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 >>>
To set a POINTER type field to
NULL, you can assign
>>> bar.values = None >>>
XXX list other conversions...
Sometimes you have instances of incompatible types. In
C, you can
cast one type into another type.
ctypes provides a
function which can be used in the same way. The
defined above accepts
POINTER(c_int) pointers or c_int arrays
values field, but not instances of other types:
>>> bar.values = (c_byte * 4)() Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance >>>
For these cases, the
cast function is handy.
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
>>> a = (c_byte * 4)() >>> cast(a, POINTER(c_int)) <ctypes.LP_c_long object at ...> >>>
cast can be used to assign to the
values field of
>>> bar = Bar() >>> bar.values = cast((c_byte * 4)(), POINTER(c_int)) >>> print bar.values 0 >>>
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