Re: [PATCH] Use ^=1 to toggle between 0 and 1

From: René Scharfe <l.s.r@web.de>
Date: Dec 14, 2023, 13:08 UTC
Message-ID: <4d0b2a5f-305b-4350-b164-44923cb250d8@web.de>
In-Reply-To: <20231213080143.GA1684525@coredump.intra.peff.net>

Am 13.12.23 um 09:01 schrieb Jeff King:

Show 6 quoted lines

> On Tue, Dec 12, 2023 at 11:30:03PM +0100, René Scharfe wrote:
>
>> I wonder if
>> it's time to use the C99 type _Bool in our code.  It would allow
>> documenting that only two possible values exist in cases like the one
>> above.  That would be even more useful for function returns, I assume.

Show 10 quoted lines

> I don't even know that we'd need much of a weather-balloon patch. I
> think it would be valid to do:
>
>   #ifndef bool
>   #define bool int
>
> to handle pre-C99 compilers (if there even are any these days). Of
> course we probably need some conditional magic to try to "#include
> <stdbool.h>" for the actual C99. I guess we could assume C99 by default
> and then add NO_STDBOOL as an escape hatch if anybody complains.

The semantics are slightly different in edge cases, so that fallback would not be fully watertight. E.g. consider:

   bool b(bool cond) {return cond == true;}
   bool b2(void) {return b(2);}

b() returns false if you give it false and true for anything else. b2() returns true.

With int as the fallback this becomes:

   int b(int cond) {return cond == 1;}
   int b2(void) {return b(2);}

Now only 1 is recognized as true, b2() returns 0 (false).

A coding rule to not compare bools could mitigate that. Or a rule to only use the values true and false in bool context and to only use logical operators on them.

René