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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: _BitInt(N)
Date: Mon, 01 Dec 2025 08:56:10 -0800
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David Brown <david.brown@hesbynett.no> writes:
[...]
> (I am not entirely sure, but I think it is standards-conforming for an
> implementation to haev BITINT_MAXWIDTH set to 64 and support all
> _BitInts up size 64, and then also support _BitInts of multiples of 64
> thereafter.  Use of _BitInt greater than BITINT_MAXWIDTH is UB in the
> standard - so an implementation can choose to give that a defined
> behaviour for specific sizes.)
[...]

No, _BitInt(N) where N > BITINT_MAXWIDTH is a constraint violation.

N3220 6.7.3.1p2 ("Constraints") :

    The parenthesized constant expression that follows the _BitInt
    keyword shall be an integer constant expression N that specifies
    the width (6.2.6.2) of the type. The value of N for unsigned
    _BitInt shall be greater than or equal to 1. The value of N
    for _BitInt shall be greater than or equal to 2.  The value of
    N shall be less than or equal to the value of BITINT_MAXWIDTH
    (see 5.2.5.3.2).

As I mentioned before, there's a proposal for C2y to allow
signed _BitInt(1).

Of course an implementation could do what you suggest as an extension.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */