[Rd] Suggestion to emphasize Rboolean is unrelated to LGLSXP in R-exts

[Tomas Kalibera]

On 1/30/25 22:07, Michael Chirico wrote:
> Hello all,
>
> The recent change (r87656) to make Rboolean map to type 'bool', not
> 'int', broke some tests & made me realize I've had totally the wrong
> impression about what Rboolean actually is, and I suspect I'm not
> alone.
>
> Till now, I've assumed that like Rbyte --> RAWSXP, Rboolean is the
> _correct_ storage type for LGLSXP, while idioms like int *ip =
> LOGICAL(...) only "happened to work" because Rboolean masks int, which
> could change at any time.
>
> Actually, it turns out Rboolean only "happened to be int" because of
> the use of 'enum', which is changing in now-and-future C standards!
>
> We made that mistake in 9 places [1], though only one happened to
> break tests. There are at least dozens of other cases on CRAN [2],[3].
>
> Here's the current exposition on Rboolean in R-exts [4]:
>
>> Further, the included header R_ext/Boolean.h has enumeration constants TRUE and FALSE of type Rboolean in order to provide a way of using “logical” variables in C consistently. This can conflict with other software: for example it conflicts with the headers in IJG’s jpeg-9 (but not earlier versions).
> I suggest embellishing this, perhaps like so:
>
>> Further, the included header R_ext/Boolean.h has enumeration constants TRUE and FALSE of type Rboolean in order to provide a way of using “logical” variables in C consistently. This has no concept of "missingness", and so is \emph{not} related to the R logical type LGLSXP. This can conflict with other software: for example it conflicts with the headers in IJG’s jpeg-9 (but not earlier versions).

  Probably worth an extra sentence making this more explicit. I've added 
one similar to your suggestion.

Tomas

> It would also be nice if this mistake could be caught by the compiler,
> but that's another issue.
>
> Michael Chirico
>
> [1] https://github.com/Rdatatable/data.table/pull/6782/files
> [2] https://github.com/search?q=org%3Acran+%2FRboolean.*%5CbLOGICAL%2F&type=code
> [3] https://github.com/search?q=org%3Acran+%2FLOGICAL.*sizeof%5B%28%5DRboolean%2F&type=code
> [4] https://cran.r-project.org/doc/manuals/r-devel/R-exts.html#Mathematical-constants
>
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