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Re: xsize and flexmember

From: Marc Nieper-Wißkirchen
Subject: Re: xsize and flexmember
Date: Thu, 30 Apr 2020 23:01:55 +0200

Am Do., 30. Apr. 2020 um 22:49 Uhr schrieb Paul Eggert <address@hidden>:
> On 4/29/20 11:39 PM, Marc Nieper-Wißkirchen wrote:
> >> #define XFLEXSIZEOF_XSIZE(type, member, n) \
> >>   (((n) <= FLEXSIZEOF (type, member, n) \
> >>     && FLEXSIZEOF (type, member, n) <= (size_t) -1) \
> >>    ? (size_t) FLEXSIZEOF (type, member, n) : (size_t) -1)

Why do you write "(n) <= FLEXSIZEOF (type, member, n)" and not "n <
FLEXSIZEOF (type, member, n)"? In case MEMBER is the first element of
TYPE, this would not indicate an overflow, would it?

> >>
> >> A couple of problems with this approach:
> >>
> >>   * It evaluates N more than once.
> >
> > Couldn't this be solved by calling a static function that would be
> > subject to be inlined?
> I don't offhand see how to get that to work if n exceeds SIZE_MAX.

My idea was:

#define XFLEXSIZEOF_XSIZE(type, member, n) xflexsizeof_xsize_bound(
FLEXSIZEOF (type, member, n), n)
static _GL_INLINE size_t xflexsizeof_xsize_bound (umaxint_t m, size_t n)
  if (n < m && m <= (size_t) -1)
    return m;
    return (size_t) -1;

> > Why would you prefer the (longer) name XFLEXSIZEOF_XSIZE vs XFLEXSIZEOF?
> It's specialized for size_t computations, and is not in general suitable for
> ptrdiff_t or other types. Also, elsewhere in Gnulib a leading "x" means the
> function signals an error if overflow occurs, and that's not what's happening
> here. I realize we have dueling conventions here, but would prefer that
> saturated size_t arithmetic have a longer prefix or suffix than just "x".

I understand. So maybe FLEXSIZEOF_XSIZE, which would at least drop the
leading "x" as we no error is signaled. :)

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