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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: how cast works?
Date: Sun, 11 Aug 2024 13:30:08 +0100
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On 11/08/2024 13:23, Thiago Adams wrote:
> Em 8/10/2024 9:10 PM, Keith Thompson escreveu:
>> 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.
> 
> [....]
> 
>> Summary:
>>
>> Conversion from any scalar type to _Bool is well defined, and must yield
>> 0 or 1.
> 
> 
> I will fix in terns of expressions types.
> 
>   - In this case cast to bool
>   - Assignment to bool
> 
>> 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.
> 
> I did a sample now..
> 
> #include <stdio.h>
> 
> int main() {
>      union {
>          int i;
>          _Bool b;
>      } data;
>      data.i = 123;
>      printf("%d", data.b);
> }
> 
> it printed 123 not 1.
> So I think the assignment and cast covers all/most cases.
> (From some previous tests I thought this was printing 1)

That's little different from this example:

  #include <stdio.h>

  int main() {
      union {
          int i;
          float b;
      } data;
      data.i = 123;
      printf("%e", data.b);
  }

I get some arbitrary float value printed. You're supposed to know what 
you are doing with unions.

It's not something I'd worry about. If you're trying to make a safer C, 
then you'd have to ban unions, or ban bools inside unions that could be 
read out as a different type, or introduce tagged unions so that runtime 
checking can be done.