memcpy

[Marc 'BlackJack' Rintsch]

On Tue, 11 Sep 2007 05:09:58 -0700, Tim wrote:

> On Sep 10, 3:31 pm, Marc 'BlackJack' Rintsch <bj_... at gmx.net> wrote:
>> On Mon, 10 Sep 2007 11:38:50 -0700, Tim wrote:
>> > How do I memcpy from a pointer to an array of floats in python?
>>
>> > I get errors: NameError: global name 'row' is not defined
>>
>> Well than the (global) name `row` is not defined.  Quite clear message,
>> isn't it?  ;-)
>>
>> > I want to be able to get the row[i] array element. In C I would
>> > normally place the address of row as the first argument.
>>
>> > cdll.msvcrt.memcpy( row, pData, 256 )
>>
>> > If I define row as the following I also get the following error:
>>
>> > row = ones( TOTAL_PARAMETER_ENTRIES, dtype=float )
>>
>> > ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know
>> > how to convert parameter 1
>>
>> You don't give enough information so we have to guess.  For example I
>> guess the `ones()` function comes from one of the packages `numeric`,
>> `numarray` or `numpy`!?
>>
>> This function returns a Python object.  You can't use arbitrary Python
>> objects with `ctypes`.  `memcpy` expects a pointer not an object.
>
> Can I initialize something in Python that I can get access to it's
> pointer?

"It's pointer"?  Then you have a pointer to a structure that represents
the Python object but still no idea what data is at that pointer.  This is
an implementation detail of the Python version and of the particular
object.

> Here is what I would like to write:
> 
> shared_memory_pointer = windll.kernel32.MapViewOfFile(hMapObject,
> FILE_MAP_ALL_ACCESS,
>             0, 0, TABLE_SHMEMSIZE)
> 
> memcpy( self.data, shared_memory_pointer, my_size )

I haven't tested but it should be possible to declare the return type of
`windll.kernel32.MapViewOfFile()` as ``ctypes.POINTER(ctypes.c_double *
256)`` and then do:

test_data = numpy.ones(1000)
shared_memory_pointer.contents[0:256] = test_data[0:256]

Ciao,
	Marc 'BlackJack' Rintsch