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:mod:`ctypes` --- A foreign function library for Python

:mod:`ctypes` is a foreign function library for Python. It provides C compatible data types, and allows calling functions in DLLs or shared libraries. It can be used to wrap these libraries in pure Python.

ctypes tutorial

Note: The code samples in this tutorial use :mod:`doctest` to make sure that they actually work. Since some code samples behave differently under Linux, Windows, or Mac OS X, they contain doctest directives in comments.

Note: Some code samples reference the ctypes :class:`c_int` type. This type is an alias for the :class:`c_long` type on 32-bit systems. So, you should not be confused if :class:`c_long` is printed if you would expect :class:`c_int` --- they are actually the same type.

Accessing functions from loaded dlls

Functions are accessed as attributes of dll objects:

>>> from ctypes import *
>>> libc.printf
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.GetModuleHandleA) # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>> print(windll.kernel32.MyOwnFunction) # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "ctypes.py", line 239, in __getattr__
    func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
>>>

Note that win32 system dlls like kernel32 and user32 often export ANSI as well as UNICODE versions of a function. The UNICODE version is exported with an W appended to the name, while the ANSI version is exported with an A appended to the name. The win32 GetModuleHandle function, which returns a module handle for a given module name, has the following C prototype, and a macro is used to expose one of them as GetModuleHandle depending on whether UNICODE is defined or not:

/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);

windll does not try to select one of them by magic, you must access the version you need by specifying GetModuleHandleA or GetModuleHandleW explicitly, and then call it with bytes or string objects respectively.

Sometimes, dlls export functions with names which aren't valid Python identifiers, like "??2@YAPAXI@Z". In this case you have to use :func:`getattr` to retrieve the function:

>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z") # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>>

On Windows, some dlls export functions not by name but by ordinal. These functions can be accessed by indexing the dll object with the ordinal number:

>>> cdll.kernel32[1] # doctest: +WINDOWS
<_FuncPtr object at 0x...>
>>> cdll.kernel32[0] # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "ctypes.py", line 310, in __getitem__
    func = _StdcallFuncPtr(name, self)
AttributeError: function ordinal 0 not found
>>>

Calling functions

You can call these functions like any other Python callable. This example uses the time() function, which returns system time in seconds since the Unix epoch, and the GetModuleHandleA() function, which returns a win32 module handle.

This example calls both functions with a NULL pointer (None should be used as the NULL pointer):

>>> print(libc.time(None)) # doctest: +SKIP
1150640792
>>> print(hex(windll.kernel32.GetModuleHandleA(None))) # doctest: +WINDOWS
0x1d000000
>>>

:mod:`ctypes` tries to protect you from calling functions with the wrong number of arguments or the wrong calling convention. Unfortunately this only works on Windows. It does this by examining the stack after the function returns, so although an error is raised the function has been called:

>>> windll.kernel32.GetModuleHandleA() # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
>>> windll.kernel32.GetModuleHandleA(0, 0) # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
>>>

The same exception is raised when you call an stdcall function with the cdecl calling convention, or vice versa:

>>> cdll.kernel32.GetModuleHandleA(None) # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
>>>

>>> windll.msvcrt.printf(b"spam") # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
>>>

To find out the correct calling convention you have to look into the C header file or the documentation for the function you want to call.

On Windows, :mod:`ctypes` uses win32 structured exception handling to prevent crashes from general protection faults when functions are called with invalid argument values:

>>> windll.kernel32.GetModuleHandleA(32) # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OSError: exception: access violation reading 0x00000020
>>>

There are, however, enough ways to crash Python with :mod:`ctypes`, so you should be careful anyway.

None, integers, bytes objects and (unicode) strings are the only native Python objects that can directly be used as parameters in these function calls. None is passed as a C NULL pointer, bytes objects and strings are passed as pointer to the memory block that contains their data (:c:type:`char *` or :c:type:`wchar_t *`). Python integers are passed as the platforms default C :c:type:`int` type, their value is masked to fit into the C type.

Before we move on calling functions with other parameter types, we have to learn more about :mod:`ctypes` data types.

Fundamental data types

:mod:`ctypes` defines a number of primitive C compatible data types :

ctypes type C type Python type
:class:`c_bool` :c:type:`_Bool` bool (1)
:class:`c_char` :c:type:`char` 1-character bytes object
:class:`c_wchar` :c:type:`wchar_t` 1-character string
:class:`c_byte` :c:type:`char` int
:class:`c_ubyte` :c:type:`unsigned char` int
:class:`c_short` :c:type:`short` int
:class:`c_ushort` :c:type:`unsigned short` int
:class:`c_int` :c:type:`int` int
:class:`c_uint` :c:type:`unsigned int` int
:class:`c_long` :c:type:`long` int
:class:`c_ulong` :c:type:`unsigned long` int
:class:`c_longlong` :c:type:`__int64` or :c:type:`long long` int
:class:`c_ulonglong` :c:type:`unsigned __int64` or :c:type:`unsigned long long` int
:class:`c_size_t` :c:type:`size_t` int
:class:`c_ssize_t` :c:type:`ssize_t` or :c:type:`Py_ssize_t` int
:class:`c_float` :c:type:`float` float
:class:`c_double` :c:type:`double` float
:class:`c_longdouble` :c:type:`long double` float
:class:`c_char_p` :c:type:`char *` (NUL terminated) bytes object or None
:class:`c_wchar_p` :c:type:`wchar_t *` (NUL terminated) string or None
:class:`c_void_p` :c:type:`void *` int or None
  1. The constructor accepts any object with a truth value.

All these types can be created by calling them with an optional initializer of the correct type and value:

>>> c_int()
c_long(0)
>>> c_wchar_p("Hello, World")
c_wchar_p('Hello, World')
>>> c_ushort(-3)
c_ushort(65533)
>>>

Since these types are mutable, their value can also be changed afterwards:

>>> i = c_int(42)
>>> print(i)
c_long(42)
>>> print(i.value)
42
>>> i.value = -99
>>> print(i.value)
-99
>>>

Assigning a new value to instances of the pointer types :class:`c_char_p`, :class:`c_wchar_p`, and :class:`c_void_p` changes the memory location they point to, not the contents of the memory block (of course not, because Python bytes objects are immutable):

>>> s = "Hello, World"
>>> c_s = c_wchar_p(s)
>>> print(c_s)
c_wchar_p('Hello, World')
>>> c_s.value = "Hi, there"
>>> print(c_s)
c_wchar_p('Hi, there')
>>> print(s)                 # first object is unchanged
Hello, World
>>>

You should be careful, however, not to pass them to functions expecting pointers to mutable memory. If you need mutable memory blocks, ctypes has a :func:`create_string_buffer` function which creates these in various ways. The current memory block contents can be accessed (or changed) with the raw property; if you want to access it as NUL terminated string, use the value property:

>>> from ctypes import *
>>> p = create_string_buffer(3)            # create a 3 byte buffer, initialized to NUL bytes
>>> print(sizeof(p), repr(p.raw))
3 b'\x00\x00\x00'
>>> p = create_string_buffer(b"Hello")     # create a buffer containing a NUL terminated string
>>> print(sizeof(p), repr(p.raw))
6 b'Hello\x00'
>>> print(repr(p.value))
b'Hello'
>>> p = create_string_buffer(b"Hello", 10) # create a 10 byte buffer
>>> print(sizeof(p), repr(p.raw))
10 b'Hello\x00\x00\x00\x00\x00'
>>> p.value = b"Hi"
>>> print(sizeof(p), repr(p.raw))
10 b'Hi\x00lo\x00\x00\x00\x00\x00'
>>>

The :func:`create_string_buffer` function replaces the :func:`c_buffer` function (which is still available as an alias), as well as the :func:`c_string` function from earlier ctypes releases. To create a mutable memory block containing unicode characters of the C type :c:type:`wchar_t` use the :func:`create_unicode_buffer` function.

Calling functions, continued

Note that printf prints to the real standard output channel, not to :data:`sys.stdout`, so these examples will only work at the console prompt, not from within IDLE or PythonWin:

>>> printf = libc.printf
>>> printf(b"Hello, %s\n", b"World!")
Hello, World!
14
>>> printf(b"Hello, %S\n", "World!")
Hello, World!
14
>>> printf(b"%d bottles of beer\n", 42)
42 bottles of beer
19
>>> printf(b"%f bottles of beer\n", 42.5)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: Don't know how to convert parameter 2
>>>

As has been mentioned before, all Python types except integers, strings, and bytes objects have to be wrapped in their corresponding :mod:`ctypes` type, so that they can be converted to the required C data type:

>>> printf(b"An int %d, a double %f\n", 1234, c_double(3.14))
An int 1234, a double 3.140000
31
>>>

Calling functions with your own custom data types

You can also customize :mod:`ctypes` argument conversion to allow instances of your own classes be used as function arguments. :mod:`ctypes` looks for an :attr:`_as_parameter_` attribute and uses this as the function argument. Of course, it must be one of integer, string, or bytes:

>>> class Bottles:
...     def __init__(self, number):
...         self._as_parameter_ = number
...
>>> bottles = Bottles(42)
>>> printf(b"%d bottles of beer\n", bottles)
42 bottles of beer
19
>>>

If you don't want to store the instance's data in the :attr:`_as_parameter_` instance variable, you could define a :class:`property` which makes the attribute available on request.

Specifying the required argument types (function prototypes)

It is possible to specify the required argument types of functions exported from DLLs by setting the :attr:`argtypes` attribute.

:attr:`argtypes` must be a sequence of C data types (the printf function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):

>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
String 'Hi', Int 10, Double 2.200000
37
>>>

Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:

>>> printf(b"%d %d %d", 1, 2, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: wrong type
>>> printf(b"%s %d %f\n", b"X", 2, 3)
X 2 3.000000
13
>>>

If you have defined your own classes which you pass to function calls, you have to implement a :meth:`from_param` class method for them to be able to use them in the :attr:`argtypes` sequence. The :meth:`from_param` class method receives the Python object passed to the function call, it should do a typecheck or whatever is needed to make sure this object is acceptable, and then return the object itself, its :attr:`_as_parameter_` attribute, or whatever you want to pass as the C function argument in this case. Again, the result should be an integer, string, bytes, a :mod:`ctypes` instance, or an object with an :attr:`_as_parameter_` attribute.

Return types

By default functions are assumed to return the C :c:type:`int` type. Other return types can be specified by setting the :attr:`restype` attribute of the function object.

Here is a more advanced example, it uses the strchr function, which expects a string pointer and a char, and returns a pointer to a string:

>>> strchr = libc.strchr
>>> strchr(b"abcdef", ord("d")) # doctest: +SKIP
8059983
>>> strchr.restype = c_char_p   # c_char_p is a pointer to a string
>>> strchr(b"abcdef", ord("d"))
b'def'
>>> print(strchr(b"abcdef", ord("x")))
None
>>>

If you want to avoid the ord("x") calls above, you can set the :attr:`argtypes` attribute, and the second argument will be converted from a single character Python bytes object into a C char:

>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> strchr(b"abcdef", b"d")
'def'
>>> strchr(b"abcdef", b"def")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: one character string expected
>>> print(strchr(b"abcdef", b"x"))
None
>>> strchr(b"abcdef", b"d")
'def'
>>>

You can also use a callable Python object (a function or a class for example) as the :attr:`restype` attribute, if the foreign function returns an integer. The callable will be called with the integer the C function returns, and the result of this call will be used as the result of your function call. This is useful to check for error return values and automatically raise an exception:

>>> GetModuleHandle = windll.kernel32.GetModuleHandleA # doctest: +WINDOWS
>>> def ValidHandle(value):
...     if value == 0:
...         raise WinError()
...     return value
...
>>>
>>> GetModuleHandle.restype = ValidHandle # doctest: +WINDOWS
>>> GetModuleHandle(None) # doctest: +WINDOWS
486539264
>>> GetModuleHandle("something silly") # doctest: +WINDOWS
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in ValidHandle
OSError: [Errno 126] The specified module could not be found.
>>>

WinError is a function which will call Windows FormatMessage() api to get the string representation of an error code, and returns an exception. WinError takes an optional error code parameter, if no one is used, it calls :func:`GetLastError` to retrieve it.

Please note that a much more powerful error checking mechanism is available through the :attr:`errcheck` attribute; see the reference manual for details.

Passing pointers (or: passing parameters by reference)

Sometimes a C api function expects a pointer to a data type as parameter, probably to write into the corresponding location, or if the data is too large to be passed by value. This is also known as passing parameters by reference.

:mod:`ctypes` exports the :func:`byref` function which is used to pass parameters by reference. The same effect can be achieved with the :func:`pointer` function, although :func:`pointer` does a lot more work since it constructs a real pointer object, so it is faster to use :func:`byref` if you don't need the pointer object in Python itself:

>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer(b'\000' * 32)
>>> print(i.value, f.value, repr(s.value))
0 0.0 b''
>>> libc.sscanf(b"1 3.14 Hello", b"%d %f %s",
...             byref(i), byref(f), s)
3
>>> print(i.value, f.value, repr(s.value))
1 3.1400001049 b'Hello'
>>>

Structures and unions

Structures and unions must derive from the :class:`Structure` and :class:`Union` base classes which are defined in the :mod:`ctypes` module. Each subclass must define a :attr:`_fields_` attribute. :attr:`_fields_` must be a list of 2-tuples, containing a field name and a field type.

The field type must be a :mod:`ctypes` type like :class:`c_int`, or any other derived :mod:`ctypes` type: structure, union, array, pointer.

Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:

>>> from ctypes import *
>>> class POINT(Structure):
...     _fields_ = [("x", c_int),
...                 ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print(point.x, point.y)
10 20
>>> point = POINT(y=5)
>>> print(point.x, point.y)
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: too many initializers
>>>

You can, however, build much more complicated structures. A structure can itself contain other structures by using a structure as a field type.

Here is a RECT structure which contains two POINTs named upperleft and lowerright:

>>> class RECT(Structure):
...     _fields_ = [("upperleft", POINT),
...                 ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print(rc.upperleft.x, rc.upperleft.y)
0 5
>>> print(rc.lowerright.x, rc.lowerright.y)
0 0
>>>

Nested structures can also be initialized in the constructor in several ways:

>>> r = RECT(POINT(1, 2), POINT(3, 4))
>>> r = RECT((1, 2), (3, 4))

Field :term:`descriptor`s can be retrieved from the class, they are useful for debugging because they can provide useful information:

>>> print(POINT.x)
<Field type=c_long, ofs=0, size=4>
>>> print(POINT.y)
<Field type=c_long, ofs=4, size=4>
>>>

Warning

:mod:`ctypes` does not support passing unions or structures with bit-fields to functions by value. While this may work on 32-bit x86, it's not guaranteed by the library to work in the general case. Unions and structures with bit-fields should always be passed to functions by pointer.

Structure/union alignment and byte order

By default, Structure and Union fields are aligned in the same way the C compiler does it. It is possible to override this behavior be specifying a :attr:`_pack_` class attribute in the subclass definition. This must be set to a positive integer and specifies the maximum alignment for the fields. This is what #pragma pack(n) also does in MSVC.

:mod:`ctypes` uses the native byte order for Structures and Unions. To build structures with non-native byte order, you can use one of the :class:`BigEndianStructure`, :class:`LittleEndianStructure`, :class:`BigEndianUnion`, and :class:`LittleEndianUnion` base classes. These classes cannot contain pointer fields.

Bit fields in structures and unions

It is possible to create structures and unions containing bit fields. Bit fields are only possible for integer fields, the bit width is specified as the third item in the :attr:`_fields_` tuples:

>>> class Int(Structure):
...     _fields_ = [("first_16", c_int, 16),
...                 ("second_16", c_int, 16)]
...
>>> print(Int.first_16)
<Field type=c_long, ofs=0:0, bits=16>
>>> print(Int.second_16)
<Field type=c_long, ofs=0:16, bits=16>
>>>

Arrays

Arrays are sequences, containing a fixed number of instances of the same type.

The recommended way to create array types is by multiplying a data type with a positive integer:

TenPointsArrayType = POINT * 10

Here is an example of an somewhat artificial data type, a structure containing 4 POINTs among other stuff:

>>> from ctypes import *
>>> class POINT(Structure):
...    _fields_ = ("x", c_int), ("y", c_int)
...
>>> class MyStruct(Structure):
...    _fields_ = [("a", c_int),
...                ("b", c_float),
...                ("point_array", POINT * 4)]
>>>
>>> print(len(MyStruct().point_array))
4
>>>

Instances are created in the usual way, by calling the class:

arr = TenPointsArrayType()
for pt in arr:
    print(pt.x, pt.y)

The above code print a series of 0 0 lines, because the array contents is initialized to zeros.

Initializers of the correct type can also be specified:

>>> from ctypes import *
>>> TenIntegers = c_int * 10
>>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
>>> print(ii)
<c_long_Array_10 object at 0x...>
>>> for i in ii: print(i, end=" ")
...
1 2 3 4 5 6 7 8 9 10
>>>

Pointers

Pointer instances are created by calling the :func:`pointer` function on a :mod:`ctypes` type:

>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>

Pointer instances have a :attr:`contents` attribute which returns the object to which the pointer points, the i object above:

>>> pi.contents
c_long(42)
>>>

Note that :mod:`ctypes` does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:

>>> pi.contents is i
False
>>> pi.contents is pi.contents
False
>>>

Assigning another :class:`c_int` instance to the pointer's contents attribute would cause the pointer to point to the memory location where this is stored:

>>> i = c_int(99)
>>> pi.contents = i
>>> pi.contents
c_long(99)
>>>

Pointer instances can also be indexed with integers:

>>> pi[0]
99
>>>

Assigning to an integer index changes the pointed to value:

>>> print(i)
c_long(99)
>>> pi[0] = 22
>>> print(i)
c_long(22)
>>>

It is also possible to use indexes different from 0, but you must know what you're doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.

Behind the scenes, the :func:`pointer` function does more than simply create pointer instances, it has to create pointer types first. This is done with the :func:`POINTER` function, which accepts any :mod:`ctypes` type, and returns a new type:

>>> PI = POINTER(c_int)
>>> PI
<class 'ctypes.LP_c_long'>
>>> PI(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: expected c_long instead of int
>>> PI(c_int(42))
<ctypes.LP_c_long object at 0x...>
>>>

Calling the pointer type without an argument creates a NULL pointer. NULL pointers have a False boolean value:

>>> null_ptr = POINTER(c_int)()
>>> print(bool(null_ptr))
False
>>>

:mod:`ctypes` checks for NULL when dereferencing pointers (but dereferencing invalid non-NULL pointers would crash Python):

>>> null_ptr[0]
Traceback (most recent call last):
    ....
ValueError: NULL pointer access
>>>

>>> null_ptr[0] = 1234
Traceback (most recent call last):
    ....
ValueError: NULL pointer access
>>>

Type conversions

Usually, ctypes does strict type checking. This means, if you have POINTER(c_int) in the :attr:`argtypes` list of a function or as the type of a member field in a structure definition, only instances of exactly the same type are accepted. There are some exceptions to this rule, where ctypes accepts other objects. For example, you can pass compatible array instances instead of pointer types. So, for POINTER(c_int), ctypes accepts an array of c_int:

>>> class Bar(Structure):
...     _fields_ = [("count", c_int), ("values", POINTER(c_int))]
...
>>> bar = Bar()
>>> bar.values = (c_int * 3)(1, 2, 3)
>>> bar.count = 3
>>> for i in range(bar.count):
...     print(bar.values[i])
...
1
2
3
>>>

In addition, if a function argument is explicitly declared to be a pointer type (such as POINTER(c_int)) in :attr:`argtypes`, an object of the pointed type (c_int in this case) can be passed to the function. ctypes will apply the required :func:`byref` conversion in this case automatically.

To set a POINTER type field to NULL, you can assign None:

>>> bar.values = None
>>>

Sometimes you have instances of incompatible types. In C, you can cast one type into another type. :mod:`ctypes` provides a :func:`cast` function which can be used in the same way. The Bar structure defined above accepts POINTER(c_int) pointers or :class:`c_int` arrays for its values field, but not instances of other types:

>>> bar.values = (c_byte * 4)()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
>>>

For these cases, the :func:`cast` function is handy.

The :func:`cast` function can be used to cast a ctypes instance into a pointer to a different ctypes data type. :func:`cast` takes two parameters, a ctypes object that is or can be converted to a pointer of some kind, and a ctypes pointer type. It returns an instance of the second argument, which references the same memory block as the first argument:

>>> a = (c_byte * 4)()
>>> cast(a, POINTER(c_int))
<ctypes.LP_c_long object at ...>
>>>

So, :func:`cast` can be used to assign to the values field of Bar the structure:

>>> bar = Bar()
>>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
>>> print(bar.values[0])
0
>>>

Incomplete Types

Incomplete Types are structures, unions or arrays whose members are not yet specified. In C, they are specified by forward declarations, which are defined later:

struct cell; /* forward declaration */

struct cell {
    char *name;
    struct cell *next;
};

The straightforward translation into ctypes code would be this, but it does not work:

>>> class cell(Structure):
...     _fields_ = [("name", c_char_p),
...                 ("next", POINTER(cell))]
...
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in cell
NameError: name 'cell' is not defined
>>>

because the new class cell is not available in the class statement itself. In :mod:`ctypes`, we can define the cell class and set the :attr:`_fields_` attribute later, after the class statement:

>>> from ctypes import *
>>> class cell(Structure):
...     pass
...
>>> cell._fields_ = [("name", c_char_p),
...                  ("next", POINTER(cell))]
>>>

Lets try it. We create two instances of cell, and let them point to each other, and finally follow the pointer chain a few times:

>>> c1 = cell()
>>> c1.name = "foo"
>>> c2 = cell()
>>> c2.name = "bar"
>>> c1.next = pointer(c2)
>>> c2.next = pointer(c1)
>>> p = c1
>>> for i in range(8):
...     print(p.name, end=" ")
...     p = p.next[0]
...
foo bar foo bar foo bar foo bar
>>>

Callback functions

:mod:`ctypes` allows to create C callable function pointers from Python callables. These are sometimes called callback functions.

First, you must create a class for the callback function. The class knows the calling convention, the return type, and the number and types of arguments this function will receive.

The :func:`CFUNCTYPE` factory function creates types for callback functions using the cdecl calling convention. On Windows, the :func:`WINFUNCTYPE` factory function creates types for callback functions using the stdcall calling convention.

Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.

I will present an example here which uses the standard C library's :c:func:`qsort` function, that is used to sort items with the help of a callback function. :c:func:`qsort` will be used to sort an array of integers:

>>> IntArray5 = c_int * 5
>>> ia = IntArray5(5, 1, 7, 33, 99)
>>> qsort = libc.qsort
>>> qsort.restype = None
>>>

:func:`qsort` must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer otherwise.

So our callback function receives pointers to integers, and must return an integer. First we create the type for the callback function:

>>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
>>>

To get started, here is a simple callback that shows the values it gets passed:

>>> def py_cmp_func(a, b):
...     print("py_cmp_func", a[0], b[0])
...     return 0
...
>>> cmp_func = CMPFUNC(py_cmp_func)
>>>

The result:

>>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +LINUX
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 5 7
py_cmp_func 1 7
>>>

Now we can actually compare the two items and return a useful result:

>>> def py_cmp_func(a, b):
...     print("py_cmp_func", a[0], b[0])
...     return a[0] - b[0]
...
>>>
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +LINUX
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
>>>

As we can easily check, our array is sorted now:

>>> for i in ia: print(i, end=" ")
...
1 5 7 33 99
>>>

Important note for callback functions:

Make sure you keep references to :func:`CFUNCTYPE` objects as long as they are used from C code. :mod:`ctypes` doesn't, and if you don't, they may be garbage collected, crashing your program when a callback is made.

Accessing values exported from dlls

Some shared libraries not only export functions, they also export variables. An example in the Python library itself is the :c:data:`Py_OptimizeFlag`, an integer set to 0, 1, or 2, depending on the :option:`-O` or :option:`-OO` flag given on startup.

:mod:`ctypes` can access values like this with the :meth:`in_dll` class methods of the type. pythonapi is a predefined symbol giving access to the Python C api:

>>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag")
>>> print(opt_flag)
c_long(0)
>>>

If the interpreter would have been started with :option:`-O`, the sample would have printed c_long(1), or c_long(2) if :option:`-OO` would have been specified.

An extended example which also demonstrates the use of pointers accesses the :c:data:`PyImport_FrozenModules` pointer exported by Python.

Quoting the docs for that value:

This pointer is initialized to point to an array of :c:type:`struct _frozen` records, terminated by one whose members are all NULL or zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.

So manipulating this pointer could even prove useful. To restrict the example size, we show only how this table can be read with :mod:`ctypes`:

>>> from ctypes import *
>>>
>>> class struct_frozen(Structure):
...     _fields_ = [("name", c_char_p),
...                 ("code", POINTER(c_ubyte)),
...                 ("size", c_int)]
...
>>>

We have defined the :c:type:`struct _frozen` data type, so we can get the pointer to the table:

>>> FrozenTable = POINTER(struct_frozen)
>>> table = FrozenTable.in_dll(pythonapi, "PyImport_FrozenModules")
>>>

Since table is a pointer to the array of struct_frozen records, we can iterate over it, but we just have to make sure that our loop terminates, because pointers have no size. Sooner or later it would probably crash with an access violation or whatever, so it's better to break out of the loop when we hit the NULL entry:

>>> for item in table:
...    print(item.name, item.size)
...    if item.name is None:
...        break
...
__hello__ 104
__phello__ -104
__phello__.spam 104
None 0
>>>

The fact that standard Python has a frozen module and a frozen package (indicated by the negative size member) is not well known, it is only used for testing. Try it out with import __hello__ for example.

Surprises

There are some edges in :mod:`ctypes` where you might expect something other than what actually happens.

Consider the following example:

>>> from ctypes import *
>>> class POINT(Structure):
...     _fields_ = ("x", c_int), ("y", c_int)
...
>>> class RECT(Structure):
...     _fields_ = ("a", POINT), ("b", POINT)
...
>>> p1 = POINT(1, 2)
>>> p2 = POINT(3, 4)
>>> rc = RECT(p1, p2)
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
>>> # now swap the two points
>>> rc.a, rc.b = rc.b, rc.a
>>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
>>>

Hm. We certainly expected the last statement to print 3 4 1 2. What happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a line above:

>>> temp0, temp1 = rc.b, rc.a
>>> rc.a = temp0
>>> rc.b = temp1
>>>

Note that temp0 and temp1 are objects still using the internal buffer of the rc object above. So executing rc.a = temp0 copies the buffer contents of temp0 into rc 's buffer. This, in turn, changes the contents of temp1. So, the last assignment rc.b = temp1, doesn't have the expected effect.

Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays doesn't copy the sub-object, instead it retrieves a wrapper object accessing the root-object's underlying buffer.

Another example that may behave different from what one would expect is this:

>>> s = c_char_p()
>>> s.value = "abc def ghi"
>>> s.value
'abc def ghi'
>>> s.value is s.value
False
>>>

Why is it printing False? ctypes instances are objects containing a memory block plus some :term:`descriptor`s accessing the contents of the memory. Storing a Python object in the memory block does not store the object itself, instead the contents of the object is stored. Accessing the contents again constructs a new Python object each time!

Variable-sized data types

:mod:`ctypes` provides some support for variable-sized arrays and structures.

The :func:`resize` function can be used to resize the memory buffer of an existing ctypes object. The function takes the object as first argument, and the requested size in bytes as the second argument. The memory block cannot be made smaller than the natural memory block specified by the objects type, a :exc:`ValueError` is raised if this is tried:

>>> short_array = (c_short * 4)()
>>> print(sizeof(short_array))
8
>>> resize(short_array, 4)
Traceback (most recent call last):
    ...
ValueError: minimum size is 8
>>> resize(short_array, 32)
>>> sizeof(short_array)
32
>>> sizeof(type(short_array))
8
>>>

This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:

>>> short_array[:]
[0, 0, 0, 0]
>>> short_array[7]
Traceback (most recent call last):
    ...
IndexError: invalid index
>>>

Another way to use variable-sized data types with :mod:`ctypes` is to use the dynamic nature of Python, and (re-)define the data type after the required size is already known, on a case by case basis.

ctypes reference

Finding shared libraries

When programming in a compiled language, shared libraries are accessed when compiling/linking a program, and when the program is run.

The purpose of the :func:`find_library` function is to locate a library in a way similar to what the compiler does (on platforms with several versions of a shared library the most recent should be loaded), while the ctypes library loaders act like when a program is run, and call the runtime loader directly.

The :mod:`ctypes.util` module provides a function which can help to determine the library to load.

The exact functionality is system dependent.

On Linux, :func:`find_library` tries to run external programs (/sbin/ldconfig, gcc, and objdump) to find the library file. It returns the filename of the library file. Here are some examples:

>>> from ctypes.util import find_library
>>> find_library("m")
'libm.so.6'
>>> find_library("c")
'libc.so.6'
>>> find_library("bz2")
'libbz2.so.1.0'
>>>

On OS X, :func:`find_library` tries several predefined naming schemes and paths to locate the library, and returns a full pathname if successful:

>>> from ctypes.util import find_library
>>> find_library("c")
'/usr/lib/libc.dylib'
>>> find_library("m")
'/usr/lib/libm.dylib'
>>> find_library("bz2")
'/usr/lib/libbz2.dylib'
>>> find_library("AGL")
'/System/Library/Frameworks/AGL.framework/AGL'
>>>

On Windows, :func:`find_library` searches along the system search path, and returns the full pathname, but since there is no predefined naming scheme a call like find_library("c") will fail and return None.

If wrapping a shared library with :mod:`ctypes`, it may be better to determine the shared library name at development time, and hardcode that into the wrapper module instead of using :func:`find_library` to locate the library at runtime.

Loading shared libraries

There are several ways to loaded shared libraries into the Python process. One way is to instantiate one of the following classes:

Instances of this class represent loaded shared libraries. Functions in these libraries use the standard C calling convention, and are assumed to return :c:type:`int`.

Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the stdcall calling convention, and are assumed to return the windows specific :class:`HRESULT` code. :class:`HRESULT` values contain information specifying whether the function call failed or succeeded, together with additional error code. If the return value signals a failure, an :class:`OSError` is automatically raised.

Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the stdcall calling convention, and are assumed to return :c:type:`int` by default.

On Windows CE only the standard calling convention is used, for convenience the :class:`WinDLL` and :class:`OleDLL` use the standard calling convention on this platform.

The Python :term:`global interpreter lock` is released before calling any function exported by these libraries, and reacquired afterwards.

Instances of this class behave like :class:`CDLL` instances, except that the Python GIL is not released during the function call, and after the function execution the Python error flag is checked. If the error flag is set, a Python exception is raised.

Thus, this is only useful to call Python C api functions directly.

All these classes can be instantiated by calling them with at least one argument, the pathname of the shared library. If you have an existing handle to an already loaded shared library, it can be passed as the handle named parameter, otherwise the underlying platforms dlopen or LoadLibrary function is used to load the library into the process, and to get a handle to it.

The mode parameter can be used to specify how the library is loaded. For details, consult the :manpage:`dlopen(3)` manpage, on Windows, mode is ignored.

The use_errno parameter, when set to True, enables a ctypes mechanism that allows to access the system :data:`errno` error number in a safe way. :mod:`ctypes` maintains a thread-local copy of the systems :data:`errno` variable; if you call foreign functions created with use_errno=True then the :data:`errno` value before the function call is swapped with the ctypes private copy, the same happens immediately after the function call.

The function :func:`ctypes.get_errno` returns the value of the ctypes private copy, and the function :func:`ctypes.set_errno` changes the ctypes private copy to a new value and returns the former value.

The use_last_error parameter, when set to True, enables the same mechanism for the Windows error code which is managed by the :func:`GetLastError` and :func:`SetLastError` Windows API functions; :func:`ctypes.get_last_error` and :func:`ctypes.set_last_error` are used to request and change the ctypes private copy of the windows error code.

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:

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.

These prefabricated library loaders are available:

For accessing the C Python api directly, a ready-to-use Python shared library object is available:

Foreign functions

As explained in the previous section, foreign functions can be accessed as attributes of loaded shared libraries. The function objects created in this way by default accept any number of arguments, accept any ctypes data instances as arguments, and return the default result type specified by the library loader. They are instances of a private class:

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.

Function prototypes

Foreign functions can also be created by instantiating function prototypes. Function prototypes are similar to function prototypes in C; they describe a function (return type, argument types, calling convention) without defining an implementation. The factory functions must be called with the desired result type and the argument types of the function.

Function prototypes created by these factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call:

The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.

paramflags must be a tuple of the same length as :attr:`argtypes`.

Each item in this tuple contains further information about a parameter, it must be a tuple containing one, two, or three items.

The first item is an integer containing a combination of direction flags for the parameter:

1
Specifies an input parameter to the function.
2
Output parameter. The foreign function fills in a value.
4
Input parameter which defaults to the integer zero.

The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.

The optional third item is the default value for this parameter.

This example demonstrates how to wrap the Windows MessageBoxA function so that it supports default parameters and named arguments. The C declaration from the windows header file is this:

WINUSERAPI int WINAPI
MessageBoxA(
    HWND hWnd ,
    LPCSTR lpText,
    LPCSTR lpCaption,
    UINT uType);

Here is the wrapping with :mod:`ctypes`:

>>> from ctypes import c_int, WINFUNCTYPE, windll
>>> from ctypes.wintypes import HWND, LPCSTR, UINT
>>> prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
>>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", None), (1, "flags", 0)
>>> MessageBox = prototype(("MessageBoxA", windll.user32), paramflags)
>>>

The MessageBox foreign function can now be called in these ways:

>>> MessageBox()
>>> MessageBox(text="Spam, spam, spam")
>>> MessageBox(flags=2, text="foo bar")
>>>

A second example demonstrates output parameters. The win32 GetWindowRect function retrieves the dimensions of a specified window by copying them into RECT structure that the caller has to supply. Here is the C declaration:

WINUSERAPI BOOL WINAPI
GetWindowRect(
     HWND hWnd,
     LPRECT lpRect);

Here is the wrapping with :mod:`ctypes`:

>>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
>>> from ctypes.wintypes import BOOL, HWND, RECT
>>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
>>> paramflags = (1, "hwnd"), (2, "lprect")
>>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)
>>>

Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.

Output parameters can be combined with the :attr:`errcheck` protocol to do further output processing and error checking. The win32 GetWindowRect api function returns a BOOL to signal success or failure, so this function could do the error checking, and raises an exception when the api call failed:

>>> def errcheck(result, func, args):
...     if not result:
...         raise WinError()
...     return args
...
>>> GetWindowRect.errcheck = errcheck
>>>

If the :attr:`errcheck` function returns the argument tuple it receives unchanged, :mod:`ctypes` continues the normal processing it does on the output parameters. If you want to return a tuple of window coordinates instead of a RECT instance, you can retrieve the fields in the function and return them instead, the normal processing will no longer take place:

>>> def errcheck(result, func, args):
...     if not result:
...         raise WinError()
...     rc = args[1]
...     return rc.left, rc.top, rc.bottom, rc.right
...
>>> GetWindowRect.errcheck = errcheck
>>>

Utility functions

Data types

This non-public class is the common base class of all ctypes data types. Among other things, all ctypes type instances contain a memory block that hold C compatible data; the address of the memory block is returned by the :func:`addressof` helper function. Another instance variable is exposed as :attr:`_objects`; this contains other Python objects that need to be kept alive in case the memory block contains pointers.

Common methods of ctypes data types, these are all class methods (to be exact, they are methods of the :term:`metaclass`):

Common instance variables of ctypes data types:

Fundamental data types

This non-public class is the base class of all fundamental ctypes data types. It is mentioned here because it contains the common attributes of the fundamental ctypes data types. :class:`_SimpleCData` is a subclass of :class:`_CData`, so it inherits their methods and attributes. ctypes data types that are not and do not contain pointers can now be pickled.

Instances have a single attribute:

Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a :attr:`restype` of :class:`c_char_p`, you will always receive a Python bytes object, not a :class:`c_char_p` instance.

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 :c:type:`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 :c:type:`char` datatype, and interprets the value as a single character. The constructor accepts an optional string initializer, the length of the string must be exactly one character.

Represents the C :c:type:`char *` datatype when it points to a zero-terminated string. For a general character pointer that may also point to binary data, POINTER(c_char) must be used. The constructor accepts an integer address, or a bytes object.

Represents the C :c:type:`double` datatype. The constructor accepts an optional float initializer.

Represents the C :c:type:`long double` datatype. The constructor accepts an optional float initializer. On platforms where sizeof(long double) == sizeof(double) it is an alias to :class:`c_double`.

Represents the C :c:type:`float` datatype. The constructor accepts an optional float initializer.

Represents the C :c:type:`signed int` datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where sizeof(int) == sizeof(long) it is an alias to :class:`c_long`.

Represents the C 8-bit :c:type:`signed int` datatype. Usually an alias for :class:`c_byte`.

Represents the C 16-bit :c:type:`signed int` datatype. Usually an alias for :class:`c_short`.

Represents the C 32-bit :c:type:`signed int` datatype. Usually an alias for :class:`c_int`.

Represents the C 64-bit :c:type:`signed int` datatype. Usually an alias for :class:`c_longlong`.

Represents the C :c:type:`signed long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`signed long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`signed short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`size_t` datatype.

Represents the C :c:type:`ssize_t` datatype.

Represents the C :c:type:`unsigned char` datatype, it interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`unsigned int` datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where sizeof(int) == sizeof(long) it is an alias for :class:`c_ulong`.

Represents the C 8-bit :c:type:`unsigned int` datatype. Usually an alias for :class:`c_ubyte`.

Represents the C 16-bit :c:type:`unsigned int` datatype. Usually an alias for :class:`c_ushort`.

Represents the C 32-bit :c:type:`unsigned int` datatype. Usually an alias for :class:`c_uint`.

Represents the C 64-bit :c:type:`unsigned int` datatype. Usually an alias for :class:`c_ulonglong`.

Represents the C :c:type:`unsigned long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`unsigned long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`unsigned short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

Represents the C :c:type:`void *` type. The value is represented as integer. The constructor accepts an optional integer initializer.

Represents the C :c:type:`wchar_t` datatype, and interprets the value as a single character unicode string. The constructor accepts an optional string initializer, the length of the string must be exactly one character.

Represents the C :c:type:`wchar_t *` datatype, which must be a pointer to a zero-terminated wide character string. The constructor accepts an integer address, or a string.

Represent the C :c:type:`bool` datatype (more accurately, :c:type:`_Bool` from C99). Its value can be True or False, and the constructor accepts any object that has a truth value.

Windows only: Represents a :c:type:`HRESULT` value, which contains success or error information for a function or method call.

Represents the C :c:type:`PyObject *` datatype. Calling this without an argument creates a NULL :c:type:`PyObject *` pointer.

The :mod:`ctypes.wintypes` module provides quite some other Windows specific data types, for example :c:type:`HWND`, :c:type:`WPARAM`, or :c:type:`DWORD`. Some useful structures like :c:type:`MSG` or :c:type:`RECT` are also defined.

Structured data types

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. :mod:`ctypes` will create :term:`descriptor`s which allow reading and writing the fields by direct attribute accesses. These are the

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_`.

Arrays and pointers

Not yet written - please see the sections :ref:`ctypes-pointers` and section :ref:`ctypes-arrays` in the tutorial.