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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: _BitInt(N)
Date: Tue, 25 Nov 2025 19:21:03 -0800
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
> bart <bc@freeuk.com> writes:
[...]
>> OK, so why are you not allowed to have _BitInt(1)? That is, a 1-bit
>> signed integer. It might only have two values of 0 and -1; doesn't
>> nobody want that particular combination?
>
> I don't know.  The language allows 1-bit signed bit-fields, so
> _BitInt(1) would make some sense, but the language requires N to
> be at least 1 for unsigned _BitInt and 2 for signed _BitInt.
>
> It doesn't bother me too much, since I'm unlikely to have a
> use for signed _BitInt(1).  But it's an arbitrary restriction.
[...]

I just learned that there's a proposal to allow _BitInt(1) in C2y.

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3699.pdf

The current restriction apparently was for historical reasons.
Prior to C23, C didn't require two's complement for signed types,
and signed _BitInt(1) doesn't make much sense for one's complement
or sign-and-magnitude (it could only hold +0 and -0).

Yes, C23 added both _BitInt and the requirement for two's complement,
but preliminary implementations of _BitInt go back several years,
and the requirements didn't catch up.  Stuff happens.

Incidentally, C23 requires BITINT_MAXWIDTH to be at least
ULLONG_WIDTH, which is at least 64.  clang/llvm sets it to 128 for
some target systems.

<OT>
https://eisenwave.github.io/cpp-proposals/bitint.html
is a proposal to add C23-style bit-precise integers to C++.
</OT>

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