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| From | Keith Thompson <Keith.S.Thompson+u@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: how cast works? |
| Date | 2024-08-10 17:10 -0700 |
| Organization | None to speak of |
| Message-ID | <87zfpj537h.fsf@nosuchdomain.example.com> (permalink) |
| References | (9 earlier) <v95sij$1arjo$3@dont-email.me> <v97eo3$i03p$2@dont-email.me> <v97p5g$lfau$1@dont-email.me> <v983ks$nglf$1@dont-email.me> <v98rgh$untn$1@dont-email.me> |
Thiago Adams <thiago.adams@gmail.com> writes:
> Em 8/10/2024 1:14 PM, Bart escreveu:
>>>
>>> Bart, Does your compiler support the `bool` type, where the value
>>> is always either 1 or 0?
>> There is a bool type, but it is treated like unsigned char, so is
>> non-conforming.
>
> I do the same in my compiler , when I transpile from C99 to C89.
> I was thinking how to make it conforming.
> For instance on each write.
>
> bool b = 123; -> unsigned char b = !!(123);
>
> The problem this does not fix unions, writing on int and reading from char.
I don't think you need to fix that.
In the following, I'll refer to _Bool. The same type is also called
bool if you have `#include <stdbool.h>` *or* if you have a C23 compiler.
It's always going to be possible to use type punning (memcpy, pointer
casting, union) to force a representation other than 00000000 or
00000001 into a _Bool object.
The standard doesn't have a rule that says a _Bool object can only have
the value 0 or 1. It says that *conversion* to _Bool yields a result of
0 or 1. And yes, you have to deal with that if you're translating C99
or later to C90, for both explicit and implicit conversions.
Suppose you do something like this:
_Bool b;
*(unsigned char*)&b = 0xff; // assume sizeof (_Bool) == 1
int i = b;
What is the value of b?
Under C23 rules, _Bool has 1 value bit and N-1 (typically 7) padding
bits. Any non-zero padding bits *either* create a trap representation
(C23 calls it a non-value representation) *or* are ignored when
determining the value of the object.
(_Bool can have either 254 trap representations or none. It's possible
that it might have some different number of trap representations, but
that's unlikely.)
If 11111111 is a trap/non-value representation, the behavior of
`int i = b;` is undefined; setting i to 255 or -1 are two of many
possible behaviors. If the padding bits are ignored, it must set i to 1.
Experiment shows that gcc sets i to 255 (implying that it's a trap
representation) while clang sets i to 1 (which could imply that it's not
a trap representation, but that's still a possible result of UB).
Summary:
Conversion from any scalar type to _Bool is well defined, and must yield
0 or 1.
It's possible to force a representation other than 0 or 1 into a _Bool
object, bypassing any value conversion.
Conversion from _Bool to any scalar type is well defined if the
operand is a _Bool object holding a representation of 0 or 1.
Conversion from _Bool to any scalar type for an object holding some
representation other than 0 or 1 either yields 0 or 1 (depending
on the low-order bit) or has undefined behavior.
--
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? Thiago Adams <thiago.adams@gmail.com> - 2024-08-08 14:57 -0300
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Re: how cast works? Bart <bc@freeuk.com> - 2024-08-12 15:36 +0100
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-09 13:57 -0400
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Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 17:12 -0700
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-09 18:29 -0400
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Re: how cast works? Bart <bc@freeuk.com> - 2024-08-13 11:34 +0100
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-13 07:51 -0400
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Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-13 12:46 -0700
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-13 21:51 +0100
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-13 16:46 -0700
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-14 00:56 +0100
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-18 03:37 -0700
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 14:29 -0700
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-09 18:35 -0400
Re: how cast works? Kaz Kylheku <643-408-1753@kylheku.com> - 2024-08-09 21:30 +0000
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 14:57 -0700
Re: how cast works? Kaz Kylheku <643-408-1753@kylheku.com> - 2024-08-09 23:14 +0000
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 16:58 -0700
Re: how cast works? Kaz Kylheku <643-408-1753@kylheku.com> - 2024-08-10 00:06 +0000
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 17:27 -0700
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-09 20:31 -0400
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-10 01:11 +0100
Re: how cast works? David Brown <david.brown@hesbynett.no> - 2024-08-13 11:23 +0200
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 17:32 -0700
Re: how cast works? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-08-09 18:35 -0400
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 17:27 -0700
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 12:23 -0700
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-09 21:31 +0100
Re: how cast works? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-08-09 13:49 -0700
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-09 22:01 +0100
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-12 00:33 -0700
Re: how cast works? Bart <bc@freeuk.com> - 2024-08-12 12:21 +0100
Re: how cast works? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-11 17:46 -0700
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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