Re: From wchar_t to char32_t

[Paul Eggert]

On 2023-07-01 07:35, Bruno Haible wrote:
> -      wchar_t wch;
> -      size_t nbytes = mbrtowc (&wch, s, n, &d->mbs);
> +      char32_t wch;
> +      size_t nbytes = mbrtoc32 (&wch, s, n, &d->mbs);
>        if (0 < nbytes && nbytes < (size_t) -2)
>          {
>            *pwc = wch;
> +          if (nbytes == (size_t) -3)
> +            nbytes = 0;
>            return nbytes;

That last change doesn't match the comment for the mbs_to_wchar 
function, which says that the function always returns a positive int. 
Callers depend on this.

Since nbytes cannot be (size_t) -3 on any known implementation, it's not 
surprising that this issue wasn't found by testing. And since it'll 
likely be a hassle to port the rest of the code to purely-theoretical 
platforms where nbytes == (size_t) -3, I suggest instead simply adding a 
comment that nbytes cannot be (size_t) -3 there. This would follow the 
advice I added to the Gnulib manual recently, here:

https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=e046d5458353f112e78893ca03d855c8a9aa2e39