Strange behaviour of ptr-ref + ptr-add(?)

[Laurent]

Hi all,

We're trying to figure out why the last case below doesn't work on Linux, but works on MacOS. Does anyone have an explanation?

The docs suggest that _double* shouldn't be different from _double for _reading_ values.

More precisely, 

#lang racket
(require ffi/unsafe)

(define N 10)
(define pt (malloc N _double 'atomic-interior))
(for ([i (in-range N)])
  (ptr-set! pt _double* i (+ 2. i)))

;; works
(for/list ([i (in-range N)])
  (ptr-ref pt _double i))

;; works
(for/list ([i (in-range N)])
  (ptr-ref (ptr-add pt i _double) _double*))

;; doesn't work!
(for/list ([i (in-range N)])
  (ptr-ref (ptr-add pt i _double) _double))

------

Output:

(2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0)
(2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0)
(2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0)

[Laurent]

Correction:

It's not Mac vs Linux, it's Racket BC (works) vs CS (doesn't work)

[Jens Axel Søgaard]

I can confirm that the bug is present in the latest snapshot of Racket CS on macOS.

FWIW  _double* is missing in this list, but I am not sure it whether it is supposed to be handled here.
https://github.com/racket/racket/blob/920c899ba866ce59a0387862286521e3cc1dabfb/racket/src/schemify/ptr-ref-set.rkt#L46

/Jens Axel

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[Laurent]

Bug report here:

https://github.com/racket/racket/issues/3173

[Matthew Flatt]

Although `_double*` isn't currently meant to be handled there ---
there's no `ptr-ref/_double` specialization --- you're right that the
problem is in the `ptr-ref/double` and similar specializations in CS.

I've pushed a repair.

Thanks for the report, Laurent!

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[Laurent]

Awesome, thanks Matthew!

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