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Re: Corrupted C99 _Bool?

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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> "das...@gmail.com" <dashley@gmail.com> writes:
>
>> C99 indicates that a _Bool is an unsigned integer type.  When a
>> variable of a type other than _Bool is assigned to a _Bool, C99
>> specifies that the _Bool receives the value of 0 if the variable was
>> 0, or 1 otherwise.
>>
>> But what does C99 say about a _Bool that is corrupted to a value other
>> than 0 or 1 (perhaps through a pointer error in another part of the
>> program)?  Will this corrupted _Bool still function correctly?
>>
>> For example:
>>
>> _Bool a = true;
>> _Bool b; /* Assume b is corrupted to "2" somehow */
>>
>> if (a && b)
>> {
>>    /* Would the compiler be able to use a bitwise */
>>    /* AND above, because it assumes all _Bools */
>>    /* must be 0 or 1, so that if (1 && 2) becomes */
>>    /* if (1 & 2) and evaluates false? */
>> }
>>
>> Thanks for any clarification.
>
> I'm reasonably sure that anything that would cause a _Bool object
> to have a value other than 0 or 1, or at least anything that tried
> to access such an object's value, would have undefined behavior.

In C99 and also in C11, an implementation may define the type _Bool
to have a size of 1, a width of CHAR_BIT, and a representation that
matches the representation of unsigned char.  I'm not aware of any
implementation that makes these choices, but they are allowed under
the rules of the respective ISO C standards (assuming I didn't miss
something, but I'm reasonably sure that I didn't).

Code running in such an implementation can easily store an object
representation, into a _Bool object, of an unsigned char with the
value 2.  Because the representations of _Bool and unsigned char
are the same, that is also the object representation of the value 2
considered as a _Bool object.

After the store, reading the _Bool object (as a _Bool) is well
defined, and must produce the value 2, because of how object
representations are turned into values.  So, in such an
implementation, 'a && b' might not be the same as 'a & b',
even though no undefined behavior has occurred.

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Thread

Corrupted C99 _Bool? "das...@gmail.com" <dashley@gmail.com> - 2023-07-11 17:32 -0700
  Re: Corrupted C99 _Bool? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-11 18:51 -0700
    Re: Corrupted C99 _Bool? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-11 19:13 -0700
    Re: Corrupted C99 _Bool? James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-07-12 02:47 -0400
    Re: Corrupted C99 _Bool? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-20 21:25 -0700
      Re: Corrupted C99 _Bool? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 01:58 -0700
        Re: Corrupted C99 _Bool? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-21 14:22 -0700
  Re: Corrupted C99 _Bool? David Brown <david.brown@hesbynett.no> - 2023-07-12 09:31 +0200
  Re: Corrupted C99 _Bool? Opus <ifonly@youknew.org> - 2023-07-12 22:10 +0200
    Re: Corrupted C99 _Bool? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-12 14:21 -0700
      Re: Corrupted C99 _Bool? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-20 22:17 -0700
    Re: Corrupted C99 _Bool? James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-07-12 18:44 -0400
      Re: Corrupted C99 _Bool? "das...@gmail.com" <dashley@gmail.com> - 2023-07-12 16:41 -0700
        Re: Corrupted C99 _Bool? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-13 01:17 +0100
        Re: Corrupted C99 _Bool? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-12 17:28 -0700
        Re: Corrupted C99 _Bool? David Brown <david.brown@hesbynett.no> - 2023-07-13 11:27 +0200
          Re: Corrupted C99 _Bool? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-13 15:18 -0700
            Re: Corrupted C99 _Bool? David Brown <david.brown@hesbynett.no> - 2023-07-14 09:07 +0200
            Re: Corrupted C99 _Bool? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-07-14 00:53 -0700
    Re: Corrupted C99 _Bool? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-20 22:09 -0700

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