"ctypes" — A foreign function library for Python
************************************************

======================================================================

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


ctypes tutorial
===============

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

Note: Some code samples reference the ctypes "c_int" type.  On
platforms where "sizeof(long) == sizeof(int)" it is an alias to
"c_long". So, you should not be confused if "c_long" is printed if you
would expect "c_int" — they are actually the same type.


Loading dynamic link libraries
------------------------------

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

You load libraries by accessing them as attributes of these objects.
*cdll* loads libraries which export functions using the standard
"cdecl" calling convention, while *windll* libraries call functions
using the "stdcall" calling convention. *oledll* also uses the
"stdcall" calling convention, and assumes the functions return a
Windows "HRESULT" error code. The error code is used to automatically
raise an "OSError" exception when the function call fails.

Changed in version 3.3: Windows errors used to raise "WindowsError",
which is now an alias of "OSError".

Here are some examples for Windows. Note that "msvcrt" is the MS
standard C library containing most standard C functions, and uses the
cdecl calling convention:

   >>> from ctypes import *
   >>> print(windll.kernel32)  
   <WinDLL 'kernel32', handle ... at ...>
   >>> print(cdll.msvcrt)      
   <CDLL 'msvcrt', handle ... at ...>
   >>> libc = cdll.msvcrt      
   >>>

Windows appends the usual ".dll" file suffix automatically.

Note: Accessing the standard C library through "cdll.msvcrt" will
  use an outdated version of the library that may be incompatible with
  the one being used by Python. Where possible, use native Python
  functionality, or else import and use the "msvcrt" module.

On Linux, it is required to specify the filename *including* the
extension to load a library, so attribute access can not be used to
load libraries. Either the "LoadLibrary()" method of the dll loaders
should be used, or you should load the library by creating an instance
of CDLL by calling the constructor:

   >>> cdll.LoadLibrary("libc.so.6")  
   <CDLL 'libc.so.6', handle ... at ...>
   >>> libc = CDLL("libc.so.6")       
   >>> libc                           
   <CDLL 'libc.so.6', handle ... at ...>
   >>>


Accessing functions from loaded dlls
------------------------------------

Functions are accessed as attributes of dll objects:

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

Note that win32 system dlls like "kernel32" and "user32" often export
ANSI as well as UNICODE versions of a function. The UNICODE version is
exported with 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
"getattr()" to retrieve the function:

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

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

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


Calling functions
-----------------

You can call these functions like any other Python callable. This
example uses the "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))  
   1150640792
   >>> print(hex(windll.kernel32.GetModuleHandleA(None)))  
   0x1d000000
   >>>

"ValueError" is raised when you call an "stdcall" function with the
"cdecl" calling convention, or vice versa:

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

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

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

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

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

There are, however, enough ways to crash Python with "ctypes", so you
should be careful anyway.  The "faulthandler" module can be helpful in
debugging crashes (e.g. from segmentation faults produced by erroneous
C library calls).

"None", integers, bytes objects and (unicode) strings are the only
native Python objects that can directly be used as parameters in these
function calls. "None" is passed as a C "NULL" pointer, bytes objects
and strings are passed as pointer to the memory block that contains
their data ("char *" or "wchar_t *").  Python integers are passed as
the 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.


Fundamental data types
----------------------

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

+------------------------+--------------------------------------------+------------------------------+
| ctypes type            | C type                                     | Python type                  |
|========================|============================================|==============================|
| "c_bool"               | "_Bool"                                    | bool (1)                     |
+------------------------+--------------------------------------------+------------------------------+
| "c_char"               | "char"                                     | 1-character bytes object     |
+------------------------+--------------------------------------------+------------------------------+
| "c_wchar"              | "wchar_t"                                  | 1-character string           |
+------------------------+--------------------------------------------+------------------------------+
| "c_byte"               | "char"                                     | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ubyte"              | "unsigned char"                            | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_short"              | "short"                                    | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ushort"             | "unsigned short"                           | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_int"                | "int"                                      | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_uint"               | "unsigned int"                             | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_long"               | "long"                                     | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ulong"              | "unsigned long"                            | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_longlong"           | "__int64" or "long long"                   | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ulonglong"          | "unsigned __int64" or "unsigned long long" | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_size_t"             | "size_t"                                   | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_ssize_t"            | "ssize_t" or "Py_ssize_t"                  | int                          |
+------------------------+--------------------------------------------+------------------------------+
| "c_float"              | "float"                                    | float                        |
+------------------------+--------------------------------------------+------------------------------+
| "c_double"             | "double"                                   | float                        |
+------------------------+--------------------------------------------+------------------------------+
| "c_longdouble"         | "long double"                              | float                        |
+------------------------+--------------------------------------------+------------------------------+
| "c_char_p"             | "char *" (NUL terminated)                  | bytes object or "None"       |
+------------------------+--------------------------------------------+------------------------------+
| "c_wchar_p"            | "wchar_t *" (NUL terminated)               | string or "None"             |
+------------------------+--------------------------------------------+------------------------------+
| "c_void_p"             | "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(140018365411392)
   >>> c_ushort(-3)
   c_ushort(65533)
   >>>

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

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

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

   >>> s = "Hello, World"
   >>> c_s = c_wchar_p(s)
   >>> print(c_s)
   c_wchar_p(139966785747344)
   >>> print(c_s.value)
   Hello World
   >>> c_s.value = "Hi, there"
   >>> print(c_s)              # the memory location has changed
   c_wchar_p(139966783348904)
   >>> print(c_s.value)
   Hi, there
   >>> print(s)                # first object is unchanged
   Hello, World
   >>>

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

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

The "create_string_buffer()" function replaces the "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.


Calling functions, continued
----------------------------

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

   >>> printf = libc.printf
   >>> printf(b"Hello, %s\n", b"World!")
   Hello, World!
   14
   >>> printf(b"Hello, %S\n", "World!")
   Hello, World!
   14
   >>> printf(b"%d bottles of beer\n", 42)
   42 bottles of beer
   19
   >>> printf(b"%f bottles of beer\n", 42.5)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   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
"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 "ctypes" argument conversion to allow instances
of your own classes be used as function arguments.  "ctypes" looks for
an "_as_parameter_" attribute and uses this as the function argument.
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 "_as_parameter_"
instance variable, you could define a "property" which makes the
attribute available on request.


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

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

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

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

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

   >>> printf(b"%d %d %d", 1, 2, 3)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   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 "from_param()" class method for them to be
able to use them in the "argtypes" sequence. The "from_param()" class
method receives the Python object passed to the function call, it
should do a typecheck or whatever is needed to make sure this object
is acceptable, and then return the object itself, its "_as_parameter_"
attribute, or whatever you want to pass as the C function argument in
this case. Again, the result should be an integer, string, bytes, a
"ctypes" instance, or an object with an "_as_parameter_" attribute.


Return types
------------

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

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

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

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

   >>> strchr.restype = c_char_p
   >>> strchr.argtypes = [c_char_p, c_char]
   >>> strchr(b"abcdef", b"d")
   'def'
   >>> strchr(b"abcdef", b"def")
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   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 "restype" attribute, if the foreign function returns
an integer.  The callable will be called with the *integer* the C
function returns, and the result of this call will be used as the
result of your function call. This is useful to check for error return
values and automatically raise an exception:

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

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

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


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

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

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

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


Structures and unions
---------------------

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

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

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

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

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

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

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

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

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

Field *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: "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 by
specifying a "_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.


Bit fields in structures and unions
-----------------------------------

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

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


Arrays
------

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

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

   TenPointsArrayType = POINT * 10

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

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

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

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

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

Initializers of the correct type can also be specified:

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


Pointers
--------

Pointer instances are created by calling the "pointer()" function on a
"ctypes" type:

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

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

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

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

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

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

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

Pointer instances can also be indexed with integers:

   >>> pi[0]
   99
   >>>

Assigning to an integer index changes the pointed to value:

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

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

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

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

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

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

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

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

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


Type conversions
----------------

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

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

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

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

   >>> bar.values = None
   >>>

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

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

For these cases, the "cast()" function is handy.

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

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

So, "cast()" can be used to assign to the "values" field of "Bar" the
structure:

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


Incomplete Types
----------------

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

   struct cell; /* forward declaration */

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

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

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

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

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

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

   >>> c1 = cell()
   >>> c1.name = "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
------------------

"ctypes" allows creating C callable function pointers from Python
callables. These are sometimes called *callback functions*.

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

The "CFUNCTYPE()" factory function creates types for callback
functions using the "cdecl" calling convention. On Windows, the
"WINFUNCTYPE()" factory function creates types for callback functions
using the "stdcall" calling convention.

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

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

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

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

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

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

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

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

The result:

   >>> qsort(ia, len(ia), sizeof(c_int), cmp_func)  
   py_cmp_func 5 1
   py_cmp_func 33 99
   py_cmp_func 7 33
   py_cmp_func 5 7
   py_cmp_func 1 7
   >>>

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

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

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

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

The function factories can be used as decorator factories, so we may
as well write:

   >>> @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
   ... def py_cmp_func(a, b):
   ...     print("py_cmp_func", a[0], b[0])
   ...     return a[0] - b[0]
   ...
   >>> qsort(ia, len(ia), sizeof(c_int), py_cmp_func)
   py_cmp_func 5 1
   py_cmp_func 33 99
   py_cmp_func 7 33
   py_cmp_func 1 7
   py_cmp_func 5 7
   >>>

Note: Make sure you keep references to "CFUNCTYPE()" objects as long
  as they are used from C code. "ctypes" doesn’t, and if you don’t,
  they may be garbage collected, crashing your program when a callback
  is made.Also, note that if the callback function is called in a
  thread created outside of Python’s control (e.g. by the foreign code
  that calls the callback), ctypes creates a new dummy Python thread
  on every invocation. This behavior is correct for most purposes, but
  it means that values stored with "threading.local" will *not*
  survive across different callbacks, even when those calls are made
  from the same C thread.


Accessing values exported from dlls
-----------------------------------

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

"ctypes" can access values like this with the "in_dll()" class methods
of the type.  *pythonapi* is a predefined symbol giving access to the
Python C api:

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

If the interpreter would have been started with "-O", the sample would
have printed "c_long(1)", or "c_long(2)" if "-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 docs for that value:

   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:
   ...     if item.name is None:
   ...         break
   ...     print(item.name.decode("ascii"), item.size)
   ...
   _frozen_importlib 31764
   _frozen_importlib_external 41499
   __hello__ 161
   __phello__ -161
   __phello__.spam 161
   >>>

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


Surprises
---------

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

Consider the following example:

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

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

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

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

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

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

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

Note: Objects instantiated from "c_char_p" can only have their value
  set to bytes or integers.

Why is it printing "False"?  ctypes instances are objects containing a
memory block plus some *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
-------------------------

"ctypes" provides some support for variable-sized arrays and
structures.

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

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

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

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

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


ctypes reference
================


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 or runtime loader 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.

ctypes.util.find_library(name)

   Try to find a library and return a pathname.  *name* is the library
   name without any prefix like *lib*, suffix like ".so", ".dylib" or
   version number (this is the form used for the posix linker 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", "objdump" and "ld") to find the library
file. It returns the filename of the library file.

Changed in version 3.6: On Linux, the value of the environment
variable "LD_LIBRARY_PATH" is used when searching for libraries, if a
library cannot be found by any other means.

Here are some examples:

   >>> 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 time, and hardcode
that into the wrapper module instead of using "find_library()" to
locate the library at runtime.


Loading shared libraries
------------------------

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

class ctypes.CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)

   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 ctypes.OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)

   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
   "HRESULT" code.  "HRESULT" values contain information specifying
   whether the function call failed or succeeded, together with
   additional error code.  If the return value signals a failure, an
   "OSError" is automatically raised.

   Changed in version 3.3: "WindowsError" used to be raised.

class ctypes.WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)

   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 "WinDLL" and "OleDLL" use the standard calling
   convention on this platform.

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

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

   Instances of this class behave like "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 *dlopen(3)* manpage.  On Windows, *mode* is
ignored.  On posix systems, RTLD_NOW is always added, and is not
configurable.

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

The function "ctypes.get_errno()" returns the value of the ctypes
private copy, and the function "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
"GetLastError()" and "SetLastError()" Windows API functions;
"ctypes.get_last_error()" and "ctypes.set_last_error()" are used to
request and change the ctypes private copy of the windows error code.

The *winmode* parameter is used on Windows to specify how the library
is loaded (since *mode* is ignored). It takes any value that is valid
for the Win32 API "LoadLibraryEx" flags parameter. When omitted, the
default is to use the flags that result in the most secure DLL load to
avoiding issues such as DLL hijacking. Passing the full path to the
DLL is the safest way to ensure the correct library and dependencies
are loaded.

Changed in version 3.8: Added *winmode* parameter.

ctypes.RTLD_GLOBAL

   Flag to use as *mode* parameter.  On platforms where this flag is
   not available, it is defined as the integer zero.

ctypes.RTLD_LOCAL

   Flag to use as *mode* parameter.  On platforms where this is not
   available, it is the same as *RTLD_GLOBAL*.

ctypes.DEFAULT_MODE

   The default mode which is used to load shared libraries.  On OSX
   10.3, this is *RTLD_GLOBAL*, otherwise it is the same as
   *RTLD_LOCAL*.

Instances of these classes have no public methods.  Functions exported
by the shared library can be accessed as attributes or by index.
Please note that accessing the function through an attribute caches
the result and therefore accessing it repeatedly returns the same
object each time.  On the other hand, accessing it through an index
returns a new object each time:

   >>> from ctypes import CDLL
   >>> libc = CDLL("libc.so.6")  # On Linux
   >>> libc.time == libc.time
   True
   >>> libc['time'] == libc['time']
   False

The following public attributes are available, their name starts with
an underscore to not clash with exported function names:

PyDLL._handle

   The system handle used to access the library.

PyDLL._name

   The name of the library passed in the constructor.

Shared libraries can also be loaded by using one of the prefabricated
objects, which are instances of the "LibraryLoader" class, either by
calling the "LoadLibrary()" method, or by retrieving the library as
attribute of the loader instance.

class ctypes.LibraryLoader(dlltype)

   Class which loads shared libraries.  *dlltype* should be one of the
   "CDLL", "PyDLL", "WinDLL", or "OleDLL" types.

   "__getattr__()" has special behavior: It allows loading a shared
   library by accessing it as attribute of a library loader instance.
   The result is cached, so repeated attribute accesses return the
   same library each time.

   LoadLibrary(name)

      Load a shared library into the process and return it.  This
      method always returns a new instance of the library.

These prefabricated library loaders are available:

ctypes.cdll

   Creates "CDLL" instances.

ctypes.windll

   Windows only: Creates "WinDLL" instances.

ctypes.oledll

   Windows only: Creates "OleDLL" instances.

ctypes.pydll

   Creates "PyDLL" instances.

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

ctypes.pythonapi

   An instance of "PyDLL" that exposes Python C API functions as
   attributes.  Note that all these functions are assumed to return C
   "int", which is of course not always the truth, so you have to
   assign the correct "restype" attribute to use these functions.

Loading a library through any of these objects raises an auditing
event "ctypes.dlopen" with string argument "name", the name used to
load the library.

Accessing a function on a loaded library raises an auditing event
"ctypes.dlsym" with arguments "library" (the library object) and
"name" (the symbol’s name as a string or integer).

In cases when only the library handle is available rather than the
object, accessing a function raises an auditing event
"ctypes.dlsym/handle" with arguments "handle" (the raw library handle)
and "name".


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:

class ctypes._FuncPtr

   Base class for C callable foreign functions.

   Instances of foreign functions are also C compatible data types;
   they represent C function pointers.

   This behavior can be customized by assigning to special attributes
   of the foreign function object.

   restype

      Assign a ctypes type to specify the result type of the foreign
      function. Use "None" for "void", a function not returning
      anything.

      It is possible to assign a callable Python object that is not a
      ctypes type, in this case the function is assumed to return a C
      "int", and the callable will be called with this integer,
      allowing further processing or error checking.  Using this is
      deprecated, for more flexible post processing or error checking
      use a ctypes data type as "restype" and assign a callable to the
      "errcheck" attribute.

   argtypes

      Assign a tuple of ctypes types to specify the argument types
      that the function accepts.  Functions using the "stdcall"
      calling convention can only be called with the same number of
      arguments as the length of this tuple; functions using the C
      calling convention accept additional, unspecified arguments as
      well.

      When a foreign function is called, each actual argument is
      passed to the "from_param()" class method of the items in the
      "argtypes" tuple, this method allows adapting the actual
      argument to an object that the foreign function accepts.  For
      example, a "c_char_p" item in the "argtypes" tuple will convert
      a string passed as argument into a bytes object using ctypes
      conversion rules.

      New: It is now possible to put items in argtypes which are not
      ctypes types, but each item must have a "from_param()" method
      which returns a value usable as argument (integer, string,
      ctypes instance).  This allows defining adapters that can adapt
      custom objects as function parameters.

   errcheck

      Assign a Python function or another callable to this attribute.
      The callable will be called with three or more arguments:

      callable(result, func, arguments)

         *result* is what the foreign function returns, as specified
         by the "restype" attribute.

         *func* is the foreign function object itself, this allows
         reusing the same callable object to check or post process the
         results of several functions.

         *arguments* is a tuple containing the parameters originally
         passed to the function call, this allows specializing the
         behavior on the arguments used.

      The object that this function returns will be returned from the
      foreign function call, but it can also check the result value
      and raise an exception if the foreign function call failed.

exception ctypes.ArgumentError

   This exception is raised when a foreign function call cannot
   convert one of the passed arguments.

On Windows, when a foreign function call raises a system exception
(for example, due to an access violation), it will be captured and
replaced with a suitable Python exception. Further, an auditing event
"ctypes.seh_exception" with argument "code" will be raised, allowing
an audit hook to replace the exception with its own.

Some ways to invoke foreign function calls may raise an auditing event
"ctypes.call_function" with arguments "function pointer" and
"arguments".


Function prototypes
-------------------

Foreign functions can also be created by instantiating function
prototypes. Function prototypes are similar to function prototypes in
C; they describe a function (return type, argument types, calling
convention) without defining an implementation.  The factory functions
must be called with the desired result type and the argument types of
the function, and can be used as decorator factories, and as such, be
applied to functions through the "@wrapper" syntax. See Callback
functions for examples.

ctypes.CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)

   The returned function prototype creates functions that use the
   standard C calling convention.  The function will release the GIL
   during the call.  If *use_errno* is set to true, the ctypes private
   copy of the system "errno" variable is exchanged with the real
   "errno" value before and after the call; *use_last_error* does the
   same for the Windows error code.

ctypes.WINFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)

   Windows only: The returned function prototype creates functions
   that use the "stdcall" calling convention, except on Windows CE
   where "WINFUNCTYPE()" is the same as "CFUNCTYPE()".  The function
   will release the GIL during the call.  *use_errno* and
   *use_last_error* have the same meaning as above.

ctypes.PYFUNCTYPE(restype, *argtypes)

   The returned function prototype creates functions that use the
   Python calling convention.  The function will *not* release the GIL
   during the call.

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

   prototype(address)

      Returns a foreign function at the specified address which must
      be an integer.

   prototype(callable)

      Create a C callable function (a callback function) from a Python
      *callable*.

   prototype(func_spec[, paramflags])

      Returns a foreign function exported by a shared library.
      *func_spec* must be a 2-tuple "(name_or_ordinal, library)". The
      first item is the name of the exported function as string, or
      the ordinal of the exported function as small integer.  The
      second item is the shared library instance.

   prototype(vtbl_index, name[, paramflags[, iid]])

      Returns a foreign function that will call a COM method.
      *vtbl_index* is the index into the virtual function table, a
      small non-negative integer. *name* is name of the COM method.
      *iid* is an optional pointer to the interface identifier which
      is used in extended error reporting.

      COM methods use a special calling convention: They require a
      pointer to the COM interface as first argument, in addition to
      those parameters that are specified in the "argtypes" tuple.

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

   *paramflags* must be a tuple of the same length as "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 "MessageBoxW"
function so that it supports default parameters and named arguments.
The C declaration from the windows header file is this:

   WINUSERAPI int WINAPI
   MessageBoxW(
       HWND hWnd,
       LPCWSTR lpText,
       LPCWSTR lpCaption,
       UINT uType);

Here is the wrapping with "ctypes":

   >>> from ctypes import c_int, WINFUNCTYPE, windll
   >>> from ctypes.wintypes import HWND, LPCWSTR, UINT
   >>> prototype = WINFUNCTYPE(c_int, HWND, LPCWSTR, LPCWSTR, UINT)
   >>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", "Hello from ctypes"), (1, "flags", 0)
   >>> MessageBox = prototype(("MessageBoxW", windll.user32), paramflags)

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

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

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

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

Here is the wrapping with "ctypes":

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

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

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

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

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

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


Utility functions
-----------------

ctypes.addressof(obj)

   Returns the address of the memory buffer as integer.  *obj* must be
   an instance of a ctypes type.

   Raises an auditing event "ctypes.addressof" with argument "obj".

ctypes.alignment(obj_or_type)

   Returns the alignment requirements of a ctypes type. *obj_or_type*
   must be a ctypes type or instance.

ctypes.byref(obj[, offset])

   Returns a light-weight pointer to *obj*, which must be an instance
   of a ctypes type.  *offset* defaults to zero, and must be an
   integer that will be added to the internal pointer value.

   "byref(obj, offset)" corresponds to this C code:

      (((char *)&obj) + offset)

   The returned object can only be used as a foreign function call
   parameter. It behaves similar to "pointer(obj)", but the
   construction is a lot faster.

ctypes.cast(obj, type)

   This function is similar to the cast operator in C. It returns a
   new instance of *type* which points to the same memory block as
   *obj*.  *type* must be a pointer type, and *obj* must be an object
   that can be interpreted as a pointer.

ctypes.create_string_buffer(init_or_size, size=None)

   This function creates a mutable character buffer. The returned
   object is a ctypes array of "c_char".

   *init_or_size* must be an integer which specifies the size of the
   array, or a bytes object which will be used to initialize the array
   items.

   If a bytes object is specified as first argument, the buffer is
   made one item larger than its length so that the last element in
   the array is a NUL termination character. An integer can be passed
   as second argument which allows specifying the size of the array if
   the length of the bytes should not be used.

   Raises an auditing event "ctypes.create_string_buffer" with
   arguments "init", "size".

ctypes.create_unicode_buffer(init_or_size, size=None)

   This function creates a mutable unicode character buffer. The
   returned object is a ctypes array of "c_wchar".

   *init_or_size* must be an integer which specifies the size of the
   array, or a string which will be used to initialize the array
   items.

   If a string is specified as first argument, the buffer is made one
   item larger than the length of the string so that the last element
   in the array is a NUL termination character. An integer can be
   passed as second argument which allows specifying the size of the
   array if the length of the string should not be used.

   Raises an auditing event "ctypes.create_unicode_buffer" with
   arguments "init", "size".

ctypes.DllCanUnloadNow()

   Windows only: This function is a hook which allows implementing in-
   process COM servers with ctypes.  It is called from the
   DllCanUnloadNow function that the _ctypes extension dll exports.

ctypes.DllGetClassObject()

   Windows only: This function is a hook which allows implementing in-
   process COM servers with ctypes.  It is called from the
   DllGetClassObject function that the "_ctypes" extension dll
   exports.

ctypes.util.find_library(name)

   Try to find a library and return a pathname.  *name* is the library
   name without any prefix like "lib", suffix like ".so", ".dylib" or
   version number (this is the form used for the posix linker option
   "-l").  If no library can be found, returns "None".

   The exact functionality is system dependent.

ctypes.util.find_msvcrt()

   Windows only: return the filename of the VC runtime library used by
   Python, and by the extension modules.  If the name of the library
   cannot be determined, "None" is returned.

   If you need to free memory, for example, allocated by an extension
   module with a call to the "free(void *)", it is important that you
   use the function in the same library that allocated the memory.

ctypes.FormatError([code])

   Windows only: Returns a textual description of the error code
   *code*.  If no error code is specified, the last error code is used
   by calling the Windows api function GetLastError.

ctypes.GetLastError()

   Windows only: Returns the last error code set by Windows in the
   calling thread. This function calls the Windows *GetLastError()*
   function directly, it does not return the ctypes-private copy of
   the error code.

ctypes.get_errno()

   Returns the current value of the ctypes-private copy of the system
   "errno" variable in the calling thread.

   Raises an auditing event "ctypes.get_errno" with no arguments.

ctypes.get_last_error()

   Windows only: returns the current value of the ctypes-private copy
   of the system "LastError" variable in the calling thread.

   Raises an auditing event "ctypes.get_last_error" with no arguments.

ctypes.memmove(dst, src, count)

   Same as the standard C memmove library function: copies *count*
   bytes from *src* to *dst*. *dst* and *src* must be integers or
   ctypes instances that can be converted to pointers.

ctypes.memset(dst, c, count)

   Same as the standard C memset library function: fills the memory
   block at address *dst* with *count* bytes of value *c*. *dst* must
   be an integer specifying an address, or a ctypes instance.

ctypes.POINTER(type)

   This factory function creates and returns a new ctypes pointer
   type. Pointer types are cached and reused internally, so calling
   this function repeatedly is cheap. *type* must be a ctypes type.

ctypes.pointer(obj)

   This function creates a new pointer instance, pointing to *obj*.
   The returned object is of the type "POINTER(type(obj))".

   Note: If you just want to pass a pointer to an object to a foreign
   function call, you should use "byref(obj)" which is much faster.

ctypes.resize(obj, size)

   This function resizes the internal memory buffer of *obj*, which
   must be an instance of a ctypes type.  It is not possible to make
   the buffer smaller than the native size of the objects type, as
   given by "sizeof(type(obj))", but it is possible to enlarge the
   buffer.

ctypes.set_errno(value)

   Set the current value of the ctypes-private copy of the system
   "errno" variable in the calling thread to *value* and return the
   previous value.

   Raises an auditing event "ctypes.set_errno" with argument "errno".

ctypes.set_last_error(value)

   Windows only: set the current value of the ctypes-private copy of
   the system "LastError" variable in the calling thread to *value*
   and return the previous value.

   Raises an auditing event "ctypes.set_last_error" with argument
   "error".

ctypes.sizeof(obj_or_type)

   Returns the size in bytes of a ctypes type or instance memory
   buffer. Does the same as the C "sizeof" operator.

ctypes.string_at(address, size=-1)

   This function returns the C string starting at memory address
   *address* as a bytes object. If size is specified, it is used as
   size, otherwise the string is assumed to be zero-terminated.

   Raises an auditing event "ctypes.string_at" with arguments
   "address", "size".

ctypes.WinError(code=None, descr=None)

   Windows only: this function is probably the worst-named thing in
   ctypes. It creates an instance of OSError.  If *code* is not
   specified, "GetLastError" is called to determine the error code. If
   *descr* is not specified, "FormatError()" is called to get a
   textual description of the error.

   Changed in version 3.3: An instance of "WindowsError" used to be
   created.

ctypes.wstring_at(address, size=-1)

   This function returns the wide character string starting at memory
   address *address* as a string.  If *size* is specified, it is used
   as the number of characters of the string, otherwise the string is
   assumed to be zero-terminated.

   Raises an auditing event "ctypes.wstring_at" with arguments
   "address", "size".


Data types
----------

class ctypes._CData

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

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

   from_buffer(source[, offset])

      This method returns a ctypes instance that shares the buffer of
      the *source* object.  The *source* object must support the
      writeable buffer interface.  The optional *offset* parameter
      specifies an offset into the source buffer in bytes; the default
      is zero.  If the source buffer is not large enough a
      "ValueError" is raised.

      Raises an auditing event "ctypes.cdata/buffer" with arguments
      "pointer", "size", "offset".

   from_buffer_copy(source[, offset])

      This method creates a ctypes instance, copying the buffer from
      the *source* object buffer which must be readable.  The optional
      *offset* parameter specifies an offset into the source buffer in
      bytes; the default is zero.  If the source buffer is not large
      enough a "ValueError" is raised.

      Raises an auditing event "ctypes.cdata/buffer" with arguments
      "pointer", "size", "offset".

   from_address(address)

      This method returns a ctypes type instance using the memory
      specified by *address* which must be an integer.

      This method, and others that indirectly call this method, raises
      an auditing event "ctypes.cdata" with argument "address".

   from_param(obj)

      This method adapts *obj* to a ctypes type.  It is called with
      the actual object used in a foreign function call when the type
      is present in the foreign function’s "argtypes" tuple; it must
      return an object that can be used as a function call parameter.

      All ctypes data types have a default implementation of this
      classmethod that normally returns *obj* if that is an instance
      of the type.  Some types accept other objects as well.

   in_dll(library, name)

      This method returns a ctypes type instance exported by a shared
      library. *name* is the name of the symbol that exports the data,
      *library* is the loaded shared library.

   Common instance variables of ctypes data types:

   _b_base_

      Sometimes ctypes data instances do not own the memory block they
      contain, instead they share part of the memory block of a base
      object.  The "_b_base_" read-only member is the root ctypes
      object that owns the memory block.

   _b_needsfree_

      This read-only variable is true when the ctypes data instance
      has allocated the memory block itself, false otherwise.

   _objects

      This member is either "None" or a dictionary containing Python
      objects that need to be kept alive so that the memory block
      contents is kept valid.  This object is only exposed for
      debugging; never modify the contents of this dictionary.


Fundamental data types
----------------------

class ctypes._SimpleCData

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

   Instances have a single attribute:

   value

      This attribute contains the actual value of the instance. For
      integer and pointer types, it is an integer, for character
      types, it is a single character bytes object or string, for
      character pointer types it is a Python bytes object or string.

      When the "value" attribute is retrieved from a ctypes instance,
      usually a new object is returned each time.  "ctypes" does *not*
      implement original object return, always a new object is
      constructed.  The same is true for all other ctypes object
      instances.

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

Subclasses of fundamental data types do *not* inherit this behavior.
So, if a foreign functions "restype" is a subclass of "c_void_p", you
will receive an instance of this subclass from the function call. Of
course, you can get the value of the pointer by accessing the "value"
attribute.

These are the fundamental ctypes data types:

class ctypes.c_byte

   Represents the C "signed char" datatype, and interprets the value
   as small integer.  The constructor accepts an optional integer
   initializer; no overflow checking is done.

class ctypes.c_char

   Represents the C "char" datatype, and interprets the value as a
   single character.  The constructor accepts an optional string
   initializer, the length of the string must be exactly one
   character.

class ctypes.c_char_p

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

class ctypes.c_double

   Represents the C "double" datatype.  The constructor accepts an
   optional float initializer.

class ctypes.c_longdouble

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

class ctypes.c_float

   Represents the C "float" datatype.  The constructor accepts an
   optional float initializer.

class ctypes.c_int

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

class ctypes.c_int8

   Represents the C 8-bit "signed int" datatype.  Usually an alias for
   "c_byte".

class ctypes.c_int16

   Represents the C 16-bit "signed int" datatype.  Usually an alias
   for "c_short".

class ctypes.c_int32

   Represents the C 32-bit "signed int" datatype.  Usually an alias
   for "c_int".

class ctypes.c_int64

   Represents the C 64-bit "signed int" datatype.  Usually an alias
   for "c_longlong".

class ctypes.c_long

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

class ctypes.c_longlong

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

class ctypes.c_short

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

class ctypes.c_size_t

   Represents the C "size_t" datatype.

class ctypes.c_ssize_t

   Represents the C "ssize_t" datatype.

   New in version 3.2.

class ctypes.c_ubyte

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

class ctypes.c_uint

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

class ctypes.c_uint8

   Represents the C 8-bit "unsigned int" datatype.  Usually an alias
   for "c_ubyte".

class ctypes.c_uint16

   Represents the C 16-bit "unsigned int" datatype.  Usually an alias
   for "c_ushort".

class ctypes.c_uint32

   Represents the C 32-bit "unsigned int" datatype.  Usually an alias
   for "c_uint".

class ctypes.c_uint64

   Represents the C 64-bit "unsigned int" datatype.  Usually an alias
   for "c_ulonglong".

class ctypes.c_ulong

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

class ctypes.c_ulonglong

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

class ctypes.c_ushort

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

class ctypes.c_void_p

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

class ctypes.c_wchar

   Represents the C "wchar_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.

class ctypes.c_wchar_p

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

class ctypes.c_bool

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

class ctypes.HRESULT

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

class ctypes.py_object

   Represents the C "PyObject *" datatype.  Calling this without an
   argument creates a "NULL" "PyObject *" pointer.

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


Structured data types
---------------------

class ctypes.Union(*args, **kw)

   Abstract base class for unions in native byte order.

class ctypes.BigEndianStructure(*args, **kw)

   Abstract base class for structures in *big endian* byte order.

class ctypes.LittleEndianStructure(*args, **kw)

   Abstract base class for structures in *little endian* byte order.

Structures with non-native byte order cannot contain pointer type
fields, or any other data types containing pointer type fields.

class ctypes.Structure(*args, **kw)

   Abstract base class for structures in *native* byte order.

   Concrete structure and union types must be created by subclassing
   one of these types, and at least define a "_fields_" class
   variable. "ctypes" will create *descriptor*s which allow reading
   and writing the fields by direct attribute accesses.  These are the

   _fields_

      A sequence defining the structure fields.  The items must be
      2-tuples or 3-tuples.  The first item is the name of the field,
      the second item specifies the type of the field; it can be any
      ctypes data type.

      For integer type fields like "c_int", a third optional item can
      be given.  It must be a small positive integer defining the bit
      width of the field.

      Field names must be unique within one structure or union.  This
      is not checked, only one field can be accessed when names are
      repeated.

      It is possible to define the "_fields_" class variable *after*
      the class statement that defines the Structure subclass, this
      allows creating data types that directly or indirectly reference
      themselves:

         class List(Structure):
             pass
         List._fields_ = [("pnext", POINTER(List)),
                          ...
                         ]

      The "_fields_" class variable must, however, be defined before
      the type is first used (an instance is created, "sizeof()" is
      called on it, and so on).  Later assignments to the "_fields_"
      class variable will raise an AttributeError.

      It is possible to define sub-subclasses of structure types, they
      inherit the fields of the base class plus the "_fields_" defined
      in the sub-subclass, if any.

   _pack_

      An optional small integer that allows overriding the alignment
      of structure fields in the instance.  "_pack_" must already be
      defined when "_fields_" is assigned, otherwise it will have no
      effect.

   _anonymous_

      An optional sequence that lists the names of unnamed (anonymous)
      fields. "_anonymous_" must be already defined when "_fields_" is
      assigned, otherwise it will have no effect.

      The fields listed in this variable must be structure or union
      type fields. "ctypes" will create descriptors in the structure
      type that allows accessing the nested fields directly, without
      the need to create the structure or union field.

      Here is an example type (Windows):

         class _U(Union):
             _fields_ = [("lptdesc", POINTER(TYPEDESC)),
                         ("lpadesc", POINTER(ARRAYDESC)),
                         ("hreftype", HREFTYPE)]

         class TYPEDESC(Structure):
             _anonymous_ = ("u",)
             _fields_ = [("u", _U),
                         ("vt", VARTYPE)]

      The "TYPEDESC" structure describes a COM data type, the "vt"
      field specifies which one of the union fields is valid.  Since
      the "u" field is defined as anonymous field, it is now possible
      to access the members directly off the TYPEDESC instance.
      "td.lptdesc" and "td.u.lptdesc" are equivalent, but the former
      is faster since it does not need to create a temporary union
      instance:

         td = TYPEDESC()
         td.vt = VT_PTR
         td.lptdesc = POINTER(some_type)
         td.u.lptdesc = POINTER(some_type)

   It is possible to define sub-subclasses of structures, they inherit
   the fields of the base class.  If the subclass definition has a
   separate "_fields_" variable, the fields specified in this are
   appended to the fields of the base class.

   Structure and union constructors accept both positional and keyword
   arguments.  Positional arguments are used to initialize member
   fields in the same order as they are appear in "_fields_".  Keyword
   arguments in the constructor are interpreted as attribute
   assignments, so they will initialize "_fields_" with the same name,
   or create new attributes for names not present in "_fields_".


Arrays and pointers
-------------------

class ctypes.Array(*args)

   Abstract base class for arrays.

   The recommended way to create concrete array types is by
   multiplying any "ctypes" data type with a positive integer.
   Alternatively, you can subclass this type and define "_length_" and
   "_type_" class variables. Array elements can be read and written
   using standard subscript and slice accesses; for slice reads, the
   resulting object is *not* itself an "Array".

   _length_

      A positive integer specifying the number of elements in the
      array. Out-of-range subscripts result in an "IndexError". Will
      be returned by "len()".

   _type_

      Specifies the type of each element in the array.

   Array subclass constructors accept positional arguments, used to
   initialize the elements in order.

class ctypes._Pointer

   Private, abstract base class for pointers.

   Concrete pointer types are created by calling "POINTER()" with the
   type that will be pointed to; this is done automatically by
   "pointer()".

   If a pointer points to an array, its elements can be read and
   written using standard subscript and slice accesses.  Pointer
   objects have no size, so "len()" will raise "TypeError".  Negative
   subscripts will read from the memory *before* the pointer (as in
   C), and out-of-range subscripts will probably crash with an access
   violation (if you’re lucky).

   _type_

      Specifies the type pointed to.

   contents

      Returns the object to which to pointer points.  Assigning to
      this attribute changes the pointer to point to the assigned
      object.
