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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: how cast works?
Date: Fri, 9 Aug 2024 18:16:19 +0200
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On 09/08/2024 01:14, Keith Thompson wrote:
> David Brown <david.brown@hesbynett.no> writes:
> [...]
>> A _Bool is always either 0 or 1.  The conversion is whatever the
>> compiler needs to give an int of value 0 or 1.
> 
> The value of a _Bool object is always either 0 or 1 *unless* the program
> does something weird.

True.  But attempting to use a _Bool object (as a _Bool) that does not 
contain either 0 or 1 is going to be undefined behaviour (at least it 
was on the platform where I saw this happen as a code bug).

> 
> C23 is a bit clearer about the representation of bool (still also called
> _Bool) than previous editions.  It states (draft N3220) that :
> 
>      The type bool shall have one value bit and (sizeof(bool)*CHAR_BIT)-1
>      padding bits.
> 
> There are several ways to force a representation other than 00000000 or
> 00000001 into a _Bool object, including a union, memset(), or type
> punning via a pointer cast.
> 
> C23 dropped the term "trap representation", replacing it with "non-value
> representation" -- a reasonable change, since accessing a trap
> representation is not guaranteed to cause the program to "perform a
> trap" (defined as "interrupt execution of the program such that no
> further operations are performed").
> 
> It doesn't specify whether setting the padding bits to 1 results in a
> non-value representation.

That's probably an implementation-defined issue, is it not?

> 
> If non-zero padding bits create a non-value representation, then
> accessing a bool object holding such a representation has undefined
> behavior.  It could, among other things, yield the same value implied by
> the representation as if it were an ordinary integer of the same size.
> 
> If there are no non-value representations, then only the
> single value bit determines the value, which is either false or true.
> 
> As you mentioned, I expect that sizeof (bool) will normally be 1, but an
> implementation could make it wider, e.g. with 1 value bit and 31 padding
> bits.
>