New in version 2.5.
ctypes
is a foreign function library for Python. It provides C compatible data types, and allows calling functions in DLLs or shared libraries. It can be used to wrap these libraries in pure Python.
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. This type is an alias for the c_long
type on 32-bit systems. So, you should not be confused if c_long
is printed if you would expect c_int
— they are actually the same type.
ctypes
exports the cdll, and on Windows windll and oledll objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects. cdll loads libraries which export functions using the standard cdecl
calling convention, while windll libraries call functions using the stdcall
calling convention. oledll also uses the stdcall
calling convention, and assumes the functions return a Windows HRESULT
error code. The error code is used to automatically raise a WindowsError
exception when the function call fails.
Here are some examples for Windows. Note that msvcrt
is the MS standard C library containing most standard C functions, and uses the cdecl calling convention:
>>> from ctypes import * >>> print windll.kernel32 <WinDLL 'kernel32', handle ... at ...> >>> print cdll.msvcrt <CDLL 'msvcrt', handle ... at ...> >>> libc = cdll.msvcrt >>>
Windows appends the usual .dll
file suffix automatically.
On Linux, it is required to specify the filename including the extension to load a library, so attribute access can not be used to load libraries. Either the 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 ...> >>>
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 strings or unicode strings respectively.
Sometimes, dlls export functions with names which aren’t valid Python identifiers, like "??2@YAPAXI@Z"
. In this case you have to use getattr()
to retrieve the function:
>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z") <_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 >>>
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 >>>
ctypes
tries to protect you from calling functions with the wrong number of arguments or the wrong calling convention. Unfortunately this only works on Windows. It does this by examining the stack after the function returns, so although an error is raised the function has been called:
>>> windll.kernel32.GetModuleHandleA() Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: Procedure probably called with not enough arguments (4 bytes missing) >>> windll.kernel32.GetModuleHandleA(0, 0) Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: Procedure probably called with too many arguments (4 bytes in excess) >>>
The same exception is raised when you call an stdcall
function with the cdecl
calling convention, or vice versa:
>>> cdll.kernel32.GetModuleHandleA(None) 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("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> WindowsError: exception: access violation reading 0x00000020 >>>
There are, however, enough ways to crash Python with ctypes
, so you should be careful anyway.
None
, integers, longs, byte strings and unicode strings are the only native Python objects that can directly be used as parameters in these function calls. None
is passed as a C NULL
pointer, byte strings and unicode strings are passed as pointer to the memory block that contains their data (char *
or wchar_t *
). Python integers and Python longs are passed as the platforms default C int
type, their value is masked to fit into the C type.
Before we move on calling functions with other parameter types, we have to learn more about ctypes
data types.
ctypes
defines a number of primitive C compatible data types:
ctypes type | C type | Python type |
---|---|---|
| bool (1) | |
| 1-character string | |
| 1-character unicode string | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| int/long | |
| float | |
| float | |
| float | |
| string or | |
| unicode or | |
| int/long or |
All these types can be created by calling them with an optional initializer of the correct type and value:
>>> c_int() c_long(0) >>> c_char_p("Hello, World") c_char_p('Hello, World') >>> c_ushort(-3) c_ushort(65533) >>>
Since these types are mutable, their value can also be changed afterwards:
>>> i = c_int(42) >>> print i c_long(42) >>> print i.value 42 >>> i.value = -99 >>> print i.value -99 >>>
Assigning a new value to instances of the pointer types 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 strings are immutable):
>>> s = "Hello, World" >>> c_s = c_char_p(s) >>> print c_s c_char_p('Hello, World') >>> c_s.value = "Hi, there" >>> print c_s c_char_p('Hi, there') >>> print s # first string is unchanged Hello, World >>>
You should be careful, however, not to pass them to functions expecting pointers to mutable memory. If you need mutable memory blocks, ctypes has a create_string_buffer()
function which creates these in various ways. The current memory block contents can be accessed (or changed) with the raw
property; if you want to access it as NUL terminated string, use the value
property:
>>> from ctypes import * >>> p = create_string_buffer(3) # create a 3 byte buffer, initialized to NUL bytes >>> print sizeof(p), repr(p.raw) 3 '\x00\x00\x00' >>> p = create_string_buffer("Hello") # create a buffer containing a NUL terminated string >>> print sizeof(p), repr(p.raw) 6 'Hello\x00' >>> print repr(p.value) 'Hello' >>> p = create_string_buffer("Hello", 10) # create a 10 byte buffer >>> print sizeof(p), repr(p.raw) 10 'Hello\x00\x00\x00\x00\x00' >>> p.value = "Hi" >>> print sizeof(p), repr(p.raw) 10 'Hi\x00lo\x00\x00\x00\x00\x00' >>>
The create_string_buffer()
function replaces the c_buffer()
function (which is still available as an alias), as well as the c_string()
function from earlier ctypes releases. To create a mutable memory block containing unicode characters of the C type wchar_t
use the create_unicode_buffer()
function.
Note that printf prints to the real standard output channel, not to sys.stdout
, so these examples will only work at the console prompt, not from within IDLE or PythonWin:
>>> printf = libc.printf >>> printf("Hello, %s\n", "World!") Hello, World! 14 >>> printf("Hello, %S\n", u"World!") Hello, World! 14 >>> printf("%d bottles of beer\n", 42) 42 bottles of beer 19 >>> printf("%f bottles of beer\n", 42.5) Traceback (most recent call last): File "<stdin>", line 1, in <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 unicode strings have to be wrapped in their corresponding ctypes
type, so that they can be converted to the required C data type:
>>> printf("An int %d, a double %f\n", 1234, c_double(3.14)) An int 1234, a double 3.140000 31 >>>
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 unicode:
>>> class Bottles(object): ... def __init__(self, number): ... self._as_parameter_ = number ... >>> bottles = Bottles(42) >>> printf("%d bottles of beer\n", bottles) 42 bottles of beer 19 >>>
If you don’t want to store the instance’s data in the _as_parameter_
instance variable, you could define a property()
which makes the data available.
It is possible to specify the required argument types of functions exported from DLLs by setting the 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("String '%s', Int %d, Double %f\n", "Hi", 10, 2.2) String 'Hi', Int 10, Double 2.200000 37 >>>
Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:
>>> printf("%d %d %d", 1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> ArgumentError: argument 2: exceptions.TypeError: wrong type >>> printf("%s %d %f\n", "X", 2, 3) X 2 3.000000 13 >>>
If you have defined your own classes which you pass to function calls, you have to implement a 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, unicode, a ctypes
instance, or an object with an _as_parameter_
attribute.
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("abcdef", ord("d")) 8059983 >>> strchr.restype = c_char_p # c_char_p is a pointer to a string >>> strchr("abcdef", ord("d")) 'def' >>> print strchr("abcdef", ord("x")) None >>>
If you want to avoid the ord("x")
calls above, you can set the argtypes
attribute, and the second argument will be converted from a single character Python string into a C char:
>>> strchr.restype = c_char_p >>> strchr.argtypes = [c_char_p, c_char] >>> strchr("abcdef", "d") 'def' >>> strchr("abcdef", "def") Traceback (most recent call last): File "<stdin>", line 1, in <module> ArgumentError: argument 2: exceptions.TypeError: one character string expected >>> print strchr("abcdef", "x") None >>> strchr("abcdef", "d") 'def' >>>
You can also use a callable Python object (a function or a class for example) as the 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 WindowsError: [Errno 126] The specified module could not be found. >>>
WinError
is a function which will call Windows FormatMessage()
api to get the string representation of an error code, and returns an exception. WinError
takes an optional error code parameter, if no one is used, it calls 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.
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('\000' * 32) >>> print i.value, f.value, repr(s.value) 0 0.0 '' >>> libc.sscanf("1 3.14 Hello", "%d %f %s", ... byref(i), byref(f), s) 3 >>> print i.value, f.value, repr(s.value) 1 3.1400001049 'Hello' >>>
Structures and unions must derive from the 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 descriptors 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.
By default, Structure and Union fields are aligned in the same way the C compiler does it. It is possible to override this behavior be specifying a _pack_
class attribute in the subclass definition. This must be set to a positive integer and specifies the maximum alignment for the fields. This is what #pragma pack(n)
also does in MSVC.
ctypes
uses the native byte order for Structures and Unions. To build structures with non-native byte order, you can use one of the BigEndianStructure
, LittleEndianStructure
, BigEndianUnion
, and LittleEndianUnion
base classes. These classes cannot contain pointer fields.
It is possible to create structures and unions containing bit fields. Bit fields are only possible for integer fields, the bit width is specified as the third item in the _fields_
tuples:
>>> class Int(Structure): ... _fields_ = [("first_16", c_int, 16), ... ("second_16", c_int, 16)] ... >>> print Int.first_16 <Field type=c_long, ofs=0:0, bits=16> >>> print Int.second_16 <Field type=c_long, ofs=0:16, bits=16> >>>
Arrays are sequences, containing a fixed number of instances of the same type.
The recommended way to create array types is by multiplying a data type with a positive integer:
TenPointsArrayType = POINT * 10
Here is an example of 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, ... 1 2 3 4 5 6 7 8 9 10 >>>
Pointer instances are created by calling the pointer()
function on a ctypes
type:
>>> from ctypes import * >>> i = c_int(42) >>> pi = pointer(i) >>>
Pointer instances have a contents
attribute which returns the object to which the pointer points, the i
object above:
>>> pi.contents c_long(42) >>>
Note that ctypes
does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:
>>> pi.contents is i False >>> pi.contents is pi.contents False >>>
Assigning another 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 >>>
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 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))] >>>
Lets try it. We create two instances of cell
, and let them point to each other, and finally follow the pointer chain a few times:
>>> c1 = cell() >>> c1.name = "foo" >>> c2 = cell() >>> c2.name = "bar" >>> c1.next = pointer(c2) >>> c2.next = pointer(c1) >>> p = c1 >>> for i in range(8): ... print p.name, ... p = p.next[0] ... foo bar foo bar foo bar foo bar >>>
ctypes
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 normal cdecl calling convention, and, on Windows, the WINFUNCTYPE factory function creates types for callback functions using the stdcall calling convention.
Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.
I will present an example here which uses the standard C library’s qsort()
function, this 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 else.
So our callback function receives pointers to integers, and must return an integer. First we create the type
for the callback function:
>>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int)) >>>
For the first implementation of the callback function, we simply print the arguments we get, and return 0 (incremental development ;-):
>>> def py_cmp_func(a, b): ... print "py_cmp_func", a, b ... return 0 ... >>>
Create the C callable callback:
>>> cmp_func = CMPFUNC(py_cmp_func) >>>
And we’re ready to go:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func) py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...> >>>
We know how to access the contents of a pointer, so lets redefine our callback:
>>> def py_cmp_func(a, b): ... print "py_cmp_func", a[0], b[0] ... return 0 ... >>> cmp_func = CMPFUNC(py_cmp_func) >>>
Here is what we get on Windows:
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func) py_cmp_func 7 1 py_cmp_func 33 1 py_cmp_func 99 1 py_cmp_func 5 1 py_cmp_func 7 5 py_cmp_func 33 5 py_cmp_func 99 5 py_cmp_func 7 99 py_cmp_func 33 99 py_cmp_func 7 33 >>>
It is funny to see that on linux the sort function seems to work much more efficiently, it is doing less comparisons:
>>> 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 >>>
Ah, we’re nearly done! The last step is to actually compare the two items and return a useful result:
>>> def py_cmp_func(a, b): ... print "py_cmp_func", a[0], b[0] ... return a[0] - b[0] ... >>>
Final run on Windows:
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) py_cmp_func 33 7 py_cmp_func 99 33 py_cmp_func 5 99 py_cmp_func 1 99 py_cmp_func 33 7 py_cmp_func 1 33 py_cmp_func 5 33 py_cmp_func 5 7 py_cmp_func 1 7 py_cmp_func 5 1 >>>
and on Linux:
>>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) py_cmp_func 5 1 py_cmp_func 33 99 py_cmp_func 7 33 py_cmp_func 1 7 py_cmp_func 5 7 >>>
It is quite interesting to see that the Windows qsort()
function needs more comparisons than the linux version!
As we can easily check, our array is sorted now:
>>> for i in ia: print i, ... 1 5 7 33 99 >>>
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.
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 Python docs: This pointer is initialized to point to an array of “struct _frozen” records, terminated by one whose members are all NULL or zero. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules.
So manipulating this pointer could even prove useful. To restrict the example size, we show only how this table can be read with ctypes
:
>>> from ctypes import * >>> >>> class struct_frozen(Structure): ... _fields_ = [("name", c_char_p), ... ("code", POINTER(c_ubyte)), ... ("size", c_int)] ... >>>
We have defined the struct _frozen
data type, so we can get the pointer to the table:
>>> FrozenTable = POINTER(struct_frozen) >>> table = FrozenTable.in_dll(pythonapi, "PyImport_FrozenModules") >>>
Since table
is a pointer
to the array of struct_frozen
records, we can iterate over it, but we just have to make sure that our loop terminates, because pointers have no size. Sooner or later it would probably crash with an access violation or whatever, so it’s better to break out of the loop when we hit the NULL entry:
>>> for item in table: ... print item.name, item.size ... if item.name is None: ... break ... __hello__ 104 __phello__ -104 __phello__.spam 104 None 0 >>>
The fact that standard Python has a frozen module and a frozen package (indicated by the negative size member) is not well known, it is only used for testing. Try it out with import __hello__
for example.
There are some edge cases 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 different from what one would expect is this:
>>> s = c_char_p() >>> s.value = "abc def ghi" >>> s.value 'abc def ghi' >>> s.value is s.value False >>>
Why is it printing False
? ctypes instances are objects containing a memory block plus some descriptors accessing the contents of the memory. Storing a Python object in the memory block does not store the object itself, instead the contents
of the object is stored. Accessing the contents again constructs a new Python object each time!
ctypes
provides some support for variable-sized arrays and structures.
The resize()
function can be used to resize the memory buffer of an existing ctypes object. The function takes the object as first argument, and the requested size in bytes as the second argument. The memory block cannot be made smaller than the natural memory block specified by the objects type, a ValueError
is raised if this is tried:
>>> short_array = (c_short * 4)() >>> print sizeof(short_array) 8 >>> resize(short_array, 4) Traceback (most recent call last): ... ValueError: minimum size is 8 >>> resize(short_array, 32) >>> sizeof(short_array) 32 >>> sizeof(type(short_array)) 8 >>>
This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:
>>> short_array[:] [0, 0, 0, 0] >>> short_array[7] Traceback (most recent call last): ... IndexError: invalid index >>>
Another way to use variable-sized data types with ctypes
is to use the dynamic nature of Python, and (re-)define the data type after the required size is already known, on a case by case basis.
When programming in a compiled language, shared libraries are accessed when compiling/linking a program, and when the program is run.
The purpose of the find_library()
function is to locate a library in a way similar to what the compiler does (on platforms with several versions of a shared library the most recent should be loaded), while the ctypes library loaders act like when a program is run, and call the runtime loader directly.
The ctypes.util
module provides a function which can help to determine the library to load.
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
, and objdump
) to find the library file. It returns the filename of the library file. Here are some examples:
>>> from ctypes.util import find_library >>> find_library("m") 'libm.so.6' >>> find_library("c") 'libc.so.6' >>> find_library("bz2") 'libbz2.so.1.0' >>>
On OS X, find_library()
tries several predefined naming schemes and paths to locate the library, and returns a full pathname if successful:
>>> from ctypes.util import find_library >>> find_library("c") '/usr/lib/libc.dylib' >>> find_library("m") '/usr/lib/libm.dylib' >>> find_library("bz2") '/usr/lib/libbz2.dylib' >>> find_library("AGL") '/System/Library/Frameworks/AGL.framework/AGL' >>>
On Windows, find_library()
searches along the system search path, and returns the full pathname, but since there is no predefined naming scheme a call like find_library("c")
will fail and return None
.
If wrapping a shared library with ctypes
, it may be better to determine the shared library name at development time, and hardcode that into the wrapper module instead of using find_library()
to locate the library at runtime.
class ctypes.CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
Instances of this class represent loaded shared libraries. Functions in these libraries use the standard C calling convention, and are assumed to return int
.
class ctypes.OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the stdcall
calling convention, and are assumed to return the windows specific 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 WindowsError
is automatically raised.
class ctypes.WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the stdcall
calling convention, and are assumed to return int
by default.
On Windows CE only the standard calling convention is used, for convenience the 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.
New in version 2.6: The use_last_error and use_errno optional parameters were added.
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:
>>> 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.
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 unicode string passed as argument into a 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 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.
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.
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.
Changed in version 2.6: The optional use_errno and use_last_error parameters were added.
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:
Specifies an input parameter to the function.
Output parameter. The foreign function fills in a value.
Input parameter which defaults to the integer zero.
The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.
The optional third item is the default value for this parameter.
This example demonstrates how to wrap the Windows MessageBoxA
function so that it supports default parameters and named arguments. The C declaration from the windows header file is this:
WINUSERAPI int WINAPI MessageBoxA( HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType);
Here is the wrapping with ctypes
:
>>> from ctypes import c_int, WINFUNCTYPE, windll >>> from ctypes.wintypes import HWND, LPCSTR, UINT >>> prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT) >>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", None), (1, "flags", 0) >>> MessageBox = prototype(("MessageBoxA", windll.user32), paramflags) >>>
The MessageBox foreign function can now be called in these ways:
>>> MessageBox() >>> MessageBox(text="Spam, spam, spam") >>> MessageBox(flags=2, text="foo bar") >>>
A second example demonstrates output parameters. The win32 GetWindowRect
function retrieves the dimensions of a specified window by copying them into RECT
structure that the caller has to supply. Here is the C declaration:
WINUSERAPI BOOL WINAPI GetWindowRect( HWND hWnd, LPRECT lpRect);
Here is the wrapping with ctypes
:
>>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError >>> from ctypes.wintypes import BOOL, HWND, RECT >>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT)) >>> paramflags = (1, "hwnd"), (2, "lprect") >>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags) >>>
Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.
Output parameters can be combined with the 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 >>>
ctypes.addressof(obj)
Returns the address of the memory buffer as integer. obj must be an instance of a ctypes type.
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.
New in version 2.6: The offset optional argument was added.
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])
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 specifying 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.
ctypes.create_unicode_buffer(init_or_size[, size])
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 specifying the size of the array if the length of the string should not be used.
If the first parameter is an 8-bit string, it is converted into a unicode string according to ctypes conversion rules.
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.
Changed in version 2.6: Windows only: find_library("m")
or find_library("c")
return the result of a call to find_msvcrt()
.
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.
New in version 2.6.
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.
New in version 2.6.
ctypes.get_last_error()
Windows only: returns the current value of the ctypes-private copy of the system LastError
variable in the calling thread.
New in version 2.6.
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_conversion_mode(encoding, errors)
This function sets the rules that ctypes objects use when converting between 8-bit strings and unicode strings. encoding must be a string specifying an encoding, like 'utf-8'
or 'mbcs'
, errors must be a string specifying the error handling on encoding/decoding errors. Examples of possible values are "strict"
, "replace"
, or "ignore"
.
set_conversion_mode()
returns a 2-tuple containing the previous conversion rules. On windows, the initial conversion rules are ('mbcs',
'ignore')
, on other systems ('ascii', 'strict')
.
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.
New in version 2.6.
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.
New in version 2.6.
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])
This function returns the string starting at memory address address. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated.
ctypes.WinError(code=None, descr=None)
Windows only: this function is probably the worst-named thing in ctypes. It creates an instance of WindowsError. If code is not specified, GetLastError
is called to determine the error code. If descr
is not specified, FormatError()
is called to get a textual description of the error.
ctypes.wstring_at(address[, size])
This function returns the wide character string starting at memory address address as unicode string. If size is specified, it is used as the number of characters of the string, otherwise the string is assumed to be zero-terminated.
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.
New in version 2.6.
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.
New in version 2.6.
from_address(address)
This method returns a ctypes type instance using the memory specified by address which must be an integer.
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.
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.
Changed in version 2.6: 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 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 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 string.
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
.
New in version 2.6.
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 2.7.
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.
New in version 2.6.
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.
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 descriptors 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 defined 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 allow 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 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_
.
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.
© 2001–2020 Python Software Foundation
Licensed under the PSF License.
https://docs.python.org/2.7/library/ctypes.html