ctypes is a foreign function library for Python. It provides C compatible data types, and allows to call functions in dlls/shared libraries. It can be used to wrap these libraries in pure Python.
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.
The exact functionality is system dependend.
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 sone 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 successfull:
>>> from ctypes.util import find_library
>>> find_library("c")
'/usr/lib/libc.dylib'
>>> find_library("m")
'/usr/lib/libm.dylib'
>>> find_library("bz2")
'/usr/lib/libbz2.dylib'
>>> find_library("AGL")
'/System/Library/Frameworks/AGL.framework/AGL'
>>>
On Windows, find_library searches along the system search path, and returns the full pathname, but since there is no predefined naming scheme a call like find_library("c") will fail and return None.
If wrapping a shared library with ctypes, it may be better to determine the shared library name at development type, and hardcode that into the wrapper module instead of using find_library to locate the library at runtime.
There are several ways to loaded shared libraries into the Python process. One way is to instantiate one of the following classes:
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 GIL is released before calling any function exported by these libraries, and reaquired afterwards.
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 shard 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.
Instances of these classes have no public methods, however __getattr__ and __getitem__ have special behaviour: functions exported by the shared library can be accessed as attributes of by index. Please note that both __getattr__ and __getitem__ cache their result, so calling them repeatedly returns the same object each time.
The following public attributes are available, their name starts with an underscore to not clash with exported function names:
Shared libraries can also be loaded by using one of the prefabricated objects, which are instances of the LibraryLoader class, either by calling the LoadLibrary method, or by retrieving the library as attribute of the loader instance.
Class which loads shared libraries. dlltype should be one of the CDLL, PyDLL, WinDLL, or OleDLL types.
__getattr__ has special behaviour: It allows to load a shared library by accessing it as attribute of a library loader instance. The result is cached, so repeated attribute accesses return the same library each time.
These prefabricated library loaders are available:
For accessing the C Python api directly, a ready-to-use Python shared library object is available:
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:
Instances of foreign functions are also C compatible data types; they represent C function pointers.
This behaviour can be customized by assigning to special attributes of the foreign function object.
Assign a ctypes type to specify the result type of the foreign function. Use None for void a function not returning anything.
It is possible to assign a callable Python object that is not a ctypes type, in this case the function is assumed to return a C int, and the callable will be called with this integer, allowing to do further processing or error checking. Using this is deprecated, for more flexible postprocessing or error checking use a ctypes data type as restype and assign a callable to the errcheck attribute.
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 to adapt 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 unicode string passed as argument into an byte string using ctypes conversion rules.
New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a from_param method which returns a value usable as argument (integer, string, ctypes instance). This allows to define adapters that can adapt custom objects as function parameters.
result is what the foreign function returns, as specified by the restype attribute.
func is the foreign function object itself, this allows to reuse the same callable object to check or postprocess the results of several functions.
arguments is a tuple containing the parameters originally passed to the function call, this allows to specialize the behaviour 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.
Foreign functions can also be created by instantiating function prototypes. Function prototypes are similar to function prototypes in C; they describe a function (return type, argument types, calling convention) without defining an implementation. The factory functions must be called with the desired result type and the argument types of the function.
Function prototypes created by the factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call.
Returns a foreign function that will call a COM method. vtbl_index is the index into the virtual function table, a small nonnegative 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 1, 2, or 3 items.
The first item is an integer containing flags for the parameter:
The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.
The optional third item is the default value for this parameter.
This example demonstrates how to wrap the Windows MessageBoxA function so that it supports default parameters and named arguments. The C declaration from the windows header file is this:
WINUSERAPI int WINAPI
MessageBoxA(
HWND hWnd ,
LPCSTR lpText,
LPCSTR lpCaption,
UINT uType);
Here is the wrapping with ctypes:
>>> from ctypes import c_int, WINFUNCTYPE, windll
>>> from ctypes.wintypes import HWND, LPCSTR, UINT
>>> prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
>>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", None), (1, "flags", 0)
>>> MessageBox = prototype(("MessageBoxA", windll.user32), paramflags)
>>>
The MessageBox foreign function can now be called in these ways:
>>> MessageBox() >>> MessageBox(text="Spam, spam, spam") >>> MessageBox(flags=2, text="foo bar") >>>
A second example demonstrates output parameters. The win32 GetWindowRect function retrieves the dimensions of a specified window by copying them into RECT structure that the caller has to supply. Here is the C declaration:
WINUSERAPI BOOL WINAPI
GetWindowRect(
HWND hWnd,
LPRECT lpRect);
Here is the wrapping with ctypes:
>>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
>>> from ctypes.wintypes import BOOL, HWND, RECT
>>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
>>> paramflags = (1, "hwnd"), (2, "lprect")
>>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)
>>>
Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.
Output parameters can be combined with the 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 >>>
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 string which will be used to initialize the array items.
If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows to specify the size of the array if the length of the string should not be used.
If the first parameter is a unicode string, it is converted into an 8-bit string according to ctypes conversion rules.
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 unicode string which will be used to initialize the array items.
If a unicode string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows to specify the size of the array if the length of the string should not be used.
If the first parameter is a 8-bit string, it is converted into an unicode string according to ctypes conversion rules.
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.
This function sets the rules that ctypes objects use when converting between 8-bit strings and unicode strings. encoding must be a string specifying an encoding, like 'utf-8' or 'mbcs', errors must be a string specifying the error handling on encoding/decoding errors. Examples of possible values are "strict", "replace", or "ignore".
set_conversion_mode returns a 2-tuple containing the previous conversion rules. On windows, the initial conversion rules are ('mbcs', 'ignore'), on other systems ('ascii', 'strict').
Common methods of ctypes data types, these are all class methods (to be exact, they are methods of the metaclass):
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 functions argtypes tuple; it must return an object that can be used as function call parameter.
All ctypes data types have a default implementation of this classmethod, normally it returns obj if that is an instance of the type. Some types accept other objects as well.
Common instance variables of ctypes data types:
Instances have a single attribute:
This attribute contains the actual value of the instance. For integer and pointer types, it is an integer, for character types, it is a single character string, for character pointer types it is a Python string or unicode string.
When the value attribute is retrieved from a ctypes instance, usually a new object is returned each time. ctypes does not implement original object return, always a new object is constructed. The same is true for all other ctypes object instances.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python string, not a c_char_p instance.
Subclasses of fundamental data types do not inherit this behaviour. 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:
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.
Structures with non-native byte order cannot contain pointer type fields, or any other data types containing pointer type fields.
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 descriptors which allow reading and writing the fields by direct attribute accesses. These are the
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 to create 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.
Structure and union subclass constructors accept both positional and named arguments. Positional arguments are used to initialize the fields in the same order as they appear in the _fields_ definition, named arguments are used to initialize the fields with the corresponding name.
It is possible to defined sub-subclasses of structure types, they inherit the fields of the base class plus the _fields_ defined in the sub-subclass, if any.
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 to access the nested fields directly, without the need to create the structure or union field.
Here is an example type (Windows):
class _U(Union):
_fields_ = [("lptdesc", POINTER(TYPEDESC)),
("lpadesc", POINTER(ARRAYDESC)),
("hreftype", HREFTYPE)]
class TYPEDESC(Structure):
_fields_ = [("u", _U),
("vt", VARTYPE)]
_anonymous_ = ("u",)
The TYPEDESC structure describes a COM data type, the vt field specifies which one of the union fields is valid. Since the u field is defined as anonymous field, it is now possible to access the members directly off the TYPEDESC instance. td.lptdesc and td.u.lptdesc are equivalent, but the former is faster since it does not need to create a temporary union instance:
td = TYPEDESC() td.vt = VT_PTR td.lptdesc = POINTER(some_type) td.u.lptdesc = POINTER(some_type)
It is possible to defined sub-subclasses of structures, they inherit the fields of the base class. If the subclass definition has a separate _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_.
Not yet written - please see pointers and arrays in the tutorial.