Re: how cast works?

Em 8/11/2024 9:30 AM, Bart escreveu:

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.

One of my tests led me to the wrong conclusion that reading a boolean value would cause the compiler to add a conversion on read.
I did something wrong..I don't remember. I will try to keep all tests next time.
But now, everything is back to normal.
   union {
         int i;
         _Bool b;
     } data;
     data.b = 123;
     printf("%d", data.b); //prints 1 as expected
   union {
         int i;
         _Bool b;
     } data;
     data.i = 123;
     printf("%d", data.b); //prints 123 as expected

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.

Something that could be done is to check in local context the last write type is the same of last read. Then we can have a warning if they are different.