How to safely cast a _pointer to _bytes?

Questions & Answers
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1

I'm using racket/draw/unsafe/cairo and a custom ffi for MagickWand C API.
I have this definition, you can see the last parameter is _bytes.

(define-mwand MagickExportImagePixels (_fun _MagickWand-pointer _size _size _size _size _string _StorageType _bytes -> _MagickBooleanType))

The cairo function I'm trying to use is

(define-cairo cairo_image_surface_get_data* (_cfun (s : _cairo_surface_t)
                                                   -> _pointer)
  #:c-id cairo_image_surface_get_data)

Which returns a pointer which then I feed to MagickExportImagePixels, for now I just added another definition called MagickExportImagePixels/pointer which is the same as the namesake function but _bytes is replaced with _pointer.

Yet I still have the question, how to cast a _pointer to _bytes correctly, I just did (cast data-pointer _pointer _bytes) and it crashed or at least I remember doing this, I also realize that _bytes in the docs is syntax, so that might be also part of a problem.

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2

Maybe something like this?

image
image1498×772 121 KB

From cairo/main.rkt at 5281c9acf2217015b96b8a6a4788b9ec6eecd179 · soegaard/cairo · GitHub

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3

Thank you for the sample code, I didn't realize there was a wrapper for cairo since I generally check the documentation index and not the package index and I also didn't realize there was two cairo_image_surface_get_data one with _bytes and one with _pointer since I want to avoid more allocations, the solution seems to be to have two functions pointing to the same c function.

4

Would it be correct to mark @soegaard 's answer as the solution?

5

The library is still new.

There is no documentation yet.

Also the library has only seen limited testing, so beware for bugs.

6

well, I'm not using that exactly, but I think I will

7

Just for the record: converting from _pointer to _bytes requires copying, and when _bytes is used without a size, the copied data's length is inferred from a terminating a nul character. The MagickExportImagePixels binding should almost certainly use _pointer instead of _bytes, since it's not dealing with nul-terminated data. It should be just as convenient, since _pointer as an argument type accepts a byte-string.

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