Re: how cast works?

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.