, arguments)
:noindex:
``result`` is what the foreign function returns, as specified
by the :attr:`restype` attribute.
``func`` is the foreign function object itself, this allows
to reuse the same callable object to check or post process
the results of several functions.
``arguments`` is a tuple containing the parameters originally
passed to the function call, this allows to specialize the
behavior on the arguments used.
The object that this function returns will be returned from the
foreign function call, but it can also check the result value
and raise an exception if the foreign function call failed.
.. exception:: ArgumentError() This exception is raised when a foreign function call cannot convert one of the passed arguments.
Foreign functions can also be created by instantiating function prototypes. Function prototypes are similar to function prototypes in C; they describe a function (return type, argument types, calling convention) without defining an implementation. The factory functions must be called with the desired result type and the argument types of the function.
.. function:: CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False) The returned function prototype creates functions that use the standard C calling convention. The function will release the GIL during the call. If *use_errno* is set to True, the ctypes private copy of the system :data:`errno` variable is exchanged with the real :data:`errno` value bafore and after the call; *use_last_error* does the same for the Windows error code.
.. function:: WINFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False) Windows only: The returned function prototype creates functions that use the ``stdcall`` calling convention, except on Windows CE where :func:`WINFUNCTYPE` is the same as :func:`CFUNCTYPE`. The function will release the GIL during the call. *use_errno* and *use_last_error* have the same meaning as above.
.. function:: PYFUNCTYPE(restype, *argtypes) The returned function prototype creates functions that use the Python calling convention. The function will *not* release the GIL during the call.
Function prototypes created by these factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call:
.. function:: prototype(address) :noindex: :module: Returns a foreign function at the specified address which must be an integer... function:: prototype(callable) :noindex: :module: Create a C callable function (a callback function) from a Python ``callable``... function:: prototype(func_spec[, paramflags]) :noindex: :module: Returns a foreign function exported by a shared library. ``func_spec`` must be a 2-tuple ``(name_or_ordinal, library)``. The first item is the name of the exported function as string, or the ordinal of the exported function as small integer. The second item is the shared library instance... function:: prototype(vtbl_index, name[, paramflags[, iid]]) :noindex: :module: Returns a foreign function that will call a COM method. ``vtbl_index`` is the index into the virtual function table, a small non-negative integer. *name* is name of the COM method. *iid* is an optional pointer to the interface identifier which is used in extended error reporting. COM methods use a special calling convention: They require a pointer to the COM interface as first argument, in addition to those parameters that are specified in the :attr:`argtypes` tuple.The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.
paramflags must be a tuple of the same length as :attr:`argtypes`.
Each item in this tuple contains further information about a parameter, it must be a tuple containing one, two, or three items.
The first item is an integer containing a combination of direction flags for the parameter:
- 1
- Specifies an input parameter to the function.
- 2
- Output parameter. The foreign function fills in a value.
- 4
- Input parameter which defaults to the integer zero.
The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.
The optional third item is the default value for this parameter.
This example demonstrates how to wrap the Windows MessageBoxA function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this:
WINUSERAPI int WINAPI
MessageBoxA(
HWND hWnd ,
LPCSTR lpText,
LPCSTR lpCaption,
UINT uType);
Here is the wrapping with ctypes:
>>> from ctypes import c_int, WINFUNCTYPE, windll
>>> from ctypes.wintypes import HWND, LPCSTR, UINT
>>> prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
>>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", None), (1, "flags", 0)
>>> MessageBox = prototype(("MessageBoxA", windll.user32), paramflags)
>>>
The MessageBox foreign function can now be called in these ways:
>>> MessageBox() >>> MessageBox(text="Spam, spam, spam") >>> MessageBox(flags=2, text="foo bar") >>>
A second example demonstrates output parameters. The win32 GetWindowRect
function retrieves the dimensions of a specified window by copying them into
RECT structure that the caller has to supply. Here is the C declaration:
WINUSERAPI BOOL WINAPI
GetWindowRect(
HWND hWnd,
LPRECT lpRect);
Here is the wrapping with ctypes:
>>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
>>> from ctypes.wintypes import BOOL, HWND, RECT
>>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
>>> paramflags = (1, "hwnd"), (2, "lprect")
>>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)
>>>
Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.
Output parameters can be combined with the :attr:`errcheck` protocol to do
further output processing and error checking. The win32 GetWindowRect api
function returns a BOOL to signal success or failure, so this function could
do the error checking, and raises an exception when the api call failed:
>>> def errcheck(result, func, args): ... if not result: ... raise WinError() ... return args ... >>> GetWindowRect.errcheck = errcheck >>>
If the :attr:`errcheck` function returns the argument tuple it receives
unchanged, ctypes continues the normal processing it does on the output
parameters. If you want to return a tuple of window coordinates instead of a
RECT instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place:
>>> def errcheck(result, func, args): ... if not result: ... raise WinError() ... rc = args[1] ... return rc.left, rc.top, rc.bottom, rc.right ... >>> GetWindowRect.errcheck = errcheck >>>
.. function:: addressof(obj) Returns the address of the memory buffer as integer. ``obj`` must be an instance of a ctypes type.
.. function:: alignment(obj_or_type) Returns the alignment requirements of a ctypes type. ``obj_or_type`` must be a ctypes type or instance.
.. function:: byref(obj[, offset])
Returns a light-weight pointer to ``obj``, which must be an
instance of a ctypes type. ``offset`` defaults to zero, and must be
an integer that will be added to the internal pointer value.
``byref(obj, offset)`` corresponds to this C code::
(((char *)&obj) + offset)
The returned object can only be used as a foreign function call
parameter. It behaves similar to ``pointer(obj)``, but the
construction is a lot faster.
.. function:: cast(obj, type) This function is similar to the cast operator in C. It returns a new instance of ``type`` which points to the same memory block as ``obj``. ``type`` must be a pointer type, and ``obj`` must be an object that can be interpreted as a pointer.
.. function:: create_string_buffer(init_or_size[, size]) This function creates a mutable character buffer. The returned object is a ctypes array of :class:`c_char`. ``init_or_size`` must be an integer which specifies the size of the array, or a string which will be used to initialize the array items. If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows to specify the size of the array if the length of the string should not be used. If the first parameter is a unicode string, it is converted into an 8-bit string according to ctypes conversion rules.
.. function:: create_unicode_buffer(init_or_size[, size]) This function creates a mutable unicode character buffer. The returned object is a ctypes array of :class:`c_wchar`. ``init_or_size`` must be an integer which specifies the size of the array, or a unicode string which will be used to initialize the array items. If a unicode string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows to specify the size of the array if the length of the string should not be used. If the first parameter is a 8-bit string, it is converted into an unicode string according to ctypes conversion rules.
.. function:: DllCanUnloadNow() Windows only: This function is a hook which allows to implement in-process COM servers with ctypes. It is called from the DllCanUnloadNow function that the _ctypes extension dll exports.
.. function:: DllGetClassObject() Windows only: This function is a hook which allows to implement in-process COM servers with ctypes. It is called from the DllGetClassObject function that the ``_ctypes`` extension dll exports.
.. function:: find_library(name) :module: ctypes.util Try to find a library and return a pathname. *name* is the library name without any prefix like ``lib``, suffix like ``.so``, ``.dylib`` or version number (this is the form used for the posix linker option :option:`-l`). If no library can be found, returns ``None``. The exact functionality is system dependent.
.. function:: find_msvcrt() :module: ctypes.util Windows only: return the filename of the VC runtype library used by Python, and by the extension modules. If the name of the library cannot be determined, ``None`` is returned. If you need to free memory, for example, allocated by an extension module with a call to the ``free(void *)``, it is important that you use the function in the same library that allocated the memory.
.. function:: FormatError([code]) Windows only: Returns a textual description of the error code. If no error code is specified, the last error code is used by calling the Windows api function GetLastError.
.. function:: GetLastError() Windows only: Returns the last error code set by Windows in the calling thread. This function calls the Windows `GetLastError()` function directly, it does not return the ctypes-private copy of the error code.
.. function:: get_errno() Returns the current value of the ctypes-private copy of the system :data:`errno` variable in the calling thread.
.. function:: get_last_error() Windows only: returns the current value of the ctypes-private copy of the system :data:`LastError` variable in the calling thread.
.. function:: memmove(dst, src, count) Same as the standard C memmove library function: copies *count* bytes from ``src`` to *dst*. *dst* and ``src`` must be integers or ctypes instances that can be converted to pointers.
.. function:: memset(dst, c, count) Same as the standard C memset library function: fills the memory block at address *dst* with *count* bytes of value *c*. *dst* must be an integer specifying an address, or a ctypes instance.
.. function:: POINTER(type) This factory function creates and returns a new ctypes pointer type. Pointer types are cached an reused internally, so calling this function repeatedly is cheap. type must be a ctypes type.
.. function:: pointer(obj) This function creates a new pointer instance, pointing to ``obj``. The returned object is of the type POINTER(type(obj)). Note: If you just want to pass a pointer to an object to a foreign function call, you should use ``byref(obj)`` which is much faster.
.. function:: resize(obj, size) This function resizes the internal memory buffer of obj, which must be an instance of a ctypes type. It is not possible to make the buffer smaller than the native size of the objects type, as given by sizeof(type(obj)), but it is possible to enlarge the buffer.
.. function:: set_conversion_mode(encoding, errors)
This function sets the rules that ctypes objects use when converting between
8-bit strings and unicode strings. encoding must be a string specifying an
encoding, like ``'utf-8'`` or ``'mbcs'``, errors must be a string specifying the
error handling on encoding/decoding errors. Examples of possible values are
``"strict"``, ``"replace"``, or ``"ignore"``.
``set_conversion_mode`` returns a 2-tuple containing the previous conversion
rules. On windows, the initial conversion rules are ``('mbcs', 'ignore')``, on
other systems ``('ascii', 'strict')``.
.. function:: set_errno(value) Set the current value of the ctypes-private copy of the system :data:`errno` variable in the calling thread to *value* and return the previous value.
.. function:: set_last_error(value) Windows only: set the current value of the ctypes-private copy of the system :data:`LastError` variable in the calling thread to *value* and return the previous value.
.. function:: sizeof(obj_or_type) Returns the size in bytes of a ctypes type or instance memory buffer. Does the same as the C ``sizeof()`` function.
.. function:: string_at(address[, size]) This function returns the string starting at memory address address. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated.
.. function:: WinError(code=None, descr=None) Windows only: this function is probably the worst-named thing in ctypes. It creates an instance of WindowsError. If *code* is not specified, ``GetLastError`` is called to determine the error code. If ``descr`` is not specified, :func:`FormatError` is called to get a textual description of the error.
.. function:: wstring_at(address) This function returns the wide character string starting at memory address ``address`` as unicode string. If ``size`` is specified, it is used as the number of characters of the string, otherwise the string is assumed to be zero-terminated.
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
:attr:`_objects`; this contains other Python objects that need to be kept alive
in case the memory block contains pointers.
Common methods of ctypes data types, these are all class methods (to be exact, they are methods of the :term:`metaclass`):
.. method:: _CData.from_buffer(source[, offset]) This method returns a ctypes instance that shares the buffer of the ``source`` object. The ``source`` object must support the writeable buffer interface. The optional ``offset`` parameter specifies an offset into the source buffer in bytes; the default is zero. If the source buffer is not large enough a ValueError is raised.
.. method:: _CData.from_buffer_copy(source[, offset]) This method creates a ctypes instance, copying the buffer from the source object buffer which must be readable. The optional ``offset`` parameter specifies an offset into the source buffer in bytes; the default is zero. If the source buffer is not large enough a ValueError is raised.
.. method:: from_address(address) This method returns a ctypes type instance using the memory specified by address which must be an integer.
.. method:: from_param(obj) This method adapts *obj* to a ctypes type. It is called with the actual object used in a foreign function call when the type is present in the foreign function's :attr:`argtypes` tuple; it must return an object that can be used as a function call parameter. All ctypes data types have a default implementation of this classmethod that normally returns ``obj`` if that is an instance of the type. Some types accept other objects as well.
.. method:: in_dll(library, name) This method returns a ctypes type instance exported by a shared library. *name* is the name of the symbol that exports the data, *library* is the loaded shared library.
Common instance variables of ctypes data types:
.. attribute:: _b_base_ Sometimes ctypes data instances do not own the memory block they contain, instead they share part of the memory block of a base object. The :attr:`_b_base_` read-only member is the root ctypes object that owns the memory block.
.. attribute:: _b_needsfree_ This read-only variable is true when the ctypes data instance has allocated the memory block itself, false otherwise.
.. attribute:: _objects This member is either ``None`` or a dictionary containing Python objects that need to be kept alive so that the memory block contents is kept valid. This object is only exposed for debugging; never modify the contents of this dictionary.
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 _CData,
so it inherits their methods and attributes. ctypes data types that are not
and do not contain pointers can now be pickled.
Instances have a single attribute:
.. attribute:: value This attribute contains the actual value of the instance. For integer and pointer types, it is an integer, for character types, it is a single character string, for character pointer types it is a Python string or unicode 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.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a :attr:`restype` of :class:`c_char_p`, you will always receive a Python string, not a :class:`c_char_p` instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions :attr:`restype` is a subclass of :class:`c_void_p`, you will
receive an instance of this subclass from the function call. Of course, you can
get the value of the pointer by accessing the value attribute.
These are the fundamental ctypes data types:
Represents the C signed char datatype, and interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.
Represents the C char datatype, and interprets the value as a single character. The constructor accepts an optional string initializer, the length of the string must be exactly one character.
Represents the C char * datatype, which must be a pointer to a zero-terminated string. The constructor accepts an integer address, or a string.
Represents the C double datatype. The constructor accepts an optional float initializer.
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 to :class:`c_double`.
Represents the C float datatype. The constructor accepts an optional float initializer.
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 to :class:`c_long`.
Represents the C 8-bit signed int datatype. Usually an alias for
:class:`c_byte`.
Represents the C 16-bit signed int datatype. Usually an alias for :class:`c_short`.
Represents the C 32-bit signed int datatype. Usually an alias for :class:`c_int`.
Represents the C 64-bit signed int datatype. Usually an alias for
:class:`c_longlong`.
Represents the C signed long datatype. The constructor accepts an optional
integer initializer; no overflow checking is done.
Represents the C signed long long datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
Represents the C signed short datatype. The constructor accepts an optional
integer initializer; no overflow checking is done.
Represents the C size_t datatype.
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.
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 for :class:`c_ulong`.
Represents the C 8-bit unsigned int datatype. Usually an alias for :class:`c_ubyte`.
Represents the C 16-bit unsigned int datatype. Usually an alias for :class:`c_ushort`.
Represents the C 32-bit unsigned int datatype. Usually an alias for :class:`c_uint`.
Represents the C 64-bit unsigned int datatype. Usually an alias for :class:`c_ulonglong`.
Represents the C unsigned long datatype. The constructor accepts an optional
integer initializer; no overflow checking is done.
Represents the C unsigned long long datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
Represents the C unsigned short datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
Represents the C void * type. The value is represented as integer. The
constructor accepts an optional integer initializer.
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.
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.
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.
Windows only: Represents a :class:`HRESULT` value, which contains success or error information for a function or method call.
Represents the C PyObject * datatype. Calling this without an argument
creates a NULL PyObject * 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.
Abstract base class for unions in native byte order.
Abstract base class for structures in big endian byte order.
Abstract base class for structures in little endian byte order.
Structures with non-native byte order cannot contain pointer type fields, or any other data types containing pointer type fields.
Abstract base class for structures in native byte order.
Concrete structure and union types must be created by subclassing one of these
types, and at least define a :attr:`_fields_` class variable. ctypes will
create :term:`descriptor`s which allow reading and writing the fields by direct
attribute accesses. These are the
.. attribute:: _fields_
A sequence defining the structure fields. The items must be 2-tuples or
3-tuples. The first item is the name of the field, the second item
specifies the type of the field; it can be any ctypes data type.
For integer type fields like :class:`c_int`, a third optional item can be
given. It must be a small positive integer defining the bit width of the
field.
Field names must be unique within one structure or union. This is not
checked, only one field can be accessed when names are repeated.
It is possible to define the :attr:`_fields_` class variable *after* the
class statement that defines the Structure subclass, this allows to create
data types that directly or indirectly reference themselves::
class List(Structure):
pass
List._fields_ = [("pnext", POINTER(List)),
...
]
The :attr:`_fields_` class variable must, however, be defined before the
type is first used (an instance is created, ``sizeof()`` is called on it,
and so on). Later assignments to the :attr:`_fields_` class variable will
raise an AttributeError.
Structure and union subclass constructors accept both positional and named
arguments. Positional arguments are used to initialize the fields in the
same order as they appear in the :attr:`_fields_` definition, named
arguments are used to initialize the fields with the corresponding name.
It is possible to defined sub-subclasses of structure types, they inherit
the fields of the base class plus the :attr:`_fields_` defined in the
sub-subclass, if any.
.. attribute:: _pack_ An optional small integer that allows to override the alignment of structure fields in the instance. :attr:`_pack_` must already be defined when :attr:`_fields_` is assigned, otherwise it will have no effect.
.. attribute:: _anonymous_
An optional sequence that lists the names of unnamed (anonymous) fields.
``_anonymous_`` must be already defined when :attr:`_fields_` is assigned,
otherwise it will have no effect.
The fields listed in this variable must be structure or union type fields.
``ctypes`` will create descriptors in the structure type that allows to
access the nested fields directly, without the need to create the
structure or union field.
Here is an example type (Windows)::
class _U(Union):
_fields_ = [("lptdesc", POINTER(TYPEDESC)),
("lpadesc", POINTER(ARRAYDESC)),
("hreftype", HREFTYPE)]
class TYPEDESC(Structure):
_anonymous_ = ("u",)
_fields_ = [("u", _U),
("vt", VARTYPE)]
The ``TYPEDESC`` structure describes a COM data type, the ``vt`` field
specifies which one of the union fields is valid. Since the ``u`` field
is defined as anonymous field, it is now possible to access the members
directly off the TYPEDESC instance. ``td.lptdesc`` and ``td.u.lptdesc``
are equivalent, but the former is faster since it does not need to create
a temporary union instance::
td = TYPEDESC()
td.vt = VT_PTR
td.lptdesc = POINTER(some_type)
td.u.lptdesc = POINTER(some_type)
It is possible to defined sub-subclasses of structures, they inherit the fields of the base class. If the subclass definition has a separate :attr:`_fields_` variable, the fields specified in this are appended to the fields of the base class.
Structure and union constructors accept both positional and keyword arguments. Positional arguments are used to initialize member fields in the same order as they are appear in :attr:`_fields_`. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize :attr:`_fields_` with the same name, or create new attributes for names not present in :attr:`_fields_`.
Not yet written - please see the sections :ref:`ctypes-pointers` and section :ref:`ctypes-arrays` in the tutorial.