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Groups > comp.lang.c > #387495
| From | Keith Thompson <Keith.S.Thompson+u@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: how cast works? |
| Date | 2024-08-11 20:14 -0700 |
| Organization | None to speak of |
| Message-ID | <87plqee8li.fsf@nosuchdomain.example.com> (permalink) |
| References | (8 earlier) <v93h12$9vom$1@dont-email.me> <87frre8v5q.fsf@nosuchdomain.example.com> <v95fcj$pv2g$1@dont-email.me> <87ttft7bei.fsf@nosuchdomain.example.com> <86frrak3hf.fsf@linuxsc.com> |
Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>
>> David Brown <david.brown@hesbynett.no> writes:
>>
>>> 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).
>>
>> It depends on whether representations with non-zero padding bits are
>> treated as trap representations (non-value representations in C23) or
>> not.
>
> In C99 and C11, iirc, the width of _Bool may be any value between
> 1 and CHAR_BIT. If the width of _Bool is greater than 1, a _Bool
> may have a well-defined value that is neither 0 or 1. My guess is
> most implementations define the width of _Bool as 1, but they don't
> have to (again, iirc, in C99 and C11).
C11 (N1570) isn't 100% clear, but I think you're right. The conversion
rank of _Bool is less than the rank of the char types. I don't see an
explicit statement that this implies that _Bool has less precision than
unsigned char, so conceivably a conforming implementation could give
_Bool a precision of 2*CHAR_BIT, but C23 has cleared this up so I'm not
going to worry about it (and it's possible I've missed something).
>>>> 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?
>>
>> I'm not sure whether it's implementation-defined or unspecified.
>> I don't see any mention of trap/non-value representations in Annex J.
>>
>> [...]
>
> 6.2.6.1 p 2;
So it's implementation-defined.
N1570 :
Except for bit-fields, objects are composed of contiguous sequences of
one or more bytes, the number, order, and encoding of which are either
explicitly specified or implementation-defined.
> J.3.13 p 1, third subpoint.
The number, order, and encoding of bytes in any object (when not
explicitly specified in this International Standard) (6.2.6.1).
(listed under Implementation-defined behavior).
Quoting the standard so that everyone else doesn't have to go look it up
(and guess which edition you're referring to). You might consider doing
that yourself.
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */
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Re: how cast works? Bart <bc@freeuk.com> - 2024-08-12 02:00 +0100
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-11 20:23 -0700
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 20:37 -0700
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-11 21:33 -0700
Re: how cast works? Ben Bacarisse <ben@bsb.me.uk> - 2024-08-12 16:57 +0100
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-12 10:04 -0700
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-12 13:35 -0700
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-09 07:57 -0300
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-09 16:25 +0100
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 12:06 -0700
Re: how cast works? David Brown <david.brown@hesbynett.no> - 2024-08-09 19:20 +0200
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-09 15:54 -0300
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-09 16:05 -0300
Re: how cast works? David Brown <david.brown@hesbynett.no> - 2024-08-09 21:43 +0200
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 13:28 -0700
Re: how cast works? David Brown <david.brown@hesbynett.no> - 2024-08-09 22:01 +0200
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-10 11:17 +0100
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-10 10:15 -0300
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-10 17:14 +0100
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-10 20:01 -0300
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-10 17:10 -0700
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-11 09:23 -0300
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-11 13:30 +0100
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-11 14:16 -0300
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-11 13:38 -0700
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-12 12:24 +0100
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 13:26 -0700
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-09 18:01 -0300
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 14:53 -0700
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 12:03 -0700
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-09 16:22 -0300
Re: how cast works? David Brown <david.brown@hesbynett.no> - 2024-08-09 00:36 +0200
Re: how cast works? Dan Purgert <dan@djph.net> - 2024-08-08 14:08 +0000
Re: how cast works? Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-08-09 02:42 +0000
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-07 13:08 -0700
Re: how cast works? Thiago Adams <thiago.adams@gmail.com> - 2024-08-08 08:35 -0300
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-08 12:39 -0700
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-08 18:40 -0400
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 17:15 -0700
Is there an audio book version (Was: how cast works?) gazelle@shell.xmission.com (Kenny McCormack) - 2024-08-08 16:19 +0000
Re: how cast works? Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-08-07 23:03 +0000
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