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

.. module:: ctypes
   :synopsis: A foreign function library for Python.
.. moduleauthor:: Thomas Heller <theller@python.net>


.. versionadded:: 2.5

``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-ctypes-tutorial:

ctypes tutorial
---------------

Note: The code samples in this tutorial use ``doctest`` to make sure that they
actually work.  Since some code samples behave differently under Linux, Windows,
or 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-loading-dynamic-link-libraries:

Loading dynamic link libraries
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``ctypes`` exports the *cdll*, and on Windows *windll* and *oledll*
objects, for loading dynamic link libraries.

You load libraries by accessing them as attributes of these objects. *cdll*
loads libraries which export functions using the standard ``cdecl`` calling
convention, while *windll* libraries call functions using the ``stdcall``
calling convention. *oledll* also uses the ``stdcall`` calling convention, and
assumes the functions return a Windows :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 ...>
   >>>

.. XXX Add section for Mac OS X.


.. _ctypes-accessing-functions-from-loaded-dlls:

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 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
   >>>


.. _ctypes-calling-functions:

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
   >>>

``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, longs, 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 and Python longs 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-fundamental-data-types:

Fundamental data types
^^^^^^^^^^^^^^^^^^^^^^

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

   +----------------------+--------------------------------+----------------------------+
   | ctypes type          | C type                         | Python type                |
   +======================+================================+============================+
   | :class:`c_char`      | ``char``                       | 1-character string         |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_wchar`     | ``wchar_t``                    | 1-character unicode string |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_byte`      | ``char``                       | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_ubyte`     | ``unsigned char``              | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_short`     | ``short``                      | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_ushort`    | ``unsigned short``             | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_int`       | ``int``                        | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_uint`      | ``unsigned int``               | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_long`      | ``long``                       | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_ulong`     | ``unsigned long``              | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_longlong`  | ``__int64`` or ``long long``   | int/long                   |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_ulonglong` | ``unsigned __int64`` or        | int/long                   |
   |                      | ``unsigned long long``         |                            |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_float`     | ``float``                      | float                      |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_double`    | ``double``                     | float                      |
   +----------------------+--------------------------------+----------------------------+
   | :class:`c_longdouble`| ``long double``                | float                      |
   +----------------------+--------------------------------+----------------------------+
   | :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/long 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.


.. _ctypes-calling-functions-continued:

Calling functions, continued
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

   >>> printf = libc.printf
   >>> printf("Hello, %s\n", "World!")
   Hello, World!
   14
   >>> printf("Hello, %S\n", u"World!")
   Hello, World!
   14
   >>> 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))
   An int 1234, a double 3.140000
   31
   >>>


.. _ctypes-calling-functions-with-own-custom-data-types:

Calling functions with your own custom data types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can also customize ``ctypes`` argument conversion to allow instances of your
own classes be used as function arguments. ``ctypes`` looks for an
: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.


.. _ctypes-specifying-required-argument-types:

Specifying the required argument types (function prototypes)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is possible to specify the required argument types of functions exported from
DLLs by setting the :attr:`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\n", "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, unicode, a ``ctypes`` instance, or an object with an
:attr:`_as_parameter_` attribute.


.. _ctypes-return-types:

Return types
^^^^^^^^^^^^

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.


.. _ctypes-passing-pointers:

Passing pointers (or: passing parameters by reference)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

``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'
   >>>


.. _ctypes-structures-unions:

Structures and unions
^^^^^^^^^^^^^^^^^^^^^

Structures and unions must derive from the :class:`Structure` and :class:`Union`
base classes which are defined in the ``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>
   >>>


.. _ctypes-structureunion-alignment-byte-order:

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.

``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.


.. _ctypes-bit-fields-in-structures-unions:

Bit fields in structures and unions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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


.. _ctypes-arrays:

Arrays
^^^^^^

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

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

   TenPointsArrayType = POINT * 10

Here is an example of 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,
   ...
   1 2 3 4 5 6 7 8 9 10
   >>>


.. _ctypes-pointers:

Pointers
^^^^^^^^

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

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

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

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

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

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

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

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

.. XXX Document dereferencing pointers, and that it is preferred over the .contents attribute.

Pointer instances can also be indexed with integers::

   >>> pi[0]
   99
   >>>

Assigning to an integer index changes the pointed to value::

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

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

Behind the scenes, the ``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
   >>>


.. _ctypes-type-conversions:

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
   >>>

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

   >>> bar.values = None
   >>>

.. XXX list other conversions...

Sometimes you have instances of incompatible types.  In C, you can cast one
type into another type.  ``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
   >>>


.. _ctypes-incomplete-types:

Incomplete Types
^^^^^^^^^^^^^^^^

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

   struct cell; /* forward declaration */

   struct {
       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,
   ...     p = p.next[0]
   ...
   foo bar foo bar foo bar foo bar
   >>>


.. _ctypes-callback-functions:

Callback functions
^^^^^^^^^^^^^^^^^^

``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,
   ...
   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.


.. _ctypes-accessing-values-exported-from-dlls:

Accessing values exported from dlls
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the ``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.


.. _ctypes-surprises:

Surprises
^^^^^^^^^

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-variable-sized-data-types:

Variable-sized data types
^^^^^^^^^^^^^^^^^^^^^^^^^

``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.


.. _ctypes-ctypes-reference:

ctypes reference
----------------


.. _ctypes-finding-shared-libraries:

Finding shared libraries
^^^^^^^^^^^^^^^^^^^^^^^^

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

The 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.


.. _ctypes-loading-shared-libraries:

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:


.. class:: CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)

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


.. class:: OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)

   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.


.. class:: WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)

   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.


.. class:: PyDLL(name, mode=DEFAULT_MODE, handle=None)

   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.

.. versionadded:: 2.6
   The ``use_last_error`` and ``use_errno`` optional parameters
   were added.

.. 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…