Re: Data_as

On Tue, Dec 30, 2014 at 11:15 AM, West, Peter <[email protected]> wrote:
>
>    pointerArrayType = POINTER( c_uint16 ) * 2      # an array of pointers type length 2
>    pointers = pointerArrayType()                           # the actual array
....
>    print pointers[ 0 ]
>    print pointers[ 1 ]
>
> This prints
> <__main__.LP_c_ushort object at 0x00000000025132C8>
> <__main__.LP_c_ushort object at 0x00000000025132C8>
>
> This suggests that the data arrays for frame1 and frame2 are at the same location.

The repr shows the address of the Python object, not the pointer value.

    >>> x, y = pointers
    >>> print x
    <__main__.LP_c_ushort object at 0x7f10648378c0>
    >>> print y
    <__main__.LP_c_ushort object at 0x7f1064837c20>

The pointer's contents attribute returns a ctypes object that can be
passed to addressof.

    >>> from ctypes import addressof
    >>> pointers[0].contents
    c_ushort(0)
    >>> addressof(pointers[0].contents)
    43254640
    >>> addressof(pointers[1].contents)
    43274656

Note that pointers[0][0] wouldn't work. That converts the c_ushort to
a Python integer.

    >>> pointers[0][0]
    0
    >>> addressof(pointers[0][0])
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: invalid type

You could also cast to a void pointer.

    >>> from ctypes import cast, c_void_p
    >>> cast(pointers[0], c_void_p)
    c_void_p(43254640)
    >>> cast(pointers[1], c_void_p)
    c_void_p(43274656)

Or use the buffer protocol instead of cast.

    >>> c_void_p.from_buffer(pointers[0])
    c_void_p(43254640)
    >>> c_void_p.from_buffer(pointers[1])
    c_void_p(43274656)

Verify against the numpy array's ctypes attribute.

    >>> frame1.ctypes.data
    43254640
    >>> frame2.ctypes.data
    43274656

    >>> frame1.ctypes._as_parameter_
    c_void_p(43254640)
    >>> frame2.ctypes._as_parameter_
    c_void_p(43274656)

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