[Numpy-discussion] make C array accessible to python without copy

Sebastian Haase haase at msg.ucsf.edu
Tue May 13 19:14:05 CDT 2003


Hi - I hope it's OK to just resent my posting from Feb 22.
I didn't get any response at the time.
Also - looking into the code again - I am wondering what the best way to
handle the reference counting would be ?
Here come my original posting:

> Short follow up:
> 1) Is it planned to support this more directly?
> 2) How much does it cost to create a buffer object if it
> uses my already allocated memory ?
> 3) Can I change the pointer so that it points to a
> different memory space WITHOUT having to recreate any
> python objects? Or would that "confuse" the buffer or
> numarray?    (We are hoping to aquire 30 images per second
> - the images should get written into a circular buffer so
> that the data can be written to disk in larger chunks -
> but the python array should always refer to the current
> image )
>
> Thanks for all the nice toys (tools) ;-)
> Sebastian Haase
>
>
>
> On Fri, 17 Jan 2003 18:16:01 -0500
>   Todd Miller <jmiller at stsci.edu> wrote:
> >Sebastian Haase wrote:
> >
> >>Hi,
> >>What is the C API to make an array that got allocated,
> >>let's say, by  a = new short[512*512],
> >>accessible to python as numarray.
> >>
> >What you want to do is not currently supported well in C.
> > The way to do what you want is:
> >
> >1.  Create a buffer object from your C++ array.  The
> >buffer object can be built such that it refers to the
> >original copy of the data.
> >
> >2.  Call  back into Python (numarray.NumArray) with your
> >buffer object as the buffer parameter.
> >
> >You can scavenge the code in NA_newAll (Src/newarray.ch)
> >for most of the callback.
> >
> >>I tried NA_New - but that seems to make a copy.
> >>I would need it to use the original memory space
> >>so that I can "observe" the array from Python WHILE
> >>the underlying C array changes (it's actually a camera
> >>image)
> >>
> >That sounds cool!
> >
> >>
> >>Thanks,
> >>Sebastian Haase





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