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15.17. ctypes — A foreign function library for Python
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.
15.17.1. ctypes tutorial
Note: The code samples in this tutorial use doctest
to make sure that they actually work. Since some code samples behave differently under Linux, Windows, or Mac OS X, they contain doctest directives in comments.
Note: Some code samples reference the ctypes c_int
type. 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.
15.17.1.1. Loading dynamic link libraries
ctypes
exports the cdll, and on Windows windll and oledll objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects. cdll loads libraries which export functions using the standard cdecl
calling convention, while windll libraries call functions using the stdcall
calling convention. oledll also uses the stdcall
calling convention, and assumes the functions return a Windows HRESULT
error code. The error code is used to automatically raise 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 ...>
>>>
15.17.1.2. Accessing functions from loaded dlls
Functions are accessed as attributes of dll objects:
>>> from ctypes import *
>>> libc.printf
<_FuncPtr object at 0x...>
>>> print windll.kernel32.GetModuleHandleA
<_FuncPtr object at 0x...>
>>> print windll.kernel32.MyOwnFunction
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "ctypes.py", line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
>>>
Note that win32 system dlls like kernel32
and user32
often export ANSI as well as UNICODE versions of a function. The UNICODE version is exported with an W
appended to the name, while the ANSI version is exported with an A
appended to the name. The win32 GetModuleHandle
function, which returns a module handle for a given module name, has the following C prototype, and a macro is used to expose one of them as GetModuleHandle
depending on whether UNICODE is defined or not:
/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
windll does not try to select one of them by magic, you must access the version you need by specifying GetModuleHandleA
or GetModuleHandleW
explicitly, and then call it with 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
>>>
15.17.1.3. Calling functions
You can call these functions like any other Python callable. This example uses the time()
function, which returns system time in seconds since the Unix epoch, and the GetModuleHandleA()
function, which returns a win32 module handle.
This example calls both functions with a NULL pointer (None
should be used as the NULL pointer):
>>> print libc.time(None)
1150640792
>>> print hex(windll.kernel32.GetModuleHandleA(None))
0x1d000000
>>>
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.
15.17.1.4. Fundamental 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 |
The constructor accepts any object with a truth value.
All these types can be created by calling them with an optional initializer of the correct type and value:
>>> c_int()
c_long(0)
>>> c_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.
15.17.1.5. Calling functions, continued
Note that printf prints to the real standard output channel, not to sys.stdout
, so these examples will only work at the console prompt, not from within IDLE or PythonWin:
>>> printf = libc.printf
>>> printf("Hello, %s\n", "World!")
Hello, World!
14
>>> printf("Hello, %S\n", u"World!")
Hello, World!
14
>>> printf("%d bottles of beer\n", 42)
42 bottles of beer
19
>>> printf("%f bottles of beer\n", 42.5)
Traceback (most recent call last):
File "<stdin>", line 1, in <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
>>>
15.17.1.6. Calling functions with your own custom data types
You can also customize ctypes
argument conversion to allow instances of your own classes be used as function arguments. ctypes
looks for an _as_parameter_
attribute and uses this as the function argument. Of course, it must be one of integer, string, or 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.
15.17.1.7. Specifying the required argument types (function prototypes)
It is possible to specify the required argument types of functions exported from DLLs by setting the argtypes
attribute.
argtypes
must be a sequence of C data types (the printf
function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf("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.
15.17.1.8. Return types
By default functions are assumed to return the C int
type. Other return types can be specified by setting the restype
attribute of the function object.
Here is a more advanced example, it uses the strchr
function, which expects a string pointer and a char, and returns a pointer to a string:
>>> strchr = libc.strchr
>>> strchr("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.
15.17.1.9. Passing pointers (or: passing parameters by reference)
Sometimes a C api function expects a pointer to a data type as parameter, probably to write into the corresponding location, or if the data is too large to be passed by value. This is also known as passing parameters by reference.
ctypes
exports the byref()
function which is used to pass parameters by reference. The same effect can be achieved with the pointer()
function, although pointer()
does a lot more work since it constructs a real pointer object, so it is faster to use byref()
if you don’t need the pointer object in Python itself:
>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer('\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'
>>>
15.17.1.10. Structures and unions
Structures and unions must derive from the Structure
and Union
base classes which are defined in the ctypes
module. Each subclass must define a _fields_
attribute. _fields_
must be a list of 2-tuples, containing a field name and a field type.
The field type must be a ctypes
type like c_int
, or any other derived ctypes
type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named x and y, and also shows how to initialize a structure in the constructor:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = [("x", c_int),
... ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print point.x, point.y
10 20
>>> point = POINT(y=5)
>>> print point.x, point.y
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: too many initializers
>>>
You can, however, build much more complicated structures. A structure can itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft and lowerright:
>>> class RECT(Structure):
... _fields_ = [("upperleft", POINT),
... ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print rc.upperleft.x, rc.upperleft.y
0 5
>>> print rc.lowerright.x, rc.lowerright.y
0 0
>>>
Nested structures can also be initialized in the constructor in several ways:
>>> r = RECT(POINT(1, 2), POINT(3, 4))
>>> r = RECT((1, 2), (3, 4))
Field 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.
15.17.1.11. Structure/union alignment and byte order
By default, Structure and Union fields are aligned in the same way the C compiler does it. It is possible to override this behavior be specifying a _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.
15.17.1.12. Bit fields in structures and unions
It is possible to create structures and unions containing bit fields. Bit fields are only possible for integer fields, the bit width is specified as the third item in the _fields_
tuples:
>>> class Int(Structure):
... _fields_ = [("first_16", c_int, 16),
... ("second_16", c_int, 16)]
...
>>> print Int.first_16
<Field type=c_long, ofs=0:0, bits=16>
>>> print Int.second_16
<Field type=c_long, ofs=0:16, bits=16>
>>>
15.17.1.13. Arrays
Arrays are sequences, containing a fixed number of instances of the same type.
The recommended way to create array types is by multiplying a data type with a positive integer:
TenPointsArrayType = POINT * 10
Here is an example of a somewhat artificial data type, a structure containing 4 POINTs among other stuff:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", c_int), ("y", c_int)
...
>>> class MyStruct(Structure):
... _fields_ = [("a", c_int),
... ("b", c_float),
... ("point_array", POINT * 4)]
>>>
>>> print len(MyStruct().point_array)
4
>>>
Instances are created in the usual way, by calling the class:
arr = TenPointsArrayType()
for pt in arr:
print pt.x, pt.y
The above code print a series of 0 0
lines, because the array contents is initialized to zeros.
Initializers of the correct type can also be specified:
>>> from ctypes import *
>>> TenIntegers = c_int * 10
>>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
>>> print ii
<c_long_Array_10 object at 0x...>
>>> for i in ii: print i,
...
1 2 3 4 5 6 7 8 9 10
>>>
15.17.1.14. Pointers
Pointer instances are created by calling the pointer()
function on a ctypes
type:
>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>
Pointer instances have a contents
attribute which returns the object to which the pointer points, the i
object above:
>>> pi.contents
c_long(42)
>>>
Note that ctypes
does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:
>>> pi.contents is i
False
>>> pi.contents is pi.contents
False
>>>
Assigning another c_int
instance to the pointer’s contents attribute would cause the pointer to point to the memory location where this is stored:
>>> i = c_int(99)
>>> pi.contents = i
>>> pi.contents
c_long(99)
>>>
Pointer instances can also be indexed with integers:
>>> pi[0]
99
>>>
Assigning to an integer index changes the pointed to value:
>>> print i
c_long(99)
>>> pi[0] = 22
>>> print i
c_long(22)
>>>
It is also possible to use indexes different from 0, but you must know what you’re doing, just as in C: You can access or change arbitrary memory locations. Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item.
Behind the scenes, the pointer()
function does more than simply create pointer instances, it has to create pointer types first. This is done with the POINTER()
function, which accepts any ctypes
type, and returns a new type:
>>> PI = POINTER(c_int)
>>> PI
<class 'ctypes.LP_c_long'>
>>> PI(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: expected c_long instead of int
>>> PI(c_int(42))
<ctypes.LP_c_long object at 0x...>
>>>
Calling the pointer type without an argument creates a NULL
pointer. NULL
pointers have a False
boolean value:
>>> null_ptr = POINTER(c_int)()
>>> print bool(null_ptr)
False
>>>
ctypes
checks for NULL
when dereferencing pointers (but dereferencing invalid non-NULL
pointers would crash Python):
>>> null_ptr[0]
Traceback (most recent call last):
....
ValueError: NULL pointer access
>>>
>>> null_ptr[0] = 1234
Traceback (most recent call last):
....
ValueError: NULL pointer access
>>>
15.17.1.15. Type conversions
Usually, ctypes does strict type checking. This means, if you have POINTER(c_int)
in the argtypes
list of a function or as the type of a member field in a structure definition, only instances of exactly the same type are accepted. There are some exceptions to this rule, where ctypes accepts other objects. For example, you can pass compatible array instances instead of pointer types. So, for POINTER(c_int)
, ctypes accepts an array of c_int:
>>> class Bar(Structure):
... _fields_ = [("count", c_int), ("values", POINTER(c_int))]
...
>>> bar = Bar()
>>> bar.values = (c_int * 3)(1, 2, 3)
>>> bar.count = 3
>>> for i in range(bar.count):
... print bar.values[i]
...
1
2
3
>>>
In addition, if a function argument is explicitly declared to be a pointer type (such as POINTER(c_int)
) in argtypes
, an object of the pointed type (c_int
in this case) can be passed to the function. ctypes will apply the required byref()
conversion in this case automatically.
To set a POINTER type field to NULL
, you can assign None
:
>>> bar.values = None
>>>
Sometimes you have instances of incompatible types. In C, you can cast one type into another type. ctypes
provides a cast()
function which can be used in the same way. The Bar
structure defined above accepts POINTER(c_int)
pointers or c_int
arrays for its values
field, but not instances of other types:
>>> bar.values = (c_byte * 4)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
>>>
For these cases, the cast()
function is handy.
The cast()
function can be used to cast a ctypes instance into a pointer to a different ctypes data type. cast()
takes two parameters, a ctypes object that is or can be converted to a pointer of some kind, and a ctypes pointer type. It returns an instance of the second argument, which references the same memory block as the first argument:
>>> a = (c_byte * 4)()
>>> cast(a, POINTER(c_int))
<ctypes.LP_c_long object at ...>
>>>
So, cast()
can be used to assign to the values
field of Bar
the structure:
>>> bar = Bar()
>>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
>>> print bar.values[0]
0
>>>
15.17.1.16. Incomplete Types
Incomplete Types are structures, unions or arrays whose members are not yet specified. In C, they are specified by forward declarations, which are defined later:
struct cell; /* forward declaration */
struct cell {
char *name;
struct cell *next;
};
The straightforward translation into ctypes code would be this, but it does not work:
>>> class cell(Structure):
... _fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in cell
NameError: name 'cell' is not defined
>>>
because the new class cell
is not available in the class statement itself. In ctypes
, we can define the cell
class and set the _fields_
attribute later, after the class statement:
>>> from ctypes import *
>>> class cell(Structure):
... pass
...
>>> cell._fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
>>>
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
>>>
15.17.1.17. Callback functions
ctypes
allows creating C callable function pointers from Python callables. These are sometimes called callback functions.
First, you must create a class for the callback function, the class knows the calling convention, the return type, and the number and types of arguments this function will receive.
The CFUNCTYPE factory function creates types for callback functions using the 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.
15.17.1.18. Accessing values exported from dlls
Some shared libraries not only export functions, they also export variables. An example in the Python library itself is the Py_OptimizeFlag
, an integer set to 0, 1, or 2, depending on the -O
or -OO
flag given on startup.
ctypes
can access values like this with the in_dll()
class methods of the type. pythonapi is a predefined symbol giving access to the Python C api:
>>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag")
>>> print opt_flag
c_long(0)
>>>
If the interpreter would have been started with -O
, the sample would have printed c_long(1)
, or c_long(2)
if -OO
would have been specified.
An extended example which also demonstrates the use of pointers accesses the PyImport_FrozenModules
pointer exported by Python.
Quoting the 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.
15.17.1.19. Surprises
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!
15.17.1.20. Variable-sized data types
ctypes
provides some support for variable-sized arrays and structures.
The resize()
function can be used to resize the memory buffer of an existing ctypes object. The function takes the object as first argument, and the requested size in bytes as the second argument. The memory block cannot be made smaller than the natural memory block specified by the objects type, a ValueError
is raised if this is tried:
>>> short_array = (c_short * 4)()
>>> print sizeof(short_array)
8
>>> resize(short_array, 4)
Traceback (most recent call last):
...
ValueError: minimum size is 8
>>> resize(short_array, 32)
>>> sizeof(short_array)
32
>>> sizeof(type(short_array))
8
>>>
This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:
>>> short_array[:]
[0, 0, 0, 0]
>>> short_array[7]
Traceback (most recent call last):
...
IndexError: invalid index
>>>
Another way to use variable-sized data types with ctypes
is to use the dynamic nature of Python, and (re-)define the data type after the required size is already known, on a case by case basis.
15.17.2. ctypes reference
15.17.2.3. Foreign functions
As explained in the previous section, foreign functions can be accessed as attributes of loaded shared libraries. The function objects created in this way by default accept any number of arguments, accept any ctypes data instances as arguments, and return the default result type specified by the library loader. They are instances of a private class:
- class
ctypes.
_FuncPtr
-
Base class for C callable foreign functions.
Instances of foreign functions are also C compatible data types; they represent C function pointers.
This behavior can be customized by assigning to special attributes of the foreign function object.
restype
-
Assign a ctypes type to specify the result type of the foreign function. Use
None
forvoid
, 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 asrestype
and assign a callable to theerrcheck
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 theargtypes
tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, ac_char_p
item in theargtypes
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.
15.17.2.4. Function prototypes
Foreign functions can also be created by instantiating function prototypes. Function prototypes are similar to function prototypes in C; they describe a function (return type, argument types, calling convention) without defining an implementation. The factory functions must be called with the desired result type and the argument types of the function.
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 realerrno
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 whereWINFUNCTYPE()
is the same asCFUNCTYPE()
. The function will release the GIL during the call. use_errno and use_last_error have the same meaning as above.
ctypes.
PYFUNCTYPE
( restype, *argtypes )-
The returned function prototype creates functions that use the Python calling convention. The function will not release the GIL during the call.
Function prototypes created by these factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call:
prototype
( address )Returns a foreign function at the specified address which must be an integer.
prototype
( callable )Create a C callable function (a callback function) from a Python callable.
prototype
( func_spec [, paramflags ] )Returns a foreign function exported by a shared library. func_spec must be a 2-tuple
(name_or_ordinal, library)
. The first item is the name of the exported function as string, or the ordinal of the exported function as small integer. The second item is the shared library instance.
prototype
( vtbl_index, name [, paramflags [, iid ] ] )Returns a foreign function that will call a COM method. vtbl_index is the index into the virtual function table, a small non-negative integer. name is name of the COM method. iid is an optional pointer to the interface identifier which is used in extended error reporting.
COM methods use a special calling convention: They require a pointer to the COM interface as first argument, in addition to those parameters that are specified in the
argtypes
tuple.The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.
paramflags must be a tuple of the same length as
argtypes
.Each item in this tuple contains further information about a parameter, it must be a tuple containing one, two, or three items.
The first item is an integer containing a combination of direction flags for the parameter:
- 1
Specifies an input parameter to the function.
- 2
Output parameter. The foreign function fills in a value.
- 4
Input parameter which defaults to the integer zero.
The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.
The optional third item is the default value for this parameter.
This example demonstrates how to wrap the Windows 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
>>>