[Numpy-discussion] Pointer to array data for passing to swig-wrapped C++

[Robert Kern]

On Mon, Aug 24, 2009 at 12:37, Angus McMorland<amcmorl at gmail.com> wrote:
> Hi all,
>
> Our lab has an in-house messaging protocol, written in C++, for
> interfacing the different components of our experimental setups, and
> allowing programs written in several different languages to talk to
> each other. I'm currently trying to write a Python interface to this
> protocol, mainly so I can show people here how good a Traits-based GUI
> would be for controlling things. I'm not familiar with interfacing C++
> and Python code, so this is a little hit and miss, and any suggestions
> on better approaches would be welcome. I have wrapped the C++ code in
> swig, and can now call the simple routines from within Python.
>
> The trouble I'm having is constructing new message data, and passing a
> reference to that data. Each message consists of a ID code telling us
> what type of message it is, a buffer of data, of which some subset is
> actually useful, and the number of bytes of the buffer that have been
> filled with useful data. I can get message data out of a message
> constructed and sent by some other implementation of the protocol by
> reading it into a numpy array with a dtype matching the structure of
> the data being sent, and calling, for example:
>
> np.fromstring(msg_data[0:num_bytes], dtype=[('a', int),('b', float)])
>
> I can't work out how to do the reverse operation: to populate the C++
> message object with data constructed in Python. The message object has
> a SetData function,which is exposed by swig, that requires a 'void *'
> pointer to the data (as well as the number of bytes being sent), and I
> thought I might be able to do something like:
>
> msg.SetData(array.data, array.nbytes)
>
> where array is another ndarray of the desired dtype, but that returns the error:
>
> TypeError: in method 'CMessage_SetData', argument 2 of type 'void *'
>
> and using array.tostring() in place of array.data above gives exactly
> the same result. Is there a convenient syntax to pass a void * pointer
> to the array's data to the swig-wrapped SetData routine, or do I have
> to write some extra wrapping code to make this happen, and what might
> that code look like roughly?

You will need an extra typemap on SetData() to make it accept a char*
on the Python side (which can be supplied as a string or preferably a
buffer object) and cast it to a void*. Someone with more recent SWIG
experience or the SWIG docs will have to tell you how to do that,
though. It should be straightforward, though.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco