Hello all
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:numpy-
> [EMAIL PROTECTED] On Behalf Of Martin Wiechert
> Sent: 15 September 2006 19:53
> To: Discussion of Numerical Python
> Subject: Re: [Numpy-discussion]PyArray_DescrConverter - alignment /
> trailingunused bytes
>
> On Friday 15 September 2006 19:14, Travis Oliphant wrote:
> > Martin Wiechert wrote:
> > > Thanks Albert! Do you also know the corresponding C-API function? It
> > > cannot be PyArray_DescrConverter (PyObject *, PyArray_Descr **), whose
> > > signature has no "align", right?
> >
> > The DescrConverter function is meant for "O&"-style conversions. It
> > can't accept an align function. We could possibly add something to the
> > converter to allow specification of alignment through the object to be
> > converted.
> >
> <snip>
> One final question. To me the repr of a dtype with gaps is a little bit
> puzzling:
>
> >>> dtype ({'names': ['a', 'b', 'c'], 'formats': ['<a4', '<f8', '<f4'],
> 'offsets': [0, 16, 24]})
> dtype([('a', '|S4'), ('', '|V12'), ('b', '<f8'), ('', '|V12'), ('c',
> '<f4')])
>
> There should be no gap between "b" and "c" but still the repr has ('',
> '|V12')
> between them. Am I missing something?
For performance reasons, compilers will typically align integers (and
probably floats) on 4-byte boundaries and apparently, doubles on 16-byte
boundaries.
Because compilers align like this, so does NumPy. This allows you to:
1. Take any kind of C struct definition
2. Convert it to a dtype
3. Create a NumPy array with this dtype
4. Pass the array's data pointer to C code
5. Cast the data pointer to a pointer to your C struct
6. Operate on the pointer to struct as if it were allocated in C
Cheers,
Albert
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