On 2025-05-16 15:48:35 +0200, Charlie Gordon wrote:
> On 16 May 2025, at 14:29, Vincent Lefevre <vinc...@vinc17.net> wrote:
> > This is portable because int64_t necessarily fits in a long long on
> > any platform.
>
> This is correct, but only since C23. Previous versions of the C
> Standard would have allowed `long long` to have a different
> representation from `int64_t` that would not support the whole set
> of values (eg: all 64-bit values except -9223372036854775808)
> causing undefined behavior when this value is cast as `long long`.
Hmmm... So you mean a compiler where "long long" is a 64-bit type
not represented in two's complement, but at the same time, which
provides int64_t, which is necessarily represented in two's
complement?
But I don't think that this is conforming even in C99, which says:
* in 6.2.5p8: "For any two integer types with the same signedness
and different integer conversion rank (see 6.3.1.1), the range
of values of the type with smaller integer conversion rank is a
subrange of the values of the other type."
* in 6.3.1.1p1: "The rank of any standard integer type shall be
greater than the rank of any extended integer type with the
same width."
So, if long long and int64_t have the same width, the conversion
rank of long long is greater than the rank of int64_t (6.3.1.1p1),
which means that all the values of int64_t are representable in
long long (6.2.5p8).
--
Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / Pascaline project (LIP, ENS-Lyon)
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